[Federal Register Volume 85, Number 251 (Thursday, December 31, 2020)]
[Rules and Regulations]
[Pages 87256-87351]
[FR Doc No: 2020-28871]

Vol. 85

Thursday,

No. 251

December 31, 2020

Part IV

Environmental Protection Agency

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40 CFR Part 50

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

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

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

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

Review of the Ozone National Ambient Air Quality Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final action.

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

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

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

Availability of Information Related to This Action

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

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

Basis for Immediate Effective Date

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

Table of Contents

    The following topics are discussed in this preamble:

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

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

Executive Summary

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

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

I. Background

A. Legislative Requirements

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

and the kind and degree of uncertainties. The selection of any
particular approach to providing an adequate margin of safety is a
policy choice left specifically to the Administrator's judgment. See
Lead Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA,
744 F.3d at 1353.
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    \3\ As used here and similarly throughout this document, the
term population (or group) refers to persons having a quality or
characteristic in common, such as a specific pre-existing illness or
a specific age or life stage. As summarized in section II.A.2.c
below, the identification of sensitive groups (called at-risk groups
or at-risk populations) involves consideration of susceptibility and
vulnerability.
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    Section 109(d)(1) of the Act requires periodic review and, if
appropriate, revision of existing air quality criteria to reflect
advances in scientific knowledge concerning the effects of the
pollutant on public health and welfare. Under the same provision, the
EPA is also to periodically review and, if appropriate, revise the
NAAQS, based on the revised air quality criteria.\4\
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    \4\ This section of the Act requires the Administrator to
complete these reviews and make any revisions that may be
appropriate ``at five-year intervals.''
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    Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate . . .'' Since the
early 1980s, this independent review function has been performed by the
CASAC of the EPA's Science Advisory Board. A number of other advisory
functions are also identified for the committee by section
109(d)(2)(C), which reads:

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

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

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

C. History of the Air Quality Criteria and Standards

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

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

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

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

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

D. Current Review of the Air Quality Criteria and Standards

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

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

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

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

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

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

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

    The CASAC comments additionally provided advice with regard to the
primary and secondary standards, as well as a number of comments
intended to improve the PA. These comments were considered in
completing that document (85 FR 31182, May 22, 2020). The CASAC advice
to the Administrator regarding the O3 standards has also
been described and considered in the PA, and in sections II and III
below. The CASAC advice on the primary standard is summarized in II.B.2
below and its advice on the secondary standard is summarized in section
III.B.1.b.
    Materials upon which this proposed decision is based, including the
documents described above, are available to the public in the docket
for the review.\16\ As in prior NAAQS reviews, the EPA is basing its
decision in this review on studies and related information included in
the air quality criteria, which have undergone CASAC and public review.
The studies assessed in the ISA \17\ and PA, and the integration of the
scientific evidence presented in them, have undergone extensive
critical review by the EPA, the CASAC, and the public. The rigor of
that review makes these studies, and their integrative assessment, the
most reliable source of scientific information on which to base
decisions on the NAAQS, decisions that all parties recognize as of
great import. Decisions on the NAAQS can have profound impacts on
public health and welfare, and NAAQS decisions should be based on
studies that have been rigorously assessed in an integrative manner not
only by the EPA but also by the statutorily mandated independent
scientific advisory committee, as well as the public review that
accompanies this process. Some commenters have referred to and
discussed individual scientific studies on the health effects of
O3 that were not included in the ISA (`` `new' studies'')
and that have not gone through this comprehensive review process. In
considering and responding to comments for which such ``new'' studies
were cited in support, the EPA has provisionally considered the cited
studies in the context of the findings of the ISA. The EPA's
provisional consideration of these studies did not and could not
provide the kind of in-depth critical review described above, but
rather was focused on determining whether they warranted reopening the
review of the air quality criteria to enable the EPA, the CASAC and the
public to consider them further.
---------------------------------------------------------------------------

