Document ID: EPA-HQ-OAR-2018-0279-0044
Agency: epa
Document Type: Proposed Rule
Title: Review of the Ozone National Ambient Air Quality Standards
Posted Date: 2020-08-14T04:00Z

[Federal Register Volume 85, Number 158 (Friday, August 14, 2020)]
[Proposed Rules]
[Pages 49830-49917]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-15453]

[[Page 49829]]

Vol. 85

Friday,

No. 158

August 14, 2020

Part V

Environmental Protection Agency

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40 CFR Part 50

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

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

[[Page 49830]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

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

Review of the Ozone National Ambient Air Quality Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed action.

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

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

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

FOR FURTHER INFORMATION CONTACT: For information or questions about the 
public hearing, please contact Ms. Regina Chappell, U.S. Environmental 
Protection Agency, Office of Air Quality Planning and Standards (OAQPS) 
(Mail Code C304-03), Research Triangle Park, NC 27711; telephone: (919) 
541-3650; email address: chappell.regina@epa.gov. For information or 
questions regarding the review of the O3 NAAQS, please 
contact Dr. Deirdre Murphy, Health and Environmental Impacts Division, 
Office of Air Quality Planning and Standards, U.S. Environmental 
Protection Agency, Mail Code C504-06, Research Triangle Park, NC 27711; 
telephone: (919) 541-0729; fax: (919) 541-0237; email: 
murphy.deirdre@epa.gov.

SUPPLEMENTARY INFORMATION: 

General Information

Participation in Virtual Public Hearings

    Please note that the EPA is deviating from its typical approach 
because the President has declared a national emergency. Due to the 
current Centers for Disease Control and Prevention (CDC) 
recommendations, as well as state and local orders for social 
distancing to limit the spread of COVID-19, the EPA cannot hold in-
person public meetings at this time. The EPA will begin pre-registering 
speakers for the hearings upon publication of this document in the 
Federal Register. To register to speak at a virtual hearing, please use 
the online registration form available at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution or contact Ms. Regina Chappell at (919) 541-3650 or by email 
at chappell.regina@epa.gov to register to speak at the virtual hearing. 
The last day to pre-register to speak at one of the hearings will be 
August 27, 2020. On August 28, 2020, the EPA will post a general agenda 
for the hearings that will list preregistered speakers in approximate 
order at: https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. The EPA will make every 
effort to follow the schedule as closely as possible on the day of each 
hearing; however, please plan for the hearing to run either ahead of 
schedule or behind schedule. Each commenter will have 5 minutes to 
provide oral testimony. The EPA may ask clarifying questions during the 
oral presentations but will not respond to the presentations at that 
time. The EPA encourages commenters to provide the EPA with a copy of 
their oral testimony electronically (via email) by emailing it to Dr. 
Deirdre Murphy and Ms. Regina Chappell. The EPA also recommends 
submitting the text of your oral testimony as written comments to the 
rulemaking docket. Written statements and supporting information 
submitted during the comment period will be considered with the same 
weight as oral testimony and supporting information presented at the 
public hearing. Please note that any updates made to any aspect of the 
hearing will be posted online at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. While the EPA expects the hearings to go forward as set 
forth above, please monitor our website or contact Ms. Regina Chappell 
at (919) 541-3650 or chappell.regina@epa.gov to determine if there are 
any updates. The EPA does not intend to publish a document in the 
Federal Register announcing updates. If you require the services of a 
translator or a special accommodation such as audio description, please 
preregister for the hearing with Ms. Regina Chappell and describe your 
needs by August 21, 2020. The EPA may not be able to arrange 
accommodations without advance notice.

Preparing Comments for the EPA

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

[[Page 49831]]

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

Availability of Information Related to This Action

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

Table of Contents

    The following topics are discussed in this preamble:

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

Executive Summary

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

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

[[Page 49833]]

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

I. Background

A. Legislative Requirements

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

[[Page 49834]]

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

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

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

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

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

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

[[Page 49835]]

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

    In light of the EPA's decision to consolidate the reconsideration 
with the review then ongoing, the D.C. Circuit proceeded with the 
litigation on the 2008 O3 NAAQS decision. On July 23, 2013, 
the court upheld the EPA's 2008 primary standard, but remanded the 2008 
secondary standard to the EPA. See Mississippi v. EPA, 744 F.3d 1334 
(D.C. Cir. 2013). With respect to the primary standard, the court 
rejected petitioners' arguments, upholding the EPA's decision. With 
respect to the secondary standard, the court held that the EPA's 
explanation for the setting of the secondary standard identical to the 
revised 8-hour primary standard was inadequate under the CAA because 
the EPA had not adequately explained how that standard provided the 
required public welfare protection.
    At the time of the court's decision, the EPA had already completed 
significant portions of its next statutorily required periodic review 
of the O3 NAAQS, which had been formally initiated in 2008, 
as summarized above. The documents developed for this review included 
the ISA,\8\ Risk and Exposure Assessments (REAs) for health and 
welfare, and PA.\9\ In late 2014, the EPA proposed to revise the 2008 
primary and secondary standards (79 FR 75234, December 17, 2014; Frey, 
2014a, Frey, 2014b, Frey, 2014c, U.S. EPA, 2014a, U.S. EPA, 2014b, U.S. 
EPA, 2014c). The EPA's final decision in this review was published in 
October 2015, establishing the now-current standards (80 FR 65292, 
October 26, 2015). In this decision, based on consideration of the 
health effects evidence on respiratory effects of O3 in at-
risk populations, the EPA revised the primary standard from a level of 
0.075 ppm to a level of 0.070 ppm, while retaining all other elements 
of the standard (80 FR 65292, October 26, 2015). The EPA's decision on 
the level for the standard was based on the weight of the scientific 
evidence and quantitative exposure/risk information. The level of the 
secondary standard was also revised from 0.075 ppm to 0.070 ppm based 
on the scientific evidence of O3 effects on welfare, 
particularly the evidence of O3 impacts on vegetation, and 
quantitative analyses available in the review.\10\ The other elements 
of the standard were retained. This decision on the secondary standard 
also incorporated the EPA's response to the D.C. Circuit's remand of 
the 2008 secondary standard in Mississippi v. EPA, 744 F.3d 1344 (D.C. 
Cir. 2013).\11\
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    \8\ The ISA serves the same purpose, in reviewing the air 
quality criteria, as the AQCD did in prior reviews.
    \9\ The PA presents an evaluation, for consideration by the 
Administrator, of the policy implications of the currently available 
scientific information, assessed in the ISA; the quantitative air 
quality, exposure or risk analyses presented in the PA and developed 
in light of the ISA findings; and related limitations and 
uncertainties. The role of the PA is to help ``bridge the gap'' 
between the Agency's scientific assessment and quantitative 
technical analyses, and the judgments required of the Administrator 
in his decisions in the review of the O3 NAAQS.
    \10\ These standards, set in 2015, are specified at 40 CFR 
50.19.
    \11\ The 2015 revisions to the NAAQS were accompanied by 
revisions to the data handling procedures, ambient air monitoring 
requirements, the air quality index and several provisions related 
to implementation (80 FR 65292, October 26, 2015).
---------------------------------------------------------------------------

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

[[Page 49836]]

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

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

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

[[Page 49837]]

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

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

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

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

D. Air Quality Information

    Ground level ozone concentrations are a mix of mostly tropospheric 
ozone and some stratospheric ozone. Tropospheric ozone is formed due to 
chemical interactions involving solar radiation and precursor 
pollutants including volatile organic compounds (VOCs) and nitrogen 
oxides (NOX). Methane (CH4) and carbon monoxide 
(CO) are also important precursors, particularly at the regional to 
global scale. The precursor emissions leading to tropospheric 
O3 formation can result from both man-made sources (e.g., 
motor vehicles and electric power generation) and natural sources 
(e.g., vegetation and wildfires). In addition, O3 that is 
created naturally in the stratosphere also contributes to O3 
levels near the surface. The stratosphere routinely mixes with the 
troposphere high above the earth's surface and, less frequently, there 
are intrusions of stratospheric air that reach deep into the 
troposphere and even to the surface. Once formed, O3 near 
the surface can be transported by winds before eventually being removed 
from the atmosphere via chemical reactions or deposition to surfaces. 
In sum, O3 concentrations are influenced by complex 
interactions between precursor emissions, meteorological conditions, 
and topographical characteristics (PA, section 2.1; ISA, Appendix 1).
    For compliance and other purposes, state and local environmental 
agencies operate O3 monitors across the U.S. and submit the 
data to the EPA. At present, there are approximately 1,300 monitors 
across the U.S. reporting hourly O3 averages during the 
times of the year when local O3 pollution can be important 
(PA, section 2.3.1).\15\ Most of this monitoring is focused on urban 
areas where precursor emissions tend to be largest, as well as 
locations directly downwind of these areas. There are also over 100 
routine monitoring sites in rural areas, including sites in the Clean 
Air Status and Trends Network (CASTNET) which is specifically focused 
on characterizing conditions in rural areas. Based on the monitoring 
data for the most recent 3-year period (2016-2018), the EPA identified 
142 counties, in which together approximately 106 million Americans 
reside where O3 design values \16\ were above 0.070, the 
level of the existing NAAQS (PA, section 2.4.1). Across these areas, 
the highest design values are typically observed in California, Texas, 
and the Northeast Corridor, locations with some of the most densely 
populated areas in the country (e.g., PA, Figure 2-8).
---------------------------------------------------------------------------

    \15\ O3 monitoring seasons vary by state from five 
months (May to September in Oregon and Washington) to all twelve 
months (in 11 states), with the most common season being March to 
October (in 27 states).
    \16\ A design value is a statistic that summarizes the air 
quality data for a given area in terms of the indicator, averaging 
time, and form of the standard. Design values can be compared to the 
level of the standard and are typically used to designate areas as 
meeting or not meeting the standard and assess progress towards 
meeting the NAAQS.
---------------------------------------------------------------------------

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

[[Page 49838]]

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

II. Rationale for Proposed Decision on the Primary Standard

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

    In presenting the rationale for the Administrator's proposed 
decision and its foundations, section II.A provides background and 
introductory information for this review of the primary O3 
standard. It includes background on the establishment of the current 
standard in 2015 (section II.A.1) and also describes the general 
approach for the current review (section II.A.2). Section II.B 
summarizes the currently available health effects evidence, focusing on 
consideration of key policy-relevant aspects. Section II.C summarizes 
the exposure and risk information for this review, drawing on the 
quantitative analyses for O3, presented in the PA. Section 
II.D presents the Administrator's proposed conclusions on the current 
standard (section II.D.3), drawing on both evidence-based and exposure/
risk-based considerations (section II.D.1) and advice from the CASAC 
(section II.D.2).

A. General Approach

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

[[Page 49839]]

health, including the health of sensitive groups.\18\
---------------------------------------------------------------------------

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

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

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

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

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

    With regard to the short-term respiratory effects that were the 
primary focus of the 2015 decision, the controlled human exposure 
studies were recognized to provide the most certain evidence indicating 
the occurrence of health effects in humans following specific 
O3 exposures (80 FR 65343, October 26, 2015; 2014 PA, 
section 3.4). These studies additionally illustrate the role of 
ventilation rate \22\ and exposure duration in eliciting responses to 
O3 exposure at the lowest studied concentrations. The 
exposure concentrations eliciting a given level of response in subjects 
at rest are higher than those eliciting a response in subjects exposed 
while at elevated ventilation, such as while exercising (2013 ISA, 
section 6.2.1.1).\23\
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    \22\ Ventilation rate (VE) is a specific technical 
term referring to breathing rate in terms of volume of air taken 
into the body per unit of time. The units for VE are 
usually liters (L) per minute (min). Another related term is 
equivalent ventilation rate (EVR), which refers to VE 
normalized by a person's body surface area in square meters (m\2\). 
Accordingly, the units for EVR are generally L/min-m\2\. For 
different activities, a person will experience different levels of 
exertion and different ventilation rates.
    \23\ In the controlled human exposure studies, the magnitude or 
severity of the respiratory effects induced by O3 is 
influenced by ventilation rate and exposure duration, as well as 
exposure concentration, with physical activity increasing 
ventilation and potential for effects. In studies of generally 
healthy adults exposed while at rest for 2 hours, 500 ppb is the 
lowest concentration eliciting a statistically significant 
O3-induced reduction in group mean lung function 
measures, while a much lower concentration produces such result when 
the study subject ventilation rates are sufficiently increased with 
exercise (2013 ISA, section 6.2.1.1). The lowest exposure 
concentration found to elicit a statistically significant 
O3-induced reduction in group mean lung function in an 
exposure of 2 hours or less was 120 ppb after a 1-hour exposure 
(continuous, very heavy exercise) of trained cyclists (2013 ISA, 
section 6.2.1.1; Gong et al., 1986) and after 2-hour exposure 
(intermittent heavy exercise) of young healthy adults (2013 ISA, 
section 6.2.1.1; McDonnell et al., 1983).
---------------------------------------------------------------------------

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

[[Page 49840]]

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

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

    The 2014 HREA developed exposure-based estimates for several 
population groups including all children and all adults. The type of 
exposure-based estimates that involved comparison of exposures to 
benchmarks was also derived for children with asthma and adults with 
asthma. The estimates of percentages of all children with exposures at 
or above benchmarks were virtually indistinguishable from the 
corresponding estimates for children with asthma.\27\ When considered 
in terms of the number of children (rather than percentages of the 
child populations), the estimates for all children were much higher 
than those for children with asthma, with the magnitude of the 
differences varying based on asthma prevalence in each study area (2014 
HREA, sections 5.3.2, 5.4.1.5 and section 5F-1). The estimates for 
percent of children experiencing an exposure at or above the benchmarks 
were higher than percent of adults due to the greater time that 
children spend outdoors and engaged in activities at elevated exertion 
(2014 HREA, section 5.3.2). Thus, consideration of the exposure-based 
results in the 2015 decision focused on the results for all children 
and children with asthma.
---------------------------------------------------------------------------

    \27\ This reflects use of the same time-location-activity diary 
pool to construct each simulated individual's time-activity series, 
which is based on the similarities observed in the available diary 
data with regard to time spent outdoors and exertion levels (2014 
HREA, sections 5.3.2 and 5.4.1.5).
---------------------------------------------------------------------------

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

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

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

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

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

[[Page 49841]]

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

    \31\ The design values in this location over the study period 
were at or somewhat below 75 ppb (Wells, 2012).
    \32\ Compared to the single-city epidemiologic studies, the 
Administrator noted additional uncertainty that applied specifically 
to interpreting air quality analyses within the context of multicity 
effect estimates for short-term O3 concentrations, where 
effect estimates for individual study cities are not presented (80 
FR 65344; October 26, 2015).
---------------------------------------------------------------------------

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

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

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

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

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

[[Page 49842]]

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

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

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

    \36\ The Administrator viewed the results of the lung function 
risk assessment, analyses of O3 air quality in locations 
of epidemiologic studies, and epidemiologic-study-based quantitative 
health risk assessment as being of less utility for selecting a 
particular standard level among a range of options (80 FR 65362, 
October 26, 2015).
    \37\ In so doing, the Administrator recognized that a standard 
level of 70 ppb would be well below the O3 exposure 
concentration documented to result in the widest range of 
respiratory effects (i.e., 80 ppb), and below the lowest 
O3 exposure concentration shown to result in the adverse 
combination of lung function decrements and respiratory symptoms (80 
FR 65363, October 26, 2015).
---------------------------------------------------------------------------

    In considering the degree of protection provided by a revised 
primary O3 standard, and the extent to which that standard 
would be expected to limit population exposures to the broad range of 
O3 exposures shown to result in health effects, the 
Administrator considered the exposure estimates from the HREA, focusing 
particularly on the estimates of two or more exposures of concern. In 
so doing,

