[Federal Register Volume 80, Number 206 (Monday, October 26, 2015)]
[Rules and Regulations]
[Pages 65292-65468]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2015-26594]



[[Page 65291]]

Vol. 80

Monday,

No. 206

October 26, 2015

Part II





Environmental Protection Agency





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40 CFR Part 50, 51, 52, et al.





 National Ambient Air Quality Standards for Ozone; Final Rule

  Federal Register / Vol. 80 , No. 206 / Monday, October 26, 2015 / 
Rules and Regulations  

[[Page 65292]]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 51, 52, 53, and 58

[EPA-HQ-OAR-2008-0699; FRL-9933-18-OAR]
RIN 2060-AP38


National Ambient Air Quality Standards for Ozone

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Based on its review of the air quality criteria for ozone 
(O3) and related photochemical oxidants and national ambient 
air quality standards (NAAQS) for O3, the Environmental 
Protection Agency (EPA) is revising the primary and secondary NAAQS for 
O3 to provide requisite protection of public health and 
welfare, respectively. The EPA is revising the levels of both standards 
to 0.070 parts per million (ppm), and retaining their indicators 
(O3), forms (fourth-highest daily maximum, averaged across 
three consecutive years) and averaging times (eight hours). The EPA is 
making corresponding revisions in data handling conventions for 
O3 and changes to the Air Quality Index (AQI); revising 
regulations for the prevention of significant deterioration (PSD) 
program to add a transition provision for certain applications; and 
establishing exceptional events schedules and providing information 
related to implementing the revised standards. The EPA is also revising 
the O3 monitoring seasons, the Federal Reference Method 
(FRM) for monitoring O3 in the ambient air, Federal 
Equivalent Method (FEM) analyzer performance requirements, and the 
Photochemical Assessment Monitoring Stations (PAMS) network. Along with 
exceptional events schedules related to implementing the revised 
O3 standards, the EPA is applying this same schedule 
approach to other future new or revised NAAQS and removing obsolete 
regulatory language for expired exceptional events deadlines. The EPA 
is making minor changes to the procedures and time periods for 
evaluating potential FRMs and equivalent methods, including making the 
requirements for nitrogen dioxide (NO2) consistent with the 
requirements for O3, and removing an obsolete requirement 
for the annual submission of Product Manufacturing Checklists by 
manufacturers of FRMs and FEMs for monitors of fine and coarse 
particulate matter. For a more detailed summary, see the Executive 
Summary below.

DATES: The final rule is effective on December 28, 2015.

ADDRESSES: EPA has established a docket for this action (Docket ID No. 
EPA-HQ-OAR-2008-0699) and a separate docket, established for the 
Integrated Science Assessment (ISA) (Docket No. EPA-HQ-ORD-2011-0050), 
which has been incorporated by reference into the rulemaking docket. 
All documents in the docket are listed on the www.regulations.gov Web 
site. Although listed in the docket index, some information is not 
publicly available, e.g., confidential business information or other 
information whose disclosure is restricted by statute. Certain other 
material, such as copyrighted material, is not placed on the Internet 
and may be viewed, with prior arrangement, at the EPA Docket Center. 
Publicly available docket materials are available either electronically 
in www.regulations.gov or in hard copy at the Air and Radiation Docket 
and Information Center, EPA/DC, WJC West Building, Room 3334, 1301 
Constitution Ave., NW., Washington, DC. The Public Reading Room is open 
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal 
holidays. The telephone number for the Public Reading Room is (202) 
566-1744 and the telephone number for the Air and Radiation Docket and 
Information Center is (202) 566-1742. For additional information about 
EPA's public docket, visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.

FOR FURTHER INFORMATION CONTACT: Ms. Susan Lyon Stone, 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) 541-1146; fax: (919) 
541-0237; email: [email protected].

SUPPLEMENTARY INFORMATION:

General Information

Availability of Related Information

    A number of the documents that are relevant to this action are 
available through the EPA's Office of Air Quality Planning and 
Standards (OAQPS) Technology Transfer Network (TTN) Web site (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html). These documents 
include the Integrated Science Assessment for Ozone (U.S. EPA, 2013), 
available at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_isa.html; the Health Risk and Exposure Assessment and the 
Welfare Risk and Exposure Assessment for Ozone, Final Reports (HREA and 
WREA, respectively; U.S. EPA, 2014a, 2014b), available at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_rea.html; and the 
Policy Assessment for the Review of the Ozone National Ambient Air 
Quality Standards (PA; U.S. EPA, 2014c), available at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_pa.html. These and 
other related documents are also available for inspection and copying 
in the EPA docket identified above.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related Control Programs
    C. Review of Air Quality Criteria and Standards for 
O3
    D. Ozone Air Quality
    E. Summary of Proposed Revisions to the O3 Standards
    F. Organization and Approach to Decisions in This O3 
NAAQS Review
II. Rationale for Decision on the Primary Standard
    A. Introduction
    1. Overview of Health Effects Evidence
    2. Overview of Human Exposure and Health Risk Assessments
    B. Need for Revision of the Primary Standard
    1. Basis for Proposed Decision
    2. Comments on the Need for Revision
    3. Administrator's Conclusions on the Need for Revision
    C. Conclusions on the Elements of a Revised Primary Standard
    1. Indicator
    2. Averaging Time
    3. Form
    4. Level
    D. Decision on the Primary Standard
III. Communication of Public Health Information
    A. Proposed Revisions to the AQI
    B. Comments on Proposed Revisions to the AQI
    C. Final Revisions to the AQI
IV. Rationale for Decision on the Secondary Standard
    A. Introduction
    1. Overview of Welfare Effects Evidence
    2. Overview of Welfare Exposure and Risk Assessment
    3. Potential Impacts on Public Welfare
    B. Need for Revision of the Secondary Standard
    1. Basis for Proposed Decision
    2. Comments on the Need for Revision
    3. Administrator's Conclusions on the Need for Revision
    C. Conclusions on Revision of the Secondary Standard
    1. Basis for Proposed Revision
    2. Comments on Proposed Revision
    3. Administrator's Conclusions on Revision
    D. Decision on the Secondary Standard
V. Appendix U: Interpretation of the Primary and Secondary NAAQS for 
O3

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    A. Background
    B. Data Selection Requirements
    C. Data Reporting and Data Handling Requirements
    D. Exceptional Events Information Submission Schedule
VI. Ambient Monitoring Related to O3 Standards
    A. Background
    B. Revisions to the Length of the Required O3 
Monitoring Seasons
    1. Proposed Changes to the Length of the Required O3 
Monitoring Seasons
    2. Comments on the Length of the Required O3 
Monitoring Seasons
    3. Final Decisions on the Length of the Required O3 
Monitoring Seasons
    C. Revisions to the PAMS Network Requirements
    1. Network Design
    2. Speciated VOC Measurements
    3. Carbonyl Measurements
    4. Nitrogen Oxides Measurements
    5. Meteorology Measurements
    6. PAMS Season
    7. Timing and Other Implementation Issues
    D. Addition of a New FRM for O3
    1. Proposed Changes to the FRM for O3
    2. Comments on the FRM for O3
    E. Revisions to the Analyzer Performance Requirements
    1. Proposed Changes to the Analyzer Performance Requirements
    2. Comments on the Analyzer Performance Requirements
VII. Grandfathering Provision for Certain PSD Permits
    A. Summary of the Proposed Grandfathering Provision
    B. Comments and Responses
    C. Final Action and Rationale
VIII. Implementation of the Revised O3 Standards
    A. NAAQS Implementation Plans
    1. Cooperative Federalism
    2. Additional New Rules and Guidance
    3. Background O3
    4. Section 110 State Implementation Plans
    5. Nonattainment Area Requirements
    B. O3 Air Quality Designations
    1. Area Designation Process
    2. Exceptional Events
    C. How do the New Source Review (NSR) requirements apply to the 
revised O3 NAAQS?
    1. NSR Requirements for Major Stationary Sources for the Revised 
O3 NAAQS
    2. Prevention of Significant Deterioration (PSD) Program
    3. Nonattainment NSR
    D. Transportation and General Conformity
    1. What are Transportation and General Conformity?
    2. When would Transportation and General Conformity apply to 
areas designated nonattainment for the revised O3 NAAQS?
    3. Impact of a Revised O3 NAAQS on a State's Existing 
Transportation and/or General Conformity SIP
    E. Regional and International Pollution Transport
    1. Interstate Transport
    2. International Transport
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health & Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect 
Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Congressional Review Act (CRA)



References

Executive Summary

    This section summarizes information about the purpose of this 
regulatory action, the major provisions of this action, and provisions 
related to implementation.

Purpose of This Regulatory Action

    Sections 108 and 109 of the Clean Air Act (CAA) govern the 
establishment, review, and revision, as appropriate, of the NAAQS to 
protect public health and welfare. The CAA requires the EPA to 
periodically review the air quality criteria--the science upon which 
the standards are based--and the standards themselves. This rulemaking 
is being conducted pursuant to these statutory requirements. The 
schedule for completing this review is established by a federal court 
order, which requires that the EPA make a final determination by 
October 1, 2015.
    The EPA completed its most recent review of the NAAQS for 
O3 in 2008. As a result of that review, EPA took four 
principal actions: (1) Revised the level of the 8-hour primary standard 
to 0.075 ppm; (2) expressed the standard to three decimal places; (3) 
revised the 8-hour secondary standard by making it identical to the 
revised primary standard; and (4) made conforming changes to the AQI.
    In subsequent litigation, the U.S. Court of Appeals for the 
District of Columbia Circuit (DC Circuit) upheld the EPA's 2008 primary 
standard but remanded the 2008 secondary standard (Mississippi v. EPA, 
744 F. 3d 1334 [D.C. Cir. 2013]). With respect to the primary standard, 
the court held that the EPA reasonably determined that the existing 
primary standard, set in 1997, did not protect public health with an 
adequate margin of safety and required revision. In upholding the EPA's 
revised primary standard, the court dismissed arguments that the EPA 
should have adopted a more stringent standard. The court remanded the 
secondary standard to the EPA after finding that the EPA's 
justification for setting the secondary standard identical to the 
revised 8-hour primary standard violated the CAA because the EPA had 
not adequately explained how that standard provided the required public 
welfare protection. In remanding the 2008 secondary standard, the court 
did not vacate it. The EPA has addressed the court's remand with this 
final action.
    This final action reflects the Administrator's conclusions based on 
a review of the O3 NAAQS that began in September 2008, and 
also concludes the EPA's reconsideration of the 2008 decision that it 
initiated in 2009 and subsequently consolidated with the current 
review. In conducting this review, the EPA has carefully evaluated the 
currently available scientific literature on the health and welfare 
effects of O3, focusing particularly on the new literature 
available since the conclusion of the previous review in 2008. Between 
2008 and 2014, the EPA prepared draft and final versions of the 
Integrated Science Assessment, the Health and Welfare Risk and Exposure 
Assessments, and the Policy Assessment. Multiple drafts of these 
documents were subject to public review and comment, and, as required 
by the CAA, were peer-reviewed by the Clean Air Scientific Advisory 
Committee (CASAC), an independent scientific advisory committee 
established pursuant to the CAA and charged with providing advice to 
the Administrator.
    The EPA proposed revisions to the primary and secondary 
O3 NAAQS on December 17, 2014 (79 FR 75234), and provided a 
3-month period for submission of comments from the public. In addition 
to written comments submitted to EPA, comments were also provided at 
public hearings held in Washington, DC, and Arlington, Texas, on 
January 29, 2015, and in Sacramento, California, on February 2, 2015. 
After consideration of public comments and the advice from the CASAC, 
the EPA has developed this final rulemaking, which is the final step in 
the review process.
    In this rulemaking, the EPA is revising the suite of standards for 
O3 to provide requisite protection of public health and 
welfare. In addition, the EPA is updating the AQI, and making changes 
in the data handling conventions and ambient air monitoring, reporting, 
and network

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design requirements to correspond with the changes to the O3 
NAAQS.

Summary of Major Provisions

    With regard to the primary standard, the EPA is revising the level 
of the standard to 0.070 ppm to provide increased public health 
protection against health effects associated with long- and short-term 
exposures. The EPA is retaining the indicator (O3), 
averaging time (8-hour) and form (annual fourth-highest daily maximum, 
averaged over 3 years) of the existing standard. This action provides 
increased protection for children, older adults, and people with asthma 
or other lung diseases, and other at-risk populations against an array 
of adverse health effects that include reduced lung function, increased 
respiratory symptoms and pulmonary inflammation; effects that 
contribute to emergency department visits or hospital admissions; and 
mortality.
    The decisions on the adequacy of the current standard and the 
appropriate level for the revised standard are based on an integrative 
assessment of an extensive body of new scientific evidence, which 
substantially strengthens what was known about O3-related 
health effects in the last review. The revised standard also reflects 
consideration of a quantitative risk assessment that estimates public 
health risks likely to remain upon just meeting the current and various 
alternative standards. Based on this information, the Administrator 
concludes that the current primary O3 standard is not 
requisite to protect public health with an adequate margin of safety, 
as required by the CAA, and that revision of the level to 0.070 ppm is 
warranted to provide the appropriate degree of increased public health 
protection for at-risk populations against an array of adverse health 
effects. In concluding that a revised primary standard set at a level 
of 0.070 ppm is requisite to protect public health with an adequate 
margin of safety, the Administrator relies on several key pieces of 
information, including: (a) A level of 0.070 ppm is well below the 
O3 exposure concentration shown to cause the widest range of 
respiratory effects (i.e., 0.080 ppm) and is below the lowest 
O3 exposure concentration shown to cause the adverse 
combination of decreased lung function and increased respiratory 
symptoms (i.e., 0.072 ppm); (b) a level of 0.070 ppm will eliminate, or 
nearly eliminate, repeated occurrence of these O3 exposure 
concentrations (this is important because the potential for adverse 
effects increases with frequency of occurrence); (c) a level of 0.070 
ppm will protect the large majority of the population, including 
children and people with asthma, from lower exposure concentrations, 
which can cause lung function decrements and airway inflammation in 
some people (i.e., 0.060 ppm); and (d) a level of 0.070 ppm will result 
in important reductions in the risk of O3-induced lung 
function decrements as well as the risk of O3-associated 
hospital admissions, emergency department visits, and mortality. In 
addition, the revised level of the primary standard is within the range 
that CASAC advised the Agency to consider.
    The EPA is also revising the level of the secondary standard to 
0.070 ppm to provide increased protection against vegetation-related 
effects on public welfare. The EPA is retaining the indicator 
(O3), averaging time (8-hour) and form (annual fourth-
highest daily maximum, averaged over 3 years) of the existing secondary 
standard. This action, reducing the level of the standard, provides 
increased protection for natural forests in Class I and other similarly 
protected areas against an array of vegetation-related effects of 
O3. The Administrator is making this decision based on 
judgments regarding the currently available welfare effects evidence, 
the appropriate degree of public welfare protection for the revised 
standard, and currently available air quality information on seasonal 
cumulative exposures that may be allowed by such a standard.
    In making this decision on the secondary standard, the 
Administrator focuses on O3 effects on tree seedling growth 
as a proxy for the full array of vegetation-related effects of 
O3, ranging from effects on sensitive species to broader 
ecosystem-level effects. Using this proxy in judging effects to public 
welfare, the Administrator has concluded that the requisite protection 
will be provided by a standard that generally limits cumulative 
seasonal exposures to 17 ppm-hours (ppm-hrs) or lower, in terms of a 3-
year W126 index. Based on air quality analyses which indicate such 
control of cumulative seasonal exposures will be achieved with a 
standard set at a level of 0.070 ppm (and the same indicator, averaging 
time, and form as the current standard), the Administrator concludes 
that a standard revised in this way will provide the requisite 
protection. In addition to providing protection of natural forests from 
growth-related effects, the revised standard is also expected to 
provide increased protection from other effects of potential public 
welfare significance, including crop yield loss and visible foliar 
injury. Thus, based on all of the information available in this review, 
the Administrator concludes that the current secondary O3 
standard is not requisite to protect public welfare as required by the 
CAA, and that this revision will provide appropriate protection against 
known or anticipated adverse effects to the public welfare.

Provisions Related to Implementation

    As directed by the CAA, reducing pollution to meet NAAQS always has 
been a shared task, one involving the federal government, states, 
tribes and local air agencies. This partnership has proved effective 
since the EPA first issued O3 standards more than three 
decades ago, and is evidenced by significantly lower O3 
levels throughout the country. To provide a foundation that helps air 
agencies build successful strategies for attaining new O3 
standards, the EPA will continue to move forward with federal 
regulatory programs, such as the final Tier 3 motor vehicle emissions 
standards. To facilitate the development of CAA-compliant 
implementation plans and strategies to attain new standards, the EPA 
intends to issue timely and appropriate implementation guidance and, 
where appropriate and consistent with the law, new rulemakings to 
streamline regulatory burdens and provide flexibility in 
implementation. Given the regional nature of O3 air 
pollution, the EPA will continue to work with states to address 
interstate transport of O3 and O3 precursors. The 
EPA also intends to work closely with states to identify locations 
affected by high background concentrations on high O3 days 
due to stratospheric intrusions of O3, wildfire 
O3 plumes, or long-range transport of O3 from 
sources outside the U.S. and ensure that the appropriate CAA regulatory 
mechanisms are employed. To this end, the EPA will be proposing 
revisions to the 2007 Exceptional Events Rule and related draft 
guidance addressing the effects of wildfires.
    In addition to revising the primary and secondary standards, this 
action is changing the AQI to reflect the revisions to the primary 
standard and also making corresponding revisions in data handling 
conventions for O3, extending the O3 monitoring 
season in 33 states, revising the requirements for the PAMS network, 
and revising regulations for the PSD permitting program to add a 
provision grandfathering certain pending permits from certain 
requirements with respect to the revised standards. The preamble also 
provides schedules and information related to implementing the revised 
standards.

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The rule also contains revisions to the schedules associated with 
exceptional events demonstration submittals for the revised 
O3 standards and other future revised NAAQS, and makes minor 
changes related to monitoring for other pollutants.

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 air 
pollutants that in her ``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 . . 
. [the Administrator] plans to issue air quality criteria . . . .'' 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(b). 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. Section 109(b)(1) 
defines a primary standard as one ``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\ A secondary standard, as defined in section 
109(b)(2), 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\ Welfare effects as defined in section 302(h) (42 U.S.C. 
7602(h)) include, but are not limited to, ``effects on soils, water, 
crops, vegetation, man-made 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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    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 Mississippi v. EPA, 744 F. 3d 1334, 1353 (D.C. Cir. 
2013); Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. 
Cir 1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 1186 
(D.C. Cir. 1981); American Farm Bureau Federation v. EPA, 559 F. 3d 
512, 533 (D.C. Cir. 2009); Association of Battery Recyclers v. EPA, 604 
F. 3d 613, 617-18 (D.C. Cir. 2010). 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 provide 
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 concentrations, see 
Lead Industries 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, the size of sensitive population(s) \3\ at risk, and the kind 
and degree of the uncertainties that must be addressed. The selection 
of any particular approach for providing an adequate margin of safety 
is a policy choice left specifically to the Administrator's judgment. 
See Lead Industries Association v. EPA, 647 F.2d at 1161-62; 
Mississippi, 744 F. 3d at 1353.
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    \3\ As used here with regard to human populations, and similarly 
throughout this document, the term ``population'' refers to people 
having a quality or characteristic in common, including a specific 
pre-existing illness or a specific age or lifestage.
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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 for these purposes. In 
so doing, the EPA may not consider the costs of implementing the 
standards. See generally, Whitman v. American Trucking Associations, 
531 U.S. 457, 465-472, 475-76 (2001). Likewise, ``[a]ttainability and 
technological feasibility are not relevant considerations in the 
promulgation of national ambient air quality standards.'' American 
Petroleum Institute v. Costle, 665 F. 2d at 1185.
    Section 109(d)(1) requires that ``not later than December 31, 1980, 
and at 5-year intervals thereafter, the Administrator shall complete a 
thorough review of the criteria published under section 108 and the 
national ambient air quality standards . . . and shall make such 
revisions in such criteria and standards and promulgate such new 
standards as may be appropriate . . . .'' Section 109(d)(2) requires 
that an 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 
1980's, the CASAC \4\ has performed this independent review function.
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    \4\ Lists of CASAC members and of members of the CASAC Ozone 
Review Panel are accessible from: http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC.
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B. Related Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of NAAQS once the EPA has established them. The EPA 
performs an oversight function, and as necessary takes actions to 
ensure CAA objectives are achieved. Under section 110 of the CAA, and 
related provisions, states 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 relevant pollutants. The states, in conjunction with the 
EPA, also administer the PSD program (CAA sections 160 to 169) which is 
a pre-construction permit program designed to prevent significant 
deterioration in air quality. In addition, federal programs provide for 
nationwide reductions in emissions of O3 precursors and 
other air pollutants through new source performance standards for 
stationary sources under section 111 of the CAA and the federal motor 
vehicle and motor vehicle fuel control program under title II of the 
CAA (sections 202

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to 250), which involves controls for emissions from mobile sources and 
controls for the fuels used by these sources. For some stationary 
sources, the national emissions standards for hazardous air pollutants 
under section 112 of the CAA may provide ancillary reductions in 
O3 precursors.
    After the EPA establishes a new or revised NAAQS, the CAA directs 
the EPA and the states to take steps to ensure that the new or revised 
NAAQS are met. One of the first steps, known as the initial area 
designations, involves identifying areas of the country that are not 
meeting the new or revised NAAQS along with the nearby areas that 
contain emissions sources that contribute to the areas not meeting the 
NAAQS. For areas designated ``nonattainment,'' the responsible states 
are required to develop SIPs to attain the standards. In developing 
their attainment plans, states first take into account projected 
emission reductions from federal and state rules that have been already 
adopted at the time of plan submittal. A number of significant emission 
reduction programs that will lead to reductions of O3 
precursors are in place today or are expected to be in place by the 
time revised SIPs will be due. Examples of such rules include the 
Nitrogen Oxides (NOX) SIP Call and Cross-State Air Pollution 
Rule (CSAPR),\5\ regulations controlling on-road and non-road engines 
and fuels, hazardous air pollutant rules for utility and industrial 
boilers, and various other programs already adopted by states to reduce 
emissions from key emissions sources. States will then evaluate the 
level of additional emission reductions needed for each nonattainment 
area to attain the O3 standards ``as expeditiously as 
practicable,'' and adopt new state regulations as appropriate. Section 
VIII of this preamble includes additional discussion of designation and 
implementation issues associated with the revised O3 NAAQS.
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    \5\ The Cross-State Air Pollution Rule was upheld by the Supreme 
Court in Environmental Protection Agency v. EME Homer City 
Generation, L.P., 134 S. Ct. 1584 (2014), and remanded to the D.C. 
Circuit for further proceedings. The D.C. Circuit issued its 
decision on remand from the Supreme Court on July 28, 2015, 
remanding CSAPR to EPA, without vacating the rule, for EPA to 
reconsider certain emission budgets for certain States (EME Homer 
City Generation, L.P. v. Environmental Protection Agency, No. 11-
1302, 2015 WL 4528137 [D.C. Cir. July 28, 2015]).
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C. Review of Air Quality Criteria and Standards for O3

    The EPA first established primary and secondary NAAQS for 
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971). The EPA 
set both primary and secondary standards at 0.08 ppm,\6\ as a 1-hour 
average of total photochemical oxidants, not to be exceeded more than 
one hour per year. The EPA based the standards on scientific 
information contained in the 1970 Air Quality Criteria for 
Photochemical Oxidants (AQCD; U.S. DHEW, 1970). The EPA initiated the 
first periodic review of the NAAQS for photochemical oxidants in 1977. 
Based on the 1978 AQCD (U.S. EPA, 1978), the EPA published proposed 
revisions to the original NAAQS in 1978 (43 FR 26962, June 22, 1978) 
and final revisions in 1979 (44 FR 8202, February 8, 1979). At that 
time, the EPA revised the level of the primary and secondary standards 
from 0.08 to 0.12 ppm and changed the indicator from photochemical 
oxidants to O3, and the form of the standards from a 
deterministic (i.e., not to be exceeded more than one hour per year) to 
a statistical form. This statistical form defined attainment of the 
standards as occurring when the expected number of days per calendar 
year with maximum hourly average concentration greater than 0.12 ppm 
equaled one or less.
---------------------------------------------------------------------------

    \6\ Although the level of the 2008 O3 standards are 
specified in the units of ppm (i.e., 0.075 ppm), O3 
concentrations are described using the units of parts per billion 
(ppb) in several sections of this notice (i.e., sections II, III, IV 
and VI) for consistency with the common convention for information 
discussed in those sections. In ppb, 0.075 ppm is equivalent to 75.
---------------------------------------------------------------------------

    Following the EPA's decision in the 1979 review, the city of 
Houston challenged the Administrator's decision arguing that the 
standard was arbitrary and capricious because natural O3 
concentrations and other physical phenomena in the Houston area made 
the standard unattainable in that area. The U.S. Court of Appeals for 
the District of Columbia Circuit (D.C. Circuit) rejected this argument, 
holding (as noted above) that attainability and technological 
feasibility are not relevant considerations in the promulgation of the 
NAAQS. 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 
this difficulty through various compliance related provisions in the 
CAA. See API v. Costle, 665 F.2d 1176, 1184-6 (D.C. Cir. 1981).
    In 1982, the EPA announced plans to revise the 1978 AQCD (47 FR 
11561; March 17, 1982), and, in 1983, the EPA initiated the second 
periodic review of the O3 NAAQS (48 FR 38009; August 22, 
1983). The EPA subsequently published the 1986 AQCD (U.S. EPA, 1986) 
and the 1989 Staff Paper (U.S. EPA, 1989). Following publication of the 
1986 AQCD, a number of scientific abstracts and articles were published 
that appeared to be of sufficient importance concerning potential 
health and welfare effects of O3 to warrant preparation of a 
Supplement (U.S. EPA, 1992). In August of 1992, under the terms of a 
court order, the EPA proposed to retain the existing primary and 
secondary standards based on the health and welfare effects information 
contained in the 1986 AQCD and its 1992 Supplement (57 FR 35542, August 
10, 1992). In March 1993, the EPA announced its decision to conclude 
this review by affirming its proposed decision to retain the standards, 
without revision (58 FR 13008, March 9, 1993).
    In the 1992 notice of its proposed decision in that review, the EPA 
announced its intention to proceed as rapidly as possible with the next 
review of the air quality criteria and standards for O3 in 
light of emerging evidence of health effects related to 6- to 8-hour 
O3 exposures (57 FR 35542, August 10, 1992). The EPA 
subsequently published the AQCD and Staff Paper for the review (U.S. 
EPA, 1996a,b). In December 1996, the EPA proposed revisions to both the 
primary and secondary standards (61 FR 65716, December 13, 1996). With 
regard to the primary standard, the EPA proposed to replace the then-
existing 1-hour primary standard with an 8-hour standard set at a level 
of 0.08 ppm (equivalent to 0.084 ppm based on the proposed data 
handling convention) as a 3-year average of the annual third-highest 
daily maximum 8-hour concentration. The EPA proposed to revise the 
secondary standard either by setting it identical to the proposed new 
primary standard or by setting it as a new seasonal standard using a 
cumulative form. The EPA completed this review in 1997 by setting the 
primary standard at a level of 0.08 ppm, based on the annual fourth-
highest daily maximum 8-hour average concentration, averaged over three 
years, and setting the secondary standard identical to the revised 
primary standard (62 FR 38856, July 18, 1997). In reaching her decision 
on the primary standard, the Administrator identified several reasons 
supporting her decision to reject a potential alternate standard set at 
0.07 ppm, including first the fact that no CASAC panel member supported 
a standard level lower than 0.08 ppm and her consideration of the 
scientific uncertainties with regard to the health effects evidence for 
exposure concentrations below 0.08 ppm. In addition to those reasons, 
the Administrator noted that a standard set

[[Page 65297]]

at a level of 0.07 ppm would be closer to peak background 
concentrations that infrequently occur in some areas due to 
nonanthropogenic sources of O3 precursors (62 FR 38856, 
38868; July 18, 1997).
    On May 14, 1999, in response to challenges by industry and others 
to the EPA's 1997 decision, the D.C. Circuit remanded the O3 
NAAQS to the EPA, finding that section 109 of the CAA, as interpreted 
by the EPA, effected an unconstitutional delegation of legislative 
authority. American Trucking Assoc. vs. EPA, 175 F.3d 1027, 1034-1040 
(D.C. Cir. 1999) (``ATA I''). In addition, the court 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. In 1999, the EPA petitioned for 
rehearing en banc on several issues related to that decision. The court 
granted the request for rehearing in part and denied it in part, but 
declined to review its ruling with regard to the potential beneficial 
effects of O3 pollution. 195 F. 3d 4, 10 (D.C Cir., 1999) 
(``ATA II''). On January 27, 2000, the EPA petitioned the U.S. Supreme 
Court for certiorari on the constitutional issue (and two other 
issues), but did not request review of the ruling regarding the 
potential beneficial health effects of O3. On February 27, 
2001, the U.S. Supreme Court unanimously reversed the judgment of the 
D.C. Circuit on the constitutional issue. Whitman v. American Trucking 
Assoc., 531 U. S. 457, 472-74 (2001) (holding that section 109 of the 
CAA does not delegate legislative power to the EPA in contravention of 
the Constitution). The Court remanded the case to the D.C. Circuit to 
consider challenges to the O3 NAAQS that had not been 
addressed by that court's earlier decisions. On March 26, 2002, the 
D.C. Circuit issued its final decision on remand, finding the 1997 
O3 NAAQS to be ``neither arbitrary nor capricious,'' and so 
denying the remaining petitions for review. American Trucking 
Associations, Inc. v. EPA, 283 F.3d 355, 379 (D.C Cir., 2002) (``ATA 
III'').
    Specifically, in ATA III, the D.C. Circuit upheld the EPA's 
decision on the 1997 O3 standard as the product of reasoned 
decision making. With regard to the primary standard, the court made 
clear that the most important support for EPA's decision to revise the 
standard was the health evidence of insufficient protection afforded by 
the then-existing standard (``the record is replete with references to 
studies demonstrating the inadequacies of the old one-hour standard''), 
as well as extensive information supporting the change to an 8-hour 
averaging time (283 F. 3d at 378). The court further upheld the EPA's 
decision not to select a more stringent level for the primary standard 
noting ``the absence of any human clinical studies at ozone 
concentrations below 0.08 [ppm]'' which supported the EPA's conclusion 
that ``the most serious health effects of ozone are `less certain' at 
low concentrations, providing an eminently rational reason to set the 
primary standard at a somewhat higher level, at least until additional 
studies become available'' (283 F. 3d at 378, internal citations 
omitted). The court also pointed to the significant weight that the EPA 
properly placed on the advice it received from CASAC (283 F. 3d at 
379). In addition, the court noted that ``although relative proximity 
to peak background O3 concentrations did not, in itself, 
necessitate a level of 0.08 [ppm], the EPA could consider that factor 
when choosing among the three alternative levels'' (283 F. 3d at 379).
    Independently of the litigation, the EPA responded to the court's 
remand to consider the potential beneficial health effects of 
O3 pollution in shielding the public from effects of UV 
radiation. The EPA provisionally determined that the information 
linking changes in patterns of ground-level O3 
concentrations to changes in relevant patterns of exposures to UV 
radiation of concern to public health was too uncertain, at that time, 
to warrant any relaxation in 1997 O3 NAAQS. The EPA also 
expressed the view that any plausible changes in UV-B radiation 
exposures from changes in patterns of ground-level O3 
concentrations would likely be very small from a public health 
perspective. In view of these findings, 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 with a call for information in 
September 2000 (65 FR 57810, September, 26, 2000). The schedule for 
completion of that review was ultimately governed by a consent decree 
resolving a lawsuit filed in March 2003 by plaintiffs representing 
national environmental and public health organizations, who maintained 
that the EPA was in breach of a nondiscretionary duty to complete 
review of the O3 NAAQS within a statutorily mandated 
deadline. In 2007, the EPA proposed to revise the level of the primary 
standard within a range of 0.075 to 0.070 ppm (72 FR 37818, July 11, 
2007). The EPA proposed to revise the secondary standard either by 
setting it identical to the proposed new primary standard or by setting 
it as a new seasonal standard using a cumulative form. Documents 
supporting these proposed decisions included the 2006 AQCD (U.S. EPA, 
2006a) and 2007 Staff Paper (U.S. EPA, 2007) and related technical 
support documents. The EPA completed the review in March 2008 by 
revising the level of the primary standard from 0.08 ppm to 0.075 ppm, 
and revising the secondary standard to be identical to the revised 
primary standard (73 FR 16436, March 27, 2008).
    In May 2008, state, public health, environmental, and industry 
petitioners filed suit challenging the EPA's final decision on the 2008 
O3 standards. On September 16, 2009, the EPA announced its 
intention to reconsider the 2008 O3 standards, 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.
    On January 2010, the EPA issued a notice of proposed rulemaking to 
reconsider the 2008 final decision (75 FR 2938, January 19, 2010). In 
that notice, the EPA proposed that further revisions of the primary and 
secondary standards were necessary to provide a requisite level of 
protection to public health and welfare. The EPA proposed to revise the 
level of the primary standard from 0.075 ppm to a level within the 
range of 0.060 to 0.070 ppm, and to revise the secondary standard to 
one with a cumulative, seasonal form. At the EPA's request, the CASAC 
reviewed the proposed rule at a public teleconference on January 25, 
2010 and provided additional advice in early 2011 (Samet, 2010, 2011). 
After considering comments from CASAC and the public, the EPA prepared 
a draft final rule, which was submitted for interagency review pursuant 
to Executive Order 12866. On September 2, 2011, consistent with the 
direction of the President, the Administrator of the Office of 
Information and Regulatory Affairs, Office of Management and Budget 
(OMB), returned the draft final rule to the EPA for further 
consideration. In view of this return and the fact that the Agency's 
next periodic review of the O3 NAAQS required under CAA 
section 109 had already begun (as announced on September 29, 2008), the 
EPA decided to consolidate the

[[Page 65298]]

reconsideration with its statutorily required periodic review.\7\
---------------------------------------------------------------------------

    \7\ This rulemaking concludes the reconsideration process. Under 
CAA section 109, the EPA is required to base its review of the NAAQS 
on the current air quality criteria, and thus the record and 
decision for this review also serve for the reconsideration.
---------------------------------------------------------------------------

    In light of the EPA's decision to consolidate the reconsideration 
with the current review, the D.C. Circuit proceeded with the litigation 
on the 2008 final decision. On July 23, 2013, the court upheld the 
EPA's 2008 primary O3 standard, but remanded the 2008 
secondary standard to the EPA (Mississippi v. EPA, 744 F. 3d 1334). 
With respect to the primary standard, the court first held that the EPA 
reasonably determined that the existing standard was not requisite to 
protect public health with an adequate margin of safety, and 
consequently required revision. Specifically, the court noted that 
there were ``numerous epidemiologic studies linking health effects to 
exposure to ozone levels below 0.08 ppm and clinical human exposure 
studies finding a causal relationship between health effects and 
exposure to ozone levels at and below 0.08 ppm'' (Mississippi v. EPA, 
744 F. 3d at 1345). The court also specifically endorsed the weight of 
evidence approach utilized by the EPA in its deliberations (Mississippi 
v. EPA, 744 F. 3d at 1344).
    The court went on to reject arguments that the EPA should have 
adopted a more stringent primary standard. Dismissing arguments that a 
clinical study (as properly interpreted by the EPA) showing effects at 
0.06 ppm necessitated a standard level lower than that selected, the 
court noted that this was a single, limited study (Mississippi v. EPA, 
744 F. 3d at 1350). With respect to the epidemiologic evidence, the 
court accepted the EPA's argument that there could be legitimate 
uncertainty that a causal relationship between O3 and 8-hour 
exposures less than 0.075 ppm exists, so that associations at lower 
levels reported in epidemiologic studies did not necessitate a more 
stringent standard (Mississippi v. EPA, 744 F. 3d at 1351-52).\8\
---------------------------------------------------------------------------

    \8\ The court cautioned, however, that ``perhaps more [clinical] 
studies like the Adams studies will yet reveal that the 0.060 ppm 
level produces significant adverse decrements that simply cannot be 
attributed to normal variation in lung function,'' and further 
cautioned that ``agencies may not merely recite the terms 
`substantial uncertainty' as a justification for their actions.'' 
Id. at 1350, 1357 (internal citations omitted).
---------------------------------------------------------------------------

    The court also rejected arguments that an 8-hour primary standard 
of 0.075 ppm failed to provide an adequate margin of safety, noting 
that margin of safety considerations involved policy judgments by the 
agency, and that by setting a standard ``appreciably below'' the level 
of the current standard (0.08 ppm), the agency had made a reasonable 
policy choice (Mississippi v. EPA, 744 F. 3d at 1351-52). Finally, the 
court rejected arguments that the EPA's decision was inconsistent with 
the CASAC's scientific recommendations because the CASAC had been 
insufficiently clear in its recommendations whether it was providing 
scientific or policy recommendations, and the EPA had reasonably 
addressed the CASAC's policy recommendations (Mississippi v. EPA, 744 
F. 3d at 1357-58).
    With respect to the secondary standard, the court held that the 
EPA's justification for setting the secondary standard identical to the 
revised 8-hour primary standard violated the CAA because the EPA had 
not adequately explained how that standard provided the required public 
welfare protection. The court thus remanded the secondary standard to 
the EPA (Mississippi v. EPA, 744 F. 3d at 1360-62).
    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. This review was formally initiated in 2008 
with a call for information in the Federal Register (73 FR 56581, Sept. 
29, 2008). On October 28-29, 2008, the EPA held a public workshop to 
discuss the policy-relevant science, which informed identification of 
key policy issues and questions to frame the review. Based in part on 
the workshop discussions, the EPA developed a draft Integrated Review 
Plan (IRP) outlining the schedule, process,\9\ and key policy-relevant 
questions that would guide the evaluation of the air quality criteria 
for O3 and the review of the primary and secondary 
O3 NAAQS. A draft of the IRP was released for public review 
and comment in September 2009 and was the subject of a consultation 
with the CASAC on November 13, 2009 (74 FR 54562; October 22, 
2009).\10\ After considering the comments received from that 
consultation and from the public, the EPA completed and released the 
IRP for the review in 2011 (U.S. EPA, 2011a).
---------------------------------------------------------------------------

    \9\ As of this review, the document developed in NAAQS reviews 
to document the air quality criteria, previously the AQCD, is the 
ISA, and the document describing the OAQPS staff evaluation, 
previously the Staff Paper, is the PA. These documents are described 
in the IRP.
    \10\ See http://yosemite.epa.gov/sab/sabproduct.nsf/WebProjectsbyTopicCASAC!OpenView for more information on CASAC 
activities related to the current O3 NAAQS review.
---------------------------------------------------------------------------

    In preparing the first draft ISA, the EPA's National Center for 
Environmental Assessment (NCEA) considered CASAC and public comments on 
the IRP, and also comments received from a workshop held on August 6, 
2010, to review and discuss preliminary drafts of key ISA sections (75 
FR 42085, July 20, 2010). In 2011, the first draft ISA was released for 
public comment and for review by CASAC at a public meeting on May 19-
20, 2011 (U.S. EPA, 2011b; 76 FR 10893, February 28, 2011; 76 FR 23809, 
April 28, 2011). Based on CASAC and public comments, NCEA prepared a 
second draft ISA, which was released for public comment and CASAC 
review (U.S. EPA, 2011c; 76 FR 60820, September 30, 2011). The CASAC 
reviewed this draft at a January 9-10, 2012, public meeting (76 FR 236, 
December 8, 2011). Based on CASAC and public comments, NCEA prepared a 
third draft ISA (U.S. EPA, 2012; 77 FR 36534, June 19, 2012), which was 
reviewed at a CASAC meeting in September 2012. The EPA released the 
final ISA in February 2013 (U.S. EPA, 2013).
    The EPA presented its plans for conducting Risk and Exposure 
Assessments (REAs) for health risk and exposure (HREA) and welfare risk 
and exposure (WREA) in two documents that outlined the scope and 
approaches for use in conducting quantitative assessments, as well as 
key issues to be addressed as part of the assessments (U.S. EPA, 2011d, 
e). The EPA released these documents for public comment in April 2011, 
and consulted with CASAC on May 19-20, 2011 (76 FR 23809, April 28, 
2011). The EPA considered CASAC advice and public comments in further 
planning for the assessments, issuing a memo that described changes to 
elements of the REA plans and brief explanations regarding them (Samet, 
2011; Wegman, 2012).
    In July 2012, the EPA made the first drafts of the Health and 
Welfare REAs available for CASAC review and public comment (77 FR 
42495, July 19, 2012; 77 FR 51798, August 27, 2012). The first draft PA 
was made available for CASAC review and public comment in August 2012 
(77 FR 42495, July 19, 2012; 77 FR 51798, August 27, 2012).\11\ The 
first

[[Page 65299]]

draft REAs and PA were the focus of a CASAC public meeting in September 
2012 (Frey and Samet, 2012a, 2012b). The second draft REAs and PA, 
prepared with consideration of CASAC advice and public comments, were 
made available for public comment and CASAC review in January 2014 (79 
FR 4694, January 29, 2014). These documents were the focus of a CASAC 
public meeting on March 25-27, 2014 (Frey, 2014a; Frey, 2014b; Frey, 
2014c). The final versions of these documents were developed with 
consideration of the comments and recommendations from CASAC, as well 
as comments from the public on the draft documents, and were released 
in August 2014 (U.S. EPA 2014a; U.S. EPA, 2014b; U.S. EPA, 2014c).
---------------------------------------------------------------------------

    \11\ The PA is prepared by the OAQPS staff. Formerly known as 
the Staff Paper, it presents a staff evaluation of the policy 
implications of the key scientific and technical information in the 
ISA and REAs for the EPA's consideration. The PA provides a 
transparent evaluation, and staff conclusions, regarding policy 
considerations related to reaching judgments about the adequacy of 
the current standards, and if revision is considered, what revisions 
may be appropriate to consider. The PA is intended to help ``bridge 
the gap'' between the agency's scientific assessments presented in 
the ISA and REAs, and the judgments required of the EPA 
Administrator in determining whether it is appropriate to retain or 
revise the NAAQS.
---------------------------------------------------------------------------

    The proposed decision (henceforth ``proposal'') on this review of 
the O3 NAAQS was signed on November 25, 2014, and published 
in the Federal Register on December 17, 2014. The EPA held three public 
hearings to provide direct opportunity for oral testimony by the public 
on the proposal. The hearings were held on January 29, 2015, in 
Arlington, Texas, and Washington, DC, and on February 2, 2015, in 
Sacramento, California. At these public hearings, the EPA heard 
testimony from nearly 500 individuals representing themselves or 
specific interested organizations. Transcripts from these hearings and 
written testimony provided at the hearings are in the docket for this 
review. Additionally, approximately 430,000 written comments were 
received from various commenters during the public comment period on 
the proposal, approximately 428,000 as part of mass mail campaigns. 
Significant issues raised in the public comments are discussed in the 
preamble of this final action. A summary of all other significant 
comments, along with the EPA's responses, can be found in a separate 
document (henceforth ``Response to Comments'') in the docket for this 
review.
    The schedule for completion of this review is governed by a court 
order resolving a lawsuit filed in January 2014 by a group of 
plaintiffs who alleged that the EPA had failed to perform its mandatory 
duty, under section 109(d)(1), to complete a review of the 
O3 NAAQS within the period provided by statute. The court 
order that governs this review, entered by the court on April 30, 2014, 
provides that the EPA will sign for publication a notice of final 
rulemaking concerning its review of the O3 NAAQS no later 
than October 1, 2015.
    As in prior NAAQS reviews, the EPA is basing its decision in this 
review on studies and related information included in the ISA, REAs and 
PA, which have undergone CASAC and public review. The studies assessed 
in the ISA 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. NAAQS 
decisions 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 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 
and welfare effects of O3 that were not included in the ISA 
(USEPA, 2013) (`` `new' studies''). 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.
    The decision to rely on studies and related information included in 
the ISA, REAs and PA, 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 NAAQS for 
O3, ``new'' studies may sometimes be of such significance 
that it is appropriate to delay a decision on revision of a NAAQS 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 and exposure pathways of 
ambient O3 made in the air quality criteria. For this 
reason, reopening the air quality criteria review would not be 
warranted even if there were time to do so under the court order 
governing the schedule for this rulemaking.
    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 CASAC and public review. The 
EPA will consider the ``new'' studies for purposes of decision making 
in the next periodic review of the O3 NAAQS, 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, CASAC, and the public. 
Further discussion of these ``new'' studies can be found in the 
Response to Comments document, which is in the docket for this 
rulemaking and also available on the web (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html).

D. Ozone Air Quality

    Ozone is formed near the earth's surface due to chemical 
interactions involving solar radiation and precursor pollutants 
including volatile organic compounds (VOCs) and NOX. Over 
longer time periods, methane (CH4) and carbon monoxide (CO) 
can also lead to O3 formation at the global scale. The 
precursor emissions leading to 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). 
Occasionally, O3 that is created naturally in the 
stratosphere can also contribute to O3 levels near 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 surface characteristics (U.S. 
EPA, 2014a).

[[Page 65300]]

    In order to continuously assess O3 air pollution levels, 
state and local environmental agencies operate O3 monitors 
at various locations and subsequently submit the data to the EPA. At 
present, there are approximately 1,400 monitors across the U.S. 
reporting hourly O3 averages during the times of the year 
when local O3 pollution can be important (U.S. EPA, 2014c, 
Section 2.1). Much of this monitoring is focused on urban areas where 
precursor emissions tend to be largest, as well as locations directly 
downwind of these areas, but there are also over 100 sites in rural 
areas where high levels of O3 can also be measured. Based on 
data from this national network, the EPA estimates that, in 2013, 
approximately 99 million Americans lived in counties where 
O3 design values \12\ were above the level of the existing 
health-based (primary) NAAQS of 0.075 ppm. High O3 values 
can occur almost anywhere within the contiguous 48 states, although the 
poorest O3 air quality in the U.S. is typically observed in 
California, Texas, and the Northeast Corridor, locations with some of 
the most densely populated areas in the country. From a temporal 
perspective, the highest daily peak O3 concentrations 
generally tend to occur during the afternoon within the warmer months 
due to higher 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 areas of regional production can occasionally result in high 
nighttime levels of O3, 2) high-elevation sites episodically 
influenced by stratospheric intrusions which can occur in other months, 
and 3) certain locations in the western U.S. where large quantities of 
O3 precursors emissions associated with oil and gas 
development can be trapped by strong inversions associated with snow 
cover during the colder months and efficiently converted to 
O3 (U.S. EPA, 2014c, Section 2.3).
---------------------------------------------------------------------------

    \12\ A design value is a statistic that describes the air 
quality status of a given location relative to the level of the 
NAAQS.
---------------------------------------------------------------------------

    One of the challenging aspects of developing plans to address high 
O3 concentrations is that the response of O3 to 
precursor reductions is nonlinear. In particular, NOX 
emissions can lead to both increases and decreases of O3. 
The net impact of NOX emissions on O3 
concentrations depends on the local quantities of NOX, VOC, 
and sunlight which interact in a set of complex chemical reactions. In 
some areas, such as certain urban centers where NOX 
emissions typically are high compared to local VOC emissions, 
NOX can suppress O3 locally. This phenomenon is 
particularly pronounced under conditions associated with low 
O3 concentrations (i.e., during cool, cloudy weather and at 
night when photochemical activity is limited or nonexistent). However, 
while NOX emissions can initially suppress O3 
levels near the emission sources, these same NOX emissions 
ultimately react to form higher O3 levels downwind when 
conditions are favorable. Photochemical model simulations suggest that, 
in general, reductions in NOX emissions in the U.S. will 
slightly increase O3 concentrations on days with lower 
O3 concentrations in close proximity to NOX 
sources (e.g., in urban core areas), while at the same time decreasing 
the highest O3 concentrations in downwind areas. See 
generally, U.S. EPA, 2014a (section 2.2.1).
    At present, both the primary and secondary NAAQS use the annual 
fourth-highest daily maximum 8-hour concentration, averaged over 3 
years, as the form of the standard. An additional metric, the W126 
exposure index, is often used to assess impacts of O3 
exposure on ecosystems and vegetation. W126 is a cumulative seasonal 
aggregate of weighted hourly O3 values observed between 8 
a.m. and 8 p.m. As O3 precursor emissions have decreased 
across the U.S., annual fourth-highest 8-hour O3 maxima have 
concurrently shown a modest downward trend. The national average change 
in annual fourth-highest daily maximum 8-hour O3 
concentrations between 2000 and 2013 was an 18% decrease. The national 
average change in the annual W126 exposure index over the same period 
was a 52% decrease. Air quality model simulations estimate that 
O3 air quality will continue to improve over the next decade 
as additional reductions in O3 precursors from power plants, 
motor vehicles, and other sources are realized.
    In addition to being affected by changing emissions, future 
O3 concentrations may also be affected by climate change. 
Modeling studies in the EPA's Interim Assessment (U.S. EPA, 2009a) that 
are cited in support of the 2009 Endangerment Finding under CAA section 
202(a) (74 FR 66496, Dec. 15, 2009) as well as a recent assessment of 
potential climate change impacts (Fann et al., 2015) project that 
climate change may lead to future increases in summer O3 
concentrations across the contiguous U.S.\13\ While the projected 
impact is not uniform, climate change has the potential to increase 
average summertime O3 concentrations by as much as 1-5 ppb 
by 2030, if greenhouse gas emissions are not mitigated. Increases in 
temperature are expected to be the principal factor in driving any 
O3 increases, although increases in stagnation frequency may 
also contribute (Jacob and Winner, 2009). If unchecked, climate change 
has the potential to offset some of the improvements in O3 
air quality, and therefore some of the improvements in public health, 
that are expected from reductions in emissions of O3 
precursors.
---------------------------------------------------------------------------

    \13\ These modeling studies are based on coupled global climate 
and regional air quality models and are designed to assess the 
sensitivity of U.S. air quality to climate change. A wide range of 
future climate scenarios and future years have been modeled and 
there can be variations in the expected response in U.S. 
O3 by scenario and across models and years, within the 
overall signal of higher summer O3 concentrations in a 
warmer climate.
---------------------------------------------------------------------------

    Another challenging aspect of this air quality issue is the impact 
from sources of O3 and its precursors beyond those from 
domestic, anthropogenic sources. Modeling analyses indicate that 
nationally the majority of O3 exceedances are predominantly 
caused by anthropogenic emissions from within the U.S. However, 
observational and modeling analyses have concluded that O3 
concentrations in some locations in the U.S. on some days can be 
substantially influenced by sources that cannot be addressed by 
domestic control measures. In particular, certain high-elevation sites 
in the western U.S. are impacted by a combination of non-U.S. sources 
like international transport, or natural sources such as stratospheric 
O3, and O3 originating from wildfire 
emissions.\14\ Ambient O3 from these non-U.S. and natural 
sources is collectively referred to as background O3. See 
generally section 2.4 of the PA (U.S. EPA, 2014c). The analyses suggest 
that, at these locations, there can be episodic events with substantial 
background contributions where O3 concentrations approach or 
exceed the level of the current NAAQS (i.e., 75 ppb). These events are 
relatively infrequent, and the EPA has policies that allow for the 
exclusion of air quality monitoring data from design value calculations 
when they are substantially affected by certain background influences.
---------------------------------------------------------------------------

    \14\ Without global greenhouse gas mitigation efforts, climate 
change is projected to dramatically increase the area burned by 
wildfires across most of the contiguous U.S., especially in the West 
(U.S. EPA, 2015 p. 72).
---------------------------------------------------------------------------

E. Summary of Proposed Revisions to the O3 Standards

    For reasons discussed in the proposal, the Administrator proposed 
to revise the

[[Page 65301]]

current primary and secondary standards for O3. With regard 
to the primary standard, the Administrator proposed to revise the level 
from 75 ppb to a level within a range from 65 to 70 ppb. The EPA 
proposed to revise the AQI for O3, consistent with revision 
to the primary standard.
    With regard to the secondary standard, the Administrator proposed 
to revise the level of the current secondary standard to within the 
range of 0.065 to 0.070 ppm, which air quality analyses indicate would 
provide cumulative, seasonal air quality or exposure values, in terms 
of 3-year average W126 index values, at or below a range of 13-17 ppm-
hours.
    The EPA also proposed to make corresponding revisions in data 
handling conventions for O3; to revise regulations for the 
PSD permitting program to add a provision grandfathering certain 
pending permits from certain requirements with respect to the proposed 
revisions to the standards; and to convey schedules and information 
related to implementing any revised standards. In conjunction with 
proposing exceptional event schedules related to implementing any 
revised O3 standards, the EPA also proposed to extend the 
new schedule approach to other future NAAQS revisions and to remove 
obsolete regulatory language associated with expired exceptional event 
deadlines for historical standards for both O3 and other 
pollutants for which NAAQS have been established. The EPA also proposed 
to make minor changes to the procedures and time periods for evaluating 
potential FRMs and equivalent methods, including making the 
requirements for NO2 consistent with the requirements for 
O3, and removing an obsolete requirement for the annual 
submission of documentation by manufacturers of certain particulate 
matter monitors.

F. Organization and Approach to Decisions in This O3 NAAQS Review

    This action presents the Administrator's final decisions in the 
current review of the primary and secondary O3 standards. 
The final decisions addressing standards for O3 are based on 
a thorough review in the ISA of scientific information on known and 
potential human health and welfare effects associated with exposure to 
O3 at levels typically found in the ambient air. These final 
decisions also take into account the following: (1) Staff assessments 
in the PA of the most policy-relevant information in the ISA as well as 
a quantitative health and welfare exposure and risk assessments based 
on that information; (2) CASAC advice and recommendations, as reflected 
in its letters to the Administrator and its discussions of drafts of 
the ISA, REAs, and PA at public meetings; (3) public comments received 
during the development of these documents, both in connection with 
CASAC meetings and separately; and (4) extensive public comments 
received on the proposed rulemaking.
    The primary standard is addressed in section II. Corresponding 
changes to the AQI are addressed in section III. The secondary standard 
is addressed in section IV. Related data handling conventions and 
exceptional events are addressed in section V. Updates to the 
monitoring regulations are addressed in section VI. Implementation 
activities, including PSD-related actions, are addressed in sections 
VII and VIII. Section IX addresses applicable statutory and executive 
order reviews.

II. Rationale for Decision on the Primary Standard

    This section presents the Administrator's final decisions regarding 
the need to revise the existing primary O3 standard and the 
appropriate revision to the level of that standard. Based on her 
consideration of the full body of health effects evidence and exposure/
risk analyses, the Administrator concludes that the current primary 
standard for O3 is not requisite to protect public health 
with an adequate margin of safety. In order to increase public health 
protection, she is revising the level of the primary standard to 70 
ppb, in conjunction with retaining the current indicator, averaging 
time and form. The Administrator concludes that such a revised standard 
will be requisite to protect public health with an adequate margin of 
safety. As discussed more fully below, the rationale for these final 
decisions draws from the thorough review in the ISA (U.S. EPA, 2013) of 
the available scientific evidence, generally published through July 
2011, on human health effects associated with the presence of 
O3 in the ambient air. This rationale also takes into 
account: (1) Analyses of O3 air quality, human exposures to 
O3, and O3-associated health risks, as presented 
and assessed in the HREA (U.S. EPA, 2014a); (2) the EPA staff 
assessment of the most policy-relevant scientific evidence and 
exposure/risk information in the PA (U.S. EPA, 2014c); (3) CASAC advice 
and recommendations, as reflected in discussions of drafts of the ISA, 
REA, and PA at public meetings, in separate written comments, and in 
CASAC's letters to the Administrator; (4) public input received during 
the development of these documents, either in connection with CASAC 
meetings or separately; and (5) public comments on the proposal notice.
    Section II.A below summarizes the information presented in the 
proposal regarding O3-associated health effects, 
O3 exposures, and O3-attributable health risks. 
Section II.B presents information related to the adequacy of the 
current primary O3 standard, including a summary of the 
basis for the Administrator's proposed decision to revise the current 
standard, public comments received on the adequacy of the current 
standard, and the Administrator's final conclusions regarding the 
adequacy of the current standard. Section II.C presents information 
related to the elements of a revised primary O3 standard, 
including information related to each of the major elements of the 
standard (i.e., indicator, averaging time, form, level). Section II.D 
summarizes the Administrator's final decisions on the primary 
O3 standard.

A. Introduction

    As discussed in section II.A of the proposal (79 FR 75243-75246, 
December 17, 2014), the EPA's approach to informing decisions on the 
primary O3 standard in the current review builds upon the 
general approaches used in previous reviews and reflects the broader 
body of scientific evidence, updated exposure/risk information, and 
advances in O3 air quality modeling now available. This 
approach is based most fundamentally on using the EPA's assessment of 
the available scientific evidence and associated quantitative analyses 
to inform the Administrator's judgments regarding a primary standard 
for O3 that is ``requisite'' (i.e., neither more nor less 
stringent than necessary) to protect public health with an adequate 
margin of safety. Specifically, it is based on consideration of the 
available body of scientific evidence assessed in the ISA (U.S. EPA, 
2013), exposure and risk analyses presented in the HREA (U.S. EPA, 
2014a), evidence- and exposure-/risk-based considerations and 
conclusions presented in the PA (U.S. EPA, 2014c), advice and 
recommendations received from CASAC (Frey, 2014a, c), and public 
comments.
    Section II.A.1 below summarizes the information presented in the 
proposal regarding O3-associated health effects. Section 
II.A.2 summarizes the information presented in the proposal regarding 
O3 exposures and O3-attributable health risks.

[[Page 65302]]

1. Overview of Health Effects Evidence
    The health effects of O3 are described in detail in the 
ISA (U.S. EPA, 2013). Based on its assessment of the health effects 
evidence, the ISA determined that a ``causal'' relationship exists 
between short-term exposure to O3 in ambient air and effects 
on the respiratory system \15\ and that a ``likely to be causal'' 
relationship exists between long-term exposure to O3 in 
ambient air and respiratory effects \16\ (U.S. EPA, 2013, pp. 1-6 to 1-
7). The ISA summarizes the longstanding body of evidence for 
O3 respiratory effects as follows (U.S. EPA, 2013, p. 1-5):
---------------------------------------------------------------------------

    \15\ In determining that a causal relationship exists for 
O3 with specific health effects, the EPA has concluded 
that ``[e]vidence is sufficient to conclude that there is a causal 
relationship with relevant pollutant exposures'' (U.S. EPA, 2013, p. 
lxiv).
    \16\ In determining a ``likely to be a causal'' relationship 
exists for O3 with specific health effects, the EPA has 
concluded that ``[e]vidence is sufficient to conclude that a causal 
relationship is likely to exist with relevant pollutant exposures, 
but important uncertainties remain'' (U.S. EPA, 2013, p. lxiv).

    The clearest evidence for health effects associated with 
exposure to O3 is provided by studies of respiratory 
effects. Collectively, a very large amount of evidence spanning 
several decades supports a relationship between exposure to 
O3 and a broad range of respiratory effects (see Section 
6.2.9 and Section 7.2.8). The majority of this evidence is derived 
from studies investigating short-term exposures (i.e., hours to 
weeks) to O3, although animal toxicological studies and 
recent epidemiologic evidence demonstrate that long-term exposure 
---------------------------------------------------------------------------
(i.e., months to years) may also harm the respiratory system.

    Additionally, the ISA determined that the relationships between 
short-term exposures to O3 in ambient air and both total 
mortality and cardiovascular effects are likely to be causal, based on 
expanded evidence bases in the current review (U.S. EPA, 2013, pp. 1-7 
to 1-8). The ISA determined that the currently available evidence for 
additional endpoints is ``suggestive'' of causal relationships with 
short-term (central nervous system effects) and long-term exposures 
(cardiovascular effects, reproductive and developmental effects, 
central nervous system effects and total mortality) to ambient 
O3.
    Consistent with emphasis in past reviews on O3 health 
effects for which the evidence is strongest, in this review the EPA 
places the greatest emphasis on studies of health effects that have 
been determined in the ISA to be caused by, or likely to be caused by, 
O3 exposures (U.S. EPA, 2013, section 2.5.2). This preamble 
section summarizes the evidence for health effects attributable to 
O3 exposures, with a focus on respiratory morbidity and 
mortality effects attributable to short- and long-term exposures, and 
cardiovascular system effects (including mortality) and total mortality 
attributable to short-term exposures (from section II.B in the 
proposal, 79 FR 75246-75271).
    The information highlighted here is based on the assessment of the 
evidence in the ISA (U.S. EPA, 2013, Chapters 4 to 8) and consideration 
of that evidence in the PA (U.S. EPA, 2014c, Chapters 3 and 4) on the 
known or potential effects on public health which may be expected from 
the presence of O3 in the ambient air. This section 
summarizes: (1) Information available on potential mechanisms for 
health effects associated with exposure to O3 (II.A.1.a); 
(2) the nature of effects that have been associated directly with both 
short- and long-term exposure to O3 and indirectly with the 
presence of O3 in ambient air (II.A.1.b); (3) considerations 
related to the adversity of O3-attributable health effects 
(II.A.1.c); and (4) considerations in characterizing the public health 
impact of O3, including the identification of ``at risk'' 
populations (II.A.1.d).
a. Overview of Mechanisms
    This section briefly summarizes the characterization of the key 
events and pathways that contribute to health effects resulting from 
O3 exposures, as discussed in the proposal (79 FR 75247, 
section II.B.1) and in the ISA (U.S. EPA, 2013, section 5.3).
    Experimental evidence elucidating modes of action and/or mechanisms 
contributes to our understanding of the biological plausibility of 
adverse O3-related health effects, including respiratory 
effects and effects outside the respiratory system (U.S. EPA, 2013, 
Chapters 6 and 7). Evidence indicates that the initial key event is the 
formation of secondary oxidation products in the respiratory tract 
(U.S. EPA, 2013, section 5.3). This mainly involves direct reactions 
with components of the extracellular lining fluid (ELF). Although the 
ELF has inherent capacity to quench (based on individual antioxidant 
capacity), this capacity can be overwhelmed, especially with exposure 
to elevated concentrations of O3 (U.S. EPA 2014c, at 3-3, 3-
9). The resulting secondary oxidation products transmit signals to the 
epithelium, pain receptive nerve fibers and, if present, immune cells 
involved in allergic responses. The available evidence indicates that 
the effects of O3 are mediated by components of ELF and by 
the multiple cell types in the respiratory tract. Oxidative stress is 
an implicit part of this initial key event.
    Secondary oxidation products initiate numerous responses at the 
cellular, tissue, and whole organ level of the respiratory system. 
These responses include the activation of neural reflexes which leads 
to lung function decrements; initiation of pulmonary inflammation; 
alteration of barrier epithelial function; sensitization of bronchial 
smooth muscle; modification of lung host defenses; airways remodeling; 
and modulation of autonomic nervous function which may alter cardiac 
function (U.S. EPA, 2013, section 5.3, Figure 5-8).
    Persistent inflammation and injury, which are observed in animal 
models of chronic and quasi-continuous exposure to O3, are 
associated with airways remodeling (see section 7.2.3 of the ISA, U.S. 
EPA, 2013). Chronic quasi-continuous exposure to O3 has also 
been shown to result in effects on the developing lung and immune 
system. Systemic inflammation and vascular oxidative/nitrosative stress 
are also key events in the toxicity pathway of O3 (U.S. EPA, 
2013, section 5.3.8). Extrapulmonary effects of O3 occur in 
numerous organ systems, including the cardiovascular, central nervous, 
reproductive, and hepatic systems (U.S. EPA, 2013, sections 6.3 to 6.5 
and sections 7.3 to 7.5).
    Responses to O3 exposure are variable within the 
population. Studies have shown a large range of pulmonary function 
(i.e., spirometric) responses to O3 among healthy young 
adults, while responses within an individual are relatively consistent 
over time. Other responses to O3 have also been 
characterized by a large degree of interindividual variability, 
including airways inflammation. The mechanisms that may underlie the 
variability in responses seen among individuals are discussed in the 
ISA (U.S. EPA, 2013, section 5.4.2). Certain functional genetic 
polymorphisms, pre-existing conditions or diseases, nutritional status, 
lifestages, and co-exposures can contribute to altered risk of 
O3-induced effects. Experimental evidence for such 
O3-induced changes contributes to our understanding of the 
biological plausibility of adverse O3-related health 
effects, including a range of respiratory effects as well as effects 
outside the respiratory system (e.g., cardiovascular effects) (U.S. 
EPA, 2013, Chapters 6 and 7).
b. Nature of Effects
    This section briefly summarizes the information presented in the 
proposal on respiratory effects attributable to short-term exposures 
(II.A.1.b.i), respiratory effects attributable to long-

[[Page 65303]]

term exposures (II.A.1.b.ii), cardiovascular effects attributable to 
short-term exposures (II.A.1.b.iii), and premature mortality 
attributable to short-term exposures (II.A.1.b.iv) (79 FR 75247, 
section II.B.2).
i. Respiratory Effects--Short-term Exposure
    Controlled human exposure, animal toxicological, and epidemiologic 
studies available in the last review provided clear, consistent 
evidence of a causal relationship between short-term O3 
exposure and respiratory effects (U.S. EPA, 2006a). Recent studies 
evaluated since the completion of the 2006 AQCD support and expand upon 
the strong body of evidence available in the last review (U.S. EPA, 
2013, section 6.2.9).
    Key aspects of this evidence are discussed below with regard to (1) 
lung function decrements; (2) pulmonary inflammation, injury, and 
oxidative stress; (3) airway hyperresponsiveness; (4) respiratory 
symptoms and medication use; (5) lung host defense; (6) allergic and 
asthma-related responses; (7) hospital admissions and emergency 
department visits; and (8) respiratory mortality.\17\
---------------------------------------------------------------------------

    \17\ CASAC concurred that these were ``the kinds of identifiable 
effects on public health that are expected from the presence of 
ozone in the ambient air'' (Frey 2014c, p. 3).
---------------------------------------------------------------------------

Lung Function Decrements
    Lung function decrements are typically measured by spirometry and 
refer to reductions in the maximal amount of air that can be forcefully 
exhaled. Forced expiratory volume in 1 second (FEV1) is a 
common index used to assess the effect of O3 on lung 
function. The ISA summarizes the currently available evidence from 
multiple controlled human exposure studies evaluating changes in 
FEV1 following 6.6-hour O3 exposures in young, 
healthy adults engaged in moderate levels of physical activity \18\ 
(U.S. EPA, 2013, section 6.2.1.1, Figure 6-1). Exposures to an average 
O3 concentration of 60 ppb results in group mean decrements 
in FEV1 ranging from 1.8% to 3.6% (Adams, 2002; Adams, 2006; 
\19\ Schelegle et al., 2009; \20\ Kim et al., 2011). The weighted 
average group mean decrement was 2.7% from these studies. In some 
analyses, these group mean decrements in lung function were 
statistically significant (Brown et al., 2008; Kim et al., 2011), while 
in other analyses they were not (Adams, 2006; Schelegle et al., 
2009).\21\ Prolonged exposure to an average O3 concentration 
of 72 ppb results in a statistically significant group mean decrement 
in FEV1 of about 6% (Schelegle et al., 2009).\22\ There is a 
smooth dose-response curve without evidence of a threshold for 
exposures between 40 and 120 ppb O3 (U.S. EPA, 2013, Figure 
6-1). When these data are taken together, the ISA concludes that ``mean 
FEV1 is clearly decreased by 6.6-hour exposures to 60 ppb 
O3 and higher concentrations in [healthy, young adult] 
subjects performing moderate exercise'' (U.S. EPA, 2013, p. 6-9).
---------------------------------------------------------------------------

    \18\ Table 6-1 of the ISA includes descriptions of the activity 
levels evaluated in controlled human exposure studies (U.S. EPA, 
2013).
    \19\ Adams (2006); (2002) both provide data for an additional 
group of 30 healthy subjects that were exposed via facemask to 60 
ppb O3 for 6.6 hours with moderate exercise. These 
subjects are described on page 133 of Adams (2006) and pages 747 and 
761 of Adams (2002). The facemask exposure is not expected to affect 
the FEV1 responses relative to a chamber exposure.
    \20\ For the 60 ppb target exposure concentration, Schelegle et 
al. (2009) reported that the actual mean exposure concentration was 
63 ppb.
    \21\ Adams (2006) did not find effects on FEV1 at 60 
ppb to be statistically significant. In an analysis of the Adams 
(2006) data, Brown et al. (2008) addressed the more fundamental 
question of whether there were statistically significant differences 
in responses before and after the 6.6 hour exposure period and found 
the average effect on FEV1 at 60 ppb to be small, but 
highly statistically significant using several common statistical 
tests, even after removal of potential outliers. Schelegle et al. 
(2009) reported that, compared to filtered air, the largest change 
in FEV1 for the 60 ppb protocol occurred after the sixth 
(and final) exercise period.
    \22\ As noted above, for the 70 ppb exposure group, Schelegle et 
al. (2009) reported that the actual mean exposure concentration was 
72 ppb.
---------------------------------------------------------------------------

    As described in the proposal (79 FR 75250), the ISA focuses on 
individuals with >10% decrements in FEV1 because (1) it is 
accepted by the American Thoracic Society (ATS) as an abnormal response 
and a reasonable criterion for assessing exercise-induced 
bronchoconstriction, and (2) some individuals in the Schelegle et al. 
(2009) study experienced 5-10% FEV1 decrements following 
exposure to filtered air. The proportion of healthy adults experiencing 
FEV1 decrements >10% following prolonged exposures to 80 ppb 
O3 while at moderate exertion ranged from 17% to 29% and 
following exposures to 60 ppb O3 ranged from 3% to 20%. The 
weighted average proportion (i.e., based on numbers of subjects in each 
study) of young, healthy adults with >10% FEV1 decrements is 
25% following exposure to 80 ppb O3 and 10% following 
exposure to 60 ppb O3, for 6.6 hours at moderate exertion 
(U.S. EPA, 2013, page 6-18 and 6-19).\23\ Responses within an 
individual tend to be reproducible over a period of several months, 
reflecting differences in intrinsic responsiveness. Given this, the ISA 
concludes that ``[t]hough group mean decrements are biologically small 
and generally do not attain statistical significance, a considerable 
fraction of exposed individuals [in the clinical studies] experience 
clinically meaningful decrements in lung function'' when exposed for 
6.6 hours to 60 ppb O3 during quasi-continuous, moderate 
exertion (U.S. EPA, 2013, section 6.2.1.1, p. 6-20).
---------------------------------------------------------------------------

    \23\ The ISA notes that by considering responses uncorrected for 
filtered air exposures, during which lung function typically 
improves (which would increase the size of the change, pre-and post-
exposure), 10% is an underestimate of the proportion of healthy 
individuals that are likely to experience clinically meaningful 
changes in lung function following exposure for 6.6 hours to 60 ppb 
O3 during quasi-continuous moderate exertion (U.S. EPA, 
2012, section 6.2.1.1).
---------------------------------------------------------------------------

    This review has marked an advance in the ability to make reliable 
quantitative predictions of the potential lung function response to 
O3 exposure, and, thus, to reasonably predict the degree of 
interindividual response of lung function to that exposure. McDonnell 
et al. (2012) and Schelegle et al. (2012) developed models, described 
in more detail in the proposal (79 FR 75250), that included 
mathematical approaches to simulate the potential protective effect of 
antioxidants in the ELF at lower ambient O3 concentrations, 
and that included a dose threshold below which changes in lung function 
do not occur. The resulting empirical models can estimate the frequency 
distribution of individual responses and summary measures of the 
distribution such as the mean or median response and the proportions of 
individuals with FEV1 decrements >10%, 15%, and 20%.\24\ The 
predictions of the models are consistent with the observed results from 
the individual controlled human exposure studies of O3-
induced FEV1 decrements (79 FR 75250-51, see also U.S. EPA, 
2013, Figures 6-1 and 6-3). CASAC agreed that these models mark a 
significant technical advance over the exposure-response modeling 
approach used for the lung function risk assessment in the last review 
and explicitly found that ``[t]he MSS model to be scientifically and 
biologically defensible'' (Frey, 2014a, pp. 8, 2). CASAC also stated 
that ``the comparison of the MSS model results to those obtained with 
the exposure-response model is of tremendous importance. Typically, the 
MSS model gives a result about a factor of three higher . . . for 
school-age children, which is expected because the MSS model includes

[[Page 65304]]

responses for a wider range of exposure protocols'' (Frey, 2014a, pp. 
8, 2).
---------------------------------------------------------------------------

    \24\ One of these models, the McDonnell-Stewart-Smith (MSS) 
model (McDonnell et al. 2012) was used to estimate the occurrences 
of lung function decrements in the HREA.
---------------------------------------------------------------------------

    Epidemiologic studies have consistently linked short-term increases 
in ambient O3 concentrations with lung function decrements 
in diverse populations and lifestages, including children attending 
summer camps, adults exercising or working outdoors, and groups with 
pre-existing respiratory diseases such as asthmatic children (U.S. EPA, 
2013, section 6.2.1.2). Some of these studies reported O3-
associated lung function decrements accompanied by respiratory symptoms 
\25\ in asthmatic children. In contrast, studies of children in the 
general population have reported similar O3-associated lung 
function decrements but without accompanying respiratory symptoms (79 
FR 75251; U.S. EPA, 2013, section 6.2.1.2). As noted in the PA (EPA, 
2014c, pp. 4-70 to 4-71), additional research is needed to evaluate 
responses of people with asthma and healthy people in the 40 to 70 ppb 
range. Further epidemiologic studies and meta-analyses of the effects 
of O3 exposure on children will help elucidate the 
concentration-response functions for lung function and respiratory 
symptom effects at lower O3 concentrations.
---------------------------------------------------------------------------

    \25\ Reversible loss of lung function in combination with the 
presence of symptoms meets ATS criteria for adversity (ATS, 2000a).
---------------------------------------------------------------------------

    Several epidemiologic panel studies \26\ reported statistically 
significant associations with lung function decrements at relatively 
low ambient O3 concentrations. For outdoor recreation or 
exercise, associations were reported in analyses restricted to 1-hour 
average O3 concentrations less than 80 ppb, down to less 
than 50 ppb. Among outdoor workers, Brauer et al. (1996) found a robust 
association with daily 1-hour max O3 concentrations less 
than 40 ppb. Ulmer et al. (1997) found a robust association in 
schoolchildren with 30-minute maximum O3 concentrations less 
than 60 ppb. For 8-hour average O3 concentrations, 
associations with lung function decrements in children with asthma were 
found to persist at concentrations less than 80 ppb in a U.S. multicity 
study (Mortimer et al., 2002) and less than 51 ppb in a study conducted 
in the Netherlands (Gielen et al., 1997).
---------------------------------------------------------------------------

    \26\ Panel studies include repeated measurements of health 
outcomes, such as respiratory symptoms, at the individual level 
(U.S. EPA, 2013, p. 1x).
---------------------------------------------------------------------------

    As described in the proposal (79 FR 75251), several epidemiologic 
panel studies provided information on potential confounding by 
copollutants and most O3 effect estimates for lung function 
were robust to adjustment for temperature, humidity, and copollutants 
such as particulate matter with mass median aerodynamic diameter less 
than or equal to 2.5 micrometers (PM2.5), particulate matter 
with mass median aerodynamic diameter less than or equal to 10 
micrometers (PM10), NO2, or sulfur dioxide 
(SO2) (Hoppe et al., 2003; Brunekreef et al., 1994; Hoek et 
al. 1993; U.S. EPA, 2013, pp. 6-67 to 6-69). Although examined in only 
a few epidemiologic studies, O3 also remained associated 
with decreases in lung function with adjustment for pollen or acid 
aerosols (79 F 75251; U.S. EPA, 2013, section 6.2.1.2).
Pulmonary Inflammation, Injury and Oxidative Stress
    As described in detail in section II.B.2.a.ii of the proposal (79 
FR 75252), O3 exposures can result in increased respiratory 
tract inflammation and epithelial permeability. Inflammation is a host 
response to injury, and the induction of inflammation is evidence that 
injury has occurred. Oxidative stress has been shown to play a key role 
in initiating and sustaining O3-induced inflammation. As 
noted in the ISA (U.S. EPA, 2013, section 6.2.3), O3 
exposures can initiate an acute inflammatory response throughout the 
respiratory tract that has been reported to persist for at least 18-24 
hours after exposure.
    Inflammation induced by exposure of humans to O3 can 
have several potential outcomes, ranging from resolving entirely 
following a single exposure to becoming a chronic inflammatory state, 
as described in detail in section II.B.2.a.ii of the proposal (79 FR 
75252) and in the ISA (U.S. EPA, 2013, section 6.2.3). Continued 
cellular damage due to chronic inflammation ``may alter the structure 
and function of pulmonary tissues'' (U.S. EPA, 2013, p. 6-161). Lung 
injury and the resulting inflammation provide a mechanism by which 
O3 may cause other more serious morbidity effects (e.g., 
asthma exacerbations) (U.S. EPA, 2013, section 6.2.3).\27\
---------------------------------------------------------------------------

    \27\ CASAC also addressed this issue: ``The CASAC believes that 
these modest changes in FEV1 are usually associated with 
inflammatory changes, such as more neutrophils in the 
bronchoalveolar lavage fluid. Such changes may be linked to the 
pathogenesis of chronic lung disease'' (Frey, 2014a p. 2).
---------------------------------------------------------------------------

    Building on the last review, recent studies continue to support the 
evidence for airway inflammation and injury with new evidence for such 
effects following exposures to lower concentrations than had been 
evaluated previously. These studies include recent controlled human 
exposure and epidemiologic studies and are discussed more below.
    An extensive body of evidence from controlled human exposure 
studies, described in section II.B.2.a.ii of the proposal, indicates 
that short-term exposures to O3 can cause pulmonary 
inflammation and increases in polymorphonuclear leukocyte (PMN) influx 
and permeability following 80-600 O3 ppb exposures, 
eosinophilic inflammation following exposures at or above 160 ppb, and 
O3-induced PMN influx following exposures of healthy adults 
to 60 ppb O3, the lowest concentration that has been 
evaluated for inflammation. A meta-analysis of 21 controlled human 
exposure studies (Mudway and Kelly, 2004) using varied experimental 
protocols (80-600 ppb O3 exposures; 1-6.6 hours exposure 
duration; light to heavy exercise; bronchoscopy at 0-24 hours post-
O3 exposure) reported that PMN influx in healthy subjects is 
linearly associated with total O3 dose.
    As with FEV1 responses to O3, inflammatory 
responses to O3 are generally reproducible within 
individuals, with some individuals experiencing more severe 
O3-induced airway inflammation than indicated by group 
averages. Unlike O3-induced decrements in lung function, 
which are attenuated following repeated exposures over several days, 
some markers of O3-induced inflammation and tissue damage 
remain elevated during repeated exposures, indicating ongoing damage to 
the respiratory system (79 FR 75252). Most controlled human exposure 
studies have reported that asthmatics experience larger O3-
induced inflammatory responses than non-asthmatics.\28\
---------------------------------------------------------------------------

    \28\ When evaluated, these studies have also reported 
O3-induced respiratory symptoms in asthmatics. 
Specifically, Scannell et al. (1996), Basha et al. (1994), and 
Vagaggini et al. (2001, 2007) reported increased symptoms in 
addition to inflammation.
---------------------------------------------------------------------------

    In the previous review (U.S. EPA, 2006a), the epidemiologic 
evidence of O3-associated changes in airway inflammation and 
oxidative stress was limited (79 FR 75253). Since then, as a result of 
the development of less invasive test methods, there has been a large 
increase in the number of studies assessing ambient O3-
associated changes in airway inflammation and oxidative stress, the 
types of biological samples collected, and the types of indicators. 
Most of these recent studies have evaluated biomarkers of inflammation 
or oxidative stress in exhaled breath, nasal lavage fluid, or induced 
sputum (U.S. EPA, 2013, section 6.2.3.2). These recent studies form a 
larger database to establish coherence with findings from controlled 
human exposure and animal

[[Page 65305]]

studies that have measured the same or related biological markers. 
Additionally, results from these studies provide further biological 
plausibility for the associations observed between ambient 
O3 concentrations and respiratory symptoms and asthma 
exacerbations.
Airway Hyperresponsiveness (AHR)
    A strong body of controlled human exposure and animal toxicological 
studies, most of which were available in the last review of the 
O3 NAAQS, report O3-induced AHR after either 
acute or repeated exposures (U.S. EPA, 2013, section 6.2.2.2). People 
with asthma often exhibit increased airway responsiveness at baseline 
relative to healthy control subjects, and asthmatics can experience 
further increases in responsiveness following exposures to 
O3. Studies reporting increased airway responsiveness after 
O3 exposure contribute to a plausible link between ambient 
O3 exposures and increased respiratory symptoms in 
asthmatics, and increased hospital admissions and emergency department 
visits for asthma (section II.B.2.a.iii, 79 FR 75254; U.S. EPA, 2013, 
section 6.2.2.2).
Respiratory Symptoms and Medication Use
    Respiratory symptoms are associated with adverse outcomes such as 
limitations in activity, and are the primary reason for people with 
asthma to use quick relief medication and to seek medical care. Studies 
evaluating the link between O3 exposures and such symptoms 
allow a direct characterization of the clinical and public health 
significance of ambient O3 exposure. Controlled human 
exposure and toxicological studies have described modes of action 
through which short-term O3 exposures may increase 
respiratory symptoms by demonstrating O3-induced AHR (U.S. 
EPA, 2013, section 6.2.2) and pulmonary inflammation (U.S. EPA, 2013, 
section 6.2.3).
    The link between subjective respiratory symptoms and O3 
exposures has been evaluated in both controlled human exposure and 
epidemiologic studies, and the link with medication use has been 
evaluated in epidemiologic studies. In the last review, several 
controlled human exposure studies reported respiratory symptoms 
following exposures to O3 concentrations at or above 80 ppb. 
In addition, one study reported such symptoms following exposures to 60 
ppb O3, though the increase was not statistically different 
from filtered air controls. Epidemiologic studies reported associations 
between ambient O3 and respiratory symptoms and medication 
use in a variety of locations and populations, including asthmatic 
children living in U.S. cities (U.S. EPA, 2013, pp. 6-1 to 6-2). In the 
current review, additional controlled human exposure studies have 
evaluated respiratory symptoms following exposures to O3 
concentrations below 80 ppb and recent epidemiologic studies have 
evaluated associations with respiratory symptoms and medication use 
(U.S. EPA, 2013, sections 6.2.1, 6.2.4).
    As noted in section II.B.2.a.iv in the proposal (79 FR 75255), the 
findings for O3-induced respiratory symptoms in controlled 
human exposure studies, and the evidence integrated across disciplines 
describing underlying modes of action, provide biological plausibility 
for epidemiologic associations observed between short-term increases in 
ambient O3 concentration and increases in respiratory 
symptoms (U.S. EPA, 2013, section 6.2.4).
    Most epidemiologic studies of O3 and respiratory 
symptoms and medication use have been conducted in children and/or 
adults with asthma, with fewer studies, and less consistent results, in 
non-asthmatic populations (U.S. EPA, 2013, section 6.2.4). The 2006 
AQCD (U.S. EPA, 2006a; U.S. EPA, 2013, section 6.2.4) concluded that 
the collective body of epidemiologic evidence indicated that short-term 
increases in ambient O3 concentrations are associated with 
increases in respiratory symptoms in children with asthma. A large body 
of single-city and single-region studies of asthmatic children provides 
consistent evidence for associations between short-term increases in 
ambient O3 concentrations and increased respiratory symptoms 
and asthma medication use in children with asthma (U.S. EPA, 2013, 
Figure 6-12, Table 6-20, section 6.2.4.1). Methodological differences, 
described in section II.B.2.a.iv of the proposal, among studies make 
comparisons across recent multicity studies of respiratory symptoms 
difficult.
    Available evidence indicates that O3-associated 
increases in respiratory symptoms are not confounded by temperature, 
pollen, or copollutants (primarily PM) (U.S. EPA, 2013, section 
6.2.4.5; Table 6-25). However, identifying the independent effects of 
O3 in some studies was complicated due to the high 
correlations observed between O3 and PM or different lags 
and averaging times examined for copollutants. Nonetheless, the ISA 
noted that the robustness of associations in some studies of 
individuals with asthma, combined with findings from controlled human 
exposure studies for the direct effects of O3 exposure, 
provide substantial evidence supporting the independent effects of 
short-term ambient O3 exposure on respiratory symptoms (U.S. 
EPA, 2013, section 6.2.4.5).
    In summary, both controlled human exposure and epidemiologic 
studies have reported respiratory symptoms attributable to short-term 
O3 exposures. In the last review, the majority of the 
evidence from controlled human exposure studies in young, healthy 
adults was for symptoms following exposures to O3 
concentrations at or above 80 ppb. Although studies that have become 
available since the last review have not reported increased respiratory 
symptoms in young, healthy adults following exposures with moderate 
exertion to 60 ppb, one recent study did report increased symptoms 
following exposure to 72 ppb O3. As was concluded in the 
last review, the collective body of epidemiologic evidence indicates 
that short-term increases in ambient O3 concentration are 
associated with increases in respiratory symptoms in children with 
asthma (U.S. EPA, 2013, section 6.2.4). Recent studies of respiratory 
symptoms and medication use, primarily in asthmatic children, add to 
this evidence. In a smaller body of studies, increases in ambient 
O3 concentration were associated with increases in 
respiratory symptoms in adults with asthma.
Lung Host Defense
    The mammalian respiratory tract has a number of closely integrated 
defense mechanisms that, when functioning normally, provide protection 
from the potential health effects of exposures to a wide variety of 
inhaled particles and microbes. Based on toxicological and human 
exposure studies, in the last review EPA concluded that available 
evidence indicates that short-term O3 exposures have the 
potential to impair host defenses in humans, primarily by interfering 
with alveolar macrophage function. Any impairment in alveolar 
macrophage function may lead to decreased clearance of microorganisms 
or nonviable particles. Compromised alveolar macrophage functions in 
asthmatics may increase their susceptibility to other O3 
effects, the effects of particles, and respiratory infections (U.S. 
EPA, 2006a).
    Relatively few studies conducted since the last review have 
evaluated the effects of O3 exposures on lung host defense. 
As presented in section II.B.2.a.v of the proposal (79 FR 75256),

[[Page 65306]]

when the available evidence is taken as a whole, the ISA concludes that 
acute O3 exposures impair the host defense capability of 
animals, primarily by depressing alveolar macrophage function and 
perhaps also by decreasing mucociliary clearance of inhaled particles 
and microorganisms. Coupled with limited evidence from controlled human 
exposure studies, this suggests that humans exposed to O3 
could be predisposed to bacterial infections in the lower respiratory 
tract.
Allergic and Asthma Related Responses
    Evidence from controlled human exposure and epidemiologic studies 
available in the last review indicates that O3 exposure 
skews immune responses toward an allergic phenotype and could also make 
airborne allergens more allergenic, as discussed in more detail in the 
proposal (79 FR 75257). Evidence from controlled human exposure and 
animal toxicology studies available in the last review indicates that 
O3 may also increase AHR to specific allergen triggers (75 
FR 2970, January 19, 2010). When combined with NO2, 
O3 has been shown to enhance nitration of common protein 
allergens, which may increase their allergenicity (Franze et al., 
2005).
Hospital Admissions and Emergency Department Visits
    The 2006 AQCD concluded that ``the overall evidence supports a 
causal relationship between acute ambient O3 exposures and 
increased respiratory morbidity resulting in increased emergency 
department visits and [hospital admissions] during the warm season'' 
\29\ (U.S. EPA, 2006a). This conclusion was ``strongly supported by the 
human clinical, animal toxicologic[al], and epidemiologic evidence for 
[O3-induced] lung function decrements, increased respiratory 
symptoms, airway inflammation, and airway hyperreactivity'' (U.S. EPA, 
2006a).
---------------------------------------------------------------------------

    \29\ Epidemiologic associations for O3 are more 
robust during the warm season than during cooler months (e.g., 
smaller measurement error, less potential confounding by 
copollutants). The rationale for focusing on warm season 
epidemiologic studies for O3 can be found at 72 FR 37838-
37840.
---------------------------------------------------------------------------

    The results of recent studies largely support the conclusions of 
the 2006 AQCD (U.S. EPA, 2013, section 6.2.7). Since the completion of 
the 2006 AQCD, relatively fewer studies, conducted in the U.S., Canada, 
and Europe, have evaluated associations between short-term 
O3 concentrations and respiratory hospital admissions and 
emergency department visits, with a growing number of studies conducted 
in Asia. This epidemiologic evidence is discussed in detail in the 
proposal (79 FR 75258) and in the ISA (U.S. EPA, 2013, section 
6.2.7).\30\
---------------------------------------------------------------------------

    \30\ The consideration of ambient O3 concentrations 
in the locations of these epidemiologic studies are discussed in 
sections II.D.1.b and II.E.4.a below, for the current standard and 
for alternative standards, respectively.
---------------------------------------------------------------------------

    In considering this body of evidence, the ISA focused primarily on 
multicity studies because they examine associations with respiratory-
related hospital admissions and emergency department visits over large 
geographic areas using consistent statistical methodologies (U.S. EPA, 
2013, section 6.2.7.1). The ISA also focused on single-city studies 
that encompassed a large number of daily hospital admissions or 
emergency department visits, included long study-durations, were 
conducted in locations not represented by the larger studies, or 
examined population-specific characteristics that may impact the risk 
of O3-related health effects but were not evaluated in the 
larger studies (U.S. EPA, 2013, section 6.2.7.1). When examining the 
association between short-term O3 exposure and respiratory 
health effects that require medical attention, the ISA distinguishes 
between hospital admissions and emergency department visits because it 
is likely that a small percentage of respiratory emergency department 
visits will be admitted to the hospital; therefore, respiratory 
emergency department visits may represent potentially less serious, but 
more common outcomes (U.S. EPA, 2013, section 6.2.7.1).
    The collective evidence across studies indicates a mostly 
consistent positive association between O3 exposure and 
respiratory-related hospital admissions and emergency department 
visits. Moreover, the magnitude of these associations may be 
underestimated to the extent members of study populations modify their 
behavior in response to air quality forecasts, and to the extent such 
behavior modification increases exposure misclassification (U.S. EPA, 
2013, Section 4.6.6). Studies examining the potential confounding 
effects of copollutants have reported that O3 effect 
estimates remained relatively robust upon the inclusion of PM and 
gaseous pollutants in two-pollutant models (U.S. EPA, 2013, Figure 6-
20, Table 6-29). Additional studies that conducted copollutant 
analyses, but did not present quantitative results, also support these 
conclusions (Strickland et al., 2010; Tolbert et al., 2007; Medina-
Ramon et al., 2006; U.S. EPA, 2013, section 6.2.7.5).\31\
---------------------------------------------------------------------------

    \31\ The ISA concluded that, ``[o]verall, recent studies provide 
copollutant results that are consistent with those from the studies 
evaluated in the 2006 O3 AQCD [(U.S. EPA, 2006[a]), 
Figure 7-12, page 7-80 of the 2006 O3 AQCD], which found 
that O3 respiratory hospital admissions risk estimates 
remained robust to the inclusion of PM in copollutant models (U.S. 
EPA, 2013, pp. 6-152 to 6-153).
---------------------------------------------------------------------------

    In the last review, studies had not evaluated the concentration-
response relationship between short-term O3 exposure and 
respiratory-related hospital admissions and emergency department 
visits. As described in the proposal in section II.B.2.a.vii (79 FR 
75257) and in the ISA (U.S. EPA, 2013, section 6.2.7.2), a preliminary 
examination of this relationship in studies that have become available 
since the last review found no evidence of a deviation from linearity 
when examining the association between short-term O3 
exposure and asthma hospital admissions (Silverman and Ito, 2010; 
Strickland et al., 2010). In addition, an examination of the 
concentration-response relationship for O3 exposure and 
pediatric asthma emergency department visits found no evidence of a 
threshold at O3 concentrations as low as 30 ppb (for daily 
maximum 8-hour concentrations) (U.S. EPA, 2013, section 6.2.7.3). 
However, in these studies there is uncertainty in the shape of the 
concentration-response curve at the lower end of the distribution of 
O3 concentrations due to the low density of data in this 
range. Further studies at low-level O3 exposures might 
reduce this uncertainty.
Respiratory Mortality
    Evidence from experimental studies indicates multiple potential 
pathways of respiratory effects from short-term O3 
exposures, which support the continuum of respiratory effects that 
could potentially result in respiratory-related mortality in adults 
(U.S. EPA, 2013, section 6.2.8).\32\ The evidence in the last review 
was inconsistent for associations between short-term O3 
concentrations and respiratory mortality (U.S. EPA, 2006a). New 
epidemiologic evidence for respiratory mortality is discussed in detail 
in the ISA (U.S. EPA, 2013, section 6.6) and summarized below. The 
majority of recent multicity studies have reported positive 
associations between short-term O3 exposures and respiratory 
mortality, particularly during the summer months (U.S. EPA, 2013, 
Figure 6-36).
---------------------------------------------------------------------------

    \32\ Premature mortality is discussed in more detail below in 
section II.A.1.b.iv.

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

[[Page 65307]]

    Recent multicity studies from the U.S. (Zanobetti and Schwartz, 
2008), Europe (Samoli et al., 2009), Italy (Stafoggia et al., 2010), 
and Asia (Wong et al., 2010), as well as a multi-continent study 
(Katsouyanni et al., 2009), reported associations between short-term 
O3 concentrations and respiratory mortality (U.S. EPA, 2013, 
Figure 6-37, page 6-259). With respect to respiratory mortality, 
summer-only analyses were consistently positive and most were 
statistically significant. All-year analyses had more mixed results, 
but most were positive.
    Of the studies evaluated, only two studies analyzed the potential 
for copollutant confounding of the O3-respiratory mortality 
relationship (Katsouyanni et al., (2009); Stafoggia et al., (2010)). 
Based on the results of these analyses, the O3 respiratory 
mortality risk estimates appear to be moderately to substantially 
sensitive (e.g., increased or attenuated) to inclusion of 
PM10. However, in the APHENA study (Katsouyanni et al., 
2009), the mostly every-6th-day sampling schedule for PM10 
in the Canadian and U.S. datasets greatly reduced their sample size and 
limits the interpretation of these results (U.S. EPA, 2013, sections 
6.2.8 and 6.2.9).
    The evidence for associations between short-term O3 
concentrations and respiratory mortality has been strengthened since 
the last review, with the addition of several large multicity studies. 
The biological plausibility of the associations reported in these 
studies is supported by the experimental evidence for respiratory 
effects.
ii. Respiratory Effects--Long-Term Exposure
    Since the last review, the body of evidence indicating the 
occurrence of respiratory effects due to long-term O3 
exposure has been strengthened. This evidence is discussed in detail in 
the ISA (U.S. EPA, 2013, Chapter 7) and summarized below for new-onset 
asthma and asthma prevalence, asthma hospital admissions, pulmonary 
structure and function, and respiratory mortality.
    Asthma is a heterogeneous disease with a high degree of temporal 
variability. The onset, progression, and symptoms can vary within an 
individual's lifetime, and the course of asthma may vary markedly in 
young children, older children, adolescents, and adults. In the 
previous review, longitudinal cohort studies that examined associations 
between long-term O3 exposures and the onset of asthma in 
adults and children indicated a direct effect of long-term 
O3 exposures on asthma risk in adults and effect 
modification by O3 in children. Since then, additional 
studies have evaluated associations with new onset asthma, further 
informing our understanding of the potential gene-environment 
interactions, mechanisms, and biological pathways associated with 
incident asthma.
    In children, the relationship between long-term O3 
exposure and new-onset asthma has been extensively studied in the 
Children's Health Study (CHS), a long-term study that was initiated in 
the early 1990's which has evaluated effects in several cohorts of 
children. For this review, recent studies from the CHS provide evidence 
for gene-environment interactions in effects on new-onset asthma by 
indicating that the lower risks associated with specific genetic 
variants are found in children who live in lower O3 
communities. Described in detail in the proposal (79 FR 75259) and in 
the ISA (U.S. EPA, 2013, section 7.2.1), these studies indicate that 
the risk for new-onset asthma is related in part to genetic 
susceptibility, as well as behavioral factors and environmental 
exposure. Cross-sectional studies by Akinbami et al. (2010) and Hwang 
et al. (2005) provide further evidence relating O3 exposures 
with asthma prevalence. Gene-environment interactions are discussed in 
detail in Section 5.4.2.1 in the ISA (U.S. EPA, 2013).
    In the 2006 AQCD (U.S. EPA, 2006a), studies on O3-
related hospital discharges and emergency department visits for asthma 
and respiratory disease mainly looked at short-term (daily) metrics. 
Recent studies continue to indicate that there is evidence for 
increases in both hospital admissions and emergency department visits 
in children and adults related to all respiratory outcomes, including 
asthma, with stronger associations in the warm months.
    In the 2006 AQCD (U.S. EPA, 2006a), few epidemiologic studies had 
investigated the effect of chronic O3 exposure on pulmonary 
function. As discussed in the proposal, epidemiologic studies of long-
term exposures in both children and adults provide mixed results about 
the effects of long-term O3 exposure on pulmonary function 
and the growth rate of lung function.
    Long-term studies in animals allow for greater insight into the 
potential effects of prolonged exposure to O3 that may not 
be easily measured in humans, such as structural changes in the 
respiratory tract. Despite uncertainties, epidemiologic studies 
observing associations of O3 exposure with functional 
changes in humans can attain biological plausibility in conjunction 
with long-term toxicological studies, particularly O3-
inhalation studies performed in non-human primates whose respiratory 
systems most closely resemble that of the human. An important series of 
studies, discussed in section 7.2.3.2 of the ISA (U.S. EPA, 2013), have 
used nonhuman primates to examine the effect of O3 alone, or 
in combination with an inhaled allergen, house dust mite antigen, on 
morphology and lung function. Animals exhibit the hallmarks of allergic 
asthma defined for humans (NHLBI, 2007). These studies and others have 
demonstrated changes in pulmonary function and airway morphology in 
adult and infant nonhuman primates repeatedly exposed to 
environmentally relevant concentrations of O3 (U.S. EPA, 
2013, section 7.2.3.2). As discussed in more detail in the proposal, 
the studies provide evidence of an O3-induced change in 
airway resistance and responsiveness and provide biological 
plausibility of long-term exposure, or repeated short-term exposures, 
to O3 contributing to the effects of asthma in children.
    Collectively, evidence from animal studies strongly suggests that 
chronic O3 exposure is capable of damaging the distal 
airways and proximal alveoli, resulting in lung tissue remodeling and 
leading to apparent irreversible changes. Potentially, persistent 
inflammation and interstitial remodeling play an important role in the 
progression and development of chronic lung disease. Further discussion 
of the modes of action that lead to O3-induced morphological 
changes and the mechanisms involved in lifestage susceptibility and 
developmental effects can be found in the ISA (U.S. EPA, 2013, section 
5.3.7, section 5.4.2.4). The findings reported in chronic animal 
studies offer insight into potential biological mechanisms for the 
suggested association between seasonal O3 exposure and 
reduced lung function development in children as observed in 
epidemiologic studies (U.S. EPA, 2013, section 7.2.3.1). Further 
research could help fill in the gaps in our understanding of the 
mechanisms involved in lifestage susceptibility and developmental 
effects in children of seasonal or long-term exposure to O3.
    A limited number of epidemiologic studies have assessed the 
relationship between long-term exposure to O3 and mortality 
in adults. The 2006 AQCD concluded that an insufficient amount of 
evidence existed ``to suggest a causal relationship between chronic 
O3 exposure and increased risk for

[[Page 65308]]

mortality in humans'' (U.S. EPA, 2006a). Though total and cardio-
pulmonary mortality were considered in these studies, respiratory 
mortality was not specifically considered.
    In a recent follow-up analysis of the American Cancer Society 
cohort (Jerrett et al., 2009), cardiopulmonary deaths were separately 
subdivided into respiratory and cardiovascular deaths, rather than 
combined as in the Pope et al. (2002) work. Increased O3 
exposure was associated with the risk of death from respiratory causes, 
and this effect was robust to the inclusion of PM2.5. 
Additionally, a recent multicity time series study (Zanobetti and 
Schwartz, 2011), which followed (from 1985 to 2006) four cohorts of 
Medicare enrollees with chronic conditions that might predispose to 
O3-related effects, observed an association between long-
term (warm season) exposure to O3 and elevated risk of 
mortality in the cohort that had previously experienced an emergency 
hospital admission due to chronic obstructive pulmonary disease (COPD). 
A key limitation of this study is the inability to control for 
PM2.5, because data were not available in these cities until 
1999.
iii. Cardiovascular Effects--Short-Term Exposure
    A relatively small number of studies have examined the potential 
effect of short-term O3 exposure on the cardiovascular 
system. The 2006 AQCD (U.S. EPA, 2006a, p. 8-77) concluded that 
``O3 directly and/or indirectly contributes to 
cardiovascular-related morbidity,'' but added that the body of evidence 
was limited. This conclusion was based on a controlled human exposure 
study that included hypertensive adult males; a few epidemiologic 
studies of physiologic effects, heart rate variability, arrhythmias, 
myocardial infarctions, and hospital admissions; and toxicological 
studies of heart rate, heart rhythm, and blood pressure.
    More recently, the body of scientific evidence available that has 
examined the effect of O3 on the cardiovascular system has 
expanded. There is an emerging body of animal toxicological evidence 
demonstrating that short-term exposure to O3 can lead to 
autonomic nervous system alterations (in heart rate and/or heart rate 
variability) and suggesting that proinflammatory signals may mediate 
cardiovascular effects. Interactions of O3 with respiratory 
tract components result in secondary oxidation product formation and 
subsequent production of inflammatory mediators, which have the 
potential to penetrate the epithelial barrier and to initiate toxic 
effects systemically. In addition, animal toxicological studies of 
long-term exposure to O3 provide evidence of enhanced 
atherosclerosis and ischemia/reperfusion (I/R) injury, corresponding 
with development of a systemic oxidative, proinflammatory environment. 
Recent experimental and epidemiologic studies have investigated 
O3-related cardiovascular events and are summarized in the 
ISA (U.S. EPA, 2013, section 6.3).
    Controlled human exposure studies discussed in previous reviews 
have not demonstrated any consistent extrapulmonary effects. In this 
review, evidence from controlled human exposure studies suggests 
cardiovascular effects in response to short-term O3 exposure 
(U.S. EPA, 2013, section 6.3.1) and provides some coherence with 
evidence from animal toxicology studies. Controlled human exposure 
studies also support the animal toxicological studies by demonstrating 
O3-induced effects on blood biomarkers of systemic 
inflammation and oxidative stress, as well as changes in biomarkers 
that can indicate the potential for increased clotting following 
O3 exposures. Increases and decreases in high frequency 
heart rate variability (HRV) have been reported. These changes in 
cardiac function observed in animal and human studies provide 
preliminary evidence for O3-induced modulation of the 
autonomic nervous system through the activation of neural reflexes in 
the lung (U.S. EPA, 2013, section 5.3.2).
    Overall, the ISA concludes that the available body of epidemiologic 
evidence examining the relationship between short-term exposures to 
O3 concentrations and cardiovascular morbidity is 
inconsistent (U.S. EPA, 2013, section 6.3.2.9).
    Despite the inconsistent evidence for an association between 
O3 concentration and cardiovascular disease (CVD) morbidity, 
mortality studies indicate a consistent positive association between 
short-term O3 exposure and cardiovascular mortality in 
multicity studies and in a multi-continent study. When examining 
mortality due to CVD, epidemiologic studies consistently observe 
positive associations with short-term exposure to O3. 
Additionally, there is some evidence for an association between long-
term exposure to O3 and mortality, although the association 
between long-term ambient O3 concentrations and 
cardiovascular mortality can be confounded by other pollutants (U.S. 
EPA, 2013). The ISA (U.S. EPA, 2013, section 6.3.4) states that taken 
together, the overall body of evidence across the animal and human 
studies is sufficient to conclude that there is likely to be a causal 
relationship between relevant short-term exposures to O3 and 
cardiovascular system effects.
iv. Premature Mortality--Short-Term Exposure
    The 2006 AQCD concluded that the overall body of evidence was 
highly suggestive that short-term exposure to O3 directly or 
indirectly contributes to nonaccidental and cardiopulmonary-related 
mortality in adults, but additional research was needed to more fully 
establish underlying mechanisms by which such effects occur (U.S. EPA, 
2006a; U.S. EPA, 2013, p. 2-18). In building on the evidence for 
mortality from the last review, the ISA states (U.S. EPA, 2013, p. 6-
261):

    The evaluation of new multicity studies that examined the 
association between short-term O3 exposures and mortality 
found evidence that supports the conclusions of the 2006 AQCD. These 
new studies reported consistent positive associations between short-
term O3 exposure and all-cause (nonaccidental) mortality, 
with associations persisting or increasing in magnitude during the 
warm season, and provide additional support for associations between 
O3 exposure and cardiovascular and respiratory mortality.

    The 2006 AQCD reviewed a large number of time-series studies of 
associations between short-term O3 exposures and total 
mortality including single- and multicity studies, and meta-analyses. 
Available studies reported some evidence for heterogeneity in 
O3 mortality risk estimates across cities and across 
studies. Studies that conducted seasonal analyses reported larger 
O3 mortality risk estimates during the warm or summer 
season. Overall, the 2006 AQCD identified robust associations between 
various measures of daily ambient O3 concentrations and all-
cause mortality, which could not be readily explained by confounding 
due to time, weather, or copollutants. With regard to cause-specific 
mortality, consistent positive associations were reported between 
short-term O3 exposure and cardiovascular mortality, with 
less consistent evidence for associations with respiratory mortality. 
The majority of the evidence for associations between O3 and 
cause-specific mortality were from single-city studies, which had small 
daily mortality counts and subsequently limited statistical power to 
detect associations. The 2006 AQCD concluded that ``the overall body of 
evidence is highly suggestive that O3 directly or indirectly 
contributes to nonaccidental and cardiopulmonary-related mortality'' 
(U.S. EPA, 2013, section 6.6.1).

[[Page 65309]]

    Recent studies have strengthened the body of evidence that supports 
the association between short-term O3 concentrations and 
mortality in adults. This evidence includes a number of studies 
reporting associations with nonaccidental as well as cause-specific 
mortality. Multi-continent and multicity studies have consistently 
reported positive and statistically significant associations between 
short-term O3 concentrations and all-cause mortality, with 
evidence for larger mortality risk estimates during the warm or summer 
months (79 FR 75262; U.S. EPA, 2013 Figure 6-27; Table 6-42). 
Similarly, evaluations of cause-specific mortality have reported 
consistently positive associations with O3, particularly in 
analyses restricted to the warm season (79 FR 75262; U.S. EPA, 2013 
Fig. 6-37; Table 6-53).
    In the previous review, multiple uncertainties remained regarding 
the relationship between short-term O3 concentrations and 
mortality, including the extent of residual confounding by 
copollutants; characterization of the factors that modify the 
O3-mortality association; the appropriate lag structure for 
identifying O3-mortality effects; and the shape of the 
O3-mortality concentration-response function and whether a 
threshold exists. Many of the studies, published since the last review, 
have attempted to address one or more of these uncertainties and are 
described in more detail in the proposal (79 FR 75262 and in the ISA 
(U.S. EPA, 2013, section 6.6.2).
    In particular, recent studies have evaluated different statistical 
approaches to examine the shape of the O3-mortality 
concentration-response relationship and to evaluate whether a threshold 
exists for O3-related mortality. These studies are detailed 
in the proposal (79 FR 75262) and in the ISA (U.S. EPA, 2013, p. 2-32). 
The ISA reaches the following overall conclusions that the 
epidemiologic studies identified in the ISA indicated a generally 
linear C-R function with no indication of a threshold but that there is 
a lack of data at lower O3 concentrations and therefore, 
less certainty in the shape of the C-R curve at the lower end of the 
distribution (U.S. EPA, 2013, p. 2-32).
c. Adversity of Effects
    In making judgments as to when various O3-related 
effects become regarded as adverse to the health of individuals, in 
previous NAAQS reviews, the EPA has relied upon the guidelines 
published by the ATS and the advice of CASAC. In 2000, the ATS 
published an official statement on ``What Constitutes an Adverse Health 
Effect of Air Pollution?'' (ATS, 2000a), which updated and built upon 
its earlier guidance (ATS, 1985). The earlier guidance defined adverse 
respiratory health effects as ``medically significant physiologic 
changes generally evidenced by one or more of the following: (1) 
Interference with the normal activity of the affected person or 
persons, (2) episodic respiratory illness, (3) incapacitating illness, 
(4) permanent respiratory injury, and/or (5) progressive respiratory 
dysfunction,'' while recognizing that perceptions of ``medical 
significance'' and ``normal activity'' may differ among physicians, 
lung physiologists and experimental subjects (ATS, 1985). The more 
recent guidance concludes that transient, reversible loss of lung 
function in combination with respiratory symptoms should be considered 
adverse.\33\ However, the committee also recommended ``that a small, 
transient loss of lung function, by itself, should not automatically be 
designated as adverse'' (ATS, 2000a, p. 670).
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    \33\ ``In 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, 2000a).
---------------------------------------------------------------------------

    There is also a more specific consideration of population risk in 
the 2000 guidance. Specifically, the committee considered that a shift 
in the risk factor distribution, and hence the risk profile of the 
exposed population, should be considered adverse, even in the absence 
of the immediate occurrence of frank illness (ATS, 2000a, p. 668). For 
example, a population of asthmatics could have a distribution of lung 
function such that no individual has a level associated with clinically 
important impairment. Exposure to air pollution could shift the 
distribution to lower levels of lung function that still do not bring 
any individual to a level that is associated with clinically relevant 
effects. However, this would be considered to be adverse because 
individuals within the population would already have diminished reserve 
function, and therefore would be at increased risk to further 
environmental insult (ATS, 2000a, p. 668).
    The ATS also concluded in its guidance that elevations of 
biomarkers such as cell numbers and types, cytokines, and reactive 
oxygen species may signal risk for ongoing injury and more serious 
effects or may simply represent transient responses, illustrating the 
lack of clear boundaries that separate adverse from nonadverse events. 
More subtle health outcomes also may be connected mechanistically to 
health effects that are clearly adverse, so that small changes in 
physiological measures may not appear clearly adverse when considered 
alone, but may be part of a coherent and biologically plausible chain 
of related health outcomes that include responses that are clearly 
adverse, such as mortality (U.S. EPA, 2014c, section 3.1.2.1).
    Application of the ATS guidelines to the least serious category of 
effects \34\ related to ambient O3 exposures, which are also 
the most numerous and, therefore, are also important from a public 
health perspective, involves judgments about which medical experts on 
CASAC panels and public commenters have in the past expressed diverse 
views. To help frame such judgments, in past reviews, the EPA has 
defined gradations of individual functional responses (e.g., decrements 
in FEV1 and airway responsiveness) and symptomatic responses 
(e.g., cough, chest pain, wheeze), together with judgments as to the 
potential impact on individuals experiencing varying degrees of 
severity of these responses. These gradations were used by the EPA in 
the 1997 O3 NAAQS review and slightly revised in the 2008 
review (U.S. EPA, 1996b, p. 59; U.S. EPA, 2007, p. 3-72; 72 FR 37849, 
July 11, 2007). These gradations and impacts are summarized in Tables 
3-2 and 3-3 in the 2007 O3 Staff Paper (U.S. EPA, 2007, pp. 
3-74 to 3-75).
---------------------------------------------------------------------------

    \34\ These include, for example, the transient and reversible 
effects demonstrated in controlled human exposure studies, such as 
lung function decrements or respiratory symptoms.
---------------------------------------------------------------------------

    For the purpose of estimating potentially adverse lung function 
decrements in active healthy people, the CASAC panel in the 2008 
O3 NAAQS review indicated that a focus on the mid to upper 
end of the range of moderate levels of functional responses is most 
appropriate (e.g., FEV1 decrements >=15% but <20%) 
(Henderson, 2006; U.S. EPA, 2007, p. 3-76). In this review, CASAC 
reiterated that the ``[e]stimation of FEV1 decrements of 
>=15% is appropriate as a scientifically relevant surrogate for adverse 
health outcomes in active healthy adults'' (Frey, 2014c, p. 3).
    For the purpose of estimating potentially adverse lung function 
decrements in people with lung disease, the CASAC panel in the 2008 
O3 NAAQS review indicated that a focus on the lower end of 
the range of moderate levels of functional responses is most 
appropriate (e.g., FEV1 decrements >=10%) (Henderson, 2006; 
U.S. EPA, 2007, p. 3-76). In their letter

[[Page 65310]]

advising the Administrator on the reconsideration of the 2008 final 
decision, CASAC stated that ``[a] 10% decrement in FEV1 can 
lead to respiratory symptoms, especially in individuals with pre-
existing pulmonary or cardiac disease. For example, people with chronic 
obstructive pulmonary disease have decreased ventilatory reserve (i.e., 
decreased baseline FEV1) such that a >= 10% decrement could 
lead to moderate to severe respiratory symptoms'' (Samet, 2011). In 
this review, CASAC provided similar advice, stating that ``[a]n 
FEV1 decrement of >= 10% is a scientifically relevant 
surrogate for adverse health outcomes for people with asthma and lung 
disease'', and that such decrements ``could be adverse for people with 
lung disease'' (Frey, 2014c, pp. 3, 7).
    In judging the extent to which these impacts represent effects that 
should be regarded as adverse to the health status of individuals, in 
previous NAAQS reviews, the EPA has also considered whether effects 
were experienced repeatedly during the course of a year or only on a 
single occasion (U.S. EPA, 2007). While some experts would judge single 
occurrences of moderate responses to be a ``nuisance,'' especially for 
healthy individuals, a more general consensus view of the adversity of 
such moderate responses emerges as the frequency of occurrence 
increases. In particular, not every estimated occurrence of an 
O3-induced FEV1 decrement will be adverse.\35\ 
However, repeated occurrences of moderate responses, even in otherwise 
healthy individuals, may be considered to be adverse since they could 
set the stage for more serious illness (61 FR 65723). The CASAC panel 
in the 1997 NAAQS review expressed a consensus view that these 
``criteria for the determination of an adverse physiological response 
were reasonable'' (Wolff, 1995). In the review completed in 2008, as in 
the current review (II.B, II.C below), estimates of repeated 
occurrences continued to be an important public health policy factor in 
judging the adversity of moderate lung function decrements in healthy 
and asthmatic people (72 FR 37850, July 11, 2007).
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    \35\ As noted above, the ATS recommended ``that a small, 
transient loss of lung function, by itself, should not automatically 
be designated as adverse'' (ATS, 2000a, p. 670).
---------------------------------------------------------------------------

d. Ozone-Related Impacts on Public Health
    The currently available evidence expands the understanding of 
populations that were identified to be at greater risk of 
O3-related health effects at the time of the last review 
(i.e., people who are active outdoors, people with lung disease, 
children and older adults and people with increased responsiveness to 
O3) and supports the identification of additional factors 
that may lead to increased risk (U.S. EPA, 2006a, section 6.3; U.S. 
EPA, 2013, Chapter 8). Populations and lifestages may be at greater 
risk for O3-related health effects due to factors that 
contribute to their susceptibility and/or vulnerability to 
O3. The definitions of susceptibility and vulnerability have 
been found to vary across studies, but in most instances 
``susceptibility'' refers to biological or intrinsic factors (e.g., 
lifestage, sex, preexisting disease/conditions) while ``vulnerability'' 
refers to non-biological or extrinsic factors (e.g., socioeconomic 
status [SES]) (U.S. EPA, 2013, p. 8-1; U.S. EPA, 2010, 2009b). In some 
cases, the terms ``at-risk'' and ``sensitive'' have been used to 
encompass these concepts more generally. In the ISA, PA, and proposal, 
``at-risk'' is the all-encompassing term used to define groups with 
specific factors that increase their risk of O3-related 
health effects.
    There are multiple avenues by which groups may experience increased 
risk for O3-induced health effects. A population or 
lifestage \36\ may exhibit greater effects than other populations or 
lifestages exposed to the same concentration or dose, or they may be at 
greater risk due to increased exposure to an air pollutant (e.g., time 
spent outdoors). A group with intrinsically increased risk would have 
some factor(s) that increases risk through a biological mechanism and, 
in general, would have a steeper concentration-risk relationship, 
compared to those not in the group. Factors that are often considered 
intrinsic include pre-existing asthma, genetic background, and 
lifestage. A group of people could also have extrinsically increased 
risk, which would be through an external, non-biological factor, such 
as socioeconomic status (SES) and diet. Some groups are at risk of 
increased internal dose at a given exposure concentration, for example, 
because of breathing patterns. This category would include people who 
work or exercise outdoors. Finally, there are those who might be placed 
at increased risk for experiencing greater exposures by being exposed 
to higher O3 concentrations. This would include, for 
example, groups of people with greater exposure to ambient 
O3 due to less availability or use of home air conditioners 
such that they are more likely to be in locations with open windows on 
high O3 days. Some groups may be at increased risk of 
O3-related health effects through a combination of factors. 
For example, children tend to spend more time outdoors when 
O3 levels are high, and at higher levels of activity than 
adults, which leads to increased exposure and dose, and they also have 
biological, or intrinsic, risk factors (e.g., their lungs are still 
developing) (U.S. EPA, 2013, Chapter 8). An at-risk population or 
lifestage is more likely to experience adverse health effects related 
to O3 exposures and/or, develop more severe effects from 
exposure than the general population. The populations and lifestages 
identified by the ISA (U.S. EPA, 2013, section 8.5) identified that 
have ``adequate'' evidence for increased O3-related health 
effects are people with certain genotypes, people with asthma, younger 
and older age groups, people with reduced intake of certain nutrients, 
and outdoor workers. These at-risk populations and lifestages are 
described in more detail in section II.B.4 of the proposal (79 FR 
75264-269).
---------------------------------------------------------------------------

    \36\ Lifestages, which in this case includes childhood and older 
adulthood, are experienced by most people over the course of a 
lifetime, unlike other factors associated with at-risk populations.
---------------------------------------------------------------------------

    One consideration in the assessment of potential public health 
impacts is the size of various population groups for which there is 
adequate evidence of increased risk for health effects associated with 
O3-related air pollution exposure (U.S. EPA, 2014c, section 
3.1.5.2). The factors for which the ISA judged the evidence to be 
``adequate'' with respect to contributing to increased risk of 
O3-related effects among various populations and lifestages 
included: Asthma; childhood and older adulthood; diets lower in 
vitamins C and E; certain genetic variants; and working outdoors (U.S. 
EPA, 2013, section 8.5). No statistics are available to estimate the 
size of an at-risk population based on nutritional status or genetic 
variability.
    With regard to asthma, Table 3-7 in the PA (U.S. EPA, 2014c, 
section 3.1.5.2) summarizes information on the prevalence of current 
asthma by age in the U.S. adult population in 2010 (Schiller et al. 
2012; children--Bloom et al., 2011). Individuals with current asthma 
constitute a fairly large proportion of the population, including more 
than 25 million people. Asthma prevalence tends to be higher in 
children than adults. Within the U.S., approximately 8.2% of adults 
have reported currently having asthma (Schiller et al., 2012) and 9.5% 
of

[[Page 65311]]

children have reported currently having asthma (Bloom et al., 
2011).\37\
---------------------------------------------------------------------------

    \37\ As noted below (II.C.3.a.ii), asthmatics can experience 
larger O3-induced respiratory effects than non-asthmatic, 
healthy adults. The responsiveness of asthmatics to O3 
exposures could depend on factors that have not been well-evaluated 
such as asthma severity, the effectiveness of asthma control, or the 
prevalence of medication use.
---------------------------------------------------------------------------

    With regard to lifestages, based on U.S. census data from 2010 
(Howden and Meyer, 2011), about 74 million people, or 24% of the U.S. 
population, are under 18 years of age and more than 40 million people, 
or about 13% of the U.S. population, are 65 years of age or older. 
Hence, a large proportion of the U.S. population (i.e., more than a 
third) is included in age groups that are considered likely to be at 
increased risk for health effects from ambient O3 exposure.
    With regard to outdoor workers, in 2010, approximately 11.7% of the 
total number of people (143 million people) employed, or about 16.8 
million people, worked outdoors one or more days per week (based on 
worker surveys).\38\ Of these, approximately 7.4% of the workforce, or 
about 7.8 million people, worked outdoors three or more days per week.
---------------------------------------------------------------------------

    \38\ The O*NET program is the nation's primary source of 
occupational information. Central to the project is the O*NET 
database, containing information on hundreds of standardized and 
occupation-specific descriptors. The database, which is available to 
the public at no cost, is continually updated by surveying a broad 
range of workers from each occupation. http://www.onetcenter.org/overview.html. http://www.onetonline.org/find/descriptor/browse/Work_Context/4.C.2/.
---------------------------------------------------------------------------

    While it is difficult to estimate the total number of people in 
groups that are at greater risk from exposure to O3, due to 
the overlap in members of the different at-risk population groups, the 
proportion of the total population at greater risk is large. The size 
of the at-risk population combined with the estimates of risk of 
different health outcomes associated with exposure to O3 can 
give an indication of the magnitude of O3 impacts on public 
health.
2. Overview of Human Exposure and Health Risk Assessments
    To put judgments about health effects into a broader public health 
context, the EPA has developed and applied models to estimate human 
exposures to O3 and O3-associated health risks. 
Exposure and risk estimates that are output from such models are 
presented and assessed in the HREA (U.S. EPA, 2014a). Section II.C of 
the proposal discusses the quantitative assessments of O3 
exposures and O3-related health risks that are presented in 
the HREA (79 FR 75270). Summaries of these discussions are provided 
below for the approach used to adjust air quality for quantitative 
exposure and risk analyses in the HREA (II.A.2.a), the HREA assessment 
of exposures to ambient O3 (II.A.2.b), and the HREA 
assessments of O3-related health risks (II.A.2.c).
a. Air Quality Adjustment
    As discussed in section II.C.1 of the proposal (79 FR 75270), the 
HREA uses a photochemical model to estimate sensitivities of 
O3 to changes in precursor emissions in order to estimate 
ambient O3 concentrations that would just meet the current 
and alternative standards (U.S. EPA, 2014a, Chapter 4).\39\ For the 15 
urban study areas evaluated in the HREA,\40\ this model-based 
adjustment approach estimates hourly O3 concentrations at 
each monitor location when modeled U.S. anthropogenic precursor 
emissions (i.e., NOX, VOC) \41\ are reduced. The HREA 
estimates air quality that just meets the current and alternative 
standards for the 2006-2008 and 2008-2010 periods.\42\
---------------------------------------------------------------------------

    \39\ The HREA uses the Community Multi-scale Air Quality (CMAQ) 
photochemical model instrumented with the higher order direct 
decoupled method (HDDM) to estimate O3 concentrations 
that would occur with the achievement of the current and alternative 
O3 standards (U.S. EPA, 2014a, Chapter 4).
    \40\ The urban study areas assessed are Atlanta, Baltimore, 
Boston, Chicago, Cleveland, Dallas, Denver, Detroit, Houston, Los 
Angeles, New York, Philadelphia, Sacramento, St. Louis, and 
Washington, DC.
    \41\ Exposure and risk analyses for most of the urban study 
areas focus on reducing U.S. anthropogenic NOX emissions 
alone. The exceptions are Chicago and Denver. Exposure and risk 
analyses for Chicago and Denver are based on reductions in emissions 
of both NOX and VOC (U.S. EPA, 2014a, section 4.3.3.1; 
Appendix 4D).
    \42\ These estimates thus reflect design values--8 hour values 
using the form of the NAAQS that meet the level of the current or 
alternative standards. These simulations are illustrative and do not 
reflect any consideration of specific control programs designed to 
achieve the reductions in emissions required to meet the specified 
standards. Further, these simulations do not represent predictions 
of when, whether, or how areas might meet the specified standards.
---------------------------------------------------------------------------

    As discussed in Chapter 4 of the HREA (U.S. EPA, 2014a), this 
approach to adjusting air quality models the physical and chemical 
atmospheric processes that influence ambient O3 
concentrations. Compared to the quadratic rollback approach used in 
previous reviews, it provides more realistic estimates of the spatial 
and temporal responses of O3 to reductions in precursor 
emissions. Because ambient NOX can contribute both to the 
formation and destruction of O3 (U.S. EPA, 2014a, Chapter 
4), the response of ambient O3 concentrations to reductions 
in NOX emissions is more variable than indicated by the 
quadratic rollback approach. This improved approach to adjusting 
O3 air quality is consistent with recommendations from the 
National Research Council of the National Academies (NRC, 2008). In 
addition, CASAC strongly supported the new approach as an improvement 
and endorsed the way it was utilized in the HREA, stating that ``the 
quadratic rollback approach has been replaced by a scientifically more 
valid Higher-order Decoupled Direct Method (HDDM)'' and that ``[t]he 
replacement of the quadratic rollback procedure by the HDDM procedure 
is important and supported by the CASAC'' (Frey, 2014a, pp. 1 and 3).
    Within urban study areas, the model-based air quality adjustments 
show reductions in the O3 levels at the upper ends of 
ambient concentrations and increases in the O3 levels at the 
lower ends of those distributions (U.S. EPA, 2014a, section 4.3.3.2, 
Figures 4-9 and 4-10).\43\ Seasonal means of daily O3 
concentrations generally exhibit only modest changes upon model 
adjustment, reflecting the seasonal balance between daily decreases in 
relatively higher concentrations and increases in relatively lower 
concentrations (U.S. EPA, 2014a, Figures 4-9 and 4-10). The resulting 
compression in the seasonal distributions of ambient O3 
concentrations is evident in all of the urban study areas evaluated, 
though the degree of compression varies considerably across areas (U.S. 
EPA, 2014a, Figures 4-9 and 4-10).
---------------------------------------------------------------------------

    \43\ It is important to note that sensitivity analyses in the 
HREA indicate that the increases in low O3 concentrations 
are smaller when NOX and VOC emissions are reduced than 
when only NOX emissions are reduced (U.S. EPA, 2014a, 
Appendix 4-D, section 4.7).
---------------------------------------------------------------------------

    As discussed in the PA (U.S. EPA, 2014c, section 3.2.1), adjusted 
patterns of O3 air quality have important implications for 
exposure and risk estimates in urban case study areas. Estimates 
influenced largely by the upper ends of the distribution of ambient 
concentrations (i.e., exposures of concern and lung function risk 
estimates, as discussed in sections 3.2.2 and 3.2.3.1 of the PA) will 
decrease with model-adjustment to the current and alternative 
standards. In contrast, seasonal risk estimates influenced by the full 
distribution of ambient O3 concentrations (i.e., 
epidemiology-based risk estimates, as discussed in section 3.2.3.2 of 
the PA) either increase or decrease in response to air quality 
adjustment, depending on the balance between the daily decreases in 
high O3

[[Page 65312]]

concentrations and increases in low O3 concentrations.\44\
---------------------------------------------------------------------------

    \44\ In addition, because epidemiology-based risk estimates use 
``area-wide'' average O3 concentrations, calculated by 
averaging concentrations across multiple monitors in urban case 
study areas (section 3.2.3.2 below), risk estimates on a given day 
depend on the daily balance between increasing and decreasing 
O3 concentrations at individual monitors.
---------------------------------------------------------------------------

    To evaluate uncertainties in air quality adjustments, the HREA 
assessed the extent to which the modeled O3 response to 
reductions in NOX emissions appropriately represent the 
trends observed in monitored ambient O3 following actual 
reductions in NOX emissions, and the extent to which the 
O3 response to reductions in precursor emissions could 
differ with emissions reduction strategies that are different from 
those used in HREA to generate risk estimates.
    To evaluate the first issue, the HREA conducted a national analysis 
evaluating trends in monitored ambient O3 concentrations 
during a time period when the U.S. experienced large-scale reductions 
in NOX emissions (i.e., 2001 to 2010). Analyses of trends in 
monitored O3 indicate that over such a time period, the 
upper end of the distribution of monitored O3 concentrations 
(i.e., indicated by the 95th percentile) generally decreased in urban 
and non-urban locations across the U.S. (U.S. EPA, 2014a, Figure 8-29). 
During this same time period, median O3 concentrations 
decreased in suburban and rural locations, and in some urban locations. 
However, median concentrations increased in some large urban centers 
(U.S. EPA, 2014a, Figure 8-28). As discussed in the HREA, these 
increases in median concentrations likely reflect the increases in 
relatively low O3 concentrations that can occur near 
important sources of NOX upon reductions in NOX 
emissions (U.S. EPA, 2014a, section 8.2.3.1). These patterns of 
monitored O3 during a period when the U.S. experienced large 
reductions in NOX emissions are qualitatively consistent 
with the modeled responses of O3 to reductions in 
NOX emissions.
    To evaluate the second issue, the HREA assessed the O3 
air quality response to reducing both NOX and VOC emissions 
(i.e., in addition to assessing reductions in NOX emissions 
alone) for a subset of seven urban study areas. As discussed in the PA 
(U.S. EPA, 2014c, section 3.2.1), the addition of VOC reductions 
generally resulted in larger decreases in mid-range O3 
concentrations (25th to 75th percentiles) (U.S. EPA, 2014a, Appendix 
4D, section 4.7).\45\ In addition, in all seven of the urban study 
areas evaluated, the increases in low O3 concentrations were 
smaller for the NOX/VOC scenarios than the NOX 
alone scenarios (U.S. EPA, 2014a, Appendix 4D, section 4.7). This was 
most apparent for Denver, Houston, Los Angeles, New York, and 
Philadelphia. Given the impacts on total risk estimates of increases in 
low O3 concentrations (discussed below), these results 
suggest that in some locations optimized emissions reduction strategies 
could result in larger reductions in O3-associated mortality 
and morbidity than indicated by HREA estimates.
---------------------------------------------------------------------------

    \45\ This was the case for all of the urban study areas 
evaluated, with the exception of New York (U.S. EPA, 2014a, Appendix 
4-D, section 4.7). In this analysis, emissions of NOX and 
VOC were reduced by equal percentages, a scenario not likely to 
reflect the optimal combination for reducing risks. In most of the 
urban study areas the inclusion of VOC emissions reductions did not 
alter the NOX emissions reductions required to meet the 
current or alternative standards. The exceptions are Chicago and 
Denver, for which the HREA risk estimates are based on reductions in 
both NOX and VOC (U.S. EPA, 2014a, section 4.3.3.1).
---------------------------------------------------------------------------

b. Exposure Assessment
    As discussed in section II.C.2 of the proposal, the O3 
exposure assessment presented in the HREA (U.S. EPA, 2014a, Chapter 5) 
provides estimates of the number and percent of people exposed to 
various concentrations of ambient O3 while at specified 
exertion levels. The HREA estimates exposures in the 15 urban study 
areas for four study groups, all school-age children (ages 5 to 18), 
asthmatic school-age children, asthmatic adults (ages 19 to 95), and 
all older adults (ages 65 to 95), reflecting the evidence indicating 
that these populations are at increased risk for O3-
attributable effects (U.S. EPA, 2013, Chapter 8; II.A.1.d, above). An 
important purpose of these exposure estimates is to provide perspective 
on the extent to which air quality adjusted to just meet the current 
O3 NAAQS could be associated with exposures to O3 
concentrations reported to result in respiratory effects.\46\ These 
analyses of exposure assessment incorporate behavior patterns, 
including estimates of physical exertion, which are critical in 
assessing whether ambient concentrations of O3 may pose a 
public health risk.\47\ In particular, exposures to ambient or near-
ambient O3 concentrations have only been shown to result in 
potentially adverse effects if the ventilation rates of people in the 
exposed populations are raised to a sufficient degree (e.g., through 
physical exertion) (U.S. EPA, 2013, section 6.2.1.1). Estimates of such 
``exposures of concern'' provide perspective on the potential public 
health impacts of O3-related effects, including effects that 
cannot currently be evaluated in a quantitative risk assessment.\48\
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    \46\ In addition, the range of modeled personal exposures to 
ambient O3 provide an essential input to the portion of 
the health risk assessment based on exposure-response functions (for 
lung function decrements) from controlled human exposure studies. 
The health risk assessment based on exposure-response information is 
discussed below (II.C.3).
    \47\ See 79 FR 75269 ``The activity pattern of individuals is an 
important determinant of their exposure. Variation in O3 
concentrations among various microenvironments means that the amount 
of time spent in each location, as well as the level of activity, 
will influence an individual's exposure to ambient O3. 
Activity patterns vary both among and within individuals, resulting 
in corresponding variations in exposure across a population and over 
time'' (internal citations omitted).
    \48\ In this review, the term ``exposure of concern'' is defined 
as a personal exposure, while at moderate or greater exertion, to 8-
hour average ambient O3 concentrations at and above 
specific benchmarks levels. As discussed below, these benchmark 
levels represent exposure concentrations at which O3-
induced health effects are known to occur, or can reasonably be 
anticipated to occur, in some individuals.
---------------------------------------------------------------------------

    The HREA estimates 8-hour exposures at or above benchmark 
concentrations of 60, 70, and 80 ppb for individuals engaged in 
moderate or greater exertion (i.e., to approximate conditions in the 
controlled human exposure studies on which benchmarks are based). 
Benchmarks reflect exposure concentrations at which O3-
induced respiratory effects are known to occur in some healthy adults 
engaged in moderate, quasi-continuous exertion, based on evidence from 
controlled human exposure studies (U.S. EPA, 2013, section 6.2; U.S. 
EPA, 2014c, section 3.1.2.1). The amount of weight to place on the 
estimates of exposures at or above specific benchmark concentrations 
depends in part on the weight of the scientific evidence concerning 
health effects associated with O3 exposures at those 
benchmark concentrations. It also depends on judgments about the 
importance, from a public health perspective, of the health effects 
that are known or can reasonably be inferred to occur as a result of 
exposures at benchmark concentrations (U.S. EPA, 2014c, sections 3.1.3, 
3.1.5).
    In considering estimates of O3 exposures of concern at 
or above benchmarks of 60, 70, and 80 ppb, the PA focuses on modeled 
exposures for school-age children (ages 5-18), including asthmatic 
school-age children, which are key at-risk populations identified in 
the ISA (U.S. EPA, 2014c, section 3.1.5). The percentages of children 
estimated to experience exposures of concern are considerably larger 
than the percentages estimated for adult populations (i.e., 
approximately 3-fold larger across urban

[[Page 65313]]

study areas) \49\ (U.S. EPA, 2014a, section 5.3.2 and Figures 5-5 to 5-
8). The larger exposure estimates for children are due primarily to the 
larger percentage of children estimated to spend an extended period of 
time being physically active outdoors when O3 concentrations 
are elevated (U.S. EPA, 2014a, sections 5.3.2 and 5.4.1).
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    \49\ HREA exposure estimates for all children and asthmatic 
children are virtually indistinguishable, in terms of the percent 
estimated to experience exposures of concern (U.S. EPA, 2014a, 
Chapter 5). Consistent with this, HREA analyses indicate that 
activity data for people with asthma is generally similar to non-
asthmatic populations (U.S. EPA, 2014a, Appendix 5G, Tables 5G2-to 
5G-5).
---------------------------------------------------------------------------

    Although exposure estimates differ between children and adults, the 
patterns of results across the urban study areas and years are similar 
among all of the populations evaluated (U.S. EPA, 2014a, Figures 5-5 to 
5-8). Therefore, while the PA highlights estimates in children, 
including asthmatic school-age children, it also notes that the 
patterns of exposures estimated for children represent the patterns 
estimated for adult asthmatics and older adults.
    Table 1 of the proposal (79 FR 75272 to 75273) summarizes key 
results from the exposure assessment. This table is reprinted below.
---------------------------------------------------------------------------

    \50\ Estimates for each urban case study area were averaged for 
the years evaluated in the HREA (2006 to 2010). Ranges reflect the 
ranges across urban study areas. Estimates smaller than 0.05% were 
rounded downward to zero (from U.S. EPA, 2014a, Tables 5-11 and 5-
12). Numbers in parentheses reflect averages across urban study 
areas, as well as over the years evaluated in the HREA.
    \51\ Numbers of children exposed in each urban case study area 
were averaged over the years 2006 to 2010. These averages were then 
summed across urban study areas. Numbers were rounded to nearest 
thousand unless otherwise indicated. Estimates smaller than 50 were 
rounded downward to zero (from U.S. EPA, 2014a, Appendix 5F Table 
5F-5).
    \52\ As discussed in section 4.3.3 of the HREA, the model-based 
air quality adjustment approach used to estimate exposures and lung 
function decrements associated with the current and alternative 
standards was unable to estimate the distribution of ambient 
O3 concentrations in New York City upon just meeting an 
alternative standard with a level of 60 ppb. Therefore, for the 60 
ppb standard level, the numbers of children and asthmatic children, 
and the ranges of percentages, reflect all of the urban study areas 
except New York.