    \16\ The docket for this review, EPA-HQ-OAR-2018-0279, has
incorporated the ISA docket (EPA-HQ-ORD-2018-0274) by reference.
Both are publicly accessible at www.regulations.gov.
    \17\ In addition to the review's opening ``Call for
Information'' (83 FR 29785, June 26, 2018), systematic review
methodologies were applied to identify relevant scientific findings
that have emerged since the 2013 ISA, which included peer reviewed
literature published through July 2011. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2011 (providing some overlap with the
cutoff date for the last ISA) and March 30, 2018. Studies published
after the literature cutoff date for this ISA were also considered
if they were submitted in response to the Call for Information or
identified in subsequent phases of ISA development, particularly to
the extent that they provide new information that affects key
scientific conclusions (ISA, Appendix 10, section 10.2). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737.
---------------------------------------------------------------------------

    This approach, and the decision to rely on studies and related
information included in the air quality criteria, which have undergone
CASAC and public review, is consistent with the EPA's practice in prior
NAAQS reviews and its interpretation of the requirements of the CAA.
Since the 1970 amendments, the EPA has taken the view that NAAQS
decisions are to be based on scientific studies and related information
that have been assessed as a part of the pertinent air quality
criteria, and the EPA has consistently followed this approach. This
longstanding interpretation was strengthened by new legislative
requirements enacted in 1977, which added section 109(d)(2) of the Act
concerning CASAC review of air quality criteria. See 71 FR 61144, 61148
(October 17, 2006, final decision on review of NAAQS for particulate
matter) for a detailed discussion of this issue and the EPA's past
practice.
    As discussed in the EPA's 1993 decision not to revise the
O3 NAAQS, ``new'' studies may sometimes be of such
significance that it is appropriate to delay a decision in a NAAQS
review and to supplement the pertinent air quality criteria so the
studies can be taken into account (58 FR at 13013-13014, March 9,
1993). In the present case, the EPA's provisional consideration of
``new'' studies concludes that, taken in context, the ``new''
information and findings do not materially change any of the broad
scientific conclusions regarding the health and welfare effects of
O3 in

ambient air made in the air quality criteria. For this reason,
reopening the air quality criteria review would not be warranted.
    Accordingly, the EPA is basing the final decisions in this review
on the studies and related information included in the O3
air quality criteria that have undergone rigorous review by the EPA,
the CASAC and the public. The EPA will consider these ``new'' studies
for inclusion in the air quality criteria for the next O3
NAAQS review, which the EPA expects to begin soon after the conclusion
of this review and which will provide the opportunity to fully assess
these studies through a more rigorous review process involving the EPA,
the CASAC, and the public.

E. Air Quality Information

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

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

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

II. Rationale for Decision on the Primary Standard

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

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

A. Introduction

    As in prior reviews, the general approach to reviewing the current
primary standard is based, most fundamentally, on using the Agency's
assessments of the current scientific evidence and associated
quantitative analyses to inform the Administrator's judgment regarding
a primary standard for photochemical oxidants that is requisite to
protect the public health with an adequate margin of safety. The EPA's
assessments are primarily documented in the ISA and PA, both of which
have received CASAC review and public comment (84 FR 50836, September
26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019;
85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging
the gap between the scientific assessments of the ISA and the judgments
required of the Administrator in his decisions on the current standard,
the PA evaluates policy implications of the assessment of the current
evidence in ISA and the quantitative exposure and risk analyses
documented extensively in appendices of the PA. In evaluating the
public health protection afforded by the current standard, the four
basic elements of the NAAQS (indicator, averaging time, level, and
form) are considered collectively.
    The final decision on the adequacy of the current primary standard
is a public health policy judgment to be made by the Administrator. In
reaching conclusions on the standard, the decision draws on the
scientific information and analyses about health effects, population
exposure and risks, as well as judgments about how to consider the
range and magnitude of uncertainties that are inherent in the
scientific evidence and analyses. This approach is based on the
recognition that the available health effects evidence generally
reflects a continuum, consisting of levels at which scientists
generally agree that health effects are likely to occur, through lower
levels at which the likelihood and magnitude of the response become
increasingly uncertain. This approach is consistent with the
requirements of the NAAQS provisions of the Clean Air Act and with how
the EPA and the courts have historically interpreted the Act
(summarized in section I.A. above). These provisions require the
Administrator to establish primary standards that, in the judgment of
the Administrator, are requisite to protect public health with an
adequate margin of safety. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that primary
standards be set at a zero-risk level, but rather at a level that
avoids unacceptable risks to public health, including the health of
sensitive groups.\20\
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    \20\ As noted in section I.A above, consideration of such
protection is focused on the sensitive group of individuals and not
a single person in the sensitive group (see S. Rep. No. 91-1196,
91st Cong., 2d Sess. 10 [1970]).
---------------------------------------------------------------------------