[[Page 49843]]

she placed the most emphasis on setting a standard that appropriately 
limits repeated occurrences of exposures at or above the 70 and 80 ppb 
benchmarks, while at elevated ventilation. She noted that a revised 
standard with a level of 70 ppb was estimated to eliminate the 
occurrence of two or more days with exposures at or above 80 ppb and to 
virtually eliminate the occurrence of two or more days with exposures 
at or above 70 ppb for all children and children with asthma, even in 
the worst-case year and location evaluated.\38\ Given the considerable 
protection provided against repeated exposures of concern for all 
benchmarks evaluated in the HREA, the Administrator judged that a 
standard with a level of 70 ppb incorporated a margin of safety against 
the adverse O3-induced effects shown to occur in the 
controlled human exposure studies (80 FR 65364, October 26, 2015).\39\
---------------------------------------------------------------------------

    \38\ Under conditions just meeting an alternative standard with 
a level of 70 ppb across the 15 urban study areas, the estimate for 
two or more days with exposures at or above 70 ppb was 0.4% of 
children, in the worst year and worst area (80 FR 65313, Table 1, 
October 26, 2015).
    \39\ In so judging, she noted that the CASAC had recognized the 
choice of a standard level within the range it recommended based on 
the scientific evidence (which is inclusive of 70 ppb) to be a 
policy judgment (80 FR 65355, October 26, 2015; Frey, 2014).
---------------------------------------------------------------------------

    While she was less confident that adverse effects would occur 
following exposures to O3 concentrations as low as 60 
ppb,\40\ as discussed above, the Administrator also considered 
estimates of exposures (while at moderate or greater exertion) for the 
60 ppb benchmark (80 FR 65363-64, October 26, 2015). In so doing, she 
recognized that while CASAC advice regarding the potential adversity of 
effects observed in studies of 60 ppb was less definitive than for 
effects observed at the next higher concentration studied, the CASAC 
did clearly advise the EPA to consider the extent to which a revised 
standard is estimated to limit the effects observed in studies of 60 
ppb exposures (80 FR 65364, October 26, 2015; Frey, 2014b). The 
Administrator's consideration of exposures at or above the 60 ppb 
benchmark, and particularly consideration of multiple occurrences of 
such exposures, was primarily in the context of considering the extent 
to which the health protection provided by a revised standard included 
a margin of safety against the occurrence of adverse O3-
induced effects (80 FR 65464, October 26, 2015). In this context, the 
Administrator noted that a revised standard with a level of 70 ppb was 
estimated to protect the vast majority of children in urban study areas 
(i.e., about 96% to more than 99% of children in individual areas) from 
experiencing two or more days with exposures at or above 60 ppb (while 
at moderate or greater exertion). Compared to the estimates for the 
then-current standard (with its level of 75 ppb), this represented a 
reduction in repeated exposures of more than 60%. Given the 
considerable protection provided against repeated exposures of concern 
for all of the benchmarks evaluated, including the 60 ppb benchmark, 
the Administrator judged that a standard with a level of 70 ppb would 
incorporate a margin of safety against the adverse O3-
induced effects shown to occur following exposures (while at moderate 
or greater exertion) to a somewhat higher concentration. The 
Administrator also judged the HREA results for one or more exposures at 
or above 60 ppb to provide further support for her somewhat broader 
conclusion that ``a standard with a level of 70 ppb would incorporate 
an adequate margin of safety against the occurrence of O3 
exposures that can result in effects that are adverse to public 
health'' (80 FR 65364, October 26, 2015).\41\
---------------------------------------------------------------------------

    \40\ The Administrator was ``notably less confident in the 
adversity to public health of the respiratory effects that have been 
observed following exposures to O3 concentrations as low 
as 60 ppb,'' based on her consideration of the ATS recommendation on 
judging adversity from transient lung function decrements alone, the 
uncertainty in the potential for such decrements to increase the 
risk of other, more serious respiratory effects in a population (per 
ATS recommendations on population-level risk), and the less clear 
CASAC advice regarding potential adversity of effects at 60 ppb 
compared to higher concentrations studied (80 FR 65363, October 26, 
2015).
    \41\ While the Administrator was less concerned about single 
occurrences of O3 exposures of concern, especially for 
the 60 ppb benchmark, she judged that estimates of one or more 
exposures of concern can provide further insight into the margin of 
safety provided by a revised standard. In this regard, she noted 
that ``a standard with a level of 70 ppb is estimated to (1) 
virtually eliminate all occurrences of exposures of concern at or 
above 80 ppb; (2) protect the vast majority of children in urban 
study areas from experiencing any exposures of concern at or above 
70 ppb (i.e., >= about 99%, based on mean estimates; Table 1); and 
(3) to achieve substantial reductions, compared to the then-current 
standard, in the occurrence of one or more exposures of concern at 
or above 60 ppb (i.e., about a 50% reduction; Table 1)'' (80 FR 
65364, October 26, 2015).
---------------------------------------------------------------------------

    In the context of considering a standard with a level of 70 ppb, 
the Administrator additionally considered the lung function risk 
estimates, epidemiologic evidence and quantitative estimates based on 
information from the epidemiologic studies. Although she placed less 
weight on these estimates and information in light of associated 
uncertainties,\42\ she judged that a standard with a level of 70 ppb 
would be expected to result in important reductions in the population-
level risk of endpoints on which these types of information are focused 
and provide associated additional public health protection, beyond that 
provided by the then-current standard (80 FR 65364, October 26, 2015).
---------------------------------------------------------------------------

    \42\ The Administrator noted important uncertainties in using 
lung function risk estimates as a basis for considering the 
occurrence of adverse effects in the population (also recognized in 
the prior review) that limited her reliance on these estimates in 
reaching judgments on health protection of a standard level of 70 
ppb versus lower levels. Additionally, with regard to epidemiologic 
studies, while the Administrator recognized there to be support for 
a standard level at least as low as 70 ppb from a single-
epidemiologic study (Mar and Koenig, 2009) that reported health 
effect associations in a location that met the then-current standard 
over the entire study period but that would have violated a revised 
standard with a level of 70 ppb, she found these studies to be of 
more limited utility for distinguishing between the appropriateness 
of health protection estimated for a standard level of 70 ppb and 
that estimated for lower levels (80 FR 65364, October 26, 2015).
---------------------------------------------------------------------------

    In summary, given her consideration of the evidence, exposure and 
risk information, advice from the CASAC, and public comments, the 
Administrator in 2015 judged a revised primary standard of 70 ppb, in 
terms of the 3-year average of annual fourth-highest daily maximum 8-
hour average O3 concentrations, to be requisite to protect 
public health, including the health of at-risk populations, with an 
adequate margin of safety (80 FR 65365, October 26, 2015).
2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the 
current primary O3 standard, or whether consideration of 
revision is appropriate, the EPA has adopted an approach in this review 
that builds upon the general approach used in the last review and 
reflects the body of evidence and information now available. 
Accordingly, the approach in this review takes into consideration the 
approach used in the last review, addressing key policy-relevant 
questions in light of currently available scientific and technical 
information. As summarized above, the Administrator's decisions in the 
prior review were based on an integration of O3 health 
effects information with judgments on the adversity and public health 
significance of key health effects, policy judgments as to when the 
standard is requisite to protect public health with an adequate margin 
of safety, consideration of CASAC advice, and consideration of public 
comments.
    Similarly, in this review, we draw on the current evidence and 
quantitative assessments of exposure pertaining to

[[Page 49844]]

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

B. Health Effects Information

    The information summarized here is based on our scientific 
assessment of the health effects evidence available in this review; 
this assessment is documented in the ISA and its policy implications 
are further discussed in the PA. In this review, as in past reviews, 
the health effects evidence evaluated in the ISA for O3 and 
related photochemical oxidants is focused on O3 (ISA, 
section IS.1.1). Ozone is concluded to be the most prevalent 
photochemical oxidant present in the atmosphere and the one for which 
there is a very large, well-established evidence base of its health and 
welfare effects. Further, ``the primary literature evaluating the 
health and ecological effects of photochemical oxidants includes ozone 
almost exclusively as an indicator of photochemical oxidants'' (ISA, 
section IS.1.1). Thus, the current health effects evidence and the 
Agency's review of the evidence, including the evidence newly available 
in this review, continues to focus on O3.
    More than 1600 studies are newly available and considered in the 
ISA, including more than 1000 health studies (ISA, Appendix 10, Figure 
10-2). As in the last review, the key evidence comes from the body of 
controlled human exposure studies that document respiratory effects in 
people exposed for short periods (6.6 to 8 hours) during quasi-
continuous exercise. Policy implications of the currently available 
evidence are discussed in the PA (as summarized in section II.D.1 
below). The subsections below briefly summarize the following aspects 
of the evidence: The nature of O3-related health effects 
(section II.B.1), the potential public health implications and 
populations at risk (section II.B.2), and exposure concentrations 
associated with health effects (section II.B.3).
1. Nature of Effects
    The evidence base available in the current review includes decades 
of extensive evidence that clearly describes the role of O3 
in eliciting an array of respiratory effects and recent evidence 
suggests the potential for relationships between O3 exposure 
and other effects. As was established in prior reviews, the most 
commonly observed effects, and those for which the evidence is 
strongest, are transient decrements in pulmonary function and 
respiratory symptoms, such as coughing and pain on deep inspiration, as 
a result of short-term exposures (ISA, section IS.4.3.1; 2013 ISA, p. 
2-26). These effects are demonstrated in the large, long-standing 
evidence base of controlled human exposure studies \43\ (1978 AQCD, 
1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, ISA). The lung function 
effects are also positively associated with ambient air O3 
concentrations in epidemiologic panel studies, available in past 
reviews, that describe these associations for outdoor workers and 
children attending summer camps in the 1980s and 1990s (2013 ISA, 
section 6.2.1.2; ISA, Appendix 3, section 3.1.4.1.3). The epidemiologic 
evidence base additionally documents associations of O3 
concentrations in ambient air with more severe health outcomes, 
including asthma-related emergency department visits and hospital 
admissions (2013 ISA, section 6.2.7; ISA, Appendix 3, sections 3.1.5.1 
and 3.1.5.2). Extensive experimental animal evidence informs a detailed 
understanding of mechanisms underlying the respiratory effects of 
short-term exposures (ISA, Appendix 3, section 3.1.11), and studies in 
animal models also provide evidence for effects of longer-term 
O3 exposure on the developing lung (ISA, Appendix 3, section 
3.2.6).
---------------------------------------------------------------------------

    \43\ The vast majority of the controlled human exposure studies 
(and all of the studies conducted at the lowest exposures) involved 
young healthy adults (typically 18-13 years old) as study subjects 
(2013 ISA, section 6.2.1.1). There are also some controlled human 
exposure studies of one to eight hours duration in older adults and 
adults with asthma, and there are still fewer controlled human 
exposure studies in healthy children (i.e., individuals aged younger 
than 18 years) or children with asthma (See, for example, PA, 
Appendix 3A, Table 3A-3).
---------------------------------------------------------------------------

    The current evidence continues to support our prior conclusion that 
short-term O3 exposure causes respiratory effects. 
Specifically, the full body of evidence continues to support the 
conclusion of a causal relationship of respiratory effects with short-
term O3 exposures and the conclusion that the relationship 
of respiratory effects with longer-term exposures is likely to be 
causal (ISA, sections IS.4.3.1 and IS.4.3.2). The current evidence base 
for short-term O3 exposure and metabolic effects,\44\ which 
was not evaluated as a separate category of effects in the last review 
when less evidence was available, is expanded by evidence newly 
available in this review. The ISA determines the current evidence 
sufficient to conclude that the relationship between short-term 
O3 exposure and metabolic effects is likely to be causal 
(ISA, section IS.4.3.3). The newly available evidence is primarily from 
experimental animal research. For other types of health effects, new 
evidence has led to different conclusions from those reached in the 
prior review. Specifically, the current evidence, particularly in light 
of the additional controlled human exposure studies, is less consistent 
than what was previously available and less indicative of 
O3-induced cardiovascular effects. This evidence has altered 
conclusions from the last review with regard to relationships between 
short-term O3 exposures and cardiovascular effects and 
mortality, such that the evidence is no longer concluded to indicate 
that the relationships are likely to be causal.\45\ Thus, while 
conclusions have changed for some effects based on the new evidence, 
the conclusions reached in the last review on respiratory effects are 
supported by the current evidence, and conclusions are also newly 
reached for an additional category of health effects.
---------------------------------------------------------------------------

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

a. Respiratory Effects
    As in the last review, the currently available evidence in this 
review supports the conclusion of a causal relationship between short-
term O3 exposure and respiratory effects (ISA, section 
IS.1.3.1). The strongest evidence for this comes from controlled human

[[Page 49845]]

exposure studies, also available in the last review, demonstrating 
O3-related respiratory effects in generally healthy 
adults.\46\ Experimental studies in animals also document an array of 
respiratory effects resulting from short-term O3 exposure 
and provide information related to underlying mechanisms (ISA, Appendix 
3, section 3.1). The potential for O3 exposure to elicit 
health outcomes more serious than those assessed in the controlled 
human exposure studies continues to be indicated by the epidemiologic 
evidence of associations of O3 concentrations in ambient air 
with increased incidence of hospital admissions and emergency 
department visits for an array of health outcomes, including asthma 
exacerbation, COPD exacerbation, respiratory infection, and 
combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5 
and 3.1.6). The strongest such evidence is for asthma-related outcomes 
and specifically asthma-related outcomes for children, indicating an 
increased risk for people with asthma and particularly children with 
asthma (ISA, Appendix 3, section 3.1.5.7).
---------------------------------------------------------------------------

    \46\ The phrases ``healthy adults'' or ``healthy subjects'' are 
used to distinguish from subjects with asthma or other respiratory 
diseases, for which there are many fewer controlled human exposure 
studies. For studies of healthy subjects ``the study design 
generally precludes inclusion of subjects with serious health 
conditions,'' such as individuals with severe respiratory diseases 
(2013 ISA, p. lx).
---------------------------------------------------------------------------

    Respiratory responses observed in human subjects exposed to 
O3 for periods of 8 hours or less, while intermittently or 
quasi-continuously, exercising, include reduced lung function,\47\ 
respiratory symptoms, increased airway responsiveness, mild 
bronchoconstriction (measured as an increase in specific airway 
resistance [sRaw]), and pulmonary inflammation, with associated injury 
and oxidative stress (ISA, Appendix 3, section 3.1.4; 2013 ISA, 
sections 6.2.1 through 6.2.4). The available mechanistic evidence, 
discussed in greater detail in the ISA, describes pathways involving 
the respiratory and nervous systems by which O3 results in 
pain-related respiratory symptoms and reflex inhibition of maximal 
inspiration (inhaling a full, deep breath), commonly quantified by 
decreases in forced vital capacity (FVC) and total lung capacity. This 
reflex inhibition of inspiration combined with mild bronchoconstriction 
contributes to the observed decrease in FEV1, the most 
common metric used to assess O3-related lung function 
effects. The evidence also indicates that the additionally observed 
inflammatory response is correlated with mild airway obstruction, 
generally measured as an increase in sRaw (ISA, Appendix 3, section 
3.1.3). As described in section II.B.3 below, the prevalence and 
severity of respiratory effects in controlled human exposure studies, 
including symptoms (e.g., pain on deep inspiration, shortness of 
breath, and cough), increases with increasing O3 
concentration, exposure duration, and ventilation rate of exposed 
subjects (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).
---------------------------------------------------------------------------

    \47\ In summarizing FEV1 responses from controlled 
human exposure studies, an O3-induced change in 
FEV1 is typically the difference between the change 
observed with O3 exposure (post-exposure FEV1 
minus pre-exposure FEV1) and what is generally an 
improvement observed with filtered air (FA) exposure (post-exposure 
FEV1 minus pre-exposure FEV1). As explained in 
the 2013 ISA, ``[n]oting that some healthy individuals experience 
small improvements while others have small decrements in 
FEV1 following FA exposure, investigators have used the 
randomized, crossover design with each subject serving as their own 
control (exposure to FA) to discern relatively small effects with 
certainty since alternative explanations for these effects are 
controlled for by the nature of the experimental design'' (2013 ISA, 
pp. 6-4 to 6-5).
---------------------------------------------------------------------------

