[Federal Register Volume 85, Number 158 (Friday, August 14, 2020)]
[Proposed Rules]
[Pages 49830-49917]
[FR Doc No: 2020-15453]

Vol. 85

Friday,

No. 158

August 14, 2020

Part V

Environmental Protection Agency

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40 CFR Part 50

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

Federal Register / Vol. 85, No. 158 / Friday, August 14, 2020 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2018-0279; FRL-10012-49-OAR]
RIN 2060-AU40

Review of the Ozone National Ambient Air Quality Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed action.

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

DATES: Comments must be received on or before October 1, 2020.
    Public hearings: The EPA will hold two virtual public hearings on
Monday, August 31, 2020, and Tuesday, September 1, 2020. Please refer
to the SUPPLEMENTARY INFORMATION section for additional information on
the public hearings.

ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2018-0279, by any of the following methods:
     Federal eRulemaking Portal: https://www.regulations.gov
(our preferred method). Follow the online instructions for submitting
comments.
     Email<EMAIL_ADDRESS>Include the Docket ID No.
EPA-HQ-OAR-2018-0279 in the subject line of the message.
     Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania
Avenue NW, Washington, DC 20460.
     Hand Delivery or Courier (by scheduled appointment only):
EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution
Avenue NW, Washington, DC 20004. The Docket Center's hours of
operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal
Holidays).
    Instructions: All submissions received must include the Docket ID
No. for this document. Comments received may be posted without change
to https://www.regulations.gov, including any personal information
provided. For detailed instructions on sending comments, see the
SUPPLEMENTARY INFORMATION section of this document. Out of an abundance
of caution for members of the public and our staff, the EPA Docket
Center and Reading Room are closed to the public, with limited
exceptions, to reduce the risk of transmitting COVID-19. Our Docket
Center staff will continue to provide remote customer service via
email, phone, and webform. We encourage the public to submit comments
via https://www.regulations.gov/ or email, as there may be a delay in
processing mail and faxes. Hand deliveries and couriers may be received
by scheduled appointment only. For further information on EPA Docket
Center services and the current status, please visit us online at
https://www.epa.gov/dockets.
    The two virtual public hearings will be held on Monday, August 31,
2020, and Tuesday, September 1, 2020. The EPA will announce further
details on the virtual public hearing website at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. Refer to the SUPPLEMENTARY INFORMATION section below
for additional information.

FOR FURTHER INFORMATION CONTACT: For information or questions about the
public hearing, please contact Ms. Regina Chappell, U.S. Environmental
Protection Agency, Office of Air Quality Planning and Standards (OAQPS)
(Mail Code C304-03), Research Triangle Park, NC 27711; telephone: (919)
541-3650; email address<EMAIL_ADDRESS>For information or
questions regarding the review of the O3 NAAQS, please
contact Dr. Deirdre Murphy, Health and Environmental Impacts Division,
Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, Mail Code C504-06, Research Triangle Park, NC 27711;
telephone:<PHONE_NUMBER>; fax:<PHONE_NUMBER>; email:
<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

General Information

Participation in Virtual Public Hearings

    Please note that the EPA is deviating from its typical approach
because the President has declared a national emergency. Due to the
current Centers for Disease Control and Prevention (CDC)
recommendations, as well as state and local orders for social
distancing to limit the spread of COVID-19, the EPA cannot hold in-
person public meetings at this time. The EPA will begin pre-registering
speakers for the hearings upon publication of this document in the
Federal Register. To register to speak at a virtual hearing, please use
the online registration form available at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution or contact Ms. Regina Chappell at<PHONE_NUMBER> or by email
at<EMAIL_ADDRESS>to register to speak at the virtual hearing.
The last day to pre-register to speak at one of the hearings will be
August 27, 2020. On August 28, 2020, the EPA will post a general agenda
for the hearings that will list preregistered speakers in approximate
order at: https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. The EPA will make every
effort to follow the schedule as closely as possible on the day of each
hearing; however, please plan for the hearing to run either ahead of
schedule or behind schedule. Each commenter will have 5 minutes to
provide oral testimony. The EPA may ask clarifying questions during the
oral presentations but will not respond to the presentations at that
time. The EPA encourages commenters to provide the EPA with a copy of
their oral testimony electronically (via email) by emailing it to Dr.
Deirdre Murphy and Ms. Regina Chappell. The EPA also recommends
submitting the text of your oral testimony as written comments to the
rulemaking docket. Written statements and supporting information
submitted during the comment period will be considered with the same
weight as oral testimony and supporting information presented at the
public hearing. Please note that any updates made to any aspect of the
hearing will be posted online at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. While the EPA expects the hearings to go forward as set
forth above, please monitor our website or contact Ms. Regina Chappell
at<PHONE_NUMBER> or<EMAIL_ADDRESS>to determine if there are
any updates. The EPA does not intend to publish a document in the
Federal Register announcing updates. If you require the services of a
translator or a special accommodation such as audio description, please
preregister for the hearing with Ms. Regina Chappell and describe your
needs by August 21, 2020. The EPA may not be able to arrange
accommodations without advance notice.

Preparing Comments for the EPA

    Follow the online instructions for submitting comments. Once
submitted to the Federal eRulemaking Portal, comments cannot be edited
or withdrawn. The EPA may publish any comment received to its public
docket. Do not submit electronically any information you consider to be
Confidential Business Information (CBI)

or other information whose disclosure is restricted by statute.
Multimedia submissions (audio, video, etc.) must be accompanied by a
written comment. The written comment is considered the official comment
and should include discussion of all points you wish to make. The EPA
will generally not consider comments or comment contents located
outside of the primary submission (i.e., on the web, the cloud, or
other file sharing system). For additional submission methods, the full
EPA public comment policy, information about CBI or multimedia
submissions, and general guidance on making effective comments, please
visit http://www2.epa.gov/dockets/commenting-epa-dockets.
    When submitting comments, remember to:
     Identify the action by docket number and other identifying
information (subject heading, Federal Register date and page number).
     Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
     Describe any assumptions and provide any technical
information and/or data that you used.
     Provide specific examples to illustrate your concerns and
suggest alternatives.
     Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
     Make sure to submit your comments by the comment period
deadline identified.

Availability of Information Related to This Action

    All documents in the dockets pertaining to this action are listed
on the www.regulations.gov website. This includes documents in the
docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2018-0279)
and a separate docket, established for the Integrated Science
Assessment (ISA) for this review (Docket ID No. EPA-HQ-ORD-2018-0274)
that has been incorporated by reference into the docket for this
proposed decision. Although listed in the index, some information is
not publicly available, e.g., CBI or other information whose disclosure
is restricted by statute. Certain other material, such as copyrighted
material, is not placed on the internet and may be viewed with prior
arrangement with the EPA Docket Center. Additionally, a number of the
documents that are relevant to this proposed decision are available
through the EPA's website at https://www.epa.gov/naaqs/ozone-o3-air-quality-standards. These documents include the Integrated Review Plan
for the Review of the Ozone National Ambient Air Quality Standards
(U.S. EPA, 2019b; hereafter IRP), available at https://www.epa.gov/naaqs/ozone-o3-standards-planning-documents-current-review, the
Integrated Science Assessment for Ozone and Related Photochemical
Oxidants (U.S. EPA, 2020a; hereafter ISA), available at https://www.epa.gov/naaqs/ozone-o3-standards-integrated-science-assessments-current-review, and the Policy Assessment for the Review of the Ozone
National Ambient Air Quality Standards (U.S. EPA, 2020b; hereafter PA),
available at https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-current-review.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related O3 Control Programs
    C. Review of the Air Quality Criteria and Standards for
O3
    D. Air Quality Information
II. Rationale for Proposed Decision on the Primary Standard
    A. General Approach
    1. Background on the Current Standard
    2. Approach for the Current Review
    B. Health Effects Information
    1. Nature of Effects
    2. Public Health Implications and At-Risk Populations
    3. Exposure Concentrations Associated With Effects
    C. Summary of Exposure and Risk Information
    1. Key Design Aspects
    2. Key Limitations and Uncertainties
    3. Summary of Exposure and Risk Estimates
    D. Proposed Conclusions on the Primary Standard
    1. Evidence- and Exposure/Risk-Based Considerations in the
Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Conclusions
III. Rationale for Proposed Decision on the Secondary Standard
    A. General Approach
    1. Background on the Current Standard
    2. Approach for the Current Review
    B. Welfare Effects Information
    1. Nature of Effects
    2. Public Welfare Implications
    3. Exposures Associated With Effects
    C. Summary of Air Quality and Exposure Information
    1. Influence of Form and Averaging Time of Current Standard on
Environmental Exposure
    2. Environmental Exposures in Terms of W126 Index
    D. Proposed Conclusions on the Secondary Standard
    1. Evidence- and Exposure/Risk-Based Considerations in the
Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Conclusions
IV. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
    I. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
    J. National Technology Transfer and Advancement Act
    K. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
    L. Determination Under Section 307(d)
V. References

Executive Summary

    This document presents the Administrator's proposed decisions in
the current review of the primary (health-based) and secondary
(welfare-based) O3 NAAQS. In so doing, this document
summarizes the background and rationale for the Administrator's
proposed decisions to retain the current standards, without revision.
In reaching his proposed decisions, the Administrator has considered
the currently available scientific evidence in the ISA, quantitative
and policy analyses presented in the PA, and advice from the Clean Air
Scientific Advisory Committee (CASAC). The EPA solicits comment on the
proposed decisions described here and on the array of issues associated
with review of these standards, including judgments of public health,
public welfare and science policy inherent in the proposed decisions,
and requests commenters also provide the rationales upon which views
articulated in submitted comments are based.
    This review of the O3 standards, required by the Clean
Air Act (CAA) on a periodic basis, was initiated in 2018. The last
review of the O3 NAAQS, completed in 2015 established the
current primary and secondary standards (80 FR 65291, October 26,
2015). In that review, the EPA significantly strengthened the primary
and secondary standards by revising both standards from 75 ppb to 70
ppb and retaining their indicators (O3), forms (fourth-
highest daily maximum,