Table 1--Summary of Estimated Exposures of Concern in All School-age Children for the Current and Alternative O3
                                         Standards in Urban Study Areas
----------------------------------------------------------------------------------------------------------------
                                                                         Average number of
                                                       Average %          children exposed     % Children--worst
    Benchmark concentration       Standard level   children  exposed     [average number of      year and worst
                                      (ppb)               \50\          asthmatic children]           area
                                                                                \51\
----------------------------------------------------------------------------------------------------------------
                                   One or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>= 80 ppb.....................                 75        0-0.3 (0.1)           27,000 [3,000]                1.1
                                               70          0-0.1 (0)              3,700 [300]                0.2
                                               65              0 (0)                  300 [0]                  0
                                               60              0 (0)             100 \52\ [0]                  0
>= 70 ppb.....................                 75      0.6-3.3 (1.9)         362,000 [40,000]                8.1
                                               70      0.1-1.2 (0.5)          94,000 [10,000]                3.2
                                               65        0-0.2 (0.1)           14,000 [2,000]                0.5
                                               60              0 (0)              1,400 [200]                0.1
>= 60 ppb.....................                 75      9.5-17 (12.2)      2,316,000 [246,000]               25.8
                                               70     3.3-10.2 (6.2)      1,176,000 [126,000]               18.9
                                               65        0-4.2 (2.1)         392,000 [42,000]                9.5
                                               60        0-1.2 (0.4)           70,000 [8,000]                2.2
----------------------------------------------------------------------------------------------------------------
                                   Two or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>= 80 ppb.....................                 75              0 (0)                600 [100]                0.1
                                               70              0 (0)                    0 [0]                  0
                                               65              0 (0)                    0 [0]                  0
                                               60              0 (0)                    0 [0]                  0
>= 70 ppb.....................                 75      0.1-0.6 (0.2)           46,000 [5,000]                2.2
                                               70          0-0.1 (0)              5,400 [600]                0.4
                                               65              0 (0)                300 [100]                  0
                                               60              0 (0)                    0 [0]                  0
>= 60 ppb.....................                 75      3.1-7.6 (4.5)         865,000 [93,000]               14.4
                                               70      0.5-3.5 (1.7)         320,000 [35,000]                9.2
                                               65        0-0.8 (0.3)           67,000 [7,500]                2.8
                                               60          0-0.2 (0)              5,100 [700]                0.3
----------------------------------------------------------------------------------------------------------------

    Uncertainties in exposure estimates are summarized in section 
II.C.2.b of the proposal (79 FR 75273). For example, due to variability 
in responsiveness, only a subset of individuals who experience 
exposures at or above a benchmark concentration can be expected to 
experience health effects.\53\ In addition, not all of these effects 
will be adverse. Given the lack of sufficient exposure-response 
information for most of the health effects that informed benchmark 
concentrations, estimates of the number of people likely to experience 
exposures at or above benchmark concentrations generally cannot be 
translated into quantitative estimates of the number of people likely 
to experience specific health effects.\54\ The PA views health-relevant 
exposures as a continuum with greater confidence and less uncertainty 
about the existence of adverse health effects at higher O3 
exposure concentrations, and less confidence and greater uncertainty as 
one considers lower exposure concentrations (e.g., U.S. EPA, 2014c,

[[Page 65314]]

sections 3.1 and 4.6). This view draws from the overall body of 
available health evidence, which indicates that as exposure 
concentrations increase, the incidence, magnitude, and severity of 
effects increases.
---------------------------------------------------------------------------

    \53\ As noted below (II.C.3.a.ii), in the case of asthmatics, 
responsiveness to O3 could depend on factors that have 
not been well-evaluated, such as asthma severity, the effectiveness 
of asthma control, or the prevalence of medication use.
    \54\ The exception to this is lung function decrements, as 
discussed below (and in U.S. EPA, 2014c, section 3.2.3.1).
---------------------------------------------------------------------------

    Another important uncertainty is that there is very limited 
evidence from controlled human exposure studies, which provided the 
basis for health benchmark concentrations for both exposures of concern 
and lung function decrements, related to clinical responses in at-risk 
populations. Compared to the healthy young adults included in the 
controlled human exposure studies, members of at-risk populations could 
be more likely to experience adverse effects, could experience larger 
and/or more serious effects, and/or could experience effects following 
exposures to lower O3 concentrations.\55\
---------------------------------------------------------------------------

    \55\ ``The CASAC further notes that clinical studies do not 
address sensitive subgroups, such as children with asthma, and that 
there is a scientific basis to anticipate that the adverse effects 
for such subgroups are likely to be more significant at 60 ppb than 
for healthy adults'' (Frey 2014a, p. 7).
---------------------------------------------------------------------------

    There are also uncertainties associated with the exposure 
modelling. These are described most fully, and their potential impact 
characterized, in section 5.5.2 of the HREA (U.S. EPA, 2013, pp. 5-72 
to 5-79). These include interpretation of activity patterns set forth 
in diaries which do not typically distinguish the basis for activity 
patterns and so may reflect averting behavior,\56\ and whether the HREA 
underestimates exposures for groups spending especially large 
proportion of time being active outdoors during the O3 
season (outdoor workers and especially active children).
---------------------------------------------------------------------------

    \56\ See EPA 2014a pp. 5-53 to 54 describing EPA's sensitivity 
analysis regarding impacts of potential averting behavior for 
school-age children on the exposure and lung function decrement 
estimate, and see also section B.2.a.i below.
---------------------------------------------------------------------------

c. Quantitative Health Risk Assessments
    As discussed in section II.C.3 of the proposal (79 FR 75274), for 
some health endpoints, there is sufficient scientific evidence and 
information available to support the development of quantitative 
estimates of O3-related health risks. In the current review, 
for short-term O3 concentrations, the HREA estimates lung 
function decrements; respiratory symptoms in asthmatics; hospital 
admissions and emergency department visits for respiratory causes; and 
all-cause mortality (U.S. EPA, 2014a). For long-term O3 
concentrations, the HREA estimates respiratory mortality (U.S. EPA, 
2014a).\57\ Estimates of O3-induced lung function decrements 
are based on exposure modeling using the MSS model (see section 
II.1.b.i.(1) above, and 79 FR 75250), combined with exposure-response 
relationships from controlled human exposure studies (U.S. EPA, 2014a, 
Chapter 6). Estimates of O3-associated respiratory symptoms, 
hospital admissions and emergency department visits, and mortality are 
based on concentration-response relationships from epidemiologic 
studies (U.S. EPA, 2014a, Chapter 7). As with the exposure assessment 
discussed above, O3-associated health risks are estimated 
for recent air quality and for ambient concentrations adjusted to just 
meet the current and alternative O3 standards, based on 
2006-2010 air quality and adjusted precursor emissions. The following 
sections summarize the discussions from the proposal on the lung 
function risk assessment (II.A.2.c.i) and the epidemiology-based 
morbidity and mortality risk assessments (II.A.2.c.ii).
---------------------------------------------------------------------------

    \57\ Estimates of O3-associated respiratory mortality 
are based on the study by Jerrett et al. (2009). This study used 
seasonal averages of 1-hour daily maximum O3 
concentrations to estimate long-term concentrations.
---------------------------------------------------------------------------

i. Lung Function Risk Assessment
    The HREA estimates risks of lung function decrements in school-aged 
children (ages 5 to 18), asthmatic school-aged children, and the 
general adult population for the 15 urban study areas. The results 
presented in the HREA are based on an updated dose-threshold model that 
estimates FEV1 responses for individuals following short-
term exposures to O3 (McDonnell et al., 2012), reflecting 
methodological improvements since the last review (II.B.2.a.i (1), 
above; U.S. EPA, 2014a, section 6.2.4). The impact of the dose 
threshold is that O3-induced FEV1 decrements 
result primarily from exposures on days with average ambient 
O3 concentrations above about 40 ppb (U.S. EPA, 2014a, 
section 6.3.1, Figure 6-9).\58\
---------------------------------------------------------------------------

    \58\ Analysis of this issue in the HREA is based on risk 
estimates in Los Angeles for 2006 unadjusted air quality. The HREA 
shows that more than 90% of daily instances of FEV1 
decrements >=10% occur when 8-hr average ambient concentrations are 
above 40 ppb for this modeled scenario. The HREA notes that the 
distribution of responses will be different for different study 
areas, years, and air quality scenarios (U.S. EPA, 2014c, Chapter 
6).
---------------------------------------------------------------------------

    Table 2 in the proposal (79 FR 75275), and reprinted below, 
summarizes key results from the lung function risk assessment. Table 2 
presents estimates of the percentages of school-aged children estimated 
to experience O3-induced FEV1 decrements 
10, 15, or 20% when air quality was adjusted to just meet 
the current and alternative 8-hour O3 standards. Table 2 
also presents the numbers of children, including children with asthma, 
estimated to experience such decrements.

 Table 2--Summary of Estimated O3-Induced Lung Function Decrements for the Current and Potential Alternative O3
                                       Standards in Urban Case Study Areas
----------------------------------------------------------------------------------------------------------------
                                                                       Number of children (5
                                   Alternative         Average %      to 18 years) [number of   % Children worst
    Lung function decrement       standard level     children \59\      asthmatic children]      year and area
                                                                                \60\
----------------------------------------------------------------------------------------------------------------
                                        One or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%.........................                 75              14-19      3,007,000 [312,000]                 22
                                               70              11-17      2,527,000 [261,000]                 20
                                               65               3-15      1,896,000 [191,000]                 18
                                               60               5-11  \61\1,404,000 [139,000]                 13
>=15%.........................                 75                3-5         766,000 [80,000]                  7
                                               70                2-4         562,000 [58,000]                  5
                                               65                0-3         356,000 [36,000]                  4
                                               60                1-2         225,000 [22,000]                  3
>=20%.........................                 75                1-2         285,000 [30,000]                2.8
                                               70                1-2         189,000 [20,000]                2.1
                                               65                0-1         106,000 [11,000]                1.4
                                               60                0-1           57,000 [6,000]                0.9
----------------------------------------------------------------------------------------------------------------

[[Page 65315]]

 
                                        Two or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%.........................                 75             7.5-12      1,730,000 [179,000]                 14
                                               70             5.5-11      1,414,000 [145,000]                 13
                                               65            1.3-8.8      1,023,000 [102,000]                 11
                                               60            2.1-6.4         741,000 [73,000]                7.3
>=15%.........................                 75            1.7-2.9         391,000 [40,000]                3.8
                                               70            0.9-2.4         276,000 [28,000]                3.1
                                               65            0.1-1.8         168,000 [17,000]                2.3
                                               60            0.2-1.0         101,000 [10,000]                1.4
>=20%.........................                 75            0.5-1.1         128,000 [13,000]                1.5
                                               70            0.3-0.8           81,000 [8,000]                1.1
                                               65              0-0.5           43,000 [4,000]                0.8
                                               60              0-0.2           21,000 [2,000]                0.4
----------------------------------------------------------------------------------------------------------------

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

    \59\ Estimates in each urban case study area were averaged for 
the years evaluated in the HREA (2006 to 2010). Ranges reflect the 
ranges across urban study areas.
    \60\ Numbers of children estimated to experience decrements in 
each study urban case study area were averaged over 2006 to 2010. 
These averages were then summed across urban study areas. Numbers 
are rounded to nearest thousand unless otherwise indicated.
    \61\ As discussed in section 4.3.3 of the HREA, the model-based 
air quality adjustment approach used to estimate risks associated 
with the current and alternative standards was unable to estimate 
the distribution of ambient O3 concentrations in New York 
City upon just meeting an alternative standard with a level of 60 
ppb. Therefore, for the 60 ppb standard level, the numbers of 
children and asthmatic children experiencing decrements, and the 
ranges of percentages of such children across study areas, reflect 
all of the urban study areas except New York City. Because of this, 
in some cases (i.e., when New York City provided the smallest risk 
estimate), the lower end of the ranges in Table 2 are higher for a 
standard level of 60 ppb than for a level of 65 ppb.
---------------------------------------------------------------------------

    Uncertainties in estimates of lung function risks are summarized in 
section II.C.3.a.ii of the proposal (79 FR 75275). In addition to the 
uncertainties noted for exposure estimates, an uncertainty which 
impacts lung function risk estimates stems from the lack of exposure-
response information in children. In the near absence of controlled 
human exposure data for children, risk estimates are based on the 
assumption that children exhibit the same lung function response 
following O3 exposures as healthy 18 year olds (i.e., the 
youngest age for which controlled human exposure data is generally 
available) (U.S. EPA, 2014a, section 6.5.3). This assumption is 
justified in part by the findings of McDonnell et al. (1985), who 
reported that children (8-11 years old) experienced FEV1 
responses similar to those observed in adults (18-35 years old) (U.S. 
EPA, 2014a, p. 3-10). In addition, as discussed in the ISA (U.S. EPA, 
2013, section 6.2.1), summer camp studies of school-aged children 
reported O3-induced lung function decrements similar in 
magnitude to those observed in controlled human exposure studies using 
adults. In extending the risk model to children, the HREA thus fixes 
the age term in the model at its highest value, the value for age 18. 
Notwithstanding the information just summarized supporting this 
approach, EPA acknowledges the uncertainty involved, and notes that the 
approach could result in either over- or underestimates of 
O3-induced lung function decrements in children, depending 
on how children compare to the adults used in controlled human exposure 
studies (U.S. EPA, 2014a, section 6.5.3).
    A related source of uncertainty is that the risk assessment 
estimates of O3-induced decrements in asthmatics used the 
exposure-response relationship developed from data collected from 
healthy individuals. Although the evidence has been mixed (U.S. EPA, 
2013, section 6.2.1.1), several studies have reported statistically 
larger, or a tendency toward larger, O3-induced lung 
function decrements in asthmatics than in non-asthmatics (Kreit et al., 
1989; Horstman et al., 1995; Jorres et al., 1996; Alexis et al., 2000). 
On this issue, CASAC noted that ``[a]sthmatic subjects appear to be at 
least as sensitive, if not more sensitive, than non-asthmatic subjects 
in manifesting O3-induced pulmonary function decrements'' 
(Frey, 2014c, p. 4). To the extent asthmatics experience larger 
O3-induced lung function decrements than the healthy adults 
used to develop exposure-response relationships, the HREA could 
underestimate the impacts of O3 exposures on lung function 
in asthmatics, including asthmatic children. The implications of this 
uncertainty for risk estimates remain unknown at this time (U.S. EPA, 
2014a, section 6.5.4), and could depend on a variety of factors that 
have not been well-evaluated, including the severity of asthma and the 
prevalence of medication use. However, the available evidence shows 
responses to O3 increase with severity of asthma (Horstman 
et al., 1995) and corticosteroid usage does not prevent O3 
effects on lung function decrements or respiratory symptoms in people 
with asthma (Vagaggini et al., 2001, 2007).
ii. Mortality and Morbidity Risk Assessments
    As discussed in section II.C.3.b of the proposal (79 FR 75276), the 
HREA estimates O3-associated risks in 12 urban study areas 
\62\ using concentration-response relationships drawn from 
epidemiologic studies. These concentration-response relationships are 
based on ``area-wide'' average O3 concentrations.\63\ The 
HREA estimates risks for the years 2007 and 2009 in order to provide 
estimates of risk for a year with generally higher O3

[[Page 65316]]

concentrations (2007) and a year with generally lower O3 
concentrations (2009) (U.S. EPA, 2014a, section 7.1.1).
---------------------------------------------------------------------------

    \62\ The 12 urban areas evaluated are Atlanta, Baltimore, 
Boston, Cleveland, Denver, Detroit, Houston, Los Angeles, New York, 
Philadelphia, Sacramento, and St. Louis.
    \63\ In the epidemiologic studies that provide the health basis 
for HREA risk assessments, concentration-response relationships are 
based on daytime O3 concentrations, averaged across 
multiple monitors within study areas. These daily averages are used 
as surrogates for the spatial and temporal patterns of exposures in 
study populations. Consistent with this approach, the HREA 
epidemiologic-based risk estimates also utilize daytime 
O3 concentrations, averaged across monitors, as 
surrogates for population exposures. In this notice, we refer to 
these averaged concentrations as ``area-wide'' O3 
concentrations. Area-wide concentrations are discussed in more 
detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
---------------------------------------------------------------------------

    In considering the epidemiology-based risk estimates, the proposal 
focuses on mortality risks associated with short-term O3 
concentrations. The proposal considers estimates of total risk (i.e., 
based on the full distributions of ambient O3 
concentrations) and estimates of risk associated with O3 
concentrations in the upper portions of ambient distributions. Both 
estimates are discussed to provide information that considers risk 
estimates based on concentration-response relationships being linear 
over the entire distribution of ambient O3 concentrations, 
and thus have the greater potential for morbidity and mortality to be 
affected by changes in relatively low O3 concentrations, as 
well as risk estimates that are associated with O3 
concentrations in the upper portions of the ambient distribution, thus 
focusing on risk from higher O3 concentrations and placing 
greater weight on the uncertainty associated with the shapes of 
concentration-response curves for O3 concentrations in the 
lower portions of the distribution. These results for O3-
associated mortality risk are summarized in Table 3 in the proposal (79 
FR 75277).
    Important uncertainties in epidemiology-based risk estimates, based 
on their consideration in the HREA and PA, are discussed in section 
II.C.3.b.ii of the proposal (79 FR 75277). Compared to estimates of 
O3 exposures of concern and estimates of O3-
induced lung function decrements (discussed above), the HREA 
conclusions reflect lower confidence in epidemiologic-based risk 
estimates (U.S. EPA, 2014a, section 9.6). In particular, the HREA 
highlights the heterogeneity in effect estimates between locations, the 
potential for exposure measurement errors, and uncertainty in the 
interpretation of the shape of concentration-response functions at 
lower O3 concentrations (U.S. EPA, 2014a, section 9.6). The 
HREA also concludes 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 is based 
on only one study, though that study is well-designed, and because of 
the uncertainty in that study about the existence and identification of 
a potential threshold in the concentration-response function (U.S. EPA, 
2014a, section 9.6).\64,65\ This section further discusses some of the 
key uncertainties in epidemiologic-based risk estimates, as summarized 
in the PA (U.S. EPA, 2014c, section 3.2.3.2), with a focus on 
uncertainties that can have particularly important implications for the 
Administrator's consideration of epidemiology-based risk estimates.
---------------------------------------------------------------------------

    \64\ The CASAC also concluded that ``[i]n light of the potential 
nonlinearity of the C-R function for long-term exposure reflecting a 
threshold of the mortality response, the estimated number of 
premature deaths avoidable for long-term exposure reductions for 
several levels need to be viewed with caution'' (Frey, 2014a, p. 3).
    \65\ There is also uncertainty about the extent to which 
mortality estimates based on the long-term metric used in the study 
by Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily 
maximum concentrations) reflects associations with long-term average 
O3 versus repeated occurrences of elevated short-term 
concentrations.
---------------------------------------------------------------------------

    The PA notes that reducing NOX emissions generally 
reduces O3-associated mortality and morbidity risk estimates 
in locations and time periods with relatively high ambient 
O3 concentrations and increases risk estimates in locations 
and time periods with relatively low concentrations (II.A, above). When 
evaluating uncertainties in epidemiologic risk estimates, the PA 
considered (1) the extent to which the modeled O3 response 
to reductions in NOX emissions appropriately represents the 
trends observed in monitored ambient O3 following actual 
reductions in NOX emissions, (2) the extent to which the 
O3 response to reductions in precursor emissions could 
differ with emissions reduction strategies that are different from 
those used in HREA to generate risk estimates, and (3) the extent to 
which estimated changes in risks in urban study areas are 
representative of the changes that would be experienced broadly across 
the U.S. population. The first two of these issues are discussed in 
section II.A.2.c above. The third issue is discussed below.
    The HREA conducted national air quality modeling analyses that 
estimated the proportion of the U.S. population living in locations 
where seasonal averages of daily O3 concentrations are 
estimated to decrease in response to reductions in NOX 
emissions, and the proportion living in locations where such seasonal 
averages are estimated to increase. Given the close relationship 
between changes in seasonal averages of daily O3 
concentrations and changes in seasonal mortality and morbidity risk 
estimates, this analysis informs consideration of the extent to which 
the risk results in urban study areas represent the U.S. population as 
a whole. This ``representativeness analysis'' indicates that the 
majority of the U.S. population lives in locations where reducing 
NOX emissions would be expected to result in decreases in 
warm season averages of daily maximum 8-hour ambient O3 
concentrations. Because the HREA urban study areas tend to 
underrepresent the populations living in such areas (e.g., suburban, 
smaller urban, and rural areas), risk estimates for the urban study 
areas are likely to understate the average reductions in O3-
associated mortality and morbidity risks that would be experienced 
across the U.S. population as a whole upon reducing NOX 
emissions (U.S. EPA, 2014a, section 8.2.3.2).
    Section 7.4 of the HREA also highlights some additional 
uncertainties associated with epidemiologic-based risk estimates (U.S. 
EPA, 2014a). This section of the HREA identifies and discusses sources 
of uncertainty and presents a qualitative evaluation of key parameters 
that can introduce uncertainty into risk estimates (U.S. EPA, 2014a, 
Table 7-4). For several of these parameters, the HREA also presents 
quantitative sensitivity analyses (U.S. EPA, 2014a, sections 7.4.2 and 
7.5.3). Of the uncertainties discussed in Chapter 7 of the HREA, those 
related to the application of concentration-response functions from 
epidemiologic studies can have particularly important implications for 
consideration of epidemiology-based risk estimates, as discussed below.
    An important uncertainty is the shape of concentration-response 
functions at low ambient O3 concentrations (U.S. EPA, 2014a, 
Table 7-4).\66\ In recognition of the ISA's conclusion that certainty 
in the shape of O3 concentration-response functions 
decreases at low ambient concentrations, the HREA provides estimates of 
epidemiology-based mortality risks for entire distributions of ambient 
O3 concentrations, as well as estimates of total mortality 
associated with various ambient O3 concentrations. The PA 
considers both types of risk estimates, recognizing greater public 
health concern for adverse O3-attributable effects at higher 
ambient O3 concentrations (which drive higher exposure 
concentrations, section 3.2.2 of the PA (U.S. EPA, 2014c)), as compared 
to lower concentrations.
---------------------------------------------------------------------------

    \66\ A related uncertainty is the existence, or not, of a 
threshold. The HREA addresses this issue for long-term O3 
by evaluating risks in models that include potential thresholds 
(II.D.2.c).
---------------------------------------------------------------------------

    A related consideration is associated with the public health 
importance of the increases in relatively low O3 
concentrations following air quality adjustment. There is uncertainty 
that relates to the assumption that the concentration response function 
for O3 is linear, such that total risk estimates are equally 
influenced by decreasing

[[Page 65317]]

high concentrations and increasing low concentrations, when the 
increases and decreases are of equal magnitude. Even on days with 
increases in relatively low area-wide average concentrations, resulting 
in increases in estimated risks, some portions of the urban study areas 
could experience decreases in high O3 concentrations. To the 
extent adverse O3-attributable effects are more strongly 
supported for higher ambient concentrations (which, as noted above, are 
consistently reduced upon air quality adjustment), the impacts on risk 
estimates of increasing low O3 concentrations reflect an 
important source of uncertainty. In addition to the uncertainties 
discussed above, the proposal also notes uncertainties related to (1) 
using concentration-response relationships developed for a particular 
population in a particular location to estimate health risks in 
different populations and locations; (2) using concentration-response 
functions from epidemiologic studies reflecting a particular air 
quality distribution to adjusted air quality necessarily reflecting a 
different (simulated) air quality distribution; (3) using a national 
concentration-response function to estimate respiratory mortality 
associated with long-term O3; and (4) unquantified 
reductions in risk that could be associated with reductions in the 
ambient concentrations of pollutants other than O3, 
resulting from control of NOX (79 FR 75277 to 75279).

B. Need for Revision of the Primary Standard

    The initial issue to be addressed in the current review of the 
primary O3 standard is whether, in view of the advances in 
scientific knowledge and additional information, it is appropriate to 
revise the existing standard. This section presents the Administrator's 
final decision on whether it is ``appropriate'' to revise the current 
standard within the meaning of section 109 (d)(1) of the CAA. Section 
II.B.1 contains a summary discussion of the basis for the proposed 
conclusions on the adequacy of the primary standard. Section II.B.2 
discusses comments received on the adequacy of the primary standard. 
Section II.B.3 presents the Administrator's final conclusions on the 
adequacy of the current primary standard.
1. Basis for Proposed Decision
    In evaluating whether it is appropriate to retain or revise the 
current standard, the Administrator's considerations build upon those 
in the 2008 review, including consideration of the broader body of 
scientific evidence and exposure and health risk information now 
available, as summarized in sections II.A to II.C (79 FR 75246-75279) 
of the proposal and section II.A above.
    In developing conclusions on the adequacy of the current primary 
O3 standard, the Administrator takes into account both 
evidence-based and quantitative exposure- and risk-based 
considerations. Evidence-based considerations include the assessment of 
evidence from controlled human exposure, animal toxicological, and 
epidemiologic studies for a variety of health endpoints. The 
Administrator focuses on health endpoints for which the evidence is 
strong enough to support a ``causal'' or a ``likely to be causal'' 
relationship, based on the ISA's integrative synthesis of the entire 
body of evidence. The Administrator's consideration of quantitative 
exposure and risk information draws from the results of the exposure 
and risk assessments presented in the HREA.
    The Administrator's consideration of the evidence and exposure/risk 
information is informed by the considerations and conclusions presented 
in the PA (U.S. EPA, 2014c). The purpose of the PA is to help ``bridge 
the gap'' between the scientific and technical information assessed in 
the ISA and HREA, and the policy decisions that are required of the 
Administrator (U.S. EPA, 2014c, Chapter 1); see also American Farm 
Bureau Federation, 559 F. 3d at 516, 521 (``[a]lthough not required by 
the statute, in practice EPA staff also develop a Staff Paper, which 
discusses the information in the Criteria Document that is most 
relevant to the policy judgments the EPA makes when it sets the 
NAAQS''). The PA's evidence-based and exposure-/risk-based 
considerations and conclusions are briefly summarized below in sections 
II.B.1.a (evidence-based considerations), II.B.1.b (exposure- and risk-
based considerations), and II.B.1.c (PA conclusions on the current 
standard). Section II.B.1.d summarizes CASAC advice to the 
Administrator and public commenter views on the current standard. 
Section II.B.1.e presents a summary of the Administrator's proposed 
conclusions concerning the adequacy of the public health protection 
provided by the current standard, and her proposed decision to revise 
that standard.
a. Evidence-Based Considerations From the PA
    In considering the available scientific evidence, the PA evaluates 
the O3 concentrations in health effects studies (U.S. EPA, 
2014c, section 3.1.4). Specifically, the PA characterizes the extent to 
which health effects have been reported for the O3 exposure 
concentrations evaluated in controlled human exposure studies, and 
effects occurring over the distributions of ambient O3 
concentrations in locations where epidemiologic studies have been 
conducted. These considerations, as they relate to the adequacy of the 
current standard, are presented in detail in section 3.1.4 of the PA 
(U.S. EPA, 2014c) and are summarized in the proposal (79 FR 75279-
75287). The PA's considerations are summarized briefly below for 
controlled human exposure, epidemiologic panel studies, and 
epidemiologic population-based studies.
    Section II.D.1.a of the proposal discusses the PA's consideration 
of the evidence from controlled human exposure and panel studies. This 
evidence is assessed in section 6.2 of the ISA (U.S. EPA, 2013) and is 
summarized in section 3.1.2 of the PA (U.S. EPA, 2014c). A large number 
of controlled human exposure studies have reported lung function 
decrements, respiratory symptoms, air inflammation, airway 
hyperresponsiveness, and/or impaired lung host defense in young, 
healthy adults engaged in moderate quasi-continuous exertion, following 
6.6-hour O3 exposures. These studies have consistently 
reported such effects following exposures to O3 
concentrations of 80 ppb or greater. In addition to lung function 
decrements, available studies have evaluated respiratory symptoms or 
airway inflammation following exposures to O3 concentrations 
below 75 ppb. Table 3-1 in the PA highlights the group mean results of 
individual controlled human exposure studies that evaluated exposures 
to O3 concentrations below 75 ppb. These studies observe the 
combination of lung function decrements and respiratory symptoms 
following exposures to O3 concentrations as low as 72 ppb, 
and lung function decrements and airway inflammation following 
exposures to O3 concentrations as low as 60 ppb (based on 
group means).
    Based on this evidence, the PA notes that controlled human exposure 
studies have reported a variety of respiratory effects in young, 
healthy adults following exposures to a wide range of O3 
concentrations for 6.6 hours, including exposures to concentrations 
below 75 ppb. In particular, the PA further notes that a recent 
controlled human exposure study reported the combination of lung 
function decrements and respiratory symptoms in healthy adults engaged 
in quasi-

[[Page 65318]]

continuous, moderate exertion following 6.6 hour exposures to 72 ppb 
O3, a combination of effects that have been classified as 
adverse based on ATS guidelines for adversity (ATS, 2000a). In 
addition, a recent study has also reported lung function decrements and 
pulmonary inflammation following exposure to 60 ppb O3. 
Sixty ppb is the lowest exposure concentration for which inflammation 
has been evaluated and reported to occur, and corresponds to the lowest 
exposure concentration demonstrated to result in lung function 
decrements large enough to be judged an abnormal response by ATS (ATS, 
2000b). The PA also notes, and CASAC agreed, that these controlled 
human exposure studies were conducted in healthy adults, while at-risk 
groups (e.g., children, people with asthma) could experience larger 
and/or more serious effects. Therefore, the PA concludes that the 
evidence from controlled human exposure studies provide support that 
the respiratory effects experienced following exposures to 
O3 concentrations lower than 75 ppb would be adverse in some 
individuals, particularly if experienced by members of at-risk 
populations (e.g., people with asthma, children).
    The PA also notes consistent results in some panel studies of 
O3-associated lung function decrements. In particular, the 
PA notes that epidemiologic panel studies in children and adults 
consistently indicate O3-associated lung function decrements 
when on-site, ambient monitored concentrations were below 75 ppb 
(although the evidence becomes less consistent at low O3 
concentrations, and the averaging periods involved ranged from 10 
minutes to 12 hours (U.S. EPA, 2014c, section 3.2.4.2)).
    Section II.D.1.b of the proposal summarizes the PA's analyses of 
monitored O3 concentrations in locations of epidemiologic 
studies. While the majority of the epidemiologic study areas evaluated 
would have violated the current standard during study periods, the PA 
makes the following observations with regard to health effect 
associations at O3 concentrations likely to have met the 
current standard:
    (1) A single-city study reported positive and statistically 
significant associations with asthma emergency department visits in 
children and adults in Seattle, a location that would have met the 
current standard over the entire study period (Mar and Koenig, 2009).
    (2) Additional single-city studies support associations with 
respiratory morbidity at relatively low ambient O3 
concentrations, including when virtually all monitored concentrations 
were below the level of the current standard (Silverman and Ito, 2010; 
Strickland et al., 2010).
    (3) Canadian multicity studies reported positive and statistically 
significant associations with respiratory morbidity or mortality when 
the majority of study cities, though not all study cities, would have 
met the current standard over the study period in each of these studies 
(Cakmak et al., 2006; Dales et al., 2006; Katsouyanni et al., 2009; 
Stieb et al., 2009).
    (4) A U.S. multicity study reported positive and statistically 
significant associations with mortality when ambient O3 
concentrations were restricted to those likely to have met the current 
O3 standard (Bell et al., 2006).
    The PA also takes into account important uncertainties in these 
analyses of air quality in locations of epidemiologic study areas. 
These uncertainties are summarized in section II.D.1.b.iii of the 
proposal. Briefly, they include the following: (1) Uncertainty in 
conclusions about the extent to which multicity effect estimates 
reflect associations with air quality meeting the current standard, 
versus air quality violating that standard; (2) uncertainty regarding 
the potential for thresholds to exist, given that regional 
heterogeneity in O3 health effect associations could obscure 
the presence of thresholds, should they exist; (3) uncertainty in the 
extent to which the PA appropriately recreated the air quality analyses 
in the published study by Bell et al. (2006); and (4) uncertainty in 
the extent to which reported health effects are caused by exposures to 
O3 itself, as opposed to other factors such as co-occurring 
pollutants or pollutant mixtures, particularly at low ambient 
O3 concentrations.\67\
---------------------------------------------------------------------------

    \67\ As noted above (section II.A.1.B.i), the ISA concludes that 
studies that examined the potential confounding effects of 
copollutants found that O3 effect estimates remained 
relatively robust upon the inclusion of PM and gaseous pollutants in 
two-pollutant models (U.S. EPA, 2013, section 6.2.7.5).
---------------------------------------------------------------------------

    In considering the analyses of monitored O3 air quality 
in locations of epidemiologic studies, as well as the important 
uncertainties in these analyses, the PA concludes that these analyses 
provide support for the occurrence of morbidity and mortality 
associated with short-term ambient O3 concentrations likely 
to meet the current O3 standard.\68\ In considering the 
evidence as a whole, the PA concludes that (1) controlled human 
exposure studies provide strong support for the occurrence of adverse 
respiratory effects following exposures to O3 concentrations 
below the level of the current standard and (2) epidemiologic studies 
provide support for the occurrence of adverse respiratory effects and 
mortality under air quality conditions that would meet the current 
standard.
---------------------------------------------------------------------------

    \68\ Unlike for the studies of short-term O3, the 
available U.S. and Canadian epidemiologic studies evaluating long-
term ambient O3 concentration metrics have not been 
conducted in locations likely to have met the current 8-hour 
O3 standard during the study period, and have not 
reported concentration-response functions that indicate confidence 
in health effect associations at O3 concentrations 
meeting the current standard (U.S. EPA, 2014c, section 3.1.4.3).
---------------------------------------------------------------------------

b. Exposure- and Risk-Based Considerations in the PA
    In order to further inform judgments about the potential public 
health implications of the current O3 NAAQS, the PA 
considers the exposure and risk assessments presented in the HREA (U.S. 
EPA, 2014c, section 3.2). Overviews of these exposure and risk 
assessments, including brief summaries of key results and 
uncertainties, are provided in section II.A.2 above. Section II.D.2 of 
the proposal summarizes key observations from the PA related to the 
adequacy of the current O3 NAAQS, based on consideration of 
the HREA exposure assessment, lung function risk assessment, and 
mortality/morbidity risk assessments (79 FR 75283).
    Section II.D.2.a of the proposal summarizes key observations from 
the PA regarding estimates of O3 exposures of concern (79 FR 
75283). Given the evidence for respiratory effects from controlled 
human exposure studies, the PA considers the extent to which the 
current standard would be estimated to protect at-risk populations 
against exposures of concern to O3 concentrations at or 
above the health benchmark concentrations of 60, 70, and 80 ppb (i.e., 
based on HREA estimates of one or more and two or more exposures of 
concern). In doing so, the PA notes the CASAC conclusion that (Frey, 
2014c, p. 6):

    The 80 ppb-8hr benchmark level represents an exposure level for 
which there is substantial clinical evidence demonstrating a range 
of ozone-related effects including lung inflammation and airway 
responsiveness in healthy individuals. The 70 ppb-8hr benchmark 
level reflects the fact that in healthy subjects, decreases in lung 
function and respiratory symptoms occur at concentrations as low as 
72 ppb and that these effects almost certainly occur in some people, 
including asthmatics and others with low lung function who are less 
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which 
ozone-

[[Page 65319]]

related effects have been observed in clinical studies of healthy 
individuals.

    For exposures of concern at or above 60 ppb, the proposal 
highlights the following key observations for air quality adjusted to 
just meet the current standard:
    (1) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 10 to 18% of children in urban study 
areas to experience one or more exposures of concern at or above 60 
ppb. Summing across urban study areas, these percentages correspond to 
almost 2.5 million children experiencing approximately 4 million 
exposures of concern at or above 60 ppb during a single O3 
season. Of these children, almost 250,000 are asthmatics.\69\
---------------------------------------------------------------------------

    \69\ As discussed in section II.C.2.b of the proposal, due to 
variability in responsiveness, only a subset of individuals who 
experience exposures at or above a benchmark concentration can be 
expected to experience adverse health effects.
---------------------------------------------------------------------------

    (2) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 3 to 8% of children in urban study 
areas to experience two or more exposures of concern to O3 
concentrations at or above 60 ppb. Summing across the urban study 
areas, these percentages correspond to almost 900,000 children 
(including almost 90,000 asthmatic children).
    (3) In the worst-case years (i.e., those with the largest exposure 
estimates), the current standard is estimated to allow approximately 10 
to 25% of children to experience one or more exposures of concern at or 
above 60 ppb, and approximately 4 to 14% to experience two or more 
exposures of concern at or above 60 ppb.


    For exposures of concern at or above 70 ppb, the PA highlights the 
following key observations for air quality adjusted to just meet the 
current standard:
    (1) On average over the years 2006 to 2010, the current standard is 
estimated to allow up to approximately 3% of children in urban study 
areas to experience one or more exposures of concern at or above 70 
ppb. Summing across urban study areas, almost 400,000 children 
(including almost 40,000 asthmatic children) are estimated to 
experience O3 exposure concentrations at or above 70 ppb 
during a single O3 season.
    (2) On average over the years 2006 to 2010, the current standard is 
estimated to allow less than 1% of children in urban study areas to 
experience two or more exposures of concern to O3 
concentrations at or above 70 ppb.
    (3) In the worst-case location and year, the current standard is 
estimated to allow approximately 8% of children to experience one or 
more exposures of concern at or above 70 ppb, and approximately 2% to 
experience two or more exposures of concern, at or above 70 ppb.

For exposures of concern at or above 80 ppb, the PA highlights the 
observation that the current standard is estimated to allow about 1% or 
fewer children in urban study areas to experience exposures of concern 
at or above 80 ppb, even in years with the highest exposure estimates.
    Uncertainties in exposure estimates are summarized in section 
II.C.2.b of the proposal (79 FR 75273), and discussed more fully in the 
HREA (U.S. EPA, 2014a, section 5.5.2) and the PA (U.S. EPA, 2014c, 
section 3.2.2). Key uncertainties include the variability in 
responsiveness following O3 exposures, resulting in only a 
subset of exposed individuals experiencing health effects, adverse or 
otherwise, and the limited evidence from controlled human exposure 
studies conducted in at-risk populations. In addition, there are a 
number of uncertainties in the exposure modelling approach used in the 
HREA, contributing to overall uncertainty in exposure estimates.
    Section II.D.2.b of the proposal summarizes key observations from 
the PA regarding the estimated risk of O3-induced lung 
function decrements (79 FR 75283 to 75284). With respect to the lung 
function decrements that have been evaluated in controlled human 
exposure studies, the PA considers the extent to which standards with 
revised levels would be estimated to protect healthy and at-risk 
populations against one or more, and two or more, moderate (i.e., 
FEV1 decrements >=10% and >=15%) and large (i.e., 
FEV1 decrements >=20%) lung function decrements. As 
discussed in section 3.1.3 of the PA (U.S. EPA, 2014c), although some 
experts would judge single occurrences of moderate responses to be a 
nuisance, especially for healthy individuals, a more general consensus 
view of the adversity of moderate lung function decrements emerges as 
the frequency of occurrence increases.
    With regard to decrements >=10%, the PA highlights the following 
key observations for air quality adjusted to just meet the current 
standard:
    (1) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 14 to 19% of children in urban study 
areas to experience one or more lung function decrements >=10%. Summing 
across urban study areas, this corresponds to approximately 3 million 
children experiencing 15 million O3-induced lung function 
decrements >=10% during a single O3 season. Of these 
children, about 300,000 are asthmatics.
    (2) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 7 to 12% of children in urban study 
areas to experience two or more O3-induced lung function 
decrements >=10%. Summing across the urban study areas, this 
corresponds to almost 2 million children (including almost 200,000 
asthmatic children) estimated to experience two or more O3-
induced lung function decrements greater than 10% during a single 
O3 season.
    (3) In the worst-case years, the current standard is estimated to 
allow approximately 17 to 23% of children in urban study areas to 
experience one or more lung function decrements >=10%, and 
approximately 10 to 14% to experience two or more O3-induced 
lung function decrements >=10%.

With regard to decrements >=15%, the PA highlights the following key 
observations for air quality adjusted to just meet the current 
standard:
    (1) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 3 to 5% of children in urban study 
areas to experience one or more lung function decrements <=15%. Summing 
across urban study areas, this corresponds to approximately 800,000 
children (including approximately 80,000 asthmatic children) estimated 
to experience at least one O3-induced lung function 
decrement <=15% during a single O3 season.
    (2) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 2 to 3% of children in urban study 
areas to experience two or more O3-induced lung function 
decrements <=15%.
    (3) In the worst-case years, the current standard is estimated to 
allow approximately 4 to 6% of children in urban study areas to 
experience one or more lung function decrements <=15%, and 
approximately 2 to 4% to experience two or more O3-induced 
lung function decrements <=15%.

    With regard to decrements <=20%, the PA highlights the following 
key observations for air quality adjusted to just meet the current 
standard:
    (1) On average over the years 2006 to 2010, the current standard is 
estimated to allow approximately 1 to 2% of children in urban study 
areas to experience one or more lung function decrements >=20%. Summing 
across

[[Page 65320]]

urban study areas, this corresponds to approximately 300,000 children 
(including approximately 30,000 asthmatic children) estimated to 
experience at least one O3-induced lung function decrement 
>=20% during a single O3 season.
    (2) On average over the years 2006 to 2010, the current standard is 
estimated to allow less than 1% of children in urban study areas to 
experience two or more O3-induced lung function decrements 
>=20%.
    (3) In the worst-case years, the current standard is estimated to 
allow approximately 2 to 3% of children to experience one or more lung 
function decrements >=20%, and less than 2% to experience two or more 
O3-induced lung function decrements >=20%.

Uncertainties in lung function risk estimates are summarized in section 
II.C.3.a of the proposal, and discussed more fully in the HREA (U.S. 
EPA, 2014a, section 6.5) and the PA (U.S. EPA, 2014c, section 3.2.3.1). 
In addition to the uncertainties noted above for exposure estimates, 
the key uncertainties associated with estimates of O3-
induced lung function decrements include the paucity of exposure-
response information in children and in people with asthma.
    Section II.D.2.c of the proposal summarizes key observations from 
the PA regarding risk estimates of O3-associated mortality 
and morbidity (79 FR 75284 to 75285). With regard to total mortality or 
morbidity associated with short-term O3, the PA notes the 
following for air quality adjusted to just meet the current standard:
    (1) When air quality was adjusted to the current standard for the 
2007 model year (the year with generally ``higher'' O3-
associated risks), 10 of 12 urban study areas exhibited either 
decreases or virtually no change in estimates of the number of 
O3-associated deaths (U.S. EPA, 2014a, Appendix 7B). 
Increases were estimated in two of the urban study areas (Houston, Los 
Angeles)\70\ (U.S. EPA, 2014a, Appendix 7B).\71\
---------------------------------------------------------------------------

    \70\ As discussed above (II.C.1), in locations and time periods 
when NOX is predominantly contributing to O3 
formation (e.g., downwind of important NOX sources, where 
the highest O3 concentrations often occur), model-based 
adjustment to the current and alternative standards decreases 
estimated ambient O3 concentrations compared to recent 
monitored concentrations (U.S. EPA, 2014a, section 4.3.3.2). In 
contrast, in locations and time periods when NOX is 
predominantly contributing to O3 titration (e.g., in 
urban centers with high concentrations of NOX emissions, 
where ambient O3 concentrations are often suppressed and 
are thus relatively low), model-based adjustment increases ambient 
O3 concentrations compared to recent monitored 
concentrations (U.S. EPA, 2014a, section 4.3.3.2). Changes in 
epidemiology-based risk estimates depend on the balance between the 
daily decreases in high O3 concentrations and increases 
in low O3 concentrations following the model-based air 
quality adjustment. Commenting on this issue, CASAC noted that 
``controls designed to reduce the peak levels of ozone (e.g., the 
fourth-highest annual MDA8) may not be effective at reducing lower 
levels of ozone on more typical days and may actually increase ozone 
levels on days where ozone concentrations are low'' (Frey 2014a, p. 
2). CASAC further noted that risk results ``suggest that the ozone-
related health risks in the urban cores can increase for some of the 
cities as ozone NAAQS alternatives become more stringent. This is 
because reductions in nitrogen oxides emissions can lead to less 
scavenging of ozone and free radicals, resulting in locally higher 
levels of ozone'' (Frey 2014c, p. 10).
    \71\ For the 2009 adjusted year (i.e., the year with generally 
lower O3 concentrations), changes in risk were generally 
smaller than in 2007 (i.e., most changes about 2% or smaller). 
Increases were estimated for Houston, Los Angeles, and New York 
City.
---------------------------------------------------------------------------

    (2) In focusing on total risk, the current standard is estimated to 
allow thousands of O3-associated deaths per year in the 
urban study areas. In focusing on the risks associated with the upper 
portions of distributions of ambient concentrations (area-wide 
concentrations <= 40, 60 ppb), the current standard is estimated to 
allow hundreds to thousands of O3-associated deaths per year 
in the urban study areas.
    (3) The current standard is estimated to allow tens to thousands of 
O3-associated morbidity events per year (i.e., respiratory-
related hospital admissions, emergency department visits, and asthma 
exacerbations).

With regard to respiratory mortality associated with long-term 
O3, the PA notes the following for air quality adjusted to 
just meet the current standard:
    (1) Based on a linear concentration-response function, the current 
standard is estimated to allow thousands of O3-associated 
respiratory deaths per year in the urban study areas.
    (2) Based on threshold models, HREA sensitivity analyses indicate 
that the number of respiratory deaths associated with long-term 
O3 concentrations could potentially be considerably lower 
(i.e., by more than 75% if a threshold exists at 40 ppb, and by about 
98% if a threshold exists at 56 ppb) (U.S. EPA, 2014a, Figure 7-9).\72\
---------------------------------------------------------------------------

    \72\ Risk estimates for respiratory mortality associated with 
long-term O3 exposures are based on the study by Jerrett 
et al. (2009) (U.S. EPA, 2014a, Chapter 7). As discussed above 
(II.B.2.b.iv) and in the PA (U.S. EPA, 2014c, section 3.1.4.3), 
Jerrett et al. (2009) reported that when seasonal averages of 1-hour 
daily maximum O3 concentrations ranged from 33 to 104 
ppb, there was no statistical deviation from a linear concentration-
response relationship between O3 and respiratory 
mortality across 96 U.S. cities (U.S. EPA, 2013, section 7.7). 
However, the authors reported ``limited evidence'' for an effect 
threshold at an O3 concentration of 56 ppb (p=0.06). In 
communications with EPA staff (Sasser, 2014), the study authors 
indicated that it is not clear whether a threshold model is a better 
predictor of respiratory mortality than the linear model, and that 
``considerable caution should be exercised in accepting any specific 
threshold.''
---------------------------------------------------------------------------

    Compared to the weight given to HREA estimates of exposures of 
concern and lung function risks, and the weight given to the evidence, 
the PA places relatively less weight on epidemiologic-based risk 
estimates. In doing so, the PA notes that the overall conclusions from 
the HREA likewise reflect less confidence in estimates of 
epidemiologic-based risks than in estimates of exposures and lung 
function risks. The determination to attach less weight to the 
epidemiologic-based estimates reflects the uncertainties associated 
with mortality and morbidity risk estimates, including the 
heterogeneity in effect estimates between locations, the potential for 
exposure measurement errors, and uncertainty in the interpretation of 
the shape of concentration-response functions at lower O3 
concentrations (U.S. EPA, 2014a, section 9.6).
    Uncertainty in the shape of concentration-response functions at 
lower O3 concentrations is particularly important to 
interpreting risk estimates given the approach used to adjust air 
quality to just meet the current standard, and potential alternative 
standards, and the resulting compression in the air quality 
distributions (i.e., decreasing high concentrations and increasing low 
concentrations) (II.A.2.a, above). Total risk estimates in the HREA are 
based on the assumption that the concentration response function for 
O3 is linear, such that total risk estimates are equally 
influenced by decreasing high concentrations and increasing low 
concentrations, when the increases and decreases are of equal 
magnitude. However, consistent with the PA's consideration of risk 
estimates, in the proposal the Administrator notes that the overall 
body of evidence provides stronger support for the occurrence of

[[Page 65321]]

O3-attributable health effects following exposures to 
O3 concentrations corresponding to the upper ends of typical 
ambient distributions (II.E.4.d of the proposal). In addition, even on 
days with increases in relatively low area-wide average concentrations, 
resulting in increases in estimated risks, some portions of the urban 
study areas could experience decreases in high O3 
concentrations. Therefore, to the extent adverse O3-
attributable effects are more strongly supported for higher ambient 
concentrations (which, as noted above, are consistently reduced upon 
air quality adjustment), the PA notes that the impacts on risk 
estimates of increasing low O3 concentrations reflect an 
important source of uncertainty.
c. PA Conclusions on the Current Standard
    Section II.D.3 of the proposal summarizes the PA conclusions on the 
adequacy of the existing primary O3 standard (79 FR 75285). 
As an initial matter, the PA concludes that reducing precursor 
emissions to achieve O3 concentrations that meet the current 
standard will provide important improvements in public health 
protection. This initial conclusion is based on (1) the strong body of 
scientific evidence indicating a wide range of adverse health outcomes 
attributable to exposures to O3 concentrations commonly 
found in the ambient air and (2) estimates indicating decreased 
occurrences of O3 exposures of concern and decreased health 
risks upon meeting the current standard, compared to recent air 
quality.
    In particular, the PA concludes that strong support for this 
initial conclusion is provided by controlled human exposure studies of 
respiratory effects, and by quantitative estimates of exposures of 
concern and lung function decrements based on information in these 
studies. Analyses in the HREA estimate that the percentages of children 
(i.e., all children and children with asthma) in urban study areas 
experiencing exposures of concern, or experiencing abnormal and 
potentially adverse lung function decrements, are consistently lower 
for air quality that just meets the current O3 standard than 
for recent air quality. The HREA estimates such reductions consistently 
across the urban study areas evaluated and throughout various portions 
of individual urban study areas, including in urban cores and the 
portions of urban study areas surrounding urban cores. These reductions 
in exposures of concern and O3-induced lung function 
decrements reflect the consistent decreases in the highest 
O3 concentrations following reductions in precursor 
emissions to meet the current standard. Thus, populations in both urban 
and non-urban areas would be expected to experience important 
reductions in O3 exposures and O3-induced lung 
function risks upon meeting the current standard.
    The PA further concludes that support for this initial conclusion 
is also provided by estimates of O3-associated mortality and 
morbidity based on application of concentration-response relationships 
from epidemiologic studies to air quality adjusted to just meet the 
current standard. These estimates are based on the assumption that 
concentration-response relationships are linear over entire 
distributions of ambient O3 concentrations, an assumption 
which has uncertainties that complicate interpretation of these 
estimates (II.A.2.c.ii). However, risk estimates for effects associated 
with short- and long-term O3 exposures, combined with the 
HREA's national analysis of O3 responsiveness to reductions 
in precursor emissions and the consistent reductions estimated for the 
highest ambient O3 concentrations, suggest that 
O3-associated mortality and morbidity would be expected to 
decrease nationwide following reductions in precursor emissions to meet 
the current O3 standard.
    After reaching the initial conclusion that meeting the current 
primary O3 standard will provide important improvements in 
public health protection, and that it is not appropriate to consider a 
standard that is less protective than the current standard, the PA 
considers the adequacy of the public health protection that is provided 
by the current standard. In considering the available scientific 
evidence, exposure/risk information, advice from CASAC (II.B.1.d, 
below), and input from the public, the PA reaches the conclusion that 
the available evidence and information clearly call into question the 
adequacy of public health protection provided by the current primary 
standard. In reaching this conclusion, the PA notes that evidence from 
controlled human exposure studies provides strong support for the 
occurrence of adverse respiratory effects following exposures to 
O3 concentrations below the level of the current standard. 
Epidemiologic studies provide support for the occurrence of adverse 
respiratory effects and mortality under air quality conditions that 
would likely meet the current standard. In addition, based on the 
analyses in the HREA, the PA concludes that the exposures and risks 
projected to remain upon meeting the current standard are indicative of 
risks that can reasonably be judged to be important from a public 
health perspective. Thus, the PA concludes that the evidence and 
information provide strong support for giving consideration to revising 
the current primary standard in order to provide increased public 
health protection against an array of adverse health effects that range 
from decreased lung function and respiratory symptoms to more serious 
indicators of morbidity (e.g., including emergency department visits 
and hospital admissions), and mortality. In consideration of all of the 
above, the PA draws the conclusion that it is appropriate for the 
Administrator to consider revision of the current primary O3 
standard to provide increased public health protection.
d. CASAC Advice
    Section II.D.4 of the proposal summarizes CASAC advice regarding 
the adequacy of the existing primary O3 standard. Following 
the 2008 decision to revise the primary O3 standard by 
setting the level at 0.075 ppm (75 ppb), CASAC strongly questioned 
whether the standard met the requirements of the CAA. In September 
2009, the EPA announced its intention to reconsider the 2008 standards, 
issuing a notice of proposed rulemaking in January 2010 (75 FR 2938). 
Soon after, the EPA solicited CASAC review of that proposed rule and in 
January 2011, solicited additional advice. This proposal was based on 
the scientific and technical record from the 2008 rulemaking, including 
public comments and CASAC advice and recommendations. As further 
described above (I.D), in the fall of 2011, the EPA did not revise the 
standard as part of the reconsideration process but decided to defer 
decisions on revisions to the O3 standards to the next 
periodic review, which was already underway. Accordingly, in this 
section we describe CASAC's advice related to the 2008 final decision 
and the subsequent reconsideration, as well as its advice on this 
current review of the O3 NAAQS that was initiated in 
September 2008.
    In April 2008, the members of the CASAC Ozone Review Panel sent a 
letter to EPA stating ``[I]n our most-recent letters to you on this 
subject--dated October 2006 and March 2007--the CASAC unanimously 
recommended selection of an 8-hour average Ozone NAAQS within the range 
of 0.060 to 0.070 parts per million [60 to 70 ppb] for the primary 
(human health-based) Ozone NAAQS'' (Henderson, 2008). In 2010, in 
response to the EPA's solicitation of advice on the EPA's

[[Page 65322]]

proposed rulemaking as part of the reconsideration, CASAC again stated 
that the current standard should be revised to provide additional 
protection to the public health (Samet, 2010):

    CASAC fully supports EPA's proposed range of 0.060-0.070 parts 
per million (ppm) for the 8-hour primary ozone standard. CASAC 
considers this range to be justified by the scientific evidence as 
presented in the Air Quality Criteria for Ozone and Related 
Photochemical Oxidants (March 2006) and Review of the National 
Ambient Air Quality Standards for Ozone: Policy Assessment of 
Scientific and Technical Information, OAQPS Staff Paper (July 2007). 
As stated in our letters of October 24, 2006, March 26, 2007 and 
April 7, 2008 to former Administrator Stephen L. Johnson, CASAC 
unanimously recommended selection of an 8-hour average ozone NAAQS 
within the range proposed by EPA (0.060 to 0.070 ppm). In proposing 
this range, EPA has recognized the large body of data and risk 
analyses demonstrating that retention of the current standard would 
leave large numbers of individuals at risk for respiratory effects 
and/or other significant health impacts including asthma 
exacerbations, emergency room visits, hospital admissions and 
mortality.

    In response to the EPA's request for additional advice on the 
reconsideration in 2011, CASAC reaffirmed their conclusion that ``the 
evidence from controlled human and epidemiological studies strongly 
supports the selection of a new primary ozone standard within the 60-70 
ppb range for an 8-hour averaging time'' (Samet, 2011, p ii). As 
requested by the EPA, CASAC's advice and recommendations were based on 
the scientific and technical record from the 2008 rulemaking. In 
considering the record for the 2008 rulemaking, CASAC stated the 
following to summarize the basis for their conclusions (Samet, 2011, 
pp. ii to iii):
    (1) The evidence available on dose-response for effects of 
O3 shows associations extending to levels within the range 
of concentrations currently experienced in the United States.
    (2) There is scientific certainty that 6.6-hour exposures with 
exercise of young, healthy, non-smoking adult volunteers to 
concentrations >=80 ppb cause clinically relevant decrements of lung 
function.
    (3) Some healthy individuals have been shown to have clinically 
relevant responses, even at 60 ppb.
    (4) Since the majority of clinical studies involve young, healthy 
adult populations, less is known about health effects in such 
potentially ozone sensitive populations as the elderly, children and 
those with cardiopulmonary disease. For these susceptible groups, 
decrements in lung function may be greater than in healthy volunteers 
and are likely to have a greater clinical significance.
    (5) Children and adults with asthma are at increased risk of acute 
exacerbations on or shortly after days when elevated O3 
concentrations occur, even when exposures do not exceed the NAAQS 
concentration of 75 ppb.
    (6) Large segments of the population fall into what the EPA terms a 
``sensitive population group,'' i.e., those at increased risk because 
they are more intrinsically susceptible (children, the elderly, and 
individuals with chronic lung disease) and those who are more 
vulnerable due to increased exposure because they work outside or live 
in areas that are more polluted than the mean levels in their 
communities.

With respect to evidence from epidemiologic studies, CASAC stated 
``while epidemiological studies are inherently more uncertain as 
exposures and risk estimates decrease (due to the greater potential for 
biases to dominate small effect estimates), specific evidence in the 
literature does not suggest that our confidence on the specific 
attribution of the estimated effects of ozone on health outcomes 
differs over the proposed range of 60-70 ppb'' (Samet, 2011, p. 10).
    Following its review of the second draft PA in the current review, 
which considers an updated scientific and technical record since the 
2008 rulemaking, CASAC concluded that ``there is clear scientific 
support for the need to revise the standard'' (Frey, 2014c, p. ii). In 
particular, CASAC noted the following (Frey, 2014c, p. 5):

    [T]he scientific evidence provides strong support for the 
occurrence of a range of adverse respiratory effects and mortality 
under air quality conditions that would meet the current standard. 
Therefore, CASAC unanimously recommends that the Administrator 
revise the current primary ozone standard to protect public 
health.\73\
---------------------------------------------------------------------------

    \73\ CASAC provided similar advice in their letter to the 
Administrator on the HREA, stating that ``The CASAC finds that the 
current primary NAAQS for ozone is not protective of human health 
and needs to be revised'' (Frey, 2014a, p. 15).

    In supporting these conclusions, CASAC judged that the strongest 
evidence comes from controlled human exposure studies of respiratory 
effects. The Committee specifically noted that ``the combination of 
decrements in FEV1 together with the statistically 
significant alterations in symptoms in human subjects exposed to 72 ppb 
ozone meets the American Thoracic Society's definition of an adverse 
health effect'' (Frey, 2014c, p. 5). CASAC further judged that ``if 
subjects had been exposed to ozone using the 8-hour averaging period 
used in the standard, adverse effects could have occurred at lower 
concentration'' and that ``the level at which adverse effects might be 
observed would likely be lower for more sensitive subgroups, such as 
those with asthma'' (Frey, 2014c, p. 5). With regard to 60 ppb 
exposures, CASAC noted that ``a level of 60 ppb corresponds to the 
lowest exposure concentration demonstrated to result in lung function 
decrements large enough to be judged an abnormal response by ATS and 
that could be adverse in individuals with lung disease'' (Frey, 2014c, 
p. 7). The CASAC further noted that ``a level of 60 ppb also 
corresponds to the lowest exposure concentration at which pulmonary 
inflammation has been reported'' (Frey, 2014c, p. 7).
    In their advice, CASAC also took note of estimates of O3 
exposures of concern and the risk of O3-induced lung 
function decrements. With regard to the benchmark concentrations used 
in estimating exposures of concern, CASAC stated the following (Frey, 
2014c, p. 6):

    The 80 ppb-8hr benchmark level represents an exposure level for 
which there is substantial clinical evidence demonstrating a range 
of ozone-related effects including lung inflammation and airway 
responsiveness in healthy individuals. The 70 ppb-8hr benchmark 
level reflects the fact that in healthy subjects, decreases in lung 
function and respiratory symptoms occur at concentrations as low as 
72 ppb and that these effects almost certainly occur in some people, 
including asthmatics and others with low lung function who are less 
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which 
ozone-related effects have been observed in clinical studies of 
healthy individuals. Based on its scientific judgment, the CASAC 
finds that the 60 ppb-8hr exposure benchmark is relevant for 
consideration with respect to adverse effects on asthmatics.

    With regard to lung function risk estimates, CASAC concluded that 
``estimation of FEV1 decrements of >=15% is appropriate as a 
scientifically relevant surrogate for adverse health outcomes in active 
healthy adults, whereas an FEV1 decrement of >=10% is a 
scientifically relevant surrogate for adverse health outcomes for 
people with asthma and lung disease'' (Frey, 2014c, p. 3). The 
Committee further concluded that ``[a]sthmatic subjects appear to be at 
least as sensitive, if not more sensitive, than non-asthmatic subjects 
in manifesting O3-induced pulmonary function decrements'' 
(Frey, 2014c, p. 4).
    Although CASAC judged that controlled human exposure studies of 
respiratory effects provide the strongest

[[Page 65323]]

evidence supporting their conclusion on the current standard, the 
Committee judged that there is also ``sufficient scientific evidence 
based on epidemiologic studies for mortality and morbidity associated 
with short-term exposure to ozone at the level of the current 
standard'' (Frey, 2014c, p. 5) and noted that ``[r]ecent animal 
toxicological studies support identification of modes of action and, 
therefore, the biological plausibility associated with the 
epidemiological findings'' (Frey, 2014c, p. 5).
e. Administrator's Proposed Decision
    Section II.D.5 in the proposal (79 FR 75287-75291) discusses the 
Administrator's proposed conclusions related to the adequacy of the 
public health protection provided by the current primary O3 
standard, resulting in her proposed decision to revise that standard. 
These proposed conclusions and her proposed decision, summarized below, 
were based on the Administrator's consideration of the available 
scientific evidence, exposure/risk information, the comments and advice 
of CASAC, and public input that had been received by the time of 
proposal.
    As an initial matter, the Administrator concluded that reducing 
precursor emissions to achieve O3 concentrations that meet 
the current primary O3 standard will provide important 
improvements in public health protection, compared to recent air 
quality. In reaching this initial conclusion, she noted the discussion 
in section 3.4 of the PA (U.S. EPA, 2014c). In particular, the 
Administrator noted that this initial conclusion is supported by (1) 
the strong body of scientific evidence indicating a wide range of 
adverse health outcomes attributable to exposures to O3 
concentrations commonly measured in the ambient air and (2) estimates 
indicating decreased occurrences of O3 exposures of concern 
and decreased O3-associated health risks upon meeting the 
current standard, compared to recent air quality. Thus, she concluded 
that it would not be appropriate in this review to consider a standard 
that is less protective than the current standard.\74\
---------------------------------------------------------------------------

    \74\ Although the Administrator noted that reductions in 
O3 precursor emissions (e.g., NOX; VOC) to 
achieve O3 concentrations that meet the current standard 
could also increase public health protection by reducing the ambient 
concentrations of pollutants other than O3 (e.g., 
PM2.5, NO2), we did not quantitatively analyze 
these effects, consistent with CASAC advice (Frey, 2014a, p.10). 
However, the Administrator is not setting the standard to address 
risks from pollutants other than O3.
---------------------------------------------------------------------------

    After reaching the initial conclusion that meeting the current 
primary O3 standard will provide important improvements in 
public health protection, and that it is not appropriate to consider a 
standard that is less protective than the current standard, the 
Administrator next considered the adequacy of the public health 
protection that is provided by the current standard. In doing so, the 
Administrator first noted that studies evaluated since the completion 
of the 2006 AQCD support and expand upon the strong body of evidence 
that, in the last review, indicated a causal relationship between 
short-term O3 exposures and respiratory health effects, the 
strongest determination under the ISA's hierarchical system for 
classifying weight of evidence for causation. Together, experimental 
and epidemiologic studies support conclusions regarding a continuum of 
O3 respiratory effects ranging from small reversible changes 
in pulmonary function, and pulmonary inflammation, to more serious 
effects that can result in respiratory-related emergency department 
visits, hospital admissions, and premature mortality. The Administrator 
further noted that recent animal toxicology studies support 
descriptions of modes of action for these respiratory effects and 
provide support for biological plausibility for the role of 
O3 in reported effects. With regard to mode of action, 
evidence indicates that antioxidant capacity may modify the risk of 
respiratory morbidity associated with O3 exposure, and that 
the inherent capacity to quench (based on individual antioxidant 
capacity) can be overwhelmed, especially with exposure to elevated 
concentrations of O3. In addition, based on the consistency 
of findings across studies and evidence for the coherence of results 
from different scientific disciplines, evidence indicates that certain 
populations are at increased risk of experiencing O3-related 
effects, including the most severe effects. These include populations 
and lifestages identified in previous reviews (i.e., people with 
asthma, children, older adults, outdoor workers) and populations 
identified since the last review (i.e., people with certain genotypes 
related to antioxidant and/or anti-inflammatory status; people with 
reduced intake of certain antioxidant nutrients, such as Vitamins C and 
E).
    The Administrator further noted that evidence for adverse 
respiratory health effects attributable to long-term \75\ O3 
exposures is much stronger than in previous reviews, and noted the 
ISA's conclusion that there is ``likely to be'' a causal relationship 
between such O3 exposures and adverse respiratory health 
effects (the second strongest causality determination). She noted that 
the evidence available in this review includes new epidemiologic 
studies using a variety of designs and analysis methods, conducted by 
different research groups in different locations, evaluating the 
relationships between long-term O3 exposures and measures of 
respiratory morbidity and mortality. New evidence supports associations 
between long-term O3 exposures and the development of asthma 
in children, with several studies reporting interactions between 
genetic variants and such O3 exposures. Studies also report 
associations between long-term O3 exposures and asthma 
prevalence, asthma severity and control, respiratory symptoms among 
asthmatics, and respiratory mortality.
---------------------------------------------------------------------------

    \75\ Based on the exposure surrogates used in recent 
epidemiologic studies of long-term O3 exposure, it is not 
possible to distinguish between the impacts of long-term 
O3 exposure and exposure to repeated short-term peaks 
over an O3 season.
---------------------------------------------------------------------------

    In considering the O3 exposure concentrations reported 
to elicit respiratory effects, the Administrator agreed with the 
conclusions of the PA and with the advice of CASAC (Frey, 2014c) that 
controlled human exposure studies provide the most certain evidence 
indicating the occurrence of health effects in humans following 
exposures to specific O3 concentrations. In particular, she 
noted that the effects reported in controlled human exposure studies 
are due solely to O3 exposures, and interpretation of study 
results is not complicated by the presence of co-occurring pollutants 
or pollutant mixtures.
    In considering the evidence from controlled human exposure studies, 
the Administrator first noted that these studies have reported a 
variety of respiratory effects in healthy adults following exposures to 
O3 concentrations of 60, 72, or 80 ppb, and higher. The 
largest respiratory effects, and the broadest range of effects, have 
been studied and reported following exposures of healthy adults to 80 
ppb O3 or higher, with most exposure studies conducted at 
these higher concentrations. She further noted that recent evidence 
includes controlled human exposure studies reporting the combination of 
lung function decrements and respiratory symptoms in healthy adults 
engaged in quasi-continuous, moderate exertion following 6.6 hour 
exposures to concentrations as low as 72 ppb, and lung function 
decrements and

[[Page 65324]]

pulmonary inflammation following exposures to O3 
concentrations as low as 60 ppb. As discussed below, compared to the 
evidence available in the last review, the Administrator viewed these 
studies as having strengthened support for the occurrence of abnormal 
and adverse respiratory effects attributable to short-term exposures to 
O3 concentrations below the level of the current standard. 
The Administrator stated that such exposures to O3 
concentrations below the level of the current standard are potentially 
important from a public health perspective, given the following:
    (1) The combination of lung function decrements and respiratory 
symptoms reported to occur in healthy adults following exposures to 72 
ppb O3 or higher, while at moderate exertion, meet ATS 
criteria for an adverse response. In specifically considering the 72 
ppb exposure concentration, CASAC noted that ``the combination of 
decrements in FEV1 together with the statistically 
significant alterations in symptoms in human subjects exposed to 72 ppb 
ozone meets the American Thoracic Society's definition of an adverse 
health effect'' (Frey, 2014c, p. 5).
    (2) With regard to 60 ppb O3, CASAC agreed that ``a 
level of 60 ppb corresponds to the lowest exposure concentration 
demonstrated to result in lung function decrements large enough to be 
judged an abnormal response by ATS and that could be adverse in 
individuals with lung disease'' (Frey, 2014c, p. 7). CASAC further 
noted that ``a level of 60 ppb also corresponds to the lowest exposure 
concentration at which pulmonary inflammation has been reported'' 
(Frey, 2014c, p. 7).
    (3) The controlled human exposure studies reporting these 
respiratory effects were conducted in healthy adults, while at-risk 
groups (e.g., children, people with asthma) could experience larger 
and/or more serious effects. In their advice to the Administrator, 
CASAC concurred with this reasoning (Frey, 2014a, p. 14; Frey, 2014c, 
p. 5).
    (4) These respiratory effects are coherent with the serious health 
outcomes that have been reported in epidemiologic studies evaluating 
exposure to O3 (e.g., respiratory-related hospital 
admissions, emergency department visits, and mortality).
    As noted above, the Administrator's proposed conclusions regarding 
the adequacy of the current primary O3 standard placed a 
large amount of weight on the results of controlled human exposure 
studies. In particular, given the combination of lung function 
decrements and respiratory symptoms following 6.6-hour exposures to 
O3 concentrations as low as 72 ppb, and given CASAC advice 
regarding effects at 72 ppb, along with ATS adversity criteria, she 
concluded that the evidence in this review supports the occurrence of 
adverse respiratory effects following exposures to O3 
concentrations lower than the level of the current standard.\76\ As 
discussed below, the Administrator further considered information from 
the broader body of controlled human exposure studies within the 
context of quantitative estimates of exposures of concern and 
O3-induced FEV1 decrements.
---------------------------------------------------------------------------

    \76\ This CASAC advice and ATS recommendations are discussed in 
more detail in section II.C.4 below (see also II.A.1.c, above).
---------------------------------------------------------------------------

    While putting less weight on information from epidemiologic studies 
than on information from controlled human exposure studies, the 
Administrator also considered what the available epidemiologic evidence 
indicates with regard to the adequacy of the public health protection 
provided by the current primary O3 standard. She noted that 
recent epidemiologic studies provide support, beyond that available in 
the last review, for associations between short-term O3 
exposures and a wide range of adverse respiratory outcomes (including 
respiratory-related hospital admissions, emergency department visits, 
and mortality) and with total mortality. Associations with morbidity 
and mortality are stronger during the warm or summer months, and remain 
robust after adjustment for copollutants.
    In considering information from epidemiologic studies within the 
context of her conclusions on the adequacy of the current standard, the 
Administrator considered the extent to which available studies support 
the occurrence of O3 health effect associations with air 
quality likely to be allowed by the current standard. Most of the 
epidemiologic studies considered by the Administrator were conducted in 
locations likely to have violated the current standard over at least 
part of the study period. However, she noted three U.S. single-city 
studies that support the occurrence of O3-associated 
hospital admissions or emergency department visits at ambient 
O3 concentrations below the level of the current standard, 
or when virtually all monitored concentrations were below the level of 
the current standard (Mar and Koenig, 2009; Silverman and Ito, 2010; 
Strickland et al., 2010) (section II.D.1 of the proposal). While the 
Administrator acknowledged greater uncertainty in interpreting air 
quality for multicity studies, she noted that O3 
associations with respiratory morbidity or mortality have been reported 
when the majority of study locations (though not all study locations) 
would likely have met the current O3 standard. When taken 
together, the Administrator reached the initial conclusion at proposal 
that single-city epidemiologic studies and associated air quality 
information support the occurrence of O3-associated hospital 
admissions and emergency department visits for ambient O3 
concentrations likely to have met the current standard, and that air 
quality analyses in locations of multicity studies provide some support 
for this conclusion for a broader range of effects, including 
mortality.
    Beyond her consideration of the scientific evidence, the 
Administrator also considered the results of the HREA exposure and risk 
analyses in reaching initial conclusions regarding the adequacy of the 
current primary O3 standard. In doing so, as noted above, 
she focused primarily on exposure and risk estimates based on 
information from controlled human exposure studies (i.e., exposures of 
concern and O3-induced lung function decrements) and placed 
relatively less weight on epidemiologic-based risk estimates.
    With regard to estimates of exposures of concern, the Administrator 
considered the extent to which the current standard provides protection 
against exposures to O3 concentrations at or above 60, 70, 
and 80 ppb. Consistent with CASAC advice (Frey, 2014c), the 
Administrator focused on children in these analyses of O3 
exposures, noting that estimates for all children and asthmatic 
children are virtually indistinguishable, in terms of the percent 
estimated to experience exposures of concern.\77\ Though she focused on 
children, she also recognized that exposures to O3 
concentrations at or above 60 or 70 ppb could be of concern for adults. 
As discussed in the HREA and PA (and II.C.2.a of the proposal), the 
patterns of exposure estimates across urban study areas, across years, 
and across air quality scenarios are similar in adults with asthma, 
older adults, all children, and children with asthma, though smaller 
percentages of adult populations are estimated to experience exposures 
of concern than children and children with asthma. Thus, the 
Administrator recognized that the exposure patterns for children across 
years, urban study areas, and air

[[Page 65325]]

quality scenarios are indicative of the exposure patterns in a broader 
group of at-risk populations that also includes asthmatic adults and 
older adults.
---------------------------------------------------------------------------

    \77\ As noted above, HREA analyses indicate that activity data 
for asthmatics is generally similar to non-asthmatics (U.S. EPA, 
2014a, Appendix 5G, Tables 5G2-to 5G-5).
---------------------------------------------------------------------------

    She further noted that while single exposures of concern could be 
adverse for some people, particularly for the higher benchmark 
concentrations (70, 80 ppb) where there is stronger evidence for the 
occurrence of adverse effects, she became increasingly concerned about 
the potential for adverse responses as the number of occurrences 
increases (61 FR 75122).\78\ In particular, she noted that repeated 
occurrences of the types of effects shown to occur following exposures 
of concern can have potentially adverse outcomes. For example, repeated 
occurrences of airway inflammation could potentially result in the 
induction of a chronic inflammatory state; altered pulmonary structure 
and function, leading to diseases such as asthma; altered lung host 
defense response to inhaled microorganisms; and altered lung response 
to other agents such as allergens or toxins (U.S. EPA, 2013, section 
6.2.3). Thus, the Administrator noted that the types of respiratory 
effects shown to occur in some individuals following exposures to 
O3 concentrations from 60 to 80 ppb, particularly if 
experienced repeatedly, provide a mode of action by which O3 
may cause other more serious effects (e.g., asthma exacerbations). 
Therefore, the Administrator placed the most weight on estimates of two 
or more exposures of concern (i.e., as a surrogate for the occurrence 
of repeated exposures), though she also considered estimates of one or 
more, particularly for the 70 and 80 ppb benchmarks.\79\
---------------------------------------------------------------------------

    \78\ The Administrator noted that not all people who experience 
an exposure of concern will experience an adverse effect (even 
members of at-risk populations). For most of the endpoints evaluated 
in controlled human exposure studies (with the exception of 
O3-induced FEV1 decrements, as discussed 
below), the number of those experiencing exposures of concern who 
will experience adverse effects cannot be reliably quantified.
    \79\ The Administrator's considerations related to estimated 
O3 exposures of concern, including her views on estimates 
of two or more and one or more such exposures, are discussed in more 
detail within the context of her consideration of public comments on 
the level of the revised standard and her final decision on level 
(II.C.4.b and II.C.4.c, below).
---------------------------------------------------------------------------

    As illustrated in Table 1 (above), the Administrator noted that if 
the 15 urban study areas evaluated in the HREA were to just meet the 
current O3 standard, fewer than 1% of children in those 
areas would be estimated to experience two or more exposures of concern 
at or above 70 ppb, though approximately 3 to 8% of children, including 
approximately 3 to 8% of asthmatic children, would be estimated to 
experience two or more exposures of concern to O3 
concentrations at or above 60 ppb \80\ (based on estimates averaged 
over the years of analysis). To provide some perspective on these 
percentages, the Administrator noted that they correspond to almost 
900,000 children in urban study areas, including about 90,000 asthmatic 
children, estimated to experience two or more exposures of concern at 
or above 60 ppb. Nationally, if the current standard were to be just 
met, the number of children experiencing such exposures would be 
larger. In the worst-case year and location (i.e., year and location 
with the largest exposure estimates), the Administrator noted that over 
2% of children are estimated to experience two or more exposures of 
concern at or above 70 ppb and over 14% are estimated to experience two 
or more exposures of concern at or above 60 ppb.
---------------------------------------------------------------------------

    \80\ Almost no children in those areas would be estimated to 
experience two or more exposures of concern at or above 80 ppb.
---------------------------------------------------------------------------

    Although, as discussed above and in section II.E.4.d of the 
proposal, the Administrator was less concerned about single occurrences 
of exposures of concern, she noted that even single occurrences can 
cause adverse effects in some people, particularly for the 70 and 80 
ppb benchmarks. Therefore, she also considered estimates of one or more 
exposures of concern. As illustrated in Table 1 (above), if the 15 
urban study areas evaluated in the HREA were to just meet the current 
O3 standard, fewer than 1% of children in those areas would 
be estimated to experience one or more exposures of concern at or above 
80 ppb (based on estimates averaged over the years of analysis). 
However, approximately 1 to 3% of children, including 1 to 3% of 
asthmatic children, would be estimated to experience one or more 
exposures of concern to O3 concentrations at or above 70 ppb 
and approximately 10 to 17% would be estimated to experience one or 
more exposures of concern to O3 concentrations at or above 
60 ppb. In the worst-case year and location, the Administrator noted 
that over 1% of children are estimated to experience one or more 
exposures of concern at or above 80 ppb, over 8% are estimated to 
experience one or more exposures of concern at or above 70 ppb, and 
about 26% are estimated to experience one or more exposures of concern 
at or above 60 ppb.
    In addition to estimated exposures of concern, the Administrator 
also considered HREA estimates of the occurrence of O3-
induced lung function decrements. In doing so, she particularly noted 
CASAC advice that ``estimation of FEV1 decrements of >=15% 
is appropriate as a scientifically relevant surrogate for adverse 
health outcomes in active healthy adults, whereas an FEV1 
decrement of >=10% is a scientifically relevant surrogate for adverse 
health outcomes for people with asthma and lung disease'' (Frey, 2014c, 
p. 3). While these surrogates provide perspective on the potential for 
the occurrence of adverse respiratory effects following O3 
exposures, the Administrator agreed with the conclusion in past reviews 
that a more general consensus view of the adversity of moderate 
responses emerges as the frequency of occurrence increases (citing to 
61 FR 65722-3) (Dec, 13, 1996). Therefore, in the proposal the 
Administrator expressed increasing concern about the potential for 
adversity as the frequency of occurrences increased and, as a result, 
she focused primarily on estimates of two or more O3-induced 
FEV1 decrements (i.e., as a surrogate for repeated 
exposures).
    When averaged over the years evaluated in the HREA, the 
Administrator noted that the current standard is estimated to allow 
about 1 to 3% of children in the 15 urban study areas (corresponding to 
almost 400,000 children) to experience two or more O3-
induced lung function decrements =15%, and to allow about 8 
to 12% of children (corresponding to about 180,000 asthmatic children) 
to experience two or more O3-induced lung function 
decrements =10%. Nationally, larger numbers of children 
would be expected to experience such O3-induced decrements 
if the current standard were to be just met. The current standard is 
also estimated to allow about 3 to 5% of children in the urban study 
areas to experience one or more decrements =15% and about 14 
to 19% of children to experience one or more decrements 
=10%. In the worst-case year and location, the current 
standard is estimated to allow 4% of children in the urban study areas 
to experience two or more decrements =15% (and 7% to 
experience one or more such decrements) and 14% of children to 
experience two or more decrements =10% (and 22% to 
experience one or more such decrements).\81\
---------------------------------------------------------------------------

    \81\ As discussed below (II.C.4), in her consideration of 
potential alternative standard levels, the Administrator placed less 
weight on estimates of the risk of O3-induced 
FEV1 decrements. In doing so, she particularly noted 
that, unlike exposures of concern, the variability in lung function 
risk estimates across urban study areas is often greater than the 
differences in risk estimates between various standard levels (Table 
2, above). Given this, and the resulting considerable overlap 
between the ranges of lung function risk estimates for different 
standard levels, although the Administrator noted her confidence in 
the lung function risk estimates themselves, she viewed them as 
providing a more limited basis than exposures of concern for 
distinguishing between the degree of public health protection 
provided by alternative standard levels.

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[[Page 65326]]

    In further considering the HREA results, the Administrator 
considered the epidemiology-based risk estimates. Compared to the 
weight given to HREA estimates of exposures of concern and lung 
function risks, she placed relatively less weight on epidemiology-based 
risk estimates. Consistent with the conclusions in the PA, her 
determination to attach less weight to the epidemiologic-based risk 
estimates reflected her consideration of key uncertainties, including 
the heterogeneity in effect estimates between locations, the potential 
for exposure measurement errors, and uncertainty in the interpretation 
of the shape of concentration-response functions for O3 
concentrations in the lower portions of ambient distributions (U.S. 
EPA, 2014a, section 9.6) (section II.D.2 of the proposal).
    The Administrator focused on estimates of total mortality risk 
associated with short-term O3 exposures.\82\ Given the 
decreasing certainty in the shape of concentration-response functions 
for area-wide O3 concentrations at the lower ends of warm 
season distributions (U.S. EPA, 2013, section 2.5.4.4), the 
Administrator focused on estimates of risk associated with 
O3 concentrations in the upper portions of ambient 
distributions. Even when considering only area-wide O3 
concentrations from these upper portions of seasonal distributions, the 
Administrator noted that the current standard is estimated to allow 
hundreds to thousands of O3-associated deaths per year in 
urban study areas (79 FR 75291 citing to section II.C.3 of the 
proposal).
---------------------------------------------------------------------------

    \82\ In doing so, she concluded that lower confidence should be 
placed in the results of the assessment of respiratory mortality 
risks associated with long-term O3 exposures, primarily 
because that analysis is based on only one study (even though that 
study is well-designed) and because of the uncertainty in that study 
about the existence and identification of a potential threshold in 
the concentration-response function (U.S. EPA, 2014a, section 9.6) 
(section II.D.2 of the proposal). CASAC also called into question 
the extent to which it is appropriate to place confidence in risk 
estimates for respiratory mortality (Frey, 2014a, p. 11).
---------------------------------------------------------------------------

    In addition to the evidence and exposure/risk information discussed 
above, the Administrator took note of the CASAC advice in the current 
review and in the 2010 proposed reconsideration of the 2008 decision 
establishing the current standard. As discussed in more detail above, 
the current CASAC ``finds that the current NAAQS for ozone is not 
protective of human health'' and ``unanimously recommends that the 
Administrator revise the current primary ozone standard to protect 
public health'' (Frey, 2014c, p. 5).
    In consideration of all of the above, the Administrator proposed 
that the current primary O3 standard is not adequate to 
protect public health, and that it should be revised to provide 
increased public health protection. This proposed decision was based on 
the Administrator's initial conclusions that the available evidence and 
exposure and risk information clearly call into question the adequacy 
of public health protection provided by the current primary standard 
and, therefore, that the current standard is not requisite to protect 
public health with an adequate margin of safety. With regard to the 
evidence, she specifically noted that (1) controlled human exposure 
studies provide support for the occurrence of adverse respiratory 
effects following exposures to O3 concentrations below the 
level of the current standard (i.e., as low as 72 ppb), and that (2) 
single-city epidemiologic studies provide support for the occurrence of 
adverse respiratory effects under air quality conditions that would 
likely meet the current standard, with multicity studies providing 
limited support for this conclusion for a broader range of effects 
(i.e., including mortality). In addition, based on the analyses in the 
HREA, the Administrator concluded that the exposures and risks 
projected to remain upon meeting the current standard can reasonably be 
judged to be important from a public health perspective. Thus, she 
reached the proposed conclusion that the evidence and information, 
together with CASAC advice based on their consideration of that 
evidence and information, provide strong support for revising the 
current primary standard in order to increase public health protection 
against an array of adverse effects that range from decreased lung 
function and respiratory symptoms to more serious indicators of 
morbidity (e.g., including emergency department visits and hospital 
admissions), and mortality.
2. Comments on the Need for Revision
    The EPA received a large number of comments, more than 430,000 
comments, on the proposed decision to revise the current primary 
O3 standard. These comments generally fell into one of two 
broad groups that expressed sharply divergent views.
    Many commenters asserted that the current primary O3 
standard is not sufficient to protect public health, especially the 
health of sensitive groups, with an adequate margin of safety. These 
commenters agreed with the EPA's proposed decision to revise the 
current standard to increase public health protection. Among those 
calling for revisions to the current primary standard were medical 
groups (e.g., American Academy of Pediatrics (AAP), American Medical 
Association, American Lung Association (ALA), American Thoracic 
Society, American Heart Association, and the American College of 
Occupational and Environmental Medicine); national, state, and local 
public health and environmental organizations (e.g., the National 
Association of County and City Health Officials, American Public Health 
Association, Physicians for Social Responsibility, Sierra Club, Natural 
Resources Defense Council, Environmental Defense Fund, Center for 
Biological Diversity, and Earthjustice); the majority of state and 
local air pollution control authorities that submitted comments (e.g., 
agencies from California Air Resources Board and Office of 
Environmental Health Hazard Assessment, Connecticut, Delaware, Iowa, 
Illinois, Maryland, Minnesota, New Hampshire, New York, North Dakota, 
Oregon, Pennsylvania, Tennessee, and Wisconsin); the National Tribal 
Air Association; State organizations (e.g., National Association of 
Clean Air Agencies (NACAA), Northeast States for Coordinated Air Use 
Management, Ozone Transport Commission). While all of these commenters 
agreed with the EPA that the current O3 standard needs to be 
revised, many supported a more protective standard than proposed by 
EPA, as discussed in more detail below (II.C.4). Many individual 
commenters also expressed similar views.
    A second group of commenters, representing industry associations, 
businesses and some state agencies, opposed the proposed decision to 
revise the current primary O3 standard, expressing the view 
that the current standard is adequate to protect public health, 
including the health of sensitive groups, and to do so with an adequate 
margin of safety. Industry and business groups expressing this view 
included the American Petroleum Institute (API), the Alliance of 
Automobile Manufacturers (AAM), the American Forest and Paper 
Association, the Dow Chemical Company, the National Association of 
Manufacturers, the

[[Page 65327]]

National Mining Association, the U.S. Chamber of Commerce (in a joint 
comment with other industry groups), and the Utility Air Regulatory 
Group (UARG). State environmental agencies opposed to revising the 
current primary O3 standard included agencies from Arkansas, 
Georgia, Louisiana, Kansas, Michigan, Mississippi, Nebraska, North 
Carolina, Ohio, Texas, Virginia, and West Virginia.
    The following sections discuss comments submitted by these and 
other groups, and the EPA's responses to those comments. Comments 
dealing with overarching issues that are fundamental to EPA's decision-
making methodology are addressed in section II.B.2.a. Comments on the 
health effects evidence, including evidence from controlled human 
exposure and epidemiologic studies, are addressed in section II.B.2.b. 
Comments on human exposure and health risk assessments are addressed in 
section II.B.2.c. Comments on the appropriate indicator, averaging 
time, form, or level of a revised primary O3 standard are 
addressed below in section II.C. In addition to the comments addressed 
in this preamble, the EPA has prepared a Response to Comments document 
that addresses other specific comments related to standard setting, as 
well as comments on implementation- and/or cost-related factors that 
the EPA may not consider as part of the basis for decisions on the 
NAAQS. This document is available for review in the docket for this 
rulemaking and through the EPA's OAQPS TTN Web site (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html).
a. Overarching Comments
    Some commenters maintained that the proposed rule (and by extension 
the final rule) is fundamentally flawed because it does not quantify, 
or otherwise define, what level of protection is ``requisite'' to 
protect the public health. These commenters asserted that ``EPA has not 
explained how far above zero-risk it believes is appropriate or how 
close to background is acceptable. EPA has failed to explain how the 
current standard is inadequate on this specific basis'' (e.g., UARG, p. 
10). These commenters further maintained that the failure to quantify a 
requisite level of protection ``drastically reduces the value of public 
participation'' since ``the public does not understand what is driving 
EPA's decision'' (e.g., UARG, p. 11).
    The EPA disagrees with these comments and notes that industry 
petitioners made virtually the same argument before the D.C. Circuit in 
ATA III, on remand from the Supreme Court, arguing that unless EPA 
identifies and quantifies a degree of acceptable risk, it is impossible 
to determine if a NAAQS is requisite (i.e., neither too stringent or 
insufficiently stringent to protect the public health). The D.C. 
Circuit rejected petitioners' argument, holding that ``[a]lthough we 
recognize that the Clean Air Act and circuit precedent require EPA 
qualitatively to describe the standard governing its selection of 
particular NAAQS, we have 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 establish a [NAAQS]''' ATA III, 
283 F. 3d at 369 (quoting NRDC v. EPA, 902 F.2d 962, 973 (D.C. Cir. 
1990)). The court went on to explain that the requirement is only for 
EPA to engage in reasoned decision-making, ``not that it definitively 
identify pollutant levels below which risks to public health are 
negligible.'' ATA III, 283 F. 3d at 370.
    Thus, the Administrator is required to exercise her judgment in the 
face of scientific uncertainty to establish the NAAQS to provide 
appropriate protection against risks to public health, both known and 
unknown. As discussed below, in the current review, the Administrator 
judges that the existing primary O3 standard is not 
requisite to protect public health with an adequate margin of safety, a 
judgment that is consistent with CASAC's conclusion that ``there is 
clear scientific support for the need to revise the standard'' (Frey, 
2014c, p. ii). Further, in section II.C.4 below, the Administrator has 
provided a thorough explanation of her rationale for concluding that a 
standard with a level of 70 ppb is requisite to protect public health 
with an adequate margin of safety, explaining the various scientific 
uncertainties which circumscribe the range of potential alternative 
standards, and how she exercised her ``judgment'' (per section 109 
(b)(1) of the CAA) in selecting a standard from within that range of 
scientifically reasonable choices. This ``reasoned decision making'' is 
what the Act requires, 283 F. 3d at 370, not the quantification 
advocated by these commenters.
    The EPA further disagrees with the comment that a failure to 
quantify a requisite level of protection impaired or impeded public 
notice and comment opportunities. In fact, the EPA clearly gave 
adequate notice of the bases both for determining that the current 
standard does not afford requisite protection,\83\ and for determining 
how the standard should be revised. In particular, the EPA explained in 
detail which evidence it considered critical, and the scientific 
uncertainties that could cause the Administrator to weight that 
evidence in various ways (79 FR 75308-75310). There were robust 
comments submitted by commenters from a range of viewpoints on all of 
these issues, an indication of the adequacy of notice. The public was 
also afforded multiple opportunities to comment to the EPA and to CASAC 
during the development of the ISA, REA, and PA. Thus, the EPA does not 
agree that lack of quantification of a risk level that is ``requisite'' 
has deprived commenters of adequate notice and opportunity to comment 
in this proceeding.
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    \83\ See 79 FR 75287-91 (noting, among other things, that 
exposure to ambient O3 concentrations below the level of 
the current standard has been associated with diminished lung 
function capacity, respiratory symptoms, and respiratory health 
effects resulting in emergency room visits or hospital admissions, 
and that a single-city epidemiologic study showed associations with 
asthma emergency department visits in an area that would have met 
the current standard over the entire study period). See also Frey 
2014c, p. 5 (CASAC reiterated its conclusion, after multiple public 
comment opportunities, that as a matter of science the current 
standard ``is not protective of public health'' and provided the 
bases for that conclusion).
---------------------------------------------------------------------------

    Various commenters maintained that it was inappropriate to revise 
the current NAAQS based on their view that natural background 
concentrations in several states are at or above O3 
concentrations associated with meeting a NAAQS set at a level less than 
75 ppb (presumably retaining the same indicator, form, and averaging 
time), making the NAAQS impossible for those states to attain and 
maintain, a result they claim is legally impermissible. In support for 
their argument, the commenters cite monitoring and modelling results 
from various areas in the intermountain west, state that EPA analyses 
provide underestimates of background O3 and conclude that 
high concentrations of background O3 \84\ exist

[[Page 65328]]

in many parts of the United States that will ``prevent attainment'' of 
a revised standard (NMA, p. 5).
---------------------------------------------------------------------------

    \84\ Background O3 can be generically defined as the 
portion of O3 in ambient air that comes from sources 
outside the jurisdiction of an area and can include natural sources 
as well as transported O3 of anthropogenic origin. EPA 
has identified two specific definitions of background O3 
relevant to this discussion: natural background (NB) and United 
States background (USB). NB is defined as the O3 that 
would exist in the absence of any manmade precursor emissions. USB 
is defined as that O3 that would exist in the absence of 
any manmade emissions inside the U.S. This includes anthropogenic 
emissions outside the U.S. as well as naturally occurring ozone. In 
many cases, the comments reference background O3 only in 
the generic sense. Unless explicitly noted otherwise, we have 
assumed all references to background in the comments are intended to 
refer to USB.
---------------------------------------------------------------------------

    The courts have clearly established that ``[a]ttainability and 
technological feasibility are not relevant considerations in the 
promulgation of [NAAQS].'' API v. EPA, 665 F. 2d 1176, 1185 (D.C. Cir. 
1981). Further, the courts have clarified that the EPA may consider 
proximity to background concentrations as a factor in the decision 
whether and how to revise the NAAQS only in the context of considering 
standard levels within the range of reasonable values supported by the 
air quality criteria and judgments of the Administrator. 79 FR 75242-43 
(citing ATA III, 283 F. 3d at 379). In this review, the overall body of 
scientific evidence and exposure/risk information, as discussed in 
Section II.B of this notice, is clear and convincing: The existing 
standard is not adequate to protect public health with an adequate 
margin of safety and that the standard needs to be revised to reflect a 
lower level to provide that protection. The EPA analyses indicate that 
there may be infrequent instances in a limited number of rural areas 
where background O3 would be appreciable but not the sole 
contributor to an exceedance of the revised NAAQS, but do not indicate 
U.S. background (USB) O3 concentrations will prevent 
attainment of a revised O3 standard with a level of 70 ppb. 
USB is defined as that O3 that would exist even in the 
absence of any manmade emissions within the United States.
    The EPA's estimates of U.S. background ozone concentrations are 
based on frequently-utilized, state-of-the-science air quality models 
and are considered reasonable and reliable, not underestimates. In 
support of their view, the commenters state that monitored (not 
modelled) ozone concentrations in remote rural locations include 
instances of 8-hour average concentrations very occasionally higher 
than 70 ppb. Monitoring data from places like the Grand Canyon and 
Yellowstone National Parks, are examples cited in comments. It is 
inappropriate to assume that monitored O3 concentrations at 
remote sites can be used as a proxy for background O3. Even 
at the most remote locations, local O3 concentrations are 
impacted by anthropogenic emissions from within the U.S. The EPA 
modeling analyses (U.S. EPA, 2014c, Figure 2-18) estimate that, on a 
seasonal basis, 10-20% of the O3 at even the most remote 
locations in the intermountain western U.S. originates from manmade 
emissions from the U.S., and thus is not part of USB. This conclusion 
is supported by commenter-submitted recent data analyses of rural 
O3 observations in Nevada and Utah (NMA, Appendices D and 
H). These analyses conclude that natural sources, international 
O3 transport, O3 transported from upwind states, 
and O3 transported from urban areas within a state all 
contributed to O3 concentrations at rural sites.\85\ Thus, 
while O3 in high-altitude, rural portions of the 
intermountain western U.S. can, at times, be substantially influenced 
by background sources such as wildfires, international transport or the 
stratosphere, measured O3 in rural locations are also 
influenced by domestic emissions and so cannot, by themselves, be used 
to estimate USB concentrations. Accordingly, the fact that 2011-2013 
design values in locations like Yellowstone National Park (66 ppb) or 
Grand Canyon National Park (72 ppb) approach or exceed 70 ppb, does not 
support the conclusion that a standard with a level of 70 ppb is 
impossible to attain.
---------------------------------------------------------------------------

    \85\ The analysis of observations in Utah notes the influence of 
domestic emissions--either from Salt Lake City (for two of the 
areas) or from Los Angeles and California (for the third of the 
areas)--on O3 concentrations at each of the locations 
included (NMA comments, Appendix E). Additionally, the analysis of 
monitoring data for Nevada also describes the influence of the 
monitoring sites by domestic emissions from other western states 
(NMA, Appendix H).
---------------------------------------------------------------------------

    To accurately estimate USB concentrations, it is necessary to use 
air quality models which can estimate how much of the O3 at 
any given location originates from sources other than manmade emissions 
within the U.S. As part of the rulemaking, the EPA has summarized a 
variety of modeling-based analyses of background O3 (U.S. 
EPA, 2013, Chapter 3) and conducted our own multi-model assessment of 
USB concentrations across the U.S. (U.S. EPA, 2014c, Chapter 2). The 
EPA analyses, which are consistent with the previously-summarized 
studies highlighted by commenters, concluded that seasonal mean daily 
maximum 8-hour average concentrations of USB O3 range from 
25-50 ppb, with the highest estimates located across the intermountain 
western U.S.
    Importantly, the modeling analyses also indicate that the highest 
O3 days (i.e., the days most relevant to the form of the 
NAAQS) generally have similar daily maximum 8-hour average USB 
concentrations as the seasonal means of this metric, but have larger 
contributions from U.S. anthropogenic sources. As summarized in the PA, 
``the highest modeled O3 site-days tend to have background 
O3 levels similar to mid-range O3 days . . . 
[T]he days with highest O3 levels have similar distributions 
(i.e. means, inter-quartile ranges) of background levels as days with 
lower values, down to approximately 40 ppb. As a result, the proportion 
of total O3 that has background origins is smaller on high O3 days 
(e.g. greater than 60 ppb) than on the more common lower O3 days that 
tend to drive seasonal means'' (U.S. EPA, 2014c, p. 2-21, emphasis 
added). When averaged over the entire U.S., the models estimate that 
the mean USB fractional contribution to daily maximum 8-hour average 
O3 concentrations above 70 ppb is less than 35 percent. U.S. 
anthropogenic emission sources are thus the dominant contributor to the 
majority of modeled O3 exceedances across the U.S. (U.S. 
EPA, 2014c, Figures 2-14 and 2-15).
    As noted in the PA, and as highlighted by the commenters based on 
existing modeling, there can be infrequent events where daily maximum 
8-hour O3 concentrations approach or exceed 70 ppb largely 
due to the influence of USB sources like a wildfire or stratospheric 
intrusion. As discussed below in Section V, the statute and EPA 
implementing regulations allow for the exclusion of air quality 
monitoring data from design value calculations when there are 
exceedances caused by certain event-related U.S. background influences 
(e.g., wildfires or stratospheric intrusions). As a result, these 
``exceptional events'' will not factor into attainability concerns.
    In sum, the EPA believes that the commenters have failed to 
establish the predicate for their argument. Uncontrollable background 
concentrations of O3 are not expected to preclude attainment 
of a revised O3 standard with a level of 70 ppb. The EPA 
also disagrees with aspects of the specific statements made by the 
commenters as support for their view that the EPA analyses have 
underestimated background O3.\86\ Thus, even assuming the 
commenters are correct that the EPA may use proximity to background as 
a justification for not revising a standard that, in the judgment of 
the Administrator, is inadequate to protect public health, the 
commenters' arguments for the justification and need to do so for this 
review are based on a flawed premise.
---------------------------------------------------------------------------

    \86\ Specific aspects of the comments on the EPA analyses are 
addressed in more detail in the RTC.
---------------------------------------------------------------------------

b. Comments on the Health Effects Evidence
    As noted above, comments on the adequacy of the current standard 
fell into two broad categories reflecting very

[[Page 65329]]

different views of the available scientific evidence. Commenters who 
expressed support for the EPA's proposed decision to revise the current 
primary O3 standard generally concluded that the body of 
scientific evidence assessed in the ISA is much stronger and more 
compelling than in the last review. These commenters also generally 
emphasized CASAC's interpretation of the body of available evidence, 
which formed an important part of the basis for CASAC's reiterated 
recommendations to revise the O3 standard to provide 
increased public health protection. In some cases, these commenters 
supported their positions by citing studies published since the 
completion of the ISA.
    The EPA generally agrees with these commenters regarding the need 
to revise the current primary O3 standard in order to 
increase public health protection though, in many cases, not with their 
conclusions about the degree of protection that is appropriate 
(II.C.4.b and II.C.4.c, below). The scientific evidence noted by these 
commenters was generally the same as that assessed in the ISA (U.S. 
EPA, 2013) and the proposal,\87\ and their interpretation of the 
evidence was often, though not always, consistent with the conclusions 
of the ISA and CASAC. The EPA agrees that the evidence available in 
this review provides a strong basis for the conclusion that the current 
O3 standard is not adequately protective of public health. 
In reaching this conclusion, the EPA places a large amount of weight on 
the scientific advice of CASAC, and on CASAC's endorsement of the 
assessment of the evidence in the ISA (Frey and Samet, 2012).
---------------------------------------------------------------------------

    \87\ As discussed in section I.C above, the EPA has 
provisionally considered studies that were highlighted by commenters 
and that were published after the ISA. These studies are generally 
consistent with the evidence assessed in the ISA, and they do not 
materially alter our understanding of the scientific evidence or the 
Agency's conclusions based on that evidence.
---------------------------------------------------------------------------

    In contrast, while commenters who opposed the proposed decision to 
revise the primary O3 standard generally focused on many of 
the same studies assessed in the ISA, these commenters highlighted 
different aspects of these studies and reached substantially different 
conclusions about their strength and the extent to which progress has 
been made in reducing uncertainties in the evidence since the last 
review. These commenters generally concluded that information about the 
health effects of concern has not changed significantly since 2008 and 
that the uncertainties in the underlying health science have not been 
reduced since the 2008 review. In some cases, these commenters 
specifically questioned the EPA's approach to assessing the scientific 
evidence and to reaching conclusions on the strength of that evidence 
in the ISA. For example, several commenters asserted that the EPA's 
causal framework, discussed in detail in the ISA, is flawed and that it 
has not been applied consistently across health endpoints. Commenters 
also noted departures from other published causality frameworks (Samet 
and Bodurow, 2008) and from the criteria for judging causality put 
forward by Sir Austin Bradford Hill (Hill, 1965).
    The EPA disagrees with comments questioning the ISA's approach to 
assessing the evidence, the causal framework established in the ISA, or 
the consistent application of that framework across health endpoints. 
While the EPA acknowledges the ISA's approach departs from assessment 
and causality frameworks that have been developed for other purposes, 
such departures reflect appropriate adaptations for the NAAQS. As with 
other ISAs, the O3 ISA uses a five-level hierarchy that 
classifies the weight of evidence for causation. In developing this 
hierarchy, the EPA has drawn on the work of previous evaluations, most 
prominently the IOM's Improving the Presumptive Disability Decision-
Making Process for Veterans (Samet and Bodurow, 2008), EPA's Guidelines 
for Carcinogen Risk Assessment (U.S. EPA, 2005), and the U.S. Surgeon 
General's smoking report (CDC, 2004). The ISA's weight of evidence 
evaluation is based on the integration of findings from various lines 
of evidence from across the health and environmental effects 
disciplines. These separate judgments are integrated into a qualitative 
statement about the overall weight of the evidence and causality. The 
ISA's causal framework has been developed over multiple NAAQS reviews, 
based on extensive interactions with CASAC and based on the public 
input received as part of the CASAC review process. In the current 
review, the causality framework, and the application of that framework 
to causality determinations in the O3 ISA, have been 
reviewed and endorsed by CASAC (Frey and Samet, 2012).
    Given these views on the assessment of the evidence in the ISA, it 
is relevant to note that many of the issues and concerns raised by 
commenters on the EPA's interpretation of the evidence, and on the 
EPA's conclusions regarding the extent to which uncertainties have been 
reduced since the 2008 review, are essentially restatements of issues 
raised during the development of the ISA, HREA, and/or PA. The CASAC 
O3 Panel reviewed the interpretation of the evidence, and 
the EPA's use of information from specific studies, in drafts of these 
documents. In CASAC's advice to the Administrator, which incorporates 
its consideration of many of the issues raised by commenters, CASAC 
approved of the scientific content, assessments, and accuracy of the 
ISA, REA, and PA, and indicated that these documents provide an 
appropriate basis for use in regulatory decision making for the 
O3 NAAQS (Frey and Samet, 2012, Frey, 2014a, Frey, 2014c). 
Therefore, the EPA's responses to many of the comments on the evidence 
rely heavily on the process established in the ISA for assessing the 
evidence, which is the product of extensive interactions with CASAC 
over a number of different reviews, and on CASAC advice received as 
part of this review of the O3 NAAQS.
    The remainder of this section discusses public comments and the 
EPA's responses, on controlled human exposure studies (II.B.2.b.i); 
epidemiologic studies (II.B.2.b.ii); and at-risk populations 
(II.B.2.b.iii).
i. Evidence From Controlled Human Exposure Studies
    This section discusses major comments on the evidence from 
controlled human exposure studies and provides the Agency's responses 
to those comments. To support their views on the adequacy of the 
current standard, commenters often highlighted specific aspects of the 
scientific evidence from controlled human exposure studies. Key themes 
discussed by these commenters included the following: (1) The adversity 
of effects demonstrated in controlled human exposure studies, 
especially studies conducted at exposure concentrations below 80 ppb; 
(2) representativeness of different aspects of the controlled human 
exposure studies for making inferences to the general population and 
at-risk populations; (3) results of additional analyses of the data 
from controlled human exposure studies; (4) evaluation of a threshold 
for effects; and (5) importance of demonstration of inflammation at 60 
ppb. This section discusses these key comment themes, and provides the 
EPA's responses. More detailed discussion of individual comments, and 
the EPA's responses, is provided in the Response to Comments document.
Adversity
    Some commenters who disagreed with the EPA's proposed decision to 
revise the current primary O3 standard disputed the Agency's 
characterization

[[Page 65330]]

of the adversity of the O3-induced health effects shown to 
occur in controlled human exposure studies. Some of these commenters 
contended that the proposal does not provide a clear definition of 
adversity or that there is confusion concerning what responses the 
Administrator considers adverse. The EPA disagrees with these comments, 
and notes that section II.E.4.d of the proposal describes the 
Administrator's proposed approach to considering the adversity of 
effects observed in controlled human exposure studies. Her final 
approach to considering the adversity of these effects, and her 
conclusions on adversity, are described in detail below (II.C.4.b, 
II.C.4.c).
    Other commenters disagreed with the EPA's judgments regarding 
adversity and expressed the view that the effects observed in 
controlled human exposure studies following 6.6-hour exposures to 
O3 concentrations below the level of the current standard 
(i.e., 75 ppb) are not adverse.\88\ This group of commenters cited 
several reasons to support their views, including that: (1) The lung 
function decrements and respiratory symptoms observed at 72 ppb in the 
study by Schelegle et al. (2009) were not correlated with each other, 
and therefore were not adverse; and (2) group mean FEV1 
decrements observed following exposures below 75 ppb are small (e.g., 
<10%, as highlighted by some commenters), transient and reversible, do 
not interfere with daily activities, and do not result in permanent 
respiratory injury or progressive respiratory dysfunction.
---------------------------------------------------------------------------

    \88\ Commenters who supported revising the primary O3 
standard often concluded that there is clear evidence for adverse 
effects following exposures to O3 concentrations at least 
as low as 60 ppb, and that such adverse effects support setting the 
level of a revised primary O3 standard at 60 ppb. These 
comments, and the EPA's responses, are discussed below within the 
context of the Administrator's decision on a revised level 
(II.C.4.b).
---------------------------------------------------------------------------

    While the EPA agrees that not all effects reported in controlled 
human exposure studies following exposures below 75 ppb can reasonably 
be considered to be adverse, the Agency strongly disagrees with 
comments asserting that none of these effects can be adverse. As an 
initial matter, the Administrator notes that, when considering the 
extent to which the current or a revised standard could allow adverse 
respiratory effects, based on information from controlled human 
exposure studies, she considers not only the effects themselves, but 
also quantitative estimates of the extent to which the current or a 
revised standard could allow such effects. Quantitative exposure and 
risk estimates provide perspective on the extent to which various 
standards could allow populations, including at-risk populations such 
as children and children with asthma, to experience the types of 
O3 exposures that have been shown in controlled human 
exposure studies to cause respiratory effects. As discussed further 
below (II.B.3, II.C.4.b, II.C.4.c), to the extent at-risk populations 
are estimated to experience such exposures repeatedly, the 
Administrator becomes increasingly concerned about the potential for 
adverse responses in the exposed population. Repeated exposures provide 
a plausible mode of action by which O3 may cause other more 
serious effects. Thus, even though the Administrator concludes there is 
important uncertainty in the adversity of some of the effects observed 
in controlled human exposure studies based on the single exposure 
periods evaluated in these studies (e.g., FEV1 decrements 
observed following exposures to 60 ppb O3, as discussed in 
sections II.C.4.b and II.C.4.c below), she judges that the potential 
for adverse effects increases as the number of exposures increases. 
Contrary to the commenters' views noted above, the Administrator 
considers the broader body of available information (i.e., including 
quantitative exposure and risk estimates) when considering the extent 
to which the current or a revised standard could allow adverse 
respiratory effects (II.B.3, II.C.4.b, II.C.4.c, below).
    In further considering commenters' views on the potential adversity 
of the respiratory effects themselves (i.e., without considering 
quantitative estimates), the EPA notes that although the results of 
controlled human exposure studies provide a high degree of confidence 
regarding the occurrence of health effects following exposures to 
O3 concentrations from 60 to 80 ppb, there are no 
universally accepted criteria by which to judge the adversity of the 
observed effects. Therefore, as in the proposal, the Administrator 
relies upon recommendations from the ATS and advice from CASAC to 
inform her judgments on adversity.
    In particular, the Administrator focuses on the ATS recommendation 
that ``reversible loss of lung function in combination with the 
presence of symptoms should be considered adverse'' (ATS, 2000a). The 
study by Schelegle et al. (2009) reported a statistically significant 
decrease in group mean FEV1 and a statistically significant 
increase in respiratory symptoms in healthy adults following 6.6-hour 
exposures to average O3 concentrations of 72 ppb. In 
considering these effects, CASAC noted that ``the combination of 
decrements in FEV1 together with the statistically 
significant alterations in symptoms in human subjects exposed to 72 ppb 
ozone meets the American Thoracic Society's definition of an adverse 
health effect'' (Frey, 2014c, p. 5).
    As mentioned above, some commenters nonetheless maintained that the 
effects observed in Schelegle et al. (2009) following exposure to 72 
ppb O3 (average concentration) were not adverse because the 
magnitudes of the FEV1 decrements and the increases in 
respiratory symptoms (as measured by the total subjective symptoms 
score, TSS) were not correlated across individual study subjects. A 
commenter submitted an analysis of the individual-level data from the 
study by Schelegle et al. (2009) to support their position. This 
analysis indicated that, while the majority of study volunteers (66%) 
did experience both lung function decrements and increased respiratory 
symptoms following 6.6-hour exposures to 72 ppb O3, some 
(33%) did not (e.g., Figure 3 in comments from Gradient).\89\ In 
addition, the study subjects who experienced relatively large lung 
function decrements did not always also experience relatively large 
increases in respiratory symptoms. These commenters interpreted the 
lack of a statistically significant correlation between the magnitudes 
of decrements and symptoms as meaning that the effects reported by 
Schelegle et al. (2009) at 72 ppb did not meet the ATS criteria for an 
adverse response.
---------------------------------------------------------------------------

    \89\ The figure provided in comments by Gradient only clearly 
illustrated the responses of 30 out of 31 subjects.
---------------------------------------------------------------------------

    However, the ATS recommendation that the combination of lung 
function decrements and symptomatic responses be considered adverse is 
not restricted to effects of a particular magnitude nor a requirement 
that individual responses be correlated. Similarly, CASAC made no such 
qualifications in its advice on the combination of respiratory symptoms 
and lung function decrements (See e.g., Frey, 2014c, p. 5). Therefore, 
as in the proposal and consistent with both CASAC advice and ATS 
recommendations, the EPA continues to conclude that the finding of both 
statistically significant decrements in lung function and significant 
increases in respiratory symptoms following 6.6-hour exposures to an 
average O3 concentration of 72 ppb provides a strong 
indication of the

[[Page 65331]]

potential for exposed individuals to experience this combination of 
effects.\90\
---------------------------------------------------------------------------

    \90\ Indeed, the finding of statistically significant decreases 
in lung function and increases in respiratory symptoms in the same 
study population indicates that, on average, study volunteers did 
experience both effects.
---------------------------------------------------------------------------

    In particular, the Administrator notes that lung function provides 
an objective measure of the respiratory response to O3 
exposure while respiratory symptoms are subjective, and as evaluated by 
Schelegle et al. (2009) were based on a TSS score. If an O3 
exposure causes increases in both objectively measured lung function 
decrements and subjective respiratory symptoms, which indicate that 
people may modify their behavior in response to the exposure, then the 
effect is properly viewed as adverse. As noted above, the commenter's 
analysis shows that the majority of study volunteers exposed to 72 ppb 
O3 in the study by Schelegle et al. (2009) did, in fact, 
experience both a decrease in lung function and an increase in 
respiratory symptoms.
    In further considering this comment, the EPA recognizes that, 
consistent with commenter's analysis, some individuals may experience 
large decrements in lung function with minimal to no respiratory 
symptoms (McDonnell et al., 1999), and vice versa. As indicated above 
and discussed in the proposal (79 FR 75289), the Administrator 
acknowledges such interindividual variability in responsiveness in her 
interpretation of estimated exposures of concern. Specifically, she 
notes that not everyone who experiences an exposure of concern, 
including for the 70 ppb benchmark, is expected to experience an 
adverse response. However, she further judges that the likelihood of 
adverse effects increases as the number of occurrences of O3 
exposures of concern increases. In making this judgment, she notes that 
the types of respiratory effects that can occur following exposures of 
concern, particularly if experienced repeatedly, provide a plausible 
mode of action by which O3 may cause other more serious 
effects.\91\ Therefore, her decisions on the primary standard emphasize 
the public health importance of limiting the occurrence of repeated 
exposures to O3 concentrations at or above those shown to 
cause adverse effects in controlled human exposure studies (II.B.3, 
II.C.4.b, II.C.4.c). The Administrator views this approach to 
considering the evidence from controlled human exposure studies as 
being consistent with commenter's analysis indicating that, while the 
majority did, not all study volunteers exposed to 72 ppb O3 
experienced the adverse combination of lung function decrements and 
respiratory symptoms following the single exposure period evaluated by 
Schelegle et al. (2009).
---------------------------------------------------------------------------

    \91\ For example, as discussed in the proposal (79 FR 75252) and 
the ISA (p. 6-76), inflammation induced by a single exposure (or 
several exposures over the course of a summer) can resolve entirely. 
However, repeated occurrences of airway inflammation could 
potentially result in the induction of a chronic inflammatory state; 
altered pulmonary structure and function, leading to diseases such 
as asthma; altered lung host defense response to inhaled 
microorganisms; and altered lung response to other agents such as 
allergens or toxins (ISA, section 6.2.3).
---------------------------------------------------------------------------

Representativeness
    A number of commenters raised issues concerning the 
representativeness of controlled human exposure studies considered by 
the Administrator in this review, based on different aspects of these 
studies. These commenters asserted that since the controlled human 
exposure studies were not representative of real-world exposures, they 
should not be relied upon as a basis for finding that the current 
standard is not adequate to protect public health. Some issues 
highlighted by commenters include: Small size of the study populations; 
unrealistic activity levels used in the studies; unrealistic exposure 
scenarios (i.e., triangular exposure protocol) used in some studies, 
including Schelegle et al. (2009); and differences in study design that 
limit comparability across studies.
    Some commenters noted that the controlled human exposure studies 
were not designed to have individuals represent portions of any larger 
group and that the impacts on a small number of people do not implicate 
the health of an entire subpopulation, particularly when the 
FEV1 decrements are small, temporary, and reversible. These 
commenters also noted that the Administrator failed to provide an 
explanation or justification for why the individuals in these studies 
can be viewed as representatives of a subpopulation. Further, they 
asserted that EPA's use of results from individuals, rather than the 
group mean responses, contradicts the intent of CAA section 109 to 
protect groups of people, not just the most sensitive individuals in 
any group (79 FR 75237).
    Consistent with CASAC advice (Frey, 2014c, p. 5), the EPA concludes 
that the body of controlled human exposure studies are sufficiently 
representative to be relied upon as a basis for finding that the 
current standard is not adequate to protect public health. These 
studies generally recruit healthy young adult volunteers, and often 
expose them to O3 concentrations found in the ambient air 
under real-world exposure conditions. As described in more detail above 
in section II.A.1.b, the evidence from controlled human exposure 
studies to date makes it clear that there is considerable variability 
in responses across individuals, even in young healthy adult 
volunteers, and that group mean responses are not representative of 
more responsive individuals. It is important to look beyond group mean 
responses to the responses of these individuals to evaluate the 
potential impact on more responsive members of the population. 
Moreover, relying on group mean changes to evaluate lung function 
responses to O3 exposures would mask the responses of the 
most sensitive groups, particularly where, as here, the group mean 
reflects responses solely among the healthy young adults who were the 
study participants. Thus, the studies of exposures below 80 ppb 
O3 show that 10% of young healthy adults experienced 
FEV1 decrements >10% following exposures to 60 ppb 
O3, and 19% experienced such decrements following exposures 
to 72 ppb (under the controlled test conditions involving moderate 
exertion for 6.6 hours). These percentages would likely have been 
higher had people with asthma or other at-risk populations been exposed 
(U.S. EPA, 2013, pp. 6-17 and 6-18; Frey 2014c, p. 7; Frey, 2014a, p. 
14).\92\
---------------------------------------------------------------------------

    \92\ See also National Environmental Development Associations 
Clean Action Project v. EPA, 686 F. 3d 803, 811 (D.C. Cir. 2012) 
(EPA drew legitimate inference that serious asthmatics would 
experience more serious health effects than clinical test subjects 
who did not have this degree of lung function impairment).
---------------------------------------------------------------------------

    Moreover, the EPA may legitimately view the individuals in these 
studies as representatives of the larger subpopulation of at-risk or 
sensitive groups. As stated in the Senate Report to the 1970 
legislation establishing the NAAQS statutory provisions, ``the 
Committee emphasizes that included among these persons whose health 
should be protected by the ambient standard are particularly sensitive 
citizens such as bronchial asthmatics and emphysematics who in the 
normal course of daily activity are exposed to the ambient environment. 
In establishing an ambient standard necessary to protect the health of 
these persons, reference should be made to a representative sample of 
persons comprising the sensitive group rather than to a single person 
in such a group. . . . For purposes of this description, a 
statistically related sample is the number of persons necessary to test 
in order to detect a deviation in the health of any person within such 
sensitive group which is attributable to the condition of the ambient 
air.'' S. Rep. No. 11-1196, 91st

[[Page 65332]]

Cong. 2d sess. at 10. As just noted above, 10% of healthy young adults 
in these studies experienced >10% FEV1 decrements following 
exposure to 60 ppb O3, and the proportion of individuals 
experiencing such decrements increases with increasing O3 
exposure concentrations. This substantial percentage certainly can be 
viewed as ``a representative sample of persons'' and as a sufficient 
number to ``detect a deviation in the health of any person within such 
sensitive group,'' especially given that it reflects the percentage of 
healthy adults who experienced decrements >10%.
    These results are consistent with estimates from the MSS model, 
which makes reliable quantitative predictions of the lung function 
response to O3 exposures, and reasonably predicts the 
magnitude of individual lung function responses following such 
exposures. As described in section II.A.2.c above, and documented in 
the HREA, when the MSS model was used to quantify the risk of 
O3-induced FEV1 decrements in 15 urban study 
areas, the current standard was estimated to allow about 8 to 12% of 
children to experience two or more O3-induced 
FEV1 decrements >=10%, and about 2 to 3% to experience two 
or more decrements >=15% (Table 2, above). These percentages correspond 
to hundreds of thousands of children in urban study areas, and tens of 
thousands of asthmatic children. While the Administrator judges that 
there is uncertainty with regard to the adversity of these 
O3-induced lung function decrements (see II.C.4.b, II.C.4.c, 
below), such risk estimates clearly indicate that they are a matter of 
public health importance on a broad scale, not isolated effects on 
idiosyncratically responding individuals.
    Other commenters considered the ventilation rates used in 
controlled human exposure studies to be unreasonably high and at the 
extreme of prolonged daily activity. Some of these commenters noted 
that these scenarios are unrealistic for sensitive populations, such as 
asthmatics and people with COPD, whose conditions would likely prevent 
them from performing the intensity of exercise, and therefore 
experiencing the ventilation rates, required to produce decrements in 
lung function observed in experimental settings.
    The EPA disagrees with these commenters. The activity levels used 
in controlled human exposure studies were summarized in Table 6-1 of 
the ISA (U.S. EPA, 2013). The exercise level in the 6.6-hour exposure 
studies by Adams (2006), Schelegle et al. (2009), and Kim et al. (2011) 
of young healthy adults was moderate and ventilation rates are 
typically targeted for 20 L/min-m\2\ BSA.\93\ Following the exposures 
to 60 ppb at this activity level, 10% of the individuals had greater 
than a 10% decrement in FEV1 (U.S. EPA, 2013, p. 6-18). 
Similar 6.6-hour exposure studies of individuals with asthma are not 
available to assess either the effects of O3 on their lung 
function or their ability to perform the required level of moderate 
exercise.
---------------------------------------------------------------------------

    \93\ Exercise consisted of alternating periods walking on a 
treadmill at a pace of 17-18 minutes per mile inclined to a grade of 
4-5% or cycling at a load of about 72 watts. Typical heart rates 
during the exercise periods were between 115-130 beats per minute. 
This activity level is considered moderate (Table 6-1, U.S. EPA, 
2013, p. 6-18).
---------------------------------------------------------------------------

    However, referring to Tables 6-9 and 6-10 of the HREA (U.S. EPA, 
2014a), between 42% and 45% of FEV1 decrements >= 10% were 
estimated to occur at exercise levels of <13 L/min-m\2\ BSA. This 
corresponds to light exercise, and this level of exercise has been used 
in a 7.6-hour study of healthy people and people with asthma exposed to 
160 ppb O3 (Horstman et al., 1995). In that study, people 
with asthma exercised with an average minute ventilation of 14.2 L/min-
m\2\ BSA. Adjusted for filtered air responses, an average 19% 
FEV1 decrement was seen in the people with asthma versus an 
average 10% FEV1 decrement in the healthy people. In 
addition, the EPA noted in the HREA that the data underlying the 
exposure assessment indicate that ``activity data for asthmatics [is] 
generally similar to [that for] non-asthmatics'' (U.S. EPA, 2014a, p. 
5-75, Tables 5G-2 and 5G-3). Thus, contrary to the commenters' 
assertion, based on both the HREA and the Horstman et al. (1995) study, 
people with respiratory disease such as asthma can exercise for a 
prolonged period under conditions where they would experience >10% 
FEV1 decrements in response to O3 exposure.
    Additionally, a number of commenters asserted that the exposure 
scenarios in Schelegle et al. (2009), which are based on a so-called 
triangular study protocol, where O3 concentrations ramp up 
and down as the study is conducted, are not directly generalizable to 
most healthy or sensitive populations because of large changes in the 
O3 concentrations from one hour to the next. Commenters 
stated that although large fluctuations in O3 are possible 
in certain locations due to meteorological conditions (e.g., in valleys 
on very hot, summer days), they believe that, in general, 
concentrations of O3 do not fluctuate by more than 20-30 ppb 
from one hour to the next. Thus, commenters suggested the Schelegle et 
al. (2009) study design could happen in a ``worst-case'' exposure 
scenario, but that the exposure protocol was not reflective of 
conditions in most cities and thus not informative with regard to the 
adequacy of the current standard.
    The EPA disagrees with the comment that these triangular exposure 
scenarios are not generalizable because of hour-to-hour fluctuations. 
Adams (2002, 2006) showed that FEV1 responses following 6.6 
hours of exposure to 60 and 80 ppb average O3 exposures do 
not differ between triangular (i.e. ramping concentration up and down) 
and square-wave (i.e. constant concentration). Schelegle et al. (2009) 
used the 80 ppb triangular protocol and a slightly modified 60 ppb 
triangular protocol (concentrations during the third and fourth hours 
were reversed) from Adams (2006). Therefore, in considering pre- to 
post-exposure changes in lung function, concerns about the hour-by-hour 
changes in O3 concentrations at 60 and 80 ppb in the 
Schelegle et al. (2009) study are unfounded.
    Finally, some commenters also stated that the Kim et al. (2011) 
study is missing critical information and its study design makes 
comparison to the other studies difficult. That is, the commenter 
suggests that data at times other than pre- and post-exposure should 
have been provided.
    The EPA disagrees with this comment. With regard to providing data 
at other time points besides pre- and post-exposure, there is no 
standard that suggests an appropriate frequency at which lung function 
should be measured in prolonged 6.6-hour exposure studies. The Adams 
(2006) study showed that lung function decrements during O3 
exposures with moderate exercise become most apparent following the 
third hour of exposure. As such, it makes little sense to measure lung 
function during the first couple hours of exposure. However, having 
data at multiple time points toward the end of an exposure can provide 
evidence that the mean post-exposure FEV1 response is not a 
single anomalous data point. The FEV1 response data for the 
3-, 4.6-, 5.6-, and 6.6-hour time points of the Kim et al. (2011) study 
are available in Figure 6 of the McDonnell et al. (2012) paper where 
they are plotted with the Adams (2006) data for 60 ppb. Similar to the 
Adams (2006) study, the responses at 5.6 hours are only marginally 
smaller than the response at 6.6 hours in the Kim et al. (2011) study. 
This indicates that the post-exposure FEV1 responses in both 
studies are consistent with responses at an earlier time point and thus 
not likely to be anomalous data.

