[Federal Register Volume 89, Number 45 (Wednesday, March 6, 2024)]
[Rules and Regulations]
[Pages 16202-16406]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-02637]



[[Page 16201]]

Vol. 89

Wednesday,

No. 45

March 6, 2024

Part III





Environmental Protection Agency





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40 CFR Parts 50, 53, and 58





Reconsideration of the National Ambient Air Quality Standards for 
Particulate Matter; Final Rule

  Federal Register / Vol. 89 , No. 45 / Wednesday, March 6, 2024 / 
Rules and Regulations  

[[Page 16202]]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

[EPA-HQ-OAR-2015-0072; FRL-8635-02-OAR]
RIN 2060-AV52


Reconsideration of the National Ambient Air Quality Standards for 
Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) 
reconsideration of the air quality criteria and the national ambient 
air quality standards (NAAQS) for particulate matter (PM), the EPA is 
revising the primary annual PM2.5 standard by lowering the 
level from 12.0 [micro]g/m\3\ to 9.0 [micro]g/m\3\. The Agency is 
retaining the current primary 24-hour PM2.5 standard and the 
primary 24-hour PM10 standard. The Agency also is not 
changing the secondary 24-hour PM2.5 standard, secondary 
annual PM2.5 standard, and secondary 24-hour PM10 
standard at this time. The EPA is also finalizing revisions to other 
key aspects related to the PM NAAQS, including revisions to the Air 
Quality Index (AQI) and monitoring requirements for the PM NAAQS.

DATES: This final rule is effective May 6, 2024.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2015-0072. All documents in the docket are 
listed on the https://www.regulations.gov website. Although listed in 
the index, some information is not publicly available, e.g., CBI or 
other information whose disclosure is restricted by statute. Certain 
other material, such as copyrighted material, is not placed on the 
internet and will be publicly available only in hard copy form. 
Publicly available docket materials are available electronically 
through https://www.regulations.gov.

FOR FURTHER INFORMATION CONTACT: Dr. Lars Perlmutt, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail Code C539-04, 
Research Triangle Park, NC 27711; telephone: (919) 541-3037; fax: (919) 
541-5315; email: [email protected].

SUPPLEMENTARY INFORMATION: 

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related PM Control Programs
    C. Review of the Air Quality Criteria and Standards for 
Particulate Matter
    1. Reviews Completed in 1971 and 1987
    2. Review Completed in 1997
    3. Review Completed in 2006
    4. Review Completed in 2012
    5. Review Initiated in 2014
    a. 2020 Proposed and Final Decisions
    b. Reconsideration of the 2020 PM NAAQS Final Action
    D. Air Quality Information
    1. Distribution of Particle Size in Ambient Air
    2. Sources and Emissions Contributing to PM in the Ambient Air
    3. Monitoring of Ambient PM
    4. Ambient Concentrations and Trends
    a. PM2.5 Mass
    b. PM2.5 Components
    c. PM10
    d. PM10-2.5
    e. UFP
    5. Characterizing Ambient PM2.5 Concentrations for 
Exposure
    a. Predicted Ambient PM2.5 and Exposure Based on 
Monitored Data
    b. Comparison of PM2.5 Fields in Estimating Exposure 
and Relative to Design Values
    6. Background PM
II. Rationale for Decisions on the Primary PM2.5 
Standards
    A. Introduction
    1. Background on the Current Standards
    2. Overview of the Health Effects Evidence
    a. Nature of Effects
    i. Mortality
    ii. Cardiovascular Effects
    iii. Respiratory Effects
    iv. Cancer
    v. Nervous System Effects
    vi. Other Effects
    b. Public Health Implications and At-Risk Populations
    c. PM2.5 Concentrations in Key Studies Reporting 
Health Effects
    i. PM2.5 Exposure Concentrations Evaluated in 
Experimental Studies
    ii. Ambient PM2.5 Concentrations in Locations of 
Epidemiologic Studies
    d. Uncertainties in the Health Effects Evidence
    3. Summary of Exposure and Risk Estimates
    a. Key Design Aspects
    b. Key Limitations and Uncertainties
    c. Summary of Risk Estimates
    B. Conclusions on the Primary PM2.5 Standards
    1. CASAC Advice
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decisions on the Primary PM2.5 Standards
III. Rationale for Decisions on the Primary PM10 Standard
    A. Introduction
    1. Background on the Current Standard
    2. Overview of Health Effects Evidence
    a. Nature of Effects
    i. Mortality
    ii. Cardiovascular Effects
    iii. Respiratory Effects
    iv. Cancer
    v. Metabolic Effects
    vi. Nervous System Effects
    B. Conclusions on the Primary PM10 Standard
    1. CASAC Advice
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decisions on the Primary PM10 Standard
IV. Communication of Public Health
    A. Air Quality Index Overview
    B. Air Quality Index Category Breakpoints for PM2.5
    1. Summary of Proposed Revisions
    a. Air Quality Index Values of 50, 100, and 150
    b. Air Quality Index Values of 200 and Above
    2. Summary of Significant Comments on Proposed Revisions
    a. Air Quality Index Values of 50, 100, and 150
    b. Air Quality Index Values of 200 and Above
    c. Other Comments
    3. Summary of Final Revisions
    C. Air Quality Index Category Breakpoints for PM10
    D. Air Quality Index Reporting
    1. Summary of Proposed Revisions
    2. Summary of Significant Comments on Proposed Revisions
    3. Summary of Final Revisions
V. Rationale for Decisions on the Secondary PM Standards
    A. Introduction
    1. Background on the Current Standards
    a. Non-Visibility Effects
    b. Visibility Effects
    2. Overview of Welfare Effects Evidence
    a. Nature of Effects
    i. Visibility
    ii. Climate
    iii. Materials
    3. Summary of Air Quality and Quantitative Information
    a. Visibility Effects
    i. Target Level of Protection in Terms of a PM2.5 
Visibility Index
    ii. Relationship Between the PM2.5 Visibility Index 
and the Current Secondary 24-Hour PM2.5 Standard
    b. Non-Visibility Effects
    B. Conclusions on the Secondary PM Standards
    1. CASAC Advice
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decisions on the Secondary PM Standards
VI. Interpretation of the NAAQS for PM
    A. Amendments to Appendix K: Interpretation of the NAAQS for 
Particulate Matter
    B. Amendments to Appendix N: Interpretation of the NAAQS for 
PM2.5
VII. Amendments to Ambient Monitoring and Quality Assurance 
Requirements

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    A. Amendment to 40 CFR Part 50 (Appendix L): Reference Method 
for the Determination of Fine Particulate Matter as PM2.5 
in the Atmosphere--Addition of the Tisch Cyclone as an Approved 
Second Stage Separator
    B. Issues Related to 40 CFR Part 53 (Reference and Equivalent 
Methods)
    C. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)
    D. Incorporating Data From Next-Generation Technologies
VIII. Clean Air Act Implementation Requirements for the Revised 
Primary Annual PM2.5 NAAQS
    A. Designation of Areas
    B. Section 110(a)(1) and (2) Infrastructure SIP Requirements
    C. Implementing the Revised Primary Annual PM2.5 
NAAQS in Nonattainment Areas
    D. Implementing the Primary and Secondary PM10 NAAQS
    E. Prevention of Significant Deterioration and Nonattainment New 
Source Review Programs for the Revised Primary Annual 
PM2.5 NAAQS
    F. Transportation Conformity Program
    G. General Conformity Program
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 14094: Modernizing Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act (NTTAA)
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations and Executive Order 14096: Revitalizing Our Nation's 
Commitment to Environmental Justice for All
    K. Congressional Review Act (CRA)
References

Executive Summary

    This document presents the Administrator's final decisions for the 
reconsideration of the 2020 final decision on the primary (health-
based) and secondary (welfare-based) National Ambient Air Quality 
Standards (NAAQS) for Particulate Matter (PM). More specifically, this 
document summarizes the background and rationale for the 
Administrator's final decisions to revise the primary annual 
PM2.5 standard by lowering the level from 12.0 [micro]g/m\3\ 
to 9.0 [micro]g/m\3\; to retain the current primary 24-hour 
PM2.5 standard (at a level of 35 [micro]g/m\3\); to retain 
the primary 24-hour PM10 standard; and, not to change the 
secondary PM standards at this time. In reaching his final decisions, 
the Administrator considered the currently available scientific 
evidence in the 2019 Integrated Science Assessment (2019 ISA) and the 
Supplement to the 2019 ISA (ISA Supplement), quantitative and policy 
analyses presented in the 2022 Policy Assessment (2022 PA), advice from 
the Clean Air Scientific Advisory Committee (CASAC), and public 
comments on the proposal. The EPA has established primary and secondary 
standards for PM2.5, which includes particles with diameters 
generally less than or equal to 2.5 [micro]m, and PM10, 
which includes particles with diameters generally less than or equal to 
10 [micro]m. The standards include two primary PM2.5 
standards: an annual average standard, averaged over three years, with 
a level of 12.0 [micro]g/m\3\, and a 24-hour standard with a 98th 
percentile form, averaged over three years, and a level of 35 [micro]g/
m\3\. It also includes a primary PM10 standard with a 24-
hour averaging time, and a level of 150 [micro]g/m\3\, not to be 
exceeded more than once per year on average over three years. Secondary 
PM standards are set equal to the primary standards, except that the 
level of the secondary annual PM2.5 standard is 15.0 
[micro]g/m\3\.
    The most recent of the PM NAAQS was completed in December 2020. In 
that review, the EPA retained the primary and secondary NAAQS, without 
revision (85 FR 82684, December 18, 2020). Following publication of the 
2020 final action, several parties filed petitions for review and 
petitions for reconsideration of the EPA's final decision.
    In June 2021, the Agency announced its decision to reconsider the 
2020 PM NAAQS final action.\1\ The EPA decided to reconsider the 
December 2020 decision because the available scientific evidence and 
technical information indicated that the current standards may not be 
adequate to protect public health and welfare, as required by the Clean 
Air Act. The EPA noted that the 2020 PA concluded that the scientific 
evidence and information called into question the adequacy of the 
primary PM2.5 standards and supported consideration of 
revising the level of the primary annual PM2.5 standard to 
below the current level of 12.0 [micro]g/m\3\ while retaining the 
primary 24-hour PM2.5 standard (U.S. EPA, 2020b). The EPA 
also noted that the 2020 PA concluded that the available scientific 
evidence and information did not call into question the adequacy of the 
primary PM10 or secondary PM standards and supported 
consideration of retaining the primary PM10 standard and 
secondary PM standards without revision (U.S. EPA, 2020b).
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    \1\ The press release for this announcement is available at: 
https://www.epa.gov/newsreleases/epa-reexamine-health-standards-harmful-soot-previous-administration-left-unchanged.
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    The final decisions presented in this document on the primary 
PM2.5 standards have been informed by key aspects of the 
available health effects evidence and conclusions contained in the 2019 
ISA and ISA Supplement, quantitative exposure/risk analyses and policy 
evaluations presented in the 2022 PA, advice from the CASAC \2\ and 
public comment received as part of this reconsideration.\3\ The health 
effects evidence newly available in this reconsideration, in 
conjunction with the full body of evidence critically evaluated in the 
2019 ISA, supports a causal relationship between long- and short-term 
exposures and mortality and cardiovascular effects, and the evidence 
supports a likely to be a causal relationship between long-term 
exposures and respiratory effects, nervous system effects, and cancer. 
The longstanding evidence base, including animal toxicological studies, 
controlled human exposure studies, and epidemiologic studies, 
reaffirms, and in some cases strengthens, the conclusions from past 
reviews regarding the health effects of PM2.5 exposures. 
Epidemiologic studies available in this reconsideration demonstrate 
generally positive, and often statistically significant, 
PM2.5 health effect associations. Such studies report 
associations between estimated PM2.5 exposures and non-
accidental, cardiovascular, or respiratory mortality; cardiovascular or 
respiratory hospitalizations or emergency room visits; and other 
mortality/morbidity outcomes (e.g., lung cancer mortality or incidence, 
asthma development). The scientific evidence available in this 
reconsideration, as evaluated in the 2019 ISA and ISA Supplement, 
includes

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a number of epidemiologic studies that use various methods to 
characterize exposure to PM2.5 (e.g., ground-based monitors 
and hybrid modeling approaches) and to evaluate associations between 
health effects and lower ambient PM2.5 concentrations. There 
are a number of recent epidemiologic studies that use varying study 
designs that reduce uncertainties related to confounding and exposure 
measurement error. The results of these analyses provide further 
support for the robustness of associations between PM2.5 
exposures and mortality and morbidity. Moreover, the Administrator 
notes that recent epidemiologic studies strengthen support for health 
effect associations at lower PM2.5 concentrations, with 
these new studies finding positive and significant associations when 
assessing exposure in locations and time periods with lower annual mean 
and 25th percentile concentrations than those evaluated in 
epidemiologic studies available at the time of previous reviews. 
Additionally, the experimental evidence (i.e., animal toxicological and 
controlled human exposure studies) strengthens the coherence of effects 
across scientific disciplines and provides additional support for 
potential biological pathways through which PM2.5 exposures 
could lead to the overt population-level outcomes reported in 
epidemiologic studies for the health effect categories for which a 
causal relationship (i.e., short- and long-term PM2.5 
exposure and mortality and cardiovascular effects) or likely to be 
causal relationship (i.e., short- and long-term PM2.5 
exposure and respiratory effects; and long-term PM2.5 
exposure and nervous system effects and cancer) was concluded.
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    \2\ In 2021, the Administrator announced his decision to 
reestablish the membership of the CASAC. The Administrator selected 
seven members to serve on the chartered CASAC, and appointed a PM 
CASAC panel to support the chartered CASAC's review of the draft ISA 
Supplement and the draft PA as a part of this reconsideration (see 
section I.C.6.b below for more information).
    \3\ More information regarding the CASAC review of the draft ISA 
Supplement and the draft PA, including opportunities for public 
comment, can be found in the following Federal Register notices: 86 
FR 54186, September 30, 2021; 86 FR 52673, September 22, 2021; 86 FR 
56263, October 8, 2021; 87 FR 958, January 7, 2022.
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    The available evidence in the 2019 ISA continues to provide support 
for factors that may contribute to increased risk of PM2.5-
related health effects including lifestage (children and older adults), 
pre-existing diseases (cardiovascular disease and respiratory disease), 
race/ethnicity, and socioeconomic status. For example, the 2019 ISA and 
ISA Supplement conclude that there is strong evidence that Black and 
Hispanic populations, on average, experience higher PM2.5 
exposures and PM2.5-related health risks than non-Hispanic 
White populations. In addition, studies evaluated in the 2019 ISA and 
ISA Supplement also provide evidence indicating that communities with 
lower socioeconomic status (SES), as assessed in epidemiologic studies 
using indicators of SES including income and educational attainment 
are, on average, exposed to higher concentrations of PM2.5 
compared to higher SES communities.
    The quantitative risk assessment, as well as policy considerations 
in the 2022 PA, also inform the final decisions on the primary 
PM2.5 standards. The risk assessment in this reconsideration 
focuses on all-cause or nonaccidental mortality associated with long- 
and short-term PM2.5 exposures. The primary analyses focus 
on exposure and risk associated with air quality that might occur in an 
area under air quality conditions that just meet the current and 
potential alternative standards. The risk assessment estimates that the 
current primary PM2.5 standards could allow a substantial 
number of PM2.5-associated premature deaths in the United 
States, and that public health improvements would be associated with 
just meeting all of the alternative (more stringent) annual and 24-hour 
standard levels modeled. Additionally, the results of the risk 
assessment suggest that for most of the U.S., the annual standard is 
the controlling standard and that revision to that standard has the 
most potential to reduce PM2.5 exposure-related risk. The 
analyses are summarized in this document and in the proposal and are 
described in detail in the 2022 PA.
    In its advice to the Administrator, in its review of the 2021 draft 
PA, the CASAC concurred that the currently available health effects 
evidence calls into question the adequacy of the primary annual 
PM2.5 standard. With regard to the primary annual 
PM2.5 standard, the majority of the CASAC concluded that the 
level of the standard should be revised within the range of 8.0 to 10.0 
[micro]g/m\3\, while the minority of the CASAC concluded that the 
primary annual PM2.5 standard should be revised to a level 
of 10.0 to 11.0 [micro]g/m\3\. With regard to the primary 24-hour 
PM2.5 standard, the CASAC did not reach consensus on the 
adequacy of the current standard. The majority of the CASAC concluded 
that the primary 24-hour PM2.5 was not adequate and that the 
level of the standard should be revised to within the range of 25 to 30 
[micro]g/m\3\, while the minority of the CASAC concluded that the 
standard was adequate and should be retained, without revision. 
Additionally, in their review of the 2019 draft PA, the CASAC did not 
reach consensus on the adequacy of the primary annual PM2.5 
standard, with the minority recommending revision and the majority 
recommending the standard be retained. In their review of the 2019 
draft PA, the CASAC reached consensus regarding the adequacy of the 
primary 24-hour PM2.5 standard, concluding that the standard 
should be retained.
    In considering how to revise the suite of primary PM2.5 
standards to provide the requisite degree of protection, the 
Administrator recognizes that the current annual standard and 24-hour 
standard, together, are intended to provide public health protection 
against the full distribution of short- and long-term PM2.5 
exposures. Further, he recognizes that changes in PM2.5 air 
quality designed to meet either the annual or the 24-hour standard 
would likely result in changes to both long-term average and short-term 
peak PM2.5 concentrations.
    As in 2012, the Administrator concludes that the most effective way 
to reduce total population risk associated with both long- and short-
term PM2.5 exposures is to set a generally controlling 
annual standard, and to provide supplemental protection against the 
occurrence of peak 24-hour PM2.5 concentrations by means of 
a 24-hour standard set at the appropriate level. Based on the current 
evidence and quantitative information, as well as consideration of 
CASAC advice and public comments, the Administrator concludes that the 
current primary annual PM2.5 standard is not adequate to 
protect public health with an adequate margin of safety. The 
Administrator notes that the CASAC was unanimous in its advice on the 
2021 draft PA regarding the need to revise the annual standard. In 
considering the appropriate level for a revised annual standard, the 
Administrator concludes that a standard set at a level of 9.0 [micro]g/
m\3\ reflects his judgment about placing the most weight on the 
strongest available evidence while appropriately weighing the 
uncertainties.
    With regard to the primary 24-hour PM2.5 standard, the 
Administrator finds the available scientific evidence and quantitative 
information to be insufficient to call into question the adequacy of 
the public health protection afforded by the current 24-hour standard. 
He further notes that a more stringent annual standard set at a level 
of 9.0 [micro]g/m\3\ is expected to reduce both average (annual) 
concentrations and peak (daily) concentrations. The Administrator also 
notes that, in their review of the 2021 draft PA, the CASAC did not 
reach consensus on whether revisions to the primary 24-hour 
PM2.5 standard are warranted at this time. He also notes 
that, in their review of the 2019 draft PA, the CASAC did reach 
consensus that the primary 24-hour PM2.5 standard should be 
retained. The Administrator concludes that the 24-hour standard should 
be retained to

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continue to provide requisite protection against short-term peak 
PM2.5 concentrations, particularly when considered in 
conjunction with the protection provided by the suite of standards and 
the decision to revise the annual standard to a level of 9.0 [micro]g/
m\3\.
    The primary PM10 standard is intended to provide public 
health protection against health effects related to exposures to 
PM10-2.5, which are particles with a diameter between 10 
[micro]m and 2.5 [micro]m. The final decision to retain the current 24-
hour PM10 standard has been informed by key aspects of the 
available health effects evidence and conclusions contained in the 2019 
ISA, the policy evaluations presented in the 2022 PA, advice from the 
CASAC and public comments. Specifically, the health effects evidence 
for PM10-2.5 exposures is somewhat strengthened since past 
reviews, although the strongest evidence still only provides support 
for a suggestive of, but not sufficient to infer, causal relationship 
with long- and short-term exposures and mortality and cardiovascular 
effects, short-term exposures and respiratory effects, and long-term 
exposures and cancer, nervous system effects, and metabolic effects. In 
reaching his final decision on the primary PM10 standard, 
the Administrator recognizes that, while the available health effects 
evidence has expanded, recent studies are subject to the same types of 
uncertainties that were judged to be important in previous reviews. He 
also recognizes that, in their review of the 2019 draft PA and the 2021 
draft PA, the CASAC generally agreed that it was reasonable to retain 
the primary 24-hour PM10 standard given the available 
scientific evidence, including retaining PM10 as the 
indicator. He concludes that the newly available evidence does not call 
into question the adequacy of the current primary PM10 
standard, and retains that standard, without revision.
    With respect to the secondary PM standards, this reconsideration 
focuses on visibility, climate, and materials effects.\4\ The 
Administrator's final decision to not change the current secondary 
standards at this time has been informed by key aspects of the 
currently available welfare effects evidence as well as the conclusions 
contained in the 2019 ISA and ISA Supplement; quantitative analyses of 
visibility impairment; policy evaluations presented in the 2022 PA; 
advice from the CASAC; and public comments. Specifically, the welfare 
effects evidence available in this reconsideration is consistent with 
the evidence available in previous reviews and supports a causal 
relationship between PM and visibility, climate, and materials effects. 
With regard to visibility effects, the Administrator notes that he 
judges that the evidence supports a target level of protection of 27 
dv. He further notes that the results of quantitative analyses of 
visibility impairment suggest that in areas that meet the current 
secondary 24-hour PM2.5 standard that estimated light 
extinction in terms of a 3-year visibility metric would be at or well 
below the target level of protection. With regard to climate and 
materials effects, while the evidence has expanded since previous 
reviews, significant limitations and uncertainties remain in the 
evidence. While the evidence has expanded since previous reviews, the 
available scientific evidence remains insufficient to allow the 
Administrator to make a reasoned judgment about what specific 
standard(s) would be requisite to protect against known or anticipated 
adverse effects to public welfare from PM's effects on materials damage 
or climate.-In their review of the 2019 draft PA and the 2021 draft PA, 
the CASAC did not recommend revising the secondary PM standards. In 
considering the available evidence and quantitative information, with 
its inherent uncertainties and limitations, the Administrator judges 
that it is appropriate not to change the secondary PM standards at this 
time.
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    \4\ Consistent with the 2016 Integrated Review Plan (U.S. EPA, 
2016), other welfare effects of PM, such as ecological effects, are 
being considered in the separate, on-going review of the secondary 
NAAQS for oxides of nitrogen, oxides of sulfur and PM. Accordingly, 
the public welfare protection provided by the secondary PM standards 
against ecological effects such as those related to deposition of 
nitrogen- and sulfur-containing compounds in vulnerable ecosystems 
is being considered in that separate review. Thus, the 
Administrator's conclusion in this reconsideration of the 2020 final 
decision is focused only and specifically on the adequacy of public 
welfare protection provided by the secondary PM standards from 
effects related to visibility, climate, and materials and hereafter 
``welfare effects'' refers to those welfare effects.
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    The final revisions to the primary annual PM2.5 NAAQS 
trigger a process under which States (and Tribes, if they choose) make 
recommendations to the Administrator regarding designations, 
identifying areas of the country that either meet or do not meet the 
new or revised PM NAAQS. Those areas that do not meet the revised PM 
NAAQS will need to develop plans that demonstrate how they will meet 
the standards. As part of these plans, states have the opportunity to 
advance environmental justice, in this case for overburdened 
communities in areas with high PM concentrations above the NAAQS, by 
using the tools described in the current PM NAAQS implementation 
guidance (80 FR 58010, 58136, August 25, 2016). The EPA is not making 
changes to any of the current PM NAAQS implementation programs in this 
final rulemaking.
    On other topics, the EPA is finalizing two sets of changes to the 
PM2.5 sub-index of the Air Quality Index (AQI). First, the 
EPA is continuing to use the approach used in the revisions to the AQI 
in 2012 (77 FR 38890, June 29, 2012) of setting the lower breakpoints 
(50, 100 and 150) based on the levels of the primary annual and 24-hour 
PM2.5 standards. In so doing, the EPA is revising the AQI 
value of 50 to 9.0 [micro]g/m\3\ and is retaining the AQI values of 100 
and 150 at 35.4 [micro]g/m\3\ and 55.4 [micro]g/m\3\, respectively. 
Second, the EPA is revising the upper AQI breakpoints (200 and above), 
and replacing the linear-relationship approach used in 1999 (64 FR 
42530, August 4, 1999) to set these breakpoints, with an approach that 
more fully considers the PM2.5 health effects evidence from 
controlled human exposure and epidemiologic studies that has become 
available in the last 20 years. The EPA is also revising the AQI values 
of 200, 300 and 500 to 125.4 [micro]g/m\3\, 225.4 [micro]g/m\3\, and 
325.4 [micro]g/m\3\, respectively. In addition, this final rule revises 
the daily reporting requirement from 5 days per week to 7 days per 
week, while also reformatting appendix G and providing clarifications.
    With regard to monitoring-related activities, the EPA finalizes 
revisions to data calculations and ambient air monitoring requirements 
for PM to improve the usefulness and appropriateness of data used in 
regulatory decision making and to better characterize air quality in 
communities that are at increased risk of PM2.5 exposure and 
health risk. These changes are found in 40 CFR part 50 (appendices K, 
L, and N), part 53, and part 58 with associated appendices (A, B, C, D, 
and E). These changes include addressing updates in data calculations, 
approval of reference and equivalent methods, updates in quality 
assurance statistical calculations to account for lower concentration 
measurements, updates to support improvements in PM methods, a revision 
to the PM2.5 network design to account for at-risk 
populations, and updates to the Probe and Monitoring Path Siting 
Criteria for NAAQS pollutants.
    In setting the NAAQS, the EPA may not consider the costs of 
implementing the standards. This was confirmed by the Supreme Court in 
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001), as discussed in section II.A of this document. As has 
traditionally been

[[Page 16206]]

done in NAAQS rulemaking, the EPA prepared a Regulatory Impact Analysis 
(RIA) to provide the public with information on the potential costs and 
benefits of attaining several alternative PM2.5 standard 
levels. In NAAQS rulemaking, the RIA is done for informational purposes 
only, and the final decisions on the NAAQS in this rulemaking are not 
based on consideration of the information or analyses in the RIA. The 
RIA fulfills the requirements of Executive Orders 14094, 13563, and 
12866. The RIA estimates the costs and monetized human health benefits 
of attaining the revised and two alternative annual PM2.5 
standard levels and one alternative 24-hour PM2.5 standard 
level. Specifically, the RIA examines the revised annual standard level 
of 9.0 [micro]g/m\3\ in combination with the current 24-hour standard 
of 35 [micro]g/m\3\ (i.e., 9.0/35 [micro]g/m\3\), as well as the 
following less and more stringent alternative standard levels: (1) An 
alternative annual standard level of 10.0 [micro]g/m\3\ in combination 
with the current 24-hour standard (i.e., 10.0/35 [micro]g/m\3\), (2) an 
alternative annual standard level of 8.0 [micro]g/m\3\ in combination 
with the current 24-hour standard (i.e., 8.0/35 [micro]g/m\3\), and (3) 
an alternative 24-hour standard level of 30 [micro]g/m\3\ in 
combination with an alternative annual standard level of 10 [micro]g/
m\3\ (i.e., 10.0/30 [micro]g/m\3\). The RIA presents estimates of the 
costs and benefits of applying illustrative national control strategies 
in 2032 after implementing existing and expected regulations and 
assessing emissions reductions to meet the current annual and 24-hour 
particulate matter NAAQS (12.0/35 [micro]g/m\3\).

I. Background

A. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment 
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the 
Administrator to identify and list certain air pollutants and then to 
issue air quality criteria for those pollutants. The Administrator is 
to list those pollutants ``emissions of which, in his judgment, cause 
or contribute to air pollution which may reasonably be anticipated to 
endanger public health or welfare''; ``the presence of which in the 
ambient air results from numerous or diverse mobile or stationary 
sources''; and for which he ``plans to issue air quality criteria. . . 
.'' (42 U.S.C. 7408(a)(1)). Air quality criteria are intended to 
``accurately reflect the latest scientific knowledge useful in 
indicating the kind and extent of all identifiable effects on public 
health or welfare which may be expected from the presence of [a] 
pollutant in the ambient air. . . .'' (42 U.S.C. 7408(a)(2)).
    Section 109 [42 U.S.C. 7409] directs the Administrator to propose 
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for 
which air quality criteria are issued [42 U.S.C. 7409(a)]. Section 
109(b)(1) defines primary standards as ones ``the attainment and 
maintenance of which in the judgment of the Administrator, based on 
such criteria and allowing an adequate margin of safety, are requisite 
to protect the public health.'' \5\ Under section 109(b)(2), a 
secondary standard must ``specify a level of air quality the attainment 
and maintenance of which, in the judgment of the Administrator, based 
on such criteria, is requisite to protect the public welfare from any 
known or anticipated adverse effects associated with the presence of 
[the] pollutant in the ambient air.'' \6\
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    \5\ The legislative history of section 109 indicates that a 
primary standard is to be set at ``the maximum permissible ambient 
air level . . . which will protect the health of any [sensitive] 
group of the population,'' and that for this purpose ``reference 
should be made to a representative sample of persons comprising the 
sensitive group rather than to a single person in such a group.'' S. 
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
    \6\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on 
welfare include, but are not limited to, ``effects on soils, water, 
crops, vegetation, manmade materials, animals, wildlife, weather, 
visibility, and climate, damage to and deterioration of property, 
and hazards to transportation, as well as effects on economic values 
and on personal comfort and well-being.''
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    In setting primary and secondary standards that are ``requisite'' 
to protect public health and welfare, respectively, as provided in 
section 109(b), the EPA's task is to establish standards that are 
neither more nor less stringent than necessary. In so doing, the EPA 
may not consider the costs of implementing the standards. See generally 
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001). Likewise, ``[a]ttainability and technological feasibility 
are not relevant considerations in the promulgation of national ambient 
air quality standards.'' American Petroleum Institute v. Costle, 665 
F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corporation v. 
EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019).
    The requirement that primary standards provide an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(D.C. Cir. 1980); American Petroleum Institute v. Costle, 665 F.2d at 
1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 
(D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 
2013). Both kinds of uncertainties are components of the risk 
associated with pollution at levels below those at which human health 
effects can be said to occur with reasonable scientific certainty. 
Thus, in selecting primary standards that include an adequate margin of 
safety, the Administrator is seeking not only to prevent pollution 
levels that have been demonstrated to be harmful but also to prevent 
lower pollutant levels that may pose an unacceptable risk of harm, even 
if the risk is not precisely identified as to nature or degree. The CAA 
does not require the Administrator to establish a primary NAAQS at a 
zero-risk level or at background concentration levels, see Lead 
Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 
F.3d at 1351, but rather at a level that reduces risk sufficiently so 
as to protect public health with an adequate margin of safety.
    In addressing the requirement for an adequate margin of safety, the 
EPA considers such factors as the nature and severity of the health 
effects involved, the size of the sensitive population(s), and the kind 
and degree of uncertainties. The selection of any particular approach 
to providing an adequate margin of safety is a policy choice left 
specifically to the Administrator's judgment. See Lead Industries Ass'n 
v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
    Section 109(d)(1) of the Act requires the review every five years 
of existing air quality criteria and, if appropriate, the revision of 
those criteria to reflect advances in scientific knowledge on the 
effects of the pollutant on public health and welfare. Under the same 
provision, the EPA is also to review every five years and, if 
appropriate, revise the NAAQS, based on the revised air quality 
criteria. Section 109(d)(1) also provides that the Administrator may 
review and revise criteria or promulgate new standards earlier or more 
frequently.
    Section 109(d)(2) addresses the appointment and advisory functions 
of an independent scientific review committee. Section 109(d)(2)(A) 
requires the Administrator to appoint this committee, which is to be 
composed of ``seven members including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air

[[Page 16207]]

pollution control agencies.'' Section 109(d)(2)(B) provides that the 
independent scientific review committee ``shall complete a review of 
the criteria . . . and the national primary and secondary ambient air 
quality standards . . . and shall recommend to the Administrator any 
new . . . standards and revisions of existing criteria and standards as 
may be appropriate. . . .'' Since the early 1980s, this independent 
review function has been performed by the Clean Air Scientific Advisory 
Committee (CASAC) of the EPA's Science Advisory Board.
    As previously noted, the Supreme Court has held that section 109(b) 
``unambiguously bars cost considerations from the NAAQS-setting 
process.'' Whitman v. Am. Trucking Associations, 531 U.S. 457, 471 
(2001). Accordingly, while some of these issues regarding which 
Congress has directed the CASAC to advise the Administrator are ones 
that are relevant to the standard setting process, others are not. 
Issues that are not relevant to standard setting may be relevant to 
implementation of the NAAQS once they are established.

B. Related PM Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under section 110, Part C, and Part D, Subparts 1 and 
4 of the CAA, and related provisions and regulations, States are to 
submit, for the EPA's approval, State implementation plans (SIPs) that 
provide for the attainment and maintenance of the NAAQS for PM through 
control programs directed to sources of the pollutants involved. The 
States, in conjunction with the EPA, also administer the prevention of 
significant deterioration of air quality program that covers these 
pollutants (see 42 U.S.C. 7470-7479). In addition, Federal programs 
provide for or result in nationwide reductions in emissions of PM and 
its precursors under Title II of the Act, 42 U.S.C. 7521-7574, which 
involves controls for motor vehicles and nonroad engines and equipment; 
the new source performance standards under section 111 of the Act, 42 
U.S.C. 7411; and the national emissions standards for hazardous 
pollutants under section 112 of the Act, 42 U.S.C. 7412.

C. Review of the Air Quality Criteria and Standards for Particulate 
Matter

1. Reviews Completed in 1971 and 1987
    The EPA first established NAAQS for PM in 1971 (36 FR 8186, April 
30, 1971), based on the original Air Quality Criteria Document (AQCD) 
(DHEW, 1969).\7\ The Federal reference method (FRM) specified for 
determining attainment of the original standards was the high-volume 
sampler, which collects PM up to a nominal size of 25 to 45 [micro]m 
(referred to as total suspended particulates or TSP). The primary 
standards were set at 260 [micro]g/m\3\, 24-hour average, not to be 
exceeded more than once per year, and 75 [micro]g/m\3\, annual 
geometric mean. The secondary standards were set at 150 [micro]g/m\3\, 
24-hour average, not to be exceeded more than once per year, and 60 
[micro]g/m\3\, annual geometric mean.
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    \7\ Prior to the review initiated in 2007 (see below), the AQCD 
provided the scientific foundation (i.e., the air quality criteria) 
for the NAAQS. Beginning in that review, the Integrated Science 
Assessment (ISA) has replaced the AQCD.
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    In October 1979 (44 FR 56730, October 2, 1979), the EPA announced 
the first periodic review of the air quality criteria and NAAQS for PM. 
Revised primary and secondary standards were promulgated in 1987 (52 FR 
24634, July 1, 1987). In the 1987 decision, the EPA changed the 
indicator for particles from TSP to PM10, in order to focus 
on the subset of inhalable particles small enough to penetrate to the 
thoracic region of the respiratory tract (including the 
tracheobronchial and alveolar regions), referred to as thoracic 
particles.\8\ The level of the 24-hour standards (primary and 
secondary) was set at 150 [micro]g/m\3\, and the form was one expected 
exceedance per year, on average over three years. The level of the 
annual standards (primary and secondary) was set at 50 [micro]g/m\3\, 
and the form was the annual arithmetic mean, averaged over three years.
---------------------------------------------------------------------------

    \8\ PM10 refers to particles with a nominal mean 
aerodynamic diameter less than or equal to 10 [micro]m. More 
specifically, 10 [micro]m is the aerodynamic diameter for which the 
efficiency of particle collection is 50 percent.
---------------------------------------------------------------------------

2. Review Completed in 1997
    In April 1994, the EPA announced its plans for the second periodic 
review of the air quality criteria and NAAQS for PM, and in 1997 the 
EPA promulgated revisions to the NAAQS (62 FR 38652, July 18, 1997). In 
the 1997 decision, the EPA determined that the fine and coarse 
fractions of PM10 should be considered separately. This 
determination was based on evidence that serious health effects were 
associated with short- and long-term exposures to fine particles in 
areas that met the existing PM10 standards. The EPA added 
new standards, using PM2.5 as the indicator for fine 
particles (with PM2.5 referring to particles with a nominal 
mean aerodynamic diameter less than or equal to 2.5 [micro]m). The new 
primary standards were as follows: (1) An annual standard with a level 
of 15.0 [micro]g/m\3\, based on the 3-year average of annual arithmetic 
mean PM2.5 concentrations from single or multiple community-
oriented monitors; \9\ and (2) a 24-hour standard with a level of 65 
[micro]g/m\3\, based on the 3-year average of the 98th percentile of 
24-hour PM2.5 concentrations at each monitor within an area. 
Also, the EPA established a new reference method for the measurement of 
PM2.5 in the ambient air and adopted rules for determining 
attainment of the new standards. To continue to address the health 
effects of the coarse fraction of PM10 (referred to as 
thoracic coarse particles or PM10-2.5, generally including 
particles with a nominal mean aerodynamic diameter greater than 2.5 
[micro]m and less than or equal to 10 [micro]m), the EPA retained the 
primary annual PM10 standard and revised the form of the 
primary 24-hour PM10 standard to be based on the 99th 
percentile of 24-hour PM10 concentrations at each monitor in 
an area. The EPA revised the secondary standards by setting them equal 
in all respects to the primary standards.
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    \9\ The 1997 annual PM2.5 standard was compared with 
measurements made at the community-oriented monitoring site 
recording the highest concentration or, if specific constraints were 
met, measurements from multiple community-oriented monitoring sites 
could be averaged (i.e., ``spatial averaging''). In the last review 
(completed in 2012) the EPA replaced the term ``community-oriented'' 
monitor with the term ``area-wide'' monitor. Area-wide monitors are 
those sited at the neighborhood scale or larger, as well as those 
monitors sited at micro- or middle-scales that are representative of 
many such locations in the same core-based statistical area (CBSA) 
(78 FR 3236, January 15, 2013).
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    Following promulgation of the 1997 PM NAAQS, petitions for review 
were filed by several parties, addressing a broad range of issues. In 
May 1999, the U.S. Court of Appeals for the District of Columbia 
Circuit (D.C. Circuit) upheld the EPA's decision to establish fine 
particle standards and to regulate coarse particle pollution, but 
vacated the 1997 PM10 standards, concluding that the EPA had 
not provided a reasonable explanation justifying use of PM10 
as an indicator for coarse particles. American Trucking Associations, 
Inc. v. EPA, 175 F. 3d 1027 (D.C. Cir. 1999). Pursuant to the D.C. 
Circuit's decision, the EPA removed the vacated 1997 PM10 
standards, and the pre-existing 1987 PM10 standards remained 
in place (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld 
the EPA's determination not to establish more stringent secondary 
standards for fine particles to address effects on visibility. American 
Trucking Associations v. EPA, 175 F. 3d at 1027.

[[Page 16208]]

    The D.C. Circuit also addressed more general issues related to the 
NAAQS, including issues related to the consideration of costs in 
setting NAAQS and the EPA's approach to establishing the levels of 
NAAQS. Regarding the cost issue, the court reaffirmed prior rulings 
holding that in setting NAAQS the EPA is ``not permitted to consider 
the cost of implementing those standards.'' American Trucking 
Associations v. EPA, 175 F. 3d at 1040-41. Regarding the levels of 
NAAQS, the court held that the EPA's approach to establishing the level 
of the standards in 1997 (i.e., both for PM and for the ozone NAAQS 
promulgated on the same day) effected ``an unconstitutional delegation 
of legislative authority.'' American Trucking Associations v. EPA, 175 
F. 3d at 1034-40. Although the court stated that ``the factors EPA uses 
in determining the degree of public health concern associated with 
different levels of ozone and PM are reasonable,'' it remanded the rule 
to the EPA, stating that when the EPA considers these factors for 
potential non-threshold pollutants ``what EPA lacks is any determinate 
criterion for drawing lines'' to determine where the standards should 
be set.
    The D.C. Circuit's holding on the cost and constitutional issues 
were appealed to the United States Supreme Court. In February 2001, the 
Supreme Court issued a unanimous decision upholding the EPA's position 
on both the cost and constitutional issues. Whitman v. American 
Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional 
issue, the Court held that the statutory requirement that NAAQS be 
``requisite'' to protect public health with an adequate margin of 
safety sufficiently guided the EPA's discretion, affirming the EPA's 
approach of setting standards that are neither more nor less stringent 
than necessary.
    The Supreme Court remanded the case to the D.C. Circuit for 
resolution of any remaining issues that had not been addressed in that 
court's earlier rulings. Id. at 475-76. In a March 2002 decision, the 
D.C. Circuit rejected all remaining challenges to the standards, 
holding that the EPA's PM2.5 standards were reasonably 
supported by the administrative record and were not ``arbitrary and 
capricious.'' American Trucking Associations v. EPA, 283 F. 3d 355, 
369-72 (D.C. Cir. 2002).
3. Review Completed in 2006
    In October 1997, the EPA published its plans for the third periodic 
review of the air quality criteria and NAAQS for PM (62 FR 55201, 
October 23, 1997). After the CASAC and public review of several drafts, 
the EPA's National Center for Environmental Assessment (NCEA) finalized 
the AQCD in October 2004 (U.S. EPA, 2004a). The EPA's Office of Air 
Quality Planning and Standards (OAQPS) finalized a Risk Assessment and 
Staff Paper in December 2005 (Abt Associates, 2005; U.S. EPA, 
2005).\10\ On December 20, 2005, the EPA announced its proposed 
decision to revise the NAAQS for PM and solicited public comment on a 
broad range of options (71 FR 2620, January 17, 2006). On September 21, 
2006, the EPA announced its final decisions to revise the primary and 
secondary NAAQS for PM to provide increased protection of public health 
and welfare, respectively (71 FR 61144, October 17, 2006). With regard 
to the primary and secondary standards for fine particles, the EPA 
revised the level of the 24-hour PM2.5 standards to 35 
[micro]g/m\3\, retained the level of the annual PM2.5 
standards at 15.0 [micro]g/m\3\, and revised the form of the annual 
PM2.5 standards by narrowing the constraints on the optional 
use of spatial averaging. With regard to the primary and secondary 
standards for PM10, the EPA retained the 24-hour standards, 
with levels at 150 [micro]g/m\3\, and revoked the annual standards. The 
then-Administrator judged that the available evidence generally did not 
suggest a link between long-term exposure to existing ambient levels of 
coarse particles and health or welfare effects. In addition, a new 
reference method was added for the measurement of PM10-2.5 
in the ambient air in order to provide a basis for approving Federal 
Equivalent Methods (FEMs) and to promote the gathering of scientific 
data to support future reviews of the PM NAAQS.
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    \10\ Prior to the review initiated in 2007, the Staff Paper 
presented the EPA staff's considerations and conclusions regarding 
the adequacy of existing NAAQS and, when appropriate, the potential 
alternative standards that could be supported by the evidence and 
information. More recent reviews present this information in the 
Policy Assessment.
---------------------------------------------------------------------------

    Several parties filed petitions for review following promulgation 
of the revised PM NAAQS in 2006. On February 24, 2009, the D.C. Circuit 
issued its opinion in the case American Farm Bureau Federation v. EPA, 
559 F. 3d 512 (D.C. Cir. 2009). The court remanded the primary annual 
PM2.5 NAAQS to the EPA because the Agency had failed to 
adequately explain why the standards provided the requisite protection 
from both short- and long-term exposures to fine particles, including 
protection for at-risk populations. Id. at 520-27. With regard to the 
standards for PM10, the court upheld the EPA's decisions to 
retain the 24-hour PM10 standard to provide protection from 
thoracic coarse particle exposures and to revoke the annual 
PM10 standard. Id. at 533-38. With regard to the secondary 
PM2.5 standards, the court remanded the standards to the EPA 
because the Agency failed to adequately explain why setting the 
secondary PM standards identical to the primary standards provided the 
required protection for public welfare, including protection from 
visibility impairment. Id. at 528-32. The EPA responded to the court's 
remands as part of the next review of the PM NAAQS, which was initiated 
in 2007 (discussed below).
4. Review Completed in 2012
    In June 2007, the EPA initiated the fourth periodic review of the 
air quality criteria and the PM NAAQS by issuing a call for information 
(72 FR 35462, June 28, 2007). Based on the NAAQS review process, as 
revised in 2008 and again in 2009,\11\ the EPA held science/policy 
issue workshops on the primary and secondary PM NAAQS (72 FR 34003, 
June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released 
the planning and assessment documents that comprise the review process 
(i.e., Integrated Review Plan, (IRP; U.S. EPA, 2008), Integrated 
Science Assessment (ISA; U.S. EPA, 2009a), Risk and Exposure Assessment 
(REA) planning documents for health and welfare (U.S. EPA, 2009b, U.S. 
EPA, 2009c), a quantitative health risk assessment (U.S. EPA, 2010a) 
and an urban-focused visibility assessment (U.S. EPA, 2010b), and a 
Policy Assessment (PA; U.S. EPA, 2011). In June 2012, the EPA announced 
its proposed decision to revise the NAAQS for PM (77 FR 38890, June 29, 
2012).
---------------------------------------------------------------------------

    \11\ The history of the NAAQS review process, including 
revisions to the process, is discussed at https://www.epa.gov/naaqs/historical-information-naaqs-review-process.
---------------------------------------------------------------------------

    In December 2012, the EPA announced its final decisions to revise 
the primary NAAQS for PM to provide increased protection of public 
health (78 FR 3086, January 15, 2013). With regard to primary standards 
for PM2.5, the EPA revised the level of the annual 
PM2.5 standard \12\ to 12.0 [micro]g/m\3\ and retained the 
24-hour PM2.5 standard, with its level of 35 [micro]g/m\3\. 
For the primary PM10 standard, the EPA retained the 24-hour 
standard to continue to provide protection against effects associated 
with short-term exposure to thoracic coarse particles (i.e., 
PM10-2.5). With regard to the secondary PM standards, the 
EPA generally retained the 24-hour

[[Page 16209]]

and annual PM2.5 standards \13\ and the 24-hour 
PM10 standard to address visibility and non-visibility 
welfare effects.
---------------------------------------------------------------------------

    \12\ The EPA also eliminated the option for spatial averaging.
    \13\ Consistent with the primary standard, the EPA eliminated 
the option for spatial averaging with the annual standard.
---------------------------------------------------------------------------

    As with previous reviews, petitioners challenged the EPA's final 
rule. Petitioners argued that the EPA acted unreasonably in revising 
the level and form of the annual standard and in amending the 
monitoring network provisions. On judicial review, the revised 
standards and monitoring requirements were upheld in all respects. NAM 
v. EPA, 750 F.3d 921 (D.C. Cir. 2014).
5. Review Initiated in 2014
    In December 2014, the EPA announced the initiation of the current 
periodic review of the air quality criteria for PM and of the 
PM2.5 and PM10 NAAQS and issued a call for 
information (79 FR 71764, December 3, 2014). On February 9 to 11, 2015, 
the EPA's NCEA and OAQPS held a public workshop to inform the planning 
for the review of the PM NAAQS (announced in 79 FR 71764, December 3, 
2014). Workshop participants, including a wide range of external 
experts as well as the EPA staff representing a variety of areas of 
expertise (e.g., epidemiology, human and animal toxicology, risk/
exposure analysis, atmospheric science, visibility impairment, climate 
effects), were asked to highlight significant new and emerging PM 
research, and to make recommendations to the Agency regarding the 
design and scope of the review. This workshop provided for a public 
discussion of the key science and policy-relevant issues around which 
the EPA structured the review of the PM NAAQS and of the most 
meaningful new scientific information that would be available in the 
review to inform understanding of these issues.
    The input received at the workshop guided the EPA staff in 
developing a draft IRP, which was reviewed by the CASAC Particulate 
Matter Panel and discussed on public teleconferences held in May 2016 
(81 FR 13362, March 14, 2016) and August 2016 (81 FR 39043, June 15, 
2016). Advice from the CASAC, supplemented by the Particulate Matter 
Panel, and input from the public were considered in developing the 
final IRP (U.S. EPA, 2016). The final IRP discusses the approaches to 
be taken in developing key scientific, technical, and policy documents 
in the review and the key policy-relevant issues that frame the EPA's 
consideration of whether the primary and/or secondary NAAQS for PM 
should be retained or revised.
    In May 2018, the then-Administrator issued a memorandum announcing 
the Agency's intention to conduct the review of the PM NAAQS in such a 
manner as to ensure that any necessary revisions were finalized by 
December 2020 (Pruitt, 2018). Following this memo, on October 10, 2018, 
the then-Administrator additionally announced that the role of 
reviewing the key assessments developed as part of the ongoing review 
of the PM NAAQS (i.e., drafts of the ISA and PA) would be performed by 
the seven-member chartered CASAC (i.e., rather than the CASAC 
Particulate Matter Panel that reviewed the draft IRP).\14\
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    \14\ Announcement available at: https://www.regulations.gov/document/EPA-HQ-OAR-2015-0072-0223.
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    The EPA released the draft ISA in October 2018 (83 FR 53471, 
October 23, 2018). The draft ISA was reviewed by the chartered CASAC at 
a public meeting held in Arlington, VA in December 2018 (83 FR 55529, 
November 6, 2018) and was discussed on a public teleconference in March 
2019 (84 FR 8523, March 8, 2019). The CASAC provided its advice on the 
draft ISA in a letter to the then-Administrator dated April 11, 2019 
(Cox, 2019a). The EPA addressed these comments in the final ISA, which 
was released in December 2019 (U.S. EPA, 2019a).
    The EPA released the draft PA in September 2019 (84 FR 47944, 
September 11, 2019). The draft PA was reviewed by the chartered CASAC 
and discussed in October 2019 at a public meeting held in Cary, NC. 
Public comments were received via a separate public teleconference (84 
FR 51555, September 30, 2019). A public meeting to discuss the 
chartered CASAC letter and response to charge questions on the draft PA 
was held in Cary, NC, in October 2019 (84 FR 51555, September 30, 
2019), and the CASAC provided its advice on the draft PA, including its 
advice on the current primary and secondary PM standards, in a letter 
to the then-Administrator dated December 16, 2019 (Cox, 2019b). With 
regard to the primary standards, the CASAC recommended retaining the 
current 24-hour PM2.5 and PM10 standards but did 
not reach consensus on the adequacy of the current annual 
PM2.5 standard. Some CASAC members expressed support for 
retaining the current primary annual PM2.5 standard while 
other members expressed support for revising that standard in order to 
increase public health protection (Cox, 2019b, p. 1 of letter). These 
views are described in greater detail in the letter to the then-
Administrator (Cox, 2019b) and in the notice of final rulemaking (85 FR 
82706-82707, December 18, 2020), as well as below. With regard to the 
secondary standards, the CASAC recommended retaining the current 
standards. In response to the CASAC's comments, the 2020 final PA 
incorporated a number of changes (Cox, 2019b, U.S. EPA, 2020b), as 
described in detail in section I.C.5 of the 2020 proposal document (85 
FR 24100, April 30, 2020).
a. 2020 Proposed and Final Actions
    On April 14, 2020, the EPA proposed to retain all of the primary 
and secondary PM standards, without revision. These proposed decisions 
were published in the Federal Register on April 30, 2020 (85 FR 24094, 
April 30, 2020). The EPA's final decision on the PM NAAQS was published 
in the Federal Register on December 18, 2020 (85 FR 82684, December 18, 
2020). In the 2020 rulemaking, the EPA retained the primary and 
secondary PM2.5 and PM10 standards, without 
revision. The then-Administrator's rationale for his decisions is 
described in more detail in section II, III, and V below, and is 
briefly summarized here.
    In reaching his final decision to retain the primary annual and 24-
hour PM2.5 standards, the then-Administrator considered the 
available scientific evidence, quantitative information, CASAC advice, 
and public comments in his supporting rationale in the 2020 final 
action (85 FR 82714, December 18, 2020). In so doing, he concluded that 
the available controlled human exposure studies did not provide support 
for additional public health protection against exposures to peak 
PM2.5 concentrations, beyond the protection provided by the 
combination of the current primary annual and 24-hour PM2.5 
standards. He also noted that the available epidemiologic studies did 
not indicate that associations in those studies are strongly influenced 
by exposures to peak concentrations in the air quality distribution and 
thus did not indicate the need for additional protection against short-
term exposures to peak PM2.5 concentrations. Accordingly, 
and taking into account consensus CASAC advice to retain the current 
primary 24-hour PM2.5 standard, the then-Administrator 
concluded the primary 24-hour PM2.5 standard should be 
retained.
    With respect to the annual PM2.5 standard, the then-
Administrator recognized that important uncertainties and limitations 
that were present in epidemiologic studies in previous

[[Page 16210]]

reviews remained in the evidence assessed in the 2019 ISA. In 
considering the epidemiologic evidence, the then-Administrator noted 
that: (1) The reported mean concentration in the majority of the key 
U.S. epidemiologic studies using ground-based monitoring data are above 
the level of the current annual standard; (2) the mean of the reported 
study means (or medians) (i.e., 13.5 [micro]g/m\3\) is above the level 
of the current primary annual PM2.5 standard of 12 [micro]g/
m\3\; (3) air quality analyses show the study means to be lower than 
their corresponding design by 10-20%; and (4) that these analyses must 
be considered in light of uncertainties inherent in the epidemiologic 
evidence. The then-Administrator further considered other available 
information, including the risk assessment, accountability studies, and 
controlled human exposure studies, and found that, in considering all 
of the evidence together along with advice from the CASAC, the suite of 
primary PM2.5 standards were requisite to protect public 
health with an adequate margin of safety, and should be retained, 
without revision.
    With regard to the primary PM10 standard, the then-
Administrator noted that the expanded body of evidence has broadened 
the range of effects that have been linked with PM10-2.5 
exposures. In light of that information, as well as continued 
uncertainties in the evidence and advice from the CASAC to retain the 
standard, the then-Administrator judged it appropriate to retain the 
primary PM10 standard to provide the requisite degree of 
public health protection against PM10-2.5 exposures, 
regardless of location, source of origin, or particle composition (85 
FR 82725, December 18, 2020).
    With regard to the secondary PM standards, the then-Administrator 
concluded that there was insufficient information available to 
establish any distinct secondary PM standards to address climate and 
materials effects of PM. For visibility effects, he found that in the 
absence of a monitoring network for direct measurement of light 
extinction, a calculated light extinction indicator that utilizes the 
IMPROVE algorithms continued to provide a reasonable basis for defining 
a target level of protection against PM-related visibility impairment. 
He further found that a visibility index with a 24-hour averaging time 
was reasonable based on its stability and suitability for representing 
subdaily periods, and a form based on the 3-year average of annual 90th 
percentile values was reasonable based on its stability and that it 
represents the median of the 20 percent worst visibility days which are 
targeted under the Regional Haze program. With regard to the level of a 
visibility index, the then-Administrator judged it appropriate to 
establish a target level of protection of 30 dv, reflecting the upper 
end of the range of visibility impairment judged to be acceptable by at 
least 50% of study participants in the available public preference 
studies, taking into consideration the variability, limitations and 
uncertainties of the public preference studies. The then-Administrator 
judged that the secondary 24-hour PM2.5 standard with its 
level of 35 [micro]g/m\3\ would provide at least the target level of 
protection for visual air quality of 30 dv which he judged appropriate. 
Accordingly, taking into consideration the advice of the CASAC to 
retain the current secondary PM standards, the then-Administrator found 
the current secondary standards provide the requisite degree of 
protection and that they should be retained (85 FR 82742, December 18, 
2020).
    Following publication of the 2020 final action, several parties 
filed petitions for review and petitions for reconsideration of the 
EPA's final decision. The petitions for review were filed in the D.C. 
Circuit and the Court consolidated the cases.\15\ Following EPA's 
decision to reconsider the 2020 final decision, the Court ordered the 
consolidated cases to be held in abeyance.
---------------------------------------------------------------------------

    \15\ See California v. EPA, (D.C. Cir., No. 21-2014 consolidated 
with Nos. 21-1027, 21-1054).
---------------------------------------------------------------------------

b. Reconsideration of the 2020 PM NAAQS Final Action
    Executive Order 13990 directed review of certain agency actions (86 
FR 7037, January 25, 2021).\16\ An accompanying fact sheet provided a 
non-exclusive list of agency actions that agency heads should review in 
accordance with that order, including the 2020 Particulate Matter NAAQS 
Decision.\17\
---------------------------------------------------------------------------

    \16\ See https://www.whitehouse.gov/briefing-room/presidential-actions/2021/01/20/executive-order-protecting-public-health-and-environment-and-restoring-science-to-tackle-climate-crisis/.
    \17\ See https://www.whitehouse.gov/briefing-room/statements-releases/2021/01/20/fact-sheet-list-of-agency-actions-for-review/.
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    On June 10, 2021, the Agency announced its decision to reconsider 
the 2020 PM NAAQS final action because the available scientific 
evidence and technical information indicate that the current standards 
may not be adequate to protect public health and welfare, as required 
by the Clean Air Act.\18\ The Administrator reached this decision in 
part based on the fact that the EPA noted that the 2020 PA concluded 
that the scientific evidence and information called into question the 
adequacy of the primary annual PM2.5 standard and supported 
revising the level to below the current level of 12.0 [micro]g/m\3\ 
while retaining the primary 24-hour PM2.5 standard (U.S. 
EPA, 2020b). The EPA also noted that the 2020 PA concluded that the 
available scientific evidence and information supported retaining the 
primary PM10 standard and secondary PM standards without 
revision (U.S. EPA, 2020b).
---------------------------------------------------------------------------

    \18\ The press release for this announcement is available at: 
https://www.epa.gov/newsreleases/epa-reexamine-health-standards-harmful-soot-previous-administration-left-unchanged.
---------------------------------------------------------------------------

    The EPA staff conclusions detailed in the 2020 PA in combination 
with the CASAC advice that informed the Administrator's decisions 
regarding the 2020 final action, studies highlighted by public comments 
on the 2020 proposal, and the numerous studies published since the 
literature cutoff date of the 2019 ISA all informed the scope of the 
reconsideration.
    In its review of the 2019 draft PA, some members of the CASAC had 
recommended that greater attention should be given to accountability 
studies and epidemiologic studies that employ alternative methods for 
confounder control (also referred to as causal inference or causal 
modeling studies) in order to ``more fully account for effects of 
confounding, measurement and estimation errors, model uncertainty, and 
heterogeneity'' in epidemiologic studies (Cox, 2019b, p. 8 of consensus 
responses). In addition, public commenters submitted a number of recent 
studies published after the literature cutoff date for the 2019 ISA 
that would have been considered within the scope of the 2019 ISA. While 
the EPA provisionally considered these studies in responding to public 
comments,\19\ it was determined that, at the time of the 2020 final 
action, these studies were generally consistent with the evidence 
assessed in the 2019 ISA (85 FR 82690, December 18, 2020; U.S. EPA, 
2020a). As such, and consistent with previous NAAQS reviews, the EPA 
concluded that the new studies did not materially change any of the 
broad scientific conclusions regarding the health and welfare effects 
of PM in ambient air made in the air quality criteria, and therefore, 
reopening of the air quality criteria was not warranted (85 FR 82691, 
December 18, 2020). However, at that time, the EPA

[[Page 16211]]

recognized that its ``provisional consideration of these studies did 
not and could not provide the kind of in-depth critical review'' (85 FR 
82690, December 18, 2020) that studies undergo in the development of an 
ISA.
---------------------------------------------------------------------------

    \19\ The list of provisionally considered studies is included in 
Appendix A to the 2020 Response to Comments document (U.S. EPA, 
2020a).
---------------------------------------------------------------------------

    In preparing to reconsider the 2020 final decision for the PM 
NAAQS, the Agency revisited the need to reopen the air quality 
criteria, given the amount of time that had passed since the literature 
cutoff date of the 2019 ISA (i.e., approximately January 2018) and the 
volume of literature that had become available, including those studies 
provisionally considered in responding to comments in 2020. In so 
doing, the EPA preliminarily concluded that at least some of these 
studies were likely to be relevant to its reconsideration of the air 
quality criteria and the PM NAAQS and that, in considering public 
comments on any proposed decisions for the reconsideration, these 
studies were likely to be raised by public commenters and would 
potentially warrant a reopening of the air quality criteria. For 
example, on February 16, 2021, the EPA received two petitions to 
reconsider the PM NAAQS. One petition objected to the EPA's provisional 
consideration of studies submitted in public comments on the 2020 
proposal and suggested that the provisional consideration was 
inadequate because the studies could be important in determining 
whether the existing standards are adequately protective. See, Petition 
for Reconsideration of National Ambient Air Quality Standards for 
Particulate Matter, submitted by American Lung Association, et al, 
dated Feb. 16, 2020. The other petition identified a number of new 
studies, including one epidemiologic study that was published after the 
provisional consideration was completed that could further inform the 
concern expressed by the CASAC that associations reported in 
epidemiologic studies do not adequately account for ``uncontrolled 
confounding and other potential sources of error and bias.'' See 
Petition for Reconsideration of ``Review of the National Ambient Air 
Quality Standards for Particulate Matter,'' submitted by the State of 
California, dated Feb. 16, 2020. This was also an uncertainty noted by 
the then-Administrator in the 2020 decision, who also recognized ``that 
methodological study designs to address confounding, such as causal 
inference methods, are an emerging field of study.'' Thus, the Agency 
concluded it was appropriate to reconsider not only the standards but 
also the air quality criteria, in light of public comments during the 
2020 PM NAAQS proposal and recent studies published since the cutoff 
date of the 2019 ISA, as reflected in petitions. In deciding to reopen 
the air quality criteria, the Agency concluded it was reasonable to 
focus on studies that were most likely to inform decisions on the 
appropriate standard, but not to reassess areas which, based on the 
assessment of available science published since the cutoff date of the 
2019 ISA and through 2021, were judged unlikely to have new information 
that would be useful for the Administrator's decision making. The 
Agency accordingly announced that, in support of the reconsideration, 
it would develop a supplement to the 2019 ISA and a revised PA.
    The EPA also explained that the draft ISA Supplement and draft PA 
would be reviewed at a public meeting by the CASAC, and the public 
would have opportunities to comment on these documents during the CASAC 
review process, as well as to provide input during the rulemaking 
through the public comment process and public hearings on the proposed 
rulemaking.
    On March 31, 2021, the Administrator announced his decision to 
reestablish the membership of the CASAC to ``ensure the agency received 
the best possible scientific insight to support our work to protect 
human health and the environment.'' \20\ Consistent with this 
memorandum, a call for nominations of candidates to the EPA's chartered 
CASAC was published in the Federal Register (86 FR 17146, April 1, 
2021). On June 17, 2021, the Administrator announced his selection of 
the seven members to serve on the chartered CASAC.21 22 
Additionally, a call for nominations of candidates to a PM-specific 
panel was published in the Federal Register (86 FR 33703, June 25, 
2021). The members of the PM CASAC panel were announced on August 30, 
2021.\23\
---------------------------------------------------------------------------

    \20\ The press release for this announcement is available at: 
https://www.epa.gov/newsreleases/administrator-regan-directs-epa-reset-critical-science-focused-federal-advisory.
    \21\ The press release for this announcement is available at: 
https://www.epa.gov/newsreleases/epa-announces-selections-charter-members-clean-air-scientific-advisory-committee.
    \22\ The list of members of the chartered CASAC and their 
biosketches are available at: https://casac.epa.gov/ords/sab/r/sab_apex/casac/mems?p14_committeeon=2021%20CASAC%20PM%20Panel&session=17433386035954
.
    \23\ The list of members of the PM CASAC panel and their 
biosketches are available at: https://casac.epa.gov/ords/sab/f?p=105:14:9979229564047:::14:P14_COMMITTEEON:2021%20CASAC%20PM%20Panel.
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    The draft ISA Supplement was released in September 2021 (U.S. EPA, 
2021a; 86 FR 54186, September 30, 2021), and included a discussion of 
the rationale and scope of the Supplement. As explained therein, the 
ISA Supplement focuses on a thorough evaluation of some studies that 
became available after the literature cutoff date of the 2019 ISA that 
could either further inform the adequacy of the current PM NAAQS or 
address key scientific topics that have evolved since the literature 
cutoff date for the 2019 ISA. In selecting the health effects to 
evaluate within the ISA Supplement, the EPA focused on health effects 
for which the evidence supported a ``causal relationship'' because 
those were the health effects that were most useful in informing 
conclusions in the 2020 PA (U.S. EPA, 2022a, section 1.2.1).\24\ 
Consistent with the rationale for the focus on certain health effects, 
in selecting the non-ecological welfare effects to evaluate within the 
ISA Supplement, the EPA focused on the non-ecological welfare effects 
for which the evidence supported a ``causal relationship'' and for 
which quantitative analyses could be supported by the evidence because 
those were the welfare effects that were most useful in informing 
conclusions in the 2020 PA.\25\ Specifically, for non-ecological 
welfare effects, the focus within the ISA Supplement is on visibility 
effects. The ISA Supplement also considers recent health effects 
evidence that addresses key scientific topics where the literature has 
evolved since the 2020 review was completed,

[[Page 16212]]

specifically since the literature cutoff date for the 2019 ISA.\26\
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    \24\ As described in section 1.2.1 of the ISA Supplement: ``In 
considering the public health protection provided by the current 
primary PM2.5 standards, and the protection that could be 
provided by alternatives, [the U.S. EPA, within the 2020 PM PA] 
emphasized health outcomes for which the ISA determined that the 
evidence supports either a `causal' or a `likely to be causal' 
relationship with PM2.5 exposures'' (U.S. EPA, 2020b). 
Although the 2020 PA initially focused on this broader set of 
evidence, the basis of the discussion on potential alternative 
standards primarily focused on health effect categories where the 
2019 PM ISA concluded a `causal relationship' (i.e., short- and 
long-term PM2.5 exposure and cardiovascular effects and 
mortality) as reflected in Figures 3-7 and 3-8 of the 2020 PA (U.S. 
EPA, 2020b).''
    \25\ As described in section 1.2.1 of the ISA Supplement: ``The 
2019 PM ISA concluded a `causal relationship' for each of the 
welfare effects categories evaluated (i.e., visibility, climate 
effects and materials effects). While the 2020 PA considered the 
broader set of evidence for these effects, for climate effects and 
material effects, it concluded that there remained `substantial 
uncertainties with regard to the quantitative relationships with PM 
concentrations and concentration patterns that limit[ed] [the] 
ability to quantitatively assess the public welfare protection 
provided by the standards from these effects' (U.S. EPA, 2020b).''
    \26\ These key scientific topics include experimental studies 
conducted at near-ambient concentrations, epidemiologic studies that 
employed alternative methods for confounder control or conducted 
accountability analyses, studies that assess the relationship 
between PM2.5 exposure and severe acute respiratory 
syndrome coronavirus 2 (SARS-CoV-2) infection and coronavirus 
disease 2019 (COVID-19) death; and in accordance with recent EPA 
goals on addressing environmental justice, studies that examine 
disparities in PM2.5 exposure and the risk of health 
effects by race/ethnicity or socioeconomic status (SES) (U.S. EPA, 
2022a, section 1.2.1).
---------------------------------------------------------------------------

    Building on the rationale presented in section 1.2.1, the ISA 
Supplement considers peer-reviewed studies published from approximately 
January 2018 through March 2021 that meet the following criteria:
 Health Effects
    [cir] U.S. and Canadian epidemiologic studies for health effect 
categories where the 2019 ISA concluded a ``causal relationship'' 
(i.e., short- and long-term PM2.5 exposure and 
cardiovascular effects and mortality).
    [ssquf] U.S. and Canadian epidemiologic studies that employed 
alternative methods for confounder control or conducted accountability 
analyses (i.e., examined the effect of a policy on reducing 
PM2.5 concentrations).
 Welfare Effects
    [cir] U.S. and Canadian studies that provide new information on 
public preferences for visibility impairment and/or developed 
methodologies or conducted quantitative analyses of light extinction.
 Key Scientific Topics
    [cir] Experimental studies (i.e., controlled human exposure and 
animal toxicological) conducted at near-ambient PM2.5 
concentrations experienced in the U.S.
    [cir] U.S.- and Canadian-based epidemiologic studies that examined 
the relationship between PM2.5 exposures and severe acute 
respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and 
coronavirus disease 2019 (COVID-19) death.
    [cir] At-Risk Populations.
    [ssquf] U.S.- and Canadian-based epidemiologic or exposure studies 
examining potential disparities in either PM2.5 exposures or 
the risk of health effects by race/ethnicity or socioeconomic status 
(SES).
    Given the narrow scope of the ISA Supplement, it is important to 
recognize that the evaluation does not encompass the full 
multidisciplinary evaluation presented within the 2019 ISA that would 
result in weight-of-evidence conclusions on causality (i.e., causality 
determinations). The ISA Supplement critically evaluates and provides 
key study-specific information for those recent studies deemed to be of 
greatest significance for informing preliminary conclusions on the PM 
NAAQS in the context of the body of evidence and scientific conclusions 
presented in the 2019 ISA.
    In developing a revised PA to support the reconsideration, the EPA 
considered the available scientific evidence, including the evidence 
presented in the 2019 ISA and ISA Supplement. The 2022 PA considered 
the quantitative and technical information presented in the 2020 PA, in 
addition to new and updated analyses conducted since the 2020 final 
decision. For those health and welfare effects for which the ISA 
Supplement evaluated recently available studies (i.e., 
PM2.5-related health effects and visibility effects), new 
updated quantitative analyses were conducted as a part of the 
development of the 2022 PA. The newly available scientific and 
technical information presented in the 2022 PA were considered in 
reaching conclusions regarding the adequacy of the current standards 
and any potential alternative standards. For those health and welfare 
effects for which newly available scientific and technical information 
were not evaluated (i.e., PM10-2.5-related health effects 
and non-visibility welfare effects), the conclusions presented in the 
2022 PA rely heavily on the information that supported the conclusions 
in the 2020 PA.
    The CASAC PM panel met at a virtual public meeting in November 2021 
to review the draft ISA Supplement (86 FR 52673, September 22, 2021). A 
virtual public meeting was then held in February 2022, and during this 
meeting the chartered CASAC considered the CASAC PM panel's draft 
letter to the Administrator on the draft ISA Supplement (87 FR 958, 
January 7, 2022).
    The chartered CASAC provided its advice on the draft ISA Supplement 
in a letter to the EPA Administrator dated March 18, 2022 (Sheppard, 
2022b). In its review of the draft ISA Supplement, the CASAC noted that 
they found ``the Draft ISA Supplement to be a well-written, 
comprehensive evaluation of the new scientific information published 
since the 2019 PM ISA'' (Sheppard, 2022b, p. 2 of letter). Furthermore, 
the CASAC stated that ``the final Integrated Science Assessment (ISA) 
Supplement . . . deserve[s] the Administrator's full consideration and 
[is] adequate for rulemaking'' (Sheppard, 2022b, p. 2 of letter). The 
CASAC generally endorsed EPA's decisions regarding the limited scope of 
the draft ISA Supplement, stating that ``this limitation [on scope] is 
appropriate for the targeted purpose of the Draft ISA Supplement'' 
although the CASAC noted it would not be appropriate for ISAs 
generally, and recommended that the EPA provide additional 
acknowledgment and explanation for the limited scope (Sheppard, 2022b, 
p. 2 of letter; see also pp. 2-3 of consensus responses). The EPA 
specifically noted in the final ISA Supplement, which was released in 
May 2022 (U.S. EPA, 2022a; hereafter referred to as the ISA Supplement 
throughout this document) that the ``targeted approach to developing 
the Supplement to the 2019 PM ISA for the purpose of reconsidering the 
2020 PM NAAQS decision does not reflect a change to EPA's approach for 
developing ISAs for NAAQS reviews.'' Thus, the evidence presented 
within the 2019 ISA, along with the targeted identification and 
evaluation of new scientific information in the ISA Supplement, 
provides the scientific basis for the reconsideration of the 2020 PM 
NAAQS final decision.
    The draft PA was released in October 2021 (86 FR 56263, October 8, 
2021). The CASAC PM panel met at a virtual public meeting in December 
2021 to review the draft PA (86 FR 52673, September 22, 2021). A 
virtual public meeting was then held in February 2022 and March 2022, 
and during this meeting the chartered CASAC considered the CASAC PM 
panel's draft letter to the Administrator on the draft PA (87 FR 958, 
January 7, 2022). The chartered CASAC provided its advice on the draft 
PA in a letter to the EPA Administrator dated March 18, 2022 (Sheppard, 
2022a). The EPA took steps to address these comments in revising and 
finalizing the PA. The 2022 PA considers the scientific evidence 
presented in the 2019 ISA and ISA Supplement and considers the 
quantitative and technical information presented in the 2020 PA, along 
with updated and newly available analyses since the completion of the 
2020 review. For those health and welfare effects for which the ISA 
Supplement evaluated recently available evidence and for which updated 
quantitative analyses were supported (i.e., PM2.5-related 
health effects and visibility effects), the 2022 PA includes 
consideration of this newly available scientific and technical 
information in reaching preliminary conclusions. For those health and 
welfare effects for which newly available scientific and technical

[[Page 16213]]

information were not evaluated (i.e., PM10-2.5-related 
health effects and non-visibility effects), the conclusions presented 
in the 2022 PA rely heavily on the information that supported the 
conclusions in the 2020 PA. The final PA was released in May 2022 (U.S. 
EPA, 2022b; hereafter referred to as the 2022 PA throughout this 
document).
    Drawing from his consideration of the scientific evidence assessed 
in the 2019 ISA and ISA Supplement and the analyses in the 2022 PA, 
including the uncertainties in the evidence and analyses, and from his 
consideration of advice from the CASAC, on January 5, 2023, the 
Administrator proposed to revise the level of the primary annual 
PM2.5 standard and to retain the primary 24-hour 
PM2.5 standard, the primary 24-hour PM10 
standard, and the secondary PM standards. These proposed decisions were 
published in the Federal Register on January 27, 2023 (88 FR 5558, 
January 27, 2023). The EPA held a multi-day virtual public hearing on 
February 21-23, 2023 (88 FR 6215, January 31, 2023). In total, the EPA 
received nearly 700,000 comments on the proposal from members of the 
public by the close of the public comment period on March 28, 2023. 
Major issues raised in the public comments are discussed throughout the 
preamble of this final action. A more detailed summary of all 
significant comments, along with the EPA's responses (henceforth 
``Response to Comments'' document), can be found in the docket for this 
rulemaking (Docket No. EPA-HQ-OAR-2015-0072).
    As in prior reviews, the EPA is basing its decision in this 
reconsideration on studies and related information in the air quality 
criteria, which have undergone CASAC and public review. These studies 
assessed in the 2019 ISA \27\ and ISA Supplement \28\ and the 2022 PA, 
and the integration of the scientific evidence presented in them, have 
undergone extensive critical review by the EPA, the CASAC, and the 
public. Decisions on the NAAQS should be based on studies that have 
been rigorously assessed in an integrative manner not only by the EPA 
but also by the statutorily mandated independent scientific advisory 
committee, as well as the public review that accompanies this process. 
It is for this reason that the EPA preliminarily concluded that the 
scientific evidence available since the completion of the 2019 ISA, 
including those raised in public comments on the proposal in 2020, 
warranted a partial reopening of the air quality criteria and prepared 
an ISA Supplement to enable the EPA, the CASAC, and the public to 
consider them further. Some commenters have referred to and discussed 
additional individual scientific studies on the health effects of PM 
that were not included in the 2019 ISA or ISA Supplement (``new 
studies'') and that have not gone through this comprehensive review 
process. In considering and responding to comments for which such 
``new'' studies were cited in support, the EPA has provisionally 
considered the cited studies in the context of the findings of the 2019 
ISA and ISA Supplement. The EPA's provisional consideration of these 
studies did not and could not provide the kind of in-depth critical 
review described above, but rather was focused on determining whether 
they warranted further reopening the review of the air quality criteria 
to enable the EPA, the CASAC, and the public to consider them further.
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    \27\ In addition to the 2020 review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), the 2019 ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009, through approximately January 
2018 (U.S. EPA, 2019a, p. ES-2). References that are cited in the 
2019 ISA, the references that were considered for inclusion but not 
cited, and electronic links to bibliographic information and 
abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
    \28\ As described above, the ISA Supplement represents an 
evaluation of recent studies that are of greatest policy relevance 
and utility to the reconsideration of the 2020 final decision on the 
PM NAAQS (U.S. EPA, 2022a).
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    This approach, and the decision to rely on the studies and related 
information in the air quality criteria, which have undergone CASAC and 
public review, is consistent with the EPA's practice in prior NAAQS 
reviews and its interpretation of the requirements of the CAA. Since 
the 1970 amendments, the EPA has taken the view that NAAQS decisions 
are to be based on scientific studies and related information that have 
been assessed as a part of the pertinent air quality criteria, and the 
EPA has consistently followed this approach. This longstanding 
interpretation was strengthened by new legislative requirements enacted 
in 1977, which added section 109(d)(2) of the Act concerning CASAC 
review of air quality criteria. See 71 FR 6114, 61148 (October 17, 
2006, final decision on review of NAAQS for particulate matter) for a 
detailed discussion of this issue and the EPA's past practice.
    As discussed in the EPA's 1993 decision not to review the 
O3 NAAQS, ``new'' studies may sometimes be of such 
significance that it is appropriate to delay a decision in a NAAQS 
review and to supplement the pertinent air quality criteria so the 
studies can be taken into account (58 FR 13013-13014, March 9, 1993). 
In the present case, the EPA decided to partially reopen the air 
quality criteria and prepared an ISA Supplement as a part of the 
reconsideration to facilitate evaluation of these studies by the EPA, 
the CASAC, and the public. The narrow scope of the ISA Supplement is 
supported by EPA's provisional consideration of ``new'' studies 
submitted in response to public comments on the 2020 proposal which 
concluded that, taken in context, the ``new'' information and findings 
do not materially change any of the broad scientific conclusions 
regarding the health and welfare effects of PM in ambient air made in 
the air quality criteria. Therefore, a full reopening of the air 
quality criteria was not warranted to assess the health and welfare 
effects of PM for purposes of the review.
    Accordingly, the EPA is basing the final decisions in this 
reconsideration on the studies and related information included in the 
PM air quality criteria (including the 2019 PM ISA and ISA Supplement) 
that have undergone rigorous review by the EPA, the CASAC, and the 
public. The EPA will consider these ``new'' studies for inclusion in 
the air quality criteria for the next PM NAAQS review, which the EPA 
expects to begin soon after the conclusion of this reconsideration and 
which will provide the opportunity to fully assess these studies 
through a more rigorous review process involving the EPA, the CASAC, 
and the public.

D. Air Quality Information

    This section provides a summary of basic information related to PM 
ambient air quality. It summarizes information on the distribution of 
particle size in ambient air (section I.D.1), sources and emissions 
contributing to PM in the ambient air (section I.D.2), monitoring 
ambient PM in the U.S. (section I.D.3), ambient PM concentrations and 
trends in the U.S. (I.D.4), characterizing ambient PM2.5 
concentrations for exposure (section I.D.5), and background PM (section 
I.D.6). Additional detail on PM air quality can be found in Chapter 2 
of the 2022 PA (U.S. EPA, 2022b).
1. Distribution of Particle Size in Ambient Air
    In ambient air, PM is a mixture of substances suspended as small 
liquid and/or solid particles (U.S. EPA, 2019a, section 2.2) and 
distinct health and welfare effects have been linked with exposures to 
particles of different sizes. Particles in the atmosphere range in size 
from less than 0.01 to more than 10 [mu]m

[[Page 16214]]

in diameter (U.S. EPA, 2019a, section 2.2). The EPA defines 
PM2.5, also referred to as fine particles, as particles with 
aerodynamic diameters generally less than or equal to 2.5 [mu]m. The 
size range for PM10-2.5, also called coarse or thoracic 
coarse particles, includes those particles with aerodynamic diameters 
generally greater than 2.5 [mu]m and less than or equal to 10 [mu]m. 
PM10, which is comprised of both fine and coarse fractions, 
includes those particles with aerodynamic diameters generally less than 
or equal to 10 [mu]m. In addition, ultrafine particles (UFP) are often 
defined as particles with a diameter of less than 0.1 [mu]m based on 
physical size, thermal diffusivity or electrical mobility (U.S. EPA, 
2019a, section 2.2). Atmospheric lifetimes are generally longest for 
PM2.5, which often remains in the atmosphere for days to 
weeks (U.S. EPA, 2019a, Table 2-1) before being removed by wet or dry 
deposition, while atmospheric lifetimes for UFP and PM10-2.5 
are shorter and are generally removed from the atmosphere within hours, 
through wet or dry deposition (U.S. EPA, 2019a, Table 2-1; U.S. EPA, 
2022b, section 2.1).
2. Sources and Emissions Contributing to PM in the Ambient Air
    PM is composed of both primary (directly emitted particles) and 
secondary particles. Primary PM is derived from direct particle 
emissions from specific PM sources while secondary PM originates from 
gas-phase precursor chemical compounds present in the atmosphere that 
have participated in new particle formation or condensed onto existing 
particles (U.S. EPA, 2019a, section 2.3). As discussed further in the 
2019 ISA (U.S. EPA, 2019a, section 2.3.2.1), secondary PM is formed in 
the atmosphere by photochemical oxidation reactions of both inorganic 
and organic gas-phase precursors. Precursor gases include sulfur 
dioxide (SO2), nitrogen oxides (NOX), and 
volatile organic compounds (VOC) (U.S. EPA, 2019a, section 2.3.2.1). 
Ammonia also plays an important role in the formation of nitrate PM by 
neutralizing sulfuric acid and nitric acid. Sources and emissions of PM 
are discussed in more detail the 2022 PA (U.S. EPA, 2022b, section 
2.1.1). Briefly, anthropogenic sources of PM include both stationary 
(e.g., fuel combustion for electricity production and other purposes, 
industrial processes, agricultural activities) and mobile (e.g., 
diesel- and gasoline-powered highway vehicles and other engine-driven 
sources) sources. Natural sources of PM include dust from the wind 
erosion of natural surfaces, sea salt, wildfires, primary biological 
aerosol particles (PBAP) such as bacteria and pollen, oxidation of 
biogenic hydrocarbons, such as isoprene and terpenes to produce 
secondary organic aerosol (SOA), and geogenic sources, such as sulfate 
formed from volcanic production of SO2. Wildland fire, which 
encompass both wildfire and prescribed fire, accounts for 44% of 
emissions of primary PM2.5 emissions (U.S. EPA, 2021b). 
Emissions from wildfire comprises 29% of primary PM2.5 
emissions.
    In recent years, the frequency and magnitude of wildfires have 
increased (U.S. EPA, 2019a). The magnitude of the public health impact 
of wildfires is substantial both because of the increase in 
PM2.5 concentrations as well as the duration of the wildfire 
smoke season, which is considered to range from May to November. 
Wildfire can make a large contribution to air pollution (including 
PM2.5), and wildfire events can threaten public safety and 
life. The impacts of wildfire events can be mitigated through 
management of wildland vegetation, including through prescribed fire. 
Prescribed fire (and some wildfires) can mimic the natural processes 
necessary to maintain fire-dependent ecosystems, minimizing 
catastrophic wildfires and the risks they pose to safety, property and 
air quality (see, e.g., 81 FR 58010, 58038, August 24, 2016). The EPA 
views the strategic use of prescribed fire as an important tool for 
reducing wildfire risk and the severity of wildfires and wildfire smoke 
(88 FR, 54118, 54126, August 9, 2023).\29\ As noted in the PM NAAQS 
proposal, agencies have efforts in place to reduce the frequency and 
severity of human-caused wildfires (88 FR 5570, January 27, 2023).
---------------------------------------------------------------------------

    \29\ See also: https://www.usda.gov/sites/default/files/documents/usda-epa-doi-cdc-mou.pdf.
---------------------------------------------------------------------------

    Wildfire events produce high PM emissions that may impact the PM 
concentrations in ambient air to the extent that the concentrations 
result in an exceedance or violation which may affect the design value 
in a given area. The EPA's Exceptional Events Rule (81 FR 68216, 
October 3, 2016) describes the process by which air agencies may 
request to exclude `event-influenced' data caused by exceptional 
events, which can include wildfires and prescribed fires on wildland. 
The EPA has issued guidance specifically addressing exceptional events 
demonstrations for both wildfires and prescribed fires on wildland. 
These documents are available on EPA's Exceptional Events Program 
website.\30\ The EPA will develop fire-related exceptional events 
implementation tools, including updates as needed to existing guidance 
to facilitate more efficient processing of PM2.5-related 
exceptional events demonstrations for both the 24-hour and annual 
standards.
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    \30\ See: https://www.epa.gov/air-quality-analysis/final-2016-exceptional-events-rule-supporting-guidance-documents-updated-faqs.
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3. Monitoring of Ambient PM
    To promote uniform application of the air quality standards set 
forth under the CAA and to achieve the degree of public health and 
welfare protection intended for the NAAQS, the EPA establishes PM 
Federal Reference Methods (FRMs) for both PM10 and 
PM2.5 in appendices J and L to 40 CFR part 50, both of which 
were amended following the 2006 and 2012 PM NAAQS reviews. The current 
PM monitoring network relies on FRMs and automated continuous Federal 
Equivalent Methods (FEMs) approved pursuant to 40 CFR part 53, in part 
to support changes necessary for implementation of the revised PM 
standards. Additionally, 40 CFR part 58, appendices A through E, detail 
the requirements to measure ambient air quality and report ambient air 
quality data and related information. More information on PM ambient 
monitoring networks is available in section 2.2 of the 2022 PA (U.S. 
EPA, 2022b).
    The PM2.5 monitoring program is one of the major ambient 
air monitoring programs with a robust, nationally consistent network of 
ambient air monitoring sites providing mass and/or chemical speciation 
measurements. 40 CFR part 58, appendix D, section 4.7 provides the 
applicable PM2.5 network design criteria. For most urban 
locations, PM2.5 monitors are sited at the neighborhood 
scale,\31\ where PM2.5 concentrations are reasonably 
homogeneous throughout an entire urban sub-region. In each CBSA with a 
monitoring requirement, at least one PM2.5 monitoring 
station representing

[[Page 16215]]

area-wide air quality is sited in an area of expected maximum 
concentration.\32\ By ensuring the area of expected maximum 
concentration in a CBSA has a site compared to both the annual and 24-
hour NAAQS, all other similar locations are thus protected. Sites that 
represent relatively unique microscale, localized hot-spot, or unique 
middle scale impact sites are only eligible for comparison to the 24-
hour PM2.5 NAAQS.
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    \31\ For PM2.5, neighborhood scale is defined at 40 
CFR part 58, appendix D, 4.7.1(c)(3) as follows: Measurements in 
this category would represent conditions throughout some reasonably 
homogeneous urban sub-region with dimensions of a few kilometers and 
of generally more regular shape than the middle scale. Homogeneity 
refers to the particulate matter concentrations, as well as the land 
use and land surface characteristics. Much of the PM2.5 
exposures are expected to be associated with this scale of 
measurement. In some cases, a location carefully chosen to provide 
neighborhood scale data would represent the immediate neighborhood 
as well as neighborhoods of the same type in other parts of the 
city. PM2.5 sites of this kind provide good information 
about trends and compliance with standards because they often 
represent conditions in areas where people commonly live and work 
for periods comparable to those specified in the NAAQS. In general, 
most PM2.5 monitoring in urban areas should have this 
scale.
    \32\ 40 CFR part 58, app. D, 4.7.1(b)(2).
---------------------------------------------------------------------------

    Under 40 CFR part 50, appendix L, and 40 CFR part 53, and 40 CFR 
part 58 appendix D there are three main methods components of the 
PM2.5 monitoring program: filter-based FRMs measuring 
PM2.5 mass, FEMs measuring PM2.5 mass, and other 
samplers used to collect the aerosol used in subsequent laboratory 
analysis for measuring PM2.5 chemical speciation. The FRMs 
are primarily used for comparison to the NAAQS, but also serve other 
important purposes, such as developing trends and evaluating the 
performance of FEMs. PM2.5 FEMs are typically continuous 
methods used to support forecasting and reporting of the Air Quality 
Index (AQI) but are also used for comparison to the NAAQS. Samplers 
that are part of the Chemical Speciation Network (CSN) and Interagency 
Monitoring of Protected Visual Environments (IMPROVE) network are used 
to provide chemical composition of the aerosol and serve a variety of 
objectives. More detail on of each of these components of the 
PM2.5 monitoring program and of recent changes to 
PM2.5 monitoring requirements are described in detail in the 
2022 PA (U.S. EPA, 2022b, section 2.2.3).
4. Ambient Concentrations and Trends
    This section summarizes available information on recent ambient PM 
concentrations in the U.S. and on trends in PM air quality. Sections 
I.D.4.a and I.D.4.b summarize information on PM2.5 mass and 
components, respectively. Section I.D.4.c summarizes information on 
PM10. Sections I.D.4.d and I.D.4.e summarize the more 
limited information on PM10-2.5 and UFP, respectively. 
Additional detail on PM air quality and trends can be found in the 2022 
PA (U.S. EPA, 2022b, section 2.3).
a. PM2.5 mass
    At monitoring sites in the U.S., annual PM2.5 
concentrations from 2017 to 2019 averaged 8.0 [mu]g/m\3\ (with the 10th 
and 90th percentiles at 5.9 and 10.0 [mu]g/m\3\, 
respectively) and the 98th percentiles of 24-hour concentrations 
averaged 21.3 [mu]g/m\3\ (with the 10th and 90th percentiles at 14.0 
and 29.7 [mu]g/m\3\, respectively) (U.S. EPA, 2022b, section 2.3.2.1). 
The highest ambient PM2.5 concentrations occur in the 
western U.S., particularly in California and the Pacific Northwest 
(U.S. EPA, 2022b, Figure 2-15). Much of the eastern U.S. has lower 
ambient concentrations, with annual average concentrations generally at 
or below 12.0 [mu]g/m\3\ and 98th percentiles of 24-hour concentrations 
generally at or below 30 [mu]g/m\3\ (U.S. EPA, 2022b, section 2.3.2.1).
    Recent ambient PM2.5 concentrations reflect the 
substantial reductions that have occurred across much of the U.S. (U.S. 
EPA, 2022b, section 2.3.2.1). From 2000 to 2019, national annual 
average PM2.5 concentrations declined from 13.5 [mu]g/m\3\ 
to 7.6 [mu]g/m\3\, a 43% decrease (U.S. EPA, 2022b, section 
2.3.2.1).\33\ These declines have occurred at urban and rural 
monitoring sites, although urban PM2.5 concentrations remain 
consistently higher than those in rural areas (Chan et al., 2018) due 
to the impact of local sources in urban areas. Analyses at individual 
monitoring sites indicate that declines in ambient PM2.5 
concentrations have been most consistent across the eastern U.S. and in 
parts of coastal California, where both annual average and 98th 
percentiles of 24-hour concentrations declined significantly (U.S. EPA, 
2022b, section 2.3.2.1). In contrast, trends in ambient 
PM2.5 concentrations have been less consistent over much of 
the western U.S., with no significant changes since 2000 observed at 
some sites in the Pacific Northwest, the northern Rockies and plains, 
and the Southwest, particularly for 98th percentiles of 24-hour 
concentrations (U.S. EPA, 2022b, section 2.3.2.1). As noted below, some 
sites in the northwestern U.S. and California, where wildfire have been 
relatively common in recent years, have experienced high concentrations 
over shorter periods (i.e., 2-hour averages).
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    \33\ See https://www.epa.gov/air-trends/particulate-matter-pm25-trends for up-to-date PM2.5 trends information.
---------------------------------------------------------------------------

    The recent deployment of PM2.5 monitors near major roads 
in large urban areas provides information on PM2.5 
concentrations near an important emissions source. For 2016-2018, Gantt 
et al. (2021) reported that 52% and 24% of the time near-road sites 
reported the highest annual and 24-hour PM2.5 design value 
\34\ in the CBSA, respectively. Of the CBSAs with the highest annual 
design values at near-road sites reported by Gantt et al. (2021), those 
design values were, on average, 0.8 [micro]g/m\3\ higher than at the 
highest measuring non-near-road sites (range is 0.1 to 2.1 [micro]g/
m\3\ higher at near-road sites). Although most near-road monitoring 
sites do not have sufficient data to evaluate long-term trends in near-
road PM2.5 concentrations, analyses of the data at one near-
road-like site in Elizabeth, NJ, \35\ show that the annual average 
near-road increment has generally decreased between 1999 and 2017 from 
about 2.0 [mu]g/m\3\ to about 1.3 [mu]g/m\3\ (U.S. EPA, 2022b, section 
2.3.2.1).
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    \34\ A design value is considered valid if it meets the data 
handling requirements given in appendix N to 40 CFR part 50.
    \35\ The Elizabeth Lab site in Elizabeth, NJ, is situated 
approximately 30 meters from travel lanes of the Interchange 13 toll 
plaza of the New Jersey Turnpike and within 200 meters of travel 
lanes for Interstate 278 and the New Jersey Turnpike.
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    Ambient PM2.5 concentrations can exhibit a diurnal cycle 
that varies due to impacts from intermittent emission sources, 
meteorology, and atmospheric chemistry. The PM2.5 monitoring 
network in the U.S. has an increasing number of continuous FEM monitors 
reporting hourly PM2.5 mass concentrations that reflect this 
diurnal variation. The 2019 ISA describes a two-peaked diurnal pattern 
in urban areas, with morning peaks attributed to rush-hour traffic and 
afternoon peaks attributed to a combination of rush hour traffic, 
decreasing atmospheric dilution, and nucleation (U.S. EPA, 2019a, 
section 2.5.2.3, Figure 2-32). Because a focus on annual average and 
24-hour average PM2.5 concentrations could mask subdaily 
patterns, and because some health studies examine PM exposure durations 
shorter than 24-hours, it is useful to understand the broader 
distribution of subdaily PM2.5 concentrations across the 
U.S. The 2022 PA presents information on the frequency distribution of 
2-hour average PM2.5 mass concentrations from all FEM 
PM2.5 monitors in the U.S. for 2017-2019. At sites meeting 
the current primary PM2.5 standards, these 2-hour 
concentrations generally remain below 10 [mu]g/m\3\, and rarely exceed 
30 [mu]g/m\3\. Two-hour concentrations are higher at sites violating 
the current standards, generally remaining below 16 [mu]g/m\3\ and 
rarely exceeding 80 [mu]g/m\3\ (U.S. EPA, 2022b, section 2.3.2.2.3). 
The extreme upper end of the distribution of 2-hour PM2.5 
concentrations is shifted higher during the warmer months, generally 
corresponding to the period of peak wildfire frequency (April to 
September) in the U.S. At sites meeting the current primary standards, 
the highest 2-hour concentrations measured rarely occur outside of the 
period of peak wildfire frequency. Most of the sites measuring

[[Page 16216]]

these very high concentrations are in the northwestern U.S. and 
California, where wildfires have been relatively common in recent years 
(see U.S. EPA, 2022b, Appendix A, Figure A-1). When the period of peak 
wildfire frequency is excluded from the analysis, the extreme upper end 
of the distribution is reduced (U.S. EPA, 2022b, section 2.3.2.2.3).
b. PM2.5 Components
    Based on recent air quality data, the major chemical components of 
PM2.5 have distinct spatial distributions. Sulfate 
concentrations tend to be highest in the eastern U.S., while in the 
Ohio Valley, Salt Lake Valley, and California nitrate concentrations 
are highest, and relatively high concentrations of organic carbon are 
widespread across most of the continental U.S. (U.S. EPA, 2022b, 
section 2.3.2.3). Elemental carbon, crustal material, and sea salt are 
found to have the highest concentrations in the northeast U.S., 
southwest U.S., and coastal areas, respectively.
    An examination of PM2.5 composition trends can provide 
insight into the factors contributing to overall reductions in ambient 
PM2.5 concentrations. The biggest change in PM2.5 
composition that has occurred in recent years is the reduction in 
sulfate concentrations due to reductions in SO2 emissions. 
Between 2000 and 2015, the nationwide annual average sulfate 
concentration decreased by 17% at urban sites and 20% at rural sites. 
This change in sulfate concentrations is most evident in the eastern 
U.S. and has resulted in organic matter or nitrate now being the 
greatest contributor to PM2.5 mass in many locations (U.S. 
EPA, 2019a, Figure 2-19). The overall reduction in sulfate 
concentrations has contributed substantially to the decrease in 
national average PM2.5 concentrations as well as the decline 
in the fraction of PM10 mass accounted for by 
PM2.5 (U.S. EPA, 2019a, section 2.5.1.1.6; U.S. EPA, 2022b, 
section 2.3.1).
c. PM10
    At long-term monitoring sites in the U.S., the 2017-2019 average of 
2nd highest 24-hour PM10 concentration was 68 [mu]g/m\3\ 
(with 10th and 90th percentiles at 28 and 124 [mu]g/m\3\, respectively) 
(U.S. EPA, 2022b, section 2.3.2.4).\36\ The highest PM10 
concentrations tend to occur in the western U.S. Seasonal analyses 
indicate that ambient PM10 concentrations are generally 
higher in the summer months than at other times of year, though the 
most extreme high concentration events are more likely in the spring 
(U.S. EPA, 2019a, Table 2-5). This is due to fact that the major 
PM10 emission sources, dust and agriculture, are more active 
during the warmer and drier periods of the year.
---------------------------------------------------------------------------

    \36\ The form of the current 24-hour PM10 standard is 
one-expected-exceedance, averaged over three years.
---------------------------------------------------------------------------

    Recent ambient PM10 concentrations reflect reductions 
that have occurred across much of the U.S. (U.S. EPA, 2022b, section 
2.3.2.4). From 2000 to 2019, 2nd highest 24-hour PM10 
concentrations have declined by about 46% (U.S. EPA, 2022b, section 
2.3.2.4).\37\ Analyses at individual monitoring sites indicate that 
annual average PM10 concentrations have generally declined 
at most sites across the U.S., with much of the decrease in the eastern 
U.S. associated with reductions in PM2.5 concentrations 
(U.S. EPA, 2022b, section 2.3.2.4). Annual 2nd highest 24-hour 
PM10 concentrations have generally declined in the eastern 
U.S., while concentrations in much of the midwest and western U.S. have 
remained unchanged or increased since 2000 (U.S. EPA, 2022b, section 
2.3.2.4).
---------------------------------------------------------------------------

    \37\ For more information, see https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat.
---------------------------------------------------------------------------

    Compared to previous reviews, data available from the NCore 
monitoring network in the current reconsideration allows a more 
comprehensive analysis of the relative contributions of 
PM2.5 and PM10-2.5 to PM10 mass. 
PM2.5 generally contributes more to annual average 
PM10 mass in the eastern U.S. than the western U.S. (U.S. 
EPA, 2022b, Figure 2-23). At most sites in the eastern U.S., the 
majority of PM10 mass is comprised of PM2.5. As 
ambient PM2.5 concentrations have declined in the eastern 
U.S. (U.S. EPA, 2022b, section 2.3.2.2), the ratios of PM2.5 
to PM10 have also declined. For sites with days having 
concurrently very high PM2.5 and PM10 
concentrations (U.S. EPA, 2022b, Figure 2-24), the PM2.5/
PM10 ratios are typically higher than the annual average 
ratios. This is particularly true in the northwestern U.S. where the 
high PM10 concentrations can occur during wildfires with 
high PM2.5 (U.S. EPA, 2022b, section 2.3.2.4).
d. PM10-2.5
    Since the 2012 review, the availability of PM10-2.5 
ambient concentration data has greatly increased because of additions 
to the PM10-2.5 monitoring capabilities to the national 
monitoring network. As illustrated in the 2022 PA (U.S. EPA, 2022b, 
section 2.3.2.5), annual average and 98th percentile 
PM10-2.5 concentrations exhibit less distinct differences 
between the eastern and western U.S. than for either PM2.5 
or PM10.
    Due to the short atmospheric lifetime of PM10-2.5 
relative to PM2.5, many of the high concentration sites are 
isolated and likely near emission sources associated with wind-blown 
and fugitive dust. The spatial distributions of annual average and 98th 
percentile concentrations of PM10-2.5 are more similar than 
that of PM2.5, suggesting that the same dust-related 
emission sources are affecting both long-term and episodic 
concentrations (U.S. EPA, 2022b, Figure 2-25). The highest 
concentrations of PM10-2.5 are in the southwest U.S. where 
widespread dry and windy conditions contribute to wind-blown dust 
emissions. Additionally, compared to PM2.5 and 
PM10, changes in PM10-2.5 concentrations have 
been small in magnitude and inconsistent in direction (U.S. EPA, 2022b, 
Figure 2-25). The majority of PM10-2.5 sites in the U.S. do 
not have a concentration trend from 2000-2019, reflecting the 
relatively consistent level of dust emissions across the U.S. during 
the same time period (U.S. EPA, 2022b, section 2.3.2.5).\38\
---------------------------------------------------------------------------

    \38\ PM from dust emissions in the National Emissions Inventory 
(NEI) remain fairly consistent from year-to-year, except when there 
are severe weather incursions or there is a dust event that 
transports or causes major local dust storms to occur (particularly 
in the western U.S.). These dust events and weather incursions 
needed to effect dust emissions on a national level are not common 
and only seldomly occur. In the emissions trends analysis presented 
in the 2022 PA (U.S. EPA, 2022b, section 2.1.1), dust is included in 
the NEI sector labeled ``miscellaneous.''
---------------------------------------------------------------------------

e. UFP
    Compared to PM2.5 mass, there is relatively little data 
on U.S. particle number concentrations, which are dominated by UFP. In 
the published literature, annual average particle number concentrations 
reaching about 20,000 to 30,000 cm\3\ have been reported in U.S. cities 
(U.S. EPA, 2019a). In addition, based on UFP measurements in two urban 
areas (New York City, Buffalo) and at a background site (Steuben 
County) in New York, there is a pronounced difference in particle 
number concentration between different types of locations (U.S. EPA, 
2022b, Figure 2-26; U.S. EPA, 2019a, Figure 2-18). Urban particle 
number counts were several times higher than at the background site, 
and the highest particle number counts in an urban area with multiple 
sites (Buffalo) were observed at a near-road location (U.S. EPA, 2022b, 
section 2.3.2.6).
    Long-term trends in UFP are not routinely available at U.S. 
monitoring

[[Page 16217]]

sites. At one background site in Illinois with long-term data 
available, the annual average particle number concentration declined 
between 2000 and 2019, closely matching the reductions in annual 
PM2.5 mass over that same period (U.S. EPA, 2022b, section 
2.3.2.6). In addition, a small number of published studies have 
examined UFP trends over time. While limited, these studies also 
suggest that UFP number concentrations have declined over time along 
with decreases in PM2.5 (U.S. EPA, 2022b, section 2.3.2.6). 
However, the relationship between changes in ambient PM2.5 
and UFPs cannot be comprehensively characterized due to the high 
variability and limited monitoring of UFPs (U.S. EPA, 2022b, section 
2.3.2.6).
5. Characterizing Ambient PM2.5 Concentrations for Exposure
    Epidemiologic studies use various methods to characterize exposure 
to ambient PM2.5. The methods used to estimate 
PM2.5 concentrations can vary from traditional methods using 
monitoring data from ground-based monitors to newer methods using more 
complex hybrid modeling approaches. Studies using hybrid modeling 
approaches aim to broaden the spatial coverage, as well as estimate 
more spatially-resolved ambient PM2.5 concentrations, by 
expanding beyond just those areas with monitors and providing estimates 
in areas that do not have ground-based monitors (i.e., areas that are 
generally less densely populated and tend to have lower 
PM2.5 concentrations) and at finer spatial resolutions 
(e.g., 1 km x 1 km grid cells). Ground-based PM2.5 monitors 
are generally sited in areas of expected maximum concentration. As 
such, the hybrid modeling approaches tend to broaden the areas captured 
in the exposure assessment, and in doing so, the studies that utilize 
these methods tend to report lower mean PM2.5 concentrations 
than monitor-based approaches. Further, other aspects of the approaches 
applied in the various epidemiologic studies to estimate 
PM2.5 exposure and/or to calculate the related study-
reported mean concentration (i.e., population weighting, trim mean 
approaches) can affect those data values. More detail related to hybrid 
modeling methods, performance of the methods, and how the reported mean 
concentrations compare across approaches is provided in section 2.3.3.2 
of the 2022 PA (U.S. EPA, 2022b). The subsections below discuss the 
characterization of PM2.5 concentrations based on monitoring 
data (I.D.5.a) and using hybrid modeling approaches (I.D.5.b).
a. Predicted Ambient PM2.5 and Exposure Based on Monitored 
Data
    Ambient concentrations of PM2.5 are often characterized 
using measurements from national monitoring networks due to the 
accuracy and precision of the measurements and the public availability 
of data. For applications requiring PM2.5 characterizations 
across large areas or provide complete coverage from the site 
measurements, data interpolation and averaging techniques (such as 
Average Nearest Neighbor tools, and area-wide or population-weighted 
averaging of monitors) are sometimes used (U.S. EPA, 2019a, chapter 3).
    For an area to meet the NAAQS, all valid design values \39\ in that 
area, including the highest annual and 24-hour design values, must be 
at or below the levels of the standards. Because the monitoring network 
siting requirements are specified to capture the high PM2.5 
concentrations (U.S. EPA, 2022b, section 2.2.3), areas meeting an 
annual PM2.5 standard with a particular level would be 
expected to have long-term average monitored PM2.5 
concentrations (i.e., averaged across space and over time in the area) 
somewhat below that standard level. This means that the 
PM2.5 design value in an area is associated with a 
distribution of PM2.5 concentrations in that area, and, 
based on monitoring siting requirements, should represent the highest 
concentration location applicable to be monitored under the 
PM2.5 NAAQS. Analyses in the 2022 PA indicate that, based on 
recent air quality in U.S. CBSAs, maximum annual PM2.5 
design values are often 10% to 20% higher than annual average 
concentrations (i.e., averaged across multiple monitors in the same 
CBSA) (U.S. EPA, 2022b, section 2.3.3.1, Figures 2-28 and 2-29). This 
difference between the maximum annual design value and the average 
concentration in an area can vary, depending on factors such as the 
number of monitors, monitor siting characteristics, and the 
distribution of ambient PM2.5 concentrations. Given that 
higher PM2.5 concentrations have been reported at some near-
road monitoring sites relative to the surrounding area (U.S. EPA, 
2022b, section 2.3.2.2.2), recent requirements for PM2.5 
monitoring at near-road locations in large urban areas (U.S. EPA, 
2022b, section 2.2.3.3) may increase the ratios of maximum design 
values to average annual design values in some areas. Such ratios may 
also depend on how the averages are calculated (i.e., averaged across 
monitors versus across modeled grid cells, as described below in 
section I.5.b). Compared to annual design values, the analysis in the 
2022 PA indicates a more variable relationship between maximum 24-hour 
PM2.5 design values and annual average concentrations (U.S. 
EPA, 2022b, section 2.3.3.1, Figure 2-29).
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    \39\ For the annual PM2.5 standard, design values are 
calculated as the annual arithmetic mean PM2.5 
concentration, averaged over 3 years. For the 24-hour standard, 
design values are calculated as the 98th percentile of the annual 
distribution of 24-hour PM2.5 concentrations, averaged 
over three years (appendix N of 40 CFR part 50).
---------------------------------------------------------------------------

b. Comparison of PM2.5 Hybrid Modeling Approaches in 
Estimating Exposure and Relative to Design Values
    Two types of hybrid approaches that have been utilized in several 
key PM2.5 epidemiologic studies in the 2019 ISA and ISA 
Supplement include neural network approaches and a satellite-based 
method with regression of residual PM2.5 with land-use and 
other variables to improve estimates of PM2.5 concentration 
in the U.S. As such, the 2022 PA further compares these two types of 
approaches across various scales (e.g., CBSA versus nationwide), taking 
into account population weighting approaches utilized in epidemiologic 
studies when estimating PM2.5 exposure (U.S. EPA, 2022b, 
section 2.3.3.2.4). Additionally, the 2022 PA assesses how average 
PM2.5 concentrations computed in epidemiologic studies using 
these hybrid surfaces compare to the maximum design values measured at 
ground-based monitors. For this assessment, the 2022 PA evaluates the 
DI2019 \40\ and HA2020 \41\ hybrid surfaces, surfaces that are used in 
several of the key epidemiologic studies in the 2022 PA. This analysis 
is intended to help inform how the magnitude of the overall study-
reported mean PM2.5 concentrations in epidemiologic studies 
may be

[[Page 16218]]

influenced by the approach used to compute that mean and how that value 
might compare to monitor reported concentrations. The PM2.5 
standards are expected to achieve a pattern of air quality through the 
attainment of a specific design value at each monitor in the monitoring 
network. As a result, it is important to be able to assess the 
relationship between monitor concentrations and patterns of air quality 
evaluated in the epidemiologic studies.
---------------------------------------------------------------------------

    \40\ This analysis includes an updated version of the surface 
used in Di et al. (2016). Predictions in Di et al. (2016) were for 
2000 to 2012 using a neural network model. The Di et al. (2019) 
study improved on that effort in several ways. First, a generalized 
additive model was used that accounted for geographic variations in 
performance to combine predictions from three models (neural 
network, random forest, and gradient boosting) to make the final 
optimal PM2.5 predictions. Second, the datasets were 
updated that were used in model training and included additional 
variables such as 12-km CMAQ modeling as predictors. Finally, more 
recent years were included in the Di et al. (2019) study.
    \41\ The HA2020 field is based on the V4.NA.03 product available 
at: https://sites.wustl.edu/acag/datasets/surface-pm2-5/. The name 
``HA2020'' comes from the references for this product (Hammer et 
al., 2020; van Donkelaar et al., 2019).
---------------------------------------------------------------------------

    In estimating exposure, some studies focus on estimating 
concentrations in urban areas, while others examine the entire U.S. or 
large portions of the country. In general, the areas that are not 
included in the CBSA-only analysis tend to be more rural or less 
densely populated areas, tend to have lower PM2.5 
concentrations, and likely correspond to those locations where 
monitoring data availability is limited or nonexistent (U.S. EPA, 
2022b, section 2.3.3.2.4, Figure 2-37). To evaluate the differences in 
mean PM2.5 concentrations across different spatial scales, 
the 2022 PA analysis compares the DI2019 and HA2020 surfaces. At the 
national scale, the two surfaces generally produce similar average 
annual PM2.5 concentrations, with the DI2019 surface being 
slightly higher compared to the HA2020 surface. The average annual 
PM2.5 concentrations are also slightly higher using the 
DI2019 surface compared to the HA2020 surface when the analyses are 
conducted for CBSAs. Also, regardless of which surface is used, the 
average annual and 3-year average of the average annual 
PM2.5 concentrations for the CBSA-only analyses are somewhat 
higher than for the nationwide analyses (4-8% higher) (U.S. EPA, 2022b, 
section 2.3.3.2.4, Table 2-5).\42\ Overall, these analyses suggest that 
there are only slight differences in the average PM2.5 
concentrations depending on the hybrid modeling method employed, though 
including other hybrid modeling methods in this comparison could result 
in larger differences.
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    \42\ For the national scale, 3-year averages of the average 
annual PM2.5 concentrations generally range from about 
5.3 [micro]g/m\3\ to 8.1 [micro]g/m\3\, compared to the CBSA scale, 
which ranges from 5.7 [micro]g/m\3\ to 8.7 [micro]g/m\3\. (U.S. EPA, 
2022b, section 2.3.3.2.4, Table 2-6).
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    The 2022 PA next evaluates how the averages of the hybrid model 
surfaces compare to regulatory design values using both the DI2019 and 
HA2020 surfaces and how population weighting influences the mean 
PM2.5 concentration.\43\ As presented in the 2022 PA, the 
results using the DI2019 and HA2020 surfaces are similar for the 
average annual PM2.5 concentrations, for each 3-year period. 
When population weighting is not applied, the average annual 
PM2.5 concentrations generally range from 7.0 to 8.6 
[micro]g/m\3\. When population weighting is applied, the average annual 
PM2.5 concentrations are slightly higher, ranging from 8.2 
to 10.2 [micro]g/m\3\. As with CBSAs versus the national comparison 
above, population weighting results in a higher average 
PM2.5 concentration than when population weighting is not 
applied (U.S. EPA, 2022b, section 2.3.3.2.4, Table 2-7). For the CBSAs 
included in the population weighted analyses, the average maximum 
annual design values generally range from 9.5 to 11.7 [micro]g/m\3\. 
The results are similar for both the DI2019 and HA2020 surfaces and the 
maximum annual PM2.5 design values measured at the monitors 
are often 40% to 50% higher than average annual PM2.5 
concentrations predicted by hybrid modeling methods when population 
weighting is not applied. However, when population weighting is 
applied, the ratio of the maximum annual PM2.5 design values 
to the predicted average annual PM2.5 concentrations are 
lower than when population weighting is not applied, with monitored 
design values generally 15% to 18% higher than population-weighted 
hybrid modeling average annual PM2.5 concentrations (U.S. 
EPA, 2022b, section 2.3.3.2.4, Table 2-7).
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    \43\ For this analysis, the 2022 PA includes CBSAs with three or 
more valid design values for the 3-year period. The regulatory 
design values for the CBSAs were calculated for each 3-year period 
for the CBSAs with 3 or more design values in each of the 3-year 
periods. Using the maximum design value for each CBSA and by each 3-
year period, the ratio of maximum design values to modeled average 
annual PM2.5 concentrations were calculated, for each 3-
year period. More details about the analytical methods used for this 
analysis are described in section A.6 of Appendix A in the 2022 PA 
(U.S. EPA, 2022b).
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6. Background PM
    In this reconsideration, background PM is defined as all particles 
that are formed by sources or processes that cannot be influenced by 
actions within the jurisdiction of concern. U.S. background PM is 
defined as any PM formed from emissions other than U.S. anthropogenic 
(i.e., manmade) emissions. Potential sources of U.S. background PM 
include both natural sources (i.e., PM that would exist in the absence 
of any anthropogenic emissions of PM or PM precursors) and 
transboundary sources originating outside U.S. borders. Background PM 
is discussed in more detail in the 2022 PA (U.S. EPA, 2022b, section 
2.4). At annual and national scales, estimated background PM 
concentrations in the U.S. are small compared to contributions from 
domestic anthropogenic sources.\44\ For example, based on zero-out 
modeling in the last review of the PM NAAQS, annual background 
PM2.5 concentrations were estimated to range from 0.5-3 
[micro]g/m\3\ across the sites examined. In addition, speciated 
monitoring data from IMPROVE sites can provide some insights into how 
contributions from different sources, including sources of background 
PM, may have changed over time. Such data suggests the estimates of 
background concentrations using speciated monitoring data from IMPROVE 
monitors are around 1-3 [micro]g/m\3\ and have not changed 
significantly since the 2012 review. Contributions to background PM in 
the U.S. result mainly from sources within North America. Contributions 
from intercontinental events have also been documented (e.g., transport 
from dust storms occurring in deserts in North Africa and Asia), but 
these events are less frequent and represent a relatively small 
fraction of background PM in most of the U.S. (U.S. EPA, 2022b, section 
2.4).
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    \44\ Sources that contribute to natural background PM include 
dust from the wind erosion of natural surfaces, sea salt, wildland 
fires, primary biological aerosol particles such as bacteria and 
pollen, oxidation of biogenic hydrocarbons such as isoprene and 
terpenes to produce secondary organic aerosols (SOA), and geogenic 
sources such as sulfate formed from volcanic production of 
SO2 and oceanic production of dimethyl-sulfide (U.S. EPA, 
2022b, section 2.4). While most of these sources release or 
contribute predominantly to fine aerosol, some sources including 
windblown dust, and sea salt also produce particles in the coarse 
size range (U.S. EPA, 2019a, section 2.3.3).
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II. Rationale for Decisions on the Primary PM2.5 Standards

    This section presents the rationale for the Administrator's 
decision to revise the primary annual PM2.5 standard down to 
a level of 9 [micro]g/m\3\ and retain the primary 24-hour 
PM2.5 standard. This rationale is based on a thorough review 
of the scientific evidence generally published through January 
2018,\45\ as evaluated in the 2019 ISA (U.S. EPA, 2019a), on the human 
health effects of PM2.5 associated with long- and short-term 
exposures \46\ to PM2.5 in

[[Page 16219]]

the ambient air. Additionally, this rationale is based on a thorough 
evaluation of some studies that became available after the literature 
cutoff date of the 2019 ISA, as evaluated in the ISA Supplement, that 
could either further inform the adequacy of the current PM NAAQS or 
address key scientific topics that have evolved since the literature 
cutoff date for the 2019 ISA, generally through March 2021 (U.S. EPA, 
2022a).\47\ The Administrator's rationale also takes into account: (1) 
The 2022 PA evaluation of the policy-relevant information in the 2019 
ISA and ISA Supplement and presentation of quantitative analyses of air 
quality and health risks; (2) CASAC advice and recommendations; and (3) 
public comments received during the development of these documents.
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    \45\ In addition to the 2020 review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), the 2019 ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009, through approximately January 
2018 (U.S. EPA, 2019a, p. ES-2). References that are cited in the 
2019 ISA, the references that were considered for inclusion but not 
cited, and electronic links to bibliographic information and 
abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
    \46\ Short-term exposures are defined as those exposures 
occurring over hours up to 1 month, whereas long-term exposures are 
defined as those exposures occurring over 1 month to years (U.S. 
EPA, 2019a, section P.3.1).
    \47\ The ISA Supplement represents an evaluation of recent 
studies that are of greatest policy relevance to the reconsideration 
of the 2020 final decision on the PM NAAQS. Specifically, the ISA 
Supplement focuses on studies of health effects for which the 
evidence in the 2019 ISA supported a ``causal relationship'' (i.e., 
short- and long-term PM2.5 exposure and mortality and 
cardiovascular effects) because those were the health effects that 
were most useful in informing conclusions in the 2020 PA. The ISA 
Supplement does not include an evaluation of studies for other 
PM2.5-related health effects (U.S. EPA, 2022a).
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    In presenting the rationale for the Administrator's decisions and 
its foundations, section II.A provides background on the general 
approach for this reconsideration and the basis for the existing 
standard, and also presents brief summaries of key aspects of the 
currently available health effects and risk information. Section II.B 
summarizes the CASAC advice and the basis for the proposed conclusions, 
addresses public comments received on the proposal and presents the 
Administrator's conclusions on the adequacy of the current standards, 
drawing on consideration of the scientific evidence and quantitative 
risk information, advice from the CASAC, and comments from the public. 
Section II.C summarizes the Administrator's decision on the primary 
PM2.5 standards.

A. Introduction

    The general approach for this reconsideration of the 2020 final 
decision on the primary PM2.5 standards is fundamentally 
based on using the EPA's assessment of the current scientific evidence 
and associated quantitative analyses to inform the Administrator's 
judgment regarding primary PM2.5 standards that protect 
public health with an adequate margin of safety. The EPA's assessments 
are primarily documented in the 2019 ISA, ISA Supplement, and 2022 PA, 
all of which have received CASAC review and public comment (83 FR 
53471, October 23, 2018; 83 FR 55529, November 6, 2018; 85 FR 4655, 
January 27, 2020; 86 FR 52673, September 22, 2021; 86 FR 54186, 
September 30, 2021; 86 FR 56263, October 8, 2021; 87 FR 958, January 7, 
2022; 87 FR 22207, April 14, 2022; 87 FR 31965, May 26, 2022). In 
bridging the gap between the scientific assessments of the 2019 ISA and 
ISA Supplement and the judgments required of the Administrator in 
determining whether the current standards provide the requisite public 
health protection, the 2022 PA evaluates policy implications of the 
evaluation of the current evidence in the 2019 ISA and ISA Supplement, 
and the risk information documented in the 2022 PA. In evaluating the 
public health protection afforded by the current standards, the four 
basic elements of the NAAQS (i.e., indicator, averaging time, level, 
and form) are considered collectively.
    The final decision on the adequacy of the current primary 
PM2.5 standards is a public health policy judgment to be 
made by the Administrator. In reaching conclusions with regard to the 
standards, the decision will draw on the scientific information and 
analyses about health effects and population risks, as well as 
judgments about how to consider the range and magnitude of 
uncertainties that are inherent in the scientific evidence and 
analyses. This approach is based on the recognition that the available 
health effects evidence generally reflects a continuum, consisting of 
levels at which scientists generally agree that health effects are 
likely to occur, through lower levels at which the likelihood and 
magnitude of the response become increasingly uncertain. This approach 
is consistent with the requirements of the NAAQS provisions of the 
Clean Air Act and with how the EPA and the courts have historically 
interpreted the Act (summarized in section I.A above). These provisions 
require the Administrator to establish primary standards that, in the 
judgment of the Administrator, are requisite to protect public health 
with an adequate margin of safety. In so doing, the Administrator seeks 
to establish standards that are neither more nor less stringent than 
necessary for this purpose. The Act does not require that primary 
standards be set at a zero-risk level, but rather at a level that 
avoids unacceptable risks to public health, including the health of 
sensitive (also referred to as ``at-risk'') groups.\48\
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    \48\ As noted in section I.A above, the legislative history 
describes such protection for the sensitive group of individuals and 
not for a single person in the sensitive group (see S. Rep. No. 91-
1196, 91st Cong, 2d Sess. 10 [1970]); see also Am. Lung Ass'n v. 
EPA, 134 F.3d 388, 389 (D.C. Cir. 1998).
---------------------------------------------------------------------------

1. Background on the Current Standards
    The current primary PM2.5 standards were retained in 
2020 based on the scientific evidence and quantitative risk information 
available at that time, as well as the then-Administrator's judgments 
regarding the available health effects evidence and the appropriate 
degree of public health protection afforded by the existing standards 
(85 FR 82718, December 18, 2020). With the 2020 decision, the then-
Administrator retained the primary annual PM2.5 standard 
with its level of 12.0 [mu]g/m\3\ and retained the primary 24-hour 
PM2.5 standard with its level of 35 [mu]g/m\3\. The key 
considerations and the then-Administrator's conclusions regarding the 
primary PM2.5 standards in the 2020 review are summarized 
below.
    The health effects evidence base available in the 2020 review 
included extensive evidence from previous reviews as well as the 
evidence that had emerged since the prior review had been completed in 
2012. This evidence base, spanning several decades, documents the 
relationship between short- and long-term PM2.5 exposure and 
mortality or serious morbidity effects. The evidence available in the 
2019 ISA reaffirmed, and in some cases strengthened, the conclusions 
from the 2009 ISA regarding the health effects of PM2.5 
exposures (U.S. EPA, 2019a). Much of the evidence came from 
epidemiologic studies conducted in North America, Europe, or Asia 
examining short-term and long-term exposures that demonstrated 
generally positive, and often statistically significant, 
PM2.5 health effect associations with a range of outcomes 
including non- accidental, cardiovascular, or respiratory mortality; 
cardiovascular- or respiratory-related hospitalizations or emergency 
department visits; and other mortality/morbidity outcomes (e.g., lung 
cancer mortality or incidence, asthma development). Experimental 
evidence, as well as evidence from panel studies, strengthened support 
for potential biological pathways through which PM2.5 
exposures could lead to health effects reported in many population-
based epidemiologic studies, including support for pathways that could 
lead to cardiovascular, respiratory, nervous system, and cancer-related 
effects.

[[Page 16220]]

Based on this evidence, the 2019 ISA concluded there to be a causal 
relationship between long- and short-term PM2.5 exposure and 
mortality and cardiovascular effects, as well as likely to be causal 
relationships between long- and short-term PM2.5 exposure 
and respiratory effects, and between long-term PM2.5 
exposure and cancer and nervous system effects (U.S. EPA, 2019a, 
section 1.7).
    Epidemiologic studies reported PM2.5 health effect 
associations with mortality and/or morbidity across multiple U.S. 
cities and in diverse populations, including in studies examining 
populations and lifestages that may be at increased risk of 
experiencing a PM2.5-related health effect (e.g., older 
adults, children). The 2019 ISA cited extensive evidence indicating 
that ``both the general population as well as specific populations and 
lifestages are at risk for PM2.5-related health effects'' 
(U.S. EPA, 2019a, p. 12-1), including children and older adults, people 
with pre-existing respiratory or cardiovascular disease, minority 
populations, and low socioeconomic status (SES) populations.
    The risk information available in the 2020 review included risk 
estimates for air quality conditions just meeting the existing primary 
PM2.5 standards, and also for air quality conditions just 
meeting potential alternative standards. The general approach to 
estimating PM2.5-associated health risks combined 
concentration-response (C-R) functions from epidemiologic studies with 
model-based PM2.5 air quality surfaces, baseline health 
incidence data, and population demographics for 47 urban areas (U.S. 
EPA, 2020b, section 3.3, Figure 3-10, Appendix C). The risk assessment 
estimated that the existing primary PM2.5 standards could 
allow a substantial number of PM2.5-associated deaths in the 
U.S. Uncertainty in risk estimates (e.g., in the size of risk 
estimates) can result from a number of factors, including assumptions 
about the shape of the C-R relationship with mortality at low ambient 
PM2.5 concentrations, the potential for confounding and/or 
exposure measurement error, and the methods used to adjust 
PM2.5 air quality.
    Consistent with the general approach routinely employed in NAAQS 
reviews, the initial consideration in the 2020 review of the primary 
PM2.5 standards was with regard to the adequacy of the 
protection provided by the existing standards.
    As an initial matter, the then-Administrator considered the range 
of scientific evidence evaluating these effects, including studies of 
at-risk populations, to inform his review of the primary 
PM2.5 standards, placing the greatest weight on evidence of 
effects for which the 2019 ISA determined there to be a causal or 
likely to be causal relationship with long- and short-term 
PM2.5 exposures (85 FR 82714-82715, December 18, 2020).
    With regard to indicator, the then-Administrator recognized that, 
consistent with the evidence available in prior reviews, the scientific 
evidence continued to provide strong support for health effects 
following short- and long-term PM2.5 exposures. He noted the 
2020 PA conclusions that the information continued to support the 
PM2.5 mass-based indicator and remained too limited to 
support a distinct standard for any specific PM2.5 component 
or group of components, and too limited to support a distinct standard 
for the ultrafine fraction. Thus, the then-Administrator concluded that 
it was appropriate to retain PM2.5 as the indicator for the 
primary standards for fine particles (85 FR 82715, December 18, 2020).
    With respect to averaging time and form, the then-Administrator 
noted that the scientific evidence continued to provide strong support 
for health effects associations with both long-term (e.g., annual or 
multi-year) and short-term (e.g., mostly 24-hour) exposures to 
PM2.5, consistent with the conclusions in the 2020 PA. In 
the 2019 ISA, epidemiologic and controlled human exposure studies 
examined a variety of PM2.5 exposure durations. 
Epidemiologic studies continued to provide strong support for health 
effects associated with short-term PM2.5 exposures based on 
24-hour PM2.5 averaging periods, and the EPA noted that 
associations with subdaily estimates are less consistent and, in some 
cases, smaller in magnitude (U.S. EPA, 2019a, section 1.5.2.1; U.S. 
EPA, 2020b, section 3.5.2.2). In addition, controlled human exposure 
and panel-based studies of subdaily exposures typically examined 
subclinical effects, rather than the more serious population-level 
effects that have been reported to be associated with 24-hour exposures 
(e.g., mortality, hospitalizations). Taken together, the 2019 ISA 
concluded that epidemiologic studies did not indicate that subdaily 
averaging periods were more closely associated with health effects than 
the 24-hour average exposure metric (U.S. EPA, 2019a, section 1.5.2.1). 
Additionally, while controlled human exposure studies provided 
consistent evidence for cardiovascular effects following 
PM2.5 exposures for less than 24 hours (i.e., <30 minutes to 
5 hours), exposure concentrations in the studies were well-above the 
ambient concentrations typically measured in locations meeting the 
existing standards (U.S. EPA, 2020b, section 3.2.3.1). Thus, these 
studies also did not suggest the need for additional protection against 
subdaily PM2.5 exposures (U.S. EPA, 2020b, section 3.5.2.2). 
Therefore, the then-Administrator judged that the 24-hour averaging 
time remained appropriate (85 FR 82715, December 18, 2020).
    With regard to the form of the 24-hour standard (98th percentile, 
averaged over three years), the then-Administrator noted that 
epidemiologic studies continued to provide strong support for health 
effect associations with short-term (e.g., mostly 24-hour) 
PM2.5 exposures (U.S. EPA, 2020b, section 3.5.2.3) and that 
controlled human exposure studies provided evidence for health effects 
following single short-term ``peak'' PM2.5 exposures. Thus, 
the evidence supported retaining a standard focused on providing 
supplemental protection against short-term peak exposures and supported 
a 98th percentile form for a 24-hour standard. The then-Administrator 
further noted that this form also provided an appropriate balance 
between limiting the occurrence of peak 24-hour PM2.5 
concentrations and identifying a stable target for risk management 
programs (U.S. EPA, 2020b, section 3.5.2.3). As such, the then-
Administrator concluded that the available information supported 
retaining the form and averaging time of the current 24-hour standard 
(98th percentile, averaged over three years) and annual standard 
(annual average, averaged over three years) (85 FR 82715, December 18, 
2020).
    With regard to the level of the standards, in reaching his final 
decision, the then-Administrator considered the large body of evidence 
presented and assessed in the 2019 ISA (U.S. EPA, 2019a), the policy-
relevant and risk-based conclusions and rationales as presented in the 
2020 PA (U.S. EPA, 2020b), advice from the CASAC, and public comments. 
In particular, in considering the 2019 ISA and 2020 PA, he considered 
key epidemiologic studies that evaluated associations between 
PM2.5 air quality distributions and mortality and morbidity, 
including key accountability studies; the availability of experimental 
studies to support biological plausibility; controlled human exposure 
studies examining effects following short-term PM2.5 
exposures; air quality analyses; and the important uncertainties and 
limitations associated with the information (85 FR 82715, December 18, 
2020).

[[Page 16221]]

    As an initial matter, the then-Administrator considered the 
protection afforded by both the annual and 24-hour standards together 
against long- and short-term PM2.5 exposures and health 
effects. The Administrator recognized that the annual standard was most 
effective in controlling ``typical'' PM2.5 concentrations 
near the middle of the air quality distribution (i.e., around the mean 
of the distribution), but also provided some control over short-term 
peak PM2.5 concentrations. On the other hand, the 24-hour 
standard, with its 98th percentile form, was most effective at limiting 
peak 24-hour PM2.5 concentrations, but in doing so also had 
an effect on annual average PM2.5 concentrations. Thus, 
while either standard could be viewed as providing some measure of 
protection against both average exposures and peak exposures, the 24-
hour and annual standards were not expected to be equally effective at 
limiting both types of exposures. Thus, consistent with previous 
reviews, the then-Administrator's consideration of the public health 
protection provided by the existing primary PM2.5 standards 
was based on his consideration of the combination of the annual and 24-
hour standards. Specifically, he recognized that the annual standard 
was more likely to appropriately limit the ``typical'' daily and annual 
exposures that are most strongly associated with the health effects 
observed in epidemiologic studies. The then-Administrator concluded 
that an annual standard (as the arithmetic mean, averaged over three 
years) remained appropriate for targeting protection against the annual 
and daily PM2.5 exposures around the middle portion of the 
PM2.5 air quality distribution. Further, recognizing that 
the 24-hour standard (with its 98th percentile form) was more directly 
tied to short-term peak PM2.5 concentrations, and more 
likely to appropriately limit exposures to such concentrations, the 
then-Administrator concluded that the current 24-hour standard (with 
its 98th percentile form, averaged over three years) remained 
appropriate to provide a balance between limiting the occurrence of 
peak 24-hour PM2.5 concentrations and identifying a stable 
target for risk management programs. However, the then-Administrator 
recognized that changes in PM2.5 air quality to meet an 
annual standard would likely result not only in lower short- and long-
term PM2.5 concentrations near the middle of the air quality 
distribution, but also in fewer and lower short-term peak 
PM2.5 concentrations. The then-Administrator further 
recognized that changes in air quality to meet a 24-hour standard, with 
a 98th percentile form, would result not only in fewer and lower peak 
24-hour PM2.5 concentrations, but also in lower annual 
average PM2.5 concentrations (85 FR 82715-82716, December 
18, 2020).
    Thus, in considering the adequacy of the 24-hour standard, the 
then-Administrator noted the importance of considering whether 
additional protection was needed against short-term exposures to peak 
PM2.5 concentrations. In examining the scientific evidence, 
he noted the limited utility of the animal toxicological studies in 
directly informing conclusions on the appropriate level of the standard 
given the uncertainty in extrapolating from effects in animals to those 
in human populations. The then-Administrator noted that controlled 
human exposure studies provided evidence for health effects following 
single, short-term PM2.5 exposures that corresponded best to 
exposures that might be experienced in the upper end of the 
PM2.5 air quality distribution in the U.S. (i.e., ``peak'' 
concentrations). However, most of these studies examined exposure 
concentrations considerably higher than are typically measured in areas 
meeting the standards (U.S. EPA, 2020b, section 3.2.3.1). In 
particular, controlled human exposure studies often reported 
statistically significant effects on one or more indicators of 
cardiovascular function following 2-hour exposures to PM2.5 
concentrations at and above 120 [mu]g/m\3\ (at and above 149 [mu]g/m\3\ 
for vascular impairment, the effect shown to be most consistent across 
studies). To provide insight into what these studies may indicate 
regarding the primary PM2.5 standards, the 2020 PA (U.S. 
EPA, 2020b, p. 3-49) noted that 2-hour ambient concentrations of 
PM2.5 at monitoring sites meeting the current standards 
almost never exceeded 32 [mu]g/m\3\. In fact, even the extreme upper 
end of the distribution of 2-hour PM2.5 concentrations at 
sites meeting the primary PM2.5 standards remained well-
below the PM2.5 exposure concentrations consistently shown 
in controlled human exposure studies to elicit effects (i.e., 99.9th 
percentile of 2-hour concentrations at these sites is 68 [mu]g/m\3\ 
during the warm season). Thus, the experimental evidence did not 
indicate the need for additional protection against exposures to peak 
PM2.5 concentrations, beyond the protection provided by the 
combination of the 24-hour and the annual standards (U.S. EPA, 2020b, 
section 3.2.3.1; 85 FR 82716, December 18, 2020).
    With respect to the epidemiologic evidence, the then-Administrator 
noted that the studies did not indicate that associations in those 
studies were strongly influenced by exposures to peak concentrations in 
the air quality distribution and thus did not indicate the need for 
additional protection against short-term exposures to peak 
PM2.5 concentrations (U.S. EPA, 2020b, section 3.5.1). The 
then-Administrator noted that this was consistent with CASAC consensus 
support for retaining the current 24-hour standard. Thus, the then-
Administrator concluded that the 24-hour standard with its level of 35 
[micro]g/m\3\ was adequate to provide supplemental protection (i.e., 
beyond that provided by the annual standard alone) against short-term 
exposures to peak PM2.5 concentrations (85 FR 82716, 
December 18, 2020).
    With regard to the level of the annual standard, the then-
Administrator recognized that the annual standard, with its form based 
on the arithmetic mean concentration, was most appropriately meant to 
limit the ``typical'' daily and annual exposures that were most 
strongly associated with the health effects observed in epidemiologic 
studies. However, the then-Administrator also noted that while 
epidemiologic studies examined associations between distributions of 
PM2.5 air quality and health outcomes, they did not identify 
particular PM2.5 exposures that cause effects and thus, they 
could not alone identify a specific level at which the standard should 
be set, as such a determination necessarily required the then-
Administrator's judgment. Thus, consistent with the approaches in 
previous NAAQS reviews, the then-Administrator recognized that any 
approach that used epidemiologic information in reaching decisions on 
what standards are appropriate necessarily required judgments about how 
to translate the information from the epidemiologic studies into a 
basis for appropriate standards. This approach included consideration 
of the uncertainties in the reported associations between daily or 
annual average PM2.5 exposures and mortality or morbidity in 
the epidemiologic studies. Such an approach is consistent with setting 
standards that are neither more nor less stringent than necessary, 
recognizing that a zero-risk standard is not required by the Clean Air 
Act (CAA) (85 FR 82716, December 18, 2020).
    The then-Administrator emphasized uncertainties and limitations 
that were present in epidemiologic studies in previous reviews and 
persisted in the 2020 review. These uncertainties

[[Page 16222]]

included exposure measurement error, potential confounding by 
copollutants, increasing uncertainty of associations at lower 
PM2.5 concentrations, and heterogeneity of effects across 
different cities or regions (85 FR 82716, December 18, 2020). The then-
Administrator also noted the advice given by the CASAC on this matter. 
As described in section I.C.5 above, the CASAC did not reach consensus 
on the adequacy of the primary annual PM2.5 standard. ``Some 
CASAC members'' expressed support for retaining the primary annual 
PM2.5 standard while ``other members'' expressed support for 
revising that standard in order to increase public health protection 
(Cox, 2019b, p. 1 of consensus letter). The CASAC members who supported 
retaining the annual standard expressed their concerns with the 
epidemiologic studies, asserting that these studies did not provide a 
sufficient basis for revising the existing standards. They also 
identified several key concerns regarding the associations reported in 
epidemiologic studies and concluded that ``while the data on 
associations should certainly be carefully considered, this data should 
not be interpreted more strongly than warranted based on its 
methodological limitations'' (Cox, 2019b, p. 8 consensus responses).
    Taking into consideration the views expressed by the CASAC members 
who supported retaining the annual standard, the then-Administrator 
recognized that epidemiologic studies examined associations between 
distributions of PM2.5 air quality and health outcomes, and 
they did not identify particular PM2.5 exposures that cause 
effects (U.S. EPA, 2020b, section 3.1.2). While the Administrator 
remained concerned about placing too much weight on epidemiologic 
studies to inform conclusions on the adequacy of the primary standards, 
he noted the approach to considering such studies in the 2012 review. 
In the 2012 review, it was noted that the evidence of an association in 
any epidemiologic study was ``strongest at and around the long-term 
average where the data in the study are most concentrated'' (78 FR 
3140, January 15, 2013). In considering the characterization of 
epidemiologic studies, the then-Administrator viewed that when 
assessing the mean concentrations of the key short-term and long-term 
epidemiologic studies in the U.S. that used ground-based monitoring 
(i.e., those studies where the mean is most directly comparable to the 
current annual standard), the majority of studies had mean 
concentrations at or above the level of the existing annual standard, 
with the mean of the study-reported means or medians equal to 13.5 
[micro]g/m\3\, a concentration level above the existing level of the 
primary annual standard of 12 [micro]g/m\3\. The then-Administrator 
further noted his caution in directly comparing the reported study mean 
values to the standard level given that study-reported mean 
concentrations, by design, are generally lower than the design value of 
the highest monitor in an area, which determines compliance. In the 
2020 PA, analyses of recent air quality in U.S. CBSAs indicated that 
maximum annual PM2.5 design values for a given three-year 
period were often 10% to 20% higher than average monitored 
concentrations (i.e., averaged across multiple monitors in the same 
CBSA) (U.S. EPA, 2020b, Appendix B, section B.7). He further noted his 
concern in placing too much weight on any one epidemiologic study but 
instead judged that it was more appropriate to focus on the body of 
studies together and therefore noted the calculation of the mean of 
study-reported means (or medians). Thus, while the then-Administrator 
was cautious in placing too much weight on the epidemiologic evidence 
alone, he noted that: (1) The reported mean concentration in the 
majority of the key U.S. epidemiologic studies using ground-based 
monitoring data were above the level of the existing annual standard; 
(2) the mean of the reported study means (or medians) (i.e., 13.5 
[micro]g/m\3\) was above the level of the current standard; \49\ (3) 
air quality analyses showed the study means to be lower than their 
corresponding design values by 10-20%; and (4) these analyses must be 
considered in light of uncertainties inherent in the epidemiologic 
evidence. When taken together, the then-Administrator judged that, even 
if it were appropriate to place more weight on the epidemiologic 
evidence, this information did not call into question the adequacy of 
the current standards (85 FR 82716-17, December 18, 2020).
---------------------------------------------------------------------------

    \49\ The median of the study-reported mean (or median) 
PM2.5 concentrations is 13.3 [micro]g/m\3\, which was 
also above the level of the existing standard.
---------------------------------------------------------------------------

    In addition to the evidence, the then-Administrator also considered 
the potential implications of the risk assessment. He noted that all 
risk assessments have limitations and that he remained concerned about 
the uncertainties in the underlying epidemiologic data used in the risk 
assessment. The then-Administrator also noted that in previous reviews, 
these uncertainties and limitations have often resulted in less weight 
being placed on quantitative estimates of risk than on the underlying 
scientific evidence itself (e.g., 78 FR 3086, 3098-99, January 15, 
2013). These uncertainties and limitations included uncertainty in the 
shapes of C-R functions, particularly at low concentrations; 
uncertainties in the methods used to adjust air quality; and 
uncertainty in estimating risks for populations, locations and air 
quality distributions different from those examined in the underlying 
epidemiologic study (U.S. EPA, 2020b, section 3.3.2.4). Additionally, 
the then-Administrator noted similar concern expressed by some members 
of the CASAC who support retaining the existing standards; they 
highlighted similar uncertainties and limitations in the risk 
assessment (Cox, 2019b). In light of all of this, the then-
Administrator judged it appropriate to place little weight on 
quantitative estimates of PM2.5-associated mortality risk in 
reaching conclusions about the level of the primary PM2.5 
standards (85 FR 82717, December 18, 2020).
    The then-Administrator additionally considered an emerging body of 
evidence from accountability studies that examined past reductions in 
ambient PM2.5 and the degree to which those reductions 
resulted in public health improvements. While the then-Administrator 
agreed with public commenters that well-designed and conducted 
accountability studies can be informative, he viewed the interpretation 
of such studies in the context of the primary PM2.5 
standards as complicated by the fact that some of the available studies 
had not evaluated PM2.5 specifically (e.g., as opposed to 
PM10 or total suspended particulates), did not show changes 
in PM2.5 air quality, or had not been able to disentangle 
health impacts of the interventions from background trends in health 
(U.S. EPA, 2020b, section 3.5.1). He further recognized that the small 
number of available studies that did report public health improvements 
following past declines in ambient PM2.5 had not examined 
air quality meeting the existing standards (U.S. EPA, 2020b, Table 3-
3). This included U.S. studies that reported increased life expectancy, 
decreased mortality, and decreased respiratory effects following past 
declines in ambient PM2.5 concentrations. Such studies 
examined ``starting'' annual average PM2.5 concentrations 
(i.e., prior to the reductions being evaluated) ranging from about 13.2 
to >20[micro]g/m\3\ (i.e., U.S. EPA, 2020b, Table 3-3). Given the lack 
of available accountability studies

[[Page 16223]]

reporting public health improvements attributable to reductions in 
ambient PM2.5 in locations meeting the existing standards, 
together with his broader concerns regarding the lack of experimental 
studies examining PM2.5 exposures typical of areas meeting 
the existing standards, the then-Administrator judged that there was 
considerable uncertainty in the potential for increased public health 
protection from further reductions in ambient PM2.5 
concentrations beyond those achieved under the existing primary 
PM2.5 standards (85 FR 82717, December 18, 2020).
    When the above considerations were taken together, the then-
Administrator concluded that the scientific evidence assessed in the 
2019 ISA, together with the analyses in the 2020 PA based on that 
evidence and consideration of CASAC advice and public comments, did not 
call into question the adequacy of the public health protection 
provided by the existing annual and 24-hour PM2.5 standards. 
In particular, the then-Administrator judged that there was 
considerable uncertainty in the potential for additional public health 
improvements from reducing ambient PM2.5 concentrations 
below the concentrations achieved under the existing primary standards 
and that, therefore, standards more stringent than the existing 
standards (e.g., with lower levels) were not supported. That is, he 
judged that more stringent standards would be more than requisite to 
protect the public health with an adequate margin of safety. This 
judgment reflected the Administrator's consideration of the 
uncertainties in the potential implications of the lower end of the air 
quality distributions from the epidemiologic studies due in part to the 
lack of supporting evidence from experimental studies and retrospective 
accountability studies conducted at PM2.5 concentrations 
meeting the existing standards (85 FR 82717, December 18, 2020).
    In reaching this conclusion in the 2020 review, the then-
Administrator judged that the existing standards provided an adequate 
margin of safety. With respect to the annual standard, the level of 12 
[micro]g/m\3\ was below the lowest ``starting'' concentration (i.e., 
13.2 [micro]g/m\3\) in the available accountability studies that showed 
public health improvements attributable to reductions in ambient 
PM2.5. In addition, while the then-Administrator placed less 
weight on the epidemiologic evidence for selecting a standard, he noted 
that the level of the annual standard was below the reported mean (and 
median) concentrations in the majority of the key U.S. epidemiologic 
studies using ground-based monitoring data (noting that these means 
tend to be 10-20% lower than their corresponding area design values 
which is the more relevant metric when considering the level of the 
standard) and below the mean of the reported means (or medians) of 
these studies (i.e., 13.5 [micro]g/m\3\). In addition, the then-
Administrator recognized that concentrations in areas meeting the 
existing 24-hour and annual standards remained well-below the 
PM2.5 exposure concentrations consistently shown to elicit 
effects in human exposure studies (85 FR 82717-82718, December 18, 
2020).
    In addition, based on the then-Administrator's review of the 
science in the 2020 review, including controlled human exposure studies 
examining effects following short-term PM2.5 exposures, the 
epidemiologic studies, and accountability studies conducted at levels 
just above the existing annual standard, he judged that the degree of 
public health protection provided by the existing annual standard is 
not greater than warranted. This judgment, together with the fact that 
no CASAC member expressed support for a less stringent standard, led 
the then- Administrator to conclude that standards less stringent than 
the existing standards (e.g., with higher levels) were also not 
supported (85 FR 82718, December 18, 2020).
    In reaching his final decision in the 2020 review, the then-
Administrator concluded that the scientific evidence and technical 
information continued to support the existing annual and 24-hour 
PM2.5 standards. This conclusion reflected the then-
Administrator's view that there were important limitations and 
uncertainties that remained in the evidence. The then-Administrator 
concluded that these limitations contributed to considerable 
uncertainty regarding the potential public health implications of 
revising the existing primary PM2.5 standards. Given this 
uncertainty, and noting the advice from some CASAC members, he 
concluded that the primary PM2.5 standards, including the 
indicators (PM2.5), averaging times (annual and 24-hour), 
forms (arithmetic mean and 98th percentile, averaged over three years) 
and levels (12.0 [micro]g/m\3\, 35 [micro]g/m\3\), when taken together, 
remained requisite to protect the public health. Therefore, in the 2020 
review, the Administrator reached the conclusion that the primary 24-
hour and annual PM2.5 standards, together, were requisite to 
protect public health from fine particles with an adequate margin of 
safety, including the health of at-risk populations, and retained the 
standards, without revision (85 FR 82718, December 18, 2020).
2. Overview of the Health Effects Evidence
    The information summarized here and further detailed in section 
II.B of the proposal (88 FR 5580, January 27, 2023), is an overview of 
the policy-relevant aspects of the health effects evidence available in 
this reconsideration; the assessment of this evidence is documented in 
the 2019 ISA (U.S. EPA, 2019a) and ISA Supplement (U.S. EPA, 2022a) and 
its policy implications are further discussed in the 2022 PA (U.S. EPA, 
2022b). While the 2019 ISA provides the broad scientific foundation for 
this reconsideration, additional literature has become available since 
the cutoff date of the 2019 ISA that expands the body of evidence 
related to mortality and cardiovascular effects for both short- and 
long-term PM2.5 exposure, which can inform the 
Administrator's judgment on the adequacy of the current primary 
PM2.5 standards. As such, the ISA Supplement builds on the 
information presented within the 2019 ISA with a targeted 
identification and evaluation of new scientific information (U.S. EPA, 
2022a, section 1.2). The ISA Supplement focuses on PM2.5 
health effects evidence where the 2019 ISA concludes a ``causal 
relationship,'' because such health effects are given the most weight 
in an Administrator's decisions in a NAAQS review. As such, in 
selecting the health effects to evaluate within the ISA Supplement 
(i.e., newly available evidence related to short- and long-term 
PM2.5 exposure and mortality and cardiovascular effects), 
the primary rationale is based on the causality determinations for 
health effect categories presented in the 2019 PM ISA, and the 
subsequent use of the health effects evidence in the 2020 PM PA. 
Specifically, U.S. and Canadian epidemiologic studies for mortality and 
cardiovascular effects, along with controlled human exposure studies 
associated with cardiovascular effects at near ambient concentrations, 
were considered to be of greatest utility in informing the 
Administrator's conclusions on the adequacy of the current primary 
PM2.5 standards. Additionally, studies examining 
associations outside the U.S. or Canada reflect air quality and 
exposure patterns that may be less typical of the U.S., and thus less 
likely to be informative for purposes of reviewing the NAAQS (U.S. EPA, 
2022b, p.1-3). While the ISA Supplement does not include information 
for health effects other than mortality and cardiovascular effects, the

[[Page 16224]]

scientific evidence for other health effect categories is evaluated in 
the 2019 ISA, which in combination with the ISA Supplement represents 
the complete scientific record for the reconsideration of the 2020 
final decision.
    The ISA Supplement also assessed accountability studies because 
these types of epidemiologic studies were part of the body of evidence 
that was a focus of the 2020 review. Accountability studies inform our 
understanding of the potential for public health improvements as 
ambient PM2.5 concentrations have declined over time. 
Further, the ISA Supplement considered studies that employed 
statistical approaches that attempt to more extensively account for 
confounders and are more robust to model misspecification (i.e., used 
alternative methods for confounder control),\50\ given that such 
studies were highlighted by the CASAC and identified in public comments 
in the 2020 review. Since the literature cutoff date for the 2019 ISA, 
multiple accountability studies and studies that employ alternative 
methods for confounder control have become available for consideration 
in the ISA Supplement and, subsequently, in this reconsideration.
---------------------------------------------------------------------------

    \50\ As noted in the ISA Supplement (U.S. EPA, 2022a, p. 1-3): 
``In the peer-reviewed literature, these epidemiologic studies are 
often referred to as causal inference studies or studies that used 
causal modeling methods. For the purposes of this Supplement, this 
terminology is not used to prevent confusion with the main 
scientific conclusions (i.e., the causality determinations) 
presented within an ISA. In addition, as is consistent with the 
weight-of-evidence framework used within ISAs and discussed in the 
Preamble to the Integrated Science Assessments, an individual study 
on its own cannot inform causality, but instead represents a piece 
of the overall body of evidence.''
---------------------------------------------------------------------------

    The ISA Supplement also considered recent health effects evidence 
that addresses key scientific issues where the literature has expanded 
since the completion of the 2019 ISA.\51\ The 2019 ISA evaluated a 
couple of controlled human exposure studies that investigated the 
effect of exposure to near-ambient concentrations of PM2.5 
(U.S. EPA, 2019a, section 6.1.10 and 6.1.13). The ISA Supplement adds 
to this limited evidence, including a recent study conducted in young 
healthy individuals exposed to near-ambient PM2.5 
concentrations (U.S. EPA, 2022a, section 3.3.1). Given the importance 
of identifying populations at increased risk of PM2.5-
related effects, the ISA Supplement also included epidemiologic or 
exposure studies that examined whether there is evidence of exposure or 
risk disparities by race/ethnicity or SES. These types of studies 
provide additional information related to factors that may increase 
risk of PM2.5-related health effects and provide additional 
evidence for consideration by the Administrator in reaching conclusions 
regarding the adequacy of the current standards. In addition, the ISA 
Supplement evaluated studies that examined the relationship between 
short- and long-term PM2.5 exposures and SARS-CoV-2 
infection and/or COVID-19 death, as these studies are a new area of 
research and were raised by a number of public commenters in the 2020 
review.
---------------------------------------------------------------------------

    \51\ As with the epidemiologic studies for long- and short-term 
PM2.5 exposure and mortality and cardiovascular effects, 
epidemiologic studies of exposure or risk disparities and SARS-CoV-2 
infection and/or COVID-19 death were limited to those conducted in 
the U.S. and Canada.
---------------------------------------------------------------------------

    The evidence presented within the 2019 ISA, along with the targeted 
identification and evaluation of new scientific information in the ISA 
Supplement, provides the scientific basis for the reconsideration of 
the 2020 final decision on the primary PM2.5 standards. The 
subsections below briefly summarize the nature of PM2.5-
related health effects (II.A.2.a), with a focus on those health effects 
for which the 2019 ISA concluded a ``causal'' or ``likely to be 
causal'' relationship, the potential public health implications and 
populations at risk (II.A.2.b), and PM2.5 concentrations in 
key studies reporting health effects (II.A.2.c).
a. Nature of Effects
    The evidence base available in the reconsideration includes decades 
of research on PM2.5-related health effects (U.S. EPA, 
2004b; U.S. EPA, 2009a; U.S. EPA, 2019a), including the full body of 
evidence evaluated in the 2019 ISA (U.S. EPA, 2019a), along with the 
targeted evaluation of recent evidence in the ISA Supplement (U.S. EPA, 
2022a). In considering the available scientific evidence, the sections 
below, and in more detail in section II.B.1 of the proposal (88 FR 
5580, January 27, 2023), summarize the relationships between long-and 
short-term PM2.5 exposures and mortality (II.A.2.a.i), 
cardiovascular effects (II.A.2.a.ii), respiratory effects 
(II.A.2.a.iii), cancer (II.A.2.a.iv), nervous system effects 
(II.A.2.a.v) and other effects (II.A.2.a.vi). For these outcomes, the 
2019 ISA concluded that the evidence supports either a ``causal'' or a 
``likely to be causal'' relationship.\52\
---------------------------------------------------------------------------

    \52\ In this reconsideration of the PM NAAQS, the EPA considers 
the full body of health evidence, placing the greatest emphasis on 
the health effects for which the evidence has been judged in the 
2019 ISA to demonstrate a ``causal'' or ``likely to be causal'' 
relationship with PM2.5 exposures.
---------------------------------------------------------------------------

i. Mortality
Long-Term PM2.5 Exposures
    In the 2012 review, the 2009 ISA reported that the evidence was 
``sufficient to conclude that the relationship between long-term 
PM2.5 exposures and mortality is causal'' (U.S. EPA, 2009a, 
p. 7-96). The strongest evidence supporting this conclusion was 
provided by epidemiologic studies, particularly those examining two 
seminal cohorts, the American Cancer Society (ACS) cohort and the 
Harvard Six Cities cohort. Analyses of the Harvard Six Cities cohort 
included evidence indicating that reductions in ambient 
PM2.5 concentrations are associated with reduced mortality 
risk (Laden et al., 2006) and increases in life expectancy (Pope et 
al., 2009). Further support was provided by other cohort studies 
conducted in North America and Europe that reported positive 
associations between long-term PM2.5 exposure and mortality 
(U.S. EPA, 2019a).
    Cohort studies, which have become available since the completion of 
the 2009 ISA and evaluated in the 2019 ISA, continue to provide 
consistent evidence of positive associations between long-term 
PM2.5 exposures and mortality. These studies add support for 
associations with all-cause and total (non-accidental) mortality,\53\ 
as well as with specific causes of mortality, including cardiovascular 
disease and respiratory disease (U.S. EPA, 2019a, section 11.2.2). 
Several of these studies conducted analyses over longer study durations 
and periods of follow-up than examined in the original ACS and Harvard 
Six Cities cohort studies and continue to report positive associations 
between long-term exposure to PM2.5 and mortality (U.S. EPA, 
2019a, section 11.2.2.1; Figures 11-18 and 11-19). In addition to 
studies focusing on the ACS and Harvard Six Cities cohorts, additional 
studies examining other cohorts also provide evidence of consistent, 
positive associations between long-term PM2.5 exposure and 
mortality across a wide range of demographic groups (e.g., age, sex, 
occupation), spatial and temporal extents, exposure assessment metrics, 
and statistical techniques (U.S. EPA, 2019a, sections 11.2.2.1, 11.2.5; 
U.S. EPA, 2022a, Table 11-8). This includes some of the largest cohort 
studies conducted to date, such as analyses of the U.S. Medicare cohort 
that includes

[[Page 16225]]

nearly 61 million enrollees and studies that control for a range of 
individual and ecological covariates, including race, age, SES, smoking 
status, body mass index, and annual weather variables (e.g., 
temperature, humidity) (U.S. EPA, 2019a).
---------------------------------------------------------------------------

    \53\ The majority of these studies examined non-accidental 
mortality outcomes, though some Medicare studies lack cause-specific 
death information and, therefore, examine total mortality.
---------------------------------------------------------------------------

