[Federal Register Volume 85, Number 251 (Thursday, December 31, 2020)]
[Rules and Regulations]
[Pages 87256-87351]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-28871]



[[Page 87255]]

Vol. 85

Thursday,

No. 251

December 31, 2020

Part IV





Environmental Protection Agency





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40 CFR Part 50





Review of the Ozone National Ambient Air Quality Standards; Final Rule

  Federal Register / Vol. 85 , No. 251 / Thursday, December 31, 2020 / 
Rules and Regulations  

[[Page 87256]]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2018-0279; FRL-10019-04-OAR]
RIN 2060-AU40


Review of the Ozone National Ambient Air Quality Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final action.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review 
of the air quality criteria and the national ambient air quality 
standards (NAAQS) for photochemical oxidants including ozone 
(O3), the EPA is retaining the current standards, without 
revision.

DATES: This final action is effective December 31, 2020.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2018-0279. Incorporated into this docket is a 
separate docket established for the Integrated Science Assessment for 
this review (Docket ID No. EPA-HQ-ORD-2018-0274). All documents in 
these dockets are listed on the www.regulations.gov website. Although 
listed in the index, some information is not publicly available, e.g., 
Confidential Business Information (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. With the exception of such 
material, publicly available docket materials are available 
electronically through https://www.regulations.gov/. Out of an 
abundance of caution for members of the public and our staff, the EPA 
Docket Center and Reading Room are closed to the public, with limited 
exceptions, to reduce the risk of transmitting COVID-19. Our Docket 
Center staff will continue to provide remote customer service via 
email, phone, and webform. For further information on EPA Docket Center 
services and the current status, please visit us online at https://www.epa.gov/dockets.

Availability of Information Related to This Action

    A number of the documents that are relevant to this action are 
available through the EPA's website at https://www.epa.gov/naaqs/ozone-o3-air-quality-standards. These documents include the Integrated Review 
Plan for the Ozone National Ambient Air Quality Standards (IRP [U.S. 
EPA, 2019b]), available at https://www.epa.gov/naaqs/ozone-o3-standards-planning-documents-current-review, the Integrated Science 
Assessment for Ozone and Related Photochemical Oxidants (ISA [U.S. EPA, 
2020a]), available at https://www.epa.gov/naaqs/ozone-o3-standards-integrated-science-assessments-current-review, the Policy Assessment 
for the Review of the Ozone National Ambient Air Quality Standards (PA 
[U.S. EPA, 2020b]), available at https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-current-review. These and other related 
documents are also available for inspection and copying in the EPA 
docket identified above.

FOR FURTHER INFORMATION CONTACT: Dr. Deirdre Murphy, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail Code C504-06, 
Research Triangle Park, NC 27711; telephone: (919) 541-0729; fax: (919) 
541-0237; email: [email protected].

SUPPLEMENTARY INFORMATION: 

Basis for Immediate Effective Date

    In accordance with section 307(d)(1)(V), the Administrator has 
designated this action as being subject to the rulemaking procedures in 
section 307(d) of the Clean Air Act (CAA). Section 307(d)(1) of the CAA 
states that: ``The provisions of section 553 through 557 * * * of Title 
5 shall not, except as expressly provided in this subsection, apply to 
actions to which this subsection applies.'' Thus, section 553(d) of the 
Administrative Procedure Act (APA), which requires publication of a 
substantive rule to be made ``not less than 30 days before its 
effective date'' subject to limited exceptions, does not apply to this 
action. In the alternative, the EPA concludes that it is consistent 
with APA section 553(d) to make this action effective December 31, 
2020.
    Section 553(d)(3) of the APA, 5 U.S.C. 553(d)(3), provides that 
final rules shall not become effective until 30 days after publication 
in the Federal Register ``except . . . as otherwise provided by the 
agency for good cause found and published with the rule.'' ``In 
determining whether good cause exists, an agency should `balance the 
necessity for immediate implementation against principles of 
fundamental fairness which require that all affected persons be 
afforded a reasonable amount of time to prepare for the effective date 
of its ruling.'' Omnipoint Corp. v. Fed. Commc'n Comm'n, 78 F.3d 620, 
630 (D.C. Cir. 1996) (quoting United States v. Gavrilovic, 551 F.2d 
1099, 1105 (8th Cir. 1977)). The purpose of this provision is to ``give 
affected parties a reasonable time to adjust their behavior before the 
final rule takes effect.'' Id.; see also Gavrilovic, 551 F.2d at 1104 
(quoting legislative history).
    The EPA is determining that in light of the nature of this action, 
good cause exists to make this final action effective immediately 
because the Agency seeks to provide regulatory certainty as soon as 
possible and the Administrator's decision to retain the current NAAQS 
does not change the status quo or impose new obligations on any person 
or entity. As a result, there is no need to provide parties additional 
time to adjust their behavior, and no person will be harmed by making 
the action immediately effective as opposed to delaying the effective 
date by 30 days. Accordingly, the EPA is making this action effective 
immediately upon publication.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related O3 Control Programs
    C. History of the Air Quality Criteria and O3 
Standards
    D. Current Review of the Air Quality Criteria and O3 
Standards
    E. Air Quality Information
II. Rationale for Decision on the Primary Standard
    A. Introduction
    1. Background on the Current Standard
    2. Overview of Health Effects Information
    3. Overview of Exposure and Risk Information
    B. Conclusions on the Primary Standard
    1. Basis for the Proposed Decision
    2. Comments on the Proposed Decision
    3. Administrator's Conclusions
    C. Decision on the Primary Standard
III. Rationale for Decision on the Secondary Standard
    A. Introduction
    1. Background on the Current Standard
    2. Overview of Welfare Effects Information
    3. Overview of Air Quality and Exposure Information
    B. Conclusions on the Secondary Standard
    1. Basis for the Proposed Decision
    2. Comments on the Proposed Decision
    3. Administrator's Conclusions
    C. Decision on the Secondary Standard
IV. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs

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    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From 
Environmental Health and Safety Risks
    I. Executive Order 13211: Actions That Significantly Affect 
Energy Supply, Distribution or Use
    J. National Technology Transfer and Advancement Act
    K. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    L. Determination Under Section 307(d)
    M. Congressional Review Act
V. References

Executive Summary

    This document presents the Administrator's decisions in the current 
review of the primary (health-based) and secondary (welfare-based) 
O3 NAAQS, to retain the current standards, without revision. 
In reaching these decisions, the Administrator has considered the 
currently available scientific evidence in the ISA, quantitative and 
policy analyses presented in the PA, advice from the Clean Air 
Scientific Advisory Committee (CASAC), and public comments on the 
proposed decision. This document provides background and summarizes the 
rationale for these decisions.
    This review of the O3 standards, required by the Clean 
Air Act (CAA) on a periodic basis, was initiated in 2018. In the last 
review, completed in 2015, the EPA significantly strengthened the 
primary and secondary O3 standards by revising the level of 
both standards from 75 parts per billion (ppb) to 70 ppb and retaining 
their indicators (O3), forms (annual fourth-highest daily 
maximum, averaged across three consecutive years) and averaging times 
(eight hours) (80 FR 65291, October 26, 2015). In subsequent litigation 
on the 2015 decisions, the U.S. Court of Appeals for the District of 
Columbia Circuit (D.C. Circuit) upheld the 2015 primary standard but 
remanded the 2015 secondary standard to the EPA for further 
justification or reconsideration. The court's remand of the secondary 
standard has been considered in reaching the decision described in this 
document on this standard, and in associated conclusions and judgments, 
also described here. Accordingly, this decision incorporates the EPA's 
response to the judicial remand of the 2015 secondary standard.
    In this review as in past reviews of the air quality criteria and 
NAAQS for O3 and related photochemical oxidants, the health 
and welfare effects evidence evaluated in the ISA is focused on 
O3. Ozone is the most prevalent photochemical oxidant in the 
atmosphere and the one for which there is a large body of scientific 
evidence on health and welfare effects. A component of smog, 
O3 in ambient air is a mixture of mostly tropospheric 
O3 and some stratospheric O3. Tropospheric 
O3 forms in the atmosphere when emissions of precursor 
pollutants, such as nitrogen oxides and volatile organic compounds 
(VOCs), interact with solar radiation. Such emissions result from man-
made sources (e.g. motor vehicles and power plants) and natural sources 
(e.g. vegetation and wildfires). In addition, O3 that is 
created naturally in the stratosphere also mixes with tropospheric 
O3 near the tropopause, and, less frequently can mix nearer 
the earth's surface.
    Based on the current health effects evidence and quantitative 
information, as well as consideration of CASAC advice and public 
comment, the Administrator concludes that the current primary standard 
is requisite to protect public health, including the health of at-risk 
populations, with an adequate margin of safety, and should be retained, 
without revision. This decision has been informed by key aspects of the 
health effects evidence newly available in this review, in conjunction 
with the full body of evidence critically evaluated in the ISA, that 
continues to support prior conclusions that short-term O3 
exposure causes and long-term O3 exposure is likely to cause 
respiratory effects. The strongest evidence continues to come from 
studies of short- and long-term O3 exposure and an array of 
respiratory health effects, including effects related to asthma 
exacerbation in people with asthma, particularly children with asthma. 
The clearest evidence comes from controlled human exposure studies, 
available at the time of the last review, of individuals exposed for 
6.6 hours during quasi-continuous exercise, that report an array of 
respiratory responses including lung function decrements and 
respiratory symptoms. Epidemiologic studies additionally describe 
consistent, positive associations between O3 exposures and 
hospital admissions and emergency department visits, particularly for 
asthma exacerbation in children. Populations and lifestages at risk 
include people with asthma, children, the elderly, and outdoor workers. 
The quantitative analyses of population exposure and risk, as well as 
policy considerations in the PA, summarized in this document and 
described in detail in the PA, also inform the decision on the primary 
standard. The general approach and methodology used for the exposure-
based assessment is similar to that used in the last review, although a 
number of updates and improvements have been implemented. These include 
a more recent period (2015-2017) of ambient air monitoring data in 
which O3 concentrations in the areas assessed are at or near 
the current standard, as well as improvements and updates to models, 
model inputs and underlying databases.
    In its advice to the Administrator, the CASAC stated that the newly 
available health effects evidence does not differ substantially from 
that available in the last review when the standard was set. Part of 
CASAC concluded that the primary standard should be retained. Another 
part of CASAC expressed concern regarding the margin of safety provided 
by the current standard, pointing to comments from the 2014 CASAC, who 
while agreeing that the evidence supported a standard level of 70 ppb, 
additionally provided policy advice expressing support for a lower 
standard. In summary, the current evidence and quantitative analyses, 
advice from the CASAC and consideration of public comments have 
informed the Administrator's judgments in reaching his decision that 
the current primary standard of 70 ppb O3, as the annual 
fourth-highest daily maximum 8-hour concentration averaged across three 
consecutive years, provides the requisite public health protection, 
with an adequate margin of safety.
    Based on the current welfare effects evidence and quantitative 
information, as well as consideration of CASAC advice and public 
comment, the Administrator concludes that the current secondary 
standard is requisite to protect the public welfare from known or 
anticipated adverse effects of O3 and related photochemical 
oxidants in ambient air, and should be retained, without revision. This 
decision has been informed by key aspects of the welfare effects 
evidence newly available in this review, in conjunction with the full 
body of evidence critically evaluated in the ISA, that supports, 
sharpens and expands somewhat on the conclusions reached in the last 
review. The currently available evidence describes an array of 
O3 effects on vegetation and related ecosystem effects, as 
well as the role of O3 in radiative forcing and subsequent 
climate-related effects. The ISA includes findings of causal or likely 
causal

[[Page 87258]]

relationships for a number of such effects with O3 in the 
ambient air. As in the last review, the strongest evidence, including 
quantitative characterizations of relationships between O3 
exposure and occurrence and magnitude of effects, is for vegetation 
effects. The scales of these effects range from the individual plant 
scale to the ecosystem scale, with potential for impacts on the public 
welfare.
    While the welfare effects of O3 vary widely with regard 
to the extent and level of detail of the available information that 
describes the exposure circumstances that may elicit them, such 
information is most advanced for plant growth-related effects. For 
example, the information on exposure metric and relationships for these 
effects with the cumulative, concentration-weighted exposure index, 
W126, is long-standing, having been first described in the 1997 review. 
Utilizing this information in reviewing the public welfare protection 
provided by the current secondary standard, reduced growth has been 
considered as proxy or surrogate for a broad array of related 
vegetation effects. Quantitative analyses of air quality and vegetation 
exposure, including in terms of the W126 index, as well as policy-
relevant considerations discussed in the PA, have also informed the 
Administrator's decision on the secondary standard. These include 
analyses of air quality monitoring data in areas meeting the current 
standard across the U.S., as well as in Class I areas, updated and 
expanded from analyses conducted in the last review. Lastly, in its 
advice to the Administrator on the secondary standard, the full CASAC 
found the current evidence to support the current standard and 
concurred with the draft PA that it should be retained without 
revision. In summary, the current evidence and quantitative analyses, 
advice from the CASAC and consideration of public comments have 
informed the Administrator's judgments in reaching his decision that 
the current secondary standard of 70 ppb O3, as the annual 
fourth-highest daily maximum 8-hour concentration averaged across three 
consecutive years, provides the requisite public welfare protection.

I. Background

A. Legislative Requirements

    Two sections of the 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.'' \1\ 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.'' \2\
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    \1\ 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).
    \2\ 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 Ass'ns, 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.'' See American Petroleum 
Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord 
Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). At 
the same time, courts have clarified the EPA may consider ``relative 
proximity to peak background . . . concentrations'' as a factor in 
deciding how to revise the NAAQS in the context of considering standard 
levels within the range of reasonable values supported by the air 
quality criteria and judgments of the Administrator. See American 
Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 (D.C. Cir. 2002), hereafter 
referred to as ``ATA III.''
    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 Ass'n 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),\3\

[[Page 87259]]

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.
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    \3\ As used here and similarly throughout this document, the 
term population (or group) refers to persons having a quality or 
characteristic in common, such as a specific pre-existing illness or 
a specific age or life stage. As summarized in section II.A.2.c 
below, the identification of sensitive groups (called at-risk groups 
or at-risk populations) involves consideration of susceptibility and 
vulnerability.
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    Section 109(d)(1) of the Act requires periodic review and, if 
appropriate, revision of existing air quality criteria to reflect 
advances in scientific knowledge concerning the effects of the 
pollutant on public health and welfare. Under the same provision, the 
EPA is also to periodically review and, if appropriate, revise the 
NAAQS, based on the revised air quality criteria.\4\
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    \4\ This section of the Act requires the Administrator to 
complete these reviews and make any revisions that may be 
appropriate ``at five-year intervals.''
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    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 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 
CASAC of the EPA's Science Advisory Board. A number of other advisory 
functions are also identified for the committee by section 
109(d)(2)(C), which reads:

    Such committee shall also (i) advise the Administrator of areas 
in which additional knowledge is required to appraise the adequacy 
and basis of existing, new, or revised national ambient air quality 
standards, (ii) describe the research efforts necessary to provide 
the required information, (iii) advise the Administrator on the 
relative contribution to air pollution concentrations of natural as 
well as anthropogenic activity, and (iv) advise the Administrator of 
any adverse public health, welfare, social, economic, or energy 
effects which may result from various strategies for attainment and 
maintenance of such national ambient air quality standards.

    As previously noted, the Supreme Court has held that section 109(b) 
``unambiguously bars cost considerations from the NAAQS-setting 
process,'' in Whitman v. American Trucking Ass'ns, 531 U.S. 457, 471 
(2001). Accordingly, while some of the issues listed in section 
109(d)(2)(C) as those on 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.\5\
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    \5\ Because some of these issues are not relevant to standard 
setting, some aspects of CASAC advice may not be relevant to EPA's 
process of setting primary and secondary standards that are 
requisite to protect public health and welfare. Indeed, were the EPA 
to consider costs of implementation when reviewing and revising the 
standards ``it would be grounds for vacating the NAAQS.'' Whitman v. 
American Trucking Ass'ns, 531 U.S. 457, 471 n.4 (2001). At the same 
time, the CAA directs CASAC to provide advice on ``any adverse 
public health, welfare, social, economic, or energy effects which 
may result from various strategies for attainment and maintenance'' 
of the NAAQS to the Administrator under section 109(d)(2)(C)(iv). In 
Whitman, the Court clarified that most of that advice would be 
relevant to implementation but not standard setting, as it 
``enable[s] the Administrator to assist the States in carrying out 
their statutory role as primary implementers of the NAAQS'' (id. at 
470 [emphasis in original]). However, the Court also noted that 
CASAC's ``advice concerning certain aspects of `adverse public 
health . . . effects' from various attainment strategies is 
unquestionably pertinent'' to the NAAQS rulemaking record and 
relevant to the standard setting process (id. at 470 n.2).
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B. Related O3 Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under sections 110 and 171 through 185 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 such standards 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 
nationwide reductions in emissions of O3 precursors and 
other air pollutants under Title II of the Act, 42 U.S.C. 7521-7574, 
which involves controls for automobile, truck, bus, motorcycle, nonroad 
engine and equipment, and aircraft emissions; the new source 
performance standards under section 111 of the Act, 42 U.S.C. 7411; and 
the national emissions standards for hazardous air pollutants under 
section 112 of the Act, 42 U.S.C. 7412.

C. History of the Air Quality Criteria and Standards

    Primary and secondary NAAQS were first established for 
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971) based on 
the air quality criteria developed in 1970 (U.S. DHEW, 1970; 35 FR 
4768, March 19, 1970). The EPA set both primary and secondary standards 
at 0.08 parts per million (ppm), as a 1-hour average of total 
photochemical oxidants, not to be exceeded more than one hour per year. 
Since that time, the EPA has reviewed the air quality criteria and 
standards a number of times, with the most recent review being 
completed in 2015.
    The EPA initiated the first periodic review of the NAAQS for 
photochemical oxidants in 1977. Based on the 1978 air quality criteria 
document (AQCD [U.S. EPA, 1978]), the EPA proposed revisions to the 
original NAAQS in 1978 (43 FR 26962, June 22, 1978) and adopted 
revisions in 1979 (44 FR 8202, February 8, 1979). At that time, the EPA 
changed the indicator from photochemical oxidants to O3, 
revised the level of the primary and secondary standards from 0.08 to 
0.12 ppm and revised the form of both standards from a deterministic 
(i.e., not to be exceeded more than one hour per year) to a statistical 
form. With these changes, attainment of the standards was defined to 
occur when the average number of days per calendar year (across a 3-
year period) with maximum hourly average O3 concentration 
greater than 0.12 ppm equaled one or less (44 FR 8202, February 8, 
1979; 43 FR 26962, June 22, 1978). Several petitioners challenged the 
1979 decision. Among those, one claimed natural O3 
concentrations and other physical phenomena made the standard 
unattainable in the Houston area.\6\ The U.S. Court of Appeals for the 
District of Columbia Circuit (D.C. Circuit) rejected this argument, 
holding (as noted in section I.A above) that attainability and 
technological feasibility are not relevant considerations in the 
promulgation of the NAAQS (American Petroleum Institute v. Costle, 665 
F.2d at 1185). The court also noted that the EPA need not tailor the 
NAAQS to fit each region or locale, pointing out that Congress was 
aware of the difficulty in meeting standards in some locations and had 
addressed it through various compliance-related provisions in the CAA 
(id. at 1184-86).
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    \6\ The EPA has determined that air quality in the area 
including Houston has attained the 1979 1-hour standard (85 FR 8411, 
February 14, 2020).
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    The next periodic reviews of the criteria and standards for 
O3 and other

[[Page 87260]]

photochemical oxidants began in 1982 and 1983, respectively (47 FR 
11561, March 17, 1982; 48 FR 38009, August 22, 1983). As part of these 
reviews, the EPA published an AQCD, a Staff Paper, and a supplement to 
the AQCD (U.S. EPA, 1986; U.S. EPA, 1989; U.S. EPA, 1992). The schedule 
for completion of this review was governed by court order. In August of 
1992, the EPA proposed to retain the existing primary and secondary 
standards (57 FR 35542, August 10, 1992). In March 1993, the EPA 
concluded this review by finalizing its proposed decision to retain the 
standards, without revision (58 FR 13008, March 9, 1993).
    In the next review of the air quality criteria and standards for 
O3 and other photochemical oxidants, for which the EPA had 
announced in August 1992 its intention to proceed rapidly, the EPA 
developed an AQCD and Staff Paper (57 FR 35542, August 10, 1992; U.S. 
EPA, 1996a; U.S. EPA, 1996b). Based on consideration of these 
assessments, the EPA proposed revisions to both the primary and 
secondary standards (61 FR 65716, December 13, 1996). The EPA completed 
this review in 1997 by revising both standards to 0.08 ppm, as the 
annual fourth-highest daily maximum 8-hour average concentration, 
averaged over three years (62 FR 38856, July 18, 1997).
    In response to challenges to the EPA's 1997 decision revising the 
standards, the D.C. Circuit remanded the standards to the EPA, finding 
that section 109 of the CAA, as interpreted by the EPA, effected an 
unconstitutional delegation of legislative authority. See American 
Trucking Ass'ns v. EPA, 175 F.3d 1027, 1034-1040 (D.C. Cir. 1999). The 
court also directed that, in responding to the remand, the EPA should 
consider the potential beneficial health effects of O3 
pollution in shielding the public from the effects of solar ultraviolet 
(UV) radiation, as well as adverse health effects (id. at 1051-53). See 
American Trucking Ass'ns v. EPA, 195 F.3d 4, 10 (D.C. Cir. 1999) 
(granting panel rehearing in part but declining to review the ruling on 
consideration of the potential beneficial effects of O3 
pollution). After granting petitions for certiorari, the U.S. Supreme 
Court unanimously reversed the judgment of the D.C. Circuit on the 
constitutional issue, holding that section 109 of the CAA does not 
unconstitutionally delegate legislative power to the EPA. See Whitman 
v. American Trucking Ass'ns, 531 U.S. 457, 472-74 (2001). The Court 
remanded the case to the D.C. Circuit to consider challenges to the 
1997 O3 NAAQS that had not yet been addressed. On remand, 
the D.C. Circuit found the 1997 O3 NAAQS to be ``neither 
arbitrary nor capricious,'' and so denied the remaining petitions for 
review. See ATA III, 283 F.3d at 379.
    Coincident with the continued litigation of the other issues, the 
EPA responded to the court's 1999 remand to consider the potential 
beneficial health effects of O3 pollution in shielding the 
public from effects of UV radiation (66 FR 57268, Nov. 14, 2001; 68 FR 
614, January 6, 2003). In 2001, the EPA proposed to leave the 1997 
primary standard unchanged (66 FR 57268, Nov. 14, 2001). After 
considering public comment on the proposed decision, the EPA published 
its final response to this remand in 2003, re-affirming the 8-hour 
primary standard set in 1997 (68 FR 614, January 6, 2003).
    The EPA initiated the fourth periodic review of the air quality 
criteria and standards for O3 and other photochemical 
oxidants with a call for information in September 2000 (65 FR 57810, 
September 26, 2000). In this review, the EPA developed an AQCD, Staff 
Paper and related technical support documents and proposed revisions to 
the primary and secondary standards (U.S. EPA, 2006; U.S. EPA, 2007; 72 
FR 37818, July 11, 2007). The review was completed in March 2008 with 
revision of the levels of both the primary and secondary standards from 
0.08 ppm to 0.075 ppm, and retention of the other elements of the prior 
standards (73 FR 16436, March 27, 2008). A number of petitioners filed 
suit challenging this decision.
    In September 2009, the EPA announced its intention to reconsider 
the 2008 O3 standards,\7\ and initiated a rulemaking to do 
so. At the EPA's request, the court held the consolidated cases in 
abeyance pending the EPA's reconsideration of the 2008 decision. In 
January 2010, the EPA issued a notice of proposed rulemaking to 
reconsider the 2008 final decision (75 FR 2938, January 19, 2010). 
Later that year, in view of the need for further consideration and the 
fact that the Agency's next periodic review of the O3 NAAQS 
required under CAA section 109 had already begun (as announced in 
September 2008),\8\ the EPA consolidated the reconsideration with its 
statutorily required periodic review.\9\
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    \7\ The press release of this announcement is available at: 
https://archive.epa.gov/epapages/newsroom_archive/newsreleases/85f90b7711acb0c88525763300617d0d.html.
    \8\ A ``Call for Information'' initiated the review (73 FR 
56581, September 29, 2008).
    \9\ This rulemaking, completed in 2015, concluded the 
reconsideration process.
---------------------------------------------------------------------------

    In light of the EPA's decision to consolidate the reconsideration 
with the review then ongoing, the D.C. Circuit proceeded with the 
litigation on the 2008 O3 NAAQS decision. On July 23, 2013, 
the court upheld the EPA's 2008 primary standard, but remanded the 2008 
secondary standard to the EPA. See Mississippi v. EPA, 744 F.3d 1334 
(D.C. Cir. 2013). With respect to the secondary standard, the court 
held that the EPA's explanation for the setting of the secondary 
standard identical to the revised 8-hour primary standard was 
inadequate under the CAA because the EPA had not adequately explained 
how that standard provided the required public welfare protection.
    At the time of the court's decision, the EPA had already completed 
significant portions of its next statutorily required periodic review 
of the O3 NAAQS, which had been formally initiated in 2008, 
as summarized above. The documents developed for this review included 
the ISA,\10\ Risk and Exposure Assessments (REAs) for health and 
welfare, and PA (Frey, 2014a, Frey, 2014b, Frey, 2014c, U.S. EPA, 2013, 
U.S. EPA, 2014a, U.S. EPA, 2014b, U.S. EPA, 2014c).\11\ In late 2014, 
the EPA proposed to revise the 2008 primary and secondary standards (79 
FR 75234, December 17, 2014). The EPA's final decision in this review 
established the now-current standards (80 FR 65292, October 26, 2015; 
40 CFR 50.19). In this decision, based on consideration of the health 
effects evidence on respiratory effects of O3 in at-risk 
populations, the EPA revised the primary standard from a level of 0.075 
ppm to a level of 0.070 ppm, while retaining all other elements of the 
standard (80 FR 65292, October 26, 2015). The EPA's decision on the 
level for the standard was based on the weight of the scientific 
evidence and quantitative exposure/risk information. The level of the 
secondary standard was also revised from 0.075 ppm to 0.070 ppm based 
on the scientific evidence of O3 effects on welfare, 
particularly the evidence of O3 impacts on vegetation, and 
quantitative analyses available in the review. The other elements of 
the standard were retained. This decision on the secondary standard 
also incorporated the EPA's response to the

[[Page 87261]]

D.C. Circuit's remand of the 2008 secondary standard in Mississippi v. 
EPA, 744 F.3d 1344 (D.C. Cir. 2013).\12\
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    \10\ The ISA, as the AQCD in prior reviews, serves the purpose 
of reviewing the air quality criteria.
    \11\ The PA presents an evaluation, for consideration by the 
Administrator, of the policy implications of the currently available 
scientific information, assessed in the ISA; the quantitative air 
quality, exposure or risk analyses presented in the PA and developed 
in light of the ISA findings; and related limitations and 
uncertainties. The role of the PA is to help ``bridge the gap'' 
between the Agency's scientific assessment and quantitative 
technical analyses, and the judgments required of the Administrator 
in his decisions in the NAAQS review.
    \12\ The 2015 revisions to the NAAQS were accompanied by 
revisions to the data handling procedures, ambient air monitoring 
requirements, the air quality index and several provisions related 
to implementation (80 FR 65292, October 26, 2015).
---------------------------------------------------------------------------

    After publication of the final rule, a number of industry groups, 
environmental and health organizations, and certain states filed 
petitions for judicial review in the D.C. Circuit. The industry and 
state petitioners argued that the revised standards were too stringent, 
while the environmental and health petitioners argued that the revised 
standards were not stringent enough to protect public health and 
welfare as the Act requires. On August 23, 2019, the court issued an 
opinion that denied all the petitions for review with respect to the 
2015 primary standard while also concluding that the EPA had not 
provided a sufficient rationale for aspects of its decision on the 2015 
secondary standard and remanding that standard to the EPA. See Murray 
Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019). The court's 
decision on the secondary standard focused on challenges to particular 
aspects of EPA's decision. The court concluded that EPA's 
identification of particular benchmarks for evaluating the protection 
the standard provided against welfare effects associated with tree 
growth loss was reasonable and consistent with CASAC's advice. However, 
the court held that EPA had not adequately explained its decision to 
focus on a 3-year average for consideration of the cumulative exposure, 
in terms of W126, identified as providing requisite public welfare 
protection, or its decision to not identify a specific level of air 
quality related to visible foliar injury. The EPA's decision not to use 
a seasonal W126 index as the form and averaging time of the secondary 
standard was also challenged, but the court did not reach a decision on 
that issue, concluding that it lacked a basis to assess the EPA's 
rationale because the EPA had not yet fully explained its focus on a 3-
year average W126 in its consideration of the standard. See Murray 
Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019). Accordingly, 
the court remanded the secondary standard to EPA for further 
justification or reconsideration. The court's remand of the secondary 
standard has been considered in reaching the decision, and associated 
conclusions and judgments, described in section III.B.3 below.
    In the August 2019 decision, the court additionally addressed 
arguments regarding considerations of background O3 
concentrations, and socioeconomic and energy impacts. With regard to 
the former, the court rejected the argument that the EPA was required 
to take background O3 concentrations into account when 
setting the NAAQS, holding that the text of CAA section 109(b) 
precluded this interpretation because it would mean that if background 
O3 levels in any part of the country exceeded the level of 
O3 that is requisite to protect public health, the EPA would 
be obliged to set the standard at the higher nonprotective level (id. 
at 622-23). Thus, the court concluded that the EPA did not act 
unlawfully or arbitrarily or capriciously in setting the 2015 NAAQS 
without regard for background O3 (id. at 624). Additionally, 
the court denied arguments that the EPA was required to consider 
adverse economic, social, and energy impacts in determining whether a 
revision of the NAAQS was ``appropriate'' under section 109(d)(1) of 
the CAA (id. at 621-22). The court reasoned that consideration of such 
impacts was precluded by Whitman's holding that the CAA ``unambiguously 
bars cost considerations from the NAAQS-setting process'' (531 U.S. at 
471, summarized in section I.A above). Further, the court explained 
that section 109(d)(2)(C)'s requirement that CASAC advise the EPA ``of 
any adverse public health, welfare, social, economic, or energy effects 
which may result from various strategies for attainment and 
maintenance'' of revised NAAQS had no bearing on whether costs are to 
be considered in setting the NAAQS (Murray Energy Corp. v. EPA, 936 
F.3d at 622). Rather, as described in Whitman and discussed further in 
section I.A above, most of that advice would be relevant to 
implementation but not standard setting (id.).

D. Current Review of the Air Quality Criteria and Standards

    In May 2018, the Administrator directed his Assistant 
Administrators to initiate this current review of the O3 
NAAQS (Pruitt, 2018). In conveying this direction, the Administrator 
further directed the EPA staff to expedite the review, implementing an 
accelerated schedule aimed at completion of the review within the 
statutorily required period (Pruitt, 2018). Accordingly, the EPA took 
immediate steps to proceed with the review. In June 2018, the EPA 
announced the initiation of the periodic reviews of the air quality 
criteria for photochemical oxidants and of the O3 NAAQS and 
issued a call for information in the Federal Register (83 FR 29785, 
June 26, 2018). Two types of information were called for: Information 
regarding significant new O3 research to be considered for 
the ISA for the review, and policy-relevant issues for consideration in 
this NAAQS review. Based in part on the information received in 
response to the call for information, the EPA developed a draft IRP, 
which was made available for consultation with the CASAC and for public 
comment (83 FR 55163, November 2, 2018; 83 FR 55528, November 6, 2018). 
Comments from the CASAC (Cox, 2018) and the public were considered in 
preparing the final IRP (U.S. EPA, 2019b).
    Under the plan outlined in the IRP and consistent with revisions to 
the process identified by the Administrator in his 2018 memo directing 
initiation of the review, the current review of the O3 NAAQS 
has progressed on an accelerated schedule (Pruitt, 2018). The EPA has 
incorporated a number of efficiencies in various aspects of the review 
process, as summarized in the IRP, to support the accelerated schedule 
(Pruitt, 2018). As one example of such an efficiency, rather than 
produce separate documents for the PA and associated quantitative 
analyses, the human exposure and health risk analyses (that inform the 
decision on the primary standard) and the air quality and exposure 
analyses (that inform the decision on the secondary standard) are 
included as appendices in the PA, along with other technical appendices 
that inform these standards decisions. The draft PA (including these 
analyses as appendices) was reviewed by the CASAC and made available 
for public comment while the draft ISA was also being reviewed by the 
CASAC and was available for public comment (84 FR 50836, September 26, 
2019; 84 FR 58711, November 1, 2019).\13\ The CASAC was assisted in its 
review by a pool of consultants with expertise in a number of fields 
(84 FR 38625, August 7, 2019). The approach employed by the CASAC in 
utilizing outside technical expertise represents an additional 
modification of the process from past reviews. Rather than join with 
some or all of the CASAC members in a CASAC review panel as has been 
common in other NAAQS reviews in the past, in this O3 NAAQS 
review (and also in the recent CASAC review of the PA for the

[[Page 87262]]

particulate matter NAAQS), the consultants comprised a pool of 
expertise that CASAC members drew on through the use of specific 
questions, posed in writing prior to the public meeting, regarding 
aspects of the documents being reviewed, obtaining subject matter 
expertise for their review in a focused, efficient and transparent 
manner.
---------------------------------------------------------------------------

    \13\ The draft ISA and draft PA were released for public comment 
and CASAC review on September 26, 2019 and October 31, 2019, 
respectively. The charges for the CASAC review summarized the 
overarching context for the document review (including reference to 
Pruitt [2018], and the CASAC's functions under section 109(d)(2)(B) 
and (C) of the Act), as well as specific charge questions for review 
of each of the documents.
---------------------------------------------------------------------------

    The CASAC discussed its review of both the draft ISA and the draft 
PA over three days at a public meeting in December 2019 (84 FR 58713, 
November 1, 2019).\14\ The CASAC discussed its draft letters describing 
its advice and comments on the documents in a public teleconference in 
early February 2020 (85 FR 4656; January 27, 2020). The letters to the 
Administrator conveying the CASAC advice and comments on the draft PA 
and draft ISA were released later that month (Cox, 2020a; Cox, 2020b).
---------------------------------------------------------------------------

    \14\ While simultaneous review of first drafts of both documents 
has not been usual in past reviews, there have been occurrences of 
the CASAC review of a draft PA (or draft REA when the process 
involved a policy assessment being included within the REA document) 
simultaneous with review of a second (or later) draft ISA (e.g., 73 
FR 19835, April 11, 2008; 73 FR 34739, June 18, 2008; 77 FR 64335, 
October 19, 2012; 78 FR 938, January 7, 2013).
---------------------------------------------------------------------------

    The letters from the CASAC and public comment on the draft ISA and 
draft PA informed completion of the final documents and further 
informed development of the Administrator's proposed and final 
decisions in this review. Comments from the CASAC on the draft ISA were 
considered by the EPA and led to a number of revisions in developing 
the final document. The CASAC review of the draft ISA and the EPA's 
consideration of CASAC comments are described in Appendix 10, section 
10.4.5 of the final ISA. In his reply to the CASAC letter conveying its 
review, ``Administrator Wheeler noted, `for those comments and 
recommendations that are more significant or cross-cutting and which 
were not fully addressed, the Agency will develop a plan to incorporate 
these changes into future O3 ISAs as well as ISAs for other 
criteria pollutant reviews' '' (ISA, p. 10-28; Wheeler, 2020). The ISA 
was completed and made available to the public in April 2020 (85 FR 
21849, April 20, 2020).\15\ Based on the rigorous scientific approach 
utilized in its development, summarized in Appendix 10 of the final 
ISA, the EPA considers the final ISA 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 [O3] in the ambient air, in varying 
quantities'' as required by the CAA (42 U.S.C. 7408(a)(2)).
---------------------------------------------------------------------------

    \15\ The ISA builds on evidence and conclusions from previous 
assessments, focusing on synthesizing and integrating the newly 
available evidence (ISA, section IS.1.1). Past assessments are 
generally cited when providing further, still relevant, details that 
informed the current assessment but are not repeated in the latest 
assessment.
---------------------------------------------------------------------------

    The CASAC comments additionally provided advice with regard to the 
primary and secondary standards, as well as a number of comments 
intended to improve the PA. These comments were considered in 
completing that document (85 FR 31182, May 22, 2020). The CASAC advice 
to the Administrator regarding the O3 standards has also 
been described and considered in the PA, and in sections II and III 
below. The CASAC advice on the primary standard is summarized in II.B.2 
below and its advice on the secondary standard is summarized in section 
III.B.1.b.
    Materials upon which this proposed decision is based, including the 
documents described above, are available to the public in the docket 
for the review.\16\ As in prior NAAQS reviews, the EPA is basing its 
decision in this review on studies and related information included in 
the air quality criteria, which have undergone CASAC and public review. 
The studies assessed in the ISA \17\ and PA, and the integration of the 
scientific evidence presented in them, have undergone extensive 
critical review by the EPA, the CASAC, and the public. The rigor of 
that review makes these studies, and their integrative assessment, the 
most reliable source of scientific information on which to base 
decisions on the NAAQS, decisions that all parties recognize as of 
great import. Decisions on the NAAQS can have profound impacts on 
public health and welfare, and NAAQS decisions 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. Some commenters have referred to and 
discussed individual scientific studies on the health effects of 
O3 that were not included in the ISA (`` `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 ISA. 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 reopening the 
review of the air quality criteria to enable the EPA, the CASAC and the 
public to consider them further.
---------------------------------------------------------------------------

    \16\ The docket for this review, EPA-HQ-OAR-2018-0279, has 
incorporated the ISA docket (EPA-HQ-ORD-2018-0274) by reference. 
Both are publicly accessible at www.regulations.gov.
    \17\ In addition to the review's opening ``Call for 
Information'' (83 FR 29785, June 26, 2018), systematic review 
methodologies were applied to identify relevant scientific findings 
that have emerged since the 2013 ISA, which included peer reviewed 
literature published through July 2011. Search techniques for the 
current ISA identified and evaluated studies and reports that have 
undergone scientific peer review and were published or accepted for 
publication between January 1, 2011 (providing some overlap with the 
cutoff date for the last ISA) and March 30, 2018. Studies published 
after the literature cutoff date for this ISA were also considered 
if they were submitted in response to the Call for Information or 
identified in subsequent phases of ISA development, particularly to 
the extent that they provide new information that affects key 
scientific conclusions (ISA, Appendix 10, section 10.2). References 
that are cited in the 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/index.cfm/project/page/project_id/2737.
---------------------------------------------------------------------------

    This approach, and the decision to rely on studies and related 
information included 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 61144, 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 revise 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 at 13013-13014, March 9, 
1993). In the present case, the EPA's provisional consideration of 
``new'' studies concludes 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 
O3 in

[[Page 87263]]

ambient air made in the air quality criteria. For this reason, 
reopening the air quality criteria review would not be warranted.
    Accordingly, the EPA is basing the final decisions in this review 
on the studies and related information included in the O3 
air quality criteria 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 O3 
NAAQS review, which the EPA expects to begin soon after the conclusion 
of this review 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.

E. Air Quality Information

    Ground level O3 concentrations are a mix of mostly 
tropospheric O3 and some stratospheric O3. 
Tropospheric O3 is formed due to chemical interactions 
involving solar radiation and precursor pollutants including VOCs and 
nitrogen oxides (NOX). Methane (CH4) and carbon 
monoxide (CO) are also important precursors, particularly at the 
regional to global scale. The precursor emissions leading to 
tropospheric O3 formation can result from both man-made 
sources (e.g., motor vehicles and electric power generation) and 
natural sources (e.g., vegetation and wildfires). In addition, 
O3 that is created naturally in the stratosphere also 
contributes to O3 in the troposphere. The stratosphere 
routinely mixes with the troposphere high above the earth's surface 
and, less frequently, there are intrusions of stratospheric air that 
reach deep into the troposphere and even to the surface. Once formed, 
O3 near the surface can be transported by winds before 
eventually being removed from the atmosphere via chemical reactions or 
deposition to surfaces. In sum, O3 concentrations are 
influenced by complex interactions between precursor emissions, 
meteorological conditions, and topographical characteristics (PA, 
section 2.1; ISA, Appendix 1).
    For compliance and other purposes, state and local environmental 
agencies operate O3 monitors across the U.S. and submit the 
data to the EPA. At present, there are approximately 1,300 monitors 
across the U.S. reporting hourly O3 averages during the 
times of the year when local O3 pollution can be important 
(PA, section 2.3.1).\18\ Most of this monitoring is focused on urban 
areas where precursor emissions tend to be largest, as well as 
locations directly downwind of these areas. There are also over 100 
routine monitoring sites in rural areas, including sites in the Clean 
Air Status and Trends Network (CASTNET) which is specifically focused 
on characterizing conditions in rural areas. Based on the monitoring 
data for the three year period from 2016 to 2018, the EPA identified 
142 counties, in which together approximately 106 million Americans 
reside where O3 design values \19\ were above 0.070 ppm, the 
level of the existing NAAQS (PA, section 2.4.1). Across these areas, 
the highest design values are typically observed in California, Texas, 
Denver, around Lake Michigan and along the Northeast Corridor, 
locations with some of the most densely populated areas in the country 
(e.g., PA, Figure 2-8).
---------------------------------------------------------------------------

    \18\ O3 monitoring seasons in each state vary from 
five months (May to September in Oregon and Washington) to year 
round (in 11 states), with March to October being most common (27 
states).
    \19\ A design value is a statistic that summarizes the air 
quality data for a given area in terms of the indicator, averaging 
time, and form of the standard. Design values can be compared to the 
level of the standard and are typically used to designate areas as 
meeting or not meeting the standard and assess progress towards 
meeting the NAAQS.
---------------------------------------------------------------------------

    From a temporal perspective, the highest O3 
concentrations tend to occur during the afternoon and within the warmer 
months of the year due to higher levels of solar radiation and other 
conducive meteorological conditions during these times. The exceptions 
to this general rule include (1) some rural sites where transport of 
O3 from upwind urban areas can occasionally result in high 
nighttime levels of O3, (2) high-elevation sites which can 
be episodically influenced by stratospheric intrusions in other months 
of the year, and (3) mountain basins in the western U.S. where large 
quantities of O3 precursors emissions associated with oil 
and gas development can be trapped in a shallow inversion layer and 
form O3 under clear, calm skies with snow cover during the 
colder months (PA, section 2.1; ISA, Appendix 1).
    Monitoring data indicate long-term reductions in peak O3 
concentrations. For example, monitoring sites operating since 1980 
indicate a 32% reduction in the national average annual fourth highest 
daily maximum 8-hour concentration from 1980 to 2018. (PA, Figure 2-
10). This has been accompanied by appreciable reductions in peak 1-hour 
concentrations, as seen by reductions in annual second highest daily 
maximum 1-hour concentrations (PA, Figure 2-17).
    Concentrations of O3 in ambient air that result from 
natural and non-U.S. anthropogenic sources are collectively referred to 
as U.S. background O3 (USB; PA, section 2.5). As in the last 
review, we generally characterize O3 concentrations that 
would exist in the absence of U.S. anthropogenic emissions (as USB). 
Findings from air quality modeling analyses performed for this review 
to investigate patterns of USB in the U.S. are largely consistent with 
conclusions reached in the last review (PA, section 2.5.4). The current 
modeling analysis indicates spatial variation in USB O3 
concentrations that is related to geography, topography and proximity 
to international borders and is also influenced by seasonal variation, 
with long-range international anthropogenic transport contributions 
peaking in the spring while U.S. anthropogenic contributions tend to 
peak in summer. The West is predicted to have higher USB concentrations 
than the East, with higher contributions from natural and international 
anthropogenic sources that exert influences in western high-elevation 
and near-border areas. The modeling predicts that for both the West and 
the East, days with the highest 8-hour concentrations of O3 
generally occur in summer and are likely to have substantially greater 
concentrations due to U.S. anthropogenic sources. While the USB 
contributions to O3 concentrations on days with the highest 
8-hour concentrations are generally predicted to come largely from 
natural sources, the modeling also indicates that some areas near the 
Mexico border may receive appreciable contributions from a combination 
of natural and international anthropogenic sources on these days. In 
such locations, the modeling suggests the potential for relatively 
infrequent events with substantial background contributions where daily 
maximum 8-hour O3 concentrations approach or exceed the 
level of the current NAAQS (i.e., 70 ppb). This contrasts with most 
monitor locations in the U.S. for which international contributions are 
predicted to be the lowest during the season with the most frequent 
occurrence of daily maximum 8-hour O3 concentrations above 
70 ppb. This is generally because, except for in near-border areas, 
larger international contributions are associated with long-distance 
transport and that is most efficient in the springtime (PA, section 
2.5.4).

II. Rationale for Decision on the Primary Standard

    This section presents the rationale for the Administrator's 
decision to retain the current primary O3 standard. This 
rationale is based on the scientific information presented in the ISA, 
on human health effects associated with

[[Page 87264]]

photochemical oxidants including O3 and pertaining to the 
presence of these pollutants in ambient air. As summarized in section 
I.D above, the ISA was developed based on a thorough review of the 
latest scientific information generally published between January 2011 
and March 2018, as well as more recent studies identified during peer 
review, submitted in response to the Call for Information, or public 
comments on the draft ISA, integrated with the information and 
conclusions from previous assessments (ISA, section IS.1.2 and Appendix 
10, section 10.2). The Administrator's rationale also takes into 
account: (1) The PA evaluation of the policy-relevant information in 
the ISA and presentation of quantitative analyses of air quality, human 
exposure and health risks; (2) CASAC advice and recommendations, as 
reflected in discussions of drafts of the ISA and PA at public meetings 
and in the CASAC's letters to the Administrator; and (3) public 
comments on the proposed decision.
    Within this section, introductory and background information is 
presented in section II.A. Section II.A.1 summarizes the 2015 
establishment of the existing standard, as background for this review. 
Section II.A.2 provides an overview of the currently available health 
effects evidence, and section II.A.3 provides an overview of the 
current exposure and risk information, drawing on the quantitative 
analyses presented in the PA. Section II.B summarizes the basis for the 
proposed decision (II.B.1), discusses public comments on the proposed 
decision (II.B.2), and presents the Administrator's considerations, 
conclusions and decision in this review of the primary standard 
(II.B.3). The decision on the current primary standard is summarized in 
section II.C.

A. Introduction

    As in prior reviews, the general approach to reviewing the current 
primary standard is based, most fundamentally, on using the Agency's 
assessments of the current scientific evidence and associated 
quantitative analyses to inform the Administrator's judgment regarding 
a primary standard for photochemical oxidants that is requisite to 
protect the public health with an adequate margin of safety. The EPA's 
assessments are primarily documented in the ISA and PA, both of which 
have received CASAC review and public comment (84 FR 50836, September 
26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019; 
85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging 
the gap between the scientific assessments of the ISA and the judgments 
required of the Administrator in his decisions on the current standard, 
the PA evaluates policy implications of the assessment of the current 
evidence in ISA and the quantitative exposure and risk analyses 
documented extensively in appendices of the PA. In evaluating the 
public health protection afforded by the current standard, 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 primary standard 
is a public health policy judgment to be made by the Administrator. In 
reaching conclusions on the standard, the decision draws on the 
scientific information and analyses about health effects, population 
exposure and 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 groups.\20\
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    \20\ As noted in section I.A above, consideration of such 
protection is focused on the sensitive group of individuals and not 
a single person in the sensitive group (see S. Rep. No. 91-1196, 
91st Cong., 2d Sess. 10 [1970]).
---------------------------------------------------------------------------

1. Background on the Current Standard
    As a result of the last O3 NAAQS review, completed in 
2015, the level of the primary standard was revised from 0.075 to 0.070 
ppm,\21\ in conjunction with retaining the existing indicator, 
averaging time, and form. This revision, establishing the current 
standard, was based on the scientific evidence and quantitative 
exposure and risk analyses available at that time, as well as the 
Administrator's judgments regarding the available health effects 
evidence, the appropriate degree of public health protection for the 
revised standard, and the available exposure and risk information 
regarding the exposures and risk that may be allowed by such a standard 
(80 FR 65292, October 26, 2015). In establishing this standard, the 
Administrator considered the extensive body of evidence spanning 
several decades documenting the causal relationship between 
O3 exposure and a broad range of respiratory effects (80 FR 
65292, October 26, 2015; 2013 ISA, p. 1-14),\22\ that had been 
augmented by evidence available since the prior review was completed in 
2008. Such effects range from small, reversible changes in pulmonary 
function and pulmonary inflammation (documented in controlled human 
exposure studies involving exposures ranging from 1 to 8 hours) \23\ to 
more serious health outcomes such as asthma-related emergency 
department visits and hospital admissions, which have been associated 
with ambient air concentrations of O3 in epidemiologic 
studies (2013 ISA, section 6.2).\24\ The 2015 decision, which provided 
increased protection for at-risk populations,\25\ such as children and

[[Page 87265]]

people with asthma, against an array of adverse health effects, drew 
upon the available scientific evidence assessed in the 2013 ISA, the 
exposure and risk information presented and assessed in the 2014 health 
REA (HREA), the consideration of that evidence and information in the 
2014 PA, the advice and recommendations of the CASAC, and public 
comments on the proposed decision (79 FR 75234, December 17, 2014).
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    \21\ Although ppm are the units in which the level of the 
standard is defined, the units, ppb, are more commonly used 
throughout this document for greater consistency with the more 
recent literature. The level of the current primary standard, 0.070 
ppm, is equivalent to 70 ppb.
    \22\ In addition to concluding there to be a causal relationship 
between short-term O3 exposures and respiratory effects, 
and that the relationship between longer-term exposure and 
respiratory effects was likely to be causal, the 2013 ISA also 
concluded there likely to be a causal relationship between short-
term exposure and mortality, as well as short-term exposure and 
cardiovascular effects, including related mortality, and that the 
evidence was suggestive of causal relationships between long-term 
exposures and total mortality, cardiovascular effects and 
reproductive, developmental effects, and between short- and long-
term exposure and nervous system effects (2013 ISA, p. 1-14, section 
2.5.2).
    \23\ Study subjects in most of the controlled human exposure 
studies are generally healthy adults.
    \24\ The evidence base also includes experimental animal studies 
that provide insight into potential modes of action, contributing to 
the coherence and robust nature of the evidence.
    \25\ As used here and similarly throughout the document, the 
term population refers to persons having a quality or characteristic 
in common, such as, and including, a specific pre-existing illness 
or a specific age or lifestage. A lifestage refers to a 
distinguishable time frame in an individual's life characterized by 
unique and relatively stable behavioral and/or physiological 
characteristics that are associated with development and growth. 
Identifying at-risk populations includes consideration of intrinsic 
(e.g., genetic or developmental aspects) or acquired (e.g., disease 
or smoking status) factors that increase the risk of health effects 
occurring with exposure to a substance (such as O3) as 
well as extrinsic, nonbiological factors, such as those related to 
socioeconomic status, reduced access to health care, or exposure.
---------------------------------------------------------------------------

    Across the different study types, the controlled human exposure 
studies, which were recognized to provide the most certain evidence 
indicating the occurrence of health effects in humans following 
specific O3 exposures, additionally document the roles of 
ventilation rate \26\ and exposure duration, in addition to exposure 
concentration, in eliciting responses to O3 exposure (80 FR 
65343, October 26, 2015; 2014 PA, section 3.4).\27\ These aspects of 
the evidence were represented in exposure-based analyses developed to 
inform the NAAQS decision with estimates of exposure and risk 
associated with air quality conditions just meeting the then-existing 
standard, and also for air quality conditions just meeting potential 
alternative standards (U.S. EPA, 2014a, hereafter 2014 HREA). The 
exposure-based analyses given greatest weight in the Administrator's 
consideration of the HREA estimates involved comparison of estimates 
for study area populations of children of exposure at elevated exertion 
to exposure benchmark concentrations (exposures of concern). The 
benchmark concentrations (60, 70 and 80 ppb) were identified from 
controlled human exposure studies (conducted with generally healthy 
adults).
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    \26\ Ventilation rate (VE) is a specific technical 
term referring to breathing rate in terms of volume of air taken 
into the body per unit of time. A person engaged in different 
activities will exert themselves at different levels and experience 
different ventilation rates.
    \27\ For example, the exposure concentrations eliciting a given 
level of response in subjects at rest are higher than those 
eliciting such response in subjects exposed while at elevated 
ventilation, such as while exercising (2013 ISA, section 6.2.1.1).
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    In weighing the health effects evidence and making judgments 
regarding the public health significance of the quantitative estimates 
of exposures and risks allowed by the then-existing standard and 
potential alternative standards considered, as well as judgments 
regarding margin of safety, the Administrator's 2015 decision 
considered the currently available information and commonly accepted 
guidelines or criteria within the public health community, including 
statements of the American Thoracic Society (ATS), an organization of 
respiratory disease specialists, advice from the CASAC, and public 
comments. In so doing, she recognized that the determination of what 
constitutes an adequate margin of safety is expressly left to the 
judgment of the EPA Administrator. See Lead Industries Ass'n v. EPA, 
647 F.2d 1130, 1161-62 (D.C. Cir 1980); Mississippi v. EPA, 744 F.3d 
1334, 1353 (D.C. Cir. 2013). In NAAQS reviews generally, evaluations of 
how particular primary standards address the requirement to provide an 
adequate margin of safety include consideration of such factors as the 
nature and severity of the health effects, the size of the sensitive 
population(s) at risk, and the kind and degree of the uncertainties 
present. Consistent with past practice and long-standing judicial 
precedent, the Administrator took the need for an adequate margin of 
safety into account as an integral part of her decision-making.
    In the decisions regarding adequacy of protection provided by the 
then-existing primary standard and on alternatives for a new revised 
standard, primary consideration was given to the evidence of 
respiratory effects from controlled human exposure studies, including 
those newly available in the review, and for which the exposure 
concentrations were at the lower end of those studied (80 FR 65342-47 
and 65362-66, October 26, 2015). This emphasis was consistent with 
comments on the strength of this evidence from the CASAC at that time 
(Frey, 2014b, p. 5). In placing weight on these studies, the 
Administrator at that time took note of the variety of respiratory 
effects reported from the studies of healthy adults engaged in quasi-
continuous exercise within a 6.6-hour exposure to O3 
concentrations of 60 ppb and higher.\28\ The lowest exposure 
concentration in such studies for which a combination of statistically 
significant reduction in lung function and increase in respiratory 
symptoms was somewhat above 70 ppb,\29\ while reduced lung function and 
increased pulmonary inflammation were reported following such exposures 
to O3 concentrations as low as 60 ppb. In considering these 
findings, the Administrator noted that the combination of 
O3-induced lung function decrements and respiratory symptoms 
met ATS criteria for an adverse response,\30\ and noted CASAC comments, 
which included a caution regarding the potential for effects in some 
groups of people, such as people with asthma, at exposure 
concentrations below those affecting healthy subjects (Frey, 2014b, pp. 
5-6; 80 FR 65343, October 26, 2015). With regard to the epidemiologic 
evidence, the Administrator noted the ISA finding that the pattern of 
effects observed across the range of exposures assessed in the 
controlled human exposure studies, increasing in severity at higher 
exposures, is coherent with (i.e., reasonably related to) the health 
outcomes reported to be associated with ambient air concentrations in 
epidemiologic studies. Additionally, while recognizing that most 
O3 epidemiologic studies reported health outcome 
associations with O3 concentrations in ambient air that 
violated the then-existing standard, the Administrator took note of a 
study that reported associations between short-term O3 
concentrations and asthma emergency department visits in children and 
adults in a U.S. location that would have met the then-existing 
standard over the entire 5-year study period (80 FR 65344, October 26, 
2015; Mar and Koenig, 2009).\31\ Taken together, the Administrator 
concluded that the scientific evidence from controlled human exposure 
and epidemiologic studies called into question the adequacy of the 
public health protection provided by the 75 ppb standard that had been 
set in 2008.
---------------------------------------------------------------------------

    \28\ The studies given primary focus were those in which 
O3 exposures occurred over the course of 6.6 hours during 
which the subjects engaged in six 50-minute exercise periods 
separated by 10-minute rest periods, with a 35-minute lunch period 
occurring after the third hour (e.g., Folinsbee et al., 1988 and 
Schelegle et al., 2009). Responses after O3 exposure were 
compared to those after filtered air exposure.
    \29\ For the 70 ppb target exposure, Schelegle et al. (2009) 
reported, based on O3 measurements during the six 50-
minute exercise periods, that the mean O3 concentration 
during the exercise portion of the study protocol was 72 ppb. Based 
on the six exercise period measurements, the time weighted average 
concentration across the full 6.6-hour exposure was 73 ppb 
(Schelegle et al., 2009).
    \30\ The most recent statement from the ATS available at the 
time of the 2015 decision stated that ``[i]n drawing the distinction 
between adverse and nonadverse reversible effects, this committee 
recommended that reversible loss of lung function in combination 
with the presence of symptoms should be considered as adverse'' 
(ATS, 2000).
    \31\ The design values in this location during the study period 
were at or somewhat below 75 ppb (Wells, 2012).
---------------------------------------------------------------------------

    In considering the exposure and risk information, the 
Administrator's 2015 decision gave particular attention to the 
exposure-based comparison-to-benchmarks analysis, focusing on the 
estimates of exposures of concern for

[[Page 87266]]

children \32\ in 15 urban study areas for air quality conditions just 
meeting the then-current standard. Consistent with the finding that 
larger percentages of children than adults were estimated to experience 
exposures at or above benchmarks, the Administrator focused on the 
results for all children and for children with asthma, noting that the 
results for these two groups, in terms of percent of the population 
group, are virtually indistinguishable (2014 HREA, sections 5.3.2, 
5.4.1.5 and section 5F-1). The Administrator placed the greatest weight 
on estimates of two or more days with occurrences of exposures at or 
above the benchmarks, in light of her increased concern about the 
potential for adverse responses with repeated occurrences of such 
exposures, noting that the types of effects shown to occur following 
exposures to O3 concentrations from 60 ppb to 80 ppb, such 
as inflammation, if occurring repeatedly as a result of repeated 
exposure, could potentially result in more severe effects (80 FR 65343, 
65345, October 26, 2015; 2013 ISA, section 6.2.3).\33\ The 
Administrator also considered estimates for single exposures at or 
above the higher benchmarks of 70 and 80 ppb (80 FR 65345, October 26, 
2015). With regard to the 60 ppb benchmark, while the Administrator 
recognized the effects reported from controlled human exposure studies 
of 60 ppb to be less severe than those for higher O3 
concentrations, she also recognized there were limitations and 
uncertainties in the evidence base with regard to unstudied population 
groups. As a result, she judged it appropriate for the standard, in 
providing an adequate margin of safety, to provide some control of 
exposures at or above the 60 ppb benchmark (80 FR 65345-65346, October 
26, 2015).
---------------------------------------------------------------------------

    \32\ Consideration focused on estimates for children, reflecting 
the finding that the estimates for percent of children experiencing 
an exposure at or above the benchmarks were higher than percent of 
adults due to the greater time children spend outdoors engaged in 
activities at elevated exertion (2014 HREA, section 5.3.2).
    \33\ In addition to recognizing the potential for continued 
inflammation to evolve into other outcomes, the 2013 ISA also 
recognized that inflammation induced by a single exposure (or 
several exposures over the course of a summer) can resolve entirely 
(2013 ISA, p. 6-76; 80 FR 65331, October 26, 2015).
---------------------------------------------------------------------------

    In considering public health implications of the exposure and risk 
information, the Administrator concluded that the exposures and risks 
projected to remain upon meeting the then-current (75 ppb) standard 
were reasonably judged important from a public health perspective. This 
conclusion was particularly based on her judgment that it is 
appropriate to set a standard that would be expected to eliminate, or 
almost eliminate, the occurrence of exposures, while at moderate 
exertion, at or above 70 and 80 ppb (80 FR 65346, October 26, 2015). In 
addition, given that in the air quality scenario for the existing 
standard, the average percent of children estimated to experience two 
or more days with exposures at or above the 60 ppb benchmark approached 
10% in some urban study areas (on average across the analysis years), 
the Administrator concluded that the existing standard did not 
incorporate an adequate margin of safety against the potentially 
adverse effects that could occur following repeated exposures at or 
above 60 ppb (80 FR 65345-46, October 26, 2015). Thus, the exposure and 
risk estimates \34\ were judged to support a conclusion that the 
existing standard was not sufficiently protective and did not 
incorporate an adequate margin of safety. In consideration of all of 
the above, as well as the CASAC advice, which included the unanimous 
recommendation ``that the Administrator revise the current primary 
ozone standard to protect public health'' (Frey, 2014b, p. 5),\35\ the 
Administrator concluded that the then-current primary O3 
standard (with its level of 75 ppb) was not requisite to protect public 
health with an adequate margin of safety, and should be revised to 
provide increased public health protection (80 FR 65346, October 26, 
2015).
---------------------------------------------------------------------------

    \34\ Although the Administrator recognized increased uncertainty 
in and placed less weight on the other types of HREA risk estimates, 
she found they supported her conclusion of public health importance 
on a broad national scale (80 FR 65347).
    \35\ The Administrator also noted that the CASAC for the prior 
review (2008) likewise recommended the standard level be revised 
below 75 ppb based on the evidence and information in the record for 
the 2008 decision (Samet, 2011; Frey and Samet, 2012).
---------------------------------------------------------------------------

    With regard to the most appropriate indicator for the revised 
standard, key considerations included the finding that O3 is 
the only photochemical oxidant (other than nitrogen dioxide) that is 
routinely monitored and for which a comprehensive database exists, and 
the consideration that, since the precursor emissions that lead to the 
formation of O3 also generally lead to the formation of 
other photochemical oxidants, measures leading to reductions in 
population exposures to O3 can generally be expected to lead 
to reductions in other photochemical oxidants (2013 ISA, section 3.6; 
80 FR 65347, October 26, 2015). The CASAC also indicated O3 
to be the appropriate indicator ``based on its causal or likely causal 
associations with multiple adverse health outcomes and its 
representation of a class of pollutants known as photochemical 
oxidants'' (Frey, 2014b, p. ii). Based on all of these considerations 
and public comments, the Administrator retained O3 as the 
indicator for the primary standard (80 FR 65347, October 26, 2015).
    With regard to averaging time, eight hours was the duration 
established in 1997 with the replacement of the then-existing 1-hour 
standard (62 FR 38856, July 18, 1997). The decision at that time was 
based on evidence from numerous controlled human exposure studies 
reporting adverse respiratory effects resulting from 6- to 8-hour 
exposures, as well as quantitative analyses indicating the control 
provided by an 8-hour averaging time of both 8-hour and 1-hour peak 
exposures and associated health risk (62 FR 38861, July 18, 1997; U.S. 
EPA, 1996b). The 1997 decision was also consistent with CASAC advice at 
that time (62 FR 38861, July 18, 1997; 61 FR 65727, December 13, 1996). 
For similar reasons, the 8-hour averaging time was retained in the 
subsequent 2008 review (73 FR 16436, March 27, 2008). In 2015, the 
decision, based on then-available health effects information, was to 
again retain the 8-hour averaging time, as appropriate for addressing 
health effects associated with short-term exposures to ambient air 
O3, and based on the conclusion that it could effectively 
limit health effects attributable to both short- and long-term 
O3 exposures (80 FR 65348, 65350, October 26, 2015).
    With regard to the form for the standard, the existing nth-high 
metric form had been established in the 1997 review, when the form was 
revised from an expected exceedance form. At that time, it was 
recognized that a concentration-based form, by giving proportionally 
more weight to years when 8-hour O3 concentrations are well 
above the level of the standard than years when concentrations are just 
above the level, better reflects the continuum of health effects 
associated with increasing O3 concentrations than does an 
expected exceedance form (80 FR 65350-65352, October 26, 2015).\36\ The 
subsequent 2008 review also

[[Page 87267]]

considered the potential value of a percentile-based form, but the EPA 
concluded that, because of the differing lengths of the monitoring 
season for O3 across the U.S., such a form would not be 
effective in ensuring the same degree of public health protection 
across the country (73 FR 16474-75, March 27, 2008). Additionally, the 
EPA recognized the importance of a form that provides stability to 
ongoing control programs and insulation from the impacts of extreme 
meteorological events that are conducive to O3 occurrence 
(73 FR 16474-16475, March 27, 2008). In the 2015 decision, based on all 
of these considerations, and including advice from the CASAC, which 
stated that this form ``provides health protection while allowing for 
atypical meteorological conditions that can lead to abnormally high 
ambient ozone concentrations which, in turn, provides programmatic 
stability'' (Frey, 2014b, p. 6), the existing form (the annual fourth-
highest daily maximum 8-hour O3 average concentration, 
averaged over three consecutive years) was retained (80 FR 65352, 
October 26, 2015).
---------------------------------------------------------------------------

    \36\ With regard to a specific concentration-based form, the 
fourth-highest daily maximum was selected in 1997, recognizing that 
a less restrictive form (e.g., fifth highest) would allow a larger 
percentage of sites to experience O3 peaks above the 
level of the standard, and would allow more days on which the level 
of the standard may be exceeded when the site attains the standard 
(62 FR 38868-38873, July 18, 1997), and there was no basis 
identified for selection of a more restrictive form (62 FR 38856, 
July 18, 1997).
---------------------------------------------------------------------------

    As for the decision on adequacy of protection provided by the 
combination of all elements of the existing standard, the 2015 decision 
to set the level of the revised standard at 70 ppb placed the greatest 
weight on the results of controlled human exposure studies and on 
quantitative analyses based on information from these studies, 
particularly analyses of O3 exposures of concern, consistent 
with CASAC advice and interpretation of the scientific evidence (80 FR 
65362, October 26, 2015; Frey, 2014b).\37\ This weighting reflected the 
recognition that controlled human exposure studies provide the most 
certain evidence indicating the occurrence of health effects in humans 
following specific O3 exposures, and, in particular, that 
the effects reported in the controlled human exposure studies are due 
solely to O3 exposures, and are not complicated by the 
presence of co-occurring pollutants or pollutant mixtures (as is the 
case in epidemiologic studies) (80 FR 65362-65363, October 26, 2015). 
With regard to this evidence, the Administrator at that time recognized 
that: (1) The largest respiratory effects, and the broadest range of 
effects, have been studied and reported following exposures to 80 ppb 
O3 or higher (i.e., decreased lung function, increased 
airway inflammation, increased respiratory symptoms, airway 
hyperresponsiveness, and decreased lung host defense); (2) exposures to 
O3 concentrations somewhat above 70 ppb have been shown to 
both decrease lung function and to result in respiratory symptoms; and 
(3) exposures to O3 concentrations as low as 60 ppb have 
been shown to decrease lung function and to increase airway 
inflammation (80 FR 65363, October 26, 2015). The Administrator also 
noted that 70 ppb was well below the O3 exposure 
concentration documented to result in the widest range of respiratory 
effects (i.e., 80 ppb), and below the lowest O3 exposure 
concentration shown in 6.6 hour exposures with quasi-continuous 
exercise to result in the combination of lung function decrements and 
respiratory symptoms (80 FR 65363, October 26, 2015).
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    \37\ The Administrator viewed the results of other quantitative 
analyses in this review--the lung function risk assessment, analyses 
of O3 air quality in locations of epidemiologic studies, 
and epidemiologic-study-based quantitative health risk assessment--
as being of less utility for selecting a particular standard level 
among a range of options (80 FR 65362, October 26, 2015).
---------------------------------------------------------------------------

    In considering the degree of protection to be provided by a revised 
standard, and the extent to which that standard would be expected to 
limit population exposures to the broad range of O3 
exposures shown to result in health effects, the Administrator focused 
particularly on the HREA estimates of two or more exposures of concern. 
Placing the most emphasis on a standard that limits repeated 
occurrences of exposures at or above the 70 and 80 ppb benchmarks, 
while at elevated ventilation, the Administrator noted that a revised 
standard with a level of 70 ppb was estimated to eliminate the 
occurrence of two or more days with exposures at or above 80 ppb and to 
virtually eliminate the occurrence of two or more days with exposures 
at or above 70 ppb for all children and children with asthma, even in 
the worst-case year and location evaluated (80 FR 65363-65364, October 
26, 2015).\38\ The Administrator's consideration of exposure estimates 
at or above the 60 ppb benchmark (focused most particularly on multiple 
occurrences), an exposure to which the Administrator was less confident 
would result in adverse effects,\39\ as discussed above, was primarily 
in the context of considering the extent to which the health protection 
provided by a revised standard included a margin of safety against the 
occurrence of adverse O3-induced effects (80 FR 65364, 
October 26, 2015). In this context, the Administrator noted that a 
revised standard with a level of 70 ppb was estimated to protect the 
vast majority of children in urban study areas (i.e., about 96% to more 
than 99% of children in individual areas) from experiencing two or more 
days with exposures at or above 60 ppb (while at moderate or greater 
exertion). This represented a more than 60% reduction in repeated 
exposures over the estimates for the then-existing standard, with its 
level of 75 ppb.
---------------------------------------------------------------------------

    \38\ Under conditions just meeting an alternative standard with 
a level of 70 ppb across the 15 urban study areas, the estimate for 
two or more days with exposures at or above 70 ppb was 0.4% of 
children, in the worst year and worst area (80 FR 65313, Table 1, 
October 26, 2015).
    \39\ The Administrator was ``notably less confident in the 
adversity to public health of the respiratory effects that have been 
observed following exposures to O3 concentrations as low 
as 60 ppb,'' based on her consideration of the ATS statement on 
judging adversity from transient lung function decrements alone, the 
uncertainty in the potential for such decrements to increase the 
risk of other, more serious respiratory effects in a population (per 
ATS recommendations on population-level risk), and the less clear 
CASAC advice regarding potential adversity of effects at 60 ppb 
compared to higher concentrations studied (80 FR 65363, October 26, 
2015).
---------------------------------------------------------------------------

    Given the considerable protection provided against repeated 
exposures of concern for all three benchmarks, including the 60 ppb 
benchmark, the Administrator judged that a standard with a level of 70 
ppb would incorporate a margin of safety against the adverse 
O3-induced effects shown to occur in the controlled human 
exposure studies following exposures (while at moderate or greater 
exertion) to a concentration somewhat higher than 70 ppb (80 FR 65364, 
October 26, 2015).\40\ The Administrator also judged the HREA estimates 
of one or more exposures (while at moderate or greater exertion) at or 
above 60 ppb to also provide support for her somewhat broader 
conclusion that ``a standard with a level of 70 ppb would incorporate 
an adequate margin of safety against the occurrence of O3 
exposures that can result in effects that are adverse to public 
health'' (80 FR 65364, October 26, 2015).\41\ Although she placed less

[[Page 87268]]

weight on the other HREA risk estimates and epidemiologic evidence for 
considering the standard level, in light of associated uncertainties, 
the Administrator judged that a standard with a level of 70 ppb would 
be expected to result in important reductions in the population-level 
risk of endpoints on which these types of information are focused and 
provide associated additional public health protection, beyond that 
provided by the then-existing standard (80 FR 65364, October 26, 2015). 
In summary, based on the evidence, exposure and risk information, 
advice from the CASAC, and public comments, the 2015 decision was to 
revise the primary standard to be 70 ppb, in terms of the 3-year 
average of annual fourth-highest daily maximum 8-hour average 
O3 concentrations, to provide the requisite protection of 
public health, including the health of at-risk populations, with an 
adequate margin of safety (80 FR 65365, October 26, 2015).
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    \40\ In so judging, she noted that the CASAC had recognized the 
choice of a standard level within the range it recommended based on 
the scientific evidence (which was inclusive of 70 ppb) to be a 
policy judgment (80 FR 65355, October 26, 2015; Frey, 2014b).
    \41\ While the Administrator was less concerned about single 
exposures, especially for the 60 ppb benchmark, she judged the HREA 
one-or-more estimates informative to margin of safety 
considerations. In this regard, she noted that ``a standard with a 
level of 70 ppb is estimated to (1) virtually eliminate all 
occurrences of exposures of concern at or above 80 ppb; (2) protect 
the vast majority of children in urban study areas from experiencing 
any exposures of concern at or above 70 ppb (i.e., >= about 99%, 
based on mean estimates; Table 1); and (3) to achieve substantial 
reductions, compared to the [then-]current standard, in the 
occurrence of one or more exposures of concern at or above 60 ppb 
(i.e., about a 50% reduction; Table 1)'' (80 FR 65364, October 26, 
2015).
---------------------------------------------------------------------------

2. Overview of Health Effects Information
    The information summarized in this section is an overview of the 
scientific assessment of the health effects evidence available in this 
review; the assessment is documented in the ISA and its policy 
implications are further discussed in the PA. In this review, as in 
past reviews, the health effects evidence evaluated in the ISA for 
O3 and related photochemical oxidants is focused on 
O3 (ISA, section IS.1.1). Ozone is concluded to be the most 
prevalent photochemical oxidant present in the atmosphere and the one 
for which there is a very large, well-established evidence base of its 
health and welfare effects (ISA, section IS.1.1). Thus, the current 
health effects evidence and the Agency's review of the evidence, 
including the evidence newly available in this review,\42\ continues to 
focus on O3. The subsections below briefly summarize the 
following aspects of the evidence: The nature of O3-related 
health effects, the potential public health implications and 
populations at risk, and exposure concentrations associated with health 
effects.
---------------------------------------------------------------------------