[[Page 65333]]

Additional Studies
    Several commenters analyzed the data from controlled human exposure 
studies, or they commented on the EPA's analysis of the data from some 
of these studies (Brown et al., 2008), to come to a different 
conclusion than the EPA's interpretation of these studies thereby 
questioning the proposed decision that the current standard is not 
adequate to protect public health. One commenter submitted an 
independent assessment of the scientific evidence and risk, and used 
this analysis to assert that there are multiple flaws in the underlying 
studies and their interpretation by the EPA. This commenter stated that 
the EPA's discussion of the spirometric responses of children and 
adolescents and older adults to O3 was misleading. They 
claimed that the EPA did not mention that ``the responses of children 
and adolescents are equivalent to those of young adults (18-35 years 
old; McDonnell et al., 1985) and that this response diminishes in 
middle-aged and older adults (Hazucha 1985).'' The EPA notes that the 
commenter misrepresented our characterization of the effect of age on 
FEV1 responses to O3 and asserted mistakenly that 
EPA did not mention diminished responses on older adults. In fact, the 
proposal clearly states that, ``Respiratory symptom responses to 
O3 exposure appears to increase with age until early 
adulthood and then gradually decrease with increasing age (U.S. EPA, 
1996b); lung function responses to O3 exposure also decline 
from early adulthood (U.S. EPA, 1996b)'' (79 FR 75267) (see also U.S. 
EPA, 2014c p. 3-82). With regard to differences between children and 
adults, it was clearly stated in the ISA (U.S. EPA, 2013, p. 6-21) that 
healthy children exposed to filtered air and 120 ppb O3 
experienced similar spirometric responses, but lesser symptoms than 
similarly exposed young healthy adults (McDonnell et al., 1985). In 
addition, the EPA's approach to modeling the effect of age on responses 
to O3 is clearly provided in the HREA (U.S. EPA, 2014a, 
Table 6-2).
    The commenter also stated that the EPA's treatment of filtered air 
responses in the dose-response curve was incorrect. They claimed that 
when creating a dose-response curve, it is most appropriate to include 
a zero-dose point and not to subtract the filtered air response from 
responses to O3. Contrary to this assertion, EPA correctly 
adjusted FEV1 responses to O3 by responses 
following filtered air, as was also done in the McDonnell et al. (2012) 
model. As indicated in the ISA (U.S. EPA, 2013, p. 6-4), the majority 
of controlled human exposure studies investigating the effects 
O3 are of a randomized, controlled, crossover design in 
which subjects were exposed, without knowledge of the exposure 
condition and in random order, to clean filtered air and, depending on 
the study, to one or more O3 concentrations. The filtered 
air control exposure provides an unbiased estimate of the effects of 
the experimental procedures on the outcome(s) of interest. Comparison 
of responses following this filtered air exposure to those following an 
O3 exposure allows for estimation of the effects of 
O3 itself on an outcome measurement while controlling for 
independent effects of the experimental procedures, such as ventilation 
rate. Thus, the commenter's approach does not provide an estimate of 
the effects of O3 alone. Furthermore, as illustrated in 
these comments, following ``long'' filtered air exposures, there is 
about a 1% improvement in FEV1. By not accounting for this 
increase in FEV1, the commenter underestimated the 
FEV1 decrement due to O3 exposure. The 
commenter's approach thus is fundamentally flawed.
    The commenter also asserted that the McDonnell et al. (2012) model 
and exposure-response (E-R) models incorrectly used only the most 
responsive people and that EPA's reliance on data from clinical trials 
that use only the most responsive people irrationally ignores large 
portions of relevant data. The EPA rejects this assertion that the 
McDonnell et al. (2012) model and the E-R analysis ignored large 
portions of relevant data. The McDonnell et al. (2012) model was fit to 
the FEV1 responses of 741 individuals to O3 and 
filtered air (i.e., reflecting all available data for O3-
induced changes in FEV1). The filtered air responses were 
subtracted from responses measured during O3 exposures. 
Subsequently, as illustrated by the figures in the McDonnell et al. 
(2012) paper and described in the text of paper, the model was fit to 
all available FEV1 data measured during the course of 
O3 exposures, including exposures shorter than 6.6 hours. 
Thus, the model predicts temporal dynamics of FEV1 response 
to any set of O3 exposure conditions that might reasonably 
be experienced in the ambient environment, predicting the mean 
responses and the distribution of responses around the mean. For the 
HREA (EPA, 2014a), the proportion of individuals, under variable 
exposure conditions, predicted to have FEV1 decrements >=10, 
15 and 20% was estimated.
    Finally, the commenter referenced the exposure-response model on p. 
6-18 of the HREA. However, they neglected to note that this was in a 
section describing the exposure-response function approach used in 
prior reviews (U.S. EPA, 2014a, starting on p. 6-17). Thus, the 
commenter confused the exposure-response model used in the last review 
with the updated approach used in this review.
    The commenter also stated that EPA did not properly consider 
O3 dose when interpreting the human clinical data. Ozone 
total dose includes three factors: duration of exposure, concentration, 
and ventilation rate. The commenter claimed the EPA emphasized only 
concentration without properly considering and communicating duration 
of exposure and ventilation rate. Further, they asserted that because 
people are not exposed to the same dose, they cannot be judged to have 
the same exposure and would therefore not be expected to respond 
consistently. The EPA rejects the claim that we emphasized only 
concentration without properly incorporating the other two factors. As 
noted in the ISA, total O3 dose does not describe the 
temporal dynamics of FEV1 responses as a function of 
concentration, ventilation rate, time and age of the exposed 
individuals (U.S. EPA, 2013, p. 6-5). Thus, the use of total 
O3 dose is antiquated and the EPA therefore conducted a more 
sophisticated analysis of FEV1 response to O3 in 
the HREA. In this review, the HREA estimates risks of lung function 
decrements in school-aged children (ages 5 to 18), asthmatic school-
aged children, and the general adult population for 15 urban study 
areas. A probabilistic model designed to account for the numerous 
sources of variability that affect people's exposures was used to 
simulate the movement of individuals through time and space and to 
estimate their exposure to O3 while occupying indoor, 
outdoor, and in-vehicle locations. That information was linked with the 
McDonnell et al. (2012) model to estimate FEV1 responses 
over time as O3 exposure concentrations and ventilation 
rates changed. As noted earlier, CASAC agreed that this approach is 
both scientifically valid and a significant improvement over approaches 
used in past O3 reviews (Frey, 2014a, p. 2).
    Several commenters criticized the EPA analysis published by Brown 
et al. (2008). One commenter suggested that the EPA needed to state why 
the Brown et al. (2008) analysis was relied on rather than Nicolich 
(2007) or Lefohn et

[[Page 65334]]

al. (2010). Further, commenters stated that the analysis of the Adams 
(2006) data in Brown et al. (2008) was flawed. Among other reasons, one 
commenter expressed the opinion that it was not appropriate for Brown 
et al. (2008) to only examine a portion of the Adams (2006) data, 
citing comments submitted by Gradient.
    The EPA disagrees with these commenters.\94\ As an initial matter, 
Nicolich (2007) was a public comment and is not a peer-reviewed 
publication that would be used to assess the scientific evidence for 
effects of O3 on lung function in the ISA (U.S. EPA, 2013). 
The Nicolich (2007) comments were specifically addressed by the EPA on 
pp. 24-25 in the Response to Comments Document for the 2007 proposed 
rule (U.S. EPA, 2008). On page A-3 of his comments, Dr. Nicolich stated 
``that the residuals are not normally distributed and the observations 
do not meet the assumptions required for the model'' and that ``the 
subject-based errors are not independently, identically and normally 
distributed and the subjects do not meet the assumptions required for 
the model.'' The EPA reasonably chose not to rely on this analysis: 
``Therefore, given that the underlying statistical assumptions required 
for his analyses were not met and that significance levels are 
questionable, in EPA's judgment the analyses presented by Dr. Nicolich 
are ambiguous'' (U.S. EPA, 2008). It is likely that the Lefohn et al. 
(2010) analysis of the Adams (2006) data would similarly not meet the 
statistical assumptions of the model (e.g., homoscedasticity). In 
contrast, recognizing the concerns related to the distribution of 
responses, Brown et al. (2008) conservatively used a nonparametric sign 
test to obtain a p-value of 0.002 for the comparison responses 
following 60 ppb O3 versus filter air. Other common 
statistical tests also showed significant effects on lung function. In 
addition, the effects of 60 ppb O3 on FEV1 
responses in Brown et al. (2008) remained statistically significant 
even following the exclusion of three potential outliers.
---------------------------------------------------------------------------

    \94\ The DC Circuit has held that EPA reasonably used and 
interpreted the Brown (2007) study in the last review. Mississippi, 
744 F. 3d at 1347. In this review, there is now additional 
corroborative evidence supporting the Brown (2007) analysis, in the 
form of further controlled human clinical studies finding health 
effects in young, healthy adults at moderate exercise at 
O3 concentrations of 60 ppb over a 6.6 hour exposure 
period.
---------------------------------------------------------------------------

    EPA disagrees with the comment stating that it was not appropriate 
for Brown et al. (2008) to only examine a portion of the Adams (2006) 
data. In fact, there is no established single manner or protocol 
decreeing that data throughout the protocol must be analyzed and 
included. Furthermore, Brown et al. (2008) was a peer-reviewed journal 
publication. CASAC also expressed favorable comments in their March 30, 
2011, letter to Administrator Jackson. With reference to a memorandum 
(Brown, 2007) that preceded the Brown et al. (2008) publication, on p. 
6 of the CASAC Consensus Responses to Charge Questions CASAC stated, 
``The results of the Adams et al. study also have been carefully 
reanalyzed by EPA investigators (Brown et. al., [2008]), and this 
reanalysis showed a statistically significant group effect on 
FEV1 after 60 ppb ozone exposure.'' On p. A-13, a CASAC 
panelist and biostatistician stated, ``Thus, from my understanding of 
the statistical analyses that have been conducted, I would argue that 
the analysis by EPA should be preferred to that of Adams for the 
specific comparison of the FEV1 effects of 0.06 ppm exposure 
relative to filtered air exposure.'' (Samet 2011, p. a-13)
Threshold
    Several commenters used the new McDonnell et al. (2012) and 
Schelegle et al. (2012) models to support their views about the 
O3 concentrations associated with a threshold for adverse 
lung function decrements. For example, one commenter who supported 
retaining the current standard noted that McDonnell et al. (2012) found 
that the threshold model fit the observed data better than the original 
(no-threshold) model, especially at earlier time points and at the 
lowest exposure concentrations. The commenter expressed the view that 
the threshold model showed that the population mean FEV1 
decrement did not reach 10% until exposures were at least 80 ppb, 
indicating that O3 exposures of 80 ppb or higher may cause 
lung function decrements and other respiratory effects.\95\
---------------------------------------------------------------------------

    \95\ Conversely, another group of commenters who supported 
revising the standard to a level of 60 ppb noted that the results of 
these models are consistent with the results of controlled human 
exposure studies finding adverse health effects at 60 ppb. These 
comments are discussed below (II.C.4.b), within the context of the 
Administrator's decision on a revised standard level.
---------------------------------------------------------------------------

    As described above in section II.A.1.b, the McDonnell et al. (2012) 
and Schelegle et al. (2012) models represent a significant 
technological advance in the exposure-response modeling approach since 
the last review, and these models indicate that a dose-threshold model 
fits the data better than a non-threshold model. However, the EPA 
disagrees that using the predicted group mean response from the 
McDonnell model provides support for retaining the current standard. As 
discussed above, the group mean responses do not convey information 
about interindividual variability, or the proportion of the population 
estimated to experience the larger lung function decrements (e.g., 10 
or 15% FEV1 decrements) that could be adverse. In fact, it 
masks this variability. These variable effects in individuals have been 
found to be reproducible. In other words, a person who has a large lung 
function response after exposure to O3 will likely have 
about the same response if exposed again in a similar manner (raising 
health concerns, as noted above). Group mean responses are not 
representative of this segment of the population that has much larger 
than average responses to O3.
Inflammation
    Some commenters asserted that the pulmonary inflammation observed 
following exposure to 60 ppb in the controlled human exposure study by 
Kim et al. (2011) was small and unlikely to result in airway damage. It 
was also suggested that this inflammation is a normal physiological 
response in all living organisms to stimuli to which people are 
normally exposed.
    The EPA recognized in the proposal (79 FR 75252) and the ISA (U.S. 
EPA, 2013, p. 6-76) that inflammation induced by a single exposure (or 
several exposures over the course of a summer) can resolve entirely. 
Thus, the inflammatory response observed following the single exposure 
to 60 ppb in the study by Kim et al. (2011) is not necessarily a 
concern. However, the EPA notes that it is also important to consider 
the potential for continued acute inflammatory responses to evolve into 
a chronic inflammatory state and to affect the structure and function 
of the lung.\96\ The Administrator considers this possibility through 
her consideration of estimated exposures of concern for the 60 ppb 
benchmark (II.B.3, II.C.4). As discussed in detail below (II.C.4.b), 
while she judges that there is uncertainty in the adversity of the 
effects shown to occur following exposures to 60 ppb O3, 
including the inflammation reported by Kim et al.

[[Page 65335]]

(2011), she gives some consideration to estimates of two or more 
exposures of concern for the 60 ppb benchmark (i.e., as a health-
protective surrogate for repeated exposures of concern at or above 60 
ppb), particularly when considering the extent to which the current and 
revised standards incorporate a margin of safety.
---------------------------------------------------------------------------

    \96\ Inflammation induced by exposure of humans to O3 
can have several potential outcomes, ranging from resolving entirely 
following a single exposure to becoming a chronic inflammatory state 
(U.S. EPA, 2013, section 6.2.3). Lung injury and the resulting 
inflammation provide a mechanism by which O3 may cause 
other more serious morbidity effects (e.g., asthma exacerbations) 
(U.S. EPA, 2013, section 6.2.3). See generally section II.A.1.a 
above.
---------------------------------------------------------------------------

ii. Evidence Fom epidemiologic studies
    This section discusses key comments on the EPA's assessment of the 
epidemiologic evidence and provides the Agency's responses to those 
comments. The focus in this section is on overarching comments related 
to the EPA's approach to assessing and interpreting the epidemiologic 
evidence as a whole. Detailed comments on specific studies, or specific 
methodological or technical issues, are addressed in the Response to 
Comments document. As discussed above, many of the issues and concerns 
raised by commenters on the interpretation of the epidemiologic 
evidence are essentially restatements of issues raised during the 
development of the ISA, HREA, and/or PA, and in many instances were 
considered by CASAC in the development of its advice on the current 
standard. The EPA's responses to these comments rely heavily on the 
process established in the ISA for assessing the evidence, and on CASAC 
advice received as part of this review of the O3 NAAQS.
    As with evidence from controlled human exposure studies, commenters 
expressed sharply divergent views on the evidence from epidemiologic 
studies, and on the EPA's interpretation of that evidence. One group of 
commenters, representing medical, public health and environmental 
organizations, and some states, generally supported the EPA's 
interpretation of the epidemiologic evidence with regard to the 
consistency of associations, the coherence with other lines of 
evidence, and the support provided by epidemiologic studies for the 
causality determinations in the ISA. These commenters asserted that the 
epidemiologic studies evaluated in the ISA provide valuable information 
supporting the need to revise the level of the current primary 
O3 standard in order to increase public health protection. 
In reaching this conclusion, commenters often cited studies (including 
a number from the past review) which they interpreted as showing health 
effect associations in locations with O3 air quality 
concentrations below the level of the current standard. A second group 
of commenters, mostly representing industry associations, businesses, 
and states opposed to revising the primary O3 standard, 
expressed the general view that while many new epidemiologic studies 
have been published since the last review of the O3 NAAQS, 
inconsistencies and uncertainties inherent in these studies as a whole, 
and in the EPA's assessment of study results, should preclude any 
reliance on them as justification for a more stringent primary 
O3 standard. To support their views, these commenters often 
focused on specific technical or methodological issues that contribute 
to uncertainty in epidemiologic studies, including the potential for 
exposure error, confounding by copollutants and by other factors (e.g., 
weather, season, disease, day of week, etc.), and heterogeneity in 
results across locations.
    The EPA agrees with certain aspects of each of these views. 
Specifically, while the EPA agrees that epidemiologic studies are an 
important part of the broader body of evidence that supports the ISA's 
causality determinations, and that these studies provide support for 
the decision to revise the current primary O3 standard, the 
Agency also acknowledges that there are important uncertainties and 
limitations associated with these epidemiologic studies that should be 
considered when reaching decisions on the current standard. Thus, 
although these studies show consistent associations between 
O3 exposures and serious health effects, including morbidity 
and mortality, and some of these studies reported such associations 
with ambient O3 concentrations below the level of the 
current standard, there are also uncertainties regarding the ambient 
O3 concentrations in critical studies, such that they lend 
only limited support to establishing a specific level for a revised 
standard. (See generally, Mississippi, 744 F. 3d at 1351 (noting that 
in prior review, EPA reasonably relied on epidemiologic information in 
determining to revise the standard but appropriately gave the 
information limited weight in determining a level of a revised 
standard); see also ATA III, 283 F. 3d at 370 (EPA justified in 
revising NAAQS when health effect associations are observed in 
epidemiologic studies at levels allowed by the current NAAQS); 
Mississippi, 744 F. 3d at 1345 (same)).
    Uncertainties in the evidence were considered by the Administrator 
in the proposal, and contributed to her decision to place less weight 
on information from epidemiologic studies than on information from 
controlled human exposure studies when considering the adequacy of the 
current primary O3 standard (see 79 FR 75281-83). Despite 
receiving less weight in the proposal, the EPA does not agree with 
commenters who asserted that uncertainties in the epidemiologic 
evidence provide a basis for concluding that the current primary 
standard does not need revision. The Administrator specifically 
considered the extent to which available studies support the occurrence 
of O3 health effect associations with air quality likely to 
be allowed by the current standard, while also considering the 
implications of important uncertainties, as assessed in the ISA and 
discussed in the PA. This consideration is consistent with CASAC 
comments on consideration of these studies in the draft PA (Frey, 
2014c, p. 5).
    Based on analyses of study area air quality in the PA, the EPA 
notes that most of the U.S. and Canadian epidemiologic studies 
evaluated were conducted in locations likely to have violated the 
current standard over at least part of the study period. Although these 
studies support the ISA's causality determinations, they provide 
limited insight into the adequacy of the public health protection 
provided by the current primary O3 standard. However, as 
discussed in the proposal, air quality analyses in the locations of 
three U.S. single-city studies provide support for the occurrence of 
O3-associated hospital admissions or emergency department 
visits at ambient O3 concentrations below the level of the 
current standard.\97\ Specifically, a U.S. single-city study reported 
associations with respiratory emergency department visits in children 
and adults in a location that would have met the current O3 
standard over the entire study period (Mar and Koenig, 2009). In 
addition, for two studies conducted in locations where the current 
standard was likely not met (i.e., Silverman and Ito, 2010; Strickland 
et al., 2010), PA analyses indicate that reported concentration-
response functions and available air quality data support the 
occurrence of O3-health effect associations on subsets of 
days with virtually all monitored ambient O3 concentrations 
below the level of the current standard (U.S. EPA, 2014c,

[[Page 65336]]

section 3.1.4.2, pp. 3-66 to 67).\98\ Thus, the EPA notes that a small 
number of O3 epidemiologic studies provide support for the 
conclusion that the current primary standard is not requisite, and that 
it should be revised to increase public health protection.
---------------------------------------------------------------------------

    \97\ As discussed in section II.E.4.d of the proposal, is the 
Administrator noted the greater uncertainty in using analyses of 
short-term O3 air quality in locations of the multicity 
studies in this review to inform decisions on the primary 
O3 standard. This is because the health information in 
these studies cannot be disaggregated by individual city. Thus, the 
multicity effect estimates reported in these studies do not provide 
clear indication of the extent to which health effects are 
associated with the ambient O3 concentrations in the 
study locations that met the current O3 standard, versus 
the ambient O3 concentrations in the study locations that 
violated the standard.
    \98\ Air quality analyses in locations of the studies by 
Silverman and Ito (2010) and Strickland et al. (2010) were used in 
the PA to inform staff conclusions on the adequacy of the current 
primary O3 standard. However, the appropriate 
interpretation of these analyses became less clear for standard 
levels below 75 ppb, as the number of days increased with monitored 
concentrations exceeding the level being evaluated (U.S. EPA, 2014c, 
Appendix 3B, Tables 3B-6 and 3B-7). Therefore, these analyses were 
not used in the PA to inform conclusions on potential alternative 
standard levels lower than 75 ppb (U.S. EPA, 2014c, Chapters 3 and 
4).
---------------------------------------------------------------------------

    As part of a larger set of comments criticizing the EPA's 
interpretation of the evidence from time series epidemiologic studies, 
some commenters objected to the EPA's reliance on the studies by 
Strickland et al. (2010), Silverman and Ito (2010), and Mar and Koenig 
(2009). These commenters highlighted what they considered to be key 
uncertainties in interpreting these studies, including uncertainties 
due to the potential for confounding by co-pollutants, aeroallergens, 
or the presence of upper respiratory infections; and uncertainties in 
the interpretation of zero-day lag models (i.e., specifically for Mar 
and Koenig, 2009).
    While the EPA agrees that there are uncertainties associated with 
interpreting the O3 epidemiologic evidence, as discussed 
above and elsewhere in this preamble, we disagree with commenters' 
assertion that these uncertainties should preclude the use of the 
O3 epidemiologic evidence in general, or the studies by 
Silverman and Ito, Strickland, or Mar and Koenig in particular, as part 
of the basis for the Administrator's decision to revise the current 
primary standard. As a general point, when considering the potential 
importance of uncertainties in epidemiologic studies, we rely on the 
broader body of evidence, not restricted to these three studies, and 
the ISA conclusions based on this evidence. The evidence, the ISA's 
interpretation of specific studies, and the use of information from 
these studies in the HREA and PA, was considered by CASAC in its review 
of drafts of the ISA, HREA, and PA. Based on the assessment of the 
evidence in the ISA, and CASAC's endorsement of the ISA conclusions, as 
well as CASAC's endorsement of the approaches to using and considering 
information from epidemiologic studies in the HREA and PA (Frey, 2014c, 
p. 5), we do not agree with these commenters' conclusions regarding the 
usefulness of the epidemiologic studies by Strickland et al. (2010), 
Silverman and Ito (2010), and Mar and Koenig (2009).
    More specifically, with regard to confounding by co-pollutants, we 
note the ISA conclusion that, in studies of O3-associated 
hospital admissions and emergency department visits ``O3 
effect estimates remained relatively robust upon the inclusion of PM . 
. . and gaseous pollutants in two-pollutant models'' (U.S. EPA, 2013, 
pp. 6-152 and 6-153). This conclusion was supported by several studies 
that evaluated co-pollutant models including, but not limited to, two 
of the studies specifically highlighted by commenters (i.e., Silverman 
and Ito, 2010; Strickland et al., 2010) (U.S. EPA, 2013, section 
6.2.7.5; Figure 6-20 and Table 6-29).
    Other potential uncertainties highlighted by commenters have been 
evaluated less frequently (e.g., confounding by allergen exposure, 
respiratory infections). However, we note that Strickland et al. (2010) 
did consider the potential for pollen (a common airborne allergen) to 
confound the association between ambient O3 and emergency 
department visits. While quantitative results were not presented, the 
authors reported that ``estimates for associations between ambient air 
pollutant concentrations and pediatric asthma emergency department 
visits were similar regardless of whether pollen concentrations were 
included in the model as covariates'' (Strickland et al., 2010, p. 
309). This suggests a limited impact of aeroallergens on O3 
associations with asthma-related emergency department visits and 
hospital admissions.
    With respect to the comment about epidemiologic studies not 
controlling for respiratory infections in the model, the EPA disagrees 
with the commenter's assertion. We recognize that asthma is a multi-
etiologic disease and that air pollutants, including O3, 
represent only one potential avenue to trigger an asthma exacerbation. 
Strickland et al. attempted to further clarify the relationship between 
short-term O3 exposures and asthma emergency department 
visits by controlling for the possibility that respiratory infections 
may lead to an asthma exacerbation. By including the daily count of 
upper respiratory visits as a covariate in the model, Strickland et al. 
were able to account for the possibility that respiratory infections 
contribute to the daily counts of asthma emergency department visits, 
and to identify the O3 effect on asthma emergency department 
visits. In models that controlled for upper respiratory infection 
visits, associations between O3 and emergency department 
visits remained statistically significant (Strickland et al., Table 4 
in published study), demonstrating a relatively limited influence of 
respiratory infections on the association observed between short-term 
O3 exposures and asthma emergency department visits, 
contrary to the commenter's claim.
    In addition, with regard to the criticism of the results reported 
by Mar and Koenig, the EPA disagrees with commenters who questioned the 
appropriateness of a zero-day lag. These commenters specifically noted 
uncertainty in the relative timing of the O3 exposure and 
the emergency department visit when they occurred on the same day. 
However, based on the broader body of evidence the ISA concludes that 
the strongest support is for a relatively immediate respiratory 
response following O3 exposures. Specifically, the ISA 
states that ``[t]he collective evidence indicates a rather immediate 
response within the first few days of O3 exposure (i.e., for 
lags days averaged at 0-1, 0-2, and 0-3 days) for hospital admissions 
and [emergency department] visits for all respiratory outcomes, asthma, 
and chronic obstructive pulmonary disease in all-year and seasonal 
analyses'' (U.S. EPA, 2013, p. 2-32). Thus, the use of a zero-day lag 
is consistent with the broader body of evidence supporting the 
occurrence of O3-associated health effects. In addition, 
while Mar and Koenig reported the strongest associations for zero-day 
lags, they also reported positive associations for lags ranging from 
zero to five days (Mar and Koenig, 2009, Table 5 in the published 
study). In considering this study, the ISA stated that Mar and Koenig 
(2009) ``found consistent positive associations across individual lag 
days'' and that ``[f]or children, consistent positive associations were 
observed across all lags . . . with the strongest associations observed 
at lag 0 (33.1% [95% CI: 3.0, 68.5]) and lag 3 (36.8% [95% CI: 6.1, 
77.2])'' (U.S. EPA, 2013, p. 6-150). Given support for a relatively 
immediate response to O3 and given the generally consistent 
results in analyses using various lags, we disagree with commenters who 
asserted that the use of a zero-day lag represents an important 
uncertainty in the interpretation of the study by Mar and Koenig 
(2009).
    Given all of the above, we do not agree with commenters who 
asserted that uncertainties in the epidemiologic evidence in general, 
or in specific key studies, should preclude the

[[Page 65337]]

Administrator from relying on those studies to inform her decisions on 
the primary O3 standard.
    Some commenters also objected to the characterization in the ISA 
and the proposal that the results of epidemiologic studies are 
consistent. These commenters contended that the purported consistency 
of results across epidemiologic studies is the result of inappropriate 
selectivity on the part of the EPA in focusing on specific studies and 
specific results within those studies. In particular, commenters 
contend that EPA favors studies that show positive associations and 
selectively ignores certain studies that report null results. They also 
cite a study published after the completion of the ISA (Goodman et al., 
2013) suggesting that, in papers where the results of more than one 
statistical model are reported, the EPA tends to report the results 
with the strongest associations.
    The EPA disagrees that it has inappropriately focused on specific 
positive studies or specific positive results within individual 
studies. The ISA appropriately builds upon the assessment of the 
scientific evidence presented in previous AQCDs and ISAs.\99\ When 
evaluating new literature, ``[s]election of studies for inclusion in 
the ISA is based on the general scientific quality of the study, and 
consideration of the extent to which the study is informative and 
policy-relevant'' (U.S. EPA, 2013, p. liii). In addition, ``the intent 
of the ISA is to provide a concise review, synthesis, and evaluation of 
the most policy-relevant science to serve as a scientific foundation 
for the review of the NAAQS, not extensive summaries of all health, 
ecological and welfare effects studies for a pollutant'' (U.S. EPA, 
2013, p. lv). Therefore, not all studies published since the previous 
review would be appropriate for inclusion in the ISA.\100\ With regard 
to the specific studies that are included in the ISA, and the analyses 
focused upon within given studies, the EPA notes that the ISA undergoes 
extensive peer review in a public setting by the CASAC. This process 
provides ample opportunity for CASAC and the public to comment on 
studies not included in the ISA, and on the specific analyses focused 
upon within individual studies. In endorsing the final O3 
ISA as adequate for rule-making purposes, CASAC agreed with the 
selection and presentation of analyses on which to base the ISA's key 
conclusions.
---------------------------------------------------------------------------

    \99\ Cf. Coalition for Responsible Regulation v. EPA, 684 F. 3d 
102, 119 (D.C. Cir. 2012) (aff'd in part and rev'd in part on other 
grounds sub. nom UARG v. EPA, S Ct. (2014)) (``EPA simply did here 
what it and other decision-makers often must do to make a science-
based judgment: it sought out and reviewed existing scientific 
evidence to determine whether a particular finding was warranted. It 
makes no difference that much of the scientific evidence in large 
part consisted of `syntheses' of individual studies and research. 
Even individual studies and research papers often synthesize past 
work in an area and then build upon it. That is how science 
works'').
    \100\ See also section II.C.4.b below responding to comments 
from environmental interests that EPA inappropriately omitted many 
studies which (in their view) support establishing a revised 
standard at a level of 60 ppb or lower. Although, as explained 
there, the EPA disagrees with these comments, the comments 
illustrate that the EPA was even-handed in its consideration of the 
epidemiologic evidence, and most certainly did not select merely 
studies favorable to the point of view of revising the current 
standard.
---------------------------------------------------------------------------

iii. Evidence Pertaining to At-Risk Populations and Lifestages
    A number of groups submitted comments on the EPA's identification 
of at-risk populations and lifestages. Some industry commenters who 
opposed revising the current standard disagreed with the EPA's 
identification of people with asthma or other respiratory diseases as 
an at-risk population for O3-attributable effects, citing 
controlled human exposure studies that did not report larger 
O3-induced FEV1 decrements in people with asthma 
than in people without asthma. In contrast, comments from medical, 
environmental, and public health groups generally agreed with the at-
risk populations identified by EPA, and also identified other 
populations that they stated should be considered at risk, including 
people of lower socio-economic status, people with diabetes or who are 
obese, pregnant women (due to reproductive and developmental effects, 
and African American, Asian, Hispanic/Latino or tribal communities. As 
support for the additional populations, these commenters cited various 
studies, including some that were not included in the ISA (which we 
have provisionally considered, as described in section I.C above).
    With regard to the former group of comments stating that the 
evidence does not support the identification of asthmatics as an at-
risk population, we disagree. As summarized in the proposal, the EPA's 
identification of populations at risk of O3 effects is based 
on a systematic approach that assesses the current scientific evidence 
across the relevant scientific disciplines (i.e., exposure sciences, 
dosimetry, controlled human exposure, toxicology, and epidemiology), 
with a focus on studies that conducted stratified analyses allowing for 
an evaluation of different populations exposed to similar O3 
concentrations within the same study design (U.S. EPA, 2013, pp. 8-1 to 
8-3). Based on this established process and framework, the ISA 
identifies individuals with asthma among the populations and lifestages 
for which there is ``adequate'' evidence to support the conclusion of 
increased risk of O3-related health effects. Other 
populations for which the evidence is adequate are individuals with 
certain genotypes, younger and older age groups, individuals with 
reduced intake of certain nutrients, and outdoor workers. These 
conclusions are based on consistency in findings across studies and 
evidence of coherence in results from different scientific disciplines.
    For example, with regard to people with asthma, the ISA notes a 
number of epidemiologic and controlled human exposure studies reporting 
larger and/or more serious effects in people with asthma than in people 
without asthma or other respiratory diseases. These include 
epidemiologic studies of lung function, respiratory symptoms, and 
medication use, as well as controlled human exposure studies showing 
larger inflammatory responses and markers indicating altered immune 
functioning in people with asthma, and also includes evidence from 
animal models of asthma that informs the EPA's interpretation of the 
other studies. We disagree with the industry commenters' focus solely 
on the results of certain studies without an integrated consideration 
of the broader body of evidence, and wider range of respiratory 
endpoints. It is such an integrated approach that supports EPA's 
conclusion that ``there is adequate evidence for asthmatics to be an 
at-risk population'' (U.S. EPA, 2013, section 8.2.2).
    We also disagree with commenters' misleading reference to various 
studies cited to support the claim that asthmatics are not at increased 
risk of O3-related health effects. One of the controlled 
human studies cited in those comments (Mudway et al. 2001) involved 
asthmatic adults who were older than the healthy controls, and it is 
well-recognized that responses to O3 decrease with age (U.S. 
EPA, 2014c, p. 3-80). Another study (Alexis et al. 2000) used subjects 
with mild asthma who are unlikely to be as responsive as people with 
more severe disease (Horstman et al., 1995) (EPA 2014c, p. 3-80). 
Controlled human exposure studies and epidemiologic studies of adults 
and children amply confirm that ``there is adequate evidence for 
asthmatics to be an at-risk population'' (U.S. EPA, 2014c, p. 3-81).

[[Page 65338]]

    We also do not agree with the latter group of commenters that there 
is sufficient evidence to support the identification of additional 
populations as at risk of O3-attributable health effects. 
Specifically with regard to pregnant women, the ISA concluded that the 
``evidence is suggestive of a causal relationship between exposures to 
O3 and reproductive and developmental effects'' including 
birth outcomes, noting that ``the collective evidence for many of the 
birth outcomes examined is generally inconsistent'' (U.S. EPA, 2013, 
pp. 7-74 and 7-75). At the time of the completion of the ISA, no 
studies had been identified that examined the relationship between 
exposure to O3 and the health of pregnant women (e.g., 
studies on pre-eclampsia, gestational hypertension). Due to the 
generally inconsistent epidemiologic evidence for effects on birth 
outcomes, the lack of studies on the health of pregnant women, and the 
lack of studies from other disciplines to provide biological 
plausibility for the effects examined in epidemiologic studies, 
pregnant women were not considered an at-risk population. Based on the 
EPA's provisional consideration of studies published since the 
completion of the ISA (I.C, above), recent studies that examine 
exposure to O3 and pre-eclampsia and other health effects 
experienced by pregnant women are not sufficient to materially change 
the ISA's conclusions on at-risk populations (I.C, above). In addition, 
as summarized in the proposal, the ISA concluded that the evidence for 
other populations was either suggestive of increased risk, with further 
investigation needed (e.g., other genetic variants, obesity, sex, and 
socioeconomic status), or was inadequate to determine if they were of 
increased risk of O3-related health effects (influenza/
infection, COPD, CVD, diabetes, hyperthyroidism, smoking, race/
ethnicity, and air conditioning use) (U.S. EPA, 2013, section 2.5.4.1). 
The CASAC has concurred with the ISA conclusions (Frey, 2014c).
c. Comments on Exposure and Risk Assessments
    This section discusses major comments on the EPA's quantitative 
assessments of O3 exposures and health risks, presented in 
the HREA and considered in the PA, and the EPA's responses to those 
comments. The focus in this section is on overarching comments related 
to the EPA's approach to assessing exposures and risks, and to 
interpreting the exposure/risk results within the context of the 
adequacy of the current primary O3 standard. More detailed 
discussion of comments and Agency responses is provided in the Response 
to Comments document. Section II.B.2.c.i discusses comments on 
estimates of O3 exposures of concern, section II.B.2.c.ii 
discusses comments on estimates of the risk of O3-induced 
lung function decrements, and section II.B.2.b.iii discusses comments 
on estimates of the risk of O3-associated mortality and 
morbidity.
i. O3 Exposures of Concern
    The EPA received a number of comments expressing divergent views on 
the estimation of, and interpretation of, O3 exposures of 
concern. In general, comments from industry, business, and some state 
groups opposed to revising the current primary O3 standard 
asserted that the approaches and assumptions that went into the HREA 
assessment result in overestimates of O3 exposures. These 
commenters highlighted several aspects of the assessment, asserting 
that the HREA overestimates the proportion of the population expected 
to achieve ventilation rates high enough to experience an exposure of 
concern; that the use of out-of-date information on activity patterns 
results in overestimates of the amount of time people spend being 
active outdoors; and that exposure estimates do not account for the 
fact that people spend more time indoors on days with bad air quality 
(i.e., they engage in averting behavior). In contrast, comments from 
medical, public health, and environmental groups that supported 
revision of the current standard asserted that the HREA assessment of 
exposures of concern, and the EPA's interpretation of exposure 
estimates, understates the potential for O3 exposures that 
could cause adverse health effects. These commenters claimed that the 
EPA's focus on 8-hour exposures understates the O3 impacts 
on public health since effects in controlled human exposure studies 
were shown following 6.6-hour exposures; that the HREA exposure 
estimates do not capture the most highly exposed populations, such as 
highly active children and outdoor workers; and that the EPA's 
interpretation of estimated exposures of concern impermissibly relies 
on the assumption that people stay indoors to avoid dangerous air 
pollution (i.e., that they engage in averting behavior).
    In considering these comments, the EPA first notes that as 
discussed in the HREA, PA, and the proposal, there are aspects of the 
exposure assessment that, considered by themselves, can result in 
either overestimates or underestimates of the occurrence of 
O3 exposures of concern. Commenters tended to highlight the 
aspects of the assessment that supported their positions, including 
aspects that were discussed in the HREA and/or the PA and that were 
considered by CASAC. In contrast, commenters tended to ignore the 
aspects of the assessment that did not support their positions. The EPA 
has carefully described and assessed the significance of the various 
uncertainties in the exposure analysis (U.S. EPA, 2014a, Table 5-10), 
noting that, in most instances, the uncertainties could result in 
either overestimates or underestimates of exposures and that the 
magnitudes of the impacts on exposure results were either ``low,'' 
``low to moderate,'' or ``moderate'' (U.S. EPA, 2014a, Table 5-10).
    Consistent with the characterization of uncertainties in the HREA, 
PA, and the proposal, the EPA agrees with some, though not all, aspects 
of these commenters' views. For example, the EPA agrees with the 
comment by groups opposed to revision that the equivalent ventilation 
rate (EVR) used to characterize individuals as at moderate or greater 
exertion in the HREA likely leads to overestimates of the number of 
individuals experiencing exposures of concern (U.S. EPA, 2014a, Table 
5-10, p. 5-79). In addition, we note that other physiological processes 
that are incorporated into exposure estimates are also identified in 
the HREA as likely leading to overestimates of O3 exposures, 
based on comparisons with the available scientific literature (U.S. 
EPA, 2014a, Table 5-10, p. 5-79). These aspects of the exposure 
assessment are estimated to have either a ``moderate'' (i.e., EVR) or a 
``low to moderate'' (i.e., physiological processes) impact on exposure 
estimates (U.S. EPA, 2014a, Table 5-10, p. 5-79). Focusing on these 
aspects of the assessment, by themselves, could lead to the conclusion 
that the HREA overstates the occurrence of O3 exposures of 
concern.
    However, the EPA notes that there are also aspects of the HREA 
exposure assessment that, taken by themselves, could lead to the 
conclusion that the HREA understates the occurrence of O3 
exposures of concern. For example, as noted above, some medical, public 
health, and environmental groups asserted that the exposure assessment 
could underestimate O3 exposures for highly active 
populations, including outdoor workers and children who spend a large 
portion of time outdoors during summer. In support of these assertions, 
commenters highlighted sensitivity analyses conducted in the HREA. 
However, as noted in the HREA (U.S. EPA, 2014a, Table 5-10), this

[[Page 65339]]

aspect of the assessment is likely to have a ``low to moderate'' impact 
on exposure estimates (i.e., a smaller impact than uncertainty 
associated with the EVR, and similar in magnitude to uncertainties 
related to physiological processes, as noted above). Therefore, when 
considered in the context of all of the uncertainties in exposure 
estimates, it is unlikely that the HREA's approach to using data on 
activity patterns leads to overall underestimates of O3 
exposures. The implications of this uncertainty are discussed in more 
detail below (II.C.4.b), within the context of the Administrator's 
decision on a revised standard level.
    In addition, medical, public health, and environmental groups also 
pointed out that the controlled human exposures studies that provided 
the basis for health effect benchmarks were conducted in healthy 
adults, rather than at-risk populations, and these studies evaluated 
6.6 hour exposures, rather than the 8-hour exposures evaluated in the 
HREA exposure analyses. They concluded that adverse effects would occur 
at lower exposure concentrations in at-risk populations, such as people 
with asthma, and if people were exposed for 8 hours, rather than 6.6 
hours. In its review of the PA, CASAC clearly recognized these 
uncertainties, which provided part of the basis for CASAC's advice to 
consider exposures of concern for the 60 ppb benchmark. For example, 
when considering the results of the study by Schelegle et al. (2009) 
for 6.6-hour exposures to an average O3 concentration of 72 
ppb, CASAC judged that if subjects had been exposed for eight hours, 
the adverse combination of lung function decrements and respiratory 
symptoms ``could have occurred'' at lower O3 exposure 
concentrations (Frey, 2014c, p. 5). With regard to at-risk populations, 
CASAC concluded that ``based on results for clinical studies of healthy 
adults, and scientific considerations of differences in responsiveness 
of asthmatic children compared to healthy adults, there is scientific 
support that 60 ppb is an appropriate exposure of concern for asthmatic 
children'' (Frey, 2014c, p. 8). As discussed below (II.B.3, II.C.4.b, 
II.C.4.c), based in large part on CASAC advice, the Administrator does 
consider exposure results for the 60 ppb benchmark.
    Thus, rather than viewing the potential implications of various 
aspects of the HREA exposure assessment in isolation, as was done by 
many commenters, the EPA considers them together, along with other 
issues and uncertainties related to the interpretation of exposure 
estimates. As discussed above, CASAC recognized the key uncertainties 
in exposure estimates, as well as in the interpretation of those 
estimates in the HREA and PA (Frey, 2014a, c). In its review of the 2nd 
draft REA, CASAC concluded that ``[t]he discussion of uncertainty and 
variability is comprehensive, appropriately listing the major sources 
of uncertainty and their potential impacts on the APEX exposure 
estimates'' (Frey, 2014a, p. 6). Even considering these and other 
uncertainties, CASAC emphasized estimates of O3 exposures of 
concern as part of the basis for their recommendations on the primary 
O3 NAAQS. In weighing these uncertainties, which can bias 
exposure results in different directions but tend to have impacts that 
are similar in magnitude (U.S. EPA, 2014a, Table 5-10), and in light of 
CASAC's advice based on its review of the HREA and the PA, the EPA 
continues to conclude that the approach to considering estimated 
exposures of concern in the HREA, PA, and the proposal reflects an 
appropriate balance, and provides an appropriate basis for considering 
the public health protectiveness of the primary O3 standard.
    The EPA disagrees with other aspects of commenters' views on HREA 
estimates of exposures of concern. For example, commenters on both 
sides of the issue objected to the EPA's handling of averting behavior 
in exposure estimates. Some commenters who supported retaining the 
current standard claimed that the HREA overstates exposures of concern 
because available time-location-activity data do not account for 
averting behavior. These commenters noted sensitivity analyses in the 
HREA that estimated fewer exposures of concern when averting behavior 
was considered. In contrast, commenters supporting revision of the 
standard criticized the EPA's estimates of exposures of concern, 
claiming that the EPA ``emphasizes the role of averting behavior, 
noting that it may result in an overestimation of exposures of concern, 
and cites this behavior (essentially staying indoors or not exercising) 
in order to reach what it deems an acceptable level of risk'' (e.g., 
ALA et al., p. 120).
    The EPA disagrees with both of these comments. In brief, the NAAQS 
must ``be established at a level necessary to protect the health of 
persons,'' not the health of persons refraining from normal activity or 
resorting to medical interventions to ward off adverse effects of poor 
air quality (S. Rep. No. 11-1196, 91st Cong. 2d Sess. at 10). On the 
other hand, ignoring normal activity patterns for a pollutant like 
O3, where adverse responses are critically dependent on 
ventilation rates, will result in a standard which provides more 
protection than is requisite. This issue is discussed in more detail 
below (II.C.4.b), within the context of the Administrator's decision on 
a revised standard level.
    These commenters also misconstrue the EPA's limited sensitivity 
analyses on impacts of averting behavior in the HREA. The purpose of 
the HREA sensitivity analyses was to provide perspective on the 
potential role of averting behavior in modifying O3 
exposures. These sensitivity analyses were limited to a single urban 
study area, a 2-day period, and a single air quality adjustment 
scenario (U.S. EPA, 2014a, section 5.4.3.3). In addition, the approach 
used in the HREA to simulate averting behavior was itself uncertain, 
given the lack of actual activity pattern data that explicitly 
incorporated this type of behavioral response. In light of these 
important limitations, sensitivity analyses focused on averting 
behavior were discussed in the proposal within the context of the 
discussion of uncertainties in the HREA assessment of exposures of 
concern (II.C.2.b in the proposal) and, contrary to the claims of some 
commenters, they were not used to support the proposed decision.
    Some industry groups also claimed that the time-location-activity 
diaries used by APEX to estimate exposures are out-of-date, and do not 
represent activity patterns in the current population. These commenters 
asserted that the use of out-of-date diary information leads to 
overestimates in exposures of concern. This issue was explicitly 
addressed in the HREA and the EPA disagrees with commenters' 
conclusions. In particular, diary data was updated in this review to 
include data from studies published as late as 2010, directly in 
response to CASAC concerns. In their review of this data, CASAC stated 
that ``[t]he addition of more recent time activity pattern data 
addresses a concern raised previously by the CASAC concerning how 
activity pattern information should be brought up to date'' (Frey, 
2014a, p. 8). As indicated in the HREA (U.S. EPA, 2014a, Appendix 5G, 
Figures 5G-7 and Figure 5G-8), the majority of diary days used in 
exposure simulations of children originate from the most recently 
conducted activity pattern studies (U.S. EPA, 2014a, Table 5-3). In 
addition, evaluations included in the HREA indicated that there were 
not major systematic differences in time-location-activity patterns 
based on information from older diaries versus those collected more 
recently (U.S. EPA,

[[Page 65340]]

2014a, Appendix 5G, Figures 5G-1 and 5G-2). Given all of the above, the 
EPA does not agree with commenters who claimed that the time-location-
activity diaries used by APEX are out-of-date, and result in 
overestimates of exposures of concern.
ii. Risk of O3-Induced FEV1 Decrements
    The EPA also received a large number of comments on the 
FEV1 risk assessment presented in chapter 6 of the HREA 
(U.S. EPA, 2014a) and summarized in the proposal (II.C.3.a in the 
proposal). Commenters representing medical, public health, and 
environmental groups generally expressed the view that these risk 
estimates support the need to revise the current primary O3 
standard in order to increase public health protection, though these 
groups also questioned some of the assumptions inherent in the EPA's 
interpretation of those risk estimates. For example, ALA et al. (p. 
127) stated that ``[t]he HREA uses a risk function derived from a 
controlled human exposure study of healthy young adults to estimate 
lung function decrements in children, including children with asthma. 
This assumption could result in an underestimate of risk.'' On this 
same issue, commenters representing industry groups opposed to revising 
the standard also asserted that assumptions about children's responses 
to O3 exposures are highly uncertain. In contrast to medical 
and public health groups, these commenters concluded that this 
uncertainty, along with others discussed below, call into question the 
use of FEV1 risk estimates to support a decision to revise 
the current primary O3 standard.
    The EPA agrees that an important source of uncertainty is the 
approach to estimating the risk of FEV1 decrements in 
children and in children with asthma based on data from healthy adults. 
However, this issue is discussed at length in the HREA and the PA, and 
was considered carefully by CASAC in its review of draft versions of 
these documents. The conclusions of the HREA and PA, and the advice of 
CASAC, were reflected in the Administrator's interpretation of 
FEV1 risk estimates in the proposal, as described below. 
Commenters have not provided additional information that changes the 
EPA's views on this issue.
    As discussed in the proposal (II.C.3.a.ii in the proposal), in the 
near absence of controlled human exposure data for children, risk 
estimates are based on the assumption that children exhibit the same 
lung function response following O3 exposures as healthy 18-
year olds (i.e., the youngest age for which sufficient controlled human 
exposure data is available) (U.S. EPA, 2014a, section 6.5.3). As noted 
by CASAC (Frey, 2014a, p. 8), this assumption is justified in part by 
the findings of McDonnell et al. (1985), who reported that children (8-
11 years old) experienced FEV1 responses similar to those 
observed in adults (18-35 years old). The HREA concludes that this 
approach could result in either over- or underestimates of 
O3-induced lung function decrements in children, depending 
on how children compare to the adults used in controlled human exposure 
studies (U.S. EPA, 2014a, section 6.5.3). With regard to people with 
asthma, although the evidence has been mixed (U.S. EPA, 2013, section 
6.2.1.1), several studies have reported statistically larger, or a 
tendency for larger, O3-induced lung function decrements in 
asthmatics than in non-asthmatics (Kreit et al., 1989; Horstman et al., 
1995; Jorres et al., 1996; Alexis et al., 2000). On this issue, CASAC 
noted that ``[a]sthmatic subjects appear to be at least as sensitive, 
if not more sensitive, than non-asthmatic subjects in manifesting 
O3-induced pulmonary function decrements'' (Frey, 2014c, p. 
4). To the extent asthmatics experience larger O3-induced 
lung function decrements than the healthy adults used to develop 
exposure-response relationships, the HREA could underestimate the 
impacts of O3 exposures on lung function in asthmatics, 
including asthmatic children (U.S. EPA, 2014a, section 6.5.4). As noted 
above, these uncertainties have been considered carefully by the EPA 
and by CASAC during the development of the HREA and PA. In addition, 
the Administrator has appropriately considered these and other 
uncertainties in her interpretation of risk estimates, as discussed 
further below (II.B.3, II.C.4.b, II.C.4.c).
    Some commenters additionally asserted that the HREA does not 
appropriately characterize the uncertainty in risk estimates for 
O3-induced lung function decrements. Commenters pointed out 
that there is statistical uncertainty in model coefficients that is not 
accounted for in risk estimates. One commenter presented an analysis of 
this uncertainty, and concluded that there is considerable overlap 
between risk estimates for standard levels of 75, 70, and 65 ppb, 
undercutting the confidence in estimated risk reductions for standard 
levels below 75 ppb.
    The Agency recognizes that there are important sources of 
uncertainty in the FEV1 risk assessment. In some cases, 
these sources of uncertainty can contribute to substantial variability 
in risk estimates, complicating the interpretation of those estimates. 
For example, as discussed in the proposal, the variability in 
FEV1 risk estimates across urban study areas is often 
greater than the differences in risk estimates between various standard 
levels (Table 2, above and 79 FR 75306 n. 164). Given this, and the 
resulting considerable overlap between the ranges of FEV1 
risk estimates for different standard levels, in the proposal the 
Administrator viewed these risk estimates as providing a more limited 
basis than exposures of concern for distinguishing between the degree 
of public health protection provided by alternative standard levels. 
Thus, although the EPA does not agree with the overall conclusions of 
industry commenters, their analysis of statistical uncertainty in risk 
estimates, and the resulting overlap between risk estimates for 
standard levels of 75, 70, and 65 ppb, tends to reinforce the 
Administrator's approach, which places greater weight on estimates of 
O3 exposures of concern than on risk estimates for 
O3-induced FEV1 decrements.
iii. Risk of O3-Associated Mortality and Morbidity
    In the proposal, the Administrator placed the greatest emphasis on 
the results of controlled human exposure studies and on quantitative 
analyses based on information from these studies, and less weight on 
mortality and morbidity risk assessments based on information from 
epidemiology studies. The EPA received a number of comments on its 
consideration of epidemiology-based risks, with some commenters 
expressing support for the Agency's approach and others expressing 
opposition.
    In general, commenters representing industry organizations or 
states opposed to revising the current primary O3 standard 
agreed with the Administrator's approach in the proposal to viewing 
epidemiology-based risk estimates, though these commenters reached a 
different conclusion than the EPA regarding the adequacy of the current 
standard. In supporting their views, these commenters highlighted a 
number of uncertainties in the underlying epidemiologic studies, and 
concluded that risk estimates based on information from such studies do 
not provide an appropriate basis for revising the current standard. For 
example, commenters noted considerable spatial heterogeneity in health 
effect associations; the potential for co-occurring pollutants (e.g., 
PM2.5) to confound O3 health effect associations;

[[Page 65341]]

and the lack of statistically significant O3 health effect 
associations in many of the individual cities evaluated as part of 
multicity analyses. In contrast, some commenters representing medical, 
public health, or environmental organizations placed greater emphasis 
than the EPA on epidemiology-based risk estimates. These commenters 
asserted that risk estimates provide strong support for a lower 
standard level, and pointed to CASAC advice to support their position.
    As in the proposal, the EPA continues to place the greatest weight 
on the results of controlled human exposure studies and on quantitative 
analyses based on information from these studies (particularly 
exposures of concern, as discussed below in II.B.3 and II.C.4), and 
less weight on risk analyses based on information from epidemiologic 
studies. In doing so, the Agency continues to note that controlled 
human exposure studies provide the most certain evidence indicating the 
occurrence of health effects in humans following specific O3 
exposures. In addition, the effects reported in these studies are due 
solely to O3 exposures, and interpretation of study results 
is not complicated by the presence of co-occurring pollutants or 
pollutant mixtures (as is the case in epidemiologic studies). The 
Agency further notes the CASAC judgment that ``the scientific evidence 
supporting the finding that the current standard is inadequate to 
protect public health is strongest based on the controlled human 
exposure studies of respiratory effects'' (Frey, 2014c, p. 5). 
Consistent with this emphasis, the HREA conclusions reflect relatively 
greater confidence in the results of the exposure and risk analyses 
based on information from controlled human exposure studies than the 
results of epidemiology-based risk analyses. As discussed in the HREA 
(U.S. EPA, 2014a, section 9.6), several key uncertainties complicate 
the interpretation of these epidemiology-based risk estimates, 
including the heterogeneity in O3 effect estimates between 
locations, the potential for exposure measurement errors in these 
epidemiologic studies, and uncertainty in the interpretation of the 
shape of concentration-response functions at lower O3 
concentrations. Commenters who opposed the EPA's approach in the 
proposal to viewing the results of quantitative analyses tended to 
highlight aspects of the evidence and CASAC advice that were considered 
by the EPA at the time of proposal and nothing in these commenters' 
views has changed those considerations. Therefore, the EPA continues to 
place the most emphasis on using the information from controlled human 
exposure studies to inform consideration of the adequacy of the primary 
O3 standard.
    However, while the EPA agrees that there are important 
uncertainties in the O3 epidemiology-based risk estimates, 
the Agency disagrees with industry commenters that these uncertainties 
support a conclusion to retain the current standard. As discussed 
below, the decision to revise the current primary O3 
standard is based on the EPA's consideration of the broad body of 
scientific evidence, quantitative analyses of O3 exposures 
and risks, CASAC advice, and public comments. While recognizing 
uncertainties in the epidemiology-based risk estimates here, and giving 
these uncertainties appropriate consideration, the Agency continues to 
conclude that these risk estimates contribute to the broader body of 
evidence and information supporting the need to revise the primary 
O3 standard.
    Some commenters opposed to revising the current O3 
standard highlighted the fact that, in a few urban study locations, 
larger risks are estimated for standard levels below 75 ppb than for 
the current standard with its level of 75 ppb. For example, TCEQ (p. 3) 
states that ``differential effects on ozone in urban areas also lead to 
the EPA's modeled increases in mortality in Houston and Los Angeles 
with decreasing ozone standards.'' These commenters cited such 
increases in estimated risk as part of the basis for their conclusion 
that the current standard should be retained.
    For communities across the U.S. (including in the Houston and Los 
Angeles areas), exposure and risk analyses indicate that reducing 
emissions of O3 precursors (NOX, VOCs) to meet a 
revised standard with a level of 70 ppb will substantially reduce the 
occurrence of adverse respiratory effects and mortality risk 
attributable to high O3 concentrations (U.S. EPA, 2014a, 
Appendix 9A; U.S. EPA, 2014c, sections 4.4.2.1 to 4.4.2.3). However, 
because of the complex chemistry governing the formation and 
destruction of O3, some NOX control strategies 
designed to reduce the highest ambient O3 concentrations can 
also result in increases in relatively low ambient O3 
concentrations. As a result of the way the EPA's epidemiology-based 
risk assessments were conducted (U.S. EPA, 2014a, Chapter 7), increases 
estimated in low O3 concentrations impacted mortality and 
morbidity risks, leading to the estimated risk increases highlighted by 
some commenters. However, while the EPA is confident that reducing the 
highest ambient O3 concentrations will result in substantial 
improvements in public health, including reducing the risk of 
O3-associated mortality, the Agency is far less certain 
about the public health implications of the changes in relatively low 
ambient O3 concentrations (79 FR at 75278/3, 75291/1, and 
75308/2). Therefore, reducing precursor emissions to meet a lower 
O3 standard is expected to result in important reductions in 
O3 concentrations from the part of the air quality 
distribution where the evidence provides the strongest support for 
adverse health effects.
    Specifically, for area-wide O3 concentrations at or 
above 40 ppb,\101\ a revised standard with a level of 70 ppb is 
estimated to reduce the number of premature deaths associated with 
short-term O3 concentrations by about 10%, compared to the 
current standard. In addition, for area-wide concentrations at or above 
60 ppb, a revised standard with a level of 70 ppb is estimated to 
reduce O3-associated premature deaths by about 50% to 
70%.\102\ The EPA views these results, which focus on the portion of 
the air quality distribution where the evidence indicates the most 
certainty regarding the occurrence of adverse O3-
attributable health effects, not only as supportive of the need to 
revise the current standard (II.B.3, below), but also as showing the 
benefits of reducing the peak O3 concentrations associated 
with air quality distributions meeting the current standard (II.C.4, 
below).
---------------------------------------------------------------------------

    \101\ The ISA concludes that there is less certainty in the 
shape of concentration-response functions for area-wide 
O3 concentrations at the lower ends of warm season 
distributions (i.e., below about 20 to 40 ppb) (U.S. EPA, 2013, 
section 2.5.4.4).
    \102\ Available experimental studies provide the strongest 
evidence for O3-induced effects following exposures to 
O3 concentrations corresponding to the upper portions of 
typical ambient distributions. In particular, as discussed above, 
controlled human exposure studies showing respiratory effects 
following exposures to O3 concentrations at or above 60 
ppb.
---------------------------------------------------------------------------

    In addition, even considering risk estimates based on the full 
distribution of ambient O3 concentrations (i.e., estimates 
influenced by decreases in higher concentrations and increases in lower 
concentrations), the EPA notes that, compared to the current standard, 
standards with lower levels are estimated to result in overall 
reductions in mortality risk across the urban study areas evaluated 
(U.S. EPA, 2014c, Figure 4-10). As discussed above (II.A.2.a, 
II.A.2.c), analyses in the HREA indicate that these overall risk 
reductions could understate the actual reductions that

[[Page 65342]]

would be experienced by the U.S. population as a whole.
    For example, the HREA's national air quality modeling analyses 
indicate that the HREA urban study areas tend to underrepresent the 
populations living in areas where reducing NOX emissions 
would be expected to result in decreases in warm season averages of 
daily maximum 8-hour ambient O3 concentrations.\103\ Given 
the strong connection between these warm season average O3 
concentrations and risk, risk estimates for the urban study areas are 
likely to understate the average reductions in O3-associated 
mortality and morbidity risks that would be experienced across the U.S. 
population as a whole upon reducing NOX emissions (U.S. EPA, 
2014a, section 8.2.3.2).
---------------------------------------------------------------------------

    \103\ Specifically, the HREA urban study areas tend to 
underrepresent populations living in suburban, smaller urban, and 
rural areas, where reducing NOX emissions would be 
expected to result in decreases in warm season averages of daily 
maximum 8-hour ambient O3 concentrations (U.S. EPA, 
2014a, section 8.2.3.2).
---------------------------------------------------------------------------

    In addition, in recognizing that the reductions in modeled 
NOX emissions used in the HREA's core analyses are meant to 
be illustrative, rather than to imply a particular control strategy for 
meeting a revised O3 NAAQS, the HREA also conducted 
sensitivity analyses in which both NOX and VOC emissions 
reductions were evaluated. In all of the urban study areas evaluated in 
these analyses, the increases in low O3 concentrations were 
smaller for the NOX/VOC emission reduction scenarios than 
the NOX only emission reduction scenario (U.S. EPA, 2014a, 
Appendix 4D, section 4.7). This was most apparent for Denver, Houston, 
Los Angeles, New York, and Philadelphia. These results suggest that in 
some locations, optimized emissions reduction strategies could result 
in larger reductions in O3-associated mortality and 
morbidity than indicated by HREA's core estimates.
    Thus, the patterns of estimated mortality and morbidity risks 
across various air quality scenarios and locations have been evaluated 
and considered extensively in the HREA and the PA, as well as in the 
proposal. Epidemiology-based risk estimates have also been considered 
by CASAC, and those considerations are reflected in CASAC's advice. 
Specifically, in considering epidemiology-based risk estimates in its 
review of the REA, CASAC stated that ``[a]lthough these estimates for 
short-term exposure impacts are subject to uncertainty, the CASAC is 
confident that that the evidence of health effects of O3 
presented in the ISA and Second Draft HREA in its totality, indicates 
that there are meaningful reductions in mean, absolute, and relative 
premature mortality associated with short-term exposures to 
O3 levels lower than the current standard'' (Frey, 2014a, p. 
3). Commenters' views on this issue are not based on new information, 
but on an interpretation of the analyses presented in the HREA that is 
different from the EPA's, and CASAC's, interpretation. Given this, the 
EPA's considerations and conclusions related to this issue, as 
described in the proposal and as summarized briefly above, remain 
valid. Therefore, the EPA does not agree with commenters who cited 
increases in estimated risk in some locations as supporting a 
conclusion that the current standard should be retained.
    For risk estimates of respiratory mortality associated with long-
term O3, several industry commenters supported placing more 
emphasis on threshold models, and including these models as part of the 
core analyses rather than as sensitivity analyses. The EPA agrees with 
these commenters that an important uncertainty in risk estimates of 
respiratory mortality associated with long-term O3 stems 
from the potential for the existence of a threshold. Based on 
sensitivity analyses included in the HREA in response to CASAC advice, 
the existence of a threshold could substantially reduce estimated 
risks. CASAC discussed this issue at length during its review of the 
REA and supported the EPA's approach to including a range of threshold 
models as sensitivity analyses (Frey, 2014a p. 3). Based in part on 
uncertainty in the existence and identification of a threshold, the 
HREA concluded that lower confidence should be placed in risk estimates 
for respiratory mortality associated with long-term O3 
exposures (U.S. EPA, 2014a, section 9.6). This uncertainty was also a 
key part of the Administrator's rationale for placing only limited 
emphasis on risk estimates for long-term O3 exposures. In 
her final decisions, discussed below (II.B.3, II.C.4.b, II.C.4.c), the 
Administrator continues to place only limited emphasis on these 
estimates. The EPA views this approach to considering risk estimates 
for respiratory mortality as generally consistent with the approach 
supported by the commenters noted above.
3. Administrator's Conclusions on the Need for Revision
    This section discusses the Administrator's conclusions related to 
the adequacy of the public health protection provided by the current 
primary O3 standard, and her final decision that the current 
standard is not requisite to protect public health with an adequate 
margin of safety. These conclusions, and her final decision, are based 
on the Administrator's consideration of the available scientific 
evidence assessed in the ISA (U.S. EPA, 2013), the exposure/risk 
information presented and assessed in the HREA (U.S. EPA, 2014a), the 
consideration of that evidence and information in the PA (U.S. EPA, 
2014c), the advice of CASAC, and public comments received on the 
proposal.
    As an initial matter, the Administrator concludes that reducing 
precursor emissions to achieve O3 concentrations that meet 
the current primary O3 standard will provide important 
improvements in public health protection, compared to recent air 
quality. In reaching this conclusion, she notes the discussion in 
section 3.4 of the PA (U.S. EPA, 2014c). In particular, the 
Administrator notes that this conclusion is supported by (1) the strong 
body of scientific evidence indicating a wide range of adverse health 
outcomes attributable to exposures to O3 at concentrations 
commonly found in the ambient air and (2) estimates indicating 
decreased occurrences of O3 exposures of concern and 
decreased O3-associated health risks upon meeting the 
current standard, compared to recent air quality. Thus, she concludes 
that it would not be appropriate in this review to consider a standard 
that is less protective than the current standard.
    After reaching the conclusion that meeting the current primary 
O3 standard will provide important improvements in public 
health protection, and that it is not appropriate to consider a 
standard that is less protective than the current standard, the 
Administrator next considers the adequacy of the public health 
protection that is provided by the current standard. In doing so, the 
Administrator first notes that studies evaluated since the completion 
of the 2006 AQCD support and expand upon the strong body of evidence 
that, in the last review, indicated a causal relationship between 
short-term O3 exposures and respiratory morbidity outcomes 
(U.S. EPA, 2013, section 2.5). This is the strongest causality finding 
possible under the ISA's hierarchical system for classifying weight of 
evidence for causation. In addition, the Administrator notes that the 
evidence for respiratory health effects attributable to long-term 
O3 exposures, including the development of asthma in 
children, is much stronger than in previous reviews, and the ISA 
concludes that there is ``likely to be'' a causal relationship

[[Page 65343]]

between such O3 exposures and adverse respiratory health 
effects (the second strongest causality finding).
    Together, experimental and epidemiologic studies support 
conclusions regarding a continuum of O3 respiratory effects 
ranging from small, reversible changes in pulmonary function, and 
pulmonary inflammation, to more serious effects that can result in 
respiratory-related emergency department visits, hospital admissions, 
and premature mortality. Recent animal toxicology studies support 
descriptions of modes of action for these respiratory effects and 
augment support for biological plausibility for the role of 
O3 in reported effects. With regard to mode of action, 
evidence indicates that the initial key event is the formation of 
secondary oxidation products in the respiratory tract, that antioxidant 
capacity may modify the risk of respiratory morbidity associated with 
O3 exposure, and that the inherent capacity to quench (based 
on individual antioxidant capacity) can be overwhelmed, especially with 
exposure to elevated concentrations of O3.
    In addition, based on the consistency of findings across studies 
and the coherence of results from different scientific disciplines, the 
available evidence indicates that certain populations are at increased 
risk of experiencing O3-related effects, including the most 
severe effects. These include populations and lifestages identified in 
previous reviews (i.e., people with asthma, children, older adults, 
outdoor workers) and populations identified since the last review 
(i.e., people with certain genotypes related to antioxidant and/or 
anti-inflammatory status; people with reduced intake of certain 
antioxidant nutrients, such as Vitamins C and E).
    In considering the O3 exposure concentrations reported 
to elicit respiratory effects, as in the proposal, the Administrator 
agrees with the conclusions of the PA that controlled human exposure 
studies provide the most certain evidence indicating the occurrence of 
health effects in humans following specific O3 exposures. In 
particular, she notes that the effects reported in controlled human 
exposure studies are due solely to O3 exposures, and 
interpretation of study results is not complicated by the presence of 
co-occurring pollutants or pollutant mixtures (as is the case in 
epidemiologic studies). Therefore, consistent with CASAC advice (Frey, 
2014c), she places the most weight on information from controlled human 
exposure studies in reaching conclusions on the adequacy of the current 
primary O3 standard.
    In considering the evidence from controlled human exposure studies, 
the Administrator first notes that these studies have reported a 
variety of respiratory effects in healthy adults following exposures to 
O3 concentrations of 60, 63,\104\ 72,\105\ or 80 ppb, and 
higher. The largest respiratory effects, and the broadest range of 
effects, have been studied and reported following exposures of healthy 
adults to 80 ppb O3 or higher, with most exposure studies 
conducted at these higher concentrations. As discussed above (II.A.1), 
the Administrator further notes that recent evidence includes 
controlled human exposure studies reporting the combination of lung 
function decrements and respiratory symptoms in healthy adults engaged 
in moderate exertion following 6.6-hour exposures to concentrations as 
low as 72 ppb, and lung function decrements and pulmonary inflammation 
following exposures to O3 concentrations as low as 60 ppb.
---------------------------------------------------------------------------

    \104\ For a 60 ppb target exposure concentration, Schelegle et 
al. (2009) reported that the actual 6.6-hour mean exposure 
concentration was 63 ppb.
    \105\ For a 70 ppb target exposure concentration, Schelegle et 
al. (2009) reported that the actual 6.6-hour mean exposure 
concentration was 72 ppb.
---------------------------------------------------------------------------

    As discussed in her response to public comments above (II.B.2.b.i), 
and in detail below (II.C.4.b, II.C.4.c), the Administrator concludes 
that these controlled human exposure studies indicate that adverse 
effects are likely to occur following exposures to O3 
concentrations below the level of the current standard. The effects 
observed following such exposures are coherent with the serious health 
outcomes that have been reported in O3 epidemiologic studies 
(e.g., respiratory-related hospital admissions, emergency department 
visits), and the Administrator judges that such effects have the 
potential to be important from a public health perspective.
    In reaching these conclusions, she particularly notes that the 
combination of lung function decrements and respiratory symptoms 
reported to occur in healthy adults following exposures to 72 ppb 
O3 meets ATS criteria for an adverse response (II.B.2.b.i, 
above). In specifically considering the 72 ppb exposure concentration, 
CASAC noted that ``the combination of decrements in FEV1 
together with the statistically significant alterations in symptoms in 
human subjects exposed to 72 ppb ozone meets the American Thoracic 
Society's definition of an adverse health effect'' (Frey, 2014c, p. 5). 
In addition, given that the controlled human exposure study reporting 
these results was conducted in healthy adults, CASAC judged that the 
adverse combination of lung function decrements and respiratory 
symptoms ``almost certainly occur in some people'' (e.g., people with 
asthma) following exposures to lower O3 concentrations 
(Frey, 2014c, p. 6).
    While the Administrator is less certain regarding the adversity of 
the lung function decrements and airway inflammation that have been 
observed following exposures as low as 60 ppb, as discussed in more 
detail elsewhere in this preamble (II.B.2.b.i, II.C.4.b, II.C.4.c), she 
judges that these effects also have the potential to be adverse, and to 
be of public health importance, particularly if they are experienced 
repeatedly. With regard to this judgment, she specifically notes the 
ISA conclusion that, while the airway inflammation induced by a single 
exposure (or several exposures over the course of a summer) can resolve 
entirely, continued inflammation could potentially result in adverse 
effects, including the induction of a chronic inflammatory state; 
altered pulmonary structure and function, leading to diseases such as 
asthma; altered lung host defense response to inhaled microorganisms; 
and altered lung response to other agents such as allergens or toxins 
(U.S. EPA, 2013, section 6.2.3). Thus, the Administrator becomes 
increasingly concerned about the potential for adverse effects at 60 
ppb O3 as the number of exposures increases, though she 
notes that the available evidence does not indicate a particular number 
of occurrences of such exposures that would be required to achieve an 
adverse respiratory effect, and that this number is likely to vary 
across the population.
    In addition to controlled human exposure studies, the Administrator 
also considers what the available epidemiologic evidence indicates with 
regard to the adequacy of the public health protection provided by the 
current primary O3 standard. She notes that recent 
epidemiologic studies provide support, beyond that available in the 
last review, for associations between short-term O3 
exposures and a wide range of adverse respiratory outcomes (including 
respiratory-related hospital admissions, emergency department visits, 
and mortality) and with total mortality. As discussed above in the EPA 
responses to public comments (II.B.2.b.ii), associations with morbidity 
and mortality are stronger during the warm or summer months, and remain 
robust after adjustment for copollutants (U.S. EPA, 2013, Chapter 6).

[[Page 65344]]

    In considering information from epidemiologic studies within the 
context of her conclusions on the adequacy of the current standard, the 
Administrator specifically considers analyses in the PA that evaluate 
the extent to which O3 health effect associations have been 
reported for air quality concentrations likely to be allowed by the 
current standard. She notes that such analyses can provide insight into 
the extent to which the current standard would allow the distributions 
of ambient O3 concentrations that provided the basis for 
these health effect associations. While the majority of O3 
epidemiologic studies evaluated in the PA were conducted in areas that 
would have violated the current standard during study periods, as 
discussed above (II.B.2.b.ii), the Administrator observes that the 
study by Mar and Koenig (2009) 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 current 
O3 standard over the entire study period.\106\ Based on 
this, she notes the conclusion from the PA that the current primary 
O3 standard would have allowed the distribution of ambient 
O3 concentrations that provided the basis for the 
associations with asthma emergency department visits reported by Mar 
and Koenig (2009) (U.S. EPA, 2014c, section 3.1.4.2).
---------------------------------------------------------------------------

    \106\ The large majority of locations evaluated in U.S. 
epidemiologic studies of long-term O3 would have violated 
the current standard during study periods, thus providing limited 
insight into the adequacy of the current standard (U.S. EPA, 2014c, 
section 3.1.4.3).
---------------------------------------------------------------------------

    In addition, even in some single-city study locations where the 
current standard was violated (i.e., those evaluated in Silverman and 
Ito, 2010; Strickland et al., 2010), the Administrator notes that PA 
analyses of reported concentration-response functions and available air 
quality data support the occurrence of O3-attributable 
hospital admissions and emergency department visits on subsets of days 
with virtually all ambient O3 concentrations below the level 
of the current standard. PA analyses of study area air quality further 
support the conclusion that exposures to the ambient O3 
concentrations present in the locations evaluated by Strickland et al. 
(2010) and Silverman and Ito (2010) could have plausibly resulted in 
the respiratory-related emergency department visits and hospital 
admissions reported in these studies (U.S. EPA, 2014c, section 
3.1.4.2). The Administrator agrees with the PA conclusion that these 
analyses indicate a relatively high degree of confidence in reported 
statistical associations with respiratory health outcomes on days when 
virtually all monitored 8-hour O3 concentrations were 75 ppb 
or below. She further agrees with the PA conclusion that although these 
analyses do not identify true design values, the presence of 
O3-associated respiratory effects on such days provides 
insight into the types of health effects that could occur in locations 
with maximum ambient O3 concentrations below the level of 
the current standard.
    Compared to the single-city epidemiologic studies discussed above, 
the Administrator notes additional uncertainty in interpreting the 
relationships between short-term O3 air quality in 
individual study cities and reported O3 multicity effect 
estimates. In particular, she judges that the available multicity 
effect estimates in studies of short-term O3 do not provide 
a basis for considering the extent to which reported O3 
health effect associations are influenced by individual locations with 
ambient O3 concentrations low enough to meet the current 
O3 standard, versus locations with O3 
concentrations that violate this standard.\107\ While such 
uncertainties limit the extent to which the Administrator bases her 
conclusions on air quality in locations of multicity epidemiologic 
studies, she does note that O3 associations with respiratory 
morbidity or premature mortality have been reported in several 
multicity studies when the majority of study locations (though not all 
study locations) would have met the current O3 standard 
(U.S. EPA, 2014c, section 3.1.4.2).
---------------------------------------------------------------------------

    \107\ As noted in the proposal (II.E.4.d), 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 the key O3 studies 
analyzed in the PA, with the exception of the study by Stieb et al. 
(2009) where none of the city-specific effect estimates for asthma 
emergency department visits were statistically significant). This 
specific uncertainty does not apply to multicity epidemiologic 
studies of long-term O3 concentrations, where multicity 
effect estimates are based on comparisons across cities. For 
example, see discussion of study by Jerrett et al. (2009) in the PA 
(U.S. EPA, 2014c, section 3.1.4.3).
---------------------------------------------------------------------------

    Looking across the body of epidemiologic evidence, the 
Administrator thus reaches the conclusion that analyses of air quality 
in study locations support the occurrence of adverse O3-
associated effects at ambient O3 concentrations that met, or 
are likely to have met, the current standard. She further concludes 
that the strongest support for this conclusion comes from single-city 
studies of respiratory-related hospital admissions and emergency 
department visits associated with short-term O3 
concentrations, with some support also from multicity studies of 
morbidity or mortality.
    Taken together, the Administrator concludes that the scientific 
evidence from controlled human exposure and epidemiologic studies calls 
into question the adequacy of the public health protection provided by 
the current standard. In reaching this conclusion, she particularly 
notes that the current standard level is higher than the lowest 
O3 exposure concentration shown to result in the adverse 
combination of lung function decrements and respiratory symptoms (i.e., 
72 ppb), and that CASAC concluded that such effects ``almost certainly 
occur in some people'' following exposures to O3 
concentrations below 72 ppb (Frey, 2014c, p. 6). While she also notes 
that the current standard level is well-above the lowest O3 
exposure concentration shown to cause respiratory effects (i.e., 60 
ppb), she has less confidence that the effects observed at 60 ppb are 
adverse (discussed in II.B.2.b.i, II.C.4.b, II.C.4.c). She further 
considers these effects, and the extent to which the current primary 
O3 standard could protect against them, within the context 
of quantitative analyses of O3 exposures (discussed below). 
With regard to the available epidemiologic evidence, the Administrator 
notes PA analyses of O3 air quality indicating that, while 
most O3 epidemiologic studies reported health effect 
associations with ambient O3 concentrations that violated 
the current standard, a small number of single-city U.S. studies 
support the occurrence of asthma-related hospital admissions and 
emergency department visits at ambient O3 concentrations 
below the level of the current standard, including one study with air 
quality that would have met the current standard during the study 
period. Some support for such O3 associations is also 
provided by multicity studies of morbidity or mortality. The 
Administrator further judges that the biological plausibility of 
associations with clearly adverse morbidity effects is supported by the 
evidence noted above from controlled human exposure studies conducted 
at, or in some cases below, typical warm-season ambient O3 
concentrations.
    Beyond her consideration of the scientific evidence, the 
Administrator also considers the results of the HREA exposure and risk 
analyses in reaching final conclusions regarding the adequacy of the 
current primary O3 standard. In doing so, consistent with

[[Page 65345]]

her consideration of the evidence, she focuses primarily on 
quantitative analyses based on information from controlled human 
exposure studies (i.e., exposures of concern and risk of O3-
induced FEV1 decrements). Consistent with the considerations 
in the PA, and with CASAC advice (Frey, 2014c), she particularly 
focuses on exposure and risk estimates in children.\108\ As discussed 
in the HREA and PA (and II.B, above), the patterns of exposure and risk 
estimates across urban study areas, across years, and across air 
quality scenarios are similar in children and adults though, because 
children spend more time being physically active outdoors and are more 
likely to experience the types of O3 exposures shown to 
cause respiratory effects, larger percentages of children are estimated 
to experience exposures of concern and O3-induced 
FEV1 decrements. Children also have intrinsic risk factors 
that make them particularly susceptible to O3-related 
effects (e.g., higher ventilation rates relative to lung volume) (U.S. 
EPA, 2013, section 8.3.1.1; see section II.A.1.d above). In focusing on 
exposure and risk estimates in children, the Administrator recognizes 
that the exposure patterns for children across years, urban study 
areas, and air quality scenarios are indicative of the exposure 
patterns in a broader group of at-risk populations that also includes 
asthmatic adults and older adults. She judges that, to the extent the 
primary O3 standard provides appropriate protection for 
children, it will also do so for adult populations,\109\ given the 
larger exposures and intrinsic risk factors in children.
---------------------------------------------------------------------------

    \108\ She focuses on estimates for all children and estimates 
for children with asthma, noting that exposure and risk estimates 
for these groups are virtually indistinguishable in terms of the 
percent estimated to experience exposures of concern or 
O3-induced FEV1 decrements (U.S. EPA, 2014c, 
sections 3.2 and 4.4.2).
    \109\ As noted below (II.C.4.2), this includes populations of 
highly active adults, such as outdoor workers. Limited sensitivity 
analyses in the HREA indicate 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 
(U.S. EPA, 2014, section 5.4.3.2, Figure 5-14).
---------------------------------------------------------------------------

    In first considering estimates of exposures of concern, the 
Administrator considers the extent to which estimates indicate that the 
current standard limits population exposures to the broader range of 
O3 concentrations shown in controlled human exposure studies 
to cause respiratory effects. In doing so, she focuses on estimates of 
O3 exposures of concern at or above the benchmark 
concentrations of 60, 70, and 80 ppb. She notes that the current 
O3 standard can provide some protection against exposures of 
concern to a range of O3 concentrations, including 
concentrations below the standard level, given that (1) with the 
current fourth-high form, most days will have concentrations below the 
standard level and that (2) exposures of concern depend on both the 
presence of relatively high ambient O3 concentrations and on 
activity patterns in the population that result in exposures to such 
high concentrations while at an elevated ventilation rate (discussed in 
detail below, II.C.4.b and II.C.4.c).
    In considering estimates of O3 exposures of concern 
allowed by the current standard, she notes that while single exposures 
of concern could be adverse for some people, particularly for the 
higher benchmark concentrations (70, 80 ppb) where there is stronger 
evidence for the occurrence of adverse effects (II.B.2.b.i, II.C.4.b, 
II.C.4.c, below), she becomes increasingly concerned about the 
potential for adverse responses as the number of occurrences 
increases.\110\ In particular, as discussed above with regard to 
inflammation, she notes that the types of lung injury shown to occur 
following exposures to O3 concentrations from 60 to 80 ppb, 
particularly if experienced repeatedly, provide a mode of action by 
which O3 may cause other more serious effects (e.g., asthma 
exacerbations). Therefore, the Administrator places the most weight on 
estimates of two or more exposures of concern (i.e., as a surrogate for 
the occurrence of repeated exposures), though she also considers 
estimates of one or more exposures for the 70 and 80 ppb benchmarks.
---------------------------------------------------------------------------

    \110\ Not all people who experience an exposure of concern will 
experience an adverse effect (even members of at-risk populations). 
For the endpoints evaluated in controlled human exposure studies, 
the number of those experiencing exposures of concern who will 
experience adverse effects cannot be reliably quantified.
---------------------------------------------------------------------------

    In considering estimates of exposures of concern, the Administrator 
first notes that if the 15 urban study areas evaluated in the HREA were 
to just meet the current O3 standard, fewer than 1% of 
children in those areas would be estimated to experience two or more 
exposures of concern at or above 70 ppb, based on exposure estimates 
averaged over the years of analysis, though up to about 2% would be 
estimated to experience such exposures in the worst-case year and 
location (i.e., year and location with the largest exposure 
estimates).\111\ Although the Administrator is less concerned about 
single occurrences of exposures of concern, she notes that even single 
occurrences could cause adverse effects in some people, particularly 
for the 70 and 80 ppb benchmarks.\112\ As illustrated in Table 1 
(above), the current standard could allow up to about 3% of children to 
experience one or more exposures of concern at or above 70 ppb, 
averaged over the years of analysis, and up to about 8% in the worst-
case year and location. In addition, in the worst-case year and 
location, the current standard could allow about 1% of children to 
experience at least one exposure of concern at or above 80 ppb, the 
highest benchmark evaluated.
---------------------------------------------------------------------------

    \111\ Virtually no children in those areas would be estimated to 
experience two or more exposures of concern at or above 80 ppb.
    \112\ That is, adverse effects are a possible outcome of single 
exposures of concern at/above 70 or 80 ppb, though the available 
information is not sufficient to estimate the likelihood of such 
effects.
---------------------------------------------------------------------------

    While the Administrator has less confidence in the adversity of the 
effects observed following exposures to 60 ppb O3 
(II.B.2.b.i, II.C.4.b, II.C.4.c), particularly for single exposures, 
she judges that the potential for adverse effects increases as the 
number of exposures of concern increases. With regard to the 60 ppb 
benchmark, she particularly notes that the current standard is 
estimated to allow approximately 3 to 8% of children in urban study 
areas, including approximately 3 to 8% of asthmatic children, to 
experience two or more exposures of concern to O3 
concentrations at or above 60 ppb, based on estimates averaged over the 
years of analysis. To provide some perspective on the average 
percentages estimated, the Administrator notes that they correspond to 
almost 900,000 children in urban study areas, including about 90,000 
asthmatic children. Nationally, if the current standard were to be just 
met, the number of children experiencing such exposures would be 
larger.
    Based on her consideration of these estimates within the context of 
her judgments on adversity, as discussed in her responses to public 
comments (II.B.2.b.i, II.C.4.b), the Administrator concludes that the 
exposures projected to remain upon meeting the current standard can 
reasonably be judged to be important from a public health perspective. 
In particular, given that the average percent of children estimated to 
experience two or more exposures of concern for the 60 ppb benchmark 
approaches 10% in some areas, even based on estimates averaged over the

[[Page 65346]]

years of the analysis, she concludes that the current standard does not 
incorporate an adequate margin of safety against the potentially 
adverse effects that can occur following repeated exposures at or above 
60 ppb. Although she has less confidence that the effects observed at 
60 ppb are adverse, compared to the effects at and above 72 ppb, she 
judges that this approach to considering the results for the 60 ppb 
benchmark is appropriate given CASAC advice, which clearly focuses the 
EPA on considering the effects observed at 60 ppb (Frey, 2014c) 
(II.C.4.b, II.C.4.c below).\113\ This approach to considering estimated 
exposures of concern is consistent with setting standards that provide 
some safeguard against dangers to human health that are not fully 
certain (i.e., standards that incorporate an adequate margin of safety) 
(See, e.g., State of Mississippi, 744 F. 3d at 1353).
---------------------------------------------------------------------------

    \113\ Though this advice is less clear regarding the adversity 
of effects at 60 ppb than CASAC's advice regarding the adversity of 
effects at 72 ppb (II.C.4.b, II.C.4.c).
---------------------------------------------------------------------------

    In addition to estimated exposures of concern, the Administrator 
also considers HREA estimates of the risk of O3-induced 
FEV1 decrements >=10 and 15%. In doing so, she particularly 
notes CASAC advice that ``estimation of FEV1 decrements of 
>=15% is appropriate as a scientifically relevant surrogate for adverse 
health outcomes in active healthy adults, whereas an FEV1 
decrement of >=10% is a scientifically relevant surrogate for adverse 
health outcomes for people with asthma and lung disease'' (Frey, 2014c, 
p. 3). The Administrator notes that while single occurrences of 
O3-induced lung function decrements could be adverse for 
some people, as discussed above (II.B.1), she agrees with the judgment 
in past reviews that a more general consensus view of the potential 
adversity of such decrements emerges as the frequency of occurrences 
increases. Therefore, as in the proposal, the Administrator focuses 
primarily on the estimates of two or more O3-induced lung 
function decrements. When averaged over the years evaluated in the 
HREA, the Administrator notes that the current standard is estimated to 
allow about 1 to 3% of children in the 15 urban study areas 
(corresponding to almost 400,000 children) to experience two or more 
O3-induced lung function decrements >=15%, and to allow 
about 8 to 12% of children (corresponding to about 180,000 asthmatic 
children) to experience two or more O3-induced lung function 
decrements >=10%.
    In further considering the HREA results, the Administrator 
considers the epidemiology-based risk estimates. As discussed in the 
proposal, compared to the weight given to HREA estimates of exposures 
of concern and lung function risks, she places relatively less weight 
on epidemiology-based risk estimates. In giving some consideration to 
these risk estimates, as discussed in the proposal and above in the 
EPA's responses to public comments (II.B.2.b.iii), the Administrator 
focuses on the risks associated with O3 concentrations in 
the upper portions of ambient distributions. In doing so, she notes the 
increasing uncertainty associated with the shapes of concentration-
response curves for O3 concentrations in the lower portions 
of ambient distributions and the evidence from controlled human 
exposure studies, which provide the strongest support for 
O3-induced effects following exposures to O3 
concentrations corresponding to the upper portions of typical ambient 
distributions (i.e., 60 ppb and above). Even when considering only 
area-wide O3 concentrations from the upper portions of 
seasonal distributions (i.e., >=40, 60 ppb, Table 3 in the proposal), 
the Administrator notes that the general magnitude of mortality risk 
estimates suggests the potential for a substantial number of 
O3-associated deaths and adverse respiratory events to occur 
nationally, even when the current standard is met (79 FR 75277 and 
II.B.2.c.iii above).
    In addition to the evidence and exposure/risk information discussed 
above, the Administrator also takes note of the CASAC advice in the 
current review, in the 2008 review and decision establishing the 
current standard, and in the 2010 reconsideration of the 2008 decision. 
As discussed in more detail above, the current CASAC ``finds that the 
current NAAQS for ozone is not protective of human health'' and 
``unanimously recommends that the Administrator revise the current 
primary ozone standard to protect public health'' (Frey, 2014c, p. 5). 
The prior CASAC O3 Panel likewise recommended revision of 
the current standard to one with a lower level due to the lack of 
protectiveness of the current standard. This earlier recommendation was 
based entirely on the evidence and information in the record for the 
2008 standard decision, which, as discussed above, has been 
substantially strengthened in the current review (Samet, 2011; Frey and 
Samet, 2012).
    In consideration of all of the above, the Administrator concludes 
that the current primary O3 standard is not requisite to 
protect public health with an adequate margin of safety, and that it 
should be revised to provide increased public health protection. This 
decision is based on the Administrator's conclusions that the available 
evidence and exposure and risk information clearly call into question 
the adequacy of public health protection provided by the current 
primary standard such that it is not appropriate, within the meaning of 
section 109(d)(1) of the CAA, to retain the current standard. With 
regard to the evidence, she particularly notes that the current 
standard level is higher than the lowest O3 exposure 
concentration shown to result in the adverse combination of lung 
function decrements and respiratory symptoms (i.e., 72 ppb), and also 
notes CASAC's advice that at-risk groups (e.g., people with asthma) 
could experience adverse effects following exposure to lower 
concentrations. In addition, while the Administrator is less certain 
about the adversity of the effects that occur following lower exposure 
concentrations, she judges that recent controlled human exposure 
studies at 60 ppb provide support for a level below 75 ppb in order to 
provide an increased margin of safety, compared to the current 
standard, against effects with the potential to be adverse, 
particularly if they are experienced repeatedly. With regard to 
O3 epidemiologic studies, she notes that while most 
available studies reported health effect associations with ambient 
O3 concentrations that violated the current standard, a 
small number provide support for the occurrence of adverse respiratory 
effects at ambient O3 concentrations below the level of the 
current standard.\114\
---------------------------------------------------------------------------

    \114\ Courts have repeatedly held that this type of evidence 
justifies an Administrator's conclusion that it is ``appropriate'' 
(within the meaning of section 109 (d)(1) of the CAA) to revise a 
primary NAAQS to provide further protection of public health. See 
e.g. Mississippi, 744 F. 3d at 1345; American Farm Bureau, 559 F. 3d 
at 525-26.
---------------------------------------------------------------------------

    Based on the analyses in the HREA, the Administrator concludes that 
the exposures and risks projected to remain upon meeting the current 
standard can reasonably be judged to be important from a public health 
perspective. In particular, this conclusion is based on her judgment 
that it is appropriate to set a standard that would be expected to 
eliminate, or almost eliminate, exposures of concern at or above 70 and 
80 ppb. In addition, given that the average percent of children 
estimated to experience two or more exposures of concern for the 60 ppb 
benchmark approaches 10% in some urban study areas, the Administrator 
concludes that the current standard does not incorporate an adequate 
margin of safety

[[Page 65347]]

against the potentially adverse effects that could occur following 
repeated exposures at or above 60 ppb. Beyond estimated exposures of 
concern, the Administrator concludes that the HREA risk estimates 
(FEV1 risk estimates, mortality risk estimates) further 
support a conclusion that the O3-associated health effects 
estimated to remain upon just meeting the current standard are an issue 
of public health importance on a broad national scale. Thus, she 
concludes that O3 exposure and risk estimates, when taken 
together, support a conclusion that the exposures and health risks 
associated with just meeting the current standard can reasonably be 
judged important from a public health perspective, such that the 
current standard is not sufficiently protective and does not 
incorporate an adequate margin of safety.
    In the next section, the Administrator considers what revisions are 
appropriate in order to set a standard that is requisite to protect 
public health with an adequate margin of safety.

C. Conclusions on the Elements of a Revised Primary Standard

    Having reached the conclusion that the current O3 
standard is not requisite to protect public health with an adequate 
margin of safety, based on the currently available scientific evidence 
and exposure/risk information, the Administrator next considers the 
range of alternative standards supported by that evidence and 
information. Consistent with her consideration of the adequacy of the 
current standard, the Administrator's conclusions on the elements of 
the primary standard are informed by the available scientific evidence 
assessed in the ISA, exposure/risk information presented and assessed 
in the HREA, the evidence-based and exposure-/risk-based considerations 
and conclusions in the PA, CASAC advice, and public comments. The 
sections below discuss the evidence and exposure/risk information, 
CASAC advice and public input, and the Administrator's proposed 
conclusions, for the major elements of the NAAQS: Indicator (II.C.1), 
averaging time (II.C.2), form (II.C.3), and level (II.C.4).
1. Indicator
    In the 2008 review, the EPA focused on O3 as the most 
appropriate indicator for a standard meant to provide protection 
against ambient photochemical oxidants. In this review, while the 
complex atmospheric chemistry in which O3 plays a key role 
has been highlighted, no alternatives to O3 have been 
advanced as being a more appropriate indicator for ambient 
photochemical oxidants. More specifically, the ISA noted that 
O3 is the only photochemical oxidant (other than 
NO2) that is routinely monitored and for which a 
comprehensive database exists (U.S. EPA, 2013, section 3.6). Data for 
other photochemical oxidants (e.g., peroxyacetyl nitrate, hydrogen 
peroxide, etc.) typically have been obtained only as part of special 
field studies. Consequently, no data on nationwide patterns of 
occurrence are available for these other oxidants; nor are extensive 
data available on the relationships of concentrations and patterns of 
these oxidants to those of O3 (U.S. EPA, 2013, section 3.6). 
In its review of the second draft PA, CASAC stated ``The indicator of 
ozone is appropriate 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, 2014c, p. ii).
    In addition, the PA notes that meeting an O3 standard 
can be expected to provide some degree of protection against potential 
health effects that may be independently associated with other 
photochemical oxidants, even though such effects are not discernible 
from currently available studies indexed by O3 alone (U.S. 
EPA, 2014c, section 4.1). That is, since the precursor emissions that 
lead to the formation of O3 generally also 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 population exposures to other 
photochemical oxidants. In considering this information, and CASAC's 
advice, the Administrator reached the proposed conclusion that 
O3 remains the most appropriate indicator for a standard 
meant to provide protection against photochemical oxidants.\115\
---------------------------------------------------------------------------

    \115\ The DC Circuit upheld the use of O3 as the 
indicator for photochemical oxidants based on these same 
considerations. American Petroleum Inst. v. Costle, 665 F. 2d 1176, 
1186 (D.C. Cir. 1981).
---------------------------------------------------------------------------

    The EPA received very few comments on the indicator of the primary 
standard. Those who did comment supported the proposed decision to 
retain O3 as the indicator, noting the rationale put forward 
in the preamble to the proposed rule. These commenters generally 
expressed support for retaining the current indicator in conjunction 
with retaining other elements of the current standard, such as the 
averaging time and form. After considering the available evidence, 
CASAC advice, and public comments, the Administrator concludes that 
O3 remains the most appropriate indicator for a standard 
meant to provide protection against photochemical oxidants. Therefore, 
she is retaining O3 as the indicator for the primary 
standard in this final rule.
2. Averaging Time
    The EPA established the current 8-hour averaging time \116\ for the 
primary O3 NAAQS in 1997 (62 FR 38856). The decision on 
averaging time in that review was based on numerous controlled human 
exposure and epidemiologic studies reporting associations between 
adverse respiratory effects and 6- to 8-hour O3 
concentrations (62 FR 38861). The EPA also noted that a standard with a 
maximum 8-hour averaging time is likely to provide substantial 
protection against respiratory effects associated with 1-hour peak 
O3 concentrations. The EPA reached similar conclusions in 
the last O3 NAAQS review and thus, the EPA retained the 8-
hour averaging time in 2008.
---------------------------------------------------------------------------

    \116\ This 8-hour averaging time reflects daily maximum 8-hour 
average O3 concentrations.
---------------------------------------------------------------------------

    In reaching a proposed conclusion on averaging time in the current 
review, the Administrator considered the extent to which the available 
evidence continues to support the appropriateness of a standard with an 
8-hour averaging time (79 FR 75292). Specifically, the Administrator 
considered the extent to which the available information indicates that 
a standard with the current 8-hour averaging time provides appropriate 
protection against short- and long-term O3 exposures. These 
considerations from the proposal are summarized below in sections 
II.C.2.a (short-term) and II.C.2.b (long-term). Section II.C.2.c 
summarizes the Administrator's proposed decision on averaging time. 
Section II.C.2.d discusses comments received on averaging time. Section 
II.C.2.e presents the Administrator's final decision regarding 
averaging time.
a. Short-Term
    As an initial consideration with respect to the most appropriate 
averaging time for the O3 NAAQS, in the proposal the 
Administrator noted that the strongest evidence for O3-
associated health effects is for respiratory effects following short-
term exposures. More specifically, the Administrator noted the ISA 
conclusion that the evidence is ``sufficient to infer a causal 
relationship'' between short-term O3 exposures and 
respiratory effects. The ISA also judges that for short-term 
O3 exposures, the evidence indicates ``likely to be causal'' 
relationships with

[[Page 65348]]

both cardiovascular effects and mortality (U.S. EPA, 2013, section 
2.5.2). Therefore, as in past reviews, the Administrator noted that the 
strength of the available scientific evidence provides strong support 
for a standard that protects the public health against short-term 
exposures to O3.
    In first considering the level of support available for specific 
short-term averaging times, the Administrator noted in the proposal the 
evidence available from controlled human exposure studies. As discussed 
in more detail in Chapter 3 of the PA, substantial health effects 
evidence from controlled human exposure studies demonstrates that a 
wide range of respiratory effects (e.g., pulmonary function decrements, 
increases in respiratory symptoms, lung inflammation, lung 
permeability, decreased lung host defense, and airway 
hyperresponsiveness) occur in healthy adults following 6.6-hour 
exposures to O3 (U.S. EPA, 2013, section 6.2.1.1). Compared 
to studies evaluating shorter exposure durations (e.g., 1-hour), 
studies evaluating 6.6-hour exposures in healthy adults have reported 
respiratory effects at lower O3 exposure concentrations and 
at more moderate levels of exertion.
    The Administrator also noted in the proposal the strength of 
evidence from epidemiologic studies that evaluated a wide variety of 
populations (e.g., including at-risk lifestages and populations, such 
as children and people with asthma, respectively). A number of 
different averaging times have been used in O3 epidemiologic 
studies, with the most common being the max 1-hour concentration within 
a 24-hour period (1-hour max), the max 8-hour average concentration 
within a 24-hour period (8-hour max), and the 24-hour average. These 
studies are assessed in detail in Chapter 6 of the ISA (U.S. EPA, 
2013). Limited evidence from time-series and panel epidemiologic 
studies comparing risk estimates across averaging times does not 
indicate that one exposure metric is more consistently or strongly 
associated with respiratory health effects or mortality, though the ISA 
notes some evidence for ``smaller O3 risk estimates when 
using a 24-hour average exposure metric'' (U.S. EPA, 2013, section 
2.5.4.2; p. 2-31). For single- and multi-day average O3 
concentrations, lung function decrements were associated with 1-hour 
max, 8-hour max, and 24-hour average ambient O3 
concentrations, with no strong difference in the consistency or 
magnitude of association among the averaging times (U.S. EPA, 2013, p. 
6-71). Similarly, in studies of short-term exposure to O3 
and mortality, Smith et al. (2009) and Darrow et al. (2011) have 
reported high correlations between risk estimates calculated using 24-
hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA, 
2013, p. 6-253). Thus, the Administrator noted that the epidemiologic 
evidence alone does not provide a strong basis for distinguishing 
between the appropriateness of 1-hour, 8-hour, and 24-hour averaging 
times.
    Considering the health information discussed above, in the proposal 
the Administrator concluded that an 8-hour averaging time remains 
appropriate for addressing health effects associated with short-term 
exposures to ambient O3. An 8-hour averaging time is similar 
to the exposure periods evaluated in controlled human exposure studies, 
including recent studies that provide evidence for respiratory effects 
following exposures to O3 concentrations below the level of 
the current standard. In addition, epidemiologic studies provide 
evidence for health effect associations with 8-hour O3 
concentrations, as well as with 1-hour and 24-hour concentrations. As 
in previous reviews, the Administrator noted that a standard with an 8-
hour averaging time (combined with an appropriate standard form and 
level) would also be expected to provide substantial protection against 
health effects attributable to 1-hour and 24-hour exposures (e.g., 62 
FR 38861, July 18, 1997). This conclusion is consistent with the advice 
received from CASAC that ``the current 8-hour averaging time is 
justified by the combined evidence from epidemiologic and clinical 
studies'' (Frey, 2014c, p. 6).
b. Long-Term
    The ISA concludes that the evidence for long-term O3 
exposures indicates that there is ``likely to be a causal 
relationship'' with respiratory effects (U.S. EPA, 2013, chapter 7). 
Thus, in this review the Administrator also considers the extent to 
which currently available evidence and exposure/risk information 
suggests that a standard with an 8-hour averaging time can provide 
protection against respiratory effects associated with longer term 
exposures to ambient O3.
    In considering this issue in the 2008 review of the O3 
NAAQS, the Staff Paper noted that ``because long-term air quality 
patterns would be improved in areas coming into attainment with an 8-hr 
standard, the potential risk of health effects associated with long-
term exposures would be reduced in any area meeting an 8-hr standard'' 
(U.S. EPA, 2007, p. 6-57). In the current review, the PA further 
evaluates this issue, with a focus on the long-term O3 
metrics reported to be associated with mortality or morbidity in recent 
epidemiologic studies. As discussed in section 3.1.3 of the PA (U.S. 
EPA, 2014c, section 4.2), much of the recent evidence for such 
associations is based on studies that defined long-term O3 
in terms of seasonal averages of daily maximum 1-hour or 8-hour 
concentrations.
    As an initial consideration, in the proposal the Administrator 
noted the risk results from the HREA for respiratory mortality 
associated with long-term O3 concentrations. These HREA 
analyses indicate that as air quality is adjusted to just meet the 
current 8-hour standard, most urban study areas are estimated to 
experience reductions in respiratory mortality associated with long-
term O3 concentrations based on the seasonal averages of 1-
hour daily maximum O3 concentrations evaluated in the study 
by Jerrett et al. (2009) (U.S. EPA, 2014a, chapter 7).\117\ As air 
quality is adjusted to meet lower alternative standard levels, for 
standards based on 3-year averages of the annual fourth-highest daily 
maximum 8-hour O3 concentrations, respiratory mortality 
risks are estimated to be reduced further in urban study areas. This 
analysis indicates that an O3 standard with an 8-hour 
averaging time, when coupled with an appropriate form and level, can 
reduce respiratory mortality reported to be associated with long-term 
O3 concentrations.
---------------------------------------------------------------------------

    \117\ Though the Administrator also notes important 
uncertainties associated with these risk estimates, as discussed in 
section II.C.3.b of the proposal.
---------------------------------------------------------------------------

    In further considering the study by Jerrett et al. (2009), in the 
proposal the Administrator noted the PA comparison of long-term 
O3 concentrations following model adjustment in urban study 
areas (i.e., adjusted to meet the current and alternative 8-hour 
standards) to the concentrations present in study cities that provided 
the basis for the positive and statistically significant association 
with respiratory mortality. As indicated in Table 4-3 of the PA (U.S. 
EPA, 2014c, section 4.2), this comparison suggests that a standard with 
an 8-hour averaging time can decrease seasonal averages of 1-hour daily 
maximum O3 concentrations, and can maintain those 
O3 concentrations below the seasonal average concentration 
where the study indicates the most confidence in the reported 
concentration-response relationship with respiratory mortality (U.S. 
EPA, 2014c, sections 4.2 and 4.4.1).

[[Page 65349]]

    The Administrator also noted in the proposal that the HREA 
conducted analyses evaluating the impacts of reducing regional 
NOX emissions on the seasonal averages of daily maximum 8-
hour O3 concentrations. Seasonal averages of 8-hour daily 
max O3 concentrations reflect long-term metrics that have 
been reported to be associated with respiratory morbidity effects in 
several recent O3 epidemiologic studies (e.g., Islam et al., 
2008; Lin et al., 2008a, 2008b; Salam et al., 2009). The HREA analyses 
indicate that the large majority of the U.S. population lives in 
locations where reducing NOX emissions would be expected to 
result in decreases in seasonal averages of daily max 8-hour ambient 
O3 concentrations (U.S. EPA, 2014a, chapter 8). Thus, 
consistent with the respiratory mortality risk estimates noted above, 
these analyses suggest that reductions in O3 precursor 
emissions in order to meet a standard with an 8-hour averaging time 
would also be expected to reduce the long-term O3 
concentrations that have been reported in recent epidemiologic studies 
to be associated with respiratory morbidity.
c. Administrator's Proposed Conclusion on Averaging Time
    In the proposal the Administrator noted that, when taken together, 
the analyses summarized above indicate that a standard with an 8-hour 
averaging time, coupled with the current fourth-high form and an 
appropriate level, would be expected to provide appropriate protection 
against the short- and long-term O3 concentrations that have 
been reported to be associated with respiratory morbidity and 
mortality. The CASAC agreed with this conclusion, stating that ``[t]he 
current 8-hour averaging time is justified by the combined evidence 
from epidemiologic and clinical studies'' and that ``[t]he 8-hour 
averaging window also provides protection against the adverse impacts 
of long-term ozone exposures, which were found to be ``likely causal'' 
for respiratory effects and premature mortality'' (Frey, 2014c, p. 6). 
Therefore, considering the available evidence and exposure risk 
information, and CASAC's advice, the Administrator proposed to retain 
the current 8-hour averaging time, and not to set an additional 
standard with a different averaging time.
d. Comments on Averaging Time
    Most public commenters did not address the issue of whether the EPA 
should consider additional or alternative averaging times. Of those who 
did address this issue, some commenters representing state agencies or 
industry groups agreed with the proposed decision to retain the current 
8-hour averaging time, generally noting the supportive evidence 
discussed in the preamble to the proposed rule. In contrast, several 
medical organizations and environmental groups questioned the degree of 
health protection provided by a standard based on an 8-hour averaging 
time. For example, one group asserted that ``[a]veraging over any time 
period, such as 8 hours, is capable of hiding peaks that may be very 
substantial if they are brief enough.''
    The EPA agrees with these commenters that an important issue in the 
current review is the appropriateness of using a standard with an 8-
hour averaging time to protect against adverse health effects that are 
attributable to a wide range of O3 exposure durations, 
including those shorter and longer than 8 hours. This is an issue that 
has been thoroughly evaluated by the EPA in past reviews, as well as in 
the current review.
    The 8-hour O3 NAAQS was originally set in 1997, as part 
of revising the then-existing standard with its 1-hour averaging time, 
and was retained in the review completed in 2008 (73 FR 16472). In both 
of these reviews, several lines of evidence and information provided 
support for an 8-hour averaging time rather than a shorter averaging 
time. For example, substantial health evidence demonstrated 
associations between a wide range of respiratory effects and 6- to 8-
hour exposures to relatively low O3 concentrations (i.e., 
below the level of the 1-hour O3 NAAQS in place prior to the 
review completed in 1997). A standard with an 8-hour averaging time was 
determined to be more directly associated with health effects of 
concern at lower O3 concentrations than a standard with a 1-
hour averaging time. In addition, results of quantitative analyses 
showed that a standard with an 8-hour averaging time can effectively 
limit both 1- and 8-hour exposures of concern, and that an 8-hour 
averaging time results in a more uniformly protective national standard 
than a 1-hour averaging time. In past reviews, CASAC has agreed that an 
8-hour averaging time is appropriate.
    In reaching her proposed decision to retain the 8-hour averaging 
time in the current review, the Administrator again considered the body 
of evidence for adverse effects attributable to a wide range of 
O3 exposure durations, including studies specifically 
referenced by public commenters who questioned the protectiveness of a 
standard with an 8-hour averaging time. For example, as noted above a 
substantial body of health effects evidence from controlled human 
exposure studies demonstrates that a wide range of respiratory effects 
occur in healthy adults following 6.6-hour exposures to O3 
(U.S. EPA, 2013, section 6.2.1.1). Compared to studies evaluating 
shorter exposure durations (e.g., 1-hour), studies evaluating 6.6-hour 
exposures in healthy adults have reported respiratory effects at lower 
O3 exposure concentrations and at more moderate levels of 
exertion. The Administrator also noted the strength of evidence from 
epidemiologic studies that evaluated a number of different averaging 
times, with the most common being the maximum 1-hour concentration 
within a 24-hour period (1-hour max), the maximum 8-hour average 
concentration within a 24-hour period (8-hour max), and the 24-hour 
average. Evidence from time-series and panel epidemiologic studies 
comparing risk estimates across averaging times does not indicate that 
one exposure metric is more consistently or strongly associated with 
respiratory health effects or mortality (U.S. EPA, 2013, section 
2.5.4.2; p. 2-31). For single- and multi-day average O3 
concentrations, lung function decrements were associated with 1-hour 
max, 8-hour max, and 24-hour average ambient O3 
concentrations, with no strong difference in the consistency or 
magnitude of association among the averaging times (U.S. EPA, 2013, p. 
6-71). Similarly, in studies of short-term exposure to O3 
and mortality, Smith et al. (2009) and Darrow et al. (2011) have 
reported high correlations between risk estimates calculated using 24-
hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA, 
2013, p. 6-253). Thus, the epidemiologic evidence does not provide a 
strong basis for distinguishing between the appropriateness of 1-hour, 
8-hour, and 24-hour averaging times.
    In addition, quantitative exposure and risk analyses in the HREA 
are based on an air quality adjustment approach that estimates hourly 
O3 concentrations, and on scientific studies that evaluated 
health effects attributable to a wide range of O3 exposure 
durations. For example, the risk of lung function decrements is 
estimated using a model based on controlled human exposure studies with 
exposure durations ranging from 2 to 7.6 hours (U.S. EPA, 2013, section 
6.2.1.1). Epidemiology-based risk estimates are based on studies that 
reported health effect associations with short-term ambient 
O3 concentrations ranging from 1-hour to 24-hours and with 
long-term seasonal average concentrations (U.S. EPA, 2014a, Table 7-2). 
Thus, the HREA estimated health

[[Page 65350]]

risks associated with a wide range of O3 exposure durations 
and the Administrator's conclusions on averaging time in the current 
review are based, in part, on consideration of these estimates.
    When taken together, the evidence and analyses indicate that a 
standard with an 8-hour averaging time, coupled with the current 
fourth-high form and an appropriate level, would be expected to provide 
appropriate protection against the short- and long-term O3 
concentrations that have been reported to be associated with 
respiratory morbidity and mortality. The CASAC agreed with this, 
stating the following (Frey, 2014c, p. 6):

    The current 8-hour averaging time is justified by the combined 
evidence from epidemiologic and clinical studies referenced in 
Chapter 4. Results from clinical studies, for example, show a wide 
range of respiratory effects in healthy adults following 6.6 hours 
of exposure to ozone, including pulmonary function decrements, 
increases in respiratory symptoms, lung inflammation, lung 
permeability, decreased lung host defense, and airway 
hyperresponsiveness. These findings are supported by evidence from 
epidemiological studies that show causal associations between short-
term exposures of 1, 8 and 24-hours and respiratory effects and 
``likely to be causal'' associations for cardiovascular effects and 
premature mortality. The 8-hour averaging window also provides 
protection against the adverse impacts of long-term ozone exposures, 
which were found to be ``likely causal'' for respiratory effects and 
premature mortality.