averaged across three consecutive years) and averaging times (eight
hours). These revisions to the NAAQS were accompanied by revisions to
the data handling procedures, ambient air monitoring requirements, the
air quality index and several provisions related to implementation (80
FR 65292, October 26, 2015). In the decision on subsequent litigation
on the 2015 decisions, the U.S. Court of Appeals for the District of
Columbia Circuit (D.C. Circuit) upheld the 2015 primary standard but
remanded the 2015 secondary standard to the EPA for further
justification or reconsideration. The court's remand of the secondary
standard has been considered in reaching the proposed decision, and the
associated proposed conclusions and judgments, described in this
document.
    In this review as in past reviews of the NAAQS for O3
and related photochemical oxidants, the health and welfare effects
evidence evaluated in the ISA is focused on O3. Ozone is the
most prevalent photochemical oxidant in the atmosphere and the one for
which there is a large body of scientific evidence on health and
welfare effects. A component of smog, O3 in ambient air is a
mixture of mostly tropospheric O3 and some stratospheric
O3. Tropospheric O3, forms in the atmosphere when
precursor emissions of pollutants, such as nitrogen oxides and volatile
organic compounds (VOCs), interact with solar radiation. Precursor
emissions result from man-made sources (e.g., motor vehicles, and power
plants) and natural sources (e.g., vegetation and wildfires). In
addition, O3 that is created naturally in the stratosphere
also mixes with tropospheric O3 near the tropopause, and,
under more limited meteorological conditions and topographical
characteristics, nearer the earth's surface.
    The proposed decision to retain the current primary standard,
without revision, has been informed by key aspects of the currently
available health effects evidence and conclusions contained in the ISA,
quantitative exposure/risk analyses and policy evaluations presented in
the PA, advice from the CASAC and public input received as part of this
ongoing review. The health effects evidence newly available in this
review, in conjunction with the full body of evidence critically
evaluated in the ISA, continues to support prior conclusions that
short-term O3 exposure causes and long-term O3
exposure likely causes respiratory effects, with evidence newly
available in this review also indicating a likely causal relationship
of short-term O3 with metabolic effects. The strongest
evidence for health effects due to ozone exposure, however, continues
to come from studies of short- and long-term ozone exposure and
respiratory health, including effects related to asthma exacerbation in
people with asthma, particularly children with asthma. The longstanding
evidence base of respiratory effects, spanning several decades,
documents the causal relationship between short-term exposure to
O3 and an array of respiratory effects. The clearest
evidence for this conclusion comes from controlled human exposure
studies, available at the time of the last review, of individuals,
exposed for 6.6 hours during quasi-continuous exercise that report an
array of respiratory responses including lung function decrements and
respiratory symptoms. Epidemiologic studies include associations
between O3 exposures and hospital admissions and emergency
department visits, particularly for asthma exacerbation in children.
People at risk include people with asthma, children, the elderly, and
outdoor workers.
    The quantitative analyses of population exposure and risk, as well
as policy considerations in the PA, also inform the proposed decision
on the primary standard. The general approach and methodology for the
exposure-based assessment used in this review is similar to that used
in the last review. However, a number of updates and improvements have
been implemented in this review which result in differences from the
analyses in the prior review. These include a more recent period (2015-
2017) of ambient air monitoring data in which O3
concentrations in the areas assessed are at or near the current
standard, as well as improvements and updates to models, model inputs
and underlying databases. The analyses are summarized in this document
and described in detail in the PA.
    Based on the current evidence and quantitative information, as well
as consideration of CASAC advice and public comment thus far in this
review, the Administrator proposes to conclude that the current primary
standard is requisite to protect public health, with an adequate margin
of safety, from effects of O3 in ambient air and should be
retained, without revision. In its advice to the Administrator, the
CASAC concurred with the draft PA that the currently available health
effects evidence is generally similar to that available in the last
review when the standard was set. Part of CASAC concluded that the
primary standard should be retained. Another part of CASAC expressed
concern regarding the margin of safety provided by the current
standard, pointing to comments from the 2014 CASAC, who while agreeing
that the evidence supported a standard level of 70 ppb, additionally
provided policy advice expressing support for a lower standard. The
advice from the CASAC has been considered by the Administrator in
proposing to conclude that the current standard, with its level of 70
ppb, provides the requisite public health protection, with an adequate
margin of safety. The EPA solicits comment on the Administrator's
proposed conclusion, and on the proposed decision to retain the
standard, without revision. The EPA also solicits comment on the array
of issues associated with review of this standard, including public
health and science policy judgments inherent in the proposed decision.
    The proposed decision to retain the current secondary standard,
without revision, has been informed by key aspects of the currently
available welfare effects evidence and conclusions contained in the
ISA, quantitative exposure/risk analyses and policy evaluations
presented in the PA, advice from the CASAC and public input received as
part of this ongoing review. The welfare effects evidence newly
available in this review, in conjunction with the full body of evidence
critically evaluated in the ISA, supports, sharpens and expands
somewhat on the conclusions reached in the last review. Consistent with
the evidence in the last review, the currently available evidence
describes an array of O3 effects on vegetation and related
ecosystem effects, as well as the role of O3 in radiative
forcing and subsequent climate-related effects. Further, evidence newly
available in this review augments more limited previously available
evidence for some additional vegetation-related effects. As in the last
review, the strongest evidence and the associated findings of causal or
likely causal relationships with O3 in ambient air, as well
as the quantitative characterizations of relationships between
O3 exposure and occurrence and magnitude of effects, are for
vegetation effects. The scales of these effects range from the
individual plant scale to the ecosystem scale, with potential for
impacts on the public welfare. While the welfare effects of
O3 vary widely with regard to the extent and level of detail
of the available information that describes the exposure circumstances
that may elicit them, such information is most advanced for growth-
related effects such as growth and yield. For example, the information

on exposure metric and relationships for these effects with the
cumulative, concentration-weighted exposure index, W126, is long-
standing, having been first described in the 1997 review. Utilizing
this information, reduced growth is considered as proxy or surrogate
for the broader array of vegetation effects in reviewing the public
welfare protection provided by the current standard.
    Quantitative analyses of air quality and exposure, including use of
the W126 index, as well as policy considerations in the PA, also inform
the proposed decision on the secondary standard. For example, analyses
of air quality monitoring data across the U.S., as well as in Class I
areas, updated and expanded from analyses conducted in the last review,
inform EPA's understanding of vegetation exposures in areas meeting the
current standard. Based on the current evidence and quantitative
information, as well as consideration of CASAC advice and public
comment thus far in this review, the Administrator proposes to conclude
that the current secondary standard is requisite to protect the public
welfare from known or anticipated adverse effects of O3 in
ambient air, and should be retained, without revision. In its advice to
the Administrator, the full CASAC concurred with the preliminary
conclusions in the draft PA that the current evidence supports
retaining the current standard without revision. The EPA solicits
comment on the Administrator's proposed conclusion that the current
standard is requisite to protect the public welfare, and on the
proposed decision to retain the standard, without revision. The EPA
also solicits comment on the array of issues associated with review of
this standard, including public welfare and science policy judgments
inherent in the proposed decision.

I. Background

A. Legislative Requirements

    Two sections of the CAA govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify and list certain air pollutants and then to issue air quality
criteria for those pollutants. The Administrator is to list those
pollutants ``emissions of which, in his judgment, cause or contribute
to air pollution which may reasonably be anticipated to endanger public
health or welfare''; ``the presence of which in the ambient air results
from numerous or diverse mobile or stationary sources''; and for which
he ``plans to issue air quality criteria. . . .'' (42 U.S.C.
7408(a)(1)). Air quality criteria are intended to ``accurately reflect
the latest scientific knowledge useful in indicating the kind and
extent of all identifiable effects on public health or welfare which
may be expected from the presence of [a] pollutant in the ambient air.
. . .'' (42 U.S.C. 7408(a)(2)).
    Section 109 [42 U.S.C. 7409] directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for
which air quality criteria are issued [42 U.S.C. 7409(a)]. Section
109(b)(1) defines primary standards as ones ``the attainment and
maintenance of which in the judgment of the Administrator, based on
such criteria and allowing an adequate margin of safety, are requisite
to protect the public health.'' \1\ Under section 109(b)(2), a
secondary standard must ``specify a level of air quality the attainment
and maintenance of which, in the judgment of the Administrator, based
on such criteria, is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
[the] pollutant in the ambient air.'' \2\
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    \1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
    \2\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on
welfare include, but are not limited to, ``effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife, weather,
visibility, and climate, damage to and deterioration of property,
and hazards to transportation, as well as effects on economic values
and on personal comfort and well-being.''
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    In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary. In so doing, the EPA
may not consider the costs of implementing the standards. See
generally, Whitman v. American Trucking Ass'ns, 531 U.S. 457, 465-472,
475-76 (2001). Likewise, ``[a]ttainability and technological
feasibility are not relevant considerations in the promulgation of
national ambient air quality standards.'' See American Petroleum
Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord
Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). At
the same time, courts have clarified the EPA may consider ``relative
proximity to peak background . . . concentrations'' as a factor in
deciding how to revise the NAAQS in the context of considering standard
levels within the range of reasonable values supported by the air
quality criteria and judgments of the Administrator. See American
Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 (D.C. Cir. 2002), hereafter
referred to as ``ATA III.''
    The requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. See Lead Industries Ass'n v. EPA, 647 F.2d 1130, 1154 (D.C.
Cir 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186;
Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C.
Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013).
Both kinds of uncertainties are components of the risk associated with
pollution at levels below those at which human health effects can be
said to occur with reasonable scientific certainty. Thus, in selecting
primary standards that include an adequate margin of safety, the
Administrator is seeking not only to prevent pollution levels that have
been demonstrated to be harmful but also to prevent lower pollutant
levels that may pose an unacceptable risk of harm, even if the risk is
not precisely identified as to nature or degree. The CAA does not
require the Administrator to establish a primary NAAQS at a zero-risk
level or at background concentration levels (see Lead Industries Ass'n
v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351),
but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
    In addressing the requirement for an adequate margin of safety, the
EPA considers such factors as the nature and severity of the health
effects involved, the size of the sensitive population(s), and the kind
and degree of uncertainties. The selection of any particular approach
to providing an adequate margin of safety is a policy choice left
specifically to the Administrator's judgment. See Lead Industries Ass'n
v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
    Section 109(d)(1) of the Act requires periodic review and, if
appropriate, revision of existing air quality criteria to reflect
advances in scientific knowledge concerning the effects of the
pollutant on public health and welfare. Under the same provision, the
EPA is also to periodically review and, if appropriate,

revise the NAAQS, based on the revised air quality criteria.\3\
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    \3\ This section of the Act requires the Administrator to
complete these reviews and make any revisions that may be
appropriate ``at five-year intervals.''
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    Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate. . . .'' Since
the early 1980s, this independent review function has been performed by
the CASAC of the EPA's Science Advisory Board. A number of other
advisory functions are also identified for the committee by section
109(d)(2)(C), which reads:

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

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

    States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under sections 110 and 171 through 185 of the CAA,
and related provisions and regulations, states are to submit, for the
EPA's approval, state implementation plans (SIPs) that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved. The states, in
conjunction with the EPA, also administer the prevention of significant
deterioration of air quality program that covers these pollutants. See
42 U.S.C. 7470-7479. In addition, federal programs provide for
nationwide reductions in emissions of O3 precursors and
other air pollutants under Title II of the Act, 42 U.S.C. 7521-7574,
which involves controls for automobile, truck, bus, motorcycle, nonroad
engine and equipment, and aircraft emissions; the new source
performance standards under section 111 of the Act, 42 U.S.C. 7411; and
the national emissions standards for hazardous air pollutants under
section 112 of the Act, 42 U.S.C. 7412.