    In addition to those cohort studies evaluated in the 2019 ISA, 
recent North American cohort studies evaluated in the ISA Supplement 
continue to examine the relationship between long-term PM2.5 
exposure and mortality and report consistent, positive, and 
statistically significant associations. These recent studies also 
utilize large and demographically diverse cohorts that are generally 
representative of the national populations in both the U.S. and Canada. 
These ``studies published since the 2019 ISA support and extend the 
evidence base that contributed to the conclusion of a causal 
relationship between long-term PM2.5 exposure and 
mortality'' (U.S. EPA, 2022a, section 3.2.2.2.1, Figure 3-19, Figure 3-
20).
    Furthermore, studies evaluated in the 2019 ISA and the ISA 
Supplement that examined cause-specific mortality expand upon previous 
research that found consistent, positive associations between 
PM2.5 exposure and specific mortality outcomes, which 
include cardiovascular and respiratory mortality, as well as other 
mortality outcomes. For cardiovascular-related mortality, the evidence 
evaluated in the ISA Supplement is consistent with the evidence 
evaluated in the 2019 ISA with recent studies reporting positive 
associations with long-term PM2.5 exposure. When evaluating 
cause-specific cardiovascular mortality, recent studies reported 
positive associations for a number of outcomes, such as ischemic heart 
disease (IHD) and stroke mortality (U.S. EPA, 2022a, Figure 3-23). 
Moreover, recent studies also provide some initial evidence that 
individuals with pre-existing health conditions, such as heart failure 
and diabetes, are at an increased risk of PM2.5-related 
health effects (U.S. EPA, 2022a, section 3.2.2.4) and that these 
individuals have a higher risk of mortality overall, which was 
previously only examined in studies that used stratified analyses 
rather than a cohort of people with an underlying health condition 
(U.S. EPA, 2022a, section 3.2.2.4). With regard to respiratory 
mortality, epidemiologic studies evaluated in the 2019 ISA and ISA 
Supplement continue to provide support for associations between long-
term PM2.5 exposure and respiratory mortality (U.S. EPA, 
2019a, section 5.2.10; U.S. EPA, 2022a, Table 3-2).
    A series of epidemiologic studies evaluated in the 2019 ISA tested 
the hypothesis that past reductions in ambient PM2.5 
concentrations are associated with increased life expectancy or a 
decreased mortality rate and report that reductions in ambient 
PM2.5 are associated with improvements in longevity (U.S. 
EPA, 2022a, section 11.2.2.5). Pope et al. (2009) conducted a cross-
sectional analysis using air quality data from 51 metropolitan areas 
across the U.S., beginning in the 1970s through the early 2000s, and 
found that a 10 [micro]g/m\3\ decrease in long-term PM2.5 
concentration was associated with a 0.61-year increase in life 
expectancy. In a subsequent analysis, the authors extended the period 
of analysis to include 2000 to 2007, a time period with lower ambient 
PM2.5 concentrations and found a decrease in long-term 
PM2.5 concentration continued to be associated with an 
increase in life expectancy, though the magnitude of the increase was 
smaller than during the earlier time period (i.e., a 10 [micro]g/m\3\ 
decrease in long-term PM2.5 concentration was associated 
with a 0.35-year increase in life expectancy) (Correia et al., 2013). 
Additional studies conducted in the U.S. or Europe similarly report 
that reductions in ambient PM2.5 are associated with 
improvements in longevity (U.S. EPA, 2022a, section 11.2.2.5).
    Since the literature cutoff date for the 2019 ISA, a few 
epidemiologic studies were published that examined the relationship 
between long-term PM2.5 exposure and life-expectancy (U.S. 
EPA, 2022a, section 3.2.1.3) and report results that are consistent 
with and expand upon the body of evidence from the 2019 ISA. For 
example, Bennett et al. (2019) reported that PM2.5 
concentrations above the lowest observed concentration (2.8 [micro]g/
m\3\) were associated with a 0.15 year decrease in national life 
expectancy for women and 0.13 year decrease in national life expectancy 
for men (U.S. EPA, 2022a, section 3.2.2.2.4, Figure 3-25). Another 
study compared participants living in areas with PM2.5 
concentrations >12 [micro]g/m\3\ to participants living in areas with 
PM2.5 concentrations <12 [micro]g/m\3\ and reported that the 
number of years of life lost due to living in areas with higher 
PM2.5 concentrations was 0.84 years over a 5-year period 
(Ward-Caviness et al., 2020; U.S. EPA, 2022a, section 3.2.2.2.4).
    Additionally, a number of accountability studies, which are 
epidemiologic studies that evaluate whether an environmental policy or 
air quality intervention resulted in reductions in ambient air 
pollution concentrations and subsequent reductions in mortality or 
morbidity, have emerged and were evaluated in the ISA Supplement (U.S. 
EPA, 2022a, section 3.2.2.3). For example, Sanders et al. (2020a) 
examined whether policy actions (i.e., the first annual 
PM2.5 NAAQS implementation rule in 2005 for the 1997 annual 
PM2.5 standard with a 3-year annual average of 15.0 [mu]g/
m\3\) reduced PM2.5 concentrations and mortality rates in 
Medicare beneficiaries between 2000-2013, and found that following 
implementation of the annual PM2.5 NAAQS, annual 
PM2.5 concentrations decreased by 1.59 [mu]g/m\3\ (95% CI: 
1.39, 1.80) which corresponded to a 0.93% reduction in mortality rates 
among individuals 65 years and older ([95% CI: 0.10%, 1.77%) in non-
attainment counties relative to attainment counties.
    The 2019 ISA also evaluated a small number of studies that used 
alternative methods for confounder control to further assess 
relationship between long-term PM2.5 exposure and mortality 
(U.S. EPA, 2019a, section 11.2.2.4). In addition, multiple 
epidemiologic studies that implemented alternative methods for 
confounder control and were published since the literature cutoff date 
of the 2019 ISA were evaluated in the ISA Supplement (U.S. EPA, 2022a, 
section 3.2.2.3). These studies used a variety of statistical methods 
including generalized propensity score (GPS), inverse probability 
weighting (IPW), and difference-in-difference (DID) to reduce 
uncertainties related to confounding bias in the association between 
long-term PM2.5 exposure and mortality. These studies 
reported consistent positive associations between long-term 
PM2.5 exposure and total mortality (U.S. EPA, 2022a, section 
3.2.2.3), and provided further support for the associations reported in 
the cohort studies referenced above.
    The 2019 ISA and ISA Supplement also evaluated the degree to which 
recent studies examining the relationship between long-term 
PM2.5 exposure and mortality addressed key policy-relevant 
issues and/or previously identified data gaps in the scientific 
evidence, including methods to estimate exposure, methods to control 
for confounding (e.g., co-pollutant confounding), the shape of the C-R 
relationship, as well as examining whether a threshold exists below 
which mortality effects do not occur. With respect to exposure 
assessment, based on its evaluation of the evidence, the 2019 ISA 
concludes that positive associations between long-term PM2.5 
exposures and mortality are robust

[[Page 16226]]

across recent analyses using various approaches to estimate 
PM2.5 exposures (e.g., based on monitors, models, satellite-
based methods, or hybrid methods that combine information from multiple 
sources) (U.S. EPA, 2019a, section 11.2.5.1). Hart et al. (2015) report 
that correction for bias due to exposure measurement error increases 
the magnitude of the hazard ratios (confidence intervals widen but the 
association remains statistically significant), suggesting that failure 
to correct for exposure measurement error could result in attenuation 
or underestimation of risk estimates.
    The 2019 ISA additionally concludes that positive associations 
between long-term PM2.5 exposures and mortality are robust 
across statistical models that use different approaches to control for 
confounders or different sets of confounders (U.S. EPA, 2019a, sections 
11.2.3 and 11.2.5), across diverse geographic regions and populations, 
and across a range of temporal periods including periods of declining 
PM concentrations (U.S. EPA, 2019a, sections 11.2.2.5 and 11.2.5.3). 
Additional evidence further demonstrates that associations with 
mortality remain robust in copollutants analyses (U.S. EPA, 2019a, 
section 11.2.3), and that associations persist in analyses restricted 
to long-term exposures (annual average PM2.5 concentrations) 
below 12 [mu]g/m\3\ (Di et al., 2017b) or 10 [mu]g/m\3\ (Shi et al., 
2016), indicating that risks are not disproportionately driven by the 
upper portions of the air quality distribution. Recent studies 
evaluated in the ISA Supplement further assess potential copollutant 
confounding and indicate that while there is some evidence of potential 
confounding of the PM2.5-mortality association by 
copollutants in some of the studies (i.e., those studies of the 
Mortality Air Pollution Associations in Low Exposure Environments 
(MAPLE) cohort), this result is inconsistent with other recent studies 
evaluated in the 2019 ISA that were conducted in the U.S. and Canada 
that found associations in both single and copollutant models (U.S. 
EPA, 2019a; U.S. EPA, 2022a, section 3.2.2.4)
    Additionally, a few studies use statistical techniques to reduce 
uncertainties related to potential confounding to further inform 
conclusions on causality for long-term PM2.5 exposure and 
mortality, as further detailed in section II.B.1.a.i of the proposal 
(88 FR 5582, January 27, 2023), studies by Greven et al. (2011), Pun et 
al. (2017), and Eum et al. (2018) completed sensitivity analyses as 
part of their Medicare cohort study in which they decompose ambient 
PM2.5 into ``spatial'' and ``spatiotemporal'' components in 
order to evaluate the potential for bias due to unmeasured spatial 
confounding. Pun et al. (2017) observed positive associations for the 
``temporal'' variation model and approximately null associations for 
the ``spatiotemporal'' variation model for all causes of death except 
for COPD mortality. The difference in the results of these two models 
for most causes of death suggests the presence of unmeasured 
confounding, though the authors do not indicate anything about the 
direction or magnitude of this bias. It is important to note that the 
``temporal'' and ``spatiotemporal'' coefficients are not directly 
comparable to the results of other epidemiologic studies when examined 
individually and can only be used in comparison with one another to 
evaluate the potential for unmeasured confounding bias. Eum et al. 
(2018) and Wu et al. (2020) also attempted to address long-term trends 
and meteorological variables as potential confounders and found that 
not adjusting for temporal trends could overestimate the association, 
while effect estimates in analyses that excluded meteorological 
variables remained unchanged compared to the main analyses. While 
results of these analyses suggest the presence of some unmeasured 
confounding, they do not indicate the direction or magnitude of the 
bias.\54\
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    \54\ In public comments on the 2019 draft PA, the authors of the 
Pun et al. (2017) study further note that ``the presence of 
unmeasured confounding. . .was expected given that we did not 
control for several potential confounders that may impact 
PM2.5-mortality associations, such as smoking, socio-
economic status (SES), gaseous pollutants, PM2.5 
components, and long-term time trends in PM2.5'' and that 
``spatial confounding may bias mortality risks both towards and away 
from the null'' (Docket ID EPA-HQ-OAR-2015-0072-0065; accessible in 
https://www.regulations.gov/).
---------------------------------------------------------------------------

    An additional important consideration in characterizing the public 
health impacts associated with PM2.5 exposure is whether C-R 
relationships are linear across the range of concentrations or if 
nonlinear relationships exist along any part of this range. Studies 
evaluated in the 2019 ISA and the ISA Supplement examine this issue, 
and continue to provide evidence of linear, no-threshold relationships 
between long-term PM2.5 exposures and all-cause and cause-
specific mortality (U.S. EPA, 2019a, section 11.2.4; U.S. EPA, 2022a, 
section 3.2.2.2.7, Table 3-6). Across the studies evaluated in the 2019 
ISA and the ISA Supplement, a variety of statistical methods have been 
used to assess whether there is evidence of deviations in linearity 
(U.S. EPA, 2019a, Table 11-7; U.S. EPA, 2022a, section 2.2.3.2). 
Studies have also conducted cut-point analyses that focus on examining 
risk at specific ambient PM2.5 concentrations. Generally, 
the evidence remains consistent in supporting a no-threshold 
relationship, and in supporting a linear relationship for 
PM2.5 concentrations >8 [mu]g/m\3\. However, uncertainties 
remain about the shape of the C-R curve at PM2.5 
concentrations <8 [mu]g/m\3\, with some recent studies providing 
evidence for either a sublinear, linear, or supralinear relationship at 
these lower concentrations (U.S. EPA, 2019a, section 11.2.4; U.S. EPA, 
2022a, section 2.2.3.2). There was also some limited evidence 
indicating that the slope of the C-R function may be steeper 
(supralinear) at lower concentrations for cardiovascular mortality 
(U.S. EPA, 2022a, section 3.1.1.2.6).
    The biological plausibility of PM2.5-attributable 
mortality is supported by the coherence of effects across scientific 
disciplines (i.e., animal toxicological, controlled human exposure 
studies, and epidemiologic) when evaluating respiratory and 
cardiovascular morbidity effects, which are some of the largest 
contributors to total (nonaccidental) mortality. The 2019 ISA outlines 
the available evidence for biologically plausible pathways by which 
inhalation exposure to PM2.5 could progress from initial 
events (e.g., pulmonary inflammation, autonomic nervous system 
activation) to endpoints relevant to population outcomes, particularly 
those related to cardiovascular diseases such as ischemic heart 
disease, stroke and atherosclerosis (U.S. EPA, 2019a, section 6.2.1), 
and to metabolic effects, including diabetes (U.S. EPA, 2019a, section 
7.3.1). The 2019 ISA notes ``more limited evidence from respiratory 
morbidity'' (U.S. EPA, 2019a, p. 11-101) such as development of chronic 
obstructive pulmonary disease (COPD) (U.S. EPA, 2019a, section 5.2.1) 
to support the biological plausibility of mortality due to long-term 
PM2.5 exposures (U.S. EPA, 2019a, section 11.2.1).
    Taken together, epidemiologic studies, including those evaluated in 
the 2019 ISA and more recent studies evaluated in the ISA Supplement, 
consistently report positive associations between long-term 
PM2.5 exposure and mortality across different geographic 
locations, populations, and analytic approaches (U.S. EPA, 2019a; U.S. 
EPA, 2022a, section 3.2.2.4). As such, these studies reduce key 
uncertainties identified in previous reviews, including those related 
to potential

[[Page 16227]]

copollutant confounding, and provide additional information on the 
shape of the C-R curve. As evaluated in the 2019 ISA, experimental and 
epidemiologic evidence for cardiovascular effects, and respiratory 
effects to a more limited degree, supports the plausibility of 
mortality due to long-term PM2.5 exposures. Overall, studies 
evaluated in the 2019 ISA support the conclusion of a causal 
relationship between long-term PM2.5 exposure and mortality, 
which is supported and extended by evidence from recent epidemiologic 
studies evaluated in the ISA Supplement (U.S. EPA, 2022a, section 
3.2.2.4).
Short-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship exists between 
short-term exposure to PM2.5 and mortality'' (U.S. EPA, 
2009a). This conclusion was based on the evaluation of both multi- and 
single-city epidemiologic studies that consistently reported positive 
associations between short-term PM2.5 exposure and non-
accidental mortality. These associations were strongest, in terms of 
magnitude and precision, primarily at lags of 0 to 1 days. Examination 
of the potential confounding effects of gaseous copollutants was 
limited, though evidence from single-city studies indicated that 
gaseous copollutants have minimal effect on the PM2.5-
mortality relationship (i.e., associations remain robust to inclusion 
of other pollutants in copollutant models). The evaluation of cause-
specific mortality found that effect estimates were larger in 
magnitude, but also had larger confidence intervals, for respiratory 
mortality compared to cardiovascular mortality. Although the largest 
mortality risk estimates were for respiratory mortality, the 
interpretation of the results was complicated by the limited coherence 
from studies of respiratory morbidity. However, the evidence from 
studies of cardiovascular morbidity provided both coherence and 
biological plausibility for the relationship between short-term 
PM2.5 exposure and cardiovascular mortality.
    Multicity studies evaluated in the 2019 ISA and the ISA Supplement 
provide evidence of primarily positive associations between daily 
PM2.5 exposures and mortality, with percent increases in 
total mortality ranging from 0.19% (Lippmann et al., 2013) to 2.80% 
(Kloog et al., 2013) \55\ at lags of 0 to 1 days in single-pollutant 
models. Whereas many studies assign exposures using data from ambient 
monitors, other studies employ hybrid modeling approaches, which 
estimate PM2.5 concentrations using data from a variety of 
sources (i.e., from satellites, land use information, and modeling, in 
addition to monitors) and enable the inclusion of less urban and more 
rural locations in analyses (e.g., Kloog et al., 2013, Lee et al., 
2015, Shi et al., 2016).
---------------------------------------------------------------------------

    \55\ As detailed in the Preface to the ISA, risk estimates are 
for a 10 [micro]g/m\3\ increase in 24-hour avg PM2.5 
concentrations, unless otherwise noted (U.S. EPA, 2019a).
---------------------------------------------------------------------------

    Some studies have expanded the examination of potential confounders 
including long-term temporal trends, weather, and co-occurring 
pollutants. Mortality associations were found to remain positive, 
although in some cases were attenuated, when using different approaches 
to account for temporal trends or weather covariates (e.g., U.S. EPA, 
2019a, section 11.1.5.1). For example, Sacks et al. (2012) examined the 
influence of model specification using the approaches for confounder 
adjustment from models employed in several multicity studies within the 
context of a common data set (U.S. EPA, 2019a, section 11.1.5.1). These 
models use different approaches to control for long-term temporal 
trends and the potential confounding effects of weather. The authors 
report that associations between daily PM2.5 and 
cardiovascular mortality were similar across models, with the percent 
increase in mortality ranging from 1.5-2.0% (U.S. EPA, 2019a, Figure 
11-4). Thus, alternative approaches to controlling for long-term 
temporal trends and for the potential confounding effects of weather 
may influence the magnitude of the association between PM2.5 
exposures and mortality but have not been found to influence the 
direction of the observed association (U.S. EPA, 2019a, section 
11.1.5.1). Taken together, the 2019 ISA and the ISA Supplement conclude 
that recent multicity studies conducted in the U.S., Canada, Europe, 
and Asia continue to provide consistent evidence of positive 
associations between short-term PM2.5 exposures and total 
mortality across studies that use different approaches to control for 
the potential confounding effects of weather (e.g., temperature) (U.S. 
EPA, 2019a, section 1.4.1.5.1; U.S. EPA, 2022a, section 3.2.1.2).
    With regard to copollutants, studies evaluated in the 2019 ISA 
provide additional evidence that associations between short-term 
PM2.5 exposures and mortality remain positive and relatively 
unchanged in copollutant models with both gaseous pollutants and 
PM10-2.5 (U.S. EPA, 2019a, section 11.1.4). Additionally, 
the low (r < 0.4) to moderate correlations (r = 0.4-0.7) between 
PM2.5 and gaseous pollutants and PM10-2.5 
increase the confidence in PM2.5 having an independent 
effect on mortality (U.S. EPA, 2019a, section 11.1.4). Consistent with 
the studies evaluated in the 2019 ISA, studies evaluated in the ISA 
Supplement that used data from more recent years also indicate that 
associations between short-term PM2.5 exposure and mortality 
remain unchanged in copollutant models. However, the evidence indicates 
that the association could be larger in magnitude in the presence of 
some copollutants such as oxidant gases (Lavigne et al., 2018; Shin et 
al., 2021).
    The generally positive associations reported with mortality are 
supported by a small group of studies employing alternative methods for 
confounder control or quasi-experimental statistical approaches (U.S. 
EPA, 2019a, section 11.1.2.1). For example, two studies by Schwartz et 
al. report associations between PM2.5 instrumental variables 
and mortality (U.S. EPA, 2019a, Table 11-2), including in an analysis 
limited to days with 24-hour average PM2.5 concentrations 
<30 [mu]g/m\3\ (Schwartz et al., 2015; Schwartz et al., 2017). In 
addition to the main analyses, these studies conducted Granger-like 
causality tests as sensitivity analyses to examine whether there was 
evidence of an association between mortality and PM2.5 after 
the day of death, which would support the possibility that unmeasured 
confounders were not accounted for in the statistical model. Neither 
study reports evidence of an association with PM2.5 after 
death (i.e., they do not indicate unmeasured confounding). Yorifuji et 
al. (2016) conducted a quasi-experimental study to examine whether a 
specific regulatory action in Tokyo, Japan (i.e., a diesel emission 
control ordinance) resulted in a subsequent reduction in daily 
mortality (Yorifuji et al., 2016). The authors reported a reduction in 
mortality in Tokyo due to the ordinance, compared to Osaka, which did 
not have a similar diesel emission control ordinance in place. In 
another study, Schwartz et al. (2018) utilized three statistical 
methods including instrumental variable analysis, a negative exposure 
control, and marginal structural models to estimate the association 
between PM2.5 and daily mortality (Schwartz et al., 2018). 
Results from this study continue to support a relationship between 
short-term PM2.5 exposure and mortality. Additional 
epidemiologic studies evaluated in the ISA Supplement that employed 
alternative methods for confounder control to examine the association 
between short-term PM2.5 exposure and

[[Page 16228]]

mortality also report consistent positive associations in studies that 
examine effects across multiple cities in the U.S. (U.S. EPA, 2022a).
    The positive associations for total mortality reported across the 
majority of studies evaluated are further supported by cause-specific 
mortality analyses, which generally report consistent, positive 
associations with both cardiovascular and respiratory mortality (U.S. 
EPA, 2019a, section 11.1.3). Recent multicity studies evaluated in the 
ISA Supplement add to the body of evidence indicating a relationship 
between short-term PM2.5 exposure and cause-specific 
mortality, with more variability in the magnitude and precision of 
associations for respiratory mortality (U.S. EPA, 2022a; Figure 3-14). 
For both cardiovascular and respiratory mortality, there has been a 
limited assessment of potential copollutant confounding, though initial 
evidence indicates that associations remain positive and relatively 
unchanged in models with gaseous pollutants and PM10-2.5, 
which further supports the copollutant analyses conducted for total 
mortality. The strong evidence for ischemic events and heart failure, 
as detailed in the assessment of cardiovascular morbidity (U.S. EPA, 
2019a, Chapter 6), provides biological plausibility for 
PM2.5-related cardiovascular mortality, which comprises the 
largest percentage of total mortality (i.e., ~33%) (NHLBI, 2017). 
Although there is evidence for exacerbations of COPD and asthma, the 
collective body of respiratory morbidity evidence provides limited 
biological plausibility for PM2.5-related respiratory 
mortality (U.S. EPA, 2019a, Chapter 5).
    In the 2009 ISA, one of the main uncertainties identified was the 
regional and city-to-city heterogeneity in PM2.5-mortality 
associations. Studies evaluated in the 2019 ISA examine both city-
specific as well as regional characteristics to identify the underlying 
contextual factors that could contribute to this heterogeneity (U.S. 
EPA, 2019a, section 11.1.6.3). Analyses focusing on effect modification 
of the PM2.5 mortality relationship by PM2.5 
components, regional patterns in PM2.5 components and city 
specific differences in composition and sources indicate some 
differences in the PM2.5 composition and sources across 
cities and regions, but these differences do not fully explain the 
observed heterogeneity. Additional studies find that factors related to 
potential exposure differences, such as housing stock and commuting, as 
well as city specific factors (e.g., land use, port volume, and traffic 
information), may also explain some of the observed heterogeneity (U.S. 
EPA, 2019a, section 11.1.6.3). Collectively, studies evaluated in the 
2019 ISA and the ISA Supplement indicate that the heterogeneity in 
PM2.5 mortality risk estimates cannot be attributed to one 
factor, but instead a combination of factors including, but not limited 
to, PM composition and sources as well as community characteristics 
that could influence exposures (U.S. EPA, 2019a, section 11.1.12; U.S. 
EPA, 2022a, section 3.2.1.2.1).
    A number of studies evaluated in the 2019 ISA and ISA Supplement 
conducted systematic evaluations of the lag structure of associations 
for the PM2.5-mortality relationship by examining either a 
series of single day or multiday lags and these studies continue to 
support an immediate effect (i.e., lag 0 to 1 days) of short-term 
PM2.5 exposures on mortality (U.S. EPA, 2019a, section 
11.1.8.1; U.S. EPA, 2022a, section 3.2.1.1). Recent studies also 
conducted analyses comparing the traditional 24-hour average exposure 
metric with a subdaily metric (i.e., 1-hour max) and provide evidence 
of a similar pattern of associations for both the 24-hour average and 
1-hour max metric, with the association larger in magnitude for the 24-
hour average metric.
    Multicity studies indicate that positive and statistically 
significant associations with mortality persist in analyses restricted 
to short-term (24-hour average PM2.5 concentrations) 
PM2.5 exposures below 35 [mu]g/m\3\ (Lee et al., 2015),\56\ 
below 30 [mu]g/m\3\ (Shi et al., 2016), and below 25 [mu]g/m\3\ (Di et 
al., 2017a), indicating that risks associated with short-term 
PM2.5 exposures are not disproportionately driven by the 
peaks of the air quality distribution. Additional studies examined the 
shape of the C-R relationship for short-term PM2.5 exposure 
and mortality and whether a threshold exists below which mortality 
effects do not occur (U.S. EPA, 2019a, section 11.1.10). These studies 
used various statistical approaches and consistently demonstrate linear 
C-R relationships with no evidence of a threshold.
---------------------------------------------------------------------------

    \56\ Lee et al. (2015) restrict exposures below 35 [mu]g/m\3\ 
only in areas with annual average concentrations <12 [mu]g/m\3\. 
Additionally, Lee et al. (2015) also report that positive and 
statistically significant associations between short-term 
PM2.5 exposures and mortality persist in analyses 
restricted to areas with long-term concentrations below 12 [mu]g/
m\3\.
---------------------------------------------------------------------------

    Moreover, recent studies evaluated in the ISA Supplement provide 
additional support for a linear, no-threshold C-R relationship between 
short-term PM2.5 exposure and mortality, with confidence in 
the shape decreasing at concentrations below 5 [micro]g/m\3\ (Shi et 
al., 2016; Lavigne et al., 2018). Recent analyses provide initial 
evidence indicating that PM2.5-mortality associations 
persist and may be stronger (i.e., a steeper slope) at lower 
concentrations (e.g., Di et al., 2017a; Figure 11-12 in U.S. EPA, 
2019). However, given the limited data available at the lower end of 
the distribution of ambient PM2.5 concentrations, the shape 
of the C-R curve remains uncertain at these low concentrations. 
Although difficulties remain in assessing the shape of the short-term 
PM2.5-mortality C-R relationship, to date, studies have not 
conducted systematic evaluations of alternatives to linearity and 
recent studies evaluated in the ISA Supplement continue to provide 
evidence of a no-threshold linear relationship, with less confidence at 
concentrations lower than 5 [micro]g/m\3\.
    Overall, epidemiologic studies evaluated in the 2019 ISA and the 
ISA Supplement build upon and extend the conclusions of the 2009 ISA 
for the relationship between short-term PM2.5 exposures and 
total mortality. Supporting evidence for PM2.5-related 
cardiovascular morbidity, and more limited evidence from respiratory 
morbidity, provide biological plausibility for mortality due to short-
term PM2.5 exposures. The primarily positive associations 
observed across studies conducted in diverse geographic locations is 
further supported by the results from copollutant analyses indicating 
robust associations, along with evidence from analyses examining the C-
R relationship. Overall, studies evaluated in the 2019 ISA support the 
conclusion of a causal relationship between short-term PM2.5 
exposure and mortality, which is further supported by evidence from 
recent epidemiologic studies evaluated in the ISA Supplement (U.S. EPA, 
2022a, section 3.2.1.4, p. 3-69).
ii. Cardiovascular Effects
Long-Term PM2.5 Exposures
    The scientific evidence reviewed in the 2009 ISA was ``sufficient 
to infer a causal relationship between long-term PM2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2009a). The strongest 
line of evidence comprised findings from several large epidemiologic 
studies of U.S. and Canadian cohorts that reported consistent positive 
associations between long-term PM2.5 exposure and 
cardiovascular mortality (Pope et al., 2004; Krewski et al., 2009; 
Miller et al.,

[[Page 16229]]

2007; Laden et al., 2006). Studies of long-term PM2.5 
exposure and cardiovascular morbidity were limited in number. 
Biological plausibility and coherence with the epidemiologic findings 
were provided by studies using genetic mouse models of atherosclerosis 
demonstrating enhanced atherosclerotic plaque development and 
inflammation, as well as changes in measures of impaired heart 
function, following 4- to 6-month exposures to PM2.5 
concentrated ambient particles (CAPs), and by a limited number of 
studies reporting CAPs-induced effects on coagulation factors, vascular 
reactivity, and worsening of experimentally induced hypertension in 
mice (U.S. EPA, 2009a).
    Consistent with the evidence assessed in the 2009 ISA, the 2019 ISA 
concludes that recent studies, together with the evidence available in 
previous reviews, support a causal relationship between long-term 
exposure to PM2.5 and cardiovascular effects. Additionally, 
recent epidemiologic studies published since the completion of the 2019 
ISA and evaluated in the ISA Supplement expands the body of evidence 
and further supports such a conclusion (U.S. EPA, 2022a). As discussed 
above (section II.A.2.a.i), results from U.S. and Canadian cohort 
studies evaluated in the 2019 ISA conducted at varying spatial and 
temporal scales and employing a variety of exposure assessment and 
statistical methods consistently report positive associations between 
long-term PM2.5 exposure and cardiovascular mortality (U.S. 
EPA, 2019, Figure 6-19, section 6.2.10). Positive associations between 
long-term PM2.5 exposures and cardiovascular mortality are 
generally robust in copollutant models adjusted for ozone, 
NO2, PM10-2.5, or SO2. In addition, 
most of the results from analyses examining the shape of the C-R 
relationship between long-term PM2.5 exposures and 
cardiovascular mortality support a linear relationship and do not 
identify a threshold below which mortality effects do not occur (U.S. 
EPA, 2019a, section 6.2.16, Table 6-52).
    The body of literature examining the relationship between long-term 
PM2.5 exposure and cardiovascular morbidity has greatly 
expanded since the 2009 ISA, with positive associations reported in 
several cohorts evaluated in the 2019 ISA (U.S. EPA, 2019a, section 
6.2). Though results for cardiovascular morbidity are less consistent 
than those for cardiovascular mortality (U.S. EPA, 2019a, section 6.2), 
studies in the 2019 ISA and the ISA Supplement provide some evidence 
for associations between long-term PM2.5 exposures and the 
progression of cardiovascular disease. Positive associations with 
cardiovascular morbidity (e.g., coronary heart disease, stroke, 
arrhythmias, myocardial infarction (MI), atherosclerosis progression) 
are observed in several epidemiologic studies (U.S. EPA, 2019a, 
sections 6.2.2 to 6.2.9; U.S. EPA, 2022a, section 3.1.2.2). 
Additionally, studies evaluated in the ISA Supplement report positive 
associations among those with pre-existing conditions, among patients 
followed after a cardiac event procedure, and among those with a first 
hospital admission for heart attacks among older adults enrolled in 
Medicare (U.S. EPA, 2022a, sections 3.1.1 and 3.1.2).
    Recent studies published since the literature cutoff date of the 
2019 ISA and evaluated in the ISA Supplement further assessed the 
relationship between long-term PM2.5 exposure and 
cardiovascular effects by conducting accountability analyses or by 
using alternative methods for confounder control in evaluating the 
association between long-term PM2.5 exposure and 
cardiovascular hospital admissions (U.S. EPA, 2022a, section 3.1.2.3). 
Studies that apply alternative methods for confounder control increase 
confidence in the relationship between long-term PM2.5 
exposure and cardiovascular effects by using methods that reduce 
uncertainties related to potential confounding through statistical and/
or study design approaches. For example, to control for potential 
confounding Wei et al. (2021) used a doubly robust additive model 
(DRAM) and found an association between long-term exposure to 
PM2.5 and cardiovascular effects, including MI, stoke, and 
atrial fibrillation, among the Medicare population. For example, an 
accountability study by Henneman et al. (2019) utilized a difference-
in-difference (DID) approach to determine the relationship between 
coal-fueled power plant emissions and cardiovascular effects and found 
that reductions in PM2.5 concentrations resulted in 
reductions of cardiovascular-related hospital admissions. Furthermore, 
several recent epidemiologic studies evaluated in the ISA Supplement 
reported that the association between long-term PM2.5 
exposure with stroke persisted after adjustment for NO2 but 
was attenuated in the model with O3 and oxidant gases 
represented by the redox weighted average of NO2 and 
O3 (U.S. EPA, 2022a, section 3.1.2.2.8). Overall, these 
studies report consistent findings that long-term PM2.5 
exposure is related to increased hospital admissions for a variety of 
cardiovascular disease outcomes among large nationally representative 
cohorts and provide additional support for a relationship between long-
term PM2.5 exposure and cardiovascular effects.
    Positive associations reported in epidemiologic studies are 
supported by toxicological evidence evaluated in the 2019 ISA. The 
positive associations reported in epidemiologic studies are supported 
by toxicological evidence for increased plaque progression in mice 
following long-term exposure to PM2.5 collected from 
multiple locations across the U.S. (U.S. EPA, 2019a, section 6.2.4.2). 
A small number of epidemiologic studies also report positive 
associations between long-term PM2.5 exposure and heart 
failure, changes in blood pressure, and hypertension (U.S. EPA, 2019a, 
sections 6.2.5 and 6.2.7). Associations with heart failure are 
supported by animal toxicological studies demonstrating decreased 
cardiac contractility and function, and increased coronary artery wall 
thickness following long-term PM2.5 exposure (U.S. EPA, 
2019a, section 6.2.5.2). Similarly, a limited number of animal 
toxicological studies demonstrating a relationship between long-term 
PM2.5 exposure and consistent increases in blood pressure in 
rats and mice are coherent with epidemiologic studies reporting 
positive associations between long-term exposure to PM2.5 
and hypertension.
    Additionally, a number of studies evaluated in the ISA Supplement 
focusing on morbidity outcomes, including those that focused on 
incidence of MI, atrial fibrillation (AF), stroke, and congestive heart 
failure (CHF), expand the evidence pertaining to the shape of the C-R 
relationship between long-term PM2.5 exposure and 
cardiovascular effects. These studies use statistical techniques that 
allow for departures from linearity (U.S. EPA, 2022a, Table 3-3), and 
generally support the evidence characterized in the 2019 ISA showing 
linear, no-threshold C-R relationship for most cardiovascular disease 
(CVD) outcomes. However, there is evidence for a sublinear or 
supralinear C-R relationship for some outcomes (U.S. EPA, 2022a, 
section 3.1.2.2.9).\57\
---------------------------------------------------------------------------

    \57\ As noted above for mortality, uncertainty in the shape of 
the C-R relationship increases near the upper and lower ends of the 
distribution due to limited data.
---------------------------------------------------------------------------

    Longitudinal epidemiologic analyses also report positive 
associations with markers of systemic inflammation (U.S. EPA, 2019a, 
section 6.2.11), coagulation (U.S. EPA, 2019a, section 6.2.12), and

[[Page 16230]]

endothelial dysfunction (U.S. EPA, 2019a, section 6.2.13). These 
results are coherent with animal toxicological studies generally 
reporting increased markers of systemic inflammation, oxidative stress, 
and endothelial dysfunction (U.S. EPA, 2019a, section 6.2.12.2 and 
6.2.14).
    In summary, the 2019 ISA concludes that there is consistent 
evidence from multiple epidemiologic studies illustrating that long-
term exposure to PM2.5 is associated with mortality from 
cardiovascular causes. Epidemiologic studies evaluated in the ISA 
Supplement provide additional evidence of positive associations between 
long-term PM2.5 exposure and cardiovascular morbidity (U.S. 
EPA, 2022a, section 3.1.2.2). Associations with coronary heart disease 
(CHD), stroke and atherosclerosis progression were observed in several 
additional epidemiologic studies, providing coherence with the 
mortality findings. Results from copollutant models generally support 
an independent effect of PM2.5 exposure on mortality. 
Additional evidence of the independent effect of PM2.5 on 
the cardiovascular system is provided by experimental studies in 
animals, which support the biological plausibility of pathways by which 
long-term exposure to PM2.5 could potentially result in 
outcomes such as CHD, stroke, CHF, and cardiovascular mortality. 
Overall, studies evaluated in the 2019 ISA support the conclusion of a 
causal relationship between long-term PM2.5 exposure and 
cardiovascular effects, which is supported and extended by evidence 
from recent epidemiologic studies evaluated in the ISA Supplement (U.S. 
EPA, 2022a, section 3.1.2.2).
Short-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship exists between 
short-term exposure to PM2.5 and cardiovascular effects'' 
(U.S. EPA, 2009a). The strongest evidence in the 2009 ISA was from 
epidemiologic studies of emergency department (ED) visits and hospital 
admissions for IHD and heart failure (HF), with supporting evidence 
from epidemiologic studies of cardiovascular mortality (U.S. EPA, 
2009a). Animal toxicological studies provided coherence and biological 
plausibility for the positive associations reported with MI, ED visits, 
and hospital admissions. These included studies reporting reduced 
myocardial blood flow during ischemia and studies indicating altered 
vascular reactivity. In addition, effects of PM2.5 exposure 
on a potential indicator of ischemia (i.e., ST segment depression on an 
electrocardiogram) were reported in both animal toxicological and 
epidemiologic panel studies.\58\ Key uncertainties from the last review 
resulted from inconsistent results across disciplines with respect to 
the relationship between short-term exposure to PM2.5 and 
changes in blood pressure, blood coagulation markers, and markers of 
systemic inflammation. In addition, while the 2009 ISA identified a 
growing body of evidence from controlled human exposure and animal 
toxicological studies, uncertainties remained with respect to 
biological plausibility.
---------------------------------------------------------------------------

    \58\ Some animal studies included in the 2009 ISA examined 
exposures to mixtures, such as motor vehicle exhaust or woodsmoke. 
In these studies, it was unclear if the resulting cardiovascular 
effects could be attributed specifically to the fine particle 
component of the mixture.
---------------------------------------------------------------------------

    Studies evaluated in the 2019 ISA provide additional support for a 
causal relationship between short-term PM2.5 exposure and 
cardiovascular effects. This includes generally positive associations 
observed in multicity epidemiologic studies of emergency department 
visits and hospital admissions for IHD, heart failure (HF), and 
combined cardiovascular-related endpoints. In particular, nationwide 
studies of older adults (65 years and older) using Medicare records 
report positive associations between PM2.5 exposures and 
hospital admissions for HF (U.S. EPA, 2019a, section 6.1.3.1). 
Moreover, recent multicity studies, published after the literature 
cutoff date of the 2019 ISA and evaluated in the ISA Supplement, are 
consistent with studies evaluated in the 2019 ISA that report positive 
association between short-term PM2.5 exposure and ED visits 
and hospital admission for IHD, heart attacks, and HF (U.S. EPA, 2022a, 
section 3.1). Epidemiologic studies conducted in single cities 
contribute some support to the causality determination, though 
associations reported in single-city studies are less consistently 
positive than in multicity studies, and include a number of studies 
reporting null associations (U.S. EPA, 2019a, sections 6.1.2 and 
6.1.3). As a whole, though, the recent body of IHD and HF epidemiologic 
evidence supports the evidence from previous ISAs reporting mainly 
positive associations between short-term PM2.5 
concentrations and emergency department visits and hospital admissions.
    Consistent with the evidence assessed in the 2019 ISA, some studies 
evaluated in the ISA Supplement report no evidence of an association 
with stroke, regardless of stroke subtype. Additionally, as in the 2019 
ISA, evidence evaluated in the ISA Supplement continues to indicate an 
immediate effect of PM2.5 on cardiovascular-related outcomes 
primarily within the first few days after exposure, and that 
associations generally persisted in models adjusted for copollutants 
(U.S. EPA, 2022a, section 3.1.1.2).
    The ISA Supplement includes additional epidemiologic studies, 
published since the literature cutoff date for the 2019 ISA, including 
accountability analyses and epidemiologic studies that employ 
alternative methods for confounder control to evaluate the association 
between short-term PM2.5 exposure and cardiovascular-related 
effects (U.S. EPA, 2022a, section 3.1.1.3). These studies employ a 
number of statistical approaches and report positive associations, 
providing additional support for a relationship between short-term 
PM2.5 exposure and cardiovascular effects, while also 
reducing uncertainties related to potential confounder bias.
    A number of controlled human exposure, animal toxicological, and 
epidemiologic panel studies provide evidence that PM2.5 
exposure could plausibly result in IHD or HF through pathways that 
include endothelial dysfunction, arterial thrombosis, and arrhythmia 
(U.S. EPA, 2019a, section 6.1.1). The most consistent evidence from 
recent controlled human exposure studies is for endothelial 
dysfunction, as measured by changes in brachial artery diameter or flow 
mediated dilation. Multiple controlled human exposure studies that 
examined the potential for endothelial dysfunction report an effect of 
PM2.5 exposure on measures of blood flow (U.S. EPA, 2019a, 
section 6.1.13.2). However, these studies report variable results 
regarding the timing of the effect and the mechanism by which reduced 
blood flow occurs (i.e., availability vs sensitivity to nitric oxide). 
In addition, some controlled human exposure studies using CAPs report 
evidence for small increases in blood pressure (U.S. EPA, 2019a, 
section 6.1.6.3). Although not entirely consistent, there is also some 
evidence across controlled human exposure studies for conduction 
abnormalities/arrhythmia (U.S. EPA, 2019a, section 6.1.4.3), changes in 
heart rate variability (HRV) (U.S. EPA, 2019a, section 6.1.10.2), 
changes in hemostasis that could promote clot formation (U.S. EPA, 
2019a, section 6.1.12.2), and increases in inflammatory cells and 
markers (U.S. EPA, 2019a, section 6.1.11.2). A recent study by Wyatt et 
al.

[[Page 16231]]

(2020), evaluated in the ISA Supplement, adds to the limited evidence 
base of controlled human exposure studies conducted at near ambient 
PM2.5 concentrations. The study, completed in healthy young 
adults subject to intermittent exercise, found some significant 
cardiovascular effects (e.g., systematic inflammation markers, 
including C-reactive protein (CRP), and cardiac repolarization). Thus, 
when taken as a whole, controlled human exposure studies are coherent 
with epidemiologic studies in that they demonstrate that short-term 
exposures to PM2.5 may result in the types of cardiovascular 
endpoints that could lead to emergency department visits and hospital 
admissions for IHD or HF, as well as mortality in some people.
    Animal toxicological studies published since the 2009 ISA and 
evaluated in the 2019 ISA also support a relationship between short-
term PM2.5 exposure and cardiovascular effects. A study 
demonstrating decreased cardiac contractility and left ventricular 
pressure in mice is coherent with the results of epidemiologic studies 
that report associations between short-term PM2.5 exposure 
and heart failure (U.S. EPA, 2019a, section 6.1.3.3). In addition, and 
as with controlled human exposure studies, there is generally 
consistent evidence in animal toxicological studies for indicators of 
endothelial dysfunction (U.S. EPA, 2019a, section 6.1.13.3). Some 
studies in animals also provide evidence for changes in a number of 
other cardiovascular endpoints following short-term PM2.5 
exposure including conduction abnormalities and arrhythmia (U.S. EPA, 
2019a, section 6.1.4.4), changes in HRV (U.S. EPA, 2019a, section 
6.1.10.3), changes in blood pressure (U.S. EPA, 2019a, section 
6.1.6.4), and evidence for systemic inflammation and oxidative stress 
(U.S. EPA, 2019a, section 6.1.11.3).
    In summary, evidence evaluated in the 2019 ISA extends the 
consistency and coherence of the evidence base evaluated in the 2009 
ISA and prior assessments. Epidemiologic studies reporting robust 
associations in copollutant models are supported by direct evidence 
from controlled human exposure and animal toxicologic studies reporting 
independent effects of PM2.5 exposures on endothelial 
dysfunction as well as endpoints indicating impaired cardiac function, 
increased risk of arrhythmia, changes in HRV, increases in BP, and 
increases in indicators of systemic inflammation, oxidative stress, and 
coagulation (U.S. EPA, 2019, section 6.1.16). For some cardiovascular 
effects, there are inconsistencies in results across some animal 
toxicological, controlled human exposure, and epidemiologic panel 
studies, though this may be due to substantial differences in study 
design and/or study populations. Overall, the results from 
epidemiologic panel, controlled human exposure, and animal 
toxicological studies, in particular those related to endothelial 
dysfunction, impaired cardiac function, ST segment depression, 
thrombosis, conduction abnormalities, and changes in blood pressure 
provide coherence and biological plausibility for the consistent 
results from epidemiologic studies observing positive associations 
between short-term PM2.5 exposures and IHD and HF, and 
ultimately cardiovascular mortality. Overall, studies evaluated in the 
2019 ISA support the conclusion of a causal relationship between short-
term PM2.5 exposure and cardiovascular effects, which is 
supported and extended by evidence from recent epidemiologic studies 
evaluated in the ISA Supplement (U.S. EPA, 2022a, section 3.1.1.4).
iii. Respiratory Effects
Long-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship is likely to 
exist between long-term PM2.5 exposure and respiratory 
effects'' (U.S. EPA, 2009a). This conclusion was based mainly on 
epidemiologic evidence demonstrating associations between long-term 
PM2.5 exposure and changes in lung function or lung function 
growth in children. Biological plausibility was provided by a single 
animal toxicological study examining pre- and post-natal exposure to 
PM2.5 CAPs, which found impaired lung development. 
Epidemiologic evidence for associations between long-term 
PM2.5 exposure and other respiratory outcomes, such as the 
development of asthma, allergic disease, and COPD; respiratory 
infection; and the severity of disease was limited, both in the number 
of studies available and the consistency of the results. Experimental 
evidence for other outcomes was also limited, with one animal 
toxicological study reporting that long-term exposure to 
PM2.5 CAPs results in morphological changes in nasal airways 
of healthy animals. Other animal studies examined exposure to mixtures, 
such as motor vehicle exhaust and woodsmoke, and effects were not 
attributed specifically to the particulate components of the mixture.
    Cohort studies evaluated in the 2019 ISA provided additional 
support for the relationship between long-term PM2.5 
exposure and decrements in lung function growth (as a measure of lung 
development), indicating a robust and consistent association across 
study locations, exposure assessment methods, and time periods (U.S. 
EPA, 2019a, section 5.2.13). This relationship was further supported by 
a retrospective study that reports an association between declining 
PM2.5 concentrations and improvements in lung function 
growth in children (U.S. EPA, 2019a, section 5.2.11). Epidemiologic 
studies also examine asthma development in children (U.S. EPA, 2019a, 
section 5.2.3), with prospective cohort studies reporting generally 
positive associations, though several are imprecise (i.e., they report 
wide confidence intervals). Supporting evidence is provided by studies 
reporting associations with asthma prevalence in children, with 
childhood wheeze, and with exhaled nitric oxide, a marker of pulmonary 
inflammation (U.S. EPA, 2019a, section 5.2.13). Additionally, the 2019 
ISA includes an animal toxicological study showing the development of 
an allergic phenotype and an increase in a marker of airway 
responsiveness supports the biological plausibility of the development 
of allergic asthma (U.S. EPA, 2019a, section 5.2.13). Other 
epidemiologic studies report a PM2.5-related acceleration of 
lung function decline in adults, while improvement in lung function was 
observed with declining PM2.5 concentrations (U.S. EPA, 
2019a, section 5.2.11). A longitudinal study found declining 
PM2.5 concentrations are also associated with an improvement 
in chronic bronchitis symptoms in children, strengthening evidence 
reported in the 2009 ISA for a relationship between increased chronic 
bronchitis symptoms and long-term PM2.5 exposure (U.S. EPA, 
2019a, section 5.2.11). A common uncertainty across the epidemiologic 
evidence is the lack of examination of copollutants to assess the 
potential for confounding. While there is some evidence that 
associations remain robust in models with gaseous pollutants, a number 
of these studies examining copollutant confounding were conducted in 
Asia, and thus have limited generalizability due to high annual 
pollutant concentrations.
    When taken together, the 2019 ISA concludes that the epidemiologic 
evidence strongly supports a relationship with decrements in lung 
function growth asthma development in children, as well as increased 
bronchitis symptoms in children with asthma. Additionally, the 
epidemiologic

[[Page 16232]]

evidence strongly supports a relationship with an acceleration of lung 
function decline in adults, and with respiratory mortality and cause-
specific respiratory mortality for COPD and respiratory infection (U.S. 
EPA, 2019a, p. 1-34). In support of the biological plausibility of 
associations reported in epidemiologic studies associated with 
respiratory health effects, animal toxicological studies evaluated in 
the 2019 ISA continue to provide direct evidence that long-term 
exposure to PM2.5 results in a variety of respiratory 
effects, including pulmonary oxidative stress, inflammation, and 
morphologic changes in the upper (nasal) and lower airways. Other 
results show that changes are consistent with the development of 
allergy and asthma, and with impaired lung development. Overall, the 
2019 ISA concludes that ``the collective evidence is sufficient to 
conclude that a causal relationship is likely to exist between long-
term PM2.5 exposure and respiratory effects'' (U.S. EPA, 
2019a, section 5.2.13).
Short-Term PM2.5 Exposures
    The 2009 ISA (U.S. EPA, 2009a) concluded that a ``causal 
relationship is likely to exist'' between short-term PM2.5 
exposure and respiratory effects. This conclusion was based mainly on 
the epidemiologic evidence demonstrating positive associations with 
various respiratory effects. Specifically, the 2009 ISA described 
epidemiologic evidence as consistently showing PM2.5-
associated increases in hospital admissions and ED visits for COPD and 
respiratory infection among adults or people of all ages, as well as 
increases in respiratory mortality. These results were supported by 
studies reporting associations with increased respiratory symptoms and 
decreases in lung function in children with asthma, though the 
epidemiologic evidence was inconsistent for hospital admissions or 
emergency department visits for asthma. Studies examining copollutant 
models showed that PM2.5 associations with respiratory 
effects were robust to inclusion of CO or SO2 in the model, 
but often were attenuated (though still positive) with inclusion of 
O3 or NO2. In addition to the copollutant models, 
evidence supporting an independent effect of PM2.5 exposure 
on the respiratory system was provided by animal toxicological studies 
of PM2.5 CAPs demonstrating changes in some pulmonary 
function parameters, as well as inflammation, oxidative stress, injury, 
enhanced allergic responses, and reduced host defenses. Many of these 
effects have been implicated in the pathophysiology for asthma 
exacerbation, COPD exacerbation, or respiratory infection. In the few 
controlled human exposure studies conducted in individuals with asthma 
or COPD, PM2.5 exposure mostly had no effect on respiratory 
symptoms, lung function, or pulmonary inflammation. Available studies 
in healthy people also did not clearly demonstrate respiratory effects 
following short-term PM2.5 exposures.
    Epidemiologic studies evaluated in the 2019 ISA continue to provide 
strong evidence for a relationship between short-term PM2.5 
exposure and several respiratory-related endpoints, including asthma 
exacerbation (U.S. EPA, 2019a, section 5.1.2.1), COPD exacerbation 
(U.S. EPA, 2019a, section 5.1.4.1), and combined respiratory-related 
diseases (U.S. EPA, 2019a, section 5.1.6), particularly from studies 
examining ED visits and hospital admissions. The generally positive 
associations between short-term PM2.5 exposure and asthma 
and COPD as well as ED visits and hospital admissions are supported by 
epidemiologic studies demonstrating associations with other 
respiratory-related effects such as symptoms and medication use that 
are indicative of asthma and COPD exacerbations (U.S. EPA, 2019a, 
sections 5.1.2.2 and 5.4.1.2). The collective body of epidemiologic 
evidence for asthma exacerbation is more consistent in children than in 
adults. Additionally, epidemiologic studies examining the relationship 
between short-term PM2.5 exposure and respiratory mortality 
provide evidence of consistent positive associations, demonstrating a 
continuum of effects (U.S. EPA, 2019a, section 5.1.9).
    Epidemiologic studies evaluated in the 2019 ISA expand the 
assessment of potential copollutant confounding evaluated in the 2009 
ISA. There is some evidence that PM2.5 associations with 
asthma exacerbation, combined respiratory-related diseases, and 
respiratory mortality remain relatively unchanged in copollutant models 
with gaseous pollutants including O3, NO2, 
SO2, and with more limited evidence for CO, as well as other 
particle sizes (i.e., PM10-2.5) (U.S. EPA, 2019a, section 
5.1.10.1).
    Insight into whether there is an independent effect of 
PM2.5 on respiratory health is also partially addressed by 
findings from animal toxicological studies evaluated in the 2019 ISA. 
Specifically, short-term exposure to PM2.5 enhanced asthma-
related responses in an animal model of allergic airways disease and 
enhanced lung injury and inflammation in an animal model of COPD (U.S. 
EPA, 2019a, sections 5.1.2.4.4 and 5.1.4.4.3). The experimental 
evidence provides biological plausibility for some respiratory-related 
endpoints, including limited evidence of altered host defense and 
greater susceptibility to bacterial infection as well as consistent 
evidence of respiratory irritant effects. However, animal toxicological 
evidence for other respiratory effects is inconsistent. A recent study 
evaluated in the ISA supplement by Wyatt et al. (2020) and conducted at 
near ambient PM2.5 concentrations, adds to the limited 
evidence base of controlled human exposure studies. The study, 
completed in healthy young adults subject to intermittent exercise, 
found some significant respiratory effects (including decrease in lung 
function), however these findings were inconsistent with the controlled 
human exposure studies evaluated in the 2019 ISA (U.S. EPA, 2019a, 
section 5.1.7.2, 5.1.2.3, and 6.1.11.2.1).
    The 2019 ISA concludes that ``[t]he strongest evidence of an effect 
of short-term PM2.5 exposure on respiratory effects is 
provided by epidemiologic studies of asthma and COPD exacerbation. 
While animal toxicological studies provide biological plausibility for 
these findings, some uncertainty remains with respect to the 
independence of PM2.5 effects'' (U.S. EPA, 2019a, p. 5-155). 
When taken together, the 2019 ISA concludes that this evidence ``is 
sufficient to conclude that a causal relationship is likely to exist 
between short-term PM2.5 exposure and respiratory effects'' 
(U.S. EPA, 2019a, p. 5-155).
iv. Cancer
    The 2009 ISA concluded that the overall body of evidence was 
``suggestive of a causal relationship between relevant PM2.5 
exposures and cancer'' (U.S. EPA, 2009a). This conclusion was based 
primarily on positive associations observed in a limited number of 
epidemiologic studies of lung cancer mortality. The few epidemiologic 
studies that had evaluated PM2.5 exposure and lung cancer 
incidence or cancers of other organs and systems generally did not show 
evidence of an association. Toxicological studies did not focus on 
exposures to specific PM size fractions, but rather investigated the 
effects of exposures to total ambient PM, or other source-based PM such 
as wood smoke. Collectively, results of in vitro studies were 
consistent with the larger body of evidence demonstrating that ambient 
PM and PM from specific combustion sources are mutagenic and genotoxic. 
However, animal inhalation studies

[[Page 16233]]

found little evidence of tumor formation in response to chronic 
exposures. A small number of studies provided preliminary evidence that 
PM exposure can lead to changes in methylation of DNA, which may 
contribute to biological events related to cancer.
    Since the completion of the 2009 ISA, additional cohort studies 
provide evidence that long-term PM2.5 exposure is positively 
associated with lung cancer mortality and with lung cancer incidence, 
and provide initial evidence for an association with reduced cancer 
survival (U.S. EPA, 2019a, section 10.2.5). Re-analyses of the ACS 
cohort using different years of PM2.5 data and follow up, 
along with various exposure assignment approaches, provide consistent 
evidence of positive associations between long-term PM2.5 
exposure and lung cancer mortality (U.S. EPA, 2019a, Figure 10-3). 
Additional support for positive associations with lung cancer mortality 
is provided by recent epidemiologic studies using individual level data 
to control for smoking status, as well as by studies of people who have 
never smoked (though such studies generally report wide confidence 
intervals due to the small number of lung cancer mortality cases within 
this population), and in additional analyses of cohorts that relied 
upon proxy measures to account for smoking status (U.S. EPA, 2019a, 
section 10.2.5.1.1). Although studies that evaluate lung cancer 
incidence, including studies of people who have never smoked, are 
limited in number, studies in the 2019 ISA generally report positive 
associations with long-term PM2.5 exposures (U.S. EPA, 
2019a, section 10.2.5.1.2). A subset of the studies focusing on lung 
cancer incidence also examined histological subtype, providing some 
evidence of positive associations for adenocarcinomas, the predominate 
subtype of lung cancer observed in people who have never smoked (U.S. 
EPA, 2019a, section 10.2.5.1.2). Associations between long-term 
PM2.5 exposure and lung cancer incidence were found to 
remain relatively unchanged, though in some cases confidence intervals 
widened, in analyses that attempted to reduce exposure measurement 
error by accounting for length of time at residential address or by 
examining different exposure assignment approaches (U.S. EPA, 2019a, 
section 10.2.5.1.2).
    To date, relatively few studies have evaluated the potential for 
copollutant confounding of the relationship between long-term 
PM2.5 exposure and lung cancer mortality or incidence. A 
small number of such studies have generally focused on O3 
and report that PM2.5 associations remain relatively 
unchanged in copollutant models (U.S. EPA, 2019a, section 10.2.5.1.3). 
However, available studies have not systematically evaluated the 
potential for copollutant confounding by other gaseous pollutants or by 
other particle size fractions (U.S. EPA, 2019a, section 10.2.5.1.3).
    Compared to total (non-accidental) mortality (U.S. EPA, 2019a, 
section 10.2.4.1.4), fewer studies have examined the shape of the C-R 
curve for cause-specific mortality outcomes, including lung cancer. 
Several studies of lung cancer mortality and incidence have reported no 
evidence of deviations from linearity in the shape of the C-R 
relationship (Lepeule et al., 2012; Raaschou-Nielsen et al., 2013; 
Puett et al., 2014), though authors provided only limited discussions 
of results (U.S. EPA, 2019a, section 10.2.5.1.4).
    In support of the biological plausibility of an independent effect 
of PM2.5 on lung cancer, the 2019 ISA notes evidence from 
experimental and epidemiologic studies demonstrating that 
PM2.5 exposure can lead to a range of effects indicative of 
mutagenicity, genotoxicity, and carcinogenicity, as well as epigenetic 
effects (U.S. EPA, 2019a, section 10.2.7). For example, both in vitro 
and in vivo toxicological studies have shown that PM2.5 
exposure can result in DNA damage (U.S. EPA, 2019a, section 10.2.2). 
Although such effects do not necessarily equate to carcinogenicity, the 
evidence that PM exposure can damage DNA, and elicit mutations, 
provides support for the plausibility of epidemiologic associations 
exhibited with lung cancer mortality and incidence. Additional 
supporting studies indicate the occurrence of micronuclei formation and 
chromosomal abnormalities (U.S. EPA, 2019a, section 10.2.2.3), and 
differential expression of genes that may be relevant to cancer 
pathogenesis, following PM2.5 exposures. Experimental and 
epidemiologic studies that examine epigenetic effects indicate changes 
in DNA methylation, providing some support that PM2.5 
exposure contributes to genomic instability (U.S. EPA, 2019a, section 
10.2.3). Overall, there is limited evidence that long-term 
PM2.5 exposure is associated with cancers in other organ 
systems, though there is some evidence that PM2.5 exposure 
may reduce survival in individuals with cancer (U.S. EPA, 2019a, 
section 10.2.7; U.S. EPA, 2022a, section 2.1.1.4.1).
    Epidemiologic evidence for associations between PM2.5 
and lung cancer mortality and incidence, together with evidence 
supporting the biological plausibility of such associations, 
contributes to the 2019 ISA's conclusion that the evidence ``is 
sufficient to conclude that a causal relationship is likely to exist 
between long-term PM2.5 exposure and cancer'' (U.S. EPA, 
2019, section 10.2.7).
v. Nervous System Effects
    Reflecting the very limited evidence available in the 2012 review, 
the 2009 ISA did not make a causality determination for long-term 
PM2.5 exposures and nervous system effects (U.S. EPA, 
2009c). Since the 2012 review, this body of evidence has grown 
substantially (U.S. EPA, 2019, section 8.2). Animal toxicological 
studies assessed in in the 2019 ISA report that long-term 
PM2.5 exposures can lead to morphologic changes in the 
hippocampus and to impaired learning and memory. This evidence is 
consistent with epidemiologic studies reporting that long-term 
PM2.5 exposure is associated with reduced cognitive function 
(U.S. EPA, 2019a, section 8.2.5). Further, while the evidence is 
limited, the presence of early markers of Alzheimer's disease pathology 
has been demonstrated in rodents following long-term exposure to 
PM2.5 CAPs. These findings support reported associations 
with neurodegenerative changes in the brain (i.e., decreased brain 
volume), all-cause dementia, or hospitalization for Alzheimer's disease 
in a small number of epidemiologic studies (U.S. EPA, 2019a, section 
8.2.6). Additionally, loss of dopaminergic neurons in the substantia 
nigra, a hallmark of Parkinson disease, has been reported in mice (U.S. 
EPA, 2019a, section 8.2.4), though epidemiologic studies provide only 
limited support for associations with Parkinson's disease (U.S. EPA, 
2019a, section 8.2.6). Overall, the lack of consideration of 
copollutant confounding introduces some uncertainty in the 
interpretation of epidemiologic studies of nervous system effects, but 
this uncertainty is partly addressed by the evidence for an independent 
effect of PM2.5 exposures provided by experimental animal 
studies.
    While the findings described above are most relevant to older 
adults, several studies of neurodevelopmental effects in children have 
also been conducted. Epidemiologic studies provided limited evidence of 
an association between PM2.5 exposure during pregnancy and 
childhood on cognitive and motor development (U.S. EPA, 2019, section 
8.2.5.2). While some studies report positive associations between long-
term

[[Page 16234]]

exposure to PM2.5 during the prenatal period and autism 
spectrum disorder (ASD) (U.S. EPA, 2019, section 8.2.7.2), the 
interpretation of these epidemiologic studies is limited due to the 
small number of studies, their lack of control for potential 
confounding by copollutants, and uncertainty related to the critical 
exposure windows. Biological plausibility is provided for the ASD 
findings by a study in mice that found inflammatory and morphologic 
changes in the corpus collosum and hippocampus, as well as 
ventriculomegaly (i.e., enlarged lateral ventricles) in young mice 
following prenatal exposure to PM2.5 CAPs.
    Taken together, the 2019 ISA concludes that studies indicate long-
term PM2.5 exposures can lead to effects on the brain 
associated with neurodegeneration (i.e., neuroinflammation and 
reductions in brain volume), as well as cognitive effects in older 
adults (U.S. EPA, 2019a, Table 1-2). Animal toxicological studies 
provide evidence for a range of nervous system effects in adult 
animals, including neuroinflammation and oxidative stress, 
neurodegeneration, cognitive effects, and effects on neurodevelopment 
in young animals. The epidemiologic evidence is more limited, but 
studies generally support associations between long-term 
PM2.5 exposure and changes in brain morphology, cognitive 
decrements and dementia. There is also initial, and limited, evidence 
for neurodevelopmental effects, particularly ASD. The consistency and 
coherence of the evidence supports the 2019 ISA's conclusion that ``the 
collective evidence is sufficient to conclude that a causal 
relationship is likely to exist between long-term PM2.5 
exposure and nervous system effects'' (U.S. EPA, 2019a, section 8.2.9).
vi. Other Effects
    For other health effect categories that were evaluated for their 
relationship with PM2.5 exposures (i.e., short-term 
PM2.5 exposure and nervous system effects and short- and 
long-term PM2.5 exposure and metabolic effects, reproduction 
and fertility, and pregnancy and birth outcomes (U.S. EPA, 2022a, Table 
ES-1), the currently available evidence is ``suggestive of, but not 
sufficient to infer, a causal relationship,'' mainly due to 
inconsistent evidence across specific outcomes and uncertainties 
regarding exposure measurement error, the potential for confounding, 
and potential modes of action (U.S. EPA, 2019a, sections 7.14, 7.2.10, 
8.1.6, and 9.1.5). The causality determination for short-term 
PM2.5 exposure and nervous system effects in the 2019 ISA 
reflects a revision to the causality determination in the 2009 ISA from 
``inadequate to infer a causal relationship,'' while this is the first-
time assessments of causality were conducted for long-term 
PM2.5 exposure and nervous system effects, as well as short- 
and long-term PM2.5 exposure and metabolic effects reflect.
    Recent studies evaluated in the 2019 ISA also further explored the 
relationship between short-and long-term UFP exposure and health 
effects. (i.e., cardiovascular effects and short-term UFP exposures; 
respiratory effects and short-term UFP exposures; and nervous system 
effects and long- and short-term exposures (U.S. EPA, 2022a, Table ES-
1). The currently available evidence is ``suggestive of, but not 
sufficient to infer, a causal relationship'' for short-term UFP 
exposure and cardiovascular and respiratory effects and for short- and 
long-term UFP exposure and nervous system effects, primarily due to 
uncertainties and limitations in the evidence, specifically, 
variability across studies in the definition of UFPs and the exposure 
metric used (U.S. EPA, 2019a, P.3.1; U.S. EPA, 2022a, section 
3.3.1.6.3). The causality determinations for the other health effect 
categories evaluated in the 2019 ISA are ``inadequate to infer a causal 
relationship.'' Additionally, this is the first time assessments of 
causality were conducted for short- and long-term UFP exposure and 
metabolic effects and long-term UFP exposure and nervous system effects 
(U.S. EPA, 2022a, Table ES-1).
    With the advent of the global COVID-19 pandemic, a number of recent 
studies evaluated in the ISA Supplement examined the relationship 
between ambient air pollution, specifically PM2.5, and SARS-
CoV-2 infections and COVID-19 deaths, including a few studies within 
the U.S. and Canada (U.S. EPA, 2022a, section 3.3.2).\59\ Some studies 
examined whether daily changes in PM2.5 can influence SARS-
CoV-2 infection and COVID-19 death (U.S. EPA, 2022a, section 3.3.2.1). 
Additionally, several studies evaluated whether long-term 
PM2.5 exposure increases the risk of SARS-CoV-2 infection 
and COVID-19 death in North America (U.S. EPA, 2022a, section 3.3.2.2). 
While there is initial evidence of positive associations with SARS-CoV-
2 infection and COVID-19 death, uncertainties remain due to 
methodological issues that may influence the results, including: (1) 
The use of ecological study design; (2) studies were conducted during 
the ongoing pandemic when the etiology of COVID-19 was still not well 
understood (e.g., specifically, there are important differences in 
COVID-19-related outcomes by a variety of factors such as race and 
SES); and (3) studies did not account for crucial factors that could 
influence results (e.g., stay-at-home orders, social distancing, use of 
masks, and testing capacity) (U.S. EPA, 2022a, chapter 5). Taken 
together, while there is initial evidence of positive associations with 
SARS-CoV-2 infection and COVID-19 death, uncertainties remain due to 
methodological issues.
---------------------------------------------------------------------------

    \59\ While there is no exact corollary within the 2019 ISA for 
these types of studies, the 2019 ISA presented evidence that 
evaluates the potential relationship between short- and long-term 
PM2.5 exposure and respiratory infection (U.S. EPA, 
2022a, section 5.1.5 and 5.2.6). Studies assessed in the 2019 ISA 
report some evidence of positive associations between short-term 
PM2.5 and hospital admissions and ED visits for 
respiratory infections, however the interpretation of these studies 
is complicated by the variability in the type of respiratory 
infection outcome examined (U.S. EPA, 2022a, Figure 5-7). In the 
2019 ISA, studies of long-term PM2.5 exposure were 
limited and while there were some positive associations reported, 
there was minimal overlap in respiratory infection outcomes examined 
across studies. Exposure to PM2.5 has been shown to 
impair host defense, specifically altering macrophage function, 
providing a biological pathway by which PM2.5 exposure 
could lead to respiratory infection (U.S. EPA, 2022a, sections 5.1.1 
and 5.1.5.) There is some additional evidence that PM2.5 
exposure can lead to decreases in an individual's immune response, 
which can subsequently facilitate replication of respiratory viruses 
(Bourdrel et al., 2021).
---------------------------------------------------------------------------

b. Public Health Implications and At-Risk Populations
    The public health implications of the evidence regarding 
PM2.5-related health effects, as for other effects, are 
dependent on the type and severity of the effects, as well as the size 
of the population affected. Such factors are discussed below in the 
context of our consideration of the health effects evidence related to 
PM2.5 in ambient air. This section also summarizes the 
current information on population groups at increased risk of the 
effects of PM2.5 in ambient air.
    The information available in this reconsideration has not altered 
our understanding of human populations at risk of health effects from 
PM2.5 exposures. As recognized in the 2020 review, the 2019 
ISA cites extensive evidence indicating that ``both the general 
population as well as specific populations and lifestages are at risk 
for PM2.5-related health effects'' (U.S. EPA, 2019a, p. 12-
1). Factors that may contribute to increased risk of PM2.5-
related health effects include lifestage (children and older adults), 
pre-existing diseases (cardiovascular disease and

[[Page 16235]]

respiratory disease), race/ethnicity, and SES.\60\
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    \60\ As described in the 2019 ISA, other factors that have the 
potential to contribute to increased risk include obesity, diabetes, 
genetic factors, smoking status, sex, diet, and residential location 
(U.S. EPA, 2019, chapter 12).
---------------------------------------------------------------------------

    Children make up a substantial fraction of the U.S. population, and 
often have unique factors that contribute to their increased risk of 
experiencing a health effect due to exposures to ambient air pollutants 
because of their continuous growth and development.\61\ Children may be 
particularly at risk for health effects related to ambient 
PM2.5 exposures compared with adults because they have (1) a 
developing respiratory system, (2) increased ventilation rates relative 
to body mass compared with adults, and (3) an increased proportion of 
oral breathing, particularly in boys, relative to adults (U.S. EPA, 
2019a, section 12.5.1.1). There is strong evidence that demonstrates 
PM2.5 associated health effects in children, particularly 
from epidemiologic studies of long-term PM2.5 exposure and 
impaired lung function growth, decrements in lung function, and asthma 
development. However, there is limited evidence from stratified 
analyses that children are at increased risk of PM2.5-
related health effects compared to adults. Additionally, there is some 
evidence that indicates that children receive higher PM2.5 
exposures than adults, and dosimetric differences in children compared 
to adults can contribute to higher doses (U.S. EPA, 2019a, section 
12.5.1.1).
---------------------------------------------------------------------------

    \61\ Children, as used throughout this document, generally 
refers to those younger than 18 years old.
---------------------------------------------------------------------------

    In the U.S., older adults, often defined as adults 65 years of age 
and older, represent an increasing portion of the population and often 
have pre-existing diseases or conditions that may compromise biological 
function. While there is limited evidence to indicate that older adults 
have higher exposures than younger adults, older adults may receive 
higher doses of PM2.5 due to dosimetric differences. There 
is consistent evidence from studies of older adults demonstrating 
generally consistent positive associations in studies examining health 
effects from short- and long-term PM2.5 exposure and 
cardiovascular or respiratory hospital admissions, emergency department 
visits, or mortality (U.S. EPA, 2019a, sections 6.1, 6.2, 11.1, 11.2, 
12.5.1.2). Additionally, several animal toxicological, controlled human 
exposure, and epidemiologic studies did not stratify results by 
lifestage, but instead focused the analyses on older individuals, and 
can provide coherence and biological plausibility for the occurrence 
among this lifestage (U.S. EPA, 2019a, section 12.5.1.2).
    Individuals with pre-existing disease may be considered at greater 
risk of an air pollution-related health effect than those without 
disease because they are likely in a compromised biological state that 
can vary depending on the disease and severity. With regard to 
cardiovascular disease, we first note that cardiovascular disease is 
the leading cause of death in the U.S., accounting for one in four 
deaths, and approximately 12% of the adult population in the U.S. has a 
cardiovascular disease (U.S. EPA, 2019a, section 12.3.1). Strong 
evidence demonstrates that there is a causal relationship between 
cardiovascular effects and long- and short-term exposures to 
PM2.5. Some of the evidence supporting this conclusion is 
from studies of panels or cohorts with pre-existing cardiovascular 
disease, which provide supporting evidence but do not directly 
demonstrate an increased risk (U.S. EPA, 2019a, section 12.3.1). 
Epidemiologic evidence indicates that individuals with pre-existing 
cardiovascular disease may be at increased risk for PM2.5-
associated health effects compared to those without pre-existing 
cardiovascular disease. While the evidence does not consistently 
support increased risk for all pre-existing cardiovascular diseases, 
there is evidence that certain pre-existing cardiovascular diseases 
(e.g., hypertension) may be a factor that increases PM2.5-
related risk. Furthermore, there is strong evidence supporting a causal 
relationship for long- and short-term PM2.5 exposure and 
cardiovascular effects, particularly for IHD (U.S. EPA, 2019a, chapter 
6, section 12.3.1).
    With regard to respiratory disease, we first note that the most 
chronic respiratory diseases in the U.S. are asthma and COPD. Asthma 
affects a substantial fraction of the U.S. population and is the 
leading chronic disease among children. COPD primarily affects older 
adults and contributes to compromised respiratory function and 
underlying pulmonary inflammation. The body of evidence indicates that 
individuals with pre-existing respiratory diseases, particularly asthma 
and COPD, may be at increased risk for PM2.5-related health 
effects compared to those without pre-existing respiratory diseases 
(U.S. EPA, 2019a, section 12.3.5). There is strong evidence indicating 
PM2.5-associated respiratory effects among those with 
asthma, which forms the primary evidence base for the likely to be 
causal relationship between short-term exposures to PM2.5 
and respiratory health effects (U.S. EPA, 2019a, section 12.3.5). For 
asthma, epidemiologic evidence demonstrates associations between short-
term PM2.5 exposures and respiratory effects, particularly 
evidence for asthma exacerbation, and controlled human exposure and 
animal toxicological studies demonstrate support for the biological 
plausibility for asthma exacerbation with PM2.5 exposures 
(U.S. EPA, 2019a, section 12.3.5.1). For COPD, epidemiologic studies 
report positive associations between short-term PM2.5 
exposures and hospital admissions and emergency department visits for 
COPD, with supporting evidence from panel studies demonstration COPD 
exacerbation. Epidemiologic evidence is supported by some experimental 
evidence of COPD-related effects, which provides support for the 
biological plausibility for COPD in response to PM2.5 
exposures (U.S. EPA, 2019a, section 12.3.5.2).
    There is strong evidence for racial and ethnic disparities in 
PM2.5 exposures and PM2.5-related health risk, as 
assessed in the 2019 ISA and with even more evidence available since 
the literature cutoff date for the 2019 ISA and evaluated in the ISA 
Supplement. There is strong evidence demonstrating that Black and 
Hispanic populations, in particular, have higher PM2.5 
exposures than non-Hispanic White populations (U.S. EPA, 2019a, Figure 
12-2; U.S. EPA, 2022a, Figure 3-38). Black populations or individuals 
that live in predominantly Black neighborhoods experience higher 
PM2.5 exposures, in comparison to non-Hispanic White 
populations. There is also consistent evidence across multiple studies 
that demonstrate increased risk of PM2.5-related health 
effects, with the strongest evidence for health risk disparities for 
mortality (U.S. EPA, 2019a, section 12.5.4). There is also evidence of 
health risk disparities for both Hispanic and non-Hispanic Black 
populations compared to non-Hispanic White populations for cause-
specific mortality and incident hypertension (U.S. EPA, 2022a, section 
3.3.3.2).
    Socioeconomic status (SES) is a composite measure that includes 
metrics such as income, occupation, or education, and can play a role 
in access to healthy environments as well as access to healthcare. SES 
may be a factor that contributes to differential risk from 
PM2.5-related health effects. Studies assessed in the 2019 
ISA and ISA Supplement provide evidence that lower SES communities are 
exposed to higher concentrations of PM2.5

[[Page 16236]]

compared to higher SES communities (U.S. EPA, 2019a, section 12.5.3; 
U.S. EPA, 2022a, section 3.3.3.1.1). Studies using composite measures 
of neighborhood SES consistently demonstrated a disparity in both 
PM2.5 exposure and the risk of PM2.5-related 
health outcomes. There is some evidence that supports associations 
larger in magnitude between mortality and long-term PM2.5 
exposures for those with low income or living in lower income areas 
compared to those with higher income or living in higher income 
neighborhoods (U.S. EPA, 2019a, section 12.5.3; U.S. EPA, 2022a, 
section 3.3.3.1.1). Additionally, evidence supports conclusions that 
lower SES is associated with cause-specific mortality and certain 
health endpoints (i.e., HI and CHF), but less so for all-cause or total 
(non-accidental) mortality (U.S. EPA, 2022a, section 3.3.3.1).
    The magnitude and characterization of a public health impact is 
dependent upon the size and characteristics of the populations 
affected, as well as the type or severity of the effects. As summarized 
above, lifestage (children and older adults), race/ethnicity and SES 
are factors that increase the risk of PM2.5-related health 
effects. The American Community Survey (ACS) for 2019 estimates that 
approximately 22% and 16% of the U.S. population are children (age<18) 
and older adults (age 65+), respectively. For all ages, non-Hispanic 
Black and Hispanic populations comprise approximately 12% and 18% of 
the overall U.S. population in 2019. Currently available information 
that helps to characterize key features of these population is included 
in the 2022 PA (U.S. EPA, 2022b, Table 3-2).
    As noted above, individuals with pre-existing cardiovascular 
disease and pre-existing respiratory disease may also be at increased 
risk of PM2.5-related health effects. Currently available 
information that helps to characterize key features of populations with 
cardiovascular or respiratory diseases or conditions is included in the 
2022 PA (U.S. EPA, 2022b, Table 3-3). The National Center for Health 
Statistics data for 2018 indicate that, for adult populations, older 
adults (e.g., those 65 years and older) have a higher prevalence of 
cardiovascular diseases compared to younger adults (e.g., those 64 
years and younger). For respiratory diseases, older adults also have a 
higher prevalence of emphysema than younger adults, and adults 44 years 
or older have a higher prevalence of chronic bronchitis. However, the 
prevalence for asthma is generally similar across all adult age groups.
    With respect to race, American Indians or Alaskan Native 
populations have the highest prevalence of all heart disease and 
coronary heart disease, while Black populations have the highest 
prevalence of hypertension and stroke. Hypertension has the highest 
prevalence across all racial groups compared to other cardiovascular 
diseases or conditions, ranging from approximately 22% to 32% of each 
racial group. Overall, the prevalence of cardiovascular diseases or 
conditions is lowest for Asians compared to Whites, Blacks, and 
American Indians or Alaskan Natives. Asthma prevalence is highest among 
Black and American Indian or Alaska Native populations, while the 
prevalence of chronic bronchitis and emphysema is generally similar 
across racial groups. Overall, the prevalence of respiratory diseases 
is lowest for Asians compared to Whites, Blacks, and American Indians 
or Alaskan Natives. With regard to ethnicity, cardiovascular and 
respiratory disease prevalence across all diseases or conditions is 
generally similar between Hispanic and non-Hispanic populations, 
although non-Hispanics have a slightly higher prevalence compared to 
Hispanics.
    Taken together, this information indicates that the groups at 
increased risk of PM2.5-related health effects represent a 
substantial portion of the total U.S. population. In evaluating the 
primary PM2.5 standards, an important consideration is the 
potential PM2.5-related public health impacts in these 
populations.
c. PM2.5 Concentrations in Key Studies Reporting Health 
Effects
    To inform conclusions on the adequacy of the public health 
protection provided by the current primary PM2.5 standards, 
the sections below summarize the 2022 PA's evaluation of the 
PM2.5 exposures, specifically the concentrations that have 
been examined in controlled human exposure studies, animal 
toxicological studies, and epidemiologic studies. The 2022 PA places 
the greatest emphasis on the health outcomes for which the 2019 ISA 
concludes that the evidence supports a ``causal'' or a ``likely to be 
causal'' relationship with short- or long-term PM2.5 
exposures (U.S. EPA, 2022b, section 3.3.3). As described in greater 
detail in section II.A.2 above, this includes short- or long-term 
PM2.5 exposures and mortality, cardiovascular effects, and 
respiratory effects and long-term PM2.5 exposures and cancer 
and nervous system effects. While the causality determinations in the 
2019 ISA are informed by studies evaluating a wide range of 
PM2.5 concentrations,\62\ the sections below summarize the 
considerations in the 2022 PA regarding the degree to which the 
evidence assessed in the 2019 ISA and ISA Supplement supports the 
occurrence of PM-related health effects at concentrations relevant to 
informing conclusions on the primary PM2.5 standards. In so 
doing, the 2022 PA focuses on the available studies that are most 
directly informative to reaching conclusions regarding the adequacy of 
the current primary PM2.5 standards (e.g., epidemiologic 
studies with annual mean PM2.5 concentrations near or below 
the level of the standard; and controlled human exposure studies at 
PM2.5 exposures that elicit consistent effects, as well as 
examining PM2.5 exposures at concentrations that are at or 
near the level of the standard).
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    \62\ As described in more detail in section 5 of the Preamble to 
the ISAs, judgments regarding causality take into consideration a 
number of aspects when evaluating the available scientific evidence 
(U.S. EPA, 2015, Table I). In reaching conclusions regarding 
causality, ``evidence is evaluated for major outcome categories or 
groups of related endpoints (e.g., respiratory effects, vegetation 
growth), integrating evidence from across disciplines, and 
evaluating the coherence of evidence across a spectrum of related 
endpoints'' (U.S. EPA, 2015, p. 24). Furthermore, ``[i]n drawing 
judgments regarding causality for the criteria air pollutants, the 
ISA focuses on evidence of effects in the range of relevant 
pollutant exposures or doses and not on determination of causality 
at any dose. Emphasis is placed on evidence of effects at doses 
(e.g., blood Pb concentration) or exposures (e.g., air 
concentrations) that are relevant to, or somewhat above, those 
currently experienced by the population. The extent to which studies 
of higher concentrations are considered varies by pollutant and 
major outcome category, but generally includes those with doses or 
exposures in the range of one to two orders of magnitude above 
current or ambient conditions to account for intra-species 
variability and toxicokinetic or toxicodynamic differences between 
experimental animals and humans. Studies that use higher doses or 
exposures may also be considered to the extent that they provide 
useful information to inform understanding of mode of action, inter-
species differences, or factors that may increase risk of effects 
for a population and if biological mechanisms have not been 
demonstrated to differ based on exposure concentration. Thus, a 
causality determination is based on weight-of-evidence evaluation 
for health or welfare effects, focusing on the evidence from 
exposures or doses generally ranging from recent ambient 
concentrations to one or two orders of magnitude above recent 
ambient concentrations'' (U.S. EPA, 2015, p. 24).
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i. PM2.5 Exposure Concentrations Evaluated in Experimental 
Studies
    Evidence for a particular PM2.5-related health outcome 
is strengthened when results from experimental studies demonstrate 
biologically plausible mechanisms through which adverse human health 
outcomes could occur (U.S. EPA, 2015, p. 20). Two types of experimental 
studies are of particular importance in understanding the effects

[[Page 16237]]

of PM exposures: controlled human exposure and animal toxicological 
studies. In such studies, investigators expose human volunteers or 
laboratory animals, respectively, to known concentrations of air 
pollutants under carefully regulated environmental conditions and 
activity levels. Thus, controlled human exposure and animal 
toxicological studies can provide information on the health effects of 
experimentally administered pollutant exposures under highly controlled 
laboratory conditions (U.S. EPA, 2015, p. 11).
    Controlled human exposure studies have reported that 
PM2.5 exposures lasting from less than one hour up to five 
hours can impact cardiovascular function,\63\ and the most consistent 
evidence from these studies is for impaired vascular function (U.S. 
EPA, 2019a, section 6.1.13.2). In addition, although less consistent, 
the 2019 ISA notes that studies examining PM2.5 exposures 
also provide evidence for increased blood pressure (U.S. EPA, 2019a, 
section 6.1.6.3), conduction abnormalities/arrhythmia (U.S. EPA, 2019a, 
section 6.1.4.3), changes in heart rate variability (U.S. EPA, 2019a, 
section 6.1.10.2), changes in hemostasis that could promote clot 
formation (U.S. EPA, 2019a, section 6.1.12.2), and increases in 
inflammatory cells and markers (U.S. EPA, 2019a, section 6.1.11.2). The 
2019 ISA concludes that, when taken as a whole, controlled human 
exposure studies demonstrate that short-term exposure to 
PM2.5 may impact cardiovascular function in ways that could 
lead to more serious outcomes (U.S. EPA, 2019a, section 6.1.16). Thus, 
such studies can provide insight into the potential for specific 
PM2.5 exposures to result in physiological changes that 
could increase the risk of more serious effects. Table 3-4 in the 2022 
PA summarizes information from the 2019 ISA and 2022 ISA supplement on 
available controlled human exposure studies that evaluate effects on 
markers of cardiovascular function following exposure to 
PM2.5 (U.S. EPA, 2022b). Most of the controlled human 
exposure studies in Table 3-4 of the 2022 PA have evaluated average 
PM2.5 concentrations at or above about 100 [micro]g/m\3\, 
with exposure durations typically up to about two hours. Statistically 
significant effects on one or more indicators of cardiovascular 
function are often, though not always, reported following 2-hour 
exposures to average PM2.5 concentrations at and above about 
120 [micro]g/m\3\, with less consistent evidence for effects following 
exposures to concentrations lower than 120 [micro]g/m\3\. Impaired 
vascular function, the effect identified in the 2019 ISA as the most 
consistent across studies (U.S. EPA, 2019a, section 6.1.13.2) is shown 
following 2-hour exposures to PM2.5 concentrations at and 
above 149 [micro]g/m\3\. Mixed results are reported in the studies that 
evaluated longer exposure durations (i.e., longer than 2 hours) and 
lower (i.e., near-ambient) PM2.5 concentrations (U.S. EPA, 
2022b, section 3.3.3.1). For example, significant effects for some 
outcomes were reported following 5-hour exposures to 24 [micro]g/m\3\ 
in Hemmingsen et al. (2015b), but not for other outcomes following 5-
hour exposures to 24 [micro]g/m\3\ in Hemmingsen et al. (2015a) and not 
following 24-hour exposures to 10.5 [micro]g/m\3\ in Br[auml]uner et 
al. (2008). Additionally, Wyatt et al. (2020) found significant effects 
for some cardiovascular (e.g., systematic inflammation markers, cardiac 
repolarization, and decreased pulmonary function) effects following 4-
hour exposures to 37.8 [micro]g/m\3\ in healthy young participants (18-
35 years, n=21) who were subject to intermittent moderate exercise. The 
higher ventilation rate and longer exposure duration in this study 
compared to most controlled human exposure studies is roughly 
equivalent to a 2-hour exposure of 75-100 [micro]g/m\3\ of 
PM2.5. Therefore, dosimetric considerations may explain the 
observed changes in inflammation in young healthy individuals. Though 
this study provides evidence of some effects at lower PM2.5 
concentrations, overall, there is inconsistent evidence for 
inflammation in other controlled human exposure studies evaluated in 
the 2019 ISA (U.S. EPA, 2019a, sections 5.1.7., 5.1.2.3.3, and 
6.1.11.2.1; U.S. EPA, 2022a, section 3.3.1).
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    \63\ In contrast, controlled human exposure studies provide 
little evidence for respiratory effects following short-term 
PM2.5 exposures (U.S. EPA, 2019a, section 5.1, Table 5-
18). Therefore, this section focuses on cardiovascular effects 
evaluated in controlled human exposure studies of PM2.5 
exposure.
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    While controlled human exposure studies are important in 
establishing biological plausibility, it is unclear how the results 
from these studies alone and the importance of the effects observed in 
these studies, should be interpreted with respect to adversity to 
public health. More specifically, impaired vascular function can signal 
an intermediate effect along the potential biological pathways for 
cardiovascular effects following short-term exposure to 
PM2.5 and show a role for exposure to PM2.5 
leading to potential worsening of IHD and heart failure followed 
potentially by ED visits, hospital admissions, or mortality (U.S. EPA, 
2019a, section 6.1 and Figure 6-1). However, just observing the 
occurrence of impaired vascular function alone does not clearly suggest 
an adverse health outcome. Additionally, associated judgments regarding 
adversity or health significance of measurable physiological responses 
to air pollutants have been informed by guidance, criteria or 
interpretative statements developed within the public health community, 
including the American Thoracic Society (ATS) and the European 
Respiratory Society (ERS), which cooperatively updated the ATS 2000 
statement What Constitutes an Adverse Health Effect of Air Pollution 
(ATS, 2000) with new scientific findings, including the evidence 
related to air pollution and the cardiovascular system (Thurston et 
al., 2017).\64\ With regard to vascular function, the ATS/ERS statement 
considers the adversity of both chronic and acute reductions in 
endothelial function. While the ATS/ERS statement concluded that 
chronic endothelial and vascular dysfunction can be judged to be a 
biomarker of an adverse health effect from air pollution, they also 
conclude that ``the health relevance of acute reductions in endothelial 
function induced by air pollution is less certain'' (Thurston et al., 
2017). This is particularly informative to our consideration of the 
controlled human exposure studies which are short-term in nature (i.e., 
generally ranging from 2- to 5-hours), including those studies that are 
conducted at near-ambient PM2.5 concentrations.
---------------------------------------------------------------------------

    \64\ The ATS/ERS described its 2017 statement as one ``intended 
to provide guidance to policymakers, clinicians and public health 
professionals, as well as others who interpret the scientific 
evidence on the health effects of air pollution for risk management 
purposes'' and further notes that ``considerations as to what 
constitutes an adverse health effect, in order to provide guidance 
to researchers and policymakers when new health effects markers or 
health outcome associations might be reported in future.'' The most 
recent policy statement by the ATS, which once again broadens its 
discussion of effects, responses and biomarkers to reflect the 
expansion of scientific research in these areas, reiterates that 
concept, conveying that it does not offer ``strict rules or 
numerical criteria, but rather proposes considerations to be weighed 
in setting boundaries between adverse and nonadverse health 
effects,'' providing a general framework for interpreting evidence 
that proposes a ``set of considerations that can be applied in 
forming judgments'' for this context (Thurston et al., 2017).
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    The 2022 PA also notes that it is important to recognize that 
controlled human exposure studies include a small number of individuals 
compared to epidemiologic studies. Additionally, these studies tend to 
include generally healthy adult individuals, who are at a lower risk of 
experiencing health effects.

[[Page 16238]]

These studies, therefore, often do not include children, older adults, 
or individuals with pre-existing conditions. As such, these studies are 
somewhat limited in their ability to inform at what concentrations 
effects may be elicited in at-risk populations.
    Nonetheless, to provide some insight into what these controlled 
human exposure studies may indicate regarding short-term exposure to 
peak PM2.5 concentrations and how concentrations relate to 
ambient PM2.5 concentrations, analyses in the 2022 PA (U.S. 
EPA, 2022b, Figure 2-19) examine monitored 2-hour PM2.5 
concentrations (the exposure window most often utilized in the 
controlled human exposure studies) at sites meeting the current primary 
PM2.5 standards to evaluate the degree to which 2-hour 
ambient PM2.5 concentrations at such locations are likely to 
exceed the 2-hour exposure concentrations in the controlled human 
exposure studies at which statistically significant effects are 
reported in multiple studies for one or more indicators of 
cardiovascular function. At sites meeting the current primary 
PM2.5 standards, most 2-hour concentrations are below 10 
[mu]g/m\3\, and almost never exceed 30 [mu]g/m\3\. The extreme upper 
end of the distribution of 2-hour PM2.5 concentrations is 
shifted higher during the warmer months (April to September), generally 
corresponding to the period of peak wildfire frequency in the U.S. At 
sites meeting the current primary PM2.5 standards, the 
highest 2-hour concentrations measured tend to occur during the period 
of peak wildfire frequency (i.e., 99.9th percentile of 2-hour 
concentrations is 62 [mu]g/m\3\ during the warm season considered as a 
whole). Most of the sites measuring these very high concentrations are 
in the northwestern U.S. and California (U.S. EPA, 2022b, Appendix A, 
Figure A-1), where wildfires have been relatively common in recent 
years. When the typical fire season is excluded from the analysis, the 
extreme upper end of the distribution is reduced (i.e., 99.9th 
percentile of 2-hour concentrations is 55 [mu]g/m\3\).\65\ Given these 
results, the 2022 PA concludes that PM2.5 exposure 
concentrations evaluated in most of these controlled human exposure 
studies are well-above the 2-hour ambient PM2.5 
concentrations typically measured in locations meeting the current 
primary standards.
---------------------------------------------------------------------------

    \65\ Similar analyses of 4-hour and 5-hour PM2.5 
concentrations are presented in Appendix A, Figure A-2 and Figure A-
3, respectively of the 2022 PA (U.S. EPA, 2022b).
---------------------------------------------------------------------------

    With respect to animal toxicological studies, the 2019 ISA relies 
on animal toxicological studies to support the plausibility of a wide 
range of PM2.5-related health effects. While animal 
toxicological studies often examine more severe health outcomes and 
longer exposure durations than controlled human exposure studies, there 
is uncertainty in extrapolating the effects seen in animals, and the 
PM2.5 exposures and doses that cause those effects, to human 
populations. The 2022 PA considers these uncertainties when evaluating 
what the available animal toxicological studies may indicate with 
regard to the current primary PM2.5 standards.
    As with controlled human exposure studies, most animal 
toxicological studies evaluated in the 2019 ISA have examined effects 
following exposure to PM2.5 well above the concentrations 
likely to be allowed by the current PM2.5 standards. Such 
studies have generally examined short-term exposures to 
PM2.5 concentrations ranging from 100 to >1,000 [mu]g/m\3\ 
and long-term exposures to concentrations from 66 to >400 [mu]g/m\3\ 
(e.g., see U.S. EPA, 2019a, Table 1-2). Two exceptions are animal 
toxicological studies reporting impaired lung development following 
long-term exposures (i.e., 24 hours per day for several months 
prenatally and postnatally) to an average PM2.5 
concentration of 16.8 [mu]g/m\3\ (Mauad et al., 2008) and increased 
carcinogenic potential following long-term exposures (i.e., 2 months) 
to an average PM2.5 concentration of 17.7 [mu]g/m\3\ 
(Cangerana Pereira et al., 2011). These two studies report serious 
effects following long-term exposures to PM2.5 
concentrations similar to the ambient concentrations reported in some 
PM2.5 epidemiologic studies (U.S. EPA, 2019a, Table 1-2), 
though still above the ambient concentrations likely to occur in areas 
meeting the current primary PM2.5 standards. However, noting 
uncertainty in extrapolating the effects seen in animals, and the 
PM2.5 exposures and doses that cause those effects to human 
populations, animal toxicological studies are of limited utility in 
informing decisions on the public health protection provided by the 
current or alternative primary PM2.5 standards. Therefore, 
the animal toxicological studies are most useful in providing further 
evidence to support the biological mechanisms and plausibility of 
various adverse effects.
ii. Ambient PM2.5 Concentrations in Locations of 
Epidemiologic Studies
    As summarized in section II.A.2.a above, epidemiologic studies 
examining associations between daily or annual average PM2.5 
exposures and mortality or morbidity represent a large part of the 
evidence base supporting several of the 2019 ISA's ``causal'' and 
``likely to be causal'' determinations. The 2022 PA considers the 
ambient PM2.5 concentrations present in areas where 
epidemiologic studies have evaluated associations with mortality or 
morbidity, and what such concentrations may indicate regarding the 
adequacy of the primary PM2.5 standards. The use of 
information from epidemiologic studies to inform conclusions on the 
primary PM2.5 standards is complicated by the fact that such 
studies evaluate associations between distributions of ambient 
PM2.5 and health outcomes, and do not identify the specific 
exposures that can lead to the reported effects. Rather, health effects 
can occur over the entire distribution of ambient PM2.5 
concentrations evaluated, and epidemiologic studies conducted to date 
do not identify a population-level threshold below which it can be 
concluded with confidence that PM2.5-associated health 
effects do not occur. Therefore, the 2022 PA evaluates the 
PM2.5 air quality distributions over which epidemiologic 
studies support health effect associations (U.S. EPA, 2022b, section 
3.3.3.2). In the absence of discernible thresholds, the 2022 PA 
considers the study-reported ambient PM2.5 concentrations 
reflecting estimated exposure with a focus around the middle portion of 
the PM2.5 air quality distribution, where the bulk of the 
observed data reside and which provides the strongest support for 
reported health effect associations. The section below, as well as in 
more detail in section II.B.3.b.i of the proposal (88 FR 5594, January 
27, 2023), describes the consideration of the key epidemiologic studies 
and observations from these studies, as evaluated in the 2022 PA (U.S. 
EPA, 2022b, section 3.3.3.2).
    As an initial matter, in considering the PM2.5 air 
quality distributions associated with mortality or morbidity in the key 
epidemiologic studies, the 2022 PA recognizes that in previous reviews, 
the decision framework used to judge adequacy of the existing 
PM2.5 standards, and what levels of any potential 
alternative standards should be considered, placed significant weight 
on epidemiologic studies that assessed associations between 
PM2.5 exposure and health outcomes that were most strongly 
supported by the body of scientific evidence. In doing so, the decision 
framework recognized that while there is no specific point in the air 
quality distribution of any

[[Page 16239]]

epidemiologic study that represents a ``bright line'' at and above 
which effects have been observed and below which effects have not been 
observed, there is significantly greater confidence in the magnitude 
and significance of observed associations for the part of the air 
quality distribution corresponding to where the bulk of the health 
events in each study have been observed, generally at or around the 
mean concentration. This is the case both for studies of daily 
PM2.5 exposures and for studies of annual average 
PM2.5 exposures (U.S. EPA, 2022b, section 3.3.3.2.1).
    As discussed further in the 2022 PA, studies of daily 
PM2.5 exposures examine associations between day-to-day 
variation in PM2.5 concentrations and health outcomes, often 
over several years (U.S. EPA, 2022b, section 3.3.3.2.1). While there 
can be considerable variability in daily exposures over a multi-year 
study period, most of the estimated exposures reflect days with ambient 
PM2.5 concentrations around the middle of the air quality 
distributions examined (i.e., ``typical'' days rather than days with 
extremely high or extremely low concentrations). Similarly, for studies 
of annual PM2.5 exposures, most of the health events occur 
at estimated exposures that reflect annual average PM2.5 
concentrations around the middle of the air quality distributions 
examined. In both cases, epidemiologic studies provide the strongest 
support for reported health effect associations for this middle portion 
of the PM2.5 air quality distribution, which corresponds to 
the bulk of the underlying data, rather than the extreme upper or lower 
ends of the distribution. Consistent with this, as noted in the 2022 PA 
(U.S. EPA, 2022b, section 3.3.1.1), several epidemiologic studies 
report that associations persist in analyses that exclude the upper 
portions of the distributions of estimated PM2.5 exposures, 
indicating that ``peak'' PM2.5 exposures are not 
disproportionately responsible for reported health effect associations.
    Thus, in considering PM2.5 air quality data from 
epidemiologic studies, consistent with approaches in the 2012 and 2020 
reviews (78 FR 3161, January 15, 2013; U.S. EPA, 2011, sections 2.1.3 
and 2.3.4.1; 85 FR 82716-82717, December 18, 2020; U.S. EPA, 2020b, 
sections 3.1.2 and 3.2.3), the 2022 PA evaluates study-reported means 
(or medians) of daily and annual average PM2.5 
concentrations as indicators for the middle portions of the air quality 
distributions, over which studies generally provide strong support for 
reported associations and for which confidence in the magnitude and 
significance of associations observed in the epidemiologic studies is 
greatest (78 FR 3101, January 15, 2013). In addition to the overall 
study means, the 2022 PA also focuses on concentrations somewhat below 
the means (e.g., 25th and 10th percentiles), when such information is 
available from the epidemiologic studies, which again is consistent 
with approaches used in previous reviews. In so doing, the 2022 PA 
notes, as in previous reviews, that a relatively small portion of the 
health events are observed in the lower part of the air quality 
distribution and confidence in the magnitude and significance of the 
associations begins to decrease in the lower part of the air quality 
distribution. Furthermore, consistent with past reviews, there is no 
single percentile value within a given air quality distribution that is 
most appropriate or ``correct'' to use to characterize where our 
confidence in associations becomes appreciably lower. However, and as 
detailed further in the 2022 PA, the range from the 25th to 10th 
percentiles is a reasonable range to consider as a region where there 
is appreciably less confidence in the associations observed in 
epidemiologic studies compared to the means (U.S. EPA, 2022b, p. 3-
69).\66\
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    \66\ As detailed in the 2011 PA, we note the interrelatedness of 
the distributional statistics and a range of one standard deviation 
around the mean which represents approximately 68% of normally 
distributed data, and in that one standard deviation below the mean 
falls between the 25th and 10th percentiles (U.S. EPA, 2011, p. 2-
71; U.S. EPA, 2005, p. 5-22).
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    In evaluating the overall study-reported means, and concentrations 
somewhat below the means from epidemiologic studies, the 2022 PA 
focuses on the form, averaging time and level of the current primary 
annual PM2.5 standard. Consistent with the approaches used 
in the 2012 and 2020 reviews (78 FR 3161-3162, January 15, 2013; 85 FR 
82716-82717, December 18, 2020), the annual standard has been utilized 
as the primary means of providing public health protection against the 
bulk of the distribution of short- and long-term PM2.5 
exposures. Thus, the evaluation of the study-reported mean 
concentrations from key epidemiologic studies lends itself best to 
evaluating the adequacy of the annual PM2.5 standard (rather 
than the 24-hour standard with its 98th percentile form). This is true 
for the study-reported means from both long-term and short-term 
exposure epidemiologic studies, recognizing that the overall mean 
PM2.5 concentrations reported in studies of short-term (24-
hour) exposures reflect averages across the study population and over 
the years of the study. Thus, mean concentrations from short-term 
exposure studies reflect long-term averages of 24-hour PM2.5 
exposure estimates. In this manner, the examination of study-reported 
means in key epidemiologic studies in the 2022 PA aims to evaluate the 
protection provided by the annual PM2.5 standard against the 
exposures where confidence is greatest for associations with mortality 
and morbidity. In addition, the protection provided by the annual 
standard is evaluated in conjunction with that provided by the 24-hour 
standard, with its 98th percentile form, which aims to provide 
supplemental protection against the short-term exposures to peak 
PM2.5 concentrations that can occur in areas with strong 
contributions from local or seasonal sources, even when overall ambient 
mean PM2.5 concentrations in an area remain relatively low.
    In focusing on the annual standard, and in evaluating the range of 
study-reported exposure concentrations for which the strongest support 
for adverse health effects exists, the 2022 PA examines exposure 
concentrations in key epidemiologic studies to determine whether the 
current primary annual PM2.5 standard provides adequate 
protection against these exposure concentrations. This means, as in 
past reviews, application of a decision framework based on assessing 
means reported in key epidemiologic studies must also consider how the 
study means were computed and how these values compare to the annual 
standard metric (including the level, averaging time and form) and the 
use of the monitor with the highest PM2.5 design value in an 
area for compliance. In the 2012 review, it was recognized that the key 
epidemiologic studies computed the study mean using an average across 
monitor-based PM2.5 concentrations. As such, the Agency 
noted that this decision framework applied an approach of using maximum 
monitor concentrations to determine compliance with the standard, while 
selecting the standard level based on consideration of composite 
monitor concentrations. Further, the Agency included analyses (Hassett-
Sipple et al., 2010; Frank, 2012) that examined the differences in 
these two metrics (i.e., maximum monitor concentrations and composite 
monitor concentrations) across the U.S. and in areas included in the 
key epidemiologic studies and found that the maximum design value in an 
area was generally higher than the monitor average across that area, 
with the difference varying

[[Page 16240]]

based on location and concentration. This information was taken into 
account in the Administrator's final decision in selecting a level for 
the primary annual PM2.5 standard the 2012 review and 
discussed more specifically in her considerations on adequate margin of 
safety.
    Consistent with the approach taken in 2012, in assessing how the 
overall mean (or median) PM2.5 concentrations reported in 
key epidemiologic studies can inform conclusions on the primary annual 
PM2.5 standard, the 2022 PA notes that the relationship 
between mean PM2.5 concentrations and the area design value 
continues to be an important consideration in evaluating the adequacy 
of the current or potential alternative annual PM2.5 
standard levels in this reconsideration. In a given area, the area 
design value is based on the monitor in an area with the highest 
PM2.5 concentrations and is used to determine compliance 
with the standard. The highest PM2.5 concentrations 
spatially distributed in the area would generally occur at or near the 
area design value monitor and the distribution of PM2.5 
concentrations would generally be lower in other locations and at 
monitors in that area. As such, when an area is meeting a specific 
annual standard level, the annual average exposures in that area are 
expected to be at concentrations lower than that level and the average 
of the annual average exposures across that area are expected (i.e., a 
metric similar to the study-reported mean values) to be lower than that 
level.\67\
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    \67\ In setting a standard level that would require the design 
value monitor to meet a level equal to the study-reported mean 
PM2.5 concentrations would generally result in lower 
concentrations of PM2.5 across the entire area, such that 
even those people living near an area design value monitor (where PM 
concentrations are generally highest) will be exposed to 
PM2.5 concentrations below the air quality conditions 
reported in the epidemiologic studies.
---------------------------------------------------------------------------

    Another important consideration is that there are a substantial 
number of different types of epidemiologic studies available since the 
2012 review, included in both the 2019 ISA and the ISA Supplement, that 
make understanding the relationship between the mean PM2.5 
concentrations and the area design value even more important (U.S. EPA, 
2019a; U.S. EPA, 2022a). While the key epidemiologic studies in the 
2012 review were all monitor-based studies, the newer studies include 
hybrid modeling approaches, which have emerged in the epidemiologic 
literature as an alternative to approaches that only use ground-based 
monitors to estimate exposure. As assessed in the 2019 ISA and ISA 
Supplement, a substantial number of epidemiologic studies used hybrid 
model-based methods in evaluating associations between PM2.5 
exposure and health effects (U.S. EPA, 2019a; U.S. EPA, 2022a). Hybrid 
model-based studies employ various fusion techniques that combine 
ground-based monitored data with air quality modeled estimates and/or 
information from satellites to estimate PM2.5 exposures.\68\ 
Additionally, hybrid modeling approaches tend to broaden the areas 
captured in the exposure assessment, and in so doing, tend to report 
lower mean PM2.5 concentrations than monitor-based 
approaches because they include more suburban and rural areas where 
concentrations are lower. While these studies provide a broader 
estimation of PM2.5 exposures compared to monitor-based 
studies (i.e., PM2.5 concentrations are estimated in areas 
without monitors), the hybrid modeling approaches result in study-
reported means that are more difficult to relate to the annual standard 
metric and to the use of maximum monitor design values to assess 
compliance. In addition, and to further complicate the comparison, when 
looking across these studies, variations exist in how exposure is 
estimated between such studies, which in turn affects how the study 
means are calculated. Two important variations across studies include: 
(1) Variability in spatial scale used (i.e., averages computed across 
the nation (or large portions of the country) versus a focus on only 
CBSAs) and (2) variability in exposure assignment methods (i.e., 
averaging across all grid cells [non-population weighting], averaging 
across a scaled-up area like a ZIP code [aspects of population 
weighting applied], and/or applying population weighting). To elaborate 
further on the variability in exposure assignment methods, studies that 
use hybrid modeling approaches can estimate PM2.5 
concentrations at different spatial resolutions, including at 1 km x 1 
km grid cells, at 12 km x 12 km grid cells, or at the census tract 
level. Mean reported PM2.5 concentrations can then be 
estimated either by averaging up to a larger spatial resolution that 
corresponds to the spatial resolution for which health data exists 
(e.g., ZIP code level) and therefore apply aspects of population 
weighting. These values are then averaged across all study locations at 
the larger spatial resolution (e.g., averaged across all ZIP codes in 
the study) over the study period, resulting in the study-reported mean 
24-hour average or average annual PM2.5 concentration. Other 
studies that use hybrid modeling methods to estimate PM2.5 
concentrations may use each grid cell to calculate the study-reported 
mean 24-hour average or average annual PM2.5 concentration. 
As such, these types of studies do not apply population weighting in 
their mean concentrations. In studies that use each grid cell to report 
a mean PM2.5 concentration and do not apply aspects of 
population weighting, the study mean may not reflect the exposure 
concentrations used in the epidemiologic study to assess the reported 
association. The impact of the differences in methods is an important 
consideration when comparing mean concentrations across studies (U.S. 
EPA, 2022b, section 3.3.3.2.1). Thus, the 2022 PA also considers the 
methods used to estimate PM2.5 concentrations, which vary 
from traditional methods using monitoring data from ground-based 
monitors \69\ to those using more complex hybrid modeling approaches 
and how these methods calculate the study-reported mean 
PM2.5 concentration.\70\
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    \68\ More detailed information about hybrid model methods and 
performance is described in section 2.3.3.2 of the 2022 PA (U.S. 
EPA, 2022b).
    \69\ In those studies that use ground-based monitors alone to 
estimate long- or short-term PM2.5 concentrations, 
approaches include: (1) PM2.5 concentrations from a 
single monitor within a city/county; (2) average of PM2.5 
concentrations across all monitors within a city/county or other 
defined study area (e.g., CBSA); or (3) population-weighted averages 
of exposures. Once the study location average PM2.5 
concentration is calculated, the study-reported long-term average is 
derived by averaging daily/annual PM2.5 concentrations 
across all study locations over the entire study period.
    \70\ Detailed information on the methods by which mean 
PM2.5 concentrations are calculated in key monitor- and 
hybrid model-based U.S. and Canadian epidemiologic studies are 
presented in Tables 3-6 through 3-9 in the 2022 PA (U.S. EPA, 
2022b).
---------------------------------------------------------------------------

    Given the emergence of the hybrid model-based epidemiologic studies 
since the 2012 review, the 2022 PA explores the relationship between 
the approaches used in these studies to estimate PM2.5 
concentrations and the impact that the different methods have on the 
study-reported mean PM2.5 concentrations. The 2022 PA 
further seeks to understand how the approaches and resulting mean 
concentrations compare across studies, as well as what the resulting 
mean values represent relative to the annual standard. In so doing, the 
2022 PA presents analyses that compare the area annual design values, 
composite monitor PM2.5 concentrations, and mean 
concentrations from two hybrid modeling approaches, including 
evaluation of the means when population weighting is applied and when 
population weighting is not

[[Page 16241]]

applied (U.S. EPA, 2022b, section 2.3.3.1).
    In the air quality analyses comparing composite monitored 
PM2.5 concentrations with annual PM2.5 design 
values in U.S. CBSAs, maximum annual PM2.5 design values 
were approximately 10% to 20% higher than annual average composite 
monitor concentrations (i.e., averaged across multiple monitors in the 
same CBSA) (sections I.D.5.a above and U.S. EPA, 2022b, section 
2.3.3.1, Figure 2-28 and Table 2-3). The difference between the maximum 
annual design value and average concentration in an area can be smaller 
or larger than this range (10-20%), depending on a variety of factors 
such as the number of monitors, monitor siting characteristics, the 
distribution of ambient PM2.5 concentrations, and how the 
average concentrations are calculated (i.e., averaged across monitors 
versus across modeled grid cells). Results of this analysis suggest 
that there will be a distribution of concentrations across an area and 
the maximum annual average monitored concentration in an area (at the 
design value monitor, used for compliance with the standard), will 
generally be 10-20% higher than the average PM2.5 
concentration across the other monitors in the area. Thus, in 
considering how the annual standard levels would relate to the study-
reported means from key monitor-based epidemiologic studies, the 2022 
PA generally concludes that an annual standard level that is no more 
than 10-20% higher than monitor-based study-reported mean 
PM2.5 concentrations would generally maintain air quality 
exposures to be below those associated with the study-reported mean 
PM2.5 concentrations, exposures for which the strongest 
support for adverse health effects occurring is available.
    The 2022 PA also evaluates data from two hybrid modeling approaches 
(DI2019 and HA2020) that have been used in several recent epidemiologic 
studies (U.S. EPA, 2022b, section 2.3.3.2.4).\71\ The analysis shows 
that the means differ when PM2.5 concentrations are 
estimated in urban areas only (CBSAs) versus when the averages were 
calculated with all or most grid cells nationwide, likely because areas 
included outside of CBSAs tend to be more rural and have lower 
estimated PM2.5 concentrations. The 2022 PA recognizes the 
importance of this variability in the means since the study areas 
included in the calculation of the mean, and more specifically whether 
a study is focused on nationwide, regional, or urban areas, will affect 
the calculation of the study mean based on how many rural areas, with 
lower estimated PM2.5 concentrations, are included in the 
study area. While the determination of what spatial scale to use to 
estimate PM2.5 concentrations does not inherently affect the 
quality of the epidemiologic study, the spatial scale can influence the 
calculated reported long-term mean concentration across the study area 
and period. The results of the analysis show that, regardless of the 
hybrid modeling approach assessed, the annual average PM2.5 
concentrations in CBSA-only analyses are 4-8% higher than for 
nationwide analyses, likely as a result of higher PM2.5 
concentrations in more densely populated areas, and exclusion of more 
rural areas (U.S. EPA, 2022b, Table 2-4). When evaluating comparisons 
between surfaces that estimate exposure using aspects of population 
weighting versus surfaces that do not calculate means using population 
weighting, surfaces that calculate long-term mean PM2.5 
concentrations with population-weighted averages have higher average 
annual PM2.5 concentrations, compared to annual 
PM2.5 concentrations in analyses that do not apply 
population weighting.\72\ Analyses show that average maximum annual 
design values are 40 to 50% higher when compared to annual average 
PM2.5 concentrations estimated without population weighting 
versus 15% to 18% higher when compared to average annual 
PM2.5 concentrations estimated with population weighting 
applied (similar to the differences observed for the composite monitor 
comparison values for the monitor-based epidemiologic studies) (U.S. 
EPA, 2022b, section 2.3.3.2.4). Given these results, it is worth noting 
that for the studies using the hybrid modeling approaches, the choice 
of methodology employed in calculating the study-reported means (i.e., 
using population weighting or not), and not a difference in estimates 
of exposure in the study itself, can produce substantially different 
study-reported mean values, where approaches that do not apply 
population weighting leading to much lower estimated mean 
PM2.5 concentrations.
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    \71\ More details on the evaluation of the two hybrid modeling 
approaches is provided in section 2.3.3.2.4 of the 2022 PA (U.S. 
EPA, 2022b).
    \72\ The annual PM2.5 concentrations for the 
population-weighted averages ranged from 8.2-10.2 [mu]g/m\3\, while 
those that do not apply population weighting ranged from 7.0-8.6 
[mu]g/m\3\. Average maximum annual design values ranged from 9.5 to 
11.7 [mu]g/m\3\.
---------------------------------------------------------------------------

    Based on these results, and similar to conclusions for the monitor-
based studies, the 2022 PA generally concludes that study-reported mean 
concentrations in the studies that employ hybrid modeling approaches 
and calculate a population-weighted mean are associated with air 
quality conditions that would be achieved by meeting annual standard 
levels that are 15-18% higher than study-reported means. Therefore, an 
annual standard level that is no more than 15-18% higher than the 
study-reported means would generally maintain air quality exposures to 
be below those associated with the study-reported mean PM2.5 
concentrations, exposures for which we have the strongest support for 
adverse health effects occurring. For the studies that utilize hybrid 
modeling approaches but do not incorporate population weighting in 
calculating the mean, the annual design values associated with these 
air quality conditions are expected to be much higher (i.e., 40-50% 
higher) and this larger difference makes it more difficult to consider 
how these studies can be used to determine the adequacy of the 
protection afforded by the current or potential alternative annual 
standards. Additionally, as noted above in studies that utilize hybrid 
modeling approaches and that do not incorporate population weighting in 
calculating the mean (e.g., use each grid cell to calculate a mean 
PM2.5 concentration), the study mean does not reflect the 
exposure concentrations used in the epidemiologic study to assess the 
reported association.
    The 2022 PA notes that while these analyses can be useful to 
informing the understanding of the relationship between study-reported 
mean concentrations and the level of the annual standard, some 
limitations of this analysis must be recognized (U.S. EPA, 2022a, 
section 3.3.3.2.1). First, the comparisons used only two hybrid 
modeling approaches. Although these two hybrid modeling surfaces have 
been used in a number of recent epidemiologic studies, they represent 
just two of the many hybrid modeling approaches that have been used in 
epidemiologic studies to estimate PM2.5 concentrations. 
These methods continue to evolve, with further development and 
improvement to prediction models that estimate PM2.5 
concentrations in epidemiologic studies. In addition to differences in 
hybrid modeling approaches, epidemiologic studies also use different 
methods to assign a population weighted average PM2.5 
concentration to their study population, and the assessment presented 
in the

[[Page 16242]]

2022 PA does not evaluate all of the potential methods that could be 
used.
    Additionally, while some of these epidemiologic studies also 
provide information on the broader distributions of exposure estimates 
and/or health events and the PM2.5 concentrations 
corresponding to the lower percentiles of those data (e.g., 25th and/or 
10th), the air quality analysis in the 2022 PA focuses on mean 
PM2.5 concentrations and a similar comparison for lower 
percentiles of data was not assessed. Therefore, any direct comparison 
of study-reported PM2.5 concentrations corresponding to 
lower percentiles and annual design values is more uncertain than such 
comparisons with the mean. Finally, air quality analysis presented in 
the 2022 PA and detailed above in section I.D.5 included two hybrid 
modeling-based approaches that used U.S.-based air quality information 
for estimating PM2.5 concentrations. As such, the analyses 
are most relevant to interpreting the study-reported mean 
concentrations from U.S. epidemiologic studies and do not provide 
additional information about how the mean exposures concentrations 
reported in epidemiologic studies in other countries would compare to 
annual design values observed in the U.S. In addition, while 
information from Canadian studies can be useful in assessing the 
adequacy of the annual standard, differences in the exposure 
environments and population characteristics between the U.S. and other 
countries can affect the study-reported mean value and its relationship 
with the annual standard level. Sources and pollutant mixtures, as well 
as PM2.5 concentration gradients, may be different between 
countries, and the exposure environments in other countries may differ 
from those observed in the U.S. Furthermore, differences in population 
characteristics and population densities can also make it challenging 
to directly compare studies from countries outside of the U.S. to a 
design value in the U.S.
    As with the experimental studies discussed above, the 2022 PA 
focuses on epidemiologic studies assessed in the 2019 ISA and ISA 
Supplement that have the potential to be most informative in reaching 
decisions on the adequacy of the primary PM2.5 standards. 
The 2022 PA focuses on epidemiologic studies that provide strong 
support for ``causal'' or ``likely to be causal'' relationships with 
PM2.5 exposures in the 2019 ISA. Further, the 2022 PA also 
focuses on the health effect associations that are determined in the 
2019 ISA and ISA Supplement to be consistent across studies, coherent 
with the broader body of evidence (e.g., including animal and 
controlled human exposure studies), and robust to potential confounding 
by co-occurring pollutants and other factors.\73\ In particular the 
2022 PA considers the U.S. and Canadian epidemiologic studies to be 
more useful for reaching conclusions on the current standards than 
studies conducted in other countries, given that the results of the 
U.S. and Canadian studies are more directly applicable for quantitative 
considerations, whereas studies conducted in other countries reflect 
different populations, exposure characteristics, and air pollution 
mixtures. Additionally, epidemiologic studies outside of the U.S. and 
Canada generally reflect higher PM2.5 concentrations in 
ambient air than are currently found in the U.S., and are less relevant 
to informing questions about adequacy of the current standards.\74\ 
However, and as noted above, the 2022 PA also recognizes that while 
information from Canadian studies can be useful in assessing the 
adequacy of the annual standard, there are still important differences 
between the exposure environments in the U.S. and Canada and 
interpreting the data (e.g., mean concentrations) from the Canadian 
studies in the context of a U.S.-based standard may present challenges 
in directly and quantitatively informing questions regarding the 
adequacy of the current or potential alternative the levels of the 
annual standard. Lastly, the 2022 PA emphasizes multicity/multistate 
studies that examine health effect associations, as such studies are 
more encompassing of the diverse atmospheric conditions and population 
demographics in the U.S. than studies that focus on a single city or 
State. Figures 3-4 through 3-7 in the 2022 PA summarize the study 
details for the key U.S. and Canadian epidemiologic studies (U.S. EPA, 
2022b, section 3.3.3.2.1).\75\
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    \73\ As described in the Preamble to the ISAs (U.S. EPA, 2015), 
``the U.S. EPA emphasizes the importance of examining the pattern of 
results across various studies and does not focus solely on 
statistical significance or the magnitude of the direction of the 
association as criteria of study reliability. Statistical 
significance is influenced by a variety of factors including, but 
not limited to, the size of the study, exposure and outcome 
measurement error, and statistical model specifications. Statistical 
significance may be informative; however, it is just one of the 
means of evaluating confidence in the observed relationship and 
assessing the probability of chance as an explanation. Other 
indicators of reliability such as the consistency and coherence of a 
body of studies as well as other confirming data may be used to 
justify reliance on the results of a body of epidemiologic studies, 
even if results in individual studies lack statistical significance. 
Traditionally, statistical significance is used to a larger extent 
to evaluate the findings of controlled human exposure and animal 
toxicological studies. Understanding that statistical inferences may 
result in both false positives and false negatives, consideration is 
given to both trends in data and reproducibility of results. Thus, 
in drawing judgments regarding causality, the U.S. EPA emphasizes 
statistically significant findings from experimental studies, but 
does not limit its focus or consideration to statistically 
significant results in epidemiologic studies.''
    \74\ This emphasis on studies conducted in the U.S. or Canada is 
consistent with the approach in the 2012 and 2020 reviews of the PM 
NAAQS (U.S. EPA, 2011, section 2.1.3; U.S. EPA, 2020b, section 
3.2.3.2.1) and with approaches taken in other NAAQS reviews. 
However, the importance of studies in the U.S., Canada, and other 
countries in informing an ISA's considerations of the weight of the 
evidence that informs causality determinations is recognized.
    \75\ The cohorts examined in the studies included in Figure 3-4 
to Figure 3-7 of the 2022 PA include large numbers of individuals in 
the general population, and often also include those populations 
identified as at-risk (i.e., children, older adults, minority 
populations, and individuals with pre-existing cardiovascular and 
respiratory disease).
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    The key epidemiologic studies identified in the 2022 PA indicate 
generally positive and statistically significant associations between 
estimated PM2.5 exposures (short- or long-term) and 
mortality or morbidity across a range of ambient PM2.5 
concentrations (U.S. EPA, 2022b, section 3.3.3.2.1), report overall 
mean (or median) PM2.5 concentrations, and include those for 
which the years of PM2.5 air quality data used to estimate 
exposures overlap entirely with the years during which health events 
are reported.\76\ Additionally, for studies that estimate 
PM2.5 exposure using hybrid modeling approaches, the 2022 PA 
also considers the approach used to estimate PM2.5 
concentrations and the approach used to validate hybrid model 
predictions when evaluating those studies as key epidemiologic studies 
\77\ and focuses on those studies that use recent methods based on 
surfaces that are with fused with monitored PM2.5

[[Page 16243]]

concentration data (U.S. EPA, 2022b, section 3.3.3.2.1).
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    \76\ For some studies of long-term PM2.5 exposures, 
exposure is estimated from air quality data corresponding to only 
part of the study period, often including only the later years of 
the health data, and are not likely to reflect the full ranges of 
ambient PM2.5 concentrations that contributed to reported 
associations. While this approach can be reasonable in the context 
of an epidemiologic study that is evaluating health effect 
associations with long-term PM2.5 exposures, under the 
assumption that spatial patterns in PM2.5 concentrations 
are not appreciably different during time periods for which air 
quality information is not available (e.g., Chen et al., 2016), the 
2022 PA focuses on the distribution of ambient PM2.5 
concentrations that could have contributed to reported health 
outcomes. Therefore, the 2022 PA identifies studies as key 
epidemiologic studies when the years of air quality data and health 
data overlap in their entirety.
    \77\ Such studies are identified as those that use hybrid 
modeling approaches for which recent methods and models were used 
(e.g., recent versions and configurations of the air quality 
models); studies that are fused with PM2.5 data from 
national monitoring networks (i.e., FRM/FEM data); and studies that 
reported a thorough model performance evaluation for core years of 
the study.
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    Figure 1 below (U.S. EPA, 2022b, Figure 3-8) highlights the overall 
mean (or median) PM2.5 concentrations reported in key U.S. 
studies that use ground-based monitors alone to estimate long- or 
short-term PM2.5 exposure.\78\ For the small subset of 
studies with available information on the broader distributions of 
underlying data, Figure 1 below also identifies the study-period 
PM2.5 concentrations corresponding to the 25th and 10th 
percentiles of health events \79\ (see Appendix B, Section B.2 of the 
2022 PA for more information). Figure 2 (U.S. EPA, 2022a, Figure 3-14) 
presents overall means of predicted PM2.5 concentrations for 
key U.S. model-based epidemiologic studies that apply aspects of 
population-weighting, and the concentrations corresponding to the 25th 
and 10th percentiles of estimated exposures or health events \80\ when 
available (see Appendix B, section B.3 for additional information).\81\
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    \78\ Canadian studies that use ground-based monitors estimate 
long- or short-term PM2.5 exposures are found in Figure 
3-9 of the 2022 PA, including concentrations corresponding to the 
25th and 10th percentiles of estimated exposures or health events, 
when available (U.S. EPA, 2022b).
    \79\ That is, 25% of the total health events occurred in study 
locations with mean PM2.5 concentrations (i.e., averaged 
over the study period) below the 25th percentiles identified in 
Figure 3-8 of the 2022 PA and 10% of the total health events 
occurred in study locations with mean PM2.5 
concentrations below the 10th percentiles identified.
    \80\ For most studies in Figure 2 below (Figure 3-14 in the 2022 
PA), 25th percentiles of exposure estimates are presented. The 
exception is Di et al. (2017b), for which Figure 2 (U.S. EPA, 2022b, 
Figure 3-14) presents the short-term PM2.5 exposure 
estimates corresponding to the 25th and 10th percentiles of deaths 
in the study population (i.e., 25% and 10% of deaths occurred at 
concentrations below these concentrations). In addition, the authors 
of Di et al. (2017b) provided population-weighted exposure values. 
The 10th and 25th percentiles of these population-weighted exposure 
estimates are 7.9 and 9.5 [mu]g/m\3\, respectively.
    \81\ Overall mean (or median) PM2.5 concentrations 
reported in key Canadian studies that use model-based approaches to 
estimate long- or short-term PM2.5 concentrations and the 
concentrations corresponding to the 25th and 10th percentiles of 
estimated exposures or health events, when available are found in 
Figure 3-9 of the 2022 PA (U.S. EPA, 2022b).

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BILLING CODE 6560-50-P
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[[Page 16245]]


[GRAPHIC] [TIFF OMITTED] TR06MR24.030

BILLING CODE 6560-50-C
    Based on its evaluation of study-reported mean concentrations, the 
2022 PA notes that key epidemiologic studies conducted in the U.S. or 
Canada report generally positive and statistically significant 
associations between estimated PM2.5 exposures (short- or 
long-term) and mortality or morbidity across a wide range of ambient 
PM2.5 concentrations (U.S. EPA, 2022b, section 3.3.3.2.1). 
The 2022 PA makes a number of observations with regard to the study-
reported PM2.5 concentrations in the key U.S. and Canadian 
epidemiologic studies.
    The 2022 PA first considers the PM2.5 concentrations 
from the key U.S. epidemiologic studies. For studies that use monitors 
to estimate PM2.5 exposures, overall mean PM2.5 
concentrations range between 9.9 [mu]g/m\3\ \82\ to 16.5 [mu]g/m\3\ 
(Figure 1 above and

[[Page 16246]]

U.S. EPA, 2022b, Figure 3-8). For key U.S. epidemiologic studies that 
use hybrid model-predicted exposures and apply aspects of population-
weighting, mean PM2.5 concentrations range from 9.3 [mu]g/
m\3\ to just above 12.2 [micro]g/m\3\ (Figure 2 above and U.S. EPA, 
2022b, Figure 3-14). In studies that average up from the grid cell 
level to the ZIP code, postal code, or census tract level, mean 
PM2.5 concentrations range from 9.8 [micro]g/m\3\ to 12.2 
[micro]g/m\3\. The one study that population-weighted the grid cell 
prior to averaging up to the ZIP code or census tract level reported 
mean PM2.5 concentrations of 9.3 [mu]g/m\3\. Based on air 
quality analyses noted above, these hybrid modelled epidemiologic 
studies are expected to report means similar to those from monitor-
based studies.
---------------------------------------------------------------------------

    \82\ This is generally consistent with, but slightly below, the 
lowest study-reported mean PM2.5 concentration from 
monitor-based studies available in the 2020 PA, which was 10.7 
[mu]g/m\3\ (U.S. EPA, 2020a, Figure 3-7).
---------------------------------------------------------------------------

    Other key U.S. epidemiologic studies that use hybrid modeling 
approaches estimate mean PM2.5 exposure by averaging each 
grid cell across the entire study area, whether that be the nation or a 
region of the country. These studies do not weight the estimated 
exposure concentrations based on population density or location of 
health events. As such, the study mean reported in these studies may 
not reflect the exposure concentrations used in the epidemiologic study 
to assess the reported association. As a result, these reported mean 
concentrations are the most different (and much lower) than the means 
reported in monitor-based studies. Due to the methodology employed in 
calculating the study-reported means and not necessarily a difference 
in estimates of exposure, these epidemiologic studies are expected to 
report some of the lowest mean values. For these studies, the reported 
mean PM2.5 concentrations range from 8.1 [mu]g/m\3\ to 11.9 
[mu]g/m\3\ (U.S. EPA, 2022b, Figure 3-14). As noted above, for studies 
that utilize hybrid modeling approaches but do not incorporate 
population weighting into the reported mean calculation, the associated 
annual design values would be expected to be much higher (i.e., 40-50% 
higher) than the study-reported means. This larger difference between 
design values and study-reported mean concentrations makes it more 
difficult to consider how these studies can be used to determine the 
adequacy of the protection afforded by the current or potential 
alternative annual standards (U.S. EPA, 2022b, section 3.3.3.2.1).
    In addition to the mean PM2.5 concentrations, a subset 
of the key U.S. epidemiologic studies report PM2.5 
concentrations corresponding to the 25th and 10th percentiles of health 
data or exposure estimates to provide insight into the concentrations 
that comprise the lower quartile of the air quality distributions. In 
studies that use monitors to estimate PM2.5 exposures, 25th 
percentiles of health events correspond to PM2.5 
concentrations (i.e., averaged over the study period for each study 
city) at or above 11.5 [micro]g/m\3\ and 10th percentiles of health 
events correspond to PM2.5 concentrations at or above 9.8 
[micro]g/m\3\ (i.e., 25% and 10% of health events, respectively, occur 
in study locations with PM2.5 concentrations below these 
values) (Figure 1 above and U.S. EPA, 2022b, Figure 3-8). Of the key 
U.S. epidemiologic studies that use hybrid modeling approaches and 
apply population-weighting to estimate long-term PM2.5 
exposures, the ambient PM2.5 concentrations corresponding to 
25th percentiles of estimated exposures are 9.1 [micro]g/m\3\ (Figure 2 
and U.S. EPA, 2022b, Figure 3-14). In key U.S. epidemiologic studies 
that use hybrid modeling approaches and apply population-weighting to 
estimate short-term PM2.5 exposures, the ambient 
concentrations corresponding to 25th percentiles of estimated 
exposures, or health events, are 6.7 [micro]g/m\3\ (Figure 2 and U.S. 
EPA, 2022b, Figure 3-14). In key U.S. epidemiologic studies that use 
hybrid modeling approaches and do not apply population-weighting to 
estimate PM2.5 exposures, the ambient concentrations 
corresponding to 25th percentiles of estimated exposures, or health 
events, range from 4.6 to 9.2 [micro]g/m\3\ (U.S. EPA, 2022b, Figure 3-
14).\83\ In the key epidemiologic studies that apply hybrid modeling 
approaches with population-weighting and with information available on 
the 10th percentile of health events, the ambient PM2.5 
concentration corresponding to that 10th percentile range from 4.7 
[micro]g/m\3\ to 7.3 [micro]g/m\3\ (Figure 2 and U.S. EPA, 2022b, 
Figure 3-14).
---------------------------------------------------------------------------

    \83\ In the one study that reports 25th percentile exposure 
estimates of 4.6 [micro]g/m\3\ (Shi et al., 2016), the authors 
report that most deaths occurred at or above the 75th percentile of 
annual exposure estimates (i.e., 10 [mu]g/m\3\). The short-term 
exposure estimates accounting for most deaths are not presented in 
the published study.
---------------------------------------------------------------------------

    The 2022 PA next considers the PM2.5 concentrations from 
the key Canadian epidemiologic studies. Generally, the study-reported 
mean concentrations in Canadian studies are lower than those reported 
in the U.S. studies for both monitor-based and hybrid model methods. 
For the majority of key Canadian epidemiologic studies that use 
monitor-based exposure, mean PM2.5 concentrations generally 
ranged from 7.0 [micro]g/m\3\ to 9.0 [micro]g/m\3\ (U.S. EPA, 2022b, 
Figure 3-9). For these studies, 25th percentiles of health events 
correspond to PM2.5 concentrations at or above 6.5 [micro]g/
m\3\ and 10th percentiles of health events correspond to 
PM2.5 concentrations at or above 6.4 [micro]g/m\3\ (U.S. 
EPA, 2022b, Figure 3-9). For the key Canadian epidemiologic studies 
that use hybrid model-predicted exposure, the mean PM2.5 
concentrations are generally lower than in U.S. model-based studies 
(U.S. EPA, 2022b, Figure 3-10), ranging from approximately 6.0 
[micro]g/m\3\ to just below 10.0 [micro]g/m\3\ (U.S. EPA, 2022b, Figure 
3-11). The majority of the key Canadian epidemiologic studies that used 
hybrid modeling were completed at the nationwide scale, while four 
studies were completed at the regional geographic spatial scale. In 
addition, all the key Canadian epidemiologic studies apply aspects of 
population weighting, where all grid cells within a postal code are 
averaged, individuals are assigned exposure at the postal code 
resolution, and study mean PM2.5 concentrations are based on 
the average of individual exposures. The majority of studies estimating 
exposure nationwide range between just below 6.0 [micro]g/m\3\ to 8.0 
[micro]g/m\3\ (U.S. EPA, 2022b, Figure 3-11). One study by Erickson et 
al. (2020) presents an analysis related immigrant status and length of 
residence in Canada versus non-immigrant populations, which accounts 
for the four highest mean PM2.5 concentrations which range 
between 9.0 [micro]g/m\3\ and 10.0 [micro]g/m\3\ (U.S. EPA, 2022b, 
Figure 3-11). The four studies that estimate exposure at the regional 
scale report mean PM2.5 concentrations that range from 7.8 
[micro]g/m\3\ to 9.8 [micro]g/m\3\ (U.S. EPA, 2022b, Figure 3-11). 
Three key Canadian epidemiologic studies report information on the 25th 
percentile of health events. In these studies, the ambient 
PM2.5 concentration corresponding to the 25th percentile is 
approximately 8.0 [micro]g/m\3\ in two studies, and 4.3 [micro]g/m\3\ 
in a third study (U.S. EPA, 2022b, Figure 3-11).
    In addition to the expanded body of evidence from the key U.S. 
epidemiologic studies discussed above, there are also a subset of 
epidemiologic studies that have emerged that further inform an 
understanding of the relationship between PM2.5 exposure and 
health effects, including studies with the highest exposures excluded 
(restricted analyses), epidemiologic studies that employed statistical 
approaches that attempt to more extensively account for confounders and 
are more robust to model misspecification (i.e., used alternative

[[Page 16247]]

methods for confounder control),\84\ and accountability studies (U.S. 
EPA, 2019a, U.S. EPA, 2021a, U.S. EPA, 2022a).
---------------------------------------------------------------------------

    \84\ As noted in the ISA Supplement (U.S. EPA, 2022a, p. 1-3): 
``In the peer-reviewed literature, these epidemiologic studies are 
often referred to as alternative methods for confounder control. For 
the purposes of this Supplement, this terminology is not used to 
prevent confusion with the main scientific conclusions (i.e., the 
causality determinations) presented within an ISA. In addition, as 
is consistent with the weight-of-evidence framework used within ISAs 
and discussed in the Preamble to the Integrated Science Assessments, 
an individual study on its own cannot inform causality, but instead 
represents a piece of the overall body of evidence.''
---------------------------------------------------------------------------

    Restricted analyses are studies that examine health effect 
associations in analyses with the highest exposures excluded, 
restricting analyses to daily exposures less than the 24-hour primary 
PM2.5 standard and annual exposures less than the annual 
PM2.5 standard. The 2022 PA presents a summary of restricted 
analyses evaluated in the 2019 ISA and ISA Supplement (U.S. EPA, 2022b, 
Table 3-10). The restricted analyses can be informative in assessing 
the nature of the association between long-term exposures (e.g., annual 
average concentrations <12.0 [micro]g/m\3\) or short-term exposures 
(e.g., daily concentrations <35 [micro]g/m\3\) when looking only at 
exposures to lower concentrations, including whether the association 
persists in such restricted analyses compared to the same analyses for 
all exposures, as well as whether the association is stronger, in terms 
of magnitude and precision, than when completing the same analysis for 
all exposures. While these studies are useful in supporting the 
confidence and strength of associations at lower concentrations, these 
studies also have inherent uncertainties and limitations, including 
uncertainty in how studies exclude concentrations (e.g., are they 
excluded at the modeled grid cell level, the ZIP code level) and in how 
concentrations in studies that restrict air quality data relate to 
design values for the annual and 24-hour standards. Further, these 
studies often do not report descriptive statistics (e.g., mean 
PM2.5 concentrations, or concentrations at other 
percentiles) that allow for additional consideration of this 
information. As such, while these studies can provide additional 
supporting evidence for associations at lower concentrations, the 2022 
PA notes that there are also limitations in how to interpret these 
studies when evaluating the adequacy of the current or potential 
alternative standards.
    Restricted analyses provide additional information on the nature of 
the association between long- or short-term exposures when analyses are 
restricted to lower PM2.5 concentrations and indicate that 
effect estimates are generally greater in magnitude in the restricted 
analyses for long- and short-term PM2.5 exposure compared to 
the main analyses. In two U.S. studies that report mean 
PM2.5 concentrations in restricted analyses and that 
estimate effects associated with long-term exposure to 
PM2.5, the effect estimates are greater in the restricted 
analyses than in the main analyses. Di et al. (2017a) and Dominici et 
al. (2019) report positive and statistically significant associations 
in analyses restricted to concentrations less than 12.0 [micro]g/m\3\ 
for all-cause mortality and effect estimates are greater in the 
restricted analyses than effect estimates reported in main analyses. In 
addition, both studies report mean PM2.5 concentrations of 
9.6 [micro]g/m\3\. While none of the U.S. studies of short-term 
exposure present mean PM2.5 concentrations for the 
restricted analyses, these studies generally have mean 24-hour average 
PM2.5 concentrations in the main analyses below 12.0 
[micro]g/m\3\, and report increases in the effect estimates in the 
restricted analyses compared to the main analyses. Additionally, in the 
one Canadian study of long-term PM2.5 exposure, Zhang et al. 
(2021) conducted analyses where annual PM2.5 concentrations 
were restricted to concentrations below 10.0 [micro]g/m\3\ and 8.8 
[micro]g/m\3\, which presumably have lower mean concentrations than the 
mean of 7.8 [micro]g/m\3\ reported in the main analyses, though 
restricted analysis mean PM2.5 concentrations are not 
reported. Effect estimates for non-accidental mortality are greater in 
analyses restricted to PM2.5 concentrations less than 10.0 
[micro]g/m\3\, but less in analyses restricted to <8.8 [micro]g/m\3\.
    The second type of studies that have recently emerged and further 
inform the consideration of the relationship between PM2.5 
exposure and health effects in the 2022 PA are those that employ 
alternative methods for confounder control. Alternative methods for 
confounder control seek to mimic randomized experiments through the use 
of study design and statistical methods to more extensively account for 
confounders and are more robust to model misspecification. The 2022 PA 
presents a summary of the studies that employ alternative methods for 
confounder control, and employ a variety of statistical methods, which 
are evaluated in the 2019 ISA and ISA Supplement (U.S. EPA, 2022b, 
Table 3-11). These studies reported consistent results among large 
study populations across the U.S. and can further inform the 
relationship between long- and short-term PM2.5 exposure and 
total mortality. Studies that employ alternative methods for confounder 
control to assess the association between long-term exposure to 
PM2.5 and mortality reduce uncertainties related to 
confounding and provide additional support for the associations 
reported in the broader body of cohort studies that examined long-term 
PM2.5 exposure and mortality.
    Lastly, there is a subset of epidemiologic studies that assess 
whether long-term reductions in ambient PM2.5 concentrations 
result in corresponding reductions in health outcomes. These include 
studies that evaluate the potential for improvements in public health, 
including reductions in mortality rates, increases in life expectancy, 
and reductions in respiratory disease as ambient PM2.5 
concentrations have declined over time. Some of these studies, 
accountability studies, provide insight on whether the implementation 
of environmental policies or air quality interventions result in 
changes/reductions in air pollution concentrations and the 
corresponding effect on health outcomes.\85\ The 2022 PA presents a 
summary of these studies, which are assessed in the 2019 ISA and ISA 
Supplement (U.S. EPA, 2022b, Table 3-12). These studies lend support 
for the conclusion that improvements in air quality are associated with 
improvements in public health.
---------------------------------------------------------------------------

    \85\ Given the nature of these studies, the majority tend to 
focus on time periods in the past during which ambient 
PM2.5 concentrations were substantially higher than those 
measured more recently (e.g., see U.S. EPA, 2022b, Figure 2-16).
---------------------------------------------------------------------------

    More specifically, of the accountability studies that account for 
changes in PM2.5 concentrations due to a policy or the 
implementation of an intervention and whether there was evidence of 
changes in associations with mortality or cardiovascular effects as a 
result of changes in annual PM2.5 concentrations, Corrigan 
et al. (2018), Henneman et al. (2019) and Sanders et al. (2020a) 
present analyses with starting PM2.5 concentrations (or 
concentrations prior to the policy or intervention) below 12.0 
[micro]g/m\3\. Henneman et al. (2019) explored changes in modeled 
PM2.5 concentrations following the retirement of coal fired 
power plants in the U.S., and found that reductions from mean annual 
PM2.5 concentrations of 10.0 [micro]g/m\3\ in 2005 to mean 
annual PM2.5 concentrations of 7.2 [micro]g/m\3\ in 2012 
from coal-fueled power plants resulted in corresponding reductions in 
the number of cardiovascular-related

[[Page 16248]]

hospital admissions, including for all cardiovascular disease, acute 
MI, stroke, heart failure, and ischemic heart disease in those aged 65 
and older. Corrigan et al. (2018) examined whether there was a change 
in the cardiovascular mortality rate before (2000-2004) and after 
(2005-2010) implementation of the first annual PM2.5 NAAQS 
implementation based on mortality data from the National Center for 
Health Statistics and reported 1.10 (95% confidence interval (CI): 
0.37, 1.82) fewer cardiovascular deaths per year per 100,000 people for 
each 1 [mu]g/m\3\ reduction in annual PM2.5 concentrations. 
When comparing whether counties met the annual PM2.5 
standard (attainment counties), there were 1.96 (95% CI: 0.77, 3.15) 
fewer cardiovascular deaths for each 1 [mu]g/m\3\ reduction in annual 
PM2.5 concentrations between the two periods for attainment 
counties, whereas in non-attainment counties (e.g., counties that did 
not meet the annual PM2.5 standard), there were 0.59 (95% 
CI: - 0.54, 1.71) fewer cardiovascular deaths between the two periods. 
And lastly, Sanders et al. (2020a) examined whether policy actions 
(i.e., the first annual PM2.5 NAAQS implementation rule in 
2005 for the 1997 annual PM2.5 standard with a 3-year annual 
average of 15 [mu]g/m\3\) reduced PM2.5 concentrations and 
mortality rates in Medicare beneficiaries between 2000-2013. They 
report evidence of changes in associations with mortality (a decreased 
mortality rate of ~0.5 per 1,000 in attainment and non-attainment 
areas) due to changes in annual PM2.5 concentrations in both 
attainment and non-attainment areas. Additionally, attainment areas had 
starting concentrations below 12.0 [micro]g/m\3\ prior to 
implementation of the annual PM2.5 NAAQS in 2005. In 
addition, following implementation of the annual PM2.5 
NAAQS, annual PM2.5 concentrations decreased by 1.59 [mu]g/
m\3\ (95% CI: 1.39, 1.80) which corresponded to a reduction in 
mortality rates among individuals 65 years and older (0.93% [95% CI: 
0.10%, 1.77%]) in non-attainment counties relative to attainment 
counties. In a life expectancy study, Bennett et al. (2019) reports 
increases in life expectancy in all but 14 counties (1325 of 1339 
counties) that have exhibited reductions in PM2.5 
concentrations from 1999 to 2015. These studies provide support for 
improvements in public health following the implementation of policies, 
including in areas with PM2.5 concentrations below the level 
of the current annual standard, as well as increases in life expectancy 
in areas with reductions in PM2.5 concentrations.
d. Uncertainties in the Health Effects Evidence
    The 2022 PA recognizes that there are a number of uncertainties and 
limitations associated with the available health effects evidence. 
Although the epidemiologic studies clearly demonstrate associations 
between long- and short-term PM2.5 exposures and health 
outcomes, several uncertainties and limitations in the health effects 
evidence remain. Epidemiologic studies evaluating short-term 
PM2.5 exposure and health effects have reported 
heterogeneity in associations between cities and geographic regions 
within the U.S. Heterogeneity in the associations observed across 
epidemiologic studies may be due in part to exposure error related to 
measurement-related issues, the use of central fixed-site monitors to 
represent population exposure to PM2.5, and a limited 
understanding of factors including exposure error related to 
measurement-related issues, variability in PM2.5 composition 
regionally, and factors that result in differential exposures (e.g., 
topography, the built environment, housing characteristics, personal 
activity patterns). Heterogeneity is expected when the methods or the 
underlying distribution of covariates vary across studies (U.S. EPA, 
2019a, p. 6-221). Studies assessed in the 2019 ISA and ISA Supplement 
have advanced the state of exposure science by presenting innovative 
methodologies to estimate PM exposure, detailing new and existing 
measurement and modeling methods, and further informing our 
understanding of the influence of exposure measurement error due to 
exposure estimation methods on the associations between 
PM2.5 and health effects reported in epidemiologic studies 
(U.S. EPA, 2019a, section 1.2.2; U.S. EPA, 2022a). Data from 
PM2.5 monitors continue to be commonly used in health 
studies as a surrogate for PM2.5 exposure, and often provide 
a reasonable representation of exposures throughout a study area (U.S. 
EPA, 2019a, section 3.4.2.2; U.S. EPA, 2022a, section 3.2.2.2.2). 
However, an increasing number of studies employ hybrid modeling methods 
to estimate PM2.5 exposure using data from several sources, 
often including satellites and models, in addition to ground-based 
monitors. These hybrid models typically have good cross-validation, 
especially for PM2.5, and have the potential to reduce 
exposure measurement error and uncertainty in the health effect 
estimates from epidemiologic models of long-term exposure (U.S. EPA, 
2019a, section 3.5; U.S. EPA, 2022a, section 2.3.3).
    While studies using hybrid modeling methods have reduced exposure 
measurement error and uncertainty in the health effect estimates, these 
studies use a variety of approaches to estimate PM2.5 
concentrations and to assign exposure to assess the association between 
health outcomes and PM2.5 exposure. This variability in 
methodology has inherent limitations and uncertainties, as described in 
more detail in section 2.3.3.1.5 of the 2022 PA, and the performance of 
the modeling approaches depends on the availability of monitoring data 
which varies by location. Factors that likely contribute to poorer 
model performance often coincide with relatively low ambient 
PM2.5 concentrations, in areas where predicted exposures are 
at a greater distance to monitors, and under conditions where the 
reliability and availability of key datasets (e.g., air quality 
modeling) are limited. Thus, uncertainty in hybrid model predictions 
becomes an increasingly important consideration as lower predicted 
concentrations are considered.
    Regardless of whether a study uses monitoring data or a hybrid 
modeling approach when estimating PM2.5 exposures, one key 
limitation that persists is associated with the interpretation of the 
study-reported mean PM2.5 concentrations and how they 
compare to design values, the metric that describes the air quality 
status of a given area relative to the NAAQS.\86\ As discussed above in 
section II.B.3.b, the overall mean PM2.5 concentrations 
reported by key epidemiologic studies reflect averaging of short- or 
long-term PM2.5 exposure estimates across location (i.e., 
across multiple monitors or across modeled grid cells) and over time 
(i.e., over several years). For monitor-based studies, the comparison 
is somewhat more straightforward than for studies that use hybrid 
modeling methods, as the monitors used to estimate exposure in the 
epidemiologic studies are generally the same monitors that are used to 
calculate design values for a given area. It is expected that areas 
meeting a PM2.5 standard with a particular level would be 
expected to have average PM2.5 concentrations (i.e., 
averaged across space and over time in the area) somewhat below that 
standard level., but the difference between the maximum annual design 
value and

[[Page 16249]]

average concentration in an area can be smaller or larger than analyses 
presented above in section I.D.5.a, likely depending on factors such as 
the number of monitors, monitor siting characteristics, and the 
distribution of ambient PM2.5 concentrations. For studies 
that use hybrid modeling methods to estimate PM2.5 
concentrations, the comparison between study-reported mean 
PM2.5 concentrations and design values is more complicated 
given the variability in the modeling methods, temporal scales (i.e., 
daily versus annual), and spatial scales (i.e., nationwide versus 
urban) across studies. Analyses above in section I.D.5.b and detailed 
more in the 2022 PA (U.S. EPA, 2022b, section 2.3.3.2.4) present a 
comparison between two hybrid modeling surfaces, which explored the 
impact of these factors on the resulting mean PM2.5 
concentrations and provided additional information about the 
relationship between mean concentrations from studies using hybrid 
modeling methods and design values. However, the results of those 
analyses only reflect two surfaces and two types of approaches, so 
uncertainty remains in understanding the relationship between estimated 
modeled PM2.5 concentrations and design values more broadly 
across hybrid modeling studies. Moreover, this analysis was completed 
using two hybrid modeling methods that estimate PM2.5 
concentrations in the U.S., thus an additional uncertainty includes 
understanding the relationship between modeled PM2.5 
concentrations and design values reported in Canada.
---------------------------------------------------------------------------

    \86\ For the annual PM2.5 standard, design values are 
calculated as the annual arithmetic mean PM2.5 
concentration, averaged over 3 years. For the 24-hour standard, 
design values are calculated as the 98th percentile of the annual 
distribution of 24-hour PM2.5 concentrations, averaged 
over three years (Appendix N of 40 CFR part 50).
---------------------------------------------------------------------------

    In addition, where PM2.5 and other pollutants (e.g., 
ozone, nitrogen dioxide, and carbon monoxide) are correlated, it can be 
difficult to distinguish whether attenuation of effects in some studies 
results from copollutant confounding or collinearity with other 
pollutants in the ambient mixture (U.S. EPA, 2019a, section 1.5.1; U.S. 
EPA, 2022a, section 2.2.1). Studies evaluated in the 2019 ISA and ISA 
Supplement further examined the potential confounding effects of both 
gaseous and particulate copollutants on the relationship between long- 
and short-term PM2.5 exposure and health effects. As noted 
in the Appendix to the 2019 ISA (U.S. EPA, 2019a, Table A-1), 
copollutant models are not without their limitations, such as instances 
for which correlations are high between pollutants resulting in greater 
copollutant confounding bias in results. However, the studies continue 
to provide evidence indicating that associations with PM2.5 
are relatively unchanged in copollutants models (U.S. EPA, 2019a, 
section 1.5.1; U.S. EPA, 2022a, section 2.2.1).
    Another area of uncertainty is associated with other potential 
confounders, beyond copollutants. Some studies have expanded the 
examination of potential confounders to not only include copollutants, 
but also systematic evaluations of the potential impact of inadequate 
control from long-term temporal trends and weather (U.S. EPA, 2019a, 
section 11.1.5.1). Analyses examining these covariates further confirm 
that the relationship between PM2.5 exposure and mortality 
is unlikely to be biased by these factors. Other studies have explored 
the use of alternative methods for confounder control to more 
extensively account for confounders and are more robust to model 
misspecification that can further inform the causality determination 
for long-term and short-term PM2.5 and mortality and 
cardiovascular effects (U.S. EPA, 2019a, section 11.2.2.4; U.S. EPA, 
2022a, sections 3.1.1.3, 3.1.2.3, 3.2.1.2, and 3.2.2.3). These studies 
indicate that bias from unmeasured confounders can occur in either 
direction, although controlling for these confounders did not result in 
the elimination of the association, but instead provided additional 
support for associations between long-term PM2.5 exposure 
and mortality when accounting for additional confounders (U.S. EPA, 
2022a, section 3.2.2.2.6).
    Another important limitation associated with the evidence is that, 
while epidemiologic studies indicate associations between 
PM2.5 and health effects, the currently available evidence 
does not identify particular PM2.5 concentrations that do 
not elicit health effects. Rather, health effects can occur over the 
entire distribution of ambient PM2.5 concentrations 
evaluated, and epidemiologic studies conducted to date do not identify 
a population-level threshold below which it can be concluded with 
confidence that PM2.5-related effects do not occur.
    Overall, evidence assessed in the 2019 ISA and ISA Supplement 
continues to indicate a linear, no-threshold C-R relationship for 
PM2.5 concentrations >8 [mu]g/m\3\. However, uncertainties 
remain about the shape of the C-R curve at PM2.5 
concentrations <8 [mu]g/m\3\, with some recent studies providing 
evidence for either a sublinear, linear, or supralinear relationship at 
these lower concentrations (U.S. EPA, 2019a, section 11.2.4; U.S. EPA, 
2022a, section 2.2.3.2).
    There are also a number of uncertainties and limitations associated 
with the experimental evidence (i.e., controlled human exposure studies 
and animal toxicological studies). With respect to controlled human 
exposure studies, the PA recognizes that these studies include a small 
number of individuals compared to epidemiologic studies. Additionally, 
these studies tend to include generally healthy adult individuals, who 
are at a lower risk of experiencing health effects. These studies, 
therefore, often do not include populations that are at increased risk 
of PM2.5-related health effects, including children, older 
adults, or individuals with pre-existing conditions. As such, these 
studies are somewhat limited in their ability to inform at what 
concentrations effects may be elicited in at-risk populations. With 
respect to animal toxicological studies, while these studies often 
examine more severe health outcomes and longer exposure durations and 
higher exposure concentrations than controlled human exposure studies, 
there is uncertainty in extrapolating the effects seen in animals, and 
the PM2.5 exposures and doses that cause those effects, to 
human populations.
    Consideration of health effects are informed by the epidemiologic, 
controlled human exposure, and animal toxicological studies. The 
evaluation and integration of the scientific evidence in the ISA 
focuses on evaluating the findings from the body of evidence across 
disciplines, including evaluating the strengths and weaknesses in the 
overall collection of studies across disciplines. Integrating evidence 
across disciplines can strengthen causal inference, such that a weak 
inference from one line of evidence can be addressed by other lines of 
evidence, and coherence of these lines of evidence can add support to a 
cause-effect interpretation of the association. Evaluation and 
integration of the evidence also includes consideration of 
uncertainties that are inherent in the scientific findings (U.S. EPA, 
2015, pp. 13-15), some of which are described above.
3. Summary of Exposure and Risk Estimates
    Beyond the consideration of the scientific evidence, discussed 
above in section II.B, the EPA also considers the extent to which new 
or updated quantitative analyses of PM2.5 air quality, 
exposure, or health risks could inform conclusions on the adequacy of 
the public health protection provided by the current primary 
PM2.5 standards. Additionally, the 2022 PA includes an at-
risk analysis that assesses PM2.5-attributable risk 
associated with PM2.5 air quality that has been adjusted to 
simulate air quality scenarios of policy

[[Page 16250]]

interest (e.g., ``just meeting'' the current or potential alternative 
standards). Drawing on the summary in section II.C of the proposal, the 
sections below provide a brief overview of key aspects of the 
assessment design (II.A.3.a), key limitations and uncertainties 
(II.A.3.b), and exposure/risk estimates (II.A.3.c).
a. Key Design Aspects
    Risk assessments combine data from multiple sources and involve 
various assumptions and uncertainties. Input data for these analyses 
includes C-R functions from epidemiologic studies for each health 
outcome and ambient annual or 24-hour PM2.5 concentrations 
for the study areas utilized in the risk assessment (U.S. EPA, 2022b, 
section 3.4.1). Additionally, quantitative and qualitative methods were 
used to characterize variability and uncertainty in the risk estimates 
(U.S. EPA, 2022b, section 3.4.1.7).
    Concentration-response functions used in the risk assessment are 
from large, multicity U.S. epidemiologic studies that evaluate the 
relationship between PM2.5 exposures and mortality. 
Epidemiologic studies and concentration-response studies that were used 
in the risk assessment to estimate risk were identified using criteria 
that take into account factors such as study design, geographic 
coverage, demographic populations, and health endpoints (U.S. EPA, 
2022b, section 3.4.1.1).\87\ The risk assessment focuses on all-cause 
or nonaccidental mortality associated with long-term and short-term 
PM2.5 exposures, for which the 2019 ISA concluded that the 
evidence provides support for a ``causal relationship'' (U.S. EPA, 
2022b, section 3.4.1.2).\88\
---------------------------------------------------------------------------

    \87\ Additional detail regarding the selection of epidemiologic 
studies and specification of C-R functions is provided in the 2022 
PA (U.S. EPA, 2022b, Appendix C, section C.1.1).
    \88\ While the 2019 ISA also found that evidence supports the 
determination of a ``causal relationship'' between long- and short-
term PM2.5 exposures and cardiovascular effects, 
cardiovascular mortality was not included as a health outcome as it 
will be captured in the estimates of all-cause mortality.
---------------------------------------------------------------------------

    As described in more detail in the 2022 PA, the risk assessment 
first estimated health risks associated with air quality for 2015 
adjusted to simulate ``just meeting'' the current primary 
PM2.5 standards (i.e., the annual standard with its level of 
12.0 [micro]g/m\3\ and the 24-hour standard with its level of 35 
[micro]g/m\3\). Air quality modeling was then used to simulate air 
quality just meeting an alternative standard with a level of 10.0 
[micro]g/m\3\ (annual) and 30 [micro]g/m\3\ (24-hour). In addition to 
the model-based approach, for the subset of 30 areas controlled by the 
annual standard linear interpolation and extrapolation were employed to 
simulate just meeting alternative annual standards with levels of 11.0 
(interpolated between 12.0 and 10.0 [mu]g/m\3\), 9.0 [mu]g/m\3\, and 
8.0 [mu]g/m\3\ (both extrapolated from 12.0 and 10.0 [mu]g/m\3\) (U.S. 
EPA, 2022b, section 3.4.1.3). The 2022 PA notes that there is greater 
uncertainty regarding whether a revised 24-hour standard (i.e., with a 
lower level) is needed to further limit ``peak'' PM2.5 
concentration exposure and whether a lower 24-hour standard level would 
most effectively reduce PM2.5-associated health risks 
associated with ``typical'' daily exposures. The risk assessment 
estimates health risks associated with air quality adjusted to meet a 
revised 24-hour standard with a level of 30 [micro]g/m\3\, in 
conjunction with estimating the health risks associated with meeting a 
revised annual standard with a level of 10.0 [micro]g/m\3\ (U.S. EPA, 
2022b, section 3.4.1.3). More details on the air quality adjustment 
approaches used in the risk assessment are described in section 3.4.1.4 
and Appendix C of the 2022 PA (U.S. EPA, 2022b).
    When selecting U.S. study areas for inclusion in the risk 
assessment, the available ambient monitors, geographic diversity, and 
ambient PM2.5 air quality concentrations were taken into 
consideration (U.S. EPA, 2022b, section 3.4.1.4). When these factors 
were applied, 47 urban study areas were identified, which include 
nearly 60 million people aged 30-99, or approximately 30% of the U.S 
population in this age range (U.S. EPA, 2022b, section 3.4.1.5, 
Appendix C, section C.1.3). Of the 47 study areas, there were 30 study 
areas where just meeting the current standards is controlled by the 
annual standard,\89\ 11 study areas where just meeting the current 
standards is controlled by the daily standard,\90\ and 6 study areas 
where the controlling standard differed depending on the air quality 
adjustment approach (U.S. EPA, 2022b, section 3.4.1.5).\91\
---------------------------------------------------------------------------

    \89\ For these areas, the annual standard is the ``controlling 
standard'' because when air quality is adjusted to simulate just 
meeting the current or potential alternative annual standards, that 
air quality also would meet the 24-hour standard being evaluated.
    \90\ For these areas, the 24-hour standard is the controlling 
standard because when air quality is adjusted to simulate just 
meeting the current or potential alternative 24-hour standards, that 
air quality also would meet the annual standard being evaluated. 
Some areas classified as being controlled by the 24-hour standard 
also violate the annual standard.
    \91\ In these 6 areas, the controlling standard depended on the 
air quality adjustment method used and/or the standard scenarios 
evaluated.
---------------------------------------------------------------------------

    In addition to the overall risk assessment, the 2022 PA also 
includes an at-risk analysis and estimates exposures and health risks 
of specific populations identified as at-risk that would be allowed 
under the current and potential alternative standards to further inform 
the Administrator's conclusions regarding the adequacy of the public 
health protection provided by the current primary PM2.5 
standards. In so doing, the 2022 PA evaluates exposure and 
PM2.5 mortality risk for older adults (e.g., 65 years and 
older), stratified for White, Black, Asian, Native American, Non-
Hispanic, and Hispanic individuals residing in the same study areas 
included in the overall risk assessment. This analysis utilizes a 
recent epidemiologic study that provides race- and ethnicity-specific 
risk coefficients (Di et al., 2017b).
b. Key Limitations and Uncertainties
    Uncertainty in risk estimates (e.g., in the size of risk estimates) 
can result from a number of factors, including the assumptions about 
the shape of the C-R function with mortality at low ambient PM 
concentrations, the potential for confounding and/or exposure 
measurement error in the underlying epidemiologic studies, and the 
methods used to adjust PM2.5 air quality. More specifically, 
the use of air quality modeling to adjust PM2.5 
concentrations are limited as they rely on model predictions, are based 
on emission changes scaled by fixed percentages, and use only two of 
the full set of possible emission scenarios and linear interpolation/
extrapolation to adjust air quality that may not fully capture 
potential non-linearities associated with real-world changes in air 
quality. Additionally, the selection of case study areas is limited to 
urban areas predominantly located CA and in the Eastern U.S. that are 
controlled by the annual standard. While the risk assessment does not 
report quantitative uncertainty in the risk estimates as exposure 
concentrations are reduced, it does provide information on the 
distribution of concentrations associated with the risk estimates when 
evaluating progressively lower alternative annual standards. Based on 
these data, as lower alternative annual standards are evaluated, larger 
proportions of the distributions in risk occur at or below 10 [mu]g/
m\3\ (at concentrations below or near most of the study-reported means 
from the key U.S. epidemiologic studies) and at or below 8 [mu]g/m\3\ 
(the concentration at which the ISA reports increasing uncertainty in 
the shape of the C-R curve based on the body of epidemiologic 
evidence).