Given all of the above, the EPA disagrees with commenters who question 
the protectiveness of an O3 standard with an 8-hour 
averaging time, particularly for an 8-hour standard with the revised 
level of 70 ppb that is being established in this review, as discussed 
below (II.C.4).
e. Administrator's Final Decision Regarding Averaging Time
    In considering the evidence and information summarized in the 
proposal and discussed in detail in the ISA, HREA, and PA; CASAC's 
views; and public comments, the Administrator concludes that a standard 
with an 8-hour averaging time can effectively limit health effects 
attributable to both short- and long-term O3 exposures. As 
was the case in the proposal, this final conclusion is based on (1) the 
strong evidence that continues to support the importance of protecting 
public health against short-term O3 exposures (e.g., <= 1-
hour to 24-hour) and (2) analyses in the HREA and PA supporting the 
conclusion that the current 8-hour averaging time can effectively limit 
long-term O3 exposures. Furthermore, the Administrator 
observes that the CASAC Panel agreed with the choice of averaging time 
(Frey, 2014c). Therefore, in the current review, the Administrator 
concludes that it is appropriate to retain the 8-hour averaging time 
and to not set a separate standard with a different averaging time in 
this final rule.
3. Form
    The ``form'' of a standard defines the air quality statistic that 
is to be compared to the level of the standard in determining whether 
an area attains that standard. The foremost consideration in selecting 
a form is the adequacy of the public health protection provided by the 
combination of the form and the other elements of the standard. In this 
review, the Administrator considers the extent to which the available 
evidence and/or information continue to support the appropriateness of 
a standard with the current form, defined by the 3-year average of 
annual fourth-highest 8-hour daily maximum O3 
concentrations. Section II.C.3.a below summarizes the basis for the 
current form. Section II.C.3.b discusses the Administrator's proposed 
decision to retain the current form. Section II.C.3.c discusses public 
comments received on the form of the primary standard. Section II.C.3.d 
discusses the Administrator's final decision on form.
a. Basis for the Current Form
    The EPA established the current form of the primary O3 
NAAQS in 1997 (62 FR 38856). Prior to that time, the standard had a 
``1-expected-exceedance'' form.\118\ An advantage of the current 
concentration-based form recognized in the 1997 review is that such a 
form better reflects the continuum of health effects associated with 
increasing ambient O3 concentrations. Unlike an expected 
exceedance form, a concentration-based form gives proportionally more 
weight to years when 8-hour O3 concentrations are well above 
the level of the standard than years when 8-hour O3 
concentrations are just above the level of the standard.\119\ The EPA 
judged it appropriate to give more weight to higher O3 
concentrations, given that available health evidence indicated a 
continuum of effects associated with exposures to varying 
concentrations of O3, and given that the extent to which 
public health is affected by exposure to ambient O3 is 
related to the actual magnitude of the O3 concentration, not 
just whether the concentration is above a specified level.
---------------------------------------------------------------------------

    \118\ For a standard with a 1-expected-exceedance form to be met 
at an air quality monitoring site, the fourth-highest air quality 
value in 3 years, given adjustments for missing data, must be less 
than or equal to the level of the standard.
    \119\ As discussed (61 FR 65731), this is because with an 
exceedance-based form, days on which the ambient O3 
concentration is well above the level of the standard are given 
equal weight to those days on which the O3 concentration 
is just above the standard (i.e., each day is counted as one 
exceedance), even though the public health impact of such days would 
be very different. With a concentration-based form, days on which 
higher O3 concentrations occur would weigh proportionally 
more than days with lower O3 concentrations since the 
actual concentrations are used directly to calculate whether the 
standard is met or violated.
---------------------------------------------------------------------------

    During the 1997 review, the EPA considered a range of alternative 
``concentration-based'' forms, including the second-, third-, fourth- 
and fifth-highest daily maximum 8-hour concentrations in an 
O3 season. The fourth-highest daily maximum was selected, 
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 38856). The EPA also considered setting a standard with 
a form that would provide a margin of safety against possible but 
uncertain chronic effects, and would provide greater stability to 
ongoing control programs.\120\ A more restrictive form was not 
selected, recognizing that the differences in the degree of protection 
afforded by the alternatives were not well enough understood to use any 
such differences as a basis for choosing the most restrictive forms (62 
FR 38856).
---------------------------------------------------------------------------

    \120\ See American Trucking Assn's v. EPA, 283 F. 3d at 374-75 
(less stable implementation programs may be less effective and would 
thereby provide less public health protection; EPA may therefore 
legitimately consider programmatic stability in determining the form 
of a NAAQS).
---------------------------------------------------------------------------

    In the 2008 review, the EPA additionally considered the potential 
value of a percentile-based form. In doing so, the EPA recognized that 
such a statistic is useful for comparing datasets of varying length 
because it samples approximately the same place in the distribution of 
air quality values, whether the dataset is several months or several 
years long. However, the EPA concluded that a percentile-based 
statistic would not be effective in ensuring the same degree of public 
health protection across the country. Specifically, a percentile-based 
form would allow more days with higher air quality values in locations 
with longer O3 seasons relative to locations with shorter 
O3 seasons. Thus, in the 2008 review, the EPA concluded that 
a form based on the nth-highest maximum O3 concentration 
would more effectively ensure that people who live in areas

[[Page 65351]]

with different length O3 seasons receive the same degree of 
public health protection.
    Based on analyses of forms specified in terms of an nth-highest 
concentration (n ranged from 3 to 5), advice from CASAC, and public 
comment, the Administrator concluded that a fourth-highest daily 
maximum should be retained (73 FR 16465, March 27, 2008). In reaching 
this decision, the Administrator recognized that ``there is not a clear 
health-based threshold for selecting a particular nth-highest daily 
maximum form of the standard'' and that ``the adequacy of the public 
health protection provided by the combination of the level and form is 
a foremost consideration'' (73 FR 16475, March 27, 2008). Based on 
this, the Administrator judged that the existing form (fourth-highest 
daily maximum 8-hour average concentration) should be retained, 
recognizing the increase in public health protection provided by 
combining this form with a lower standard level (i.e., 75 ppb).
    The Administrator also recognized that it is important to have a 
form that provides stability with regard to implementation of the 
standard. In the case of O3, for example, he noted the 
importance of a form insulated from the impacts of extreme 
meteorological events that are conducive to O3 formation. 
Such events could have the effect of reducing public health protection, 
to the extent they result in frequent shifts in and out of attainment 
due to meteorological conditions. The Administrator noted that such 
frequent shifting could disrupt an area's ongoing implementation plans 
and associated control programs (73 FR 16474, March 27, 2008). In his 
final decision, the Administrator judged that a fourth-high form 
``provides a stable target for implementing programs to improve air 
quality'' (id. at 16475).
b. Proposed Decision on Form
    In the proposal for the current review, the Administrator 
considered the extent to which newly available information provides 
support for the current form (79 FR 75293). In so doing, she took note 
of the conclusions of prior reviews summarized above. She recognized 
the value of an nth-high statistic over that of an expected exceedance 
or percentile-based form in the case of the O3 standard, for 
the reasons summarized above. The Administrator additionally took note 
of the importance of stability in implementation to achieving the level 
of protection specified by the NAAQS. Specifically, she noted that to 
the extent areas engaged in implementing the O3 NAAQS 
frequently shift from meeting the standard to violating the standard, 
it is possible that ongoing implementation plans and associated control 
programs could be disrupted, thereby reducing public health protection.
    In light of this, while giving foremost consideration to the 
adequacy of public health protection provided by the combination of all 
elements of the standard, including the form, the Administrator 
considered particularly the findings from prior reviews with regard to 
the use of the nth-high metric. As noted above, the EPA selected the 
fourth-highest daily maximum, recognizing the public health protection 
provided by this form, when coupled with an appropriate averaging time 
and level, and recognizing that such a form can provide stability for 
implementation programs. In the proposal the Administrator concluded 
that the currently available evidence and information do not call into 
question these conclusions from previous reviews. In reaching this 
initial conclusion, the Administrator noted that CASAC concurred that 
the O3 standard should be based on the fourth-highest, daily 
maximum 8-hour average value (averaged over 3 years), stating 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, 2014c, p. 6). Thus, a standard with the current fourth-high 
form, coupled with a level lower than 75 ppb as discussed below, would 
be expected to increase public health protection relative to the 
current standard while continuing to provide stability for 
implementation programs. Therefore, the Administrator proposed to 
retain the current fourth-highest daily maximum form for an 
O3 standard with an 8-hour averaging time and a revised 
level.
c. Public Comments on Form
    Several commenters focused on the stability of the standard to 
support their positions regarding form. Some industry associations and 
state agencies support changing to a form that would allow a larger 
number of exceedances of the standard level than are allowed by the 
current fourth-high form. In some cases, these commenters argued that a 
standard allowing a greater number of exceedances would provide the 
same degree of public health protection as the current standard. Some 
commenters advocated a percentile-based form, such as the 98th 
percentile. These commenters cited a desire for consistency with short-
term standards for other criteria pollutants (e.g., PM2.5, 
NO2), as well as a desire to allow a greater number of 
exceedances of the standard level, thus making the standard less 
sensitive to fluctuations in background O3 concentrations 
and to extreme meteorological events.
    Other commenters submitted analyses purporting to indicate that a 
fourth-high form provides only a small increase in stability, relative 
to forms that allow fewer exceedances of the standard level (i.e., 
first-high, second-high). These commenters also called into question 
the degree of health protection achieved by a standard with a fourth-
high form and a level in the proposed range (i.e., 65 to 70 ppb). They 
pointed out that a fourth-high form will, by definition, allow 3 days 
per year, on average, with 8-hour O3 concentrations above 
the level of the standard. Commenters further stated that ``[i]f ozone 
levels on these peak days are appreciably higher than on the fourth-
highest day, given EPA's acknowledged concerns regarding single or 
multiple (defined by EPA as 2 or more) exposures to elevated ozone 
concentrations, EPA must account for the degree of under-protection in 
setting the level of the NAAQS'' (e.g., ALA et al., p. 138).
    For the reasons discussed in the proposal, and summarized above, 
the EPA disagrees with commenters who supported a percentile-based 
form, such as the 98th percentile, for the O3 NAAQS. As 
noted above, a percentile-based statistic would not be effective in 
ensuring the same degree of public health protection across the 
country. Rather, a percentile-based form would allow more days with 
higher air quality values in locations with longer O3 
seasons relative to locations with shorter O3 seasons. Thus, 
as in the 2008 review, in the current review the EPA concludes that a 
form based on the nth-highest maximum O3 concentration would 
more effectively ensure that people who live in areas with different 
length O3 seasons receive the same degree of public health 
protection.
    In considering various nth-high values, as in past reviews (e.g., 
73 FR 16475, March 27, 2008), the EPA recognizes that there is not a 
clear health-based threshold for selecting a particular nth-highest 
daily maximum form. Rather, the primary consideration is the adequacy 
of the public health protection provided by the combination of all of 
the elements of the standard, including the form. Environmental and 
public health commenters are correct that a standard with the current 
fourth-high form will allow 3 days per year, on average, with 8-hour 
O3 concentrations higher than the standard level. However, 
the EPA disagrees with these

[[Page 65352]]

commenters' assertion that using a fourth-high form results in a 
standard that is under-protective. The O3 exposure and risk 
estimates that informed the Administrator's consideration of the degree 
of public health protection provided by various standard levels were 
based on air quality that ``just meets'' various standards with the 
current 8-hour averaging time and fourth-high, 3-year average form 
(U.S. EPA, 2014a, section 4.3.3). Therefore, air quality adjusted to 
meet various levels of the standard with the current form and averaging 
time will include days with concentrations above the level of the 
standard, and these days contribute to exposure and risk estimates. In 
this way, the Administrator has reasonably considered the public health 
protection provided by the combination of all of the elements of the 
standard, including the fourth-high form.
    In past reviews, EPA selected the fourth-highest daily maximum form 
in recognition of the public health protection provided by this form, 
when coupled with an appropriate averaging time and level, and 
recognizing that such a form can provide stability for ongoing 
implementation programs. As noted above, some commenters submitted 
analyses suggesting that a fourth-high form provides only a small 
increase in stability, relative to a first- or second-high form. The 
EPA has conducted analyses of ambient O3 monitoring data to 
further consider these commenters' assertions regarding stability. The 
EPA's analyses of nth-high concentrations ranging from first-high to 
fifth-high have been summarized in a memo to the docket (Wells, 2015a). 
Consistent with commenters' analyses, Wells (2015a) indicates a 
progressive decrease in the variability of O3 
concentrations, and an increase in the stability of those 
concentrations, as ``n'' increases. Based on these analyses, there is 
no clear threshold for selecting a particular nth-high form based on 
stability alone. Rather, as in past reviews, the decision on form in 
this review focuses first and foremost on the Administrator's judgments 
on public health protection, with judgments regarding stability of the 
standard being a legitimate, but secondary consideration. The 
Administrator's final decision on form is discussed below.
d. Administrator's Final Decision Regarding Form
    In reaching a final decision on the form of the primary 
O3 standard, as described in the proposal and above, the 
Administrator recognizes that there is not a clear health-based 
rationale for selecting a particular nth-highest daily maximum form. 
Her foremost consideration is the adequacy of the public health 
protection provided by the combination of all of the elements of the 
standard, including the form. In this regard, the Administrator 
recognizes the support from analyses in previous reviews, and from the 
CASAC in the current review, for the conclusion that the current 
fourth-high form of the standard, when combined with a revised level as 
discussed below, provides an appropriate balance between public health 
protection and a stable target for implementing programs to improve air 
quality. In particular, she notes that the CASAC concurred that the 
O3 standard should be based on the fourth-highest, daily 
maximum 8-hour average value (averaged over 3 years), stating 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, 2014c, p. 6). Based on these considerations, and on 
consideration of public comments on form as discussed above, the 
Administrator judges it appropriate to retain the current fourth-high 
form (fourth-highest daily maximum 8-hour O3 concentration, 
averaged over 3 years) in this final rule.
4. Level
    This section summarizes the basis for the Administrator's proposed 
decision to revise the current standard level (II.C.4.a); discusses 
public comments, and the EPA's responses, on that proposed decision 
(II.C.4.b); and presents the Administrator's final decision regarding 
the level of the primary O3 standard (II.C.4.c).
a. Basis for the Administrator's Proposed Decision on Level
    In conjunction with her proposed decisions to retain the current 
indicator, averaging time, and form (II.C.1 to II.C.3, above), the 
Administrator proposed to revise the level of the primary O3 
standard to within the range of 65 to 70 ppb. In proposing this range 
of standard levels, as discussed in section II.E.4 of the proposal, the 
Administrator carefully considered the scientific evidence assessed in 
the ISA (U.S. EPA, 2013); the results of the exposure and risk 
assessments in the HREA (U.S. EPA, 2014a); the evidence-based and 
exposure-/risk-based considerations and conclusions in the PA (U.S. 
EPA, 2014c); CASAC advice and recommendations, as reflected in CASAC's 
letters to the Administrator and in public discussions of drafts of the 
ISA, HREA, and PA (Frey and Samet, 2012; Frey, 2014 a, c); and public 
input received during the development of these documents.
    The Administrator's proposal to revise the standard level built 
upon her proposed conclusion that the overall body of scientific 
evidence and exposure/risk information calls into question the adequacy 
of public health protection afforded by the current primary 
O3 standard, particularly for at-risk populations and 
lifestages. In reaching proposed conclusions on alternative levels for 
the primary O3 standard, the Administrator considered the 
extent to which various alternatives would be expected to protect the 
public, including at-risk populations, against the wide range of 
adverse health effects that have been linked with short- or long-term 
O3 exposures.
    As was the case for her consideration of the adequacy of the 
current primary O3 standard (II.B.3, above), the 
Administrator placed the greatest weight on the results of controlled 
human exposure studies and on exposure and risk analyses based on 
information from these studies. In doing so, she noted that controlled 
human exposure studies provide the most certain evidence indicating the 
occurrence of health effects in humans following exposures to specific 
O3 concentrations. The effects reported in these studies are 
due solely to O3 exposures, and interpretation of study 
results is not complicated by the presence of co-occurring pollutants 
or pollutant mixtures (as is the case in epidemiologic studies). She 
further noted the CASAC judgment that ``the scientific evidence 
supporting the finding that the current standard is inadequate to 
protect public health is strongest based on the controlled human 
exposure studies of respiratory effects'' (Frey, 2014c, p. 5).
    In considering the evidence from controlled human exposure studies, 
the Administrator first noted that the largest respiratory effects, and 
the broadest range of effects, have been studied and reported following 
exposures to 80 ppb O3 or higher, with most exposure studies 
conducted at these higher concentrations. Exposures of healthy adults 
to O3 concentrations of 80 ppb or higher have been reported 
to decrease lung function, increase airway inflammation, increase 
respiratory symptoms, result in airway hyperresponsiveness, and 
decrease lung host defenses. The Administrator further noted that 
O3 exposure concentrations as low as 72 ppb have been shown 
to both decrease lung function and increase respiratory

[[Page 65353]]

symptoms (Schelegle et al., 2009),\121\ a combination that meets the 
ATS criteria for an adverse response, and that exposures as low as 60 
ppb have been reported to decrease lung function and increase airway 
inflammation.
---------------------------------------------------------------------------

    \121\ As noted above, for the 70 ppb target exposure 
concentration, Schelegle et al. (2009) reported that the actual mean 
exposure concentration was 72 ppb.
---------------------------------------------------------------------------

    Based on this evidence, the Administrator reached the initial 
conclusion that the results of controlled human exposure studies 
strongly support setting the level of a revised O3 standard 
no higher than 70 ppb. In reaching this conclusion, she placed a large 
amount of weight on the importance of setting the level of the standard 
well below 80 ppb, the exposure concentration at which the broadest 
range of effects have been studied and reported, and below 72 ppb, the 
lowest exposure concentration shown to result in the adverse 
combination of lung function decrements and respiratory symptoms. She 
placed significant weight on this combination of effects, as did CASAC, 
in making judgments regarding the potential for adverse responses.
    In further considering the potential public health implications of 
a standard with a level of 70 ppb, the Administrator also considered 
quantitative estimates of the extent to which such a standard would be 
expected to limit population exposures to the broader range of 
O3 concentrations shown in controlled human exposure studies 
to cause respiratory effects. In doing so, she focused on estimates of 
O3 exposures of concern at or above the benchmark 
concentrations of 60, 70, and 80 ppb. The Administrator judged that the 
evidence supporting the occurrence of adverse respiratory effects is 
strongest for exposures at or above the 70 and 80 ppb benchmarks. 
Therefore, she placed a large amount of emphasis on the importance of 
setting a standard that limits exposures of concern at or above these 
benchmarks.
    The Administrator expressed less confidence that adverse effects 
will occur following exposures to O3 concentrations as low 
as 60 ppb. In reaching this conclusion, she highlighted the fact that 
statistically significant increases in respiratory symptoms, combined 
with lung function decrements, have not been reported following 
exposures to 60 or 63 ppb O3, though several studies have 
evaluated the potential for such effects (Kim et al., 2011; Schelegle 
et al., 2009; Adams, 2006).\122\ The proposal specifically stated that 
``[t]he Administrator has decreasing confidence that adverse effects 
will occur following exposures to O3 concentrations below 72 
ppb. In particular, compared to O3 exposure concentrations 
at or above 72 ppb, she has less confidence that adverse effects will 
occur following exposures to O3 concentrations as low as 60 
ppb'' (79 FR 73304-05).
---------------------------------------------------------------------------

    \122\ In the study by Schelegle, for the 60 ppb target exposure 
concentration, study authors reported that the actual mean exposure 
concentration was 63 ppb.
---------------------------------------------------------------------------

    However, she noted the possibility for adverse effects following 
such exposures given that: (1) CASAC judged the adverse combination of 
lung function decrements and respiratory symptoms ``almost certainly 
occur in some people'' following exposures to O3 
concentrations below 72 ppb (though CASAC did not specify or otherwise 
indicate how far below) (Frey, 2014c, p. 6); (2) CASAC indicated the 
moderate lung function decrements (i.e., FEV1 decrements >= 
10%) that occur in some healthy adults following exposures to 60 ppb 
O3 could be adverse to people with lung disease; and (3) 
airway inflammation has been reported following exposures as low as 60 
ppb O3. She also took note of CASAC advice that the 
occurrence of exposures of concern at or above 60 ppb is an appropriate 
consideration for people with asthma (Frey, 2014c, p. 6). Therefore, 
while the Administrator expressed less confidence that adverse effects 
will occur following exposures to O3 concentrations as low 
as 60 ppb, compared to 70 ppb and above, based on the evidence and 
CASAC advice she also gave some consideration to exposures of concern 
for the 60 ppb benchmark.
    Due to interindividual variability in responsiveness, the 
Administrator further noted that not every occurrence of an exposure of 
concern will result in an adverse effect, and that repeated occurrences 
of some of the effects demonstrated following exposures of concern 
could increase the likelihood of adversity (U.S. EPA, 2013, section 
6.2.3). Therefore, the Administrator was most concerned about 
protecting at-risk populations against repeated occurrences of 
exposures of concern. Based on the above considerations, the 
Administrator focused on the extent to which a revised standard with a 
level of 70 ppb would be expected to protect populations from 
experiencing two or more O3 exposures of concern (i.e., as a 
surrogate for repeated exposures).
    As illustrated in Table 1 in the proposal (and Table 1 above), the 
Administrator noted that, in urban study areas, 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 and above 
80 ppb and to virtually eliminate the occurrence of two or more 
exposures of concern to O3 concentrations at and above 70 
ppb, even in the worst-case urban study area and year evaluated. Though 
the Administrator acknowledged greater uncertainty with regard to the 
occurrence of adverse effects following exposures to 60 ppb, she noted 
that a revised standard with a level of 70 ppb would also be expected 
to protect the large majority of children in the urban study areas 
(i.e., about 96% to more than 99% of children in individual urban study 
areas) from experiencing two or more exposures of concern at or above 
the 60 ppb benchmark. Compared to the current standard, this represents 
a reduction of more than 60%.\123\
---------------------------------------------------------------------------

    \123\ The Administrator judged that the evidence is less 
compelling, and indicates greater uncertainty, with regard to the 
potential for adverse effects following single occurrences of 
O3 exposures of concern. While acknowledging this greater 
uncertainty, she noted that a standard with a level of 70 ppb would 
also be expected to virtually eliminate all occurrences (including 
single occurrences) of exposures of concern at or above 80 ppb, even 
in the worst-case year and location. She also judged that such a 
standard will achieve important reductions, compared to the current 
standard, in the occurrence of one or more exposures of concern at 
or above 70 and 60 ppb.
---------------------------------------------------------------------------

    In further evaluating the potential public health impacts of a 
standard with a level of 70 ppb, the Administrator also considered the 
HREA estimates of O3-induced lung function decrements. To 
inform her consideration of these decrements, the Administrator took 
note of CASAC advice that ``estimation of FEV1 decrements of 
>= 15% is appropriate as a scientifically relevant surrogate for 
adverse health outcomes in active healthy adults, whereas an 
FEV1 decrement of >= 10% is a scientifically relevant 
surrogate for adverse health outcomes for people with asthma and lung 
disease'' (Frey, 2014c, p. 3).
    Although these FEV1 decrements provide perspective on 
the potential for the occurrence of adverse respiratory effects 
following O3 exposures, the Administrator agreed with the 
conclusion in past reviews that a more general consensus view of the 
adversity of moderate responses emerges as the frequency of occurrence 
increases (61 FR 65722-3, Dec, 13, 1996). Specifically, she judged that 
not every estimated occurrence of an O3-induced 
FEV1 decrement will be adverse and

[[Page 65354]]

that repeated occurrences of moderate responses could lead to more 
serious illness. Therefore, the Administrator noted increasing concern 
about the potential for adversity as the number of occurrences 
increases and, as a result, she focused primarily on estimates of two 
or more O3-induced FEV1 decrements (i.e., as a 
surrogate for repeated exposures).\124\
---------------------------------------------------------------------------

    \124\ In the proposal, the Administrator further judged that it 
would not be appropriate to set a standard that is intended to 
eliminate all O3-induced FEV1 decrements. She 
noted that this is consistent with CASAC advice, which did not 
include a recommendation to set the standard level low enough to 
eliminate all O3-induced FEV1 decrements 
= 10 or 15% (Frey, 2014c).
---------------------------------------------------------------------------

    The Administrator noted that a revised O3 standard with 
a level of 70 ppb is estimated to protect about 98 to 99% of children 
in urban study areas from experiencing two or more O3-
induced FEV1 decrements =15%, and about 89 to 94% 
from experiencing two or more decrements =10%. She judged 
that these estimates reflect important risk reductions, compared to the 
current standard. Given these estimates, as well as estimates of one or 
more decrements per season (about which she was less concerned (79 FR 
75290, December 17, 2014)), the Administrator concluded that a revised 
standard with a level of 70 ppb would be expected to provide 
substantial protection against the risk of O3-induced lung 
function decrements, and would be expected to result in important 
reductions in such risks, compared to the current standard. The 
Administrator further noted, however, that the variability in lung 
function risk estimates across urban study areas is often greater than 
the differences in risk estimates between various standard levels 
(Table 2, above). Given this, and the resulting considerable overlap 
between the ranges of lung function risk estimates for different 
standard levels, in the proposal the Administrator viewed lung function 
risk estimates as providing a more limited basis than exposures of 
concern for distinguishing between the degrees of public health 
protection provided by alternative standard levels (79 FR 75306 n. 
164).
    In next considering the additional protection that would be 
expected from standard levels below 70 ppb, the Administrator evaluated 
the extent to which a standard with a level of 65 ppb would be expected 
to further limit O3 exposures of concern and O3-
induced lung function decrements. In addition to eliminating almost all 
exposures of concern to O3 concentrations at or above 80 and 
70 ppb, even in the worst-case years and locations, the Administrator 
noted that a revised standard with a level of 65 ppb would be expected 
to protect more than 99% of children in urban study areas from 
experiencing two or more exposures of concern at or above 60 ppb and to 
substantially reduce the occurrence of one or more such exposures, 
compared to the current standard. With regard to O3-induced 
lung function decrements, an O3 standard with a level of 65 
ppb is estimated to protect about 98% to more than 99% of children from 
experiencing two or more O3-induced FEV1 
decrements =15% and about 91 to 99% from experiencing two or 
more decrements =10%.\125\
---------------------------------------------------------------------------

    \125\ Although the Administrator was less concerned about the 
public health implications of single O3-induced lung 
function decrements, she also noted that a revised standard with a 
level of 65 ppb is estimated to reduce the risk of one or more 
O3-induced decrements per season, compared to the current 
standard.
---------------------------------------------------------------------------

    Taken together, the Administrator concluded that the evidence from 
controlled human exposure studies, and the information from 
quantitative analyses that draw upon these studies, provide strong 
support for standard levels from 65 to 70 ppb. In particular, she based 
this conclusion on the fact that such standard levels would be well 
below the O3 exposure concentration shown to result in the 
widest range of respiratory effects (i.e., 80 ppb),\126\ and below the 
lowest O3 exposure concentration shown to result in the 
adverse combination of lung function decrements and respiratory 
symptoms (i.e., 72 ppb). A standard with a level from 65 to 70 ppb 
would also be expected to result in important reductions, compared to 
the current standard, in the occurrence of O3 exposures of 
concern for all of the benchmarks evaluated (i.e., 60, 70, and 80 ppb) 
and in the risk of O3-induced lung function decrements 
=10 and 15%.
---------------------------------------------------------------------------

    \126\ Although the widest range of effects have been evaluated 
following exposures to 80 ppb O3, there is no evidence 
that 80 ppb is a threshold for these effects.
---------------------------------------------------------------------------

    In further considering the evidence and exposure/risk information, 
the Administrator considered the extent to which the epidemiologic 
evidence also provides support for standard levels from 65 to 70 ppb. 
In particular, the Administrator noted analyses in the PA (U.S. EPA, 
2014c, section 4.4.1) indicating that a revised standard with a level 
of 65 or 70 ppb would be expected to maintain distributions of short-
term ambient O3 concentrations below those present in the 
locations of all the single-city epidemiologic studies of hospital 
admissions or emergency department visits analyzed. She concluded that 
a revised standard with a level at least as low as 70 ppb would result 
in improvements in public health, beyond the protection provided by the 
current standard, in the locations of the single-city epidemiologic 
studies that reported significant health effect associations.\127\
---------------------------------------------------------------------------

    \127\ The Administrator also concluded that analyses in the HREA 
and PA indicate that a standard with an 8-hour averaging time, 
coupled with the current fourth-high form and a level from 65 to 70 
ppb, would be expected to provide increased protection, compared to 
the current standard, against the long-term O3 
concentrations that have been reported to be associated with 
respiratory morbidity or mortality (79 FR 75293; 75308).
---------------------------------------------------------------------------

    The Administrator noted additional uncertainty in interpreting air 
quality in locations of multicity epidemiologic studies of short-term 
O3 for the purpose of evaluating alternative standard levels 
(II.D.1 and U.S. EPA, 2014c, section 4.4.1). While acknowledging this 
uncertainty, and therefore placing less emphasis on these analyses of 
study location air quality, she noted that PA analyses suggest that 
standard levels of 65 or 70 ppb would require reductions, beyond those 
required by the current standard, in ambient O3 
concentrations present in several of the locations that provided the 
basis for statistically significant O3 health effect 
associations in multicity studies.
    In further evaluating information from epidemiologic studies, the 
Administrator considered the HREA's epidemiology-based risk estimates 
for O3-associated morbidity or mortality (U.S. EPA, 2014a, 
Chapter 7). Compared to the weight given to the evidence from 
controlled human exposure studies, and to HREA estimates of exposures 
of concern and lung function risks, she placed relatively less weight 
on epidemiology-based risk estimates. In doing so, she noted that the 
overall conclusions from the HREA likewise reflect relatively less 
confidence in estimates of epidemiology-based risks than in estimates 
of exposures of concern and lung function risks.
    In considering epidemiology-based risk estimates, the Administrator 
focused on risks associated with O3 concentrations in the 
upper portions of ambient distributions, given the greater uncertainty 
associated with the shapes of concentration-response curves for 
O3 concentrations in the lower portions of ambient 
distributions (i.e., below about 20 to 40 ppb depending on the 
O3 metric, health endpoint, and study population) (U.S. EPA, 
2013, section 2.5.4.4). The Administrator further noted that 
experimental studies provide the strongest evidence for O3-
induced effects following exposures to O3 concentrations 
corresponding to the upper portions of typical ambient

[[Page 65355]]

distributions. In particular, as discussed above, she noted controlled 
human exposure studies showing respiratory effects following exposures 
to O3 concentrations at or above 60 ppb (79 FR 75308, 
December 17, 2014). Therefore, in considering risks associated with 
O3 concentrations in the upper portions of ambient 
distributions, the Administrator focused on the extent to which revised 
standards with levels of 70 or 65 ppb are estimated to reduce the risk 
of premature deaths associated with area-wide O3 
concentrations at or above 40 ppb and 60 ppb.
    Given all of the above evidence, exposure/risk information, and 
advice from CASAC, the Administrator proposed to revise the level of 
the current primary O3 standard to within the range of 65 to 
70 ppb. In considering CASAC advice on the range of standard levels, 
the Administrator placed a large amount of weight on CASAC's conclusion 
that there is adequate scientific evidence to consider a range of 
levels for a primary standard that includes an upper end at 70 ppb. She 
also noted that although CASAC expressed concern about the margin of 
safety at a level of 70 ppb, it further acknowledged that the choice of 
a level within the range recommended based on scientific evidence is a 
policy judgment (Frey, 2014c, p. ii). While she agreed with CASAC that 
it is appropriate to consider levels below 70 ppb, as reflected in her 
range of proposed levels from 65 to 70 ppb, for the reasons discussed 
above she also concluded that a standard level as high as 70 ppb, which 
CASAC concluded could be supported by the scientific evidence, could 
reasonably be judged to be requisite to protect public health with an 
adequate margin of safety.
    In considering the appropriateness of standard levels below 65 ppb, 
the Administrator noted the conclusions of the PA and the advice of 
CASAC that it would be appropriate for her to consider standard levels 
as low as 60 ppb. In making the decision to not propose levels below 65 
ppb, she focused on CASAC's rationale for a level of 60 ppb, which 
focused on the importance of limiting exposures to O3 
concentrations as low as 60 ppb (Frey, 2014c, p. 7). As discussed 
above, the Administrator agreed that it is appropriate to consider the 
implications of a revised standard level for estimated exposures of 
concern at or above 60 ppb. She noted that standards within the 
proposed range of 65 to 70 ppb would be expected to substantially limit 
the occurrence of exposures of concern to O3 concentrations 
at or above 60 ppb, particularly the occurrence of two or more 
exposures. When she further considered that not all exposures of 
concern lead to adverse effects, and that the NAAQS are not meant to be 
zero-risk or background standards, the Administrator judged that 
alternative standard levels below 65 ppb are not needed to further 
reduce such exposures.
b. Comments on Level
    A number of groups representing medical, public health, or 
environmental organizations; some state agencies; and many individuals 
submitted comments on the appropriate level of a revised primary 
O3 standard.\128\ Virtually all of these commenters 
supported setting the standard level within the range recommended by 
CASAC (i.e., 60 to 70). Some expressed support for the overall CASAC 
range, without specifying a particular level within that range, while 
others expressed a preference for the lower part of the CASAC range, 
often emphasizing support for a level of 60 ppb. Some of these 
commenters stated that if the EPA does not set the level at 60 ppb, 
then the level should be set no higher than 65 ppb (i.e., the lower 
bound of the proposed range of standard levels).
---------------------------------------------------------------------------

    \128\ In general, commenters who expressed the view that the EPA 
should retain the current O3 NAAQS (i.e., commenters 
representing industry and business groups, and some states) did not 
provide comments on alternative standard levels. As a result, this 
section focuses primarily on comments from commenters who expressed 
support for the proposed decision to revise the current primary 
O3 standard.
---------------------------------------------------------------------------

    To support their views on the level of a revised standard, some 
commenters focused on overarching issues related to the statutory 
requirements for the NAAQS. For example, some commenters maintained 
that the primary NAAQS must be set at a level at which there is an 
absence of adverse effects in sensitive populations. While this 
argument has some support in the case law and in the legislative 
history to the 1970 CAA (see Lead Industries Ass'n v. EPA, 647 F. 2d 
1147, 1153 (D.C. Cir. 1980)), it is well established that the NAAQS are 
not meant to be zero risk standards. See Lead Industries v. EPA, 647 
F.2d at 1156 n.51; Mississippi v. EPA, 744 F. 3d at 1351. From the 
inception of the NAAQS standard-setting process, the EPA and the courts 
have acknowledged that scientific uncertainties in general, and the 
lack of clear thresholds in pollutant effects in particular, preclude 
any such definitive determinations. Lead Industries, 647 F. 2d at 1156 
(setting standard at a level which would remove most but not all sub-
clinical effects). Likewise, the House report to the 1977 amendments 
addresses this question (H. Rep. 95-294, 95th Cong. 1st sess. 127): 
\129\
---------------------------------------------------------------------------

    \129\ Similarly, Senator Muskie remarked 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''. 123 Cong. Rec. S9423 (daily ed. June 
10, 1977).

    Some have suggested that since the standards are to protect 
against all known or anticipated effects and since no safe threshold 
can be established, the ambient standards should be set at zero or 
background levels. Obviously, this no-risk philosophy ignores all 
economic and social consequences and is impractical. This is 
particularly true in light of the legal requirement for mandatory 
---------------------------------------------------------------------------
attainment of the national primary standards within 3 years.

    Thus, post-1970 jurisprudence makes clear the impossibility, and 
lack of legal necessity, for NAAQS removing all health risk. See 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)'').
    In this review, EPA is setting a standard based on a careful 
weighing of available evidence, including a weighing of the strengths 
and limitations of the evidence and underlying scientific uncertainties 
therein. The Administrator's choice of standard level is rooted in her 
evaluation of the evidence, which reflects her legitimate uncertainty 
as to the O3 concentrations at which the public would 
experience adverse health effects. 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).

[[Page 65356]]

    In an additional overarching comment, some commenters also 
fundamentally objected to the EPA's consideration of exposure estimates 
in reaching conclusions on the primary O3 standard. These 
commenters' general assertion was that NAAQS must be established so as 
to be protective, with an adequate margin of safety, regardless of the 
activity patterns that feed into exposure estimates. They contended 
that ``[a]ir quality standards cannot rely on avoidance behavior in 
order to protect the public health and sensitive groups'' and that 
``[i]t would be unlawful for EPA to set the standard at a level that is 
contingent upon people spending most of their time indoors'' (e.g., ALA 
et al., p. 124). To support these comments, for example, ALA et al. 
analyzed ambient monitoring data from Core-Based Statistical Areas 
(CBSAs) with design values between 66-70 ppb (Table 17, pp. 145-151 in 
ALA et al.) and 62-65 ppb (Table 18, pp. 153-154 in ALA et al.) and 
pointed out that there are many more days with ambient concentrations 
above the benchmark levels than were estimated in the EPA's exposure 
analysis (i.e., at and above the benchmark level of 60, 70 and 80 ppb).
    The EPA disagrees with these commenters' conclusions regarding the 
appropriateness of considering exposure estimates, and notes that NAAQS 
must be ``requisite'' (i.e., ``sufficient, but not more than 
necessary'' (Whitman, 531 U.S. at 473)) to protect the ``public 
health'' (``the health of the public'' (Whitman, 531 U.S. at 465)). 
Estimating exposure patterns based on extensive available data \130\ is 
a reasonable means of ascertaining that standards are neither under- 
nor over-protective, and that standards address issues of public health 
rather than health issues pertaining only to isolated individuals.\131\ 
Behavior patterns are critical in assessing whether ambient 
concentrations of O3 may pose a public health risk.\132\ 
Exposures to ambient or near-ambient O3 concentrations have 
only been shown to result in potentially adverse effects if the 
ventilation rates of people in the exposed populations are raised to a 
sufficient degree (e.g., through physical exertion) (U.S. EPA, 2013, 
section 6.2.1.1).\133\ Ignoring whether such elevated ventilation rates 
are actually occurring, as advocated by these commenters, would not 
provide an accurate assessment of whether the public health is at risk. 
Indeed, a standard established without regard to behavior of the public 
would likely lead to a standard which is more stringent than necessary 
to protect the public health.
---------------------------------------------------------------------------

    \130\ The CHAD database used in the HREA's exposure assessment 
contains over 53,000 individual daily diaries including time-
location-activity patterns for individuals of both sexes across a 
wide range of ages (U.S. EPA, 2014a, Chapter 5).
    \131\ CASAC generally agreed with the EPA's methodology for 
characterizing exposures of concern (Frey, 2014a, pp. 5-6).
    \132\ See 79 FR 75269 (``The activity pattern of individuals is 
an important determinant of their exposure. Variation in 
O3 concentrations among various microenvironments means 
that the amount of time spent in each location, as well as the level 
of activity, will influence an individual's exposure to ambient 
O3. Activity patterns vary both among and within 
individuals, resulting in corresponding variations in exposure 
across a population and over time'' (internal citations omitted).
    \133\ For healthy young adults exposed at rest for 2 hours, 500 
ppb is the lowest O3 concentration reported to produce a 
statistically significant O3-induced group mean 
FEV1 decrement (U.S. EPA, 2013, section 6.2.1.1).
---------------------------------------------------------------------------

    While setting the primary O3 standard based only on 
ambient concentrations, without consideration of activity patterns and 
ventilation rates, would likely result in a standard that is over-
protective, the EPA also concludes that setting a standard based on the 
assumption that people will adjust their activities to avoid exposures 
on high-pollution days would likely result in a standard that is under-
protective. The HREA's exposure assessment does not make this latter 
assumption.\134\ The time-location-activity diaries that provided the 
basis for exposure estimates reflect actual variability in human 
activities. While some diary days may reflect individuals spending less 
time outdoors than would be typical for them, it is similarly likely 
that some days reflect individuals spending more time outdoors than 
would be typical. Considering the actual variability in time-location-
activity patterns is at the least a permissible way of identifying 
standards that are neither over- nor under-protective.\135\
---------------------------------------------------------------------------

    \134\ The EPA was aware of the possibility of averting behavior 
during the development of the HREA, and that document includes 
sensitivity analyses to provide perspective on the potential role of 
averting behavior in modifying O3 exposures. As discussed 
further above (II.B.2.c), these sensitivity analyses were limited 
and the results were discussed in the proposal within the context of 
uncertainties in the HREA assessment of exposures of concern.
    \135\ See Mississippi, 744 F. 3d 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 . . .))''
---------------------------------------------------------------------------

    Further, the EPA sees nothing in the CAA that prohibits 
consideration of the O3 exposures that could result in 
effects of public health concern. While a number of judicial opinions 
have upheld the EPA's decisions in other NAAQS reviews to place little 
weight on particular risk or exposure analyses (i.e., because of 
scientific uncertainties in those analyses), none of these opinions 
have suggested that such analyses are irrelevant because actual 
exposure patterns do not matter. See, e.g. Mississippi, 744 F. 3d at 
1352-53; ATA III, 283 F. 3d at 373-74. Therefore, because behavior 
patterns are critical in assessing whether ambient concentrations of 
O3 may pose a public health risk, the EPA disagrees with the 
views expressed by these commenters objecting to the consideration of 
O3 exposures in reaching decisions on the primary 
O3 standard.
    In addition to these overarching comments, a number of commenters 
supported their views on standard level by highlighting specific 
aspects of the scientific evidence, exposure/risk information, and/or 
CASAC advice. Key themes expressed by these commenters included the 
following: (1) Controlled human exposure studies provide strong 
evidence of adverse lung function decrements and airway inflammation in 
healthy adults following exposures to O3 concentrations as 
low as 60 ppb, and at-risk populations would be likely to experience 
more serious effects or effects at even lower concentrations; (2) 
epidemiologic studies provide strong evidence for associations with 
mortality and morbidity in locations with ambient O3 
concentrations below 70 ppb, and in many cases in locations with 
concentrations near and below 60 ppb; (3) quantitative analyses in the 
HREA are biased such that they understate O3 exposures and 
risks, and the EPA's interpretation of lung function risk estimates is 
not appropriate and not consistent with other NAAQS; and (4) the EPA 
must give deference to CASAC advice, particularly CASAC's policy advice 
to set the standard level below 70 ppb. The next sections discuss 
comments related to each of these points, and provide the EPA's 
responses to those comments. More detailed discussion of individual 
comments, and the EPA's responses, is provided in the Response to 
Comments document.
i. Effects in Controlled Human Exposure Studies
    Some commenters who advocated for a level of 60 ppb (or absent 
that, for 65 ppb) asserted that controlled human exposure studies have 
reported adverse respiratory effects in healthy adults following 
exposures to O3 concentrations as low as 60 ppb. These 
commenters generally based their conclusions on the demonstration of 
FEV1 decrements >= 10% and increased airway inflammation 
following exposures of healthy adults to 60 ppb O3. They 
concluded that even more serious effects would occur in at-risk

[[Page 65357]]

populations exposed to 60 ppb O3, and that such populations 
would experience adverse effects following exposures to O3 
concentrations below 60 ppb.
    While the EPA agrees that information from controlled human 
exposure studies conducted at 60 ppb can help to inform the 
Administrator's decision on the standard level, the Agency does not 
agree that this information necessitates a level below 70 ppb. In fact, 
as discussed in the proposal, a revised O3 standard with a 
level of 70 ppb can be expected to provide substantial protection 
against the effects shown to occur following various O3 
exposure concentrations, including those observed following exposures 
to 60 ppb. This is because the degree of protection provided by any 
NAAQS is due to the combination of all of the elements of the standard 
(i.e., indicator, averaging time, form, level). In the case of the 
fourth-high form of the O3 NAAQS, which the Administrator is 
retaining in the current review (II.C.3), the large majority of days in 
areas that meet the standard will have 8-hour O3 
concentrations below the level of the standard, with most days well 
below the level. Therefore, as discussed in the proposal, in 
considering the degree of protection provided by an O3 
standard with a particular level, it is important to consider the 
extent to which that standard would be expected to limit population 
exposures of concern to the broader range of O3 exposure 
concentrations shown in controlled human exposure studies to result in 
health effects. The Administrator's consideration of such exposures of 
concern is discussed below (II.C.4.c).
    Another important part of the Administrator's consideration of 
exposure estimates is the extent to which she judges that adverse 
effects could occur following specific O3 exposures. While 
controlled human exposure studies provide a high degree of confidence 
regarding the extent to which specific health effects occur following 
exposures to O3 concentrations from 60 to 80 ppb, the 
Administrator notes that there are no universally accepted criteria by 
which to judge the adversity of the observed effects. Therefore, in 
making judgments about the extent to which the effects observed in 
controlled human exposure studies have the potential to be adverse, the 
Administrator considers the recommendations of ATS and advice from 
CASAC (II.A.1.c, above).
    As an initial matter, with regard to the effects shown in 
controlled human exposure studies following O3 exposures, 
the Administrator notes the following:
    1. The largest respiratory effects, and the broadest range of 
effects, have been studied and reported following exposures to 80 ppb 
O3 or higher, with most exposure studies conducted at these 
higher concentrations. Specifically, 6.6-hour exposures of healthy 
young adults to 80 ppb O3, while engaged in quasi-
continuous, moderate exertion, can decrease lung function, increase 
airway inflammation, increase respiratory symptoms, result in airway 
hyperresponsiveness, and decrease lung host defenses.
    2. Exposures of healthy young adults for 6.6 hours to O3 
concentrations as low as 72 ppb, while engaged in quasi-continuous, 
moderate exertion, have been shown to both decrease lung function and 
result in respiratory symptoms.
    3. Exposures of healthy young adults for 6.6 hours to O3 
concentrations as low as 60 ppb, while engaged in quasi-continuous, 
moderate exertion, have been shown to decrease lung function and to 
increase airway inflammation.
    To inform her judgments on the potential adversity to public health 
of these effects reported in controlled human exposure studies, as in 
the proposal, the Administrator considers the ATS recommendation that 
``reversible loss of lung function in combination with the presence of 
symptoms should be considered adverse'' (ATS, 2000a). She notes that 
this combination of effects has been shown to occur following 6.6-hour 
exposures to O3 concentrations at or above 72 ppb. In 
considering these effects, CASAC observed that ``the combination of 
decrements in FEV1 together with the statistically 
significant alterations in symptoms in human subjects exposed to 72 ppb 
ozone meets the American Thoracic Society's definition of an adverse 
health effect'' (Frey, 2014c, p. 5).
    Regarding the potential for adverse effects following exposures to 
lower concentrations, the Administrator notes the CASAC judgment that 
the adverse combination of lung function decrements and respiratory 
symptoms ``almost certainly occur in some people'' following exposures 
to O3 concentrations below 72 ppb (Frey, 2014c, p. 6). In 
particular, when commenting on the extent to which the study by 
Schelegle et al. (2009) suggests the potential for adverse effects 
following O3 exposures below 72 ppb, CASAC judged that:

    [I]f subjects had been exposed to ozone using the 8-hour 
averaging period used in the standard [rather than the 6.6-hour 
exposures evaluated in the study], adverse effects could have 
occurred at lower concentration. Further, in our judgment, the level 
at which adverse effects might be observed would likely be lower for 
more sensitive subgroups, such as those with asthma (Frey, 2014c, p. 
5).

    Though CASAC did not provide advice as to how far below 72 ppb 
adverse effects would likely occur, the Administrator agrees that such 
effects could occur following exposures at least somewhat below 72 ppb.
    The Administrator notes that while adverse effects could occur 
following exposures at least somewhat below 72 ppb, the combination of 
statistically significant increases in respiratory symptoms and 
decrements in lung function has not been reported following 6.6-hour 
exposures to average O3 concentrations of 60 ppb or 63 ppb, 
though studies have evaluated the potential for such effects (Adams, 
2006; Schelegle et al., 2009; Kim et al., 2011). In the absence of this 
combination, the Administrator looks to additional ATS recommendations 
and CASAC advice in order to inform her judgments regarding the 
potential adversity of the effects that have been observed following 
O3 exposures as low as 60 ppb.
    With regard to ATS, she first notes the recommendations that ``a 
small, transient loss of lung function, by itself, should not 
automatically be designated as adverse'' and that ``[f]ew . . . 
biomarkers have been validated sufficiently that their responses can be 
used with confidence to define the point at which a response should be 
equated to an adverse effect warranting preventive measures'' (ATS, 
2000a).\136\ Based on these recommendations, compared to effects 
following exposures at or above 72 ppb, the Administrator has less 
confidence in the adversity of the respiratory effects that have been 
observed following exposures to 60 or 63 ppb.
---------------------------------------------------------------------------

    \136\ With regard to this latter recommendation, as discussed 
above (II.A.1.c), the ATS concluded that elevations of biomarkers 
such as cell numbers and types, cytokines, and reactive oxygen 
species may signal risk for ongoing injury and more serious effects 
or may simply represent transient responses, illustrating the lack 
of clear boundaries that separate adverse from nonadverse events.
---------------------------------------------------------------------------

    She further notes that some commenters who advocated for a level of 
60 ppb also focused on ATS recommendations regarding population-level 
risks. These commenters specifically stated that lung function 
decrements ``may be adverse in terms of `population risk,' where 
exposure to air pollution increases the risk to the population even 
though it might not harm lung function to a degree that is, on its own, 
`clinically important' to an individual'' (e.g., ALA et al., p. 118). 
These commenters asserted that the EPA

[[Page 65358]]

has not appropriately considered the potential for such population-
level risk. Contrary to the views expressed by these commenters, the 
Administrator carefully considers the potential for population risk, 
particularly within the context of the ATS recommendation that ``a 
shift in the risk factor distribution, and hence the risk profile of 
the exposed population, should be considered adverse, even in the 
absence of the immediate occurrence of frank illness'' (ATS, 2000a). 
Given that exposures to 60 ppb O3 have been shown in 
controlled human exposure studies to cause transient and reversible 
decreases in group mean lung function, the Administrator notes the 
potential for such exposures to result in similarly transient and 
reversible shifts in the risk profile of an exposed population. 
However, in contrast to commenters who advocated for a level of 60 ppb, 
the Administrator also notes that the available evidence does not 
provide information on the extent to which a short-term, transient 
decrease in lung function in a population, as opposed to a longer-term 
or permanent decrease, could affect the risk of other, more serious 
respiratory effects (i.e., change the risk profile of the population). 
This uncertainty, together with the additional ATS recommendations 
noted above, indicates to the Administrator that her judgment that 
there is uncertainty in the adversity of the effects shown to occur at 
60 ppb is consistent with ATS recommendations.\137\
---------------------------------------------------------------------------

    \137\ ATS provided additional recommendations to help inform 
judgments regarding the adversity of air pollution-related effects 
(e.g., related to ``quality of life''), though it is not clear 
whether, or how, such recommendations should be applied to the 
respiratory effects observed in controlled human exposure studies 
following 6.6-hour O3 exposures (ATS, 200a, p. 672).
---------------------------------------------------------------------------

    With regard to CASAC advice, the Administrator notes that, while 
CASAC clearly advised the EPA to consider the health effects shown to 
occur following exposures to 60 ppb O3, its advice regarding 
the adversity of those effects is less clear. In particular, she notes 
that CASAC was conditional about whether the lung function decrements 
observed in some people at 60 ppb (i.e., FEV1 decrements >= 
10%) are adverse. Specifically, CASAC stated that these decrements 
``could be adverse in individuals with lung disease'' (Frey, 2014c, p. 
7, emphasis added) and that they provide a ``surrogate for adverse 
health outcomes for people with asthma and lung disease'' (Frey, 2014c, 
p. 3, emphasis added). Further, CASAC did not recommend considering 
standard levels low enough to eliminate O3-induced 
FEV1 decrements >= 10% (Frey, 2014c). With regard to the 
full range of effects shown to occur at 60 ppb (i.e., FEV1 
decrements, airway inflammation), CASAC stated that exposures of 
concern for the 60 ppb benchmark are ``relevant for consideration'' 
with respect to people with asthma (Frey, 2014c, p. 6, italics added). 
In addition, ``[t]he CASAC concurs with EPA staff regarding the finding 
based on scientific evidence that a level of 60 ppb corresponds to the 
lowest exposure concentration demonstrated to result in lung function 
decrements large enough to be judged an abnormal response by ATS and 
that could be adverse in individuals with lung disease'' (Frey, 2014c, 
p. 7, italics added). The Administrator contrasts these statements with 
CASAC's clear advice that ``the combination of decrements in 
FEV1 together with the statistically significant alterations 
in symptoms in human subjects exposed to 72 ppb ozone meets the 
American Thoracic Society's definition of an adverse health effect'' 
(Frey, 2014c, p. 5).
    Based on her consideration of all of the above recommendations and 
advice noted above, the Administrator judges that, compared to exposure 
concentrations at and above 72 ppb, there is greater uncertainty with 
regard to the adversity of effects shown to occur following 
O3 exposures as low as 60 ppb. However, based on the effects 
that have been shown to occur at 60 ppb (i.e., lung function 
decrements, airway inflammation), and CASAC advice indicating the 
importance of considering these effects (though its advice regarding 
the adversity of effects at 60 ppb is less clear), she concludes that 
it is appropriate to give some consideration to the extent to which a 
revised standard could allow such effects.
    In considering estimates of exposures of concern for the 60, 70, 
and 80 ppb benchmarks within the context of her judgments on adversity, 
the Administrator notes that, due to interindividual variability in 
responsiveness, not every occurrence of an exposure of concern will 
result in an adverse effect. As discussed above (II.B.2.b.i), this 
point was highlighted by some commenters who opposed revision of the 
current standard, based on their analysis of effects shown to occur 
following exposures to 72 ppb O3. This point was also 
highlighted by some commenters who advocated for a level of 60 ppb, 
based on the discussion of O3-induced inflammation in the 
proposal. In particular, this latter group of commenters highlighted 
discussion from the proposal indicating that ``[i]nflammation induced 
by a single O3 exposure can resolve entirely but, as noted 
in the ISA (U.S. EPA, 2013, p. 6-76), `continued acute inflammation can 
evolve into a chronic inflammatory state''' (e.g., ALA et al., p. 48). 
Consistent with these comments, and with her consideration of estimated 
exposurs of concern in the proposal, the Administrator judges that the 
types of respiratory effects that can occur following exposures of 
concern, particularly if experienced repeatedly, provide a plausible 
mode of action by which O3 may cause other more serious 
effects. Because of this, as in the proposal, the Administrator is most 
concerned about protecting against repeated occurrences of exposures of 
concern.
    The Administrator's consideration of estimated exposures of concern 
is discussed in more detail below (II.C.4.b.iv, II.C.4.c). In summary, 
contrary to the conclusions of commenters who advocated for a level of 
60 ppb, the Administrator judges that a revised standard with a level 
of 70 ppb will effectively limit the occurrence of the O3 
exposures for which she is most confident in the adversity of the 
resulting effects (i.e., based on estimates for the 70 and 80 ppb 
benchmarks). She further concludes that such a standard will provide 
substantial protection against the occurrence of O3 
exposures for which there is greater uncertainty in the adversity of 
effects (i.e., based on estimates for the 60 ppb benchmark).
    As noted above, commenters also pointed out that benchmark 
concentrations are based on studies conducted in healthy adults, 
whereas at-risk populations are likely to experience more serious 
effects and effects at lower O3 exposure concentrations. In 
considering this issue, the EPA notes CASAC's endorsement of 60 ppb as 
the lower end of the range of benchmarks for evaluation, and its advice 
that ``the 60 ppb-8hr exposure benchmark is relevant for consideration 
with respect to adverse effects on asthmatics'' (Frey, 2014c, p. 6). As 
discussed in detail below (II.C.4.c), the Administrator has carefully 
considered estimated exposures of concern for the 60 ppb benchmark. In 
addition, though the available information does not support the 
identification of specific benchmarks below 60 ppb that could be 
appropriate for consideration for at-risk populations, and though CASAC 
did not recommend consideration of any such benchmarks, the EPA expects 
that a revised standard with a level of 70 ppb will also reduce the 
occurrence of exposures to O3 concentrations at least 
somewhat below 60 ppb (U.S. EPA,

[[Page 65359]]

2014a, Figures 4-9 and 4-10).\138\ Thus, even if some members of at-
risk populations may experience effects following exposures to 
O3 concentrations somewhat below 60 ppb, a revised level of 
70 ppb would be expected to reduce the occurrence of such 
exposures.\139\ Therefore, the EPA has considered O3 
exposures that could be relevant for at-risk populations such as 
children and people with asthma, and does not agree that controlled 
human exposure studies reporting respiratory effects in healthy adults 
following exposures to 60 ppb O3 necessitate a standard 
level below 70 ppb.
---------------------------------------------------------------------------

    \138\ Air quality analyses in the HREA indicate that reducing 
the level of the primary standard from 75 ppb to 70 ppb will result 
in reductions in the O3 concentrations in the upper 
portions of ambient distributions. This includes 8-hour ambient 
O3 concentrations at, and somewhat below, 60 ppb (U.S. 
EPA, 2014a, Figures 4-9 and 4-10).
    \139\ The uncertainty associated with the potential adversity of 
any such effects would be even greater than that discussed above for 
the 60 ppb benchmark.
---------------------------------------------------------------------------

ii. Epidemiologic Studies
    Commenters representing environmental and public health 
organizations also highlighted epidemiologic studies that, in their 
view, provide strong evidence for associations with mortality and 
morbidity in locations with ambient O3 concentrations near 
and below 60 ppb. These commenters focused both on the epidemiologic 
studies evaluated in the PA's analyses of study location air quality 
(U.S. EPA, 2014c, Chapter 4) and on studies that were not explicitly 
analyzed in the PA, and in some cases on studies that were not included 
in the ISA.
    The EPA agrees that epidemiologic studies can provide perspective 
on the degree to which O3-associated health effects have 
been identified in areas with air quality likely to have met various 
standards. However, as discussed below, we do not agree with the 
specific conclusions drawn by these commenters regarding the 
implications of epidemiologic studies for the standard level. As an 
initial matter in considering epidemiologic studies, the EPA notes its 
decision, consistent with CASAC advice, to place the most emphasis on 
information from controlled human exposure studies (II.B.2 and II.B.3, 
above). This decision reflects the greater certainty in using 
information from controlled human exposure studies to link specific 
O3 exposures with health effects, compared to using air 
quality information from epidemiologic studies of O3 for 
this purpose.
    While being aware of the uncertainties discussed above 
(II.B.2.b.ii), in considering what epidemiologic studies can tell us, 
the EPA notes analyses in the PA (U.S. EPA, 2014c, section 4.4.1) 
indicating that a revised standard with a level at or below 70 ppb 
would be expected to maintain distributions of short-term ambient 
O3 concentrations below those present in the locations of 
all of the single-city epidemiologic studies analyzed. As discussed in 
the PA (U.S. EPA, 2014c, section 4.4.1), this includes several single-
city studies conducted in locations that would have violated the 
current standard, and the study by Mar and Koenig (2009) that reported 
positive and statistically significant associations with respiratory 
emergency department visits with children and adults in a location that 
would have met the current standard over the entire study period, but 
would have violated a standard with a level of 70 ppb.\140\ While these 
analyses provide support for a level at least as low as 70 ppb, the 
Administrator judges that they do not provide a compelling basis for 
distinguishing between the appropriateness of 70 ppb and lower standard 
levels.
---------------------------------------------------------------------------

    \140\ As noted above (II.B.2.b.ii and II.B.3), the studies by 
Silverman and Ito (2010) and Strickland et al. (2010) provided 
support for the Administrator's decision to revise the current 
primary O3 standard, but do not provide insight into the 
appropriateness of specific standard levels below 75 ppb.
---------------------------------------------------------------------------

    As in the proposal, the EPA acknowledges additional uncertainty in 
interpreting air quality in locations of multicity epidemiologic 
studies of short-term O3 for the purpose of evaluating 
alternative standard levels (U.S. EPA, 2014c, sections 3.1.4.2, 4.4.1). 
In particular, the PA concludes that interpretation of such air quality 
information is complicated by uncertainties in the extent to which 
multicity effect estimates (i.e., which are based on combining 
estimates from multiple study locations) can be attributed to ambient 
O3 in the subset of study locations that would have met a 
particular standard, versus O3 in the study locations that 
would have violated the standard. While giving only limited weight to 
air quality analyses in these study areas because of this uncertainty, 
the EPA also notes PA analyses indicating that a standard level at or 
below 70 ppb would require additional reductions, beyond those required 
by the current standard, in the ambient O3 concentrations 
that provided the basis for statistically significant O3 
health effect associations in multicity epidemiologic studies. As was 
the case for the single-city studies, and contrary to the views 
expressed by the commenters noted above, the Administrator judges that 
these studies do not provide a compelling basis for distinguishing 
between the appropriateness of alternative standard levels at or below 
70 ppb.
    In some cases, commenters highlighted studies that were assessed in 
the 2008 review of the O3 NAAQS, but were not included in 
the ISA in the current review. These commenters asserted that such 
studies support the occurrence of O3 health effect 
associations in locations with air quality near or, in some cases, 
below 60 ppb. Specifically, commenters highlighted a number of studies 
included in the 2007 Staff Paper that were not included in the ISA, 
claiming that these studies support a standard level below 70 ppb, and 
as low as 60 ppb.
    As an initial matter with regard to these studies, the EPA notes 
that the focus of the ISA is on assessing the most policy-relevant 
scientific evidence. In the current review, the ISA considered over 
1,000 new studies that have been published since the last review. Thus, 
it is not surprising that, as the body of evidence has been 
strengthened since the last review, some of the studies considered in 
the last review are no longer among the most policy relevant. However, 
based on the information included in the 2007 Staff Paper, the EPA does 
not agree that the studies highlighted by commenters provide compelling 
support for a level below 70 ppb. In fact, as discussed in the Staff 
Paper in the last review (U.S. EPA, 2007, p. 6-9; Appendix 3B), the 
O3 concentrations reported for these studies, and the 
concentrations highlighted by commenters, were based on averaging 
across multiple monitors in study areas. Given that the highest monitor 
in an area is used to determine whether that area meets or violates the 
NAAQS, the averaged concentrations reported in the Staff Paper are thus 
not appropriate for direct comparison to the level of the O3 
standard. When the Staff Paper considered the O3 
concentrations measured at individual monitors for the subset of these 
study areas with particularly low concentrations, they were almost 
universally found to be above, and in many cases well above, even the 
current standard level of 75 ppb.\141\ Based on the above

[[Page 65360]]

considerations, and consistent with the Administrator's overall 
decision to place less emphasis on air quality in locations of 
epidemiologic studies to select a standard level, the EPA disagrees 
with commenters who asserted that epidemiologic studies included in the 
last review, but not cited in the ISA or PA in this review, necessitate 
a level below 70 ppb. In fact, the EPA notes that these studies are 
consistent with the majority of the U.S. studies evaluated in the PA in 
the current review, in that most were conducted in locations that would 
have violated the current O3 NAAQS over at least part of the 
study periods.
---------------------------------------------------------------------------

    \141\ For one study conducted in Vancouver, where data from 
individual monitors did indicate ambient concentrations below the 
level of the current standard (Vedal et al., 2003), the Staff Paper 
noted that the study authors questioned whether O3, other 
gaseous pollutants, and PM in this study may be acting as surrogate 
markers of pollutant mixes that contain more toxic compounds, 
``since the low measured concentrations were unlikely, in their 
opinion, to cause the observed effects'' (U.S. EPA, 2007, p. 6-16). 
The Staff Paper further noted that another study conducted in 
Vancouver failed to find statistically significant associations with 
O3 (Villeneuve et al., 2003).
---------------------------------------------------------------------------

iii. Exposure and Risk Assessments
    Some commenters supporting levels below 70 ppb also asserted that 
quantitative analyses in the HREA are biased such that they understate 
O3 exposures of concern and risks of O3-induced 
FEV1 decrements. Many of these comments are discussed above 
within the context of the adequacy of the current standard 
(II.B.2.b.i), including comments pointing out that exposure and risk 
estimates are based on information from healthy adults rather than at-
risk populations; comments noting that the exposure assessment 
evaluates 8-hour O3 exposures rather than the 6.6-hour 
exposures used in controlled human exposure studies; and comments 
asserting that the EPA's exposure and risk analyses rely on people 
staying indoors on high pollution days (i.e., averting behavior).
    As discussed in section II.B.2.b.i above, while the EPA agrees with 
certain aspects of these commenters' assertions, we do not agree with 
their overall conclusions. In particular, there are aspects of the 
HREA's quantitative analyses that, if viewed in isolation, would tend 
to either overstate or understate O3 exposures and/or health 
risks. While commenters tended to focus on those aspects of the 
assessments that support their position, they tended to ignore aspects 
of the assessments that do not support their position (points that were 
often raised by commenters on the other side of the issue). Rather than 
viewing the potential implications of these aspects of the HREA 
assessments in isolation, the EPA considers them together, along with 
other issues and uncertainties related to the interpretation of 
exposure and risk estimates.
    For example, some commenters who advocated for a level below 70 ppb 
asserted that the exposure assessment could underestimate O3 
exposures for highly active populations, including outdoor workers and 
children who spend a large portion of time outdoors during summer. In 
support of these assertions, commenters highlighted sensitivity 
analyses conducted in the HREA. However, as noted in the HREA (U.S. 
EPA, 2014a, Table 5-10), this aspect of the assessment is likely to 
have only a ``low to moderate'' impact on the magnitude of exposure 
estimates. To put this magnitude in perspective, HREA sensitivity 
analyses conducted in a single urban study area indicate that, 
regardless of whether exposure estimates for children are based on all 
available diaries or on a subset of diaries restricted to simulate 
highly exposed children, a revised standard with a level of 70 ppb is 
estimated to protect more than 99% of children from experiencing two or 
more exposures of concern at or above 70 ppb (U.S. EPA, 2014a, Chapter 
5 Appendices, Figure 5G-9).142 143 In contrast to the focus 
of commenters who supported a level below 70 ppb, other aspects of 
quantitative assessments, some of which were highlighted by commenters 
who opposed revising the current standard (II.B.2), tend to result in 
overestimates of O3 exposures. These aspects are 
characterized in the HREA as having either a ``low,'' a ``low-to-
moderate,'' or a ``moderate'' impact on the magnitudes of exposure 
estimates.
---------------------------------------------------------------------------

    \142\ More specifically, based on all children's diaries, just 
under 0.1% of children are estimated to experience two or more 
exposures of concern at or above 70 ppb. Based on simulated profiles 
of highly exposed children, this estimate increased to just over 
0.1% (U.S. EPA, 2014a, Chapter 5 Appendices, Figure 5G-9).
    \143\ In addition, 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 worst-case 
cities and years (i.e., cities and years with the highest exposure 
estimates) (U.S. EPA, 2014, section 5.4.3.2, Figure 5-14).
---------------------------------------------------------------------------

    In its reviews of the HREA and PA, CASAC recognized many of the 
uncertainties and issues highlighted by commenters. Even considering 
these uncertainties, CASAC endorsed the approaches adopted by the EPA 
to assess O3 exposures and health risks, and CASAC used 
exposure and risk estimates as part of the basis for their 
recommendations on the primary O3 NAAQS (Frey, 2014c). Thus, 
as discussed in section II.B.2.b.i above, the EPA disagrees with 
commenters who claim that the aspects of the quantitative assessments 
that they highlight lead to overall underestimates of exposures or 
health risks.\144\
---------------------------------------------------------------------------

    \144\ As discussed in II.B.2.b above, in weighing the various 
uncertainties, which can bias exposure results in different 
directions but tend to have impacts that are similar in magnitude 
(U.S. EPA, 2014a, Table 5-10), and in light of CASAC's advice based 
on its review of the HREA and the PA, the EPA continues to conclude 
that the approach to considering estimated exposures of concern in 
the HREA, PA, and the proposal reflects an appropriate balance, and 
provides an appropriate basis for considering the public health 
protectiveness of the primary O3 standard.
---------------------------------------------------------------------------

    Some commenters further contended that the level of the primary 
O3 standard should be set below 70 ppb in order to 
compensate for the use of a form that allows multiple days with 
concentrations higher than the standard level. These groups submitted 
air quality analyses to support their point that the current fourth-
high form allows multiple days per year with ambient O3 
concentrations above the level of the standard. While the EPA does not 
dispute the air quality analyses submitted by these commenters, and 
agrees that fourth-high form allows multiple days per year with ambient 
O3 concentrations above the level of the standard (3 days 
per year, on average over a 3-year period), the Agency disagrees with 
commenters' assertion that, because of this, the level of the primary 
O3 standard should be set below 70 ppb. As discussed above 
(II.A.2), the quantitative assessments that informed the 
Administrator's proposed decision, presented in the HREA and considered 
in the PA and by CASAC, estimated O3 exposures and health 
risks associated with air quality that ``just meets'' various standards 
with the current 8-hour averaging time and fourth-high, 3-year average 
form. Thus, in considering the degree of public health protection 
appropriate for the primary O3 standard, the Administrator 
has considered quantitative exposure and risk estimates that are based 
a fourth-high form, and therefore on a standard that, as these 
commenters point out, allows multiple days per year with ambient 
O3 concentrations above the level of the standard.
iv. CASAC Advice
    Many commenters, including those representing major medical, public 
health, or environmental groups; some state agencies; and a large 
number of individual commenters, focused on CASAC advice in their 
rationale supporting levels below 70 ppb, and as low as 60 ppb. These 
commenters generally asserted that the EPA must

[[Page 65361]]

give deference to CASAC. In some cases, these commenters expressed 
strong objections to a level of 70 ppb, noting CASAC policy advice that 
such a level would provide little margin of safety.
    The EPA agrees that CASAC advice is an important consideration in 
reaching a decision on the standard level (see e.g. CAA section 307 
(d)(3)),\145\ though not with commenters' conclusion that CASAC advice 
necessitates a standard level below 70 ppb. As discussed above 
(II.C.4.a), the Administrator carefully considered CASAC advice in the 
proposal, and she judged that her proposed decision to revise the level 
to within the range of 65 to 70 ppb was consistent with CASAC advice, 
based on the available science.
---------------------------------------------------------------------------

    \145\ The EPA notes, of course, that the CAA places the 
responsibility for judging what standard is requisite with the 
Administrator and only requires that, if her decision differs in 
important ways from CASAC's advice, she explain her reasoning for 
differing.
---------------------------------------------------------------------------

    As in the proposal, in her final decision on level the 
Administrator notes CASAC's overall conclusion that ``based on the 
scientific evidence from clinical studies, epidemiologic studies, 
animal toxicology studies, as summarized in the ISA, the findings from 
the exposure and risk assessments as summarized in the HREA, and the 
interpretation of the implications of all of these sources of 
information as given in the Second Draft PA . . . there is adequate 
scientific evidence to recommend a range of levels for a revised 
primary ozone standard from 70 ppb to 60 ppb'' (Frey, 2014c, p. 8). 
Thus, CASAC used the health evidence and exposure/risk information to 
inform its range of recommended standard levels, a range that included 
an upper bound of 70 ppb based on the scientific evidence, and it did 
not use the evidence and information to recommend setting the primary 
O3 standard at any specific level within the range of 70 to 
60 ppb. In addition, CASAC further stated that ``the choice of a level 
within the range recommended based on scientific evidence [i.e., 70 to 
60 ppb] is a policy judgment under the statutory mandate of the Clean 
Air Act'' (Frey, 2014c, p. ii).
    In addition to its advice based on the scientific evidence, CASAC 
offered the ``policy advice'' to set the level below 70 ppb, stating 
that a standard level of 70 ppb ``may not meet the statutory 
requirement to protect public health with an adequate margin of 
safety'' (Frey, 2014c, p. ii). In supporting its policy advice to set 
the level below 70 ppb, CASAC noted the respiratory effects that have 
been shown to occur in controlled human exposure studies following 
exposures from 60 to 80 ppb O3, and the extent to which 
various standard levels are estimated to allow the occurrence of 
population exposures that can result in such effects (Frey, 2014c, pp. 
7-8).
    The EPA agrees that an important consideration when reaching a 
decision on level is the extent to which a revised standard is 
estimated to allow the types of exposures shown in controlled human 
exposure studies to cause respiratory effects. In reaching her final 
decision that a level of 70 ppb is requisite to protect public health 
with an adequate margin of safety (II.C.4.c, below), the Administrator 
carefully considers the potential for such exposures and effects. In 
doing so, she emphasizes the importance of setting a standard that 
limits the occurrence of the exposures about which she is most 
concerned (i.e., those for which she has the most confidence in the 
adversity of the resulting effects, which are repeated exposures of 
concern at or above 70 or 80 ppb, as discussed above in II.C.4.b.i). 
Based on her consideration of information from controlled human 
exposure studies in light of CASAC advice and ATS recommendations, the 
Administrator additionally judges that there is important uncertainty 
in the extent to which the effects shown to occur following exposures 
to 60 ppb O3 are adverse to public health (discussed above, 
II.C.4.b.i and II.C.4.b.iii). However, based on the effects that have 
been shown to occur, CASAC advice indicating the importance of 
considering these effects, and ATS recommendations indicating the 
potential for adverse population-level effects (II.C.4.b.i, 
II.C.4.b.iii), she concludes that it is appropriate to give some 
consideration to the extent to which a revised standard could allow the 
respiratory effects that have been observed following exposures to 60 
ppb O3.
    When considering the extent to which a revised standard could allow 
O3 exposures that have been shown in controlled human 
exposures studies to result in respiratory effects, the Administrator 
is most concerned about protecting the public, including at-risk 
populations, against repeated occurrences of such exposures of concern 
(II.C.4.b.i, above). In considering the appropriate metric for 
evaluating repeated occurrences of exposures of concern, the 
Administrator acknowledges that it is not clear from the evidence, or 
from the ATS recommendations, CASAC advice, or public comments, how 
particular numbers of exposures of concern could impact the seriousness 
of the resulting effects, especially at lower exposure concentrations. 
Therefore, the Administrator judges that focusing on HREA estimates of 
two or more exposures of concern provides a health-protective approach 
to considering the potential for repeated occurrences of exposures of 
concern that could result in adverse effects. She notes that other 
possible metrics for considering repeated occurrences of exposures of 
concern (e.g., 3 or more, 4 or more, etc.) would result in smaller 
exposure estimates.
    As discussed further below (II.C.4.c), the Administrator notes 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 (Table 1, above). For the 70 ppb 
benchmark, this reflects about a 90% reduction in the number of 
children estimated to experience two or more exposures of concern, 
compared to the current standard.\146\ Even considering the worst-case 
urban study area and worst-case year evaluated in the HREA, a standard 
with a level of 70 ppb is estimated to protect more than 99% of 
children from experiencing two or more exposures of concern to 
O3 concentrations at or above 70 ppb (Table 1).
---------------------------------------------------------------------------

    \146\ Percent reductions in this section refer to reductions in 
the number of children in HREA urban study areas (averaged over the 
years evaluated in the HREA) estimated to experience exposures of 
concern, based on the information in Table 1 above.
---------------------------------------------------------------------------

    Though the Administrator judges that there is greater uncertainty 
with regard to the occurrence of adverse effects following exposures as 
low as 60 ppb, she notes that a revised standard with a level of 70 ppb 
is estimated to protect the vast majority of children in urban study 
areas (i.e., about 96% to more than 99% in individual areas) from 
experiencing two or more exposures of concern at or above 60 ppb. 
Compared to the current standard, this represents a reduction of more 
than 60% in exposures of concern for the 60 ppb benchmark (Table 1). 
Given the Administrator's uncertainty regarding the adversity of the 
effects following exposures to 60 ppb O3, and her health-
protective approach to considering repeated occurrences of exposures of 
concern, the Administrator judges that this degree of protection is 
appropriate and that it reflects substantial protection against the 
occurrence of O3-induced effects, including effects for 
which she judges the adversity to public health is uncertain.

[[Page 65362]]

    While being less concerned about single occurrences of exposures of 
concern, especially at lower exposure concentrations, the Administrator 
also notes 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 >= about 99% of children in urban study areas from 
experiencing any exposures of concern at or above 70 ppb; and (3) to 
achieve substantial reductions (i.e., about 50%), compared to the 
current standard, in the occurrence of one or more exposures of concern 
at or above 60 ppb (Table 1).
    Given the information and advice noted above (and in II.C.4.b.i, 
II.C.4.b.iii), the Administrator judges that a revised standard with a 
level of 70 ppb will effectively limit the occurrence of the 
O3 exposures for which she has the most confidence in the 
adversity of the resulting effects (i.e., based on estimates for the 70 
and 80 ppb benchmarks). She further judges that such a standard will 
provide a large degree of protection against O3 exposures 
for which there is greater uncertainty in the adversity of effects 
(i.e., those observed following exposures to 60 ppb O3), 
contributing to the margin of safety of the standard. See Mississippi, 
744 F. 3d at 1353 (``By requiring an `adequate margin of safety', 
Congress was directing EPA to build a buffer to protect against 
uncertain and unknown dangers to human health''). Given the 
considerable protection provided against repeated exposures of concern 
for all of the benchmarks evaluated, including the 60 ppb benchmark, 
the Administrator judges that a standard with a level of 70 ppb will 
provide an adequate margin of safety against the adverse O3-
induced effects shown to occur following exposures at or above 72 ppb, 
and judged by CASAC likely to occur following exposures somewhat below 
72 ppb.\147\
---------------------------------------------------------------------------

    \147\ As discussed above (II.C.4.b.i), when commenting on the 
extent to which the study by Schelegle et al. (2009) suggests the 
potential for adverse effects following O3 exposures 
below 72 ppb, CASAC stated the following: ``[I]f subjects had been 
exposed to ozone using the 8-hour averaging period used in the 
standard [rather than the 6.6-hour exposures evaluated in the 
study], adverse effects could have occurred at lower concentration. 
Further, in our judgment, the level at which adverse effects might 
be observed would likely be lower for more sensitive subgroups, such 
as those with asthma'' (Frey, 2014c, p. 5).
---------------------------------------------------------------------------

    Contrary to the conclusions of commenters who advocated for a level 
below 70 ppb, the Administrator notes that her final decision is 
consistent with CASAC's advice, based on the scientific evidence, and 
with CASAC's focus on setting a revised standard to further limit the 
occurrence of the respiratory effects observed in controlled human 
exposure studies, including effects observed following exposures to 60 
ppb O3. Given her judgments and conclusions discussed above, 
and given that the CAA reserves the choice of the standard that is 
requisite to protect public health with an adequate margin of safety 
for the judgment of the EPA Administrator, she disagrees with 
commenters who asserted that CASAC advice necessitates a level below 70 
ppb, and as low as 60 ppb. The Administrator's final conclusions on 
level are discussed in more detail below (II.C.4.c).
c. Administrator's Final Decision Regarding Level
    Having carefully considered the public comments on the appropriate 
level of the primary O3 standard, as discussed above and in 
the Response to Comments document, the Administrator believes her 
scientific and policy judgments in the proposal remain valid. In 
conjunction with her decisions to retain the current indicator, 
averaging time, and form (II.C.1 to II.C.3, above), the Administrator 
is revising the level of the primary O3 standard to 70 ppb. 
In doing so, she is selecting a primary O3 standard that is 
requisite to protect public health with an adequate margin of safety, 
in light of her judgments based on an interpretation of the scientific 
evidence and exposure/risk information that neither overstates nor 
understates the strengths and limitations of that evidence and 
information and the appropriate inferences to be drawn therefrom.
    The Administrator's decision to revise the level of the primary 
O3 standard to 70 ppb builds upon her conclusion that the 
overall body of scientific evidence and exposure/risk information calls 
into question the adequacy of public health protection afforded by the 
current standard, particularly for at-risk populations and lifestages 
(II.B.3).\148\ Consistent with the proposal, her decision on level 
places the greatest emphasis 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. As in the proposal, and as discussed further below, she views 
the results of the lung function risk assessment, analyses of 
O3 air quality in locations of epidemiologic studies, and 
epidemiology-based quantitative health risk assessments as providing 
information in support of her decision to revise the current standard, 
but a more limited basis for selecting a particular standard level 
among a range of options. See Mississippi, 744 F. 3d at 1351-52 
(studies can legitimately support a decision to revise the standard, 
but not provide sufficient information to justify their use in setting 
the level of a revised standard).
---------------------------------------------------------------------------

    \148\ At-risk populations include people with asthma; children 
and older adults; people who are active outdoors, including outdoor 
workers; people with certain genetic variants; and people with 
reduced intake of certain nutrients.
---------------------------------------------------------------------------

    Given her consideration of the evidence, exposure/risk information, 
advice from CASAC, and public comments, the Administrator judges that a 
standard with a level of 70 ppb is requisite to protect public health 
with an adequate margin of safety. She notes that the determination of 
what constitutes an adequate margin of safety is expressly left to the 
judgment of the EPA Administrator. See Lead Industries Association v. 
EPA, 647 F.2d at 1161-62; Mississippi, 744 F. 3d at 1353. She further 
notes that in evaluating how particular standards address the 
requirement to provide an adequate margin of safety, it is appropriate 
to consider such factors as the nature and severity of the health 
effects, the size of sensitive population(s) at risk, and the kind and 
degree of the uncertainties present (I.B, above). Consistent with past 
practice and long-standing judicial precedent, the Administrator takes 
the need for an adequate margin of safety into account as an integral 
part of her decision-making on the appropriate level, averaging time, 
form, and indicator of the standard.\149\
---------------------------------------------------------------------------

    \149\ See, e.g. NRDC v. EPA, 902 F. 2d 962, 973-74 (D.C. Cir. 
1990).
---------------------------------------------------------------------------

    In considering the need for an adequate margin of safety, the 
Administrator notes that a standard with a level of 70 ppb 
O3 would be expected to provide substantial improvements in 
public health, including for at-risk groups such as children and people 
with asthma. The following paragraphs summarize the basis for the 
Administrator's conclusion that a revised primary O3 
standard with a level of 70 ppb is requisite to protect the public 
health with an adequate margin of safety.
    As an initial matter, consistent with her conclusions on the need 
for revision of the current standard (II.B.3), in reaching a decision 
on level the Administrator places the most weight on information from 
controlled human exposure studies. In doing so, she notes that 
controlled human exposure studies provide the most certain evidence 
indicating the occurrence of health

[[Page 65363]]

effects in humans following specific O3 exposures. In 
particular, she notes that the effects reported in controlled human 
exposure studies are due solely to O3 exposures, and 
interpretation of study results is not complicated by the presence of 
co-occurring pollutants or pollutant mixtures (as is the case in 
epidemiologic studies). The Administrator also observes that her 
emphasis on information from controlled human exposure studies is 
consistent with CASAC's advice and interpretation of the scientific 
evidence (Frey, 2014c).
    With regard to the effects shown in controlled human exposure 
studies following specific O3 exposures, as discussed in 
more detail above (II.B, II.C.4.b.i), the Administrator notes 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, AHR, and decreased 
lung host defense); (2) exposures to O3 concentrations as 
low as 72 ppb have been shown to both decrease lung function and 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.
    While such controlled human exposure studies provide a high degree 
of confidence regarding the occurrence of health effects following 
exposures to O3 concentrations from 60 to 80 ppb, there are 
no universally accepted criteria by which to judge the adversity of the 
observed effects. To inform her judgments on the potential adversity to 
public health of effects reported in controlled human exposure studies, 
the Administrator considers ATS recommendations and CASAC advice, as 
described in detail above (II.B.2, II.C.4.b.i, II.C.4.b.iii, 
II.C.4.b.iv). Based on her consideration of such recommendations and 
advice, the Administrator is confident that the respiratory effects 
that have been observed following exposures to 72 ppb O3 or 
above can be adverse. In addition, she judges that adverse effects are 
likely to occur following exposures somewhat below 72 ppb (II.C.4.b.i). 
However, as described above (II.C.4.b.i, II.C.4.b.iii, II.C.4.b.iv), 
the Administrator is 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, given her 
consideration of the following: (1) ATS recommendations indicating 
uncertainty in judging adversity based on lung function decrements 
alone; (2) uncertainty in the extent to which a short-term, transient 
population-level decrease in FEV1 would increase the risk of 
other, more serious respiratory effects in that population (i.e., per 
ATS recommendations on population-level risk); and (3) compared to 72 
ppb, CASAC advice is less clear regarding the potential adversity of 
effects at 60 ppb.
    Taken together, the Administrator concludes that the evidence from 
controlled human exposure studies provides strong support for her 
conclusion that a revised standard with a level of 70 ppb is requisite 
to protect the public health with an adequate margin of safety. She 
bases this conclusion, in part, on the fact that such a standard level 
would be well below the O3 exposure concentration shown to 
result in the widest range of respiratory effects (i.e., 80 ppb), and 
below the lowest O3 exposure concentration shown to result 
in the adverse combination of lung function decrements and respiratory 
symptoms (i.e., 72 ppb). See Lead Industries, 647 F. 2d at 1160 
(setting NAAQS at level well below the level where the clearest adverse 
effects occur, and at a level eliminating most ``sub-clinical effects'' 
provides an adequate margin of safety).
    As discussed above (II.C.4.b.i), the Administrator also notes that 
a revised O3 standard with a level of 70 ppb can provide 
substantial protection against the broader range of O3 
exposure concentrations that have been shown in controlled human 
exposure studies to result in respiratory effects, including exposure 
concentrations below 70 ppb. The degree of protection provided by any 
NAAQS is due to the combination of all of the elements of the standard 
(i.e., indicator, averaging time, form, level) and, in the case of the 
fourth-high form of the revised primary O3 standard 
(II.C.3), the large majority of days in areas that meet the revised 
standard will have 8-hour O3 concentrations below 70 ppb, 
with most days having 8-hour O3 concentrations well below 
this level. In addition, the degree of protection provided by the 
O3 NAAQS is also dependent on the extent to which people 
experience health-relevant O3 exposures in locations meeting 
the NAAQS. As discussed above, for a pollutant like O3 where 
adverse responses are critically dependent on ventilation rates, the 
Administrator notes that it is important to consider activity patterns 
in the exposed population. Not considering activity patterns, and 
corresponding ventilation rates, can result in a standard that provides 
more protection than is requisite. Therefore, as discussed in the 
proposal, in considering the degree of protection provided by a revised 
primary O3 standard, the Administrator considers the extent 
to which that standard would be expected to limit population exposures 
of concern (i.e., which take into account activity patterns and 
estimated ventilation rates) to the broader range of O3 
exposure concentrations shown to result in health effects.
    Due to interindividual variability in responsiveness, the 
Administrator notes that not every occurrence of an exposure of concern 
will result in an adverse effect (II.C.4.b.i). Moreover, repeated 
occurrences of some of the effects demonstrated following exposures of 
concern could increase the likelihood of adversity (U.S. EPA, 2013, 
Section 6.2.3, p. 6-76). In particular, she notes that the types of 
respiratory effects that can occur following exposures of concern, 
particularly if experienced repeatedly, provide a plausible mode of 
action by which O3 may cause other more serious effects. 
Therefore, as in the proposal, the Administrator is most concerned 
about protecting at-risk populations against repeated occurrences of 
exposures of concern. In considering the appropriate metric for 
evaluating repeated occurrences of exposures of concern, the 
Administrator acknowledges that it is not clear from the evidence, or 
from the ATS recommendations, CASAC advice, or public comments, how 
particular numbers of exposures of concern could impact the seriousness 
of the resulting effects, especially at lower exposure concentrations. 
Therefore, the Administrator judges that focusing on HREA estimates of 
two or more exposures of concern provides a health-protective approach 
to considering the potential for repeated occurrences of exposures of 
concern that could result in adverse effects.
    Based on her consideration of adversity discussed above, the 
Administrator places the most emphasis on setting a standard that 
appropriately limits repeated occurrences of exposures of concern at or 
above the 70 and 80 ppb benchmarks. She notes 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.
    While she is less confident that adverse effects will occur 
following exposures to O3 concentrations as low as 60 ppb, 
as discussed above, the

[[Page 65364]]

Administrator judges that it is also appropriate to consider estimates 
of exposures of concern for the 60 ppb benchmark. Consistent with this 
judgment, although CASAC advice regarding the potential adversity of 
effects at 60 ppb was less definitive than for effects at 72 ppb, CASAC 
did clearly advise the EPA to consider the extent to which a revised 
standard is estimated to limit the effects observed following 60 ppb 
exposures (Frey, 2014c). Therefore, the Administrator considers 
estimated exposures of concern for the 60 ppb benchmark, particularly 
considering the extent to which the health protection provided by a 
revised standard includes a margin of safety against the occurrence of 
adverse O3-induced effects. The Administrator notes that a 
revised standard with a level of 70 ppb is 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 
exposures of concern at or above 60 ppb. Compared to the current 
standard, this represents a reduction of more than 60%.
    Given the considerable protection provided against repeated 
exposures of concern for all of the benchmarks evaluated, including the 
60 ppb benchmark, the Administrator judges that a standard with a level 
of 70 ppb will incorporate a margin of safety against the adverse 
O3-induced effects shown to occur following exposures at or 
above 72 ppb, and judged likely to occur following exposures somewhat 
below 72 ppb.
    While the Administrator is less concerned about single occurrences 
of O3 exposures of concern, especially for the 60 ppb 
benchmark, she judges that estimates of one or more exposures of 
concern can provide further insight into the margin of safety provided 
by a revised standard. In this regard, she notes 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 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). The Administrator judges that these results provide further 
support for her conclusion that a standard with a level of 70 ppb will 
incorporate an adequate margin of safety against the occurrence of 
O3 exposures that can result in effects that are adverse to 
public health.
    The Administrator additionally judges that a standard with a level 
of 70 ppb would be expected to result in important reductions, compared 
to the current standard, in the population-level risk of O3-
induced lung function decrements (>=10%, >=15%) in children, including 
children with asthma. Specifically, a revised standard with a level of 
70 ppb is estimated to reduce the risk of two or more O3-
induced decrements by about 30% and 20% for decrements >=15 and 10%, 
respectively (Table 2, above). However, as discussed above 
(II.C.4.b.i), the Administrator judges that there are important 
uncertainties in using lung function risk estimates as a basis for 
considering the occurrence of adverse effects in the population given 
(1) the ATS recommendation that ``a small, transient loss of lung 
function, by itself, should not automatically be designated as 
adverse'' (ATS, 2000a); (2) uncertainty in the extent to which a 
transient population-level decrease in FEV1 would increase 
the risk of other, more serious respiratory effects in that population 
(i.e., per ATS recommendations on population-level risk); and (3) that 
CASAC did not advise considering a standard that would be estimated to 
eliminate O3-induced lung function decrements >=10 or 15% 
(Frey, 2014c). Moreover, as at proposal, the Administrator notes that 
the variability in lung function risk estimates across urban study 
areas is often greater than the differences in risk estimates between 
various standard levels (Table 2, above).\150\ Given this, and the 
resulting considerable overlap between the ranges of lung function risk 
estimates for different standard levels, the Administrator puts limited 
weight on the lung function risk estimates for distinguishing between 
the degrees of public health protection provided by alternative 
standard levels. Therefore, the Administrator judges that while a 
standard with a level of 70 ppb would be expected to result in 
important reductions, compared to the current standard, in the 
population-level risk of O3-induced lung function decrements 
(10%, 15%) in children, including children with asthma, she 
also judges that estimated risks of O3-induced lung function 
decrements provide a more limited basis than exposures of concern for 
distinguishing between the appropriateness of the health protection 
afforded by a standard level of 70 ppb versus lower levels.
---------------------------------------------------------------------------

    \150\ For example, the average percentage of children estimated 
to experience two or more decrements >=10% ranges from approximately 
6 to 11% for a standard level of 70 ppb, up to about 9% for a level 
of 65 ppb, and up to about 6% for a level of 60 ppb (Table 2, 
above).
---------------------------------------------------------------------------

    The Administrator also considers the epidemiologic evidence and the 
quantitative risk estimates based on information from epidemiologic 
studies. As discussed in the proposal, and above in the EPA's responses 
to significant comments, although the Administrator acknowledges the 
important uncertainties in using the O3 epidemiologic 
studies as a basis for selecting a standard level, she notes that these 
studies can provide perspective on the degree to which O3-
associated health effects have been identified in areas with air 
quality likely to have met various standards. Specifically, the 
Administrator notes analyses in the PA (U.S. EPA, 2014c, section 4.4.1) 
indicating that a revised standard with a level of 70 ppb would be 
expected to require additional reductions, beyond those required by the 
current standard, in the short- and long-term ambient O3 
concentrations that provided the basis for statistically significant 
O3 health effect associations in both the single-city and 
multicity epidemiologic studies evaluated. As discussed above in the 
response to comments, while the Administrator concludes that these 
analyses support a level at least as low as 70 ppb, based on a study 
reporting health effect associations in a location that met the current 
standard over the entire study period but that would have violated a 
revised standard with a level of 70 ppb,\151\ she further judges that 
they are of more limited utility for distinguishing between the 
appropriateness of the health protection estimated for a standard level 
of 70 ppb and the protection estimated for lower levels. Thus, the 
Administrator notes that a revised standard with a level of 70 ppb will 
provide additional public health protection, beyond that provided by 
the current standard, against the clearly adverse effects reported in

[[Page 65365]]

epidemiologic studies. She judges that a standard with a level of 70 
ppb strikes an appropriate balance between setting the level to require 
reductions in the ambient O3 concentrations associated with 
statistically significant health effects in epidemiologic studies, 
while not being more protective than necessary in light of her 
considerable uncertainty in the extent to which studies clearly show 
O3-attributable effects at lower ambient O3 
concentrations. This judgment is consistent with the Administrator's 
conclusions based on information from controlled human exposure 
studies, as discussed above.
---------------------------------------------------------------------------

    \151\ As discussed above (II.B.2.c.ii and II.B.3), the study by 
Mar and Koenig (2009) reported positive and statistically 
significant associations with respiratory emergency department 
visits in a location that would have met the current standard over 
the entire study period, but violated a standard with a level of 70 
ppb. In addition, air quality analyses in the locations of two 
additional studies highlighted in sections II.B.2 and II.B.3 
(Silverman and Ito, 2010; Strickland et al., 2010) were used in the 
PA to inform staff conclusions on the adequacy of the current 
primary O3 standard. However, they did not provide 
insight into the appropriateness of standard levels below 75 ppb 
and, therefore, these analyses were not used to inform conclusions 
on potential alternative standard levels lower than 75 ppb (U.S. 
EPA, 2014c, Chapters 3 and 4). See Mississippi, 744 F. 3d at 1352-53 
(study appropriate for determining causation may not be probative 
for determining level of a revised standard).
---------------------------------------------------------------------------

    With regard to epidemiology-based risk estimates, the Administrator 
takes note of the CASAC conclusion that ``[a]lthough the estimates for 
short-term exposure impacts are subject to uncertainty, the data 
supports a conclusion that there are meaningful reductions in mean 
premature mortality associated with ozone levels lower than the current 
standard'' (Frey, 2014a, p. 10). While she concludes that epidemiology-
based risk analyses provide only limited support for any specific 
standard level, consistent with CASAC advice the Administrator judges 
that, compared to the current standard, a revised standard with a level 
of 70 ppb will result in meaningful reductions in the mortality and 
respiratory morbidity risk that is associated with short-or long-term 
ambient O3 concentrations.
    Given all of the evidence and information discussed above, the 
Administrator judges that a standard with a level of 70 ppb is 
requisite to protect public health with an adequate margin of safety, 
and that a level below 70 ppb would be more than ``requisite'' to 
protect the public health. In reaching this conclusion, she notes that 
a decision to set a lower level would place a large amount of emphasis 
on the potential public health importance of (1) further reducing the 
occurrence of O3 exposures of concern, though the exposures 
about which she is most concerned are estimated to be almost eliminated 
with a level of 70 ppb, and lower levels would be expected to achieve 
virtually no additional reductions in these exposures (see Table 1, 
above); (2) further reducing the risk of O3-induced lung 
function decrements 10 and 15%, despite having less 
confidence in judging the potential adversity of lung function 
decrements alone and the considerable overlap between risk estimates 
for various standard levels that make it difficult to distinguish 
between the risk reductions achieved; (3) further reducing ambient 
O3 concentrations, relative to those in locations of 
epidemiologic studies, though associations have not been reported for 
air quality that would have met a standard with a level of 70 ppb 
across all study locations and over entire study periods, and despite 
her consequent judgment that air quality analyses in epidemiologic 
study locations are not informative regarding the additional degree of 
public health protection that would be afforded by a standard set at a 
level below 70 ppb; and (4) further reducing epidemiology-based risk 
estimates, despite the important uncertainties in those estimates. As 
discussed in this section and in the responses to significant comments 
above, the Administrator does not agree that it is appropriate to place 
significant weight on these factors or to use them to support the 
appropriateness of standard levels below 70 ppb O3. Compared 
to an O3 standard level of 70 ppb, the Administrator 
concludes that the extent to which lower standard levels could result 
in further public health improvements becomes notably less certain.
    Thus, having carefully considered the evidence, information, CASAC 
advice, and public comments relevant to her decision on the level of 
the primary O3 standard, as discussed above and in the 
Response to Comments document, the Administrator is revising the level 
of the primary O3 standard to 70 ppb. She is mindful that 
the selection of a primary O3 standard that is requisite to 
protect public health with an adequate margin of safety requires 
judgments based on an interpretation of the scientific evidence and 
exposure/risk information that neither overstate nor understate the 
strengths and limitations of that evidence and information and the 
appropriate inferences to be drawn therefrom. Her decision places the 
greatest emphasis 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. As in the 
proposal, and as discussed above, she views the results of the lung 
function risk assessment, analyses of O3 air quality in 
locations of epidemiologic studies, and epidemiology-based quantitative 
health risk assessments as providing information in support of her 
decision to revise the current standard, but a more limited basis for 
selecting a particular standard level among a range of options.
    In making her decision to revise the level of the primary 
O3 standard to 70 ppb, the Administrator judges that a 
revised standard with a level of 70 ppb strikes the appropriate balance 
between limiting the O3 exposures about which she is most 
concerned and not going beyond what would be required to effectively 
limit such exposures. Specifically, the Administrator judges it 
appropriate to set a standard estimated to eliminate, or almost 
eliminate, repeated occurrences of exposures of concern for the 70 and 
80 ppb benchmarks. She further judges that a lower standard level would 
not be appropriate given that lower levels would be expected to achieve 
virtually no additional reductions in repeated occurrences of exposures 
of concern for these benchmarks. For the 60 ppb benchmark, a level of 
70 ppb is estimated to protect the vast majority of children (including 
children with asthma) in urban study areas from experiencing two or 
more exposures of concern, reflecting important reductions in such 
exposures compared to the current standard and indicating that the 
revised primary O3 standard provides an adequate margin of 
safety. Given these results, including the considerable protection 
provided against repeated exposures of concern for the 60 ppb 
benchmark, the Administrator judges that a standard with a level of 70 
ppb incorporates an adequate margin of safety against the occurrence of 
adverse O3-induced effects.
    For all of the above reasons, the Administrator concludes that a 
primary O3 standard with an 8-hour averaging time; a 3-year 
average, fourth-high form; and a level of 70 ppb is requisite to 
protect public health, including the health of at-risk populations, 
with an adequate margin of safety. Therefore, in this final rule she is 
setting the level of the primary O3 standard at 70 ppb.

D. Decision on the Primary Standard

    For the reasons discussed above, and taking into account 
information and assessments presented in the ISA, HREA, and PA, the 
advice and recommendations of the CASAC Panel, and the public comments, 
the Administrator has decided to revise the existing 8-hour primary 
O3 standard. Specifically, the Administrator is revising the 
level of the primary O3 standard to 70 ppb. The revised 8-
hour primary standard, with a level of 70 ppb, would be met at an 
ambient air monitoring site when the 3-year average of the annual 
fourth-highest daily maximum 8-hour average O3 concentration 
is less than or equal to 70 ppb. Data handling conventions are 
specified in the new Appendix U that is adopted, as discussed in 
section V below.

[[Page 65366]]

    At this time, EPA is also promulgating revisions to the Air Quality 
Index (AQI) for O3 to be consistent with the revisions to 
the primary O3 standard and the health information evaluated 
in this review of the standards. These revisions are discussed below in 
section III.

III. Communication of Public Health Information

    Information on the public health implications of ambient 
concentrations of criteria pollutants is currently made available 
primarily through EPA's AQI program. The AQI has been in use since its 
inception in 1999 (64 FR 42530). It provides accurate, timely, and 
easily understandable information about daily levels of pollution. It 
is designed to tell individual members of the public how clean or 
unhealthy their air is, whether health effects might be a concern, and, 
if so, measures individuals can take to reduce their exposure to air 
pollution.\152\ See CAA section 127. The AQI focuses on health effects 
individuals may experience within a few hours or days after breathing 
unhealthy air. The AQI establishes a nationally uniform system of 
indexing pollution concentrations for O3, CO, 
NO2, PM and SO2. The AQI converts pollutant 
concentrations in a community's air to a number on a scale from 0 to 
500. Reported AQI values enable the public to know whether air 
pollution concentrations in a particular location are characterized as 
good (0-50), moderate (51-100), unhealthy for sensitive groups (101-
150), unhealthy (151-200), very unhealthy (201-300), or hazardous (301-
500). The AQI index value of 100 typically corresponds to the level of 
the short-term NAAQS for each pollutant. For the 2008 O3 
NAAQS, an 8-hour average concentration of 75 ppb corresponds to an AQI 
value of 100. An AQI value greater than 100 means that a pollutant is 
in one of the unhealthy categories (i.e., unhealthy for sensitive 
groups, unhealthy, very unhealthy, or hazardous) on a given day; an AQI 
value at or below 100 means that a pollutant concentration is in one of 
the satisfactory categories (i.e., moderate or good). An additional 
consideration in selecting breakpoints is for each category to span at 
least a 15 ppb range to allow for more accurate air pollution 
forecasting. Decisions about the pollutant concentrations at which to 
set the various AQI breakpoints, that delineate the various AQI 
categories, draw directly from the underlying health information that 
supports the NAAQS review.
---------------------------------------------------------------------------

    \152\ EPA issued the AQI in 1999, updating the previous 
Pollutant Standards Index (PSI) to send ``a clear and consistent 
message to the public by providing nationally uniform information on 
air quality.'' The rule requires metropolitan areas of 350,000 and 
larger to report the AQI [and associated health effects] daily; all 
other AQI-related activities--including real-time ozone and particle 
pollution reporting, next-day air quality forecasting and action 
days--are voluntary and are carried out at the discretion of state, 
local and tribal air agencies. In the 1999 rule, we acknowledged 
these other programs, noting, for example, that while states 
primarily use the AQI ``to provide general information to the public 
about air quality and its relationship to public health,'' some 
state, local or tribal agencies use the index to call ``action 
days.'' Action days encourage additional steps, usually voluntary, 
that the public, business or industry could take to reduce emissions 
when higher levels of pollution are forecast to occur. As the 1999 
rule notes, agencies may have several motivations for calling action 
days, including: providing health information to the public; 
attaining or maintaining NAAQS attainment status; meeting specific 
emission reduction targets; and managing or reducing traffic 
congestion. State, local and tribal agencies should consider whether 
non-voluntary emissions or activity curtailments are necessary (as 
opposed to a suite of voluntary measures) for days when the AQI is 
forecasted to be on the lower end of the moderate category.
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A. Proposed Revisions to the AQI

    Recognizing the importance of revising the AQI in a timely manner 
to be consistent with any revisions to the NAAQS, EPA proposed 
conforming changes to the AQI, in connection with the Agency's proposed 
decision on revisions to the O3 NAAQS. These conforming 
changes included setting the 100 level of the AQI at the same level as 
the revised primary O3 NAAQS and also making adjustments 
based on health information from this NAAQS review to AQI breakpoints 
at the lower end of each range (i.e., AQI values of 50, 150, 200 and 
300). The EPA did not propose to change the level at the top of the 
index (i.e., AQI value of 500) that typically is set equal to the 
Significant Harm Level (40 CFR 51.16), which would apply to state 
contingency plans.
    The EPA proposed to revise the AQI for O3 by setting an 
AQI value of 100 equal to the level of the revised O3 
standard (65-70 ppb). The EPA also proposed to revise the following 
breakpoints: an AQI value of 50 to within a range from 49-54 ppb; an 
AQI value of 150 to 85 ppb; an AQI value of 200 to 105 ppb, and an AQI 
value of 300 to 200 ppb. All these levels are averaged over 8 hours. 
The EPA proposed to set an AQI value of 50, the breakpoint between the 
good and moderate categories, at 15 ppb below the value of the proposed 
standard, i.e. to within a range from 49 to 54 ppb. The EPA took 
comment on what level within this range to select, recognizing that 
there is no health message for either at-risk or healthy populations in 
the good category. Thus, the level selected should be below the lowest 
concentration (i.e., 60 ppb) that has been shown in controlled human 
exposure studies of young, healthy adults exposed to O3 
while engaged in quasi-continuous moderate exercise for 6.6 hours to 
cause moderate lung function decrements (i.e., FEV1 
decrements >= 10%, which could be adverse to people with lung disease) 
and airway inflammation.\153\ The EPA proposed to set an AQI value of 
150, the breakpoint between the unhealthy for sensitive groups and 
unhealthy categories, at 85 ppb. At this level, controlled human 
exposure studies of young, healthy adults indicate that up to 25% of 
exposed people are likely to have moderate lung function decrements 
(i.e., 25% have FEV1 decrements >= 10%; 12% have 
FEV1 decrements >= 15%) and up to 7% are likely to have 
large lung function decrements (i.e., FEV1 decrements >= 
20%) (McDonnell et al., 2012; Figure 7). Large lung function decrements 
would likely interfere with normal activity for many healthy people. 
For most people with lung disease, large lung function decrements would 
not only interfere with normal activity but would increase the 
likelihood that they would seek medical treatment (72 FR 37850, July 
11, 2007). The EPA proposed to set an AQI value of 200, the breakpoint 
between the unhealthy and very unhealthy categories, at 105 ppb. At 
this level, controlled human exposure studies of young, healthy adults 
indicate that up to 38% of exposed people are likely to have moderate 
lung function decrements (i.e., 38% have FEV1 decrements >= 
10%; 22% have FEV1 decrements >= 15%) and up to 13% are 
likely to have large lung function decrements (i.e., FEV1 
decrements >= 20%). The EPA proposed to set an AQI value of 300, the 
breakpoint between the very unhealthy and hazardous categories, at 200 
ppb. At this level, controlled human exposure studies of healthy adults 
indicate that up to 25% of exposed individuals are likely to have large 
lung function decrements (i.e., FEV1 decrements >= 20%), 
which would interfere with daily activities for many of them and likely 
cause people with lung disease to seek medical attention.
---------------------------------------------------------------------------

    \153\ Exposures to 50 ppb have not been evaluated 
experimentally, but are estimated to potentially affect only a small 
proportion of healthy adults and with only a half to a third of the 
moderate to large lung function decrements observed at 60 ppb 
(McDonnell et al., 2012; Figure 7).
---------------------------------------------------------------------------

    EPA stated that the proposed breakpoints reflect an appropriate 
balance between reflecting the health evidence that is the basis for 
the proposed primary O3 standard and providing category 
ranges that are large enough to be forecasted accurately, so

[[Page 65367]]

that the new AQI for O3 can be implemented more easily in 
the public forum for which the AQI ultimately exists. However, the EPA 
recognized alternative approaches to viewing the evidence and 
information and solicited comment on the proposed revisions to the AQI.
    With respect to reporting requirements (40 CFR part 58, section 
58.50), EPA proposed to revise 40 CFR part 58, section 58.50 (c) to 
determine the areas subject to AQI reporting requirements based on the 
latest available census figures, rather than the most recent decennial 
U.S. census.\154\ This change is consistent with our current practice 
of using the latest population figures to make monitoring requirements 
more responsive to changes in population.
---------------------------------------------------------------------------

    \154\ Under 40 CFR 58.50, any MSA with a population exceeding 
350,000 is required to report AQI data.
---------------------------------------------------------------------------

B. Comments on Proposed Revisions to the AQI

    EPA received many comments on the proposed changes to the AQI. 
Three issues came up in the comments, including: (1) Whether the AQI 
should be revised at all, even if the primary standard is revised; (2) 
whether an AQI value of 100 should be set equal to the level of the 
primary standard and the other breakpoints adjusted accordingly; and, 
(3) whether the AQI reporting requirements should be based on the 
latest available census figures rather than the most recent decennial 
census.
    With respect to the first issue, some industry commenters stated 
that the AQI should not be revised at all, even if the level of the 
primary O3 standard is revised. In support of this position, 
these commenters stated that the proposed conforming changes to the AQI 
would lower O3 levels in each category, and would mean that 
air quality that is actually improving would be reported as less 
healthy. According to commenters, the revised AQI would fail to capture 
these improvements and potentially mislead the public into thinking 
that air quality has degraded and that EPA and state regulators are not 
doing their jobs. These commenters noted that there is no requirement 
to revise the AQI, and that the CAA does not tie the AQI to the 
standards, stating that the purpose of section 319(a) of the CAA is to 
provide a consistent, uniform means of gauging air quality. These 
commenters further asserted that EPA's proposed changes run counter to 
that uniformity by changing the air quality significance of a given 
index value and category and that retention of the current AQI 
breakpoints would allow continued uniform information on air quality. 
Commenters stated that it is important that the EPA clearly 
communicates that the immediate increases in moderate rated days are 
due to AQI breakpoint adjustment and not due to a sudden decline in air 
quality. One commenter estimated the increased proportion of days in 
the moderate category and above in 10 metropolitan areas for 2013 and 
also for 2025 for 4 cities from the original 10 that were estimated to 
attain a standard below 70 ppb, to compare with 2013. This commenter 
noted that the change in the proposed AQI breakpoint between ``good'' 
and ``moderate'' would result in a larger number of days that did not 
meet the ``good'' criteria. They went further to claim that the change 
in breakpoints would result in fewer ``good'' days in the year 2025 
(using the new breakpoint) than occurred in 2013 (using the old 
breakpoints) despite substantial improvement in air quality over that 
time period.
    On the other hand, state and local agencies and their 
organizations, environmental and medical groups, and members of the 
public overwhelmingly supported revising the AQI when the level of the 
standard is revised. Even state agencies that did not support revising 
the standard, expressed support for revising the AQI at the same time 
as the standard, if the standard is revised.
    Recognizing the importance of the AQI as a communication tool that 
allows members of the public to take exposure reduction measures when 
air quality poses health risks, the EPA agrees with these comments 
about revising the AQI at the same time as the primary standard. The 
EPA agrees with state and local agency commenters that its historical 
approach of setting an AQI value of 100 equal to the level of the 
revised 8-hour primary O3 standard is appropriate, both from 
a public health and a communication perspective.
    EPA disagrees with commenters who stated that the AQI should not be 
linked to the primary standards. As noted in the August 4, 1999, 
rulemaking (64 FR 149, 42531) that established the current AQI, the EPA 
established the nationally uniform air quality index, called the 
Pollutant Standards Index (PSI), in 1976 to meet the needs of state and 
local agencies with the following advantages: It sends a clear and 
consistent message to the public by providing nationally uniform 
information on air quality; it is keyed as appropriate to the NAAQS and 
the Significant Harm Level which have a scientific basis relating air 
quality and public health; it is simple and easily understood by the 
public; it provides a framework for reflecting changes to the NAAQS; 
and it can be forecasted to provide advance information on air quality. 
Both the PSI and AQI have historically been normalized across 
pollutants by defining an index value of 100 as the numerical level of 
the short-term (i.e., averaging time of 24-hours or less) primary NAAQS 
for each pollutant. Moreover, this approach does not mislead the 
public. Since the establishment of the AQI, the EPA and state and local 
air agencies and organizations have developed experience in educating 
the public about changes in the standards and, concurrently, related 
changes to AQI breakpoints and advisories. When the standards change, 
EPA and state and local agencies have tried to help the public 
understand that air quality is not getting worse, it's that the health 
evidence underlying the standards and the AQI has changed. EPA's Air 
Quality System (AQS), the primary repository for air quality monitoring 
data, is also adjusted to reflect the revised breakpoints. 
Specifically, all historical AQI values in AQS are recomputed with the 
revised breakpoints, so that all data queries and reports downstream of 
AQS will show appropriate trends in AQI values over time.\155\
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    \155\ Although we do not contest the assertion that the new AQI 
breakpoints will lead to fewer green days in the near future, we do 
not agree that commenters' analysis sufficiently demonstrates that 
there would be fewer green days in 2025 than in 2013. In their 
analysis, they compared observed 2013 data with modeled 2025 data 
without doing any model performance evaluation for AQI categories or 
comparison of current year modeled and observed data. The current 
year observations are not directly comparable to the future-year 
modeling data without some such evaluation and, as such, we cannot 
support their quantitative conclusions.
---------------------------------------------------------------------------

    In general, commenters who supported revising the AQI when the 
standard is revised, also supported setting an AQI value of 100 equal 
to the level of the 8-hour primary O3 standard. The EPA 
agrees with these commenters. With respect to an AQI value of 100, the 
EPA is taking final action to set an AQI value of 100 equal to the 
level of the 8-hour primary standard at 70 ppb O3.
    With respect to proposed changes to other AQI breakpoints, some 
state and local agency commenters expressed general support for all the 
changes in O3 breakpoints (in Table 2 of Appendix G). In 
addition, we received a few comments specifically about the breakpoint 
between the good and moderate categories. One state expressed the view 
that forecasting the AQI for O3 is not an exact science, so 
it is important to provide a range large enough to reasonably predict 
O3

[[Page 65368]]

concentrations for the following day (>= 20 ppb). Although not 
supporting revision of the standard, this state recommended that if the 
primary standard was revised to 70 ppb, the lower end of moderate 
category should be set at 50 ppb to allow for a 20 ppb spread in that 
category. Several commenters recommending a breakpoint between the good 
and moderate categories of no higher than 50 ppb stated that this 
breakpoint should be set on health information, pointing to 
epidemiologic data and the World Health organization guidelines. The 
Agency agrees that AQI breakpoints should take into consideration 
health information when possible, and also that it is important for AQI 
categories to span ranges large enough to support accurate forecasting. 
The EPA is setting the breakpoint at the lower end of the moderate 
category at 55 ppb, which is 15 ppb below the level of the standard of 
70 ppb. This is consistent with past practice of making a proportional 
adjustment to this AQI breakpoint, relative to an AQI value of 100 
(i.e., 70 ppb), and also retains the current practice of providing a 15 
ppb range in the moderate category to allow for accurate forecasting. 
This level is below the lowest concentration (i.e., 60 ppb) that has 
been shown in controlled human exposure studies of healthy adults to 
cause moderate lung function decrements (i.e., FEV1 
decrements >= 10%, which could be adverse to people with lung disease), 
large lung function decrements (i.e., FEV1 decrements >= 
20%) in a small proportion of people, and airway inflammation, 
notwithstanding the Administrator's judgment that there is uncertainty 
in the adversity of the effects shown to occur at 60 ppb.
    We received fewer comments on proposed changes to the AQI values of 
150, 200 and 300. Again, some state and local agency commenters 
expressed general support for proposed changes to the AQI. Some states 
specifically supported these breakpoints. However, a commenter 
suggested setting an AQI value at the lower end of the unhealthy 
category, at a level much lower than 85 ppb, since they state that it 
is a key threshold that is often used in air quality action day 
programs as a trigger to encourage specific behavior modifications or 
reduce emissions of O3 precursors (e.g., by taking public 
transportation to work). This commenter stated that setting the 
breakpoint at 85 ppb would, in the Agency's own rationale, not require 
the triggering of these pollution reduction measures until air quality 
threatened to impact 25% of people exposed. We disagree with this 
commenter because EPA does not have any requirements for voluntary 
programs. State and local air agencies have discretion to set the 
trigger for voluntary action programs at whatever level they choose, 
and they are currently set at different levels, not just at the 
unhealthy breakpoint specified in the comment. For example, Houston, 
Galveston and Brazoria TX metropolitan area calls ozone action days 
when air quality reaches the unhealthy for sensitive groups category. 
For more information about action days programs across the U.S. see the 
AirNow Web site (www.airnow.gov) and click on the link to AirNow Action 
Days. The unhealthy category represents air quality where there are 
general population-level effects. We believe that setting the 
breakpoint between the unhealthy for sensitive groups and unhealthy 
categories, at 85 ppb where, as discussed in section IIIA above, 
controlled human exposure studies of young, healthy adults exposed to 
O3 while engaged in quasi-continuous moderate exercise for 
6.6 hours indicate that up to 25% of exposed people are likely to have 
moderate lung function decrements and up to 7% are likely to have large 
lung function decrements (McDonnell et al., 2012; Figure 7) is 
appropriate. A smaller proportion of inactive or less active 
individuals would be expected to experience lung function decrements at 
85 ppb. Moreover, a breakpoint at 85 ppb allows for category ranges 
large enough for accurate forecasting. Accordingly, the EPA is adopting 
the proposed revisions to the AQI values of 150, 200 and 300.
    As noted earlier, the EPA proposed to revise 40 CFR part 58, 
section 58.50(c) to determine the areas subject to AQI reporting 
requirements based on the latest available census figures, rather than 
the most recent decennial U.S. census.
    A total of five state air monitoring agencies provided comments on 
this proposed change. Four agencies supported the proposal. One state 
commenter did not support the proposal, noting that the change would 
unnecessarily complicate AQI reporting and possibly increase reporting 
burdens in an unpredictable manner.
    The EPA notes that the majority of monitoring network minimum 
requirements listed in Appendix D to Part 58 include a reference to 
``latest available census figures.'' Minimum network requirements for 
O3, PM2.5, SO2, and NO2 all 
include this language in the regulatory text and monitoring agencies 
have successfully adopted these processes into their planning 
activities and the subsequent revision of their annual monitoring 
network plans which are posted for public review. Annual population 
estimates are easily obtainable from the U.S. Census Bureau and the EPA 
does not believe the burden in tracking these annual estimates is 
excessive or complicated.\156\ Although the changes in year to year 
estimates are typically modest, there are MSAs that are approaching (or 
have recently exceeded) the 350,000 population AQI reporting limit and 
there is great value in having the AQI reported for these areas when 
the population threshold is exceeded versus waiting potentially up to 
10 years for a revision to the decennial census. Accordingly, the EPA 
is finalizing the proposed revision to 40 CFR part 58, section 58.50(c) 
to require the AQI reporting requirements to be based on the latest 
available census figures.
---------------------------------------------------------------------------

    \156\ http://www.census.gov/popest/data/metro/totals/2014/CBSA-EST2014-alldata.html.
---------------------------------------------------------------------------

    One state requested additional guidance on the frequency of 
updating the AQI reporting threshold, and recommended linking the AQI 
reporting requirement evaluation with the annual air monitoring network 
plan requirements, and recommended requiring AQI reporting to begin no 
later than January 1 of the following year. The EPA notes that the 
census bureau estimates appear to be released around July 1 of each 
year which would not provide sufficient time for monitoring agencies to 
incorporate AQI reporting in their annual plans for that year, which 
are also due by July 1 each year. EPA believes that it should be 
unnecessary for monitoring agencies to wait until the implementation of 
the following year's annual plan (i.e., approximately 18 months later) 
to begin AQI reporting. Accordingly, EPA is not at this time including 
a specific deadline for commencement of AQI reporting for newly-subject 
areas in 40 CFR part 58, but will work with agencies to implement 
additional AQI reporting as needed to ensure that information is being 
disseminated in a timely fashion.

C. Final Revisions to the AQI

    For the reasons discussed above, the EPA is revising the AQI for 
O3 by setting an AQI value of 100 equal to 70 ppb, 8-hour 
average, the level of the revised primary O3 standard. The 
EPA is also revising the following breakpoints: An AQI value of 50 is 
set at 54 ppb; an AQI value of 150 is set at 85 ppb; an AQI value of 
200 is set at 105 ppb; and an AQI value of 300 is set at 200 ppb. All 
of these levels are averaged over 8 hours. The revisions to all of the

[[Page 65369]]

breakpoints are based on estimated health outcomes at relevant ambient 
concentrations and to allow for each category to span at least a 15-20 
ppb category range to allow for more accurate air pollution 
forecasting. The EPA believes that the revised breakpoints provide a 
balance between adjustments to reflect the health information 
supporting the revised O3 standard and providing category 
ranges that are large enough to be forecasted accurately, so that the 
AQI can be implemented more easily in the public forum for which the 
AQI ultimately exists. With respect to AQI reporting requirements (40 
CFR part 58, section 58.50), the EPA is revising 40 CFR part 58, 
section 58.50(c) to make the AQI reporting requirements based on the 
latest available census figures, rather than the most recent decennial 
U.S. census. This change is consistent with our current practice of 
using the latest population figures to make monitoring requirements 
more responsive to changes in population.

IV. Rationale for Decision on the Secondary Standard

A. Introduction

    This section (IV) presents the rationale for the Administrator's 
decisions regarding the need to revise the current secondary standard 
for O3, and the appropriate revision. Based on her 
consideration of the full body of welfare effects evidence and related 
analyses, including the evidence of effects associated with cumulative 
seasonal exposures of the magnitudes allowed by the current standard, 
the Administrator has concluded that the current secondary standard for 
O3 does not provide the requisite protection of public 
welfare from known or anticipated adverse effects. She has decided to 
revise the level of the current secondary standard to 0.070 ppm, in 
conjunction with retaining the current indicator, averaging time and 
form.
    The Administrator has made this decision based on judgments 
regarding the currently available welfare effects evidence, the 
appropriate degree of public welfare protection for the revised 
standard, and currently available air quality information on seasonal 
cumulative exposures that may be allowed by such a standard. In so 
doing, she has focused on O3 effects on tree seedling growth 
as a proxy for the full array of vegetation-related effects of 
O3, ranging from effects on sensitive species to broader 
ecosystem-level effects. Using this proxy in judging effects to public 
welfare, the Administrator has concluded that the requisite protection 
from adverse effects to public welfare will be provided by a standard 
that limits cumulative seasonal exposures to 17 ppm-hrs or lower, in 
terms of a 3-year W126 index, in nearly all instances, and she has also 
concluded that such control of cumulative seasonal exposures may be 
achieved by revising the level of the current standard to 70 ppb. Based 
on all of these considerations, the Administrator has decided that a 
secondary standard with a level of 0.070 ppm, and the current form and 
averaging time, will provide the requisite protection of public welfare 
from known or anticipated adverse effects.
    As discussed more fully below, this decision is based on a thorough 
review, in the ISA, of the latest scientific information on 
O3-induced environmental effects. This decision also takes 
into account (1) staff assessments in the PA of the most policy-
relevant information in the ISA regarding evidence of adverse effects 
of O3 to vegetation and ecosystems, information on 
biologically-relevant exposure metrics, WREA analyses of air quality, 
exposure, and ecological risks and associated ecosystem services, and 
staff analyses of relationships between levels of a W126-based metric 
and a metric based on the form and averaging time of the current 
standard summarized in the PA and in the proposal notice; (2) CASAC 
advice and recommendations; and (3) public comments received during the 
development of these documents, either in connection with CASAC 
meetings or separately, and on the proposal notice.
    This decision draws on the ISA's integrative synthesis of the 
entire body of evidence, generally published through July 2011, on 
environmental effects associated with the presence of O3 and 
related photochemical oxidants in the ambient air (U.S. EPA, 2013, ISA 
chapters 9-10), and includes more than four hundred new studies that 
build on the extensive evidence base from the last review. In addition 
to reviewing the most recent scientific information as required by the 
CAA, this rulemaking incorporates the EPA's response to the judicial 
remand of the 2008 secondary O3 standard in State of 
Mississippi v. EPA, 744 F. 3d 1334 (D.C. Cir. 2013) and, in accordance 
with the court's decision in that case, fully explains the 
Administrator's conclusions as to the level of air quality that 
provides the requisite protection of public welfare from known or 
anticipated adverse effects. In drawing conclusions on the secondary 
standard, the decision described in this rulemaking is a public welfare 
policy judgment made by the Administrator. The Administrator's decision 
draws upon the available scientific evidence for O3-
attributable welfare effects and on analyses of exposures and public 
welfare risks based on impacts to vegetation, ecosystems and their 
associated services, as well as judgments about the appropriate weight 
to place on the range of uncertainties inherent in the evidence and 
analyses. As described in sections IV.B.3 and IV.C.3 below, such 
judgments in the context of this review include 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 those related to the variability in 
occurrence of such effects in areas of the U.S., especially areas of 
particular public welfare significance; and on the extent to which such 
effects in such areas may be considered adverse to public welfare.
    Information related to vegetation and ecosystem effects, 
biologically relevant exposure indices, and vegetation exposure and 
risk assessments were summarized in sections IV.A through IV.C of the 
proposal (79 FR at 75314-75329), respectively, and key observations 
from the proposal are briefly outlined in sections IV.A.1 to IV.A.3 
below. Subsequent sections of this preamble provide a more complete 
discussion of the Administrator's rationale, in light of key issues 
raised in public comments, for concluding that the current standard is 
not requisite to protect public welfare from known or anticipated 
adverse effects (section IV.B), and that it is appropriate to revise 
the current secondary standard to provide additional public welfare 
protection by revising the level while retaining the current indicator, 
form and averaging time (section IV.C). A summary of the final 
decisions on revisions to the secondary standard is presented in 
section IV.D.
1. Overview of Welfare Effects Evidence
a. Nature of Effects
    In the more than fifty years that have followed identification of 
O3's phytotoxic effects, extensive research has been 
conducted both in and outside of the U.S. to examine the impacts of 
O3 on plants and their associated ecosystems (U.S. EPA, 
1978, 1986, 1996a, 2006a, 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

[[Page 65370]]

for plant growth, reproduction, and/or yield. For seed-bearing plants, 
these reproductive effects will culminate in reduced seed production or 
yield (U.S. EPA, 1996a, pp. 5-28 and 5-29). Recent studies, assessed in 
the ISA, together with this longstanding and well-established 
literature on O3-related vegetation effects, further 
contribute to the coherence and consistency of the vegetation effects 
evidence (U.S. EPA, 2013, chapter 9).
    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 (U.S. EPA, 2013, p. 1-
15). Such effects at the plant scale can also be linked to an array of 
effects at larger spatial scales, with the currently available evidence 
indicating that ``ambient O3 exposures can affect ecosystem 
productivity, crop yield, water cycling, and ecosystem community 
composition'' (U.S. EPA, 2013, p. 1-15; Chapter 9, section 9.4). The 
current body of O3 welfare effects evidence confirms and 
strengthens support for the conclusions reached in the last review on 
the nature of O3-induced welfare effects and is summarized 
in the ISA as follows (U.S. EPA, 2013, p. 1-8).

    The welfare effects of O3 can be observed across 
spatial scales, starting at the subcellular and cellular level, then 
the whole plant and finally, ecosystem-level processes. Ozone 
effects at small spatial scales, such as the leaf of an individual 
plant, can result in effects along a continuum of larger spatial 
scales. These effects include altered rates of leaf gas exchange, 
growth, and reproduction at the individual plant level, and can 
result in broad changes in ecosystems, such as productivity, carbon 
storage, water cycling, nutrient cycling, and community composition.

    Based on assessment of this extensive body of science, the EPA has 
determined that, with respect to vegetation and ecosystems, a causal 
relationship exists between exposure to O3 in ambient air 
and visible foliar injury effects on vegetation, reduced vegetation 
growth, reduced productivity in terrestrial ecosystems, reduced yield 
and quality of agricultural crops and alteration of below-ground 
biogeochemical cycles (U.S. EPA, 2013, Table 1-2). In consideration of 
the evidence of O3 exposure and alterations in stomatal 
performance, ``which may affect plant and stand transpiration and 
therefore possibly affecting hydrological cycling,'' the ISA concludes 
that ``[a]lthough the direction of the response differed among 
studies,'' the evidence is sufficient to conclude a likely causal 
relationship between O3 exposure and the alteration of 
ecosystem water cycling (U.S. EPA, 2013, section 2.6.3). The evidence 
is also sufficient to conclude a likely causal relationship between 
O3 exposure and the alteration of community composition of 
some terrestrial ecosystems (U.S. EPA, 2013, section 2.6.5). Related to 
the effects on vegetation growth, productivity and, to some extent, 
below-ground biogeochemical cycles, the EPA has additionally determined 
that a likely causal relationship exists between exposures to 
O3 in ambient air and reduced carbon sequestration (also 
termed carbon storage) in terrestrial ecosystems (U.S. EPA, 2013, p. 1-
10 and section 2.6.2). Modeling studies available in this review 
consistently found negative impacts of O3 on carbon 
sequestration, although the severity of impact was influenced by 
``multiple interactions of biological and environmental factors'' (U.S. 
EPA, 2013, p. 2-39).
    Ozone in the troposphere is also a major greenhouse gas and 
radiative forcing agent,\157\ with the ISA formally concluding that 
``the evidence supports a causal relationship between changes in 
tropospheric O3 concentrations and radiative forcing'' (U.S. 
EPA, 2013, p. 1-13 and section 2.7.1). While tropospheric O3 
has been ranked third in importance after carbon dioxide and methane, 
there are ``large uncertainties in the magnitude of the radiative 
forcing estimate attributed to tropospheric O3, making the 
impact of tropospheric O3 on climate more uncertain than the 
effect of the longer-lived greenhouse gases'' (U.S. EPA, 2013, p. 2-
47). The ISA notes that ``[e]ven with these uncertainties, global 
climate models indicate that tropospheric O3 has contributed 
to observed changes in global mean and regional surface temperatures'' 
and concludes that ``[a]s a result of such evidence presented in 
climate modeling studies, there is likely to be a causal relationship 
between changes in tropospheric O3 concentrations and 
effects on climate'' (U.S. EPA, 2013, p. 2-47).\158\ The ISA 
additionally states that ``[i]mportant uncertainties remain regarding 
the effect of tropospheric O3 on future climate change'' 
(U.S. EPA, 2013, p. 10-31).
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    \157\ As described in the ISA, ``[r]adiative forcing by a 
greenhouse gas or aerosol 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 that substance'' (U.S. EPA, 
2013, p. 1-13).
    \158\ Climate responses, including increased surface 
temperature, have downstream climate-related ecosystem effects (U.S. 
EPA, 2013, p. 10-7). As noted in section I.D above, such effects may 
include an increase in the area burned by wildfires, which, in turn, 
are sources of O3 precursor emissions.
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b. Vegetation Effects
    Given the strong evidence base and the findings of causal or likely 
causal relationships with O3 in ambient air, including the 
quantitative assessments of relationships between O3 
exposure and occurrence and magnitude of effects, this review has given 
primary consideration to three main kinds of vegetation effects, some 
of which contribute to effects at scales beyond the plant level, such 
as at the ecosystem level and on ecosystem services. The three kinds of 
effects are addressed below in the following order: 1) Visible foliar 
injury, 2) impacts on tree growth, productivity and carbon storage, and 
3) crop yield loss.
    Visible foliar injury resulting from exposure to O3 has 
been well characterized and documented over several decades of research 
on many tree, shrub, herbaceous, and crop species (U.S. EPA, 2013, p. 
1-10; U.S. EPA, 2006a, 1996a, 1986, 1978). Ozone-induced visible foliar 
injury symptoms on certain plant species, such as black cherry, yellow-
poplar and common milkweed, are considered diagnostic of exposure to 
O3 based on the consistent association established with 
experimental evidence (U.S. EPA, 2013, p. 1-10). The evidence has found 
that visible foliar injury occurs only when sensitive plants are 
exposed to elevated O3 concentrations in a predisposing 
environment; a major modifying factor is the amount of available soil 
moisture during the year (U.S. EPA, 2013, section 9.4.2).
    The significance of O3 injury at the leaf and whole 
plant levels depends on an array of factors, and therefore, it is 
difficult to quantitatively relate visible foliar injury symptoms to 
vegetation effects such as individual tree growth, or effects at 
population or ecosystem levels (U.S. EPA, 2013, p. 9-39). The ISA notes 
that visible foliar injury ``is not always a reliable indicator of 
other negative effects on vegetation'' (U.S. EPA, 2013, p. 9-39). 
Factors that influence the significance to the leaf and whole plant 
include the amount of total leaf area affected, age of plant, size, 
developmental stage, and degree of functional redundancy among the 
existing leaf area (U.S. EPA, 2013, section 9.4.2). Although there 
remains a lack of robust exposure-response functions that would allow 
prediction of visible foliar injury severity and incidence under 
varying air quality and environmental conditions, ``[e]xperimental 
evidence has clearly

[[Page 65371]]

established a consistent association of visible injury with 
O3 exposure, with greater exposure often resulting in 
greater and more prevalent injury'' (U.S. EPA, 2013, section 9.4.2, p. 
9-41).
    By far the most extensive field-based dataset of visible foliar 
injury incidence is that obtained by the U.S. Forest Service Forest 
Health Monitoring/Forest Inventory and Analysis (USFS FHM/FIA) 
biomonitoring network program (U.S. EPA, 2013, section 9.4.2.1; Smith, 
2012; Coulston et al., 2007). A recently published trend analysis of 
data from the sites located in 24 states of the northeast and north 
central U.S. for the 16-year period from 1994 through 2009 (Smith, 
2012) describes evidence of visible foliar injury occurrence in the 
field as well as some insight into the influence of changes in air 
quality and soil moisture on visible foliar injury and the difficulty 
inherent in predicting foliar injury response under different air 
quality and soil moisture scenarios (Smith, 2012; U.S. EPA, 2013, 
section 9.4.2.1). Study results showed that incidence and severity of 
foliar injury were dependent on local site conditions for soil moisture 
availability and O3 exposure (U.S. EPA, 2013, p. 9-41). 
Although the study indicated that moderate O3 exposures 
continued to cause visible foliar injury at sites throughout the study 
area, there was an overall declining trend in the incidence of visible 
foliar injury as peak O3 concentrations declined (U.S. EPA, 
2013, p. 9-40).
    Ozone has been shown to affect a number of important U.S. tree 
species with respect to growth, productivity, and carbon storage. 
Ambient O3 concentrations have long been known to cause 
decreases in photosynthetic rates and plant growth. As discussed in the 
ISA, research published since the 2006 AQCD substantiates prior 
conclusions regarding O3-related effects on forest tree 
growth, productivity and carbon storage, and further strengthens the 
support for those conclusions. A variety of factors in natural 
environments can either mitigate or exacerbate predicted O3-
plant interactions and are recognized sources of uncertainty and 
variability. Such factors include multiple genetically influenced 
determinants of O3 sensitivity, changing sensitivity to 
O3 across vegetative growth stages, co-occurring stressors 
and/or modifying environmental factors (U.S. EPA, 2013, section 9.4.8). 
In considering of the available evidence, the ISA states, ``previous 
O3 AQCDs concluded that there is strong evidence that 
exposure to O3 decreases photosynthesis and growth in 
numerous plant species'' and that ``[s]tudies published since the 2008 
review support those conclusions'' (U.S. EPA, 2013, p. 9-42). The 
available studies come from a variety of different study types that 
cover an array of different species, effects endpoints, levels of 
biological organization and exposure methods and durations. The 
O3-induced effects at the scale of the whole plant may 
translate to the ecosystem scale, with changes in productivity and 
carbon storage. As stated in the ISA, ``[s]tudies conducted during the 
past four decades have demonstrated unequivocally that O3 
alters biomass allocation and plant reproduction'' (U.S. EPA, 2013, p. 
1-10).
    The strong evidence of O3 impacts on trees includes 
robust exposure-response (E-R) functions for reduced growth, termed 
relative biomass loss (RBL),\159\ in seedlings of 11 species. These 
functions were developed under the National Health and Environmental 
Effects Research Laboratory-Western Ecology Division program, a series 
of experiments that used open top chambers (OTCs) to investigate 
seedling growth response for a single growing season under a variety of 
O3 exposures (ranging from near background to well above 
current ambient concentrations) and growing conditions (U.S. EPA, 2013, 
section 9.6.2; Lee and Hogsett, 1996). The evidence from these studies 
shows that there is a wide range in sensitivity across the studied 
species in the seedling growth stage over the course of a single 
growing season, with some species being extremely sensitive and others 
being very insensitive over the range of cumulative O3 
exposures studied (U.S. EPA, 2014c, Figure 5-1). At the other end of 
the organizational spectrum, field-based studies of species growing in 
natural stands have compared observed plant responses across a number 
of different sites and/or years when exposed to varying ambient 
O3 exposure conditions. For example, a study conducted in 
forest stands in the southern Appalachian Mountains during a period 
when O3 concentrations exceeded the current standard found 
that the cumulative effects of O3 decreased seasonal stem 
growth (measured as a change in circumference) by 30-50 percent for 
most of the examined tree species (i.e., tulip poplar, black cherry, 
red maple, sugar maple) in a high-O3 year in comparison to a 
low-O3 year (U.S. EPA, 2013, section 9.4.3.1; McLaughlin et 
al., 2007a). The study also reported that high ambient O3 
concentrations can increase whole-tree water use and in turn reduce 
late-season streamflow (McLaughlin et al., 2007b; U.S. EPA, 2013, p. 9-
43).
---------------------------------------------------------------------------

    \159\ These functions for RBL estimate reduction in a year's 
growth as a percentage of that expected in the absence of 
O3 (U.S. EPA, 2013, section 9.6.2; U.S. EPA, 2014b, 
section 6.2).
---------------------------------------------------------------------------

    The magnitude of O3 impact on ecosystem productivity and 
on forest composition can vary among plant communities based on several 
factors, including the type of stand or community in which the 
sensitive species occurs (e.g., single species versus mixed canopy), 
the role or position of the species in the stand (e.g., dominant, sub-
dominant, canopy, understory), and the sensitivity of co-occurring 
species and environmental factors (e.g., drought and other factors). 
For example, recent studies found O3 to have little impact 
on white fir, but to greatly reduce growth of ponderosa pine in 
southern California locations, with associated reductions in ponderosa 
pine abundance in the community, and to cause decreased net primary 
production of most forest types in the mid-Atlantic region, with only 
small impacts on spruce-fir forest (U.S. EPA, 2013, section 9.4.3.4).
    There is previously and newly available evidence of the potential 
for O3 to alter biomass allocation and plant reproduction in 
seasons subsequent to exposure (U.S. EPA, 2013, section 9.4.3). For 
example, several studies published since the 2006 AQCD further 
demonstrate that O3 can alter the timing of flowering and 
the number of flowers, fruits and seeds in herbaceous and woody plant 
species (U.S. EPA, 2013, section 9.4.3.3). Further, limited evidence in 
previous reviews reported that vegetation effects from a single year of 
exposure to elevated O3 could be observed in the following 
year. For example, growth affected by a reduction in carbohydrate 
storage in one year may result in the limitation of growth in the 
following year. Such ``carry-over'' effects have been documented in the 
growth of some tree seedlings and in roots (U.S. EPA, 2013, section 
9.4.8; Andersen et al., 1997). In the current review, additional field-
based evidence expands the EPA's understanding of the consequences of 
single and multi-year O3 exposures in subsequent years.
    A number of studies were conducted at a planted forest at the Aspen 
free-air carbon-dioxide and ozone enrichment (FACE) experiment site in 
Wisconsin. These studies, which occurred in a field setting (more 
similar to natural forest stands than OTC studies), observed tree 
growth responses when grown in single or two species stands within 30-m 
diameter rings and exposed over a period of ten years to existing 
ambient conditions and elevated O3

[[Page 65372]]

concentrations. Some studies indicate the potential for carry-over 
effects, such as those showing that the effects of O3 on 
birch seeds (reduced weight, germination, and starch levels) could lead 
to a negative impact on species regeneration in subsequent years, and 
that the O3-attributable effect of reduced aspen bud size 
might have been related to the observed delay in spring leaf 
development. These effects suggest that elevated O3 
exposures have the potential to alter carbon metabolism of 
overwintering buds, which may have subsequent effects in the following 
year (Darbah, et al., 2008, 2007; Riikonen et al., 2008; U.S. EPA, 
2013, section 9.4.3). Other studies found that, in addition to 
affecting tree heights, diameters, and main stem volumes in the aspen 
community, elevated O3 over a 7-year study period was 
reported to increase the rate of conversion from a mixed aspen-birch 
community to a community dominated by the more tolerant birch, leading 
the authors to conclude that elevated O3 may alter intra- 
and inter-species competition within a forest stand (U.S. EPA, 2013, 
section 9.4.3; Kubiske et al., 2006; Kubiske et al., 2007). These 
studies confirm earlier FACE results of aspen growth reductions from 
exposure to elevated O3 during the first seven years of 
stand growth and of cumulative biomass impacts associated with changes 
in annual production in studied tree communities (U.S. EPA, 2013, 
section 9.4.3; King et al., 2005).
    Robust and well-established E-R functions for RBL are available for 
11 tree species: black cherry, Douglas fir, loblolly pine, ponderosa 
pine, quaking aspen, red alder, red maple, sugar maple, tulip poplar, 
Virginia pine, and white pine (U.S. EPA, 2013; U.S. EPA, 2014c). While 
these 11 species represent only a small fraction (0.8 percent) of the 
total number of native tree species in the contiguous U.S. (1,497), 
this small 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 (U.S. EPA, 2013, 
section 9.6.2; U.S. EPA, 2014b, section 6.2, Figure 6-2, Table 6-1). 
Supporting the E-R functions for each of these species are studies in 
OTCs, with most species studied multiple times under a wide range of 
exposure and/or growing conditions, with separate E-R functions 
developed for each combination of species, exposure condition and 
growing condition scenario (U.S. EPA, 2013, section 9.6.1). Based on 
these separate E-R functions, species-specific composite E-R functions 
have been developed and successfully used to predict the biomass loss 
response from tree seedling species over a range of cumulative exposure 
conditions (U.S. EPA, 2013, section 9.6.2). These 11 composite 
functions, as well as the E-R function for eastern cottonwood (derived 
from a field study in which O3 and climate conditions were 
not controlled),\160\ are described in the ISA and graphed in the WREA 
to illustrate the predicted responses of these species over a wide 
range of cumulative exposures (U.S. EPA, 2014b, section 6.2, Table 6-1 
and Figure 6-2; U.S. EPA, 2013, section 9.6.2). For some of these 
species, the E-R function is based on a single study (e.g., red maple), 
while for other species there were as many as 11 studies available 
(e.g., ponderosa pine). In total, the E-R functions developed for these 
12 species (the 11 with robust composite E-R functions plus eastern 
cottonwood) reflect 52 tree seedling studies. A stochastic analysis in 
the WREA, summarized in section IV.C of the proposal, indicates the 
potential for within-species variability in these relationships for 
each species. Consideration of biomass loss estimates in the PA and in 
discussions below, however, is based on conventional methods and 
focuses on estimates for the 11 species for which the robust datasets 
from OTC experiments are available, in consideration of CASAC advice.
---------------------------------------------------------------------------

    \160\ The CASAC cautioned the EPA against placing too much 
emphasis on the eastern cottonwood data. In comments on the draft 
PA, the CASAC stated that the eastern cottonwood response data from 
a single study ``receive too much emphasis,'' explaining that these 
``results are from a gradient study that did not control for ozone 
and climatic conditions and show extreme sensitivity to ozone 
compared to other studies'' and that ``[a]lthough they are important 
results, they are not as strong as those from other experiments that 
developed E-R functions based on controlled ozone exposure'' (Frey, 
2014c, p. 10).
---------------------------------------------------------------------------

    The ``detrimental effect of O3 on crop production has 
been recognized since the 1960s'' (U.S. EPA, 2013, p. 1-10, section 
9.4.4). On the whole, the newly available evidence supports and 
strengthens previous conclusions that exposure to O3 reduces 
growth and yield of crops. The ISA describes average crop yield loss 
reported across a number of recently published meta-analyses and 
identifies several new exposure studies that support prior findings for 
a variety of crops of decreased yield and biomass with increased 
O3 exposure (U.S. EPA, 2013, section 9.4.4.1, Table 9-17). 
Studies have also ``linked increasing O3 concentration to 
decreased photosynthetic rates and accelerated aging in leaves, which 
are related to yield'' and described effects of O3 on crop 
quality, such as nutritive quality of grasses, macro- and micronutrient 
concentrations in fruits and vegetable crops and cotton fiber quality 
(U.S. EPA, 2013, p. 1-10, section 9.4.4). The findings of the newly 
available studies do not change the basic understanding of 
O3-related crop yield loss since the last review and little 
additional information is available in this review on factors that 
influence associations between O3 levels and crop yield loss 
(U.S. EPA, 2013, section 9.4.4.). However, the evidence available in 
this review continues to support the conclusion that O3 in 
ambient air can reduce the yield of major commodity crops in the U.S. 
Further, the recent evidence increases our confidence in the use of 
crop E-R functions based on OTC experiments to characterize the 
quantitative relationship between ambient O3 concentrations 
and yield loss (U.S. EPA, 2013, section 9.4.4).
    The new evidence has strengthened support for previously 
established E-R functions for 10 crops (barley, field corn, cotton, 
kidney bean, lettuce, peanut, potato, grain sorghum, soybean and winter 
wheat), reducing two important areas of uncertainty, especially for 
soybean, as summarized in more detail in section IV.A of the proposal. 
The established E-R functions for relative yield loss (RYL)\161\ were 
developed from OTC-type experiments from the National Crop Loss 
Assessment Network (NCLAN) (U.S. EPA, 2013, section 9.6.3; U.S. EPA, 
2014b, section 6.2; U.S. EPA, 2014c, Figure 5-4 and section 6.3). With 
regard to the first area of uncertainty reduced, evaluations in the ISA 
found that yield loss in soybean from O3 exposure at the 
SoyFACE (Soybean Free Air Concentration Enrichment) field experiment 
was reliably predicted by soybean E-R functions developed from NCLAN 
data (U.S. EPA, 2013, section 9.6.3.1),\162\ demonstrating a robustness 
of the NCLAN-based E-R functions for predicting relative yield loss 
from O3 exposure. A second area of uncertainty that was 
reduced is that regarding the

[[Page 65373]]

application of the NCLAN E-R functions to more recent cultivars 
currently growing in the field. Recent studies, especially those 
focused on soybean, provide little evidence that crops are becoming 
more tolerant of O3 (U.S. EPA, 2006a; U.S. EPA, 2013, 
sections 9.6.3.1 and 9.6.3.4 and p. 9-59). The ISA comparisons of NCLAN 
and SoyFACE data referenced above also ``confirm that the response of 
soybean yield to O3 exposure has not changed in current 
cultivars'' (U.S. EPA, 2013, p. 9-59; section 9.6.3.1). Additionally, a 
recent assessment of the relationship between soybean yield loss and 
O3 in ambient air over the contiguous area of Illinois, 
Iowa, and Indiana found a relationship that correlates well with 
previous results from FACE- and OTC-type experiments (U.S. EPA, 2013, 
section 9.4.4.1).
---------------------------------------------------------------------------

    \161\ These functions for RYL estimate reduction in a year's 
growth as a percentage of that expected in the absence of 
O3 (U.S. EPA, 2013, section 9.6.2; U.S. EPA, 2014b, 
section 6.2).
    \162\ The NCLAN program, which was undertaken in the early to 
mid-1980s, assessed multiple U.S. crops, locations, and 
O3 exposure levels, using consistent methods, to provide 
the largest, most uniform database on the effects of O3 
on agricultural crop yields (U.S. EPA 1996a; U.S. EPA, 2006a; U.S. 
EPA, 2013, sections 9.2, 9.4, and 9.6, Frey, 2014c, p. 9). The 
SoyFACE experiment was a chamberless (or free-air) field-based 
exposure study conducted in Illinois from 2001--2009 (U.S. EPA, 
2013, section 9.2.4).
---------------------------------------------------------------------------

c. Biologically Relevant Exposure Metric
    In assessing biologically based indices of exposure pertinent to 
O3 effects on vegetation, the ISA states the following (U.S. 
EPA, 2013, p. 2-44).