C. Review of the Air Quality Criteria and Standards for O3

    Primary and secondary NAAQS were first established for
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971) based on
the air quality criteria developed in 1970 (U.S. DHEW, 1970; 35 FR
4768, March 19, 1970). The EPA set both primary and secondary standards
at 0.08 parts per million (ppm), as a 1-hour average of total
photochemical oxidants, not to be exceeded more than one hour per year
based on the scientific information in the 1970 air quality criteria
document (AQCD). Since that time, the EPA has reviewed the air quality
criteria and standards a number of times, with the most recent review
being completed in 2015.
    The EPA initiated the first periodic review of the NAAQS for
photochemical oxidants in 1977. Based on the 1978 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 changed the indicator
from photochemical oxidants to O3, revised the level of the
primary and secondary standards from 0.08 to 0.12 ppm and revised the
form of both standards from a deterministic (i.e., not to be exceeded
more than one hour per year) to a statistical form. With these changes,
attainment of the standards was defined to occur when the average
number of days per calendar year (across a 3-year period) with maximum
hourly average O3 concentration greater than 0.12 ppm
equaled one or less (44 FR 8202, February 8, 1979; 43 FR 26962, June
22, 1978). Several petitioners challenged the 1979 decision. Among
those, one claimed natural O3 concentrations and other
physical phenomena made the standard unattainable in the Houston area.
The U.S. Court of Appeals for the District of Columbia Circuit (D.C.
Circuit) rejected this argument, holding (as noted in section I.A
above) that attainability and technological feasibility are not
relevant considerations in the promulgation of the NAAQS (American
Petroleum Institute v. Costle, 665 F.2d at 1185). The court also noted
that the EPA need not tailor the NAAQS to fit each region or locale,
pointing out that Congress was aware of the difficulty in meeting
standards in some locations and had addressed it through various
compliance-related provisions in the CAA (id. at 1184-86).
    The next periodic reviews of the criteria and standards for
O3 and other photochemical oxidants began in 1982 and 1983,
respectively (47 FR 11561, March 17, 1982; 48 FR 38009, August 22,
1983). The EPA subsequently published the 1986 AQCD, 1989 Staff Paper,
and a supplement to the 1986 AQCD (U.S. EPA, 1986; U.S. EPA, 1989; U.S.
EPA, 1992). In August of 1992, the EPA proposed to retain the existing
primary and secondary standards (57 FR 35542, August 10, 1992). In
March 1993, the EPA concluded this review by finalizing its proposed
decision to retain the standards, without revision (58 FR 13008, March
9, 1993).
    In the 1992 decision in that review, the EPA announced its
intention to proceed rapidly with the next review of the air quality
criteria and standards for O3 and other photochemical
oxidants

(57 FR 35542, August 10, 1992). The EPA subsequently published the AQCD
and Staff Paper for that next review (U.S. EPA, 1996a; U.S. EPA,
1996b). In December 1996, the EPA proposed revisions to both the
primary and secondary standards (61 FR 65716, December 13, 1996). The
EPA completed this review in 1997 by revising the primary and secondary
standards to 0.08 ppm, as the annual fourth-highest daily maximum 8-
hour average concentration, averaged over three years (62 FR 38856,
July 18, 1997).
    In response to challenges to the EPA's 1997 decision, the D.C.
Circuit remanded the 1997 O3 NAAQS to the EPA, finding that
section 109 of the CAA, as interpreted by the EPA, effected an
unconstitutional delegation of legislative authority. See American
Trucking Ass'ns v. EPA, 175 F.3d 1027, 1034-1040 (D.C. Cir. 1999). The
court also directed that, in responding to the remand, the EPA should
consider the potential beneficial health effects of O3
pollution in shielding the public from the effects of solar ultraviolet
(UV) radiation, as well as adverse health effects (id. at 1051-53). See
American Trucking Ass'ns v. EPA,195 F.3d 4, 10 (D.C. Cir. 1999)
(granting panel rehearing in part but declining to review the ruling on
consideration of the potential beneficial effects of O3
pollution). After granting petitions for certiorari, the U.S. Supreme
Court unanimously reversed the judgment of the D.C. Circuit on the
constitutional issue, holding that section 109 of the CAA does not
unconstitutionally delegate legislative power to the EPA. See Whitman
v. American Trucking Ass'ns, 531 U.S. 457, 472-74 (2001). The Court
remanded the case to the D.C. Circuit to consider challenges to the
1997 O3 NAAQS that had not yet been addressed. On remand,
the D.C. Circuit found the 1997 O3 NAAQS to be ``neither
arbitrary nor capricious,'' and so denied the remaining petitions for
review. See ATA III, 283 F.3d at 379.
    Coincident with the continued litigation of the other issues, the
EPA responded to the court's 1999 remand to consider the potential
beneficial health effects of O3 pollution in shielding the
public from effects of UV radiation (66 FR 57268, Nov. 14, 2001; 68 FR
614, January 6, 2003). In 2001, the EPA proposed to leave the 1997
primary standard unchanged (66 FR 57268, Nov. 14, 2001). After
considering public comment on the proposed decision, the EPA published
its final response to this remand in 2003, re-affirming the 8-hour
primary standard set in 1997 (68 FR 614, January 6, 2003).
    The EPA initiated the fourth periodic review of the air quality
criteria and standards for O3 and other photochemical
oxidants with a call for information in September 2000 (65 FR 57810,
September 26, 2000). Documents developed for the review included the
2006 AQCD (U.S. EPA, 2006) and 2007 Staff Paper (U.S. EPA, 2007) and
related technical support documents. In 2007, the EPA proposed
revisions to the primary and secondary standards (72 FR 37818, July 11,
2007). The EPA completed the review in March 2008 by revising the
levels of both the primary and secondary standards from 0.08 ppm to
0.075 ppm while retaining the other elements of the prior standards (73
FR 16436, March 27, 2008). A number of petitioners filed suit
challenging this decision.
    In September 2009, the EPA announced its intention to reconsider
the 2008 O3 standards,\5\ and initiated a rulemaking to do
so. At the EPA's request, the court held the consolidated cases in
abeyance pending the EPA's reconsideration of the 2008 decision. In
January 2010, the EPA issued a notice of proposed rulemaking to
reconsider the 2008 final decision (75 FR 2938, January 19, 2010).
Later that year, in view of the need for further consideration and the
fact that the Agency's next periodic review of the O3 NAAQS
required under CAA section 109 had already begun (as announced on
September 29, 2008),\6\ the EPA consolidated the reconsideration with
its statutorily required periodic review.\7\
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    \5\ The press release of this announcement is available at:
https://archive.epa.gov/epapages/newsroom_archive/newsreleases/85f90b7711acb0c88525763300617d0d.html.
    \6\ The ``Call for Information'' initiating the new review was
announced in the Federal Register (73 FR 56581, September 29, 2008).
    \7\ This rulemaking, completed in 2015, concluded the
reconsideration process.
---------------------------------------------------------------------------

    In light of the EPA's decision to consolidate the reconsideration
with the review then ongoing, the D.C. Circuit proceeded with the
litigation on the 2008 O3 NAAQS decision. On July 23, 2013,
the court upheld the EPA's 2008 primary standard, but remanded the 2008
secondary standard to the EPA. See Mississippi v. EPA, 744 F.3d 1334
(D.C. Cir. 2013). With respect to the primary standard, the court
rejected petitioners' arguments, upholding the EPA's decision. With
respect to the secondary standard, the court held that the EPA's
explanation for the setting of the secondary standard identical to the
revised 8-hour primary standard was inadequate under the CAA because
the EPA had not adequately explained how that standard provided the
required public welfare protection.
    At the time of the court's decision, the EPA had already completed
significant portions of its next statutorily required periodic review
of the O3 NAAQS, which had been formally initiated in 2008,
as summarized above. The documents developed for this review included
the ISA,\8\ Risk and Exposure Assessments (REAs) for health and
welfare, and PA.\9\ In late 2014, the EPA proposed to revise the 2008
primary and secondary standards (79 FR 75234, December 17, 2014; Frey,
2014a, Frey, 2014b, Frey, 2014c, U.S. EPA, 2014a, U.S. EPA, 2014b, U.S.
EPA, 2014c). The EPA's final decision in this review was published in
October 2015, establishing the now-current standards (80 FR 65292,
October 26, 2015). In this decision, based on consideration of the
health effects evidence on respiratory effects of O3 in at-
risk populations, the EPA revised the primary standard from a level of
0.075 ppm to a level of 0.070 ppm, while retaining all other elements
of the standard (80 FR 65292, October 26, 2015). The EPA's decision on
the level for the standard was based on the weight of the scientific
evidence and quantitative exposure/risk information. The level of the
secondary standard was also revised from 0.075 ppm to 0.070 ppm based
on the scientific evidence of O3 effects on welfare,
particularly the evidence of O3 impacts on vegetation, and
quantitative analyses available in the review.\10\ The other elements
of the standard were retained. This decision on the secondary standard
also incorporated the EPA's response to the D.C. Circuit's remand of
the 2008 secondary standard in Mississippi v. EPA, 744 F.3d 1344 (D.C.
Cir. 2013).\11\
---------------------------------------------------------------------------

    \8\ The ISA serves the same purpose, in reviewing the air
quality criteria, as the AQCD did in prior reviews.
    \9\ The PA presents an evaluation, for consideration by the
Administrator, of the policy implications of the currently available
scientific information, assessed in the ISA; the quantitative air
quality, exposure or risk analyses presented in the PA and developed
in light of the ISA findings; and related limitations and
uncertainties. The role of the PA is to help ``bridge the gap''
between the Agency's scientific assessment and quantitative
technical analyses, and the judgments required of the Administrator
in his decisions in the review of the O3 NAAQS.
    \10\ These standards, set in 2015, are specified at 40 CFR
50.19.
    \11\ The 2015 revisions to the NAAQS were accompanied by
revisions to the data handling procedures, ambient air monitoring
requirements, the air quality index and several provisions related
to implementation (80 FR 65292, October 26, 2015).
---------------------------------------------------------------------------

    After publication of the final rule, a number of industry groups,
environmental and health organizations, and certain states filed
petitions for judicial review in the D.C. Circuit. The