    Within the evidence base from controlled human exposure studies, 
the majority of studies involve healthy adult subjects (generally 18 to 
35 years), although there are studies involving subjects with asthma, 
and a limited number of studies, generally of durations shorter than 
four hours, involving adolescents and adults older than 50 years. A 
summary of salient observations of O3 effects on lung 
function, based on the controlled human exposure study evidence 
reviewed in the 1996 and 2006 AQCDs, and recognized in the 2013 ISA, 
continues to pertain to this evidence base as it exists today: ``(1) 
young healthy adults exposed to >=80 ppb ozone develop significant 
reversible, transient decrements in pulmonary function and symptoms of 
breathing discomfort if minute ventilation (Ve) or duration of exposure 
is increased sufficiently; (2) relative to young adults, children 
experience similar spirometric responses [i.e., as measured by 
FEV1 and/or FVC] but lower incidence of symptoms from 
O3 exposure; (3) relative to young adults, ozone-induced 
spirometric responses are decreased in older individuals; (4) there is 
a large degree of inter-subject variability in physiologic and 
symptomatic responses to O3, but responses tend to be 
reproducible within a given individual over a period of several months; 
and (5) subjects exposed repeatedly to O3 for several days 
experience an attenuation of spirometric and symptomatic responses on 
successive exposures, which is lost after about a week without 
exposure'' (ISA, Appendix 3, section 3.1.4.1.1, p. 3-11).\48\
---------------------------------------------------------------------------

    \48\ 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).
---------------------------------------------------------------------------

    The evidence is most well established with regard to the effects, 
reversible with the cessation of exposure, that are associated with 
short-term exposures of several hours. For example, the evidence 
indicates a rapid recovery from O3-induced lung function 
decrements (e.g., reduced FEV1) and respiratory symptoms 
(2013 ISA, section 6.2.1.1). However, in some cases, such as after 
exposure to higher concentrations such as 300 ppb, the recovery phase 
may be slower and involve a longer time period (e.g., at least 24 
hours). Repeated daily exposure studies at such higher concentrations 
also have found FEV1 response 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).
    With regard to airway inflammation and the potential for repeated 
occurrences to contribute to further effects, 2013 ISA indicates that 
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). 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 (ISA, Appendix 3, section 3.1.4.3.1).
    The extensive evidence base for O3 health effects, 
compiled over several decades, continues to indicate respiratory 
responses to short exposures as the most sensitive effects of 
O3. Such

[[Page 49846]]

effects are well documented in controlled human exposure studies, most 
of which involve healthy adult study subjects. These studies have 
documented an array of respiratory effects, including reduced lung 
function, respiratory symptoms, increased airway responsiveness, and 
inflammation, in study subjects following 1- to 8-hour exposures, 
primarily while exercising. Such effects 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, due to the latter characteristic, 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, p. IS-22; 2013 ISA, section 6.2.9; 
2006 AQCD, section 8.4.2). Further, 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 (as recognized below in sections II.B.2 and II.C). The 
increased significance of effects in people with asthma and risk of 
increased exposure for children is illustrated by the epidemiologic 
findings of positive associations between O3 exposure and 
asthma-related ED 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.
    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 for O3 exposures 
to increase susceptibility to infectious diseases through effects on 
defense mechanisms of the respiratory tract (ISA, section 3.1.7.3; 2013 
ISA, section 6.2.5). The evidence base regarding respiratory infections 
and associated effects has been augmented in this review by a number of 
epidemiologic studies reporting positive associations between short-
term O3 concentrations and emergency department visits for a 
variety of respiratory infection endpoints (ISA, Appendix 3, section 
3.1.7).
    Although the long-term exposure conditions that may contribute to 
further respiratory effects are less well understood, the conclusion 
based on the current evidence base remains that the relationship for 
such exposure conditions with respiratory effects is likely to be 
causal (ISA, section IS.4.3.2). Most notably, 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, Appendix 3, sections 3.2.4.1.3 and 3.2.6). The 
biological plausibility of such a role for O3 has been 
indicated by animal toxicological evidence on biological mechanisms 
(ISA, Appendix 3, sections 3.2.3 and 3.2.4.1.2). Specifically, the 
animal evidence, including the nonhuman primate studies of early life 
O3 exposure, indicates that such exposures can cause 
``structural and functional changes that could potentially contribute 
to airway obstruction and increased airway responsiveness,'' which are 
hallmarks of asthma (ISA, Appendix 3, section 3.2.6, p. 3-113).
    Overall, the respiratory effects evidence newly available in this 
review is generally consistent with the evidence base in the last 
review (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). 
Thus, our current understanding of the respiratory effects of 
O3 is similar to that in the last review.
    One aspect of the evidence that has been augmented concerns 
pulmonary function in adults older than 50 years of age. Previously 
available evidence in this age group indicated smaller O3-
related decrements in middle-aged adults (35 to 60 years) than in 
adults 35 years of age and younger (2006 AQCD, p. 6-23; 2013 ISA, p. 6-
22; ISA, Appendix 3, section 3.1.4.1.1.2). A recent multicenter study 
of 55- to 70-year old subjects (average age of 60 years), conducted for 
a 3-hour duration involving alternating 15-minute rest and exercise 
periods and a 120 ppb exposure concentration, reported a statistically 
significant O3 FEV1 response (ISA, Appendix 3, 
section 3.1.4.1.1.2; Arjomandi et al., 2018). While there is not a 
study in younger adults of precisely comparable design, the mean 
response for the 55- to 70-year olds, 1.2% O3-related 
FEV1 decrement, is lower than results for somewhat 
comparable exposures in adults aged 18 to 35 years, suggesting somewhat 
reduced responses to O3 exposure in this older age group 
(ISA, Appendix 3, section 3.1.4.1.1.2; Arjomandi et al., 2018; Adams, 
2000; Adams, 2006b).\49\ Such a reduced response in middle-aged and 
older adults compared to young adults is consistent with conclusions in 
previous reviews (2013 ISA, section 6.2.1.1; 2006 AQCD, section 6.4).
---------------------------------------------------------------------------

    \49\ For the same exposure concentration of 120 ppb, Adams 
(2006b) observed an average 3.2%, statistically significant, 
O3-related FEV1 decrement in young adults 
(average age 23 years) at the end of the third hour of an 8-hour 
protocol that alternated 30 minutes of exercise and rest, with the 
equivalent ventilation rate (EVR) averaging 20 L/min-m\2\ during the 
exercise periods (versus 15 to 17 L/min-m\2\ in.Arjomandi et 
al.[2018]). For the same concentration with a lower EVR during 
exercise (17 L/min-m\2\), although with more exercise, Adams (2000) 
observed a 4%, statistically significant, O3-related 
FEV1 decrement in young adults (average age 22 years) 
after the third hour of a 6.6-hour protocol (alternating 50 minutes 
exercise and 10 minutes rest).
---------------------------------------------------------------------------

    The strongest evidence of O3-related health effects, as 
was the case in the last review, continues to be that for respiratory 
effects of O3 (ISA, section ES.4.1). Among the newly 
available studies, there are several controlled human exposure studies 
that investigated lung function effects of higher exposure 
concentrations (e.g., 100 to 300 ppb) in healthy individuals younger 
than 35 years old, with findings generally consistent with previous 
studies (ISA, Appendix 3, section 3.1.4.1.1.2, p. 3-17). No studies are 
newly available in this review of 6.6-hour controlled human exposures 
(with exercise) to O3 concentrations below those previously 
studied.\50\ The newly available animal toxicological studies augment 
the previously available information concerning mechanisms underlying 
the effects documented in experimental studies. Newly available 
epidemiologic studies of hospital admissions and emergency department 
visits for a variety of respiratory outcomes supplement the previously 
available evidence with additional findings of consistent associations 
with O3 concentrations across a number of study locations 
(ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5, 3.1.6.1.1, 3.1.7.1 and 
3.1.8). These studies include a number that report positive 
associations for asthma-related outcomes, as well as a few for COPD-
related outcomes. Together these studies in the current epidemiologic 
evidence base continue to indicate the potential for O3 
exposures to contribute to such serious health outcomes, particularly 
for people with asthma.
---------------------------------------------------------------------------

    \50\ The recent 3-hour study of 55- to 70-year old subjects 
included a target exposure of 70 ppb, as well as 120 ppb, with only 
the latter eliciting a statistically significant FEV1 
decrement in this age group of subjects (ISA, Appendix 3, section 
3.1.4.1.1.2).

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[[Page 49847]]

b. Other Effects
    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. For some of these other categories 
of effects, the evidence now available has contributed to changes in 
conclusions reached in the last review. 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. Additionally, 
newly available evidence has also led to conclusions for another 
category, metabolic effects, for which formal causal determinations 
were previously not articulated.
    The ISA finds the evidence for metabolic effects sufficient to 
conclude that the relationship with short-term O3 exposures 
is likely to be causal (ISA, section IS.4.3.3). The evidence of 
metabolic effects of O3 comes primarily from experimental 
animal study findings that short-term O3 exposure can impair 
glucose tolerance, increase triglyceride levels and elicit fasting 
hyperglycemia, and increase hepatic gluconeogenesis (ISA, Appendix 5, 
section 5.1.8 and Table 5-3). The exposure conditions from these 
studies generally involve much higher O3 concentrations than 
those commonly occurring in areas of the U.S. where the current 
standard is met. For example, the animal studies include 4-hour 
concentrations of 400 to 800 ppb (ISA, Appendix 5, Tables 5-8 and 5-
10). The concentration in the available controlled human exposure study 
is similarly high, at 300 ppb; this study reported increases in two 
biochemicals suggestive of some liver biomarkers and no change in a 
number of other biochemicals associated with metabolic effects (ISA, 
sections 5.1.3, 5.1.5 and 5.1.8, Table 5-3). A limited number of 
epidemiologic studies is also available (ISA, section IS.4.3.3; 
Appendix 5, sections 5.1.3 and 5.1.8).
    The ISA additionally concludes that the evidence is suggestive of, 
but not sufficient to infer, a causal relationship between long-term 
O3 exposures and metabolic effects (ISA, section 
IS.4.3.6.2). As with metabolic effects and short-term O3, 
the primary evidence is from experimental animal studies in which the 
exposure concentrations are appreciably higher than those commonly 
occurring in the U.S. For example, the animal studies include exposures 
over several weeks to concentrations of 250 ppb and higher (ISA, 
Appendix 5, section 5.2.3.1.1). The somewhat limited epidemiologic 
evidence related to long-term O3 concentrations and 
metabolic effects includes studies reporting increased odds of being 
overweight or obese or having metabolic syndrome and increased hazard 
ratios for diabetes incidence with increased O3 
concentrations (ISA, Appendix 5, sections 5.2.3.4.1, 5.2.5 and 5.2.9, 
Tables 5-12 and 5-15).
    With regard to cardiovascular effects and total (nonaccidental) 
mortality and short-term O3 exposures, the conclusions 
regarding the potential for a causal relationship have changed from 
what they were in the last review after integrating the previously 
available evidence with newly available evidence. 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). This reflects several aspects of the current evidence 
base: (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) that still remain. Although 
there exists consistent or generally 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). Related to the updated evidence for cardiovascular 
effects, the evidence for short-term O3 concentrations and 
mortality is also updated (ISA, section 4.3.5 and Appendix 6, section 
6.1.8). While epidemiologic studies show positive associations between 
short-term O3 concentrations and total (nonaccidental) and 
cardiovascular mortality (and there are some studies reporting 
associations that remain after controlling for PM10 and 
NO2), the full evidence base does not describe a continuum 
of effects that could lead to cardiovascular mortality.\51\ The 
category of total mortality includes all contributions to mortality, 
including both respiratory and cardiovascular mortality, as well as 
other causes of death, such as cancer or other chronic diseases. The 
evidence base supporting a continuum of effects of short-term 
O3 concentrations that could potentially lead to respiratory 
mortality is more consistent and coherent as compared to that for 
cardiovascular mortality (ISA, sections 3.1.11 and 4.1.17; 2013 ISA, 
section 6.2.8). However, because cardiovascular mortality is the 
largest contributor to total mortality, the relatively limited 
biological plausibility and coherence within and across disciplines for 
cardiovascular effects (including mortality) is the dominant factor 
which contributes to a revised causality determination for total 
mortality (ISA, section IS.4.3.5). The ISA concludes that the currently 
available evidence for cardiovascular effects and total mortality is 
suggestive of, but not sufficient to infer, a causal relationship with 
short-term (as well as long-term) O3 exposures (ISA, 
sections IS.4.3.4 and IS.4.3.5).
---------------------------------------------------------------------------

    \51\ Due to findings from controlled human exposure studies 
examining clinical endpoints (e.g., blood pressure) that do not 
indicate an O3 effect and from epidemiologic studies 
examining cardiovascular-related hospital admissions and ED visits 
that do not find positive associations, a continuum of effects that 
could lead to cardiovascular mortality is not apparent (ISA, 
Appendices 4 and 6).
---------------------------------------------------------------------------

    For other health effect categories, conclusions in this review are 
largely unchanged from those in the last review. 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, as was the case in the last review (ISA, section 
IS.4.3.6.5 and Table IS-1). Additionally, the evidence is inadequate to 
determine if a causal relationship exists between O3 
exposure and cancer (ISA, section IS.4.3.6.6 and Table IS-1).
2. Public Health Implications and At-Risk Populations
    The public health implications of the evidence regarding 
O3-related health

[[Page 49848]]

effects, as for other effects, are dependent on the type and severity 
of the effects, as well as the size of the population affected. Such 
factors are discussed here in the context of our consideration of the 
health effects evidence related to O3 in ambient air. 
Additionally, we summarize the currently available information related 
to judgments or interpretative statements developed by public health 
experts, particularly experts in respiratory health. This section also 
summarizes the current information on population groups at increased 
risk of the effects of O3 in ambient air.
    With regard to O3 in ambient air, the potential public 
health impacts relate most importantly to the role of O3 in 
eliciting respiratory effects, the category of effects that the ISA 
concludes to be causally related to O3 exposure (short-
term). 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.B.1 above; 2013 ISA, section 6.2.7; ISA, 
Appendix 3, sections 3.1.5.1 and 3.1.5.2).
    The clinical significance of individual responses to O3 
exposure depends on the health status of the individual, the magnitude 
of the changes in pulmonary function, the severity of respiratory 
symptoms, and the duration of the response. With regard to pulmonary 
function, the greater impact of larger decrements on affected 
individuals can be described. For example, 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 for most healthy individuals, 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 
for more of such individuals (e.g., 2014 PA, p. 3-53; 2006 AQCD, Table 
8-2).
    In addition to the difference in severity or magnitude of specific 
effects in healthy people, the same reduction in FEV1 or 
increase in inflammation or airway responsiveness in a healthy group 
and a group with asthma may increase the risk of a more severe effect 
in the group with asthma. For 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., 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. In summary, consideration of differences in 
magnitude or severity, and also the relative transience or persistence 
of such 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).
    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 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 ATS 
described the statement as not offering ``strict rules or numerical 
criteria,'' but rather proposing ``principles to be used in weighing 
the evidence and setting boundaries,'' and stated that ``the placement 
of dividing lines should be a societal judgment'' (ATS, 2000). 
Similarly, the most recent policy statement by the ATS, which once 
again broadens its discussion of effects, responses and biomarkers to 
reflect the expansion of scientific research in these areas, reiterates 
that concept, conveying that it does not offer ``strict rules or 
numerical criteria, but rather proposes considerations to be weighed in 
setting boundaries between adverse and nonadverse health effects,'' 
providing a general framework for interpreting evidence that proposes a 
``set of considerations that can be applied in forming judgments'' for 
this context (Thurston et al., 2017).
    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 such lung function changes in the absence of respiratory symptoms in 
individuals with pre-existing compromised function, such as that 
resulting from asthma (Thurston et al., 2017). More specifically, the 
recent ATS statement expresses the view that the occurrence of ``small 
lung function changes'' in individuals with pre-existing compromised 
function, such as asthma, ``should be considered adverse . . . even 
without accompanying respiratory symptoms'' (Thurston et al., 2017). In 
keeping with the intent of these statements to avoid specific criteria, 
neither statement provides more specific descriptions of such 
responses, such as with regard to magnitude, duration or frequency, for 
consideration of such conclusions. The earlier ATS statement, in 
addition to emphasizing clinically relevant effects, also emphasized 
both the need to consider changes in ``the risk profile of the exposed 
population,'' and effects on the portion of the population that may 
have a diminished reserve that puts its members at potentially 
increased risk if affected by another agent (ATS, 2000). These 
concepts, including the consideration of the magnitude of effects 
occurring in just a subset of study subjects, continue to be recognized 
as important in the more recent ATS statement (Thurston et al., 2017) 
and continue to be relevant to the evidence base for O3.
    The information newly available in this review has not altered our 
understanding of human populations at particular risk of health effects 
from O3 exposures (ISA, section IS.4.4). For example, as 
recognized in prior reviews, people with asthma are the key population 
at risk of O3-related effects. The respiratory effects 
evidence, extending decades into the past and augmented by new studies 
in this review, supports this conclusion (ISA, sections IS.4.3.1). For 
example, numerous epidemiological studies document associations with 
O3 with asthma exacerbation. Such studies