    The main conclusions from the 1996 and 2006 O3 AQCDs 
[Air Quality Criteria Documents] regarding indices based on ambient 
exposure remain valid. These key conclusions can be restated as 
follows: ozone effects in plants are cumulative; higher 
O3 concentrations appear to be more important than lower 
concentrations in eliciting a response; plant sensitivity to 
O3 varies with time of day and plant development stage; 
[and] quantifying exposure with indices that cumulate hourly 
O3 concentrations and preferentially weight the higher 
concentrations improves the explanatory power of exposure/response 
models for growth and yield, over using indices based on mean and 
peak exposure values.

    The long-standing body of available evidence upon which these 
conclusions are based includes a wealth of information on aspects of 
O3 exposure that are important in influencing plant response 
(U.S. EPA, 1996a; U.S. EPA, 2006a; U.S. EPA, 2013). Specifically, a 
variety of ``factors with known or suspected bearing on the exposure-
response relationship, including concentration, time of day, respite 
time, frequency of peak occurrence, plant phenology, predisposition, 
etc.,'' have been identified (U.S. EPA, 2013, section 9.5.2). In 
addition, the importance of the duration of the exposure and the 
relatively greater importance of higher concentrations over lower 
concentrations in determining plant response to O3 have been 
consistently well documented (U.S. EPA, 2013, section 9.5.3). Based on 
improved understanding of the biological basis for plant response to 
O3 exposure, a large number of ``mathematical approaches for 
summarizing ambient air quality information in biologically meaningful 
forms for O3 vegetation effects assessment purposes'' have 
been developed (U.S. EPA, 2013, section 9.5.3), including those that 
cumulate exposures over some specified period while weighting higher 
concentrations more than lower (U.S. EPA, 2013, section 9.5.2). As with 
any summary statistic, these exposure indices retain information on 
some, but not all, characteristics of the original observations.
    Based on extensive review of the published literature on different 
types of exposure-response metrics, including comparisons between 
metrics, the EPA has focused on cumulative, concentration-weighted 
indices, recognizing them as the most appropriate biologically based 
metrics to consider in this context (U.S. EPA, 1996a; U.S. EPA, 1996b; 
U.S. EPA, 2006a; U.S. EPA, 2013). In the last two reviews of the 
O3 NAAQS, the EPA concluded that the risk to vegetation 
comes primarily from cumulative exposures to O3 over a 
season or seasons \163\ and focused on metrics intended to characterize 
such exposures: SUM06 \164\ in the 1997 review (61 FR 65716, December 
13, 1996) and W126 in the 2008 review (72 FR 37818, July 11, 2007). 
Although in both reviews the policy decision was made not to revise the 
form and averaging time of the secondary standard, the Administrator, 
in both cases, also concluded, consistent with CASAC advice, that a 
cumulative, seasonal index was the most biologically relevant way to 
relate exposure to plant growth response (62 FR 38856, July 18, 1997; 
73 FR 16436, March 27, 2008). This approach for characterizing 
O3 exposure concentrations that are biologically relevant 
with regard to potential vegetation effects received strong support 
from CASAC in the last review and again in this review, including 
strong support for use of such a metric as the form for the secondary 
standard (Henderson, 2006, 2008; Samet, 2010; Frey, 2014c).
---------------------------------------------------------------------------

    \163\ In describing the form as ``seasonal,'' the EPA is 
referring generally to the growing season of O3-sensitive 
vegetation, not to the seasons of the year (i.e., spring, summer, 
fall, winter).
    \164\ The SUM06 index is a threshold-based approach described as 
the sum of all hourly O3 concentrations greater or equal 
to 0.06 ppm observed during a specified daily and seasonal time 
window (U.S. EPA, 2013, section 9.5.2). The W126 index is a non-
threshold approach, described more fully below.
---------------------------------------------------------------------------

    Alternative methods for characterizing O3 exposure to 
predict plant response have, in recent years, included flux models, 
which some researchers have claimed may ``better predict vegetation 
responses to O3 than exposure-based approaches'' because 
they estimate the ambient O3 concentration that actually 
enters the leaf (i.e., flux or deposition). However, the ISA notes that 
``[f]lux calculations are data intensive and must be carefully 
implemented'' (U.S. EPA, 2013, p. 9-114). Further, the ISA states, 
``[t]his uptake-based approach to quantify the vegetation impact of 
O3 requires inclusion of those factors that control the 
diurnal and seasonal O3 flux to vegetation (e.g., climate 
patterns, species and/or vegetation-type factors and site-specific 
factors)'' (U.S. EPA, 2013, p. 9-114). In addition to these data 
requirements, each species has different amounts of internal 
detoxification potential that may protect species to differing degrees. 
The lack of detailed species- and site-specific data required for flux 
modeling in the U.S. and the lack of understanding of detoxification 
processes have continued to make this technique less viable for use in 
vulnerability and risk assessments at the national scale in the U.S. 
(U.S. EPA, 2013, section 9.5.4).
    Therefore, consistent with the ISA conclusions regarding the 
appropriateness of considering cumulative exposure indices that 
preferentially weight higher concentrations over lower for predicting 
O3 effects of concern based on the well-established 
conclusions and supporting evidence described above, and in light of 
continued CASAC support, we continue to focus on cumulative 
concentration-weighted indices as the most biologically relevant 
metrics for consideration of O3 exposures eliciting 
vegetation-related effects. Quantifying exposure in this way ``improves 
the explanatory power of exposure/response models for growth and yield 
over using indices based on mean and peak exposure values'' (U.S. EPA, 
2013, section 2.6.6.1, p. 2-44). In this review, as in the last review, 
we use the W126-based cumulative, seasonal metric (U.S. EPA, 2013, 
sections 2.6.6.1 and 9.5.2) for consideration of the effects evidence 
and in the exposure and risk analyses in the WREA.
    This metric, commonly called the W126 index, is a non-threshold 
approach described as the sigmoidally weighted sum of all hourly 
O3 concentrations observed during a specified daily and 
seasonal time window, where each hourly O3 concentration is 
given a weight that increases from zero to one with increasing 
concentration (U.S. EPA, 2014c, p. 5-6; U.S. EPA 2013, p. 9-101).

[[Page 65374]]

The first step in calculating the seasonal W126 index, as described and 
considered in this review, is to sum the weighted ambient O3 
concentrations during daylight hours (defined as 8:00 a.m. to 8:00 
p.m.) within each calendar month, resulting in monthly index values 
(U.S. EPA, 2014b, pp. 4-5 to 4-6). As more completely described in the 
WREA, the monthly W126 index values are calculated from hourly 
O3 concentrations as follows:
[GRAPHIC] [TIFF OMITTED] TR26OC15.000

where N is the number of days in the month, d is the day of the month 
(d = 1, 2, . . ., N), h is the hour of the day (h = 0, 1, . . ., 23), 
and Cdh is the hourly O3 concentration observed 
on day d, hour h, in parts per million. The seasonal W126 index value 
for a specific year is the maximum sum of the monthly index values for 
three consecutive months. Three-year W126 index values are calculated 
by taking the average of seasonal W126 index values for three 
consecutive years (U.S. EPA, 2014b, pp. 4-5 to 4-6; Wells, 2014a).
2. Overview of Welfare Exposure and Risk Assessment
    This section outlines the information presented in section IV.C of 
the proposal regarding the WREA conducted for this review, which built 
upon similar analyses performed in the last review. The WREA focuses 
primarily on analyses related to two types of effects on vegetation: 
Reduced growth (biomass loss) in both trees and agricultural crops, and 
foliar injury. The assessments of O3-associated reduced 
growth in native trees and crops (specifically, RBL and RYL, 
respectively) include analysis of associated changes in related 
ecosystem services, including pollution removal, carbon sequestration 
or storage, and hydrology, as well as economic impacts on the forestry 
and agriculture sectors of the economy. The foliar injury assessments 
include cumulative analyses of the proportion of USFS biosite index 
scores \165\ above zero (or five, in a separate set of analyses) with 
increasing W126 exposure index estimates, with and without 
consideration of soil moisture conditions. The implications of visible 
foliar injury in national parks were considered in a screening level 
assessment and three case studies.\166\
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    \165\ Sampling sites in the FIA/FHM O3 biomonitoring 
program, called ``biosites'', are plots of land on which data are 
collected regarding the incidence and severity of visible foliar 
injury on a variety of O3-sensitive plant species. 
Biosite index scores are derived from these data (U.S. EPA, 2014b, 
section 7.2.1).
    \166\ All of the analyses are described in detail in the WREA 
and summarized in the PA and in section IV.C of the proposal (U.S. 
EPA, 2014a; U.S. EPA, 2014b; 79 FR 75324-75329, December 17, 2014).
---------------------------------------------------------------------------

    Growth-related effects were assessed for W126-based exposure 
estimates in five scenarios of national-scale \167\ air quality: Recent 
conditions (2006 to 2008), the existing secondary standard, and W126 
index values of 15 ppm-hrs, 11 ppm-hrs, and 7 ppm-hrs, using 3-year 
averages (U.S. EPA, 2014b, chapter 4). For each of these scenarios, 3-
year average W126 exposure index values were estimated for 12 kilometer 
(km) by 12 km grid cells in a national-scale spatial surface. The 
method for creating these grid cell estimates generally involved two 
steps (summarized in Table 5-4 of the PA).
---------------------------------------------------------------------------

    \167\ Although the scenarios and the grid cell O3 
concentrations on which they are based were limited to the 
contiguous U.S., we have generally used the phrase ``national-
scale'' in reference to the WREA scenarios and surfaces.
---------------------------------------------------------------------------

    The first step in creating the grid cell estimates for each 
scenario was calculation of the average W126 index value (across the 
three years) at each monitor location. For the recent conditions 
scenario, this value was based on unadjusted O3 
concentrations from monitoring data. For the other four scenarios, the 
W126 index value for each monitor location was calculated from model-
adjusted hourly O3 concentrations. The adjusted 
concentrations were based on model-predicted relationships between 
O3 at each monitor location and reductions in 
NOX. Adjustments were applied independently for each of the 
nine U.S. regions (see U.S. EPA, 2014b, section 4.3.4.1).\168\ The 
existing standard scenario was created first, with the result being a 
national dataset for which the highest monitor location in each U.S. 
region had a design value equal to the level of the current 
standard.\169\ The W126 scenarios were created from the hourly 
concentrations used to create the existing standard scenario, with 
model-based adjustments made at all monitor sites in those regions with 
a site not already at or below the target W126 value for that scenario 
(U.S. EPA, 2014b, section 4.3.4.1).\170\
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    \168\ The U.S. regions referenced here and in section IV.C below 
are NOAA climate regions, as shown in Figure 2B-1 of the PA.
    \169\ The adjustment results in broad regional reductions in 
O3 and includes reductions in O3 at some 
monitors that were already at or below the target level. These 
reductions do not represent an optimized control scenario, but 
rather characterize one potential distribution of air quality across 
a region that meets the scenario target (U.S. EPA, 2014b, sections 
4.3.4.2 and 4.4).
    \170\ In regions where the air quality adjustment was applied, 
it was based on emissions reductions determined necessary for the 
highest monitor in that region to just equal the existing standard 
or the W126 target for the scenario. Concentrations at all other 
monitor locations in the region were also adjusted based on the same 
emissions reductions assumptions.
---------------------------------------------------------------------------

    After completing step one for all the scenarios, the second step 
involved creating the national-scale spatial surfaces (composed of 3-
year W126 index values at grid cell centroids). These were created by 
applying the Voronoi Neighbor Averaging (VNA) spatial interpolation 
technique to the monitor-location, 3-year W126 index values (described 
in step 1).\171\ This step of creating the gridded spatial surfaces 
resulted in further reduction of the highest values in each modeling 
region, as demonstrated by comparing the W126 index values from steps 
one and two for the existing standard scenario. After the step-one 
adjustment of the monitor location concentrations such that the highest 
location in each NOAA region just met the existing standard (using 
relationships mentioned above), the maximum 3-year average W126 values 
in the nine regions ranged from 18.9 ppm-hrs in the West region to 2.6 
ppm-hrs in the Northeast region (U.S. EPA, 2014b, Table 4-3). After 
application of the VNA technique in the second step, however, the 
highest 3-year average W126 values across the national surface grid 
cells, which were in the Southwest region, were below 15 ppm-hrs (U.S. 
EPA, 2014b, Figure 4-7).\172\
---------------------------------------------------------------------------

    \171\ The VNA technique is described in the WREA (U.S. EPA, 
2014b, Appendix 4A).
    \172\ Thus, it can be seen that application of the VNA 
interpolation method to estimate W126 index values at the centroid 
of every 12 km x 12 km grid cell rather than only at each monitor 
location results in a lowering of the highest values in each region.
---------------------------------------------------------------------------

    All of the assessments based on growth impacts relied on the W126 
index estimates from the national-scale spatial surfaces (created from 
the 3-year average monitor location values as described above). Among 
the analyses related to visible foliar injury, a small component of the 
screening-level

[[Page 65375]]

national park assessment and also the three national park case studies 
involved summarizing 3-year W126 index estimates from the four air 
quality scenarios. However, the visible foliar injury cumulative 
proportion analyses and a component of the national park screening-
level assessment relied on national-scale spatial surfaces of single-
year, unadjusted W126 index values created for each year from 2006 
through 2010 using the VNA interpolation technique applied to the 
monitor location index values for these years (U.S. EPA, 2014b, section 
4.3.2, Appendix 4A).
    Because the W126 estimates generated for the different air quality 
scenarios assessed are inputs to the vegetation risk analyses for tree 
biomass and crop yield loss, and also used in some components of the 
visible foliar injury assessments, limitations and uncertainties in the 
air quality analyses, which are discussed in detail in the WREA and 
some of which are mentioned here, are propagated into those analyses 
(U.S. EPA, 2014b, chapters 4 and 8 and section 8.5, Table 4-5). An 
important uncertainty in the analyses is the application of regionally 
determined emissions reductions to meet the existing standard (U.S. 
EPA, 2014b, section 8.5.1). The model adjustments are based on 
emissions reductions in NOx and characterize only one potential 
distribution of air quality across a region when all monitor locations 
meet the standard, as well as for the W126 scenarios (U.S. EPA, 2014b, 
section 4.3.4.2).\173\
---------------------------------------------------------------------------

    \173\ The adjustment is applied to all monitor locations in each 
region. In this way, the adjustment results in broad regional 
reductions in O3 and includes reductions in O3 
at some monitors that were already meeting or below the target 
level. Thus, the adjustments performed to develop a scenario meeting 
a target level at the highest monitor in each region did result in 
substantial reduction below the target level in some areas of the 
region. This result at the monitors already well below the target 
indicates an uncertainty with regard to air quality expected from 
specific control strategies that might be implemented to meet a 
particular target level.
---------------------------------------------------------------------------

    An additional uncertainty related to the W126 index estimates in 
the national surfaces for each air quality scenario, and to the 
estimates for the single-year surfaces used in the visible foliar 
injury cumulative analysis, comes with the creation of the national-
scale spatial surfaces of grid cells from the monitor-location 
O3 data.\174\ In general, spatial interpolation techniques 
perform better in areas where the O3 monitoring network is 
denser. Therefore, the W126 index values estimated using this technique 
in rural areas in the West, Northwest, Southwest, and West North 
Central regions where there are few or no monitors (U.S. EPA, 2014b, 
Figure 2-1) are more uncertain than those estimated for areas with 
denser monitoring. Further, as described above, this interpolation 
method generally underpredicts the highest W126 exposure index values. 
Due to the important influence of higher exposures in determining risks 
to plants, the potential for the VNA interpolation approach to dampen 
peak W126 index values could result in an underestimation of risks to 
vegetation in some areas.\175\
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    \174\ Some uncertainty is inherent in any approach to 
characterizing O3 air quality over broad geographic areas 
based on concentrations at monitor locations.
    \175\ In the visible foliar injury dataset used for the 
cumulative analysis, underestimation of W126 index values at sites 
with injury would contribute to overestimates of the cumulative 
proportion of sites with injury plotted for the lower W126 values.
---------------------------------------------------------------------------

    The vegetation analyses performed in the WREA, along with key 
observations, insights, uncertainties and limitations were summarized 
in sections IV.C.2 through IV.C.3 of the proposal. Highlights for the 
three categories of biomass loss and foliar injury assessments are 
summarized here.
a. Tree Growth, Productivity and Carbon Storage
    These assessments rely on the species-specific E-R functions 
described in section IV.A.1.b above. For the air quality scenarios 
described above, the WREA applied the species-specific E-R functions to 
develop estimates of O3-associated RBL and associated 
effects on productivity, carbon storage and associated ecosystem 
services (U.S. EPA, 2014b, Chapter 6). More specifically, the WREA 
derived species-specific and weighted RBL estimates for grid cells 
across the continental U.S. and summarized the estimates by counties 
and national parks. Additional WREA case study analyses focused on 
selected urban areas. The WREA estimates indicate substantial 
heterogeneity in plant responses to O3, both within species 
(e.g., study-specific variation), between species, and across regions 
of the U.S. National variability in the estimates (e.g., eastern vs 
western U.S.) is influenced by there being different sets of resident 
species (with different E-R functions) in different areas of the U.S., 
as well as differences in number of national parks and O3 
monitors. For example, the eastern U.S. has different resident species 
compared to the western U.S., and the eastern U.S. has far more such 
species. Additionally, there are more national parks in the western 
than the eastern U.S., yet fewer O3 monitors (U.S. EPA, 
2014b, chapter 8).
    Relative biomass loss nationally (across all of the air quality 
surface grid cells) was estimated for each of the 12 studied species 
from the composite E-R functions for each species described above and 
information on the distribution of those species across the U.S. (U.S. 
EPA, 2014b, section 6.2.1.3 and Appendix 6A). In consideration of CASAC 
advice (summarized in section IV.A.1.b above), the WREA derived RBL and 
weighted RBL (wRBL) estimates separately, both with and without the 
eastern cottonwood, and the PA and proposal gave primary focus to 
analyses that exclude cottonwood. These analyses provided estimates of 
per-species and cross-species RBL in the different air quality 
scenarios. Air quality scenario estimates were also developed in terms 
of proportion of basal area affected at different magnitudes of RBL. 
The wRBL analysis integrated the species-specific estimates, providing 
an indication of potential magnitude of ecological effect possible in 
some ecosystems. The county analyses also included analyses focused on 
the median species response. The WREA also used the E-R functions to 
estimate RBL across tree lifespans and the resulting changes in 
consumer and producer/farmer economic surplus in the forestry and 
agriculture sectors of the economy. Case studies in five urban areas 
provided comparisons across air quality scenarios of estimates for 
urban tree pollutant removal and carbon storage or sequestration.
    The array of uncertainties associated with estimates from these 
tree RBL analyses are summarized in the proposal and described in 
detail in the WREA, including the potential for the air quality 
scenarios to underestimate the higher W126 index values and associated 
implications for the RBL-related estimates, as referenced above.
b. Crop Yield Loss
    These assessments rely on the species-specific E-R functions 
described in section IV.A.1.b above. For the different air quality 
scenarios, the WREA applied the species-specific E-R functions to 
develop estimates of O3 impacts related to crop yield, 
including annual yield losses estimated for 10 commodity crops grown in 
the U.S. and how these losses affect producer and consumer economic 
surpluses (U.S. EPA, 2014b, sections 6.2, 6.5). The WREA derived 
estimates of crop RYL nationally and in a county-specific analysis, 
relying on information regarding crop distribution (U.S. EPA, 2014b, 
section 6.5). As with the tree analyses described above, the county 
analysis included estimates based on

[[Page 65376]]

the median O3 response across the studied crop species (U.S. 
EPA, 2014b, section 6.5.1, Appendix 6B).
    Overall effects on agricultural yields and producer and consumer 
surplus depend on the ability of producers/farmers to substitute other 
crops that are less O3 sensitive, and the responsiveness, or 
elasticity, of demand and supply (U.S. EPA, 2014b, section 6.5). The 
WREA discusses multiple areas of uncertainty associated with the crop 
yield loss estimates, including those associated with the model-based 
adjustment methodology as well as those associated with the projection 
of yield loss using the Forest and Agriculture Sector Optimization 
Model (with greenhouse gases) at the estimated O3 
concentrations (U.S. EPA, 2014b, Table 6-27, section 8.5). Because the 
W126 index estimates generated in the air quality scenarios are inputs 
to the vegetation risk analyses for crop yield loss, any uncertainties 
in the air quality scenario estimation of W126 index values are 
propagated into those analyses (U.S. EPA, 2014b, Table 6-27, section 
8.5). Therefore, the air quality scenarios in the crop yield analyses 
have the same uncertainties and limitations as in the biomass loss 
analyses (summarized above), including those associated with the model-
based adjustment methodology (U.S. EPA, 2014b, section 8.5).
c. Visible Foliar Injury
    The WREA presents a number of analyses of O3-related 
visible foliar injury and associated ecosystem services impacts (U.S. 
EPA, 2014b, Chapter 7). In the initial analysis, the WREA used the 
biomonitoring site data from the USFS FHM/FIA Network (USFS, 
2011),\176\ associated soil moisture data during the sample years, and 
national surfaces of ambient air O3 concentrations based on 
spatial interpolation of monitoring data from 2006 to 2010 in a 
cumulative analysis of the proportion of biosite records with any 
visible foliar injury, as indicated by a nonzero biosite index score 
(U.S. EPA, 2014b, section 7.2). This analysis was done for all records 
together, and also for subsets based on soil moisture conditions 
(normal, wet or dry).
---------------------------------------------------------------------------

    \176\ Data were not available for several western states 
(Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New 
Mexico, Oklahoma, and portions of Texas).
---------------------------------------------------------------------------

    In each cumulative analysis, the biosite records were ordered by 
W126 index and then, moving from low to high W126 index, the records 
were cumulated into a progressively larger dataset. With the addition 
of each new data point (composed of biosite index score and W126 index 
value for a biosite and year combination) to the cumulative dataset, 
the percentage of sites with a nonzero biosite index score was derived 
and plotted versus the W126 index estimate for the just added data 
point. The cumulative analysis for all sites indicates that (1) as the 
cumulative set of sites grows with addition of sites with progressively 
higher W126 index values, the proportion of the dataset for which no 
foliar injury was recorded changes (increases) noticeably prior to 
about 10 ppm-hrs (10.46 ppm-hrs), and (2) as the cumulative dataset 
grows still larger with the addition of records for higher W126 index 
estimates, the proportion of the cumulative dataset with no foliar 
injury remains relatively constant (U.S. EPA, 2014b, Figure 7-10). The 
data for normal moisture years are very similar to the dataset as a 
whole, with an overall proportion of about 18 percent for presence of 
any foliar injury. The data for relatively wet years have a much higher 
proportion of biosites showing injury, approximately 25% when all data 
are included, and a proportion of approximately 20% when data for W126 
index estimates up to about 5-8 ppm-hrs are included (U.S. EPA, 2014b, 
Figure 7-10).\177\ The overall proportion showing injury for the subset 
for relatively dry conditions is much lower, less than 15% for the 
subset (U.S. EPA, 2014b, section 7.2.3, Figures 7-10). While these 
analyses indicate the potential for foliar injury to occur under 
conditions that meet the current standard, the extent of foliar injury 
that might be expected under different exposure conditions is unclear 
from these analyses.
---------------------------------------------------------------------------

    \177\ As discussed in section IV.C.2 below, as the cumulative 
set increases, with increasing W126 values, the overall prevalence 
of visible foliar injury in the cumulative set is more and more 
influenced by data for the lower W126 values. Accordingly, the 
``leveling off'' observed above ~10 ppm-hrs in the `all sites' 
analysis likely reflects the counterbalancing of visible foliar 
injury occurrence at the relatively fewer higher O3 sites 
by the larger representation within the subset of the lower W126 
conditions associated with which there is lower occurrence or extent 
of foliar injury.
---------------------------------------------------------------------------

    Criteria derived from the cumulative analyses were then used in two 
additional analyses. The national-scale screening-level assessment 
compared W126 index values estimated within 214 national parks using 
the VNA technique described above for the individual years from 2006 to 
2010 with benchmark criteria developed from the biosite data analysis 
(U.S. EPA, 2014b, Appendix 7A and section 7.3). Separate case study 
analyses described visits, as well as visitor uses and expenditures for 
three national parks, and the 3-year W126 index estimates in those 
parks for the four air quality scenarios (U.S. EPA, 2014b, section 
7.4). Uncertainties associated with these analyses, included those 
associated with the W126 index estimates, are discussed in the WREA, 
sections 7.5 and 8.5.3, and in WREA Table 7-24, and also summarized in 
the PA (e.g., U.S. EPA, 2014c, section 6.3).
3. Potential Impacts on Public Welfare
    As provided in the CAA, section 109(b)(2), 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.'' Effects on welfare include, but are not limited to, 
``effects on soils, water, crops, vegetation, man-made 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'' 
(CAA section 302(h)). The secondary standard is not meant to protect 
against all known or anticipated O3-related 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 8312, January 15, 2013; see also 73 FR 16496, March 
27, 2008). Thus, 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. In the current review, the Administrator's judgment is 
informed by conclusions drawn with regard to adversity of effects to 
public welfare in decisions on secondary O3 standards in 
past reviews.
    As indicated by the Administrator in the 2008 decision, the degree 
to which O3 effects on vegetation should be considered to be 
adverse to the public welfare depends on the intended use of the 
vegetation and the significance of the vegetation to the public welfare 
(73 FR 16496, March 27, 2008). Such judgments regarding public welfare 
significance in the last O3 NAAQS decision gave particular 
consideration to O3 effects in areas with special federal 
protections, 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). For example, in reaching his conclusion regarding the 
need for revision of the secondary standard in the 2008 review, the 
Administrator took

[[Page 65377]]

note of ``a number of actions taken by Congress to establish public 
lands that are set aside for specific uses that are 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'' (73 FR 16496, March 27, 2008). As 
further recognized in the 2008 notice, ``[s]uch public lands that are 
protected areas of national interest include national parks and 
forests, wildlife refuges, and wilderness areas'' (73 FR 16496, March 
27, 2008).\178\ \179\ Such areas include Class I areas\180\ which are 
federally mandated to preserve certain air quality related values. 
Additionally, as the Administrator recognized, ``States, Tribes and 
public interest groups also set aside areas that are intended to 
provide similar benefits to the public welfare, for residents on State 
and Tribal lands, as well as for visitors to those areas'' (73 FR 
16496, March 27, 2008). The Administrator took note of 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).
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    \178\ For example, the National Park Service Organic Act of 1916 
established the National Park Service (NPS) and, in describing the 
role of the NPS with regard to ``Federal areas known as national 
parks, monuments, and reservations'', stated that the ``fundamental 
purpose'' for these federal areas ``is to conserve the scenery and 
the natural and historic objects and the wild life therein and to 
provide for the enjoyment of the same in such manner and by such 
means as will leave them unimpaired for the enjoyment of future 
generations.'' 16 U.S.C. 1.
    \179\ As a second example, 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).
    \180\ 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 six thousand 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.
---------------------------------------------------------------------------

    The concept described in the 2008 notice regarding the degree to 
which effects on vegetation in specially protected areas, such as those 
identified above, may be judged adverse also applies beyond the species 
level to the ecosystem level, such that judgments can depend on the 
intended use\181\ for, or service (and value) of, the affected 
vegetation, ecological receptors, ecosystems and resources and the 
significance of that use to the public welfare (73 FR 16496, March 27, 
2008). Uses or services provided by areas that have been afforded 
special protection can flow in part or entirely from the vegetation 
that grows there. Aesthetic value and outdoor recreation depend, at 
least in part, on the perceived scenic beauty of the environment (U.S. 
EPA, 2014b, chapters 5 and 7). Further, analyses have reported that the 
American public values--in monetary as well as nonmonetary ways--the 
protection of forests from air pollution damage. In fact, studies that 
have assessed willingness-to-pay for spruce-fir forest protection in 
the southeastern U.S. from air pollution and insect damage have found 
that values held by the survey respondents for the more abstract 
services (existence, option and bequest)\182\ were greater than those 
for recreation or other services (U.S. EPA, 2014b, Table 5-6; Haefele 
et al., 1991; Holmes and Kramer, 1995).
---------------------------------------------------------------------------

    \181\ Ecosystem services have been defined as ``the benefits 
that people obtain from ecosystems'' (U.S. EPA, 2013, Preamble, p. 
1xxii; UNEP, 2003) and thus are an aspect of the use of a type of 
vegetation or ecosystem. Similarly, a definition used for the 
purposes of the EPA benefits assessments states that ecological 
goods and services are the ``outputs of ecological functions or 
processes that directly or indirectly contribute to social welfare 
or have the potential to do so in the future'' and that ``[s]ome 
outputs may be bought and sold, but most are not marketed'' (U.S. 
EPA, 2006b). Ecosystem services analyses were one of the tools used 
in the last review of the secondary standards for oxides of nitrogen 
and sulfur to inform the decisions made with regard to adequacy and 
as such, were used in conjunction with other considerations in the 
discussion of adversity to public welfare (77 FR 20241, April 3, 
2012).
    \182\ 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 spatial, temporal and social dimensions of public welfare 
impacts are also influenced by the type of service affected. For 
example, a national park can provide direct recreational services to 
the thousands of visitors that come each year, but also provide an 
indirect value to the millions who may not visit but receive 
satisfaction from knowing it exists and is preserved for the future 
(U.S. EPA, 2014b, chapter 5, section 5.5.1). Similarly, ecosystem 
services can be realized over a range of temporal scales. An evaluation 
of adversity to the public welfare might also consider the likelihood, 
type, and magnitude of the effect, as well as the potential for 
recovery and any uncertainties relating to these conditions, as stated 
in the preamble of the 2012 final notice of rulemaking on the secondary 
standards for oxides of nitrogen and sulfur (77 FR 20232, April 3, 
2012).
    The three main categories of effects on vegetation discussed in 
section IV.A.1.b above differ with regard to aspects important to 
judging their public welfare significance. Judgments regarding crop 
yield loss, for example, depend on considerations related to the heavy 
management of agriculture in the U.S., while judgments regarding the 
other categories of effects generally relate to considerations 
regarding forested areas. For example, while both tree growth-related 
effects and visible foliar injury have the potential to be significant 
to the public welfare through impacts in Class I and other protected 
areas, they differ in how they might be significant and with regard to 
the clarity of the data that describe the relationship between the 
effect and the services potentially affected.
    With regard to effects on tree growth, reduced growth is associated 
with effects on an array of ecosystem services including reduced 
productivity, altered forest and forest community (plant, insect and 
microbe) composition, reduced carbon storage and altered water cycling 
(U.S. EPA, 2013, Figure 9-1, sections 9.4.1.1 and 9.4.1.2; U.S. EPA, 
2014b, section 6.1). For example, forest or forest 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 (U.S. EPA, 2013, sections 9.4.3 and 9.4.3.1). 
Depending on the type and location of the affected ecosystem, services 
benefitting the public in other ways can be affected as well. For 
example, other services valued by people that can be affected by 
reduced tree growth, productivity and carbon storage include aesthetic 
value, food, fiber, timber, other forest products, habitat, 
recreational opportunities, climate and water regulation, erosion 
control, air pollution removal, and desired fire regimes (U.S. EPA 
2013, sections 9.4.1.1 and 9.4.1.2; U.S. EPA, 2014b, section 6.1, 
Figure 6-1, section 6.4, Table 6-13). Further, impacts on some of these 
services (e.g., forest or forest community composition) may be 
considered of greater public welfare significance when occurring in 
Class I or other protected areas.
    Consideration of the magnitude of tree growth effects that might 
cause or contribute to adverse effects for trees, forests, forested 
ecosystems or the public welfare is complicated by aspects

[[Page 65378]]

of, or limitations in, the available information. For example, the 
evidence on tree seedling growth effects, deriving from the E-R 
functions for 11 species (described in section IV.A.1 above), provides 
no clear threshold or breakpoint in the response to O3 
exposure. Additionally, there are no established relationships between 
magnitude of tree seedling growth reduction and forest ecosystem 
impacts and, as noted in section IV.A.1.b above, other factors can 
influence the degree to which O3-induced growth effects in a 
sensitive species affect forest and forest community composition and 
other ecosystem service flows from forested ecosystems. These include 
(1) the type of stand or community in which the sensitive species is 
found (i.e., single species versus mixed canopy); (2) the role or 
position the species has in the stand (i.e., dominant, sub-dominant, 
canopy, understory); (3) the O3 sensitivity of the other co-
occurring species (O3 sensitive or tolerant); and (4) 
environmental factors, such as soil moisture and others. The lack of 
such established relationships complicates judgments as to the extent 
to which different estimates of impacts on tree seedling growth would 
indicate significance to the public welfare and thus be an important 
consideration in the level of protection for the secondary standard.
    During the 1997 review of the secondary standard, views related to 
this issue were provided by a 1996 workshop of 16 leading scientists in 
the context of discussing their views for a secondary O3 
standard (Heck and Cowling, 1997). In their consideration of tree 
growth effects as an indicator for forest ecosystems and crop yield 
reduction as an indicator of agricultural systems, the workshop 
participants identified annual percentages, of RBL for forest tree 
seedlings and RYL for agricultural crops, considered important to their 
judgments on the standard. With regard to forest ecosystems and 
seedling growth effects as an indicator, the participants selected a 
range of 1-2% RBL per year ``to avoid cumulative effects of yearly 
reductions of 2%.'' With regard to crops, they indicated an interest in 
protecting against crop yield reductions of 5% RYL yet noted 
uncertainties surrounding such a percentage which led them to 
identifying 10% RYL for the crop yield endpoint (Heck and Cowling, 
1997). The workshop report provides no explicit rationale for the 
percentages identified (1-2% RBL and 5% or 10% RYL); nor does it 
describe their connection to ecosystem impacts of a specific magnitude 
or type, nor to judgments on significance of the identified effects for 
public welfare, e.g., taking into consideration the intended use and 
significance of the affected vegetation (Heck and Cowling, 1997). In 
recognition of the complexity of assessing the adversity of tree growth 
effects and effects on crop yield in the broader context of public 
welfare, the EPA's consideration of those effects in both the 1997 and 
2008 reviews extended beyond the consideration of various benchmark 
responses for the studied species, and, with regard to crops, 
additionally took note of their extensive management (62 FR 38856, July 
18, 1997; 73 FR 16436, March 27, 2008).
    While, as noted above, public welfare benefits of forested lands 
can be particular to the type of area in which the forest occurs, some 
of the potential public welfare benefits associated with forest 
ecosystems are not location dependent. A potentially extremely valuable 
ecosystem service provided by forested lands is carbon storage, a 
regulating service that is ``of paramount importance for human 
society'' (U.S. EPA, 2013, section 2.6.2.1 and p. 9-37). As noted 
above, the EPA has concluded that this ecosystem service has a likely 
causal relationship with O3 in ambient air. The service of 
carbon storage is potentially important to the public welfare no matter 
in what location the sensitive trees are growing or what their intended 
current or future use. In other words, the benefit exists as long as 
the tree is growing, regardless of what additional functions and 
services it provides. Another example of locations potentially 
vulnerable to O3-related impacts but not necessarily 
identified for such protection might be 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 species 
identified by the USFS and 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 but whose 
vulnerability to O3 impacts has not been quantitatively 
characterized (U.S. EPA, 2014b, Chapter 6).
    As noted above, in addition to tree growth-related effects, 
O3-induced visible foliar injury also has the potential to 
be significant to the public welfare through impacts in Class I and 
other similarly protected areas. Visible foliar injury is a visible 
bioindicator of O3 exposure in species sensitive to this 
effect, with the injury affecting the physical appearance of the plant. 
Accordingly visible foliar injury surveys are used by federal land 
managers as tools in assessing potential air quality impacts in Class I 
areas. These surveys may focus on plant species that have been 
identified as potentially sensitive air quality related values (AQRVs) 
due to their sensitivity to O3-induced foliar injury (USFS, 
NPS, FWS, 2010). An AQRV is defined by the National Park Service as a 
``resource, as identified by the [federal land manager] for one or more 
Federal areas that may be adversely affected by a change in air 
quality,'' and the resource ``may include visibility or a specific 
scenic, cultural, physical, biological, ecological, or recreational 
resource identified by the [federal land manager] for a particular 
area'' (USFS, NPS, USFWS, 2010).\183\ No criteria have been 
established, however, regarding a level or prevalence of visible foliar 
injury considered to be adverse to the affected vegetation, and, as 
noted in section IV.A.1.b above, there is not a clear relationship 
between visible foliar injury and other effects, such as reduced growth 
and productivity.\184\ Thus, key considerations with regard to public 
welfare significance of this endpoint

[[Page 65379]]

have related to qualitative consideration of the plant's aesthetic 
value in protected forested areas. Depending on the extent and 
severity, O3-induced visible foliar injury might be expected 
to have the potential to impact the public welfare in scenic and/or 
recreational areas during the growing season, particularly in areas 
with special protection, such as Class I areas.
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    \183\ The identification, monitoring and assessment of AQRVs 
with regard to an adverse effect is an approach used for assessing 
the potential for air pollution impacts in Class I areas from 
pending permit actions (USFS, NPS, USFWS, 2010). An adverse impact 
is recognized by the National Park Service as one that results in 
diminishment of the Class I area's national significance or the 
impairment of the ecosystem structure or functioning, as well as 
impairment of the quality of the visitor experience (USFS, NPS, 
USFWS, 2010). Federal land managers make such adverse impact 
determinations on a case-by-case basis, using technical and other 
information that they provide for consideration by permitting 
authorities. The National Park Service has developed a document 
describing an overview of approaches related to assessing projects 
under the National Environmental Policy Act and other planning 
initiatives affecting the National Park System (http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011-01-14.pdf).
    \184\ The National Park Service identifies various ranges of 
W126 index values in providing approaches for assessing air quality-
related impacts of various development projects which appear to be 
based on the 1996 workshop report (Heck and Cowling, 1997), and may, 
at the low end, relate to a benchmark derived for the highly 
sensitive species, black cherry, for growth effects (10% RBL), 
rather than visible foliar injury (Kohut, 2007; Lefohn et al., 
1997). As noted in section IV.A.1.b above, visible foliar injury is 
not always a reliable indicator of other negative effects on 
vegetation (U.S. EPA, 2013, p. 9-39). We also note that the USFS 
biomonitoring analyses of visible foliar injury biomonitoring data 
commonly make use of a set of biosite index categories for which 
risk assumptions have been assigned, providing a relative scale of 
possible impacts (Campbell et al, 2007); however, little information 
is available on the studies, effects and judgments on which these 
categories are based.
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    The ecosystem services most likely to be affected by O3-
induced visible foliar injury (some of which are also recognized above 
for tree growth-related effects) are cultural services, including 
aesthetic value and outdoor recreation. In addition, several tribes 
have indicated that many of the species identified as O3 
sensitive (including bioindicator species) are culturally significant 
(U.S. EPA, 2014c, Table 5-1). The geographic extent of protected areas 
that may be vulnerable to such public welfare effects of O3 
is potentially appreciable. Sixty-six plant species that occur on U.S. 
National Park Service (NPS) and U.S. Fish and Wildlife Service lands 
\185\ have been identified as sensitive to O3-induced 
visible foliar injury, and some also have particular cultural 
importance to some tribes (U.S. EPA, 2014c, Table 5-1 and Appendix 5-A; 
U.S. EPA, 2014b, section 6.4.2). Not all species are equally sensitive 
to O3, however, and quantitative E-R relationships for 
O3 exposure and other important effects, such as seedling 
growth reduction, are only available for a subset of 12 of the 66, as 
summarized in section IV.A.1.b above. A diverse array of ecosystem 
services has been identified for these twelve species (U.S. EPA, 2014c, 
Table 5-1). Two species in this group that are slightly more sensitive 
than the median for the group with regard to effects on growth are the 
ponderosa pine and quaking aspen (U.S. EPA, 2014b, section 6.2), the 
ranges for which overlap with many lands that are protected or 
preserved for enjoyment of current and future generations (consistent 
with the discussion above on Class I and other protected areas), 
including such lands located in the west and southwest regions of the 
U.S. where ambient O3 concentrations and associated 
cumulative seasonal exposures can be highest (U.S. EPA, 2014c, Appendix 
2B).\186\
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    \185\ See http://www2.nature.nps.gov/air/Pubs/pdf/flag/NPSozonesensppFLAG06.pdf.
    \186\ Basal area for resident species in national forests and 
parks are available in files accessible at: http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml. Basal area is generally 
described as the area of ground covered by trees.
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    With regard to agriculture-related effects, the EPA has recognized 
other complexities, stating that the degree to which O3 
impacts on vegetation that could occur in areas and on species that are 
already heavily managed to obtain a particular output (such as 
commodity crops or commercial timber production) would impair the 
intended use at a level that might be judged adverse to the public 
welfare has been less clear (73 FR 16497, March 27, 2008). As noted in 
section IV.B.2 of the proposal, 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. These 
competing impacts on producers and consumers complicate consideration 
of these effects in terms of potential adversity to the public welfare 
(U.S. EPA, 2014c, sections 5.3.2 and 5.7). When agricultural impacts or 
vegetation effects in other areas are contrasted with the emphasis on 
forest ecosystem effects in Class I and similarly protected areas, it 
can be seen that the Administrator has in past reviews judged the 
significance to the public welfare of O3-induced effects on 
sensitive vegetation growing within the U.S. to differ depending on the 
nature of the effect, the intended use of the sensitive plants or 
ecosystems, and the types of environments in which the sensitive 
vegetation and ecosystems are located, with greater significance 
ascribed to areas identified for specific uses and benefits to the 
public welfare, such as Class I areas, than to areas for which such 
uses have not been established (FR 73 16496-16497, March 27, 2008).
    In summary, several considerations are recognized as important to 
judgments on the public welfare significance of the array of effects of 
different O3 exposure conditions on vegetation. While there 
are complexities associated with the consideration of the magnitude of 
key vegetation effects that might be concluded to be adverse to 
ecosystems and associated services, there are numerous locations where 
O3-sensitive tree species are present that may be vulnerable 
to impacts from O3 on tree growth, productivity and carbon 
storage and their associated ecosystems and services. Cumulative 
exposures that may elicit effects and the significance of the effects 
in specific situations can vary due to differences in exposed species 
sensitivity, the importance 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. These factors 
contribute to the complexity of the Administrator's judgments regarding 
the adversity of known and anticipated effects to the public welfare.

B. Need for Revision of the Secondary Standard

    The initial issue to be addressed in this review of the secondary 
standard for O3 is whether, in view of the currently 
available scientific evidence, exposure and risk information and air 
quality analyses, as reflected in the record, the standard should be 
retained or revised. In drawing conclusions on adequacy of the current 
O3 secondary standard, the Administrator has taken into 
account both evidence-based and quantitative exposure- and risk-based 
considerations, as well as advice from CASAC and public comment. 
Evidence-based considerations draw upon the EPA's assessment and 
integrated synthesis of the scientific evidence from experimental and 
field studies evaluating welfare effects related to O3 
exposure, with a focus on policy-relevant considerations, as discussed 
in the PA. Air quality analyses inform these considerations with regard 
to cumulative, seasonal exposures occurring in areas of the U.S. that 
meet the current standard. Exposure- and risk-based considerations draw 
upon the EPA assessments of risk of key welfare effects, including 
O3 effects on forest growth, productivity, carbon storage, 
crop yield and visible foliar injury, expected to occur in model-based 
scenarios for the current standard, with appropriate consideration of 
associated uncertainties.
    In evaluating whether it is appropriate to revise the current 
standard, the Administrator's considerations build on the general 
approach used in the last review, as summarized in section IV.A of the 
proposal, and reflect the body of evidence and information available 
during this review. The approach used is based on an integration of the 
information on vegetation effects associated with exposure to 
O3 in ambient air, as well as policy judgments on the 
adversity of such effects to public welfare and on when the standard is 
requisite to protect public welfare from known or anticipated adverse 
effects. Such judgments are informed by air quality and related 
analyses, quantitative assessments, when available, and qualitative 
assessment of impacts that could not be quantified. The Administrator 
has taken into

[[Page 65380]]

account both evidence of effects on vegetation and ecosystems and 
public uses of these entities that may be important to the public 
welfare. The decision on adequacy of the protection provided by the 
current standard has also considered the 2013 remand of the secondary 
standard by the D.C. Circuit such that this decision incorporates the 
EPA's response to this remand.
    Section IV.B.1 below summarizes the basis for the proposed decision 
by the Administrator that the current secondary standard should be 
revised. Significant comments received from the public on the proposal 
are discussed in section IV.B.2 and the Administrator's final decision 
is described in section IV.B.3.
1. Basis for Proposed Decision
    In evaluating whether it was appropriate to propose to retain or 
revise the current standard, as discussed in section IV.D of the 
proposal, the Administrator carefully considered the assessment of the 
current evidence in the ISA, findings of the WREA, including associated 
limitations and uncertainties, considerations and staff conclusions and 
associated rationales presented in the PA, views expressed by CASAC, 
and public comments that had been offered up to that point. In the 
paragraphs below, we summarize the proposal presentation of the PA 
considerations with regard to adequacy of the current secondary 
standard, advice from the CASAC, and the Administrator's proposed 
conclusions, drawing from section IV.D of the proposal, where a fuller 
discussion is presented.
a. Considerations and Conclusions in the PA
    The PA evaluation is based on the longstanding evidence for 
O3 effects and the associated conclusions in the current 
review of causal and likely causal relationships between O3 
in ambient air and an array of welfare effects at a range of biological 
and ecological scales of organization, as summarized in section IV.A.1 
above (and described in detail in the ISA). Drawing from the ISA and 
CASAC advice, the PA emphasizes the strong support in the evidence for 
the conclusion that effects on vegetation are attributable to 
cumulative seasonal O3 exposures, taking note of the 
improved ``explanatory power'' (for effects on vegetation) of the W126 
index over other exposure metrics, as summarized in section IV.A.1.c 
above. The PA further recognizes the strong basis in the evidence for 
the conclusion that it is appropriate to use a cumulative seasonal 
exposure metric, such as the W126 index, to judge impacts of 
O3 on vegetation; related effects on ecosystems and 
services, such as carbon storage; and the level of public welfare 
protection achieved for such effects (U.S. EPA, 2014c, p. 5-78). As a 
result, based on the strong support in the evidence and advice from 
CASAC in the current and past reviews, the PA concludes that the most 
appropriate and biologically relevant way to relate O3 
exposure to plant growth, and to determine what would be adequate 
protection for public welfare effects attributable to the presence of 
O3 in ambient air, is to characterize exposures in terms of 
a cumulative seasonal form, and in particular the W126 metric (U.S. 
EPA, 2014c, pp. 5-7 and 5-78). Accordingly, in considering the evidence 
with regard to level of protection provided by the current secondary 
standard, the PA considers air quality data and exposure-response 
relationships for vegetation effects, particularly those related to 
forest tree growth, productivity and carbon storage, in terms of the 
W126 index (U.S. EPA, 2014c, section 5.2; 79 FR 75330-75333, December 
17, 2014).
    In considering the extent to which such growth-related effects 
might be expected to occur under conditions that meet the current 
secondary standard, the PA focused particularly on tree seedling RBL 
estimates for the 11 species for which robust E-R functions have been 
developed, noting the CASAC concurrence with use of O3-
related tree biomass loss as a surrogate for related effects extending 
to the ecosystem scale (U.S. EPA, 2014c, p. 5-80, Frey, 2014c, p. 10). 
The PA evaluation relied on RBL estimates for these 11 species derived 
using the robust OTC-based E-R functions, noting that analyses newly 
performed in this review have reduced the uncertainty associated with 
using OTC E-R functions to predict tree growth effects in the field 
(U.S. EPA, 2014c, section 5.2.1; U.S. EPA, 2013, section 9.6.3.2).
    In considering the RBL estimates for different O3 
conditions associated with the current standard, the PA focused 
primarily on the median of the species-specific (composite) E-R 
functions. In so doing, in the context of considering the adequacy of 
protection afforded by the current standard, the PA takes note of 
CASAC's view regarding a 6% median RBL (Frey, 2014c, p. 12). Based on 
the summary of RBL estimates in the PA, the PA notes that the median 
species RBL estimate, across the 11 estimates derived from the robust 
species-specific E-R functions, is at or above 6% for W126 index values 
of 19 ppm-hrs and higher (U.S. EPA, 2014c, Tables 6-1 and 5C-3).
    In recognition of the potential significance to public welfare of 
vegetation effects in Class I areas, the proposal described in detail 
findings of the PA analysis of the occurrence of O3 
concentrations associated with the potential for RBL estimates above 
benchmarks of interest in Class I areas that meet the current standard, 
focusing on 22 Class I areas for which air quality data indicated the 
current standard was met and cumulative seasonal exposures, in terms of 
a 3-year average W126 index, were at or above 15 ppm-hrs (79 FR 75331-
75332, Table 7, December 17, 2014; U.S. EPA, 2014c, Table 5-2). The PA 
noted that W126 index values (both annual and 3-year average values) in 
many such areas, distributed across multiple states and NOAA climatic 
regions, were above 19 ppm-hrs. The highest 3-year average value was 
over 22 ppm-hrs and the highest annual value was over 27 ppm-hrs, 
exposure values for which the corresponding median species RBL 
estimates markedly exceed 6%, which CASAC has termed ``unacceptably 
high'' (U.S. EPA, 2014c, section 5.2). The PA additionally considered 
the species-specific RBL estimates for two tree species (quaking aspen 
and ponderosa pine) that are found in many of these Class I areas and 
that have a sensitivity to O3 exposure that places them 
slightly more sensitive than the median of the group for which robust 
E-R functions have been established (U.S. EPA, 2014c, sections 5.2 and 
5.7). As further summarized in the proposal, the PA describes the 
results of this analysis, particularly in light of advice from CASAC 
regarding the significance of the 6% RBL benchmark, as evidence of the 
occurrence in Class I areas, during periods when the current standard 
is met, of cumulative seasonal O3 exposures of a magnitude 
for which the tree growth impacts indicated by the associated RBL 
estimates might reasonably be concluded to be important to public 
welfare (79 FR 75332; U.S. EPA, 2014c, sections 5.2.1 and 5.7).
    The proposal also noted that the PA additionally considered 
findings of the WREA analyses of O3 effects on tree growth 
and an array of ecosystem services provided by forests, including 
timber production, carbon storage and air pollution removal (79 FR 
75332-75333; U.S. EPA, 2014b, sections 6.2-6.8; U.S. EPA, 2014c, 
section 5.2). While recognizing that these analyses provide 
quantitative estimates of impacts on tree growth and associated 
services for several different air quality scenarios,

[[Page 65381]]

the PA takes note of the large uncertainties associated with these 
analyses (see U.S. EPA, 2014b, Table 6-27) and the potential for these 
findings to underestimate the response at the national scale. While 
noting the potential usefulness of considering predicted and 
anticipated impacts to these services in assessing the extent to which 
the current information supports or calls into question the adequacy of 
the protection afforded by the current standard, the PA also recognizes 
significant uncertainties associated with the absolute magnitude of the 
estimates for these ecosystem service endpoints which limited the 
weight staff placed on these results (U.S. EPA, 2014c, sections 5.2 and 
5.7).
    As described in the proposal, the PA also considered O3 
effects on crops, taking note of the extensive and long-standing 
evidence of the detrimental effect of O3 on crop production, 
which continues to be confirmed by evidence newly available in this 
review (79 FR 75333; U.S. 2014c, sections 5.3 and 5.7). With regard to 
consideration of the quantitative impacts of O3 exposures 
under exposure conditions associated with the current standard, the PA 
focused on RYL estimates that had strong support in the current 
evidence (as characterized in the ISA, section 9.6) in light of CASAC 
comments regarding RYL benchmarks (Frey, 2014c, pp. iii and 14). In 
considering such evidence-based analyses, as well as the exposure/risk-
based information for crops, the PA notes the CASAC comments regarding 
the use of crop yields as a surrogate for consideration of public 
welfare impacts, which noted that ``[c]rops provide food and fiber 
services to humans'' and that ``[e]valuation of market-based welfare 
effects of O3 exposure in forestry and agricultural sectors 
is an appropriate approach to take into account damage that is adverse 
to public welfare'' (Frey, 2014c, p. 10; U.S. EPA, 2014c, section 5.7). 
The PA additionally notes, however, as recognized in section IV.A.3 
above that the determination of the point at which O3-
induced crop yield loss becomes adverse to the public welfare is still 
unclear, given that crops are heavily managed (e.g., with fertilizer, 
irrigation) for optimum yields, have their own associated markets and 
that benefits can be unevenly distributed between producers and 
consumers (79 FR 75322; U.S. EPA, 2014c, sections 5.3 and 5.7).
    With regard to visible foliar injury, as summarized in the 
proposal, the PA recognizes the long-standing evidence that has 
established that O3 causes diagnostic visible foliar injury 
symptoms on studied bioindicator species and also recognizes that such 
O3-induced impacts have the potential to impact the public 
welfare in scenic and/or recreational areas, with visible foliar injury 
associated with important cultural and recreational ecosystem services 
to the public, such as scenic viewing, wildlife watching, hiking, and 
camping, that are of significance to the public welfare and enjoyed by 
millions of Americans every year, generating millions of dollars in 
economic value (U.S. EPA, 2014b, section 7.1). In addition, several 
tribes have indicated that many of the O3-sensitive species 
(including bioindicator species) are culturally significant (U.S. EPA, 
2014c, Table 5-1). Similarly, the PA notes CASAC comments that 
``visible foliar injury can impact public welfare by damaging or 
impairing the intended use or service of a resource,'' including 
through ``visible damage to ornamental or leafy crops that affects 
their economic value, yield, or usability; visible damage to plants 
with special cultural significance; and visible damage to species 
occurring in natural settings valued for scenic beauty or recreational 
appeal'' (Frey, 2014c, p. 10). Given the above, and taking note of 
CASAC views, the PA recognizes visible foliar injury as an important 
O3 effect which, depending on severity and spatial extent, 
may reasonably be concluded to be of public welfare significance, 
especially when occurring in nationally protected areas, such as 
national parks and other Class I areas.
    As summarized in the proposal, the PA additionally takes note of 
the evidence described in the ISA regarding the role of soil moisture 
conditions that can decrease the incidence and severity of visible 
foliar injury under dry conditions (U.S. EPA, 2014c, sections 5.4 and 
5.7). As recognized in the PA, this area of uncertainty complicates 
characterization of the potential for visible foliar injury and its 
severity or extent of occurrence for given air quality conditions and 
thus complicates identification of air quality conditions that might be 
expected to provide a specific level of protection from this effect 
(U.S. EPA, 2014c, sections 5.4 and 5.7). While noting the uncertainties 
associated with describing the potential for visible foliar injury and 
its severity or extent of occurrence for any given air quality 
conditions, the PA notes the occurrence of O3-induced 
visible foliar injury in areas, including federally protected Class I 
areas that meet the current standard, and suggests it may be 
appropriate to consider revising the standard for greater protection. 
In so doing, however, the PA recognizes that the degree to which 
O3-induced visible foliar injury would be judged important 
and potentially adverse to public welfare is uncertain (U.S. EPA, 
2014c, section 5.7).
    As noted in the proposal, with regard to other welfare effects, for 
which the ISA determined a causal or likely causal relationships with 
O3 in ambient air, such as alteration of ecosystem water 
cycling and changes in climate, the PA concludes there are limitations 
in the available information that affect our ability to consider 
potential impacts of air quality conditions associated with the current 
standard.
    Based on the considerations described in the PA, summarized in the 
proposal and outlined here, the PA concludes that the currently 
available evidence and exposure/risk information call into question the 
adequacy of the public welfare protection provided by the current 
standard and provide support for considering potential alternative 
standards to provide increased public welfare protection, especially 
for sensitive vegetation and ecosystems in federally protected Class I 
and similarly protected areas. In this conclusion, staff gives 
particular weight to the evidence indicating the occurrence in Class I 
areas that meet the current standard of cumulative seasonal 
O3 exposures associated with estimates of tree growth 
impacts of a magnitude that may reasonably be considered important to 
public welfare.
b. CASAC Advice
    The proposal also summarized advice offered by the CASAC in the 
current review, based on the updated scientific and technical record 
since the 2008 rulemaking. The CASAC stated that it ``[supports] the 
conclusion in the Second Draft PA that the current secondary standard 
is not adequate to protect against current and anticipated welfare 
effects of ozone on vegetation'' (Frey, 2014c, p. iii) and that the PA 
``clearly demonstrates that ozone-induced injury may occur in areas 
that meet the current standard'' (Frey, 2014c, p. 12). The CASAC 
further stated ``[w]e support the EPA's continued emphasis on Class I 
and other protected areas'' (Frey, 2014c, p. 9). Additionally, the 
CASAC indicated support for the concept of ecosystem services ``as part 
of the scope of characterizing damage that is adverse to public 
welfare'' and ``concur[red] 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

[[Page 65382]]

pollution removal, and hydrologic and fire regime stabilization'' 
(Frey, 2014c, p. 9). Similar to comments from CASAC in the last review, 
and comments on the proposed reconsideration, the current CASAC also 
endorsed the PA discussions and conclusions on biologically relevant 
exposure metrics and the focus on the W126 index accumulated over a 12-
hour period (8 a.m.-8 p.m.) over the 3-month summation period of a year 
resulting in the maximum value (Frey, 2014c, p. iii).
    In addition, CASAC stated that ``relative biomass loss for tree 
species, crop yield loss, and visible foliar injury are appropriate 
surrogates for a wide range of damage that is adverse to public 
welfare,'' listing an array of related ecosystem services (Frey, 2014c, 
p. 10). With respect to RBL for tree species, CASAC states that it is 
appropriate to identify in the PA ``a range of levels of alternative 
W126-based standards that include levels that aim for not greater than 
2% RBL for the median tree species'' and that a median tree species RBL 
of 6% is ``unacceptably high'' (Frey, 2014c, pp. 13 and 14). With 
respect to crop yield loss, CASAC points to a benchmark of 5%, stating 
that a crop RYL for median species over 5% is ``unacceptably high'' and 
described crop yield as a surrogate for related services (Frey, 2014c, 
p. 13).
c. Administrator's Proposed Conclusions
    At the time of proposal, the Administrator took into account the 
information available in the current review with regard to the nature 
of O3-related effects on vegetation and the adequacy of 
protection provided by the current secondary standard. The 
Administrator recognized the appropriateness and usefulness of the W126 
metric in evaluating O3 exposures of potential concern for 
vegetation effects, additionally noting support conveyed by CASAC for 
such a use for this metric. Further, the Administrator took particular 
note of (1) the PA analysis of the magnitude of tree seedling growth 
effects (biomass loss) estimated for different cumulative, seasonal, 
concentration-weighted exposures in terms of the W126 metric; (2) the 
monitoring analysis in the PA of cumulative exposures (in terms of W126 
index) occurring in locations where the current standard is met, 
including those locations in or near Class I areas, and associated 
estimates of tree seedling growth effects; and (3) the analyses in the 
WREA illustrating the geographic distribution of tree species for which 
E-R functions are available and estimates of O3-related 
growth impacts for different air quality scenarios, taking into account 
the identified potential for the WREA's existing standard scenario to 
underestimate the highest W126-based O3 values that would be 
expected to occur.
    With regard to considering the adequacy of public welfare 
protection provided by the current secondary standard at the time of 
proposal, the Administrator focused first on welfare effects related to 
reduced native plant growth and productivity in terrestrial systems, 
taking note of the following: (a) The ISA conclusion of a causal 
relationship between O3 in the ambient air and these welfare 
effects, and supporting evidence related to O3 effects on 
vegetation growth and productivity, including the evidence from OTC 
studies of tree seedling growth that support robust E-R functions for 
11 species; (b) the evidence, described in section IV.D.1 of the 
proposal and summarized above, of the occurrence of cumulative seasonal 
O3 exposures for which median species RBL estimates are of a 
magnitude that CASAC has termed ``unacceptably high'' in Class I areas 
during periods where the current standard is met; (c) actions taken by 
Congress to establish public lands that are 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 for the enjoyment of 
future generations, such as national parks and forests, wildlife 
refuges, and wilderness areas (many of which have been designated Class 
I areas); and (d) PA conclusions that the current information calls 
into question the adequacy of the current standard, based particularly 
on impacts on tree growth (and the potential for associated ecosystem 
effects), estimated for Class I area conditions meeting the current 
standard, that are reasonably concluded to be important from a public 
welfare standpoint in terms of both the magnitude of the vegetation 
effects and the significance to public welfare of such effects in such 
areas.
    At the time of proposal, the Administrator also recognized the 
causal relationships between O3 in the ambient air and 
visible foliar injury, reduced yield and quality of agricultural crops, 
and alteration of below-ground biogeochemical cycles associated with 
effects on growth and productivity. As to visible foliar injury, she 
took note of the complexities and limitations in the evidence base 
regarding characterizing air quality conditions with respect to the 
magnitude and extent of risk for visible foliar injury, and she 
additionally recognized the challenges of associated judgments with 
regard to adversity of such effects to public welfare. In taking note 
of the conclusions with regard to crops, she recognized the complexity 
of considering adverse O3 impacts to public welfare due to 
the heavy management common for achieving optimum yields and market 
factors that influence associated services and additionally took note 
of the PA conclusions that placing emphasis on the protection afforded 
to trees inherently also recognizes a level of protection afforded for 
crops.
    Based on her consideration of the conclusions in the PA, and with 
particular weight given to PA findings pertaining to tree growth-
related effects, as well as with consideration of CASAC's conclusion 
that the current standard is not adequate, the Administrator proposed 
to conclude that the current standard is not requisite to protect 
public welfare from known or anticipated adverse effects and that 
revision is needed to provide the requisite public welfare protection, 
especially for sensitive vegetation and ecosystems in federally 
protected Class I areas and in other areas providing similar public 
welfare benefits. The Administrator further concluded that the 
scientific evidence and quantitative analyses on tree growth-related 
effects provide strong support for consideration of alternative 
standards that would provide increased public welfare protection beyond 
that afforded by the current O3 secondary standard. She 
further noted that a revised standard would provide increased 
protection for other growth-related effects, including for carbon 
storage and for areas for which it is more difficult to determine 
public welfare significance, as recognized in section IV.B.2 of the 
proposal, as well as other welfare effects of O3, including 
visible foliar injury and crop yield loss.
2. Comments on the Need for Revision
    In considering comments on the need for revision, we first note the 
advice and recommendations from CASAC with regard to the adequacy of 
the current standard. In its review of the second draft PA, CASAC 
stated that it ``supports the scientific conclusion in the Second Draft 
PA that the current secondary standard is not adequate to protect 
against current and anticipated welfare effects of ozone on 
vegetation'' (Frey, 2014c).
    General comments received from the public on the proposal that are 
based on relevant factors and either supported or opposed the proposed 
decision to revise

[[Page 65383]]

the current O3 secondary standard are addressed in this 
section. Comments on specific issues or information that relate to 
consideration of the appropriate elements of a revised secondary 
standard are addressed below in section IV.C. Other specific comments 
related to standard setting, as well as general comments based on 
implementation-related factors that are not a permissible basis for 
considering the need to revise the current standard, are addressed in 
the Response to Comments document.
    Public comments on the proposal were divided with regard to support 
for the Administrator's proposed decision to revise the current 
secondary standard. Many state and local environmental agencies or 
government bodies, tribal agencies and organizations, and environmental 
organizations agreed with the EPA's proposed conclusion on the need to 
revise the current standard, stating that the available scientific 
information shows that O3-induced vegetation and ecosystem 
effects are occurring under air quality conditions allowed by the 
current standard and, therefore, provides a strong basis and support 
for the conclusion that the current secondary standard is not adequate. 
In support of their view, these commenters relied on the entire body of 
evidence available for consideration in this review, including evidence 
assessed previously in the 2008 review. These commenters variously 
pointed to the information and analyses in the PA and the conclusions 
and recommendations of CASAC as providing a clear basis for concluding 
that the current standard does not provide adequate protection of 
public welfare from O3-related effects. Many of these 
commenters generally noted their agreement with the rationale provided 
in the proposal with regard to the Administrator's proposed conclusion 
on adequacy of the current standard, and some gave additional emphasis 
to several aspects of that rationale, including the appropriateness of 
the EPA's attention to sensitive vegetation and ecosystems in Class I 
areas and other public lands that provide similar public welfare 
benefits and of the EPA's reliance on the strong evidence of impacts to 
tree growth and growth-related effects.
    Comments from tribal organizations additionally noted that many 
Class I areas are of sacred value to tribes or provide treaty-protected 
benefits to tribes, including the exercise of gathering rights. Tribal 
organizations also noted the presence in Class I areas of large numbers 
of culturally important plant species, which they indicate to be 
impacted by air quality conditions allowed by the current standard. The 
impacts described include visible foliar injury, loss in forest growth 
and crop yield loss, which these groups describe as especially 
concerning when occurring on lands set aside for the benefit of the 
public or that are of sacred value to tribes or provide treaty-
protected benefits to tribes.
    As described in section IV.B.3 below, the EPA generally agrees with 
the view of these commenters regarding the need for revision of the 
current secondary standard and with CASAC that the evidence provides 
support for the conclusions that the current secondary standard is not 
adequate to protect public welfare from known or anticipated adverse 
effects, particularly with respect to effects on vegetation.
    A number of industries, industry associations, or industry 
consultants, as well as some state governors, attorneys general and 
environmental agencies, disagreed with the EPA's proposed conclusion on 
the adequacy of the current standard and recommended against revision. 
In support of their position, these commenters variously stated that 
the available evidence is little changed from that available at the 
time of the 2008 decision, and that the evidence is too uncertain, 
including with regard to growth-related effects and visible foliar 
injury, to support revision, and does not demonstrate adverse effects 
to public welfare for conditions associated with the current standard, 
with some commenters stating particularly that the EPA analysis of 
Class I areas did not document adverse effects to public welfare. They 
also cited the WREA modeling analyses as indicating that any welfare 
improvements associated with a revised standard would be marginal; in 
particular, compared to the benefits of achieving the current standard. 
Further, they state that, because of long-range transport of 
O3 and precursors, it is not appropriate for the EPA to draw 
conclusions about the level of protection offered by the current 
standard based on current air quality conditions; in support of this 
view, these commenters point to different modeling analyses as 
demonstrating that under conditions where the current standard is met 
throughout the U.S., the associated W126 values would all be below the 
upper end of the range proposed as providing requisite public welfare 
protection and nearly all below the lower end of 13 ppm-hrs.
    As an initial matter, we note that, as noted in sections I.C and 
IV.A above, the EPA's 2008 decision on the secondary standard was 
remanded back to the Agency because in setting the 2008 secondary 
standard, the EPA failed to specify what level of air quality was 
requisite to protect public welfare from known or anticipated adverse 
effects or explain why any such level would be requisite. So, in 
addressing the court remand, the EPA has more explicitly considered the 
extent to which protection is provided from known or anticipated 
effects that the Administrator may judge to be adverse to public 
welfare, and has described how the air quality associated with the 
revised standard would provide requisite public welfare protection, 
consistent with CAA section 109(b)(2) and the court's decision 
remanding the 2008 secondary standard. In undertaking this review, 
consistent with the direction of the CAA, the EPA has considered the 
current air quality criteria.
    While we recognize, as stated in the proposal, that the evidence 
newly available in this review is largely consistent with the evidence 
available at the time of the last review (completed in 2008) with 
regard to the welfare effects of O3, we disagree with the 
commenters' interpretations of the evidence and analyses available in 
this review and with their views on the associated uncertainties. As 
summarized in section IV.A above, the ISA has determined causal 
relationships to exist between several vegetation and ecosystem 
endpoints and O3 in ambient air (U.S. 2013, section 9.7). 
The ISA characterized the newly available evidence as largely 
consistent with and supportive of prior conclusions, as summarized in 
section IV.A above. This is not to say, however, that there is no newly 
available evidence and information in this review or that it is 
identical to that available in the last review. In some respects, the 
newly available evidence has strengthened the evidence available in the 
last review and reduced important uncertainties. As summarized in 
section IV.A.1.b above, newly available field studies confirm the 
cumulative effects and effects on forest community composition over 
multiple seasons. Additionally, among the newly available evidence for 
this review are analyses documented in the ISA that evaluate the RBL 
and RYL E-R functions for aspen and soybean, respectively, with 
experimental datasets that were not used in the derivation of the 
functions (U.S. 2013, section 9.6.3). These evaluations confirm the 
pertinence of the tree seedling RBL estimates for aspen, a species with 
sensitivity roughly midway in the range of sensitivities for the 
studied species, across multiple years in older trees.

[[Page 65384]]

With regard to crops, the ISA evaluations demonstrate a robustness of 
the E-R functions to predict O3-attributable RYL and confirm 
the relevance of the crop RYL estimates for more recent cultivars 
currently growing in the field. Together, the information newly 
available in this review confirms the basis for the E-R functions and 
strengthens our confidence in interpretations drawn from their use in 
other analyses newly available in this review that have been described 
in the WREA and PA.
    With regard to comments on uncertainties associated with estimates 
of RBL, we first note that these established, robust E-R functions, 
which the EPA gave particular emphasis in this review, are available 
for seedling growth for 11 tree species native to the U.S., as 
summarized in section IV.A.1.b above and described in the proposal. 
These E-R functions are based on studies of multiple genotypes of 11 
tree species grown for up to three years in multiple locations across 
the U.S. (U.S. EPA, 2013, section 9.6.1). We have recognized the 
uncertainty regarding the extent to which the studied species encompass 
the O3 sensitive species in the U.S. and also the extent to 
which they represent U.S. vegetation as a whole (U.S. EPA, 2014b, 
section 6.9). However, the studied species include both deciduous and 
coniferous trees with a wide range of sensitivities and species native 
to every region across the U.S. and in most cases are resident across 
multiple states and NOAA climatic regions (U.S. EPA, 2014b, Appendix 
6A). While the CASAC stated that there is ``considerable uncertainty in 
extrapolating from the [studied] forest tree species to all forest tree 
species in the U.S.,'' it additionally expressed the view that it 
should be anticipated that there are highly sensitive vegetation 
species for which we do not have E-R functions and others that are 
insensitive.\187\ In so doing, the CASAC stated that it ``should not be 
assumed that species of unknown sensitivity are tolerant to ozone'' and 
``[i]t is more appropriate to assume that the sensitivity of species 
without E-R functions might be similar to the range of sensitivity for 
those species with E-R functions'' (Frey, 2014c, p. 11). Accordingly, 
we disagree with commenters' view that effects on these species are not 
appropriate considerations for evaluation of the adequacy of the 
current standard.
---------------------------------------------------------------------------

    \187\ Use of RBL estimates in the proposal, and in this final 
decision, focuses on the RBL for the studied species as a surrogate 
for a broad array of growth-related effects of potential public 
welfare significance, consistent with the CASAC advice.
---------------------------------------------------------------------------

    In support of their view that RBL estimates are too uncertain to 
inform a conclusion that the current standard is not adequately 
protective of public welfare, some commenters state that some of the 11 
E-R functions are based on as few as one study. The EPA agrees that 
there are two species for which there is only one study supporting the 
E-R function (Virginia pine and red maple). We also note, however, that 
those two species are appreciably less sensitive than the median (Lee 
and Hogsett, 1996; U.S. EPA, 2014c, Table 5C-1). Thus, in the relevant 
analyses, they tend to influence the median toward a relatively less 
(rather than more) sensitive response. Further, there are four species 
for which the E-R functions are based on more than five studies,\188\ 
contrary to the commenters' claims of there being no functions 
supported by that many studies. That said, the EPA has noted the 
relatively greater uncertainty in the species for which fewer studies 
are available, and it is in consideration of such uncertainties that 
the EPA focused in the proposal on the median E-R function across the 
11 species, rather than a function for a species much more (or less) 
sensitive than the median. The EPA additionally notes that it gave less 
emphasis to the E-R function available for one species, eastern 
cottonwood, based on CASAC advice that the study results supporting 
that E-R function were not as strong as the results of the other 
experiments that support the other, robust E-R functions and that the 
eastern cottonwood study results showed extreme sensitivity to 
O3 compared to other studies (Frey, 2014c, p. 10). 
Accordingly, the EPA has appropriately considered the strength of the 
scientific evidence and the associated uncertainties in considering 
revision of the secondary standard.
---------------------------------------------------------------------------

    \188\ These four species, aspen, Douglas fir, ponderosa pine and 
red alder, range broadly in sensitivities that fall above, below and 
at the median for the 11 species (Lee and Hogsett, 1996; U.S. EPA, 
2014c, Table 5C-1).
---------------------------------------------------------------------------

    Other commenters stated that the scientific evidence does not 
support revising the NAAQS, pointing to uncertainty related to 
interpretation of the RBL estimates (based on tree seedling studies) 
with regard to effects on older tree lifestages. Some of these 
commenters' claim that mature canopy trees experience reduced 
O3 effects. The EPA agrees that the quantitative information 
for O3 growth effects on older tree lifestages is available 
for a more limited set of species than that available for tree 
seedlings. We note, however, that this is an area for which there is 
information newly available in this review. A detailed analysis of 
study data for seedlings and older lifestages of aspen shows close 
agreement between the O3-attributable reduced growth 
observed in the older trees and reductions predicted from the seedling 
E-R function (U.S. EPA, 2013, section 9.6.3.2; discussed in the PA, 
section 5.2.1 as noted in the proposal, p. 75330). This finding, newly 
available in this review and documenting impacts on mature trees, 
improves our confidence in conclusions drawn with regard to the 
significance of RBL estimates for this species, which is prevalent 
across multiple regions of the U.S.\189\ It is also noteworthy that 
this species is generally more sensitive to O3 effects on 
growth than the median of the 11 species with robust E-R functions (as 
shown in U.S. EPA 2014c, Table 5C-1). Other newly available studies, 
summarized in section IV.A.1.b above and section IV.B.1.b of the 
proposal, provide additional evidence of O3 impacts on 
mature trees, including a meta-analysis reporting older trees to be 
more affected by O3 than younger trees (U.S. EPA, 2013, p. 
9-42; Wittig et al., 2007). We additionally note that CASAC 
``concur[red] 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'' additionally stating that ``[b]iomass loss may 
also have indirect process-related effects such as on nutrient and 
hydrologic cycles'' leading them to conclude that ``[t]herefore, 
biomass loss is a scientifically valid surrogate of a variety of 
adverse effects to public welfare'' (Frey, 2014c, p. 10).
---------------------------------------------------------------------------

    \189\ The WREA notes a few additional, limited analyses using 
modeling tools and data from previous publications that indicate 
there may be species-specific differences in the extent of 
similarities between seedling and adult growth response to 
O3, with some species showing greater and some lesser 
response for seedlings as compared to mature tree, but a general 
comparability (U.S. EPA 2014b, section 6.2.1.1 and p. 6-67).
---------------------------------------------------------------------------

    As noted in section IV.A above and discussed below, the 
Administrator's final decision on the adequacy of the current standard 
draws upon, among other things, the available evidence and quantitative 
analyses as well as judgments about the appropriate weight to place on 
the range of uncertainties inherent in the evidence and analyses. The 
strengthening in this review, as compared with the last review, of the 
basis for the robust E-R functions for tree seedling RBL, as well as 
other newly available quantitative analyses,

[[Page 65385]]

will, accordingly, contribute to judgments made by the Administrator 
with regard to these effects in reaching her final decisions in this 
review.
    Amongst the newly available information in this review is a new 
analysis describing W126-based exposures occurring in counties 
containing Class I areas for which monitoring data indicated compliance 
with the current standard. The PA gave particular attention to this 
analysis in consideration of the adequacy of the current standard, and 
this analysis was also described in the proposal (U.S. EPA, 2014c, 
Appendix 5B and pp. 5-27 to 5-29; 79 FR 75331-75332, December 17, 
2014). Some of the commenters who disagreed with the EPA's conclusion 
on adequacy of the current standard variously stated that this analysis 
does not demonstrate growth effects are occurring in Class I areas and 
that the analysis is too uncertain for reliance on by the Administrator 
in her judgment on adequacy of the current standard. While the EPA 
agrees with commenters that data on the occurrence of growth effects in 
the areas and time periods identified are not part of this analysis, we 
note that this is because such data have not been collected and 
consequently cannot be included. As a result, the EPA has utilized 
measurements of O3 in or near these areas in combination 
with the established E-R functions to estimate the potential for growth 
impacts in these areas under conditions where the current standard is 
met. The EPA additionally notes that species for which E-R functions 
have been developed have been documented to occur within these areas 
(see Table 3).
    The EPA disagrees with commenters regarding the appropriateness of 
this analysis for the Administrator's consideration. This analysis 
documents the occurrence of cumulative growing season exposures in 
these ecosystems which the EPA and CASAC have interpreted, through the 
use of the established E-R functions for tree seedling growth effects 
summarized in section IV.A.1.b above (and described in the ISA, PA and 
proposal), as indicating the potential for growth effects of 
significance in these protected areas. To the extent that these 
comments imply that the Administrator may only consider welfare effects 
that are certain in judging the adequacy of the current standard, we 
note that section 109(b)(2) of the CAA plainly provides for 
consideration of both known and anticipated adverse effects in 
establishing or revising secondary NAAQS.
    In support of some commenters' view that this analysis is too 
uncertain to provide a basis for the Administrator's proposed 
conclusion that the current standard is not adequate, one commenter 
observed that the O3 monitors used for six of the 22 Class I 
areas in the analysis, although in the same county, were sited outside 
of the Class I areas. This was the case due to the analysis being 
focused on the highest monitor in the county that met the current 
standard. To clarify the presentation, however, we have refocused the 
presentation, restricting it to data for monitors sited in or within 15 
kilometers of a Class I area,\190\ and note that the results are little 
changed, continuing to call into question the adequacy of the current 
standard. As shown in Table 3, the dataset in the refocused 
presentation, which now spans 1998 up through 2013, includes 17 Class I 
areas for which monitors were identified in this manner. For context, 
we note that this represents nearly a quarter of the Class I areas for 
which there are O3 monitors within 15 km.\191\
---------------------------------------------------------------------------

    \190\ The 15 km distance was selected as a natural breakpoint in 
distance of O3 monitoring sites from Class I areas and as 
still providing similar surroundings to those occurring in the Class 
I area. We note that given the strict restrictions on structures and 
access within some of these areas, it is common for monitors 
intended to collect data pertaining to air quality in these types of 
areas to be sited outside their boundaries.
    \191\ There is an O3 monitor within fewer than 15% of 
all Class I areas, and fewer than half of all Class I areas have a 
monitor within 15 km.
---------------------------------------------------------------------------

    In recognition of the influence that other environmental factors 
can exert in the natural environment on the relationship between 
ambient O3 exposures and RBL, potentially modifying the 
impact predicted by the E-R functions, the PA and proposal took 
particular note of the occurrence of 3-year average W126 index values 
at or above 19 ppm-hrs. In the re-focused analysis in Table 3, there 
are 11 areas, distributed across four states in two NOAA climatic 
regions, for which the 3-year W126 exposure index values ranged at or 
above 19 ppm-hrs, a value for which the corresponding median species 
RBL estimate for a growing season's exposure is 6%, a magnitude termed 
``unacceptably high'' by CASAC (Frey, 2014c, p. 13). The highest 3-year 
W126 index values in these 11 areas ranged from 19.0 up to 22.2 ppm-
hrs, a cumulative seasonal exposure for which the median species RBL 
estimate is 9% for a single growing season. The annual W126 index 
values range above 19 ppm-hrs in 15 of the areas in the re-focused 
table provided here; these areas are distributed across six states (AZ, 
CA, CO, KY, SD, UT) and four regions (West, Southwest, West North 
Central and Central).\192\ The highest index values in the areas with 
annual index values above 19 ppm-hrs range from 19.1 to 26.9 ppm-hrs. 
As is to be expected from the focus on a smaller dataset, the number of 
states with 1-year W126 index values above 19 ppm-hrs is smaller in the 
refocused analysis (15 as compared to 20), although the number of 
regions affected is the same. More importantly, however, the number of 
areas with 3-year W126 index values at or above 19 ppm-hrs is the same, 
11 Class I areas across two regions, supporting the prior conclusions.
---------------------------------------------------------------------------

    \192\ This compares to 20 areas in eight states and four regions 
in the earlier analysis.

Table 3--O3 Concentrations for Class I Areas During Period From 1998 to 2013 That Met the Current Standard and Where 3-Year Average W126 Index Value Was
                                                                 at or Above 15 ppm-hrs
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Design   3-Year average W126 (ppm-                           Number of 3-
 Class I area (distance away, if monitor is            State/ County            value     hrs)* (# >= 19 ppm-hrs,   Annual W126 (ppm-hrs)*       year
          not at/within boundaries)                                             (ppb)*            range)           (# >= 19 ppm-hrs, range)    periods
--------------------------------------------------------------------------------------------------------------------------------------------------------
Bridger Wilderness Area \QA, DF\ (8.9 km)...  WY/Sublette...................      70-72                16.2-17.0                 13.9-18.8             4
Canyonlands National Park \QA, DF, PP\......  UT/San Juan...................      70-73  15.4-19.5 (2, 19.1-19.5)   9.6-23.6 (4, 19.2-23.6)            8
Chiricahua National Monument \DF, PP\ (12     AZ/Cochise....................      69-73       15.2-19.8 (1, 19.8)  11.7-21.9 (2, 19.8-21.9)           10
 km).
Grand Canyon National Park \QA, DF, PP\.....  AZ/Coconino...................      68-74  15.3-22.2 (7, 19.1-22.2)  10.1-26.9 (6, 19.8-26.9)           12
Desolation Wilderness \PP\ (3.9 km).........  CA/El Dorado..................         75            19.8 (1, 19.8)  15.6-22.9 (2, 21.0-22.9)            1

[[Page 65386]]

 
Lassen Volcanic National Park \DF, PP\......  CA/Shasta.....................      72-74                15.3-15.6        11.5-19.1 (1, 19.1)            2
Mammoth Cave National Park \BC, C, LP, RM,    KY/Edmonson...................         74                     15.7        12.3-22.0 (1, 22.0)            1
 SM, VP, YP\ (0.1 km).
Maroon Bells-Snowmass Wilderness Area \QA,    CO/Gunnison...................      68-73       15.6-20.2 (1, 20.2)  13.0-23.8 (3, 21.3-23.8)            8
 DF\ (0.8 km).
Mazatzal Wilderness \DF, PP\ (10.9 km)......  AZ/Maricopa...................      74-75       17.8-19.9 (1, 19.9)  10.3-26.2 (3, 19.7-26.2)            2
Mesa Verde National Park \DF\...............  CO/Montezuma..................      67-73       15.4-20.7 (1, 20.7)  10.7-23.4 (4, 19.5-23.4)           11
Petrified Forest National Park \C\..........  AZ/Navajo.....................         70                15.4-16.9                 12.7-18.6             2
Rocky Mountain National Park \QA, DF, PP\     CO/Larimer....................      73-74                15.3-18.4    8.3-26.2 (4, 19.4-26.2)            5
 (0.9 km).
Saguaro National Park \DF, PP\ (0.1 km)**...  AZ/Pima.......................      69-74       15.4-19.0 (1, 19.0)   7.3-22.9 (3, 19.6-22.9)            6
                                              AZ/Gila.......................      72-75  16.6-20.9 (2, 19.0-20.9)  13.8-25.5 (4, 19.0-25.5)            5
Superstition Wilderness Area \PP\ (6.3, 14.9  AZ/Maricopa...................      70-75         15-20.2 (1, 20.2)   6.3-23.9 (4, 19.6-23.9)            4
 km and 7.2 km)**.
                                              AZ/Pinal......................      72-75       15.3-21.1 (1, 21.1)  10.2-24.7 (4, 21.4-24.7)            7
Weminuche Wilderness Area \QA, DF, PP\ (14.9  CO/La Plata...................      70-74       15.1-19.1 (1, 19.1)  10.8-21.0 (2, 20.8-21.0)            6
 km).
Wind Cave National Park \QA, PP\............  SD/Custer.....................         70                     15.4        12.3-20.5 (1, 20.5)            1
Zion National Park \QA, DF, PP\ (3.6 km)....  UT/Washington.................      70-73  17.0-20.1 (2, 19.4-20.1)  14.2-23.2 (3, 19.8-23.2)            6
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Based on hourly O\3\ concentration data retrieved from AQS on June 25, 2014, and additional CASTNET data downloaded from http://java.epa.gov/castnet/epa_jsp/prepackageddata.jsp on June 25, 2014. Design values shown above are derived in accordance with Appendix P to 40 CFR Part 50. Annual W126 index
  values are derived as described in section IV.A.1 above; three consecutive year annual values are averaged for 3-year averages. Prior to presentation,
  both types of W126 index values are rounded to one decimal place. The full list of monitoring site identifiers and individual statistics is available
  in the docket for this rulemaking.
** No monitor was sited within these Areas and multiple monitors were sited within 15 km. Data for the closest monitor per county are presented.
Superscript letters refer to species present for which E-R functions have been developed. QA=Quaking Aspen, BC=Black Cherry, C=Cottonwood, DF=Douglas
  Fir, LP=Loblolly Pine, PP=Ponderosa Pine, RM=Red Maple, SM=Sugar Maple, VP=Virginia Pine, YP=Yellow (Tulip) Poplar. Sources include USDA-NRCS (2014,
  http://plants.usda.gov), USDA-FS (2014, http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml) UM-CFCWI (2014, http://www.wilderness.net/printFactSheet.cfm?WID=583), NPS (http://www.nps.gov/pefo/planyourvisit/upload/Common-Plants-Site-Bulletin-sb-2013.pdf) and Phillips and Comus (2000).

    As support for their view that the Class I area analysis is too 
uncertain to provide a basis for the Administrator's proposed 
conclusion that the current standard is not adequate, some commenters 
stated that forests in Class I areas were composed of mature trees and 
that the tree seedling E-R functions do not predict growth impacts in 
mature forests. The EPA disagrees with the commenters' statement that 
Class I areas are only made up of mature trees. Seedlings exist 
throughout forests as part of the natural process of replacing aging 
trees and overstory trees affected by periodic disturbances.\193\ 
Seedlings also tend to occur in areas affected by natural disturbances, 
such as fires, insect infestations and flooding, and such disturbances 
are common in many natural forests. As noted above, information newly 
available in this review strengthens our understanding regarding 
O3 effects on mature trees for aspen, an important and 
O3-sensitive species (U.S. EPA, 2013, section 9.6.3.2).
---------------------------------------------------------------------------

    \193\ Basic information on forest processes, including the role 
of seedlings is available at: http://www.na.fs.fed.us/stewardship/pubs/NE_forest_regeneration_handbook_revision_130829_desktop.pdf.
---------------------------------------------------------------------------

    One commenter additionally stated that the EPA has not shown 
reduced biomass to be adverse to public welfare, variously citing 
individual studies, most of which are not considering O3, as 
support for their view that such an effect of O3 may not 
occur in the environment and may be of no significance if it does. With 
regard to the occurrence of O3-related reduced growth in the 
field, we note the strength of the evidence from field OTC studies on 
which the E-R functions are based, and evidence from comparative 
studies with open-air chamberless control treatments suggests that 
characteristics particular to the OTC did not significantly affect 
plant response (U.S. EPA, 2013, p. 9-5). Thus, we view the OTC systems 
as combining aspects of controlled exposure systems with field 
conditions to facilitate a study providing data that represent the role 
of the studied pollutant in a natural system.
    Further, we disagree with the commenters on the significance of 
O3-attributable reduced growth in natural ecosystems. Even 
in the circumstances cited by the commenter (e.g., subsequent to large-
scale disturbances, nutrient limited system, multigeneration exposure), 
O3 can affect growth of seedlings and older trees, with the 
potential for effects on ecosystem productivity, handicapping the 
sensitive species and affecting community dynamics and associated 
community composition, as well as ecosystem hydrologic cycles (U.S. 
EPA, 2013, p. 1-8). For example, two recent studies report on the role 
of O3 exposure in affecting water use in a mixed deciduous 
forest and indicated that O3 increased water use in the 
forest and also reduced growth rate (U.S. EPA, 2013, p. 9-43, 
McLaughlin, 2007a, 2007b). Contrary to the lesser effects implied by 
the commenters, the authors of these two studies noted implications of 
their findings with regard to the potential for effects to be amplified 
under conditions of increased temperature and associated reduced water 
availability (McLaughlin, 2007a). We additionally note comments from

[[Page 65387]]

the CASAC, summarized above, in which it concurs with a focus on 
biomass loss and the use of RBL estimates, calling biomass loss in 
trees a ``relevant surrogate for damage to tree growth'' that affects 
an array of ecosystem services (Frey, 2014c, p. 10), and identifies 6% 
RBL as ``unacceptably high'' (Frey, 2014c, p. 13). The evidence we 
presented includes evidence related to RBL estimates above that 
benchmark. Thus, while we agree that some reductions in tree growth may 
not be concluded to be adverse to public welfare, we disagree with 
commenters that we have not presented the evidence, which includes RBL 
estimates well above the 6% magnitude identified by CASAC, that 
supports the Administrator's judgments on adversity that may be 
indicated by such estimates and her conclusion that adequate protection 
is not provided by the current standard, as described in section IV.B.3 
below.
    Some commenters disagree with the EPA's consideration of the Class 
I areas analysis, stating that it is not appropriate for the EPA to 
evaluate the level of protection offered by the current primary 
O3 standard under current conditions due to the long-range 
transport of O3 and O3 precursors to Class I 
areas from upwind non-attainment areas. It is the view of these 
commenters that once the upwind areas make emissions reductions to 
attain the current standard, downwind areas will see improvements in 
air quality and decreasing W126 levels. In support of this view, 
commenters point to several modeling analyses. Some commenters point to 
air quality modeling conducted by an environmental consultant that 
projects all sites to have W126 index values below 13 ppm-hrs when 
emissions are adjusted such that all upwind monitors are modeled to 
meet the current standard. Detailed methodology, results and references 
for the commenter's modeling analysis were not provided, precluding a 
thorough evaluation and comparison to the EPA's modeling. While the EPA 
agrees that transport of O3 and O3 precursors can 
affect downwind monitors, we disagree with commenters regarding the 
conclusions that are appropriate to draw from modeling simulations for 
the reasons noted below.
    As support for their view that the current standard provides 
adequate protection, some commenters pointed to estimates drawn from 
the EPA's air quality modeling performed for the RIA, stating that this 
modeling for an alternative standard level of 70 ppb indicates ``only a 
handful'' of monitoring sites approaching as high as 13 ppm-hrs as a 3-
year average (e.g., UARG, p. 76). These commenters further point to the 
WREA modeling, noting that those estimates project that attainment of 
the current standard would result in only 5 sites above 15 ppm-hrs. 
Based on these statements, these commenters state that the current 
standard is likely to provide conditions with no site having a monitor 
over 17 ppm-hrs and a ``minimal number'' likely exceeding 13 ppm-hrs 
(e.g., UARG, p. 77). We disagree with commenters' interpretation of the 
modeling information from the two different assessments. As we 
summarized in section IV.C.1 of the proposal with regard to the WREA 
modeling, the modeling estimates are each based on a single set of 
precursor emissions reductions that are estimated to achieve the 
desired target conditions, which is also the case for the RIA 
modeling\194\ (U.S. EPA, 2014c, pp. 5-40 to 5-41; see also section 
1.2.2 of the 2014 RIA).
---------------------------------------------------------------------------

    \194\ Although commenters cite to both analyses as if providing 
the same information, there are many differences in specific aspects 
of the RIA approach from that of the WREA, which derive, at least in 
part, from their very different purposes. The RIA is not developed 
for consideration in the NAAQS review. Rather, it is intended to 
provide insights and analysis of an illustrative control strategy 
that states might adopt to meet the revised standard. The EPA does 
not consider this analysis informative to consideration of the 
protection provided by the current standard, and the results of the 
RIA have not been considered in the EPA's decisions on the 
O3 standards.
---------------------------------------------------------------------------

    As noted in section IV.A.2 above, and in the proposal, the model-
adjusted air quality in the WREA scenario for the current standard does 
not represent an optimized control scenario that just meets the current 
standard, but rather characterizes one potential distribution of air 
quality across a region when all monitor locations meet the standard 
(79 FR 75322; U.S. EPA, 2014b, section 4.3.4.2). Alternate precursor 
emissions reductions would be expected to produce different patterns of 
O3 concentrations and associated differences in W126 index 
values. Specifically, the precursor emissions reductions scenarios 
examined in the WREA focuses on regional reductions over broad areas 
rather than localized cuts that may focus more narrowly on areas 
violating the current standard (U.S. EPA, 2014b, p. 4-35). The 
assumption of regionally determined across-the-board emissions 
reductions is a source of potential uncertainty with the potential to 
overestimate W126 scenario benefits (U.S. EPA, 2014b, Table 4-5 [row 
G]). The application of emissions reductions to all locations in each 
region to bring down the highest monitor in the region to meet the 
current standard could potentially lead to W126 index underestimates at 
some locations, as noted in the WREA: ``[w]hile the scenarios 
implemented in this analysis show that [] bringing down the highest 
monitor in a region would lead to reductions below the targeted level 
through the rest of the region, to the extent that the regional 
reductions from on-the-books controls are supplemented with more local 
controls the additional benefit may be overestimated'' (U.S. EPA, 
2014b, p. 4-36; U.S. EPA, 2014c, pp. 5-40 to 5-41). This point was 
emphasized by CASAC in their comments on the 2nd draft WREA. CASAC 
noted that, ``[m]eeting a target level at the highest monitor requires 
substantial reductions below the targeted level through the rest of the 
region'' and stated that ``[t]his artificial simulation does not 
represent an actual control strategy and may conflate differences in 
control strategies required to meet different standards'' (Frey, 2014b, 
p. 2).
    Due to the uncertainty about what actual future emissions control 
strategies might be and their associated emissions reductions, and the 
impact such uncertainty might have on modeling estimates involving 
reductions from recent conditions, we believe it is important to place 
weight on ambient air monitoring data for recent conditions in drawing 
conclusions regarding W126 index values that would be expected in areas 
that meet the current standard. The analysis of air quality data for 
Class I areas described in the proposal, and updated in Table 3 above 
(1998-2013), indicates the occurrence of 3-year W126 exposure index 
values well above 19 ppm-hrs, a cumulative exposure value for which 
CASAC termed the associated median RBL estimate ``unacceptably high,'' 
in multiple Class I areas that meet the current standard (79 FR 75312, 
December 17, 2014, Table 7; updated in Table 3 above). Additionally, 
analysis of recent air quality data (2011-2013) for all locations 
across the U.S. indicates 10 monitor locations distributed across two 
NOAA climatic regions that meet the current standard and at which 3-
year W126 index values are above 19 ppm-hrs, with the highest values 
extending up to 23 ppm-hrs (Wells, 2015b).
    In support of their view that the EPA's modeling supports the 
conclusion that W126 index values of interest are achieved under the 
current secondary standard, some commenters additionally state that the 
W126 values in the WREA are overestimated in unmonitored rural areas 
due to the much greater prevalence of urban monitors across the U.S. 
The EPA

[[Page 65388]]

disagrees with this conclusion. In order to estimate O3 
concentrations in grid cells across a national-scale spatial surface, 
the WREA applied the VNA spatial interpolation technique after applying 
the HDDM technique to adjust O3 concentrations at monitoring 
sites based on the emissions reductions necessary to just meet the 
current standard. In estimating concentrations in unmonitored areas, 
the VNA method considers only the ``neighboring'' monitors, using an 
inverse distance squared weighting formula, which assigns the greatest 
influence to the nearest neighboring monitor (U.S. EPA, 2014b, p. 4A-
6). By this approach, monitors in less-densely monitored areas 
contribute to the concentration estimates over much larger areas than 
do monitors in more-densely monitored areas. In an urban area, 
neighboring monitors may be quite close to one another, such that any 
one monitor may only be influencing concentration estimates for a 
handful of spatial grid cells in the immediate vicinity. By contrast, 
monitors in rural areas may influence hundreds of grid cells. A 
specific example of this is the monitor in Great Basin National Park in 
eastern Nevada. The VNA algorithm assigns very high weights to this 
monitor for all of the grid cells covering a 100 km radius around it, 
simply because there are no other monitors in that area and it is the 
closest. On the other hand, a monitor near downtown Las Vegas may only 
get a high weight for, and thus exert influence on the concentration 
estimate in, the one grid cell containing it. We agree with the 
commenter that urban monitors may influence the spatial surface for 
some distance away from the urban areas, although the influence wanes 
with increasing distance from that area and decreasing distance to the 
next closest monitor. As we lack data for the intervening locations, 
however, we have no reason to conclude that the VNA surface is 
overestimating the W126 index values. Further, as was summarized in 
section IV.A.2 above, and in the WREA, the PA and the proposal (U.S. 
EPA, 2014b, Table 6-27, section 8.5; U.S. EPA, 2014c, p. 5-49; 79 FR 
75323, December 17, 2014), the VNA approach results in a lowering of 
the highest W126 index values at monitoring sites, which contributes to 
underestimates of the highest W126 index values in each region.
    In support of their view that the current standard is adequate, 
some industry commenters additionally cite WREA analyses for the 
current standard scenario, including the W126 index estimates in 
national parks, as showing that the current standard provides more than 
adequate protection, with alternative scenarios providing only marginal 
and increasingly uncertain benefits. As we noted in the proposal and 
section IV.A.2 above, there are an array of uncertainties associated 
with the W126 index estimates, in the current standard scenario and in 
the other scenarios, which, as they are inputs to the vegetation risk 
analyses, are propagated into those analyses (79 FR 75323; December 17, 
2014). As a result, consistent with the approach in the proposal, the 
Administrator has not based her decision with regard to adequacy of the 
current standard in this review on these air quality scenario analyses.
    In support of their view that the current standard provides 
adequate protection and should not be revised, some commenters 
described their concerns with any consideration of visible foliar 
injury in the decision regarding the secondary standard. These 
commenters variously stated that visible foliar injury cannot be 
reliably evaluated for adversity given lack of available information, 
is not an adverse effect on public welfare that must be addressed 
through a secondary standard, and is not directly relatable to growth 
suppression (and the EPA's use of RBL captures that effect anyway). 
Additionally, some state that any associated ecosystem services effects 
are not quantifiable. In sum, the view of these commenters is that it 
is not appropriate for the Administrator to place any weight on this 
O3 effect in determining the adequacy of the current 
standard. As an initial matter, the EPA agrees with the comment that 
the current evidence does not include an approach for relating visible 
foliar injury to growth suppression,\195\ as recognized in section 
IV.A.1.b above. Further, we note that, similar to decisions in past 
O3 reviews, the Administrator's proposed decision in this 
review recognized the ``complexities and limitations in the evidence 
base regarding characterizing air quality conditions with respect to 
the magnitude and extent of risk for visible foliar injury'' and the 
``challenges of associated judgments with regard to adversity of such 
effects to public welfare'' (79 FR 75336; December 17, 2014). Contrary 
to the implications of the commenters, although the Administrator took 
into consideration the potential for adverse effects on public welfare 
from visible foliar injury, she placed weight primarily on growth-
related effects of O3, both in her proposed decision on 
adequacy and with regard to proposed judgments on what revisions would 
be appropriate. Although visible foliar injury may impact the public 
welfare and accordingly has the potential to be adverse to the public 
welfare (as noted in section IV.B.2 of the proposal), the Administrator 
placed less weight on visible foliar injury considerations in 
identifying what revisions to the standard would be appropriate to 
propose. In considering these effects for this purpose, she recognized 
``significant challenges'' in light of ``the variability and the lack 
of clear quantitative relationship with other effects on vegetation, as 
well as the lack of established criteria or objectives that might 
inform consideration of potential public welfare impacts related to 
this vegetation effect'' (79 FR 75349; December 17, 2014). As 
summarized in section IV.A.1.a above, the evidence demonstrates a 
causal relationship of O3 with visible foliar injury. 
Accordingly, we note that the uncertainty associated with visible 
foliar injury is not with regard to whether O3 causes 
visible foliar injury. Rather, the uncertainty is, as discussed in 
sections IV.A.1.b and IV.A.3 above, with the lack of established, 
quantitative exposure-response functions that document visible foliar 
injury severity and incidence under varying air quality and 
environmental conditions and information to support associated 
judgments on the significance of such responses with regard to 
associated public welfare impacts. As with the Administrator's proposed 
decisions on the standard, such considerations also informed her final 
decisions, described in sections IV.B.3 and IV.C.3 below.
---------------------------------------------------------------------------

    \195\ The current evidence indicates that``[t]he significance of 
O3 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'' and ``in some 
cases, visible foliar symptoms have been correlated with decreased 
vegetative growth . . . and with impaired reproductive function'' 
(U.S. EPA, 2013, p. 9-39). The ISA concludes, however, ``it is not 
presently possible to determine, with consistency across species and 
environments, what degree of injury at the leaf level has 
significance to the vigor of the whole plant'' (U.S. EPA, 2013, p. 
9-39).
---------------------------------------------------------------------------

    In support of their view that the current standard should be 
retained, some commenters note the WREA finding for the current 
standard scenario of no U.S. counties with RYL estimates at or above 
5%, the RYL value emphasized by CASAC and state that policy reasons 
provide support for not focusing on crops in the decision; other 
commenters state that additional studies on crops and air quality are 
needed. As

[[Page 65389]]

described previously in this section, and in section IV.A.2 above, an 
aspect of uncertainties associated with the WREA air quality scenarios, 
including the current standard scenario, is underestimation of the 
highest W126 index values, contributing to underestimates in the 
effects associated with the current standard scenario. The EPA agrees 
with commenters that additional studies on crops and air quality will 
be useful to future reviews. Additionally, however, as noted above, the 
Administrator's proposed conclusion on adequacy of the current 
standard, as well as her final decision described in section IV.B.3 
below, gives less weight to consideration of effects on agricultural 
crops in recognition of the complicating role of heavy management in 
that area.
    Lastly, we note that many commenters cited the costs of compliance 
as supporting their view that the standard should not be revised, 
although as we have described in section I.B above, the EPA may not 
consider the costs of compliance in determining what standard is 
requisite to protect public welfare from known or anticipated adverse 
effects.
3. Administrator's Conclusions on the Need for Revision
    Having carefully considered the advice from CASAC and public 
comments, as discussed above, the Administrator believes that the 
fundamental scientific conclusions on the welfare effects of 
O3 in ambient air reached in the ISA and summarized in the 
PA and in section IV.B of the proposal remain valid. Additionally, the 
Administrator believes the judgments she reached in the proposal 
(section IV.D.3) with regard to consideration of the evidence and 
quantitative assessments and advice from CASAC remain appropriate. 
Thus, as described below, the Administrator concludes that the current 
secondary standard is not requisite to protect public welfare from 
known and anticipated adverse effects associated with the presence of 
O3 in the ambient air and that revision is needed to provide 
additional protection.
    In considering the adequacy of the current secondary O3 
standard, the Administrator has carefully considered the available 
evidence, analyses and conclusions contained in the ISA, including 
information newly available in this review; the information, 
quantitative assessments, considerations and conclusions presented in 
the PA; the advice and recommendations from CASAC; and public comments. 
The Administrator gives primary consideration to the evidence of growth 
effects in well-studied tree species and information, presented in the 
PA and represented with a narrower focus in section IV.B.2 above, on 
cumulative exposures occurring in Class I areas when the current 
standard is met. This information indicates the occurrence of exposures 
associated with Class I areas during periods when the current standard 
is met for which associated estimates of growth effects, in terms of 
the tree seedling RBL in the median species for which E-R functions 
have been established, extend above a magnitude considered to be 
``unacceptably high'' by CASAC. This analysis estimated such cumulative 
exposures occurring under the current standard for nearly a dozen 
areas, distributed across two NOAA climatic regions of the U.S. The 
Administrator gives particular weight to this analysis, given its focus 
in Class I areas. Such an emphasis on lands afforded special government 
protections, such as national parks and forests, wildlife refuges, and 
wilderness areas, some of which are designated Class I areas under the 
CAA, is consistent with such emphasis in the 2008 revision of the 
secondary standard (73 FR 16485, March 27, 2008). As noted in section 
IV.A above, Congress has set such lands aside for specific uses that 
are 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. The Administrator 
additionally recognizes 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.
    As noted in prior reviews, judgments regarding effects that are 
adverse to public welfare consider the intended use of the ecological 
receptors, resources and ecosystems affected. Thus, the Administrator 
recognizes that the median RBL estimate for the studied species is a 
quantitative tool within a larger framework of considerations 
pertaining to the public welfare significance of O3 effects 
on the public welfare. Such considerations include effects that are 
associated with effects on growth and that the ISA has determined to be 
causally or likely causally related to O3 in ambient air, 
yet for which there are greater uncertainties affecting our estimates 
of impacts on public welfare. These other effects include reduced 
productivity in terrestrial ecosystems, reduced carbon sequestration in 
terrestrial ecosystems, alteration of terrestrial community 
composition, alteration of below-grown biogeochemical cycles, and 
alteration of terrestrial ecosystem water cycles, as summarized in 
section IV.A.1. Thus, in her attention to CASAC's characterization of a 
6% estimate for tree seedling RBL in the median studied species as 
``unacceptably high'', the Administrator, while mindful of 
uncertainties with regard to the magnitude of growth impact that might 
be expected in mature trees, is also mindful of related, broader, 
ecosystem-level effects for which our tools for quantitative estimates 
are more uncertain and those for which the policy foundation for 
consideration of public welfare impacts is less well established. She 
finds her consideration of tree growth effects consistent with CASAC 
advice regarding consideration of O3-related biomass loss as 
a surrogate for the broader array of O3 effects at the plant 
and ecosystem levels.
    The Administrator also recognizes that O3-related 
effects on sensitive vegetation can occur in other areas that have not 
been afforded special federal protections, including effects on 
vegetation growing in managed city parks and residential or commercial 
settings, such as ornamentals used in urban/suburban landscaping or 
vegetation grown in land use categories that are heavily managed for 
commercial production of commodities such as timber. In her 
consideration of the evidence and quantitative information of 
O3 effects on crops, the Administrator recognizes the 
complexity of considering adverse O3 impacts to public 
welfare due to the heavy management common for achieving optimum yields 
and market factors that influence associated services. In so doing, she 
notes that her judgments that place emphasis on the protection of 
forested ecosystems inherently also recognize a level of protection for 
crops. Additionally, for vegetation used for residential or commercial 
ornamental purposes, the Administrator believes that there is not 
adequate information specific to vegetation used for those purposes, 
but notes that a secondary standard revised to provide protection for 
sensitive natural vegetation and ecosystems would likely also provide 
some degree of protection for such vegetation.
    The Administrator also takes note of the long-established evidence 
of consistent association of the presence of visible foliar injury with 
O3 exposure and the currently available information that 
indicates the occurrence of visible foliar injury in sensitive species 
of

[[Page 65390]]

vegetation during recent air quality in public forests across the U.S. 
She additionally notes the PA conclusions regarding difficulties in 
quantitatively relating visible foliar injury symptoms to vegetation 
effects such as growth or related ecosystem effects. As at the time of 
the last review, the Administrator believes that the degree to which 
such effects should be considered to be adverse depends on the intended 
use of the vegetation and its significance. The Administrator also 
believes that the significance of O3-induced visible foliar 
injury depends on the extent and severity of the injury and takes note 
of studies in the evidence base documenting increased severity and/or 
prevalence with higher O3 exposures. However, the 
Administrator takes note of limitations in the available information 
with regard to judging the extent to which the extent and severity of 
visible foliar injury occurrence associated with conditions allowed by 
the current standard may be considered adverse to public welfare.
    Based on these considerations, and taking into consideration the 
advice and recommendations of CASAC, the Administrator concludes that 
the protection afforded by the current secondary O3 standard 
is not sufficient and that the standard needs 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. The 
Administrator additionally recognizes 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. Given 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 areas 
contain O3-sensitive vegetation, the Administrator further 
concludes that it is appropriate to revise the secondary standard in 
part to provide increased protection against O3-caused 
impairment to vegetation and ecosystems in such areas, which have been 
specially protected to provide public welfare benefits. She further 
notes that a revised standard would provide increased protection for 
other growth-related effects, including for crop yield loss, reduced 
carbon storage and for areas for which it is more difficult to 
determine public welfare significance, as recognized in section IV.A.3 
above, as well other welfare effects of O3, such as visible 
foliar injury.

C. Conclusions on Revision of the Secondary Standard

    The elements of the standard--indicator, averaging time, form, and 
level--serve to define the standard and are considered collectively in 
evaluating the welfare protection afforded by the secondary standard. 
Section IV.C.1 below summarizes the basis for the proposed revision. 
Significant comments received from the public on the proposal are 
discussed in section IV.C.2 and the Administrator's final decision on 
revisions to the secondary standard is described in section IV.C.3.
1. Basis for Proposed Revision
    At the time of proposal, in considering what revisions to the 
secondary standard would be appropriate, the Administrator considered 
the ISA conclusions regarding the weight of the evidence for a range of 
welfare effects associated with O3 in ambient air and 
associated areas of uncertainty; quantitative risk and exposure 
analyses in the WREA for different adjusted air quality scenarios and 
associated limitations and uncertainties; staff evaluations of the 
evidence, exposure/risk information and air quality information in the 
PA; additional air quality analyses of relationships between air 
quality metrics based on form and averaging time of the current 
standards and a cumulative seasonal exposure index; CASAC advice; and 
public comments received as of that date in the review. In the 
paragraphs below, we summarize the proposal presentation with regard to 
key aspects of the PA considerations, advice from the CASAC, air 
quality analyses of different air quality metrics and the 
Administrator's proposed conclusions, drawing from section IV.E of the 
proposal.
a. Considerations and Conclusions in the PA
    As summarized in the proposal, in identifying alternative secondary 
standards appropriate to consider in this review, the PA focused on 
standards based on a cumulative, seasonal, concentration-weighted form 
consistent with the CASAC advice in the current and last review. Based 
on conclusions of the ISA, as also summarized in section IV.A above, 
the PA considered a cumulative, seasonal, concentration-weighted 
exposure index to provide the most scientifically defensible approach 
for characterizing vegetation response to ambient O3 and 
comparing study findings, as well as for defining indices for 
vegetation protection, as summarized in the proposal section IV.E.2.a. 
With regard to the appropriate index, the PA considered the evidence 
for a number of different such indices, as described in the proposal, 
and noted the ISA conclusion that the W126 index has some important 
advantages over other similarly weighted indices. The PA additionally 
considered the appropriate diurnal and seasonal exposure periods in a 
given year by which to define the seasonal W126 index and based on the 
evidence in the ISA and CASAC advice, as summarized in the proposal, 
decided on the 12-hour daylight window (8:00 a.m. to 8:00 p.m.) and the 
3-consecutive-month period providing the maximum W126 index value.
    Based on these considerations, the PA concluded it to be 
appropriate to retain the current indicator of O3 and to 
consider a secondary standard form that is an average of the seasonal 
W126 index values (derived as described in section IV.A.1.c above) 
across three consecutive years (U.S. EPA, 2014c, section 6.6). In so 
doing, the PA recognized that there is limited information to discern 
differences in the level of protection afforded for cumulative growth-
related effects by potential alternative W126-based standards of a 
single-year form as compared to a 3-year form (U.S. EPA, 2014c, pp. 6-
30). The PA concluded a 3-year form to be appropriate for a standard 
intended to provide the desired level of protection from longer-term 
effects, including those associated with potential compounding, and 
that such a form might be concluded to contribute to greater stability 
in air quality management programs, and thus, greater effectiveness in 
achieving the desired level of public welfare protection than might 
result from a single-year form. (U.S. EPA, 2014c, section 6.6).
    As summarized in the proposal, the PA noted that, due to the 
variability in the importance of the associated ecosystem services 
provided by different species at different exposures and in different 
locations, as well as differences in associated uncertainties and 
limitations, it is essential to consider the species present and their 
public welfare significance, together with the magnitude of the ambient 
concentrations in drawing conclusions regarding the significance or 
magnitude of public welfare impacts. Therefore, in development of the 
PA conclusions, staff took note of the complexity of judgments to be 
made by the Administrator regarding the adversity of known and 
anticipated effects to the

[[Page 65391]]

public welfare and recognized that the Administrator's ultimate 
judgments on the secondary standard will most appropriately reflect an 
interpretation of the available scientific evidence and exposure/risk 
information that neither overstates nor understates the strengths and 
limitations of that evidence and information. In considering an 
appropriate range of levels to consider for an alternative standard, 
the PA primarily considered tree growth, crop yield loss, and visible 
foliar injury, as well as impacts on the associated ecosystem services, 
while noting key uncertainties and limitations.
    In specifically evaluating exposure levels, in terms of the W126 
index, as to their appropriateness for consideration in this review 
with regard to providing the desired level of vegetation protection for 
a revised secondary standard, the PA focused particularly on RBL 
estimates for the median across the 11 tree species for which robust E-
R functions are available. Table 4 below presents these estimates (U.S. 
EPA, 2014c, Appendix 5C, Table 5C-3; also summarized in Table 8 of the 
proposal). In so doing and recognizing the longstanding, strong 
evidence base supporting these relationships, the PA also noted 
uncertainties regarding inter-study variability for some species, as 
well as with regard to the extent to which tree seedling E-R functions 
can be used to represent mature trees. As summarized in the proposal, 
the PA conclusions on a range of W126 levels appropriate to consider 
are based on specific advice from CASAC with regard to median tree 
seedling RBL estimates that might be considered unacceptably high (6%), 
as well as its judgment on a RBL benchmark (2%) for identification of 
the lower end of a W126 index value range for consideration that might 
give more emphasis to the more sensitive tree seedlings (Frey, 2014c, 
p. 14).\196\
---------------------------------------------------------------------------

    \196\ The CASAC provided several comments related to 2% RBL for 
tree seedlings both with regard to its use in summarizing WREA 
results and with regard to consideration of the potential 
significance of vegetation effects, as summarized in sections IV.D.2 
and IV.E.3 of the proposal.

         Table 4--Tree Seedling Biomass Loss and Crop Yield Loss Estimated for O3 Exposure Over a Season
----------------------------------------------------------------------------------------------------------------
                                       Tree seedling biomass loss A                  Crop yield loss B
  W126 index value for exposure  -------------------------------------------------------------------------------
             period                  Median value     Individual species     Median value     Individual species
----------------------------------------------------------------------------------------------------------------
23 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 4/10
                                   7.6% loss.          species.            8.8% loss.          species
                                                      <= 5% loss: 4/11                        >5,<10% loss: 1/10
                                                       species.                                species
                                                      <=10% loss: 8/11                        >10,<20% loss: 4/
                                                       species.                                10 species
                                                      <=15% loss: 10/11                       >20: 1/10 species
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
22 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 4/10
                                   7.2% loss.          species.            8.2% loss.          species
                                                      <= 5% loss: 4/11                        >5,<10% loss: 1/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 4/
                                                       species.                                10 species
                                                      <=15% loss: 10/11                       >20: 1/10 species
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
21 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 4/10
                                   6.8% loss.          species.            7.7% loss.          species
                                                      <= 5% loss: 4/11                        >5,<10% loss: 3/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 3/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
20 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 5/10
                                   6.4% loss.          species.            7.1% loss.          species
                                                      <= 5% loss: 5/11                        >5,<10% loss: 3/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
19 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 5/10
                                   6.0% loss.          species.            6.4% loss.          species
                                                      <=5% loss: 5/11                         >5, <10% loss: 3/
                                                       species.                                10 species
                                                      <=10% loss: 7/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
18 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 5/10
                                   5.7% loss.          species.            5.7% loss.          species
                                                      <= 5% loss: 5/11                        >5,<10% loss: 3/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
17 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 5/10
                                   5.3% loss.          species.            5.1% loss.          species
                                                      <=5% loss: 5/11                         >5, <10% loss: 3/
                                                       species.                                10 species
                                                      <=10% loss: 9/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
16 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 5/10
                                   4.9% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 6/11                        >5,<10% loss: 4/10
                                                       species.                                species
                                                      <=10% loss: 10/11                       >10,<20% loss: 1/
                                                       species.                                10 species
                                                      >30% loss: 1/11
                                                       species.
15 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 6/10
                                   4.5% loss.          species.            <=5.0% loss.        species
                                                      <=5% loss: 6/11                         >5, <10% loss: 4/
                                                       species.                                10 species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
14 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 6/10
                                   4.2% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 6/11                        >5,<10% loss: 4/10
                                                       species.                                species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
13 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 6/10
                                   3.8% loss.          species.            <=5.0% loss.        species
                                                      <5% loss: 7/11                          >5, <10% loss: 4/
                                                       species.                                10 species
                                                      <10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.

[[Page 65392]]

 
12 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 8/10
                                   3.5% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 8/11                        >5,<10% loss: 2/10
                                                       species.                                species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
11 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 9/10
                                   3.1% loss.          species.            <=5.0% loss.        species
                                                      <=5% loss: 8/11                         >5, <10% loss: 1/
                                                       species.                                10 species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
10 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 9/10
                                   2.8% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 9/11                        >5,<10% loss: 1/10
                                                       species.                                species
                                                      <10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
9 ppm-hrs.......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: all
                                   2.4% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
8 ppm-hrs.......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: all
                                   2.0% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 10/11
                                                       species.
                                                      >15% loss: 1/11
                                                       species.
7 ppm-hrs.......................  Median species w.   <= 2% loss: 7/11    Median species w.   <= 5% loss: all
                                   <2.0% loss.         species.            <=5.0% loss.        species
                                                      <=5% loss: 10/11
                                                       species.
                                                      >15% loss: 1/11
                                                       species.
----------------------------------------------------------------------------------------------------------------
\A\ Estimates here are based on the E-R functions for 11 species described in the WREA, section 6.2 and
  discussed in the PA, section 5.2.1. The cottonwood was excluded to address CASAC comments (Frey, 2014c; U.S.
  EPA, 2014b, U.S. EPA, 2014c, Appendix 6F). The median is the median of the 11 composite E-R functions (U.S.
  EPA, 2014c, Appendix 5C).
\B\ Estimates here are based on the 10 E-R functions for crops described in the WREA, section 6.2 and discussed
  in the PA, section 5.3.1. The median is the median of the 10 composite E-R functions (U.S. EPA, 2014b; U.S.
  EPA, 2014c, Appendix 5C).