industry and state petitioners argued that the revised standards were
too stringent, while the environmental and health petitioners argued
that the revised standards were not stringent enough to protect public
health and welfare as the Act requires. On August 23, 2019, the court
issued an opinion that denied all the petitions for review with respect
to the 2015 primary standard while also concluding that the EPA had not
provided a sufficient rationale for aspects of its decision on the 2015
secondary standard and remanding that standard to the EPA. See Murray
Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019). The court's
decision on the secondary standard focused on challenges to particular
aspects of EPA's decision. The court concluded that EPA's
identification of particular benchmarks for evaluating the protection
the standard provided against welfare effects associated with tree
growth loss was reasonable and consistent with CASAC's advice. However,
the court held that EPA had not adequately explained its decision to
focus on a 3-year average for consideration of the cumulative exposure,
in terms of W126, identified as providing requisite public welfare
protection, or its decision to not identify a specific level of air
quality related to visible foliar injury. The EPA's decision not to use
a seasonal W126 index as the form and averaging time of the secondary
standard was also challenged, but the court did not reach that issue,
concluding that it lacked a basis to assess the EPA's rationale on this
point because the EPA had not yet fully explained its focus on a 3-year
average W126 in its consideration of the standard. See Murray Energy
Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019). Accordingly, the
court remanded the secondary standard to EPA for further justification
or reconsideration. The court's remand of the secondary standard has
been considered in reaching the proposed decision, and associated
proposed conclusions and judgments, described in section III.D.3 below.
    In the August 2019 decision, the court additionally addressed
arguments regarding considerations of background O3
concentrations, and socioeconomic and energy impacts. With regard to
the former, the court rejected the argument that the EPA was required
to take background O3 concentrations into account when
setting the NAAQS, holding that the text of CAA section 109(b)
precluded this interpretation because it would mean that if background
O3 levels in any part of the country exceeded the level of
O3 that is requisite to protect public health, the EPA would
be obliged to set the standard at the higher nonprotective level (id.
at 622-23). Thus, the court concluded that the EPA did not act
unlawfully or arbitrarily or capriciously in setting the 2015 NAAQS
without regard for background O3 (id. at 624). Additionally,
the court denied arguments that the EPA was required to consider
adverse economic, social, and energy impacts in determining whether a
revision of the NAAQS was ``appropriate'' under section 109(d)(1) of
the CAA (id. at 621-22). The court reasoned that consideration of such
impacts was precluded by Whitman's holding that the CAA ``unambiguously
bars cost considerations from the NAAQS-setting process'' (531 U.S. at
471, summarized in section 1.2 above). Further, the court explained
that section 109(d)(2)(C)'s requirement that CASAC advise the EPA ``of
any adverse public health, welfare, social, economic, or energy effects
which may result from various strategies for attainment and
maintenance'' of revised NAAQS had no bearing on whether costs are to
be considered in setting the NAAQS (Murray Energy Corp. v. EPA, 936
F.3d at 622). Rather, as described in Whitman and discussed further in
section I.A above, most of that advice would be relevant to
implementation but not standard setting (id.).
    In May 2018, the Administrator directed his Assistant
Administrators to initiate this current review of the O3
NAAQS (Pruitt, 2018). In conveying this direction, the Administrator
further directed the EPA staff to expedite the review, implementing an
accelerated schedule aimed at completion of the review within the
statutorily required period (Pruitt, 2018). Accordingly, the EPA took
immediate steps to proceed with the review. In June 2018, the EPA
announced the initiation of the current periodic review of the air
quality criteria for photochemical oxidants and the O3 NAAQS
and issued a call for information in the Federal Register (83 FR 29785,
June 26, 2018). Two types of information were called for: Information
regarding significant new O3 research to be considered for
the ISA for the review, and policy-relevant issues for consideration in
this NAAQS review. Based in part on the information received in
response to the call for information, the EPA developed a draft IRP,
which was made available for consultation with the CASAC and for public
comment (83 FR 55163, November 2, 2018; 83 FR 55528, November 6, 2018).
Comments from the CASAC (Cox, 2018) and the public were considered in
preparing the final IRP (U.S. EPA, 2019b).
    Under the plan outlined in the IRP and consistent with revisions to
the process identified by the administrator in his 2018 memo directing
initiation of the review, the current review of the O3 NAAQS
is progressing on an accelerated schedule (Pruitt, 2018). The EPA is
incorporating a number of efficiencies in various aspects of the review
process, as summarized in the IRP, to support completion within the
statutorily required period (Pruitt, 2018). As one example of such an
efficiency, rather than produce two separate documents, the exposure
and risk analyses for the primary standard are included as an appendix
in the PA, along with a number of other technical appendices. The draft
PA (including these analyses as appendices) was reviewed by the CASAC
and made available for public comment while the draft ISA was also
being reviewed by the CASAC and was available for public comment (84 FR
50836, September 26, 2019; 84 FR 58711, November 1, 2019).\12\ The
CASAC was assisted in its review by a pool of consultants with
expertise in a number of fields (84 FR 38625, August 7, 2019). The
approach employed by the CASAC in utilizing outside technical expertise
represents an additional modification of the process from past reviews.
Rather than join with some or all of the CASAC members in a CASAC
review panel as has been common in other NAAQS reviews in the past, in
this O3 NAAQS review (and also in the recent CASAC review of
the PA for the particulate matter NAAQS), the consultants comprised a
pool of expertise that CASAC members drew on through the use of
specific questions, posed in writing prior to the public meeting,
regarding aspects of the documents being reviewed, obtaining subject
matter expertise for its document review in a focused, efficient and
transparent manner.
---------------------------------------------------------------------------

    \12\ The draft ISA and draft PA were released for public comment
and CASAC review on September 26, 2019 and October 31, 2019,
respectively. The charges for the CASAC review summarized the
overarching context for the document review (including reference to
Pruitt [2018], and the CASAC's role under section 109(d)(2)(C) of
the Act), as well as specific charge questions for review of each of
the documents.
---------------------------------------------------------------------------

    The CASAC discussed its review of both the draft ISA and the draft
PA over three days at a public meeting in December 2019 (84 FR 58713,
November 1, 2019).\13\ The CASAC discussed its

draft letters describing its advice and comments on the documents in a
public teleconference in early February 2020 (85 FR 4656; January 27,
2020). The letters to the Administrator conveying the CASAC advice and
comments on the draft PA and draft ISA were released later that month
(Cox, 2020a, Cox, 2020b).
---------------------------------------------------------------------------

    \13\ While simultaneous review of first drafts of both documents
has not been usual in past reviews, there have been occurrences of
the CASAC review of a draft PA (or draft REA when the process
involved a policy assessment being included within the REA document)
simultaneous with review of a second (or later) draft ISA (e.g., 73
FR 19835, April 11, 2008; 73 FR 34739, June 18, 2008; 77 FR 64335,
October 19, 2020; 78 FR 938, January 7, 2013).
---------------------------------------------------------------------------

    The letters from the CASAC and public comment on the draft ISA and
draft PA have informed completion of the final documents and further
inform development of the Administrator's proposed decision in this
review. Comments from the CASAC on the draft ISA have been considered
by the EPA and led to a number of revisions in developing the final
document. The CASAC review and the EPA's consideration of CASAC
comments are described in Appendix 10, section 10.4.5 of the final ISA.
In his reply to the CASAC letter conveying its review, ``Administrator
Wheeler noted, `for those comments and recommendations that are more
significant or cross-cutting and which were not fully addressed, the
Agency will develop a plan to incorporate these changes into future
Ozone ISAs as well as ISAs for other criteria pollutant reviews' ''
(ISA, p. 10-28; Wheeler, 2020). The ISA was completed and made
available to the public in April 2020 (85 FR 21849, April 20, 2020).
Based on the rigorous scientific approach utilized in its development,
summarized in Appendix 10 of the final ISA, the EPA considers the final
ISA to ``accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on public
health or welfare which may be expected from the presence of
[O3] in the ambient air, in varying quantities'' as required
by the CAA (42 U.S.C. 7408(a)(2)).
    The CASAC comments additionally provided advice with regard to the
primary and secondary standards, as well as a number of comments
intended to improve the PA. These comments were considered in
completing that document, which was completed in May 2020 (85 FR 31182,
May 22, 2020). The CASAC advice to the Administrator regarding the
O3 standards has also been described and considered in the
PA, and in sections II and III below. The CASAC advice on the primary
standard is summarized in II.D.2 below and its advice on the secondary
standard is summarized in section III.D.2.
    Materials upon which this proposed decision is based, including the
documents described above, are available to the public in the docket
for the review.\14\ Following a public comment period on the proposed
decision, a final decision in the review is projected for late in 2020.
---------------------------------------------------------------------------

    \14\ The docket for the current O3 NAAQS review is
identified as EPA-HQ-OAR-2018-0279. This docket has incorporated the
ISA docket (EPA-HQ-ORD-2018-0274) by reference. Both dockets are
publicly accessible at www.regulations.gov.
---------------------------------------------------------------------------

D. Air Quality Information

    Ground level ozone concentrations are a mix of mostly tropospheric
ozone and some stratospheric ozone. Tropospheric ozone is formed due to
chemical interactions involving solar radiation and precursor
pollutants including volatile organic compounds (VOCs) and nitrogen
oxides (NOX). Methane (CH4) and carbon monoxide
(CO) are also important precursors, particularly at the regional to
global scale. The precursor emissions leading to tropospheric
O3 formation can result from both man-made sources (e.g.,
motor vehicles and electric power generation) and natural sources
(e.g., vegetation and wildfires). In addition, O3 that is
created naturally in the stratosphere also contributes to O3
levels near the surface. The stratosphere routinely mixes with the
troposphere high above the earth's surface and, less frequently, there
are intrusions of stratospheric air that reach deep into the
troposphere and even to the surface. Once formed, O3 near
the surface can be transported by winds before eventually being removed
from the atmosphere via chemical reactions or deposition to surfaces.
In sum, O3 concentrations are influenced by complex
interactions between precursor emissions, meteorological conditions,
and topographical characteristics (PA, section 2.1; ISA, Appendix 1).
    For compliance and other purposes, state and local environmental
agencies operate O3 monitors across the U.S. and submit the
data to the EPA. At present, there are approximately 1,300 monitors
across the U.S. reporting hourly O3 averages during the
times of the year when local O3 pollution can be important
(PA, section 2.3.1).\15\ Most of this monitoring is focused on urban
areas where precursor emissions tend to be largest, as well as
locations directly downwind of these areas. There are also over 100
routine monitoring sites in rural areas, including sites in the Clean
Air Status and Trends Network (CASTNET) which is specifically focused
on characterizing conditions in rural areas. Based on the monitoring
data for the most recent 3-year period (2016-2018), the EPA identified
142 counties, in which together approximately 106 million Americans
reside where O3 design values \16\ were above 0.070, the
level of the existing NAAQS (PA, section 2.4.1). Across these areas,
the highest design values are typically observed in California, Texas,
and the Northeast Corridor, locations with some of the most densely
populated areas in the country (e.g., PA, Figure 2-8).
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    \15\ O3 monitoring seasons vary by state from five
months (May to September in Oregon and Washington) to all twelve
months (in 11 states), with the most common season being March to
October (in 27 states).
    \16\ A design value is a statistic that summarizes the air
quality data for a given area in terms of the indicator, averaging
time, and form of the standard. Design values can be compared to the
level of the standard and are typically used to designate areas as
meeting or not meeting the standard and assess progress towards
meeting the NAAQS.
---------------------------------------------------------------------------

    From a temporal perspective, the highest daily peak O3
concentrations generally tend to occur during the afternoon and within
the warmer months of the year due to higher levels of solar radiation
and other conducive meteorological conditions during these times. The
exceptions to this general rule include (1) some rural sites where
transport of O3 from upwind urban areas can occasionally
result in high nighttime levels of O3, (2) high-elevation
sites which can be episodically influenced by stratospheric intrusions
in other months of the year, and (3) mountain basins in the western
U.S. where large quantities of O3 precursors emissions
associated with oil and gas development can be trapped in a shallow
inversion layer and form O3 under clear, calm skies with
snow cover during the colder months (PA, section 2.1; ISA, Appendix 1).
    Monitoring data indicate long-term reductions in short-term
O3 concentrations. For example, monitoring sites operating
since 1980 indicate a 32% reduction in the national average annual
fourth highest daily maximum 8-hour concentration from 1980 to 2018.
(PA, Figure 2-10). This has been accompanied by appreciable reductions
in peak 1-hour concentrations (PA, Figure 2-17).
    Concentrations of O3 in ambient air that result from
natural and non-U.S. anthropogenic sources are collectively referred to
as U.S. background O3 (USB; PA, section 2.5). As in the last
review, we generally characterize O3 concentrations that
would exist in the absence of U.S. anthropogenic emissions as U.S.
background (USB). Findings from modeling analyses performed for this
review to investigate

patterns of USB in the U.S. are largely consistent with conclusions
reached in the last review (PA, section 2.5.4). The current modeling
analysis indicates spatial variation in USB O3 that is
related to geography, topography and proximity to international borders
and is also influenced by seasonal variation, with long-range
international anthropogenic transport contributions peaking in the
spring while U.S. anthropogenic contributions tend to peak in summer.
The West is predicted to have higher USB concentrations than the East,
with higher contributions from natural and international anthropogenic
sources that exert influences in western high-elevation and near-border
areas. The modeling predicts that for both the West and the East, days
with the highest 8-hour concentrations of O3 generally occur
in summer and are likely to have substantially greater concentrations
due to U.S. anthropogenic sources. While the USB contributions to
O3 concentrations on days with the highest 8-hour
concentrations are generally predicted to come largely from natural
sources, the modeling also indicates that a small area near the Mexico
border may receive appreciable contributions from a combination of
natural and international anthropogenic sources on these days. In such
locations, the modeling suggests the potential for episodic and
relatively infrequent events with substantial background contributions
where daily maximum 8-hour O3 concentrations approach or
exceed the level of the current NAAQS (i.e., 70 ppb). This contrasts
with most monitor locations in the U.S. for which international
contributions are predicted to be the lowest during the season with the
most frequent occurrence of daily maximum 8-hour O3
concentrations above 70 ppb. This is generally because, except for in
near-border areas, larger international contributions are associated
with long-distance transport and that is most efficient in the
springtime (PA, section 2.5.4).