[[Page 49849]]

indicate the associations to be strongest for populations of children 
which is consistent with their generally greater time outdoors while at 
elevated exertion. Together, these considerations indicate people with 
asthma, including particularly children with asthma, to be at 
relatively greater risk of O3-related effects than other 
members of the general population (ISA, section IS.4.4.2 and Appendix 
3).\52\
---------------------------------------------------------------------------

    \52\ Populations or lifestages can be at increased risk of an 
air pollutant-related health effect due to one or more of a number 
of factors. These factors can be intrinsic, such as physiological 
factors that may influence the internal dose or toxicity of a 
pollutant, or extrinsic, such as sociodemographic, or behavioral 
factors.
---------------------------------------------------------------------------

    With respect to people with asthma, the limited evidence from 
controlled human exposure studies (which are primarily in adult 
subjects) indicates similar magnitude of FEV1 decrements as 
in people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Across 
other respiratory effects of O3 (e.g., increased respiratory 
symptoms, increased airway responsiveness and increased lung 
inflammation), the evidence has also found the observed responses to 
generally not differ due to the presence of asthma, although the 
evidence base is more limited with regard to study subjects with asthma 
(ISA, Appendix 3, section 3.1.5.7). However, the features of asthma 
(e.g., increased airway responsiveness) contribute to a risk of asthma-
related responses, such as asthma exacerbation in response to asthma 
triggers, which may increase the risk of more severe health outcomes 
(ISA, section 3.1.5). For example, a particularly strong and consistent 
component of the epidemiologic evidence is the appreciable number of 
epidemiologic studies that demonstrate associations between ambient 
O3 concentrations and hospital admissions and emergency 
department visits for asthma (ISA, section IS.4.4.3.1). \53\ We 
additionally recognize that in these studies, the strongest 
associations (e.g., highest effect estimates) or associations more 
likely to be statistically significant are those for childhood age 
groups, which are recognized in section II.C.1 as age groups most 
likely to spend time outdoors during afternoon periods (when 
O3 may be highest) and at activity levels corresponding to 
those that have been associated with respiratory effects in the human 
exposure studies (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).\54\ 
The epidemiologic studies of hospital admissions and emergency 
department visits are augmented by a large body of individual-level 
epidemiologic panel studies that demonstrated associations of short-
term ozone concentrations with respiratory symptoms in children with 
asthma. Additional support comes from epidemiologic studies that 
observed ozone-associated increases in indicators of airway 
inflammation and oxidative stress in children with asthma (ISA, section 
IS.4.3.1). Together, this evidence continues to indicate the increased 
risk of population groups with asthma (ISA, Appendix 3, section 
3.1.5.7).
---------------------------------------------------------------------------

    \53\ In addition to asthma exacerbation, the epidemiologic 
evidence also includes findings of positive associations of 
increased O3 concentrations with hospital admissions or 
emergency department visits for COPD exacerbation and other 
respiratory diseases (ISA, Appendix 3, sections 3.1.6.1.3 and 
3.1.8).
    \54\ There is limited data on activity patterns by health 
status. An analysis in the 2014 HREA indicated that asthma status 
had little to no impact on the percent of people participating in 
outdoor activities during afternoon hours, the amount of time spent, 
and whether they performed activities at elevated exertion levels 
(2014 HREA, section 5.4.1.5). Based on an updated evaluation of 
recent activity pattern data we found children, for days having some 
time spent outdoors spend, on average, approximately 2\1/4\ hours of 
afternoon time outdoors, 80% of which is at a moderate or greater 
exertion level, regardless of their asthma status (see Appendix 3D, 
section 3D.2.5.3). Adults, for days having some time spent outdoors, 
also spend approximately 2\1/4\ hours of afternoon time outdoors 
regardless of their asthma status but the percent of afternoon time 
at moderate or greater exertion levels for adults (about 55%) is 
lower than that observed for children.
---------------------------------------------------------------------------

    Children, and also outdoor adult workers, are at increased risk 
largely due to their generally greater time spent outdoors while at 
elevated exertion rates (including in the summer when O3 
levels may be higher). This behavior makes them more likely to be 
exposed to O3 in ambient air, under conditions contributing 
to increased dose due to greater air volumes taken into the lungs (2013 
ISA, section 5.2.2.7). In light of the evidence summarized in the prior 
paragraph, children and outdoor workers with asthma may be at increased 
risk of more severe outcomes, such as asthma exacerbation. Further, 
there is experimental evidence from early life exposures of nonhuman 
primates that indicates potential for effects in childhood when human 
respiratory systems are under development (ISA, section IS.4.4.4.1). 
Overall, the evidence available in the current review, while not 
increasing our knowledge about susceptibility of these population 
groups, is consistent with that in the last review.
    Older adults have also been identified as being at increased risk. 
That identification, based on the assessment in the 2013 ISA, was based 
largely on studies of short-term O3 exposure and mortality, 
which are part of the larger evidence base that is now concluded to be 
suggestive, but not sufficient to infer a causal relationship (ISA, 
sections IS.4.3.5 and IS.4.4.4.2, Appendix 4, section 4.1.16.1 and 
4.1.17).\55\ Other evidence available in the current review adds little 
to the evidence available at the time of the last review for older 
adults (ISA, sections IS.4.4.2 and IS.4.4.4.2).
---------------------------------------------------------------------------

    \55\ As noted in the ISA, ``[t]he majority of evidence for older 
adults being at increased risk of health effects related to ozone 
exposure comes from studies of short-term ozone exposure and 
mortality evaluated in the 2013 Ozone ISA'' (ISA, p. IS-52).
---------------------------------------------------------------------------

    The ISA in the last review concluded that the information available 
at the time for low socioeconomic status (SES) as a factor associated 
with the risk of O3-related health effects, provided 
suggestive evidence of potentially increased risk (2013 ISA, section 
8.3.3 and p. 8-37). The 2013 ISA concluded that ``[o]verall, evidence 
is suggestive of SES as a factor affecting risk of O3-
related health outcomes based on collective evidence from epidemiologic 
studies of respiratory hospital admissions but inconsistency among 
epidemiologic studies of mortality and reproductive outcomes,'' 
additionally stating that ``[f]urther studies are needed to confirm 
this relationship, especially in populations within the U.S.'' (2013 
ISA, p. 8-28). The evidence available in the current review adds little 
to the evidence available at the time of the last review in this area 
(ISA, section IS.4.4.2 and Table IS-10). The ISA in the last review 
additionally identified a role for dietary anti-oxidants such as 
vitamins C and E in influencing risk of O3-related effects, 
such as inflammation, as well as a role for genetic factors to also 
confer either an increased or decreased risk (2013 ISA, sections 8.1 
and 8.4.1). No newly available evidence has been evaluated that would 
inform or change these prior conclusions (ISA, section IS.4.4 and Table 
IS-10).
    The magnitude and characterization of a public health impact is 
dependent upon the size and characteristics of the populations 
affected, as well as the type or severity of the effects. As summarized 
above, a key population most at risk of health effects associated with 
O3 in ambient air is people with asthma. The National Center 
for Health Statistics data for 2017 indicate that approximately 7.9% of 
the U.S. populations has asthma (CDC, 2019; PA, Table 3-1). This is one 
of the principal populations that the primary O3 NAAQS is 
designed to protect (80 FR 65294, October 26, 2015).
    The age group for which the prevalence documented by these data is 
greatest is children aged five to 19 years old, with 9.7% of children 
aged five to

[[Page 49850]]

14 and 9.4% of children aged 15 to 19 years old having asthma (CDC, 
2019, Tables 3-1 and 4-1; PA, Table 3-1). In 2012 (the most recent year 
for which such an evaluation is available), asthma was the leading 
chronic illness affecting children (Bloom et al., 2013). The prevalence 
is greater for boys than girls (for those less than 18 years of age). 
Among populations of different races or ethnicities, black non-Hispanic 
children aged five to 14 have the highest prevalence, at 16.1%. Asthma 
prevalence is also increased among populations in poverty. For example, 
11.7% of people living in households below the poverty level have 
asthma compared to 7.3%, on average, of those living above it (CDC, 
2019, Tables 3-1 and 4-1; PA, Table 3-1). Population groups with 
relatively greater asthma prevalence might be expected to have a 
relatively greater potential for O3-related health 
impacts.\56\
---------------------------------------------------------------------------

    \56\ As summarized in section II.A.1 above, the current standard 
was set to protect at-risk populations, which include people with 
asthma. Accordingly, populations with asthma living in areas not 
meeting the standard would be expected to be at increased risk of 
effects than others in those areas.
---------------------------------------------------------------------------

    Children under the age of 18 account for 16.7% of the total U.S. 
population, with 6.2% of the total population being children under 5 
years of age (U.S. Census Bureau, 2019). Based on a prior analysis of 
data from the Consolidated Human Activity Database (CHAD) \57\ in the 
2014 HREA, children ages 4-18 years old, for days having some time 
spent outdoors, were found to more frequently spend time outdoors 
compared to other age groups (e.g., adults aged 19-34) spending more 
than 2 hours outdoors, particularly during the afternoon and early 
evening (e.g., 12:00 p.m. through 8:00 p.m.) (2014 HREA, section 5G-
1.2). These results were confirmed by additional analyses of CHAD data 
reported in the ISA, noting greater participation in afternoon outdoor 
events for children ages 6-19 years old during the warm season months 
compared to other times of the day (ISA, Appendix 2, section 2.4.1, 
Table 2-1). The 2014 HREA also found that children ages 4-18 years old 
spent 79% of their outdoor time at moderate or greater exertion (2014 
HREA, section 5G-1.4). Further analyses performed for this review using 
the most recent version of CHAD generated similar results (PA, Appendix 
3D, section 3D.2.5.3 and Figure 3D-9). Each of these analyses indicate 
children participate more frequently and spend more afternoon time 
outdoors than all other age groups while at elevated exertion, and 
consistently do so when considering the most important influential 
factors such as day-of-week and outdoor temperature. Given that 
afternoon time outdoors and elevated exertion were determined most 
important in understanding the fraction of the population that might 
experience O3 exposures of concern (e.g., 2014 HREA, section 
5.4.2), they may be at greater risk of effects due to increased 
exposure to O3 in ambient air.
---------------------------------------------------------------------------

    \57\ The CHAD provides time series data on human activities 
through a database system of collected human diaries, or daily time 
location activity logs.
---------------------------------------------------------------------------

    About one third of workers were required to perform outdoor work in 
2018 (Bureau of Labor Statistics, 2019). Jobs in construction and 
extraction occupations and protective service occupations required more 
than 90% of workers to spend at least part of their workday outdoors 
(Bureau of Labor Statistics, 2017). Other employment sectors, including 
installation, maintenance and repair occupations and building and 
grounds cleaning and maintenance operations, also had a high percentage 
of employees who spent part of their workday outdoors (Bureau of Labor 
Statistics, 2017). These occupations often include physically demanding 
tasks and involve increased ventilation rates which when combined with 
exposure to O3, may increase the risk of health effects.
3. Exposure Concentrations Associated With Effects
    As at the time of the last review, the EPA's conclusions regarding 
exposure concentrations of O3 associated with respiratory 
effects reflect the extensive longstanding evidence base of controlled 
human exposure studies of short-term O3 exposures of people 
with and without asthma (ISA, Appendix 3). These studies have 
documented an array of respiratory effects, including reduced lung 
function, respiratory symptoms, increased airway responsiveness, and 
inflammation, in study subjects following 1- to 8-hour exposures, 
primarily while exercising. The severity of observed responses, the 
percentage of individuals responding, and strength of statistical 
significance at the study group level have been found to increase with 
increasing exposure (ISA; 2013 ISA; 2006 AQCD). Factors influencing 
exposure include activity level or ventilation rate, exposure 
concentration, and exposure duration (ISA; 2013 ISA; 2006 AQCD). For 
example, evidence from studies with similar duration and exercise 
aspects (6.6-hour duration with six 50-minute exercise periods) 
demonstrates an exposure-response relationship for O3-
induced reduction in lung function (ISA, Appendix 3, Figure 3-3; PA, 
Figure 3-2).58 59
---------------------------------------------------------------------------

    \58\ For a subset of the studies included in PA, Figure 3-2 
(those with face mask rather than chamber exposures), there is no 
O3 exposure during some of the 6.6-hour experiment (e.g., 
during the lunch break). Thus, while the exposure concentration 
during the exercise periods is the same for the two types of 
studies, the time-weighted average (TWA) concentration across the 
full 6.6-hour period differs slightly. For example, in the facemask 
studies of 120 ppb, the TWA across the full 6.6-hour experiment is 
109 ppb (PA, Appendix 3A, Table 3A-2).
    \59\ The relationship also exists for size of FEV1 
decrement with alternative exposure or dose metrics, including total 
inhaled O3 and intake volume averaged concentration.
---------------------------------------------------------------------------

    The current evidence, including that newly available in this 
review, does not alter the scientific conclusions reached in the last 
review on exposure duration and concentrations associated with 
O3-related health effects. These conclusions were largely 
based on the body of evidence from the controlled human exposure 
studies. A limited number of controlled human exposure studies are 
newly available in the current review, with none involving lower 
exposure concentrations than those previously studied or finding 
effects not previously reported (ISA, Appendix 3, section 3.1.4).\60\
---------------------------------------------------------------------------

    \60\ No 6.6-hour studies are newly available in this review 
(ISA, Appendix 3, section 3.1.4.1.1). Rather, the newly available 
controlled human exposure studies are generally for exposures of 
three hours or less, and in nearly all instances involve exposure 
(while at elevated exertion) to concentrations above 100 ppb (ISA, 
Appendix 3, section 3.1.4).
---------------------------------------------------------------------------

    The extensive evidence base for O3 health effects, 
compiled over several decades, continues to indicate respiratory 
responses to short-term exposures as the most sensitive effects of 
O3. As summarized in section II.B.1 above, an array of 
respiratory effects is well documented in controlled human exposure 
studies of subjects exposed for 1 to 8 hours, primarily while 
exercising. The risk of more severe health outcomes associated with 
such effects is increased in people with asthma as illustrated by the 
epidemiologic findings of positive associations between O3 
exposure and asthma-related ED visits and hospital admissions.
    The magnitude of respiratory response (e.g., size of lung function 
reductions and magnitude of symptom scores) documented in the 
controlled human exposure studies is influenced by ventilation rate, 
exposure duration, and exposure concentration. When performing physical 
activities requiring elevated exertion, ventilation rate is increased, 
leading to greater potential for health effects due to an increased 
internal dose (2013 ISA, section 6.2.1.1, pp. 6-5 to 6-11). 
Accordingly, the exposure concentrations eliciting a