    \42\ More than 1600 studies are newly available and considered 
in the ISA, including more than 1000 health studies (ISA, Appendix 
10, Figure 10-2).
---------------------------------------------------------------------------

a. Nature of Effects
    The evidence base available in the current review includes decades 
of extensive evidence that clearly describes the role of O3 
in eliciting an array of respiratory effects and recent evidence 
indicates the potential for relationships between O3 
exposure and metabolic effects. As was established in prior reviews, 
the effects for which the evidence is strongest are transient 
decrements in pulmonary function and respiratory symptoms, such as 
coughing and pain on deep inspiration, as a result of short-term 
exposures particularly when breathing at elevated rates (ISA, section 
IS.4.3.1; 2013 ISA, p. 2-26). These effects are demonstrated in the 
large, long-standing evidence base of controlled human exposure studies 
\43\ (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, ISA). The 
epidemiologic evidence base documents consistent, positive associations 
of O3 concentrations in ambient air with lung function 
effects in panel studies (2013 ISA, section 6.2.1.2; ISA, Appendix 3, 
section 3.1.4.1.3), and with more severe health outcomes, including 
asthma-related emergency department visits and hospital admissions 
(2013 ISA, section 6.2.7; ISA, Appendix 3, sections 3.1.5.1 and 
3.1.5.2). Extensive experimental animal evidence informs a detailed 
understanding of mechanisms underlying the short-term respiratory 
effects, and studies in animal models describe effects of longer-term 
O3 exposure on the developing lung (ISA, Appendix 3, 
sections 3.1.11 and 3.2.6).
---------------------------------------------------------------------------

    \43\ The vast majority of the controlled human exposure studies 
(and all of the studies conducted at the lowest exposures) involved 
young healthy adults (typically 18-35 years old) as study subjects 
(2013 ISA, section 6.2.1.1). There are also some controlled human 
exposure studies of one to eight hours duration in older adults and 
adults with asthma, and there are still fewer controlled human 
exposure studies in healthy children (i.e., individuals aged younger 
than 18 years) or children with asthma (See, for example, PA, 
Appendix 3A, Table 3A-3).
---------------------------------------------------------------------------

    The full body of evidence continues to support the conclusions of a 
causal relationship of respiratory effects with short-term 
O3 exposures and of a relationship of respiratory effects 
with longer-term exposures that is likely to be causal (ISA, sections 
IS.4.3.1 and IS.4.3.2). Further, the ISA determines that the 
relationship between short-term O3 exposure and metabolic 
effects \44\ is likely to be causal, based primarily on newly available 
experimental animal evidence (ISA, section IS.4.3.3). The newly 
available evidence, particularly from controlled human exposure studies 
of cardiovascular endpoints, has altered conclusions from the last 
review with regard to relationships between short-term O3 
exposures and cardiovascular effects and mortality, such that the 
evidence no longer supports conclusions that the relationships are 
likely to be causal.\45\
---------------------------------------------------------------------------

    \44\ The term metabolic effects is used in the ISA to refer 
metabolic syndrome (a collection of risk factors including 
alterations in glucose and insulin homeostasis, high blood pressure, 
adiposity, elevated triglycerides and low high density lipoprotein 
cholesterol), diabetes, metabolic disease mortality, and indicators 
of metabolic syndrome that include peripheral inflammation, liver 
function, neuroendocrine signaling, and serum lipids (ISA, section 
IS.4.3.3).
    \45\ The currently available evidence for cardiovascular, 
reproductive and nervous system effects, as well as mortality, is 
``suggestive of, but not sufficient to infer'' a causal relationship 
with short- or long-term O3 exposures (ISA, Table IS-1). 
The evidence is inadequate to infer the presence or absence of a 
causal relationship between long-term O3 exposure and 
cancer (ISA, section IS.4.3.6.6).
---------------------------------------------------------------------------

    With regard to respiratory effects from short-term O3 
exposure, the strongest evidence comes from controlled human exposure 
studies, also available in the last review, demonstrating 
O3-related respiratory effects in generally healthy adults 
(ISA, section IS.1.3.1).\46\ As in the last review, the key evidence 
comes from the body of controlled human exposure studies that document 
respiratory effects in people exposed for short periods (6.6 to 8 
hours) during quasi-continuous exercise.\47\ The potential for 
O3 exposure to elicit health outcomes more serious than 
those assessed in the controlled human exposure studies continues to be 
indicated by the epidemiologic evidence of associations of 
O3 concentrations in ambient air with increased incidence of 
hospital admissions and emergency department visits for an array of 
health outcomes, including asthma exacerbation, chronic obstructive 
pulmonary disease (COPD) exacerbation, respiratory infection, and 
combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5 
and 3.1.6). The strongest such evidence is for asthma-related outcomes 
and specifically asthma-related outcomes for children, indicating an 
increased risk for people with asthma and particularly children with 
asthma (ISA, Appendix 3, section 3.1.5.7).
---------------------------------------------------------------------------

    \46\ The phrases ``healthy adults'' or ``healthy subjects'' are 
used to distinguish from subjects with asthma or other respiratory 
diseases because the ``the study design generally precludes 
inclusion of subjects with serious health conditions,'' such as 
individuals with severe respiratory diseases (2013 ISA, p. lx).
    \47\ A quasi-continuous exercise protocol is common to these 
controlled exposure studies where study subjects complete six 50-
minute periods of exercise, each followed by 10-minute periods of 
rest (e.g., ISA, Appendix 3, section 3.1.4.1.1, and p. 3-11; 2013 
ISA, section 6.2.1.1).
---------------------------------------------------------------------------

    Respiratory responses observed in human subjects exposed to 
O3 for periods of 8 hours or less, while intermittently or 
quasi-continuously exercising, include lung function decrements (e.g., 
based on forced expiratory volume in one second [FEV1] 
measurements),\48\ respiratory symptoms,

[[Page 87269]]

increased airway responsiveness, mild bronchoconstriction (measured as 
an increase in specific airway resistance [sRaw]), and pulmonary 
inflammation, with associated injury and oxidative stress (ISA, 
Appendix 3, section 3.1.4; 2013 ISA, sections 6.2.1 through 6.2.4). The 
available mechanistic evidence, discussed in greater detail in the ISA, 
describes pathways involving the respiratory and nervous systems by 
which O3 results in pain-related respiratory symptoms and 
reflex inhibition of maximal inspiration (inhaling a full, deep 
breath), commonly quantified by decreases in forced vital capacity 
(FVC) and total lung capacity. This reflex inhibition of inspiration 
combined with mild bronchoconstriction contributes to the observed 
decrease in FEV1, the most common metric used to assess 
O3-related lung function effects. The evidence also 
indicates that the additionally observed inflammatory response is 
correlated with mild airway obstruction, generally measured as an 
increase in sRaw (ISA, Appendix 3, section 3.1.3). As described below, 
the prevalence and severity of respiratory effects in controlled human 
exposure studies, including symptoms (e.g., pain on deep inspiration, 
shortness of breath, and cough), increases with increasing 
O3 concentration, exposure duration, and ventilation rate of 
exposed subjects (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).
---------------------------------------------------------------------------

    \48\ In summarizing FEV1 responses from controlled 
human exposure studies as ``decrements'', an O3-induced 
change in FEV1 is typically the difference between the 
change observed with O3 exposure ([post-exposure 
FEV1 minus pre-exposure FEV1] divided by pre-
exposure FEV1) and what is generally an improvement 
observed with filtered air (FA) exposure ([post-exposure 
FEV1 minus pre-exposure FEV1] divided by pre-
exposure FEV1).
---------------------------------------------------------------------------

    Within the evidence base from controlled human exposure studies, 
the majority of studies involve healthy adult subjects (generally 18 to 
35 years), although there are studies involving subjects with asthma, 
and a limited number of studies, generally of durations shorter than 
four hours, involving adolescents and adults older than 50 years. A 
summary of salient observations of O3 effects on lung 
function, based on the controlled human exposure study evidence 
reviewed in the 1996 and 2006 AQCDs, and recognized in the 2013 ISA, 
continues to pertain to this evidence base as it exists today: ``(1) 
young healthy adults exposed to >=80 ppb ozone develop significant 
reversible, transient decrements in pulmonary function and symptoms of 
breathing discomfort if minute ventilation (Ve) or duration of exposure 
is increased sufficiently; (2) relative to young adults, children 
experience similar spirometric responses [i.e., as measured by 
FEV1 and/or FVC] but lower incidence of symptoms from 
O3 exposure; (3) relative to young adults, ozone-induced 
spirometric responses are decreased in older individuals; (4) there is 
a large degree of inter-subject variability in physiologic and 
symptomatic responses to O3, but responses tend to be 
reproducible within a given individual over a period of several months; 
and (5) subjects exposed repeatedly to O3 for several days 
experience an attenuation of spirometric and symptomatic responses on 
successive exposures, which is lost after about a week without 
exposure'' (ISA, Appendix 3, section 3.1.4.1.1, p. 3-11).\49\ Repeated 
daily exposure studies at higher concentrations, such as 300 ppb, have 
found FEV1 responses to be enhanced on the second day of 
exposure. This enhanced response is absent, however, with repeated 
exposure at lower concentrations, perhaps as a result of a more 
complete recovery or less damage to pulmonary tissues (2013 ISA, 
section pp. 6-13 to 6-14; Folinsbee et al., 1994).
---------------------------------------------------------------------------

    \49\ A spirometric response refers to a change in the amount of 
air breathed out of the body (forced expiratory volumes) and the 
associated time to do so (e.g., FEV1).
---------------------------------------------------------------------------

    With regard to airway inflammation and the potential for repeated 
occurrences to contribute to further effects, O3-induced 
respiratory tract inflammation ``can have several potential outcomes: 
(1) Inflammation induced by a single exposure (or several exposures 
over the course of a summer) can resolve entirely; (2) continued acute 
inflammation can evolve into a chronic inflammatory state; (3) 
continued inflammation can alter the structure and function of other 
pulmonary tissue, leading to diseases such as fibrosis; (4) 
inflammation can alter the body's host defense response to inhaled 
microorganisms, particularly in potentially at-risk populations such as 
the very young and old; and (5) inflammation can alter the lung's 
response to other agents such as allergens or toxins'' (2013 ISA, p. 6-
76; ISA Appendix 3, section 3.1.5.6). With regard to O3-
induced increases in airway responsiveness, the controlled human 
exposure study evidence for healthy adults generally indicates 
resolution within 18 to 24 hours after exposure, with slightly longer 
persistence in some individuals (ISA, Appendix 3, section 3.1.4.3.1; 
2013 ISA, p. 6-74; Folinsbee and Hazucha, 2000).
    The array of O3-associated respiratory effects, 
including reduced lung function, respiratory symptoms, increased airway 
responsiveness, and inflammation are of increased significance to 
people with asthma given aspects of the disease that contribute to a 
baseline status that includes chronic airway inflammation and greater 
airway responsiveness than people without asthma (ISA, section 3.1.5). 
For example, O3 exposure of a magnitude that increases 
airway responsiveness may put such people at potential increased risk 
for prolonged bronchoconstriction in response to asthma triggers (ISA, 
Appendix 3, p. 3-7, 3-28; 2013 ISA, section 6.2.9; 2006 AQCD, section 
8.4.2). The increased significance of effects in people with asthma and 
risk of increased exposure for children (from greater frequency of 
outdoor exercise) \50\ is illustrated by the epidemiologic findings of 
positive associations between O3 exposure and asthma-related 
emergency department visits and hospital admissions for children with 
asthma. Thus, the evidence indicates O3 exposure to increase 
the risk of asthma exacerbation, and associated outcomes, in children 
with asthma.
---------------------------------------------------------------------------

    \50\ Children are the age group most likely to be outdoors at 
activity levels corresponding to those that have been associated 
with respiratory effects in the human exposure studies (PA, Appendix 
3D, section 3D.2.5.3), as recognized in section II.A.2.b below.
---------------------------------------------------------------------------

    With regard to an increased susceptibility to infectious diseases, 
the experimental animal evidence continues to indicate, as described in 
the 2013 ISA and past AQCDs, the potential role for O3 
exposures through effects on defense mechanisms of the respiratory 
tract (ISA, section 3.1.7.3; 2013 ISA, section 6.2.5). The evidence 
base regarding respiratory infections and associated effects has been 
augmented in this review by a number of epidemiologic studies reporting 
positive associations between short-term O3 concentrations 
and emergency department visits for a variety of respiratory infection 
endpoints (ISA, Appendix 3, section 3.1.7).
    Although the long-term exposure conditions that may contribute to 
further respiratory effects are less well understood, experimental 
studies, including with nonhuman infant primates, have provided 
evidence relating O3 exposure to asthma-like effects, and 
epidemiologic cohort studies have reported associations of 
O3 concentrations in ambient air with asthma development in 
children (ISA, IS.4.3.2 and Appendix 3, sections 3.2.4.1.3 and 3.2.6). 
The biological plausibility of such a role for O3 has been 
indicated by animal toxicological

[[Page 87270]]

evidence on biological mechanisms (ISA, Appendix 3, sections 3.2.3 and 
3.2.4.1.2).
    Overall, the respiratory effects evidence newly available in this 
review is consistent with the evidence base in the last review, 
supporting a generally similar understanding of the respiratory effects 
of O3 (ISA, Appendix 3, section 3.1.4). A few recent studies 
provide insights in previously unexamined areas, both with regard to 
human study groups and animal models for different effects, while other 
studies confirm and provide depth to prior findings with updated 
protocols and techniques (ISA, Appendix 3, sections 3.1.11 and 3.2.6). 
Newly available epidemiologic studies of hospital admissions and 
emergency department visits for a variety of respiratory outcomes 
supplement the previously available evidence with additional findings 
of consistent associations with O3 concentrations across a 
number of study locations (ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5, 
3.1.6.1.1, 3.1.7.1 and 3.1.8). These studies include a number that 
report positive associations for asthma-related outcomes, as well as a 
few for COPD-related outcomes. Together these epidemiologic studies 
continue to indicate the potential for O3 exposures to 
contribute to such serious health outcomes, particularly for people 
with asthma.
    As was the case for the evidence available in the last review, the 
currently available evidence for health effects other than those of 
O3 exposures on the respiratory system is more uncertain 
than that for respiratory effects.\51\ Further, the evidence now 
available has contributed to changes in conclusions for some of these 
effects. For example, the current evidence for cardiovascular effects 
and mortality, expanded from that in the last review, is no longer 
considered sufficient to conclude that the relationships of short-term 
exposure with these effects are likely to be causal (ISA, sections 
IS.4.3.4 and IS.4.3.5). These changes stem from newly available 
evidence in combination with the uncertainties recognized for the 
evidence available in the last review.\52\ Although there exists 
largely consistent evidence for a limited number of O3-
induced cardiovascular endpoints in animal toxicological studies and 
cardiovascular mortality in epidemiologic studies, there is a general 
lack of coherence between these results and findings in controlled 
human exposure and epidemiologic studies of cardiovascular health 
outcomes (ISA, section IS.1.3.1, Appendix 6, section 6.1.8). The 
relationships are now characterized as suggestive of, but not 
sufficient to infer, a causal relationship (ISA, Appendix 4, section 
4.1.17; Appendix 6, section 6.1.8).
---------------------------------------------------------------------------

    \51\ For example, the available evidence for reproductive and 
developmental effects, as well as for effects on the nervous system, 
is suggestive of, but not sufficient to infer, a causal relationship 
(ISA, section IS.4.3.6.5 and Table IS-1).
    \52\ These aspects of the current evidence base include: (1) A 
now-larger body of controlled human exposure studies providing 
evidence that is not consistent with a cardiovascular effect in 
response to short-term O3 exposure; (2) a paucity of 
epidemiologic evidence indicating more severe cardiovascular 
morbidity endpoints (e.g., emergency department visits and hospital 
visits for cardiovascular endpoints including myocardial 
infarctions, heart failure or stroke) that could connect the 
evidence for impaired vascular and cardiac function from animal 
toxicological studies with the evidence from epidemiologic studies 
of cardiovascular mortality; and (3) the remaining uncertainties and 
limitations recognized in the 2013 ISA (e.g., lack of control for 
potential confounding by copollutants in epidemiologic studies) 
still remain.
---------------------------------------------------------------------------

    With regard to metabolic effects of short-term O3 
exposures, the evidence comes primarily from experimental animal study 
findings, with a limited number of epidemiologic studies (ISA, section 
IS.4.3.3 and Appendix 5, section 5.1.8 and Table 5-3). The exposure 
conditions from the animal studies generally involve much higher 
O3 concentrations (e.g., 4-hour concentrations of 400 to 800 
ppb [ISA, Appendix 5, Tables 5-8 and 5-10]) than those commonly 
occurring in areas of the U.S. where the current standard is met, and 
the concentration in the available controlled human exposure study is 
similarly high, at 300 ppb (ISA, sections 5.1.3, 5.1.5 and 5.1.8, Table 
5-3). The evidence for metabolic effects and long-term exposures is 
concluded to be suggestive of, but not sufficient to infer, a causal 
relationship (ISA, section IS.4.3.6.2).
b. Public Health Implications and At-Risk Populations
    The public health implications of the evidence regarding 
O3-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. Judgments or interpretative statements 
developed by public health experts, particularly experts in respiratory 
health, also inform consideration of public health implications.
    With regard to O3 in ambient air, the potential public 
health impacts relate most importantly to respiratory effects. 
Controlled human exposure studies have documented reduced lung 
function, respiratory symptoms, increased airway responsiveness, and 
inflammation, among other effects, in healthy adults exposed while at 
elevated ventilation, such as while exercising. Ozone effects in 
individuals with compromised respiratory function, such as individuals 
with asthma, are plausibly related to emergency department visits and 
hospital admissions for asthma which have been associated with ambient 
air concentrations of O3 in epidemiologic studies (as 
summarized in section II.A.2.a above; 2013 ISA, section 6.2.7; ISA, 
Appendix 3, sections 3.1.5.1 and 3.1.5.2).
    The clinical significance of individual responses to O3 
exposure depends on the health status of the individual, the magnitude 
of the responses, the severity of respiratory symptoms, and the 
duration of the response. While a particular reduction in 
FEV1 or increase in inflammation or airway responsiveness 
may not be of concern for a healthy group, it may increase the risk of 
a more severe effect in a group with asthma. As a more specific 
example, the same increase in inflammation or airway responsiveness in 
individuals with asthma could predispose them to an asthma exacerbation 
event triggered by an allergen to which they may be sensitized (e.g., 
ISA, Appendix 3, section 3.1.5.6.1; 2013 ISA, sections 6.2.3 and 
6.2.6). Duration and frequency of documented effects is also reasonably 
expected to influence potential adversity and interference with normal 
activity.\53\ In summary, consideration of differences in magnitude or 
severity, and also the relative transience or persistence of the 
responses (e.g., FEV1 changes) and respiratory symptoms, as 
well as pre-existing sensitivity to effects on the respiratory system, 
and other factors, are important to characterizing implications for 
public health effects of an air pollutant such as O3 (ATS, 
2000; Thurston et al., 2017).
---------------------------------------------------------------------------

    \53\ For example, for most healthy individuals moderate effects 
on pulmonary function, such as transient FEV1 decrements 
smaller than 20% or transient respiratory symptoms, such as cough or 
discomfort on exercise or deep breath, would not be expected to 
interfere with normal activity, while larger effects on pulmonary 
function (e.g., FEV1 decrements of 20% or larger lasting 
longer than 24 hours) and/or more severe respiratory symptoms are 
more likely to interfere with normal activity (e.g., PA, p. 3-30; 
2006 AQCD, Table 8-2).
---------------------------------------------------------------------------

    Decisions made in past reviews of the O3 primary 
standard and 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 ATS, an 
organization of respiratory disease specialists, as well as

[[Page 87271]]

the advice from the CASAC. The ATS released its initial statement 
(titled Guidelines as to What Constitutes an Adverse Respiratory Health 
Effect, with Special Reference to Epidemiologic Studies of Air 
Pollution) in 1985 and updated it in 2000 (ATS, 1985; ATS, 2000). The 
ATS described its 2000 statement, considered in the last review of the 
O3 standard, as being intended to ``provide guidance to 
policy makers and others who interpret the scientific evidence on the 
health effects of air pollution for the purposes of risk management'' 
(ATS, 2000). The recent statement further notes 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). Similarly, in the 2000 statement, the ATS describes it as 
proposing ``principles to be used in weighing the evidence and setting 
boundaries'' and states that ``the placement of dividing lines should 
be a societal judgment'' (ATS, 2000). The ATS explicitly states that it 
does ``not attempt to provide an exact definition or fixed list of 
health impacts that are, or are not, adverse,'' providing instead ``a 
number of generalizable `considerations''' (ATS, 2000). The ATS state 
there ``cannot be precise numerical criteria, as broad clinical 
knowledge and scientific judgments, which can change over time, must be 
factors in determining adversity'' (ATS, 2000).
    With regard to pulmonary function decrements, the earlier ATS 
statement concluded that ``small transient changes in forced expiratory 
volume in 1 s[econd] (FEV1) alone were not necessarily 
adverse in healthy individuals but should be considered adverse when 
accompanied by symptoms'' (ATS, 2000). The more recent ATS statement 
continues to support this conclusion and also gives weight to findings 
of small lung function changes in the absence of respiratory symptoms 
in individuals with pre-existing compromised function, such as that 
resulting from asthma (Thurston et al., 2017). In keeping with the 
intent of these statements to avoid specific criteria, neither 
statement provides more specific descriptions of such responses, such 
as with regard to magnitude, duration or frequency, for consideration 
of such conclusions. The earlier ATS statement, in addition to 
emphasizing clinically relevant effects, also emphasized both the need 
to consider changes in ``the risk profile of the exposed population,'' 
and effects on the portion of the population that may have a diminished 
reserve that puts its members at potentially increased risk if affected 
by another agent (ATS, 2000). These concepts, including the 
consideration of the magnitude of effects occurring in just a subset of 
study subjects, continue to be recognized as important in the more 
recent ATS statement (Thurston et al., 2017) and continue to be 
relevant to the evidence base for O3.
    The information newly available in this review regarding 
O3 exposure and health effects among sensitive populations, 
thoroughly evaluated in the ISA, has not altered our understanding of 
human populations at particular risk of health effects from 
O3 exposures (ISA, section IS.4.4). The respiratory effects 
evidence, extending decades into the past and augmented by new studies 
in this review, supports the conclusion that ``individuals with pre-
existing asthma are at greater risk of ozone-related health effects 
based on the substantial and consistent evidence within epidemiologic 
studies and the coherence with toxicological studies'' (ISA, p. IS-57). 
Numerous epidemiologic studies document associations of O3 
with asthma exacerbation. Such studies indicate the associations to be 
strongest for populations of children which is consistent with their 
generally greater time outdoors while at elevated exertion. Together, 
these considerations indicate people with asthma, including 
particularly children with asthma, to be at relatively greater risk of 
O3-related effects than other members of the general 
population (ISA, section IS.4.4.2 and Appendix 3).\54\
---------------------------------------------------------------------------

    \54\ Populations or lifestages can be at increased risk of an 
air pollutant-related health effect due to one or more of a number 
of factors. These factors can be intrinsic, such as physiological 
factors that may influence the internal dose or toxicity of a 
pollutant, or extrinsic, such as sociodemographic, or behavioral 
factors.
---------------------------------------------------------------------------

    With respect to people with asthma, the limited evidence from 
controlled human exposure studies (which are primarily in adult 
subjects) indicates similar magnitude of FEV1 decrements as 
in people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Across 
studies of other respiratory effects of O3 (e.g., increased 
respiratory symptoms, increased airway responsiveness and increased 
lung inflammation), the responses observed in study subjects generally 
do not differ due to the presence of asthma, although the evidence base 
is more limited with regard to study subjects with asthma (ISA, 
Appendix 3, section 3.1.5.7). However, the features of asthma (e.g., 
increased airway responsiveness) contribute to a risk of asthma-related 
responses, such as asthma exacerbation in response to asthma triggers, 
which may increase the risk of more severe health outcomes (ISA, 
section 3.1.5). For example, a particularly strong and consistent 
component of the epidemiologic evidence is the appreciable number of 
epidemiologic studies that demonstrate associations between ambient 
O3 concentrations and hospital admissions and emergency 
department visits for asthma (ISA, section IS.4.4.3.1). The strongest 
associations (e.g., highest effect estimates) or associations more 
likely to be statistically significant are those for childhood age 
groups, which are age groups most likely to spend time outdoors during 
afternoon periods (when O3 may be highest) and at activity 
levels corresponding to those that have been associated with 
respiratory effects in the human exposure studies (ISA, Appendix 3, 
sections 3.1.4.1 and 3.1.4.2).\55\ The epidemiologic studies of 
hospital admissions and emergency department visits are augmented by a 
large body of individual-level epidemiologic panel studies that 
demonstrated associations of short-term ozone concentrations with 
respiratory symptoms in children with asthma. Additional support comes 
from epidemiologic studies that observed O3-associated 
increases in indicators of airway inflammation and oxidative stress in 
children with asthma (ISA, section IS.4.3.1). Together, this evidence 
continues to indicate the increased risk of population groups with 
asthma,

[[Page 87272]]

including particularly, children (ISA, Appendix 3, section 3.1.5.7).
---------------------------------------------------------------------------

    \55\ Evaluations of activity pattern data in current and last 
review indicate children to more frequently spend time outdoors 
during afternoon and early evening hours, while at moderate or 
greater exertion level, than other age groups (PA, Appendix 3D, 
section 3D.2.5.3, including Figure 3D-9; 2014 HREA, section 5.4.1.5 
and Appendix 5G, section 5G-1.4). For example, for days with some 
time spent outdoors, children spend, on average, approximately 2\1/
4\ hours of afternoon time outdoors, 80% of which is at a moderate 
or greater exertion level, regardless of their asthma status (PA, 
Appendix 3D, section 3D.2.5.3). Adults, for days having some time 
spent outdoors, also spend approximately 2\1/4\ hours of afternoon 
time outdoors regardless of their asthma status but the percent of 
afternoon time at moderate or greater exertion levels for adults 
(about 55%) is lower than that observed for children. Such analyses 
also note greater participation in outdoor events during the 
afternoon, compared to other times of day, for children ages 6 
through 19 years old during the warm season months (ISA, Appendix 2, 
section 2.4.1, Table 2-1). Analyses of the limited activity pattern 
data by health status do not indicate asthma status to have 
appreciable impact (PA, Appendix 3D, section 3D.2.5.3; 2014 HREA, 
section 5.4.1.5).
---------------------------------------------------------------------------

    Children, and also outdoor adult workers, are at increased risk 
largely due to their generally greater time spent outdoors while at 
elevated exertion rates (including in summer afternoons and early 
evenings when O3 levels may be higher). This behavior makes 
them more likely to be exposed to O3 in ambient air, under 
conditions contributing to increased dose, e.g., elevated ventilation 
taking greater air volumes into the lungs \56\ (2013 ISA, section 
5.2.2.7). In light of the evidence summarized in the prior paragraph, 
children and outdoor workers with asthma may be at increased risk of 
more severe outcomes, such as asthma exacerbation. Further, there is 
experimental evidence from early life exposures of nonhuman primates 
that indicates potential for effects in childhood when human 
respiratory systems are under development \57\ (ISA, section 
IS.4.4.4.1). Overall, the evidence available in the current review, 
while not increasing our knowledge about susceptibility or at-risk 
status of these population groups, is consistent with that in the last 
review (ISA, section IS.4.4).\58\
---------------------------------------------------------------------------

    \56\ Additionally, compared to adults, children have higher 
ventilation rates relative to their lung volume which tends to 
increase the dose normalized to lung surface area. (ISA, p. IS-60).
    \57\ Human lung development begins during the fetal period and 
continues into early adulthood. This continued development comprises 
an extended window of potential vulnerability to O3 (ISA, 
p. 3-99).
    \58\ Evidence available in the current review for older adults, 
a population identified as at risk in the last review, adds little 
to the evidence previously available (ISA, sections IS.4.4.2 and 
IS.4.4.4.2). The ISA notes, however, that ``[t]he majority of 
evidence for older adults being at increased risk of health effects 
related to ozone exposure comes from studies of short-term ozone 
exposure and mortality evaluated in the 2013 Ozone ISA'' (ISA, p. 
IS-52). Such studies are part of the larger evidence base that is 
now concluded to be suggestive, but not sufficient to infer a causal 
relationship of O3 with mortality (ISA, sections IS.4.3.5 
and IS.4.4.4.2, Appendix 4, section 4.1.16.1 and 4.1.17).
---------------------------------------------------------------------------

    The ISA also expressly considered the evidence regarding 
O3 exposure and health effects among populations with 
several other potential risk factors. As in the last review, the 
evidence for low income and minority populations, remains 
``suggestive'' of increased risk, and includes several inconsistencies 
(ISA, Tables IS-9 and IS-10).\59\ The ISA in the last review 
additionally identified a role for dietary anti-oxidants such as 
vitamins C and E in influencing risk of O3-related effects, 
such as inflammation, as well as a role for genetic factors to also 
confer either an increased or decreased risk (2013 ISA, sections 8.1 
and 8.4.1). No newly available evidence has been evaluated that would 
inform or change these prior conclusions (ISA, section IS.4.4 and Table 
IS-10).
---------------------------------------------------------------------------

    \59\ The 2013 ISA concluded that the overall evidence is 
suggestive of socioeconomic economic status (SES) as a factor 
affecting risk of O3-related health outcomes ``based on 
collective evidence from epidemiologic studies of respiratory 
hospital admissions but inconsistency among epidemiologic studies of 
mortality and reproductive outcomes,'' additionally stating that 
``[f]urther studies are needed to confirm this relationship, 
especially in populations within the U.S.'' (2013 ISA, p. 8-28). The 
evidence available in the current review adds little to the evidence 
available at the time of the last review in this area (ISA, section 
IS.4.4.2 and Table IS-10). Other factors for which the evidence 
remains suggestive of an influence on risk status are being male or 
being female and pre-existing obesity (ISA, Table IS-10).
---------------------------------------------------------------------------

    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, a population most at risk of health effects associated with 
O3 in ambient air is people with asthma. The National Center 
for Health Statistics data for 2017 indicate that approximately 7.9% of 
the U.S. populations has asthma (CDC, 2019; PA, Table 3-1) and this is 
one of the principal populations that the primary O3 NAAQS 
is designed to protect (80 FR 65294, October 26, 2015). Children under 
the age of 18 account for 16.7% of the total U.S. population, with 6.2% 
of the total population being children under 5 years of age (U.S. 
Census Bureau, 2019). Another at-risk population group, also due to 
time and activity outdoors, is outdoor workers.\60\ Population groups 
with relatively greater asthma prevalence, such as populations in 
poverty and children \61\ (CDC, 2019, Tables 3-1 and 4-1; PA, Table 3-
1), might be expected to have a relatively greater potential for 
O3-related health impacts.\62\
---------------------------------------------------------------------------

    \60\ For example, jobs in construction and extraction 
occupations and protective service occupations, as well as 
installation, maintenance and repair occupations and building and 
grounds cleaning and maintenance operations, had high percentages of 
employees who spent part of their workday outdoors (Bureau of Labor 
Statistics, 2017). Such jobs often include physically demanding 
tasks and involve increased ventilation rates, increasing the 
potential for exposure to O3.
    \61\ In 2017 and 2018, the prevalence of asthma in children 0 to 
17 years old was 8.4% and 7.5% respectively (CDC, 2019).
    \62\ As the current standard was set to protect at-risk 
populations, such as people with asthma, populations with asthma 
living in areas not meeting the standard would be expected to be at 
greater risk of effects than others in those areas.
---------------------------------------------------------------------------

c. Exposure Concentrations Associated With Effects
    The extensive evidence base for O3 health effects, 
compiled over several decades, continues to indicate respiratory 
responses to short-term exposures as the most sensitive effects. As at 
the time of the last review, our conclusions regarding O3 
exposure concentrations associated with respiratory effects reflect the 
extensive longstanding evidence base of controlled human exposure 
studies of short-term exposures of people with and without asthma (ISA, 
Appendix 3). As summarized in section II.A.2.a above, these studies 
have documented an array of respiratory effects, including reduced lung 
function, respiratory symptoms, increased airway responsiveness, and 
inflammation, in study subjects following 1- to 8-hour exposures, 
primarily while exercising.\63\
---------------------------------------------------------------------------

    \63\ The risk of more severe health outcomes associated with 
such effects is increased in people with asthma as illustrated by 
the epidemiologic findings of positive associations between 
O3 exposure and asthma-related ED visits and hospital 
admissions.
---------------------------------------------------------------------------

    The current evidence, including that newly available in this 
review, does not alter the scientific conclusions reached in the last 
review on exposure duration and concentrations associated with 
O3-related health effects. These conclusions were largely 
based on the body of evidence from the controlled human exposure 
studies. A limited number of controlled human exposure studies are 
newly available in the current review, with none involving lower 
exposure concentrations than those previously studied or finding 
effects not previously reported (ISA, Appendix 3, section 3.1.4).\64\
---------------------------------------------------------------------------

    \64\ The newly available 3-hour controlled human exposure 
studies (involving intermittent exercise) reported statistically 
significant respiratory response at 120 ppb in adults 55 to 70 years 
old (ISA, Appendix 3, section 3.1.4; PA, Appendix 3A, Table 3A-3).
---------------------------------------------------------------------------

    The severity of observed responses, the percentage of individuals 
responding, and strength of statistical significance at the study group 
level have been found to increase with increasing exposure (ISA; 2013 
ISA; 2006 AQCD). For example, the magnitude of respiratory response 
(e.g., size of lung function reductions and magnitude of symptom 
scores) documented in the controlled human exposure studies is 
influenced by ventilation rate, exposure duration, and exposure 
concentration. When performing physical activities requiring elevated 
exertion, ventilation rate is increased, leading to greater potential 
for health effects due to an increased internal dose (2013 ISA, section 
6.2.1.1, pp. 6-5 to 6-11). Accordingly, the exposure concentrations 
eliciting a given level of response after a given exposure duration is 
lower for subjects exposed while at elevated ventilation, such as while 
exercising (2013 ISA, pp.

[[Page 87273]]

6-5 to 6-6; ISA Appendix 3, section 3.1.4.2). For example, in studies 
of healthy young adults exposed while at rest for 2 hours, 500 ppb is 
the lowest concentration eliciting a statistically significant 
O3-induced group mean lung function decrement, while a 1- to 
2-hour exposure to 120 ppb produces a statistically significant 
response in lung function when the ventilation rate of the group of 
study subjects is sufficiently increased with exercise (2013 ISA, pp. 
6-5 to 6-6).\65\
---------------------------------------------------------------------------

    \65\ The lowest exposure concentration that has elicited a 
statistically significant O3-induced reduction in group 
mean lung function in an exposure of 2 hours or less is 120 ppb, 
occurring in trained cyclists after a 1-hour exposure during 
continuous, very heavy exercise (2013 ISA, section 6.2.1.1; Gong et 
al., 1986) and in young healthy adults after a 2-hour exposure 
during intermittent heavy exercise (2013 ISA, section 6.2.1.1; 
McDonnell et al., 1983).
---------------------------------------------------------------------------

    The exposure conditions (e.g., duration and exercise) given primary 
focus in the past several O3 NAAQS reviews are those of the 
6.6-hour study design, which involves six 50-minute exercise periods 
during which subjects maintain a moderate level of exertion to achieve 
a ventilation rate of approximately 20 L/min per m\2\ body surface area 
while exercising.\66\ The 6.6 hours of exposure in these quasi-
continuous exercise studies has generally occurred in an enclosed 
chamber and the study design includes three hours in each of which is a 
50-minute exercise period and a 10-minute rest period, followed by a 
35-minute lunch (rest) period, which is followed by three more hours of 
exercise and rest, as before lunch.\67\ Most of these studies performed 
to date involve exposure maintained at a constant (unchanging) 
concentration for the full duration, although a subset of studies have 
concentrations that vary (generally in a stepwise manner) across the 
exposure period and are selected so as to achieve a specific target 
concentration as the exposure average.\68\
---------------------------------------------------------------------------

    \66\ Ventilation rate (VE) is a specific technical 
term referring to breathing rate in terms of volume of air taken 
into the body per unit of time. The units for VE are 
usually liters (L) per minute (min). Another related term is 
equivalent ventilation rate (EVR), which refers to VE 
normalized by a person's body surface area in square meters (m\2\). 
Accordingly, the units for EVR are generally L/min per m\2\.
    \67\ A few studies have involved exposures by facemask rather 
than freely breathing in a chamber. To date, there is little 
research differentiating between exposures conducted with a facemask 
and in a chamber since the pulmonary responses of interest do not 
seem to be influenced by the exposure mechanism. However, similar 
responses have been seen in studies using both exposure methods at 
higher O3 concentrations (Adams, 2002; Adams, 2003). In 
the facemask designs, there is a short period of zero O3 
exposure, such that the total period of exposure is closer to 6 
hours than 6.6 (Adams, 2000; Adams, 2002; Adams, 2003).
    \68\ In these studies, the exposure concentration changes for 
each of the six hours in which there is exercise and the 
concentration during the 35-minute lunch is the same as in the prior 
(third) hour with exercise. For example, in the study by Adams 
(2006), the protocol for the 6.6-hour period is as follows: 60 
minutes at 40 ppb, 60 minutes at 70 ppb, 95 minutes at 90 ppb, 60 
minutes at 70 ppb, 60 minutes at 50 ppb and 60 minutes at 40 ppb.
---------------------------------------------------------------------------

    Evidence from studies with similar duration and quasi-continuous 
exercise aspects (6.6-hour duration with six 50-minute exercise 
periods) demonstrates an exposure-response (E-R) relationship for 
O3-induced reduction in lung function (Table 1; ISA, 
Appendix 3, Figure 3-3 PA, Figure 3-2).\69\ No studies of the 6.6-hour 
design are newly available in this review. The previously available 
studies of this design document statistically significant 
O3-induced reduction in lung function (FEV1) and 
increased pulmonary inflammation in young healthy adults exposed to 
O3 concentrations as low as 60 ppb. Statistically 
significant group mean changes in FEV1, also often 
accompanied by statistically significant increases in respiratory 
symptoms, become more consistent across such studies of exposures to 
higher O3 concentrations, such as somewhat above 70 ppb (73 
ppb),\70\ and 80 ppb (Table 1 and Appendix 3A, Table 3A-1). The lowest 
exposures concentration for which these studies document a 
statistically significant increase in respiratory symptoms is somewhat 
above 70 ppb, at 73 ppb (Schelegle et al., 2009). In the 6.6-hour 
studies, the group means of O3-induced \71\ FEV1 
reductions for target exposure concentrations at or below 70 ppb are 
approximately 6% or lower (Table 1). For example, the group means of 
O3-induced FEV1 decrements reported in these 
studies that are statistically significantly different from the 
responses in filtered air are 6.1% for 70 ppb and 1.7% to 3.5% for 60 
ppb (Table 1).
---------------------------------------------------------------------------

    \69\ The relationship also exists for size of FEV1 
decrement with alternative exposure or dose metrics, including total 
inhaled O3 and intake volume averaged concentration (ISA, 
Appendix 3).
    \70\ The design for the study on which the 70 ppb benchmark 
concentration is based, Schelegle et al. (2009), involved varying 
concentrations across the full exposure period, with a 35-minute 
lunch period following the third exposure hour during which the 
exposure concentration remains the same as in the third hour. The 
study reported the average O3 concentration measured 
during each of the six exercise periods. The mean concentration 
across these six values is 72 ppb. The time weighted average for the 
full 6.6-hour exposure period, based on the six reported 
measurements and the study design, is 73 ppb (Schelegle et al., 
2009). Other 6.6-hour studies have not reported measured 
concentrations for each exposure, but have generally reported an 
exposure concentration precision at or tighter than 3 ppb (e.g., 
Adams 2006).
    \71\ Consistent with the ISA and 2013 ISA, the phrase 
``O3-induced'' decrement or reduction in lung function or 
FEV1 refers to the percent change from pre-exposure 
measurement of the O3 exposure minus the percent change 
from pre-exposure measurement of the filtered air exposure (2013 
ISA, p. 6-4).
---------------------------------------------------------------------------

    The group mean O3-induced FEV1 decrements 
generally increase with increasing O3 exposures, reflecting 
increases in both the number of the individuals experiencing 
FEV1 reductions and the magnitude of the FEV1 
reduction (Table 1; ISA, Appendix 3, Figure 3-3; PA, Figure 3-2). For 
example, following 6.6-hour exposures to a lower concentration (40 
ppb), for which decrements were not statistically significant at the 
group mean level, none of 60 subjects across two separate studies 
experienced an O3-induced FEV1 reduction as large 
as 15% or more (Table 1; PA, Appendix 3D, Table 3D-19). The group mean 
O3-induced FEV1 decrements generally increase 
with increasing O3 exposures, reflecting increases in both 
the number of the individuals experiencing FEV1 reductions 
and the magnitude of the FEV1 reduction (Table 1; ISA, 
Appendix 3, Figure 3-3; PA, Figure 3-2). For example, following 6.6-
hour exposures to a lower concentration (40 ppb), for which decrements 
were not statistically significant at the group mean level, none of 60 
subjects across two separate studies experienced an O3-
induced FEV1 reduction as large as 15% or more (Table 1; PA, 
Appendix 3D, Table 3D-19). Across the four experiments (with number of 
subjects ranging from 30 to 59) that have reported results for a 60 ppb 
target exposure,\72\ the number of subjects experiencing this magnitude 
of FEV1 reduction (at or above 15%) varied (zero of 30, one 
of 59, two of 31 and two of 30 exposed subjects), while, together, they 
represent 3% of all 150 subjects. This percentage of subjects (with 
reductions of 15% or more) increased to 10% (three of 31 subjects) for 
the study at 73 ppb (70 ppb target) (PA, Appendix 3D, Table 3D-19; 
Schelegle et al., 2009), and is higher still (16%) in a variable 
exposure study at 80 ppb (PA, Appendix 3D, Table 3D-20; Schelegle et 
al., 2009). In addition to illustrating the E-R relationship, these 
findings also illustrate the considerable variability in magnitude of 
responses observed among study subjects (ISA, Appendix 3, section 
3.1.4.1.1; 2013 ISA, p. 6-13).\73\
---------------------------------------------------------------------------

    \72\ For these four experiments, the average concentration 
across the 6.6 hour period ranged from 60 to 63 ppb (PA, Appendix 
3A, Table 3A-2).
    \73\ With regard to decrements at or above 10%, the percentages 
of study subjects with such a response increased from 7% of the 150 
subjects of the four studies with target exposures of 60 ppb 
(average exposure ranged from 60 to 63) to 19% for the study at 73 
ppb to more than 32% in one variable exposure study of 80 ppb (PA, 
Appendix 3D, Table 3D-20).

[[Page 87274]]



                                  Table 1--Summary of 6.6-Hour Controlled Human Exposure Study-Findings, Healthy Adults
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                       O3 target exposure concentration         Statistically        O3-induced group mean
              Endpoint                                \A\                  significant effect \B\         response \B\                  Study
--------------------------------------------------------------------------------------------------------------------------------------------------------
FEV1 Reduction......................  120 ppb...........................  Yes.....................  -10.3% to -15.9% \C\...  Horstman et al. 1990; Adams
                                                                                                                              2002; Folinsbee et al.
                                                                                                                              (1988); Folinsbee et al.
                                                                                                                              (1994); Adams, 2002; Adams
                                                                                                                              2000; Adams and Ollison
                                                                                                                              1997.\D\
                                      100 ppb...........................  Yes.....................  -8.5% to -13.9% \C\....  Horstman et al., 1990;
                                                                                                                              McDonnell et al., 1991.\D\
                                      87 ppb............................  Yes.....................  -12.2%.................  Schelegle et al., 2009.
                                      80 ppb............................  Yes.....................  -7.5%..................  Horstman et al., 1990.
                                                                                                    -7.7%..................  McDonnell et al., 1991.
                                                                                                    -6.5%..................  Adams, 2002.
                                                                                                    -6.2% to -5.5% \C\.....  Adams, 2003.
                                                                                                    -7.0% to -6.1% \C\.....  Adams, 2006.
                                                                                                    -7.8%..................  Schelegle et al., 2009.
                                                                          ND \E\..................  -3.5%..................  Kim et al., 2011.\F\
                                      70 ppb............................  Yes.....................  -6.1%..................  Schelegle et al., 2009.
                                      60 ppb............................  Yes \G\.................  -2.9%..................  Adams, 2006; Brown et al.,
                                                                                                    -2.8%..................   2008.
                                                                          Yes.....................  -1.7%..................  Kim et al., 2011.
                                                                          No......................  -3.5%..................  Schelegle et al., 2009.
                                      40 ppb............................  No......................  -1.2%..................  Adams, 2002.
                                                                          No......................  -0.2%..................  Adams, 2006.
Increased Respiratory Symptoms......  120 ppb...........................  Yes.....................  Increased symptom        Horstman et al. 1990; Adams
                                                                                                     scores.                  2002; Folinsbee et al.
                                                                                                                              1988; Folinsbee et al.
                                                                                                                              1994; Adams, 2002; Adams
                                                                                                                              2000; Adams and Ollison
                                                                                                                              1997; Horstman et al.,
                                                                                                                              1990; McDonnell et al.,
                                                                                                                              1991; Schelegle et al.,
                                                                                                                              2009; Adams, 2003; Adams,
                                                                                                                              2006.\H\
                                      100 ppb...........................  Yes.....................
                                      87 ppb............................  Yes.....................
                                      80 ppb............................  Yes.....................
                                      70 ppb............................  Yes.....................
                                      60 ppb............................  No......................  .......................  Adams, 2006; Kim et al.,
                                      40 ppb............................  No......................                            2011; Schelegle et al.,
                                                                                                                              2009; Adams, 2002.\H\
Airway Inflammation.................  80 ppb............................  Yes.....................  Multiple indicators \I\  Devlin et al., 1991; Alexis
                                                                                                                              et al., 2010.
                                      60 ppb............................  Yes.....................  Increased neutrophils..  Kim et al., 2011.
Increased Airway Resistance and       120 ppb...........................  Yes.....................  Increased..............  Horstman et al., 1990;
 Responsiveness.                                                                                                              Folinsbee et al., 1994 (O3
                                                                                                                              induced sRaw not
                                                                                                                              reported).
                                      100 ppb...........................  Yes.....................                           Horstman et al., 1990.
                                      80 ppb............................  Yes.....................                           Horstman et al., 1990.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ This refers to the average concentration across the six exercise periods as targeted by authors. This differs from the time-weighted average
  concentration for the full exposure periods (targeted or actual). For example, as shown in Appendix 3A, Table 3A-2, in chamber studies implementing a
  varying concentration protocol with targets of 0.03, 0.07, 0.10, 0.15, 0.08 and 0.05 ppm, the exercise period average concentration is 0.08 ppm while
  the time weighted average for the full exposure period (based on targets) is 0.082 ppm due to the 0.6 hour lunchtime exposure between periods 3 and 4.
  In some cases this also differs from the exposure period average based on study measurements. For example, based on measurements reported in Schelegle
  et al., (2009), the full exposure period average concentration for the 70 ppb target exposure is 73 ppb, and the average concentration during exercise
  is 72 ppb.
\B\ Statistical significance based on the O3 compared to filtered air response at the study group mean (rounded here to decimal).
\C\ Ranges reflect the minimum to maximum FEV1 decrements across multiple exposure designs and studies. Study-specific values and exposure details
  provided in the PA, Appendix 3A, Tables 3A-1 and 3A-2, respectively.
\D\ Citations for specific FEV1 findings for exposures above 70 ppb are provided in PA, Appendix 3A, Table 3A-1.
\E\ ND (not determined) indicates these data have not been subjected to statistical testing.
\F\ The data for 30 subjects exposed to 80 ppb by Kim et al. (2011) are presented in Figure 5 of McDonnell et al. (2012).
\G\ Adams (2006) reported FEV1 data for 60 ppb exposure by both constant and varying concentration designs. Subsequent analysis of the FEV1 data from
  the former found the group mean O3 response to be statistically significant (p < 0.002) (Brown et al., 2008; 2013 ISA, section 6.2.1.1). The varying-
  concentration design data were not analyzed by Brown et al., 2008.
\H\ Citations for study-specific respiratory symptoms findings are provided in the PA, Appendix 3A, Table 3A-1.
\I\ Increased numbers of bronchoalveolar neutrophils, permeability of respiratory tract epithelial lining, cell damage, production of proinflammatory
  cytokines and prostaglandins (ISA, Appendix 3, section 3.1.4.4.1; 2013 ISA, section 6.2.3.1).

    For shorter exposure periods (e.g., one to two hours), with heavy 
intermittent or very heavy continuous exercise, higher exposure 
concentrations, ranging up from 80 ppb up to 400 ppb, have been studied 
(ISA, section 3.1; 2013 ISA, section 6.2.1.1; 2006 AQCD, chapter 6; PA, 
Appendix 3A, Table 3A-3). Across these shorter-duration studies (which 
involved ventilation rates 2-3 times greater than in the prolonged 
[6.6- or 8-hour] exposure studies) the lowest exposure concentration 
for which statistically significant respiratory effects were reported 
is 120 ppb, for a 1-hour exposure combined with continuous very heavy 
exercise and a 2-hour exposure with intermittent heavy exercise. As 
recognized above, the increased ventilation rate associated with 
increased exertion increases the

[[Page 87275]]

amount of O3 entering the lung, where depending on dose and 
the individual's susceptibility, it may cause respiratory effects (2013 
ISA, section 6.2.1.1). Thus, for exposures involving a lower exertion 
level, a comparable response would not be expected to occur without a 
longer exposure duration (ISA, Appendix 3, Figure 3-3; PA, Appendix 3A, 
Table 3A-1).
    With regard to the epidemiologic studies reporting associations 
between O3 and respiratory health outcomes such as asthma-
related emergency department visits and hospitalizations, these studies 
are generally focused on investigating the existence of a relationship 
between O3 occurring in ambient air and specific health 
outcomes. Accordingly, while as a whole, this evidence base of 
epidemiologic studies provides strong support for the conclusions of 
causality,\74\ these studies provide less information on details of the 
specific O3 exposure circumstances that may be eliciting 
health effects associated with such outcomes, and whether these occur 
under air quality conditions that meet the current standard.\75\ 
Further, the vast majority of these studies were conducted in locations 
and during time periods that would not have met the current 
standard.\76\ The extent to which reported associations with health 
outcomes in the resident populations in these studies are influenced by 
the periods of higher concentrations during times that did not meet the 
current standard is unknown. While this does not lessen their 
importance in the evidence base documenting the causal relationship 
between O3 and respiratory effects, it means they are less 
informative in considering O3 exposure concentrations 
occurring under air quality conditions allowed by the current standard.
---------------------------------------------------------------------------

    \74\ Combined with the coherent evidence from experimental 
studies, the epidemiologic studies ``can support and strengthen 
determinations of the causal nature of the relationship between 
health effects and exposure to ozone at relevant ambient air 
concentrations'' (ISA, p. ES-17).
    \75\ For example, these studies generally do not measure 
personal exposures of the study population or track individuals in 
the population with a defined exposure to O3 alone.
    \76\ Consistent with the evaluation of the epidemiologic 
evidence of associations between O3 exposure and 
respiratory health effects in the ISA, this focuses on those studies 
conducted in the U.S. and Canada as including populations and air 
quality characteristics that may be most relevant to circumstances 
in the U.S. (ISA, Appendix 3, section 3.1.2). Among the 
epidemiologic studies finding a statistically significant positive 
relationship of short- or long-term O3 concentrations 
with respiratory effects, there are no single-city studies conducted 
in the U.S. in locations with ambient air O3 
concentrations that would have met the current standard for the 
entire duration of the study (ISA, Appendix 3, Tables 3-13, 3-14, 3-
39, 3-41, 3-42 and Appendix 6, Tables 6-5 and 6-8; PA, Appendix 3B, 
Table 3B-1). There are two single city studies conducted in Canada 
that include locations for which the highest-monitor design values 
calculated in the PA fell below 70 ppb, at 65 and 69 ppb (PA, 
Appendix 3B, Table 3B-1; Kousha and Rowe, 2014; Villeneuve et al., 
2007). These studies did not include analysis of correlations with 
other co-occurring pollutants or of the strength of the associations 
when accounting for effects of copollutants in copollutant models 
(ISA, Appendix 3, Tables 3-14 and 3-39).
---------------------------------------------------------------------------

    With regard to the experimental animal evidence (largely in 
rodents) and exposure conditions associated with respiratory effects, 
the exposure concentrations are generally much greater than those 
examined in the controlled human exposure studies (summarized above), 
and higher than concentrations commonly occurring in ambient air in 
areas of the U.S. where the current standard is met. This is also true 
for the small number of early life studies in nonhuman primates that 
reported O3 to contribute to asthma-like effects in infant 
primates.\77\ The exposures eliciting the effects in these studies 
included multiple 5-day periods with O3 concentrations of 
500 ppb over 8-hours per day (ISA, Appendix 3, section 3.2.4.1.2).
---------------------------------------------------------------------------

    \77\ These studies indicate that sufficient early-life 
O3 exposure can cause structural and functional changes 
that could potentially contribute to airway obstruction and 
increased airway responsiveness (ISA, Table IS-10, p. 3-92 and p.3-
113).
---------------------------------------------------------------------------

    Thus, as in the last review the exposures given greatest attention 
in this review, particularly with regard to considering O3 
exposures expected under air quality conditions that meet the current 
standard, are those informed by the controlled human exposure studies. 
The full body of evidence continues to indicate respiratory effects as 
the effects associated with lowest exposures, with conditions of 
exposure (duration, ventilation rate, as well as concentration) 
influencing dose and associated response. Evidence for other categories 
of effects does not indicate effects at comparably low exposures.\78\
---------------------------------------------------------------------------

    \78\ For example, the evidence base for metabolic effects is 
comprised primarily of experimental animal studies, and generally 
involve much higher O3 concentrations (400-800 ppb, [ISA, 
Appendix 5, Table 5-87]) than those examined in the controlled human 
exposure studies of respiratory effects (and much higher than 
concentrations commonly occurring in ambient air in areas of the 
U.S. where the current standard is met). There are only two 
epidemiologic studies reporting statistically significant positive 
associations of O3 with metabolic effects (e.g., changes 
in glucose, insulin, metabolic clearance), both based in Asian 
countries, in which there is a potential for appreciable differences 
from the U.S. in air quality patterns, limiting their usefulness for 
informing our understanding of exposure concentrations and 
conditions eliciting such effects in the U.S. (ISA, Appendix 5, 
section 5.1).
---------------------------------------------------------------------------

3. Overview of Exposure and Risk Information
    Consideration of the scientific evidence available in the current 
review, as at the time of the last review, is informed by results from 
quantitative analyses of estimated population exposure and consequent 
risk of respiratory effects. These analyses in this review have focused 
on exposure-based risk analyses, producing two types of risk metrics. 
The first metric estimates population occurrences of daily maximum 7-
hour average exposure concentrations (during periods of elevated 
breathing rates) at or above concentrations of potential concern 
(benchmark concentrations). The second metric (lung function risk) uses 
E-R information for O3 exposures and FEV1 
decrements to estimate the portion of the simulated at-risk population 
expected to experience one or more days with an O3-related 
FEV1 decrement of at least 10%, 15% or 20%. Both of these 
metrics were used to characterize health risk associated with 
O3 exposures among the simulated population during periods 
of elevated breathing rates. Similar risk metrics were also derived in 
the 2014 HREA for the last review and the associated estimates informed 
the Administrator's 2015 decision on the current standard (80 FR 65292, 
October 26, 2015).
    The currently available evidence in this review continues to 
demonstrate a causal relationship between short-term O3 
exposures and respiratory effects, with the current evidence base for 
respiratory effects largely consistent with that for the last review, 
as summarized in section II.A.2 above. Accordingly, the exposure-based 
analyses performed in this review, summarized below, are conceptually 
similar to those in the last review while also incorporating a number 
of updates that contribute to reduced uncertainty. Drawing on the 
summary in section II.C of the proposal, while giving relatively 
greater focus on the comparison-to-benchmarks analysis, the short 
sections below provide an 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
    Exposure and risk estimates were derived for air quality conditions 
just meeting the current primary O3 standard, and for two 
additional scenarios reflecting conditions just meeting design values 
just lower and just higher than the level of the current

[[Page 87276]]

standard (65 and 75 ppb).\79\ The analyses estimated population 
exposure and risk for simulated populations in eight urban study areas 
which represent a variety of circumstances with regard to population 
exposure to short-term concentrations of O3 in ambient air. 
The areas (Atlanta, Boston, Dallas, Detroit, Philadelphia, Phoenix, 
Sacramento and St. Louis) range in total population size from 
approximately two to eight million and are distributed across seven 
regions of the U.S.: Northeast, Southeast, Central, East North Central, 
South, Southwest and West (PA, Appendix 3D, Table 3D-1). Study-area-
specific characteristics contribute to variation in the estimated 
magnitude of exposure and associated risk across the urban study areas 
that reflect an array of air quality, meteorological, and population 
exposure conditions. The current set of study areas, streamlined 
compared to the 15-area set in the last review, was chosen to ensure it 
reflects the full range of air quality and exposure variation expected 
in major urban areas in the U.S. with air quality that just meets the 
current standard. Seven of the eight study areas were also included in 
the 2014 HREA; the eighth study area (Phoenix) is newly added in the 
current assessment to insure representation of a large city in the 
southwest. Additionally, the O3 concentrations simulated in 
these areas are somewhat nearer the current standard than was the case 
for the 2014 HREA (PA, Appendix 3C, Table 3C and 2014 HREA, Table 4-1). 
This contributes to a reduction in the uncertainty associated with 
development of the air quality scenarios of interest, particularly the 
one reflecting air quality conditions that just meet the current 
standard.
---------------------------------------------------------------------------

    \79\ All analyses are summarized more fully in the PA section 
3.4 and Appendices 3C and 3D.
---------------------------------------------------------------------------

    With regard to the objectives for the analysis approach, the 
analyses and the use of a case study approach are intended to provide 
assessments of air quality scenarios, including particularly one just 
meeting the current standard, for a diverse set of areas and associated 
exposed populations. These analyses are not intended to provide a 
comprehensive national assessment (PA, section 3.4.1). Nor is the 
objective to present an exhaustive analysis of exposure and risk in the 
areas that currently just meet the current standard and/or of exposure 
and risk associated with air quality adjusted to just meet the current 
standard in areas that currently do not meet the standard. Rather, the 
purpose is to assess, based on current tools and information, the 
potential for exposures and risks beyond those indicated by the 
information available at the time the standard was established. 
Accordingly, use of this approach recognizes that capturing an 
appropriate diversity in study areas and air quality conditions \80\ is 
an important aspect of the role of the exposure and risk analyses in 
informing the Administrator's conclusions on the public health 
protection afforded by the current standard.
---------------------------------------------------------------------------

    \80\ A broad variety of spatial and temporal patterns of 
O3 concentrations can exist when ambient air 
concentrations just meet the current standard. These patterns will 
vary due to many factors including the types, magnitude, and timing 
of emissions in a study area, as well as local factors, such as 
meteorology and topography. We focused our current assessment on 
specific study areas having ambient air concentrations close to 
conditions that reflect air quality that just meets the current 
standard. Accordingly, assessment of these study areas is more 
informative to evaluating the health protection provided by the 
current standard than would be an assessment that included areas 
with much higher and much lower concentrations.
---------------------------------------------------------------------------

    Consistent with the health effects evidence in this review 
(summarized in section II.A.2 above), the focus of the quantitative 
assessment is on short-term exposures of individuals in the population 
during times when they are breathing at an elevated rate. Exposure and 
risk are characterized for four population groups. Two are populations 
of school-aged children, aged 5 to 18 years: All children and children 
with asthma; two are populations of adults: All adults and adults with 
asthma. Estimates for adults, in terms of percentages, are generally 
lower due to the lesser amount and frequency of time spent outdoors at 
elevated exertion (PA, Appendix 3D, section 3D.3.2). The exception is 
outdoor workers who, due to the requirements of their job, spend more 
time outdoors at elevated exertion. For a number of reasons, including 
the appreciable data limitations (e.g., related to specific durations 
of time spent outdoors and activity data), and associated uncertainties 
summarized in Table 3D-64 of Appendix 3D of the PA, the group was not 
simulated in these analyses, a decision also made for past exposure 
assessments.\81\ Asthma prevalence estimates for the full populations 
in the eight study areas range from 7.7 to 11.2%; the rates for 
children in these areas range from 9.2 to 12.3% (PA, Appendix 3D, 
section 3D.3.1).
---------------------------------------------------------------------------

    \81\ Limited exploratory analyses of a hypothetical outdoor 
worker population in the 2014 HREA (single study area, single year) 
for the 75 ppb air quality scenario estimated an appreciably greater 
portion of this population to experience exposures at or above 
benchmark concentrations than the full adult or child populations 
simulated, although there are a number of uncertainties associated 
with the estimates due to appreciable limitations in the data 
underlying the analyses (2014 HREA, section 5.4.3.2). It is expected 
that if an approach similar to that used in the 2014 HREA had been 
used for this assessment a generally similar pattern might be 
observed, although with somewhat lower overall percentages based on 
the comparison of current estimates with estimates from the 2014 
HREA (PA, Appendix 3D, section 3D.3.2.4).
---------------------------------------------------------------------------

    The approach for this analysis incorporates an array of models and 
data (PA, section 3.4.1). Ambient air O3 concentrations were 
estimated in each study area for the air quality conditions of interest 
by adjusting hourly ambient air concentrations, from monitoring data 
for the years 2015-2017, using a photochemical model-based approach and 
then applying a spatial interpolation technique to produce air quality 
surfaces with high spatial and temporal resolution (PA, Appendix 3C). 
The final products were datasets of ambient air O3 
concentration estimates with high temporal and spatial resolution 
(hourly concentrations in 500 to 1,700 census tracts) for each of the 
eight study areas (PA, section 3.4.1 and Appendix 3C, section 3C.7) 
representing the three air quality scenarios assessed.
    Population exposures were estimated using the EPA's Air Pollutant 
Exposure model (APEX) version 5, which probabilistically generates a 
large sample of hypothetical individuals from population demographic 
and activity pattern databases and simulates each individual's 
movements through time and space to estimate their time series of 
O3 exposures occurring within indoor, outdoor, and in-
vehicle microenvironments (PA, Appendix 3D, section 3D.2).\82\ The APEX 
model accounts for the most important factors that contribute to human 
exposure to O3 from ambient air, including the temporal and 
spatial distributions of people and ambient air O3 
concentrations throughout a study area, the variation of ambient air-
related O3 concentrations within various microenvironments 
in which people conduct their daily activities, and the effects of 
activities involving different levels of exertion on breathing rate (or 
ventilation rate) for the exposed individuals of different sex, age, 
and body mass in the study area (PA, Appendix 3D, section 3D.2).\83\ By 
incorporating individual activity

[[Page 87277]]

patterns, and estimating physical exertion for each exposure event, the 
model addresses an important determinant of their exposure (2013 ISA, 
section 4.4.1).\84\ For each exposure event, the APEX model tracks 
activity performed, ventilation rate, exposure concentration, and 
duration for all simulated individuals throughout the assessment 
period, and then utilizes the time-series of exposure events in 
derivation of the exposure and risk estimates.
---------------------------------------------------------------------------

    \82\ The APEX model has a history of application, evaluation, 
and progressive model development in estimating human exposure, 
dose, and risk for reviews of NAAQS for gaseous pollutants, 
including the last review of the O3 NAAQS (U.S. EPA, 
2008; U.S. EPA, 2009; U.S. EPA, 2010; U.S. EPA, 2014a; U.S. EPA, 
2018).
    \83\ The APEX model generates each simulated person or profile 
by probabilistically selecting values for a set of profile 
variables, including demographic variables, health status and 
physical attributes (e.g., residence with air conditioning, height, 
weight, body surface area), and activity-specific ventilation rate 
(PA, Appendix 3D, section 3D.2).
    \84\ To represent personal time-location-activity patterns of 
simulated individuals, the APEX model draws from the consolidated 
human activity database (CHAD) developed and maintained by the EPA 
(McCurdy, 2000; U.S. EPA, 2019a). The CHAD provides data on human 
activities through a database system of human diaries or daily time 
series or daily time location activity logs collected in surveys at 
city, state, and national levels. Included are personal attributes 
of survey participants (e.g., age, sex), along with the locations 
they visited, activities performed throughout a day, time-of-day the 
activities occurred and activity duration (PA, Appendix 3D, section 
3D.2.5.1).
---------------------------------------------------------------------------

    The general approach and methodology for the exposure-based 
assessment used in this review is similar to that used in the last 
review, although a number of updates and improvements, related to the 
air quality, exposure, and risk aspects of the assessment, have been 
implemented (Appendices 3C and 3D). These include (1) a more recent 
period (2015-2017) of ambient air monitoring data in which 
O3 concentrations in the eight study areas are at or near 
the current standard; (2) the most recent version of the photochemical 
air quality model, CAMx (comprehensive air quality model with 
extensions), with updates to the treatment of atmospheric chemistry and 
physics within the model; (3) a significantly expanded CHAD, that now 
has nearly 180,000 diaries, with over 25,000 school aged children; (4) 
updated National Health and Nutrition Examination Survey data (2009-
2014), which are the basis for the age- and sex-specific body weight 
distributions used to specify the individuals in the modeled 
populations; (5) updated algorithms used to estimate age- and sex-
specific resting metabolic rate, a key input to estimating a simulated 
individual's activity-specific ventilation (or breathing) rate; (6) 
updates to the ventilation rate algorithm itself; and (7) an approach 
that better matches the simulated exposure estimates with the 6.6-hour 
duration of the controlled human exposure studies and with the study 
subject ventilation rates. Further, the current APEX model uses the 
most recent U.S. Census demographic and commuting data (2010), NOAA 
Integrated Surface Hourly meteorological data to reflect the assessment 
years studied (2015-2017), and updated estimates of asthma prevalence 
for all census tracts in all study areas based on 2013-2017 National 
Health Interview Survey and Behavioral Risk Factor Surveillance System 
data. Additional details are described in the PA (e.g., PA, section 
3.4.1, Appendices 3C and 3D).
    The comparison-to-benchmarks analysis characterizes the extent to 
which individuals in at-risk populations could experience O3 
exposures, while engaging in their daily activities, with the potential 
to elicit the effects reported in controlled human exposure studies for 
concentrations at or above specific benchmark concentrations. Results 
are characterized through comparison of exposure concentrations to 
three benchmark concentrations of O3: 60, 70, and 80 ppb. 
These are based on the three lowest concentrations targeted in studies 
of 6- to 6.6-hour exposures, with quasi-continuous exercise, and that 
yielded different occurrences, of statistical significance, and 
severity of respiratory effects, as summarized in section II.A.2.c 
above and section II.C.1 of the proposal (PA, section 3.3.3; PA, 
Appendix 3A, section 3A.1; PA, Appendix 3D, section 3D.2.8.1). The 
lowest benchmark, 60 ppb, represents the lowest exposure concentration 
for which controlled human exposure studies have reported statistically 
significant respiratory effects, as summarized in section II.A.2.c 
above. Exposure to approximately 70 ppb averaged over 6.6 hours 
resulted in a larger group mean lung function decrement, as well as a 
statistically significant increase in prevalence of respiratory 
symptoms (Table 1; ISA, Appendix 3, Figure 3-3 and section 3.1.4.1.1; 
Schelegle et al., 2009). Studies of exposures to approximately 80 ppb 
have reported larger lung function decrements at the study group mean 
than following exposures to 60 or 70 ppb, in addition to an increase in 
airway inflammation, increased respiratory symptoms, increased airway 
responsiveness, and decreased resistance to other respiratory effects 
(ISA, Appendix 3, sections 3.1.4.1--3.1.4.4; PA, Figure 3-2 and section 
3.3.3).
    The APEX-generated exposure concentrations for comparison to these 
benchmark concentrations is the average of concentrations encountered 
by an individual while at an activity level that elicits the specified 
elevated ventilation rate. The incidence of such exposures above the 
benchmark concentrations are summarized for each simulated population, 
study area, and air quality scenario in Appendix 3D of the PA.
    The lung function risk analysis estimates the extent to which 
individuals in exposed populations could experience O3-
induced lung function decrements of different sizes in two different 
ways. The population-based E-R function approach uses quantitative 
descriptions of the E-R relationships for study group incidence of 
different magnitudes of lung function decrements based on individual 
study subject observations (PA, Appendix 3D, section 3D.2.8.2.1). The 
individual-based McDonnell-Smith-Stewart (MSS) model uses quantitative 
estimates of biological processes identified as important in eliciting 
the different sizes of decrements at the individual level, with a 
factor that also provides a representation of intra- and inter-
individual response variability (PA, Appendix 3D, section 3D.2.8.2.2; 
McDonnell et al., 2013). The two approaches, summarized in sections 
II.C and II.D.1 of the proposal and described in detail in Appendix 3D 
of the PA, utilize evidence from the 6.6-hour controlled human exposure 
studies in different ways, and accordingly, differ in strengths, 
limitations and uncertainties.
    While the lung function risk analysis focuses only on the specific 
O3 effect of FEV1 reduction, the comparison-to-
benchmark analysis, with its use of multiple benchmark concentrations, 
provides for risk characterization of the array of respiratory effects 
elicited by O3 exposure, the type and severity of which 
increase with increased exposure concentration. In this way, the 
comparison-to-benchmark analysis (involving comparison of daily maximum 
7-hour average exposure concentrations that coincide with 7-hour 
average elevated ventilation rates at or above the target rate to 
benchmark concentrations) provides perspective on the extent to which 
the air quality being assessed could be associated with discrete 
exposures to O3 concentrations reported to result in an 
array of respiratory effects. For example, estimates of such exposures 
can indicate the potential for O3-related effects in the 
exposed population, including effects for which we do not have E-R 
functions that could be used in quantitative risk analyses. Thus, the 
comparison-to-benchmark analysis provides for a broader risk 
characterization with consideration of the array of O3-
related respiratory effects.
b. Key Limitations and Uncertainties
    Uncertainty in the exposure and risk analyses was characterized 
using a

[[Page 87278]]

largely qualitative approach adapted from the World Health Organization 
approach for characterizing uncertainty in exposure assessment (WHO, 
2008) augmented by several quantitative sensitivity analyses for key 
aspects of the assessment approach (PA, section 3.4.4 and Appendix 3D, 
section 3D.3.4). This characterization and associated analyses build on 
information generated from a previously conducted quantitative 
uncertainty analysis of population-based O3 exposure 
modeling (Langstaff, 2007), considering the various types of data, 
algorithms, and models that together yield exposure and risk estimates 
for the eight study areas. In this way, we considered the limitations 
and uncertainties underlying these data, algorithms, and models and the 
extent of their influence on the resultant exposure/risk estimates 
using the general approach applied in past risk and exposure 
assessments for O3, nitrogen dioxide, carbon monoxide, and 
sulfur dioxide (U.S. EPA, 2008; U.S. EPA, 2010; U.S. EPA, 2014a; U.S. 
EPA, 2018).
    Key uncertainties and limitations in data and tools that affect the 
quantitative estimates of exposure and risk and their interpretation in 
the context of considering the current standard are summarized here. 
These include uncertainty related to estimation of the concentrations 
in ambient air for the current standard and the additional air quality 
scenarios; lung function risk approaches that rely, to varying extents, 
on extrapolating from controlled human exposure study conditions to 
lower exposure concentrations, lower ventilation rates, and shorter 
durations; and characterization of risk for particular population 
groups that may be at greatest risk, particularly for people with 
asthma, and particularly children with asthma. Areas in which 
uncertainty has been reduced by new or updated information or methods 
include the use of updated air quality modeling, with a more recent 
model version and model inputs, applied to study areas with design 
values near the current standard, as well as updates to several inputs 
to the exposure model, including changes to the exposure duration to 
better match those in the controlled human exposure studies and an 
alternate approach to characterizing periods of activity while at 
moderate or greater exertion for simulated individuals.
    With regard to the analysis approach overall, two updates since the 
2014 HREA reduce uncertainty in the results. The first relates to 
identifying when simulated individuals may be at moderate or greater 
exertion, with the new approach reducing the potential for 
overestimation of the number of people achieving the associated 
ventilation rate, which was an important uncertainty in the 2014 HREA. 
Additionally, the current analysis focus on exposures of 7 hours 
duration better represents the 6.6-hour exposures from the controlled 
human exposure studies (than the 8-hour exposure durations used for the 
2014 HREA and prior assessments).
    Additional aspects of the analytical design pertaining to both 
exposure-based risk metrics include the estimation of ambient air 
O3 concentrations for the air quality scenarios, and main 
components of the exposure modeling. Uncertainties include the modeling 
approach used to adjust ambient air concentrations to meet the air 
quality scenarios of interest and the method used to interpolate 
monitor concentrations to census tracts. While the adjustment to 
conditions near, just above, or just below the current standard is an 
important area of uncertainty, the size of the adjustment needed to 
meet a given air quality scenario is minimized with the selection of 
study areas for which recent O3 design values were near the 
level of the current standard. Also, more recent data are used as 
inputs for the air quality modeling, such as more recent O3 
concentration data (2015-2017), meteorological data (2016) and 
emissions data (2016), as well as a recently updated air quality 
photochemical model which includes state-of-the-science atmospheric 
chemistry and physics (PA, Appendix 3C). Further, the number of ambient 
monitors sited in each of the eight study areas provides a reasonable 
representation of spatial and temporal variability for the air quality 
conditions simulated in those areas. Among other key aspects, there is 
uncertainty associated with the simulation of study area populations 
(and at-risk populations), including those with particular physical and 
personal attributes. As also recognized in the 2014 HREA, exposures 
could be underestimated for some population groups that are frequently 
and routinely outdoors during the summer (e.g., outdoor workers, 
children). In addition, longitudinal activity patterns do not exist for 
these and other potentially important population groups (e.g., those 
having respiratory conditions other than asthma), limiting the extent 
to which the exposure model outputs reflect information that may be 
particular to these groups. Important uncertainties in the approach 
used to estimate energy expenditure (i.e., metabolic equivalents of 
work or METs used to estimate ventilation rates), include the use of 
longer-term average MET distributions to derive short-term estimates, 
along with extrapolating adult observations to children. Both of these 
approaches are reasonable based on the availability of relevant data 
and appropriate evaluations conducted to date, and uncertainties 
associated with these steps are somewhat reduced in the current 
analyses (compared to the 2014 HREA) because of the added specificity, 
and use of redeveloped METs distributions (based on newly available 
information), which is expected to more realistically estimate 
activity-specific energy expenditure.
    There are some uncertainties that apply to the estimation of lung 
function risk and not to the comparison-to-benchmarks analysis. For 
example, both lung function risk approaches utilized in the risk 
analyses incorporate some degree of extrapolation beyond the exposure 
circumstances evaluated in the controlled human exposure studies. 
Accordingly, the uncertainty in the lung function risk estimates 
increases with decreasing exposure concentration and is particularly 
increased for concentrations below those evaluated in controlled human 
exposure studies (85 FR 49857-49859, PA, section 3.4.4 and Appendix 3D, 
section 3D.3.4). The two lung function risk approaches differ in how 
they extrapolate beyond the controlled human exposure study conditions 
and in the impact on the estimates. The E-R function approach generates 
nonzero predictions from the full range of nonzero concentrations for 
7-hour average durations in which the average exertion levels meets or 
exceeds the target. The MSS model, which draws on evidence-based 
concepts of how human physiological processes respond to O3, 
extrapolates beyond the controlled experimental conditions with regard 
to exposure concentration, duration and ventilation rate (both 
magnitude and duration). Differences in percent of the risk estimates 
for days for which the highest 7-hour average concentration is below 
the lowest 6.6-hour exposure concentration tested, as presented in the 
PA, Tables 3-6 and 3-7, illustrate the impact.
    An overarching area of uncertainty, remaining from the last review 
and important to consideration of the exposure and risk analysis 
results, relates to the underlying health effects evidence base. 
Although the quantitative analysis focuses on the evidence providing 
the ``strongest evidence'' of O3 respiratory effects (ISA,

[[Page 87279]]

p. IS-1), the controlled human exposure studies, and on the array of 
respiratory responses documented in those studies, evidence is lacking 
from controlled human exposure studies at the lower concentrations 
(e.g., 60, 70 and 80 ppb) for children and for people of any age with 
asthma. While the limited evidence informing our understanding of 
potential risk to people with asthma is uncertain, it indicates the 
potential for this group, given their disease status, to be at great 
risk, as summarized in section II.A.2 above. Such a conclusion is 
consistent with the epidemiologic study findings of positive 
associations of O3 concentrations with asthma-related ED 
visits and hospital admissions (and the higher effect estimates from 
these studies).
c. Summary of Exposure and Risk Estimates
    The benchmark-based risk metric results are summarized in terms of 
the percent of the simulated populations of all children and children 
with asthma estimated to experience at least one day per year \85\ with 
a 7-hour average exposure concentration at or above the different 
benchmark concentrations while breathing at elevated rates under air 
quality conditions just meeting the current standard (Table 2). Given 
the recognition of people with asthma as an at-risk population and the 
relatively greater amount and frequency of time spent outdoors at 
elevated exertion of children, this summary focuses on the estimates 
from the comparison-to-benchmarks analysis for children, including 
children with asthma, which were the focus of the Administrator's 
proposed decision. Under air quality conditions just meeting the 
current standard, less than 0.1% of any study area's children with 
asthma, on average, were estimated to experience any days per year with 
a 7-hour average exposure at or above 80 ppb, while breathing at 
elevated rates (Table 3; PA, section 3.4 and Appendix 3D). With regard 
to the 70 ppb benchmark, the study areas' estimates for children with 
asthma range up to 0.7 percent (0.6% for all children), on average 
across the 3-year period, and range up to 1.0% in a single year. 
Approximately 3% to nearly 9% of each study area's simulated children 
with asthma, on average across the 3-year period, are estimated to 
experience one or more days per year with a 7-hour average exposure at 
or above 60 ppb. This range is very similar for the populations of all 
children.
---------------------------------------------------------------------------

    \85\ While the duration of an O3 season for each year 
may vary across the study areas, for the purposes of the exposure 
and risk analyses, the O3 season in each study area is 
considered synonymous with a year. These seasons capture the times 
during the year when concentrations are elevated (80 FR 65419-65420, 
October 26, 2015).
---------------------------------------------------------------------------

    Regarding multiday occurrences, the analyses indicate that no 
children would be expected to experience more than a single day with a 
7-hour average exposure at or above 80 ppb in any year simulated in any 
location (Table 2). For the 70 ppb benchmark, the estimate is less than 
0.1% of any area's children (on average across 3-year period), both 
those with asthma and all children. The estimates for the 60 ppb 
benchmark are slightly higher, with up to 3% of children estimated to 
experience more than a single day with a 7-hour average exposure at or 
above 60 ppb, on average (and more than 4% in the highest year across 
all eight study area locations).
    Framed from the perspective of estimated protection provided by the 
current standard, these results indicate that, in the single year with 
the highest concentrations across the 3-year period, 99% of the 
population of children with asthma would not be expected to experience 
such a day with an exposure at or above the 70 ppb benchmark; 99.9% 
would not be expected to experience such a day with exposure at or 
above the 80 ppb benchmark. The estimates, on average across the 3-year 
period, indicate that over 99.9%, 99.3% and 91.2% of the population of 
children with asthma would not be expected to experience a day with a 
7-hour average exposure while at elevated ventilation that is at or 
above 80 ppb, 70 ppb and 60 ppb, respectively (Table 1). Further, more 
than approximately 97% of all children or children with asthma are 
estimated to be protected against multiple days of exposures at or 
above 60 ppb.