II. Rationale for Proposed Decision on the Primary Standard

    This section presents the rationale for the Administrator's
proposed decision to retain the current primary O3 standard.
This rationale is based on a thorough review of the latest scientific
information generally published between January 2011 and March 2018, as
well as more recent studies identified during peer review or by public
comments (ISA, section IS.1.2),\17\ integrated with the information and
conclusions from previous assessments and presented in the ISA, on
human health effects associated with photochemical oxidants including
O3 and pertaining to their presence in ambient air. The
Administrator's rationale also takes into account: (1) The PA
evaluation of the policy-relevant information in the ISA and
presentation of quantitative analyses of air quality, human exposure
and health risks; (2) CASAC advice and recommendations, as reflected in
discussions of drafts of the ISA and PA at public meetings and in the
CASAC's letters to the Administrator; and (3) public comments received
during the development of these documents.
---------------------------------------------------------------------------

    \17\ In addition to the review's opening ``Call for
Information'' (83 FR 29785, June 26, 2018), systematic review
methodologies were applied to identify relevant scientific findings
that have emerged since the 2013 ISA, which included peer reviewed
literature published through July 2011. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2011 (providing some overlap with the
cutoff date for the last ISA) and March 30, 2018. Studies published
after the literature cutoff date for this ISA were also considered
if they were submitted in response to the Call for Information or
identified in subsequent phases of ISA development, particularly to
the extent that they provide new information that affects key
scientific conclusions (ISA, Appendix 10, section 10.2). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737.
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    In presenting the rationale for the Administrator's proposed
decision and its foundations, section II.A provides background and
introductory information for this review of the primary O3
standard. It includes background on the establishment of the current
standard in 2015 (section II.A.1) and also describes the general
approach for the current review (section II.A.2). Section II.B
summarizes the currently available health effects evidence, focusing on
consideration of key policy-relevant aspects. Section II.C summarizes
the exposure and risk information for this review, drawing on the
quantitative analyses for O3, presented in the PA. Section
II.D presents the Administrator's proposed conclusions on the current
standard (section II.D.3), drawing on both evidence-based and exposure/
risk-based considerations (section II.D.1) and advice from the CASAC
(section II.D.2).

A. General Approach

    The past and current approaches described below are both based,
most fundamentally, on using the EPA's assessments of the current
scientific evidence and associated quantitative analyses to inform the
Administrator's judgment regarding a primary standard for photochemical
oxidants that is requisite to protect the public health with an
adequate margin of safety. The EPA's assessments are primarily
documented in the ISA and PA, all of which have received CASAC review
and public comment (84 FR 50836, September 26, 2019; 84 FR 58711,
November 1, 2019; 84 FR 58713, November 1, 2019; 85 FR 21849, April 20,
2020; 85 FR 31182, May 22, 2020). In bridging the gap between the
scientific assessments of the ISA and the judgments required of the
Administrator in his decisions on the current standard, the PA
evaluates policy implications of the evaluation of the current evidence
in ISA and the quantitative exposure and risk analyses documented in
appendices of the PA. In evaluating the public health protection
afforded by the current standard, the four basic elements of the NAAQS
(indicator, averaging time, level, and form) are considered
collectively.
    The final decision on the adequacy of the current primary standard
is a public health policy judgment to be made by the Administrator. In
reaching conclusions with regard to the standard, the decision will
draw on the scientific information and analyses about health effects,
population exposure and risks, as well as judgments about how to
consider the range and magnitude of uncertainties that are inherent in
the scientific evidence and analyses. This approach is based on the
recognition that the available health effects evidence generally
reflects a continuum, consisting of levels at which scientists
generally agree that health effects are likely to occur, through lower
levels at which the likelihood and magnitude of the response become
increasingly uncertain. This approach is consistent with the
requirements of the NAAQS provisions of the Clean Air Act and with how
the EPA and the courts have historically interpreted the Act
(summarized in section I.A. above). These provisions require the
Administrator to establish primary standards that, in the judgment of
the Administrator, are requisite to protect public health with an
adequate margin of safety. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that primary
standards be set at a zero-risk level, but rather at a level that
avoids unacceptable risks to public

health, including the health of sensitive groups.\18\
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    \18\ As noted in section I.A above, the legislative history
describes such protection for the sensitive group of individuals and
not for a single person in the sensitive group (see S. Rep. No. 91-
1196, 91st Cong., 2d Sess. 10 [1970]).
---------------------------------------------------------------------------

    The subsections below provide background and introductory
information. Background on the establishment of the current standard in
2015, including the rationale for that decision, is summarized in
section II.A.1. This is followed, in section II.A.2, by an overview of
the general approach for the current review of the 2015 standard.
Following this introductory section and subsections, the subsequent
sections summarize current information and analyses, including that
newly available in this review. The Administrator's proposed
conclusions on the standard set in 2015, based on the current
information, are provided in section II.D.3.
1. Background on the Current Standard
    The current primary standard was set in 2015 based on the
scientific evidence and quantitative exposure and risk analyses
available at that time, and on the Administrator's judgments regarding
the available scientific evidence, the appropriate degree of public
health protection for the revised standard, and the available exposure
and risk information regarding the exposures and risk that may be
allowed by such a standard (80 FR 65292, October 26, 2015). The 2015
decision revised the level of the primary standard from 0.075 to 0.070
ppm,\19\ in conjunction with retaining the indicator (O3),
averaging time (eight hours), and form (annual fourth-highest daily
maximum 8-hour average concentration, averaged across three consecutive
years). This action provided increased protection for at-risk
populations,\20\ such as children and people with asthma, against an
array of adverse health effects. The 2015 decision drew upon the
available scientific evidence assessed in the 2013 ISA, the exposure
and risk information presented and assessed in the 2014 health REA
(HREA), the consideration of that evidence and information in the 2014
PA, the advice and recommendations of the CASAC, and public comments on
the proposed decision (79 FR 75234, December 17, 2014).
---------------------------------------------------------------------------

    \19\ Although ppm are the units in which the level of the
standard is defined, the units, ppb, are more commonly used
throughout this document for greater consistency with their use in
the more recent literature. The level of the current primary
standard, 0.070 ppm, is equivalent to 70 ppb.
    \20\ As used here and similarly throughout the document, the
term population refers to persons having a quality or characteristic
in common, such as, and including, a specific pre-existing illness
or a specific age or lifestage. A lifestage refers to a
distinguishable time frame in an individual's life characterized by
unique and relatively stable behavioral and/or physiological
characteristics that are associated with development and growth.
Identifying at-risk populations includes consideration of intrinsic
(e.g., genetic or developmental aspects) or acquired (e.g., disease
or smoking status) factors that increase the risk of health effects
occurring with exposure to a substance (such as O3) as
well as extrinsic, nonbiological factors, such as those related to
socioeconomic status, reduced access to health care, or exposure.
---------------------------------------------------------------------------

    The health effects evidence base available in the 2015 review
included extensive evidence from previous reviews as well as the
evidence that had emerged since the prior review had been completed in
2008. This evidence base, spanning several decades, documents the
causal relationship between exposure to O3 and a broad range
of respiratory effects (2013 ISA, p. 1-14). Such effects range from
small, reversible changes in pulmonary function and pulmonary
inflammation (documented in controlled human exposure studies involving
exposures ranging from 1 to 8 hours) to more serious health outcomes
such as emergency department visits and hospital admissions, which have
been associated with ambient air concentrations of O3 in
epidemiologic studies (2013 ISA, section 6.2). In addition to extensive
controlled human exposure and epidemiologic studies, the evidence base
includes experimental animal studies that provide insight into
potential modes of action for these effects, contributing to the
coherence and robust nature of the evidence. Based on this evidence,
the 2013 ISA concluded there to be a causal relationship between short-
term O3 exposures and respiratory effects, and also
concluded that the relationship between longer-term exposure and
respiratory effects was likely to be causal (2013 ISA, p. 1-14).\21\
---------------------------------------------------------------------------

    \21\ The 2013 ISA also concluded there likely to be causal
relationship between short-term exposure and mortality, as well as
short-term exposure and cardiovascular effects, including related
mortality, and that the evidence was suggestive of causal
relationships between long-term O3 exposures and total
mortality, cardiovascular effects and reproductive and developmental
effects, and between short-term and long-term O3 exposure
and nervous system effects (2013 ISA, section 2.5.2).
---------------------------------------------------------------------------

    With regard to the short-term respiratory effects that were the
primary focus of the 2015 decision, the controlled human exposure
studies were recognized to provide the most certain evidence indicating
the occurrence of health effects in humans following specific
O3 exposures (80 FR 65343, October 26, 2015; 2014 PA,
section 3.4). These studies additionally illustrate the role of
ventilation rate \22\ and exposure duration in eliciting responses to
O3 exposure at the lowest studied concentrations. The
exposure concentrations eliciting a given level of response in subjects
at rest are higher than those eliciting a response in subjects exposed
while at elevated ventilation, such as while exercising (2013 ISA,
section <IP_ADDRESS>).\23\
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    \22\ Ventilation rate (VE) is a specific technical
term referring to breathing rate in terms of volume of air taken
into the body per unit of time. The units for VE are
usually liters (L) per minute (min). Another related term is
equivalent ventilation rate (EVR), which refers to VE
normalized by a person's body surface area in square meters (m\2\).
Accordingly, the units for EVR are generally L/min-m\2\. For
different activities, a person will experience different levels of
exertion and different ventilation rates.
    \23\ In the controlled human exposure studies, the magnitude or
severity of the respiratory effects induced by O3 is
influenced by ventilation rate and exposure duration, as well as
exposure concentration, with physical activity increasing
ventilation and potential for effects. In studies of generally
healthy adults exposed while at rest for 2 hours, 500 ppb is the
lowest concentration eliciting a statistically significant
O3-induced reduction in group mean lung function
measures, while a much lower concentration produces such result when
the study subject ventilation rates are sufficiently increased with
exercise (2013 ISA, section <IP_ADDRESS>). The lowest exposure
concentration found to elicit a statistically significant
O3-induced reduction in group mean lung function in an
exposure of 2 hours or less was 120 ppb after a 1-hour exposure
(continuous, very heavy exercise) of trained cyclists (2013 ISA,
section <IP_ADDRESS>; Gong et al., 1986) and after 2-hour exposure
(intermittent heavy exercise) of young healthy adults (2013 ISA,
section <IP_ADDRESS>; McDonnell et al., 1983).
---------------------------------------------------------------------------