[[Page 49851]]

given level of response after a given exposure duration is lower for 
subjects exposed while at elevated ventilation, such as while 
exercising (2013 ISA, pp. 6-5 to 6-6). For example, in studies of 
healthy young adults exposed while at rest for 2 hours, 500 ppb is the 
lowest concentration eliciting a statistically significant 
O3-induced group mean lung function decrement, while a 1- to 
2-hour exposure to 120 ppb produces a statistically significant 
response in lung function when the ventilation rate of the group of 
study subjects is sufficiently increased with exercise (2013 ISA, pp. 
6-5 to 6-6).
    The exposure conditions (e.g., duration and exercise) given primary 
focus in the past several reviews are those of the 6.6-hour study 
design, which involves six 50-minute exercise periods during which 
subjects maintain a moderate level of exertion to achieve a ventilation 
rate of approximately 20 L/min per m\2\ body surface area while 
exercising. The 6.6 hours of exposure in these studies has generally 
occurred in an enclosed chamber and the study design includes three 
hours in each of which is a 50-minute exercise period and a 10-minute 
rest period, followed by a 35-minute lunch (rest) period, which is 
followed by three more hours of exercise and rest, as before lunch.\61\ 
Most of these studies performed to date involve exposure maintained at 
a constant (unchanging) concentration for the full duration, although a 
subset of studies have concentrations that vary (generally in a 
stepwise manner) across the exposure period and are selected so as to 
achieve a specific target concentration as the exposure average.\62\ No 
studies of the 6.6-hour design are newly available in this review. The 
previously available studies of this design document statistically 
significant O3-induced reduction in lung function 
(FEV1) and increased pulmonary inflammation in young healthy 
adults exposed to O3 concentrations as low as 60 ppb. 
Statistically significant group mean changes in FEV1, also 
often accompanied by statistically significant increases in respiratory 
symptoms, become more consistent across such studies of exposures to 
higher O3 concentrations, such as 70 ppb and 80 ppb (Table 
1; PA, Appendix 3A, Table 3A-1). The lowest exposures concentration for 
which these studies document a statistically significant increase in 
respiratory symptoms is somewhat above 70 ppb (Schelegle et al., 
2009).\63\
---------------------------------------------------------------------------

    \61\ A few studies have involved exposures by facemask rather 
than freely breathing in a chamber. To date, there is little 
research differentiating between exposures conducted with a facemask 
and in a chamber since the pulmonary responses of interest do not 
seem to be influenced by the exposure mechanism. However, similar 
responses have been seen in studies using both exposure methods at 
higher O3 concentrations (Adams, 2002; Adams, 2003). In 
the facemask designs, there is a short period of zero O3 
exposure, such that the total period of exposure is closer to 6 
hours than 6.6 (Adams, 2000; Adams, 2002; Adams, 2003).
    \62\ In these studies, the exposure concentration changes for 
each of the six hours in which there is exercise and the 
concentration during the 35-minute lunch is the same as in the prior 
(third) hour with exercise. For example, in the study by Adams, 
2006a), the protocol for the 6.6-hour period is as follows: 60 
minutes at 40 ppb, 60 minutes at 70 ppb, 95 minutes at 90 ppb, 60 
minutes at 70 ppb, 60 minutes at 50 ppb and 60 minutes at 40 ppb.
    \63\ Measurements are reported in this study for each of the six 
50-minute exercise periods, for which the mean is 72 ppb (Schelegle 
et al., 2009). Based on these data, the time-weighted average 
concentration across the full 6.6-hour duration was 73 ppb 
(Schelegle et al., 2009). The study design includes a 35-minute 
lunch period following the third exposure hour during which the 
exposure concentration remains the same as in the third hour.
---------------------------------------------------------------------------

    In the 6.6-hour studies, the group means of O3-induced 
\64\ FEV1 reductions for exposure concentrations below 80 
ppb are at or below 6% (Table 1). For example, the group means of 
O3-induced FEV1 decrements reported in these 
studies that are statistically significantly different from the 
responses in filtered air are 6.1% for 70 ppb and 1.7% to 3.5% for 60 
ppb (Table 1). The group mean O3-induced FEV1 
decrements generally increase with increasing O3 exposures, 
reflecting increases in both the number of the individuals experiencing 
FEV1 reductions and the magnitude of the FEV1 
reduction (Table 1; ISA, Figure 3-3; PA, Figure 3-2). For example, 
following 6.6-hour exposures to a lower concentration (40 ppb), for 
which decrements were not statistically significant at the group mean 
level, none of 60 subjects across two separate studies experienced an 
O3-induced FEV1 reduction as large as 15% or more 
(Table 1; PA, Appendix 3D, Table 3D-19). Across the four experiments 
(with number of subjects ranging from 30 to 59) that have reported 
results for 60 ppb target exposure, the number of subjects experiencing 
this magnitude of FEV1 reduction (at or above 15%) varied 
(zero of 30, one of 59, two of 31 and two of 30 exposed subjects). This 
response increased to three of 31 subjects for the study with a 70 ppb 
target concentration (PA, Appendix 3D, Table 3D-19; Schelegle et al., 
2009). In addition to illustrating the E-R relationship, these findings 
also illustrate the considerable variability in magnitude of responses 
observed among study subjects (ISA, Appendix 3, section 3.1.4.1.1; 2013 
ISA, p. 6-13).
---------------------------------------------------------------------------

    \64\ Consistent with the ISA and 2013 ISA, the phrase 
``O3-induced'' decrement or reduction in lung function or 
FEV1 refers to the percent change from pre-exposure 
measurement of the O3 exposure minus the percent change 
from pre-exposure measurement of the filtered air exposure (2013 
ISA, p. 6-4).

                                  Table 1--Summary of 6.6-Hour Controlled Human Exposure Study-Findings, Healthy Adults
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                       O3 target  exposure concentration        Statistically        O3-induced group mean
              Endpoint                                \A\                  significant  effect \B\        response \B\                  Study
--------------------------------------------------------------------------------------------------------------------------------------------------------
FEV1 Reduction......................  120 ppb...........................  Yes.....................  -10.3% to -15.9% \C\...  Horstman et al. (1990);
                                                                                                                              Adams (2002); Folinsbee et
                                                                                                                              al. (1988); Folinsbee et
                                                                                                                              al. (1994); Adams, 2002;
                                                                                                                              Adams (2000); Adams and
                                                                                                                              Ollison (1997).\D\
                                      100 ppb...........................  Yes.....................  -8.5% to -13.9% \C\....  Horstman et al., 1990;
                                                                                                                              McDonnell et al., 1991.\D\
                                      87 ppb............................  Yes.....................  -12.2%.................  Schelegle et al., 2009.
                                      80 ppb............................  Yes.....................  -7.5%..................  Horstman et al., 1990.
                                                                                                    -7.7%..................  McDonnell et al., 1991.
                                                                                                    -6.5%..................  Adams, 2002.
                                                                                                    -6.2% to -5.5% \C\.....  Adams, 2003.
                                                                                                    -7.0% to -6.1% \C\.....  Adams, 2006a.
                                                                                                    -7.8%..................  Schelegle et al., 2009.
                                                                          ND \E\..................  -3.5%..................  Kim et al., 2011.\F\
                                      70 ppb............................  Yes.....................  -6.1%..................  Schelegle et al., 2009.

[[Page 49852]]

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

    For shorter exposure periods, ranging from one to two hours, higher 
exposure concentrations, ranging up from 80 ppb up to 400 ppb, have 
been studied (ISA, section 3.1; 2013 ISA, section 6.2.1.1; 2006 AQCD; 
PA, Appendix 3A, Table 3A-3). In these studies, some exposure protocols 
have included heavy intermittent or very heavy continuous exercise, 
which results in 2-3 times greater ventilation rate than in the 
prolonged (6.6- or 8-hour) exposure studies, which only incorporate 
moderate quasi-continuous exercise.\65\ Across these shorter-duration 
studies, the lowest exposure concentration for which statistically 
significant respiratory effects were reported is 120 ppb, for a 1-hour 
exposure combined with continuous very heavy exercise and a 2-hour 
exposure with intermittent heavy exercise. As recognized above, the 
increased ventilation rate associated with increased exertion increases 
the amount of O3 entering the lung, where depending on dose 
and the individual's susceptibility, it may cause respiratory effects 
(2013 ISA, section 6.2.1.1). Thus, for exposures involving a lower 
exertion level, a comparable response would not be expected to occur 
without a longer duration at this concentration (120 ppb), as is 
illustrated by the 6.6-hour study results for this concentration (ISA, 
Appendix 3, Figure 33; PA, Appendix 3A, Table 3A-1).
---------------------------------------------------------------------------

    \65\ A quasi-continuous exercise protocol is common to the 
prolonged exposure studies where study subjects complete six 50-
minute periods of exercise, each followed by 10-minute periods of 
rest (e.g., ISA, Appendix 3, section 3.1.4.1.1, and p. 3-11; 2013 
ISA, section 6.2.1.1).
---------------------------------------------------------------------------

    With regard to the epidemiologic studies reporting associations 
between O3 and respiratory health outcomes such as asthma-
related emergency department visits and hospitalizations, these studies 
are generally focused on investigating the existence of a relationship 
between O3 occurring in ambient air and specific health 
outcomes. Accordingly, while as a whole, this evidence base of 
epidemiologic studies provides strong support for the conclusions of 
causality, as summarized in section II.B.1 above,\66\ these studies 
provide less information on details of the specific O3 
exposure circumstances that may be eliciting health effects associated 
with such outcomes, and whether these occur under conditions that meet 
the current standard. For example, these studies generally do not 
measure personal exposures of the study population or track individuals 
in the population with a defined exposure to O3 alone. 
Further, the vast majority of these studies were conducted in locations 
and during time periods that would not have met the current 
standard.\67\ While this does not

[[Page 49853]]

lessen their importance in the evidence base documenting the causal 
relationship between O3 and respiratory effects, it means 
they are less informative in considering O3 exposure 
concentrations occurring under air quality conditions allowed by the 
current standard.
---------------------------------------------------------------------------

    \66\ Combined with the coherent evidence from experimental 
studies, the epidemiologic studies ``can support and strengthen 
determinations of the causal nature of the relationship between 
health effects and exposure to ozone at relevant ambient air 
concentrations'' (ISA, p. ES-17).
    \67\ Consistent with the evaluation of the epidemiologic 
evidence of associations between O3 exposure and 
respiratory health effects in the ISA, this summary focuses on those 
studies conducted in the U.S. and Canada to provide a focus on study 
populations and air quality characteristics that may be most 
relevant to circumstances in the U.S. (ISA, Appendix 3, section 
3.1.2).
---------------------------------------------------------------------------

    Among the epidemiologic studies finding a statistically significant 
positive relationship of short- or long-term O3 
concentrations with respiratory effects, there are no single-city 
studies conducted in the U.S. in locations with ambient air 
O3 concentrations that would have met the current standard 
for the entire duration of the study (ISA, Appendix 3, Tables 3-13, 3-
14, 3-39, 3-41, 3-42 and Appendix 6, Tables 6-5 and 6-8; PA, Appendix 
3B, Table 3B-1). There are (among this large group of studies) two 
single city studies conducted in western Canada that include locations 
for which the highest-monitor design values calculated in the PA fell 
below 70 ppb, at 65 and 69 ppb (PA, Appendix 3B, Table 3B-1; Kousha and 
Rowe, 2014; Villeneuve et al., 2007). These studies did not include 
analysis of correlations with other co-occurring pollutants or of the 
strength of the associations when accounting for effects of 
copollutants in copollutant models (ISA, Tables 3-14 and 3-39). Thus, 
the studies pose significant limitations with regard to informing 
conclusions regarding specific O3 exposure concentrations 
and elicitation of such effects. There is also a handful of multicity 
studies conducted in the U.S. or Canada in which the O3 
concentrations in a subset of the study locations and for a portion of 
the study period appear to have met the current standard (PA, Appendix 
3B). Concentrations in other portions of the study area or study 
period, however, do not meet the standard, or data were not available 
in some cities for the earlier years of the study period when design 
values for other cities in the study were well above 70 ppb. The extent 
to which reported associations with health outcomes in the resident 
populations in these studies are influenced by the periods of higher 
concentrations during times that did not meet the current standard is 
unknown. Additionally, with regard to multicity studies, the reported 
associations were based on the combined dataset from all cities, 
complicating interpretations regarding the contribution of 
concentrations in the small subset of locations that would have met the 
current standard compared to that from the larger number of locations 
that would have violated the standard (Appendix 3B).\68\ Further, given 
that populations in the single city or multicity studies may have also 
experienced longer-term, variable and uncharacterized exposure to 
O3 (as well as to other ambient air pollutants), 
``disentangling the effects of short-term ozone exposure from those of 
long-term ozone exposure (and vice-versa) is an inherent uncertainty in 
the evidence base'' (ISA, p. IS-87 [section IS.6.1]). While given the 
depth and breadth of the evidence base for O3 respiratory 
effects, such uncertainties do not change our conclusions regarding the 
causal relationship between O3 and respiratory effects, they 
affect the extent to which the two studies mentioned here (conducted in 
conditions that may have met the current standard) can inform our 
conclusions regarding the potential for O3 concentrations 
allowed by the current standard to contribute to health effects.
---------------------------------------------------------------------------

    \68\ As recognized in the last review, ``multicity studies do 
not provide a basis for considering the extent to which reported 
O3 health effects associations are influenced by 
individual locations with ambient [air] O3 concentrations 
low enough to meet the current O3 standard versus 
locations with O3 concentrations that violate this 
standard'' (80 FR 64344, October 26, 2015).
---------------------------------------------------------------------------

    With regard to the experimental animal evidence and exposure 
conditions associated with respiratory effects, concentrations are 
generally much greater than those examined in the controlled human 
exposure studies, summarized in section II.B.1 above, and higher than 
concentrations commonly occurring in ambient air in areas of the U.S. 
where the current standard is met. In addition to being true for the 
various rodent studies, this is also true for the small number of early 
life studies in nonhuman primates that reported O3 to 
contribute to asthma-like effects in infant primates. The exposures 
eliciting the effects in these studies included multiple 5-day periods 
with O3 concentrations of 500 ppb over 8-hours per day (ISA, 
Appendix 3, section 3.2.4.1.2).
    With regard to short-term O3 and metabolic effects, the 
category of effects for which the ISA concludes there likely to be a 
causal relationship with O3, the evidence base is comprised 
primarily of experimental animal studies, as summarized in section 
II.B.1 above (ISA, Appendix 5, section 5.1). The exposure conditions 
from these animal studies generally involve much higher O3 
concentrations than those examined in the controlled human exposure 
studies of respiratory effects (and much higher than concentrations 
commonly occurring in ambient air in areas of the U.S. where the 
current standard is met). For example, the animal studies include 4-
hour concentrations of 400 to 800 ppb (ISA, Appendix 5, Table 5-
87).\69\ The two epidemiologic studies reporting statistically 
significant positive associations of O3 with metabolic 
effects (e.g., changes in glucose, insulin, metabolic clearance) are 
based in Taiwan and South Korea, respectively.\70\ Given the potential 
for appreciable differences in air quality patterns between Taiwan and 
South Korea and the U.S., as well as differences in other factors that 
might affect exposure (e.g., activity patterns), those studies are of 
limited usefulness for informing our understanding of exposure 
concentrations and conditions eliciting such effects in the U.S. (ISA, 
Appendix 5, section 5.1).
---------------------------------------------------------------------------