   Table 2--Percent and Number of Simulated Children and Children With Asthma Estimated To Experience at Least One or More Days per Year With a 7-Hour
           Average Exposure At or Above Indicated Concentration While Breathing at an Elevated Rate in Areas Just Meeting the Current Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                 One or more days                Two or more days                Four or more days
                                                         -----------------------------------------------------------------------------------------------
              Exposure concentration (ppb)                  Average per    Highest in a     Average per    Highest in a     Average per    Highest in a
                                                               year         single year        year         single year        year         single year
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                Children with asthma--percent of simulated population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................  0 \B\-<0.1 \C\             0.1               0               0               0               0
>=70....................................................         0.2-0.7             1.0            <0.1             0.1               0               0
>=60....................................................         3.3-8.8            11.2         0.6-3.2             4.9        <0.1-0.8             1.3
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                               --number of individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................            0-67             202               0               0               0               0
>=70....................................................         93-1145            1616            3-39             118               0               0
>=60....................................................       1517-8544           11776        282-2609            3977          23-637            1033
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                    All children--percent of simulated population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................      0 \B\-<0.1             0.1               0               0               0               0
>=70....................................................         0.2-0.6             0.9            <0.1             0.1          0-<0.1            <0.1
>=60....................................................         3.2-8.2            10.6         0.6-2.9             4.3        <0.1-0.7             1.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                               --number of individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................           0-464            1211               0               0               0               0
>=70....................................................        727-8305           11923          16-341             660             0-5              14

[[Page 87280]]

 
>=60....................................................     14928-69794           96261      2601-24952           36643        158-5997            9554
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each study area were averaged across the 3-year assessment period. Ranges reflect the ranges of averages.
\B\ A value of zero (0) means that there were no individuals estimated to have the selected exposure in any year.
\C\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).

    These estimates are of generally similar magnitude to those which 
were the focus in the 2015 decision establishing the current standard 
(Table 3; PA, sections 3.1 and 3.4, Appendix 3D, section 3D.3.2.4, 
Table 3D-38).\86\ The differences observed are generally slight, likely 
reflecting influences of a number of the differences in the 
quantitative modeling and analyses performed in the current assessment 
from those for the 2014 HREA, summarized in section II.A.3.a above 
(e.g., 2015-2017 vs. 2006-2010 distribution of ambient air 
O3 concentrations, better matching of simulated exposure 
estimates with the 6.6-hour duration of the controlled human exposure 
studies and with the study subject ventilation rates). Much larger 
differences are seen between different air quality scenario results for 
the same benchmark. For example, for the 70 ppb benchmark, the 
differences between the 75 ppb and current standard scenario (or 
between the 65 ppb and current standard scenarios) in either assessment 
are appreciably larger than the slight differences between the two 
assessments for any one air quality scenario.
---------------------------------------------------------------------------

    \86\ For example, the 2015 decision to set the standard level at 
70 ppb noted that ``a revised standard with a level of 70 ppb is 
estimated to eliminate the occurrence of two or more exposures of 
concern to O3 concentrations at or above 80 ppb and to 
virtually eliminate the occurrence of two or more exposures of 
concern to O3 concentrations at or above 70 ppb for all 
children and children with asthma, even in the worst-case year and 
location evaluated'' (80 FR 65363, October 26, 2015). This statement 
remains true for the current assessment (Table 3). For the 60 ppb 
benchmark, on which the 2015 decision placed relatively greater 
weight for multiple (versus single) occurrences of exposures at or 
above it, the Administrator at that time noted the 2014 HREA 
estimates for the 70 ppb air quality scenario that estimated 0.5 to 
3.5% of children to experience multiple such occurrences on average 
across the study areas, stating that the now-current standard ``is 
estimated to protect the vast majority of children in urban study 
areas . . . from experiencing two or more exposures of concern at or 
above 60 ppb'' (80 FR 65364, October 26, 2015). The corresponding 
estimates, on average across the 3-year period in the current 
assessments, are remarkably similar at 0.6 to 2.9% (Table 3).

 Table 3--Comparison of Current Assessment and 2014 HREA (All Study Areas) for Percent of Children Estimated To
   Experience at Least One, or Two, Days With an Exposure At or Above Benchmarks While at Moderate or Greater
                                                    Exertion
----------------------------------------------------------------------------------------------------------------
                                           Estimated average % of simulated    Estimated average % of simulated
                                          children with at least one day per    children with at least two days
                                              year at or above benchmark        per year at or above benchmark
    Air Quality Scenario (DV,\C\ ppb)         (highest in single season)          (highest in single season)
                                         -----------------------------------------------------------------------
                                           Current PA \A\     2014 HREA \B\    Current PA \A\     2014 HREA \B\
----------------------------------------------------------------------------------------------------------------
                                   Benchmark Exposure Concentration of 80 ppb
----------------------------------------------------------------------------------------------------------------
75......................................      <0.1 \A\-0.3       0-0.3 (1.1)     0-<0.1 (<0.1)           0 (0.1)
                                                     (0.6)
70......................................      0-<0.1 (0.1)       0-0.1 (0.2)             0 (0)             0 (0)
65......................................     0-<0.1 (<0.1)             0 (0)             0 (0)             0 (0)
----------------------------------------------------------------------------------------------------------------
                                   Benchmark Exposure Concentration of 70 ppb
----------------------------------------------------------------------------------------------------------------
75......................................     1.1-2.0 (3.4)     0.6-3.3 (8.1)     0.1-0.3 (0.7)     0.1-0.6 (2.2)
70......................................     0.2-0.6 (0.9)     0.1-1.2 (3.2)        <0.1 (0.1)       0-0.1 (0.4)
65......................................       0-0.2 (0.2)       0-0.2 (0.5)     0-<0.1 (<0.1)             0 (0)
----------------------------------------------------------------------------------------------------------------
                                   Benchmark Exposure Concentration of 60 ppb
----------------------------------------------------------------------------------------------------------------
75......................................   6.6-15.7 (17.9)   9.5-17.0 (25.8)     1.7-8.0 (9.9)    3.1-7.6 (14.4)
70......................................    3.2-8.2 (10.6)   3.3-10.2 (18.9)     0.6-2.9 (4.3)     0.5-3.5 (9.2)
65......................................     0.4-2.3 (3.7)       0-4.2 (9.5)    <0.1-0.3 (0.5)       0-0.8 (2.8)
----------------------------------------------------------------------------------------------------------------
\A\ For the current analysis, calculated percent is rounded to the nearest tenth decimal using conventional
  rounding. Values equal to zero are designated by ``0'' (there are no individuals exposed at that level).
  Small, non-zero values that do not round upwards to 0.1 (i.e., <0.05) are given a value of ``<0.1''.
\B\ For the 2014 HREA. calculated percent was rounded to the nearest tenth decimal using conventional rounding.
  Values that did not round upwards to 0.1 (i.e., <0.05) were given a value of ``0''.
\C\ The monitor location with the highest concentrations in each area had a design value just equal to the
  indicated value.


[[Page 87281]]

B. Conclusions on the Primary Standard

    In drawing conclusions on the adequacy of the current primary 
standard, in view of the advances in scientific knowledge and 
additional information now available, the Administrator has considered 
the currently available health effects evidence and exposure/risk 
information. He additionally has considered the evidence base, 
information, and policy judgments that were the foundation of the last 
review, to the extent they remain relevant in light of the currently 
available information. The Administrator has taken into account both 
evidence-based and exposure- and risk-based considerations discussed in 
the PA, 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, particularly that from controlled 
human exposure studies and epidemiologic studies evaluating health 
effects related to O3 exposures as presented in the ISA, 
with a focus on policy-relevant considerations as discussed in the PA 
(summarized in sections II.B and II.D.1 of the proposal and section 
II.A.2 above). The exposure- and risk-based considerations draw from 
the results of the quantitative analyses presented and considered in 
the PA (as summarized in section II.C of the proposal and section 
II.A.3 above).
    The consideration of the evidence and exposure/risk information in 
the PA informed the Administrator's proposed conclusions and judgments 
in this review, and his associated proposed decision. Section II.B.1 
below briefly summarizes the basis for the Administrator's proposed 
decision, drawing from section II.D of the proposal. Section II.B.1.a 
provides a brief overview of key aspects of the policy evaluations 
presented in the PA, and the advice and recommendations of the CASAC 
are summarized in section II.B.1.b. An overview of the Administrator's 
proposed conclusions is presented in section II.B.1.c. Public comments 
on the proposed decision are addressed in section II.B.2, and the 
Administrator's conclusions and decision in this review regarding the 
adequacy of the current primary standard and whether any revisions are 
appropriate are described in section II.B.3.
1. Basis for Proposed Decision
a. Policy-Relevant Evaluations in the PA
    The main focus of the policy-relevant considerations in the PA is 
consideration of the question: Does the currently available scientific 
evidence- and exposure/risk-based information support or call into 
question the adequacy of the protection afforded by the current primary 
O3 standard? The PA response to this overarching question 
takes into account discussions that address the specific policy-
relevant questions for this review, focusing first on consideration of 
the evidence, as evaluated in the ISA, including that newly available 
in this review, and the extent to which it alters key conclusions 
supporting the current standard. The PA also considers the quantitative 
exposure and risk estimates drawn from the exposure/risk analyses 
(presented in detail in Appendices 3C and 3D of the PA), including 
associated limitations and uncertainties, and the extent to which they 
may indicate different conclusions from those in the last review 
regarding the magnitude of risk, as well as level of protection from 
adverse effects, associated with the current standard. The PA 
additionally considers the key aspects of the evidence and exposure/
risk estimates that were emphasized in establishing the current 
standard, as well as the associated public health policy judgments and 
judgments about the uncertainties inherent in the scientific evidence 
and quantitative analyses that are integral to the Administrator's 
consideration of whether the currently available information supports 
or calls into question the adequacy of the current primary 
O3 standard (PA, section 3.5).
    As summarized in section II.D.1 of the proposal, based on the 
evidence in the ISA, the PA concludes that the respiratory effects 
evidence newly available in this review is consistent with the evidence 
base in the last review, supporting a generally similar understanding 
of the respiratory effects of O3 (PA, section 3.5.4; ISA, 
Appendix 3). As was the case for the evidence available in the last 
review, the currently available evidence for health effects other than 
those of O3 exposures on the respiratory system is more 
uncertain than that for respiratory effects. Such effects include 
metabolic effects, for which the evidence available in this review is 
sufficient to conclude there to likely be a causal relationship with 
short-term O3 exposures and suggestive of, but not 
sufficient to infer, such a relationship between long-term 
O3 exposure (ISA, section IS.1.3.1). These new 
determinations are based on evidence largely from experimental animal 
studies, that is newly available in this review (ISA, Appendix 5). 
Additionally, newly available evidence regarding cardiovascular effects 
and mortality, in combination with uncertainties in the previously 
available evidence that had been identified in the last review, 
contributes to conclusions that the evidence is suggestive of, but not 
sufficient to infer, causal relationships with O3 exposures 
(ISA, Appendix 4, section 4.1.17 and Appendix 6, section 6.1.8). As in 
the last review, the evidence is also suggestive of such relationships 
for reproductive and developmental effects, and nervous system effects 
(ISA, section IS.1.3.1).
    In evaluating the policy implications of the current evidence, the 
PA observes that within the respiratory effects evidence base, the most 
certain evidence comes from controlled human exposure studies, the 
majority of which involve healthy adult subjects (generally 18 to 35 
years), although there are studies (generally not at the lowest studied 
exposures) involving subjects with asthma, and a limited number of 
studies, generally of durations shorter than four hours, involving 
adolescents and adults older than 50 years. Respiratory responses 
observed in human subjects exposed to O3 for periods of 8 
hours or less, while intermittently or quasi-continuously exercising, 
include lung function decrements (e.g., based on FEV1 
measurements), respiratory symptoms, increased airway responsiveness, 
mild bronchoconstriction (measured as an increase in sRaw), and 
pulmonary inflammation, with associated injury and oxidative stress 
(ISA, Appendix 3, section 3.1.4; 2013 ISA, sections 6.2.1 through 
6.2.4). Newly available epidemiologic studies of hospital admissions 
and emergency department visits for a variety of respiratory outcomes 
supplement the previously available evidence with additional findings 
of consistent associations with O3 concentrations across a 
number of study locations (ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5, 
3.1.6.1.1, 3.1.7.1 and 3.1.8). Together, the clinical and 
epidemiological bodies of evidence, in combination with the insights 
gained from the experimental animal evidence, continue to indicate the 
potential for O3 exposures to contribute to serious health 
outcomes and to indicate the increased risk of population groups with 
asthma, including particularly, children (ISA, Appendix 3, section 
3.1.5.7).
    The PA concludes that the newly available evidence in this review 
does not alter conclusions from the last review on exposure duration 
and concentrations associated with O3-related effects, 
observing that the 6.6-hour controlled human exposure studies

[[Page 87282]]

of respiratory effects remain the focus for our consideration of 
exposure circumstances associated with O3 health effects. 
The PA additionally recognizes that while the evidence clearly 
demonstrates that short-term O3 exposures cause respiratory 
effects, as was the case in the last review, uncertainties remain in 
several aspects of our understanding of these effects. These include 
uncertainties related to exposures likely to elicit effects (and the 
associated severity and extent) in population groups not studied, or 
less well studied (including individuals with asthma and children) and 
also the severity and prevalence of responses to short (e.g., 6.6- to 
8-hour) O3 exposures, at and below 60 ppb, while at 
increased exertion levels.
    The PA additionally includes exposure/risk analyses of air quality 
scenarios in eight study areas, with a focus on the scenario for air 
quality that just meets the current standard, as described in section 
II.C of the proposal and summarized in section II.A.3 above. In 
considering the results of these analyses, the PA gives particular 
emphasis to the comparison-to-benchmarks analysis, which provides a 
characterization of the extent to which population exposures to 
O3 concentrations, similar to those evaluated in controlled 
human exposure studies, have the potential to occur in areas of the 
U.S. when air quality just meets the current standard (PA, section 
3.4). The policy evaluations of the exposure/risk analyses focus on 
children and children with asthma as key at-risk populations, and 
consideration of the potential for one versus multiple exposures to 
occur. The PA recognizes that consideration of differences in magnitude 
or severity of responses (e.g., FEV1 changes) including the 
relative transience or persistence of the responses and respiratory 
symptoms, as well as pre-existing sensitivity to effects on the 
respiratory system, and other factors, are important to characterizing 
implications for public health effects of an air pollutant such as 
O3 (PA, sections 3.3.2, 3.4.5 and 3.5).
    In summary, the PA concludes that the newly available health 
effects evidence, critically assessed in the ISA as part of the full 
body of evidence, reaffirms conclusions on the respiratory effects 
recognized for O3 in the last review on which the current 
standard is based. The PA additionally draws on the quantitative 
exposure and risk estimates for conditions just meeting the current 
standard (PA, sections 3.4 and 3.5.2). Limitations and uncertainties 
associated with the available information remain (PA, sections 3.5.1 
and 3.5.2). The PA recognizes that the newly available quantitative 
exposure/risk estimates for conditions just meeting the current 
standard indicate a generally similar level of protection for at-risk 
populations from respiratory effects, as that described in the last 
review for the now-current standard (section II.A.3, Table 3, above; 
PA, sections 3.1 and 3.4, Appendix 3D, section 3D.3.2.4, Table 3D-38). 
Collectively, in consideration of the evidence and quantitative 
exposure/risk information available in the current review, as well as 
advice from the CASAC, the PA concludes that it is appropriate to 
consider retaining the current primary standard of 0.070 ppm 
O3, as the fourth-highest daily maximum 8-hour concentration 
averaged across three years, without revision.
b. CASAC Advice in This Review
    In comments on the draft PA, the CASAC agreed with the draft PA 
findings that the health effects evidence newly available in this 
review does not substantially differ from that available in the 2015 
review, stating that, ``[t]he CASAC agrees that the evidence newly 
available in this review that is relevant to setting the ozone standard 
does not substantially differ from that of the 2015 Ozone NAAQS 
review'' (Cox, 2020a, Consensus Responses to Charge Questions p. 12). 
With regard to the adequacy of the current standard, views of 
individual CASAC members differed. Part of the CASAC ``agree with the 
EPA that the available evidence does not call into question the 
adequacy of protection provided by the current standard, and thus 
support retaining the current primary standard'' (Cox, 2020a, p. 1). 
Another part of the CASAC indicated its agreement with the previous 
CASAC's advice, based on review of the 2014 draft PA, that a primary 
standard with a level of 70 ppb may not be protective of public health 
with an adequate margin of safety, including for children with asthma 
(Cox, 2020a, p. 1 and Consensus Responses to Charge Questions p. 
12).\87\ Additional comments from the CASAC in the ``Consensus 
Responses to Charge Questions'' on the draft PA attached to the CASAC 
letter provide recommendations on improving the presentation of the 
information on health effects and exposure and risk estimates in 
completing the final PA. The EPA considered these comments, making a 
number of revisions to address them in completing the PA. The comments 
from the CASAC also took note of uncertainties that remain in this 
review of the primary standard and identified a number of additional 
areas for future research and data gathering that would inform the next 
review of the primary O3 NAAQS (Cox, 2020a, Consensus 
Responses to Charge Questions p. 14). The recommendations from the 
CASAC were considered in the proposed decision and have been considered 
by the Administrator in his decision in this review, summarized in 
section II.B.3 below.
---------------------------------------------------------------------------

    \87\ In the last review, the advice from the prior CASAC 
included a range of recommended levels for the standard, with the 
CASAC concluding that ``there is adequate scientific evidence to 
recommend a range of levels for a revised primary ozone standard 
from 70 ppb to 60 ppb'' (Frey, 2014b, p. ii). In so doing, the prior 
CASAC noted that ``[i]n reaching its scientific judgment regarding a 
recommended range of levels for a revised ozone primary standard, 
the CASAC focused on the scientific evidence that identifies the 
type and extent of adverse effects on public health'' and further 
acknowledged ``that the choice of a level within the range 
recommended based on scientific evidence is a policy judgment under 
the statutory mandate of the Clean Air Act'' (Frey, 2014b, p. ii). 
The prior CASAC then described that its ``policy advice [emphasis 
added] is to set the level of the standard lower than 70 ppb within 
a range down to 60 ppb, taking into account [the Administrator's] 
judgment regarding the desired margin of safety to protect public 
health, and taking into account that lower levels will provide 
incrementally greater margins of safety'' (Frey, 2014b, p. ii).
---------------------------------------------------------------------------

c. Administrator's Proposed Conclusions
    In reaching conclusions on the adequacy and appropriateness of 
protection provided by the current primary standard and his proposed 
decision to retain the standard, the Administrator carefully 
considered: (1) The assessment of the current evidence and conclusions 
reached in the ISA; (2) the currently available exposure and risk 
information, including associated limitations and uncertainties, 
described in detail in the PA; (3) the considerations and staff 
conclusions and associated rationales presented in the PA, including 
consideration of commonly accepted guidelines or criteria within the 
public health community, including the ATS, an organization of 
respiratory disease specialists; (4) the advice and recommendations 
from the CASAC; and (5) public comments that had been offered up to 
that point (85 FR 49830, August 14, 2020). In so doing, he considered 
the evidence base on health effects associated with exposure to 
photochemical oxidants, including O3, in ambient air, noting 
the health effects evidence newly available in this review, and the 
extent to which it alters key scientific conclusions in the last 
review. He additionally considered the quantitative exposure and risk 
estimates

[[Page 87283]]

developed in this review, including associated limitations and 
uncertainties, and what they indicate regarding the magnitude of risk, 
as well as level of protection from adverse effects, associated with 
the current standard. The Administrator also considered the key aspects 
of the evidence and exposure/risk estimates from the 2015 review that 
were emphasized in establishing the standard at that time. Further, he 
considered uncertainties in the current evidence and the exposure/risk 
information, as a part of public health judgments that are essential 
and integral to his decision on the adequacy of protection provided by 
the standard, similar to the judgments made in establishing the current 
standard. Such judgments include public health policy judgments and 
judgments about the uncertainties inherent in the scientific evidence 
and quantitative analyses. The Administrator drew on the considerations 
and conclusions in the current PA, taking note of key aspects of the 
associated rationale, and he considered the advice and conclusions of 
the CASAC, including particularly its overall agreement that the 
currently available evidence does not substantially differ from that 
which was available in the 2015 review when the current standard was 
established.
    As an initial matter, the Administrator recognized the continued 
support in the current evidence for O3 as the indicator for 
photochemical oxidants, taking note that no newly available evidence 
has been identified in this review regarding the importance of 
photochemical oxidants other than O3 with regard to 
abundance in ambient air, and potential for health effects. For such 
reasons, described with more specificity in the ISA and PA and 
summarized in the proposal, he proposed to conclude it is appropriate 
for O3 to continue to be the indicator for the primary 
standard for photochemical oxidants and focused on the current 
information for O3 (85 FR 49830, August 14, 2020).
    With regard to O3 health effects, the Administrator 
recognized the long-standing evidence that has established there to be 
a causal relationship between respiratory effects and short-term 
O3 exposures. He recognized that the strongest and most 
certain evidence for this conclusion, as in the last review, is that 
from controlled human exposure studies that report an array of 
respiratory effects in study subjects (which are largely generally 
healthy adults) engaged in quasi-continuous or intermittent exercise. 
He also recognized the supporting experimental animal and epidemiologic 
evidence, including the epidemiologic studies reporting positive 
associations for asthma-related hospital admissions and emergency 
department visits, which are strongest for children, with short-term 
O3 exposures (85 FR 49830, August 14, 2020).
    Regarding the current evidence and EPA conclusions for populations 
at increased risk of O3-related health effects (ISA, section 
4.4), the Administrator took particular note of the robust evidence 
that continues to identify people with asthma as being at increased 
risk of O3 related respiratory effects, including 
specifically asthma exacerbation and associated health outcomes, and 
also children, particularly due to their generally greater time 
outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections 
IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). 
Based on this evidence and related factors, the Administrator proposed 
to conclude it appropriate to give particular focus to people with 
asthma and children (population groups for which the evidence of 
increased risk is strongest) in evaluating whether the current standard 
provides requisite protection based on the judgment that such a focus 
will also provide protection of other population groups, identified in 
the ISA, for which the current evidence is less robust and clear as to 
the extent and type of any increased risk, and the exposure 
circumstances that may contribute to it.
    The Administrator additionally recognized newly available evidence 
and conclusions regarding O3 exposures and metabolic 
effects. In so doing, he also noted that the basis for the conclusions 
is largely experimental animal studies in which the exposure 
concentrations were well above those in the controlled human exposure 
studies for respiratory effects, and also above those likely to occur 
in areas of the U.S. that meet the current standard. In light of these 
considerations, he further proposed to judge the current standard to be 
protective of such circumstances, leading him to continue to focus on 
respiratory effects in evaluating whether the current standard provides 
requisite protection (85 FR 49830, August 14, 2020).
    With regard to exposure circumstances of interest for respiratory 
effects, the Administrator focused particularly on the 6.6-hour 
controlled human exposure studies involving exposure, with quasi-
continuous exercise, that examine exposures from 60 to 80 ppb. In so 
doing, he recognized that this information on exposure concentrations 
that have been found to elicit effects in exercising study subjects is 
unchanged from what was available in the last review. He additionally 
recognized that while, as a whole, the epidemiologic studies of 
associations between O3 and respiratory effects and health 
outcomes (e.g., asthma-related hospital admission and emergency 
department visits) provide strong support for the conclusions of 
causality, they are less useful for his consideration of the potential 
for O3 exposures associated with air quality conditions 
allowed by the current standard to contribute to such health outcomes, 
taking note of the scarcity of U.S. studies conducted in locations in 
which and during time periods when the current standard would have been 
met (85 FR 49830, August 14, 2020).
    In reaching his proposed decision to retain the 2015 standard, the 
Administrator took note of several aspects of the rationale by which it 
was established, giving weight to the considerations summarized here. 
The 2015 decision considered the breadth of the O3 
respiratory effects evidence, recognizing the relatively greater 
significance of effects reported for exposures while at elevated 
exertion to average O3 concentrations at and above 80 ppb, 
as well as to the greater array of effects elicited. The decision also 
recognized the significance of effects observed at the next lower 
studied exposures (slightly above 70 ppb) that included both lung 
function decrements and respiratory symptoms. The standard level was 
set to provide a high level of protection from such exposures. The 
decision additionally emphasized consideration of lower exposures down 
to 60 ppb, particularly with regard to consideration of a margin of 
safety in setting the standard. In this context, the 2015 decision 
identified the appropriateness of a standard that provided a degree of 
control of multiple or repeated occurrences of exposures, while at 
elevated exertion, at or above 60 ppb (80 FR 65365, October 26, 
2015).\88\ The controlled human exposure study evidence as a whole 
provided context for consideration of the 2014 HREA estimates for the

[[Page 87284]]

comparison-to-benchmarks analysis (80 FR 65363, October 26, 2015). The 
current Administrator proposed to similarly consider the currently 
available exposure and risk analyses in this review (85 FR 49830, 
August 14, 2020).
---------------------------------------------------------------------------

    \88\ With the 2015 decision, the prior Administrator judged 
there to be uncertainty in the adversity of the effects shown to 
occur following exposures to 60 ppb O3, including the 
inflammation reported by the single study at the level, and 
accordingly placed greater weight on estimates of multiple, versus 
single, exposures for the 60 ppb benchmark, particularly when 
considering the extent to which the current and revised standards 
incorporate a margin of safety (80 FR 65344-45, October 26, 2015). 
She based this, at least in part, on consideration of effects at 
this exposure level, the evidence for which remains the same in the 
current review, and she considered this information in judgments 
regarding the 2014 HREA estimates for the 60 ppb benchmark.
---------------------------------------------------------------------------

    The Administrator also recognized some uncertainty, reflecting 
limitations in the evidence base, with regard to the exposure levels 
eliciting effects (as well as the severity of the effects) in some 
population groups not well represented in the available controlled 
human exposure studies, such as children and individuals with asthma. 
In so doing, the Administrator recognizes that the controlled human 
exposure studies, primarily conducted in healthy adults, on which the 
depth of our understanding of O3-related health effects is 
based, provide limited, but nonetheless important information with 
regard to responses in people with asthma or in children. Additionally, 
some aspects of our understanding continue to be limited, as in the 
2015 review; among these aspects are the risk posed to these less 
studied population groups by 7-hour exposures with exercise to 
concentrations as low as 60 ppb that are estimated in the exposure 
analyses. Collectively, these aspects of the evidence and associated 
uncertainties contribute to a recognition that for O3, as 
for other pollutants, the available evidence base in a NAAQS review 
generally reflects a continuum, consisting of ambient 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.
    As in the 2015 decision, the Administrator's proposed decision in 
this review recognized that the exposure and risk estimates developed 
from modeling exposures to O3 in ambient air are critically 
important to consideration of the potential for exposures and risks of 
concern under air quality conditions of interest, and consequently are 
critically important to judgments on the adequacy of public health 
protection provided by the current standard. Thus taking into 
consideration related information, limitations and uncertainties 
recognized in the proposal, the Administrator considered the exposure 
and risk estimates across the eight study areas (with their array of 
exposure conditions) for air quality conditions just meeting the 
current standard. In light of factors recognized above and summarized 
in section II.D.4 of the proposal, the Administrator, in his 
consideration of the exposure and risk analyses, focused in the 
proposal on the results for children and children with asthma. In 
considering the public health implications of estimated occurrences of 
exposures, while at increased exertion, at or above the three benchmark 
concentrations (60, 70, and 80 ppb), the Administrator considered the 
effects reported in controlled human exposure studies of this range of 
concentrations during 6.6 hours of quasi-continuous exercise. While the 
Administrator noted reduced uncertainty in several aspects of the 
exposure and risk approaches as compared to the analyses in the last 
review, he recognized the relatively greater uncertainty associated 
with the lung function risk estimates compared to the results of the 
comparison-to-benchmarks analysis. In light of these uncertainties, as 
well as the recognition that the comparison-to-benchmarks analysis 
provides for characterization of risk for the broad array of 
respiratory effects compared to a narrower focus limited to lung 
function decrements, the Administrator focused in the proposal 
primarily on the estimates of exposures at or above different benchmark 
concentrations that represent different levels of significance of 
O3-related effects, both with regard to the array of effects 
and severity of individual effects (85 FR 49830, August 14, 2020).
    In his consideration of the exposure analysis estimates for 
exposures at or above the different benchmark concentrations (with 
reduced associated uncertainty compared to the analysis available in 
2015) and based on the greater severity of responses reported in 
controlled human exposures, with quasi-continuous exercise, at and 
above 73 ppb, the Administrator focused in the proposal first on the 
higher two benchmark concentrations (which at 70 and 80 ppb are, 
respectively, slightly below and above this level) and the estimates 
for one-or-more-day occurrences. In this context, he proposed to judge 
it desirable that the standard provide a high level of protection 
against one or more occurrences of days with exposures, while breathing 
at an elevated rate, to concentrations at or above 70 ppb. With regard 
to the 60 ppb benchmark, the Administrator gave greater weight to 
estimates of occurrences of two or more (rather than one or more) days 
with an exposure at or above that benchmark, taking note of the lesser 
severity of responses observed in studies of the lowest benchmark 
concentration of 60 ppb and other considerations summarized in the 
proposal, including potential risks for at-risk populations. Based on 
this weighting of the exposure analysis results for the eight urban 
study areas, the Administrator noted what was indicated by the exposure 
estimates for air quality conditions just meeting the current standard 
with regard to protection for the simulated at-risk populations. Some 
97% to more than 99% of all children (including those with asthma), on 
average, and more than 95% in the single highest year, are estimated to 
be protected from experiencing two or more days with exposures at or 
above 60 ppb while at elevated exertion. More than 99% of children with 
asthma (and of all children), on average per year, are estimated to be 
protected from a day or more with an exposure at or above 70 ppb. 
Lastly, the percentage (for both population groups) for at least one 
day with such an exposure at or above 80 ppb is 99.9% or more in each 
of the three years simulated, with no simulated children estimated to 
experience more than a single such day. The Administrator proposed to 
judge that protection from this set of exposures provides a strong 
degree of protection to at-risk populations, such as children with 
asthma. In so doing, he found that the updated exposure and risk 
analyses continue to support a conclusion of a high level of 
protection, including for at-risk populations, from O3-
related effects of exposures that might be expected with air quality 
conditions that just meet the current standard (85 FR 49830, August 14, 
2020).
    In reaching his proposed conclusion, the Administrator additionally 
took note of the comments and advice from the CASAC, including the 
CASAC conclusion that the newly available evidence does not 
substantially differ from that available in the last review, and the 
conclusion expressed by part of the CASAC, that the currently available 
evidence supports retaining the current standard (85 FR 49873, August 
14, 2020). He also noted that another part of the CASAC indicated its 
agreement with the prior CASAC comments on the 2014 draft PA, in which 
the prior CASAC opined that a standard set at 70 ppb may not provide an 
adequate margin of safety (Cox, 2020a, p. 1). With regard to the latter 
view (that referenced 2014 comments from the prior CASAC), the 
Administrator additionally noted that the 2014 advice from the prior 
CASAC also concluded that the scientific evidence supported a range of 
standard levels that included 70 ppb and recognized the choice of a 
level within its recommended range to be ``a policy judgment under the 
statutory mandate

[[Page 87285]]

of the Clean Air Act'' (Frey, 2014b, p. ii).\89\
---------------------------------------------------------------------------

    \89\ This 2014 advice was considered in the last review's 
decision to establish the current standard with a level of 70 ppb 
(80 FR 65362, October 26, 2015).
---------------------------------------------------------------------------

    In reflecting on all of the information currently available, the 
Administrator also considered the extent to which the currently 
available information might indicate support for a less stringent 
standard, noting that the CASAC advice did not convey support for such 
a standard. He additionally considered the current exposure and risk 
estimates for the air quality scenario for a design value just above 
the level of the current standard (at 75 ppb), in comparison to the 
scenario for the current standard, with its level of 70 ppb. In so 
doing, he found the markedly increased estimates of exposures to the 
higher benchmarks under air quality for a higher standard level to be 
of concern and indicative of less than the requisite protection. Thus, 
in light of considerations raised in the proposal, including the need 
for an adequate margin of safety, the Administrator proposed to judge 
that a less stringent standard would not be appropriate to consider (85 
FR 49830, August 14, 2020).
    Similarly, the Administrator also considered whether it would be 
appropriate to consider a more stringent standard that might be 
expected to result in reduced O3 exposures. As an initial 
matter in this regard, he considered the advice from the CASAC 
(summarized in section II.B.1.b above). With regard to the CASAC 
advice, he noted that while part of the Committee concluded that the 
evidence supported retaining the current standard without revision, 
another part of the Committee reiterated advice from the prior CASAC, 
which while including the current standard level among the range of 
recommended standard levels, also provided policy advice to set the 
standard at a lower level (85 FR 49873, August 14, 2020). In 
considering the reference to the 2014 CASAC advice, the Administrator 
noted the slight differences of the current exposure and risk estimates 
from the 2014 HREA estimates considered by the prior CASAC. The 
Administrator additionally recognized the PA finding that the factors 
contributing to these differences, which include the use of air quality 
data reflecting concentrations much closer to the now-current standard 
than was the case in the 2015 review, also contribute to a reduced 
uncertainty in the estimates. Thus, he noted that the current exposure 
analysis estimates indicate the current standard to provide appreciable 
protection against multiple days with a maximum exposure at or above 60 
ppb. He considered this in the context of the adequacy of protection 
provided by the standard and of the CAA requirement that the standard 
protect public health, including the health of at-risk populations, 
with an adequate margin of safety, and proposed to conclude that the 
current standard provides an adequate margin of safety, and that a more 
stringent standard is not needed (85 FR 49873, August 14, 2020).
    In light of all of the above, including advice from the CASAC, the 
Administrator proposed to judge the current exposure and risk analysis 
results to describe appropriately strong protection of at-risk 
populations from O3-related health effects. Thus, based on 
his consideration of the evidence and exposure/risk information, 
including that related to the lowest exposures studied and the 
associated uncertainties, the Administrator proposed to judge that the 
current standard provides the requisite protection, including an 
adequate margin of safety, and thus should be retained, without 
revision (85 FR 49874, August 14, 2020). In so doing, he recognized 
that the protection afforded by the current standard can only be 
assessed by considering its elements collectively, including the 
standard level of 70 ppb, the averaging time of eight hours and the 
form of the annual fourth-highest daily maximum concentration averaged 
across three years. The Administrator proposed to judge that the 
current evidence presented in the ISA and considered in the PA, as well 
as the current air quality, exposure and risk information presented and 
considered in the PA, provide continued support to these elements, as 
well as to the current indicator.
    In summary, in the proposal the Administrator recognized that the 
ISA found the newly available health effects evidence, critically 
assessed in the ISA as part of the full body of evidence, consistent 
with the conclusions on the respiratory effects recognized for 
O3 in the last review. He additionally noted that the 
evidence newly available in this review, such as that related to 
metabolic effects, does not include information indicating a basis for 
concern for exposure conditions associated with air quality conditions 
meeting the current standard. Further, the Administrator noted the 
quantitative exposure and risk estimates for conditions just meeting 
the current standard that indicate a high level of protection for at-
risk populations from respiratory effects. Collectively, these 
considerations (including those discussed more completely in the 
proposal) provided the basis for the Administrator's proposed judgments 
regarding the public health protection provided by the current primary 
standard of 0.070 ppm O3, as the fourth-highest daily 
maximum 8-hour concentration averaged across three years. On this 
basis, the Administrator proposed to conclude that the current standard 
is requisite to protect the public health with an adequate margin of 
safety, and that it is appropriate to retain the standard without 
revision (85 FR 49874, August 14, 2020).
2. Comments on the Proposed Decision
    Over 50,000 individuals and organizations indicated their views in 
public comments on the proposed decision. Most of these are associated 
with mass mail campaigns or petitions. Approximately 40 separate 
submissions were also received from individuals, and 75 from 
organizations and groups of organizations; forty elected officials also 
submitted comments. Among the organizations commenting were state and 
local agencies and organizations of state agencies, organizations of 
health professionals and scientists, environmental and health 
protection advocacy organizations, industry organizations and 
regulatory policy-focused organizations. The comments on the proposed 
decision to retain the current primary standard are addressed here. 
Those in support of the proposed decision are addressed in section 
II.B.2.a and those in disagreement are addressed in section II.B.2.b. 
Comments related to aspects of the process followed in this review of 
the O3 NAAQS (described in section I.D above), as well as 
comments related to other legal, procedural or administrative issues, 
and those related to issues not germane to this review are addressed in 
the separate Response to Comments document.
a. Comments in Support of Proposed Decision
    Of the commenters supporting the Administrator's proposed decision 
to retain the current primary standard, without revision, all generally 
note their agreement with the rationale provided in the proposal, with 
the CASAC conclusion that the current evidence is generally consistent 
with that available in the last review, and with the CASAC members that 
conclude the evidence does not call into question the adequacy of the 
current standard. Some commenters further remarked that the primary 
standard was upheld in the litigation following its 2015

[[Page 87286]]

establishment (Murray Energy Corp. v. EPA, 936 F.3d 597 [D.C. Cir. 
2019]) and that this review is based largely on the same body of 
respiratory effects evidence. These commenters all find the process for 
the review to conform to Clean Air Act requirements and the proposed 
decision to retain the current standard to be well supported, noting 
that the there are no new controlled human exposure studies (of the 
type given primary focus in the establishment of the current standard) 
and concurring with the proposed judgment that at-risk populations are 
protected with an adequate margin of safety. Some commenters also 
variously cited EPA statements that the recent metabolic studies, as 
well as the epidemiologic and toxicological studies newly available in 
this review for other health endpoints, do not demonstrate effects of 
O3 when the current standard is met and thus do not call 
into question the protection provided by the standard. The EPA agrees 
with these commenters' conclusion on the current standard.
    Further, these comments concur with the EPA's consideration of 
epidemiologic and toxicological studies of respiratory effects, and 
with the weight the proposed decision placed on the evidence for other 
effects, including metabolic and cardiovascular effects, and total 
mortality. Some of these comments also express the view that health 
benefits of a more restrictive O3 standard are highly 
uncertain, while such a standard would likely cause an increase in 
nonattainment areas and socioeconomic impacts that the EPA should 
consider and find to outweigh the uncertain benefits. While, as 
discussed in section II.B.3 below, the Administrator does not find a 
more stringent standard necessary to provide requisite public health 
protection, he does not consider the number of nonattainment areas or 
economic impacts of alternate standards in reaching this judgment.\90\ 
As summarized in section I.A. above, in setting primary and secondary 
standards that are ``requisite'' to protect public health and welfare, 
respectively, as provided in section 109(b), the EPA may not consider 
the costs of implementing the standards. See generally, Whitman v. 
American Trucking Ass'ns, 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 Corp. v. EPA, 936 
F.3d 597, 623-24 [D.C. Cir. 2019]). Arguments such as the views on 
socioeconomic impacts expressed by these commenters have been rejected 
by the courts, as summarized in section I.A above, including in Murray 
Energy, with the reasoning that consideration of such impacts was 
precluded by Whitman's holding that the ``plain text of the Act 
`unambiguously bars cost considerations from the NAAQS-setting process' 
'' (Murray Energy Corp. v. EPA, 936 F.3d at 621, quoting Whitman, 531 
U.S. at 471).
---------------------------------------------------------------------------

    \90\ Comments related to implementation programs are not 
addressed here because, as described in section I.A above, this 
action is being taken pursuant to CAA section 109(d)(1) and relevant 
case law. Accordingly, concerns related to implementation of the 
existing or an alternate standard are outside the scope of this 
action.
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    We also note that some commenters that stated their support for 
retaining the current standard without revision additionally claimed 
that, based on the results of the exposure and risk analyses in this 
review, the current standard provides somewhat more public health 
protection than the EPA recognized in the 2015 decision establishing 
it. As support for this view, these commenters cite conclusions 
(including those in the PA) that the exposure and risk estimates are 
equivalent or slightly lower than those from the 2014 HREA. In 
generally agreeing with the commenters' observation with regard to the 
differences in exposure/risk estimates from analyses in this review 
compared to those from 2014, we note that the current exposure/risk 
estimates, while based on conceptually similar approaches to those used 
in the 2014 HREA, reflect a number of improvements to input data and 
modeling approaches, summarized in section II.A.3 above, which have 
reduced uncertainties. These updated analyses inform the 
Administrator's judgments in this review.
b. Comments in Disagreement With Proposed Decision
    Of the commenters that disagreed with the proposal to retain the 
current standard, some recommend tightening the standard, while one 
submission recommends a less stringent standard. The commenters 
supporting a less stringent standard generally assert that the current 
standard is overprotective, stating that information they provide 
supports returning to the pre-2015 standard of 75 ppb and/or revising 
the form from the 4th highest daily maximum to the seventh highest 
daily maximum. The commenters that recommended a more stringent 
standard describe a need for revision to provide greater public health 
protection, generally claiming that the current standard is inadequate 
and does not provide an adequate margin of safety for potentially 
vulnerable groups. We address these sets of comments in turn below.
(i) Comments in Disagreement With Proposed Decisions--Calling for Less 
Stringent Standard
    The commenters recommending revision to a less stringent standard 
generally expressed the view that the current standard is more 
stringent than necessary to protect public health. In support of this 
view the commenters argue (1) that in this review the EPA 
``discredited'' a cardiovascular mortality study on which commenters 
assert the 2015 decision had placed especially heavy weight; (2) that 
in light of limitations they assert for the exposure and risk estimate 
analyses conducted in this review, a 75 ppb standard would meet 2015 
objectives; and, (3) that additional factors they identify indicate 
that the current standard of 70 ppb is too close to background levels 
while a standard of 75 ppb or one with a form that uses the seventh 
(versus fourth) highest daily maximum 8-hour O3 
concentration would not be.
    With regard to the first argument, the EPA knows of no 
cardiovascular mortality study, much less any health study, that was 
relied on in the 2015 review that has been discredited, and the 
commenters provide no citation for such a study. To the extent that the 
commenter may be intending to refer to the difference of the current 
review from the 2015 review with regard to the Agency's causality 
determinations for cardiovascular effects and all-cause mortality, we 
note that these changes did not involve ``discrediting'' of any studies 
in the 2013 ISA. Rather, as summarized in section II.A.2.a above, since 
the time of the last review the controlled human exposure study 
evidence base has been appreciably expanded from one study to several, 
none of which report O3-induced cardiovascular endpoints. 
This update to the evidence base for cardiovascular effects, which also 
includes epidemiologic studies, has contributed to a change in the 
weight of evidence that supports the Agency's causality determinations 
for both cardiovascular effects and mortality. To the extent that the 
commenters intend to suggest that these changes in causality 
determinations indicate that the current standard is more stringent 
than necessary to protect public health, the Agency disagrees. The 
Administrator's reasons for concluding that the current standard 
provides the requisite public

[[Page 87287]]

health protection are explained in section II.B.3 below.
    With regard to the risk and exposure analyses, the comment argues 
that 2019 O3 ambient air monitoring data for locations 
meeting a design value of 75 ppb indicate that a 75 ppb standard could 
achieve comparable exposure estimates to those derived for air quality 
just meeting the current standard by the EPA's exposure/risk analyses. 
The comment also asserts that uncertainty in the controlled human 
exposure evidence base with regard to children with asthma suggests 
``some latitude'' is needed in the risk calculations. The analysis 
provided in the comment appears to focus on counties in designated 
nonattainment areas with 2019 design values ranging from 71 to 75. For 
these counties, the commenters' analysis appears to sum the population 
of the subset of these counties with at least one daily maximum 8-hour 
average concentration in 2019 falling in the range from 73 to 79 ppb 
(and, separately, the population of counties with at least one such 
value above 80 ppb). From these population counts, the analysis derives 
estimates of the subpopulations of children with asthma spending 
afternoons outdoors (using national estimates for representation of 
children in the total population, of children with asthma in the total 
child population, and of children in asthma spending afternoons 
outdoors using analysis of CHAD diaries for children). The analysis 
divides the two values by the commenters' estimate of children with 
asthma in the U.S. (304 million [total population of the U.S.] x 10.5% 
[percentage representing children] x 9.7% [percentage representing 
children with asthma]).
    There are many aspects of the analysis submitted with the comment 
that are not focused on the objective of estimating exposures of 
concern that might be expected to be experienced by at-risk populations 
in U.S. areas that just meet a standard with an alternative level of 75 
ppb. As just one example of these aspects, the denominator in the final 
step of the commenters' calculation is inflated by population counts 
for areas of the U.S. excluded from the commenters' analysis (with this 
larger population multiplied by a national estimates of percent that 
are children, 10.5%, and a national estimate of percent of children 
that have asthma, 9.7%), yielding a percentage of unclear relevance to 
consideration of exposures occurring in areas just meeting an 
alternative standards of 75 ppb. If the population of the nonattainment 
areas on which the commenters' focus is substituted in the calculation 
for the total population of the U.S. as the denominator (29.5 million x 
10.5% x 9.7% = 146,664), with the commenters' estimates of children in 
those areas that may experience an exposure at or above 80 ppb (4,788) 
or below 80 ppb and at or above 73 ppb (12,641), the percentages are 
3.3% and 8.6%, respectively (and the percentage for at or above 73 ppb 
would be 5.8%).\91\ Thus, contrary to the commenters' assertion, their 
analytical approach, with use of a denominator that reflects the 
commenters' focus areas, results in higher estimates of the percentage 
of at-risk children that may experience particular exposures of concern 
in areas meeting a 75 ppb standard than does the EPA's analysis, which 
takes into account a number of factors in much greater detail (e.g., 
through the use of exposure modeling and human activity data to 
estimate time series contributing to 7-hour exposure periods with 
average O3 concentrations at or above benchmarks), and 
focuses on temporal and spatial patterns of air quality in areas just 
meeting a standard of 75 ppb. The commenters analysis is not focused on 
the factors that are key determinants of population exposures of 
concern, leading to results that are inconsistent with and less 
informative than the findings of EPA's more detailed, extensive and 
technically sound exposure and risk analyses (summarized in section 
II.A.3 above and Appendices 3C and 3D of the PA). Based on 
consideration of these analyses, among other factors, as described in 
section II.B.3 below, the EPA disagrees that the available evidence and 
quantitative analyses supports the conclusion that the current standard 
is overprotective and that a standard of 75 ppb would protect public 
health with an adequate margin of safety.
---------------------------------------------------------------------------

    \91\ The EPA's exposure and risk analyses estimate <.1 to 0.3% 
of children with asthma might be expected to experience at least one 
exposure, while at increased exertion, at or above 80 ppb, on 
average across a 3-year period in areas just meeting a potential 
alternative standard of 75 ppb (85 FR 49865,Table 4, August 14, 
2020). For the 70 ppb benchmark, these percentages are 1.1 to 2.0%.
---------------------------------------------------------------------------

    In support of the commenters' additional argument that the current 
standard is too close to background and that a 75 ppb standard (or a 
standard using the seventh highest form) would not be, the commenters 
(1) state that just because a D.C. Circuit decision has stated that EPA 
is not required to take U.S. background O3 (USB) into 
consideration in NAAQS decisions does not mean that such considerations 
are precluded; (2) cite the lower number of counties (and associated 
population) that would be in nonattainment for a 75 ppb standard as 
compared to the current standard (while also suggesting that revision 
of the form to a seventh highest would appropriately allow for 
additional high O3 days due to wildfires); and (3) suggest 
that the EPA is underestimating USB by a factor of three.
    With regard to the legal point, the EPA agrees that while it is not 
required to take USB into account in NAAQS decisions, it may do so when 
such consideration is consistent with the Clean Air Act and prior court 
decisions. The EPA is not relying on consideration of background 
O3 levels to support its decision in this review. Moreover, 
given the differences in public health protection, as noted in the 
Administrator's proposed conclusions and described in his conclusions 
in section II.B.3 below, we do not believe that we could use proximity 
to background concentrations as a basis for revising the current 70 ppb 
standard to a potential 75 ppb standard.\92\ On the commenters' second 
point, the EPA notes that the number of counties that would or would 
not be in nonattainment, the size of population living in them, and the 
increasing number of days for high O3 due to wildfires are 
not relevant factors in judging whether a particular standard is 
requisite under the Clean Air Act. Regardless of such implications of a 
decision to retain or revise a NAAQS, the key consideration for the 
review of a primary standard is whether the standard is judged to 
provide the requisite protection of public health with an adequate 
margin of safety.\93\ The commenters have provided no evidence 
suggesting that the current standard provides more than the requisite 
public health protection under the CAA or indicating that an alternate 
standard

[[Page 87288]]

with a level of 75 ppb or with a seventh highest form would provide 
requisite protection. For these reasons, we do not find these comments 
persuasive in supporting consideration of revising the current standard 
to an alternate standard with a level of 75 ppb or with a seventh 
highest form.
---------------------------------------------------------------------------

    \92\ Taken together, the EPA generally understands prior court 
decisions addressing consideration of background O3 in 
NAAQS reviews to hold that while the Agency may not establish a 
NAAQS that is outside the range of reasonable values supported by 
the air quality criteria and the judgments of the Administrator 
because of proximity to background concentrations, it is not 
precluded from considering relative proximity to background 
O3 as one factor in selecting among standards that are 
within that range (American Trucking Ass'ns v. EPA, 283 F.3d 355, 
379 [D.C. Cir. 2002]; Murray Energy v. EPA, 936 F.3d at 622-624; 
American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 [D.C. 
Cir. 1982]).
    \93\ Comments related to implementation programs are not 
addressed here because, as described in section I.A above, this 
action is being taken pursuant to CAA section 109(d)(1) and relevant 
case law. Furthermore, leaving the NAAQS unaltered will not require 
the EPA to make new air quality designations, nor require States or 
authorized tribes to undertake new planning or control efforts. 
Accordingly, concerns related to implementation of the existing or 
an alternate standard are outside the scope of this action.
---------------------------------------------------------------------------

    With regard to USB, the commenters present an argument focused on 
an urban/``rural'' comparison and one focused on a 1-month analysis of 
O3 concentrations in response to population mobility changes 
attributed to restrictions placed to manage infections of Corona virus 
19 disease (COVID-19). We find there to be limitations in both 
arguments that undercut the conclusions reached by the commenter. As a 
result, we disagree that the observations made by the commenters 
support their statements regarding USB and with the implication that 
they contradict the EPA's findings from the detailed and extensive 
analyses presented in the PA (PA, section 2.5 and Appendix 2B).
    With regard to the urban/``rural'' comparison, the commenters' 
first cite EPA's analysis in the PA which indicated, based on daily 
maximum 8-hour (MDA8) concentrations for the nation as a whole, that 
from one quarter (10 out of 42 ppb) to one third (14 out of 45 ppb) of 
average MDA8 concentrations in spring and summer, respectively, are 
derived from anthropogenic sources. They then state that differences in 
monthly mean MDA8 concentration between two sets of monitoring sites in 
the Philadelphia metropolitan area that they identify as the three 
highest and the three most rural was 3.3 ppb in April 2020. The 
commenters suggest that this amount is much smaller than the 10 to 14 
ppb that EPA estimated to be from anthropogenic sources. Based on these 
two statements, they contend that USB is being underestimated by a 
factor of three.
    We find the commenters' analysis to have several flaws that 
undercut their conclusion. First, the difference between the two sets 
of sites, all of which fall in the Philadelphia metropolitan area, are 
not indicative of either USA (i.e., U.S. anthropogenic) or USB 
contributions. There is no evidence that this difference is indicative 
of either USB or USA, and it is especially anomalous given that the 
commenters' analysis is based on 2020 data (affected by reduced 
emissions during the reduced travel during the initial months of the 
COVID-19 epidemic in the U.S.) while EPA's is based on 2016 data. 
Second, the authors cite a country-wide seasonal average despite the 
fact that the U.S. anthropogenic contributions are clearly higher in 
the nonattainment area (than a U.S. average) being referenced. Further, 
the conclusions about USB underestimation appear quantitatively 
incorrect and to perhaps confuse USA and USB in the calculations. Even 
if all USA anthropogenic contributions cited (10 USA and 30 USB of 
total 40 ppb) in spring of 2016 were actually USB, the underestimation 
of USB would be 25% at most (0 USA and 40 USB of total 40 ppb; (40-30)/
40 = 25%), thus it is unclear how the commenter concluded a factor of 
three (300%) under-estimation of USB. In addition, the commenter's 
dataset is for the Philadelphia-Wilmington-Atlantic City CSA, where 
O3 more frequently exceeds the level of the standard in May 
through September (e.g., PA, Appendix 3C, Figure 3C-79), months that 
have lower USB and higher US anthropogenic than month of April, which 
the commenters analyzed. Finally, the commenter has focused on low 
concentration days (averaging ~40-45ppb) that the PA shows tend to be 
different than high days (PA, section 2.5 and Appendix 2B).
    The second argument is based on data on Apple Mobility data \94\ 
and O3 and NO2 concentrations for the period from 
3/22/2020 to 4/20/2020 (when transportation activity was affected by 
the behavioral changes in response to COVID-19) and differences from 
the same period in prior years. Based on the differences, the 
commenters conclude that O3 concentrations were less 
responsive to the 40 to 60% reduction in mobility than were 
NO2 concentrations (7% vs 22% difference), indicating to the 
commenters that society is reaching a period of diminishing returns of 
actions to control O3 concentrations. We note, however, that 
the period of the commenters' analysis is April, while the majority of 
days with MDA8 greater than 70 ppb in the Philadelphia nonattainment 
area occur in May to September. In the mid to late summer period, local 
production of O3 is increased (see PA section 2.5.3.2) and 
MDA8 concentrations in the Philadelphia nonattainment area more 
frequently are above the level of the standard. Thus, the analysis does 
not support the commenters' argument for a less stringent standard.\95\
---------------------------------------------------------------------------

    \94\ https://covid19.apple.com/mobility.
    \95\ We also note, contrary to the commenters' premise, 
NO2 and/or NOX are not conserved over a day. 
Rather, the overall lifetime of NOX is on the order of 
six hours. Further, while the commenter describes the ``local'' 
nature of O3, it is well established that O3 
has a large transport component. The diurnal pattern of 
O3 concentrations highlighted on this point is likely 
illustrating O3 concentrations subject to local 
NOX-titration rather than purely local formation as 
suggested by the commenters.
---------------------------------------------------------------------------

(ii) Comments in Disagreement With Proposed Decision and Calling for 
More Stringent Standard
    Among the commenters that disagree with the proposed decision and 
call for a more stringent standard, most express concerns regarding the 
process for reviewing the criteria and standards in this review and 
assert that the proposal must be withdrawn, and a new review conducted. 
The commenters expressing the view that a more stringent standard is 
needed variously cite a number of concerns. Some state that EPA cannot, 
as some commenters imply it does, simply base its decision on a 
judgment that the available evidence is similar to that when the 
standard was established in a prior review, and some argue that the 
available health effects evidence indicates that adverse health effects 
occur from exposures allowed by the current standard. Further some 
commenters express their views that the combined consideration of the 
complete evidence base indicates that sensitive or vulnerable 
populations are not protected by the current standard; and/or that the 
standard does not provide an adequate margin of safety. Additionally, 
in support of their view that the standard should be made more 
stringent, some commenters disagree with the conclusions of the 
exposure and risk analyses, characterizing the analyses as deficient, 
and contending that other quantitative analyses they cite indicate 
health impacts that would be avoided by a lower standard level. Most of 
the commenters advocating a more stringent standard recommend revision 
of the level to a value at or below 60 ppb and others support a level 
at or below 65 ppb. Some of these commenters additionally note they had 
raised similar concerns during the 2015 review.\96\ Some commenters 
also express the view that the EPA should establish a separate long-
term standard.
---------------------------------------------------------------------------

    \96\ We note that comments raised in the prior review were fully 
considered in reaching the decision in that review. Such comments 
are addressed in the decision and associated Response to Comments 
(80 FR 65292, October 26, 2020; U.S. EPA, 2015). To the extent that 
commenters are raising similar issues in support of their comments 
on the proposed decision in this review, we have addressed them in 
the current decision, based on the information now available.
---------------------------------------------------------------------------

    With regard to the process by which this review has been conducted, 
we disagree with the commenters that it is arbitrary and capricious or 
that it does not comport with legislative requirements. The review 
process, summarized in section I.D, implemented a number of features, 
some of which have been employed in past reviews and others which have 
not,

[[Page 87289]]

and several which represent efficiencies in consideration of the 
statutorily required time frame for completion of the review. The 
comments that raise concerns regarding specific aspects of the process 
are addressed in the separate Response to Comments document. As 
indicated there, the EPA disagrees with these comments. The EPA finds 
the review to have been lawfully conducted, the process reasonably 
explained, and thus finds no reason to withdraw the proposal.
    We disagree with some commenters' contention that the EPA based its 
proposed decision simply on the similarity of the health effects 
evidence to that available in the last review. While the health effects 
information is generally similar to that available in the last review, 
particularly with regard to respiratory effects (the effects causally 
related to O3 exposure), the current health effects evidence 
base includes hundreds of new health studies. Based on consideration of 
the full evidence base, including that the newly available in the 
current review, the EPA has reached different conclusions regarding 
some categories of effects (as summarized in II.A.2.a above). The EPA's 
observation that the nature of the evidence has not substantially 
changed with regard to effects causally related to O3 
exposure, was not, as implied by the comment, the primary consideration 
in the Administrator's proposed decision. The Administrator considered 
a number of factors in reaching his proposed decision, including the 
full extent of the currently available health effects evidence, and the 
details in which it is, and is not, similar to the last review, which 
has led to conclusions similar to prior conclusions for some categories 
of O3 effects and resulted in changes to others (85 FR 
49868-49874, August 14, 2020).\97\ Further, in reaching his final 
decision in this review, as described in section II.B.3 below, he has 
again considered the currently available information, now in light of 
the public comments received on the proposal, among other factors.\98\ 
In sum, while we have noted the similarities in the health effects 
information between this review and the last review (particularly for 
respiratory effects), we have engaged in independent analysis and 
assessment of the health effects information in this review, and the 
Administrator has exercised his independent judgment based on the 
current health effects assessment, in combination with current 
exposure/risk information, advice from the CASAC and public comment. 
Thus, contrary to the suggestion by these commenters, the decision on 
the primary standard has been made in consideration of the current 
health effects evidence, current analyses of air quality, exposure and 
risk, advice from the CASAC, and public comments, consistent with 
requirements under the CAA.
---------------------------------------------------------------------------

    \97\ As just one example, the causal determinations for 
cardiovascular effects and total mortality in this review differ 
from those made in the last review, as described in section 
II.A.2.a.
    \98\ In so doing, to the extent the current evidence before the 
Administrator continues to support or reinforce conclusions reached 
in prior reviews, he may reasonably reach those same conclusions.
---------------------------------------------------------------------------

    In support of their position that the available health effects 
evidence indicates that O3 exposures occurring in areas that 
meet the current standard are causing adverse effects, some commenters 
cite studies that investigate associations of O3 
concentrations and effects, such as respiratory effects, mortality, and 
preterm birth.\99\ These studies include some already evaluated in the 
air quality criteria,100 101 some published subsequent to 
the literature cutoff date for the ISA, and some which some commenters 
claim the EPA arbitrarily dismissed or inconsistently weighed in 
reaching the proposed decision.\102\ As discussed in I.D above, we have 
provisionally considered these ``new'' studies that have not already 
been evaluated in the air quality criteria and that were cited by 
commenters in support of their comments on the proposed decision (Luben 
et al., 2020). Based on this consideration, we conclude that these 
studies do not materially change the broad conclusions of the ISA with 
regard to these health effects, including the conclusions that there is 
a causal relationship of short-term respiratory effects with 
O3 exposures; a relationship of long-term respiratory 
effects with O3 exposure that is likely to be causal; 
evidence that is suggestive of, but not sufficient to infer, causal 
relationships of cardiovascular effects and total mortality with short- 
or long-term O3 exposure; evidence that is suggestive of, 
but not sufficient to infer, causal relationships of central nervous 
system effects with short- or long-term O3 exposure; and, 
evidence that is suggestive of, but not sufficient to infer, causal 
relationships of reproductive and developmental effects with long-term 
O3 exposure (ISA, section IS.1.3.1). Nor do we find that 
these studies warrant reopening the air quality criteria for further 
review (Luben et al., 2020). Thus, we do not find these publications to 
be contrary to the discussions and associated conclusions in the PA and 
proposal or to indicate the current standard to be inadequate. We 
disagree that studies cited by commenters show these categories of 
effects to be caused by O3 exposures associated with 
O3 air quality that meets the current standard. We continue 
to focus on the studies of respiratory effects as most important to the 
Administrator's judgments concerning the public health protection 
provided by the current standard.
---------------------------------------------------------------------------

    \99\ With regard to effects other than respiratory effects, 
studies cited by these commenters include studies of cardiovascular 
effects (Day et al., 2017; Shin et al., 2019; Wang et al., 2019b), 
all-cause mortality (Bell et al., 2014; Cohen et al., 2017; Di et 
al., 2017a, b), neurological effects (Cleary et al., 2018), and 
reproductive and developmental effects (Wallace et al., 2016; 
Lavigne et al., 2016; Salam et al., 2005; Steib et al, 2019; 
Morello-Frosch et al., 2010).
    \100\ In updating the air quality criteria in the current 
review, the current ISA evaluates relevant scientific literature 
published since the 2013 ISA, integrating with key information and 
judgments contained in the 2013 Ozone ISA and previous assessments 
(ISA, p. lxix; 2013 ISA; U.S. EPA, 2006; U.S. EPA, 1996a; U.S. EPA, 
1982; U.S. EPA, 1986; U.S. EPA 1978; NAPCA, 1969).
    \101\ This commenter cited a epidemiologic study (Day et al., 
2017) that had been among studies of short term O3 and 
cardiovascular effects excluded from the draft ISA due to location, 
however this study was considered by the EPA in response to advice 
from the CASAC on the draft ISA (Luben, 2020). This consideration of 
these studies did not change EPA's analysis of the weight of 
evidence from that described in the draft ISA, thus supporting the 
causality determination for cardiovascular effects described in the 
final ISA (ISA, section IS.4.3).
    \102\ We note that one study identified by a commenter to 
support their view that O3 concentrations allowed by the 
current standard is causing health effects does not include 
O3 among the pollutants it examines (Gan et al., 2014). 
Accordingly, we do not find the study to provide support to the 
commenter's point.
---------------------------------------------------------------------------

    The epidemiologic studies of respiratory effects identified by the 
commenters include some investigating associations of O3 
exposure with hospital admissions or emergency department visits for 
respiratory outcomes, or with various respiratory effects for selected 
population groups. Studies of O3 and respiratory effects 
cited by these commenters in support of their comment include studies 
that have already been evaluated in the air quality criteria (Goodman 
et al., 2017; O'Lenick et al., 2017; Jerrett et al., 2009; Lin et al., 
2008; Islam et al., 2009; Galizia et al., 1999; Peters et al., 1999; 
Wendt et al., 2014), and also several ``new'' studies, including four 
that investigate a relationship between O3 and COVID-19 
(Ware et al., 2016; Strosnider et al., 2019; Wang et al., 2019a; 
Adhikari and Yin, 2020; Zhu et al., 2020; Zoran et al., 2020; Petroni 
et al., 2020).\103\ We do not

[[Page 87290]]

find these studies to contradict any of the scientific conclusions on 
respiratory effects described in the ISA.
---------------------------------------------------------------------------

    \103\ As discussed in section I.D above, the ``new'' studies 
identified by commenters have not been through the comprehensive 
CASAC and public review process that the air quality criteria went 
through. To address these comments, we have provisionally considered 
these studies, as discussed in I.D above, and found they do not 
materially change the broad scientific conclusions of the ISA with 
regard to respiratory effects, or warrant re-opening the air quality 
criteria for further review (Luben et al., 2020).
---------------------------------------------------------------------------

    With regard to the four studies on COVID-19, we disagree with the 
commenters that they provide evidence that O3 exposure 
contributes to COVID-19 incidence, much less that they indicate that 
O3 concentrations occurring when the current standard is met 
would do so. These studies investigate an association between 
O3 and COVID-19 cases or deaths. We note, however, that the 
time-series study design used in three of these studies (Zhu et al., 
2020 [incorrectly cited by some commenters as Yongiian et al, 2020]; 
Adhikari and Yin, 2020; Zoran et al., 2020) is not appropriate for 
infectious disease cases, which do not follow a Poisson distribution, 
as they increase exponentially with community spread. The fourth study, 
an ecological study (Petroni et al. 2020), is also limited by its study 
design, which is susceptible to confounding or other biases related to 
ecologic fallacy,\104\ as well as its manner of assigning exposure to 
the population.\105\ Further, the time periods in none of the four 
studies is long enough to rule out a coincidental increase in the 
community spread of COVID-19 with the increased O3 
concentrations expected with the beginning of O3 season in 
these areas (e.g., March-April). Lastly, the biological basis by which 
a gaseous pollutant such as O3 would be expected to 
contribute to incidence of this disease is unclear.\106\ Thus, we do 
not find these studies to support a conclusion that O3 
exposure causes COVID-19 morbidity or mortality.\107\
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    \104\ Ecologic fallacy is a specific type of bias that results 
when group- or population-level data are used to estimate 
individual-level risks in an epidemiologic study
    \105\ This study uses 2016 summertime average O3 as a 
surrogate for O3 from 3/1/2020 to 7/11/2020 (Petroni et 
al., 2020). Yet COVID-19 cases did not surge in many parts of the 
U.S. until late summer or fall 2020. To the extent these areas 
(e.g., rural upper midwest) have lower O3 concentrations 
than areas of the country where COVID-19 cases surged earlier (e.g., 
New York City), a correlation between O3 concentrations 
and COVID-19 deaths would be overestimated.
    \106\ While there may be correlations between O3 
concentrations and COVID-19 cases and deaths, they could be 
explained by coincidental timing of the COVID-19 community 
transmission period in New York City and Milan with the early part 
of the O3 seasons in those areas, and neither the 
investigators or commenters provide evidence supporting an 
alternative plausible basis (Adhikari and Yin, 2020; Zoran et al., 
2020).
    \107\ While the full evidence base indicates the potential for 
O3 to increase susceptibility to some respiratory infections, the 
studies cited by commenters do not provide evidence that short-term 
or long-term O3 exposure increases susceptibility to 
COVID-19.
---------------------------------------------------------------------------