[[Page 16251]]

    Similarly, the at-risk analysis is also subject to many of these 
same uncertainties noted above. Additionally, the at-risk analysis 
included C-R functions from only one study (Di et al., 2017b), which 
reported associations between long-term PM2.5 exposures and 
mortality, stratified by race/ethnicity, in populations age 65 and 
older, as opposed to the multiple studies used in the overall risk 
assessment to convey risk estimate variability. These and other sources 
of uncertainty in the overall risk assessment and the at-risk analyses 
are characterized in more depth in the 2022 PA (U.S. EPA, 2022b, 
section 3.4.1.7, section 3.4.1.8, Appendix C, section C.3).
c. Summary of Risk Estimates
    Although limitations in the underlying data and approaches lead to 
some uncertainty regarding estimates of PM2.5-associated 
risk, the risk assessment estimates that the current primary 
PM2.5 standards could allow a substantial number of 
PM2.5-associated deaths in the U.S. For example, when air 
quality in the 47 study areas is adjusted to simulate just meeting the 
current standards, the risk assessment estimates up to 45,100 deaths in 
2015 are attributable to long-term PM2.5 exposures 
associated with just meeting the current annual and 24-hour 
PM2.5 standards (U.S. EPA, 2022b, section 3.4.2.1). 
Additionally, as described in more detail in the 2022 PA, the at-risk 
analysis suggests that a lower annual standard level (i.e., below 12 
[micro]g/m\3\ and down as low as 8 [micro]g/m\3\) will help to reduce 
PM2.5 exposure and may also help to mitigate exposure and 
risk disparities in populations identified as particularly at-risk for 
adverse effects from PM exposures (i.e., minority populations).
    Compared to the current annual standard, meeting a revised annual 
standard with a lower level is estimated to reduce PM2.5-
associated health risks in the 30 study areas controlled by the annual 
standard by about 7-9% for a level of 11.0 [micro]g/m\3\, 15-19% for a 
level of 10.0 [micro]g/m\3\, 22-28% for a level of 9.0 [micro]g/m\3\, 
and 30-37% for a level of 8.0 [micro]g/m\3\) (U.S. EPA, 2022b, Table 3-
17). Meeting a revised annual standard with a lower level may also help 
to mitigate exposure and risk disparities in populations identified as 
particularly at-risk for adverse effects from PM exposures (i.e., 
minority populations) in simulated scenarios just meeting alternative 
annual standards. However, though reduced, disparities by race and 
ethnicity persist even at an alternative annual standard level of 8 
[micro]g/m\3\, the lowest alternative annual standard included in the 
risk assessment (U.S. EPA, 2022b, section 3.4.2.4).
    Revising the level of the 24-hour standard to 30 [mu]g/m\3\ is 
estimated to lower PM2.5-associated risks across a more 
limited population and number of areas than revising the annual 
standard (U.S. EPA, 2022, section 3.4.2.4). Risk reduction predictions 
are largely confined to areas located in the western U.S., several of 
which are also likely to experience risk reductions upon meeting a 
revised annual standard. In the 11 areas controlled by the 24-hour 
standard, when air quality is simulated to just meet the current 24-
hour standard, PM2.5 exposures are estimated to be 
associated with as many as 2,570 deaths annual. Compared to just 
meeting the current standard, air quality just meeting an alternative 
24-hour standard level of 30 [micro]g/m\3\ is associated with 
reductions in estimated risk of 9-13% (U.S. EPA, 2022b, section 
3.4.2.3).

B. Conclusions on the Primary PM2.5 Standards

    In drawing conclusions on the adequacy of the current primary 
PM2.5 standards, in view of the advances in scientific 
knowledge and additional information now available, the Administrator 
has considered the evidence base, information, and policy judgments 
that were the foundation of the 2012 and 2020 reviews and reflects upon 
the body of evidence and information newly available in this 
reconsideration. In so doing, the Administrator has taken into account 
both evidence-based and risk-based considerations, as well as advice 
from the CASAC and public comments. Evidence-based considerations draw 
upon the EPA's integrated assessment of the scientific evidence of 
health effects related to PM2.5 exposure presented in the 
2019 ISA and ISA Supplement (summarized in the proposal in sections 
II.B (88 FR 5580, January 27, 2023) and II.D.2.a (88 FR 5609, January 
27, 2023), and also in section II.A.2 above) to address key policy-
relevant questions in the reconsideration. Similarly, the risk-based 
considerations draw upon the assessment of population exposure and risk 
(summarized in the proposal in sections II.C (88 FR 5605, January 27, 
2023) and II.D.2.b (88 FR 5614, January 27, 2023), and also in section 
II.A.3 above) in addressing policy-relevant questions focused on the 
potential for PM2.5 exposures associated with mortality 
under air quality conditions just meeting the current and potential 
alternative standards.
    The approach to reviewing the primary standards is consistent with 
requirements of the provisions of the CAA related to the review of the 
NAAQS and with how the EPA and the courts have historically interpreted 
the CAA. As discussed in section I.A above, these provisions require 
the Administrator to establish primary standards that, in the 
Administrator's judgment, are requisite (i.e., neither more nor less 
stringent than necessary) to protect public health with an adequate 
margin of safety. Consistent with the Agency's approach across all 
NAAQS reviews, the EPA's approach to informing these judgments is based 
on a recognition that the available health effects evidence generally 
reflects a continuum that includes ambient air exposures for which 
scientists generally agree that health effects are likely to occur 
through lower levels at which the likelihood and magnitude of response 
become increasingly uncertain. The CAA does not require the 
Administrator to establish a primary standard at a zero-risk level or 
at background concentration levels, but rather at a level that reduces 
risk sufficiently so as to protect public health, including the health 
of sensitive groups, with an adequate margin of safety.
    The decisions on the adequacy of the current primary 
PM2.5 standards described below is a public health policy 
judgment by the Administrator that draws on the scientific evidence for 
health effects, quantitative analyses of population exposures and/or 
health risks, and judgments about how to consider the uncertainties and 
limitations that are inherent in the scientific evidence and 
quantitative analyses. The four basic elements of the NAAQS (i.e., 
indicator, averaging time, form, and level) have been considered 
collectively in evaluating the public health protection afforded by the 
current standards.
    Section II.B.2 below briefly summarizes the basis for the 
Administrator's proposed decision, drawing from section II.D.3 of the 
proposal (88 FR 5617, January 27, 2023). The advice and recommendations 
of the CASAC and public comments on the proposed decision are addressed 
below in sections II.B.1 and II.B.3, respectively. The Administrator's 
final conclusions in this reconsideration regarding the adequacy of the 
current primary PM2.5 standards and whether any revisions 
are appropriate are described in section II.B.4.
1. CASAC Advice
    As part of its review of the 2019 draft PA, the CASAC provided 
advice on the adequacy of the public health protection afforded by the 
current primary PM2.5 standards. Its advice is documented in

[[Page 16252]]

a letter sent to the EPA Administrator (Cox, 2019b). In this letter, 
the committee recommended retaining the current 24-hour 
PM2.5 standard but did not reach consensus on whether the 
scientific and technical information support retaining or revising the 
current annual standard. In particular, though the CASAC agreed that 
there is a long-standing body of health evidence supporting 
relationships between PM2.5 exposures and various health 
outcomes, including mortality and serious morbidity effects, individual 
CASAC members ``differ[ed] in their assessments of the causal and 
policy significance of these associations'' (Cox, 2019b, p. 8 of 
consensus responses). Drawing from this evidence, ``some CASAC 
members'' expressed support for retaining the current annual standard 
while ``other members'' expressed support for revising that standard in 
order to increase public health protection (Cox, 2019b, p.1 of letter). 
These views are summarized below.
    The CASAC members who supported retaining the current annual 
standard expressed the view that substantial uncertainty remains in the 
evidence for associations between PM2.5 exposures and 
mortality or serious morbidity effects. These committee members 
asserted that ``such associations can reasonably be explained in light 
of uncontrolled confounding and other potential sources of error and 
bias'' (Cox, 2019b, p. 8 of consensus responses). They noted that 
associations do not necessarily reflect causal effects, and they 
contended that recent epidemiologic studies assessed in the 2019 ISA 
that report positive associations at lower estimated exposure 
concentrations mainly confirm what was anticipated or already assumed 
in setting the 2012 NAAQS. In particular, they concluded that such 
studies have some of the same limitations as prior studies and do not 
provide new information calling into question the existing standard. 
They further asserted that ``accountability studies provide potentially 
crucial information about whether and how much decreasing 
PM2.5 causes decreases in future health effects'' (Cox, 
2019b, p. 10 of consensus responses), and they cited recent reviews 
(i.e., Henneman et al., 2017; Burns et al., 2019) to support their 
position that in such studies, ``reductions of PM2.5 
concentrations have not clearly reduced mortality risks'' (Cox, 2019b, 
p. 8 of consensus responses). Thus, the committee members who supported 
retaining the current annual standard advise that, ``while the data on 
associations should certainly be carefully considered, this data should 
not be interpreted more strongly than warranted based on its 
methodological limitations'' (Cox, 2019b, p. 8 of consensus responses).
    These members of the CASAC further concluded that the quantitative 
risk assessment included in the 2019 draft PA does not provide a valid 
basis for revising the current standards. This conclusion was based on 
concerns that (1) ``the risk assessment treats regression coefficients 
as causal coefficients with no justification or validation provided for 
this decision;'' (2) the estimated regression concentration-response 
functions ``have not been adequately adjusted to correct for 
confounding, errors in exposure estimates and other covariates, model 
uncertainty, and heterogeneity in individual biological (causal) 
[concentration-response] functions;'' (3) the estimated concentration-
response functions ``do not contain quantitative uncertainty bands that 
reflect model uncertainty or effects of exposure and covariate 
estimation errors;'' and (4) ``no regression diagnostics are provided 
justifying the use of proportional hazards . . . and other modeling 
assumptions'' (Cox, 2019b, p. 9 of consensus responses). These 
committee members also contended that details regarding the derivation 
of concentration-response functions, including specification of the 
beta values and functional forms, were not well-documented, hampering 
the ability of readers to evaluate these design details. Thus, these 
members ``think that the risk characterization does not provide useful 
information about whether the current standard is protective'' (Cox, 
2019b, p. 11 of consensus responses).
    Drawing from their evaluation of the evidence and the risk 
assessment in the 2019 draft PA, these committee members concluded that 
``the Draft PM PA does not establish that new scientific evidence and 
data reasonably call into question the public health protection 
afforded by the . . . 2012 PM2.5 annual standard'' (Cox, 
2019b, p.1 of letter).
    In contrast, ``[o]ther members of CASAC conclude[d] that the weight 
of the evidence, particularly reflecting recent epidemiology studies 
showing positive associations between PM2.5 and health 
effects at estimated annual average PM2.5 concentrations 
below the current standard, does reasonably call into question the 
adequacy of the 2012 annual PM2.5 [standard] to protect 
public health with an adequate margin of safety'' (Cox, 2019b, p.1 of 
letter). The committee members who supported this conclusion noted that 
the body of health evidence for PM2.5 not only includes the 
repeated demonstration of associations in epidemiologic studies, but 
also includes support for biological plausibility established by 
controlled human exposure and animal toxicology studies. They pointed 
to recent studies demonstrating that the associations between 
PM2.5 and health effects occur in a diversity of locations, 
in different time periods, with different populations, and using 
different exposure estimation and statistical methods. They concluded 
that ``the entire body of evidence for PM health effects justifies the 
causality determinations made in the Draft PM ISA'' (Cox, 2019b, p. 8 
of consensus responses).
    The members of the CASAC who supported revising the current annual 
standard particularly emphasized recent findings of associations with 
PM2.5 in areas with average long-term PM2.5 
concentrations below the level of the annual standard and studies that 
show positive associations even when estimated exposures above 12 
[mu]g/m\3\ are excluded from analyses. They found it ``highly 
unlikely'' that the extensive body of evidence indicating positive 
associations at low estimated exposures could be fully explained by 
confounding or by other non-causal explanations (Cox, 2019b, p. 8 of 
consensus responses). They additionally concluded that ``the risk 
characterization does provide a useful attempt to understand the 
potential impacts of alternate standards on public health risks'' (Cox, 
2019b, p. 11 of consensus responses). These CASAC members concluded 
that the available evidence reasonably calls into question the 
protection provided by the current primary PM2.5 standards 
and supports revising the annual standard to increase that protection 
(Cox, 2019b).
    As a part of this reconsideration, the CASAC reviewed the 2021 
draft PA (developed to support the reconsideration as described in 
section I.C.5 above). As a part of their review of the 2021 draft PA, 
the CASAC provided advice on the adequacy of the current primary 
PM2.5 standards. The range of views summarized here 
generally reflects differing judgments as to the relative weight to 
place on various types of evidence, the risk-based information, and the 
associated uncertainties, as well as differing judgments about the 
importance of various PM2.5-related health effects from a 
public health perspective.
    In its comments on the 2021 draft PA, the CASAC stated that: 
``[o]verall the CASAC finds the Draft PA to be well-

[[Page 16253]]

written and appropriate for helping to `bridge the gap' between the 
agency's scientific assessments and quantitative technical analyses, 
and the judgments required of the Administrator in determining whether 
it is appropriate to retain or revise the National Ambient Air Quality 
Standards (NAAQS)'' (Sheppard, 2022a, p. 1 of consensus letter). The 
CASAC also stated that the ``[d]raft PA adequately captures and 
appropriately characterizes the key aspects of the evidence assessed 
and integrated in the 2019 ISA and Draft ISA Supplement of 
PM2.5-related health effects'' (Sheppard, 2022b, p. 2 of 
consensus letter). The CASAC also stated that ``[t]he interpretation of 
the risk assessment for the purpose of evaluating the adequacy of the 
current primary PM2.5 annual standard is appropriate given 
the scientific findings presented'' (Sheppard, 2022a, p. 2 of consensus 
letter).
    With regard to the adequacy of the current primary annual 
PM2.5 standard, ``all CASAC members agree that the current 
level of the annual standard is not sufficiently protective of public 
health and should be lowered'' (Sheppard, 2022a, p. 2 of consensus 
letter). Additionally, ``the CASAC reached consensus that the 
indicator, form, and averaging time should be retained, without 
revision'' (Sheppard, 2022a, p. 2 of consensus letter). With regard to 
the level of the primary annual PM2.5 standard, the CASAC 
had differing recommendations for the appropriate range for an 
alternative level. The majority of the CASAC ``judge[d] that an annual 
average in the range of 8-10 [mu]g/m\3\'' was most appropriate, while 
the minority of the CASAC members stated that ``the range of the 
alternative standard of 10-11 [mu]g/m\3\ is more appropriate'' 
(Sheppard, 2022a, p. 16 of consensus responses). The CASAC did 
highlight, however, that ``the alternative standard level of 10 [mu]g/
m\3\ is within the range of acceptable alternative standards 
recommended by all CASAC members, and that an annual standard below 12 
[mu]g/m\3\ is supported by a larger and coherent body of evidence'' 
(Sheppard, 2022a, p. 16 of consensus responses).
    In reaching conclusions on a recommended range of 8-10 [mu]g/m\3\ 
for the primary annual PM2.5 standard, the majority of the 
CASAC placed weight on various aspects of the available scientific 
evidence and quantitative risk assessment information discussed in the 
2021 draft PA (Sheppard, 2022a, p. 16 of consensus responses). In 
particular, these members cited recent U.S.- and Canadian-based 
epidemiologic studies that show positive associations between 
PM2.5 exposure and mortality with study-reported mean 
concentrations below 10 [mu]g/m\3\. Further, these members also noted 
that the lower portions of the air quality distribution (i.e., 
concentrations below the mean) provide additional information to 
support associations between health effects and PM2.5 
concentrations lower than the reported long-term mean concentration. In 
addition, the CASAC members recognized that the available evidence has 
not identified a threshold concentration, below which an association no 
longer remains, pointing to the conclusion in the draft ISA Supplement 
that the ``evidence remains clear and consistent in supporting a no-
threshold relationship, and in supporting a linear relationship for 
PM2.5 concentrations >8 [mu]g/m\3\'' (Sheppard, 2022a, p. 16 
of consensus responses). Finally, these CASAC members placed weight on 
the at-risk analysis as providing support for protection of at-risk 
demographic groups, including minority populations.
    In recommending a range of 10-11 [mu]g/m\3\ for the primary annual 
PM2.5 standard, the minority of the CASAC emphasized that 
there were few key epidemiologic studies that reported positive and 
statistically significant health effects associations for 
PM2.5 air quality distributions with overall mean 
concentrations below 9.6 [mu]g/m\3\ (Sheppard, 2022a, p. 17 of 
consensus responses). In so doing, the minority of the CASAC 
specifically noted the variability in the relationship between study-
reported means and area annual design values based on the methods 
utilized in the studies, noting that design values are generally higher 
than area average exposure levels. Further, the minority of the CASAC 
stated that ``uncertainties related to copollutants and confounders 
make it difficult to justify a recommendation below 10-11 [mu]g/m\3\'' 
(Sheppard, 2022a, p. 17 of consensus responses). Finally, the minority 
of the CASAC placed less weight on the risk assessment results, noting 
large uncertainties, including the approaches used for adjusting air 
quality to simulate just meeting the current and alternative standards.
    With regard to the current primary 24-hour PM2.5 
standard, in their review of the 2021 draft PA, the CASAC did not reach 
consensus regarding the adequacy of the public health protection 
provided by the current standard. As described further below, the 
majority of the CASAC members concluded ``that the available evidence 
calls into question the adequacy of the current 24-hour standard'' 
(Sheppard, 2022a, p. 3 of consensus letter), while the minority of the 
CASAC members agreed with ``the EPA's preliminary conclusion [in the 
draft PA] to retain the current 24-hour PM2.5 standard 
without revision'' (Sheppard, 2022a, p. 4 of consensus letter). The 
CASAC recommended that in future reviews, the EPA should also consider 
alternative forms for the primary 24-hour PM2.5 standard. 
Specifically, the CASAC ``suggests considering a rolling 24-hour 
average and examining alternatives to the 98th percentile of the 3-year 
average,'' pointing to concerns that computing 24-hour average 
PM2.5 concentrations using the current midnight-to-midnight 
timeframe could potentially underestimate the effects of high 24-hour 
exposures, especially in areas with wood-burning stoves and wintertime 
stagnation (Sheppard, 2022a, p. 18 of consensus responses).
    As noted above, the majority of the CASAC favored revising the 
level of the primary 24-hour PM2.5 standard, suggesting that 
a range of 25-30 [mu]g/m\3\ would be adequately protective. In so 
doing, the majority of the CASAC placed weight on the available 
epidemiologic evidence, including epidemiologic studies that restricted 
analyses to 24-hour PM2.5 concentrations below 25 [mu]g/
m\3\. These members also placed weight on results of controlled human 
exposure studies with exposures close to the current standard, which 
they note provide support for the epidemiologic evidence to lower the 
standard. These members noted the limitations in using controlled human 
exposure studies alone in considering the adequacy of the 24-hour 
standard, recognizing that controlled human exposure studies 
preferentially recruit less susceptible individuals and have a typical 
exposure duration shorter than 24 hours. These members also placed 
``greater weight on the scientific evidence than on the values 
estimated by the risk assessment,'' citing their concerns that the risk 
assessment ``may not adequately capture areas with wintertime 
stagnation and residential wood-burning where the annual standard is 
less likely to be protective'' (Sheppard, 2022a, p. 17 of consensus 
responses). Furthermore, these CASAC members ``also are less confident 
that the annual standard could adequately protect against health 
effects of short-term exposures'' (Sheppard, 2022a, p. 17 of consensus 
responses).
    The minority of the CASAC agreed with the EPA's preliminary 
conclusion in the 2021 draft PA to retain the current primary 24-hour 
PM2.5 standard. In so doing, the minority of the CASAC 
placed greater weight on the risk assessment, noting that the risk

[[Page 16254]]

assessment accounts for both the level and the form of the current 
standard and the manner by which attainment with the standard is 
determined. Further, the minority of the CASAC stated that the ``risk 
assessment indicates that the annual standard is the controlling 
standard across most of the urban study areas evaluated and revising 
the level of the 24-hour standard is estimated to have minimal impact 
on the PM2.5-associated risks'' and therefore, ``the annual 
standard can be used to limit both long- and short-term 
PM2.5 concentrations'' (Sheppard, 2022a, p. 18 of consensus 
responses). Further, the minority of the CASAC placed more weight on 
the controlled human exposure studies, which show ``effects at 
PM2.5 concentrations well above those typically measured in 
areas meeting the current standards'' and which suggest that ``the 
current standards are providing adequate protection against these 
exposures'' (Sheppard, 2022a, p. 18 of consensus responses).
    While the CASAC members expressed differing opinions on the 
appropriate revisions to the current standards, they did ``find that 
both primary standards, 24-hour and annual, are critical to protect 
public health given the evidence on detrimental health outcomes at both 
short-term and long-term exposures including peak events'' (Sheppard, 
2022a, p. 13 of consensus responses). The comments from the CASAC also 
took note of uncertainties that remain in this reconsideration of the 
primary PM2.5 standards and they identified a number of 
additional areas for future research and data gathering and 
dissemination that would inform future reviews of the primary 
PM2.5 NAAQS (Sheppard, 2022a, pp. 14-15 of consensus 
responses).
2. Basis for the Proposed Decision
    In reaching his proposed decisions to revise the level of the 
primary annual PM2.5 standard from its current level of 12.0 
[micro]g/m\3\ to within the range of 9.0 to 10.0 [micro]g/m\3\, and to 
retain the current primary 24-hour PM2.5 standard (88 FR 
5558, January 27, 2023), the Administrator carefully considered the 
assessment of the current evidence and conclusions reached in the 2019 
ISA and ISA Supplement; the currently available exposure and risk 
information, including associated limitations and uncertainties, 
described in detail in the 2022 PA; the considerations and staff 
conclusions and associated rationales presented in the 2022 PA; the 
advice and recommendations from the CASAC; and public comments that had 
been offered up to that point (88 FR 5558, January 27, 2023).
    In reaching his proposed conclusions on whether the currently 
available scientific evidence and quantitative risk-based information 
support or call into question the adequacy of the public health 
protection afforded by the current primary PM2.5 standards, 
and as is the case with NAAQS reviews in general, the extent to which 
the current primary PM2.5 standards are judged to be 
adequate will depend on a variety of factors, including science policy 
and public health policy judgments to be made by the Administrator on 
the strength and uncertainties of the scientific evidence. The factors 
relevant to judging the adequacy of the standards also include the 
interpretation of, and decisions as to the weight to place on, 
different aspects of the results of the risk assessment for the study 
areas included and the associated uncertainties. Thus, in reaching 
proposed conclusions of the current standards, the Administrator 
recognized that such a determination depends in part on judgments 
regarding aspects of the evidence and risk estimates, and judgments 
about the degree of protection that is requisite to protect public 
health with an adequate margin of safety.
    The Administrator's full rationale for his proposed conclusions is 
presented in section II.D.3 of proposal (88 FR 5658, January 27, 2023), 
but is also briefly summarized here. In reaching the proposed decision 
to revise the annual standard level to 9-10 [micro]g/m\3\, the 
Administrator placed weight on the full body of scientific information. 
He noted that the 2019 ISA finds that exposure to PM2.5 
causes mortality and cardiovascular effects and is likely to cause 
respiratory effects, cancer, and nervous system effects as detailed 
further in section II.B.1 of the proposal. As detailed further in 
section II.B.4 of the proposal, he additionally noted that the 2019 ISA 
identifies at-risk populations at greater risk of health effects from 
exposure to PM2.5, including children, older adults, people 
with pre-existing respiratory or cardiovascular disease, minority 
populations, and low socioeconomic status (SES) populations.
    The Administrator also recognized that epidemiologic studies 
provide the strongest scientific evidence when evaluating the adequacy 
of the level of the annual standard. He noted that there is no specific 
point in the air quality distribution of any epidemiologic study that 
represents a `bright line' at and above which effects have been 
observed and below which effects have not been observed. In his 
proposed decision, he noted previous decision-making frameworks, which 
placed weight on values at or near the study-reported mean 
PM2.5 concentrations, which is where the most confidence in 
the reported association of the epidemiologic study exists. He further 
noted that there are a number of epidemiologic studies available in 
this reconsideration that use new PM2.5 exposure estimation 
techniques (e.g., hybrid modeling) that were not used in epidemiologic 
studies that were available in previous reviews. These recent 
epidemiologic studies that use new exposure estimation techniques 
report long-term mean PM2.5 concentrations that are well 
below corresponding design values, which is an important consideration 
in reaching decisions on the level of the annual PM2.5 
standard.
    In reaching his proposed decision, the Administrator noted that a 
level of 9-10 [micro]g/m\3\ would near or below the reported 25th 
percentiles in key U.S. based epidemiologic studies, while also 
recognizing that he has less confidence in the magnitude and 
significance of the association at even lower percentiles (e.g., 10th 
percentile), where even fewer health events are observed. The 
Administrator also noted that a proposed level of 9-10 [micro]g/m\3\ 
would be near the mean PM2.5 reported in Canadian based 
studies, though he also recognized that there are a number of factors 
associated with the studies in Canada (e.g., exposure environments) 
that make it more difficult to compare mean concnetrations from 
Canadian studies to design values, which determine compliance with the 
standard in the U.S.
    The Administrator took note of additional pieces of scientific 
evidence, which were not available in previous reviews, including 
restricted analyses, which support that the association seen in 
epidemiologic studies does not just occur from the peaks of the 
exposure distribution. Additionally, he notes that a level of 9-10 
[micro]g/m\3\ would be below the starting concentration in newly 
available accountability studies, though he did note that it is more 
difficult to interpret these studies in the context of selecting the 
level of the annual PM2.5 standard.
    Further, the Administrator took into consideration the advice of 
the CASAC, noting that all members included 10 [micro]g/m\3\ in their 
recommended range, and that the proposed range of 9-10 [micro]g/m\3\ 
for the level of the primary annual PM2.5 standard was 
within the range recommended by the majority of the CASAC.
    In reaching the proposed conclusion of a range between 9-10 
[micro]g/m\3\, the Administrator noted that a level as high

[[Page 16255]]

as 11 [micro]g/m\3\ might not provide an adequate margin of safety, 
given that 11 [micro]g/m\3\ was well above many of the epidemiologic 
study-reported mean PM2.5 concentrations. Additionally, the 
Administrator noted the uncertainties associated with the scientific 
and quantitative information supporting a level as low as 8 [micro]g/
m\3\, which call into question the potential public health improvements 
of a standard below 9 [micro]g/m\3\. The Administrator specifically 
noted the lack of key U.S. studies with mean concentrations below 9.3 
[micro]g/m\3\ and he further noted that the risk assessment suggests 
that the risk remaining under a standard of 8 [micro]g/m\3\ would occur 
at very low concentrations (e.g., mainly 7 [micro]g/m\3\ and below).
    As such, the Administrator's proposed decision noted that the 
current PM2.5 annual standard did not adequately provide 
requisite protection against exposures to PM2.5 and that a 
proposed range of 9-10 [micro]g/m\3\ would provide an adequate margin 
of safety.
    In his proposed decision to retain the current primary 24-hour 
PM2.5 standard with a level of 35 [micro]g/m\3\, the 
Administrator first considered the scientific information related to 
short-term exposures to PM2.5 and health effects. He noted 
that the controlled human exposure studies are the strongest line of 
evidence for informing his conclusions regarding the adequacy of the 
current 24-hour standard. In so doing, the Administrator recognized 
that controlled human exposure studies are conducted with healthy adult 
volunteers and that these studies do not include individuals who may be 
at increased risk of PM2.5-related health effects (i.e., 
children, older adults, people with pre-existing diseases). He also 
noted that the effects observed in the controlled human exposure 
studies (e.g., changes in vascular function) are not effects that are 
judged to be clearly adverse. He recognized the most consistent 
evidence of effects in these studies occurs at higher concentrations 
(e.g., >120 [micro]g/m\3\) following 1-5 hour exposures, and that one 
study observed effects at concentrations as low as 38 [micro]g/m\3\ 
following 4-hour exposures. However, the Administrator reiterated that 
these studies do not tell us at exactly what concentrations an adverse 
effect might occur, especially for at-risk populations. As noted above 
in section II.A.2.c, controlled human exposure studies tend to include 
generally healthy adult individuals who are at a lower risk of 
experiencing health effects, and often do not include at-risk 
populations (e.g., children, older adults, or individuals with pre-
existing conditions). As such, the Administrator recognized that these 
studies are somewhat limited in their ability to inform at what 
concentrations effects may be elicited in in at-risk populations. The 
Administrator also considered air quality analyses in the 2022 PA that 
demonstrate that there will be very few, if any, days with 
PM2.5 concentrations at levels evaluated in controlled human 
exposure studies that are associated with effects in areas that meet 
the current primary 24-hour PM2.5 standard.
    The Administrator also noted that as, in previous PM NAAQS reviews, 
the protection provided by the suite of standards (e.g., annual and 24-
hour standards) is evaluated together. He noted that the annual 
standard is the controlling standard in most areas of the country. He 
also considered air quality analyses in the 2022 PA that suggest that 
revision of the annual standard to a level between 9-10 [micro]g/m\3\ 
would also control 24-hour PM2.5 concentrations in most 
areas to, or below, 30 [micro]g/m\3\. Finally, the Administrator noted 
the agreement with the advice from the minority of CASAC and 
additionally noted the limited rationale and evidence provided by the 
majority CASAC's recommendation to support revision of the 24-hour 
standard. As such, the Administrator proposed to retain the current 24-
hour standard with its level of 35 [micro]g/m\3\.
    Additionally, the Administrator proposed to conclude that it is 
appropriate to retain all other elements (i.e., indicator, averaging 
time, and form) of the annual and 24-hour standards.
3. Comments on the Proposed Decision
    With respect to the adequacy of the primary annual PM2.5 
standard, a number of commenters, primarily those from industry and 
industry groups, non-governmental organizations, and some State and 
local governments, disagree with the EPA's proposed decision to revise 
the level of the primary annual PM2.5 standard. These 
commenters generally expressed the view that the current standards 
provide the requisite degree of public health protection and should be 
retained, consistent with the 2020 final decision. In supporting their 
view, these commenters assert that the scientific evidence available in 
this reconsideration is essentially unchanged since the 2020 final 
decision and that the additional scientific evidence and quantitative 
risk information available for the reconsideration does not support 
strengthening the primary annual PM2.5 standard. These 
commenters also assert that uncertainties associated with the available 
scientific evidence have not changed since the 2020 final decision, and 
they note that these uncertainties were essential factors in the then-
Administrator's decision to retain the primary annual PM2.5 
standard. These commenters argue that, while the current Administrator 
acknowledges these uncertainties, he does not place enough weight on 
them in reaching his conclusions regarding the current standard. The 
commenters specifically highlight uncertainties related to exposure 
misclassification, confounding, and other sources of potential bias, 
which they claim supports retaining the current level of the annual 
standard. These commenters also note that these uncertainties were 
emphasized by the minority of the CASAC in their review of the 2021 
draft PA, and the commenters further suggest that the lack of consensus 
from the CASAC on the appropriate level for the primary annual 
PM2.5 standard show that the research is unclear. The 
commenters contend that there is not support in this reconsideration 
for deviating from the then-Administrator's decision in 2020.
    In contrast, other commenters, primarily from public health and 
environmental organizations, some State and local elected 
representatives, and some State and local government agencies agree 
with the EPA's proposed decision that the primary annual 
PM2.5 standard is not adequate. These commenters support 
revising the level of the primary annual PM2.5 standard and 
emphasize that the available scientific evidence, in particular 
epidemiologic studies, along with the CASAC's advice in their review 
for the 2021 draft PA, provide strong support for the proposed 
decision. In particular, these commenters agree with the EPA's 
conclusions about the strength of the scientific evidence, including 
uncertainties, and they emphasize that the CASAC reached consensus in 
their review of the 2021 draft PA that the current primary annual 
PM2.5 standard is not adequate. Some of these commenters 
also note that a revised primary annual PM2.5 standard would 
result in significant public health benefits by reducing morbidity and 
mortality associated with PM2.5 exposure, especially for at-
risk populations.
    The EPA agrees with commenters that the primary annual 
PM2.5 standard is not adequate. The EPA recognizes the 
longstanding body of health evidence supporting relationships between 
PM2.5 exposures (short- and long-term) and both mortality 
and serious morbidity effects. The evidence available in this 
reconsideration (i.e., the studies

[[Page 16256]]

assessed in the 2019 ISA and ISA Supplement summarized above in section 
II.A.2.a) reaffirms, and in some cases strengthens, the conclusions 
from the 2009 ISA regarding the health effects of PM2.5 
exposures. As noted above, epidemiologic studies demonstrate generally 
positive and often statistically significant associations between 
PM2.5 exposures and health effects. Such studies report 
associations between estimated PM2.5 exposures and non-
accidental, cardiovascular, or respiratory mortality; cardiovascular or 
respiratory hospitalizations or emergency room visits; and other 
mortality/morbidity outcomes (e.g., lung cancer mortality or incidence, 
asthma development). Recent experimental evidence, as well as evidence 
from epidemiologic panel studies, strengthens support for potential 
biological pathways through which PM2.5 exposures could lead 
to the serious effects reported in many population-level epidemiologic 
studies, including support for pathways that could lead to 
cardiovascular, respiratory, nervous system, and cancer-related 
effects. Moreover, these recent epidemiologic studies strengthen 
support for health effect associations at PM2.5 
concentrations lower than in those evaluated in epidemiologic studies 
available at the time of previous reviews.
    Additionally, as discussed in more detail in section I.C.5.b above, 
the ISA Supplement focused on studies that were most likely to inform 
decisions on the appropriate standard, but not to reassess areas that, 
based on the assessment of available science published since the cutoff 
date of the 2019 ISA and through 2021, were judged unlikely to have new 
information that would be useful for the Administrator's decision 
making. The ISA Supplement included U.S. and Canadian epidemiologic 
studies for health effect categories where the 2019 ISA concluded a 
causal relationship (i.e., short- and long-term PM2.5 
exposure and cardiovascular effects and mortality), as well as U.S. and 
Canadian epidemiologic studies that employed alternative methods for 
confounder control or conducted accountability analyses (i.e., studies 
that examined the effect of a policy on reducing PM2.5 
concentrations). These studies, summarized in section II.A.2.a above, 
examine both short- and long-term PM2.5 exposure and 
cardiovascular effects and mortality. Additionally, studies that employ 
alternative methods for confounder control, as described in II.A.2.a 
above and in Table 3-11 and of the 2022 PA (U.S. EPA, 2022b), use a 
variety of statistical methods to control for confounding bias. These 
studies consistently report positive associations, which further 
supports the broader body of epidemiologic evidence for both 
cardiovascular effects and mortality.
    In addition, there are epidemiologic studies that provide 
supplemental information for consideration in reaching conclusions that 
the current suite of PM2.5 standards is not adequate. These 
studies include analyses that restrict annual average PM2.5 
concentrations to concentrations below 12 [micro]g/m\3\ and provide 
support for positive and statistically significant associations with 
mortality and cardiovascular morbidity at mean PM2.5 
concentrations below the current level of the primary annual 
PM2.5 standard (described above in section II.A.2.c.ii and 
in Table 3-10 of the 2022 PA (U.S. EPA, 2022b)). Recent accountability 
studies that have starting annual PM2.5 concentrations at or 
below 12 [micro]g/m\3\ suggest public health improvements may occur at 
concentrations below 12 [micro]g/m\3\. These studies indicate positive 
and statistically significant associations with mortality and morbidity 
(e.g., cardiovascular hospital admissions) and reductions in 
PM2.5 concentrations in ambient air (described above in 
section II.A.2.c.ii and in Table 3-12 of the 2022 PA (U.S. EPA, 
2022b)).
    Thus, in considering the available scientific evidence to inform 
conclusions on the adequacy of the primary PM2.5 standards, 
the Administrator recognizes that the 2019 ISA and the ISA Supplement 
together provides a strong scientific foundation for concluding that 
the current primary PM2.5 standards are not adequate.
    In addition to the scientific evidence above, the risk assessment 
estimates that the current primary annual PM2.5 standard 
could allow a substantial number of deaths in the U.S. Although the 
Administrator recognizes that while the risk estimates can help to 
place the evidence for specific health effects into a broader public 
health context, they should be considered along with the inherent 
uncertainties and limitations of such analyses when informing judgments 
about the potential for additional public health protection associated 
with PM2.5 exposures and related health effects. The 
Administrator takes into consideration these uncertainties, which are 
described in more detail in section II.A.3.b above, but notes that the 
general magnitude of risk estimates supports the potential for 
significant public health impacts, particularly for lower alternative 
annual standard levels.
    In the CASAC's review of the 2019 draft PA, the CASAC did not reach 
consensus on whether the current annual standard is adequate, with the 
majority of the CASAC recommending that the annual standard be retained 
and the minority of the CASAC recommending that the standard be 
revised. In their review of the 2021 draft PA, the CASAC unanimously 
recommended that the current annual standard is not sufficiently 
protective of public health (Sheppard, 2022a, p. 2 of consensus 
letter).
    The EPA disagrees with the commenters who state that the available 
scientific and quantitative information available in this 
reconsideration does not provide support for the current Administrator 
to reach a different decision than the then-Administrator reached in 
the 2020 final action. The EPA agrees with these commenters that there 
are uncertainties associated with the currently available scientific 
evidence. The EPA has considered these uncertainties extensively both 
in reaching conclusions in the 2022 PA (U.S. EPA, 2022b, sections 
3.4.3, 3.6.1, and 4.6.3) and in the proposal (88 FR 5604, 5609, January 
27, 2023), and the EPA addresses more detailed public comments about 
these uncertainties, including those related to copollutant 
confounding, unmeasured confounding, and temporal and spatiotemporal 
confounding, in the Response to Comments document. However, we disagree 
with the commenters that the evidence does not provide support for the 
Administrator's conclusion that the current primary annual 
PM2.5 standard is not adequate to protect public health with 
an adequate margin of safety, and should be revised. As described 
above, epidemiologic studies in the 2019 ISA and the ISA Supplement 
support and extend the evidence evaluated in the 2009 ISA, through 
studies conducted in diverse populations and geographic locations, 
using various statistical models and approaches to control for 
potential confounders, and using a variety of exposure assessment 
methodologies. Therefore, the consistent, positive associations 
reported across studies (U.S. EPA, 2019a, Figures 11-1 and 11-18; U.S. 
EPA, 2022a) are unlikely to be to be the result of unmeasured 
confounding and other biases are unlikely to account for the consistent 
positive associations observed across epidemiologic studies.
    Additionally, this reconsideration includes epidemiologic studies 
that were not before the then-Administrator for consideration in 
reaching his final decisions at the time of the 2020 decision and that 
specifically evaluate

[[Page 16257]]

confounding using alternative methods for confounder control). These 
recent epidemiologic studies provide support for the current 
Administrator's conclusion that the suite of primary PM2.5 
standards are not adequate. While confounding was an uncertainty noted 
by the then-Administrator in the 2020 decision, he recognized ``that 
methodological study designs to address confounding, such as causal 
inference methods, are an emerging field of study'' (85 FR 82710, 
December 18, 2020). The ISA Supplement considered studies that employed 
statistical approaches that attempt to more extensively account for 
confounders and are more robust to model misspecification (i.e., used 
alternative methods for confounder control),\92\ given that such 
studies were highlighted by the CASAC in their review of the 2019 draft 
PA and identified in public comments on the 2020 proposal. Since the 
literature cutoff date for the 2019 ISA, multiple studies that employ 
alternative methods for confounder control have become available for 
consideration in the ISA Supplement and, subsequently, in this 
reconsideration. For example, one study before the Administrator in 
this reconsideration that was not available in the 2019 ISA is Schwartz 
et al. (2021), which used a causal modeling approach focused on 
exposure changes and controls for measured confounders by design in 
order to evaluate the association between long-term PM2.5 
exposure and mortality in the Medicare population. The study authors 
found significant associations of PM2.5 with increased 
mortality rates using a causal modeling approach robust to omitted 
confounding. The results of this study and other studies in the ISA 
Supplement that employ alternative methods to control for confounders 
lend support to the robustness of positive associations between 
PM2.5 exposure and multiple morbidity and mortality 
endpoints exhibited across epidemiologic studies, and also indicate 
that unmeasured confounding and other biases are unlikely to account 
for the consistent positive associations observed across epidemiologic 
studies (U.S. EPA, 2022b, sections 3.1.1.3, 3.1.2.3, 3.2.1.3, and 
3.2.2.3).
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    \92\ As noted in the ISA Supplement: ``In the peer-reviewed 
literature, these epidemiologic studies are often referred to as 
causal inference studies or studies that used causal modeling 
methods. For the purposes of this Supplement, this terminology is 
not used to prevent confusion with the main scientific conclusions 
(i.e., the causality determinations) presented within an ISA. In 
addition, as is consistent with the weight-of-evidence framework 
used within ISAs and discussed in the Preamble to the Integrated 
Science Assessments, an individual study on its own cannot inform 
causality, but instead represents a piece of the overall body of 
evidence'' (U.S. EPA, 2022a, p. 1-3).
---------------------------------------------------------------------------

    Further, the EPA disagrees with the commenters who argue that the 
Administrator did not appropriately consider the strengths and 
limitations of the health evidence in reaching his decision to revise 
the current primary annual PM2.5 standard in this 
reconsideration. In reaching his proposed decision, the Administrator 
considered the entire body of evidence and how to appropriately weigh 
the uncertainties associated with the health evidence (88 FR 5617, 
January 27, 2023). Such an approach is consistent with setting 
standards that are neither more nor less stringent than necessary, 
recognizing that ``Congress provided that the Administrator is to use 
his judgment in setting air quality standards precisely to permit him 
to act in the face of uncertainty,'' the Administrator must set 
standards on ``the frontiers of scientific and medical knowledge'' and 
``Congress directed the Administrator to err on the side of caution in 
making the necessary decisions.'' Lead Indus. Ass'n, Inc. v. EPA, 647 
F.2d 1130, 1155 & n.50 (D.C. Cir. 1980) (quoting H.R. Rep. No. 95-294, 
at 50). As such, a determination of identifying a specific level at 
which the standard should be set necessarily requires the 
Administrator's judgement (e.g., weighing the uncertainties and margin 
of safety).
    Additionally, the EPA disagrees with the commenters that contend 
that there is no basis in this reconsideration for deviating from the 
previous Administrator's decision in 2020. It is well-established that 
in CAA section 109 Congress specifically left the determination of the 
requisite NAAQS to the judgment of the Administrator and, moreover, 
that ``decisions about the appropriate NAAQS level must `necessarily . 
. . rest largely on policy judgments.' '' Mississippi v. EPA, 744 F.3d 
1344, 1357 (D.C. Cir. 2013) (quoting Lead Industries Ass'n v. EPA, 647 
F.2d 1130, 1147 (D.C. Cir. 1980)). As the Court of Appeals for the D.C. 
Circuit has noted, ``Every time EPA reviews a NAAQS, it (presumably) 
does so against contemporary policy judgments and the existing corpus 
of scientific knowledge.'' Id., at 1343.
    In this reconsideration, both the existing corpus of scientific 
knowledge as well as the Administrator's policy judgments about how to 
interpret and weigh that evidence to protect public health with an 
adequate margin of safety have changed. The expansion of the air 
quality criteria to encompass additional studies, information and 
analyses in the ISA Supplement and 2022 PA, as well as the additional 
consideration of the scientific record by the CASAC and the public 
provided the Administrator with significant additional information on 
which to base his decision.\93\ In addition, in this reconsideration, 
the Administrator is reaching different judgments about how to weigh 
the epidemiologic evidence, including the uncertainties in the 
scientific evidence, and how to ensure an adequate margin of safety to 
protect against uncertain harms, compared to the approach in the 2020 
final decision. For example, as discussed in greater detail above in 
section II.A.1 and in the 2020 notice of final rulemaking (85 FR 82717, 
December 18, 2020), in considering the epidemiologic evidence as part 
of his decision to retain the current primary annual PM2.5 
standard in the 2020 decision, the then-Administrator placed weight on 
the mean of the study-reported means (or medians) (i.e., 13.5 [micro]g/
m\3\) from key U.S. epidemiologic studies that are monitor-based being 
above the level of the current primary annual PM2.5 standard 
of 12.0 [micro]g/m\3\. By contrast, in this reconsideration, the 
current Administrator has taken an approach more similar to how the EPA 
has considered study-reported mean PM2.5 concentrations 
relative to the level of the primary annual PM2.5 standard 
in other recent PM NAAQS reviews. In so doing, in reaching his decision 
to revise the level of the primary annual PM2.5 standard to 
9.0 [micro]g/m\3\, he is using an approach that places weight on 
selecting a level for the standard that is below the study-reported 
mean PM2.5 concentrations reported in key U.S. epidemiologic 
studies, including recent epidemiologic studies that use hybrid model-
based methods, as well as being near or below the 25th percentile 
PM2.5 concentrations in those key U.S. epidemiologic studies 
that report these concentrations.
---------------------------------------------------------------------------

    \93\ The EPA notes that, in considering the additional 
scientific evidence available in this reconsideration, one member of 
the CASAC who reviewed both the 2019 draft PA and the 2021 draft PA 
found that the available scientific and quantitative information 
available in this reconsideration supported revising the level of 
the primary annual PM2.5 standard, whereas he recommended 
retaining the standard during the review of the 2019 draft PA.
---------------------------------------------------------------------------

    As such and further detailed in section II.B.4 below, in 
considering the adequacy of the current primary PM standards in this 
reconsideration, the Administrator has carefully considered the: (1) 
Policy-relevant evidence and conclusions contained in the 2019 ISA and 
2022 ISA Supplement; (2) the quantitative information presented and

[[Page 16258]]

assessed in the 2022 PA; (3) the evaluation of this evidence, the 
quantitative information, and the rationale and conclusions presented 
in the 2022 PA; (4) the advice and recommendations from the CASAC; and 
(5) public comments. The Administrator concludes that the current suite 
of primary PM2.5 standards are not adequate to protect 
public health with an adequate margin of safety.
    The four basic elements of the NAAQS (indicator, averaging time, 
form, and level) are considered collectively in evaluating the health 
protection afforded by a standard. The EPA received relatively few 
comments on the averaging time and form for the primary 
PM2.5 standards, but those who did provide comments on these 
elements were primarily from public health and environmental 
organizations, State and local elected representatives, and State and 
local government agencies. Some commenters assert that the current 24-
hour averaging time for the primary 24-hour PM2.5 standard 
does not adequately protect against short-term peaks. These commenters 
further state that the 24-hour averaging time protects against chronic 
exposures but does not adequately protect against serious acute risks 
from certain sources such as prescribed burning. Also, a few commenters 
explicitly recommend that a subdaily averaging time would be more 
appropriate, although none of the commenters recommended a specific 
averaging time for consideration. Additionally, some commenters cite to 
the CASAC's advice in their review of the 2021 draft PA that future 
reviews of the PM NAAQS should include evaluation of alternative forms 
and averaging times of the current primary 24-hour PM2.5 
standard.
    The EPA disagrees with commenters that the current primary 24-hour 
PM2.5 standard, with its 24-hour averaging time, does not 
adequately protect against short-term peaks and disagrees that that 
there is sufficient information to conclude that a subdaily averaging 
time would be more appropriate than a 24-hour averaging time. The EPA 
has reviewed the currently available scientific evidence and finds that 
it does not indicate that alternative averaging times would be more 
appropriate for the primary PM2.5 standards. Accordingly, 
the EPA concludes that it is appropriate to retain both the annual and 
24-hour averaging times for standards meant to protect against long- 
and short-term PM2.5.
    As noted in the proposal, the 2019 ISA and ISA Supplement found 
that the scientific evidence continues to provide strong support for 
health effect associations with both long-term (e.g., annual or multi-
year) and short-term (e.g., mostly 24-hour) exposures to 
PM2.5. Epidemiologic studies continue to provide strong 
support for health effects associated with short-term PM2.5 
exposures based on 24-hour PM2.5 averaging periods, and we 
note that subdaily effect estimates are less consistent and, in some 
cases, smaller in magnitude (88 FR 5618, January 27, 2023). Controlled 
human exposure and panel-based studies of subdaily exposures typically 
examine subclinical effects rather than the more serious population-
level effects that have been reported to be associated with 24-hour 
exposures (e.g., mortality, hospitalizations). Collectively, the 2019 
ISA concludes that epidemiologic studies do not indicate that subdaily 
averaging periods are more closely associated with health effects than 
the 24-hour average exposure metric (U.S. EPA, 2019a, section 1.5.2.1). 
Additionally, the EPA notes that while recent controlled human exposure 
studies provide consistent evidence for cardiovascular effects 
following PM2.5 exposures for less than 24 hours (i.e., <30 
minutes to 5 hours), exposure concentrations in these studies are well-
above the ambient concentrations typically measured in locations 
meeting the current standards (U.S. EPA, 2022a, section 3.3.3.1). 
Therefore, this information does not indicate that a revision to the 
averaging time is needed to provide additional protection against 
subdaily PM2.5 exposures, beyond that provided by the 
current primary standards. This conclusion is also supported by the 
advice given to EPA by the CASAC in their review of the 2021 draft PA, 
which reached consensus that averaging times for the standards should 
be retained, without revision (Sheppard, 2022a, p. 2 of consensus 
letter).\94\ For all of these reasons, the Administrator concludes that 
the currently available evidence does not support considering 
alternatives to the annual and 24-hour averaging times for standards 
meant to protect against long- and short-term PM2.5 
exposures.
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    \94\ In providing advice on the 2019 draft PA, the CASAC did not 
weigh in specifically on the averaging time of the primary 24-hour 
PM2.5 standard but did recommend that the standard be 
retained because the available evidence does not call into question 
its adequacy (Cox, 2019b, p. 3 of consensus letter).
---------------------------------------------------------------------------

    Multiple commenters, primarily from public health and environmental 
organizations, recommend revising the form of the primary 24-hour 
PM2.5 standard to a 99th percentile to provide increased 
public health protection against peak PM2.5 exposures, 
particularly for at-risk populations. These commenters express concern 
that the current 98th percentile form allows 7 exceedances per year and 
contend that a 99th percentile form that would allow half that number 
is more appropriate. Commenters also cite to the CASAC's advice in 
their review of the 2021 draft PA, which recommended that the EPA 
consider alternative percentiles for the form of the primary 24-hour 
PM2.5 standard in the future.
    The EPA disagrees that the current 98th percentile form does not 
provide the requisite public health protection against peak 
PM2.5 exposures and concludes that the 98th percentile, 
averaged over three years, remains appropriate for the primary 24-hour 
PM2.5 standard. As noted in previous reviews and in the 
proposal, the EPA has set both an annual standard and a 24-hour 
standard to provide protection from health effects associated with both 
long- and short-term exposures to PM2.5 (62 FR 38667, July 
18, 1997; 88 FR 5620, January 27, 2023). With respect to the form of 
the 24-hour standard, as described just above, the epidemiologic 
studies continue to provide strong support for health effect 
associations with short-term (e.g., mostly 24-hour) PM2.5 
exposures and controlled human exposure studies provide evidence for 
health effects following single short-term ``peak'' PM2.5 
exposures (88 FR 5619, January 27, 2023). Both the 98th and the 99th 
percentile form provide a very high degree of control of peak 
concentrations. As the commenters point out, a 99th percentile would 
reduce the number of allowable exceedances to four days per year. The 
EPA anticipates, however, that such a revision to the form would make 
the attainment status of an area more subject to change from 
unpredictable nonanthropogenic factors, such as meteorological events. 
The EPA has often noted that frequent shifts in attainment status that 
are unrelated to long-term air quality trends is inconsistent with 
providing a stable target for air quality planning and risk management 
programs, which in turn provides for the most effective public health 
protection in the long run (78 FR 3127, January 15, 2013; 80 FR 65351, 
October 26, 2015). Thus, the EPA's interest in an appropriate degree of 
stability is to ensure that the State air quality programs are 
effective in controlling pollution and that the public health 
protections of the standard are achieved. As discussed above, while 
recent controlled human exposure studies provide consistent evidence 
for cardiovascular effects following PM2.5

[[Page 16259]]

exposures for less than 24 hours (i.e., < 30 minutes to 5 hours), 
exposure concentrations in these studies are well-above the ambient 
concentrations typically measured in locations meeting the current 
standards (U.S. EPA, 2022a, section 3.3.3.1), and the 98th percentile 
form is very effective at limiting occurrences of exposures of concern. 
Taking into consideration the available scientific information and 
quantitative information, the EPA therefore concludes that the 98th 
percentile form provides an appropriate balance between limiting the 
occurrence of peak 24-hour PM2.5 concentrations and 
identifying a stable target for risk management programs. This 
conclusion is also supported by the advice given to the EPA by the 
CASAC in their review of the 2021 draft PA, where they reached 
consensus that the form for the standards should be retained, without 
revision (Sheppard, 2022a, p. 2 of consensus letter).\95\
---------------------------------------------------------------------------

    \95\ The CASAC did not provide advice or recommendations 
regarding the forms of the primary PM2.5 standards in 
their review of the 2019 draft PA (Cox, 2019b).
---------------------------------------------------------------------------

    Additionally, the EPA recognizes the CASAC's advice in their review 
of the 2021 draft PA, where they recommended ``that in future reviews, 
the EPA provide a more comprehensive assessment of the 24-hour standard 
that includes the form as well as the level'' (Sheppard, 2022a, p. 4 of 
consensus letter). This advice is reflected in the proposal by the EPA, 
which noted ``that it would be appropriate to gather additional air 
quality and scientific information and further consider these issues in 
future reviews'' (88 FR 5619, January 27, 2023). The EPA will consider 
the information provided by the commenters regarding the form of the 
24-hour PM2.5 standard in the next review of the PM NAAQS.
    A number of commenters who support revising the level of the 
primary annual PM2.5 standard, particularly those who 
support a revised level of 8 [micro]g/m\3\, disagree with how the EPA 
has emphasized the mean PM2.5 concentrations reported in key 
epidemiologic studies to inform conclusions on the level of the primary 
PM2.5 standard. These commenters argue that, in this 
reconsideration, the EPA is arbitrarily emphasizing uncertainties in 
key epidemiologic studies in the focus on mean concentrations. Many of 
these commenters recommend that the EPA consider the full distribution 
of PM2.5 concentrations from the key epidemiologic studies 
in reaching conclusions on the appropriate level for the primary annual 
PM2.5 standards, in particular concentrations below the 
mean, such as the 25th percentile. In supporting this view, commenters 
point to the CASAC's advice in their review of the 2021 draft PA, where 
the majority of the CASAC stated that the ``use of the mean to define 
where the data provide the most evidence is conservative since robust 
data clearly indicate effects below the mean in concentration-response 
functions'' (Sheppard, 2022a, p. 16 of consensus responses), and that 
``[e]pidemiologic studies require consideration of distribution around 
the mean of exposure to identify effects and thus lower levels than the 
mean must be considered as part of the range where the data provide 
higher confidence'' (Sheppard, 2022a, p. 13 of consensus responses).
    As an initial matter, consistent with some previous approaches and 
as detailed by the Administrator in reaching conclusions on the level 
of the primary annual PM2.5 standard in section II.B.4 
below, the EPA considers the long-term study-reported mean 
PM2.5 concentrations from key epidemiologic studies and sets 
the level of the standard to somewhat below the lowest long-term mean 
PM2.5 concentration. Additionally, as discussed further 
below, the EPA also considers the available information from a subset 
of epidemiologic studies that report exposure estimates or health 
events at the 25th and 10th percentiles of PM2.5 
concentrations. The Administrator gives some weight to the 25th 
percentile data, although he recognizes that his confidence in the 
magnitude and significance in the reported concentrations, and their 
ability to inform decisions on the appropriate level of the annual 
standard, decreases with reduced data (below the mean) and diminishes 
further at percentiles that are even further below the mean and the 
25th percentile. Therefore, the Administrator places weight on the 
reported 25th percentiles concentrations, rather than the reported 10th 
percentile concentrations, for the subset of studies that report lower 
percentile PM2.5 concentrations in reaching his conclusions 
regarding the appropriate level for the primary annual PM2.5 
standard.
    In considering the available scientific evidence to reach decisions 
on the adequacy of the suite of primary PM2.5 standards, the 
EPA notes that in previous PM NAAQS reviews (including the 1997, 2006 
and 2012 reviews), evidence-based approaches were used that focused on 
identifying standard levels near or somewhat below long-term mean 
concentrations reported in key epidemiologic studies. These approaches 
were supported by the CASAC in previous reviews and were supported in 
this reconsideration by the CASAC in their review of the 2021 draft 
PA.\96\
---------------------------------------------------------------------------

    \96\ The Administrator notes that, in their review of the 2021 
draft PA, a majority of members of the CASAC noted that there are 
some limitations for this approach ``for the purpose of informing 
the adequacy of the standards'' (Sheppard, 2022a, p. 8 of consensus 
responses) and advised that future reviews should include evaluation 
of other metrics, including the distribution of concentrations 
reported in epidemiologic studies and in analyses restricting 
concentrations to below the current standard level. The 
Administrator also notes that, in their review of the 2019 draft PA, 
the CASAC lacked consensus on the inferences to be drawn from the 
epidemiologic evidence, with a majority of CASAC having concerns 
about confounding, error and bias and concluding that newer studies 
did not provide a basis for revising the current standards, while a 
minority concluded that the evidence, including more recent studies 
showing associations in areas with average long-term 
PM2.5 concentrations below the current annual standard, 
supported their conclusion that the current standards are inadequate 
(Cox, 2019b, pp. 8-9 of consensus responses).
---------------------------------------------------------------------------

    In considering the available scientific evidence, the EPA notes the 
strength of the epidemiologic evidence which includes multiple studies 
that consistently report positive associations for short- and long-term 
PM2.5 exposures and mortality and cardiovascular effects. 
Some available studies also use a variety of statistical methods to 
control for confounding bias and report similar associations, which 
further supports the broader body of epidemiologic evidence for both 
mortality and cardiovascular effects. Additionally, the EPA notes that 
recent epidemiologic studies strengthen support for health effect 
associations at PM2.5 concentrations lower than in those 
evaluated in epidemiologic studies available at the time of previous 
reviews.
    While these epidemiologic studies evaluate associations between 
distributions of ambient PM2.5 concentrations and health 
outcomes, they do not identify the specific exposures that led to the 
reported effects. As such, there is no specific point in the air 
quality distribution of any epidemiologic study that represents a 
``bright line'' at and above which effects have been observed and below 
which effects have not been observed.
    Studies of daily PM2.5 exposures examine associations 
between day-to-day variation in PM2.5 concentrations and 
health outcomes, often over several years. While there can be 
considerable variability in daily exposures over a multi-year study 
period, most of the estimated exposures reflect days with

[[Page 16260]]

ambient PM2.5 concentrations around the middle of the air 
quality distributions examined (i.e., ``typical'' days rather than days 
with extremely high or extremely low concentrations). Similarly, for 
studies of annual PM2.5 exposures, most of the health events 
occur at estimated exposures that reflect annual average 
PM2.5 concentrations around the middle of the air quality 
distributions examined. In both cases, epidemiologic studies provide 
the strongest support for reported health effect associations for this 
middle portion of the PM2.5 air quality distribution, which 
corresponds to the bulk of the underlying data, rather than the extreme 
upper or lower ends of the distribution. Therefore, in the absence of 
discernible thresholds, long-term study-reported means--that is, the 
study-reported ambient PM2.5 concentrations in the 
epidemiologic studies that reflect estimated exposures with a focus 
around the middle portion of the PM2.5 air quality 
distribution where the bulk of the observed data reside--provide the 
strongest support for reported health effect associations in 
epidemiologic studies.
    Based on the air quality criteria for this reconsideration, as 
described in the 2019 ISA, ISA Supplement, 2022 PA and the proposal, 
the EPA believes it is appropriate to continue to use the mean 
PM2.5 concentrations from the key epidemiologic studies to 
inform conclusions regarding the appropriate level for the primary 
annual PM2.5 standard.
    There are a large number of key epidemiologic studies available in 
this reconsideration to inform conclusions regarding the level of the 
primary annual PM2.5 standard. For the key U.S. 
epidemiologic studies, the study-reported mean PM2.5 
concentrations range from 9.9-16.5 [mu]g/m\3\ for monitor-based studies 
(Figure 1 above) and range from 9.3-12.2 [mu]g/m\3\ for hybrid 
modeling-based studies (Figure 2 above).
    In addition to the study-reported mean PM2.5 
concentrations, the EPA agrees with the CASAC's advice in their review 
of the 2021 draft PA and public comments that information on other 
percentiles below the mean can also be informative, and the EPA notes 
that the CASAC advised that for the purpose of informing the adequacy 
of the standards, future reviews should include an evaluation of other 
metrics, including the distribution of concentrations reported in 
epidemiologic studies (Sheppard, 2022a, p. 9 of consensus responses). 
As such, in reaching conclusions in this reconsideration, the EPA takes 
note of the additional study-reported PM2.5 concentrations 
below the means (e.g., 25th and 10th percentiles) that are available 
from a limited subset of key U.S. epidemiologic studies. As shown in 
Figures 1 and 2 above, six key U.S. epidemiologic studies report 
information on other percentiles (e.g., 10th and 25th percentiles of 
PM2.5 concentrations or 10th and 25th percentiles of 
PM2.5 concentrations associated with health events) that are 
below the mean.\97\ Three of the studies are monitor-based and three 
are hybrid model-based.
---------------------------------------------------------------------------

    \97\ The Wang et al. (2017) study only reports the 25th 
percentile of the estimated PM2.5 concentrations, not the 
10th percentile.
---------------------------------------------------------------------------

    The key U.S. epidemiologic studies that report percentiles below 
the mean that are monitor based are older studies. These studies 
included smaller numbers of people than the newer hybrid model-based 
studies. For the three older, monitor-based studies, because the 
cohorts were smaller in size, a relatively smaller portion of the 
health events were observed in the lower part of the air quality 
distribution. As such, our confidence in the magnitude and significance 
of the associations begins to decrease in the lower part of the air 
quality distribution of those older, monitor-based studies.
    The three newer, hybrid model-based studies have larger cohort 
sizes than the older, monitor-based studies and, as noted by 
commenters, have more health events in the lower part of the air 
quality distribution. For these reasons, the EPA notes that we have 
more confidence in the reported association at concentrations lower 
than the reported mean in these more recent hybrid model-based studies, 
particularly at the 25th percentile compared to the 10th percentile. 
While the cohort sizes in the more recent, hybrid model-based studies 
are larger than the older, monitor-based studies, the EPA notes that 
the 10th percentiles are well below the middle portion of the air 
quality distribution for which we have the greatest confidence, and as 
noted above, our confidence in the magnitude and significance of 
associations in the lower parts of the air quality distribution begins 
to decrease. While we have more confidence in the lower percentiles 
because of the larger cohort sizes in the more recent hybrid model-
based studies, we also have more confidence in the 25th percentiles 
than in the 10th percentiles, which are further from the means and 
closer to the lower end of the air quality distribution.
    In considering how the six studies that report percentiles lower 
than the mean can be used to inform conclusions regarding the level of 
the primary annual PM2.5 standard, the EPA first notes that 
the three monitor-based epidemiologic studies (Bell et al., 2008; 
Franklin et al., 2007; Zanobetti and Schwartz, 2009) report 25th 
percentile concentrations that are at or above 11.5 [micro]g/m\3\. For 
two of the more recent hybrid model-based studies (Di et al., 2017b; 
Wang et al., 2017), the 25th percentile of estimated PM2.5 
concentrations are just above 9 [micro]g/m\3\, while one study (Di et 
al., 2017a) reports a PM2.5 concentrations corresponding to 
25th percentiles of health events of just below 7 [micro]g/m\3\. For 
the Di et al. (2017a) study, the 25th percentile PM2.5 
concentration (6.7 [micro]g/m\3\) is based on the PM2.5 
concentration at which the 25th percentile of deaths occur in the 
study, while the reported mean (11.6 [micro]g/m\3\) is based on 
estimated PM2.5 exposure concentrations. Additionally, the 
25th percentiles of the other two recently available hybrid model-based 
studies (Di et al., 2017b; Wang et al., 2017) are based on estimated 
PM2.5 concentrations. As such, the PM2.5 
concentration at which the 25th percentile of health events occur may 
be different from the estimated 25th percentile PM2.5 
concentration in this study (Di et al., 2017a), creating an uncertain 
basis for comparison with the studies by Di et al. (2017b) and Wang et 
al. (2017). The 25th percentiles from these studies, in particular 
those that are more recently available, help to inform the 
Administrator's judgments regarding the appropriate level for the 
primary annual PM2.5 standard.
    Some commenters disagree with the EPA's consideration of the 
relationship between mean PM2.5 concentrations reported in 
the key epidemiologic studies and design values to inform conclusions 
on the appropriate level for the primary annual PM2.5 
standards. Commenters contend that setting the level of the primary 
annual standard below the design values in the epidemiologic studies, 
rather than below the study-reported mean concentrations, might keep 
overall mean PM2.5 concentrations throughout an area below 
the study-reported means but allow PM2.5 concentrations in 
some parts of the area, including near the ``design value monitor'' to 
remain above the study-reported mean PM2.5 concentrations, 
which are the concentrations where the evidence of health effects is 
strongest. Commenters contend that such a decision framework would not 
result in a standard that would provide requisite protection with an 
adequate margin of safety,

[[Page 16261]]

particularly for at-risk populations. These commenters further support 
this view by citing the CASAC's advice in their review of the 2021 
draft PA, where the majority of CASAC stated that ``even if a design 
value is somewhat higher than the area average, it reflects actual 
exposure levels and thus any portion of the population living near the 
design value monitor does experience exposures at that level and 
consequent health effects of exposure to that higher concentration'' 
(Sheppard, 2022a, p. 14 of consensus responses). Additionally, these 
commenters suggest that the EPA should not deviate from the approach 
taken in the 2012 review, which was to set the standard at a level 
``somewhat below'' the lowest mean PM2.5 concentration in 
the key epidemiologic studies.
    To the extent that commenters are suggesting that the EPA is 
setting the level of the primary annual PM2.5 standard below 
the design values in the epidemiologic studies, rather than below the 
study-reported mean PM2.5 concentrations, we disagree with 
the commenters. In reaching conclusions on the level of the primary 
annual PM2.5 standard, the EPA considers the long-term 
study-reported mean PM2.5 concentrations from key 
epidemiologic studies and sets the level of the standard to somewhat 
below the lowest long-term mean PM2.5 concentration, not 
below the design values in the epidemiologic studies. Additionally, the 
EPA also considers the available information from a subset of 
epidemiologic studies that report exposure estimates or health events 
at the 25th and 10th percentiles of PM2.5 concentrations. 
The EPA particularly considers the 25th percentile data, while 
recognizing that our confidence in the magnitude and significance in 
the reported concentrations, and the ability of the lower percentile 
PM2.5 concentrations to inform decisions on the appropriate 
level of the annual standard, decreases with reduced data (below the 
mean) and diminishes further at percentiles that are even further below 
the mean and the 25th percentile.
    However, the EPA notes that it is important to understand, and to 
not ignore, the relationship between the study-reported mean 
PM2.5 concentrations reported in key epidemiologic studies 
and the area design value. As an initial matter, the NAAQS consists of 
all four elements of the standard (indicator, averaging time, form, and 
level) and setting a standard that is requisite to protect public 
health includes consideration of all four elements together. Following 
implementation of the NAAQS, the design value is the metric used to 
determine compliance with the standard and is the statistic that 
describes the air quality status of a given location relative to the 
level of the primary annual PM2.5 NAAQS. The design value is 
different from the study-reported mean PM2.5 concentrations. 
This is because the study-reported mean PM2.5 concentrations 
are an annual average PM2.5 concentration, similar to the 
level of the standard, but the epidemiologic studies do not report 
statistics that take into account the other elements of the standard 
(i.e., averaging time and form). Therefore, when considering the 
appropriate revisions to the annual PM2.5 standard, the EPA 
must consider the protection provided by a revised standard taking into 
account all of the elements of the standard, not just the annual 
average PM2.5 concentration alone.
    In considering the annual standard, and in assessing the range of 
study-reported exposure concentrations for which we have the strongest 
support for adverse health effects observed in epidemiologic studies, 
the EPA focuses on whether the current primary annual PM2.5 
standard provides adequate protection against these exposure 
concentrations or if the level of the standard should be revised to 
provide the appropriate public health protection. This means that, as 
in some previous reviews, it is important to consider how the study 
means were computed and how these concentrations compare to the annual 
standard metric (including the level, averaging time and form) which 
must be met at the monitor with the highest PM2.5 design 
value in an area for compliance with the NAAQS. This approach is based 
on the application of a decision framework based on assessing means (as 
well as the lower distribution of reported PM2.5 
concentration, as noted above) reported in key epidemiologic studies. 
In the 2012 review, the available key epidemiologic studies computed 
the mean PM2.5 concentrations using an average across 
monitor-based PM2.5 concentrations. As such, at that time, 
the decision framework used an approach based on maximum monitor 
concentrations to determine compliance with the standard, while 
selecting the standard level based on consideration of composite 
monitor concentrations (i.e., selecting the standard level of 12.0 
[micro]g/m\3\ was just below the long-term study-reported mean 
PM2.5 concentrations in key epidemiologic studies). Further, 
the EPA conducted analyses that examined the differences in these two 
metrics (i.e., maximum monitor concentrations, which is how compliance 
with the standard is assessed and composite monitor concentrations, 
which is how key epidemiologic studies report their mean 
concentrations) across the U.S. and in areas included in the key 
epidemiologic studies and found that the maximum design value in an 
area was generally higher than the monitor average across that area, 
with the amount of difference between the two metrics varying based on 
location and concentration (Hassett-Sipple et al., 2010; Frank, 2012). 
This information was taken into account by the then-Administrator's 
final decision in selecting a level of 12.0 [micro]g/m\3\ for the 
primary annual PM2.5 standard in the 2012 review and 
discussed more specifically in her considerations on adequate margin of 
safety.
    The relationship between the mean PM2.5 concentrations 
and the area design value continues to be an important consideration in 
evaluating the adequacy of the current or potential alternative annual 
standard levels in this reconsideration. Again, in a given area, the 
area design value is based on the monitor in an area with the highest 
PM2.5 concentrations and is used to determine compliance 
with the standard, including the averaging time and form of the 
standard (i.e., an annual average over 3-years must not exceed the 
level of the of the annual PM2.5 standard). The highest 
PM2.5 concentrations spatially distributed in the area would 
generally occur at or near the area design value monitor and the 
distribution of PM2.5 concentrations would generally be 
lower in other locations and at monitors in that area. As such, when an 
area is meeting a specific annual standard level (e.g., 9.0 [micro]g/
m\3\), we would expect the annual average exposures (i.e., a metric 
similar to the study-reported mean values) in that area to be at 
concentrations lower than that level (e.g., lower than 9.0 [micro]g/
m\3\).
    However, as described in section II.A.2.c.ii, we note that there 
are a substantial number of different types of epidemiologic studies 
available since the 2012 review, as assessed in both the 2019 ISA and 
the ISA Supplement, that make understanding the relationship between 
the mean PM2.5 concentrations and the area design value an 
even more important consideration in this reconsideration (U.S. EPA, 
2019a; U.S. EPA, 2022a). While the key epidemiologic studies in the 
2012 review were all monitor-based studies, the recent epidemiologic 
studies in this reconsideration include hybrid modeling approaches that 
have emerged in the epidemiologic literature as an

[[Page 16262]]

alternative to approaches that only use ground-based monitors to 
estimate PM2.5 exposure. As assessed in the 2019 ISA and ISA 
Supplement, a substantial number of epidemiologic studies used hybrid 
model-based methods in evaluating associations between PM2.5 
exposure and health effects. Hybrid model-based studies employ various 
fusion techniques that combine ground-based monitored data with air 
quality modeled estimates and/or information from satellites to 
estimate PM2.5 exposures. While these studies provide a 
broader estimation of PM2.5 exposures compared to monitor-
based studies (i.e., PM2.5 concentrations are estimated in 
areas without monitors), the hybrid modeling approaches result in 
study-reported means that are more difficult to relate to the annual 
standard metric and to the maximum monitor design values used to assess 
compliance. In addition, to further complicate the comparison, when 
looking across these studies, we find variations in how exposure is 
estimated between such studies, and thus, how the study means are 
calculated. Two important variations across studies include: (1) 
Variability in spatial scale used (i.e., averages computed across the 
national (or large portions of the country) versus a focus on only 
CBSAs); and (2) variability in exposure assignment methods (i.e., 
averaging across all grid cells, averaging across a scaled-up area like 
a ZIP code, and population weighting). The differences in these 
approaches can result in studies reporting different study means, even 
though the association between PM2.5 exposure and health 
effects outcomes are similar.
    To emphasize the importance of the differences between the studies, 
we revisit the simplified example in the State of Georgia from the 2022 
PA that evaluates monitors and hybrid modeling approaches, noting that 
this example is useful to exhibit how the differences in the methods 
used to estimate exposure can lead to differences in the reported mean 
concentrations (U.S. EPA, 2022b, p. 3-71). In this example, for all 
monitors within the Atlanta-Sandy Springs-Roswell CBSA, the average 
PM2.5 concentration is 9.3 [micro]g/m\3\, while the area 
design value (based on the highest monitored PM2.5 
concentration in the area) is 10.4 [micro]g/m\3\. This comparison helps 
to illustrate the fact that composite monitor values tend to be 
somewhat lower than the highest area monitor values, consistent with 
the key points made in the 2012 review. This example also illustrates 
how monitors are sited to represent the higher concentrations within 
the area and that the area's annual design value, which is used for 
compliance with the standard, is calculated based on the highest 
monitor in the area. Next, in this example, mean PM2.5 
concentrations were calculated using similar approaches to those used 
in hybrid modeling-based epidemiologic studies to compute study-
reported means, including (1) the average concentration across the 
entire State of Georgia; (2) the population-weighted average across the 
entire State; (3) the average concentration across the Atlanta-Sandy 
Springs-Roswell CBSA; and (4) the population-weighted average across 
the Atlanta-Sandy Springs-Roswell CBSA. At the urban level (e.g., 
Atlanta-Sandy Springs-Roswell CBSA), the average PM2.5 
concentration when taking the mean of all grid cells is 9.2 [micro]g/
m\3\, whereas the population-weighted mean is 9.6 [micro]g/m\3\. Across 
Georgia, the average PM2.5 concentration using the hybrid 
approach and averaged across each grid cell is 8.3 [micro]g/m\3\, which 
is lower than the population-weighted statewide average of 9.1 
[micro]g/m\3\. While this is a simple example completed in one State 
and one CBSA, it suggests that the lowest mean values tend to result 
from the approaches that use concentrations from all or most grid cells 
(e.g., did not apply population weighting), both urban and rural, 
across the study area to compute the mean. Higher mean values are 
observed when the approach focuses on the urban areas alone or when the 
approach incorporates population weighting. Overall, this example 
suggests that the means from studies using hybrid modeling approaches 
are generally lower than the means from monitor-based approaches, and 
means from both approaches are lower than the annual design values for 
the same area. Population weighting tends to increase the calculated 
mean concentration, likely because more densely populated areas also 
tend to have higher PM2.5 concentrations. In other words, 
this simplified example exhibits how not all reported mean 
PM2.5 concentrations from key epidemiologic studies are the 
same; some reported means are from monitored studies and some reported 
means are from hybrid modeling studies, while some reported means 
include only urban areas, and other reported means include both urban 
and rural areas, and some reported means include aspects of population 
weighting while others do not.
    As detailed above in section I.D.5, in the air quality analyses 
comparing composite monitored PM2.5 concentrations with 
annual PM2.5 design values in U.S. CBSAs, maximum annual 
PM2.5 design values were approximately 10% to 20% higher 
than annual average composite monitor concentrations (i.e., averaged 
across multiple monitors in the same CBSA). Based on these results, 
this analysis suggests that there will be a distribution of 
concentrations and the maximum annual average monitored concentration 
in an area (at the design value monitor, used for compliance with the 
standard), will generally be 10-20% higher than the average across the 
other monitors in the area. Thus, in considering how the annual 
standard levels would relate to the study-reported means from monitor-
based studies, we can generally conclude that an annual standard level 
that is no more than 10-20% higher than monitor-based study-reported 
mean PM2.5 concentrations would generally maintain air 
quality exposures to be below those associated with the study-reported 
mean PM2.5 concentrations, exposures for which we have the 
strongest support for adverse health effects occurring.
    Air quality analyses described in section I.D.5 above also consider 
information from the epidemiologic studies that utilized the hybrid 
modeling approaches. Analyses show that average maximum annual design 
values are 40-50% higher when compared to annual average 
PM2.5 concentrations estimated without population weighting 
and are 15-18% higher when compared to average annual PM2.5 
concentrations with population weighting applied. Given these results, 
it is worth noting that for the studies using the hybrid modeling 
approaches, the choice of methodology employed in calculating the 
study-reported means (i.e., using population weighting versus not 
applying aspects of population weighting), and not a difference in 
estimates of exposure in the study itself, can produce substantially 
different study-reported mean values, with the approach that does not 
employ population weighting producing a much lower reported mean 
PM2.5 concentration. Therefore, the impact of the 
differences in methods is an important consideration when comparing 
mean concentrations across studies.
    Because of the differences in the methods employed by the key 
epidemiologic studies, and as demonstrated by the example and air 
quality analyses above, the application of any decision framework that 
considers the study-reported mean PM2.5 concentrations, and 
evaluates whether the current annual standard provides adequate 
protection against these reported exposure concentrations, is more 
complicated than the

[[Page 16263]]

approaches used in past reviews. As such, the EPA disagrees with 
commenters who argue that the EPA's consideration of the relationship 
between mean PM2.5 concentrations reported in key 
epidemiologic studies and design values is not appropriate and should 
be ignored.
    In considering the information from the epidemiologic studies, 
while the EPA does not dispute the reported associations of 
epidemiologic studies in hybrid modeling studies that report long-term 
mean concentrations and do not apply aspects of population weighting, 
using the reported long-term mean concentration from these studies in 
informing an appropriate level of the annual PM2.5 standard 
is more uncertain. Given this, hybrid modeling studies that do not 
apply aspects of population weighting provide less information on 
conclusions regarding the appropriate level of the primary annual 
PM2.5 standard. In support of this, some commenters also 
noted this consideration and suggested that the Administrator place 
lower weight on U.S. studies that did not use population weighting.
    In considering the relationship between study-reported mean 
PM2.5 concentrations and the design values, the EPA agrees 
with commenters that setting the level of the primary annual standard 
below the design values, rather than below the study-reported mean 
concentrations, might allow PM2.5 concentrations in some 
part of the area near the design value monitor to remain above the 
study-reported mean PM2.5concentration, where evidence of 
health effects is strongest. As discussed in the proposal and in 
section II.B.4 below, the Administrator specifically notes that that 
the highest PM2.5 concentrations spatially distributed in 
the area would generally occur at or near the area design value monitor 
and that PM2.5 concentrations will be equal to or lower at 
other monitors in the area. Furthermore, since monitoring strategies 
aim to site monitors in areas with higher PM2.5 
concentrations, monitored areas will generally have higher 
concentrations compared to areas without monitors. Therefore, by 
setting the level of the standard to 9.0 [micro]g/m\3\ and just below 
the lowest study-reported mean PM2.5 concentration (e.g., 
9.3 [micro]g/m\3\), the highest possible design value in a given area 
would be just below the study-reported mean PM2.5 
concentration, the concentration where we have the most confidence in 
the reported health effect association, and we anticipate that, based 
on our assessment of air quality data, the distribution of 
PM2.5 concentrations would decrease even further with 
distance from the highest monitor (i.e., the ``design value monitor'') 
(see, for example, U.S. EPA, 2022a, section 2.3.3.2.4 and pp. 3-71 to 
3-77). The Administrator further notes that when an epidemiologic study 
reports a mean PM2.5 concentration that reflects the average 
of annual average monitor-based concentrations across an area, the area 
design value will generally be higher than the study-reported mean. 
Similarly, he observes that when a study reports a mean that reflects 
the average of annual average concentrations estimated at across an 
area using a hybrid modeling approach, the area design value will 
generally be higher. As such, by evaluating the difference between the 
study-reported mean PM2.5 concentrations and design values, 
the Administrator seeks to set the level of the standard below the 
lowest study-reported mean, while ensuring that the primary annual 
PM2.5 standard, including its averaging time and form, 
provides protection against the exposures associated with health 
effects observed in the key epidemiologic studies.
    Additionally, the EPA disagrees with commenters who contend that 
the approach taken may allow PM2.5 near the design value 
monitor to remain above the study-reported mean PM2.5 
concentrations. In following this approach of setting the annual 
standard level somewhat below the lowest reported mean PM2.5 
concentration, setting a standard level that requires the design value 
monitor (which is the highest monitor in an area) to be just below the 
lowest study-reported mean across key studies will generally result in 
distributions of even lower concentrations of PM2.5 across 
the entire area, such that even those people living near an area design 
value monitor (where PM2.5 concentrations are generally 
highest) will be exposed to PM2.5 concentrations below the 
PM2.5 concentrations reported in the epidemiologic studies 
where there is the highest confidence of an association. In their 
review of the 2021 draft PA, the majority of the CASAC had some 
concerns about the approach for comparing study means and design 
values, questioning whether such an approach would provide adequate 
protection for people who live in areas with higher concentrations, 
such as those living in areas with higher concentrations (e.g., near 
the design value monitor) (Sheppard, 2022a, p. 8 of consensus 
responses). The minority of the CASAC, in considering the relationship 
between the study-reported mean PM2.5 concentration and 
design values, stated that ``the form of the standard and the way 
attainment with the standard is determined (i.e., highest design value 
in the CBSA) are important factors when determining the appropriate 
level for the standard'' and noted that that design values are 
generally higher than area average exposure levels (Sheppard, 2022a, p. 
17 of consensus responses). For all of the reasons discussed above, and 
consistent with the minority of the CASAC's advice in their review of 
the 2021 draft PA, we disagree with the commenters that areas near the 
design value monitors would be expected to experience PM2.5 
concentrations above the study-reported mean concentrations.
    Several commenters assert that epidemiologic studies that restrict 
PM2.5 concentration to below 12 [micro]g/m\3\ provide 
additional support for revising the level of the primary annual 
PM2.5 standard to 8 [micro]g/m\3\. Some commenters disagree 
with the EPA's assertion that the studies that employ restricted 
analyses do not provide enough information to understand how the 
studies were restricted to certain PM2.5 concentrations, 
with commenters providing additional information on the methods for 
restricted analyses. The commenters state that for the long-term 
studies at issue here, the study authors simply examined their database 
that linked subjects to long-term PM2.5 concentrations above 
12 [mu]g/m\3\, removed those data from the analysis, and reran the 
analysis. Additionally, one commenter provided an explanation of how 
the restricted analyses were conducted in studies for which he was an 
author. The commenter notes that for each year a subject was in the 
study, annual PM2.5 concentrations were assigned at the ZIP 
code level. If they moved, they were assigned the ZIP code level 
PM2.5 concentration for the new ZIP code. The commenter 
notes that these restricted analyses only included subjects whose 
annual PM2.5 exposure never exceeded that restricted 
concentration for any year of follow-up in the study. The commenter 
suggested that the EPA may be concerned as to how PM2.5 
concentrations in restricted analyses related to a design value since 
these are exposures for individuals who may have relocated during the 
study but argue that that is not the point. The commenters assert that 
while the analyses were restricted to people never exposed above 
certain concentrations over longer periods of time, the actual 
PM2.5 exposure was one year of exposure in most of these 
studies. Commenters also suggest that, since the

[[Page 16264]]

EPA has deviated from its approach from the 2012 review for considering 
study-reported mean PM2.5 concentrations, the EPA should 
dismiss its concerns regarding being able to relate the mean 
PM2.5 concentrations from these studies to design values.
    First, the EPA agrees with commenters that studies that employ 
restricted analyses can be used for informing conclusions regarding the 
appropriate level of the primary annual PM2.5 standard. 
However, the EPA disagrees that the information provided by the 
commenters provides a sufficient basis for an annual standard level of 
8 [mu]g/m\3\. Restricted analyses provide additional support for 
effects at lower concentrations, exhibiting associations for mean 
concentrations presumably below the mean concentrations for the main 
analyses. However, even though commenters note that any individual with 
exposures over the restricted analyses is excluded from restricted 
analyses, uncertainties remain with regard to how the mean 
PM2.5 concentrations in restricted analyses compare to 
design values, particularly in light of the removal of entire ZIP codes 
from analyses. Design values are calculated based on all measured 
PM2.5 concentrations. When an analysis is restricted below a 
certain level, some parts of the air quality distribution are removed, 
but comparing the restricted mean to a design value is not possible 
because these are two different metrics. For example, in a study that 
restricts concentrations below 12 [micro]g/m\3\, that represents only 
part of the air quality distribution, whereas a design value for that 
study area would include all PM2.5 concentrations, not just 
the ones below 12 [micro]g/m\3\. Therefore, in contrast to means from 
the main (unrestricted) analysis, it is not possible to compare mean 
concentrations from restricted analyses to design values. Further, it 
is unclear how one could evaluate such a relationship between design 
values and mean PM2.5 concentrations from studies that use 
restricted analyses because the standard is set based on all of its 
elements (indicator, averaging time, form, and level) and removing 
PM2.5 concentrations from the calculation of the design 
value for such a comparison would result in a metric that is no longer 
a design value that would provide the intended protection of the 
standard. This leads to greater uncertainty in how to use the mean 
PM2.5 concentrations from these studies that use restricted 
analyses in a similar decision framework as the epidemiologic studies 
that report long-term mean PM2.5 concentrations for health 
effect associations for the full distribution of PM2.5 
concentrations.
    As described in reaching his conclusions in the section below, the 
Administrator judges that, despite these uncertainties and limitations, 
studies that use restricted analyses can provide supplemental 
information for consideration in reaching conclusions regarding both 
the adequacy and level of the standard. He notes two studies (Di et 
al., 2017b and Dominici et al., 2019) are available in this 
reconsideration that report means in their restricted analyses 
(restricting annual average PM2.5 exposure below 12 [mu]g/
m\3\) and used population-weighted approaches to estimate 
PM2.5 exposures and these studies report mean 
PM2.5 concentrations of 9.6 [micro]g/m\3\. He recognizes 
that these studies are just one line of evidence for consideration and 
that along with the broader evidence base, including the key 
epidemiologic studies, these studies provide support that the level of 
the primary annual PM2.5 standard should be set below 10 
[micro]g/m\3\.
    We disagree with the commenters that concerns about relating the 
mean PM2.5 concentrations from restricted analyses to design 
values are not valid. As an initial matter, restricted analyses were 
not available and did not inform the 2012 decision to revise the annual 
PM2.5 standard level to 12.0 [micro]g/m\3\. The approach in 
2012 in revising the annual standard was to set the level to somewhat 
below the mean of key epidemiologic studies. As noted above, while the 
EPA believes that restricted analyses can help inform conclusions 
regarding the adequacy and the level of the primary annual 
PM2.5 standard, in the context of placing the studies in a 
decision framework to inform the appropriate level of the annual 
PM2.5 standard, the EPA has not deviated from its approach 
from the 2012 review. Given that restricted analyses are new since the 
2012 review, the EPA disagrees with commenters that uncertainties 
associated with these studies should not be considered, and that these 
studies should be used in a similar manner to their main analyses in 
taking an approach to set a level of the standard somewhat below the 
lowest long-term reported mean PM2.5 concentration. 
Specifically, as detailed above there are uncertainties and limitations 
associated with relating the mean PM2.5 concentrations from 
these studies to design values for studies that use restricted 
analyses, and many of these studies did not expressly report a mean 
PM2.5 concentration for the restricted analysis which makes 
it impossible to make such a comparison.
    Several commenters contend that in considering the accountability 
studies, the EPA inappropriately reached conclusions regarding the 
level of the primary annual PM2.5 standard based on the 
starting PM2.5 concentrations of these studies, rather than 
the ending concentrations (i.e., concentrations after a policy was 
implemented). The commenters assert that these studies provide support 
for revising the level of the primary annual PM2.5 standard 
to below the proposed range of 9-10 [micro]g/m\3\ to protect public 
health with an adequate margin of safety.
    Accountability studies examine the effect of a policy on reducing 
PM2.5 concentrations in ambient air and evaluate whether 
such reductions were observed to also lead to reductions in 
PM2.5- associated health outcomes (e.g., mortality). 
Additionally, accountability studies can reduce uncertainties related 
to residual confounding of temporal and spatial factors (U.S. EPA, 
2022a, p. 3-25). Prior to implementation of the policies, three 
accountability studies newly available in this reconsideration and 
assessed in the ISA Supplement, report mean PM2.5 
concentrations below the level of the current annual standard level 
(12.0 [micro]g/m\3\) and ranged from 10.0 [micro]g/m\3\ to 11.1 
[micro]g/m\3\ (Sanders et al., 2020b; Corrigan et al., 2018; and 
Henneman et al., 2019). These studies suggest that public health 
improvements may occur following the implementation of a policy that 
reduces annual average PM2.5 concentrations below the level 
of the current standard of 12.0 [micro]g/m\3\, and potentially below 
the lowest ``starting'' concentrations in these studies of 10.0 
[micro]g/m\3\. However, while the small number of studies may provide 
limited information related to informing the adequacy and level of the 
annual PM2.5 standard, we note that accountability studies 
are only one line of evidence, and that these studies provide 
supplemental information for consideration in addition to the full body 
of evidence. Further, the EPA does not believe it would be appropriate 
to determine the level of the standard by reference to ending 
concentrations in accountability studies. Accountability studies are 
most informative in demonstrating that public health improvements may 
occur following the implementation of a policy that reduces annual 
average PM2.5 concentrations below the level of the current 
standard of 12.0 [micro]g/m\3\, and potentially below the lowest 
``starting'' concentrations in these studies of 10.0 [micro]g/m\3\. 
However, the EPA finds the available information from accountability 
studies is too limited to support a conclusion that the

[[Page 16265]]

appropriate level at which to set the primary annual PM2.5 
standard would be equal to the ending concentrations of those studies, 
as the commenters suggest. These studies demonstrate that there are 
reductions in health outcomes when PM2.5 concentrations are 
reduced in these studies from the starting concentration to the ending 
concentration, but do not provide support for health effect 
associations at or below the ending concentrations that would warrant a 
more stringent standard.
    Commenters disagree with the Administrator placing less weight on 
the epidemiologic studies conducted in Canada when reaching conclusions 
regarding the level of the primary annual PM2.5 standard. 
These commenters argue that the Canadian epidemiologic studies provide 
support for setting the level at the lowest end of the proposed range 
(i.e., 8 [micro]g/m\3\) because they report mean PM2.5 
concentrations, in some cases, below 8 [micro]g/m\3\. Commenters 
disagree with the EPA's reasoning for placing less weight on the 
Canadian epidemiologic studies, suggesting it conflicts with the 
approaches in previous PM NAAQS reviews and arguing that the findings 
of the Canadian epidemiologic studies can be directly translated into a 
primary annual PM2.5 standard. Additionally, while the 
commenters disagree with the EPA's approach for considering the study-
reported mean PM2.5 concentrations and design values in 
general, they note that the CASAC, in their review of the 2021 PA, 
noted that ``while there may be no design value in Canada, there are 
data that indicate what a U.S. design value would be if an area average 
like that found in the Canadian studies were to occur in the U.S.'' 
(Sheppard, 2022a, p. 13 of consensus responses). The commenters contend 
that the EPA failed to acknowledge this advice from the CASAC, 
specifically noting that the majority of the CASAC highlighted Canadian 
epidemiologic studies as a part of their rationale for revising the 
level of the primary annual PM2.5 standard to within the 
range of 8-10 [micro]g/m\3\.
    In considering the information from the epidemiologic studies in 
reaching his conclusions, the Administrator considered the full body of 
evidence, including studies conducted in the U.S. and Canada. However, 
as described in the proposal and in section II.B.4 below, the 
Administrator also recognizes that the exposure environments in the 
U.S. are different from those in Canada. In particular, the U.S. 
population density is approximately 43 people per square kilometer in 
the contiguous U.S.\98\ compared to Canada, which has one of the lowest 
population densities on the Earth with 4.2 people per square kilometer 
(Statistics Canada, 2023). This difference in population density 
between the U.S. and Canada was not as apparent, and did not need to be 
highlighted, in the 2012 review given that the available Canadian 
epidemiologic studies used population-weighting and focused on urban 
areas where monitors were available and population densities were more 
comparable with those in the U.S. Given this, the study-reported mean 
concentrations from U.S. and Canadian studies in the 2012 review were 
very similar. The recent epidemiologic evidence available in this 
reconsideration, however, includes studies that utilize approaches that 
highlight the importance of considering the differences between the two 
exposure environments in the U.S. versus Canada. When focusing on the 
recently available Canadian monitor-based epidemiologic studies in this 
reconsideration, the information indicates that these studies, unlike 
the studies available in the 2012 review, do not apply population 
weighting (e.g., Lavigne et al., 2018; Liu et al., 2019). As noted in 
responding to other public comments above, the absence of population 
weighting is an important consideration that limits the utility of 
these studies in informing the appropriate level of the primary annual 
PM2.5 standard. In addition, there are recently available 
studies in the 2019 ISA and ISA Supplement that expand the geographical 
extent of the epidemiologic study areas by estimating exposure 
concentrations in areas where there are no monitors. To do this, these 
studies use either a statistical extrapolation of monitored values or 
use air quality modeling and other forms of data (e.g., hybrid model-
based approaches). For these Canadian studies, the EPA notes two 
important considerations in using the information to directly translate 
to policy decisions regarding the level of the annual standard in the 
U.S. The first is that in incorporating a larger portion of Canada into 
these recent studies, more rural areas are included, and as such, the 
population densities and exposure environment differences become more 
important. The second is that in analyses that evaluate and validate 
hybrid models, there is less certainty in PM2.5 exposure 
estimates in more rural areas, which are further from air quality 
monitors and where PM2.5 concentrations in the ambient air 
tend to be lower (U.S. EPA, 2022b, pp. 2-51 and 2-63). Additionally, it 
is unclear what portion of the PM2.5 concentrations from 
rural areas are contributing to the study reported mean. Given this, 
studies that incorporate more rural areas into the epidemiologic 
studies highlight the importance of considering the differences between 
the population exposures in the studies themselves and in the U.S. 
versus Canadian study areas, as well as the influence these differences 
have on the interpretation of the epidemiologic study results. For 
these reasons, while the Canadian epidemiologic studies provide 
additional support for associations between PM2.5 
concentrations and health effects, the long-term means from Canadian 
epidemiologic studies are a less certain basis for informing the EPA's 
selection of the annual standard level, given that it is a U.S.-based 
standard.
---------------------------------------------------------------------------

    \98\ All of the key U.S. epidemiologic studies considered in 
this reconsideration focus on all or subsections of the continental 
U.S.
---------------------------------------------------------------------------

    With respect to the CASAC's advice in their review of the 2021 
draft PA, the EPA recognizes that the majority of the CASAC pointed to 
the Canadian studies as supporting their recommendation to revise the 
annual standard level to within the range of 8-10 [micro]g/m\3\. 
However, the EPA also notes that the CASAC did not advise the EPA to 
revise the annual standard to a level that was below the study-reported 
means in the key Canadian epidemiologic studies. Indeed, the CASAC 
noted that some of the Canadian studies showed associations below 8 
[micro]g/m\3\, but did not recommend that the Administrator consider 
levels below 8 [micro]g/m\3\ for the annual standard. Further, based on 
the CASAC's advice, the Administrator is not excluding Canadian studies 
from his consideration in this reconsideration, but he is considering 
them in light of the limitations and challenges presented and in the 
context of the full body of available scientific evidence.
    Lastly, the EPA disagrees with commenters that the findings of the 
Canadian epidemiologic studies can be directly translated into a 
primary annual PM2.5 standard based on the evaluation of the 
relationship between U.S. study-reported mean PM2.5 
concentrations and U.S. design values. It is unclear whether the 
relationship between U.S. study-reported mean PM2.5 
concentrations and U.S. design values (which, in the case of U.S. 
hybrid model-based studies, indicates that design values are 15-18% 
greater than area mean PM2.5 concentrations) would apply to 
the Canadian epidemiologic studies and their reported mean 
PM2.5

[[Page 16266]]

concentrations, given that these studies generally report lower 
PM2.5 concentrations than the U.S.-based studies. As such, 
interpreting the study-reported mean concentrations from the Canadian 
studies in the context of a U.S.-based standard may present challenges 
in directly and quantitatively informing decisions regarding potential 
alternative levels of the annual standard, particularly noting the 
different in exposure relationships in the U.S. versus Canada given the 
large difference in population densities between the two countries. 
Further, as mentioned above, while the CASAC advised the EPA to 
consider the Canadian studies as relevant evidence and found that 
placing weight on the Canadian studies supported their recommendation 
to revise the annual standard level to within the range of 8-10 
[micro]g/m\3\, the lower end of their recommended range for the level 
of the annual standard did not extend below the lower study-reported 
means from those studies.
    Commenters who supported retaining and revising the primary annual 
PM2.5 standard both raised concerns regarding how the EPA 
used the scientific evidence and quantitative risk assessment related 
to disparities in PM2.5 exposure and risk in informing 
conclusions on the standard. Commenters who supported retaining the 
standard assert that the available scientific evidence that 
demonstrates disparities for minority populations do not support 
revising the standard, noting that these studies are in areas that tend 
to have large minority populations and more sources of PM. These 
commenters contend that because the studies conclude that minority 
populations experience more effects than others living in the same area 
that something other than PM2.5 concentrations in ambient 
air is causing the disproportionate impact on minority populations, 
providing proximity to a source as an example. The commenters note that 
it is unclear how a national standard will reduce exposure disparities 
for population groups living in the same area, and further assert that 
studies of exposure disparities among minority populations were 
considered in reaching the 2020 final decision to retain the standards.
    Conversely, commenters who support revising the standard assert 
that the at-risk analyses conducted in the 2022 PA provide support for 
revising the primary annual PM2.5 standard to a level of 8 
[mu]g/m\3\. In particular, these commenters state that the at-risk 
analysis demonstrated that while disparities in mortality risk remain 
at a standard level of 9.0 [mu]g/m\3\, disparities in exposure are 
significantly reduced for an alternative standard level of 8.0 [mu]g/
m\3\ (U.S. EPA, 2022b, p. 3-162).
    As discussed in section I above, the primary (health-based) NAAQS 
are established at a level that is requisite to protect public health, 
including the health of sensitive or at-risk groups, with an adequate 
margin of safety.\99\ In so doing, decisions on the NAAQS are based on 
an explicit and comprehensive assessment of the current scientific 
evidence and associated risk analyses. More specifically, the EPA 
expressly considers the available information regarding health effects 
among at-risk populations in decisions on the primary NAAQS. Where 
populations with disparities in exposure and risk are among the at-risk 
populations, the decision on the standards is based on providing 
requisite protection for these and other at-risk populations and 
lifestages.
---------------------------------------------------------------------------

    \99\ The legislative history of section 109 indicates that a 
primary standard is to be set at ``the maximum permissible ambient 
air level . . . which will protect the health of any [sensitive] 
group of the population,'' and that for this purpose ``reference 
should be made to a representative sample of persons comprising the 
sensitive group rather than to a single person in such a group.'' S. 
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970); see also, e.g., 
Am. Lung Ass'n v. EPA, 134 F.3d 388, 389 (D.C. Cir. 1998) (``If a 
pollutant adversely affects the health of these sensitive 
individuals, EPA must strengthen the entire national standard'').
---------------------------------------------------------------------------

    The Administrator expressly considered the available information 
regarding health effects among at-risk populations in reaching the 
proposed decisions that the current primary annual PM2.5 
standard is not requisite to protect public health with an adequate 
margin of safety, and should be revised. The 2019 ISA and ISA 
Supplement identified children, older adults, people with pre-existing 
diseases (cardiovascular disease and respiratory disease), minority 
populations, and low SES populations as at-risk populations. The 
Administrator is thus, in his final decision, establishing primary 
PM2.5 standards which, in his judgment, will provide 
protection for these at-risk populations, including minority 
populations, with an adequate margin of safety.
    With respect to the risk assessment, while the EPA notes that the 
analyses support the conclusion that the primary PM2.5 
standards are not adequate, as detailed further in the proposal and 
above in section II.A.3, the EPA also cautions against an over-
interpretation of the absolute results. The quantitative risk 
assessment provides estimates of PM2.5-attributable 
mortality based on input data that include C-R functions from 
epidemiologic studies that do not quantitatively account for 
uncertainties in associations between PM2.5 exposure and 
health effects at lower concentrations and are based on an air quality 
adjustment approach that incorporates proportional decreases in 
PM2.5 concentrations to meet lower alternative standard 
levels. As a result, simulated air quality improvements used in the 
risk assessment will always lead to proportional decreases in risk 
(i.e., each additional [micro]g/m\3\ reduction produces additional 
benefits with no clear stopping point), without considering the 
substantially greater uncertainties associated with the relationship 
between PM2.5 exposures and health effects at lower 
concentrations.
    The same is true for the new at-risk analysis in the risk 
assessment presented in the 2022 PA that is based on a recent 
epidemiologic study that is available in this reconsideration that 
provides mortality risk coefficients for older adults (i.e., 65 years 
and older) based on PM2.5 exposure and stratified by racial 
and ethnic demographics. Generally, the results of at-risk analyses can 
vary greatly depending on the inputs to the analyses, including the 
representativeness of the populations and demographics captured by the 
study areas that are a part of the analyses, as well as the available 
C-R functions from epidemiologic studies that stratify by race and 
ethnicity and the air quality adjustment approaches that are used to 
simulate air quality at different standard levels. In fact, for this 
at-risk analysis, the results are even more uncertain than similar 
estimates from the overall risk assessment due to additional sources of 
uncertainty specific to the at-risk analysis, such as using C-R 
functions derived from smaller epidemiologic sample sizes along with 
the sources of uncertainty that apply to the overall risk assessment 
(U.S. EPA, 2022b, section 3.4.1.8). Additionally, in characterizing at-
risk populations, the at-risk analysis only used one of the air quality 
adjustment approaches used in the overall risk assessment, which 
decreases the potential representativeness of the PM2.5 
concentrations across the study areas (U.S. EPA, 2022b, section 
3.4.1.8). Lastly, this at-risk analysis relies on the stratified risk 
coefficients from only one epidemiologic study.\100\ For these reasons, 
the Administrator places little

[[Page 16267]]

weight on the absolute results of the risk assessment, including the 
at-risk analysis, for purposes of selecting the level of the annual 
standard that is requisite.
---------------------------------------------------------------------------

    \100\ Additional information on all available at-risk 
epidemiologic studies in this reconsideration are available in 
section 3.4 and Appendix C of the 2022 PA (U.S. EPA, 2022b, section 
3.4, Figure 3-17, and Appendix C, section C.3.2).
---------------------------------------------------------------------------

    While there are substantial uncertainties in the absolute results 
of the quantitative risk assessment, the EPA also notes that recent 
scientific evidence evaluated in the ISA Supplement, which built upon 
the 2019 PM ISA conclusions, found that the evidence ``[c]ontinue[s] to 
support disparities in PM2.5 exposure and health risks by 
race and ethnicity'' while studies of SES ``provide additional support 
indicating there may be disparities in PM2.5 exposure and 
health risk by SES'' (U.S. EPA, 2022a, p. 5-4). Thus, in light of the 
statutory requirement to provide protection for at-risk populations, it 
is not surprising that the stratified population results of the risk 
assessment suggest that meeting a revised standard would result in 
higher risk reductions for minority and low SES populations.
    In conclusion, the EPA recognizes that the at-risk analysis was 
based on one epidemiologic study that stratified by race/ethnicity for 
older adults (e.g., 65+ years old) and that there is increasing 
uncertainty in quantitative estimates of stratified risk estimates at 
the lower end of the range of standard levels assessed. Moreover, the 
EPA finds that the goal of the NAAQS is to provide the requisite 
protection to at-risk groups, and where minority populations are 
included among the at-risk groups, providing requisite protection to 
minority populations will also result in protecting the public health 
of other populations. Thus, in setting the NAAQS to protect the health 
of at-risk groups with an adequate margin of safety, the Administrator 
is selecting the standard that will provide requisite protection, 
including for minority populations and other at-risk populations, which 
also generally results in protecting the public health of other 
populations and reducing risk disparities.
    A number of commenters, primarily from industries and industry 
groups and some States, support the EPA's proposed decision to retain 
the primary 24-hour PM2.5 standard. Many of these commenters 
contend that the available scientific evidence and quantitative 
information has not significantly changed since the 2020 final decision 
and note that important uncertainties remain. The commenters agree with 
the EPA's conclusions regarding the controlled human exposure studies 
and their relationship to short-term peak PM2.5 
concentrations in ambient air. These commenters also noted the primary 
annual and 24-hour PM2.5 standards work together to provide 
public health protection, with the 98th percentile form of the 24-hour 
standard effectively limiting peak daily concentrations. The commenters 
agree with the EPA that the current suite of standards maintain 
subdaily concentrations below the higher concentrations in controlled 
human exposure studies where more consistent health effects are 
observed. Commenters also agree with the EPA's conclusions that the 
epidemiologic studies are not useful for informing decisions on the 
level of the primary 24-hour PM2.5 standard because the 
standard focuses on reducing peak exposures with its 98th percentile 
form, while the epidemiologic studies often focus on the mean or median 
as the percentile for which associations with short-term exposures are 
observed. These commenters also agree with the EPA's focus on U.S.-
based studies because of differences compared to Canadian studies. The 
commenters also generally agree with the Administrator's judgment that 
it was appropriate to place less weight on the risk assessment, noting 
that the annual standard is controlling in most areas of the country 
and revising the annual standard would have the most potential to 
reduce risk related to PM2.5 exposures and would reduce both 
average (annual) and peak (daily) PM2.5 concentrations. 
Finally, these commenters note that the CASAC did not reach consensus 
on whether the current primary 24-hour PM2.5 standard should 
be revised, and they agree with the minority of the CASAC's 
recommendation in their review of the 2021 draft PA that the primary 
24-hour primary PM2.5 standard should be retained. These 
commenters also note the CASAC's support in their review of the 2019 
draft PA for retaining the primary 24-hour PM2.5 standard.
    A number of commenters, primarily from public health and 
environmental organizations and some States, oppose the EPA's proposed 
decision to retain the primary 24-hour PM2.5 standard. These 
commenters support revising the level of the primary 24-hour 
PM2.5 standard, contending that a more stringent standard is 
necessary to provide requisite public health protection with an 
adequate margin of safety, particularly for at-risk groups. In so 
doing, these commenters place weight on the same aspects of the 
available scientific evidence as the majority of the CASAC in their 
review of the 2021 draft PA, and generally advocate for revising the 
level of the standard to within the range of 25-30 [micro]g/m\3\ as 
recommended by the majority of the CASAC. Some of these commenters 
support a level no higher than 25 [micro]g/m\3\ and others support a 
level of 20 [micro]g/m\3\. These commenters generally cite to the 
available scientific evidence, including evidence of disproportionate 
exposures and risks for certain at-risk groups, and the CASAC's advice 
in support for their recommendation. Some of these commenters also 
suggest that decisions regarding the primary 24-hour PM2.5 
standard should not be related to decisions on the primary annual 
PM2.5 standard.
    As an initial matter, the EPA disagrees with commenters who suggest 
that decisions regarding the primary 24-hour PM2.5 standard 
should not be related to decisions on the primary annual 
PM2.5 standard. In reviewing the adequacy of the public 
health protection afforded by the primary PM2.5 standards, 
the Administrator's consistent past practice has been to evaluate the 
combination of the annual and 24-hour standards together. In 2012, the 
then-Administrator concluded that the most effective and efficient way 
to reduce total population risk associated with both long- and short-
term PM2.5 exposures was to set a generally controlling 
annual standard, and to provide supplemental protection by means of a 
24-hour standard set at the appropriate level. In so doing, the then-
Administrator explicitly recognized that potential air quality changes 
associated with meeting a revised annual standard (with a level of 12 
[micro]g/m\3\) would result in lowering risks associated with both 
long- and short-term PM2.5 exposures by lowering the overall 
distribution of air quality concentrations, and that retaining a 24-
hour standard at the appropriate level would ensure an adequate margin 
of safety against short-term effects in areas with high peak-to-mean 
ratios (78 FR 3163, January 15, 2013). In this reconsideration, also, 
the Administrator considers it appropriate to rely on the annual 
standard (arithmetic mean, averaged over three years) for targeting 
protection against both long- and short-term PM2.5 
exposures, noting that the annual standard is typically controlling, 
while the 24-hour standard (98th percentile, averaged over three years) 
can provide supplemental protection against the occurrence of peak 24-
hour PM2.5 concentrations (U.S. EPA, 2022b, section 3.6.3). 
Further, the Administrator notes that, as in the 2012 review, changes 
in PM2.5 air quality to meet a revised annual standard would

[[Page 16268]]

affect the entire distribution of long- and short-term concentrations, 
thus likely resulting not only in lower short- and long-term 
PM2.5 concentrations near the middle of the air quality 
distribution, but also in fewer and lower short-term peak 
PM2.5 concentrations.\101\ Thus, the Administrator continues 
to conclude it is appropriate to consider whether the annual and 24-
hour standards together provide requisite protection of public health, 
rather than considering each standard in isolation.
---------------------------------------------------------------------------

    \101\ Similarly, the Administrator recognizes that changes in 
air quality to meet a 24-hour standard, would result not only in 
fewer and lower peak 24-hour PM2.5 concentrations, but 
also in lower annual average PM2.5 concentrations. 
However, as noted in 2012, an approach that relied on setting the 
level of the 24-hour standard such that the 24-hour standard was 
generally controlling would be less effective and result in less 
uniform protection across the U.S. than an approach that focuses on 
setting a generally controlling annual standard (78 FR 3163, January 
15, 2013).
---------------------------------------------------------------------------

    Regarding the appropriate basis for determining the level of the 
24-hour standard, a number of commenters who support revising the 
primary 24-hour PM2.5 standard to a lower level contend that 
the EPA should not rely on the controlled human exposure studies in 
evaluating the adequacy of the public health protection afforded by the 
primary 24-hour PM2.5 standard. These commenters support 
this view by citing the CASAC comments in their review of the 2019 
draft PA which advised that controlled human exposure studies have 
limitations that may impact their ability to inform conclusions on the 
adequacy of the public health protection afforded by the primary 24-
hour PM2.5 standard. Commenters noted that these studies do 
not include the most vulnerable populations and often involve exposure 
to only one pollutant to elicit a response, and therefore are not 
representative of real-world exposures.
    Other commenters support the EPA's use of the controlled human 
exposure studies to inform the adequacy of the public health protection 
and note that the 24-hour standard must at least provide protection 
against the health effects observed in controlled human exposure 
studies. Some of the commenters cite the Wyatt et al. (2020) study that 
demonstrated cardiovascular effects following 2-hour exposures to 120 
[micro]g/m\3\ and 4-hour exposures to 37.8 [micro]g/m\3\. Some of these 
commenters contend that the current primary 24-hour PM2.5 
standard allows PM2.5 exposures comparable to those observed 
to elicit effects in the controlled human exposure studies, and 
therefore, the EPA must revise the level of the current standard to 
protect public health. To support this view, some commenters submitted 
an analysis of monitoring data from 2017-2020, which compares the 
number of days per year where maximum daily PM2.5 
concentrations exceed 120 [micro]g/m\3\ and 37.8 [micro]g/m\3\.
    Additionally, other commenters assert that the EPA should focus 
less on peak PM2.5 concentrations ``typically measured'' in 
areas meeting the current primary PM2.5 standards even if 
they do not exceed the concentrations in the controlled human exposure 
studies because, in their view, the standard needs to protect against 
atypical exposures to atypical peak PM2.5 concentrations. 
These commenters conclude that, when considered together, the 
controlled human exposure studies and the epidemiologic studies warrant 
strengthening the level of the primary 24-hour PM2.5 
standard.
    The EPA generally disagrees with commenters who contend that it is 
inappropriate to rely on the controlled human exposures studies in 
evaluating the adequacy of the public health protection afforded by the 
primary 24-hour PM2.5 standard. The Agency considers these 
studies informative both for establishing biological plausibility and 
for determining an appropriate level for the 24-hour standard. When 
looking to the experimental studies, the EPA finds that the 2019 ISA 
and ISA Supplement included controlled human exposure studies that 
report statistically significant effects on one or more indicators of 
cardiovascular function following 2-hour exposures to PM2.5 
concentrations at and above 120 [mu]g/m\3\ (and at and above 149 [mu]g/
m\3\ for vascular impairment, the effect shown to be most consistent 
across studies). As noted in the 2019 ISA, these studies are important 
in establishing biological plausibility for PM2.5 exposures 
causing more serious health effects, such as those seen in short-term 
exposure epidemiologic studies, and they provide support that more 
adverse effects may be experienced following longer exposure durations 
and/or exposure to higher concentrations. Additionally, one controlled 
human exposure study assessed in the ISA Supplement reports evidence of 
some effects for cardiovascular markers at lower PM2.5 
concentrations, 4-hour exposures to 37.8 [micro]g/m\3\ (Wyatt et al., 
2020). However, there is inconsistent evidence for inflammation in 
other controlled human exposure studies evaluated in the 2019 ISA. The 
EPA notes that although the controlled human exposure studies do not 
provide a threshold below which no effects occur, the observed effects 
in these controlled human exposures studies are ones that signal an 
intermediate effect in the body, likely due to short-term exposure to 
PM2.5, and typically would not, by themselves, be judged as 
adverse (88 FR 5620, January 27, 2023) 102 103
---------------------------------------------------------------------------

    \102\ Judgments regarding adversity or health significance of 
measurable physiological responses to air pollutants have been 
informed by guidance, criteria or interpretative statements 
developed within the public health community, including the American 
Thoracic Society (ATS) and the European Respiratory Society (ERS), 
which cooperatively updated the ATS 2000 statement What Constitutes 
an Adverse Health Effect of Air Pollution (ATS, 2000) with new 
scientific findings, including the evidence related to air pollution 
and the cardiovascular system (Thurston et al., 2017).
    \103\ The ATS/ERS described its 2017 statement as one ``intended 
to provide guidance to policymakers, clinicians and public health 
professionals, as well as others who interpret the scientific 
evidence on the health effects of air pollution for risk management 
purposes'' and further notes that ``considerations as to what 
constitutes an adverse health effect, in order to provide guidance 
to researchers and policymakers when new health effects markers or 
health outcome associations might be reported in future.'' The most 
recent policy statement by the ATS, which once again broadens its 
discussion of effects, responses and biomarkers to reflect the 
expansion of scientific research in these areas, reiterates that 
concept, conveying that it does not offer ``strict rules or 
numerical criteria, but rather proposes considerations to be weighed 
in setting boundaries between adverse and nonadverse health 
effects,'' providing a general framework for interpreting evidence 
that proposes a ``set of considerations that can be applied in 
forming judgments'' for this context (Thurston et al., 2017).
---------------------------------------------------------------------------

    The EPA notes that the majority of the CASAC, in their review of 
the 2021 draft PA, commented that these controlled human exposure 
studies generally do not include populations with substantially 
increased risk from exposure to PM2.5, such as children, 
older adults, or those with more severe underlying illness, and often 
involve exposure to only one pollutant to elicit a response. However, 
both the majority and the minority of the CASAC explained that, even 
taking into consideration their limitations, the controlled human 
exposure studies provide some support for assessing the adequacy of the 
24-hour standard.\104\
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    \104\ In their review of the 2021 draft PA, the majority of the 
CASAC advised that ``evidence of effects from controlled human 
exposure studies with exposures close to the current standard 
support epidemiologic evidence for lowering the standard'' 
(Sheppard, 2022a, p. 4 of consensus letter). The minority of the 
CASAC also advised that it was appropriate to place ``more emphasis 
on the controlled human exposure studies, showing effects at 
PM2.5 concentrations well above those typically measured 
in areas meeting the current standards'' (Sheppard, 2022a, p. 4 of 
consensus letter), in evaluating adequacy of the 24-hour standard.
---------------------------------------------------------------------------

    The EPA agrees with the CASAC that the controlled human exposure 
studies generally do not include populations with substantially 
increased risk from exposure to PM2.5, like children, older 
adults, or those with pre-existing severe illness, like cardiovascular 
effects. As such, and as an initial note, these

[[Page 16269]]

studies are therefore somewhat limited in their ability to inform at 
what concentrations effects may be elicited in at-risk populations. In 
spite of this limitation, the EPA also agrees with the CASAC, that even 
taking into consideration the limitations of the controlled human 
exposure studies, these studies can provide some support for evaluating 
the adequacy of the 24-hour standard. However, the EPA further notes 
that while the controlled human exposure studies are important in 
establishing biological plausibility, the health outcomes observed in 
these controlled human exposure studies are often ``intermediate'' 
outcomes (i.e., not always clearly adverse) and therefore it is unclear 
how the importance of the effects observed in the studies should be 
interpreted with respect to adversity to public health. The EPA finds 
that it is appropriate to consider these study limitations in assessing 
the information provided by controlled human exposure studies in 
evaluating the adequacy of the primary 24-hour PM2.5 
standard.
    The EPA agrees with commenters that the primary 24-hour 
PM2.5 standard must at least provide protection against the 
health effects consistently observed in controlled human exposure 
studies. As discussed in the proposal, the EPA looks at whether the 
exposures that elicit a response following exposure to PM2.5 
in the controlled human exposure studies occur under recent air quality 
conditions in areas meeting the current standards. Based on these air 
quality analyses, the EPA concludes that these types of exposures very 
rarely occur when the current standards are being met.
    The EPA did receive multiple comments questioning these results and 
the approach in the EPA's analyses. For example, some commenters 
submitted an analysis of monitoring data from 2017-2020, which compares 
the number of days per year where maximum daily PM2.5 
concentrations exceed 120 [micro]g/m\3\ and 37.8 [micro]g/m\3\ and 
evaluate the number of days subset by groups of monitors with 4-year 
average PM2.5 concentrations close to the levels of 
combinations of current and proposed annual (+/- 0.2 [micro]g/m\3\) and 
24-hour (+/-2 [micro]g/m\3\) PM2.5 standards. To support 
their view that the primary PM2.5 standards should be 
revised, the commenters describe decreases in days per monitor per year 
with 2-hour maximum concentrations greater than 120 [micro]g/m\3\ and 
4-hour maximum concentrations greater than 37.8 [micro]g/m\3\ when 
comparing monitors that achieve close to 10 and 30 [micro]g/m\3\ versus 
monitors that meet close to 8 [micro]g/m\3\ and 25 [micro]g/m\3\. The 
commenters noted decreases in the number of days per monitor per year 
with 2-hour maximum concentrations over 120 [micro]g/m\3\ and 4-hour 
max concentration over 37.8 [micro]g/m\3\ were also seen when comparing 
monitors close to achieving 24-hour standards with levels of 35 
[micro]g/m\3\ versus 25 [micro]g/m\3\.
    First, the EPA notes that this analysis submitted by commenters was 
limited to a very small number of monitors and did not include a 
national perspective. Second, the EPA notes that this analysis focused 
on number of days (rather than the number of times) where there was a 
2-hour maximum concentration over 120 [micro]g/m\3\ or a 4-hour max 
concentration over 37.8 [micro]g/m\3\. In order to evaluate the 
protection provided by the current 24-hour standard against peak 
exposures, including exposures with 2-hour concentrations greater than 
120 [micro]g/m\3\ and 4-hour concentrations greater than 37.8 [micro]g/
m\3\, the EPA considers it more informative and appropriate from a 
public health perspective to assess the number of times a subdaily 
exposure of concern occurs in a year, rather than the number of days on 
which they occur because the former identifies more potential exposures 
of concern and provides more information about the scale and scope of 
the occurrences of those exposures. Lastly, the analyses allowed 
monitors somewhat above the standards to be included. Therefore, it is 
unclear whether the exceedances of the 2-hour or 4-hour benchmarks 
would still have occurred if the area had actually been meeting the 
current primary PM2.5 standards. However, in considering the 
analyses submitted by the commenters, the EPA conducted new analyses 
\105\ that looked at all individual monitors across the U.S. and 
evaluated the percentage of times the monitors experienced a 2-hour 
maximum concentration over 120 [micro]g/m\3\ or a 4-hour max 
concentration over 37.8 [micro]g/m\3\ when that monitor was meeting the 
current standards. Further, given that the Administrator concludes that 
the level of the current primary annual PM2.5 is not 
adequate and that it should be revised to 9.0 [micro]g/m\3\, the new 
analysis evaluates the percentage of times during a recent 3-year 
period (i.e. 2019-2021) that individual monitors experienced a 2-hour 
maximum concentration over 120 [micro]g/m\3\ or a 4-hour max 
concentration over 37.8 [micro]g/m\3\ when that monitor was meeting the 
current primary 24-hour PM2.5 standard with its level of 35 
[micro]g/m\3\ and a revised primary annual PM2.5 standard of 
9.0 [micro]g/m\3\.
---------------------------------------------------------------------------

    \105\ Jones et al. (2023). Comparison of Occurrence of 
Scientifically Relevant Air Quality Observations Between Design 
Value Groups. Memorandum to the Rulemaking Docket for the Review of 
the National Ambient Air Quality Standards for Particulate Matter 
(EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    In evaluating the results from the new analyses, it is important to 
keep in mind that the 2019 ISA and ISA Supplement concluded that the 
most consistent evidence from the controlled human exposures studies is 
for impaired vascular function following 2-hour exposures to average 
PM2.5 concentrations at and above about 120 [micro]g/m\3\, 
with less consistent evidence for effects following exposures to 
concentrations lower than 120 [micro]g/m\3\. The new analyses show that 
across all monitors, on average, only 0.029 percent of 2-hour 
observations reach PM2.5 concentrations higher than 120 
[micro]g/m\3\ in areas meeting the current 24-hour standard and a 
revised annual standard of 9.0 [micro]g/m\3\. Further, recognizing that 
one purpose of the 24-hour standard is to protect against exposure in 
areas with high peak-to-mean ratios, when assessing the monitors 
individually across the U.S. under these same conditions, the monitors 
reporting the highest PM2.5 concentrations have only 0.47 
percent of 2-hour observations reach PM2.5 concentrations 
higher than 120 [micro]g/m\3\.
    Additionally, the analyses also evaluated the frequency of 
reporting a 4-hour maximum concentration over 37.8 [micro]g/m\3\ when 
monitors were meeting the current 24-hour standard and a revised annual 
standard of 9.0 [micro]g/m\3\. For this part of the analysis, the EPA 
finds that across all monitors, on average, only 0.41 percent of 4-hour 
observations reach PM2.5 concentrations higher than 37.8 
[micro]g/m\3\ in areas meeting the current 24-hour standard and a 
revised annual standard of 9.0 [micro]g/m\3\. Further, when assessing 
the monitors individually across the U.S. under these same conditions, 
the monitors reporting the highest PM2.5 concentrations have 
only 2.6 percent of 4-hour observations reach PM2.5 
concentrations higher than 37.8 [micro]g/m\3\. Thus, the EPA disagrees 
with commenters that the current primary 24-hour PM2.5 
standard typically allows PM2.5 exposures at or above those 
observed to cause health effects in controlled human exposure studies. 
Furthermore, the EPA notes that in light of the small number of 
occurrences and the intermediate nature of the effects observed in 
Wyatt et al. (2020) at concentrations of 37.8 [micro]g/m\3\ (i.e., 
effects that typically would not, by themselves, be judged as adverse), 
there is substantial basis to doubt whether further improvements in 
public health

[[Page 16270]]

would be achieved by further reducing these exposures. In drawing this 
conclusion, the EPA notes the lack of evidence of effects from 
controlled human exposure studies at levels below the current 24-hour 
standard and the fact that the results of Wyatt et al. (2020) are 
inconsistent with other currently available studies, and this study 
only observes intermediate effects.
    In response to commenters that cited the majority of the CASAC's 
view that, in general, ``[t]here is . . . less confidence that the 
annual standard could adequately protect against health effects of 
short-term exposures'' (Sheppard, 2022a, p. 4 of consensus letter), the 
EPA disagrees with the majority of CASAC, noting that the results of 
the EPA's analysis suggest that high peak concentrations are extremely 
infrequent in areas meeting an annual standard of 9.0 [micro]g/m\3\, 
occurring less than 0.029-0.41 percent of the time (for 2-hour 
concentrations >120 [micro]g/m\3\ and 4-hour concentrations >37.8 
[micro]g/m\3\, respectively). This suggests that in most locations, 
even the upper tail of the distribution would be controlled quite well 
under a revised annual standard. With regard to the likelihood that the 
current standards would allow peak concentrations that are clearly of 
concern from a health perspective, therefore, the EPA concludes that 
such occurrences are extremely infrequent--and will be even less 
frequent under the improved air quality conditions associated with 
meeting a revised annual PM2.5 standard of 9.0 [micro]g/
m\3\.
    A number of commenters who support revising the primary 24-hour 
PM2.5 standard to a lower level suggest that the available 
epidemiologic evidence provides support for such a revision. To support 
their view, the commenters note that the currently available evidence, 
including a number of epidemiologic studies that demonstrate 
associations between short-term PM2.5 exposures and health 
effects, provides support for causal relationships for short-term 
PM2.5 exposures and health effects as described in the 2019 
ISA and ISA Supplement. The commenters further note that the available 
epidemiologic studies include diverse populations that are broadly 
representative of the U.S. population, including at-risk populations, 
which they assert is an advantage over the controlled human exposure 
studies and the risk assessment, which are not as broadly 
representative.
    These commenters highlight a number of specific epidemiologic 
studies that they suggest provide support for revising the level of the 
24-hour standard. Additionally, commenters contend that there are 
epidemiologic studies using restricted analyses that show that positive 
and statistically significant associations between short-term 
PM2.5 exposure and mortality persist at daily mean 
concentrations below 25 [micro]g/m\3\. The commenters also cite several 
studies that provide no evidence of a threshold. These commenters also 
point to the CASAC advice in their review of the 2021 draft PA, where 
the majority of the CASAC cited epidemiologic studies using restricted 
analyses as offering support for revision. The commenters argue that 
the EPA cannot base discretion on uncertainties related to the methods 
used in restricted analyses in the epidemiologic studies. In so doing, 
these commenters disagree with the EPA that it is important to take 
into consideration that these studies do not consider the form or 
averaging time of the 24-hour standard. Finally, the commenters claim 
that while the EPA stated that the study-reported means from 
epidemiologic studies that use restricted analyses are more useful for 
identifying impacts from typical 24-hour exposures than for peak 24-
hour exposures, the commenters assert that the studies also indicate 
that there are health risks at relatively high concentrations below the 
current level of the primary 24-hour PM2.5 standard that 
must be addressed.
    As noted by the commenters, epidemiologic studies that show 
positive and statistically significant associations between short-term 
PM2.5 exposure and mortality provide support for the causal 
determination in the 2019 ISA. The EPA also agrees that the available 
epidemiologic studies include diverse populations that are broadly 
representative of the U.S. population, including at-risk populations. 
Further, the EPA agrees that studies evaluated in the 2019 ISA and the 
ISA Supplement continue to provide evidence of linear, no-threshold 
concentration-response relationships, but with less certainty in the 
shape of the curve at lower concentrations (i.e., below about 8 
[micro]g/m\3\), with some recent studies providing evidence for either 
a sublinear, linear, or supralinear relationship at these lower 
concentrations (U.S. EPA, 2019a, section 11.2.4; U.S. EPA, 2022a, 
section 2.2.3.2).
    However, findings of positive, significant associations in short-
term epidemiologic studies do not directly indicate that short-term 
effects would occur in areas meeting the 24-hour standard and 
therefore, do not directly address the question of whether the current 
24-hour standard is adequate. While short-term epidemiologic studies 
evaluate associations between distributions of ambient PM2.5 
and health outcomes, they do not identify the specific exposures (i.e., 
a specific 24-hour concentration) that can lead to the reported 
effects. Short-term epidemiologic studies evaluate the association 
between day-to-day variation in daily (24-hour) PM2.5 
exposure and health endpoints (e.g., mortality) to understand how these 
changes in air pollution concentrations are associated with changes in 
health outcomes. But these studies do not report daily concentrations; 
rather, they report the long-term mean concentration of the 24-hour 
PM2.5 concentrations over the entire multi-year period of 
the study, and typically report their results as a relative risk (e.g., 
for each 10 [micro]g/m\3\ increase in PM2.5, the risk of 
mortality or cardiovascular hospital admissions increases by a certain 
percentage, across the full range of the 24-hour PM2.5 
concentrations in the study). This means that there is no specific 
point in the air quality distribution of any epidemiologic study that 
represents a ``bright line'' at and above which effects have been 
observed and below which effects have not been observed. Nor, as noted 
above, do these studies allow for any direct inferences about health 
impacts associated with the short-term ``peak'' exposures that the 
primary 24-hour standard is designed to protect against. While there 
can be considerable variability in daily exposures over a multi-year 
study period, most of the estimated exposures in these epidemiologic 
studies reflect days with ambient PM2.5 concentrations 
around the mean or middle of the air quality distributions examined 
(i.e., ``typical'' days rather than days with extremely high or 
extremely low concentrations). This is true of long-term epidemiologic 
studies as well. The difference between epidemiologic studies examining 
associations with long-term exposures and short-term exposures is 
comparing different levels of exposure over different exposure 
durations (i.e., long-term studies exposures are defined as those that 
are annual or multi-year, while short-term exposures are defined as 
those that are mostly 24-hour) (U.S. EPA, 2019a, section P.3.1). Thus, 
in both cases, and in the absence of a discernible threshold, 
epidemiologic studies of short-term and long-term exposures provide the 
strongest support and confidence for reported health effect 
associations around the middle portion of the PM2.5 air 
quality distribution (e.g., the study-reported mean PM2.5

[[Page 16271]]

concentration), which corresponds to the bulk of the underlying data, 
rather than at the extreme upper or lower ends of the distribution. 
However, the difference between the annual standard and the 24-hour 
standard, aside from averaging times, is that the form of the annual 
standard is a mean PM2.5 concentration, which is based on 
the bulk of the air quality data, while the form of the 24-hour 
standard is a 98th percentile form, which is based on peak 
concentrations. Both long-term and short-term epidemiologic studies are 
informative for determining the appropriate level of the annual 
PM2.5 standard, which is designed to control ``typical'' 
daily exposures and risks, because these studies most often report 
long-term mean (or median) PM2.5 concentrations that are 
representative of ``typical'' exposures that are associated with health 
effects. In contrast, while the short-term epidemiologic studies 
examine health effects associated with shorter exposure durations 
(e.g., mostly 24-hour exposures), these studies are less informative 
for determining the appropriate level of the 24-hour PM2.5 
standard because these studies do not report the 98th percentile 
PM2.5 concentrations,\106\ which is more directly comparable 
to the form of the 24-hour standard. Additionally, if the 98th 
percentile of data were reported, the EPA would consider the peak 
concentrations observed in these studies (which by definition rarely 
occur) in conjunction with other supporting evidence. However, as 
already noted, there is an absence of new information in this 
reconsideration (either from controlled human exposure studies or 
epidemiologic studies) suggesting that peak concentrations just below 
the level of the current 24-hour standard (with its level of 35 
[micro]g/m\3\) are associated with adverse effects. Instead, the 
evidence links risk to more typical daily exposures near the middle of 
the air quality distribution--exposures most effectively controlled 
through a strengthening of the annual standard. As noted in the 2012 
final rule, ``reducing the annual standard is the most efficient way to 
reduce the risks from short-term exposures . . . as the bulk of the 
risk comes from the large number of days across the bulk of the air 
quality distribution, not the relatively small number of days with peak 
concentrations'' (78 FR 3156, January 15, 2013).
---------------------------------------------------------------------------

    \106\ In the 2022 PA, the EPA has identified a number of key 
areas for additional research and data collection for 
PM2.5, based on the uncertainties and limitations that 
remain in the scientific evidence and technical information. In 
addition to research and data collection, the EPA specifically 
highlights additional information that could be reported in the 
epidemiologic studies that may help inform future reviews of the 
primary PM2.5 standards, including additional descriptive 
statistics in the upper percentiles of the air quality distribution 
(i.e., from the 95th to the 99th percentile), as well as the number 
of days of concentrations and/or health events within each of these 
percentiles (U.S. EPA, 2022a, section 3.7).
---------------------------------------------------------------------------

    As noted above, in evaluating the adequacy of the current 
standards, the EPA has consistently considered the annual standard 
(based on arithmetic mean concentrations) and 24-hour standard (based 
on 98th percentile concentrations) together in evaluating the public 
health protection provided by the standards against the full 
distribution of short- and long-term PM2.5 exposures. 
Moreover, the EPA has previously noted that the annual standard is 
generally controlling in most parts of the country, providing an 
effective and efficient way to reduce total population risk to both 
long- and short-term PM2.5 exposures, while the 24-hour 
standard, with its 98th percentile form, provides supplemental 
protection, particularly for areas with high peak-to-mean ratios of 24-
hour PM2.5 concentrations (78 FR 3158, January 15, 2013). In 
such areas, annual average PM2.5 concentrations could be 
quite low, and the 24-hour standard provides a means of ensuring 
control of episodic peaks possibly associated with strong local or 
seasonal sources, or PM2.5-related effects that may be 
associated with shorter-than daily exposure periods. The approach taken 
in evaluating the adequacy and alternative levels of the annual 
standard has been to evaluate the long-term mean PM2.5 
concentrations of both long-term and short-term key epidemiologic 
studies, where we have the most confidence in the reported health 
effects association, while also giving some consideration to lower 
percentiles of the air quality distribution (e.g., 25th percentiles). 
However, using a similar approach to evaluate the adequacy of the 
current and any potential alternative levels of the 24-hour standard 
with short-term epidemiologic studies, as the majority of CASAC and 
some commenters are suggesting, presents challenges.
    Short-term epidemiologic studies, including those that use 
restricted analyses, often report metrics that include mean 
PM2.5 concentrations, with some studies also reporting lower 
percentiles, such as the 25th percentile. As previously noted above, 
for studies of daily PM2.5 exposure, which examine 
associations between day-to-day variation in PM2.5 
concentrations and health outcomes, often over several years, most of 
the estimated exposures reflect days with ambient PM2.5 
concentrations around the middle of the air quality distributions 
examined (i.e., the mean or median). However, there is not a metric or 
statistic reported in short-term epidemiologic studies that allows for 
a direct comparison to the current 24-hour PM2.5 standard 
and its 98th percentile form. While a 98th percentile of 
PM2.5 concentrations is a metric that might be more closely 
compared to the 24-hour standard level, 98th percentile 
PM2.5 concentrations were not reported in key epidemiologic 
studies. Consistent with the Administrator's final decision in 2012, 
the EPA notes that even if 98th percentile values were reported, it 
would be inappropriate to focus on these concentrations without also 
considering the impact of a revised annual standard on short-term 
concentrations, since many areas would be expected to experience 
decreasing short- and long-term PM2.5 concentrations in 
response to a revised annual standard (78 FR 3156, January 15, 2013). 
Furthermore, in light of the scarcity of days at the very upper end of 
the distribution, and to avoid placing undue reliance on the peak 
concentrations observed in these studies (which by definition rarely 
occur), the EPA finds that such values would need to be considered in 
conjunction with other supporting evidence. In addition, as described 
above, the other lines of evidence available for consideration by the 
EPA do not indicate that the current primary 24-hour standard requires 
revision to protect public health with an adequate margin of safety. 
The EPA notes again the lack of corroborating evidence from controlled 
human exposure studies. While the EPA agrees with the CASAC that the 
controlled human exposure studies are limited in their ability to speak 
to the concentrations at which effects may be elicited in at-risk 
populations, as discussed above the lowest concentration associated 
with effects is 37.8 [micro]g/m\3\ and the effects observed were 
``intermediate'' outcomes that are not by themselves considered 
adverse. We also note that, as detailed in section II.A.2.a above, the 
study that observed intermediate effects at concentrations of 37.8 
[micro]g/m\3\ was evaluated in the ISA Supplement and the results of 
this study were inconsistent with the controlled human exposure studies 
evaluated in the 2019 ISA. Additionally, as noted above, the EPA finds 
that across all monitors, on average, only 0.41 percent of 4-hour 
observations reach PM2.5 concentrations higher than 38 
[micro]g/m\3\ in areas meeting the current 24-hour

[[Page 16272]]

standard and a revised annual standard of 9.0 [micro]g/m\3\. Given the 
rarity of these occurrences and the fact that the effects associated 
with exposures to this PM2.5 concentration have not been 
found to be adverse in and of themselves, the EPA finds it reasonable 
to conclude that this pattern of air quality will protect at-risk 
populations, even though such populations were not in the study groups. 
The EPA concludes that further evidence would be needed at specific 
short-term (i.e., hourly or daily) concentrations below the level of 
the current 24-hour standard to support any revision to the current 24-
hour standard.
    With regard to the data that are available from the short-term 
epidemiologic studies (which, as noted, do not include 98th percentile 
values), the EPA considers it inappropriate to utilize the study-
reported means from the short-term epidemiologic evidence to assess the 
adequacy of the 24-hour standard, with its 98th percentile form, 
considering that the study-reported mean concentrations do not provide 
meaningful insight regarding the frequency or health significance of 
peak concentrations occurring during the study period. As indicated in 
the 2022 PA, the study-reported means of short-term epidemiologic 
studies do not serve a purpose in determining a level at which we can 
confidently attribute effects to the impact of ``peak'' exposures. The 
24-hour standard is intended to provide supplemental protection against 
short-term peak exposures and while there is a general relationship 
between mean concentrations and 98th percentile concentrations in 
individual locations, such relationships vary by location and there is 
not an established relationship that can be relied upon to predict 98th 
percentile concentrations based on mean PM2.5 concentrations 
reported in multi-city epidemiologic studies. Instead, mean 
concentrations from short-term epidemiologic studies are more useful in 
addressing questions regarding the effects of ``typical'' or average 
24-hour exposures, which are addressed through the annual standard. For 
this reason, the EPA does consider the mean concentrations of short-
term studies (as well as the means from the long-term studies) in 
evaluating the level of the annual standard, which the EPA recognizes 
as the generally controlling standard for both long- and short-term 
exposures. However, the EPA does not agree with commenters that it is 
appropriate to use means from short-term epidemiologic studies as the 
basis for a decision-making framework to determine the adequacy of the 
current 24-hour standard, with its 98th percentile form.
    As described in the proposal (88 FR 5613, January 27, 2023), the 
2022 PA also noted the epidemiologic studies that restrict 24-hour 
average PM2.5 concentrations to values of less than 35 
[micro]g/m\3\, and in some cases less than 25 [micro]g/m\3\, and annual 
average PM2.5 concentrations less than 12 [micro]g/m\3\. 
Restricted analyses use a subset of data from their main analyses and 
conduct an epidemiologic study with health events that occur at 
concentrations below a certain concentration (e.g., 25 [micro]g/m\3\). 
While some of these studies do not report the mean PM2.5 
concentration for the restricted analysis, the mean of the restricted 
analysis is presumably less than the mean PM2.5 
concentration in the main analysis. Restricted analyses from long-term 
and short-term exposure epidemiologic studies are informative in 
providing support that the health effects associations are not driven 
by just the upper peaks of the PM2.5 air quality 
distributions and provide support for revision to the level of the 
annual PM2.5 standard. Short-term restricted analyses also 
report positive associations between short-term PM2.5 
exposure and morbidity and mortality. As an example, in a restricted 
analysis evaluating the association between short-term exposures and 
PM2.5 concentrations less than 25 [micro]g/m\3\, Di et al. 
(2017a) removed 6.3 percent of the data from their main analyses, 
(i.e., all PM2.5 concentrations greater than 25 [micro]g/
m\3\), and still found a positive and significant association between 
short-term PM2.5 exposure and mortality. This study provides 
additional support that the association between short-term exposure to 
PM2.5 and mortality in the main epidemiologic analysis is 
not driven by the upper peaks of the PM2.5 air quality 
distribution, which in turn supports the conclusion that lowering the 
entire distribution of air quality concentrations through a revised 
annual standard is an appropriate means of protecting against adverse 
effects from short-term exposure, as discussed further below.
    In their review of the 2021 draft PA, the majority of the CASAC 
highlighted three U.S.-based epidemiologic studies that restricted 24-
hour average PM2.5 concentrations below 25 [mu]g/m\3\ as a 
part of their rationale for recommending that the EPA revise the level 
of the primary 24-hour PM2.5 standard. Similarly, in 
evaluating positive associations in restricted analyses, some 
commenters also suggest that because an association exists at 24-hour 
concentrations below 25 [micro]g/m\3\, the 24-hour standard level 
should be set at the concentration at which the analysis was restricted 
(e.g., 25 [micro]g/m\3\). However, the EPA notes that neither the CASAC 
nor public commenters provided any detail regarding, how, in their 
view, these studies demonstrate that the level of the current 24-hour 
standard is not adequate, and/or how these studies demonstrate what 
revised level of the 24-hour standard would provide requisite public 
health protection with an adequate margin of safety. The EPA considers 
that such an approach would have several important limitations. First, 
the approach assumes that a specific point on the air quality 
distribution (e.g., the point at which the analysis was restricted) is 
where health effects are exhibited and where we have the most 
confidence in the reported association. However, in addition to the 
limitations associated with the short-term epidemiologic studies 
outlined above, the EPA does not agree that it would be appropriate to 
identify the requisite level of the primary 24-hour PM2.5 
standard based on the specific concentration at which the analyses 
restrict their studies. The choice to restrict the data at a particular 
concentration is in effect arbitrary, and does not establish that any 
particular effects are attributable to that concentration as opposed to 
other concentrations within the restricted analysis.
    Further, these restricted analyses do not report the 
PM2.5 concentration at the 98th percentile of data or other 
metrics relating to the upper end of the distribution that could 
provide information about health risks associated with peak exposures. 
For example, the CASAC does not provide a discussion of what the 
comparable 98th percentile concentration is in the distribution of 
remaining 24-hour PM2.5 concentrations of restricted 
analyses (because such data is not reported by the study authors) and 
what degree of confidence the Administrator should place on those upper 
percentile values (e.g., 98th percentile values). In order to identify 
a level of the 24-hour standard based on associations between the 
``upper end'' of exposures, either in the unrestricted or the 
restricted analyses, and adverse health effects, it would be necessary 
to have both greater detail on the distribution of air quality in the 
study and greater confidence in the reported association at the peak 
concentrations such as the 98th percentile--in other words, a better 
understanding of how specific 24-hour concentrations correspond to the 
frequency and total number of observed health events in the study.
    Further, the EPA notes that when resulting analyses based on the

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restricted dataset continue to find positive associations between the 
remaining air quality distribution and health effects, it suggests that 
the relationship was in fact not driven primarily by the upper tail 
(now removed from the dataset) but rather by lower portions of the 
distribution of air quality. In other words, we have no confidence that 
the remaining upper end of the air quality distribution is driving the 
remaining associations reported in the restricted analyses, as opposed 
to the vast array of health events at and around the mean 
PM2.5 concentration. In fact, it is reasonable to conclude 
that to effectively address the health effects observed in the study, 
it is necessary to control not just the peak concentrations but to 
reduce the bulk of the exposures (occurring near the mean), a task more 
effectively achieved, as noted above through a tightening of the annual 
standard, which has the effect of shifting the entire distribution of 
PM2.5 concentrations downward (both peaks and means). 
Therefore, while the EPA agrees that both short- and long-term 
epidemiologic studies that completed restricted analyses and reported 
the resulting study means could be used to inform conclusions regarding 
the adequacy of the annual standard, given that the resulting study 
means (when reported) could be evaluated in the context of the decision 
framework described above for informing decisions on the level of the 
annual standard, the EPA considers that current short-term 
epidemiologic studies that restrict analyses are subject to the same 
limitations outlined above for current short-term epidemiologic studies 
in how they can be used in a decision-making framework to inform the 
adequacy and alternative level of the primary 24-hour PM2.5 
standard. As such, while the available short-term epidemiologic studies 
that restrict their analyses are useful for informing conclusions 
regarding the strength of the associations for health outcomes, they 
are not, as currently designed, as useful for informing conclusions 
regarding the adequacy of the current primary 24-hour PM2.5 
standard. In reaching this conclusion, the EPA notes that the majority 
of the CASAC did not address the limitations of these studies outlined 
in the 2021 draft PA, particularly in the context of the 24-hour 
standard with its 98th percentile form. Among the future research needs 
identified by the EPA in the 2022 final PA, the Agency noted a number 
of gaps in the currently available information reported in the 
epidemiologic studies of short-term exposure, including ``descriptive 
statistics of PM2.5 concentrations at individual percentiles 
from the 95th percentile to the 99th percentile, as well as the number 
of days of concentrations and/or health events within each of these 
percentiles'' and other descriptive statistics and details regarding 
analytical design in studies employing restricted analyses (U.S. EPA, 
2022b, pp. 3-225 to 3-226). Such information could significantly 
improve the EPA's ability to draw conclusions from these studies with 
regard to the adequacy of the current primary 24-hour PM2.5 
standard.
    Due to the limitations and uncertainties outlined above, in 
reaching his decision on the primary 24-hour PM2.5 standard, 
the Administrator judges that the information from currently available 
short-term epidemiologic studies, including those that use restricted 
analyses, is inadequate to inform decisions regarding the adequacy of 
the current 24-hour standard. Additionally, consistent with the final 
decision in 2012, the EPA continues to view an approach that focuses on 
setting a generally controlling annual standard as the most effective 
and efficient way to reduce total population risk associated with both 
long- and short-term PM2.5 exposures. Potential air quality 
changes associated with meeting an annual standard level of 9.0 
[micro]g/m\3\ will result in lowering risk associated with both long- 
and short-term PM2.5 exposure by lowering the overall air 
quality distribution. As discussed above, reducing the annual standard 
is the most efficient way to reduce the risks from short-term exposures 
identified in the epidemiologic studies, as the available evidence 
suggests the bulk of the risk comes from the large number of days 
across the bulk of the air quality distribution, not the relatively 
small number of days with peak concentrations. However, as in the 2012 
review, the Administrator recognizes that an annual standard alone 
would not be expected to offer sufficient protection with an adequate 
margin of safety against the effects of short-term PM2.5 
exposures in all parts of the country, particularly in areas with high 
peak-to-mean ratios, and concludes that it is appropriate to continue 
to provide supplemental protection by means of a 24-hour standard. In 
so doing, the Administrator concludes that retaining the level of the 
primary 24-hour PM2.5 standard of 35 [micro]g/m\3\ will 
provide requisite protection against short-term peak PM2.5 
concentrations, in conjunction with a revised annual standard level of 
9.0 [micro]g/m\3\.
4. Administrator's Conclusions
    This section summarizes the Administrator's considerations and 
conclusions related to the adequacy of the current primary 
PM2.5 standards and presents his decision to revise the 
primary annual PM2.5 standard to a level of 9.0 [micro]g/
m\3\ and retain the primary 24-hour PM2.5 standard. In 
establishing primary standards under the Act that are ``requisite'' to 
protect public health with an adequate margin of safety, the 
Administrator is seeking to establish standards that are neither more 
nor less stringent than necessary for this purpose. He recognizes that 
the requirement to provide an adequate margin of safety was intended to 
address uncertainties associated with inconclusive scientific and 
technical information and to provide a reasonable degree of protection 
against hazards that research has not yet identified. However, the Act 
does not require that primary standards be set at a zero-risk level; 
rather, the NAAQS must be sufficiently protective, but not more 
stringent than necessary.
    Given these requirements, the Administrator's final decision in 
this reconsideration is a public health policy judgment drawing upon 
scientific and technical information examining the health effects of 
PM2.5 exposures, including how to consider the range and 
magnitude of uncertainties inherent in that information. This public 
health policy judgment is based on an interpretation of the scientific 
and technical information that neither overstates nor understates its 
strengths and limitations, nor the appropriate inferences to be drawn, 
and is informed by the Administrator's consideration of advice from the 
CASAC and public comments received on the proposal.
    The initial issue to be addressed in the reconsideration of the 
primary PM2.5 standards is whether, in view of the advances 
in scientific knowledge and other information reflected in the 2019 
ISA, ISA Supplement, and 2022 PA, the current standards are requisite 
to protect public health with an adequate margin of safety. In 
considering the adequacy of the current suite of primary 
PM2.5 standards, the Administrator has considered the large 
body of evidence presented and assessed in the 2019 ISA and ISA 
Supplement, the conclusions presented in the 2022 PA, the views 
expressed by the CASAC, and public comments. The Administrator has 
taken into account both evidence- and risk-based considerations in 
developing final conclusions on the adequacy of the current primary 
PM2.5 standards. The Administrator has additionally