    With regard to secondary standard revisions appropriate to consider 
in this review, as summarized in the proposal, the PA concluded it to 
be appropriate to consider a W126-based secondary standard with index 
values within the range of 7 to 17 ppm-hrs and a form averaged over 3 
years (U.S. EPA, 2014c, section 6.7). The PA additionally recognized 
the role of policy judgments required of the Administrator with regard 
to the public welfare significance of identified effects, the 
appropriate weight to assign the range of uncertainties inherent in the 
evidence and analyses, and ultimately, in identifying the requisite 
protection for the secondary O3 standard.
    The PA additionally recognized that to the extent the Administrator 
finds it useful to consider the public welfare protection that might be 
afforded by revising the level of the current standard, this is 
appropriately judged by evaluating the impact of associated 
O3 exposures in terms of the cumulative seasonal W126-based 
index, an exposure metric considered appropriate for evaluating impacts 
on vegetation (U.S. EPA, 2014c, section 6.7). Accordingly, the PA 
included several air quality data analyses that might inform such 
consideration (U.S. EPA, 2014c, section 6.4). Additional air quality 
analyses were performed subsequent to the PA, described in the proposal 
and are summarized below.
b. CASAC Advice
    Advice received from the CASAC during the current review, similar 
to that in the last review, recommended retaining O3 as the 
indicator, while also recommending consideration of a secondary 
standard with a revised form and averaging time based on the W126 index 
(Frey, 2014c, p. iii). The CASAC concurred with the 12-hour period (8 
a.m. to 8 p.m.) and 3-month summation period resulting in the maximum 
W126 index value, as described in the PA, while recommending a somewhat 
narrower range of levels from 7 ppm-hrs to 15 ppm-hrs. While the CASAC 
recommended a W126 index limited to a single year, in contrast with the 
PA's conclusion that it was appropriate to consider the W126 index 
averaged across three years, it also noted that the Administrator may 
prefer, as a policy matter, to base the secondary standard on a 3-year 
averaging period. In such a case, the CASAC recommended revising 
downward the level for such a metric to avoid a seasonal W126 index 
value above a level in their recommended range in any given year of the 
3-year period, indicating an upper end of 13 ppm-hrs as an example for 
such a 3-year average W126 index range (Frey, 2014c, p. iii and iv).
c. Air Quality Analyses
    The proposal additionally summarized several analyses of air 
quality that considered relationships between metrics based on a 3-year 
W126 index and based on the form and averaging time of the current 
standard, the ``fourth-high'' metric (U.S. EPA, 2014c, Chapter 2, 
Appendix 2B and section 6.4; Wells, 2014a), as well as describing the 
uncertainties and limitations associated with these analyses. The 
proposal concluded that these analyses suggest that, depending on the 
level, a standard of the current averaging time and form can be 
expected to control cumulative seasonal O3 exposures to such 
that they may meet specific 3-year average W126 index values. The 
fourth-high and W126 metrics, and changes in the two metrics over the 
past decade, were found to be highly correlated (U.S. EPA, 2014c, 
section 6.4 and Appendix 2B; Wells, 2014a). From these analyses, it was 
concluded that future control programs designed to help meet a standard 
based on the fourth-high metric are also expected to result in 
reductions in values of the W126 metric (Wells, 2014a). Further, the 
second analysis also found that the Southwest and West NOAA climatic 
regions, which showed the greatest potential for sites to measure 
elevated cumulative, seasonal O3 exposures without the 
occurrence of elevated daily maximum 8-hour average O3 
concentrations, exhibited the greatest reduction in W126 metric value 
per unit reduction in fourth-high metric (Wells, 2014a, Figures 5b and 
12 and Table 6).

[[Page 65393]]

    Analyses of the most recent periods studied in the two analyses 
(2009-2011 and 2011-2013) had similar findings regarding the highest 
W126 metric values occurring at monitoring sites that meet alternative 
levels of the fourth-high metric (U.S. EPA, 2014c, section 6.4; Wells, 
2014a). In both analyses, the highest W126 metric values were in the 
Southwest and West NOAA climatic regions. In both analyses, no 
monitoring sites for which the fourth-high metric was at or below 70 
ppb had a W126 metric value above 17 ppm-hrs (U.S. EPA, 2014c, Figure 
2B-3b; Wells, 2014a, Table 4). All U.S. regions were represented in 
these subsets. In the 2011-2013 subset of sites for which the fourth-
high metric was at or below a potential alternative primary standard 
level of 65 ppb, no monitoring sites had W126 metric values above 11 
ppm-hrs (Wells, 2014a, Table 4).
d. Administrator's Proposed Conclusions
    At the time of proposal, the Administrator concluded it to be 
appropriate to continue to use O3 as the indicator for a 
secondary standard that is intended to address effects associated with 
exposure to O3 alone and in combination with related 
photochemical oxidants. While the complex atmospheric chemistry in 
which O3 plays a key role has been highlighted in this 
review, no alternatives to O3 have been advanced as being a 
more appropriate surrogate for ambient photochemical oxidants and their 
effects on vegetation. The CASAC agreed that O3 should be 
retained as the indicator for the standard (Frey, 2014c, p. iii). In 
proposing to retain O3 as the indicator, the Administrator 
recognized that measures leading to reductions in ecosystem exposures 
to O3 would also be expected to reduce exposures to other 
photochemical oxidants.
    The Administrator proposed to retain the current averaging time and 
form and to revise the level of the current secondary standard to a 
level within the range of 0.065 to 0.070 ppm. She based this proposal 
on her provisional conclusions regarding the level of cumulative 
seasonal O3 exposures that would provide the requisite 
protection against known or anticipated adverse effects to the public 
welfare and on a policy option that would provide this level of 
protection. With regard to the former, the Administrator concluded that 
in judging the extent of public welfare protection that might be 
afforded by a revised standard and whether it meets the appropriate 
level of protection, it is appropriate to use a cumulative, seasonal 
concentration-weighted exposure metric. For this purpose, the 
Administrator concluded it to be appropriate to use the W126 index 
value, averaged across three years, with each year's value identified 
as that for the 3-month period yielding the highest seasonal value and 
with daily O3 exposures within a 3-month period cumulated 
for the 12-hour period from 8:00 a.m. to 8:00 p.m.
    To identify the range of cumulative seasonal exposures, in terms of 
the W126 index, expected to be associated with the appropriate degree 
of public welfare protection, the Administrator gave primary 
consideration to growth-related impacts, using tree seedling RBL 
estimates for a range of W126 exposure index values and CASAC advice 
regarding such estimates. Additionally taking into account judgments on 
important uncertainties and limitations inherent in the current 
available scientific evidence and quantitative assessments, and 
judgments regarding the extent to which different RBL estimates might 
be considered indicative of effects adverse to public welfare, the 
Administrator proposed that ambient O3 concentrations 
resulting in cumulative seasonal O3 exposures of a level 
within the range from 13 ppm-hrs to 17 ppm-hrs, in terms of a W126 
index averaged across three consecutive years, would provide the 
requisite protection against known or anticipated adverse effects to 
the public welfare. In identifying policy options for a revised 
secondary standard that would control exposures to such an extent, the 
Administrator considered the results of air quality analyses that 
examined the responsiveness of cumulative exposures (in terms of the 
W126 index) to O3 reductions in response to the current and 
prior standard for which the form and averaging time are summarized as 
a fourth-high metric, and also examined the extent to which cumulative 
exposures (in terms of the W126 index) may be limited by alternative 
levels of a metric based on the current standard averaging time and 
form. Based on the results of these analyses, she proposed that 
revision of the level of the current secondary standard to within the 
range of 0.065 to 0.070 ppm would be expected to provide the requisite 
public welfare protection, depending on final judgments concerning such 
requisite protection.
2. Comments on Proposed Revision
    Significant comments from the public regarding revisions to the 
secondary standard are addressed in the subsections below. We first 
discuss comments related to our consideration of growth-related effects 
and visible foliar injury in identifying appropriate revisions to the 
standard (sections IV.C.2.a and IV.C.2.b). Next, we address comments 
related to the use of the W126 metric in evaluating vegetation effects 
and public welfare protection and comments related to the form and 
averaging time for the revised standard (sections IV.C.2.c and 
IV.C.2.d). Comments on revisions to the level of the standard are 
described in section IV.C.2.e, and those related to the way in which 
today's rulemaking addresses the 2013 court remand are addressed in 
section IV.C.2.f. Other significant comments related to consideration 
of a revised secondary standard, and that are based on relevant 
factors, are addressed in the Response to Comments document.
a. Consideration of Growth-Related Effects
    In considering public comments received on the consideration of 
growth-related effects of O3 in the context of the proposed 
decision on a revised secondary standard, we first note related advice 
and comments from the CASAC provided during development of the PA, 
stating, as summarized in section IV.B.1.b above, that ``relative 
biomass loss for tree species, crop yield loss, and visible foliar 
injury are appropriate surrogates for a wide range of damage that is 
adverse to public welfare'' (Frey, 2014c, p. 10). Additionally, in the 
context of different standard levels they considered appropriate for 
the EPA to consider, CASAC stated that it is appropriate to ``include[] 
levels that aim for not greater than 2% RBL for the median tree 
species'' and that a median tree species RBL of 6% is ``unacceptably 
high'' (Frey, 2014c, p. 14).\197\ With respect to crop yield loss, 
CASAC points to a benchmark of 5%, stating that a crop RYL for median 
species over 5% is ``unacceptably high'' (Frey, 2014c, p. 13).
---------------------------------------------------------------------------

    \197\ The CASAC made this comment while focusing on Table 6-1 in 
the second draft PA and the entry for 17 ppm-hrs (Frey, 2014c, p. 
14). That table was revised for inclusion in the final PA in 
consideration of CASAC comments on the E-R function for eastern 
cottonwood, and after that revision, the median RBL estimate for 17 
ppm-hrs in the final table (see Table 4 above) is below the value of 
6% that CASAC described in this way.
---------------------------------------------------------------------------

    In addition, regarding consideration of RBL benchmarks for tree 
seedlings, the CASAC stated that ``[a] 2% biomass loss is an 
appropriate scientifically based value to consider as a benchmark of 
adverse impact for long-lived perennial species such as trees, because 
effects are cumulative over multiple

[[Page 65394]]

years'' (Frey, 2014c, p. 14).\198\ With regard to this benchmark, the 
CASAC also commented that ``it is appropriate to identify a range of 
levels of alternative W126-based standards that includes levels that 
aim for not greater than 2% RBL for the median tree species'' in the PA 
(Frey, 2014c, p. 14). The CASAC noted that the ``level of 7 ppm-hrs is 
the only level analyzed for which the relative biomass loss for the 
median tree species is less than or equal to 2 percent,'' indicating 
that 7 ppm was appropriate as a lower bound for the recommended range 
(Frey, 2014c, p. 14).\199\
---------------------------------------------------------------------------

    \198\ The CASAC provided several comments related to 2% RBL for 
tree seedlings both with regard to its use in summarizing WREA 
results and with regard to consideration of the potential 
significance of vegetation effects, as summarized in sections IV.D.2 
and IV.E.3 of the proposal.
    \199\ The CASAC made this comment while focusing on Table 6-1 in 
the second draft PA, which included odd-numbered W126 index values 
and in which the median RBL values were based on 12 species. That 
table was revised for inclusion in the final PA in consideration of 
CASAC comments on the E-R function for eastern cottonwood, such that 
the median RBL species estimate for both 7 ppm-hrs and 8 ppm-hrs are 
less than or equal to 2.0% in the final table (see Table 4 above and 
Table 5C-3 of the final PA).
---------------------------------------------------------------------------

    With regard to consideration of effects on crops, in addition to 
their comments regarding a median species RYL over 5% yield loss, noted 
above (Frey, 2014c, p. 13), the CASAC further noted that ``[c]rop loss 
appears to be less sensitive than these other indicators, largely 
because of the CASAC judgment that a 5% yield loss represents an 
adverse impact, and in part due to more opportunities to alter 
management of annual crops'' (Frey, 2014c, p. 14).
    Comments from the public with regard to how the EPA considered 
growth-related effects in the proposed decision on a revised secondary 
standard varied. Generally, those commenters who recommended against 
revision of the standard expressed the view that RBL estimates based on 
the established E-R functions for the 11 studied species, and their 
pertinence to mature trees, were too uncertain to serve as a basis for 
judgments regarding public welfare protection afforded by the secondary 
standard. The EPA generally disagrees with this view, as discussed in 
section IV.B.2 above, and addressed in more detail in the Response to 
Comments document.
    Some commenters also took note of the unclear basis for CASAC's 2% 
benchmark, stating that the CASAC advice on this point is ``not wholly 
scientific,'' given that it referenced the 1996 workshop, which 
provided little specificity as to scientific basis for such a 
benchmark; based on this, the commenters described this CASAC advice as 
a policy judgment and described the important role of the EPA's 
judgment in such instances. As noted in section IV.E.3 of the proposal, 
we generally agree with these commenters regarding the unclear 
scientific basis for the 2% value. Consistent with this advice from 
CASAC, however, the range of levels for a revised secondary standard 
that the PA concluded was appropriate for the Administrator to consider 
did include a level for which the estimated median RBL across the 11 
studied tree species would be 2%, as well as a level for which the 
median RBL would be below 2% (U.S. EPA, 2014c, section 6.7 and Tables 
6-1 and 5C-3), and, as described in the proposal, the Administrator 
considered the conclusions of the PA in reaching her proposed decision 
that it was appropriate to consider a range for the revised secondary 
standard that did not focus on this benchmark. The Administrator has 
further considered and explained any differences from CASAC's 
recommendations on this point in her final decision, as described in 
section IV.C.3 below.
    Some of the state and local environmental agencies and 
organizations and environmental groups that supported the EPA's 
proposed decision to revise the secondary standard additionally 
indicated their view that the EPA should give more weight to growth-
related effects by setting the standard at a level for which the 
estimated RBL would be at or below 2% in the median studied species. In 
support of this recommendation, the commenters cited the CASAC advice 
and stated that the EPA's rationale deviates from that advice with 
regard to consideration of RBL. In so doing, the commenters implied 
incorrectly that the EPA's proposal did not put the most weight on the 
median RBL. In fact, in considering RBL as a metric for growth effects, 
the Administrator's proposed conclusions focused solely on the median 
RBL estimates, indicating that appreciable weight was given to growth-
related effects and on the median RBL. Additionally, the commenters 
implied that the EPA misconstrued the CASAC comment on 6% RBL to 
indicate that it was acceptable. Yet, the proposal notes CASAC's view 
that a 6% RBL is ``unacceptably high'' nine times, and, in section 
IV.B.3 above, the Administrator takes note of this view in reaching the 
decision that the current standard should be revised. The EPA considers 
this statement from CASAC, provided in the context of considering 
effects related to different W126 index values, to be of a different 
nature than CASAC advice discussed above that options for the EPA 
consideration ``include'' a level that aims for median RBL at or below 
2%.
    The comments that state that the standard should control cumulative 
exposures to levels for which the estimated median species RBL is at or 
below 2% provided little rationale beyond citing to CASAC advice. We 
note, however, that the CASAC did not specify that the revised 
secondary standard be set to limit cumulative exposures to that extent. 
Nor, in identifying a range of alternatives for the EPA to consider, 
did CASAC recommend that the EPA consider only W126 index levels 
associated with median RBL estimates at or below 2%. Rather, the CASAC 
stated that ``it is appropriate to identify a range of levels of 
alternative W126-based standards that includes {emphasis added{time}  
levels that aim for not greater than 2% RBL for the median tree 
species'' (Frey, 2014c, p. 14) and seven of the nine levels in the 
CASAC-recommended range of W126 index levels were associated with 
higher RBL estimates (as shown in Table 4 above).
    In citing to CASAC advice, commenters quoted the CASAC 
characterization of a 2% RBL as ``an appropriate scientifically based 
value to consider as a benchmark of adverse impact for long-lived 
perennial species such as trees, because effects are cumulative over 
multiple years'' (Frey, 2014, p. 14). Presumably to indicate reasoning 
for this statement, the subsequent sentence in the same CASAC letter 
referenced findings for biomass loss in aspen exposed to elevated 
O3 over seven years, citing Wittig et al., 2009. As noted in 
the proposal, however, the way in which these findings would provide a 
basis for CASAC's view with regard to 2% is unclear, as the original 
publication that is the source for the 7-year biomass loss value (King, 
et al., 2005) and which is cited in Wittig et al. (2009) indicates 
yearly RBL values during this 7-year exposure that are each well above 
2%, and, in fact, are all above 20% (King, et al., 2005). In the same 
paragraph, the CASAC letter additionally referenced the report of the 
1996 workshop sponsored by the Southern Oxidants Study group (Heck and 
Cowling, 1997, noted in section IV.A.3 above). The workshop report 
identified 1-2% per year growth reduction (based on a stated interest 
in avoiding 2% cumulative effects) as an appropriate endpoint for 
consideration of growth effects in trees, although an explicit 
rationale for the identified percentages is not provided

[[Page 65395]]

(Frey, 2014c, p. 14).\200\ Like the 1996 workshop, the CASAC describes 
2% RBL as providing the basis for consideration of 7 ppm-hrs, the lower 
end of their recommended W126 range (Frey, 2014c, p. 14). As a result, 
the specific scientific basis for judging a value of 2% RBL in the 
median studied species as an appropriate benchmark of adverse impact 
for trees and other long-lived perennials is not clear, which, as 
described in the proposal, contributed to the Administrator noting the 
greater uncertainty regarding the extent to which estimates of benefits 
in terms of ecosystem services and reduced effects on vegetation at 
O3 exposures below her identified range of 13 to 17 ppm-hrs 
might be judged significant to the public welfare.
---------------------------------------------------------------------------

    \200\ The report of the 1996 workshop provides no more explicit 
rationale for the percentages identified or specification with 
regard to number or proportion of species for which such percentages 
should be met (Heck and Cowling, 1997).
---------------------------------------------------------------------------

    Some commenters recommended revision of the standard to 7 ppm-hrs 
as a W126 form stating that such a change is needed to protect against 
climate change. In so doing, one commenter expressed the view that the 
relatively lesser weight the EPA placed on the WREA estimates of carbon 
storage (in terms of CO2) in consideration of a proposed 
revision to the secondary standard is inconsistent with the emphasis 
that the EPA placed on CO2 emissions reductions estimated 
for the proposed Clean Power Plan (79 FR 34830, 34931-33). As support 
for this view of inconsistency, the commenter compared the WREA 30-year 
estimate of the amount of CO2 removed from the air and 
stored in vegetation with estimated reductions in CO2 
emissions from power plants over a 4-year period. We note, however, 
some key distinctions between the two types of estimates which 
appropriately lead to different levels of emphasis by the EPA in the 
two actions. First, we note that the lengths of time pertaining to the 
two estimates that the commenter states to be ``roughly equal'' (e.g., 
ALA et al., p. 211) differ by more than a factor of seven (4 years 
compared to 30). Second, the CPP estimates are for reductions in 
CO2 produced and emitted from power plants, while the WREA 
estimates are for amounts of CO2 removed from the air and 
stored in vegetation as a result of plant photosynthesis occurring 
across the U.S. This leads to two important differences. The first is 
whether a ton of additional carbon uptake by plants is equal to a ton 
of reduced emissions from fossil fuels. This is still an active area of 
discussion due in part to the potentially transient nature of the 
carbon storage in vegetation. The second is that there are much 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. 
Therefore, as summarized in section IV.C.3 below, the Administrator is 
judging, as at the time of proposal, that the quantitative 
uncertainties are too great to support identification of a revised 
standard based specifically on the WREA quantitative estimates of 
carbon storage benefits to climate. In so doing, she notes that a 
revised standard, established primarily based on other effects for 
which our quantitative estimates are less uncertain, can be expected to 
also provide increased protection in terms of carbon storage.
b. Consideration of Visible Foliar Injury
    In considering public comments received on the EPA's consideration 
of visible foliar injury in its decision on a revised secondary 
standard, the EPA first notes related advice and comments from the 
CASAC received during development of the PA. The CASAC stated that 
``[w]ith respect to the secondary standard, the CASAC concurs with the 
EPA's identification of adverse welfare effects related to . . . damage 
to resource use from foliar injury'' (Frey, 2014, p. iii). In its 
comments on levels of a W126-based standard, the CASAC, seemingly in 
reference to the WREA visible foliar injury analyses, additionally 
stated that ``[a] level below 10 ppm-hrs is required to reduce foliar 
injury'' (Frey, 2014, pp. iii and 15), with ``W126 values below 10 ppm-
hr required to reduce the number of sites showing visible foliar 
injury'' (Frey, 2014, p. 14).
    Public comments were generally split between two views, either that 
visible foliar injury was not appropriate to consider in decisions 
regarding the standard, based on variously identified reasons, or that 
it should be considered and it would lead the EPA to focus on a W126 
value below approximately 10 ppm-hrs. Comments of the former type are 
discussed in section IV.B.2 above, with, in some cases, additional 
detail in the Response to Comments document. Commenters expressing the 
latter view variously cite CASAC advice and figures from the WREA 
cumulative analysis of USFS biosite data with WREA W126 index value 
estimates. The EPA disagrees that only a reduction in cumulative 
exposures to W126 index values below 10 ppm-hrs will affect the 
occurrence or extent of visible foliar injury. In so doing, we note 
that the extensive evidence, which is summarized in the ISA (including 
studies of the USFS biomonitoring program), analyses in the 2007 Staff 
Paper and also observations based on the WREA dataset do not support 
this conclusion.
    The evidence regarding visible foliar injury as an indicator of 
O3 exposure is well established and generally documents a 
greater extent and severity of visible foliar injury with higher 
O3 exposures and a modifying role of soil moisture 
conditions (U.S. EPA, 2013, section 9.4.2). As stated in the ISA, 
``[v]isible foliar injury resulting from exposure to O3 has 
been well characterized and documented over several decades of research 
on many tree, shrub, herbaceous and crop species'' and ``[o]zone-
induced visible foliar injury symptoms on certain bioindicator plant 
species are considered diagnostic as they have been verified 
experimentally'' (U.S. EPA, 2013, p. 9-41). Further, a recent study 
highlighted in the ISA, which analyzed trends in the incidence and 
severity of foliar injury, reported a declining trend in the incidence 
of foliar injury as peak O3 concentrations declined (U.S. 
EPA, 2013, p. 9-40; Smith, 2012). Another study available in this 
review that focused on O3-induced visible foliar injury in 
forests of west coast states observed that both percentage of biosites 
with injury and average biosite index were higher for sites with 
average cumulative O3 concentrations above 25 ppm-hrs in 
terms of SUM06 (may correspond to W126 of approximately 21 ppm-hrs 
[U.S. EPA, 2007, p. 8-26, Appendix 7B]) as compared to groups of sites 
with lower average cumulative exposure concentrations, with much less 
clear differences between the two lower exposure groups (Campbell et 
al., 2007, Figures 27 and 28 and p. 30). A similar finding was reported 
in the 2007 Staff Paper which reported on an analysis that showed a 
smaller percentage of injured sites among the group of sites with 
O3 exposures below a SUM06 metric of 15 ppm-hrs or a fourth-
high metric of 74 ppb as compared to larger groups that also included 
sites with SUM06 values up to 25 ppm-hrs or fourth-high metric up to 84 
ppb, respectively (U.S. EPA 2007, pp. 7-63 to 7-64).
    With regard to the comments referencing the WREA cumulative 
analysis of USFS FHM/FIA biosite data or related CASAC comments, we 
note some clarification of this analysis. This analysis does not show, 
as implied by the comments, that at W126 index values above 10 ppm-hrs, 
there is little change with increasing W126 index in

[[Page 65396]]

the proportion of records with any visible foliar injury (biosite index 
above 0). As the analysis is a cumulative analysis, each point graphed 
in the analysis includes the records for the same and lower W126 index 
values, so the analysis does not compare results for groups of records 
with differing, non-overlapping W126 index values. Rather, the points 
represent groups with records (and W126 index values) in common and the 
number of records in the groups is greater for higher W126 index values 
(U.S. EPA, 2014b, section 7.2). Additionally, we note that the pattern 
observed in the cumulative analysis is substantially influenced by the 
large number of records for which the W126 index estimates are at or 
below 11 ppm-hrs, more than two thirds of the dataset (Smith and 
Murphy, 2015, Table 1).
    To more fully address the comments related to this WREA analysis, 
we have drawn several additional observations from the WREA dataset, 
re-presenting the same data in a different format in a technical 
memorandum to the docket (Smith and Murphy, 2015). Contrary to the 
implication of the statements from the commenters and CASAC that no 
reduction in the occurrence of visible foliar injury can be achieved 
with exposures above 10 ppm-hrs, both the proportion of records with 
injury and the average biosite index are lower for groups of records 
with W126 index estimates at or below 17 ppm-hrs compared to the group 
for the highest W126 index range. This is true when considered 
regardless of soil moisture conditions (all records), as well as for 
dry, normal and wet records, separately (Smith and Murphy, 2015, Table 
2). The pattern of the two measures across record groups with lower 
W126 index values differs with moisture level, with the wetter than 
normal records generally showing decreasing proportions of injured 
sites and decreasing average biosite index with lower W126 index 
values, while little difference in these measures is seen among the 
middle W126 values although they are lower than the highest W126 index 
group and higher than the lowest W126 index group (Smith and Murphy, 
2015, Table 2). In summary, the EPA disagrees with commenters, noting 
that the available information, including additional observations from 
the WREA dataset, indicate declines in the occurrence of visible foliar 
injury across decreasing W126 index values that are higher than 10 ppm-
hrs.
c. Use of W126 Metric in Evaluating Vegetation Effects and Public 
Welfare Protection
    In considering public comments received on the EPA's use of the 
W126 exposure index in its decision on a revised secondary standard, 
the EPA first notes related advice and comments from the CASAC received 
during development of the PA. Although we recognize that CASAC's 
comments on the W126 index were provided in the context of its 
recommendation for a secondary standard of that form, we find them to 
also relate to our use of the W126 metric in evaluating the magnitude 
and extent of vegetation effects that might be expected and conversely 
the level of protection that might be provided under different air 
quality conditions. In comments on the first draft PA, the CASAC stated 
that ``discussions and conclusions on biologically relevant exposure 
metrics are clear and compelling and the focus on the W126 form is 
appropriate'' (Frey and Samet, 2012a). With regard to specific aspects 
of the W126 index, the CASAC concurred with the second draft PA focus 
on ``the biologically-relevant W126 index accumulated over a 12-hour 
period (8 a.m.-8 p.m.) over the 3-month summation period of a single 
year resulting in the maximum value of W126'' (Frey, 2014c, p. iii).
    The CASAC advice on levels of the W126 index on which to focus for 
public welfare protection recommended a level within the range of 7 
ppm-hrs to 15 ppm-hrs (Frey, 2014c, p. iii). We note, however, as 
summarized in section IV.E.3 of the proposal, that this advice was 
provided in the context of the CASAC review of the second draft PA, 
which concluded that a range from 7 to 17 ppm-hrs was appropriate to 
consider. In considering the upper end of this range, the CASAC 
consulted Table 6-1 of the second draft PA which indicated for a W126 
index value of 17 ppm-hrs an RBL estimate of 6%, a magnitude that CASAC 
described as ``unacceptably high'' and that contributed to a lack CASAC 
support for W126 exposures values higher than 15 ppm-hrs (Frey, 2014c, 
p. 14; U.S. EPA 2014d, Table 6-1). As noted in section IV.E.3 of the 
proposal, revisions to the RBL estimate table in the final PA, which 
were made in consideration of other CASAC comments, have resulted in 
changes to the median species RBL estimate associated with each W126 
index value, such that the median species RBL estimate for a W126 index 
value of 17 ppm-hrs in this table in the final PA was 5.3%, rather than 
the ``unacceptably high'' value of 6% (U.S. EPA, 2014c, Table 6-1; U.S. 
EPA, 2014d, Table 6-1; Frey, 2014c, p. 14).\201\ Additionally, the 
CASAC recognized that the Administrator may, as a policy matter, prefer 
to use a 3-year average, and stated that in that case, the range of 
levels should be revised downward (Frey, 2014c, p. iii-iv).
---------------------------------------------------------------------------

    \201\ We additionally note that the median species RBL estimate 
for 17 ppm-hrs in the final PA is nearly identical to the estimate 
for 15 ppm-hrs (the value corresponding to the upper end of the 
CASAC-identified range) that was in the second draft PA (5.2%) which 
was the subject of the CASAC review (U.S. EPA, 2014c, Table 6-1; 
U.S. EPA, 2014d, Table 6-1).
---------------------------------------------------------------------------

    The majority of comments on the W126 index concurred with its use 
for assessing O3 exposures, while some commenters 
additionally expressed the view that this index should be used as the 
form of the secondary standard (as discussed in section IV.C.2.d 
below). Most submissions from state and local environmental agencies or 
governments, as well as organizations of state agencies, that provided 
comments on the magnitude of cumulative exposure, in terms of the W126 
index, appropriate to consider for a revised secondary standard, 
recommended that the EPA focus on an index value within the EPA's 
proposed range of 13 to 17 ppm-hrs, as did the industry commenters. 
These commenters variously noted their agreement with the rationale 
provided by the EPA in the proposal or cited to CASAC comments, 
including for a downward adjustment of its recommended values if a 3-
year average W126 was used rather than a single year index. Some other 
commenters, including two groups of environmental organizations, 
submitted comments recommending a focus on a W126 index level as low as 
7 ppm-hrs based on reasons generally focused on consideration of 
visible foliar injury.
    Some aspects of these comments have been addressed in sections 
IV.C.2.a and IV.C.2.b above. In the Response to Comments document, we 
have additionally addressed other comments that recommend a focus on 
W126 index values for specific reasons other than generally citing the 
CASAC recommended range. Further, in her consideration of a target 
level of protection for the revised secondary standard in section 
IV.C.3 below, the Administrator has considered comments from the CASAC 
regarding the basis for their recommended range.
    An additional comment from an organization of western state air 
quality managers indicated a concern with the use of W126 for 
vegetation in arid and high altitude regions, such as those in the 
western states, which the

[[Page 65397]]

commenter hypothesized may have reduced sensitivity. The commenters did 
not provide evidence of this hypothesis, calling for further research 
in order to characterize the sensitivity of vegetation in such areas. 
The EPA agrees that additional research would be useful in more 
completely characterizing the response of species in such areas, as 
well as other less well studied areas, but does not find support in the 
currently available evidence for the commenter's suggestion that 
species in arid and high altitude regions may be less sensitive than 
those in other areas.\202\
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    \202\ For example, we note that among the 11 species for which 
robust E-R functions have been established for O3 effects 
on tree seedling growth, the sensitivity of ponderosa pine, a 
species occurring in arid and high altitude regions of the western 
U.S., is similar to the median (U.S. EPA, 2014c, Table 5C-1).
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    Among the small number of commenters recommending against using the 
W126 metric to assess O3 exposure, a few expressed the view 
that some other, not-yet-identified cumulative exposure metric should 
be used. These commenters cited a variety of concerns that they state 
are not addressed by the W126 index: that plant exposure to and uptake 
of O3 are not always equivalent because of variations in 
stomatal conductance and plant defenses and their respective diel 
patterns, which will also influence plant response; that the duration 
between harmful O3 exposures affects the plant's ability to 
repair damage; and, that night-time exposures may be important. These 
commenters do not identify an alternative to the W126 index that they 
conclude to better represent exposures relevant to considering 
O3 effects on vegetation and particularly for growth 
effects. The EPA has considered the items raised by these commenters, 
recognizing some as areas of uncertainty (U.S. EPA, 2013, pp. 9-109 to 
9-113), yet has concluded that based on the information available at 
this time, exposure indices that cumulate and differentially weight the 
higher hourly average concentrations while also including the ``mid-
level'' values offer the most appropriate approach for use in 
developing response functions and comparing studies of O3 
effects on vegetation (U.S. EPA, 2013, p. 9-117). When considering the 
response of vegetation to O3 exposures represented by the 
threshold (e.g., SUM06) and non-threshold (e.g., W126) indices, the ISA 
notes that ``the W126 metric does not have a cut-off in the weighting 
scheme as does SUM06 and thus it includes consideration of potentially 
damaging exposures below 60 ppb'' and that ``[t]he W126 metric also 
adds increasing weight to hourly concentrations from about 40 ppb to 
about 100 ppb'' (U.S. EPA, 2013, p. 9-104). This aspect of W126 is one 
way it differs from cut-off metrics such as the SUM06 where all 
concentrations above 60 ppb are treated equally and is identified by 
the ISA as ``an important feature of the W126 since as hourly 
concentrations become higher, they become increasingly likely to 
overwhelm plant defenses and are known to be more detrimental to 
vegetation'' (U.S. EPA, 2013, p. 9-104). Further, we note the 
concurrence by CASAC with the EPA's focus on the W126 exposure index, 
as noted above.
    Some commenters also raised concerns regarding the sensitivity of 
vegetation in desert areas where plants take in ambient air during 
nighttime rather than daylight hours, such that little exposure occurs 
from 8 a.m. to 8 p.m., stating that the W126 index as defined by the 
EPA to cumulate hourly O3 from 8 a.m. to 8 p.m. may result 
in an overly stringent exposure level in areas with such vegetation. 
The EPA recognizes that plants, such as cacti, that commonly occur in 
desert systems exhibit a particular type of metabolism (referred to as 
CAM photosynthesis) such that they only open their stomata at night 
(U.S. EPA, 2013, p. 9-109). We note, however, that few if any 
O3 exposure studies of these species are available \203\ to 
further inform our characterization of these species' responses to 
O3, and we have no basis on which to conclude that an 
exposure level based on the studied species and a daylight exposure 
metric would be overly or underly stringent in areas where only species 
utilizing CAM photosynthesis occur. As summarized above, the CASAC 
advice concurred with the use of an 8am to 8pm diurnal period for the 
W126 exposure index. Thus, we conclude that for our purposes in this 
review the focus on daylight hours is appropriate. Our use of the W126 
index in this review has been for purposes of characterizing the 
potential harm and conversely the potential protection that might be 
afforded from the well-characterized effects of O3 on 
vegetation, while recognizing associated uncertainties and limitations. 
We note that different ecosystems across the U.S. will be expected to 
be of varying sensitivities with regard to the effects of 
O3. For example, large water bodies without vegetation 
extending above the water's surface would be expected to be less 
sensitive than forests of sensitive species. The EPA notes, however, 
that the NAAQS are set with applicability to all ambient air in the 
U.S., such that the secondary O3 standard provides 
protection in areas across the U.S. regardless of site-specific aspects 
of vegetation sensitivity to O3. In considering the evidence 
on O3 and associated welfare effects, we recognize 
variability in sensitivity that may relate to a number of factors, as 
discussed in the ISA (U.S. EPA, 2013, section 9.4.8). This variability 
is among the Administrator's considerations in setting the secondary 
standard for O3 that is requisite to protect public welfare 
against anticipated or known adverse effects.
---------------------------------------------------------------------------

    \203\ No O3 exposure studies on cacti or other 
species that utilize CAM photosynthesis are reported in the ISA 
(U.S. EPA, 2013).
---------------------------------------------------------------------------

    Further, some commenters who agreed with a focus on the W126 
exposure index also stated that the EPA's definition of the index for 
the daylight hours of 8 a.m. to 8 p.m. and a 3-month period was not 
appropriate, stating that derivation of the W126 metric should involve 
summing concentrations for all 24 hours in each day and all months in 
each year to avoid underestimating O3 exposure that the 
commenters viewed as pertinent. Support for the EPA's definition of the 
W126 index, with which CASAC concurred (Frey, 2014c, p. iii), is based 
on the assessment of the evidence in the ISA (U.S. 2013, section 
9.5.3.2) and the context for use of the W126 index in relating 
O3 exposure to magnitude and/or extent of O3 
response. This context has a particular focus on growth effects for the 
purposes of judging the potential for public welfare impacts, as well 
as the level of protection, associated with different exposure 
circumstances. We note that the ISA stated there is a lack of 
information that would allow consideration of the extent to which 
nocturnal exposures that may be of interest occur (U.S. EPA, 2013, p. 
9-109). Additionally, in our use of the W126 index, we are relying on 
E-R functions based on studies that were generally of 3-month duration 
and involved controlled exposures during the daylight period. 
Accordingly we have relied on the E-R function derived for 12-hour and 
3-month W126 indices, as described in section IV.A.1 above. To apply 
these E-R functions to the W126 estimates derived using 24 hours-per-
day index values would inaccurately represent the response observed in 
the study (producing an overestimate). Similarly, with regard to the 3-
month duration, ``[d]espite the possibility that plants may be exposed 
to ambient O3 longer than 3 months in some locations, there 
is generally a lack of exposure experiments conducted for longer than

[[Page 65398]]

3 months'' (U.S. EPA, 2014c, p. 9-112). Thus, in consideration of the 
lack of support in the current evidence for characterizing exposure for 
purposes of estimating RBL based on cumulative exposures derived from a 
combination of daytime and nighttime exposures and consideration of 
year-round O3 concentrations across the U.S., we disagree 
with the commenters' view of the appropriateness of using an exposure 
index based on 24-hour, year-round O3 concentrations.
    The commenters supporting the use of the W126 exposure index were 
divided with regard to whether the EPA should focus on an annual index 
or one averaged over three years. Some of the commenters indicating 
support for the EPA's proposed focus on a 3-year average W126 index 
stated that this was appropriate in light of the wide variations in 
W126 index values that can occur on a year-to-year basis as a result of 
the natural variation of climatic conditions that have a direct impact 
on O3 formation; in their view, these factors are mitigated 
by use of a 3-yr average, which thus provides ``stability'' in the 
assessment dampening out the natural variation of climatic conditions 
that have a direct impact on O3 formation. Others noted that 
use of a 3-year average may be supported as matter of policy. We 
generally concur with the relevance of these points, among others, to a 
focus on the 3-year average W126. Other commenters expressed the view 
that the EPA should focus on an annual W126 index, generally making 
these comments in the context of expressing their support for a 
secondary standard with a W126 form. These commenters variously cited 
CASAC advice and its rationale for preferring a single year W126 form, 
stated that vegetation damage occurs on an annual basis, and/or 
questioned the EPA's statements of greater confidence in conclusions as 
to O3 impacts based on a 3-year average exposure metric.
    The EPA agrees with commenters that, as discussed in the PA and the 
proposal, depending on the exposure conditions, O3 can 
contribute to measurable effects on vegetation in a single year. We 
additionally recognize that, as described in the PA and proposal, there 
is generally a greater significance for effects associated with 
multiple-year exposures. The proposal described a number of 
considerations raised in the PA as influencing the Administrator's 
decision to focus on a 3-year average W126 index (79 FR 75347, December 
17, 2014). These included, among others, the observation of a greater 
significance for effects associated with multiple-year exposures, and 
the uncertainties associated with consideration of annual effects 
relative to multiple-year effects.
    Further, we note that among the judgments contributing to the 
Administrator's decision on the level of protection appropriate for the 
secondary standard are judgments regarding 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 and judgments on the extent to which such 
effects in such areas may be considered adverse to public welfare (79 
FR 75312, December 17, 2014). Thus, conclusions regarding the extent to 
which the size and/or prevalence of effects on vegetation in a single 
year and any ramifications for future years represent an adverse effect 
to the public welfare, conclusions that are also inherently linked to 
overall magnitudes of exposures, are dependent on the Administrator's 
judgment. Accordingly, the decision regarding the need to focus on a 1-
year or 3-year W126 index value is also a judgment of the 
Administrator, informed by the evidence, staff evaluations and advice 
from CASAC, as described in section IV.C.3 below.
d. Form and Averaging Time
    In considering comments received on the proposed form for the 
revised standard, the EPA first notes the advice and comments from the 
CASAC, received in its review of the second draft PA. Similar to its 
advice in the last review, the CASAC recommended ``establishing a 
revised form of the secondary standard to be the biologically relevant 
W126 index'' (Frey, 2014c, p. iii). With regard to its reasons for this 
view, the CASAC cites the PA in stating that it ``concurs with the 
justification in [section 5.7] that the form of the standard should be 
changed from the current 8-hr form to the cumulative W126 index'' 
(Frey, 2014c, p. 12). In addressing specific aspects of this index, the 
CASAC concurred with the EPA's focus on the 3-month period with the 
highest index value and further states that ``[a]ccumulation over the 
08:00 a.m.-08:00 p.m. daytime 12-hour period is a scientifically 
acceptable and recommended means of generalizing across latitudes and 
seasons'' (Frey, 2014c, p. 13). As section 5.7 of the PA discusses the 
W126 index in the context of the support in the evidence for use of the 
W126 exposure index for assessing impacts of O3 on 
vegetation and the extent of protection from such impacts, we interpret 
CASAC's statement on this point to indicate that the basis for CASAC's 
view with regard to the form for the secondary standard relates to the 
appropriateness of the W126 exposure index for those assessment 
purposes.\204\ \205\
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    \204\ Section 5.7 of the PA states that ``the evidence continues 
to provide a strong basis for concluding that it is appropriate to 
judge impacts of O3 on vegetation, related effects and 
services, and the level of public welfare protection achieved, using 
a cumulative, seasonal exposure metric, such as the W126-based 
metric,'' references the support of CASAC for a W126-based secondary 
standard, and then concludes that ``based on the consistent and 
well-established evidence described above, . . . the most 
appropriate and biologically relevant way to relate O3 
exposure to plant growth, and to determine what would be adequate 
protection for public welfare effects attributable to the presence 
of O3 in the ambient air, is to characterize exposures in 
terms of a cumulative seasonal form, and in particular the W126 
metric'' (U.S. EPA, 2014c, p. 5-78).
    \205\ The CASAC also mentioned its support for revising the 
secondary standard to a W126 index-based form in its review of 
Chapter 6 of the second draft PA (Frey, 2014c, p. 13). Similar to 
section 5.7, in that chapter of the PA staff concluded that 
``specific features associated with the W126 index still make it the 
most appropriate and biologically relevant cumulative concentration-
weighted form for use in the context of the secondary O3 NAAQS 
review'' (U.S. EPA, 2014c, p. 6-5) and also concluded that ``it is 
appropriate to consider a revised secondary standard in terms of the 
cumulative, seasonal, concentration-weighted form, the W126 index'' 
(U.S. EPA, 2014c, p. 6-57).
---------------------------------------------------------------------------

    The public comments on the form for a revised secondary standard 
were divided. Most of the state and local environmental agencies or 
governments, and all of the tribal agencies and organizations that 
provided comments on the form for the secondary standard concurred with 
the EPA's proposed decision, as did the industry commenters. These 
commenters generally indicated agreement with the rationale provided in 
the proposal that drew from the EPA analyses of recent air quality data 
examining relationships at sites across the U.S. between values of the 
fourth-high metric (the current design value) and values of a 3-year 
average W126-based metric, stating that this analysis showed that a 
standard in the form of the fourth-high metric, as proposed, can 
provide air quality consistent with or below the range of 3-year W126 
exposure index values identified in the proposal. Some commenters 
additionally stated that the choice of form was a policy decision for 
the EPA and that little or no additional protection of public welfare 
would be gained by adopting a W126-based form. Some of these commenters 
provided analyses of data for their state or region that further 
supported this view. As

[[Page 65399]]

described in section IV.C.3 below, the EPA generally agrees with these 
commenters.
    Some commenters, including a regional organization of state 
agencies and two groups of environmental organizations, submitted 
comments recommending revision of the standard to a cumulative, 
seasonal form based on the W126 index. In support of their position, 
these commenters generally cited CASAC advice, variously additionally 
indicating their view that the standard form should be a metric 
described as biologically relevant, and that the existing form, with a 
level in the proposed range, would not provide adequate ecosystem 
protection. Some commenters additionally suggested that the EPA cannot 
lawfully retain the form and averaging time that were initially 
established for purposes of the primary standard when the EPA has 
identified the W126 index as a metric appropriate for judging 
vegetation-related effects on public welfare. With regard to the EPA 
air quality analyses, summarized in the proposal, of the W126 index 
values at sites where O3 concentrations met different levels 
of fourth-high metric, some of these commenters stated that the 
analyses showed widespread variation in W126 values for each fourth-
high metric examined. Further, some commenters disagreed with the EPA 
that the analyses indicated that a revised standard level within the 
proposed range would be expected to limit W126 exposures in the future 
to the extent suggested by the analyses of data from the past.
    We agree with public commenters and CASAC regarding the 
appropriateness of the W126 index (the sum of hourly concentrations 
over a specified period) as a biologically relevant metric for 
assessing exposures of concern for vegetation-related public welfare 
effects, as discussed in the proposal, PA and ISA. Accordingly, we 
agree that this metric is appropriate for use in considering the 
protection that might be expected to be afforded by potential 
alternative secondary standards, as discussed in section IV.C.2.c 
above. We disagree with commenters, however, that use of the W126 
metric for this purpose dictates that we must establish a secondary 
standard with a W126 index form.
    In support of this position, we note 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.\206\ Across reviews for the various NAAQS pollutants, we 
have used a variety of exposure metrics to evaluate the protection 
afforded by the standards. 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. Some examples of exposure metrics used to evaluate 
health impacts in primary standard reviews include the concentration of 
lead in blood of young children and a 5-minute exposure concentration 
for sulfur dioxide. In contrast, the health-based standards for these 
two pollutants are the 3-month concentration of lead in total suspended 
particles and the average across three years of the 99th percentile of 
1-hour daily maximum concentration of sulfur dioxide in ambient air, 
respectively (73 FR 66964, November 12, 2008; 75 FR 35520, June 22, 
2010). In somewhat similar manner, in the 2012 PM review, the EPA 
assessed the extent to which the existing 24-hour secondary standard 
for PM2.5, expressed as a 24-hour concentration (of 
PM2.5 mass per cubic meter of air) not to be exceeded more 
than once per year on average over three years, could provide the 
desired protection from effects on visibility in terms of the 90th 
percentile, 24-hour average PM2.5 light extinction, averaged 
over three years, based on speciated PM2.5 mass 
concentrations and relative humidity data (79 FR 3086, January 15, 
2013). Additionally, in the case of the screening-level risk analyses 
in the 2008 review of the secondary standard for lead, concentrations 
of lead in soil, surface water and sediment were evaluated to assess 
the potential for welfare effects related to lead deposition from air, 
while the standard is expressed in terms of the concentration of lead 
in particles suspended in air (73 FR 67009, November 12, 2008).
---------------------------------------------------------------------------

    \206\ The term design value is commonly used to refer to the 
metric for the standard. Consistent with the summary in section I.D 
above, a design value is the statistic that describes the air 
quality of a given location in terms of the indicator, form and 
averaging time of the standard such that it can then be compared to 
the level of the standard.
---------------------------------------------------------------------------

    Further, depending on the evidence base, some NAAQS reviews may 
consider multiple exposure metrics in assessing risks associated with a 
particular pollutant in ambient air in order to judge the adequacy of 
an existing standard in providing the required level of protection. And 
a standard with an averaging time of one duration may provide 
protection against effects elicited by exposures of appreciably shorter 
or longer durations. For example, in the current review of the primary 
O3 standard, as described in section II above, we have 
considered the potential for effects associated with both short- and 
long-term exposures and concluded, based on a combination of air 
quality and risk analyses and the health effects evidence, that the 
existing standard with its short (8-hour) averaging time provides 
control of both the long and short term exposures (e.g., from one hour 
to months or years) that may be of concern to public health. Similarly, 
during the 1996 review of the NO2 primary standard, while 
health effects were recognized to result from both long-term and short-
term exposures to NO2, the primary standard, which was a 
long-term (annual) standard, was concluded to provide the requisite 
protection against both long- and short-term exposures (61 FR 52852, 
Oct 8 1996). In the subsequent review of the NO2 primary 
standard in which the available air quality information indicated that 
the annual standard was not providing the needed control of the shorter 
term exposures, an additional short-term standard was established (75 
FR 6474, February 9, 2010).
    Thus, we note that different metrics may logically, reasonably, and 
for technically sound reasons, be used in assessing exposures of 
concern or characterizing risk as compared to the metric of the 
standard which is used to control air quality to provide the desired 
degree of protection. That is, 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 secondary standard for O3, 
the EPA agrees that, for the reasons summarized in section IV.A.1 above 
and described in the ISA, the W126 index--and not an 8-hour daily 
maximum concentration that has relevance in human health risk 
characterization, as described in section II above--is the appropriate 
metric for assessing exposures of concern for vegetation, 
characterizing risk to public welfare, and evaluating what air quality 
conditions might provide the desired degree of public welfare 
protection. We disagree, however, that the secondary standard must be 
established using that same metric.
    Moreover, we note that the CAA does not require that the secondary 
O3 standard be established in a specific

[[Page 65400]]

form. 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 is appropriate, in combination with the other aspects 
of the standard (averaging time, 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.\207\
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    \207\ 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]).
---------------------------------------------------------------------------

    With regard to the commenter's emphasis on advice from CASAC on the 
form of the secondary standard, the EPA agrees with the importance of 
giving such advice careful consideration. The EPA further notes, 
however, that the Administrator is not legally precluded from departing 
from CASAC's recommendations, when she has provided an explanation of 
the reasons for such differences.\208\ Accordingly, in reaching 
conclusions on the revised secondary standard in this review, the 
Administrator has given careful consideration to the CASAC advice in 
this review and, when she has differed from CASAC recommendations, she 
has fully explained the reasons and judgments that led her to a 
different conclusion, as described in section IV.C.3 below.
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    \208\ 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'').
---------------------------------------------------------------------------

    In disagreeing with the EPA's conclusions drawn from analyses of 
recent air quality data on the extent to which cumulative seasonal 
exposures might be limited to within or below the identified 3-year 
average W126 index values by controlling air quality using different 
values for the fourth-high metric, one group of environmental 
organizations emphasized the range of W126 index values that occur at 
monitors with concentrations at or below specific values for the 
fourth-high metric. For monitor observations for which the fourth-high 
metric was at or below 70 ppb, this commenter group stated that some 
sites have 3-year average W126 index values above 17 ppm-hrs and noted 
a maximum 3-year W126 index value of 19.1 ppm-hrs, while additionally 
noting occurrences of other W126 values above the CASAC range of 7 to 
15 ppm-hrs. This commenter additionally stated that the air quality 
data ``do not support a claim of congruence'' between the fourth-high 
and W126 metrics (e.g., ALA et al., p. 196), that there is no basis for 
concluding that there is some fundamental underlying relationship that 
assures meeting the fourth-high metric will mean meeting any of the 
W126 options, and that the relationship between the metrics is non-
linear with significant spread in the data (citing visual inspection of 
a graph).
    The EPA does not agree with the commenter's statements regarding 
the relationship between the two metrics.\209\ We have not, as stated 
by the commenter, claimed there to be ``congruence'' between the two 
metrics (e.g., ALA et al., p. 196), or that the two metrics coincide 
exactly. Rather, at any location, values of both metrics are a 
reflection of the temporal distribution of hourly O3 
concentrations across the year and both vary in response to changes in 
that distribution. While the EPA's air quality analysis shows that the 
specific relationship differs among individual sites, it documents an 
overall strong, positive, non-linear relationship between the two 
metrics (Wells, 2014a, p. 6, Figures 5a and 5b; Wells, 2015b). Further, 
this analysis finds the amount of year-to-year variability in the two 
metrics tended to decrease over time with decreasing O3 
concentrations, especially for the W126 metric, as described in section 
IV.E.4 of the proposal (Wells, 2014a; Wells, 2015b).
---------------------------------------------------------------------------

    \209\ The EPA additionally notes that commenters contradict 
their own assertion when, after stating their view that no 
relationship exists between the 4th high and W126 metrics, the 
commenter then states that there is a nonlinear relationship and yet 
then relies on a predicted linear relationship to estimate W126 
values occurring when air quality meets different values for the 4th 
high metric at 11 national parks.
---------------------------------------------------------------------------

    With regard to the highest 3-year average W126 exposure index 
values that might reasonably be expected in the future in areas where a 
revised standard with a fourth-high form is met, we disagree with the 
commenters as to the significance of the W126 index value of 19.1 ppm-
hrs in the 13-year dataset. This value, for a site during the period 
2006-2008, is the only occurrence at or above 19 ppm-hrs in the nearly 
4000 3-year W126 index values--across the 11 3-year periods extending 
back in time from 2013--for which the fourth-high metric for the same 
monitor location is at or below 70 ppb. This is clearly an isolated 
occurrence.
    In considering this comment, we have expanded the technical 
memorandum that was available at the time of proposal (Wells, 2014a). 
The expanded memorandum describes the same air quality analyses for 3-
year periods from 2001 through 2013 as the 2014 memorandum, and 
includes additional summary tables for all 3-year periods from 2001 
through 2013 as well as tables for the most recent period, 2011-2013 
(Wells, 2015b). After the 3-year W126 index value of 19 ppm-hrs, the 
next three highest 3-year average W126 index values, which are the only 
other such values above 17 ppm-hrs in the 13-year dataset, and which 
also occur during periods in the past, round to 18 ppm-hrs (Wells, 
2015b). Additionally, we note that reductions in the fourth-high metric 
over the 13-year period analyzed are strongly associated with 
reductions in the cumulative W126 index (Wells, 2014a, Figure 11, Table 
6; Wells, 2015b). Specifically, the regression analysis of changes in 
W126 index between the 2001-2003 period and the 2011-2013 period with 
changes in the fourth-high metric across the same periods indicates a 
fairly linear and positive relationship between reductions of the two 
types of metrics, with, on average, a change of approximately 0.7 ppm-
hr in the W126 index per ppb change in the fourth-high metric value. 
From this information we conclude that W126 exposures above 17 ppm-hrs 
at sites for which the fourth-high metric is at or below 70 ppb would 
be expected to continue to be rare in the future, particularly as steps 
are taken to meet a 70 ppb standard.
    With regard to the comment that the relationship between the two 
metrics varies across locations, the EPA agrees that there is variation 
in cumulative seasonal O3 exposure (in terms of a 3-year 
average W126 index) among locations that are at or below the same 
fourth-high metric. As noted in the proposal, the analysis illustrates 
this variation, with the locations in the West and Southwest NOAA 
climatic regions tending to have the highest cumulative seasonal 
exposures for the same fourth-high metric value. In considering 
expectations for the future in light of this observation, however, we 
note that

[[Page 65401]]

the regional regressions of reductions in W126 metric with reductions 
in the fourth-high metric indicate that the Southwest and West regions, 
which had the greatest potential for sites having 3-year W126 index 
values greater than the various W126 values of interest when fourth-
high values are less than or equal to the various fourth-high metric 
values of interest, also exhibited the greatest reduction in the W126 
index values per unit reduction in the fourth-high values (Wells, 
2015b). Thus, in considering the potential for occurrences of values 
above 17 ppm-hrs in the future in areas that meet a fourth-high of 70 
ppb, the EPA notes that the analysis indicates that those areas that 
exhibited the greatest likelihood of occurrence of a 3-year W126 index 
above a level of interest (e.g., the commenters' example in the 
Southwest region of a value of 19.1 ppm-hrs [2006-2008] in comparison 
to the W126 level of 17 ppm-hrs) also exhibit the greatest improvement 
in W126 per unit decrease in fourth-high metric.\210\ It is expected 
that future control programs designed to meet a standard with a fourth-
high form would provide similar improvements in terms of the W126 
metric.
---------------------------------------------------------------------------

    \210\ Additionally, O3 levels at any location are 
influenced by upwind precursor emissions, and many rural areas, 
including the site referenced by the commenter, are impacted by 
precursor emissions from upwind urban areas, such that as emissions 
are reduced to meet a revised standard in the upwind locations, 
reductions in those upwind emissions will contribute to reductions 
at the downwind sites (Wells, 2014a; ISA, pp. 3-129 to 3-133).
---------------------------------------------------------------------------

    As part of their rationale in support of revising the current form 
and averaging time, one commenter pointed to the regional variation in 
the highest W126 index values expected at sites that just meet a 
fourth-high metric of 70 ppb, based on the EPA's analysis of recent air 
quality data available at the time of the proposal (Wells, 2014a). This 
commenter observed that, while in some U.S. regions, locations that 
meet a potential alternative standard with the current form and a level 
of 70 ppb also have 3-year average W126 index values no higher than 17 
ppm-hrs, the highest W126 index values in other parts of the country 
are lower. As a result, the commenter concluded that such a standard 
would result in regionally differing levels of welfare protection. The 
commenter additionally states that, for extreme values, a W126 form for 
the secondary standard would also offer different levels of protection, 
although with the primary standard setting the upper boundary for such 
values.
    The EPA recognizes that a standard with the current form might be 
expected to result in regionally differing distributions of W126 
exposure index values (including different maximum values) depending on 
precursor sources, local meteorology, and patterns of O3 
formation. Variation in exposures is to be expected with any standard 
(secondary or primary) of any form. In fact, variation in exposures and 
any associated variation in welfare or health risk is generally an 
inherent aspect of the Administrator's judgment on a specific standard, 
and any associated variation in welfare or health protection may play a 
role in the Administrator's judgment with regard to public welfare or 
public health protection objectives for a national standard. In 
considering the comment, however, we have focused only on the extent to 
which the commenter's conclusion that a secondary standard of the 
current form and averaging time would provide regionally varying 
welfare protection might indicate that the specified air quality is 
more (or less) than necessary to achieve the purposes of the standard. 
In so doing, we additionally respond to a separate comment that the EPA 
needs to address how the revised secondary standard is neither more or 
less than necessary to protect the public welfare.
    The CAA requirement in establishing a standard is that it be set at 
a level of air quality that is requisite, meaning ``sufficient, but not 
more than necessary'' (Whitman v. American Trucking Ass'ns, 531 U.S. 
457, 473 [2001]). We note that the air quality that is specified by the 
revised primary standard has been concluded to be ``necessary'' and it 
may be reasonable and appropriate to consider the stringency of the 
secondary standard in light of what is identified as ``necessary'' for 
the primary standard. The EPA considered the stringency of the 
O3 secondary standard in this way in the 1979 decision (44 
FR 8211, February 8, 1979), which was upheld in subsequent litigation 
(API v Costle, 665 F.2d 1176 [D.C. Cir. 1991]). We note that, in 
similar manner, the commenter considered public welfare protection that 
might be afforded by the primary standard in noting that the primary 
standard would be expected to provide welfare protection from extreme 
values.\211\
---------------------------------------------------------------------------

    \211\ As described earlier in this section, the EPA has also 
considered the air quality specified by one secondary standard in a 
decision on the need for a second secondary standard. In the 
decision not to adopt a second PM2.5 secondary standard 
specific to visibility-related welfare effects, the Administrator, 
after describing the public welfare protection objective related to 
visibility effects, considered analyses that related air quality 
associated with the existing secondary standard to that expected for 
the proposed visibility-focused secondary standard. From these 
analyses, she concluded sufficient protection against visibility 
effects would be provided by the existing standard, and to the 
extent that the existing standard would provide more protection than 
had been her objective for such effects, adoption of a second 
secondary standard focused on visibility would not change that 
result (78 FR 3227-3228, January 15, 2013). This decision responded 
to a court remand of the prior EPA decision that visibility 
protection would be afforded by a secondary standard set equal to 
the primary standard based on the court's conclusion that the EPA 
had not adequately described the Administrator's objectives for 
visibility-related public welfare protection under the standard 
(American Farm Bureau, 559 F.3d at 530-531).
---------------------------------------------------------------------------

    In addressing the remand of the 2008 secondary standard in this 
rulemaking, as discussed in section IV.C.2.e below, the EPA recognizes 
that it must explain the basis for concluding that the standard 
selected by the Administrator specifies air quality that will provide 
the degree of public welfare protection needed from the secondary 
standard (Mississippi v. EPA, 744 F.3d 1334, 1360-61 [D.C. Cir. 2013]). 
In this review, the Administrator describes the degree or level of 
public welfare protection needed from the secondary standard and fully 
explains the basis for concluding that the standard selected specifies 
air quality that will provide that degree of protection. If the 
Administrator concludes that the level of air quality specified by the 
primary standard would provide sufficient protection against known or 
anticipated adverse public welfare effects, the EPA believes that a 
secondary standard with that indicator, level, form and averaging time 
could be considered to be requisite. If the level of air quality that 
areas will need to achieve or maintain for purposes of the primary 
standard also provides a level of air quality that is adequate to 
provide the level of protection identified for the secondary standard, 
there would be little purpose in requiring the EPA to establish a less 
stringent secondary standard. For these reasons, the expectation of 
regionally differing cumulative exposures under a secondary standard of 
the current form and averaging time does not lead us to conclude that 
the air quality specified by such a standard would be more (or less) 
than necessary (and thus not requisite) for the desired level of public 
welfare protection.
e. Revisions to the Standard Level
    Some comments specifically addressed the level for a revised 
secondary standard of the current form and averaging time. Of the 
comments that addressed this, some from states or industry groups 
generally supported a level within the proposed range, frequently 
specifying the upper end of the range (70 ppb), while comments

[[Page 65402]]

from tribes and tribal organizations, and a few others, recommended a 
level no higher than 65 ppb. The Administrator has considered such 
comments in reaching her decision on the appropriate revisions to the 
standard, described in section IV.C.3. Detailed aspects of these 
comments are discussed in the Response to Comments document.
f. 2013 Court Remand and Levels of Protection
    Both industry groups and a group of environmental advocacy 
organizations submitted comments on the extent to which the proposal 
addressed the July 2013 remand of the secondary standard by the U.S. 
Court of Appeals for the D.C. Circuit. The former generally concluded 
that the proposal had adequately addressed the remand, while the latter 
expressed the view that the EPA had failed to comply with the court's 
remand because it had failed to identify the target levels of 
vegetation protection for which the proposed range of standards would 
provide the requisite protection, claiming that the identified W126 
index range of 13-17 ppm-hrs was not based on a proposed level of 
protection against biomass loss, carbon storage loss, or foliar injury 
that the EPA had identified as requisite for public welfare.
    We agree with the comments that state that we have addressed the 
court's remand. More specifically, with this rulemaking, including 
today's decision and the Administrator's conclusions described in 
section IV.C.3 below, the EPA has fully addressed the remand of the 
2008 secondary O3 standard. In Mississippi v. EPA, the D.C. 
Circuit remanded the 2008 secondary O3 standard to the EPA 
for reconsideration because it had not adequately explained why that 
standard provided the requisite public welfare protection. 744 F.3d 
1334, 1360-61 (D.C. Cir. 2013). In doing so, the court relied on the 
language of CAA section 109(b)(2), and the court's prior decision, 
American Farm Bureau Federation v. EPA, 559 F.3d 512, 528-32 (D.C. Cir. 
2009), which came to the same conclusion for the 2006 secondary 
PM2.5 standard. Both decisions recognize that the plain 
language of section 109(b)(2) requires the EPA to ``specify a level of 
air quality the maintenance of which . . . is requisite to protect the 
public welfare from any known or anticipated adverse effects'' 
(Mississippi, 744 F.3d at 1360 [citing American Farm Bureau, 559 F.3d 
at 530]). Further, explaining that it was insufficient for the EPA 
``merely to compare the level of protection afforded by the primary 
standard to possible secondary standards and to find the two roughly 
equivalent'' (Mississippi, 744 F.3d at 1360), the court rejected the 
EPA's justification for setting the secondary standard equivalent to 
the primary standard because that justification was based on comparing 
the protection from the primary standard to that expected from one 
possible standard with a cumulative, seasonal form (21 ppm-hrs) without 
stating that such a cumulative seasonal standard would be requisite to 
protect welfare or explaining why that would be so. Because the EPA had 
``failed to determine what level of protection was `requisite to 
protect the public welfare'' (Mississippi, 744 F.3d at 1362), the court 
found that the EPA's rationale failed to satisfy the requirements of 
the Act.
    Today's rulemaking both satisfies the requirements of section 
109(b)(2) of the Act and addresses the issues raised in the court's 
remand. In this rulemaking, the Administrator has established a revised 
secondary standard that replaces the remanded 2008 secondary standard. 
In so doing, based on her consideration of the currently available 
evidence and quantitative exposure and air quality information, as well 
as advice from CASAC and input from public comments, the Administrator 
has described the requisite public welfare protection for the secondary 
standard and explained how the standard selected specifies air quality 
that will provide that protection. As explained in detail in IV.C.3 
below, in this review the Administrator is describing the public 
welfare protection she finds requisite in terms of seedling RBL in the 
median species, which serves as a surrogate for a broader array of 
O3 effects at the plant and ecosystem levels. This 
description of the desired protection sufficiently articulates the 
standard that the Administrator is using to evaluate welfare 
protection. Further, the Administrator has considered air quality 
analyses in determining how to achieve the air quality conditions 
associated with the desired protection. Based on these analyses, the 
Administrator is determining that revising the level of the secondary 
standard to 70 ppb, while retaining the current form, averaging time, 
and indicator, specifies a level of air quality that will provide the 
requisite public welfare protection.
    To the extent the comments suggest that the EPA is required in 
establishing a standard to identify a precise and quantified level of 
public welfare protection that is requisite with respect to every 
potentially adverse public welfare impact (e.g., visible foliar injury, 
crop yield loss) that is considered in establishing the standard, we 
disagree. While the D.C. Circuit has required the EPA to 
``qualitatively describe the standard governing its selection of 
particular NAAQS,'' it has 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'' 
(ATA III, 283 F.3d at 369 [internal marks and citations omitted]). That 
is, the EPA must ``engage in reasoned decision-making,'' but is not 
required to ``definitively identify pollutant levels below which risks 
to public health are negligible'' (ATA III, 283 F.3d at 370). This 
principle recognizes that the Act requires the EPA to establish NAAQS 
even when the risks or effects of a pollutant cannot be quantified or 
precisely identified because of scientific uncertainty concerning such 
effects at atmospheric concentrations (ATA III, 283 F.3d at 370). 
Though these decisions specifically address setting a primary standard 
under CAA section 109(b)(1), we believe the same principles apply to 
the parallel provision in section 109(b)(2) governing secondary 
standards. Accordingly, while the EPA recognizes that it must explain 
the basis for concluding that the standard selected by the 
Administrator specifies air quality that will provide the protection 
against adverse effects on public welfare needed from the secondary 
standard (Mississippi v. EPA, 744 F.3d 1334, 1360-61 [D.C. Cir. 2013]), 
the CAA does not require the EPA to precisely quantify the measure of 
protection that is necessary to protect the public welfare in 
establishing a secondary standard. In light of the Administrator's 
description of the desired public welfare protection in IV.C.3 below, 
which has both qualitative and quantitative components, the EPA is not 
required to further reduce this description to a precise, quantitative 
target level of vegetation protection. Moreover, nothing in the CAA or 
in case law requires the EPA to identify a target level of protection 
for any particular public welfare effect, such as vegetation effects, 
but rather leaves the Administrator discretion in judging how to 
describe the public welfare protection that she concludes is requisite. 
In IV.C.3 below, the Administrator explains her reasoning for giving 
primary focus to growth-related effects in describing the requisite 
welfare protection, rather than to other welfare effects such as foliar 
injury, for which there are more uncertainties and less predictability 
with respect to the severity of the effects that would be expected from 
varying O3 exposures in the natural environment

[[Page 65403]]

and the significance of the associated impacts to public welfare.
3. Administrator's Conclusions on Revision
    In reaching her decision on the appropriate revisions to the 
secondary standard, the Administrator has drawn on (1) the ISA 
conclusions regarding the weight of the evidence for a range of welfare 
effects associated with O3 in ambient air, quantitative 
findings regarding air quality and ecosystem exposures associated with 
such effects, and associated limitations and uncertainties; (2) staff 
evaluations in the PA of the evidence summarized in the ISA, the 
exposure/risk information developed in the WREA and analyses of air 
quality monitoring information; (3) additional air quality analyses of 
relationships between air quality metrics based on form and averaging 
time of the current standard and the W126 cumulative seasonal exposure 
index; (4) CASAC advice; and (5) consideration of public comments. 
After giving careful consideration to all of this information, the 
Administrator believes that the conclusions and policy judgments 
supporting her proposed decision remain valid.
    The Administrator concludes it is appropriate to continue to use 
O3 as the indicator for a secondary standard intended to 
address adverse effects to public welfare associated with exposure to 
O3 alone and in combination with related photochemical 
oxidants. In this review, no alternatives to O3 have been 
advanced as being a more appropriate surrogate for ambient 
photochemical oxidants. Advice from CASAC concurs with the 
appropriateness of retaining the current indicator. Thus, as is the 
case for the primary standard (discussed above in section II.C.1), the 
Administrator has decided to retain O3 as the indicator for 
the secondary standard. In so doing, she recognizes that measures 
leading to reductions in ecosystem exposures to O3 would 
also be expected to reduce exposures to other photochemical oxidants.
    In her decision on the other elements of the standard, the 
Administrator has considered the body of evidence and information in a 
systematic fashion, giving appropriate consideration to the important 
findings of the ISA as to the effects of O3 in ambient air 
that may present risks to the public welfare, measures of exposure best 
formulated for assessment of these effects, associated evidence 
regarding ecosystem exposures and air quality associated with such 
effects; judgments regarding the weight to place on strengths, 
limitations and uncertainties of this full body of information; and 
public welfare policy judgments on the appropriate degree of protection 
and the form and level of a revised standard that will provide such 
protection. In reaching her decision, the Administrator recognizes that 
the Act does not require that NAAQS be set at zero-risk or background 
levels, but rather at levels that reduce risk sufficiently to protect 
public welfare from known or anticipated adverse effects. In addition, 
we note that the elements of the standard (indicator, level, form, and 
averaging time) are considered together in assessing the protection 
provided by a new or revised standard, and the EPA's approach for 
considering the elements of a new or revised standard is part of the 
exercise of the judgment of the Administrator.
    As an initial matter, the Administrator recognizes the robustness 
of the longstanding evidence, described in the ISA, of O3 
effects on vegetation and associated terrestrial ecosystems. The newly 
available studies and analyses have strengthened the evidence for the 
current review that provides the foundation for the Administrator's 
consideration of O3 effects, associated public welfare 
protection objectives, and the revisions to the current standard needed 
to achieve those objectives. In light of the extensive evidence base in 
this regard, the Administrator focuses on protection against adverse 
public welfare effects of O3 related effects on vegetation. 
In so doing, she takes note of effects that compromise plant function 
and productivity, with associated effects on ecosystems. She is 
particularly concerned about such effects in natural ecosystems, such 
as those in areas with protection designated by Congress for current 
and future generations, as well as areas similarly set aside by states, 
tribes and public interest groups with the intention of providing 
similar benefits to the public welfare. She additionally recognizes 
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.
    A central issue in this review of the secondary standard, as in the 
last review (completed in 2008), has been consideration of the role for 
a cumulative seasonal exposure index. In the last review, the 
Administrator proposed such an index as one of two options for the form 
of a revised standard. The Administrator's decision in that review was 
to retain the existing form and averaging time, while revising the 
standard level to provide the desired level of protection. As described 
in section IV.A above, this decision was remanded to the EPA in 2013 by 
the DC Circuit. In the current review, the ISA evaluates the evidence 
and concludes that, among the approaches investigated, quantifying 
exposure with a cumulative seasonal index best captures the aspects of 
exposure that relate to effects on vegetation, particularly those 
related to growth and yield. The PA considered this finding both in the 
context of assessing potential impacts, and, conversely, the protection 
from such impacts that might be realized, as well as in the context of 
using a cumulative seasonal exposure index as a form for the secondary 
standard. In the proposal, the Administrator focused on the former 
context, as an exposure index, while additionally soliciting comment on 
use of the index as the form for the revised standard. Advice from 
CASAC, all of which was received prior to the proposal, has largely 
emphasized the latter context, and that was also the focus of some 
comments.
    In considering revisions to the secondary standard that will 
specify a level of air quality to provide the necessary public welfare 
protection, the Administrator focuses on use of a cumulative seasonal 
exposure index, including specifically the W126 index as defined in the 
proposal, for assessing exposure, both for making judgments with regard 
to the potential harm to public welfare posed by conditions allowed by 
various levels of air quality and for making the associated judgments 
regarding the appropriate degree of protection against such potential 
harm. In so doing, the Administrator takes note of the conclusions in 
the ISA and PA, with which the CASAC concurred, that, based on the 
currently available evidence, a cumulative seasonal concentration-
weighted index best captures the aspects of ecosystem exposure to 
O3 in ambient air that impact vegetation. In considering the 
public comments in this area, she notes the broad support for use of 
such a metric as an exposure index, with many additionally supporting 
its use as the form for a revised standard, in light of CASAC advice on 
that point. Thus, based on the substantial support in the evidence and 
CASAC advice, and in consideration of public comments, the 
Administrator concludes that it is appropriate to use such a cumulative 
seasonal concentration-weighted index for purposes of assessing the 
potential

[[Page 65404]]

public welfare risks, and similarly, for assessing the potential 
protection achieved against such risks on a national scale.
    The Administrator has considered conclusions of the ISA and PA, as 
well as advice from CASAC and public comments, regarding different 
cumulative, concentration-weighted metrics, and different temporal 
definitions of aspects of these metrics. The Administrator takes note 
of the PA conclusions in support of the W126 exposure index, recognized 
by the ISA for its strength in weighting potentially damaging 
O3 concentrations that contributes to the advantages it 
offers over other weighted cumulative indices. With regard to the 
relevant definitions for the temporal aspects of this index, 
conclusions in the ISA and PA, and such considerations in the last 
review, have led to a focus on a maximum 3-month, 12-hour index, 
defined by the 3-consecutive-month period within the O3 
season with the maximum sum of W126-weighted hourly O3 
concentrations during the period from 8:00 a.m. to 8:00 p.m. each day 
(as explained in section IV.A.1.c above). The Administrator takes note 
of the support in the ISA and PA, as well as CASAC recommendations for 
consideration of the W126 index defined in this way. While recognizing 
that no one definition of an exposure metric used for the assessment of 
protection for multiple effects at a national scale will be exactly 
tailored to every species or each vegetation type, ecosystem and region 
of the country, as discussed in section IV.C.2 above, the Administrator 
judges that on balance, a W126 index derived in this way, and averaged 
over three years, as discussed below, will be appropriate for such 
purposes.
    In considering the appropriate exposure index to facilitate 
assessment of the level of protection afforded to the public welfare by 
alternative secondary standards in the proposal, the Administrator 
concluded that a 3-year average W126 index was appropriate for these 
purposes. A number of considerations raised in the PA influenced the 
Administrator's conclusion at the time of proposal, in combination with 
public welfare judgments regarding 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 and judgments on the extent to which such effects in such 
areas may be considered adverse to public welfare (79 FR 76347, 75312, 
December 17, 2014,). Some comments were received from the public on 
this aspect of the proposed decision, as discussed in section IV.C.2 
above, and have been considered in the conclusions reached here.
    The Administrator continues to place weight on key aspects raised 
in the PA and summarized in the proposal on the appropriateness of 
considering a 3-year average index. The Administrator notes the PA 
consideration of the potential for multiple consecutive years of 
critical O3 exposures to result in larger impacts on 
forested areas than intermittent occurrences of such exposures due to 
the potential for compounding effects on tree growth. The Administrator 
additionally notes the evidence, as considered in the PA and summarized 
in the proposal, for some perennial species of some effects associated 
with a single year's exposure of a critical magnitude that may have the 
potential for some ``carry over'' of effects on plant growth or 
reproduction in the subsequent season. Further, the Administrator notes 
the occurrence of visible foliar injury and growth or yield loss in 
annual plants or crops associated with exposures of a critical 
magnitude. While the Administrator appreciates that the scientific 
evidence documents the effects on vegetation resulting from individual 
growing season exposures of specific magnitude, including those that 
can affect the vegetation in subsequent years, she is also mindful, 
both of the strengths and limitations of the evidence, and of the 
information on which to base her judgments with regard to adversity of 
effects on the public welfare. The Administrator also recognizes 
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.
    As she did for the proposal, the Administrator has considered 
advice from CASAC in this area, including the CASAC comments that it 
favors a W126-based secondary standard with a single year form, that 
its recommended range of levels relates to such a form, and that a 
lower range (e.g., with 13 ppm-hrs at the upper end) would pertain to a 
3-year form. The Administrator also notes CASAC's recognition that her 
decision on use of a 3-year average over a single-year W126 index may 
be a matter of policy. While recognizing the potential for effects on 
vegetation associated with a single-year exposure, the Administrator 
concludes that use of a 3-year average metric can address the potential 
for adverse effects to public welfare that may relate to shorter 
exposure periods, including a single year.
    While the Administrator recognizes the scientific information and 
interpretations, as well as CASAC advice, with regard to a single-year 
exposure index, she also takes note of uncertainties associated with 
judging the degree of vegetation impacts for annual effects that would 
be adverse to public welfare. Even in the case of annual crops, the 
assessment of public welfare significance is unclear for the reasons 
discussed below related to agricultural practices. The Administrator is 
also mindful of the variability in ambient air O3 
concentrations from year to year, as well as year-to-year variability 
in environmental factors, including rainfall and other meteorological 
factors, that influence the occurrence and magnitude of O3-
related effects in any year, and contribute uncertainties to 
interpretation of the potential for harm to public welfare over the 
longer term. As noted above, the Administrator also recognizes 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. Based on all of these 
considerations, the Administrator recognizes greater confidence in 
judgments related to public welfare impacts based on a 3-year average 
metric. Accordingly, the considerations identified here lead the 
Administrator to conclude it is appropriate to use an index averaged 
across three years for judging public welfare protection afforded by a 
revised secondary standard.
    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 has 
given 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) CASAC's views 
regarding the strength of the evidence and its adequacy to inform 
judgments on public welfare protection. The Administrator recognizes 
that such judgments include judgments about the interpretation of the 
evidence and other information, such as the quantitative analyses of 
air quality monitoring,

[[Page 65405]]

exposure and risk. She also recognizes 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. 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 (as described in section IV.A.3 above). 
The Administrator additionally recognizes 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.
    The Administrator finds the coherence and strength of the weight of 
evidence concerning effects on vegetation from the large body of 
available literature compelling. The currently available evidence 
addresses a broad array of O3-induced effects on a variety 
of tree species across a range of growth stages (i.e., seedlings, 
saplings and mature trees) using diverse field-based (e.g., free air, 
gradient and ambient) and OTC exposure methods. The Administrator gives 
particular attention to the effects related to native tree growth and 
productivity, recognizing their relationship to a range of ecosystem 
services, including forest and forest community composition. She is 
also mindful of the significance of community composition changes, 
particularly in protected areas, such as Class I areas. At the same 
time, she recognizes, while the evidence strongly supports conclusions 
regarding O3 impacts on growth and the evidence showing 
effects on tree seedlings, as well as on older trees, there are 
limitations in our ability to predict impacts in the environment or to 
estimate air quality or exposures that will avoid such impacts. Such 
limitations relate to the variability of environmental factors or 
characteristics that can influence the extent of O3 effects.
    In recognition of the CASAC advice and the potential for adverse 
public welfare effects, the Administrator has considered the nature and 
degree of effects of O3 on the public welfare. In so doing, 
the Administrator recognizes that the significance to the public 
welfare of O3-induced effects on sensitive vegetation 
growing within the U.S. can vary, depending on the nature of the 
effect, the intended use of the sensitive plants or ecosystems, and the 
types of environments in which the sensitive vegetation and ecosystems 
are located. Any given O3-related effect on vegetation and 
ecosystems (e.g., biomass loss, visible foliar injury), therefore, may 
be judged to have a different degree of impact on the public depending, 
for example, on whether that effect occurs in a Class I area, a 
residential or commercial setting, or elsewhere. The Administrator 
notes that such a distinction is supported by CASAC advice in this 
review. In her judgment, like those of the Administrator in the last 
review, it is appropriate that this variation in the significance of 
O3-related vegetation effects should be taken into 
consideration in making judgments with regard to the level of ambient 
O3 concentrations that is requisite to protect the public 
welfare from any known or anticipated adverse effects. As a result, the 
Administrator concludes that of those known and anticipated 
O3-related vegetation and ecosystem effects identified and 
discussed in this notice, particular significance should be ascribed to 
those that may occur on sensitive species that are known to or are 
likely to occur in federally protected areas such as Class I areas or 
on lands set aside by states, tribes and public interest groups to 
provide similar benefits to the public welfare, for residents on those 
lands, as well as visitors to those areas.
    Likewise, the Administrator also notes that less protection related 
to growth effects may be called for in the case of other types of 
vegetation or vegetation associated with other uses or services. For 
example, the maintenance of adequate agricultural crop yields is 
extremely important to the public welfare and currently involves the 
application of intensive management practices. With respect to 
commercial production of commodities, the Administrator notes that 
judgments about the extent to which O3-related effects on 
commercially managed vegetation are adverse from a public welfare 
perspective are particularly difficult to reach, given that the 
extensive management of such vegetation (which, as CASAC noted, may 
reduce yield variability) may also to some degree mitigate potential 
O3-related effects. The management practices used on these 
lands are highly variable and are designed to achieve optimal yields, 
taking into consideration various environmental conditions. In 
addition, changes in yield of commercial crops and commercial 
commodities, such as timber, may affect producers and consumers 
differently, further complicating the question of assessing overall 
public welfare impacts. Thus, the Administrator concludes, while 
research on agricultural crop species remains useful in illuminating 
mechanisms of action and physiological processes, information from this 
sector on O3-induced effects is considered less useful in 
informing judgments on what specific standard would provide the 
appropriate public welfare protection. In so doing, the Administrator 
notes that a standard revised to increase protection for forested 
ecosystems would also be expected to provide some increased protection 
for agricultural crops and other commercial commodities, such as 
timber.
    The Administrator also recognizes that O3-related 
effects on sensitive vegetation can occur in other areas that have not 
been afforded special federal or other protections, including effects 
on vegetation growing in managed city parks and residential or 
commercial settings, such as ornamentals used in urban/suburban 
landscaping or vegetation grown in land use categories involving 
commercial production of commodities, such as timber. For vegetation 
used for residential or commercial ornamental purposes, the 
Administrator believes that there is not adequate information at this 
time to establish a secondary standard based specifically on impairment 
of these categories of vegetation, but notes that a secondary standard 
revised to provide protection for sensitive natural vegetation and 
ecosystems would likely also provide some degree of protection for such 
vegetation.
    Based on the above considerations, in identifying the appropriate 
level of protection for the secondary standard, the Administrator finds 
it appropriate to focus on sensitive trees and other native species 
known or anticipated to occur in protected areas such as Class I areas 
or on other lands set aside by the Congress, states, tribes and public 
interest groups to provide similar benefits to the public welfare, for 
residents on those lands, as well as visitors to those areas. In light 
of their public welfare significance, the Administrator gives 
particular weight to protecting such vegetation and ecosystems. Given 
the reasons for the special protection afforded such areas (identified 
in section I.A.3 above), she recognizes the importance of protecting 
these natural forests from O3-induced impacts, including 
those related to O3 effects on growth, and including those 
extending in scale from individual plants to the ecosystem. The 
Administrator also recognizes that the impacts identified for 
O3 range from those for which the public welfare 
significance may be more easily judged, but for which quantitative 
relationships

[[Page 65406]]

with O3 in ambient air are less well established, such as 
impacts on forest community composition in protected wilderness areas, 
carbon storage and other important ecosystem services, to specific 
plant-level effects, such as growth impacts (in terms of RBL) in tree 
seedlings, for which our quantitative estimates are more robust.
    For considering the appropriate public welfare protection objective 
for a revised standard, the Administrator finds appropriate and useful 
the estimates of tree seedling growth impacts (in terms of RBL) 
associated with a range of W126-based index values developed from the 
robust E-R functions for 11 tree species, that were described in the PA 
and proposal and are summarized in Table 4 above. In making judgments 
based on those observations, however, the Administrator has considered 
the broader evidence base and public welfare implications, including 
associated strengths, limitations and uncertainties. Thus, in drawing 
on estimates from this table, she is not making judgments simply about 
a specific magnitude of growth effect in seedlings that would be 
acceptable or unacceptable in the natural environment. Rather, the 
Administrator is using the estimates in the table, as suggested by 
CASAC and emphasized by some commenters, as a surrogate or proxy for 
consideration of the broader array of vegetation-related effects of 
potential public welfare significance, that include effects on growth 
of individual sensitive species and extend to ecosystem-level effects, 
such as community composition in natural forests, particularly in 
protected public lands, as well as forest productivity. In so doing, 
she notes that CASAC similarly viewed biomass loss as ``a 
scientifically valid surrogate of a variety of adverse effects to 
public welfare'' (Frey, 2014c, p. 10). Thus, in considering the 
appropriate level of public welfare protection for the revised 
standard, the Administrator gives primary attention to the relationship 
between W126 exposures and estimates of RBL in tree seedlings in Table 
4, finding this to be a useful quantitative tool to inform her 
judgments in this matter.
    In considering the RBL estimates in Table 4 above (drawn from the 
final PA), the Administrator takes note of comments from CASAC that 
also give weight to these relationships in formulating its advice and 
notes the CASAC comments on specific RBL values (Frey, 2014c). In so 
doing, she considers and contrasts comments and their context on RBL 
estimates of 2% and 6% for the median studied species.
    With regard to the CASAC advice regarding 2% RBL for the median 
studied tree species, the Administrator notes, as an initial matter, 
the unclear basis for such a focus, as described in section IV.C.2 
above and in the proposal. Further, she notes that the CASAC advice 
related to this RBL value was that it would be appropriate for the 
range of levels identified in the PA for the Administrator's 
consideration to ``include[] levels that aim for not greater than 2% 
RBL for the median tree species'' (Frey, 2014c, p. 14). As described in 
the proposal, the range identified in the PA, which the Administrator 
considered, extended down to W126 index levels for which the estimated 
RBL in the median tree species is less than or equal to 2%, consistent 
with the CASAC advice. In addition, the Administrator notes that only 
the lowest portion of this range (7-8 ppm-hrs) corresponds to an 
estimated RBL for the median tree species of less than or equal to 2%, 
with the remainder of CASAC's range (up to 15 ppm-hrs) associated with 
higher median RBL estimates. Thus, the Administrator understands CASAC 
to have identified 2% RBL for the median tree species as a benchmark 
falling within, and at one end of, the range of levels of protection 
that the CASAC considers appropriate for the revised standard to 
provide. However, the fact that the CASAC range included levels for 
which the RBL estimates were appreciably greater than 2% indicates that 
CASAC did not judge it necessary that the revised standard be based on 
the 2% RBL benchmark. Accordingly, the Administrator proposed revisions 
to the secondary standard based on options related to higher RBL 
estimates and associated exposures. After also considering public 
comments, the Administrator continues to consider the uncertainty 
regarding the extent to which associated effects on vegetation at lower 
O3 exposures would be adverse to public welfare to be too 
great to provide a foundation for public welfare protection objectives 
for a revised secondary standard.
    With regard to the CASAC comments on a 6% RBL estimate, the 
Administrator takes particular note of their characterization of this 
level of effect in the median studied species as ``unacceptably high'' 
(Frey, 2014c, pp. iii, 13, 14). These comments were provided in the 
context of CASAC's considering the significance of effects associated 
with a range of alternatives for the secondary standard. Moreover, the 
range recommended by CASAC excluded W126 index values for which the 
median species was estimated to have a 6% RBL,\212\ based on the 
information before CASAC at the time (Frey, 2014c, p. 12-13). 
Accordingly, the EPA interprets these comments regarding 6% RBL to be 
of a different nature than the CASAC advice regarding a 2% median RBL, 
both because these two comments are framed to address different 
questions and because CASAC treated them differently in its recommended 
range.
---------------------------------------------------------------------------

    \212\ As summarized in IV.C.2 above (and noted in section IV.E.3 
of the proposal), revisions to this table in the final PA, made in 
consideration of other CASAC comments, have resulted in changes to 
the median species RBL estimates such that the median species RBL 
estimate for a W126 index value of 17 ppm-hrs in this table in the 
final PA (5.3%) is nearly identical to the median species estimate 
for 15 ppm-hrs (the value corresponding to the upper end of the 
CASAC-identified range) in the second draft PA (5.2%), the review of 
which was the context for CASAC's advice on this point (Frey, 
2014c). The median RBL estimate ranges from 5.3% to 3.8% across the 
range of W126 exposures (17 ppm-hrs to 13 ppm-hrs) that the 
Administrator proposed to conclude would provide the appropriate 
public welfare protection for a revised secondary standard.
---------------------------------------------------------------------------

    In the Administrator's consideration of the RBL estimates to inform 
judgments on O3 exposures of concern to public welfare and 
the appropriate protection that the secondary standard should provide 
from such exposures, she has given particular consideration to the 
current evidence for the relationship of reduced growth of sensitive 
tree species with ecosystem effects (as described in the ISA), CASAC's 
view of 6% RBL for the median studied species as unacceptably high, and 
the role of the Administrator's judgments regarding public welfare 
impacts of effects in specially protected natural systems, such as 
Class I areas. With regard to a point of focus among the median RBL 
estimates extending below 6% for purposes of judging the appropriate 
public welfare protection objectives for a revised secondary standard, 
the Administrator is mindful of the CASAC advice to consider lower 
levels if using a 3-year average, rather than annual, W126 index value.
    In considering the CASAC advice, the Administrator notes that her 
judgments on a 3-year average index focus on the level of confidence in 
conclusions that might be drawn with regard to single as compared to 
multiple year impacts, as described above. For example, the 
Administrator, while recognizing the strength of the evidence with 
regard to quantitative characterization of O3 effects on 
growth of tree seedlings and crops, and in addition to noting the 
additional difficulties for assessing the welfare impacts of 
O3 on crops, takes note of the uncertainty associated with

[[Page 65407]]

drawing conclusions with regard to the extent to which small percent 
reductions in annual growth contribute to adverse effects on public 
welfare and the role of annual variability in environmental factors 
that affect plant responses to O3. Moreover, as explained 
above, the Administrator concludes that concerns related to the 
possibility of a single unusually damaging year, inclusive of those 
described by the CASAC, can be addressed through use of a 3-year 
average metric. Thus, similar to the CASAC's view that a lower level 
would be appropriate with a 3-year form, the Administrator considers it 
appropriate to focus on a standard that would generally limit 
cumulative exposures to those for which the median RBL estimate would 
be somewhat lower than 6%.
    In focusing on cumulative exposures associated with a median RBL 
estimate somewhat below 6%, the Administrator considers the 
relationships in Table 4, noting that the median RBL estimate is 6% for 
a cumulative seasonal W126 exposure index of 19 ppm-hrs. Considering 
somewhat lower values, the median RBL estimate is 5.7% (which rounds to 
6%) for a cumulative seasonal W126 exposure index of 18 ppm-hrs and the 
median RBL estimate is 5.3% (which rounds to 5%) for 17 ppm-hrs. In 
light of her decision that it is appropriate to use a 3-year cumulative 
exposure index for assessing vegetation effects (described above), 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 concludes it is 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. In 
reaching this conclusion, based on the current information to inform 
consideration of vegetation effects and their potential adversity to 
public welfare, she additionally judges that the RBL estimates 
associated with 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.
    While giving primary consideration to growth effects using the 
surrogate of RBL estimates based on tree seedling effects, the 
Administrator also recognizes the longstanding and robust evidence of 
O3 effects on crop yield. She takes note of CASAC 
concurrence with the PA description of such effects as of public 
welfare significance and agrees. As recognized in the proposal, the 
maintenance of adequate agricultural crop yields is extremely important 
to the public welfare. Accordingly, research on agricultural crop 
species remains important for further illumination of mechanisms of 
action and physiological processes. Given that the extensive management 
of such vegetation, which as CASAC noted may reduce yield variability, 
may also to some degree mitigate potential O3-related 
effects, however, judgments about the extent to which O3-
related effects on crop yields are adverse from a public welfare 
perspective are particularly difficult to reach. Further, management 
practices for agricultural crops are highly variable and generally 
designed to achieve optimal yields, taking into consideration various 
environmental conditions. As a result of this extensive role of 
management in optimizing crop yield, the Administrator notes the 
potential for greater uncertainty with regard to estimating the impacts 
of O3 exposure on agricultural crop production than that 
associated with O3 impacts on vegetation in natural forests. 
For all of these reasons, the Administrator is not giving the same 
weight to CASAC's statement regarding crop yield loss as a surrogate 
for adverse effects on public welfare, or the magnitude that would 
represent an adverse impact to public welfare, as to the CASAC's 
comments on RBL as a surrogate for an array of growth-related effects. 
Similarly, given the considerations summarized above and in the 
proposal, the Administrator concludes that agricultural crops do not 
have the same need for additional protection from the NAAQS as forested 
ecosystems and finds protection of public welfare from crop yield 
impacts to be a less important consideration in this review for the 
reasons identified, including the extensive management of crop yields 
and the dynamics of agricultural markets. Thus, the Administrator is 
not giving a primary focus to crop yield loss in selecting a revised 
secondary standard. She notes, however, that a standard revised to 
increase protection for forested ecosystems would also be expected to 
provide some increased protection for agricultural crops.
    The Administrator has additionally considered the evidence and 
analyses of visible foliar injury. In so doing, the Administrator notes 
the ISA conclusion that ``[e]xperimental evidence has clearly 
established a consistent association of visible injury with 
O3 exposure, with greater exposure often resulting in 
greater and more prevalent injury'' (U.S. EPA, 2013, section 9.4.2, p. 
9-41). The Administrator also recognizes the potential for this effect 
to affect the public welfare in the context of affecting values 
pertaining to natural forests, particularly those afforded special 
government protection, as discussed in section IV.A.3 above. However, 
she recognizes significant challenges in judging the specific extent 
and severity at which such effects should be considered adverse to 
public welfare, in light of the variability in the occurrence of 
visible foliar injury and the lack of clear quantitative relationships 
with other effects on vegetation, as well as the lack of established 
criteria or objectives that might inform consideration of potential 
public welfare impacts related to this vegetation effect.
    Further, the Administrator takes note of the range of evidence on 
visible foliar injury and the various related analyses, including 
additional observations drawn from the WREA biosite dataset in response 
to comments, as summarized in section IV.C.2 above. In so doing, she 
does not agree with CASAC's comment that a level of W126 exposure below 
10 ppm-hrs is required to reduce foliar injury, noting some lack of 
clarity in the WREA and PA presentations of the WREA cumulative 
proportion analysis findings and their meaning (described in section 
IV.C.2.b above). She notes that the additional observations summarized 
in section IV.C.2 above indicate declines in proportions of sites with 
any visible foliar injury and biosite index scores with reductions in 
cumulative W126 exposure across a range of values extending at the high 
end well above 20 ppm-hrs, down past and including 17 ppm-hrs. In 
considering this information, however, the Administrator takes note of 
the current lack of robust exposure-response functions that would allow 
prediction of visible foliar injury severity and incidence under 
varying air quality and environmental conditions, as recognized in 
section IV.A.1.b above. Thus, while the Administrator notes that the 
evidence is not conducive to use for identification of a specific 
quantitative public welfare protection objective, due to uncertainties 
and complexities described in sections IV.A.1.b and IV.A.3 above, she 
concludes that her judgments above, reached with a focus on RBL 
estimates, would also be expected to provide an additional

[[Page 65408]]

desirable degree of protection against visible foliar injury in 
sensitive vegetation. Accordingly, she considers a conclusion on the 
appropriateness of selecting a standard that will generally limit 
cumulative exposures above 17 ppm-hrs to be additionally supported by 
evidence for visible foliar injury, while not based on specific 
consideration of this effect.
    With the public welfare protection objectives identified above in 
mind, the Administrator turns to her consideration of form and level 
for the revised secondary standard. In considering whether the current 
form should be retained or revised in order to provide the appropriate 
degree of public welfare protection, the Administrator has considered 
the analyses of air quality data from the last 13 years that describe 
the cumulative exposures, in terms of a 3-year W126 index, occurring at 
monitoring sites across the U.S. when the air quality metric at that 
location, in terms of the current standard's form and averaging time, 
is at or below different alternative levels. The Administrator notes 
both the conclusions drawn from analyses of the strong, positive 
relationship between these metrics and the findings that indicate the 
amount of control provided by the fourth-high metric.
    The Administrator has also considered advice from CASAC and public 
commenters that support revision of the form to the W126 exposure 
index. The Administrator concurs with the underlying premise that 
O3 effects on vegetation are most directly assessed using a 
cumulative seasonal exposure index, specifically the W126 exposure 
index. The Administrator additionally recognizes, based on analyses of 
the last 13 years of monitoring data, and consideration of modeling 
analyses with associated limitations and uncertainties, that cumulative 
seasonal exposures appear to have a strong relationship with design 
values based on the current form and averaging time. She additionally 
notes the correlation of reductions in W126 index values with 
reductions in precursor emissions over the past decade that were 
targeted at meeting the current O3 standards (with fourth-
high form), which indicate the control of cumulative seasonal exposures 
that can be achieved with a standard of the current form and averaging 
time.
    With regard to recommendations from the CASAC that the form for the 
revised secondary standard should be the biologically relevant exposure 
metric, and related comments from the public indicating that the 
secondary standard must have such a form, the Administrator disagrees. 
In so doing, she notes that CAA section 109 does not impose such a 
requirement on the form or averaging time for the NAAQS, as explained 
in IV.C.2 above. She further notes that the averaging time and form of 
primary standards are often not the same as the exposure metrics used 
in reviews of primary standards, in which specific information on 
quantitative relationships between different exposure metrics and 
health risk is more often available than it is in reviews of secondary 
NAAQS. As discussed in section IV.C.2 above, with examples, a primary 
standard with a particular averaging time and form may provide the 
requisite public health protection from health effects that are most 
appropriately assessed using an exposure metric of a different 
averaging time and form and indicator, and the same principle can apply 
when establishing or revising secondary standards. The Administrator 
recognizes that the exposure metric and the standard metric can be 
quite similar, as in the case of consideration of short-term health 
effects with the primary O3 standard. She also notes, 
however, as illustrated by the examples described in section IV.C.2 
above, that it is not uncommon for the EPA to retain or adopt elements 
of an existing standard that the Administrator judges in combination 
across all elements, including in some cases a revised level, to 
provide the requisite protection under the Act, even if those elements 
do not neatly correspond to the exposure metric. Accordingly, she 
concludes that the Act does not require that the secondary 
O3 standard be revised to match the exposure metric 
identified as biologically relevant in this review, as long as the 
revised standard provides the degree of protection required under CAA 
section 109(b)(2).
    Based on the considerations described here, including the use of an 
exposure metric that CASAC has agreed to be biologically relevant and 
appropriate, related considerations summarized in the proposal with 
regard to air quality analyses and common uses of exposure metrics in 
other NAAQS reviews, the Administrator finds that, in combination with 
a revised level, the current form and averaging time for a revised 
secondary standard can be expected to provide the desired level of 
public welfare protection. Accordingly, she next turns to the important 
consideration of a level that, in combination with the form and 
averaging time, will yield a standard that specifies the requisite air 
quality for protection of public welfare. In so doing, she has 
recognized the recommendation by CASAC for revision of the form and 
averaging time and provided the basis for her alternative view, as 
described above. Further, in the context of the Administrator's 
decision on objectives for public welfare protection of a revised 
secondary standard, and with consideration of the advice from CASAC on 
levels for a W126-based standard, the Administrator has also reached 
the conclusion, as described above, that in order to provide the 
appropriate degree of public welfare protection, the revised secondary 
standard should restrict cumulative seasonal exposures to 17 ppm-hrs or 
lower, in terms of a 3-year average W126 index, in nearly all 
instances. Thus, the Administrator finds it appropriate to revise the 
standard level to one that, in combination with the form and averaging 
time, will exert this desired degree of control for cumulative seasonal 
exposures.
    In considering a revised standard level, the Administrator has, in 
light of public comments, revisited the information she considered in 
reaching her proposed decision on a level within the range of 65 to 70 
ppb, and additional information or insights conveyed with public 
comments. The primary focus of the Administrator's considerations in 
reaching her proposed decision was the multi-faceted analysis of air 
quality data from 2001 through 2013 documented in the technical memo in 
the docket (Wells, 2014a), as well as the earlier analyses and related 
information described in the PA (as summarized in section IV.E.4 of the 
proposal). This analysis describes the occurrences of 3-year W126 index 
values of a magnitude from 17 ppm-hrs through 7 ppm-hrs at monitor 
locations where O3 concentrations met different alternative 
standards with the current form and averaging time, and has been 
expanded in consideration of public comments to present in summary form 
the more extensive historical dataset accompanying this analysis 
(Wells, 2015b). Focusing first on the air quality analyses for the most 
recent period for which data are available (2011-2013) and with the 
protection objectives identified above in mind, the Administrator 
observes that across the sites meeting the current standard of 75 ppb, 
the analysis finds 25 sites distributed across different NOAA climatic 
regions with 3-year average W126 index values above 17 ppm-hrs, with 
the values at nearly half of the sites extending above 19 ppm-hrs, with 
some well above. In comparison, she observes that across sites meeting 
an alternative

[[Page 65409]]

standard of 70 ppb, the analysis for the period from 2011-2013 finds no 
occurrences of W126 metric values above 17 ppm-hrs and less than a 
handful of occurrences that equal 17 ppm-hrs. The more than 500 
monitors that would meet an alternative standard of 70 ppb during the 
2011-2013 period are distributed across all nine NOAA climatic regions 
and 46 of the 50 states (Wells, 2015b and associated dataset in the 
docket).
    The Administrator notes that some public commenters, who disagreed 
with her proposed decision on form and averaging time, emphasized past 
occurrences of cumulative W126 exposure values above the range 
identified in the proposal (of 13 to 17 ppm-hrs). For example, these 
commenters emphasize data from farther back across the full time period 
of the dataset analyzed in the technical memorandum (2001-2013), 
identifying a value of 19.1 ppm-hrs at a monitor for which the fourth-
high metric is 70 ppb for the 3-year period of 2006-2008. The 
Administrator notes, as discussed in section IV.C.2 above, that this 
was one of fewer than a handful of isolated occurrences of sites for 
which the fourth-high was at or below 70 ppb and the W126 index value 
was above 17 ppm-hrs, all but one of which were below 19 ppm-hrs. The 
Administrator additionally recognizes her underlying objective of a 
revised secondary standard that would limit cumulative exposures in 
nearly all instances to those for which the median RBL estimate would 
be somewhat lower than 6%. She observes that the single occurrence of 
19 ppm-hrs identified by the commenter among the nearly 4000 3-year 
W126 index values from across the most recently available 11 3-year 
periods of data at monitors for which the fourth-high metric is at or 
below 70 ppb is reasonably regarded as an extremely rare and isolated 
occurrence (Wells, 2015b). As such, it is unclear whether it would 
recur, particularly as areas take further steps to reduce O3 
to meet revised primary and secondary standards. Further, based on the 
currently available information, the Administrator does not judge RBL 
estimates associated with marginally higher exposures in isolated, rare 
instances to be indicative of adverse effects to the public welfare. 
Thus, the Administrator concludes that a standard with a level of 70 
ppb and the current form and averaging time may 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.
    The Administrator recognizes that any standard intended to exert a 
very high degree of control on cumulative seasonal exposures, with the 
objective of limiting exposures above 17 ppm-hrs across the U.S., in 
nearly all instances, will, due to regional variation in meteorology 
and sources of O3 precursors, result in cumulative seasonal 
exposures well below 17 ppm-hrs in many areas. Even implementation of a 
standard set in terms of the cumulative seasonal exposure metric, while 
limiting the highest exposures, would, due to regional variation in 
meteorology and sources of O3 precursors, result in many 
areas with much lower exposures. Such variation in exposures occurring 
under a specific standard is not unexpected and the overall 
distribution of exposures estimated to occur with air quality 
conditions associated with different alternative standards is a routine 
part of the consideration of public health protection in reviews of 
primary standards, and can also play a role in the review of secondary 
standards. For these reasons, and in light of the discussion in section 
IV.C.2.d above on consideration of ``necessary'' protection, the 
Administrator notes that an expectation of differing exposures is not, 
in itself, a basis for concluding that the air quality would be more 
(or less) than necessary (and thus not requisite) for the desired level 
of public welfare protection.
    The Administrator has also considered the protection afforded by a 
revised standard against other effects studied in this review, such as 
visible foliar injury and reduced yield for agricultural crops, and 
also including those associated with climate change. While noting the 
evidence supporting a relationship of O3 in ambient air with 
climate forcing effects, as concluded in the ISA, the Administrator 
judges the quantitative uncertainties to be too great to support 
identification of a standard specific to such effects such that she 
concludes it is more important to focus, as she has done above, on 
setting a standard based on providing protection against vegetation-
related effects which would be expected to also have positive 
implications for climate change protection through the protection of 
ecosystem carbon storage.
    The Administrator additionally considers the extent of control for 
cumulative seasonal exposures exerted by a revised standard level of 65 
ppb, the lower end of the proposed range. In focusing on the air 
quality analyses for the most recent 3-year period for which data are 
available, the Administrator observes that across the sites meeting a 
fourth-high metric of 65 ppb, the analysis finds no occurrences of W126 
metric values above 11 ppm-hrs and 35 occurrences of a value between 7 
ppm-hrs and 11 ppm-hrs, scattered across NOAA climatic regions. The 
Administrator finds these magnitudes of cumulative seasonal exposures 
to extend appreciably below the objectives she identified above for 
affording public welfare protection. In considering this alternative 
level, she additionally notes that data for only 276 monitors (less 
than 25 percent of the total with valid fourth-high and W126 metric 
values) were at or below a fourth-high value of 65 ppb during the 
period from 2011-2013. In so noting, she recognizes the appreciably 
smaller and less geographically extensive dataset available and the 
associated uncertainty for conclusions based on such an analysis.
    Thus, based on the support provided by currently available 
information on air quality, the evidence base of O3 effects 
on vegetation and her public welfare policy judgments, and after 
carefully taking the above comments and considerations into account, 
fully considering the scientific views of the CASAC, and also taking 
note of CASAC's policy views, the Administrator has decided to retain 
the current indicator, form and averaging time and to revise the 
secondary standard level to 70 ppb. In the Administrator's judgment, 
based on the currently available evidence and quantitative exposure and 
air quality information, a standard set at this level, in combination 
with the currently specified form, averaging time and indicator would 
be requisite to protect the public welfare from known or anticipated 
adverse effects. A standard set at this level provides an appreciable 
increase in protection compared to the current standard. The 
Administrator judges that such a 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 believes that a standard set at 70 ppb would be 
sufficient to protect public welfare from known or anticipated adverse 
effects and believes that a lower standard would be more than what is 
necessary to provide such protection. This judgment by the 
Administrator appropriately recognizes

[[Page 65410]]

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. 
Accordingly, the Administrator concludes that it is appropriate to 
revise the level for the secondary standard to 70 ppb (0.070 ppm), in 
combination with retaining the current form, indicator, and averaging 
time, in order to specify the level of air quality that provides the 
requisite protection to the public welfare from any known or 
anticipated adverse effects associated with the presence of 
O3 in the ambient air.

D. 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 and 
recommendations of CASAC, and the public comments, as well as public 
welfare judgments, the Administrator is revising the level of the 
current secondary standard. Specifically, the Administrator has decided 
to revise the level of the secondary standard to a level of 0.070 ppm, 
in conjunction with retaining the current indicator, averaging time and 
form. Accordingly the revised secondary standard is 0.070 ppm 
O3, as the annual fourth-highest daily maximum 8-hour 
average concentration, averaged over three years.

V. Appendix U: Interpretation of the Primary and Secondary NAAQS for 
O3

A. Background

    The EPA is finalizing the proposed Appendix U to 40 CFR part 50: 
Interpretation of the Primary and Secondary National Ambient Air 
Quality Standards for Ozone. The proposed Appendix U addressed the 
selection of ambient O3 monitoring data to be used in making 
comparisons with the NAAQS, data reporting and data handling 
conventions for comparing ambient O3 monitoring data with 
the level of the NAAQS, and data completeness requirements. The EPA 
solicited public comment on four elements where the proposed Appendix U 
differed from Appendix P to 40 CFR part 50, which addressed data 
handling conventions for the previous O3 NAAQS. These 
included the following: (1) the addition of a procedure to combine data 
collected from two or more O3 monitors operating 
simultaneously at the same physical location, (2) the addition of a 
provision allowing the Regional Administrator to approve ``site 
combinations'', or the combination of data from two nearby monitoring 
sites for the purpose of calculating a valid design value, (3) a change 
from the use of one-half of the method detection limit (\1/2\ MDL) to 
zero (0.000 ppm) as the substitution value in 8-hour average data 
substitution tests, and 4) a new procedure for calculating daily 
maximum 8-hour average O3 concentrations for the revised 
NAAQS.
    The EPA is also finalizing, as proposed, exceptional events 
scheduling provisions in 40 CFR 50.14 that will apply to the submission 
of information supporting claimed exceptional events affecting 
pollutant data that are intended to be used in the initial area 
designations for any new or revised NAAQS. The new scheduling 
provisions will apply to initial area designations for the 2015 
O3 NAAQS.

B. Data Selection Requirements

    The EPA proposed this section in Appendix U to clarify which data 
are to be used in comparisons with the revised O3 NAAQS. The 
EPA is finalizing this section in Appendix U as proposed.
    First, the EPA proposed to combine data at monitoring sites with 
two or more O3 monitoring instruments operating 
simultaneously into a single site-level data record for determining 
compliance with the NAAQS, and proposed an analytical approach to 
perform this combination (79 FR 75351-75352, December 17, 2014). 
Several commenters supported the EPA's proposed approach, including the 
State of Iowa, where 15 of the 20 monitoring sites currently operating 
two O3 monitors simultaneously are located. Commenters 
supporting the proposal noted that a similar approach is already being 
used for lead and particulate monitoring, and that the proposed 
approach will help states meet data completeness requirements.
    A few commenters supported the EPA's proposed approach with the 
additional restrictions that the monitoring instruments must use 
identical methods and be operated by the same monitoring agency. The 
EPA notes that at the time of this rulemaking, all monitors reporting 
O3 concentration data to the EPA for regulatory use were 
FEMs. All current O3 FEMs use an ultraviolet photometry 
sampling methodology and have been found to meet the performance 
criteria in 40 CFR part 53. Therefore, the EPA has no reason to believe 
that O3 concentration data should not be combined across 
monitoring methods at the site level. Regarding the commenters' 
suggestion that data should not be combined when two or more monitors 
at the same site are operated by different monitoring agencies, the EPA 
is aware of only one instance where this presently occurs. In this 
instance, the monitors have been assigned distinct site ID numbers in 
the AQS database, so that data will not be combined across these 
monitors. Should future instances arise where two or more monitoring 
agencies decide to operate O3 monitors at the same site, the 
EPA encourages these agencies to work together to establish a plan for 
how the data collected from these monitors should be used in regulatory 
decision making.
    One state objected to combining data across monitors because the 
secondary monitors at their sites were used only for quality assurance 
purposes and data from these monitors should not be combined with data 
reported from the primary monitors. The EPA notes that concentration 
data collected to meet quality assurance requirements (i.e. precision 
and bias data) are reported and stored in a separate location within 
the AQS database and are not used for determining compliance with the 
NAAQS. The required quality assurance data are derived from 
O3 standards and not from a separate O3 monitor. 
However, if a separate O3 monitor is used strictly for 
quality assurance purposes and does not meet the applicable monitoring 
requirements, it can be distinguished in AQS in such a manner that data 
from the secondary monitor would not be combined with data from the 
primary monitor.
    Another commenter objected to the proposal because it would reduce 
the total number of comparisons made with the NAAQS. While this is 
true, the number of physical locations being compared with the NAAQS 
will not decrease under the proposed approach, and in fact may increase 
due to additional sites meeting the data completeness requirements.
    Finally, two commenters submitted similar comments citing the EPA's 
evaluation of collocated O3 monitoring data and precision 
data in the ISA (U.S. EPA, 2013, section 3.5.2), and stated that 
although the median differences in concentrations reported by the pairs 
of monitoring instruments were near zero, the extreme values were close 
to +/- 3.5%. The commenter argued that since the O3 NAAQS 
are based on the fourth-highest annual value, data should not be 
combined across monitors because of the imprecision in the extreme 
values. The EPA disagrees, noting that the data presented in the ISA 
are based on hourly concentrations, while design values for the 
O3 NAAQS are based on a 3-year average of 8-hour average 
concentrations. Thus, the random variability in the hourly 
O3 concentration data due to monitoring

[[Page 65411]]

imprecision will be reduced when concentrations are averaged for 
comparison with the NAAQS. Additionally, the precision data are 
typically collected at concentrations at or above the level of the 
NAAQS, thus the EPA expects that the level of precision documented in 
the ISA analysis is consistent with the level of precision in the 
fourth-highest daily maximum concentrations used for determining 
compliance with the NAAQS.
    The EPA is finalizing this addition in Appendix U as proposed. In 
addition, the AQS database will be updated to require state agencies to 
designate a primary monitor at O3 monitoring sites that 
report data under more than one Pollutant Occurrence Code (POC), a 
numeric indicator in AQS used to identify individual monitoring 
instruments. O3 design value calculations in AQS will be 
updated so that the data will automatically be combined across POCs at 
a site, and a single design value will be reported for each site. The 
EPA notes that the substitution approach described above will only be 
applied to design value calculations for the revised O3 
standards, and that design values for previous O3 standards 
will continue to be calculated at the monitor level, in accordance with 
the applicable appendices of 40 CFR part 50.
    Second, the EPA proposed to add a provision in Appendix U that 
would allow the Regional Administrator to approve ``site 
combinations'', or to combine data across two nearby monitors for the 
purpose of calculating a valid design value. Although data handling 
appendices for previous O3 standards do not explicitly 
mention site combinations, the EPA has approved over 100 site 
combinations since the promulgation of the first 8-hour O3 
NAAQS in 1997. Thus, the EPA's intention in proposing this addition was 
merely to codify an existing convention, and to improve transparency by 
implementing site combinations in AQS design value calculations.
    Public commenters unanimously supported this proposed addition. Two 
commenters suggested that the EPA should require monitoring agencies to 
provide technical documentation supporting the similarities between 
sites approved for combining data, including a requirement for 
simultaneous monitoring whenever possible. One state requested that the 
EPA provide more detailed acceptability criteria for approving site 
combinations, while another state urged the EPA not to create a 
regulatory burden by prescribing detailed requirements codified in 
regulations.
    The EPA is finalizing this addition as proposed in Appendix U. The 
EPA believes that approval of site combinations should be handled on a 
case-by-case basis, and that any requests for supporting documentation 
should be left to the discretion of the Regional Administrator. The EPA 
may issue future guidance providing general criteria for determining an 
acceptable level of similarity in air quality concentrations between 
monitored locations, but is not prescribing detailed criteria for 
approval of site combinations in this rulemaking.
    Additionally, the AQS database will be updated with new fields for 
monitoring agencies to request site combinations, and an additional 
field indicating Regional Administrator approval. All pre-existing site 
combinations will be initially entered into the database as having 
already been approved by the Regional Administrator. Since this 
provision has already been used in practice under previous 
O3 standards, site combinations will be applied to AQS 
design value calculations for both the revised O3 standards 
and previous O3 standards.

C. Data Reporting and Data Handling Requirements

    First, the EPA proposed a change in Appendix U to the pre-existing 
8-hour average data substitution test (40 CFR part 50, Appendix P, 
section 2.1) which is used to determine if a site would have had a 
valid 8-hour average greater than the NAAQS when fewer than 6 hourly 
O3 concentration values are available for a given 8-hour 
period. The EPA proposed to change the value substituted for the 
missing hourly concentrations from one-half of the method detection 
limit of the O3 monitoring instrument (\1/2\ MDL) to zero 
(0.000 ppm).
    Several commenters supported the proposed change, stating that the 
use of a constant substitution value instead of \1/2\ MDL, which can 
vary across O3 monitoring methods, would simplify design 
value calculations. One commenter noted that with a substitution value 
of zero, the data substitution test for an 8-hour average value greater 
than the NAAQS is equivalent to a sum of hourly O3 
concentrations greater than 0.567 ppm (i.e., if the sum is 0.568 ppm or 
higher, the resulting 8-hour average must be at least 0.071 ppm, which 
is greater than the revised O3 NAAQS of 0.070 ppm). Finally, 
one commenter opposed the proposed change in favor of some type of 
mathematical or statistical interpolation approach, but did not provide 
a specific recommendation.
    The EPA is finalizing the proposed change in Appendix U, with the 
addition of a short clause making note of the equivalent summation 
approach described above. The purpose of the data substitution test is 
to identify 8-hour periods that do not meet the requirements for a 
valid 8-hour average, yet the reported hourly concentration values are 
so high that the NAAQS would have been exceeded regardless of the 
magnitude of the missing concentration values. The EPA believes that 
zero, being the lowest measured O3 concentration physically 
possible, is the most appropriate value to substitute in this 
situation. Additionally, the EPA does not support the use of 
interpolation or other means of filling in missing monitoring data for 
O3 NAAQS comparisons. Such an approach would be contrary to 
the EPA's long-standing policy of using only quality-assured and 
certified ambient air quality measurement data to determine compliance 
with the O3 NAAQS.
    Second, the EPA proposed a new procedure in Appendix U for 
determining daily maximum 8-hour O3 concentrations for the 
revised NAAQS.\213\ The EPA proposed to determine the daily maximum 8-
hour O3 concentration based on 17 consecutive moving 8-hour 
periods in each day, beginning with the 8-hour period from 7:00 a.m. to 
3:00 p.m., and ending with the 8-hour period from 11:00 p.m. to 7:00 
a.m. In addition, the EPA proposed that a daily maximum value would be 
considered valid if 8-hour averages were available for at least 13 of 
the 17 consecutive moving 8-hour periods, or if the daily maximum value 
was greater than the level of the NAAQS. This procedure is designed to 
eliminate ``double counting'' exceedances of the NAAQS based on 
overlapping 8-hour periods from two consecutive days with up to 7 hours 
in common, which was allowed under previous 8-hour O3 NAAQS. 
A dozen public commenters expressed support for the proposed procedure, 
including several states.
---------------------------------------------------------------------------

    \213\ This procedure will be adopted only for the revised 
O3 NAAQS. Design values for the 1997 8-hour O3 
NAAQS and the 2008 8-hour O3 NAAQS will continue to be 
calculated according to Appendix I and Appendix P of 40 CFR part 50, 
respectively.
---------------------------------------------------------------------------

    One regional air quality management organization and three of its 
member states submitted similar comments stating that they agreed with 
the principle of eliminating ``double counting'' exceedances of the 
NAAQS

[[Page 65412]]

based on overlapping 8-hour periods, but suggested an alternative 
calculation procedure that would accomplish the same objective. The 
alternative procedure iteratively finds the highest 8-hour period in a 
given year, then removes this 8-hour period and all other 8-hour 
periods associated with that day, including any overlapping 8-hour 
periods on adjacent days, from the data until a daily maximum value is 
determined for each day of the year with sufficient monitoring data. 
The EPA examined a similar iterative procedure in a previous data 
analysis supporting the proposal (Wells, 2014b, Method 1). The EPA 
compared this procedure to the procedure proposed by the commenters 
using the data from the original analysis and found the resulting daily 
maximum 8-hour values to be nearly identical (Wells, 2015a). 
Additionally, the commenters' procedure suffers from the same 
limitations the EPA identified previously in the original analysis: 
added complexity in design value calculations, longer computational 
time, and challenges to real-time O3 data reporting systems, 
which would have to re-calculate daily maximum 8-hour values for the 
entire year each time the system was updated with new data.
    Three states submitted comments stating that they agreed with the 
proposed calculation procedure, but disagreed with the proposed 
requirements for determining a valid daily maximum 8-hour O3 
concentration. These states were primarily concerned that the proposed 
requirements would only allow a monitoring site to have four missing 8-
hour averages during a day before the entire day would be invalidated, 
compared with six missing 8-hour averages allowed previously. Two of 
these states also stated concerns that the proposed requirements would 
be more difficult to meet while maintaining compliance with existing 
monitoring requirements such as biweekly quality assurance checks. The 
EPA compared annual data completeness rates calculated using the 
Appendix U requirements to annual data completeness rates calculated 
using the requirements under the previous O3 standards 
across all U.S. monitoring sites based on data from 2004-2013 (Wells, 
2015a). The national mean annual data completeness rate was 0.1% higher 
under the proposed Appendix U requirements than under the previous 
O3 standards, and the national median annual data 
completeness rates were identical. In addition, the EPA notes that the 
Appendix U requirements allow for biweekly quality assurance checks and 
other routine maintenance to be performed between 5:00 a.m. and 9:00 
a.m. local time without affecting data completeness. Thus, the EPA does 
not believe that the proposed daily data completeness requirements in 
Appendix U will be more difficult for monitoring agencies to meet.
    Finally, two public commenters opposed the proposed procedures for 
determining daily maximum 8-hour concentrations. These commenters 
expressed similar concerns, primarily that not considering 8-hour 
periods starting midnight to 6:00 a.m. is less protective of public 
health than the procedure used to determine daily maximum 8-hour 
concentrations for the previous O3 standards. The EPA 
believes that this approach provides the appropriate degree of 
protection for public health, noting that the hourly concentrations 
from midnight to 7:00 a.m. are covered under the 8-hour period from 
11:00 p.m. to 7:00 a.m., which is included in the design value 
calculations proposed in Appendix U. At the same time, the proposed 
approach ensures that individual hourly concentrations may not 
contribute to multiple exceedances of the NAAQS, which the EPA believes 
is inappropriate given that people are only exposed once.
    The EPA is finalizing as proposed in Appendix U the procedure for 
determining daily maximum 8-hour concentrations. The EPA does not 
believe that daily maximum 8-hour concentrations for two consecutive 
days should be based on overlapping 8-hour periods, since the exposures 
experienced by individuals only occur once. The EPA believes that the 
new procedure will avoid this outcome while continuing to make use of 
all hourly concentrations in determining attainment of the standards, 
without introducing unnecessary complexity into design value 
calculations, and without creating additional difficulties for 
monitoring agencies to meet the data completeness requirements.

D. Exceptional Events Information Submission Schedule

    The ``Treatment of Data Influenced by Exceptional Events; Final 
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events 
Rule and codified at 40 CFR 50.14, contains generic deadlines for an 
air agency to submit to the EPA specified information about exceptional 
events and associated air pollutant concentration data. As discussed in 
this section and in more detail in the O3 NAAQS proposal, 
without revisions to 40 CFR 50.14, an air agency may not be able to 
flag and submit documentation for some relevant data either because the 
generic deadlines may have already passed by the time a new or revised 
NAAQS is promulgated or because the generic deadlines require 
submission of documentation at least 12 months prior to the date by 
which the EPA must make a regulatory decision, which may be before air 
agencies have collected some of the potentially affected data. Specific 
to the revised O3 NAAQS, revisions to 40 CFR 50.14 are 
needed because it is not possible for air agencies to flag and submit 
documentation for any exceptional events that occur in October through 
December of 2016 by 1 year before the designations are made in October 
2017, as is required by the existing generic schedule.
    The EPA is finalizing exceptional events scheduling provisions in 
40 CFR 50.14, as proposed and as supported by multiple commenters, that 
will apply to the submission of information supporting claimed 
exceptional events affecting pollutant data that are intended to be 
used in the initial area designations for any new or revised NAAQS. The 
new scheduling provisions will apply to initial area designations for 
the revised O3 NAAQS. The provisions that we are 
promulgating use a ``delta schedule'' that calculates the timelines 
associated with flagging data potentially influenced by exceptional 
events, submitting initial event descriptions and submitting 
exceptional events demonstrations based on the promulgation date of a 
new or revised NAAQS. The general data flagging deadlines in the 
Exceptional Events Rule at 40 CFR 50.14(c)(2)(iii) and the general 
schedule for submission of demonstrations at 40 CFR 50.14(c)(3)(i) 
continue to apply to data used in regulatory decisions other than those 
related to the initial area designations process under a new or revised 
NAAQS.\214\
---------------------------------------------------------------------------

    \214\ The EPA intends to consider changes to these retained 
scheduling requirements as part of the planned notice and comment 
rulemaking revisions to the 2007 Exceptional Events Rule.
---------------------------------------------------------------------------

    The EPA acknowledges the concern raised by several commenters that 
a strengthened O3 NAAQS may result in numerous 
demonstrations for exceptional events occurring between 2014 and 2016, 
the data years that the EPA will presumably use for initial area 
designation decisions made in October 2017.\215\ Commenters noted that 
the proposed schedule is particularly burdensome for agencies needing 
to submit exceptional events packages for

[[Page 65413]]

the third year to be used in a 3-year design value (i.e., 2016 data). 
Several commenters recommended that the EPA either establish no defined 
schedule for data flagging and exceptional events demonstration 
submittal or allow a minimum of 2 years from the setting of any new or 
revised NAAQS for air agencies to provide a complete exceptional events 
demonstration. Given the CAA requirement that the EPA follow a 2-year 
designations schedule, the EPA cannot remove submittal schedules 
entirely for data influenced by exceptional events or provide a minimum 
2-year period from the setting of a new or revised NAAQS for 
documentation submittal. Neither of these options would ensure that the 
EPA has time to consider event-influenced data in initial area 
designation decisions. Rather, the EPA is promulgating in this action 
an exceptional events schedule that provides air agencies with the 
maximum amount of time available to prepare exceptional events 
demonstrations and will still allow the EPA sufficient time to consider 
such exceptional events demonstrations in the designations process in 
advance of the date by which the EPA must send 120-day notification 
letters to states.\216\ The EPA recognizes that the schedule 
promulgated in this action is compressed, particularly for the third 
year of data to be used in a 3-year design value, and we will work 
cooperatively with air agencies to accommodate this scenario.
---------------------------------------------------------------------------

    \215\ Governors may also use 2013 data to formulate their 
recommendations regarding designations.
    \216\ See Section VIII.B for additional detail on the initial 
area designations process for the revised O3 NAAQS.
---------------------------------------------------------------------------

    Under the schedule promulgated in this action and assuming initial 
area designation decisions in October 2017 for the revised 
O3 NAAQS, affected air agencies would need to flag data, 
submit initial event descriptions and submit demonstrations for 
exceptional events occurring in 2016 by May 31, 2017. This schedule 
provides approximately 5 months between the EPA's receipt of the 
demonstration package and the expected date of designation decisions 
and approximately 1 month between the EPA's receipt of a package and 
the date by which the EPA must notify states and tribes of intended 
modifications to the Governors' recommendations for designations (i.e., 
120-day letters).
    While, for the third year of data anticipated to be used in a 3-
year design value for the revised O3 NAAQS, the promulgated 
schedule provides for demonstration submission 5 months after the end 
of the calendar year, the EPA expects that most submitting agencies 
will have additional time to prepare documentation as we expect the 
majority of potential O3-related exceptional events to occur 
during the warmer months (e.g., March through October). Additionally, 
the EPA will soon propose rule revisions to the 2007 Exceptional Events 
Rule and will release through a Federal Register Notice of Availability 
a draft guidance document to address Exceptional Events Rule criteria 
for wildfires that could affect O3 concentrations. We expect 
to promulgate Exceptional Events Rule revisions and finalize the new 
guidance document before the October 2016 date by which states, and any 
tribes that wish to do so, are required to submit their initial 
designation recommendations for the revised O3 NAAQS. 
Considered together, the EPA believes the exceptional events scheduling 
dates promulgated in this action, the upcoming Exceptional Events Rule 
revisions, the forthcoming guidance, and the existing guidance and 
examples of submitted demonstrations currently on the EPA's exceptional 
events Web site at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events, will help air agencies submit 
information in a timely manner.
    Applying the ``delta schedule'' promulgated in this action for air 
quality data collected in 2013 through 2014 that could be influenced by 
exceptional events and be considered during the initial area 
designations process for the revised O3 NAAQS, results in 
extending to July 1, 2016, the otherwise applicable generic deadlines 
of July 1, 2014, and July 1, 2015, respectively, for flagging data and 
providing an initial description of an event (40 CFR 50.14(c)(2)(iii)). 
The schedule promulgated in this action also results in a July 1, 2016, 
date for flagging data and providing an initial description of an event 
for air quality data collected in 2015. The July 1, 2016, date for data 
collected in 2015 is the same as that which would apply under the 
existing generic deadline in the 2007 Exceptional Events Rule. Under 
the schedule promulgated in this action, October 1, 2016 is the 
deadline for submitting exceptional events demonstrations for data 
years 2013 through 2015. As noted previously, under the schedule 
promulgated in this action, affected air agencies would need to flag, 
submit initial event descriptions and submit demonstrations for 
exceptional events occurring in 2016 by May 31, 2017. The EPA believes 
these revisions will provide adequate time for air agencies to review 
potential O3 exceptional events influencing compliance with 
the revised O3 NAAQS, to notify the EPA by flagging the 
relevant data and providing an initial event description in AQS, and to 
submit documentation to support exceptional events demonstrations. The 
schedule revisions promulgated in this action will also allow the EPA 
to consider and act on the submitted information during the initial 
area designation process.
    While the EPA will make every effort to designate areas for any new 
or revised NAAQS on a 2-year schedule, the EPA recognizes that under 
some circumstances we may need up to an additional year for the 
designations process to ensure that air agencies and the EPA base 
designations decisions on complete and sufficient information. The 
promulgated schedule accounts for the possibility that the EPA might 
announce after promulgating a new or revised NAAQS that we are 
extending the designations schedule beyond 2 years using authority 
provided in CAA section 107(d)(B)(i). If the EPA determines that we 
will follow a 3-year designation schedule, the deadline is 2 years and 
7 months after promulgation of a new or revised NAAQS for states to 
flag data influenced by exceptional events, submit initial event 
descriptions and submit exceptional events demonstrations for the last 
year of data that will be used in the designations (e.g., if the EPA 
were to designate areas in October 2018, the exceptional events 
submittal deadline for 2017 data would be May 31, 2018). If the EPA 
notifies states and tribes of a designations schedule between 2 and 3 
years, the deadline for states to flag data affected by exceptional 
events, submit initial event descriptions, and submit exceptional 
events demonstrations associated with data from the last year to be 
considered would be 5 months prior to the date specified for 
designation decisions.
    Therefore, using the authority provided in CAA section 319(b)(2) 
and in the 2007 Exceptional Events Rule at 40 CFR 50.14(c)(2)(vi), the 
EPA is modifying the schedule for flagging data and submitting 
exceptional events demonstrations considered for initial area 
designations by replacing the deadlines and information in Table 1 in 
40 CFR 50.14 with the deadlines and information presented in Table 5. 
As we did in the O3 NAAQS proposal, we are also providing 
Table 6 to illustrate how the promulgated schedule might apply to the 
designations process for the revised O3 NAAQS and to 
designations

[[Page 65414]]

processes for other future new or revised NAAQS.\217\
---------------------------------------------------------------------------

    \217\ The range of dates identified in Table 6 is illustrative 
of the dates for the revised O3 NAAQS. Users could 
increment these dates by any constant number (for example by 6 years 
for a hypothetical NAAQS promulgated in 2021) to develop a table 
with dates relevant to NAAQS promulgated in the future.
---------------------------------------------------------------------------

    Additionally, in conjunction with promulgating exceptional events 
schedules for initial area designations for new or revised NAAQS, the 
EPA, as proposed, is removing obsolete regulatory language in 40 CFR 
50.14(c)(2)(iv) and (v) and 40 CFR 50.14(c)(3)(ii) and (iii) associated 
with exceptional events schedules for all historical standards.