    The exposure and risk information available in the 2015 review
included exposure and risk estimates for air quality conditions just
meeting the then-existing standard, and also for air quality conditions
just meeting potential alternative standards (U.S. EPA, 2014a,
hereafter 2014 HREA). Estimates were derived for two exposure-based
analyses, as well as for an analysis based on epidemiologic study
associations. The first of the exposure-based analyses involved
comparison of population exposure estimates at elevated exertion to
exposure benchmark concentrations (exposures of concern).\24\ These
benchmark concentrations are based on exposure concentrations from
controlled human exposure studies in which lung function changes and
other effects were measured in healthy, young adult volunteers exposed
to O3 while engaging in quasi-continuous moderate physical
activity for a defined period (generally 6.6 hours).\25\ The second

exposure-based analysis provided population risk estimates of the
occurrence of days with O3-attributable lung function
reductions of varying magnitudes by using the exposure-response (E-R)
information in the form of E-R functions or other quantitative
descriptions of biological processes.\26\ In the epidemiologic study-
based analysis, risk estimates were also derived from ambient air
concentrations using concentration-response (C-R) functions derived
from epidemiologic studies. These latter estimates were given less
weight by the Administrator in her decision on the standard in light of
conclusions reached in the 2014 PA and the HREA, which reflected lower
confidence in these estimates (80 FR 65316-17, October 26, 2015).
---------------------------------------------------------------------------

    \24\ The benchmark concentrations to which exposure
concentrations experienced while at moderate or greater exertion
were compared were 60, 70 and 80 ppb.
    \25\ The studies given primary focus were those for which
O3 exposures occurred over the course of 6.6 hours during
which the subjects engaged in six 50-minute exercise periods
separated by 10-minute rest periods, with a 35-minute lunch period
occurring after the third hour (e.g., Folinsbee et al., 1988 and
Schelegle et al., 2009). Responses after O3 exposure were
compared to those after filtered air exposure.
    \26\ The E-R information and quantitative models derived from it
are based on controlled human exposure studies.
---------------------------------------------------------------------------

    The 2014 HREA developed exposure-based estimates for several
population groups including all children and all adults. The type of
exposure-based estimates that involved comparison of exposures to
benchmarks was also derived for children with asthma and adults with
asthma. The estimates of percentages of all children with exposures at
or above benchmarks were virtually indistinguishable from the
corresponding estimates for children with asthma.\27\ When considered
in terms of the number of children (rather than percentages of the
child populations), the estimates for all children were much higher
than those for children with asthma, with the magnitude of the
differences varying based on asthma prevalence in each study area (2014
HREA, sections 5.3.2, <IP_ADDRESS> and section 5F-1). The estimates for
percent of children experiencing an exposure at or above the benchmarks
were higher than percent of adults due to the greater time that
children spend outdoors and engaged in activities at elevated exertion
(2014 HREA, section 5.3.2). Thus, consideration of the exposure-based
results in the 2015 decision focused on the results for all children
and children with asthma.
---------------------------------------------------------------------------

    \27\ This reflects use of the same time-location-activity diary
pool to construct each simulated individual's time-activity series,
which is based on the similarities observed in the available diary
data with regard to time spent outdoors and exertion levels (2014
HREA, sections 5.3.2 and <IP_ADDRESS>).
---------------------------------------------------------------------------

    In weighing the 2013 ISA conclusions with regard to the health
effects evidence and making judgments regarding the public health
significance of the quantitative estimates of exposures and risks
allowed by the then-existing standard and potential alternative
standards considered, as well as judgments regarding margin of safety,
the Administrator considered the currently available information and
commonly accepted guidelines or criteria within the public health
community, including statements of the American Thoracic Society (ATS),
an organization of respiratory disease specialists,\28\ advice from the
CASAC and public comments. In so doing, she recognized that the
determination of what constitutes an adequate margin of safety is
expressly left to the judgment of the EPA Administrator. See Lead
Industries Ass'n v. EPA, 647 F.2d 1130, 1161-62 (D.C. Cir. 1980);
Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). In NAAQS
reviews generally, evaluations of how particular primary standards
address the requirement to provide an adequate margin of safety include
consideration of such factors as the nature and severity of the health
effects, the size of the sensitive population(s) at risk, and the kind
and degree of the uncertainties present. Consistent with past practice
and long-standing judicial precedent, the Administrator took the need
for an adequate margin of safety into account as an integral part of
her decision-making.
---------------------------------------------------------------------------

    \28\ In this regard, the 2014 PA considered statements issued by
the ATS that had also been considered in prior reviews (ATS, 2000;
ATS, 1985).
---------------------------------------------------------------------------

    In the 2015 decision, the Administrator first addressed the
adequacy of protection provided by the then-existing primary standard
and decided that the standard should be revised. Considerations related
to that decision are summarized in section II.A.1.a below. The
considerations and decisions on the revisions to the then-existing
standard in order to provide the requisite protection under the Act,
including an adequate margin of safety, are summarized in section
II.A.1.b.
a. Considerations Regarding Adequacy of the Prior Standard
    In the decision that the primary standard that existed at the time
of the last review should be revised, the Administrator at that time
gave primary consideration to the evidence of respiratory effects from
controlled human exposure studies, including those newly available in
the review, and for which the exposure concentrations were at the lower
end of those studied (80 FR 65343, October 26, 2015). This emphasis was
consistent with comments from the CASAC at that time on the strength of
this evidence (Frey, 2014b, p. 5). In placing weight on these studies,
the Administrator took note of the variety of respiratory effects
reported from the studies of healthy adults engaged in six 50-minute
periods of moderate exertion within a 6.6-hour exposure to
O3 concentrations of 60 ppb and higher. The lowest exposure
concentration in such studies for which a combination of statistically
significant reduction in lung function and increase in respiratory
symptoms was reported was 72 ppb (during the exercise periods),\29\
while reduced lung function and increased pulmonary inflammation were
reported following such exposures to O3 concentrations as
low as 60 ppb. In considering these findings, the Administrator noted
that the combination of O3-induced lung function decrements
and respiratory symptoms met ATS criteria for an adverse response.\30\
She additionally noted the CASAC comments on this point and also its
caution that these study findings were for healthy adults and thus
indicated the potential for such effects in some groups of people, such
as people with asthma, at lower exposure concentrations (Frey, 2014b,
pp. 5-6; 80 FR 65343, October 26, 2015).
---------------------------------------------------------------------------

    \29\ For the 70 ppb target exposure, Schelegle et al. (2009)
reported, based on O3 measurements during the six 50-
minute exercise periods, that the mean O3 concentration
during the exercise portion of the study protocol was 72 ppb. Based
on the measurements for the six exercise periods, the time weighted
average concentration across the full 6.6-hour exposure was 73 ppb
(Schelegle et al., 2009).
    \30\ The most recent statement from the ATS available at the
time of the 2015 decision stated that ``[i]n drawing the distinction
between adverse and nonadverse reversible effects, this committee
recommended that reversible loss of lung function in combination
with the presence of symptoms should be considered as adverse''
(ATS, 2000).
---------------------------------------------------------------------------

    The 2013 ISA indicated that the pattern of effects observed across
the range of exposures assessed in the controlled human exposure
studies, increasing with severity at higher exposures, is coherent with
(i.e., reasonably related to) the health outcomes reported to be
associated with ambient air concentrations in epidemiologic studies
(e.g., respiratory-related hospital admissions, emergency department
visits). With regard to the available epidemiologic studies, while
analyses of O3 air quality in the 2014 PA indicated that
most O3 epidemiologic studies reported health effect
associations with O3 concentrations in ambient air that
violated the then-current (75 ppb) standard, the Administrator took
particular note of a study that reported associations

between short-term O3 concentrations and asthma emergency
department visits in children and adults in a U.S. location that would
have met the then-current standard over the entire 5-year study period
(80 FR 65344, October 26, 2015; Mar and Koenig, 2009).\31\ While
uncertainties limited the Administrator's conclusions on air quality in
locations of multicity epidemiologic studies,\32\ in looking across the
body of epidemiologic evidence, the Administrator reached the
conclusion that analyses of air quality in some study locations
supported the occurrence of adverse O3-associated effects at
O3 concentrations in ambient air that met, or are likely to
have met, the then-current standard (80 FR 65344, October 26, 2016).
Taken together, the Administrator concluded that the scientific
evidence from controlled human exposure and epidemiologic studies
called into question the adequacy of the public health protection
provided by the 75 ppb standard that had been set in 2008.
---------------------------------------------------------------------------

    \31\ The design values in this location over the study period
were at or somewhat below 75 ppb (Wells, 2012).
    \32\ Compared to the single-city epidemiologic studies, the
Administrator noted additional uncertainty that applied 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 (80
FR 65344; October 26, 2015).
---------------------------------------------------------------------------

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

    \33\ In addition to recognizing the potential for continued
inflammation to evolve into other outcomes, the 2013 ISA also
recognized that inflammation induced by a single exposure (or
several exposures over the course of a summer) can resolve entirely
(2013 ISA, p. 6-76; 80 FR 65331, October 26, 2015).
---------------------------------------------------------------------------

    In considering the exposure estimates from the 2014 HREA with
regard to public health implications, the Administrator concluded that
the exposures and risks projected to remain upon meeting the then-
current (75 ppb) standard could reasonably be judged to be important
from a public health perspective. In particular, this conclusion was
based on her judgment that it is appropriate to set a standard that
would be expected to eliminate, or almost eliminate, the occurrence of
exposures, while at moderate exertion, at or above 70 and 80 ppb (80 FR
65346, October 26, 2015). In addition, given that the average percent
of children estimated to experience two or more days with exposures at
or above the 60 ppb benchmark approaches 10% in some urban study areas
(on average across the analysis years), the Administrator concluded
that the then-current standard did not incorporate an adequate margin
of safety against the potentially adverse effects that could occur
following repeated exposures at or above 60 ppb (80 FR 65345-46,
October 26, 2015). Further, although the Administrator recognized
increased uncertainty in and placed less weight on the HREA estimates
for lung function risk and for the epidemiologic-study-based risk
analyses, she found them supportive of a conclusion that the
O3-associated health effects estimated to remain upon just
meeting the then-current standard are an issue of public health
importance on a broad national scale. Thus, she concluded that
O3 exposure and risk estimates, taken together, supported a
conclusion that the exposures and health risks associated with just
meeting the then-current standard could reasonably be judged to be of
public health significance, such that the then-current standard was not
sufficiently protective and did not incorporate an adequate margin of
safety.
    In consideration of all of the above, as well as the CASAC advice,
which included the unanimous recommendation ``that the Administrator
revise the current primary ozone standard to protect public health''
(Frey, 2014b, p. 5),\34\ the Administrator concluded that the then-
current primary O3 standard (with its level of 75 ppb) was
not requisite to protect public health with an adequate margin of
safety, and that it should be revised to provide increased public
health protection. This decision was based on the Administrator's
conclusions that the available evidence and exposure and risk
information clearly called into question the adequacy of public health
protection provided by the then-current primary standard such that it
was ``not appropriate, within the meaning of section 109(d)(1) of the
CAA, to retain the current standard'' (80 FR 65346, October 26, 2015).
---------------------------------------------------------------------------

    \34\ The Administrator also noted that CASAC for the prior,
2008, review likewise recommended revision of the standard to one
with a level below 75 ppb. This earlier recommendation was based
entirely on the evidence and information in the record for the 2008
decision, which had been expanded in the 2015 review (Samet, 2011;
Frey and Samet, 2012).
---------------------------------------------------------------------------

b. Considerations for the Revised Standard
    With regard to the most appropriate indicator for a revised
standard, the Administrator considered findings and assessments in the
2013 ISA and 2014 PA, as well as advice from the CASAC and public
comment. These include the finding that O3 is the only
photochemical oxidant (other than nitrogen dioxide) that is routinely
monitored and for which a comprehensive database exists, and the
consideration that, since the precursor emissions that lead to the
formation of O3 also generally lead to the formation of
other photochemical oxidants, measures leading to reductions in
population exposures to O3 can generally be expected to lead
to reductions in other photochemical oxidants (2013 ISA, section 3.6;
80 FR