[[Page 16274]]

considered the associated public health policy judgments and judgments 
about the uncertainties inherent in the scientific evidence and 
quantitative analyses that are integral to the conclusions on the 
adequacy of the current primary PM2.5 standards.
    In evaluating the adequacy of the current standards, the 
Administrator first recognizes the longstanding body of health evidence 
supporting relationships between PM2.5 exposures (short- and 
long-term) and mortality and serious morbidity effects. The evidence 
available in this reconsideration (i.e., that assessed in the 2019 ISA 
and ISA Supplement) and summarized above in section II.A.2.a reaffirms, 
and in some cases strengthens, the conclusions from the 2009 ISA 
regarding the health effects of PM2.5 exposures. Recent 
epidemiologic studies demonstrate generally positive and often 
statistically significant associations between PM2.5 
exposures and a number of health effects, including non-accidental, 
cardiovascular, or respiratory mortality; cardiovascular or respiratory 
hospitalizations or emergency room visits; and other mortality/
morbidity outcomes (e.g., lung cancer mortality or incidence, asthma 
development). Recent controlled human exposure and animal toxicological 
studies, as well as evidence from epidemiologic panel studies, 
strengthens support for potential biological pathways through which 
PM2.5 exposures could lead to the serious effects reported 
in many population-level epidemiologic studies, including support for 
pathways that could lead to cardiovascular, respiratory, nervous 
system, and cancer-related effects. In considering the available 
scientific evidence, and consistent with approaches employed in past 
NAAQS reviews, the Administrator places the most weight on evidence 
supporting ``causal'' or ``likely to be causal'' relationship with long 
or short-term PM2.5 exposures. In addition, the 
Administrator also takes note of those populations identified to be at 
greater risk of PM2.5-related health effects, as 
characterized in the 2019 ISA and ISA Supplement, and the potential 
public health implications.
    In evaluating what existing or revised standards may be requisite 
to protect public health, as described above in section II.A.2, the 
Administrator's approach recognizes that the current annual standard 
(based on arithmetic mean concentrations) and 24-hour standard (based 
on 98th percentile concentrations), together, are intended to provide 
public health protection against the full distribution of short- and 
long-term PM2.5 exposures. This approach recognizes that 
changes in PM2.5 air quality designed to meet either the 
annual or the 24-hour standard would likely result in changes to both 
long-term average and short-term peak PM2.5 concentrations.
    Further, consistent with the approach adopted in 2012, the 
Administrator concludes that the most effective and efficient way to 
reduce total population risk associated with both long- and short-term 
PM2.5 exposures is to set a generally controlling annual 
standard, and to provide supplemental protection against the occurrence 
of peak 24-hour PM2.5 concentrations by means of a 24-hour 
standard set at the appropriate level. In reaching this conclusion, the 
Administrator explicitly recognizes that air quality changes associated 
with meeting a revised annual standard would result in lowering risks 
associated with both long- and short-term PM2.5 exposures by 
lowering the overall distribution of air quality concentrations, 
leading to not only in lower short- and long-term PM2.5 
concentrations near the middle of the air quality distribution, but 
also in fewer and lower short-term peak PM2.5 
concentrations. Similarly, the Administrator recognizes that changes in 
air quality to meet a 24-hour standard, would result not only in fewer 
and lower peak 24-hour PM2.5 concentrations, but also in 
lower annual average PM2.5 concentrations. However, as noted 
in 2012, he also recognizes that an approach that relies on setting the 
level of the 24-hour standard such that the 24-hour standard is 
generally controlling would be less effective and result in less 
uniform protection across the U.S. than an approach that focuses on 
setting a generally controlling annual standard. Thus, he concludes 
that relying on a revised annual standard as the controlling standard 
will reduce aggregate risks associated with both long- and short-term 
exposures more consistently than a generally controlling 24-hour 
standard. He further concludes that retaining a 24-hour standard at the 
appropriate level will ensure an adequate margin of safety against 
short-term effects in areas with high peak-to-mean ratios.
    In light of his focus on the annual standard as the generally 
controlling standard, in considering whether the primary 
PM2.5 standards are adequate, the Administrator first 
considers information available to inform his final conclusions 
regarding the primary annual PM2.5 standard. In so doing, he 
notes that in this reconsideration, a large number of key U.S. 
epidemiologic studies report positive and statistically significant 
associations for air quality distributions with overall mean 
PM2.5 concentrations that are well below the current level 
of the annual standard of 12.0 [mu]g/m\3\. He further recognizes that 
there is additional scientific evidence assessed in the 2019 ISA and 
newly assessed in this reconsideration in the ISA Supplement that can 
provide supplemental information to inform his decisions. In addition 
to the key U.S. epidemiologic studies, the Administrator also 
recognizes that key Canadian epidemiologic studies also demonstrate 
positive and statistically significant associations at concentrations 
below 12 [mu]g/m\3\. He also recognizes that epidemiologic studies that 
restrict annual average PM2.5 concentrations to below 12 
[mu]g/m\3\ also provide support for positive and statistically 
significant associations at lower mean PM2.5 concentrations, 
as do accountability studies that also suggest public health 
improvements may occur at concentrations below 12 [mu]g/m\3\.
    With regard to the available scientific evidence to inform his 
final decisions on the adequacy of the current 24-hour standard, the 
Administrator finds that there is less information available to support 
decisions on the 24-hour standard than that summarized above for the 
annual standard. The Administrator first notes that controlled human 
exposure studies, including those newly available in this 
reconsideration, demonstrate effects following short-term 
PM2.5 exposures at concentrations higher than the current 
24-hour standard. The Administrator also considers air quality analyses 
conducted in the 2022 PA and in responding to public comments, as 
described above in section II.B.3, that evaluate PM2.5 
concentrations in ambient air for similar durations to the controlled 
human exposure studies. As noted above, these air quality analyses 
indicate that the current 24-hour standard, particularly in conjunction 
with the revised level of the annual standard, provides a high degree 
of protection against subdaily PM2.5 concentrations that 
have been shown to elicit effects in controlled human exposure studies. 
The Administrator considers a limited number of available epidemiologic 
studies that report associations with health effects when the analyses 
are restricted to daily PM2.5 concentrations below 35 [mu]g/
m\3\. As described above, although these studies are useful in 
demonstrating that health effects are associated with exposure to daily 
PM2.5 concentrations in the lower part of the air quality 
distribution, they do not provide information about health effects 
associated with the short-term

[[Page 16275]]

``peak'' exposures that the 24-hour standard is designed to protect 
against. Accordingly, these studies have limited relevance in informing 
a decision about the appropriate level of the 24-hour standard.
    In addition to the scientific evidence, the Administrator also 
considers the information from the risk assessment. In so doing, he 
notes that the risk assessment estimates that the current primary 
annual PM2.5 standard could allow a substantial number of 
deaths in the U.S. With respect to the 24-hour standard, the 
Administrator recognizes that there are only a small number of study 
areas where the 24-hour standard is controlling and changes in the 24-
hour standard level are estimated to have a much smaller impact on 
public health. The Administrator recognizes that while the risk 
estimates can help to place the evidence for specific health effects 
into a broader public health context, they should be considered along 
with the inherent uncertainties and limitations of such analyses when 
informing judgments about the potential for additional public health 
protection associated with PM2.5 exposure and related health 
effects. While the Administrator recognizes that these uncertainties 
are important, he also notes that the general magnitude of the risk 
estimates provide support for significant public health impacts, 
particularly for lower alternative annual standard levels.
    In reaching his final conclusions regarding the adequacy of the 
primary PM2.5 standards, the Administrator also considers 
the CASAC's advice and recommendations, as well as public comments. 
With respect to the CASAC's advice, the Administrator recognizes that, 
in their review of the 2021 draft PA, the CASAC reached consensus that 
the current primary annual PM2.5 standard is not adequate 
and that it is not sufficiently protective of public health. The 
Administrator also takes note of the CASAC's advice in their review of 
the 2019 draft PA, where the CASAC did not reach consensus on the 
adequacy of the primary annual PM2.5 standard, with the 
minority recommending revision and the majority recommending the 
standard be retained. Furthermore, he recognizes that in reviewing the 
2019 draft PA, the CASAC reached consensus regarding the adequacy of 
the primary 24-hour PM2.5 standard, concluding that the 
standard should be retained. Conversely, in their review of the 2021 
draft PA, the majority of the CASAC advised that the current primary 
24-hour PM2.5 standard is not adequate and recommended 
revising the level of the standard, while the minority of the CASAC 
concluded that the standard was adequate and should be retained. 
However, in considering the advice of the CASAC collectively in the 
context of this reconsideration, the Administrator recognizes that the 
2021 draft PA included scientific evidence and quantitative risk 
information that was not available in the 2019 draft PA, and therefore, 
the advice and recommendations of the CASAC in their review of the 2021 
draft PA are based on consideration of the full body of scientific 
evidence available in this reconsideration, including the evidence 
evaluated in the 2019 ISA and the ISA Supplement.
    The Administrator recognizes that much of the scientific evidence 
available in this reconsideration was also available in the 2019 ISA 
and was considered by the then-Administrator when he decided that the 
current primary PM2.5 standards are requisite to protect 
public health with an adequate margin of safety. However, as described 
in section I.C.5.b above, in reaching his decision to reconsider the 
2020 final decision, the Administrator also recognized that there were 
a number of studies published since the literature cutoff date of the 
2019 ISA that were raised by some members of the CASAC in their review 
of the 2019 draft PA, in public comments on the 2020 proposal, and in 
the petitions for reconsideration. As such, the expansion of the air 
quality criteria in this reconsideration to encompass both the 2019 ISA 
and the additional scientific evidence evaluated in the ISA Supplement, 
along with evidence and updated quantitative analyses in the 2022 PA 
also provided an expanded record for the CASAC's review and public 
comments as a part of this reconsideration. Taken together, the 2019 
ISA, ISA Supplement, and 2022 PA, along with the CASAC's advice and 
recommendations and public comments, provide the Administrator with 
additional information for consideration in reaching his final 
conclusions in this reconsideration. As a result, the record before him 
notably expands upon and strengthens the basis for the conclusions of 
the 2019 ISA while reducing some uncertainties that were identified in 
the 2020 final action.
    In considering the available information in this reconsideration, 
the current Administrator reached different conclusions regarding the 
appropriate weight to place on certain aspects of the evidence than the 
then-Administrator in the 2020 final decision. For example, in reaching 
his conclusions on the primary annual PM2.5 standard in 
2020, the then-Administrator concluded that it was appropriate to place 
more weight on epidemiologic studies that used ground-based monitors 
and to place less weight on the studies that used hybrid model-based 
approaches, citing to increased uncertainties associated with this new 
and emerging approach to estimating exposure. In placing more weight on 
the key U.S. monitor-based studies, the then-Administrator noted that 
the majority of these studies had mean concentrations at or above the 
level of the annual standard (12.0 [micro]g/m\3\). However, unlike the 
approach for considering such studies in the 2012 review, the then-
Administrator concluded that it was appropriate to consider the study-
reported means collectively, and in so doing, he placed weight on the 
average of the study-reported means (or medians) across the U.S. 
monitor-based studies of 13.5 [micro]g/m\3\, and noted that this 
concentration was above the level of the standard (85 FR 82717, 
December 18, 2020). In contrast, in this reconsideration, the current 
Administrator judges that it is appropriate to consider the individual 
study-reported mean PM2.5 concentrations from not only the 
U.S. monitor-based epidemiologic studies, but also the U.S. hybrid 
model-based epidemiologic studies, which are an advancement in the 
available science since the completion of the 2009 ISA. The current 
Administrator also adopts an approach similar to some previous 
approaches for the PM NAAQS in which he judges it most appropriate to 
set the level of the standard to somewhat below the lowest long-term 
study-reported mean PM2.5 concentration reported in key U.S. 
epidemiologic studies, which is 9.3 [micro]g/m\3\. The study that 
reports the long-term mean PM2.5 concentration of 9.3 
[micro]g/m\3\ is newly available in this reconsideration and is 
evaluated in the ISA Supplement. In the 2019 ISA, the lowest long-term 
study-reported mean PM2.5 concentrations for U.S.-based 
studies that use ground-based monitors and hybrid model-based 
approaches are 9.9 [micro]g/m\3\ and 10.7 [micro]g/m\3\, respectively. 
In judging that it is appropriate to consider both monitor- and hybrid 
model-based epidemiologic studies and that it is appropriate to adopt 
an approach to set the level of the standard to somewhat below the 
lowest long-term mean PM2.5 concentration, the current 
Administrator judges that the available scientific evidence--evaluated 
in both the 2019 ISA and in the ISA Supplement--provide support for his 
conclusion that that current primary

[[Page 16276]]

PM2.5 standard is not adequate and should be revised.
    In addition to adopting a different approach than the previous 
Administrator for considering the long-term mean PM2.5 
concentrations from key U.S. epidemiologic studies (one more consistent 
with the approach of the EPA in other prior reviews), the current 
Administrator both has information newly available in this 
reconsideration before him and is reaching different conclusions about 
how to weigh the evidence before him in reaching his final conclusions. 
For example, in reaching his final decision in 2020, the then-
Administrator was concerned about placing too much weight on 
epidemiologic studies to inform his conclusions on the adequacy of the 
primary PM2.5 standards, noting that the epidemiologic 
studies do not identify particular PM2.5 concentrations that 
cause effects and cannot alone identify a specific level at which to 
set the standard. In so doing, the then-Administrator placed greater 
weight on the uncertainties and limitations associated with the 
epidemiologic studies, including exposure measurement error, potential 
confounding by copollutants, increased uncertainty of associations at 
lower PM2.5 concentrations, and heterogeneity of effects 
across different cities or regions (85 FR 82716, December 18, 2020). 
The Administrator recognizes that in reaching these judgments, the 
then-Administrator took into consideration the views of some members of 
the CASAC, who, in their advice on the 2019 draft PA, expressed the 
view that the current PM NAAQS should be retained because reported 
associations between short- and long-term PM2.5 exposures 
and adverse health outcomes ``can reasonably be explained in light of 
uncontrolled confounding and other potential sources of error and 
bias'' (Cox, 2019b, p. 8 of consensus responses).
    In this reconsideration, the current Administrator notes that the 
ISA Supplement evaluates additional studies that employed statistical 
approaches that attempted to more extensively account for confounders 
and are more robust to model misspecification (i.e., used alternative 
methods for confounder control, which are sometimes referred to as 
causal modeling or causal inference methods) that build upon those 
studies available and evaluated in the 2019 ISA (U.S. EPA, 2019, 
sections 11.1.2.1 and 11.2.2.4). These studies report consistent 
positive associations between long-term and short-term PM2.5 
exposure and total mortality and cardiovascular effects (U.S. EPA, 
2022a, section 3.2.2.3). In considering the epidemiologic evidence 
evaluated in the 2019 ISA, along with the newly available studies 
evaluated in the ISA Supplement, the current Administrator also 
recognizes that there are uncertainties and limitations associated with 
the epidemiologic studies, but judges that it is appropriate to place 
less weight on these uncertainties than the then-Administrator placed 
on them in reaching his final decision in 2020, given the strength of 
the longstanding large body of epidemiologic evidence, employing a 
variety of study designs, that demonstrates associations between long- 
and short-term PM2.5 exposures and health effects across 
multiple U.S. cities and in diverse populations, including in studies 
examining populations and lifestages that may be at comparatively 
higher risk of experiencing a PM2.5-related health effect 
(e.g., older adults, children).
    In reaching this final decision, the Administrator recognizes he is 
differing not only with the prior Administrator but also with the 
advice some members of the CASAC provided during their review of the 
2019 draft PA. Specifically, taking into consideration the strength of 
the evidence providing support for causality determinations, the advice 
of other members of the CASAC and the need to protect public health 
with an adequate margin of safety, the current Administrator disagrees 
with these members of CASAC regarding the weight to be given to 
epidemiologic evidence ``based on its methodological limitations'' 
(Cox, 2019b, p. 8 of consensus responses), such as the possibility 
``that such associations could reasonably be explained by uncontrolled 
confounding and other potential sources of error and bias'' (Cox, 
2019b, p. 8 of consensus responses).
    As another example of information that was not available to the 
CASAC in providing advice to the Administrator in reaching his final 
decision in 2020, the then-Administrator noted in his final decision 
that, while some members of the CASAC and public commenters highlighted 
a number of accountability studies that examined past reductions in 
ambient PM2.5 concentrations and the degree to which those 
reductions have resulted in public health improvements, the small 
number of available accountability studies did not examine air quality 
with starting concentrations meeting the primary annual 
PM2.5 standard of 12.0 [micro]g/m\3\. The then-
Administrator took into consideration the absence of such 
accountability studies, as part of his consideration of the full body 
of scientific evidence, in reaching his judgment that there was 
considerable uncertainty in the potential for increased public health 
protection from further reductions in ambient PM2.5 
concentrations beyond those achieved under the existing primary 
PM2.5 NAAQS (85 FR 82717, December 18, 2020). However, there 
are several accountability studies available since the literature 
cutoff date of the 2019 ISA and evaluated in the ISA Supplement in this 
reconsideration that have starting concentrations (or concentrations 
prior to the policy or intervention) below 12.0 [micro]g/m\3\ (Corrigan 
et al, 2018; Henneman et al., 2019; Sanders et al., 2020a). The current 
Administrator concludes that, while the number of available 
accountability studies is limited, he recognizes that these studies 
provide supplemental information for consideration for informing 
decisions on the appropriate level of the primary annual 
PM2.5 standard along with the full body of evidence.
    As EPA has frequently noted throughout this document, the extent to 
which the current primary PM2.5 standards are judged to be 
adequate depends in part on science policy and public health policy 
judgments to be made by the Administrator on the strength and 
uncertainties of the scientific evidence, such as how to consider 
epidemiologic evidence and the need for an adequate margin of safety in 
setting the standards. Thus, it would be pure speculation to guess 
whether the then-Administrator would have reached the same or different 
conclusions in the 2020 final decision had the record before him 
included the newly available information in this reconsideration.\107\ 
However, the current Administrator concludes that, for the reasons 
explained herein that, in his judgment, based on the record before him 
in this reconsideration, it is necessary and appropriate to revise the 
primary annual PM2.5 NAAQS to provide requisite protection 
of public health with an adequate margin of safety.
---------------------------------------------------------------------------

    \107\ The EPA notes that, in considering the additional 
scientific evidence available in this reconsideration, one member of 
the CASAC who reviewed both the 2019 draft PA and the 2021 draft PA 
found that the available scientific and quantitative information 
available in this reconsideration supported revising the level of 
the primary annual PM2.5 standard to within the range of 
10-11 [micro]g/m\3\, whereas he recommended retaining the standard 
during the review of the 2019 draft PA.
---------------------------------------------------------------------------

    Based on the available scientific evidence and quantitative 
information, as well as consideration of the CASAC's advice and public 
comments, the Administrator concludes that the

[[Page 16277]]

current primary annual PM2.5 standard is not adequate to 
protect public health with an adequate margin of safety. In addition, 
he finds the available information insufficient to call into question 
the adequacy of the public health protection afforded by the current 
primary 24-hour PM2.5 standard.
    In considering how to revise the current suite of primary 
PM2.5 standards in order to achieve the requisite protection 
for public health, with an adequate margin of safety, against long- and 
short-term PM2.5 exposures the Administrator considers the 
four basic elements of the NAAQS (indicator, averaging time, form, and 
level) collectively. With respect to indicator, the Administrator 
recognizes that the scientific evidence in this reconsideration, as in 
previous reviews, continues to provide strong support for health 
effects associated with PM2.5 mass. He notes the 2022 PA 
conclusion that the available information continues to support the 
PM2.5 mass-based indicator and remains too limited to 
support a distinct standard for any specific PM2.5 component 
or group of components, and too limited to support a distinct standard 
for the ultrafine fraction of PM (U.S. EPA, 2022b, section 3.6.3.2.1). 
In its advice on the adequacy of the current primary PM2.5 
standards in their review of the 2021 draft PA, the CASAC reached 
consensus that the PM2.5 mass-based indicator should be 
retained, without revision (Sheppard, 2022a, p. 2 of consensus 
letter).\108\ Additionally, there was no information in the public 
comments that provided a rationale for an alternative indicator. For 
all of these reasons, the Administrator concludes that it is 
appropriate to retain PM2.5 mass as the indicator for the 
primary standards for fine particles.
---------------------------------------------------------------------------

    \108\ The CASAC did not provide advice or recommendations 
regarding the indicator of the primary PM2.5 standards in 
their review of the 2019 draft PA (Cox, 2019b).
---------------------------------------------------------------------------

    Consistent with his proposed conclusions regarding averaging time, 
the Administrator notes that the scientific evidence continues to 
provide strong support for health effect associations with both long- 
and short-term PM2.5 exposures (88 FR 5618, January 27, 
2023). Epidemiologic studies continue to provide strong support for 
health effects associated with short-term PM2.5 exposures 
based on 24-hour averaging periods, and associations in epidemiologic 
studies with subdaily estimates are less consistent and, in some cases, 
smaller in magnitude (88 FR 5618, January 27, 2023). Taken together, 
the 2019 ISA concludes that epidemiologic studies do not indicate that 
subdaily averaging periods are more closely associated with health 
effects than the 24-hour average exposure metric (U.S. EPA, 2019a, 
section 1.5.2.1). In addition, controlled human exposure and panel-
based studies of subdaily exposures typically examine subclinical 
effects rather than the more serious population-level effects that have 
been reported to be associated with 24-hour exposures (e.g., mortality, 
hospitalizations). While recent controlled human exposure studies 
provide consistent evidence for cardiovascular effects following 
PM2.5 exposures for less than 24 hours (i.e., <30 minutes to 
5 hours), air quality analyses have shown that the current averaging 
times can effectively protect against the exposure concentrations in 
these studies. This information does not indicate that a revision to 
the averaging time is necessary to provide additional protection 
against subdaily PM2.5 exposures, beyond that provided by 
the current primary annual and 24-hour PM2.5 standards. The 
Administrator also notes that this conclusion is also support by the 
CASAC's advice in their review of the 2021 draft PA where they reached 
consensus that averaging times for the primary PM2.5 
standards should be retained, without revision (Sheppard, 2022a, p. 2 
of consensus letter).\109\ The Administrator also considers the 
relatively few public comments received that support a subdaily 
averaging time, but concludes that the currently available information 
does not provide support for an alternate averaging time. Consistent 
with his proposed decision, the Administrator concludes that it is 
appropriate to retain the annual and 24-hour averaging times for the 
primary PM2.5 standards to protect against long- and short-
term PM2.5 exposures.
---------------------------------------------------------------------------

    \109\ The CASAC did not provide advice or recommendations 
regarding the averaging times of the primary PM2.5 
standards in their review of the 2019 draft PA (Cox, 2019b).
---------------------------------------------------------------------------

    With regard to form, the Administrator first notes that the EPA has 
set both an annual standard and a 24-hour standard to provide 
protection from health effects associated with both long- and short-
term exposures to PM2.5 (62 FR 38667, July 18, 1997; 88 FR 
5620, January 27, 2023). With regard to the form of the annual 
standard, the Administrator recognizes that a large majority of the 
recently available epidemiologic studies continue to report 
associations between health effects and annual average PM2.5 
concentrations. These studies of annual average PM2.5 
concentrations provide support for retaining the current form of the 
annual standard to provide protection against long- and short-term 
PM2.5 exposures. In its review of the 2021 draft PA, the 
CASAC reached consensus that the form of the annual standard (i.e., 
annual mean, averaged over 3 years) should be retained, without 
revision (Sheppard, 2022a, p. 2 of consensus letter).\110\ The 
Administrator also notes that there were no public comments that 
recommended an alternative form for the primary annual PM2.5 
standard.
---------------------------------------------------------------------------

    \110\ The CASAC did not provide advice or recommendations 
regarding the forms of the primary PM2.5 standards in 
their review of the 2019 draft PA (Cox, 2019b).
---------------------------------------------------------------------------

    With regard to the form of the 24-hour standard (98th percentile, 
averaged over three years), epidemiologic studies continue to provide 
strong support for health effect associations with short-term (e.g., 
mostly 24-hour) PM2.5 exposures, and controlled human 
exposure studies provide evidence for health effects following single 
short-term ``peak'' PM2.5 exposures (88 FR 5618, January 27, 
2023). Therefore, the Administrator concludes that the evidence 
supports retaining a standard focused on providing supplemental 
protection against short-term peak exposures and supports a 98th 
percentile form for a 24-hour standard, in combination with a primary 
annual PM2.5 standard with its annual mean averaged over 
three years form. As described in the proposal and in responding to 
comments in section II.B.3 above, the Administrator further notes that 
the 98th percentile, averaged over three years, form also provides an 
appropriate balance between limiting the occurrence of peak 24-hour 
PM2.5 concentrations and identifying a stable target for 
risk management programs (U.S. EPA, 2022b, section 3.6.3.2.3). 
Furthermore, the Administrator notes that the multi-year percentile 
form (i.e., averaged over three years) offers greater stability to the 
air quality management process by reducing the possibility that 
statistically unusual indicator values will lead to transient 
violations of the standard. This conclusion is also supported by the 
CASAC's advice in their review of the 2021 draft PA, where they reached 
consensus that the form for the primary PM2.5 standards 
should be retained, without revision (Sheppard, 2022a, p. 2 of 
consensus letter).\111\
---------------------------------------------------------------------------

    \111\ The CASAC did not provide advice or recommendations 
regarding the forms of the primary PM2.5 standards in 
their review of the 2019 draft PA (Cox, 2019b).
---------------------------------------------------------------------------

    The Administrator also recognizes that the CASAC recommended that 
in future reviews, the EPA also consider alternative forms for the 
primary 24-hour PM2.5 standard (Sheppard, 2022a,

[[Page 16278]]

p. 18 of consensus responses). Based on the CASAC's advice, the 
proposal solicited comment on alternatives to the current form for 
consideration in future reviews (88 FR 5619, January 27, 2023). The 
Administrator recognizes that there were a limited number of public 
comments related to the form of the primary PM2.5 standards 
as discussed in section II.D.3 above and in the Response to Comments 
document, and notes that, the EPA will consider the information 
provided by the commenters regarding the form of the 24-hour 
PM2.5 standard in the next review of the PM NAAQS. 
Consistent with his proposed decision, in considering the information 
summarized above, the Administrator concludes that it is appropriate to 
retain the forms of the current annual and 24-hour PM2.5 
standards.
    In considering how to revise the current suite of PM2.5 
standards to provide the requisite public health protection with an 
adequate margin of safety, the Administrator next evaluates the 
appropriate levels of the primary PM2.5 standards, beginning 
with the annual PM2.5 standard. In having carefully 
considered public comments related to the primary annual 
PM2.5 standard, the Administrator believes that the 
fundamental conclusions regarding the scientific evidence and 
quantitative information that supported his proposed conclusions (as 
described in the 2019 ISA, ISA Supplement, 2022 PA, and the proposal) 
remain valid. In considering the level at which the primary annual 
PM2.5 standard should be set, the Administrator considers 
the entire body of evidence and information, giving appropriate weight 
to each part of that body of evidence and information. He continues to 
place the greatest weight in this reconsideration on the available 
scientific evidence that provides support for associations between 
health effects and long- and short-term PM2.5 exposures. In 
conjunction with his decisions to retain the current indicator, 
averaging time, and form as described above, the Administrator is 
revising the level of the primary annual PM2.5 standard to 
9.0 [micro]g/m\3\. In so doing, he is selecting a primary annual 
PM2.5 standard that, together with the primary 24-hour 
PM2.5 standard, provides requisite public health protection 
with an adequate margin of safety, based on his judgments about and 
interpretation of the scientific evidence and quantitative risk 
information.
    The Administrator's decision to revise the level of the primary 
annual PM2.5 standard to 9.0 [micro]g/m\3\ builds upon his 
conclusion that the overall body of scientific evidence and 
quantitative risk information calls into question the adequacy of 
public health protection afforded by the current standard, particularly 
for at-risk populations. Consistent with his consideration of the 
available information in reaching his proposed decisions, the 
Administrator's final decision on the level of the primary annual 
PM2.5 standard places the greatest emphasis on key U.S. 
epidemiologic studies that report associations between long- and short-
term PM2.5 exposures and mortality and morbidity. As in the 
proposal, and as discussed further below, he views additional 
epidemiologic studies (i.e., studies that employ alternative methods 
for confounding control, studies that employ restricted analyses, and 
accountability studies), the controlled human exposure studies, and the 
risk assessment as providing supplemental information in support of his 
decision to revise the current annual standard, but recognizes that 
some of these lines of evidence and information provide a more limited 
basis for selecting a particular standard level among a range of 
options. See Mississippi, 744 F. 3d at 1351-52 (studies can 
legitimately support a decision to revise the standard, but not provide 
sufficient information to justify their use in setting the level of a 
revised standard).
    Given his consideration of the scientific evidence, quantitative 
risk information, advice from the CASAC, and public comments, the 
Administrator judges that a primary annual PM2.5 standard 
with a level of 9.0 [micro]g/m\3\ is requisite to protect public health 
with an adequate margin of safety. He notes that the determination of 
what constitutes an adequate margin of safety is expressly left to the 
judgment of the EPA Administrator. See Lead Industries Association v. 
EPA, 647 F.2d at 1161-62; Mississippi, 744 F.3d at 1353. He further 
notes that in evaluating how particular standards address the 
requirement to provide an adequate margin of safety, it is appropriate 
to consider such factors as the nature and severity of the health 
effects, the size of the at-risk populations, and the kind and degree 
of the uncertainties present. In considering the need for an adequate 
margin of safety, the Administrator notes that a primary annual 
PM2.5 standard with a level of 9.0 [micro]g/m\3\ would be 
expected to provide substantial improvements in public health compared 
to the current annual standard, including for at-risk groups such as 
children, older adults, people with preexisting conditions, minority 
populations, and low SES populations.
    Consistent with his conclusions on the need for revision of the 
current annual standard, in reaching a decision on level, the 
Administrator places the most weight on information from epidemiologic 
studies. In so doing, the Administrator notes that these studies 
provide consistent evidence of positive and statistically significant 
associations between long- and short-term exposure to PM2.5 
and mortality and morbidity (88 FR 5624, January 27, 2023). The 
Administrator recognizes that placing weight on the information from 
the epidemiologic studies allows for examination of the entire 
population, including those that may be at comparatively higher risk of 
experiencing a PM2.5-related health effects (e.g., children, 
older adults, minority populations) (88 FR 5624, January 27, 2023). The 
Administrator also recognizes that recent epidemiologic studies 
continue to support a no-threshold relationship, meaning that there is 
no ``bright line'' below which no effects have been found. These 
studies also support a linear relationship between health effects and 
PM2.5 exposures at PM2.5 concentrations greater 
than 8 [mu]g/m\3\, though uncertainties remain about the shape of the 
C-R curve at PM2.5 concentrations less than 8 [mu]g/m\3\, 
with some recent studies providing evidence for either a sublinear, 
linear, or supralinear relationship at these lower concentrations (U.S. 
EPA, 2019a, section 11.2.4; U.S. EPA, 2022a, section 2.2.3.2; 88 FR 
5625, January 27, 2023).
    As at the time of proposal, the Administrator notes that some 
recent epidemiologic studies have adopted a broad range of approaches 
to examine confounding and the results of those examinations support 
the robustness of reported associations seen in epidemiologic studies. 
These include studies that employ alternative methods for confounder 
control and studies that evaluate the uncertainty related to exposure 
measurement error, both of which continue to support associations 
between PM2.5 exposures and health effects while taking 
approaches to address uncertainties.
    In considering the epidemiologic evidence, the Administrator judges 
that, in reaching his decision on an appropriate level for the annual 
standard that will protect public health with an adequate margin of 
safety, in the absence of any discernible population-level thresholds, 
and in recognizing the need to weigh uncertainties associated with the 
epidemiologic evidence, it is most appropriate to examine where the 
evidence of associations observed in the

[[Page 16279]]

epidemiologic studies is strongest and, conversely, to place less 
weight where he has less confidence in the associations observed in the 
epidemiologic studies. As at the time of proposal, the Administrator 
notes that in previous reviews, evidence-based approaches noted that 
the evidence of an association in any epidemiologic study is 
``strongest at and around the long-term average where the data in the 
study are most concentrated'' (78 FR 3140, January 15, 2013). Given 
this, these approaches focused on identifying standard levels near or 
somewhat below long-term mean concentrations reported in key 
epidemiologic studies. These approaches were supported by previous 
CASAC advice as well as the CASAC's advice in their review of the 2021 
draft PA as a part of this reconsideration.
    Additionally, the Administrator acknowledges that in the 2020 final 
action, the then-Administrator decided to retain the standard based in 
part on concerns about placing reliance on the epidemiologic studies 
and his judgment that even if he did rely on them, the majority of the 
studies had means or medians, as well as the mean of all of the key 
study-reported means or medians, above the level of the current annual 
standard. However, after considering the evidence, the advice of CASAC, 
and public comments the Administrator judges that this approach is 
insufficient to protect public health with an adequate margin of 
safety. The Administrator's decision to reach a different judgment 
about the appropriate level of the annual standard reflects the updated 
and expanded scientific record available to the Administrator in this 
reconsideration, as well as the additional advice from the CASAC and 
the public comments based on this newly available information. In 
addition, the Administrator observes the decision in this action to 
place weight on the epidemiologic studies, and to revise the annual 
primary standard to a level below the lowest long-term mean in the 
U.S.-based epidemiologic studies, is consistent with the EPA's past 
practice in PM NAAQS reviews.
    In this reconsideration, the Administrator is considering the 
scientific record which has been expanded and updated since the 2020 
final action, as well as the additional advice from the CASAC and the 
public comments that are based on the newly available information that 
expands upon the information previously available. In addition, the 
Administrator is exercising his judgment about how to interpret and 
weigh the expanded evidence in a way that is more consistent with the 
approaches used in prior PM NAAQS reviews. As a result, the 
Administrator has concluded on reconsideration that the level of the 
primary annual standard is not adequate and should be revised to 
protect public health with an adequate margin of safety.
    Consistent with his proposed decisions, in reaching conclusions on 
the level of the primary annual PM2.5 standard, the 
Administrator considers the long-term \112\ study-reported mean 
PM2.5 concentrations from key long- and short-term 
epidemiologic studies and sets the level of the standard to somewhat 
below the lowest long-term mean PM2.5 concentration.\113\ He 
notes that in previous PM NAAQS reviews (including the 1997, 2006 and 
2012 reviews), evidence-based approaches focused on identifying 
standard levels near or somewhat below long-term mean concentrations 
reported in key long- and short-term epidemiologic studies. These 
approaches were supported by the CASAC in previous reviews and were 
supported in this reconsideration by the CASAC in their review of the 
2021 draft PA. In considering the available scientific evidence to 
inform such an approach, the Administrator notes the strength of the 
epidemiologic evidence which includes multiple studies that 
consistently report positive associations for short- and long-term 
PM2.5 exposure and mortality and cardiovascular effects. 
Some available studies also use a variety of statistical methods to 
control for confounding bias and report similar associations, which 
further supports the broader body of epidemiologic evidence for both 
mortality and cardiovascular effects. Additionally, he notes that 
recent epidemiologic studies available for consideration in reaching 
his final decision strengthen support for health effect associations at 
PM2.5 concentrations lower than in those evaluated in 
epidemiologic studies available at the time of previous reviews. The 
Administrator does recognize, however, that while these epidemiologic 
studies evaluate associations between distributions of ambient 
PM2.5 concentrations and health outcomes, they do not 
identify the specific exposures that led to the reported effects. As 
such, he notes that there is no specific point in the air quality 
distribution of any epidemiologic study that represents a ``bright 
line'' at and above which effects have been observed and below which 
effects have not been observed. The Administrator further notes that 
the epidemiologic studies provide the strongest support for reported 
health effect associations for this middle portion of the 
PM2.5 air quality distribution, which corresponds to the 
bulk of the underlying data, rather than the extreme upper or lower 
ends of the distribution, and concludes that the long-term study-
reported means from both long- and short-term studies provide the 
strongest support for reported health effect associations in 
epidemiologic studies. For these reasons, as described in the proposal 
and in responding to public comments in section II.B.3 above, the 
Administrator concludes that it is appropriate to continue to employ an 
approach that focuses on the mean PM2.5 concentrations from 
the key epidemiologic studies to inform his conclusions regarding the 
appropriate level for the primary annual PM2.5 standard.
---------------------------------------------------------------------------

    \112\ ``Long-term'' represents PM2.5 exposures and 
concentrations that are annual or multi-year.
    \113\ As described in section II.A.2.c above, key epidemiologic 
studies are those that report overall mean (or median) 
PM2.5 concentrations and for which the years of 
PM2.5 air quality data used to estimate exposures overlap 
entirely with the years during which health events are reported.
---------------------------------------------------------------------------

    In adopting such an approach, the Administrator considers the long-
term mean concentrations reported in two types of key epidemiologic 
studies: (1) Monitor-based studies \114\ (epidemiologic studies that 
used ground-based monitors to estimate exposure, similar to approaches 
used in past reviews), and (2) hybrid modeling-based studies \115\ 
(epidemiologic studies that used hybrid modeling approaches and apply 
aspects of population weighting to estimate exposures). In reaching 
conclusions regarding the level of a standard that would provide 
requisite protection with an adequate margin of safety, the 
Administrator recognizes that he must use his judgment regarding the 
appropriate weight to place on the available evidence and technical 
information, including uncertainties. As shown in Figures 1 and 2 
above, for the key U.S. monitor-based epidemiologic studies,

[[Page 16280]]

the study-reported mean concentrations range from 9.9-16.5 [mu]g/m\3\, 
and for the key U.S. hybrid modeling-based epidemiologic studies, the 
mean concentrations range from 9.3-12.2 [mu]g/m\3\. The Administrator 
also recognizes that, in their review of the 2021 draft PA, both the 
majority and minority of the CASAC emphasized the epidemiologic studies 
in support of their recommendations for the level of the annual 
standard, but they weighed the studies in different ways (Sheppard, 
2022a, p. 16-17 of consensus responses).
---------------------------------------------------------------------------

    \114\ Reported mean PM2.5 concentrations in monitor-
based studies are averaged across monitors in each study area with 
multiple monitors, referred to as a composite monitor concentration, 
in contrast to the highest concentration monitored in the study 
area, referred to as a maximum monitor concentration (i.e., the 
``design value'' concentration), which is used to determine whether 
an area meets a given standard.
    \115\ Studies that use hybrid modeling approaches employ methods 
to estimate ambient PM2.5 concentrations across large 
geographical areas, including areas without monitors, and thus, when 
compared to monitor-based studies, require additional information to 
inform the relationship between the estimated PM2.5 
concentrations across an area and the maximum monitor design values 
used to assess compliance.
---------------------------------------------------------------------------

    Based on this information, and in considering the CASAC's advice in 
their review of the 2021 draft PA, the Administrator judges that it is 
appropriate to set the level of the primary PM2.5 standard 
at least as low as the lowest mean PM2.5 concentration from 
these key U.S.-based epidemiologic studies, which is 9.3 [micro]g/m\3\. 
The Administrator additionally notes that setting the annual standard 
level at 9.0 [micro]g/m\3\, which is below the lowest study-reported 
mean PM2.5 concentration of 9.3 [micro]g/m\3\, would be 
expected to shift the distribution of PM2.5 concentrations 
in an area such that the area's highest monitor would generally be at 
or below 9.0 [micro]g/m\3\ annually, when meeting the annual standard. 
In this situation, the resulting average or mean PM2.5 
concentration for the entire area (measured across a number of 
monitors) would be even further below the study-reported means,\116\ 
and will provide adequate protection not only in areas where the 
highest allowable concentrations would be expected (i.e., near design 
value monitors) but also in other parts of the area where 
PM2.5 concentrations would be expected to be maintained even 
lower.
---------------------------------------------------------------------------

    \116\ Analyses in the 2022 PA suggest that the highest monitored 
value would be expected to be greater than the study-reported mean 
values by 10-20% for monitor-based studies and 15-18% for hybrid 
modeling studies that apply aspects of population weighting.
---------------------------------------------------------------------------

    As noted above, however, the Administrator must exercise his 
judgment regarding the appropriate weight to place on the available 
scientific evidence and quantitative information, including 
uncertainties, in determining what level of the annual standard is 
sufficient to protect public health with an adequate margin of safety. 
In so doing, he considers other information available in this 
reconsideration to inform his judgments, including study-reported 
PM2.5 concentrations at lower percentiles in key 
epidemiologic studies, supplemental information from other types of 
epidemiologic studies, study-reported PM2.5 concentrations 
from key Canadian epidemiologic studies, and the results from the 
quantitative risk assessment.
    In weighing the evidence in considering the requisite level of the 
annual standard, the Administrator also takes into account additional 
information from the key long- and short-term U.S. epidemiologic 
studies available that provide study-reported PM2.5 
concentrations below the mean and, in particular, the subset of 
epidemiologic studies that report 25th and 10th percentile 
concentrations. Consistent with his proposed conclusions, as well as 
the CASAC's advice in their review of the 2021 draft PA and public 
comments, the Administrator judges that it is appropriate to place some 
weight on these lower percentiles in reaching his conclusions on the 
level of the primary annual standard. There are six key U.S. 
epidemiologic studies that report information on other percentiles 
(e.g., 10th and 25th percentiles of PM2.5 concentrations or 
10th and 25th percentiles of PM2.5 concentrations associated 
with health events) that are below the mean.\117\ In considering the 
information from these studies, the Administrator first notes that the 
three older, monitor-based studies that report lower percentiles of 
PM2.5 concentrations have smaller cohort sizes than the 
three hybrid model-based studies. Thus, the Administrator recognizes 
that the older, monitor-based studies had a relatively smaller portion 
of the health events that were observed in the lower part of the air 
quality distribution because of the generally smaller size of the 
cohorts. He further notes that the recent hybrid model-based studies 
have larger cohort sizes than the older, monitor-based studies, and 
therefore, have more health events in the lower part of the air quality 
distribution. Because of the larger cohort sizes and having a larger 
portion of health events that are observed across the air quality 
distribution, the Administrator has more confidence in the magnitude 
and significance of the associations in the lower parts of the air 
quality distribution for the recent, hybrid model-based studies 
compared to the older, monitor-based studies. Given this, the 
Administrator judges that it is appropriate to place weight on the 25th 
percentile concentrations reported in the recently available hybrid 
model-based studies in reaching his conclusions regarding the 
appropriate level for the primary annual PM2.5 standard. 
However, the Administrator also recognizes that his confidence in the 
magnitude and significance in the reported concentrations, and their 
ability to inform decisions on the appropriate level of the annual 
standard, starts to diminish at percentiles that are even further below 
the mean and the 25th percentile. For these reasons, the Administrator 
places weight on the reported 25th percentiles concentrations in the 
recent hybrid model-based studies, rather than the reported 10th 
percentile concentrations, in reaching his conclusions regarding the 
appropriate level for the primary annual PM2.5 standard.
---------------------------------------------------------------------------

    \117\ The Wang et al. (2017) study only reports the 25th 
percentile of the estimated PM2.5 concentrations, not the 
10th percentile.
---------------------------------------------------------------------------

    In considering the information from these studies, as described in 
section II.A.2.c and in responding to public comments in section II.B.3 
above, the Administrator notes that there are two hybrid model-based 
studies with large cohort sizes that apply population weighting and 
report lower percentile values. These studies are Di et al. (2017b) and 
Wang et al. (2017) and the reported 25th percentile concentration is 
9.1 [micro]g/m\3\ for both studies.\118\ In considering these studies, 
the Administrator concludes that it is appropriate to place weight on 
the 25th percentile concentrations of these newer hybrid model-based 
studies (of 9.1 [micro]g/m\3\) such that setting the level of the 
standard near these 25th percentile concentrations would provide 
requisite protection. The Administrator observes that an annual 
standard level of 9.0 [micro]g/m\3\ would be near the reported 25th 
percentile concentrations in these studies.
---------------------------------------------------------------------------

    \118\ There is a third hybrid model-based study, as described in 
the 2022 PA and in section II.B.3 above in responding to public 
comments, but it is not referenced here because it reports a 25th 
percentile PM2.5 concentration based on the 25th 
percentile of health events that occur in the study (Di et al., 
2017a) rather than report the 25th percentile based on air quality 
concentrations.
---------------------------------------------------------------------------

    As at the time of proposal, the Administrator also takes note of 
the study-reported long-term mean PM2.5 concentrations in 
long- and short-term Canadian epidemiologic studies, which ranged from 
6.9 to 13.3 [micro]g/m\3\ for monitor-based studies and 5.9 to 9.8 
[micro]g/m\3\ for hybrid model-based studies. While the Administrator 
notes that these studies provide additional support for associations 
between PM2.5 concentrations and health effects, he is also 
mindful that there are important differences between the exposure 
environments in the U.S. and Canada and that interpreting the data 
(e.g., study-reported mean concentrations)

[[Page 16281]]

from the Canadian studies in the context of a U.S.-based standard may 
present challenges in directly and quantitatively informing decisions 
regarding potential alternative levels of the annual standard. For 
example, in terms of people per square kilometer, the U.S. population 
density is nearly 10 times in the contiguous U.S. compared to Canada. 
As described in more detail in responding to public comments in section 
II.B.3 above, in this reconsideration, the Administrator recognizes 
that this difference in population density between the U.S. and Canada 
is more apparent than in previous reviews because the studies available 
in this reconsideration use different approaches than those previously 
available. In the 2012 review, the available Canadian epidemiologic 
studies used population-weighting and focused on urban areas where 
monitors were available and population densities were more comparable 
with those in the U.S., and at that time, the U.S. and Canadian studies 
reported similar mean PM2.5 concentrations. However, in this 
reconsideration, the Administrator takes note that for the new Canadian 
epidemiologic studies: (1) The Canadian monitor-based studies available 
in this reconsideration do not apply population weighting as the 
previously available studies did; and (2) some of the studies now use 
hybrid modeling approaches for estimating exposure. The Administrator 
recognizes that these differences are important to consider in reaching 
conclusions on how these Canadian epidemiologic studies should be 
interpreted regarding decisions on the requisite level of the primary 
annual PM2.5 standard. Specifically, the Administrator notes 
that the more recent Canadian studies that use hybrid modeling 
incorporate larger portions of the country, and therefore include more 
rural areas. The more rural areas that are included in the study using 
the hybrid modeling approaches, the more important it is to consider 
how the population densities and exposure environments differ between 
the U.S. and Canada. Additionally, the Administrator notes that for 
hybrid modeling-based studies there is less certainty in 
PM2.5 exposure estimates in more rural areas, which are 
further from air quality monitors and where PM2.5 
concentrations in the ambient air tend to be lower. For these hybrid 
model-based studies, the portion of the rural areas that are 
contributing to the study-reported mean PM2.5 concentrations 
in these studies is unclear. For these reasons, the Administrator 
concludes that it is important to consider the differences between the 
population exposures in the U.S. and Canadian study areas and how these 
differences influence the interpretation of the epidemiologic study 
results.
    Thus, the Administrator considers the Canadian studies to inform 
his judgments on what level for the annual standard is requisite in 
light of the limitations and challenges presented. The Administrator 
also recognizes that the majority of the CASAC in their review of the 
2021 draft PA, as well as a number of public commenters, place weight 
on the Canadian epidemiologic studies in recommending that the level of 
the primary annual PM2.5 standard be revised to 8-10 
[micro]g/m\3\. The Administrator further notes while the majority of 
the CASAC advised the EPA to consider the Canadian studies in revising 
the annual standard level to within the range of 8.0-10.0 [micro]g/
m\3\, they did not advise the EPA to set the annual standard level 
below the study-reported means from those studies. Given these 
considerations, the Administrator judges that it is appropriate to set 
the level of annual standard within the range of 8-10 [micro]g/m\3\ to 
be consistent with the majority of the CASAC's advice in their 
consideration of these studies.
    The Administrator also recognizes that information from 
epidemiologic studies that included analyses that restrict annual 
average PM2.5 concentrations to concentrations below the 
level of the current annual standard can be useful for informing 
conclusions regarding the appropriate level of the primary annual 
PM2.5 standard. In so doing, he particularly notes the two 
key U.S. epidemiologic studies (Di et al., 2017b and Dominici et al., 
2019) that restrict annual average PM2.5 concentrations to 
less than 12 [micro]g/m\3\ and report positive and statistically 
significant associations with all-cause mortality and mean 
PM2.5 concentrations of 9.6 [micro]g/m\3\. He also considers 
these results along with the uncertainties and limitations associated 
with studies that restricted analyses below certain PM2.5 
concentrations. As described in responding to comments in section 
II.B.3 above, uncertainties associated with how the studies exclude 
PM2.5 concentrations from the analyses (e.g., at what 
spatial resolution are concentrations being excluded), make it 
difficult to understand how to interpret the results of the restricted 
analyses in the context of the approach employed in this 
reconsideration, which takes into consideration the relationship 
between mean PM2.5 concentrations and design values.
    The Administrator also recognizes that, in their review of the 2021 
draft PA, the CASAC noted that epidemiologic studies that restrict 
analyses below certain PM2.5 concentrations represent one 
area for which the evidence has expanded in this reconsideration, 
stating that these studies provide support for mortality effects at 
concentrations below the current PM NAAQS (Sheppard, 2022a, p. 5 of 
consensus responses). In their recommendations on alternative levels 
for the primary annual PM2.5 standard, the majority of the 
CASAC cited to studies that restrict PM2.5 concentrations to 
below 12 [micro]g/m\3\ as a part of their rationale for supporting a 
level within the range of 8-10 [micro]g/m\3\ (Sheppard, 2022a p. 16 of 
consensus responses). Additionally, the Administrator notes that some 
members of the CASAC, in their review of the 2019 draft PA, concluded 
that the epidemiologic studies that restrict analyses below 12 
[micro]g/m\3\ and show positive associations with health effects, along 
with other aspects of the scientific evidence, provide support for 
their conclusion that the primary annual PM2.5 standard is 
not adequate (Cox, 2019b, p. 9 of consensus responses). Furthermore, 
the Administrator takes note of public commenters who also noted that 
the epidemiologic studies that restrict PM2.5 concentrations 
to below the current standard provide support, along with the other 
available information, for lowering the level of the primary annual 
PM2.5 standard. In considering the studies that include 
restricted analyses, along with the CASAC's advice and public comments 
on these types of studies, the Administrator concludes that, although 
there are inherent uncertainties associated with this limited body of 
evidence, these studies that apply restricted analyses provide support 
for serious effects (e.g., mortality) at concentrations below 10.0 
[micro]g/m\3\. Given this, the Administrator concludes that it is 
appropriate to place some weight on these studies, and in doing so, 
notes that a standard level of 9.0 [micro]g/m\3\ would be below the 
reported mean PM2.5 concentrations of 9.6 [micro]g/m\3\ in 
these studies and would, thus, be expected to provide protection 
against exposures related to these reported mean concentrations.
    The Administrator also takes into consideration recent U.S. 
accountability studies, which assess the health effects associated with 
actions that improve air quality (e.g., air quality policies or 
implementation of an intervention). These types of studies can also 
reduce uncertainties related to residual confounding of temporal and 
spatial factors (U.S. EPA, 2022a, p. 3-25). The

[[Page 16282]]

Administrator notes that in the 2020 review, the available 
accountability studies had ``starting'' annual average PM2.5 
concentrations (i.e., mean concentration prior to reductions being 
evaluated) from 13.2-31.5 [micro]g/m\3\, and the then-Administrator 
cited the lack of accountability studies in areas where the 
``starting'' concentration met the current primary PM2.5 
standards as part of his rationale for retaining the standards. As at 
the time of proposal, the current Administrator notes that in three 
studies newly available in this reconsideration and assessed in the ISA 
Supplement, prior to implementation of the policies, mean 
PM2.5 concentrations in these studies were below the level 
of the current annual standard level (12.0 [micro]g/m\3\) and ranged 
from 10.0 [micro]g/m\3\ to 11.1 [micro]g/m\3\. These studies report 
positive and significant associations between mortality and 
cardiovascular morbidity and reductions in ambient PM2.5 
following the implementation of a policy (Henneman et al., 2019; 
Corrigan et al., 2018; Sanders et al., 2020a; 88 FR 5627, January 27, 
2023). These studies suggest that public health improvements may occur 
following the implementation of a policy that reduces annual average 
PM2.5 concentrations below the level of the current standard 
of 12.0 [micro]g/m\3\. The Administrator recognizes that in their 
review of the 2021 draft PA, the CASAC noted that the availability of 
recent accountability studies was one area where the evidence had been 
strengthened and that the studies assessed in the ISA Supplement 
provide evidence of mortality effects at annual average 
PM2.5 concentrations below the current NAAQS (Sheppard, 
2022a, p. 5 of consensus responses). The Administrator recognizes that 
the CASAC also concluded that, along with other lines of evidence, the 
accountability studies with starting concentrations below the levels of 
the current standards are appropriate to consider for informing 
conclusions on alternative standard levels (Sheppard, 2022a, p. 13 of 
consensus responses). The Administrator also notes the advice of the 
CASAC in their review of the 2019 draft ISA, where they suggested that 
accountability studies be taken into account and such studies provide 
potentially crucial information about whether and how much decreasing 
PM2.5 causes decreases in future health effects, which 
reflects the primary purpose of the NAAQS (Cox, 2019b, p. 8 and 10 of 
consensus responses). The Administrator also notes that in their review 
of the 2019 draft ISA, some members of the CASAC cautioned against 
placing more weight on the data from accountability studies based on 
the methodological limitations of the studies (Cox, 2019b, p. 8 of 
consensus responses). The Administrator notes that the CASAC did not 
explicitly cite to accountability studies in their reviews of the 2019 
draft PA or 2021 draft PA as support for their recommendations on the 
adequacy of the primary annual PM2.5 standard or potential 
alternative standard levels. A number of public commenters who support 
revising the level of the standard to 8 [micro]g/m\3\ cite these 
accountability studies, along with the broader evidence base, as 
support for a more protective standard. The Administrator, in 
considering the evidence, the advice from the CASAC, and public 
comment, first recognizes that accountability studies are just one line 
of evidence to be considered in the broader evaluations of the 
information available to inform conclusions on the level of the 
standard. In so doing, he notes that public health improvements may 
occur following the implementation of a policy that reduces annual 
average PM2.5 concentrations below the level of the current 
standard of 12.0 [micro]g/m\3\, and potentially below the lowest 
``starting'' concentrations in these studies of 10.0 [micro]g/m\3\. 
However, the Administrator concludes that the limited number of 
accountability studies provide limited information for informing 
decisions on the appropriate level of the primary annual 
PM2.5 standard but recognizes that these studies provide 
supplemental information for consideration along with the full body of 
evidence. Taken together, the Administrator notes a revised annual 
standard level of 9.0 [micro]g/m\3\ is at or below the lowest starting 
concentration of these accountability studies (i.e., 10.0 [micro]g/
m\3\), and judges that it is appropriate to place some weight on these 
studies, particularly for informing his public policy judgments 
regarding an adequate margin of safety.
    In addition to his consideration of and conclusions regarding the 
available scientific evidence, the Administrator also considers the 
results of the quantitative risk assessment to inform his conclusions 
regarding the appropriate level for the primary annual PM2.5 
standard. The Administrator recognizes that the risk estimates can help 
to place the evidence for specific health effects into a broader public 
health context, but should be considered along with the inherent 
uncertainties and limitations of such analyses when informing judgments 
about the potential for additional public health protection associated 
with PM2.5 exposure and related health effects. The 
Administrator recognizes that the overall risk assessment estimates 
suggest that the current primary annual PM2.5 standard could 
allow a substantial number of PM2.5-associated deaths in the 
U.S. The Administrator also recognizes that the CASAC concurred with 
the 2021 draft PA's assessment that meaningful risk reductions will 
result from lowering the annual PM2.5 standard (Sheppard, 
2022a, p. 16 of consensus responses).
    Additionally, with respect to the results of the quantitative risk 
assessment, the Administrator recognizes that the 2022 PA also provides 
information on the distribution of concentrations associated with the 
estimated mortality risk at each alternative standard level assessed 
(U.S. EPA, 2022b, sections 3.4.2.2 and 3.6.2.2, Figure 3-18 and 3-19). 
When meeting an annual standard of 9.0 [micro]g/m\3\ at the design 
value monitor, the exposure concentrations within an area are estimated 
to be below 9 [micro]g/m\3\, with the majority of those exposures being 
at concentrations of below 8 [micro]g/m\3\. The Administrator notes 
that this range of concentrations is below the lowest means in the key 
long- and short-term epidemiologic studies (concentrations at which the 
evidence is the strongest in supporting an association between exposure 
to PM2.5 and adverse health effects observed in the key 
epidemiologic studies available in this reconsideration). Thus, the 
Administrator concludes that the results of the quantitative risk 
assessment suggest that a revised annual standard level of 9.0 
[micro]g/m\3\ is estimated to reduce PM2.5 exposures to fall 
within the range of concentrations in which there is the most 
confidence in the associations and thus, confidence that estimated risk 
reductions will actually occur.
    The Administrator also notes the information provided by the 
quantitative risk assessment on the distribution of concentrations 
associated with the estimated mortality risk for a higher annual 
standard level of 10.0 [micro]g/m\3\ and a lower standard level of 8.0 
[micro]g/m\3\ (U.S. EPA, 2022b, sections 3.4.2.2 and 3.6.2.2, Figure 3-
18 and 3-19). The Administrator finds that, for an annual standard 
level of 10.0 [micro]g/m\3\, the quantitative risk assessment estimates 
that the standard would allow multiple exposures at concentrations 
above the lowest means in the key epidemiologic studies, and therefore, 
calls into question whether a standard level of 10.0 [micro]g/m\3\ 
would provide enough public health protection. Additionally, the 
Administrator also finds that, for a lower annual standard level of 8.0 
[micro]g/

[[Page 16283]]

m\3\, the quantitative risk assessment estimates the exposure 
concentrations to be below 8 [micro]g/m\3\, with the majority of those 
exposures being at concentrations of below 7 [micro]g/m\3\. The 
Administrator observes that the majority of exposure concentrations 
under this air quality scenario are estimated to fall outside of the 
range of concentrations in which he has the most confidence in the 
associations and that the additional risk reductions will actually 
occur.
    Recognizing and building upon the above considerations and 
judgments, and with consideration of advice from the CASAC and public 
comment, the Administrator concludes that the current body of 
scientific evidence and quantitative risk assessment support his 
judgment that the level of the primary annual PM2.5 standard 
should be revised to a level of 9.0 [micro]g/m\3\. Revising the level 
of the primary annual PM2.5 standard will, in the 
Administrator's judgment, provide requisite public health protection 
with an adequate margin of safety.
    The Administrator recognizes that placing weight on the information 
from the epidemiologic studies allows for examination of the entire 
population, including those that may be at comparatively higher risk of 
experiencing a PM2.5-related health effects (e.g., children, 
older adults, minority populations) (88 FR 5624, January 27, 2023). In 
considering the epidemiologic evidence, the Administrator judges that, 
in reaching his decision on an appropriate level for the annual 
standard that will protect public health with an adequate margin of 
safety, in the absence of any discernible population-level thresholds, 
and in recognizing the need to weigh uncertainties associated with the 
epidemiologic evidence, it is most appropriate to examine where the 
evidence of associations observed in the epidemiologic studies is 
strongest and, conversely, to place less weight where he has less 
confidence in the associations observed in the epidemiologic studies. 
The Administrator notes that in previous reviews, evidence-based 
approaches noted that the evidence of an association in any 
epidemiologic study is ``strongest at and around the long-term average 
where the data in the study are most concentrated'' (78 FR 3140, 
January 15, 2013). These approaches were supported by previous CASAC 
advice as well as the CASAC's advice in their review of the 2021 draft 
PA as a part of this reconsideration. Given this, the Administrator 
notes that in revising the annual PM2.5 standard to a level 
of 9.0 [micro]g/m\3\, he is setting the standard at a level below the 
long-term mean PM2.5 concentrations in the key long- and 
short-term epidemiologic studies, including the lowest study reported 
mean of 9.3 [micro]g/m\3\, following an approach that is consistent 
with previous PM NAAQS reviews. The Administrator additionally notes 
that air quality analyses in the 2022 PA demonstrate that areas meeting 
a revised annual standard of 9.0 [micro]g/m\3\ would be expected to 
shift the distribution of PM2.5 exposure concentrations in 
an area such that the area's highest monitor would generally be at or 
below 9.0 [micro]g/m\3\ annually, and most of the resulting 
PM2.5 concentrations across the area would be even further 
below the study-reported means.119 120 Thus, a standard 
level of 9.0 [micro]g/m\3\ is expected to provide sufficient protection 
not only in areas where the highest allowable concentration would be 
located (i.e., near design value monitors) but also in other parts of 
the area where PM2.5 concentrations would be expected to be 
maintained even lower.
---------------------------------------------------------------------------

    \119\ Analyses in the 2022 PA suggest that the highest monitored 
value would be expected to be greater than the study-reported mean 
values by 10-20% for monitor-based studies and 15-18% for hybrid 
modeling studies that apply aspects of population weighting (U.S. 
EPA, 2022b, section 2.3.3.2.4).
    \120\ The risk assessment in the 2022 PA used air quality 
adjustments to simulate just meeting the current primary 
PM2.5 standards, as well as alternative standard levels 
(U.S. EPA, 2022b, section 3.4.1.4 and Appendix C, section C.1.4).
---------------------------------------------------------------------------

    Furthermore, the Administrator recognizes the CASAC's advice in 
their review of the 2021 draft PA, as well as public comments, that 
weight should be placed on study-reported PM2.5 
concentrations that are somewhat below the mean, particularly for some 
of the newer epidemiologic studies with larger cohort sizes. In 
weighing uncertainties associated with using these data to inform a 
revised annual standard level, as well as noting the limited studies 
for which this information is available, the Administrator judges that 
some weight should be placed on these data, but they should not receive 
the same weight as the study-reported mean concentrations. Thus, the 
Administrator concludes that it would be appropriate to set the annual 
standard level near the 25th percentile PM2.5 concentrations 
in the two newer key epidemiologic studies for which these values were 
reported. In doing so, the Administrator notes that a decision to 
revise the annual standard to 9.0 [micro]g/m\3\ would set a level of 
the standard near and somewhat below the reported 25th percentile 
PM2.5 concentrations of 9.1 [micro]g/m\3\ in these two more 
recent hybrid model-based studies.
    The Administrator also takes note of the study-reported long-term 
mean PM2.5 concentrations in the key Canadian epidemiologic 
studies. While the Administrator notes that these studies provide 
additional support for associations between PM2.5 
concentrations and health effects, he is also mindful that there are 
important differences between the exposure environments in the U.S. and 
Canada that affect interpretation of the data in the context of 
informing decisions regarding potential alternative levels of the 
annual standard. The Administrator also recognizes that the majority of 
the CASAC in their review of the 2021 draft PA, as well as a number of 
public commenters, placed weight on the Canadian epidemiologic studies 
in recommending that the level of the primary annual PM2.5 
standard be revised to 8-10 [micro]g/m\3\. The Administrator notes that 
a decision to revise the annual standard to 9.0 [micro]g/m\3\ would set 
the level of the standard within the range of levels recommended by the 
majority of CASAC in their consideration of these studies.
    Additionally, the Administrator also considers the information 
provided by epidemiologic studies that use restricted analyses, as well 
as accountability studies. With respect to the restricted analyses, the 
Administrator, in considering the CASAC's advice in their review of the 
2021 draft PA and many public comments on these types of studies, 
concludes that, although there are inherent uncertainties associated 
with this limited body of evidence, the studies that apply restricted 
analyses provide support for serious effects (e.g., mortality) at 
concentrations below 10.0 [micro]g/m\3\. Additionally, in considering 
accountability studies, the Administrator concludes that while the 
small number of these studies provide limited information for informing 
decisions on the appropriate level of the primary annual 
PM2.5 standard, these studies provide supplemental 
information for consideration along with the full body of evidence. The 
Administrator further notes that these studies suggest that public 
health improvements may occur following the implementation of a policy 
that reduces annual average PM2.5 concentrations below the 
level of the current standard of 12.0 [micro]g/m\3\, and potentially 
below the lowest ``starting'' concentrations in these studies of 10.0 
[micro]g/m\3\. Taken together, the Administrator judges that it is 
appropriate to place some weight on these types of studies, 
particularly for informing his public policy judgments regarding an 
adequate margin

[[Page 16284]]

of safety, and notes that a revised annual standard level of 9.0 
[micro]g/m\3\ is below the lowest starting concentration of the 
accountability studies (i.e., 10.0 [micro]g/m\3\), and below the 
concentration at which studies that apply restricted analyses provide 
support for serious effects (i.e., 9.6 [micro]g/m\3\).
    The Administrator also judges that the results of the quantitative 
risk assessment provide support for a primary annual PM2.5 
standard with a level of 9.0 [micro]g/m\3\. The results of the risk 
assessment suggest that when meeting an annual standard of 9.0 
[micro]g/m\3\, PM2.5 exposures are maintained below 9 
[micro]g/m\3\ at the design value monitor, with the majority of those 
exposures being at concentrations below 8 [micro]g/m\3\. Thus, the 
Administrator notes that an annual standard level of 9.0 [micro]g/m\3\ 
would be expected to provide protection from exposures where he has the 
greatest confidence in the associations between health effects and 
PM2.5 exposures (i.e. the long-term mean PM2.5 
concentrations in the key U.S. epidemiologic studies, of which the 
lowest is 9.3 [micro]g/m\3\) and would provide an adequate margin of 
safety by maintaining most PM2.5 exposures even further 
below 9.0 [micro]g/m\3\.
    When considering adequate margin of safety, the Administrator notes 
that in his decision to revise the annual standard level to 9.0 
[micro]g/m\3\, he is placing weight on the information from the 
epidemiologic studies which allows for examination of the entire 
population, including those that may be at comparatively higher risk of 
experiencing a PM2.5-related health effects (e.g., children, 
older adults, minority populations). Additionally, as discussed above, 
the Administrator also recognizes that setting the annual standard 
level at 9.0 [micro]g/m\3\, which is below concentrations at which the 
evidence is the strongest in supporting an association between exposure 
to PM2.5 and adverse health effects observed in the key 
epidemiologic studies available in this reconsideration, would be 
expected to shift the distribution of PM2.5 exposure 
concentrations in an area such that the area's highest monitor would 
generally be at or below 9.0 [micro]g/m\3\ annually, and most of the 
resulting PM2.5 concentrations across the area would be even 
lower. In considering these air quality relationships, the 
Administrator judges that a revised annual standard level of 9.0 
[micro]g/m\3\ would provide requisite protection with adequate margin 
of safety, for all populations, including those most at-risk.
    In reaching this conclusion, the Administrator recognizes that in 
establishing primary standards under the Act that are requisite to 
protect public health with an adequate margin of safety, he is seeking 
to establish standards that are neither more nor less stringent than 
necessary for this purpose. The Act does not require that primary 
standards be set at a zero-risk level or to protect the most sensitive 
individual, but rather at a level that avoids unacceptable risks to 
public health. In this context, the Administrator's conclusion is that 
revised primary annual standard, in conjunction with the 24-hour 
standard, provides the appropriate degree of protection, and that more 
or less stringent standards would not be requisite.
    In considering the requirement for an adequate margin of safety, 
the Administrator notes that the determination of what constitutes an 
adequate margin of safety is expressly left to the judgment of the EPA 
Administrator. See Lead Industries Association v. EPA, 647 F.2d at 
1161-62; Mississippi, 744 F.3d at 1353. He further notes that in 
evaluating how particular standards address the requirement to provide 
an adequate margin of safety, it is appropriate to consider such 
factors as the nature and severity of the health effects, the size of 
sensitive population(s) at risk, and the kind and degree of the 
uncertainties present. Consistent with past practice and long-standing 
judicial precedent, and as described in this section, the Administrator 
takes the need for an adequate margin of safety into account as an 
integral part of his decision making on a standard. See, e.g., NRDC v. 
EPA, 902 F. 2d 962, 973-74 (D.C. Cir. 1990).
    Given all of the evidence and information discussed above, the 
Administrator judges that a standard with a level of 9.0 [micro]g/m\3\ 
is requisite to protect public health with an adequate margin of 
safety. In so doing, he first recognizes that a less stringent standard 
would allow the occurrence of higher long- and short-term 
PM2.5 concentrations at a level at or above the mean 
PM2.5 concentrations in key U.S. epidemiologic studies. That 
is, a less stringent standard would be expected to allow more 
PM2.5 exposures at concentrations at or above which the key 
U.S. epidemiologic studies have reported associations between mean 
PM2.5 concentrations and serious health effects and would 
deviate from some past approaches for selecting the appropriate level 
of the annual standard. A less stringent standard would also not 
provide requisite protection with an adequate margin of safety against 
PM2.5 exposures in the lower percentiles of the air quality 
distribution (i.e., 25th percentile) for which associations with health 
effects have been observed in a limited number of epidemiologic 
studies. Furthermore, the Administrator notes that the primary annual 
and 24-hour PM2.5 standards, together, are intended to 
provide public health protection against the full distribution of long- 
and short-term PM2.5 exposures. As noted above, the 
Administrator recognizes that the changes in PM2.5 air 
quality designed to meet a less stringent annual standard would likely 
result in higher exposures across the distribution of air quality, 
including both higher average (or typical) concentrations as well as 
higher short-term peak PM2.5 concentrations. Taking into 
consideration both the full evidence base for associations of 
PM2.5 with mortality and other adverse health effects, 
including the reported mean PM2.5 concentrations from key 
long- and short-term U.S. epidemiologic studies, information from 
epidemiologic studies that report 25th percentile PM2.5 
concentrations, supplemental information from other epidemiologic 
studies (i.e., epidemiologic studies that use restricted analyses, 
accountability studies, and Canadian epidemiologic studies), and the 
results of the risk assessment, as well as the advice from the CASAC 
and public comments, the Administrator concludes that a less stringent 
standard would allow risks of mortality and other adverse health 
effects that are too great, and thus would not provide sufficient 
protection for public health as required by the CAA.
    Additionally, in considering a less stringent standard, the 
Administrator recognizes that through its control of long- and short-
term PM2.5 concentrations, the annual standard provides a 
margin of safety for less well-studied exposure levels and population 
groups for which the evidence is limited or lacking. In so doing, he 
recognizes that our understanding of the relationships between the 
presence of a pollutant in ambient air and associated health effects is 
based on a broad body of information encompassing not only more 
established aspects of the evidence, such as the conclusion that long- 
and short-term exposures to PM2.5 are causally related to 
mortality and cardiovascular effects and likely to be causally related 
to respiratory effects, but also aspects with which there may be 
substantial uncertainty. In particular, the Administrator notes that 
there are other categories of effects with causality determinations 
that are suggestive of, but not sufficient to infer, a causal

[[Page 16285]]

relationship between PM2.5 exposure and health outcomes. 
These include, but are not limited t,o short-term exposure and nervous 
system effects, as well as long- and short-term exposure and pregnancy 
and birth outcomes, where the evidence is less certain but which 
represent potentially substantial additional risk to public health from 
exposure to PM2.5. He recognizes the CAA requirement that 
requires primary standards to provide an adequate margin of safety was 
intended to address uncertainties associated with inconclusive 
scientific and technical information as well as to provide a reasonable 
degree of protection against hazards that research has not yet 
identified and in his judgment, the primary NAAQS must be set at a 
level that is adequately protective against these and other effects 
which research has not yet identified. Thus, even if the Administrator 
had somewhat greater concerns about the possibility of confounding, 
error and bias in the epidemiologic studies, which reduced his 
confidence in finding that PM2.5 is causally related to 
mortality and cardiovascular effects, he would still find it 
appropriate to set the primary NAAQS below the means of key U.S. 
epidemiologic studies given the strength of the evidence providing 
support for the association, as well as additional evidence linking 
PM2.5 to other endpoints of substantial public health 
concern, and the need to protect public health with an adequate margin 
of safety. In considering the uncertainties in both the epidemiologic 
evidence and the controlled human exposures studies, the Administrator 
recognizes that collectively, the health effects evidence generally 
reflects a continuum, consisting of levels at which scientists 
generally agree that health effects are likely to occur, through lower 
levels at which the likelihood and magnitude of the response become 
increasingly uncertain. In light of these uncertainties, the 
Administrator recognizes that the CAA requirement that primary 
standards provide an adequate margin of safety, as summarized in 
section I.A above, is intended to address uncertainties associated with 
inconclusive scientific and technical information, as well as to 
provide a reasonable degree of protection against hazards that research 
has not yet identified. The Administrator has taken the need to provide 
for an adequate margin of safety into account as an integral part of 
his decision-making on the appropriate standards in setting the 
standard at a level below the level where available epidemiologic 
studies, which include diverse populations that are broadly 
representative of the U.S. population including at-risk populations, 
have provided the strongest evidence supporting effects, and in other 
ways as well. For example, consideration of a margin of safety is 
reflected in the approach of setting the level of the annual standard 
near and somewhat below the 25th percentile PM2.5 
concentrations from key U.S. epidemiologic studies (i.e., 9.1 [micro]g/
m\3\), as well as recognition that attaining a design value will 
generally result in significantly broader and greater improvements of 
air quality across an area (including but certainly not limited to 
areas near the design value monitor) (U.S. EPA, 2022a, sections 
2.3.3.2.4 and 3.3.3.2.1, Table 3-5). Based on all of the considerations 
noted here, and considering the current body of evidence, including the 
associated limitations and uncertainties, in combination with the 
exposure/risk information, the Administrator concludes that a less 
stringent standard than the current standard would not provide the 
requisite protection of public health, including an adequate margin of 
safety.
    Having concluded that a less stringent standard would not provide 
the requisite protection of public health, the Administrator next 
considers whether a more stringent standard would be appropriate. In so 
doing, he notes that a decision to set the level of the annual standard 
to below 9.0 [micro]g/m\3\ would place a large amount of the emphasis 
on potential public health importance of further reducing the 
occurrence of PM2.5 concentrations of concern, though the 
exposures about which he is most concerned are well controlled with an 
annual standard level of 9.0 [micro]g/m\3\, as demonstrated by the 
quantitative risk assessment. Such a decision would also place greater 
weight on (1) further reducing ambient PM2.5 concentrations 
relative to those observed in long-and short-term epidemiologic 
studies, including those that he had judged to have significant 
uncertainties, including Canadian studies, studies using restricted 
analyses, and accountability studies; (2) shifting the air quality 
distribution in areas such that the highest exposure concentrations are 
reduced to below PM2.5 concentrations observed in 
epidemiologic studies to be in the 25th or lower percentile, for which 
the evidence is limited; and (3) further shifting exposure 
concentrations to those shown at the lower end of the distribution in 
the quantitative risk assessment, despite the important uncertainties 
in the overall risk assessment. As discussed in this section and in 
responses to significant comments above and in the Response to Comments 
document, the Administrator has concluded that placing a large emphasis 
on these factors and revising the standard to a level below 9.0 
[micro]g/m\3\ would result in a standard that is more stringent than 
the evidence indicates to be sufficient to protect public health with 
an adequate margin of safety. Compared to a primary annual 
PM2.5 standard set at a level of 9.0 [micro]g/m\3\, the 
Administrator concludes that the extent to which lower standard levels 
could result in further public health improvements becomes notably less 
certain.
    Thus, having carefully considered the scientific evidence, 
quantitative information, CASAC advice, and public comments relevant to 
his decision on the level of the primary annual PM2.5 
standard, as discussed above and in the Response to Comments document, 
the Administrator is revising the level of the primary annual 
PM2.5 standard to 9.0 [micro]g/m\3\. In the Administrator's 
judgment, based on the currently available evidence and information, an 
annual standard set at this level and using the specified indicator, 
averaging time, and form, in conjunction with the other primary PM 
standards, would be requisite to protect public health with an adequate 
margin of safety. The Administrator judges that such a standard would 
protect, with an adequate margin of safety, the health of at-risk 
populations, including children, older adults, those with pre-existing 
cardiovascular and respiratory diseases, minority populations, and low 
SES populations. The Administrator believes that a standard set at 9.0 
[micro]g/m\3\ would be sufficient to protect public health with a 
margin of safety, and believes that a lower standard would be more than 
what is necessary to provide this degree of protection. This judgment 
by the Administrator appropriately considers the degree of protection 
that is neither more nor less stringent than necessary for this purpose 
and recognizes that the CAA does not require that primary standards be 
set at a zero-risk level, but rather at a level that reduces risk 
sufficiently so as to protect public health with an adequate margin of 
safety.
    In reaching his conclusions on adequacy of the current suite of 
primary PM2.5 standards, based on consideration of the 
available scientific evidence and quantitative information, the CASAC's 
advice and public comments, the Administrator finds that the available 
information is insufficient to call into question the adequacy of the 
public

[[Page 16286]]

health protection afforded by the current primary 24-hour 
PM2.5 standard. As described earlier in this section, the 
Administrator concludes that it is appropriate to retain the current 
indicator (PM2.5), averaging time (24-hour), and form (98th 
percentile, averaged over three years) for the primary 24-hour 
PM2.5 standard and below explains the basis for his final 
decision that is also appropriate to retain the current level of the 
primary 24-hour PM2.5 standard.
    In reaching his conclusion to retain the current primary 24-hour 
PM2.5 standard the Administrator does so in light of the 
conclusion that the epidemiologic evidence supports associations 
between short- and long-term PM2.5 exposures and adverse 
health effects, but that the epidemiologic evidence does not identify 
specific concentrations at which those effects occur and the 
Administrator has greatest confidence in effects where the bulk of the 
data is reported (i.e., the mean PM2.5 concentration, with 
some consideration for the 25th percentile of the air quality 
distribution). Thus, in considering the epidemiologic evidence, the 
Administrator concludes it is appropriate to focus on setting a 
generally controlling annual standard as the most effective and 
efficient way to reduce total population risk associated with both 
long- and short-term PM2.5 exposures, and that it is 
appropriate to revise the level of the annual standard level to 9.0 
[micro]g/m\3\. In addition to the epidemiologic evidence, the 
Administrator also considers the available controlled human exposure 
studies, which provide evidence for health effects following single, 
short-term PM2.5 exposures to concentrations that typically 
correspond to upper end of the PM2.5 air quality 
distribution in the U.S. (i.e., ``peak'' concentrations). In so doing, 
the Administrator notes that these studies report statistically 
significant effects on one or more indicators of cardiovascular 
function following 2-hour exposures to PM2.5 concentrations 
at and above 120 [mu]g/m\3\ and at and above 149 [mu]g/m\3\ for 
vascular impairment, the effect shown to be most consistent across 
studies. In particular, the Administrator notes that a single study is 
assessed in the ISA Supplement that reports effects following 4-hour 
exposures at 37.8 [micro]g/m\3\, although the results of this study are 
inconsistent with the results of the controlled human exposure studies 
assessed in the 2019 ISA. Along with the inconsistent results from the 
controlled human exposure studies, the Administrator also recognizes 
that effects observed in these studies are intermediate effects which 
are not typically considered adverse and that the study participants 
were healthy individuals. Taking into consideration the available 
scientific evidence, including the uncertainties and limitations, along 
with the CASAC's advice, the Administrator concludes that it is 
appropriate to maintain a primary 24-hour PM2.5 standard to 
protect against peak exposures.
    Thus, the Administrator considers what primary 24-hour 
PM2.5 standard is requisite to provide supplemental 
protection against peak exposures. While having confidence that the 
revised annual standard will result in lowering risk associated with 
both long- and short-term PM2.5 exposure by lowering the 
overall air quality distribution, as in the 2012 review, the 
Administrator recognizes that an annual standard alone would not be 
expected to offer sufficient protection with an adequate margin of 
safety against the effects of short-term PM2.5 exposures in 
all parts of the country. Therefore, he continues to conclude that it 
is appropriate to continue to provide supplemental protection by means 
of a 24-hour standard, in conjunction with a revised annual standard 
level of 9.0 [micro]g/m\3\.
    In considering the available scientific evidence assessed in the 
2019 ISA and ISA Supplement, the Administrator first considers the 
controlled human exposure studies for informing his decisions on the 
primary 24-hour PM2.5 standard. In so doing, he notes that 
in their review of the 2021 draft PA, the majority of CASAC members 
expressed the view that controlled human exposure studies are not the 
best evidence to use for justifying retaining the 24-hour standard 
without revision, in part because these studies preferentially recruit 
less susceptible individuals and have a typical exposure duration much 
shorter than 24 hours. Thus, in the view of the majority, ``the 
evidence of effects from controlled human exposure studies with 
exposures close to the current 24-hour standard supports 
epidemiological evidence for lowering the standard'' (Sheppard, 2022a, 
p. 3-4 of consensus letter). In reviewing the controlled human exposure 
studies, the Administrator agrees with the majority of CASAC that these 
controlled human exposure studies generally do not include populations 
with substantially increased risk from exposure to PM2.5, 
such as children, older adults, or those with more severe underlying 
illness. However, he disagrees with any conclusion that they should not 
be used to inform a decision about the adequacy of the current 
standard. The Administrator finds the information available from these 
studies to be useful, noting that the recently available controlled 
human exposure studies provide evidence for health effects following 
single, short-term exposures to PM2.5 concentrations that 
are greater than those allowed under the current standard. The results 
of the controlled human exposure studies are inconsistent, particularly 
at lower PM2.5 concentrations, but some studies do report 
statistically significant effects on one or more indicators of 
cardiovascular function following 2-hour exposures to PM2.5 
concentrations at and above 120 [mu]g/m\3\ (and at and above 149 [mu]g/
m\3\ for vascular impairment, the effect shown to be most consistent 
across studies). Additionally, one controlled human exposure study 
assessed in the ISA Supplement reports evidence of some effects for 
cardiovascular markers following 4-hour exposures to 37.8 [micro]g/m\3\ 
(Wyatt et al., 2020). However, there is inconsistent evidence for 
inflammation in other controlled human exposure studies evaluated in 
the 2019 ISA. The Administrator finds these studies are important in 
establishing biological plausibility for PM2.5 exposures 
causing more serious health effects, such as those seen in short-term 
exposure epidemiologic studies, and they provide support that more 
adverse effects may be experienced following longer exposure durations 
and/or exposure to higher concentrations. As described in more detail 
in responding to public comments in section II.B.3 above, he notes that 
although the controlled human exposure studies do not provide a 
threshold below which no effects occur, the observed effects in these 
controlled human exposures studies are ones that signal an intermediate 
effect in the body, likely due to short-term exposure to 
PM2.5, and typically would not, by themselves, be judged as 
adverse. As noted in sections II.A.2 and II.B.3 above, associated 
judgments regarding adversity or health significance of measurable 
physiological responses to air pollutants in previous NAAQS reviews 
have been informed by guidance, criteria or interpretative statements 
developed within the public health community. This type of information 
on adversity of effects is particularly informative to the 
Administrator's judgments regarding the adversity of the effects 
observed in the controlled human exposure studies which are short-term 
in nature (i.e., generally ranging from 2- to 5-hours), including those 
studies that are

[[Page 16287]]

conducted at near-ambient PM2.5 concentrations. Based on the 
observation that the effects observed in Wyatt et al. (2020) are not by 
themselves adverse, and the fact that the findings of this study are 
inconsistent with other currently available evidence regarding the 
level at which effects are observed, the Administrator disagrees with 
the view expressed by the majority of CASAC that this study supports 
epidemiologic evidence for lowering the 24-hour standard.
    Consistent with his approach in reaching his proposed decision and 
taking into consideration these points as well as balancing these 
limitations (i.e., that the health outcomes observed in these 
controlled human exposure studies are not clearly adverse and that the 
studies generally do not include those at increased risk from 
PM2.5 exposure), the Administrator still considers it 
appropriate to ensure that the 24-hour PM2.5 standard 
provides protection against health effects consistently observed in the 
controlled human exposure studies. He next examines the air quality 
analyses, described in more detail in section II.A.c.i above, to assess 
whether during recent air quality conditions, areas meeting the current 
standards would experience PM2.5 concentrations reported in 
these controlled human exposure studies. He observes that air quality 
analyses demonstrate that the PM2.5 exposures shown to cause 
consistent effects in the controlled human exposure studies are well 
above the ambient concentrations typically measured in locations 
meeting the current primary standards, and therefore suggest that the 
current primary PM2.5 standards provide protection against 
these ``peak'' concentrations. In fact, at air quality monitoring sites 
meeting the current primary PM2.5 standards (i.e., the 24-
hour standard of 35 [mu]g/m\3\ and the annual standard of 12 [mu]g/
m\3\), the 2-hour concentrations generally remain below 10 [mu]g/m\3\, 
and rarely exceed 30 [mu]g/m\3\. Though two-hour concentrations are 
higher at monitoring sites violating the current standards, they 
generally remain below 16 [mu]g/m\3\ and rarely exceed 80 [mu]g/m\3\, 
still below concentrations in CHE studies where consistent effects are 
observed (e.g., greater than 120 [mu]g/m\3\) (U.S. EPA, 2022b, section 
2.3.2.2.3, Figure 2-19, and section 3.3.3.1). Additionally, and in 
response to public comments, the Administrator notes additional air 
quality analyses conducted by the EPA,\121\ that provide a more refined 
analysis of whether areas that meet the current standards experience 
peak concentrations reported in controlled human exposure studies. He 
notes that 2-hour observations greater than 120 [mu]g/m\3\ and 4-hour 
observations greater than 38 [mu]g/m\3\ rarely occur (e.g., 0.025% of 
rolling 2-hour observations are greater than 120 [mu]g/m\3\ and 0.78% 
of rolling 4-hour observations greater than 38 [mu]g/m\3\). Based on 
this information, the Administrator finds that the current suite of 
standards maintains subdaily concentrations of PM2.5 in 
ambient air far below the exposure concentrations in controlled human 
exposure studies where consistent effects have been observed, and notes 
that while these studies generally do not include the most at-risk 
individuals, the exposure concentrations in these studies also do not 
elicit adverse effects.
---------------------------------------------------------------------------

    \121\ Jones et al. (2023). Comparison of Occurrence of 
Scientifically Relevant Air Quality Observations Between Design 
Value Groups. Memorandum to the Rulemaking Docket for the Review of 
the National Ambient Air Quality Standards for Particulate Matter 
(EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    Further, in light of the Administrator's emphasis on the annual 
standard as the controlling standard, with the 24-hour standard 
providing supplemental protection against peak concentrations, he next 
considers the potential impact of a revised annual standard of 9.0 
[micro]g/m\3\ on the occurrence of peak sub-daily PM2.5 
concentrations. Specifically, the Administrator takes note of the new 
air quality analyses \122\ where he observes that lower percentages of 
concentrations greater than 120 [micro]g/m\3\ and 38 [micro]g/m\3\ 
occur in areas meeting an annual standard of 9.0 [micro]g/m\3\ and a 
24-hour standard of 35 [micro]g/m\3\, versus an annual standard of 12.0 
[micro]g/m\3\ and a 24-hour standard of 35 [micro]g/m\3\. Thus, he 
concludes that an annual standard that is controlling across most areas 
of the country will continue to effectively limit peak daily 
concentrations in conjunction with the existing 24-hour standard, with 
its level of 35 [micro]g/m\3\ and 98th percentile form, which continues 
to provide supplemental protection against peak concentrations.
---------------------------------------------------------------------------

    \122\ Jones et al. (2023). Comparison of Occurrence of 
Scientifically Relevant Air Quality Observations Between Design 
Value Groups. Memorandum to the Rulemaking Docket for the Review of 
the National Ambient Air Quality Standards for Particulate Matter 
(EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    In addition, the Administrator also notes that the majority of the 
CASAC in their review of the 2021 draft PA, as well as a number of 
public commenters, support their recommendation to revise the current 
24-hour standard by pointing to ``substantial epidemiologic evidence 
from both morbidity and mortality studies'' which ``includes three U.S. 
air pollution studies with analyses restricted to 24-hour 
concentrations below 25 [mu]g/m\3\'' (Sheppard, 2022a, p. 17 consensus 
responses). The Administrator notes that the epidemiologic evidence 
available in this reconsideration, including the studies that restrict 
short-term PM2.5 exposures (i.e., 24-hour PM2.5 
concentrations) to levels below 25 [mu]g/m\3\, provides support for 
positive and statistically significant associations between short-term 
exposure to PM2.5 and all-cause mortality (Di et al., 2017a) 
and CVD hospital admissions (deSouza et al., 2021; Di et al., 2017a). 
He agrees that these studies help to provide additional support for 
reaching conclusions on causality in the 2019 ISA. He further agrees 
that the available epidemiologic studies provide important information 
that it is appropriate to consider in this reconsideration, including 
information on associations between health effects and PM2.5 
exposures in diverse populations that are broadly representative of the 
U.S. population, and include populations identified as at-risk (e.g., 
older adults, minority populations), as well as evidence of linear, no-
threshold concentration-response relationships in those associations, 
although with less certainty in the shape of the curve at long-term 
average concentrations below about 8 [mu]g/m\3\.
    However, the Administrator also notes significant limitations in 
the currently available epidemiologic information that limit his 
ability to draw conclusions from the key short-term studies, including 
those that employ restricted analyses, to inform his decision regarding 
the level of the 24-hour PM2.5 standard. As a result of 
these limitations, the Administrator does not find that the short-term 
epidemiologic studies, or the other evidence such as the controlled 
human exposure studies or the risk assessment, provide a sufficient 
justification for revising the 24-hour standard.
    First, he notes that short-term epidemiologic studies examine 
associations between day-to-day variations in PM2.5 
concentrations and health outcomes, often over multi-year study 
periods. As such, these studies report long-term mean 24-hour 
PM2.5 concentrations (e.g., mean 24-hour PM2.5 
concentrations over multi-year study periods), rather than at specific 
points in the distribution (i.e., 90th or 98th percentile 24-hour 
concentrations) at which effects occur. Further, he notes

[[Page 16288]]

that while there can be considerable variability in daily exposures 
over a multi-year study period, the bulk of the observations reflect 
days with ambient PM2.5 concentrations in the middle of the 
air quality distribution (i.e., ``typical'' days rather than days with 
extremely low or extremely high concentrations). As a result, the 
results of these studies are more directly applicable to decisions 
regarding the annual standard (which is based on the long-term mean of 
both short- and long-term epidemiologic studies), and the fact that 
they do not report other air quality statistics, such as the 98th 
percentile concentrations which might be more directly compared to the 
level of the 24-hour standard, makes them less useful for informing 
decisions on the 24-hour standard. As discussed in responding to 
comments above, the form of the annual standard is based on the annual 
mean PM2.5 concentration averaged over three years,\123\ 
which makes it better suited as a basis for controlling air quality to 
avoid effects observed in both long-term and short-term epidemiologic 
studies. By contrast, the form of the 24-hour standard is the 98th 
percentile averaged over three years, which makes it appropriate for 
controlling short-term peak concentrations. However, based on the 
available air quality information, including distribution statistics of 
PM2.5 concentrations and health events reported in the 
short-term epidemiologic studies, these studies are too limited in 
their ability to identify health effects attributable to specific 
short-term peak concentrations that are necessary to evaluate whether 
the 24-hour standard with its 98th percentile form should be revised 
(e.g., restricted epidemiologic studies do not report the number or the 
percentile of health events or the percentile of PM2.5 
concentrations across the highest part of the restricted air quality 
distribution, including the 98th percentile). Thus, the Administrator 
does not consider it appropriate to use the reported means from short-
term studies to determine the appropriate level for a 24-hour standard 
with a 98th percentile form.
---------------------------------------------------------------------------

    \123\ The annual mean is calculated by averaging daily values in 
a calendar quarter and then averaging calendar quarters. See 40 CFR 
part 50 Appendix N, section 4.4.
---------------------------------------------------------------------------

    Similarly, the Administrator does not consider the results of the 
restricted analyses to be well suited to informing the choice of level 
for a 24-hour standard. Restricted analyses use a subset of data from 
their main analyses to evaluate health events that occur at 
concentrations below a certain concentration (e.g., 25 [mu]g/m\3\). The 
Administrator notes that the associations between the health effects 
(e.g., mortality and cardiovascular morbidity) and PM2.5 
concentrations remain even after excluding higher concentrations in the 
restricted analyses, and he also recognizes that the magnitude of the 
effect is generally greater in the restricted analyses compared to the 
associations reported in the main analysis. He considers such analyses 
to be informative in indicating that the health effects association 
reported in the main (unrestricted) analysis are not driven only by the 
upper peaks of the PM2.5 air quality distribution, but 
rather persist at lower portions of the distribution (consistent with 
his emphasis on the annual standard, which is focused on exposures near 
the mean concentration, where the bulk of the exposure distribution is 
concentrated). Indeed, he notes that if peak concentrations were the 
principal driver of health effects associated with PM2.5 
exposure, one might expect the associations to become weaker as the 
upper portion of the data is excluded in the restricted analyses, which 
is not what is reported by the analyses (e.g., the restricted analyses 
generally report associations that are greater in magnitude compared to 
the main analyses). However, he disagrees with the assertion by the 
CASAC in their review of the 2021 draft PA and some public commenters 
that it would be appropriate to focus on the specific PM2.5 
concentration (e.g., 25 or 30 [mu]g/m\3\) at which the analysis was 
restricted as the basis for choosing a 24-hour standard level. The 
Administrator recognizes that in restricted analyses, while an 
association continues to persist across the full range of the air 
quality distribution, and that the cutpoint concentration at which the 
analysis was restricted (e.g., 25 or 30 [micro]g/m\3\) becomes the 
maximum PM2.5 concentration in the distribution, he also 
notes that these studies do not provide information related to the 
distribution of health events and PM2.5 concentrations, and 
as such, he is more uncertain where the bulk of the data are and where 
he has confidence in the reported association.\124\ He notes that no 
evidence exists to support a conclusion that the PM2.5 
concentration chosen as the cutpoint in a restricted analysis has any 
bearing on the concentration at which effects are likely to occur (or 
not occur). He notes that, as with long-term studies, the evidence does 
not suggest there is a specific point in the air quality distribution 
of these short-term studies that represents a ``bright line'' at and 
above which effects have been observed and below which effects have not 
been observed. In order to identify a level of the 24-hour standard 
based on associations between the ``upper end'' of exposures, either in 
the unrestricted or the restricted analyses, and adverse health 
effects, it would be necessary to have a better understanding of how 
specific 24-hour concentrations correspond to the frequency and total 
number of observed health events in the study. Currently, such 
information, including 98th percentile statistics, are not reported in 
the key short-term epidemiologic studies (and if they were reported, 
the Administrator would have to carefully consider how to weigh the 
data). As such, in reaching his decision on the primary 24-hour 
PM2.5 standard, the Administrator judges that the currently 
available information from short-term epidemiologic studies, including 
those that employ restricted analyses, does not provide a sufficient 
basis to revise the current 24-hour standard, given that the 24-hour 
standard focuses on reducing ``peak'' exposures (with its 98th 
percentile form), but rather that such information supports his 
judgment that it is appropriate to focus on revising the annual 
standard for purposes of reducing all exposures, across the entire 
distribution of air quality, to increase public health protection.
---------------------------------------------------------------------------

    \124\ These studies do not report information about the 
distribution of the health events and PM2.5 
concentrations (e.g., means, medians, other percentiles) in the 
restricted analyses.
---------------------------------------------------------------------------

    In reaching final decisions regarding the adequacy of the primary 
24-hour PM2.5 standard, the Administrator continues to view 
an approach that focuses on setting a generally controlling annual 
standard as the most effective and efficient way to reduce total 
population risk associated with both long- and short-term 
PM2.5 exposures. Additionally, he emphasizes that 
improvements in air quality associated with meeting an annual standard 
level of 9.0 [micro]g/m\3\ will result in lowering risk associated with 
both long- and short-term PM2.5 exposure by lowering the 
overall air quality distribution. The Administrator concludes that 
reducing the annual standard is the most efficient way to reduce the 
risks from short-term exposures identified in the epidemiologic 
studies, as the available evidence suggests the bulk of the risk comes 
from the large number of days across the bulk of the air quality 
distribution, not the relatively small number of days with peak 
concentrations. However, as in the 2012

[[Page 16289]]

review, the Administrator recognizes that an annual standard alone 
would not be expected to offer sufficient protection with an adequate 
margin of safety against the effects of short-term PM2.5 
exposures in all parts of the country and concludes that, in 
conjunction with a revised annual standard level of 9.0 [micro]g/m\3\, 
it is appropriate to continue to provide supplemental protection by 
means of a 24-hour standard, particularly for areas with high peak-to-
mean ratios possibly associated with strong local or seasonal sources.
    In selecting the level of a 24-hour standard designed to provide 
supplemental protection against peak exposures (in conjunction with a 
revised annual standard of 9.0 [micro]g/m\3\), the Administrator 
considers the information from the controlled human exposure studies 
and the EPA's analysis of peak concentrations observed in areas meeting 
the current standard of 35 [micro]g/m\3\ in conjunction with a revised 
standard of 9.0 [micro]g/m\3\ to be of particular relevance. He notes 
the controlled human exposure evidence includes studies reporting 
effects on one or more indicators of cardiovascular function following 
2-hour exposures at and above 120 [micro]g/m\3\, including effects 
reported at and above 149 [micro]g/m\3\ for vascular impairment, the 
effect shown to be most consistent across studies, and less consistent 
effects at lower concentrations, including a single study at near 
ambient concentrations (Wyatt et al., 2020) reporting effects following 
4-hour exposures at 37.8 [micro]g/m\3\. He recognizes that the effects 
observed (in those studies that observed effects) are ones that signal 
an intermediate effect in the body, likely due to short-term exposure 
to PM2.5, and typically would not, by themselves, be judged 
as adverse, and the study participants were healthy individuals.
    He notes in particular that, in the EPA's analysis, in areas 
meeting the current 24-hour standard and the revised annual standard 
0.029 percent of 2-hour observations and 0.41 percent of 4-hour 
observations reach PM2.5 concentrations higher than 120 
[micro]g/m\3\ and 37.8 [micro]g/m\3\, respectively. He also notes the 
lack of evidence of effects from controlled human exposure studies at 
levels below the current 24-hour standard and the fact that the results 
of Wyatt et al. (2020) are inconsistent with other available studies, 
as well as the intermediate nature of effects observed in this study. 
In his judgment, the small number of occurrences of peak exposures 
indicate that, in conjunction with a revised annual standard of 9.0 
[micro]g/m\3\, the current 24-hour standard of 35 [micro]g/m\3\ remains 
requisite to protect public health with an adequate margin of safety, 
and that there is substantial basis to doubt whether further 
improvements in public health would be achieved by further reducing 
these exposures. Furthermore, the Administrator concludes that due to 
the limitations and uncertainties outlined above, the information from 
recent short-term epidemiologic studies, including those that use 
restricted analyses, is inadequate to inform decisions regarding the 
adequacy of the current 24-hour standard. Thus, in reaching his 
decision on the primary 24-hour PM2.5 standard, the 
Administrator concludes that currently available evidence does not call 
into question the adequacy of the current standard.
    In addition to the scientific evidence, the Administrator also 
considers the risk assessment in evaluating the appropriate level of 
the 24-hour PM2.5 standard. The risk assessment indicates 
that the annual standard is the controlling standard across most of the 
urban study areas evaluated (i.e., when air quality related to the 
annual average PM2.5 concentrations decrease, daily average 
PM2.5 concentrations are also expected to decrease). When 
air quality is adjusted to just meet an alternative 24-hour standard 
level of 30 [mu]g/m\3\ in the areas where the 24-hour standard is 
controlling, the risk assessment estimates reductions in 
PM2.5-associated risks across a more limited population and 
number of areas compared to when air quality is adjusted to simulate 
alternative levels for the annual standard (i.e., where the annual 
standard is controlling), and these predictions are largely confined to 
areas located in the western U.S., several of which are also likely to 
experience risk reductions upon meeting a revised annual standard. With 
respect to the CASAC's advice in their review of the 2021 draft PA, the 
Administrator notes that the minority of CASAC advised that these 
results suggest that the annual standard can be used to limit both 
long- and short-term PM2.5 concentrations and views these 
risk assessment results as supporting the conclusion that the current 
24-hour standard is adequate (Sheppard, 2022a, p. 4 of consensus 
letter). In contrast, the majority of CASAC members in their review of 
the 2021 draft PA, as well as a number of public commenters that 
support revision of the 24-hour standard, placed greater weight on the 
evidence-based considerations (e.g. scientific evidence, like the 
restricted analyses) than on the values estimated by the risk 
assessment, noting the potential for uncertainties in how the risk 
assessment was able to ``capture areas with wintertime stagnation and 
residential wood-burning where the annual standard is less likely to be 
protective'' (Sheppard, 2022a, p. 4 of consensus letter).
    In considering the application of the risk assessment to judgments 
about the adequacy of the current primary 24-hour PM2.5 
standard, the Administrator again notes that the risk assessment 
analyses of PM2.5-attributable mortality use input data that 
include C-R functions from epidemiologic studies that have no threshold 
and a linear C-R relationship down to zero, as well an air quality 
adjustment approach that incorporates proportional decreases in 
PM2.5 concentrations to meet lower standard levels. As such, 
the Administrator notes that this quantitative approach does not 
incorporate any elements of uncertainty in associations of health 
effects at lower concentrations and that simulated air quality 
improvements will always lead to proportional decreases in risk (i.e., 
each additional [micro]g/m\3\ reduction produces additional benefits 
with no clear stopping point at any PM2.5 concentration). 
Therefore, the Administrator recognizes that while the risk estimates 
can help to place the evidence for specific health effects into a 
broader public health context, the results should be considered along 
with the inherent uncertainties and limitations of such analyses when 
informing judgments about the potential for additional public health 
protection associated with PM2.5 exposure and related health 
effects. Further, the Administrator notes additionally that air quality 
analyses have also been considered in looking at the adequacy of the 
24-hour standard in controlling peak PM2.5 concentrations of 
potential concern,\125\ and that those analyses included monitoring 
information from across the entire U.S., specifically highlighting 
areas with higher peak concentrations and including areas impacted by 
wintertime stagnation and residential wood-burning. Thus, while the 
risk assessment may have focused on a subset of areas across the U.S. 
based on the study area selection criteria, the Administrator is 
considering a broader set of information in reaching his conclusions 
regarding the appropriateness of the current 24-

[[Page 16290]]

hour standard to control peak concentrations.
---------------------------------------------------------------------------

    \125\ Jones et al. (2023). Comparison of Occurrence of 
Scientifically Relevant Air Quality Observations Between Design 
Value Groups. Memorandum to the Rulemaking Docket for the Review of 
the National Ambient Air Quality Standards for Particulate Matter 
(EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    The Administrator also considers the advice from the CASAC in their 
reviews of the 2019 draft PA and 2021 draft PA. In their review of the 
2019 draft PA, the CASAC ``agrees with the EPA and finds that the 
available evidence does not call into question the adequacy of public 
health protection afforded by the current 24-hour PM2.5 
standard and concurs that it be retained'' (Cox, 2019b, p. 3 of 
letter). He also notes that in their review of the 2021 draft PA, the 
CASAC did not reach consensus on whether the current 24-hour standard 
is adequate, with the majority of the CASAC recommending that the 24-
hour standard be revised and the minority of the CASAC recommending 
that the standard be retained. The majority of the CASAC members 
further stated that ``[t]here is also less confidence that the annual 
standard could adequately protect against health effects of short-term 
exposures. A range of 25-30 [mu]g/m\3\ for the 24-hour PM2.5 
standard would be adequately protective'' (Sheppard, 2022a, p. 4 of 
consensus letter). The Administrator also acknowledges that some public 
commenters agreed with the majority of the CASAC in supporting a 
revision to the level of the 24-hour standard to a range between 25-30 
[micro]g/m\3\. These commenters cite a number of reasons, including: 
(1) Results from controlled human exposure studies at near ambient 
concentrations; (2) aspects of the scientific evidence, including 
restricted analyses that report positive and significant associations 
below 35 [micro]g/m\3\; and (3) quantitative risk analyses that show 
decreasing risk with decreasing PM2.5 concentrations. In 
responding to these comments, the Administrator recognizes that some 
commenters have different interpretations of the evidence, air quality 
information, and quantitative results from the risk assessment in this 
review and would make different judgments about the weight to place on 
the relative strength and limitations of the currently available 
scientific evidence and information and how such information could be 
used in making public health policy decisions on the 24-hour standard. 
However, as outlined above, the Administrator has carefully considered 
the information available from controlled human exposure studies and 
short-term epidemiologic studies, and weighed the strengths and 
limitations of this evidence in formulating his decisions. Furthermore, 
as discussed above the Administrator has noted significant 
uncertainties and limitations inherent in the risk estimates, as well 
as noting that very few areas were included. In addition, he has given 
careful consideration to the majority of the CASAC's advice in their 
review of the 2021 draft PA, but has drawn different conclusions with 
respect to how currently available evidence and air quality information 
inform the selection of level for the 24-hour primary PM2.5 
standard.
    In considering the advice of the majority of CASAC, the 
Administrator notes that a decision to set the level of the 24-hour 
standard to below 35 [micro]g/m\3\ would place a large amount of 
emphasis on the potential public health importance of further reducing 
the occurrence of peak PM2.5 concentrations. However, the 
Administrator concludes that there is insufficient basis to conclude 
that a more stringent standard to further reduce peak concentrations is 
needed or would benefit public health. As discussed above, he judges 
that the PM2.5 exposures in controlled human exposure 
studies that correspond to peak concentrations will already be well 
controlled via the combination of the revised annual standard, with a 
level of 9.0 [micro]g/m\3\, and the 24-hour standard with its level 35 
[micro]g/m\3\ and its 98th percentile form. Taking into consideration 
the inconsistent results reported in controlled human exposure studies, 
the intermediate nature of the health effects observed in the 
controlled human exposure studies that are not typically considered 
adverse, the health status of the study participants, and how 
infrequently peak concentrations of potential concern are anticipated 
to occur in areas meeting the revised primary annual PM2.5 
standard, he judges that the current 24-hour standard is requisite to 
protect against the effects reported in these studies with an adequate 
margin of safety. Likewise, he judges that neither the epidemiologic 
studies (including the studies that use restricted analyses) nor the 
risk assessment provide a sufficient basis for revising the 24-hour 
standard. As discussed above, the epidemiologic studies, including 
short-term studies and those with restricted analyses, are not well-
suited for identifying a level for a 24-hour standard to address health 
effects associated with peak concentrations. The restricted analyses 
support the conclusion that the health effects associated with 
PM2.5 is not associated primarily with exposure to higher 
concentrations of the main analyses, but like other epidemiologic 
studies they typically report only long-term mean 24-hour 
concentrations (e.g., restricted epidemiologic studies do not report 
the number or the percentile of health events or the percentile of 
PM2.5 concentrations across the highest part of the 
restricted air quality distribution, including the 98th percentile) and 
do not identify any particular concentration within the air quality 
distribution above which effects have been observed and below which 
effects have not been observed. Similarly, the risk assessment 
highlights that the annual standard is controlling across much of the 
U.S. and is generally more effective at reducing risk than the 24-hour 
standard and, taking into account the limitations and assumptions of 
the risk assessment discussed above, does not provide a basis for 
revising the 24-hour standard. For the reasons discussed herein, the 
Administrator judges that the uncertainties as to whether there would 
be public health benefits from a more stringent 24-hour standard are 
too great to justify revising the standard.
    Thus, having carefully considered the scientific evidence, 
quantitative information, CASAC advice, and public comments, the 
Administrator is retaining the current primary 24-hour PM2.5 
standard, with its level of to 35 [micro]g/m\3\ and its 98th percentile 
form. In the Administrator's judgment, based on the currently available 
evidence and information, a 24-hour standard set at this level and 
using the specified indicator, averaging time, and form would be 
requisite to protect public health with an adequate margin of safety, 
in conjunction with the annual standard. As noted, in evaluating the 
adequacy of the current standards, the Administrator focuses on 
evaluating the public health protection afforded by the annual and 24-
hour standards, taken together, against adverse health effects 
associated with long- or short-term PM2.5 exposures. A 24-
hour standard set at a level of 35 [micro]g/m\3\, in conjunction with a 
revised annual standard level of 9.0 [micro]g/m\3\, in the judgment of 
the Administrator, provides an appropriate level of public health 
protection, for both long- and short-term PM2.5 exposures. 
The Administrator believes that a 24-hour standard set at 35 [micro]g/
m\3\ would continue to be sufficient to protect public health with a 
margin of safety, and believes that a lower standard would be more than 
what is necessary to provide this degree of protection when considered 
in conjunction with a revised annual standard. The Administrator 
concludes the current 24-hour standard at a level of 35 [micro]g/m\3\, 
in conjunction with a revised annual standard level of 9.0 [micro]g/
m\3\, will provide appropriate protection

[[Page 16291]]

in areas in which the long-term mean concentrations are already 
relatively low (i.e., below 9 [micro]g/m\3\) but where there may be 
elevated short-term peak PM2.5 concentrations, often 
associated with strong local or seasonal sources. This judgment by the 
Administrator appropriately considers the degree of protection that is 
neither more nor less stringent than necessary for this purpose and 
recognizes that the CAA does not require that primary standards be set 
at a zero-risk level, but rather at a level that reduces risk 
sufficiently so as to protect public health with an adequate margin of 
safety.
    In making this decision to retain the current level of the primary 
PM2.5 24-hour standard at 35 [micro]g/m\3\ in conjunction 
with revising the annual standard level from 12.0 [micro]g/m\3\ to 9.0 
[micro]g/m\3\, given all of the evidence and information discussed 
above, the Administrator judges that the revised suite of primary 
PM2.5 standards and the rationale supporting these levels 
appropriately reflects consideration of the strength of the available 
evidence and other information and its associated uncertainties as well 
as the advice of CASAC and consideration of public comments. He 
additionally judges that this suite of primary PM2.5 
standards is requisite to protect public health, including at-risk 
populations, with an adequate margin of safety from effects associated 
with long and short-term exposures to fine particles. This judgment by 
the Administrator appropriately considers the requirement for standards 
that are requisite to protect public health but are neither more nor 
less stringent than necessary.

C. Decisions on the Primary PM2.5 Standards

    For the reasons discussed above, and taking into account the 
information and assessments presented in the 2019 ISA and ISA 
Supplement, the scientific and quantitative risk information in the 
2022 PA, the advice and recommendations of the CASAC, and public 
comments, the Administrator revises the current suite of primary 
PM2.5 standards. Specifically, the Administrator revises the 
level of the primary annual PM2.5 standard to 9.0 [micro]g/
m\3\ while retaining its form, indicator and averaging time. In 
conjunction with revising the primary annual PM2.5 standard 
level to provide protection from effects associated with long- and 
short-term PM2.5 exposures, the Administrator retains the 
level of 35 [micro]g/m\3\ and the 98th percentile form, indicator and 
averaging time of the primary 24-hour PM2.5 standard to 
continue to provide supplemental protection for areas with high peak 
PM2.5 concentrations. The Administrator concludes that this 
suite of standards is requisite to protect public health with an 
adequate margin of safety against health effects potentially associated 
with long- and short-term PM2.5 exposures.

III. Rationale for Decisions on the Primary PM10 Standard

    This section presents the rationale for the Administrator's 
decision to retain the existing primary PM10 standard. This 
decision is based on a thorough review of the latest scientific 
information, published through January 2018 \126\ and evaluated in the 
2019 ISA, on human health effects associated with PM10-2.5 
in ambient air. As described in section I above and in section 1.2 of 
the ISA Supplement, the scope of the updated scientific evaluation of 
the health effects evidence is based on those PM size fractions, 
exposure durations, and health effects category combinations where the 
2019 ISA concluded a causal relationship exists (U.S. EPA, 2019a, U.S. 
EPA, 2022b).). Therefore, because the 2019 ISA did not conclude a 
causal relationship for PM10-2.5 for any exposure durations 
or health effect categories, the ISA Supplement does not include an 
evaluation of additional studies for PM10-2.5. As a result, 
the 2019 ISA continues to serve as the scientific foundation for 
assessing the adequacy of the primary PM10 standard in this 
reconsideration of the 2020 final decision (U.S. EPA, 2019a, section 
1.7; U.S. EPA, 2022a). The Administrator's decision also takes into 
account the 2022 PA evaluation of the policy-relevant information in 
the 2019 ISA, CASAC advice and recommendations, and public comments.
---------------------------------------------------------------------------

    \126\ In addition to the review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), the 2019 ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009, through approximately January 
2018 (U.S. EPA, 2019a, p. ES-2). References cited in the 2019 ISA, 
the references considered for inclusion but not cited, and 
electronic links to bibliographic information and abstracts can be 
found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's final decision 
and its foundations, Section III.A provides background on the 2020 
final decision to retain the primary PM10 and a brief 
summary of key aspects of the currently available health effects 
information. Section III.B summarizes the CASAC advice and the 
Administrator's proposed conclusions to retain the existing primary 
PM10 standard, addresses public comments received on the 
proposal, and presents the Administrator's conclusions on the adequacy 
of the current standard, drawing on consideration of information in the 
2019 ISA and the 2022 PA, advice from the CASAC, and comments from the 
public. Section III.C summarizes the Administrator's decision on the 
primary PM10 standard.

A. Introduction

    The general approach for this reconsideration of the 2020 final 
decision on the primary PM10 standard relies on the 
scientific information available for this review, as well as the 
Administrator's judgments regarding the available public health effects 
evidence, and the appropriate degree of public health protection for 
the existing standards. With the 2020 decision, the then-Administrator 
retained the existing primary 24-hour PM10 standard, with 
its level of 150 [micro]g/m\3\ and its one-expected-exceedance form on 
average over three years, to continue to provide public health 
protection against short-term exposures to PM10-2.5 (85 FR 
82725, December 18, 2020).
1. Background on the Current Standard
    Consistent with the 2009 ISA, the 2019 ISA concluded that the 
available epidemiologic, controlled human exposure, and animal 
toxicological studies, including uncertainties, provided support for 
the causality determinations of ``suggestive of, but not sufficient to 
infer, a causal relationship'' between short-term exposures to 
PM10-2.5 and cardiovascular effects, respiratory effects, 
and mortality (U.S. EPA, 2019a, section 1.4.2). The 2019 ISA also 
reached the conclusion that the evidence supports a ``suggestive of, 
but not sufficient to infer, a causal relationship'' between short-term 
PM10-2.5 exposures and metabolic effects, an endpoint that 
was not evaluated in the 2009 ISA (U.S. EPA, 2019a, section 1.4.2).
    Compared to the 2009 ISA, the 2019 ISA includes expanded evidence 
for the relationships between long-term exposures and cardiovascular 
effects, metabolic effects, nervous system effects, cancer, and 
mortality. The 2019 ISA concluded that the small number of 
epidemiologic and experimental studies, including uncertainties, 
contribute to the determination that, ``the evidence is suggestive of, 
but not sufficient to infer, a causal relationship between long-term 
PM10-2.5 exposure and cardiovascular effects, metabolic 
effects, nervous system effects, cancer, and mortality and cancer (U.S. 
EPA, 2019a, p. 10-87). For long-term exposures and cardiovascular 
effects,

[[Page 16292]]

cardiovascular effects, and cancer, this is an upgrade from the 
``inadequate to infer the presence or absence of a causal 
relationship'' conclusions in the 2009 ISA (U.S. EPA, 2019a, section 
1.4.2). This determination is also the first for long-term exposures 
and metabolic effects, as the 2009 ISA did not include metabolic 
effects as an endpoint (U.S. EPA, 2019a section 1.4.2).
    In considering the available body of evidence, it was noted in the 
2020 review there were considerable uncertainties and limitations 
associated with the experimental evidence for PM2.5 
exposures and health effects, and as such more weight was placed on the 
available epidemiologic evidence. Therefore, the primary focus in the 
2020 review was on multi-city and single-city epidemiologic studies 
that evaluated associations between short-term PM10-2.5 and 
mortality, cardiovascular effects (hospital admissions and emergency 
department visits, as well as blood pressure and hypertension), and 
respiratory effects. Despite differences in the approaches \127\ used 
to estimate ambient PM10-2.5 concentrations, the majority of 
the studies reported positive, though often not statistically 
significant, associations with short-term PM10-2.5 
exposures. Most PM10-2.5 effect estimates remained positive 
in copollutant models that included either gaseous pollutants or other 
particulate matter size fractions (e.g., PM2.5). In U.S. 
study locations likely to have met the PM10 standard during 
the study period, a few studies reported positive associations between 
PM10-2.5 and mortality that were statistically significant 
and remained so in copollutant models (U.S. EPA, 2019a). In addition to 
the epidemiologic studies, there were a small number of controlled 
human exposure studies evaluated in the 2019 ISA that reported 
alterations in heart rate variability or increased pulmonary 
inflammation following short-term exposure to PM10-2.5, 
providing some support for the associations in the epidemiologic 
studies. Animal toxicological studies examined the effect of short-term 
PM10-2.5 exposures using non-inhalation (e.g., intratracheal 
instillation) route.\128\ Therefore, these studies provided limited 
evidence for the biological plausibility of PM10-2.5-induced 
effects (U.S. EPA, 2019a). Although the scientific evidence available 
in the 2019 ISA expanded the understanding of health effects associated 
with PM10-2.5 exposures, a number of important uncertainties 
remained. These uncertainties, and their implications for interpreting 
the scientific evidence, include the following:
---------------------------------------------------------------------------

    \127\ As discussed further below, methods employed by the 
epidemiologic studies to estimate ambient PM10-2.5 
concentrations include: (1) Calculating the difference between 
PM10 and PM2.5 at co-located monitors, (2) 
calculating the difference between county-wide averages of monitored 
PM10 and PM2.5 based on monitors that are not 
necessarily co-located, and (3) direct measurement of 
PM10-2.5 using a dichotomous sampler (U.S. EPA, 2019a, 
section 1.4.2).
    \128\ Non-inhalation exposure experiments (i.e., intratracheal 
[IT] instillation) are informative for size fractions (e.g., 
PM10-2.5) that cannot penetrate the airway of a study 
animal and may provide information relevant to biological 
plausibility and dosimetry (U.S. EPA, 2019a, section A-12).
---------------------------------------------------------------------------

     The potential for confounding by copollutants, notably 
PM2.5, was addressed with copollutant models in a relatively 
small number of PM10-2.5 epidemiologic studies (U.S. EPA, 
2019a). This was particularly important given the relatively small body 
of experimental evidence (i.e., controlled human exposure and animal 
toxicological studies) available to support the independent effect of 
PM10-2.5 on human health. This increases the uncertainty 
regarding the extent to which PM10-2.5 itself, rather than 
one or more copollutants, is responsible for the mortality and 
morbidity effects reported in epidemiologic studies.
     There was greater spatial variability in 
PM10-2.5 concentrations than PM2.5 
concentrations, resulting in the potential for increased exposure error 
for PM10-2.5 (U.S. EPA, 2019a). Available measurements did 
not provide sufficient information to adequately characterize the 
spatial distribution of PM10-2.5 concentrations (U.S. EPA, 
2019a). The limitations in estimates of ambient PM10-2.5 
concentrations ``would tend to increase uncertainty and make it more 
difficult to detect effects of PM10-2.5 in epidemiologic 
studies'' (U.S. EPA, 2019a).
     Estimation of PM10-2.5 concentrations over 
which reported health outcomes occur remain highly uncertain. When 
compared with PM2.5, there is uncertainty spanning all 
epidemiologic studies examining associations with PM10-2.5 
including deficiencies in the existing monitoring networks, the lack of 
a systematic evaluation of the various methods used to estimate 
PM10-2.5 concentrations and the resulting uncertainty in the 
spatial as well as the temporal variability in PM10-2.5 
concentration (U.S. EPA, 2019a).). Given these limitations in routine 
monitoring, epidemiologic studies employed a number of different 
approaches for estimating PM10-2.5 concentrations, including 
(1) calculating the difference between PM10 and 
PM2.5 at co-located monitors, (2) calculating the difference 
between county-wide averages of monitored PM10 and 
PM2.5 based on monitors that are not necessarily co-located, 
and (3) direct measurement of PM10-2.5 using a dichotomous 
sampler (U.S. EPA, 2019a, section 1.4.2). Given the relatively small 
number of PM10-2.5 monitoring sites, the relatively large 
spatial variability in ambient PM10-2.5 concentrations, the 
use of different approaches to estimating ambient PM10-2.5 
concentrations across epidemiologic studies, and the limitations 
inherent in such estimates, the distributions of PM10-2.5 
concentrations over which reported health outcomes occur remain highly 
uncertain (U.S. EPA, 2019a).
    There was relatively little information available to characterize 
potential exposure differences that may inform the apparent variability 
in associations between short-term PM10-2.5 exposures and 
health effects across study locations (U.S. EPA, 2019a). Specifically, 
the potential spatial and temporal variability in PM10-2.5 
exposures complicates the interpretation of results between study 
locations as well as the relative lack of information on the chemical 
and biological composition of PM10-2.5 (U.S. EPA, 2009a U.S. 
EPA, 2019a).
    In reaching his decision in 2020 to retain the existing 24-hour 
primary PM10 standard, the then-Administrator specifically 
noted that, while the health effects evidence was somewhat expanded 
since the prior reviews, the overall conclusions in the 2019 ISA, 
including uncertainties and limitations, were generally consistent with 
what was considered in the 2012 review (85 FR 82725, December 18, 
2020). In addition, the then-Administrator recognized that there were 
still a number of uncertainties and limitations associated with the 
available evidence.
    With regard to the evidence on PM10-2.5-related health 
effects, the then-Administrator noted that epidemiologic studies 
continued to report positive associations with mortality and morbidity 
in cities across North America, Europe, and Asia, where 
PM10-2.5 sources and composition were expected to vary 
widely. While significant uncertainties remained in the 2020 review, 
the then-Administrator recognized that this expanded body of evidence 
had broadened the range of effects that have been linked with 
PM10-2.5 exposures. The studies evaluated in the 2019 ISA 
expanded the scientific foundation presented in the 2009 ISA and led to 
revised causality determinations (and new determinations) for long-term 
PM10-2.5

[[Page 16293]]

exposures and mortality, cardiovascular effects, metabolic effects, 
nervous system effects, and cancer (85 FR 82726, December 18, 2020). 
Drawing from his consideration of this evidence, the then-Administrator 
concluded that the scientific information available since the time of 
the last review supported a decision to maintain a primary 
PM10 standard to provide public health protection against 
PM10-2.5 exposures, regardless of location, source of 
origin, or particle composition (85 FR 82726, December 18, 2020). With 
regard to uncertainties in the available evidence, the then-
Administrator first noted that a number of limitations were identified 
in the 2012 review related to: (1) Estimates of ambient 
PM10-2.5 concentrations used in epidemiologic studies; (2) 
limited evaluation of copollutant models to address the potential for 
confounding; and (3) limited experimental studies supporting biological 
plausibility for PM10-2.5-related effects. Despite the 
expanded body of evidence for PM10-2.5 exposures and health 
effects, the then-Administrator recognized that uncertainties in the 
2020 review continued to include those associated with the exposure 
estimates used in epidemiologic studies, the independence of the 
PM10-2.5 health effect associations, and the biologically 
plausible pathways for PM10-2.5 health effects (85 FR 82726, 
December 18, 2020). These uncertainties contributed to the 2019 ISA 
determinations that the evidence is at most ``suggestive of, but not 
sufficient to infer'' causal relationships (85 FR 82726, December 18, 
2020). In considering the available evidence in his basis for the 
decision, the then-Administrator emphasized evidence supporting 
``causal'' and ``likely to be causal'' relationships, and therefore, 
judged that the PM10-2.5-related health effects evidence 
provided an uncertain scientific foundation for making standard-setting 
decisions. He further judged limitations in the evidence raised 
questions as to whether additional public health improvements would be 
achieved by revising the existing PM10 standard (85 FR 
24126, April 30, 2020). In the 2020 decision, for all of the reasons 
discussed above and recognizing the CASAC conclusion that the evidence 
provided support for retaining the current standard, the then-
Administrator concluded that it was appropriate to retain the existing 
primary PM10 standard, without revision. His decision was 
consistent with the CASAC advice related to the primary PM10 
standard. Specifically, the CASAC agreed with the 2020 PA conclusions 
that, while these effects are important, the ``evidence does not call 
into question the adequacy of the public health protection afforded by 
the current primary PM10 standard'' and ``supports 
consideration of retaining the current standard in this review'' (Cox, 
2019b, p. 3 of consensus letter). Thus, the then-Administrator 
concluded that the primary PM10 standard (in all of its 
elements (i.e., indicator, averaging time, form, and level)) was 
requisite to protect public health with an adequate margin of safety 
against effects that have been associated with PM10-2.5. In 
light of this conclusion, the EPA retained the existing PM10 
standard.
2. Overview of the Health Effects Evidence
    The information summarized here is based on the scientific 
assessment of the health effects evidence available in this 
reconsideration; this evaluation is documented in the 2019 ISA and its 
policy implications are discussed further in the 2022 PA. As noted 
above, the ISA Supplement does not include an evaluation of studies for 
PM10-2.5, and the 2019 ISA continues to serve as the 
scientific foundation for this reconsideration.
a. Nature of Effects
    For the health effect categories and exposure duration combinations 
evaluated, the 2019 ISA concludes that the evidence supports causality 
determinations for PM10-2.5 that are at most ``suggestive 
of, but not sufficient to infer, a causal relationship''. While the 
evidence supporting the causal nature of relationships between exposure 
to PM10-2.5 has been strengthened for some health effect 
categories since the completion of the 2009 ISA, the 2019 ISA concludes 
that overall ``the uncertainties in the evidence identified in the 2009 
ISA have, to date, still not been addressed'' (U.S. EPA, 2019a, section 
1.4.2, p. 1-41; U.S. EPA, 2022b, section 4.3.1). Specifically, 
epidemiologic studies available in the 2012 review relied on various 
methods to estimate PM10-2.5 concentrations, and these 
methods had not been systematically compared to evaluate spatial and 
temporal correlations in PM10-2.5 concentrations. Methods 
included: (1) Calculating the difference between PM10 and 
PM2.5 concentrations at co-located monitors, (2) calculating 
the difference between county-wide averages of monitored 
PM10- and PM2.5-based on monitors that are not 
necessarily co-located, and (3) direct measurement of 
PM10-2.5 using a dichotomous sampler (U.S. EPA, 2019a, 
section 1.4.2). As described in the 2019 ISA, there continues to be 
variability across epidemiologic studies in the approaches used to 
estimate PM10-2.5 concentrations. Additionally, some studies 
estimate long-term PM10-2.5 exposures as the difference 
between PM10 and PM2.5 concentrations based on 
information from spatiotemporal or land use regression (LUR) models, in 
addition to monitors. The various methods used to estimate 
PM10-2.5 concentrations have not been systematically 
evaluated (U.S. EPA, 2019a, section 3.3.1.1), contributing to 
uncertainty regarding the spatial and temporal correlations in 
PM10-2.5 concentrations across methods and in the 
PM10-2.5 exposure estimates used in epidemiologic studies 
(U.S. EPA, 2019a, section 2.5.1.2.3). Given the greater spatial and 
temporal variability of PM10-2.5 and the lower number of 
PM10-2.5 monitoring sites, compared to PM2.5, 
this uncertainty is particularly important for the coarse size 
fraction. Beyond the uncertainty associated with PM10-2.5 
exposure estimates in epidemiologic studies, the limited information on 
the potential for confounding by copollutants and the limited support 
available for the biological plausibility of health effects following 
PM10-2.5 exposures also continue to contribute to 
uncertainty in the PM10-2.5 health evidence. Uncertainty 
related to potential confounding stems from the relatively small number 
of epidemiologic studies that have evaluated PM10-2.5 health 
effect associations in copollutants models with both gaseous pollutants 
and other PM size fractions. On the other hand, uncertainty related to 
the biological plausibility of effects attributed to 
PM10-2.5 exposures results from the small number of 
controlled human exposure and animal toxicological studies that have 
evaluated the health effects of experimental PM10-2.5 
inhalation exposures. The evidence supporting the 2019 ISA's 
``suggestive of, but not sufficient to infer, a causal relationship'' 
causality determinations for PM10-2.5, including 
uncertainties in this evidence, is summarized below in sections 
III.B.1.a through III.B.1.f.
i. Mortality
    Due to the dearth of studies examining the association between 
long-term PM10-2.5 exposure and mortality, the 2009 ISA 
concluded that the evidence was ``inadequate to determine if a causal 
relationship exists'' (U.S. EPA, 2009a). As reported in the 2019 ISA, 
some cohort studies conducted in the U.S. and Europe report positive 
associations between long-term PM10-2.5 exposure and total 
(nonaccidental)

[[Page 16294]]

mortality, though results are inconsistent across studies (U.S. EPA, 
2019a, Table 11-11). The examination of copollutant models in these 
studies remains limited and, when included, PM10-2.5 effect 
estimates are often attenuated after adjusting for PM2.5 
(U.S. EPA, 2019a, Table 11-11). Across studies, PM10-2.5 
exposure concentrations are estimated using a variety of approaches, 
including direct measurements from dichotomous samplers, calculating 
the difference between PM10 and PM2.5 
concentrations measured at collocated monitors, and calculating 
difference of area-wide concentrations of PM10 and 
PM2.5. As discussed above, temporal and spatial correlations 
between these approaches have not been evaluated, contributing to 
uncertainty regarding the potential for exposure measurement error 
(U.S. EPA, 2019a, section 3.3.1.1 and Table 11-11). The 2019 ISA 
concludes that this uncertainty ``reduces the confidence in the 
associations observed across studies'' (U.S. EPA, 2019a, p. 11-125). 
The 2019 ISA additionally concludes that the evidence for long-term 
PM10-2.5 exposures and cardiovascular effects, respiratory 
morbidity, and metabolic disease provide limited biological 
plausibility for PM10-2.5-related mortality (U.S. EPA, 
2019a, sections 11.4.1 and 11.4). Taken together, the 2019 ISA 
concludes that, ``this body of evidence is suggestive, but not 
sufficient to infer, that a causal relationship exists between long-
term PM10-2.5 exposure and total mortality'' (U.S. EPA, 
2019a, p. 11-125).
    With regard to short-term PM10-2.5 exposures and 
mortality, the 2009 ISA concluded that the evidence is ``suggestive of 
a causal relationship between short-term exposure to 
PM10-2.5 and mortality'' (U.S. EPA, 2009a). The 2019 ISA 
included multicity epidemiologic studies conducted primarily in Europe 
and Asia that continue to provide consistent evidence of positive 
associations between short-term PM10-2.5 exposure and total 
(nonaccidental) mortality (U.S. EPA, 2019a, Table 11-9). Although these 
studies contribute to increasing confidence in the PM10-2.5-
mortality relationship, the use of various approaches to estimate 
PM10-2.5 exposures continues to contribute uncertainty to 
the associations observed. Recent studies expand the assessment of 
potential copollutant confounding of the PM10-2.5-mortality 
relationship and provide evidence that PM10-2.5 associations 
generally remain positive in copollutant models, though associations 
are attenuated in some instances (U.S. EPA, 2019a, section 11.3.4.1, 
Figure 11-28, Table 11-10). The 2019 ISA concludes that, overall, the 
assessment of potential copollutant confounding is limited due to the 
lack of information on the correlation between PM10-2.5 and 
gaseous pollutants and the small number of locations in which 
copollutant analyses have been conducted. Associations with cause-
specific mortality (i.e., cardiovascular and respiratory mortality) 
provide some support for associations with total (nonaccidental) 
mortality, though associations with respiratory mortality are more 
uncertain (i.e., wider confidence intervals) and less consistent (U.S. 
EPA, 2019a, section 11.3.7). The 2019 ISA concludes that the evidence 
for PM10-2.5-related cardiovascular effects provides only 
limited support for the biological plausibility of a relationship 
between short-term PM10-2.5 exposure and cardiovascular 
mortality (U.S. EPA, 2019a, section 11.3.7). Based on the overall 
evidence, the 2019 ISA concludes that, ``this body of evidence is 
suggestive, but not sufficient to infer, that a causal relationship 
exists between short-term PM10-2.5 exposure and total 
mortality'' (U.S. EPA, 2019a, p. 11-120).
ii. Cardiovascular Effects
    In the 2009 ISA, the evidence describing the relationship between 
long-term exposure to PM10-2.5 and cardiovascular effects 
was characterized as ``inadequate to infer the presence or absence of a 
causal relationship.'' The limited number of epidemiologic studies 
reported contradictory results and experimental evidence demonstrating 
an effect of PM10-2.5 on the cardiovascular system was 
lacking (U.S. EPA, 2019a, section 6.4).
    The evidence relating long-term PM10-2.5 exposures to 
cardiovascular mortality remains limited, with no consistent pattern of 
associations across studies and, as discussed above, uncertainty 
stemming from the use of various approaches to estimate 
PM10-2.5 concentrations (U.S. EPA, 2019a, Table 6-70). The 
evidence for associations with cardiovascular morbidity has grown and, 
while results across studies are not entirely consistent, some 
epidemiologic studies report positive associations with ischemic heart 
disease (IHD) and MI (U.S. EPA, 2019a, Figure 6-34); stroke (U.S. EPA, 
2019a, Figure 6-35); atherosclerosis (U.S. EPA, 2019a, section 6.4.5); 
venous thromboembolism (VTE) (U.S. EPA, 2019a, section 6.4.7); and 
blood pressure and hypertension (U.S. EPA, 2019a, Section 6.4.6). 
PM10-2.5 cardiovascular mortality effect estimates are often 
attenuated, but remain positive, in copollutants models that adjust for 
PM2.5. For morbidity outcomes, associations are inconsistent 
in copollutant models that adjust for PM2.5, NO2, 
and chronic noise pollution (U.S. EPA, 2019a, p. 6-276). The lack of 
toxicological evidence for long-term PM10-2.5 exposures 
represents a data gap (U.S. EPA, 2019a, section 6.4.10), resulting in 
the 2019 ISA conclusion that ``evidence from experimental animal 
studies is of insufficient quantity to establish biological 
plausibility'' (U.S. EPA, 2019a, p. 6-277). Based largely on the 
observation of positive associations in some epidemiologic studies, the 
2019 ISA concludes that ``evidence is suggestive of, but not sufficient 
to infer, a causal relationship between long-term PM10-2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2019a, p. 6-277).
    With regard to short-term PM10-2.5 exposures and 
cardiovascular effects, the 2009 ISA found that the available evidence 
for short-term PM10-2.5 exposure and cardiovascular effects 
was ``suggestive of a causal relationship.'' This conclusion was based 
on several epidemiologic studies reporting associations between short-
term PM10-2.5 exposure and cardiovascular effects, including 
IHD hospitalizations, supraventricular ectopy, and changes in heart 
rate variability (HRV). In addition, dust storm events resulting in 
high concentrations of crustal material were linked to increases in 
total cardiovascular disease emergency department visits and hospital 
admissions. However, the 2009 ISA noted the potential for exposure 
measurement error primarily due to the different methods used across 
studies to estimate PM10-2.5 concentrations and copollutant 
confounding in these epidemiologic studies. In addition, there was only 
limited evidence of cardiovascular effects from a small number of 
experimental studies (e.g. animal toxicological studies and controlled 
human exposure studies) that examined short-term PM10-2.5 
exposures (U.S. EPA, 2009a, section 6.2.12.2). In the 2019 ISA, key 
uncertainties included the potential for exposure measurement error, 
copollutant confounding, and limited evidence of biological 
plausibility for cardiovascular effects following inhalation exposure 
(U.S. EPA, 2019a, section 6.3.13).
    The evidence for short-term PM10-2.5 exposure and 
cardiovascular outcomes has expanded since the 2009 ISA, though 
important uncertainties remain. The 2019 ISA notes that there are a 
small number of epidemiologic studies reporting positive associations 
between short-term exposure to PM10-2.5 and cardiovascular-
related morbidity

[[Page 16295]]

outcomes. However, the 2019 ISA notes that there is limited evidence to 
support that these associations are biologically plausible, or 
independent of copollutant confounding. The 2019 ISA also concludes 
that it remains unclear how the approaches used to estimate 
PM10-2.5 concentrations in epidemiologic studies compare 
amongst one another and subsequently how exposure measurement error 
varies between each method. Specifically, it is unclear how well-
correlated PM10-2.5 concentrations are both temporally and 
spatially across these methods and therefore whether exposure 
measurement error varies across these methods. Taken together, the 2019 
ISA concludes that ``the evidence is suggestive of, but not sufficient 
to infer, a causal relationship between short-term PM10-2.5 
exposures and cardiovascular effects'' (U.S. EPA, 2019a, p. 6-254).
iii. Respiratory Effects
    With regard to short-term PM10-2.5 exposures and 
respiratory effects, the 2009 ISA (U.S. EPA, 2009a) concluded that the 
relationship between short-term exposure to PM10-2.5 and 
respiratory effects is ``suggestive of a causal relationship'' based on 
a small number of epidemiologic studies observing associations with 
some respiratory effects and limited evidence from experimental studies 
to support biological plausibility. Epidemiologic findings were 
consistent for respiratory infection and combined respiratory-related 
diseases, but not for COPD. Studies were characterized by overall 
uncertainty in the exposure assignment approach and limited information 
regarding potential copollutant confounding. Controlled human exposure 
studies of short-term PM10-2.5 exposures found no lung 
function decrements and inconsistent evidence for pulmonary 
inflammation. Animal toxicological studies were limited to those using 
non-inhalation (e.g., intra-tracheal instillation) routes of 
PM10-2.5 exposure.
    Recent epidemiologic findings consistently link PM10-2.5 
exposure to asthma exacerbation and respiratory mortality, with some 
evidence that associations remain positive (though attenuated in some 
studies of mortality) in copollutant models that include 
PM2.5 or gaseous pollutants. Epidemiologic studies provide 
limited evidence for positive associations with other respiratory 
outcomes, including COPD exacerbation, respiratory infection, and 
combined respiratory-related diseases (U.S. EPA, 2019a, Table 5-36). As 
noted above for other endpoints, an uncertainty in these epidemiologic 
studies is the lack of a systematic evaluation of the various methods 
used to estimate PM10-2.5 concentrations and the resulting 
uncertainty in the spatial and temporal variability in 
PM10-2.5 concentrations compared to PM2.5 (U.S. 
EPA, 2019a, sections 2.5.1.2.3 and 3.3.1.1). Specifically, the existing 
monitoring networks do not provide a good characterization of how well 
correlated concentrations are both spatially and temporally across the 
PM10-2.5 estimation methods and overall spatial and temporal 
patterns in PM10-2.5 concentrations. Taken together, the 
2019 ISA concludes that ``the collective evidence is suggestive of, but 
not sufficient to infer, a causal relationship between short-term 
PM10-2.5 exposure and respiratory effects'' (U.S. EPA, 
2019a, p. 5-270).
iv. Cancer
    In the 2012 review, little information was available from studies 
of cancer following inhalation exposures to PM10-2.5. Thus, 
the 2009 ISA determined the evidence was ``inadequate to evaluate the 
relationship between long-term PM10-2.5 exposures and 
cancer'' (U.S. EPA, 2009a). The scientific information evaluated in the 
2019 ISA of long-term PM10-2.5 exposure and cancer remains 
limited, with a few recent epidemiologic studies reporting positive, 
but imprecise, associations with lung cancer incidence (U.S. EPA, 
2019a). Moreover, uncertainty remains in these studies with respect to 
exposure measurement error due to the use of PM10-2.5 
predictions that have not been validated by monitored 
PM10-2.5 concentrations (U.S. EPA, 2019a, sections 3.3.2.3 
and 10.3.4). Relatively few experimental studies of PM10-2.5 
have been conducted, though available studies indicate that 
PM10-2.5 exhibits two key characteristics of carcinogens: 
genotoxicity and oxidative stress. While limited, such experimental 
studies provide some evidence of biological plausibility for the 
findings in a small number of epidemiologic studies (U.S. EPA, 2019a, 
section 10.3.4).
    Taken together, the small number of epidemiologic and experimental 
studies, along with uncertainty with respect to exposure measurement 
error, contribute to the determination in the 2019 ISA that, ``the 
evidence is suggestive of, but not sufficient to infer, a causal 
relationship between long-term PM10-2.5 exposure and 
cancer'' (U.S. EPA, 2019a, p. 10-87).
v. Metabolic Effects
    The 2009 ISA did not make a causality determination for 
PM10-2.5-related metabolic effects. One epidemiologic study 
in the 2019 ISA reports an association between long-term 
PM10-2.5 exposure and incident diabetes, while additional 
cross-sectional studies report associations with effects on glucose or 
insulin homeostasis (U.S. EPA, 2019a, section 7.4). As discussed above 
for other outcomes, uncertainties with the epidemiologic evidence 
include the potential for copollutant confounding and exposure 
measurement error due to the different methods used across studies to 
estimate PM10-2.5 concentrations (U.S. EPA, 2019a, Tables 7-
14 and 7-15). The evidence base to support the biological plausibility 
of metabolic effects following PM10-2.5 exposures is 
limited, but a cross-sectional study that investigated biomarkers of 
insulin resistance and systemic and peripheral inflammation may support 
a pathway leading to type 2 diabetes (U.S. EPA, 2019a, sections 7.4.1 
and 7.4.3). Based on the expanded, though still limited evidence base, 
the 2019 ISA concludes that, ``[o]verall, the evidence is suggestive 
of, but not sufficient to infer, a causal relationship between [long]-
term PM10-2.5 exposure and metabolic effects'' (U.S. EPA, 
2019a, p. 7-56).
vi. Nervous System Effects
    The 2009 ISA did not make a causality determination for 
PM10-2.5-related nervous system effects. In the 2019 ISA, 
available epidemiologic studies report associations between 
PM10-2.5 and impaired cognition and anxiety in adults in 
longitudinal analyses (U.S. EPA, 2019a, Table 8-25, section 8.4.5). 
Associations of long-term exposure with neurodevelopmental effects are 
not consistently reported in children (U.S. EPA, 2019a, sections 8.4.4 
and 8.4.5). Uncertainties in these studies include the potential for 
copollutant confounding, as no studies examined copollutants models 
(U.S. EPA, 2019a, section 8.4.5), and for exposure measurement error, 
given the use of various methods to estimate PM10-2.5 
concentrations (U.S. EPA, 2019a, Table 8-25). In addition, there is 
limited animal toxicological evidence supporting the biological 
plausibility of nervous system effects (U.S. EPA, 2019a, sections 8.4.1 
and 8.4.5). Overall, the 2019 ISA concludes that, ``the evidence is 
suggestive of, but not sufficient to infer, a causal relationship'' 
between long-term PM10-2.5 exposure and nervous system 
effects (U.S. EPA, 2019a, p. 8-75).

[[Page 16296]]

B. Conclusions on the Primary PM10 Standard

    In drawing conclusions on the adequacy of the current primary 
PM10 standard, in view of the advances in scientific 
knowledge and additional information now available, the Administrator 
has considered the evidence base, information, and policy judgments 
that were the foundation of the 2020 review and reflects upon the body 
of information and evidence available in this reconsideration. In so 
doing, the Administrator has taken into account both evidence-based and 
quantitative information-based considerations, as well as advice from 
the CASAC and public comments. Evidence-based considerations draw upon 
the EPA's integrated synthesis of the scientific evidence from animal 
toxicologic, controlled human exposure, and epidemiologic studies 
evaluating health effects related to exposures to PM10-2.5 
as presented in the 2019 ISA and discussed in section III.A.2. In 
addition to the evidence, the Administrator has weighed a range of 
policy-relevant considerations as discussed in the 2022 PA and 
summarized in sections III.B and III.C of the proposal and summarized 
in section III.B.2 below. These considerations, along with the advice 
from the CASAC (section III.B.1) and public comments (section III.B.3), 
are discussed below. A more detailed summary of all significant 
comments, along with the EPA's responses in the Response to Comments 
document, can be found in the docket for this rulemaking (Docket No. 
EPA-HQ-OAR-2015-00072). This document is available for review in the 
docket for this rulemaking and through EPA's NAAQS website (link). The 
Administrator's conclusions in this reconsideration regarding the 
adequacy of the current primary PM10 standard and whether 
any revisions are appropriate are described in section III.B.4.
1. CASAC Advice
    As described in section I.X, the EPA decided to prepare a revised 
PA for the reconsideration of the 2020 final decision. The CASAC's 
advice on the 2019 draft PA and the 2021 draft PA was documented in 
letters to the prior and current Administrators (Cox, 2019b; Sheppard, 
2022a) and is summarized below. In reviewing both the 2019 draft PA and 
the 2021 draft PA, the CASAC agreed with the EPA's preliminary 
conclusion that the available scientific evidence, including its 
uncertainties and limitations, does not call into question the adequacy 
of the current primary PM10 standard and that the standard 
should be retained, without revision.
    In its review of the 2019 draft PA, the CASAC concurred with the 
overall preliminary conclusion that it is appropriate to consider 
retaining the current primary PM10 standard, without 
revision. In their agreement with the conclusions in the 2019 draft PA, 
the CASAC stated that ``that key uncertainties identified in the last 
review remain'' (Cox, 2019b) and that ``none of the identified health 
outcomes linked to PM10-2.5'' were judged to be causal or 
likely to be causal (Cox, 2019b, p. 12 of consensus responses). 
Moreover, to reduce these uncertainties in future reviews, the CASAC 
recommended improvements to PM10-2.5 exposure assessment, 
including a more extensive network for direct monitoring of the 
PM10-2.5 fraction (Cox, 2019b, p. 13 of consensus 
responses). The CASAC also recommended additional controlled human 
exposure and animal toxicological studies of the PM10-2.5 
fraction to improve the understanding of biological mechanisms and 
pathways (Cox, 2019b, p. 13 of consensus responses). Overall, the CASAC 
agreed with the EPA's preliminary conclusion in the 2019 draft PA that 
``. . . the available evidence does not call into question the adequacy 
of the public health protection afforded by the current primary 
PM10 standard and that evidence supports consideration of 
retaining the current standard in this review'' (Cox, 2019b, p. 3 of 
letter).
    In its review of the 2021 draft PA, the CASAC provided advice on 
the adequacy of the current primary PM10 standard in the 
context of its review of the revised PA for this reconsideration 
(Sheppard, 2022a) \129\.) \130\. In this context, the CASAC supported 
the preliminary conclusion in the 2021 draft PA that the evidence 
reviewed in the 2019 ISA does not call into question the public health 
protection provided by the current primary PM10 standard 
against PM10-2.5 exposures and concurs with the 2021 draft 
PA's overall preliminary conclusion that it is appropriate to consider 
retaining the current primary PM10 standard (Sheppard, 
2022a, p. 4 of consensus letter). Additionally, the CASAC concurred 
that ``. . . at this time, PM10 is an appropriate choice as 
the indicator for PM10-2.5'' and ``that it is important to 
retain the level of protection afforded by the current PM10 
standard'' (Sheppard, 2022a, p. 4 of consensus letter). The CASAC also 
recognized uncertainties associated with the scientific evidence, 
including ``compared to PM2.5 studies, the more limited 
number of epidemiology studies with positive statistically significant 
findings, and the difficulty in extracting the sole contribution of 
coarse PM to observed adverse health effects'' (Sheppard, 2022a, p. 19 
of consensus responses).
---------------------------------------------------------------------------

    \129\ As described in section I.C.5.b above, the scope of the 
ISA Supplement did not include consideration of studies of health 
effects associated with exposure to PM10-2.5. Therefore, 
the information and conclusions presented in the 2022 PA are very 
similar to those in the 2020 PA.
    \130\ As described in section I.C.5.b above, the scope of the 
ISA Supplement did not include consideration of studies of health 
effects associated with exposure to PM10-2.5. Therefore, 
the information and conclusions presented in the 2022 PA are very 
similar to those in the 2020 PA.
---------------------------------------------------------------------------

    The CASAC recommended several areas for additional research to 
reduce uncertainties in the PM10-2.5 exposure estimates used 
in the epidemiologic studies, to evaluate the independence of 
PM10-2.5 health effect associations, to evaluate the 
biological plausibility of PM10-2.5-related effects, and to 
increase the number of studies examining PM10-2.5-related 
health effects in at-risk populations (Sheppard, 2022a, p. 20 of 
consensus responses). Furthermore, the CASAC ``recognizes a need for, 
and supports investment in research and deployment of measurement 
systems to better characterize PM10-2.5'' and to ``provide 
information that can improve public health'' (Sheppard, 2022a, p. 20 of 
consensus responses).
2. Basis for the Proposed Decision
    At the time of the proposal, the Administrator carefully considered 
the assessment of the current evidence and conclusions reached in the 
2019 ISA, considerations and staff conclusions and associated 
rationales presented in the 2020 PA and 2022 PA, and advice and 
recommendations of the CASAC (88 FR 5634, January 27, 2023). Consistent 
with previous reviews, the Administrator first considered the available 
scientific evidence for PM10-2.5-related exposures and 
health effects, as evaluated in the 2019 ISA. As an initial matter, the 
Administrator recognized that the scientific evidence for 
PM10-2.5-related effects available in this reconsideration 
is the same body of evidence that was available at the time of the 2020 
review, as evaluated in the 2019 ISA and summarized in section III.A.2 
above. The 2019 ISA concludes that the evidence supports ``suggestive 
of, but not sufficient to infer'' causal relationships between short- 
and long-term exposures to PM10-2.5 and cardiovascular 
effects, cancer, and mortality and long-term PM10-2.5 
exposures and metabolic effects and nervous system effects (U.S. EPA, 
2019a). The Administrator noted that

[[Page 16297]]

the evidence for several PM10-2.5-related health effects has 
expanded since the completion of the 2009 ISA, but important 
uncertainties remain. The uncertainties in the epidemiologic studies 
contribute to the determinations in the 2019 ISA that the evidence for 
short and long-term PM10-2.5 exposures and mortality, 
cardiovascular effects, metabolic effects, nervous system effects, and 
cancer is ``suggestive of, but not sufficient to infer'' causal 
relationships (U.S. EPA, 2019a; U.S. EPA, 2022b, section 4.3.1). 
Drawing from the evidence evaluated in the 2019 ISA and consideration 
of the scientific evidence in the 2022 PA, the Administrator noted 
that, consistent with previous reviews, the 2019 ISA and the 2022 PA 
highlight a number of uncertainties associated with the evidence, 
including: (1) PM10-2.5 exposure estimates used in 
epidemiologic studies, (2) independence of PM10-2.5 health 
effect associations, and (3) biological plausibility of the 
PM10-2.5-related effects. These uncertainties contribute to 
the determinations in the 2019 ISA that the evidence for short-term 
PM10-2.5 exposures and key health effects is ``suggestive 
of, but not sufficient to infer'' causal relationships. In considering 
the available scientific evidence, consistent with approaches employed 
in past NAAQS reviews, the Administrator placed the most weight on 
evidence supporting ``causal'' and ``likely to be causal'' 
relationships. In so doing, he noted that the available evidence for 
short- and long-term PM10-2.5 exposures and health effects 
does not support causality determinations of a ``causal relationship'' 
or ``likely to be causal relationship.'' Furthermore, the Administrator 
recognized that, because of the uncertainties and limitations in the 
evidence base, the 2022 PA does not include a quantitative assessment 
of PM10-2.5 exposures and risk that might further inform 
decisions regarding the adequacy of the current 24-hour primary 
PM10 standard. Therefore, in light of the 2019 ISA 
conclusions that the evidence supports ``suggestive of, but not 
sufficient to infer'' causal relationships. The Administrator judged 
that there are substantial uncertainties that raise questions regarding 
the degree to which additional public health improvements would be 
achieved by revising the existing PM10 standard. In 
considering the available evidence for long-term PM10-2.5 
exposures, the Administrator noted that there is limited evidence that 
would support consideration of an annual standard to provide protection 
against such effects, in conjunction with the current primary 24-hour 
PM10 standard. He preliminarily concluded that the current 
primary 24-hour PM2.5 standard that reduces 24-hour 
exposures also likely reduces long-term average exposures, and 
therefore provides some margin of safety against the health effects 
associated with long-term PM10-2.5 exposures.
    In reaching his proposed decision on the adequacy of the current 
primary 24-hour PM10 standard, the Administrator also 
considered advice from the CASAC. As noted above in section III.B.1, 
the CASAC recognized uncertainties associated with the scientific 
evidence and agreed with the 2019 draft PA and 2021 draft PA 
conclusions that the scientific evidence does not call into question 
the adequacy of the primary PM10 standard and supports 
consideration of retaining the current standard.
    When considering the above information together, the Administrator 
proposed to conclude that the available scientific evidence continues 
to support a PM10 standard to provide some measure of 
protection against PM10-2.5 exposures. Additionally, he 
recognized that there are important uncertainties and limitations 
associated with the available evidence for PM10-2.5-related 
health effects, for both short and long-term exposure, as evaluated in 
the 2019 ISA. Consistent with the decisions in the previous reviews, 
the Administrator proposed to conclude that these limitations lead to 
considerable uncertainty regarding the potential public health 
implications of revising the level of the current primary 24-hour 
PM10 standard. Thus, based on his consideration of the 
evidence and associated uncertainties and limitations for 
PM10-2.5-related health effects and his consideration of 
CASAC advice on the primary PM10 standard, the Administrator 
proposed to retain the current primary PM10 standard, 
without revision.
3. Comments on the Proposed Decision
    Of the public comments received on the proposal, very few 
commenters provided comments on the primary PM10 standard. 
Of those commenters who did provide comments on the primary 
PM10 standard, the majority agree with the EPA's proposed 
decision to retain the primary PM10 standard. In so doing, 
these commenters agree with the EPA's rationale regarding the available 
scientific information, including uncertainties and limitations, for 
informing decisions on the standard. These commenters state that no new 
scientific evidence or quantitative information has emerged since the 
2020 decision to retain the current standard. Furthermore, these 
commenters note that the EPA did not evaluate any new scientific 
evidence related to PM10-2.5 exposures and health effects as 
a part of the 2022 ISA Supplement developed for this reconsideration, 
nor did the revised 2022 PA consider any new or different information 
from the 2020 PA, and therefore, the EPA reached the same conclusion as 
is the 2020 PA that the current standard is adequate and should be 
retained. This group includes industries and industry groups, as well 
as some State and local governments. All of these commenters generally 
note their agreements with the rationale provided in the proposal and 
the CASAC concurrence with the 2021 draft PA conclusion that the 
available information does not call into question the adequacy of the 
current standard, and therefore, does not support revision and that the 
current standard should be retained.
    Some commenters, including those from environmental and public 
health organizations and groups, some states, and individuals, 
disagreed with the Administrator's proposed decision to retain the 
current primary PM10 standard. These commenters recommend 
that the EPA revise the primary PM10 standard to a lower 
level to provide increased public health protection, citing to the 
available scientific evidence, as well as the proposed revision to the 
primary PM2.5 standard.
    Commenters who disagreed with the proposal to retain the current 
standard state that revision to the primary PM10 standard is 
necessary to protect public health with an adequate margin of safety. 
In their recommendations for revising the standard, some commenters 
contend that the current standard, with its indicator of 
PM10 to target exposures to PM10-2.5, has become 
less protective as ambient concentrations of PM2.5 have been 
reduced with revisions to that standard. These commenters assert that 
the current primary PM10 standard allows increased exposure 
to PM10-2.5 in ambient air because retaining the primary 
PM10 would allow proportionately more PM10-2.5 
mass as the PM2.5 standard has been revised downward. 
Moreover, in support of their recommendations, the commenters note that 
the available evidence of PM10-2.5-related health effects 
has been expanded and strengthened since the time of the last review. 
Taken together, the commenters contend that the primary PM10 
standard should be revised and failure to do so would be arbitrary and 
capricious. Some of these

[[Page 16298]]

commenters assert that the level of the primary PM10 
standard should be revised to 140 or 145 [micro]g/m\3\, concurrent with 
a strengthened primary 24-hour PM2.5 standard, while other 
commenters recommend revising the level of the standard to within the 
range of 65-75 [micro]g/m\3\, to provide increased public health 
protection.
    We disagree with the commenters that the primary PM10 
standard should be revised because of reductions in ambient 
concentrations of PM2.5. As an initial matter, we note that 
overall, ambient concentrations of both PM10 and 
PM2.5 have declined significantly over time. Ambient 
concentrations of PM10 have declined by 46% across the U.S. 
from 2000 to 2019,\131\ while PM2.5 concentrations in 
ambient air have declined by 43% during this same time period.\132\ As 
noted in the 2022 PA (p. 2-41), the majority of PM10-2.5 
sites have generally remained steady and do not exhibit a trend of 
increasing or decreasing concentrations during this time period, 
reflecting the relatively consistent level of dust emission across the 
U.S. from 2000 to 2019 (U.S. EPA, 2022b).
---------------------------------------------------------------------------

    \131\ PM10 concentrations presented as the annual 
second maximum 24-hour concentration (in [micro]g/m\3\) at 262 sites 
in the U.S. For more information, see: https://www.epa.gov/air-
trends/particulate-matter-pm10-trends
    \132\ PM2.5 concentrations presented as the 
seasonally-weighted annual average concentration (in [micro]g/m\3\) 
at 406 sites in the U.S. For more information, see: https://www.epa.gov/air-trends/particulate-matter-pm25-trends
---------------------------------------------------------------------------

    The 2019 ISA provides a comparison of the relative contribution of 
PM2.5 and PM10-2.5 to PM10 
concentrations by region and season using the more comprehensive 
monitoring data from the NCore network available in this 
reconsideration (U.S. EPA, 2019, section 2.5.1.1.4). The data indicate 
that, for urban areas, there are roughly equivalent amounts of 
PM2.5 and PM10-2.5 contributing to 
PM10 in ambient air, while rural locations have a slightly 
higher contribution of PM10-2.5 contributing to 
PM10 concentrations than PM2.5 (U.S. EPA, 2019, 
section 2.5.1.1.4, Table 2-7). There is generally a greater 
contribution from the PM2.5 fraction in the East and a 
greater contribution from the PM10-2.5 fraction in the West 
and Midwest.
    The EPA recognizes that when the primary annual PM2.5 
standard was revised from 15.0 [micro]g/m\3\ to 12.0 [micro]g/m\3\ 
while leaving the 24-hour PM2.5 standards unchanged at 35 
[micro]g/m\3\ and the 24-hour PM10 standard unchanged at 150 
[micro]g/m\3\, the PM10-2.5 fraction of PM10 
could increase in some areas as the PM2.5 fraction decreases 
(78 FR 3085, March 03, 2013). As described in the 2019 ISA, 
PM10 has become considerably coarser across the U.S. 
compared to similar observations in the 2009 ISA such that, in urban 
areas, the mass of the coarse fraction of PM is similar to or greater 
than the mass of the fine fraction of PM (U.S. EPA, 2019, section 
2.5.1.1.4; U.S. EPA, 2009c). However, in considering recent air quality 
data, the EPA notes that in most areas of the country PM2.5 
and PM10 concentrations have declined and are well below 
their respective 24-hour standards. While the contribution of fine and 
coarse PM to PM10 mass concentrations may vary spatially and 
temporally, based on the trends in recent air quality data, the 
Administrator concludes that the current primary 24-hour 
PM10 standard is maintaining air quality at level that 
provides requisite protection against PM10-2.5. That is, 
recent air quality data does not suggest that PM10-2.5 
concentrations have been increasing as PM2.5 concentrations 
have been decreasing. In considering the available PM10-2.5 
health effects evidence in this reconsideration, there continue to be 
significant uncertainties and limitations, specifically with respect to 
the exposure assessment methods used to estimate PM10-2.5 
concentrations, that make it difficult to fully assess the public 
health implications of revising the primary PM10 standard 
even considering the possibility for additional variability in the 
relative ratio of PM2.5 to PM10-2.5 in current 
PM10 air quality across the U.S. As described in detail 
above in section III.A.2 and in the proposal (85 FR 5558, January 27, 
2023), the uncertainties and limitations in the health effects evidence 
for PM10-2.5 contributed to the determinations in the 2019 
ISA that the evidence for key PM10-2.5 health effects is 
``suggestive of, but not sufficient to infer, a causal relationship'' 
or ``inadequate to infer the presence, or absence of a causal 
relationship'' (U.S. EPA, 2019a). While the evidence base for 
PM10-2.5-related health effects has somewhat expanded since 
the 2009 ISA, the Administrator concludes that the evidence remains too 
limited to inform judgments regarding whether a more protective primary 
PM10 standard is warranted at this time.
    Beyond the uncertainties and limitations associated with the 
available scientific evidence, the EPA also notes that, while the NCore 
monitoring network has been expanded since the time of the last review, 
epidemiologic studies available in this review do not use 
PM10-2.5 NCore data in evaluating associations between 
PM10-2.5 in ambient air and long- or short-term exposures. 
In the absence of such evidence, the public health implications of 
changes in ambient PM10-2.5 concentrations as 
PM2.5 concentrations decrease remain unclear. Therefore, the 
EPA continues to recognize this as an area for future research, to 
address the existing uncertainties (U.S. EPA, 2022b, section 4.6), and 
inform future reviews of the PM NAAQS. Taken together, as at the time 
of proposal, the Administrator concludes that these and other 
limitations in the PM10-2.5 evidence raised questions as to 
whether additional public health improvements would be achieved by 
revising the existing PM10 standard, particularly when 
considering such judgments along with his decision to retain the 
current primary 24-hour PM2.5 standard. Therefore, the EPA 
does not agree with the commenters that the currently available air 
quality information or scientific evidence support revisions to the 
primary PM10 standard in this reconsideration.
    Consistent with their comments on the 2020 proposal, some 
commenters disagreed with the Administrator's proposed conclusion to 
retain the current primary PM10 standard, primarily focusing 
their comments on the need for revisions to the form of the standard or 
the level of the standard. With regard to comments on the form of the 
standard, some commenters assert that the EPA should revise the 
standard by adopting a separate form (or a ``compliance threshold'' in 
their words)--the 99th percentile, averaged over three years--for the 
primary PM10 standard for continuous monitors, which provide 
data every day, while maintaining the current form of the standard (one 
exceedance, averaged over three years) for 1-in-6 samplers, given the 
increased use of continuous monitoring and to ease the burden of 
demonstrating exceptional events. These commenters, in support of their 
comment, contend that the 99th percentile would effectively change the 
form from the 2nd highest to the 4th highest and would allow no more 
than three exceedances per year, averaged over three years. These 
commenters additionally highlight the EPA's decision in the 1997 review 
to adopt a 99th percentile form, averaged over three years, citing to 
advantages of a percentile-based form in the Administrator's rationale 
in that review. The comments further assert that a 99th percentile form 
for the primary PM10 standard is still more conservative 
than the form for other short-term NAAQS (e.g., PM2.5 and 
NO2).