  Table 5--Schedule for Flagging and Documentation Submission for Data
  Influenced by Exceptional Events for Use in Initial Area Designations
------------------------------------------------------------------------
                                           Exceptional events deadline
  Exceptional events/Regulatory action             schedule \d\
------------------------------------------------------------------------
Flagging and initial event description   If state and tribal initial
 deadline for data years 1, 2 and 3 \a\.  designation recommendations
                                          for a new/revised NAAQS are
                                          due August through January,
                                          then the flagging and initial
                                          event description deadline
                                          will be the July 1 prior to
                                          the recommendation deadline.
                                          If state and tribal
                                          recommendations for a new/
                                          revised NAAQS are due February
                                          through July, then the
                                          flagging and initial event
                                          description deadline will be
                                          the January 1 prior to the
                                          recommendation deadline.
Exceptional events demonstration         No later than the date that
 submittal deadline for data years 1, 2   state and tribal
 and 3 \a\.                               recommendations are due to the
                                          EPA.
Flagging, initial event description and  By the last day of the month
 exceptional events demonstration         that is 1 year and 7 months
 submittal deadline for data year 4 \b\   after promulgation of a new/
 and, where applicable, data year 5 \c\.  revised NAAQS, unless either
                                          option a or b applies.
                                         a. If the EPA follows a 3-year
                                          designation schedule, the
                                          deadline is 2 years and 7
                                          months after promulgation of a
                                          new/revised NAAQS.
                                         b. If the EPA notifies the
                                          state/tribe that it intends to
                                          complete the initial area
                                          designations process according
                                          to a schedule between 2 and 3
                                          years, the deadline is 5
                                          months prior to the date
                                          specified for final
                                          designations decisions in such
                                          EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
  considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
  consider when it makes final area designations for a new/revised NAAQS
  under the standard designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
  consider when it makes final area designations for a new/revised NAAQS
  under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
  quality monitoring data in AQS is annually on May 1 of the year
  following the year of data collection as specified in 40 CFR
  58.15(a)(2). In some cases, however, air agencies may choose to
  certify a prior year's data in advance of May 1 of the following year,
  particularly if the EPA has indicated its intent to promulgate final
  designations in the first 8 months of the calendar year. Data
  flagging, initial event description and exceptional events
  demonstration deadlines for ``early certified'' data will follow the
  deadlines for ``year 4'' and ``year 5'' data.


[[Page 65415]]

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[[Page 65416]]

VI. Ambient Monitoring Related to O3 Standards

A. Background

    The EPA proposed to revise the state-by-state O3 
monitoring seasons; the PAMS monitoring requirements; the FRM for 
measuring O3; and the FEM performance requirement 
specifications for automated O3 analyzers. The EPA also 
proposed to make additional minor changes to the FEM analyzer 
performance testing requirements for NO2 and particulate 
matter in part 53.
    The EPA is finalizing changes to the length of the required 
O3 monitoring season for 32 states and the District of 
Columbia. Section VI.B of this preamble provides an overview of the 
proposed changes to the length of the required O3 monitoring 
seasons, a summary of significant public comments and our responses, 
and a summary of the final decisions made to the O3 
monitoring seasons for each state.
    The EPA is finalizing changes to the PAMS monitoring requirements 
in 40 CFR part 58, Appendix D Section 5. Section VI.C of this preamble 
provides background on the PAMS program and current monitoring 
requirements, a summary of the proposed changes to the PAMS 
requirements, a summary of significant public comments and our 
responses, and a summary of the changes to the PAMS requirements in 
this final rule.
    The EPA is finalizing changes to the FRM for O3 in 
Section VI.D of this preamble and to the associated FEM performance 
requirement specifications for automated O3 analyzers in 
Section VI.E. A summary of significant public comments and our 
responses are provided and a summary of the final changes to the FRM 
and FEM requirements in this final rule. The EPA is also finalizing 
minor additional changes to Part 53 including conforming changes to the 
FEM performance testing requirements in Table B-1 and Figure B-5 for 
NO2; extending the period of time for the Administrator to 
take action on a request for modification of a FRM or FEM from 30 days 
to 90 days in part 53.14; and removing an obsolete provision for 
manufacturers to submit Product Manufacturing Checklists for fine and 
coarse particulate matter monitors in part 53.9.

B. Revisions to the Length of the Required O3 Monitoring 
Seasons

    Unlike the ambient monitoring requirements in 40 CFR part 58 for 
other criteria pollutants that mandate year-round monitoring at State 
and Local Air Monitoring Stations (SLAMS), O3 monitoring is 
only required during the seasons of the year that are conducive to 
O3 formation. These seasons vary in length from place-to-
place as the conditions conducive to the formation of O3 
(i.e., seasonally-dependent factors such as ambient temperature, 
strength of solar insolation, and length of day) differ by location. In 
some locations, conditions conducive to O3 formation are 
limited to the summer months of the year. In other states with warmer 
climates (e.g., California, Nevada, and Arizona), the currently 
required O3 season is year-round. Elevated levels of winter-
time O3 have also been measured in some western states where 
precursor emissions can interact with sunlight off the snow cover under 
very shallow, stable boundary layer conditions (U.S. EPA 2013).
    The EPA has determined that the proposed lengthening of the 
O3 monitoring seasons in 32 states and the District of 
Columbia is appropriate. Ambient O3 concentrations in these 
areas could approach or exceed the level of the NAAQS, more frequently 
and during more months of the year compared with the current season 
lengths. It is important to monitor for O3 during the 
periods when ambient concentrations could approach the level of the 
NAAQS to ensure that the public is informed when exposure to 
O3 could reach or has reached a level of concern.
    The EPA completed an analysis to address whether extensions of 
currently required monitoring seasons are appropriate (Rice, 2014). In 
this analysis, we used all available data in AQS, including data from 
monitors that collected O3 data year-round during 2010-2013. 
More than half of O3 monitors are voluntarily operated on a 
year-round basis by monitoring agencies. We determined the number of 
days where one or more monitors had a daily maximum 8-hour 
O3 average equal to or above 0.060 ppm in the months outside 
each state's current O3 monitoring season and the pattern of 
those days in the out-of-season months. We believe that a threshold of 
0.060 ppm, taking into consideration reasonable uncertainty, serves as 
an appropriate indicator of ambient conditions that may be conducive to 
the formation of O3 concentrations that approach or exceed 
the NAAQS. We also considered regional consistency, particularly for 
those states with little available data. We note that seasonal 
O3 patterns vary year-to-year due primarily to highly 
variable meteorological conditions conducive to the formation of 
elevated O3 concentrations early or late in the season in 
some years and not others. The EPA believes it is important that 
O3 monitors operate during all periods when there is a 
reasonable possibility of ambient levels approaching the level of the 
NAAQS.
    Basing O3 monitoring season requirements on the goal of 
ensuring monitoring when ambient O3 levels approach or 
exceed the level of the NAAQS supports established monitoring network 
objectives described in Appendix D of Part 58, including the 
requirement to provide air pollution data to the general public in a 
timely manner \218\ and to support comparisons of an area's air 
pollution levels to the NAAQS. The operation of O3 monitors 
during periods of time when ambient levels approach or exceed the level 
of the NAAQS ensures that unusually sensitive people and sensitive 
groups are alerted to O3 levels of potential health concern 
allowing them to take precautionary measures. The majority of 
O3 monitors in the U.S. report to AIRNOW,\219\ as well as to 
state-operated Web sites and automated phone reporting systems. These 
programs support many objectives including real-time air quality 
reporting to the public, O3 forecasting, and the 
verification of real-time air quality forecast models.
---------------------------------------------------------------------------

    \218\ Public reporting requirements are detailed in 40 CFR part 
58 Appendix G, Uniform Air Quality Index (AQI) and Daily Reporting.
    \219\ See http://airnow.gov/.
---------------------------------------------------------------------------

1. Proposed Changes to the Length of the Required O3 
Monitoring Seasons
    The EPA proposed to extend the length of the required O3 
monitoring season in 32 states and the District of Columbia. The 
proposed changes were an increase of one month for 22 states 
(Connecticut, Delaware, Idaho, Illinois, Iowa, Kansas, Maryland, 
Massachusetts, Minnesota, Missouri, Nebraska, New Hampshire, New 
Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, 
South Carolina, Texas (northern portion only), Virginia, and West 
Virginia) and the District of Columbia, an increase of one and one half 
months for Wisconsin, an increase of two months for four states 
(Indiana, Michigan, Montana, and North Dakota), an increase of four 
months for Florida and South Dakota, an increase of five months for 
Colorado, and an increase of seven months for Utah. For Wyoming, we 
proposed to add three months at the beginning of the season and remove 
one month at the end of the season, resulting in a net increase of two 
months. Ozone season requirements are currently split by Air Quality 
Control Region (AQCR) in Louisiana and Texas. We proposed lengthening 
the required season in the northern part of Texas (AQCR 022, 210,

[[Page 65417]]

211, 212, 215, 217, and 218) by one month and leaving the year-round 
O3 season in the southern part of Texas (AQCRs 106, 153, 
213, 214, and 216) unchanged. No changes were proposed for the AQCRs in 
Louisiana. As noted earlier, in a few states with limited available 
data and few exceedance days outside the currently-required season 
(Iowa, Missouri, and West Virginia), the proposed changes were made by 
considering supporting information from the surrounding states. These 
changes involved the proposed addition of one month (March) to the 
currently-required O3 seasons for these states.
    The EPA also proposed that O3 monitors at all National 
Core Multipollutant Monitoring Stations (NCore) be operated year-round, 
January through December, regardless of the length of the required 
O3 season for the remainder of the SLAMS within each state.
    We noted that the EPA Regional Administrators have previously 
approved deviations from the required O3 monitoring seasons 
as allowed by paragraph 4.1(i) of 40 CFR part 58, Appendix D. We 
proposed to retain the rule language permitting such deviations from 
the required O3 monitoring seasons, but note that finalized 
changes to O3 monitoring season requirements would revoke 
all existing Regional Administrator-granted waiver approvals. As 
appropriate, monitoring agencies could seek new approvals for seasonal 
deviations. Any seasonal deviations based on the Regional 
Administrator's waiver of requirements must be described in the state's 
annual monitoring network plan and updated in the AQS.
    Given the timing of the final rulemaking and any associated burden 
on state/local monitoring agencies to implement the extended 
O3 seasons, we proposed that implementation of the revised 
O3 seasons would become effective at SLAMS (including NCore 
sites) on January 1, 2017. We solicited comment on whether the revised 
seasons could be implemented beginning January 1, 2016, for all 
monitors or for a subset of monitors, such as those currently operating 
year-round or on a schedule that corresponds to the proposed 
O3 season.
2. Comments on the Length of the Required O3 Monitoring 
Seasons
    We received several comments on the proposed revisions to 
O3 monitoring seasons. Several commenters supported the 
proposed O3 season length changes and agreed that 
O3 monitoring seasons should reflect the times of year when 
O3 may approach or exceed the level of the NAAQS. A few 
commenters noted the complexities that would arise in the 
implementation of multi-state planning agreements if states that shared 
an MSA had different required O3 monitoring seasons. Two 
state agencies that supported season length changes also recommended 
changes to neighboring states' O3 seasons. New York 
recommended that Connecticut's proposed O3 season be further 
extended (adding the month of October) to match the proposed season in 
New York (March-October) because they share a major MSA and 
nonattainment area, and the highest design value monitor in the 
nonattainment area is often in Connecticut. The results from the EPA's 
analysis did not support the addition of October for Connecticut. The 
EPA recognizes that there may be value in having a consistent 
O3 season across multi-state planning areas. We recommend 
that monitoring agency representatives from New York and Connecticut 
contact their respective EPA Regional Office to jointly develop a 
monitoring plan to provide coverage of the MSA for a longer period of 
time. Consistent with the results from the EPA's analysis and 
consistent with our proposal, the EPA is finalizing the March-October 
season in New York and the March-September season in Connecticut.
    Although no changes were proposed for Arkansas, the Arkansas 
Department of Environmental Quality recommended that the O3 
season in the nonattainment area that includes Crittenden County, 
Arkansas (March-November) be consistent with the O3 seasons 
in Tennessee (March-October) and Mississippi (March-October) by either 
shortening the O3 season in Arkansas or lengthening the 
O3 season by one month in Tennessee and Mississippi. Based 
on the results from the EPA's analysis and consistent with our 
proposal, the EPA is not finalizing any changes to the current 
O3 seasons in Arkansas, Tennessee, or Mississippi. There is 
currently one monitor operating in Crittenden County. We recommend that 
Arkansas work with their EPA Regional Administrator to consider a 
waiver for the monitor(s) in Crittenden County to allow a deviation 
(shortened season) from the required O3 season if the agency 
demonstrates that such a deviation is appropriate for consistency in 
the nonattainment area.
    Two commenters noted the need to extend seasons to capture 
wintertime O3 events. One commenter urged the EPA to extend 
monitoring to year-round in the intermountain west (specifically 
Wyoming) to adequately capture summer and winter O3 problem 
days and noted especially two monitors in the Pinedale area of Wyoming 
that should be operated year-round. The EPA's analysis showed that 
there were no days that were >= 0.060 ppm in Wyoming for the months of 
October-December and that the Wyoming Department of Environmental 
Quality is currently operating about 70% of their O3 
monitors year-round including all O3 monitors in Sublette 
County, which includes the Pinedale area. Another commenter supported 
lengthening the seasons for states in the western U.S. where wintertime 
O3 could be an issue in light of the unique and growing 
O3 pollution problems caused by oil and gas development 
activities. They also recommended that the EPA expand the O3 
monitoring season to year-round for North Dakota, South Dakota, and 
Montana beyond what was proposed. The number of observed days that were 
>= 0.060 ppm in the months outside the season proposed for these states 
(one day for North Dakota and no days observed for South Dakota and 
Montana) do not support a further extension to the length of the 
O3 monitoring season beyond what was proposed. These states 
are already operating a large percentage of their monitors year-round 
(89% in North Dakota, 100% in South Dakota, and 78% in Montana). The 
EPA is finalizing the seasons as proposed in Wyoming (January-
September), North Dakota (March-September), South Dakota (March-
October), and Montana (April-September). The EPA encourages these 
states to continue year-round operation of their monitors to determine 
what areas are affected by elevated levels of winter-time 
O3.
    The commenters who opposed lengthening the O3 monitoring 
seasons noted concerns with the threshold (0.060 ppm) used as the basis 
for the changes and the length of time (2010-2013) for which ambient 
data were retrieved and analyzed. Many of those with concerns 
recommended that levels in the proposed range (e.g., 0.065 ppm or 0.070 
ppm) or the current NAAQS level of 0.075 ppm be used as the appropriate 
threshold for determining the O3 season. With regard to the 
0.060 ppm threshold used, this value is consistent with the 85 percent 
threshold used to require additional O3 monitoring based on 
Appendix D requirements, which include the MSA population and design 
value.\220\ As noted previously, year-to-year variability occurs in 
seasonal O3 patterns based on highly variable and 
unpredictable meteorological

[[Page 65418]]

conditions, which can support the formation of early or late season 
elevated O3 concentrations in some years and not in other 
years. This threshold serves as an appropriate indicator of ambient 
conditions that may be conducive to the formation of O3 
concentrations that approach or exceed the level of the NAAQS.
---------------------------------------------------------------------------

    \220\ See 40 CFR part 58, appendix D, Table D-2.
---------------------------------------------------------------------------

    Certain logistical complexities were noted if longer seasons were 
required, including site access during winter and the challenge of 
getting the monitoring equipment ready in time. Four states noted 
concerns with operator safety and anticipated their inability to access 
sites due to early spring snowfall. The EPA agrees that site access 
could be an issue depending on weather conditions and notes that 
specific site monitoring season deviations may be appropriate. We 
suggest that this be addressed through the monitoring season waiver 
process with the EPA Regional Administrator. Any deviations based on 
the Regional Administrator's waiver of requirements must be described 
in the state's annual monitoring network plan and updated in AQS.
    Several commenters had concerns about the additional cost and 
resources needed to expand the O3 monitoring seasons. There 
was some disagreement with the EPA's total annual average cost estimate 
of $230,000 which took into account the number of O3 
monitors already operating year-round across the country. Commenters 
noted specifically that the proposed extension of required monitoring 
seasons would increase operational costs and potentially impact the 
resources available for other monitoring efforts. The added cost of 
operating O3 monitors over a longer period was noted by some 
commenters, referencing both the cost of staff to operate the monitors, 
as well as the additional wear and tear those O3 monitors 
would experience over a longer operational period. They noted that 
extending their required monitoring season by adding the month of March 
would increase staffing requirements for monitor operation and quality 
assurance. They also noted that the life expectancy of equipment would 
be reduced due to increased wear and tear. The EPA acknowledges that 
operational costs for O3 monitoring networks will 
incrementally increase in states where required seasons have been 
lengthened. We encourage monitoring agencies to review available 
technology and operational procedures to institute practices that could 
potentially reduce such costs, such as the automation of quality 
control and calibration checks and remote access to evaluate monitor 
operations. As noted earlier, all states operated at least a portion of 
their O3 monitoring network outside of the required 
O3 season during the 2010-2013 data period and reported the 
data to AQS. In addition, many states are operating more than the 
minimum number of monitors required to support the basic monitoring 
objectives described in 40 CFR part 58, Appendix D. Some states have a 
large percentage of their total O3 monitors operating 
outside the currently-required O3 season and some states 
have a small percentage. In situations where states are already 
operating a large number of their O3 monitors outside their 
current O3 season, the actual cost increase will be less. In 
cases where states have a small number of monitors operating outside 
their current O3 season, in addition to automation and 
remote access, those states could investigate with their Regional 
Administrator the process in 40 CFR part 58.14 for reducing the total 
number of operating monitors that are above the number required by 40 
CFR, part 58, appendix D to offset the cost of extending the 
O3 monitoring season in their state.
    Two commenters had concerns about the 4-year period of time 
evaluated in the EPA's analysis and noted that the 4-year period of 
time evaluated does not take into account meteorological anomalies and 
other weather induced situations and is not consistent with the 3 years 
used to calculate design values. One state agency's comments referenced 
their own analysis showing concentrations going back 20 years. They 
noted that 2010 was an unusual year and inclusion of such an unusual 
year in the 4-year period (2010-2013) of the EPA's analysis provides 
too much weight on those data. As noted earlier, year-to-year 
variability occurs in seasonal O3 patterns based on variable 
meteorological conditions and given the impracticality of forecasting 
such conditions that affect O3 photochemistry, the EPA 
believes it is important that O3 monitors operate when there 
is a reasonable possibility of ambient levels approaching the level of 
the NAAQS. Another state agency commented that 4 years appeared to be 
an unusual number of years given that design values are based on 3 
years. To support the proposed rule in 2014, the EPA's analysis of 
O3 seasons began in 2013. At that time the EPA's analysis 
considered the most recent 3 years of certified data (2010-2012) and 
updated the analysis to add a fourth year (2013) when the data were 
quality-assured, certified, and available in AQS. We used 4 years of 
data, including the most recent year (2013) to include an additional 
year of potentially-variable meteorological conditions to propose 
changes to the seasons. The EPA treated all years equally and did not 
put any more weight on the 2010 data than any of the other years used 
in the analysis. The EPA believes that using recently-available data 
across multiple years to capture varying meteorological conditions was 
appropriate to support the decisions on extending the O3 
seasons. One commenter disagreed with the EPA's definition of year-
round (at least 20 daily observations in all 12 months of at least 1 
year of the 4-year period). The definition of year-round was used to 
estimate the number of monitors being operated outside a state's 
required O3 season and also used for the EPA's Information 
Collection Request (ICR). All available data in AQS were used for the 
O3 season analysis, including data from year-round monitors.
    Two commenters noted that ``regional consistency'' is not a 
scientific reason and is not needed for making changes to the 
O3 seasons. One commenter noted that significant 
geographical, meteorological and demographic differences exist between 
neighboring states that may not warrant identical monitoring seasons. 
The EPA notes that regional consistency was considered, but only 
important for a few states where little data were available and the 
neighboring states had more available data and a sufficient number of 
days that were >= 0.060 ppm to support the proposed O3 
season changes. Regional consistency was not important for other 
states.
    Some commenters expressed support for the proposed requirement that 
NCore O3 sites operate year-round. They questioned whether 
data from NCore stations outside the O3 season will be used 
for designations and requested that the EPA exclude those data from the 
designations process. Consistent with the designations process for all 
criteria pollutants, the states, tribes, and the EPA use all data 
available in AQS that meet the quality assurance requirements in 40 CFR 
part 58, Appendix A for the designations process. Given that 
O3 data from NCore stations will meet these requirements, 
there is no rational basis for excluding these data from comparison to 
the NAAQS. Accordingly, such data from NCore stations cannot be 
excluded and will be treated in a manner equivalent to all other 
O3 data in AQS. The EPA expects that the highest 
O3 values will occur during the required O3 
season; therefore, we don't anticipate that NCore data from the out-of-
season months will contribute to the design value used in

[[Page 65419]]

the designations process. The EPA is finalizing the requirement for 
year-round O3 monitoring at NCore stations.
    The EPA Regional Administrators have previously approved deviations 
from the required O3 monitoring seasons through rulemakings 
(64 FR 3028, January 20, 1999; 67 FR 57332, September 10, 2002; and 69 
FR 52836, August 30, 2004). The current ambient monitoring rule, in 
paragraph 4.1(i) of 40 CFR part 58, Appendix D (71 FR 61319, October 
17, 2006), allows the EPA Regional Administrators to approve changes to 
the O3 monitoring season without rulemaking. The EPA is 
retaining the rule language allowing such deviations from the required 
O3 monitoring seasons without rulemaking. In the finalized 
revision to paragraph 4.1(i) of 40 CFR part 58, Appendix D, the EPA is 
clarifying the minimum considerations that should be taken into account 
when reviewing requests, and clarifying that changes to the 
O3 seasons finalized in this rule revoke all previously 
approved seasonal deviations. The EPA clarifies that all O3 
season waivers will be revoked when this final rule becomes effective. 
We encourage monitoring agencies with existing waivers to engage their 
EPA Regions as soon as possible to evaluate whether new or continued 
waivers are appropriate given the level of the revised O3 
NAAQS.
    We received three comments for and three comments against early 
implementation of the revised O3 seasons by the start of the 
applicable O3 season in each state by January 1, 2016. Those 
commenters in favor of early implementation of the revised 
O3 seasons are already operating a large percentage of 
O3 monitors year-round or outside the current O3 
monitoring season in their state. Those commenters against early 
implementation cited concerns with the need for additional time to 
implement the revised O3 seasons, especially in areas where 
access in order to service and support the monitoring equipment may be 
problematic during winter weather conditions, and the undue burden on 
already constrained state resources. One commenter noted that given the 
date for the final rule (October 1, 2015) that there is insufficient 
time for public review of their annual monitoring network plan due July 
1, 2015, for early implementation in 2016. The EPA encourages those 
agencies who are able to implement the O3 season changes 
early to do so by the start of the applicable O3 season in 
their state in 2016. However, taking into consideration the timing and 
potential burden on monitoring agencies, the EPA is finalizing the 
requirement for implementing the revised O3 seasons no later 
than the start of the applicable O3 monitoring season in 
2017, as proposed.
3. Final Decisions on the Length of the Required O3 
Monitoring Seasons
    Final changes to the required O3 monitoring seasons are 
summarized in this section as well as in revised Table D-3 in 40 CFR 
part 58, Appendix D.
    Detailed state-by-state technical information has been placed in 
the docket to document the basis for the EPA's decision on each state. 
This information includes state-by-state maps and number of days that 
were >= 0.060 ppm; distribution charts of the number of days that were 
>= 0.060 ppm by month and state; and detailed information regarding AQS 
site IDs, dates and concentrations of all occurrences of the 8-hour 
daily maximum of at least 0.060 ppm between 2010 and 2013. Summaries 
have also been prepared for each state including the former and 
proposed O3 monitoring seasons.
    No changes to the required O3 monitoring season were 
proposed or finalized for these states: Alabama, Alaska, Arizona, 
Arkansas, California, Georgia, Hawaii, Kentucky, Northern Louisiana 
(AQCR \221\ 019, 022), Southern Louisiana (AQCR 106), Maine, 
Mississippi, Nevada, New Mexico, Oklahoma, Oregon, Tennessee, Southern 
Texas (AQCR 106, 153, 213, 214, 216), Vermont, Washington, Puerto Rico, 
Virgin Islands, Guam, and American Samoa. All existing O3 
season deviations or waivers are revoked.
---------------------------------------------------------------------------

    \221\ Air Quality Control Region.
---------------------------------------------------------------------------

    Changes to the required O3 monitoring seasons are 
finalized as follows for these states and the District of Columbia and 
all existing O3 season deviations or waivers are revoked.
    Colorado: Proposed addition of January, February, October, 
November, and December is finalized. The required season is revised to 
January-December.
    Connecticut: Proposed addition of March is finalized, revising 
season to March-September.
    Delaware: Proposed addition of March is finalized, revising season 
to March-October.
    District of Columbia: Proposed addition of March is finalized, 
revising season to March-October.
    Florida: Proposed addition of January, February, November, and 
December is finalized. The required season is revised to January-
December.
    Idaho: Proposed addition of April is finalized, revising season to 
April-September.
    Illinois: Proposed addition of March is finalized, revising season 
to March-October.
    Indiana: Proposed addition of March and October, revising season to 
March-October.
    Iowa: Proposed addition of March is finalized, revising season to 
March-October.
    Kansas: Proposed addition of March is finalized, revising season to 
March-October.
    Maryland: Proposed addition of March is finalized, revising season 
to March-October.
    Massachusetts: Proposed addition of March is finalized, revising 
season to March-September.
    Michigan: Proposed addition of March and October is finalized, 
revising season to March-October.
    Minnesota: Proposed addition of March is finalized, revising season 
to March-October.
    Missouri: Proposed addition of March is finalized, revising season 
to March-October.
    Montana: Proposed addition of April and May is finalized, revising 
season to April-September.
    Nebraska: Proposed addition of March is finalized, revising season 
to March-October.
    New Hampshire: Proposed addition of March is finalized, revising 
season to March-September.
    New Jersey: Proposed addition of March is finalized, revising 
season to March-October.
    New York: Proposed addition of March is finalized, revising season 
to March-October.
    North Carolina: Proposed addition of March is finalized, revising 
season to March-October.
    North Dakota: Proposed addition of March and April is finalized, 
revising season to March-September.
    Ohio: Proposed addition of March is finalized, revising season to 
March-October.
    Pennsylvania: Proposed addition of March is finalized, revising 
season to March-October.
    Rhode Island: Proposed addition of March is finalized, revising 
season to March-September.
    South Carolina: Proposed addition of March is finalized, revising 
season to March-October.
    South Dakota: Proposed addition of March, April, May, and October 
is finalized, revising season to March-October.
    Texas (Northern AQCR 022, 210, 211, 212, 215, 217, 218): Proposed 
addition of November is finalized, revising season to March-November.
    Utah: Proposed addition of January, February, March, April, 
October,

[[Page 65420]]

November, and December is finalized. The required season is revised to 
January-December.
    Virginia: Proposed addition of March is finalized, revising season 
to March-October.
    West Virginia: Proposed addition of March is finalized, revising 
season to March--October.
    Wisconsin: Proposed addition of March and April 1--15 is finalized, 
revising season to March--October 15.
    Wyoming: Proposed addition of January, February, March, and removal 
of October is finalized, revising season to January--September.
    Finally, we are finalizing the required O3 monitoring 
season for all NCore stations to be year-round (January--December) 
regardless of the required monitoring season for the individual state 
in which the NCore station is located.

C. Revisions to the PAMS Network Requirements

    Section 182 (c)(1) of the CAA required the EPA to promulgate rules 
for enhanced monitoring of O3, NOX, and VOCs for 
nonattainment areas classified as serious (or above) to obtain more 
comprehensive and representative data on O3 air pollution. 
In addition, Section 185B of the CAA required the EPA to work with the 
National Academy of Sciences (NAS) to conduct a study on the role of 
O3 precursors in tropospheric O3 formation and 
control. As a result of this study, the NAS issued the report entitled, 
``Rethinking the Ozone Problem in Urban and Regional Air Pollution'', 
(NAS, 1991).
    In response to the CAA requirements and the recommendations of the 
NAS report, on February 12, 1993 (58 FR 8452), the EPA revised the 
ambient air quality surveillance regulations to require PAMS in each 
O3 nonattainment area classified as serious, severe, or 
extreme (``PAMS areas''). As noted in the EPA's Technical Assistance 
Document (TAD) for Sampling and Analysis of Ozone Precursors (U.S. EPA, 
1998), the current objectives of the PAMS program are to: (1) Provide a 
speciated ambient air database that is both representative and useful 
in evaluating control strategies and understanding the mechanisms of 
pollutant transport by ascertaining ambient profiles and distinguishing 
among various individual volatile organic compounds (VOCs); (2) provide 
local, current meteorological and ambient data to serve as initial and 
boundary condition information for photochemical grid models; (3) 
provide a representative, speciated ambient air database that is 
characteristic of source emission impacts to be used in analyzing 
emissions inventory issues and corroborating progress toward 
attainment; (4) provide ambient data measurements that would allow 
later preparation of unadjusted and adjusted pollutant trends reports; 
(5) provide additional measurements of selected criteria pollutants for 
attainment/nonattainment decisions and to construct NAAQS maintenance 
plans; and (6) provide additional measurements of selected criteria and 
non-criteria pollutants to be used for evaluating population exposure 
to air toxics as well as criteria pollutants.
    The original requirements called for two to five fixed sites per 
PAMS area depending on the area's population. Four types of PAMS sites 
were identified including upwind (Type 1), maximum precursor emission 
rate (Type 2), maximum O3 concentration (Type 3), and 
extreme downwind (Type 4) sites. Each PAMS site was required to measure 
O3, nitrogen oxide (NO), NO2, speciated VOCs, 
selected carbonyl compounds, and selected meteorological parameters. In 
addition, upper air meteorological monitoring was required at one site 
in each PAMS area.
    In the October 17, 2006 monitoring rule (71 FR 61236), the EPA 
revised the PAMS requirements to only require two sites per PAMS area. 
The intent of the revision was to ``allow PAMS monitoring to be more 
customized to local data needs rather than meeting so many specific 
requirements common to all subject O3 nonattainment areas; 
the changes also gave states the flexibility to reduce the overall size 
of their PAMS programs--within limits--and to use the associated 
resources for other types of monitoring they consider more useful.'' In 
addition to reducing the number of required sites per PAMS area, the 
2006 revisions also limited the requirement for carbonyl measurements 
(specifically formaldehyde, acetaldehyde, and acetone) to areas 
classified as serious or above for the 8-hour O3 standards. 
This change was made in recognition of carbonyl sampling issues which 
were believed to cause significant uncertainty in the measured 
concentrations.
    Twenty-two areas were classified as serious or above O3 
nonattainment at the time the PAMS requirements were promulgated in 
1993. On July 18, 1997 (62 FR 38856), the EPA revised the averaging 
time of the O3 NAAQS from a 1-hour averaging period to an 8-
hour averaging period. On June 15, 2005 (70 FR 44470), the EPA revoked 
the 1-hour; however, PAMS requirements were identified as requirements 
that had to be retained in the anti-backsliding provisions included in 
that action. Therefore, PAMS requirements continue to be applicable to 
areas that were classified as serious or above nonattainment for the 1-
hour O3 standards as of June 15, 2004. Currently, 25 areas 
are subject to the PAMS requirements with a total of 75 sites. As will 
be discussed in detail later, the current PAMS sites are concentrated 
in the Northeast U.S. and California with relatively limited coverage 
in the rest of the country (Cavender, 2014).
    The first PAMS sites began operation in 1994, and have been in 
operation for over 20 years. Since the start of the program, there have 
been many changes to the nature and scope of the O3 problem 
in the U.S. as well as to our understanding of it. The O3 
standards has been revised multiple times since the PAMS program was 
first implemented. On July 18, 1997, the EPA revised the O3 
NAAQS to a level of 0.08 parts per million (ppm), with a form based on 
the 3-year average of the annual fourth-highest daily maximum 8-hour 
average O3 concentration. On March 28, 2008 (73 FR 16436), 
the EPA revised the O3 standards to a level of 0.075 ppm, 
with a form based on the 3-year average of the annual fourth-highest 
daily maximum 8-hour average O3 concentration. These changes 
in the level and form of the O3 NAAQS, along with notable 
decreases in O3 levels in most parts of the U.S., have 
changed the landscape of O3 NAAQS violations in the U.S. At 
the time of the first round of designations for the 8-hour standards 
(June 15, 2005), only 5 areas were classified as serious or above for 
the 8-hour standards as compared to 22 areas that were classified as 
serious or above for the 1-hour standards. While the number of serious 
and above areas decreased, the number of nonattainment areas remained 
nearly the same. In addition to the change in the landscape of 
O3 nonattainment issues, much of the equipment used at PAMS 
sites is outdated and in need of replacement. New technologies have 
been developed since the inception of the PAMS program that should be 
considered for use in the network to simplify procedures and improve 
data quality. For these reasons, the EPA determined that it would be 
appropriate to re-evaluate the PAMS program as explained below.
    In 2011, the EPA initiated an effort to re-evaluate the PAMS 
requirements in light of changes in the needs of PAMS data users and 
the improvements in monitoring technology. The EPA consulted with the 
Clean Air Science Advisory Committee (CASAC), Air

[[Page 65421]]

Monitoring and Methods Subcommittee (AMMS) to seek advice on potential 
revisions to the technical and regulatory aspects of the PAMS program; 
including changes to required measurements and associated network 
design requirements. The EPA also requested advice on appropriate 
technology, sampling frequency, and overall program objectives in the 
context of the most recently revised O3 NAAQS and changes to 
atmospheric chemistry that have occurred over the past 10-15 years in 
the significantly impacted areas. The CASAC AMMS met on May 16 and May 
17, 2011, and provided a report with their advice on the PAMS program 
on September 28, 2011 (U.S. EPA, 2011f). In addition, the EPA met 
multiple times with the National Association of Clean Air Agencies 
(NACAA) Monitoring Steering Committee (MSC) to seek advice on the PAMS 
program. The MSC includes monitoring experts from various State and 
local agencies actively engaged in ambient air monitoring and many 
members of the MSC have direct experience with running PAMS sites. 
Specific advice obtained from the CASAC AMMS and the MSC that was 
considered in making the proposed changes to the PAMS requirements is 
discussed in the appropriate sections below.
    Based on the findings of the PAMS evaluation and the consultations 
with the CASAC AMMS and NACAA MSC, the EPA proposed to revise several 
aspects of the PAMS monitoring requirements including changes in (1) 
network design, (2) VOC sampling, (3) carbonyl sampling, (4) nitrogen 
oxides sampling, and (5) meteorology measurements. The following 
paragraphs summarize the proposed changes, the comments received, and 
the final changes and supporting rationale.
1. Network Design
    As discussed above, the current PAMS network design calls for two 
sites (a Type 2, and a Type 1 or Type 3) per PAMS area. In their report 
(U.S EPA, 2011f), the CASAC AMMS found ``that the existing uniform 
national network design model for PAMS is outdated and too resource 
intensive,'' and recommended ``that greater flexibility for network 
design and implementation of the PAMS program be transferred to state 
and local monitoring agencies to allow monitoring, research, and data 
analysis to be better tailored to the specific needs of each 
O3 problem area.'' While stating that the current PAMS 
objectives were appropriate, the AMMS report also stated that 
``objectives may need to be revised to include both a national and 
regional focus because national objectives may be different from 
regional objectives.'' The NACAA MSC also advised the EPA that the 
existing PAMS requirements were too prescriptive and may hinder state 
efforts to collect other types of data that were more useful in 
understanding their local O3 problems.
    The EPA agrees with CASAC that the PAMS objectives include both 
local and national objectives, and believes that the current PAMS 
network design is no longer suited for meeting either sets of 
objectives. As part of the PAMS evaluation, it was determined that at 
the national level the primary use of the PAMS data has been to 
evaluate photochemical model performance. Due to the locations of the 
current PAMS areas and the current network design, existing PAMS sites 
are clustered along the northeast and west coasts leading to 
significant redundancy in these areas and very limited coverage 
throughout the remainder of the country (Cavender, 2014). The resulting 
uneven spatial coverage greatly limits the value of the PAMS data for 
evaluation of model performance. CASAC (U.S. EPA, 2011f) noted the 
spatial coverage issue and advised that the EPA should consider 
requiring PAMS measurements in areas in addition to ``areas classified 
as serious and above for the O3 NAAQS to improve spatial 
coverage.'' The EPA also agrees with CASAC and NACAA that the PAMS 
requirements should be revised to provide monitoring agencies greater 
flexibility in meeting local objectives.
    The EPA proposed changes to the network design requirements to 
better serve both national and local objectives. The EPA proposed a two 
part network design. The first part of the design included a network of 
fixed sites (``required PAMS sites'') intended to support O3 
model development and the tracking of trends of important O3 
precursor concentrations. The second part of the network design 
required states with O3 non-attainment areas to develop and 
implement Enhanced Monitoring Plans (EMPs) which were intended to allow 
monitoring agencies the needed flexibility to implement additional 
monitoring capabilities to suit the needs of their area.
    To implement the fixed site portion of the network design, the EPA 
proposed to require PAMS measurements at any existing NCore site in an 
O3 nonattainment area in lieu of the current PAMS network 
design requirements.\222\ The NCore network is a multi-pollutant 
monitoring network consisting of 80 sites (63 urban, 17 rural) sited in 
typical neighborhood scale locations and supports multiple air quality 
objectives including some of the objectives of the PAMS program 
including the development and evaluation of photochemical models 
(including both PM2.5 and O3 models), development 
and evaluation of control strategies, and the tracking of regional 
precursor trends.
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    \222\ The EPA noted that the proposed change would expand the 
PAMS applicability beyond that required in 182(c)(1) of the CAA. 
Thus, in this final rule, the EPA is relying on the authority 
provided in Sections 103(c), 110(a)(2)(B), 114(a) and 301(a)(1) of 
the CAA to expand the PAMS applicability to areas other than those 
that are serious or above O3 nonattainment.
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    The EPA recognized that in limited situations existing NCore sites 
may not be the most appropriate locations for making PAMS measurements. 
For example, an existing PAMS site in an O3 nonattainment 
area may be sited at a different location than the existing NCore site. 
In this case, it may be appropriate to continue monitoring at the 
existing PAMS site to support ongoing research and to maintain trends 
information. To account for these situations, the EPA also proposed to 
provide the EPA Regional Administrator the authority to approve an 
alternative location for a required PAMS site where appropriate. The 
EPA also solicited comments on alternative frameworks using other 
benchmarks such as attainment status or population to ensure an 
appropriately sized fixed PAMS monitoring network. The EPA received 
several comments on the proposed changes to the network design, 
primarily from state and local monitoring agencies. The following 
paragraphs summarize the major comments made on the proposed network 
design, our response, and final network design requirements.
    Most commenters agreed with the need to revise the existing network 
design. One commenter agreed that ``requiring PAMS monitoring at 
already existing NCore locations will benefit national and local 
objectives to understand ozone formation and would also provide 
significant cost efficiencies.'' Another commenter stated that they 
supported the proposed changes, ``especially the flexibility provided 
by EMPs designed to meet local objectives and achieve a better 
understanding of photochemical precursors.'' Another commenter 
supporting the changes stated that the ``proposed network revision will 
provide states the flexibility to use their resources effectively.'' 
One commenter stated that the proposed changes ``reflect a more 
efficient use of state and local monitoring resources by availing

[[Page 65422]]

monitoring agencies of existing NCore infrastructure to fulfill PAMS 
requirements.''
    A number of concerns were also raised with the proposed network 
design. Several commenters stated that the proposal ``would drastically 
reduce the PAMS network in the Northeast.'' One commenter stated that 
``this is not acceptable for the Northeast and Mid-atlantic Corridor, 
which requires monitoring of the complex transport from multiple large 
metropolitan areas in the region.'' One commenter recognized that the 
EPA had intended to allow states to use EMPs to address upwind and 
downwind data needs, but raised concerns that states with historically 
important upwind and downwind sites in the Ozone Transport Region \223\ 
(OTR) may not be required to develop an EMP since those sites would be 
in states that are attaining the O3 NAAQS. One commenter 
suggested that ``the EPA consider the entire OTR when designing a PAMS 
network rather than pockets of nonattainment areas in the region.'' The 
EPA agrees that the reduction of sites in the OTR is a potential issue 
and that many important existing PAMS sites would not be part of the 
required PAMS sites based on the proposed network design. As noted by 
several commenters, the EPA intended the state directed EMPs to give 
states flexibility in determining data needed to understand local 
O3 formation, including transport in the Northeast. However, 
the EPA also agrees that as proposed many states in the OTR would not 
be required to develop EMPs and, therefore, may not be provided PAMS 
resources. To address these concerns and ensure adequate network 
coverage in the OTR, the EPA is adding a requirement that all states in 
the OTR develop and implement an EMP regardless of O3 
attainment status. This change will help ensure that an EMP appropriate 
for the entire OTR can be implemented.
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    \223\ Section 184(c) of the CAA establishes the OTR as comprised 
of the states of Connecticut, Delaware, Maine, Maryland, 
Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, 
Rhode Island, Vermont, and Consolidated Metropolitan Statistical 
Area that includes the District of Columbia.
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    Concerns were raised by some states that existing NCore sites may 
not be the most appropriate location for making PAMS measurements. One 
commenter noted that their NCore site was inland but that their ``most 
significant ozone problems occur along the shoreline due to transport 
along the lake'', and that ``the NCore site cannot provide insight into 
these important lakeshore ozone processes.'' Another commenter stated 
that ``while it was laudable to leverage sites where data is already 
being collected, it is unclear whether NCore sites adequately meet the 
objectives of the PAMS program'', and that ``the current NCore network 
may not be adequate to depict boundary conditions or areas of maximum 
emissions.'' One commenter stated that ``in some nonattainment areas an 
NCore site may be an appropriate location for a PAMS monitor, but in 
other areas it would be preferable to install the PAMS monitoring in a 
location downwind of a source region where higher ozone exposures 
occur'' and that ``State and local boundaries should not be part of the 
network design criteria.'' One commenter noted that while the EPA had 
proposed to allow waivers, it was unclear if waivers would be allowed 
where the alternative site was in a different CBSA or state than the 
required PAMS site. As stated in our proposal, the EPA recognizes that 
in some cases existing PAMS sites (or other sites) may be better suited 
to meet local and national data needs. For this reason, we had proposed 
to allow waivers in these situations. We do agree that it is 
appropriate in some cases to allow these waivers to cross CBSA and 
state boundaries. Therefore, we have added specific language to the 
final waiver provisions to clarify that waivers can be allowed to cross 
CBSA and state boundaries. Where a monitoring agency receives a waiver 
from siting a monitor in reliance on a monitor operated by a different 
monitoring agency (e.g., across state lines), the waiver will be 
conditioned on the monitor being properly included in the other 
agency's network plan, and operated in accordance with the requirements 
of Part 58, including the relevant appendices.
    In addition to the concerns raised about closing important existing 
PAMS sites discussed above, some commenters raised concerns that many 
of the newly required PAMS sites would be in locations that were 
expected to attain the revised O3 NAAQS soon after the new 
sites would be installed. One commenter noted that ``requiring marginal 
nonattainment areas to install PAMS sites would result in a large 
undertaking at an area that would most likely be back in attainment at 
or around the time the PAMS site started collecting data.'' One 
commenter stated that by tying the network requirement to NAAQS 
attainment ``threatens to underserve areas that are very close to 
exceeding the revised ozone NAAQS and results in significant gaps in 
the spatial coverage of the PAMS network'' and ``has the potential to 
introduce undesirable uncertainty on the size and spatial extent of the 
PAMS network over the long term.'' Another commenter was concerned that 
the proposed network would be unstable, and would experience frequent 
changes as areas came into attainment or went out of attainment thus 
reducing the value of the data collected, and resulting in inefficient 
use of resources. One commenter noted that ``a more stable monitoring 
network design will allow for the examination of trends from spatially 
robust, long running sites and will allow states to firmly establish 
the infrastructure costs.''
    The EPA noted in the proposal that the size and locations of the 
proposed required PAMS network is sensitive to the level of the revised 
O3 NAAQS and future O3 concentrations. We 
recognize and agree that if current downward trends in O3 
concentrations continue, many initially required sites may no longer be 
required to make PAMS measurements soon after the sites were installed. 
Non-required sites could be closed, soon after being installed, at the 
state's discretion. We agree this would result in an inefficient use of 
resources. We also note that if these sites were closed following a 
potential reclassification to attainment, the loss of those sites could 
lead to a network with poor spatial coverage. Therefore, the EPA is 
making changes to the proposed revisions to the network design to 
improve the stability of the fixed site network. As explained below, 
the final requirements are based on options for which we requested 
comments in the proposal and the comments we have received.
    We requested comments on additional options to define the fixed 
PAMS network component of the new network design. These options were 
further discussed in a memorandum to the docket (Cavender, 2014). One 
option discussed was to require PAMS measurements at all NCore sites 
irrespective of the O3 attainment status of the area. One 
commenter noted that ``requiring PAMS monitoring at all NCore sites, 
regardless of ozone attainment status, provides the most spatially 
robust and stable monitoring network.'' We noted that this requirement 
would result in a network of approximately 80 sites, which would be 
larger than the current network. In the supporting memorandum, we noted 
that a fixed network of 80 sites would strain existing resources and 
would not allow adequate resources to implement the state directed 
EMPs.
    Another option discussed in the proposal included requiring PAMS 
measurements at NCore sites in O3

[[Page 65423]]

nonattainment areas with a population greater than 1,000,000. We noted 
that this option would result in a network of between 31 and 37 sites 
depending on the level of the revised O3 NAAQS. We also 
noted that focusing the applicability of PAMS to those NCore sites in 
larger CBSAs would still provide the desired improvement in geographic 
distribution while reducing the number of required sites down to a 
level that would provide sufficient resources to implement the state-
directed EMP portion of the network. One commenter stated that they 
``supported a 1,000,000 population threshold because it would help 
prioritize resources to areas based on the greatest human health 
impacts.'' In addition, a number of commenters, while not commenting on 
the need for a population limit, did raise concerns about their ability 
to acquire and retain staff with the necessary expertise to collect 
PAMS measurements in less urbanized areas. As with the proposed network 
design, we recognize that the total number of sites and the ultimate 
spatial coverage under this option is also sensitive to changes in 
O3 concentrations. If current downward trends in 
O3 concentrations continue, many initially required sites 
would not be required soon after they were installed. As with the 
proposed option, this option could result in an unstable network 
resulting in an inefficient use of resources and inadequate spatial 
coverage to meet the network goals discussed above.
    Upon further consideration and in response to the comments 
received, we are finalizing a network design that includes a 
requirement for states to make PAMS measurements at all NCore sites in 
CBSAs with a population of 1,000,000 people or more, irrespective of 
O3 attainment status. We believe this requirement will 
result in an appropriately sized network (roughly 40 sites) that will 
provide adequate spatial coverage to meet national model evaluation 
needs (Cavender, 2015). Redundancy is greatly reduced while important 
network coverage is added in the midwest, southeast, and mountain west. 
The improved spatial coverage will also strengthen the EPA's ability to 
track trends in precursor concentrations regionally.
    Because the network requirement is not tied to attainment status, 
this final requirement will ensure network stability and allows for 
more efficient use of available resources. This final requirement also 
removes uncertainty as to applicability and aids planning and logistics 
involved with implementing the new requirements. Monitoring agencies 
can determine the applicability of the fixed site requirements to their 
areas today, and begin to make plans for investments in equipment, 
shelter improvements, and staffing and training needs necessary to 
implement the fixed site requirements without having to wait for the 
designations process to be completed. In addition, this final 
requirement should alleviate concerns raised by monitoring agencies in 
more rural locations over the ability to attract and retain staff with 
the skills necessary to make PAMS measurements.
    By adding the PAMS measurements to existing NCore sites, 
significant efficiencies can be obtained which should further reduce 
the costs of the fixed site network as NCore sites currently make many 
of the PAMS measurements. Furthermore, adding the additional PAMS 
measurements (e.g., speciated VOCs, carbonyls, and mixing height) to 
existing NCore sites will improve our ability to assess other 
pollutants (e.g., air toxics and PM2.5).
    Although, as discussed in comment and summarized above, we believe 
there are good reasons for not tying the requirement for fixed PAMS 
sites to O3 attainment status, we continue to believe that 
requiring PAMS measurements in areas that historically have had low 
O3 concentrations is unlikely to provide data of significant 
value to warrant the expense and effort of making such measurements. 
Therefore, we have included a provision that would allow a monitoring 
agency to obtain a waiver, based on Regional Administrator approval, in 
instances where CBSA-wide O3 design values are equal to or 
less than 85% of the 8-hour O3 NAAQS and where the site is 
not considered an important upwind or downwind site for other 
nonattainment areas. The EPA selected 85% as the threshold for this 
waiver provision as it has been used historically to identify locations 
needing additional monitoring for both the O3 and 
PM2.5 NAAQS. The EPA will work with the monitoring agencies 
and the Regions to help ensure consistent implementation of this waiver 
provision.
    The second part of the proposed PAMS network design included 
monitoring agency directed enhanced O3 monitoring activities 
intended to provide data needed to understand an area's specific 
O3 issues. To implement this part of the PAMS network 
design, the EPA proposed to add a requirement for states with 
O3 nonattainment areas to develop an EMP. The purpose of the 
EMP was to improve monitoring for ambient concentrations of 
O3, NOX, total reactive nitrogen (NOy) 
\224\, VOC, and meteorology. The EPA suggested that types of activities 
that might be included in the state's EMP could include additional PAMS 
sites (e.g., upwind or downwind sites), additional O3 and 
NOX monitoring, ozonesondes or other aloft measurements, 
rural measurements, mobile PAMS sites, additional meteorological 
measurements, and episodic or intensive studies. The intent of the EMPs 
is to allow monitoring agencies flexibility in determining and 
collecting the information they need to understand their specific 
O3 problems.
---------------------------------------------------------------------------

    \224\ NOy includes NO, NO2, and other 
oxidized nitrogen compounds (NOz).
---------------------------------------------------------------------------

    We received comments on the proposed requirement for an EMP in 
states with O3 nonattainment areas. Most comments supported 
the requirement, but other comments raised a number of concerns. A 
number of commenters questioned the need for EMPs in Marginal and 
Moderate O3 nonattainment areas. They noted that in most 
cases, Marginal O3 nonattainment areas were expected to come 
into compliance without state-specific controls. One commenter stated 
that ``nonattainment areas projected to attain the standard without 
additional state-level actions may not need the PAMS resources and 
additional monitoring to develop a better understanding of their ozone 
issues.'' One commenter noted that ``marginal ozone nonattainment areas 
are given only a few requirements because it is assumed that the areas 
will reach attainment within three years.'' Another commenter stated 
``requiring enhanced monitoring for any marginal or moderate area 
should only be implemented where such analyses show the need for this 
data.'' The EPA agrees that based on current trends in O3 
concentrations and the EPA's own projections, states in Marginal 
nonattainment areas likely will comply with the revised NAAQS without 
additional state-directed controls, and as such, an EMP is not 
necessary in Marginal O3 attainment areas. Accordingly, the 
EPA is finalizing a requirement for EMPs in areas classified as 
Moderate or above O3 nonattainment and, thereby, removing 
the applicability of the requirement for Marginal areas. We believe 
this final requirement will provide the desired flexibility to allow 
states to identify enhanced monitoring needs while focusing resources 
for EMPs in areas of greater need of enhanced monitoring data.
    Commenters expressed concerns over the lack of detail on what an 
approvable EMP would entail. As proposed, the

[[Page 65424]]

EMPs would be reviewed and approved by the EPA Regional Administrator 
as part of the annual monitoring plan review process. One commenter 
recommended that the ``EPA detail the requirements of the EMPs for 
ozone nonattainment areas in future implementation guidance.'' One 
commenter stated that the ``EPA should provide some coordination 
between regional offices and technical guidance to state agencies that 
would be of assistance in developing and executing the EMPs.'' The 
requirements for the EMPs were intentionally left quite general in 
order to maximize the flexibility for states in identifying their 
specific data needs. Regional approval of the plans is required to 
ensure the enhanced monitoring planned will be commensurate with grant 
funds provided for EMPs. Nonetheless, the EPA understands the need for 
guidance on developing EMPs and commits to working with monitoring 
agencies and the regions to develop appropriate guidance on developing 
and reviewing EMPs.
2. Speciated VOC Measurements
    Measurement of speciated VOCs important to O3 formation 
is a key aspect of the PAMS program. The existing PAMS requirements 
allow for a number of options in measuring speciated VOCs at PAMS sites 
which include (1) hourly measurements using an automatic gas 
chromatograph (``autoGC''), (2) eight 3-hour samples daily using 
canisters, or (3) one morning and one afternoon sample with a 3-hour or 
less averaging time daily using canisters plus continuous Total Non-
methane Hydrocarbon (TNMHC) measurements.
    The EPA believes that the current options provided for VOC 
measurement limit the comparative value of the data being collected, 
and proposed that required PAMS sites must measure and report hourly 
speciated VOCs, which effectively would require them to use an autoGC 
to measure VOCs in lieu of canisters. More complete and consistent 
speciated VOC data nationally would better help meet certain objectives 
of the PAMS program described above (e.g., a speciated ambient air 
database useful in evaluating control strategies, analyzing emissions 
inventory issues, corroborating progress toward attainment, and 
evaluating population exposure to air toxics). Furthermore, as noted by 
the CASAC AMMS, hourly VOC data are ``particularly useful in evaluating 
air quality models and performing diagnostic emission attribution 
studies. These data can be provided on a near real-time basis and 
presented along with other precursor species (e.g., oxides of nitrogen 
and carbon monoxide) collected over similar averaging times.'' Longer 
time-averaged data are of significantly lower value for model 
evaluation. In addition, creating consistent monitoring requirements 
across the network would provide better data for analyzing regional 
trends and spatial patterns.
    At the time the original PAMS requirements were promulgated, the 
canister options were included because the EPA recognized that the 
technologies necessary to measure hourly average speciated VOCs 
concentrations were relatively new and may not have been suitable for 
broad network use. At that time, GCs designed for laboratory use were 
equipped with auto-samplers designed to ``trap'' the VOC compounds from 
a gas sample, and then ``purge'' the compounds onto the GC column. The 
EPA did not believe that autoGCs were universally appropriate due to 
the technical skill and effort necessary at that time to properly 
operate an autoGC.
    While the basic principles of autoGC technology have not changed, 
the hardware and software of modern autoGCs are greatly improved over 
that available at the time of the original PAMS requirements. Based on 
advice from the CASAC AMMS, the EPA initiated an evaluation of current 
autoGCs potentially suitable for use in the PAMS network. Based on the 
preliminary results, the EPA believes that typical site operators, with 
appropriate training, will have the skill necessary to operate a modern 
autoGC successfully. Considering the advances in autoGC technology, the 
added value obtained from hourly data, and the proposed move of PAMS 
measurements to NCore sites in O3 nonattainment areas, the 
EPA proposed to require hourly speciated VOC sampling at all PAMS 
sites. The EPA noted that this proposed requirement would effectively 
prevent the use of canisters to collect speciated VOCs at the required 
PAMS sites but that canister sampling may continue to be an appropriate 
method for collecting speciated VOCs at other locations as part of 
discretionary monitoring designed within the EMPs.
    While the EPA believes that the proposed transition to hourly 
speciated VOC sampling is the appropriate strategy to take advantage of 
improved technology and to broaden the utility of collected data, we 
are also mindful of the additional rigidity that the proposed mandatory 
use of autoGCs may have for monitoring agencies, especially those that 
have experience with and have established effective and reliable 
canister sampling programs. Therefore, the EPA requested comment on the 
proposed requirement for hourly VOC sampling as well as the range of 
alternatives that might be appropriate in lieu of a strict requirement.
    The EPA received a number of comments on the requirement to measure 
hourly VOCs at required PAMS sites. Many commenters agreed with 
requiring hourly VOC data. One commenter agreed that ``hourly VOC data 
collection is the most appropriate and useful for PAMS monitors'' and 
that ``it is only appropriate to approve an alternative data collection 
interval if it is believed that the high ozone in an area is due to 
other pollutants, such as NOX or methane.'' One commenter 
stated they ``supported the movement towards hourly PAMS VOC speciated 
measurements with flexibility to use canisters if programmatic or 
logistical needs indicate.''
    However, some commenters raised concerns with the hourly VOC 
requirement. Some commenters questioned if autoGCs would be capable of 
measuring important VOC species in their environment. One commenter 
noted that in their location (high desert) ``the largest VOC present in 
our inventory is creosote, a compound not commonly measured with this 
instrumentation.'' One commenter stated that the ``Southeastern United 
States is dominated by biogenic VOC emissions'' and questioned ``the 
benefits of an autoGC in understanding ozone formation in any potential 
nonattainment area in our State.'' \225\ Some questioned the detection 
capabilities of autoGCs as compared to canister sampling. One commenter 
found that the method detection limit (MDL) for their canister sampling 
was ``consistently equal to or less than the autoGC instrumentation'' 
based on the EPA's autoGC evaluation laboratory report (RTI, 2014). 
Another commenter noted that the MDLs for many of the compounds and 
systems reported in the laboratory report were too high to be useful at 
PAMS sites. Another commenter stated that they found that ``retention-
time shifts made it difficult for instant identification of chemical 
peaks'' and that ``states should be allowed the flexibility to continue 
using canisters instead of autoGC.''
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    \225\ The EPA notes that isoprene (the dominant biogenic 
compound in the Southeast) is well measured using autoGCs. The EPA 
is also evaluating the potential of modern autoGC's to measure alpha 
and beta pinene; however that work is not complete.
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    As noted in the preamble, and the comments received, the EPA is 
currently completing an evaluation of

[[Page 65425]]

commercially available autoGCs. A copy of the report for the laboratory 
phase of the study is available in the docket (RTI, 2014). As noted in 
the laboratory report, the MDL estimates made for the laboratory study 
were not conducted according to normal MDL testing procedures and as 
such the results should only be used to compare the various instruments 
being tested against each other.\226\ As part of the evaluation, the 
EPA identified the manufacturer's specifications for MDL. Most of the 
systems that are being evaluated have a manufacturer's estimated MDL in 
the range of 0.1 ppb to 0.5 ppb. Based on the evaluation of MDL 
capabilities and typical ambient concentrations of O3 
precursors, the EPA believes that autoGCs are an appropriate method for 
gathering VOC data at most urban locations. However, canister sampling 
may be more appropriate in locations with low VOC concentrations.
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    \226\ Several factors combined to result in the high relative 
MDL estimates reported in laboratory report. The MDL testing in the 
laboratory was conducted during concurrent tests for interferences 
from humidity and temperature. In addition, the MDL testing was 
conducted at relatively high concentrations compared to the 
concentrations testing would be conducted at for conventional MDL 
testing. Finally, as noted in the laboratory report, a number of 
instruments were having technical difficulties during the testing 
which greatly impacted their MDL results. The EPA is continuing the 
autoGC evaluation and has conducted a field study during the summer 
of 2015. A final report is expected in early 2016.
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    For the reasons discussed above and in the proposed rule, the EPA 
is finalizing a requirement for hourly speciated VOC measurements at 
required PAMS sites. The EPA believes that hourly VOC measurements will 
provide a more complete and consistent speciated VOC database to help 
meet the PAMS program objectives described above. Hourly VOC data are 
particularly useful in evaluating air quality models and performing 
diagnostic emission attribution studies. Longer time-averaged data are 
of lower value for model evaluation. Consistent monitoring requirements 
across the network will provide better data for analyzing regional 
trends and spatial patterns.
    However, the EPA agrees that there may be locations where an autoGC 
may not be the most appropriate method for VOC measurement and that it 
is appropriate to allow for canister sampling in limited situations. 
Accordingly, the EPA is adding a waiver option (to be approved by the 
EPA Regional Administrator) to allow three 8-hour average samples every 
3rd day as an alternative in cases where VOCs are not well measured by 
autoGC due to low concentrations of target compounds or where the 
predominant VOC compounds cannot be measured using autoGC technology 
(e.g., creosote in high desert environments). This alternative sampling 
frequency was selected to be consistent with the sampling frequency 
selected for carbonyls, which is discussed later in this preamble.
3. Carbonyl Measurements
    Carbonyls include a number of compounds important to O3 
formation that cannot currently be measured using the autoGCs or 
canisters used at PAMS sites to measure speciated VOCs. The current 
method for measuring carbonyls in the PAMS program is Compendium Method 
TO-11A (U.S. EPA, 1999). In this method, carbonyl compounds are 
adsorbed and converted into stable hydrazones using 
dinitrophenylhydrazine (DNPH) cartridges. These cartridges are then 
analyzed for the individual carbonyl compounds using liquid 
chromatography (LC) techniques. Three carbonyls are currently required 
to be measured in the PAMS program--formaldehyde, acetaldehyde, and 
acetone.
    In 2006, the EPA revised the PAMS requirements such that carbonyl 
sampling was only required in areas classified as serious or above 
nonattainment for O3 under the 8-hour O3 standard 
which effectively reduced the applicability of carbonyl sampling to a 
few areas in California. This change was made in recognition that there 
were a number of issues with Method TO-11A that raised concerns with 
the uncertainty in the carbonyl data being collected. These issues 
include interferences (humidity and O3) and breakthrough 
(i.e., overloading of the DNPH cartridge) at high concentrations. While 
solutions for these issues have been investigated, these improvements 
have not been incorporated into Method TO-11A.
    A recent evaluation of the importance of VOCs and carbonyls to 
O3 formation determined that carbonyls, especially 
formaldehyde, are very important to O3 formation (Cavender, 
2013). CASAC AMMS (U.S. EPA, 2011f) also noted the importance of 
carbonyls stating that ``There are many compelling scientific reasons 
to measure carbonyls. They are a very important part of O3 
chemistry almost everywhere.'' Although the EPA recognizes the issues 
that have been raised about the current method of measuring carbonyls, 
due to the importance of carbonyls to understanding O3 
chemistry, the EPA proposed to require all required PAMS sites to 
measure carbonyls.
    Several commenters agreed with the need for carbonyl data at PAMS 
sites. However, a number of commenters questioned the proposed 
frequency of eight 3-hour samples every day during the PAMS sampling 
season (June through August). Several commenters indicated that the 
frequency was too high. One commenter noted that the requirement would 
require 800 samples per season at each PAMS site and pointed out that 
this requirement, which was required at the inception of the PAMS 
program in the 1990s was ``found to be prohibitively expensive, 
technically unsustainable, and qualitatively compromised.'' Another 
commenter stated that ``this level of sampling would require a 
substantial amount of agency resources and seems unduly burdensome.'' A 
number of commenters also questioned the commercial availability of an 
8-channel carbonyl sampler that would be needed to take eight 3-hour 
samples daily. In light of the comments and upon further review, the 
EPA agrees that the proposed frequency is unduly burdensome and is 
finalizing a requirement with a lower frequency.
    A number of alternative frequencies were suggested in the comments. 
Several commenters suggested a frequency of three 8-hour samples on 
either a 1-in-6 day or 1-in-3 day basis. Another commenter suggested a 
frequency of eight 3-hour samples on a 1 in 6 day basis. The EPA notes 
that sampling on a 1-in-6 day frequency would lead to as little as 15 
sampling days per PAMS sampling season. The EPA believes that 15 
sampling days is too few to provide a meaningful representation of 
carbonyl concentrations over the PAMS sampling period. A sampling 
frequency of 1-in-3 days would lead to 30 sampling days per season with 
each day of the week being represented at least 4 times per sampling 
season. With regards to samples per day, a 3-hour sampling duration 
provides a better diurnal representation of carbonyl sampling compared 
with an 8-hour sampling duration; however 8-hour sampling can provide 
information useful for evaluating diurnal differences in carbonyl 
concentrations. Upon further consideration and in light of the comments 
received, the EPA is finalizing a carbonyl sampling requirement with a 
frequency of three 8-hour samples on a 1-in-3 day basis. This final 
requirement will result in approximately 90 samples per PAMS sampling 
season which the EPA believes is not unduly burdensome and

[[Page 65426]]

will provide a reasonable representation of carbonyl concentrations.
    A number of commenters noted the ongoing development of continuous 
formaldehyde instruments, and recommended that EPA allow for continuous 
formaldehyde measurements as an alternative to the manual cartridge 
based TO-11A method. The EPA agrees that continuous formaldehyde, with 
the ability to obtain hourly averaged measurements, would be a 
significantly more valuable that the longer averaged measurements. As a 
result, the EPA has added an option to allow for continuous 
formaldehyde as an alternative to the carbonyl measurements using TO-
11A.
4. Nitrogen Oxides Measurements
    It is well known that NO and NO2 play important roles in 
O3 formation (U.S. EPA, 2013, Section 3.2.2). Under the 
current network design, Type 2 PAMS sites are required to measure 
NOX (which by definition is the sum of NO and 
NO2), and Types 1, 3, and 4 sites are required to measure 
NOy. NCore sites are currently required to measure 
NOy but are not required to measure NO2 
separately.
    In conventional NOX analyzers, NO2 is 
determined as the difference between the measured NO and NOX 
concentrations. However, due to the non-selective reduction of oxidized 
nitrogen compounds by the molybedenum converter used in conventional 
NOX monitors, the NO2 measurement made by 
conventional NOX monitors can be biased high due to the 
varying presence of NOz compounds that may be reported as 
NO2. The unknown bias from the NOz compounds is undesirable 
when attempting to understand O3 chemistry.
    Improvements in reactive nitrogen measurements have been made since 
the original PAMS requirements were promulgated that allow for improved 
NO2 measurements. Selective photolytic converters have been 
developed that are not significantly biased by NOz compounds (Ryerson 
et al., 2000). Monitors using photolytic converters are commercially 
available and have been approved as FEMs for the measurement of 
NO2. In addition, methods that directly read NO2 
have been developed that allow for very accurate readings of 
NO2 without some of the issues inherent to the ``difference 
method'' used in converter-based NOX analyzers. However, 
these direct reading NO2 analyzers generally do not provide 
an NO estimate, and would need to be paired with a converter-based 
NOX monitor or NOy monitor in order to also 
measure NO.
    As discussed above, the EPA is finalizing a PAMS network design 
such that PAMS measurements will be required at existing NCore sites in 
CBSAs with a population of 1,000,000 people or more. NCore sites 
currently are required to measure NO and NOy. NCore sites 
are not currently required to measure NO2. Due to the 
importance of accurate NO2 data to the understanding of 
O3 formation, the EPA proposed to require NO2 
measurements at required PAMS sites. Since existing NCore sites 
currently measure NOy, either a direct reading 
NO2 analyzer or a photolytic-converter NOX 
analyzer could be used to meet the proposed requirement. The EPA 
believes conventional NOX analyzers would not be appropriate 
for making PAMS measurements due to the uncertainty caused by 
interferences from NOz compounds.
    A number of commenters questioned the need for both NOy 
and NO2 measurements at PAMS sites. One commenter stated 
that ``in dense urban areas an NO/NO2/NOX 
instrument may be adequate but in a more rural area an NO/
NOy instrument may be preferable.'' Another commenter stated 
that due to the size of the grid cells used in grid models that ``the 
impact of NOz interferences would be very small compared to other 
modeling uncertainties such as emission inventories and mixing 
heights.'' Another commenter suggested that ``EPA should provide clear 
and specific guidance on how agencies can request that the 
NOy monitoring be eliminated from the NCore suite based on 
comparative data between the NO2 and NOy 
monitors.''
    The comments suggest that the model's ability to simulate the 
partitioning of reactive nitrogen is unimportant because there may be 
other errors in the model. The EPA believes that measurements should be 
routinely collected so that it can be demonstrated that the chemistry, 
meteorology, and emissions in the model are all of sufficient 
reliability for use in informing air quality management decisions. 
Monitoring sites rarely fall into simple categories of urban or rural, 
and the speciation of NOy varies considerably as a function 
of meteorology and time of day at a given site. The state-of-the-
science in regulatory air quality modeling is such that accurate 
measurements of key O3 precursors must be available to 
demonstrate the credibility of the model predictions. The increased 
availability of special field study observations is leading to 
increased scrutiny of the chemical mechanisms used in regulatory 
modeling. Comprehensive and accurate measurement sites are needed to 
demonstrate the adequacy of the models and to respond to these 
challenges.
    Measurements of NO, NO2, and NOy 
concentrations are critical to understanding atmospheric aging and 
photochemistry. These measurements will provide essential information 
about whether NOy compounds are fresh or aged which is 
important for understanding both local photochemistry (i.e. through 
indicator ratios to distinguish NOX vs VOC limited 
conditions) as well as for characterizing transport from upwind 
regions. These evaluations may be conducted using observations, box 
modeling or through complex photochemical grid based modeling. Accurate 
speciated and total NOy measurements are necessary for all 
three types of analysis. For these reasons, the EPA is finalizing the 
requirement for required PAMS sites to measure true NO2 in 
addition to NO and NOy.
5. Meteorology Measurements
    The current PAMS requirements require monitoring agencies to 
collect surface meteorology at all required PAMS sites. As noted in the 
EPA's Technical Assistance Document (U.S. EPA, 1998) for the PAMS 
program, the PAMS requirements do not provide specific surface 
meteorological parameters to be monitored. As part of the 
implementation efforts for the original PAMS program, a list of 
recommended parameters was developed and incorporated into the TAD 
which includes wind direction, wind speed, temperature, humidity, 
atmospheric pressure, precipitation, solar radiation, and ultraviolet 
(UV) radiation. Currently, NCore sites are required to measure the 
above parameters with the exceptions of atmospheric pressure, 
precipitation, solar radiation, and UV radiation. In recognition of the 
importance of these additional measurements for understanding 
O3 formation, the EPA proposed to specify that 
required PAMS sites are required to collect wind direction, wind speed, 
temperature, humidity, atmospheric pressure, precipitation, solar 
radiation, and UV radiation. Since NCore sites are currently required 
to measure several of these surface meteorological parameters, the net 
impact of the proposal was to add the requirement for the monitoring of 
atmospheric pressure, precipitation, solar radiation, and UV radiation 
at affected NCore sites. The EPA received no significant comments on 
this portion of the proposal, and therefore is finalizing the 
requirement as proposed.

[[Page 65427]]

    The existing PAMS requirements also require the collection of upper 
air meteorological measurements at one site in each PAMS area. The term 
upper air meteorological is not well defined in the existing PAMS 
requirements. As part of the implementation efforts for the original 
PAMS program, mixing height was added to the PAMS TAD as a recommended 
meteorological parameter to be monitored. Most monitoring agencies 
installed radar profilers to meet the requirement to collect upper air 
meteorology. Radar profilers provide data on wind direction and speed 
at multiple heights in the atmosphere. Radio acoustic sounding system 
(RASS) profilers are often included with radar profilers to obtain 
atmospheric temperature at multiple heights in the atmosphere and to 
estimate mixing height. The EPA recognizes that the upper air data on 
wind speed and wind direction from radar profilers can be very useful 
in O3 modeling. However, many of the current PAMS radar 
profilers are old and in need of replacement or expensive maintenance. 
In addition, the cost to install and operate radar profilers at all 
required PAMS sites would be prohibitive. Therefore, the EPA did not 
propose to add upper air wind speed and direction as required 
meteorological parameters to be monitored at required PAMS sites. Where 
monitoring agencies find the radar profiler data valuable, continued 
operation of existing radar profilers or the installation of new radar 
profilers would be appropriate to consider as part of the state's EMP.
    As discussed above, mixing height is one upper air meteorological 
measurement that has historically been measured at PAMS sites. A number 
of methods can be used to measure mixing height in addition to radar 
profiler technology discussed above. Recent developments in ceilometer 
technology allow for the measurement of mixing height by changes in 
particulate concentrations at the top of the boundary layer (Eresmaa et 
al., 2006). Ceilometers provide the potential for continuous mixing 
height data at a fraction of the cost of radar profilers. Due to the 
importance of mixing height measurements for O3 modeling, 
the EPA proposed to add the requirement for monitoring agencies to 
measure mixing height at required PAMS sites.
    A number of commenters questioned the need for mixing height 
measurements at PAMS sites. One commenter stated, ``the photochemical 
modeling community has a long history of relying upon National Weather 
Service measurements for mixing height.'' Another commenter stated that 
``in some areas of the country the models used to predict mixing height 
are adequate, but in other mountainous or marine areas model-predicted 
mixing height data is inadequate.'' Accurate estimates of mixing height 
are important for appropriately characterizing concentrations of 
O3 and O3 precursors. Mixing height is also 
important for characterizing how modeled O3 may change as a 
result of changing NOX and VOC concentrations. For instance, 
if the modeled mixing height is too low causing unrealistically high 
concentration of NOX, then O3 destruction could 
be predicted when O3 production may be happening in the 
atmosphere. When this or the opposite situation exists in modeling it 
may lead O3 response to emissions changes that are less 
reliable for air quality planning purposes. While models are believed 
to do a reasonable job of predicting mixing height during the day, 
there is considerably more uncertainty in predicting this parameter 
during morning and evening transition periods and at night. Model 
O3 predictions are particularly sensitive to mixing height 
during the time periods for which uncertainty in this parameter is 
greatest.
    Several commenters noted that nearby National Oceanic and 
Atmospheric Administration (NOAA) Automated Surface Observing System 
(ASOS) sites may be a better alternative for collection of mixing 
height data. As indicated in the proposal, the EPA is aware of the 
network of ceilometers operated by NOAA as part of ASOS. The EPA has 
been in discussions with NOAA regarding the potential for these systems 
to provide the needed mixing height data. However, the ASOS ceilometers 
are not currently equipped to provide mixing height data and NOAA has 
no current plans to measure continuous mixing height in the future. 
Nonetheless, the EPA will continue to work with NOAA to determine if 
the ASOS ceilometers can be upgraded to meet the need for mixing height 
data, and included proposed regulatory language that will allow states 
a waiver to use nearby mixing height data from ASOS (or other sources) 
to meet the requirement to collect mixing height data at required PAMS 
sites when such data are suitable and available.
    The EPA is finalizing the requirement for the measurement of mixing 
height at required PAMS sites due to the importance of mixing height in 
O3 modeling. A waiver option, to be approved by the Regional 
Administrator, is also being included to allow mixing height 
measurements to be obtained from other nearby sites (e.g., NOAA ASOS 
sites).
6. PAMS Season
    Currently, PAMS measurements are required to be taken during the 
months of June, July, and August. This 3-month period is referred to as 
the ``PAMS Season.'' As part of the PAMS re-evaluation, the EPA 
considered changes to the PAMS season. The 3-month PAMS season was 
originally selected to represent the most active period for 
O3 formation. However, the EPA notes that in many areas the 
highest O3 concentrations are observed outside of the PAMS 
season. As an example, the highest O3 concentrations in the 
mountain-west often occur during the winter months. Data collected 
during the current PAMS season would have limited value in 
understanding winter O3 episodes.
    The CASAC AMMS (U.S. EPA, 2011f) noted in their report to the EPA 
that ``it would be desirable to extend the PAMS monitoring season 
beyond the current June, July, August sampling period.'' But that ``the 
monitoring season should not be mandated and rigid; it should be 
flexible and adopted and coordinated on a regional airshed basis.'' The 
EPA agrees with CASAC on the need for flexibility in determining when 
PAMS measurements should be taken to meet local monitoring needs but 
also agrees with CASAC that the flexibility ``should not conflict with 
national goals for the PAMS program.'' A significant benefit of the 
standard PAMS season is that it ensures data availability from all PAMS 
sites for national- or regional-scale modeling efforts.
    While the EPA agrees with the potential benefit of extending the 
availability of PAMS measurements outside of the current season, we 
also considered the burden of requiring monitoring agencies to operate 
additional PAMS measurements (e.g., hourly speciated VOC) for periods 
that in some cases, might be much longer than the current 3-month 
season, for example, if the PAMS season was extended to match each 
state's required O3 monitoring season. Being mindful of the 
potential burden associated with a lengthening of the PAMS season as 
well as the potential benefits of the additional data, the EPA proposed 
to maintain the current 3-month PAMS monitoring season for required 
PAMS sites rather than extending the PAMS season to other periods where 
elevated O3 may be expected. No significant comments were 
received on the proposed PAMS season, and as such, for the reasons 
stated here and in the proposal, the EPA is not changing the 3-month 
PAMS season of June, July, and August.

[[Page 65428]]

    The EPA believes that the 3-month PAMS season will provide a 
consistent data set of O3 and O3 precursor 
measurements for addressing the national PAMS objectives. Monitoring 
agencies are strongly encouraged to consider collecting PAMS 
measurements in additional periods beyond the required PAMS season as 
part of their EMP. The monitoring agencies should consider factors such 
as the periods of expected peak O3 concentrations and 
regional consistency when determining potential expansion of their 
specific monitoring periods beyond the required PAMS season.
7. Timing and Other Implementation Issues
    The EPA recognizes that the changes to the PAMS requirements will 
require resources and a reasonable timeline in order to be successfully 
implemented. The PAMS program is funded, in part, as part of the EPA's 
section 105 grants. The EPA believes that the current national funding 
level of the PAMS program is sufficient to support these final changes, 
but changes in the distribution of PAMS funds will need to be made. The 
network design changes will require some monitoring agencies to start 
collection of new PAMS measurements, while other monitoring agencies 
will see reductions in PAMS measurement requirements. The EPA will work 
with the NAACA, AAPCA, and other monitoring agencies to develop an 
appropriate PAMS grant distribution strategy.
    In addition to resources, the affected monitoring agencies will 
need time to implement the revised PAMS requirements. For the required 
PAMS sites, monitoring agencies can determine now which NCore sites 
will be required to make PAMS measurements based on readily available 
census data. However, monitoring agencies will still need time to 
evaluate and seek approval for alternative sites or alternative VOC 
methods. In addition, monitoring agencies will need time to make 
capital investments (primarily for the installation of autoGCs, 
NO2 monitors, and ceilometers), prepare appropriate QA 
documents, and develop the expertise needed to successfully collect 
PAMS measurements via training or otherwise. In order to ensure 
monitoring agencies have adequate time to plan and successfully 
implement the revised PAMS requirements, the EPA is requiring that 
monitoring agencies identify their plans to implement the PAMS 
measurements at NCore sites in their Annual Network Plan due July 1, 
2018, and to begin making PAMS measurements at NCore sites by June 1, 
2019. The EPA believes some monitoring agencies may be able to begin 
making PAMS measurements sooner than June 2019 and encourages early 
deployment where possible.
    Monitoring agencies will need to wait until O3 
designations are made to officially determine the applicability of the 
EMP requirement. The EPA proposed to allow two years after designations 
to develop EMPs, and that the EMPs would be submitted as part of their 
Annual Network Plan. Several commenters stated that due to the level of 
planning and coordination required for the EMPs, that the plans should 
instead be included as part of the 5-year network assessment. While the 
EPA agrees that the EMPs will require a substantial amount of planning 
and coordination, the next 5 year network assessment will not be due 
until July 1, 2020--nearly 5 years from the date of this final 
rulemaking. The EPA believes that it would be inappropriate to wait 5-
years from the date of this rulemaking to develop plans for enhanced 
O3 monitoring. In addition, the EPA believes that the first 
round of EMP development should receive additional focus and review 
that may not be afforded as part of the larger network assessment. 
Finally, most monitoring agencies will be aware of their likely 
O3 attainment status well in advance of the official 
designations. In order to ensure timely development of the initial 
EMPs, the EPA is requiring affected monitoring agencies to submit their 
initial EMPs no later than two years following designations. States in 
the OTR do not need to wait until designations to determine EMP 
applicability and may not be classified as Moderate or above. As such, 
the final rule includes a requirement for states in the OTR to submit 
their initial EMPs by October 1, 2019 (which is consistent with the 
expected timeline for the remaining EMPs). However, subsequent review 
and revisions to the EMPs are to be made as part of the 5-year network 
assessments beginning with the assessments due in 2025.

D. Addition of a New FRM for O3

    The use of FRM analyzers for the collection of air monitoring data 
provides uniform, reproducible measurements of concentrations of 
criteria pollutants in ambient air. FRMs for various pollutants are 
described in several appendixes to 40 CFR part 50. For most gaseous 
criteria pollutants (including O3 in Appendix D of part 50), 
the FRM is described as a particular measurement principle and 
calibration procedure to be implemented, with further reference to 
specific analyzer performance requirements specified in 40 CFR part 53.
    The EPA allows new or alternative monitoring technologies--
identified as FEMs--to be used in lieu of FRMs, provided that such 
alternative methods produce measurements closely comparable to 
corresponding FRM measurements. Part 53 sets forth the specific 
performance requirements as well as the performance test procedures 
required by the EPA for determining and designating both FRM and FEM 
analyzers by brand and model.
    To be used in a determination of compliance with the O3 
NAAQS, ambient O3 monitoring data must be obtained using 
either a FRM or a FEM, as defined in parts 50 and 53. For 
O3, nearly all the monitoring methods currently used by 
state and local monitoring agencies are FEM (not FRM) continuous 
analyzers that utilize an alternative measurement principle based on 
quantitative measurement of the absorption of UV light by 
O3. This type of O3 analyzer was introduced into 
monitoring networks in the 1980s and has since become the predominant 
type of method used because of its all-optoelectronic design and its 
ease of installation and operation.
    The existing O3 FRM specifies a measurement principle 
based on quantitative measurement of chemiluminescence from the 
reaction of ambient O3 with ethylene (ET-CL). Ozone 
analyzers based on this FRM principle were once widely deployed in 
monitoring networks, but now they are no longer used for routine 
O3 field monitoring because readily available UV-type FEMs 
are substantially less difficult to install and operate. In fact, the 
extent of the utilization of UV-type FEMs over FRMs for O3 
monitoring is such that FRM analyzers have now become commercially 
unavailable. The last new commercial FRM analyzer was designated by the 
EPA in 1979. The current list of all approved FRMs and FEMs capable of 
providing ambient O3 data for use in NAAQS attainment 
decisions may be found on the EPA's Web site and in the docket for this 
action (U.S. EPA, 2014e). However, that list does not indicate whether 
or not each listed method is still commercially available.
1. Proposed Changes to the FRM for O3
    Although the existing O3 FRM is still a technically 
sound methodology, the lack of commercially available FRM O3 
analyzers severely impedes the use of FRM analyzers, which are needed 
for quality control purposes and as the standard to which candidate 
FEMs are

[[Page 65429]]

required to be compared. Therefore, the EPA proposed to establish a new 
FRM measurement technique for O3 based on NO-
chemiluminescence (NO-CL) methodology. This new chemiluminescence 
technique is very similar to the existing ET-CL methodology with 
respect to operating principle, so the EPA proposed to incorporate it 
into the existing O3 FRM as a variation of the existing ET-
CL methodology, coupled with the same existing FRM calibration 
procedure.
    A revised Appendix D to 40 CFR part 50 was proposed to include both 
the original ET-CL methodology as well as the new NO-CL methodology, 
such that use of either measurement technique would be acceptable for 
implementation in commercial FRM analyzers. Currently, two 
O3 analyzer models (from the same manufacturer) employing 
the NO-CL methodology have been designated by the EPA as FEMs and would 
qualify for re-designation as FRMs under the revised O3 FRM. 
The rationale for selecting the new NO-CL FRM methodology, including 
what other methodologies were also considered, and additional 
information to support its selection are discussed in the preamble to 
the proposal for this action (79 FR 75366-75368). No substantive change 
was proposed to the existing O3 FRM calibration procedure, 
which would be applicable to both chemiluminescence FRM methodologies.
    The proposed FRM in part 50, Appendix D also included numerous 
editorial changes to provide clarification of some provisions, some 
revised wording, additional details, and a more refined numbering 
system and format consistent with that of two other recently revised 
FRMs (for SO2 and CO).
    As noted in the proposal, there is substantial similarity between 
the new and previously existing FRM measurement techniques, and 
comparative field data show excellent agreement between ambient 
O3 measurements made with the two techniques (U.S. EPA 
2014f). Therefore, the EPA believes that there will be no significant 
impact on the comparability between existing ambient O3 
monitoring data based on the original ET-CL methodology and new 
monitoring data that may be based on the NO-CL methodology.
    The proposed FRM retains the original ET-CL methodology, so all 
existing FEMs, which were designated under part 53 based on 
demonstrated comparability to that ET-CL methodology, will retain their 
FEM designations. Thus, there will be no negative consequences or 
disruption to monitoring agencies, which will not be required to make 
any changes to their O3 monitors due to the revised 
O3 FRM. New FEMs would be designated under part 53, based on 
demonstrated acceptable comparability to either FRM methodology.
2. Comments on the FRM for O3
    Comments that were received from the public on the proposed new 
O3 FRM technique are addressed in this section. Most 
commenters expressed general support for the proposed changes, although 
a few commenters expressed some concerns. The most significant issue 
discussed in comments was the relatively small but nevertheless 
potentially significant interference of water vapor observed in the ET-
CL technique. As some comments pointed out, this interference is 
positive and could possibly affect NAAQS attainment decisions. The 
available NO-CL FEM analyzers include a sample dryer, which minimizes 
this interference. As noted previously, very few, if any, ET-CL FRM 
analyzers are still in operation. The ET-CL (with and without a sample 
dryer), the proposed NO-CL FRM, and all designated FEM analyzers have 
demonstrated compliance with the substantially reduced water vapor 
interference equivalent limit specified in 40 CFR part 53.
    The proposed FRM mentioned the need for a sample air dryer for both 
ET-CL and NO-CL FRM analyzers. In response to these comments, the 
wording of the ET-CL FRM has been augmented to clarify the requirement 
for a dryer in all newly designated FRMs (the only change being made by 
the EPA to the existing ET-CL FRM as proposed). Also, the interference 
equivalent limit for water vapor in part 53 was proposed to be 
substantially reduced from the current 0.02 ppm to 0.002 ppm. The 
interference equivalent test for water vapor applicable to the new NO-
CL candidate FRM analyzers (specified in Table B-3 of part 53) was 
proposed to be more stringent than the corresponding existing test for 
ET-CL FRM analyzers by requiring that water vapor be mixed with 
O3. This mixing requirement was not part of the existing 
test for ET-CL candidate analyzers (denoted by footnote 3 in Table B-
3). However, in further response to these commenters' concerns, the EPA 
has modified Table B-3 to extend this water vapor mixing requirement to 
newly designated ET-CL analyzers, as well. These measures should insure 
that potential water vapor interference is minimized in all newly 
designated FRM analyzers.
    Several comments indicated concern that currently-designated FEM 
analyzers retain their designation without retesting if the new FRM 
were promulgated. The current ET-CL FRM is being retained; therefore, 
it is not necessary to make these new requirements retroactive to 
existing designated FEM analyzers. The existing FEM analyzers will not 
be required to be retested, and their FEM designation will be retained 
so that there will be no disruption to current monitoring networks.
    Although beyond the scope of this rulemaking, other comments 
concerned potential hazards of the NO compressed gas supply required 
for NO-CL analyzer operation, and the current non-availability of a 
photolytic converter to provide an alternative source of NO from a less 
hazardous nitrous oxide gas supply. With regard to the photolytic 
converter, the EPA would approve such a converter as a source of NO if 
requested by an FRM analyzer manufacturer, upon demonstration of 
adequate functionality.
    A few commenters liked the ``scrubberless UV absorption'' (SL-UV) 
measurement technique. The EPA has identified the SL-UV method as a 
potentially advantageous candidate for the O3 FRM, but could 
not propose adopting it until additional test and performance 
information becomes available. A related comment requested 
clarification that promulgation of the proposed revised FRM would not 
preclude future consideration of other O3 measurement 
techniques such as SL-UV. In response, the EPA can always consider new 
technologies for FRMs under 40 CFR 53.16 (Supersession of reference 
methods). However, a revised or amended FRM that included the SL-UV 
technique, as set forth in Appendix D of 40 CFR part 50, would have to 
be promulgated as part of a future rulemaking, before a SL-UV analyzer 
could be approved as an FRM under 40 CFR part 53.
    One comment suggested that the value for the absorption cross 
section of O3 at 254 nm used by the FRM's calibration 
procedure should be changed. The comment indicated that the nearly 2% 
difference effectively lowers the O3 NAAQS by that amount. 
Using the corrected value would resolve much of the difference observed 
between O3 measurements calibrated against the UV standard 
reference photometer versus those calibrated using NO gas phase 
titration and it would allow the EPA to adopt the less complex and more 
economical Gas Phase Titration (GPT) technique as the primary 
calibration standard for the

[[Page 65430]]

FRM. The EPA will await the results of further studies determining the 
value of the O3 cross section at 254 nm before making a 
change to the calibration procedures and will not finalize changes to 
the calibration procedures in this final rule.

E. Revisions to the Analyzer Performance Requirements

1. Proposed Changes to the Analyzer Performance Requirements
    In close association with the proposed O3 FRM, the EPA 
also proposed changes to the associated analyzer performance 
requirements for designation of FRMs and FEMs for O3, as set 
forth in 40 CFR part 53. These changes were largely confined to Table 
B-1, which specifies performance requirements for FRM and FEM analyzers 
for SO2, CO, O3, and NO2, and to Table 
B-3, which specifies test concentrations for the various interfering 
agent (interferent) tests. Minor changes were also proposed for Figure 
B-5 and the general provisions in subpart A of part 53. All of these 
proposed changes are described and discussed more fully in the preamble 
to the proposal for this action (79 FR 75368-75369).
    Modest changes proposed for Table B-3 would add new interferent 
test concentrations specifically for NO-CL O3 analyzers, 
which include a test for NO2 interference.
    Several changes to Table B-1 were proposed. Updated performance 
requirements for ``standard range'' analyzers were proposed to be more 
consistent with current O3 analyzer performance 
capabilities, including reduced limits for noise allowance, lower 
detectable limit (LDL), interference equivalent, zero drift, span 
drift, and lag, rise, and fall times. The previous limit on the total 
of all interferents was proposed to be withdrawn as unnecessary and to 
be consistent with that same change made previously for SO2 
and CO analyzers. Also, the span drift limit at 20% of the upper range 
limit (URL) was proposed to be withdrawn because it has similarly been 
shown to be unnecessary and to maintain consistency with that same 
change made previously for SO2 and CO analyzers.
    The form of the precision limits at both 20% and 80% of the URL was 
proposed to be changed from ppm to percent. The proposed new limits (in 
percent) were set to be equivalent to the previously existing limits 
(in ppm) and thus remain effectively unchanged. This change in form of 
the precision limits in Table B-1 has been previously made for 
SO2 and CO analyzers, and was proposed to extend also to 
analyzers for NO2, (again with equivalent limits) for 
consistency and to simplify Table B-1 across all types of analyzers to 
which the table applies. A new footnote proposed for Table B-1 
clarifies the new form for precision limits as ``standard deviation 
expressed as percent of the URL.'' Also proposed was a revision to 
Figure B-5 (Calculation of Zero Drift, Span Drift, and Precision) to 
reflect the changes proposed in the form of the precision limits and 
the withdrawal of the limits for total interference equivalent.
    Concurrent with the proposed changes to the performance 
requirements for candidate O3 analyzers, the EPA conducted a 
review of all designated FRM and FEM O3 analyzers currently 
in production or being used, and verified that all meet the proposed 
new performance requirements. Therefore, none would require withdrawal 
or cancellation of their current FRM or FEM respective designations.
    Finally, the EPA proposed new, optional, ``lower range'' 
performance limits for O3 analyzers operating on measurement 
ranges lower (i.e., more sensitive) than the standard range specified 
in Table B-1. The new performance requirements are listed in a new 
``lower range'' column in Table B-1 and will provide for more stringent 
performance in applications where more sensitive O3 
measurements are needed.
    Two minor changes were proposed to the general, administrative 
provisions in Subpart A of part 53. These include an increase in the 
time allowed for the EPA to process requests for approval of 
modifications to previously designated FRMs and FEMs in 53.14 and the 
withdrawal of a requirement for annual submission of Product 
Manufacturing Checklists associated with FRMs and FEMs for 
PM2.5 and PM10-2.5 in 53.9. No comments were 
received on these proposed changes and the EPA will be finalizing these 
revisions in this rulemaking.
2. Comments on the Analyzer Performance Requirements
    Several comments were received related to the proposed changes to 
the analyzer performance requirements of part 53, and most were 
supportive. Comments from a few monitoring agencies suggested that the 
more stringent performance requirements proposed might be difficult to 
achieve or would increase monitor maintenance and cost. The EPA is also 
clarifying that these requirements apply only to the performance 
qualification requirements for designations of new FRM and FEM 
analyzers and will have no impact on a monitoring agency's operation of 
existing O3 analyzers.
    More specific comments from an analyzer manufacturer pointed out 
that the proposed lower limits for noise and LDL may be too stringent, 
the former because low-cost portable analyzers may have shorter 
absorption cells, and the latter because of limitations of current 
calibration technology. After further consideration of available 
analyzer performance data in light of these comments, the EPA agrees 
and is changing the noise limits from the proposed values of 1 ppb and 
0.5 ppb (for the standard and lower ranges, respectively) to 2.5 ppb 
and 1 ppb (respectively). The EPA is also changing the LDL limit from 
the proposed values of 3 ppb and 1 ppb (respectively) to 5 ppb and 2 
ppb (respectively). These new limits are still considerably more 
stringent than the previous limits (for the standard range) and are 
also consistent with those recommended by the commenter and the current 
performance capabilities of existing analyzer/calibration technology.
    This commenter also pointed out that the proposed lower limit for 
12-hour zero drift, together with the way the prescribed test is 
carried out, resulted in the test being dominated by analyzer noise 
rather than drift. The EPA agrees with this comment in general but 
believes that further study is needed before any specific changes can 
be proposed for the 12-hour zero drift test, particularly since any 
such changes would affect analyzers for other gaseous pollutants, as 
well.
    Other comments suggested that there was no need for the proposed 
new, low-range performance requirements, because of cost and that 
available calibrators would be inadequate for calibration of such low 
ranges. The EPA disagrees with these comments and believes, as noted in 
the proposal preamble, that there is a definite need for low-level 
O3 measurements in some applications and that suitable 
calibration for such low-level measurement ranges can be adequately 
carried out. As stated previously, the new ``low range'' specifications 
for O3 analyzers are optional.
    Several comments pointed out some typographical errors related to 
footnotes in Table B-3, as proposed; these errors have been corrected 
in the version of Table B-3 being finalized today.
    EPA is finalizing the proposed amendments to both the O3 
FRM in Appendix D of part 50 and provisions in part 53, modified as 
described above, in response to the comments received.

[[Page 65431]]

VII. Grandfathering Provision for Certain PSD Permits

    This section addresses the grandfathering provision for certain 
Prevention of Significant Deterioration (PSD) permit applications that 
is being finalized in this rule. Section VIII.C of this preamble 
contains a description of the PSD and Nonattainment New Source Review 
(NNSR) permitting programs and additional discussion of the 
implementation of those programs for the O3 NAAQS.

A. Summary of the Proposed Grandfathering Provision

    The EPA proposed to amend the PSD regulations to add a transition 
plan that would address the extent to which the revised O3 
NAAQS will apply to pending PSD permit applications. This transition 
plan is reflected in a grandfathering provision that applies to permit 
applications that meet certain milestones in the review process prior 
to either the signature date or effective date of the revised 
O3 NAAQS. Absent such a grandfathering provision in the 
EPA's regulations, the EPA interprets section 165(a)(3)(B) of the CAA 
and the implementing PSD regulations at 40 CFR 52.21(k)(1) and 
51.166(k)(1) to require that PSD permit applications include a 
demonstration that emissions from the proposed facility will not cause 
or contribute to a violation of any NAAQS that is in effect as of the 
date the PSD permit is issued. The proposal included a grandfathering 
provision that would enable eligible PSD applications to make the 
demonstration that the proposed project would not cause or contribute 
to a violation of any NAAQS with respect to the O3 NAAQS in 
effect at the time the relevant permitting benchmark for grandfathering 
was reached, rather than the revised O3 NAAQS. We proposed 
that the grandfathering provision would apply specifically to either of 
two categories of pending PSD permit applications: (1) Applications for 
which the reviewing authority has formally determined that the 
application is complete on or before the signature date of the final 
rule revising the O3 NAAQS; and (2) applications for which 
the reviewing authority has first published a public notice of the 
draft permit or preliminary determination before the effective date of 
the revised NAAQS.
    In the proposal, we also noted that for sources subject to the 
federal PSD program under 40 CFR 52.21, the EPA and air agencies that 
have been delegated authority to implement the federal PSD program for 
the EPA would apply the grandfathering provision to any PSD application 
that satisfies either of the two criteria that make an application 
eligible for grandfathering. Accordingly, if a particular application 
does not qualify under the first criterion based on a complete 
application determination, it may qualify under the second criterion 
based on a public notice announcing the draft permit or preliminary 
determination. Conversely, a source may qualify for grandfathering 
under the first criterion, even if it does not satisfy the second.
    The EPA also proposed revisions to the PSD regulations at 40 CFR 
51.166 that would afford air agencies that issue PSD permits under a 
SIP-approved PSD permit program the discretion to adopt provisions into 
the SIP that allow for grandfathering of pending PSD permits under the 
same circumstances as set forth in the federal PSD regulations. With 
regard to implementing the grandfathering provision, we also explained 
that air agencies with EPA-approved PSD programs in their SIPs would 
have additional flexibility for implementing the proposed 
grandfathering provision to the extent that any alternative approach is 
at least as stringent as the federal provision. In addition, the 
proposal recognized that some air agencies do not make formal 
completeness determinations; thus, only the latter criterion based on 
the issuance of a public notice would be relevant in such cases and the 
state could elect to adopt only that criterion into its SIP. 
Accordingly, the EPA proposed to add a grandfathering provision to 40 
CFR 51.166 containing the same two criteria as proposed for 40 CFR 
52.21.

B. Comments and Responses

    Many of the comments supported the concept of grandfathering. Some 
of these comments, mostly by state and local air agencies, supported 
the grandfathering provision as proposed. Many others recommended 
alternative approaches to grandfathering based on several different 
dates. Several comments recommended that air agencies be allowed to 
grandfather certain PSD permit applications and issue a PSD permit 
based on the 2008 O3 NAAQS after the area is designated 
nonattainment for the revised O3 NAAQS. An opposing set of 
comments, representing a coalition of eight environmental groups and 
one health advocacy group, strongly objected to the proposal for 
grandfathering, claiming that the EPA did not have any authority under 
the CAA to exempt or grandfather permit applicants from the statutory 
PSD permitting requirements. We are addressing some of these comments 
below and others in the Response to Comment Document that is included 
in the docket for this rule.
    Comments that recommended broadening the scope of the proposed 
grandfathering provision suggested a variety of approaches. Some air 
agency and industry comments recommended that the EPA adopt a 
grandfathering provision applicable only to those PSD applications for 
which the reviewing authority has determined the application to be 
complete on or before the signature date of the revised NAAQS. Other 
air agency and industry comments recommended that grandfathered status 
be determined only on the basis of whether the relevant permitting 
milestone has been achieved by the effective date of the revised NAAQS.
    The EPA disagrees with these comments; the final rule uses separate 
dates for the two grandfathering milestones, as proposed. If the 
effective date of the revised NAAQS were used as the date for the 
complete application milestone, this could lead to pressure on state 
permitting authorities to prematurely issue completeness determinations 
in order to qualify for the grandfathering provision in the time period 
between signature of this final rule and the effective date. Using the 
signature date of the revised O3 NAAQS as the date for the 
grandfathering milestone based on the completeness determination is 
thus intended to help preserve the integrity of the completeness 
determination process. Permit applications that have not yet been 
determined complete can be supplemented or revised to address the 
revised O3 standards before the completeness determination 
is issued. Conversely, the amount and type of work required for a 
preliminary determination or a draft permit reduces the risk that such 
a document would be released prematurely merely to qualify for 
grandfathering. Similarly, because these documents are released for the 
purpose of providing an adequate opportunity for public participation 
in the permitting process, it would not behoove a reviewing authority 
to precipitately release such documents merely to satisfy the 
grandfathering milestone. Accordingly, the EPA does not have the same 
concerns about using the effective date of this final rule for the 
preliminary determination or draft permit milestone and further finds 
it reasonable to provide additional time for satisfying this milestone. 
Moreover, using the proposed milestones and corresponding dates is 
consistent with the milestones and corresponding dates that were used 
in the grandfathering provisions for the 2012 PM2.5 NAAQS.

[[Page 65432]]

    Several other comments recommended that the grandfathering 
provision apply to all PSD applications for which a final PSD permit 
will be issued prior to the effective date of the area designations for 
the revised NAAQS. Some of these comments explained that without some 
transition provisions in the final rule, it may be impossible for a 
source to demonstrate attainment if the current ambient air monitoring 
data indicates a revised, lowered standard is not being met. The 
comments also suggested that the extended period for grandfathering a 
source from the revised NAAQS would provide states with additional time 
to establish offset banks or similar systems for new nonattainment 
areas.
    Other comments recommended that air agencies be allowed to 
grandfather either all or certain PSD permit applications received 
before the effective date of the final nonattainment designations for 
the revised O3 NAAQS. These comments supported allowing air 
agencies to issue PSD permits to grandfathered sources even after the 
area in which the source proposes to locate is designated nonattainment 
for the revised O3 NAAQS. One comment saw this as being 
necessary because the development of the regulatory framework that will 
support the revised NAAQS, such as development of a credit market or 
even a transition into NNSR permitting, does not instantaneously 
accompany the revised standard. Hence, the comment added that 
``[d]uring the Interim Period (the time between the revision of the 
NAAQS rule and development of the regulatory framework) the project may 
be unable to secure offsets and no offsets would be available for 
purchase.'' Another comment explained that the extended period for 
grandfathering sources from the revised O3 NAAQS was needed 
to ``minimize disruption to complex projects that may have been under 
development since before the EPA published the proposed NAAQS 
revision.'' This comment noted the ``PSD projects commonly undergo 
years of engineering and other development resources before an air 
permit application can be prepared.''
    The EPA does not agree with the comments recommending that the EPA 
use a date after the effective date of the revised O3 NAAQS 
as the date by which the permit application must reach the relevant 
milestone to qualify for grandfathering. The EPA does not believe it is 
appropriate to unreasonably or unnecessarily delay implementation of 
these revised standards under the PSD program. As explained in more 
detail below, the purpose of the grandfathering provision is to provide 
a reasonable transition mechanism for certain PSD applications and the 
EPA believes that the milestones proposed and finalized here strike the 
appropriate balance in providing for such a reasonable transition. 
Moreover, in some cases, some of these recommended approaches could 
enable a situation where a PSD permit would be issued to a source 
during a future period when the area is designated nonattainment for 
the revised O3 NAAQS. As explained below, the EPA does not 
believe that this specific outcome is permissible under the CAA.
    The EPA does not agree with the comments suggesting that the 
grandfathering provision should be expanded to apply to any PSD 
application received before the effective date of the final 
nonattainment designations for the revised O3 NAAQS. Because 
the process for reviewing PSD permit applications and issuing a final 
PSD permit is time consuming, such an approach could allow issuance of 
PSD permits to grandfathered sources even after the area in which the 
source proposes to locate is designated nonattainment for the revised 
O3 NAAQS. The EPA does not agree that grandfathering should 
be extended in a way that would allow a source located in an area 
designated as nonattainment for a pollutant at the time of permit 
issuance to obtain a PSD permit for that pollutant rather than a NNSR 
permit. The EPA does not interpret the CAA or its implementing 
regulations to allow such an outcome. The PSD requirements under CAA 
section 165 only apply in areas designated attainment or unclassifiable 
for the pollutant. Alabama Power v. Costle, 636 F.2d 323, 365-66, 368 
(D.C. Cir. 1980). Accordingly, the PSD implementing regulations at 40 
CFR 52.21(i)(2) contain an exemption that provides that the substantive 
PSD requirements shall not apply to a pollutant if the owner or 
operator demonstrates that the facility is located in an area 
designated nonattainment for that pollutant under CAA section 107 of 
the Act. See also 40 CFR 51.166(i)(2) (allowing for the same exemption 
in SIP-approved PSD permitting programs). In addition, under CAA 
section 172(c)(5) implementation plans must require that permits issued 
to new or modified stationary sources ``anywhere in the nonattainment 
area'' meet the requirements of CAA section 173, which contains the 
NNSR permit requirements. See 40 CFR part 51, Appendix S, IV.A 
(providing that, if a major new source or major modification that would 
locate in an area designated as nonattainment for a pollutant for which 
the source or modification would be major, approval to construct may be 
granted only if the specific conditions for NNSR are met, including 
obtaining emission offsets and an emission limitation that specifies 
the lowest achievable emissions rate). Moreover, given the adverse air 
quality conditions that already exist in a nonattainment area and the 
congressional directive to reach attainment as expeditiously as 
practicable, construction of a major stationary source that 
significantly increases emissions in such an area should be expected to 
address all of the NNSR requirements, which are designed to ensure that 
a new or modified major stationary source will not interfere with 
reasonable progress toward attainment, even if this could cause delay 
to the permit applicant.
    With respect to the comments that suggested the effective date of 
the NAAQS should be used as the date for both milestones, the EPA does 
not agree that such a change is necessary. The purpose of the 
grandfathering provision is to provide a reasonable transition 
mechanism in the following circumstances: first, the PSD application is 
one for which both the applicant and the reviewing authority have 
committed substantial resources; and, second, this situation is one 
where the need to satisfy the demonstration requirement under CAA 
section 165(a)(3) could impact the reviewing authority's ability to 
meet the statutory deadline for issuing a permit within one year of the 
completeness determination. In situations where the reviewing authority 
has not yet issued a completeness determination as of the signature 
date of the revised O3 NAAQS, both the permit applicant and 
the reviewing authority have sufficient notice of the revised standard 
so that it can be addressed before the completeness determination is 
issued and the one-year clock begins to run. The grandfathering 
provision issued in this rulemaking is crafted to draw a reasonable 
balance that accommodates the requirements under both CAA sections 
165(a)(3) and 165(c). Any modification of the dates further than is 
necessary to accommodate these concerns could upset this balance.
    With respect to the comments that suggested adopting a 
grandfathering provision applicable only to those PSD applications for 
which the reviewing authority has determined the application to be 
complete on or before the signature date of the revised NAAQS, the EPA 
is not making this change because we understand that not all reviewing 
authorities issue formal completeness determinations. Including

[[Page 65433]]

a grandfathering provision based on the publication of a public notice 
of the draft permit or preliminary determination provides a reasonable 
transition mechanism for PSD applications in situations where the 
reviewing authority does not issue formal completeness determinations, 
but the applicant and the reviewing authority have both committed 
substantial resources to the pending permit application at the time the 
revisions to the O3 NAAQS are finalized.
    An opposing set of comments--submitted by a consortium of eight 
environmental groups and one health advocacy group--challenged the 
proposed grandfathering provision on the basis that the EPA did not 
have the legal authority to grandfather sources from PSD requirements. 
These commenters argued that the plain language of CAA section 165 
forecloses the EPA's proposed approach and raised several other legal 
considerations. The EPA disagrees with these comments, including the 
interpretations of the CAA that they offer. As summarized in the 
rationale for the final action below in section VII.C of this preamble, 
the EPA believes that the CAA provides it authority and discretion to 
establish a PSD grandfathering provision such as the one being adopted 
today through a rulemaking process. The EPA is providing a further, 
detailed analysis fully responding to this set of comments, as well as 
other comments related to the grandfathering provision, in the Response 
to Comment Document in the docket for this rule.

C. Final Action and Rationale

    After consideration and evaluation of all the public comments 
received on the grandfathering provision, the EPA is finalizing this 
provision as proposed, with minor revisions that enhance the clarity of 
the grandfathering provision, without changing its substantive effect. 
While these revisions lead to slight differences in wording for the 
grandfathering provision for the 2012 PM2.5 NAAQS and the 
grandfathering provision finalized in this rulemaking, those 
differences are not intended to create a different meaning; rather, the 
grandfathering provision finalized in this rulemaking is intended to 
have the same substantive effect and meaning for the revised 
O3 standards as the grandfathering provision for the 2012 
PM2.5 NAAQS had for the revised PM standards. Other than 
those clarifying revisions, this final rule includes the same rule 
language for the grandfathering provision as previously proposed for 
the PSD regulations at 40 CFR 52.21(i)(12) and 51.166(i)(11), 
respectively. The provision in the final rule reflects the same two 
milestones and corresponding dates as the proposed grandfathering 
provision. Thus, under the grandfathering provision as finalized, 
either of the following two categories of pending PSD permit 
applications would be eligible for grandfathering: (1) Applications for 
which the reviewing authority has formally determined that the 
application is complete on or before the signature date of the revised 
O3 NAAQS, or (2) applications for which the reviewing 
authority has first published a notice of a draft permit or preliminary 
determination before the effective date of the revised O3 
NAAQS. The EPA believes that it continues to be appropriate to include 
the two proposed milestones for pending permit applications to be 
eligible for grandfathering. While a completeness determination is 
often the first event, some air agencies do not determine applications 
complete as part of their permit process.
    Under 40 CFR 52.21, a permit application may qualify for 
grandfathering under either of the two sets of milestones and dates 
contained in the provision. Where the EPA is the reviewing authority, 
the EPA intends to apply the grandfathering provision to PSD applicants 
pursuant to PSD regulations at 40 CFR 52.21 primarily through the use 
of the completeness determination milestone because the EPA Regional 
Offices make a formal completeness determination for any PSD 
application that they receive and review. The EPA is including the 
second criterion in 40 CFR 52.21 so that pending applications can still 
qualify for grandfathering under the second criterion if any air agency 
that incorporates 40 CFR 52.21 into a SIP-approved program does not 
make formal completeness determinations as part of its permit review 
process.
    The EPA is also amending the PSD regulations at 40 CFR 51.166 to 
enable states and other air agencies that issue PSD permits under SIP-
approved PSD programs to adopt a comparable grandfathering provision. 
Nevertheless, such air agencies have discretion to not grandfather PSD 
applications or to apply grandfathering under their approved PSD 
programs in another manner as long as that program is at least as 
stringent as the provision being added to 40 CFR 51.166. Accordingly, 
an air agency may elect to rely on both sets of milestones and dates or 
it may grandfather on the sole basis of only one set. However, the EPA 
anticipates that once a decision is made concerning the use of either 
set of milestones and dates, the air agency will apply grandfathering 
consistently to all pending PSD permit applications.
    As explained in more detail in the proposal, absent a regulatory 
grandfathering provision, the EPA interprets section 165(a)(3)(B) of 
the CAA and the implementing PSD regulations at 40 CFR 52.21(k)(1) and 
51.166(k)(1) to require that PSD permit applications include a 
demonstration that emissions from the proposed facility will not cause 
or contribute to a violation of any NAAQS that is in effect as of the 
date the PSD permit is issued. However, reading CAA section 
165(a)(3)(B) in context with other provisions of the Act and the 
legislative history, the EPA interprets the Act to provide the EPA with 
authority to establish grandfathering provisions through regulation. 
The EPA has explained its interpretation of its authority to promulgate 
grandfathering provisions in previous rulemaking actions, most recently 
in the rule establishing the grandfathering provision for the 2012 
PM2.5 NAAQS (78 FR 3086, 3254-56, January 15, 2013), as well 
as in the proposal for this final action. The EPA is providing 
additional discussion of this authority in the Response to Comment 
Document contained in the docket for this final action.
    To summarize briefly, the addition of this grandfathering provision 
is permissible under the discretion provided by the CAA for the EPA to 
craft a reasonable implementation regulation that balances competing 
objectives of the statutory PSD program found in CAA section 165. 
Specifically, section 165(a)(3) requires a permit applicant to 
demonstrate that its proposed project will not cause or contribute to a 
violation of any NAAQS, while section 165(c) requires that a PSD permit 
be granted or denied within one year after the permitting authority 
determines the application for such permit to be complete. Section 
109(d)(1) of the CAA requires the EPA to review existing NAAQS and make 
appropriate revisions every five years. When these provisions are 
considered together, a statutory ambiguity arises concerning how the 
requirements under CAA section 165(a)(3)(B) should be applied to a 
limited set of pending PSD permit applications when the O3 
NAAQS is revised. The Act does not clearly address how the requirements 
of CAA section 165(a)(3)(B) should be met for PSD permit applications 
that are pending when the NAAQS are revised, particularly when the EPA 
also determines that complying with the

[[Page 65434]]

demonstration requirement for the revised NAAQS could hinder compliance 
with the requirement under section 165(c) to issue a permit within one 
year of the completeness determination for a certain subset of pending 
permits. The CAA also does not address how the requirements of CAA 
sections 165(a)(3) and 165(c) should be balanced in light of the 
statutory requirement to review the NAAQS every five years. As Congress 
has not spoken precisely to this issue, the EPA has the discretion to 
apply a permissible interpretation of the Act that balances the 
statutory requirements to make a decision on a permit application 
within one year and to ensure the new and modified sources will only be 
authorized to construct after showing they can meet the substantive 
permitting criteria. See Chevron, U.S.A., Inc. v. Natural Res. Def. 
Council, Inc., 467 U.S. 837, 843-44 (1984).
    In addressing these gaps in the CAA and the tension that may arise 
in section 165 in these circumstances, the EPA also applies CAA section 
301, where the Administrator is authorized ``to prescribe such 
regulations as are necessary to carry out his functions under this 
chapter.'' Sections 165(a)(3) and 165(c) of the CAA make clear that the 
interests behind CAA section 165 include both protection of air quality 
and timely decision-making on pending permit applications. The 
legislative history illustrates congressional intent to avoid delays in 
permit processing. S. Rep. No. 94-717, at 26 (1976) (``nothing could be 
more detrimental to the intent of this section and the integrity of 
this Act than to have the process encumbered by bureaucratic delay''). 
Thus, when read in combination, these provisions of the CAA provide the 
EPA with the discretion to issue regulations to grandfather pending 
permit applications from having to address a revised NAAQS where 
necessary to achieve both CAA objectives--to protect the NAAQS and to 
avoid delays in processing PSD permit applications. Accordingly, the 
EPA is seeking in this action to balance the requirements in the CAA to 
make a decision on a permit application within one year and to ensure 
that new and modified sources will only be authorized to construct 
after showing they can meet the substantive permitting criteria that 
apply to them. The EPA is achieving this balance by determining through 
rulemaking which O3 NAAQS apply to certain permit 
applications that are pending when the EPA finalizes the revisions to 
the O3 NAAQS in this final rule. We are clarifying, for the 
limited purpose of satisfying the requirements under section 
165(a)(3)(B) for those permits, which O3 NAAQS are 
applicable to those permit applications and must be addressed in the 
source's demonstration that its emissions do not cause or contribute to 
a violation of the NAAQS.
    This approach is consistent with a recent opinion by the U.S. Court 
of Appeals for the Ninth Circuit, which recognized the EPA's 
traditional exercise of grandfathering authority through rulemaking. 
The court observed that this approach was consistent with the statutory 
requirement to ``enforce whatever regulations are in effect at the time 
the agency makes a final decision'' because it involved identifying 
``an operative date, incident to setting the new substantive standard, 
and the grandfathering of pending permit applications was explicitly 
built into the new regulations.'' Sierra Club v. EPA, 762 F.3d 971, 983 
(9th Cir. 2014). As discussed in more detail in the EPA's Response to 
Comment Document contained in the docket for this rule, this case 
supports the EPA's action in this rulemaking. The court favorably 
discussed prior adoption of regulatory grandfathering provisions that 
are similar to the action in this rulemaking, such as the 
grandfathering provision that the EPA promulgated when revising the 
PM2.5 NAAQS that became effective in 2013. See id. at 982-
83.\227\
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    \227\ This case specifically involved an action by the EPA to 
issue an individual PSD permit, which grandfathered a specific 
permit applicant from certain requirements without any revision to 
the regulations that were in effect. The court's reasoning in this 
case distinguishes that type of permit-specific grandfathering from 
establishing grandfathering provisions through a rulemaking process. 
While the court was not persuaded that there was a conflict between 
the requirements of sections 165(a)(3) and 165(c) of the CAA that 
supported the permit-specific grandfathering at issue in that case, 
it did not extend that uncertainty to its discussion of the EPA's 
rulemaking authority. In fact, in its favorable discussion of the 
EPA's authority to grandfather pending permit applications through 
regulation, the court noted that the power of an administrative 
agency ``to administer a congressionally created and funded program 
necessarily requires the formulation of policy and the making of 
rules to fill any gap left, implicitly or explicitly, by Congress'' 
though ``such decision cannot be made on an ad hoc basis.'' Sierra 
Club v. EPA, 762 F.3d 971, 983 (9th Cir. 2014) (internal quotations 
and marks omitted). This indicates that the court believed there is 
a gap in the CAA that supports including grandfathering provisions 
in regulations.
---------------------------------------------------------------------------

    This adoption of a grandfathering provision in this action is also 
consistent with previous actions in which the EPA has recognized that 
the CAA provides discretion for the EPA to establish grandfathering 
provisions for PSD permit applications through regulations. Some 
examples of previous references to the EPA's authority to grandfather 
certain applications through rulemaking include 45 FR 52683, August 7, 
1980; 52 FR 24672, July 1, 1987; and most recently 78 FR 3086, January 
15, 2013.
    This grandfathering provision does not apply to any applicable PSD 
requirements related to O3 other than the requirement to 
demonstrate that the proposed source does not cause or contribute to a 
violation of the revised O3 NAAQS. Sources with projects 
qualifying under the grandfathering provision will be required to meet 
all the other applicable PSD requirements, including applying BACT to 
all applicable pollutants, demonstrating that emissions from the 
proposed facility will not cause or contribute to a violation of the 
O3 NAAQS in effect at the time of the relevant 
grandfathering milestone, and addressing any Class I area and 
additional O3-related impacts in accordance with the 
applicable PSD requirements. In addition, this grandfathering provision 
would not apply to any permit application for a new or modified major 
stationary source of O3 located in an area designated 
nonattainment for O3 on the date the permit is issued.

VIII. Implementation of the Revised O3 Standards

    This section provides background information for understanding the 
implications of the revised O3 NAAQS and describes the EPA's 
plans for providing revised rules or additional guidance on some 
subjects in a timely manner to assist states with their implementation 
efforts under the requirements of the CAA. This section also describes 
existing EPA rules, interpretations of CAA requirements, and other EPA 
guidance relevant to implementation of the revised O3 NAAQS. 
Relevant CAA provisions that provide potential flexibility with regard 
to meeting implementation timelines are highlighted and discussed. This 
section also contains a discussion of how existing requirements to 
reduce the impact on O3 concentrations from the stationary 
source construction in permit programs under the CAA are affected by 
the revisions to the O3 NAAQS. These are the PSD and 
Nonattainment New Source Review (NNSR) programs. As discussed in 
section VII of this preamble, to facilitate a smooth transition to the 
PSD requirements for the revised O3 NAAQS, the EPA is 
finalizing as part of this rulemaking a grandfathering provision that 
applies to certain PSD permit applications that are pending and have 
met certain milestones in the permitting process

[[Page 65435]]

when the revised O3 NAAQS is signed or before the effective 
date of the revised O3 NAAQS, depending on the milestone.
    In the preamble for the O3 NAAQS proposal, the EPA 
solicited comments on several issues related to implementing the 
revised O3 NAAQS that the agency anticipated addressing in 
future guidance or regulatory actions, but for which the EPA was not at 
that time proposing any action. The EPA received numerous comments on 
those and other implementation issues. Consistent with what the EPA 
indicated in the O3 NAAQS proposal (79 FR 75370), the agency 
is not responding to the implementation comments that are not related 
to a specific proposal. However, the EPA intends to take these comments 
under advisement as the agency develops rules and guidance to assist 
with implementation of the revised NAAQS. Because the EPA did 
specifically propose and is finalizing provisions in the regulations 
addressing grandfathering for certain PSD permit applications and 
requirements, as discussed in section VII of this preamble, the EPA is 
responding to comments on the proposed PSD grandfathering provisions.