    With regard to the commenters' claims that effects other than 
respiratory effects (see above) are occurring as a result of 
O3 concentrations allowed by the current standard, we note 
that the standard is exceeded in nearly all of the locations and time 
periods analyzed in these studies.\108\ Although some studies analyzed 
multiple cities or locations in which the current standard was met 
during some time periods, air quality during other time periods or 
locations in the dataset does not meet the current standard. As noted 
in past reviews, compared to single-city studies, there is additional 
uncertainty in interpreting relationships between O3 air 
quality in individual study cities and reported O3 multicity 
effect estimates. Specifically, as recognized in section II.A.2.c 
above, the available multicity effect estimates in studies of short-
term O3 do not provide a basis for considering the extent to 
which O3 health effect associations are influenced by 
individual locations with ambient O3 concentrations low 
enough to meet the current O3 standards versus locations 
with O3 concentrations that violate this standard (85 FR 
49853, August 14, 2020; 80 FR 65344, October 26, 2015).\109\ Thus, 
based on this information and the full health effects evidence base for 
O3, we disagree with commenters about the implications of 
the cited epidemiologic studies regarding health risks of O3 
exposures resulting from the O3 concentrations in ambient 
air allowed by the current standard.
---------------------------------------------------------------------------

    \108\ Locations and time periods analyzed in these studies 
include three large metropolitan areas in Texas before 2012 (Goodman 
et al., 2017); Atlanta, Dallas and St. Louis from 2002 to 2008 
(O'Lenick et al., 2017); large cities across the U.S. from late 
1970s through 2000 (Jerrett et al., 2009); New York State, primarily 
during the 1990s (Lin et al., 2008); U.S. location chosen for O3 
concentrations not meeting the standard (Galizia and Kinney, 1999); 
a set of southern California communities during period (1990s) 
recognized to be exceeding the NAAQS (Peters et al., 1999; Islam et 
al., 2009); Houston metropolitan area during 2005 to 2007 (Wendt et 
al., 2014); multiple locations including St. Louis, Memphis and 
Atlanta 2003 through 2012 (Ware et al., 2016); six U.S. metropolitan 
areas, including Los Angeles, Baltimore and New York City, from 1999 
thru 2018 (Wang et al., 2019a); and 894 U.S. counties, including 
those for New York City and Los Angeles, 2001 to 2014 (Strosnider et 
al., 2019). Air quality data and design values derived by the U.S. 
indicate that the current 70 ppb standard was not met throughout the 
study period, or, for multicity studies for which single-city 
analyses not performed, was not met in all cities throughout the 
study (PA, Appendix 3B and Excell files available at: https://www.epa.gov/air-trends/air-quality-design-values).
    \109\ This uncertainty applies specifically to interpreting air 
quality analyses within the context of multicity effect estimates 
for short-term O3 concentrations, where effect estimates 
for individual study cities are not presented, as is the case for 
some of the multicity studies identified by commenters (85 FR 49870, 
August 14, 2020).
---------------------------------------------------------------------------

    Protection of Sensitive Groups: Commenters expressing the view that 
the current standard does not protect sensitive or at-risk populations, 
variously state that the EPA does not consider risks to a number of 
population groups the commenters identify as at higher risk for 
O3-related health effects, and that retaining the current 
standard ``creates additional and unacceptable risks'' for Black and 
low-income communities. Further, some commenters express the views that 
together the evidence from controlled human exposure studies and from 
epidemiologic studies indicates adverse effects associated with 
exposures allowed by the current standard; and that the EPA has not 
appropriately considered a number of aspects of the evidence related to 
risks to people with asthma.
    Some commenters, in addition to contending that the current 
standard will not protect populations for which the EPA has concluded 
there is adequate evidence for identification of increased risk (e.g., 
people with asthma, children, and outdoor workers), additionally assert 
that the current standard will not protect populations of color, 
American Indian/American Native groups, low SES communities, people of 
any age with respiratory issues other than asthma, diabetes or atrial 
fibrillation and pregnant women. As described in section I.A. above, 
primary NAAQS are intended to protect the public health, including at-
risk populations, with an adequate margin of safety. Accordingly, in 
reviewing the air quality criteria, the EPA evaluates the evidence with 
regard to factors that place some populations at increased risk of harm 
from the subject pollutant. In this review, the populations for which 
the evidence indicates increased risk include people with asthma, 
children and outdoor workers, among other groups, as summarized in 
section II.A.2.b above (ISA, section IS.4.4).
    In support of their argument that individuals with atrial 
fibrillation are at increased risk of O3-related health 
effects, the commenter cited a study of O3 exposure and 
total mortality that has been evaluated in the ISA (Medina-Ramon and 
Schwartz, 2008). It was initially evaluated in the last review and 
explicitly discussed again as part of the evidence base available in 
the current review (ISA, section 6.1.5.2 and Table IS-10; 2013 ISA, 
sections 6.6.2.2 and 8.2.4). Based on consideration of that study and 
others investigating a potential for increased risk among populations 
with cardiovascular disease

[[Page 87291]]

(CVD), the 2013 ISA concluded that the evidence was ``inadequate to 
classify pre-existing CVD as a potential at-risk factor for 
O3-related health effects'' (2013 ISA, sections 8.2.4). In 
the current review, while a limited number of recent studies add to the 
evidence available in the 2013 ISA,\110\ collectively the evidence 
remains inadequate to conclude whether individuals with pre-existing 
CVD are at greater risk of O3-related health effects (ISA, 
Table IS-10, section IS.4.4.3.5). Thus, the evidence does not support 
the commenters assertion that populations with atrial fibrillation are 
at increased risk of O3-related effects and that the current 
standard does not protect these groups.
---------------------------------------------------------------------------

    \110\ Further, we note that a more recent study than that cited 
by the commenters investigated the potential for an association of 
O3-related mortality risk with individuals with atrial 
fibrillation and observed no evidence of an association (ISA, 
Appendix 6, p. 6-11).
---------------------------------------------------------------------------

    The commenters who contend pregnant women are at increased risk do 
not provide supporting evidence, and the ISA does not reach such a 
conclusion based on the currently available evidence. Further, the ISA 
determined the evidence to be suggestive of, but not sufficient to 
infer, a causal relationship between O3 exposure and 
reproductive effects (ISA, section IS.4.3.6.3). Thus, we disagree with 
the commenters that pregnant women may be at increased risk of 
O3-related effects and disagree that the current standard 
does not protect these groups.
    With regard to a potential for increased risk of O3-
related health effects based upon race or ethnicity, including American 
Indians or Native Americans), the available evidence is inadequate to 
make such a determination (ISA, section IS.4.4, Tables IS-9 and IS-
10).\111\ Additionally, the evidence of increased O3 risk 
based on SES has been evaluated in the ISA and concluded to be 
``suggestive,'' but the evidence is limited by inconsistencies (ISA, 
section IS.4.4). Thus, contrary to the view expressed by some 
commenters, the EPA has considered this factor in this review and the 
evidence was not adequate to identify SES as a risk factor for 
O3 related health effects. As noted by the commenters, the 
evidence for low SES populations is ``suggestive'' of increased risk 
(ISA, section IS.4.4), in part because it includes several 
inconsistencies (as summarized in section II.A.2.b above), including 
studies that did not find O3-related risk to be higher in 
lower SES communities.\112\ While we agree with the commenters that 
populations of some particular races or ethnic backgrounds or with low 
SES have higher rates of some health conditions, including asthma,\113\ 
the available evidence is not adequate to conclude an increased risk 
status based solely on racial, ethnic or income variables alone (ISA, 
section IS.4.4). Thus, we disagree with commenters that EPA has 
arbitrarily not considered such factors in reaching the decision on the 
primary standard.
---------------------------------------------------------------------------

    \111\ In support of their view that O3-related risk 
is increased in Black populations, some commenters cite a study 
published after the ISA (Gharibi et al., 2019). We have 
provisionally considered this study, as described in section I.D. 
above, and found that it does not materially affect the broad 
conclusions in the ISA, including those regarding the adequacy of 
evidence for finding an influence on O3-related risk of 
different categories of population status, or warrant reopening the 
air quality criteria for further review (Luben et al., 2020).
    \112\ We note that two studies described by one commenter as 
indicating that those with low SES or who live in low SES 
communities face higher risk of hospital admissions and emergency 
department visits related to O3 pollution have been 
evaluated by the EPA and found not to report such findings (2013 
ISA, section 8.3.3; ISA, Table IS-10). In the first, a study of 
O3 exposure and respiratory hospital admissions in 10 
Canadian cities (Cakmak et al., 2006) ``no consistent trend in the 
effect was seen across quartiles of income,'' and the second, a 
study of O3 exposure and asthma hospital admissions and 
emergency visits (Burra et al., 2009), ``reported inverse effects 
for all levels of SES'' (2013 ISA, p. 8-27; ISA, Table IS-10).
    \113\ This is noted in the PA and proposal with regard to Black 
non-Hispanic and several Hispanic population groups (PA, Table 3-1). 
As some commenters note, this is also the case for American Indian 
and Native American population groups. Based on the recently 
available, 2016-2018 National Health Interview Survey, while just 
under 8% of the U.S. population is estimated to have asthma, the 
estimate is more than 10% for American Indian or Native American 
populations in the U.S. (https://www.cdc.gov/asthma/most_recent_national_asthma_data.htm; document identifier EPA-HQ-
OAR-2018-0279-0086).
---------------------------------------------------------------------------

    Some commenters further claim that tribal populations and 
communities of color are at increased risk of O3-related 
health effects due to increased impacts of COVID-19. We disagree with 
commenters that the studies they cite provide support for the role of 
O3 exposure in the observed increase in prevalence. The 
studies cited simply describe greater prevalence of COVID-19 among such 
communities and do not investigate and therefore do not provide 
evidence for a role for O3 exposure.\114\ An additional 
study cited by one commenter in support of their statement that people 
with COVID-19 are more susceptible to effects of O3, does 
not include any analyses with O3 among its analyses (Wu et 
al, 2020). With regard to diabetes, we note that the evidence related 
to a potential for this to affect risk of O3-related effects 
has been explicitly evaluated and found to be inadequate, thus 
indicating a lack of basis in the evidence for the statement by some 
commenters that diabetes prevalence in a community increases the risk 
of O3-related effects (ISA, Table IS-10).
---------------------------------------------------------------------------

    \114\ The commenter cites Price-Haywood et al. (2020), Stokes et 
al. (2020), Millett et al. (2020), Killerby et al. (2020), and Gold 
et al. (2020). These studies present information regarding COVID-19 
cases, hospitalizations and/or deaths among various population 
groups, but they do not investigate association of those occurrences 
with O3.
---------------------------------------------------------------------------

    Additionally, commenters that contend that retaining the current 
standard ``creates additional and unacceptable risks'' for minority and 
low-income populations variously cite higher rates of asthma and other 
preexisting conditions in these populations and higher levels of 
pollution.\115\ In making this claim, these commenters state that non-
Hispanic Blacks have been found to be more likely to live in counties 
with higher O3 pollution. To the extent that such patterns 
in the distribution of certain population groups and O3 
concentrations result in these populations residing in areas that do 
not currently meet the current standard, we note that they are at 
greater risk than populations residing in areas that meet the current 
standard, and implementing the standard will reduce their risks. But we 
disagree with the commenters' conclusion that retaining the current 
standard, without any change, creates additional risks for these 
populations.
---------------------------------------------------------------------------

    \115\ In making their argument, these commenters do not provide 
any explanation for why retaining the existing standard (i.e., 
making no regulatory change) would create additional risk for these 
populations. Rather, these commenters seem to be describing 
differences in predicted risk or mortality of air quality associated 
with a lower standard level and that of the current standard. In 
that way, they are claiming that retaining the current standard 
``creates'' additional risk. We address comments advocating a lower 
standard based on commenter-cited risk estimates (e.g., mortality) 
further below.
---------------------------------------------------------------------------

    Thus, contrary to statements by some commenters, the EPA's proposed 
decision to retain the current standard did consider evidence regarding 
risk to and thus protection of specific populations, such as those of 
particular races or ethnicities or low-income populations. The proposed 
decision, and the Administrator's decision described in section II.B.3 
below, are based on consideration of the currently available evidence, 
particularly that with regard to populations that may be at greater 
risk of O3-related health effects than the general 
population. As described in section II.B.3 below, the Administrator 
judges that by basing his decision on consideration of these 
populations, including adults and children with asthma, the at-risk 
population groups for which the

[[Page 87292]]

evidence is strongest and most extensive, will also provide protection 
for other at-risk populations for which the evidence is less certain 
and less complete.
    The commenters who express the view that the current standard does 
not provide sufficient protection of people with asthma raise concerns 
with the EPA's consideration of this group and O3-related 
effects. Further, some commenters state that the EPA has not adequately 
explained how its approach for decision-making in this review protects 
at-risk populations, such as people with asthma. Such commenters state 
that the EPA does not explain how the proposed decision accounts for 
the greater vulnerability of people with asthma, given the attention to 
evidence from controlled human exposure studies of largely healthy 
subjects. Some commenters contend that the EPA arbitrarily focuses on 
lung function decrements and respiratory symptoms ahead of lung 
inflammation, and/or that the EPA has not rationally considered the 
most recent ATS statement with regard to consideration of effects in 
people with respiratory disease, such as asthma (which the commenters 
describe as a difference from past reviews).
    We disagree with these commenters. In this review, as in past 
reviews, the EPA has fully considered the health effects evidence in 
this review, including for sensitive populations, such as people with 
asthma, and explained its conclusions regarding the adequacy of public 
health protection offered by the current standard, including for such 
populations. Thus, the decision in this review, as described in section 
II.B.3 below, is based on the current scientific information. Further, 
our approach in this review does not differ appreciably from our 
approach in the last review. This approach is consistent with the 
applicable legal requirements for this review, including with 
provisions of the CAA related to the review of the NAAQS, and with how 
the EPA and the courts have historically interpreted the CAA. The 
approach is based fundamentally on the current health effects evidence 
in the ISA and quantitative analyses of exposure and risk in the PA. 
The policy implications of this information, along with guidance, 
criteria or interpretive statements developed within the public health 
community, including, also, statements from the ATS, in addition to 
advice from the CASAC are evaluated in the PA for consideration by the 
Administrator. The PA evaluations inform the Administrator's public 
health policy judgments and conclusions. Thus, as in past reviews, the 
Administrator's decision on the adequacy of the current primary 
standard draws upon the scientific evidence for health effects, 
quantitative analyses of population exposures and health risks, CASAC 
advice, and judgments about how to consider the uncertainties and 
limitations that are inherent in the scientific evidence and 
quantitative analyses, as well as public comments on the proposed 
decision.
    As described in section II.B.3 below, key aspects of the evidence 
informing the Administrator's decision-making in this review include: 
(1) The causal relationship of O3 with respiratory effects, 
based on the full health effects evidence base, including both the 
controlled human exposure studies conducted primarily in largely 
healthy adult subjects, and the epidemiologic studies of health 
outcomes for people with asthma, and particularly children with asthma; 
(2) the increased risk to children and people with asthma, among other 
groups (3) the respiratory effects reported at the lowest exposures in 
the controlled human exposure studies; and (4) features of asthma that 
contribute to the susceptibility of people with asthma to 
O3-related effects. As a whole, the evidence base in this 
NAAQS review generally reflects a continuum, consisting of exposure 
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. As summarized 
in section I.A above, 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. The Administrator's consideration of the 
scientific evidence is informed by the quantitative estimates of 
exposure and risk for air quality allowed by the current standard, and 
associated judgments on the adequacy of public health protection 
provided by the current standard are informed by advice from the CASAC 
and statements from ATS on adversity.
    With regard to the most recent ATS statement, the commenters' claim 
that the EPA does not adequately consider the implications of the 
sentence that ``small lung function changes should be considered 
adverse in individuals with extant compromised function, such as that 
resulting from asthma, even without accompanying respiratory symptoms'' 
and to consider the importance of examining effects in susceptible 
subsets of broader populations (Thurston et al., 2017). We disagree. 
The ATS statements (from the initial statement in 1985 to the recent 
2017 statement) and their role in primary O3 standard 
reviews, summarized in section II.A.2.b above, occupy a prominent role 
in consideration of public health implications in the PA and the 
proposal (PA, section 3.3.2; 85 FR 49848, 49866, 49871, August 14, 
2020), and the Administrator considers them in his decision, as 
described in section II.B.3 below. The PA presentation includes 
summaries of the purpose and intentions articulated by the ATS, and of 
the evolution and areas of consistency across the statements. The PA 
gave particular attention to the ATS emphasis on consideration of the 
significance or adversity of effects, particularly for more susceptible 
individuals. It recognized both the 2000 ATS statement concluding that 
``small transient changes in forced expiratory volume in 1 s[econd] 
(FEV1) alone were not necessarily adverse in healthy 
individuals, but should be considered adverse when accompanied by 
symptoms'' (ATS, 2000), and also the more recent statement that also 
gives weight to findings of such lung function changes in the absence 
of respiratory symptoms in individuals with pre-existing compromised 
function, such as that resulting from asthma (Thurston et al., 2017). 
With regard to population risk (another aspect of the ATS statement 
cited by commenters), the PA and proposal summarized the 2000 and 2017 
ATS statements, recognizing that the 2017 statement references and 
further describes concepts described in the 2000 statement, such as its 
discussion of considering effects on the portion of the population that 
may have a diminished reserve that puts its members at potentially 
increased risk if affected by another agent (ATS, 2000).\116\ As 
described in section II.B.3 below, the Administrator considers the ATS 
statements in reaching his conclusions in this review.
---------------------------------------------------------------------------

    \116\ The sentence in the 2017 statement of which one commenter 
quoted only a part, ``As discussed in the previous ATS statement, a 
small but statistically significant mean reduction in 
FEV1 in a population means that some people had larger 
reductions, with the likelihood that reductions in a subset of 
susceptible subjects can have passed a threshold for clinical 
importance'' This paragraph goes on to note that a study in which 
the mean decrement is about 3%, included two subjects with 
decrements greater than 10% (Thurston et al., 2017).
---------------------------------------------------------------------------

    In support of their claim that the EPA has not appropriately 
considered the ATS statements, some commenters

[[Page 87293]]

additionally take issue with the EPA's use of the number of subjects 
experiencing at least a 15% FEV1 decrement in its 
description in the proposal of the increased response evident by 
comparing from the lowest exposure levels studied (40 ppb) up to 70 ppb 
(85 FR 49851, August 14, 2020). These comments also state that EPA did 
not discuss the clinical significance of FEV1 decrements of 
10% or higher for people with existing lung disease, while stating that 
the ATS statement mentions this magnitude of decrement. The ATS 
statement references decrements at or above 10% in illustrating a point 
about variation of subject responses beyond a group mean, noting that 
while the mean of an exposed group of study subjects may be small, some 
group members have larger reductions and can have passed a threshold 
for clinical importance. It does not provide a discussion of thresholds 
of clinical importance.\117\ In claiming that EPA's discussion on this 
represents a difference from the last review, the commenters cite the 
2014 HREA and state that we have not considered FEV1 
decrements at or above 10% in the current review, however this is not 
the case.\118\ Furthermore, the PA states that the mid- to upper-end of 
the range of moderate levels of functional responses and higher (i.e., 
FEV1 decrements >=15% and >=20%) are included to generally 
represent potentially adverse lung function decrements in active 
healthy adults, while for people with asthma or lung disease, a focus 
on moderate functional responses (FEV1 decrements down to 
10%) may be appropriate (PA, Appendix 3D, p. 3D-76).
---------------------------------------------------------------------------

    \117\ With regard to 10% as a magnitude decrement, the prior ATS 
statement noted that the EPA had graded this ``mild'' in a prior 
review, while noting that such a grading has not been evaluated 
against other measures (ATS, 2000). In this review, as in past 
reviews, the EPA has summarized study results with regard to 
multiple magnitudes of lung function decrement, including 10%, 
recognizing that 10% has been used in clinical settings to detect a 
FEV1 change likely indicative of a response rather than 
intrasubject variability, e.g., for purposes of identifying subjects 
with responses to increased ventilation (Dryden, 2010). For example, 
the PA in the current review provides such a summary (PA, Appendix 
3D, p. 3D-77).
    \118\ Contrary to this claim, the lung function risk analysis in 
the current review (which is an update of the very same analysis in 
the 2014 HREA to which the commenters cite) presents the results for 
exactly the same categories of lung function decrement (at/above 
10%, at/above 15% and at/above 20%) as in the 2014 HREA (e.g., PA, 
Table 3-4).
---------------------------------------------------------------------------

    In objecting to the EPA's approach to considering the ATS 
statement, these commenters cite a reference to the ATS statement in 
CASAC's advice as additional evidence that the EPA approach to 
considering the ATS statement is arbitrary.\119\ This comment was made 
within the context of the CASAC comments on the draft PA that 
emphasized the need to improve discussion of the susceptibility of 
people with asthma, including giving attention to the occurrence of 
lung function decrements in susceptible groups, specifically children 
with asthma. This section of the CASAC letter also cautions against too 
great a focus on lung function decrements and emphasizes the need for 
fuller consideration of respiratory effects that are likely to be 
important in people with asthma due to features of that disease. In 
consideration of these comments, the final PA includes an improved 
discussion on the unique vulnerability of people with asthma (PA, 
sections 3.3.1.1, 3.3.2, 3.3.4, and 3.5.1) that contributes to due 
consideration of this population group in decision-making on the 
primary O3 standard. Further, in considering the exposure 
and risk analysis results, we recognize the comparison-to-benchmarks 
analysis as providing a more robust consideration of risk to sensitive 
groups as it provides the ability to consider O3 effects 
more broadly, with each benchmark representing the array of effects, at 
different severities, associated with that exposure level. This is one 
of the reasons (consistent with the CASAC advice) that this analysis 
(rather than the lung function risk analysis) receives greater emphasis 
in the PA, consistent with the CASAC advice in this area.
---------------------------------------------------------------------------

    \119\ The citation provided by the commenters is the CASAC 
letter on the draft PA; in this letter the CASAC cites the ATS 
statement in making a comment on the draft PA indicating that the 
concept that lung function decrements in the absence of symptoms do 
not represent an adverse health effect should not apply to the 
susceptible group of children with asthma (Cox, 2020a, Consensus 
Responses to Charge Questions, pp. 8-9).
---------------------------------------------------------------------------

    In light of the above discussion, we note that the PA, the 
proposal, and the decision described in section II.B.3 below, focus 
specifically on consideration of people with asthma, and particularly 
children with asthma. While the evidence regarding the susceptibility 
of people with asthma to the effects of O3 is robust, our 
understanding of the exposures at which various effects (of varying 
severity) would be elicited is less defined. For example, the inherent 
characteristics of asthma contribute to a risk of asthma-related 
responses, such as asthma exacerbation in response to asthma triggers, 
which may increase the risk of more severe health outcomes (ISA, 
section 3.1.5). This is supported by the strong and consistent 
epidemiologic evidence that demonstrates associations between ambient 
O3 concentrations and hospital admissions and emergency 
department visits for asthma (ISA, section IS.4.4.3.1). In moving to 
consideration of the potential specific exposure scenarios (e.g., 
multiple-hour exposures to 60 to 80 ppb O3 during quasi-
continuous exercise), we note that the evidence is for largely healthy 
adult subjects. With regard to lung function decrements, the limited 
evidence from controlled human exposure studies (primarily at higher 
exposures and in adult subjects) indicates similar magnitude of 
O3-related FEV1 decrements for people with as for 
people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Further, 
across other respiratory effects of O3 (e.g., increased 
respiratory symptoms, increased airway responsiveness and increased 
lung inflammation), the evidence has also found the observed responses 
to generally not differ due to the presence of asthma, although the 
evidence base is more limited with regard to study subjects with asthma 
(ISA, Appendix 3, section 3.1.5.7). Thus, in light of the uncertainties 
in the evidence base with respect to people with asthma and exposures 
eliciting effects and the severity of those effects, other aspects of 
the evidence are informative to the necessary judgments. Accordingly, 
the advice from the CASAC and the statements from the ATS are important 
to the judgments made by the Administrator in basing his decision on 
the current evidence and ensuring a primary standard that protects at-
risk populations, such as people with asthma.
    Contrary to the claim from some commenters, our consideration of 
effects in people with asthma did not focus solely on lung function 
responses. As noted above, we recognize that the inherent 
characteristics of asthma as a disease provide the potential for 
O3 exposures to trigger asthmatic responses, such as through 
causing an increase in airway responsiveness. Based on the available 
evidence, we consider the potential for such a response to be greater, 
in general, at relatively higher, versus lower, exposure 
concentrations, noting 80 ppb to be the lowest exposure concentration 
at which increased airway responsiveness has been reported in generally 
healthy adults. We recognize that this evidence and the evidence 
represented by the three benchmark concentrations used in the exposure/
risk analyses (60, 70 and 80 ppb) is for largely healthy adults and 
does not include data for people with

[[Page 87294]]

asthma. In reaching his decision in this review, the Administrator 
gives additional consideration to the effects of particular concern for 
people with asthma, such as asthma exacerbation, in light of the 
limitations of the evidence represented by the benchmarks in this 
regard, as discussed in section II.B.3 below.
    In support of their view that the EPA gives too little weight to 
effects reported in studies of 60 ppb, some commenters assert that the 
EPA arbitrarily focused on the evidence for lung function decrements 
and respiratory symptoms, and does not explain how the proposed 
decision protects against the harm posed by inflammatory responses to 
O3. In making this statement they cite the study by Kim et 
al. (2011) and discussions in the ISA regarding studies documenting the 
role of O3 in eliciting inflammatory responses and regarding 
possible conceptual mechanisms by which inflammatory responses can 
contribute to other effects (including cardiovascular effects). In so 
doing, they contend that exposures lower than those for which the 
current standard is intended can cause inflammation resulting in 
permanent lung damage and the development of severe lung disease. They 
additionally state that airway inflammation of O3 is of 
particular concern for people with asthma as airway inflammation is a 
feature in the definition of asthma.
    Contrary to the view of some commenters, the Administrator has 
given significant consideration (in the proposal and in section II.B.3 
below) to the exposure estimates for the 60 ppb benchmark. In 
considering the O3 inflammatory response, we note that 
inflammation induced by a single exposure (or several exposures over 
the course of a summer) can resolve entirely (2013 ISA, p. 6-76). Thus, 
the inflammatory response observed following the single exposure to 60 
ppb in the study by Kim et al. (2011) of largely healthy subjects is 
not necessarily an adverse response.\120\ We further consider the 
comments from the CASAC regarding airway inflammation as an important 
aspect of asthma, including the CASAC's description of increased airway 
inflammation in people with asthma as having the potential to increase 
the risk of an asthma exacerbation. As described in section II.B.3 
below, the Administrator also considers this, while noting also the 
lack of evidence from studies of people with asthma at 60 ppb. In so 
doing, he recognizes that, due to interindividual variability in 
responsiveness, both in regard to O3 and in regard to asthma 
exacerbation triggering events, not every occurrence of an exposure 
considered to have the potential to increase airway inflammation will 
result in such an adverse effect. We find it important to note, 
however, that continued acute inflammation can contribute to a chronic 
inflammatory state, with the potential to affect the structure and 
function of the lung (2013 ISA, p. 6-76; ISA, sections 3.1.4.4.2 and 
3.1.5.6.2).\121\ In light of this evidence, the Administrator, in his 
consideration of the exposure/risk estimates of exposures at or above 
the 60 ppb benchmark (described in section II.B.3 below), is less 
concerned about such estimates representing a single occurrence, and 
gives weight to estimates of multiple occurrences and their associated 
greater risk. Thus, rather than a sole focus on a single exposure level 
or type of effect (such as lung function decrements), the Administrator 
considers the quantitative estimates for all three benchmarks (with 
regard to single and multiple occurrences), recognizing that they 
represent differing levels of significance and severity of 
O3-related effects, both with regard to the array of effects 
and severity of each type of effects, as well as the implications for 
the at-risk populations, including people with asthma. The comparison-
to-benchmarks analysis provides for this full characterization of risk 
for the broad array of respiratory effects, including inflammation and 
airway responsiveness, thus avoiding an inadequate and narrower focus, 
e.g., limited to lung function decrements (85 FR 49872, August 14, 
2020).
---------------------------------------------------------------------------

    \120\ One commenter contends that inflammation is apparent from 
short-term O3 exposures ranging from 12 to 35 ppb, based 
on air quality metrics reported in some epidemiologic studies, such 
as mean 24-hour averages or monthly averages of 8-hour 
concentrations (ISA, Table 4-28). The commenter implies that such 
values for these metrics are lower than the level of the standard 
(70 ppb) means that exposures allowed by the standard are causing 
outcomes analyzed in the study. However, none of the metrics for 
which values are cited by the commenter are in terms of design 
values for the current standard, such that a direct comparison of 
the values is not meaningful.
    \121\ The currently available evidence does not support the 
implication of the commenters that the inflammatory response 
reported in some individuals after a 6.6-hour exposure to 60 ppb, 
during quasi-continuous exercise (as in Kim et al., 2011), causes 
permanent lung damage or development of severe lung disease. While 
the experimental animal evidence indicates the potential for 
repeated exposures to elevated concentrations (e.g., at or above 500 
ppb over multiple days) can contribute to other effects in animal 
models or to other asthmatic responses in animal models of asthma, 
the full evidence base for single exposures to lower concentrations 
does not provide such a finding (ISA, sections 3.1.4.4, 3.1.4.4.2 
and 3.1.5.6.2; 2013 ISA, section 6.2.3). Thus, the potential for 
effects reported from 6.6-hour exposures to 60 ppb O3, 
during quasi-continuous exercise, including the inflammation 
reported by Kim et al. (2011) to contribute to adverse health 
effects is uncertain. Newly available evidence in this review does 
not reduce this uncertainty or provide a contradiction to conclusion 
regarding the implications of inflammation induced by single or 
isolated exposures (ISA, Appendix 3).
---------------------------------------------------------------------------

    Contrary to the commenters' claims, the Administrator, in reaching 
his proposed decision, and in his final decision, as described in 
section II.B.3 below, placed primary focus on what the evidence 
indicates with regard to health effects in the at-risk population of 
people with asthma, particularly children with asthma, and on results 
of the exposure and risk analysis for this population. In so doing, he 
recognizes key aspects of the evidence, as summarized in section 
II.A.2.a above, that indicate the array of O3-associated 
respiratory effects to be of increased significance to people with 
asthma given aspects of the disease that may put such peoples at 
increased risk for prolonged bronchoconstriction in response to asthma 
triggers. The increased significance of effects in people with asthma 
and risk of increased exposure for children (from greater frequency of 
outdoor exercise) is illustrated by the epidemiologic findings of 
positive associations between O3 exposure and asthma-related 
emergency department visits and hospital admissions for children with 
asthma. In this context, the Administrator focuses on the breadth of 
O3 respiratory effects evidence at the lowest exposures 
tested in the controlled human exposure studies which provide the most 
certain evidence, considering this in light of the fuller evidence base 
which provides a foundation for necessary judgments in light of 
uncertainties.
    Thus, we disagree with commenters that we have not considered the 
full body of evidence and quantitative information available in this 
review with regard to exposures that might be expected to elicit 
effects in at-risk populations. In so doing, as summarized in section 
II.A.2.a above, section II.B.1.a of the proposal, and the PA, we 
recognize that the currently available evidence supports the conclusion 
of a causal relationship between short-term O3 exposure and 
respiratory effects, with the strongest evidence coming from controlled 
human exposure studies that document subtle reversible effects in 6.6-
hour exposures of largely healthy adult subjects, engaged in quasi-
continuous exercise, to average concentrations as low as 60 ppb. The 
epidemiologic evidence of associations of O3 concentrations 
in ambient air with

[[Page 87295]]

increased incidence of hospital admissions and emergency department 
visits for an array of respiratory health outcomes further indicates 
the potential for O3 exposures to elicit health outcomes 
more serious than those assessed in the experimental studies, 
particularly for children with asthma, and the evidence base of such 
epidemiologic studies as a whole provides strong support for the 
conclusion of causality for respiratory effects.\122\ Further as 
described in the PA and the proposal and summarized in section II.A.2.a 
above, very few of these studies were conducted in locations during 
periods when the current standard was met. While some commenters cite 
the low values of some of the air quality metrics analyzed in such 
studies, those metrics are not in the form of the design value for the 
current standard and so, contrary to commenters' assertion, cannot show 
that serious health effects are occurring under air quality conditions 
allowed by the current 70 ppb standard.
---------------------------------------------------------------------------

    \122\ As described in section II.A.2.c above and in the PA, 
these studies generally do not detail the specific exposure 
circumstances eliciting such effects.
---------------------------------------------------------------------------

    Protection With an Adequate Margin of Safety: Some commenters 
expressed the view that the current standard does not provide an 
adequate margin of safety variously argue that the EPA is ignoring 
precedent and CAA requirements for considering scientific uncertainty 
in its judgments regarding an adequate margin of safety, and that 
statements from the prior CASAC and new evidence suggests that the 
current 70 ppb standard provides little margin of safety for protection 
of sensitive subpopulations from harm. These commenters generally 
advocate revision to a 60 ppb standard to address this concern. In 
support of their views, some state that the EPA is ignoring findings of 
a statistically significant lung function response to 6.6-hour exposure 
to 60 ppb during quasi-continuous exercise while others cite the EPA 
consideration of epidemiologic evidence, claiming that the EPA is 
inappropriately using identified uncertainties as a basis for not 
revising the standard. We disagree with these characterizations.
    As an initial matter, we note that, contrary to the statements made 
by these commenters, the Administrator, in reaching his proposed 
decision, as in reaching his final decision, has considered legal 
precedent and CAA requirements for a primary standard that protects 
public health, including the health of sensitive groups, with an 
adequate margin of safety. With regard to scientific uncertainty, as 
summarized in section I.A above, the CAA 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 Ass'n 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). Thus, in considering whether the primary 
standard includes an adequate margin of safety, the Administrator is 
seeking to ensure that the standard not only prevents pollution levels 
that have been demonstrated to be harmful but also prevents lower 
pollutant levels that may pose an unacceptable risk of harm, even if 
the risk is not precisely identified as to nature or degree. In so 
doing, however, 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 the proposed decision, as in the decision described in section 
II.B.3 below, the Administrator's consideration of the kind and degree 
of uncertainties associated with the current information (as some of 
the factors the EPA considers in addressing the requirement for an 
adequate margin of safety) involved a number of judgments. With regard 
to his consideration of the epidemiologic evidence, for example, the 
Administrator recognizes that, as a whole, investigations of 
associations between O3 and respiratory effects and health 
outcomes (e.g., asthma-related hospital admission and emergency 
department visits) provide strong support for the overarching 
conclusion of that O3 causes respiratory effects, and its 
risks to people with asthma. In his consideration of O3 
exposures of concern, the Administrator, agrees with staff evaluations 
in the PA, that such studies available in this review are less 
informative to his judgments related to air quality conditions allowed 
by the current standard (85 FR 49870, August 14, 2020). For example, as 
summarized in section II.A.2.c above, none of the U.S. studies that 
show associations between O3 and the clearly adverse health 
outcomes of hospital admissions or emergency department visits for 
respiratory causes were based in locations during time periods when the 
current standard was always met (PA, section 3.3.3). While there were 
two such studies based in single cities in Canada, as discussed above, 
the interpretation of individual single-city results is complicated by 
the presence of co-occurring pollutants or pollutant mixtures (PA, 
section 3.3.3).\123\ Thus, as in reaching his decision in this review, 
the Administrator has fully considered conclusions reached in the ISA 
regarding the epidemiologic evidence and the policy evaluations in the 
PA, and does not find the currently available epidemiologic studies to 
provide insights regarding exposure concentrations associated with 
health outcomes that might be expected under air quality conditions 
that meet the current standard (85 FR 49870, August 14, 2020). Thus, 
the EPA's decision on the standard in this review fully and 
appropriately considers the full evidence base, including the 
epidemiologic evidence, and associated uncertainties and limitations.
---------------------------------------------------------------------------

    \123\ Accordingly, uncertainties remain with regard to the 
independent role of O3 exposures in eliciting the 
reported health outcomes analyzed, and in the absence of analyses 
that might reduce such uncertainties (e.g., analyses of the presence 
and effects of co-occurring pollutants).
---------------------------------------------------------------------------

    With regard to the controlled human exposure studies, and the 
nature and degree of effects that might be expected at exposures lower 
than those studied or in unstudied population groups, the Administrator 
has considered first what the evidence base indicates with regard to 
the lowest exposures as well as differences and similarities between 
the studied populations and the less well studied population groups 
recognized to be at increased risk. In so doing, he considers the 
findings of statistically significant respiratory responses in the 
studies of 60 ppb exposures in largely healthy subjects, particularly 
in his consideration of the exposure and risk estimates. For example, 
in reaching his decision in section II.B.3 below, as for his proposed 
decision, he finds it appropriate to consider the level of protection 
provided by the current standard from single exposures, but to give 
greater weight to multiple exposures, in judging adequacy of the margin 
of safety provided by the current standard.\124\ Such considerations

[[Page 87296]]

contributed to the Administrator's proposed judgments with regard to 
the requisite level of protection needed to protect at-risk populations 
with an adequate margin of safety, as required by the Act and 
consistent with the factors recognized in the relevant case law. Thus, 
consistent with the CAA requirements and prior judicial decisions, the 
Administrator based his proposed decision, and bases his final decision 
(as summarized in II.B.4 below) on the scientific evidence, our current 
understanding of it, and his judgments concerning associated 
uncertainties, both those associated with inconclusive scientific and 
technical information, and those associated with hazards that research 
has not yet identified. These judgments, along with the factors 
recognized above that the EPA generally considers in each NAAQS review, 
contribute to his reasoned decision making in this review, as described 
in section II.B.3 below.
---------------------------------------------------------------------------

    \124\ Contrary to implications of some commenters, this judgment 
by the current Administrator is consistent with that made by the 
prior Administrator in establishing the current standard, as seen 
from the summary of the prior Administrator's judgment in that 
regard that was summarized in the proposal and that these commenters 
cite:
    Further, while the Administrator recognized the effects 
documented in the controlled human exposure studies for exposures to 
60 ppb to be less severe than those associated with exposures to 
higher O3 concentrations, she also recognized there to be 
limitations and uncertainties in the evidence base with regard to 
unstudied population groups. As a result, she judged it appropriate 
for the standard, in providing an adequate margin of safety, to 
provide some control of exposures at or above the 60 ppb benchmark 
(80 FR 65345-65346, October 26, 2015). [85 FR 49841, August 14, 
2020]
---------------------------------------------------------------------------

    With regard to advice provide by CASAC in the last review as a 
general matter, we disagree with the commenters' presumption that it is 
necessary for EPA to address in this review each statement a prior 
CASAC made in a prior review. The Clean Air Act does not impose such a 
requirement. We further note that a prior CASAC's advice would be based 
on review of the prior air quality criteria, exposure/risk analyses and 
standard, as well as considerations pertinent in the prior review 
(which may, depending on the issue, differ from the pertinent evidence, 
information and considerations before a current CASAC). We note, 
however, that this specific advice from the prior CASAC on the adequacy 
of the margin of safety was cited by part of the CASAC in the current 
review. As summarized in the proposal and in section II.B.1.b above, 
while the prior CASAC advised that the size of the margin of safety 
provided varied across different standard levels within the range from 
70 to 60 ppb that the prior CASAC recommended, it found a level of 70 
ppb could be supported by the scientific evidence. Further, the prior 
CASAC recognized, as summarized in section II.B.1.b above, that with 
regard to the ``size'' of the margin of safety, the selection of any 
particular approach to providing an adequate margin of safety is a 
policy choice left specifically to the Administrator's judgment (Lead 
Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 
F.3d at 1353; section I.A above). Thus, in reaching his proposed 
decision, the Administrator explicitly considered the advice provided 
by the prior CASAC to the extent it was represented in advice from the 
current CASAC as emphasized by part of the current CASAC (85 FR 49873, 
August 14, 2020), and he has similarly again considered it reaching his 
final decision, as described in section II.B.3 below, in light of 
public comments raising it.
    Some commenters also express the view that EPA is using limited 
data in sensitive population groups as an excuse for not establishing a 
level at which there is ``an absence of adverse effect'' in sensitive 
groups. In support of their view, some commenters claim that the EPA 
has not addressed a statement of the prior CASAC regarding the 
potential for lung function decrements and respiratory symptoms to 
occur in people with asthma with exposure to 70 ppb (while at elevated 
exertion). Contrary to this claim,\125\ the EPA considered in the last 
review the point made by the prior CASAC in the statement highlighted 
by the commenters. The statement from the prior CASAC that the 
commenters reference was provided in the CASAC review of a draft PA in 
the last review, fully considered in completing the final 2014 PA, and, 
along with the totality of the prior CASAC advice, taken into account 
in establishing the current primary standard (80 FR 65292, October 26, 
2020). It is not necessary for the EPA to address in this review each 
statement a prior CASAC made in a prior review.
---------------------------------------------------------------------------

    \125\ The context for this statement is in considering the 
benchmark concentrations utilized in the exposure-to-benchmarks 
analysis of the 2014 HREA and reflecting on responses reported in 
controlled human exposure studies of healthy subjects exposed for 
6.6 hours with quasi-continuous exercise. With regard to the 
responses reported from exposure to 72 ppb, on average across the 
exercise periods, the prior CASAC stated its view ``that these 
effects almost certainly occur in some people, including asthmatics 
and others with low lung function . . . at levels of 70 ppb and 
below'' (Frey, 2014b, p. 6).
---------------------------------------------------------------------------

    We agree with commenters who express the view that to protect 
sensitive populations from effects reported in some largely healthy 
subjects from the 6.6 hour exposure to 73 ppb (with quasi-continuous 
exercise), the standard should provide protection against somewhat 
lower exposures. As summarized in section II.B.3 below, this is an 
objective the Administrator identifies for the standard and, based on 
the exposure/risk estimates, he finds the standard to provide strong 
protection from such exposures (and associated risk of such effects). 
In addition, in highlighting this isolated statement from the last 
review, the commenters fail to distinguish CASAC advice on the primary 
standard from consideration of the exposure benchmark for comparison to 
a multi-hour exposure while engaged in quasi-continuous exercise.
    With regard to the prior CASAC's scientific and policy advice on 
the primary standard,\126\ the prior CASAC concluded that the 
scientific evidence supported a range of standard levels that included 
70 ppb, and also recognized the choice of a level within that range to 
be ``a policy judgment under the statutory mandate of the Clean Air 
Act'' (85 FR 49873).\127\ We further note that the current CASAC 
concludes in this review that newly available evidence relevant to 
standard setting does not substantially differ from that available in 
the last review (Cox, 2020a, Consensus Responses to Charge Questions p. 
12; 85 FR 49873, August 14, 2020). As discussed further below, we note 
that 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.
---------------------------------------------------------------------------

    \126\ In their 2014 advice, the prior CASAC concluded by 
explicitly stating ``our policy advice is to set the level of the 
standard lower than 70 ppb within a range down to 60 ppb, taking 
into account your judgment regarding the desired margin of safety to 
protect public health.''
    \127\ 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.''
---------------------------------------------------------------------------

    Some commenters also state that the primary NAAQS must be set at a 
level at which there is an absence of adverse effects in sensitive 
populations. While the EPA agrees that the NAAQS must be set to protect 
sensitive populations with an adequate margin of safety, it is well 
established that the NAAQS are not

[[Page 87297]]

meant to be zero risk standards. See Lead Industries v. EPA, 647 F.2d 
at 1156 n.51; ATA III, 283 F. 3d at 360 (``[t]he lack of a threshold 
concentration below which these pollutants are known to be harmless 
makes the task of setting primary NAAQS difficult, as EPA must select 
standard levels that reduce risks sufficiently to protect public health 
even while recognizing that a zero-risk standard is not possible''); 
Mississippi, 744 F. 3d at 1351 (same); see also id. at 1343 
(``[d]etermining what is `requisite' to protect the `public health' 
with an `adequate' margin of safety may indeed require a contextual 
assessment of acceptable risk. See Whitman, 531 U.S. at 494-95 (Breyer 
J. concurring)''). As the Court of Appeals for the D.C. Circuit said in 
reviewing the 2015 O3 NAAQS, ``the primary standard for a 
non-threshold pollutant like ozone is not required to produce zero 
risk, and `[t]he task of determining what standard is `requisite' to 
protect the qualitative value of public health or what margin of safety 
is `adequate' to protect sensitive subpopulations necessarily requires 
the exercise of policy judgment.' '' Murray Energy, 936 F.3d at 610 
(quoting Mississippi v. EPA).\128\ The Administrator's judgments in 
this review are rooted in his evaluation of the evidence, which 
reflects the scientific uncertainty as to the O3 
concentrations at which sensitive subpopulations would experience 
adverse health effects, and his judgments weigh both the risks and the 
uncertainties. This is a legitimate, and well recognized, exercise of 
``reasoned decision-making.'' ATA III. 283 F. 3d at 370; see also id. 
at 370 (``EPA's inability to guarantee the accuracy or increase the 
precision of the . . . NAAQS in no way undermines the standards' 
validity. Rather, these limitations indicate only that significant 
scientific uncertainty remains about the health effects of fine 
particulate matter at low atmospheric concentration. . . .''); 
Mississippi, 744 F. 3d at 1352-53 (appropriate for EPA to balance 
scientific uncertainties in determining level of revised O3 
NAAQS).
---------------------------------------------------------------------------

    \128\ The legislative history of the Clean Air Act provides 
further support for these holdings, as do the statutory deadlines 
for attainment. See H. Rep. 95-294, 95th Cong. 1st sess. 127, 123 
Cong. Rec. S9423 (daily ed. June 10, 1977) (statement of Senator 
Muskie during the floor debates on the 1977 Amendments that ``there 
is no such thing as a threshold for health effects. Even at the 
national primary standard level, which is the health standard, there 
are health effects that are not protected against.''
---------------------------------------------------------------------------

    Exposure/risk Analyses: In expressing the view that the standard 
should be made more stringent, some commenters disagree with EPA 
conclusions based on the exposure/risk analyses, and point to other 
analyses that they state show that a lower standard level (e.g. 65 ppb 
or lower) would avoid important health effects. These commenters' 
claims of deficiencies with the exposure/risk analyses include claims 
that the study area selection is not explained, that population size of 
the study areas analyzed is too small to support conclusions and does 
not include particular areas; that the analysis does not include 
adults, and other groups of interest, and that selection of study areas 
with air quality close to the current standard contributed to 
underestimates of population exposures. We disagree with these 
commenters' claims.
    With regard to study area selection and population size for the 
analysis, we note that an exposure and risk analysis based on eight 
study areas, all of which are major metropolitan areas provides a 
robust foundation for population exposure estimates. The eight study 
areas included reflect the full range of air quality and exposure 
variation expected across major urban areas in the U.S and seven 
different NOAA climate regions (PA, section 3.4.1).\129\ This number of 
areas (8) and combined population size (more than 45 million in the 
combined metropolitan areas [PA, Appendix 3D, Table 3D-1]) are much 
larger than similar analyses in recent NAAQS reviews for other 
pollutants (e.g., sulfur dioxide [U.S. 2018]), and not that dissimilar 
to similar analyses in past O3 NAAQS reviews.\130\ Some 
commenters claim that the exclusion of specific urban areas in which 
O3 concentrations are much higher than those analyzed 
resulted in underestimates of exposure. We disagree with this claim as 
the air quality analyzed across all study areas was adjusted to just 
meet the current standard (or alternative scenarios). Thus, an urban 
area that currently has O3 concentrations well in exceedance 
of the current standard would not necessarily have been found to have 
higher exposure estimates if it were simulated to have air quality just 
meeting the current standard. Such estimates would, however, have 
greater uncertainty, which is the reason such study areas as those 
identified by commenters (e.g., Los Angeles) were excluded. Areas 
included were those for which only small adjustments were required for 
the air quality to just meet the current standard (and alternative 
scenarios), yielding reduced uncertainty (e.g., given the need for 
larger air quality adjustments to achieve conditions that just meet the 
current standard) in these estimates compared to those from the 2014 
HREA (PA, sections 3.4.1 and 3.4.4, and Appendix 3D). In selecting such 
areas, however, we considered a number of other characteristics in 
order to achieve a varied set of study areas, including with regard to 
air quality patterns.\131\ This variation contributed to variation in 
exposure estimates, even for the same air quality scenario (PA, 
Appendix 3D, Tables 3D-26, 3D-28 and 3D-30). Thus, in addition to 
focusing on study areas with ambient air concentrations close to 
conditions that reflect air quality that just meets the current 
standard that would be more informative to evaluating the health 
protection provided by the current standard than areas with much higher 
(or much lower) concentrations, the approach employed recognizes that 
capturing an appropriate diversity in study areas and air quality 
conditions (that reflect the current standard scenario) is an important 
aspect of the role of the exposure and risk analyses in informing the 
Administrator's conclusions on the public health protection afforded by 
the current standard.
---------------------------------------------------------------------------

    \129\ Contrary to the commenters' assertion of a lack of 
explanation for the study areas included in the analyses, the PA 
describes the study area selection criteria and process, including 
steps taken to include adequate representation of diverse 
conditions. As observed in the PA, seven of the eight study areas 
were also included in the 2014 HREA, and the eighth study area 
(Sacramento) was newly added in the current review to insure 
representation of a large city in the southwest (PA, section 3.4.1 
and Appendix 3D, section 3D.2.1). Clarification on this point in the 
final PA was responsive to the only CASAC comment on completeness of 
the description of study area selection (Cox, 2020a). We disagree 
with the implication by some commenters that each review's analyses 
must focus on the same areas. There is no such requirement under the 
Act, and such a view ignores the need to consider the current 
information in each review in planning appropriate analyses.
    \130\ For example, the exposure assessment for the 1997 
O3 NAAQS review included nine urban study areas, for 
which the combined population simulated was 41.7 million. The 
exposure assessment for the current review included eight urban 
study areas with a combined simulated population size of 
approximately 39 million (PA, p. 3D-96; U.S. EPA, 1996b, p. 76). We 
additionally note the focus on analysis results in terms of 
population percentages rather than population counts.
    \131\ A broad variety of spatial and temporal patterns of O3 
concentrations can exist when ambient air concentrations just meet 
the current standard. These patterns will vary due to many factors 
including the types, magnitude, and timing of emissions in a study 
area, as well as local factors, such as meteorology and topography.
---------------------------------------------------------------------------

    Contrary to one commenter's assertion that adults were not included 
in the exposure assessment, the populations assessed included two adult 
populations groups: All adults and

[[Page 87298]]

adults with asthma.\132\ The results for these groups and all of the 
populations assessed are presented in detail in the PA (PA, Appendix 
3D). As described in the PA and proposal, the estimates for adults as a 
percentage of the study populations were generally lower than those for 
children. Thus, we focused discussion on the estimates for children, 
including particularly children with asthma. As recognized by the 
Administrator in section II.B.3 below, his judgments on the adequacy of 
protection provided by the current standard takes into account 
protection provided to the U.S. population, including those population 
groups at increased risk, which includes children and people of all 
ages with asthma, among other groups.
---------------------------------------------------------------------------

    \132\ Further, contrary to the implication of one comment, the 
exposure/risk analyses did not exclude athletes, hikers and others 
who exercise outdoors, using their full lung capacity, a group the 
commenter characterizes as at increased risk. In fact, it is just 
such individuals who are most likely, depending on their locations, 
to experience exposures of concern due to their high exertion 
levels. As described in the PA, the comparison to benchmarks 
analysis identifies the portion of the exposed population whose 7-
hour average concentration, while at moderate or greater exertion, 
is at or above the benchmarks (PA, section 3.4 and Appendix 3D).
---------------------------------------------------------------------------

    Some commenters claimed that the EPA should have separately 
assessed exposure for certain additional population subgroups, such as 
children at summer camps, adults with lung impairments other than 
asthma or outdoor workers. As an initial matter, we recognize 
appreciably increased uncertainty regarding key aspects of the 
information necessary for such simulations for all of these groups. Of 
the three groups, only outdoor workers are identified as an at-risk 
population in this review (ISA, Table IS-10), and accordingly this 
group was explicitly considered in designing the exposure 
analyses.\133\ The information available, however, was considered to be 
too uncertain to produce estimates for this population, as a separate 
group, with confidence. As described in the PA, important uncertainties 
exist in generating the simulated activity patterns for this group, 
including the limited number of CHAD diary days available for outdoor 
workers, assignment of diaries to proper occupation categories, and in 
approximating number of days/week and hours/day outdoors, among other 
pertinent aspects (PA, Appendix 3D, Table 3D-64). We note that these 
appreciable data limitations and associated uncertainties were also 
recognized in the 2014 HREA in which a limited sensitivity analyses was 
conducted for this subgroup. Those limited analyses, conducted for a 
single area with air quality just meeting the prior 75 ppb standard, 
indicated that when diaries were selected to mimic exposures that could 
be experienced by outdoor workers, the percentages of modeled 
individuals estimated to experience exposures of concern were generally 
similar to the percentages estimated for children (i.e., using the full 
database of diary profiles) in the urban study areas and years with the 
largest exposure estimates (2014 HREA, section 5.4.3.2, Figure 5-
14).\134\ Accordingly, in this review, in recognition of the data 
limitations that remain in the current review,\135\ outdoor workers 
were not assessed as a separate population group, and in light of our 
consideration of conclusions from the sensitivity analyses in the last 
review, we have generally given primary focus to the estimates for the 
populations of children.
---------------------------------------------------------------------------

    \133\ With regard to the other two groups, we note the ISA 
explicitly evaluated evidence for people with the lung disease, 
COPD, and concluded the evidence was inadequate to determine whether 
this lung impairment confers increased risk of O3 related 
effects (ISA, Table IS-10). With regard to children at summer camp, 
we note that to the extent that the behaviors of such children 
(e.g., exercising outdoors) are represented in the CHAD, they are 
represented among the at-risk populations of children and children 
with asthma that were simulated in the exposure/risk analyses.
    \134\ Similarly, the EPA also did not conduct an exposure 
analysis for outdoor workers in the 2008 review and instead focused 
on children since it was judged that school aged children presented 
the greatest likelihood of being outdoors and exposed under moderate 
exertion averaged over the critical time period based on prior 
analysis findings. Thus, while as recognized in multiple reviews, 
outdoor workers are also at risk, the EPA has focused, in past 
reviews as in the current one, on children, the population group for 
which the analysis estimates in terms of percentage of population 
are greatest (PA, section 3.4.2). Accordingly, providing protection 
for this population group will provide protection for other at-risk 
populations as well.
    \135\ In support of their view that estimates should have been 
derived for outdoor workers, one group of commenters cites a study 
on research priorities for assessing climate change impacts on 
outdoor workers (Moda et al., 2019). We note, that other than being 
focused on outdoor workers and recognizing there to be significant 
research needed for impacts assessment, this paper has little 
relevance in this review. The paper is focused on climate change 
impacts in tropical developing countries with a focus on sub-Saharan 
Africa and does not discuss exposure modeling of outdoor workers or 
O3.
---------------------------------------------------------------------------

    In summary, we disagree with comments stating that the exposure/
risk analyses were deficient and do not provide support for their 
conclusions. As summarized above, in planning and conducting the 
exposure/risk analyses, we have appropriately considered issues raised 
by the commenters, such that the analyses reasonably reflect current 
understanding, information, tools and methodologies. Further, in 
presenting the analyses in the PA, we have recognized any associated 
limitations and uncertainties in an uncertainty characterization that 
utilized a largely qualitative approach adapted from the World Health 
Organization approach (and commonly utilized in NAAQS exposure/risk 
assessments, as discussed in the PA and proposal [85 FR 49857, August 
14, 2020]), accompanied by a number of quantitative sensitivity 
analyses. This characterization and accompanying analyses build on 
previously conducted work in the 2014 HREA and provide a transparent 
and explicit recognition of strengths, limitations and uncertainties of 
the current exposure/risk analysis that were described the PA, 
considered in the proposal and also in the Administrator's decision 
described in section II.B.3 below. Thus, the exposure/risk analyses 
conducted for this review appropriately and soundly reflect current 
information and methodologies; and we have interpreted their results 
appropriately in light of any associated limitations and uncertainties.
    With regard to other quantitative analyses identified by some 
commenters and described as showing health impacts that would be 
avoided by a more stringent standard (e.g. with a level of 65 ppb or 
lower), we note that these analyses utilize epidemiologic study effect 
estimates as concentration-response functions to generate predictions 
of the occurrence of health outcomes, primarily mortality, under 
different air quality conditions (characterized by the metric used in 
the epidemiologic study).\136\ As an initial matter, we note that our 
understanding of the relationship between O3 exposures and 
total mortality is different in this review than it was in the last 
review, based on the more extensive evidence base now available. As 
summarized in section II.A.2.a above, and noted earlier in this 
section, while our conclusion in the last review was that the 
relationship of O3 exposure with mortality was likely to be 
causal, the current evidence base does not support that conclusion 
because of limited evidence for cardiovascular mortality, which is by 
far the largest contributor to total mortality. Rather, the EPA has 
concluded the evidence in

[[Page 87299]]

this review to be suggestive of, but not sufficient to infer, causal 
relationships of total mortality with short- or long-term O3 
exposure, as summarized in section II.A.2.a above (ISA, Appendix 6). 
Thus, we do not find the weight the commenter is suggesting we place on 
predictions of total mortality from the epidemiologic study based risk 
analyses cited by commenters to appropriately reflect the current 
evidence base for O3 and mortality, or the evidence base for 
O3 and cardiovascular effects (the primary contributor to 
mortality in the U.S.).
---------------------------------------------------------------------------

    \136\ The analyses cited by these commenters include Cromar et 
al (2019) and OTC (2020). To address these comments, we have 
provisionally considered the documents, as discussed in I.D above, 
and found they do not materially change the broad scientific 
conclusions of the ISA with regard to respiratory effects, or 
warrant re-opening the air quality criteria for further review 
(Luben et al., 2020). Further, some of these commenters reference 
epidemiologic study based risk, analyses in the 2014 HREA.
---------------------------------------------------------------------------

    With regard to estimates of avoided respiratory mortality from the 
analyses cited by these commenters, we note that, while the 
epidemiologic studies that are inputs to the quantitative analyses 
cited by the commenters are part of the evidence base that supports our 
conclusion of a causal relationship between short-term O3 
exposure and respiratory effects, there are uncertainties inherent in 
the derivation of estimates of mortality ascribed to O3 
exposures using effect estimates from these studies. For example, in 
planning for analyses in this review, the IRP recognized several 
important uncertainties associated with aspects of the O3 
epidemiologic study-based approach used in the 2014 HREA (one of the 
analyses cited by commenters), and similar to the approach used in 
other analyses cited by commenters, that the EPA concluded to have a 
moderate or greater impact on risk estimates (IRP, Appendix 5A). Such 
uncertainties include complications posed by the presence of co-
occurring pollutants or pollutant mixtures, as well as those involving 
the correlation of population O3 exposures and ambient air 
monitor concentrations (including the use of area wide average 
O3 concentrations) and uncertainties in the derived 
concentration-response functions (IRP, Appendix 5A; PA, Appendix 34D, 
section 3D.1.4). Specifically with regard to the 2014 HREA estimates of 
respiratory mortality, the EPA has recognized uncertainty about the 
extent to which mortality estimates based on the long-term metric in 
Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily maximum 
concentrations) reflect associations with long-term average 
O3 versus repeated occurrences of elevated short-term 
concentrations; and given potential nonlinearity of the C-R function to 
reflect a threshold of the mortality response, these estimates should 
be viewed with caution (IRP, Appendix 5A). Accordingly, the 2014 HREA 
concluded that lower confidence should be placed in the results of the 
assessment of respiratory mortality risks associated with long-term 
O3, primarily because that analysis relies on just one study 
(Jerrett et al., 2009), and because of the uncertainty in that study 
about the existence and identification of a potential threshold in the 
concentration-response function (2014 HREA, section 9.6; 80 FR 65316, 
October 26, 2015). The other analysis cited by the commenters for 
predictions of respiratory mortality is also based its estimates on 
Jerrett et al. (2009). Thus, we find the conclusions regarding 
uncertainty and low confidence recognized for the 2014 HREA estimates 
to also apply to the other analysis by commenters and disagree with the 
conclusions reached by these commenters on this analysis. Further, we 
do not find that the 2014 HREA or other analyses cited by the 
commenters, in combination with the full body of evidence currently 
available, support a conclusion of significant health outcomes for 
O3 air quality that meets the current standard.
    Long-term Standard Consideration: In support of their view that EPA 
should establish an additional primary standard that targets long-term 
exposure, some commenters stated that recent epidemiologic studies 
indicate causal linkages between long-term exposures and adverse health 
outcomes, while also suggesting there was support for such a standard 
in a statement made by the CASAC in reviewing the draft PA. With regard 
to the epidemiologic studies, these commenters cite several studies 
published after the literature cut-off date for the ISA \137\ that they 
describe as showing linkages of long-term O3 exposure to a 
number of outcomes, including mortality, smokers progression to COPD, 
hospital admissions for acute respiratory disease syndrome and 
emergency department visits (Dominici et al., 2019; Seltzer et al., 
2020; Limaye and Knowlton, 2020; Dedoussi et al., 2020; Lim et al., 
2019; Paulin et al,. 2020; \138\ Rhee et al., 2019; Strosnider et al., 
2019).\139\ We have provisionally considered these ``new'' studies in 
addressing these comments consistent with section I.D above (Luben et 
al., 2020). Of the studies focused on mortality that these commenters 
cite as providing new information in support of a long-term standard, 
just three represent new evidence related to investigation of 
associations of long-term O3 exposure with mortality (Lim et 
al., 2019) or respiratory morbidity (Paulin et al., 2020 and Rhee et 
al., 2019).\140\ The study by Lim et al. (2019) analyzes associations 
between long-term O3 exposure and respiratory mortality in a 
U.S. population of older adults in the U.S., reporting a positive 
association with an effect estimate lower than a previously published 
study included in the ISA. These results contribute to the evidence 
base for respiratory effects, e.g., with an additional high-quality 
study of a previously studied population group (Lim et al., 2019) or 
with studies investigating additional populations and respiratory 
outcomes (Paulin et al., 2020; Rhee et al., 2019), albeit with 
limitations,\141\ without reducing uncertainties in the evidence base 
as a whole. These studies are

[[Page 87300]]

generally consistent with the evidence assessed in the ISA, and they do 
not materially change the broad conclusions in the ISA regarding the 
scientific evidence.
---------------------------------------------------------------------------

    \137\ These commenters also assert that some other studies 
published after the ISA cut-off date were arbitrarily included in 
the ISA, citing just a single study (Garcia et al., 2019). Contrary 
to implication by the commenters, such an occurrence is clearly 
described in the ISA, which states ``[s]tudies published after the 
literature cutoff date for this review were also considered if they 
were submitted in response to the Call for Information or identified 
in subsequent phases of ISA development . . ., particularly to the 
extent that they provide new information that affects key scientific 
conclusions'' (ISA, Appendix 10, p. 10-1).
    \138\ Although the commenters submitted a document that appears 
to be an unpublished draft of an earlier manuscript of this paper, 
to which they assigned a 2019 publication date and a very slightly 
different title (rather than the published paper, it is the 
published study, Paulin et al., (2020) that we have provisionally 
considered (Luben et al., 2020).
    \139\ Some commenters imply that projections of increasing 
O3 concentrations in response to climate change in the 
future will ``heighten'' long-term O3 concentrations and 
chronic exposures and indicate a need for a long-term standard. In 
making this claim, they cite an analysis of air quality projected in 
2045 through 2055 (Nassikas et al., 2020) and an evaluation of the 
effects of climate change on air quality including O3 
concentrations. (Archer et al., 2019). The former ``new'' study has 
been provisionally considered and found not to materially affect the 
broad scientific conclusions regarding the air quality criteria 
documented in the ISA or to warrant reopening the air quality 
criteria (Luben et al., 2020) As neither is evaluating health 
effects associated with air quality under the current standard, we 
do not find these studies informative to consideration of a need for 
a long-term standard to protect public health.
    \140\ Two others (Dedoussi et al 2020; Seltzer et al, 2020) are 
quantitative assessments that estimate O3 impacts based 
on use of effect estimates from previously published studies that 
are included in the ISA, another (Dominici et al., 2019) is the full 
technical report from the Health Effects Institute, the main results 
of which were previously published in studies that are included in 
the ISA, and a fourth (Limaye and Knowlton., 2020) is commentary on 
a previously published study that is included in the ISA. One other 
study cited by the commenters is focused on short-term O3 
exposures, not long-term O3 exposure as indicated by the 
commenters (Strosnider et al., 2019)
    \141\ While studies by Paulin et al. (2020) and Rhee et al. 
(2019) provide evidence for a novel population sub-group (smokers) 
or endpoint (e.g., acute respiratory distress syndrome, ARDS), each 
study has limitations. For example, the cross-sectional design of 
Paulin et al. (2020) is a major limitation, while limitations 
associated with Rhee et al. (2019) relate to linking long-term 
exposure with hospital admissions for ARDS based on exposure timing 
and the mechanism for acute vs. chronic development of disease, and 
to power in the study (e.g., very low hospital admission counts per 
year per ZIP code [Rhee et al., 2019, Table 2]).
---------------------------------------------------------------------------

    We additionally note that the O3 concentrations did not 
meet the current standard in all locations and time periods analyzed in 
these three multicity studies. Although two of these studies include 
some locations across the U.S. in which the current standard was likely 
met during some portions of the study period, air quality during other 
time periods of locations in the dataset did not meet the current 
standard.\142\ Further, the multicity effect estimates in these studies 
do not provide a basis for considering the extent to which 
O3 health effect associations are influenced by individual 
locations with ambient O3 concentrations low enough to meet 
the current O3 standards versus locations with O3 
concentrations that violate this standard. Thus, while these more 
recent studies may be consistent with the existing evidence base 
evaluated in the ISA, they do not provide a basis for conclusions 
regarding whether the O3 exposures occurring under air 
quality conditions allowed by the current standard may be eliciting the 
effects analyzed.
---------------------------------------------------------------------------

    \142\ The studies by Lim et al (2019) and Rhee et al (2019) 
include zip codes across the entire U.S., while Paulin et al (2020) 
includes the cities of Baltimore, Maryland, New York City, New York, 
Los Angeles and San Francisco, California, Ann Arbor, Michigan, Salt 
Lake City, Utah and Winston-Salem, North Carolina. The study time 
periods include ten or more years extending from the early 2000s to 
the late 2010s; a period within which the design values for most of 
those identified cities and many other U.S. metropolitan areas 
exceeded the level of the current standard (as seen by the design 
values presented for those areas during those time periods at 
https://www.epa.gov/air-trends/air-quality-design-values).
---------------------------------------------------------------------------

    We additionally note that while epidemiologic studies evaluate the 
relationship between health effects and specific O3 
concentrations during a defined study period and the generally 
consistent and coherent associations observed in these epidemiologic 
studies contribute to the causality determinations and conclusions 
regarding the causal nature of the effect of O3 exposure on 
health effects, ``they do not provide information about which averaging 
times or exposure metrics may be eliciting the health effects under 
study'' (ISA, section IS.6.1). As noted in the ISA, ``disentangling the 
effects of short-term ozone exposure from those of long-term ozone 
exposure (and vice-versa) is an inherent uncertainty in the evidence 
base,'' as ``the populations included in epidemiologic studies have 
long-term, variable, and uncharacterized exposures to ozone and other 
ambient pollutants'' (ISA, section IS.6.1). As summarized in the 
proposal, however, we have also considered the toxicological studies of 
effects associated with long-term exposures and note that they involve 
much higher exposures than those occurring at the current standard (85 
FR, 49853, August 14, 2020).
    Lastly, we disagree with the commenters' implication that the EPA 
has not addressed a CASAC recommendation. The commenters appear to be 
asserting that the CASAC recommended that EPA consider a long-term 
standard. However, the CASAC did not make such a recommendation (Cox, 
2020a). In making this assertion, the commenter cites a comment the 
CASAC makes with regard to a sentence in the draft PA that is drawn 
from the Administrator's conclusions section of the 2015 decision. 
Rather than ignoring this CASAC comment, as asserted by the commenters, 
we made a revision to that section of the PA (moving the statement from 
the draft PA to a footnote in the final PA with the objective of 
retaining an accurate description of a consideration related to that 
2015 decision, while lessening the potential for confusion of a 2015 
consideration with considerations in the current review).\143\ 
Notwithstanding sentences pertaining to the last review, we note the PA 
evaluates the information in the current review with regard to the 
protection offered by the current standard (and that the Administrator 
considered the PA evaluation, as well as the CASAC advice in his 
proposed decision [summarized in section II.B.1.c above] as in his 
final decision below). We further note that the description of the 
Administrator's conclusion in the last review, which is also summarized 
in the proposal (and in section II.A.1 above), does not describe health 
effects associated with long-term average concentrations likely under 
the current standard.
---------------------------------------------------------------------------

    \143\ In its comments regarding the 2015 statement, the CASAC 
and its consultants stated that controls that reduce peak 
O3 concentrations will not consistently reduce mean 
O3 concentrations. We don't disagree with this statement, 
and we note that we did not make a statement to the contrary in 
either the proposal or this final decision document.
---------------------------------------------------------------------------

    Further, in considering an implication of the commenters' claim 
that a ``long-term standard'' is needed in order to provide protection 
from health effects that may be elicited by long-term exposures to 
O3, we note that the impact of standards with short 
averaging times, such as eight hours, is not limited to reducing short-
term exposures. This is because a reduction in magnitude of short-term 
exposure concentrations (e.g., daily maximum 8-hour concentrations) is 
also a reduction in exposure to such concentrations over the longer 
term. For example, a standard, such as the current one, that limits 
daily maximum 8-hour concentrations to not exceed 70 ppb as a 3-year 
average of the annual fourth highest value, in addition to limiting the 
magnitude of concentrations to which a population is exposed in eight 
hour periods, also limits the frequency to which the population is 
exposed to such concentrations over the long term. That is, the 
reduction in frequency of the higher concentrations reduces exposures 
to such concentrations over the short and long term. Thus, given that, 
as indicated by the current and established evidence, the O3 
concentrations most likely to contribute to health effects are the 
higher concentrations, the current standard provides control of 
exposures to such concentrations over both the short and long term. In 
light of all of the considerations raised here, we disagree with the 
commenters assertion of the need for a long-term O3 
standard.
4. Administrator's Conclusions
    Having carefully considered the public comments, as discussed 
above, the Administrator believes that the fundamental scientific 
conclusions on effects of photochemical oxidants, including 
O3, in ambient air that were reached in the ISA and 
summarized in the PA, and estimates of potential O3 
exposures and risks described in the PA, and summarized above and in 
sections II.B and II.C of the proposal, remain valid. Additionally, the 
Administrator believes the judgments he proposed to reach in the 
proposal (section II.D.3) with regard to the evidence and the 
quantitative exposure/risk information remain appropriate. Thus, as 
described below, the Administrator concludes that the current primary 
O3 standard provides the requisite protection of the public 
health with an adequate margin of safety, including for at-risk 
populations, and should be retained. In considering the adequacy of the 
current primary O3 standard, the Administrator has carefully 
considered the assessment of the available health effects evidence and 
conclusions contained in the ISA; the evaluation of policy-relevant 
aspects of the evidence and quantitative analyses in the PA (summarized 
in sections II.A.2 and II.A.3 above); the advice and recommendations 
from the CASAC (summarized in section II.B.1.b above); and public 
comments (discussed in section II.B.2 above and in the separate RTC 
document).