65347, October 26, 2015). The CASAC indicated its view that
O3 is the appropriate indicator ``based on its causal or
likely causal associations with multiple adverse health outcomes and
its representation of a class of pollutants known as photochemical
oxidants'' (Frey, 2014c, p. ii). Based on all of these considerations
and public comments, the Administrator concluded that O3
remained the most appropriate indicator for a standard meant to provide
protection against photochemical oxidants in ambient air, and she
retained O3 as the indicator for the primary standard (80 FR
65347, October 26, 2015).
    The 8-hour averaging time for the primary O3 standard
was established in 1997 with the decision to replace the then-existing
1-hour standard with an 8-hour standard (62 FR 38856, July 18, 1997).
The decision in that review was based on evidence from numerous
controlled human exposure studies of healthy adults of adverse
respiratory effects resulting from 6- to 8-hour exposures, as well as
quantitative analyses indicating the control provided by an 8-hour
averaging time of both 8-hour and 1-hour peak exposures and associated
health risk (62 FR 38861, July 18, 1997; U.S. EPA, 1996b). The 1997
decision was also consistent with advice from the CASAC (62 FR 38861,
July 18, 1997; 61 FR 65727, December 13, 1996). The EPA reached similar
conclusions in the subsequent 2008 review in which the 8-hour averaging
time was retained (73 FR 16436, March 27, 2008). In the review
completed in 2015, the Administrator concluded, in consideration of the
then-available health effects information, that an 8-hour averaging
time remained appropriate for addressing health effects associated with
short-term exposures to ambient air O3 and that it could
effectively limit health effects attributable to both short- and long-
term O3 exposures (80 FR 65348, October 26, 2015). Thus, she
found it appropriate to retain this averaging time (80 FR 65350,
October 26, 2015).
    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 additionally considered
the appropriateness of retaining the nth-high metric as the form for
the revised standard (80 FR 65350-65352, October 26, 2015). In so
doing, she considered findings from prior reviews, including the 1997
review, in which it was recognized that a concentration-based form, by
giving proportionally more weight to years when 8-hour O3
concentrations are well above the level of the standard than years when
concentrations are just above the level, better reflects the continuum
of health effects associated with increasing O3
concentrations than does an expected exceedance form, which had been
the form of the standard prior to 1997.\35\ Although the subsequent
2008 review considered the potential value of a percentile-based form,
the EPA concluded at that time that, because of the differing lengths
of the monitoring season for O3 across the U.S., a
percentile-based statistic would not be effective in ensuring the same
degree of public health protection across the country (73 FR 16474-75,
March 27, 2008). The 2008 review additionally recognized the importance
of a form that provides stability to ongoing control programs and
insulation from the impacts of extreme meteorological events that are
conducive to O3 occurrence (73 FR 16474-16475, March 27,
2008). Based on all of these considerations, and including advice from
the CASAC, which stated that this form ``provides health protection
while allowing for atypical meteorological conditions that can lead to
abnormally high ambient ozone concentrations which, in turn, provides
programmatic stability'' (Frey, 2014b, p. 6), the 2015 decision was to
retain the existing form (the annual fourth-highest daily maximum 8-
hour O3 average concentration, averaged over three
consecutive years), without revision (80 FR 65352, October 26, 2015).
---------------------------------------------------------------------------

    \35\ With regard to a specific concentration-based form, the
fourth-highest daily maximum was selected in 1997, recognizing that
a less restrictive form (e.g., fifth highest) would allow a larger
percentage of sites to experience O3 peaks above the
level of the standard, and would allow more days on which the level
of the standard may be exceeded when the site attains the standard
(62 FR 38868-38873, July 18, 1997), and there was no basis
identified for selection of a more restrictive form (62 FR 38856,
July 18, 1997).
---------------------------------------------------------------------------

    The 2015 decision to set the level of the revised primary
O3 standard at 70 ppb built upon the Administrator's
conclusion (summarized in section II.A.1.a above) that the overall body
of scientific evidence and exposure/risk information called into
question the adequacy of the public health protection afforded by the
then-current standard, particularly for at-risk populations and
lifestages (80 FR 65362, October 26, 2015). In her decision on level,
the Administrator placed the greatest weight on the results of
controlled human exposure studies and on quantitative analyses based on
information from these studies, particularly analyses of O3
exposures of concern.\36\ In so doing, the Administrator noted that
controlled human exposure studies provide the most certain evidence
indicating the occurrence of health effects in humans following
specific O3 exposures, noting in particular that the effects
reported in the controlled human exposure studies are due solely to
O3 exposures, and are not complicated by the presence of co-
occurring pollutants or pollutant mixtures (as is the case in
epidemiologic studies). The Administrator's emphasis on the information
from the controlled human exposure studies was consistent with the
CASAC's advice and interpretation of the scientific evidence (80 FR
65362, October 26, 2015; Frey, 2014b). In this regard, the
Administrator recognized that: (1) The largest respiratory effects, and
the broadest range of effects, have been studied and reported following
exposures to 80 ppb O3 or higher (i.e., decreased lung
function, increased airway inflammation, increased respiratory
symptoms, airway hyperresponsiveness, and decreased lung host defense);
(2) exposures to O3 concentrations somewhat above 70 ppb
have been shown to both decrease lung function and to result in
respiratory symptoms; and (3) exposures to O3 concentrations
as low as 60 ppb have been shown to decrease lung function and to
increase airway inflammation (80 FR 65363, October 26, 2015). The
Administrator also considered both ATS recommendations and CASAC advice
to inform her judgments on the potential adversity to public health
associated with O3 effects reported in controlled human
exposure studies (80 FR 65363, October 26, 2015).\37\
---------------------------------------------------------------------------

    \36\ The Administrator viewed the results of the lung function
risk assessment, analyses of O3 air quality in locations
of epidemiologic studies, and epidemiologic-study-based quantitative
health risk assessment as being of less utility for selecting a
particular standard level among a range of options (80 FR 65362,
October 26, 2015).
    \37\ In so doing, the Administrator recognized that a standard
level of 70 ppb would be well below the O3 exposure
concentration documented to result in the widest range of
respiratory effects (i.e., 80 ppb), and below the lowest
O3 exposure concentration shown to result in the adverse
combination of lung function decrements and respiratory symptoms (80
FR 65363, October 26, 2015).
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    In considering the degree of protection provided by a revised
primary O3 standard, and the extent to which that standard
would be expected to limit population exposures to the broad range of
O3 exposures shown to result in health effects, the
Administrator considered the exposure estimates from the HREA, focusing
particularly on the estimates of two or more exposures of concern. In
so doing,

she placed the most emphasis on setting a standard that appropriately
limits repeated occurrences of exposures at or above the 70 and 80 ppb
benchmarks, while at elevated ventilation. She noted that a revised
standard with a level of 70 ppb was estimated to eliminate the
occurrence of two or more days with exposures at or above 80 ppb and to
virtually eliminate the occurrence of two or more days with exposures
at or above 70 ppb for all children and children with asthma, even in
the worst-case year and location evaluated.\38\ Given the considerable
protection provided against repeated exposures of concern for all
benchmarks evaluated in the HREA, the Administrator judged that a
standard with a level of 70 ppb incorporated a margin of safety against
the adverse O3-induced effects shown to occur in the
controlled human exposure studies (80 FR 65364, October 26, 2015).\39\
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    \38\ Under conditions just meeting an alternative standard with
a level of 70 ppb across the 15 urban study areas, the estimate for
two or more days with exposures at or above 70 ppb was 0.4% of
children, in the worst year and worst area (80 FR 65313, Table 1,
October 26, 2015).
    \39\ In so judging, she noted that the CASAC had recognized the
choice of a standard level within the range it recommended based on
the scientific evidence (which is inclusive of 70 ppb) to be a
policy judgment (80 FR 65355, October 26, 2015; Frey, 2014).
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    While she was less confident that adverse effects would occur
following exposures to O3 concentrations as low as 60
ppb,\40\ as discussed above, the Administrator also considered
estimates of exposures (while at moderate or greater exertion) for the
60 ppb benchmark (80 FR 65363-64, October 26, 2015). In so doing, she
recognized that while CASAC advice regarding the potential adversity of
effects observed in studies of 60 ppb was less definitive than for
effects observed at the next higher concentration studied, the CASAC
did clearly advise the EPA to consider the extent to which a revised
standard is estimated to limit the effects observed in studies of 60
ppb exposures (80 FR 65364, October 26, 2015; Frey, 2014b). The
Administrator's consideration of exposures at or above the 60 ppb
benchmark, and particularly consideration of multiple occurrences of
such exposures, was primarily in the context of considering the extent
to which the health protection provided by a revised standard included
a margin of safety against the occurrence of adverse O3-
induced effects (80 FR 65464, October 26, 2015). In this context, the
Administrator noted that a revised standard with a level of 70 ppb was
estimated to protect the vast majority of children in urban study areas
(i.e., about 96% to more than 99% of children in individual areas) from
experiencing two or more days with exposures at or above 60 ppb (while
at moderate or greater exertion). Compared to the estimates for the
then-current standard (with its level of 75 ppb), this represented a
reduction in repeated exposures 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 judged that a standard with a level of 70 ppb would
incorporate a margin of safety against the adverse O3-
induced effects shown to occur following exposures (while at moderate
or greater exertion) to a somewhat higher concentration. The
Administrator also judged the HREA results for one or more exposures at
or above 60 ppb to provide further support for her somewhat broader
conclusion that ``a standard with a level of 70 ppb would incorporate
an adequate margin of safety against the occurrence of O3
exposures that can result in effects that are adverse to public
health'' (80 FR 65364, October 26, 2015).\41\
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    \40\ The Administrator was ``notably less confident in the
adversity to public health of the respiratory effects that have been
observed following exposures to O3 concentrations as low
as 60 ppb,'' based on her consideration of the ATS recommendation on
judging adversity from transient lung function decrements alone, the
uncertainty in the potential for such decrements to increase the
risk of other, more serious respiratory effects in a population (per
ATS recommendations on population-level risk), and the less clear
CASAC advice regarding potential adversity of effects at 60 ppb
compared to higher concentrations studied (80 FR 65363, October 26,
2015).
    \41\ While the Administrator was less concerned about single
occurrences of O3 exposures of concern, especially for
the 60 ppb benchmark, she judged 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 noted
that ``a standard with a level of 70 ppb is estimated to (1)
virtually eliminate all occurrences of exposures of concern at or
above 80 ppb; (2) protect the vast majority of children in urban
study areas from experiencing any exposures of concern at or above
70 ppb (i.e., >= about 99%, based on mean estimates; Table 1); and
(3) to achieve substantial reductions, compared to the then-current
standard, in the occurrence of one or more exposures of concern at
or above 60 ppb (i.e., about a 50% reduction; Table 1)'' (80 FR
65364, October 26, 2015).
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    In the context of considering a standard with a level of 70 ppb,
the Administrator additionally considered the lung function risk
estimates, epidemiologic evidence and quantitative estimates based on
information from the epidemiologic studies. Although she placed less
weight on these estimates and information in light of associated
uncertainties,\42\ she judged that a standard with a level of 70 ppb
would be expected to result in important reductions in the population-
level risk of endpoints on which these types of information are focused
and provide associated additional public health protection, beyond that
provided by the then-current standard (80 FR 65364, October 26, 2015).
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    \42\ The Administrator noted important uncertainties in using
lung function risk estimates as a basis for considering the
occurrence of adverse effects in the population (also recognized in
the prior review) that limited her reliance on these estimates in
reaching judgments on health protection of a standard level of 70
ppb versus lower levels. Additionally, with regard to epidemiologic
studies, while the Administrator recognized there to be support for
a standard level at least as low as 70 ppb from a single-
epidemiologic study (Mar and Koenig, 2009) that reported health
effect associations in a location that met the then-current standard
over the entire study period but that would have violated a revised
standard with a level of 70 ppb, she found these studies to be of
more limited utility for distinguishing between the appropriateness
of health protection estimated for a standard level of 70 ppb and
that estimated for lower levels (80 FR 65364, October 26, 2015).
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    In summary, given her consideration of the evidence, exposure and
risk information, advice from the CASAC, and public comments, the
Administrator in 2015 judged a revised primary standard of 70 ppb, in
terms of the 3-year average of annual fourth-highest daily maximum 8-
hour average O3 concentrations, to be requisite to protect
public health, including the health of at-risk populations, with an
adequate margin of safety (80 FR 65365, October 26, 2015).
2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the
current primary O3 standard, or whether consideration of
revision is appropriate, the EPA has adopted an approach in this review
that builds upon the general approach used in the last review and
reflects the body of evidence and information now available.
Accordingly, the approach in this review takes into consideration the
approach used in the last review, addressing key policy-relevant
questions in light of currently available scientific and technical
information. As summarized above, the Administrator's decisions in the
prior review were based on an integration of O3 health
effects information with judgments on the adversity and public health
significance of key health effects, policy judgments as to when the
standard is requisite to protect public health with an adequate margin
of safety, consideration of CASAC advice, and consideration of public
comments.
    Similarly, in this review, we draw on the current evidence and
quantitative assessments of exposure pertaining to