A. NAAQS Implementation Plans

1. Cooperative Federalism
    As directed by the CAA, reducing pollution to meet national air 
quality standards always has been a shared task, one involving the 
federal government, states, tribes and local air quality management 
agencies. The EPA develops regulations and strategies to reduce 
pollution on a broad scale, while states and tribes are responsible for 
implementation planning and any additional emission reduction measures 
necessary to bring specific areas into attainment. The agency supports 
implementation planning with technical resources, guidance, and program 
rules where necessary, while air quality management agencies use their 
knowledge of local needs and opportunities in designing emission 
reduction strategies that will work best for their industries and 
communities.
    This partnership has proved effective since the EPA first issued 
O3 standards more than three decades ago. For example, 101 
areas were designated as nonattainment for the 1-hour O3 
standards issued in 1979. As of the end of 2014, air quality in all but 
one of those areas meets the 1-hour standards. The EPA strengthened the 
O3 standards in 1997, shifting to an 8-hour standard to 
improve public health protection, particularly for children, the 
elderly, and other sensitive individuals. The 1997 standards drew 
significant public attention when they were proposed, with numerous 
parties voicing concerns about states' ability to comply. However, 
after close collaboration between the EPA, states, tribes and local 
governments to reduce O3-forming pollutants, significant 
progress has been made. Air quality in 108 of the original 115 areas 
designated as nonattainment for the 1997 O3 NAAQS now meets 
those standards. Air quality in 18 of the original 46 areas designated 
as nonattainment for the 2008 O3 NAAQS now meets those 
standards.
    The revisions to the primary and secondary O3 NAAQS 
discussed in sections II.D and IV.D of this preamble trigger a process 
under which states \228\ make recommendations to the Administrator 
regarding area designations. Then, the EPA promulgates the final area 
designations. States also are required to review capacity and 
authorities in their existing SIPs to ensure the CAA requirements 
associated with the new standards can be carried out, and modify or 
supplement their existing SIPs as needed. The O3 NAAQS 
revisions also apply to the transportation conformity and general 
conformity determinations, and affect which preconstruction permitting 
requirements apply to sources of O3 precursor emissions, and 
the nature of those requirements.
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    \228\ This and all subsequent references to ``state'' are meant 
to include state, local, and tribal agencies responsible for the 
implementation of an O3 control program.
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    The EPA has regulations in place addressing the general 
requirements for SIPs, and there are also provisions in these existing 
rules that cover O3 SIPs (40 CFR part 51). States likewise 
have provisions in their existing SIPs to address air quality for 
O3 and to implement the existing O3 NAAQS. In the 
course of the past 45 years of regulating criteria pollutants, 
including O3, the EPA has also provided general guidance on 
the development of SIPs and administration of construction permitting 
programs, as well as specific guidance on implementing the 
O3 NAAQS in some contexts under the CAA and the EPA 
regulations.
    The EPA has considered the extent to which existing EPA regulations 
and guidance are sufficient to implement the revised standards. The CAA 
does not require that the EPA promulgate new implementing regulations 
or issue new guidance for states every time that a NAAQS is revised. 
Likewise, the CAA does not require the issuance of additional 
implementing regulations or guidance by the EPA before a revised NAAQS 
becomes effective. It is important to note that the existing EPA 
regulations in 40 CFR part 51 applicable to SIPs generally and to 
particular pollutants, including O3 and O3 
precursors, continue to apply unless and until they are updated. 
Accordingly, the discussion below provides the EPA's current thoughts 
about the extent to which revisions to existing regulations and 
additional guidance are appropriate to aid in the implementation of the 
revised O3 NAAQS.
2. Additional New Rules and Guidance
    The EPA has received comments from a variety of states and 
organizations asking for rules and guidance associated with a revised 
NAAQS to be issued in a timely manner. As explained above, and 
consistent with the proposal, the EPA is not responding to these 
comments at this time because they are not related to any changes to 
existing regulations that EPA proposed in this rule. Moreover, although 
issuance of such rules and guidance is not a part of the NAAQS review 
process, National Ass'n of Manufacturers v. EPA, 750 F. 3d 921, 926-27 
(D.C. Cir. 2014), toward that end, the EPA intends to develop 
appropriate revisions to necessary implementation rules and provide 
additional guidance in time frames that are useful to states when 
developing implementation plans that meet CAA requirements.
    Certain requirements under the PSD preconstruction permit review 
program apply immediately to a revised NAAQS upon the effective date of 
that NAAQS, unless the EPA has established a grandfathering provision 
through rulemaking. To ensure a smooth transition to a revised 
O3 NAAQS, the EPA is finalizing a grandfathering provision 
similar to the provision finalized in the 2012 PM2.5 NAAQS 
Rule. See section VII.C of this preamble for more details on the PSD 
program and the final grandfathering provision.
    Promulgation or revision of the NAAQS starts a clock for the EPA to 
designate areas as either attainment or nonattainment. State 
recommendations for area designations are due to the EPA within 12 
months of promulgating or revising the NAAQS. In an effort to allow 
states to make more informed recommendations for these particular 
standards, the EPA intends to issue additional guidance concerning the 
designations process for these standards within four months of 
promulgation of the NAAQS, or approximately eight months before state 
recommendations are due. The EPA generally completes

[[Page 65436]]

area designations two years after promulgation of a NAAQS. See section 
VIII.B of this preamble for additional information on the initial area 
designation process.
    Under CAA section 110, a NAAQS revision triggers the review and, as 
necessary, revision of SIPs to be submitted within three years of 
promulgation of a revised NAAQS. These SIPs are referred to as 
``infrastructure SIPs.'' The EPA issued general guidance on submitting 
infrastructure SIPs on September 13, 2013.\229\ It should be noted that 
this guidance did not address certain state planning and emissions 
control requirements related to interstate pollution transport. This 
guidance remains relevant for the revised O3 NAAQS. See 
section VIII.A.4 of this preamble for additional information on 
infrastructure SIPs.
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    \229\ See memorandum from Stephen D. Page to Regional Air 
Directors, ``Guidance on Infrastructure State Implementation Plan 
(SIP) Elements under Clean Air Act Sections 110(a)(1) and 
110(a)(2)'' September 13, 2013, which is available at http://www3.epa.gov/airquality/urbanair/sipstatus/docs/Guidance_on_Infrastructure_SIP_Elements_Multipollutant_FINAL_Sept_2013.pdf.
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    While much of the existing rules and guidance for prior ozone 
standards remains applicable to the new standards, the EPA intends to 
propose to adopt revised rules on some subjects to facilitate air 
agencies' efforts to implement the revised O3 NAAQS within 
one year after the revised NAAQS is established. The rules would 
address nonattainment area classification methodologies and attainment 
dates, attainment plan and NNSR SIP submission due dates, and any other 
necessary revisions to existing regulations for other required 
implementation programs. The EPA anticipates finalizing these rules by 
the time areas are designated nonattainment. Finalizing rules and 
guidance on these subjects by this time would assist air quality 
management agencies with development of any CAA-required SIPs 
associated with nonattainment areas. See section VIII.A.5 of this 
preamble for additional information on nonattainment SIPs and section 
VIII.C.3 for additional information on nonattainment New Source Review 
requirements applicable to new major sources and major modifications of 
existing sources.
3. Background O3
    The EPA and state, local and tribal air agencies, strive to 
determine how to most effectively and efficiently use the CAA's various 
provisions to provide required public health and welfare protection 
from the harmful effects of O3. In most cases, reducing man-
made emissions of NOX and VOCs within the U.S. will reduce 
O3 formation and provide additional health and welfare 
protection. The EPA recognizes, however, that there can be infrequent 
events where daily maximum 8-hour O3 concentrations approach 
or exceed 70 ppb largely due to the influence of wildfires or 
stratospheric intrusions, which contribute to U.S. background (USB) 
levels but may also qualify for consideration under the Exceptional 
Events Rule. See section I.D; but see section II.A.2.a above 
(percentage of anthropogenic O3 tends to increase on high 
O3 days relative to percentage of background, including in 
intermountain west).
    The term ``background'' O3 is often used to refer to 
O3 that originates from natural sources of O3 
(e.g., wildfires and stratospheric O3 intrusions) and 
O3 precursors, as well as from man-made international 
emissions of O3 precursors. Using the term generically, 
however, can lead to confusion as to what sources of O3 are 
being considered. Relevant to the O3 implementation 
provisions of the CAA, we define background O3 the same way 
the EPA defines USB: O3 that would exist in the absence of 
any man-made emissions inside the U.S.
    While the great majority of modeled O3 exceedances have 
local and regional emissions as their primary cause, there can be 
events where O3 levels approach or exceed the concentration 
level of the revised O3 standards in large part due to 
background sources. These cases of high USB levels on high 
O3 days typically result from stratospheric intrusions of 
O3 or wildfire O3 plumes. These events are 
infrequent and the CAA contains provisions that can be used to help 
deal, in particular, with stratospheric intrusion and wildfire events 
with O3 contributions of this magnitude, including providing 
varying degrees of regulatory relief for air agencies and potential 
regulated entities. The EPA intends to work closely with states to 
identify affected locations and ensure that the appropriate regulatory 
mechanisms are employed.
    Statutory and regulatory relief associated with U.S. background 
O3 may include: \230\
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    \230\ Note that the relief mechanisms discussed here do not 
include the CAA's interstate transport provisions found in sections 
110(a)(2)(D) and 126. The interstate transport provisions are 
intended to address the cross-state transport of O3 and 
O3 precursor emissions from man-made sources within the 
continental U.S. rather than background O3 as it is 
defined in this section. As noted in section II.A.2.a above, many of 
the instances where commenters pointed to remote monitored locations 
having O3 exceedances due to background O3 in 
fact reflected sizeable contributions from domestic sources, 
including interstate contributions (including from the Los Angeles 
Basin and other California locations).
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     Relief from designation as a nonattainment area through 
exclusion of data affected by exceptional events;
     Relief from the more stringent requirements of higher 
nonattainment area classifications through treatment as a rural 
transport area, through exclusion of data affected by exceptional 
events, or through international transport provisions;
     Relief from having to demonstrate attainment and having to 
adopt more than reasonable controls on local sources through 
international transport provisions.
    Further discussion of these mechanisms is provided in sections 
VIII.B.2 (exceptional events), VIII.B.1 (rural transport areas), and 
VIII.E.2 (international transport).
    Although these relief mechanisms require some level of assessment 
or demonstration by a state and/or the EPA to invoke, they have been 
used successfully in the past under appropriate circumstances. For 
example, the EPA has historically acted on every exceptional events 
demonstration that has affected a regulatory decision regarding initial 
area designations. See e.g., Idaho: West Silver Valley Nonattainment 
Area--Area Designations for the 2012 primary annual PM2.5 
NAAQS Technical Support Document, pp. 10-14, December 2014. For the 
revised O3 standards, the areas that would most likely need 
to use the mechanisms discussed in this section as part of attaining 
the revised O3 standards are locations in the western U.S. 
where we have estimated the largest seasonal average values of 
background O3 occur. We expect some of these areas to use 
the provisions in the Exceptional Events Rule during the designations 
process for the revised O3 standards. The EPA will then give 
priority to exceptional events demonstrations submitted by air agencies 
with areas whose designation decision could be influenced by the 
exclusion of data under the Exceptional Events Rule. In addition, as 
discussed in more detail in sections V.D and VIII.B.2 of this action, 
to streamline the exceptional events process, the EPA will soon propose 
revisions to the 2007 Exceptional Events Rule and will release through 
a Federal Register Notice of Availability a draft guidance document to 
address Exceptional Events Rule criteria for wildfires that could 
affect O3 concentrations. We expect to

[[Page 65437]]

promulgate Exceptional Events Rule revisions and finalize the new 
guidance document before the October 2016 date by which states, and any 
tribes that wish to do so, are required to submit their initial 
designation recommendations for the revised O3 NAAQS.
4. Section 110 State Implementation Plans
    The CAA section 110 specifies the general requirements for SIPs. 
Within three years after the promulgation of revised NAAQS (or such 
shorter period as the Administrator may prescribe \231\) each state 
must adopt and submit ``infrastructure'' SIPs to the EPA to address the 
requirements of section 110(a)(1) and (2), as applicable. These 
``infrastructure SIP'' submissions establish the basic state programs 
to implement, maintain, and enforce revised NAAQS and provide 
assurances of state resources and authorities. States are required to 
develop and maintain an air quality management infrastructure that 
includes enforceable emission limitations, a permitting program, an 
ambient monitoring program, an enforcement program, air quality 
modeling capabilities, and adequate personnel, resources, and legal 
authority. Because the revised primary NAAQS and secondary NAAQS are 
identical, the EPA does not at present discern any need for there to be 
any significant substantive difference in the infrastructure SIP 
elements for the two standards and thus believes it would be more 
efficient for states and the EPA if each affected state submits a 
single section 110 infrastructure SIP that addresses both standards at 
the same time (i.e., within three years of promulgation of the 
O3 NAAQS). Accordingly the EPA is not extending the SIP 
deadline for purposes of a revised secondary standard.
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    \231\ While the CAA allows the EPA to set a shorter time for 
submission of these SIPs, the EPA does not currently intend to do so 
for this revision to the O3 NAAQS.
---------------------------------------------------------------------------

    It is the responsibility of each state to review its air quality 
management program's compliance with the infrastructure SIP provisions 
in light of each new or revised NAAQS. Most states have revised and 
updated their infrastructure SIPs in recent years to address 
requirements associated with the 2008 O3 NAAQS. We expect 
that the result of these prior updates is that, in most cases, states 
will already have adequate state regulations previously adopted and 
approved into the SIP to address a particular requirement with respect 
to the revised O3 NAAQS. For such portions of the state's 
infrastructure SIP submission, the state may provide a 
``certification'' specifying that certain existing provisions in the 
SIP are adequate to meet applicable requirements. Although the term 
``certification'' does not appear in the CAA as a type of 
infrastructure SIP submittal, the EPA sometimes uses the term in the 
context of infrastructure SIPs, by policy and convention, to refer to a 
state's SIP submission. If a state determines that its existing EPA-
approved SIP provisions are adequate in light of the revised 
O3 NAAQS with respect to a given infrastructure SIP element 
(or sub-element), then the state may make a ''certification'' that the 
existing SIP contains provisions that address those requirements of the 
specific CAA section 110(a)(2) infrastructure elements. In the case of 
a certification, the submittal does not have to include another copy of 
the relevant provision (e.g., rule or statute) itself. Rather, the 
submission may provide citations to the already SIP-approved state 
statutes, regulations, or non-regulatory measures, as appropriate, 
which meet the relevant CAA requirement. Like any other SIP submission, 
such certification can be made only after the state has provided 
reasonable notice and opportunity for public hearing. This ``reasonable 
notice and opportunity for public hearing'' requirement for 
infrastructure SIP submittals appears at section 110(a), and it 
comports with the more general SIP requirement at section 110(l) of the 
CAA. Under the EPA's regulations at 40 CFR part 51, if a public hearing 
is held, an infrastructure SIP submission must include documentation by 
the state that the public hearing was held in accordance with the EPA's 
procedural requirements for public hearings. See 40 CFR part 51, 
Appendix V, paragraph 2.1(g), and 40 CFR 51.102. In the event that a 
state's existing SIP does not already meet applicable requirements, 
then the infrastructure SIP submission must include the modifications 
or additions to the state's SIP in order to update it to meet the 
relevant elements of section 110(a)(2).
5. Nonattainment Area Requirements
    Part D of the CAA describes the various program requirements that 
apply to states with nonattainment areas for different NAAQS. Clean Air 
Act Section 182 (found in subpart 2 of part D) includes the specific 
SIP requirements that govern the O3 program, and supplements 
the more general nonattainment area requirements in CAA sections 172 
and 173. Under CAA section 182, states generally are required to submit 
attainment demonstration SIPs within three or four years after the 
effective date of area designations promulgated by the EPA, depending 
on the classification of the area.\232\ These SIP submissions need to 
show how the nonattainment area will attain the primary O3 
standard ``as expeditiously as practicable,'' but no later than within 
the relevant time frame from the effective date of designations 
associated with the classification of the area.
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    \232\ Section 181(a)(1) of the CAA establishes classification 
categories for areas designated nonattainment for the primary 
O3 NAAQS. These categories range from ``Marginal,'' the 
lowest O3 classification with the fewest requirements 
associated with it, to ``Extreme,'' the highest classification with 
the most required programs. Areas with worse O3 problems 
are given more time to attain the NAAQS and more associated emission 
control requirements.
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    The EPA believes that the overall framework and policy approach of 
the implementation rules associated with the 2008 O3 NAAQS 
provide an effective and appropriate template for the general approach 
states would follow in planning for attainment of the revised 
O3 standard.\233\ However, to assist with the implementation 
of the revised O3 standards, the EPA intends to develop and 
propose an additional O3 NAAQS Implementation Rule that will 
address certain subjects specific to the new O3 NAAQS 
finalized here. This will include establishing air quality thresholds 
associated with each nonattainment area classification (i.e., Marginal, 
Moderate, etc.), associated attainment deadlines, and deadlines for 
submitting attainment planning SIP elements (e.g., RACT for major 
sources, RACT VOC control techniques guidelines, etc.). The rulemaking 
will also address whether to revoke the 2008 O3 NAAQS, and 
to impose appropriate anti-backsliding requirements to ensure that the 
protections afforded by that standard are preserved. The EPA intends to 
propose this implementation rule within one year after the revised 
O3 NAAQS is promulgated, and finalize this implementation 
rule by no later than the time the area designations process is 
finalized (approximately two years after promulgation of the revised 
O3 NAAQS).
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    \233\ Implementation of the 2008 National Ambient Air Quality 
Standards for Ozone: State Implementation Plan Requirements; Final 
Rule (80 FR 12264; March 6, 2015) and Implementation of the 2008 
National Ambient Air Quality Standards for Ozone: Nonattainment Area 
Classifications Approach, Attainment Deadlines and Revocation of the 
1997 Ozone Standards for Transportation Conformity Purposes (77 FR 
30160; May 21, 2012).
---------------------------------------------------------------------------

    We know that developing the implementation plans that outline the 
steps a nonattainment area will take to

[[Page 65438]]

meet an air quality standard requires a significant amount of work on 
the part of state, tribal or local air agencies. The EPA routinely 
looks for ways to reduce this workload, including assisting with air 
quality modeling by providing inputs such as emissions, meteorological 
and boundary conditions; and sharing national-scale model results that 
states can leverage in their development of attainment demonstrations.

B. O3 Air Quality Designations

1. Area Designation Process
    After the EPA establishes or revises a NAAQS, the CAA directs the 
EPA and the states to take steps to ensure that the new or revised 
NAAQS is met. One of the first steps, known as the initial area 
designations, involves identifying areas of the country that either 
meet or do not meet the new or revised NAAQS, along with any nearby 
areas that contribute to areas that do not meet the new or revised 
NAAQS.
    Section 107(d)(1) of the CAA provides that, ``By such date as the 
Administrator may reasonably require, but not later than 1 year after 
promulgation of a new or revised national ambient air quality standard 
for any pollutant under section 109, the Governor of each state shall . 
. . submit to the Administrator a list of all areas (or portions 
thereof) in the state'' that designates those areas as nonattainment, 
attainment, or unclassifiable. The EPA must then promulgate the area 
designations according to a specified process, including procedures to 
be followed if the EPA intends to modify a state's initial 
recommendation.
    Clean Air Act Section 107(d)(1)(B)(i) further provides, ``Upon 
promulgation or revision of a national ambient air quality standard, 
the Administrator shall promulgate the designations of all areas (or 
portions thereof) . . . as expeditiously as practicable, but in no case 
later than 2 years from the date of promulgation of the new or revised 
national ambient air quality standard. Such period may be extended for 
up to one year in the event the Administrator has insufficient 
information to promulgate the designations.'' By no later than 120 days 
prior to promulgating area designations, the EPA is required to notify 
states of any intended modifications to their recommendations that the 
EPA may deem necessary. States then have an opportunity to demonstrate 
why any proposed modification is inappropriate. Whether or not a state 
provides a recommendation, the EPA must timely promulgate the 
designation that the agency deems appropriate.
    While section 107 of the CAA specifically addresses states, the EPA 
intends to follow the same process for tribes to the extent 
practicable, pursuant to CAA section 301(d) regarding tribal authority 
and the Tribal Authority Rule (63 FR 7254, February 12, 1998). To 
provide clarity and consistency in doing so, the EPA issued a 2011 
guidance memorandum on working with tribes during the designation 
process.\234\
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    \234\ Page, S. (2011). Guidance to Regions for Working with 
Tribes during the National Ambient Air Quality Standards (NAAQS) 
Designations Process, Memorandum from Stephen D. Page, Director, EPA 
Office of Air Quality Planning and Standards to Regional Air 
Directors, Regions I-X, December 20, 2011. Available: http://www.epa.gov/ttn/oarpg/t1/memoranda/20120117naaqsguidance.pdf.
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    As discussed in sections II and IV of this preamble, the EPA is 
revising both the primary and secondary O3 NAAQS. 
Accordingly, the EPA intends to complete designations for both NAAQS 
following the standard 2-year process discussed above. In accordance 
with section 107(d)(1) of the CAA, state Governors (and tribes, if they 
choose) should submit their initial designation recommendations for a 
revised primary and secondary NAAQS by 1 year after October 1, 2015. If 
the EPA intends to modify any state recommendation, the EPA would 
notify the appropriate state Governor (or tribal leader) no later than 
120 days prior to making final designation decisions. A state or tribe 
that believes the modification is inappropriate would then have the 
opportunity to demonstrate to the EPA why it believes its original 
recommendation (or a revised recommendation) is more appropriate. The 
EPA would take any additional input into account in making the final 
designation decisions.
    The CAA defines an area as nonattainment if it is violating the 
NAAQS or if it is contributing to a violation in a nearby area. 
Consistent with previous area designations processes, the EPA intends 
to use area-specific analysis of multiple factors to support area 
boundary decisions. The EPA intends to evaluate information related to 
the following factors for designations: air quality data, emissions and 
emissions-related data, meteorology, geography/topography, and 
jurisdictional boundaries. Additional guidance on the designation 
process and how these factors may be evaluated and inform the process 
will be issued by the EPA early in 2016 to assist states in developing 
their recommendations.
    Areas that are designated as nonattainment are also classified at 
the time of designation by operation of law according to the severity 
of their O3 problem. The classification categories are 
Marginal, Moderate, Serious, Severe, and Extreme. Ozone nonattainment 
areas are subject to specific mandatory measures depending on their 
classification. As indicated previously, the thresholds for the 
classification categories will be established in a future O3 
implementation rule.
    Clean Air Act section 182(h) authorizes the EPA Administrator to 
determine that an area designated nonattainment can be treated as a 
rural transport area. Regardless of its classification, a rural 
transport area is deemed to have fulfilled all O3-related 
planning and control requirements if it meets the CAA's requirements 
for areas classified Marginal, which is the lowest classification 
specified in the CAA. In accordance with the statute, a nonattainment 
area may qualify for this determination if it meets the following 
criteria:
     The area does not contain emissions sources that make a 
significant contribution to monitored O3 concentrations in 
the area, or in other areas; and
     The area does not include and is not adjacent to a 
Metropolitan Statistical Area.
    Historically, the EPA has listed four nonattainment areas as rural 
transport areas under this statutory provision.\235\ The EPA has not 
issued separate written guidance to further elaborate on the 
interpretation of these CAA qualification criteria. However, the EPA 
developed draft guidance in 2005 that explains the kinds of technical 
analyses that states could use to establish that transport of 
O3 and/or O3 precursors into the area is so 
overwhelming that the contribution of local emissions to an observed 8-
hour O3 concentration above the level of the NAAQS is 
relatively minor and determine that emissions within the area do not 
make a significant contribution to the O3 concentrations 
measured in the area or in other areas.\236\ While this guidance

[[Page 65439]]

was not prepared specifically for rural transport areas, it could be 
useful to states for developing technical information to support a 
request that the EPA treat a specific O3 nonattainment area 
as a rural transport area. The EPA will work with states to ensure 
nonattainment areas eligible for treatment as rural transport areas are 
identified.
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    \235\ For the 1979 1-hour O3 standard, Door County 
Area, Wisconsin; Edmonson County Area, Kentucky; Essex County Area 
(Whiteface Mountain), New York; and Smyth County Area (White Top 
Mountain), Virginia were recognized by the EPA as rural transport 
areas. No rural transport areas were recognized for the 1997 or 2008 
8-hour O3 standards.
    \236\ U.S. Environmental Protection Agency (2005). Criteria For 
Assessing Whether an Ozone Nonattainment Area is Affected by 
Overwhelming Transport [Draft EPA Guidance]. U.S. Environmental 
Protection Agency, Research Triangle Park, NC. June 2005. Available 
at http://www.epa.gov/scram001/guidance/guide/owt_guidance_07-13-05.pdf.
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2. Exceptional Events
    During the initial area designations process, the EPA intends to 
evaluate multiple factors, including air quality data, when identifying 
and determining boundaries for areas of the country that meet or do not 
meet the revised O3 NAAQS. In some cases, these data may be 
influenced by exceptional events. Under the Exceptional Events Rule, an 
air agency can request and the EPA can agree to exclude data associated 
with event-influenced exceedances or violations of a NAAQS, including 
the revised O3 NAAQS, provided the event meets the statutory 
requirements in section 319(b) of the CAA, which requires that:
     the event ``affects air quality;''
     the event ``is not reasonably controllable or 
preventable;''
     the event is ``caused by human activity that is unlikely 
to recur at a particular location or [is] a natural event,'' \237\ and
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    \237\ A natural event is further described in 40 CFR 50.1(k) as 
``an event in which human activity plays little or no direct causal 
role.''
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     that ``a clear causal relationship must exist between the 
measured exceedances of a [NAAQS] and the exceptional event. . . .''
    The EPA's implementing regulations, the Exceptional Events Rule, 
further specify certain requirements for air agencies making 
exceptional events demonstrations.\238\
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    \238\ 72 FR 13,560 (March 22, 2007), ``Treatment of Data 
Influenced by Exceptional Events,'' Final Rule; see also 40 CFR 
parts 50 and 51.
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    The ISA contains discussions of natural events that may contribute 
to O3 or O3 precursors. These include 
stratospheric O3 intrusion and wildfire events.\239\ As 
indicated above, to satisfy the exceptional events requirements and to 
qualify for data exclusion under the Exceptional Events Rule, an air 
agency must develop and submit a demonstration, including evidence, 
addressing each of the identified criteria. The extent to which a 
stratospheric O3 intrusion event or a wildfire event 
contributes to O3 levels can be uncertain, and in most cases 
requires detailed analyses to determine.
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    \239\ The preamble to the Exceptional Events Rule (72 FR 13560) 
identifies both stratospheric O3 intrusions and wildfires 
as natural events that could also qualify as exceptional events 
under the CAA and Exceptional Event Rule criteria. Note that 
O3 resulting from routine natural emissions from 
vegetation, microbes, animals and lightning are not exceptional 
events authorized for exclusion under the section 319 of the CAA.
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    Strong stratospheric O3 intrusion events, most prevalent 
at high elevation sites during winter or spring, can be identified 
based on measurements of low relative humidity, evidence of deep 
atmospheric mixing, and a low ratio of CO to O3 based on 
ambient measurements. Accurately determining the extent of weaker 
intrusion events remains challenging (U.S. EPA 2013, p. 3-34). Although 
states have submitted only a few exceptional events demonstrations for 
stratospheric O3 intrusion, the EPA recently approved a 
demonstration from Wyoming for a June 2012 stratospheric O3 
event.\240\
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    \240\ U.S. EPA (2014) Treatment of Data Influenced by 
Exceptional Events: Examples of Reviewed Exceptional Event 
Submissions. U.S. Environmental Protection Agency, Research Triangle 
Park, NC, available at http://www.epa.gov/ttn/analysis/exevents.htm.
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    While stratospheric O3 intrusions can increase monitored 
ground-level ambient O3 concentrations, wildfire plumes can 
either suppress or enhance O3 depending upon a variety of 
factors including fuel type, combustion stage, plume chemistry, aerosol 
effects, meteorological conditions and distance from the fire (Jaffe 
and Wigder, 2012). As a result, determining the impact of wildfire 
emissions on specific O3 observations is challenging. The 
EPA recently approved an exceptional events demonstration for wildfires 
affecting 1-hour O3 levels in Sacramento, California in 2008 
that successfully used a variety of analytical tools (e.g., regression 
modeling, back trajectories, satellite imagery, etc.) to support the 
exclusion of O3 data affected by large fires.\241\
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    \241\ U.S. EPA (2014) Treatment of Data Influenced by 
Exceptional Events: Examples of Reviewed Exceptional Event 
Submissions. U.S. Environmental Protection Agency, Research Triangle 
Park, NC. Examples of O3-related exceptional event 
submissions, available at http://www.epa.gov/ttn/analysis/exevents.htm.
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    In response to previously expressed stakeholder feedback regarding 
implementation of the Exceptional Events Rule and specific stakeholder 
concerns regarding the burden of exceptional events demonstrations, the 
EPA is currently engaged in a rulemaking process to amend the 
Exceptional Events Rule. As part of an upcoming notice and comment 
rulemaking effort (and related activities, including the issuance of 
relevant guidance documents), the EPA sees opportunities to standardize 
best practices for collaboration between the EPA and air agencies, 
clarify and simplify demonstrations, and improve tools and consistency.
    Additionally, the EPA intends to develop guidance to address 
implementing the Exceptional Events Rule criteria for wildfires that 
could affect ambient O3 concentrations. Wildfire emissions 
are a component of background O3 (Jaffe and Wigder, 2012) 
and in some locations can significantly contribute to periodic high 
O3 levels (Emery, 2012). The threat from wildfires can be 
mitigated through management of wildland vegetation. Planned and 
managed fires are one tool that land managers can use to reduce fuel 
load, unnatural understory and tree density, thus helping to reduce the 
risk of catastrophic wildfires. Allowing some wildfires to continue and 
the thoughtful use of prescribed fire can influence the occurrence of 
catastrophic wildfires, which may reduce the probability of fire-
induced smoke impacts and subsequent health effects. Thus, appropriate 
use of prescribed fire may help manage the contribution of wildfires to 
both background and periodic peak O3 air pollution. Several 
commenters expressed concern that the revised O3 NAAQS could 
limit the future use of prescribed fire. Under the current Exceptional 
Events Rule, prescribed fires meeting the rule criteria may also 
qualify as exceptional events. The EPA intends to further clarify the 
Exceptional Events Rule criteria for prescribed fire on wildland in its 
upcoming rulemaking.
    The EPA is committed to working with federal land managers, other 
federal agencies, tribes and states to effectively manage prescribed 
fire use to reduce the impact of wildfire-related emissions on 
O3 through policies and regulations implementing these 
standards.

C. How do the New Source Review (NSR) requirements apply to the revised 
O3 NAAQS?

1. NSR Requirements for Major Stationary Sources for the Revised 
O3 NAAQS
    The CAA, at parts C and D of title I, contains preconstruction 
review and permitting programs applicable to new major stationary 
sources and major modifications of existing major sources. The 
preconstruction review of each new major stationary source and major 
modification applies on a pollutant-specific basis, and the 
requirements that apply for each pollutant depend on whether the area 
in which the source is situated is designated as attainment (or

[[Page 65440]]

unclassifiable) or nonattainment for that pollutant. In areas 
designated attainment or unclassifiable for a pollutant, the PSD 
requirements under part C apply to construction at major sources. In 
areas designated nonattainment for a pollutant, the NNSR requirements 
under part D apply to major source construction. Collectively, those 
two sets of permit requirements are commonly referred to as the ``major 
New Source Review'' or ``major NSR'' programs.
    Until an area is formally designated with respect to the revised 
O3 NAAQS, the NSR provisions applicable under that area's 
current designation for the 2008 O3 NAAQS (including any 
applicable anti-backsliding requirements) will continue to apply. That 
is, for areas designated as attainment/unclassifiable for the 2008 
O3 NAAQS, PSD will apply for new major stationary sources 
and major modifications that trigger major source permitting 
requirements for O3; areas designated nonattainment for the 
2008 O3 NAAQS must comply with the NNSR requirements for new 
major stationary sources and major modifications that trigger major 
source permitting requirements for O3. When the new 
designations for the revised O3 NAAQS become effective, 
under the current rules, those designations will generally serve to 
determine whether PSD or NNSR applies to O3 and its 
precursors. The PSD regulations at 40 CFR 51.166(i)(2) and 52.21(i)(2) 
provide that the substantive PSD requirements do not apply for a 
particular pollutant if the owner or operator of the new major 
stationary source or major modification demonstrates that the area in 
which the source is located is designated nonattainment for that 
pollutant under CAA section 107. Thus, new major sources and 
modifications will generally be subject to the PSD program requirements 
for O3 if they are locating in an area that does not have a 
current nonattainment designation under CAA section 107 for 
O3. These rules further provide that nonattainment 
designations for a revoked NAAQS, as contained in 40 CFR part 81, are 
not viewed as current designations under CAA section 107 for purposes 
of determining the applicability of such PSD requirements.\242\
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    \242\ This description of paragraph (i)(2) of the PSD 
regulations at 40 CFR 51.166 and 52.21 reflects revisions made in 
the final 2008 O3 NAAQS SIP Requirements Rule. See 80 FR 
12264 at 12287 (March 6, 2015).
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    The EPA's major NSR regulations define the term ``regulated NSR 
pollutant'' to include any pollutant for which a NAAQS has been 
promulgated and any pollutant identified in EPA regulations as a 
constituent or precursor to such pollutant.\243\ Both the PSD and NNSR 
regulations identify VOC and NOX as precursors to 
O3. Accordingly, the major NSR programs for O3 
are applied to emissions of VOC and NOX as precursors of 
O3.\244\
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    \243\ The definition of ``regulated NSR pollutant'' is found in 
the PSD regulations at 40 CFR 51.166(b)(49) and 52.21(b)(50), and in 
the NNSR regulations at 40 CFR 51.165(a)(1)(xxxvii).
    \244\ VOC and NOX are defined as precursors of ozone 
in the PSD regulations at 40 CFR 51.166(b)(49)(i)(b)(1) and 
52.21(b)(50)(i)(b)(1), and in the NNSR regulations at 40 CFR 
51.165(a)(1)(xxxvii)(B) and (C)(1) and part 51, Appendix S, 
II.A.31(ii)(b)(1).
---------------------------------------------------------------------------

2. Prevention of Significant Deterioration (PSD) Program
    The statutory requirements for a PSD permit program set forth under 
part C of title I of the CAA (sections 160 through 169) are addressed 
by the EPA's PSD regulations found at 40 CFR 51.166 (minimum 
requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD 
permitting program for permits issued under the EPA's federal 
permitting authority). Both sets of regulations already apply for 
O3 when the area is designated attainment or unclassifiable 
for O3 and when the new source or modification triggers PSD 
requirements for O3.
    For PSD, a ``major stationary source'' is one that emits or has the 
potential to emit 250 tons per year (tpy) or more of any regulated NSR 
pollutant, unless the new or modified source is classified under a list 
of 28 source categories contained in the statutory definition of 
``major emitting facility'' in section 169(1) of the CAA. For those 28 
source categories, a ``major stationary source'' is one that emits or 
has the potential to emit 100 tpy or more of any regulated NSR 
pollutant. A ``major modification'' is a physical change or a change in 
the method of operation of an existing major stationary source that 
results first, in a significant emissions increase of a regulated NSR 
pollutant for the project, and second, in a significant net emissions 
increase of that pollutant at the source. See 40 CFR 51.166(b)(2)(i), 
40 CFR 52.21(b)(2)(i).
    Among other things, for each regulated NSR pollutant emitted or 
increased in significant amounts, the PSD program requires a new major 
stationary source or a major modification to apply Best Available 
Control Technology and to conduct an air quality impact analysis to 
demonstrate that the proposed source or project will not cause or 
contribute to a violation of any NAAQS or PSD increment (see CAA 
section 165(a)(3)-(4), 40 CFR 51.166(j)-(k), 40 CFR 52.21(j)-(k)). The 
PSD requirements may also include, in appropriate cases, an analysis of 
potential adverse impacts on Class I areas (see CAA sections 162 and 
165).\245\ The EPA has generally interpreted the requirement for an air 
quality impact analysis under CAA section 165(a)(3) and the 
implementing regulations to include a requirement to demonstrate that 
emissions from the proposed facility will not cause or contribute to a 
violation of any NAAQS that is in effect as of the date a PSD permit is 
issued.\246\ See, e.g., 73 FR 28321, 28324, 28340 (May 16, 2008); 78 FR 
3253 (Jan. 15, 2013); Memorandum from Stephen D. Page, Director, Office 
of Air Quality Planning & Standards, ``Applicability of the Federal 
Prevention of Significant Deterioration Permit Requirements to New and 
Revised National Ambient Air Quality Standards'' (April 1, 2010). 
Consistent with this interpretation, the demonstration required under 
CAA section 165(a)(3) and 40 CFR 51.166(k) and 52.21(k) will apply to 
any revised O3 NAAQS when such NAAQS become effective, 
except to the extent that a pending permit application is subject to a 
grandfathering provision that the EPA establishes through rulemaking. 
In addition, the other existing requirements of the PSD program will 
remain applicable to O3 after the revised O3 
NAAQS takes effect.
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    \245\ Congress established certain Class I areas in section 
162(a) of the CAA, including international parks, national 
wilderness areas, and national parks that meet certain criteria. 
Such Class I areas, known as mandatory federal Class I areas, are 
afforded special protection under the CAA. In addition, states and 
tribal governments may establish Class I areas within their own 
political jurisdictions to provide similar special air quality 
protection.
    \246\ An exception occurs in cases where the EPA has included a 
grandfathering provision in its PSD regulations for a particular 
pollutant. The EPA historically has exercised its discretion to 
transition the implementation of certain new requirements through 
grandfathering, under appropriate circumstances, either by 
rulemaking or through a case-by-case determination for a specific 
permit application. In 2014, the United States Court of Appeals for 
the Ninth Circuit vacated a decision by the EPA to issue an 
individual PSD permit grandfathering a permit applicant from certain 
requirements. See Sierra Club v. EPA, 762 F.3d 971 (9th Cir. 2014). 
In light of that decision, the EPA is no longer asserting authority 
to grandfather permit applications on a case-by-case basis. This 
decision is addressed in more detail in the discussion of the 
grandfathering provisions that the EPA is issuing through this 
rulemaking in section VII of this preamble.
---------------------------------------------------------------------------

    Because the complex chemistry of O3 formation in the 
atmosphere poses significant challenges for the assessing the impacts 
of individual stationary sources on O3 formation, the EPA's 
judgment historically has been that it is not technically sound to 
designate a

[[Page 65441]]

specific air quality model that must be used in the PSD permitting 
process to make this demonstration for O3. To address 
ambient impacts of emissions from proposed individual stationary 
sources on O3, the EPA proposed amendments to Appendix W to 
40 CFR part 51 in July 2015 that would, among other things, revise the 
Appendix W provisions relating to the analytical techniques for 
demonstrating that an individual PSD source or modification does not 
cause or contribute to a violation of the O3 NAAQS (80 FR 
45340, July 29, 2015). Until any revisions are finalized and in effect, 
PSD permit applicants should continue to follow the current provisions 
in the applicable regulations and Appendix W in order to demonstrate 
that a proposed source or modification does not cause or contribute to 
a violation of the O3 NAAQS.
a. What transition plan is the EPA providing for implementing the PSD 
requirements for the revised O3 NAAQS?
    In this rulemaking, the EPA is amending the PSD regulations at 40 
CFR 51.166 and 40 CFR 52.21 to include a grandfathering provision that 
will allow reviewing authorities to continue to review certain pending 
PSD permit applications in accordance with the O3 NAAQS that 
was in effect when a specific permitting milestone was reached, rather 
than the revised O3 NAAQS. The EPA is finalizing the 
grandfathering provision as proposed with two trigger dates--the 
signature date of the revised O3 NAAQS rule for complete 
applications and the effective date of the revised O3 NAAQS 
for a draft permit or preliminary determination. A more detailed 
discussion of the final provision, comments received and our responses 
to those comments is provided in section VII of this preamble, which 
addresses this change to the PSD regulations, as well as the Response 
to Comment Document contained in the docket for this rulemaking.
b. What screening and compliance demonstration tools are used to 
implement the PSD program?
    The EPA has historically allowed the use of screening and 
compliance demonstration tools to help facilitate the implementation of 
the NSR program by reducing the source's burden and streamlining the 
permitting process for circumstances where the emissions or ambient 
impacts of a particular pollutant could be considered de minimis. For 
example, the EPA has established significant emission rates, or SERs, 
that are used as screening tools to determine when a pollutant would be 
considered to be emitted in a significant amount and, accordingly, when 
the NSR requirements should be applied to that pollutant. See 40 CFR 
51.166(b)(23) and 52.21(b)(23). For O3, the EPA established 
a SER of 40 tpy for emissions of each O3 precursor--VOC and 
NOX. For PSD, the O3 SER applies independently to 
emissions of VOC and NOX (emissions of precursors are not 
added together) to determine when the proposed major stationary source 
or major modification must undergo PSD review for that precursor and 
whether individual PSD requirements, such as BACT, apply to that 
precursor.\247\
---------------------------------------------------------------------------

    \247\ See In re Footprint Power Salem Harbor Development, LP, 16 
E.A.D ___, PSD Appeal No. 14-02, at 20-25 (EAB, Sept. 2, 2014) 
(including description of EPA's position on application of BACT to 
ozone precursors) available at http://yosemite.epa.gov/oa/EAB_Web_Docket.nsf/PSD+Permit+Appeals+(CAA)?OpenView.
---------------------------------------------------------------------------

    In the context of the PSD air quality impact analysis, the EPA has 
also used a value called a significant impact level (SIL) as a 
compliance demonstration tool. The SIL, expressed as an ambient 
concentration of a pollutant, may be used first to determine the 
geographical scope of the ambient impact analysis that must be 
completed for the applicable pollutant to satisfy the air quality 
demonstration requirement under CAA section 165(a)(3). A second use is 
to guide the determination of whether the impact of the source is 
considered to cause or contribute to a violation of any NAAQS. The EPA 
has not established a SIL for O3. The EPA is currently 
considering development of a SIL for O3 through either 
guidance or a rulemaking process. Such a SIL would complement proposed 
revisions to Appendix W mentioned above (80 FR 45340, July 29, 2015) 
and would assist in the implementation of the PSD air quality analysis 
requirement for protection of the O3 NAAQS. However, the EPA 
is not making revisions in this rulemaking to address the PSD air 
quality analysis for O3. Until any rulemaking to amend 
existing PSD regulations for O3 is completed, permitting 
decisions should continue to be based on the existing provisions in the 
applicable regulations.
    Several commenters addressed statements that the EPA made 
concerning screening tools for O3 in the preamble to the 
O3 NAAQS proposal. These statements were not linked to any 
proposed amendments to EPA regulations. Aside from adopting the 
grandfathering provision addressed in section VII of this preamble, the 
EPA is not revising the PSD requirements for O3 in this 
final rule. Therefore, the EPA is not responding to those comments at 
this time, consistent with the EPA's general approach to comments on 
implementation topics described above.
c. Other PSD Transition Issues
    The EPA anticipates that the existing O3 air quality in 
some areas currently designated attainment of unclassifiable for 
O3 will not meet the revised O3 NAAQS upon its 
effective date and that some of these areas will ultimately be 
designated ``nonattainment'' for the revised O3 NAAQS 
through the formal area designation process set forth under the CAA 
(see section VIII.B above). However, until the EPA issues such 
nonattainment designations, proposed new major sources and major 
modifications situated in any area designated attainment or 
unclassifiable for the 2008 O3 NAAQS will continue to be 
required to address O3 in a PSD permit.\248\ As mentioned 
above, the PSD permitting program requires that proposed new major 
stationary sources and major modifications must demonstrate that the 
emissions from the proposed source or modification will not cause or 
contribute to a violation of any NAAQS. In the notice of proposed 
rulemaking, the EPA provided information concerning its views on the 
possibility that some PSD permit applications could satisfy the air 
quality analysis requirements for O3 by obtaining air 
quality offsets (called PSD offsets).\249\ Several commenters expressed 
concern that without some transition provisions in the final rule 
exempting PSD permit applications for sources located in such areas 
from meeting the air quality analysis requirements for the revised 
O3 NAAQS, such applications might not be able to satisfy the 
demonstration requirement, as the current ambient air monitoring data 
indicate the revised lower standards are not being met. The 
O3 NAAQS proposal included no proposed revisions to PSD 
regulations on this

[[Page 65442]]

topic and the EPA is not making any revisions to the PSD requirements 
for O3 in this action to address this issue. Therefore, the 
EPA is not responding to those comments at this time, consistent with 
its general approach to comments on implementation topics described 
above. However, to help address this concern raised by commenters, the 
EPA is considering issuing additional guidance on how PSD offsets can 
be implemented.
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    \248\ Any proposed major stationary source or major modification 
subject to PSD for O3 that does not receive its PSD 
permit by the effective date of a new O3 nonattainment 
designation for the area where the source would locate would then be 
required to satisfy all of the applicable NNSR preconstruction 
permit requirements for O3, even if such source had been 
grandfathered under the PSD regulations from the demonstration 
requirement under CAA section 165(a)(3) for O3.
    \249\ The EPA has historically recognized in regulations and 
through other actions that sources applying for PSD permits may have 
the option of utilizing offsets as part of the required PSD 
demonstration under CAA section 165(a)(3)(B). See, e.g., In re 
Interpower of New York, Inc., 5 E.A.D. 130, 141 (EAB 1994) 
(describing an EPA Region 2 PSD permit that relied in part on 
offsets to demonstrate the source would not cause or contribute to a 
violation of the NAAQS). 52 FR 24698 (July 1, 1987); 78 FR 3261-62 
(Jan. 15, 2013).
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3. Nonattainment NSR
    Part D of title I of the CAA includes preconstruction review and 
permitting requirements for new major stationary sources and major 
modifications when they locate in areas designated nonattainment for a 
particular pollutant. The relevant part D requirements are typically 
referred to as the nonattainment NSR (NNSR) program. The EPA 
regulations for the NNSR program are contained at 40 CFR 51.165, 52.24 
and part 51 Appendix S. The EPA's minimum requirements for a NNSR 
program to be approvable into a SIP are contained in 40 CFR 51.165. 
Appendix S to 40 CFR part 51 contains an interim NNSR program. This 
interim program enables implementation of NNSR permitting in 
nonattainment areas that lack a SIP-approved NNSR permitting program 
for the particular nonattainment pollutant, and the interim program can 
be applied during the time between the date of the relevant 
nonattainment designation and the date on which the EPA approves into 
the SIP a NNSR program or additional components of an NNSR program for 
a particular pollutant.\250\ This interim program is commonly known as 
the Emissions Offset Interpretative Rule, and is applicable to all 
criteria pollutants, including O3.\251\
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    \250\ See Appendix S, Part I; 40 CFR 52.24(k).
    \251\ As appropriate, certain NNSR requirements under 40 CFR 
51.165 or Appendix S can also apply to sources and modifications 
located in areas that are designated attainment or unclassifiable in 
the Ozone Transport Region. See, e.g., CAA 184(b)(2), 40 CFR 
52.24(k).
---------------------------------------------------------------------------

    The EPA is not modifying any existing NNSR requirements in this 
rulemaking. Under the CAA, area designations for new or revised NAAQS 
are addressed subsequent to the effective date of the new or revised 
NAAQS. If the EPA determines that any revisions to the existing NNSR 
requirements, including those in Appendix S, are appropriate, the EPA 
expects, at a later date contemporaneous with the designation process 
for the revised O3 NAAQS, to propose those revisions. If any 
changes are proposed to Appendix S requirements, the EPA anticipates 
that it would intend for those changes to become effective no later 
than the effective date of the area designations. This timing would 
allow air agencies that lack an approved NNSR program for O3 
to use the relevant Appendix S provisions to issue NNSR permits 
addressing O3 on and after the effective date of 
designations of new nonattainment areas for O3 until such 
time as a NNSR program for O3 is approved into the SIP.\252\
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    \252\ States with SIP-approved NNSR programs for O3 
should evaluate that program to determine whether they can continue 
to issue permits under their approved program or whether revisions 
to their program are necessary to address the revised O3 
NAAQS.
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    For NNSR, new major stationary sources and major modifications for 
O3 must comply with the Lowest Achievable Emission Rate 
(LAER) requirements as defined in the CAA and NNSR rules, and must 
perform other analyses and satisfy other requirements under section 173 
of the CAA. For example, under CAA section 173(c) emissions reductions, 
known as emissions offsets, must be secured to offset the increased 
emissions of the air pollutant (including the relevant precursors) from 
the new or modified source by an equal or greater reduction, as 
applicable, of such pollutant. The appropriate emissions offset needed 
for a particular source will depend upon the classification for the 
O3 nonattainment area in which the source or modification 
will locate, such that areas with more severe nonattainment 
classifications have more stringent offset requirements. This ranges 
from 1.1:1 for areas classified as Marginal to 1.5:1 for areas 
classified as Extreme. See, e.g., CAA section 182, 40 CFR 51.165(a)(9) 
and 40 CFR part 51 Appendix S section IV.G.2.
    To facilitate continued economic development in nonattainment 
areas, many states have established offset banks or registries.\253\ 
Such banks or registries can help new or modified major stationary 
source owners meet offset requirements by streamlining identification 
and access to available emissions reductions. Some states have 
established offset banks to help ensure a consistent method for 
generating, validating and transferring NOX and VOC offsets. 
Offsets in these areas are generated by emissions reductions that meet 
specific creditability criteria set forth by the SIP consistent with 
the EPA regulations. See 40 CFR 51.165(a)(3)(ii)(A)-(J) and part 51 
Appendix S section IV.C. The EPA received comments expressing concern 
about the limited availability of offsets in nonattainment areas. Since 
the EPA did not propose, and is not finalizing, any amendments related 
to the NNSR offset provisions, the EPA is not responding to those 
comments at this time, consistent with the EPA's general approach to 
comment on implementation topics as described above.
---------------------------------------------------------------------------

    \253\ See, for example, emission reduction credit banking 
programs in Ohio (OAC Chapter 3745-1111) and California (H&SC 
Section 40709).
---------------------------------------------------------------------------

D. Transportation and General Conformity

1. What are transportation and general conformity?
    Conformity is required under CAA section 176(c) to ensure that 
federal actions are consistent with (``conform to'') the purpose of the 
SIP. Conformity to the purpose of the SIP means that federal activities 
will not cause new air quality violations, worsen existing violations, 
or delay timely attainment of the relevant NAAQS or interim reductions 
and milestones. Conformity applies to areas that are designated 
nonattainment, and those nonattainment areas redesignated to attainment 
with a CAA section 175A maintenance plan after 1990 (``maintenance 
areas'').
    The EPA's Transportation Conformity Rule (40 CFR 51.390 and part 
93, subpart A) establishes the criteria and procedures for determining 
whether transportation activities conform to the SIP. These activities 
include adopting, funding or approving transportation plans, 
transportation improvement programs (TIPs) and federally supported 
highway and transit projects. For further information on conformity 
rulemakings, policy guidance and outreach materials, see the EPA's Web 
site at http://www.epa.gov/otaq/stateresources/transconf/index.htm. The 
EPA may issue future transportation conformity guidance as needed to 
implement a revised O3 NAAQS.
    With regard to general conformity, the EPA first promulgated 
general conformity regulations in November 1993. (40 CFR part 51, 
subpart W, 40 CFR part 93, subpart B) Subsequently the EPA finalized 
revisions to the general conformity regulations on April 5, 2010. (75 
FR 17254-17279). Besides ensuring that federal actions not covered by 
the transportation conformity rule will not interfere with the SIP, the 
general conformity program also fosters communications between federal 
agencies and state/local air quality agencies, provides for public 
notification of and access to federal agency conformity determinations, 
and allows for air quality review of

[[Page 65443]]

individual federal actions. More information on the general conformity 
program is available at http://www.epa.gov/air/genconform/.
2. When would transportation and general conformity apply to areas 
designated nonattainment for the revised O3 NAAQS?
    Transportation and general conformity apply one year after the 
effective date of nonattainment designations for the revised 
O3 NAAQS. This is because CAA section 176(c)(6) provides a 
1-year grace period from the effective date of initial designations for 
any revised NAAQS before transportation and general conformity apply in 
areas newly designated nonattainment for a specific pollutant and 
NAAQS.
3. Impact of a Revised O3 NAAQS on a State's Existing 
Transportation and/or General Conformity SIP
    In this final rule, the EPA is revising the O3 NAAQS, 
but is not making specific changes to its transportation or general 
conformity regulations. Therefore, states should not need to revise 
their transportation and/or general conformity SIPs. While we are not 
making any revisions to the general conformity regulations at this 
time, we recommend, when areas develop SIPs for a revised O3 
NAAQS, that state and local air quality agencies work with federal 
agencies with large emitting activities that are subject to the general 
conformity regulations to establish an emissions budget for those 
facilities and activities in order to facilitate future conformity 
determinations under the conformity regulations. Finally, states with 
existing conformity SIPs and new nonattainment areas may also need to 
revise their conformity SIPs in order to ensure the state regulations 
apply in any newly designated areas.
    Because significant tracts of land under federal management may be 
included in nonattainment area boundaries, the EPA encourages state and 
local air quality agencies to work with federal agencies to assess and 
develop emissions budgets that consider emissions from projects subject 
to general conformity, including emissions from fire on wildland, in 
any baseline, modeling and SIP attainment inventory. Where appropriate, 
states, land managers, and landowners may also consider developing 
plans to ensure that fuel accumulations are addressed Information is 
available from DOI and USDA Forest Service on the ecological role of 
fire and on smoke management programs and basic smoke management 
practices.\254\
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    \254\ USDA Forest Service and Natural Resources Conservation 
Service, Basic Smoke Management Practices Tech Note, October 2011, 
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1046311.pdf.
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    If this is the first time that transportation conformity will apply 
in a state, such a state is required by the statute and EPA regulations 
to submit a SIP revision that addresses three specific transportation 
conformity requirements that address consultation procedures and 
written commitments to control or mitigation measures associated with 
conformity determinations for transportation plans, TIPs or projects. 
(40 CFR 51.390) Additional information and guidance can be found in the 
EPA's ``Guidance for Developing Transportation Conformity State 
Implementation Plans'' (http://www.epa.gov/otaq/stateresources/transconf/policy/420b09001.pdf).

E. Regional and International Pollution Transport

1. Interstate Transport
    The CAA contains provisions that specifically address and require 
regulation of the interstate transport of air pollution that does not 
otherwise qualify for data exclusion under the Act's exceptional events 
provisions. As previously noted, emissions from events, such as 
wildfires, may qualify as exceptional events and may be transported 
across jurisdictional boundaries. The EPA intends to address the 
transport of event-related emissions in our upcoming proposed revisions 
to the Exceptional Events Rule and draft guidance document addressing 
the Exceptional Events Rule criteria for wildfires that could affect 
O3 concentrations. The EPA encourages affected air agencies 
to coordinate with their EPA regional office to identify approaches to 
evaluate the potential impacts of transported event-related emissions 
and determine the most appropriate information and analytical methods 
for each area's unique situation.
    CAA section 110(a)(2)(D)(i)(I), Interstate Transport--CAA section 
110(a)(2)(D)(i)(I) requires states to develop and implement a SIP to 
address the interstate transport of emissions. Specifically, this 
provision requires the SIP to prohibit ``any source or other type of 
emissions activity within the state'' that would ``significantly 
contribute to nonattainment'' of any NAAQS in another state, or that 
would ``interfere with maintenance'' of any NAAQS in another state. 
When EPA promulgates or revises a NAAQS, each state is required to 
submit a SIP addressing this interstate transport provision within 3 
years.
    CAA section 126, Interstate Transport--CAA section 126(b) provides 
states and political subdivisions with a mechanism to petition the 
Administrator for a finding that ``any major source or group of 
stationary sources emits or would emit any air pollution in violation 
of the prohibition of [CAA section 110(a)(2)(D)(i)(I)].'' \255\ Where 
the EPA makes such finding, the source is allowed to operate beyond a 
3-month period after such finding only if the EPA establishes emissions 
limitations and a compliance schedule designated to bring the source 
into compliance as expeditiously as practicable, but no later than 
three years after such finding. This mechanism is available to downwind 
states and political subdivisions, regardless of designation status, 
that would be affected by emissions from upwind states.
---------------------------------------------------------------------------

    \255\ The text of section 126 codified in the United States Code 
cross references section 110(a)(2)(D)(ii) instead of section 
110(a)(2)(D)(i). The courts have confirmed that this is a 
scrivener's error and the correct cross reference is to section 
110(a)(2)(D)(i), See Appalachian Power Co. v. EPA, 249 F.3d 1032, 
1040-44 (D.C. Cir. 2001).
---------------------------------------------------------------------------

2. International Transport
    The agency is active in work to reduce the international transport 
of O3 and other pollutants that can contribute to 
``background'' O3 levels in the U.S. Under the Convention on 
Long-Range Transboundary Air Pollution (LRTAP) of the United Nations 
Economic Commission for Europe, the U.S. has been a party to the 
Protocol to Abate Acidification, Eutrophication, and Ground-level Ozone 
(known as the Gothenburg Protocol) since 2005. The U.S. is also active 
in the LRTAP Task Force for Hemispheric Transport of Air Pollution. The 
U.S. has worked bilaterally with Canada under the US-Canada Air Quality 
Agreement to adopt an Ozone Annex to address transboundary 
O3 impacts and continues to work with China on air quality 
management activities. This work includes supporting China's efforts to 
rapidly deploy power plant pollution controls that can achieve 
NOX reductions of at least 80 to 90%. The U.S. also 
continues to work bilaterally with Mexico on the Border 2020 program to 
support efforts to improve environmental conditions in the border 
region. One of the main goals of the program is to reduce air 
pollution, including emissions that can cause transboundary 
O3 impacts.

[[Page 65444]]

    Clean Air Act section 179B recognizes the possibility that certain 
nonattainment areas may be impacted by O3 or O3 
precursor emissions from international sources beyond the regulatory 
jurisdiction of the state. The EPA's science review suggests that the 
influence of international sources on U.S. O3 levels will be 
largest in locations that are in the immediate vicinity of an 
international border with Canada or Mexico. The science review also 
cites two recent studies which indicate that intercontinental transport 
of pollution, along with other natural sources and local pollutant 
sources, can affect O3 air quality in the western U.S. under 
specific conditions. (U.S. EPA 2013, p. 3-140). Section 179B allows 
states to consider in their attainment plans and demonstrations whether 
an area might meet the O3 NAAQS by the attainment date ``but 
for'' emissions contributing to the area originating outside the U.S. 
If a state is unable to demonstrate attainment of the NAAQS in such an 
area impacted by international transport after adopting all reasonably 
available control measures (e.g., RACM, including RACT, as required by 
CAA section 182(b)), the EPA can nonetheless approve the CAA-required 
state attainment plan and demonstration using the authority in section 
179B.
    When the EPA approves this type of attainment plan and 
demonstration, and there would be no adverse consequence for a finding 
that the area failed to attain the NAAQS by the relevant attainment 
date. States can also avoid potential sanctions and FIPs that would 
otherwise apply for failure to submit a required SIP submission or 
failure to submit an approvable SIP submission. For example, section 
179B explicitly provides that the area shall not be reclassified to the 
next highest classification or required to implement a section 185 
penalty fee program if a state meets the applicable criteria.
    Section 179B authority does not allow an area to avoid a 
nonattainment designation or for the area to be classified with a lower 
classification than is indicated by actual ambient air quality. Section 
179B also does not provide for any relaxation of mandatory emissions 
control measures (including contingency measures) or the prescribed 
emissions reductions necessary to achieve periodic emissions reduction 
progress requirements. In this way, section 179B insures that states 
will take actions to mitigate the public health impacts of exposure to 
ambient levels of pollution that violate the NAAQS by imposing 
reasonable control measures on the sources that are within the 
jurisdiction of the state while also authorizing EPA to approve such 
attainment plans and demonstrations even though they do not fully 
address the public health impacts of international transport. Also, 
generally, monitoring data influenced by international transport may 
not be excluded from regulatory determinations. However, depending on 
the nature and scope of international emissions events affecting air 
quality in the U.S., the event-influenced data may qualify for 
exclusion under the Exceptional Events Rule. The EPA encourages 
affected air agencies to coordinate with their EPA regional office to 
identify approaches to evaluate the potential impacts of international 
transport and to determine the most appropriate information and 
analytical methods for each area's unique situation. The EPA will also 
work with states that are developing attainment plans for which section 
179B is relevant, and ensure the states have the benefit of the EPA's 
understanding of international transport of ozone and ozone precursors.
    The EPA has used section 179B authority previously to approve 
attainment plans for Mexican border areas in El Paso, TX 
(O3, PM10, and CO plans); and Nogales, AZ 
(PM10 plan). The 24-hour PM10 attainment plan for 
Nogales, AZ, was approved by EPA as sufficient to demonstrate 
attainment of the NAAQS by the Moderate classification deadline, but 
for international emissions sources in the Nogales Municipality, Mexico 
area (77 FR 38400, June 27, 2012).
    States are encouraged to consult with their EPA Regional Office to 
establish appropriate technical requirements for these analyses.

IX. 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

    This action is an economically significant regulatory action that 
was submitted to the Office of Management and Budget (OMB) for review. 
Any changes made in response to OMB recommendations have been 
documented in the docket. The EPA prepared an analysis of the potential 
costs and benefits associated with this action. This analysis is 
contained in the document, Regulatory Impact Analysis of the Final 
National Ambient Air Quality Standards for Ground-Level Ozone, October 
2015. A copy of the analysis is available in the RIA docket (EPA-HQ-
OAR-2013-0169) and the analysis is briefly summarized here. The RIA 
estimates the costs and monetized human health and welfare benefits of 
attaining three alternative O3 NAAQS nationwide. 
Specifically, the RIA examines the alternatives of 65 ppb and 70 ppb. 
The RIA contains illustrative analyses that consider a limited number 
of emissions control scenarios that states and Regional Planning 
Organizations might implement to achieve these alternative 
O3 NAAQS. However, the CAA and judicial decisions make clear 
that the economic and technical feasibility of attaining ambient 
standards are not to be considered in setting or revising NAAQS, 
although such factors may be considered in the development of state 
plans to implement the standards. Accordingly, although an RIA has been 
prepared, the results of the RIA have not been considered in issuing 
this final rule.

B. Paperwork Reduction Act

    The information collection requirements in this final rule have 
been submitted for approval to the Office of Management and Budget 
(OMB) under the Paperwork Reduction Act (PRA). The information 
collection requirements are not enforceable until OMB approves them. 
The Information Collection Request (ICR) document prepared by the EPA 
for these revisions has been assigned EPA ICR #2313.04.
    The information collected and reported under 40 CFR part 58 is 
needed to determine compliance with the NAAQS, to characterize air 
quality and associated health and ecosystems impacts, to develop 
emission control strategies, and to measure progress for the air 
pollution program. We are extending the length of the required 
O3 monitoring season in 32 states and the District of 
Columbia and the revised O3 monitoring seasons will become 
effective on January 1, 2017. We are also revising the PAMS monitoring 
requirements to reduce the number of required PAMS sites while 
improving spatial coverage, and requiring states in moderate or above 
O3 non-attainment areas and the O3 transport 
region to develop an enhanced monitoring plan as part of the PAMS 
requirements. Monitoring agencies will need to comply with the PAMS 
requirements by June 1, 2019. In addition, we are revising the 
O3 FRM to establish a new, additional technique for 
measuring O3 in the ambient air. It will be

[[Page 65445]]

incorporated into the existing O3 FRM, using the same 
calibration procedure in Appendix D of 40 CFR part 50. We are also 
making changes to the procedures for testing performance 
characteristics and determining comparability between candidate FEMs 
and reference methods.
    For the purposes of ICR number 2313.04, the burden figures 
represent the burden estimate based on the requirements contained in 
this rule. The burden estimates are for the 3-year period from 2016 
through 2018. The implementation of the PAMS changes will occur beyond 
the time frame of this ICR with implementation occurring in 2019. The 
cost estimates for the PAMS network (including revisions) will be 
captured in future routine updates to the Ambient Air Quality 
Surveillance ICR that are required every 3 years by OMB. The addition 
of a new FRM in 40 CFR part 50 and revisions to the O3 FEM 
procedures for testing performance characteristics in 40 CFR part 53 
does not add any additional information collection requirements.
    The ICR burden estimates are associated with the changes to the 
O3 seasons in this final rule. This information collection 
is estimated to involve 158 respondents for a total cost of 
approximately $24,597,485 (total capital, labor, and operation and 
maintenance) plus a total burden of 339,930 hours for the support of 
all operational aspects of the entire O3 monitoring network. 
The labor costs associated with these hours are $20,209,966. Also 
included in the total are other costs of operations and maintenance of 
$2,254,334 and equipment and contract costs of $2,133,185. The actual 
labor cost increase to expand the O3 monitoring seasons is 
$2,064,707. In addition to the costs at the state, local, and tribal 
air quality management agencies, there is a burden to EPA of 41,418 
hours and $2,670,360. Burden is defined at 5 CFR 1320.3(b). State, 
local, and tribal entities are eligible for state assistance grants 
provided by the federal government under the CAA which can be used for 
related activities. An agency may not conduct or sponsor, and a person 
is not required to respond to, a collection of information unless it 
displays a currently valid OMB control number. The OMB control numbers 
for EPA's regulations in 40 CFR are listed in 40 CFR part 9.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
rule establishes 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 at 1044-45 (NAAQS 
do not have significant impacts upon small entities because NAAQS 
themselves impose no regulations upon small entities). Similarly, the 
revisions to 40 CFR part 58 address the requirements for states to 
collect information and report compliance with the NAAQS and will not 
impose any requirements on small entities. Similarly, the addition of a 
new FRM in 40 CFR part 50 and revisions to the FEM procedures for 
testing in 40 CFR part 53 will not impose any requirements on small 
entities.

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain an unfunded federal mandate of $100 
million or more as described in UMRA, 2 U.S.C. 1531-1538, and does not 
significantly or uniquely affect small governments. The revisions to 
the O3 NAAQS impose no enforceable duty on any state, local, 
or tribal governments or the private sector beyond those duties already 
established in the CAA. The expected costs associated with the 
monitoring requirements are described in the EPA's ICR document, and 
these costs are not expected to exceed $100 million in the aggregate 
for any year.
    Furthermore, as indicated previously, in setting NAAQS the EPA 
cannot consider the economic or technological feasibility of attaining 
ambient air quality standards, although such factors may be considered 
to a degree in the development of state plans to implement the 
standards (see American Trucking Associations v. EPA, 175 F. 3d at 1043 
[noting that because the EPA is precluded from considering costs of 
implementation in establishing NAAQS, preparation of a RIA pursuant to 
the UMRA would not furnish any information which the court could 
consider in reviewing the NAAQS]). With regard to the sections of the 
rule preamble discussing implementation of the revisions to the 
O3 NAAQS, the CAA imposes the obligation for states to 
submit SIPs to implement the NAAQS for O3. To the extent the 
EPA's discussion of implementation topics in this final rule may 
reflect some interpretations of those requirements, those 
interpretations do not impose obligations beyond the duties already 
established in the CAA and thus do not constitute a federal mandate for 
purposes of UMRA. The EPA is also adopting a grandfathering provision 
for certain PSD permits in this action, as described above. However, 
that provision does not impose any mandate on any state, local, or 
tribal government or the private sector, but rather provides relief 
from requirements that would otherwise result from the new standards. 
In addition, the EPA is not requiring states to revise their SIPs to 
include such a provision.

E. 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.

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

    This action does not have tribal implications as specified in 
Executive Order 13175. It does not have a substantial direct effect on 
one or more Indian tribes. This rule provides increased protection from 
adverse effects of ozone for the entire country, including for 
sensitive populations, and tribes are not obligated to adopt or 
implement any NAAQS. In addition, tribes are not obligated to conduct 
ambient monitoring for O3 or to adopt the ambient monitoring 
requirements of 40 CFR part 58. Even if this action were determined to 
have tribal implications within the meaning of Executive Order 13175, 
it will neither impose substantial direct compliance costs on tribal 
governments, nor preempt tribal law. Thus, consultation under Executive 
Order 13175 was not required.
    Nonetheless, consistent with the ``EPA Policy on Consultation and 
Coordination with Indian Tribes'', the EPA offered government-to-
government consultation on the proposed rule. No tribe requested 
government-to-government consultation with the EPA on this rule. In 
addition, the EPA conducted outreach to tribal environmental 
professionals, which included participation in the Tribal Air call 
sponsored by the National Tribal Air Association, and two other calls 
available to tribal environmental professionals. During the public 
comment period we received comments on the proposed rule from seven 
tribes and three tribal organizations.

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

    This action is subject to Executive Order 13045 because it is an

[[Page 65446]]

economically significant regulatory action as defined by Executive 
Order 12866, and the EPA believes that the environmental health risk 
addressed by this action may have a disproportionate effect on 
children. The rule will establish uniform NAAQS for O3; 
these standards are designed to protect public health with an adequate 
margin of safety, as required by CAA section 109. However, the 
protection offered by these standards may be especially important for 
children because children, especially children with asthma, along with 
other at-risk populations \256\ such as all people with lung disease 
and people active outdoors, are at increased risk for health effects 
associated with exposure to O3 in ambient air. Because 
children are considered an at-risk lifestage, we have carefully 
evaluated the environmental health effects of exposure to O3 
pollution among children. Discussions of the results of the evaluation 
of the scientific evidence, policy considerations, and the exposure and 
risk assessments pertaining to children are contained in sections II.B 
and II.C of this preamble.
---------------------------------------------------------------------------

    \256\ As used here and similarly throughout this document, the 
term population refers to people having a quality or characteristic 
in common, including a specific pre-existing illness or a specific 
age or lifestage.
---------------------------------------------------------------------------

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

    This action is not a ``significant energy action'' because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this rule is to 
establish revised NAAQS for O3, establish an additional FRM, 
revise FEM procedures for testing, and revises air quality surveillance 
requirements. The rule does not prescribe specific pollution control 
strategies by which these ambient standards and monitoring revisions 
will be met. Such strategies will be developed by states on a case-by-
case basis, and the EPA cannot predict whether the control options 
selected by states will include regulations on energy suppliers, 
distributors, or users. Thus, the EPA concludes that this rule is not 
likely to have any adverse energy effects and does not constitute a 
significant energy action as defined in Executive Order 13211.

I. National Technology Transfer and Advancement Act

    This rulemaking involves environmental monitoring and measurement. 
Consistent with the Agency's Performance Based Measurement System 
(PBMS), the EPA is not requiring the use of specific, prescribed 
analytical methods. Rather, the Agency is allowing the use of any 
method that meets the prescribed performance criteria. Ambient air 
concentrations of O3 are currently measured by the FRM in 40 
CFR part 50, Appendix D (Measurement Principle and Calibration 
Procedure for the Measurement of Ozone in the Atmosphere) or by FEM 
that meet the requirements of 40 CFR part 53. Procedures are available 
in part 53 that allow for the approval of an FEM for O3 that 
is similar to the FRM. Any method that meets the performance criteria 
for a candidate equivalent method may be approved for use as an FEM. 
This approach is consistent with EPA's PBMS. The PBMS approach is 
intended to be more flexible and cost-effective for the regulated 
community; it is also intended to encourage innovation in analytical 
technology and improved data quality. The EPA is not precluding the use 
of any method, whether it constitutes a voluntary consensus standard or 
not, as long as it meets the specified performance criteria.

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

    The EPA believes that this action will not have disproportionately 
high and adverse human health or environmental effects on minority 
populations, low-income populations or indigenous peoples. The action 
described in this notice is to strengthen the NAAQS for O3.
    The primary NAAQS are established at a level that is requisite to 
protect public health, including the health of sensitive or at-risk 
groups, with an adequate margin of safety. The NAAQS decisions are 
based on an explicit and comprehensive assessment of the current 
scientific evidence and associated exposure/risk analyses. More 
specifically, EPA expressly considers the available information 
regarding health effects among at-risk populations, including that 
available for low-income populations and minority populations, in 
decisions on NAAQS. Where low-income populations or minority 
populations are among the at-risk populations, the decision on the 
standard is based on providing protection for these and other at-risk 
populations and lifestages. Where such populations are not identified 
as at-risk populations, a NAAQS that is established to provide 
protection to the at-risk populations would also be expected to provide 
protection to all other populations, including low-income populations 
and minority populations.
    The ISA, HREA, and PA for this review, which include identification 
of populations at risk from O3 health effects, are available 
in the docket, EPA-HQ-OAR-2008-0699. The information on at-risk 
populations for this NAAQS review is summarized and considered earlier 
in this preamble (see section II.A). This final rule increases the 
level of environmental protection for all affected populations without 
having any disproportionately high and adverse human health or 
environmental effects on any population, including any minority 
populations, low-income populations or indigenous peoples. This rule 
establishes uniform national standards for O3 in ambient air 
that, in the Administrator's judgment, protect public health, including 
the health of sensitive groups, with an adequate margin of safety.
    Although it is part of a separate docket (EPA-HQ-OAR-2013-0169) and 
is not part of the rulemaking record for this action, EPA has prepared 
a RIA of this decision. As part of the RIA, a demographic analysis was 
conducted. While, as noted in the RIA, the demographic analysis is not 
a full quantitative, site-specific exposure and risk assessment, that 
analysis examined demographic characteristics of persons living in 
areas with poor air quality relative to the proposed standard. 
Specifically, Chapter 9, section 9.10 (page 9-7) and Appendix 9A of the 
RIA describe this proximity and socio-demographic analysis. This 
analysis found that in areas with poor air quality relative to the 
revised standard,\257\ the representation of minority populations was 
slightly greater than in the U.S. as a whole. Because the air quality 
in these areas does not currently meet the revised standard, 
populations in these areas would be expected to benefit from 
implementation of the strengthened standard, and, thus, would be more 
affected by strategies to attain the revised standard. This analysis, 
which evaluates the potential implications for minority populations and 
low-income populations of future air pollution control actions that 
state and local agencies may consider in implementing the revised 
O3 NAAQS described in this decision notice are discussed in 
Appendix 9A of the RIA. The RIA is available on the Web, through the 
EPA's Technology Transfer Network Web site at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html and

[[Page 65447]]

in the RIA docket (EPA-HQ-OAR-2013-0169). As noted above, although an 
RIA has been prepared, the results of the RIA have not been considered 
in issuing this final rule.
---------------------------------------------------------------------------

    \257\ This refers to monitored areas with O3 design 
values above the revised and alternative standards.
---------------------------------------------------------------------------

K. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule 
report to each House of the Congress and to the Comptroller General of 
the United States. This action is a ``major rule'' as defined by 5 
U.S.C. 804(2).

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carcinogen risk

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assessment. Risk Assessment Forum, U.S. EPA, Washington, DC. EPA/
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for ozone and related photochemical oxidants. U.S. Environmental 
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U.S. Environmental Protection Agency (1996b). Review of national 
ambient air quality standards for ozone: Assessment of scientific 
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information on effects of ozone on health and vegetation: Supplement 
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U.S. Environmental Protection Agency (1989). Review of the National 
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U.S. Environmental Protection Agency (1986). Air quality criteria 
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U.S. Environmental Protection Agency (1978). Air quality criteria 
for ozone and other photochemical oxidants. Washington, DC. EPA/600/
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Network.
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Service. (2010). Federal land managers' air quality related values 
work group (FLAG): Phase I report--revised (2010). Natural Resource 
Report NPS/NRPC/NRR--2010/232. National Park Service, Denver, CO. 
http://www.nature.nps.gov/air/Pubs/pdf/flag/FLAG_2010.pdf.
U.S. National Park Service. (2003). Ozone Sensitive Plant Species on 
National Park Service and U.S. Fish and Wildlife Service Lands: 
Results of a June 24-25, 2003 Workshop. Baltimore, MD. http://www.nature.nps.gov/air/pubs/pdf/baltfinalreport1.pdf.
U.S. National Park Service. (2006). Ozone Sensitive Plant Species, 
by Park, November 2006. http://www.nature.nps.gov/air/Permits/ARIS/docs/Ozone_Sensitive_ByPark_3600.pdf.
Ulmer, C; Kopp, M; Ihorst, G; Frischer, T; Forster, J; Kuehr, J. 
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summer of 1994 on pulmonary function of schoolchildren. Pediatr 
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0496(199705)23:5<344::AID-PPUL63.0.CO;2-K.
Vagaggini, B; Cianchetti, S; Bartoli, M; Ricci, M; Bacci, E; Dente, 
FL; Di Franco, A; Paggiaro, P. (2007). Prednisone blunts airway 
neutrophilic inflammatory response due to ozone exposure in 
asthmatic subjects. Respiration 74:61-58. http://dx.doi.org/10.1159/000096078.
Vagaggini, B; Taccola, M; Conti, I; Carnevali, S; Cianchetti, S; 
Bartoli, ML; Bacci, E; Dente, FL; Di Franco, A; Giannini, D; 
Paggiaro, PL. (2001). Budesonide reduces neutrophilic but not 
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List of Subjects

40 CFR Part 50

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

40 CFR Part 51

    Environmental protection, Administrative practices and

[[Page 65452]]

procedures, Air pollution control, Intergovernmental relations.

40 CFR Part 52

    Environmental Protection, Administrative practices and procedures, 
Air pollution control, Incorporation by reference, Intergovernmental 
relations.

40 CFR Part 53

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

40 CFR Part 58

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

    Dated: October 1, 2015.
Gina McCarthy,
Administrator.
    For the reasons set forth in the preamble, chapter I of title 40 of 
the Code of Federal Regulations is amended as follows:

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

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

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

0
2. Amend Sec.  50.14 by:
0
a. Revising paragraphs (c)(2)(iii) and (vi) and (c)(3)(i); and
0
b. Removing and reserving paragraphs (c)(2)(iv) and (v) and (c)(3)(ii) 
and (iii).
    The revisions read as follows:


Sec.  50.14  Treatment of air quality monitoring data influenced by 
exceptional events.

* * * * *
    (c) * * *
    (2) * * *
    (iii) Flags placed on data as being due to an exceptional event 
together with an initial description of the event shall be submitted to 
EPA not later than July 1st of the calendar year following the year in 
which the flagged measurement occurred, except as allowed under 
paragraph (c)(2)(vi) of this section.
* * * * *
    (vi) Table 1 identifies the data submission process for a new or 
revised NAAQS. This process shall apply to those data that will or may 
influence the initial designation of areas for any new or revised 
NAAQS.

  Table 1--Schedule for Flagging and Documentation Submission for Data
  Influenced by Exceptional Events for Use in Initial Area Designations
------------------------------------------------------------------------
   Exceptional events/regulatory    Exceptional events deadline schedule
              action                                 \d\
------------------------------------------------------------------------
Flagging and initial event          If state and tribal initial
 description deadline for data       designation recommendations for a
 years 1, 2 and 3.\a\.               new/revised NAAQS are due August
                                     through January, then the flagging
                                     and initial event description
                                     deadline will be the July 1 prior
                                     to the recommendation deadline. If
                                     state and tribal recommendations
                                     for a new/revised NAAQS are due
                                     February through July, then the
                                     flagging and initial event
                                     description deadline will be the
                                     January 1 prior to the
                                     recommendation deadline.
Exceptional events demonstration    No later than the date that state
 submittal deadline for data years   and tribal recommendations are due
 1, 2 and 3.\a\.                     to EPA.
Flagging, initial event             By the last day of the month that is
 description and exceptional         1 year and 7 months after
 events demonstration submittal      promulgation of a new/revised
 deadline for data year 4 \b\ and,   NAAQS, unless either option a or b
 where applicable, data year 5.\c\.  applies.
                                    a. If the EPA follows a 3-year
                                     designation schedule, the deadline
                                     is 2 years and 7 months after
                                     promulgation of a new/revised
                                     NAAQS.
                                    b. If the EPA notifies the state/
                                     tribe that it intends to complete
                                     the initial area designations
                                     process according to a schedule
                                     between 2 and 3 years, the deadline
                                     is 5 months prior to the date
                                     specified for final designations
                                     decisions in such EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
  considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
  consider when it makes final area designations for a new/revised NAAQS
  under the standard designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
  consider when it makes final area designations for a new/revised NAAQS
  under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
  quality monitoring data in AQS is annually on May 1 of the year
  following the year of data collection as specified in 40 CFR
  58.15(a)(2). In some cases, however, air agencies may choose to
  certify a prior year's data in advance of May 1 of the following year,
  particularly if the EPA has indicated its intent to promulgate final
  designations in the first 8 months of the calendar year. Data
  flagging, initial event description and exceptional events
  demonstration deadlines for ``early certified'' data will follow the
  deadlines for ``year 4'' and ``year 5'' data.

    (3) Submission of demonstrations. (i) Except as allowed under 
paragraph (c)(2)(vi) of this section, a State that has flagged data as 
being due to an exceptional event and is requesting exclusion of the 
affected measurement data shall, after notice and opportunity for 
public comment, submit a demonstration to justify data exclusion to EPA 
not later than the lesser of 3 years following the end of the calendar 
quarter in which the flagged concentration was recorded or 12 months 
prior to the date that a regulatory decision must be made by EPA. A 
State must submit the public comments it received along with its 
demonstration to EPA.
* * * * *

0
3. Section 50.19 is added to read as follows:


Sec.  50.19  National primary and secondary ambient air quality 
standards for ozone.

    (a) The level of the national 8-hour primary ambient air quality 
standard for ozone (O3) is 0.070 parts per million (ppm), 
daily maximum 8-hour average, measured by a reference method based on 
appendix D to this part and designated in accordance with part 53 of 
this chapter or an equivalent method designated in accordance with part 
53 of this chapter.
    (b) The 8-hour primary O3 ambient air quality standard 
is met at an ambient air quality monitoring site when the 3-year 
average of the annual fourth-highest daily maximum 8-hour average 
O3 concentration is less than or equal to 0.070 ppm, as 
determined in accordance with appendix U to this part.
    (c) The level of the national secondary ambient air quality 
standard for O3 is 0.070 ppm, daily maximum 8-hour

[[Page 65453]]

average, measured by a reference method based on appendix D to this 
part and designated in accordance with part 53 of this chapter or an 
equivalent method designated in accordance with part 53 of this 
chapter.
    (d) The 8-hour secondary O3 ambient air quality standard 
is met at an ambient air quality monitoring site when the 3-year 
average of the annual fourth-highest daily maximum 8-hour average 
O3 concentration is less than or equal to 0.070 ppm, as 
determined in accordance with appendix U to this part.

0
4. Revise appendix D to part 50 to read as follows:

Appendix D to Part 50--Reference Measurement Principle and Calibration 
Procedure for the Measurement of Ozone in the Atmosphere 
(Chemiluminescence Method)

    1.0 Applicability.
    1.1 This chemiluminescence method provides reference 
measurements of the concentration of ozone (O3) in 
ambient air for determining compliance with the national primary and 
secondary ambient air quality standards for O3 as 
specified in 40 CFR part 50. This automated method is applicable to 
the measurement of ambient O3 concentrations using 
continuous (real-time) sampling and analysis. Additional quality 
assurance procedures and guidance are provided in 40 CFR part 58, 
appendix A, and in Reference 14.
    2.0 Measurement Principle.
    2.1 This reference method is based on continuous automated 
measurement of the intensity of the characteristic chemiluminescence 
released by the gas phase reaction of O3 in sampled air 
with either ethylene (C2H4) or nitric oxide 
(NO) gas. An ambient air sample stream and a specific flowing 
concentration of either C2H4 (ET-CL method) or 
NO (NO-CL method) are mixed in a measurement cell, where the 
resulting chemiluminescence is quantitatively measured by a 
sensitive photo-detector. References 8-11 describe the 
chemiluminescence measurement principle.
    2.2 The measurement system is calibrated by referencing the 
instrumental chemiluminescence measurements to certified 
O3 standard concentrations generated in a dynamic flow 
system and assayed by photometry to be traceable to a National 
Institute of Standards and Technology (NIST) standard reference 
photometer for O3 (see Section 4, Calibration Procedure, 
below).
    2.3 An analyzer implementing this measurement principle is shown 
schematically in Figure 1. Designs implementing this measurement 
principle must include: an appropriately designed mixing and 
measurement cell; a suitable quantitative photometric measurement 
system with adequate sensitivity and wavelength specificity for 
O3; a pump, flow control, and sample conditioning system 
for sampling the ambient air and moving it into and through the 
measurement cell; a sample air dryer as necessary to meet the water 
vapor interference limit requirement specified in subpart B of part 
53 of this chapter; a means to supply, meter, and mix a constant, 
flowing stream of either C2H4 or NO gas of 
fixed concentration with the sample air flow in the measurement 
cell; suitable electronic control and measurement processing 
capability; and other associated apparatus as may be necessary. The 
analyzer must be designed and constructed to provide accurate, 
repeatable, and continuous measurements of O3 
concentrations in ambient air, with measurement performance that 
meets the requirements specified in subpart B of part 53 of this 
chapter.
    2.4 An analyzer implementing this measurement principle and 
calibration procedure will be considered a federal reference method 
(FRM) only if it has been designated as a reference method in 
accordance with part 53 of this chapter.
    2.5 Sampling considerations. The use of a particle filter on the 
sample inlet line of a chemiluminescence O3 FRM analyzer 
is required to prevent buildup of particulate matter in the 
measurement cell and inlet components. This filter must be changed 
weekly (or at least often as specified in the manufacturer's 
operation/instruction manual), and the sample inlet system used with 
the analyzer must be kept clean, to avoid loss of O3 in 
the O3 sample air prior to the concentration measurement.
    3.0 Interferences.
    3.1 Except as described in 3.2 below, the chemiluminescence 
measurement system is inherently free of significant interferences 
from other pollutant substances that may be present in ambient air.
    3.2 A small sensitivity to variations in the humidity of the 
sample air is minimized by a sample air dryer. Potential loss of 
O3 in the inlet air filter and in the air sample handling 
components of the analyzer and associated exterior air sampling 
components due to buildup of airborne particulate matter is 
minimized by filter replacement and cleaning of the other inlet 
components.
    4.0 Calibration Procedure.
    4.1 Principle. The calibration procedure is based on the 
photometric assay of O3 concentrations in a dynamic flow 
system. The concentration of O3 in an absorption cell is 
determined from a measurement of the amount of 254 nm light absorbed 
by the sample. This determination requires knowledge of (1) the 
absorption coefficient ([alpha]) of O3 at 254 nm, (2) the 
optical path length (l) through the sample, (3) the transmittance of 
the sample at a nominal wavelength of 254 nm, and (4) the 
temperature (T) and pressure (P) of the sample. The transmittance is 
defined as the ratio I/I0, where I is the intensity of 
light which passes through the cell and is sensed by the detector 
when the cell contains an O3 sample, and I0 is 
the intensity of light which passes through the cell and is sensed 
by the detector when the cell contains zero air. It is assumed that 
all conditions of the system, except for the contents of the 
absorption cell, are identical during measurement of I and 
I0. The quantities defined above are related by the Beer-
Lambert absorption law,
[GRAPHIC] [TIFF OMITTED] TR26OC15.002

Where:

[alpha] = absorption coefficient of O3 at 254 nm = 308 
4 atm-1 cm-1 at 0 [deg]C and 760 torr,\1, 2, 3, 4, 5, 6, 
7\
c = O3 concentration in atmospheres, and
l = optical path length in cm.

    A stable O3 generator is used to produce 
O3 concentrations over the required calibration 
concentration range. Each O3 concentration is determined 
from the measurement of the transmittance (I/I0) of the 
sample at 254 nm with a photometer of path length l and calculated 
from the equation,
[GRAPHIC] [TIFF OMITTED] TR26OC15.003


[[Page 65454]]


    The calculated O3 concentrations must be corrected 
for O3 losses, which may occur in the photometer, and for 
the temperature and pressure of the sample.
    4.2 Applicability. This procedure is applicable to the 
calibration of ambient air O3 analyzers, either directly 
or by means of a transfer standard certified by this procedure. 
Transfer standards must meet the requirements and specifications set 
forth in Reference 12.
    4.3 Apparatus. A complete UV calibration system consists of an 
O3 generator, an output port or manifold, a photometer, 
an appropriate source of zero air, and other components as 
necessary. The configuration must provide a stable O3 
concentration at the system output and allow the photometer to 
accurately assay the output concentration to the precision specified 
for the photometer (4.3.1). Figure 2 shows a commonly used 
configuration and serves to illustrate the calibration procedure, 
which follows. Other configurations may require appropriate 
variations in the procedural steps. All connections between 
components in the calibration system downstream of the O3 
generator must be of glass, Teflon, or other relatively inert 
materials. Additional information regarding the assembly of a UV 
photometric calibration apparatus is given in Reference 13. For 
certification of transfer standards which provide their own source 
of O3, the transfer standard may replace the 
O3 generator and possibly other components shown in 
Figure 2; see Reference 12 for guidance.
    4.3.1 UV photometer. The photometer consists of a low-pressure 
mercury discharge lamp, (optional) collimation optics, an absorption 
cell, a detector, and signal-processing electronics, as illustrated 
in Figure 2. It must be capable of measuring the transmittance, I/
I0, at a wavelength of 254 nm with sufficient precision 
such that the standard deviation of the concentration measurements 
does not exceed the greater of 0.005 ppm or 3% of the concentration. 
Because the low-pressure mercury lamp radiates at several 
wavelengths, the photometer must incorporate suitable means to 
assure that no O3 is generated in the cell by the lamp, 
and that at least 99.5% of the radiation sensed by the detector is 
254 nm radiation. (This can be readily achieved by prudent selection 
of optical filter and detector response characteristics.) The length 
of the light path through the absorption cell must be known with an 
accuracy of at least 99.5%. In addition, the cell and associated 
plumbing must be designed to minimize loss of O3 from 
contact with cell walls and gas handling components. See Reference 
13 for additional information.
    4.3.2 Air flow controllers. Air flow controllers are devices 
capable of regulating air flows as necessary to meet the output 
stability and photometer precision requirements.
    4.3.3 Ozone generator. The ozone generator used must be capable 
of generating stable levels of O3 over the required 
concentration range.
    4.3.4 Output manifold. The output manifold must be constructed 
of glass, Teflon, or other relatively inert material, and should be 
of sufficient diameter to insure a negligible pressure drop at the 
photometer connection and other output ports. The system must have a 
vent designed to insure atmospheric pressure in the manifold and to 
prevent ambient air from entering the manifold.
    4.3.5 Two-way valve. A manual or automatic two-way valve, or 
other means is used to switch the photometer flow between zero air 
and the O3 concentration.
    4.3.6 Temperature indicator. A device to indicate temperature 
must be used that is accurate to 1 [deg]C.
    4.3.7 Barometer or pressure indicator. A device to indicate 
barometric pressure must be used that is accurate to 2 
torr.
    4.4 Reagents.
    4.4.1 Zero air. The zero air must be free of contaminants which 
would cause a detectable response from the O3 analyzer, 
and it must be free of NO, C2H4, and other 
species which react with O3. A procedure for generating 
suitable zero air is given in Reference 13. As shown in Figure 2, 
the zero air supplied to the photometer cell for the I0 
reference measurement must be derived from the same source as the 
zero air used for generation of the O3 concentration to 
be assayed (I measurement). When using the photometer to certify a 
transfer standard having its own source of O3, see 
Reference 12 for guidance on meeting this requirement.
    4.5 Procedure.
    4.5.1 General operation. The calibration photometer must be 
dedicated exclusively to use as a calibration standard. It must 
always be used with clean, filtered calibration gases, and never 
used for ambient air sampling. A number of advantages are realized 
by locating the calibration photometer in a clean laboratory where 
it can be stationary, protected from the physical shock of 
transportation, operated by a responsible analyst, and used as a 
common standard for all field calibrations via transfer standards.
    4.5.2 Preparation. Proper operation of the photometer is of 
critical importance to the accuracy of this procedure. Upon initial 
operation of the photometer, the following steps must be carried out 
with all quantitative results or indications recorded in a 
chronological record, either in tabular form or plotted on a 
graphical chart. As the performance and stability record of the 
photometer is established, the frequency of these steps may be 
reduced to be consistent with the documented stability of the 
photometer and the guidance provided in Reference 12.
    4.5.2.1 Instruction manual. Carry out all set up and adjustment 
procedures or checks as described in the operation or instruction 
manual associated with the photometer.
    4.5.2.2 System check. Check the photometer system for integrity, 
leaks, cleanliness, proper flow rates, etc. Service or replace 
filters and zero air scrubbers or other consumable materials, as 
necessary.
    4.5.2.3 Linearity. Verify that the photometer manufacturer has 
adequately established that the linearity error of the photometer is 
less than 3%, or test the linearity by dilution as follows: Generate 
and assay an O3 concentration near the upper range limit 
of the system or appropriate calibration scale for the instrument, 
then accurately dilute that concentration with zero air and re-assay 
it. Repeat at several different dilution ratios. Compare the assay 
of the original concentration with the assay of the diluted 
concentration divided by the dilution ratio, as follows
[GRAPHIC] [TIFF OMITTED] TR26OC15.004

Where:

E = linearity error, percent
A1 = assay of the original concentration
A2 = assay of the diluted concentration
R = dilution ratio = flow of original concentration divided by the 
total flow

    The linearity error must be less than 5%. Since the accuracy of 
the measured flow-rates will affect the linearity error as measured 
this way, the test is not necessarily conclusive. Additional 
information on verifying linearity is contained in Reference 13.
    4.5.2.4 Inter-comparison. The photometer must be inter-compared 
annually, either directly or via transfer standards, with a NIST 
standard reference photometer (SRP) or calibration photometers used 
by other agencies or laboratories.
    4.5.2.5 Ozone losses. Some portion of the O3 may be 
lost upon contact with the photometer cell walls and gas handling 
components. The magnitude of this loss must be determined and used 
to correct the calculated O3 concentration. This loss 
must not exceed 5%. Some guidelines for quantitatively determining 
this loss are discussed in Reference 13.
    4.5.3 Assay of O3 concentrations. The operator must 
carry out the following steps to properly assay O3 
concentrations.
    4.5.3.1 Allow the photometer system to warm up and stabilize.
    4.5.3.2 Verify that the flow rate through the photometer 
absorption cell, F, allows the cell to be flushed in a reasonably 
short period of time (2 liter/min is a typical flow). The precision 
of the measurements is inversely related to the time required for 
flushing, since the photometer drift error increases with time.
    4.5.3.3 Ensure that the flow rate into the output manifold is at 
least 1 liter/min greater than the total flow rate required by the 
photometer and any other flow demand connected to the manifold.

[[Page 65455]]

    4.5.3.4 Ensure that the flow rate of zero air, Fz, is at least 1 
liter/min greater than the flow rate required by the photometer.
    4.5.3.5 With zero air flowing in the output manifold, actuate 
the two-way valve to allow the photometer to sample first the 
manifold zero air, then Fz. The two photometer readings must be 
equal (I = I0).

    Note:  In some commercially available photometers, the operation 
of the two-way valve and various other operations in section 4.5.3 
may be carried out automatically by the photometer.

    4.5.3.6 Adjust the O3 generator to produce an 
O3 concentration as needed.
    4.5.3.7 Actuate the two-way valve to allow the photometer to 
sample zero air until the absorption cell is thoroughly flushed and 
record the stable measured value of Io.
    4.5.3.8 Actuate the two-way valve to allow the photometer to 
sample the O3 concentration until the absorption cell is 
thoroughly flushed and record the stable measured value of I.
    4.5.3.9 Record the temperature and pressure of the sample in the 
photometer absorption cell. (See Reference 13 for guidance.)
    4.5.3.10 Calculate the O3 concentration from equation 
4. An average of several determinations will provide better 
precision.
[GRAPHIC] [TIFF OMITTED] TR26OC15.005

Where:

[O3]OUT = O3 concentration, ppm
[alpha] = absorption coefficient of O3 at 254 nm = 308 
atm-1 cm-1 at 0[deg] C and 760 torr
l = optical path length, cm
T = sample temperature, K
P = sample pressure, torr
L = correction factor for O3 losses from 4.5.2.5 = (1-
fraction of O3 lost).

    Note:  Some commercial photometers may automatically evaluate 
all or part of equation 4. It is the operator's responsibility to 
verify that all of the information required for equation 4 is 
obtained, either automatically by the photometer or manually. For 
``automatic'' photometers which evaluate the first term of equation 
4 based on a linear approximation, a manual correction may be 
required, particularly at higher O3 levels. See the 
photometer instruction manual and Reference 13 for guidance.

    4.5.3.11 Obtain additional O3 concentration standards 
as necessary by repeating steps 4.5.3.6 to 4.5.3.10 or by Option 1.
    4.5.4 Certification of transfer standards. A transfer standard 
is certified by relating the output of the transfer standard to one 
or more O3 calibration standards as determined according 
to section 4.5.3. The exact procedure varies depending on the nature 
and design of the transfer standard. Consult Reference 12 for 
guidance.
    4.5.5 Calibration of ozone analyzers. Ozone analyzers must be 
calibrated as follows, using O3 standards obtained 
directly according to section 4.5.3 or by means of a certified 
transfer standard.
    4.5.5.1 Allow sufficient time for the O3 analyzer and 
the photometer or transfer standard to warm-up and stabilize.
    4.5.5.2 Allow the O3 analyzer to sample zero air 
until a stable response is obtained and then adjust the 
O3 analyzer's zero control. Offsetting the analyzer's 
zero adjustment to +5% of scale is recommended to facilitate 
observing negative zero drift (if any). Record the stable zero air 
response as ``Z''.
    4.5.5.3 Generate an O3 concentration standard of 
approximately 80% of the desired upper range limit (URL) of the 
O3 analyzer. Allow the O3 analyzer to sample 
this O3 concentration standard until a stable response is 
obtained.
    4.5.5.4 Adjust the O3 analyzer's span control to 
obtain the desired response equivalent to the calculated standard 
concentration. Record the O3 concentration and the 
corresponding analyzer response. If substantial adjustment of the 
span control is necessary, recheck the zero and span adjustments by 
repeating steps 4.5.5.2 to 4.5.5.4.
    4.5.5.5 Generate additional O3 concentration 
standards (a minimum of 5 are recommended) over the calibration 
scale of the O3 analyzer by adjusting the O3 
source or by Option 1. For each O3 concentration 
standard, record the O3 concentration and the 
corresponding analyzer response.
    4.5.5.6 Plot the O3 analyzer responses (vertical or 
Y-axis) versus the corresponding O3 standard 
concentrations (horizontal or X-axis). Compute the linear regression 
slope and intercept and plot the regression line to verify that no 
point deviates from this line by more than 2 percent of the maximum 
concentration tested.
    4.5.5.7 Option 1: The various O3 concentrations 
required in steps 4.5.3.11 and 4.5.5.5 may be obtained by dilution 
of the O3 concentration generated in steps 4.5.3.6 and 
4.5.5.3. With this option, accurate flow measurements are required. 
The dynamic calibration system may be modified as shown in Figure 3 
to allow for dilution air to be metered in downstream of the 
O3 generator. A mixing chamber between the O3 
generator and the output manifold is also required. The flow rate 
through the O3 generator (Fo) and the dilution air flow 
rate (FD) are measured with a flow or volume standard that is 
traceable to a NIST flow or volume calibration standard. Each 
O3 concentration generated by dilution is calculated 
from:
[GRAPHIC] [TIFF OMITTED] TR26OC15.006

Where:

[O3]'OUT = diluted O3 
concentration, ppm
FO = flow rate through the O3 generator, liter/min
FD = diluent air flow rate, liter/min

    Note:  Additional information on calibration and pollutant 
standards is provided in Section 12 of Reference 14.

    5.0 Frequency of Calibration.
    5.1 The frequency of calibration, as well as the number of 
points necessary to establish the calibration curve, and the 
frequency of other performance checking will vary by analyzer; 
however, the minimum frequency, acceptance criteria, and subsequent 
actions are specified in Appendix D of Reference 14: Measurement 
Quality Objectives and Validation Templates. The user's quality 
control program shall provide guidelines for initial establishment 
of these variables and for subsequent alteration as operational 
experience is accumulated. Manufacturers of analyzers should include 
in their instruction/operation manuals information and guidance as 
to these variables and on other matters of operation, calibration, 
routine maintenance, and quality control.
    6.0 References.
1. E.C.Y. Inn and Y. Tanaka, ``Absorption coefficient of Ozone in 
the Ultraviolet and Visible Regions'', J. Opt. Soc. Am., 43, 870 
(1953).
2. A. G. Hearn, ``Absorption of Ozone in the Ultraviolet and Visible 
Regions of the Spectrum'', Proc. Phys. Soc. (London), 78, 932 
(1961).
3. W. B. DeMore and O. Raper, ``Hartley Band Extinction Coefficients 
of Ozone in the Gas Phase and in Liquid Nitrogen, Carbon Monoxide, 
and Argon'', J. Phys. Chem., 68, 412 (1964).
4. M. Griggs, ``Absorption Coefficients of Ozone in the Ultraviolet 
and Visible Regions'', J. Chem. Phys., 49, 857 (1968).
5. K. H. Becker, U. Schurath, and H. Seitz, ``Ozone Olefin Reactions 
in the Gas Phase. 1. Rate Constants and Activation Energies'', Int'l 
Jour. of Chem. Kinetics, VI, 725 (1974).
6. M. A. A. Clyne and J. A. Coxom, ``Kinetic Studies of Oxy-halogen 
Radical Systems'', Proc. Roy. Soc., A303, 207 (1968).
7. J. W. Simons, R. J. Paur, H. A. Webster, and E. J. Bair, ``Ozone 
Ultraviolet Photolysis. VI. The Ultraviolet Spectrum'', J. Chem. 
Phys., 59, 1203 (1973).
8. Ollison, W.M.; Crow, W.; Spicer, C.W. ``Field testing of new-
technology

[[Page 65456]]

ambient air ozone monitors.'' J. Air Waste Manage. Assoc., 63 (7), 
855-863 (2013).
9. Parrish, D.D.; Fehsenfeld, F.C. ``Methods for gas-phase 
measurements of ozone, ozone precursors and aerosol precursors.'' 
Atmos. Environ., 34 (12-14), 1921-1957(2000).
10. Ridley, B.A.; Grahek, F.E.; Walega, J.G. ``A small, high-
sensitivity, medium-response ozone detector suitable for 
measurements from light aircraft.'' J. Atmos. Oceanic Technol., 9 
(2), 142-148(1992).
11. Boylan, P., Helmig, D., and Park, J.H. ``Characterization and 
mitigation of water vapor effects in the measurement of ozone by 
chemiluminescence with nitric oxide.'' Atmos. Meas. Tech. 7, 1231-
1244 (2014).
12. Transfer Standards for Calibration of Ambient Air Monitoring 
Analyzers for Ozone, EPA publication number EPA-454/B-13-004, 
October 2013. EPA, Office of Air Quality Planning and Standards, 
Research Triangle Park, NC 27711. [Available at www.epa.gov/ttnamti1/files/ambient/qaqc/OzoneTransferStandardGuidance.pdf.]
13. Technical Assistance Document for the Calibration of Ambient 
Ozone Monitors, EPA publication number EPA-600/4-79-057, September, 
1979. [Available at www.epa.gov/ttnamti1/files/ambient/criteria/4-79-057.pdf.]
14. QA Handbook for Air Pollution Measurement Systems--Volume II. 
Ambient Air Quality Monitoring Program. EPA-454/B-13-003, May 2013. 
[Available at http://www.epa.gov/ttnamti1/files/ambient/pm25/qa/QA-Handbook-Vol-II.pdf.]
[GRAPHIC] [TIFF OMITTED] TR26OC15.007


[[Page 65457]]


[GRAPHIC] [TIFF OMITTED] TR26OC15.008


[[Page 65458]]


[GRAPHIC] [TIFF OMITTED] TR26OC15.009


0
5. Add appendix U to Part 50 to read as follows:

Appendix U to Part 50--Interpretation of the Primary and Secondary 
National Ambient Air Quality Standards for Ozone

1. General

    (a) This appendix explains the data handling conventions and 
computations necessary for determining whether the primary and 
secondary national ambient air quality standards (NAAQS) for ozone 
(O3) specified in Sec.  50.19 are met at an ambient 
O3 air quality monitoring site. Data reporting, data 
handling, and computation procedures to be used in making 
comparisons between reported O3 concentrations and the 
levels of the O3 NAAQS are specified in the following 
sections.
    (b) Whether to exclude or retain the data affected by 
exceptional events is determined by the requirements under 
Sec. Sec.  50.1, 50.14 and 51.930.
    (c) The terms used in this appendix are defined as follows:
    8-hour average refers to the moving average of eight consecutive 
hourly O3 concentrations measured at a site, as explained 
in section 3 of this appendix.
    Annual fourth-highest daily maximum refers to the fourth highest 
value measured at a site during a year.
    Collocated monitors refers to the instance of two or more 
O3 monitors operating at the same physical location.
    Daily maximum 8-hour average O3 concentration refers to the 
maximum calculated 8-hour average value measured at a site on a 
particular day, as explained in section 3 of this appendix.
    Design value refers to the metric (i.e., statistic) that is used 
to compare ambient O3 concentration data measured at a 
site to the NAAQS in order to determine compliance, as explained in 
section 4 of this appendix.
    Minimum data completeness requirements refer to the amount of 
data that a site is required to collect in order to make a valid 
determination that the site is meeting the NAAQS.
    Monitor refers to a physical instrument used to measure ambient 
O3 concentrations.
    O3 monitoring season refers to the span of time 
within a year when individual states are required to measure ambient 
O3 concentrations, as listed in Appendix D to part 58 of 
this chapter.
    Site refers to an ambient O3 air quality monitoring 
site.
    Site data record refers to the set of hourly O3 
concentration data collected at a site for use in comparisons with 
the NAAQS.
    Year refers to calendar year.

2. Selection of Data for use in Comparisons With the Primary and 
Secondary Ozone NAAQS

    (a) All valid hourly O3 concentration data collected 
using a federal reference method specified in Appendix D to this 
part, or an equivalent method designated in accordance with part 53 
of this chapter, meeting all applicable requirements in part 58 of 
this chapter, and submitted to EPA's Air Quality System (AQS) 
database or otherwise available to EPA, shall be used in design 
value calculations.
    (b) All design value calculations shall be implemented on a 
site-level basis. If data are reported to EPA from collocated 
monitors, those data shall be combined into a single site data 
record as follows:
    (i) The monitoring agency shall designate one monitor as the 
primary monitor for the site.
    (ii) Hourly O3 concentration data from a secondary 
monitor shall be substituted into

[[Page 65459]]

the site data record whenever a valid hourly O3 
concentration is not obtained from the primary monitor. In the event 
that hourly O3 concentration data are available for more 
than one secondary monitor, the hourly concentration values from the 
secondary monitors shall be averaged and substituted into the site 
data record.
    (c) In certain circumstances, including but not limited to site 
closures or relocations, data from two nearby sites may be combined 
into a single site data record for the purpose of calculating a 
valid design value. The appropriate Regional Administrator may 
approve such combinations after taking into consideration factors 
such as distance between sites, spatial and temporal patterns in air 
quality, local emissions and meteorology, jurisdictional boundaries, 
and terrain features.

3. Data Reporting and Data Handling Conventions

    (a) Hourly average O3 concentrations shall be 
reported in parts per million (ppm) to the third decimal place, with 
additional digits to the right of the third decimal place truncated. 
Each hour shall be identified using local standard time (LST).
    (b) Moving 8-hour averages shall be computed from the hourly 
O3 concentration data for each hour of the year and shall 
be stored in the first, or start, hour of the 8-hour period. An 8-
hour average shall be considered valid if at least 6 of the hourly 
concentrations for the 8-hour period are available. In the event 
that only 6 or 7 hourly concentrations are available, the 8-hour 
average shall be computed on the basis of the hours available, using 
6 or 7, respectively, as the divisor. In addition, in the event that 
5 or fewer hourly concentrations are available, the 8-hour average 
shall be considered valid if, after substituting zero for the 
missing hourly concentrations, the resulting 8-hour average is 
greater than the level of the NAAQS, or equivalently, if the sum of 
the available hourly concentrations is greater than 0.567 ppm. The 
8-hour averages shall be reported to three decimal places, with 
additional digits to the right of the third decimal place truncated. 
Hourly O3 concentrations that have been approved under 
Sec.  50.14 as having been affected by exceptional events shall be 
counted as missing or unavailable in the calculation of 8-hour 
averages.
    (c) The daily maximum 8-hour average O3 concentration 
for a given day is the highest of the 17 consecutive 8-hour averages 
beginning with the 8-hour period from 7:00 a.m. to 3:00 p.m. and 
ending with the 8-hour period from 11:00 p.m. to 7:00 a.m. the 
following day (i.e., the 8-hour averages for 7:00 a.m. to 11:00 
p.m.). Daily maximum 8-hour average O3 concentrations 
shall be determined for each day with ambient O3 
monitoring data, including days outside the O3 monitoring 
season if those data are available.
    (d) A daily maximum 8-hour average O3 concentration 
shall be considered valid if valid 8-hour averages are available for 
at least 13 of the 17 consecutive 8-hour periods starting from 7:00 
a.m. to 11:00 p.m. In addition, in the event that fewer than 13 
valid 8-hour averages are available, a daily maximum 8-hour average 
O3 concentration shall also be considered valid if it is 
greater than the level of the NAAQS. Hourly O3 
concentrations that have been approved under Sec.  50.14 as having 
been affected by exceptional events shall be included when 
determining whether these criteria have been met.
    (e) The primary and secondary O3 design value 
statistic is the annual fourth-highest daily maximum 8-hour 
O3 concentration, averaged over three years, expressed in 
ppm. The fourth-highest daily maximum 8-hour O3 
concentration for each year shall be determined based only on days 
meeting the validity criteria in 3(d). The 3-year average shall be 
computed using the three most recent, consecutive years of ambient 
O3 monitoring data. Design values shall be reported in 
ppm to three decimal places, with additional digits to the right of 
the third decimal place truncated.

4. Comparisons With the Primary and Secondary Ozone NAAQS

    (a) The primary and secondary national ambient air quality 
standards for O3 are met at an ambient air quality 
monitoring site when the 3-year average of the annual fourth-highest 
daily maximum 8-hour average O3 concentration (i.e., the 
design value) is less than or equal to 0.070 ppm.
    (b) A design value greater than the level of the NAAQS is always 
considered to be valid. A design value less than or equal to the 
level of the NAAQS must meet minimum data completeness requirements 
in order to be considered valid. These requirements are met for a 3-
year period at a site if valid daily maximum 8-hour average 
O3 concentrations are available for at least 90% of the 
days within the O3 monitoring season, on average, for the 
3-year period, with a minimum of at least 75% of the days within the 
O3 monitoring season in any one year.
    (c) When computing whether the minimum data completeness 
requirements have been met, meteorological or ambient data may be 
sufficient to demonstrate that meteorological conditions on missing 
days were not conducive to concentrations above the level of the 
NAAQS. Missing days assumed less than the level of the NAAQS are 
counted for the purpose of meeting the minimum data completeness 
requirements, subject to the approval of the appropriate Regional 
Administrator.
    (d) Comparisons with the primary and secondary O3 
NAAQS are demonstrated by examples 1 and 2 as follows:

                                               Example 1--Site Meeting the Primary and Secondary O3 NAAQS
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                           Percent valid
                                                          days within O3    1st highest     2nd highest     3rd highest     4th highest     5th highest
                          Year                              monitoring     daily max  8-   daily max  8-   daily max  8-   daily max  8-   daily max  8-
                                                           season (Data   hour O3  (ppm)  hour O3  (ppm)  hour O3  (ppm)  hour O3  (ppm)  hour O3  (ppm)
                                                           completeness)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2014....................................................             100           0.082           0.080           0.075           0.069           0.068
2015....................................................              96           0.074           0.073           0.065           0.062           0.060
2016....................................................              98           0.070           0.069           0.067           0.066           0.060
Average.................................................              98  ..............  ..............  ..............           0.065
--------------------------------------------------------------------------------------------------------------------------------------------------------

    As shown in Example 1, this site meets the primary and secondary 
O3 NAAQS because the 3-year average of the annual fourth-
highest daily maximum 8-hour average O3 concentrations 
(i.e., 0.065666 ppm, truncated to 0.065 ppm) is less than or equal 
to 0.070 ppm. The minimum data completeness requirements are also 
met (i.e., design value is considered valid) because the average 
percent of days within the O3 monitoring season with 
valid ambient monitoring data is greater than 90%, and no single 
year has less than 75% data completeness.

                                          Example 2--Site Failing to Meet the Primary and Secondary O3 O3 NAAQS
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                           Percent valid
                                                          days within O3    1st highest     2nd highest     3rd highest     4th highest     5th highest
                          Year                              monitoring     daily max  8-   daily max  8-   daily max  8-   daily max  8-   daily max  8-
                                                           season (Data   hour O3  (ppm)  hour O3  (ppm)  hour O3  (ppm)  hour O3  (ppm)  hour O3  (ppm)
                                                           completeness)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2014....................................................              96           0.085           0.080           0.079           0.074           0.072

[[Page 65460]]

 
2015....................................................              74           0.084           0.083           0.072           0.071           0.068
2016....................................................              98           0.083           0.081           0.081           0.075           0.074
Average.................................................              89  ..............  ..............  ..............           0.073
--------------------------------------------------------------------------------------------------------------------------------------------------------

    As shown in Example 2, this site fails to meet the primary and 
secondary O3 NAAQS because the 3-year average of the 
annual fourth-highest daily maximum 8-hour average O3 
concentrations (i.e., 0.073333 ppm, truncated to 0.073 ppm) is 
greater than 0.070 ppm, even though the annual data completeness is 
less than 75% in one year and the 3-year average data completeness 
is less than 90% (i.e., design value would not otherwise be 
considered valid).

PART 51--REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF 
IMPLEMENTATION PLANS

0
6. The authority citation for part 51 continues to read as follows:

    Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

Subpart I---Review of New Sources and Modifications

0
8. Amend Sec.  51.166 by adding paragraph (i)(11) to read as follows:


Sec.  51.166  Prevention of significant deterioration of air quality.

* * * * *
    (i) * * *
    (11) The plan may provide that the requirements of paragraph (k)(1) 
of this section shall not apply to a permit application for a 
stationary source or modification with respect to the revised national 
ambient air quality standards for ozone published on October 26, 2015 
if:
    (i) The reviewing authority has determined the permit application 
subject to this section to be complete on or before October 1, 2015. 
Instead, the requirements in paragraph (k)(1) of this section shall 
apply with respect to the national ambient air quality standards for 
ozone in effect at the time the reviewing authority determined the 
permit application to be complete; or
    (ii) The reviewing authority has first published before December 
28, 2015 a public notice of a preliminary determination or draft permit 
for the permit application subject to this section. Instead, the 
requirements in paragraph (k)(1) of this section shall apply with 
respect to the national ambient air quality standards for ozone in 
effect at the time of first publication of a public notice of the 
preliminary determination or draft permit.
* * * * *

PART 52--APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS

0
8. The authority citation for part 52 continues to read as follows:

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

0
9. Amend Sec.  52.21 by adding paragraph (i)(12) to read as follows:


Sec.  52.21  Prevention of significant deterioration of air quality.

* * * * *
    (i) * * *
    (12) The requirements of paragraph (k)(1) of this section shall not 
apply to a permit application for a stationary source or modification 
with respect to the revised national ambient air quality standards for 
ozone published on October 26, 2015 if:
    (i) The Administrator has determined the permit application subject 
to this section to be complete on or before October 1, 2015. Instead, 
the requirements in paragraph (k)(1) of this section shall apply with 
respect to the national ambient air quality standards for ozone in 
effect at the time the Administrator determined the permit application 
to be complete; or
    (ii) The Administrator has first published before December 28, 2015 
a public notice of a preliminary determination or draft permit for the 
permit application subject to this section. Instead, the requirements 
in paragraph (k)(1) of this section shall apply with respect to the 
national ambient air quality standards for ozone in effect on the date 
the Administrator first published a public notice of a preliminary 
determination or draft permit.
* * * * *

PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS

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

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

Subpart A--General Provisions


Sec.  53.9  [Amended]

0
11. Amend Sec.  53.9 by removing paragraph (i).

0
12. Amend Sec.  53.14 by revising paragraph (c) introductory text to 
read as follows:


Sec.  53.14  Modification of a reference or equivalent method.

* * * * *
    (c) Within 90 calendar days after receiving a report under 
paragraph (a) of this section, the Administrator will take one or more 
of the following actions:
* * * * *

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

0
13. Amend Sec.  53.23 by revising paragraph (e)(1)(vi) to read as 
follows:


Sec.  53.23  Test procedures.

* * * * *
    (e) * * *
    (1) * * *
    (vi) Precision: Variation about the mean of repeated measurements 
of the same pollutant concentration, denoted as the standard deviation 
expressed as a percentage of the upper range limits.\258\
---------------------------------------------------------------------------

    \258\ NO2 precision in Table B-1 is also changed to 
percent to agree with the calculation specified in 53.23(e)(10)(vi).
---------------------------------------------------------------------------

* * * * *

0
14. Revise Table B-1 to Subpart B of Part 53 to read as follows:

[[Page 65461]]



                                                    Table B-1 to Subpart B of Part 53--Performance Limit Specifications for Automated Methods
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                         SO2                             O3                              CO
                                                          ------------------------------------------------------------------------------------------------   NO2  (Std.     Definitions and test
       Performance parameter              Units \1\                         Lower range 2                   Lower range 2                   Lower range 2      range)            procedures
                                                           Std. range \3\         3        Std. range \3\         3        Std. range \3\         3
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range..........................  ppm..................           0-0.5            <0.5           0-0.5            <0.5            0-50             <50           0-0.5  Sec. 53.23(a)
2. Noise..........................  ppm..................           0.001          0.0005          0.0025           0.001             0.2             0.1           0.005  Sec. 53.23(b)
3. Lower detectable limit.........  ppm..................           0.002           0.001           0.005           0.002             0.4             0.2           0.010  Sec. 53.23(c)
4. Interference equivalent
    Each interferent..............  ppm..................          0.005     minus>0.005     minus>0.005     minus>0.005       minus>1.0       minus>0.5      minus>0.02
    Total, all interferents.......  ppm..................               -               -               -               -               -               -            0.04  Sec. 53.23(d)
5. Zero drift, 12 and 24 hour.....  ppm..................          0.004     minus>0.002     minus>0.004     minus>0.002       minus>0.5       minus>0.3      minus>0.02
6. Span drift, 24 hour
    20% of upper range limit......  Percent..............               -               -               -               -               -               -          20.0
    80% of upper range limit......  Percent..............          3.0       minus>3.0       minus>3.0       minus>3.0       minus>2.0       minus>2.0       minus>5.0
7. Lag time.......................  Minutes..............               2               2               2               2             2.0             2.0              20  Sec. 53.23(e)
8. Rise time......................  Minutes..............               2               2               2               2             2.0             2.0              15  Sec. 53.23(e)
9. Fall time......................  Minutes..............               2               2               2               2             2.0             2.0              15  Sec. 53.23(e)
10. Precision
    20% of upper range limit......                                      -               -               -               -               -               -                  Sec. 53.23(e)
                                    Percent \5\..........               2               2               2               2             1.0             1.0               4  Sec. 53.23(e)
    80% of upper range limit......                                      -               -               -               -               -               -                  Sec. 53.23(e)
                                    Percent \5\..........               2               2               2               2             1.0             1.0               6  Sec. 53.23(e)
                                                                                                                                                                           Sec. 53.23(e)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas. Percent means percent of the upper
  measurement range limit.
\2\ Tests for interference equivalent and lag time do not need to be repeated for any lower range provided the test for the standard range shows that the lower range specification (if
  applicable) is met for each of these test parameters.
\3\ For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct suitable tests to demonstrate their
  function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are within specifications under all applicable conditions. For candidate analyzers
  with operator-selectable time constants or smoothing filters, conduct calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be
  included in the FRM or FEM designation.
\4\ For nitric oxide interference for the SO2 UVF method, interference equivalent is 0.0003 ppm for the lower range.
\5\ Standard deviation expressed as percent of the URL.


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[[Page 65463]]

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[[Page 65464]]

[GRAPHIC] [TIFF OMITTED] TR26OC15.012


[[Page 65465]]

[GRAPHIC] [TIFF OMITTED] TR26OC15.013


[[Page 65466]]

* * * * *

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

0
17. Amend Sec.  53.32 by revising paragraph (g)(1)(iii) to read as 
follows:


Sec.  53.32  Test procedures for methods for SO2, CO, 
O3, and NO2.

* * * * *
    (g) * * *
    (1) * * *
    (iii) The measurements shall be made in the sequence specified in 
table C-2 of this subpart.
* * * * *


Figure E-2 to Subpart E of Part 53  [Removed]

0
18. Amend subpart E by removing figure E-2 to subpart E of part 53.

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

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

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

Subpart B--Monitoring Network

0
20. Amend Sec.  58.10 by adding paragraphs (a)(9) through (11) to read 
as follows:


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

    (a) * * *
    (9) The annual monitoring network plan shall provide for the 
required O3 sites to be operating on the first day of the 
applicable required O3 monitoring season in effect on 
January 1, 2017 as listed in Table D-3 of appendix D of this part.
    (10) A plan for making Photochemical Assessment Monitoring Stations 
(PAMS) measurements, if applicable, in accordance with the requirements 
of appendix D paragraph 5(a) of this part shall be submitted to the EPA 
Regional Administrator no later than July 1, 2018. The plan shall 
provide for the required PAMS measurements to begin by June 1, 2019.
    (11) An Enhanced Monitoring Plan for O3, if applicable, 
in accordance with the requirements of appendix D paragraph 5(h) of 
this part shall be submitted to the EPA Regional Administrator no later 
than October 1, 2019 or two years following the effective date of a 
designation to a classification of Moderate or above O3 
nonattainment, whichever is later.
* * * * *

0
21. Section Sec.  58.11 is amended by revising paragraph (c) to read as 
follows:


Sec.  58.11  Network technical requirements.

* * * * *
    (c) State and local governments must follow the network design 
criteria contained in appendix D to this part in designing and 
maintaining the SLAMS stations. The final network design and all 
changes in design are subject to approval of the Regional 
Administrator. NCore and STN network design and changes are also 
subject to approval of the Administrator. Changes in SPM stations do 
not require approvals, but a change in the designation of a monitoring 
site from SLAMS to SPM requires approval of the Regional Administrator.
* * * * *

0
22. Amend Sec.  58.13 by adding paragraphs (g) and (h) to read as 
follows:


Sec.  58.13  Monitoring network completion.

* * * * *
    (g) The O3 monitors required under appendix D, section 
4.1 of this part must operate on the first day of the applicable 
required O3 monitoring season in effect January 1, 2017.
    (h) The Photochemical Assessment Monitoring sites required under 40 
CFR part 58 Appendix D, section 5(a) must be physically established and 
operating under all of the requirements of this part, including the 
requirements of appendix A, C, D, and E of this part, no later than 
June 1, 2019.

Subpart F--Air Quality Index Reporting

0
23. Amend Sec.  58.50 by revising paragraph (c) to read as follows:


Sec.  58.50  Index reporting.

* * * * *
    (c) The population of a metropolitan statistical area for purposes 
of index reporting is the latest available U.S. census population.

Subpart G--Federal Monitoring

0
24. Amend appendix D to part 58, under section 4, by revising section 
4.1(i) and table D-3 to appendix D of part 58, and by revising section 
5 to read as follows:

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

* * * * *

4. Pollutant-Specific Design Criteria for SLAMS Sites

* * * * *
    4.1 * * *
    (i) Ozone monitoring is required at SLAMS monitoring sites only 
during the seasons of the year that are conducive to O3 
formation (i.e., ``ozone season'') as described below in Table D-3 
of this appendix. These O3 seasons are also identified in 
the AQS files on a state-by-state basis. Deviations from the 
O3 monitoring season must be approved by the EPA Regional 
Administrator. These requests will be reviewed by Regional 
Administrators taking into consideration, at a minimum, the 
frequency of out-of-season O3 NAAQS exceedances, as well 
as occurrences of the Moderate air quality index level, regional 
consistency, and logistical issues such as site access. Any 
deviations based on the Regional Administrator's waiver of 
requirements must be described in the annual monitoring network plan 
and updated in AQS. Changes to the O3 monitoring season 
requirements in Table D-3 revoke all previously approved Regional 
Administrator waivers. Requests for monitoring season deviations 
must be accompanied by relevant supporting information. Information 
on how to analyze O3 data to support a change to the 
O3 season in support of the 8-hour standard for the 
entire network in a specific state can be found in reference 8 to 
this appendix. Ozone monitors at NCore stations are required to be 
operated year-round (January to December).

Table D-3 \1\ to Appendix D of part 58. Ozone Monitoring Season by state
------------------------------------------------------------------------
              State                   Begin Month          End Month
------------------------------------------------------------------------
Alabama.........................  March.............  October.
Alaska..........................  April.............  October.
Arizona.........................  January...........  December.
Arkansas........................  March.............  November.
California......................  January...........  December.
Colorado........................  January...........  December.
Connecticut.....................  March.............  September.
Delaware........................  March.............  October.
District of Columbia............  March.............  October.

[[Page 65467]]

 
Florida.........................  January...........  December.
Georgia.........................  March.............  October.
Hawaii..........................  January...........  December.
Idaho...........................  April.............  September.
Illinois........................  March.............  October.
Indiana.........................  March.............  October.
Iowa............................  March.............  October.
Kansas..........................  March.............  October.
Kentucky........................  March.............  October.
Louisiana (Northern) AQCR 019,    March.............  October.
 022.
Louisiana (Southern) AQCR 106...  January...........  December.
Maine...........................  April.............  September.
Maryland........................  March.............  October.
Massachusetts...................  March.............  September.
Michigan........................  March.............  October.
Minnesota.......................  March.............  October.
Mississippi.....................  March.............  October.
Missouri........................  March.............  October.
Montana.........................  April.............  September.
Nebraska........................  March.............  October.
Nevada..........................  January...........  December.
New Hampshire...................  March.............  September.
New Jersey......................  March.............  October.
New Mexico......................  January...........  December.
New York........................  March.............  October.
North Carolina..................  March.............  October.
North Dakota....................  March.............  September.
Ohio............................  March.............  October.
Oklahoma........................  March.............  November.
Oregon..........................  May...............  September.
Pennsylvania....................  March.............  October.
Puerto Rico.....................  January...........  December.
Rhode Island....................  March.............  September.
South Carolina..................  March.............  October.
South Dakota....................  March.............  October.
Tennessee.......................  March.............  October.
Texas (Northern) AQCR 022, 210,   March.............  November.
 211, 212, 215, 217, 218.
Texas (Southern) AQCR 106, 153,   January...........  December.
 213, 214, 216.
Utah............................  January...........  December.
Vermont.........................  April.............  September.
Virginia........................  March.............  October.
Washington......................  May...............  September.
West Virginia...................  March.............  October.
Wisconsin.......................  March.............  October 15.
Wyoming.........................  January...........  September.
American Samoa..................  January...........  December.
Guam............................  January...........  December.
Virgin Islands..................  January...........  December.
------------------------------------------------------------------------
\1\ The required O3 monitoring season for NCore stations is January
  through December.

* * * * *

5. Network Design for Photochemical Assessment Monitoring Stations 
(PAMS) and Enhanced Ozone Monitoring

    (a) State and local monitoring agencies are required to collect 
and report PAMS measurements at each NCore site required under 
paragraph 3(a) of this appendix located in a CBSA with a population 
of 1,000,000 or more, based on the latest available census figures.
    (b) PAMS measurements include:
    (1) Hourly averaged speciated volatile organic compounds (VOCs);
    (2) Three 8-hour averaged carbonyl samples per day on a 1 in 3 
day schedule, or hourly averaged formaldehyde;
    (3) Hourly averaged O3;
    (4) Hourly averaged nitrogen oxide (NO), true nitrogen dioxide 
(NO2), and total reactive nitrogen (NOy);
    (5) Hourly averaged ambient temperature;
    (6) Hourly vector-averaged wind direction;
    (7) Hourly vector-averaged wind speed;
    (8) Hourly average atmospheric pressure;
    (9) Hourly averaged relative humidity;
    (10) Hourly precipitation;
    (11) Hourly averaged mixing-height;
    (12) Hourly averaged solar radiation; and
    (13) Hourly averaged ultraviolet radiation.
    (c) The EPA Regional Administrator may grant a waiver to allow 
the collection of required PAMS measurements at an alternative 
location where the monitoring agency can demonstrate that the 
alternative location will provide representative data useful for 
regional or national scale modeling and the tracking of trends in 
O3 precursors. The alternative location can be outside of 
the CBSA or outside of the monitoring agencies jurisdiction. In 
cases where the alternative location crosses jurisdictions the 
waiver will be contingent on the monitoring agency responsible for 
the alternative location including the required PAMS measurements in 
their annual monitoring plan required under Sec.  58.10 and 
continued successful collection of PAMS measurements at the 
alternative location. This waiver can be revoked in cases where the 
Regional Administrator determines the PAMS measurements are not 
being collected at the alternate location in compliance with 
paragraph (b) of this section.
    (d) The EPA Regional Administrator may grant a waiver to allow 
speciated VOC measurements to be made as three 8-hour averages on 
every third day during the PAMS

[[Page 65468]]

season as an alternative to 1-hour average speciated VOC 
measurements in cases where the primary VOC compounds are not well 
measured using continuous technology due to low detectability of the 
primary VOC compounds or for logistical and other programmatic 
constraints.
    (e) The EPA Regional Administrator may grant a waiver to allow 
representative meteorological data from nearby monitoring stations 
to be used to meet the meteorological requirements in paragraph 5(b) 
where the monitoring agency can demonstrate the data is collected in 
a manner consistent with EPA quality assurance requirements for 
these measurements.
    (f) The EPA Regional Administrator may grant a waiver from the 
requirement to collect PAMS measurements in locations where CBSA-
wide O3 design values are equal to or less than 85% of 
the 8-hour O3 NAAQS and where the location is not 
considered by the Regional Administrator to be an important upwind 
or downwind location for other O3 nonattainment areas.
    (g) At a minimum, the monitoring agency shall collect the 
required PAMS measurements during the months of June, July, and 
August.
    (h) States with Moderate and above 8-hour O3 
nonattainment areas and states in the Ozone Transport Region as 
defined in 40 CFR 51.900 shall develop and implement an Enhanced 
Monitoring Plan (EMP) detailing enhanced O3 and 
O3 precursor monitoring activities to be performed. The 
EMP shall be submitted to the EPA Regional Administrator no later 
than October 1, 2019 or two years following the effective date of a 
designation to a classification of Moderate or above O3 
nonattainment, whichever is later. At a minimum, the EMP shall be 
reassessed and approved as part of the 5-year network assessments 
required under 40 CFR 58.10(d). The EMP will include monitoring 
activities deemed important to understanding the O3 
problems in the state. Such activities may include, but are not 
limited to, the following:
    (1) Additional O3 monitors beyond the minimally 
required under paragraph 4.1 of this appendix,
    (2) Additional NOX or NOy monitors beyond 
those required under 4.3 of this appendix,
    (3) Additional speciated VOC measurements including data 
gathered during different periods other than required under 
paragraph 5(g) of this appendix, or locations other than those 
required under paragraph 5(a) of this appendix, and
    (4) Enhanced upper air measurements of meteorology or pollution 
concentrations.
* * * * *

0
25. Appendix G of Part 58 is amended by revising table 2 to read as 
follows:

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

* * * * *

                                        TABLE 2--BREAKPOINTS FOR THE AQI
----------------------------------------------------------------------------------------------------------------
                                 These breakpoints                                        Equal these AQI's
----------------------------------------------------------------------------------------------------------------
                           PM2.5        PM10
O3 (ppm) 8- O3 (ppm) 1-  ([micro]g/  ([micro]g/  CO (ppm) 8-  SO2 (ppb)   NO2 (ppb)
   hour       hour\1\    m\3\) 24-    m\3\) 24-     hour       1-hour      1-hour        AQI         Category
                            hour        hour
----------------------------------------------------------------------------------------------------------------
0.000-0.05  --          0.0--12.0    0-54        0.0-4.4     0-35        0-53        0-50        Good.
 4
0.055-0.07  --          12.1--35.4   55-154      4.5-9.4     36-75       54-100      51-100      Moderate.
 0
0.071-0.08  0.125-0.16  35.5--55.4   155-254     9.5-12.4    76-185      101-360     101-150     Unhealthy for
 5           4                                                                                    Sensitive
                                                                                                  Groups.
0.086-0.10  0.165-0.20  \3\ 55.5--   255-354     12.5-15.4   \4\ 186-    361-649     151-200     Unhealthy.
 5           4           150.4                                304
0.106-0.20  0.205-0.40  \3\ 150.5--  355-424     15.5-30.4   \4\ 305-    650-1249    201-300     Very Unhealthy.
 0           4           250.4                                604
0.201-      0.405-0.50  \3\ 250.5--  425-504     30.5-40.4   \4\ 605-    1250-1649   301-400     Hazardous.
 (\2\)       4           350.4                                804
(\2\)       0.505-0.60  \3\ 350.5--  505-604     40.5-50.4   \4\ 805-    1650-2049   401-500
             4           500.4                                1004
----------------------------------------------------------------------------------------------------------------
\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small
  number of areas where an AQI based on 1-hour ozone values would be more precautionary. In these cases, in
  addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the
  maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>301). AQI values > 301 are calculated with 1-hour O3
  concentrations.
\3\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
\4\ 1-hr SO2 values do not define higher AQI values (>=200). AQI values of 200 or greater are calculated with 24-
  hour SO2 concentration.

[FR Doc. 2015-26594 Filed 10-23-15; 8:45 am]
 BILLING CODE 6560-50-P