[[Page 87301]]

    In the discussion below, the Administrator considers the key 
aspects of the evidence and exposure/risk estimates important to his 
judgment regarding the adequacy of protection provided by the current 
standard. First, the Administrator considers the evidence base on 
health effects associated with exposure to photochemical oxidants, 
including O3, in ambient air. He additionally considers the 
quantitative exposure and risk estimates developed in this review, 
including associated limitations and uncertainties, and what they 
indicate regarding the magnitude of risk, as well as degree of 
protection from adverse health effects, associated with the current 
standard. He additionally considers uncertainties in the evidence and 
the exposure/risk information, as a part of public health judgments 
that are essential and integral to his decision on the adequacy of 
protection provided by the standard. Such judgments include public 
health policy judgments and judgments about the implications of the 
uncertainties inherent in the scientific evidence and quantitative 
analyses. The Administrator draws on the PA considerations, and PA 
conclusions in the current review, taking note of key aspects of the 
rationale presented for those conclusions. Further, the Administrator 
considers the advice and conclusions of the CASAC, including 
particularly its overall agreement that the currently available 
evidence does not substantially differ from that which was available in 
the 2015 review when the current standard was established.
    As an initial matter, the Administrator recognizes the continued 
support in the current evidence for O3 as the indicator for 
photochemical oxidants (as recognized in section II.D.1 of the 
proposal). As recognized in the proposal, no newly available evidence 
has been identified in this review regarding the importance of 
photochemical oxidants other than O3 with regard to 
abundance in ambient air, and potential for health effects, and the 
``the primary literature evaluating the health and ecological effects 
of photochemical oxidants includes ozone almost exclusively as an 
indicator of photochemical oxidants'' (ISA, p. IS-3). Accordingly, the 
information relating health effects to photochemical oxidants in 
ambient air is also focused on O3. Thus, the Administrator 
concludes it is appropriate for O3 to continue to be the 
indicator for the primary standard for photochemical oxidants.
    With regard to the extensive evidence base for health effects of 
O3, the Administrator gives particular attention to the 
longstanding evidence of respiratory effects causally related to short-
term O3 exposures (summarized in section II.A.2.a above). He 
recognizes that the strongest and most certain evidence for this 
conclusion, as in the last review, is that from controlled human 
exposure studies that report an array of respiratory effects in study 
subjects (largely generally healthy adults) engaged in quasi-continuous 
or intermittent exercise. He additionally recognizes the supporting 
experimental animal and epidemiologic evidence. In so doing, he takes 
note of the epidemiologic evidence of positive associations for 
increased incidence of hospital admissions and emergency department 
visits for an array of respiratory outcomes, with the strongest such 
evidence being for asthma-related outcomes and specifically asthma-
related outcomes for children, with short-term O3 exposures. 
As a whole, this strong evidence base continues to demonstrate a causal 
relationship between short-term O3 exposures and respiratory 
effects, including in people with asthma. The Administrator also notes 
the ISA conclusion that the relationship between long-term exposures 
and respiratory effects is likely to be causal. These conclusions are 
also consistent with the conclusions in the last review and reflect a 
general similarity in the underlying evidence base for such effects.
    With regard to conclusions regarding the health effects evidence 
that differ from those in the last review, the Administrator recognizes 
the new conclusions regarding metabolic effects, cardiovascular effects 
and mortality (as summarized in section II.A.2.a above; ISA, Table ES-
1). As an initial matter, he takes note of the fact that while the 2013 
ISA considered the evidence available in the last review sufficient to 
conclude that the relationships for short-term O3 exposure 
with cardiovascular health effects and mortality were likely to be 
causal, that conclusion is no longer supported by the now more 
expansive evidence base which the current ISA determines to be 
suggestive of, but not sufficient to infer, a causal relationship for 
these health effect categories (ISA, Appendix 4, section 4.1.17; 
Appendix 6, section 6.1.8). Further, the Administrator recognizes the 
new ISA determination that the relationship between short-term 
O3 exposure and metabolic effects is likely to be causal 
(ISA, section IS.4.3.3). In so doing, he takes note that the basis for 
this conclusion is largely experimental animal studies in which the 
exposure concentrations were well above those in the controlled human 
exposure studies for respiratory effects as well as above those likely 
to occur in areas of the U.S. that meet the current standard (as 
summarized in section II.A.2.c above). Thus, while recognizing the 
ISA's conclusion regarding this potential hazard of O3, he 
also recognizes that the evidence base is largely focused on 
circumstances of elevated concentrations above those occurring in areas 
that meet the current standard. In light of these considerations, he 
judges the current standard to be protective of such circumstances 
leading him to continue to focus on respiratory effects in his 
evaluation of whether the current standard provides requisite 
protection.
    With regard to populations at increased risk of O3-
related health effects, the Administrator notes the populations and 
lifestages identified in the ISA and summarized in section II.A.2.b 
above. In so doing, he takes note of the longstanding and robust 
evidence that supports identification of people with asthma as being at 
increased risk of O3-related respiratory effects, including 
specifically asthma exacerbation and associated health outcomes, and 
also children, particularly due to their generally greater time 
outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections 
IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). This 
tendency of children to spend more time outdoors while at elevated 
exertion than other age groups, including in the summer when 
O3 levels may be higher, makes them more likely to be 
exposed to O3 in ambient air under conditions contributing 
to increased dose due to greater air volumes taken into the lungs. 
Based on these considerations, the Administrator concludes it is 
appropriate to give particular focus to people with asthma and 
children, population groups for which the evidence of increased risk is 
strongest, in evaluating whether the current standard provides 
requisite protection. He judges that such a focus will also provide 
protection of other potentially at-risk population groups, identified 
in the ISA, for which the current evidence is less robust and clear as 
to the extent and type of any increased risk, and the exposure 
circumstances that may contribute to it.
    With regard to exposures of interest for respiratory effects, the 
Administrator refers to the controlled human exposure studies of 6.6-
hour exposures, with

[[Page 87302]]

quasi-continuous exercise,\144\ to concentrations ranging from as low 
as approximately 40 ppb to 120 ppb (as considered in the PA, and 
summarized in sections II.A.2.c above). He also notes that, as in the 
last review, these studies, and particularly those that examine 
exposures from 60 to 80 ppb, are the primary focus of the PA 
consideration of exposure circumstances associated with O3 
health effects important to the Administrator's judgments regarding the 
adequacy of the current standard. The Administrator further recognizes 
that this information on exposure concentrations that have been found 
to elicit effects in exercising study subjects is unchanged from what 
was available in the last review.
---------------------------------------------------------------------------

    \144\ These studies employ a 6.6-hour protocol that includes six 
50-minute periods of exercise at moderate or greater exertion.
---------------------------------------------------------------------------

    With regard to the epidemiologic studies of respiratory effects, 
the Administrator recognizes that, as a whole, these investigations of 
associations between O3 and respiratory effects and health 
outcomes (e.g., asthma-related hospital admission and emergency 
department visits) provide strong support for the conclusions of 
causality (as summarized in section II.A.2.a above). He additionally 
takes note of the PA observation that these studies are generally 
focused on investigating the existence of relationships between 
O3 in ambient air and specific health outcomes and not on 
detailing the specific exposure circumstances eliciting such effects 
(PA, section 3.3.3). In so doing, he takes note of the PA conclusions 
in this regard, including the scarcity of U.S. studies \145\ conducted 
in locations in which and during time periods when the current standard 
would have been met (as summarized in sections II.A.2.c above).\146\ He 
also recognizes the additional considerations raised in the PA and 
summarized in section II.A.2.c above regarding information on exposure 
concentrations in these studies during times and locations that would 
not have met the current standard, including considerations such as 
complications in disentangling specific O3 exposures that 
may be eliciting effects (PA, section 3.3.3; ISA, p. IS-86 to IS-88). 
He takes note that such considerations do not lessen the importance of 
these studies in the evidence base documenting the causal relationship 
between O3 and respiratory effects. With regard to his 
consideration of exposure concentrations associated with O3 
air quality conditions that meet the current standard, based on 
information cited here and discussed in the PA and section II.B.2.b(ii) 
above, he judges these studies that are available in the current review 
to be less informative. Thus, the Administrator agrees with the PA 
conclusions in consideration of this evidence from controlled human 
exposure and epidemiologic studies (as assessed in the ISA and 
summarized in the PA), and in consideration of public comments in 
section II.B.2.b(ii) above, that the evidence base in this review does 
not include new evidence of respiratory effects associated with 
appreciably different exposure circumstances than the evidence 
available in the last review, including particularly any circumstances 
that would also be expected to be associated with air quality 
conditions likely to occur under the current standard. In light of 
these considerations, he finds it appropriate to focus on the studies 
of 6.6-hour exposures with quasi-continuous exercise, and particularly 
on study results for concentrations ranging from 60 to 80 ppb.
---------------------------------------------------------------------------

    \145\ Consistent with the evaluation of the epidemiologic 
evidence of associations between short-term O3 exposure 
and respiratory health effects in the ISA, we focus on those studies 
conducted in the U.S. and Canada, and most particularly in the U.S., 
to provide a focus on study populations and air quality 
characteristics that are most relevant to circumstances in the U.S. 
(PA, p. 3-45).
    \146\ Among the epidemiologic studies finding a statistically 
significant positive relationship of short- or long-term 
O3 concentrations with respiratory effects, there are no 
single-city studies conducted in the U.S. in locations with ambient 
air O3 concentrations that would have met the current 
standard for the entire duration of the study. Nor is there a U.S. 
multicity study for which all cities met the standard for the entire 
study period. The extent to which reported associations with health 
outcomes in the resident populations in these studies are influenced 
by the periods of higher concentrations during times that did not 
meet the current standard is unknown. These and additional 
considerations are summarized in section II.A.2.c above and in the 
PA.
---------------------------------------------------------------------------

    In considering the significance of responses documented in these 
studies and the full evidence base for his purposes in judging 
implications of the current information on public health protection 
provided by the current standard, notes that the responses reported 
from exposures ranging from 60 to 80 ppb are transient and reversible 
in the study subjects. In so doing, he also notes that these studies 
are in largely healthy adult subjects, that such data are lacking at 
these exposure levels for children and people with asthma, and that the 
evidence indicates that such responses, if repeated or sustained, 
particularly in people with asthma, pose risks of effects of greater 
concern, including asthma exacerbation, as cautioned by the CASAC.\147\ 
The Administrator also takes note of statements from the ATS 
(summarized in section II.A.2.b above), as well as judgments made by 
the EPA in considering similar effects (and ATS statements) in previous 
NAAQS reviews (80 FR 65343, October 26, 2015). With regard to the ATS 
statements, including the one newly available in this review (Thurston 
et al., 2017), the Administrator recognizes the role of such 
statements, as described by the ATS, as proposing principles or 
considerations for weighing the evidence rather than offering ``strict 
rules or numerical criteria'' (ATS, 2000, Thurston et al., 2017).
---------------------------------------------------------------------------

    \147\ The CASAC noted that ``[a]rguably the most important 
potential adverse effect of acute ozone exposure in a child with 
asthma is not whether it causes a transient decrement in lung 
function, but whether it causes an asthma exacerbation'' and that 
increases in airway inflammation also have the potential to increase 
the risk for an asthma exacerbation. The CASAC further cautioned 
with regard to repeated episodes of such responses, e.g., airway 
inflammation, indicating that they have the potential to contribute 
to irreversible reductions in lung function (Cox, 2020a, Consensus 
Responses to Charge Questions pp. 7-8).
---------------------------------------------------------------------------

    The more recent ATS statement is generally consistent with the 
prior statement (that was considered in the last O3 NAAQS 
review) and the attention that statement gives to at-risk or vulnerable 
population groups, while also broadening the discussion of effects, 
responses and biomarkers to reflect the expansion of scientific 
research in these areas. In this way, the most recent statement updates 
the prior statement, while retaining previously identified 
considerations, including, for example, its emphasis on consideration 
of vulnerable populations, thus expanding upon (e.g., with some 
increased specificity), while retaining core consistency with, the 
earlier ATS statement (Thurston et al., 2017; ATS, 2000). One example 
of this increased specificity that was raised in public comments and 
discussed in section II.B.2 above, is in the discussion of small 
changes in lung function (in terms of FEV1) in people with 
compromised function, such as people with asthma (Thurston et al., 
2017). In considering these statements, the Administrator notes that, 
in keeping with the intent of these statements to avoid specific 
criteria, the statements, in discussing what constitutes an adverse 
health effect, do not comprehensively describe all the biological 
responses raised, e.g., with regard to magnitude, duration or frequency 
of small pollutant-related changes in lung function. In so doing, he 
also recognizes the limitations in the current evidence base with 
regard to our

[[Page 87303]]

understanding of these aspects of such changes that may be associated 
with exposure concentrations of interest. Notwithstanding these 
limitations and associated uncertainties, he takes note of the emphasis 
of the ATS statement on consideration of individuals with pre-existing 
compromised function, such as that resulting from asthma (an emphasis 
which is reiterated and strengthened in the current statement), and 
agrees that these are important considerations in his judgment on the 
adequacy of protection provided by the current standard for at-risk 
populations, as recognized below.
    The Administrator recognizes some uncertainty, reflecting 
limitations in the evidence base, with regard to the exposure levels 
eliciting effects (as well as the severity of the effects) in some 
population groups not included in the available controlled human 
exposure studies, such as children and individuals with asthma. In so 
doing, the Administrator recognizes that the controlled human exposure 
studies, primarily conducted in healthy adults, on which the depth of 
our understanding of O3-related health effects is based, in 
combination with the larger evidence base, informs our conceptual 
understanding of O3 responses in people with asthma and in 
children. Aspects of our understanding continue to be limited, however, 
including with regard to the risk of particular effects and associated 
severity for these less studied population groups that may be posed by 
7-hour exposures with exercise to concentrations as low as 60 ppb that 
are estimated in the exposure analyses. Collectively, these aspects of 
the evidence and associated uncertainties contribute to the 
Administrator's recognition that for O3, as for other 
pollutants, the available evidence base in a NAAQS review 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 in the evidence, as well as those 
associated with the exposure and risk analyses, the Administrator notes 
that, as is the case in NAAQS reviews in general, his decision 
regarding the primary O3 standard in this review depends on 
a variety of factors, including his science policy judgments and public 
health policy judgments. These factors include judgments regarding 
aspects of the evidence and exposure/risk estimates, such as judgments 
concerning his interpretation of the different benchmark 
concentrations, in light of the available evidence and of associated 
uncertainties, as well as judgments on the public health significance 
of the effects that have been observed at the exposures evaluated in 
the health effects evidence. These judgments are rooted in his 
interpretation of the evidence, which reflects a continuum of health-
relevant exposures, with less confidence and greater uncertainty in the 
existence of adverse health effects as one considers lower 
O3 exposures. The factors relevant to judging the adequacy 
of the standards also include the interpretation of, and decisions as 
to the relative weight to place on, different aspects of the results of 
the exposure and risk assessment for the eight areas studied and the 
associated uncertainties. Together, factors described here inform the 
Administrator's judgment about the degree of protection that is 
requisite to protect public health with an adequate margin of safety, 
including the health of sensitive groups, and, accordingly, his 
conclusion that the current standard is requisite to protect public 
health with an adequate margin of safety.
    As in prior O3 NAAQS reviews, the Administrator 
considers the exposure estimates developed from modeling exposures to 
O3 in ambient air in this review to be critically important 
to consideration of the potential for exposures and risks of concern 
under air quality conditions of interest, and consequently important to 
his judgments on the adequacy of public health protection provided by 
the current standard. The exposure/risk analysis provides a framework 
within which to consider implications of the health effects evidence 
with regard to protection afforded by the current standard. In his 
consideration of the exposure/risk estimates, the Administrator places 
greater weight and gives primary attention to the comparison-to-
benchmarks analysis. This focus reflects his recognition of multiple 
factors, including the relatively greater uncertainty associated with 
the lung function risk estimates compared to the results of the 
comparison-to-benchmarks analysis. Additionally, he recognizes that, as 
noted in the PA, the comparison-to-benchmarks analysis provides for 
characterization of risk for the broad array of respiratory effects 
documented in the controlled human exposure studies. Accordingly, this 
analysis facilitates consideration of an array of respiratory effects, 
including but not limited to lung function decrements. Accordingly, the 
Administrator focuses primarily on the estimates of exposures at or 
above different benchmark concentrations that represent different 
levels of significance of O3-related effects, both with 
regard to the array of effects and severity of individual effects. In 
so doing, he notes that this assures his consideration of the 
protection provided by the standard from the array of respiratory 
effects documented in the currently available evidence base.
    In considering the public health implications of estimated 
occurrences of exposures (while at increased exertion) to the three 
benchmark concentrations (60, 70 and 80 ppb), the Administrator 
considers the respiratory effects reported in controlled human exposure 
studies of this range of concentrations (during quasi-continuous 
exercise). Accordingly, the controlled human exposure study evidence 
base, as a whole, provides context for consideration of the exposure/
risk estimates. The Administrator recognizes the three benchmarks to 
represent exposure conditions associated with different levels of 
respiratory response in the subjects studied and to inform his 
judgments on different levels of risk that might be posed to unstudied 
members of at-risk populations. The highest benchmark concentration (80 
ppb) represents an exposure where multiple controlled human exposure 
studies involving 6.6-hour exposures during quasi-continuous exercise 
demonstrate a range of O3-related respiratory effects 
including inflammation and airway responsiveness, as well as 
respiratory symptoms and lung function decrements in healthy adult 
subjects. Findings for this O3 exposure include: A 
statistically significant increase in multiple types of respiratory 
inflammation indicators in multiple studies; statistically 
significantly increased airway resistance and responsiveness; 
statistically significant FEV1 decrements; and statistically 
significant increases in respiratory symptoms (Table 1). In one 
variable exposure study for which this (80 ppb) was the exposure period 
average concentration, the study subject mean FEV1 decrement 
was nearly 8%, with individual decrements of 15% or greater (moderate 
or greater) in 16% of subjects and decrements of 10% or greater in 32% 
of subjects (Schelegle et al 2009); the percentages of individual 
subjects with decrements great than 10 or 15% were lower in other 
studies for this exposure. The second benchmark (70 ppb) represents an 
exposure level below the lowest exposures that have reported both 
statistically significant FEV1

[[Page 87304]]

decrements \148\ and increased respiratory symptoms (reported at 73 
ppb, Schelegle et al 2009) or statistically significant increases in 
airway resistance and responsiveness (reported at 80 ppb, Horstman et 
al., 1990). The lowest benchmark (60 ppb) represents still lower 
exposure, and a level for which findings from controlled human exposure 
studies of largely healthy subjects have included: Statistically 
significant decrements in lung function (with mean decrements ranging 
from 1.7% to 3.5% across the four studies with average exposures of 60 
to 63 ppb \149\), but not respiratory symptoms; and, a statistically 
significant increase in a biomarker of airway inflammatory response 
relative to filtered air exposures in one study (Kim et al, 2011).
---------------------------------------------------------------------------

    \148\ The study group mean lung function decrement for the 73 
ppb exposure was 6%, with individual decrements of 15% or greater 
(moderate or greater) in about 10% of subjects and decrements of 10% 
or greater in 19% of subjects. Decrements of 20% or greater were 
reported in 6.5% of subjects (Schelegle et al., 2009; PA, Table 3-2 
and Appendix 3D, Table 3D-20). In studies of 80 ppb exposure, the 
percent of study subjects with individual FEV1 decrements 
of this size ranged up to nearly double this (PA, Appendix 3D, Table 
3D-20).
    \149\ Among subjects in all four of these studies, individual 
FEV1 decrements of at least 15% were reported in 3% of 
subjects, with 7% of subjects reported to have decrements at or 
above a lower value of 10% (PA, Appendix 3D, Table 3D-20).
---------------------------------------------------------------------------

    In turning to the exposure/risk analysis results, the Administrator 
considers the evidence represented by these benchmarks noting that due 
to differences among individuals in responsiveness, not all people 
experiencing such exposures experience a response, such as a lung 
function decrement, as illustrated by the percentages cited above. 
Further, among those that experience a response, not all will 
experience an adverse effect. Accordingly, the Administrator notes that 
not all people estimated to experience an exposure of 7-hour duration 
while at elevated exertion above even the highest benchmark would be 
expected to experience an adverse effect, even members of at-risk 
populations. With these considerations in mind, he notes that while 
single occurrences could be adverse for some people, particularly for 
the higher benchmark concentration where the evidence base is stronger, 
the potential for adverse response increases with repeated occurrences 
(as cautioned by the CASAC).\150\ In so doing, he also notes that while 
the exposure/risk analyses provide estimates of exposures of the at-
risk population to concentrations of potential concern, they do not 
provide information on how many of such populations will have an 
adverse health outcome. Accordingly, in considering the exposure/risk 
analysis results, while giving due consideration to occurrences of one 
or more days with an exposure at or above a benchmark, particularly the 
higher benchmarks, he judges multiple occurrences to be of greater 
concern than single occurrences.
---------------------------------------------------------------------------

    \150\ For example, for people with asthma, the risk of an asthma 
exacerbation event may be expected to increase with repeated 
occurrences of lung function decrements of 10% or 15% as compared to 
a single occurrence.
---------------------------------------------------------------------------

    In this context, the Administrator considers the exposure risk 
estimates, focusing first on the results for the highest benchmark 
concentration (80 ppb), which represents an exposure well established 
to elicit an array of responses in sensitive individuals among study 
groups of largely healthy adult subjects, exposed while at elevated 
exertion. Similar to judgments of past Administrators, the current 
Administrator judges these effects in combination and severity to 
represent adverse effects for individuals in the population group 
studied, and to pose risk of adverse effects for individuals in at-risk 
populations, most particularly people with asthma, as noted above. 
Accordingly, he judges that the primary standard should provide 
protection from such exposures. In considering the exposure/risk 
estimates, he focuses on the results for children, and children with 
asthma, given the higher frequency of exposures of potential concern 
for children compared to adults, in terms of percent of the population 
groups.\151\ The exposure/risk estimates indicate more than 99.9% to 
100% of children and children with asthma, on average across the three 
years, to be protected from one or more occasions of exposure at or 
above this level; the estimate is 99.9% of children with asthma and of 
all children for the highest year and study area (Table 2). Further, no 
children in the simulated populations (zero percent) are estimated to 
be exposed more than once (two or more occasions) in the 3-year 
simulation to 7-hr concentrations, while at elevated exertion, at or 
above 80 ppb (Table 2). These estimates indicate strong protection 
against exposures of at-risk populations that have been demonstrated to 
elicit a wide array of respiratory responses in multiple studies.
---------------------------------------------------------------------------

    \151\ This finding relates to children's greater frequency and 
duration of outdoor activity, as well as their greater activity 
level while outdoors (PA, section 3.4.3).
---------------------------------------------------------------------------

    The Administrator next considers the results for the second 
benchmark concentration (70 ppb), which is just below the lowest 
exposure concentration (73 ppb) for which a study has reported a 
combination of a statistically significant increase in respiratory 
symptoms and statistically significant lung function decrements in 
sensitive individuals in a study group of largely healthy adult 
subjects, exposed while at elevated exertion (Schelegle et al., 2009). 
Recognizing the lack of evidence for people with asthma from studies at 
80 ppb and 73 ppb, as well as the emphasis in the ATS statement on the 
vulnerability of people with compromised respiratory function, such as 
people with asthma, the Administrator judges it appropriate that the 
standard protect against exposure, particularly multiple occurrences of 
exposure, to somewhat lower levels. In so doing, he notes that the 
exposure/risk estimates indicate more than 99% of children with asthma, 
and of all children, to be protected from one or more occasions in a 
year, on average, of 7-hour exposures to concentrations at or above 70 
ppb, while at elevated exertion (Table 2). The estimate is 99% of 
children with asthma for the highest year and study area (Table 2). 
Further, he notes that 99.9% of these groups are estimated to be 
protected from two or more such occasions, and 100% from still more 
occasions. These estimates also indicate strong protection of at-risk 
populations against exposures similar to those demonstrated to elicit 
lung function decrements and increased respiratory symptoms in healthy 
subjects, a response described as adverse by the ATS.
    In consideration of the exposure/risk results for the lowest 
benchmark (60 ppb), the Administrator notes that the lung function 
decrements in controlled human exposure studies of largely healthy 
adult subjects exposed while at elevated exertion to concentrations of 
60 ppb, although statistically significant, are much reduced from that 
observed in the next higher studied concentration (73 ppb), both at the 
mean and individual level, and are not reported to be associated with 
increased respiratory symptoms in healthy subjects.\152\ In light of 
these results and the transient nature of the responses, the 
Administrator does not judge these responses to represent adverse 
effects for generally healthy individuals. However, he further 
considers these findings specifically with regard to protection of at-
risk populations, such as people with asthma. In so doing, he notes 
that such data are lacking for at-risk groups, such as people with 
asthma, and considers the evidence and

[[Page 87305]]

comments from the CASAC regarding the need to consider endpoints of 
particular importance for this population group, such as risk of asthma 
exacerbation and prolonged inflammation. He takes note of comments from 
the CASAC (and also noted in the ATS statement) that small lung 
function decrements in this at-risk group may contribute to a risk of 
asthma exacerbation, an outcome described by the CASAC as ``arguably 
the most important potential adverse effect'' of O3 exposure 
for a child with asthma. Thus, he judges it important for the standard 
to provide protection that reduces such risks. However, he recognizes 
gaps in our ability to predict risk of such events at the low 
concentrations such as those represented by the lowest benchmark in the 
exposure/risk analysis. With regard to the inflammatory response he 
notes the evidence, discussed in section II.B.2 above, indicating the 
role of repeated occurrences of inflammation in contributing to 
severity of response. Thus, he finds repeated occurrences of exposure 
events of potential concern to pose greater risk than single events, 
leading him to place greater weight to exposure/risk estimates for 
multiple occurrences.
---------------------------------------------------------------------------

    \152\ The response for the 60 ppb studies is also somewhat lower 
than that for the 63 ppb study (Table 1; PA, Appendix 3D, Table 3D-
20).
---------------------------------------------------------------------------

    In light of the uncertainties associated with the lack of 
controlled human exposure data for people with asthma, particularly 
with regard to the extent to which the lower exposure concentrations 
studied in generally healthy adults might be expected to elicit 
asthmatic responses in this at-risk population, the Administrator notes 
that the CASAC also recognized this, describing the gap in clinical 
studies to be a ``key knowledge gap'' important to considerations of 
margin of safety for the standard. The Administrator further notes that 
the CAA 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. This approach is consistent with the requirements 
of the NAAQS provisions of the CAA 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 groups.\153\
---------------------------------------------------------------------------

    \153\ As noted in section I.A above, consideration of such 
protection is focused on the sensitive group of individuals and not 
a single person in the sensitive group (see S. Rep. No. 91-1196, 
91st Cong., 2d Sess. 10 [1970]).
---------------------------------------------------------------------------

    Thus, in this context, and given that the 70 ppb benchmark 
represents an exposure level somewhat below the lowest exposure 
concentration for which both statistically significant lung function 
decrements and increased respiratory symptoms have been reported in 
largely healthy adult subjects, the Administrator considers the 
exposure/risk estimates for the third benchmark of 60 ppb to be 
informative most particularly to his judgments on an adequate margin of 
safety. In that context, the Administrator turns to the third benchmark 
concentration (60 ppb). In so doing, he takes note that these estimates 
indicate more than 96% to more than 99% of children with asthma to be 
protected from more than one occasion in a year (two or more), on 
average, of 7-hour exposures to concentrations at or above this level, 
while at elevated exertion (Table 2). Additionally, the analysis 
estimates more than 90% of all children, on average across the three 
years, to be protected from one or more occasions of exposure at or 
above this level. The Administrator finds this to indicate an 
appropriate degree of protection from such exposures.
    The Administrator additionally takes note of the new finding in 
this review of evidence of a likely to be causal relationship between 
O3 and metabolic effects. In so doing, he notes the lack of 
evidence that would suggest such effects to be associated with 
exposures likely to occur with air quality conditions meeting the 
current standard, as discussed in section II.A.2.c above. Thus, he 
judges the current standard to provide protection from effects other 
than respiratory effects, for which the evidence is less certain. 
Accordingly, the Administrator concludes that the standard does not 
need to be revised to provide additional protection from such effects.
    In reflecting on all of the information currently available, the 
Administrator considers the extent to which the currently available 
information might indicate support for a less stringent standard. He 
recognizes the advice from the CASAC, which generally indicates support 
for retaining the current standard without revision or for revision to 
a more stringent level based on additional consideration of the margin 
of safety for at-risk populations. He notes that the CASAC advice did 
not convey support for a less stringent standard. He additionally 
considers the current exposure and risk estimates for the air quality 
scenario for a design value just above the level of the current 
standard (at 75 ppb), in comparison to the scenario for the current 
standard, as summarized in section II.A.3 above. In so doing, he finds 
the markedly increased estimates of exposures to the higher benchmarks 
under air quality for a higher standard level to be of concern and 
indicative of less than the requisite protection (Table 2). Thus, in 
light of the considerations raised here, including the need for an 
adequate margin of safety, the Administrator judges that a less 
stringent standard would not be appropriate.
    The Administrator additionally considers whether it would be 
appropriate to consider a more stringent standard that might be 
expected to result in reduced O3 exposures. As an initial 
matter, he considers the advice from the CASAC. With regard to the 
CASAC advice, while part of the Committee concluded the evidence 
supported retaining the current standard without revision, another part 
of the Committee reiterated advice from the prior CASAC, which while 
including the current standard level among the range of recommended 
standard levels, also provided policy advice to set the standard at a 
lower level. In considering this advice now in this review, as it was 
raised by part of the current CASAC, the Administrator notes the slight 
differences of the current exposure and risk estimates from the 2014 
HREA estimates for the lowest benchmark, which were those considered by 
the prior CASAC (Table 4). For example, while the 2014 HREA estimated 
3.3 to 10.2% of children, on average, to experience one or more days 
with an exposures at or above 60 ppb (and as many as 18.9% in a single 
year), the comparable estimates for the current analyses are lower, 
particularly at the upper end (3.2 to 8.2% and 10.6%). While the 
estimates for two or more days with occurrences at or above 60 ppb, on 
average across the assessment period, are more similar between the two 
assessments, the current estimate for the single highest year is much 
lower (9.2 versus 4.3%). The Administrator additionally recognizes the 
PA finding that the factors contributing to these differences, which 
includes the use of air quality data reflecting concentrations much 
closer to the now-current standard than was the case in the 2015 
review, also contribute to a reduced

[[Page 87306]]

uncertainty in the current estimates, as summarized in section II.A.3 
above (PA, sections 3.4 and 3.5). Thus, he notes that the current 
exposure analysis estimates indicate the current standard to provide 
appreciable protection against multiple days with a maximum exposure at 
or above 60 ppb. In the context of his consideration of the adequacy of 
protection provided by the standard and of the CAA requirement that the 
standard protect public health, including the health of at-risk 
populations, with an adequate margin of safety, the Administrator 
concludes, in light of all of the considerations raised here, that the 
current standard provides appropriate protection, and that a more 
stringent standard would be more than requisite to protect public 
health.
    In light of all of the above, including advice from the CASAC, the 
Administrator finds the current exposure and risk analysis results to 
describe appropriately strong protection of at-risk populations from 
exposures associated with O3-related health effects. 
Therefore, based on his consideration of the evidence and exposure/risk 
information, including that related to the lowest exposures studied in 
controlled human exposure studies, and the associated uncertainties, 
the Administrator judges that the current standard provides the 
requisite protection of public health, including an adequate margin of 
safety, and thus should be retained, without revision. Accordingly, he 
concludes that a more stringent standard is not needed to provide 
requisite protection and that the current standard provides the 
requisite protection of public health under the Act. With regard to key 
aspects of the specific elements of the standard, the Administrator 
recognizes the support in the current evidence base for O3 
as the indicator for photochemical oxidants. In so doing, he notes the 
ISA conclusion that O3 is the most abundant of the 
photochemical oxidants in the atmosphere and the one most clearly 
linked to human health effects. He additionally recognizes the control 
exerted by the 8-hour averaging time on associated exposures of 
importance for O3-related health effects. Lastly, with 
regard to form and level of the standard, the Administrator takes note 
of the exposure and risk results as discussed above and the level of 
protection that they indicate the elements of the current standard to 
provide. Beyond his recognition of this support in the available 
information for the elements of the current standard, the Administrator 
has considered the elements collectively in evaluating the health 
protection afforded by the current standard. For all of the reasons 
discussed above, the Administrator concludes that the current primary 
O3 standard (in all of its elements) is requisite to protect 
public health with an adequate margin of safety, including the health 
of at-risk populations, and thus should be retained, without revision.

C. Decision on the Primary Standard

    For the reasons discussed above and taking into account information 
and assessments presented in the ISA and PA, the advice from the CASAC, 
and consideration of public comments, the Administrator concludes that 
the current primary O3 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.

III. Rationale for Decision on the Secondary Standard

    This section presents the rationale for the Administrator's 
decision to retain the current secondary O3 standard. This 
rationale is based on the scientific information presented in the ISA, 
on welfare effects associated with photochemical oxidants including 
O3 and pertaining to the presence of these pollutants in 
ambient air. As summarized in section I.D above, the ISA was developed 
based on a thorough review of the latest scientific information 
generally published between January 2011 and March 2018, as well as 
more recent studies identified during peer review or by public comments 
on the draft ISA integrated with the information and conclusions from 
previous assessments (ISA, section IS.1.2 and Appendix 10, section 
10.2). The Administrator's rationale also takes into account: (1) The 
PA evaluation of the policy-relevant information in the ISA and 
presentation of quantitative analyses of air quality, exposure, and 
risk; (2) CASAC advice and recommendations, as reflected in discussions 
of drafts of the ISA and PA at public meetings, and in the CASAC's 
letters to the Administrator; (3) public comments on the proposed 
decision; and also (4) the August 2019 decision of the D.C. Circuit 
remanding the secondary standard established in the last review to the 
EPA for further justification or reconsideration. See Murray Energy 
Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019).
    Within this section, introductory and background information is 
presented in section III.A. Section III.A.1 summarizes the 2015 
establishment of the existing standard, as background for this review. 
Sections III.A.2 and III.A.3 provide overviews of the currently 
available welfare effects evidence and current air quality and 
environmental exposure information, respectively. Section III.B 
summarizes the basis for the proposed decision (III.B.1), including 
CASAC advice, discusses public comments on the proposed decision 
(III.B.2), and presents the Administrator's considerations, conclusions 
and decision in this review of the secondary standard (III.B.3). The 
decision is summarized in section III.C.

A. Introduction

    As in prior reviews, the general approach to reviewing the current 
secondary standard is based, most fundamentally, on using the Agency's 
assessments of the current scientific evidence and associated 
quantitative analyses to inform the Administrator's judgment regarding 
a secondary standard for photochemical oxidants that is 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 ISA and PA, both of which 
have received CASAC review and public comment (84 FR 50836, September 
26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019; 
85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging 
the gap between the scientific assessments of the ISA and the judgments 
required of the Administrator in his decisions on the current standard, 
the PA evaluates policy implications of the assessment of the current 
evidence in the ISA and the quantitative air quality, exposure and risk 
analyses and information documented in the PA. In evaluating the public 
welfare protection afforded by the current standard, 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 
standard is a public welfare policy judgment to be made by the 
Administrator. In reaching conclusions on the standard, the decision 
draws on the scientific information and analyses about welfare effects, 
environmental exposure and risks, 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

[[Page 87307]]

scientists generally 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 the 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.
    This decision on the secondary O3 standard also 
considers the August 2019 decision by the D.C. Circuit and issues 
raised by the court in its remand of the 2015 standard to the EPA such 
that the decision in this review incorporates the EPA's response to the 
court's remand. The opinion issued by the court concluded, in relevant 
part, that EPA had not provided a sufficient rationale for aspects of 
its 2015 decision on the secondary standard. See Murray Energy Corp. v. 
EPA, 936 F.3d 597 (D.C. Cir. 2019). Accordingly, the court remanded 
that standard to EPA for further justification or reconsideration, 
particularly in relation to its decision to focus on a 3-year average 
for consideration of the cumulative exposure for vegetation, in terms 
of W126, identified as providing requisite public welfare protection, 
and its decision to not identify a specific level of air quality 
related to visible foliar injury.\154\ Thus, in addition to considering 
the currently available welfare effects evidence and quantitative air 
quality, exposure and risk information, the decision described here, 
and the associated conclusions and judgments, also consider the court's 
remand. In consideration of the court remand, for example, certain 
analyses in this review are expanded compared with those conducted in 
the last review, issues raised in the remand have been discussed, and 
additional explanation of rationales for conclusions on these points is 
provided in this review.
---------------------------------------------------------------------------

    \154\ The EPA's decision not to use a seasonal W126 index as the 
form and averaging time of the secondary standard was also 
challenged in this case, but the court did not reach a decision on 
that issue, concluding that it lacked a basis to assess the EPA's 
rationale on this point because the EPA had not yet fully explained 
its focus on a 3-year average W126 in its consideration of the 
standard. See Murray Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. 
Cir. 2019).
---------------------------------------------------------------------------

1. Background on the Current Standard
    As a result of the last O3 review, completed in 2015, 
the level of the secondary standard was revised to 0.070 ppm, in 
conjunction with retaining the indicator, averaging time and form. This 
revision, establishing the current standard, was based on the 
scientific evidence and technical analyses available at that time, as 
well as the Administrator's judgments regarding the available welfare 
effects evidence, the appropriate degree of public welfare protection 
for the revised standard, and available air quality information on 
seasonal cumulative exposures that may be allowed by such a standard 
(80 FR 65292, October 26, 2015). In establishing this standard, the 
Administrator considered the extensive welfare effects evidence base 
compiled from more than fifty years of extensive research on the 
phytotoxic effects of O3, conducted both in and outside of 
the U.S., that documents the impacts of O3 on plants and 
their associated ecosystems (U.S. EPA, 1978, 1986, 1996, 2006, 2013). 
As was established in prior reviews, O3 can interfere with 
carbon gain (photosynthesis) and allocation of carbon within the plant, 
making fewer carbohydrates available for plant growth, reproduction, 
and/or yield (U.S. EPA, 1996b, pp. 5-28 and 5-29).\155\ The 2015 
decision drew upon: (1) The available scientific evidence assessed in 
the 2013 ISA; (2) assessments in the 2014 PA of the most policy-
relevant information in the 2013 ISA regarding evidence of adverse 
effects of O3 to vegetation and ecosystems, information on 
biologically-relevant exposure metrics, 2014 welfare REA (WREA) 
analyses of air quality, exposure, and ecological risks and associated 
ecosystem services, and staff analyses of relationships between levels 
of a W126-based exposure index \156\ and potential alternative standard 
levels in combination with the form and averaging time of the existing 
standard; (3) additional air quality analyses of the W126 index and 
design values based on the form and averaging time of the existing 
standard; (4) CASAC advice and recommendations; and (5) public comments 
received during the development of these documents and on the 2014 
proposal (80 FR 65292, October 26, 2015). In addition to reviewing the 
most recent scientific information as required by the CAA, the 2015 
rulemaking also incorporated the EPA's response to the judicial remand 
of the 2008 secondary O3 standard in Mississippi v. EPA, 744 
F.3d 1334 (D.C. Cir. 2013) and, in light of the court's decision in 
that case, explained the Administrator's conclusions as to the level of 
air quality judged to provide the requisite protection of public 
welfare from known or anticipated adverse effects.
---------------------------------------------------------------------------

    \155\ In addition to concluding there to be causal relationships 
between O3 and visible foliar injury, reduced vegetation 
growth, reduced productivity, reduced growth and yield of 
agricultural crops, and alteration of below-ground biogeochemical 
cycles, the 2013 ISA also concluded there likely to be a causal 
relationships between O3 and reduced carbon sequestration 
in terrestrial ecosystems, alteration of terrestrial ecosystem water 
cycling and alteration of terrestrial community composition (2013 
ISA, p. lxviii and Table 9-19). The 2013 ISA also found there to be 
a causal relationship between changes in tropospheric O3 
concentrations and radiative forcing, and likely to be a causal 
relationship between tropospheric O3 concentrations and 
effects on climate as quantified through surface temperature 
response (2013 ISA, section 10.5).
    \156\ The W126 index is a cumulative seasonal metric described 
as the sigmoidally weighted sum of all hourly O3 
concentrations during a specified daily and seasonal time window, 
with each hourly O3 concentration given a weight that 
increases from zero to one with increasing concentration (80 FR 
65373-74, October 26, 2015). The units for W126 index values are 
ppm-hours (ppm-hrs).
---------------------------------------------------------------------------

    Across the different types of studies, the strongest evidence for 
effects from O3 exposure on vegetation was from controlled 
exposure studies of many species of vegetation (2013 ISA, p. 1-15). 
Primary consideration in the decision was given to the studies of 
O3 exposures that reduced growth in tree seedlings from 
which E-R functions of seasonal relative biomass loss (RBL) have been 
established (80 FR 65385-86, 65389-90, October 26, 2015). The 
Administrator considered the effects of O3 on tree seedling 
growth, as suggested by the CASAC, as a surrogate or proxy for the 
broader array of vegetation-related effects of O3, ranging 
from effects on sensitive species to broader ecosystem-level effects 
(80 FR 65369, 65406, October 26, 2015). The metric used for quantifying 
effects on tree seedling growth in the review was RBL, with the 
evidence base providing robust and established E-R functions for 
seedlings of 11 tree species (80 FR 65391-92, October 26, 2015; 2014 
PA, Appendix 5C).\157\ The Administrator used this metric in her 
judgments on O3 effects on the public welfare. In this 
context, exposure was evaluated in terms of the W126 cumulative 
seasonal

[[Page 87308]]

exposure index, an index supported by the evidence in the 2013 ISA for 
this purpose and that was consistent with advice from the CASAC (2013 
ISA, section 9.5.3, p. 9-99; 80 FR 65375, October 26, 2015).
---------------------------------------------------------------------------

    \157\ These functions for RBL estimate the reduction in a year's 
growth as a percentage of that expected in the absence of 
O3 (2013 ISA, section 9.6.2; 2014 WREA, section 6.2).
---------------------------------------------------------------------------

    The 2015 decision was a public welfare policy judgment made by the 
Administrator, that drew upon the available scientific evidence for 
O3-attributable welfare effects and on quantitative analyses 
of exposures and public welfare risks, as well as judgments about the 
appropriate weight to place on the range of uncertainties inherent in 
the evidence and analyses. Included in this decision were judgments on 
the weight to place on the evidence of specific vegetation-related 
effects estimated to result across a range of cumulative seasonal 
concentration-weighted O3 exposures; on the weight to give 
associated uncertainties, including uncertainties of predicted 
environmental responses (based on experimental study data); variability 
in occurrence of the specific effects in areas of the U.S., especially 
in areas of particular public welfare significance; and on the extent 
to which such effects in such areas may be considered adverse to public 
welfare. For example, in considering the public welfare protection 
provided by the then-existing standard, the Administrator gave primary 
consideration to an analysis of cumulative seasonal exposures in or 
near Class I areas,\158\ which are lands that Congress set aside for 
specific uses intended to provide benefits to the public welfare, 
including lands that are to be protected so as to conserve the scenic 
value and the natural vegetation and wildlife within such areas, and to 
leave them unimpaired for the enjoyment of future generations.\159\ The 
decision additionally recognized that states, tribes and public 
interest groups also set aside areas that are intended to provide 
similar benefits to the public welfare for residents on those lands, as 
well as for visitors to those areas (80 FR 65390, October 26, 2015). In 
recognizing that her judgments regarding effects that are adverse to 
the public welfare consider the intended use of the natural resources 
and ecosystems affected, the Administrator utilized the RBL as a 
quantitative tool within a larger framework of considerations 
pertaining to the public welfare significance of O3 effects 
(80 FR 65389, October 26, 2015; 73 FR 16496, March 27, 2008).
---------------------------------------------------------------------------

    \158\ Areas designated as Class I include all international 
parks, national wilderness areas which exceed 5,000 acres in size, 
national memorial parks which exceed 5,000 acres in size, and 
national parks which exceed 6,000 acres in size, provided the park 
or wilderness area was in existence on August 7, 1977. Other areas 
may also be Class I if designated as Class I consistent with the 
CAA.
    \159\ This emphasis on such lands was consistent with a similar 
emphasis in the 2008 review of the standard (73 FR 16485, March 27, 
2008).
---------------------------------------------------------------------------

    In the Administrator's consideration of the adequacy of public 
welfare protection afforded by the existing standard, she gave 
particular attention to the air quality analysis for Class I areas that 
estimated cumulative exposures, in terms of 3-year average W126 index 
values, at and above 19 ppm-hrs, to have occurred under the standard in 
nearly a dozen areas distributed across two NOAA climatic regions of 
the U.S (80 FR 65385-86, October 26, 2015). The Administrator took note 
of these occurrences of exposures in Class I areas during periods when 
the existing standard was met, for which the associated estimates of 
growth effects across the species with E-R functions extend above a 
magnitude considered to be ``unacceptably high'' by the CASAC (80 FR 
65385-65386, 65389-65390, October 26, 2015).\160\ Based on this 
analysis and the considerations summarized above, including 
consideration of CASAC advice and public comment, the Administrator 
concluded that the protection afforded by the then-existing standard 
was not sufficient and that the standard needed to be revised to 
provide additional protection from known and anticipated adverse 
effects to public welfare, related to effects on sensitive vegetation 
and ecosystems, most particularly those occurring in Class I areas, and 
also in other areas set aside by states, tribes and public interest 
groups to provide similar benefits to the public welfare. In so doing, 
she further noted that a revised standard would provide increased 
protection for other growth-related effects, including relative yield 
loss (RYL) of crops, reduced carbon storage, and types of effects for 
which it is more difficult to determine public welfare significance, as 
well as other welfare effects of O3, such as visible foliar 
injury \161\ (80 FR 65390, October 26, 2015).
---------------------------------------------------------------------------

    \160\ The Administrator focused on the median RBL estimate 
across the eleven tree species for which robust established E-R 
functions were available and took note of the CASAC's consideration 
of RBL estimates presented in the 2014 draft PA, in which it 
characterized an estimate of 6% RBL in the median studied species as 
being ``unacceptably high,'' (Frey, 2014b).
    \161\ As described in the ISA, ``[t]ypical types of visible 
injury to broadleaf plants include stippling, flecking, surface 
bleaching, bifacial necrosis, pigmentation (e.g., bronzing), and 
chlorosis or premature senescence'' and ``[t]ypical visible injury 
symptoms for conifers include chlorotic banding, tip burn, flecking, 
chlorotic mottling, and premature senescence of needles'' (ISA, 
Appendix 8, p. 8-13).
---------------------------------------------------------------------------

    In light of the judicial remand of the 2008 secondary O3 
standard referenced above, the 2015 decision on selection of a revised 
secondary standard first considered the available evidence and 
quantitative analyses in the context of an approach for considering and 
identifying public welfare objectives for the revised standard (80 FR 
65403-65408, October 26, 2015). In light of the extensive evidence base 
of O3 effects on vegetation and associated terrestrial 
ecosystems, the Administrator focused on protection against adverse 
public welfare effects of O3-related effects on vegetation, 
giving particular attention to such effects in natural ecosystems, such 
as those in areas with protection designated by Congress, and areas 
similarly set aside by states, tribes and public interest groups, with 
the intention of providing benefits to the public welfare for current 
and future generations.\162\
---------------------------------------------------------------------------

    \162\ The Administrator additionally recognized that providing 
protection for this purpose will also provide a level of protection 
for other vegetation that is used by the public and potentially 
affected by O3 including timber, produce grown for 
consumption, and horticultural plants used for landscaping (80 FR 
65403, October 26, 2015).
---------------------------------------------------------------------------

    In reaching a conclusion on the amount of public welfare protection 
from the presence of O3 in ambient air that is appropriate 
to be afforded by a revised secondary standard, the Administrator gave 
particular consideration to the following: (1) The nature and degree of 
effects of O3 on vegetation, including her judgments as to 
what constitutes an adverse effect to the public welfare; (2) the 
strengths and limitations of the available and relevant information; 
(3) comments from the public on the Administrator's proposed decision, 
including comments related to identification of a target level of 
protection; and (4) the CASAC's views regarding the strength of the 
evidence and its adequacy to inform judgments on public welfare 
protection. The Administrator recognized that such judgments should 
neither overstate nor understate the strengths and limitations of the 
evidence and information nor the appropriate inferences to be drawn as 
to risks to public welfare, and that the choice of the appropriate 
level of protection is a public welfare policy judgment entrusted to 
the Administrator under the CAA taking into account both the available 
evidence and the uncertainties (80 FR 65404-05, October 26, 2015).\163\
---------------------------------------------------------------------------

    \163\ The CAA does not require that a secondary standard be 
protective of all effects associated with a pollutant in the ambient 
air but rather those known or anticipated effects judged adverse to 
the public welfare (CAA section 109).

---------------------------------------------------------------------------

[[Page 87309]]

    With regard to the extensive evidence of welfare effects of 
O3, including visible foliar injury and crop RYL, the RBL 
information available for seedlings of a set of 11 tree species was 
judged to be more useful (particularly in a role as surrogate for the 
broader array of vegetation-related effects) in informing judgments 
regarding the nature and severity of effects associated with different 
air quality conditions and associated public welfare significance (80 
FR 65405-06, October 26, 2015). With regard to visible foliar injury, 
while the Administrator recognized the potential for this effect to 
affect the public welfare in the context of affecting value ascribed to 
natural forests, particularly those afforded special government 
protection, she also recognized limitations in the available 
information that might inform consideration of potential public welfare 
impacts related to this vegetation effect noting the significant 
challenges in judging the specific extent and severity at which such 
effects should be considered adverse to public welfare (80 FR 65407, 
October 26, 2015).\164\ Similarly, while O3-related growth 
effects on agricultural and commodity crops had been extensively 
studied and robust E-R functions developed for a number of species, the 
Administrator found this information less useful in informing her 
judgments regarding an appropriate level of public welfare protection 
(80 FR 65405, October 26, 2015).\165\ Thus, and in light of the 
extensive evidence base in this regard, the Administrator focused on 
the information related to trees and growth impacts in identifying the 
public welfare objectives for the revised secondary standard.
---------------------------------------------------------------------------

    \164\ These limitations included the lack of established E-R 
functions that would allow prediction of visible foliar injury 
severity and incidence under varying air quality and environmental 
conditions, a lack of consistent quantitative relationships linking 
visible foliar injury with other O3-induced vegetation 
effects, such as growth or related ecosystem effects, and a lack of 
established criteria or objectives relating reports of foliar injury 
with public welfare impacts (80 FR 65407, October 26, 2015).
    \165\ With respect to commercial production of commodities, the 
Administrator noted the difficulty in discerning the extent to which 
O3-related effects on commercially managed vegetation are 
adverse from a public welfare perspective, given that the extensive 
management of such vegetation (which, as the CASAC noted, may reduce 
yield variability) may also to some degree mitigate potential 
O3-related effects. Management practices are highly 
variable and are designed to achieve optimal yields, taking into 
consideration various environmental conditions. Further, changes in 
yield of commercial crops and commercial commodities, such as 
timber, may affect producers and consumers differently, complicating 
the assessment of overall public welfare effects still further (80 
FR 65405, October 26, 2015).
---------------------------------------------------------------------------

    Accordingly, consideration of the appropriate public welfare 
protection objective for a revised standard focused on the estimates of 
tree seedling growth impacts (in terms of RBL) for a range of W126 
index values, developed from the E-R functions for 11 tree species (80 
FR 65391-92, Table 4, October 26, 2015). The Administrator also 
incorporated into her considerations the broader evidence base 
associated with forest tree seedling biomass loss, including other less 
quantifiable effects of potentially greater public welfare 
significance. That is, in drawing on these RBL estimates, the 
Administrator was not simply making judgments about a specific 
magnitude of growth effect in seedlings that would be acceptable or 
unacceptable in the natural environment. Rather, though mindful of 
associated uncertainties, the Administrator used the RBL estimates as a 
surrogate or proxy for consideration of the broader array of related 
vegetation-related effects of potential public welfare significance, 
which included effects on individual species and extending to 
ecosystem-level effects (80 FR 65406, October 26, 2015). This broader 
array of vegetation-related effects included those for which public 
welfare implications are more significant but for which the tools for 
quantitative estimates were more uncertain.
    In using the RBL estimates as a proxy, the Administrator focused 
her attention on a revised standard that would generally limit 
cumulative exposures to those for which the median RBL estimate for 
seedlings of the 11 species with robust and established E-R functions 
would be somewhat below 6% (80 FR 65406-07, October 26, 2015). In so 
doing, she noted that the median RBL estimate was 6% for a cumulative 
seasonal W126 exposure index of 19 ppm-hrs (80 FR 65391-92, Table 4, 
October 26, 2015).\166\ Given the information on median RBL at 
different W126 exposure levels, using a 3-year cumulative exposure 
index for assessing vegetation effects,\167\ the potential for single-
season effects of concern, and CASAC comments on the appropriateness of 
a lower value for a 3-year average W126 index, the Administrator 
concluded it was appropriate to identify a standard that would restrict 
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances (80 FR 65407, October 26, 
2015). Based on such information, available at that time, to inform 
consideration of vegetation effects and their potential adversity to 
public welfare, the Administrator additionally judged that the RBL 
estimates associated with marginally higher exposures in isolated, rare 
instances were not indicative of effects that would be adverse to the 
public welfare, particularly in light of variability in the array of 
environmental factors that can influence O3 effects in 
different systems and uncertainties associated with estimates of 
effects associated with this magnitude of cumulative exposure in the 
natural environment (80 FR 65407, October 26, 2015).
---------------------------------------------------------------------------

    \166\ When stated to the first decimal place, the median RBL was 
6.0% for a cumulative seasonal W126 exposure index of 19 ppm-hrs. 
For 18 ppm-hrs, the median RBL estimate was 5.7%, which rounds to 
6%, and for 17 ppm-hrs, the median RBL estimate was 5.3%, which 
rounds to 5% (80 FR 65407, October 26, 2015).
    \167\ Based on a number of considerations, the Administrator 
recognized greater confidence in judgments related to public welfare 
impacts based on a 3-year average metric than a single-year metric, 
and consequently concluded it to be appropriate to use a seasonal 
W126 index averaged across three years for judging public welfare 
protection afforded by a revised secondary standard. For example, 
she recognized uncertainties associated with interpretation of the 
public welfare significance of effects resulting from a single-year 
exposure, and that the public welfare significance of effects 
associated with multiple years of critical exposures are potentially 
greater than those associated with a single year of such exposure. 
She additionally concluded that use of a 3-year average metric could 
address the potential for adverse effects to public welfare that may 
relate to shorter exposure periods, including a single year (80 FR 
65404, October 26, 2015).
---------------------------------------------------------------------------

    Using these objectives, the Administrator's decision regarding a 
revised standard was based on extensive air quality analyses that 
included the most recently available data (monitoring year 2013) and 
extended back more than a decade (80 FR 65408, October 26, 2015; Wells, 
2015). These analyses evaluated the cumulative seasonal exposure levels 
in locations meeting different alternative levels for a standard of the 
existing form and averaging time. Based on these analyses, the 
Administrator judged that the desired level of public welfare 
protection, considered in terms of cumulative exposure (quantified as 
the W126 index), could be achieved by a standard with a revised level 
in combination with the existing form and averaging time (80 FR 65408, 
October 26, 2015).
    In the most recent period of air quality data (2011-2013), across 
the more than 800 monitor locations meeting the existing standard (with 
its level of 75 ppb), the 3-year average W126 index values were above 
17 ppm-hrs in 25 sites distributed across different NOAA climatic 
regions, and

[[Page 87310]]

above 19 ppm-hrs at nearly half of these sites, with some well above 
(Wells, 2015). In comparison, among the more than 500 sites meeting an 
alternative standard of 70 ppb across 46 of the 50 states, there were 
no occurrences of a W126 value above 17 ppm-hrs and fewer than five 
occurrences that equaled 17 ppm-hrs (Wells, 2015 and associated dataset 
[document identifier, EPA-HQ-OAR-2008-0699-4325]). For the full air 
quality dataset (extending back to 2001), among the nearly 4000 
instances where a monitoring site met a standard level of 70 ppb, the 
Administrator noted that there was only ``a handful of isolated 
occurrences'' of 3-year W126 index values above 17 ppm-hrs, ``all but 
one of which were below 19 ppm-hrs'' (80 FR 65409, October 26, 2015). 
The Administrator concluded that that single value of 19.1 ppm-hrs 
(just equaling 19, when rounded), observed at a monitor for the 3-year 
period of 2006-2008, was reasonably regarded as an extremely rare and 
isolated occurrence, and, as such, it was unclear whether it would 
recur, particularly as areas across the U.S. took further steps to 
reduce O3 to meet revised primary and secondary standards. 
Further, based on all of the then available information, as noted 
above, the Administrator did not judge RBL estimates associated with 
marginally higher exposures in isolated, rare instances to be 
indicative of adverse effects to the public welfare. The Administrator 
concluded that a standard with a level of 70 ppb and the existing form 
and averaging time would be expected to limit cumulative exposures, in 
terms of a 3-year average W126 exposure index, to values at or below 17 
ppm-hrs, in nearly all instances, and accordingly, to eliminate or 
virtually eliminate cumulative exposures associated with a median RBL 
of 6% or greater (80 FR 65409, October 26, 2015). Thus, using RBL as a 
proxy in judging effects to public welfare, the Administrator judged 
that such a standard with a level of 70 ppb would provide the requisite 
protection from adverse effects to public welfare by limiting 
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances, and decided to revise the 
standard level to 70 ppb.
    In summary, the Administrator judged that the revised standard 
would protect natural forests in Class I and other similarly protected 
areas against an array of adverse vegetation effects, most notably 
including those related to effects on growth and productivity in 
sensitive tree species. The Administrator additionally judged that the 
revised standard would be sufficient to protect public welfare from 
known or anticipated adverse effects. This judgment by the 
Administrator appropriately recognized that the CAA 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. Thus, based on the conclusions 
drawn from the air quality analyses which demonstrated a strong, 
positive relationship between the 8-hour and W126 metrics and the 
findings that indicated the significant amount of control provided by 
the fourth-high metric, the evidence base of O3 effects on 
vegetation and her public welfare policy judgments, as well as public 
comments and CASAC advice, the Administrator decided to retain the 
existing form and averaging time and revise the level to 0.070 ppm, 
judging that such a standard would provide the requisite protection to 
the public welfare from any known or anticipated adverse effects 
associated with the presence of O3 in ambient air (80 FR 
65409-10, October 26, 2015).
2. Overview of Welfare Effects Information
    The information summarized here is an overview of the scientific 
assessment of the welfare effects evidence available in this review; 
this assessment is documented in the ISA and its policy implications 
are further discussed in the PA. As in past reviews, the welfare 
effects evidence evaluated in the ISA for O3 and related 
photochemical oxidants is focused on O3 (ISA, p. IS-3). 
Ozone is the most prevalent photochemical oxidant present in the 
atmosphere and the one for which there is a very large, well-
established evidence base of its health and welfare effects (ISA, p. 
IS-3). Thus, the current welfare effects evidence and the Agency's 
review of the evidence, including the evidence newly available in this 
review,\168\ continues to focus on O3. The subsections below 
briefly summarize the following aspects of the evidence: the nature of 
O3-related welfare effects, the potential public welfare 
implications, and exposure concentrations associated with effects.
---------------------------------------------------------------------------

    \168\ More than 1600 studies are newly available and considered 
in the ISA, including nearly 600 studies on welfare effects (ISA, 
Appendix 10, Figure 10-2).
---------------------------------------------------------------------------

a. Nature of Effects
    The welfare effects evidence base available in the current review 
includes more than sixty years of extensive research on the phytotoxic 
effects of O3 and subsequent effects on associated 
ecosystems (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, 2020 
ISA). As described in past reviews, O3 can interfere with 
carbon gain (photosynthesis) and allocation of carbon within the plant, 
making fewer carbohydrates available for plant growth, reproduction, 
and/or yield (2013 ISA, p. 1-10; 1996 AQCD, pp. 5-28 and 5-29). As 
described in the 2013 ISA, the strongest evidence for effects from 
O3 exposure on vegetation is from controlled exposure 
studies, which ``have clearly shown that exposure to O3 is 
causally linked to visible foliar injury, decreased photosynthesis, 
changes in reproduction, and decreased growth'' in many species of 
vegetation (2013 ISA, p. 1-15). Such effects at the plant scale can 
also be linked to an array of effects at larger spatial scales (and 
higher levels of biological organization), with the evidence available 
in the last review indicating that ``O3 exposures can affect 
ecosystem productivity, crop yield, water cycling, and ecosystem 
community composition'' (2013 ISA, p. 1-15, Chapter 9, section 9.4). 
Beyond its effects on plants, the 2013 ISA also recognized 
O3 in the troposphere as a major greenhouse gas (ranking 
behind carbon dioxide and methane in importance), with associated 
radiative forcing and effects on climate, and recognized the 
accompanying ``large uncertainties in the magnitude of the radiative 
forcing estimate . . . making the impact of tropospheric O3 
on climate more uncertain than the effect of the longer-lived 
greenhouse gases'' (2013 ISA, sections 10.3.4 and 10.5.1 [p. 10-30]).
    The evidence newly available in this review supports, sharpens and 
expands somewhat on the conclusions reached in the last review (ISA, 
Appendices 8 and 9). Consistent with the evidence in the last review, 
the currently available evidence describes an array of O3 
effects on vegetation and related ecosystem effects, as well as the 
role of O3 in radiative forcing and subsequent climate-
related effects. The ISA concludes there to be causal relationships 
between O3 and visible foliar injury,\169\ reduced 
vegetation growth and reduced plant reproduction,\170\ as well as 
reduced

[[Page 87311]]

yield and quality of agricultural crops, reduced productivity in 
terrestrial ecosystems, alteration of terrestrial community 
composition,\171\ and alteration of belowground biogeochemical cycles 
(ISA, section IS.5). The current ISA also concludes there likely to be 
a causal relationship between O3 and alteration of ecosystem 
water cycling, reduced carbon sequestration in terrestrial ecosystems, 
and with increased tree mortality (ISA, section IS.5). Additionally, 
evidence newly available in this review augments more limited 
previously available evidence related to insect interactions with 
vegetation, contributing to the ISA conclusion that the evidence is 
sufficient to infer that there are likely to be causal relationships 
between O3 exposure and alteration of plant-insect signaling 
(ISA, Appendix 8, section 8.7) and of insect herbivore growth and 
reproduction (ISA, Appendix 8, section 8.6). Thus, conclusions reached 
in the last review continue to be supported by the current evidence 
base and conclusions are also reached in a few new areas based on the 
now expanded evidence.
---------------------------------------------------------------------------

    \169\ Evidence continues to indicate that ``visible foliar 
injury usually occurs when sensitive plants are exposed to elevated 
ozone concentrations in a predisposing environment,'' with a major 
factor for such an environment being the amount of soil moisture 
available to the plant (ISA, Appendix 8, p. 8-23; 2013 ISA, section 
9.4.2).
    \170\ The 2013 ISA did not include a separate causality 
determination for reduced plant reproduction. Rather, it was 
included with the conclusion of a causal relationship with reduced 
vegetation growth (ISA, Table IS-12).
    \171\ The 2013 ISA had concluded alteration of terrestrial 
community composition to be likely causally related to O3 
based on the then available information (ISA, Table IS-12).
---------------------------------------------------------------------------

    As in the last review, the strongest evidence and the associated 
findings of causal or likely causal relationships with O3 in 
ambient air, and the quantitative characterizations of relationships 
between O3 exposure and occurrence and magnitude of effects 
are for vegetation effects. Visible foliar injury has long been used as 
a bioindicator of O3 exposure, although it is not always a 
reliable indicator of other negative effects on vegetation (ISA, 
sections IS.5.1.2 and 8.2). Effects of O3 on physiology of 
individual plants at the cellular level, such as through photosynthesis 
and carbon allocation, can impact plant growth and reproduction (ISA, 
section IS.5.1.2). The scales of these effects range from the 
individual plant scale to the ecosystem scale, with potential for 
impacts on the public welfare (as discussed in section III.A.2.b 
below). The effects of O3 on plants and plant populations 
have implications for ecosystems. Effects at the ecosystem scale 
include reduced terrestrial productivity and carbon storage, and 
altered terrestrial community composition, as well as impacts on 
ecosystem functions, such as belowground biogeochemical cycles and 
ecosystem water cycling (ISA, Appendix 8, sections 8.11 and 8.9).
    Ozone welfare effects also extend beyond effects on vegetation and 
associated biota due to it being a major greenhouse gas and radiative 
forcing agent.\172\ The current evidence, augmented since the 2013 ISA, 
continues to support a causal relationship between the global abundance 
of O3 in the troposphere and radiative forcing, and a likely 
causal relationship between the global abundance of O3 in 
the troposphere and effects on temperature, precipitation, and related 
climate variables \173\ (ISA, section IS.5.2 and Appendix 9; Myhre et 
al., 2013). Uncertainty in the magnitude of radiative forcing estimated 
to be attributed to tropospheric O3 contributes to the 
relatively greater uncertainty associated with climate effects of 
tropospheric O3 compared to such effects of the well mixed 
greenhouse gases, such as carbon dioxide and methane (ISA, section 
IS.6.2.2).
---------------------------------------------------------------------------

    \172\ Radiative forcing is a metric used to quantify the change 
in balance between radiation coming into and going out of the 
atmosphere caused by the presence of a particular substance (ISA, 
Appendix 9, section 9.1.3.3).
    \173\ Effects on temperature, precipitation, and related climate 
variables were referred to as ``climate change'' or ``effects on 
climate'' in the 2013 ISA (ISA, p. IS-82; 2013 ISA, pp. 1-14 and 10-
31).
---------------------------------------------------------------------------

    Lastly, the evidence regarding tropospheric O3 and UV-B 
shielding (shielding of ultraviolet radiation at wavelengths of 280 to 
320 nanometers) was evaluated in the 2013 ISA and determined to be 
inadequate to draw a causal conclusion (2013 ISA, section 10.5.2). The 
current ISA concludes there to be no new evidence since the 2013 ISA 
relevant to the question of UV-B shielding by tropospheric 
O3 (ISA, IS.1.2.1 and Appendix 9, section 9.1.3.4).
b. Public Welfare Implications
    The secondary standard is to ``specify a level of air quality the 
attainment and maintenance of which in the judgment of the 
Administrator . . . is requisite to protect the public welfare from any 
known or anticipated adverse effects associated with the presence of 
such air pollutant in the ambient air'' (CAA, section 109(b)(2)). As 
recognized in prior reviews of secondary standards, the secondary 
standard is not meant to protect against all known or anticipated 
O3-related welfare effects, but rather those that are judged 
to be adverse to the public welfare, and a bright line determination of 
adversity is not required in judging what is requisite (78 FR 3212, 
January 15, 2013; 80 FR 65376, October 26, 2015; see also 73 FR 16496, 
March 27, 2008). The significance of each type of welfare effect with 
regard to potential effects on the public welfare depends on the type 
and severity of effects, as well as the extent of such effects on the 
affected environmental entity, and on the societal use of the affected 
entity and the entity's significance to the public welfare. Such 
factors have been considered in the context of judgments and 
conclusions made in some prior reviews regarding public welfare 
effects. For example, judgments regarding public welfare significance 
in the last two O3 NAAQS decisions gave particular attention 
to O3 effects in areas with special federal protections 
(such as Class I areas), and lands set aside by states, tribes and 
public interest groups to provide similar benefits to the public 
welfare (73 FR 16496, March 27, 2008; 80 FR 65292, October 26, 
2015).\174\ In the 2015 review, the EPA recognized the ``clear public 
interest in and value of maintaining these areas in a condition that 
does not impair their intended use and the fact that many of these 
lands contain O3-sensitive species'' (73 FR 16496, March 27, 
2008).
---------------------------------------------------------------------------

    \174\ For example, the fundamental purpose of parks in the 
National Park System ``is to conserve the scenery, natural and 
historic objects, and wild life in the System units and to provide 
for the enjoyment of the scenery, natural and historic objects, and 
wild life in such manner and by such means as will leave them 
unimpaired for the enjoyment of future generations'' (54 U.S.C. 
100101). Additionally, the Wilderness Act of 1964 defines designated 
``wilderness areas'' in part as areas ``protected and managed so as 
to preserve [their] natural conditions'' and requires that these 
areas ``shall be administered for the use and enjoyment of the 
American people in such manner as will leave them unimpaired for 
future use and enjoyment as wilderness, and so as to provide for the 
protection of these areas, [and] the preservation of their 
wilderness character . . .'' (16 U.S.C. 1131(a) and (c)). Other 
lands that benefit the public welfare include national forests which 
are managed for multiple uses including sustained yield management 
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3); 
16 U.S.C. 1601(d)(1)).
---------------------------------------------------------------------------

    Judgments regarding effects on the public welfare can depend on the 
intended use for, or service (and value) of, the affected vegetation, 
ecological receptors, ecosystems and resources and the importance of 
that use to the public welfare (73 FR 16496, March 27, 2008; 80 FR 
65377, October 26, 2015). Uses or services provided by areas that have 
been afforded special protection can flow in part or entirely from the 
vegetation that grows there. Ecosystem services range from those 
directly related to the natural functioning of the ecosystem to 
ecosystem uses for human recreation or profit, such as through the 
production of lumber or fuel (Costanza et al., 2017; ISA, section 
IS.5.1). Services of aesthetic value and outdoor recreation depend, at 
least in part, on

[[Page 87312]]

the perceived scenic beauty of the environment. Additionally, public 
surveys have indicated that Americans rank as very important the 
existence of resources, the option or availability of the resource and 
the ability to bequest or pass it on to future generations (Cordell et 
al., 2008).
    The different types of O3 effects on vegetation 
recognized in section III.A.2.a above differ with regard to aspects 
important to judging their public welfare significance. For example, in 
the case of effects on crop yield, such judgments may consider aspects 
such as the heavy management of agriculture in the U.S., while 
judgments for other categories of effects may generally relate to 
considerations regarding natural areas, including specifically those 
areas that are not managed for harvest. In this context, it may be 
important to consider that O3 effects on tree growth and 
reproduction could, depending on severity, extent and other factors, 
lead to effects on a larger scale including reduced productivity, 
altered forest and forest community (plant, insect and microbe) 
composition, reduced carbon storage and altered ecosystem water cycling 
(ISA, section IS.5.1.8.1; 2013 ISA, Figure 9-1, sections 9.4.1.1 and 
9.4.1.2). For example, the composition of vegetation or of terrestrial 
community composition can be affected through O3 effects on 
growth and reproductive success of sensitive species in the community, 
with the extent of compositional changes dependent on factors such as 
competitive interactions (ISA, section IS.5.1.8.1; 2013 ISA, sections 
9.4.3 and 9.4.3.1). Impacts on some of these characteristics (e.g., 
forest or forest community composition) may be considered of greater 
public welfare significance when occurring in Class I or other 
protected areas, due to value for particular services that the public 
places on such areas.
    Agriculture and silviculture provide ecosystem services with clear 
public welfare benefits. With regard to agriculture-related effects of 
O3, however, there are complexities in this consideration 
related to areas and plant species that are heavily managed to obtain a 
particular output (such as commodity crops or commercial timber 
production). In light of this, the degree to which O3 
impacts on agriculturally important vegetation would impair the 
intended use at a level that might be judged adverse to the public 
welfare has been less clear (80 FR 65379, October 26, 2015; 73 FR 
16497, March 27, 2008). While having sufficient crop yields is of high 
public welfare value, important commodity crops are typically heavily 
managed to produce optimum yields. Moreover, based on the economic 
theory of supply and demand, increases in crop yields would be expected 
to result in lower prices for affected crops and their associated 
goods, which would primarily benefit consumers. Analyses in past 
reviews have described how these competing impacts on producers and 
consumers complicate consideration of these effects in terms of 
potential adversity to the public welfare (2014 WREA, sections 5.3.2 
and 5.7).
    Other ecosystem services valued by people that can be affected by 
reduced tree growth, productivity and associated forest effects include 
aesthetic value; provision of food, fiber, timber, other forest 
products, habitat, and recreational opportunities; climate and water 
regulation; erosion control; air pollution removal, and desired fire 
regimes (PA, Figure 4-2; ISA, section IS.5.1; 2013 ISA, sections 
9.4.1.1 and 9.4.1.2). In considering such services in past reviews, the 
Agency has given particular attention to effects in natural ecosystems, 
indicating that a protective standard, based on consideration of 
effects in natural ecosystems in areas afforded special protection, 
would also ``provide a level of protection for other vegetation that is 
used by the public and potentially affected by O3 including 
timber, produce grown for consumption and horticultural plants used for 
landscaping'' (80 FR 65403, October 26, 2015). For example, locations 
potentially vulnerable to O3-related impacts might include 
forested lands, both public and private, where trees are grown for 
timber production. Forests in urbanized areas also provide a number of 
services that are important to the public in those areas, such as air 
pollution removal, cooling, and beautification. There are also many 
other tree species, such as various ornamental and agricultural species 
(e.g., Christmas trees, fruit and nut trees), that provide ecosystem 
services that may be judged important to the public welfare.
    With its effect on the physical appearance of plants, visible 
foliar injury has the potential to be significant to the public 
welfare, depending on its severity and spatial extent, by impacting 
aesthetic or scenic values and outdoor recreation in Class I and other 
similarly protected areas valued by the public. To assess evidence of 
injury to plants in forested areas on national and regional scales, the 
U.S. Forest Service (USFS) conducted surveys of the occurrence and 
severity of visible foliar injury on sensitive (bioindicator) species 
at biomonitoring sites across most of the U.S., beginning in 1994 (in 
eastern U.S.) and extending through 2011 (Smith et al., 2003; Coulston 
et al., 2003). At these sites (biosites), a national protocol, 
including verification and quality assurance procedures and a scoring 
system, was implemented. The resultant biosite index (BI) scores may be 
described with regard to one of several categories ranging from little 
or no foliar injury to severe injury (e.g., Smith et al., 2003; 
Campbell et al., 2007; Smith et al., 2007; Smith, 2012).\175\ However, 
the available information does not yet address or describe the 
relationships expected to exist between some level of injury severity 
(e.g., little, low/light, moderate or severe) and/or spatial extent 
affected and scenic or aesthetic values. This gap impedes consideration 
of the public welfare implications of different injury severities, and 
accordingly judgments on the potential for public welfare significance. 
That notwithstanding, while minor spotting on a few leaves of a plant 
may easily be concluded to be of little public welfare significance, 
some level of severity and widespread occurrence of visible foliar 
injury, particularly if occurring in specially protected areas, where 
the public can be expected to place value (e.g., for recreational 
uses), might reasonably be concluded to impact the public welfare.
---------------------------------------------------------------------------

    \175\ Authors of studies presenting USFS biomonitoring program 
data have suggested what might be ``assumptions of risk'' (e.g., for 
the forest resource) related to scores in these categories, e.g., 
none, low, moderate and high for BI scores of zero to five, five to 
15, 15 to 25 and above 25, respectively (e.g., Smith et al., 2003; 
Smith et al., 2012. For example, maps of localized moderate to high 
risk areas may be used to identify areas where more detailed 
evaluations are warranted (Smith et al., 2012).
---------------------------------------------------------------------------