[[Page 16299]]

    First, the EPA has long recognized that the form is an integral 
part of the NAAQS and must be selected together with the other elements 
(i.e., indicator, averaging time, level) of the NAAQS to ensure the 
appropriate stringency and requisite degree of public health 
protection. Thus, if the EPA were to change the form according to the 
monitoring method it would be establishing two different NAAQS, varying 
based on the monitoring method. The EPA has not done this to date, did 
not propose such an approach, and declines to adopt it for the final 
rule, as we believe such a decision in this final rule is beyond the 
scope of the proposal, and that each PM standard should have a single 
form, indicator, level and averaging time, chosen by the Administrator 
as necessary and appropriate. While certain continuous monitors may be 
established and approved as a Federal Equivalent Method (FEM) for 
PM10, as an alternative to a Federal Reference Method (FRM), 
the use of an FEM is intended as an alternative means of determining 
compliance with the NAAQS, not as authorizing a different NAAQS.
    Even if the commenters had asked that the change in form be made 
without regard to monitoring method, the EPA does not believe such a 
change would be warranted. The change in form for continuous monitors 
suggested by the commenters, without also lowering the level of such a 
standard, would allow more exceedances and thereby reduce the public 
health protection provided against exposures to PM10-2.5 in 
ambient air, resulting in a less stringent primary PM10 
standard than the current standard. These commenters have not provided 
new evidence or analyses to support their conclusion that an 
appropriate degree of public health protection could be achieved by 
allowing the use of an alternative form (i.e., 99th percentile), while 
retaining the other elements of the standard.
    With regard to the commenters' assertion that an alternate form of 
the standard would ease the burden of demonstrating exceptional events, 
the EPA recognizes, consistent with the CAA, that it may be appropriate 
to exclude monitoring data influenced by ``exceptional'' events when 
making certain regulatory determinations. However, the EPA notes that 
the cost of implementation of the standards may not be considered by 
the EPA in reviewing the standards. The EPA continues to update and 
develop documentation and tools to facilitate the implementation of the 
2016 Exceptional Events Rule, including new PM2.5 
implementation focused products under development that are intended to 
assist air agencies with the development of demonstrations for specific 
types of exceptional events. With regard to the commenters' specific 
concerns for wildfires or high winds, the EPA released updated guidance 
documents on the preparation of exceptional event demonstrations 
related to wildfires in September 2016, high wind dust events in April 
2019, and prescribed fires in August 2019. These guidance documents 
outline the regulatory requirements and provide examples for air 
agencies preparing demonstrations for wildfires, high wind dust, and 
prescribed fire events. For all of the reasons discussed above, the EPA 
does not agree with the commenters that the form of the primary 
PM10 standard should be revised to a 99th percentile for 
continuous monitors.
4. Administrator's Conclusions
    This section summarizes the Administrator's considerations and 
conclusions related to the current primary PM10 standard. In 
establishing primary standards under the Act that are ``requisite'' to 
protect the public health with an adequate margin of safety, the 
Administrator is seeking to establish standards that are neither more 
nor less stringent than necessary for this purpose. In so doing, the 
Administrator notes that his final decision in this reconsideration is 
a public health policy judgment that draws upon scientific information, 
as well as judgments about how to consider the range and magnitude of 
uncertainties that are inherent in the information. Accordingly, he 
recognizes that his decision requires judgments based on the 
interpretation of the evidence that neither overstates nor understates 
the strength or limitations of the evidence nor the appropriate 
inferences to be drawn. He recognizes, as described in section I.A 
above, that the Act does not require that primary standards be set at a 
zero-risk level; rather, the NAAQS must be sufficient but not more 
stringent than necessary to protect public health, including the health 
of sensitive groups with an adequate margin of safety.
    Given these requirements, and consistent with the primary 
PM2.5 standards discussed above (section II.C.3), the 
Administrator's final decision in this reconsideration of the current 
primary PM10 standard will be a public health policy 
judgment that draws upon the scientific information examining the 
health effects of PM10-2.5 exposures, including how to 
consider the range and magnitude of uncertainties inherent in that 
information. The Administrator's final decision is based on an 
interpretation of the scientific evidence that neither overstates nor 
understates its strengths and limitations, nor the appropriate 
inferences to be drawn.
    Having carefully considered advice from the CASAC and public 
comments, as discussed above, the Administrator notes that the 
fundamental scientific conclusions on health effects of 
PM10-2.5 in ambient air that were reached in the 2019 ISA 
and summarized in the 2020 PA and 2022 PA remain valid. Additionally, 
the Administrator believes the judgments he proposed (85 FR 5558, 
January 27, 2023) with regard to the evidence remain appropriate. 
Further, in considering the adequacy of the current primary 
PM10 standard in this reconsideration, the Administrator has 
carefully considered the policy-relevant evidence and conclusions 
contained in the 2019 ISA; the rationale and conclusions presented in 
the 2020 PA and 2022 PA; the advice and recommendations from the CASAC 
in their reviews of the 2019 draft PA and 2021 draft PA; and public 
comments, as addressed in section III.B.3 above and in the RTC 
document. In the discussion below, the Administrator gives weight to 
the conclusions in the 2020 PA and 2022 PA, with which the CASAC has 
concurred, as summarized in section III.C of the proposal and takes 
note of the key aspects of the rationale for those conclusions that 
contribute to his decision in this review. In considering this 
information, the Administrator concludes that the preliminary 
conclusions and policy judgments supporting his proposed decision 
remain valid, and that the current primary PM10 standard 
provides requisite protection of public health with an adequate margin 
of safety and should be retained. In considering the 2020 PA and 2022 
PA evaluations and conclusions, the Administrator notes that, while the 
health effects evidence is somewhat expanded since the 2009 ISA as 
described in section III.A.2 above, the overall conclusions are 
generally consistent with those reached in the 2009 ISA (U.S. EPA, 
2020b, section 4.4). In so doing, he additionally notes that the CASAC 
supported the preliminary conclusion in the 2019 draft PA and 2021 
draft PA that the evidence reviewed in the 2019 ISA does not call into 
question the public health protection provided by the current primary 
PM10 standard against PM10-2.5 exposures and 
concurs that it is appropriate to consider retaining the current 
primary PM10 standard (Cox,

[[Page 16300]]

2019b, p. 13 of consensus responses; Sheppard, 2022a, p. 4 of consensus 
letter).
    As noted below, the scientific evidence for PM10-2.5-
related health effects has expanded somewhat since the 2012 review, in 
particular for long-term exposures. The Administrator recognizes, 
however, that there are a number of uncertainties and limitations 
associated with the available information, as described in the proposal 
(85 FR 5558, January 27, 2023) and below. With regard to the current 
evidence on PM10-2.5-related health effects, the 
Administrator takes note of recent epidemiologic studies that continue 
to report positive associations with mortality and morbidity in cities 
across North America, Europe, and Asia, where PM10-2.5 
sources and composition are expected to vary widely. While significant 
uncertainties remain, as described below, the Administrator recognizes 
that this expanded body of evidence has broadened the range of effects 
that have been linked with PM10-2.5 exposures. These studies 
provide an important part of the scientific foundation supporting the 
2019 ISA's revised causality determinations (and new determinations) 
for long-term PM10-2.5 exposures and mortality, 
cardiovascular effects, metabolic effects, nervous system effects, and 
cancer (U.S. EPA, 2019a; U.S. EPA, 2022b, section 4.2). Drawing from 
his consideration of this evidence, the Administrator concludes that 
the available scientific information supports a decision to maintain a 
primary PM10 standard to provide public health protection 
against PM10-2.5 exposures, regardless of location, source 
of origin, or particle composition. With regard to uncertainties in the 
evidence, the Administrator first notes that a number of limitations 
were identified in the 2012 review related to: (1) Estimates of ambient 
PM10-2.5 concentrations used in epidemiologic studies; (2) 
limited evaluation of copollutant models to address the potential for 
confounding; and (3) limited experimental studies supporting biological 
plausibility for PM10-2.5-related effects. Despite the 
expanded body of evidence for PM10-2.5 exposures and health 
effects assessed in the 2019 ISA, the Administrator recognizes that 
uncertainties remain, similar to those in the 2012 review. As 
summarized in section III.A.2 above and in responding to public 
comments, uncertainties in the available scientific evidence continue 
to include those associated with the exposure estimates used in 
epidemiologic studies, the independence of the PM10-2.5 
health effect associations, and the biologically plausible pathways for 
PM10-2.5 health effects (U.S. EPA, 2022b, section 4.3). 
These uncertainties contribute to the 2019 ISA determinations that the 
evidence is ``suggestive of, but not sufficient to infer'' causal 
relationships (U.S. EPA, 2019a). The Administrator recognizes that the 
NAAQS must allow for a margin of safety but also places emphasis on 
evidence supporting ``causal'' or ``likely to be causal'' relationships 
(as described in sections II.A.2 and III.A.2 above). Finding that there 
is too much uncertainty that a more stringent standard would improve 
public health, the Administrator judges that the available evidence 
provides support for his conclusion that the current standard provides 
the requisite level of protection from the effects of 
PM10-2.5. In making this judgment, the Administrator 
considers whether this level of protection is more than what is 
requisite and whether a less stringent standard would be appropriate to 
consider. He notes that there continues to be uncertainty associated 
with the evidence, as reflected by the ``suggestive of, but not 
sufficient to infer'' causal determinations. The Administrator 
recognizes that the CAA requirement that primary standards provide an 
adequate margin of safety, as summarized in section I.A above, is 
intended to address uncertainties associated with inconclusive 
scientific evidence and technical information, as well as to provide a 
reasonable degree of protection against hazards that research has not 
yet identified. In light of these considerations and the current body 
of evidence, including uncertainties and limitations, the Administrator 
concludes that a less stringent standard would not provide the 
requisite protection of public health, including an adequate margin of 
safety. The Administrator also considers whether the level of 
protection associated with the current standard is less than what is 
requisite and whether a more stringent standard would be appropriate to 
consider. In so doing, the Administrator considers, as discussed above, 
the level of protection offered from exposures for which public health 
implications are less clear. In so doing, he again notes the 
significant uncertainties and limitations that persist in the 
scientific evidence. In particular, he notes limitations in the 
approaches used to estimate ambient PM10-2.5 concentrations 
in epidemiologic studies, limited examination of the potential for 
confounding by co-occurring pollutants, and limited support for the 
biological plausibility of the serious effects reported in many 
epidemiologic studies that are reflected by the ``suggestive of, but 
not sufficient to infer'' causal determinations. Thus, in light of the 
currently available information, including the uncertainties and 
limitations of the evidence base available to inform his judgments 
regarding protection against PM10-2.5-related effects, the 
Administrator does not find it appropriate to increase the stringency 
of the standard in order to provide the requisite public health 
protection. Rather, he judges it appropriate to maintain the level of 
protection provided by the current primary PM10 standard for 
PM10-2.5 exposures and he does not judge that the available 
information and the associated uncertainties indicate the need for a 
greater level of public health protection.
    In reaching his conclusions on the primary PM10 
standard, the Administrator also considers advice from the CASAC. In 
their comments, the CASAC noted that uncertainties that were identified 
in the 2012 review persist in the evidence for PM10-2.5-
related health effects (Cox, 2019b, p. 13 of consensus responses; 
Sheppard, 2022a, p. 4 of consensus letter) In considering these 
comments, the Administrator takes note of the CASAC consideration of 
the evidence, and associated uncertainties, and its conclusion that the 
evidence reviewed in the 2019 ISA does not call into question the 
adequacy of the public health protection afforded by the current 
primary PM10 standard (Cox, 2019b, p. 3 of letter; Sheppard, 
2022a, p. 4 of consensus letter). The Administrator further notes the 
unanimous conclusions of the CASAC that evidence supports consideration 
of retaining the current primary PM10 standard (Cox, 2019b, 
p. 3 of consensus letter; Sheppard, 2022a, p. 4 of consensus letter). 
In addition to the CASAC's advice, the Administrator also considers 
public comments, the majority of which supported retaining the primary 
PM10 standard, citing to and agreeing with the 
Administrator's rationale for his proposed decision. The Administrator 
also recognizes that a few public commenters supported revising the 
primary PM10 standard in order to provide increased 
protection against PM10-2.5-related health effects.
    The Administrator also notes that the scientific record for his 
decision on the primary PM10 standard is the same as the 
record before the then-Administrator in 2020, as the scope of the ISA 
Supplement focused on health effect categories where the 2019 ISA 
concluded a causal relationship (i.e.,

[[Page 16301]]

short- and long-term PM2.5 exposure and cardiovascular 
effects and mortality). Therefore, because no health outcome categories 
for short- or long-term PM10-2.5 exposure in the 2019 ISA 
were greater than ``suggestive of, but not sufficient to infer, a 
causal relationship'', the ISA Supplement did not evaluate studies 
published after the literature cutoff date of the 2019 ISA related to 
PM10-2.5 exposures and health effects. The Administrator 
further notes his decision is consistent with the decision of the prior 
Administrator in 2020 to retain the primary PM10 standard.
    With regard to the indicator for the primary PM10 
standard, the Administrator recognizes that the 2022 PA notes that the 
evidence continues to support retaining the PM10 indicator 
to provide public health protection against PM10-2.5-related 
effects. He notes that, consistent with the approaches in previous 
reviews, a standard with a PM10 mass-based indicator, in 
conjunction with a PM2.5 mass-based standard, will result in 
controlling allowable concentrations of PM10-2.5. The 
Administrator also takes note of the 2019 ISA comparison that showed 
that the relative contribution of PM2.5 and 
PM10-2.5 to PM10 concentrations can vary across 
the U.S. by region and season, with urban locations having a somewhat 
higher contribution of PM2.5 contributing to PM10 
concentrations than PM10-2.5 (U.S. EPA, 2019a, section 
2.5.1.1.4, Table 2-7). In these urban locations, where PM2.5 
concentrations are somewhat higher than in rural locations, the 
toxicity of the PM10 may be higher due to contaminating 
PM2.5. Further, although uncertainties with the evidence 
persist, the strongest health effects evidence associated with 
PM10-2.5 comes from epidemiologic studies conducted in urban 
areas. He also notes that the CASAC agreed with the EPA's conclusions 
that a PM10 indicator remained appropriate (Cox, 2019b, p. 
13 of consensus responses; Sheppard, 2022a, p. 4 of letter). In light 
of this information, the Administrator concludes that the 
PM10 indicator remains appropriate and provides protection 
from exposure to all coarse PM, regardless of location, source of 
origin, or particle composition.
    Similarly, with regard to averaging time, form, and level of the 
standard, the Administrator takes note of uncertainties in the 
available evidence and information and continues to find that the 
current standard, as defined by in all of its elements, is requisite. 
As an initial matter, the Administrator notes that the current primary 
PM10 standard, with its level of 150 [micro]g/m\3\, 24-hour 
averaging time, not to be exceeded more than once per year on average 
over three years, is intended to protect against short-term peak 
PM10-2.5 exposures. In so doing, while the Administrator 
notes that changes in PM2.5 concentrations in ambient air 
can influence the contribution of the fine and coarse fractions to 
PM10 mass, such that reductions in PM2.5 
concentrations can lead to more allowable PM10-2.5 under the 
current primary PM10 standard, he recognizes that there is 
no new information available in this reconsideration to suggest that 
the public health protection provided by the current standard is not 
requisite or that a more stringent standard is warranted at this time. 
The Administrator concludes that, particularly in light of his decision 
to retain the primary 24-hour PM2.5 standard with its level 
of 35 [micro]g/m\3\ as described in section II.B.4 above, the primary 
PM10 standard would be expected to maintain 
PM10-2.5 concentrations in ambient air below those that have 
been considered to be associated with serious health effects in past 
NAAQS reviews. The Administrator also notes that while the scientific 
evidence available in the 2019 ISA has expanded since the completion of 
the 2009 ISA, he concludes that this information does not provide 
support for the causal or likely to be causal relationships upon which 
he places the greatest weight in considering the adequacy of the 
current standards. He further concludes that the uncertainties and 
limitations of the scientific evidence, along with the absence of 
information to inform a quantitative exposure or risk assessment, make 
it difficult to reach decisions regarding whether a more protective 
standard is warranted at this time. He has additionally considered the 
public comments regarding revisions to these elements of the standard 
and continues to judge that the existing level and the existing form, 
in all its aspects, together with the other elements of the existing 
standard provide an appropriate level of public health protection. For 
all of the reasons discussed above and recognizing the CASAC's 
conclusion that the current evidence provides support for retaining the 
current standard, the Administrator concludes that the current primary 
PM10 standard (in all of its elements) is requisite to 
protect public health with an adequate margin of safety from effects of 
PM10-2.5 in ambient air and should be retained without 
revision.

C. Decision on the Primary PM10 Standard

    For the reasons discussed above and considering information and 
assessments presented in the 2019 ISA and the 2022 PA, the advice from 
the CASAC, and public comments, the Administrator concludes that the 
current primary PM10 standard is requisite to protect public 
health with an adequate margin of safety, including the health of at-
risk populations, and is retaining the current standard without 
revision.

IV. Communication of Public Health

A. Air Quality Index Overview

    Information about the public health implications of ambient 
concentrations of criteria pollutants is communicated to the public 
using the Air Quality Index (AQI) reported on the EPA's AirNow 
website.\133\ The current AQI has been in use since its inception in 
1999.\134\ It provides useful, timely, and easily understandable 
information about the daily degree of pollution. The goal of the AQI is 
to establish a nationally uniform system of indexing pollution 
concentrations for ozone, carbon monoxide, nitrogen dioxide, PM, and 
sulfur dioxide. The AQI is recognized internationally as a proven tool 
to effectively communicate air quality information to the public as 
demonstrated by the fact that many countries have created similar 
indices based on the AQI.
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    \133\ See http://www.airnow.gov/.
    \134\ In 1976, the EPA established a nationally uniform air 
quality index, then called the Pollutant Standard Index (PSI), for 
use by State and local agencies on a voluntary basis (41 FR 37660, 
September 7, 1976; 52 FR 24634, July 1,1987). In August 1999, the 
EPA adopted revisions to this air quality index (64 FR 42530, August 
4, 1999) and renamed the index the AQI.
---------------------------------------------------------------------------

    The AQI converts an individual pollutant concentration in a 
community's air to a number on a scale from 0 to 500. Reported AQI 
values for specific pollutants enable the public to know whether air 
pollution levels in a particular location are characterized as good (0-
50), moderate (51-100), unhealthy for sensitive groups (101-150), 
unhealthy (151-200), very unhealthy (201-300), or hazardous (301+). 
Across criteria pollutants, the AQI value of 100 typically corresponds 
to the level of the short-term (e.g., 24-hour, 8-hour, or 1-hour 
standard) NAAQS for each pollutant. Below an index value of 100, an 
intermediate value of 50 is defined either as the level of the annual 
standard if an annual standard has been established (e.g., 
PM2.5, nitrogen dioxide), a

[[Page 16302]]

concentration equal to one-half the value of the 24-hour standard used 
to define an index value of 100 (e.g., carbon monoxide), or a 
concentration based directly on health effects evidence (e.g., ozone). 
An AQI value greater than 100 means that a pollutant is in one of the 
unhealthy categories (i.e., unhealthy for sensitive groups, unhealthy, 
very unhealthy, or hazardous). An AQI value at or below 100 means that 
a pollutant concentration is in one of the satisfactory categories 
(i.e., moderate or good). The scientific evidence on pollutant-related 
health effects for each NAAQS review support decisions related to 
pollutant concentrations at which to set the various AQI breakpoints, 
which delineate the AQI categories for each individual pollutant (i.e., 
the pollutant concentrations corresponding to index values of 150, 200, 
300, and 500). The AQI is reported three ways by the EPA and State, 
local and Tribal agencies, all of which are useful and complementary. 
The daily AQI is reported for the previous day and used to observe 
trends in community air quality, the AQI forecast helps people plan 
their outdoor activities for the next day, and the near-real-time AQI, 
or NowCast AQI, tells people whether it is a good time for outdoor 
activity.
    Historically, State and local agencies have primarily used the AQI 
to provide general information to the public about air quality and its 
relationship to public health. For more than two decades, many State 
and local agencies, as well as the EPA and other Federal agencies, have 
been developing new and innovative programs and initiatives to provide 
more information related to air quality and health messaging to the 
public in a more timely way. These initiatives, including air quality 
forecasting, near real-time data reporting through the AirNow website, 
use of data from air quality sensors on the EPA and U.S. Forest 
Service's (USFS) Fire and Smoke Map, and air quality action day 
programs, provide useful, up-to-date, and timely information to the 
public about air pollution and its health effects. Such information can 
help the public learn when their well-being may be compromised, so they 
can take actions to avoid or to reduce exposures to ambient air 
pollution at concentrations of concern. This information can also 
encourage the public to take actions that will reduce air pollution on 
days when concentrations are projected to be of concern to local 
communities (e.g., air quality action day programs can encourage 
individuals to drive less or carpool).

B. Air Quality Index Category Breakpoints for PM2.5

    Recognizing the scientific information available and current AQI 
reporting practices, the EPA proposed several revisions to the AQI 
PM2.5 breakpoints. EPA solicited and received comments on 
these proposed revisions. Upon reviewing the information in the 
proposal and considering the comments received EPA is making final 
revisions to the AQI category breakpoints for PM2.5. This 
section summarizes the proposed revisions, which can be read in full in 
the proposal (88 FR 5638, January 27, 2023), significant comments, and 
final revisions.
1. Summary of Proposed Revisions
    One purpose of the AQI is to communicate to the public when air 
quality is poor and thus when they should consider taking actions to 
reduce their exposures. The higher the AQI value, the higher the level 
of air pollution and the greater the health concern. In recognition of 
the scientific information available that is informing the 
reconsideration of the 2020 final decision on the primary 
PM2.5 standards, including a number of new controlled human 
exposure and epidemiologic studies published since the completion of 
the 2009 ISA, as well as additional epidemiologic studies from other 
peer reviewed documents that evaluate the health effects of wildfire 
smoke exposure and that can inform the selection of AQI breakpoints at 
higher PM2.5 concentrations,\135\ the EPA proposed to make 
two sets of changes to the PM2.5 sub-index of the AQI. 
First, the EPA proposed to continue to use the approach used in the 
revisions to the AQI in 2012 (77 FR 38890, June 29, 2012) of setting 
the lower breakpoints (50, 100 and 150) to be based on the levels of 
the primary PM2.5 annual and 24-hour standards and proposed 
to revise the lower breakpoints to be consistent with changes to the 
primary PM2.5 standards that are part of this 
reconsideration. Second, the EPA proposed to revise the upper AQI 
breakpoints (200 and above) and to replace the linear-relationship 
approach used in 1999 to set these breakpoints, with an approach that 
more fully considers the PM2.5 health effects evidence from 
controlled human exposure and epidemiologic studies that have become 
available in the last 20 years (64 FR 42530, August 4, 1999).
---------------------------------------------------------------------------

    \135\ In evaluating the scientific evidence available to inform 
decisions regarding the AQI breakpoints, the EPA considered studies 
that were included as a part of the 2019 ISA and ISA Supplement, but 
also considered other studies that were not included as a part of 
the review of the air quality criteria. The ISAs have specific 
criteria for study inclusion and consideration in reaching 
conclusions regarding causal relationships, and some studies that 
may not have met those criteria (e.g., epidemiologic studies that 
evaluate the health effects of wildfire smoke exposure that would 
have higher PM2.5 concentrations, which are outside of 
the scope of the ISA) were identified as studies that could be used 
to inform decisions on the AQI, particularly for the upper 
breakpoints.
---------------------------------------------------------------------------

a. Air Quality Index Values of 50, 100 and 150
    With respect to the lower AQI breakpoints in the proposal (88 FR 
5638, January 27, 2023), the EPA proposed to conclude that it is 
appropriate to continue setting these breakpoints to be consistent with 
the primary annual and 24-hour PM2.5 standard levels. The 
lowest AQI value of 50 provides the breakpoint between the ``good'' and 
``moderate'' categories. At and below this concentration, air quality 
is considered ``good'' for everyone. Above this concentration, in the 
``moderate'' category, the AQI contains advisories for unusually 
sensitive individuals. The EPA has historically set this breakpoint at 
the level of the primary annual PM2.5 standard. In doing so, 
the EPA has recognized that: (1) The annual standard is set to provide 
protection to the public, including at-risk populations, from 
PM2.5 concentrations, which, when experienced on average for 
a year, have the potential to result in adverse health effects; and (2) 
the AQI exposure period represents a shorter exposure period (e.g., 24-
hour (or less)) while focusing on the most sensitive individuals. The 
EPA saw no basis for deviating from this approach in this 
reconsideration. Thus, the EPA proposed to set the AQI value of 50 at a 
daily (i.e., 24-hour) average concentration equal to the level of the 
primary annual PM2.5 standard that is promulgated.
    The historical approach to setting an AQI value of 100, which is 
the breakpoint between the ``moderate'' and ``unhealthy for sensitive 
groups'' categories, and above which advisories are generated for 
sensitive groups, is to set it at the same level as the primary 24-hour 
PM2.5 standard. In so doing, the EPA has recognized that the 
primary 24-hour PM2.5 standard is set to provide protection 
to the public, including at-risk populations, from short-term exposures 
to PM2.5 concentrations that have the potential to result in 
adverse health effects. Given this, it is appropriate to generate 
advisories for sensitive groups at concentrations above this level. In 
the past, State, local, and Tribal air quality agencies have expressed 
strong support for this approach (78 FR 3086, January 15, 2013). The 
EPA saw no basis to deviate

[[Page 16303]]

from this approach in this reconsideration. In the proposal (88 FR 
5638, January 27, 2023), the EPA proposed to retain the current primary 
24-hour PM2.5 standard with its level of 35 [mu]g/m\3\ but 
took comment on revising the level of that standard to 25 [mu]g/m\3\ 
(section II.D.3.b). Thus, the EPA proposed to retain the AQI value of 
100 set at the level of the current primary 24-hour PM2.5 
standard concentration of 35 [mu]g/m\3\ (i.e., 24-hour average).
    With respect to an AQI value of 150, which is the breakpoint 
between the ``unhealthy for sensitive groups'' and ``unhealthy 
categories,'' this breakpoint concentration in this reconsideration is 
based upon the considering the same health effects information, as 
assessed in the 2019 ISA and ISA Supplement and described in section II 
above, that informs the proposed decisions on the level of the 24-hour 
standard and the AQI value of 100. Previously, the Agency has used a 
proportional adjustment in which the AQI value of 150 was set 
proportionally to the AQI value of 100. This proportional adjustment 
inherently recognizes that the available epidemiologic studies provide 
no evidence of discernible thresholds, below which effects do not occur 
in either sensitive groups or in the general population, that could 
inform conclusions regarding concentrations at which to set this 
breakpoint. Given that the epidemiologic evidence continues to be the 
most relevant health effects evidence for informing this range of AQI 
values, the EPA saw no basis to deviate from this approach in this 
reconsideration. Therefore, the EPA proposed to set an AQI value of 150 
proportionally, depending on the breakpoint concentration of the AQI 
value of 100 (i.e., 55.4 for a 24-hour standard of 35 [mu]g/m\3\).
b. Air Quality Index Values of 200 and Above
    In the proposal (88 FR 5639, January 27, 2023), the EPA summarized 
the history of setting the AQI values of 300 and above in the 1999 rule 
(64 FR 42530, August 4, 1999) and established breakpoints for 
PM2.5 in that range. In general, the AQI values between 100 
and 500 were based on PM2.5 concentrations that generally 
reflected a linear relationship between increasing index values and 
increasing PM2.5 concentrations.\136\ It was found that this 
linear relationship was generally consistent with the health effects 
evidence, which suggested that as PM2.5 concentrations 
increase, increasingly larger numbers of people are likely to 
experience serious health effects in this range of PM2.5 
concentrations (64 FR 42536, August 4, 1999). For the AQI breakpoint of 
500, the concentration was based on the method used to establish a 
previously existing PM10 breakpoint that was informed by 
studies conducted in London using the British Smoke method, which uses 
a different particle size cutpoint as noted in the proposal (88 FR 
5639, January 27, 2023). Due to limited ambient PM2.5 
monitoring data available at that time, the decision on the 500 value 
concentration for PM2.5 was based on the stated assumption 
that PM concentrations measured by the British Smoke method were 
approximately equivalent to PM2.5 concentrations (64 FR 
42530, August 4, 1999). Given that the British Smoke method has a 
larger particle size cutpoint than the current PM2.5 
monitoring method, which has a cutpoint of 2.5 microns, a concentration 
of 500 [mu]g/m\3\ based on the British Smoke method would be equivalent 
to a lower PM2.5 concentration. With respect to the upper 
breakpoints of the AQI, the EPA has historically been concerned about 
establishing these upper breakpoints using evidence based on larger 
size fractions of PM, given that PM2.5 is the indicator for 
the AQI. While monitoring data for higher PM2.5 
concentrations in ambient air has been available for many years, the 
health effects evidence has only recently become available for 
consideration in informing decisions on the upper breakpoints of the 
AQI.
---------------------------------------------------------------------------

    \136\ The AQI breakpoint at 150 was originally set in 1999 to be 
linearly related to the concentrations at the 100 and 500 
breakpoints but then revised in 2012 to be proportional to the AQI 
breakpoint concentration at 100 (78 FR 3181, January 15, 2013).
---------------------------------------------------------------------------

    As part of this reconsideration, the EPA recognized that the health 
effects evidence associated with PM2.5 exposure has greatly 
expanded in recent years. Multiple controlled human exposure studies 
have become available that provide information about health effects 
across a range of concentrations. While many of the new studies 
evaluated in the 2019 ISA focused on examining health effects 
associated with exposure to lower PM2.5 concentrations, 
there are also several new controlled human exposure studies that 
provide information about the health effects observed in study 
participants at concentrations well above the standard levels. 
Additionally, there are also epidemiologic studies now available and 
evaluated in other Agency peer-reviewed documents that can inform 
health effects associated with higher PM2.5 concentrations 
(U.S. EPA, 2021b).\137\ Thus, the EPA concluded that it is appropriate 
to reevaluate the upper AQI breakpoints, taking into account the 
expanded body of scientific evidence, particularly given several new 
epidemiologic studies conducted during high pollution events like 
wildfires and multiple controlled human exposure studies. While it 
remains unclear the exact PM2.5 concentrations at which 
specific health effects occur, the more recent studies do provide more 
refined information about the concentration range in which these 
effects might occur in some populations. These studies provide support 
for coherence of effects across scientific disciplines and potentially 
biologically plausible pathways for the overt population-level health 
effects observed in epidemiologic studies. Therefore, taking into 
account the short exposure time period in these studies (e.g., 1-6 
hours) and that the studies generally do not include at-risk (or 
sensitive) populations, but rather young, healthy adults, these 
studies, in conjunction with information from epidemiologic studies, 
the EPA preliminarily concluded it would be appropriate to be more 
cautionary and offer advisories to the public for reducing exposures at 
lower concentrations than recommended with the current AQI breakpoints.
---------------------------------------------------------------------------

    \137\ In this reconsideration, the controlled human exposure 
studies were evaluated in the 2019 ISA, whereas the epidemiologic 
studies of wildfire smoke exposures were included in the EPA 
Comparative Assessment of the Impacts of Prescribed Fire Versus 
Wildfire (CAIF): A Case Study in the Western U.S. (U.S. EPA 2021b).
---------------------------------------------------------------------------

    The AQI value of 200 is the breakpoint between the ``unhealthy'' 
and ``very unhealthy'' categories. At AQI values above 200, the AQI 
would be providing a health warning that the risk of anyone 
experiencing a health effect following short-term exposures to these 
PM2.5 concentrations has increased. To inform proposed 
decisions on this breakpoint, the EPA takes note of studies indicating 
the potential for respiratory or cardiovascular effects that are on 
their own representative of or are on the biologically plausible 
pathway to more serious health outcomes (e.g., emergency department 
visits, hospital admissions). The controlled human exposure studies 
evaluated in the 2009 and 2019 ISAs provide evidence of inflammation as 
well as cardiovascular effects in healthy subjects at and above 120 
[micro]g/m\3\. For example, Ramanathan et al. (2016) observed a 
transient reduction in antioxidant/anti-inflammatory function after 
exposing healthy young subjects to a mean concentration of 150 
[micro]g/m\3\ of PM2.5 for 2 hours. Urch et al.

[[Page 16304]]

(2010) also reported increased markers of inflammation when exposing 
both asthmatic and non-asthmatic subjects to a mean concentration of 
140 [micro]g/m\3\ of PM2.5 for 3 hours. In studies 
specifically examining cardiovascular effects, Ghio et al. (2000) and 
Ghio et al. (2003) exposed healthy subjects to a mean concentration of 
120 [micro]g/m\3\ for 2 hours and reported significantly increased 
levels of fibrinogen, a marker of coagulation that increases during 
inflammation. Sivagangabalan et al. (2011) exposed healthy subjects to 
a mean concentration of 150 [micro]g/m\3\ of PM2.5 for 2 
hours and noted an increased QT interval (3.4  1.4) 
indicating some evidence for conduction abnormalities, an indicator of 
possible arrhythmias. Lastly, Brook et al. (2009) reported a transient 
increase of 2.9 mm Hg in diastolic blood pressure in healthy subjects 
during the 2-hour exposure to a mean concentration of 148 [micro]g/m\3\ 
of PM2.5.
    In addition to epidemiologic studies evaluated in the 2019 ISA that 
analyzed exposures at ambient PM2.5 concentrations, there 
are a number of recent epidemiologic studies focusing on wildfire smoke 
that have become available that were evaluated in the EPA's recently 
released peer-reviewed assessment on wildland fire (U.S. EPA, 2021b). 
One of these studies, Hutchinson et al. (2018), conducted a 
bidirectional case-crossover analysis to examine associations between 
wildfire-specific PM2.5 exposure and respiratory-related 
healthcare encounters (i.e., ED visits, inpatient hospital admissions, 
and outpatient visits) prior and during the 2007 San Diego wildfires. 
This study found positive and significant associations to 
PM2.5 exposures and respiratory-related healthcare 
encounters. Further, during the initial 5-day period of the wildfire 
event, the study observed that there was evidence of increases in a 
number of respiratory-related outcomes particularly ED visits for 
asthma, upper respiratory infection, respiratory symptoms, acute 
bronchitis, and all respiratory-related visits (Hutchinson et al., 
2018). When examining the air quality during the wildfire event, 
PM2.5 concentrations were highest during the initial five 
days of the wildfire, with 24-hour average PM2.5 
concentrations of 89.1 [micro]g/m\3\ across all zip codes and with the 
highest 24-hour average of 160 [micro]g/m\3\ on the first day 
(Hutchinson et al., 2018).
    When considering this collective body of evidence from controlled 
human exposure and epidemiologic studies, the Agency proposed to set an 
AQI value of 200 at a daily (i.e., 24-hour average) concentration of 
PM2.5 of 125 [mu]g/m\3\. As discussed above and in the 
proposal (88 FR 5640, January 27, 2023), this concentration is at the 
lower end of the concentrations consistently shown to be associated 
with respiratory and cardiovascular effects in controlled human 
exposure studies following short-term exposures (e.g., 2-3 hours) and 
in young, healthy adults (Ghio et al., 2000; Ghio et al., 2003; Urch et 
al., 2010; Ramanathan et al., 2016; Sivagangabalan et al., 2011; and 
Brook et al., 2009) and also within the range of 5-day average and 
maximum concentrations observed to be associated with respiratory-
related outcomes following exposure to wildfire smoke (Hutchinson et 
al., 2018).
    The AQI value of 300 denotes the breakpoint between the ``very 
unhealthy'' and ``hazardous'' categories, and thus marks the beginning 
of the ``hazardous'' AQI category. At AQI values above 300, the AQI 
provides a health warning that everyone is likely to experience effects 
following short-term exposures to these PM2.5 
concentrations. To inform decisions on this AQI breakpoint, the EPA 
takes note of controlled human exposure studies that consistently show 
subclinical effects which are often associated with more severe 
cardiovascular outcomes. As discussed above, Brook et al. (2009) 
reported a transient increase of 2.9 mm Hg in diastolic blood pressure 
in healthy subjects during the 2-hour exposure to a mean concentration 
of 148 [micro]g/m\3\ of PM2.5. Bellavia et al. (2013) 
exposed healthy subjects to an average PM2.5 concentration 
of 242 [micro]g/m\3\ for 2 hours and reported increased systolic blood 
pressure (2.53 mm Hg). Tong et al. (2015) exposed healthy subjects to 
an average PM2.5 concentration of 253 [micro]g/m\3\ for 2 
hours and observed a significant increase in diastolic blood pressure 
(2.1 mm Hg) and a nonsignificant increase in systolic blood pressure 
(2.5 mm Hg). Lucking et al. (2011) reported impaired vascular function 
and increased potential for coagulation when exposing healthy subjects 
to diesel exhaust (DE) with an average PM2.5 concentration 
of 320 [micro]g/m\3\ for a duration of 1 hour.\138\ These studies all 
provided evidence of impaired vascular function, including 
vasodilatation impairment and increased thrombus formation, with Tong 
et al. (2015), Bellavia et al. (2013), Brook et al. (2009) all 
reporting increases in blood pressure. Additionally, Behbod et al. 
(2013) reported increased inflammatory markers following a 2-hour 
exposure to an average PM2.5 concentration of 250 [micro]g/
m\3\ in healthy subjects.
---------------------------------------------------------------------------

    \138\ Although participants in Lucking et al. (2011) were 
exposed to diesel exhaust (DE), the authors also conducted analyses 
using a particle trap, and as noted in the 2019 ISA, this type of 
study design allows for the assessment of the role of 
PM2.5 on the health effects observed by removing PM from 
the DE mixture.
---------------------------------------------------------------------------

    In addition to the controlled human exposure studies discussed 
above, the epidemiologic study conducted by DeFlorio-Barker et al. 
(2019) examined the relationship between wildfire smoke and 
cardiopulmonary hospitalizations among adults 65 years of age and older 
from 2008-2010 in 692 U.S. counties. The authors reported a 2.22% 
increase in all-cause respiratory hospitalizations on wildfire smoke 
days for a 10 [micro]g/m\3\ increase in 24-hour average 
PM2.5 concentrations (DeFlorio-Barker et al., 2019). The 
maximum 24-hour average concentration in this study on wildfire smoke 
days was 212.5 [micro]g/m\3\ (DeFlorio-Barker et al., 2019). In 
considering this study, the EPA notes the increased probability that 
even healthy adults experience effects at this maximum exposure 
concentration, particularly given that this maximum concentration is 
near the exposure concentrations in controlled human exposure studies 
that consistently reported evidence of impaired vascular function and 
several that reported increases in blood pressure in healthy adults 
following 2-hour exposures.
    Based on the information discussed above and in the proposal (88 FR 
5640, January 27, 2023), the EPA proposed to revise the 300 level of 
the AQI, which marks the beginning of the ``hazardous'' AQI category, 
to a concentration that is consistent with the PM2.5 
concentrations associated with health effects as reported in the 
controlled human exposure (Brook et al., 2009; Bellavia et al., 2013; 
Tong et al., 2015; Behbod et al., 2013) and epidemiologic studies 
(DeFlorio-Barker et al. (2019). Specifically, the Agency proposed to 
set an AQI value of 300 at a daily (i.e., 24-hour average) 
PM2.5 concentration of 225 [mu]g/m\3\. This concentration 
falls between the 2-hour average concentrations reported in controlled 
human exposure studies found to be consistently associated, in healthy 
adults, with impaired vascular function and/or increases in blood 
pressure, which could both be a precursor to more severe cardiovascular 
effects following short-term (1- to 2-hour) exposures, and the maximum 
24-hour average PM2.5 concentrations on wildfire smoke days 
reported in the epidemiologic study conducted by DeFlorio-Barker et al. 
(2019).

[[Page 16305]]

c. Air Quality Index Value of 500
    Lastly, the EPA also proposed revisions to the 500 value of the 
AQI. The 500 value of the AQI is within the ``hazardous'' category but 
is specified and used to calculate the slope of the AQI values in the 
``hazardous category'' above and below AQI values of 500. In the past, 
this breakpoint had a very prominent role in determining the current 
upper AQI values given that it was used as part of the linear 
relationship with the concentration at the AQI value of 100 to 
determine the AQI values of 200 and 300 in 1999 (64 FR 42530, August 4, 
1999).
    As discussed above and in the proposal (88 FR 5641, January 27, 
2023), the current breakpoint concentration for the 500 value of the 
AQI was set in 1999 at a 24-hour average PM2.5 concentration 
of 500 [mu]g/m\3\ and was based on studies conducted in London using 
the British Smoke method, which used a different particle size cutpoint 
and likely overestimated the PM2.5 concentration. In looking 
to improve upon that approach, the EPA considered several recent 
controlled human exposure studies that observe health effects that are 
on the biologically plausible pathway to more severe cardiovascular 
outcomes and note that these seem to follow exposures to high 
PM2.5 concentrations that are well above those typically 
observed in ambient air. More specifically, in controlled human 
exposure studies, Vieira et al. (2016a) and Vieira et al. (2016b) 
exposed healthy subjects and subjects with heart failure to diesel 
exhaust (DE) with a mean PM2.5 concentration of 325 
[micro]g/m\3\ for 21 minutes and reported decreased stroke volume, and 
increased arterial stiffness (an indicator of endothelial dysfunction) 
in both healthy and heart failure subjects.\139\ Also as summarized 
above and discussed in the proposal (88 FR 5641, January 27, 2023), 
Lucking et al. (2011) exposed healthy subjects to DE with a mean 
PM2.5 concentration of 320 [micro]g/m\3\ for 1 hour.\140\ 
Epidemiologic studies have linked the types of cardiovascular effects 
observed in these controlled human exposure studies with the 
exacerbation of ischemic heart disease (IHD) and heart failure as well 
as myocardial infarction (MI) and stroke.
---------------------------------------------------------------------------

    \139\ These effects were attenuated when the DE was filtered, to 
reduce PM2.5 concentrations, indicating the effects were 
likely associated with PM2.5 exposure.
    \140\ When applying a particle trap, PM2.5 
concentrations were reduced, and effects associated with 
cardiovascular function including impaired vascular function, as 
measured by vasodilatation and thrombus formation were attenuated 
indicating associations with PM2.5.
---------------------------------------------------------------------------

    In addition to the controlled human exposure studies discussed in 
the proposal (88 FR 5641, January 27, 2023) and summarized above, 
recent epidemiologic studies examining the relationship between 
concentrations of PM2.5 during wildfires and respiratory 
health also informed the proposed decisions on the concentration for 
the AQI value of 500. As discussed in the proposal (88 FR 5641, January 
27, 2023) and summarized earlier in this section, Hutchinson et al. 
(2018) reported increases in a number of respiratory-related ED visits 
for asthma, upper respiratory infection, respiratory symptoms, acute 
bronchitis, and all combined respiratory-related visits based on data 
from Medi-Cal claims for emergency department presentations, inpatient 
hospitalizations, and outpatient visits during the initial 5-day period 
of the 2007 San Diego fire. During the initial 5-day window, 
PM2.5 concentrations were found to be at their highest with 
the 95th percentile of 24-hour average concentrations of 333 [micro]g/
m\3\.
    Although studies of short-term (i.e., daily) exposures to wildfire 
smoke are more informative in considering alternative level for the AQI 
value of 500 since they mirror the 24-hour exposure timeframe, 
additional information from epidemiologic studies of longer-term 
exposures (i.e., over many weeks) during wildfire events can provide 
supporting information. As discussed in the proposal (88 FR 5641, 
January 27, 2023) and summarized here, Orr et al. (2020) conducted a 
longitudinal study that reported exposure to wildfire smoke from a 
multi-month fire resulted in reduced lung function in subsequent years 
and concluded that exposure to high PM2.5 concentrations 
during a multi-week fire event may lead to health consequences, such as 
declines in lung function. During the 2017 wildfire event (August 1 to 
September 19, 2017), Orr et al. (2020) reported that many days during 
the multi-month fire had PM2.5 concentrations above 300 
[micro]g/m\3\, resulting in a daily average PM2.5 
concentration of 220.9 [mu]g/m\3\ with a maximum PM2.5 
concentration of 638 [micro]g/m\3\.
    The controlled human exposure studies provide biological 
plausibility for results of epidemiologic studies that document 
increases in respiratory-related health care events during the 
wildfires. The collective evidence from controlled human exposure and 
epidemiologic studies, which includes decreases in stroke volume, 
increased arterial stiffness, impaired vascular function and 
respiratory-related healthcare encounters provide health-based evidence 
that informed the proposed decisions on the level of the AQI value of 
500. Given the concentrations observed in these studies, the Agency 
proposed to revise the AQI value of 500 to a level set at a daily 
(i.e., 24-hour average) PM2.5 concentration of 325 [mu]g/
m\3\. This concentration is at or below the lowest concentrations 
observed in the controlled human exposure studies associated with more 
severe effects discussed above and also at the low end of the daily 
concentrations observed in the epidemiologic studies conducted by 
Hutchinson et al. (2018) and Orr et al. (2020).
    Table 1 below summarizes the proposed breakpoints for the 
PM2.5 sub-index.

                                Table 1--Proposed Breakpoints for PM2.5 Sub-Index
----------------------------------------------------------------------------------------------------------------
                                                                                     Proposed breakpoints ([mu]g/
                            AQI category                              Index values      m\3\, 24-hour average)
----------------------------------------------------------------------------------------------------------------
Good...............................................................            0-50               0.0-(9.0-10.0)
Moderate...........................................................          51-100              (9.1-10.1)-35.4
Unhealthy for Sensitive Groups.....................................         101-150                    35.5-55.4
Unhealthy..........................................................         151-200                   55.5-125.4
Very Unhealthy.....................................................         201-300                  125.5-225.4

[[Page 16306]]

 
Hazardous \1\......................................................            301+                        225.5
----------------------------------------------------------------------------------------------------------------
\1\ AQI values between breakpoints are calculated using equation 1 in appendix G. For AQI values in the
  hazardous category, AQI values greater than 500 should be calculated using equation 1 and the PM2.5
  concentration specified for the AQI value of 500.

2. Summary of Significant Comments on Proposed Revisions
    The EPA received many comments on the proposed changes to the 
PM2.5 AQI breakpoints. Many commenters generally supported 
all the proposed revisions to the AQI breakpoints based on the 
revisions to the primary annual and daily PM2.5 standards 
and recent scientific evidence discussed in the proposal (88 FR 5558, 
January 27, 2023). However, we received specific comments on proposed 
revisions to the breakpoints in the lower end of the AQI, related to 
their linkage to the annual and daily PM2.5 standards, and 
proposed revisions to the breakpoints at the upper end of the AQI, 
based on EPA's interpretation of available health effects evidence.
a. Air Quality Index Values of 50, 100, and 150
    Some commenters agreed with using the historical approach of 
setting the 50, 100 and 150 breakpoints of the AQI to be consistent 
with the primary PM2.5 standards. Some cited the reason that 
this approach creates consistent communication with respect to air 
quality and the standards, and this is how the other AQI sub-indices 
are set. A few commenters disagreed with the historical approach and 
suggested instead that the 50 breakpoint of the AQI should not be 
revised at all, or that the 50 and 100 breakpoints of the AQI should be 
supported directly by health data similar to the basis for the proposed 
200, 300 and 500 breakpoints.
    The few commenters that disagreed with the historical approach of 
the 50 breakpoint of the AQI noted that setting a short-term breakpoint 
to annual standard was not logical since it is a long-term standard and 
not meant to be interpreted for short-term messaging with the AQI, in 
particular when reported hourly via the NowCast. These commenters also 
noted that additional studies are needed to identify the health impacts 
of short-term exposures at low concentrations. They also noted that 
lowering the 50 breakpoint of the AQI in conjunction with the annual 
standard may cause confusion with the public because some State 
programs and policy decisions are connected to the AQI while others are 
based on PM concentrations, which could lead to inconsistent messaging 
reducing the public's trust. These comments were supported by noting 
that revised breakpoints could lead to more moderate days than in the 
past, but the monitor values would be the same as before when the 
commenters considered it ``healthy,'' possibly eroding trust in air 
agencies' messaging. Commenters also noted if the breakpoints are 
revised, the public will not visually be able to detect the difference 
between what was considered a good AQI day versus a now moderate AQI 
day.
    The EPA disagrees with these commenters. With respect to setting a 
short-term breakpoint to the level of a much longer-term (annual) 
standard, setting the lower AQI breakpoints at the level of the annual 
and daily PM2.5 standards for communication purposes was 
discussed in the proposed reconsideration (88 FR 5558, January 27, 
2023) and previously supported by State organizations in the 2012 PM 
Final Rule (77 FR 38890, June 29, 2012). Both the AQI and the Pollutant 
Standards Index, which came before it, have historically been 
normalized across pollutants by defining an index value of 50 and 100 
as the numerical level of the annual (when defined) and short-term 
(i.e., averaging time of 24-hours or less) primary NAAQS for each 
pollutant. This approach clearly communicates the air quality to the 
public. The EPA considers this approach to be appropriate given the 
available evidence and structure of the standard. As discussed in 
section II.B above and in the notice of final rulemaking for the 2012 
review (77 FR 38890, June 29, 2012), the primary annual and 24-hour 
PM2.5 standards work together in concert to provide public 
health protection. The annual PM2.5 standard is generally 
viewed as the principal means of providing public health protection 
against ``typical'' daily and annual PM2.5 exposures, while 
the 24-hour PM2.5 standard is generally viewed as a means of 
providing protection against short-term exposures to ``peak'' 
PM2.5 concentrations, such as can occur in areas with strong 
contributions from local or seasonal sources, even when annual average 
PM2.5 concentrations remain relatively low. Because the 
annual standard provides public health protection for typical daily 
PM2.5 exposures, the EPA thinks it is appropriate to use 
that level for the 50 breakpoint of the AQI and describe daily air 
quality at and below the level of the annual standard ``Good.'' Since 
an annual standard allows for days with air quality above that level, 
it is appropriate to call days just above it ``Moderate.'' If the 50 
breakpoint of the AQI was set at a level above the annual standard, it 
would be possible for the majority of days to be called ``good'' in a 
year when an area exceeds the annual standard. This could cause 
confusion with the public about air quality if the general perception 
is that local air quality is ``good,'' but the area fails to meet the 
annual standard. In addition, the EPA continues to find it appropriate 
to use the NowCast with the PM2.5 AQI index to provide more 
real-time information to the public. As discussed in the AQI Technical 
Assistance Document, while the NowCast algorithm is approximating a 24-
hour average exposure, it can reflect concentrations observed over 
shorter averaging times when air quality is changing rapidly (U.S. EPA, 
2018a). The EPA continues to consider the use of the primary annual 
standard level suitable in the NowCast given the health evidence 
supporting the standard and given that the reported concentrations are 
an approximation of ``typical'' daily exposure. Additionally, the EPA 
reflects the nature of the NowCast in the associated health messaging.
    With regard to the commenter stating the public may not be able to 
visually detect a difference in the air quality, the EPA notes that the 
AQI is intended to be a communication tool for public awareness 
precisely because it is generally difficult for the public to visually 
judge air quality risks when air pollution is ``moderate.'' Moreover, 
since the establishment of the AQI, the EPA and State and local air 
agencies and organizations have developed experience in educating the 
public about changes in the standards and,

[[Page 16307]]

concurrently, related changes to AQI breakpoints and advisories. When 
the standards change, the EPA and State and local agencies have sought 
to help the public understand that air quality is not getting worse, 
it's that the health evidence underlying the standards and the AQI has 
changed. The EPA's Air Quality System (AQS), the primary repository for 
air quality monitoring data, is also adjusted to reflect the revised 
breakpoints. Specifically, all historical AQI values in AQS are 
recomputed with the revised breakpoints, so that all data queries and 
reports downstream of AQS will show appropriate trends in AQI values 
over time. If any State, local or Tribal air agency is concerned that 
people are or will be confused on a moderate AQI day, then they could 
use the communication information that has been developed with this 
rulemaking.
    Some commenters stated that the AQI should not necessarily be 
linked to the primary PM2.5 standards. One example is the 
comment that if the annual standard is not lowered to 8 [micro]g/m\3\, 
the EPA should lower the 50 breakpoint of the AQI to that level to 
better inform the public of the need for behavioral modifications to 
reduce the harm to health from PM2.5 exposure. Similar to 
the reasons discussed above, the EPA concludes that setting the 50 
breakpoint of the AQI at the level of the annual PM2.5 
standard is appropriate from a health perspective and for communication 
purposes. The Administrator has judged the primary annual standard (in 
conjunction with the other primary standards) as revised in this final 
action to be requisite to protect public health with an adequate margin 
of safety, based on the health evidence discussed in section II.A.2. 
Setting the 50 breakpoint lower than the annual standard also has the 
potential to cause confusion with the public since it does not reflect 
the standards and the Administrator's judgments about the standards as 
well.
    With regard to the 100 breakpoint of the AQI, several commenters 
expressed the view that the level of the 24-hour PM2.5 
standard and an AQI value of 100 should be set at 25 [mu]g/m\3\ based 
on the body of evidence and lower end of the range recommended by 
CASAC. These commenters noted that if the current 24-hour standard and 
AQI value of 100 is retained at 35 [mu]g/m\3\ then the public will not 
be able to make informed decisions about actions to take to protect 
their health. Many of these commenters further recommended that the AQI 
value of 100 should be lowered to 25 [mu]g/m\3\ even if the standard is 
retained. Commenters expressed the view that this would more adequately 
allow the public to take health-protective actions.
    The EPA disagrees with these commenters and notes that many State, 
Tribal and local air agencies have expressed strong support for 
aligning the 100 breakpoint of the AQI with the short-term 24-hour 
primary PM2.5 standards as discussed in the proposal (88 FR 
5558, January 27, 2023). The EPA agrees with the view, expressed by 
State, local and Tribal entities, that aligning the lower breakpoints 
with the standards enables clear communication of the standards. This 
alignment approach is also utilized in the other AQI sub-indices lower 
breakpoints and taking a different approach with the PM2.5 
AQI could cause confusion. Additionally, the Administrator has judged 
that it is appropriate to retain the 24-hour standard at a level of 35 
[mu]g/m\3\ (in conjunction with the other primary standards) to protect 
public health with an adequate margin of safety, based on the health 
evidence discussed in section II.A.2. Thus, EPA disagrees that it is 
necessary or appropriate to set the 100 breakpoint at a lower 
concentration to provide further information to the public. The 50 
breakpoint, which is set at a level below 25 [mu]g/m\3\, will continue 
to provide information to members of the public particularly concerned 
about exposures to PM2.5. As with the 50 breakpoint, 
aligning the breakpoint with the standard both reflects the 
Administrator's judgment about the health risks and eliminates the 
potential to cause confusion in the public about those risks.
b. Air Quality Index Values of 200 and Above
    Some commenters supported the proposed revisions to the 200, 300 
and 500 breakpoints that recognize the expanded body of scientific 
evidence, particularly several new epidemiologic studies conducted 
during high pollution events such as wildfires and multiple controlled 
human exposure studies. A few commenters agreed with incorporating the 
expanded body of scientific evidence into the 200, 300 and 500 
breakpoints, but suggested a modified linear approach between 200 (115 
[mu]g/m\3\) and 500 (312 [mu]g/m\3\, setting the 300 breakpoint to 187 
[mu]g/m\3\) based on recent epidemiologic wildfire smoke studies.
    Other commenters disagreed with the proposed revisions and 
suggested the EPA should continue using the previous breakpoints that 
follow the 1999 linear approach (64 FR 42530, August 4, 1999), because 
not changing the breakpoints would simplify communications. A few 
commenters stated the proposed revisions to the AQI upper breakpoints 
are not justified because the scientific evidence supporting the 
revisions is inadequate. To support this view, the commenters suggest 
that only three epidemiologic studies were used in determining the 
upper breakpoints and none of them were representative of potential 
effects in the general public; of the 13 studies cited only three were 
near the proposed revised breakpoints; four of the studies involved 
exposure to PM from diesel and traffic pollution, which is different 
than PM from wildfire smoke; and the data supporting the revisions only 
indicated ``mild'' health effects that were mostly in sensitive 
populations.
    The EPA agrees with the majority of commenters that supported 
utilizing the expanded body of scientific evidence to revise the 200, 
300 and 500 breakpoints of the AQI. The EPA appreciates the suggestion 
of using a revised linear approach from 200 to 500. But rather than 
using the available evidence to only set the breakpoint of 500, the EPA 
finds it appropriate to set the breakpoints for 200, 300 and 500 using 
an evidence-based approach, by relying on information presented in both 
controlled human exposure studies and epidemiologic studies that 
examine relationships between high PM2.5 exposure episodes 
(i.e., periods of wildfire smoke) and various health outcomes. Setting 
these breakpoints based directly on health effects evidence, which can 
be communicated, is more useful and appropriate than using a linear 
approach, because it can better describe the potential health effects 
and symptoms which also helps the public better understand why more 
health protective actions are needed. By its nature, a linear approach 
does not evaluate and identify associated health effects and risk 
factors.
    The EPA disagrees with the commenters that expressed the view that 
these upper breakpoints should not be revised based largely on the 
numerous peer-reviewed studies published since the 200, 300 and 500 
breakpoints were originally established in 1999 (64 FR 42530, August 4, 
1999). As discussed in the proposal (88 FR 5641, January 27, 2023), the 
rationale behind the proposed revisions is rooted in the fact the upper 
AQI breakpoints are based on outdated scientific evidence. 
Specifically, the traditional linear approach was predicated on the 500 
value of the AQI, which was estimated using health studies that used 
the British Smoke Method. The British Smoke Method is based on a 
particle size fraction (4.5 microns) that is larger

[[Page 16308]]

than PM2.5. Given that the British Smoke method has a larger 
particle size cutpoint than the current PM2.5 monitoring 
method, which has a cutpoint of 2.5 microns, a concentration of 500 
[mu]g/m\3\ based on the British Smoke method would be equivalent to a 
lower PM2.5 concentration (88 FR 5641, January 27, 2023). 
The combination of a larger particle size fraction informing previous 
decisions around upper AQI breakpoints and more recent scientific 
evidence than the London Fog Episode, on the potential health 
consequences of what we currently consider to be high PM2.5 
exposures, provides the underlying basis for revising the upper 
breakpoints to better inform the public about air quality to allow the 
public to take health protective actions as appropriate. Moreover, as 
discussed above, until recently there was limited information upon 
which to base the breakpoints between 150 and 500, so the linear 
approach was a reasonable substitute. While not changing the 
breakpoints may be easier because there is no change to communicate, 
using a health-based approach is more appropriate, because it helps the 
public better understand that more health protective actions are 
needed.
    The Agency disagrees that the scientific evidence discussed in the 
proposal is inadequate to revise the 200, 300 and 500 breakpoints of 
the AQI (88 FR 5640, January 27, 2023). The EPA disagrees that these 
studies should not be considered because they ``indicated mild health 
effects in sensitive populations.'' The EPA notes that many of the 
subclinical effects discussed in the proposal (88 FR 5640, January 27, 
2023) that informed the breakpoints are on the biologically plausible 
pathway (see 2019 ISA, section 6.1.1 and Figure 6-1) to more severe 
cardiovascular outcomes, such as ED visits, hospital admissions, and 
death as depicted in the large number of epidemiologic studies 
evaluated in the 2019 ISA and ISA Supplement. From a public health 
perspective, the purpose of the AQI is to inform the public when air 
quality could adversely affect their health. The scientific evidence 
informed revisions to the breakpoints at the upper end of the AQI allow 
it to better reflect the risk of experiencing health effects at higher 
PM2.5 concentrations. In addition, the EPA disagrees with 
the commenter that the effects reported at these higher concentrations 
were observed only in sensitive populations as these effects were also 
reported in healthy populations (Ghio et al., 2000; Ghio et al., 2003; 
Urch et al., 2010; Ramanathan et al., 2016; Sivagangabalan et al., 
2011; Brook et al., 2009; Bellavia et al. (2013); Tong et al. (2015); 
Behbod et al. (2013); Vieira et al. (2016a) Vieira et al. (2016b); and 
Lucking et al. (2011)).
c. Other Comments
    The EPA received a few additional comments on elements of the 
PM2.5 AQI, including the averaging time. Some commenters 
expressed the view that the 24-hour averaging time was not useful when 
informing the public how to protect their health, particularly during 
rapidly changing conditions such as wildfire smoke events. Instead, 
they suggested a subdaily averaging time of 1-3 hours would be more 
effective because it more closely aligns with how people breathe.
    A few of these commenters suggested that instead of changing the 
AQI averaging time, which aligns with the short-term standard, the EPA 
could create a public health warning system for unhealthy 
PM2.5 levels. The commenters noted that aligning the AQI 
averaging time with the short-term standard could be useful for 
consistent communication with the standards and attainment but 
suggested that a subdaily warning system could better allow the public 
to take health protective actions.
    The EPA disagrees that a shorter averaging period for the 
PM2.5 AQI sub-index would be better. The health effects 
evidence supporting a subdaily metric is limited and inconsistent. As 
part of its review of the health effects evidence, the 2019 ISA 
evaluated whether a subdaily metric would be more closely related to 
health effects. Most epidemiologic studies that examined the 
relationship between short-term PM2.5 exposures and health 
effects evaluated an exposure metric averaged over 24-hours. Some 
recent studies, focusing on respiratory and cardiovascular effects and 
mortality, have examined whether there is evidence that subdaily 
exposure metrics are more closely related to health effects than a 
traditional 24-hour average metric. After evaluating this limited newer 
evidence, the 2019 ISA concluded that ``collectively, the available 
evidence does not indicate that subdaily averaging periods for 
PM2.5 are more closely associated with health effects than 
the 24-hour avg exposure metric,'' (2019 ISA, chapter 1, section 
1.5.2.1, pp. 146-147; U.S. EPA, 2022a).
    In addition, there are communication benefits to aligning the 
averaging time of the AQI with the daily standard, as some of these 
commenters note, such as providing consistent messages about when it 
may be beneficial for people to take actions to reduce PM2.5 
exposures. Furthermore, with regard to an additional warning system, 
the EPA is concerned that having two air quality communication systems 
operating at the same time would likely be confusing to the public and 
reduce the effectiveness of the systems.
    At the same time, the EPA recognizes that when air quality is 
rapidly changing, such as during wildfire smoke events, reporting 
information based on a 24-hour metric may not be as useful for the 
public as reporting more frequently would be. The EPA has balanced 
concerns about being able to provide timely communication of air 
quality hazards when conditions are changing quickly with the goal of 
limiting the number of air quality communications systems and its 
judgment that the evidence supports a 24-hour-based metric linked to 
the daily standard by establishing the NowCast, which takes into 
consideration subdaily PM2.5 concentrations and provides a 
near real-time AQI value based on the AQI colors and scale. 
Specifically, the NowCast shows air quality conditions for the most 
current hour of PM2.5 data available by using a calculation 
that involves multiple hours of past data. As noted in the AQI 
Technical Assistance Document, the NowCast currently uses longer 
averages during periods of stable air quality and shorter averages 
(down to a 3-hour average) when air quality is changing rapidly, such 
as during a wildfire (U.S. EPA, 2018a). As discussed further in section 
IV.D.2 of this notice, the EPA uses the NowCast to approximate the 
complete daily AQI (24-hour average) during any given hour. This means 
the subdaily NowCast is approximating a 24-hour average exposure, which 
aligns with the health evidence and the existing AQI communications 
network, while also being capable of communicating rapidly changing 
conditions to the public.
3. Summary of Final Revisions
    Upon reviewing and considering the comments on the proposed 
revisions (summarized above in Section IV.C) along with the scientific 
evidence outlined in the proposal (88 FR 5639, January 27, 2023) and 
summarized above in section IV.A, the EPA is finalizing the proposed 
changes to the AQI.
    Thus, as discussed in section IV of the preamble (88 FR 5639, 
January 27, 2023) to the proposed rule, the EPA is taking final action 
to revise the AQI value of 50 to 9.0 [mu]g/m\3\, 24-hour average, 
consistent with the final decision on the primary annual 
PM2.5 standard level as summarized in section II.C of the

[[Page 16309]]

preamble to the final rule; retain the AQI value of 100 at 35 [mu]g/
m\3\, 24-hour average, consistent with the final decision on the 
primary 24-hour PM2.5 standard level as summarized in 
section II.C of the preamble to the final rule; and retain the AQI 
value of 150 at 55 [mu]g/m\3\, 24-hour average. The EPA is also taking 
action to revise the AQI value of 200 to 125 [mu]g/m\3\, 24-hour 
average; 300 to 225 [mu]g/m\3\, 24-hour average; and 500 to 325 [mu]g/
m\3\, 24-hour average, consistent with the rationale discussed above 
and the health evidence discussed in section IV of the preamble (88 FR 
5639, January 27, 2023) to the proposed rule. The EPA has prepared 
communications materials to assist States with adjusting to the revised 
AQI and looks forward to working with, and learning from the 
experiences of, State, local, and Tribal governments in implementing 
these changes.

C. Air Quality Index Category Breakpoints for PM10

    The EPA proposed to retain the PM10 sub-index of the AQI 
consistent with the proposed decision to retain the primary 
PM10 standard, and consistent with the health effects 
information that supports this proposed decision, as discussed in 
section III.D of the proposal (88 FR 5632, January 27, 2023). EPA did 
not receive comments on this and is taking final action to retain the 
PM10 sub-index of the AQI for the reasons stated in the 
preamble to the proposed rule (88 FR 5642, January 27, 2023).

D. Air Quality Index Reporting

    With respect to the reporting requirements for the AQI and as noted 
in the proposal (88 FR 5642, January 27, 2023) there have been many 
technological advances in air quality monitoring and data reporting 
since the appendix G to 40 CFR part 58 was last revised in 1999. 
Federal, State, local, and Tribal agencies have used these changes to 
make health information and air quality data more readily available and 
easier to access. Given this, it is useful to update the reporting 
requirements and recommendations to match current practices and ensure 
the public has the most useful and timely information to take health-
protective behaviors.
1. Summary of Proposed Revisions
    Currently, appendix G defines daily reporting as five days per 
week. When this reporting requirement was originated in 1999 the 
technology available at that time was not sufficient to calculate and 
report the AQI more than five days per week without requiring 
additional staffing on the weekends. Since that time, advances in 
technology have allowed for reporting seven days per week automatically 
without expending additional resources on weekends. As a result, most 
State, local, and Tribal air agencies now report the AQI seven days per 
a week. Given these technological advances and noting that reporting 
agencies currently report the AQI seven days per week, the EPA proposed 
that State, local, and Tribal agencies that report the AQI be required 
to report it seven days a week, ensuring that the members of the public 
continue to have access to daily air quality and health information 
that they can use to take steps to protect their health.
    Improvements in monitoring networks and modeling capabilities have 
also enabled the ability to report the AQI in near real-time. This 
allows State, local, and Tribal air agencies to provide timely air 
quality information to the public for making health-protective 
decisions and to help satisfy AQI reporting requirements. The 
availability of near real-time AQI data also allows for more timely 
responses by the public when air quality conditions are changing 
rapidly, such as during wildfire smoke events. Subdaily reporting of 
the AQI can be critical when there are rapidly change conditions and/or 
high pollution events so that the public is able to make informed 
decisions to protect their health. Many State, local, and Tribal air 
agencies currently report the AQI hourly to ensure that the public has 
access to accurate and timely information. In recognition of these 
advances, and to continue to provide for near-real time AQI reporting 
that the public has come to rely on, the EPA proposed to recommend that 
State, local, and Tribal agencies report the AQI in near-real time.
    In lieu of or along with reporting the near-real-time AQI directly 
to the public, most State/local and Tribal agencies submit hourly air 
quality data to the EPA. The EPA and some State, local and Tribal air 
quality agencies use this near-real-time data to create products for 
use by the public, weather service providers and the media as discussed 
in the proposal (88 FR 5643, January 27, 2023). To continue to ensure 
the availability of the products that the public and many stakeholders 
rely upon, the EPA proposed to recommend that State, local, and Tribal 
air quality agencies submit hourly data to the EPA's air quality 
database. Submitting hourly data to the EPA for use on the AirNow 
website and in other products also enables State, local, and Tribal air 
quality agencies to meet the recommendation to report the AQI in near-
real-time.
    In addition to the proposed updates to the reporting requirements 
and recommendations for near-real-time reporting and data submission 
recommendations, the Agency also proposed reformatting the question-
and-answer format used in appendix G to align with the current standard 
formatting used in the Code of Federal Regulations. In proposing to 
update the format, the EPA did not reopen the language that has merely 
been moved or rearranged as there are no substantive changes.
    Another change the EPA proposed to make to appendix G is with 
regard to Table 2--Breakpoints for the AQI for purposes of clarity. As 
discussed in the proposal (88 FR 5642, January 27, 2023) and summarized 
here, the EPA proposed to collapse the two rows presented for the 
Hazardous Category into one. The two rows in the current table specify 
pollutant concentrations for two AQI ranges within the Hazardous 
category (301-400 and 401-500), with an intermediate break at 400. The 
400 breakpoint for all criteria pollutants in the current Table 2 is 
set at the proportional pollutant concentration approximately halfway 
between the Index values of 300 and 500. In proposing updated AQI 
breakpoints for PM2.5, the EPA considered adjusting the 400 
breakpoint similarly. However, the EPA concluded that collapsing the 
two rows into a single range (301-500) would provide a more transparent 
and easy-to-follow presentation of the pollutant concentrations 
corresponding to the AQI range for the Hazardous category. Moreover, 
collapsing the Hazardous category into a single row in Table 2 has no 
substantive effect on the Emergency Episode program in 40 CFR part 51, 
appendix L. Thus, the EPA proposed to remove the breakpoint of 400 from 
the table in appendix G but this change would not substantively affect 
the derivation of the AQI for any pollutant.
    In addition, the EPA proposed to move some information currently in 
appendix G into the Technical Assistance Document for the Reporting of 
Daily Air Quality, or TAD (U.S. EPA, 2018a), so that it can be updated 
in a more timely manner to reflect current scientific and health 
effects evidence and current communication methods, thereby assisting 
State, local, and Tribal agencies in providing accurate and timely 
information to the public. Information that was proposed to be moved 
from appendix G to the TAD included the definitions of the sensitive 
(at-risk) populations for each pollutant.

[[Page 16310]]

This definition is typically evaluated and updated, as warranted, in 
most NAAQS reviews, even if the standard is not revised. Generally, if 
the standard is not revised in a review of the NAAQS, then appendix G 
is also not revised. Moving the definitions of sensitive groups to the 
TAD allows them to be updated even when a NAAQS is not revised to be 
consistent with the definitions of the sensitive (at-risk) populations 
identified in the ISA for that NAAQS review. Also, the proposal (88 FR 
5642, January 27, 2023) recognized that the ways that air quality and 
health information is supplied to the news media and public changes 
regularly and thus proposed that information about suggested approaches 
for public communication be taken out of appendix G and discussed in 
the TAD.
2. Summary of Significant Comments on the Proposed Revisions
    The EPA received many comments on the proposed changes to AQI 
reporting, many of which supported the proposed revisions. EPA 
discusses several of the topics that received the most attention from 
commenters below. Discussion of other comments received on the proposed 
changes to the AQI can be found in section IV of the Responses to 
Significant Comments on the 2023 Proposed Reconsideration of the 
National Ambient Air Quality Standards for Particulate Matter.
    Most commenters expressed support for revising the definition of 
``daily reporting'' from five days a week to seven days a week. A 
commenter did not support this change and recommended the EPA maintain 
the definition of daily as five days per week, noting that State and 
local air agencies do not routinely work seven days per week and would 
not be available to perform quality control of this data and report it 
reliably on weekends.
    The EPA appreciates the support for this proposed revision and 
disagrees that the proposed change would require personnel to perform 
quality control of AQI data on weekends. 40 CFR part 58 Appendix D 
defines continuous monitoring requirements for agencies participating 
in the State/Local Air Monitoring Stations (SLAMS) network, and 
Appendix G states that agencies `` . . . must use concentration data 
from State/Local Air Monitoring Stations (SLAMS) required by 40 CFR 
58.10'' when reporting the AQI. Therefore, as noted in Appendix D and 
G, Agencies are required to report the AQI using monitors within SLAMS, 
which are not subject to daily quality control/validation.
    A few commenters noted that the proposal preamble language 
mentioned AQI is reported three ways (88 FR 5637, 5638, January 27, 
2023): ``The AQI is reported three ways all of which are useful and 
complementary. The daily AQI is reported for the previous day and used 
to observe trends in community air quality, the AQI forecast helps 
people plan their outdoor activities for the next day, and the near-
real-time AQI, or NowCast AQI, tells people whether it is a good time 
for outdoor activity.'' These commenters suggested that the NowCast is 
being codified in 40 CFR part 58 Appendix G as a method of calculating 
the AQI, which they oppose, saying that codifying its use is 
inappropriate given the shortest averaging period of the 
PM2.5 NAAQS remains at 24-hours. Some stated that NowCast 
values have no direct correlation to the AQI calculation methodology 
codified in 40 CFR part 58 Appendix G. These commenters say that 
codifying the NowCast would impose a significant burden on States' 
forecasting staff.
    However, some other commenters noted they appreciate the public-
friendly format and near real-time data the NowCast provides and use it 
in their clinical encounters with patients. One air agency recognized 
the importance of the NowCast near real-time AQI during high pollution 
events and suggested the EPA should provide more ``concrete'' health 
messaging for these short-term spikes.
    The EPA disagrees that the preamble language proposed to codify the 
NowCast or to impose a burden on reporting agencies. The preamble to 
the proposed rule references the AQI being reported in three ways and 
it does so because the EPA and many State, local and Tribal air quality 
agencies already report it these three ways. However, text included in 
the preamble is generally explanatory and does not alter regulatory 
provisions. Comments that State that EPA is codifying the NowCast into 
Appendix G are incorrect. Further, in proposed revisions to 40 CFR part 
58 Appendix G, the EPA recommended, but did not propose to require, the 
use of air quality forecasts and a subdaily AQI. Consistent with the 
proposal, the EPA is therefore not finalizing any additional 
requirement or burden on States' forecasting staff relative to 
forecasts or a subdaily AQI.
    The EPA disagrees with the comment that the NowCast values have no 
direct correlation to the AQI calculation methodology codified in 40 
CFR part 58 Appendix G. As noted in the AQI Technical Assistance 
Document (Technical Assistance Document for the Reporting of Daily Air 
Quality--the Air Quality Index (AQI)), the NowCast algorithm is based 
on the AQI methodology but provides more real-time information to the 
public (U.S. EPA, 2018a). While the NowCast algorithm is approximating 
a 24-hour average exposure, it can reflect concentrations observed over 
shorter averaging times when air quality is changing rapidly (U.S. EPA, 
2018a). The EPA reflects the nature of the NowCast in the health 
messaging provided there.
    As noted in the above discussion of the AQI, air quality can change 
quickly during the day. A central purpose of the AQI is to help the 
public know when it is prudent to take action to reduce their exposure 
to pollution. Accordingly, the EPA developed the NowCast to estimate 
the 24-hour AQI for the current hour to give people information and 
tools to reduce their exposures to protect their health, particularly 
when air quality may be changing. The NowCast gives people the 
knowledge and ability to take timely action. They can use this 
information to reduce their exposure--reducing exposures if 
PM2.5 is high only during a few hours a day will help reduce 
a person's 24-hour exposure--or be active when air quality is better.
    The first NowCast method was developed in 2003 and was designed so 
``current conditions'' represent the 24-hour PM2.5 standard 
as closely as possible. This method proved to be slow to respond during 
rapid air quality changes. In 2013, the EPA developed an updated 
NowCast method for PM2.5 \141\ that responds more quickly to 
rapidly changing air quality conditions, such as those we see during 
wildfires, to make air quality alerts more timely. We analyzed millions 
of data points in developing this NowCast method and presented this 
information to State, local and Tribal air agencies. The updated 
NowCast, which is still in use, was launched August 1, 2013, on 
AirNow.gov. It was designed to represent a shorter average (target 3-
hour) when air quality is changing rapidly, in part because 3-hour 
averages from some continuous monitors are more stable than 1-hour 
averages. The NowCast reflects a longer-term (12-hour) average when air 
quality is stable.
---------------------------------------------------------------------------

    \141\ U.S. EPA. (2013). Transitioning to a New NowCast Method. 
Presentation available in the Rulemaking Docket for the Review of 
the National Ambient Air Quality Standards for Particulate Matter 
(EPA-HQ-OAR-2015-0072), at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    After evaluating the 2013 NowCast method, the EPA concluded that it 
matched the desired characteristics. The NowCast method responds to 
rapid changes in air quality yet still reflects a

[[Page 16311]]

longer-term average when air quality is stable; will work in any 
location with adequate air quality data and for any air quality 
situation; gives people the best possible estimate of a 24-hour 
exposure; allows the EPA to caution people in time for them to take 
protective action and reduce their 24-hour exposure; and ensures that 
AQI maps on AirNow more closely match what people see.
    The AQI is designed to allow people to reduce their exposure when 
pollution levels are higher and be active outdoors when pollution 
levels are lower. Since air quality almost always changes during the 
day, that level of granularity is not possible with a 24-hour forecast. 
If the public has only the 24-hour forecast, they may miss the times to 
be active outdoors when air quality is better and may be active 
outdoors when air quality is worse.
    Also as noted above, many entities appreciate the near real-time 
reporting of the AQI that the NowCast provides and suggested more 
specific messaging is needed. The EPA appreciates this insight and will 
continue to consider ways to communicate air quality information most 
effectively to the public. For example, in light of recent wildfire 
events, the EPA worked with the USFS to pilot the AirNow Fire and Smoke 
Map.
3. Summary of Final Revisions
    Upon reviewing and considering the comments on the proposed 
revisions (summarized above in Section IV.C) along with the rationale 
outlined in the proposal (88 FR 5638, January 27, 2023) and summarized 
above in section IV.C, the EPA is finalizing the proposed changes to 
the AQI reporting requirements. Thus, as discussed in section IV of the 
preamble to the proposed rule, the EPA is taking final action to 
require the AQI be reported seven days a week; recommend that State, 
local, and Tribal agencies report the AQI in near-real time; recommend 
that State, local, and Tribal air quality agencies submit hourly data 
to the EPA's air quality database; reformat appendix G to align with 
the current standard formatting used in the Code of Federal 
Regulations; collapse the two rows in Table 2 presented for the 
Hazardous Category into one by removing the 400 breakpoint; and move 
some information currently in appendix G into the Technical Assistance 
Document for the Reporting of Daily Air Quality, or TAD (U.S. EPA, 
2018a) such as including the definitions of the sensitive (at-risk) 
populations for each pollutant and suggested approaches for public 
communication as stated in the revised Appendix G.
    Table 2 below summarizes the breakpoints for the PM2.5 
sub-index.

                                    Table 2--Breakpoints for PM2.5 Sub-Index
----------------------------------------------------------------------------------------------------------------
                                                                                      Breakpoints  ([mu]g/m\3\,
                            AQI category                              Index values         24-hour average)
----------------------------------------------------------------------------------------------------------------
Good...............................................................            0-50                      0.0-9.0
Moderate...........................................................          51-100                     9.1-35.4
Unhealthy for Sensitive Groups.....................................         101-150                    35.5-55.4
Unhealthy..........................................................         151-200                   55.5-125.4
Very Unhealthy.....................................................         201-300                  125.5-225.4
Hazardous \1\......................................................            301+                        225.5
----------------------------------------------------------------------------------------------------------------
\1\ AQI values between breakpoints are calculated using equation 1 in appendix G. For AQI values in the
  hazardous category, AQI values greater than 500 should be calculated using equation 1 and the PM2.5
  concentration specified for the AQI value of 500.

V. Rationale for Decisions on the Secondary PM Standards

    This section presents the rationale for the Administrator's 
decision that no change to the current secondary PM standards is 
required at this time to provide requisite protection against the 
public welfare effects of PM within the scope of this reconsideration 
(i.e., visibility, climate, and materials effects).\142\ This decision 
is based on a thorough review of the scientific evidence generally 
published through December 2017,\143\ as presented in the 2019 ISA 
(U.S. EPA, 2019a), on the non-ecological public welfare effects of PM 
pertaining to the presence of PM in ambient air, specifically 
visibility, climate, and materials effects. Additionally, this decision 
is based on a thorough evaluation of some studies that became available 
after the literature cutoff date of the 2019 ISA that could either 
further inform the adequacy of the current PM NAAQS or address key 
scientific topics that have evolved since the literature cutoff date 
for the 2019 ISA, generally through March 2021, as presented in the ISA 
Supplement \144\ (U.S. EPA, 2022a). The selection of welfare effects 
evaluated within the ISA Supplement was based on the causality 
determinations reported in the 2019 ISA and the subsequent use of 
scientific evidence in the 2020 PA.\145\

[[Page 16312]]

Specifically, for welfare effects, the focus within the ISA Supplement 
is on visibility effects. The ISA Supplement does not include an 
evaluation of studies on climate or materials effects. The 
Administrator's decision also takes into account the 2022 PA evaluation 
of the policy-relevant information in the 2019 ISA and ISA Supplement 
and presentation of quantitative analysis of air quality related to 
visibility impairment; CASAC advice and recommendations, as reflected 
in discussions of the drafts of the ISA Supplement and 2022 PA at 
public meetings and in the CASAC's letters to the Administrator; and 
public comments received on the proposal.
---------------------------------------------------------------------------

    \142\ Consistent with the 2016 Integrated Review Plan (U.S. EPA, 
2016), other welfare effects of PM, including ecological effects, 
are being considered in the separate, on-going review of the 
secondary NAAQS for oxides of nitrogen, oxides of sulfur and PM. 
Accordingly, the public welfare protection provided by the secondary 
PM standards against ecological effects such as those related to 
deposition of nitrogen- and sulfur-containing compounds in 
vulnerable ecosystems is being considered in that separate review. 
Thus, the Administrator's decision in this reconsideration will be 
focused only and specifically on the adequacy of public welfare 
protection provided by the secondary PM standards from effects 
related to visibility, climate, and materials and hereafter 
``welfare effects'' refers to non-ecological welfare effects (i.e., 
visibility, climate, and materials effects).
    \143\ In addition to the 2020 review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), the 2019 ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009 through approximately January 
2018 (U.S. EPA, 2019a, p. ES-2). References that are cited in the 
2019 ISA, the references that were considered for inclusion but not 
cited, and electronic links to bibliographic information and 
abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
    \144\ As described in more detail in the ISA Supplement, ``the 
scope of this Supplement provides specific criteria for the types of 
studies considered for inclusion within the Supplement. 
Specifically, studies must be peer reviewed and published between 
approximately January 2018 and March 2021'' (U.S. EPA, 2022a, 
section 1.2.2).
    \145\ As described in section 1.2.1 of the ISA Supplement, ``the 
selection of welfare effects to evaluate within this Supplement is 
based on the causality determinations reported in the 2019 PM ISA 
and the subsequent use of scientific evidence in the 2020 PM PA. The 
2019 PM ISA concluded a causal relationship for each of the welfare 
effects categories evaluated (i.e., visibility, climate effects, and 
materials effects). While the 2020 PM PA considered the broader set 
of evidence for these effects, for climate effects and material 
effects, it concluded that there remained `substantial uncertainties 
with regard to the quantitative relationships with PM concentrations 
and concentration patterns that limit[ed] [the] ability to 
quantitatively assess the public welfare protection provided by the 
standards from these effects (U.S. EPA, 2020b). Given these 
uncertainties and limitations, the basis of the discussion on 
conclusions regarding the secondary standards in the 2020 PM PA 
primarily focused on visibility effects. Therefore, this Supplement 
focuses only on visibility effects in evaluating newly available 
scientific information and is limited to studies conducted in the 
U.S. and Canada'' (U.S. EPA, 2022a, section 1.2.1).
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's final decision 
and its foundations, section V.A provides background on the 2020 final 
decision to retain the secondary PM standards (section V.A.1), and also 
provides brief summaries of key aspects of the currently available 
welfare effects evidence (section V.A.2) and quantitative information 
(section V.A.3). Section V.B summarizes the CASAC's advice (section 
V.B.1) and the proposed conclusions (section V.B.2), addresses public 
comments received on the proposal (section V.B.3), and presents the 
Administrator's conclusions on the adequacy of the current standards 
(section V.B.4), drawing on consideration of the available scientific 
and quantitative information, advice from the CASAC, and comments from 
the public. Section V.C summarizes the Administrator's decision on the 
secondary PM standards.

A. Introduction

    The general approach for this reconsideration of the 2020 final 
decision on the secondary PM standards relies on the EPA's assessments 
of the current scientific evidence and associated quantitative analyses 
to inform the Administrator's judgments regarding secondary standards 
that are requisite to protect the public welfare from known or 
anticipated adverse effects associated with the pollutant's presence in 
the ambient air. The EPA's assessments are primarily documented in the 
2019 ISA, ISA Supplement, and 2022 PA, which builds on the 2020 PA, all 
of which have received CASAC review and public comment (83 FR 53471, 
October 23, 2018; 83 FR 55529, November 6, 2018; 85 FR 4655, January 
27, 2020; 86 FR 52673, September 22, 2021; 86 FR 54186, September 30, 
2021; 86 FR 56263, October 8, 2021; 87 FR 958, January 7, 2022; 87 FR 
22207, April 14, 2022; 87 FR 31965, May 26, 2022). In bridging the gap 
between the scientific assessments of the 2019 ISA and ISA Supplement 
and the judgments required of the Administrator in determining whether 
the current standards provide the requisite public welfare protection, 
the 2022 PA evaluates policy implications of the evaluation of the 
current evidence in the 2019 ISA and ISA Supplement, and the 
quantitative information documented in the 2022 PA. In evaluating the 
public welfare protection afforded by the current standards against PM-
related effects within the scope of this reconsideration, the four 
basic elements of the NAAQS (indicator, averaging time, level, and 
form) are considered collectively.
    The final decision on the adequacy of the current secondary 
standards is a public welfare policy judgment to be made by the 
Administrator. In reaching conclusions with regard to the standard, the 
decision draws on the scientific information and analyses about welfare 
effects, and associated public welfare significance, as well as 
judgments about how to consider the range and magnitude of 
uncertainties that are inherent in the scientific evidence and 
analyses. This approach is based on the recognition that the available 
evidence generally reflects a continuum that includes ambient air 
exposures at which scientists agree that effects are likely to occur 
through lower levels at which the likelihood and magnitude of responses 
become increasingly uncertain. This approach is consistent with the 
requirements of the provisions of the Clean Air Act related to the 
review of NAAQS and with how the EPA and the courts have historically 
interpreted the Act. These provisions require the Administrator to 
establish secondary standards that, in the judgment of the 
Administrator, are requisite to protect public welfare from known or 
anticipated adverse effects associated with the presence of the 
pollutant in the ambient air. In so doing, the Administrator seeks to 
establish standards that are neither more nor less stringent than 
necessary for this purpose. The Act does not require that standards be 
set at a zero-risk level, but rather at a level that reduces risk 
sufficiently so as to protect the public welfare from known or 
anticipated adverse effects.
1. Background on the Current Standards
    The current secondary PM standards were retained in 2020 based on 
the scientific and technical information available at that time, as 
well as the then-Administrator's judgments regarding the available 
welfare effects evidence, the appropriate degree of public welfare 
protection for the existing standards, and available air quality 
information on visibility impairment that may be allowed by such a 
standard (85 FR 82684, December 18, 2020). With the 2020 decision, the 
then-Administrator retained the secondary 24-hour PM2.5 
standard, with its level of 35 [micro]g/m\3\, the annual 
PM2.5 standard, with its level of 15.0 [micro]g/m\3\, and 
the 24-hour PM10 standard, with its level of 150 [micro]g/
m\3\. The subsections below focus on the key considerations and the 
then-Administrator's conclusions in the 2020 final decision for climate 
and materials effects (section V.A.1.a) and visibility effects (section 
V.A.2.b).
a. Non-Visibility Effects
    In light of the robust evidence base, the 2019 ISA concluded there 
to be causal relationships between PM and climate effects and materials 
effects (U.S. EPA, 2019a, sections 13.3.9 and 13.4.2). The 2020 final 
decision was based on a thorough review in the 2019 ISA of the 
scientific information on PM-induced climate and materials effects. The 
decision also took into account: (1) Assessments in the 2020 PA of the 
most policy-relevant information in the 2019 ISA regarding evidence of 
adverse effects of PM to climate and materials, (2) uncertainties in 
the available evidence to inform a quantitative assessment of PM-
related climate and materials effects, (3) CASAC advice and 
recommendations, and (4) public comments received during the 
development of these documents and on the proposal document.
    In considering non-visibility welfare effects in the 2020 decision, 
the then-Administrator concluded that, while it is important to 
maintain an appropriate degree of control of fine and coarse particles 
to address non-visibility welfare effects, ``it is generally 
appropriate to retain the existing standards and that there is 
insufficient information to establish any distinct secondary PM 
standards to address climate and materials effects of PM'' (85 FR 
82744, December 18, 2020).
    With regard to climate, the then-Administrator recognized that 
there were a number of improvements and refinements to climate models 
since the 2012 review. However, while the evidence continued to support 
a causal relationship between PM and climate effects, the then-
Administrator noted that significant limitations continued to exist 
related to quantifying the contributions of direct and indirect

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effects of PM and PM components on climate forcing (U.S. EPA, 2020b, 
sections 5.2.2.1.1 and 5.4). He also recognized that the models 
continued to exhibit considerable variability in estimates of PM-
related climate impacts at regional scales (e.g., ~100 km) as compared 
to simulations at global scales. Therefore, the resulting uncertainty 
led the then-Administrator to conclude in the 2020 decision that the 
available scientific information remained insufficient to quantify 
climate impacts associated with particular concentrations of PM in 
ambient air (U.S. EPA, 2020b, section 5.2.2.2.1) or to evaluate or 
consider a level of PM air quality in the U.S. to protect against 
climate effects and that there was insufficient information available 
to base a national ambient standard on climate impacts (85 FR 82744, 
December 18, 2020).
    With regard to materials effects, the then-Administrator noted that 
the evidence available in the 2019 ISA continued to support a causal 
relationship between materials effects and PM deposition (U.S. EPA, 
2019a, section 13.4). He recognized that the deposition of fine and 
coarse particles to materials can lead to physical damage and/or 
impaired aesthetic qualities. Particles can contribute to materials 
damage by adding to the natural weathering processes and by promoting 
the corrosion of metals, the degradation of building materials, and the 
weakening of material components. While some new information was 
available in the 2019 ISA, the information was from studies primarily 
conducted outside of the U.S. in areas where PM concentrations in 
ambient air are higher than those observed in the U.S. (U.S. EPA, 
2020b, section 13.4). Additionally, the information assessed in the 
2019 ISA did not support quantitative analyses of PM-related materials 
effects in the 2020 PA (U.S. EPA, 2020b, section 5.2.2.2.2). Given the 
limited amount of information available and its inherent uncertainties 
and limitations, the Administrator concluded that he was unable to 
relate soiling or damage to specific levels of PM in ambient air or to 
evaluate or consider a level of air quality to protect against such 
materials effects, and that there was insufficient information 
available to support a distinct national ambient standard based on 
materials effects (85 FR 82744, December 18, 2020).
    In reviewing the 2019 draft PA, the CASAC agreed with staff 
conclusions that, while these effects are important, ``the available 
evidence does not call into question the protection afforded by the 
current secondary PM standards'' and recommended that the secondary 
standards ``should be retained'' (Cox, 2019b, p. 3 of letter). In 
reaching a final decision in 2020, for all of the reasons discussed 
above and recognizing the CASAC conclusion that the evidence provided 
support for retaining the current secondary PM standards, the then-
Administrator concluded that it was appropriate to retain the existing 
secondary PM standards, without revision. For climate and materials 
effects, this conclusion reflected his judgment that, although it 
remains important to maintain secondary PM2.5 and 
PM10 standards to provide some degree of control over long- 
and short-term concentrations of both fine and coarse particles, there 
was insufficient information to establish distinct secondary PM 
standards to address non-visibility PM-related welfare effects (85 FR 
82744, December 18, 2020).
b. Visibility Effects
    The 2019 ISA concluded that, ``the evidence is sufficient to 
conclude that a causal relationship exists between PM and visibility 
impairment'' (U.S. EPA, 2019a, section 13.2.6). The 2020 decision on 
the adequacy of the secondary standards with regard to visibility 
effects was a public welfare policy judgment made by the then-
Administrator, which drew upon the available scientific evidence for 
PM-related visibility effects and on analyses of visibility impairment, 
as well as judgments about the appropriate weight to place on the range 
of uncertainties inherent in the evidence and analyses. The 2020 final 
decision was based on a thorough review in the 2019 ISA of the 
scientific information on PM-related visibility effects. The decision 
also took into account: (1) Assessments in the 2020 PA of the most 
policy-relevant information in the 2019 ISA regarding evidence of 
adverse effects of PM on visibility; (2) air quality analyses of the 
PM2.5 visibility index and design values based on the form 
and averaging time of the existing secondary 24-hour PM2.5 
standard; (3) CASAC advice and recommendations; and (4) public comments 
received during the development of these documents and on the 2020 
proposal document.
    In considering the visibility effects in the 2020 review, the then-
Administrator noted the long-standing body of evidence for PM-related 
visibility impairment. This evidence, which is based on the fundamental 
relationship between light extinction and PM mass, demonstrated that 
ambient PM can impair visibility in both urban and remote areas, and 
had changed very little since the 2012 review (U.S. EPA, 2019a, section 
13.1; U.S. EPA, 2009a, section 9.2.5). The evidence related to public 
perception of visibility impairment was from studies from four areas in 
North America.\146\ These studies provided information to inform our 
understanding of levels of visibility impairment that the public judged 
to be ``acceptable'' (U.S. EPA, 2010b; 85 FR 24131, April 30, 2020). In 
considering these public preference studies, the then-Administrator 
noted that no new visibility studies conducted in the U.S. were 
discussed in the 2019 ISA, and there was little newly available 
information with regard to acceptable levels of visibility impairment 
in the U.S. The Administrator recognized that visibility impairment can 
have implications for people's enjoyment of daily activities and their 
overall well-being, and therefore, considered the degree to which the 
current secondary standards protect against PM-related visibility 
impairment.
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    \146\ Preference studies were available in four urban areas. 
Three western preference studies were available, including one in 
Denver, Colorado (Ely et al., 1991), one in the lower Fraser River 
valley near Vancouver, British Columbia, Canada (Pryor, 1996), and 
one in Phoenix, Arizona (BBC Research & Consulting, 2003). A pilot 
focus group study was also conducted for Washington, DC (Abt 
Associates, 2001), and a replicate study with 26 participants was 
also conducted for Washington, DC (Smith and Howell, 2009). More 
details about these studies are available in Appendix D of the 2022 
PA (U.S. EPA, 2022b).
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    Consistent with the 2012 review, in the 2020 review, the then-
Administrator first concluded that a target level of protection for a 
secondary PM standard is most appropriately defined in terms of a 
visibility index that directly takes into account the factors (i.e., 
species composition and relative humidity) that influence the 
relationship between PM2.5 in ambient air and PM-related 
visibility impairment. In defining a target level of protection, the 
then-Administrator considered the specific aspects of such an index, 
including the appropriate indicator, averaging time, form and level (78 
FR 82742-82744, December 18, 2020).
    First, with regard to indicator, the then-Administrator noted that 
in the 2012 review, the EPA used an index based on estimates of light 
extinction by PM2.5 components calculated using an adjusted 
version of the IMPROVE algorithm, which allows the estimation of the 
light extinction using routinely monitored components of 
PM2.5, PM10-2.5 mass, and estimates of relative 
humidity. The then-Administrator recognized that, while there have been 
some revisions to the IMPROVE algorithm since the time of the 2012

[[Page 16314]]

review, our fundamental understanding of the relationship between PM in 
ambient air and light extinction had changed little and the various 
IMPROVE algorithms appropriately reflected this relationship across the 
U.S. In the absence of a monitoring network for direct measurement of 
light extinction, he concluded that a calculated light extinction 
indicator that utilizes the IMPROVE algorithms continued to provide a 
reasonable basis for defining a target level of protection against PM-
related visibility impairment (78 FR 82742-82744, December 18, 2020).
    In further defining the characteristics of a visibility index, the 
then-Administrator next considered the appropriate averaging time, 
form, and level of the index. Given the available scientific 
information the review, and in considering the CASAC's advice and 
public comments, the then-Administrator concluded that, consistent with 
the decision in the 2012 review, a visibility index with a 24-hour 
averaging time and a form based on the 3-year average of annual 90th 
percentile values remained reasonable. With regard to the averaging 
time and form of such an index, the Administrator noted analyses 
conducted in the last review that demonstrated relatively strong 
correlations between 24-hour and subdaily (i.e., 4-hour average) 
PM2.5 light extinction (78 FR 3226, January 15, 2013), 
indicating that a 24-hour averaging time is an appropriate surrogate 
for the subdaily time periods of the perception of PM-related 
visibility impairment and the relevant exposure periods for segments of 
the viewing public. This decision in the 2020 review also recognized 
that a 24-hour averaging time may be less influenced by atypical 
conditions and/or atypical instrument performance (78 FR 3226, January 
15, 2013). The then-Administrator recognized that there was no new 
information to support updated analyses of this nature, and therefore, 
he believed these analyses continued to provide support for 
consideration of a 24-hour averaging time for a visibility index in 
this review. With regard to the statistical form of the index, the 
Administrator noted that, consistent with the 2012 review: (1) A multi-
year percentile form offers greater stability from the occasional 
effect of interannual meteorological variability (78 FR 3198, January 
15, 2013; U.S. EPA, 2011, p. 4-58); (2) a 90th percentile represents 
the median of the distribution of the 20 percent worst visibility days, 
which are targeted in Federal Class I areas by the Regional Haze 
Program; and (3) public preference studies did not provide information 
to identify a different target than that identified for Federal Class I 
areas (U.S. EPA, 2011, p. 4-59). Therefore, the then-Administrator 
judged that a visibility index based on estimates of light extinction, 
with a 24-hour averaging time and a 90th percentile form, averaged over 
three years, remained appropriate (78 FR 82742-82744, December 18, 
2020).
    With regard to the level of a visibility index, consistent with the 
2012 review, the then-Administrator judged that it was appropriate to 
establish a target level of protection of 30 deciviews 
(dv),147 148 reflecting the upper end of the range of 
visibility impairment judged to be acceptable by at least 50% of study 
participants in the available public preference studies (78 FR 3226, 
January 15, 2013). The 2011 PA identified a range of levels from 20 to 
30 dv based on the responses in the public preference studies available 
at that time (U.S. EPA, 2011, section 4.3.4). At the time of the 2012 
review, the then-Administrator noted a number of uncertainties and 
limitations in public preference studies, including the small number of 
stated preference studies available, the relatively small number of 
study participants, the extent to which the study participants may not 
be representative of the broader study area population in some of the 
studies, and the variations in the specific materials and methods used 
in each study. In considering the available preference studies in 2012, 
with their inherent uncertainties and limitations, the then-
Administrator concluded that the substantial degree of variability and 
uncertainty in the public preference studies should be reflected in a 
target level of protection based on the upper end of the range of 
candidate protection levels (CPLs).
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    \147\ Deciview (dv) refers to a scale for characterizing 
visibility that is defined directly in terms of light extinction. 
The deciview scale is frequently used in the scientific and 
regulatory literature on visibility.
    \148\ For comparison, 20 dv, 25 dv, and 30 dv are equivalent to 
64, 112, and 191 megameters (Mm-1), respectively.
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    Given that there were no new preference studies in the 2019 ISA, 
the then-Administrator's judgments in 2020 were based on the same 
studies, with the same range of levels, available in the 2012 review. 
The 2020 PA (U.S. EPA, 2020b, section 5.5), discussed a number of 
limitations and uncertainties associated with these studies. In 
considering the scientific information, with its uncertainties and 
limitations, as well as public comments on the level of the target 
level of protection against visibility impairment, the then-
Administrator concluded that it was appropriate to again use a level of 
30 dv for the visibility index (78 FR 82742-82744, December 18, 2020).
    Having concluded that the protection provided by a standard defined 
in terms of a PM2.5 visibility index, with a 24-hour 
averaging time, and a 90th percentile form, averaged over 3 years, set 
at a level of 30 dv, was requisite to protect public welfare with 
regard to visual air quality, the Administrator next considered the 
degree of protection from visibility impairment afforded by the 
existing suite of secondary PM standards.
    In this context, the then-Administrator considered the updated 
analyses of visibility impairment presented in the 2020 PA (U.S. EPA, 
2020b, section 5.2.1.2), which reflected a number of improvements since 
the 2012 review. Specifically, the updated analyses examined multiple 
versions of the IMPROVE equation, including the version incorporating 
revisions since the time of the 2012 review. These updated analyses 
provided a further understanding of how variation in the inputs to the 
algorithms affect the estimates of light extinction (U.S. EPA, 2020b, 
Appendix D). Additionally, for a subset of monitoring sites with 
available PM10-2.5 data, the updated analyses better 
characterized the influence of coarse PM on light extinction than in 
the 2012 review (U.S. EPA, 2020b, section 5.2.1.2).
    The results of the updated analyses in the 2020 PA were consistent 
with those from the 2012 review. Regardless of which version of the 
IMPROVE equation was used, the analyses demonstrated that, based on 
2015-2017 data, the 3-year visibility metric was at or below about 30 
dv in all areas meeting the current 24-hour PM2.5 standard, 
and below 25 dv in most of those areas. In locations with available 
PM10-2.5 monitoring, which met both the current 24-hour 
secondary PM2.5 and PM10 standards, 3-year 
visibility index metrics were at or below 30 dv regardless of whether 
the coarse fraction was included as an input to the algorithm for 
estimating light extinction (U.S. EPA, 2020b, section 5.2.1.2). While 
the inclusion of the coarse fraction had a relatively modest impact on 
the estimates of light extinction, the then-Administrator recognized 
the continued importance of the PM10 standard given the 
potential for larger impacts on light extinction in areas with higher 
coarse particle concentrations, which were not included in the analyses 
in the 2020 PA due to a lack of available data (U.S. EPA, 2019a, 
section 13.2.4.1; U.S. EPA, 2020b, section 5.2.1.2). He

[[Page 16315]]

noted that the air quality analyses showed that all areas meeting the 
existing 24-hour PM2.5 standard, with its level of 35 
[micro]g/m\3\, had visual air quality at least as good as 30 dv, based 
on the visibility index. Thus, the secondary 24-hour PM2.5 
standard would likely be controlling relative to a 24-hour visibility 
index set at a level of 30 dv. Additionally, areas would be unlikely to 
exceed the target level of protection for visibility of 30 dv without 
also exceeding the existing secondary 24-hour PM2.5 
standard. Thus, the then-Administrator judged that the 24-hour 
PM2.5 standard provided sufficient protection in all areas 
against the effects of visibility impairment, i.e., that the existing 
24-hour PM2.5 standard would provide at least the target 
level of protection for visual air quality of 30 dv which he judged 
appropriate (78 FR 82742-82744, December 18, 2020).
2. Overview of Welfare Effects Evidence
    The information summarized here is based on the scientific 
assessment of the welfare effects evidence available in this 
reconsideration; this assessment is documented in the 2019 ISA and ISA 
Supplement and its policy implications are further discussed in the 
2022 PA. While the 2019 ISA provides the broad scientific foundation 
for this reconsideration, additional literature has become available 
since the cutoff date of the 2019 ISA that expands the body of evidence 
related to visibility effects that can inform the Administrator's 
judgment on the adequacy of the current secondary PM standards. As 
such, the ISA Supplement builds on the information in the 2019 ISA with 
a targeted identification and evaluation of new scientific information 
regarding visibility effects. As described in the ISA Supplement and 
the 2022 PA, the selection of welfare effects to evaluate within the 
ISA Supplement were based on the causality determinations reported in 
the 2019 ISA and the subsequent use of scientific evidence in the 2020 
PA (U.S. EPA, 2019a, section 1.2; U.S. EPA, 2022a, section 1.4.2). The 
ISA Supplement focuses on U.S. and Canadian studies that provide new 
information on public preferences for visibility impairment and/or 
developed new methodologies or conducted quantitative analyses of light 
extinction (U.S. EPA, 2022a, section 1.2). Such studies of visibility 
effects and quantitative relationships between visibility impairment 
and PM in ambient air were considered to be of greatest utility in 
informing the Administrator's conclusions on the adequacy of the 
current secondary PM standards. The visibility effects evidence 
presented within the 2019 ISA, along with the targeted identification 
and evaluation of new scientific information in the ISA Supplement, 
provides the scientific basis for the reconsideration of the 2020 final 
decision on the secondary PM standards for visibility effects. For 
climate and materials effects, the 2020 PA concluded that there were 
substantial uncertainties associated with the quantitative 
relationships with PM concentrations and the concentration patterns 
that limited the ability to quantitatively assess the public welfare 
protection provided by the standards from these effects. Therefore, the 
evaluation of the information related to these effects draws heavily 
from the 2019 ISA and 2020 PA. The subsections below briefly summarize 
the nature of PM-related visibility (section V.B.1.a), climate (section 
V.B.1.b), and materials (section V.B.1.c) effects.
a. Nature of Effects
    Visibility impairment can have implications for people's enjoyment 
of daily activities and for their overall sense of well-being (U.S. 
EPA, 2009a, section 9.2). The strongest evidence for PM-related 
visibility impairment comes from the fundamental relationship between 
light extinction and PM mass (U.S. EPA, 2009a), which confirms a well-
established ``causal relationship exists between PM and visibility 
impairment'' (U.S. EPA, 2009a, p. 2-28). Beyond its effects on 
visibility, the 2009 ISA also identified a causal relationship 
``between PM and climate effects, including both direct effects of 
radiative forcing and indirect effects that involve cloud and feedbacks 
that influence precipitation formation and cloud lifetimes'' (U.S. EPA, 
2009a, p. 2-29). The evidence also supports a causal relationship 
between PM and effects on materials, including soiling effects and 
materials damage (U.S. EPA, 2009a, p. 2-31).
    The evidence available in this reconsideration is consistent with 
the evidence available at the time of the 2012 and 2020 reviews and 
supports the conclusions of causal relationships between PM and 
visibility, climate, and materials effects (U.S. EPA, 2019a, chapter 
13). Evidence newly available in this reconsideration augments the 
previously available evidence of the relationship between PM and 
visibility impairment (U.S. EPA, 2019a, section 13.2; U.S. EPA, 2022a, 
section 4), climate effects (U.S. EPA, 2019a, section 13.3), and 
materials effects (U.S. EPA, 2019a, section 13.4).
i. Visibility
    The fundamental relationship between light extinction and PM mass, 
and the EPA's understanding of this relationship, has changed little 
since the 2009 ISA (U.S. EPA, 2009a). The combined effect of light 
scattering and absorption by particles and gases is characterized as 
light extinction, i.e., the fraction of light that is scattered or 
absorbed per unit of distance in the atmosphere.\149\ Light extinction 
is measured in units of 1/distance, which is often expressed in the 
technical literature as visibility per megameter (abbreviated 
Mm-1). Higher values of light extinction (usually given in 
units of Mm-1 or dv) correspond to lower visibility. When PM 
is present in the air, its contribution to light extinction is 
typically much greater than that of gases (U.S. EPA, 2019a, section 
13.2.1). The impact of PM on light scattering depends on particle size 
and composition, as well as relative humidity. All particles scatter 
light, as described by the Mie theory, which relates light scattering 
to particle size, shape, and index of refraction (U.S. EPA, 2019a, 
section 13.2.3; Mie, 1908, Van de Hulst, 1981). Fine particles scatter 
more light than coarse particles on a per unit mass basis and include 
sulfates, nitrates, organics, light-absorbing carbon, and soil (Malm et 
al., 1994). Hygroscopic particles like ammonium sulfate, ammonium 
nitrate, and sea salt increase in size as relative humidity increases, 
leading to increased light scattering (U.S. EPA, 2019a, section 
13.2.3).
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    \149\ All particles scatter light and, although a larger 
particle scatters more light than a similarly shaped smaller 
particle of the same composition, the light scattered per unit of 
mass is greatest for particles with diameters from ~0.3-1.0 [micro]m 
(U.S. EPA, 2009a, section 2.5.1; U.S. EPA, 2019a, section 13.2.1). 
Particles with hygroscopic components (e.g., particulate sulfate and 
nitrate) contribute more to light extinction at higher relative 
humidity than at lower relative humidity because they change size in 
the atmosphere in response to relative humidity.
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    As at the time of the 2012 and 2020 reviews, direct measurements of 
PM light extinction, scattering, and absorption continue to be 
considered more accurate for quantifying visibility than PM mass-based 
estimates because measurements do not depend on assumptions about 
particle characteristics (e.g., size, shape, density, component 
mixture, etc.) (U.S. EPA, 2019a, section 13.2.2.2). Measurements of 
light extinction can be made with high time resolution, allowing for 
characterization of subdaily temporal patterns of visibility 
impairment. A number of measurement methods have been used for 
visibility impairment (e.g.,

[[Page 16316]]

transmissometers, integrating nephelometers, teleradiometers, 
telephotometers, and photography and photographic modeling), although 
each of these methods has its own strengths and limitations (U.S. EPA, 
2019a, Table 13-1). While some recent research confirms and adds to the 
body of knowledge regarding direct measurements as is described in the 
2019 ISA and ISA Supplement, no major new developments have been made 
with these measurement methods since prior reviews (U.S. EPA, 2019a, 
section 13.2.2.2; U.S. EPA, 2022a, section 4.2).
    In the absence of a robust monitoring network for the routine 
measurement of light extinction across the U.S., estimation of light 
extinction based on existing PM monitoring can be used. The theoretical 
relationship between light extinction and PM characteristics, as 
derived from Mie theory (U.S. EPA, 2019a, Equation 13.5), can be used 
to estimate light extinction by combining mass scattering efficiencies 
of particles with particle concentrations (U.S. EPA, 2019a, section 
13.2.3; U.S. EPA, 2009a, sections 9.2.2.2 and 9.2.3.1). This estimation 
of light extinction is consistent with the method used in previous 
reviews. The algorithm used to estimate light extinction, known as the 
IMPROVE algorithm,\150\ provides for the estimation of light extinction 
(bext), in units of Mm\-1\, using routinely monitored 
components of fine (PM2.5) and coarse (PM10-2.5) 
PM. Relative humidity data are also needed to estimate the contribution 
by liquid water that is in solution with the hygroscopic components of 
PM. To estimate each component's contribution to light extinction, 
their concentrations are multiplied by extinction coefficients and are 
additionally multiplied by a water growth factor that accounts for 
their expansion with moisture. Both the extinction efficiency 
coefficients and water growth factors of the IMPROVE algorithm have 
been developed by a combination of empirical assessment and theoretical 
calculation using particle size distributions associated with each of 
the major aerosol components (U.S. EPA, 2019a, sections 13.2.3.1 and 
13.2.3.3).
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    \150\ The algorithm is referred to as the IMPROVE algorithm as 
it was developed specifically to use monitoring data generated at 
IMPROVE network sites and with equipment specifically designed to 
support the IMPROVE program and was evaluated using IMPROVE optical 
measurements at the subset of monitoring sites that make those 
measurements (Malm et al., 1994).
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    At the time of the 2012 review, two versions of the IMPROVE 
algorithm were available in the literature--the original IMPROVE 
algorithm (Lowenthal and Kumar, 2004, Malm and Hand, 2007, Ryan et al., 
2005) and the revised IMPROVE algorithm (Pitchford et al., 2007). As 
described in detail in the 2022 PA (U.S. EPA, 2022b, section 5.3.1.1) 
and the 2019 ISA (U.S. EPA, 2019a, section 13.2.3), the algorithm has 
been further evaluated and refined since the time of the 2012 review 
(Lowenthal and Kumar, 2016), particularly for PM characteristics and 
relative humidity in remote areas. All three versions of the IMPROVE 
algorithm were considered in evaluating visibility impairment in this 
reconsideration.
    Consistent with the evidence available at the time of the 2012 and 
2020 reviews, our understanding of public perception of visibility 
impairment comes from visibility preference studies conducted in four 
areas in North America.\151\ The detailed methodology for these studies 
are described in the 2022 PA (U.S. EPA, 2022b, section 5.3.1.1), the 
2019 ISA (U.S. EPA, 2019a), and the 2009 ISA (U.S. EPA, 2019a). In 
summary, the study participants were queried regarding multiple images 
that were either photographs of the same location and scenery that had 
been taken on different days on which measured extinction data were 
available or digitized photographs onto which a uniform ``haze'' had 
been superimposed. Results of the studies indicated a wide range of 
judgments on what study participants considered to be acceptable 
visibility across the different study areas, depending on the setting 
depicted in each photograph. Based on the results of the four cities, a 
range encompassing the PM2.5 visibility index values from 
images that were judged to be acceptable by at least 50 percent of 
study participants across all four of the urban preference studies was 
identified (U.S. EPA, 2010b, p. 4-24; U.S. EPA, 2020b, Figure 5-2). 
Much lower visibility (considerably more haze resulting in higher 
values of light extinction) was considered acceptable in Washington, 
DC, than was in Denver, and 30 dv reflected the level of impairment 
that was determined to be ``acceptable'' by at least 50 percent of 
study participants (78 FR 3226-3227, January 15, 2013).
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    \151\ Preference studies were available in four urban areas in 
the last review: Denver, Colorado (Ely et al., 1991), Vancouver, 
British Columbia, Canada (Pryor, 1996), Phoenix, Arizona (BBC 
Research & Consulting, 2003), and Washington, DC (Abt Associates, 
2001; Smith and Howell, 2009).
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    Since the completion of the 2009 and 2019 ISAs, there has been only 
one public preference study that has become available in the U.S. This 
study uses images of the Grand Canyon, AZ, described in the ISA 
Supplement (U.S. EPA, 2022a). The Grand Canyon study, conducted by Malm 
et al. (2019), has a similar study design to that used in the public 
preference studies discussed above; however, there are several 
important differences that make it difficult to directly compare the 
results of the Malm et al. (2019) study with other public preference 
studies. As an initial matter, the Grand Canyon study was conducted in 
a Federal Class I area, as opposed to in an urban area, with a scene 
depicted in the photographs that did not include urban features.\152\ 
We recognize that public preferences with respect to visibility in 
Federal Class 1 areas may well differ from visibility preferences in 
urban areas and other contexts, although there is currently a lack of 
information to on such questions. Further, the Malm et al. (2019) study 
also used a much lower range of superimposed ``haze'' than the 
preference studies discussed above.\153\ It is unclear whether the 
participant preferences are a function in part of the range of 
potential values presented, such that the participant preferences for 
the Grand Canyon were generally lower \154\ than the other preference 
studies in part because of the lower range of superimposed ``haze'' for 
the images in that study, or if their preferences would vary if 
presented with images with a range of superimposed ``haze'' more 
comparable to the levels used in the other studies (i.e., more ``haze'' 
superimposed on the images).
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    \152\ The Grand Canyon study used a single scene looking west 
down the canyon with a small landscape feature of a 100-km-distant 
mountain (Mount Trumbull), along with other closer landscape 
features. The scenes presented in the previously available 
visibility preference studies are presented in more detail in Table 
D-9 in the 2022 PA (U.S. EPA, 2022b, Appendix D).
    \153\ The Grand Canyon study superimposed light extinction 
ranging from 3 dv to 20 dv on the image slides shown to participants 
compared to the previously available preference studies. In those 
studies, the visibility ranges presented were as low as 9 dv and as 
high as 45 dv. The visibility ranges presented in the previously 
available visibility preference studies are described in more detail 
in Table D-9 in the 2022 PA (U.S. EPA, 2022b, Appendix D).
    \154\ In the Grand Canyon study, the level of impairment that 
was determined to be ``acceptable'' by at least 50 percent of study 
participants was 7 dv (Malm et al., 2019).
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    The Malm et al. (2019) study also explored alternate methods for 
evaluating ``acceptable'' levels of visual air quality from the 
preference studies, including the use of scene-specific visibility 
indices as potential indicators of visibility levels as perceived by 
the observer (Malm et al., 2019). In addition to measures of 
atmospheric haze, such

[[Page 16317]]

as atmospheric extinction, used in previously available preference 
studies, other indices for visual air quality include color and 
achromatic contrast of single landscape figures, average and equivalent 
contrast of an entire scene, edge detection algorithms such as the 
Sobel index, and just-noticeable difference or change indexes. The 
results reported by Malm et al. (2019) suggest that scene-dependent 
metrics, such as contrast, may be useful alternate predictors of 
preference levels compared to universal metrics like light extinction 
(U.S. EPA, 2022a, section 4.2.1). This is because extinction alone is 
not a measure of ``haze,'' but of light attenuation per unit distance, 
and visible ``haze'' is dependent on both light extinction and distance 
to a landscape feature (U.S. EPA, 2022a, section 4.2.1). However, there 
are very few studies available that use scene-dependent metrics (i.e., 
contrast) to evaluate public preference information, which makes it 
difficult to evaluate them as an alternative to the light extinction 
approach.
ii. Climate
    The available evidence continues to support the conclusion of a 
causal relationship between PM and climate effects (U.S. EPA, 2019a, 
section 13.3.9). Since the 2012 review, climate impacts have been 
extensively studied and recent research reinforces and strengthens the 
evidence evaluated in the 2009 ISA. Recent evidence provides greater 
specificity about the details of radiative forcing effects \155\ and 
increases the understanding of additional climate impacts driven by PM 
radiative effects. The Intergovernmental Panel on Climate Change (IPCC) 
assesses the role of anthropogenic activity in past and future climate 
change, and since the completion of the 2009 ISA, has issued the Fifth 
IPCC Assessment Report (AR5; IPCC, 2013), which summarizes any key 
scientific advances in understanding the climate effects of PM since 
the previous report. As in the 2009 ISA, the 2019 ISA draws 
substantially on the IPCC report to summarize climate effects. As 
discussed in more detail in the 2022 PA (U.S. EPA, 2022b, section 
5.3.2.1.1), the general conclusions are similar between the IPCC AR4 
and AR5 reports with regard to effects of PM on global climate. 
Consistent with the evidence available in the 2012 review, the key 
components, including sulfate, nitrate, organic carbon (OC), black 
carbon (BC), and dust, that contribute to climate processes vary in 
their reflectivity, forcing efficiencies, and direction of forcing. 
Since the completion of the 2009 ISA, the evidence base has expanded 
with respect to the mechanisms of climate responses and feedbacks to PM 
radiative forcing; however, the recently published literature assessed 
in the 2019 ISA does not reduce the considerable uncertainties that 
continue to exist related these mechanisms.
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    \155\ Radiative forcing (RF) for a given atmospheric constituent 
is defined as the perturbation in net radiative flux, at the 
tropopause (or the top of the atmosphere) caused by that 
constituent, in watts per square meter (Wm\-2\), after allowing for 
temperatures in the stratosphere to adjust to the perturbation but 
holding all other climate responses constant, including surface and 
tropospheric temperatures (Fiore et al., 2015; Myhre et al., 2013). 
A positive forcing indicates net energy trapped in the Earth system 
and suggests warming of the Earth's surface, whereas a negative 
forcing indicates net loss of energy and suggests cooling (U.S. EPA, 
2019a, section 13.3.2.2).
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    As described in the proposal (88 FR 5650, January 27, 2023), PM has 
a very heterogeneous distribution globally and patterns of forcing tend 
to correlate with PM loading, with the greatest forcings centralized 
over continental regions. The climate response to this PM forcing, 
however, is more complicated since the perturbation to one climate 
variable (e.g., temperature, cloud cover, precipitation) can lead to a 
cascade of effects on other variables. While the initial PM radiative 
forcing may be concentrated regionally, the eventual climate response 
can be much broader spatially or be concentrated in remote regions, and 
may be quite complex, affecting multiple climate variables with 
possible differences in the direction of the forcing in different 
regions or for different variables (U.S. EPA, 2019a, section 13.3.6). 
The complex climate system interactions lead to variation among climate 
models, which have suggested a range of factors that can influence 
large-scale meteorological processes and may affect temperature, 
including local feedback effects involving soil moisture and cloud 
cover, changes in the hygroscopicity of the PM, and interactions with 
clouds (U.S. EPA, 2019a, section 13.3.7). As a result, there remains 
insufficient evidence to related climate effects to specific PM levels 
in ambient air or to establish a quantitative relationship between PM 
and climate effects, particularly at a regional scale. Further research 
is needed to better characterize the effects of PM on regional climate 
in the U.S. before PM climate effects can be quantified.
iii. Materials
    Consistent with the evidence assessed in the 2009 ISA, the 
available evidence continues to support the conclusion that there is a 
causal relationship between PM deposition and materials effects. 
Effects of deposited PM, particularly sulfates and nitrates, to 
materials include both physical damage and impaired aesthetic 
qualities, generally involving soiling and/or corrosion (U.S. EPA, 
2019a, section 13.4.2). Because of their electrolytic, hygroscopic, and 
acidic properties and their ability to sorb corrosive gases, particles 
contribute to materials damage by adding to the effects of natural 
weathering processes, by potentially promoting or accelerating the 
corrosion of metals, degradation of painted surfaces, deterioration of 
building materials, and weakening of material components.\156\ There is 
a limited amount of recently available data for consideration in this 
review from studies primarily conducted outside of the U.S. on 
buildings and other items of cultural heritage. However, these studies 
involved concentrations of PM in ambient air greater than those 
typically observed in the U.S. (U.S. EPA, 2019a, section 13.4).
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    \156\ As discussed in the 2019 ISA (U.S. EPA, 2019a, section 
13.4.1), corrosion typically involves reactions of acidic PM (i.e., 
acidic sulfate or nitrate) with material surfaces, but gases like 
SO2 and nitric acid (HNO3) also contribute. 
Because ``the impacts of gaseous and particulate N and S wet 
deposition cannot be clearly distinguished'' (U.S. EPA, 2019a, p. 
13-1), the assessment of the evidence in the 2019 ISA considers the 
combined impacts.
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    Building on the evidence available in the 2009 ISA, and as 
described in detail in the proposal (88 FR 5650, January 27, 2023) and 
in the 2019 ISA (U.S. EPA, 2019a, section 13.4), research has 
progressed on (1) the theoretical understanding of soiling of items of 
cultural heritage; (2) the quantification of degradation rates and 
further characterization of factors that influence damage of stone 
materials; (3) materials damage from PM components besides sulfate and 
black carbon and atmospheric gases besides SO2; (4) methods 
for evaluating soiling of materials by PM mixtures; (5) PM-attributable 
damage to other materials, including glass and photovoltaic panels; (6) 
development of dose-response relationships for soiling of building 
materials; and (7) damage functions to quantify material decay as a 
function of pollutant type and load. While the evidence of PM-related 
materials effects has expanded somewhat since the completion of the 
2009 ISA, there remains insufficient evidence to relate soiling or 
damage to specific PM levels in ambient air or to establish a 
quantitative relationship between PM and materials degradation. The 
recent evidence assessed in the 2019 ISA is generally similar to the 
evidence available in the 2009 ISA, including

[[Page 16318]]

associated limitations and uncertainties and a lack of evidence to 
inform quantitative relationships between PM and materials effects, 
therefore leading to similar conclusions about the PM-related effects 
on materials.
3. Summary of Air Quality and Quantitative Information
    Beyond the consideration of the scientific evidence, as discussed 
in section V.A.2 above, quantitative analyses of PM air quality, when 
available, can also inform conclusions on the adequacy of the public 
welfare protection provided by the current secondary PM standards.
a. Visibility Effects
    In the 2012 and 2020 reviews, quantitative analyses for PM-related 
visibility effects focused on daily visibility impairment, given the 
short-term nature of PM-related visibility effects. The evidence and 
information available in this reconsideration continues to provide 
support for the short-term (i.e., hourly or daily) nature of PM-related 
visibility impairment. As such, the quantitative analyses presented in 
the 2022 PA continue to focus on daily visibility impairment and 
utilize a two-phase assessment approach for visibility impairment, 
consistent with the approaches taken in past reviews. First, the 2022 
PA considers the appropriateness of the elements (indicator, averaging 
time, form, and level) of the visibility index for providing protection 
against PM-related visibility effects. Second, recent air quality was 
used to evaluate the relationship between the current secondary 24-hour 
PM2.5 standard and the visibility index. The information 
available since the 2012 review includes an updated equation for 
estimating light extinction, summarized in the 2022 PA (U.S. EPA, 
2022b, section 5.3.1.1) and described in the 2019 ISA (U.S. EPA, 2019a, 
section 13.2.3.3), as well as more recent air monitoring data, that 
together allow for development of an updated assessment of PM-related 
visibility impairment in study locations in the U.S.
    i. Target Level of Protection in Terms of a PM2.5 
Visibility Index
    In evaluating the adequacy of the current secondary PM standards, 
the 2022 PA first evaluates the appropriateness of the elements 
(indicator, averaging time, form, and level) identified for a 
visibility index to protect against visibility effects. In previous 
reviews, the visibility index as set at a level of 30 dv, with 
estimated light extinction as the indicator, a 24-hour averaging time, 
and a 90th percentile form, averaged over three years.
    With regard to an indicator for the visibility index, the 2022 PA 
recognizes the lack of availability of methods and an established 
network for directly measuring light extinction (U.S. EPA, 2022b, 
section 5.3.1.1). Therefore, consistent with previous reviews, the 2022 
PA concludes that a visibility index based on estimates of light 
extinction by PM2.5 components derived from an adjusted 
version of the original IMPROVE algorithm to be the most appropriate 
indicator for the visibility index in this reconsideration. As 
described in section 5.3.1.1 of the 2022 PA, the IMPROVE algorithm 
estimates light extinction using routinely monitored components of 
PM2.5 and PM10-2.5, along with estimates of 
relative humidity (U.S. EPA, 2022b, section 5.3.1.1).
    With regard to averaging time, the 2022 PA notes that the evidence 
continues to provide support for the short-term nature of PM-related 
visibility effects. Given that there is no new information available 
regarding the time periods during which visibility impairment occurs or 
public preferences related to specific time periods for visibility 
impairment, the 2022 PA concludes that it is appropriate to continue to 
focus on daily visibility impairment. In so doing, the 2022 PA relies 
on analyses that were conducted in the 2012 review that showed 
relatively strong correlations between 24-hour and subdaily (i.e., 4-
hour average) PM2.5 light extinction that indicated that a 
24-hour averaging time is an appropriate surrogate for the subdaily 
time periods relevant for visual perception (U.S. EPA, 2011, Figures G-
4 and G-5; Frank, 2012). These analyses continue to provide support for 
a 24-hour averaging time for the visibility index in this 
reconsideration. Consistent with previous reviews, the 2022 PA also 
notes that the 24-hour averaging time may be less influenced by 
atypical conditions and/or atypical instrument performance than a 
subdaily averaging time (85 FR 82740, December 18, 2020; 78 FR 3226, 
January 15, 2013).
    With regard to the form for the visibility index, the available 
information continues to provide support for a 3-year average of annual 
90th percentile values. Given that there is no new information to 
inform selection of an alternate form, as in previous reviews, the 2022 
PA notes that the 3-year average form provides stability from the 
occasional effect of inter-annual meteorological variability that can 
result in unusually high pollution levels for a particular year (85 FR 
82741, December 18, 2020; 78 FR 3198, January 15, 2013; U.S. EPA, 2011, 
p. 4-58). In so doing, the 2022 PA considers the evaluation in the 2010 
Urban-Focused Visibility Assessment (UFVA) of three different 
statistical forms: 90th, 95th, and 98th percentiles (U.S. EPA, 2010b, 
Chapter 4).). In considering this evaluation of statistical forms from 
the 2010 UFVA, consistent with the 2011 PA, the 2022 PA notes that the 
Regional Haze Program targets the 20 percent most impaired days for 
visibility improvements in visual air quality in Federal Class I areas 
and that the median of the distribution of these 20 percent most 
impaired days would be the 90th percentile. The 2011 PA also noted that 
strategies that are implemented so that 90 percent of days would have 
visual air quality that is at or below the level of the visibility 
index would reasonably be expected to lead to improvements in visual 
air quality for the 20 percent most impaired days. Additionally, as in 
the 2011 PA, the 2022 PA recognizes that the available public 
preference studies do not address frequency of occurrence of different 
levels of visibility (U.S. EPA, 2022b, section 5.3.1.2). Therefore, the 
analyses and consideration for the form of a visibility index from the 
2011 PA continue to provide support for a 90th percentile form, 
averaged across three years, in defining the characteristics of a 
visibility index in this reconsideration.
    With regard to the level for the visibility index, the 2022 PA 
recognizes that there is an additional public preference study (Malm et 
al., 2019) available in this reconsideration. As noted above, however, 
this study differs from the previously available public preference 
studies in several ways, which makes it difficult to integrate this 
newly available study with the previously available studies. Most 
significantly, this study was evaluated public preferences for 
visibility in the Grand Canyon, perhaps the most notable Class I area 
in the country for visibility purposes. Therefore, the 2022 PA 
concludes that the Grand Canyon study is not directly comparable to the 
other available preferences studies and public preferences of 
visibility impairment in the Malm et al. (2019) study are not 
appropriate to consider in identifying a range of levels for the target 
level of protection against visibility impairment for this 
reconsideration of the secondary PM NAAQS.

[[Page 16319]]

    Therefore, the 2022 PA continues to rely on the same studies \157\ 
and the range of 20 to 30 dv identified from those studies in previous 
reviews. With regard to selecting the appropriate target level of 
protection for visibility impairment within this range, the 2022 PA 
notes that in previous reviews, a level at the upper end of the range 
(i.e., 30 dv) was selected given the uncertainties and limitations 
associated with the public preference studies (U.S. EPA, 2022b, section 
5.3.1.1). However, the 2022 PA also recognizes that (1) the degree of 
protection provided by a secondary PM NAAQS is not determined solely by 
any one element of the standard but by all elements (i.e., indicator, 
averaging time, form, and level) being considered together, and (2) 
decisions regarding the adequacy of the current secondary standards is 
a public welfare policy judgment to be made by the Administrator. As 
such, the Administrator may judge that a target level of protection 
below the upper end of the range (i.e., less than 30 dv) is 
appropriate, depending on his public welfare policy judgments, which 
draw upon the available scientific evidence for PM-related visibility 
effects and on analyses of visibility impairment, as well as judgments 
about the appropriate weight to place on the range of uncertainties 
inherent in the evidence and analyses.
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    \157\ As noted above, the available public preference studies 
include those conducted in Denver, Colorado (Ely et al., 1991), 
Vancouver, British Columbia, Canada (Pryor, 1996), Phoenix, Arizona 
(BBC Research & Consulting, 2003), and Washington, DC (Abt 
Associates, 2001; Smith and Howell, 2009).
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    In considering the available public preference studies, consistent 
with past reviews, the 2022 PA concludes that it is reasonable to 
consider a range of 20 to 30 dv for selecting a target level of 
protection, including a high value of 30 dv, a midpoint value of 25 dv, 
and a low value of 20 dv. A target level of protection at or in the 
upper end of the range would focus on the Washington, DC, preference 
study results (Abt Associates, 2001; Smith and Howell, 2009), which 
identified 30 dv as the level of impairment that was determined to be 
``acceptable'' by at least 50 percent of study participants. The public 
preferences of visibility impairment in the Washington, DC, study are 
likely to be generally representative of urban areas that do not have 
valued scenic elements (e.g., mountains) in the distant background. 
This would be more representative of areas in the middle of the country 
and many areas in the eastern U.S., as well as possibly some areas in 
the western U.S.
    A target level of protection in the middle of the range would be 
most closely associated with the level of impairment that was 
determined to be ``acceptable'' by at least 50 percent of study 
participants in the Phoenix, AZ, study (BBC Research & Consulting, 
2003), which was 24 dv. This study, while methodologically similar to 
the other public preference studies, included participants that were 
selected as a representative sample of the Phoenix area population 
\158\ and used computer-generated images to depict specific uniform 
visibility impairment conditions. This study yielded the best results 
of the four public preference studies in terms of the least noisy 
preference results and the most representative selection of 
participants. Therefore, based on this study, the use of 25 dv to 
represent a midpoint within the range of target levels protection is 
well supported.
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    \158\ The other preference studies did not include populations 
that were necessarily representative of the population in the area 
for which the images being judged. For example, in the Denver, CO, 
study, participants were from intact groups (i.e., those who were 
meeting for other reasons) and were asked to provide a period of 
time during a regularly scheduled meeting to participate in the 
study (Ely et al., 1991). As another example, in the British 
Columbia, Canada, study, participants were recruited from 
undergraduate and graduate students enrolled in classes at the 
University of British Columbia's Department of Geography (Pryor, 
1996).
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    A target level of protection at or just above the lower end of the 
range would focus on the Denver, CO, study, but may not be as strongly 
supported as higher levels within the range (Ely et al., 1991). Older 
studies, such as those conducted in Denver, CO (Ely et al., 1991), and 
British Columbia, Canada (Pryor, 1996), used photographs that were 
taken at different times of the day and on different days to capture a 
range of light extinction levels needed for the preference studies. 
Compared to studies that used computer-generated images (i.e., those in 
Phoenix, AZ, and Washington, DC) there was more variability in scene 
appearance in these older studies that could affect preference rating 
and includes uncertainties associated with using ambient measurements 
to represent sight path-averaged light extinction values rather than 
superimposing a computer-generated amount of haze onto the images. When 
using photographs, the intrinsic appearance of the scene can change due 
to meteorological conditions (i.e., shadow patterns and cloud 
conditions) and spatial variations in ambient air quality that can 
result in ambient light extinction measurement not being representative 
of the sight-path-averaged light extinction. Computer-generated images, 
such as those generated with WinHaze, do not introduce such 
uncertainties, as the same base photograph is used (i.e., there is no 
intrinsic change in scene appearance) and the modeled haze that is 
superimposed on the photograph is determined based on uniform light 
extinction throughout the scene.
    In addition to differences in preferences that may arise from 
photographs versus computer-generated images, urban visibility 
preference may differ by location, and such differences may arise from 
differences in the cityscape scene that is depicted in the images. 
These differences are related to the perceived value of objects and 
scenes that are included in the image, as objects at a greater distance 
have a greater sensitivity to perceived visibility changes as light 
extinction is changed compared to similar scenes with objects at 
shorter distances. For example, a person (regardless of their location) 
evaluating visibility in an image with more scenic elements such as 
mountains or natural views may value better visibility conditions in 
these images compared to the same level of visibility impairment in an 
image that only depicts urban features such as buildings and roads. 
That is, if a person was shown the same level of visibility impairment 
in two images depicting different scenes--one with mountains in the 
background and urban features in the foreground and one with no 
mountains in the background and nearby buildings in the image without 
mountains in the distance--may find the amount of haze to be 
unacceptable in the image with the mountains in the distance because of 
a greater perceived value of viewing the mountains, while finding the 
amount of haze to be acceptable in the image with the buildings because 
of a lesser value of viewing the cityscape or an expectation that such 
urban areas may generally have higher levels of haze in general. This 
is consistent when comparing the differences between the Denver, CO, 
study results (which found the 50% acceptance criteria occurred at the 
best visual air quality levels among the four cities) and the 
Washington, DC, results (which found the 50% acceptability criteria 
occurred at the worst visual air quality levels among the four cities). 
These results may occur because the most prominent and picturesque 
feature of the cityscape of Denver is the visible snow-covered 
mountains in the distance, while the prominent and

[[Page 16320]]

picturesque features of the Washington, DC, cityscape are buildings 
relatively nearby without prominent and/or valued scenic features that 
are more distant. Given these variabilities in preferences it is 
unclear to what extent, the available evidence provides strong support 
for a target level of protection at the lower end of the range. Future 
studies that reduce sources of noisiness and uncertainty in the results 
could provide more information that would support selection of a target 
level of protection at or just above the lower end of the range.
    Taken together, the 2022 PA concludes that available information 
continues to support a visibility index with estimated light extinction 
as the indicator, a 24-hour averaging time, and a 90th percentile form, 
averaged over three years, with a level within the range of 20 to 30 
dv.
ii. Relationship Between the PM2.5 Visibility Index and the 
Current Secondary 24-Hour PM2.5 Standard
    The 2022 PA presents quantitative analyses based on recent air 
quality that evaluate the relationship between recent air quality and 
calculated light extinction. As in previous reviews, these analyses 
explored this relationship as an estimate of visibility impairment in 
terms of the 24-hour PM2.5 standard and the visibility 
index. Generally, the results of the updated analyses are similar to 
those based on the data available at the time of the 2012 and 2020 
reviews (U.S. EPA, 2022b, section 5.3.1.2). As discussed in section 
V.C.1.a above, the 2022 PA concludes that the available evidence 
continues to support a visibility index with estimated light extinction 
as the indicator, a 24-hour averaging time, and a 90th percentile form, 
averaged over three years, with a level within the range of 20 to 30 
dv. These analyses evaluate visibility impairment in the U.S. under 
recent air quality conditions, particularly those conditions that meet 
the current standards, and the relative influence of various factors on 
light extinction. Given the relationship of visibility with short-term 
PM, we focus particularly on the short-term PM standards.\159\ Compared 
to the 2012 review, updated analyses incorporate several refinements, 
including (1) the evaluation of three versions of the IMPROVE equation 
to calculate light extinction (U.S. EPA, 2022b, Appendix D, Equations 
D-1 through D-3) in order to better understand the influence of 
variability in equation inputs; \160\ (2) the use of 24-hour relative 
humidity data, rather than monthly average relative humidity as was 
used in the 2012 review (U.S. EPA, 2022b, section 5.3.1.2, Appendix D); 
and (3) the inclusion of the coarse fraction in the estimation of light 
extinction (U.S. EPA, 2022b, section 5.3.1.2, Appendix D). The analyses 
in the reconsideration are updated from the 2012 and 2020 reviews and 
include 60 monitoring sites that measure PM2.5 and 
PM10 and are geographically distributed across the U.S. in 
both urban and rural areas (U.S. EPA, 2022b, Appendix D, Figure D-1).
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    \159\ The analyses presented in the 2022 PA focus on the 
visibility index and the current secondary 24-hour PM2.5 
standard with a level of 35 [micro]g/m\3\. However, we recognize 
that all three secondary PM standards influence the PM 
concentrations associated with the air quality distribution. As 
noted in section V.A.1 above, the current secondary PM standards 
include the 24-hour PM2.5 standard, with its level of 35 
[micro]g/m\3\, the annual PM2.5 standard, with its level 
of 15.0 [micro]g/m\3\, and the 24-hour PM10 standard, 
with its level of 150 [micro]g/m\3\. With regard to the annual 
PM2.5 standard, we note that all 60 areas included in the 
analyses meet the current secondary annual PM standard (U.S. EPA, 
2022b, Table D-7).
    \160\ While the PM2.5 monitoring network has an 
increasing number of continuous FEM monitors reporting hourly 
PM2.5 mass concentrations, there continue to be data 
quality uncertainties associated with providing hourly 
PM2.5 mass and component measurements that could be input 
into IMPROVE equation calculations for subdaily visibility 
impairment estimates. As detailed in the 2022 PA, there are 
uncertainties associated with the precision and bias of 24-hour 
PM2.5 measurements (U.S. EPA, 2022b, p. 2-18), as well as 
to the fractional uncertainty associated with 24-hour PM component 
measurements (U.S. EPA, 2022b, p. 2-21). Given the uncertainties 
present when evaluating data quality on a 24-hour basis, the 
uncertainty associated with subdaily measurements may be even 
greater. Therefore, the inputs to these light extinction 
calculations are based on 24-hour average measurements of 
PM2.5 mass and components, rather than subdaily 
information.
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    When light extinction was calculated using the revised IMPROVE 
equation, in areas that meet the current 24-hour PM2.5 
standard for the 2017-2019 time period, all sites have light extinction 
estimates at or below 26 dv (U.S. EPA, 2022b, Figure 5-3). For the four 
locations that exceed the current 24-hour PM2.5 standard, 
light extinction estimates range from 22 dv to 27 dv (U.S. EPA, 2022b, 
Figure 5-3). These findings are consistent with the findings of the 
analyses using the same IMPROVE equation in the 2012 review with data 
from 102 sites with data from 2008-2010 and in the 2020 review with 
data from 67 sites with data from 2015-2017. The analyses presented in 
the 2022 PA indicate similar findings to those from the analyses in the 
2012 and 2020 reviews, i.e., the updated quantitative analysis shows 
that the 3-year visibility metric was no higher than 30 dv \161\ at 
sites meeting the current secondary PM standards, and at most such 
sites the 3-year visibility index values are much lower (e.g., an 
average of 20 dv across the 60 sites).\162\
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    \161\ A 3-year visibility metric with a level of 30 dv would be 
at the upper end of the range of levels identified from the public 
preference studies.
    \162\ When light extinction is calculated using the original 
IMPROVE equation, all 60 sites have 3-year visibility metrics below 
30 dv, 58 sites are at or below 25 dv, and 26 sites are at or below 
20 dv (see U.S. EPA, 2022b, Appendix D, Table D-3).
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    When light extinction was calculated using the revised IMPROVE 
equation,\163\ the resulting 3-year visibility metrics are nearly 
identical to light extinction estimates calculated using the original 
IMPROVE equation (U.S. EPA, 2022b, Figure 5-4), but some sites are just 
slightly higher. Using the revised IMPROVE equation, for those sites 
that meet the current 24-hour PM2.5 standard, the 3-year 
visibility metric is at or below 26 dv. For the four locations that 
exceed the current 24-hour PM2.5 standard, light extinction 
estimates range from 22 dv to 29 dv (U.S. EPA, 2022b, Figure 5-4). 
These results are similar to those for light extinction calculated 
using the original IMPROVE equation,\164\ and those from previous 
reviews.
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    \163\ As described in more detail in the 2022 PA, the revised 
IMPROVE equation divides PM components into smaller and larger sizes 
of particles in PM2.5, with separate mass scattering 
efficiencies and hygroscopic growth functions for each size category 
(U.S. EPA, 2022b, section 5.3.1.1).
    \164\ When light extinction is calculated using the revised 
IMPROVE equation, all 60 sites have 3-year visibility metrics below 
30 dv, 56 sites are at or below 25 dv, and 26 sites are at or below 
20 dv (see U.S. EPA, 2022b, Appendix D, Table D-3).
---------------------------------------------------------------------------

    When light extinction was calculated using the refined equation 
from Lowenthal and Kumar (2016), the resulting 3-year visibility 
metrics are slightly higher at all sites compared to light extinction 
estimates calculated using the original IMPROVE equation (U.S. EPA, 
2022b, Figure 5-5).\165\ These higher estimates are to be expected, 
given the higher OC multiplier included in the IMPROVE equation from 
Lowenthal and Kumar (2016), which reflects the use of data from remote 
areas with higher concentrations of organic PM when validating the 
equation. As such, it is important to note that the Lowenthal and Kumar 
(2016) version of the equation may overestimate light extinction in 
non-remote areas, including the urban areas in the updated analyses in 
this reconsideration.
---------------------------------------------------------------------------

    \165\ When light extinction is calculated using the Lowenthal 
and Kumar IMPROVE equation, 59 sites have 3-year visibility metrics 
below 30 dv, 45 sites are at or below 25 dv, and 15 sites are at or 
below 20 dv. The one site with a 3-year visibility metric of 32 dv 
exceeds the secondary 24-hour PM2.5 standard, with a 
design value of 56 [micro]g/m\3\ (see U.S. EPA, 2022b, Appendix D, 
Table D-3).
---------------------------------------------------------------------------

    Nevertheless, when light extinction is calculated using the 
Lowenthal and

[[Page 16321]]

Kumar (2016) equation for those sites that meet the current 24-hour 
PM2.5 standard, the 3-year visibility metric is generally at 
or below 28 dv. For those sites that exceed the current 24-hour 
PM2.5 standard, three of these sites have a 3-year 
visibility metric ranging between 26 dv and 30 dv, while one site in 
Fresno, California that exceeds the current 24-hour PM2.5 
standard and has a 3-year visibility index value of 32 dv (compared to 
29 dv when light extinction is calculated with the original IMPROVE 
equation) (see U.S. EPA, 2022b, Appendix D, Table D-3). At this site, 
it is likely that the 3-year visibility metric using the Lowenthal and 
Kumar (2016) equation would be below 30 dv if PM2.5 
concentrations were reduced such that the 24-hour PM2.5 
level of 35 [micro]g/m\3\ was attained.
    In considering visibility impairment under recent air quality 
conditions, the 2022 PA recognizes that the differences in the inputs 
to equations estimating light extinction can influence the resulting 
values. For example, given the varying chemical composition of 
emissions from different sources, the 2.1 multiplier for converting OC 
to organic matter (OM) in the Lowenthal and Kumar (2016) equation may 
not be appropriate for all source types. At the time of the 2012 
review, the EPA judged that a 1.6 multiplier was more appropriate, for 
the purposes of estimating visibility index at sites across the U.S., 
than the 1.4 or 1.8 multipliers used in the original and revised 
IMPROVE equations, respectively. A multiplier of 1.8 or 2.1 would 
account for the more aged and oxygenated organic PM that tends to be 
found in more remote regions than in urban regions, whereas a 
multiplier of 1.4 may underestimate the contribution of organic PM 
found in remote regions when estimating light extinction (78 FR 3206, 
January 15, 2013; U.S. EPA, 2012, p. IV-5). The available scientific 
information and results of the air quality analyses indicate that it 
may be appropriate to select inputs to the IMPROVE equation (e.g., the 
multiplier for OC to OM) on a regional basis rather than a national 
basis when calculating light extinction. This is especially true when 
comparing sites with localized PM sources (such as sites in urban or 
industrial areas) to sites with PM derived largely from biogenic 
precursor emissions (that contribute to widespread secondary organic 
aerosol formation), such as those in the southeastern U.S. The 2022 PA 
notes, however, that conditions involving PM from such different 
sources have not been well studied in the context of applying a 
multiplier to estimate light extinction, contributing uncertainty to 
estimates of light extinction for such conditions.
    At the time of the 2012 review, the EPA noted that PM2.5 
is the size fraction of PM responsible for most of the visibility 
impairment in urban areas (77 FR 38980, June 29, 2012). Data available 
at the time of the 2012 review suggested that, generally, 
PM10-2.5 was a minor contributor to visibility impairment 
most of the time (U.S. EPA, 2010b) although the coarse fraction may be 
a major contributor in some areas in the desert southwestern region of 
the U.S. Moreover, at the time of the 2012 review, there were few data 
available from PM10-2.5 monitors to quantify the 
contribution of coarse PM to calculated light extinction. Since that 
time, an expansion in PM10-2.5 monitoring efforts has 
increased the availability of data for use in estimating light 
extinction with both PM2.5 and PM10-2.5 
concentrations included as inputs in the equations. The analysis in the 
2020 PA addressed light extinction at 20 of the 67 PM2.5 
sites where collocated PM10-2.5 monitoring data were 
available. Since that time, PM10-2.5 monitoring data are 
available at more locations and the analyses presented in the 2022 PA 
include those for light extinction estimated with coarse and fine PM at 
all 60 sites. Generally, the contribution of the coarse fraction to 
light extinction at these sites is minimal, contributing less than 1 dv 
to the 3-year visibility metric (U.S. EPA, 2020b, section 5.2.1.2). 
However, the 2022 PA notes that in the updated quantitative analyses, 
only a few sites were in locations that would be expected to have high 
concentrations of coarse PM, such as the Southwest. These results are 
consistent with those in the analyses in the 2019 ISA, which found that 
mass scattering from PM10-2.5 was relatively small (less 
than 10%) in the eastern and northwestern U.S., whereas mass scattering 
was much larger in the Southwest (more than 20%) particularly in 
southern Arizona and New Mexico (U.S. EPA, 2019a, section 13.2.4.1, p. 
13-36).
    Overall, the findings of these updated quantitative analyses are 
generally consistent with those in the 2012 and 2020 reviews. The 3-
year visibility metric was generally below 26 dv in most areas that 
meet the current 24-hour PM2.5 standard. Small differences 
in the 3-year visibility metric were observed between the variations of 
the IMPROVE equation, which may suggest that it may be more appropriate 
to use one version over another in different regions of the U.S. based 
on PM characteristics such as particle size and composition to more 
accurately estimate light extinction.
b. Non-Visibility Effects
    Consistent with the evidence available at the time of the 2012 and 
2020 reviews, and as described in detail in the 2022 PA (U.S. EPA, 
2022b, section 5.3.2.2), the data remain insufficient to conduct 
quantitative analyses for PM effects on climate and materials. For PM-
related climate effects, as explained in more detail in the proposal 
(88 FR 5654, January 27, 2023), our understanding of PM-related climate 
effects is still limited by significant key uncertainties. The recently 
available evidence does not appreciably improve our understanding of 
the spatial and temporal heterogeneity of PM components that contribute 
to climate forcing (U.S. EPA, 2022b, sections 5.3.2.1.1 and 5.5). 
Significant uncertainties also persist related to quantifying the 
contributions of PM and PM components to the direct and indirect 
effects on climate forcing, such as changes to the pattern of rainfall, 
changes to wind patterns, and effects on vertical mixing in the 
atmosphere (U.S. EPA, 2022b, sections 5.3.2.1.1 and 5.5). Additionally, 
while improvements have been made to climate models since the 
completion of the 2009 ISA, the models continue to exhibit variability 
in estimates of the PM-related climate effects on regional scales 
(e.g., ~100 km) compared to simulations at the global scale (U.S. EPA, 
2022b, sections 5.3.2.1.1 and 5.5). While our understanding of climate 
forcing on a global scale is somewhat expanded since the 2012 review, 
significant limitations remain to quantifying potential adverse PM-
related climate effects in the U.S. and how they would vary in response 
to incremental changes in PM concentrations across the U.S. As such, 
while recent research is available on climate forcing on a global 
scale, the remaining limitations and uncertainties are significant, and 
the recent global scale research does not translate directly for use at 
regional spatial scales. Therefore, the evidence does not provide a 
clear understanding at the necessary spatial scales for quantifying the 
relationship between PM mass in ambient air and the associated climate-
related effects in the U.S. that would be necessary to evaluate or 
consider a level of air quality to protect against such effects and for 
informing consideration of a national PM standard on climate in this 
reconsideration (U.S. EPA, 2022b, section 5.3.2.2.1; U.S. EPA, 2019a, 
section 13.3).

[[Page 16322]]

    For PM-related materials effects, as explained in more detail in 
the 2022 PA (U.S. EPA, 2022b, section 5.3.2.2), the available evidence 
has been somewhat expanded to include additional information about the 
soiling process and the types of materials impacted by PM. This 
evidence provides some limited information to inform dose-response 
relationships and damage functions associated with PM, although most of 
these studies were conducted outside of the U.S. where PM 
concentrations in ambient air are typically above those observed in the 
U.S. (U.S. EPA, 2022b, section 5.3.2.1.2; U.S. EPA, 2019a, section 
13.4). The evidence on materials effects characterized in the 2019 ISA 
also includes studies examining effects of PM on the energy efficiency 
of solar panels and passive cooling building materials, although the 
evidence remains insufficient to establish quantitative relationships 
between PM in ambient air and these or other materials effects (U.S. 
EPA, 2022b, section 5.3.2.1.2). While the available evidence assessed 
in the 2019 ISA is somewhat expanded since the time of the 2012 review, 
quantitative relationships have not been established for PM-related 
soiling and corrosion and frequency of cleaning or repair that further 
the understanding of the public welfare implications of materials 
effects (U.S. EPA, 2022b, section 5.3.2.2.2; U.S. EPA, 2019a, section 
13.4). Therefore, there is insufficient information to inform 
quantitative analyses assessing materials effects to inform 
consideration of a national PM standard on materials in this 
reconsideration (U.S. EPA, 2022b, section 5.3.2.2.2; U.S. EPA, 2019a, 
section 13.4).

B. Conclusions on the Secondary PM Standards

    In drawing conclusions on the adequacy of the current secondary PM 
standards, in view of the advances in scientific knowledge and 
additional information now available, the Administrator has considered 
the evidence base, information, and policy judgments that were the 
foundation of the 2020 decision and reflects upon the body of 
information and evidence available in this reconsideration. In so 
doing, the Administrator has taken into account both evidence-based and 
quantitative information-based considerations, as well as advice from 
the CASAC and public comments. Evidence-based considerations draw upon 
the EPA's assessment and integrated synthesis of the scientific 
evidence from studies evaluating welfare effects related to visibility, 
climate, and materials associated with PM in ambient air as discussed 
in the 2022 PA (summarized in sections V.B and V.D.2 of the proposal, 
section V.A.2 above). The quantitative information-based considerations 
draw from the results of the quantitative analyses of visibility 
impairment presented in the 2022 PA (as summarized in section V.C of 
the proposal and V.A.3 above) and consideration of these results in the 
2022 PA.
    Consideration of the scientific evidence and quantitative 
information in the 2022 PA and by the Administrator is framed by 
consideration of a series of policy-relevant questions. Section V.B.2 
below summarizes the rationale for the Administrators proposed 
decision, drawing from section V.D.3 of the proposal. The advice and 
recommendations of the CASAC and public comments on the proposed 
decision are addressed below in sections V.B.1 and V.B.3, respectively. 
The Administrator's conclusions in this reconsideration regarding the 
adequacy of the secondary PM standards and whether any revisions are 
appropriate are described in section V.D.4.
1. CASAC Advice
    In comments on the 2019 draft PA, the CASAC concurred with the 
staff's overall preliminary conclusions that it is appropriate to 
consider retaining the current secondary standards without revision 
(Cox, 2019b). The CASAC ``finds much of the information . . . on 
visibility and materials effects of PM2.5 to be useful, 
while recognizing that uncertainties and controversies remain about the 
best ways to evaluate these effects'' (Cox, 2019b, p. 13 of consensus 
responses). Regarding climate, while the CASAC agreed that research on 
PM-related effects has expanded since the 2012 review, it also 
concluded that ``there are still significant uncertainties associated 
with the accurate measurement of PM to the direct and indirect effects 
of PM on climate'' (Cox, 2019b, pp. 13-14 of consensus responses). The 
committee recommended that the EPA summarize the ``current scientific 
knowledge and quantitative modeling results for effects of reducing 
PM2.5'' on several climate-related outcomes (Cox, 2019b, p. 
14 of consensus responses), while also recognizing that ``it is 
appropriate to acknowledge uncertainties in climate change impacts and 
resulting welfare impacts in the United States of reductions in 
PM2.5 levels'' (Cox, 2019b, p. 14 of consensus responses). 
When considering the overall body of scientific evidence and technical 
information for PM-related effects on visibility, climate, and 
materials, the CASAC agreed with the EPA's preliminary conclusions in 
the 2019 draft PA, stating that ``the available evidence does not call 
into question the protection afforded by the current secondary PM 
standards and concurs that they should be retained'' (Cox, 2019b, p. 3 
of letter).
    In this reconsideration, the CASAC provided its advice regarding 
the current secondary PM standards in the context of its review of the 
2021 draft PA (Sheppard, 2022a). In its comments on the 2021 draft PA, 
the CASAC first recognized that the scientific evidence is sufficient 
to support a causal relationship between PM and visibility effects, 
climate effects and materials effects.
    With regard to visibility effects, the CASAC recognized that the 
identification of a target level of protection for the visibility index 
is based on a limited number of studies and suggested that ``additional 
region- and view-specific visibility preference studies and data 
analyses are needed to support a more refined visibility target'' 
(Sheppard, 2022a, p. 21 of consensus responses). While the CASAC did 
not recommend revising either the target level of protection for the 
visibility index or the level of the current 24-hour PM2.5 
standard, they did state that a visibility index of 30 deciviews 
``needs to be justified'' and ``[i]f a value of 20-25 deciviews is 
deemed to be an appropriate visibility target level of protection, then 
a secondary 24-hour PM2.5 standard in the range of 25-35 
[micro]g/m\3\ should be considered'' (Sheppard, 2022a, p. 21 of 
consensus responses).
    The CASAC also recognized the limited availability of monitoring 
methods and networks for directly measuring light extinction. As such, 
they suggest that ``[a] more extensive technical evaluation of the 
alternatives for visibility indicators and practical measurement 
methods (including the necessity for a visibility FRM) is need for 
future reviews'' (Sheppard, 2022a, p. 22 of consensus letter). The 
majority of the CASAC ``recommend[ed] that an FRM for a directly 
measured PM2.5 light extinction indicator be developed'' to 
inform the consideration of the protection afforded by the secondary PM 
standards against visibility impairment, the minority of the CASAC 
``believe that a light extinction FRM is not necessary to set a 
secondary standard protective of visibility'' (Sheppard, 2022a, p. 22 
of consensus responses).