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

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

    Across the different study types, the controlled human exposure
studies, which were recognized to provide the most certain evidence
indicating the occurrence of health effects in humans following
specific O3 exposures, additionally document the roles of
ventilation rate \26\ and exposure duration, in addition to exposure
concentration, in eliciting responses to O3 exposure (80 FR
65343, October 26, 2015; 2014 PA, section 3.4).\27\ These aspects of
the evidence were represented in exposure-based analyses developed to
inform the NAAQS decision with estimates of exposure and risk
associated with air quality conditions just meeting the then-existing
standard, and also for air quality conditions just meeting potential
alternative standards (U.S. EPA, 2014a, hereafter 2014 HREA). The
exposure-based analyses given greatest weight in the Administrator's
consideration of the HREA estimates involved comparison of estimates
for study area populations of children of exposure at elevated exertion
to exposure benchmark concentrations (exposures of concern). The
benchmark concentrations (60, 70 and 80 ppb) were identified from
controlled human exposure studies (conducted with generally healthy
adults).
---------------------------------------------------------------------------

    \26\ Ventilation rate (VE) is a specific technical
term referring to breathing rate in terms of volume of air taken
into the body per unit of time. A person engaged in different
activities will exert themselves at different levels and experience
different ventilation rates.
    \27\ For example, the exposure concentrations eliciting a given
level of response in subjects at rest are higher than those
eliciting such response in subjects exposed while at elevated
ventilation, such as while exercising (2013 ISA, section <IP_ADDRESS>).
---------------------------------------------------------------------------

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

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

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

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

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

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

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

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

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

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

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

    \37\ The Administrator viewed the results of other quantitative
analyses in this review--the lung function risk assessment, analyses
of O3 air quality in locations of epidemiologic studies,
and epidemiologic-study-based quantitative health risk assessment--
as being of less utility for selecting a particular standard level
among a range of options (80 FR 65362, October 26, 2015).
---------------------------------------------------------------------------

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

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

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

weight on the other HREA risk estimates and epidemiologic evidence for
considering the standard level, in light of associated uncertainties,
the Administrator judged that a standard with a level of 70 ppb would
be expected to result in important reductions in the population-level
risk of endpoints on which these types of information are focused and
provide associated additional public health protection, beyond that
provided by the then-existing standard (80 FR 65364, October 26, 2015).
In summary, based on the evidence, exposure and risk information,
advice from the CASAC, and public comments, the 2015 decision was to
revise the primary standard to be 70 ppb, in terms of the 3-year
average of annual fourth-highest daily maximum 8-hour average
O3 concentrations, to provide the requisite protection of
public health, including the health of at-risk populations, with an
adequate margin of safety (80 FR 65365, October 26, 2015).
---------------------------------------------------------------------------

    \40\ In so judging, she noted that the CASAC had recognized the
choice of a standard level within the range it recommended based on
the scientific evidence (which was inclusive of 70 ppb) to be a
policy judgment (80 FR 65355, October 26, 2015; Frey, 2014b).
    \41\ While the Administrator was less concerned about single
exposures, especially for the 60 ppb benchmark, she judged the HREA
one-or-more estimates informative to margin of safety
considerations. In this regard, she noted that ``a standard with a
level of 70 ppb is estimated to (1) virtually eliminate all
occurrences of exposures of concern at or above 80 ppb; (2) protect
the vast majority of children in urban study areas from experiencing
any exposures of concern at or above 70 ppb (i.e., >= about 99%,
based on mean estimates; Table 1); and (3) to achieve substantial
reductions, compared to the [then-]current standard, in the
occurrence of one or more exposures of concern at or above 60 ppb
(i.e., about a 50% reduction; Table 1)'' (80 FR 65364, October 26,
2015).
---------------------------------------------------------------------------

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

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

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

    \43\ The vast majority of the controlled human exposure studies
(and all of the studies conducted at the lowest exposures) involved
young healthy adults (typically 18-35 years old) as study subjects
(2013 ISA, section <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).
---------------------------------------------------------------------------