the public health risk of O3 in ambient air. In considering
the scientific and technical information here, we consider both the
information available at the time of the last review and information
newly available since the last review, including that which has been
critically analyzed and characterized in the current ISA. The
quantitative exposure and risk analyses provide a context for
interpreting the evidence of respiratory effects in people breathing at
elevated rates and the potential public health significance of
exposures associated with air quality conditions that just meet the
current standard. The overarching purpose of these analyses is to
inform the Administrator's conclusions on the public health protection
afforded by the current primary standard, with an important focus on
the potential for exposures and risks beyond those indicated by the
information available at the time the standard was established.

B. Health Effects Information

    The information summarized here is based on our scientific
assessment of the health effects evidence available in this review;
this assessment is documented in the ISA and its policy implications
are further discussed in the PA. In this review, as in past reviews,
the health effects evidence evaluated in the ISA for O3 and
related photochemical oxidants is focused on O3 (ISA,
section IS.1.1). Ozone is concluded to be the most prevalent
photochemical oxidant present in the atmosphere and the one for which
there is a very large, well-established evidence base of its health and
welfare effects. Further, ``the primary literature evaluating the
health and ecological effects of photochemical oxidants includes ozone
almost exclusively as an indicator of photochemical oxidants'' (ISA,
section IS.1.1). Thus, the current health effects evidence and the
Agency's review of the evidence, including the evidence newly available
in this review, continues to focus on O3.
    More than 1600 studies are newly available and considered in the
ISA, including more than 1000 health studies (ISA, Appendix 10, Figure
10-2). As in the last review, the key evidence comes from the body of
controlled human exposure studies that document respiratory effects in
people exposed for short periods (6.6 to 8 hours) during quasi-
continuous exercise. Policy implications of the currently available
evidence are discussed in the PA (as summarized in section II.D.1
below). The subsections below briefly summarize the following aspects
of the evidence: The nature of O3-related health effects
(section II.B.1), the potential public health implications and
populations at risk (section II.B.2), and exposure concentrations
associated with health effects (section II.B.3).
1. Nature of Effects
    The evidence base available in the current review includes decades
of extensive evidence that clearly describes the role of O3
in eliciting an array of respiratory effects and recent evidence
suggests the potential for relationships between O3 exposure
and other effects. As was established in prior reviews, the most
commonly observed effects, and those for which the evidence is
strongest, are transient decrements in pulmonary function and
respiratory symptoms, such as coughing and pain on deep inspiration, as
a result of short-term exposures (ISA, section IS.4.3.1; 2013 ISA, p.
2-26). These effects are demonstrated in the large, long-standing
evidence base of controlled human exposure studies \43\ (1978 AQCD,
1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, ISA). The lung function
effects are also positively associated with ambient air O3
concentrations in epidemiologic panel studies, available in past
reviews, that describe these associations for outdoor workers and
children attending summer camps in the 1980s and 1990s (2013 ISA,
section <IP_ADDRESS>; ISA, Appendix 3, section <IP_ADDRESS>.3). The epidemiologic
evidence base additionally documents associations of O3
concentrations in ambient air with more severe health outcomes,
including asthma-related emergency department visits and hospital
admissions (2013 ISA, section 6.2.7; ISA, Appendix 3, sections <IP_ADDRESS>
and <IP_ADDRESS>). Extensive experimental animal evidence informs a detailed
understanding of mechanisms underlying the respiratory effects of
short-term exposures (ISA, Appendix 3, section 3.1.11), and studies in
animal models also provide evidence for effects of longer-term
O3 exposure on the developing lung (ISA, Appendix 3, section
3.2.6).
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    \43\ The vast majority of the controlled human exposure studies
(and all of the studies conducted at the lowest exposures) involved
young healthy adults (typically 18-13 years old) as study subjects
(2013 ISA, section <IP_ADDRESS>). There are also some controlled human
exposure studies of one to eight hours duration in older adults and
adults with asthma, and there are still fewer controlled human
exposure studies in healthy children (i.e., individuals aged younger
than 18 years) or children with asthma (See, for example, PA,
Appendix 3A, Table 3A-3).
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    The current evidence continues to support our prior conclusion that
short-term O3 exposure causes respiratory effects.
Specifically, the full body of evidence continues to support the
conclusion of a causal relationship of respiratory effects with short-
term O3 exposures and the conclusion that the relationship
of respiratory effects with longer-term exposures is likely to be
causal (ISA, sections IS.4.3.1 and IS.4.3.2). The current evidence base
for short-term O3 exposure and metabolic effects,\44\ which
was not evaluated as a separate category of effects in the last review
when less evidence was available, is expanded by evidence newly
available in this review. The ISA determines the current evidence
sufficient to conclude that the relationship between short-term
O3 exposure and metabolic effects is likely to be causal
(ISA, section IS.4.3.3). The newly available evidence is primarily from
experimental animal research. For other types of health effects, new
evidence has led to different conclusions from those reached in the
prior review. Specifically, the current evidence, particularly in light
of the additional controlled human exposure studies, is less consistent
than what was previously available and less indicative of
O3-induced cardiovascular effects. This evidence has altered
conclusions from the last review with regard to relationships between
short-term O3 exposures and cardiovascular effects and
mortality, such that the evidence is no longer concluded to indicate
that the relationships are likely to be causal.\45\ Thus, while
conclusions have changed for some effects based on the new evidence,
the conclusions reached in the last review on respiratory effects are
supported by the current evidence, and conclusions are also newly
reached for an additional category of health effects.
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    \44\ The term metabolic effects is used in the ISA to refer
metabolic syndrome (a collection of risk factors including high
blood pressure, elevated triglycerides and low high density
lipoprotein cholesterol), diabetes, metabolic disease mortality, and
indicators of metabolic syndrome that include alterations in glucose
and insulin homeostasis, peripheral inflammation, liver function,
neuroendocrine signaling, and serum lipids (ISA, section IS.4.3.3).
    \45\ The currently available evidence for cardiovascular,
reproductive and nervous system effects, as well as mortality, is
``suggestive of, but not sufficient to infer'' a causal relationship
with short- or long-term O3 exposures (ISA, Table IS-1).
The evidence is inadequate to infer the presence or absence of a
causal relationship between long-term O3 exposure and
cancer (ISA, section IS.<IP_ADDRESS>).
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a. Respiratory Effects
    As in the last review, the currently available evidence in this
review supports the conclusion of a causal relationship between short-
term O3 exposure and respiratory effects (ISA, section
IS.1.3.1). The strongest evidence for this comes from controlled human

exposure studies, also available in the last review, demonstrating
O3-related respiratory effects in generally healthy
adults.\46\ Experimental studies in animals also document an array of
respiratory effects resulting from short-term O3 exposure
and provide information related to underlying mechanisms (ISA, Appendix
3, section 3.1). The potential for O3 exposure to elicit
health outcomes more serious than those assessed in the controlled
human exposure studies continues to be indicated by the epidemiologic
evidence of associations of O3 concentrations in ambient air
with increased incidence of hospital admissions and emergency
department visits for an array of health outcomes, including asthma
exacerbation, COPD exacerbation, respiratory infection, and
combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5
and 3.1.6). The strongest such evidence is for asthma-related outcomes
and specifically asthma-related outcomes for children, indicating an
increased risk for people with asthma and particularly children with
asthma (ISA, Appendix 3, section <IP_ADDRESS>).
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    \46\ The phrases ``healthy adults'' or ``healthy subjects'' are
used to distinguish from subjects with asthma or other respiratory
diseases, for which there are many fewer controlled human exposure
studies. For studies of healthy subjects ``the study design
generally precludes inclusion of subjects with serious health
conditions,'' such as individuals with severe respiratory diseases
(2013 ISA, p. lx).
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    Respiratory responses observed in human subjects exposed to
O3 for periods of 8 hours or less, while intermittently or
quasi-continuously, exercising, include reduced lung function,\47\
respiratory symptoms, increased airway responsiveness, mild
bronchoconstriction (measured as an increase in specific airway
resistance [sRaw]), and pulmonary inflammation, with associated injury
and oxidative stress (ISA, Appendix 3, section 3.1.4; 2013 ISA,
sections 6.2.1 through 6.2.4). The available mechanistic evidence,
discussed in greater detail in the ISA, describes pathways involving
the respiratory and nervous systems by which O3 results in
pain-related respiratory symptoms and reflex inhibition of maximal
inspiration (inhaling a full, deep breath), commonly quantified by
decreases in forced vital capacity (FVC) and total lung capacity. This
reflex inhibition of inspiration combined with mild bronchoconstriction
contributes to the observed decrease in FEV1, the most
common metric used to assess O3-related lung function
effects. The evidence also indicates that the additionally observed
inflammatory response is correlated with mild airway obstruction,
generally measured as an increase in sRaw (ISA, Appendix 3, section
3.1.3). As described in section II.B.3 below, the prevalence and
severity of respiratory effects in controlled human exposure studies,
including symptoms (e.g., pain on deep inspiration, shortness of
breath, and cough), increases with increasing O3
concentration, exposure duration, and ventilation rate of exposed
subjects (ISA, Appendix 3, sections <IP_ADDRESS> and <IP_ADDRESS>).
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    \47\ In summarizing FEV1 responses from controlled
human exposure studies, an O3-induced change in
FEV1 is typically the difference between the change
observed with O3 exposure (post-exposure FEV1
minus pre-exposure FEV1) and what is generally an
improvement observed with filtered air (FA) exposure (post-exposure
FEV1 minus pre-exposure FEV1). As explained in
the 2013 ISA, ``[n]oting that some healthy individuals experience
small improvements while others have small decrements in
FEV1 following FA exposure, investigators have used the