[[Page 16323]]

    With regard to climate, the CASAC noted that ``there is a causal 
relationship between PM and climate change, but large uncertainties 
remain'' and recommended additional research (Sheppard, 2022a, p. 22 of 
consensus responses). With respect to materials damage, the CASAC noted 
that ``[q]uantitative information on the relationship between PM and 
material damage is lacking'' and suggested some additional studies and 
research approaches that could provide additional information on the 
effects of PM on materials and the quantitative assessment of the 
relationship between materials effects and PM in ambient air (Sheppard, 
2022a, p. 23 of consensus responses).
2. Basis for the Proposed Decision
    In reaching his proposed conclusions, the Administrator first 
recognized that, consistent with the scope of this reconsideration, his 
decision in this reconsideration will be focused only and specifically 
on the adequacy of public welfare protection provided by the secondary 
PM standards from effects related to visibility, climate, and 
materials. He then considered the assessment of the current evidence 
and conclusions reached in the 2019 ISA and ISA Supplement; the 
currently available quantitative information, including associated 
limitations and uncertainties, described in detail and characterized in 
the 2022 PA; considerations and staff conclusions and associated 
rationales presented in the 2022 PA; and the advice and recommendations 
from the CASAC (88 FR 5655, January 27, 2023).
    With respect to visibility, the Administrator noted the 
longstanding body of evidence that demonstrates a causal relationship 
between ambient PM and effects on visibility (U.S. EPA, 2019a, section 
13.2). and that visibility impairment can have implications for 
people's enjoyment of daily activities and for their overall sense of 
well-being. Therefore, as in previous reviews, he considered the degree 
to which the current secondary standards protect against PM-related 
visibility impairment. In so doing, and consistent with previous 
reviews, the Administrator considered the protection provided by the 
current secondary standards against PM-related visibility impairment in 
conjunction with the Regional Haze Program \166\ for protecting 
visibility in Class I areas,\167\ which together would be expected to 
achieve appropriate visual air quality across all areas (88 FR 5658, 
January 27, 2023). The Administrator proposed to conclude that 
addressing visibility impairment in Class I areas is beyond the scope 
of the secondary PM NAAQS and that setting the secondary PM NAAQS at a 
level that would remedy visibility impairment in Class I areas would 
result in standards that are more stringent than is requisite.
---------------------------------------------------------------------------

    \166\ The Regional Haze Program was established by Congress 
specifically to achieve ``the prevention of any future, and the 
remedying of existing, impairment of visibility in mandatory Class I 
areas, which impairment results from man-made air pollution,'' and 
that Congress established a long-term program to achieve that goal 
(CAA section 169A).
    \167\ In adopting section 169A, Congress set a goal of 
eliminating anthropogenic visibility impairment at Class I areas, as 
well as a framework for achieving that goal which extends well 
beyond the planning process and timeframe for attaining secondary 
NAAQS. Thus, the Regional Haze Program will continue to contribute 
to reductions in visibility impairment in Class I areas.
---------------------------------------------------------------------------

    In further considering what standards are requisite to protect 
against adverse public welfare effects from visibility impairment, the 
Administrator adopted an approach consistent with the approach used in 
previous reviews (88 FR 5645, January 27, 2023). That is, he first 
identified an appropriate target level of protection in terms of a PM 
visibility index that accounts for the factors that influence the 
relationship between particles in the ambient air and visibility (i.e., 
size fraction, species composition, and relative humidity). He then 
considered air quality analyses examining the relationship between this 
PM visibility index and the current secondary 24-hour PM2.5 
standard in locations meeting the current 24-hour PM2.5 and 
PM10 standards (U.S. EPA, 2022b, section 5.3.1.2; 88 FR 
5650, January 27, 2023).
    To identify a target level of protection, the Administrator first 
considered the characteristics of the visibility index and defines its 
elements (indicator, averaging time, form, and level). With regard to 
the indicator for the visibility index, the Administrator recognized 
that there is a lack of availability of methods and an established 
network for directly measuring light extinction, consistent with the 
conclusions reached in the 2022 PA (U.S. EPA, 2022b, section 5.3.1.1) 
and with the CASAC's recommendation for additional research on direct 
measurement methods for light extinction in their review of the 2021 
draft PA (Sheppard, 2022a, p. 22 of consensus responses). Consistent 
with the approaches used in reaching decisions in 2012 and 2020, given 
the lack of such monitoring data, the Administrator preliminarily 
judged that estimated light extinction, as calculated using one or more 
versions of the IMPROVE algorithms, continues to be the most 
appropriate indicator for the visibility index in this reconsideration 
(88 FR 5659, January 27, 2023).
    In further defining the characteristics of a visibility index based 
on estimates of light extinction, the Administrator considered the 
appropriate averaging time, form, and level of the index. With regard 
to the averaging time and form, the Administrator noted that in 
previous reviews, a 24-hour averaging time was selected and the form 
was defined as the 3-year average of annual 90th percentile values. The 
Administrator recognized that the evidence available in this 
reconsideration and described in the 2022 PA continue to provide 
support for the short-term nature of PM-related visibility effects. 
Considering the available analyses of 24-hour and subdaily 
PM2.5 light extinction, and noting that the CASAC did not 
provide advice or recommendations with regard to the averaging time of 
the visibility index, the Administrator preliminarily judged that the 
24-hour averaging time continues to be appropriate for the visibility 
index (88 FR 5659, January 27, 2023).
    With regard to the form of the visibility index, the Administrator 
noted that, consistent with the approach taken in other NAAQS, 
including the current secondary 24-hour PM2.5 NAAQS, a 
multi-year percentile form offers greater stability to the air quality 
management process by reducing the possibility that statistically 
unusual indicator values will lead to transient violations of the 
standard. Using a 3-year average provides stability from the occasional 
effects of inter-annual meteorological variability that can result in 
unusually high pollution levels for a particular year (88 FR 5659, 
January 27, 2023). In considering the percentile that would be 
appropriate with the 3-year average, the Administrator first noted that 
the Regional Haze Program targets the 20% most impaired days for 
improvements in visual air quality in Class I areas.\168\ Based on 
analyses examining 90th, 95th, and 98th percentile forms, the 
Administrator preliminarily judged that a focus similar to the Regional 
Haze Program focused on improving the 20% most impaired days suggest 
that the 90th percentile, which represents the median of the 20% most 
impaired days, such that 90% of days have visual air quality that is at 
or below the target level of protection of the visibility

[[Page 16324]]

index, would be reasonably expected to lead to improvements in visual 
air quality for the 20% most impaired days (88 FR 5659, January 27, 
2023). In the analyses of percentiles, the results suggest that a 
higher percentile value could have the effect of limiting the 
occurrence of days with peak PM-related light extinction in areas 
outside of Federal Class I areas to a greater degree. However, the 
Administrator preliminarily concluded that it is appropriate to balance 
concerns about focusing on the group of most impaired days with 
concerns about focusing on the days with peak visibility impairment. 
Additionally, the Administrator noted that the CASAC did not provide 
advice or recommendations related to the form of the visibility index. 
Therefore, the Administrator preliminarily judged that it remains 
appropriate to define a visibility index in terms of a 24-hour 
averaging time and a form based on the 3-year average of annual 90th 
percentile values (88 FR 5659, January 27, 2023).
---------------------------------------------------------------------------

    \168\ As noted above, the Administrator viewed the Regional Haze 
Program as a complement to the secondary PM NAAQS, and thus took 
into consideration its approach to improving visibility in 
considering how to address visibility outside of Class I areas.
---------------------------------------------------------------------------

    With regard to the level of the visibility index, the Administrator 
first noted that the scientific evidence that is available to inform 
the level of the visibility index is largely the same as in previous 
reviews, and continues to provide support for a level within the range 
of 20 to 30 dv (88 FR 5659-5660, January 27, 2023). The Administrator 
recognized that significant uncertainties and limitations remained, in 
particular those related to the public preference studies, including 
methodological differences between the studies, and that the available 
studies may not capture the full range of visibility preferences in the 
U.S. population (88 FR 5659-5660, January 27, 2023). The Administrator 
also noted that, in their review of the 2021 draft PA, the CASAC 
recognized that a judgment regarding the appropriate target level of 
protection for the visibility index is based on a limited number of 
visibility preference studies, with studies conducted in the western 
U.S. reporting public preferences for visibility impairment associated 
with the lower end of the range of levels, while studies conducted in 
the eastern U.S. reporting public preferences associated with the upper 
end of the range (Sheppard, 2022a, p. 21 of consensus responses). The 
Administrator noted that there have long been significant questions 
about how to set a national standard for visibility that is not 
overprotective for some areas of the U.S. In establishing the Regional 
Haze Program to improve visibility in Class I areas, Congress noted 
that ``as a matter of equity, the national ambient air quality 
standards cannot be revised to adequately protect visibility in all 
areas of the country.'' H.R. Rep. 95-294 at 205. Thus, in reaching his 
proposed conclusion, the Administrator recognized that there are 
substantial uncertainties and limitations in the public preference 
studies that should be considered when selecting a target level of 
protection for the visibility index and took the uncertainties and 
variability inherent in the public preference studies into account. In 
so doing, the Administrator first preliminarily judged that, consistent 
with similar judgments in past reviews, it is appropriate to recognize 
that the secondary 24-hour PM2.5 standard is intended to 
address visibility impairment across a wide range of regions and 
circumstances, and that the current standard works in conjunction with 
the Regional Haze Program to improve visibility, and therefore, it is 
appropriate to establish a target level of protection based on the 
upper end of the range of levels. In considering the information 
available in this reconsideration and the CASAC's advice, the 
Administrator proposed to conclude that the protection provided by a 
visibility index based on estimated light extinction, a 24-hour 
averaging time, and a 90th percentile form, averaged over 3 years, set 
at a level of 30 dv (the upper end of the range of levels) would be 
requisite to protect public welfare with regard to visibility 
impairment (88 FR 5660, January 27, 2023).
    In preliminarily concluding that it remains appropriate in this 
reconsideration to define the target level of protection in terms of a 
visibility index based on estimated light extinction as described above 
(i.e., with a 24-hour averaging time; a 3-year, 90th percentile form; 
and a level of 30 dv), the Administrator next considered the degree of 
protection from visibility impairment afforded by the existing 
secondary standards. He considered the updated analyses of PM-related 
visibility impairment presented in the 2022 PA (U.S. EPA, 2022b, 
section 5.3.1.2), which reflect several improvements over the analyses 
conducted in the 2012 review. Specifically, the updated analyses 
examine multiple versions of the IMPROVE algorithm, including the 
version incorporating revisions since the 2012 review (section 
V.B.1.a), which provides an improved understanding of how variation in 
equation inputs impacts calculated light extinction (U.S. EPA, 2022b, 
Appendix D). In addition, unlike the analyses in the 2012 review and 
the 2020 PA, all of the sites included in the analyses had 
PM10-2.5 data available, which allows for better 
characterization of the influence of the coarse fraction on light 
extinction (U.S. EPA, 2022b, section 5.3.1.2).
    The Administrator noted that the results of these updated analyses 
are consistent with the results from the 2012 and 2020 reviews (88 FR 
5660, January 27, 2023). Regardless of the IMPROVE equation used, these 
analyses demonstrate that the 3-year visibility metric is at or below 
28 dv in all areas meeting the current 24-hour PM2.5 
standard (section V.C.1.b). Given the results of these analyses, the 
Administrator preliminarily concluded that the updated scientific 
evidence and technical information support the adequacy of the current 
secondary PM2.5 and PM10 standards to protect 
against PM-related visibility impairment. While the inclusion of the 
coarse fraction had a relatively modest impact on calculated light 
extinction in the analyses presented in the 2022 PA, he nevertheless 
recognized the continued importance of the PM10 standard 
given the potential for larger impacts in locations with higher coarse 
particle concentrations, such as in the southwestern U.S., for which 
only a few sites met the criteria for inclusion in the analyses in the 
2022 PA (U.S. EPA, 2019a, section 13.2.4.1; U.S. EPA, 2022b, section 
5.3.1.2).
    With regard to the adequacy of the secondary 24-hour 
PM2.5 standard, the Administrator noted that the CASAC 
stated that ``[i]f a value of 20-25 deciviews is deemed to be an 
appropriate visibility target level of protection, then a secondary 24-
hour PM2.5 standard in the range of 25-35 [micro]g/m\3\ 
should be considered'' (Sheppard, 2022a, p. 21 of consensus responses). 
The Administrator recognized that the CASAC recommended that the 
Administrator provide additional justification for a visibility index 
target of 30 dv but did not specifically recommend that he choose an 
alternative level for the visibility index. The Administrator 
considered the CASAC's advice, together with the available scientific 
evidence and quantitative information, in reaching his proposed 
conclusions. He recognized conclusions regarding the appropriate weight 
to place on the scientific and technical information examining PM-
related visibility impairment including how to consider the range and 
magnitude of uncertainties inherent in that information is a public 
welfare policy judgment left to the Administrator. As such, the 
Administrator noted his conclusion on

[[Page 16325]]

the appropriate visibility index (i.e., with a 24-hour averaging time; 
a 3-year, 90th percentile form; and a level of 30 dv) and his 
conclusions regarding the quantitative analyses of the relationship 
between the visibility index and the current secondary 24-hour 
PM2.5 standard. In so doing, he proposed to conclude that 
the current secondary standards provide requisite protection against 
PM-related visibility effects (88 FR 5661, January 27, 2023).
    In reaching his proposed conclusions, the Administrator also 
recognized that the available evidence on visibility impairment 
generally reflects a continuum and that the public preference studies 
did not identify a specific level of visibility impairment that would 
be perceived as ``acceptable'' or ``unacceptable'' across the whole 
U.S. population. However, he noted that a judgment regarding the 
appropriate target level of protection would take into consideration 
the appropriate weight to place on the individual public preference 
studies. In so doing, he noted that placing more weight on the public 
preference study from Washington, DC, could provide support for a 
target level of protection at or near 30 dv, whereas placing more 
weight on the public preference study performed in the Phoenix, AZ, 
study could provide support for a target level of protection below 30 
dv and down to 25 dv. While the Administrator noted that, in their 
review of the 2021 draft PA, the CASAC did not recommend revising the 
level of the current 24-hour PM2.5 standard, the 
Administrator recognized that they did recommend greater justification 
for a target level of protection of 30 dv, and noted that if a target 
level of protection of 20-25 dv was identified, then a secondary 24-
hour PM2.5 standard in the range of 25-35 [mu]g/m\3\ should 
be considered (Sheppard, 2022a, p. 21 of consensus responses). For 
these reasons, the Administrator solicited comment on his proposed 
decision to retain the current secondary 24-hour PM2.5 
standard, as well as the appropriateness of a target level of 
protection for visibility below 30 dv and as low as 25 dv, and on 
revising the level of the current secondary 24-hour PM2.5 
standard to a level as low as 25 [micro]g/m\3\.
    With respect to climate effects, the Administrator recognized that 
a number of improvements and refinements have been made to climate 
models since the time of the 2012 review. However, despite continuing 
research and the strong evidence supporting a causal relationship with 
climate effects (U.S. EPA, 2019a, section 13.3.9), the Administrator 
noted that there are still significant limitations in quantifying the 
contributions of the direct and indirect effects of PM and PM 
components on climate forcing (U.S. EPA, 2022b, sections 5.3.2.1.1 and 
5.5). He also recognized that models continue to exhibit considerable 
variability in estimates of PM-related climate impacts at regional 
scales (e.g., ~100 km), compared to simulations at the global scale 
(U.S. EPA, 2022b, sections 5.3.2.1.1 and 5.5). As noted above, the 
CASAC recognized a causal relationship between PM and climate effects 
but also the large uncertainties associated with quantitatively 
assessing such effects, particularly on a national level in the context 
of a U.S.-based standard. These uncertainties led the Administrator to 
preliminarily conclude that the scientific information available in 
this reconsideration remains insufficient to quantify, with confidence, 
the impacts of ambient PM on climate in the U.S. (U.S. EPA, 2022b, 
section 5.3.2.2.1) and that there is insufficient information at this 
time to revise the current secondary PM standards or to promulgate a 
distinct secondary standard to address PM-related climate effects (88 
FR 5661, January 27, 2023).
    With respect to materials effects, the Administrator noted that the 
available evidence continues to support the conclusion that there is a 
causal relationship with PM deposition (U.S. EPA, 2019a, section 13.4). 
He recognized that deposition of particles in the fine or coarse 
fractions can result in physical damage and/or impaired aesthetic 
qualities. Particles can contribute to materials damage by adding to 
the effects of natural weathering processes and by promoting the 
corrosion of metals, the degradation of painted surfaces, the 
deterioration of building materials, and the weakening of material 
components. While some recent evidence on materials effects of PM is 
available in the 2019 ISA, the Administrator noted that this evidence 
is primarily from studies conducted outside of the U.S. in areas where 
PM concentrations in ambient air are higher than those observed in the 
U.S. (U.S. EPA, 2019a, section 13.4). The CASAC also noted the lack of 
quantitative information relating PM and material effects. Given the 
limited amount of information on the quantitative relationships between 
PM and materials effects in the U.S., and uncertainties in the degree 
to which those effects could be adverse to the public welfare, the 
Administrator preliminarily judged that the scientific information 
available in this reconsideration remains insufficient to quantify, 
with confidence, the public welfare impacts of ambient PM on materials 
and that there is insufficient information at this time to revise the 
current secondary PM standards or to promulgate a distinct secondary 
standard to address PM-related materials effects (88 FR 5661, January 
27, 2023).
    Taken together, the Administrator proposed to conclude that the 
scientific and technical information for PM-related visibility 
impairment, climate impacts, and materials effects, with its attendant 
uncertainties and limitations, supports the current level of protection 
provided by the secondary PM standards as being requisite to protect 
against known and anticipated adverse effects on public welfare. For 
visibility impairment, this proposed conclusion reflected his 
consideration of the evidence for PM-related light extinction, together 
with his consideration of updated analyses of the protection provided 
by the current secondary PM2.5 and PM10 
standards. For climate and materials effects, this conclusion reflected 
his preliminary judgment that, although it remains important to 
maintain secondary PM2.5 and PM10 standards to 
provide some degree of control over long- and short-term concentrations 
of both fine and coarse particles, it is generally appropriate not to 
change the existing secondary standards at this time and that it is not 
appropriate to establish any distinct secondary PM standards to address 
PM-related climate and materials effects at this time. As such, the 
Administrator recognized that current suite of secondary standards 
(i.e., the 24-hour PM2.5, 24-hour PM10, and 
annual PM2.5 standards) together provide such control for 
both fine and coarse particles and long- and short-term visibility and 
non-visibility (e.g., climate and materials) \169\ effects related to 
PM in ambient air. His proposed conclusions on the secondary standards 
were consistent with advice from the CASAC, which noted substantial 
uncertainties remain in the scientific evidence for climate and 
materials effects. Thus, based on his consideration of the evidence and 
analyses for PM-related welfare effects, as described above, and his 
consideration of CASAC advice on the secondary standards, the 
Administrator proposed not to change those standards (i.e., the current 
24-hour and annual PM2.5 standards, 24-hour PM10 
standard) at this time (88 FR 5662, January 27, 2023).
---------------------------------------------------------------------------

    \169\ As noted earlier, other welfare effects of PM, such as 
ecological effects, are being considered in the separate, on-going 
review of the secondary NAAQS for oxides of nitrogen, oxides of 
sulfur and PM.

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[[Page 16326]]

3. Comments on the Proposed Decision
    Of the public comments received on the proposal, very few were 
specific to the secondary PM standards. Of those commenters who did 
provide comments on the secondary PM standards, the majority support 
the Administrator's proposed decision to retain the current standards. 
Some commenters disagree with the Administrator's proposed conclusion 
to retain the current secondary standards, primarily focusing their 
comments on the need for a revised standard to protect against 
visibility impairment. In addition to the comments addressed in this 
notice, the EPA has prepared a Response to Comments document that 
addresses other specific comments related to setting the secondary PM 
standards. This document is available for review in the docket for this 
rulemaking and through the EPA's NAAQS website (https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards).
    We first note that some commenters raise questions about the 
protection provided by the secondary PM standards for ecological 
effects (e.g., effects on ecosystems, ecosystem services, or species). 
However, consistent with the 2016 IRP and as described in the proposal 
(88 FR 5643, January 27, 2023), other welfare effects of PM, such as 
the ecological effects identified by commenters, are being considered 
as part of the separate, ongoing review of the secondary standards for 
oxides of sulfur, oxides of nitrogen and PM, and thus, those comments 
are beyond the scope of this action.
    Of the comments addressing the proposed decision for the secondary 
PM standards, many of the commenters support the Administrator's 
proposed decision to retain the current secondary PM standards, without 
revision. This group includes industries and industry groups and State 
and local governments and organizations. All of these commenters 
generally note their agreement with the rationale provided in the 
proposal, with a focus on the strength of the available scientific 
evidence for PM-related welfare effects. Most also recognize that the 
scientific evidence and quantitative information available in this 
reconsideration have not substantially altered our previous 
understanding of PM-related effects on non-ecological welfare effects 
(i.e., visibility, climate, and materials) and do not call into 
question the adequacy of the current secondary standards. They find the 
proposed decision not to change the standards at this time to be well 
supported and a reasonable exercise of the Administrator's public 
welfare policy judgment under the CAA. The EPA agrees with these 
comments regarding the adequacy of the current secondary PM standards 
and the lack of support for revision of these standards at this time.
    The EPA received relatively few comments on the proposed decision 
that it is not appropriate to establish any distinct secondary PM 
standards to address PM-related climate effects. Several commenters 
agree that the available scientific evidence provides support for the 
2019 conclusion that there is a causal relationship between PM and 
climate effects, and the commenters also agree with the EPA that the 
currently available information is not sufficient for supporting 
quantitative analyses for the climate effects of PM in ambient air. 
These commenters support the Administrator's proposed decision not to 
set a distinct standard for climate.
    There were also very few commenters who commented on the proposed 
decision that it is not appropriate to establish any distinct secondary 
PM standards to address PM-related materials effects. As with comments 
on climate effects, commenters generally agree with the EPA that the 
evidence is not sufficient to support quantitative analyses for PM-
related materials effects. However, some commenters contend that EPA 
failed to explain in the proposal how the current standard is 
appropriate to protect materials from the effects of PM. These 
commenters disagree with the EPA's conclusion that quantitative 
relationships have not been established for PM-related soiling and 
corrosion and frequency of cleaning or repair of materials, and cite to 
several studies conducted outside the U.S. that they contend that the 
EPA should consider since the same materials are present in the U.S. 
They further contend that, in discussing the available scientific 
evidence in the 2019 ISA for studies conducted outside of the U.S., the 
EPA did not provide references to these studies and, therefore, the 
public is unable to comment on these studies. They further State that 
EPA failed to consider the following information: (1) Recent work 
related to soiling of photovoltaic modules and other surfaces, and; (2) 
damage and degradation resulting from oxidant concentrations and solar 
radiation for a number of materials, including polymeric materials, 
plastic, paint, and rubber. These commenters further assert that the 
EPA failed to propose a standard that provides requisite protection 
against materials effects attributable to PM.
    As an initial matter, we note that the commenters submitted the 
same comments related to materials effects during the 2020 review. 
Consistent with our response in the 2020 notice of final rulemaking (85 
FR 82737, December 18, 2020), we disagree with the commenters that the 
EPA failed to consider the relevant scientific information about 
materials effects available in this reconsideration. The 2019 ISA 
considered and included studies related to materials effects of PM, 
including studies conducted in and outside of the U.S., on newly 
studied materials including photovoltaic modules that were published 
prior to the cutoff date for the literature search.\170\ These include 
the Besson et al. (2017) study referenced by the commenters (U.S. EPA, 
2019a, section 13.4.2). The Gr[oslash]ntoft et al. (2019) study 
referenced by the same commenters was published after the cutoff date 
for the literature search for the 2019 ISA. However, the EPA 
provisionally considered new studies in responding to comments in the 
2020 review, including the new studies highlighted by the commenters in 
their comments on the 2020 notice of proposed rulemaking, in the 
context of the findings of the 2019 ISA (see Appendix in U.S. EPA, 
2020a).\171\ Based on the provisional consideration, the EPA concluded 
in the 2020 review that the new studies are not sufficient to alter the 
conclusions reached in the 2019 ISA regarding PM and materials effects. 
For example, the Gr[oslash]ntoft et al. (2019) study was based on 
European air pollution which as the EPA has noted has higher 
concentrations (as well as diversity in sources, such as light duty 
diesel engines) compared to the U.S.. Thus, the EPA did not find it 
necessary or appropriate to reopen the air quality criteria to consider 
this study because it would not have been an adequate basis on which to 
set a NAAQS. As discussed in section I, when the EPA decided to 
reconsider the standards, it also decided to reopen the air quality 
criteria to a limited degree, based on its judgment that certain new 
studies were likely to be useful in reconsidering the standards.

[[Page 16327]]

Based on the provisional consideration in the 2020 review and the 
significant data gaps that existed at that time, the EPA did not 
include these studies within the scope of the 2022 ISA Supplement 
because, although these studies provide additional support for PM-
related materials, the studies would not support quantitative analyses 
or alternative conclusions regarding these effects. As described in 
section I.C.5.b above, the ISA Supplement focuses on a thorough 
evaluation of some studies that became available after the literature 
cutoff date of the 2019 ISA that could either further inform the 
adequacy of the current PM NAAQS or address key scientific topics that 
have evolved since the literature cutoff date for the 2019 ISA. In 
developing the ISA Supplement, the EPA focused on the non-ecological 
welfare effects for which the evidence supported a ``causal 
relationship'' and for which quantitative analyses could be supported 
by the evidence because those were the welfare effects that were most 
useful in informing conclusions in the 2020 PA. While the 2020 PA 
considered the broader set of evidence for materials effects, it 
concluded that there remained `substantial uncertainties with regard to 
the quantitative relationships with PM concentrations and concentration 
patterns that limit[ed] [the] ability to quantitatively assess the 
public welfare protection provided by the standards from these effects' 
(U.S. EPA, 2020b).'' Therefore, the ISA Supplement did not include an 
evaluation of scientific evidence for PM-related materials effects. 
However, the EPA has once again provisionally considered new studies in 
this reconsideration, including the studies highlighted by the 
commenters, in the context of the 2019 ISA and concludes that, as in 
the 2020 review, these studies are not sufficient to alter the 
conclusions reached in the 2019 ISA regarding PM and materials effects 
or to provide sufficient information on which to base a secondary 
NAAQS. The EPA agrees there is a causal relationship between the 
presence of PM in the ambient air and materials effects, but to set a 
standard, the EPA needs not only to understand at what point materials 
effects become adverse to public welfare but to be able to relate 
specific concentrations of ambient PM to those levels of materials 
effects. Given the significant gaps in the evidence, particularly given 
that the majority of the recent evidence has been conducted outside of 
the U.S., establishing any quantitative relationships between particle 
size, concentration, chemical components, and specific measures of 
materials damage, such as frequency of painting or repair of materials, 
the EPA finds the evidence is insufficient to support a secondary NAAQS 
to protect against materials effects.
---------------------------------------------------------------------------

    \170\ As noted earlier in section V, the 2019 ISA ``identified 
and evaluated studies and reports that that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009, and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019a, Appendix, p. 
A-3).
    \171\ As discussed in section I.D, the EPA has provisionally 
considered studies that were highlighted by commenters and that were 
published after the 2019 ISA. These studies are generally consistent 
with the evidence assessed in the 2019 ISA, and they do not 
materially alter our understanding of the scientific evidence or the 
Agency's conclusions based on that evidence.
---------------------------------------------------------------------------

    With regard to studies conducted outside of the U.S., including 
those referenced by the commenters, as described in the proposal, in 
reaching his proposed conclusion, the Administrator recognized that 
while there was some newly available information related to materials 
effects of PM included in the 2019 ISA, ``this evidence is primarily 
from studies conducted outside of the U.S. in areas where PM 
concentrations in ambient air are higher than those observed in the 
U.S. (U.S. EPA, 2019a, section 13.4)'' (88 FR 5661, January 27, 2023). 
We disagree with the commenters that EPA did not provide references for 
these studies, nor that the lack of references inhibited the public's 
ability to provide comment on this proposed conclusion. First, the 
reference to section 13.4 in the 2019 ISA is a direct citation to the 
evaluation of newly available studies on PM-related materials effects, 
which includes citations for all materials effects evidence considered 
in the 2020 review and in this reconsideration. Second, section 
5.3.2.1.2 of the 2022 PA considers the available scientific evidence 
for PM-related materials effects--including citations to the studies 
newly available in the 2019 ISA--and how that evidence informs 
conclusions regarding the adequacy of the standard (U.S. EPA, 2022b, 
section 5.3.2.1.2). Therefore, the EPA disagrees that the proposal 
failed to provide the proper references to the studies conducted 
outside of the U.S., and that the public was not provided the 
opportunity to provide comment on these studies.
    Moreover, we disagree with the commenters that the EPA failed to 
consider quantitative information from studies available in this 
reconsideration. As detailed in sections 5.3.2.1.2 and 5.3.2.2 of the 
2022 PA, and consistent with the information available in the 2020 
review, a number of new studies are available that apply new methods to 
characterize PM-related effects on previously studied materials; 
however, the evidence remains insufficient to relate soiling or damage 
to specific levels of PM in ambient air or to establish quantitative 
relationships between PM and materials degradation. The uncertainties 
in the evidence identified in the 2012 review persist in the evidence 
in the 2020 review and in this reconsideration, with significant 
uncertainties and limitations to establishing quantitative 
relationships between particle size, concentration, chemical 
components, and frequency of painting or repair of materials. While 
some new evidence is available in the 2019 ISA, overall, the data are 
insufficient to conduct quantitative analyses for PM-related materials 
effects. Quantitative relationships have not been established between 
characteristics of PM and frequency of repainting or cleaning of 
materials, including photovoltaic panels and other energy-efficient 
materials, that would help inform our understanding of the public 
welfare implications of soiling in the U.S. (U.S. EPA, 2022b, section 
5.3.2.2.2; U.S. EPA, 2019a, section 13.4). Similarly, the information 
does not support quantitative analyses between microbial deterioration 
of surfaces and the contribution of carbonaceous PM to the formation of 
black crusts that contribute to soiling (U.S. EPA, 2022b, section 
5.3.2.2.2; U.S. EPA, 2019a, section 13.4). We also note that 
quantitative relationships are difficult to assess, in particular those 
characterized using damage functions as these approaches depend on 
human perception of the level of soiling deemed to be acceptable and 
evidence in this area remains limited in this reconsideration (U.S. 
EPA, 2022b, section 5.3.2.1.2). Additionally, we note the CASAC's 
concurrence with conclusions in the 2020 PA (Cox, 2019b, p. 13 of 
consensus responses) and the 2022 PA (Sheppard, 2022a, p. 23 of 
consensus responses) that uncertainties remain about the best way to 
evaluate materials effects of PM in ambient air. Further, no new 
studies are available in this reconsideration to link human perception 
of reduced aesthetic appeal of buildings and other objects to materials 
effects and PM in ambient air. Finally, uncertainties remain about 
deposition rates of PM in ambient air to surfaces and the interaction 
of PM with copollutants on these surfaces (U.S. EPA, 2022b, section 
5.6).
    With respect to the commenters' assertion that the EPA failed to 
consider information related to materials damage and degradation from 
oxidant concentrations and solar radiation for a variety of materials, 
we first note that, even assuming these sources of materials damage are 
within the scope of this review of the PM NAAQS, the commenter did not 
provide any references to the scientific studies that they suggest that 
the EPA did not consider. Despite the lack of a list of specific 
references from the commenter, we note that the 2019 ISA considered a 
number of studies that examined the

[[Page 16328]]

relationships between PM and several of the materials listed by the 
commenters (e.g., paint, plastic, rubber). However, as described in the 
2022 PA, these studies did not provide additional information regarding 
quantitative relationships between PM and materials that could inform 
quantitative analyses (U.S. EPA, 2022b, sections 5.3.2.1.2 and 
5.3.2.2.2), nor did they alter conclusions regarding the adequacy of 
the current standard (U.S. EPA, 2022b, section 5.5).
    As summarized above and in the proposal, the evidence in the 2020 
review and in this reconsideration for PM-related effects on materials 
is not substantively changed from that in the 2012 review. There 
continues to be a lack of evidence related to materials effects that 
establishes quantitative relationships and supports quantitative 
analyses of PM-related materials soiling or damage. While the 
information available in the 2020 review and in this reconsideration 
continues to support a causal relationship between PM in ambient air 
and materials effects (U.S. EPA, 2019a, section 13.4), the EPA is 
unable to relate soiling or damage to specific levels of PM in ambient 
air and is unable to evaluate or consider a level of air quality to 
protect against such materials effects. Although the EPA did not 
propose a distinct level of air quality or a national standard based on 
air quality impacts (88 FR 5662, January 27, 2023), we did identify 
data gaps that prevented us from doing so. The EPA identified a number 
of key uncertainties and areas of future research (U.S. EPA, 2022b, 
section 5.6) that may inform consideration of the materials effects of 
PM in ambient air in future reviews of the PM NAAQS. The EPA notes that 
one commenter objected to the Administrator's proposed conclusion in 
the proposal (88 FR 5661, January 27, 2023) that in light of the 
available evidence for PM-related impacts on climate and on materials 
that it is appropriate not to change the existing secondary standards 
at this time. The EPA has explained, in both the proposal and this 
final action, the basis for its conclusion that there is insufficient 
evidence to identify any particular secondary standard or standards 
that would provide requisite protection against climate effects or 
materials damage. The EPA acknowledges that, as a result, the adoption 
of any distinct secondary PM standards for those effects would be 
inconsistent with the requirements of the CAA. The EPA is clarifying 
that it is not basing its decisions on secondary standards in this 
reconsideration to address these welfare effects because it has 
concluded that the available scientific evidence is insufficient to 
allow the Administrator to make a reasoned judgment about what specific 
standard(s) would be requisite to protect against known or anticipated 
adverse effects to public welfare from PM-related materials damage or 
climate effects.
    Some commenters agree with the Administrator's proposed conclusion 
that a target level of protection for visibility of 30 dv and the level 
of the secondary 24-hour PM2.5 standard of 35 [micro]g/m\3\ 
continues to be adequate to protect visibility, highlighting 
improvements in visibility in the U.S. Other commenters who disagree 
with the proposed decision indicated support for a more stringent 
standard for visibility impairment, although some of these commenters 
did not necessarily specify the alternative standard that would, in 
their judgment, address their concerns related to various aspects of 
the EPA's proposal, including the available public preference studies, 
specific aspects of the visibility index, and the target level of 
protection identified by the Administrator. Rather, most commenters 
focused on particular aspects of the visibility metric underlying the 
current secondary 24-hour PM2.5 standard, including the 
form, averaging time, and target level of protection necessary to 
protect against visibility impairment.
    With regard to the commenters' assertion that the current secondary 
standards are inadequate to protect the public welfare from PM-related 
visibility impairment, the EPA disagrees that the currently available 
information is sufficient to suggest that a more stringent standard is 
warranted. The EPA identified and addressed in great detail the 
limitations and uncertainties associated with the public preference 
studies as a part of the 2012 review (78 FR 3210, January 15, 2013). 
Given that the evidence related to public preferences has not 
substantially changed since the 2012 review, the EPA reiterated the 
limitations and uncertainties inherent in the evidence as a part of the 
2020 PA (U.S. EPA, 2020b, section 5.5), as well as in the 2022 PA for 
this reconsideration (U.S. EPA, 2022b, section 5.6). The 2022 PA 
highlights key uncertainties associated with public perception of 
visibility impairment and identifies areas for future research to 
inform future PM NAAQS reviews, including those raised by the 
commenters (U.S. EPA, 2022b, section 5.6). Specifically, the EPA agrees 
with commenters that there are several areas where additional 
information would reduce uncertainty in our interpretation of the 
available information for purposes of characterizing visibility 
impairment. As described in more detail in the 2020 PA (U.S. EPA, 
2020b, p. 5-41) and the 2022 PA (U.S. EPA, 2022b, p. 5-53), briefly, 
these areas include: (1) Expanding the number and geographic coverage 
of preference studies in urban, rural, and Class I areas; (2) 
evaluating visibility preferences of the U.S. population today, given 
that the preference studies were conducted more than 15 years ago, 
during which time air quality in the U.S. has improved; (3) accounting 
for the influence of varying study methods may have on an individual's 
response as to what level of visibility impairment is acceptable, and; 
(4) information on people's judgments on acceptable visibility based on 
factors that can influence their perception of visibility (e.g., 
duration of impairment experiences, time of day, frequency of 
impairment).
    However, the EPA disagrees with the commenters that the current 
secondary PM standards are inadequate and should be made more stringent 
because of the limitations and uncertainties associated with the 
available public preference studies. The EPA does not view the 
limitations of the preference studies and other available evidence as 
so significant as to render the EPA unable to identify a secondary 
standard to protect against the adverse effects of PM on visibility, 
but the EPA also does not believe that the limitations themselves mean 
that the standards are inadequate. In fact, there is a limited amount 
of recently available scientific evidence to further inform our 
understanding of public preferences and visibility impairment is 
recognized by the Administrator in reaching his proposed decision not 
to change the current secondary PM standards at this time, given that 
the evidence base is largely the same as at the time of the 2012 and 
2020 reviews.
    These same commenters further contend that the EPA failed to use 
the latest science to develop a visibility index, stating that the EPA 
failed to consider the contrast of distance methodology employed in a 
recent meta-analysis of available preference studies (Malm et al., 
2019). Commenters claim that the EPA draws conclusions from the Malm et 
al. (2019) study about how to relate contrast to acceptable visibility 
preferences in the 2022 ISA Supplement, yet ignores the findings of the 
study and fails to consider the ``contrast of distance'' methodology in 
the 2022 PA and the proposal, thereby, in their view, departing from 
the CASAC's advice to consider this evidence in setting the secondary

[[Page 16329]]

standard. Finally, the commenters assert that the EPA did not explain 
why the available public preference studies are adequate for analysis 
using a light extinction approach but not using the contrast of 
distance approach, and that such differential treatment is arbitrary.
    We disagree with the commenters that the EPA did not use the latest 
science in evaluating the visibility index, and that the EPA failed to 
consider the contrast of distance methodology used in Malm et al. 
(2019). As the commenters state, the Malm et al. (2019) study was 
included in the ISA Supplement (U.S. EPA, 2022a, section 4.2.1). 
However, the EPA disagrees with the assertion that the ISA Supplement 
reached conclusions about how to relate contrast to acceptable 
visibility preferences. The ISA Supplement provided an overview of the 
Malm et al. (2019) study, stating that ``[t]he main conclusion of this 
study was that the level of acceptable visual air quality is more 
consistent across studies using metrics that evaluate the distinction 
of an object from a background than using metrics that evaluate the 
greatest distance at which an object can be observed.'' Furthermore, 
the statements that the commenters are referencing in support of this 
statement (i.e., U.S. EPA, 2022b, pp. 4-5-4-6) are in fact the 
conclusions of the study itself, rather than conclusions of the EPA. 
For example, the ISA Supplement notes that ``Malm et al. (2019) 
suggested that scene-dependent metrics like contrast, which integrate 
the effects of bext along the sight paths between observers 
and landscape features, are better predictors of preference levels than 
universal metrics like light extinction.'' The suggestion that the 
contrast of distance methodology is a better predictor than light 
extinction is one of the study authors, not the EPA. The EPA has not 
reached a conclusion on whether contrast of distance methodology would 
be a more appropriate indicator for a visibility index than estimated 
light extinction because the EPA finds that there is insufficient 
information in the record at this time to support that it is practical 
to evaluate, much less adopt, the contrast of distance methodology on a 
national basis. Specifically, the Malm et al. (2019) study does not 
provide as a part of their publication the specific input values to the 
equation to calculate the contrast of distance associated with the 
available public preference studies (e.g., sight paths from the 
images), nor do the preference studies present or make publicly 
available these data in their publications. In the absence of 
additional studies or publicly available data to further evaluate the 
contrast of distance methodology, the EPA is unable to consider 
contrast of distance as an alternative to estimated light extinction in 
this reconsideration, although we note that it may be appropriate to 
evaluate it more closely in future reviews.
    In reaching conclusions regarding the appropriate indicator for the 
visibility index, the 2022 PA specifically notes ``that limited new 
research is available on methods of characterizing visibility or on how 
visibility is valued by the public, such as visibility preference 
studies. Thus, while limited new research has further informed our 
understanding of the influence of atmospheric components of 
PM2.5 on light extinction, the available evidence to inform 
consideration of the public welfare implications of PM-related 
visibility impairment remains relatively unchanged'' (U.S. EPA, 2022b, 
p. 5-50). The EPA again notes in the proposal that ``there are very few 
studies available that use scene-dependent metrics (i.e., contrast) to 
evaluate public preference information, which makes it difficult to 
evaluate them as an alternative to the light extinction approach'' (88 
FR 5649-5650, January 27, 2023). To further expand on this statement, 
the Malm et al. (2019) study does not provide enough information to 
replicate the results of their contrast of distance approach to allow 
for a comprehensive evaluation of the potential use of this methodology 
in considering the results of the public preference studies for 
determining the target level of protection for visibility.
    Some commenters suggests that the methodology could be approximated 
by simply ensuring that people could always see distant scenic 
elements, and that characterizing typical average and/or maximal 
viewing distances cross different geographical areas and regions would 
be a straightforward Geographical Information Systems (GIS) exercise. 
The EPA disagrees that this assessment would be straightforward, given 
the lack of data establishing viewing distances in the available 
scientific record and the diversity of distance to scenic elements 
across different areas and regions of the U.S., and finds that this 
approach is also not practical to adopt in this reconsideration. 
Finally, while the Malm et al. (2019) study is using an alternative 
approach for evaluating public preferences and acceptability, we note 
that this study is evaluating the same public preference studies that 
have been available for the past several decades. For these reasons, 
the EPA disagrees with the commenters' allegation that the EPA ignored 
the findings of the Malm et al. (2019) study and failed to consider the 
contrast of distance methodology in the 2022 PA and the proposal, and 
ignored the CASAC's advice to consider this study. The ISA Supplement 
and the 2022 PA considered the Malm et al. (2019) study, along with the 
full body of available scientific evidence, and took into account the 
uncertainties and limitations associated with the evidence for 
visibility preferences, in reaching conclusions regarding the adequacy 
of the secondary 24-hour PM2.5 standard (U.S. EPA, 2022b, 
pp. 5-24-5.25, 5-50).
    Several comments in support of revising the secondary 24-hour 
PM2.5 standard to protect against visibility generally 
recommend revisions to the elements of the standard and visibility 
index (indicator, averaging time, form, and level) consistent with 
those supported by the CASAC and public comments in previous PM NAAQS 
reviews. Some commenters assert that the EPA's approach in the 2022 PA 
and in the proposal for this reconsideration did not evaluate options 
for alternative secondary PM standards and thereby is flawed. We 
address comments on the elements of a visibility index and a revised 
standard for visibility effects below.
    As an initial matter, the EPA disagrees to the extent commenters 
are suggesting that the PA is legally required to analyze options for 
alternative standards. The PA is a document developed by the EPA in 
order to assist the Administrator and the CASAC in reaching conclusions 
regarding the adequacy of the current standards, and its scope is 
determined by the EPA. Moreover, the 2022 PA did assess a wide range of 
information relevant to the Administrator's decision and considered a 
range of potential standards.
    First, in developing the 2022 PA and in responding to CASAC's 
advice and recommendations during its review of the 2021 draft PA, the 
EPA expanded upon its discussion of determining the target level of 
protection for the visibility index and considered the extent to which 
the available scientific information would alter regarding the 
visibility index and the appropriate target level of protection against 
PM-related visibility effects (U.S. EPA, 2022b, pp. 5-27-5-29). This 
detailed discussion expands the consideration of the target level of 
protection for the visibility index presented in the 2020 PA (U.S. EPA, 
2020b) and the 2021 draft PA (U.S. EPA, 2021c), neither of which 
specifically considered the elements of the visibility index in 
determining the appropriate target level of protection. In

[[Page 16330]]

considering the available information in the 2022 PA, the EPA concluded 
that the available information continued to provide support for a 
visibility index with a level of 30 dv, with estimated light extinction 
as the indicator, a 24-hour averaging time, and a 90th percentile form, 
averaged over three years.
    Additionally, in summarizing the air quality and quantitative 
information in the proposal for this reconsideration, the EPA further 
expands upon the discussion added to the 2022 PA related to the target 
level of protection in terms of a PM2.5 visibility index. In 
so doing, the EPA considers even more extensively the available public 
preference studies and quantitative analyses (88 FR 5651-5652, January 
27, 2023). In particular, there is a more detailed discussion of the 
public preference studies, including the levels of impairment 
determined to be ``acceptable'' by at least 50 percent of study 
participants and the methodologies used in the studies, including 
uncertainties and limitations associated with the methodologies (88 FR 
5652, January 27, 2023). In reaching a proposed decision regarding the 
adequacy of the secondary PM standards, as well as the appropriate 
target level of protection for the visibility index, the Administrator 
considered the available scientific evidence and quantitative analyses, 
as well as judgments about how to consider the range and magnitude of 
uncertainties that are inherent in the scientific evidence and 
analyses. In so doing, the Administrator proposed to conclude that the 
protection provided by a visibility index based on estimated light 
extinction, a 24-hour averaging time, and a 90th percentile form, 
averaged over 3 years, set at a level of 30 dv would be requisite to 
protect public welfare with regard to visibility impairment (88 FR 
5660, January 27, 2023).
    Having provisionally concluded that it was appropriate to define 
the target level of protection in terms of a visibility index based on 
estimated light extinction as described above (i.e., with a 24-hour 
averaging time; a 3-year, 90th percentile form; and a level of 30 dv), 
the Administrator next considered the degree of protection from 
visibility afforded by the current secondary PM standards. In so doing, 
he considered the updated analyses of PM-related visibility impairment 
presented in the 2022 PA (U.S. EPA, 2022b, section 5.3.1.2) and 
described in more detail in the proposal (88 FR 5656, January 27, 
2023), which included estimating light extinction using multiple 
versions of the IMPROVE algorithm and inclusion of PM10-2.5 
data at all sites to allow for better characterization of the influence 
of the coarse fraction of PM on light extinction. The Administrator 
noted that the results of the analyses in the 2022 PA were consistent 
with those from the 2012 and 2020 reviews. He also recognized that, 
regardless of the IMPROVE equation that was used, the analyses 
demonstrated that the 3-year visibility metric is at or below 28 dv in 
all areas meting the current 24-hour PM2.5 standard (88 FR 
5657, January 27, 2023). The Administrator also noted that, in their 
review of the 2021 draft PA, the CASAC stated that ``[i]f a value of 
20-25 deciviews is deemed to be an appropriate visibility target level 
of protection, then a secondary 24-hour standard in the range of 25-35 
[micro]g/m\3\ should be considered (Sheppard, 2022a, p. 21 of consensus 
responses). The Administrator recognized that while the CASAC 
recommended that additional justification be provided for a visibility 
index target level of protection of 30 dv, they did not specifically 
recommend that he choose an alternative level for the visibility index. 
Therefore, the Administrator considered the available scientific 
evidence, quantitative information, and the CASAC's advice in reaching 
his proposed conclusions. The Administrator recognized conclusions 
regarding the appropriate weight to place on the scientific and 
technical information, including how to consider the range and 
magnitude of uncertainties inherent in that information, is a public 
welfare policy judgment left to the Administrator. As such, the 
Administrator noted his preliminary conclusion on the appropriate 
visibility index (i.e., with a 24-hour averaging time; a 3-year, 90th 
percentile form; and a level of 30 dv) and his preliminary conclusions 
regarding the quantitative analyses of the relationship between the 
visibility index and the current secondary 24-hour PM2.5 
standard. In so doing, he proposed to conclude that the current 
secondary standards provide requisite protection against PM-related 
visibility effects (88 FR 5661, January 27, 2023).
    However, the Administrator additionally recognized that the 
available evidence on visibility impairment generally reflects a 
continuum and that the public preference studies did not identify a 
specific level of visibility impairment that would be perceived as 
``acceptable'' or ``unacceptable'' across the whole U.S. population. He 
noted a judgment of a target level of protection, below 30 dv and down 
to 25 dv, could be supported if more weight was put on the public 
preference study performed in the Phoenix, AZ, study (BBC Research & 
Consulting, 2003). As described above, while the Administrator noted 
that the CASAC did not recommend revising the level of the current 24-
hour PM2.5 standard in their review of the 2021 draft PA, 
they did state that, should an alternative level be considered for the 
visibility index, revisions to the secondary 24-hour PM2.5 
standard should also be considered (Sheppard, 2022a, p. 21 of consensus 
responses). Thus, the Administrator solicited comment on the 
appropriateness of a target level of protection for visibility below 30 
dv and down as low as 25 dv, and of revising the level of the current 
secondary 24-hour PM2.5 standard to a level as low as 25 
[micro]g/m\3\ (88 FR 5662, January 27, 2023), and the Administrator 
considered these public comments in reaching his final decision on the 
secondary standards. Thus, the EPA disagrees that the 2022 PA and the 
proposal did not adequately consider options for revising the secondary 
PM NAAQS.
    With regard to the elements of the visibility index, in considering 
the adequacy of the current secondary 24-hour PM2.5 standard 
to protect against visibility impairment, as described in the proposal 
(88 FR 5658-5660, January 27, 2023), the Administrator first defined an 
appropriate target level of protection in terms of a PM visibility 
index. In considering the information available in this reconsideration 
and the CASAC's advice, the Administrator proposed to conclude that the 
protection provided by a visibility index based on estimated light 
extinction, a 24-hour averaging time, and 90th percentile form, 
averaged over 3 years, set at a level of 30 dv, would be requisite to 
protect public welfare with regard to visibility impairment (88 FR 
5660, January 27, 2023).
    In defining this target level of protection, the Administrator 
first considered the indicator of such an index. He noted that, given 
the lack of availability of methods and an established network for 
directly measuring light extinctions, a visibility index based on 
estimates of light extinction by PM2.5 components derived 
from an adjusted version of the original IMPROVE algorithm would be 
most appropriate, consistent with the 2012 and 2020 reviews. As 
described in the proposal (88 FR 5649, January 27, 2023) and above 
(section V.A.2), the IMPROVE algorithm estimates light extinction using 
routinely monitored components of PM2.5 and 
PM10-2.5, along with estimates of relative humidity. The

[[Page 16331]]

Administrator, while recognizing that some revisions to the IMRPOVE 
algorithm were newly available in the 2020 review, noted that the 
fundamental relationship between ambient PM and light extinction has 
changed very little and the different versions of the IMPROVE 
algorithms can appropriately reflect this relationship across the U.S. 
(88 FR 5658-5659, January 27, 2023). As such, he judged that defining a 
target level of protection in terms of estimated light extinction 
continues to be a reasonable approach in this reconsideration.
    Some commenters who criticized the EPA's interpretation and 
application of the Malm et al. (2019) study also contend that an 
indicator based on the contrast of distance would be a significant 
improvement over the current indicator for the visibility index and 
would more accurately evaluate public preferences. However, as 
described in the 2022 PA (U.S. EPA, 2022b, section 5.3.1.1), while 
scene-dependent metrics, such as contrast, may be useful alternative 
predictors of preferences compared to universal metrics like light 
extinction, there are a very limited number of studies that use such 
metrics to evaluate public preferences of visibility impairment and 
there is a lack of scientific evidence that supports one metric over 
another. Moreover, the EPA finds that even if the Administrator agreed 
that the contrast of distance methodology was an improvement over light 
extinction, there is insufficient information available to evaluate and 
adopt contrast of distance as an indicator for a national visibility 
target at this time. While, in its review of the 2021 draft PA the 
CASAC suggested that the EPA consider this method in developing the 
secondary PM standards, the CASAC also noted that ``more extensive 
technical evaluation of the alternatives for visibility indicators and 
practical measurement methods'' is needed to inform future reviews of 
the secondary PM standards (Sheppard, 2022a, p. 22 of consensus 
responses). The CASAC did not recommend using a different indicator for 
this reconsideration, with the majority of CASAC members reiterated 
past advice recommending development of a visibility FRM for a directly 
measured PM2.5 light extinction indicator (Sheppard, 2022a, 
p. 22 of consensus responses), a recommendation that was supported by 
other public commenters as well, and the minority of the CASAC 
suggested that such an FRM is not necessary. For these reasons, the EPA 
does not consider it feasible or appropriate to define the visibility 
index in terms of a contrast of distance indicator at this time.
    With regard to averaging time, some commenters suggested to the EPA 
that a secondary standard with a different form than the primary 
standard may be a more relevant for welfare effects. While they do not 
recommend a specific alternative form, the commenters point to CASAC 
advice in past reviews where the CASAC stated that a subdaily standard 
based on daylight hours better reflects visibility impairment.
    In defining the characteristics of a visibility index, the EPA 
continues to believe that a 24-hour averaging time is reasonable. This 
is in part based on analyses conducted in the 2012 review that showed 
relatively strong correlations between 24-hour and subdaily (i.e., 4-
hour average) PM2.5 light extinction (88 FR 5659, January 
27, 2023; 85 FR 82740, December 18, 2020; 78 FR 3226, January 15, 
2013), indicating that a 24-hour averaging time is an appropriate 
surrogate for the subdaily time periods relevant for visual perception. 
The EPA believes that these analyses continue to provide support for a 
24-hour averaging time for the visibility index in this 
reconsideration. The EPA also recognizes that the longer averaging time 
may be less influenced by atypical conditions and/or atypical 
instrument performance (88 FR 5659, January 27, 2023; 85 FR 82740, 
December 18, 2020; 78 FR 3226, January 15, 2013). When taken together, 
the available scientific information and updated analyses of calculated 
light extinction available in this reconsideration continue to support 
that a 24-hour averaging time is appropriate when defining a target 
level of protection against visibility impairment in terms of a 
visibility index.
    Moreover, the EPA disagrees with commenters that a secondary 
PM2.5 standard with a 24-hour averaging time does not 
provide requisite protection against the public welfare impacts of 
visibility impairment. At the time of the 2012 review, the EPA 
recognized that hourly or subdaily (i.e., 4- to 6-hour) averaging 
times, within daylight hours and excluding hours with high relative 
humidity, are more directly related to the short-term nature of 
visibility impairment and the relevant viewing periods for segments of 
the viewing public than a 24-hour averaging time. At the time of the 
2012 review, the EPA agreed that a subdaily averaging time would 
generally be preferable. However, the Agency noted significant data 
quality uncertainties associated with the instruments that would 
provide hourly PM2.5 mass concentrations necessary to inform 
a subdaily averaging time. These uncertainties, as described in the 
2012 review, included short-term variability in hourly data from 
available continuous monitoring methods, which would prohibit 
establishing a subdaily averaging time (78 FR 3209, January 15, 2013). 
For all of these reasons, and consistent with the 2020 review, the EPA 
continues to believe that a subdaily averaging time is not supported by 
the information available in this reconsideration.
    With regard to the form of the visibility index, some commenters 
contend that the form used in evaluating visibility impairment is not 
appropriate. First, commenters contend that the EPA incorrectly stated 
that the CASAC did not provide advice on the 3-year, 90th percentile 
form of the visibility index and that the CASAC specifically 
recommended that the EPA further justify the metric and form, and by 
not doing so, the proposal arbitrarily departs from the CASAC's 
recommendations. The commenters also contend that the EPA fails to 
explain how averaging the form over three years is protective given 
that the public does not perceive visibility in three-year averages.
    We disagree with the commenters that the EPA departed from the 
CASAC's recommendations that ``[t]he final PA should provide a robust 
justification for the daily light extinction percentile used in the 
analysis'' (Sheppard, 2022a, p. 22 of consensus responses). In this 
statement, the CASAC did not make explicit recommendations for 
revisions to the form of the visibility index, as the commenters 
assert, but rather requested additional justification for the 
percentile selected for the visibility index in the 2022 PA. In 
response to the CASAC's recommendation after reviewing the 2021 draft 
PA, the EPA included a new section in the 2022 PA that explicitly 
discusses the elements (i.e., indicator, averaging time, form, and 
level) of the visibility index, including additional justification for 
the conclusions regarding the appropriate elements for the index (U.S. 
EPA, 2022b, pp. 5-27-5-29). In so doing, the 2022 PA recognizes that 
there is no new information available in this reconsideration to inform 
selection of an alternative form of the visibility index, and 
therefore, relied on the analyses presented in the 2010 UFVA that 
evaluated the different statistical forms of the visibility index. The 
2022 PA also discusses the approach to improving visual air quality in 
Federal Class I areas as a part of the Regional Haze Program (U.S. EPA, 
2022b, p. 5-28). Furthermore, as reflected in responding to public 
comments below, and in

[[Page 16332]]

reaching his final conclusions in section V.B.4 below, the 
Administrator further considers the available scientific and 
quantitative information, the CASAC's advice, and public comments in 
informing his final conclusions regarding the appropriate target level 
of protection for the visibility index. With regard to the commenters' 
assertion that the EPA did not justify why averaging the form over 
three years is protective, we agree with the commenters that people do 
not perceive visibility impairment in three year averages. As described 
in the 2022 PA, visibility-related effects and perceived impairment are 
often associated with short-term PM concentrations, and therefore, the 
focus of the visibility analyses is centered on the adequacy of the 24-
hour PM2.5 standard (U.S. EPA, 2022b, p. 5-29). However, as 
described in the 2022 PA, the 3-year average form provides stability 
from the occasional effect of inter-annual meteorological variability 
that can result in unusually high pollution levels for a particular 
year (U.S. EPA, 2022b, p. 5-28). Occasional meteorological variability 
is of particular concern for the visibility index, which can be 
impacted by not only PM concentrations in ambient air but also relative 
humidity. The D.C. Circuit has previously recognized that it is 
legitimate for the EPA to consider overall stability of the standard 
and its resulting promotion of overall effectiveness of NAAQS control 
programs in setting a standard. See American Trucking Ass'ns v. 
Whitman, 283 F.3d 355, 375-76 (D.C. Cir. 2002). The 2022 PA concluded 
that the available information continues to provide support for a 90th 
percentile form, averaged over three years, and the inclusion of 
additional justification for the elements of the visibility index 
responds to the CASAC's recommendation (U.S. EPA, 2022b, section 
5.3.1.2).
    Some commenters suggest that the 90th percentile form is too low 
and would result in 36 days being excluded annually, presuming that the 
public only finds it objectionable when visibility is worse than the 
standard on 37 or more days per year. The commenters also contend that 
the EPA's approach of using a 90th percentile form for the visibility 
index is inconsistent with the goals of the Regional Haze Program. In 
so doing, the commenters note that the Regional Haze Rule focuses on 
improving conditions on the worst days, while they argue that a 90th 
percentile form for the visibility index would ignore the 36 worst 
visibility days, rather than identifying them and reducing pollution on 
those days.
    In reaching conclusions regarding the appropriate form of the 
visibility index, the EPA is following the same approach employed in 
past reviews of the secondary PM NAAQS, including those in the 2012 and 
2020 rulemakings. In reaching conclusions regarding the appropriate 
form of the visibility index in the 2011 PA, the EPA considered the 
percentile forms of the visibility index assessed in the 2010 PA (i.e., 
90th, 95th, 98th) along with the approach for improving visual air 
quality under the Regional Haze Program. In so doing, the 2011 PA notes 
that the Regional Haze Program targets the 20% most impaired days for 
improvements in visual air quality in Federal Class I areas (i.e., the 
days more impaired than the 80th percentile). The 2011 PA recognized 
that to increase the likelihood of improving visual air quality on the 
worst days, the form of the visibility index should be set well above 
the 80th percentile. The 2011 PA further concluded that a 90th 
percentile form would represent the median of the distribution of the 
20% most impaired days, and meeting a visibility index with a 90th 
percentile form would mean that 90% of the days have visual air quality 
that is at or below the level of the visibility index and would 
reasonably expected to lead to improvements in visual air quality for 
the 20% most impaired days (U.S. EPA, 2011, p. 4-59). The 2022 PA noted 
that there is no new information from public preference studies that 
would inform the Administrator's consideration of the appropriate form 
for the visibility target index, and reached conclusions consistent 
with those of 2011 PA. However, as discussed below, the EPA disagrees 
that a focus on the 90th percentile ``ignores'' any days with worse 
visibility. It is possible to examine past patterns of air quality to 
judge the relationship between the 90th percentile and higher 
percentiles, and to assess whether achieving a 90th percentile 
visibility target will also result in air quality improvements, where 
necessary, at higher percentiles. Based on its assessment of past air 
quality and potential alternative percentiles for the form, the EPA 
judged that a 90th percentile would appropriately achieve improved air 
quality both above and below that percentile.
    Some commenters suggest that the analyses conducted in the 2010 
UFVA are based on a different metric than the 24-hour average being 
considered in the reconsideration, that the analyses are outdated and 
irrelevant. Therefore, the commenters assert that relying on the 
analyses in the 2010 UFVA is not a rational justification for the use 
of a 90th percentile for the visibility index in this reconsideration. 
Moreover, these commenters state that, in past reviews, both the EPA 
and the CASAC have considered and recommended a 98th percentile form, 
but the proposal does not consider the 98th percentile.

[[Page 16333]]

    These commenters assert that the 2010 UFVA was not considering the 
same metric under consideration here. However, the EPA was citing to 
the 2010 UFVA for the conclusion that there are correlations between 
different statistical forms of the visibility index. To confirm whether 
these correlations occur under recent air quality, we conducted 
additional air quality analyses evaluating the visibility index using 
the current percentile form (i.e., 90th) and two alternative forms 
(i.e., 95th and 98th).\172\ While a higher percentile form would 
further limit the number of days with peak PM-related light extinction, 
the analyses confirm that a 90th percentile form is effective in 
limiting visibility impairment at higher percentiles. Based on these 
analyses, depending on which version of the IMPROVE equation is used to 
estimate light extinction, the differences in the 3-year averages of 
estimated light extinction for the 90th, 95th, and 98th percentile 
forms are small. For example, in areas that meet the current 24-hour 
PM2.5 standard, for light extinction estimated using the 
original IMPROVE equation, all sites have light extinction estimates 
for a 90th percentile form at or below 26 dv, for a 95th or 98th 
percentile form at or below 29 dv.\173\ In most locations, when 
estimating light extinction based on the original IMPROVE equation, the 
difference between a 95th or 98th percentile form and a 90th percentile 
form is generally less than 3 dv.\174\ As noted in previous reviews, a 
change of 1 to 2 dv in light extinction under many viewing conditions 
will be perceived as a small, but noticeable, change in the appearance 
of a scene, regardless of the initial amount of visibility impairment 
(88 FR 5657, January 27, 2023; U.S. EPA, 2004b; U.S. EPA, 2010b). Thus, 
differences between a 90th percentile form and a 95th or 98th 
percentile form remain small, and for any of these forms of the 
visibility index, the estimated light extinction based on the original 
IMPROVE equation in areas meeting the current secondary 24-hour 
PM2.5 standard is below the upper end of the range of the 
levels considered for the visibility index (i.e., below 30 dv).
---------------------------------------------------------------------------

    \172\ Gantt, B., and Hagan, N. (2023). Analysis of Percentile 
Forms of the Visibility Index. Memorandum to the Rulemaking Docket 
for the Review of the National Ambient Air Quality Standards for 
Particulate Matter (EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
    \173\ Gantt, B., and Hagan, N. (2023). Analysis of Percentile 
Forms of the Visibility Index. Memorandum to the Rulemaking Docket 
for the Review of the National Ambient Air Quality Standards for 
Particulate Matter (EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
    \174\ Gantt, B., and Hagan, N. (2023). Analysis of Percentile 
Forms of the Visibility Index. Memorandum to the Rulemaking Docket 
for the Review of the National Ambient Air Quality Standards for 
Particulate Matter (EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    Some commenters disagree with the EPA's proposed conclusion that a 
level of 30 dv is appropriate for the visibility index and support a 
lower level in order to provide increased protection against visibility 
impairment. Commenters who support a revised level for the visibility 
index state that a target level of protection of 30 dv would mean that 
less than 10% of participants in the public preference studies, other 
than the Washington, DC, study, would accept visibility conditions 
above 29 dv. These commenters further suggest that a 75% acceptability, 
rather than 50% acceptability, is requisite to protect visibility 
sources, which would be on average a level of 21 dv when using the 
light extinction method or 18 dv when using the contrast of distance 
method. These commenters argue that, based on the available 
information, a target level of protection for the visibility index of 
approximately 20 dv would be more appropriate, and therefore, the level 
of the secondary 24-hour PM2.5 standard should be 
strengthened to 25 [mu]g/m\3\. Other commenters who support a revised 
level for the visibility index suggest that public preference studies 
with longer sight paths to distant landscape features or with lower 
target levels than those in the Washington, DC study, such as the 
Phoenix study, would support a lower level. These commenters support 
revising the target level of protection for the visibility index to a 
25 dv, and revising the level of the secondary 24-hour PM2.5 
standard to a level as low as 25 [mu]g/m\3\, suggesting that in low 
relative humidity environments, 25 dv is consistent with 
PM2.5 concentrations of less than 25 [mu]g/m\3\.
    Some commenters state that EPA's justification for setting a target 
level of protection at the upper end of the 20 to 30 dv range is 
arbitrary. These commenters state that the EPA's reliance on the 
standard operating in many regions and circumstances as support for the 
upper end of the range is irrational and illegal. Moreover, these 
commenters contend that EPA provided no rational connection between the 
Regional Haze Program and the proposed decision to set the target level 
of protection at the upper end of the range. They suggest that the EPA 
proposed to rely exclusively on the Regional Haze Program to protect 
visibility in Class I areas and to give visibility in these areas no 
weight in considering the secondary PM standard and that it is not 
rational to entirely ignore visibility in Class I areas when setting 
the secondary standard. These commenters assert that the Regional Haze 
Program provides no rational basis for a target level of protection at 
the upper end of the range, nor does the EPA identify one.
    Some commenters contend that the EPA failed to justify the adequacy 
of the current secondary annual PM2.5 standard, noting that 
the secondary 24-hour and annual PM2.5 standards work 
together to provide protection against short- and long-term effects of 
PM2.5. These commenters point to CASAC comments on the 2021 
draft PA and the comments of an individual CASAC member's support for 
strengthening the secondary annual PM2.5 standard to provide 
increased protection against climate and materials effects over time. 
They contend that EPA arbitrarily failed to discuss the secondary 
annual PM2.5 standard not only in the proposal, but also in 
the 2022 PA and in the 2020 final decision.
    The EPA recognizes that the selection of the target level of 
protection for the visibility index is fundamentally a public welfare 
policy judgment for the Administrator. The Administrator is tasked by 
the CAA to judge when visibility impairment becomes an adverse effect 
on public welfare. It is clear that visibility impairment can become 
adverse to public welfare, but the Administrator does not consider that 
every deciview of impairment is adverse to public welfare. In 
considering the point at which visibility impairment becomes adverse to 
public welfare, such that the attainment of the secondary PM NAAQS 
would prevent the adverse effect, the Administrator gives weight to the 
public preference studies as to when visibility impairment is 
unacceptable. At the same time, the Administrator recognizes the 
limitations of these studies, which have been detailed in the proposal 
and the 2022 PA. Similarly, the EPA discussed the Regional Haze program 
in the proposal to highlight that there is a distinct program to 
protect against visibility impairment in Class I areas, and the 
existence of that program is relevant to the Administrator's judgment 
about the level of visibility impairment that is adverse to public 
welfare under CAA 109(d), because in determining what is requisite the 
Administrator is primarily considering visibility impairment outside of 
Class I areas.

[[Page 16334]]

    In considering how to use the results of the public preference 
studies, the Administrator concludes that a 50th acceptability 
criterion is an appropriate tool. The Administrator's task is to set 
standards that are neither more stringent nor less stringent than 
necessary, and a 50% acceptability criterion seems most appropriate to 
use in judging when visibility impairments become adverse, because it 
should more closely represent when the median person would find the 
impairment to be adverse. The Administrator notes this conclusion is 
consistent with the approach adopted in the Denver study by Ely et al. 
(1991) where the 50% acceptability criterion for urban visibility was 
first presented. This study discussed the use of the 50% acceptability 
criteria as a reasonable basis for setting a standard to protect 
visibility in urban areas. In doing so, Ely et al. (1991) noted that 
the 50% acceptability criterion divided the slides into two groups--
those judged acceptable and those judged unacceptable by a majority of 
people in the study--and therefore, was reasonable since it defines the 
point where the majority of the study participants began to judge 
levels of visibility impairment as unacceptable (Ely et al., 1991).
    In considering the appropriate target level of protection, we next 
look to the available public preference studies, noting that the 
selecting of the range of 20 to 30 dv for the target level of 
protection for the visibility index is informed by the 50% 
acceptability values from these studies. The Denver, CO, (Ely et al., 
1991) and British Columbia, Canada, (Pryor, 1996) studies met the 50% 
acceptability criteria at 20 dv and 19-23 dv, respectively (U.S. EPA, 
2022b, Table D-8). As described in the proposal, these studies used 
photographs that were taken at different times of the day and on 
different days to capture a range of light extinction levels needed for 
the preference studies (88 FR 5652, January 27, 2023). Compared to 
studies that used computer-generated images (i.e., those in Phoenix, 
AZ, and Washington, DC) there was more variability in scene appearance 
in these older studies that could affect preference rating and includes 
uncertainties associated with using ambient measurements to represent 
sight path-averaged light extinction values rather than superimposing a 
computer-generated amount of haze onto the images. When using 
photographs, the intrinsic appearance of the scene can change due to 
meteorological conditions (i.e., shadow patterns and cloud conditions) 
and spatial variations in ambient air quality that can result in 
ambient light extinction measurement not being representative of the 
sight-path-averaged light extinction. Computer-generated images, such 
as those generated with WinHaze, do not introduce such uncertainties, 
as the same base photograph is used (i.e., there is no intrinsic change 
in scene appearance) and the modeled haze that is superimposed on the 
photograph is determined based on uniform light extinction throughout 
the scene. Because of the uncertainties and limitations associated with 
the Denver, CO, and British Columbia, Canada, the EPA concludes that it 
is appropriate to place less weight on these studies, and to instead 
focus on the public preference studies that were designed to reduce 
these uncertainties and limitations.
    In so doing, we focus on the public preference studies that use 
computer-generated images (i.e., those in the Phoenix, AZ, and 
Washington, DC) studies. As described in the proposal, the use of 
computer-generated images have less variability in scene appears than 
in those studies that use photographs taken on different days and at 
different times of the days (i.e., those in the Denver, CO, study) that 
would be likely to influence preference rating and introduces 
uncertainties associated with using ambient measurements to present 
sight path-averaged light extinction values rather than superimposing a 
computer-generated amount of haze onto the images (88 FR 5652, January 
27, 2023).
    The Phoenix, AZ, public preference study (BBC Research & 
Consulting, 2003) had several strengths compared to some of the other 
public preference studies. The Phoenix, AZ, study had the largest 
number of participants (385 in 27 separate focus group sessions) of all 
of the public preference studies, with a sample group designed to be 
demographically representative of the Phoenix population at that time. 
The age range in the Phoenix study was also more inclusive (18-65+), 
with the distribution of the study participants corresponding 
reasonably well to the overall age distribution in the 2000 U.S. Census 
for the Phoenix area (BBC Research & Consulting, 2003). Furthermore, 
the 21 images used in the Phoenix, AZ, study were developed using the 
WinHaze software with visual air quality ranging from 15 to 35 dv, and 
the view was toward the southwest, including downtown Phoenix, with the 
Sierra Estrella Mountains in the background at a distance of 25 miles. 
This study had the least noisy preference results, perhaps because a 
larger, more representative group of participants combined with the use 
of computer-generated images resulted in the smoother distribution of 
responses of ``acceptable'' visual air quality. Based on the EPA's 
evaluation of the public preference studies in the 2012 review, the 50% 
``acceptable'' criteria was met at approximately 24 dv (U.S. EPA, 2010, 
Table 2-3).
    We also consider the public preferences for the Washington, DC, 
studies (Abt Associates, 2001; Smith and Howell, 2009). The 2001 
Washington, DC study included nine participants, and the 2009 
Washington, DC, study replicated the 2001 study with 26 additional 
participants. Similar to the Phoenix study, the Washington, DC, studies 
also had the strength of having the 20 images included in the study 
generated using WinHaze with visual air quality ranging from 9 to 45 
dv. The study depicted a scene of a panoramic view of the Potomac 
River, the National Mall, and downtown Washington, DC. All of the 
distinct buildings in the scene were within four miles and the higher 
elevations in the background were less than 10 miles from where the 
image was taken from the Arlington National Cemetary in Virginia. The 
50% ``acceptable'' criteria was met at approximately 29 dv (U.S. EPA, 
2010, Table 2-3).
    As described in more detail in the proposal, visibility preferences 
can vary by location, and such differences may arise based on the 
differences in the cityscape scene that is depicted in the images (88 
FR 5652, January 27, 2023). In considering the geographical differences 
between the public preference studies, we recognize that the 
methodological differences between the studies may influence the 
resulting ``acceptable'' level of visibility impairment. In the 
Phoenix, AZ, study, the image depicted mountains in the background and 
urban features in the foreground, whereas the Washington, DC, study 
depicted nearby buildings in the image without mountains in the 
distance. As an initial matter, we note that the object of interest to 
the study participant could differ across the studies based on the 
scenes included in the images being evaluated--with the mountains being 
of greater interest in the images in the Phoenix, AZ, study, despite 
also depicting buildings that are similar to those shown and presumed 
to be of interest in the images in the Washington, DC, study (88 FR 
5652, January 27, 2023). We also agree with the commenters that the 
distance between the object of interest and the camera is an important 
consideration in

[[Page 16335]]

evaluating the public preference studies. Objects at greater distances 
from the camera location (such as those in the Phoenix, AZ, study which 
had a maximum distance of 42 km (U.S. EPA, 2022b, Table D-8)) have a 
greater sensitivity to light extinction, which alone could explain 
differences in preferences but coupled with an object of greater 
interest results in lower acceptable levels of visibility impairment. 
Conversely, objects at closer distances from the camera location (such 
as those in the Washington, DC, study which had a maximum distance of 8 
km (U.S. EPA, 2022b, Table D-8)) have less sensitivity to light 
extinction, which coupled with objects of interest (compared to the 
mountainous views in the Phoenix, AZ, study) result in higher 
acceptable levels of visibility impairment. These studies clearly 
demonstrate that there are differences in the public preferences across 
the studies depending on the images that are used, in particular the 
object of interest to the study participant depicted in the image and 
the distance of the sight path to the object, and that such differences 
can influence preference results.
    However, we note that these uncertainties and limitations have 
persisted from past reviews, and there is very little new information 
to inform conclusions regarding the interpretation of these results 
with regard to the target level of protection. In selecting a target 
level of protection, and in considering the CASAC's advice in their 
review of the 2021 draft PA and public comments, we conclude that it is 
appropriate to consider the information from the public preference 
studies in Washington, DC, and Phoenix, AZ, and in so doing, that it is 
appropriate to place weight on both of these studies in reaching 
conclusions on the appropriate target level of protection. The EPA 
recognizes that the scenes depicted in these two studies are different 
and may influence public preferences of visibility impairment, but 
notes these studies can be considered together as providing information 
about different areas across the U.S. with variations in the scenes 
that people are likely to most commonly encounter. The scene depicted 
in the images used in the Washington, DC, study have a mix of 
buildings, landmarks, and open space. On the other hand, the scene 
depicted in the Phoenix, AZ, study included a mix of buildings in the 
foreground and with more distant mountains in the background. The 
Administrator considers it appropriate to consider these studies 
together because in combination, they provide a greater diversity of 
scenes, which is more likely to be representative of scenes people 
typically experience around the country (e.g., not only in eastern 
metropolitan statistical areas, but also in western areas with 
different vistas). In considering these two studies together, the EPA 
recognizes that, first, the ``object of interest'' is a subjective 
judgment left to the participants of the public preference studies, and 
second, the images in these two studies may differ in terms of 
sensitivity to changes in light extinction because of the distance 
between the object of interest in the scene and the camera. As noted by 
the public commenters, the sight path for the images in the public 
preference studies is an important consideration in reaching 
conclusions regarding the appropriate target level of protection for 
the visibility index. In addition, the Administrator judges that giving 
weight to multiple studies is a more appropriate approach than focusing 
on a single study, particularly where the study design (including the 
representativeness of the participants and the scenes depicted in the 
images) may be important for interpreting the results of the public 
preference studies for informing conclusions regarding the visibility 
index. Given these considerations and taking into consideration public 
comments on the target level of protection for the visibility index, 
the Administrator recognizes that it is more appropriate to consider a 
broader range of public preferences, reflecting a broader range of 
scenes, by putting significant weight on both the Washington, DC, and 
Phoenix, AZ, studies. In so doing, he reaches the conclusion that it 
would be appropriate to identify secondary PM standards that generally 
limit visibility impairment to a level between the two studies.
    The Administrator next considers what target level of protection 
would be appropriate based on the available information from these 
public preference studies. He first recognizes that, in the 2012 and 
2020 final decisions, the then-Administrators selected a target level 
of protection of 30 dv, based on the upper end of the range. In so 
doing, the then-Administrators judged that it was appropriate to place 
more weight on the uncertainties associated with the public preference 
studies in reaching their conclusions. However, in this 
reconsideration, the current Administrator, while continuing to 
recognize that substantial uncertainties remain and that there is 
relatively limited new information regarding public preferences of 
visibility impairment, judges that it is important to balance the 
weight placed on uncertainties with the strength of the scientific 
evidence. As such, the Administrator concludes that it is appropriate 
to consider a target level of protection within the range of 20 to 30 
dv. He further concludes that in selecting a target level within that 
range it is appropriate to place weight on both the mid-point of the 
range, as supported by the study in Phoenix, AZ, as well as the upper 
end, as supported by the Washington, DC, study. The Administrator notes 
that these two studies both employ similar methodologies that are 
subject to fewer uncertainties than older public preference studies 
(including their use of WinHaze to reduce uncertainties in the 
preference solicitations) although he notes that the Phoenix, AZ, study 
yielded the best results of the four public preference studies in terms 
of the least noisy preference results and the most representative 
selection of participants. Furthermore, he notes the differences 
between the scenes used for each study and finds that consideration of 
these studies together is more appropriate in selecting a national 
target for visibility protection than considering either study alone. 
Thus, in considering this information, along with the uncertainties and 
limitations of the public preference studies, the Administrator judges 
that it would be appropriate to select a target level of protection 
based on placing equal weight on the upper end of the range (i.e., 30 
dv) and the middle of the range (i.e., 24 dv based on the Phoenix, AZ, 
study) in order to identify a nationwide target for protection against 
visibility impairment. In so doing, the Administrator concludes that a 
visibility index with a target level of protection of 27, defined in 
terms of estimated light extinction, with a 24-hour averaging time and 
a 3-year, 90th percentile form, would provide adequate protection 
against PM-related visibility effects on public welfare. Such a target 
level of protection balances the information from two key studies 
reflecting different participant preferences for different vistas in 
different parts of the country, appropriately weighting both near-field 
and more distant landscape features that may be of importance to public 
perceptions of visibility.
    The Administrator notes that the available evidence indicates that 
the relationship between PM and light extinction is complex, depending 
on factors such as PM composition, size fraction, and age of the 
particles in ambient air, as well as relative

[[Page 16336]]

humidity. These factors can vary across the country based on 
differences in regional influences, as well as meteorological 
conditions that can vary spatially and temporally in different areas. 
The Administrator also recognizes that this variability, coupled with 
the age of the PM depending on the distance from the source to the 
monitor location, also complicates the selection of which IMPROVE 
equation is most appropriate in different areas, although he notes that 
different IMPROVE equations will yield similar, but not identical, 
results. In so doing, the Administrator takes note of the figures 
presented in the 2022 PA, which depict the comparisons using the 
original IMPROVE equation (Figure 5-3), the revised IMPROVE equation 
(Figure 5-4), and the Lowenthal & Kumar equation (Figure 5-6), as well 
as the estimated light extinction values for the three different 
equations presented in Table D-7.
    The Administrator notes that when light extinction is calculated 
using the original IMPROVE equation, all 60 sites have 3-year 
visibility metrics below 28 dv, 58 sites are at or below 25 dv, 26 
sites are at or below 20 dv, and of the two sites above 25 dv one is at 
26 dv and the other has a 24-hour PM2.5 design value of 56 
[mu]g/m\3\ (i.e., well above the current 24-hour standard). Results are 
similar for other IMPROVE equations.\175\ Based on these analyses, and 
consistent with the results of similar analyses in the 2012 review and 
the 2020 PA, the Administrator concludes that the current secondary 24-
hour PM2.5 standard, with its level of 35 [mu]g/m\3\, 
maintains the visibility index below 27 dv, and in fact, the current 
standard maintains air quality such that many areas have visibility 
index values that range between 15 and 25 dv for all three IMPROVE 
equations. In the areas that meet the secondary 24-hour 
PM2.5 standard, all locations were below 27 dv when using 
the original and revised IMPROVE equation and all but three locations 
were at or below 27 dv when using the Lowenthal & Kumar IMPROVE 
equation. Three locations (two in California and one in Utah) had air 
quality that was at 28 dv when the Lowenthal & Kumar IMPROVE equation 
was used. As described in more detail in section V.A.1.3, we recognize 
that there are differences in the inputs for the three IMPROVE 
equations that can influence the resulting estimated light extinction 
values. The higher multiplier for converting OC to OM in the Lowenthal 
& Kumar IMPROVE equation (i.e., a multiplier of 2.1) may be more 
appropriate in more remote locations where there is more aged and 
oxygenated organic PM than in urban locations. The three locations with 
air quality at 28 dv are all in urban areas (downtown Los Angeles, CA; 
Rubidoux, CA; Salt Lake City, UT) and tend to have higher levels of 
nitrate and OC, especially during the wintertime when peak 
PM2.5 concentrations typically occur. In these locations, it 
may be more appropriate to use either the original or revised IMPROVE 
equation, which have multipliers of 1.4 and 1.8, respectively, in order 
to refine the inputs such that estimated light extinction in these 
locations is more accurately characterized based on site-specific 
characteristics.
---------------------------------------------------------------------------

    \175\ When light extinction is calculated using the revised 
IMPROVE equation, all 60 sites have 3-year visibility metrics below 
28 dv, 56 sites are at or below 25 dv, and 26 sites are at or below 
20 dv. When light extinction is calculated using the Lowenthal and 
Kumar IMPROVE equation, 59 sites have 3-year visibility metrics 
below 28 dv, 45 sites are at or below 25 dv, and 15 sites are at or 
below 20 dv. The one site with a 3-year visibility metric of 32 dv 
exceeds the secondary 24-hour PM2.5 standard, with a 
design value of 56 [mu]g/m\3\ (see U.S. EPA, 2022b, Appendix D, 
Table D-3).
---------------------------------------------------------------------------

    We also note that the four areas that exceed the secondary 24-hour 
PM2.5 standard also generally had air quality that was below 
27 dv in terms of the visibility index, with only two locations 
experiencing a visibility index above 27 dv. One location that exceeds 
the secondary 24-hour PM2.5 standard had a visibility index 
of 29 dv using the original IMPROVE equation, while two locations were 
30 and 32 dv using the Lowenthal & Kumar IMPROVE equation. We believe 
attainment and maintenance of the secondary 24-hour PM2.5 
standard will result in improved air quality in these areas, such that 
the visibility index values for these areas will decrease even further.
    The Administrator recognizes that in concluding that it is 
appropriate to identify secondary PM standards that generally limit 
visibility impairment to as low as 27 dv in terms of the visibility 
index, the current secondary PM standards continue to provide 
protection against visibility impairment associated with a visibility 
index as low as, or even lower than, 27 dv. In so doing, he notes that 
when meeting the current 24-hour PM2.5 standard, all sites 
have a visibility index at or below 27 dv with the original and revised 
IMPROVE equations, and all but three sites at or below 27 dv with the 
Lowenthal and Kumar IMPROVE equation. Furthermore, the Administrator 
notes that this conclusion is consistent with the CASAC's advice who, 
in their review the 2021 draft PA, stated that ``[i]f a value of 20-25 
deciviews is deemed to be an appropriate visibility target level of 
protection, then a secondary 24-hour PM2.5 standard in the 
range of 25-35 [mu]g/m\3\ should be considered'' (Sheppard, 2022a, p. 
21 of consensus responses).
    Thus, the Administrator concludes that weight on both the upper end 
of the range of target levels of protection for the visibility index 
identified in previous reviews and the mid-point of the range, as 
presented by the Phoenix, AZ, public preference study, and focusing on 
a target level of protection of 27 dv, he still judges the current 
secondary 24-hour PM2.5 standard requisite to achieve that 
target because the standard generally maintains the visibility index at 
or below 27 dv such that more stringent standards are not warranted.
    The EPA agrees with the commenters that the secondary PM standards 
work together to provide protection against short- and long-term 
effects of both fine and coarse particles (U.S. EPA, 2022b, section 
5.5; 88 FR 5661, January 27, 2023). However, the EPA disagrees with 
commenters that we failed to discuss the secondary annual 
PM2.5 standard in the proposal, 2022 PA, and the 2020 final 
notice and that we failed to justify the adequacy of the secondary 
annual PM2.5 standard. As described in the 2022 PA and the 
proposal, we recognize that PM2.5 is the size fraction of PM 
responsible for most of the visibility impairment in urban areas (U.S. 
EPA, 2022b, section 5.3.1.2; 88 FR 5654, January 27, 2023). Analyses in 
the 2019 ISA found that mass scattering from PM10-2.5 was 
relatively small (less than 10%) in the eastern and northwestern U.S., 
whereas mass scattering was much larger in the Southwest (more than 
20%), particularly in southern Arizona and New Mexico (U.S. EPA, 2019, 
section 13.2.4.1, p. 13-36). Given the relationship between visibility 
and PM2.5 along with the short-term nature of visibility 
effects, we focus more on the adequacy of the secondary 24-hour 
PM2.5 standard for providing protection against visibility 
impairment (U.S. EPA, 2022b, section 5.3.1.2; 88 FR 5653, January 27, 
2023). In reaching his proposed conclusions, the Administrator clearly 
states that he ``recognizes that the current suite of secondary 
standards (i.e., the 24-hour PM2.5, 24-hour PM10, 
and annual PM2.5 standards) together provide . . . control 
for both fine and coarse particulates and long- and short-term 
visibility and non-visibility (e.g., climate and materials) effects 
related to PM in ambient air'' (88 FR 5661, January 27, 2023). Thus, by 
explaining how the secondary standards work together to provide 
protection from adverse effects, why we focus on the secondary 24-hour 
PM2.5 standard as

[[Page 16337]]

most relevant to visibility impairment, and how the Administrator 
selected the target level of protection for the visibility index, we 
have addressed the CASAC's request to support the proposed decision to 
revise the secondary 24-hour PM2.5 standard while retaining 
the secondary annual PM2.5 standard. The commenters also 
cite to an individual CASAC member's comments for the review of the 
2021 draft PA who stated ``[f]or the limited scope of this 
reconsideration review, I see no reason to not simply set the Secondary 
equal to the Primary PM Standards, whatever they may be'' (Sheppard, 
2022a, p. A-3). This CASAC member did not provide a supporting 
rationale for revising the secondary standards to levels equal to the 
primary standards. Although areas across the country are required to 
attain both the primary and secondary PM2.5 standards so air 
quality is unaffected by the Administrator's decision not to revise the 
secondary standards to be equal to the primary standards, as described 
in responding to comments above, the CAA provisions require the 
Administrator to establish secondary standards that, in the judgment of 
the Administrator, are requisite to protect public welfare from known 
or anticipated adverse effects associated with the presence of the 
pollutant in ambient air. In so doing, the Administrator seeks to 
establish standards that are neither more nor less stringent than 
necessary for this purpose. The Act does not require that standards be 
set at a zero-risk level, but rather at a level that reduces risk 
sufficiently so as to protect the public welfare from known or 
anticipated adverse effects. The final decision on the adequacy of the 
current secondary standards is a public welfare policy judgment to be 
made by the Administrator. In reaching his proposed and final decisions 
regarding the adequacy of the current secondary PM standards, the 
Administrator considered the available scientific information and 
analyses about welfare effects, and associated public welfare 
significance, as well as judgments about how to consider the range and 
magnitude of uncertainties that are inherent in the scientific evidence 
and analyses. In so doing, the Administrator concluded that the 
currently available scientific evidence and quantitative analyses, 
including uncertainties and limitations, do not call into question the 
adequacy of the current secondary PM standards and that the current 
secondary PM standards should be retained, without revision. The 
Administrator's judgments and decisions on the primary and secondary 
standards are independent and consider different aspects of the 
available scientific evidence and information in reaching conclusions 
regarding the adequacy of the standards in protecting against PM-
related health and welfare effects.
4. Administrator's Conclusions
    This section summarizes the Administrator's considerations and 
conclusions related to the current secondary PM2.5 and 
PM10 standards and presents the rationale for his decision 
that no change is required for those standards at this time. The CAA 
provisions require the Administrator to establish secondary standards 
that, in the judgment of the Administrator, are requisite to protect 
public welfare from known or anticipated adverse effects associated 
with the presence of the pollutant in the ambient air. In so doing, the 
Administrator seeks to establish standards that are neither more nor 
less stringent than necessary for this purpose. The Act does not 
require that standards be set at a zero-risk level, but rather at a 
level that reduces risk sufficiently so as to protect the public 
welfare from known or anticipated adverse effects. The final decision 
on the adequacy of the current secondary standards is a public welfare 
policy judgment to be made by the Administrator. The decision should 
draw on the scientific information and analyses about welfare effects, 
and associated public welfare significance, as well as judgments about 
how to consider the range and magnitude of uncertainties that are 
inherent in the scientific evidence and analyses. This approach is 
based on the recognition that the available evidence generally reflects 
a continuum that includes ambient air exposures at which scientists 
agree that effects are likely to occur through lower levels at which 
the likelihood and magnitude of responses become increasingly 
uncertain. This approach is consistent with the requirements of the 
provisions of the Clean Air Act related to the review of NAAQS and with 
how the EPA and the courts have historically interpreted the Act.
    Given these requirements, the Administrator's final decision in 
this reconsideration is a public welfare policy judgment that draws 
upon the scientific and technical information examining PM-related 
visibility impairment, climate effects and materials effects, including 
how to consider the range and magnitude of uncertainties inherent in 
that information. The Administrator recognizes that his final decision 
is based on an interpretation of the scientific evidence and technical 
analyses that neither overstates nor understates their strengths and 
limitations, or the appropriate inferences to be drawn. In particular, 
the Administrator notes that the assessment of when visibility 
impairment is adverse to public welfare requires a public welfare 
policy judgment informed by available scientific and quantitative 
information.
    In considering the adequacy of the current secondary PM standards 
in this reconsideration, the Administrator has carefully considered 
the: (1) Policy-relevant evidence and conclusions contained in the 2019 
ISA and 2022 ISA Supplement; (2) the quantitative information presented 
and assessed in the 2022 PA; (3) the evaluation of this evidence, the 
quantitative information, and the rationale and conclusions presented 
in the 2022 PA; (4) the advice and recommendations from the CASAC; and 
(5) public comments. In the discussion below, the Administrator gives 
weight to the 2022 PA conclusions, with which the CASAC generally 
concurred during their review of the 2019 draft PA and 2021 draft PA, 
as summarized in section IV.B.1 of the 2020 final notice and section 
V.D.1 of the 2022 proposal, and takes note of key aspects of the 
rationale for those conclusions that contribute to his decision in this 
reconsideration. After giving careful consideration to all of this 
information, the Administrator judges that no change is required for 
the secondary PM standards at this time.
    In considering the 2022 PA evaluations and conclusions, the 
Administrator takes note of the overall conclusions that the non-
ecological welfare effects evidence and quantitative information are 
generally consistent with what was considered in the 2020 final 
decision and in the 2012 review (U.S. EPA, 2022b, section 5.5). The 
scientific evidence for non-ecological welfare effects in this 
reconsideration is largely the same as that available in the 2019 ISA 
and 2020 PA. As described in section I.C.5.b above, the 2022 ISA 
Supplement included a limited number of newly available studies on PM-
related visibility effects. This newly available evidence on visibility 
effects, along with the full body of non-ecological welfare effects 
evidence assessed in the 2019 ISA, reaffirms conclusions on the 
visibility, climate, and materials effects recognized in the 2020 final 
decision and in the 2012 review, including key conclusions on which the 
standards are based. Further, as discussed in more

[[Page 16338]]

detail above, the updated quantitative analyses of visibility 
impairment for areas meeting the current standards in the 2022 PA 
support the adequacy of the current secondary PM standards to protect 
against PM-related visibility impairment. The Administrator also 
recognizes that uncertainties and limitations continue to be associated 
with the available scientific evidence and quantitative information.
    With regard to the current evidence on visibility effects, as 
summarized in the 2022 PA and discussed in detail in the 2019 ISA and 
ISA Supplement, the Administrator notes the long-standing body of 
evidence for PM-related visibility impairment. As in previous reviews, 
this evidence continues to demonstrate a causal relationship between PM 
in ambient air and effects on visibility (U.S. EPA, 2019a, section 
13.2). The Administrator recognizes that visibility impairment can have 
implications for people's enjoyment of daily activities and for their 
overall sense of well-being. Therefore, as in previous reviews, he 
considers the degree to which the current secondary standards protect 
against PM-related visibility impairment and the degree to which PM-
related visibility impairment is adverse to public welfare. In 
particular, in recognizing the short-term nature of visibility 
impairment along with the fact that PM2.5 is the size 
fraction that contributes most to light extinction, the Administrator 
especially focuses on the adequacy of the current secondary 24-hour 
PM2.5 standard in providing protection against PM-related 
visibility effects judged to be adverse. The Administrator also 
considers the protection provided by the current secondary 24-hour 
PM2.5 standard against PM-related visibility impairment in 
conjunction with the Regional Haze Program as a means of achieving 
appropriate levels of protection against PM-related visibility 
impairment in urban, suburban, rural, and Federal Class I areas across 
the U.S. Programs implemented to meet the secondary PM standards, along 
with the requirements of the Regional Haze Program established for 
protecting against visibility impairment in Class I areas, would be 
expected to improve visual air quality across all areas of the country.
    As described in the proposal (88 FR 5658, January 27, 2023), the 
Administrator recognizes that the Regional Haze Program was established 
by Congress specifically to achieve ``the prevention of any future, and 
the remedying of existing, impairment of visibility in mandatory Class 
I areas, which impairment results from man-made air pollution,'' and 
that Congress established a long-term program to achieve that goal (CAA 
section 169A). In adopting section 169, Congress set a goal of 
eliminating anthropogenic visibility impairment at Class I areas, as 
well as a framework for achieving that goal which extends well beyond 
the planning process and timeframe for attaining the secondary PM 
NAAQS. Recognizing that the Regional Haze Program will continue to 
contribute to reductions in visibility impairment in Class I areas, 
consistent with his proposed conclusions, the Administrator concludes 
that addressing visibility impairment in Class I areas is largely 
beyond the scope of the secondary PM standards and that setting the 
secondary 24-hour PM2.5 standard at a level that would 
remedy visibility impairment in Class I areas would result in standards 
that are more stringent than is requisite.
    In further considering what standards are requisite to protect 
against adverse public welfare effects from visibility impairment, the 
Administrator concludes that it is appropriate to use an approach 
consistent with the approach used past reviews (88 FR 5650, January 27, 
2023). He first identifies an appropriate target level of protection in 
terms of a PM visibility index that takes into account the factors that 
influence the relationship between PM in ambient air and visibility 
(i.e., size fraction, species composition, and relative humidity). He 
then considers the air quality analyses conducted in the 2022 PA that 
examine the relationship between the PM visibility index and the 
current secondary 24-hour PM2.5 standard in locations that 
meet the current 24-hour PM2.5 and PM10 standards 
(U.S. EPA, 2022b, section 5.3.1.2).
    In reaching conclusions regarding the target level of protection, 
the Administrator first considers the characteristics of the visibility 
index and defines its elements (indicator, averaging time, form, and 
level). With regard to the indicator for the visibility index, the 
Administrator continues to recognize that, consistent with the 
conclusions of the 2022 PA and the CASAC's advice in their review of 
the 2021 draft PA, there is a lack of availability of methods and an 
established network for directly measuring light extinction. Therefore, 
the Administrator concludes that it continues to be appropriate to 
using an index based on estimates of light extinction by 
PM2.5 components based on the IMPROVE algorithm. In so 
doing, the Administrator recognizes that the fundamental understanding 
of the relationship between ambient PM and light extinction has 
generally changed very little over time; however, several versions of 
the IMPROVE equation have been developed and evaluated that could be 
used to estimate light extinction. As at the time of the proposal, the 
Administrator recognizes that the results of the quantitative analyses 
in the 2022 PA that examined three versions of the IMPROVE equation 
indicate that there are very small differences in estimates of light 
extinction between the equations, and that it is not always clear that 
one version of the IMPROVE equation is more appropriate for estimating 
light extinction across the U.S. than other versions of the IMPROVE 
algorithm (88 FR 5659, January 27, 2023). He also recognizes that the 
selection of inputs to the IMPROVE equation (e.g., the multiplier for 
OC to OM) may be more appropriate on a regional basis rather than a 
national basis when calculating light extinction, and notes the CASAC's 
advice that PM-visibility relationships are region specific (Sheppard, 
2022a, p. 21 of consensus responses). The Administrator further notes 
that neither the CASAC nor public commenters recommended a specific 
IMPROVE equation or an approach for using different IMPROVE equations 
across the U.S. Therefore, given the absence of a robust monitoring 
network to directly measure light extinction, the Administrator 
concludes that light estimated light extinction, as calculated using 
one or more versions of the IMPROVE algorithms, continues to be the 
most appropriate indicator for the visibility index.
    Having reached the conclusion that estimated light extinction is 
the appropriate indicator for the visibility index, the Administrator 
next considers the appropriate averaging time and form of the index. 
With regard to the averaging time and form, the Administrator notes 
that in previous reviews, a 24-hour averaging time was selected and the 
form was defined as the 3-year average of annual 90th percentile 
values. As at the time of proposal, the Administrator recognizes that 
the available information continues to provide support for the short-
term nature of visibility effects. He further recognizes that no new 
information is available in this reconsideration to inform his 
conclusions regarding averaging time, and therefore, he considers past 
analyses of 24-hour and subdaily PM2.5 light extinction to 
inform his conclusions on averaging time. As described in the proposal 
(88 FR 5659, January 27, 2023) and in responding to comments in section 
V.B.3 above, prior

[[Page 16339]]

analyses demonstrated that there are strong correlations between 24-
hour and subdaily (i.e., 4-hour average) PM2.5 light 
extinction, indicating that a 24-hour averaging time is an appropriate 
surrogate for the subdaily time periods associated with when 
individuals experience visibility impairment and that a longer 
averaging time may also be less influenced by atypical conditions and/
or atypical instrument performance. The Administrator also notes that 
the CASAC did not provide advice or recommendations with regard to the 
averaging time of the visibility index, although some public commenters 
referenced CASAC advice in past reviews that a subdaily standard based 
on daylight hours would better reflect the public welfare effects of 
public perceptions of visibility impairment than a 24-hour standard. 
However, in considering the available scientific and quantitative 
information, as well as the CASAC's advice in their reviews of the 2019 
draft PA and 2021 draft PA, the Administrator concludes that the 24-
hour averaging time continues to be appropriate for the visibility 
index because it is an appropriate surrogate for subdaily time periods 
and results in a more stable target.
    With regard to the form of the visibility index, the Administrator 
notes the approach in other NAAQS that a multi-year percentile form 
offers greater stability to the air quality management process by 
reducing the possibility that statistically unusual indicator values 
will lead to transient violations of the standard. He recognizes that 
using a 3-year average provides stability from the occasional effects 
of inter-annual meteorological variability (including relative 
humidity) that can result in unusually high pollution levels for a 
particular year (88 FR 5659, January 27, 2023) and recognizes that a 
stable standard contributes to the benefits of the NAAQS by ensuring 
that attainment strategies are designed to address non-transient 
problems and achieve durable air quality improvements. For these 
reasons, he concludes that a 3-year average continues to be 
appropriate.
    In considering the percentile that would be appropriate with the 3-
year average, the Administrator recognizes that there is very little 
new information available in this reconsideration to inform selection 
of an alternative form of the visibility index and that the appropriate 
form requires the exercise of public welfare policy judgment. In 
selecting the appropriate target level of protection for the visibility 
index, the Administrator is required to assess when visibility 
impairment becomes adverse to public welfare, weighing both the degree 
of visibility impairment (in dv) and the frequency of such impairment 
(through the form). As with the mass-based PM air quality standard, the 
target level of protection for the visibility index must be selected in 
conjunction with the form to determine the appropriate stringency. In 
so doing, consistent with approaches in past reviews, the Administrator 
first notes that the Regional Haze Program targets the 20% most 
impaired days for improvements in visual air quality in Class I areas, 
which are the days above the 80th percentile form of the visibility 
index. The Administrator concludes that a percentile form set at the 
80th percentile would not be likely to sufficiently improve visual air 
quality on the worst days based on the visibility index. In considering 
the information available in past reviews regarding the form of the 
visibility index, as well as the analysis of alternative forms based on 
recent air quality discussed above, the Administrator notes that a 90th 
percentile form would represent the median of the distribution of the 
20% most impaired days, and meeting a visibility index with a 90th 
percentile form would reasonably be expected to lead to improvements in 
visual air quality for days both above and below the 90th percentile 
(88 FR 5660, January 27, 2023). In reaching his conclusion that a 90th 
percentile would appropriately achieve improved air quality both above 
and below that percentile, the Administrator took into consideration 
assessments of air quality data and potential alternative percentiles 
for the form. The Administrator further notes that, consistent with the 
conclusions in the 2011 PA and 2020 PA, the 2022 PA concluded that 
there is no new information from public preference studies that would 
suggest that a 90th percentile form is not appropriate. The 
Administrator also considers air quality analyses described above in 
responding to public comments regarding the percentile form of the 
visibility index. In particular, the Administrator notes that while a 
higher percentile form (i.e., 95th or 98th) would somewhat further 
limit the number of days with peak PM-related light extinction, the 
differences in the 3-year averages of estimated light extinction for 
the 90th, 95th, and 98th percentile forms are small. For example, he 
notes that for the original IMPROVE equation, in areas that meet the 
current 24-hour PM2.5 standard, all sites have light 
extinction estimates for a 90th percentile form at or below 26 dv, and 
for a 95th or 98th percentile form light extinction estimates are at or 
below 29 dv.\176\ He further notes that, in most locations when 
estimating light extinction based on the original IMPROVE equation, the 
difference between a 95th or 98th percentile form and a 90th percentile 
form is generally less than 3 dv.\177\ Moreover, the Administrator 
concludes that a 90th percentile form achieves a very high degree of 
control but appropriately targets the group of worst days, rather than 
the few very worst days. Based on the available information and these 
analyses, the Administrator concludes that the information does not 
indicate that it would be appropriate to consider limiting the 
occurrence of days with peak PM-related light extinction to a greater 
degree, nor did the CASAC provide advice or recommendations related to 
the form of the visibility index. Therefore, the Administrator judges 
that it remains appropriate to define a visibility index in terms of a 
24-hour averaging time and form based on the 3-year average of annual 
90th percentile values.
---------------------------------------------------------------------------

    \176\ Gantt, B., and Hagan, N. (2023). Analysis of Percentile 
Forms of the Visibility Index. Memorandum to the Rulemaking Docket 
for the Review of the National Ambient Air Quality Standards for 
Particulate Matter (EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
    \177\ Gantt, B., and Hagan, N. (2023). Analysis of Percentile 
Forms of the Visibility Index. Memorandum to the Rulemaking Docket 
for the Review of the National Ambient Air Quality Standards for 
Particulate Matter (EPA-HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    With regard to the level of the visibility index, as at the time of 
proposal, the Administrator continues to recognize that there is very 
little new information available to inform his judgment regarding the 
range of levels of visibility impairment judged to be acceptable by at 
least 50% of study participants in the visibility preference 
studies,\178\ and therefore, the range of 20 to 30 dv identified in the 
2022 PA remains appropriate for considering the level of the visibility 
index. The Administrator also recognizes that the uncertainties and 
limitations associated with the public preferences identified in the 
2012 and 2020 reviews continue to persist, and that these limitations 
and uncertainties contributed to the decisions in 2012 and 2020 that a 
level at the upper end of the range (i.e., 30 dv) was selected. The 
Administrator specifically notes that, while the studies

[[Page 16340]]

are methodologically similar, there are a number of factors that can 
influence comparability across the studies and that the available 
studies may not capture the full range of visibility preferences in the 
U.S. population, as described in more detail in section V.D.3 of the 
2022 proposal (88 FR 5659-5660, January 27, 2023). The Administrator 
also notes the CASAC's advice in their review of the 2021 draft PA that 
there are a limited number of visibility preference studies available 
to inform the Administrator's judgment regarding the appropriate target 
level of protection for the visibility index (Sheppard, 2022a, p. 21 of 
consensus responses). In considering the available information, 
including uncertainties and limitation, and the CASAC's advice, the 
Administrator proposed to conclude that it is appropriate to consider a 
target level of protection for the visibility index within the range of 
20 to 30 dv, and that establishing a target level of protection at the 
upper end of the range was appropriate. In so doing, the Administrator 
proposed to conclude that the protection provided by a visibility index 
based on estimated light extinction, a 24-hour averaging time, and a 
90th percentile form, averaged over 3 years, set to a level of 30 dv 
would be requisite to protect public welfare with regard to visibility 
impairment.
---------------------------------------------------------------------------

    \178\ For reasons stated above and described in the 2022 PA and 
proposal, the Administrator does not find it appropriate to use the 
most recent preference study based on the Grand Canyon study area 
(Malm et al., 2019) for purposes of identifying a target level of 
protection for the visibility index.
---------------------------------------------------------------------------

    However, at the time of proposal, the Administrator recognized that 
the available evidence on visibility impairment generally reflects a 
continuum and that the public preference studies do not provide 
information about the specific level for which visibility impairment 
would be ``acceptable'' or ``unacceptable'' across the country, and 
that alternative target levels of protection could be supported. At 
that time, in soliciting public comments, the Administrator recognized 
that other interpretations, assessments, and judgments based on the 
available welfare effects evidence for this reconsideration could be 
possible (88 FR 5662, January 27, 2023).
    With regard to the appropriate target level of protection for the 
visibility index, the Administrator first notes that while the public 
preference studies were conducted in several geographical areas across 
the U.S., and they provide insight into regional preferences for 
visibility impairment, none of the studies identify a specific level of 
visibility impairment that would be perceived as ``acceptable'' or 
``unacceptable'' across the whole U.S. population. He also noted that 
there have been significant questions about how to set a standard for 
visibility that is neither overprotective nor underprotective for some 
areas of the U.S. As described in the proposal (88 FR 5660, January 27, 
2023), in establishing the Regional Haze Program to improve visibility 
in Class I areas, Congress noted that ``as a matter of equity, the 
national ambient air quality standards cannot be revised to adequately 
protect visibility in all areas of the country.'' H.R. Rep. 95-294 at 
205. For the reasons noted above, in reaching his proposed decision 
regarding visibility impairment, the Administrator recognized that he 
is not seeking to set a standard that would eliminate visibility 
impairment in Class I areas, but significant uncertainties remain 
regarding how to judge when visibility impairment becomes adverse to 
public welfare across the range of daily outdoor activities for 
Americans across the country.
    In reaching final conclusions regarding the available information, 
along with the CASAC's advice and public comments, the Administrator 
again considers what constitutes an appropriate target level of 
protection, and in particular considers whether a target level of 
protection below 30 dv is warranted. In so doing, he first notes the 
variability in public preferences of visibility impairment as 
demonstrated by the available public preferences, which support a range 
of potential target levels of protection for the visibility index from 
20 to 30 dv. He also notes that this range informed the 2012 and 2020 
then-Administrators final decisions that a target level of protection 
at the upper end of the range (i.e., 30 dv) would be most appropriate, 
given the uncertainties and limitations associated with the public 
preference studies. As described in in section V.B.3 above in 
responding to public comments, the Administrator recognizes that a 
number of factors can influence public preferences across studies, in 
particular due to the types of scenes depicted in the images as well as 
the distances at which the objects of interest are located from the 
camera. Furthermore, the Administrator recognizes the small number of 
public preference studies currently available makes precise 
interpretations of their results challenging for determining a 
nationally appropriate target level of visibility protection. The 
Administrator also recognizes that the CASAC, in their review of 2021 
draft PA, reiterated that PM-visibility relationships are region-
specific based on aerosol composition, and that several public 
commenters emphasized the importance of the sight path distance in the 
images when considering how to interpret the public preference studies.
    In this reconsideration, the Administrator judges that in 
determining when visibility impairment becomes adverse to public 
welfare for purposes of the secondary NAAQS, while continuing to 
recognize that substantial uncertainties remain and that there is 
relatively limited new information regarding public preferences of 
visibility impairment, it is important to balance the weight placed on 
uncertainties with the strength of the scientific evidence. In so 
doing, the Administrator first concludes that, consistent with previous 
reviews and his proposed decision, it remains appropriate to consider a 
target level of protection within the range of 20 to 30 dv. However, in 
further considering the available scientific and quantitative 
information, CASAC advice, and public comments, he further concludes 
that in selecting a target level within that range it is appropriate to 
place weight on both the middle of the range, as supported by the study 
in Phoenix, AZ, as well as the upper end, as supported by the 
Washington, DC, study. In so doing, he notes that the Washington, DC, 
and Phoenix, AZ, studies employ similar methodologies that are subject 
to fewer uncertainties than older public preference studies (including 
their use of WinHaze to reduce uncertainties in the preference 
solicitations) although he does note that the Phoenix, AZ, study 
yielded the best results of the four public preference studies in terms 
of the least noisy preference results and the most representative 
selection of participants. Further, the Administrator judges that this 
approach would take into account scenes that are similar to both the 
Washington, DC, study and Phoenix, AZ, study, which would be more 
representative of the ``typical'' scenes encountered across more areas 
of the U.S. than an approach that places weight on just one study or on 
studies conducted in certain geographical areas of the country. In 
considering this information, along with the uncertainties and 
limitations of the public preference studies, the Administrator judges 
that it would be appropriate to select a target level of protection 
based on placing equal weight on the upper end of the range (i.e., 30 
dv) and the middle of the range (i.e., 24 dv based on the Phoenix, AZ, 
study) in order to provide protection against visibility impairment in 
different geographical areas of the U.S. For these reasons, the 
Administrator concludes that a visibility index with a target level of 
protection of 27 dv,

[[Page 16341]]

defined in terms of estimated light extinction, with a 24-hour 
averaging time and a 3-year, 90th percentile form, would provide 
adequate protection against PM-related visibility effects. In reaching 
this conclusion, the Administrator judges that such a target level of 
protection balances the information from these two key public 
preference studies in such a way appropriately weighs both near-field 
and more distant landscape features that may be of importance to public 
perceptions of visibility.
    In further considering the appropriate target level of protection 
for the visibility index, the Administrator again recognizes the 
complexity of the relationship between PM and light extinction which is 
dependent on a number of factors, including PM composition, size 
fraction, and age of the particles in ambient air, as well as relative 
humidity. As noted in responding to comments above, these factors can 
vary geographically across the U.S. and local or regional 
meteorological conditions can also vary spatially and temporally. These 
factors are critical inputs to the IMPROVE equation and can influence 
the resulting estimated light extinction such that it is not a 
straightforward comparison between estimated light extinction in one 
area of the country versus another. Moreover, the Administrator 
recognizes that there is variability in estimated light extinction 
depending on the version of the IMPROVE equation that is used. As 
described in more detail in the 2022 PA and the proposal, and in 
reaching his decisions on the indicator of the visibility index above, 
the Administrator notes that the 2022 PA concluded that one version of 
the IMPROVE equation is not more accurate or precise in estimating 
light extinction, and that difference in locations may support the 
selection of inputs into the IMPROVE equation or of the appropriate 
IMPROVE equation to estimate light extinction on a regional basis 
rather than on a national basis.
    In considering the available information, including variations in 
both public preferences of visibility impairment and estimates of light 
extinction using one or more IMPROVE equation, as well as the CASAC's 
advice in their review of the 2019 draft PA and 2021 draft PA and 
public comments, the Administrator judges that a target level of 
protection of 27 dv would be appropriate. In so doing, he concludes 
that a target level of protection above 27 dv would not provide 
adequate protection against PM-related visibility impairment based on 
the 50% acceptability values when both the Washington, DC, and Phoenix, 
AZ, studies are considered. However, he also notes that when 
considering the 50% acceptability values from studies conducted in 
different areas of the U.S. and with different scenes and images 
depicted, the available public preference studies do not provide a 
``bright line'' at and above which visibility impairment is considered 
adverse to public welfare. He further recognizes that, as discussed 
just above, there are a number of region-specific factors that can 
influence light extinction, and thereby influence visibility 
impairment, as well as variations in public preferences of visibility 
impairment based on the available studies, that complicate selection of 
a single target level of protection that would be appropriate for a 
national visibility index. While the Administrator recognizes that the 
uncertainties and limitations associated with public preferences of 
visibility and estimating light extinction have persisted over the last 
several PM NAAQS reviews, he also recognizes that in reaching 
conclusions regarding the appropriate target level of protection for 
the visibility index also involves public welfare policy judgments 
regarding how to appropriately consider the particular uncertainties 
around identifying when visibility impairment becomes adverse to public 
welfare, and the limitations on relying on the public preference 
studies.
    The Administrator also places weight on the high degree of spatial 
and temporal variability in PM composition and relative humidity across 
the U.S. in considering a target level of protection. This approach of 
establishing a target level of protection that takes into account 50% 
acceptability values from both eastern and western sites is a more 
appropriate basis for determining the requisite level of protection 
against known or anticipated adverse effects on public welfare across 
diverse locations, i.e., a standard that is neither more nor less 
stringent than necessary nationwide. Specifically, the Administrator 
judges that a target level of protection for the visibility index 
focused on maintaining estimated light extinction between the upper end 
of the range of the target levels of protection (i.e., 30 dv based on 
the Washington, DC, study) and the middle of the range (i.e., 24 dv 
based on the Phoenix, AZ, study) to be more appropriate for a 
nationwide standard to protect against visibility impairment compared 
to a value derived from one location or one type of scene alone. For 
these reasons, in selecting a target level of protection, the 
Administrator concludes that a target level of protection somewhere 
between the upper end and middle of the range is appropriate because he 
judges that this approach, in conjunction with the Regional Haze 
program, is sufficient, but not more stringent than necessary, to 
protect against adverse effects on public welfare. Thus, he concludes a 
secondary 24-hour PM2.5 NAAQS should be evaluated based on 
its ability to provide protection against visibility impairment 
associated with estimated light extinction of 27 dv based on estimated 
light extinction, a 24-hour averaging time, and a 90th percentile form, 
averaged over 3 years.
    Having concluded that it is appropriate to identify a target level 
of protection in terms of a visibility index based on estimated light 
extinction as described above, the Administrator next considers the 
degree of protection from visibility impairment afforded by the current 
secondary PM standards. He considers the updated analyses of PM-related 
visibility impairment presented in the 2022 PA (U.S. EPA, 2022b, 
section 5.3.1.2) and described in section V.B.1.a of the proposal, and 
notes that the results of the analyses are consistent with the results 
from the 2012 and 2020 reviews.
    Taking into consideration the full body of scientific evidence and 
technical information concerning the known and anticipated effects of 
PM on visibility impairment, the Administrator concludes that the 
current secondary PM2.5 and PM10 standards are 
requisite to protect against PM-related visibility impairment. While 
the inclusion of the coarse fraction had a relatively modest impact on 
calculated light extinction in the analyses presented in the 2022 PA, 
he recognizes the continued importance of the PM10 standard 
given the potential for larger impacts in locations with higher coarse 
particle concentrations, such as in the southwestern U.S., for which 
only a few sites met the criteria for inclusion in the analyses in the 
2022 PA (U.S. EPA, 2019a, section 13.2.4.1; U.S. EPA, 2022b, section 
5.3.1.2).
    With regard to the adequacy of the secondary 24-hour 
PM2.5 standard, the Administrator notes that, in their 
review of the 2021 draft PA, the CASAC stated that ``[i]f a value of 
20-25 deciviews is deemed to be an appropriate visibility target level 
of protection, then a secondary 24-hour PM2.5 standard in 
the range of 25-35 [micro]g/m\3\ should be considered'' (Sheppard, 
2022a, p. 21 of consensus responses). The Administrator recognizes that 
the CASAC recommended that the Administrator provide additional 
justification for a visibility index target

[[Page 16342]]

of 30 dv but did not specifically recommend that he choose an 
alternative level for the visibility index. The Administrator carefully 
considered the advice of CASAC and the public comments and concluded 
that a lower target level of visibility was appropriate in order to 
properly reflect both a broader set of studies and a broader range of 
vistas that were the subject of those studies. However, in their review 
of the 2021 draft PA, the CASAC recognized that even a visibility index 
target in the range of 20-25 dv could still warrant retention of the 
current secondary 24-hour PM2.5 standard. The Administrator 
also considers the advice from the CASAC in their review of the 2019 
draft PA, who ``recogniz[ed] that uncertainties. . .remain about the 
best way to evaluate'' PM-related visibility effects (Cox, 2019b, p. 13 
consensus responses). The Administrator considered the CASAC's advice, 
together with the available scientific evidence and quantitative 
information, in reaching his conclusions.
    The Administrator recognizes that conclusions regarding the 
appropriate weight to place on the scientific and technical information 
examining PM-related visibility impairment, including how to consider 
the range and magnitude of uncertainties inherent in that information, 
is a public welfare policy judgment left to the Administrator. In 
reaching his final decision in 2020, the then-Administrator noted that 
the available evidence regarding visibility effects had changed very 
little since the 2012 review, specifically recognizing that, as 
evaluated in the 2019 ISA, there were no new visibility studies that 
were conducted in the U.S. and there was little new information 
available with regard to acceptable levels of visibility impairment in 
the U.S. (85 FR 82742, December 18, 2020). As such, the then-
Administrator concluded that the protection provided by a standard 
defined in terms of a PM2.5 visibility index, with a 24-hour 
averaging time, a 90th percentiles form averaged over three years, set 
at a level of 30 dv, was requisite to protect public welfare against 
visibility impairment (85 FR 82743, December 18, 2020). He also 
recognized that there was some new information to inform quantitative 
analyses of light extinction, but that the results of the analyses 
conducted in the 2020 PA were consistent with those from the 2012 
review. The then-Administrator recognized that the analyses 
demonstrated that the 3-year visibility metric was at or below about 30 
dv in all areas that met the current secondary 24-hour PM2.5 
standard, and was below 25 dv in most of those areas (85 FR 82743, 
December 18, 2020). Therefore, the Administrator judged that the 
secondary 24-hour PM2.5 standard provided sufficient 
protection for visual air quality of 30 dv, which he judged appropriate 
(88 FR 82744, December 18, 2020). In this reconsideration, the ISA 
Supplement evaluated newly available studies on public preferences for 
visibility impairment and/or development methodologies or conducted 
quantitative analyses of light extinction. In considering the available 
scientific and quantitative information, including that newly available 
in this reconsideration, the current Administrator reached the same 
preliminary conclusions in the notice of proposed rulemaking regarding 
the 3-year visibility index and the current secondary PM standards as 
the then-Administrator in the 2020 final decision. However, in light of 
public comments on the proposal, the Administrator has further 
considered the available scientific evidence and information, as well 
as the CASAC's advice regarding visibility effects in their review of 
the 2021 draft PA. In so doing, the Administrator judges that it is 
appropriate to place more weight on certain aspects of the evidence 
that he had placed less weight on in reaching his proposed conclusions 
(i.e., he focused on the both the middle and the upper end of the range 
of the 50% acceptability values from the available public preference 
studies). As such, the Administrator notes his conclusion on the 
appropriate visibility index (i.e., with a 24-hour averaging time; a 3-
year, 90th percentile form; and a level of 27 dv), which takes into 
account the regional variations in public preferences and equations for 
estimating light extinction, and his conclusions regarding the 
quantitative analyses of the relationship between the visibility index 
and the current secondary 24-hour PM2.5 standard. In so 
doing, the Administrator concludes that the current secondary standards 
provide requisite protection against PM-related visibility effects.
    With respect to climate effects, as at the time of proposal, the 
Administrator recognizes that a number of improvements and refinements 
have been made to climate models since the time of the 2012 review. 
However, despite continuing research and the strong evidence supporting 
a causal relationship with climate effects (U.S. EPA, 2019a, section 
13.3.9), the Administrator notes that there are still significant 
limitations in quantifying the contributions of the direct and indirect 
effects of PM and PM components on climate forcing (U.S. EPA, 2022b, 
sections 5.3.2.1.1 and 5.5). He also recognizes that models continue to 
exhibit considerable variability in estimates of PM-related climate 
impacts at regional scales (e.g., ~100 km), compared to simulations at 
the global scale (U.S. EPA, 2022b, sections 5.3.2.1.1 and 5.5). 
Moreover, the effects of PM on climate are diverse as well as 
uncertain. Depending on the circumstances, the radiative forcing 
effects of PM in the atmosphere can vary, such that positive forcing 
could result in warming of the Earth's surface, whereas a negative 
forcing could result in cooling (U.S. EPA, 2019a, section 13.3.2.2). 
The resulting uncertainty leads the Administrator to conclude that the 
scientific information available in this reconsideration remains 
insufficient to quantify, with confidence, the impacts of ambient PM on 
climate in the U.S. (U.S. EPA, 2022b, section 5.3.2.2.1) and that there 
is not an adequate scientific basis to link attainment of any 
particular PM concentration in ambient air in the U.S. to specific 
climate effects. Consequently, the Administrator judges that there is 
insufficient information at this time to revise the current secondary 
PM standards or to promulgate a distinct secondary standard to address 
PM-related climate effects.
    With respect to materials effects, the Administrator notes that the 
available evidence continues to support the conclusion that there is a 
causal relationship with PM deposition (U.S. EPA, 2019a, section 13.4). 
He recognizes that deposition of particles in the fine or coarse 
fractions can result in physical damage and/or impaired aesthetic 
qualities. Particles can contribute to materials damage by adding to 
the effects of natural weathering processes and by promoting the 
corrosion of metals, the degradation of painted surfaces, the 
deterioration of building materials, and the weakening of material 
components. While some recent evidence on materials effects of PM is 
available in the 2019 ISA, the Administrator notes that this evidence 
is primarily from studies conducted outside of the U.S. in areas where 
PM concentrations in ambient air are higher than those observed in the 
U.S. (U.S. EPA, 2019a, section 13.4). Given the limited amount of 
information on the quantitative relationships between PM and materials 
effects in the U.S., and uncertainties in the degree to which those 
effects could be adverse to the public welfare, the Administrator 
judges that the available scientific information

[[Page 16343]]

remains insufficient to quantify, with confidence, the public welfare 
impacts of ambient PM on materials and that there is insufficient 
information at this time to revise the current secondary PM standards 
or to promulgate a distinct secondary standard to address PM-related 
materials effects.
    Taken together, the Administrator concludes that the scientific and 
quantitative information for PM-related non-ecological welfare effects 
(i.e., visibility, climate, and materials),\179\ along with the 
uncertainties and limitations, supports the current level of protection 
provided by the secondary PM standards as being requisite to protect 
against known and anticipated adverse effects on public welfare. For 
visibility impairment, this conclusion reflects his consideration of 
the evidence for PM-related light extinction, together with his 
consideration of updated air quality analyses of the relationship 
between the visibility index and the current secondary 24-hour 
PM2.5 standard and the protection provided by the current 
secondary PM2.5 and PM10 standards. For climate 
and materials effects, this conclusion reflects his judgment that, 
although it remains important to maintain secondary PM2.5 
and PM10 standards to provide some degree of control over 
long- and short-term concentrations of both fine and coarse particles, 
it is appropriate not to change the existing secondary standards at 
this time and that it is not appropriate to establish any distinct 
secondary PM standards to address PM-related climate and materials 
effects at this time. As such, the Administrator recognizes that 
current suite of secondary standards (i.e., the 24-hour 
PM2.5, 24-hour PM10, and annual PM2.5 
standards) together provide such control for both fine and coarse 
particles and long- and short-term visibility and non-visibility (e.g., 
climate and materials) effects related to PM in ambient air. His 
conclusions on the secondary standards are consistent with advice from 
the CASAC, which noted substantial uncertainties remain in the 
scientific evidence for climate and materials effects, as well as the 
majority of public comments on the secondary PM standards. Thus, based 
on his consideration of the evidence and analyses for PM-related 
welfare effects, as described above, and his consideration of CASAC 
advice and public comments on the secondary standards, the 
Administrator concludes that it is appropriate not to change those 
standards (i.e., the current 24-hour and annual PM2.5 
standards, 24-hour PM10 standard) at this time.
---------------------------------------------------------------------------

    \179\ As noted earlier, other welfare effects of PM, such as 
ecological effects, are being considered in the separate, on-going 
review of the secondary NAAQS for oxides of nitrogen, oxides of 
sulfur and PM.
---------------------------------------------------------------------------

C. Decision on the Secondary PM Standards

    For the reasons discussed above and taking into account information 
and assessments presented in the 2019 ISA, ISA Supplement, and 2022 PA, 
advice from the CASAC, and consideration of public comments, the 
Administrator concludes that the current secondary PM standards are 
requisite to protect public welfare from known or anticipated adverse 
effects and is not changing the standards at this time.

VI. Interpretation of the NAAQS for PM

    The EPA is finalizing revisions on data calculations in appendix K 
for PM10 and appendix N for PM2.5. Revisions to 
appendix K make the PM10 data handling procedures for the 
24-hour PM10 standards more consistent with those of other 
NAAQS pollutants and codify existing practices. Revisions to appendix N 
update references to the revision(s) of the standards and change data 
handling provisions related to combining data from nearby monitoring 
sites to codify existing practices that are currently being implemented 
as the EPA standard operating procedures.

A. Amendments to Appendix K: Interpretation of the NAAQS for 
Particulate Matter

    The EPA proposed to modify its data handling procedures for the 24-
hour PM10 standard in appendix K to part 50 (88 FR 5662, 
January 27, 2023). The proposed modifications include: (1) Revising 
design value calculations to be on a site-level basis, (2) codifying 
site combinations to maintain a continuous data record, and (3) 
clarifying daily validity requirements for continuous monitors. The 
purpose of these modifications is to make the data handling procedures 
for the 24-hour PM10 standard more consistent with those of 
other NAAQS pollutants and codify existing practices that are currently 
being implemented as EPA standard operating procedures.
    The EPA received few comments on these proposed appendix K 
revisions, the majority of which were supportive.
    One commenter was not supportive of the proposed appendix K 
revision to site-level PM10 design values, asserting that it 
would amount to an imposition of a more stringent PM10 
standard due to the potential high bias of FEMs. The EPA disagrees with 
this assertion because site-level design values would combine data from 
any high biased FEM with other monitors at the site rather than 
calculate a monitor-level design value with data solely from that high-
biased FEM. The EPA tested the impact of calculating site-level 
PM10 design values for the 2019-2021 period by assigning the 
lowest parameter occurrence code as the primary monitor and calculating 
site-level design values. Most resulting site-level design values were 
either identical to or in-between the multiple monitor-level design 
values at the site. Combining data from two or more monitors also has 
the benefit of increasing the number of valid sample days at many 
sites. For the 2019-2021 test period, approximately 10% of the sites 
with more than one monitor went from having multiple invalid design 
values to a single valid design value.
    One commenter was not supportive of a footnote in the preamble of 
the NPRM stating that in the absence of a designated primary monitor at 
a given site, the default primary monitor would be one with the most 
complete data record (88 FR 5662, January 27, 2023). Because the 
procedure for calculating PM10 design values on a site-level 
basis being finalized here will require monitoring agencies to 
designate a primary monitor for each site in their annual network plans 
(88 FR 5694, January 27, 2023; App. K, 1.0(b)), the EPA agrees with the 
commenter that this footnote was unnecessary.
    Therefore, the EPA is finalizing these appendix K revisions as 
proposed.

B. Amendments to Appendix N: Interpretation of the NAAQS for PM2.5

    The EPA proposed to modify its data handling procedures for the 
annual and 24-hour PM2.5 standards in appendix N to part 50 
(88 FR 5663, January 27, 2023). These proposed revisions include: (1) 
Updating references to the revisions of the standards rather than 
stating the specific level, and (2) codifying site combinations to 
maintain a continuous data record. The purpose of both modifications is 
to codify existing practices that are currently being implemented as 
the EPA standard operating procedures.
    The EPA received few comments on these revisions in the proposed 
rule, with most supportive of the appendix N revisions.
    Although the EPA did not propose or request comment on this issue, 
one commenter suggested that appendix N be revised to only allow data 
from the primary monitor to be used in PM2.5 NAAQS 
designations asserting that it would add flexibility. The EPA disagrees 
with the commenter's assertion that this would add flexibility because 
it could force agencies to run

[[Page 16344]]

their FRMs on a daily schedule or potentially lead to invalid design 
values if manual sampling interruptions or laboratory issues impact FRM 
data completeness. This change would also be undesirable because it 
could reduce by two-thirds the number of days used in calculations for 
the annual and 24-hour PM2.5 design values at many sites.
    Therefore, the EPA is finalizing these appendix N revisions as 
proposed.

VII. Amendments to Ambient Monitoring and Quality Assurance 
Requirements

    The EPA is finalizing revisions to ambient air monitoring 
requirements for PM to improve the usefulness of and appropriateness of 
data used in regulatory decision making. These changes focus on ambient 
monitoring requirements found in 40 CFR parts 50 (appendix L), 53, and 
58 with associated appendices (A, B, C, D, and E). These changes 
include addressing updates in the approval of reference and equivalent 
methods, updates in quality assurance statistical calculations to 
account for lower concentration measurements, updates to support 
improvements in PM methods, a revision to the PM2.5 network 
design to account for at-risk populations, and updates to the Probe and 
Monitoring Path Siting Criteria for NAAQS pollutants. The EPA also took 
comment on how to incorporate data from next generation technologies 
into Agency efforts. A summary of the comments received is included in 
this section.

A. Amendment to 40 CFR Part 50 (Appendix L): Reference Method for the 
Determination of Fine Particulate Matter as PM2.5 in the Atmosphere--
Addition of the Tisch Cyclone as an Approved Second Stage Separator

    The EPA proposed a change to the FRM for PM2.5 (40 CFR 
part 50, appendix L), the addition of an alternative PM2.5 
particle size separator to that of the Well Impactor Ninety-Six (WINS) 
and the Very Shape Cut Cyclone (VSCC) size separators (88 FR 5663, 
January 27, 2023). The new separator is the TE-PM2.5C 
cyclone manufactured by Tisch Environmental Inc.,\180\ Cleves Ohio, 
which has been shown to have performance equivalent to that of the 
originally specified WINS impactor with regards to aerodynamic cutpoint 
and PM2.5 concentration measurement. In addition, the new 
TE-PM2.5C has a significantly longer service interval than 
the WINS and is comparable to that of the VSCC separator. Generally, 
the TE-PM2.5C is also physically interchangeable with the 
WINS and VSCC where both are manufactured for the same sampler. The 
proposed change would allow either the WINS, VSCC, or TE-
PM2.5C to be used in a PM2.5 FRM sampler. As is 
the case for the WINS and VSCC, the TE-2.5C is now also an approved 
size separator for candidate PM2.5 FEMs. Currently, the EPA 
has designated one PM2.5 sampler configured with TE-
PM2.5C separator as a Class II PM2.5 equivalent 
method and one as a PM10-2.5 equivalent method. Upon 
promulgation of this change to appendix L, these instruments would be 
redesignated as PM2.5 and PM10-2.5 FRMs, 
respectively. Owners of such samplers should contact the sampler 
manufacturer to receive a new reference method label for the samplers.
---------------------------------------------------------------------------

    \180\ Mention of commercial names does not constitute EPA 
endorsement.
---------------------------------------------------------------------------

    The EPA received only one comment regarding this proposed change, 
which was supportive. Therefore, the EPA is finalizing this change to 
Appendix L as proposed.

B. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

    The EPA proposed to clarify the regulations associated with FRM and 
FEM applications for review by the EPA (88 FR 5664, January 27, 2023). 
Revisions were also proposed in instances where current regulatory 
specifications are no longer pertinent and require updating. In 
addition, the EPA proposed to correct a compiled a list of noted minor 
errors in the regulations associated with the testing requirements and 
acceptance criteria for FRMs and FEMs in part 53. These errors are 
typically not associated with the content of Federal Register documents 
but often relate to transcription errors and typographical errors in 
the electronic CFR (eCFR) and printed versions of the CFR.
1. Update to Program Title and Delivery Address for FRM and FEM 
Applications
    The EPA proposed a change to 40 CFR 53.4(a) to update the delivery 
address for FRM and FEM Applications and Modification Requests, as well 
as update the name of the program responsible for their review (88 FR 
5664, January 27, 2023). These revisions are due solely to 
organizational changes and do not affect the structure or role of the 
Reference and Equivalent Methods Designation Program in reviewing new 
FRM and FEM application requests and requests to modify existing 
designated instruments. The EPA received no comments on this revision 
and, therefore, the EPA is finalizing this revision as proposed.
2. Requests for Delivery of a Candidate FRM or FEM Instrument
    The EPA proposed a change to 40 CFR 53.4(d), which currently allows 
the EPA to request only candidate PM2.5 FRMs and Class II or 
Class III equivalent methods for testing purposes as part of the 
applicant review process (88 FR 5664, January 27, 2023). The EPA 
proposed to revise this section to enable requesting any candidate FRM, 
FEM, or a designated FRM or FEM associated with a Modification Request, 
regardless of NAAQS pollutant type or metric. The EPA received no 
comments on these revisions; therefore, the EPA is finalizing this 
revision as proposed.
3. Amendments to Requirements for Submission of Materials in 40 CFR 
53.4(b)(7) for Language and Format
    The EPA proposed a change to 40 CFR 53.4(b)(7) to specify that all 
written FRM and FEM application materials must be submitted to the EPA 
in English in MS Word format and that submitted data must be submitted 
in MS Excel format (88 FR 5664, January 27, 2023). The EPA received no 
comments on these revisions; therefore, the EPA is finalizing this 
section as proposed.
4. Amendment to Designation of Reference and Equivalent Methods
    The EPA proposed a change to 40 CFR 53.8(a) to clarify the terms of 
new FRM and FEM methods to ensure that candidate samplers and analyzers 
are not publicly announced, marketed, or sold until the EPA's approval 
has been formally announced in the Federal Register (88 FR 5664, 
January 27, 2023). The EPA received no comments on these revisions; 
therefore, the EPA is finalizing this section as proposed.
5. Amendment to One Test Field Campaign Requirement for Class III 
PM2.5 FEMs
    The EPA proposed a change to 40 CFR 53.35(b)(1)(ii)(D) that 
involves field comparability tests for candidate Class III 
PM2.5 FEMs, including the requirement that a total of five 
field campaigns must be conducted at four separate sites, A, B, C, and 
D (88 FR 5664, January 27, 2023). The existing Site D specifications 
require that the site ``shall be in a large city east of the 
Mississippi River, having characteristically high sulfate 
concentrations and high humidity levels.'' However, dramatic decreases 
in ambient sulfate concentration make it difficult for applicants to 
routinely meet the high sulfate concentration requirement. Therefore, 
the EPA proposed to revise the Site D specifications to read ``shall be 
in a large

[[Page 16345]]

city east of the Mississippi River, having characteristically high 
humidity levels.'' Only one comment was received on this proposed 
revision, which was supportive. Therefore, the EPA is finalizing the 
revision to 40 CFR 53.35(b)(1)(ii)(D), as proposed.
6. Amendment to Use of Monodisperse Aerosol Generator
    The EPA proposed a change to 40 CFR 53.61(g), 53.62(e), and Table 
F-1 that involves the wind tunnel evaluation of candidate 
PM10 inlets and candidate PM2.5 fractionators 
under static conditions, which requires the generation and use of 
monodisperse calibration aerosols of specified aerodynamic sizes (88 FR 
5664, January 27, 2023). In the current regulations, the TSI 
Incorporated Vibrating Orifice Aerosol Generator (VOAG) is the only 
monodisperse generator that is approved for this purpose. However, TSI 
Incorporated no longer manufacturers nor supports the VOAG. Therefore, 
a commercially available monodisperse aerosol generator (Model 1520 
Fluidized Monodisperse Aerosol Generator, MSP Corporation, Shoreview, 
MN) has been added to list of approved generators for this purpose. No 
comments were received on this revision; therefore, the EPA is 
finalizing this revision as proposed.
7. Corrections to 40 CFR Part 53 (Reference and Equivalent Methods)
    Certain provisions of 40 CFR 53.14, Modification of a reference or 
equivalent method, incorrectly state an EPA response deadline of 30 
days for receipt of modification materials in response to an EPA 
notice. Per a 2015 amendment (80 FR 65460, 65416, Oct. 26, 2015), all 
EPA response deadlines for modifications of reference or equivalent 
methods are 90 days from day of receipt. Thus, the EPA proposed a 
correction to specify the correct 90-day deadline (88 FR 5664, January 
27, 2023).
    Requirements for Reference and Equivalent Methods for Air 
Monitoring of Criteria Pollutants identifies the applicable 40 CFR part 
50 appendices and 40 CFR part 53 subparts for each criteria pollutant. 
The four rows in the section for PM10-2.5 erroneously do not 
include the footnote instruction that the aforementioned pollutant 
alternative Class III requirements may be substituted in regard to 
Appendix O to Part 50--Reference Method for the Determination of Coarse 
Particulate Matter as PM10-2.5 in the Atmosphere.
    Table B-1 specifies that the interference equivalent for each 
interferent is 0.005 ppm for both the standard-range and 
lower-range limits, with the exception of nitric oxide (NO) for the 
lower-range limit per note 4. When testing the lower range of 
SO2, the limit for NO is 0.003 ppm, therefore, 
an incorrect lower limit (0.0003) is currently stated in 
note 4 for this exception to the SO2 lower range limit. 
Thus, the EPA proposed a correction to Table B-1 to specify the correct 
limit in note 4 (88 FR 5664, January 27, 2023).
    After the EPA received an inquiry regarding the interaction of NO 
and O3, the EPA investigated the interferent testing 
requirements stated by 40 CFR part 53, subpart B. The EPA has 
determined that during the 2011 SO2 amendment and subsequent 
2015 O3 amendment, several typographical errors were 
introduced into Table B-3, the most significant of which is the 
omission of note 3, which instructs the applicant to not mix the 
pollutant with the interferent. Thus, the EPA proposed revisions to 
Table B-3 to correct these errors (88 FR 5664, January 27, 2023).
    Additionally, appendix A to subpart B of part 53 provides figures 
depicting optional forms for reporting test results. Figure B-3 lists 
an incorrect formula: the lower detectible limit section is missing the 
proper operator in the LDL calculation formula and Figure B-5 lists an 
incorrect calculation metric, and there is a typesetting error in the 
calculation of the standard deviation. The EPA proposed to correct the 
typesetting errors and noted other errors to be corrected in several 
formulas provided throughout Sec.  53.43 (88 FR 5664, January 27, 
2023).
    The EPA proposed a revision to 40 CFR 53.43(a)(2)(xvi), 
53.43(b)(2)(iv), and 53.43(b)(2)(iv) to correct typographical errors in 
equations.
    The EPA proposed a revision to Table C-4 of part 53 Subpart C (88 
FR 5700). This change is related to field comparability tests of 
candidate PM2.5, PM10-2.5, and PM10 
FEMs, which requires testing at wide range of ambient concentrations. 
For this reason, Table C-4 specifies a minimum number of valid sample 
sets to be conducted at specified high concentrations. However, due to 
the dramatic decrease in ambient PM concentrations in the past two 
decades, these number of valid test days at high concentrations has 
been difficult to achieve. Accordingly, the EPA proposed to revise the 
testing specifications for high concentration events in Table C-4 to 
reflect current levels of ambient PM for all three PM metrics. In 
addition to the revision of the ambient PM concentration specifications 
to Table C-4, there are also several entry errors that required 
correction.
    The EPA received no comments on these proposed revisions; 
therefore, the EPA is finalizing the changes as proposed.

C. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)

1. Quality Assurance Requirements for Monitors Used in Evaluations for 
National Ambient Air Quality Standards
    In the proposal, the EPA described how we evaluated the quality 
system as part of the PM NAAQS reconsideration (88 FR 5665, January 27, 
2023). In this section, the EPA identified several areas for 
improvement in steadily declining average ambient PM2.5 
concentrations across the country and the final decision to revise 
primary annual PM2.5 NAAQS described in section II above. We 
assessed PM2.5 concentration data across a range of values 
to determine if any changes to the statistical calculations used to 
evaluate the data quality in the PM2.5 network were 
warranted. This section describes the EPA's assessment, comments 
received, and the EPA's final decisions on the proposed changes. Other 
changes in this section include clarifications and other improvements 
that will facilitate consistency and the operation of quality assurance 
programs by State, local, and Tribal (SLT) agencies nationwide.
a. Quality System Requirements
    The EPA reconsidered the appendix A, section 2.3.1.1 goal for 
acceptable measurement uncertainty (88 FR 5665, January 27, 2023) for 
automated and manual PM2.5 methods for total bias. The 
existing total bias goal is an upper 90 percent confidence limit for 
the coefficient of variation (CV) of 10 percent and 10 
percent for total bias. The intent of the proposal was to investigate 
if this bias goal is still realistic given updated precision and bias 
statistic. The EPA received one comment that bias reevaluation may be 
premature, since the final NAAQS standard had not yet been determined 
at the time of the proposal. The EPA acknowledges this comment but 
clarifies that the proposed new bias statistic was evaluated at a range 
of levels including the range of proposed PM2.5 standards in 
the technical memorandum, ``Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network.'' \181\ Considering the

[[Page 16346]]

justification in the technical memorandum and the lack of adverse 
comments regarding this part of the proposal, the EPA is retaining the 
appendix A, section 2.3.1.1, goal for acceptable measurement 
uncertainty for automated and manual PM2.5 methods for total 
bias.
---------------------------------------------------------------------------

    \181\ Noah, G. (2023). Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network. Memorandum to the Rulemaking Docket for the Review of the 
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    The EPA also proposed to update and clarify ambient air monitoring 
requirements found in 40 CFR part 58, appendix A, section 2.6.1 
pertaining to EPA Protocol Gas standards used for ambient air 
monitoring and the Ambient Air Protocol Gas Verification Program (PGVP) 
(88 FR 5665, January 27, 2023). The EPA proposed to revise appendix A 
to clarify that in order to participate in the Ambient Air PGVP, 
producers of Protocol Gases must adhere to the requirements of 40 CFR 
75.21(g), and only regulatory ambient air monitoring programs may 
submit cylinders for assay verification to the EPA Ambient Air PGVP. 
The EPA received mixed comments in support of and in opposition to this 
proposed revision. The sole commenter opposing the proposed revision 
indicated that the proposed PGVP requirements would be additional and 
is concerned with an increased resource burden. But the EPA responds 
that the PGVP requirements that were proposed to be added are 
consistent with the existing PGVP requirements in 40 CFR 75.21(g), and 
PGVP has been defined as a regulatory requirement since 2016 (81 FR 
17263, March 28, 2016), so the proposed part 58 changes are not 
``additional'' to existing regulations. After consideration of the 
comments, the EPA is finalizing the update and clarification of ambient 
air monitoring requirements found in appendix A, section 2.6.1 
pertaining to EPA Protocol Gas standards used for ambient air 
monitoring and the Ambient Air PGVP as proposed.
b. Measurement Quality Check Requirements
    The EPA proposed to remove section 3.1.2.2 from appendix A, which 
allows NO2 compressed gas standards to be used to generate 
audit standards (88 FR 5665, January 27, 2023). The EPA received one 
comment supporting this change. As a result of the comment received and 
other general supportive comments regarding quality assurance, the EPA 
is finalizing the removal of section 3.1.2.2 from appendix A as 
proposed.
    The EPA proposed to revise the requirement in Appendix A, section 
3.1.3.3 changing the National Performance Audit Program (NPAP) 
requirement for annual verification of gaseous standards to the ORD-
recommended certification periods identified in Table 2-3 of the EPA 
Traceability Protocol for Assay and Certification of Gaseous 
Calibration Standards (appendix A, section 6.0(4)) (88 FR 5665). The 
EPA received one comment supporting this change. As a result of the 
comment received and other general supportive comments regarding 
quality assurance, the EPA is finalizing the updated NPAP gaseous 
certification requirement in section 3.1.3.3 as proposed.
    The EPA proposed to adjust the minimum value required by appendix 
A, section 3.2.4, to be considered valid sample pairs for the 
PM2.5 Performance Evaluation Program (PEP) from 3 [mu]g/m\3\ 
to 2 [mu]g/m\3\ (88 FR 5665, January 27, 2023). The EPA received 
comments in support and against the change. In the only opposing 
comment, the commenter expressed concern that the method detection 
limit (MDL) for PM2.5 is 2 [mu]g/m\3\. The commenter also 
indicated that the MDL ``typically has minimal value per the definition 
of the MDL.'' 40 CFR part 50, appendix L states, ``The lower detection 
limit of the mass concentration measurement range is estimated to be 
approximately 2 [mu]g/m\3\, based on noted mass changes in field blanks 
in conjunction with the 24 m\3\ nominal total air sample volume 
specified for the 24-hour sample.'' The EPA notes that field blanks 
currently average less than 10 [mu]g nationally, and when divided by 
the 24 m\3\ nominal total air sample volume specified for a 24-hour 
sample, the result is 0.4 [mu]g/m\3\. The appendix L MDL referenced by 
the commenter was part of the 1997 PM NAAQS rulemaking (62 FR 38652, 
July 18, 1997); current data shows that the MDL is substantially lower 
than the EPA's original estimate. After review of the comments, and in 
consideration of the recently calculated detection limit for the 
PM2.5 FRM that is substantially lower than our original 
estimate,\182\ the EPA is finalizing the revised minimum value for 
valid sample pairs for the PM2.5 Performance Evaluation 
Program (PEP) from 3 [mu]g/m\3\ to 2 [mu]g/m\3\ in appendix A, section 
3.2.4 as proposed.
---------------------------------------------------------------------------

    \182\ See the EPA's PM2.5 Data Quality Dashboard 
available at https://sti-r-shiny.shinyapps.io/QVA_Dashboard/.
---------------------------------------------------------------------------

c. Calculations for Data Quality Assessments
    The EPA proposed to change Equations 6 and 7 of appendix A, section 
4.2.1 that are used to calculate the Collocated Quality Control Sampler 
Precision Estimate for PM10, PM2.5 and Pb (88 FR 5666, January 27, 
2023). The proposed new statistics are designed to address the high 
imprecision values that result from using these calculations to compare 
low concentrations that are now more routinely observed in the 
networks. The EPA received several comments in support of this change 
in general, but some commenters indicated that they believed there was 
an error in the new calculation that may result in high imprecision 
from the calculation of the equation. The EPA reviewed the technical 
memorandum and confirmed that a multiplier of 100 was unintentionally 
left in the proposed relative difference equation, Equation 6. Also, 
equation 6 was corrected from a normalized percent difference to a 
normalized relative percent difference that is appropriate for 
comparing collocated pairs at low concentrations. The technical 
memorandum titled ``Task 16 on PEP/NPAP Task Order: Bias and Precision 
DQOs for the PM2.5 Ambient Air Monitoring Network'' has been 
amended to correct the error and is included in the docket for this 
action.\183\
---------------------------------------------------------------------------

    \183\ Noah, G. (2023). Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network. Memorandum to the Rulemaking Docket for the Review of the 
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    Equation 6 as proposed at 88 FR 5666 (January 27, 2023) was:

[[Page 16347]]

[GRAPHIC] [TIFF OMITTED] TR06MR24.031

    As a result of the positive comments received and the correction to 
the equation made in response to some comments, the EPA is finalizing 
the updated Equation 6 as described and is finalizing Equation 7 as 
proposed for the calculation of the Collocated Quality Control Sampler 
Precision Estimate for PM10, PM2.5, and Pb in section 4.2.1.
    The EPA proposed to update the appendix A, section 4.2.5, Equation 
8, calculation for the Performance Evaluation Program Bias Estimate for 
PM2.5 (88 FR 5666-67, January 27, 2023). Because average 
ambient PM concentrations across the nation have steadily declined 
since the promulgation of the PM2.5 standard, the EPA 
proposed to replace the current percent difference equation with a 
relative difference equation. The EPA received several comments in 
support of this change in general, but some commenters identified a 
potential error in the new calculation that resulted in an artificially 
high estimate, which they do not support. The EPA reviewed the 
technical memorandum and discovered that a multiplier of 100 was left 
in the new relative difference equation used in the bias equation. The 
technical memorandum, ``Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network'' has been amended to correct the error and is included in the 
docket.\184\ The proposed Equation 8 proposed at 88 FR 5667 (January 
27, 2023) was:
---------------------------------------------------------------------------

    \184\ Noah, G. (2023). Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network. Memorandum to the Rulemaking Docket for the Review of the 
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
[GRAPHIC] [TIFF OMITTED] TR06MR24.032

    As a result of the supportive comments received and the correction 
to the equation in response to some comments, the EPA is updating and 
finalizing Equation 8 as described for the calculation for the 
Performance Evaluation Program Bias Estimate for PM2.5, in 
section 4.2.5.
d. References
    The EPA proposed to update the references and hyperlinks in 
appendix A, section 6 (88 FR 5667, January 27, 2023) to provide 
accuracy in identifying and locating essential supporting documentation 
and delete references to historical documents that do not represent 
current practices. The EPA received only favorable comments, and as a 
result, the EPA is finalizing the updated the references and hyperlinks 
in appendix A, section 6, as proposed.
    The EPA also proposed to add a footnote to Table A-1 of part 58, 
appendix A--Minimum Data Assessment Requirements for NAAQS Related 
Criteria Pollutant Monitors (88 FR 5669, January 27, 2023). The 
proposed footnote clarifies the allowable time (i.e., every two weeks, 
once a month, once a quarter, once every six months, or distributed 
over all four quarters depending on the check) between checks and 
encourages monitoring organizations to perform data assessments at 
regular intervals. The EPA received two comments regarding this 
proposed footnote. One commenter indicated that this change is 
inconsistent with the QA Handbook for Air Pollution Measurement 
Systems: ``Volume II: Ambient Air Quality Monitoring Program QA 
Handbook.'' The EPA agrees with the commenter; because the QA Handbook 
is guidance,

[[Page 16348]]

the EPA will revise it after this action is finalized to be consistent 
with the updated CFR provision. Another commenter does not support the 
addition of the footnote due to concerns about limiting flexibility. In 
response, the EPA reiterates that the proposed revision is intended to 
clarify intent and does not make any changes to the required 
frequencies or acceptance criteria for data assessment. A ``weight of 
evidence'' narrative is still found in 40 CFR part 58, appendix A, 
section 1.2.3. As a result of the comments received and the rationale 
discussed above, the EPA is finalizing the addition of the new footnote 
to Table A-1 of part 58, appendix A--Minimum Data Assessment 
Requirements for NAAQS Related Criteria Pollutant Monitors as proposed.
2. Quality Assurance Requirements for Prevention of Significant 
Deterioration (PSD) Air Monitoring
    The EPA proposed to revise appendix B, Quality Assurance 
Requirements for Prevention of Significant Deterioration (PSD) Air 
Monitoring (88 FR 5667, January 27, 2023), in parallel to the proposal 
to revise appendix A. Thus, this section of the proposal included 
similar detail and proposed revisions related to evaluating quality 
system statistical calculations for PM2.5, clarifications 
and other improvements that would facilitate consistency and the 
operation of quality assurance programs for PSD by SLT agencies 
nationwide.
a. Quality System Requirements
    The EPA reconsidered the goal in appendix B, section 2.3.1.1 for 
acceptable measurement uncertainty for automated and manual 
PM2.5 methods for total bias (88 FR 5668, January 27, 
2023).\185\ The current total bias goal is an upper 90 percent 
confidence limit for the coefficient of variation (CV) of 10 percent 
and 10 percent for total bias. The EPA's intent was to 
investigate if this goal is still realistic given updated precision and 
bias statistics. The EPA received one comment that bias reevaluation 
may be premature, since the final NAAQS standard had not yet been 
determined at the time of the proposal. The EPA acknowledges this 
comment but clarifies that the proposed new bias statistic was 
evaluated at a range of levels including the proposed range of 
PM2.5 standards in the technical memorandum, ``Task 16 on 
PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 
Ambient Air Monitoring Network.'' \186\ Considering the justification 
in the technical memorandum and the lack of adverse comments regarding 
the substantive proposal, the EPA is retaining the appendix B, section 
2.3.1.1, goal for acceptable measurement uncertainty for automated and 
manual PM2.5 methods for total bias.
---------------------------------------------------------------------------

    \185\ In the proposal, in section VII.C.2 Quality Assurance 
Requirements for Prevention of Significant Deterioration (PSD) Air 
Monitoring (88 FR 5667-69), the EPA inadvertently referred to 
``appendix A'' in the section rather than the correct ``appendix 
B.'' The EPA's intent to have proposed changes to appendix B on 
these pages is made clear by the section header, the Table of 
Contents on page 5559, and the proposed regulatory text for appendix 
B on pages 5707-08. See, e.g., id. at p.5668 (preamble erroneously 
states that the EPA proposed to change appendix A, section 2.6.1); 
id. at p.5668 (preamble erroneously states that the EPA proposed to 
adjust the minimum value required by appendix A, section 3.2.4).
    \186\ Noah, G. (2023). Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network. Memorandum to the Rulemaking Docket for the Review of the 
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    The EPA also proposed to update and clarify ambient air monitoring 
requirements found in 40 CFR part 58, appendix B, section 2.6.1 
pertaining to EPA Protocol Gas standards used for ambient air 
monitoring and the Ambient Air PGVP (88 FR 5668, January 27, 2023). The 
EPA proposed to revise appendix B to clarify that in order to 
participate in the Ambient Air PGVP, producers of Protocol Gases must 
adhere to the requirements of 40 CFR 75.21(g), and only regulatory 
ambient air monitoring programs may submit cylinders for assay 
verification to the EPA Ambient Air PGVP. The EPA received comments in 
support of and in opposition to this proposed revision. The commenter 
opposing the revision indicated that the proposed PGVP requirements 
would be additional and is concerned with an increased resource burden. 
However, the EPA disagrees with the commenter because that the proposed 
PGVP requirements are consistent with the existing PGVP requirements in 
40 CFR 75.21(g). PGVP has been defined as a regulatory requirement 
since 2016 (81 FR 17263, March 28, 2016), so the proposed part 58 
changes are not ``additional'' to existing regulations. After 
consideration of the comments, the EPA is finalizing the update and 
clarification of ambient air monitoring requirements found in appendix 
B, section 2.6.1 pertaining to EPA Protocol Gas standards used for 
ambient air monitoring and the Ambient Air PGVP as proposed.
b. Measurement Quality Check Requirements
    The EPA proposed to remove section 3.1.2.2 from appendix B, which 
allows NO2 compressed gas standards to be used to generate 
audit standards (88 FR 5668, January 27, 2023). The EPA received one 
comment supporting this change. As a result of the comment received and 
other general supportive comments regarding quality assurance, the EPA 
is finalizing the removal of section 3.1.2.2 from appendix B as 
proposed.
    The EPA proposed to revise the requirement in Appendix B, section 
3.1.3.3 changing the National Performance Audit Program (NPAP) 
requirement for annual verification of gaseous standards to the ORD-
recommended certification periods identified in Table 2-3 of the EPA 
Traceability Protocol for Assay and Certification of Gaseous 
Calibration Standards (appendix B, section 6.0(4)) (88 FR 5668, January 
27, 2023). The EPA received one comment supporting this change. As a 
result of the comment received and other general supportive comments 
regarding quality assurance, the EPA is finalizing the updated NPAP 
gaseous certification requirement in section 3.1.3.3 as proposed.
    The EPA proposed to adjust the minimum value required by appendix 
B, section 3.2.4, to be considered valid sample pairs for the 
PM2.5 Performance Evaluation Program (PEP) from 3 [mu]g/m\3\ 
to 2 [mu]g/m\3\ (88 FR 5668, January 27, 2023). The EPA received 
comments in support and against the change. In the only opposing 
comment, the commenter expressed concern that the method detection 
limit (MDL) for PM2.5 is 2 [mu]g/m\3\. The commenter also 
indicated that the MDL ``typically has minimal value per the definition 
of the MDL.'' 40 CFR part 50, appendix L states, ``The lower detection 
limit of the mass concentration measurement range is estimated to be 
approximately 2 [mu]g/m\3\, based on noted mass changes in field blanks 
in conjunction with the 24 m\3\ nominal total air sample volume 
specified for the 24-hour sample''. The EPA notes that field blanks 
currently average less than 10 [mu]g nationally, and when divided by 
the 24 m\3\ nominal total air sample volume specified for a 24-hour 
sample, the result is 0.4 [mu]g/m\3\. The appendix L MDL referenced by 
the commenter was part of the 1997 PM NAAQS rulemaking more than 20 
years ago (62 FR 38652, July 18, 1997); current data shows that the MDL 
is substantially lower than EPA's original estimate. After review of 
the comments, and in consideration of the recently calculated

[[Page 16349]]

detection limit for the PM2.5 FRM that is substantially 
lower than our original estimate, the EPA is revising the minimum value 
for valid sample pairs for the PM2.5 Performance Evaluation 
Program (PEP) from 3 [mu]g/m\3\ to 2 [mu]g/m\3\ in appendix B, section 
3.2.4 as proposed.
c. Calculations for Data Quality Assessments
    The EPA proposed to change Equations 6 and 7 of appendix B, section 
4.2.1 used for calculating the Collocated Quality Control Sampler 
Precision Estimate for PM10, PM2.5 and Pb (88 FR 5707, January 27, 
2023). These new statistics are designed to address the high 
imprecision values that result from using these calculations to compare 
low concentrations that are now more routinely observed in the 
networks. The EPA received several comments in support of this change 
in general, but a couple commenters indicated that there could be an 
error in the new calculation that resulted in high imprecision from the 
calculation of the equation. The EPA reviewed the technical memorandum 
and discovered that a multiplier of 100 was unintentionally left in the 
proposed relative difference equation, Equation 6. Also, equation 6 was 
corrected from a normalized percent difference to a normalized relative 
percent difference that is appropriate for comparing collocated pairs 
at low concentrations. The technical memorandum titled ``Task 16 on 
PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 
Ambient Air Monitoring Network'' was amended to correct the error and 
is included in the docket.\187\
---------------------------------------------------------------------------

    \187\ Noah, G. (2023). Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network. Memorandum to the Rulemaking Docket for the Review of the 
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
[GRAPHIC] [TIFF OMITTED] TR06MR24.033

    As a result of the positive comments received and the correction to 
the equation made in response to those comments, the EPA is finalizing 
the update to Equation 6 and retaining Equation 7 as proposed for the 
calculation of the Collocated Quality Control Sampler Precision 
Estimate for PM10, PM2.5 and Pb in section 4.2.1.
    The EPA proposed to update the appendix B, section 4.2.5, Equation 
8, calculation for the Performance Evaluation Program Bias Estimate for 
PM2.5 (88 FR 5668-59, January 27, 2023). Because average 
ambient PM concentrations across the nation have steadily declined 
since the promulgation of the PM2.5 standard, the EPA 
proposed to replace the current percent difference equation with a 
relative difference equation. The EPA received several comments in 
support of this change in general, but some commenters identified a 
potential error in the new calculation that resulted in an artificially 
high estimate, which they do not support. The EPA reviewed the 
technical memorandum and discovered that a multiplier of 100 was left 
in the new relative difference equation used in the bias equation. The 
technical memorandum, ``Task 16 on PEP/NPAP Task Order: Bias and 
Precision DQOs for the PM2.5 Ambient Air Monitoring 
Network'' has been amended to correct the error and is included in the 
docket. The proposed Equation 8 (88 FR 5669, January 27, 2023) was:

[[Page 16350]]

[GRAPHIC] [TIFF OMITTED] TR06MR24.034

    As a result of the supportive comments received and the correction 
to the equation in response to some comments, the EPA is updating and 
finalizing Equation 8 as described for the calculation for the 
Performance Evaluation Program Bias Estimate for PM2.5, in 
section 4.2.5.
d. References
    The EPA proposed to update the references and hyperlinks in 
appendix B, section 6 (88 FR 5669, January 27, 2023) to provide 
accuracy in identifying and locating essential supporting documentation 
and delete references to historical documents that do not represent 
current practices. The EPA received only favorable comments, and as a 
result, the EPA is finalizing the updated the references and hyperlinks 
in appendix B, section 6, as proposed.
    The EPA also proposed to add a footnote to Table B-1 of part 58, 
appendix B--Minimum Data Assessment Requirements for NAAQS Related 
Criteria Pollutant PSD Monitors (88 FR 5669, January 27, 2023). The 
proposed footnote clarifies the allowable time (i.e., every two weeks, 
once a month, once a quarter, once every six months, or distributed 
over all four quarters depending on the check) between checks and 
encourages monitoring organizations to perform data assessments at 
regular intervals. The EPA received two comments regarding this 
proposal. One commenter indicated that this change is inconsistent with 
the QA Handbook. The EPA agrees with the commenter; because the QA 
Handbook is guidance, the EPA will revise it after this action is 
finalized to be consistent with the updated CFR provision. Another 
commenter does not support the addition of the footnote due to concerns 
about limiting flexibility. In response, the EPA reiterates that the 
proposed revision is intended to clarify intent and does not make any 
changes to the required frequencies or acceptance criteria for data 
assessment. A ``weight of evidence'' narrative is still found in 40 CFR 
part 58, appendix B, section 1.2.3. As a result of the comments 
received and the rationale discussed above, the EPA is adding the new 
footnote to Table B-1 of part 58, appendix B--Minimum Data Assessment 
Requirements for NAAQS Related Criteria Pollutant PSD Monitors as 
proposed.
3. Amendments to PM Ambient Air Quality Methodology
a. Revoking Approved Regional Methods (ARMs)
    The EPA proposed to remove provisions for approval and use of 
Approved Regional Methods (ARMs) throughout parts 50 and 58 of the CFR 
(88 FR 5669, January 27, 2023). ARMs are continuous PM2.5 
methods that have been approved specifically within a State or local 
air agency monitoring network for purposes of comparison to the NAAQS 
and to meet other monitoring objectives. Currently, there are no 
approved ARMs. There are, however, more than a dozen approved Federal 
Equivalent Methods (FEMs) for PM2.5. These approved FEMs are 
eligible for comparison to the NAAQS and to meet other monitoring 
objectives.
    The EPA received comments from multiple State air programs in 
support of the proposal to remove provisions for approval and use of 
ARMs. One commenter cites that there are multiple FEMs available for 
monitoring agencies to work with and that the agency was never able to 
get a candidate ARM to meet the requirements for approval. With the 
availability of multiple FEMs that now work in the monitoring agency's 
network, the commenting agency does not anticipate the need to ever 
pursue an ARM in the future and, therefore, suggests that the ARM 
provision is no longer needed. Another commenter strongly supported the 
proposed changes to remove the ARM provisions. The EPA also received 
comments from a few agencies that supported retaining the ARM 
provisions instead. One commenter cited the need to consider the rapid 
advancement of various new technologies and that, in some cases, 
approved continuous FEMs may have shortcomings, meaning that losing the 
ability to propose an ARM in the future may limit useful alternative 
options to monitoring agencies. Another commenter suggested that the 
removal of the ARM would take away the ability and right to use locally 
derived correction factors.
    After considering the comments for and against removing the 
provisions for ARMs, the EPA believes it is most appropriate to remove 
the ARM provisions. As described in the proposal, when the EPA first 
proposed the process for approving and using ARMs, there were no 
continuous FEMs approved. There are now over a dozen approved 
PM2.5 continuous FEMs and no approved ARMs. Therefore, the 
EPA is finalizing the removal of ARMs throughout 40 CFR parts 50 and 58 
as proposed.
b. Calibration of PM Federal Equivalent Methods (FEMs)
    The EPA proposed to modify its specifications for PM FEMs in 
appendix C to Part 58 (88 FR 5670-73, January 27, 2023). Specifically, 
the EPA proposed that valid State, local, and Tribal (SLT) air 
monitoring data from Federal Reference Methods (FRMs) generated in 
routine networks and submitted to the EPA may be used to improve the PM 
concentration measurement performance of approved FEMs. This approach, 
initiated by instrument manufacturers, would be implemented as a 
national solution in factory calibrations of approved FEMs through a 
firmware update. This could apply to any PM FEM methods (i.e., 
PM10, PM2.5, and PM10-2.5).
    The EPA proposed this modification because there are some approved 
PM FEMs that are not currently meeting bias measurement quality 
objectives (MQOs) when evaluating data nationally as described in the 
2022 PA (U.S. EPA, 2022b, section 2.2.3.1), meaning that an update to 
factory calibrations may be appropriate; however, there is no clearly 
defined process to update the calibration of FEMs. While there are 
several types of data available to use as the reference for such 
updates (e.g., routinely operated FRMs, audit program FRMs, and 
chemical speciation sampler

[[Page 16351]]

data), we proposed to use routinely operated SLT FRMs as the basis of 
comparison upon which to calibrate FEMs. The goal of updating factory 
calibrations would be to increase the number of routinely operating 
FEMs meeting bias MQOs across the networks in which they are operated. 
While there are other approaches that could improve data comparability 
between PM FEMs and collocated FRMs, the EPA believes that the proposed 
modification to calibrate PM FEMs represents the most reliable approach 
to update FEM factory calibrations, since the existing FRM network data 
that meet MQOs would be used to set updated factory calibrations.
    While the Agency proposed to add this language to more expressly 
define a process to update factory calibrations of approved PM FEMs, 
the EPA believes that the existing rules for updating approved FRMs and 
FEMs found at 40 CFR 53.14 may also continue to be utilized for this 
purpose, as appropriate. 40 CFR 53.14 allows instrument manufactures to 
submit to the EPA a ``Modification of a reference or equivalent 
method.'' Submitting a modification request may be appropriate to 
ensure an approved FEM continues to meet 40 CFR 53.9, ``Conditions of 
designation.'' Specifically, 40 CFR 53.9(c) requires that, ``Any 
analyzer, PM10 sampler, PM2.5 sampler, or 
PM10-2.5 sampler offered for sale as part of an FRM or FEM 
shall function within the limits of the performance specifications 
referred to in Sec.  53.20(a), Sec.  53.30(a), Sec.  53.35, Sec.  
53.50, or Sec.  53.60, as applicable, for at least 1 year after 
delivery and acceptance when maintained and operated in accordance with 
the manual referred to in Sec.  53.4(b)(3).'' Thus, instrument 
manufacturers are encouraged to seek improvements to their approved FEM 
methods as needed to continue to meet data quality needs as operated 
across the network.
    There are several technical components to EPA's proposed 
modification, including: the reference data to be used in the 
calibrations; implementing as a national solution in factory 
calibrations of approved FEMs through firmware updates; application to 
any PM FEM methods (i.e., PM10, PM2.5, and 
PM10-2.5); the appropriate range of data to be used to 
develop and test new factory calibrations, from just the most 
representative concentrations up to all available concentrations; the 
representative set of geographic locations that can be used; whether 
outliers may be included or not included; that new factory calibrations 
should be developed using data from at least 2 years and tested on data 
from a separate year or years; that updates to factory calibrations can 
occur as often as needed; that calibrations should be evaluated by 
monitoring agencies as part of routine data assessments, e.g., during 
certification of data and 5-year assessments; the EPA's recognition 
that only data from existing operating sites is available; and finally, 
that an updated factory calibration does not have to work with the 
original field study data submitted that led to the original FEM 
designation.
    With the proposed modification, the EPA solicited input on these 
technical issues as well as the overall approach and any alternatives 
that could lead to more sites meeting the bias MQO with automated FEMs, 
especially for those sites that are near the level of the primary 
annual PM2.5 NAAQS, as proposed to be revised in section II 
above. In response, the EPA received comments from about two dozen 
entities, most of which were SLT air programs or Multi-Jurisdictional 
Organizations (MJOs) comprised of these entities.
    Overall, there was broad and strong support from a majority of 
commenters for the proposed requirement to use FRM data generated in 
routine networks and submitted to the EPA to update factory 
calibrations included as part of approved FEMs. There were a smaller 
number of critical comments on the proposed process as well as some 
commentors that supported the proposed requirement but also provided 
additional suggestions for the EPA's consideration. Below, we address 
each of the areas on which the EPA requested comment regarding the 
calibration of PM FEMs, as well as a few additional areas where 
multiple commenters offered input on other areas related to our 
proposal.
    A majority of the commenters on the proposed PM FEM calibration 
process support the process to use valid State, local, and Tribal FRM 
data generated in routine networks and submitted to the EPA to improve 
the PM concentration measurement performance of approved FEMs. Some 
commenters suggested that this action is needed to ensure that data 
reported from FRMs and FEMs are comparable and correction methods 
applied to data from FEM monitors are defensible across the national PM 
monitoring network. Others stated that they agree with the EPA that 
this is a critical step in the right direction to account for the 
discrepancies between PM2.5 FRM data and PM2.5 
FEM data. Some commented that applying corrections includes a 
recognition that, while different measurement principles may produce 
differences in the resulting data, having an approach that minimizes 
bias is extremely important. Finally, some stated their belief that a 
correction factor is necessary to preserve data integrity with the FRM.
    The EPA also received comments suggesting ways that the PM FEM 
correction could be performed, including through detailed analysis of 
data; by having PM FEM instrument manufacturers evaluate nationally 
available valid FRM data to update factory calibrations; and, by having 
the instrument manufacturers implement calibration adjustments at the 
factory.
    The EPA also received supportive comments on the PM FEMs 
calibration relating to comparability to the NAAQS. For example, a 
commenter stated that it is important to ensure bias MQOs are met for 
FEMs run at sites potentially affected by revised standards as well as 
the need to accurately designate areas as attaining or not attaining 
the NAAQS. There were comments supporting the correction of PM FEM data 
as helping the EPA and SLT monitoring programs continue to evolve 
toward more automated methods. For example, one commenter appreciates 
the EPA's support for the ongoing move from filter-based 
PM2.5 FRMs to use of continuous FEMs, stating that they 
concur with the EPA's assessment that there is monitoring bias between 
FRMs and FEMs, and commending EPA for recognizing ongoing data quality 
issues for FEMs and for taking action to improve these issues in 
collaboration with instrument manufacturers and SLT agencies.
    A small number of commenters were critical of the proposed FEM 
calibration approach. One commenter noted that EPA should further 
examine the handling of FEM PM2.5 data when used for 
comparison to the NAAQS. In response, we note that monitoring agencies 
and the EPA will continue to examine the comparability and use of FEM 
data used in comparison to the NAAQS. Another commenter suggested that 
the calibration process for a designated PM monitor should not be 
altered following Class III designation approval. The EPA disagrees as 
we believe it is appropriate for FEMs to be calibrated with routinely 
operated FRMs, because doing so is an efficient way to work towards FEM 
data meeting the bias MQO across the networks in which the FEMs are 
currently being operated. Also, having continuous PM FEMs meeting bias 
MQOs allows the use of the data in a variety of other ways that 
manually operated FRMs samplers cannot support. Another commenter 
stated that, if a particular FEM designated make or model of

[[Page 16352]]

instruments fails to meet MQOs, then that make or model should be 
removed from the designations altogether. The EPA agrees and clarifies 
that the modification would not prevent removal of FEM designation from 
a make or model of instrument under the existing 40 CFR 53.11--
Cancellation of reference or equivalent method designation. This may be 
appropriate if there are no other solutions to improve the method such 
that it achieves bias MQOs.
    A few commenters provided specific recommendations for how the 
regulatory language could be improved. These included comments that the 
new regulatory language proposed for 40 CFR part 58, appendix C, 
section 2.2 must ensure consistency and transparency when requesting 
changes to the factory calibration; that the EPA should incorporate 
binding regulatory language in 40 CFR part 58, appendix C, section 2.2 
(i.e., it currently lacks ``shall'' or ``must'') to ensure the language 
is not open to inconsistency and does not provide unique deference to 
instrument manufacturers without a mechanism for transparent 
communication of the changes being made and the supporting technical 
analysis. A commenter also requested that the EPA define the core 
requirements needed to ensure all requests for updating factory 
calibrations are required to follow the same process, using data of the 
same known quality, and evaluating the effectiveness of the resulting 
correction factors consistently.
    In response to these comments, while the EPA agrees that the 
proposed regulatory language for 40 CFR part 58, appendix C, section 
2.2 must ensure consistency and transparency when entities request 
changes to factory calibrations, the EPA disagrees that the regulations 
cannot also provide some flexibility. For example, we believe that a 
degree of flexibility is appropriate regarding whether outliers in the 
data to be used for factory calibration should or should not be 
included, the range of data to be included, and in utilizing collocated 
FRM and FEM data for updated calibrations from a representative set of 
geographic areas in which it is produced. The EPA believes that the 
proposal defined the core requirements needed to ensure all requests 
for updating FEM factory calibrations will follow the same process, 
using data of the same known quality and evaluating the effectiveness 
of the resulting correction factors consistently.
    In its proposal, the EPA identified that while there are several 
types of data available to use as the reference for FEM calibration 
updates, including data from routinely operated FRMs, audit program 
FRMs, and PM2.5 chemical speciation samplers, the EPA 
proposed to use routinely operated State, local, and Tribal FRMs as the 
basis of comparison upon which to calibrate FEMs (88 FR 5670-71, 
January 27, 2023). Importantly, routine SLT agency FRM data form the 
largest portion of the monitored air quality data used in epidemiologic 
studies that are being used to inform proposed decisions regarding the 
adequacy of the public health protection afforded by the primary 
PM2.5 NAAQS, as discussed in section II above.
    Overall, there was broad and strong support for utilizing 
collocated FRM data from routine SLT networks to provide calibrations 
of the continuous FEMs. For example, several commenters agree that 
valid SLT air monitoring data generated in routine networks and 
submitted to the EPA will improve the PM concentration measurement 
performance of approved FEMs. Another commenter provided support for PM 
FEM instrument manufacturers to evaluate nationally available valid FRM 
data as well as other data sets such as the performance evaluation 
audit program to update factory calibrations. The EPA believes that the 
routinely operated PM FRMs represent the best and largest source of 
data to calibrate continuous PM FEMs, and that performance evaluation 
audit program data should be kept independent of the calibration 
process. This will mean that assessments of the routine monitoring 
operations, including both the FRM and any future updated PM FEMs, will 
appropriately remain independent in evaluating whether updated methods 
are meeting bias MQOs. The EPA is, therefore, finalizing its approach 
to use routinely operated SLT FRMs as the basis of comparison upon 
which to calibrate continuous PM FEMs as proposed.
    Regarding the EPA's proposed requirement to utilize factory 
calibrations (88 FR 5670-71, January 27, 2023), several commenters 
agreed that factory calibrations provide the best option to improve PM 
FEMs. For example, one commenter stated that the correction factors are 
necessary to preserve data integrity with the FRM, and they support the 
proposal that the approach be initiated by instrument manufacturers and 
implemented as a national solution through firmware updates.
    Regarding the proposed requirement that calibrations be initiated 
by instrument manufacturers (88 FR 5671, January 27, 2023), most 
commenters were supportive of the proposed approach that recalibration 
of FEM PM instruments be initiated by instrument manufacturers. For 
example, one commenter stated they support allowing instrument 
companies submit improvements to their existing FEMs, as vendors should 
be encouraged to improve their methods. Another commenter noted that 
having a methodology initiated by the manufacturer will have nationwide 
consistency. A few of commenters recommended that SLT air agencies 
should have the additional ability to petition the EPA Administrator to 
initiate factory calibrations of FEMs to better meet MQOs when data 
collected by their agencies indicate disparities, because the 
monitoring agencies are responsible for the quality of the data from 
the specific makes and models of instrumentation used in their 
networks. While the EPA believes that, in most cases, the instrument 
companies should be the ones to initiate the process for calibration of 
FEMs to routinely operated FRMs, we agree with the commenters who 
suggested that other options should be available, including allowing 
monitoring agencies or MJOs to work independently or together to pursue 
improvements to designated FEMs. However, the EPA believes that any 
such improvements initiated by monitoring agencies or MJOs should still 
be facilitated through the responsible instrument company. Also, any 
such effort to improve data quality should be employed across all the 
networks in which the methods are operated and not limited to the 
networks operated by the agency(s) pursuing such improvements.
    Regarding how frequently factory calibrations should be updated, 
our proposal identified that it would be most appropriate to not define 
a specific time period for updates; rather, updates should be based on 
whether or not quality data is being produced across a given network 
(88 FR 5672, January 27, 2023). Regarding this issue, one commenter 
recommended that instrument manufacturers be required to evaluate and, 
if necessary, adjust PM FEMs factory calibrations on an ongoing basis 
at regular intervals. The EPA notes that while it does not have the 
authority to require instrument companies to evaluate the quality of 
data from operating FEMs under 40 CFR part 58, the EPA does routinely 
participate in conferences and workshops and makes assessments of data 
quality specific to instrument makes and models publicly available. The 
EPA also regularly summarizes relevant FRM and FEM data

[[Page 16353]]

quality in documents such as the 2022 PA (U.S. EPA, 2022b). Therefore, 
consistent with the proposal, we are not finalizing any specifics 
regarding how frequently factory calibrations should be updated but 
commit to continue to routinely provide information to SLT agencies 
regarding FEM data quality.
    The EPA proposed that the calibration of FEMs could apply to any of 
the PM FEM method indicators (i.e., PM10, PM2.5, 
and PM10-2.5) (88 FR 5670, January 27, 2023). The EPA 
received only supportive comments. All comments that included a 
discussion of three PM metrics support their inclusion for calibration 
of PM FEMs. Therefore, the EPA is finalizing the inclusion of all three 
PM indicators (i.e., PM10, PM2.5, and 
PM10-2.5) as proposed.
    The EPA proposed that either all data available or a range of data 
up to 125% of the 24-hour NAAQS for the PM indicator of interest may be 
used to establish new factory calibrations, (88 FR 5671-73, January 27, 
2023). The EPA received many comments supportive of the proposal and 
one comment offering a different approach on the range of data to use. 
One commenter recommends that the EPA should consider using all 
``validated'' data because how these instruments behave under normal 
operating ranges may be just as important as how they behave when 
monitoring conditions are low or elevated, and that the full range of 
data should be used when determining the appropriate level of the 
standard, just as the full range of data is used in determining if an 
area is attaining the standard. In response to this comment, the EPA 
believes that making allowances for some flexibilities will increase 
the likelihood of instrument companies pursuing such improvements. 
Also, even though there is flexibility, the EPA will still be able to 
evaluate the appropriateness of a range of concentration data included 
as part of each application submitted. Also, the EPA notes that in 
certain circumstances, States do petition the EPA to set aside data 
under the Exceptional Events Rule (Sec.  50.14, ``Treatment of air 
quality monitoring data influenced by exceptional events''). Where 
approved, exceptional event data are set aside from use in regulatory 
decisions. Thus, there is a process to set aside certain high 
concentration data for certain purposes. Therefore, the EPA is 
finalizing the provision that factory calibrations may be based on a 
range of valid data as proposed.
    The EPA solicited comment on the representative set of geographic 
locations to use in the calibration of FEMs compared to collocated FEMs 
(88 FR 5671, January 27, 2023). Most commenters were supportive of the 
approach of using representative sites in SLT networks from across the 
country. For example, several commenters provided their support for PM 
FEM instrument manufacturers to evaluate nationally available valid FRM 
data to update factory calibrations. Commenters disagreeing with a 
national geographic approach preferred to allow local solutions to 
correct data. For example, one commenter suggested having a local or 
regional option because PM instruments are impacted by, and respond 
differently to, a variety of local factors, including relative 
humidity, temperature, concentration levels, and particle composition. 
The EPA agrees that there are challenges in the response of PM FEMs to 
a variety of local factors; however, this can be true of many methods 
and are not specific to PM FEMs and, therefore, does not provide a 
reason to reject this approach in this instance. Another commenter 
stated that the proposed national correction factor is a ``flawed 
concept,'' suggesting that it is ``widely understood throughout the 
monitoring community that monitors perform best with a local correction 
factor.'' This commentor offered no record or citation supporting this 
point. The EPA counters that while monitoring agencies may 
statistically correct data from a PM continuous monitor for AQI 
purposes (40 CFR part 58, appendix G), there are both examples of well 
performing statistically corrected PM continuous monitors being used 
for AQI purposes; however, without proper attention and updates, there 
are also examples of poorly performing ones. Finally, another commenter 
believes that a national correction factor cannot possibly incorporate 
data to represent all the scenarios across the nation that have an 
impact on monitor performance and data quality. Although the EPA agrees 
that there are a variety of local scenarios that could affect monitor 
performance, the overall benefits of having nationally consistent 
measurement of PM concentrations and national calibration of data 
outweigh the potential advantages of locally specific calibrations.
    Several commenters also disagreed with using local and regional 
calibrations of data, including some monitoring agencies that asserted 
being unable to reinvest in the operation of FRMs that would be 
required to locally calibrate their own PM FEMs. Further, every 
approved PM FEM method designated today is effectively calibrated 
through demonstration of field testing in the areas in which it was 
required to be tested (40 CFR 53.35(b)(1)). Moreover, the EPA proposed 
to require instrument manufacturers to demonstrate that they can 
improve the number of sites meeting bias MQOs by initiating a 
recalibration of an FEM. Thus, the use of a national set of sites where 
the methods are operated is essentially a fine-tuning of the PM FEMs 
performance across all sites where it is used.
    After considering all the comments received, the EPA believes it is 
appropriate to finalize as proposed with a representative set 
geographic locations at SLT sites to calibrate PM FEMs. Identification 
of such sites would be made by the applicant of the planned updated 
calibration, subject to EPA approval, and submitted to the EPA in 
accordance with the requirements and application instructions in 40 CFR 
part 58, appendix C, sections 2.2 and 2.7. The EPA encourages early 
communication between an applicant seeking a method update and the EPA 
to facilitate the most appropriate sites are included in any updated 
application of the methods calibration.
    The EPA proposed that instrument companies may, but are not 
required to, check for and exclude any potential outliers that may 
exist in the validated State, local, and Tribal agency network data 
available from AQS that would be used to establish new factory 
calibrations. The EPA received two comments regarding potential outlier 
approaches. One commenter disagreed with the proposed approach and 
instead recommended the use of all ``validated'' data, because how 
these instruments behave under normal operating ranges may be just as 
important as how they behave when monitoring conditions are low or 
elevated. The EPA acknowledges this point; however, the proposal on 
outliers allows flexibility in using standard outlier tests if needed 
to include or exclude such data as part of the calibration process. 
Ultimately, the true test of success for an updated method calibration 
will be that a higher number of sites are meeting bias MQOs in the 
areas in which the method is used, which will include all routine valid 
data including any potential outliers. Another commenter asserted 
concerns with the ability of instrument manufacturers to analyze data 
within individual monitoring agencies. The EPA disagrees with the 
commenter because decisions whether to include or exclude outliers 
should be flexible and made on a case-by-case basis. Moreover, the 
expected substantially larger dataset from routinely operated 
collocated FRMs and FEMs compared to what was

[[Page 16354]]

used in the original FEM designation testing (Sec.  53.35 Test 
procedure for Class II and Class III methods for PM2.5 and 
PM10-2.5) will minimize the effect of any potential 
outliers.
    In contrast to these two comments, the EPA received many comments 
supportive of the proposed outlier approach overall. Therefore, the EPA 
is finalizing this part of the proposal that instrument companies may, 
but are not required to, check for and exclude any potential outliers 
that may exist in the validated State, local, and Tribal agency network 
data available from AQS that would be used to establish new factory 
calibrations.
    Several commenters offered input on statistical criteria and 
initial testing requirements for approval of candidate PM FEMs and the 
role of instrument manufacturers in this process. The EPA did not 
propose any changes related to these issues; however, these comments 
have been considered below.
    One commenter suggested that data quality objectives, bias, and 
precision estimators for different monitoring methods should be based 
on averages at both national and regional levels for purposes of 
comparison. Another commenter asked to strengthen the criteria for 
Class 3 Equivalency standards for candidate PM instrumentation. On 
testing requirements, one commenter recommended that the EPA consider 
updating the 40 CFR part 53 process for approving FEMs so that the 
testing process more closely reflects the regulatory deployment and 
data handling that generates NAAQS-comparable data. Another commenter 
asked that the results from ``summer'' and ``winter'' field evaluations 
not be averaged together because it allows agencies to minimize the 
error of biased instruments by averaging poor results with data often 
biased in the other direction. The same commenter also recommended that 
candidate instruments data sets should not be averaged together as is 
done currently where data from triplicate instruments are averaged for 
each day. Another commenter asked that the EPA require FEM field 
comparability tests in the northwest (e.g., in EPA Region 10) in areas 
where particulate derived from biomass predominates to ensure that 
certified instruments will perform reliably in regions influenced by 
these sources. Related to the different measurement principles and the 
instrument companies' role in PM FEMs, one commenter noted that FEMs 
may never align perfectly with the FRMs due to the use of different 
measurement principles. Another commenter asked that manufacturers of 
FEM instruments be held accountable for ensuring that they continue to 
meet FEM criteria, whether through calibration updates and/or follow-up 
evaluations. Another commenter suggested that instrument manufacturers 
should be required to further evaluate the FEM monitoring data at 
defined intervals including, but not limited to, the 2-year and 5-year 
approval anniversaries.
    The EPA did not propose to make modifications to the statistical 
criteria or testing requirements; however, we did solicit comment on 
any alternatives that would lead to more sites meeting the bias MQO 
with automated FEMs, especially for those sites that are near the level 
of the primary annual PM2.5 NAAQS as proposed (88 FR 5672-
73, January 27, 2023). While the comments requesting that the 
statistical criteria be strengthened may have merit, doing so would not 
address the large inventory of already deployed PM FEMs used throughout 
the country. Also, without performing a detailed Data Quality Objective 
(DQO) design process, it is unclear how changing one or more 
statistical criteria would help improve the number of sites meeting the 
bias MQO now or in the future. Similarly, while the comments asking for 
changes to the locations of testing may also have merit, the EPA 
believes this could be a deterrent for instrument manufactures to seek 
additional improvements since more testing would be required, at least 
for candidate methods. Regarding the comment on the different 
measurement principles, the EPA concurs that different measurement 
principles may never align perfectly. Also, the EPA notes that the 
Agency has longstanding goals for acceptable measurement uncertainty of 
automated and manual PM2.5 methods in 40 CFR part 58, 
appendix A, section 2.3.1.1. Therefore, while having different 
measurement principles align is useful, meeting the goal for acceptable 
measurement uncertainty is the objective.
    Regarding the comments related to the instrument companies' role in 
PM FEMs, the EPA notes that FEMs are already required to meet 40 CFR 
53.9, ``Conditions of designation.'' Specifically, 40 CFR 53.9(c) 
requires that, ``Any analyzer, PM10 sampler, 
PM2.5 sampler, or PM10-2.5 sampler offered for 
sale as part of an FRM or FEM shall function within the limits of the 
performance specifications referred to in Sec.  53.20(a), Sec.  
53.30(a), Sec.  53.35, Sec.  53.50, or Sec.  53.60, as applicable, for 
at least 1 year after delivery and acceptance when maintained and 
operated in accordance with the manual referred to in Sec.  
53.4(b)(3).'' The EPA does not have the authority to require instrument 
manufacturers to further evaluate the FEM monitoring data at defined 
intervals, including but not limited to the 2-year and 5-year approval 
anniversaries, as one commenter suggested.
    In addition to these few recommendations, the EPA received many 
comments supportive of the proposal that valid State, local, and Tribal 
air monitoring data from FRMs generated in routine networks and 
submitted to the EPA may be used to improve the PM concentration 
measurement performance of approved FEMs; therefore, consistent with 
the proposal we are not finalizing any updates to the statistical 
criteria, testing requirements, or requirements on instrument 
manufactures as proposed.
    The EPA proposed that any new factory calibration should be 
developed using data from at least 2 years and tested on a separate 
year(s) of data (88 FR 5672, January 27, 2023). Comments on this part 
of the proposal were generally supportive. One commenter requested that 
at least a 3-year dataset, rather than the proposed 2 years, be used 
for a representative design value comparison of the FEM and FRM 
datasets to be evaluated. Another commenter pointed out that as large a 
data set as possible should be used, but EPA should not limit it to 
only data collected by instruments that have operated for more than 2 
years.
    In response to these comments, the EPA notes the broad support for 
the proposal as written. Also, the EPA notes that the 2-year period for 
using data to develop a factory calibration is a minimum, and that more 
years may be used as appropriate. Therefore, the EPA is finalizing its 
approach that any new factory calibration should be developed using 
data from at least 2 years and tested on a separate year(s) of data as 
proposed.
    The EPA proposed several aspects of the FEM calibration on which we 
did not receive specific comments, including a provision that FEM 
methods should be evaluated by monitoring agencies as part of routine 
data assessments, such as during certification of data and 5-year 
assessments; the fact that the EPA recognizes only data from existing 
operating sites are available for use in factory calibrations; and 
recognition that an updated factory calibration does not have to work 
with the original field study data submitted that led to the 
designation as an FEM. With the broad general support from commenters 
summarized above, the EPA is finalizing each of these

[[Page 16355]]

individual aspects of the FEM calibration as proposed.
    In the proposal, the EPA identified that we should expect a lag 
between the date when an already designated method is approved with a 
new factory calibration as an updated method by the EPA and when it can 
be implemented in the field. The EPA solicited comment on how to 
approach the data produced during this lag. Commenters provided input 
not only on how to address data during the lag, but also regarding how 
to address data already collected prior to a method update that has the 
potential to be used in regulatory decision making, particularly where 
such collected data do not meet the bias MQO. In response to this 
solicitation of comment, there was a consistent recommendation that 
calibrations of data associated with method updates should be applied 
to all relevant PM data prior to the EPA using it for designations 
under a final NAAQS.
    While the EPA appreciates these comments and recognizes their 
support for retroactive data correction, at this time and following 
this final rule, monitoring agencies should continue to report PM FEM 
data as measured. This component of this final rule is focused only on 
revising 40 CFR part 53, appendix C to implement an updated calibration 
for approved PM FEMs. The issue of how prior and future monitoring data 
will be used in the implementation of this NAAQS, such as for 
designations, and for air quality regulatory programs is outside the 
scope of this rulemaking and will therefore be addressed by the EPA in 
a subsequent relevant action or actions.
    The EPA received comments on whether updates to PM FEM methods 
should be required to be implemented or there would flexibility in when 
and if a monitoring agency implemented them. The commenters asked that 
EPA be flexible in allowing the use of updated method correction 
factors intended to improve the data comparability between the FRMs and 
FEMs.
    In most cases, the EPA expects that updating the FEMs will result 
in improved data quality and more sites meeting bias MQOs; however, the 
EPA is not finalizing an update requirement in this action. Monitoring 
agencies can assess their data and make decisions on an update based on 
whether they are meeting the bias MQOs. Such decisions on whether or 
not to update a method may efficiently be included in those agencies' 
annual monitoring network plans under 40 CFR 58.10, ``Annual monitoring 
network plan and periodic assessment,'' which are already subject to 
EPA Regional office approval. In some circumstances, it is possible the 
original PM FEM may be revised in a manner where only the updated 
method has an active approved designation. In these cases, monitoring 
agencies would need to address updating their PM FEM in a timely 
manner.
    The EPA solicited input on any alternative approaches that could 
lead to more sites meeting the bias MQO with automated PM FEMs, 
especially for those sites that are near the level of the primary 
annual PM2.5 NAAQS as proposed to be revised in section II 
above. A few commentors provided input on potential options for 
alternative approaches and several others offered input on how a local 
or regional calibration of an FEM could work. Among alternative 
approaches, one commenter suggested that manufacturers of FEMs could 
provide settings that would allow for adjustments to make FEM data more 
``FRM-like.'' Another commenter suggested working with the 
manufacturers of FEM equipment to diagnose the cause of the bias and 
then to address it appropriately.
    The EPA received several comments on how to implement a local or 
regional calibration of FEMs. One commenter suggested that EPA could 
allow for SLT agencies to adjust FEM data to be more ``FRM-like'' prior 
to submitting data to AQS. Another commenter suggested using a rolling 
3-month linear regression based on a comparison of FEM data to 
PM2.5 levels measured by a 1-in-6-day FRM. Another commenter 
recommended that the EPA allow the application of a correction factor 
that is from an area with a similar climate and other conditions. 
Another commenter suggested that, for metropolitan statistical areas 
(MSAs) where the re-calibrated FEMs still do not meet equivalency 
criteria, monitoring agencies should be able to use the rolling linear 
regression technique to further calibrate the FEMs within an MSA. 
Another commenter suggested that developing a simple linear regression 
could establish the relationship between FEM data and FRM data and be 
used to adjust the FEM data at each site where they are collocated. 
Another commenter suggested that averaging the results within a MSA and 
applying it on an MSA basis with the previous 2 years of data could 
provide an adjustment method for sites without a collocated FRM. 
Another commenter identified that a regional correction factor 
potentially could improve instrument accuracy to biomass sources, which 
are a large component of PM in many communities.
    Among the alternative approaches suggested, having settings that 
would allow for adjustments to make FEM data more ``FRM-like'' has 
merit, but assuming this was within a PM FEM itself, it would need to 
be separately incorporated into each make and model of FEM. If EPA were 
to pursue this alternative approach, the suggestion could be 
incorporated into a future regulatory action as a potential condition 
of designation because, without having the opportunity to thoughtfully 
consider how every step of such an approach would need to work, 
including what such requirements would look like and how potential 
settings adjustments would be made, it is not appropriate for the EPA 
to require the availability of such settings now, nor would it address 
the inventory of currently available PM FEMs already operating.
    Regarding the suggestion that the EPA and SLTs should work with the 
manufacturers of FEM equipment to diagnose the cause of any biases and 
then to address them appropriately, the EPA supports this 
recommendation, but does not believe a regulatory change is required to 
allow the monitoring community (EPA and SLTs) to work with instrument 
manufacturers in this way.
    Regarding the several comments on how to implement a local or 
regional calibration of FEMs, the EPA acknowledges the desire for this 
flexibility but believes that any such provisions for local or regional 
calibration of FEMs would need to be thoroughly thought out and 
proposed for consideration across the monitoring community. While 
several commenters support such an approach, the EPA also received 
adverse comments on the potential for local and regional calibration of 
PM FEMs instead of national. Most of the criticism of local and 
regional calibration of PM FEMs centered on both the lack of existing 
operating PM FRMs in commenters' networks and monitoring agencies' 
inability to staff the higher number of operating FRMs that would have 
to be collocated with PM FEMs to calibrate. Thus, the commenters that 
oppose local and regional calibrations of data prefer to utilize the 
national calibration of FEM data as proposed. Acknowledging all of 
these viewpoints, the EPA believes that it would not be appropriate to 
institute such an approach at this time. As discussed throughout this 
section, this final rule, the EPA is embarking on a new national 
approach to calibration of FEMs where valid State, local, and

[[Page 16356]]

Tribal air monitoring data from FRMs generated in routine networks and 
submitted to the EPA may be used to improve the PM concentration 
measurement performance of approved FEMs. The EPA and the community of 
SLT monitoring agencies can further consider other solutions to 
improving PM FEM methods, including local and regional scale 
calibration of FEMs, in a future review of the PM NAAQS.
    In summary, the EPA is finalizing its proposal to allow valid 
State, local, and Tribal air monitoring data from PM FRMs and FEMs 
generated in routine networks and submitted to the EPA to update 
factory calibrations included as part of approved FEMs (40 CFR part 58, 
appendix C, sections 2.2 and 2.7). This approach, which will typically 
be initiated by instrument manufacturers but can also be spurred by 
monitoring agencies, MJOs of monitoring agencies, and the EPA itself, 
is to be implemented as a national solution in factory calibrations of 
approved FEMs through a firmware update, subject to EPA approval. FEM 
calibrations can apply to any PM FEM methods (i.e., PM10, 
PM2.5, and PM10-2.5). As part of this process, 
the EPA is finalizing that a range of data based on the most 
representative concentrations up to all available concentrations may be 
used in developing and testing a new factory calibration; that a 
representative set of geographic locations can be used; that outliers 
may be included or not included; that a new factory calibration should 
be developed using data from at least 2 years and tested on a separate 
year(s) of data; that updates to factory calibrations can occur as 
often as needed and should be evaluated by monitoring agencies as part 
of routine data assessments such as during certification of data and 5-
year assessments; that the EPA recognizes only data from existing 
operating sites is available; and that an updated factory calibration 
does not have to work with the original field study data submitted that 
led to the designation as an FEM. The EPA is finalizing this approach 
as proposed with the intention of having more sites meet the bias MQOs 
with automated PM FEMs.
4. Revisions to the PM2.5 Monitoring Network Design Criteria 
To Address At-Risk Communities
    To enhance protection of air quality in communities subject to 
disproportionate air pollution risk, particularly in light of the 
proposed range for a revised primary annual PM2.5 standard, 
the EPA proposed to modify the PM2.5 monitoring network 
design criteria to include an environmental justice (EJ) factor that 
accounts for proximity of at-risk populations (i.e., those identified 
in the 2019 ISA and ISA Supplement as being at increased risk of 
adverse health effects from PM2.5 exposures to sources of 
concern), consistent with the statutory requirement that the NAAQS 
protect the health of at-risk populations (88 FR 5673, January 27, 
2023). Specifically, the EPA proposed to modify the existing 
requirement at 40 CFR part 58, appendix D, section 4.7.1(b)(3)): ``For 
areas with additional required SLAMS, a monitoring station is to be 
sited in an area of poor air quality,'' to additionally address at-risk 
communities with a focus on anticipated exposures from local sources of 
emissions. The scientific evidence evaluated in the 2019 ISA and ISA 
Supplement indicates that sub-populations at potentially greater risk 
from PM2.5 exposures include children, lower socioeconomic 
status (SES) \188\ populations, minority populations (particularly 
Black populations), and people with certain preexisting diseases 
(particularly cardiovascular disease and asthma). The EPA proposed that 
communities with relatively higher proportions of sub-populations at 
greater risk from PM2.5 exposure within the jurisdiction of 
a State or local monitoring agency should be considered ``at-risk 
communities'' for these purposes.
---------------------------------------------------------------------------

    \188\ SES is a composite measure that includes metrics such as 
income, occupation, and education, and can play a role in 
populations' access to healthy environments and healthcare.
---------------------------------------------------------------------------

    The PM2.5 network design criteria have led to a robust 
national network of PM2.5 monitoring stations. These 
monitoring stations are largely in Core-Based Statistical Areas (CBSAs) 
\189\ across the country that include many PM2.5 monitoring 
sites in at-risk communities. Many of the epidemiologic studies 
evaluated in the 2019 ISA and ISA Supplement, including those that 
provide evidence of disparities in PM2.5 exposure and health 
risk in minority populations and low-SES populations, often use data 
from these existing PM2.5 monitoring sites. However, we 
anticipate that with the more protective annual NAAQS finalized in 
section II above, characterizing localized air quality issues around 
local emission sources may become even more important. The EPA believes 
that adding a network design requirement to locate monitors in at-risk 
communities will improve our characterization of exposures for at-risk 
communities where localized air quality issues may contribute to air 
pollution exposures. Requiring that PM2.5 monitoring 
stations be sited in at-risk communities will allow other methods to be 
operated alongside PM2.5 measurements to support multiple 
monitoring objectives per 40 CFR part 58, appendix D, section 1.1. The 
EPA believes that it is appropriate to formalize the monitoring 
network's characterization of PM2.5 concentrations in 
communities at increased risk to provide such areas with the level of 
protection intended with the PM2.5 NAAQS. The addition of 
this requirement will also lead to enhanced local data that will allow 
air quality regulators help communities reduce exposures and inform 
future implementation and reviews of the NAAQS.
---------------------------------------------------------------------------

    \189\ Metropolitan and Micropolitan Statistical Areas are 
collectively referred to as ``Core-Based Statistical Areas.'' 
Metropolitan statistical areas have at least one urbanized area of 
50,000 or more population, plus adjacent territory that has a high 
degree of social and economic integration with the core as measured 
by commuting ties. Micropolitan statistical areas are a set of 
statistical areas that have at least one urban cluster of at least 
10,000 but less than 50,000 population, plus adjacent territory that 
has a high degree of social and economic integration with the core 
as measured by commuting ties.
---------------------------------------------------------------------------

    The EPA received comments concerning the proposed requirement to 
modify the PM2.5 monitoring network design criteria to 
include an EJ factor that accounts for the proximity of populations at 
increased risk of adverse health effects from PM2.5 
exposures to sources of concern. Commenters included State, local, and 
Tribal air agencies and multijurisdictional organizations (MJOs) 
comprised of those agencies; industry and industry groups; other 
Federal, State, and local government entities; public health, medical, 
and environmental nongovernmental organizations (NGOs); and private 
citizens. The EPA proposed to require that sites located in at-risk 
communities (particularly those whose air quality is potentially 
affected by local sources of concern) should nonetheless meet the 
requirements to be considered representative of ``areawide'' air 
quality as this is consistent with all other minimally required sites. 
There were several other technical components of the proposed 
requirement for which we asked for comment, including: how to identify 
at-risk communities; the PM sources of concern important to consider; 
the datasets that can be used to identify communities with high 
exposures; the most useful measurement methods to collocate with 
PM2.5 in at-risk communities; and the timeline to implement 
any new or moved sites.
    Overall, most commenters were very supportive of the EPA's proposed 
modification to the PM2.5 monitoring

[[Page 16357]]

network design criteria to include an EJ factor that accounts for 
proximity of populations at increased risk of adverse health effects 
from PM2.5 exposures to sources of concern. A few commenters 
offered detailed supporting comments. For example, one commenter 
recommended targeting investment in regulatory monitors in EJ 
communities, opining that there is presently a lack of equitable 
distribution of these monitors in low-income and minority communities. 
Another commenter supports the inclusion of an EJ factor in 
PM2.5 monitoring network design criteria as a means to 
assess whether disparities in exposure are reduced in the future. The 
EPA appreciates the support for the proposed requirement and 
acknowledges the desirability of a goal to assess if disparities in 
exposure are reduced in the future as a result of these monitoring 
efforts.
    Some commenters were generally supportive of the proposed 
requirement but suggested that the EPA should recast the approach in a 
more specific way or offered additional examples of sources of concern. 
For example, one commenter stated that PM2.5 emissions from 
residential and commercial wood burning result in localized hotspots 
that are often not revealed by community air monitoring. Another 
commenter asked that the EPA adopt a strategy to monitor EJ communities 
near both larger well-known point sources of PM2.5 and along 
traffic corridors as well as smaller sources that, when taken together, 
may create a large amount of emissions and health harms in the area. 
Another commenter stated that the national network of monitors operated 
by the EPA captures data used for generalized modeling, but overall 
monitoring is not as granular as one would expect, especially in urban 
areas. For instance, the commenter suggested that EPA could monitor 
suspected ``hot spots'' (e.g., residential development adjacent to 
highways and active construction sites) to better manage and mitigate 
PM2.5 pollution at their sites of origin, and that more 
extensive and granular monitoring data would also facilitate essential 
research and inform future evaluations and adjustments of the NAAQS. 
The EPA acknowledges these comments identifying other sources of 
concern, and we address these and other potential sources of concern 
below.
    Among adverse comments, a few commenters stated that ``at-risk 
communities'' is not well defined. The EPA disagrees and directs those 
commenters to the numerous places where this definition is covered, 
including in Section II.B.2 of the proposal where we explained the term 
related to a variety of at-risk populations (88 FR 5591-92, January 27, 
2023) as well as section 12.5 of the 2019 ISA (U.S. EPA, 2019a) and 
section 3.3.3 of the ISA Supplement (U.S. EPA, 2022a). Other commenters 
oppose the addition of the proposed monitoring because they feel it 
would reduce flexibility for agencies in deciding where they should 
site monitors, advocating that monitoring agencies should be afforded 
maximum flexibility to identify where to site monitors for at-risk 
areas. Because the EPA recognizes the challenges cited by these 
commenters related to establishing new ambient air monitoring stations, 
the EPA is finalizing the modified requirement on PM2.5 
monitoring network design criteria intended to address at-risk 
communities that allows flexibility regarding which EJ communities 
should be monitored. Finally, one commenter asked that the EPA clarify 
a specific metric to judge how to site monitors in at-risk communities. 
Instead, the EPA believes it is appropriate for agencies to recommend 
what they believe to be the most important things to consider for their 
sites to meet the PM2.5 network design requirements and, 
thus, applying a new metric could take away from local priorities for 
at-risk communities.
    A few commenters asked that the EPA require more monitoring than 
proposed. One commenter stated that it would be more beneficial to 
overburdened communities if air monitoring were required in all at-risk 
communities. A few commenters asked that EPA require additional 
monitoring for attainment of PM2.5 NAAQS in EJ communities. 
In response to these comments, the EPA supports the SLT agencies' 
initiatives to conduct additional monitoring beyond the minimum 
monitoring requirements and network design criteria. In addition, the 
EPA supports agencies' use of alternative datasets such as sensors and 
sensors networks, satellites, and other non-regulatory monitoring where 
appropriate for non-regulatory data uses. The EPA notes that many 
monitoring agencies already operate more monitoring sites than are 
minimally required, and we expect this to continue as agencies consider 
siting monitors in at-risk communities.
    However, the EPA also received substantial concerns from monitoring 
agencies about their resource constraints, including staffing to 
support any potential new monitoring. The EPA also notes that the 
existing and robust network of almost 1,000 PM2.5 sites 
nationally is designed to continue to protect all populations at the 
level of the NAAQS discussed in section II of this final action by 
always having at least one site in the area of expected maximum 
concentration for each CBSA where monitoring is required. As a result 
of the revisions to the annual PM2.5 NAAQS being finalized 
in this action, a small number of new monitoring sites will also be 
required under EPA's current minimum monitoring requirements. With the 
monitoring network design changes finalized in this rule, many of these 
existing and new sites will form an important sub-component of the 
PM2.5 network by better characterizing air quality in at-
risk communities, particularly with respect to sources of concern.
    The EPA concludes that the requirements in this final rule for 
siting of monitoring in at-risk communities will meaningfully improve 
the PM2.5 monitoring network and its characterization of air 
quality in at-risk communities, without placing substantial new 
resource burdens on States and their monitoring agencies that would be 
associated with requirements for additional monitoring sites. 
Therefore, the EPA is finalizing this part of the proposed action 
without requiring additional monitoring sites beyond what would be 
associated with the revised annual PM2.5 NAAQS described in 
section II as they pertain to the minimum requirements associated with 
Table D-5 of Appendix D to Part 58--PM2.5 Minimum Monitoring 
Requirements.
    A few commenters asked that the EPA enhance monitoring in smaller 
cities and rural areas. One commenter asked for the EPA to extend the 
proposed monitoring network to Micropolitan Statistical Areas with 
populations of 10,000-50,000 and to rural areas. Another commenter 
pointed out that current air quality monitoring networks focus on urban 
and densely populated areas; therefore, rural areas are often not 
captured in this existing monitoring infrastructure, despite well-
documented examples of high PM concentration in rural communities. The 
commenter believes this results in inadequate assessment of air 
pollution exposures for a substantial segment of the U.S. population. 
The EPA disagrees that there needs to be additional requirements for 
small CBSA's and rural areas. Regarding these comments, the EPA points 
out that we have a long-standing requirement for each State to monitor 
at background and transport sites (40 CFR part 58, appendix D, section 
4.7.3--Requirement for PM2.5 Background and Transport 
Sites). Also, if an agency deems it appropriate to do so, monitoring 
coverage of rural areas can be accomplished with other tools

[[Page 16358]]

such as sensors and sensors networks, satellites, and other non-
regulatory monitoring. Although there may be short-term high exposures 
in rural areas, there is no evidence that long-term averages are higher 
in rural areas compared to urban areas with significantly higher 
density of populations and emissions. For smaller cities or rural areas 
that may have concentrations near the level of the PM2.5 
NAAQS finalized in section II above, monitoring agencies are encouraged 
to monitor and address emissions as appropriate.
    Some commenters disagree that the proposed revision to the 
PM2.5 monitoring network design criteria to address at-risk 
communities is needed. One commenter stated that including an EJ factor 
is not necessary because the current network is designed to protect all 
citizens. Another commenter stated that EJ factors could be cumbersome 
to implement. Another commenter asserted the proposal to add SLAMS in 
at-risk communities with higher PM2.5 concentrations might 
create more granular data and provide for a greater margin of safety 
for those communities and monitors in such a way that data from those 
areas could misrepresent the larger area represented by the network. In 
response to the comment on the current network protecting all citizens, 
the EPA agrees that by measuring in the community with the highest 
concentration of PM2.5 we protect other citizens; however, 
as stated in the proposal, the EPA believes that adding a requirement 
for sites with an EJ factor near sources of concern will enhance the 
overall network to the benefit of all citizens. Also, we anticipate 
that with the more protective annual NAAQS finalized in section II 
above, characterizing localized air quality issues will become even 
more important around local emission sources. As for EJ factors being 
cumbersome to implement, the EPA disagrees because there are many such 
locations already operating successfully in the current network. 
Regarding the comment that sites in at-risk communities may 
misrepresent the larger area represented by a particular network, the 
EPA notes that pursuant to 40 CFR part 58, minimally required sites in 
a given network are to represent area-wide air quality; therefore, 
sites in at-risk communities, by definition, would be representative of 
the communities within the network in which they are sited for the 
level of protection intended under the annual PM2.5 NAAQS.
    In the proposal, the EPA identified that, in light of the evidence 
of increased risk to at-risk communities, it would be appropriate to 
better characterize exposures for communities in proximity to local 
sources of concern (88 FR 5673-76, January 27, 2023). Thus, the EPA 
proposed that enhanced networks should include representation of at-
risk communities living near emission sources of concern (e.g., major 
ports, rail yards, airports, industrial areas, or major transportation 
corridors). The EPA requested comment on the types of sources of 
concern most important to consider. In addition to supporting the types 
of sources the EPA identified in the proposal, commenters also 
identified several additional localized sources such as railroads, 
stationary sources, transportation facilities, and communities with 
high numbers of wood stoves.
    A few commenters suggested the inclusion of sources that are often 
considered line and/or area sources, e.g., traffic corridors and 
emissions from federally regulated facilities, military installations, 
and national forests. Commenters also identified other sources usually 
associated with long-range transport such as smoke from wildfire and 
prescribed fires and long-distance transport of PM, for example from 
Saharan dust and other international transport. As explained in the 
proposal, the site with the highest expected PM2.5 is 
already required to have a monitor by our long-standing requirement 
that monitors be placed ``. . . in the area of expected maximum 
concentration'' (Sec.  58.1 and appendix D, section 4.7.1(b)(1)). The 
EPA expects that both sites with the expected maximum concentration and 
sites specifically placed in at-risk communities would be impacted by 
any long-range transport in the area. Therefore, the EPA believes any 
emphasis on the sources of concern should prioritize localized sources, 
including point, area, and line sources of concern impacting the at-
risk community of interest. Therefore, based upon the comments, the EPA 
is finalizing a broader example list of sources of concern to include 
localized sources such as point sources and transportation facilities, 
since these are the most commonly expected additional sources of 
concern. In response to the other sources of concern suggested by 
commenters, the EPA notes that while it has provided examples, the 
siting of monitors in EJ communities would not be limited to these 
examples. Thus, the revised set of examples would include ``a major 
industrial area, point source(s), port, rail yard, airport, or other 
transportation facility or corridor.'' In finalizing this modified list 
of examples, the EPA is not looking to prioritize one type of source 
category over another; rather, we intend to further illustrate the 
types of localized sources of pollution that might impact at-risk 
communities such that the siting of monitors nearby may be appropriate.
    One commenter noted that the proposal may have unintentionally 
taken out the requirement related to specific design criteria for 
PM2.5 in 40 CFR part 58, appendix D, 4.7.1(b)(3) that, for 
an area with a requirement for an additional SLAMS monitor, it should 
``be sited in an area of poor air quality.'' Thus, the language as 
proposed neither requires that such monitors be sited in areas of poor 
air quality, nor does it require that the monitor be sited in an area 
that is anticipated to experience poor air quality from unspecified 
(and thus potentially relatively insignificant) sources in the area. 
The EPA agrees that this was not our intention; the EPA wants to 
protect populations in at-risk communities by ensuring they are 
protected by the NAAQS when there are sources of concern that may be 
impacting them (i.e., not insignificant sources). Thus, the EPA is 
reinstating this requirement in the network design language and 
combining it with the examples of the types of localized sources of 
concern: ``For areas with additional required SLAMS, a monitoring 
station is to be sited in an at-risk community with poor air quality, 
particularly where there are anticipated effects from sources in the 
area (e.g., a major industrial area, point source(s), port, rail yard, 
airport, or other transportation facility or corridor).''
    To ensure minimally required monitoring sites appropriately 
represent exposures in at-risk communities, the EPA proposed that sites 
represent ``area-wide'' air quality near local sources of concern (88 
FR 5674, January 27, 2023). Sites representing ``area-wide'' air 
quality are those monitors sited at neighborhood, urban, and regional 
scales, as well as those monitors sited at either micro- or middle-
scale that are identified as being representative of many such 
locations in the same Metropolitan Statistical Area (MSA).\190\ Most 
existing--as well as new or moved sites--are expected to be 
neighborhood-scale, which means that the monitoring stations would 
typically represent conditions throughout some reasonably homogeneous 
urban sub-region with dimensions of a few kilometers per part 58, 
appendix D, section 4.7.1(c)(3). Additionally, as described in Sec.  
58.30,

[[Page 16359]]

sites representing ``area-wide'' air quality have a long-standing 
applicability to both the annual and 24-hour PM2.5 NAAQS. 
Our proposed requirement for siting monitors in communities 
representing ``area-wide'' air quality is consistent with other network 
design objectives pursuant to which we seek to have monitors located 
where people live, work, and play.
---------------------------------------------------------------------------

    \190\ MSA means a CBSA associated with at least one urbanized 
area of 50,000 population or greater. The central-county, plus 
adjacent counties with a high degree of integration, comprise the 
area.
---------------------------------------------------------------------------

    The EPA received a few comments on its proposed requirement that 
minimally required sites represent ``area-wide'' air quality. One 
commenter stated that the inclusion of a provision for EJ would narrow 
the location of monitors to certain communities that may not best 
represent ``areawide'' air quality. Another commenter asked the EPA to 
consider removing requirements that sites be area-wide, since 24-hour 
and annual averaging times would miss short, elevated pollution events. 
A couple commenters had concerns with the difference in the scale of 
representation between EJ monitors using small scale and other NAAQS 
monitors using area-wide scale, in that area-wide scale would not 
protect those most at risk. However, another commenter agreed with the 
EPA that sites representing at-risk communities should represent area-
wide air quality. In addition to these comments, the EPA received many 
comments with support for its proposed modifications to the network 
design criteria as whole.
    Regarding whether narrowing the location to certain communities may 
not best represent ``area-wide'' air quality, the EPA notes that sites 
are either identified as being area-wide or not; the EPA did not 
suggest it was seeking a best ``area-wide'' location. In response to 
the comment that area-wide site may miss short, elevated pollution 
events, the EPA is aware that there can be local, short-term spikes in 
PM2.5 concentrations. However, the network design criteria 
associated with minimally required sites is applicable to both the 
annual and 24-hour PM2.5 NAAQS, and the EPA believes it is 
appropriate to continue to ensure all minimally required sites have the 
most utility and remain applicable to both forms of the 
PM2.5 NAAQS. The identification of unique micro- and middle-
scale sites was directed at discretionary efforts of any monitoring 
agency, with the recognition that such sites, (i.e., relatively unique 
micro-scale, or localized hot spot, or unique middle-scale impact 
sites), are not applicable to the annual NAAQS as described in Sec.  
58.30--Special consideration for data comparison to the NAAQS.
    After considering all the comments on this topic, the EPA is 
finalizing this part of the modification to the network design criteria 
to maintain, consistent with our long-standing network design criteria, 
that all minimally required sites are to represent area-wide air 
quality.
    In addition to using data from the robust network of almost 1,000 
PM2.5 sites for NAAQS and AQI purposes, having a stable 
network of long-term sites is especially valuable to examine trends and 
to inform long-term health and epidemiology studies that support 
reviews of the PM NAAQS. Therefore, while we proposed to add a 
PM2.5 network design criterion to address at-risk 
communities, many sites are likely already in valuable locations 
meeting one of the existing network design criteria (i.e., being in an 
area-wide area of expected maximum concentration or collocated with 
near-road sites) and supporting multiple monitoring objectives. Also, 
in many communities, there may already be sites meeting the network 
design criterion we proposed for at-risk communities. Thus, 
acknowledging the value of having long-term data from a consistent set 
of network sites, the EPA believes that moving sites should be 
minimized, especially in MSAs with a small number of sites. However, 
because a small number of new sites are expected to be required due to 
the existing minimum monitoring requirements (40 CFR part 58, appendix 
D, Table D-5) \191\ and the revised primary annual PM2.5 
NAAQS detailed in section II, and because sites occasionally have to be 
moved--due to, for example, loss of access to a site or a site no 
longer meeting siting criteria--the EPA believes it is appropriate to 
prioritize establishing sites in at-risk communities near sources of 
concern, whenever new sites are established, whether because it is a 
new site or a replacement for a prior site that must be moved. The EPA 
accordingly proposed that annual monitoring network plans (40 CFR 
58.10(a)(1)) and 5-year assessments (40 CFR 58.10(d)) that include any 
of the few new sites that will be required include a commitment to 
examine the ability of existing and proposed sites to support air 
quality characterization for areas with at-risk populations in the 
community and the objective discussed herein.
---------------------------------------------------------------------------

    \191\ Gantt, B. (2022). Analyses of Minimally Required 
PM2.5 Sites Under Alternative NAAQS. Memorandum to the 
Rulemaking Docket for the Review of the National Ambient Air Quality 
Standards for Particulate Matter (EPA-HQ-OAR-2015-0072). Available 
at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    In the proposal, the EPA identified that assessing and prioritizing 
at-risk communities for monitoring can be accomplished through several 
approaches (88 FR 5675). The most critical aspect of prioritizing which 
communities to monitor is their representation of the at-risk 
populations described earlier in this section. The other major 
consideration is whether the community is near a source or sources of 
concern. While many CBSAs have one or more sources of concern described 
above, some CBSAs will not have a quantity of emissions from sources of 
concern that result in an elevated level of measured PM2.5 
concentrations in surrounding communities. The siting criteria to be 
``in the area of expected maximum concentration,'' Sec.  58.1 & 
appendix D, section 4.7.1(b)(1) ensures there is a monitoring site in 
the community with the highest exposure in each CBSA with a monitoring 
requirement. Some CBSAs may also have a requirement to collocate a 
PM2.5 monitor at a near-road NO2 station. 
Therefore, the EPA believes that for cases where an additional 
PM2.5 site is required, we should include a criterion that 
the site be in an at-risk community when there are no sources of 
concern identified in that CBSA, or such sources do exist but are not 
expected to lead to elevated levels of measured PM2.5 
concentrations.
    In its proposal, the EPA highlighted that tools such as the EPA's 
EJSCREEN \192\ are available to identify the at-risk communities 
intended for monitoring as part of the proposed revision to the 
PM2.5 network design criteria (88 FR 5675-76, January 27, 
2023). The EPA solicited comment on other tools and/or datasets that 
can be utilized to identify at-risk communities. In addition to support 
for using EJSCREEN, commenters identified several other options to 
identify at-risk communities intended for monitoring as part of the 
proposed revision to the PM2.5 network design criteria. 
Among similar tools, one commentor suggesting using 
CalEnviroScreen.\193\ Commenters also identified different options for 
models including InMAP,\194\ satellite-derived models that can be 
employed to help identify EJ communities, and hybrid models. A few 
commenters also suggested using sensors and sensor networks such as the 
BlueSky \195\ and PurpleAir \196\ sensors.
---------------------------------------------------------------------------

    \192\ See: https://www.epa.gov/ejscreen.
    \193\ See: https://oehha.ca.gov/calenviroscreen.
    \194\ See: https://inmap.run/#home.
    \195\ Mention of commercial names does not constitute EPA 
endorsement.
    \196\ Mention of commercial names does not constitute EPA 
endorsement.
---------------------------------------------------------------------------

    The EPA supports the use of other State and local tools designed to 
help identify the at-risk communities that

[[Page 16360]]

should be monitored to meet the revised network design criteria. The 
EPA additionally agrees with commenters that the use of models as well 
as sensors and sensor networks may be appropriate and helpful in 
identifying the most appropriate at-risk communities in which to locate 
monitors.
    For at-risk communities, monitoring agencies need data that can 
best inform where there may be elevated levels of exposures from 
sources of concern. While we use FRMs and FEMs to determine compliance 
with the NAAQS, data from these methods will only be available at 
existing sites. However, there are several additional datasets 
available that may be useful in evaluating the potential for elevated 
levels of exposure to communities near sources of concern. In the 
proposal, EPA identified potential non-regulatory monitoring datasets 
such as CSN, IMPROVE, and AQI non-regulatory PM2.5 
continuous monitors; modeling data that utilizes emission inventory and 
meteorological data; emerging sensor networks such as those that 
comprise EPA and the USFS's Fire and Smoke Map; \197\ and satellites 
that measure radiance and, with computational algorithms, can be used 
to estimate PM2.5 from aerosol optical depth (AOD) (88 FR 
5675-76, January 27, 2023). The EPA solicited comment on datasets most 
useful to identify communities with high exposures for PM2.5 
NAAQS (i.e., annual or 24-hour). In addition to providing information 
about datasets that can inform the NAAQS comparison, commenters 
additionally identified several types of datasets that may be useful to 
identify where there may be elevated levels of exposures from sources 
of concern. These datasets include satellite measurements, sensors, and 
sensor network data, which may all be useful to find hot spots in 
communities. Commenters also identified EJScreen and CalEnviroScreen, 
which are screening and mapping tools that utilize several datasets. 
Another commenter stated that to better understand exposure differences 
in disadvantaged communities, shorter measurement intervals should be 
measured and reported.
---------------------------------------------------------------------------

    \197\ See: https://fire.airnow.gov/.
---------------------------------------------------------------------------

    In considering the datasets identified in the proposal as well as 
the ones commenters provided, the EPA believes all the datasets have 
value to help inform where there may be elevated levels of exposures 
from sources of concern. However, each of them may also have 
limitations and, therefore, users should be careful not to rely solely 
on one dataset versus another for all purposes. Fortunately, many of 
the available datasets are becoming easier to work with and more 
accessible, which will allow interested parties and monitoring agencies 
the opportunity to efficiently review the datasets and determine best 
applicability. For all of these reasons, the EPA is not finalizing a 
requirement to use a specific dataset or tool to identify at risk 
communities; however, whatever datasets a monitoring agency elects to 
use, its plan to use such data for purposes of meeting the network 
design requirements will be subject to EPA approval as part of the 40 
CFR 58.10 annual monitoring network plan. Regarding the comment 
recommending shorter measurement intervals in measuring and reporting 
data to better understand exposure differences in disadvantaged 
communities, the EPA agrees and generally supports use of continuous 
methods. While we generally support use of continuous methods, approved 
filter-based technologies and methods also provide valuable air quality 
information. Therefore, the EPA is not requiring the use of automated 
continuous methods beyond what is already required in 40 CFR part 58, 
appendix D, section 4.7.2--Requirement for Continuous PM2.5 
Monitoring.
    The monitoring methods appropriate for use at required 
PM2.5 sites in at-risk communities are FRMs and automated 
continuous FEMs (88 FR 5675-76, January 27, 2023). These are the 
methods eligible to compare to the PM2.5 NAAQS, which is the 
primary objective for collecting this data. There are several other 
monitoring objectives that would benefit from the use of automated 
continuous FEMs. For example, having hourly data available from 
automated continuous FEMs would allow sites to provide data in near-
real time to support forecasting and near real-time reporting of the 
AQI. Automated continuous methods are also useful to support evaluation 
of other methods such as low-cost sensors. When used in combination 
with on-site wind speed and wind direction measurements, automated FEMs 
can provide useful pollution roses, which help in identifying the 
origin of emissions that affect a community. Additionally, when 
collocated with continuous carbon methods such as an aethalometer, 
automated FEMs can help identify potential local carbon sources 
contributing to increased exposure in the community. While either FRMs 
or automated FEMs may be used at a site for comparison to the 
PM2.5 NAAQS, the EPA supports use of automated continuous 
FEMs at sites in at-risk communities.
    The EPA requested comment on the measurement methods most useful to 
collocate with PM2.5 in at-risk communities (88 FR 5675-76, 
January 27, 2023), and a few commenters provided input. One commenter 
recommended that the EPA should employ supplemental technologies and 
systems to increase coverage of the regulatory monitoring network and 
obtain more complete data to further protect public health and address 
environmental injustice in air pollution exposure. Another commenter 
recommended that the EPA invest in community-led monitoring and mobile 
air quality monitoring with a goal of recording block-level 
variabilities in data. And another commenter cited the value of 
community-deployed PM2.5 monitoring.
    The EPA appreciates the comments provided on the measurement 
methods most useful to collocate with PM2.5 monitoring sites 
in at-risk communities. Because the use of methods beyond the required 
PM2.5 FRMs or FEMs or other criteria pollutant measurements 
meeting a NAAQS monitoring requirement is voluntary, the establishment 
of PM2.5 NAAQS comparable sites in at-risk communities will 
allow for collaboration at multiple levels. The EPA strongly encourages 
such collaboration with impacted communities, and the measurement 
methods discussed here should be considered for use as appropriate.
    In the proposal, the EPA identified that, to meet the revised 
network design criteria, there will be only a few new sites 
required,\198\ plus any potentially moved sites in cases where an 
existing site lease is lost or otherwise requires relocation (88 FR 
5675-76, January 27, 2023). To handle these new or relocated sites, the 
EPA proposed to build upon our existing regulatory process for 
selecting and approving these sites under 40 CFR 58.10 (88 FR 5676, 
January 27, 2023). In the proposal, we stated it would be appropriate 
to provide at least 12 months from the effective date of the final rule 
to allow monitoring agencies to initiate planning to implement these 
measures by seeking input from communities and other interested parties 
and considering whether to revise their PM2.5 networks

[[Page 16361]]

or explain how their existing networks meet the objectives of the 
proposed modification to the network design criteria. Thus, the EPA 
proposed that monitoring agencies should address their approach to the 
question of whether any new or moved sites are needed and identify the 
potential communities in which the agencies are considering adding 
monitoring, if applicable, as well as identifying how they intend to 
meet the revised criteria for PM2.5 network design to 
address at-risk communities in the agencies' annual monitoring network 
plans due to each applicable EPA Regional office no later than July 1, 
2024 (see 40 CFR 58.10). Specifics on the resulting new or moved sites 
for PM2.5 network design to address at-risk communities were 
proposed to be detailed in the annual monitoring network plans due to 
each applicable EPA Regional office no later than July 1, 2025 (40 CFR 
58.10). The EPA proposed that any new or moved sites would be required 
to be implemented and fully operational no later than 24 months from 
the date of approval of a plan or January 1, 2027, whichever comes 
first, but the EPA solicited comment on whether less time is needed 
(e.g., 12 months from plan approval and/or January 1, 2026).
---------------------------------------------------------------------------

    \198\ Gantt, B. (2022). Analyses of Minimally Required 
PM2.5 Sites Under Alternative NAAQS. Memorandum to the 
Rulemaking Docket for the Review of the National Ambient Air Quality 
Standards for Particulate Matter (EPA-HQ-OAR-2015-0072). Available 
at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    The EPA received a few comments on its proposed timeline for 
monitoring agencies to identify, propose, and ultimately bring any new 
or moved sites online. One commenter asked that the timeline give 
states more time to start or move sites. A few commenters asked that 
the EPA only require meeting a timeline for identifying whether any new 
or moved sites are needed after the EPA has provided the monitoring 
agencies with guidance on the priority of the potential at-risk 
communities. One of those commenters further requests that the EPA 
allow at least 24 months from the date of approval of a Sec.  58.10 
monitoring plan identifying any relocation of monitoring sites or 
establishment of new monitoring sites to implement any changes to the 
network, citing the need for more time to work with local officials, 
procure monitoring equipment, and contract for services, all of which 
can cause significant delays in establishing a monitoring site. Another 
commenter asked that the EPA remain attentive to the challenges that 
States, and air agencies face regarding recruiting and retaining the 
specialized staff needed to support their existing regulatory 
monitoring networks and the capital resources needed to implement and 
sustain new monitoring stations in areas that are clearly meeting the 
existing PM NAAQS or any revised PM NAAQS. Another commenter stated 
that the July 1, 2024, timeline for a network evaluation this complex 
is insufficient, noting that they submit their draft annual monitoring 
network plan for public review and comment in mid-April for 30 days. 
Because the final plan is due July 1 and must include all comments and 
responses and describe any changes based on those comments, the 
timeline does not take these requirements into consideration by 
allowing for the more extensive assessment of changes that may be 
needed to meet the proposed new monitoring requirements. The commenter 
stated that it would be appropriate to provide at least 12 months from 
the effective date of this final rule for monitoring agencies to 
initiate planning to implement these measures, seek input, consider 
revisions to their PM2.5 networks, and explain how their 
existing networks meets the objectives of the final rule. The commenter 
notes that that SLT agencies should be provided a minimum of 18 months 
after the final recommendation is published to add this information to 
their Sec.  58.10 annual monitoring network plans. Another commenter 
encourages the EPA to retain the proposed deadline for any newly 
required monitoring stations in at-risk communities to be operational 
(i.e., 24 months after the July 2025 network plan approval or January 
1, 2027, whichever is earlier). While the need for this data is urgent, 
the commenter stated that the process for procuring instrumentation, 
securing leases, and building permits, and other logistics in 
constructing new monitoring sites can take a significant amount of 
time, some of which are outside of agencies' control.
    As stated earlier, the EPA received strong support for our proposal 
to modify the PM2.5 monitoring network design criteria to 
include an EJ factor that accounts for proximity of populations at 
increased risk of adverse health effects from PM2.5 
exposures to sources of concern from a wide range of commenters. A few 
commenters support the timeline proposed, a few others support starting 
any new or moved sites sooner than proposed, while other commenters 
asked for more time or offered conditions regarding how to establish an 
appropriate timeline.
    The EPA disagrees with the commenter that suggested the EPA should 
only require agencies to meet a timeline to identify whether any new or 
moved sites are needed after the EPA has provided the monitoring 
agencies with guidance on the priority of the potential at-risk 
communities, because the regulatory text provides all the guidance 
required for agencies to begin this process. As we explained above, the 
EPA does not anticipate that many new or moved sites will be required 
based on the final rule because we think most sites are already in 
suitable locations and long-term sites are highly valued. Also, 
monitoring agencies have discretion to provide to the EPA their 
recommendations regarding how they intend to meet the modifications to 
the PM2.5 monitoring network design criteria to include an 
EJ factor that accounts for proximity of populations at increased risk 
of adverse health effects from PM2.5 exposures to sources of 
concern. Overall, the EPA believes that having sites in the areas of 
expected maximum concentrations will best ensure that all communities 
are protected. Since there may be multiple choices for sites in EJ 
areas near sources of concern, the EPA acknowledges that there may be 
many locations that can meet the revised PM2.5 network 
design criteria. While, as we explained earlier, we want such sites to 
also be in areas of poor air quality, the sites in the area of maximum 
concentration will ensure that all communities are protected, there can 
be more flexibility afforded in the selection amongst at-risk 
communities to meet the revised requirements, since any alternative at-
risk communities would already be protected.
    The EPA considered both the concerns and support for the timeline 
proposed and clarifies that the component of the proposed requirement 
regarding the need to identify potential new sites or an intention to 
move sites to be included in the annual monitoring network plan due to 
EPA on July 1, 2024, would be satisfied with a statement of intent to 
pursue a new site per the revised network design criteria and in 
consideration of the minimum monitoring requirements. While monitoring 
agencies may provide as much detail as they deem appropriate regarding 
the revised PM2.5 network design criteria in their annual 
monitoring network plans due on July 1, 2024, there is no expectation 
that any details on site-specific information would be included at that 
stage. We encourage agencies to provide their initial thinking on the 
communities they are most interested in monitoring pursuant to the 
revised network design criteria. Therefore, the EPA is finalizing the 
timeline as proposed, including the provision that monitoring agencies 
report their intention to add or move sites, where required, in their 
annual monitoring network plans due to each applicable EPA Regional 
office no later than July 1, 2024 (40 CFR 58.10). The

[[Page 16362]]

monitoring agencies will then provide specifics on any new or moved 
sites for PM2.5 network design to address at-risk 
communities in the annual monitoring network plans due to each 
applicable EPA Regional office no later than July 1, 2025 (40 CFR 
58.10). And any new or moved sites shall be implemented and fully 
operational no later than 24 months from the date of approval of a 
Sec.  58.10 plan, or January 1, 2027, whichever comes first.
    In summary, the EPA is finalizing modifications to the 
PM2.5 network design criteria to include an EJ factor to 
address at-risk communities with a focus on exposures from sources of 
concern in areas of poor air quality. While this modification to the 
PM2.5 network design requires sites to be located in at-risk 
communities, particularly those whose air quality is potentially 
affected by local sources of concern, such sites must still meet the 
requirement for being considered ``area-wide'' air quality. In 
finalizing this modification to the PM2.5 network design 
requirement, the EPA is making two changes in the final rule response 
to the comments received. First, the EPA is broadening our examples of 
``sources of concern'' to include localized sources such as point 
sources and major transportation facilities or corridors. Second, the 
EPA is reinstating ``poor air quality'' in our requirement for the 
modified network design criteria, meaning the revised PM2.5 
network design requirement now states: ``For areas with additional 
required SLAMS, a monitoring station is to be sited in an at-risk 
community with poor air quality, particularly where there are 
anticipated effects from sources in the area (e.g., a major industrial 
area, point source(s), port, rail yard, airport, or other 
transportation facility or corridor).'' All other aspects of the 
PM2.5 network design requirements are being finalized as 
proposed.
5. Revisions to Probe and Monitoring Path Siting Criteria
    The EPA proposed changes to monitoring requirements in the Appendix 
E--Probe and Monitoring Path Siting Criteria for Ambient Air Quality 
Monitoring (88 FR 5676-78, January 27, 2023). Since 2006, the EPA 
finalized multiple rule revisions to establish siting requirements for 
PM10-2.5 and O3 monitoring sites (71 FR 2748, 
January 17, 2006), Near-Road NO2 monitoring sites (75 FR 
6535, February 9, 2010), Near-Road CO monitoring sites (76 FR 54342, 
August 31, 2011), and Near-Road PM2.5 monitoring sites (78 
FR 3285, January 15, 2013). Through these previous revisions to the 
regulatory text, some requirements were inadvertently omitted, and, 
over time, the clarity of this appendix was reduced through those 
omissions that, in a few instances, led to unintended and conflicting 
regulatory requirements. The EPA proposed to reinstate portions of 
previous Probe and Monitoring Path Siting Criteria Requirements from 
previous rulemakings, where appropriate, to restore the original 
intent.
    The EPA only received a few comments on the proposed rulemaking 
pertaining to the proposed changes regarding probe and monitoring path 
siting criteria for ambient air quality monitoring, most of which were 
supportive of the proposed revisions. One commenter noted that the 
image for Figure E-1 in Appendix E to part 58 was distorted and of 
extremely poor quality, rendering the text in places almost unreadable 
(88 FR 5712, January 27, 2023). The EPA makes several references to 
Figure E-1, which provides detailed information needed for assessing a 
range of acceptable probe distances from roadways based on a monitor's 
spatial scale. The commenter also stated that a higher quality image is 
needed for the figure so that agencies can fully interpret the figure 
to the extent that EPA requires. The EPA agrees with the commenter that 
a higher quality image for Figure E-1 is important and needed. Based on 
this comment, the EPA is finalizing the revision to Figure E-1 to 
clearly communicate the requirements of appendix E.
    The EPA is revising appendix E in its entirety as proposed (88 FR 
5709-5717, January 27, 2023) for clarity and as described in detail 
below.
a. Separate Section for Open Path Monitoring Requirements
    The EPA proposed to relocate all open path monitor siting criteria 
requirements to a separate section in appendix E from those 
requirements for siting samplers and monitors that utilize probe inlets 
(88 FR 5676, January 27, 2023). Separate sections for these distinct 
monitoring method types allows the EPA to more clearly articulate 
minimum technical siting requirements for each.
    The EPA received one supportive comment to adopt this change and 
received no adverse comments. Another commenter stated the regulatory 
text of the proposal improves the clarity of the appendix but 
encouraged the EPA to break the summary tables down further into more 
manageable components (perhaps by pollutant). The commenter stated that 
summary tables for the proposed appendix continue to be a ``jumbled 
mess of regulatory requirements.'' The EPA agrees that the summary 
tables E-3 and E-6 in the proposal could be improved further. Also, the 
EPA found that footnote 3 of Table E-6 in the proposed rule was 
incomplete and corrected this editorial error.
    Therefore, the EPA is making editorial changes to both summary 
tables E-3 and E-6 and finalizing the remainder of the language as 
proposed with the open path monitor siting criteria requirements placed 
into a separate section of the appendix.
b. Distance Precision for Spacing Offsets
    The EPA proposed to require that when rounding is performed to 
assess compliance with these siting requirements, the distance 
measurements will be rounded such as to retain at least two significant 
figures (88 FR 5676, January 27, 2023). The EPA proposed to communicate 
this rounding requirement in the regulatory text using footnotes in the 
tables of this appendix.
    The EPA received two supportive comments and no adverse comments 
regarding this proposed change. While supportive of the proposal, one 
of the two supporting comments suggested it would be clearer if EPA 
explicitly defined a decimal in the distance values and round to the 
nearest tenths place for these assessments. The EPA disagrees with this 
recommendation because in some cases it would be more restrictive and 
burdensome than the proposed requirement that was intended to provide 
both clarity and flexibility. Therefore, the EPA is finalizing the 
language as proposed.
c. Summary Table of Probe Siting Criteria
    The EPA proposed to provide additional specificity and flexibility 
to the summary table for probe siting criteria by changing the ``>'' 
(greater than) symbols to ``>='' (greater than or equal to) symbols in 
the summary table E-4 (88 FR 5676, January 27, 2023). Because one 
commenter pointed out to the EPA that in the prior version of the rule 
there was no table E-4, as a clerical matter, we have renumbered this 
summary table to table E-3 in the final rule. This proposed minor 
revision to the summary table more clearly expresses the EPA's intent 
that the distance offsets provided in the summary tables in appendix E 
are acceptable for NAAQS compliance monitoring.
    The EPA received one comment supporting the proposal. The EPA 
received no adverse comments. Because

[[Page 16363]]

one commenter pointed out to the EPA that in the prior version of the 
rule there was no table E-4, as a clerical matter, we have renumbered 
this summary table to table E-3 in the final rule. Therefore, the EPA 
is updating the table numbering and otherwise finalizing the tables as 
proposed.
d. Spacing From Minor Sources
    The EPA proposed to clarify and provide flexibility regarding 
siting monitors near minor sources by changing a requirement to a goal 
(88 FR 5676-77, January 27, 2023). To accomplish this, the EPA proposed 
to replace the ``must'' in the regulation with a ``should.'' While the 
EPA proposed to change this requirement to a goal, the EPA reiterated 
in the proposal that it recommends that sites with minor sources be 
avoided whenever practicable and probe inlets should be spaced as far 
from minor sources as possible when alternative monitoring stations are 
not suitable.
    The EPA received one comment supporting the proposed revision and 
received no adverse comments. Therefore, the EPA is finalizing the 
language as proposed.
e. Spacing From Obstructions and Trees
    The EPA proposed to clarify and redefine that the minimum arc 
required to be free of obstructions for a probe inlet or monitoring 
path is 270-degrees and that probe inlets must be no closer than 10-
meters to the driplines of any trees (88 FR 5677, January 27, 2023). 
These changes were proposed because of inconsistencies introduced into 
the rule with the 2006 rulemaking. Both are discussed in more detail in 
the following sections.
    The majority of comments received were supportive of these proposed 
siting amendments and clarifications. Two commenters were not 
supportive of this proposal. One adverse comment focused on the 
potential that site modifications would be required if the minimum arc 
required to be free of obstructions for a probe inlet is 270-degrees. 
The second adverse comment pertained to the proposal to clarify 
distance requirements from tree driplines. The commenter stated they 
would expect significant challenges in meeting the proposed 20-meter 
tree dripline distance. This comment is not a substantive negative 
comment because the 20-meter distance provided in the proposal is a 
goal and not a requirement. As such, monitoring organizations should 
not expect additional challenges in meeting the probe siting 
requirements. One supportive commenter on the 270-degree minimum arc 
proposal also requested that the EPA acknowledge that some cases exist 
where monitoring is desired or necessary to protect the public health, 
but siting criteria cannot be met.
    Based on the only two negative comments received from monitoring 
agencies or organizations, one of which was not substantive, the EPA 
believes most sites already meet these proposed requirements related to 
the arc and distance from dripline. However, the EPA also acknowledges 
that there may be limited cases where this proposed revision may 
require site modifications, and some sites may not be able to be 
achieve the proposed siting requirements, even with modifications to 
the site. For cases where long-term trend sites or monitors that 
determine the design value for their area cannot reasonably meet these 
regulatory siting requirements, the EPA encourages monitoring 
organizations to work with their respective EPA Regional offices to 
determine if a waiver from this siting criteria would be appropriate 
under appendix E, section 10.
    These siting requirements are discussed in more detail below in 
sections VII.B.5.f and VII.B.5.h.
f. Reinstating Minimum 270-Degree Arc and Clarified 180-Degree Arc
    The EPA proposed to correct identified inconsistencies in the 270-
degree requirement for unrestricted airflow to the probe inlet by 
reinstating the requirement stated in appendix E, paragraph 4(b), and 
to clarify that the continuous 180-degree minimum arc of unrestricted 
airflow provision is reserved for monitors sited on the side of a 
building or wall to comply with network design criteria requirements 
specified in appendix D of part 58 (88 FR 5677, January 27, 2023).
    The EPA received two comments regarding this proposal, with one 
being supportive and one being negative. The adverse comment focused on 
the potential that site modifications would be required if this 
revision was made. The commenter supporting the proposal also requested 
that the EPA acknowledge that some cases exist where monitoring is 
desired or necessary to protect the public health, but siting criteria 
cannot be met. The EPA agrees with both commenters and acknowledges 
that there does exist limited cases where this proposal would require 
site modifications and some sites may not be able to be achieve the 
proposed siting requirement even with modifications to the site. For 
these cases, and especially when long-term trend sites or monitors that 
determine the design value for their area cannot reasonably meet these 
regulatory siting requirements, the EPA encourages monitoring 
organizations to work with their respective EPA Regional Offices to 
determine if a waiver from this siting criteria is appropriate through 
the provisions found in Section 10 of this appendix.
    Based on the EPA only receiving a single negative comment regard 
the 270-degree and 180-degree provisions the EPA thinks most sites 
already meet these proposed requirements. Additionally, as stated 
above, the EPA is also retaining waiver provisions from these siting 
requirements for the remaining cases that can be exercised when 
appropriate. Therefore, the EPA is finalizing the language as proposed.
g. Obstacles That Act as Obstructions
    The EPA proposed to clarify the definitions of ``obstructions'' and 
``obstacles'' in the regulatory text (88 FR 5677, January 27, 2023). 
Stating that, ``[o]bstructions to the air flow of the probe inlet are 
those obstacles that are horizontally closer than twice the vertical 
distance the obstacle protrudes above the probe inlet and can be 
reasonably thought to scavenge reactive gases or to restrict the 
airflow for any pollutant,'' the EPA proposed to reiterate that the EPA 
does not generally consider objects or obstacles such as flag poles or 
site towers used for NOy convertors and meteorological 
sensors, etc., to be deemed obstructions.
    The EPA received one comment supporting the proposal and received 
no adverse comments. Therefore, the EPA is finalizing the definitions 
as proposed.
h. 10-Meter Tree Dripline Requirement
    The EPA proposed to reconcile the conflicting requirements in 5(a) 
and the prior table E-4 footnote 3 by clarifying that the probe inlet 
must always be no closer than 10 meters to the tree dripline (88 FR 
5677, January 27, 2023). The EPA also proposed to reinstate the goal 
``that monitor probe inlets should be at least 20-meters from the 
driplines of trees,'' a goal that was inadvertently omitted during 
previous rule revisions. In addition, the EPA proposed to clarify that 
if a tree or group of trees is considered an ``obstruction,'' section 
4(a) will apply.
    As described above, the majority of comments received were 
supportive of the EPA proposed amendments and clarification, with two 
commenters focused on the possibility that monitoring agencies may not 
be able to meet the revised siting requirements. Specific to the 
proposed dripline requirement, the EPA reiterates that the

[[Page 16364]]

20-meter tree dripline offset is not a requirement, but rather a goal. 
Monitoring programs should as much as practicable attempt to meet this 
20-meter tree dripline offset goal but are only required to be at least 
10 meters removed from tree driplines. If these requirements cannot be 
met, the EPA encourages monitoring organizations to contact their 
respective EPA Regional offices to determine if a waiver from this 
siting criteria would be appropriate under appendix E, section 10.
    Another commenter recommended that the proposal should also include 
an elevation specification. For instance, if a monitor is on the roof 
of a shelter, a tree below that roof should not be considered an 
obstruction no matter the distance to the dripline. The EPA considers 
this scenario to occur in practice only rarely. The EPA agrees that 
when the overall tree height is less than the height of the probe 
inlet, the tree is not obstructing the airflow to the probe inlet. 
However, a tree in such proximity to the probe inlet in many cases is 
not likely to remain at a height lower than the probe inlet. The EPA 
considers a scenario such as this to be best addressed in the waiver 
provisions of this appendix due both to the rarity of this occurring as 
well as the need for the EPA to periodically reassess whether tree 
growth has adversely impacted the site conditions.
    For these reasons, the EPA is finalizing the language as proposed.
i. Spacing Requirement for Microscale Monitoring
    The EPA proposed to require that microscale sites for any pollutant 
shall have no trees or shrubs blocking the line-of-sight fetch between 
the monitor's probe inlet and the source under investigation (88 FR 
5677, January 27, 2023). This proposed revision would bring consistency 
between near-road monitoring stations and other microscale monitoring.
    The EPA received one comment on this proposed requirement 
expressing concerns regarding its practicality and legality. The 
commenter stated agencies may at times want to site a monitor close to 
a source, but the closest location will have trees in the line of sight 
on private property. Additionally, in some cases, the trees may have 
been planted for the purpose of reducing off-property emissions from a 
source such as a Concentrated Animal Feeding Operation (CAFO). The 
commenter further stated that the proposal mandates that State agencies 
order the removal of trees from private property to collect valid data.
    The EPA disagrees that the proposed requirement is impractical or 
unlawful. The proposed requirement would not require, mandate, or 
otherwise empower monitoring agencies to force the removal of trees on 
private property. The EPA agrees with the commenter that trees may at 
times be planted as part of control strategies to reduce offsite 
emissions and thus protect the public, but the EPA disagrees with the 
commenter that the trees must be removed to perform ambient air 
monitoring in these locations. Rather, if trees or shrubs block the 
line-of-sight fetch between the monitor's probe inlet and the source 
under investigation, it is the EPA's position that, for most cases, a 
microscale designation does not accurately reflect the monitoring scale 
for this location, and instead the EPA would recommend that the 
monitoring scale be designated to a more representative monitoring 
scale such as middle scale or neighborhood scale.
    Moreover, for cases where long-term trend sites or monitors that 
determine the design value for an area cannot reasonably meet this 
regulatory siting requirement, the EPA encourages monitoring 
organizations to work with their respective EPA Regional offices to 
determine if a waiver from this siting criteria may be appropriate 
under appendix E, section 10.
    For these reasons, the EPA is finalizing the language as proposed.
j. Waiver Provisions
    The EPA proposed to maintain the appendix E, section 10 waiver 
provisions in the current regulation for siting criteria, but to modify 
section 10.3 to require that waivers from the probe-siting criteria 
must be reevaluated and renewed minimally every 5 years (88 FR 5677-78, 
January 27, 2023).
    The EPA received one comment supporting the proposal and no adverse 
comments. Therefore, the EPA is finalizing the language as proposed.
k. Acceptable Probe Materials
    The EPA proposed to expand the list of acceptable probe materials 
for sampling reactive gases in appendix E, section 9, from just 
borosilicate glass and fluorinated ethylene propylene (FEP) 
Teflon[supreg], or their equivalents. The EPA proposed to add 
polyvinylidene fluoride (PVDF), also known as Kynar[supreg], 
polytetrafluoroethylene (PTFE), and perfluoroalkoxy (PFA) to the list 
of approved materials for efficiently transporting gaseous criteria 
pollutants, and the use of Nafion\TM\ upstream of ozone analyzers (88 
FR 5678, January 27, 2023). Mention of trade names or commercial 
products does not constitute endorsement.
    The EPA received two comments supporting the proposal and received 
no adverse comments. Therefore, the EPA is finalizing the language as 
proposed.

D. Incorporating Data From Next Generation Technologies

    In the proposal, the EPA requested comment on how to incorporate 
data from next generation technologies into Agency efforts (88 FR 5678-
80, January 27, 2023). The near real-time integration of data from 
PM2.5 continuous monitors, sensors, and satellites has 
allowed the EPA to use data in certain informational applications such 
as EPA and USFS's Fire and Smoke Map.\199\ This mapping product uses 
Application Program Interfaces (APIs) where data sets are automatically 
shared on prespecified computer servers. Given the success of the Fire 
and Smoke Map, the EPA indicated interest in exploring the use of next-
generation technologies to develop additional approaches, products, and 
applications to help address important non-regulatory air quality data 
needs. Therefore, the EPA solicited comment on the most important data 
uses and data sets to consider in such future initiatives. Such 
approaches and/or products could utilize historical or near real-time 
data. The EPA sought this input and prioritization on use of next 
generation technologies to help improve the utility of data to better 
support air quality management to improve public health and the 
environment.
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    \199\ Available at https://fire.airnow.gov/.
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    The EPA received comments from about two dozen entities on its 
request for comments on how to incorporate data from next generation 
technologies. The entities that provided comment included federal 
agencies; representatives of industry and industry groups; public 
health, medical, and environmental organizations; State, local and 
related multi-state organizations involved in air program management; 
Tribes and Tribal organizations involved in air program management; and 
other State and local governments.
    While there were some differences across commenters, a majority of 
the commenters support use of next generation data for non-regulatory 
purposes, but not for regulatory decision making due to their inherent 
uncertainties and limitations. The EPA also received comments from some 
environmental organizations support using alternative data for 
regulatory decision making.

[[Page 16365]]

    Many commenters pointed out that they are already successfully 
using sensor data and networks in supplemental and informational 
applications and support further expansion of these capabilities. 
Across many commenters, there was support for using next generation 
data as ``fit for purpose,'' filling in gaps, finding hot spots, 
identifying and addressing EJ concerns, and evaluating and informing 
network siting. The EPA acknowledges the successful examples of sensor 
data and networks for non-regulatory purposes. A few commenters support 
expanding the use of sensor data to provide real-time AQI; the EPA is 
interested in this use of next generation data as well. A few 
commenters pointed the need for the EPA to work closely with them and 
their communities to understand and use next generation data, while 
others expressed a desire for help developing best practices around 
collecting and using next generation data, developing products with 
data analysis/visualization, and developing appropriate QA/QC for 
sensor data. The EPA acknowledges each of these requests and expects to 
continue to work closely with SLTs and other stakeholders to understand 
and develop information on the collection and use of next generation 
data.
    A few commenters offered more detailed comments. Some recommended 
that the EPA repropose implementation provisions related to next 
generation technologies with greater clarity to provide for meaningful 
comment. For example, the use of low-cost sensor and satellite data 
could be used in drawing nonattainment area boundaries or identifying 
sources for emissions control, but doing so would be such a significant 
change from prior EPA policy that it warrants a more specific proposal, 
beyond the scope of this request for comment. In response to this 
comment, the EPA notes it did not propose or change the use of non-
regulatory measurement data as part of this proposal, but instead 
opened an opportunity to comment about the use of next generation 
technologies.
    Another commenter stated that while low-cost sensor data can be 
invaluable for some purposes, the potentially overwhelming amount of 
data produced by sensors may present additional challenges to 
communities without the resources or expertise to analyze it. Cost is 
another concern associated with some next generation technologies of 
which some communities may not be aware, as the initial cost of the 
sensor alone is not indicative of the total cost of operation, which 
can include costs of internet access and servers. The EPA appreciates 
the need to consider all the costs of implementing and maintaining 
sensor data.
    Another commenter stated that having a dense sensor network 
collocated with FRMs and FEMs could help ensure timely maintenance of 
the regulatory measurements in the event there appears to be a 
divergence of data. The EPA appreciates the comment that emphasizes how 
sensors could be used to complement the FRM and FEM data with regard to 
ensuring timely maintenance.
    Another commentor strongly opposes incorporating sensor data into 
any EPA systems unless robust quality assurance (QA) practices are 
widely established and managed by qualified personnel. The EPA agrees 
that QA is necessary, and notes that the ``fit for purpose'' aspect of 
using sensor data will inform the appropriate QA associated with the 
intended use of such data.
    In summary, the EPA invited comment on how we should consider 
incorporating data from next generation technologies into our air 
monitoring efforts. In seeking comment on this topic, the EPA did not 
propose to add, edit, or delete any regulatory language associated with 
the PM NAAQS. The EPA received comments from a variety of entities that 
largely support using next generation data for a variety of purposes 
that supplement, but cannot replace, the measurement data from 
monitoring methods required (i.e., FRMs and FEMs) for regulatory 
decision making. Across many commenters, there was support for using 
next generation technologies and data as ``fit for purpose,'' filling 
in gaps, finding hot spots, identifying, and addressing EJ concerns, 
and evaluating and informing network siting. Quality assurance of the 
data will be an important component in the use of next generation 
technology data. The EPA will consider these comments as it continues 
its work with the co-regulated community comprised of SLT agencies and 
other stakeholders to understand and use next generation data and joint 
efforts to manage the nation's ambient air.

VIII. Clean Air Act Implementation Requirements for the Revised Primary 
Annual PM2.5 NAAQS

    The EPA's revision to the primary annual PM2.5 NAAQS 
discussed in section II above triggers a number of implementation 
related activities that were described in the NPRM. The two most 
immediate implementation impacts following a final new or revised NAAQS 
are related to stationary source permitting and the initial area 
designations process. Permitting implications are discussed below in 
section VIII.E. With regard to initial area designations, the EPA is 
separately issuing a memorandum regarding the Initial Area Designations 
for the Revised Primary Annual Fine Particle National Ambient Air 
Quality Standard Memorandum (the ``Annual PM2.5 NAAQS 
Designations Memorandum'') that will provide information about the 
statutory schedule for the designations process. For other 
implementation related implications, please refer back to the NPRM 
section VIII.
    The NPRM also referred to the PM2.5 State Implementation 
Plan (SIP) Requirements Rule (81 FR 58010, August 24, 2016), which 
specifies planning requirements for areas designated as nonattainment 
for purposes of the PM2.5 NAAQS and includes a number of key 
recommendations for areas to consider implications of environmental 
justice through the attainment planning process, consistent with the 
identification of at-risk groups in the 2019 ISA and ISA Supplement and 
the statutory requirement to protect the health of at-risk groups. As 
stated in the NPRM, State and local air agencies are encouraged to 
consider how they might develop implementation plans that encourage 
early emission reductions.

A. Designation of Areas

    As discussed in section II, with respect to the PM2.5 
NAAQS, the EPA is finalizing: (1) Revisions to the level of the primary 
annual PM2.5 NAAQS and retaining the current primary 24-hour 
PM2.5 NAAQS (section II.B.4); and (2) no change to the 
current secondary annual and 24-hour PM2.5 NAAQS at this 
time (section V.B.4). Upon promulgation of a new or revised NAAQS, 
States and the EPA must initiate the process for initial designations.
    The timeline for initial area designations begins with promulgation 
of the revised primary annual PM2.5 NAAQS, as stated in the 
CAA section 107(d)(1)(B)(i). Through this process, which provides for 
input from States and others at various stages, the EPA identifies 
areas of the country that either meet or do not meet the revised 
primary annual PM2.5 NAAQS, along with the nearby areas 
contributing to NAAQS violations. The following includes additional 
information regarding the designations process described in the CAA.
    Section 107(d)(1) of the CAA states that, ``By such date as the 
Administrator may reasonably require, but not later than 1 year after 
promulgation of a new or revised national ambient air quality standard 
for any pollutant under section

[[Page 16366]]

109, the Governor of each State shall . . . submit to the Administrator 
a list of all areas (or portions thereof) in the State'' and make 
recommendations for whether the EPA should designate those areas as 
nonattainment, attainment, or unclassifiable.\200\ The CAA provides the 
EPA with discretion to require States to submit their designations 
recommendations within a reasonable amount of time not exceeding one 
additional year.\201\ Section 107(d)(1)(A) of the CAA also states that 
``the Administrator may not require the Governor to submit the required 
list sooner than 120 days after promulgating a new or revised national 
ambient air quality standard.'' Section 107(d)(1)(B)(i) further 
provides, ``Upon promulgation or revision of a NAAQS, the Administrator 
shall promulgate the designations of all areas (or portions thereof) . 
. . as expeditiously as practicable, but in no case later than 2 years 
from the date of promulgation. Such period may be extended for up to 
one year in the event the Administrator has insufficient information to 
promulgate the designations.'' With respect to the NAAQS setting 
process, courts have interpreted the term ``promulgation'' to be 
signature and widespread dissemination of a final rule.\202\
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    \200\ While the CAA says ``designating'' with respect to the 
Governor's letter, in the full context of the CAA section it is 
clear that the Governor actually makes a recommendation to which the 
EPA must respond via a specified process if the EPA does not accept 
it.
    \201\ In certain circumstances in which the Administrator has 
insufficient information to promulgate area designations within two 
years from the promulgation of the NAAQS, CAA section 
107(d)(1)(B)(i) provides that the EPA may extend the designations 
schedule by up to one year.
    \202\ API v. Costle, 609 F.2d 20 (D.C. Cir. 1979).
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    If the EPA agrees with the designations recommendation of the 
State, then it may proceed to promulgate the designations for such 
areas. If, however, the EPA disagrees with the State's recommendation, 
then the EPA may elect to make modifications to the recommended 
designations. By no later than 120 days prior to promulgating the final 
designations, the EPA is required to notify States of any intended 
modifications to the State designation recommendations for any areas or 
portions thereof, including the boundaries of areas, as the EPA may 
deem necessary. States then have an opportunity to comment on the EPA's 
intended modification and tentative designation decision. If a State 
elects not to provide designation recommendations for any area, then 
the EPA must itself promulgate the designation that it deems 
appropriate.
    While section 107(d) of the CAA specifically addresses the 
designations process for States, the EPA intends to follow the same 
process for Tribes to the extent practicable, pursuant to section 
301(d) of the CAA regarding Tribal authority, and the Tribal Authority 
Rule (63 FR 7254, February 12, 1998). To provide clarity and 
consistency in doing so, the EPA issued a guidance memorandum to our 
Regional Offices on working with Tribes during the designations 
process.\203\
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    \203\ ``Guidance to Regions for Working with Tribes during the 
National Ambient Air Quality Standards (NAAQS) Designations 
Process,'' December 20, 2011, Memorandum from Stephen D. Page to 
Regional Air Directors, Regions 1-X available at https://www.epa.gov/sites/default/files/2017-02/documents/12-20-11_guidance_to_regions_for_working_with_tribes_naaqs_designations.pdf
.
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    Consistent with the process used in previous area designations 
efforts, the EPA will evaluate each area on a case-by-case basis 
considering the specific facts and circumstances unique to the area to 
support area boundary decisions for the revised standard. The EPA 
intends to issue a designations memorandum which will provide 
information regarding the designations process. In broad overview, the 
EPA has historically used area-specific analyses to support 
nonattainment area boundary recommendations and final boundary 
determinations by evaluating factors such as air quality data, 
emissions and emissions-related data (e.g., population density and 
degree of urbanization, traffic and commuting patterns), meteorology, 
geography/topography, and jurisdictional boundaries. We expect to 
follow a similar process when establishing area designations for this 
revised PM2.5 NAAQS. CAA section 107(d) explicitly requires 
that the EPA designate as nonattainment not only the area that is 
violating the pertinent standard, but also those nearby areas that 
contribute to the violation in the violating area. In the 
PM2.5 NAAQS Designations Memorandum, the EPA intends to 
include information regarding consideration of federal land boundaries 
that may be fully or partially included within the bounds of a county 
otherwise identified as nonattainment.
    As with past revisions of the PM2.5 NAAQS, the EPA 
intends to make the designations decisions for the revised primary 
annual PM2.5 NAAQS based on the most recent three years of 
quality-assured, certified air quality data in the EPA's Air Quality 
System (AQS). Accordingly, the EPA recommends that States base their 
initial area designation recommendations on the most current available 
three years of complete and certified air quality data at the time of 
the recommendations. The EPA will then base the final designations on 
the most recent three consecutive years of complete, certified air 
quality monitoring data available at the time of final 
designations.\204\
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    \204\ In certain circumstances in which the Administrator has 
insufficient information to promulgate area designations within two 
years from the promulgation of a new or revised NAAQS, CAA section 
107(d)(1)(B)(i) provides the EPA may extend the designations 
schedule by up to one year.
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    Monitoring data are currently available from numerous existing 
PM2.5 Federal Equivalent Methods (FEM) and Federal Reference 
Methods (FRM) sites to determine violations of the revised primary 
annual PM2.5 NAAQS. As described in section VII.C.3.b, the 
EPA took comment on how to deal with cases where an FEM is approved by 
the EPA with an update and when it can be implemented in the field. The 
EPA took comment on how to approach the data produced during this lag 
and received input from over a dozen commenters. The commenters asked 
that the EPA be flexible in allowing the use of updated method 
correction factors intended to improve the data comparability between 
the FRMs and FEMs. The EPA will address any data correction issues 
between the FRMs and FEMs through a future Notice of Data Availability 
(NOA).
    Consistent with past practice and as noted in the NPRM, the EPA 
intends to provide additional information concerning the designations 
process, including information about the schedule and recommendations 
for determining area boundaries in the forthcoming Annual 
PM2.5 NAAQS Designations Memorandum. Other topics addressed 
in this memorandum include the schedule for preparing and submitting 
exceptional events initial notification and exceptional events 
demonstrations relevant to the designations process, and information 
related to wildfire and prescribed fire on wildlands as it pertains to 
initial area designations, as well as addressing back-correction of PM 
FEM data when a method has an approved factory calibration as part of a 
method update. The Annual PM2.5 NAAQS Designations 
Memorandum is intended to assist States and Tribes in formulating their 
area recommendations.\205\
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    \205\ See: https://www.epa.gov/particle-pollution-designations.
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    As discussed in the proposal, the ``Treatment of Data Influenced by 
Exceptional Events; Final Rule,'' (81 FR 68216, October 3, 2016) and 
codified at 40 CFR 50.1, 40 CFR 50.14, and 40 CFR 51.930, contains 
instructions and requirements for air agencies that may

[[Page 16367]]

flag air quality data for certain days in the Air Quality System due to 
potential impacts from exceptional events (i.e., such as prescribed 
fires on wildland, wildfires, or high wind dust storms). Accordingly, 
for purposes of initial area designations for a new or revised NAAQS, 
an air agency may submit to the EPA an exceptional events demonstration 
with supporting information and analyses for each monitoring site and 
day the air agency claims the EPA should exclude from design value 
calculations for designations purposes.
    The EPA has provided tools to assist air agencies in preparing 
adequate exceptional events demonstrations.\206\ Further, the EPA will 
continue to work with air agencies as they identify exceptional events 
that may influence decisions related to the initial area designations 
process, and to prepare and submit exceptional events demonstrations if 
appropriate. Importantly, air quality monitoring data may be influenced 
by emissions from prescribed fires on wildland and wildfires. The EPA's 
Exceptional Events Rule provides for both of these types of events to 
be considered as exceptional events, provided the affected air agencies 
submit exceptional events demonstrations that meet the procedural and 
technical requirements of the EPA's Exceptional Events Rule. To that 
end, the EPA has issued guidance addressing development of exceptional 
events demonstrations for both wildfire and prescribed fires on 
wildland.\207\ In light of the growing frequency and severity of 
wildfire events, and expected increases in the application of 
prescribed fire as a means to achieve long-term reductions in high 
severity wildfire risk and associated smoke impacts, the EPA seeks to 
ensure that the Agency's exceptional events process provides an 
efficient and clear pathway for excluding data that may be affected by 
such events in a manner that is consistent with the Clean Air Act and 
the public health objectives of the NAAQS. Accordingly, the EPA is 
continuing to explore opportunities to develop additional tools that 
could assist air agencies in preparing exceptional events 
demonstrations for wildfires and prescribed fires on wildland. In 
addition, EPA intends to continue engaging with the U.S. Department of 
Agriculture, U.S. Department of the Interior, air agencies, and other 
stakeholders on these issues. For more information regarding the 
exceptional events demonstration submission deadlines for the area 
designations process, please see Table 2 to 40 CFR 50.14(c)(2)(vi)--
``Schedule for Initial Notification and Demonstration Submission for 
Data Influenced by Exceptional Events for Use in Initial Area 
Designations.''
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    \206\ See the EPA's Exceptional Events homepage at https://www.epa.gov/air-quality-analysis/treatment-air-quality-data-influenced-exceptional-events-homepage-exceptional.
    \207\ See EPA's ``Final Guidance on the Preparation of 
Exceptional Events Demonstrations for Wildfire Events that May 
Influence Ozone Concentrations and EPA's Exceptional Events 
Guidance: Prescribed Fire on Wildland that May Influence Ozone and 
Particulate Matter Concentrations,'' found on EPA's Exceptional 
Events homepage at https://www.epa.gov/air-quality-analysis/treatment-air-quality-data-influenced-exceptional-events-homepage-exceptional.
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B. Section 110(a)(1) and (2) Infrastructure SIP Requirements

    As discussed in the NPRM, the CAA directs States to address basic 
SIP requirements to implement, maintain, and enforce the NAAQS. Under 
CAA sections 110(a)(1) and (2), states are required to have State 
implementation plans that provide the necessary air quality management 
infrastructure that provides for the implementation, maintenance, and 
enforcement of the NAAQS. After the EPA promulgates a new or revised 
NAAQS, States are required to make a new SIP submission to establish 
that they meet the necessary structural requirements for such new or 
revised NAAQS or make changes to do so. The EPA refers to this type of 
SIP submission as an ``infrastructure SIP submission.'' Under CAA 
section 110(a)(1), all States are required to make these infrastructure 
SIP submissions within three years after the effective date of a new or 
revised primary standard. While the CAA authorizes the EPA to set a 
shorter time for States to make these SIP submissions, the EPA is 
requiring submission of infrastructure SIPs within three years of the 
effective date of this revised primary annual PM2.5 NAAQS.
    The EPA has provided general guidance to States concerning its 
interpretation of these requirements of CAA section 110(a)(1) and (2) 
in the context of infrastructure SIP submissions for a new or revised 
NAAQS.\208\ The EPA encourages States to use this guidance when 
developing their infrastructure SIPs for this revised primary annual 
PM2.5 NAAQS.
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    \208\ See ``Guidance on Infrastructure State Implementation Plan 
(SIP) Elements under Clean Air Act Sections 110(a)(1) and 
110(a)(2)'' September 2013, Memorandum from Stephen D. Page to 
Regional Air Directors, Regions 1-10.
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    As a reminder, the EPA notes that States are not required to 
address nonattainment plan requirements for purposes of the revised 
primary annual PM2.5 NAAQS on the same schedule as 
infrastructure SIP requirements. The EPA interprets the CAA such that 
two elements identified in section 110(a)(2) are not subject to the 3-
year submission deadline of section 110(a)(1) and thus States are not 
required to address them in the context of an infrastructure SIP 
submission. The elements pertain to part D, in title I of the CAA, 
which addresses additional SIP requirements for nonattainment areas. 
Therefore, for the reasons explained below, the following section 
110(a)(2) elements are considered by the EPA to be outside the scope of 
infrastructure SIP actions: (1) The portion of section 110(a)(2)(C), 
programs for enforcement of control measures and for construction or 
modification of stationary sources that applies to permit programs 
applicable in designated nonattainment areas (known as ``nonattainment 
new source review'') under part D; and (2) section 110(a)(2)(I), which 
requires a SIP submission pursuant to part D, in its entirety.
    Accordingly, the EPA does not expect States to address the 
requirement for a new or revised NAAQS in the infrastructure SIP 
submissions to include regulations or emissions limits developed 
specifically for attaining the relevant standard in areas designated 
nonattainment for the revised primary annual PM2.5 NAAQS. 
States are required to submit infrastructure SIP submissions for the 
revised primary annual PM2.5 NAAQS before they will be 
required to submit nonattainment plan SIP submissions to demonstrate 
attainment with the same NAAQS. States are required to submit 
nonattainment plan SIP submissions to provide for attainment and 
maintenance of a revised primary annual PM2.5 NAAQS within 
18 months from the effective date of nonattainment area designations as 
required under CAA section 189(a)(2)(B). The EPA reviews and acts upon 
these later SIP submissions through a separate process. For this 
reason, the EPA does not expect States to address new nonattainment 
area emissions controls per section 110(a)(2)(I) in their 
infrastructure SIP submissions.
    One of the required infrastructure SIP elements is that each State 
SIP must contain adequate provisions to prohibit, consistent with the 
provisions of title I of the CAA, emissions from within the State that 
will significantly contribute to nonattainment in, or interfere with 
maintenance by, any other State of the primary or secondary NAAQS.\209\ 
This

[[Page 16368]]

element is often referred to as the ``good neighbor'' or ``interstate 
transport'' provision.\210\ The provision has two prongs: significant 
contribution to nonattainment (prong 1), and interference with 
maintenance (prong 2). The EPA and States must give independent 
significance to prong 1 and prong 2 when evaluating downwind air 
quality problems under CAA section 110(a)(2)(D)(i)(I).\211\ Further, 
case law has established that the EPA and States must implement 
requirements to meet interstate transport obligations in alignment with 
the applicable statutory attainment schedule of the downwind areas 
impacted by upwind-state emissions.\212\ Thus, the EPA anticipates that 
States will need to address interstate transport obligations associated 
with this revised PM NAAQS, in alignment with the provisions of subpart 
4 of part D of the CAA, as discussed in more detail in section VIII.C 
below. Specifically, States must implement any measures required to 
address interstate transport obligations as expeditiously as 
practicable and no later than the next statutory attainment date, i.e., 
for this NAAQS revision as expeditiously as practicable, but no later 
than the end of the sixth calendar year following nonattainment area 
designations. See CAA section 188(c). States may find it efficient to 
make SIP submissions to address the interstate transport provisions 
separately from other infrastructure SIP elements.
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    \209\ CAA section 110(a)(2)(D)(i)(I).
    \210\ CAA section 110(a)(2)(D)(i)(II) also addresses certain 
interstate effects that states must address and thus is also 
sometimes referred to as relating to ``interstate transport.''
    \211\ See North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir. 
2008).
    \212\ See id. 911-13. See also Wisconsin v. EPA, 938 F.3d 303, 
313-20 (D.C. Cir. 2019); Maryland v. EPA, 958 F.3d 1185, 1203-04 
(D.C. Cir. 2020).
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    Each State has the authority and responsibility to review its air 
quality management program's existing SIP provisions in light of a new 
or revised NAAQS to determine if any revisions are necessary to 
implement the new or revised NAAQS. Most States have revised and 
updated their SIPs in recent years to address requirements associated 
with other revised NAAQS. For certain infrastructure elements, some 
States may believe they already have adequate State regulations adopted 
and approved into the SIP to address a particular requirement with 
respect to the revised primary annual PM2.5 NAAQS.
    If a State determines that existing SIP-approved provisions are 
adequate in light of this revised primary annual PM2.5 NAAQS 
with respect to a given infrastructure SIP element (or sub-element), 
then the State may make an infrastructure SIP submission ``certifying'' 
that the existing State's existing EPA approved SIP already contains 
provisions that address one or more specific section 110(a)(2) 
infrastructure elements.\213\ In the case of such a submission, the 
State does not have to include a copy of the relevant provision (e.g., 
rule or statute) itself. Rather, this certification submission should 
provide citations to the SIP-approved State statutes, regulations, or 
non-regulatory measures, as appropriate, in or referenced by the 
already EPA-approved SIP that meet particular infrastructure SIP 
element requirements. The State's infrastructure SIP submission should 
also include an explanation as to how the State has determined that 
those existing provisions meet the relevant requirements.
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    \213\ A ``certification'' approach would not be appropriate for 
the interstate pollution control requirements of CAA section 
110(a)(2)(D)(i).
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    Like any other SIP submission, that State can make such an 
infrastructure SIP submission certifying that it has already met some 
or all of the applicable requirements only after it has provided 
reasonable notice and opportunity for public hearing. This ``reasonable 
notice and opportunity for public hearing'' requirement for 
infrastructure SIP submissions is to meet the requirements of CAA 
sections 110(a) and 110(l). Under the EPA's regulations at 40 CFR part 
51, if a public hearing is held, an infrastructure SIP submission must 
include a certification by the State that the public hearing was held 
in accordance with the EPA's procedural requirements for public 
hearings. See 40 CFR part 51, appendix V, section 2.1(g), and see 40 
CFR 51.102.
    In consultation with the EPA's Regional office, a State should 
follow all applicable EPA regulations governing infrastructure SIP 
submissions in 40 CFR part 51--e.g., subpart I (Review of New Sources 
and Modifications), subpart J (Ambient Air Quality Surveillance), 
subpart K (Source Surveillance), subpart L (Legal Authority), subpart M 
(Intergovernmental Consultation), subpart O (Miscellaneous Plan Content 
Requirements), subpart P (Protection of Visibility), and subpart Q 
(Reports). For the EPA's general criteria for infrastructure SIP 
submissions, refer to 40 CFR part 51, appendix V, Criteria for 
Determining the Completeness of Plan Submissions. For additional 
information on infrastructure SIP submission requirements, refer to the 
EPA's 2013 guidance entitled ``Guidance on Infrastructure State 
Implementation Plan (SIP) Elements under Clean Air Act Sections 
110(a)(1) and 110(a)(2).'' The EPA recommends that States 
electronically submit their infrastructure SIPs to the EPA through the 
State Plan Electronic Collaboration System (SPeCS),\214\ an online 
system available through the EPA's Central Data Exchange.
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    \214\ https://cdx.epa.gov/.
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C. Implementing Revised Primary Annual PM2.5 NAAQS in Nonattainment 
Areas

    As discussed in the NPRM, the EPA issued a SIP Requirements Rule 
for implementing the PM2.5 NAAQS (81 FR 58010, August 24, 
2016) (PM2.5 SIP Requirements Rule). It provides guidance 
and establishes additional regulatory requirements for States regarding 
development of attainment plans for nonattainment areas for the 1997, 
2006, and 2012 revisions of the PM2.5 NAAQS. The guidance 
and regulations in the SIP Requirements Rule also apply to any States 
for which the EPA promulgates nonattainment area designations for the 
new revised primary annual PM2.5 NAAQS.
    The PM2.5 SIP Requirements Rule provides comprehensive 
information regarding nonattainment plan requirements including, among 
other things: nonattainment area emissions inventories; policies 
regarding PM2.5 precursor pollutants (i.e., SO2, 
NOX, VOC, and ammonia); control strategies (such as 
reasonably available control measures and reasonably available control 
technology for direct PM2.5 and relevant precursors); air 
quality modeling; attainment demonstrations; reasonable further 
progress requirements; quantitative milestones; and contingency 
measures. Information provided in the PM2.5 SIP Requirements 
Rule is supplemented by other EPA documents, including guidance on 
emissions inventory development (80 FR 8787, February 19, 2015; U.S. 
EPA, 2017), optional PM2.5 precursor demonstrations (U.S. 
EPA, 2019b),\215\ and guidance on air quality modeling for meeting air 
quality goals for the ozone and PM2.5 NAAQS and regional 
haze program (U.S. EPA, 2018b).
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    \215\ Provides guidance on developing demonstrations under 
section 189(e) intended to show that a certain PM2.5 
precursor in a particular nonattainment area does not significantly 
contribute to PM2.5 concentrations that exceed the 
standard.
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    As stated in the NPRM, the PM2.5 SIP Requirements Rule 
provides recommendations to States regarding consideration of 
environmental justice in the context of PM2.5 attainment

[[Page 16369]]

planning. Some of the considerations for States include: (1) 
Identifying areas with overburdened communities where more ambient 
monitoring may be warranted; (2) targeting emissions reductions that 
may be needed to attain the PM2.5 NAAQS; and (3) increasing 
opportunities for meaningful involvement for overburdened populations 
(see 88 FR 5558, 5684, January 27, 2023; 80 FR 58010, 58136, August 25, 
2016). In light of the identification of at-risk populations for this 
reconsideration, the EPA encourages States to consider these and other 
factors as part of their attainment plan SIP development process.
    The PM2.5 SIP Requirements Rule outlines some examples 
of how States can elect to implement these recommendations.\216\ For 
instance, States can use modeling and screening tools to better 
understand where sources of PM2.5 or PM2.5 
precursor emissions are located and identify areas that may be 
candidates for additional ambient monitoring. Furthermore, once these 
target areas are identified, States can prioritize direct 
PM2.5 or PM2.5 precursor control measures and 
enforcement strategies in these areas to reduce ambient 
PM2.5 and achieve the NAAQS. As articulated in the NPRM and 
the PM2.5 SIP Requirements Rule, the EPA recognizes that 
States have flexibility under the CAA to concentrate State resources on 
controlling sources of PM2.5 emissions in light of 
environmental justice considerations (see 88 FR 5558, 5684, January 27, 
2023; 81 FR 58010, 58137, August 24, 2016). Moreover, States can 
establish opportunities to bolster meaningful involvement in a number 
of ways, such as communicating in appropriate languages, ensuring 
access to draft SIPs and other information, and developing enhanced 
notice-and-comment opportunities, as appropriate (see 88 FR 5558, 5684, 
January 27, 2023; 80 FR 58010, 58136, August 25, 2016).
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    \216\ For more information on the EPA's recommendations and 
examples, see 81 FR 58010, 58137, August 24, 2016.
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    As previously mentioned, the PM2.5 SIP Requirements Rule 
provides guidance and regulatory requirements for remaining 
nonattainment areas for the 1997, 2006, and 2012 revisions of the 
PM2.5 NAAQS, as well as for nonattainment areas designated 
pursuant to any future revisions of the PM2.5 NAAQS, 
including the revised annual PM2.5 NAAQS being finalized in 
this action. The EPA is not making any changes to the current 
PM2.5 SIP Requirements Rule.

D. Implementing the Primary and Secondary PM10 NAAQS

    As summarized in sections III.B.4 and V.B.4 above, the EPA is 
retaining the current primary and secondary 24-hour PM10 
NAAQS to protect against the health effects associated with short-term 
exposures to thoracic coarse particles and against the welfare effects 
considered in this reconsideration (i.e., visibility, climate, and 
materials effects). The EPA is retaining the existing implementation 
strategy for meeting the CAA requirements for the PM10 
NAAQS. States and emissions sources should continue to follow the 
existing regulations and guidance for implementing the current 
standards.\217\
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    \217\ CAA Sections 110(a) and 172 contain general nonattainment 
planning provisions, regarding the public review, adoption, 
submittal, and content of implementation plans. CAA Section 189 
specifies additional plan provisions for particulate matter 
nonattainment areas. General Preamble for the Implementation of 
Title I of the Clean Air Act Amendments of 1990 provides a detailed 
discussion of the EPA's interpretation of the Title I requirements 
(57 FR 13498, April 16, 1992; 59 FR 41998, August 16, 1994).
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E. Prevention of Significant Deterioration and Nonattainment New Source 
Review Programs for the Revised Primary Annual PM2.5 NAAQS

    The CAA, at parts C and D of title I, contains preconstruction 
review and permitting programs applicable to new major stationary 
sources and major modifications of existing major sources. The 
preconstruction review of each new major stationary source and major 
modification applies on a pollutant-specific basis, and the 
requirements that apply for each pollutant depend on whether the area 
in which the source is situated is designated as attainment (or 
unclassifiable) or nonattainment for that pollutant. In areas 
designated attainment or unclassifiable for a pollutant, the Prevention 
of Significant Deterioration (PSD) requirements under part C apply to 
construction at major sources. In areas designated nonattainment for a 
pollutant, the Nonattainment New Source Review (NNSR) requirements 
under part D apply to construction at major sources. Collectively, 
those two sets of permit requirements are commonly referred to as the 
``major New Source Review'' or ``major NSR'' programs.
    Until the EPA designates an area with respect to the revised 
primary annual PM2.5 NAAQS, the NSR provisions applicable 
under an area's current designation for the 1997, 2006, and 2012 
PM2.5 NAAQS would continue to apply. See 40 CFR 51.166(i)(2) 
and 52.21(i)(2). That is, for areas designated as attainment/
unclassifiable for the 1997, 2006, and 2012 PM2.5 NAAQS, PSD 
will apply to new major stationary sources and major modifications that 
trigger major source permitting requirements for PM2.5. For 
areas designated nonattainment for the 1997, 2006, or 2012 
PM2.5 NAAQS, NNSR requirements will apply for new major 
stationary sources and major modifications that trigger major source 
permitting requirements for PM2.5. When the initial area 
designations for this revised primary annual PM2.5 NAAQS 
become effective, those designations will further determine whether PSD 
or NNSR applies to PM2.5 in a particular area, depending on 
the designation status. New major sources and major modifications will 
be subject to the PSD program requirements for PM2.5 if they 
are located in an area that does not have a current nonattainment 
designation under CAA section 107 for PM2.5.\218\
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    \218\ 40 CFR 51.166(i)(2) and 52.21(i)(2).
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    Under the PSD program, the permit applicant must demonstrate that 
the new or modified source emissions increase does not cause or 
contribute to a NAAQS violation. In 2017, the EPA revised the Guideline 
on Air Quality Models (published as appendix W to 40 CFR part 51) to 
address primary and secondary PM2.5 impacts in making this 
demonstration. The EPA has since provided associated technical 
guidance, models and tools, such as the recent ``Final Guidance for 
Ozone and Fine Particulate Matter Permit Modeling'' (July 29, 
2022).\219\ Additionally, in light of this NAAQS revision, the EPA is 
updating its guidance that provides recommended significant impact 
levels (SILs) for PM2.5 and expects that an updated SIL for 
the revised primary annual PM2.5 NAAQS will be available

[[Page 16370]]

on or before the effective date of the final NAAQS.
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    \219\ On July 29, 2022, the EPA issued ``Final Guidance for 
Ozone and Fine Particulate Matter Permit Modeling,'' available at 
https://www.epa.gov/system/files/documents/2022-07/Guidance_for_O3_PM25_Permit_Modeling.pdf. This guidance provides the 
EPA's recommendations for how a stationary source seeking a PSD 
permit may demonstrate that it will not cause or contribute to a 
violation of the National Ambient Air Quality Standards for Ozone 
and PM2.5 and PSD increments for PM2.5, as 
required under section 165(a)(3) of the Clean Air Act and 40 CFR 
51.166(k) and 52.21(k). The EPA has also previously issued two 
technical guidance documents for use in conducting these 
demonstrations: ``Guidance on the Development of Modeled Emission 
Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for 
Ozone and PM2.5 under the PSD Permitting Program,'' 
available at https://www.epa.gov/sites/default/files/2020-09/documents/epa-454_r-19-003.pdf, and ``Guidance on the Use of Models 
for Assessing the Impacts of Emissions from Single Sources on the 
Secondarily Formed Pollutants: Ozone and PM2.5,'' 
available at https://www.epa.gov/sites/default/files/2020-09/documents/epa-454_r-16-005.pdf.
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    The statutory requirements for a PSD permit program set forth under 
part C of title I of the CAA (sections 160 through 169) are addressed 
by the EPA's PSD regulations found at 40 CFR 51.166 (minimum 
requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD 
permitting program for permits issued under the EPA's Federal 
permitting authority). These regulations already apply to 
PM2.5 in areas that are designated attainment or 
unclassifiable for PM2.5 whenever a proposed new major 
source or major modification triggers PSD requirements for 
PM2.5.
    For PSD, a ``major stationary source'' is one with the potential to 
emit 250 tons per year (tpy) or more of any regulated NSR pollutant, 
unless the new or modified source is classified under a list of 28 
source categories contained in the statutory definition of ``major 
emitting facility'' in section 169(1) of the CAA. For those 28 source 
categories, a ``major stationary source'' is one with the potential to 
emit 100 tpy or more of any regulated NSR pollutant. A ``major 
modification'' is a physical change or a change in the method of 
operation of an existing major stationary source that results, first, 
in a significant emissions increase of a regulated NSR pollutant and, 
second, in a significant net emissions increase of that pollutant. See 
40 CFR 51.166(b)(2)(i), 40 CFR 52.21(b)(2)(i). The EPA PSD regulations 
define the term ``regulated NSR pollutant'' to include any pollutant 
for which a NAAQS has been promulgated and any pollutant identified by 
the EPA as a constituent or precursor to such pollutant. See 40 CFR 
51.166(b)(49), 40 CFR 52.21(b)(50). These regulations identify 
SO2 and NOX as precursors to PM2.5 in 
attainment and unclassifiable areas. See 40 CFR 51.166(b)(49)(i)(b), 40 
CFR 52.21(b)(50)(i)(b).\220\ Thus, for PM2.5, the PSD 
program currently requires the review and control of emissions of 
direct PM2.5 emissions and SO2 and NOX 
(as precursors to PM2.5), absent a demonstration otherwise 
for NOX. Among other things, for each regulated NSR 
pollutant emitted or increased in a significant amount, the PSD program 
requires a new major stationary source or a major modification to apply 
the ``best available control technology'' (BACT) to limit emissions and 
to conduct an air quality impact analysis to demonstrate that the 
proposed major stationary source or major modification will not cause 
or contribute to a violation of any NAAQS or PSD increment.\221\ See 
CAA section 165(a)(3) and (4), 40 CFR 51.166(j) and (k), 40 CFR 
52.21(j) and (k). The PSD requirements may also include, in appropriate 
cases, an analysis of potential adverse impacts on Class I areas. See 
CAA sections 162(a) and 165(d), 40 CFR 51.166(p); 40 CFR 
52.21(p)).\222\ The EPA developed the Guideline on Air Quality Models 
and other documents to, among other things, provide methods and 
guidance for demonstrating that increased emissions from construction 
will not cause or contribute to exceedances of the PM2.5 
NAAQS and PSD increments for PM2.5.\223\
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    \220\ Sulfur dioxide is a precursor to PM2.5 in all 
attainment and unclassifiable areas. NOX is presumed to 
be a precursor to PM2.5 in all attainment and 
unclassifiable areas, unless a state or the EPA demonstrates that 
emissions of NOX from sources in a specific area are not 
a significant contributor to that area's ambient PM2.5 
concentrations. VOC is presumed not to be a precursor to 
PM2.5 in any attainment or unclassifiable area, unless a 
state or the EPA demonstrates that emissions of VOC from sources in 
a specific area are a significant contributor to that area's ambient 
PM2.5 concentrations.
    \221\ By establishing the maximum allowable level of ambient 
pollutant concentration increase in a particular area, an increment 
defines ``significant deterioration'' of air quality in that area. 
Increments are defined by the CAA as maximum allowable increases in 
ambient air concentrations above a baseline concentration and are 
specified in the PSD regulations by pollutant and area 
classification (Class I, II and III). 40 CFR 51.166(c), 40 CFR 
52.21(c); 75 FR 64864 (October 20, 2010).
    \222\ Congress established certain Class I areas in section 
162(a) of the CAA, including international parks, national 
wilderness areas, and national parks that meet certain criteria. 
Such Class I areas, known as mandatory Federal Class I areas, are 
afforded special protection under the CAA. In addition, States and 
Tribal governments may establish Class I areas within their own 
political jurisdictions to provide similar special air quality 
protection.
    \223\ See 40 CFR part 51, appendix W; 82 FR 5182 (January 17, 
2017); See also U.S. EPA, 2021d. The EPA provided an initial version 
of the 2021 guidance for public comment on February 10, 2020. Upon 
consideration of the comments received, and consistent with 
Executive Order 13990, the EPA revised the initial draft guidance 
and posted the revised version for additional public comment.
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    Upon the effective date of the revised primary annual 
PM2.5 NAAQS, the demonstration required under CAA Section 
165(a)(3), and the associated regulations, must include the revised 
primary annual PM2.5 NAAQS. In past NAAQS revision rules, 
including the 2012 PM2.5 NAAQS (78 FR 3086, January 15, 
2013) and 2015 Ozone NAAQS (80 FR 65292, October 26, 2015), the EPA 
included limited provision that exempted certain sources with pending 
PSD permit applications (those that had reached a particular stage in 
the permitting process at the time the revised NAAQS was promulgated or 
became effective) from the requirement to demonstrate that the proposed 
emissions increases would not cause or contribute to a violation of the 
revised NAAQS.\224\ In August 2019, the U.S. Court of Appeals for the 
D.C. Circuit vacated the exemption provision in the PSD rules for the 
2015 Ozone NAAQS, finding that the provision contradicted ``Congress's 
`express policy choice' not to allow construction which will `cause or 
contribute to' nonattainment of `any' effective NAAQS, regardless of 
when they are adopted or when a permit was completed.'' Murray Energy 
Corp. v. EPA, 936 F.3d 597, 627 (D.C. Cir. 2019).\225\ Based on that 
court decision, the EPA is not establishing any PSD permitting 
exemption provision in this action. Some commenters requested that the 
EPA provide the same kind of relief for pending PSD permit applications 
by extending the effective date of this new revised NAAQS beyond the 60 
days that the EPA has traditionally used for such rules. Such comments 
are addressed in the Response to Comments portion of this action. The 
EPA is making this revised primary annual PM2.5 NAAQS 
effective in 60 days.
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    \224\ This exemption was referred to as ``grandfathering'' in 
the 2015 Ozone NAAQS and the D.C. Circuit's Murray Energy Corp. 
decision on that exemption. See 80 FR 65292, 65431 (October 26, 
2015); Murray Energy Corp. v. EPA, 936 F.3d 597, 627 (D.C. Cir. 
2019). The EPA refers to this ``grandfathering'' provision in this 
action as an exemption provision.
    \225\ While the specifics of this case involved the 2015 ozone 
NAAQS, the case was based upon an interpretation of CAA section 
165(a) and therefore applies equally to any PSD permitting exemption 
provision for a new or revised NAAQS.
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    The EPA anticipates that the existing PM2.5 air quality 
in some areas will not be in attainment with the revised primary annual 
PM2.5 NAAQS, and the EPA will designate these areas as 
nonattainment at a later date, consistent with the designation process 
described in the preceding sections. However, until such nonattainment 
designation occurs, proposed new major sources and major modifications 
located in any area currently designated attainment or unclassifiable 
for all preexisting PM2.5 NAAQS will continue to be subject 
to the PSD program requirements for PM2.5. Any proposed 
major stationary source or major modification triggering PSD 
requirements for PM2.5 that does not receive its PSD permit 
by the effective date of a new nonattainment designation for the area 
where the source would locate would then be required to satisfy 
applicable NNSR preconstruction permit requirements for 
PM2.5.
    In areas where air pollution exceeds the level of the revised 
primary annual PM2.5 NAAQS, a PSD permit applicant must 
demonstrate that the source or modification will not cause or

[[Page 16371]]

contribute to a violation of the NAAQS. Section 165(a)(3)(B) of the CAA 
states that a proposed source may not construct unless it demonstrates 
that it will not cause or contribute to a violation of any NAAQS. This 
statutory requirement is implemented through a provision contained in 
the PSD regulations at 40 CFR 51.166(k) and 52.21(k).\226\ If a source 
cannot make this demonstration, or if its initial air quality impact 
analysis shows that the source's impact would cause or contribute to a 
violation, the reviewing authority may not issue a PSD permit to that 
source. However, a PSD permit applicant may be able to make this 
demonstration if it compensates for the adverse impact that would 
otherwise cause or contribute to a violation of the NAAQS. In contrast 
to the NSR requirements for nonattainment areas, the PSD regulations do 
not explicitly specify remedial actions that a prospective source must 
take to address such a situation, but the EPA has historically 
recognized that sources applying for PSD permits may utilize offsetting 
reductions in emissions as part of the required PSD demonstration under 
CAA section 165(a)(3)(B).\227\
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    \226\ 40 CFR 51.166(k) states that SIPs must require that the 
owner or operator of the proposed source or modification demonstrate 
that allowable emission increases from the proposed source or 
modification, in conjunction with all other applicable emissions 
increases or reductions (including secondary emissions), would not 
cause or contribute to air pollution in violation of: (i) Any 
national ambient air quality standard in any air quality control 
region; or (ii) any applicable maximum allowable increase over the 
baseline concentration in any area.
    \227\ See, e.g., Memorandum from Stephen D. Page, Director, 
Office of Air Quality Planning and Standards to Regional Air 
Division Directors, Guidance Concerning Implementation of the 1-hour 
SO2 NAAQS for the Prevention of Significant Deterioration 
Program. August 23, 2010. Office of Air Quality Planning and 
Standards U.S. EPA, Research Triangle Park. Available at: https://www.epa.gov/sites/default/files/2015-07/documents/appwso2.pdf; 44 FR 
3274, 3278, January 16, 1979; See also In re Interpower of New York, 
Inc., 5 E.A.D. 130, 141 (EAB 1994) (describing an EPA Region 2 PSD 
permit that relied in part on offsets to demonstrate the source 
would not cause or contribute to a violation of the NAAQS). 52 FR 
24634, 24684, July 1, 1987; 78 FR 3085, 3261-62, January 15, 2013. 
The EPA has recognized the ability of sources to obtain offsets in 
the context of PSD though the PSD provisions of the Act do not 
expressly reference offsets as the NNSR provisions of the Act do. 
See 80 FR 65292, 65441, October 26, 2015.
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    Part D of title I of the CAA includes preconstruction review and 
permitting requirements applicable to new major stationary sources and 
major modifications located in areas designated nonattainment for a 
pollutant for which the EPA has established a NAAQS (i.e., a criteria 
pollutant). The relevant part D requirements are typically referred to 
as the nonattainment NSR (NNSR) program. The EPA's regulations for the 
NNSR program are contained in 40 CFR 51.165 and 52.24 and part 51, 
appendix S. Specifically, the EPA has developed minimum program 
requirements for a NNSR program that is approvable in a SIP, and those 
requirements, which include requirements for PM2.5, are 
contained in 40 CFR 51.165. In addition, 40 CFR part 51, appendix S, 
contains requirements constituting an interim NNSR program. This 
interim program enables NNSR permitting in nonattainment areas by 
States that lack a SIP-approved NNSR permitting program during the time 
between the date of the relevant designation and the date that the EPA 
approves into the SIP a NNSR program. See 40 CFR part 51, appendix S, 
section I; 40 CFR 52.24(k).
    For NNSR, ``major stationary source'' is generally defined as a 
source with the potential to emit at least 100 tpy of the regulated NSR 
pollutant for which the area is designated nonattainment. In some 
cases, however, the CAA and the NNSR regulations define ``major 
stationary source'' for NNSR in terms of a lower rate dependent on the 
pollutant and degree of nonattainment in the area. For purposes of the 
PM2.5NAAQS, in addition to the general threshold level of 
100 tpy in Moderate PM2.5 nonattainment areas, a lower major 
source threshold of 70 tpy applies in Serious PM2.5 
nonattainment areas pursuant to subpart 4 of part D, title I of the 
CAA. See 40 CFR 51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 CFR part 51, 
appendix S, II.A.4(i)(a)(7) and (8).
    Under the NNSR program, direct PM2.5 emissions and 
emissions of each PM2.5 precursor are considered separately 
in accordance with the applicable major source threshold. For example, 
the threshold for Serious PM2.5 nonattainment areas is 70 
tpy of direct PM2.5, as well as for the PM2.5 
precursors SO2, NOX, VOC, and ammonia.\228\ See 
40 CFR 51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 CFR part 51, appendix 
S, II.A.4.(i)(a)(7) and (8). A source qualifies as major for 
nonattainment NSR in a PM2.5 nonattainment area if it emits 
or has the potential to emit direct PM2.5 or any 
PM2.5 precursor in an amount equal to or greater than the 
applicable threshold.
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    \228\ All of these pollutants are identified as precursors to 
PM2.5 in NNSR regulations. See 40 CFR 
51.165(a)(1)(xxxvii)(C)(2). No significant emission rate is 
established by the EPA for ammonia, and states are required to 
define ``significant'' for ammonia for their respective areas unless 
the state pursues the optional precursor demonstration to exclude 
ammonia from planning requirements. See 40 CFR 51.165(a)(1)(x)(F); 
40 CFR 51.165(a)(13).
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    For modifications, NNSR applies to proposed physical changes or 
changes in the method of operation of an existing stationary source 
where (1) the source is major for the nonattainment pollutant (or a 
precursor for that pollutant) and (2) the physical change or change in 
the method of operation of a major stationary source results, first, in 
a significant emissions increase of a regulated NSR pollutant and, 
second, in a significant net emissions increase of that same 
nonattainment pollutant (or same precursor for that pollutant). See 40 
CFR 51.165(a)(1)(v)(A); 40 CFR part 51, appendix S, II.A.5.(i). For 
example, to qualify as a major modification for SO2 (as a 
PM2.5 precursor) in a Moderate PM2.5 
nonattainment area, the existing source would have to have the 
potential to emit 100 tpy or more of SO2, and the project 
would have to result in an increase in SO2 emissions of 40 
tpy or more. See 40 CFR 51.165(a)(1)(x)(A).
    New major stationary sources and major modifications for 
PM2.5 subject to NNSR must comply with the ``lowest 
achievable emission rate'' (LAER), as defined in the CAA and NNSR 
rules. Such sources must also perform other analyses and obtain 
emission offsets, as required under section 173 of the CAA and 
applicable regulations.
    Following the promulgation of this revised primary annual 
PM2.5 NAAQS, some new areas may be designated nonattainment 
for PM2.5. Where a State does not have an existing NNSR 
program or where the current NNSR program does not apply to 
PM2.5, that State will be required to submit the necessary 
SIP revisions to ensure that new major stationary sources and major 
modifications for PM2.5 or a PM2.5 precursor 
undergo preconstruction review pursuant to the NNSR program. States 
with designated nonattainment areas for the revised primary annual 
PM2.5 NAAQS are required to make SIP submissions to meet 
nonattainment plan requirements within 18 months from the effective 
date of designations, as required under CAA section 189(a)(2)(B). 
States that have existing NNSR program requirements that cannot be 
interpreted to apply at the time of designation to the revised primary 
annual PM2.5 NAAQS may, in the interim, issue permits in 
accordance with the applicable nonattainment permitting requirements 
contained in 40 CFR part 51, appendix S, which would apply to the 
revised primary annual PM2.5 NAAQS upon its effective date. 
See 73 FR 28321, 28340, May 16, 2008.
    Finally, the EPA has released several documents that discuss air 
permitting

[[Page 16372]]

and environmental justice, including, for example, a memorandum \229\ 
and attached permitting principles.\230\ The EPA recommends that PSD 
and NNSR permitting authorities review this memorandum and the 
principles and consider applying them in their air permitting actions 
as appropriate to help identify, analyze, and address environmental 
justice concerns in those air permitting actions to help ensure that 
the NAAQS achieve their intended health benefits for at-risk 
populations.
---------------------------------------------------------------------------

    \229\ Memorandum from Joseph Goffman, Principal Deputy Assistant 
Administrator, Office of Air and Radiation, to Air and Radiation 
Division Directors, ``Principles for Addressing Environmental 
Justice in Air Permitting'' (December 22, 2022), available at 
https://www.epa.gov/caa-permitting/ej-air-permitting-principles-addressing-environmental-justice-concerns-air.
    \230\ Id., Attachment, ``EJ in Air Permitting: Principles for 
Addressing Environmental Justice Concerns in Air Permitting'' 
(December 2022), available at https://www.epa.gov/caa-permitting/ej-air-permitting-principles-addressing-environmental-justice-concerns-air.
---------------------------------------------------------------------------

F. Transportation Conformity Program

    Transportation conformity is required under CAA section 176(c) to 
ensure that transportation plans, transportation improvement programs 
(TIPs) and federally supported highway and transit projects will not 
cause or contribute to any new air quality violation, increase the 
frequency or severity of any existing violation, or delay timely 
attainment or any required interim emissions reductions or other 
milestones. Transportation conformity applies to areas that are 
designated as nonattainment or nonattainment areas that have been 
redesignated to attainment with an approved CAA section 175A 
maintenance plan (i.e., maintenance areas) for transportation-related 
criteria pollutants: carbon monoxide, ozone, NO2, 
PM2.5, and PM10. Transportation conformity for 
the revised primary annual PM2.5 NAAQS does not apply until 
one year after the effective date of nonattainment designations for 
that NAAQS. See CAA section 176(c)(6) and 40 CFR 93.102(d)). The EPA's 
Transportation Conformity Rule \231\ establishes the criteria and 
procedures for determining whether transportation activities conform to 
the SIP. No changes are being made to the transportation conformity 
rule in this final rulemaking. The EPA notes that the transportation 
conformity rule already addresses the PM2.5 and 
PM10 NAAQS. However, in the future, the EPA intends to 
review the need to issue or revise guidance describing how the current 
conformity rule applies in nonattainment and maintenance areas for the 
revised primary annual PM2.5 NAAQS, as needed.
---------------------------------------------------------------------------

    \231\ 40 CFR part 93, subpart A.
---------------------------------------------------------------------------

G. General Conformity Program

    The conformity requirement under CAA section 176(c) ensures that 
federal activities implemented by federal agencies will not interfere 
with a State's ability to attain and maintain the NAAQS. Under CAA 
176(c)(1), the requirement prohibits Federal agencies from approving, 
permitting, licensing, or funding activities that do not conform to the 
purpose of the applicable SIP for the control and prevention of air 
pollution. See CAA 176(c)(1)(A). Under CAA 176(c)(1)(B), conformity to 
an implementation plan means that federal activities will not cause or 
contribute to any new violations of the NAAQS, increase the frequency 
or severity of any existing NAAQS violation, or delay timely attainment 
or any required interim emissions reductions or other milestones 
contained in the applicable SIP.
    The general conformity program \232\ implements CAA section 
176(c)(4)(A), and the criteria and procedures for determining 
conformity of federal activities to the applicable SIP are established 
under 40 CFR part 93 subpart B, sections 93.150 through 93.165. General 
Conformity applies to federal activities that (1) would cause emissions 
of relevant criteria or precursor pollutants to originate within 
nonattainment areas or areas that have been redesignated to attainment 
with an approved CAA section 175A maintenance plan (i.e., maintenance 
areas), as set forth under 40 CFR 93.153, and (2) are not Federal 
Highway Administration (FHWA) or Federal Transit Administration (FTA) 
transportation projects as defined in 40 CFR 93.101 under the 
transportation conformity requirements. See 40 CFR 93.153. General 
conformity for the revised primary annual PM2.5 NAAQS does 
not apply until one year after the effective date of the nonattainment 
designation for that NAAQS. See 40 CFR 93.153(k).
---------------------------------------------------------------------------

    \232\ 40 CFR part 93 subpart B.
---------------------------------------------------------------------------

    With regard to issues regarding prescribed fires, which were 
addressed earlier in this action, here is some additional information 
regarding prescribed fires and General Conformity regulations. Under 
the General Conformity regulations at 40 CFR 93.153(c)(4), a conformity 
evaluation is not required to support a decision by a federal agency to 
conduct or carry out prescribed burning when the burn is consistent 
with the terms of a land management plan or other plan that includes 
the prescribed burn at issue, where the overall plan that includes the 
burn was previously evaluated under 40 CFR part 93 subpart B by the 
responsible federal agency, and the agency found the plan conforms 
under CAA paragraphs 176(c)(1)(A) and (1)(B). This assumes the burn at 
issue will be conducted by meeting any conditions specified as 
necessary for meeting conformity in the agency's decision to approve 
the plan. Alternatively, a presumption of conformity applies also under 
40 CFR 93.153(i)(2) for prescribed fires conducted in accordance with a 
Smoke Management Program that meets the requirements of the EPA's 1998 
Interim Air Quality Policy on Wildland and Prescribed Fires or an 
equivalent replacement EPA policy. The preamble to the Exceptional 
Events Rule explains that the EPA adapted language associated with the 
six basic components of a certifiable Smoke Management Program for 
exceptional events purposes from the 1998 Interim Air Quality Policy on 
Wildland and Prescribed Fires (see, e.g., 81 FR 68216, 68252 (including 
footnote 75), 68256, October 2, 2016). The Exceptional Events Rule at 
40 CFR 50.14(a)(3)(ii)(A) also indicates that certain requirements 
within the Exceptional Events Rule can be satisfied if a prescribed 
fire is conducted under a certified Smoke Management Program or using 
appropriate basic smoke management practices such as those identified 
in Table 1 to 40 CFR 50.14 (see e.g., 81 FR 68216, 68250-68257, 68277-
68278, October 3, 2016).
    No changes are being made to the general conformity regulations in 
this final rulemaking and the EPA notes that the courts recognize the 
regulations constitute control for the established PM2.5 and 
PM10 NAAQS. However, in the future, the EPA intends to 
review the need to issue or revise guidance describing how the current 
General Conformity regulations apply within nonattainment and 
maintenance areas for the revised primary annual PM2.5 
NAAQS, as needed.\233\
---------------------------------------------------------------------------

    \233\ Further, the EPA's current Unified Agenda and Regulatory 
Plan includes its intention to issue a proposed rule to amend the 
General Conformity Regulations. The EPA intends to address in that 
regulatory action topics regarding prescribed fire, including 
consideration of smoke management approaches such as those discussed 
in the Exceptional Events Rule, among other topics. See, e.g., 
https://www.reginfo.gov/public/do/eAgendaViewRule?pubId=202310&RIN=2060-AV28.
---------------------------------------------------------------------------

IX. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive orders 
can be

[[Page 16373]]

found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 14094: Modernizing Regulatory Review

    This action is ``significant regulatory action'' as defined under 
section 3(f)(1) of Executive Order 12866, as amended by Executive Order 
14094. Accordingly, the EPA submitted this action to the Office of 
Management and Budget (OMB) for review. Documentation of any changes 
made in response to the Executive Order 12866 review is available in 
the docket. The EPA prepared an illustrative analysis of the potential 
costs and benefits associated with this action. This analysis, 
``Regulatory Impact Analysis for the Reconsideration of the National 
Ambient Air Quality Standards for Particulate Matter,'' is available in 
the Regulatory Impact Analysis (RIA) docket (EPA-HQ-OAR-2019-0587) and 
briefly summarized below. However, the CAA and judicial decisions make 
clear that the economic and technical feasibility of attaining ambient 
standards are not to be considered in setting or revising NAAQS, 
although such factors may be considered in the development of State 
plans to implement the standards. Accordingly, although an RIA has been 
prepared, the results of the RIA have not been considered in issuing 
this final rule.
    The RIA estimates the costs and monetized human health benefits in 
2032, after implementing existing and expected regulations and 
assessing emissions reductions to meet the current primary annual and 
24-hour particulate matter NAAQS (12/35 [mu]g/m\3\), associated with 
applying national control strategies for the revised annual and 24-hour 
standard levels of 9/35 [mu]g/m\3\, as well as the following less and 
more stringent alternative standard levels: (1) A less stringent 
alternative annual standard level of 10 [mu]g/m\3\ in combination with 
the current 24-hour standard (i.e., 10/35 [mu]g/m\3\), (2) a more 
stringent alternative annual standard level of 8 [mu]g/m\3\ in 
combination with the current 24-hour standard (i.e., 8/35 [mu]g/m\3\), 
and (3) a more stringent alternative 24-hour standard level of 30 
[mu]g/m\3\ in combination with an annual standard level of 10 [mu]g/
m\3\ (i.e., 10/30 [mu]g/m\3\). Table 3 provides a summary of the 
estimated monetized benefits, costs, and net benefits associated with 
applying national control strategies toward reaching the revised and 
alternative standard levels.

  Table 3--Estimated Monetized Benefits, Costs, and Net Benefits of the Illustrative Control Strategies Applied
  Toward the Primary Revised and Alternative Annual and Daily Standard Levels of 10/35 [mu]g/m\3\, 10/30 [mu]g/
                         m\3\, 9/35 [mu]g/m\3\, and 8/35 [mu]g/m\3\ in 2032 for the U.S.
                                               [Millions of 2017$]
----------------------------------------------------------------------------------------------------------------
                                         10/35               10/30               9/35                8/35
----------------------------------------------------------------------------------------------------------------
Benefits \a\....................  $8,500 and $17,000  $10,000 and         $22,000 and         $48,000 and
                                                       $21,000.            $46,000.            $99,000.
Costs \b\.......................  $200..............  $340..............  $590..............  $1,500.
                                 -------------------------------------------------------------------------------
    Net Benefits................  $8,300 and $17,000  $9,900 and $21,000  $22,000 and         $46,000 and
                                                                           $46,000.            $97,000.
----------------------------------------------------------------------------------------------------------------
Notes: Rows may not appear to add correctly due to rounding. We provide a snapshot of costs and benefits in
  2032, using the best available information to approximate social costs and social benefits recognizing
  uncertainties and limitations in those estimates. The estimated costs and monetized human health benefits
  associated with applying national control strategies do not fully account for all the emissions reductions
  needed to reach the final and more stringent alternative standard levels for some standard levels analyzed.
\a\ We assume that there is a cessation lag between the change in PM exposures and the total realization of
  changes in mortality effects. Specifically, we assume that some of the incidences of premature mortality
  related to PM2.5 exposures occur in a distributed fashion over the 20 years following exposure, which affects
  the valuation of mortality benefits at different discount rates. Similarly, we assume there is a cessation lag
  between the change in PM exposures and both the development and diagnosis of lung cancer. The benefits are
  associated with two point estimates from two different epidemiologic studies, and we present the benefits
  calculated at a real discount rate of 3 percent. The monetized benefits exclude additional health and welfare
  benefits that could not be quantified.
\b\ The costs are annualized using a 7 percent interest rate.

B. Paperwork Reduction Act (PRA)

    This action does not impose any new information collection burden 
under the PRA. OMB has previously approved the information collection 
activities contained in the existing regulations and has assigned OMB 
control number 2060-0084. The data collected through this information 
collection consist of ambient air concentration measurements for the 
seven air pollutants with national ambient air quality standards (i.e., 
ozone, sulfur dioxide, nitrogen dioxide, lead, carbon monoxide, 
PM2.5 and PM10), ozone precursors, air toxics, 
meteorological variables at a select number of sites, and other 
supporting measurements. Accompanying the pollutant concentration data 
are quality assurance/quality control data and air monitoring network 
design information. The EPA and others (e.g., State and local air 
quality management agencies, tribal entities, environmental 
organizations, academic institutions, industrial groups) use the 
ambient air quality data for many purposes including informing the 
public and other interested parties of an area's air quality, judging 
an area's air quality in comparison with the established health or 
welfare standards, evaluating an air quality management agency's 
progress in achieving or maintaining air pollutant levels below the 
national and local standards, developing and revising State 
Implementation Plans (SIPs), evaluating air pollutant control 
strategies, developing or revising national control policies, providing 
data for air quality model development and validation, supporting 
enforcement actions, documenting episodes and initiating episode 
controls, assessing air quality trends, and conducting air pollution 
research.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
final rule establishes national standards for allowable concentrations 
of PM in ambient air as required by section 109 of the CAA. See also 
American Trucking Associations v. EPA, 175 F.3d 1027, 1044-45 (D.C. 
Cir. 1999) (NAAQS do not have significant impacts upon small entities 
because NAAQS themselves impose no regulations upon small entities), 
rev'd in part on other grounds, Whitman v. American Trucking 
Associations, 531 U.S. 457 (2001).