    The full body of evidence continues to support the conclusions of a
causal relationship of respiratory effects with short-term
O3 exposures and of a relationship of respiratory effects
with longer-term exposures that is likely to be causal (ISA, sections
IS.4.3.1 and IS.4.3.2). Further, the ISA determines that the
relationship between short-term O3 exposure and metabolic
effects \44\ is likely to be causal, based primarily on newly available
experimental animal evidence (ISA, section IS.4.3.3). The newly
available evidence, particularly from controlled human exposure studies
of cardiovascular endpoints, has altered conclusions from the last
review with regard to relationships between short-term O3
exposures and cardiovascular effects and mortality, such that the
evidence no longer supports conclusions that the relationships are
likely to be causal.\45\
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    \44\ The term metabolic effects is used in the ISA to refer
metabolic syndrome (a collection of risk factors including
alterations in glucose and insulin homeostasis, high blood pressure,
adiposity, elevated triglycerides and low high density lipoprotein
cholesterol), diabetes, metabolic disease mortality, and indicators
of metabolic syndrome that include peripheral inflammation, liver
function, neuroendocrine signaling, and serum lipids (ISA, section
IS.4.3.3).
    \45\ The currently available evidence for cardiovascular,
reproductive and nervous system effects, as well as mortality, is
``suggestive of, but not sufficient to infer'' a causal relationship
with short- or long-term O3 exposures (ISA, Table IS-1).
The evidence is inadequate to infer the presence or absence of a
causal relationship between long-term O3 exposure and
cancer (ISA, section IS.<IP_ADDRESS>).
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    With regard to respiratory effects from short-term O3
exposure, the strongest evidence comes from controlled human exposure
studies, also available in the last review, demonstrating
O3-related respiratory effects in generally healthy adults
(ISA, section IS.1.3.1).\46\ As in the last review, the key evidence
comes from the body of controlled human exposure studies that document
respiratory effects in people exposed for short periods (6.6 to 8
hours) during quasi-continuous exercise.\47\ The potential for
O3 exposure to elicit health outcomes more serious than
those assessed in the controlled human exposure studies continues to be
indicated by the epidemiologic evidence of associations of
O3 concentrations in ambient air with increased incidence of
hospital admissions and emergency department visits for an array of
health outcomes, including asthma exacerbation, chronic obstructive
pulmonary disease (COPD) exacerbation, respiratory infection, and
combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5
and 3.1.6). The strongest such evidence is for asthma-related outcomes
and specifically asthma-related outcomes for children, indicating an
increased risk for people with asthma and particularly children with
asthma (ISA, Appendix 3, section <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 because the ``the study design generally precludes
inclusion of subjects with serious health conditions,'' such as
individuals with severe respiratory diseases (2013 ISA, p. lx).
    \47\ A quasi-continuous exercise protocol is common to these
controlled exposure studies where study subjects complete six 50-
minute periods of exercise, each followed by 10-minute periods of
rest (e.g., ISA, Appendix 3, section <IP_ADDRESS>.1, and p. 3-11; 2013
ISA, section <IP_ADDRESS>).
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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 lung function decrements (e.g.,
based on forced expiratory volume in one second [FEV1]
measurements),\48\ 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 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>).
---------------------------------------------------------------------------

    \48\ In summarizing FEV1 responses from controlled
human exposure studies as ``decrements'', an O3-induced
change in FEV1 is typically the difference between the
change observed with O3 exposure ([post-exposure
FEV1 minus pre-exposure FEV1] divided by pre-
exposure FEV1) and what is generally an improvement
observed with filtered air (FA) exposure ([post-exposure
FEV1 minus pre-exposure FEV1] divided by pre-
exposure FEV1).
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    Within the evidence base from controlled human exposure studies,
the majority of studies involve healthy adult subjects (generally 18 to
35 years), although there are studies involving subjects with asthma,
and a limited number of studies, generally of durations shorter than
four hours, involving adolescents and adults older than 50 years. A
summary of salient observations of O3 effects on lung
function, based on the controlled human exposure study evidence
reviewed in the 1996 and 2006 AQCDs, and recognized in the 2013 ISA,
continues to pertain to this evidence base as it exists today: ``(1)
young healthy adults exposed to >=80 ppb ozone develop significant
reversible, transient decrements in pulmonary function and symptoms of
breathing discomfort if minute ventilation (Ve) or duration of exposure
is increased sufficiently; (2) relative to young adults, children
experience similar spirometric responses [i.e., as measured by
FEV1 and/or FVC] but lower incidence of symptoms from
O3 exposure; (3) relative to young adults, ozone-induced
spirometric responses are decreased in older individuals; (4) there is
a large degree of inter-subject variability in physiologic and
symptomatic responses to O3, but responses tend to be
reproducible within a given individual over a period of several months;
and (5) subjects exposed repeatedly to O3 for several days
experience an attenuation of spirometric and symptomatic responses on
successive exposures, which is lost after about a week without
exposure'' (ISA, Appendix 3, section <IP_ADDRESS>.1, p. 3-11).\49\ Repeated
daily exposure studies at higher concentrations, such as 300 ppb, have
found FEV1 responses to be enhanced on the second day of
exposure. This enhanced response is absent, however, with repeated
exposure at lower concentrations, perhaps as a result of a more
complete recovery or less damage to pulmonary tissues (2013 ISA,
section pp. 6-13 to 6-14; Folinsbee et al., 1994).
---------------------------------------------------------------------------