    \69\ Resting rats and resting human subjects exposed to the same 
concentration receive similar O3 doses (ISA, section 
3.1.4.1.2; Hatch et al., 2013). Further, the exposure concentration 
in the single controlled human exposure study of metabolic effects 
(e.g., 300 ppb for two hours of intermittent moderate to heavy 
exercise [Miller et al., 2016]) is also well above exposures 
examined in the 6.6- to 8-hour respiratory effect studies (ISA, 
Appendix 5, Table 5-7).
    \70\ Of the epidemiologic studies discussed in the ISA that 
investigate associations between short-term O3 exposure 
and metabolic effects, two are conducted in the U.S. and they report 
either a null or negative association of metabolic markers with 
O3 concentration (ISA, Appendix 5, Tables 5-6 and 5-9).
---------------------------------------------------------------------------

C. Summary of Exposure and Risk Information

    Our consideration of the scientific evidence available in the 
current review, as at the time of the last review, is informed by 
results from quantitative analyses of estimated population exposure and 
consequent risk of respiratory effects. These analyses in this review 
have focused on exposure-based risk analyses. Estimates from such 
analyses, particularly the comparison of daily maximum exposures to 
benchmark concentrations reflecting exposures at which respiratory 
effects have been observed in controlled human exposure studies, were 
most informative to the Administrator's decision in the last review (as 
summarized in section II.A.1 above). This largely reflected the 
conclusion that ``controlled human exposure studies provide the most 
certain evidence indicating the occurrence of health effects in humans 
following specific O3 exposures,'' and recognition that 
``effects reported in controlled human exposure studies are due solely 
to O3 exposures, and interpretation of

[[Page 49854]]

study results is not complicated by the presence of co-occurring 
pollutants or pollutant mixtures (as is the case in epidemiologic 
studies)'' (80 FR 65343, October 26, 2015).\71\ The focus in this 
review on exposure-based analyses reflects both the emphasis given to 
these types of analyses and the characterization of their uncertainties 
in the last review, and also the availability of new or updated 
information, models, and tools that address those uncertainties (IRP, 
Appendix 5A).
---------------------------------------------------------------------------

    \71\ In the last review, the Administrator placed relatively 
less weight on the air quality epidemiologic-based risk estimates, 
in recognition of an array of uncertainties, including, for example, 
those related to exposure measurement error (80 FR 65316, 65346, 
October 26, 2015; 79 FR 75277-75279, December 17, 2014; 2014 HREA, 
sections 3.2.3.2 and 9.6). Further, importantly in this review, the 
causal determinations for short-term O3 with mortality in 
the current ISA differ from the 2013 ISA. The current determinations 
for both short-term and long-term O3 exposure (as 
summarized in section II.B.1 above) are that the evidence is 
``suggestive'' but not sufficient to infer causal relationships for 
O3 with mortality (ISA, Table IS-1).
---------------------------------------------------------------------------

    The longstanding evidence continues to demonstrate a causal 
relationship between short-term O3 exposures and respiratory 
effects, with the current evidence base for respiratory effects is 
largely consistent with that for the last review, as summarized in 
section II.B above. Accordingly, the exposure-based analyses performed 
in this review, summarized below, are conceptually similar to those in 
the last review. Section II.C.1 summarizes key aspects of the 
assessment design, including the study areas, populations simulated, 
the conceptual approach, modeling tools, benchmark concentrations and 
exposure and risk metrics derived. Key limitations and uncertainties 
associated with the assessment are identified in section II.C.2 and the 
exposure and risk estimates are summarized in section II.C.3. An 
overarching focus of these analyses is whether the current exposure and 
risk information alters overall conclusions reached in the last review 
regarding health risk estimated to result from exposure to 
O3 in ambient air, and particularly for air quality 
conditions that just meet the current standard.
1. Key Design Aspects
    The analyses of O3 exposures and risk summarized here 
inform our understanding of the protection provided by the current 
standard from effects that the health effects evidence indicates to be 
elicited in some portion of exercising people exposed for several hours 
to elevated O3 concentrations. The analyses estimated 
population exposure and risk for simulated populations in eight urban 
study areas: Atlanta, Boston, Dallas, Detroit, Philadelphia, Phoenix, 
Sacramento and St. Louis. In addition to deriving exposure and risk 
estimates for air quality conditions just meeting the current primary 
O3 standard, estimates were also derived for two additional 
scenarios reflecting conditions just meeting design values just lower 
and just higher than the level of the current standard (65 and 75 
ppb).\72\
---------------------------------------------------------------------------

    \72\ All analyses are summarized more fully in the PA section 
3.4 and Appendices 3C and 3D.
---------------------------------------------------------------------------

    The eight study areas represent a variety of circumstances with 
regard to population exposure to short-term concentrations of 
O3 in ambient air. The areas range in total population size 
from approximately two to eight million and are distributed across 
seven of the nine climate regions of the U.S.: Northeast, Southeast, 
Central, East North Central, South, Southwest and West (PA, Appendix 
3D, Table 3D-1). The set of eight study areas is streamlined compared 
to the 15-area set in the last review and was chosen to ensure it 
reflects the full range of air quality and exposure variation expected 
in major urban areas in the U.S. with air quality that just meets the 
current standard (2014 HREA, section 3.5). Accordingly, while seven of 
the eight study areas were also included in the 2014 HREA, the eighth 
study area is newly added in the current assessment to insure 
representation of a large city in the southwest. Additionally, the 
years simulated reflect more recent emissions and atmospheric 
conditions subsequent to data used in the 2014 HREA, and therefore 
represent O3 concentrations somewhat nearer the current 
standard than was the case for study areas included in the 2014 HREA 
(Appendix 3C, Table 3C and 2014 HREA, Table 4-1). This contributes to a 
reduction in the uncertainty associated with development of the air 
quality scenarios of interest, particularly the one reflecting air 
quality conditions that just meet the current standard. Study-area-
specific characteristics contribute to variation in the estimated 
magnitude of exposure and associated risk across the urban study areas 
(e.g., combined statistical areas that include urban and suburban 
populations) that reflect an array of air quality, meteorological, and 
population exposure conditions.
    With regard to the objectives for the analysis approach, the 
analyses and the use of a case study approach are intended to provide 
assessments of an air quality scenario just meeting the current 
standard for a diverse set of areas and associated exposed populations. 
These analyses are not intended to provide a comprehensive national 
assessment (PA, section 3.4.1). Nor is the objective to present an 
exhaustive analysis of exposure and risk in the areas that currently 
just meet the current standard and/or of exposure and risk associated 
with air quality adjusted to just meet the current standard in areas 
that currently do not meet the standard. Rather, the purpose is to 
assess, based on current tools and information, the potential for 
exposures and risks beyond those indicated by the information available 
at the time the standard was established. Accordingly, use of this 
approach recognizes that capturing an appropriate diversity in study 
areas and air quality conditions (that reflect the current standard 
scenario) \73\ is an important aspect of the role of the exposure and 
risk analyses in informing the Administrator's conclusions on the 
public health protection afforded by the current standard.
---------------------------------------------------------------------------

    \73\ A broad variety of spatial and temporal patterns of 
O3 concentrations can exist when ambient air 
concentrations just meet the current standard. These patterns will 
vary due to many factors including the types, magnitude, and timing 
of emissions in a study area, as well as local factors, such as 
meteorology and topography. We focused our current assessment on 
specific study areas having ambient air concentrations close to 
conditions that reflect air quality that just meets the current 
standard. Accordingly, assessment of these study areas is more 
informative to evaluating the health protection provided by the 
current standard than would be an assessment that included areas 
with much higher and much lower concentrations.
---------------------------------------------------------------------------

    Consistent with the health effects evidence in this review 
(summarized in section II.B.1 above), the focus of the quantitative 
assessment is on short-term exposures of individuals in the population 
during times when they are breathing at an elevated rate. Exposure and 
risk are characterized for four population groups. Two are populations 
of school-aged children, aged 5 to 18 years: \74\ All children and 
children with asthma; two are populations of adults: All adults and 
adults with asthma. Asthma prevalence in each study area is estimated 
using regional, national, and state level prevalence information, as 
well as U.S. census tract-level population data and demographic 
information related to age, sex, and family income to represent 
expected spatial variability in asthma prevalence within and across the 
eight study areas. Asthma prevalence estimates for the full populations 
in the eight study areas

[[Page 49855]]

range from 7.7 to 11.2%; the rates for children in these areas range 
from 9.2 to 12.3% (PA, Appendix 3D, section 3D.3.1).
---------------------------------------------------------------------------

    \74\ The child population group focuses on ages 5 to 18 in 
recognition of data limitations and uncertainties, including those 
related to accurately simulating activities performed and estimating 
physiological attributes, as well as challenges in asthma diagnoses 
for children younger than 5 years old.
---------------------------------------------------------------------------

    The approach for this analysis incorporates an array of models and 
data (PA, section 3.4.1). Ambient air O3 concentrations were 
estimated using an approach that relies on a combination of ambient air 
monitoring data, atmospheric photochemical modeling, and statistical 
methods (PA, Appendix 3C). Population exposure and risk modeling is 
employed to estimate exposures and related lung function risk resulting 
from the estimated ambient air O3 concentrations (PA, 
Appendix 3D). While the lung function risk analysis focuses only on the 
specific O3 effect of FEV1 reduction, the 
comparison-to-benchmark approach, with its use of multiple benchmark 
concentrations, provides for risk characterization of the array of 
respiratory effects elicited by O3 exposure, the type and 
severity of which increase with increased exposure concentration.
    Ambient air O3 concentrations were estimated in each 
study area for the air quality conditions of interest by adjusting 
hourly ambient air concentrations, from monitoring data for the years 
2015-2017, using a photochemical model-based approach and then applying 
a spatial interpolation technique to produce air quality surfaces with 
high spatial and temporal resolution (PA, Appendix 3C).\75\ The final 
product were datasets of ambient air O3 concentration 
estimates with high temporal and spatial resolution (hourly 
concentrations in 500 to 1,700 census tracts) for each of the eight 
study areas (PA, section 3.4.1 and Appendix 3C, section 3C.7) 
representing the three air quality scenarios (just meeting the current 
standard, and the 65 ppb and 75 ppb scenarios).
---------------------------------------------------------------------------

    \75\ A similar approach was used to develop the air quality 
scenarios for the 2014 HREA.
---------------------------------------------------------------------------

    Population exposures were estimated using the EPA's Air Pollutant 
Exposure model (APEX) version 5, which probabilistically generates a 
large sample of hypothetical individuals from population demographic 
and activity pattern databases and simulates each individual's 
movements through time and space to estimate their time series of 
O3 exposures occurring within indoor, outdoor, and in-
vehicle microenvironments (PA, Appendix 3D, section 3D.2).\76\ The APEX 
model accounts for the most important factors that contribute to human 
exposure to O3 from ambient air, including the temporal and 
spatial distributions of people and ambient air O3 
concentrations throughout a study area, the variation of ambient air-
related O3 concentrations within various microenvironments 
in which people conduct their daily activities, and the effects of 
activities involving different levels of exertion on breathing rate (or 
ventilation rate) for the exposed individuals of different sex, age, 
and body mass in the study area (PA, Appendix 3D, section 3D.2). The 
APEX model generates each simulated person or profile by 
probabilistically selecting values for a set of profile variables, 
including demographic variables, health status and physical attributes 
(e.g., residence with air conditioning, height, weight, body surface 
area), and activity-specific ventilation rate (PA, Appendix 3D, section 
3D.2).
---------------------------------------------------------------------------

    \76\ The APEX model estimates population exposure using a 
stochastic, event-based microenvironmental approach. This model has 
a history of application, evaluation, and progressive model 
development in estimating human exposure, dose, and risk for reviews 
of NAAQS for gaseous pollutants, including the last review of the 
O3 NAAQS (U.S. EPA, 2008; U.S. EPA, 2009; U.S. EPA, 2010; 
U.S. EPA, 2014a; U.S. EPA, 2018).
---------------------------------------------------------------------------

    The activity patterns of individuals are an important determinant 
of their exposure (2013 ISA, section 4.4.1). By incorporating 
individual activity patterns,\77\ the model estimates physical exertion 
associated with each exposure event. This aspect of the exposure 
modeling is critical in estimating exposure, ventilation rate, 
O3 intake (dose), and health risk resulting from ambient air 
concentrations of O3.\78\ Because of variation in 
O3 concentrations among the different microenvironments in 
which individuals are active, the amount of time spent in each 
location, as well as the exertion level of the activity performed, will 
influence an individual's exposure to O3 from ambient air 
and potential for adverse health effects. Activity patterns vary both 
among and within individuals, resulting in corresponding variations in 
exposure across a population and over time (2013 ISA, section 4.4.1; 
2020 ISA, Appendix 2, section 2.4). For each exposure event, the APEX 
model tracks activity performed, ventilation rate, exposure 
concentration, and duration for all simulated individuals throughout 
the assessment period. The time-series of exposure events serves as the 
basis for calculating exposure and risk metrics of interest.
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    \77\ To represent personal time-location-activity patterns of 
simulated individuals, the APEX model draws from the consolidated 
human activity database (CHAD) developed and maintained by the EPA 
(McCurdy, 2000; U.S. EPA, 2019a). The CHAD is comprised of data from 
several surveys that collected activity pattern data at city, state, 
and national levels. Included are personal attributes of survey 
participants (e.g., age, sex), along with the locations they 
visited, activities performed throughout a day, time-of-day the 
activities occurred and activity duration (PA, Appendix 3D, section 
3D.2.5.1).
    \78\ Indoor sources are generally minor in comparison to 
O3 from ambient air (ISA, Appendix 2, section 2.1) and 
are not accounted for by the exposure modeling in this assessment.
---------------------------------------------------------------------------

    As in the last review, the quantitative analyses for this review 
uses the APEX model estimates of population exposures for simulated 
individuals breathing at elevated rates \79\ to characterize health 
risk based on information from the controlled human exposure studies on 
the incidence of lung function decrements in study subjects who are 
exposed over multiple hours while intermittently or quasi-continuously 
exercising (PA, Appendix 3D, section 3D.2.8). In drawing on this 
evidence base for this purpose, the assessment has given primary focus 
to the well-documented controlled human exposure studies for 6.6-hour 
average exposure concentrations ranging from 40 ppb to 120 ppb (ISA, 
Appendix 3, Figure 3-3; PA, Figure 3-2 and Appendix 3A, Table 3A-1). 
Health risk is characterized in two ways, producing two types of risk 
metrics: One that compares population exposures involving elevated 
exertion to benchmark concentrations (that are specific to elevated 
exertion exposures), and the second that estimates population 
occurrences of ambient air O3-related lung function 
decrements. The first risk metric is based on comparison of estimated 
daily maximum 7-hour average exposure concentrations for individuals 
breathing at elevated rates to concentrations of potential concern 
(benchmark concentrations). The second metric (lung function risk) uses 
E-R information for O3 exposures and FEV1 
decrements to estimate the portion of the simulated at-risk population 
expected to experience one or more days with an O3-related 
FEV1 decrement of at least 10%, 15% and 20%. Both of these 
metrics are used to characterize health risk associated with 
O3 exposures among the simulated population during periods 
of elevated breathing rates. Similar risk metrics were also derived in 
the 2014 HREA for the last review and the associated estimates informed 
the Administrator's 2015 decision on the current standard (80 FR 65292, 
October 26, 2015).
---------------------------------------------------------------------------

    \79\ Based on minute-by-minute activity levels, and 
physiological characteristics of the simulated person, APEX 
estimates an equivalent ventilation rate, by normalizing the 
simulated individuals' activity-specific ventilation rate to their 
body surface area (PA, Appendix 3D, section 3D.2.2.3.3).