    The tropospheric O3-related effects of radiative forcing 
and subsequent effects on temperature, precipitation and related 
climate also have important public welfare implications, although their 
quantitative evaluation in response to O3 concentrations in 
the U.S. is complicated by ``[c]urrent limitations in climate modeling 
tools, variation across models, and the need for more comprehensive 
observational data on these effects'' (ISA, section IS.6.2.2). An 
ecosystem service provided by forested lands is carbon sequestration or 
storage (ISA, section IS.5.1.4 and Appendix 8, section 8.8.3; 2013 ISA, 
section 2.6.2.1 and p. 9-37) \176\, which has an extremely valuable 
role in counteracting the impact of greenhouse gases on radiative 
forcing and related climate effects on the public welfare. Accordingly, 
the

[[Page 87313]]

service of carbon storage can be of paramount importance to the public 
welfare no matter in what location the trees are growing or what their 
intended current or future use (e.g., 2013 ISA, section 9.4.1.2). This 
benefit exists as long as the trees are growing, regardless of what 
additional functions and services it provides.
---------------------------------------------------------------------------

    \176\ While carbon sequestration or storage also occurs for 
vegetated ecosystems other than forests, it is relatively larger in 
forests given the relatively greater biomass for trees compared to 
other plants.
---------------------------------------------------------------------------

    Categories of effects newly identified as likely causally related 
to O3 in ambient air, such as alteration of plant-insect 
signaling and insect herbivore growth and reproduction, also have 
potential public welfare implications (e.g., given the role of the 
plant-insect signaling process in pollination and seed dispersal). 
Uncertainties and limitations in the current evidence (e.g., summarized 
in sections III.B.3.c and III.D.1 of the proposal) preclude an 
assessment of the extent and magnitude of O3 effects on 
these endpoints, which thus also precludes an evaluation of the 
potential for associated public welfare implications.
    In summary, several considerations are recognized as important to 
judgments on the public welfare significance of the array of welfare 
effects of different O3 exposure conditions. These include 
uncertainties and limitations associated with the magnitude of key 
welfare effects that might be concluded to be adverse to ecosystems and 
associated services. Additionally, the presence of O3-
sensitive tree species may contribute to a vulnerability of numerous 
locations to public welfare impacts from O3 related to tree 
growth, productivity and carbon storage and their associated ecosystems 
and services. Other important considerations include the exposure 
circumstances that may elicit effects and the potential for the 
significance of the effects to vary in specific situations due to 
differences in sensitivity of the exposed species, the severity and 
associated significance of the observed or predicted O3-
induced effect, the role that the species plays in the ecosystem, the 
intended use of the affected species and its associated ecosystem and 
services, the presence of other co-occurring predisposing or mitigating 
factors, and associated uncertainties and limitations.
c. Exposures Associated With Effects
    The welfare effects identified in section III.A.2.a above vary 
widely with regard to the extent and level of detail of the available 
information that describes the O3 exposure circumstances 
that may elicit the effects. The information on exposure metric and E-R 
relationships for effects related to vegetation growth is long-
standing, having been first described in the 1997 review, while such 
information is much less established for visible foliar injury. The 
evidence base for other categories of effects is also lacking in 
information that might support characterization of potential impacts of 
changes in O3 concentrations.
(i) Growth-Related Effects
    The long-standing body of vegetation effects evidence includes a 
wealth of information on aspects of O3 exposure that 
influence its effects on plant growth and yield, and that has been 
described in the scientific assessments across the last several decades 
(1996 AQCD; 2006 AQCD; 2013 ISA; 2020 ISA). A variety of factors have 
been investigated, and a number of mathematical approaches have been 
developed for summarizing O3 exposure for the purpose of 
assessing effects on vegetation, including several that cumulate 
exposures over some specified period while weighting higher more than 
lower concentrations (2013 ISA, sections 9.5.2 and 9.5.3; ISA, Appendix 
8, section 8.2.2.2). Over this period, the EPA's scientific assessments 
have focused on the use of a cumulative, seasonal \177\ concentration-
weighted index when considering the growth-related effects evidence and 
when analyzing exposures for purposes of reaching conclusions on the 
secondary standard. Such metrics have included SUM06,\178\ in the past, 
and more recently (since the 2008 review), the focus has been on the 
W126-based, seasonal metric, termed the ``W126 index'' \179\ (ISA, 
section IS.3.2, Appendix 8, sections 8.1 and 8.13).
---------------------------------------------------------------------------

    \177\ The ``seasonal'' descriptor refers to the duration of the 
period quantified (3 months) rather than a specific season of the 
year.
    \178\ The SUM06 index received attention across past 
O3 NAAQS reviews. It is the seasonal sum of hourly 
concentrations at or above 0.06 ppm during a specified daily time 
window (2006 AQCD, p. AX9-161; 2013 ISA, section 9.5.2).
    \179\ The W126 index is described in section III.B.3.a(i) of the 
proposal (85 FR 49887, August 14, 2020) and in the PA (PA, Appendix 
4D, section 4D.2.2).
---------------------------------------------------------------------------

    Quantifying exposure using cumulative, concentration-weighted 
indices of exposure, such as the W126 index, has been found to improve 
the explanatory power of E-R models for growth and yield over using 
indices based only on mean and peak exposure values (ISA, section 
IS.5.1.9, p. IS-79; 2013 ISA, section 2.6.6.1, p. 2-44). The most well-
analyzed datasets in such evaluations are two detailed datasets 
established two decades ago, one for seedlings of 11 tree species \180\ 
and one for 10 crops (e.g., Lee and Hogsett, 1996, Hogsett et al., 
1997). These datasets, which include species-specific seedling growth 
and crop yield response information across multiple seasonal cumulative 
exposures, were used to develop robust quantitative E-R functions to 
predict growth reduction relative to a zero-O3 setting (RBL) 
in seedlings of the tree species \181\ and similarly, E-R functions for 
predicting RYL for a set of 10 common crops (ISA, Appendix 8, section 
8.13.2; 2013 ISA, section 9.6.2).
---------------------------------------------------------------------------

    \180\ In total, the 11 species-specific composite E-R functions 
are based on 51 tree seedling studies or experiments, many of which 
employed open top chambers, an established experimental approach 
(PA, Appendix 4A, section 4A.1.1; ISA, section 8.1.2.1.2). For six 
of the 11 species, this function is based on just one or two 
studies, while for other species there were as many as 11 studies 
available.
    \181\ While the 11 species represent only a small fraction of 
the total number of native tree species in the contiguous U.S., this 
subset includes eastern and western species, deciduous and 
coniferous species, and species that grow in a variety of ecosystems 
and represent a range of tolerance to O3 (PA, Appendix 
4B; 2013 ISA, section 9.6.2).
---------------------------------------------------------------------------

    The tree seedling E-R functions were derived from data for multiple 
studies documenting effects on tree seedling growth under a variety of 
O3 exposures \182\ and growing conditions. Importantly the 
data included hourly concentrations recorded across the duration of the 
exposure, which allowed for derivation of various metrics that were 
analyzed for association with reduced growth (2013 ISA, section 9.6.2; 
Lee and Hogsett, 1996). In producing E-R functions of consistent 
duration across the experiments, the E-R functions were derived first 
based on the exposure duration of the experiment \183\ and then 
normalized to 3-month (seasonal) periods \184\ (see Lee and Hogsett, 
1996, section I.3; PA, Appendix 4A). The species-specific composite E-R 
functions developed from the experiment-specific functions indicate the 
wide variation in growth

[[Page 87314]]

sensitivity of the studied tree species at the seedling stage (PA, 
Appendix 4A, section 4A.1.1).
---------------------------------------------------------------------------

    \182\ Across the experiments for the 11 tree species, the 
exposure levels assessed are more extensive for relatively higher 
seasonal exposures (e.g., at/above a SUM06 of 30 ppm-hrs). Across 
these experiments, there is more limited representation of lower 
cumulative exposure levels, such as SUM06 values below those that 
may correspond to a W126 index of 20 ppm-hrs. These lowest levels 
did not always yield a statistically significant effect (PA, section 
4.5.1.2 and Appendix 4A; 85 FR 49901, August 14, 2020).
    \183\ The exposure durations varied from periods of 82 to 140 
days over a single year to periods of 180 to 555 days across two 
years (Lee and Hogsett, 1996; PA, Appendix 4A, Table 4A-5).
    \184\ Underlying the adjustment is a simplifying assumption of 
uniform W126 distribution across the exposure periods and of a 
linear relationship between duration of cumulative exposure in terms 
of the W126 index and plant growth response (85 FR 49901; August 14, 
2020; PA). Some functions for experiments that extended over two 
seasons were derived by distributing responses observed at the end 
of two seasons of varying exposures equally across the two seasons 
(e.g., essentially applying the average to both seasons).
---------------------------------------------------------------------------

    Since the initial set of tree seedling growth studies were 
completed, several additional studies, focused on aspen, have been 
published based on the Aspen FACE experiment in a planted forest in 
Wisconsin; the findings were consistent with earlier open top chamber 
(OTC) studies \185\ (ISA, Appendix 8, section 8.13.2). Newly available 
studies that investigated growth effects of O3 exposures are 
also consistent with the existing evidence base, and generally involve 
particular aspects of the effect rather than expanding the conditions 
under which plant species, particularly tree species, have been 
assessed (ISA, section IS.5.1.2). These publications include a 
compilation of previously available studies on plant biomass response 
to O3; the compilation reports linear regressions conducted 
on the associated varying datasets. Based on these regressions, this 
study describes distributions of sensitivity to O3 effects 
on biomass across many tree and grassland species, including 17 species 
native to the U.S. and 65 introduced species (ISA, Appendix 8, section 
8.13.2; van Goethem et al., 2013). Additional information is needed to 
more completely describe O3 exposure response relationships 
for these species in the U.S.\186\
---------------------------------------------------------------------------

    \185\ These studies included experiments that used OTCs to 
investigate tree seedling growth response and crop yield over a 
growing season under a variety of O3 exposures and 
growing conditions (2013 ISA, section 9.6.2; Lee and Hogsett, 1996).
    \186\ The studies compiled in this publication included at least 
21 days exposure above 40 ppb O3 (expressed as AOT40 
[seasonal sum of the difference between an hourly concentration 
above 40 ppb and 40 ppb]); and had a maximum hourly concentration 
that was no higher than 100 ppb (van Goethem et al., 2013). The 
publication does not report study-specific exposure durations, 
details of biomass response measurements or hourly O3 
concentrations, making it less useful for describing E-R 
relationships that might support estimation of specific impacts 
associated with air quality conditions meeting the current standard 
(e.g., 2013 ISA, p. 9-118).
---------------------------------------------------------------------------

(ii) Visible Foliar Injury
    Current evidence ``continues to show a consistent association 
between visible injury and ozone exposure,'' while also recognizing the 
role of modifying factors such as soil moisture and time of day (ISA, 
section IS.5.1.1). The ISA summarizes several recently available 
studies that continue to document that O3 elicits visible 
foliar injury in many plant species. As in the prior review, the 
evidence in the current review, while documenting that elevated 
O3 conditions in ambient air generally results in visible 
foliar injury in sensitive species (when in a predisposing 
environment),\187\ does not include a quantitative description of the 
relationship of incidence or severity of visible foliar injury in 
natural areas of the U.S. with specific metrics of seasonal 
O3 exposure.
---------------------------------------------------------------------------

    \187\ As a major modifying factor is the amount of soil moisture 
available to a plant, dry periods decrease the incidence and 
severity of ozone-induced visible foliar injury, such that the 
incidence of visible foliar injury is not always higher in years and 
areas with higher ozone, especially with co-occurring drought (ISA, 
Appendix 8, p. 8-23; Smith, 2012; Smith et al., 2003).
---------------------------------------------------------------------------

    Although studies of the incidence of visible foliar injury in 
national forests, wildlife refuges, and similar areas have often used 
cumulative indices (e.g., SUM06) to investigate variations in incidence 
of foliar injury, studies also suggest an additional role for metrics 
focused on peak concentrations (ISA; 2013 ISA; 2006 AQCD; Hildebrand et 
al., 1996; Smith, 2012). Other studies have indicated this uncertainty 
regarding the influential metric(s), e.g., by recognizing the need for 
research to help develop a ``better linkage between air levels and 
visible injury'' (Campbell et al., 2007).\188\ Some studies of visible 
foliar injury incidence data have investigated such a role for peak 
concentrations quantified by an O3 exposure index that is a 
count of hourly concentrations (e.g., in a growing season) above a 
threshold concentration of 100 ppb, N100 (e.g., Smith, 2012; Smith et 
al., 2012). For example, a study describing injury patterns over 16 
years at USFS biosites in 24 states in the Northeast and North Central 
regions, in the context of the SUM06 index and N100 metrics, suggested 
that there may be a threshold exposure needed for injury to occur, and 
that the number of hours of elevated O3 concentrations 
during the growing season (such as what is captured by a metric like 
N100) may be more important than cumulative exposure in determining the 
occurrence of foliar injury (Smith, 2012).\189\ This finding is 
consistent with statistical analyses of seven years of visible foliar 
injury data from a wildlife refuge in the mid-Atlantic area (Davis and 
Orendovici, 2006).\190\
---------------------------------------------------------------------------

    \188\ In considering their findings, the authors expressed the 
view that ``[a]lthough the number of sites or species with injury is 
informative, the average biosite injury index (which takes into 
account both severity and amount of injury on multiple species at a 
site) provides a more meaningful measure of injury'' for their 
assessment at a statewide scale (Campbell et al., 2007).
    \189\ Although the ISA and past assessments have not described 
extensive evaluations of specific peak concentration metrics such as 
the N100, in summarizing this study in the last review, the ISA 
observed that ``[o]verall, there was a declining trend in the 
incidence of foliar injury as peak O3 concentrations declined'' 
(2013 ISA, p. 9-40).
    \190\ The models evaluated included several with cumulative 
exposure indices alone. These included SUM60 (i.e., SUM06 in ppb), 
SUM0, and SUM80 (SUM08 in ppb), but not W126. They did not include a 
model with W126 that did not also include N100. Across all of the 
models evaluated, the model with the best fit to the data was found 
to be the one that included N100 and W126, along with the drought 
index (Davis and Orendovici, 2006).
---------------------------------------------------------------------------

    The established significant role of higher or peak O3 
concentrations, as well as pattern of their occurrence, in plant 
responses has also been noted in prior ISAs or AQCDs. The evidence has 
included studies that use indices to summarize the incidence of injury 
on bioindicator species present at specific monitored sites, as well as 
experimental studies that assess varying O3 treatments on 
cultured stands of different tree species (2013 ISA, section 9.5.3.1; 
2006 AQCD, p. AX9-169; Oksanen and Holopainen, 2001; Yun and Laurence, 
1999). In identifying support for such O3 metrics with 
regard to foliar injury as the response, the 2013 ISA and 2006 AQCD 
both cite studies that support the ``important role that peak 
concentrations, as well as the pattern of occurrence, plays in plant 
response to O3'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-
169).
    A recent study (by Wang et al. [2012]) involved a statistical 
modeling analysis on a subset of the years of USFS BI data that were 
described in Smith (2012). This analysis tested a number of models for 
their ability to predict the presence of visible foliar injury (a 
nonzero biosite score), regardless of severity, and generally found 
that the type of O3 exposure metric (e.g., SUM06 versus 
N100) made only a small difference, although the models that included 
both a cumulative index (SUM06) and N100 had a just slightly better fit 
(Wang et al., 2012). Based on their investigation of 15 different 
models, using differing combinations of several types of potential 
predictors, the study authors concluded that they were not able to 
identify environmental conditions under which they ``could reliably 
expect plants to be damaged'' (Wang et al., 2012). This is indicative 
of the current state of knowledge, in which there remains a lack of 
established quantitative functions describing E-R relationships that 
would allow prediction of visible foliar injury severity and incidence 
under varying air quality and other environmental conditions.
    The information related to O3 exposures associated with 
visible foliar injury of varying severity available in this review also 
includes quantitative

[[Page 87315]]

presentations of the dataset (developed by the EPA in the last review) 
of USFS BI scores, collected during the years 2006 through 2010 at 
locations in 37 states. In developing this dataset, the BI scores were 
combined with estimates of soil moisture \191\ and estimates of 
seasonal cumulative O3 exposure in terms of W126 index \192\ 
(PA, Appendix 4C). This dataset includes more than 5,000 records of 
which more than 80 percent have a BI score of zero (indicating a lack 
of visible foliar injury). While the estimated W126 index assigned to 
records in this dataset ranges from zero to somewhat above 50 ppm-hrs, 
more than a third of all the records (and also of records with BI 
scores above zero or five) \193\ are at sites with W126 index estimates 
below 7 ppm-hrs. In an extension of analyses developed in the last 
review, the presentation in the PA \194\ describes the BI scores for 
the records in this dataset in relation to the W126 index estimate for 
each record, using ``bins'' of increasing W126 index values. The PA 
presentation utilizes the BI score breakpoints in the scheme used by 
the USFS to categorize severity. This presentation indicates that, 
across the W126 bins, there is variation in both the incidence of 
particular magnitude BI scores and in the average score per bin. In 
general, however, the greatest incidence of records with BI scores 
above zero, five, or higher--and the highest average BI score--occurs 
with the highest W126 bin, i.e., the bin for W126 index estimates 
greater than 25 ppm-hrs (PA, Appendix 4C, Table 4C-6).
---------------------------------------------------------------------------

    \191\ This dataset, including associated uncertainties and 
limitations in the assignment of soil moisture categories (dry, wet 
or normal), such as the substantial spatial variation in soil 
moisture and large size of NOAA climate divisions, is described in 
the PA, Appendix 4C.
    \192\ The W126 index estimates assigned to the biosite locations 
were developed for 12 kilometer (km) by 12 km cells in a national-
scale spatial grid for each year. A spatial interpolation technique 
was applied to annual W126 values derived from O3 
measurements at ambient air monitoring locations for the years of 
the BI data (PA, Appendix 4C, sections 4.C.2 and 4C.5).
    \193\ One third (33%) of scores above 15 are at sites with W126 
below 7 ppm-hrs (PA, Appendix 4C, Table 4C-3).
    \194\ Beyond the presentation of a statistical analysis 
developed in the last review, the PA presentations are primarily 
descriptive (as compared to statistical) in recognition of the 
limitations and uncertainties of the dataset (PA, Appendix 4C, 
section 4C.5).
---------------------------------------------------------------------------

    Overall, the dataset described in the PA generally indicates the 
risk of injury, and particularly injury considered at least light, 
moderate or severe, to be higher at the highest W126 index values, with 
appreciable variability in the data for the lower bins (PA, Appendix 
4C). This appears to be consistent with the conclusions of the detailed 
quantitative analysis studies, summarized above, that the pattern is 
stronger at higher O3 concentrations. A number of factors 
may contribute to the observed variability in BI scores and lack of a 
clear pattern with W126 index bin; among other factors, these may 
include uncertainties in assignment of W126 estimates and soil moisture 
categories to biosite locations, variability in biological response 
among the sensitive species monitored, and the potential role of other 
aspects of O3 air quality not captured by the W126 index. 
Thus, the dataset has limitations affecting associated conclusions, and 
uncertainty remains regarding the tools for and the appropriate metric 
(or metrics) for quantifying O3 exposures, as well as 
perhaps for quantifying soil moisture conditions, with regard to their 
influence on extent and/or severity of injury in sensitive species in 
natural areas, as quantified via BI scores (Davis and Orendovici, 2006, 
Smith et al., 2012; Wang et al., 2012).
(iii) Other Effects
    With regard to radiative forcing and subsequent climate effects 
associated with the global tropospheric abundance of O3, the 
newly available evidence in this review does not provide more detailed 
quantitative information regarding response to O3 
concentrations at the national scale. Rather, it is noted that ``the 
heterogeneous distribution of ozone in the troposphere complicates the 
direct attribution of spatial patterns of temperature change to ozone 
induced [radiative forcing]'' and there are ``ozone climate feedbacks 
that further alter the relationship between ozone [radiative forcing] 
and temperature (and other climate variables) in complex ways'' (ISA, 
Appendix 9, section 9.3.1, p. 9-19). Further, ``precisely quantifying 
the change in surface temperature (and other climate variables) due to 
tropospheric ozone changes requires complex climate simulations that 
include all relevant feedbacks and interactions'' (ISA, section 9.3.3, 
p. 9-22). Yet, there are limitations in current climate modeling 
capabilities for O3; an important one is representation of 
important urban- or regional-scale physical and chemical processes, 
such as O3 enhancement in high-temperature urban situations 
or O3 chemistry in city centers where NOX is 
abundant. Such limitations impede our ability to quantify the impact of 
incremental changes in O3 concentrations in the U.S. on 
radiative forcing and subsequent climate effects.
    With regard to tree mortality, the evidence available in the last 
several reviews included field studies of pollution gradients that 
concluded O3 damage to be an important contributor to tree 
mortality although ``several confounding factors such as drought, 
insect outbreak and forest management'' were identified as potential 
contributors (2013 ISA, p. 9-81, section 9.4.7.1). Among the newly 
available studies, there is only limited experimental evidence that 
isolates the effect of O3 on tree mortality \195\ and might 
be informative regarding O3 concentrations of interest in 
the review, and evidence is lacking regarding exposure conditions 
closer to those occurring under the current standard and any 
contribution to tree mortality.
---------------------------------------------------------------------------

    \195\ Of the three new studies on tree mortality described in 
the ISA is another field study of a pollution gradient that, like 
such studies in prior reviews, recognizes O3 exposures as 
one of several contributing environmental and anthropogenic 
stressors (ISA, p. 8-55).
---------------------------------------------------------------------------

    With regard to alteration of herbivore growth and reproduction, 
although ``[t]here are multiple studies demonstrating ozone effects on 
fecundity and growth in insects that feed on ozone-exposed 
vegetation'', ``no consistent directionality of response is observed 
across studies and uncertainties remain in regard to different plant 
consumption methods across species and the exposure conditions 
associated with particular severities of effects'' (ISA, pp. ES-18). 
The evidence for alteration of plant-insect signaling draws on new 
research yielding clear evidence of O3 modification of 
volatile plant signaling compounds and behavioral responses of insects 
to the modified chemical signals (ISA, section IS.6.2.1). The evidence 
includes a relatively small number of plant species and plant-insect 
associations and is limited to short controlled exposures, posing 
limitations for consideration of the potential for associated impacts 
to be elicited by air quality conditions that meet the current standard 
(ISA, section IS.6.2.1 and Appendix 8, section 8.7).
    For categories of vegetation-related effects that were recognized 
in past reviews, other than growth and visible foliar injury (e.g., 
reduced plant reproduction, reduced productivity in terrestrial 
ecosystems, alteration of terrestrial community composition and 
alteration of below-ground biogeochemical cycles), the newly available 
evidence includes a variety of studies that quantify exposure of 
varying duration in various countries using a variety of metrics (ISA, 
Appendix 8, sections 8.4, 8.8 and 8.10).

[[Page 87316]]

The ISA also describes publications that analyze and summarize 
previously published studies. For example, a meta-analysis of 
reproduction studies categorized the reported O3 exposures 
into bins of differing magnitude, grouping differing concentration 
metrics and exposure durations together, and performed statistical 
analyses investigating associations with an O3-related 
effect (ISA, Appendix 8, section 8.4.1). While such studies continue to 
support conclusions of O3 ecological hazards, they do not 
improve capabilities for characterizing the likelihood of such effects 
under patterns of environmental O3 concentrations occuring 
with air quality conditions that meet the current standard (e.g., 
factors such as variation in exposure assessments and limitations in 
response information preclude detailed analysis for such conditions), 
as discussed further in the PA.
    As at the time of the last review, growth impacts, most 
specifically as evaluated by RBL for tree seedlings and RYL for crops, 
remain the type of vegetation-related effects for which we have the 
best understanding of exposure conditions likely to elicit them. 
Accordingly, as was the case in the last review, the quantitative 
analyses of exposures occurring under air quality that meets the 
current standard, summarized below, are focused primarily on the W126 
index, given its established relationship with growth effects.
3. Overview of Air Quality and Exposure Information
    The air quality and exposure analyses developed in this review, 
like those in the last review, are of two types: (1) W126-based 
cumulative exposure estimates in Class I areas; and (2) analyses of 
W126-based exposures and their relationship with the current standard 
for all U.S. monitoring locations (PA, Appendix 4D). We recognize 
relatively lower uncertainty associated with the use of these types of 
analyses (compared to the national or regional-scale modeling analyses 
performed in the last review) to inform a characterization of 
cumulative O3 exposure (in terms of the W126 index) 
associated with air quality just meeting the current standard (IRP, 
section 5.2.2). As in the last review, the lower uncertainty of these 
air quality monitoring-based analyses contributes to their value in 
informing the current review.
    The analyses conducted in this review focus on design values (3-
year average annual fourth-highest 8-hour daily maximum concentration, 
also termed the ``4th max metric'') and W126 index values (in terms of 
the 3-year average) for the recent 2016 to 2018 period and across the 
historical record back to 2000 (PA, Section 4.3). These analyses are 
based primarily on the hourly air monitoring data that were reported to 
EPA from O3 monitoring sites nationwide and in or near Class 
1 areas.\196\
---------------------------------------------------------------------------

    \196\ Across the seventeen 3-year periods from 2000-2002 to 
2016-2018, the number of monitoring sites with sufficient data for 
calculation of valid design values and W126 index values (across the 
3-year design value period) ranged from a low of 992 in 2000-2002 to 
a high of 1119 in 2015-2017 (PA, Section 4.3).
---------------------------------------------------------------------------

a. Influence of Form and Averaging Time of Current Standard on 
Environmental Exposure
    The findings of the quantitative analyses in this review of 
relationships between air quality in terms of the form and averaging 
time of the current standard and environmental exposures in terms of 
the W126 index are similar to those based on the data available during 
the last review (PA, Appendix 4D, section 4D.2.2).\197\ As previously, 
the current analysis of data spanning 19 years and including seventeen 
3-year periods documented a positive nonlinear relationship between 
cumulative seasonal exposure (quantified using the W126 index) and 
design values (based on the form and averaging time of the current 
standard). In the current analysis, which revealed the variability in 
the annual W126 index values across a 3-year period to be relatively 
low,\198\ the positive nonlinear relationship is shown for both the 
average W126 index across the 3-year design value period and for W126 
index values for individual years within the period (PA, Figure 4-7; 
Appendix 4D, section 4D.3.1.2). That is, W126 index values (in a single 
year or averaged across years) are lower at monitoring sites with lower 
design values. This is seen both for design values above the standard 
and across lower design values, indicating the effectiveness of the 
averaging time and form of the current standard at controlling W126-
based cumulative exposures.
---------------------------------------------------------------------------

    \197\ In 2015 the Administrator concluded that, with revision of 
the standard level, the existing form and averaging time provided 
the control of cumulative seasonal exposure circumstances needed for 
the public welfare protection desired (80 FR 65408, October 26, 
2015).
    \198\ This evaluation, performed for all U.S. monitoring sites 
with sufficient data available in the most recent 3-year period, 
2016 to 2018, indicates the extent to which the three single-year 
W126 index values within a 3-year period deviate from the average 
for the period. Across the full set of sites, regardless of W126 
index magnitude (or whether or not the current standard is met), 
single-year W126 index values differ less than 15 ppm-hrs from the 
average for the 3-year period (PA, Appendix 4D, Figure 4D-6). For 
the approximately 850 sites meeting the current standard, over 99% 
of single-year W126 index values differ from the 3-year average by 
no more than 5 ppm-hrs, and 87% by no more than 2 ppm-hrs (PA, 
Appendix 4D, Figure 4D-7).
---------------------------------------------------------------------------

    Further, analysis of the relationship between trends or long-term 
changes in design value and long-term changes in W126 index shows there 
to be a positive, linear relationship at monitoring sites across the 
U.S. (PA, Appendix 4D, section 4D.3.2.3). The existence of this 
relationship means that a change in the design value at a monitoring 
site was generally accompanied by a similar change in the W126 index. 
The relationship varies across the NOAA climate regions, with the 
greatest change in W126 index per unit change in design value observed 
in the Southwest and West regions, the regions which had the highest 
W126 index values at sites meeting the current standard (PA, Appendix 
4D, Figures 4D-6 and 4D-14, Table 4D-12). Thus, this analysis indicates 
that going forward, as design values are reduced in areas that are 
presently not meeting the current standard, the W126 index in those 
areas would also be expected to decline and the greatest improvement in 
W126 index per unit decrease in design value would be expected in the 
Southwest and West regions (PA, Appendix 4D, section 4D.3.2.3 and 
4D.5). The overall trend showing reductions in the W126 index 
concurrent with reductions in the design value metric for the current 
standard is positive whether the W126 index is expressed in terms of 
the average across the 3-year design value period or the annual value 
(PA, Appendix 4D, section 4D.3.2.3).
    The available air quality information also indicates that the 
current standard's form and averaging time exerts control on other 
vegetation exposures of potential concern, such as days with 
particularly high O3 concentrations that may contribute to 
visible foliar injury. The current form and averaging time, by their 
very definition, limit occurrences of such concentrations. This is 
demonstrated by reductions in daily maximum 8-hour concentrations, as 
well as in the frequency of elevated 1-hour concentrations, including 
concentrations at or above 100 ppb, with decreasing design values (PA, 
Figure 2-11, Appendix 2A, section 2A.2). As the form and averaging time 
of the secondary standard have not changed since 1997, the analyses 
have been able to assess the amount of control exerted by these aspects 
of the standard, in combination with reductions in the standard level 
(i.e.,

[[Page 87317]]

from 0.08 ppm in 1997 to 0.075 ppm in 2008 to 0.070 ppm in 2015), on 
cumulative seasonal exposures in terms of W126 index, and on the 
magnitude of short-term peak concentrations. These analyses indicate 
that the long-term reductions in the design values, presumably 
associated with implementation of the revised standards, were 
accompanied by reductions in W126 index, as well as in short-term peak 
concentrations.
b. Environmental Exposures in Terms of W126 Index
    The analyses summarized here describe the nature and magnitude of 
vegetation exposures associated with conditions meeting the current 
standard at sites across the U.S., particularly in specially protected 
areas, such as Class I areas. Given the evidence indicating the W126 
index to be strongly related to growth effects and its use in the E-R 
functions for tree seedling RBL (as summarized in section III.A.2.c 
above), exposure is quantified using the W126 metric. These analyses 
include a particular focus on monitoring sites in or near Class I 
areas,\199\ in light of the greater public welfare significance of many 
O3 related impacts in such areas, as described in section 
III.A.2.b above, and consider both recent air quality (2016-2018) and 
the air quality record since 2000 (PA, Appendix 4D). As was the case in 
the last review, the currently available quantitative information 
continues to indicate appreciable control of seasonal W126 index-based 
cumulative exposure at all sites with air quality meeting the current 
standard.
---------------------------------------------------------------------------

    \199\ This includes monitors sited within Class I areas or the 
closest monitoring site within 15 km of the area boundary.
---------------------------------------------------------------------------

    Among sites nationwide meeting the current standard in the recent 
period of 2016 to 2018, there are none with a W126 index, based on the 
3-year average, above 19 ppm-hrs, and just one with such a value above 
17 ppm-hrs (Table 4).\200\ Additionally, the full historical dataset 
includes no occurrences of a 3-year average W126 index above 19 ppm-hrs 
for sites meeting the current standard, and just eight occurrences of a 
W126 index above 17 ppm-hrs (less than 0.1% of the dataset), with the 
highest such occurrence just equaling 19 ppm-hrs (Table 4; PA, Appendix 
4D, section 4D.3.2.1).
---------------------------------------------------------------------------

    \200\ Rounding conventions are described in detail in the PA, 
Appendix 4D, section 4D.2.2.
---------------------------------------------------------------------------

    With regard to Class I areas, the updated air quality analyses 
include data from sites in or near 65 Class I areas. The findings for 
these sites, which are distributed across all nine NOAA climate regions 
in the contiguous U.S., as well as Alaska and Hawaii, mirror the 
findings for the analysis of all U.S. sites in the dataset. Among the 
Class I area sites meeting the current standard (i.e., having a design 
value at or below 70 ppb) in the most recent period of 2016 to 2018, 
there are none with a W126 index (averaged over the design value 
period) above 17 ppm-hrs (Table 4). The historical dataset includes 
just seven occurrences (all dating from the 2000-2010 period) of a 
Class I area site meeting the current standard and having a 3-year 
average W126 index above 17 ppm-hrs, and no such occurrences above 19 
ppm-hrs (Table 4).
    The W126 index values at sites that do not meet the current 
standard are much higher, with values at such sites ranging as high as 
approximately 60 ppm-hrs (PA, Appendix 4D, Figure 4D-3). Among all 
sites across the U.S. that do not meet the current standard in the 2016 
to 2018 period, more than a quarter have average W126 index values 
above 19 ppm-hrs and a third exceed 17 ppm-hrs (Table 4). A similar 
situation exists for Class I area sites (Table 4). For example, out of 
the 11 Class I area sites with design values above 70 ppb during the 
most recent period, eight sites had a 3-year average W126 index above 
19 ppm-hrs (with a maximum value of 47 ppm-hrs) and for nine, it was 
above 17 ppm-hrs (Table 4; PA, Appendix 4D, Table 4D-17).

 Table 4--Distribution of 3-Yr Average Seasonal W126 Index for Sites in Class I Areas and Across U.S. That Meet the Current Standard and For Those That
                                                                         Do Not
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                   Number of occurrences or site-DVs \A\
                                                 -------------------------------------------------------------------------------------------------------
                                                                   In Class I areas                      Across all monitoring sites (urban and rural)
                 3-year periods                  -------------------------------------------------------------------------------------------------------
                                                                           W126 (ppm-hrs)                                      W126 (ppm-hrs)
                                                     Total    ---------------------------------------    Total    --------------------------------------
                                                                   >19          >17          <=17                      >19          >17          <=17
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                        At sites that meet the current standard (design value at or below 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018.......................................           47            0            0           47          849            0            1          848
All from 2000 to 2018...........................          498            0            7          491        8,292            0            8        8,284
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                          At sites that exceed the current standard (design value above 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018.......................................           11            8            9            2          273           78           91          182
All from 2000 to 2018...........................          362          159          197          165       10,695        2,317        3,174        7,521
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Counts presented here are drawn from the PA, Appendix D, Tables 4D-1, 4D-4, 4D-5, 4D-6, 4D-9, 4D-10 and 4D-13 through 16.

B. Conclusions on the Secondary Standard

    In drawing conclusions on the adequacy of the current secondary 
standard, in view of the advances in scientific knowledge and 
additional information now available, the Administrator has considered 
the currently available welfare effects evidence and air quality and 
ecological exposure information. He additionally has considered the 
evidence base, information, and policy judgments that were the 
foundation of the last review, to the extent they remain relevant in 
light of the currently available information. The Administrator has 
taken into account both evidence-based and air quality and exposure-
based considerations discussed in the PA, as well as advice from the 
CASAC and

[[Page 87318]]

public comments. Evidence-based considerations draw upon the EPA's 
assessment and integrated synthesis of the scientific evidence as 
presented in the ISA, with a focus on policy-relevant considerations as 
discussed in the PA (summarized in sections III.B and III.D.1 of the 
proposal and section III.A.2 above). The air quality and exposure-based 
considerations draw from the results of the quantitative air quality 
analyses presented and considered in the PA (as summarized in section 
III.C of the proposal and section III.A.3 above). The Administrator 
additionally considered the August 2019 decision of the D.C. Circuit 
remanding the 2015 secondary standard for further justification or 
reconsideration.
    The consideration of the evidence and air quality/exposure 
information in the PA informed the Administrator's proposed conclusions 
and judgments in this review, and his associated proposed decision. 
Section III.B.1 below briefly summarizes the basis for the 
Administrator's proposed decision, drawing from section III.D of the 
proposal. Section III.B.1.a provides a brief overview of key aspects of 
the policy evaluations presented in the PA, and the advice and 
recommendations of the CASAC are summarized in III.B.1.b. An overview 
of the Administrator's proposed conclusions is presented in section 
III.B.1.c. Public comments on the proposed decision are addressed below 
in sections III.B.2 and the Administrator's conclusions and decision in 
this review regarding the adequacy of the current secondary standard 
and whether any revisions are appropriate are described in section 
III.B.3.
1. Basis for Proposed Decision
a. Policy-Relevant Evaluations in the PA
    The main focus of the policy-relevant considerations in the PA is 
consideration of the question: Does the currently available scientific 
evidence- and air quality and environmental exposure-based information 
support or call into question the adequacy of the protection afforded 
by the current secondary O3 standard? The PA response to 
this overarching question takes into account discussions that address 
the specific policy-relevant questions for this review, focusing first 
on consideration of the evidence, as evaluated in the ISA, including 
that newly available in this review. The PA also considers the 
quantitative information available in this review that relates 
O3 environmental exposures to vegetation responses 
(presented in Appendices 4A and 4C of the PA) and the air quality 
analyses that investigate relationships between air quality that meets 
the current standard and cumulative and peak exposures (presented in 
detail in Appendix 4D of the PA). The PA additionally discusses the key 
aspects of the evidence and exposure/risk estimates that were 
emphasized in establishing the current standard, and key aspects of the 
2019 court remand on the standard. In so doing, the PA also considers 
associated public welfare policy judgments and judgments about the 
uncertainties inherent in the scientific evidence and quantitative 
analyses that are integral to the Administrator's consideration of 
whether the currently available information supports or calls into 
question the adequacy of the current secondary O3 standard 
(PA, section 3.5).
    Key policy-relevant considerations identified by the PA included 
the following. The new information available is consistent with that 
available in the last review for the principal effects for which the 
evidence is strongest (e.g., growth, reproduction, and related larger-
scale effects, as well as, visible foliar injury) and for key aspects 
of the decision in that review. The currently available information 
does not provide established quantitative relationships and tools for 
estimating incidence and severity of visible foliar injury in protected 
areas across the U.S. or provide information linking extent and 
severity of injury to aesthetic values that might be useful for 
considering public welfare implications. Further, the currently 
available evidence for forested locations across the U.S., such as 
studies of USFS biosites, does not indicate widespread incidence of 
significant visible foliar injury. Additionally, the evidence regarding 
RBL and air quality in areas meeting the current standard does not 
appear to call into question the adequacy of protection. For other 
vegetation-related effects that the ISA newly concludes likely to be 
causally related to O3, the new information does not provide 
us an indication of the extent to which such effects might be 
anticipated to occur in areas that meet the current standard of a 
significance reasonably judged significant to public welfare. Thus, the 
PA does not find the current information for these newly identified 
categories to call into question the adequacy of the current standard. 
Similarly, the current information regarding O3 contribution 
to radiative forcing or effects on temperature, precipitation and 
related climate variables is not strengthened from that available in 
the last review, including with regard to uncertainties that limit 
quantitative evaluations. Based on such considerations, discussed in 
detail in the PA, it concludes that the currently available evidence 
and quantitative exposure/risk information does not call into question 
the adequacy of the current secondary standard such that it is 
appropriate to consider retaining the current standard without 
revision. In so doing, it recognized that, as is the case in NAAQS 
reviews in general, the extent to which the Administrator judges the 
current secondary O3 standard to be adequate will depend on 
a variety of factors, including science policy judgments and public 
welfare policy judgments.
b. CASAC Advice in This Review
    In comments on the draft PA, the CASAC concurred with the PA 
conclusions, stating that it ``finds, in agreement with the EPA, that 
the available evidence does not reasonably call into question the 
adequacy of the current secondary ozone standard and concurs that it 
should be retained'' (Cox, 2020a, p. 1). The CASAC additionally stated 
that it ``commends the EPA for the thorough discussion and rationale 
for the secondary standard'' (Cox, 2020a, p. 2). The CASAC also 
provided comments particular to the consideration of climate and 
growth-related effects.
    With regard to O3 effects on climate, the CASAC 
recommended quantitative uncertainty and variability analyses, with 
associated discussion (Cox, 2020a, p. 2 and Consensus Responses to 
Charge Questions p. 22).\201\ With regard to growth-related effects and 
consideration of the evidence in quantitative exposure analyses, it 
stated that the W126 index ``appears reasonable and scientifically 
sound,'' ``particularly [as] related to growth effects'' (Cox, 2020a, 
Consensus Responses to Charge Questions p. 16). Additionally, with 
regard to the prior Administrator's expression of greater confidence in 
judgments related to public welfare impacts based on a seasonal W126 
index estimated by a three-year average and accordingly relying on that 
metric the CASAC expressed the view that this ``appears of reasonable 
thought and scientifically

[[Page 87319]]

sound'' (Cox, 2020a, Consensus Responses to Charge Questions p. 19). 
Further, the CASAC stated that ``RBL appears to be appropriately 
considered as a surrogate for an array of adverse welfare effects and 
based on consideration of ecosystem services and potential for impact 
to the public as well as conceptual relationships between vegetation 
growth effects and ecosystem scale effects'' and that it agrees ``that 
biomass loss, as reported in RBL, is a scientifically-sound surrogate 
of a variety of adverse effects that could be exerted to public 
welfare,'' concurring that this approach is not called into question by 
the current evidence which continues to support ``the use of tree 
seedling RBL as a proxy for the broader array of vegetation related 
effects, most particularly those related to growth that could be 
impacted by ozone'' (Cox, 2020a, Consensus Responses to Charge 
Questions p. 21). The CASAC additionally concurred that the strategy of 
a secondary standard that generally limits 3-year average W126 index 
values somewhat below those associated with a 6% RBL in the median 
species is ``scientifically reasonable'' and that, accordingly, a W126 
index target value of 17 ppm-hrs for generally restricting cumulative 
exposures ``is still effective in particularly protecting the public 
welfare in light of vegetation impacts from ozone'' (Cox, 2020a, 
Consensus Responses to Charge Questions p. 21).
---------------------------------------------------------------------------

    \201\ As recognized in the ISA, ``[c]urrent limitations in 
climate modeling tools, variation across models, and the need for 
more comprehensive observational data on these effects represent 
sources of uncertainty in quantifying the precise magnitude of 
climate responses to ozone changes, particularly at regional 
scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p. 9-
22). These complexities impede our ability to consider specific 
O3 concentrations in the U.S. with regard to specific 
magnitudes of impact on radiative forcing and subsequent climate 
effects.
---------------------------------------------------------------------------

    With regard to the court's remand of the 2015 secondary standard to 
the EPA for further justification or reconsideration (``particularly in 
relation to its decision to focus on a 3-year average for consideration 
of the cumulative exposure, in terms of W126, identified as providing 
requisite public welfare protection, and its decision to not identify a 
specific level of air quality related to visible foliar injury''), 
while the CASAC stated that it was not clear whether the draft PA had 
fully addressed this concern (Cox, 2020a, Consensus Responses to Charge 
Questions p. 21), it described there to be a solid scientific 
foundation for the current secondary standard and also commented on 
areas related to the remand. With regard to support in the information 
available in the current review for the focus on the 3-year average 
W126 index in assessing different patterns of air quality using median 
tree seedling RBL, in addition to the comments summarized above, the 
CASAC concluded, in considering the approach used in the last review, 
that reliance on the 3-year average and associated judgments in doing 
so ``appears of reasonable thought and scientifically sound'' (Cox, 
2020a, Consensus Responses to Charge Questions p. 19). Further, while 
recognizing the existence of established E-R functions that relate 
cumulative seasonal exposure of varying magnitudes to various 
incremental reductions in expected tree seedling growth (in terms of 
RBL) and in expected crop yield, the CASAC letter also noted that while 
decades of research also recognizes visible foliar injury as an effect 
of O3, ``uncertainties continue to hamper efforts to 
quantitatively characterize the relationship of its occurrence and 
relative severity with ozone exposures'' (Cox, 2020a, Consensus 
Responses to Charge Questions p. 20). In summary, the CASAC stated that 
the approach described in the draft PA to considering the evidence for 
welfare effects ``is laid out very clearly, thoroughly discussed and 
documented, and provided a solid scientific underpinning for the EPA 
conclusion leaving the current secondary standard in place'' (Cox, 
2020a, Consensus Responses to Charge Questions p. 22).
c. Administrator's Proposed Conclusions
    In reaching conclusions on the adequacy and appropriateness of 
protection provided by the current secondary standard and his proposed 
decision to retain the standard, the Administrator carefully 
considered: (1) The assessment of the available welfare effects 
evidence and conclusions contained in the ISA, with supporting details 
in the 2013 ISA and past AQCDs; (2) the evaluation of policy-relevant 
aspects of the evidence and quantitative analyses in the PA; (3) the 
advice and recommendations from the CASAC; (4) the August 2019 decision 
of the D.C. Circuit remanding the secondary standard established in the 
last review to the EPA for further justification or reconsideration; 
and (5) public comments that had been received up to that point (85 FR 
49830, August 14, 2020). In considering the evidence base on welfare 
effects associated with exposure to photochemical oxidants, including 
O3, in ambient air, he noted the newly available evidence, 
and the extent to which it alters key scientific conclusions from the 
last review. He additionally considered the quantitative analyses 
developed in this review, and their associated limitations and 
uncertainties, with regard to what they indicate regarding the 
protection provided by the current standard. Key aspects of the 
evidence and air quality and exposure information emphasized in 
establishing the current standard were also considered. Further, he 
considered uncertainties in the evidence and quantitative information 
as a part of public welfare policy judgments that are essential and 
integral to his decision on the adequacy of protection provided by the 
standard. In considering the CASAC advice, he noted the CASAC 
characterization of the ``thorough discussion and rationale for the 
secondary standard'' presented in the PA (Cox, 2020a, p. 2), and also 
considered the Committee's overall agreement that the currently 
available evidence does not call into question the adequacy of the 
current standard and that it should be retained (Cox, 2020a, p. 1).
    As an initial matter, the Administrator recognized the continued 
support in the current evidence for O3 as the indicator for 
photochemical oxidants, noting that no newly available evidence has 
been identified in this review on the importance of photochemical 
oxidants other than O3 with regard to abundance in ambient 
air and potential for welfare effects. For such reasons, described with 
more specificity in the ISA and PA and summarized in the proposal, he 
proposed to conclude it to be appropriate to retain O3 as 
the indicator for the secondary NAAQS for photochemical oxidants and he 
focused on the current information for O3.
    With regard to the currently available welfare effects evidence, 
the Administrator recognized that, consistent with the evidence in the 
last review, the currently available evidence describes an array of 
effects on vegetation and related ecosystem effects causally or likely 
to be causally related to O3 in ambient air, as well as the 
causal relationship of tropospheric O3 in radiative forcing 
and subsequent likely causally related effects on temperature, 
precipitation and related climate variables. The evidence for three 
additional categories of effects was newly determined in this review to 
be sufficient to infer likely causal relationships with O3. 
However, the Administrator did not find the evidence for these effects 
to be informative to his proposed decision in review of the standard. 
For example, the Administrator noted the PA did not find the current 
evidence to indicate air quality under the current standard to cause 
increased tree mortality, and, accordingly, he found it appropriate to 
focus on more sensitive effects, such as tree seedling growth, in his 
review of the standard. With regard to the two insect-related 
categories of effects with new ISA determinations (alteration of plant-
insect signaling and alteration of

[[Page 87320]]

insect herbivore growth and reproduction), the Administrator noted the 
associated uncertainties in the evidence that preclude a full 
understanding of key aspects of the effects and indicate there to be 
insufficient information to judge the current standard inadequate based 
on these effects as described in the proposal.
    In considering the evidence documenting tropospheric O3 
as a greenhouse gas causally related to radiative forcing, and likely 
causally related to subsequent effects on variables such as temperature 
and precipitation, the Administrator took note of the limitations and 
uncertainties in the evidence base that affect characterization of the 
extent of any relationships between O3 concentrations in 
ambient air in the U.S. and climate-related effects. He found this to 
preclude quantitative characterization of climate responses to changes 
in O3 concentrations in ambient air at regional (versus 
global) scales. This lack of quantitative tools precluding important 
analyses and the resulting uncertainty led the Administrator to 
conclude there to be insufficient information available for these 
effects in the current review to support judging the existing standard 
inadequate or to identify an appropriate revision.
    With regard to visible foliar injury, the Administrator recognized 
that, depending on its severity and spatial extent, as well as the 
location(s) and intended use(s), the impact of visible foliar injury on 
the physical appearance of plants has the potential to be significant 
to the public welfare. For example, depending on its extent and 
severity, its occurrence in specially protected natural areas may 
affect aesthetic and recreational values, such as the aesthetic value 
of scenic vistas in protected natural areas (e.g., national parks and 
wilderness areas). While recognizing there to be a paucity of 
information that relates incidence or severity of injury on vegetation 
in public lands to impacts on the public welfare (e.g., related to 
recreational services), the Administrator noted the USFS BI scoring 
scheme, and proposed to judge that occurrence of the lower categories 
of BI scores does not pose concern for the public welfare, but that 
findings of BI scores categorized as ``moderate to severe'' injury by 
the USFS scheme would be an indication of visible foliar injury 
occurrence that, depending on extend and severity, may raise public 
welfare concerns.
    While recognizing that important uncertainties remain in the 
understanding of the O3 exposure conditions that will elicit 
visible foliar injury of varying severity and extent in natural areas, 
the Administrator took note of the evidence indicating a general 
association of injury incidence and severity with cumulative exposure 
metrics, including the W126 index, and also an influence of peak 
concentrations, as well as the quantitative analyses in the PA of USFS 
biosite data and of air quality monitoring data. In the PA analysis of 
biosite scores, the incidence of nonzero BI scores, and particularly of 
relatively higher scores, such as those indicative of ``moderate to 
severe'' injury in the USFS scheme, appear to markedly increase only 
with W126 index values above 25 ppm-hrs. The Administrator noted that 
such a magnitude of W126 index (either as a 3-year average or in a 
single year) is not seen to occur at monitoring locations in or near 
Class I areas where the current standard is met (and such a W126 index, 
in a single year, has occurred only once in 19 years of monitoring data 
at sites across the U.S.), and that values above 17 or 19 ppm-hrs are 
rare (PA, Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3; 85 
FR 49911, August 14, 2020). The Administrator further took note of the 
PA consideration of the USFS publications that identify an influence of 
peak concentrations on BI scores (beyond an influence of cumulative 
exposure) and the PA observation of the appreciable control of peak 
concentrations exerted by the form and averaging time of the current 
standard, as evidenced by the air quality analyses which document 
reductions in 1-hour daily maximum concentrations with declining design 
values. Based on these considerations, the Administrator agreed with 
the PA finding that the current standard provides control of air 
quality conditions that contribute to increased BI scores and to scores 
of a magnitude indicative of ``moderate to severe'' foliar injury. 
Based on his consideration of PA findings that areas that meet the 
current standard are unlikely to have BI scores reasonably considered 
to be impacts of public welfare significance, the Administrator further 
proposed to conclude that the current standard provides sufficient 
protection of natural areas, including particularly protected areas 
such as Class I areas, from O3 concentrations in the ambient 
air that might be expected to elicit visible foliar injury of such an 
incidence and severity as would reasonably be judged adverse to the 
public welfare.
    With regard to the welfare effects of reduced plant growth or 
yield, the Administrator recognized that the evidence base continues to 
indicate growth-related effects as sensitive welfare effects, with the 
potential for ecosystem-scale ramifications. While recognizing 
associated uncertainties, the Administrator took note of the PA 
conclusion and CASAC advice that the approach taken in the last review 
of using estimates of O3 impacts on tree seedling growth (in 
terms of RBL) as a surrogate for comparable information on other 
species and lifestages, as well as a proxy or surrogate for other 
vegetation-related effects, including larger-scale effects, continues 
to appear to be a reasonable judgment in the current review (85 FR 
49910, August 14, 2020; PA, section 4.5.3). These estimates were 
medians based on the established E-R functions for 11 tree species. In 
light of this and the lack of an alternative metric or approach being 
indicated by the current evidence, the Administrator found it 
appropriate to adopt this approach in the current review.
    The Administrator additionally took note of considerations in the 
PA regarding aspects of the derivation of the tree seedling E-R 
functions that he found informative to his consideration of issues 
discussed in the court's remand of the 2015 secondary standard with 
respect to use of a 3-year average W126. In this context, the 
Administrator considered whether aspects of this evidence support 
making judgments using the E-R functions with W126 index derived as an 
average across multiple years. He noted that such averaging would have 
some conceptual similarity to the assumptions underlying the adjustment 
made to develop seasonal W126 E-R functions from exposures that 
extended over multiple seasons (or less than a single season).\202\ The 
Administrator also noted uncertainties in regard to estimated RBL at 
lower cumulative exposure levels, given the more limited data and fewer 
findings of statistical significance supporting the functions at the 
relatively lower cumulative exposure levels most commonly

[[Page 87321]]

associated with the current standard (e.g., at or below 17 ppm-hrs). 
The Administrator additionally took note of the PA summary of different 
comparisons that had been performed in the 2013 ISA and the current ISA 
of RBL estimated via the aspen E-R function using either a cumulative 
average multi-year W126 index (2013 ISA) or a single-year W126 index 
(current ISA) with RBL estimates derived directly from aspen growth 
information in a multi-year O3 exposure study. In this 
context, he noted the PA finding that consideration of these two 
different comparisons illustrate the variability inherent in the 
magnitude of growth impacts of O3 and in the quantitative 
relationship of O3 exposure and RBL,\203\ while also 
providing general agreement of predictions (based on either metric) 
with observations. In light of these considerations, the Administrator 
recognized that such factors as identified in the proposal, including 
the currently available evidence and its recognized limitations, 
variability and uncertainties, support a conclusion that it is 
reasonable to use a seasonal RBL averaged over multiple years, such as 
a 3-year average (85 FR 49910, August 14, 2020). The Administrator 
additionally took note of the CASAC advice reaffirming the EPA's focus 
on a 3-year average W126, concluding such a focus to be reasonable and 
scientifically sound. In light of these considerations, the 
Administrator found there to be support for use of an average seasonal 
W126 index derived from multiple years (with their representation of 
variability in environmental factors), concluding the use of such 
averaging to provide an appropriate representation of the evidence and 
attention to considerations summarized above. In so doing, he found 
that a reliance on single year W126 estimates for reaching judgments 
with regard to magnitude of O3-related RBL and associated 
judgments of public welfare protection would ascribe a greater 
specificity and certainty to such estimates than supported by the 
current evidence. Thus, he proposed to conclude that it is appropriate 
to use a seasonal W126 averaged over a 3-year period, which is the 
design value period for the current standard, to estimate median RBL 
using the established E-R functions for purposes in this review of 
considering the public welfare protection provided by the standard.
---------------------------------------------------------------------------

    \202\ The E-R functions for the 11 species were derived in terms 
of a seasonal W126 index from experiments that varied in duration 
from less than three months to many more. Underlying the adjustments 
made to derive the functions for a 3-month season duration are 
simplifying assumptions of uniform W126 distribution over the 
exposure period and linear relationship between cumulative exposure 
duration and response. Averaging of seasonal W126 across three 
years, with its reduction of the influence of annual variations in 
seasonal W126, would give less influence to RBL estimates derived 
from such potentially variable representations of W126, thus 
providing an estimate of W126 considered more suitably paired with 
the E-R functions.
    \203\ For example, there is variability associated with tree 
growth in the natural environment (e.g., related to variability in 
plant, soil, meteorological and other factors), as well as 
variability associated with plant responses to O3 
exposures in the natural environment (85 FR 49910, August 14, 2020).
---------------------------------------------------------------------------

    In reaching his proposed conclusions and judgments related to the 
use of RBL as a surrogate for the broad array of vegetation-related 
effects, the Administrator recognized a number of important public 
welfare policy judgments. The Administrator proposed to conclude that 
the current evidence base and available information (qualitative and 
quantitative) continue to support consideration of the potential for 
O3-related vegetation impacts in terms of the RBL estimates 
from established E-R functions as a quantitative tool within a larger 
framework of considerations pertaining to the public welfare 
significance of O3 effects. He judged the framework to 
include consideration of effects that are associated with effects on 
vegetation, and particularly those that conceptually relate to growth, 
and that are causally or likely causally related to O3 in 
ambient air, yet for which there are greater uncertainties affecting 
estimates of impacts on public welfare. In his consideration of the 
adequacy of protection provided by the current standard, the 
Administrator also noted judgments of the prior Administrator in 
considering the public welfare significance of small magnitude 
estimates of RBL and associated unquantified potential for larger-scale 
related effects. In light of CASAC advice and based on the current 
evidence as evaluated in the PA, the Administrator proposed to conclude 
that the approach or framework initially described with the 2015 
decision, with its focus on controlling air quality such that 
cumulative exposures at or above 19 ppm-hrs, in terms of a 3-year 
average W126 index, are isolated and rare, is appropriate for a 
secondary standard that provides the requisite public welfare 
protection and proposed to use such an approach in this review (85 FR 
49911, August 14, 2020).
    With this approach and protection target in mind, the Administrator 
considered the analyses of air quality at sites across the U.S., 
particularly including those sites in or near Class I areas. In 
virtually all design value periods and all locations at which the 
current standard was met (i.e., in more than 99.9% of such instances) 
across the 19 years of the data analyzed, the 3-year average W126 
metric was at or below 17 ppm-hrs. Further, in all such design value 
periods and locations the 3-year average W126 index was at or below 19 
ppm-hrs. The Administrator additionally considered the protection 
provided by the current standard from the occurrence of O3 
exposures within a single year with potentially damaging consequences, 
such as a significantly increased incidence of areas with visible 
foliar injury that might be judged moderate to severe. In so doing, he 
noted the PA findings that incidence of sites with BI scores above 15 
(termed ``moderate to severe injury'' by the USFS categorization 
scheme) markedly increases with W126 index estimates above 25 ppm-hrs, 
and the scarcity of single-year W126 index values above 25 ppm-hrs at 
sites that meet the current standard, with just a single occurrence 
across all U.S. sites with design values meeting the current standard 
in the 19-year historical dataset dating back to 2000 (PA, section 4.4 
and Appendix 4D). In light of the evidence indicating that peak short-
term concentrations (e.g., of durations as short as one hour) may also 
play a role in the occurrence of visible foliar injury, the 
Administrator additionally recognized the control of peak 1-hour 
concentrations provided by the form and averaging time of the current 
standard and noted there to be less than one day per site with a 
maximum hourly concentration at or above 100 ppb (PA, Appendix 2A, 
section 2A.2). In consideration of these findings, the Administrator 
proposed to judge that the current standard provides adequate 
protection from air quality conditions with the potential to be adverse 
to the public welfare (85 FR 49912, August 14, 2020).
    In reaching his proposed decision, the Administrator gave primary 
attention to the principal effects of O3 as recognized in 
the current ISA, the 2013 ISA and past AQCDs, and for which the 
evidence is strongest (e.g., growth, reproduction, and related larger-
scale effects, as well as visible foliar injury). With respect to the 
currently available information related to O3-related 
visible foliar injury, the Administrator considered air quality 
analyses that may be informative with regard to air quality conditions 
associated with appreciably increased incidence and severity of BI 
scores at USFS biomonitoring sites, noting that this information does 
not indicate a potential for public welfare impacts of concern under 
air quality conditions that meet the current standard. In light of 
these and other considerations discussed more completely in the 
proposal, and with particular attention to Class I and other areas 
afforded special protection, the Administrator proposed to conclude 
that the evidence regarding visible foliar injury and air quality in 
areas meeting the current standard indicates that the current standard 
provides adequate protection for this effect.

[[Page 87322]]

    The Administrator additionally considered O3 effects on 
crop yield, taking note of the long-standing evidence, qualitative and 
quantitative, of the reducing effect of O3 on the yield of 
many crops, as summarized in the PA and current ISA and characterized 
in detail in past reviews (e.g., 2013 ISA, 2006 AQCD, 1997 AQCD, 2014 
WREA). In so doing, he recognized that not every effect on crop yield 
will be adverse to public welfare and in the case of crop yield effects 
in particular there are a number of complexities related to the heavy 
management of many crops to obtain a particular output for commercial 
purposes, and to other factors, that contribute uncertainty to 
predictions of potential O3-related public welfare impacts, 
as summarized in sections III.B.2 and III.D.1 of the proposal (PA, 
sections 4.5.1.3 and 4.5.3). Thus, in judging the extent to which the 
median RYL estimated for the W126 index values generally occurring in 
areas meeting the current standard would be expected to be of public 
welfare significance, he recognized the potential for a much larger 
influence of extensive management of such crops, and also considered 
other factors recognized in the PA and proposal, including similarities 
in median estimates of RYL and RBL (PA, sections 4.5.1.3 and 4.5.3). 
With this context, the information for crop yield effects did not lead 
the Administrator to identify this endpoint as requiring separate 
consideration or to provide a more appropriate focus for the standard 
than RBL, in its role as a proxy or surrogate for the broader array of 
vegetation-related effects, as discussed above. Rather, in light of 
these considerations, he proposed to judge that a decision based on RBL 
as a proxy for other vegetation-related effects will provide adequate 
protection against crop related effects. In light of the current 
information and considerations discussed more completely in the 
proposal, the Administrator further proposed to conclude that the 
evidence regarding RBL, and its use as a proxy or surrogate for the 
broader array of vegetation-related effects, in combination with air 
quality in areas meeting the current standard, provide adequate 
protection for these effects (85 FR 49912, August 14, 2020).
    In reaching his proposed conclusion on the current standard, the 
Administrator also considered the extent to which the current 
information may provide support for an alternative standard, proposing 
to conclude that the appreciably greater occurrence of higher levels of 
cumulative exposure, in terms of the W126 index, as well as an 
appreciably greater occurrence of peak concentrations (both hourly and 
8-hour average concentrations) in areas that do not meet the current 
standard (e.g., areas meeting a higher standard level), would not 
provide the appropriate protection of public welfare in light of the 
potential for adverse effects on the public welfare. The Administrator 
also considered an alternative based solely on the W126 metric, as was 
considered in the last review, based on such a concentration-weighted, 
cumulative exposure metric having been identified as quantifying 
exposure in a way that relates to reduced plant growth (ISA, Appendix 
8, section 8.13.1). While recognizing a role for W126 index in 
quantifying exposure to develop estimates of RBL that the Administrator 
considers appropriately used as a proxy or surrogate for the broader 
array of vegetation-related effects, he notes that the evidence 
indicates there to be aspects of O3 air quality not captured 
by measures of cumulative exposure like W126 index that may pose a risk 
of harm to the public welfare (e.g., risk of visible foliar injury 
related to peak concentrations). Thus, in light of the information 
available in this review, the Administrator proposed to conclude that 
such an alternative standard in terms of a W126 index would be less 
likely to provide sufficient protection against such occurrences and 
accordingly would not provide the requisite control of aspects of air 
quality that pose risk to the public welfare.
    In summary, the Administrator recognized that his proposed decision 
on the public welfare protection afforded by the current secondary 
O3 standard from identified O3-related welfare 
effects, and from their potential to present adverse effects to the 
public welfare, is based in part on judgments regarding uncertainties 
and limitations in the available information, such as those identified 
above. In this context, he considered what the available evidence and 
quantitative information indicated with regard to the protection 
provided from the array of O3 welfare effects, finding it to 
not indicate the current standard to allow air quality conditions with 
implications of concern for the public welfare. He additionally took 
note of the advice from the CASAC in this review. Based on all of the 
above considerations, described in more detail in the proposal, 
including his consideration of the currently available evidence and 
quantitative exposure/risk information, the Administrator proposed to 
conclude that the current secondary standard provides the requisite 
protection against known or anticipated effects to the public welfare, 
and thus that the current standard should be retained, without 
revision.
3. Comments on the Proposed Decision
    Over 50,000 individuals and organizations indicated their views in 
public comments on the proposed decision. Most of these are associated 
with mass mail campaigns or petitions. Approximately 40 separate 
submissions were also received from individuals, and 75 from 
organizations and groups of organizations; 40 elected officials also 
submitted comments. Among the organizations commenting were state and 
local agencies and organizations of state agencies, organizations of 
health professionals and scientists, environmental and health 
protection advocacy organizations, industry organizations and 
regulatory policy-focused organizations. The comments on the proposed 
decision to retain the current secondary standard are addressed here. 
Those in support of the proposed decision are addressed in section 
III.B.2.a and those in disagreement are addressed in section III.B.2.b. 
Comments related to aspects of the process followed in this review of 
the O3 NAAQS (described in section I.D above), as well as 
comments related to other legal, procedural or administrative issues, 
and those related to issues not germane to this review are addressed in 
the separate Response to Comments document.
a. Comments in Support of Proposed Decision
    Of the comments supporting the Administrator's proposed decision to 
retain the current secondary standard without revision, all generally 
state that the record supports the proposed decision, and note the 
CASAC conclusion that the current evidence is generally consistent with 
that available in the last review, and the CASAC conclusion that the 
evidence does not call into question the adequacy of the current 
standard and should be retained. In support of their views, some of 
these commenters state that new evidence is lacking that might call 
into question the objective for the standard to generally protect 
against cumulative exposures associated with median RBL estimates above 
6%. They additionally state that the proposed decision appropriately 
addresses the Murray Energy remand issues. Further, these commenters 
conclude that the available evidence with regard to areas meeting the 
current standard does not call into question the adequacy of protection 
provided by the current standard from

[[Page 87323]]

the array of vegetation effects, including in Class I areas. Lastly, 
these commenters find the EPA's proposed judgments regarding the 
uncertainties associated with predicting responses of climate-related 
effects to changes in O3 concentrations across the U.S., as 
well as the limitations in the availability of tools for such purposes, 
to be appropriate and well supported. The EPA agrees with these 
comments.
    Some of these comments also express the view that welfare benefits 
of a more restrictive O3 standard are highly uncertain, 
while such a standard would likely cause socioeconomic impacts that the 
EPA should consider and find to outweigh the uncertain benefits. While 
as discussed in section III.B.3 below, the Administrator does not find 
a more stringent secondary standard requisite to protect the public 
welfare, he does not consider economic impacts of alternate standards 
in reaching this judgment. As summarized in section I.A. above, in 
setting primary and secondary standards that are ``requisite'' to 
protect public health and welfare, respectively, as provided in section 
109(b), the EPA may not consider the costs of implementing the 
standards. See generally Whitman v. American Trucking Ass'ns, 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.'' See American 
Petroleum Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); 
accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 
2019). Arguments such as the views on socioeconomic impacts expressed 
by these commenters have been rejected by the courts, as summarized in 
section I.A above, including in the recent Murray Energy decision, with 
the reasoning that consideration of such impacts was precluded by 
Whitman's holding that the ``plain text of the Act unambiguously bars 
cost considerations from the NAAQS-setting process'' (Murray Energy 
Corp. v. EPA, 936 F.3d at 621, quoting Whitman [531 U.S. at 471]).
b. Comments in Disagreement With Proposed Decision
    Among those submitting comments that disagreed with the proposed 
decision to retain the current secondary standard, or that raised 
concerns with the basis for the decision, most of these commenters 
expressed concerns regarding the process for reviewing the criteria and 
standards and state that the proposal must be withdrawn, and a new 
review conducted. Most of these commenters also disagree with the EPA's 
proposed conclusion that the current standard, with its current 
averaging time and form, provides the requisite public welfare 
protection from known or anticipated adverse public welfare effects 
associated with the array of O3-related effects, and 
generally state that the standard should be revised to be in terms of a 
single-year W126 index. Among the claims made in describing the basis 
for their view, these commenters claim that EPA failed to describe the 
basis for its proposed conclusion; to explain why a standard using the 
W126 index was not proposed, consistent with 2014 advice from the 
former CASAC, and to address the issues raised by court remand of the 
2015 standard. Some commenters expressing the view that the standard 
should be revised also express the view that an additional standard 
should be established to protect from O3 effects on climate.
    With regard to the process by which this review has been conducted, 
we disagree with the commenters that claim that it is arbitrary and 
capricious or that it does not comport with legislative requirements. 
The review process, summarized in section I.D, implemented a number of 
features, some of which have been employed in past reviews and others 
which have not, and several which represent efficiencies in 
consideration of the statutorily required time frame for completion of 
the review. The comments received that raise concerns regarding 
specific aspects of the process are addressed in the separate Response 
to Comments document. As indicated there, the EPA disagrees with these 
comments. The EPA finds the review to have been lawfully conducted and 
the process reasonably explained. Accordingly, the EPA is not 
withdrawing the proposal and restarting the review.
(i) Metric for Standard
    The premise of many of the comments expressing disagreement with 
the proposed decision is that the secondary standard must be a 
``biologically relevant'' metric, which they identify to be the W126 
index. Similarly, some commenters assert that EPA cannot lawfully or 
rationally set a secondary standard using the metric of the current 
standard, which is also the metric used for the primary standard, 
claiming that this contradicts EPA's recognition of the relevance of 
the W126 index as an exposure metric for assessing the level of 
protection from welfare effects, such as RBL. These commenters also 
claim that this approach arbitrarily disregards the recommendations of 
the prior CASAC, and, in doing so, imply that EPA must establish a W126 
based standard because of prior CASAC advice.
    We disagree with these commenters. The Clean Air Act includes no 
requirements with respect to what metrics should be used to establish 
the secondary standards. As is clear from the text of Section 109(b)(2) 
of the CAA, the critical test for NAAQS is whether they achieve the 
requisite protection. In so doing, it is not uncommon for the form and 
averaging time of a NAAQS to differ from exposure metrics most relevant 
to assessment of particular effects. These exposure metrics are based 
on the health or welfare effects evidence for the specific pollutant 
and commonly, in assessments for primary standards, on established 
exposure-response relationships or health-based benchmarks (doses or 
exposures of concern) for effects associated with specific exposure 
circumstances. Evidence for this is found in the common use, in 
assessments conducted for NAAQS reviews, of exposure metrics that 
differ in a variety of ways from the ambient air concentration metrics 
of those standards.\204\ Across reviews for the various NAAQS 
pollutants over the years, the EPA has used a variety of exposure 
metrics to evaluate the protection afforded by the standards (see 
examples identified at 80 FR 65399-65400, October 26, 2015). Further, a 
single standard may provide protection from multiple different effects, 
the protection for which may be assessed using different exposure 
metrics. One standard may also provide protection from multiple 
pathways of exposure. Both the primary and secondary Pb standards 
provide examples of this. While these standards are expressed in terms 
of the concentration of lead in particles suspended in air, different 
exposure metrics have been used to evaluate the protection provided by 
the Pb standards. The salient exposure metric for assessment of 
protection provided by the primary standard has been blood Pb, while 
for the secondary standard, concentrations of lead in soil, surface 
water and sediment are pertinent, and have been evaluated to assess the 
potential for welfare effects related to lead deposition from air (73 
FR 67009, November 12, 2008). In somewhat similar manner, the exposure 
metric used to evaluate health impacts in the primary sulfur dioxide 
standard review includes a 5-minute exposure

[[Page 87324]]

concentration. In contrast, the health-based standard for this 
pollutant is the average across three years of the 99th percentile of 
1-hour daily maximum concentration of sulfur dioxide in ambient air (75 
FR 35520, June 22, 2010; 84 FR 9866, March 18, 2019).
---------------------------------------------------------------------------

    \204\ The term design value, defined above, is used in this 
discussion to refer to the metric for the standard.
---------------------------------------------------------------------------

    We disagree with the comment that a secondary standard with the 
same form and averaging time as the primary standard does not comply 
with the CAA. The CAA does not require that the secondary standard be 
established in a specific form or averaging time. The Act, at Section 
109(b)(2), provides only that any secondary NAAQS ``shall specify a 
level of air quality the attainment and maintenance of which in the 
judgment of the Administrator, based on [the air quality] criteria, is 
requisite to protect the public welfare from any known or anticipated 
adverse effects associated with the presence of such air pollutant in 
the ambient air. . . . [S]econdary standards may be revised in the same 
manner as promulgated.'' The EPA interprets this provision to leave it 
considerable discretion to determine whether a particular form and 
averaging time are appropriate, in combination with the other aspects 
of the standard (level and indicator), for specifying the air quality 
that provides the requisite protection, and to determine whether, once 
a standard has been established in a particular form, that form must be 
revised. Moreover, nothing in the Act or the relevant case law 
precludes the EPA from establishing a secondary standard equivalent to 
the primary standard in some or all respects, as long as the Agency has 
engaged in reasoned decision-making.\205\
---------------------------------------------------------------------------

    \205\ In fact, the D.C. Circuit has upheld secondary NAAQS that 
were identical to the corresponding primary standard for the 
pollutant (e.g., ATA III, 283 F.3d at 375, 380 [D.C. Cir. 2002, 
upholding secondary standards for PM2.5 and O3 
that were identical to primary standards]).
---------------------------------------------------------------------------