[[Page 16374]]

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain an unfunded mandate of $100 million or 
more as described in the Unfunded Mandates Reform Act (UMRA), 2 U.S.C. 
1531-1538, and does not significantly or uniquely affect small 
governments. Furthermore, as indicated previously, in setting a NAAQS 
the EPA cannot consider the economic or technological feasibility of 
attaining ambient air quality standards, although such factors may be 
considered to a degree in the development of State plans to implement 
the standards. See also American Trucking Associations v. EPA, 175 F. 
3d at 1043 (noting that because the EPA is precluded from considering 
costs of implementation in establishing NAAQS, preparation of the RIA 
pursuant to the Unfunded Mandates Reform Act would not furnish any 
information that the court could consider in reviewing the NAAQS).

E. Executive Order 13132: Federalism

    This action will not have substantial direct effects on the States, 
on the relationship between the National Government and the States, or 
on the distribution of power and responsibilities among the various 
levels of government. However, the EPA recognizes that States will have 
a substantial interest in this action and any future revisions to 
associated requirements.

F. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    This action does not have Tribal implications, as specified in 
Executive Order 13175. It does not have a substantial direct effect on 
one or more Indian Tribes as Tribes are not obligated to adopt or 
implement any NAAQS. In addition, Tribes are not obligated to conduct 
ambient monitoring for PM or to adopt the ambient monitoring 
requirements of 40 CFR part 58. Thus, Executive Order 13175 does not 
apply to this action. However, consistent with the EPA Policy on 
Consultation and Coordination with Indian Tribes, the EPA offered 
consultation to all 574 Federally Recognized Tribes during the 
development of this action. Although no Tribes requested consultation, 
the EPA provided informational meetings including an informational 
meeting with the Pueblo de San Ildefonso and provided information on 
the monthly National Tribal Air Association calls.

G. Executive Order 13045: Protection of Children From Environmental 
Health Risks and Safety Risks

    Executive Order 13045 directs federal agencies to include an 
evaluation of the health and safety effects of the planned regulation 
on children in federal health and safety standards and explain why the 
regulation is preferable to potentially effective and reasonably 
feasible alternatives. This action is subject to Executive Order 13045 
because it is a significant regulatory action under section 3(f)(1) of 
Executive Order 12866, and the EPA believes that the environmental 
health or safety risk addressed by this action may have a 
disproportionate effect on children. Accordingly, we have evaluated the 
environmental health or safety effects of PM exposures on children. The 
protection offered by these standards may be especially important for 
children because childhood represents a lifestage associated with 
increased susceptibility to PM-related health effects. Because children 
have been identified as a susceptible population, we have carefully 
evaluated the environmental health effects of exposure to PM pollution 
among children. Children make up a substantial fraction of the U.S. 
population, and often have unique factors that contribute to their 
increased risk of experiencing a health effect due to exposures to 
ambient air pollutants because of their continuous growth and 
development. As described in the 2019 Integrated Science Assessment, 
children may be particularly at risk for health effects related to 
ambient air PM2.5 exposures compared with adults because 
they have (1) a developing respiratory system, (2) increased 
ventilation rates relative to body mass compared with adults, and (3) 
an increased proportion of oral breathing, particularly in boys, 
relative to adults. More detailed information on the evaluation of the 
scientific evidence and policy considerations pertaining to children, 
including an explanation for why the Administrator judges the revised 
standards to be requisite to protect public health, including the 
health of children, with an adequate margin of safety, are contained in 
section II.A.2. ``Overview of the Health Effects Evidence'', section 
II.A.2.b ``Public Health Implications and At-Risk Populations'' and 
II.B ``Conclusions on the Primary PM2.5 Standards'' of this 
preamble. Copies of all documents have been placed in the public docket 
for this action. The Administrator judges that revising the primary 
annual PM2.5 standard to a level of 9.0 [micro]g/m\3\ and 
retaining the primary 24-hour PM2.5 standard provides 
requisite public health protection with an adequate margin of safety, 
including for children. Furthermore, the Policy on Children's Health 
also applies to this action. Information on how the Policy was applied 
is described in section II.A.2 ``Overview of the Health Effects 
Evidence'', section II.A.2.b ``Public Health Implications and At-Risk 
Populations'' and II.B ``Conclusions on the Primary PM2.5 
Standards'' of this preamble.

H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this action is to revise 
level of the primary annual PM2.5 NAAQS. The action does not 
prescribe specific pollution control strategies by which these ambient 
standards and monitoring revisions will be met. Such strategies will be 
developed by States on a case-by-case basis, and the EPA cannot predict 
whether the control options selected by States will include regulations 
on energy suppliers, distributors, or users. Thus, the EPA concludes 
that this action does not constitute a significant energy action as 
defined in Executive Order 13211.

I. National Technology Transfer and Advancement Act (NTTAA)

    This rulemaking involved environmental monitoring or measurement. 
The EPA has decided it will continue to use the existing indicators for 
fine (PM2.5) and coarse (PM10) particles. The 
indicator for fine particles is measured using the Reference Method for 
the Determination of Fine Particulate Matter as PM2.5 in the 
Atmosphere (appendix L to 40 CFR part 50), which is known as the 
PM2.5 FRM, and the indicator for coarse particles is 
measured using the Reference Method for the Determination of 
Particulate Matter as PM10 in the Atmosphere (appendix J to 
40 CFR part 50), which is known as the PM10 FRM.
    To the extent feasible, the EPA employs a Performance-Based 
Measurement System (PBMS), which does not require the use of specific, 
prescribed analytic methods. The PBMS is defined as a set of processes 
wherein the data quality needs, mandates or limitations of a program or 
project are specified and serve as criteria for selecting appropriate 
methods to meet those needs in a cost-effective manner.

[[Page 16375]]

It is intended to be more flexible and cost effective for the regulated 
community; it is also intended to encourage innovation in analytical 
technology and improved data quality. Though the FRM defines the 
particular specifications for ambient monitors, there is some 
variability with regard to how monitors measure PM, depending on the 
type and size of PM and environmental conditions. Therefore, it is not 
practically possible to fully define the FRM in performance terms to 
account for this variability. Nevertheless, our approach in the past 
has resulted in multiple brands of monitors being approved as FRM for 
PM, and we expect this to continue. Also, the FRMs described in 40 CFR 
part 50 and the equivalency criteria described in 40 CFR part 53, 
constitute a performance-based measurement system for PM, since methods 
that meet the field testing and performance criteria can be approved as 
FEMs. Since finalized in 2006 (71 FR 61236, October 17, 2006) the new 
field and performance criteria for approval of PM2.5 
continuous FEMs has resulted in the approval of 13 approved FEMs. In 
summary, for measurement of PM2.5 and PM10, the 
EPA relies on both FRMs and FEMs, with FEMs relying on a PBMS approach 
for their approval. The EPA is not precluding the use of any other 
method, whether it constitutes a voluntary consensus standard or not, 
as long as it meets the specified performance criteria.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations and 
Executive Order 14096: Revitalizing Our Nation's Commitment to 
Environmental Justice for All

    The EPA believes that the human health or environmental conditions 
associated with the primary PM2.5 NAAQS that exist prior to 
this action result in or have the potential to result in 
disproportionate and adverse human health or environmental effects on 
communities with environmental justice concerns. There is strong 
evidence for racial and ethnic disparities in PM2.5 
exposures and PM2.5-related health risk, as assessed in the 
2019 Integrated Science Assessment and with even more evidence 
available since the literature cutoff date for the 2019 Integrated 
Science Assessment and evaluated in the Supplement to the 2019 
Integrated Science Assessment. There is strong evidence demonstrating 
that Black and Hispanic populations, in particular, have higher 
PM2.5 exposures than non-Hispanic White populations. Black 
populations or individuals that live in predominantly Black 
neighborhoods experience higher PM2.5 exposures, in 
comparison to non-Hispanic White populations. There is also consistent 
evidence across multiple studies that demonstrate increased risk of 
PM2.5-related health effects, with the strongest evidence 
for health risk disparities for mortality. There is also evidence of 
health risk disparities for both Hispanic and non-Hispanic Black 
populations compared to non-Hispanic White populations for cause-
specific mortality and incident hypertension.
    Socioeconomic status (SES) is a composite measure that includes 
metrics such as income, occupation, or education, and can play a role 
in access to healthy environments as well as access to healthcare. SES 
may be a factor that contributes to differential risk from 
PM2.5-related health effects. Studies assessed in the 2019 
Integrated Science Assessment and Supplement to the 2019 Integrated 
Science Assessment provide evidence that lower SES communities are 
exposed to higher concentrations of PM2.5 compared to higher 
SES communities. Studies using composite measures of neighborhood SES 
consistently demonstrated a disparity in both PM2.5 exposure 
and the risk of PM2.5-related health outcomes. There is some 
evidence that supports associations larger in magnitude between 
mortality and long-term PM2.5 exposures for those with low 
income or living in lower income areas compared to those with higher 
income or living in higher income neighborhoods. Additionally, evidence 
supports conclusions that lower SES is associated with cause-specific 
mortality and certain health endpoints (i.e., HI and CHF), but less so 
for all-cause or total (non-accidental) mortality.
    The EPA believes that this action is likely to reduce existing 
disproportionate and adverse effects on communities with environmental 
justice concerns.
    The EPA additionally identified and addressed environmental justice 
concerns by providing opportunities for public input on the proposed 
decisions. The EPA held a multi-day virtual public hearing for the 
public to provide oral testimony and there was a 60-day public comment 
period for the proposed action. As described in section II.A.3 above, 
the EPA conducted a risk assessment to support this action that 
included an at-risk analysis that evaluates exposure and 
PM2.5 mortality risk for older adults (e.g., 65 years and 
older), stratified for White, Black, Asian, Native American, Non-
Hispanic, and Hispanic individuals. This at-risk analysis found that 
compared to a primary annual PM2.5 standard with a level of 
12.0 [micro]g/m\3\, meeting a revised annual standard with a level of 
9.0 [micro]g/m\3\ is estimated to reduce PM2.5-associated 
health risks in the 30 study areas controlled by the annual standard by 
about 22-28% and is expected to reduce disparities in exposure and risk 
among these populations.
    The information supporting this Executive Order review is is 
contained in sections II.A.2, II.B.3.a, II.B.3.c, II.B.2, and II.B.4. 
of this preamble and also in the 2019 Integrated Science Assessment, 
Supplement to the 2019 Integrated Science Assessment, and 2022 Policy 
Assessment. The EPA has carefully evaluated the potential impacts on 
minority populations and low SES populations as discussed in sections 
II.A.2, II.A.3, II.B.2, and II.B.4 of this preamble. The 2019 
Integrated Science Assessment, Supplement to the Integrated Science 
Assessment, and 2022 Policy Assessment contain the evaluation of the 
scientific evidence, quantitative risk analyses and policy 
considerations that pertain to these populations. These documents are 
available in the public docket for this action.

K. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule 
report to each House of the Congress and to the Comptroller General of 
the United States. This action meets the criteria set forth in 5 U.S.C. 
804(2).

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Abt Associates, Inc. (2005). Particulate matter health risk 
assessment for selected urban areas: Draft report. EPA Contract No. 
68-D-03-002. U.S. Environmental Protection Agency. Research Triangle 
Park, NC. Available at: http://www3.epa.gov/ttn/naaqs/standards/pm/data/PMrisk20051220.pdf.
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Medicine 161(2): 665-673.
BBC Research & Consulting (2003). Phoenix area visibility survey. 
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Schwartz, J, Koutrakis, P, Silverman, F and Gold, DR (2013). 
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Bell, ML, Ebisu, K, Peng, RD, Walker, J, Samet, JM, Zeger, SL and 
Dominic, F (2008). Seasonal and regional short-term effects of fine 
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Bellavia, A, Urch, B, Speck, M, Brook, RD, Scott, JA, Albetti, B, 
Behbod, B, North, M, Valeri, L, Bertazzi, PA, Silverman, F, Gold, D 
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Bennett, JE, Tamura-Wicks, H, Parks, RM, Burnett, RT, Pope, CA, 
Bechle, MJ, Marshall, JD, Danaei, G and Ezzati, M (2019). 
Particulate matter air pollution and national and county life 
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16(7): e1002856.
Besson, P, Mu[ntilde]oz, C, Ram[iacute]rez-Sagner, G, Salgado, M, 
Escobar, R and Platzer, W (2017). Long-Term Soiling Analysis for 
Three Photovoltaic Technologies in Santiago Region. IEEE Journal of 
Photovoltaics 7(6): 1755-1760.
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and Technical Information, OAQPS Staff Paper. Office of Air Quality 
Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-05-005a. December 2005. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1009MZM.txt.
U.S. EPA (2008). Integrated Review Plan for the National Ambient Air 
Quality Standards for Particulate Matter. Office of Research and 
Development, National Center for Environmental Assessment; Office of 
Air Quality Planning and Standards, Health and Environmental Impacts 
Division. Research Triangle Park, NC. U.S. EPA. EPA 452/R-08-004. 
March 2008. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1001FB9.txt.
U.S. EPA (2009a). Particulate Matter National Ambient Air Quality 
Standards: Scope and Methods Plan for Health Risk and Exposure 
Assessment. Office of Air Quality Planning and Standards, Health and 
Environmental Impacts Division. Research Triangle Park, NC. U.S. 
EPA. EPA-452/P-09-002. February 2009. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100FLWP.txt.
U.S. EPA (2009b). Particulate Matter National Ambient Air Quality 
Standards: Scope and Methods Plan for Urban Visibility Impact 
Assessment. Office of Air Quality Planning and Standards, Health and 
Environmental Impacts Division. Research Triangle Park, NC. U.S. 
EPA. EPA-452/P-09-001. February 2009. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100FLUX.txt.
U.S. EPA (2009c). Integrated Science Assessment for Particulate 
Matter (Final Report). Office of Research and Development, National 
Center for Environmental Assessment. Research Triangle Park, NC. 
U.S. EPA. EPA-600/R-08-139F. December 2009. Available at: https://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=216546.
U.S. EPA (2010a). Quantitative Health Risk Assessment for 
Particulate Matter (Final Report). Office of Air Quality Planning 
and Standards, Health and Environmental Impacts Division. Research 
Triangle Park, NC. U.S. EPA. EPA-452/R-10-005. June 2010. Available 
at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1007RFC.txt.
U.S. EPA (2010b). Particulate Matter Urban-Focused Visibility 
Assessment (Final Document). Office of Air Quality Planning and 
Standards, Health and Environmental Impacts Division. Research 
Triangle Park, NC. U.S. EPA. EPA-452/R-10-004. July 2010. Available 
at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100FO5D.txt.
U.S. EPA (2011). Policy Assessment for the Review of the Particulate 
Matter National Ambient Air Quality Standards. Office of Air Quality 
Planning and Standards, Health and Environmental Impacts Division. 
Research Triangle Park, NC. U.S. EPA. EPA-452/R-11-003. April 2011. 
Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100AUMY.txt.
U.S. EPA (2012). Responses to Significant Comments on the 2012 
Proposed Rule on the National Ambient Air Quality Standards for 
Particulate Matter (June 29, 2012; 77 FR 38890). Research Triangle 
Park, NC. U.S. EPA. Docket ID No. EPA-HQ-OAR-2007-0492. Available 
at: https://www3.epa.gov/ttn/naaqs/standards/pm/data/20121214rtc.pdf.
U.S. EPA (2015). Preamble to the integrated science assessments. 
U.S. Environmental Protection Agency, Office of Research and 
Development, National Center for Environmental Assessment, RTP 
Division. Research Triangle Park, NC. U.S. EPA. EPA/600/R-15/067. 
November 2015. Available at: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310244.
U.S. EPA (2016). Integrated review plan for the national ambient air 
quality standards for particulate matter. Office of Air Quality 
Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-16-005. December 2016. Available at: https://www3.epa.gov/ttn/naaqs/standards/pm/data/201612-final-integrated-review-plan.pdf.
U.S. EPA (2017). Emissions Inventory Guidance for Implementation of 
Ozone and Particulate Matter National Ambient Air Quality Standards 
(NAAQS) and Regional Haze Regulations. Office of Air Quality 
Planning and Standards, Office of Air and Radiation. Research 
Triangle Park, NC. U.S. EPA. U.S. EPA-454/B-17-002. Available at: 
https://www.epa.gov/sites/default/files/2017-07/documents/ei_guidance_may_2017_final_rev.pdf.
U.S. EPA (2018a). Technical Assistance Document (TAD) for the 
Reporting of Daily Air Quality--the Air Quality Index (AQI). U.S. 
Environmental Protection Agency, Office of Air Quality Planning and 
Standards. Research Triangle Park, NC. U.S. EPA. EPA 454/B-18-007. 
September 2018. Available at: https://www.airnow.gov/sites/default/files/2020-05/aqi-technical-assistance-document-sept2018.pdf.
U.S. EPA (2018b). Modeling Guidance for Demonstrating Air Quality 
Goals for Ozone, PM2.5, and Regional Haze. Office of Air 
Quality Planning and Standards, Air Quality Policy Division. 
Research Triangle Park, NC. U.S. EPA. EPA 454/R-18-009. November 
2018. Available at: https://www.epa.gov/sites/default/files/2020-10/documents/o3-pm-rh-modeling_guidance-2018.pdf.
U.S. EPA (2019a). Integrated Science Assessment (ISA) for 
Particulate Matter (Final Report). U.S. Environmental Protection 
Agency, Office of Research and Development, National Center for 
Environmental Assessment. Washington, DC. U.S. EPA. EPA/600/R-19/
188. December 2019. Available at: https://www.epa.gov/naaqs/particulate-matter-pm-standards-integrated-science-assessments-current-review.
U.S. EPA (2019b). PM2.5 Precursor Demonstration Guidance. 
Office of Air Quality Planning and Standards, Air Quality Policy 
Division. Research Triangle Park, NC. U.S. EPA. EPA-454/R-19-004. 
May 2019. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi/P100YD1Q.PDF?Dockey=P100YD1Q.PDF.
U.S. EPA (2020a). Responses to significant comments on the 2020 
proposed rule on the National Ambient Air Quality Standards for 
particulate matter (April 30, 2020; 85 FR 24094). EPA-HQ-OAR-2015-
0072. Available at: https://www.epa.gov/sites/production/files/2020-12/documents/pm_naaqs_response_to_comments_final.pdf.
U.S. EPA (2020b). Policy Assessment for the Review of the National 
Ambient Air Quality Standards for Particulate Matter. Office of Air 
Quality Planning and Standards, Health and Environmental Impacts 
Division. Research Triangle Park, NC. U.S. EPA. EPA-452/R-20-002. 
January 2020. Available at: https://www.epa.gov/system/files/documents/2021-10/final-policy-assessment-for-the-review-of-the-pm-naaqs-01-2020.pdf.
U.S. EPA (2021a). Supplement to the 2019 Integrated Science 
Assessment for Particulate Matter (External Review Draft). U.S. 
Environmental Protection Agency, Office of Research and Development, 
Center for Public Health and Environmental Assessment. Research 
Triangle Park, NC. U.S. EPA. EPA/600/R-21/198. December 2019. 
Available at: https://www.epa.gov/naaqs/particulate-matter-pm-standards-integrated-science-assessments-current-review.
U.S. EPA (2021b). Comparative Assessment of the Impacts of 
Prescribed Fire Versus Wildfire (CAIF): A Case Study in the Western 
U.S. U.S. Environmental Protection Agency. Washington, DC. U.S. EPA. 
EPA/600/R-21/197.
U.S. EPA (2021c). Policy Assessment for the Review of the National 
Ambient Air Quality Standards for Particulate Matter (External 
Review Draft). Office of Air Quality Planning and Standards, Health 
and Environmental Impacts Division. Research Triangle Park, NC. U.S. 
EPA. EPA-452/P-21-001. October 2021. Available at: https://
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Permit Modeling. U.S. Environmental Protection Agency, Office of Air 
Quality Planning and Standards, Air Quality Assessment Division. 
Research Triangle Park, NC. U.S. EPA. EPA-454/P-21-001. September 
2021.
U.S. EPA (2022a). Policy Assessment for the Reconsideration of the 
National Ambient Air Quality Standards for Particulate Matter. 
Office of Air Quality Planning and Standards, Health and 
Environmental Impacts Division. Research Triangle Park, NC. U.S. 
EPA. EPA-452/R-22-004. May 2022. Available at: https://www.epa.gov/system/files/documents/2022-05/Final%20Policy%20Assessment%20for%20the%20Reconsideration%20of%20the%20PM%20NAAQS_May2022_0.pdf.
U.S. EPA (2022b). Supplement to the 2019 Integrated Science 
Assessment for Particulate Matter (Final Report). U.S. Environmental 
Protection Agency, Office of Research and Development, Center for 
Public Health and Environmental Assessment. Research Triangle Park, 
NC. U.S. EPA. EPA/600/R 22/028. May 2022. Available at: https://www.epa.gov/naaqs/particulate-matter-pm-standards-integrated-science-assessments-current-review.
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List of Subjects

40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

40 CFR Part 53

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Reporting and recordkeeping requirements.

40 CFR Part 58

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Reporting and 
recordkeeping requirements.

Michael S. Regan,
Administrator.

    For the reasons set forth in the preamble, chapter I of title 40 of 
the Code of Federal Regulations is amended as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

0
1. The authority citation for part 50 continues to read as follows:

    Authority:  42 U.S.C. 7401, et seq.

0
2. Add Sec.  50.20 to read as follows:


Sec.  50.20  National primary ambient air quality standards for PM2.5.

    (a) The national primary ambient air quality standards for 
PM2.5 are 9.0 micrograms per cubic meter ([micro]g/m\3\) 
annual arithmetic mean concentration and 35 [micro]g/m\3\ 24-hour 
average concentration measured in the ambient air as PM2.5 
(particles with an aerodynamic diameter less than or equal to a nominal 
2.5 micrometers) by either:
    (1) A reference method based on appendix L to this part and 
designated in accordance with part 53 of this chapter; or
    (2) An equivalent method designated in accordance with part 53 of 
this chapter.
    (b) The primary annual PM2.5 standard is met when the 
annual arithmetic mean concentration, as determined in accordance with 
appendix N to this part, is less than or equal to 9.0 [micro]g/m\3\.
    (c) The primary 24-hour PM2.5 standard is met when the 
98th percentile 24-hour concentration, as determined in accordance with 
appendix N to this part, is less than or equal to 35 [micro]g/m\3\.

0
3. Amend appendix K to part 50 by:
0
a. In section 1.0 revising paragraph (b);
0
b. In section 2.3 adding paragraph (d); and
0
c. In section 3.0 adding paragraphs (a) and (b).
    The revision and additions read as follows:

Appendix K to Part 50--Interpretation of the National Ambient Air 
Quality Standards for Particulate Matter

1.0 General

* * * * *
    (b) The terms used in this appendix are defined as follows:
    Average refers to the arithmetic mean of the estimated number of 
exceedances per year, as per section 3.1 of this appendix.
    Collocated monitors refer to two or more air measurement 
instruments for the same parameter (e.g., PM10 mass) 
operated at the same site location, and whose placement is 
consistent with part 53 of this chapter. For purposes of considering 
a combined site record in this appendix, when two or more monitors 
are operated at the same site, one

[[Page 16381]]

monitor is designated as the ``primary'' monitor with any additional 
monitors designated as ``collocated.'' It is implicit in these 
appendix procedures that the primary monitor and collocated 
monitor(s) are all reference or equivalent methods; however, it is 
not a requirement that the primary and collocated monitors utilize 
the same specific sampling and analysis method.
    Combined site data record is the data set used for performing 
computations in this appendix and represents data for the primary 
monitors augmented with data from collocated monitors according to 
the procedure specified in section 3.0(a) of this appendix.
    Daily value for PM10 refers to the 24-hour average 
concentration of PM10 calculated or measured from 
midnight to midnight (local time).
    Exceedance means a daily value that is above the level of the 
24-hour standard after rounding to the nearest 10 [micro]g/m\3\ 
(i.e., values ending in 5 or greater are to be rounded up).
    Expected annual value is the number approached when the annual 
values from an increasing number of years are averaged, in the 
absence of long-term trends in emissions or meteorological 
conditions.
    Primary monitors are suitable monitors designated by a State or 
local agency in their annual network plan as the default data source 
for creating a combined site data record. If there is only one 
suitable monitor at a particular site location, then it is presumed 
to be a primary monitor.
    Year refers to a calendar year.
* * * * *

2.3 Data Requirements

* * * * *
    (d) 24-hour average concentrations will be computed from 
submitted hourly PM10 concentration data for each 
corresponding day of the year and the result will be stored in the 
first, or start, hour (i.e., midnight, hour `0') of the 24-hour 
period. A 24-hour average concentration shall be considered valid if 
at least 75 percent of the hourly averages (i.e., 18 hourly values) 
for the 24-hour period are available. In the event that fewer than 
all 24 hourly average concentrations are available (i.e., fewer than 
24 but at least 18), the 24-hour average concentration shall be 
computed on the basis of the hours available using the number of 
available hours within the 24-hour period as the divisor (e.g., the 
divisor is 19 if 19 hourly values are available). 24-hour periods 
with 7 or more missing hours shall also be considered for 
computations in this appendix if, after substituting zero for all 
missing hourly concentrations, the resulting 24-hour average daily 
value exceeds the level of the 24-hour standard specified in Sec.  
50.6 after rounding to the nearest 10 [micro]g/m\3\.
* * * * *

3.0 Computational Equations for the 24-Hour Standards

    (a) All computations shown in this appendix shall be implemented 
on a site-level basis. Site level concentration data shall be 
processed as follows:
    (1) The default dataset for PM10 mass concentrations 
for a site shall consist of the measured concentrations recorded 
from the designated primary monitor(s). All daily values produced by 
the primary monitor are considered part of the site record.
    (2) If a daily value is not produced by the primary monitor for 
a particular day, but a value is available from a single collocated 
monitor, then that collocated monitor value shall be considered part 
of the combined site data record. If daily value data is available 
from two or more collocated monitors, the average of those 
collocated values shall be used as the daily value. The data record 
resulting from this procedure is referred to as the ``combined site 
data record.''
    (b) In certain circumstances, including but not limited to site 
closures or relocations, data from two nearby sites may be combined 
into a single site data record for the purpose of calculating a 
valid design value. The appropriate Regional Administrator may 
approve such combinations if the Regional Administrator determines 
that the measured concentrations do not differ substantially between 
the two sites, taking into consideration factors such as distance 
between sites, spatial and temporal patterns in air quality, local 
emissions and meteorology, jurisdictional boundaries, and terrain 
features.
* * * * *

0
4. Amend appendix L to part 50 by revising section 7.3.4 and adding 
section 7.3.4.5 to read as follows:

Appendix L to Part 50--Reference Method for the Determination of Fine 
Particulate Matter as PM[bdi2].[bdi5] in the Atmosphere

* * * * *
    7.3.4 Particle size separator. The sampler shall be configured 
with one of the three alternative particle size separators described 
in this section. One separator is an impactor-type separator (WINS 
impactor) described in sections 7.3.4.1, 7.3.4.2, and 7.3.4.3 of 
this appendix. One alternative separator is a cyclone-type separator 
(VSCC\TM\) described in section 7.3.4.4 of this appendix. The other 
alternative separator is also a cyclone-type separator (TE-
PM2.5C) described in section 7.3.4.5 of this appendix.
* * * * *
    7.3.4.5 A second cyclone-type separator is identified as a Tisch 
TE-PM2.5C Cyclone particle size separator specified as 
part of EPA-designated reference method RFPS-1014-219 and as 
manufactured by Tisch Environmental Incorporated, 145 S. Miami 
Avenue, Village of Cleves, Ohio 45002.
* * * * *

0
5. Amend appendix N to part 50 by:
0
a. In section 1.0 revising paragraph (a);
0
b. In section 3.0 adding paragraph (d)(3);
0
c. In section 4.1 revising paragraph (a); and
0
d. In section 4.2 revising paragraph (a).
    The addition and revisions read as follows.

Appendix N to Part 50--Interpretation of the National Ambient Air 
Quality Standards for PM[bdi2].[bdi5]

1.0 General

    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the national ambient air 
quality standards (NAAQS) for PM2.5 are met, specifically 
the primary and secondary annual and 24-hour PM2.5 NAAQS 
specified in Sec. Sec.  50.7, 50.13, 50.18, and 50.20. 
PM2.5 is defined, in general terms, as particles with an 
aerodynamic diameter less than or equal to a nominal 2.5 
micrometers. PM2.5 mass concentrations are measured in 
the ambient air by a Federal Reference Method (FRM) based on 
appendix L to this part, as applicable, and designated in accordance 
with part 53 of this chapter or by a Federal Equivalent Method (FEM) 
designated in accordance with part 53 of this chapter. Only those 
FRM and FEM measurements that are derived in accordance with part 58 
of this chapter (i.e., that are deemed ``suitable'') shall be used 
in comparisons with the PM2.5 NAAQS. The data handling 
and computation procedures to be used to construct annual and 24-
hour NAAQS metrics from reported PM2.5 mass 
concentrations, and the associated instructions for comparing these 
calculated metrics to the levels of the PM2.5 NAAQS, are 
specified in sections 2.0, 3.0, and 4.0 of this appendix.
* * * * *

3.0 Requirements for Data Use and Data Reporting for Comparisons With 
the NAAQS for PM[bdi2].[bdi5]

* * * * *
    (d) * * *
    (3) In certain circumstances, including but not limited to site 
closures or relocations, data from two nearby sites may be combined 
into a single site data record for the purpose of calculating a 
valid design value. The appropriate Regional Administrator may 
approve such site combinations if the Regional Administrator 
determines that the measured concentrations do not differ 
substantially between the two sites, taking into consideration 
factors such as distance between sites, spatial and temporal 
patterns in air quality, local emissions and meteorology, 
jurisdictional boundaries, and terrain features.
* * * * *

4.1 Annual PM[bdi2].[bdi5] NAAQS

    (a) Levels of the primary and secondary annual PM2.5 
NAAQS are specified in Sec. Sec.  50.7, 50.13, 50.18, and 50.20 as 
applicable.
* * * * *

4.2 Twenty-Four-Hour PM[bdi2].[bdi5] NAAQS

    (a) Levels of the primary and secondary 24-hour PM2.5 
NAAQS are specified in Sec. Sec.  50.7, 50.13, 50.18, and 50.20 as 
applicable.
* * * * *

[[Page 16382]]

PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS

0
6. The authority citation for part 53 continues to read as follows:

    Authority:  Sec. 301(a) of the Clean Air Act (42 U.S.C. 
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat. 
1713, unless otherwise noted.

Subpart A--General Provisions

0
7. Amend Sec.  53.4 by:
0
a. Revising paragraph (a);
0
b. Adding paragraph (b)(7); and
0
c. Revising paragraph (d).
    The revisions and addition read as follows:


Sec.  53.4  Applications for reference or equivalent method 
determinations.

    (a) Applications for FRM or FEM determinations and modification 
requests of existing designated instruments shall be submitted to: U.S. 
Environmental Protection Agency, Director, Center for Environmental 
Measurement and Modeling, Reference and Equivalent Methods Designation 
Program (MD-D205-03), 109 T.W. Alexander Drive, P.O. Box 12055, 
Research Triangle Park, North Carolina 27711 (commercial delivery 
address: 4930 Old Page Road, Durham, North Carolina 27703).
* * * * *
    (b) * * *
    (7) All written materials for new FRM and FEM applications and 
modification requests must be submitted in English in MS Word format. 
For any calibration certificates originally written in a non-English 
language, the original non-English version of the certificate must be 
submitted to EPA along with a version of the certificate translated to 
English. All laboratory and field data associated with new FRM and FEM 
applications and modification requests must be submitted in MS Excel 
format. All worksheets in MS Excel must be unprotected to enable full 
inspection as part of the application review process.
* * * * *
    (d) For candidate reference or equivalent methods or for designated 
instruments that are the subject of a modification request, the 
applicant, if requested by EPA, shall provide to EPA a representative 
sampler or analyzer for test purposes. The sampler or analyzer shall be 
shipped free on board (FOB) destination to Director, Center for 
Environmental Measurements and Modeling, Reference and Equivalent 
Methods Designation Program (MD D205-03), U.S. Environmental Protection 
Agency, 4930 Old Page Road, Durham, North Carolina 27703, scheduled to 
arrive concurrently with or within 30 days of the arrival of the other 
application materials. This sampler or analyzer may be subjected to 
various tests that EPA determines to be necessary or appropriate under 
Sec.  53.5(f), and such tests may include special tests not described 
in this part. If the instrument submitted under this paragraph (d) 
malfunctions, becomes inoperative, or fails to perform as represented 
in the application before the necessary EPA testing is completed, the 
applicant shall be afforded the opportunity to repair or replace the 
device at no cost to the EPA. Upon completion of EPA testing, the 
sampler or analyzer submitted under this paragraph (d) shall be 
repacked by EPA for return shipment to the applicant, using the same 
packing materials used for shipping the instrument to EPA unless 
alternative packing is provided by the applicant. Arrangements for, and 
the cost of, return shipment shall be the responsibility of the 
applicant. The EPA does not warrant or assume any liability for the 
condition of the sampler or analyzer upon return to the applicant.

0
8. Amend Sec.  53.8 by revising paragraph (a) to read as follows:


Sec.  53.8  Designation of reference and equivalent methods.

    (a) A candidate method determined by the Administrator to satisfy 
the applicable requirements of this part shall be designated as an FRM 
or FEM (as applicable) by and upon publication of the designation in 
the Federal Register. Applicants shall not publicly announce, market, 
or sell the candidate sampler and analyzer as an approved FRM or FEM 
(as applicable) until the designation is published in the Federal 
Register.
* * * * *


0
9. Amend Sec.  53.14 by revising paragraphs (c)(4), (5), and (6) to 
read as follows:


Sec.  53.14  Modification of a reference or equivalent method.

* * * * *
    (c) * * *
    (4) Send notice to the applicant that additional information must 
be submitted before a determination can be made and specify the 
additional information that is needed (in such cases, the 90-day period 
shall commence upon receipt of the additional information).
    (5) Send notice to the applicant that additional tests are 
necessary and specify which tests are necessary and how they shall be 
interpreted (in such cases, the 90-day period shall commence upon 
receipt of the additional test data).
    (6) Send notice to the applicant that additional tests will be 
conducted by the Administrator and specify the reasons for and the 
nature of the additional tests (in such cases, the 90-day period shall 
commence 1 calendar day after the additional tests are completed).
* * * * *

0
10. Revise table A-1 to subpart A of part 53 to read as follows:

  Table A-1 to Subpart A of Part 53--Summary of Applicable Requirements for Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                  Applicable                         Applicable subparts of this part
    Pollutant          Reference or     Manual or automated  appendix of part 50 -----------------------------------------------------------------------
                        equivalent                             of this  chapter        A           B           C           D           E           F
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2..............  Reference..........  Manual.............  A-2
                                        Automated..........  A-1                    [check]     [check]
                   Equivalent.........  Manual.............  A-1                    [check]   ..........    [check]
                                        Automated..........  A-1                    [check]     [check]     [check]
CO...............  Reference..........  Automated..........  C                      [check]     [check]
                   Equivalent.........  Manual.............  C                      [check]   ..........    [check]
                                        Automated..........  C                      [check]     [check]     [check]
O3...............  Reference..........  Automated..........  D                      [check]     [check]
                   Equivalent.........  Manual.............  D                      [check]   ..........    [check]
                                        Automated..........  D                      [check]     [check]     [check]
NO2..............  Reference..........  Automated..........  F                      [check]     [check]
                   Equivalent.........  Manual.............  F                      [check]   ..........    [check]
                                        Automated..........  F                      [check]     [check]     [check]

[[Page 16383]]

 
Pb...............  Reference..........  Manual.............  G
                   Equivalent.........  Manual.............  G                      [check]   ..........    [check]
                                        Automated..........  G                      [check]   ..........    [check]
PM10-Pb..........  Reference..........  Manual.............  Q
                   Equivalent.........  Manual.............  Q                      [check]   ..........    [check]
                                        Automated..........  Q                      [check]   ..........    [check]
PM10.............  Reference..........  Manual.............  J                      [check]   ..........  ..........    [check]
                   Equivalent.........  Manual.............  J                      [check]   ..........    [check]     [check]
                                        Automated..........  J                      [check]   ..........    [check]     [check]
PM2.5............  Reference..........  Manual.............  L                      [check]   ..........  ..........  ..........    [check]
                   Equivalent Class I.  Manual.............  L                      [check]   ..........    [check]   ..........    [check]
                   Equivalent Class II  Manual.............  L \1\                  [check]   ..........        \2\   ..........    [check]         1 2
                                                                                                            [check]                             [check]
                   Equivalent Class     Automated..........  L \1\                  [check]   ..........    [check]   ..........    [check]         \1\
                    III.                                                                                                                        [check]
PM10-2.5.........  Reference..........  Manual.............  L,\2\ O                [check]   ..........  ..........  ..........    [check]
                   Equivalent Class I.  Manual.............  L,\2\ O                [check]   ..........    [check]   ..........    [check]
                   Equivalent Class II  Manual.............  L,\2\ O                [check]   ..........        \2\   ..........    [check]       \1,2\
                                                                                                            [check]                             [check]
                   Equivalent Class     Automated..........  \1\ L, 1 2 O           [check]   ..........    [check]   ..........    [check]         \1\
                    III.                                                                                                                        [check]
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator.
\2\ Alternative Class III requirements may be substituted.

Subpart B--Procedures for Testing Performance Characteristics of 
Automated Methods for SO2, CO, O3, and NO2

0
11. Amend table B-1 to subpart B of part 53 by revising footnote 4 to 
read as follows:
Table B-1 to Subpart B of Part 53--Performance Limit Specifications for 
Automated Methods
* * * * *
    \4\ For nitric oxide interference for the SO2 
ultraviolet fluorescence (UVF) method, interference equivalent is 
0.003 ppm for the lower range.
* * * * *


0
12. Revise table B-3 to subpart B of part 53 to read as follows:

[[Page 16384]]



                                                               Table B-3 to Subpart B of Part 53--Interferent Test Concentration 1
                                                                                       [Parts per million]
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                   Hydro-
   Pollutant      Analyzer type    chloric   Ammonia   Hydrogen   Sulfur    Nitrogen   Nitric     Carbon    Ethylene    Ozone    M-xylene    Water     Carbon    Methane   Ethane    Naphthalene
                       \2\          acid               sulfide    dioxide   dioxide     oxide    dioxide                                     vapor    monoxide
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...........  Ultraviolet       ........  ........    \5\ 0.1  \4\ 0.14        0.5       0.5  .........  .........       0.5        0.2    20,000  .........  ........  ........      \6\ 0.05
                 fluorescence.
SO2...........  Flame             ........  ........       0.01  \4\ 0.14  .........  ........        750  .........  ........  .........       \3\         50  ........  ........  ............
                 photometric.                                                                                                                20,000
SO2...........  Gas               ........  ........        0.1  \4\ 0.14  .........  ........        750  .........  ........  .........       \3\         50  ........  ........  ............
                 chromatography.                                                                                                             20,000
SO2...........  Spectrophotometr       0.2       0.1        0.1  \4\ 0.14        0.5  ........        750  .........       0.5  .........  ........  .........  ........  ........  ............
                 ic-wet chemical
                 (pararosanaline
                 ).
SO2...........  Electrochemical.       0.2       0.1        0.1  \4\ 0.14        0.5       0.5  .........        0.2       0.5  .........       \3\  .........  ........  ........  ............
                                                                                                                                             20,000
SO2...........  Conductivity....       0.2       0.1  .........  \4\ 0.14        0.5  ........        750  .........  ........  .........  ........  .........  ........  ........  ............
SO2...........  Spectrophotometr  ........  ........  .........  \4\ 0.14        0.5       0.5  .........  .........       0.5        0.2  ........  .........  ........  ........  ............
                 ic-gas phase,
                 including DOAS.
O3............  Ethylene          ........  ........    \3\ 0.1  ........  .........  ........        750  .........  \4\ 0.08  .........       \3\  .........  ........  ........  ............
                 Chemiluminescen                                                                                                             20,000
                 ce.
O3............  NO-               ........  ........    \3\ 0.1  ........        0.5  ........        750  .........  \4\ 0.08  .........       \3\  .........  ........  ........  ............
                 chemiluminescen                                                                                                             20,000
                 ce.
O3............  Electrochemical.  ........   \3\ 0.1  .........       0.5        0.5  ........  .........  .........  \4\ 0.08  .........       \3\  .........  ........  ........  ............
                                                                                                                                             20,000
O3............  Spectrophotometr  ........   \3\ 0.1  .........       0.5        0.5   \3\ 0.5  .........  .........  \4\ 0.08  .........  ........  .........  ........  ........  ............
                 ic-wet chemical
                 (potassium
                 iodide).
O3............  Spectrophotometr  ........  ........  .........       0.5        0.5   \3\ 0.5  .........  .........  \4\ 0.08       0.02    20,000  .........  ........  ........  ............
                 ic-gas phase,
                 including
                 ultraviolet
                 absorption and
                 DOAS.
CO............  Non-dispersive    ........  ........  .........  ........  .........  ........        750  .........  ........  .........    20,000     \4\ 10  ........  ........  ............
                 Infrared.
CO............  Gas               ........  ........  .........  ........  .........  ........  .........  .........  ........  .........    20,000     \4\ 10  ........       0.5  ............
                 chromatography
                 with flame
                 ionization
                 detector.
CO............  Electrochemical.  ........  ........  .........  ........  .........       0.5  .........        0.2  ........  .........    20,000     \4\ 10  ........  ........  ............
CO............  Catalytic         ........       0.1  .........  ........  .........  ........        750        0.2  ........  .........    20,000     \4\ 10       5.0       0.5  ............
                 combustion-
                 thermal
                 detection.
CO............  IR fluorescence.  ........  ........  .........  ........  .........  ........        750  .........  ........  .........    20,000     \4\ 10  ........       0.5  ............
CO............  Mercury           ........  ........  .........  ........  .........  ........  .........        0.2  ........  .........  ........     \4\ 10  ........       0.5  ............
                 replacement-UV
                 photometric.
NO2...........  Chemiluminescent  ........   \3\ 0.1  .........       0.5    \4\ 0.1       0.5  .........  .........  ........  .........    20,000  .........  ........  ........  ............
NO2...........  Spectrophotometr  ........  ........  .........       0.5    \4\ 0.1       0.5        750  .........       0.5  .........  ........  .........  ........  ........  ............
                 ic-wet chemical
                 (azo-dye
                 reaction).
NO2...........  Electrochemical.       0.2   \3\ 0.1  .........       0.5    \4\ 0.1       0.5        750  .........       0.5  .........    20,000         50  ........  ........  ............
NO2...........  Spectrophotometr  ........   \3\ 0.1  .........       0.5    \4\ 0.1       0.5  .........  .........       0.5  .........    20,000         50  ........  ........  ............
                 ic-gas phase.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Concentrations of interferent listed must be prepared and controlled to 10 percent of the stated value.
\2\ Analyzer types not listed will be considered by the Administrator as special cases.
\3\ Do not mix interferent with the pollutant.
\4\ Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
\5\ If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.
\6\ If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full-scale response, reattach the scrubber, and evaluate
  response for interference.


[[Page 16385]]

0
13. Amend appendix A to subpart B of part 53 by revising figures B-3 
and B-5 to read as follows:

Appendix A to Subpart B of Part 53--Optional Forms for Reporting Test 
Results

* * * * *

Figure B-3 to Appendix A to Subpart B of Part 53--Form for Test Data 
and Calculations for Lower Detectable Limit (LDL) and Interference 
Equivalent (IE) (see Sec.  53.23(c) and (d))

LDL Interference Test Data

Applicant--------------------------------------------------------------
Analyzer---------------------------------------------------------------
Date-------------------------------------------------------------------
Pollutant--------------------------------------------------------------
[GRAPHIC] [TIFF OMITTED] TR06MR24.035

* * * * *

Figure B-5 to Appendix A to Subpart B of Part 53--Form for Calculating 
Zero Drift, Span Drift and Precision (see Sec.  53.23(e))

Calculation of Zero Drift, Span Drift, and Precision

Applicant--------------------------------------------------------------
Analyzer---------------------------------------------------------------
Date-------------------------------------------------------------------
Pollutant--------------------------------------------------------------
[GRAPHIC] [TIFF OMITTED] TR06MR24.036


[[Page 16386]]


* * * * *

Subpart C--Procedures for Determining Comparability Between 
Candidate Methods and Reference Methods

0
14. Amend Sec.  53.35 by revising paragraph (b)(1)(ii)(D) to read as 
follows:


Sec.  53.35  Test procedure for Class II and Class III methods for 
PM2.5 and PM10-2.5.

* * * * *
    (b) * * *
    (1) * * *
    (ii) * * *
    (D) Site D shall be in a large city east of the Mississippi River, 
having characteristically high humidity levels.
* * * * *

0
15. Revise table C-4 to subpart C of part 53 to read as follows:

                    Table C-4 to Subpart C of Part 53--Test Specifications for PM10, PM2.5, and PM10-2.5 Candidate Equivalent Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                 PM2.5                                           PM10-2.5
          Specification                  PM10        ---------------------------------------------------------------------------------------------------
                                                            Class I            Class II            Class III           Class II            Class III
--------------------------------------------------------------------------------------------------------------------------------------------------------
Acceptable concentration range    5-300.............  3-200.............  3-200.............  3-200.............  3-200.............  3-200.
 (Rj), [micro]g/m\3\.
Minimum number of test sites....  2.................  1.................  2.................  4.................  2.................  4.
Minimum number of candidate       3.................  3.................  3\1\..............  3\1\..............  3\1\..............  3.\1\
 method samplers or analyzers
 per site.
Number of reference method        3.................  3.................  3\1\..............  3\1\..............  3\1\..............  3.\1\
 samplers per site.
Minimum number of acceptable
 sample sets per site for PM10
 methods:
    Rj < 20 [micro]g/m\3\.......  3.................  ..................  ..................  ..................  ..................  ..................
    Rj > 20 [micro]g/m\3\.......  3.................  ..................  ..................  ..................  ..................  ..................
        Total...................  10................  ..................  ..................  ..................  ..................  ..................
Minimum number of acceptable
 sample sets per site for PM2.5
 and PM10-2.5 candidate
 equivalent methods:
    Rj < 15 [micro]g/m\3\ for 24- ..................  3.................  3.................  3.................  3.................  3.
     hr or Rj < 8 [micro]g/m\3\
     for 48-hr samples..
    Rj > 15 [micro]g/m\3\ for 24- ..................  3.................  3.................  3.................  3.................  3.
     hr or Rj > 8 [micro]g/m\3\
     for 48-hr samples.
    Each season.................  ..................  10................  23................  23................  23................  23.
        Total, each site........  ..................  10................  23................  23 (46 for two-     23................  23 (46 for two-
                                                                                               season sites).                          season sites).
Precision of replicate reference  5 [mu]g/m\3\ or     2 [mu]g/m\3\ or     10%\2\............  10%\2\............  10%\2\............  10%.\2\
 method measurements, PRj or       7%..                5%..
 RPRj, respectively; RP for
 Class II or III PM2.5 or PM10-
 2.5, maximum.
Precision of PM2.5 or PM10-2.5    ..................  ..................  10%\2\............  15%\2\............  15%\2\............  15%.\2\
 candidate method, CP, each site.
Slope of regression relationship  1 0.10  1 0.05  1 0.10  1 0.10  1 0.10  1 0.12.
Intercept of regression           0 5...  0 1...  Between: 13.55--    Between: 15.05--    Between: 62.05--    Between: 70.50--
 relationship, [micro]g/m\3\.                                              (15.05 x slope),    (17.32 x slope),    (70.5 x slope),     (82.93 x slope),
                                                                           but not less        but not less        but not less        but not less
                                                                           than--1.5; and      than--2.0; and      than--3.5; and      than--7.0; and
                                                                           16.56--(15.05 x     15.05--(13.20 x     78.95--(70.5 x      70.50--(61.16 x
                                                                           slope), but not     slope), but not     slope), but not     slope), but not
                                                                           more than +1.5.     more than +2.0.     more than +3.5.     more than +7.0.
                                                                         -------------------------------------------------------------------------------
Correlation of reference method   >= 0.97...........  >= 0.97...........  >= 0.93--for CCV <= 0.4;
 and candidate method                                                     >= 0.85 + 0.2 x CCV--for 0.4 <= CCV <= 0.5;
 measurements.                                                            >= 0.95--for CCV >= 0.5
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Some missing daily measurement values may be permitted; see test procedure.
\2\ Calculated as the root mean square over all measurement sets.


[[Page 16387]]

Subpart D--Procedures for Testing Performance Characteristics of 
Methods for PM10

0
16. Amend Sec.  53.43 by revising the formula in paragraph (a)(2)(xvi) 
and the formula in paragraph (c)(2)(iv) to read as follows:


Sec.  53.43  Test procedures.

    (a) * * *
    (2) * * *
    (xvi) * * *
    [GRAPHIC] [TIFF OMITTED] TR06MR24.037
    
* * * * *
    (c) * * *
    (2) * * *
    (iv) * * *
    [GRAPHIC] [TIFF OMITTED] TR06MR24.038
    
    if Cj is below 80 [micro]g/m\3\, or
    [GRAPHIC] [TIFF OMITTED] TR06MR24.039
    
    if Cj is above 80 [micro]g/m\3\.

Subpart E--Procedures for Testing Physical (Design) and Performance 
Characteristics of Reference Methods and Class I and Class II 
Equivalent Methods for PM2.5 or PM10-2.5

0
17. Amend Sec.  53.51 by revising paragraph (d)(2) to read as follows:


Sec.  53.51  Demonstration of compliance with design specifications and 
manufacturing and test requirements.

* * * * *
    (d) * * *
    (2) VSCC and TE-PM2.5C separators. For samplers and 
monitors utilizing the BGI VSCC or Tisch TE-PM2.5C particle 
size separators specified in sections 7.3.4.4 and 7.3.4.5 of appendix L 
to part 50 of this chapter, respectively, the respective manufacturers 
shall identify the critical dimensions and manufacturing tolerances for 
the separator, devise appropriate test procedures to verify that the 
critical dimensions and tolerances are maintained during the 
manufacturing process, and carry out those procedures on each separator 
manufactured to verify conformance of the manufactured products. The 
manufacturer shall also maintain records of these tests and their test 
results and submit evidence that this procedure is incorporated into 
the manufacturing procedure, that the test is or will be routinely 
implemented, and that an appropriate procedure is in place for the 
disposition of units that fail this tolerance tests.
* * * * *

Subpart F--Procedures for Testing Performance Characteristics of 
Class II Equivalent Methods for PM2.5

0
18. Amend Sec.  53.61 by revising paragraph (g) introductory text, the 
first sentence of paragraph (g)(1), the first sentence of (g)(1)(i), 
(g)(2)(i) and adding paragraph (g)(2)(iii) to read as follows:


 Sec.  53.61   Test conditions.

* * * * *
    (g) Vibrating Orifice Aerosol Generator (VOAG) and Flow-Focusing 
Monodisperse Aerosol Generator (FMAG) conventions. This section 
prescribes conventions regarding the use of the vibrating orifice 
aerosol generator (VOAG) and the flow-focusing monodisperse aerosol 
generator (FMAG) for the size-selective performance tests outlined in 
Sec. Sec.  53.62, 53.63, 53.64, and 53.65.
    (1) Particle aerodynamic diameter. The VOAG and FMAG produce near-
monodisperse droplets through the controlled breakup of a liquid jet. * 
* *
    (i) The physical diameter of a generated spherical particle can be 
calculated from the operational parameters of the VOAG and FMAG as:
* * * * *
    (2) * * *
    (i) Solid particle tests performed in this subpart shall be 
conducted using particles composed of ammonium fluorescein. For use in 
the VOAG or FMAG, liquid solutions of known volumetric concentration 
can be prepared by diluting fluorescein powder 
(C2OH12O5, FW = 332.31, CAS 2321-07-5) 
with aqueous ammonia. Guidelines for preparation of fluorescein 
solutions of the desired volume concentration (Cvol) are 
presented in Vanderpool and Rubow (1988) (Reference 2 in appendix A to 
this subpart). For purposes of converting particle physical diameter to 
aerodynamic diameter, an ammonium fluorescein particle density of 1.35 
g/cm\3\ shall be used.
* * * * *
    (iii) Calculation of the physical diameter of the particles 
produced by the VOAG and FMAG requires

[[Page 16388]]

knowledge of the liquid solution's volume concentration 
(Cvol). Because uranine is essentially insoluble in oleic 
acid, the total particle volume is the sum of the oleic acid volume and 
the uranine volume. The volume concentration of the liquid solution 
shall be calculated as:
[GRAPHIC] [TIFF OMITTED] TR06MR24.040

Where:
Vu = uranine volume, ml;
Voleic = oleic acid volume, ml;
Vsol = total solution volume, ml;
Mu = uranine mass, g;
Pu = uranine density, g/cm\3\;
Moleic = oleic acid mass, g; and
Poleic = oleic acid density, g/cm\3\.
* * * * *

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

0
19. The authority citation for part 58 continues to read as follows:

    Authority:  42 U.S.C. 7403, 7405, 7410, 7414, 7601, 7611, 7614, 
and 7619.

Subpart A--General Provisions

0
20. Amend Sec.  58.1 by:
0
a. Removing the definition for ``Approved regional method (ARM)''; and
0
b. Revising the definition for ``Traceable.''
    The revision reads as follows:


Sec.  58.1  Definitions.

* * * * *
    Traceable means a measurement result from a local standard whereby 
the result can be related to the International System of Units (SI) 
through a documented unbroken chain of calibrations, each contributing 
to the measurement uncertainty. Traceable measurement results must be 
compared and certified, either directly or via not more than one 
intermediate standard, to a National Institute of Standards and 
Technology (NIST)-certified reference standard. Examples include but 
are not limited to NIST Standard Reference Material (SRM), NIST-
traceable Reference Material (NTRM), or a NIST-certified Research Gas 
Mixture (RGM). Traceability to the SI through other National Metrology 
Institutes (NMIs) in addition to NIST is allowed if a Declaration of 
Equivalence (DoE) exists between NIST and that NMI.
* * * * *

Subpart B--Monitoring Network

0
21. Amend Sec.  58.10 by:
0
a. Revising paragraphs (a)(1) and (b)(10) and (13);
0
b. Adding paragraph (b)(14); and
0
c. Revising paragraph (d).
    The revisions and addition read as follows:


Sec.  58.10  Annual monitoring network plan and periodic network 
assessment.

    (a)(1) Beginning July 1, 2007, the State, or where applicable 
local, agency shall submit to the Regional Administrator an annual 
monitoring network plan which shall provide for the documentation of 
the establishment and maintenance of an air quality surveillance system 
that consists of a network of SLAMS monitoring stations that can 
include FRM and FEM monitors that are part of SLAMS, NCore, CSN, PAMS, 
and SPM stations. The plan shall include a statement of whether the 
operation of each monitor meets the requirements of appendices A, B, C, 
D, and E to this part, where applicable. The Regional Administrator may 
require additional information in support of this statement. The annual 
monitoring network plan must be made available for public inspection 
and comment for at least 30 days prior to submission to the EPA and the 
submitted plan shall include and address, as appropriate, any received 
comments.
* * * * *
    (b) * * *
    (10) Any monitors for which a waiver has been requested or granted 
by the EPA Regional Administrator as allowed for under appendix D or 
appendix E to this part. For those monitors where a waiver has been 
approved, the annual monitoring network plan shall include the date the 
waiver was approved.
* * * * *
    (13) The identification of any PM2.5 FEMs used in the 
monitoring agency's network where the data are not of sufficient 
quality such that data are not to be compared to the national ambient 
air quality standards (NAAQS). For required SLAMS where the agency 
identifies that the PM2.5 Class III FEM does not produce 
data of sufficient quality for comparison to the NAAQS, the monitoring 
agency must ensure that an operating FRM or filter-based FEM meeting 
the sample frequency requirements described in Sec.  58.12 or other 
Class III PM2.5 FEM with data of sufficient quality is 
operating and reporting data to meet the network design criteria 
described in appendix D to this part.
    (14) The identification of any site(s) intended to address being 
sited in an at-risk community where there are anticipated effects from 
sources in the area as required in section 4.7.1(b)(3) of appendix D to 
this part. An initial approach to the question of whether any new or 
moved sites are needed and to identify the communities in which they 
intend to add monitoring for meeting the requirement in this paragraph 
(b)(14), if applicable, shall be submitted in accordance with the 
requirements of section 4.7.1(b)(3) of appendix D to this part, which 
includes submission to the EPA Regional Administrator no later than 
July 1, 2024. Specifics on the resulting proposed new or moved sites 
for PM2.5 network design to address at-risk communities, if 
applicable, would need to be detailed in annual monitoring network 
plans due to each applicable EPA Regional office no later than July 1, 
2025. The plan shall provide for any required sites to be operational 
no later than 24 months from date of approval of a plan or January 1, 
2027, whichever comes first.
* * * * *
    (d) The State, or where applicable local, agency shall perform and 
submit to the EPA Regional Administrator an assessment of the air 
quality surveillance system every 5 years to determine, at a minimum, 
if the network meets the monitoring objectives defined in appendix D to 
this part, whether new sites are needed, whether existing sites are no 
longer needed and can be terminated, and whether new technologies are 
appropriate for incorporation into the ambient air monitoring network. 
The network assessment must consider the ability of existing and 
proposed sites to support air quality characterization for areas with 
relatively high populations of

[[Page 16389]]

susceptible individuals (e.g., children with asthma) and other at-risk 
populations, and, for any sites that are being proposed for 
discontinuance, the effect on data users other than the agency itself, 
such as nearby States and Tribes or health effects studies. The State, 
or where applicable local, agency must submit a copy of this 5-year 
assessment, along with a revised annual network plan, to the Regional 
Administrator. The assessments are due every 5 years beginning July 1, 
2010.
* * * * *

0
22. Amend Sec.  58.11 by revising paragraphs (a)(2) and (e) to read as 
follows:


Sec.  58.11  Network technical requirements.

    (a) * * *
    (2) Beginning January 1, 2009, State and local governments shall 
follow the quality assurance criteria contained in appendix A to this 
part that apply to SPM sites when operating any SPM site which uses an 
FRM or an FEM and meets the requirements of appendix E to this part, 
unless the Regional Administrator approves an alternative to the 
requirements of appendix A with respect to such SPM sites because 
meeting those requirements would be physically and/or financially 
impractical due to physical conditions at the monitoring site and the 
requirements are not essential to achieving the intended data 
objectives of the SPM site. Alternatives to the requirements of 
appendix A may be approved for an SPM site as part of the approval of 
the annual monitoring plan, or separately.
* * * * *
    (e) State and local governments must assess data from Class III 
PM2.5 FEM monitors operated within their network using the 
performance criteria described in table C-4 to subpart C of part 53 of 
this chapter, for cases where the data are identified as not of 
sufficient comparability to a collocated FRM, and the monitoring agency 
requests that the FEM data should not be used in comparison to the 
NAAQS. These assessments are required in the monitoring agency's annual 
monitoring network plan described in Sec.  58.10(b) for cases where the 
FEM is identified as not of sufficient comparability to a collocated 
FRM. For these collocated PM2.5 monitors, the performance 
criteria apply with the following additional provisions:
    (1) The acceptable concentration range (Rj), [micro]g/m\3\ may 
include values down to 0 [micro]g/m\3\.
    (2) The minimum number of test sites shall be at least one; 
however, the number of test sites will generally include all locations 
within an agency's network with collocated FRMs and FEMs.
    (3) The minimum number of methods shall include at least one FRM 
and at least one FEM.
    (4) Since multiple FRMs and FEMs may not be present at each site, 
the precision statistic requirement does not apply, even if precision 
data are available.
    (5) All seasons must be covered with no more than 36 consecutive 
months of data in total aggregated together.
    (6) The key statistical metric to include in an assessment is the 
bias (both additive and multiplicative) of the PM2.5 
continuous FEM(s) compared to a collocated FRM(s). Correlation is 
required to be reported in the assessment, but failure to meet the 
correlation criteria, by itself, is not cause to exclude data from a 
continuous FEM monitor.

0
23. Amend Sec.  58.12 by revising paragraph (d)(1):


Sec.  58.12  Operating schedules.

* * * * *
    (d) * * *
    (1)(i) Manual PM2.5 samplers at required SLAMS stations 
without a collocated continuously operating PM2.5 monitor 
must operate on at least a 1-in-3 day schedule unless a waiver for an 
alternative schedule has been approved per paragraph (d)(1)(ii) of this 
section.
    (ii) For SLAMS PM2.5 sites with both manual and 
continuous PM2.5 monitors operating, the monitoring agency 
may request approval for a reduction to 1-in-6 day PM2.5 
sampling or for seasonal sampling from the EPA Regional Administrator. 
Other requests for a reduction to 1-in-6 day PM2.5 sampling 
or for seasonal sampling may be approved on a case-by-case basis. The 
EPA Regional Administrator may grant sampling frequency reductions 
after consideration of factors (including but not limited to the 
historical PM2.5 data quality assessments, the location of 
current PM2.5 design value sites, and their regulatory data 
needs) if the Regional Administrator determines that the reduction in 
sampling frequency will not compromise data needed for implementation 
of the NAAQS. Required SLAMS stations whose measurements determine the 
design value for their area and that are within plus or minus 10 
percent of the annual NAAQS, and all required sites where one or more 
24-hour values have exceeded the 24-hour NAAQS each year for a 
consecutive period of at least 3 years are required to maintain at 
least a 1-in-3 day sampling frequency until the design value no longer 
meets the criteria in this paragraph (d)(1)(ii) for 3 consecutive 
years. A continuously operating FEM PM2.5 monitor satisfies 
the requirement in this paragraph (d)(1)(ii) unless it is identified in 
the monitoring agency's annual monitoring network plan as not 
appropriate for comparison to the NAAQS and the EPA Regional 
Administrator has approved that the data from that monitor may be 
excluded from comparison to the NAAQS.
    (iii) Required SLAMS stations whose measurements determine the 24-
hour design value for their area and whose data are within plus or 
minus 5 percent of the level of the 24-hour PM2.5 NAAQS must 
have an FRM or FEM operate on a daily schedule if that area's design 
value for the annual NAAQS is less than the level of the annual 
PM2.5 standard. A continuously operating FEM or 
PM2.5 monitor satisfies the requirement in this paragraph 
(d)(1)(iii) unless it is identified in the monitoring agency's annual 
monitoring network plan as not appropriate for comparison to the NAAQS 
and the EPA Regional Administrator has approved that the data from that 
monitor may be excluded from comparison to the NAAQS. The daily 
schedule must be maintained until the referenced design values no 
longer meets the criteria in this paragraph (d)(1)(iii) for 3 
consecutive years.
    (iv) Changes in sampling frequency attributable to changes in 
design values shall be implemented no later than January 1 of the 
calendar year following the certification of such data as described in 
Sec.  58.15.
* * * * *

0
24. Revise Sec.  58.15 to read as follows:


Sec.  58.15  Annual air monitoring data certification.

    (a) The State, or where appropriate local, agency shall submit to 
the EPA Regional Administrator an annual air monitoring data 
certification letter to certify data collected by FRM and FEM monitors 
at SLAMS and SPM sites that meet criteria in appendix A to this part 
from January 1 to December 31 of the previous year. The head official 
in each monitoring agency, or his or her designee, shall certify that 
the previous year of ambient concentration and quality assurance data 
are completely submitted to AQS and that the ambient concentration data 
are accurate to the best of her or his knowledge, taking into 
consideration the quality assurance findings. The annual data 
certification letter is due by May 1 of each year.
    (b) Along with each certification letter, the State shall submit to 
the Regional Administrator an annual

[[Page 16390]]

summary report of all the ambient air quality data collected by FRM and 
FEM monitors at SLAMS and SPM sites. The annual report(s) shall be 
submitted for data collected from January 1 to December 31 of the 
previous year. The annual summary serves as the record of the specific 
data that is the object of the certification letter.
    (c) Along with each certification letter, the State shall submit to 
the Regional Administrator a summary of the precision and accuracy data 
for all ambient air quality data collected by FRM and FEM monitors at 
SLAMS and SPM sites. The summary of precision and accuracy shall be 
submitted for data collected from January 1 to December 31 of the 
previous year.

Subpart C--Special Purpose Monitors

0
25. Amend Sec.  58.20 by revising paragraphs (b) through (e) to read as 
follows:


Sec.  58.20  Special purpose monitors (SPM).

* * * * *
    (b) Any SPM data collected by an air monitoring agency using a 
Federal reference method (FRM) or Federal equivalent method (FEM) must 
meet the requirements of Sec. Sec.  58.11 and 58.12 and appendix A to 
this part or an approved alternative to appendix A. Compliance with 
appendix E to this part is optional but encouraged except when the 
monitoring agency's data objectives are inconsistent with the 
requirements in appendix E. Data collected at an SPM using a FRM or FEM 
meeting the requirements of appendix A must be submitted to AQS 
according to the requirements of Sec.  58.16. Data collected by other 
SPMs may be submitted. The monitoring agency must also submit to AQS an 
indication of whether each SPM reporting data to AQS monitor meets the 
requirements of appendices A and E.
    (c) All data from an SPM using an FRM or FEM which has operated for 
more than 24 months are eligible for comparison to the relevant NAAQS, 
subject to the conditions of Sec. Sec.  58.11(e) and 58.30, unless the 
air monitoring agency demonstrates that the data came from a particular 
period during which the requirements of appendix A, appendix C, or 
appendix E to this part were not met, subject to review and EPA 
Regional Office approval as part of the annual monitoring network plan 
described in Sec.  58.10.
    (d) If an SPM using an FRM or FEM is discontinued within 24 months 
of start-up, the Administrator will not base a NAAQS violation 
determination for the PM2.5 or ozone NAAQS solely on data 
from the SPM.
    (e) If an SPM using an FRM or FEM is discontinued within 24 months 
of start-up, the Administrator will not designate an area as 
nonattainment for the CO, SO2, NO2, or 24-hour 
PM10 NAAQS solely on the basis of data from the SPM. Such 
data are eligible for use in determinations of whether a nonattainment 
area has attained one of these NAAQS.
* * * * *

0
26. Amend appendix A to part 58 by:
0
a. Revising section 2.6.1 and adding sections 2.6.1.1 and 2.6.1.2;
0
b. Removing section 3.1.2.2 and redesignating sections 3.1.2.3, 
3.1.2.4, 3.1.2.5, and 3.1.2.6 as sections 3.1.2.2, 3.1.2.3, 3.1.2.4, 
and 3.1.2.5, respectively;
0
c. Revising sections 3.1.3.3, 3.2.4, 4.2.1, and 4.2.5; and
0
d. In section 6 revising References (1), (4), (6), (7), (9), (10), and 
(11) and table A-1.
    The revisions and additions read as follows:

Appendix A to Part 58--Quality Assurance Requirements for Monitors used 
in Evaluations of National Ambient Air Quality Standards

* * * * *
    2.6.1 Gaseous pollutant concentration standards (permeation 
devices or cylinders of compressed gas) used to obtain test 
concentrations for CO, SO2, NO, and NO2 must 
be EPA Protocol Gases certified in accordance with one of the 
procedures given in Reference 4 of this appendix.
    2.6.1.1 The concentrations of EPA Protocol Gas standards used 
for ambient air monitoring must be certified with a 95-percent 
confidence interval to have an analytical uncertainty of no more 
than 2.0 percent (inclusive) of the certified 
concentration (tag value) of the gas mixture. The uncertainty must 
be calculated in accordance with the statistical procedures defined 
in Reference 4 of this appendix.
    2.6.1.2 Specialty gas producers advertising certification with 
the procedures provided in Reference 4 of this appendix and 
distributing gases as ``EPA Protocol Gas'' for ambient air 
monitoring purposes must adhere to the regulatory requirements 
specified in 40 CFR 75.21(g) or not use ``EPA'' in any form of 
advertising. Monitoring organizations must provide information to 
the EPA on the specialty gas producers they use on an annual basis. 
PQAOs, when requested by the EPA, must participate in the EPA 
Ambient Air Protocol Gas Verification Program at least once every 5 
years by sending a new unused standard to a designated verification 
laboratory.
* * * * *
    3.1.3.3 Using audit gases that are verified against the NIST 
standard reference methods or special review procedures and 
validated per the certification periods specified in Reference 4 of 
this appendix (EPA Traceability Protocol for Assay and Certification 
of Gaseous Calibration Standards) for CO, SO2, and 
NO2 and using O3 analyzers that are verified 
quarterly against a standard reference photometer.
* * * * *
    3.2.4 PM2.5 Performance Evaluation Program (PEP) 
Procedures. The PEP is an independent assessment used to estimate 
total measurement system bias. These evaluations will be performed 
under the national performance evaluation program (NPEP) as 
described in section 2.4 of this appendix or a comparable program. A 
prescribed number of Performance evaluation sampling events will be 
performed annually within each PQAO. For PQAOs with less than or 
equal to five monitoring sites, five valid performance evaluation 
audits must be collected and reported each year. For PQAOs with 
greater than five monitoring sites, eight valid performance 
evaluation audits must be collected and reported each year. A valid 
performance evaluation audit means that both the primary monitor and 
PEP audit concentrations are valid and equal to or greater than 2 
[micro]g/m3. Siting of the PEP monitor must be consistent with 
section 3.2.3.4(c) of this appendix. However, any horizontal 
distance greater than 4 meters and any vertical distance greater 
than one meter must be reported to the EPA regional PEP coordinator. 
Additionally for every monitor designated as a primary monitor, a 
primary quality assurance organization must:
* * * * *
    4.2.1 Collocated Quality Control Sampler Precision Estimate for 
PM10, PM2.5, and Pb. Precision is estimated 
via duplicate measurements from collocated samplers. It is 
recommended that the precision be aggregated at the PQAO level 
quarterly, annually, and at the 3-year level. The data pair would 
only be considered valid if both concentrations are greater than or 
equal to the minimum values specified in section 4(c) of this 
appendix. For each collocated data pair, calculate ti, 
using equation 6 to this appendix:

[[Page 16391]]

[GRAPHIC] [TIFF OMITTED] TR06MR24.041

    Where Xi is the concentration from the primary 
sampler and Yi is the concentration value from the audit 
sampler. The coefficient of variation upper bound is calculated 
using equation 7 to this appendix:
[GRAPHIC] [TIFF OMITTED] TR06MR24.042

    Where k is the number of valid data pairs being aggregated, and 
X\2\0.1,k-1 is the 10th percentile of a chi-squared 
distribution with k-1 degrees of freedom. The factor of 2 in the 
denominator adjusts for the fact that each ti is 
calculated from two values with error.
* * * * *
    4.2.5 Performance Evaluation Programs Bias Estimate for 
PM2.5. The bias estimate is calculated using the PEP 
audits described in section 3.2.4. of this appendix. The bias 
estimator is based on, si, the absolute difference in 
concentrations divided by the square root of the PEP concentration.
[GRAPHIC] [TIFF OMITTED] TR06MR24.043

* * * * *

6. References

(1) American National Standard Institute--Quality Management Systems 
For Environmental Information And Technology Programs--Requirements 
With Guidance For Use. ASQ/ANSI E4-2014. February 2014. Available 
from ANSI Webstore https://webstore.ansi.org/.
* * * * *
(4) EPA Traceability Protocol for Assay and Certification of Gaseous 
Calibration Standards. EPA-600/R-12/531. May, 2012. Available from 
U.S. Environmental Protection Agency, National Risk Management 
Research Laboratory, Research Triangle Park NC 27711. https://www.epa.gov/nscep.
* * * * *
(6) List of Designated Reference and Equivalent Methods. Available 
from U.S. Environmental Protection Agency, Center for Environmental 
Measurements and Modeling, Air Methods and Characterization 
Division, MD-D205-03, Research Triangle Park, NC 27711. https://www.epa.gov/amtic/air-monitoring-methods-criteria-pollutants.
(7) Transfer Standards for the Calibration of Ambient Air Monitoring 
Analyzers for Ozone. EPA-454/B-13-004 U.S. Environmental Protection 
Agency, Research Triangle Park, NC 27711, October, 2013. https://www.epa.gov/sites/default/files/2020-09/documents/ozonetransferstandardguidance.pdf.
* * * * *
(9) Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume 1--A Field Guide to Environmental Quality Assurance. 
EPA-600/R-94/038a. April 1994. Available from U.S. Environmental 
Protection Agency, ORD Publications Office, Center for Environmental 
Research Information (CERI), 26 W. Martin Luther King Drive, 
Cincinnati, OH 45268. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance#documents.
(10) Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume II: Ambient Air Quality Monitoring Program Quality 
System Development. EPA-454/B-13-003. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance#documents.
(11) National Performance Evaluation Program Standard Operating 
Procedures. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance#npep.

                Table A-1 to Section 6 of Appendix A--Minimum Data Assessment Requirements for NAAQS Related Criteria Pollutant Monitors
--------------------------------------------------------------------------------------------------------------------------------------------------------
               Method                   Assessment method           Coverage           Minimum  frequency    Parameters  reported   AQS assessment type
--------------------------------------------------------------------------------------------------------------------------------------------------------
Gaseous Methods (CO, NO2, SO2, O3):

[[Page 16392]]

 
    One-Point QC for SO2, NO2, O3,   Response check at       Each analyzer.........  Once per 2 weeks \5\.  Audit concentration    One-Point QC.
     CO.                              concentration 0.005-                                                   \1\ and measured
                                      0.08 ppm SO2, NO2,                                                     concentration.\2\.
                                      O3, and.
                                     0.5 and 5 ppm CO......
Annual performance evaluation for    See section 3.1.2 of    Each analyzer.........  Once per year........  Audit concentration    Annual PE.
 SO2, NO2, O3, CO.                    this appendix.                                                         \1\ and measured
                                                                                                             concentration \2\
                                                                                                             for each level.
NPAP for SO2, NO2, O3, CO..........  Independent Audit.....  20% of sites each year  Once per year........  Audit                  NPAP.
                                                                                                             concentration\1\ and
                                                                                                             measured
                                                                                                             concentration \2\
                                                                                                             for each level.
Particulate Methods:
    Continuous \4\ method--          Collocated samplers...  15%...................  1-in-12 days.........  Primary sampler        No Transaction
     collocated quality control                                                                              concentration and      reported as raw
     sampling PM2.5.                                                                                         duplicate sampler      data.
                                                                                                             concentration.\3\.
    Manual method--collocated        Collocated samplers...  15%...................  1-in-12 days.........  Primary sampler        No Transaction
     quality control sampling PM10,                                                                          concentration and      reported as raw
     PM2.5, Pb-TSP, Pb-PM10.                                                                                 duplicate sampler      data.
                                                                                                             concentration.\3\.
    Flow rate verification PM10      Check of sampler flow   Each sampler..........  Once every month \5\.  Audit flow rate and    Flow Rate
     (low Vol) PM2.5, Pb-PM10.        rate.                                                                  measured flow rate     Verification.
                                                                                                             indicated by the
                                                                                                             sampler.
    Flow rate verification PM10      Check of sampler flow   Each sampler..........  Once every quarter     Audit flow rate and    Flow Rate
     (High-Vol), Pb-TSP.              rate.                                           \5\.                   measured flow rate     Verification.
                                                                                                             indicated by the
                                                                                                             sampler.
    Semi-annual flow rate audit      Check of sampler flow   Each sampler..........  Once every 6 months    Audit flow rate and    Semi Annual Flow Rate
     PM10, TSP, PM10-2.5, PM2.5, Pb-  rate using                                      \5\.                   measured flow rate     Audit.
     TSP, Pb-PM10.                    independent standard.                                                  indicated by the
                                                                                                             sampler.
    Pb analysis audits Pb-TSP, Pb-   Check of analytical     Analytical............  Once each quarter \5\  Measured value and     Pb Analysis Audits.
     PM10.                            system with Pb audit                                                   audit value (ug Pb/
                                      strips/filters.                                                        filter) using AQS
                                                                                                             unit code 077.
    Performance Evaluation Program   Collocated samplers...  (1) 5 valid audits for  Distributed over all   Primary sampler        PEP.
     PM2.5.                                                   primary QA orgs, with   4 quarters \5\.        concentration and
                                                              <=5 sites.                                     performance
                                                             (2) 8 valid audits for                          evaluation sampler
                                                              primary QA orgs, with                          concentration.
                                                              >5 sites.
                                                             (3) All samplers in 6
                                                              years.
    Performance Evaluation Program   Collocated samplers...  (1) 1 valid audit and   Distributed over all   Primary sampler        PEP.
     Pb-TSP, Pb-PM10.                                         4 collocated samples    4 quarters \5\.        concentration and
                                                              for primary QA orgs,                           performance
                                                              with <=5 sites.                                evaluation sampler
                                                             (2) 2 valid audits and                          concentration.
                                                              6 collocated samples                           Primary sampler
                                                              for primary QA orgs                            concentration and
                                                              with >5 sites.                                 duplicate sampler
                                                                                                             concentration.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Effective concentration for open path analyzers.
\2\ Corrected concentration, if applicable for open path analyzers.
\3\ Both primary and collocated sampler values are reported as raw data.
\4\ PM2.5 is the only particulate criteria pollutant requiring collocation of continuous and manual primary monitors.
\5\ EPA's recommended maximum number of days that should exist between checks to ensure that the checks are routinely conducted over time and to limit
  data impacts resulting from a failed check.

* * * * *

0
27. Amend appendix B to part 58 by:
0
a. Revising section 2.6.1 and adding sections 2.6.1.1 and 2.6.1.2;
0
b. Removing and reserving section 3.1.2.2;

[[Page 16393]]

0
c. Revising sections 3.1.3.3 and 3.2.4;
0
d. Adding sections 3.2.4.1 through 3.2.4.3;
0
e. Revising sections 4.2.1, and 4.2.5; and
0
f. In section 6 revising References (1), (4), (6), (7), (9), (10), and 
(11) and table B-1.
    The revisions and additions read as follows:

Appendix B to Part 58--Quality Assurance Requirements for Prevention of 
Significant Deterioration (PSD) Air Monitoring

* * * * *
    2.6.1 Gaseous pollutant concentration standards (permeation devices 
or cylinders of compressed gas) used to obtain test concentrations for 
CO, SO2, NO, and NO2 must be EPA Protocol Gases 
certified in accordance with one of the procedures given in Reference 4 
of this appendix.
    2.6.1.1 The concentrations of EPA Protocol Gas standards used for 
ambient air monitoring must be certified with a 95-percent confidence 
interval to have an analytical uncertainty of no more than 2.0 percent (inclusive) of the certified concentration (tag 
value) of the gas mixture. The uncertainty must be calculated in 
accordance with the statistical procedures defined in Reference 4 of 
this appendix.
    2.6.1.2 Specialty gas producers advertising certification with the 
procedures provided in Reference 4 of this appendix and distributing 
gases as ``EPA Protocol Gas'' for ambient air monitoring purposes must 
adhere to the regulatory requirements specified in 40 CFR 75.21(g) or 
not use ``EPA'' in any form of advertising. The PSD PQAOs must provide 
information to the PSD reviewing authority on the specialty gas 
producers they use (or will use) for the duration of the PSD monitoring 
project. This information can be provided in the QAPP or monitoring 
plan but must be updated if there is a change in the specialty gas 
producers used.
* * * * *
    3.1.3.3 Using audit gases that are verified against the NIST 
standard reference methods or special review procedures and validated 
per the certification periods specified in Reference 4 of this appendix 
(EPA Traceability Protocol for Assay and Certification of Gaseous 
Calibration Standards) for CO, SO2, and NO2 and 
using O3 analyzers that are verified quarterly against a 
standard reference photometer.
* * * * *
    3.2.4 PM2.5 Performance Evaluation Program (PEP) 
Procedures. The PEP is an independent assessment used to estimate total 
measurement system bias. These evaluations will be performed under the 
NPEP as described in section 2.4 of this appendix or a comparable 
program. Performance evaluations will be performed annually within each 
PQAO. For PQAOs with less than or equal to five monitoring sites, five 
valid performance evaluation audits must be collected and reported each 
year. For PQAOs with greater than five monitoring sites, eight valid 
performance evaluation audits must be collected and reported each year. 
A valid performance evaluation audit means that both the primary 
monitor and PEP audit concentrations are valid and equal to or greater 
than 2 [micro]g/m3. Siting of the PEP monitor must be consistent with 
section 3.2.3.4(c) of this appendix. However, any horizontal distance 
greater than 4 meters and any vertical distance greater than one meter 
must be reported to the EPA regional PEP coordinator. Additionally for 
every monitor designated as a primary monitor, a primary quality 
assurance organization must:
    3.2.4.1 Have each method designation evaluated each year; and,
    3.2.4.2 Have all FRM and FEM samplers subject to a PEP audit at 
least once every 6 years, which equates to approximately 15 percent of 
the monitoring sites audited each year.
    3.2.4.3 Additional information concerning the PEP is contained in 
Reference 10 of this appendix. The calculations for evaluating bias 
between the primary monitor and the performance evaluation monitor for 
PM2.5 are described in section 4.2.5 of this appendix.
* * * * *
    4.2.1 Collocated Quality Control Sampler Precision Estimate for 
PM10, PM2.5, and Pb. Precision is estimated via 
duplicate measurements from collocated samplers. It is recommended that 
the precision be aggregated at the PQAO level quarterly, annually, and 
at the 3-year level. The data pair would only be considered valid if 
both concentrations are greater than or equal to the minimum values 
specified in section 4(c) of this appendix. For each collocated data 
pair, calculate ti, using equation 6 to this appendix:
[GRAPHIC] [TIFF OMITTED] TR06MR24.044

    Where Xi is the concentration from the primary sampler 
and Yi is the concentration value from the audit sampler. 
The coefficient of variation upper bound is calculated using equation 7 
to this appendix:
[GRAPHIC] [TIFF OMITTED] TR06MR24.045

    Where k is the number of valid data pairs being aggregated, and 
X\2\0.1,k-1 is the 10th percentile of a chi-squared 
distribution with k-1 degrees of freedom. The factor of 2 in the 
denominator adjusts for the fact that

[[Page 16394]]

each ti is calculated from two values with error.
* * * * *
    4.2.5 Performance Evaluation Programs Bias Estimate for 
PM2.5. The bias estimate is calculated using the PEP audits 
described in section 3.2.4. of this appendix. The bias estimator is 
based on, si, the absolute difference in concentrations 
divided by the square root of the PEP concentration.
[GRAPHIC] [TIFF OMITTED] TR06MR24.046

* * * * *

6. References

(1) American National Standard Institute--Quality Management Systems 
For Environmental Information And Technology Programs--Requirements 
With Guidance For Use. ASQ/ANSI E4-2014. February 2014. Available 
from ANSI Webstore https://webstore.ansi.org/.
* * * * *
(4) EPA Traceability Protocol for Assay and Certification of Gaseous 
Calibration Standards. EPA-600/R-12/531. May, 2012. Available from 
U.S. Environmental Protection Agency, National Risk Management 
Research Laboratory, Research Triangle Park NC 27711. https://www.epa.gov/nscep.
* * * * *
(6) List of Designated Reference and Equivalent Methods. Available 
from U.S. Environmental Protection Agency, Center for Environmental 
Measurements and Modeling, Air Methods and Characterization 
Division, MD-D205-03, Research Triangle Park, NC 27711. https://www.epa.gov/amtic/air-monitoring-methods-criteria-pollutants.
(7) Transfer Standards for the Calibration of Ambient Air Monitoring 
Analyzers for Ozone. EPA-454/B-13-004 U.S. Environmental Protection 
Agency, Research Triangle Park, NC 27711, October, 2013. https://www.epa.gov/sites/default/files/2020-09/documents/ozonetransferstandardguidance.pdf.
* * * * *
(9) Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume 1--A Field Guide to Environmental Quality Assurance. 
EPA-600/R-94/038a. April 1994. Available from U.S. Environmental 
Protection Agency, ORD Publications Office, Center for Environmental 
Research Information (CERI), 26 W. Martin Luther King Drive, 
Cincinnati, OH 45268. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance#documents.
(10) Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume II: Ambient Air Quality Monitoring Program Quality 
System Development. EPA-454/B-13-003. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance#documents.
(11) National Performance Evaluation Program Standard Operating 
Procedures. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance#npep.

              Table B-1 to Section 6 of Appendix B- Minimum Data Assessment Requirements for NAAQS Related Criteria Pollutant PSD Monitors
--------------------------------------------------------------------------------------------------------------------------------------------------------
               Method                  Assessment  method           Coverage           Minimum  frequency    Parameters  reported   AQS  Assessment type
--------------------------------------------------------------------------------------------------------------------------------------------------------
Gaseous Methods (CO, NO2, SO2, O3):
    One-Point QC for SO2, NO2, O3,   Response check at       Each analyzer.........  Once per 2 weeks\5\..  Audit                  One-Point QC.
     CO.                              concentration 0.005-                                                   concentration\1\ and
                                      0.08 ppm SO2, NO2,                                                     measured
                                      O3, & 0.5 and 5 ppm                                                    concentration\2\.
                                      CO.
    Quarterly performance            See section 3.1.2 of    Each analyzer.........  Once per quarter\5\..  Audit                  Annual PE.
     evaluation for SO2, NO2, O3,     this appendix.                                                         concentration\1\ and
     CO.                                                                                                     measured
                                                                                                             concentration\2\ for
                                                                                                             each level.
    NPAP for SO2, NO2, O3, CO\3\...  Independent Audit.....  Each primary monitor..  Once per year........  Audit                  NPAP.
                                                                                                             concentration\1\ and
                                                                                                             measured
                                                                                                             concentration\2\ for
                                                                                                             each level.
Particulate Methods:
    Collocated sampling PM10,        Collocated samplers...  1 per PSD Network per   Every 6 days or every  Primary sampler        No Transaction
     PM2.5, Pb.                                               pollutant.              3 days if daily        concentration and      reported as raw
                                                                                      monitoring required.   duplicate sampler      data.
                                                                                                             concentration\4\.
    Flow rate verification PM10,     Check of sampler flow   Each sampler..........  Once every month\5\..  Audit flow rate and    Flow Rate
     PM2.5, Pb.                       rate.                                                                  measured flow rate     Verification.
                                                                                                             indicated by the
                                                                                                             sampler.
    Semi-annual flow rate audit      Check of sampler flow   Each sampler..........  Once every 6 months    Audit flow rate and    Semi Annual Flow Rate
     PM10, PM2.5, Pb.                 rate using                                      or beginning, middle   measured flow rate     Audit.
                                      independent standard.                           and end of             indicated by the
                                                                                      monitoring\5\.         sampler.

[[Page 16395]]

 
    Pb analysis audits Pb-TSP, Pb-   Check of analytical     Analytical............  Each quarter\5\......  Measured value and     Pb Analysis Audits.
     PM10.                            system with Pb audit                                                   audit value (ug Pb/
                                      strips/filters.                                                        filter) using AQS
                                                                                                             unit code 077 for
                                                                                                             parameters:.
                                                                                                            14129--Pb (TSP) LC
                                                                                                             FRM/FEM.
                                                                                                            85129--Pb (TSP) LC
                                                                                                             Non-FRM/FEM..
    Performance Evaluation Program   Collocated samplers...  (1) 5 valid audits for  Over all 4             Primary sampler        PEP.
     PM2.5\3\.                                                PQAOs with <= 5         quarters\5\.           concentration and
                                                              sites..                                        performance
                                                             (2) 8 valid audits for                          evaluation sampler
                                                              PQAOs with > 5 sites..                         concentration.
                                                             (3) All samplers in 6
                                                              years.
    Performance Evaluation Program   Collocated samplers...  (1) 1 valid audit and   Over all 4             Primary sampler        PEP.
     Pb \3\.                                                  4 collocated samples    quarters\5\.           concentration and
                                                              for PQAOs, with <=5                            performance
                                                              sites..                                        evaluation sampler
                                                             (2) 2 valid audits and                          concentration.
                                                              6 collocated samples                           Primary sampler
                                                              for PQAOs with >5                              concentration and
                                                              sites..                                        duplicate sampler
                                                                                                             concentration.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Effective concentration for open path analyzers.
\2\ Corrected concentration, if applicable for open path analyzers.
\3\ NPAP, PM2.5, PEP, and Pb-PEP must be implemented if data is used for NAAQS decisions otherwise implementation is at PSD reviewing authority
  discretion.
\4\ Both primary and collocated sampler values are reported as raw data
\5\ A maximum number of days should be between these checks to ensure the checks are routinely conducted over time and to limit data impacts resulting
  from a failed check.


0
28. Amend appendix C to part 58 by:
0
a. Adding sections 2.2 and 2.2.1 through 2.2.19;
0
b. Removing and reserving sections 2.4, 2.4.1;
0
c. Removing sections 2.4.1.1 through 2.4.1.7; and
0
d. Revising section 2.7.1.
    The additions and revision reads as follows:

Appendix C to Part 58--Ambient Air Quality Monitoring Methodology

* * * * *
    2.2 PM10, PM2.5, or PM10-2.5 
continuous FEMs with existing valid designations may be calibrated 
using network data from collocated FRM and continuous FEM data under 
the following provisions:
    2.2.1 Data to demonstrate a calibration may include valid data 
from State, local, or Tribal air agencies or data collected by 
instrument manufacturers in accordance with 40 CFR 53.35 or other 
data approved by the Administrator.
    2.2.2 A request to update a designated methods calibration may 
be initiated by the instrument manufacturer of record or the EPA 
Administrator. State, local, Tribal, and multijusistincional 
organizations of these entities may work with an instrument 
manufacture to update a designated method calibration.
    2.2.3 Requests for approval of an updated PM10, 
PM2.5, or PM10-2.5 continuous FEM calibration 
must meet the general submittal requirements of section 2.7 of this 
appendix.
    2.2.4 Data included in the request should represent a subset of 
representative locations where the method is operational. For cases 
with a small number of collocated FRMs and continuous FEMs sites, an 
updated candidate calibration may be limited to the sites where both 
methods are in use.
    2.2.5 Data included in a candidate method updated calibration 
may include a subset of sites where there is a large grouping of 
sites in one part of the country such that the updated calibration 
would be representative of the country as a whole.
    2.2.6 Improvements should be national in scope and ideally 
implemented through a firmware change.
    2.2.7 The goal of a change to a methods calibration is to 
increase the number of sites meeting measurements quality objectives 
of the method as identified in section 2.3.1.1 of appendix A to this 
part.
    2.2.8 For meeting measurement quality objectives (MQOs), the 
primary objective is to meet the bias goal as this statistic will 
likely have the most influence on improving the resultant data 
collected.
    2.2.9 Precision data are to be included, but so long as 
precision data are at least as good as existing network data or meet 
the MQO referenced in section 2.2.8 of this appendix, no further 
work is necessary with precision.
    2.2.10 Data available to use may include routine primary and 
collocated data.
    2.2.11 Audit data may be useful to confirm the performance of a 
candidate updated calibration but should not be used as the basis of 
the calibration to keep the independence of the audit data.
    2.2.12 Data utilized as the basis of the updated calibration may 
be obtained by accessing EPA's AQS database or future analogous EPA 
database.
    2.2.13 Years of data to use in a candidate method calibration 
should include two recent years where we are past the certification 
period for the previous year's data, which is May 1 of each year.
    2.2.14 Data from additional years is to be used to test an 
updated calibration such that the calibration is independent of the 
test years of interest. Data from these additional years need to 
minimally demonstrate that a larger number of sites are expected to 
meet bias MQO especially at sites near the level of the NAAQS for 
the PM indicator of interest.
    2.2.15 Outliers may be excluded using routine outlier tests.
    2.2.16 The range of data used in a calibration may include all 
data available or alternatively use data in the range from the 
lowest measured data available up to 125% of the 24-hour NAAQS for 
the PM indicator of interest.

[[Page 16396]]

    2.2.17 Other improvements to a PM continuous method may be 
included as part of a recommended update so long as appropriate 
testing is conducted with input from EPA's Office of Research and 
Development (ORD) Reference and Equivalent (R&E) Methods Designation 
program.
    2.2.18 EPA encourages early communication by instrument 
manufacturers considering an update to a PM method. Instrument 
companies should initiate such dialogue by contacting EPA's ORD R&E 
Methods Designation program. The contact information for this can be 
found at 40 CFR 53.4.
    2.2.19 Manufacturers interested in improving instrument's 
performance through an updated factory calibration must submit a 
written modification request to EPA with supporting rationale. 
Because the testing requirements and acceptance criteria of any 
field and/or lab tests can depend upon the nature and extent of the 
intended modification, applicants should contact EPA's R&E Methods 
Designation program for guidance prior to development of the 
modification request.
* * * * *
    2.7.1 Requests for approval under sections 2.2, 2.4, 2.6.2, or 
2.8 of this appendix must be submitted to: Director, Center for 
Environmental Measurement and Modeling, Reference and Equivalent 
Methods Designation Program (MD-D205-03), U.S. Environmental 
Protection Agency, P.O. Box 12055, Research Triangle Park, North 
Carolina 27711.

0
29. Amend appendix D to part 58 by revising sections 1 and 1.1(b), the 
introductory text before the table in section 4.7.1(a), and sections 
4.7.1(b)(3) and 4.7.2 to read as follows:

Appendix D to Part 58--Network Design Criteria for Ambient Air Quality 
Monitoring

* * * * *

1. Monitoring Objectives and Spatial Scales

    The purpose of this appendix is to describe monitoring 
objectives and general criteria to be applied in establishing the 
required SLAMS ambient air quality monitoring stations and for 
choosing general locations for additional monitoring sites. This 
appendix also describes specific requirements for the number and 
location of FRM and FEM sites for specific pollutants, NCore 
multipollutant sites, PM10 mass sites, PM2.5 
mass sites, chemically-speciated PM2.5 sites, and 
O3 precursor measurements sites (PAMS). These criteria 
will be used by EPA in evaluating the adequacy of the air pollutant 
monitoring networks.
    1.1 * * *
    (b) Support compliance with ambient air quality standards and 
emissions strategy development. Data from FRM and FEM monitors for 
NAAQS pollutants will be used for comparing an area's air pollution 
levels against the NAAQS. Data from monitors of various types can be 
used in the development of attainment and maintenance plans. SLAMS, 
and especially NCore station data, will be used to evaluate the 
regional air quality models used in developing emission strategies, 
and to track trends in air pollution abatement control measures' 
impact on improving air quality. In monitoring locations near major 
air pollution sources, source-oriented monitoring data can provide 
insight into how well industrial sources are controlling their 
pollutant emissions.
* * * * *
    4.7.1 * * *
    (a) State and where applicable, local, agencies must operate the 
minimum number of required PM2.5 SLAMS sites listed in 
table D-5 to this appendix. The NCore sites are expected to 
complement the PM2.5 data collection that takes place at 
non-NCore SLAMS sites, and both types of sites can be used to meet 
the minimum PM2.5 network requirements. For many State 
and local networks, the total number of PM2.5 sites 
needed to support the basic monitoring objectives of providing air 
pollution data to the general public in a timely manner, support 
compliance with ambient air quality standards and emission strategy 
development, and support for air pollution research studies will 
include more sites than the minimum numbers required in table D-5 to 
this appendix. Deviations from these PM2.5 monitoring 
requirements must be approved by the EPA Regional Administrator.
* * * * *
    (b) * * *
    (3) For areas with additional required SLAMS, a monitoring 
station is to be sited in an at-risk community with poor air 
quality, particularly where there are anticipated effects from 
sources in the area (e.g., a major industrial area, point source(s), 
port, rail yard, airport, or other transportation facility or 
corridor).
* * * * *
    4.7.2 Requirement for Continuous PM2.5 Monitoring. 
The State, or where appropriate, local agencies must operate 
continuous PM2.5 analyzers equal to at least one-half 
(round up) the minimum required sites listed in table D-5 to this 
appendix. At least one required continuous analyzer in each MSA must 
be collocated with one of the required FRM/FEM monitors, unless at 
least one of the required FRM/FEM monitors is itself a continuous 
FEM monitor in which case no collocation requirement applies. State 
and local air monitoring agencies must use methodologies and quality 
assurance/quality control (QA/QC) procedures approved by the EPA 
Regional Administrator for these required continuous analyzers.
* * * * *

0
30. Revise appendix E to part 58 to read as follows:

Appendix E to Part 58--Probe and Monitoring Path Siting Criteria for 
Ambient Air Quality Monitoring

1. Introduction
2. Monitors and Samplers with Probe Inlets
3. Open Path Analyzers
4. Waiver Provisions
5. References

1. Introduction

1.1 Applicability

    (a) This appendix contains specific location criteria applicable 
to ambient air quality monitoring probes, inlets, and optical paths 
of SLAMS, NCore, PAMS, and other monitor types whose data are 
intended to be used to determine compliance with the NAAQS. These 
specific location criteria are relevant after the general location 
has been selected based on the monitoring objectives and spatial 
scale of representation discussed in appendix D to this part. 
Monitor probe material and sample residence time requirements are 
also included in this appendix. Adherence to these siting criteria 
is necessary to ensure the uniform collection of compatible and 
comparable air quality data.
    (b) The probe and monitoring path siting criteria discussed in 
this appendix must be followed to the maximum extent possible. It is 
recognized that there may be situations where some deviation from 
the siting criteria may be necessary. In any such case, the reasons 
must be thoroughly documented in a written request for a waiver that 
describes whether the resulting monitoring data will be 
representative of the monitoring area and how and why the proposed 
or existing siting must deviate from the criteria. This 
documentation should help to avoid later questions about the 
validity of the resulting monitoring data. Conditions under which 
the EPA would consider an application for waiver from these siting 
criteria are discussed in section 4 of this appendix.
    (c) The pollutant-specific probe and monitoring path siting 
criteria generally apply to all spatial scales except where noted 
otherwise. Specific siting criteria that are phrased with ``shall'' 
or ``must'' are defined as requirements and exceptions must be 
granted through the waiver provisions. However, siting criteria that 
are phrased with ``should'' are defined as goals to meet for 
consistency but are not requirements.

2. Monitors and Samplers with Probe Inlets

2.1 Horizontal and Vertical Placement

    (a) For O3 and SO2 monitoring, and for 
neighborhood or larger spatial scale Pb, PM10, 
PM10-2.5, PM2.5, NO2, and CO sites, 
the probe must be located greater than or equal to 2.0 meters and 
less than or equal to 15 meters above ground level.
    (b) Middle scale CO and NO2 monitors must have 
sampler inlets greater than or equal to 2.0 meters and less than or 
equal to 15 meters above ground level.
    (c) Middle scale PM10-2.5 sites are required to have 
sampler inlets greater than or equal to 2.0 meters and less than or 
equal to 7.0 meters above ground level.
    (d) Microscale Pb, PM10, PM10-2.5, and 
PM2.5 sites are required to have sampler inlets greater 
than or equal to 2.0 meters and less than or equal to 7.0 meters 
above ground level.
    (e) Microscale near-road NO2 monitoring sites are 
required to have sampler inlets greater than or equal to 2.0 meters 
and less than or equal to 7.0 meters above ground level.
    (f) The probe inlets for microscale carbon monoxide monitors 
that are being used to

[[Page 16397]]

measure concentrations near roadways must be greater than or equal 
to 2.0 meters and less than or equal to 7.0 meters above ground 
level. Those probe inlets for microscale carbon monoxide monitors 
measuring concentrations near roadways in downtown areas or urban 
street canyons must be greater than or equal to 2.5 meters and less 
than or equal to 3.5 meters above ground level. The probe must be at 
least 1.0 meter vertically or horizontally away from any supporting 
structure, walls, parapets, penthouses, etc., and away from dusty or 
dirty areas. If the probe is located near the side of a building or 
wall, then it should be located on the windward side of the building 
relative to the prevailing wind direction during the season of 
highest concentration potential for the pollutant being measured.

2.2 Spacing From Minor Sources

    (a) It is important to understand the monitoring objective for a 
particular site in order to interpret this requirement. Local minor 
sources of a primary pollutant, such as SO2, lead, or 
particles, can cause high concentrations of that particular 
pollutant at a monitoring site. If the objective for that monitoring 
site is to investigate these local primary pollutant emissions, then 
the site will likely be properly located nearby. This type of 
monitoring site would, in all likelihood, be a microscale-type of 
monitoring site. If a monitoring site is to be used to determine air 
quality over a much larger area, such as a neighborhood or city, a 
monitoring agency should avoid placing a monitor probe inlet near 
local, minor sources, because a plume from a local minor source 
should not be allowed to inappropriately impact the air quality data 
collected at a site. Particulate matter sites should not be located 
in an unpaved area unless there is vegetative ground cover year-
round, so that the impact of windblown dusts will be kept to a 
minimum.
    (b) Similarly, local sources of nitric oxide (NO) and ozone-
reactive hydrocarbons can have a scavenging effect causing 
unrepresentatively low concentrations of O3 in the 
vicinity of probes for O3. To minimize these potential 
interferences from nearby minor sources, the probe inlet should be 
placed at a distance from furnace or incineration flues or other 
minor sources of SO2 or NO. The separation distance 
should take into account the heights of the flues, type of waste or 
fuel burned, and the sulfur content of the fuel.

2.3 Spacing From Obstructions

    (a) Obstacles may scavenge SO2, O3, or 
NO2, and can act to restrict airflow for any pollutant. 
To avoid this interference, the probe inlet must have unrestricted 
airflow pursuant to paragraph (b) of this section and should be 
located at a distance from obstacles. The horizontal distance from 
the obstacle to the probe inlet must be at least twice the height 
that the obstacle protrudes above the probe inlet. An obstacle that 
does not meet the minimum distance requirement is considered an 
obstruction that restricts airflow to the probe inlet. The EPA does 
not generally consider objects or obstacles such as flag poles or 
site towers used for NOy convertors and meteorological sensors, etc. 
to be deemed obstructions.
    (b) A probe inlet located near or along a vertical wall is 
undesirable because air moving along the wall may be subject to 
removal mechanisms. A probe inlet must have unrestricted airflow 
with no obstructions (as defined in paragraph (a) of this section) 
in a continuous arc of at least 270 degrees. An unobstructed 
continuous arc of 180 degrees is allowable when the applicable 
network design criteria specified in appendix D of this part require 
monitoring in street canyons and the probe is located on the side of 
a building. This arc must include the predominant wind direction for 
the season of greatest pollutant concentration potential. For 
particle sampling, there must be a minimum of 2.0 meters of 
horizontal separation from walls, parapets, and structures for 
rooftop site placement.
    (c) A sampling station with a probe inlet located closer to an 
obstacle than required by the criteria in this section should be 
classified as middle scale or microscale, rather than neighborhood 
or urban scale, since the measurements from such a station would 
more closely represent these smaller scales.
    (d) For near-road monitoring stations, the monitor probe shall 
have an unobstructed air flow, where no obstacles exist at or above 
the height of the monitor probe, between the monitor probe and the 
outside nearest edge of the traffic lanes of the target road 
segment.

2.4 Spacing From Trees

    (a) Trees can provide surfaces for SO2, 
O3, or NO2 adsorption or reactions and 
surfaces for particle deposition. Trees can also act as obstructions 
in locations where the trees are between the air pollutant sources 
or source areas and the monitoring site and where the trees are of a 
sufficient height and leaf canopy density to interfere with the 
normal airflow around the probe inlet. To reduce this possible 
interference/obstruction, the probe inlet should be 20 meters or 
more from the drip line of trees and must be at least 10 meters from 
the drip line of trees. If a tree or group of trees is an obstacle, 
the probe inlet must meet the distance requirements of section 2.3 
of this appendix.
    (b) The scavenging effect of trees is greater for O3 
than for other criteria pollutants. Monitoring agencies must take 
steps to consider the impact of trees on ozone monitoring sites and 
take steps to avoid this problem.
    (c) Beginning January 1, 2024, microscale sites of any air 
pollutant shall have no trees or shrubs located at or above the 
line-of-sight fetch between the probe and the source under 
investigation, e.g., a roadway or a stationary source.

2.5 Spacing From Roadways

   Table E-1 to Section 2.5 of Appendix E--Minimum Separation Distance
 Between Roadways and Probes for Monitoring Neighborhood and Urban Scale
          Ozone (O3) and Oxides of Nitrogen (NO, NO2, NOX, NOy)
------------------------------------------------------------------------
                                                               Minimum
  Roadway average daily traffic, vehicles per     Minimum     distance\1
                      day                        distance\1      2 3\
                                                3\ (meters)    (meters)
------------------------------------------------------------------------
<=1,000.......................................           10           10
10,000........................................           10           20
15,000........................................           20           30
20,000........................................           30           40
40,000........................................           50           60
70,000........................................          100          100
>=110,000.....................................          250          250
------------------------------------------------------------------------
\1\ Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table
  values based on the actual traffic count./TNOTE>
\2\ Applicable for ozone monitors whose placement was not approved as of
  December 18, 2006.
\3\ All distances listed are expressed as having 2 significant figures.
  When rounding is performed to assess compliance with these siting
  requirements, the distance measurements will be rounded such as to
  retain at least two significant figures.

2.5.1 Spacing for Ozone Probes

    In siting an O3 monitor, it is important to minimize 
destructive interferences from sources of NO, since NO readily 
reacts with O3. Table E-1 of this appendix provides the 
required minimum separation distances between a roadway and a probe 
inlet for various ranges of daily roadway traffic. A sampling site 
with a monitor probe located closer to a roadway than allowed by the 
Table E-1 requirements should be classified as middle scale or 
microscale, rather than neighborhood or urban scale, since the 
measurements from such a site would more closely represent these 
smaller scales.

2.5.2 Spacing for Carbon Monoxide Probes

    (a) Near-road microscale CO monitoring sites, including those 
located in downtown areas, urban street canyons, and other near-road 
locations such as those adjacent to highly trafficked roads, are 
intended to provide a measurement of the influence of the immediate 
source on the pollution exposure on the adjacent area.
    (b) Microscale CO monitor probe inlets in downtown areas or 
urban street canyon locations shall be located a minimum distance of 
2.0 meters and a maximum distance of 10 meters from the edge of the 
nearest traffic lane.
    (c) Microscale CO monitor probe inlets in downtown areas or 
urban street canyon locations shall be located at least 10 meters 
from an intersection, preferably at a midblock location. Midblock 
locations are preferable to intersection locations because 
intersections represent a much smaller portion of downtown space 
than do the streets between

[[Page 16398]]

them. Pedestrian exposure is probably also greater in street canyon/
corridors than at intersections.
    (d) Neighborhood scale CO monitor probe inlets in downtown areas 
or urban street canyon locations shall be located according to the 
requirements in Table E-2 of this appendix.

  Table E-2 to Section 2.5.2 of Appendix E--Minimum Separation Distance
  Between Roadways and Probes for Monitoring Neighborhood Scale Carbon
                                Monoxide
------------------------------------------------------------------------
                                                   Minimum distance 1 2
Roadway average daily traffic, vehicles per day          (meters)
------------------------------------------------------------------------
<=10,000.......................................                       10
15,000.........................................                       25
20,000.........................................                       45
30,000.........................................                       80
40,000.........................................                      115
50,000.........................................                      135
>=60,000.......................................                      150
------------------------------------------------------------------------
1 Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table
  values based on the actual traffic count.
2 All distances listed are expressed as having 2 significant figures.
  When rounding is performed to assess compliance with these siting
  requirements, the distance measurements will be rounded such as to
  retain at least two significant figures.

2.5.3 Spacing for Particulate Matter (PM2.5, PM2.5	10, PM10, Pb) 
Inlets

    (a) Since emissions associated with the operation of motor 
vehicles contribute to urban area particulate matter ambient levels, 
spacing from roadway criteria are necessary for ensuring national 
consistency in PM sampler siting.
    (b) The intent is to locate localized hot-spot sites in areas of 
highest concentrations, whether it be caused by mobile or multiple 
stationary sources. If the area is primarily affected by mobile 
sources and the maximum concentration area(s) is judged to be a 
traffic corridor or street canyon location, then the monitors should 
be located near roadways with the highest traffic volume and at 
separation distances most likely to produce the highest 
concentrations. For microscale traffic corridor sites, the location 
must be greater than or equal 5.0 meters and less than or equal to 
15 meters from the major roadway. For the microscale street canyon 
site, the location must be greater than or equal 2.0 meters and less 
than or equal to 10 meters from the roadway. For the middle scale 
site, a range of acceptable distances from the roadway is shown in 
Figure E-1 of this appendix. This figure also includes separation 
distances between a roadway and neighborhood or larger scale sites 
by default. Any PM probe inlet at a site, 2.0 to 15 meters high, and 
further back than the middle scale requirements will generally be 
neighborhood, urban or regional scale. For example, according to 
Figure E-1 of this appendix, if a PM sampler is primarily influenced 
by roadway emissions and that sampler is set back 10 meters from a 
30,000 ADT (average daily traffic) road, the site should be 
classified as microscale, if the sampler's inlet height is between 
2.0 and 7.0 meters. If the sampler's inlet height is between 7.0 and 
15 meters, the site should be classified as middle scale. If the 
sampler is 20 meters from the same road, it will be classified as 
middle scale; if 40 meters, neighborhood scale; and if 110 meters, 
an urban scale.
[GRAPHIC] [TIFF OMITTED] TR06MR24.047

2.5.4 Spacing for Nitrogen Dioxide (NO2) Probes

    (a) In siting near-road NO2 monitors as required in 
section 4.3.2 of appendix D of this part, the monitor probe shall be 
as near as practicable to the outside nearest edge of the traffic 
lanes of the target road segment but shall not be located at a 
distance greater than 50 meters, in the horizontal, from the outside 
nearest edge of the traffic lanes of the target road segment. Where 
possible, the near-road NO2 monitor probe should be 
within 20 meters of the target road segment.
    (b) In siting NO2 monitors for neighborhood and 
larger scale monitoring, it is important to minimize near-road 
influences. Table E-1 of this appendix provides the required minimum 
separation distances between a roadway and a probe inlet for various 
ranges of daily roadway traffic. A site with a monitor probe located 
closer to a roadway than allowed by the Table E-1 requirements 
should be classified

[[Page 16399]]

as microscale or middle scale rather than neighborhood or urban 
scale.

2.6 Probe Material and Pollutant Sampler Residence Time

    (a) For the reactive gases (SO2, NO2, and 
O3), approved probe materials must be used for monitors. 
Studies25 34 have been conducted to determine the 
suitability of materials such as polypropylene, polyethylene, 
polyvinyl chloride, Tygon[supreg], aluminum, brass, stainless steel, 
copper, borosilicate glass, polyvinylidene fluoride (PVDF), 
polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), and 
fluorinated ethylene propylene (FEP) for use as intake sampling 
lines. Of the above materials, only borosilicate glass, PVDF, PTFE, 
PFA, and FEP have been found to be acceptable for use as intake 
sampling lines for all the reactive gaseous pollutants. Furthermore, 
the EPA \25\ has specified borosilicate glass, FEP Teflon[supreg], 
or their equivalents as the only acceptable probe materials for 
delivering test atmospheres in the determination of reference or 
equivalent methods. Therefore, borosilicate glass, PVDF, PTFE, PFA, 
FEP, or their equivalents must be the only material in the sampling 
train (from probe inlet to the back of the monitor) that can be in 
contact with the ambient air sample for reactive gas monitors. 
Nafion\TM\, which is composed primarily of PTFE, can be considered 
equivalent to PTFE; it has been shown in tests to exhibit virtually 
no loss of ozone at 20-second residence times.\35\
    (b) For volatile organic compound (VOC) monitoring at PAMS, FEP 
Teflon[supreg] is unacceptable as the probe material because of VOC 
adsorption and desorption reactions on the FEP Teflon[supreg]. 
Borosilicate glass, stainless steel, or their equivalents are the 
acceptable probe materials for VOC and carbonyl sampling. Care must 
be taken to ensure that the sample residence time is kept to 20 
seconds or less.
    (c) No matter how nonreactive the sampling probe material is 
initially, after a period of use, reactive particulate matter is 
deposited on the probe walls. Therefore, the time it takes the gas 
to transfer from the probe inlet to the sampling device is critical. 
Ozone in the presence of nitrogen oxide (NO) will show significant 
losses, even in the most inert probe material, when the residence 
time exceeds 20 seconds.\26\ Other studies 27 28indicate 
that a 10-second or less residence time is easily achievable. 
Therefore, sampling probes for all reactive gas monitors for 
SO2, NO2, and O3 must have a sample 
residence time less than 20 seconds.

2.7 Summary

    Table E-3 of this appendix presents a summary of the general 
requirements for probe siting criteria with respect to distances and 
heights. Table E-3 requires different elevation distances above the 
ground for the various pollutants. The discussion in this appendix 
for each of the pollutants describes reasons for elevating the 
monitor or probe inlet. The differences in the specified range of 
heights are based on the vertical concentration gradients. For 
source oriented and near-road monitors, the gradients in the 
vertical direction are very large for the microscale, so a small 
range of heights are used. The upper limit of 15 meters is specified 
for the consistency between pollutants and to allow the use of a 
single manifold for monitoring more than one pollutant.

                    Table E-3 to Section 2.7 of Appendix E--Summary of Probe Siting Criteria
----------------------------------------------------------------------------------------------------------------
                                                                   Horizontal or     Distance
                                                       Height    vertical distance  from drip
                                                        from      from supporting    line of     Distance from
           Pollutant                  Scale \9\      ground to  structures \1\ \8\   trees to  roadways to probe
                                                     probe \8\    to probe inlet    probe \8\     \8\ (meters)
                                                      (meters)       (meters)        (meters)
----------------------------------------------------------------------------------------------------------------
SO2\2 3 4 5\...................  Middle,                2.0-15               >=1.0       >=10  N/A.
                                  Neighborhood,
                                  Urban, and
                                  Regional.
 CO3 4 6.......................  Micro [downtown or    2.5-3.5                                 2.0-10 for
                                  street canyon                                                 downtown areas
                                  sites].                                                       or street canyon
                                                                                                microscale.
                                 Micro [Near-Road      2.0-7.0               >=1.0       >=10  <=50 for near-
                                  sites].                                                       road microscale.
                                 Middle and             2.0-15                                 See Table E-2 of
                                  Neighborhood.                                                 this appendix
                                                                                                for middle and
                                                                                                neighborhood
                                                                                                scales.
O32 3 4........................  Middle,                2.0-15               >=1.0       >=10  See Table E-1.
                                  Neighborhood,
                                  Urban, and
                                  Regional.
                                 Micro.............    2.0-7.0                                 <=50 for near-
                                                                                                road micro-
                                                                                                scale.
NO22 3 4.......................  Middle,                2.0-15               >=1.0       >=10  See Table E-1.
                                  Neighborhood,
                                  Urban, and
                                  Regional.
PAMS2 3 4 Ozone precursors.....  Neighborhood and       2.0-15               >=1.0       >=10  See Table E-1.
                                  Urban.
PM, Pb 2 3 4 7.................  Micro.............    2.0-7.0
                                 Middle,                2.0-15   >=2.0 (horizontal       >=10  See Figure E-1.
                                  Neighborhood,                     distance only)
                                  Urban and
                                  Regional.
----------------------------------------------------------------------------------------------------------------
N/A--Not applicable.
\1\ When a probe is located on a rooftop, this separation distance is in reference to walls, parapets, or
  penthouses located on the roof.
\2\ Should be greater than 20 meters from the dripline of tree(s) and must be 10 meters from the dripline.
\3\ Distance from sampler or probe inlet to obstacle, such as a building, must be at least twice the height the
  obstacle protrudes above the sampler or probe inlet. Sites not meeting this criterion may be classified as
  microscale or middle scale (see paragraphs 2.3(a) and 2.3(c)).
\4\ Must have unrestricted airflow in a continuous arc of at least 270 degrees around the probe or sampler; 180
  degrees if the probe is on the side of a building or a wall for street canyon monitoring.
\5\ The probe or sampler should be away from minor sources, such as furnace or incineration flues. The
  separation distance is dependent on the height of the minor source emission point(s), the type of fuel or
  waste burned, and the quality of the fuel (sulfur, ash, or lead content). This criterion is designed to avoid
  undue influences from minor sources.
\6\ For microscale CO monitoring sites, the probe must be >=10 meters from a street intersection and preferably
  at a midblock location.
\7\ Collocated monitor inlets must be within 4.0 meters of each other and at least 2.0 meters apart for flow
  rates greater than 200 liters/min or at least 1.0 meter apart for samplers having flow rates less than 200
  liters/min to preclude airflow interference, unless a waiver has been granted by the Regional Administrator
  pursuant to paragraph 3.3.4.2(c) of appendix A of part 58. For PM2.5, collocated monitor inlet heights should
  be within 1.0 meter of each other vertically.
\8\ All distances listed are expressed as having 2 significant figures. When rounding is performed to assess
  compliance with these siting requirements, the distance measurements will be rounded such as to retain at
  least two significant figures.
\9\ See section 1.2 of appendix D for definitions of monitoring scales.

3. Open Path Analyzers

3.1 Horizontal and Vertical Placement

    (a) For all O3 and SO2 monitoring sites 
and for neighborhood or larger spatial scale NO2, and CO 
sites, at least 80 percent of the monitoring path must be located 
greater than or equal 2.0 meters and less than or equal to 15 meters 
above ground level.
    (b) Middle scale CO and NO2 sites must have 
monitoring paths greater than or equal 2.0 meters and less than or 
equal to 15 meters above ground level.
    (c) Microscale near-road monitoring sites are required to have 
monitoring paths greater than or equal 2.0 meters and less than or 
equal to 7.0 meters above ground level.
    (d) For microscale carbon monoxide monitors that are being used 
to measure concentrations near roadways, the monitoring path must be 
greater than or equal 2.0 meters and less than or equal to 7.0 
meters above ground level. If the microscale carbon monoxide 
monitors measuring concentrations near roadways are in downtown 
areas or urban street canyons, the monitoring path must be greater 
than or equal 2.5 meters and less than or equal to 3.5 meters above 
ground level and at least 90

[[Page 16400]]

percent of the monitoring path must be at least 1.0 meter vertically 
or horizontally away from any supporting structure, walls, parapets, 
penthouses, etc., and away from dusty or dirty areas. If a 
significant portion of the monitoring path is located near the side 
of a building or wall, then it should be located on the windward 
side of the building relative to the prevailing wind direction 
during the season of highest concentration potential for the 
pollutant being measured.

3.2 Spacing From Minor Sources

    (a) It is important to understand the monitoring objective for a 
particular site in order to interpret this requirement. Local minor 
sources of a primary pollutant, such as SO2 can cause 
high concentrations of that particular pollutant at a monitoring 
site. If the objective for that monitoring site is to investigate 
these local primary pollutant emissions, then the site will likely 
be properly located nearby. This type of monitoring site would, in 
all likelihood, be a microscale type of monitoring site. If a 
monitoring site is to be used to determine air quality over a much 
larger area, such as a neighborhood or city, a monitoring agency 
should avoid placing a monitoring path near local, minor sources, 
because a plume from a local minor source should not be allowed to 
inappropriately impact the air quality data collected at a site.
    (b) Similarly, local sources of nitric oxide (NO) and ozone-
reactive hydrocarbons can have a scavenging effect causing 
unrepresentatively low concentrations of O3 in the 
vicinity of monitoring paths for O3. To minimize these 
potential interferences from nearby minor sources, at least 90 
percent of the monitoring path should be at a distance from furnace 
or incineration flues or other minor sources of SO2 or 
NO. The separation distance should take into account the heights of 
the flues, type of waste or fuel burned, and the sulfur content of 
the fuel.

3.3 Spacing From Obstructions

    (a) Obstacles may scavenge SO2, O3, or 
NO2, and can act to restrict airflow for any pollutant. 
To avoid this interference, at least 90 percent of the monitoring 
path must have unrestricted airflow and should be located at a 
distance from obstacles. The horizontal distance from the obstacle 
to the monitoring path must be at least twice the height that the 
obstacle protrudes above the monitoring path. An obstacle that does 
not meet the minimum distance requirement is considered an 
obstruction that restricts airflow to the monitoring path. The EPA 
does not generally consider objects or obstacles such as flag poles 
or site towers used for NOy convertors and meteorological sensors, 
etc. to be deemed obstructions.
    (b) A monitoring path located near or along a vertical wall is 
undesirable because air moving along the wall may be subject to 
removal mechanisms. At least 90 percent of the monitoring path for 
open path analyzers must have unrestricted airflow with no 
obstructions (as defined in paragraph (a) of this section) in a 
continuous arc of at least 270 degrees. An unobstructed continuous 
arc of 180 degrees is allowable when the applicable network design 
criteria specified in appendix D of this part require monitoring in 
street canyons and the monitoring path is located on the side of a 
building. This arc must include the predominant wind direction for 
the season of greatest pollutant concentration potential.
    (c) Special consideration must be given to the use of open path 
analyzers given their inherent potential sensitivity to certain 
types of interferences and optical obstructions. A monitoring path 
must be clear of all trees, brush, buildings, plumes, dust, or other 
optical obstructions, including potential obstructions that may move 
due to wind, human activity, growth of vegetation, etc. Temporary 
optical obstructions, such as rain, particles, fog, or snow, should 
be considered when siting an open path analyzer. Any of these 
temporary obstructions that are of sufficient density to obscure the 
light beam will negatively affect the ability of the open path 
analyzer to continuously measure pollutant concentrations. 
Transient, but significant obscuration of especially longer 
measurement paths, could occur as a result of certain meteorological 
conditions (e.g., heavy fog, rain, snow) and/or aerosol levels that 
are of a sufficient density to prevent the open path analyzer's 
light transmission. If certain compensating measures are not 
otherwise implemented at the onset of monitoring (e.g., shorter path 
lengths, higher light source intensity), data recovery during 
periods of greatest primary pollutant potential could be 
compromised. For instance, if heavy fog or high particulate levels 
are coincident with periods of projected NAAQS-threatening pollutant 
potential, the representativeness of the resulting data record in 
reflecting maximum pollution concentrations may be substantially 
impaired despite the fact that the site may otherwise exhibit an 
acceptable, even exceedingly high, overall valid data capture rate.
    (d) A sampling station with a monitoring path located closer to 
an obstacle than required by the criteria in this section should be 
classified as middle scale or microscale, rather than neighborhood 
or urban scale, since the measurements from such a station would 
more closely represent these smaller scales.
    (e) For near-road monitoring stations, the monitoring path shall 
have an unobstructed air flow, where no obstacles exist at or above 
the height of the monitoring path, between the monitoring path and 
the outside nearest edge of the traffic lanes of the target road 
segment.

3.4 Spacing From Trees

    (a) Trees can provide surfaces for SO2, 
O3, or NO2 adsorption or reactions. Trees can 
also act as obstructions in locations where the trees are located 
between the air pollutant sources or source areas and the monitoring 
site, and where the trees are of a sufficient height and leaf canopy 
density to interfere with the normal airflow around the monitoring 
path. To reduce this possible interference/obstruction, at least 90 
percent of the monitoring path should be 20 meters or more from the 
drip line of trees and must be at least 10 meters from the drip line 
of trees. If a tree or group of trees could be considered an 
obstacle, the monitoring path must meet the distance requirements of 
section 3.3 of this appendix.
    (b) The scavenging effect of trees is greater for O3 
than for other criteria pollutants. Monitoring agencies must take 
steps to consider the impact of trees on ozone monitoring sites and 
take steps to avoid this problem.
    (c) Beginning January 1, 2024, microscale sites of any air 
pollutant shall have no trees or shrubs located at or above the 
line-of-sight fetch between the monitoring path and the source under 
investigation, e.g., a roadway or a stationary source.
    3.5 Spacing from Roadways

   Table E-4 of Section 3.5 of Appendix E--Minimum Separation Distance
  Between Roadways and Monitoring Paths for Monitoring Neighborhood and
    Urban Scale Ozone (O3) and Oxides of Nitrogen (NO, NO2, NOX, NOy)
------------------------------------------------------------------------
                                                     Minimum    Minimum
                                                     distance   distance
  Roadway average daily traffic, vehicles per day     \1 3\     \1 2 3\
                                                     (meters)   (meters)
------------------------------------------------------------------------
<=1,000...........................................         10         10
10,000............................................         10         20
15,000............................................         20         30
20,000............................................         30         40
40,000............................................         50         60
70,000............................................        100        100
>=110,000.........................................        250        250
------------------------------------------------------------------------
\1\ Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table
  values based on the actual traffic count.
\2\ Applicable for ozone open path monitors whose placement was not
  approved as of December 18, 2006.
\3\ All distances listed are expressed as having 2 significant figures.
  When rounding is performed to assess compliance with these siting
  requirements, the distance measurements will be rounded such as to
  retain at least two significant figures.

3.5.1 Spacing for Ozone Monitoring Paths

    In siting an O3 open path analyzer, it is important 
to minimize destructive interferences form sources of NO, since NO 
readily reacts with O3. Table E-4 of this appendix 
provides the required minimum separation distances between a roadway 
and at least 90 percent of a monitoring path for various ranges of 
daily roadway traffic. A monitoring site with a monitoring path 
located closer to a roadway than allowed by the Table E-4 
requirements should be classified as microscale or middle scale, 
rather than neighborhood or urban scale, since the measurements from 
such a site would more closely represent these smaller scales. The 
monitoring path(s) must not cross over a roadway with an average 
daily traffic count of 10,000 vehicles per day or more. For 
locations where a monitoring path crosses a roadway with fewer than 
10,000 vehicles per day, monitoring agencies must consider the 
entire segment of the monitoring path in the area of potential 
atmospheric interference from automobile emissions. Therefore, this

[[Page 16401]]

calculation must include the length of the monitoring path over the 
roadway plus any segments of the monitoring path that lie in the 
area between the roadway and minimum separation distance, as 
determined from Table E-4 of this appendix. The sum of these 
distances must not be greater than 10 percent of the total 
monitoring path length.

3.5.2 Spacing for Carbon Monoxide Monitoring Paths

    (a) Near-road microscale CO monitoring sites, including those 
located in downtown areas, urban street canyons, and other near-road 
locations such as those adjacent to highly trafficked roads, are 
intended to provide a measurement of the influence of the immediate 
source on the pollution exposure on the adjacent area.
    (b) Microscale CO monitoring paths in downtown areas or urban 
street canyon locations shall be located a minimum distance of 2.0 
meters and a maximum distance of 10 meters from the edge of the 
nearest traffic lane.
    (c) Microscale CO monitoring paths in downtown areas or urban 
street canyon locations shall be located at least 10 meters from an 
intersection, preferably at a midblock location. Midblock locations 
are preferable to intersection locations because intersections 
represent a much smaller portion of downtown space than do the 
streets between them. Pedestrian exposure is probably also greater 
in street canyon/corridors than at intersections.
    (d) Neighborhood scale CO monitoring paths in downtown areas or 
urban street canyon locations shall be located according to the 
requirements in Table E-5 of this appendix.

   Table E-5 Section 3.5.2 of Appendix E--Minimum Separation Distance
 Between Roadways and Monitoring Paths for Monitoring Neighborhood Scale
                             Carbon Monoxide
------------------------------------------------------------------------
                                                              Minimum
     Roadway average daily traffic, vehicles per day      distance \1 2\
                                                             (meters)
------------------------------------------------------------------------
<=10,000................................................              10
15,000..................................................              25
20,000..................................................              45
30,000..................................................              80
40,000..................................................             115
50,000..................................................             135
>=60,000................................................             150
------------------------------------------------------------------------
\1\ Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table
  values based on the actual traffic count.
\2\ All distances listed are expressed as having 2 significant figures.
  When rounding is performed to assess compliance with these siting
  requirements, the distance measurements will be rounded such as to
  retain at least two significant figures.

3.5.3 Spacing for Nitrogen Dioxide (NO2) Monitoring Paths

    (a) In siting near-road NO2 monitors as required in 
section 4.3.2 of appendix D of this part, the monitoring path shall 
be as near as practicable to the outside nearest edge of the traffic 
lanes of the target road segment but shall not be located at a 
distance greater than 50 meters, in the horizontal, from the outside 
nearest edge of the traffic lanes of the target road segment.
    (b) In siting NO2 open path monitors for neighborhood 
and larger scale monitoring, it is important to minimize near-road 
influences. Table E-5 of this appendix provides the required minimum 
separation distances between a roadway and at least 90 percent of a 
monitoring path for various ranges of daily roadway traffic. A site 
with a monitoring path located closer to a roadway than allowed by 
the Table E-4 requirements should be classified as microscale or 
middle scale rather than neighborhood or urban scale. The monitoring 
path(s) must not cross over a roadway with an average daily traffic 
count of 10,000 vehicles per day or more. For locations where a 
monitoring path crosses a roadway with fewer than 10,000 vehicles 
per day, monitoring agencies must consider the entire segment of the 
monitoring path in the area of potential atmospheric interference 
form automobile emissions. Therefore, this calculation must include 
the length of the monitoring path over the roadway plus any segments 
of the monitoring path that lie in the area between the roadway and 
minimum separation distance, as determined from Table E-5 of this 
appendix. The sum of these distances must not be greater than 10 
percent of the total monitoring path length.

3.6 Cumulative Interferences on a Monitoring Path

    The cumulative length or portion of a monitoring path that is 
affected by minor sources, trees, or roadways must not exceed 10 
percent of the total monitoring path length.

3.7 Maximum Monitoring Path Length

    The monitoring path length must not exceed 1.0 kilometer for 
open path analyzers in neighborhood, urban, or regional scale. For 
middle scale monitoring sites, the monitoring path length must not 
exceed 300 meters. In areas subject to frequent periods of dust, 
fog, rain, or snow, consideration should be given to a shortened 
monitoring path length to minimize loss of monitoring data due to 
these temporary optical obstructions. For certain ambient air 
monitoring scenarios using open path analyzers, shorter path lengths 
may be needed in order to ensure that the monitoring site meets the 
objectives and spatial scales defined in appendix D to this part. 
The Regional Administrator may require shorter path lengths, as 
needed on an individual basis, to ensure that the SLAMS sites meet 
the appendix D requirements. Likewise, the Administrator may specify 
the maximum path length used at NCore monitoring sites.

3.8 Summary

    Table E-6 of this appendix presents a summary of the general 
requirements for monitoring path siting criteria with respect to 
distances and heights. Table E-6 requires different elevation 
distances above the ground for the various pollutants. The 
discussion in this appendix for each of the pollutants describes 
reasons for elevating the monitoring path. The differences in the 
specified range of heights are based on the vertical concentration 
gradients. For source oriented and near-road monitors, the gradients 
in the vertical direction are very large for the microscale, so a 
small range of heights are used. The upper limit of 15 meters is 
specified for the consistency between pollutants and to allow the 
use of a monitoring path for monitoring more than one pollutant.

                 Table E-6 Section 3.8 of Appendix E--Summary of Monitoring Path Siting Criteria
----------------------------------------------------------------------------------------------------------------
                                                                  Horizontal or
                                                                    vertical
                                                   Height from    distance from   Distance from   Distance from
                                    Maximum       ground to 80%    supporting     trees to 90%     roadways to
          Pollutant             monitoring path   of monitoring   structures 2    of monitoring  monitoring path
                                  length 9 10       path 1 8        to 90% of       path 1 8       1 8 (meters)
                                                    (meters)       monitoring       (meters)
                                                                    path 1 8
                                                                    (meters)
----------------------------------------------------------------------------------------------------------------
SO2 3 4 5 6..................  <= 300 m for              2.0-15           >=1.0            >=10  N/A.
                                Middle.
                               <= 1.0 km for
                                Neighborhood,
                                Urban, and
                                Regional.
CO4 5 7......................  <= 300 m for             2.5-3.5           >=1.0            >=10  2.0-10 for
                                Micro [downtown                                                   downtown areas
                                or street                                                         or street
                                canyon sites].                                                    canyon
                                                                                                  microscale.
                               <= 300 m for             2.0-7.0                                  <=50 for near-
                                Micro [Near-                                                      road
                                Road sites].                                                      microscale.

[[Page 16402]]

 
                               <= 300 m for              2.0-15                                  See Table E-5
                                Middle.                                                           of this
                                                                                                  appendix for
                                                                                                  middle and
                                                                                                  neighborhood
                                                                                                  scales.
                               <= 1.0 km for
                                Neighborhood.
O33 4 5......................  <= 300 m for
                                Middle.
                               <= 1.0 km for             2.0-15           >=1.0            >=10  See Table E-4.
                                Neighborhood,
                                Urban, and
                                Regional.
NO23 4 5.....................  Between 50 m-300         2.0-7.0                                  <=50 for near-
                                m for Micro                                                       road micro-
                                (Near-Road).                                                      scale.
                               <= 300 m for                               >=1.0            >=10
                                Middle.
                               <= 1.0 km for             2.0-15                                  See Table E-4.
                                Neighborhood,
                                Urban, and
                                Regional.
PAMS3 4 5 Ozone precursors...  <= 1.0 km for             2.0-15           >=1.0            >=10  See Table E-4.
                                Neighborhood
                                and Urban.
----------------------------------------------------------------------------------------------------------------
N/A--Not applicable.
\1\ Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring,
  middle, neighborhood, urban, and regional scale NO2 monitoring, and all applicable scales for monitoring SO2,
  O3, and O3 precursors.
\2\ When the monitoring path is located on a rooftop, this separation distance is in reference to walls,
  parapets, or penthouses located on roof.
\3\ At least 90 percent of the monitoring path should be greater than 20 meters from the dripline of tree(s) and
  must be 10-meters from the dripline.
\4\ Distance from 90 percent of monitoring path to obstacle, such as a building, must be at least twice the
  height the obstacle protrudes above the monitoring path. Sites not meeting this criterion may be classified as
  microscale or middle scale (see text).
\5\ Must have unrestricted airflow 270 degrees around at least 90 percent of the monitoring path; 180 degrees if
  the monitoring path is adjacent to the side of a building or a wall for street canyon monitoring.
\6\ The monitoring path should be away from minor sources, such as furnace or incineration flues. The separation
  distance is dependent on the height of the minor source's emission point (such as a flue), the type of fuel or
  waste burned, and the quality of the fuel (sulfur, ash, or lead content). This criterion is designed to avoid
  undue influences from minor sources.
\7\ For microscale CO monitoring sites, the monitoring path must be >=10. meters from a street intersection and
  preferably at a midblock location.
\8\ All distances listed are expressed as having 2 significant figures. When rounding is performed to assess
  compliance with these siting requirements, the distance measurements will be rounded such as to retain at
  least two significant figures.
\9\ See section 1.2 of appendix D for definitions of monitoring scales.
\10\ See section 3.7 of this appendix.

4. Waiver Provisions

    Most sampling probes or monitors can be located so that they 
meet the requirements of this appendix. New sites, with rare 
exceptions, can be located within the limits of this appendix. 
However, some existing sites may not meet these requirements and may 
still produce useful data for some purposes. The EPA will consider a 
written request from the State, or where applicable local, agency to 
waive one or more siting criteria for some monitoring sites 
providing that the State or their designee can adequately 
demonstrate the need (purpose) for monitoring or establishing a 
monitoring site at that location.
    4.1 For a proposed new site, a waiver may be granted only if 
both the following criteria are met:
    4.1.1 The proposed new site can be demonstrated to be as 
representative of the monitoring area as it would be if the siting 
criteria were being met.
    4.1.2 The monitor or probe cannot reasonably be located so as to 
meet the siting criteria because of physical constraints (e.g., 
inability to locate the required type of site the necessary distance 
from roadways or obstructions).
    4.2 For an existing site, a waiver may be granted if either the 
criterion in section 4.1.1 or the criterion in 4.1.2 of this 
appendix is met.
    4.3 Cost benefits, historical trends, and other factors may be 
used to add support to the criteria in sections 4.1.1 and 4.1.2 of 
this appendix; however, by themselves, they will not be acceptable 
reasons for the EPA to grant a waiver. Written requests for waivers 
must be submitted to the Regional Administrator. Granted waivers 
must be renewed minimally every 5 years and ideally as part of the 
network assessment as defined in Sec.  58.10(d). The approval date 
of the waiver must be documented in the annual monitoring network 
plan to support the requirements of Sec.  58.10(a)(1) and 
58.10(b)(10).

5. References

    1. Bryan, R.J., R.J. Gordon, and H. Menck. Comparison of High 
Volume Air Filter Samples at Varying Distances from Los Angeles 
Freeway. University of Southern California, School of Medicine, Los 
Angeles, CA. (Presented at 66th Annual Meeting of Air Pollution 
Control Association. Chicago, IL. June 24-28, 1973. APCA 73-158.)
    2. Teer, E.H. Atmospheric Lead Concentration Above an Urban 
Street. Master of Science Thesis, Washington University, St. Louis, 
MO. January 1971.
    3. Bradway, R.M., F.A. Record, and W.E. Belanger. Monitoring and 
Modeling of Resuspended Roadway Dust Near Urban Arterials. GCA 
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Relationship Between Monitor Height and Measured Particulate Levels 
in Seven U.S. Urban Areas. U.S. Environmental Protection Agency, 
Research Triangle Park, NC. (Presented at 70th Annual Meeting of Air 
Pollution Control Association, Toronto, Canada. June 20-24, 1977. 
APCA 77-13.4.)
    5. Harrison, P.R. Considerations for Siting Air Quality Monitors 
in Urban Areas. City of Chicago, Department of Environmental 
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Pollution Control Association, Chicago, IL. June 24-28, 1973. APCA 
73-161.)
    6. Study of Suspended Particulate Measurements at Varying 
Heights Above Ground. Texas State Department of Health, Air Control 
Section, Austin, TX. 1970. p.7.
    7. Rodes, C.E. and G.F. Evans. Summary of LACS Integrated 
Pollutant Data. In: Los Angeles Catalyst Study Symposium. U.S. 
Environmental Protection Agency, Research Triangle Park, NC. EPA 
Publication No. EPA-600/4-77-034. June 1977.
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Particulate Problem: Volume 1, National Assessment. GCA Technology 
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Research Triangle Park, NC. EPA Publication No. EPA-450/3-75-024. 
June 1976.
    9. Pace, T.G. Impact of Vehicle-Related Particulates on TSP 
Concentrations and Rationale for Siting Hi-Vols in the Vicinity of 
Roadways. OAQPS, U.S. Environmental Protection Agency, Research 
Triangle Park, NC. April 1978.
    10. Ludwig, F.L., J.H. Kealoha, and E. Shelar. Selecting Sites 
for Monitoring Total Suspended Particulates. Stanford Research 
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EPA-450/3-77-018. June 1977, revised December 1977.
    11. Ball, R.J. and G.E. Anderson. Optimum Site Exposure Criteria 
for SO2 Monitoring. The Center for the Environment and 
Man,

[[Page 16403]]

Inc., Hartford, CT. Prepared for U.S. Environmental Protection 
Agency, Research Triangle Park, NC. EPA Publication No. EPA-450/3-
77-013. April 1977.
    12. Ludwig, F.L. and J.H.S. Kealoha. Selecting Sites for Carbon 
Monoxide Monitoring. Stanford Research Institute, Menlo Park, CA. 
Prepared for U.S. Environmental Protection Agency, Research Triangle 
Park, NC. EPA Publication No. EPA-450/3-75-077. September 1975.
    13. Ludwig, F.L. and E. Shelar. Site Selection for the 
Monitoring of Photochemical Air Pollutants. Stanford Research 
Institute, Menlo Park, CA. Prepared for U.S. Environmental 
Protection Agency, Research Triangle Park, NC. EPA Publication No. 
EPA-450/3-78-013. April 1978.
    14. Lead Analysis for Kansas City and Cincinnati, PEDCo 
Environmental, Inc., Cincinnati, OH. Prepared for U.S. Environmental 
Protection Agency, Research Triangle Park, NC. EPA Contract No. 66-
02-2515, June 1977.
    15. Barltrap, D. and C.D. Strelow. Westway Nursery Testing 
Project. Report to the Greater London Council. August 1976.
    16. Daines, R. H., H. Moto, and D. M. Chilko. Atmospheric Lead: 
Its Relationship to Traffic Volume and Proximity to Highways. 
Environ. Sci. and Technol., 4:318, 1970.
    17. Johnson, D. E., et al. Epidemiologic Study of the Effects of 
Automobile Traffic on Blood Lead Levels, Southwest Research 
Institute, Houston, TX. Prepared for U.S. Environmental Protection 
Agency, Research Triangle Park, NC. EPA-600/1-78-055, August 1978.
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Development, U.S. Environmental Protection Agency, Washington, DC 
EPA-600/8-83-028 aF-dF, 1986, and supplements EPA-600/8-89/049F, 
August 1990. (NTIS document numbers PB87-142378 and PB91-138420.)
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and Lead from an Expressway, Ph.D. Dissertation, University of 
Cincinnati, Cincinnati, OH. 1972.
    20. Wechter, S.G. Preparation of Stable Pollutant Gas Standards 
Using Treated Aluminum Cylinders. ASTM STP. 598:40-54, 1976.
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and Sulfur Dioxide Adsorption On and Description From Glass, Plastic 
and Metal Tubings. J. Air Poll. Con. Assoc. 17:753, 1976.
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Monitoring Equipment. U.S. NTIS. p. 202, 249, 1971.
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Air Quality Measurement. ISA Transactions, 14:281-291, 1975.
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with Plastic and Metallic Materials in a Dynamic Flow System. 
Intern. Jour. Air and Water Poll., 4:70-78, 1961.
    25. Code of Federal Regulations. 40 CFR 53.22, July 1976.
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on Analysis of Atmospheric Nitrogen Oxides and Ozone, Anal. Chem., 
43:1890, 1971.
    27. Slowik, A.A. and E.B. Sansone. Diffusion Losses of Sulfur 
Dioxide in Sampling Manifolds. J. Air. Poll. Con. Assoc., 24:245, 
1974.
    28. Yamada, V.M. and R.J. Charlson. Proper Sizing of the 
Sampling Inlet Line for a Continuous Air Monitoring Station. 
Environ. Sci. and Technol., 3:483, 1969.
    29. Koch, R.C. and H.E. Rector. Optimum Network Design and Site 
Exposure Criteria for Particulate Matter, GEOMET Technologies, Inc., 
Rockville, MD. Prepared for U.S. Environmental Protection Agency, 
Research Triangle Park, NC. EPA Contract No. 68-02-3584. EPA 450/4-
87-009. May 1987.
    30. Burton, R.M. and J.C. Suggs. Philadelphia Roadway Study. 
Environmental Monitoring Systems Laboratory, U.S. Environmental 
Protection Agency, Research Triangle Park, N.C. EPA-600/4-84-070 
September 1984.
    31. Technical Assistance Document for Sampling and Analysis of 
Ozone Precursors. Atmospheric Research and Exposure Assessment 
Laboratory, U.S. Environmental Protection Agency, Research Triangle 
Park, NC 27711. EPA 600/8-91-215. October 1991.
    32. Quality Assurance Handbook for Air Pollution Measurement 
Systems: Volume IV. Meteorological Measurements. Atmospheric 
Research and Exposure Assessment Laboratory, U.S. Environmental 
Protection Agency, Research Triangle Park, NC 27711. EPA 600/4-90-
0003. August 1989.
    33. On-Site Meteorological Program Guidance for Regulatory 
Modeling Applications. Office of Air Quality Planning and Standards, 
U.S. Environmental Protection Agency, Research Triangle Park, NC 
27711. EPA 450/4-87-013. June 1987F.
    34. Johnson, C., A. Whitehill, R. Long, and R. Vanderpool. 
Investigation of Gaseous Criteria Pollutant Transport Efficiency as 
a Function of Tubing Material. U.S. Environmental Protection Agency, 
Research Triangle Park, NC 27711. EPA/600/R-22/212. August 2022.
    35. Hannah Halliday, Cortina Johnson, Tad Kleindienst, Russell 
Long, Robert Vanderpool, and Andrew Whitehill. Recommendations for 
Nationwide Approval of Nafion\TM\ Dryers Upstream of UV-Absorption 
Ozone Analyzers. U.S. Environmental Protection Agency, Research 
Triangle Park, NC 27711. EPA/600/R-20/390. November 2020.


0
31. Revise appendix G to part 58 to read as follows:

Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily 
Reporting

1. General Information
2. Reporting Requirements
3. Data Handling

1. General Information

    1.1 AQI Overview. The AQI is a tool that simplifies reporting 
air quality to the public in a nationally uniform and easy to 
understand manner. The AQI converts concentrations of pollutants, 
for which the EPA has established a national ambient air quality 
standard (NAAQS), into a uniform scale from 0-500. These pollutants 
are ozone (O3), particulate matter (PM2.5, 
PM10), carbon monoxide (CO), sulfur dioxide 
(SO2), and nitrogen dioxide (NO2). The scale 
of the index is divided into general categories that are associated 
with health messages.

2. Reporting Requirements

    2.1 Applicability. The AQI must be reported daily for a 
metropolitan statistical area (MSA) with a population over 350,000. 
When it is useful and possible, it is recommended, but not required 
for an area to report a sub-daily AQI as well.
    2.2 Contents of AQI Report.
    2.2.1 Content of AQI Report Requirements. An AQI report must 
contain the following:
    a. The reporting area(s) (the MSA or subdivision of the MSA).
    b. The reporting period (the day for which the AQI is reported).
    c. The main pollutant (the pollutant with the highest index 
value).
    d. The AQI (the highest index value).
    e. The category descriptor and index value associated with the 
AQI and, if choosing to report in a color format, the associated 
color. Use only the following descriptors and colors for the six AQI 
categories:

           Table 1 to Section 2 of Appendix G--AQI Categories
------------------------------------------------------------------------
      For this AQI         Use this descriptor      And this color \1\
------------------------------------------------------------------------
0 to 50................  ``Good''...............  Green.
51 to 100..............  ``Moderate''...........  Yellow.
101 to 150.............  ``Unhealthy for          Orange.
                          Sensitive Groups''.
151 to 200.............  ``Unhealthy''..........  Red.
201 to 300.............  ``Very Unhealthy''.....  Purple.
301 and above..........  ``Hazardous''..........  Maroon\1\.
------------------------------------------------------------------------
\1\Specific color definitions can be found in the most recent reporting
  guidance (Technical Assistance Document for the Reporting of Daily Air
  Quality), which can be found at https://www.airnow.gov/publications/air-quality-index/technical-assistance-document-for-reporting-the-daily-aqi/.


[[Page 16404]]

    f. The pollutant specific sensitive groups for any reported 
index value greater than 100. The sensitive groups for each 
pollutant are identified as part of the periodic review of the air 
quality criteria and the NAAQS. For convenience, the EPA lists the 
relevant groups for each pollutant in the most recent reporting 
guidance (Technical Assistance Document for the Reporting of Daily 
Air Quality), which can be found at https://www.airnow.gov/publications/air-quality-index/technical-assistance-document-for-reporting-the-daily-aqi/.
    2.2.2 Contents of AQI Report When Applicable. When appropriate, 
the AQI report may also contain the following, but such information 
is not required:
    a. Appropriate health and cautionary statements.
    b. The name and index value for other pollutants, particularly 
those with an index value greater than 100.
    c. The index values for sub-areas of your MSA.
    d. Causes for unusually high AQI values.
    e. Pollutant concentrations.
    f. Generally, the AQI report applies to an area's MSA only. 
However, if a significant air quality problem exists (AQI greater 
than 100) in areas significantly impacted by the MSA but not in it 
(for example, O3 concentrations are often highest 
downwind and outside an urban area), the report should identify 
these areas and report the AQI for these areas as well.
    2.3. Communication, Timing, and Frequency of AQI Report. The 
daily AQI must be reported 7 days per week and made available via 
website or other means of public access. The daily AQI report 
represents the air quality for the previous day. Exceptions to this 
requirement are in section 2.4 of this appendix.
    a. Reporting the AQI sub-daily is recommended, but not required, 
to provide more timely air quality information to the public for 
making health-protective decisions.
    b. Submitting hourly data in real-time to the EPA's AirNow (or 
future analogous) system is recommended, but not required, and 
assists the EPA in providing timely air quality information to the 
public for making health-protective decisions.
    c. Submitting hourly data for appropriate monitors (referenced 
in section 3.2 of this appendix) satisfies the daily AQI reporting 
requirement because the AirNow system makes daily and sub-daily AQI 
reports widely available through its website and other communication 
tools.
    d. Forecasting the daily AQI provides timely air quality 
information to the public and is recommended but not required. Sub-
daily forecasts are also recommended, especially when air quality is 
expected to vary substantially throughout the day, like during 
wildfires. Long-term (multi-day) forecasts can also be made 
available when useful.
    2.4. Exceptions to Reporting Requirements.
    a. If the index value for a particular pollutant remains below 
50 for a season or year, then it may be excluded from the 
calculation of the AQI in section 3 of this appendix.
    b. If all index values remain below 50 for a year, then the AQI 
may be reported at the discretion of the reporting agency. In 
subsequent years, if pollutant levels rise to where the AQI would be 
above 50, then the AQI must be reported as required in section 2 of 
this appendix.
    c. As previously mentioned in section 2.3 of this appendix, 
submitting hourly data in real-time from appropriate monitors 
(referenced in section 3.2 of this appendix) to the EPA's AirNow (or 
future analogous) system satisfies the daily AQI reporting 
requirement.

3. Data Handling.

    3.1 Relationship of AQI and pollutant concentrations. For each 
pollutant, the AQI transforms ambient concentrations to a scale from 
0 to 500. As appropriate, the AQI is associated with the NAAQS for 
each pollutant. In most cases, the index value of 100 is associated 
with the numerical level of the short-term standard (i.e., averaging 
time of 24-hours or less) for each pollutant. The index value of 50 
is associated with the numerical level of the annual standard for a 
pollutant, if there is one, at one-half the level of the short-term 
standard for the pollutant or at the level at which it is 
appropriate to begin to provide guidance on cautionary language. 
Higher categories of the index are based on the potential for 
increasingly serious health effects to occur following exposure and 
increasing proportions of the population that are likely to be 
affected. The reported AQI corresponds to the pollutant with the 
highest calculated AQI. For the purposes of reporting the AQI, the 
sub-indexes for PM10 and PM2.5 are to be 
considered separately. The pollutant responsible for the highest 
index value (the reported AQI) is called the ``main'' pollutant for 
that day.
    3.2 Monitors Used for AQI Reporting. Concentration data from 
State/Local Air Monitoring Station (SLAMS) or parts of the SLAMS 
required by 40 CFR 58.10 must be used for each pollutant except PM. 
For PM, calculate and report the AQI on days for which air quality 
data has been measured (e.g., from continuous PM2.5 
monitors required in appendix D to this part). PM measurements may 
be used from monitors that are not reference or equivalent methods 
(for example, continuous PM10 or PM2.5 
monitors). Detailed guidance for relating non-approved measurements 
to approved methods by statistical linear regression is referenced 
here:
    Reference for relating non-approved PM measurements to approved 
methods (Eberly, S., T. Fitz-Simons, T. Hanley, L. Weinstock., T. 
Tamanini, G. Denniston, B. Lambeth, E. Michel, S. Bortnick. Data 
Quality Objectives (DQOs) For Relating Federal Reference Method 
(FRM) and Continuous PM2.5 Measurements to Report an Air 
Quality Index (AQI). U.S. Environmental Protection Agency, Research 
Triangle Park, NC. EPA-454/B-02-002, November 2002).
    3.3 AQI Forecast. The AQI can be forecasted at least 24-hours in 
advance using the most accurate and reasonable procedures 
considering meteorology, topography, availability of data, and 
forecasting expertise. The guidance document, ``Guidelines for 
Developing an Air Quality (Ozone and PM2.5) Forecasting 
Program,'' can be found at https://www.airnow.gov/publications/weathercasters/guidelines-developing-air-quality-forecasting-program/.
    3.4 Calculation and Equations.
    a. The AQI is the highest value calculated for each pollutant as 
follows:
    i. Identify the highest concentration among all of the monitors 
within each reporting area and truncate as follows:

(A) Ozone--truncate to 3 decimal places
PM2.5--truncate to 1 decimal place
PM10--truncate to integer
CO--truncate to 1 decimal place
SO2--truncate to integer
NO2--truncate to integer

    (B) [Reserved]
    ii. Using table 2 to this appendix, find the two breakpoints 
that contain the concentration.
    iii. Using equation 1 to this appendix, calculate the index.
    iv. Round the index to the nearest integer.


                                              Table 2 to Section 3.4 of Appendix G--Breakpoints for the AQI
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                  These breakpoints                                                            Equal these AQI's
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                           PM2.5      PM10 ([micro]g/
   O3 (ppm) 8-hour      O3 (ppm) 1-     ([micro]g/    m\3\)  24-hour   CO  (ppm) 8-   SO2  (ppb)  1-  NO2  (ppb)  1-        AQI            Category
                          hour\1\      m\3\) 24-hour                       hour            hour            hour
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.000-0.054.........  ..............         0.0-9.0            0-54         0.0-4.4            0-35            0-53            0-50  Good.
0.055-0.070.........  ..............        9.1-35.4          55-154         4.5-9.4           36-75          54-100          51-100  Moderate.
0.071-0.085.........     0.125-0.164       35.5-55.4         155-254        9.5-12.4          76-185         101-360         101-150  Unhealthy for
                                                                                                                                       Sensitive Groups.
0.086-0.105.........     0.165-0.204      55.5-125.4         255-354       12.5-15.4     \3\ 186-304         361-649         151-200  Unhealthy.
0.106-0.200.........     0.205-0.404    125.5--225.4         355-424       15.5-30.4     \3\ 305-604        650-1249         201-300  Very Unhealthy.

[[Page 16405]]

 
0.201-(\2\).........          0.405+          225.5+            425+           30.5+        \3\ 605+           1250+            301+  \4\ Hazardous.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour
  ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be
  calculated, and the maximum of the two values reported.
\2\ 8-hour O3 concentrations do not define higher AQI values (>301). AQI values > 301 are calculated with 1-hour O3 concentrations.
\3\ 1-hr SO2 concentrations do not define higher AQI values (>=200). AQI values of 200 or greater are calculated with 24-hour SO2 concentration.
\4\ AQI values between breakpoints are calculated using equation 1 to this appendix. For AQI values in the hazardous category, AQI values greater than
  500 should be calculated using equation 1 and the concentration specified for the AQI value of 500. The AQI value of 500 are as follows: O3 1-hour--
  0.604 ppm; PM2.5 24-hour--325.4 [micro]g/m\3\; PM10 24-hour--604 [micro]g/m\3\; CO ppm--50.4 ppm; SO2 1-hour--1004 ppb; and NO2 1-hour--2049 ppb.

    b. If the concentration is equal to a breakpoint, then the index 
is equal to the corresponding index value in table 2 to this 
appendix. However, equation 1 to this appendix can still be used. 
The results will be equal. If the concentration is between two 
breakpoints, then calculate the index of that pollutant with 
equation 1. It should also be noted that in some areas, the AQI 
based on 1-hour O3 will be more precautionary than using 
8-hour values (see footnote 1 to table 2). In these cases, the 1-
hour values as well as 8-hour values may be used to calculate index 
values and then use the maximum index value as the AQI for 
O3.
[GRAPHIC] [TIFF OMITTED] TR06MR24.048

Where:

Ip = the index value for pollutantp.
Cp = the truncated concentration of 
pollutantp.
BPHi = the breakpoint that is greater than or equal to 
Cp.
BPLo = the breakpoint that is less than or equal to 
Cp.
IHi = the AQI value corresponding to BPHi.
Ilo = the AQI value corresponding to BPLo.

    c. If the concentration is larger than the highest breakpoint in 
table 2 to this appendix then the last two breakpoints in table 2 
may be used when equation 1 to this appendix is applied.
Example:

    d. Using table 2 and equation 1 to this appendix, calculate the 
index value for each of the pollutants measured and select the one 
that produces the highest index value for the AQI. For example, if a 
PM10 value of 210 [micro]g/m\3\ is observed, a 1-hour 
O3 value of 0.156 ppm, and an 8-hour O3 value 
of 0.130 ppm, then do this:
    i. Find the breakpoints for PM10 at 210 [micro]g/m\3\ 
as 155 [micro]g/m\3\ and 254 [micro]g/m\3\, corresponding to index 
values 101 and 150;
    ii. Find the breakpoints for 1-hour O3 at 0.156 ppm 
as 0.125 ppm and 0.164 ppm, corresponding to index values 101 and 
150;
    iii. Find the breakpoints for 8-hour O3 at 0.130 ppm 
as 0.116 ppm and 0.374 ppm, corresponding to index values 201 and 
300;
    iv. Apply equation 21 to this appendix for 210 [micro]g/m\3\, 
PM10:
[GRAPHIC] [TIFF OMITTED] TR06MR24.049

    v. Apply equation 3 to this appendix for 0.156 ppm, 1-hour 
O3:
[GRAPHIC] [TIFF OMITTED] TR06MR24.050

    vi. Apply equation 4 to this appendix for 0.130 ppm, 8-hour 
O3:
[GRAPHIC] [TIFF OMITTED] TR06MR24.051


[[Page 16406]]


    vii. Find the maximum, 206. This is the AQI. A minimal AQI 
report could read: ``Today, the AQI for my city is 206, which is 
Very Unhealthy, due to ozone.'' It would then reference the 
associated sensitive groups.

[FR Doc. 2024-02637 Filed 3-5-24; 8:45 am]
BILLING CODE 6560-50-P