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[[Page 49856]]

    The general approach and methodology for the exposure-based 
assessment used in this review is similar to that used in the last 
review. However, a number of updates and improvements, related to the 
air quality, exposure, and risk aspects of the assessment, have been 
implemented in this review which result in differences from the 
analyses in the prior review (Appendices 3C and 3D). These include (1) 
a more recent period (2015-2017) of ambient air monitoring data in 
which O3 concentrations in the eight study areas are at or 
near the current standard; (2) the most recent CAMx model, with updates 
to the treatment of atmospheric chemistry and physics within the model; 
(3) a significantly expanded CHAD, that now has nearly 180,000 diaries, 
with over 25,000 school aged children; (4) updated National Health and 
Nutrition Examination Survey data (2009-2014), which are the basis for 
the age- and sex-specific body weight distributions used to specify the 
individuals in the modeled populations; (5) updated algorithms used to 
estimate age- and sex-specific resting metabolic rate, a key input to 
estimating a simulated individual's activity-specific ventilation (or 
breathing) rate; (6) updates to the ventilation rate algorithm itself; 
and (7) an approach that better matches the simulated exposure 
estimates with the 6.6-hour duration of the controlled human exposure 
studies and with the study subject ventilation rates. Further, the 
current APEX model uses the most recent U.S. Census demographic and 
commuting data (2010), NOAA Integrated Surface Hourly meteorological 
data to reflect the assessment years studied (2015-2017), and updated 
estimates of asthma prevalence for all census tracts in all study areas 
based on 2013-2017 National Health Interview Survey and Behavioral Risk 
Factor Surveillance System data. Additional details are described in 
the PA (e.g., PA, section 3.4.1, Appendices 3C and 3D).
    The exposure-to-benchmark comparison characterizes the extent to 
which individuals in at-risk populations could experience O3 
exposures, while engaging in their daily activities, with the potential 
to elicit the effects reported in controlled human exposure studies for 
concentrations at or above specific benchmark concentrations. Results 
are characterized using three benchmark concentrations of 
O3: 60, 70, and 80 ppb. These are based on the three lowest 
concentrations targeted in studies of 6- to 6.6-hour exposures, with 
quasi-continuous exercise, and that yielded different occurrences, of 
statistical significance, and severity of respiratory effects (PA, 
section 3.3.3; PA, Appendix 3A, section 3A.1; PA, Appendix 3D, section 
3D.2.8.1). The lowest benchmark, 60 ppb, represents the lowest exposure 
concentration for which controlled human exposure studies have reported 
statistically significant respiratory effects. At this concentration, 
there is evidence of a statistically significant decrease in lung 
function and increase in markers of airway inflammation (ISA, Appendix 
3, section 3.1.4.1.1; Brown et al., 2008; Adams, 2006a). Exposure to 
approximately 70 ppb \80\ averaged over 6.6 hours resulted in a larger 
group mean lung function decrement, as well as an increase in 
prevalence of respiratory symptoms over what was observed for 60 ppb 
(Table 1; ISA, Appendix 3, Figure 3-3 and section 3.1.4.1.1; Schelegle 
et al., 2009). Studies of exposures to approximately 80 ppb have 
reported larger lung function decrements at the study group mean than 
following exposures to 60 or 70 ppb, in addition to an increase in 
airway inflammation, increased respiratory symptoms, increased airway 
responsiveness, and decreased resistance to other respiratory effects 
(Table 1; ISA, Appendix 3, sections 3.1.4.1 through 3.1.4.4; PA, Figure 
3-2 and section 3.3.3;). The APEX-generated exposure concentrations for 
comparison to these benchmark concentrations is the average of 
concentrations encountered by an individual while at an activity level 
that elicits the specified elevated ventilation rate.\81\ The incidence 
of such exposures above the benchmark concentrations are summarized for 
each simulated population, study area, and air quality scenario as 
discussed in section II.C.3 below.
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    \80\ The design for the study on which the 70 ppb benchmark 
concentration is based, Schelegle et al. (2009), involved varying 
concentrations across the full exposure period. The study reported 
the average O3 concentration measured during each of the 
six exercise periods. The mean concentration across these six values 
is 72 ppb. The 6.6-hour time weighted average based on the six 
reported measurements and the study design is 73 ppb (Schelegle et 
al., 2009). Other 6.6-hour studies have not reported measured 
concentrations for each exposure, but have generally reported an 
exposure concentration precision at or tighter than 3 ppb (e.g., 
Adams, 2006a).
    \81\ For this assessment, the APEX model averages the 
ventilation rate (VE) and simultaneously occurring 
exposure concentration for every simulated individual (based on the 
activities performed) over 7-hour periods using their time-series of 
exposure events. To reasonably extrapolate the VE of the 
controlled human study subjects (i.e., adults having a specified 
body size and related lung capacity), who were engaging in quasi-
continuous exercise during the study period, to individuals having 
varying body sizes (e.g., children with smaller size and related 
lung capacity), an equivalent ventilation rate (EVR) was calculated 
by normalizing the VE (L/min) by body surface area 
(m\2\). Then, daily maximum 7-hour exposure concentrations 
associated with 7-hour average EVR at or above the target of 17.3 
 1.2 L/min-m\2\ (i.e., the value corresponding to 
average EVR across the 6.6-hour study duration in the controlled 
human exposure studies) are compared to the benchmark concentrations 
(PA, Appendix 3D, section 3D.2.8.1).
---------------------------------------------------------------------------

    The lung function risk analysis provides estimates of the extent to 
which individuals in the populations could experience decrements in 
lung function. Estimates were derived for risk of experiencing a day 
with a lung function decrement at or above three different magnitudes, 
i.e., FEV1 reductions of at least 10%, 15%, and 20%. Lung 
function decrement risk was estimated by two different approaches, 
which utilize the evidence from the 6.6-hour controlled human exposure 
studies in different ways.\82\ One, the population-based E-R function 
risk approach, uses quantitative descriptions of the E-R relationships 
for study group incidence of the different magnitudes of lung function 
decrements based on the individual study subject observations (PA, 
Appendix 3D, section 3D.2.8.2.1). The second, the individual-based 
McDonnell-Smith-Stewart model (MSS; McDonnell et al., 2013), uses 
quantitative descriptions of biological processes identified as 
important in eliciting the different sizes of decrements at the 
individual level, with a factor that also provides a representation of 
intra- and inter-individual response variability (PA, Appendix 3D, 
section 3D.2.8.2.2). These two approaches involve different uses of the 
health effects evidence, with each accordingly, differing in their 
strengths, limitations and uncertainties.
---------------------------------------------------------------------------

    \82\ In so doing, the approaches also estimate responses 
associated with unstudied exposure circumstances and population 
groups in different ways.
---------------------------------------------------------------------------

    The E-R functions used for estimating the risk of lung function 
decrements at or above three sizes were developed from the individual 
study subject measurements of O3-related FEV1 
decrements from the 6.6-hour controlled human exposure studies 
targeting mean exposure concentrations from 120 ppb down to 40 ppb (PA, 
Appendix 3D, Table 3D-19; PA, Appendix 3A, Figure 3A-1). Functions were 
developed from the study results in terms of percent of study subjects 
experiencing O3-related decrements equal to at least 10%, 
15% or 20%.\83\ The functions indicate the

[[Page 49857]]

fraction of the population experiencing a particular decrement as a 
function of the exposure concentration experienced while at the target 
ventilation rate. This type of risk model, which has been used in risk 
assessments since the 1997 O3 NAAQS review, was last updated 
with the recently available study data (PA, Appendix 3D, section 
3D.2.8.2.1). In this review, the E-R functions are applied to the APEX 
estimates of daily maximum 7-hour average exposure concentrations 
concomitant with the target ventilation level estimated by APEX, with 
the results presented in terms of number of individuals in the 
simulated populations (and percent of the population) estimated to 
experience a day (or more) with a lung function decrement at or above 
10%, 15% or 20%.
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    \83\ Across the exposure range from 40 to 120 ppb, the 
percentage of exercising study subjects with asthma estimated to 
have at least a 10% O3 related FEV1 decrement 
increases from 0 to 7% (a statistically non-significant response at 
exposures of 40 ppb) up to approximately 50 to 70% at exposures of 
120 ppb (PA, Appendix 3D, Section 3D.2.8.2.1, Table 3D-19).
---------------------------------------------------------------------------

    The MSS model, also used for estimating the risk of lung function 
decrements, was developed using the extensive database from controlled 
human exposure studies that has been compiled over the past several 
decades, and biological concepts based on that evidence (McDonnell et 
al., 2012; McDonnell et al., 2013). The model mathematically estimates 
the magnitude of FEV1 decrement as a function of inhaled O3 
dose (based on concentration & ventilation rate) over the time period 
of interest (PA, Appendix 3D, section 3D.2.8.2.2). The simulation of 
decrements is dynamic, based on a balance between predicted development 
of the decrement in response to inhaled dose and predicted recovery 
(using a decay factor). This model was first applied in combination 
with the APEX model to generate lung function risk estimates in the 
last review (80 FR 65314, October 26, 2015) and has been updated since 
then based on the most recent study by its developers (McDonnell et 
al., 2013). In this review, the model is applied to the APEX estimates 
of exposure concentration and ventilation for every exposure event 
experienced by each simulated individual. The model then utilizes its 
mathematical predictions of lung function response to inhaled dose and 
predicted recovery to estimate the magnitude of O3 response 
across the sequence of exposure events in each individual's day. Each 
occurrence of decrements reaching magnitudes of interest (e.g., 10%, 
15% and 20%) is tallied. Thus, results are reported using the same 
metrics as for the E-R function, i.e., number of individuals in the 
simulated populations (and percent of the population) estimated to 
experience a day (or more) per simulation period with a lung function 
decrement at or above 10%, 15% and 20%.
    The comparison-to-benchmark analysis (involving comparison of daily 
maximum 7-hour average exposure concentrations that coincide with 7-
hour average elevated ventilation rates at or above the target to 
benchmark concentrations) provides perspective on the extent to which 
the air quality being assessed could be associated with discrete 
exposures to O3 concentrations reported to result in 
respiratory effects. For example, estimates of such exposures can 
indicate the potential for O3-related effects in the exposed 
population, including effects for which we do not have E-R functions 
that could be used in quantitative risk analyses (e.g., airway 
inflammation). Thus, the comparison-to-benchmark analysis provides for 
a broader risk characterization with consideration of the array of 
O3-related respiratory effects. For this reason, as well as 
the uncertainties associated with the lung function risk estimates, as 
summarized below, the summary of estimates in section II.C.3 below 
focuses primarily on results for the comparison-to-benchmark analysis.
2. Key Limitations and Uncertainties
    Uncertainty in the current exposure and risk analyses was 
characterized using a largely qualitative approach adapted from the 
World Health Organization (WHO) approach for characterizing uncertainty 
in exposure assessment (WHO, 2008) augmented by several quantitative 
sensitivity analyses for key aspects of the assessment approach 
(described in detail in Appendix 3D of the PA).\84\ This 
characterization and associated analyses builds on information 
generated from a previously conducted quantitative uncertainty analysis 
of population-based O3 exposure modeling (Langstaff, 2007). 
In so doing, the characterization considers the various types of data, 
algorithms, and models that together yield exposure and risk estimates 
for the eight study areas. In this way, the limitations and 
uncertainties underlying these data, algorithms, and models and the 
extent of their influence on the resultant exposure/risk estimates are 
considered. Consistent with the WHO (2008) uncertainty guidance, the 
overall impact of the uncertainty is scaled by qualitatively assessing 
the extent or magnitude of the impact of the uncertainty as implied by 
the relationship between the source of the uncertainty and the exposure 
and risk output. The characterization in the current assessment also 
evaluates the direction of influence, indicating how the source of 
uncertainty was judged to affect the exposure and risk estimates, e.g., 
likely to over- or under-estimate (PA, Appendix 3D, section 3D.3.4.1).
---------------------------------------------------------------------------

    \84\ The approach used has been applied in REAs for past NAAQS 
reviews for O3, NOX, CO and sulfur oxides 
(U.S. EPA, 2008; U.S. EPA, 2010; U.S. EPA, 2014a; U.S. EPA, 2018).
---------------------------------------------------------------------------

    Several areas of uncertainty are identified as particularly 
important to considering the exposure and risk estimates. There are 
also several areas where new or updated information have reduced 
uncertainties since the last review. Some of these areas pertain to 
estimates for both types of risk metrics, and some pertain more to one 
type of estimate versus the other. There are also differences in the 
uncertainties that pertain to each of the two approaches used for the 
lung function risk metric.
    An overarching and important area of uncertainty, which remains 
from the last review, and is important to our consideration of the 
exposure and risk analysis results relates to the underlying health 
effects evidence base. This analysis focuses on the evidence base 
described as providing the ``strongest evidence'' of O3 
respiratory effects (ISA, p. IS-1), the controlled human exposure 
studies, and on the array of respiratory responses documented in those 
studies (e.g., lung function decrements, respiratory symptoms, 
increased airway responsiveness and inflammation). However, we 
recognize the lack of evidence from controlled human exposure studies 
at the lower concentrations of greatest interest (e.g., 60, 70 and 80 
ppb) for children and for people of any age with asthma. While the 
limited evidence that informs our understanding of potential risk to 
people with asthma is uncertain, it indicates some potential for them 
to have lesser reserve to protect against such effects than other 
population groups under similar exposure circumstances, as summarized 
in section II.B above. Thus, the health effects reported in controlled 
human exposure studies of healthy adults may be contribute to more 
severe outcomes in people with asthma. Such a conclusion is consistent 
with the epidemiologic study findings of positive associations of 
O3 concentrations with asthma-related ED visits and hospital 
admissions (and the higher effect estimates from these studies), as 
referenced in section II.B. above and presented in detail in the ISA. 
Further, with regard to lung function decrements, information is 
lacking on the factors contributing to increased