    Thus, we note that particular metrics may logically, reasonably, 
and for technically or scientifically sound reasons, be used in 
assessing exposures of concern or characterizing risk. The purpose, and 
use, of exposure metrics is different from the purpose, and use, of 
metrics for the standard, and as a result the metrics may differ from 
one use to the other. Exposure metrics are used to assess the likely 
occurrence and/or frequency and extent of effects under different air 
quality conditions, while the air quality standards are intended to 
control air quality to the extent requisite to protect from the 
occurrence of public health or welfare effects judged to be adverse. In 
this review of the O3 secondary standard, the EPA agrees 
that based on evidence summarized in section III.A above, metrics such 
as the W126 index are appropriate for assessing exposures of concern 
for vegetation, characterizing risk to public welfare, and evaluating 
what air quality conditions might provide the appropriate degree of 
public welfare protection. We disagree, however, that the secondary 
standard must be established using those same metrics. Rather, when the 
Administrator judges that a standard using a different metric provides 
the requisite protection, in light of his consideration of all the 
elements of the standard together, he may reasonably establish or 
retain such a standard.
    With regard to the commenter's emphasis on recommendations from the 
CASAC on the form of the secondary standard, the EPA generally agrees 
with the importance of giving such recommendations careful 
consideration. However, it is not necessary for EPA to address in this 
review each statement a prior CASAC made in a prior review. In 
addition, if a recommendation of a prior CASAC is raised in a 
subsequent review (e.g., in public comments or as a focus in court 
decision being addressed), it is reasonable for the Agency to consider 
it in the context both of the current review and of consideration of 
all the other now available scientific, technical and policy-relevant 
information, including advice from the current CASAC. We note that in 
this review of the secondary standard, the current CASAC, based on its 
review of the information and analyses available in the current review, 
concurs with retention of a secondary standard with a metric that 
differs from commonly used vegetation exposure metrics, such as the 
W126 index (Cox, 2020a). We further note, under the relevant provisions 
of the CAA and case law interpreting them, the Administrator is never 
bound by the CASAC's conclusions but rather may depart from them when 
he has provided an explanation of the reasons for such 
differences.\206\ While the EPA does not interpret the requirements of 
CAA sections 307(d)(3) and 307(d)(6)(A) to apply to every 
recommendation it has received from a prior CASAC, even assuming there 
are some circumstances in which EPA were required to comply with the 
requirements of CAA section 307(d)(3) and (6)(A) with respect to 
particular recommendations from a prior CASAC, these same principles 
would apply. Thus, the Administrator would not be bound to follow those 
recommendations, but rather could depart from them when he had 
explained his reasons for doing so. Accordingly, in reaching 
conclusions on the revised secondary standard in this review, the 
Administrator has given careful consideration to the current CASAC 
advice in this review and to issues raised by the prior CASAC that are 
subject to the Murray Energy remand. When he has differed from those 
CASAC recommendations, the reasons and judgments that led to a 
different conclusion are explained, as summarized in this section and 
in section III.B.3 below. Consistent with his consideration of all 
significant issues raised in public comments, the Administrator has 
also considered the issues raised by commenters that have also been 
raised by a prior CASAC, together with the Agency's responses to those 
comments, as summarized in this section and in section III.B.3 below.
---------------------------------------------------------------------------

    \206\ See CAA sections 307(d)(3) and 307(d)(6)(A); see also 
Mississippi v. EPA, 744 F.3d 1334, 1354 (D.C. Cir. 2013) (``Although 
EPA is not bound by CASAC's recommendations, it must fully explain 
its reasons for any departure from them''); id. at 1358 (noting 
CASAC, like EPA, exercises both scientific judgment and public 
health policy judgment). Selection of a metric for the standard is a 
public health or public welfare policy judgment about what standards 
will control air quality to the extent judged requisite to protect 
from adverse public health or welfare effects.
---------------------------------------------------------------------------

    The current air quality analyses demonstrate the successfulness of 
the current form and averaging time in controlling cumulative 
exposures, in terms of W126. These extensive air quality analyses, 
presented in the PA and summarized in the proposal, are based on data 
collected across the U.S. over a time span of nearly 20 years (85 FR 
49892-49895, 49903-49904, August 14, 2020). One of these analyses 
describes the positive, linear relationship between long-term changes 
in the O3 design value and long-term changes in the W126 
index at monitoring sites across the U.S.\207\ This positive, linear 
relationship exists for the O3 design value with both a 3-
year average and single-year W126 index (PA, Appendix 4D, Figure 4D-
11). The existence of this relationship means that a change (e.g., 
reduction) in the design value at a monitoring site was generally 
accompanied by a similar change (e.g., reduction) in the W126 index, 
both in the 3-year average and in the single-year values. As the form 
and averaging time of the secondary standard have not changed since 
1997, the analyses performed have been able to assess the amount of 
control exerted by these aspects of the standard, in combination

[[Page 87325]]

with reductions in the level (i.e., from 80 ppb in 1997 to 75 ppb in 
2008 to 70 ppb in 2015) on cumulative seasonal exposures in terms of 
W126 index. The analyses have found that the reductions in design 
value, presumably associated with implementation of the revised 
standards, have been accompanied by reductions in cumulative seasonal 
exposures in terms of W126 index (PA, section 4.4.1). Further, while 
the formulation of the W126 metric gives more weight to higher 
concentrations (in the context of its focus on cumulative exposure), it 
is much less effective at curbing elevated hourly concentrations (that 
can be important in altering plant growth and yield) than the current 
design value metric, as discussed in section III.B.2.b(ii) below.
---------------------------------------------------------------------------

    \207\ This analysis focuses on the relationship between changes 
(at each monitoring site) in the 3-year design value across the 17 
design value periods from 2000-2002 to 2016-2018 and changes in the 
W126 index over the same period (PA, Appendix 4D, section 4D.3.2.3).
---------------------------------------------------------------------------

    In expressing the view that the secondary standard should be in 
terms of a W126 index, some commenters describe the EPA's statements 
regarding the protection from cumulative exposures that is provided by 
the current form and averaging time to be ``incidental'' and 
``happenstance,'' which leads them to claim the EPA's findings of 
protection to be arbitrary. In support of their view, the commenters 
quote a statement of the prior CASAC cautioning against interpreting 
the W126 index levels in the W126 index scenario created for the 2014 
WREA, by first adjusting air quality to meet the then-existing fourth 
maximum standard of 75 ppb, to be representative of implementation of a 
W126 index standard. The issue described by the prior CASAC related to 
the application to all monitoring sites of the precursor reduction 
necessary for the highest monitoring site in a region to just meet the 
scenario target; the prior CASAC's concern was that actual 
implementation of the target as a standard would not necessarily yield 
such reductions. We disagree with the commenters that this is relevant 
to the air quality analysis in the current review, in which we simply 
observe the W126 index values that exist in reality at sites that have 
met the existing secondary standard. Contrary to the context for the 
prior CASAC's caution, the analysis in the current review is not 
showing the results of a theoretical scenario created by modeling 
theoretical precursor reductions estimated for attaining a particular 
W126-based or fourth high standard. Rather, we are observing what the 
W126-based cumulative exposure is at ambient air monitoring sites that 
meet the current secondary standard. Thus, regardless of the labels 
assigned by the commenter to the findings of the air quality analyses 
in the current review, these analyses clearly document the success of 
the existing standard (with its fourth maximum form and 8-hour 
averaging time) in controlling exposure in terms of the W126 index.
    Thus, in light of this evidence, the EPA disagrees with the 
commenters who express the view that to provide the requisite 
protection the secondary standard must be a W126 index standard. In 
assessing the air quality necessary to provide the requisite degree of 
protection, particularly for growth and related vegetation and 
ecosystem effects, the Agency has recognized the importance of 
cumulative exposures, but also the significance of higher peak 
exposures (as summarized in section III.B.2.b(ii) below) that can be 
characterized through other metrics (e.g., N100). As a result, in 
assessing the protection provided by the current standard, the Agency 
has focused on the W126 index, expressed in terms of the average of 
three consecutive years (in light of considerations discussed below), 
as a metric for cumulative exposure, but has also considered the 
frequency and magnitude of elevated single-year W126 index values, and 
of elevated hourly O3 concentrations (as discussed further 
below).
(ii) Protection Against Unusually Damaging Years
    In the last review, the Administrator relied on the 70 ppb standard 
(as the fourth highest daily maximum 8-hour average concentration 
averaged over three consecutive years) to achieve a level of air 
quality that would restrict cumulative seasonal exposures to 17 ppm-hrs 
or lower, in terms of a 3-year average W126 value, in nearly all 
instances. The Murray Energy court found in relevant part that the EPA 
had not explained why that level of protection was requisite, in light 
of certain comments from the CASAC in 2014 recommending that EPA base a 
standard on a one-year W126 metric, in part to limit exposures in 
single unusually damaging years.\208\ In responding to the remand,\209\ 
we are explaining in this document that the EPA is looking to prevent 
the damaging effects of O3 on tree growth as a proxy for 
public welfare effects related to the broad array of O3's 
vegetation-related effects conceptually related to growth effects, 
including ecosystem-level effects (as discussed in section III.B.2.b(v) 
below). In this review, in assessing the air quality requisite to 
prevent adverse effects on public welfare from these effects, the EPA 
is not relying solely on maintaining a particular 3-year W126 value. 
Rather, we are considering air quality patterns that are associated 
with meeting the current standard, including control of peak hourly 
concentrations, and the exposures that would be expected under the 
current standard, including in terms of W126 values, particularly those 
averaged over a 3-year period. The EPA is explaining the grounds for 
our conclusion that use of the 3-year average W126 index is a 
reasonable basis for assessing protection from RBL, but also that the 
Administrator is using other exposure information in reaching the 
conclusion that retention of the existing standard (with its form and 
averaging time of the fourth highest annual daily maximum 8-hour 
average concentration, averaged over three years) provides the needed 
protection of RBL, including from what the Murray Energy court noted 
that the prior CASAC termed ``unusually damaging years.''
---------------------------------------------------------------------------

    \208\ The prior CASAC comments on this matter were in the 
context of its recommendation for a secondary standard in the form 
of a single-year W126 index, which as discussed below would be 
expected to provide relatively less control against high-
concentration years compared with the current secondary standard. 
The prior CASAC additionally commented that it ``favor[ed] a single-
year period'' which it stated would ``provide more protection for 
annual crops and for the anticipated cumulative effects on perennial 
species.'' The prior CASAC continued on to state that if the 
Administrator preferred, instead, to establish a secondary standard 
as a 3-year average W126 index, as a policy matter, the level should 
be revised downward (Frey, 2014b, p. iii). The prior CASAC stated 
the purpose for this step would be to be protecting ``against single 
unusually damaging years that will be obscured in the average'' 
(Frey, 2014b, p. 13).
    \209\ The Agency intends this decision, associated analyses 
conducted for this review in consideration of issues raised by the 
court's remand, and the discussions herein to constitute its 
response to the Murray Energy remand on this issue.
---------------------------------------------------------------------------

    In disagreeing with the EPA's proposed decision, some commenters 
object to the EPA's use of a 3-year average W126 index in assessing 
different patterns of air quality using median tree seedling RBL as a 
surrogate for an array of vegetation-related effects, particularly 
those related to growth and productivity. In so doing, these commenters 
variously claim that this use of a 3-year average W126 index (rather 
than a single-year W126 index) is inconsistent with recommendations 
from the prior CASAC, does not address the court remand on this point, 
and that it is inadequate to protect vegetation from high years or 
years with hourly O3 concentrations that can be most 
important in eliciting adverse effects.
    The EPA disagrees with these commenters and notes that it has taken 
such concerns, as well as the court's remand, into account in the final 
decision. In evaluating the air quality

[[Page 87326]]

conditions allowed by the current standard, the EPA has focused on the 
W126 metric averaged over 3 years as the most appropriate measure of 
cumulative exposure for consideration of adverse effects on public 
welfare, but EPA has also considered other relevant exposure 
information, including higher exposures that might be expected to occur 
in an ``unusually damaging year.'' The Administrator's decision on the 
adequacy of protection provided by the current standards is based on 
the full scope of exposure information he has considered.
    The EPA concludes that the 3-year average W126 index is a 
reasonable metric for assessing the level of protection provided by the 
current standard from cumulative seasonal exposures related to RBL, 
while noting that our evaluation for the protection provided by the 
current standard has also been informed by our consideration of other 
metrics (as described further below). In reaching this conclusion, we 
have taken into account the available evidence base and air quality 
analyses, with a focus on two types of considerations, as well as 
consideration of the context for RBL as a proxy for an array of other 
vegetation effects (discussed in section III.B.2.b(v) below). The first 
of the two consideration types concerns the E-R functions and their use 
with a 3-year average W126 index, and the second concerns the control 
by the W126 index metric of exposures that might be termed ``unusually 
damaging.'' With regard to the first, we find our use of the 3-year 
average W126 index appropriate in light of the approach used in 
deriving the E-R functions from the underlying data (from exposures of 
varying durations, including of multiple years), and the evidence 
available for evaluating these functions across multiyear 
exposures.\210\ Additionally, with regard to the second consideration, 
we recognize limitations associated with a reliance solely on W126 
index as a metric to control exposures that might be termed ``unusually 
damaging.'' For example, two different air quality patterns for which 
the associated W126 index is the same may have very different incidence 
of elevated O3 concentrations, and accordingly pose 
different risks to vegetation. As discussed below, however, the 
occurrence of such concentrations (and any associated risk of damage) 
are controlled by the current secondary standard.
---------------------------------------------------------------------------

    \210\ Additionally, as described in section III.B.1.c above and 
III.B.2.b(v) below, the EPA's identification of 17 ppm-hrs for a 
target W126 index of 17 ppm-hrs (e.g., versus 18 ppm-hrs) was in 
consideration of the prior CASAC recommendation for considering a 
``lower'' level ppm-hrs.
---------------------------------------------------------------------------

    In light of this evidence, and recognizing the role for both peak 
and cumulative exposures in eliciting growth and related vegetation and 
ecosystem effects, the EPA concludes that focusing solely on W126 index 
(either in terms of a single year or 3-year average) in considering the 
public welfare protection provided by the current standard would not be 
considering all the relevant scientific information. To the extent that 
the prior CASAC advised that the EPA should focus solely on single-year 
W126 index values in evaluating the protection provided by the 
secondary standard, the EPA disagrees that this would provide the 
needed protection, for the reasons explained more fully below. In this 
regard, we additionally note that the current CASAC concluded that 
focusing on three-year average W126 index values in considering the 
public welfare protection offered by the secondary standard ``appears 
of reasonable thought and scientifically sound'' (Cox, 2020a, p. 19).
    With regard to the established tree seedling E-R functions, we note 
there are aspects of the datasets and methodology on which the E-R 
functions are based which provide support for a 3-year average 
approach. As summarized in section III.A.2.c(i) above, in deriving the 
E-R functions from studies of durations that varied from shorter than 
90 days to multiple years or growing seasons, the results were 
normalized to the duration of a single 90-day seasonal period (PA, 
section 4.5.1.2 and Appendix 4A, pp. 4A-28 to 4A-29 and footnote 17). 
Inherent in this approach is an assumption that the growth impacts 
relate generally to the cumulative O3 exposure across the 
multiple growing seasons, i.e., with little additional influence 
related to any year to year differences in the exposures. As discussed 
in the proposal, the use of a 3-year average in assessing RBL using the 
established tree seedling E-R functions is compatible with the 
normalization step taken to derive functions for a seasonal 90-day 
period from the underlying data with its varying exposure durations (85 
FR 49901, August 14, 2020).
    This concept of the importance of cumulative multiyear 
O3 exposure to multiyear impacts, and its representation as 
an average, is also reflected in the evaluation of the predicted growth 
impacts compared to observations from the multiyear study of 
O3 impacts on aspen by King et al (2005), as presented in 
the 2013 and 2020 ISAs and summarized in the PA (PA, Section 4.5.1.2). 
The ISAs considered the 6-year experimental dataset of O3 
exposures and aspen growth effects with regard to correspondence of E-R 
function predictions with study observations (2020 ISA, Appendix 8, 
section 8.13.2 and Figure 8-17; 2013 ISA, section 9.6.3.2, Table 9-15, 
Figure 9-20). The analysis in the 2013 ISA compared observed reductions 
in growth for each of the six years to those predicted by applying the 
established E-R function for Aspen to cumulative multi-year average 
W126 index values (2013 ISA, section 9.6.3.2).211 212 The 
evaluation in the 2020 ISA applied the E-R functions to the single-year 
W126 index for each year rather than the cumulative multi-year W126 
(2020 ISA, Appendix 8, Figure 8-17), with this approach indicating a 
somewhat less tight fit to the experimental observations (2020 ISA, 
Appendix 8, p. 8-192),\213\ Both ISAs reach similar conclusions 
regarding general support for the E-R functions across a multiyear 
study of trees in naturalistic settings (ISA, Appendix 8, section 
8.13.3 and p. 8-192; 2013 ISA, p. 9-135).
---------------------------------------------------------------------------

    \211\ For example, the growth impact estimate for year 1 used 
the W126 index for year 1; the estimate for year 2 used the average 
of W126 index in year 1 and W126 index in year 2; the estimate for 
year 3 used the average of W126 index in years 1, 2 and 3; and so 
on.
    \212\ One finding of this evaluation was that ``the function 
based on one year of growth was shown to be applicable to subsequent 
years'' (2013 ISA, p. 9-135).
    \213\ Based on information drawn from Figure 8-17 in the 2020 
ISA, the correlation metric (r\2\) for the percent difference 
(estimated vs observed biomass) and year of growth can be estimated 
to be approximately 0.7, while using values reported in Table 9-15 
of the 2013 ISA (which are plotted in Figure 9-20), the r\2\ for 
predicted O3 impact versus observed impact is 0.99 and 
for the percent difference versus year is approximately 0.85.
---------------------------------------------------------------------------

    Based on all of the above considerations, the EPA finds the 
evidence to support a 3-year average W126 index for use in assessing 
the level of protection provided by the current standard from 
cumulative seasonal exposures related to RBL of concern based on the 
established E-R functions. As discussed in section III.B.3 below, the 
EPA additionally finds the 3-year average metric to be reasonable in 
the context of the use of RBL as a proxy to represent an array of 
vegetation-related effects. In the discussion immediately below, we 
additionally and specifically address the issue of protection from 
``unusually damaging years'' of vegetation exposure.
    With regard to the comment that cited a recommendation from the 
prior CASAC on protection of vegetation

[[Page 87327]]

against ``unusually damaging years'' and the part of the court remand 
referencing that CASAC recommendation, we have considered the CASAC 
discussion using this term, in the context of the court remand. Use of 
this term by the prior CASAC occurs in the 2014 letter on the second 
draft PA in the 2015 review (Frey, 2014b). Most prominently, the prior 
CASAC defined as damage ``injury effects that reach sufficient 
magnitude as to reduce or impair the intended use or value of the plant 
to the public, and thus are adverse to public welfare'' (Frey, 2014b, 
p. 9). The prior CASAC additionally provided advice with regard to 
surrogate metrics for judging such ``damage,'' e.g., use of RBL for 
judging effects on trees and their related functions and ecosystem 
services, use of crop RYL for judging public welfare effects of crop 
effects (Frey, 2014b, p. 10). We also note that the context for the 
prior CASAC's use of the phrase ``unusually damaging years'' is in 
considering the form and averaging time for a revised secondary 
standard in terms of a W126 index (Frey, 2014b, p. 13), which as 
discussed below is relatively less controlling of high-concentration 
years, rather than in the context of the current secondary standard and 
its fourth highest daily maximum 8-hour metric.
    While the prior CASAC did not provide any specificity or details as 
to the exposure circumstances and damage intended by its more general 
phrasing, nor did it cite to specific evidence in scientific 
publications, we agree with the general concept that particular air 
quality patterns in a year may pose particular risk of vegetation 
damage, in terms of both or either growth-related effects or visible 
foliar injury (discussed in section III.B.2(iii) below). Across past 
O3 NAAQS reviews, the air quality criteria for vegetation 
effects have emphasized the risk posed to vegetation from higher hourly 
average O3 concentrations (e.g., ``[h]igher concentrations 
appear to be more important than lower concentrations in eliciting a 
response'' [ISA, p. 8-180]; ``higher hourly concentrations have greater 
effects on vegetation than lower concentrations'' [2013 ISA, p. 91-4] 
``studies published since the 2006 O3 AQCD do not change 
earlier conclusions, including the importance of peak concentrations, . 
. . in altering plant growth and yield'' [2013 ISA, p. 9-117]). In 
fact, the EPA has recognized the W126 index for E-R models for growth 
and yield (in the current and prior ISA and prior AQCD) in part due to 
its preferential weighting of higher concentrations (ISA, p. 8-130).
    We note, however, that while the W126 index weights higher hourly 
concentrations, it cannot, given its definition as an index that sums 
three months of weighted hourly concentrations into a single value, 
always differentiate between air quality patterns with high peak 
concentrations and those without such concentrations. This is 
illustrated by the following two hypothetical examples. In the first 
example, two air quality monitors have a similar pattern of generally 
lower average hourly concentrations, but differ in the occurrence of 
higher concentrations (e.g., hourly concentrations at or above 100 
ppb). The W126 index describing these two monitors would differ. In the 
second example, one monitor has appreciably more hourly concentrations 
above 100 ppb compared to a second monitor; but the second monitor has 
higher average hourly concentrations than the first. In the second 
example, the two monitors may have the same W126 index, even though the 
air quality patterns observed at those monitors are quite different, 
particularly with regard to the higher concentrations, which have been 
recognized to be important in eliciting responses (as noted above).
    Thus, the EPA disagrees with a view implied by many of the 
commenters (who object to the EPA's proposed decision) that the sole 
focus for assessing public welfare protection, related to vegetation 
damage, and air quality control provided by the secondary standard 
should be on the W126 index. This view ignores both the limitations of 
the W126 index itself in distinguishing among different patterns of 
hourly O3 concentrations and the fact that the current 
secondary standard has, by virtue of its form, a metric that does. With 
regard to these limitations of the W126 index, as described above, two 
different locations or years may have different patterns of hourly 
concentrations but the same W126 index value. This was recognized in 
the study by Lefohn et al. (1997), which observed the appreciable 
differences between the prevalence of hourly concentrations at or above 
100 ppb in exposures on which the E-R functions are based and those 
common in ambient air.\214\
---------------------------------------------------------------------------

    \214\ For example, many of the experimental exposures of 
elevated O3 on which the established E-R functions for 
the 11 tree seedling species are based, had hundreds of hours of 
O3 concentrations above 100 ppb, far more than are common 
in (unadjusted) ambient air, including in areas that meet the 
current standard (Lefohn et al. 1997; PA, Appendix 2A, section 2A.2; 
Wells, 2020). Similarly, the experimental exposures in studies 
supporting some of the established E-R functions for 10 crop species 
also include many hours with hourly O3 concentrations at 
or above 100 ppb (Lefohn and Foley, 1992).
---------------------------------------------------------------------------

    This potential for such a difference in peak concentrations between 
two different locations with the same W126 index was noted by one 
commenter who objected to the EPA's focus on a 3-year average W126 
index in assessing RBL and advocated use of a single-year W126 index. 
This commenter stated that the same 3-year average could be maintained 
in two different locations in which the annual exposure may differ due 
to ``variability of the higher hourly average concentrations associated 
with vegetation effects.'' In emphasizing the higher hourly average 
concentrations associated with effects, the commenter cited the support 
provided by the evidence for the San Bernardino National Forest, 
described in the 2013 ISA and prior CDs (e.g., 2013 ISA, section 
9.5.3.1). We agree with this point and additionally note that this 
point also applies to two locations with the same single-year W126 
index, given its definition (as noted above).
    Given the mathematics inherent in calculation of the W126 index, 
while the metric is useful for comparing cumulative exposures, it can 
conceal peak concentrations that can be of concern (as described 
above). More specifically, one year or location could have few, or even 
no, hourly concentrations above 100 ppb \215\ and the second could have 
many such concentrations; yet each of the two years or locations could 
have the identical W126 index (e.g., equal to 25 or 17 or 10 ppm-hrs, 
or some other value). However, as can be seen by the historical ambient 
air monitoring dataset of O3 concentrations, the form of the 
current standard limits the occurrence of such elevated concentrations, 
e.g., at or above 100 ppb (PA, Appendix 2A, section 2A.2; Wells, 2020).
---------------------------------------------------------------------------

    \215\ The value of 100 ppb is used here as it has been in some 
studies focused on O3 effects on vegetation, simply as an 
indicator of elevated or peak hourly O3 concentrations 
(e.g., Lefohn et al. 1997, Smith, 2012; Davis and Orendovici, 2006; 
Kohut, 2007a). Values of 95 ppb and 110 ppb have also been 
considered in this way (2013 ISA, section 9.5.3.1).
---------------------------------------------------------------------------

    Analyses of hourly concentrations for different air quality 
scenarios developed in consideration of the remand and such comments 
(and documented in a technical memorandum to the docket) show the form 
and averaging time of the existing standard to be much more effective 
than the W126 index in limiting the number of hours with O3 
concentrations at or above 100 ppb (N100) and in limiting the number of 
days with any such hours (Wells,

[[Page 87328]]

2020).\216\ For example, during the recent design value period (2016-
2018), across all sites that met the current standard, few sites had 
any hours at or above 100 ppb in a year (6% in the highest year, Wells, 
2020, Table 2).\217\ Among the sites with any such hours, the vast 
majority had fewer than five such hours (99.5% in the highest year, 
Wells, 2020, Table 2), with none having more than ten such hours,\218\ 
and no site having more than three days in any one year with any such 
concentrations (Wells, 2020, Figures 4 and 5). In comparison, sites 
with an annual W126 index below 15 ppm-hrs recorded nearly 40 hourly 
concentrations at or above 100 ppb, and as many as seven days with such 
a concentration (Wells, 2020, e.g., Figures 10 and 11).\219\ A similar 
pattern is seen using the historical dataset extending back to 2000. 
This historical dataset also shows the appreciable reductions in peak 
concentrations (via either the N100 or D100 metric) that have been 
achieved in the U.S. as air quality has improved under O3 
standards of the existing form and averaging time (Wells, 2020, Figures 
12 and 13). Thus, based on the findings of both the analyses in the PA 
(PA, Appendix 2A) and the additional analyses (Wells, 2020), the EPA 
disagrees with the commenter that the proposed decision ignores the 
importance of elevated hourly O3 concentrations in eliciting 
effects on vegetation. Rather, the proposed decision, and final 
decision to retain the existing standard, which controls peak 
concentrations and also cumulative seasonal exposure in terms of W126 
index, explicitly considers this importance and address it in a way 
that is more effective than a standard expressed in terms of the W126 
index would be, even based on a single-year W126 well below 17 ppm-hrs 
(as shown in the additional air quality analyses [Wells, 2020]).
---------------------------------------------------------------------------

    \216\ The impact of the current form of the standard on 
occurrence of elevated hourly concentrations is also seen by a 
recent study submitted with comments (Neufeld et al., 2019). For 
example, the frequency of episodes defined by three consecutive 
hours at or above 60 ppb, as well as the magnitude of W126 index, 
has appreciably declined at locations within and immediately 
adjacent to the Smoky Mountains National Park, and the periods of 
respite from elevated episodes has appreciably increased (Neufeld et 
al., 2019). This was found for low elevation sites, and also high 
elevation Park sites, which generally have higher levels (Neufeld et 
al., 2019).
    \217\ In these analyses the N100 and D100 metrics are based on 
counts of hourly O3 concentrations at or above 100 ppb 
across the consecutive 3-month period with the highest total (Wells, 
2020). The metric D100 is the count of days with an hour at or above 
100 ppb.
    \218\ We note that we are not intending to ascribe specific 
significance to five days with an hour at or above 100 ppb or ten 
hours such, per se. Rather, these are used simply as reference 
points to facilitate comparison to illustrate the point that such 
high concentrations, which based on toxicological principles, pose 
greater risk to biota than lower concentrations (e.g., ``[h]igher 
concentrations appear to be more important than lower concentrations 
in eliciting a response'' [ISA, p. 8-180]; ``higher hourly 
concentrations have greater effects on vegetation than lower 
concentrations'' [2013 ISA, p. 91-4] ``studies published since the 
2006 O3 AQCD do not change earlier conclusions, including 
the importance of peak concentrations, . . . in altering plant 
growth and yield'' [2013 ISA, p. 9-117]).
    \219\ We also note the higher percentages of sites with an N100 
above five among sites meeting a single-year W126 index of 7 ppm-hrs 
than sites meeting the current standard (Wells, 2020, Table 2). 
Sites with an annual W126 index of 7 ppm-hrs also record a greater 
percentage of sites with more than two days with an hour at or above 
100 ppb (Wells, 2020, Table 2).
---------------------------------------------------------------------------

    In summary, we find that a 3-year average is appropriate for use in 
assessing protection for RBL based on the established tree seedling E-R 
functions, in light of the discussion above, while also finding it 
important to consider additional aspects of O3 air quality, 
that influence vegetation exposures of potential concern, in reaching 
conclusions about the adequacy of the current standard. We disagree 
with the commenters and the prior CASAC that focus on a single year 
W126 index is needed to protect against years with O3 
concentrations with the potential to be ``unusually damaging,'' Rather, 
as described here, the metric of the current standard provides strong 
protection against elevated hourly concentrations that might contribute 
to ``unusually damaging'' years with the potential to be adverse to the 
public welfare, as well as providing protection against effects of 
cumulative exposures seen in experimental studies. Accordingly, we 
disagree with those commenters that express the view that the current 
standard does not provide such protection.
(iii) Visible Foliar Injury
    In support of their disagreement with the EPA's proposed decision, 
some commenters express the view that the EPA's proposed conclusion 
that the current standard provides sufficient protection from an 
incidence and severity of visible foliar injury that would reasonably 
be judged adverse to the public welfare is unlawful. These commenters 
variously claim that EPA analyses are flawed, arbitrary, and ignore 
conclusions and judgments of the prior CASAC; cite some studies that 
they state indicate a threshold for foliar injury lower than 25 or 17 
ppm-hrs; claim that the EPA must, yet does not, identify a level of 
injury that is adverse; state that the EPA does not explain its use of 
USFS biosite scores in this regard, and state that the EPA does not 
adequately address the Murray Energy remand related to these effects. 
With regard to the latter, the Agency intends this decision, associated 
analyses conducted for this review in consideration of issues raised by 
the court remand, and the discussions herein to constitute its response 
to the Murray Energy remand on these effects.
    With regard to EPA's analyses of the current information on 
O3-related visible foliar injury, some commenters claim that 
the EPA needs to and has not adequately explained why it disagrees with 
the conclusions and judgments of the prior CASAC in comments on the 
2014 draft PA regarding a W126 index value of 10 ppm-hrs. As an initial 
matter, we note that in discussing this topic, these commenters 
conflate the prior CASAC's scientific evidence-based recommendations on 
the secondary standard with its judgments of scientific information in 
the context of its policy recommendations. In its letter on the draft 
PA, the prior CASAC explicitly separates into two separate paragraphs 
its scientific judgment based recommendations to the Administrator on 
the standard from its additional policy recommendations, with this 
statement regarding visible foliar injury occurring in the second 
paragraph (that addresses policy recommendations) (Frey, 2014b, p. 
iii).\220\ Thus, we

[[Page 87329]]

reasonably interpreted the statement by the prior CASAC as simply 
indicating a consideration of the prior CASAC in reaching its decision 
on the recommended range of levels, stated multiple times in the same 
letter and including levels higher than 10 ppm-hrs, that the Committee 
thought might be useful (e.g., as a ``policy recommendation'') to the 
Administrator in exercising the discretion granted him under the Act 
for specifying a secondary standard (Frey, 2014b, p. iii). The prior 
CASAC statement regarding a W126 index value of 10 ppm-hrs, is related 
to visible foliar injury at biosites, and, more specifically, is based 
on its consideration of an EPA cumulative analysis of a biomonitoring 
dataset presented in the 2013 draft WREA.\221\ This analysis, the 
dataset for which is further described in Appendix 3C of the PA for the 
current review, does not show, as implied by the 2014 CASAC comments, 
that, in considering sites with W126 index values from highest to 
lowest, there is no reduction in prevalence of sites with visible 
foliar injury above a W126 index of 10 ppm-hrs (i.e., there are not 
differences in the occurrence of injury across higher values).\222\ The 
2014 WREA analysis could not and was not addressing this issue.
---------------------------------------------------------------------------

    \220\ The first paragraph, conveying scientific judgment 
provides a range of levels for a revised standard (Frey, 2014b, p. 
iii). The second begins by noting that the ``scientific judgment'' 
regarding a revised secondary standard, in prior paragraph, are 
based on the scientific evidence. Midway through that paragraph, as 
shown below, the prior CASAC turns to its policy recommendations, in 
which it relates various W126 index values in different ways to 
various effect categories, including crop yield loss, foliar injury, 
and relative biomass loss (Frey, 2014b, p. iii). Given that the 
prior CASAC recommended multiple times in this letter a standard 
level range that extends higher than 10 ppm-hrs (to 15 ppm-hrs), the 
fact that the sentence regarding visible foliar injury in the 
version of this second paragraph that appears within the attachment 
to the letter begins with the phrase ``[b]ased on its scientific 
judgment'' cannot reasonably be interpreted to be overriding the 
Committee's scientific advice on the standard. Rather, the prior 
CASAC appears to be implying that to the extent the Administrator 
judges, as a matter of public welfare policy, it important to 
consider such a focus on foliar injury, the prior CASAC's scientific 
judgment is that 10 ppm-hrs is required to reduce it (Frey, 2014b, 
pp. iii and 15). In relevant part, the second paragraph reads:
    In reaching its scientific judgment regarding the indicator, 
form, summation time, and range of levels for a revised secondary 
standard, the CASAC has focused on the scientific evidence for the 
identification of the kind and extent of adverse effects on public 
welfare. The CASAC acknowledges that the choice of a level within 
the range recommended based on scientific evidence is a policy 
judgment under the statutory mandate of the Clean Air Act. . . . As 
a policy recommendation, separate from its advice above regarding 
scientific findings, the CASAC advises that a level . . . below 10 
ppm-hrs is required to reduce foliar injury. A level of 7 ppm-hrs . 
. . offers additional protection against crop yield loss and foliar 
injury. . . . Thus, lower levels within the recommended range offer 
a greater degree of protection of more endpoints than do higher 
levels within the range. (Frey, 2014b, p. iii, [emphasis added]).
    \221\ In reference to the 2013 draft WREA cumulative frequency 
analysis (e.g., 2013 draft WREA, Figures 7-9 to 7-12), a 2014 CASAC 
comment cited by commenters states that ``W126 values below 10 ppm-
hrs [are] required to reduce the number of sites showing visible 
foliar symptoms'' (Frey, 2014b, p. 14).
    \222\ We note that in light of, and subsequent to, the prior 
CASAC's 2014 letter in the last review, the EPA had considered the 
extensive evidence documented in the 2013 ISA, as well as analyses 
of USFS data in the 2008 and 2015 reviews, including technical memos 
developed after the prior CASAC provided its 2014 advice (80 FR 
65376, 65395-96, October 26, 2015). In the current review, the now 
expanded available data and analyses augment the support for EPA's 
conclusions in this regard.
---------------------------------------------------------------------------

    The 2014 WREA analysis is a cumulative analysis of the proportion 
of records with nonzero BI scores; each point graphed in the analysis 
includes the records for the same and lower W126 index values. Not only 
is the analysis silent with regard to severity of injury, but it also 
does not compare the incidence of visible foliar injury for records of 
differing W126 index values. Rather, each point in the cumulative 
frequency figure represents all the records included in the group (thus 
far), which increase by one with each new point (moving through 
dataset). Where the record added to the group has the same W126 index 
value as the prior included record, the point is at the same location 
along the x-axis, but at a slightly higher location along the y-axis 
(if it has a nonzero BI), thus contributing to an increase in the 
proportion of sites (the metric assessed on the y-axis). Thus, where 
there are many records with quite similar W126 index values, the points 
do not appreciably move along the x-axis, yet when they have a nonzero 
BI score, they are placed higher along the y-axis (as each represents 
another nonzero record in the dataset, thus increasing the proportion 
of records). At such a location along the x-axis, an inflection occurs 
(i.e., a location along the x-axis for which each additional record had 
the same or quite similar W126 index as the prior record such that the 
point is at a similar location on the x-axis but contributes to 
increasing values along the y-axis). As the addition of each new record 
makes the dataset larger, such increases (or decreases for zero BI 
records) become progressively smaller (along the y-axis), making such 
changes or inflections less pronounced at higher W126 index values. 
Accordingly, given the much greater representation in the dataset of 
relatively lower W126 index records (some two thirds of the dataset has 
W126 index values at/below 11 ppm-hrs), the prominent inflection point 
noted by the prior CASAC on the cumulative frequency graph occurs 
around 11 ppm-hrs, and the figure from the 2014 WREA shows only small 
changes in the height of the line with increasing W126 index. This does 
not mean that records with higher W126 index values have no greater 
occurrence of foliar injury than values below 11 ppm-hrs; in fact, they 
do, most particularly the records with W126 index values above 25 ppm-
hrs (PA, Figure 4-5). Thus, we disagree with the prior CASAC statement 
that W126 index values below 10 ppm-hrs are required for any reduction 
in visible foliar injury and with the suggestion that the WREA 
cumulative analysis supports such a conclusion. Given that the 
statement by the prior CASAC did not provide any information to 
indicate another basis for its statement and because the 2014 WREA 
analysis cannot and does not address this issue, we conclude that the 
prior CASAC's statement lacks scientific support. Based on this 
conclusion, the Administrator does not find this statement from the 
prior CASAC informative to his consideration of the adequacy of the 
protection provided by the current standard for adverse public welfare 
effects related to visible foliar injury (discussed in section III.B.3 
below).
    Unlike the 2014 WREA cumulative frequency analysis, the 
presentations in the PA for this review allow for comparison of injury 
incidence, and severity, at distinctly different exposures. As can be 
seen by graphs of the distribution of nonzero BI scores for bins of 
increasing W126 index estimates, the greatest representation of nonzero 
BI scores occurs in the bin with the highest W126 index estimates, 
which for the normal soil moisture category is above 25 ppm-hrs (PA, 
Figure 4-5). In disagreeing with the EPA's observations from this 
analysis, these commenters express the view that the higher percentage 
at the higher W126 index level is not meaningful because there are 
fewer records for the higher W126 index levels. While we agree that 
there are fewer records in the higher W126 index bins, as noted above, 
we disagree that there are too few records in those bins to support 
some interpretation for some soil moisture categories (such as the 
normal or dry categories), although for other soil moisture categories 
(i.e., wet), the small sample size does limit interpretation. Sample 
size in each bin was considered in the PA analysis and was recognized 
as placing a limitation on interpretation of patterns for the wet soil 
moisture category. Contrary to these commenters' view that EPA provides 
no reason for giving little focus to the higher W126 index bins for the 
wet soil moisture category, the PA explains that interpretations of 
patterns across the higher W126 bins are limited for the wet soil 
moisture category, noting that the number of records in each of the 
W126 bins above 13 ppm-hrs comprise less than 1% of the records 
available for that soil moisture category (PA, Appendix 4C, section 
4C.6). Thus, we agree with these commenters that sample size is an 
important consideration in reaching conclusions from this dataset, and, 
contrary to the commenters' assertion of providing no valid reasons 
with regard to the EPA's lesser emphasis on the wet soil moisture 
category, the proposal stated that the PA observations focused 
primarily on the records for the normal or dry soil moisture categories 
explicitly in recognition of those categories having adequate sample 
size which the bins above 13 ppm-hrs did not for the wet soil moisture 
category (85 FR 49890, August 14, 2020). While the dataset includes an 
extremely small number of records in the wet soil moisture category 
that fall into the higher W126 index bins

[[Page 87330]]

(just 18 distributed across the three W126 index bins above 13 ppm-
hrs),\223\ there are more than 550 records categorized as normal soil 
moisture distributed across all five bins for W126 index above 13 ppm-
hrs, more than 40 in each bin (PA, Appendix 4C, Table 4C-4). To the 
extent that the commenters are suggesting that the EPA is disregarding 
data for sites categorized as wet soil moisture, we disagree. In 
recognition of the role of soil moisture in contributing to a condition 
``necessary for visible foliar injury to occur,'' the PA analysis 
presents BI scores separated into groups based on categorization 
related to soil moisture (ISA, Appendix 8, p. 8-13; 85 FR 49881; PA, 
pp. 4-40 to 4-41). The EPA thus considered the available evidence for 
all of the soil moisture categories, but with regard to any patterns 
evidenced for the higher W126 index bins (above 13 ppm-hrs), the EPA 
reasonably explained its focus on two of the three categories (the 
normal or dry soil moisture categories), and lesser attention to the 
third category (wet soil moisture) due to the extremely small number of 
records in that category that fall into the higher W126 index bins.
---------------------------------------------------------------------------

    \223\ The records for the wet soil moisture category in the 
higher W126 bins are more limited than the other categories, with 
nearly 90% of the wet soil moisture records falling into the bins 
for W126 index at or below 9 ppm-hrs, limiting interpretations for 
higher W126 bins (PA, Appendix 4C, Table 4C.4 and section 4C.6). The 
number of records in each of the W126 bins above 13 ppm-hrs (sample 
size ranging from zero to 9) comprise less than 1% of the wet soil 
moisture category. Accordingly, the PA observations focused 
primarily on the records for the normal or dry soil moisture 
categories, for which all W126 index in the analysis, including 
those above 13 ppm-hrs, are better represented (85 FR 49890, August 
14, 2020). For the wet soil moisture category, we agree with the 
commenter's statement that ``higher percentage at higher levels 
isn't necessarily meaningful, because there are fewer sites with any 
data at those levels,'' however note that there is much greater 
representation of the normal and dry soil moisture categories in 
each of the higher bins, extending to the highest bins, than is the 
case for the wet soil moisture category bins.
---------------------------------------------------------------------------

    Further, in addition to incidence of sites with any injury, the PA 
presentations indicate that the severity of injury is also highest in 
records for the highest W126 index values, appreciably higher that it 
is in all of the lower W126 index bins. For normal soil moisture 
category, the median BI score across the nonzero records in the highest 
W126 bin (greater than 25 ppm-hrs) is just over 10 (with an average 
over 15), compared to well below 5 (averages below 7) for each of the 
lower W126 bins (PA, Figure 4-5, Appendix 4C, Table 4C-5). Both of 
these observations are consistent with an E-R relationship of 
O3 with visible foliar injury, while the variability 
observed across the full dataset, in addition to perhaps indicating 
limitations in some aspects of the dataset (e.g., categorization by 
soil moisture, among others [PA, Appendix 4C, section 4C.5]), no doubt 
also indicates the role of other factors that have not been completely 
accounted for. Given the evidence from controlled experiments 
documented across many years, the lack of noticeable change in 
incidence or severity across lower W126 index values may, as recognized 
in the PA, relate to a number of factors, including uncertainties in 
the assignment of W126 index estimates to the biosite locations and the 
soil moisture categorization of sites, as well as potential for 
differences in individual plant responses in controlled experiments 
from plant communities in natural environmental settings. Although such 
factors may contribute to an unclear pattern at lower exposures, 
precluding reaching conclusions regarding O3-related 
response across the lower W126 index bins, the observed response for 
the highest bin clearly indicates an O3-related response for 
W126 index values above 25 ppm-hrs.
    Some commenters question the significance EPA ascribes to its 
observation that the BI scores are appreciably higher for records in 
the highest W126 index bin, cryptically characterizing the observation 
as describing a ``derivative of a derivative.'' Yet, this observation 
is simply focused on the response (e.g., incidence of BI score greater 
than 0 or 5 or 15) exhibited across the range of exposure levels 
evaluated. The EPA makes this observation in assessing the dataset as 
to whether an E-R relationship is exhibited and if so, at what part of 
the exposure range is there a noticeable increase in response. This 
assessment, in combination with related evidence, then informs the 
Agency's conclusions regarding O3 exposure circumstances 
that influence BI scores, as well as levels of W126 for which such an 
influence is indicated.\224\ The commenters quote the prior CASAC as 
characterizing the 2014 WREA analysis as ``a change in the E-R slope,'' 
\225\ but, as discussed in detail above, the 2014 WREA figure is 
presenting a cumulative frequency analysis, which, by its design, does 
not show ``a change in the E-R slope.'' Such an analysis, because 
responses are not compared among distinct and discrete exposures, as 
explained above, is not well described as an exposure-response 
assessment (i.e., an analysis of responses occurring across a range of 
different exposures). This is in contrast to the current PA 
presentation of BI scores across bins of increasing W126 index, which 
presents the occurrence of responses, quantified by magnitude of BI 
score, associated with multiple different exposures (presented as 
bins). Thus, the EPA finds the current analyses in the PA, and not the 
cumulative frequency analysis in the 2014 WREA, to be informative to 
the consideration of relationships between extent of visible foliar 
injury and W126 index, and finds the 2014 WREA analysis to be 
mistakenly interpreted by the commenters.
---------------------------------------------------------------------------

    \224\ Such information informs the Administrator's consideration 
of the currently available evidence and the extent to which it can 
inform his judgments on O3 air quality associated with 
visible foliar injury of such an extent and severity in the 
environment as to indicate adverse effects to the public welfare. 
Such judgments, as discussed further below, rely on information on 
relationships between different O3 air quality metrics 
and injury incidence and severity as well as factors influencing the 
public welfare significance of different incidence and severity of 
foliar injury in vegetated areas valued by the public (e.g., as 
summarized in section III.A.2.b).
    \225\ This characterization was made in the 2014 letter 
providing the prior CASAC's review of the second draft WREA. As 
noted by some commenters, the letter goes on to state, ``[b]ased on 
this E-R slope change, 10 ppm-hrs is a reasonable candidate level 
for consideration in the WREA, along with other levels'' (Frey, 
2014c, p. 7). Although the EPA did not examine the specific value of 
10 ppm-hrs in the 2014 WREA, as observed by these commenters, the 
EPA did consider this recommendation in the 2015 decision, contrary 
to the claim of the commenters (80 FR 65395-96, October 26, 2020).
---------------------------------------------------------------------------

    Further some commenters, who object to the Administrator's proposed 
focus on BI scores above 15 for his consideration of visible foliar 
injury that may be adverse to the public welfare, additionally suggest 
that EPA should give weight to all nonzero BI scores in considering the 
appropriate protection against this effect for the standard. As an 
initial matter, contrary to the implication of the commenters that any 
amount of visible foliar injury is adverse to the affected plant, we 
note the long-standing conclusions that visible foliar injury ``is not 
always a reliable indicator of other negative effects on vegetation,'' 
such as growth and reproduction, and the ``significance of ozone injury 
at the leaf and whole-plant levels depends on how much of the total 
leaf area of the plant has been affected, as well as the plant's age, 
size, developmental stage, and degree of functional redundancy, among 
the existing leaf area'' (ISA, p. 8-24; 2013 ISA, section 9.4.2). 
Further, we disagree with the further implication of these commenters 
that any occurrence of a nonzero BI score in the PA dataset can be used 
to identify O3 exposure conditions that are adverse to the 
public

[[Page 87331]]

welfare. As discussed in section III.A.2.b above, a number of factors 
influence the public welfare implications of visible foliar injury, and 
as discussed further below, the Administrator has taken these into 
account in his decision making regarding the protection from such 
effects that should be afforded by the secondary standard.
    These commenters additionally claim that the USFS dataset indicates 
a clear relationship between the W126 metric and foliar injury. While 
we agree that the dataset provides some support for the conclusion of a 
greater incidence of nonzero BI scores and higher scores for the 
highest W126 bin, a change in response is not evident across the full 
range of W126 index levels (for records of similar soil moisture 
category), thus suggesting a limitation of the dataset in its ability 
to describe the E-R relationship of BI scores with W126 index. As 
discussed in the PA, limitations in the dataset (e.g., with regard to 
assignment of W126 index estimates to biosite records and the approach 
for accounting for the role of soil moisture) may be contributing to 
the lack of a clearly delineated E-R relationship of injury occurrence 
and BI score with W126 index across a range of W126 index values, such 
that a clear shape for a relationship between these variables is not 
evident with this dataset, and may be contributing to uncertainties in 
this regard. It is with the increase in W126 for the last bin (>25 ppm-
hrs) that the accompanying noticeable increase in response provides 
increased confidence in that response (BI scores) being related to a 
particular magnitude of the O3 metric. It is this 
consideration which leads to the emphasis that EPA's conclusions from 
this analysis place on W126 index above 25 ppm-hrs, albeit with a 
recognition of some associated uncertainty.
    Regarding the Administrator's judgment of the extent and severity 
of visible foliar injury that may be adverse to the public welfare, 
some commenters state that the EPA must, and has not, considered the 
full USFS dataset, including records for which the BI scores are below 
5, and they express the view that the USFS data indicate injury (i.e., 
a nonzero BI score) to be occurring at W126 index values as low as 3 
ppm-hrs. In so doing, they note the occurrence of scores above 15 in 
the lowest bin (W126 index below 7 ppm-hrs). These commenters note that 
a third of all records with a BI above 15 are in the lowest W126 index 
bin (W126 <7 ppm-hrs) and more than 500 records with nonzero BI are in 
higher bins, seemingly intending this as support of their view that the 
EPA should identify a W126 of 7 ppm-hrs as a target level for visible 
foliar protection. However, this line of logic seems to ignore the fact 
that this bin also has over a third of the records with a BI above zero 
(PA, Table 4C-4), a fact which would seem contrary to these commenters' 
position that 7 ppm-hrs would protect against such scores. All three of 
these observations are likely due to the fact that this bin contains 
42% of all records and the most records of any bin, by far (PA, 
Appendix 4C, Table 4C-4). Accordingly, the more important observation 
with regard to the extent of conclusions supported by the dataset on 
the role of W126 index in influencing BI scores is that the proportion 
of records in the lowest W126 bin that have scores above 15, 5 or 0 is 
appreciably less that in the highest W126 index bin (PA, Appendix 4C, 
Table 4C-6). The fact that there is not a clear pattern of increasing 
proportion across the intervening (and full set of) bins indicates 
there to be factors unaccounted-for in this dataset with regard to the 
O3 exposure circumstances and the environmental 
circumstances that together elicit increased scores in vegetated areas.
    In considering the PA analyses of the biosite dataset in light of 
these comments, we first note that, as described in the PA, the USFS 
dataset includes a broad assortment of BI scores, extending down to 
zero, occurring across the range of W126 estimates applied to the 
records (PA, Appendix 4C, Figure 4C-3). Contrary to the statement by 
these commenters, the EPA has considered the full dataset. The PA 
documents the various ways in which this is done, and the proposal 
discusses key observations from this dataset to inform the 
Administrator's judgment on adversity to public welfare (PA, section 
4.3.3.2, 4.5.1.2 and Appendix 4C; 85 FR 49889-90, 49903, August 14, 
2020). For example, the lack of clear BI score response to W126 across 
the range of lower values is consistent with findings of published 
studies of the USFS biomonitoring data which find that W126 index alone 
may not be sufficient to characterize the O3 conditions 
contributing to injury levels that may be of interest (e.g., Smith et 
al., 2012; Smith, 2012; 85 FR 49888-49889, August 14, 2020). Similar to 
the discussion above, these studies suggest a role for the occurrence 
of elevated hourly concentrations and a focus solely on W126 index may 
miss this. This consideration of the larger evidence base for visible 
foliar injury and associated USFS biomonitoring findings is important 
to judging the findings of analyses of the BI dataset and their 
informativeness to the Administrator's needs in judging public welfare 
adversity. Based on a detailed evaluation of the currently available 
record regarding such data, the EPA recognizes the need to consider 
factors beyond just W126 index in considering O3 conditions 
most influential in the incidence and extent of visible foliar injury.
    With regard to lower ``thresholds,'' the commenters simply cite a 
set of studies that describe visible foliar injury observations in 
bioindicator species and for which estimates of W126 index for a prior 
time period are below 25 ppm-hrs. The first group of these studies 
focus on naturally occurring plants in locations during which the 
current standard (with its level of 70 ppb) is not met.\226\ As 
discussed above, the current standard limits the occurrence of elevated 
concentrations which, as discussed above, is suggested to be important 
in the occurrence of visible foliar injury in sites of the USFS biosite 
monitoring program, and such elevated concentrations are much more 
prevalent in areas that do not meet the current standard (e.g., PA, 
Appendix 4A, section 2A.2; Wells [2020]). Thus, this group of studies 
do not provide sufficient information to characterize the O3 
exposure circumstances that may be eliciting the observed responses. 
Nor are they informative with regard to consideration of the incidence 
and extent or severity of injury that may occur under air quality 
conditions allowed by the current standard. Two other examples raised 
by commenters (but without complete study citations), appear to relate 
to leaf injury assessed in potted plants either outdoors but watered 
daily or maintained in greenhouse conditions. The injury assessed is at 
the individual plant level, making implications with regard to natural 
vegetation communities unclear, and the extent to which either finding 
in artificial conditions might represent such plant responses in 
natural environmental conditions is unknown. These commenters 
additionally note what they describe as ``threshold values'' reported 
in a National Park Service publication (Kohut, 2020). This publication 
includes three ``injury thresholds'' in terms of three assessment 
metrics, with one being a 3-month W126 index and a second in terms of

[[Page 87332]]

SUM06.\227\ For each metric, three ranges of ``thresholds'' are 
presented (for different purposes). The ranges for SUM06 come from a 
1996 workshop report (Heck and Cowling, 1997). The ranges for W126 
index are based on a W126 index conversion of the SUM06 ranges. One of 
the ranges is labeled as pertaining to foliar injury as a response, 
yet, the publication cited does not provide data on foliar injury in 
relation to that range, nor do publications cited by the former 
publication. As we can best discern based on cited and related 
publications, it appears to at the lower end relate to a benchmark 
derived for growth effects (10% RBL) in the highly sensitive species, 
black cherry, rather than visible foliar injury (Kohut, 2007b; Lefohn 
et al., 1997; 80 FR 65378, October 26, 2015). Thus, contrary to the 
commenters' assertion, the range for W126 index (labeled as pertaining 
to foliar injury) does not appear to provide a threshold based on 
evidence for visible foliar injury.
---------------------------------------------------------------------------

    \226\ For example, valid design values include: (1) 73 (2002) 
and 72 (2003) at monitoring site 450190046, (2) 91 (2002), 94 
(2003), and 88 (2004) at 230090102; (3) 77 ppb (2004) at 261530001, 
and (4) 90 (2002 and 2003) at 340010005.
    \227\ We note that the third assessment approach utilizes a 
combination of a W126 index metric with the N100 metric, 
illustrating the consideration by the National Park Service of the 
role of peak concentrations in posing risk of visible foliar injury 
(Kohut, 2020).
---------------------------------------------------------------------------

    Some commenters (citing page 4C-18 of the PA), express confusion 
over how EPA can state there to be an incomplete understanding of the 
relationships influencing severity of visible foliar injury while also 
using the USFS scores to inform the Administrator's judgments regarding 
conditions that may be adverse to the public welfare. We see no 
contradiction in this. Rather, it is this recognition of an incomplete 
understanding, including the recognition of uncertainty in ``specific 
aspects of [the influences of environmental/genetic factors] on the 
relationship between O3 exposures, the most appropriate 
exposure metrics, and the occurrence or severity of visible foliar 
injury'' (PA, Appendix 4C, p. 4C-18), that leads the EPA to place 
greatest weight on the most clear findings from the USFS data. With 
regard to the PA presentation, with its recognized uncertainties and 
limitations, such a finding is the obviously increased prevalence and 
severity of visible foliar injury for records with W126 index estimates 
above 25 ppm-hrs.
    Further, in considering public welfare implications of 
O3 related visible foliar injury, the EPA continues to 
recognize that the occurrence of visible foliar injury has the 
potential to be adverse to the public welfare (e.g., as summarized in 
section III.A.2.b above and section III.B.2 of the proposal). However, 
as noted in the proposal, the EPA does not find that any small 
discoloring on a single leaf of a plant (which might yield a quite low, 
nonzero BI score in the USFS system) is reasonably considered adverse 
to the public welfare. Thus, findings such as those raised by 
commenters of injury on individual plants in controlled conditions, 
while providing support to the conclusion of a causal relationship 
between O3 exposure and visible foliar injury (ISA, Appendix 
8, Table 8-3), are less informative to the Administrator's judgment on 
adequacy of the protection provided by the current standard from 
adverse effects to the public welfare. Rather, the USFS biosite 
monitoring data provide information that is more useful for such a 
judgment because this monitoring program, as summarized in section 
III.A.2.b above (and III.B.3.b of the proposal), and the scale of its 
objectives which focus on natural settings in the U.S. and forests as 
opposed to individual plants is better suited for the Administrator's 
consideration with regard to the public welfare protection afforded by 
the current standard. In this context, as described in section III.B.3 
below, the Administrator judges that very low BI scores, such as those 
less than 5, described by the USFS scheme as ``little or no foliar 
injury'' do not pose concern for the public welfare.\228\
---------------------------------------------------------------------------

    \228\ Studies that consider such data for purposes of 
identifying areas of potential impact to the forest resource suggest 
this category corresponds to ``none'' with regard to ``assumption of 
risk'' (Smith et al., 2007; Smith et al., 2012).
---------------------------------------------------------------------------

    Lastly, we disagree with the comment that the Act requires the EPA 
to specify ``a level'' of injury that is adverse. The Court of Appeals 
for the D.C. Circuit has held that ``the Agency may sometimes need to 
articulate the level of threat to the population it considers 
tolerable; but there is no separate methodological requirement under 
Sec.  109 that the Administrator establish a measure of the risk to 
safety it considers adequate to protect public health every time it 
establishes a standard pursuant to Sec.  109.'' See Nat. Res. Def. 
Council, Inc. v. EPA, 902 F.2d 962, 973 (D.C. Cir. 1990), opinion 
vacated in part on other grounds, 921 F.2d 326 (D.C. Cir. 1991). The 
same principle applies for consideration of the protection of public 
welfare in the context of establishing or reviewing secondary 
standards. The court later confirmed that it ``expressly rejected the 
notion that the Agency must `establish a measure of the risk to safety 
it considers adequate to protect public health every time it 
establishes a [NAAQS].'' See ATA III, 283 F.3d at 369 (D.C. Cir. 2002) 
(quoting Natural Res. Def. Council, Inc. v. EPA, 902 F.2d 962, 973 
[D.C. Cir. 1990]). As is recognized by the courts and by EPA and CASAC 
across NAAQS reviews, the judgment of the Administrator, in addition to 
being based on the scientific evidence, depends on a variety of 
factors, including science policy judgments and public welfare policy 
judgments. As noted by the case law and also in section III.B.2.b(iv) 
below, the EPA is not required under the Act to identify individual 
levels of adversity or set separate standards for every type of effect 
that may be caused by a pollutant in ambient air, as long as it has 
engaged in reasoned decision making in determining that a particular 
standard provides the requisite protection. Thus, it is common for one 
NAAQS to provide protection for multiple effects, with the most 
sensitive effect influencing the stringency of the standard and 
accordingly leading to protection that is adequate for other, less 
sensitive effects. Given the significant uncertainties which are 
present in every NAAQS review, it is enough for the Administrator to 
set standards that specify a level of air quality that will be 
``tolerable,'' (NRDC, 902 F.2d at 973), and ``qualitatively to describe 
the standard governing its selection of particular NAAQS'' (ATA III, 
283 F.3d at 369). In reviewing each standard, the EPA gives due 
consideration to each of the effects that are relevant for that 
standard in considering whether the standard provides adequate 
protection from the type, magnitude or extent of such effects known or 
anticipated to be adverse to the public welfare. In the case of visible 
foliar injury, as discussed in section III.B.3 below, the Administrator 
has considered the available scientific evidence, with associated 
uncertainties and limitations, in reaching his decision that the 
current secondary standard provides adequate public welfare protection 
for this effect.
(iv) Crop Yield Effects
    Some commenters object to the proposed conclusions with regard to 
the protection provided by the existing secondary standard from adverse 
effects on the public welfare related to O3 effects on crop 
yield, expressing the view that the EPA must specify ``a level'' to 
protect the public welfare against crop yield reductions and that not 
doing so is unlawful and arbitrary. These commenters' additionally 
object to the Administrator's proposed judgment that a decision based 
on RBL as a proxy for other vegetation-related effects will also 
provide adequate protection against crop related effects, indicating 
their view that EPA does not

[[Page 87333]]

adequately explain the basis for this judgment. These commenters 
additionally claim that the prior CASAC described 5.1% RYL as 
constituting an adverse welfare effect and express the view that the 
EPA arbitrarily and unlawfully does not ``give effect to'' the prior 
CASAC's recommendation.
    We disagree with the implication of these commenters that, in 
judging adequacy of protection provided by the current standard for a 
particular effect, it is per se unlawful to conclude that the air 
quality achieved by the current standard provides adequate protection 
for that particular effect, even if the greater attention in reviewing 
the current standard is on another effect. The EPA is not precluded 
from reaching such a conclusion as long as the Agency has engaged in 
reasoned decision-making in doing so.\229\ In reaching his proposed 
conclusions regarding the extent to which the current standard provides 
appropriate protection from O3 effects on crop yield that 
may be adverse to the public welfare, as in his conclusions described 
in section III.B.3 below, the Administrator recognizes the long-
standing evidence of O3 effects on crop yield and the 
established E-R functions for which RYL estimates for the median crop 
species are presented in the PA (PA, Appendix 3A). He also considers 
factors that might be important to his judgments related to the 
requisite protection for a secondary standard that protects against 
adverse effects to the public welfare. In this context he judges that 
the median RYL estimated for air quality that achieves his RBL-related 
objectives for the current standard does not constitute an adverse 
effect on public welfare and thus concludes that the current standard 
also provides adequate protection for crop yield-related effects. Given 
that the decision on adequacy of protection is a judgment of the 
Administrator and that the Clean Air Act does not require a particular 
approach for reaching such judgments, we disagree with the commenters 
to the extent that they suggest that it is per se unlawful for the 
Administrator to use such an approach. The circumstances for his use of 
this approach include particular aspects of the information available 
on O3-related crop yield effects and other factors important 
to judgments on public welfare effects related to crop yield effects.
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    \229\ Section 109(b)(2) of the CAA provides only that any 
secondary standard ``shall specify a level of air quality the 
attainment and maintenance of which in the judgment of the 
Administrator, based on [the air quality ] criteria, is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of such air pollutant in the 
ambient air.''
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    In reaching his decision in this review, as described in section 
III.B.3 below, the Administrator has also considered public comments on 
these issues, including that regarding a prior CASAC statement. The 
comment regarding the prior CASAC appears to draw on a judgment of the 
prior CASAC that a median RYL of 5% ``represents an adverse impact'' 
(Frey, 2014b, p. 14). The prior CASAC provided no clear scientific 
foundation for this judgment. While we infer this judgment to draw on 
discussion at a 1996 workshop,\230\ neither the prior CASAC nor the 
workshop summary provides any explicit rationale for identification of 
5% (with regard to RYL), or any description of a connection of an 
estimated 5% RYL to broader impacts of a specific magnitude or type, or 
to judgments on significance of a 5% RYL to the public welfare. Thus 
the EPA disagrees with the commenters regarding the weight to give the 
prior CASAC statement and, as described below, respectfully disagrees 
with the prior CASAC on this statement.
---------------------------------------------------------------------------

    \230\ The first reference to 5% RYL by the prior CASAC (in the 
2015 O3 NAAQS review) appears to be in its letter on the 
first draft PA (Frey and Samet, 2012). In that letter, the prior 
CASAC identifies 5% RYL as a factor on which levels for a W126 index 
secondary standard should be based, although no rationale is 
provided for this recommendation. In a letter attachment, comments 
from an individual member point to a 1996 workshop (2014 PA, pp. 6-
15 through 6-17; Heck and Cowling, 1997). As summarized in the 2015 
O3 decision, the 1996 workshop participants (16 leading 
scientists, discussing their views for a secondary O3 
standard) indicated an interest in protecting against crop yield 
reductions of 5% yet noted uncertainties surrounding such a 
percentage which led them to identify 10% RYL (80 FR 65378, October 
26, 2015). In their emphasis on 5%, the 2012 comments from the 
individual prior CASAC member expressed the view that the ability to 
estimate 5% RYL has improved (Frey and Samet, 2012, p. A-54). 
Neither the individual prior CASAC member nor the 1997 workshop 
report provide any explicit rationale for the percentages identified 
or any description of their connection to ecosystem impacts of a 
specific magnitude or type, or to judgments on significance of the 
identified effects for public welfare (80 FR 65378, October 26, 
2015; Heck and Cowling, 1997).
---------------------------------------------------------------------------

    In reaching his judgment regarding whether the current standard 
provides the requisite public welfare protection, as described in 
section III.B.3 below, the Administrator considers the extent to which 
a specific estimate of RYL may be indicative of adverse effects to the 
public welfare. In so doing, he notes that the secondary standard is 
not intended to protect against all known or anticipated O3-
related effects, but rather those that are judged to be adverse to the 
public welfare, and that a bright-line determination of adversity is 
not required in judging what is requisite. In his decision described 
below, the Administrator also notes that the determination of the 
extent of RYL estimated from experimental O3 exposures that 
should be judged adverse to the public welfare is not clear, in light 
of the extensive management of agricultural crops that occurs to elicit 
optimum yields (e.g., through irrigation and usage of soil amendments, 
such as fertilizer). Further, in considering effects on the public 
welfare that may relate to agricultural markets, we note that 
detrimental impacts on crops, as well as beneficial impacts, can be 
unevenly distributed between producers and consumers, complicating 
conclusions with regard to relative adversity. In light of such 
considerations, the Administrator, while finding consideration of the 
RYL estimate for the median crop species informative to his judgment on 
the adequacy of the protection provided by the current standard for 
this effect, does not find such an RYL estimate of 5% to represent an 
adverse effect to the public welfare, as described more fully in 
section III.B.3 below. For these reasons, and for the reasons discussed 
earlier in this section, including those regarding advice from a prior 
CASAC, the EPA also disagrees with the commenters' assertion that the 
Administrator is arbitrarily and unlawfully failing to ``give effect 
to'' the prior CASAC's recommendation.
    Further, we disagree with the comment that the Act requires the EPA 
to specify ``a level'' to protect the public welfare against crop yield 
reductions. As discussed in greater detail in section III.B.2.b(iii) 
above, the EPA is not required under the Act to set separate standards 
for every type of effect that may be caused by a pollutant in ambient 
air, as long as it has engaged in reasoned decision making in 
determining that a particular standard provides the requisite 
protection. Thus, it is common for one NAAQS to provide protection for 
multiple effects, with the most sensitive effect influencing the 
stringency of the standard and accordingly leading to protection that 
is adequate for other less sensitive effects. As discussed further in 
section III.B.2.b(iii) above, in reviewing each standard, the EPA gives 
due consideration to each effect relevant for that standard in 
considering whether the standard provides adequate protection from the 
type, magnitude or extent of such effects known or anticipated to be 
adverse to the public welfare. In the case of crop yield loss, as 
discussed in section III.B.3 below, the Administrator has considered 
the magnitude of RYL that may be

[[Page 87334]]

associated with W126 index values that occur under the current standard 
and, based on the current information with regard to the RYL estimates, 
notes that these estimates are generally no higher than 5.1% and 
predominantly well below that. In so doing, he has also considered 
factors such as those raised above, and in light of all of these 
considerations, he judges that a RYL of 5.1% does not represent an 
adverse effect to the public welfare. Thus, the Administrator judges 
that the current standard provides adequate protection of the public 
welfare for crop yield loss related effects.
(v) RBL
    In objecting to the EPA's proposed decision, some commenters 
disagree with the target level of protection identified based on use of 
RBL. In so doing, such commenters variously claim that a 3-year average 
of 17 ppm-hrs is ``ill-suited'' to protect against adverse impacts to 
the public welfare; that 6% RBL is too high to protect the public 
welfare; that use of a 3-year average instead of a single year W126 
index is needed; and, that EPA must focus a target on exposures that 
would avoid 2% RBL, citing comments from the prior CASAC on the second 
draft PA in the 2015 review, and claiming that a focus on a W126 index 
of 7 ppm-hrs is needed for that. With regard to the EPA's use of 6% in 
considering the adequacy of protection related to RBL, these commenters 
recognize that Murray Energy rejected an argument that EPA's prior 
reliance on 6% (in the 2015 decision) was arbitrary based on the record 
in that case (Murray Energy, 936 F.3d at 615-16). In pressing their 
views, however, the commenters state that nothing in Murray Energy 
prevents EPA from revising its prior determination based on the 
scientific evidence and CASAC advice.
    With respect to the latter point, the EPA agrees that the 
Administrator's decision in this review must take into account the 
currently available scientific evidence and advice from the CASAC, and 
that the Agency is not bound by the Administrator's conclusions in the 
prior review. As summarized in the proposal for the current review, in 
the proposal, the Administrator took the currently available scientific 
evidence and advice from the CASAC into account, while also choosing to 
consider the judgments and decision made by the prior Administrator in 
that Administrator's consideration of RBL related targets for 
cumulative seasonal exposure. He did so, in light of the welfare 
effects evidence and air quality information now available, as well as 
the advice from the current CASAC reflecting its concurrence that 
implementation of the prior Administrator's approach or framework is 
``still effective'' in protecting the public welfare from vegetation 
effects of O3 (Cox, 2020a, Consensus Responses to Charge 
Questions p. 21). As described in section III.B.3 below, after 
considering the public comments on this point, he is taking a similar 
approach in reaching his decision in this review.
    With regard to the commenters' objection to the EPA's use of a 3-
year average in assessing RBL, we note, as an initial matter, that the 
EPA's focus on a 3-year average of 17 ppm-hrs as a target level relates 
to an RBL estimate of 5.3%, a value that was also chosen in 2015 in 
recognition of the prior CASAC advice both with regard to 6% RBL and 
about considering a lower W126 index target for a 3-year average due to 
the prior CASAC's concern about ``unusually damaging years.'' In the 
current review, the CASAC has explicitly considered the EPA's 
interpretation of 6% in identifying a target of 17 ppm-hrs as a 3-year 
average, and expressed its view that this target ``is still effective 
in particularly protecting the public welfare in light of vegetation 
impacts from ozone'' (Cox, 2020a, Consensus Responses to Charge 
Questions p. 21). Accordingly, the EPA disagrees with the comments that 
6% RBL and a 3-year average W126 index target of 17 ppm-hrs are too 
high to inform the Administrator's judgments on O3 air 
quality that protects the public welfare; rather, the Administrator 
continues to find this useful in informing his judgments regarding the 
public welfare protection provided by the standard, together with a 
broader consideration of air quality patterns associated with meeting 
the current standard, such as control of peak hourly concentrations, as 
described in section III.B.3 below. Further, we refer to the discussion 
above of how the existing standard, with its current averaging time and 
form provides the protection from the occurrence of elevated hourly 
concentrations that may characterize what the prior CASAC described as 
``unusually damaging years.'' As discussed above, the available air 
quality data demonstrate the strong protection provided by the current 
standard from elevated concentrations that may occur in some years. As 
noted above, these analyses indicate that while the current form and 
averaging time of the existing standard provides control of these 
concentrations and the associated peak exposures, reliance solely on a 
standard in the form of the W126 index based standard, as advocated by 
the commenters, even with a level as low as 7 ppm-hrs cannot be relied 
on to provide it.
    In support of their view that the EPA must focus on avoiding 2% RBL 
with a W126 index of 7 ppm-hrs, these commenters provide little 
rationale beyond citing a comment by the prior CASAC made in the last 
review. In so doing, the commenters assert that because the prior CASAC 
had noted that 7 ppm-hrs was the only W126 index level for which the E-
R functions yielded a RBL for the median tree species that was less 
than or equal to 2%, the EPA must protect against 2% RBL and adopt a 
W126 index level of 7 ppm-hrs. We disagree. As an initial matter, we 
note our discussion above regarding the EPA's consideration in this 
review of advice from a prior CASAC, including prior CASAC statements 
that are raised by commenters, such as those noted here. Further, in 
making the statement that the commenters' cite, the prior CASAC did not 
reach the same conclusion as the commenters with regard to the extent 
to which a revised secondary standard should limit cumulative exposures 
and associated estimates of RBL, such that the prior CASAC did not 
recommend that the EPA consider only W126 index levels associated with 
median RBL estimates at or below 2%.\231\ See Murray Energy, 936 F.36 
at 615-16 (noting that ``CASAC did not identify 2% growth loss as the 
only sufficiently protective level'' but merely recommended ``2% as the 
lower end of a range of permissible target levels'' to be considered). 
In fact, seven of the nine W126 index levels in the range recommended 
by the prior CASAC (7 to 15 ppm-hrs [Frey, 2014b]) are associated with 
RBL estimates higher than 2% (PA, Appendix 4A). As a basis for their 
assertion that the secondary standard should protect against a median 
RBL of 2%, these commenters additionally oddly declare that after three 
years, a 2% RBL per year ``becomes 6%.'' There is no evidence in the 
record, and the commenter provides no evidence, that would support 
their declaration that without a tripling in exposure, the 
O3-attributable reduction in annual growth (the RBL) would 
triple.\232\ Nor is there

[[Page 87335]]

evidence that would support an alternative interpretation of the 
commenters' statement as a claim that a tree experiencing a 2% RBL per 
year is reduced in absolute biomass by 6% after three years.\233\
---------------------------------------------------------------------------

    \231\ Additionally, an explicit scientific rationale for 2% is 
not provided by the former CASAC. Nor is it provided in the workshop 
report referenced by the prior CASAC in its discussion, as further 
discussed in the 2015 decision (80 FR 65394, October 26, 2015; Frey, 
2014b, p. 14).
    \232\ It is unclear by what logic the commenters conclude that 
RBL, a metric describing the effect of the O3 exposure in 
a single year, can be modified by the RBL in a prior year.
    \233\ The fallacy of such interpretations can be seen in the 
presentation of above-ground biomass from a multiyear study of 
O3 exposure of aspen that varies little over six years. 
Across the six years, the above-ground biomass of the trees 
receiving elevated O3 exposure is 25%, 30%, 29%, 29%, 31% 
and 29% lower than the reference trees (2013 ISA, Table 9-14; 2020 
ISA, Figure 8-17).
---------------------------------------------------------------------------

    Some commenters who disagree with the proposed decision also 
express the view that the EPA has ``proposed'' to use RBL functions for 
trees as a proxy for all vegetation effects. Based on this view, these 
commenters variously assert that the EPA is failing to comply with its 
obligation under the Clean Air Act that a secondary standard protect 
the public welfare from ``any known or anticipated adverse effects''; 
that the EPA's approach is not the same as the prior CASAC's discussion 
of RBL as a surrogate; that the EPA is contravening its statutory 
obligation by using one adverse effect as a surrogate for another 
without showing that prevention of the former will prevent the latter; 
and that, based on the commenters' interpretation of a statement made 
by the prior CASAC, a standard that allows tree growth loss above 2% 
cannot protect against visible foliar injury. As an initial matter, we 
note that the citation provided by the commenters for their statement 
that the ``EPA proposes'' to use RBL functions as a proxy for the broad 
array of O3 vegetation-related effects does not include such 
a ``proposal.'' Rather the commenters' citation points to the 
background section of the proposal which simply summarizes the concept 
of RBL as a proxy or surrogate which was employed in the last review 
and which was described by the prior CASAC (85 FR 49899, August 14, 
2020). In describing use of RBL as a proxy or surrogate, the proposal 
(and the PA) use several phrases, ranging from ``for consideration of 
the broader array of vegetation-related effects of potential public 
welfare significance, that included effects on growth of individual 
sensitive species and extended to ecosystem-level effects, such as 
community composition in natural forests, particularly in protected 
public lands, as well as forest productivity'' (85 FR 49878, August 14, 
2020), to shorter phrases, such as ``for the broad array of vegetation 
related effects that extend to the ecosystem scale'' (85 FR 49911, 
August 14, 2020).
    We disagree with these commenters that the way the EPA uses RBL as 
a ``proxy'' or ``surrogate'' is contrary to law, and with their 
contention that the EPA uses one adverse effect as a surrogate for 
another without showing that prevention of the former will prevent the 
latter. As described in the Administrator's decision below, the most 
precise use of RBL as a surrogate or proxy is in the target level of 
protection for cumulative seasonal exposure (17 ppm-hrs as a 3-year 
average W126 index). This use relates specifically to public welfare 
effects related to O3 effects on growth of individual 
sensitive species and related effects, including ecosystem-level 
effects, such as community composition in natural forests, particularly 
in protected public lands, as well as forest productivity (as discussed 
in the PA, section 4.5.1.2). In fact, the ISA describes (or relies on) 
conceptual relationships among such effects in considering causality 
determinations for ecosystem-scale effects such as altered terrestrial 
community composition and reduced productivity, as well as reduced 
carbon sequestration, in terrestrial ecosystems (ISA, Appendix 8, 
sections 8.8 and 8.10). Beyond these relationships of plant-level 
effects and ecosystem-level effects,\234\ RBL can be appropriately 
described as a scientifically valid surrogate of a variety of welfare 
effects based on consideration of ecosystem services and the potential 
for adverse impacts on public welfare, as well as conceptual 
relationships between vegetation growth-related effects (including 
carbon allocation) and ecosystem-scale effects (PA, pp. 4-75 and 4-76). 
Both the prior CASAC and the current CASAC recognized this (Frey, 
2014b, pp. iii, 9-10; \235\ Cox, 2020a, Consensus Responses to Charge 
Questions pp. 18 and 21 \236\). As was discussed in the proposal, the 
information available in this review provides continued support for the 
use of tree seedling RBL as a proxy for the broad array of vegetation-
related effects conceptually related to growth effects, a conclusion 
with which the CASAC agreed (85 FR 49899,49906, August 14, 20202).\237\
---------------------------------------------------------------------------

    \234\ As summarized in the ISA, O3 can mediate 
changes in plant carbon budgets (affecting carbon allocation to 
leaves, stems, roots and other biomass pools) contributing to growth 
impacts, and altering ecosystem properties such as productivity, 
carbon sequestration and biogeochemical cycling. In this way, 
O3 mediated changes in carbon allocation can ``scale up'' 
to population, community and ecosystem-level effects including 
changes in soil biogeochemical cycling, increased tree mortality, 
shifts in community composition, changes in species interactions, 
declines in ecosystem productivity and carbon sequestration and 
alteration of ecosystem water cycling (ISA, section 8.1.3).
    \235\ The prior CASAC 2014 letter on the second draft PA in that 
review stated the following (Frey, 2014b, p. 9-10):
    For example, CASAC concurs that trees are important from a 
public welfare perspective because they provide valued services to 
humans, including aesthetic value, food, fiber, timber, other forest 
products, habitat, recreational opportunities, climate regulation, 
erosion control, air pollution removal, and hydrologic and fire 
regime stabilization. Damage effects to trees that are adverse to 
public welfare occur in such locations as national parks, national 
refuges, and other protected areas, as well as to timber for 
commercial use. The CASAC concurs that biomass loss in trees is a 
relevant surrogate for damage to tree growth that affects ecosystem 
services such as habitat provision for wildlife, carbon storage, 
provision of food and fiber, and pollution removal. Biomass loss may 
also have indirect process-related effects such as on nutrient and 
hydrologic cycles. Therefore, biomass loss is a scientifically valid 
surrogate of a variety of adverse effects to public welfare.
    \236\ The CASAC letter on the draft PA in the current review 
stated the following (Cox, 2020a, Consensus Responses to Charge 
Questions p. 18):
    The RBL appears to be appropriately considered as a surrogate 
for an array of adverse welfare effects and based on consideration 
of ecosystem services and potential for impacts to the public as 
well as conceptual relationships between vegetation growth effects 
and ecosystem scale effects. Biomass loss is a scientifically sound 
surrogate of a variety of adverse effects that could be exerted to 
public welfare. . . . In the previous review, the Administrator used 
RBL as a surrogate for consideration of the broader array of 
vegetation related effects of potential welfare significance that 
included effects of growth of individual sensitive species and 
extended to ecosystem level effects such as community composition in 
natural forests, particularly in protected public lands (80 FR 
65406, October 26, 2015). The EPA believes, and the CASAC concurs, 
that information available in the present review does not call into 
question this approach, indicating there continues to be support for 
the use of tree seedling RBL as a proxy for the broader array of 
vegetation-related effects, most particularly those related to 
growth.
    \237\ Further, the EPA lacks sufficient information in the air 
quality criteria to identify requisite air quality for these 
effects.
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    As recognized in the proposal (and PA) there are two other 
vegetation effect categories with extensive evidence bases (which 
include analyses that assess the influence of cumulative seasonal 
exposure); these are crop yield loss and visible foliar injury. As 
discussed above, the consideration of protection provided by the 
current standard for the former goes beyond the target focused on RBL 
and includes aspects of the evidence specific to those effects. As 
described above and in section III.B.3 below, the EPA is concluding 
that the level of protection is adequate to protect the public welfare 
from effects related to crop yield loss. With regard to the latter, 
contrary to the commenter's assertion, the EPA is not claiming that 
protection focused on RBL provides protection for visible foliar 
injury. The EPA's consideration of visible foliar injury is described 
earlier in this section and in section III.B.3 below.