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

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

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

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

evidence on biological mechanisms (ISA, Appendix 3, sections 3.2.3 and
<IP_ADDRESS>.2).
    Overall, the respiratory effects evidence newly available in this
review is consistent with the evidence base in the last review,
supporting a generally similar understanding of the respiratory effects
of O3 (ISA, Appendix 3, section 3.1.4). A few recent studies
provide insights in previously unexamined areas, both with regard to
human study groups and animal models for different effects, while other
studies confirm and provide depth to prior findings with updated
protocols and techniques (ISA, Appendix 3, sections 3.1.11 and 3.2.6).
Newly available epidemiologic studies of hospital admissions and
emergency department visits for a variety of respiratory outcomes
supplement the previously available evidence with additional findings
of consistent associations with O3 concentrations across a
number of study locations (ISA, Appendix 3, sections <IP_ADDRESS>.3, 3.1.5,
<IP_ADDRESS>.1, <IP_ADDRESS> and 3.1.8). These studies include a number that
report positive associations for asthma-related outcomes, as well as a
few for COPD-related outcomes. Together these epidemiologic studies
continue to indicate the potential for O3 exposures to
contribute to such serious health outcomes, particularly for people
with asthma.
    As was the case for the evidence available in the last review, the
currently available evidence for health effects other than those of
O3 exposures on the respiratory system is more uncertain
than that for respiratory effects.\51\ Further, the evidence now
available has contributed to changes in conclusions for some of these
effects. For example, the current evidence for cardiovascular effects
and mortality, expanded from that in the last review, is no longer
considered sufficient to conclude that the relationships of short-term
exposure with these effects are likely to be causal (ISA, sections
IS.4.3.4 and IS.4.3.5). These changes stem from newly available
evidence in combination with the uncertainties recognized for the
evidence available in the last review.\52\ Although there exists
largely consistent evidence for a limited number of O3-
induced cardiovascular endpoints in animal toxicological studies and
cardiovascular mortality in epidemiologic studies, there is a general
lack of coherence between these results and findings in controlled
human exposure and epidemiologic studies of cardiovascular health
outcomes (ISA, section IS.1.3.1, Appendix 6, section 6.1.8). The
relationships are now characterized as suggestive of, but not
sufficient to infer, a causal relationship (ISA, Appendix 4, section
4.1.17; Appendix 6, section 6.1.8).
---------------------------------------------------------------------------

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

    With regard to metabolic effects of short-term O3
exposures, the evidence comes primarily from experimental animal study
findings, with a limited number of epidemiologic studies (ISA, section
IS.4.3.3 and Appendix 5, section 5.1.8 and Table 5-3). The exposure
conditions from the animal studies generally involve much higher
O3 concentrations (e.g., 4-hour concentrations of 400 to 800
ppb [ISA, Appendix 5, Tables 5-8 and 5-10]) than those commonly
occurring in areas of the U.S. where the current standard is met, and
the concentration in the available controlled human exposure study is
similarly high, at 300 ppb (ISA, sections 5.1.3, 5.1.5 and 5.1.8, Table
5-3). The evidence for metabolic effects and long-term exposures is
concluded to be suggestive of, but not sufficient to infer, a causal
relationship (ISA, section IS.<IP_ADDRESS>).
b. Public Health Implications and At-Risk Populations
    The public health implications of the evidence regarding
O3-related health effects, as for other effects, are
dependent on the type and severity of the effects, as well as the size
of the population affected. Judgments or interpretative statements
developed by public health experts, particularly experts in respiratory
health, also inform consideration of public health implications.
    With regard to O3 in ambient air, the potential public
health impacts relate most importantly to respiratory effects.
Controlled human exposure studies have documented reduced lung
function, respiratory symptoms, increased airway responsiveness, and
inflammation, among other effects, in healthy adults exposed while at
elevated ventilation, such as while exercising. Ozone effects in
individuals with compromised respiratory function, such as individuals
with asthma, are plausibly related to emergency department visits and
hospital admissions for asthma which have been associated with ambient
air concentrations of O3 in epidemiologic studies (as
summarized in section II.A.2.a above; 2013 ISA, section 6.2.7; ISA,
Appendix 3, sections <IP_ADDRESS> and <IP_ADDRESS>).
    The clinical significance of individual responses to O3
exposure depends on the health status of the individual, the magnitude
of the responses, the severity of respiratory symptoms, and the
duration of the response. While a particular reduction in
FEV1 or increase in inflammation or airway responsiveness
may not be of concern for a healthy group, it may increase the risk of
a more severe effect in a group with asthma. As a more specific
example, the same increase in inflammation or airway responsiveness in
individuals with asthma could predispose them to an asthma exacerbation
event triggered by an allergen to which they may be sensitized (e.g.,
ISA, Appendix 3, section <IP_ADDRESS>.1; 2013 ISA, sections 6.2.3 and
6.2.6). Duration and frequency of documented effects is also reasonably
expected to influence potential adversity and interference with normal
activity.\53\ In summary, consideration of differences in magnitude or
severity, and also the relative transience or persistence of the
responses (e.g., FEV1 changes) and respiratory symptoms, as
well as pre-existing sensitivity to effects on the respiratory system,
and other factors, are important to characterizing implications for
public health effects of an air pollutant such as O3 (ATS,
2000; Thurston et al., 2017).
---------------------------------------------------------------------------

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

    Decisions made in past reviews of the O3 primary
standard and associated judgments regarding adversity or health
significance of measurable physiological responses to air pollutants
have been informed by guidance, criteria or interpretative statements
developed within the public health community, including the ATS, an
organization of respiratory disease specialists, as well as

the advice from the CASAC. The ATS released its initial statement
(titled Guidelines as to What Constitutes an Adverse Respiratory Health
Effect, with Special Reference to Epidemiologic Studies of Air
Pollution) in 1985 and updated it in 2000 (ATS, 1985; ATS, 2000). The
ATS described its 2000 statement, considered in the last review of the
O3 standard, as being intended to ``provide guidance to
policy makers and others who interpret the scientific evidence on the
health effects of air pollution for the purposes of risk management''
(ATS, 2000). The recent statement further notes that it does not offer
``strict rules or numerical criteria, but rather proposes
considerations to be weighed in setting boundaries between adverse and
nonadverse health effects,'' providing a general framework for
interpreting evidence that proposes a ``set of considerations that can
be applied in forming judgments'' for this context (Thurston et al.,
2017). Similarly, in the 2000 statement, the ATS describes it as
proposing ``principles to be used in weighing the evidence and setting
boundaries'' and states that ``the placement of dividing lines should
be a societal judgment'' (ATS, 2000). The ATS explicitly states that it
does ``not attempt to provide an exact definition or fixed list of
health impacts that are, or are not, adverse,'' providing instead ``a
number of generalizable `considerations''' (ATS, 2000). The ATS state
there ``cannot be precise numerical criteria, as broad clinical
knowledge and scientific judgments, which can change over time, must be
factors in determining adversity'' (ATS, 2000).
    With regard to pulmonary function decrements, the earlier ATS
statement concluded that ``small transient changes in forced expiratory
volume in 1 s[econd] (FEV1) alone were not necessarily
adverse in healthy individuals but should be considered adverse when
accompanied by symptoms'' (ATS, 2000). The more recent ATS statement
continues to support this conclusion and also gives weight to findings
of small lung function changes in the absence of respiratory symptoms
in individuals with pre-existing compromised function, such as that