[[Page 87336]]

    With regard to the two newly identified categories of insect-
related effects, the Administrator finds there to be insufficient 
information to judge the current standard inadequate based on these 
effects, as discussed in section III.B.3 below. He does not claim that 
RBL provides a surrogate for these effects. However, he notes that the 
available information in the air quality criteria does not indicate a 
greater sensitivity of such effects as compared to O3 
effects on vegetation growth, and that he lacks sufficient information 
in the air quality criteria to identify requisite air quality for these 
effects.
(vi) W126 Index in Areas Meeting Current Standard
    In objecting to the proposed decision, one group of commenters 
disagree with EPA's findings regarding the W126 index levels in areas 
that meet the current standard. In so doing, these commenters claim 
that the EPA is mistaken to claim that in virtually all design value 
periods and locations at which the current standard was met across the 
period covered by the historical dataset the 3-year W126 index was at 
or below 17 ppm-hrs because they variously assert there are either 25 
or 21 such occurrences, and they further assert there to be either 50 
occurrences of a single-year W126 index at or above 19 ppm-hrs or 52 
occurrences of a single-year W126 index above 19 ppm-hrs. These counts 
are in disagreement with the air quality analyses documented in 
Appendix 4D of the PA. For example, out of 8,292 values across nearly 
20 years for U.S. ambient air monitoring sites, distributed across all 
nine climate regions, with air quality that meets the current standard, 
there are just eight occurrences of a 3-year W126 index value above 17 
ppm-hrs (PA, Appendix 4D, Tables 4D-10 and 4D-7). This means that 99.9% 
of the records (virtually all) were at or below 17 ppm-hrs. While the 
details of each step of the analyses in the PA are extensively 
documented, including data handling, rounding conventions and data 
acceptability criteria (PA, Appendix 4D, section4D.2), the lack of 
documentation provided by the commenters and their conflicting claims 
(indicated above) leave the EPA to hypothesize that the reason for the 
disagreements include differences with regard to these details, such as 
those regarding rounding conventions. As described in the PA, W126 
values ``were rounded to the nearest unit ppm-hr for applications 
requiring direct comparison to a W126 level,'' a convention intended to 
provide consistency in the precision of the comparison as the W126 
levels for comparison were also in whole numbers (PA, Appendix 4D, 
section 4D.2.2). With the rounding conventions applied in the PA, there 
are eight 3-year W126 index values greater than 17 ppm-hrs (i.e., equal 
to 18 or 19). It may be that the commenters counted unrounded 3-year 
W126 index values as low as 17.01 as being greater than 17 ppm-hrs, 
although the reason for them providing two conflicting counts is 
unclear. Similarly with regard to the counts for single-year W126 index 
values above 19 ppm-hrs, the commenters may have counted unrounded 
single-year index values as low as 19.01 ppm-hrs as being greater than 
19 ppm-hrs. Thus, we find the commenters criticism of the EPA's 
characterization of the findings of the air quality analyses, as well 
as the commenters' counts, to be unfounded.
    Some commenters claim EPA pays inadequate attention to the 
relatively few occurrences of single-year W126 index values at or above 
19 ppm-hrs that have occurred at sites meeting the current standard 
since 2002 and that the standard must be set to avoid such occurrences. 
The EPA disagrees with these commenters, as described below, after 
carefully considering the relatively few occurrences of W126 index 
values at or above 19 ppm-hrs, including single-year values. In so 
doing, we have given particular focus on Class I areas, recognizing the 
attention given to such areas by the Administrator in judging the 
potential for effects adverse to the public welfare, a focus recognized 
by the CASAC and with which the prior CASAC explicitly concurred (Cox, 
2020a; Frey, 2014b, p. 9).
    Among the nearly 500 values for monitoring sites in or near Federal 
Class I areas across the U.S., during periods from 2000 through 2018 
when the current standard was met, there are no occurrences of a 3-year 
average W126 index above 19 ppm-hrs (PA, Table 4-1). Across this same 
period in the same Class I locations, there are just 15 occurrences of 
single-year W126 index values above 19 ppm-hrs, all of which date prior 
to 2013 (PA, Appendix 4D, section 4D.3.2.4). All of these occurrences 
are below 25 ppm-hrs. Thus, in addition to their being relatively few 
occurrences of a single-year W126 above 19 ppm-hrs in/near a Class I 
area in the 19-year dataset, none of them (the most recent of which was 
in 2012) is higher than 25 ppm-hrs; in fact, the highest is 23 ppm-hrs 
(PA, Appendix 4D, section 4D.3.2.4).
    We have also considered the full 19-year dataset for locations 
beyond those in or near Class I areas, noting that, at other sites 
across the U.S., occurrences of single-year W126 index above 19 ppm-hrs 
(22) were predominantly in urban areas, including Los Angeles, and the 
highest values were just equal to 25 ppm-hrs, or, in one instance, just 
equal to 26 ppm-hrs, when rounded (85 FR 49895, 49904, August 14, 2020; 
PA, sections 4.4 and 4.5, Appendix 4D). In considering the potential 
risk posed by these scattered occurrences, largely in urban areas, with 
none since 2012 in or near a Class I areas, we additionally consider 
the data on peak hourly concentrations also discussed above (Wells, 
2020). Together, these data indicate the control provided by the 
current standard in areas that are of particular focus in protecting 
the public welfare, on the extent and frequency of occurrence of 
cumulative exposures in terms of the W126 index (and of peak 
concentrations) of a magnitude of potential concern. As discussed in 
section III.B.3 below, the Administrator does not find the air quality 
patterns allowed by the current standard, as indicated by these 
analyses, to pose a risk of adverse effects to the public welfare.
    In their criticism of the EPA's air quality analyses, one commenter 
claims that the analyses are difficult to evaluate for California and 
other West region states and suggest that California sites brought into 
compliance with the existing standard would still have elevated W126 
index values, similar to sites in the Southwest region. We disagree 
with the commenter's claim that the air quality analyses suggest that 
compliance with the existing standard would not reduce the W126 index 
values at California sites. In making their claim, the commenters cite 
Figures 4D-4 and 4D-5 of the PA. These figures, however, simply 
document W126 index at sites with various design values at one point in 
time (2016-2018). They do not describe analyses of trends over time, 
with changes in air quality. Yet, that very issue was the subject of 
separate regression analyses in the PA (PA, Appendix 4D, section 
4D.3.2.3). These analyses show that the Southwest region, which had 
highest W126 index values at sites meeting the current standard, also 
exhibited the greatest improvement in the W126 metric values per unit 
decrease in their design value (slope of 0.93) over the nearly 20 year 
period analyzed. The pattern is very similar for the West region (with 
a slope of 0.80), with the exception of three sites (in downtown LA); 
however, the design values for these sites are above 100 ppb, making 
such projections quite uncertain (PA, Appendix 4D, section 4D.3.2.3).

[[Page 87337]]

(vii) Climate Effects
    In support of their disagreement with the EPA's proposed decision, 
some commenters claim that EPA needs to establish a standard to protect 
from radiative forcing and related climate effects. In so doing, they 
stated that EPA cannot rely on uncertainty by retaining the existing 
standard and instead, given the uncertainties recognized in the ISA, 
which they suggest could mean current information underestimates 
O3 climate related impacts, the Administrator should 
strengthen the existing standard or establish an additional standard. 
Some commenters additionally assert that the EPA has failed to address 
a recommendation from CASAC regarding a quantitative analysis, while 
also criticizing EPA conclusions regarding a carbon storage analysis in 
the last review. The EPA disagrees with the commenters that the 
available information is sufficient to identify such a standard that 
could be judged to provide the requisite protection under the Act, and 
notes that the commenters do not submit or describe such information; 
nor do the commenters identify a standard that they claim would provide 
such protection.
    With regard to the CASAC recommendation cited by some commenters, 
we note in its review of the draft PA, the CASAC recommended changes to 
the PA to ``more thoroughly address effects of ozone on climate 
change,'' that would include some quantitation, such as estimates of 
climate change related to a change in O3 (Cox 2020a, 
Consensus Responses to Charge Questions p. 22). In consideration of 
this advice, the final PA included additional discussion on the 
available information and tools related to such estimates. As discussed 
below, we conclude that this information, as documented in the ISA, 
does not provide a foundation with which to derive such estimates as 
might pertain to O3 and public health and welfare 
considerations relevant to decisions on the NAAQS.\238\
---------------------------------------------------------------------------

    \238\ In raising EPA's conclusions on a carbon storage analysis 
in the last review, some commenters repeat their comments in the 
last review that claimed that the relatively lesser weight the EPA 
placed on the 2014 WREA estimates of carbon storage (in terms of 
CO2) was inconsistent with the emphasis the EPA placed on 
CO2 emissions reductions estimated for another regulatory 
action. The commenters overlook, however, key distinctions between 
the two types of estimates in the two different analyses which 
appropriately led the EPA to recognize much greater uncertainty in 
the WREA estimates and accordingly give them less weight. While the 
WREA estimates were for amounts of CO2 removed from the 
air and stored in vegetation as a result of plant photosynthesis 
occurring across the U.S., the estimates for the other action were 
for reductions in CO2 produced and emitted from power 
plants (79 FR 34830, 34931-33). The potentially transient nature of 
carbon storage in vegetation makes a ton of additional carbon uptake 
by plants in the former arguably unequal to a ton of reduced 
emissions from fossil fuels. Further, there are appreciably larger 
uncertainties involved in attempting to quantify the additional 
carbon uptake by plants which requires complex modeling of 
biological and ecological processes and their associated sources of 
uncertainty, and there is no new information available in the 
current review that would reduce such uncertainties in quantitative 
estimates of carbon storage benefits to climate. In recognizing the 
public welfare value of ecosystem carbon storage, we additionally 
note, however, that protection provided by the current standard from 
vegetation effects (and RBL) also provides a degree of protection in 
terms of carbon storage.
---------------------------------------------------------------------------

    As recognized in the proposal and summarized in section III.A.2 
above, there are a number of limitations and uncertainties that affect 
our ability to characterize the extent of any relationships between 
O3 concentrations in ambient air in the U.S. and climate-
related effects, thus precluding a quantitative characterization of 
climate responses to changes in O3 concentrations in ambient 
air at regional (vs global) scales. While evidence supports a causal 
relationship between the global abundance of O3 in the 
troposphere and radiative forcing, and a likely causal relationship 
between the global abundance of O3 in the troposphere and 
effects on temperature, precipitation, and related climate variables 
(ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013), the non-
uniform distribution of O3 (spatially and temporally) makes 
the development of quantitative relationships between the magnitude of 
such effects and differing O3 concentrations in the U.S. 
challenging (ISA, Appendix 9). Additionally, ``the heterogeneous 
distribution of ozone in the troposphere complicates the direct 
attribution of spatial patterns of temperature change to ozone induced 
[radiative forcing]'' and there are ``ozone climate feedbacks that 
further alter the relationship between ozone [radiative forcing] and 
temperature (and other climate variables) in complex ways'' (ISA, 
Appendix 9, section 9.3.1, p. 9-19). Thus, various uncertainties 
``render the precise magnitude of the overall effect of tropospheric 
ozone on climate more uncertain than that of the well-mixed GHGs'' and 
``[c]urrent limitations in climate modeling tools, variation across 
models, and the need for more comprehensive observational data on these 
effects represent sources of uncertainty in quantifying the precise 
magnitude of climate responses to ozone changes, particularly at 
regional scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p. 
9-22). For example, current limitations in modeling tools include 
``uncertainties associated with simulating trends in upper tropospheric 
ozone concentrations'' (ISA, section 9.3.1, p. 9-19), and uncertainties 
such as ``the magnitude of [radiative forcing] estimated to be 
attributed to tropospheric ozone'' (ISA, section 9.3.3, p. 9-22). 
Further, ``precisely quantifying the change in surface temperature (and 
other climate variables) due to tropospheric ozone changes requires 
complex climate simulations that include all relevant feedbacks and 
interactions'' (ISA, section 9.3.3, p. 9-22). For example, an important 
limitation in current climate modeling capabilities for O3 
is representation of important urban- or regional-scale physical and 
chemical processes, such as O3 enhancement in high-
temperature urban situations or O3 chemistry in city centers 
where NOx is abundant. Because of such limitations we cannot quantify 
or judge the impact of incremental changes in O3 
concentrations in the U.S. on radiative forcing and subsequent climate 
effects.
    Thus, as discussed in section III.B.3 below, the significant 
limitations and uncertainties summarized here together preclude 
identification of an O3 standard that could be judged to 
provide requisite protection of the public welfare from adverse effects 
linked to O3 influence on radiative forcing, and related 
climate effects. Contrary to the commenters' charge that the lack of a 
quantitative analysis of climate-related effects due to recognition of 
such limitations and uncertainties is unlawful and arbitrary, the 
information available in this review is insufficient to judge the 
existing standard inadequate or to identify an appropriate revision 
based on O3-related climate effects. In the face of 
insufficient evidence for such conclusions, it might, on the contrary, 
be judged unlawful and arbitrary for the Agency to perform guesswork to 
assert a particular new standard provided requisite protection for this 
category of effects. The EPA agrees with the commenters that ``perfect 
information'' is not required. However, information that provides for 
assessment of how the current and potential alternative or additional 
standards would affect O3-related climate impacts is 
lacking. As noted in the ISA, few studies have even attempted to 
estimate ``climate response to changes in tropospheric ozone 
concentrations alone in the future atmosphere,'' and effects of 
O3 on radiative forcing and climate depend on many factors 
other than tropospheric ozone concentrations, including ``changes in a 
suite of climate-sensitive

[[Page 87338]]

factors, such as the water vapor content of the atmosphere'' (ISA, p. 
9-20; Myhre et al., 2013). Thus, as discussed in section III.B.3 below, 
while the Administrator recognizes that the evidence supports a 
relationship of tropospheric O3 with climate effects, he 
judges the quantitative uncertainties to be too great to support 
identification of a standard specific to such effects.
4. Administrator's Conclusions
    Based on the large body of evidence concerning the welfare effects, 
and potential for public welfare impacts, of exposure to O3 
in ambient air, and taking into consideration the attendant 
uncertainties and limitations of the evidence, the Administrator 
concludes that the current secondary O3 standard provides 
the requisite protection against known or anticipated adverse effects 
to the public welfare, and should therefore be retained, without 
revision. In reaching these conclusions, the Administrator has 
carefully considered the assessment of the available welfare effects 
evidence and conclusions contained in the ISA, with supporting details 
in the 2013 ISA and past AQCDs; the evaluation of policy-relevant 
aspects of the evidence and quantitative analyses in the PA (summarized 
in sections III.A.2 and III.A.3 above); the advice and recommendations 
from the CASAC (summarized in section III.B.1.b above); and public 
comments (as discussed in section III.B.2 above and the separate 
Response to Comments document), as well as the August 2019 decision of 
the D.C. Circuit remanding the secondary standard established in the 
last review to the EPA for further justification or reconsideration.
    In considering the currently available information in this review 
of the O3 secondary standard, the Administrator recognizes 
the longstanding evidence base for vegetation-related effects, 
augmented in some aspects since the last review, described in section 
III.A.2.a above. The currently available evidence describes an array of 
effects on vegetation and related ecosystem effects causally or likely 
to be causally related to O3 in ambient air, as well as the 
causal relationship of tropospheric O3 in radiative forcing 
and subsequent likely causally related effects on temperature, 
precipitation and related climate variables. The Administrator also 
takes note of the quantitative analyses and policy evaluations 
documented in the PA that, with CASAC advice and consideration of 
public comment, inform the judgments required of him in reaching his 
decision on a secondary standard that provides the requisite protection 
under the Act.
    As an initial matter, the Administrator recognizes the continued 
support in the current evidence for O3 as the indicator for 
photochemical oxidants (as recognized in section III.B.1.c above). In 
so doing, he notes that no newly available evidence has been identified 
in this review regarding the importance of photochemical oxidants other 
than O3 with regard to abundance in ambient air, and 
potential for welfare effects, and that, as stated in the current ISA, 
``the primary literature evaluating the health and ecological effects 
of photochemical oxidants includes ozone almost exclusively as an 
indicator of photochemical oxidants'' (ISA, section IS.1.1). Thus, the 
Administrator recognizes that, as was the case for previous reviews, 
the evidence base for welfare effects of photochemical oxidants does 
not indicate an importance of any other photochemical oxidants. For 
these reasons, described with more specificity in the ISA and PA, he 
proposes to conclude it is appropriate to retain O3 as the 
indicator for the secondary NAAQS for photochemical oxidants (85 FR 
49896, August 14, 2020).
    In his review of the existing secondary O3 standard, in 
light of the evidence base and quantitative analyses available today, 
the Administrator has given particular attention to consideration of 
the issues raised by the August 2019 court remand, and related issues 
raised in public comment, as well as analyses that were conducted or 
updated in this review in consideration of the remand and related 
public comment. In so doing, he has also given careful consideration of 
the form and averaging time of the current standard and its ability to 
control the patterns of O3 concentrations that contribute to 
environmental exposures of potential concern to the public welfare. 
Further, he has considered what is indicated by the information 
currently available with regard to exposure metrics, supported by the 
current evidence, for assessing potential risks posed to vegetation, 
and protection provided from such exposures. Additionally, with regard 
to visible foliar injury, he has considered the current evidence in the 
ISA in combination with quantitative information and policy evaluations 
in the PA, advice from the CASAC and public comment, in making 
judgments regarding adequacy of the protection provided by the current 
standard from adverse effects to the public welfare related to this 
effect. Before turning to these issues, discussed, in turn, below in 
the context of the EPA's understanding of the information now available 
in the current review, he addresses two endpoints newly identified in 
this review, as well as tropospheric O3 effects related to 
climate.
    With regard to the two insect-related categories of effects with 
new ISA determinations in this review, the Administrator takes note of 
the conclusions that the current evidence is sufficient to infer likely 
causal relationships of O3 with alterations of plant-insect 
signaling and insect herbivore growth and reproduction (as summarized 
in section III.A.2.a above). He additionally recognizes the PA finding 
that uncertainties in the current evidence, as summarized in section 
III.A.2 above, preclude a full understanding of such effects, the air 
quality conditions that might elicit them, the potential for impacts in 
a natural ecosystem. Accordingly, the Administrator notes a lack of 
clarity in the characterization of these effects, and a lack of 
important quantitative information to consider such effects in this 
context such that it is not feasible to relate different patterns of 
O3 concentrations with specific risks of such alterations. 
As a result, the Administrator concludes there is insufficient 
information to judge how particular ambient air concentrations of 
O3 relate to the degree of impacts on public welfare related 
to these effects. Thus, he concludes there is insufficient information 
to judge the current standard inadequate or to identify an appropriate 
revision based on these effects.
    Before focusing further on the key vegetation-related effects 
identified above, the Administrator first considers the strong evidence 
documenting tropospheric O3 as a greenhouse gas causally 
related to radiative forcing, and likely causally related to subsequent 
effects on variables such as temperature and precipitation. In so 
doing, he takes note of the limitations and uncertainties in the 
evidence base that affect characterization of the extent of any 
relationships between O3 concentrations in ambient air in 
the U.S. and climate-related effects, and preclude quantitative 
characterization of climate responses to changes in O3 
concentrations in ambient air at regional or national (vs global) 
scales, as summarized in sections III.A.2 above. As a result, he 
recognizes the lack of important quantitative tools with which to 
consider such effects in this context such that it is not feasible to 
relate different patterns of O3 concentrations at the 
regional (or national) scale in the U.S. with specific risks of 
alterations in temperature, precipitation and other

[[Page 87339]]

climate-related variables. The Administrator finds that these 
significant limitations and uncertainties together preclude his 
identification of an O3 standard reasonably judged to 
provide requisite protection of the public welfare from adverse effects 
linked to O3 influence on radiative forcing, and related 
climate effects. Thus, the Administrator concludes that the information 
available in this review is insufficient to judge the existing standard 
inadequate or to identify an appropriate revision based on 
O3-related climate effects.
    The Administrator turns now to vegetation-related effects, the 
evidence for which as a whole is extensive, spans several decades, and 
supports the Agency's conclusions of causal or likely to be causal 
relationship for O3 in ambient air with an array of effect 
categories. These categories include reduced vegetation growth, 
reproduction, crop yield, productivity and carbon sequestration in 
terrestrial systems; increased tree mortality; alteration of 
terrestrial community composition, belowground biogeochemical cycles 
and ecosystem water cycling; and visible foliar injury (ISA, Appendix 
8). As an initial matter, the Administrator notes the new ISA 
determination that the current evidence is sufficient to infer likely 
causal relationships of O3 with increased tree mortality. 
With regard to the current evidence for this effect, the Administrator 
notes that the evidence does not indicate a potential for O3 
concentrations that occur in locations that meet the current standard 
to cause increased tree mortality, as summarized in section III.A.2.a 
above (PA, section 4.3.1). Accordingly, he finds it appropriate to 
focus on more sensitive effects, such as tree seedling growth, in his 
review of the standard. Thus, in considering the adequacy of protection 
provided by the current standard from adverse effects to the public 
welfare related to these effects, the Administrator begins by 
considering vegetation growth and conceptually related effects with a 
focus on RBL (described in section III.B.2 above), then turns to a 
specific consideration of crop yield loss and lastly, to consideration 
of visible foliar injury.
    With regard to vegetation growth and related effects, the 
Administrator has considered discussions in the PA and in response to 
public comments in section III.B.2 above, and finds it appropriate for 
identification of the requisite protection to extend beyond 
consideration of a magnitude of growth effects, per se, that he may 
judge adverse to the public welfare. Rather, the Administrator extends 
his consideration beyond that, judging it appropriate to consider 
reduced growth (i.e., RBL) as a proxy for an array of other vegetation-
related effects to the public welfare. As discussed in section III.B.2 
above, these categories of effects include reduced vegetation growth, 
reproduction, productivity and carbon sequestration in terrestrial 
systems, and also alteration of terrestrial community composition, 
belowground biogeochemical cycles and ecosystem water cycling. In 
adopting RBL as a proxy for this array of effects, the Administrator 
notes that such a use is consistent with advice from CASAC, and that 
RBL was also adopted as a proxy for this array of effects by the prior 
Administrator, in consideration of advice from the prior CASAC.
    In assessments of RBL estimated from O3 exposure, the 
Administrator takes note of the PA consideration of the established E-R 
relationships for RBL in tree seedlings of 11 species with 
O3 exposures in terms of W126 index (PA, Appendix 4A). In so 
doing, he agrees with the PA conclusion regarding 6% RBL, with which 
the CASAC concurred, as described in sections III.B.1.b and III.B.2 
above), and judges that for his use of RBL as a proxy, maintaining 
O3 concentrations such that associated estimates of RBL fall 
below 6%, as a median across the 11 species represented by the 
established E-R relationships would assure the appropriate protection. 
In making these judgments, he observes that they were also adopted by 
the prior Administrator, with consideration of advice from the prior 
CASAC.
    Further, based on considerations discussed in the PA, advice from 
CASAC and discussion in section III.B.2 above, Administrator has 
considered the use of RBL in his judgment of the public welfare 
protection provided by the secondary standard. Based on those 
considerations, including uncertainties in the E-R relationships and 
their use in the way described here, the Administrator judges it 
appropriate for the standard to protect against W126 index values 
associated with a median RBL at or above 6% (while also controlling 
peak hourly concentrations, as discussed below). Based on this 
judgment, in addition to a recognition of uncertainty in these 
estimates (in light of the discussion in section III.B.2.b(ii) above 
regarding the appropriate duration or averaging for the W126 index 
metric) he concludes it appropriate for the standard to generally 
control exposures in terms of W126 index to a level of 17 ppm-hrs, 
recognizing that the RBL estimated for such a W126 index value is 5.3%, 
a value appreciably below 6%.
    With regard to the appropriate O3 exposure metric to 
employ in assessing adequacy of air quality control in protecting 
against RBL, the Administrator has considered the discussions in the 
PA, and in response to public comments in section III.B.2 above 
regarding the available evidence and air quality analyses. He has also 
considered this in the context of the court remand with regard to the 
EPA's use of a 3-year average W126 index to assess protection from RBL 
and the court's reference to advice from the prior CASAC on protection 
against ``unusually damaging years'' (described in section III.B.2 
above). In so doing, the Administrator considers below the extent of 
conceptual similarities of the 3-year average W126 index with some 
aspects of the derivation approach for the established E-R functions, 
the context of RBL as a proxy (as recognized above), and limitations 
associated with a reliance solely on W126 index as a metric to control 
exposures that might be termed ``unusually damaging.''
    With regard to the established E-R functions used to describe the 
relationship of RBL with O3 in terms of a seasonal W126 
index, the Administrator recognizes that the E-R functions were derived 
mathematically from studies of different exposure durations (varying 
from shorter than one to multiple growing seasons) by applying 
adjustments so that they would yield estimates normalized to the same 
period of time (season), such that the estimates may represent average 
impact for a season, as summarized in section III.A.1.c(ii) above (PA, 
section 4.5.1.2, Appendix 4A, Attachment 1). He notes the compatibility 
of W126 index averaged over multiple growing seasons or years with 
these adjustments. He also notes the exposure levels represented in the 
data underlying the E-R functions are somewhat limited with regard to 
the relatively lower cumulative exposure levels most commonly 
associated with the current standard (e.g., at or below 17 ppm-hrs), 
indicating additional uncertainty for application to such levels. 
Further, he notes the PA observation that some of the underlying 
studies did not find statistically significant effects of O3 
at the lower exposure levels, indicating some uncertainty in 
predictions of an O3-related RBL at those levels, as 
summarized in section III.A.1.c(ii) above (PA, section 4.5.1.2). He 
additionally notes the differing patterns of hourly concentrations of 
the elevated exposure levels in the datasets from which the E-R 
functions from the patterns in ambient

[[Page 87340]]

air meeting the current standard across the U.S. today, as summarized 
in section III.B.2.b(ii). With these considerations regarding the E-R 
functions and their underlying datasets in mind, he also takes note of 
year-to-year variability of factors other than O3 exposures 
that affect tree growth in the natural environment (e.g., related to 
variability in soil moisture, meteorological, plant-related and other 
factors), that have the potential to affect O3 E-R 
relationships (ISA, Appendix 8, section 3.12; 2013 ISA section 9.4.8.3; 
PA, sections 4.3 and 4.5). Based on these considerations, the 
Administrator finds there to be a consistency of his use of the W126 
index averaged over multiple years with the approach used in deriving 
the E-R function, and with other factors that may affect growth in the 
natural environment.
    In light of such considerations, the Administrator agrees with the 
PA finding that several factors contribute uncertainty and some 
resulting imprecision or inexactitude to RBL estimated from single-year 
seasonal W126 index values, as discussed in section III.D.1.b(ii) of 
the proposal (85 FR 49900-01, August 14, 2020; PA sections 4.5.1.2 and 
4.5.3). The Administrator additionally recognizes the qualitative and 
conceptual nature of our understanding, in many cases, of relationships 
of O3 effects on plant growth and productivity with larger-
scale impacts, such as those on populations, communities and 
ecosystems. From these considerations, he judges that use of a seasonal 
RBL averaged over multiple years, such as a 3-year average, is 
reasonable, and provides a more stable and well-founded RBL estimate 
for his purposes as a proxy to represent the array of vegetation-
related effects identified above. The Administrator additionally takes 
note of the CASAC advice agreeing with the EPA's focus on a 3-year 
average W126 for this purpose, concluding such a focus to be reasonable 
and scientifically sound, as summarized in section III.B.1.b above. In 
light of these considerations, the Administrator finds there to be 
support for use of an average seasonal W126 index derived from multiple 
years (with their representation of variability in environmental 
factors), concluding the use of such averaging to provide an 
appropriate representation of the evidence and attention to 
considerations summarized above. In so doing, he finds that sole 
reliance on single year W126 estimates for reaching judgments with 
regard to magnitude of O3 related RBL and associated 
judgments of public welfare protection would ascribe a greater 
specificity and certainty to such estimates than supported by the 
current evidence. Rather, he finds it appropriate, for purposes of 
considering public welfare protection from effects for which RBL is 
used as a proxy, to primarily consider W126 index in terms of a 3-year 
average metric.
    In his consideration of the appropriateness of using a 3-year 
average W126 metric, the Administrator additionally takes note of the 
discussion in section III.B.2 above with regard to protection against 
``unusually damaging years,'' a caution raised by the prior CASAC in 
considering a secondary standard in terms of a 3-year average W126 
index (and an issue raised in the court remand). With regard to this 
caution, the Administrator finds informative the discussion in section 
III.B.2 above regarding the extent to which a standard in terms of a 
W126 metric might be expected to control exposure circumstances of 
concern (e.g., for growth effects, among others). This discussion and 
its focus on air quality analyses in the PA and additional analyses 
conducted in consideration of public comment investigate the annual 
occurrence of elevated hourly O3 concentrations which may 
contribute to vegetation exposures of concern (PA, Appendix 2A, section 
2A.2; Wells, 2020).\239\
---------------------------------------------------------------------------

    \239\ The ISA references the longstanding recognition of the 
risk posed to vegetation of peak hourly O3 concentrations 
(e.g., ``[h]igher concentrations appear to be more important than 
lower concentrations in eliciting a response'' [ISA, p. 8-180]; 
``higher hourly concentrations have greater effects on vegetation 
than lower concentrations'' [2013 ISA, p. 91-4] ``studies published 
since the 2006 O3 AQCD do not change earlier conclusions, 
including the importance of peak concentrations, . . . in altering 
plant growth and yield'' [2013 ISA, p. 9-117]).
---------------------------------------------------------------------------

    These air quality analyses illustrate limitations of the W126 index 
for purposes of controlling peak concentrations, and also the strengths 
of the current standard in this regard. As discussed more fully in 
section III.B.2.b(ii) above, the W126 index cannot, by virtue of its 
definition, always differentiate between air quality patterns with high 
peak concentrations and those without such concentrations. This is 
demonstrated in the air quality analyses which show that the form and 
averaging time of the existing standard is much more effective than the 
W126 index in limiting peak concentrations (e.g., hourly O3 
concentrations at or above 100 ppb) and in limiting number of days with 
any such hours (Wells, 2020, e.g., Figures 4, 5, 8, 9 compared to 
Figures 6, 7, 10 and 11). A similar finding is evidence in the 
historical data extending back to 2000. These data show the appreciable 
reductions in peak concentrations that have been achieved in the U.S. 
as air quality has improved under O3 standards of the 
existing form and averaging time (Wells, 2020, Figures 12 and 13). From 
these analyses, the Administrator concludes that the form and averaging 
time of the current standard is effective in controlling peak hourly 
concentrations and that a W126 index based standard would be much less 
effective in providing the needed protection against years with such 
elevated and potentially damaging hourly concentrations. Thus, in light 
of the current evidence and quantitative air quality analyses, the 
Administrator notes that the W126 index, by its very definition, does 
not provide specificity with regard to year-to-year variability in 
elevated hourly O3 concentrations with the potential to 
contribute to the ``unusually damaging years'' that the prior CASAC 
identified for increased concern. In so doing, he disagrees with the 
statement of the prior CASAC that a single-year W126 index would 
necessarily provide protection from such years. Further, he judges that 
a standard based on either a 3-year or a single-year W126 index would 
not be expected to provide effective control of the peak concentrations 
that may contribute to ``unusually damaging years'' for vegetation.
    Thus, in considering the extent of protection provided by the 
current standard, in addition to considering seasonal W126 averaged 
over a 3-year period to estimate median RBL using the established E-R 
functions, the Administrator finds it appropriate to also consider 
other metrics, including peak hourly concentrations. While he 
recognizes that the evidence does not indicate a particular threshold 
number of hours at or above 100 ppb (or another reference point for 
elevated concentrations), he takes particular note of the evidence of 
greater impacts from higher concentrations (particularly with increased 
frequency) and of the air quality analyses that document variability in 
such concentrations for the same W126 index value. In light of these 
considerations, he judges such a multipronged approach to be needed to 
ensure appropriate consideration of exposures of concern and the 
associated protection from them afforded by the secondary standard. 
Thus, the Administrator concludes that use of a seasonal W126 averaged 
over a 3-year period, which is the design value period for the current 
standard, to estimate median RBL using the established E-R functions, 
in combination with a

[[Page 87341]]

broader consideration of air quality patterns, such as peak hourly 
concentrations, is appropriate for considering the public welfare 
protection provided by the standard.
    In the discussion above, the Administrator recognizes a number of 
public welfare policy judgments important to his review of the current 
standard that include the appropriateness of the W126 index, averaged 
across a 3-year period, for assessing the extent of protection afforded 
by the standard from cumulative seasonal O3 exposures. In 
reflecting on these judgments, the current evidence presented in the 
ISA and the associated evaluations in the PA, the Administrator 
concludes that the currently available information supports such 
judgments, additionally noting the CASAC concurrence with regard to the 
scientific support for these judgments (Cox 2020a, Consensus Responses 
to Charge Questions p. 21). Accordingly, the Administrator concludes 
that the current evidence base and available information (qualitative 
and quantitative) continues to support consideration of the potential 
for O3-related vegetation impacts in terms of the RBL 
estimates from established E-R functions as a quantitative tool within 
a larger framework of considerations pertaining to the public welfare 
significance of O3 effects. Such consideration includes 
effects that are associated with effects on vegetation, and 
particularly those that conceptually relate to growth, and that are 
causally or likely causally related to O3 in ambient air, 
yet for which there are greater uncertainties affecting estimates of 
impacts on public welfare. The Administrator additionally notes that 
this approach to weighing the available information in reaching 
judgments regarding the secondary standard additionally takes into 
account uncertainties regarding the magnitude of growth impact that 
might be expected in mature trees (e.g., compared to seedlings), and of 
related, broader, ecosystem-level effects for which the available tools 
for quantitative estimates are more uncertain and those for which the 
policy foundation for consideration of public welfare impacts is less 
well established.
    In his consideration of the adequacy of protection provided by the 
current standard, the Administrator does not consider every possible 
instance of an effect on vegetation growth from O3 to be 
adverse to public welfare, although he recognizes that, depending on 
factors including extent and severity, such vegetation-related effects 
have the potential to be adverse to public welfare. Comments from the 
current CASAC, in the context of its review of the draft PA, expressed 
the view that the strategy described by the prior Administrator for the 
secondary standard established in 2015 with its focus on limiting 3-
year average W126 index values somewhat below those associated with a 
6% RBL in the median species and associated W126 index target of 17 
ppm-hrs (in terms of a 3-year average), at or below which the 2015 
standard was expected to generally restrict cumulative seasonal 
exposure, is ``still scientifically reasonable'' and ``still effective 
in particularly protecting the public welfare in light of vegetation 
impacts from ozone'' (Cox, 2020a, Consensus Responses to Charge 
Questions p. 21). In light of this advice and based on the current 
evidence as evaluated in the PA, the Administrator judges that this 
approach or framework, with its focus on controlling cumulative 
seasonal exposures associated with an RBL of 6% or greater, by limiting 
air quality in terms of a 3-year average W126 index, to or below a 
target of 17 ppm-hrs, in combination with a broader consideration of 
air quality patterns, such as control of peak hourly concentrations, 
associated with meeting the current standard, is appropriate for his 
use in this review. In so doing, he additional notes the isolated, rare 
occurrences in locations meeting the current standard of such exposures 
at 19 ppm-hrs. Based on the current information to inform consideration 
of vegetation effects and their potential adversity to public welfare, 
he additionally judges that the RBL estimates associated with such 
marginally higher exposures in isolated, rare instances are not 
indicative of effects that would be adverse to the public welfare, 
particularly in light of variability in the array of environmental 
factors that can influence O3 effects in different systems 
and uncertainties associated with estimates of effects associated with 
this magnitude of cumulative exposure in the natural environment.
    With regard to O3 effects on crop yield, the 
Administrator, as an initial matter, takes note of the long-standing 
evidence, qualitative and quantitative, of the reducing effect of 
O3 on the yield of many crops, as summarized in the PA and 
current ISA and characterized in detail in past reviews (e.g., 2013 
ISA, 2006 AQCD, 1997 AQCD, 2014 WREA). He additionally notes the 
established E-R functions for 10 crops and the estimates of RYL derived 
from them, as presented in the PA (PA, Appendix 4A, section 4A.1, Table 
4A-4), and the potential public welfare significance of reductions in 
crop yield, as summarized in section III.A.2.b above. In so doing, 
however, he additionally recognizes that not every effect on crop yield 
will be adverse to public welfare. In the case of crops in particular 
there are a number of complexities related to the heavy management of 
many crops to obtain a particular output for commercial purposes, and 
related to other factors, that the Administrator takes into 
consideration in evaluating potential O3-related public 
welfare impacts, as summarized in section III.B.2.b(iv) above (PA, 
sections 4.5.1.3 and 4.5.3).
    Similarly, the Administrator concludes that the extensive 
management of agricultural crops that occurs to elicit optimum yields 
(e.g., through irrigation and usage of soil amendments, such as 
fertilizer) is relevant in evaluating the extent of RYL estimated from 
experimental O3 exposures that should be judged adverse to 
the public welfare. He considers these opportunities in crop management 
for market objectives, as well as complications in judging relative 
adversity that relate to market responses and their effects on 
producers and consumers in evaluating the potential impact on public 
welfare of estimated crop yield losses. Further, the Administrator 
takes note of the conclusion of the CASAC that the available evidence 
does not call into question the adequacy of the current secondary 
standard and that it should be retained (Cox 2020a, p.1).
    The Administrator also considered the public comments, discussed in 
section III.B.2.b(iv) above, suggesting that the proposed decision was 
not giving adequate consideration to crop yield effects and that his 
decision should consider a statement by the prior CASAC, raised in 
public comments, that a 5% RYL estimate, as the median based on the 10 
E-R functions, ``represents an adverse impact.'' With regard to the 
prior CASAC statement, he notes the discussion in section III.B.2.b(iv) 
above regarding the unclear basis for the prior CASAC judgment, both 
with regard to a connection of an estimated 5% RYL to broader impacts 
and to judgments on significance of a 5% RYL to the public welfare. In 
considering the adequacy of protection of the public welfare from 
effects related to crop yield loss, the Administrator considers the air 
quality analyses and the W126 index levels commonly occurring in areas 
that meet the current standard. In so doing, he notes that W126 index 
values (3-year average) were at or below 17 ppm-hrs in virtually all 
monitoring sites with air quality meeting the current standard.

[[Page 87342]]

Based on the established E-R functions, the median RYL estimate 
corresponding to 17 ppm-hrs is 5.1%. In considering single-year index 
values, as discussed in section III.B.2.b(vi), the vast majority are 
similarly low (with more than 99% less than or equal to 17 ppm-hrs), 
and the higher values predominantly occur in urban areas. The 
Administrator additionally takes note of the discussion in section 
III.B.2.b(ii) above regarding the role of elevated hourly 
concentrations in effects on vegetation growth and yield. In so doing, 
in addition to his consideration of W126 index occurring in areas that 
meet the current standard, he also takes note of the control of 
elevated hourly O3 concentrations that is exerted by the 
current standard.
    In light of all of the above, in reaching his judgment regarding 
public welfare implications of the W126 index values summarized here 
(and associated estimated RYL), including the isolated and rare 
occurrence of somewhat higher values, the Administrator notes that the 
secondary standard is not intended to protect against all known or 
anticipated O3-related effects, but rather those that are 
judged to be adverse to the public welfare. He also takes into 
consideration the extensive management of agricultural crops, and the 
complexities associated with identifying adverse public welfare effects 
for market-traded goods (where producers and consumers may be impacted 
differently). Based on all of these factors, the Administrator 
disagrees with the prior CASAC statement that an estimated median RYL 
of 5% represents an adverse impact and further judges that an estimated 
median RYL of 5.1%, based on experimental exposures, would not 
constitute an adverse effect on public welfare. Accordingly, the 
Administrator notes that the current standard generally maintains air 
quality at a W126 index below 17 ppm-hrs, with few exceptions, and 
accordingly would limit the estimated RYL (based on experimental 
O3 exposures) to this degree. Therefore, he concludes that 
the current standard provides adequate protection of public welfare 
related to crop yield loss and does not need to be revised to provide 
additional protection against this effect. In so doing, the 
Administrator notes the conclusions by the current CASAC that the 
evidence supports retaining the current standard, without revision.
    Turning to consideration of visible foliar injury and protection 
afforded by the secondary standard from associated impacts to the 
public welfare, the Administrator takes note of the long-standing and 
well-established evidence base, updated in the ISA for this review, and 
of policy-relevant analyses presented in the PA to inform his judgments 
regarding a secondary standard that provides appropriate protection of 
the public welfare from this effect. In so doing, he has also taken 
into account issues raised by public comments, both with regard to our 
understanding of relationships between O3 exposure 
circumstances and extent and severity of injury in natural areas across 
the U.S., and with regard to the extent of our understanding of the 
relationship of injury extent and severity to public welfare effects 
anticipated to be adverse, and the Murray Energy remand.
    In considering public welfare implications of this effect, he notes 
the potential for this effect, when of a significant extent and 
severity, to reduce aesthetic and recreational values, such as the 
aesthetic value of scenic vistas in protected natural areas including 
national parks and wilderness areas, as well as other areas similarly 
protected by state and local governments for similar public uses. Based 
on these considerations, the Administrator recognizes that, depending 
on its severity and spatial extent, as well as the location(s) and the 
associated intended use, the impact of visible foliar injury on the 
physical appearance of plants has the potential to be significant to 
the public welfare. In this regard, he agrees with the PA statement 
that cases of widespread and relatively severe injury during the 
growing season (particularly when sustained across multiple years and 
accompanied by obvious impacts on the plant canopy) might reasonably be 
expected to have the potential to adversely impact the public welfare 
in scenic and/or recreational areas, particularly in areas with special 
protection, such as Class I areas (PA, sections 4.3.2 and 4.5.1). In so 
doing, the Administrator notes that the secondary standard is not meant 
to protect against all known or anticipated O3-related 
welfare effects, but rather those that are judged to be adverse to the 
public welfare, and further notes that there are not established 
measures for when such welfare effects should be judged adverse to the 
public welfare. Rather, the level of protection from known or 
anticipated adverse effects to public welfare that is requisite for the 
secondary standard is a public welfare policy judgment to be made by 
the Administrator.
    While recognizing there to be a paucity of established approaches 
for interpreting specific levels of severity and extent of foliar 
injury in natural areas with regard to impacts on the public welfare 
(e.g., related to recreational services), the Administrator recognizes 
that injury to whole stands of trees of a severity apparent to the 
casual observer (e.g., when viewed as a whole from a distance) would 
reasonably be expected to affect recreational values and thus pose a 
risk of adverse effects to the public welfare. He further notes that 
the available information does not provide for specific 
characterization of the incidence and severity that would not be 
expected to be apparent to the casual observer, nor for clear 
identification of the pattern of O3 concentrations that 
would provide for such a situation.
    In this context, the Administrator takes note of the system 
developed by the USFS for its monitoring program \240\ to categorize BI 
scores of visible foliar injury at biosites (sites with O3-
sensitive vegetation assessed for visible foliar injury) in natural 
vegetated areas by severity levels (described in section III.A.2.c(ii) 
above). While recognizing that quantitative analyses and evidence are 
lacking that might support a more precise conclusion with regard to a 
magnitude of BI score coupled with an extent of occurrence that might 
be specifically identified as adverse to the public welfare, the 
Administrator also takes note of the D.C. Circuit's holding that 
substantial uncertainty about the level at which visible foliar injury 
may become adverse to public welfare does not necessarily provide a 
basis for declining to evaluate whether the existing standard provides 
requisite protection against such effects. See Murray Energy Corp. v. 
EPA, 936 F.3d 597, 619-20 (D.C. Cir. 2019). In this context, he 
considers the discussion in the PA and in sections III.A.2.b, III.A.2.c 
and III.B.2 above regarding the USFS biosite monitoring program. He 
finds the scale of this program's objectives, which focus on natural 
settings in the U.S. and forests as opposed to individual plants, to be 
suited for his consideration with regard to the public welfare 
protection afforded by the current standard, and consequently, he finds 
the data and analyses generated by the program informative in such 
considerations.
---------------------------------------------------------------------------

    \240\ During the period from 1994 (beginning in eastern U.S.) 
through 2011, the USFS conducted surveys of the occurrence and 
severity of visible foliar injury on sensitive species at sites 
across most of the U.S. following a national protocol.
---------------------------------------------------------------------------

    In this context, he takes note of the USFS system, including its 
descriptors for BI scores of differing magnitude intended for that 
Agency's consideration in identifying areas of potential impact to 
forest resources. As described in section III.A.2.b(iii) above, very 
low BI scores (at or below 5) are

[[Page 87343]]

described by the USFS scheme as ``little or no foliar injury'' (Smith 
et al., 2007; Smith et al., 2012).\241\ The Administrator notes that BI 
scores above 15 are categorized as moderate to severe (and scores above 
25 as severe). In so doing, in light of considerations raised in the PA 
and consideration of public comment, he recognizes the lower categories 
of BI scores as indicative of injury of generally lesser risk to the 
natural area or to public enjoyment, which he judges unlikely to be 
indicative of injury of such a magnitude or extent as to pose risk of 
adverse effects to the public welfare. Thus, the Administrator reaches 
the conclusion that occurrence of the lower categories of BI scores 
does not pose concern for the public welfare, but that findings of BI 
scores categorized as ``moderate to severe'' injury by the USFS scheme 
would be an indication of visible foliar injury occurrence that, 
depending on extent and severity, may raise public welfare concerns. In 
this framework, the Administrator considers the PA evaluations of the 
currently available information and what it indicates with regard to 
patterns of air quality of concern for such an occurrence, and the 
extent to which they are expected to occur in areas that meet the 
current standard.
---------------------------------------------------------------------------

    \241\ Studies that consider such data for purposes of 
identifying areas of potential impact to the forest resource suggest 
this category corresponds to ``none'' with regard to ``assumption of 
risk'' (Smith et al., 2007; Smith et al., 2012).
---------------------------------------------------------------------------

    In so doing, the Administrator takes particular note of the USFS 
biosite monitoring program studies of the occurrence, extent and 
severity of visible foliar injury in indicator species in defined plots 
or biosites in natural areas across the U.S. These studies of data for 
USFS biosites (sites with O3-sensitive vegetation assessed 
for visible foliar injury) have often summarized O3 
concentrations in terms of cumulative exposure metrics (e.g., SUM06 or 
W126 index). Some of these studies, particularly those examining such 
data across multiple years and multiple regions of the U.S., have 
reported that variation in cumulative O3 exposure, in terms 
of such metrics, does not completely explain the patterns of occurrence 
and severity of injury observed. Although the availability of detailed 
analyses that have explored multiple exposure metrics and other 
influential variables is limited, multiple studies have indicated a 
potential role for an additional metric, one related to the occurrence 
of days with relatively high concentrations (e.g., number of days with 
a 1-hour concentration at or above 100 ppb), as summarized in section 
III.A.2.c above (PA, section 4.5.1.2). Thus, the Administrator takes 
note of this evidence indicating an influence of peak concentrations on 
BI scores (beyond an influence of cumulative exposure). He also finds 
noteworthy the extensive evidence of trends across the past nearly 20 
years that indicate reductions in severity of visible foliar injury 
that parallel reductions in peak concentrations that have been 
suggested to be influential in the severity of visible foliar 
injury.\242\
---------------------------------------------------------------------------

    \242\ For example, the PA describes findings from USFS studies 
that have concluded a ``declining risk of probable impact'' over the 
16-year period of the program, especially after 2002 (e.g., Smith, 
2012), and the parallel national reductions in O3 
concentrations from 2000 through 2018 in terms of cumulative 
seasonal exposures, as well as in peak O3 concentrations 
such as the annual fourth highest daily maximum 8-hour concentration 
and also the occurrence of 1-hour concentrations above 100 ppb (PA, 
Figure 2-11, Appendix 2A, Tables 2A-2 to 2A-4, and Appendix 4D, 
Figure 4D-9).
---------------------------------------------------------------------------

    Further, the Administrator considers the PA analysis of a dataset 
developed from USFS biosite index scores, combined with W126 estimates 
and soil moisture categories, summarized in section III.A.2.c above. In 
so doing, he takes note of the PA observation that important 
uncertainties remain in the understanding of the O3 exposure 
conditions that will elicit visible foliar injury of varying severity 
and extent in natural areas, and particularly in light of the other 
environmental variables that influence its occurrence, and of the 
recognition by the CASAC that ``uncertainties continue to hamper 
efforts to quantitatively characterize the relationship of [visible 
foliar injury] occurrence and relative severity with ozone exposures'' 
(Cox 2020a, Consensus Responses to Charge Questions, p. 20). 
Notwithstanding, and while being mindful of, such uncertainties with 
regard to predictive O3 metric or metrics and a quantitative 
function relating them to incidence and severity of visible foliar 
injury in natural areas (as also noted in the USFS studies referenced 
above), the Administrator takes note of the PA finding that the 
incidence of nonzero BI scores, and, particularly of relatively higher 
scores (such as scores above 15 which are indicative of ``moderate to 
severe'' injury in the USFS scheme) appears to markedly increase only 
with W126 index values above 25 ppm-hrs, as summarized in section 
III.B.2.b above (PA, section 4.3.3 and Appendix 4C).
    In light of these observations, the Administrator finds the current 
evidence to be incomplete with regard to information to support a 
quantitative characterization of air quality that would be anticipated 
to result in visible foliar injury of an extent and severity to cause 
adverse effects to the public welfare. The Administrator also considers 
discussion in the court's remand of the 2015 standard with regard to 
visible foliar injury (Murray Energy Corp. v EPA, 936 F.3d at 619-20). 
The court concluded that the EPA had failed to offer a reasoned 
explanation for deciding not to specify a level of air quality to 
protect against adverse effects related to visible foliar injury. In 
particular, the court stated that the EPA had not explained why it was 
unable to choose such a level although the prior CASAC had provided 
advice with regard to a specific level. The EPA's disagreement with the 
prior CASAC on its identified level is explained in section III.B.2 
above, as is the reason why the EPA did not find the analysis on which 
the prior CASAC based its advice to be appropriate for such a 
conclusion.\243\ This and other associated issues raised by the court 
have been raised in public comments on the proposal for this action and 
are addressed in section III.B.2 above.
---------------------------------------------------------------------------

    \243\ As discussed in section III.B.2.b, the cumulative 
frequency graph relied on by the CASAC does not present biosite 
scores for comparison at different cumulative exposure levels. 
Accordingly, it does not provide the type of analysis that is needed 
for the EPA to reach a conclusion about the extent of protection 
that different patterns of O3 concentrations would 
provide against visible foliar injury of an extent and severity as 
to pose risk of adverse effects to the public welfare.
---------------------------------------------------------------------------

    Based on the evidence and quantitative analyses available in the 
present review, and advice from the current CASAC, the Administrator 
considers the question of a level of air quality that would provide 
protection against visible foliar injury related effects known or 
anticipated to cause adverse effects to the public welfare. Based on 
the evidence and associated quantitative analyses in this review, the 
Administrator's judgment reflects his recognition of less confidence 
and greater uncertainty in the existence of adverse public welfare 
effects with lower O3 exposures. In this context, the 
Administrator judges that W126 index values at or below 25 ppm-hrs, 
when in combination with infrequent occurrences of hourly 
concentrations at or above 100 ppb, would not be anticipated to pose 
risk of visible foliar injury of an extent and severity so as to be 
adverse to the public welfare.
    With these conclusions in mind, the Administrator considers the air 
quality analyses presented in the PA and the additional analyses 
developed in response to public comment. In so doing, he notes that a 
W126 index above

[[Page 87344]]

25 ppm-hrs (either as a 3-year average or in a single year) is not seen 
to occur at monitoring locations (including in or near Class I areas) 
where the current standard is met, and that, in fact, values above 17 
or 19 ppm-hrs are rare, as summarized in section III.A.3 above (PA, 
Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3). Further, the 
Administrator takes note of the PA consideration of the USFS 
publications that identify an influence of peak concentrations on BI 
scores (beyond an influence of cumulative exposure) and the PA 
observation of the appreciable control of peak concentrations exerted 
by the form and averaging time of the current standard, as evidenced by 
the air quality analyses which document reductions in 1-hour daily 
maximum concentrations with declining design values. He also notes, as 
discussed above, the uncommonness of days with any hours at or above 
100 ppb at monitoring sites that meet the current standard, as well as 
the minimal number of hours on any such days (Wells, 2020). Based on 
these considerations, the Administrator concludes that the current 
standard provides control of air quality conditions that contribute to 
increased BI scores and to scores of a magnitude indicative of 
``moderate to severe'' foliar injury.
    The Administrator further takes note of the PA finding that the 
current information, particularly in locations meeting the current 
standard or with W126 index estimates likely to occur under the current 
standard, does not indicate a significant extent and degree of injury 
(e.g., based on analyses of BI scores in the PA, Appendix 4C) or 
specific impacts on recreational or related services for areas, such as 
wilderness areas or national parks. Thus, he gives credence to the 
associated PA conclusion that the evidence indicates that areas that 
meet the current standard are unlikely to have BI scores reasonably 
considered to be impacts of public welfare significance. Based on all 
of the considerations raised here, the Administrator concludes that the 
current standard provides sufficient protection of natural areas, 
including particularly protected areas such as Class I areas, from 
O3 concentrations in the ambient air that might be expected 
to elicit visible foliar injury of such an incidence and severity as 
would reasonably be judged adverse to the public welfare.
    With a primary focus on RBL in its role as proxy, the Administrator 
further considers the analyses available in this review of recent air 
quality at sites across the U.S., particularly including those sites in 
or near Class I areas, and also the analyses of historical air quality. 
In so doing, the Administrator recognizes that these analyses are 
distributed across all nine NOAA climate regions and 50 states, 
although some geographic areas within specific regions and states may 
be more densely covered and represented by monitors than others, as 
summarized in section III.C of the proposal (PA, Appendix 4D). The 
Administrator notes that the findings from both the analysis of the air 
quality data from the most recent period and from the larger analysis 
of historical air quality data extending back to 2000, as presented in 
the PA and summarized in section III.A.3 above, are consistent with the 
air quality analyses available in the last review. That is, in 
virtually all design value periods and all locations at which the 
current standard was met across the 19 years and 17 design value 
periods (in more than 99.9% of such observations), the 3-year average 
W126 metric was at or below 17 ppm-hrs. Further, in all such design 
value periods and locations the 3-year average W126 index was at or 
below 19 ppm-hrs. The Administrator additionally considers the 
protection provided by the current standard from the occurrence of 
O3 exposures within a single year with potentially damaging 
consequences, such as a significantly increased incidence of areas with 
visible foliar injury that might be judged moderate to severe, as 
discussed in section III.B.2 above. In so doing, he takes notes of the 
PA analyses, summarized in section III.A.2.c above, of USFS BI scores, 
giving particular focus to scores above 15, termed ``moderate to severe 
injury'' by the USFS categorization scheme, as described in section 
III.A.2.b above (PA, sections 4.3.3.2, 4.5.1.2 and Appendix 4C). He 
notes the PA finding that incidence of sites with BI scores above 15 
markedly increases with W126 index estimates above 25 ppm-hrs. In this 
context, he additionally takes note of the air quality analysis finding 
of a scarcity of single-year W126 index values above 25 ppm-hrs at 
sites that meet the current standard, with just a single occurrence 
across all U.S. sites with design values meeting the current standard 
in the 19-year historical dataset dating back to 2000 (PA, section 4.4 
and Appendix 4D). Further, in light of the evidence indicating that 
peak short-term concentrations (e.g., of durations as short as one 
hour) may also play a role in the occurrence of visible foliar injury, 
the Administrator additionally takes note of the air quality analyses 
in the PA and in the additional analysis documented in Wells (2020). 
These analyses of data from the past 20 years show a declining trend in 
1-hour daily maximum concentrations mirroring the declining trend in 
design values, supporting the PA conclusion that the form and averaging 
time of the current standard provides appreciable control of peak 1-
hour concentrations. Furthermore, these analyses indicate there to be 
only a few days among sites meeting the current standard, with hourly 
concentrations at or above 100 ppb (just seven in the period from 2000 
through 2018) (Wells, 2020). In light of these findings from the air 
quality analyses and considerations in the PA, both with regard to 3-
year average W126 index values at sites meeting the current standard 
and the rarity of such values at or above 19 ppm-hrs, and with regard 
to single-year W126 index values at sites meeting the current standard, 
and the rarity of such values above 25 ppm-hrs, as well as with regard 
to the appreciable control of 1-hour daily maximum concentrations, the 
Administrator judges that the current standard provides adequate 
protection from air quality conditions with the potential to be adverse 
to the public welfare.
    In reaching his conclusion on the current secondary O3 
standard, the Administrator recognizes, as is the case in NAAQS reviews 
in general, his decision depends on a variety of factors, including 
science policy judgments and public welfare policy judgments, as well 
as the currently available information. With regard to the current 
review, the Administrator gives primary attention to the principal 
effects of O3 as recognized in the current ISA, the 2013 ISA 
and past AQCDs, and for which the evidence is strongest (e.g., growth, 
reproduction, and related larger-scale effects, as well as, visible 
foliar injury). With regard to growth and the categories of effects 
identified above for which RBL has been identified for use as a proxy, 
based on all of the considerations above, including the discussion of 
air quality immediately above, the Administrator judges the current 
standard to provide adequate protection for air quality conditions with 
the potential to be adverse to the public welfare. Further, with regard 
to visible foliar injury, the Administrator concludes that the 
currently available information on visible foliar injury and with 
regard to air quality analyses that may be informative with regard to 
air quality conditions associated with appreciably increased incidence 
and severity of BI scores at USFS biomonitoring sites, and with 
particular attention to Class I and other areas afforded special 
protection,

[[Page 87345]]

indicates the current standard to provide adequate protection from 
visible foliar injury of an extent or severity that might be 
anticipated to be adverse to the public welfare.
    In summary, the Administrator has based his decision on the public 
welfare protection afforded by the secondary O3 standard 
from identified O3-related welfare effects, and from their 
potential to present adverse effects to the public welfare, on 
judgments regarding what the available evidence, quantitative 
information, and associated uncertainties and limitations (such as 
those identified above) indicate with regard to the protection provided 
from the array of O3 welfare effects. He finds that, as a 
whole, this information, as summarized above, and presented in detail 
in the ISA and PA, does not indicate the current standard to allow air 
quality conditions with implications of concern for the public welfare. 
He has additionally considered the advice from the CASAC in this 
review, including its finding ``that the available evidence does not 
reasonably call into question the adequacy of the current secondary 
ozone standard and concurs that it should be retained'' (Cox, 2020a, p. 
1), and well as public comment on the proposed decision. Based on all 
of the above considerations, including his consideration of the 
currently available evidence and quantitative exposure/risk 
information, the Administrator concludes that the current secondary 
standard is requisite to protect the public welfare from known or 
anticipated adverse effects of O3 and related photochemical 
oxidants in ambient air, and thus that the current standard should be 
retained, without revision.

C. Decision on the Secondary Standard

    For the reasons discussed above and taking into account information 
and assessments presented in the ISA and PA, the advice from the CASAC, 
and consideration of public comments, the Administrator concludes that 
the current secondary O3 standard is requisite to protect 
the public welfare from known or anticipated adverse effects, and is 
retaining the current standard without revision.

IV. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders 
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 13563: Improving Regulation and Regulatory Review

    The Office of Management and Budget (OMB) has determined that this 
action is a significant regulatory action and it was submitted to OMB 
for review. Any changes made in response to OMB recommendations have 
been documented in the docket. Because this action does not change the 
existing O3 NAAQS, it does not impose costs or benefits 
relative to the baseline of continuing with the current NAAQS in 
effect. EPA has thus not prepared a Regulatory Impact Analysis for this 
action.

B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs

    This action is not an Executive Order 13771 regulatory action. 
There are no quantified cost estimates for this action because EPA is 
retaining the current standards.

C. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under 
the PRA. There are no information collection requirements directly 
associated with a decision to retain a NAAQS without any revision under 
section 109 of the CAA, and this action retains the existing 
O3 NAAQS without any revisions.

D. 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 
action retains, without revision, existing national standards for 
allowable concentrations of O3 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).

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely 
affect small governments. This action imposes no enforceable duty on 
any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

    This action does not have federalism implications. It 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.

G. 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. This action does not change existing 
regulations; it retains the existing O3 NAAQS, without 
revision. Executive Order 13175 does not apply to this action.

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

    This action is not subject to Executive Order 13045 because it is 
not economically significant as defined in Executive Order 12866. The 
health effects evidence and risk assessment information for this 
action, which focuses on children and people (of all ages) with asthma 
as key at-risk populations, is summarized in section II.A.2 and II.A.3 
above and described in the ISA and PA, copies of which are in the 
public docket for this action.

I. Executive Order 13211: Actions That Significantly Affect Energy 
Supply, Distribution or Use

    This action is not a ``significant energy action'' for purposes of 
Executive Order 13211. The action is not likely to have a significant 
adverse effect on the supply, distribution, or use of energy. This 
action retains the current O3 NAAQS. This decision does not 
change existing requirements. The Administrator of the Office of 
Information and Regulatory Affairs has not otherwise designated this 
action as a significant energy action. Thus, this decision does not 
constitute a significant energy action as defined in Executive Order 
13211.

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

K. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations

    The EPA believes that this action does not have disproportionately 
high and

[[Page 87346]]

adverse human health or environmental effects on minority, low-income 
populations and/or indigenous peoples, as specified in Executive Order 
12898 (59 FR 7629, February 16, 1994). The action described in this 
document is to retain without revision the existing O3 NAAQS 
based on the Administrator's conclusions that the existing primary 
standard protects public health, including the health of sensitive 
groups, with an adequate margin of safety, and that the existing 
secondary standard protects public welfare from known or anticipated 
adverse effects. As discussed in section II above, the EPA expressly 
considered the available information regarding health effects among at-
risk populations in reaching the decision that the existing standard is 
requisite.

L. Determination Under Section 307(d)

    Section 307(d)(1)(V) of the CAA provides that the provisions of 
section 307(d) apply to ``such other actions as the Administrator may 
determine.'' Pursuant to section 307(d)(1)(V), the Administrator 
determines that this action is subject to the provisions of section 
307(d).

M. Congressional Review Act

    The EPA will submit a rule report to each House of the Congress and 
to the Comptroller General of the United States. This action is not a 
``major rule'' as defined by 5 U.S.C. 804(2).

V. References

Adams, WC (2000). Ozone dose-response effects of varied equivalent 
minute ventilation rates. J Expo Anal Environ Epidemiol 10(3): 217-
226.
Adams, WC (2002). Comparison of chamber and face-mask 6.6-hour 
exposures to ozone on pulmonary function and symptoms responses. 
Inhal Toxicol 14(7): 745-764.
Adams, WC (2003). Comparison of chamber and face mask 6.6-hour 
exposure to 0.08 ppm ozone via square-wave and triangular profiles 
on pulmonary responses. Inhal Toxicol 15(3): 265-281.
Adams, WC (2006). Comparison of chamber 6.6-h exposures to 0.04-0.08 
PPM ozone via square-wave and triangular profiles on pulmonary 
responses. Inhal Toxicol 18(2): 127-136.
Adams, WC and Ollison, WM (1997). Effects of prolonged simulated 
ambient ozone dosing patterns on human pulmonary function and 
symptomatology. Air & Waste Management Association, Pittsburgh, PA.
Adhikari, A and Yin, J (2020). Short-Term Effects of Ambient Ozone, 
PM2.5, and Meteorological Factors on COVID-19 Confirmed 
Cases and Deaths in Queens, New York. Int J Env Res Public Health 
17(11): 4047.
Alexis, NE, Lay, JC, Hazucha, M, Harris, B, Hernandez, ML, Bromberg, 
PA, Kehrl, H, Diaz-Sanchez, D, Kim, C, Devlin, RB and Peden, DB 
(2010). Low-level ozone exposure induces airways inflammation and 
modifies cell surface phenotypes in healthy humans. Inhal Toxicol 
22(7): 593-600.
Archer, CL. Brodie, JF and Rauscher, SA (2019). Global Warming Will 
Aggravate Ozone Pollution in the U.S. Mid-Atlantic. J Appl Meteorol 
Climatol 58(6): 1267-78.
ATS (1985). Guidelines as to what constitutes an adverse respiratory 
health effect, with special reference to epidemiologic studies of 
air pollution. Am Rev Respir Dis 131(4): 666-668.
ATS (2000). What constitutes an adverse health effect of air 
pollution? Am J Respir Crit Care Med 161(2): 665-673.
Bell, ML, Zanobetti, A and Dominici F (2014). Who Is More Affected 
by Ozone Pollution? A Systematic Review and Meta-Analysis. Am J Epi 
180(1): 15-28.
Brown, JS, Bateson, TF and McDonnell, WF (2008). Effects of exposure 
to 0.06 ppm ozone on FEV1 in humans: a secondary analysis of 
existing data. Environ Health Perspect 116(8): 1023-1026.
Bureau of Labor Statistics (2017). U.S. Department of Labor, The 
Economics Daily, Over 90 percent of protective service and 
construction and extraction jobs require work outdoors. Available 
at: https://www.bls.gov/opub/ted/2017/over-90-percent-of-protective-service-and-construction-and-extraction-jobs-require-work-outdoors.htm (visited August 27, 2019).
Burra, TA, Moineddin, R, Agha, MM, and Glazier, RH (2009). Social 
disadvantage, air pollution, and asthma physician visits in Toronto, 
Canada. Environ Res 109(5):567-74.
Cakmak, S, Dales, RE and Judek, S (2006). Respiratory health effects 
of air pollution gases: Modification by education and income. Arch 
Environ Occup Health 61: 5-10.
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[[Page 87351]]

List of Subjects in 40 CFR Part 50

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

Andrew Wheeler,
Administrator.

[FR Doc. 2020-28871 Filed 12-30-20; 8:45 am]
BILLING CODE 6560-50-P