[[Page 49858]]

susceptibility to O3-induced lung function decrements among 
some people. Thus, there is uncertainty regarding the interpretation of 
the exposure and risk estimates and the extent to which they represent 
the populations at greatest risk of O3-related respiratory 
effects.
    Aspects of the analytical design that pertain to both exposure-
based risk metrics include the estimation of ambient air O3 
concentrations for the assessed air quality scenarios, as well as the 
main components of the exposure modeling. Key uncertainties include the 
modeling approach used to adjust ambient air concentrations to meet the 
air quality scenarios of interest and the method used to interpolate 
monitor concentrations to census tracts. While the adjustment to 
conditions near, just above, or just below the current standard is an 
important area of uncertainty, the approach used has taken into account 
the currently available information and selected study areas having 
design values near the level of the current standard to minimize the 
size of the adjustment needed to meet a given air quality scenario. The 
approach also uses more recent data as inputs for the air quality 
modeling, such as more recent O3 concentration data (2015-
2017), meteorological data (2016) and emissions data (2016), as well as 
a recently updated air quality photochemical model which includes 
state-of-the-science atmospheric chemistry and physics (PA, Appendix 
3C). Further, the number of ambient monitors sited in each of the eight 
study areas provides a reasonable representation of spatial and 
temporal variability in those areas for the air quality conditions 
simulated. Among other key aspects, there is uncertainty associated 
with the simulation of study area populations (and at-risk 
populations), including those with particular physical and personal 
attributes. As also recognized in the 2014 HREA, exposures could be 
underestimated for some population groups that are frequently and 
routinely outdoors during the summer (e.g., outdoor workers, children). 
In addition, longitudinal activity patterns do not exist for these and 
other potentially important population groups (e.g., those having 
respiratory conditions other than asthma), thus limiting the extent to 
which the exposure model outputs reflect information that may be 
particular to these groups. Important uncertainties in the approach 
used to estimate energy expenditure (i.e., metabolic equivalents of 
work or METs), which are ultimately used to estimate ventilation rates, 
include the use of longer-term average MET distributions to derive 
short-term estimates, along with extrapolating adult observations to 
children. Both of these approaches are reasonable based on the 
availability of relevant data and appropriate evaluations conducted to 
date, and uncertainties associated with these steps are somewhat 
reduced in the current analyses (compared to the 2014 HREA) because of 
the added specificity and redevelopment of METs distributions, based on 
information newly available in this review, is expected to more 
realistically estimate activity-specific energy expenditure.
    With regard to the aspects of the two risk metrics, there are some 
uncertainties that apply to the estimation of lung function risk and 
not to the comparison-to-benchmarks analysis. Both lung function risk 
approaches utilized in the risk analyses incorporate some degree of 
extrapolation beyond the exposure circumstances evaluated in the 
controlled human exposure studies. This is the case in different ways 
and with differing impacts for the two approaches. One way in which 
both approaches extrapolate beyond the exposure studies concerns 
estimates of lung function risk derived for exposure concentrations 
below those represented in the evidence base. The approaches provide 
this in recognition of the potential for lung function decrements to be 
greater in unstudied at-risk population groups than is evident from the 
available studies. Accordingly, the uncertainty in the lung function 
risk estimates increases with decreasing exposure concentration and is 
particularly increased for concentrations below those evaluated in 
controlled human exposure studies.
    There are differences between the two lung function risk approaches 
in how they extrapolate beyond the controlled human exposure study 
conditions and in the impact on the estimates (with somewhat smaller 
differences for multiple day estimates).\85\ The E-R function approach 
generates nonzero predictions from the full range of nonzero 
concentrations for 7-hour average durations in which the average 
exertion levels meets or exceeds the target. The MSS model, which draws 
on evidence-based concepts of how human physiological processes respond 
to O3, extrapolates beyond the controlled experimental 
conditions with regard to exposure concentration, duration and 
ventilation rate (both magnitude and duration). The difference between 
the two models in the impact of the differing extents of extrapolation 
is illustrated by differences in the percent of the risk estimates for 
days for which the highest 7-hour average concentration is below the 
lowest 6.6-hour exposure concentration tested (PA, Tables 3-6 and 3-7). 
For example, with the E-R model, 3 to 6% of the risk to children of 
experiencing at least one day with decrements greater than 20% (for 
single years in three study areas) is associated with exposure 
concentrations below 40 ppb (the lowest concentration studied in the 
controlled human exposure studies, and at which no decrements of this 
severity occurred in any study subjects). This is in comparison to 25% 
to nearly 40% of MSS model estimates of decrements greater than 20% 
deriving from exposures below 40 ppb. The MSS model also used 
ventilation rates lower than those used for the E-R function risk 
approach (which are based on the controlled human exposure study 
conditions), contributing to relatively greater risks estimated by the 
MSS model.\86\
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    \85\ This is largely because the percent contribution to low-
concentration risk for two or more decrement days predicted by the 
E-R approach is, by design, greater than the corresponding 
contribution to low-concentration risk for one or more days. This 
also occurs because the MSS model estimates risk from a larger 
variety of exposure and ventilation conditions (PA, Tables 3-6 and 
3-7, Appendix 3D, sections 3D.3.4.2.3 and 3D.3.4.2.4).
    \86\ Limiting the MSS model results to estimates for individuals 
with at least the same exertion level achieved by study subjects 
(>=17.3 L/min-m\2\), reduces the risks of experiencing at least one 
lung function decrement by an amount between 24 to 42%. (PA, 
Appendix 3D, Table 3D-69).
---------------------------------------------------------------------------

    Many of the uncertainties previously identified as part of the 2014 
HREA as unique to the MSS model also remain as important uncertainties 
in the current assessment. For example, the extrapolation of the MSS 
model age parameter down to age 5 (from the age range of the 18- to 35-
year old study subjects to which the model was fit) is an important 
uncertainty given that children are an at-risk population in this 
assessment. There is also uncertainty in estimating the frequency and 
magnitude of lung function decrements as a result of the statistical 
form and parameters used for the MSS model inter- and intra-individual 
variability terms (PA, Appendix 3D, section 3D.3.4). As a whole, the 
differences between the two lung function risk approaches and the 
estimates generated by these approaches indicate appreciably greater 
uncertainty for the MSS model estimates than the E-R function estimates 
(PA, section 3.4.4

[[Page 49859]]

and Tables 3-6 and 3-7).\87\ In light of the uncertainties summarized 
here for the MSS model (and discussed in detail in Appendix 3D, section 
3D.3.4 of the PA), the lung function risk estimates summarized in 
section II.C.3 below are those derived using the E-R approach.
---------------------------------------------------------------------------

    \87\ The E-R function risk approach conforms more closely to the 
circumstances of the 6.6-hour controlled human exposure studies, 
such that the 7-hour duration and moderate or greater exertion level 
are necessary for nonzero risk. This approach does, however, use a 
continuous function which predicts responses for exposure 
concentrations below those studied down to zero. As a result, 
exposures below those studied in the controlled human exposures will 
result in a fraction of the population being estimated by the E-R 
function to experience a lung function decrement (albeit to an 
increasingly small degree with decreasing exposures). The MSS model, 
which has been developed based on a conceptualization intended to 
reflect a broader set of controlled human exposure studies (e.g., 
including studies of exposures to higher concentrations for shorter 
durations), does not require a 7-hour duration for estimation of a 
response, and lung function decrements are estimated for exertion 
below moderate or greater levels, as well as for exposure 
concentrations below those studied (PA, Appendix 3D, section 
3D.3.4.2; 2014 HREA section 6.3.3). These differences in the models, 
accordingly, result in differences in the extent to which they 
reflect the particular conditions of the available controlled human 
exposure studies and the frequency and magnitude of the measured 
responses.
---------------------------------------------------------------------------

    Two updates to the analysis approach since the 2014 HREA reduce 
uncertainty in the results. The first is related to the approach to 
identifying when simulated individuals may be at moderate or greater 
exertion. The approach used in the current review reduces the potential 
for overestimation of the number of people achieving the associated 
ventilation rate, an important uncertainty identified in the 2014 HREA. 
Additionally, the current analysis focuses on exposures of 7 hours 
duration to better represent the 6.6-hour exposures from the controlled 
human exposure studies (than the 8-hour exposure durations used for the 
2014 HREA and prior assessments).
    In summary, among the multiple uncertainties and limitations in 
data and tools that affect the quantitative estimates of exposure and 
risk and their interpretation in the context of considering the current 
standard, several are particularly important, some of which are similar 
to those recognized in the last review. These include uncertainty 
related to estimation of the concentrations in ambient air for the 
current standard and the additional air quality scenarios; lung 
function risk approaches that rely, to varying extents, on 
extrapolating from controlled human exposure study conditions to lower 
exposure concentrations, lower ventilation rates, and shorter 
durations; and characterization of risk for particular population 
groups that may be at greatest risk, particularly for people with 
asthma, and particularly children. Areas in which uncertainty has been 
reduced by new or updated information or methods include the use of 
more refined air quality modeling based on selection of study areas 
with design values near the current standard and a more recent model 
and model inputs, as well as updates to several inputs to the exposure 
model including changes to the exposure duration to better match those 
in the controlled human exposure studies and an alternate approach to 
characterizing periods of activity while at moderate or greater 
exertion for simulated individuals.
3. Summary of Exposure and Risk Estimates
    Exposure and risk estimates for the eight urban study areas are 
summarized here, with a focus on the estimates for air quality 
conditions adjusted to just meet the current standard. The analyses in 
this review include two types of risk estimates for the 3-year 
simulation in each study area: (1) The number and percent of simulated 
people experiencing exposures at or above the particular benchmark 
concentrations of interest in a year, while breathing at elevated 
rates; and (2) the number and percent of people estimated to experience 
at least one O3-related lung function decrement 
(specifically, FEV1 reductions of a magnitude at or above 
10%, 15% or 20%) in a year and the number and percent of people 
estimated to experience multiple lung function decrements associated 
with O3 exposures.
    The benchmark-based risk metric results are summarized in terms of 
the percent of the simulated populations of all children and children 
with asthma estimated to experience at least one day per year \88\ with 
a 7-hour average exposure concentration at or above the different 
benchmark concentrations while breathing at elevated rates under air 
quality conditions just meeting the current standard (Table 2). 
Estimates for adults, in terms of percentages, are generally lower due 
to the lesser amount and frequency of time spent outdoors at elevated 
exertion (PA, Appendix 3D, section 3D.3.2). The exception is outdoor 
workers who, due to the requirements of their job, spend more time 
outdoors. Targeted analyses of outdoor workers in the 2014 HREA (single 
study area, single year) estimated an appreciably greater portion of 
this population to experience exposures at or above benchmark 
concentration than the full adult or child populations (2014 HREA, 
section 5.4.3.2) although there are a number of uncertainties 
associated with these estimates due to appreciable limitations in the 
data underlying the analyses. For a number of reasons, including the 
appreciable data limitations (e.g., related to specific durations of 
time spent outdoors and activity data), and associated uncertainties 
summarized in Table 3D-64 of Appendix 3D of the PA, the group was not 
simulated in the current analyses.\89\
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    \88\ While the duration of an O3 season for each year 
may vary across the study areas, for the purposes of the exposure 
and risk analyses, the O3 season in each study area is 
considered synonymous with a year. These seasons capture the times 
during the year when concentrations are elevated (80 FR 65419-65420, 
October 26, 2015).
    \89\ It is expected that if an approach similar to that used in 
the 2014 HREA were used for this assessment the distribution of 
exposures (single day and multiday) would be similar to that 
estimated in the 2014 HREA (e.g., 2014 HREA, Figure 5-14), although 
with slightly lower overall percentages (and based on the comparison 
of current estimates with estimates from the 2014 HREA) (PA, 
Appendix 3D, section 3D.3.2.4).
---------------------------------------------------------------------------

    Given the recognition of people with asthma as an at-risk 
population and the relatively greater amount and frequency of time 
spent outdoors at elevated exertion of children, we focus here on the 
estimates for children, including children with asthma. Under air 
quality conditions just meeting the current standard, approximately 
less than 0.1% of any area's children with asthma, on average, were 
estimated to experience any days per year with a 7-hour average 
exposure at or above 80 ppb, while breathing at elevated rates (Table 
2). With regard to the 70 ppb benchmark, the study areas' estimates for 
children with asthma are as high as 0.7 percent (0.6% for all 
children), on average across the 3-year period, and range up to 1.0% in 
a single year. Approximately 3% to nearly 9% of each study area's 
simulated children with asthma, on average across the 3-year period, 
are estimated to experience one or more days per year with a 7-hour 
average exposure at or above 60 ppb. This range is very similar for the 
populations of all children.
    Regarding multiday occurrences, the analyses indicate that no 
children would be expected to experience more than a single day with a 
7-hour average exposure at or above 80 ppb in any year simulated in any 
location (Table 2). For the 70 ppb benchmark, the estimate is less than 
0.1% of any area's children (on average across 3-year period), both 
those with asthma and all children. The estimates for the 60 ppb 
benchmark are slightly higher, with up to 3% of

[[Page 49860]]

children estimated to experience more than a single day with a 7-hour 
average exposure at or above 60 ppb, on average (and more than 4% in 
the highest year across all eight study area locations).
    These estimates for the analyses in the current review, while based 
on conceptually similar approaches to those used in the 2014 HREA, also 
reflect the updates and revisions to those approaches that have been 
implemented since that time. The range of estimates across the study 
areas from the current assessment for air quality conditions simulated 
to just meet the current standard are similar, although the upper end 
of the ranges is slightly lower in some cases, to the estimates for 
these same populations in the 2014 HREA. For example, for air quality 
conditions just meeting the now-current standard, the 2014 HREA 
estimated 0.1 to 1.2% of all children across the study areas to 
experience, on average, at least one day with exposure at or above 70 
ppb, while at elevated ventilation, compared to the comparable 
estimates of 0.2 to 0.6% from the current analyses (PA, Appendix 3D, 
section 3D.3.2.4, Table 3D-38). There are a number of differences 
between the quantitative modeling and analyses performed in the current 
assessment and the 2014 HREA that likely contribute to the small 
differences in estimates between the two assessments (e.g., 2015-2017 
vs. 2006-2010 distribution of ambient air O3 concentrations, 
better matching of simulated exposure estimates with the 6.6-hour 
duration of the controlled human exposure studies and with the study 
subject ventilation rates).

   Table 2--Percent and Number of Simulated Children and Children With Asthma Estimated To Experience at Least One or More Days per Year With a 7-Hour
           Average Exposure at or Above Indicated Concentration While Breathing at an Elevated Rate in Areas Just Meeting the Current Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                 One or more days                Two or more days                Four or more days
                                                         -----------------------------------------------------------------------------------------------
              Exposure concentration (ppb)                  Average per    Highest in a     Average per    Highest in a     Average per    Highest in a
                                                               year         single year        year         single year        year         single year
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                Children With Asthma--Percent of Simulated Population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................  0 \B\-<0.1 \C\            0.1%               0               0               0               0
>=70....................................................         0.2-0.7            1.0%            <0.1             0.1               0               0
>=60....................................................         3.3-8.8            11.2         0.6-3.2             4.9        <0.1-0.8             1.3
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                     Children With Asthma--Number of Individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................            0-67             202               0               0               0               0
>=70....................................................        93-1,145           1,616            3-39             118               0               0
>=60....................................................     1,517-8,544          11,776      282--2,609           3,977          23-637           1,033
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                    All Children--Percent of Simulated Population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................      0 \B\-<0.1             0.1               0               0               0               0
>=70....................................................         0.2-0.6             0.9            <0.1             0.1          0-<0.1            <0.1
>=60....................................................         3.2-8.2            10.6         0.6-2.9             4.3        <0.1-0.7             1.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                         All Children--Number of Individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................           0-464           1,211               0               0               0               0
>=70....................................................       727-8,305          11,923          16-341             660             0-5              14
>=60....................................................   14,928-69,794          96,261    2,601-24,952          36,643       158-5,997           9,554
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each study area were averaged across the 3-year assessment period. Ranges reflect the ranges of averages.
\B\ A value of zero (0) means that there were no individuals estimated to have the selected exposure in any year.
\C\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).

    In framing these same exposure estimates from the perspective of 
estimated protection provided by the current standard, these results 
indicate that, in the single year with the highest concentrations 
across the 3-year period, 99% of the population of children with asthma 
would not be expected to experience such a day with an exposure at or 
above the 70 ppb benchmark; 99.9% would not be expected to experience 
such a day with exposure at or above the 80 ppb benchmark. The 
estimates, on average across the 3-year period, indicate that over 
99.9%, 99.3% and 91.2% of the population of children with asthma would 
not be expected to experience a day with a 7-hour average exposure 
while at elevated ventilation that is at or above 80 ppb, 70 ppb and 60 
ppb, respectively (Table 2, above). Further, more than approximately 
97% of all children or children with asthma are estimated to be 
protected against multiple days of exposures at or above 60 ppb. These 
estimates are of a magnitude roughly consistent with the level of 
protection that was described in establishing the current standard in 
2015 (PA, section 3.1).
    With regard to lung function risk estimated using the population-
based E-R function approach, the estimates for children with asthma are 
similar to those for all children, but with the higher end of the 
ranges for the eight study areas being just slightly higher in some 
cases (Table 3). For example, on average between 0.5 to 0.9% (and at 
most 1.0%) of children with asthma are estimated to have at least one 
day per year with a 15% (or larger) FEV1 decrement. When 
considering the same decrement for all children, on average the 
estimate is between 0.5 to 0.8% (and at most 0.9%). Somewhat larger 
differences are seen when comparing single-day occurrences of 10% (or 
larger) FEV1 decrements for the two population groups, but 
again, differing by only a few tenths of a percent (e.g., at most, 3.6% 
percent of children with asthma versus 3.3% of all children).
    Regarding multi-day occurrences, the analyses find that very few 
children are estimated to experience 15% (or larger) FEV1 
decrements (i.e., on the order of a few tenths of a percent). For 
example, at most 0.6% and 0.2% of all children (and children with 
asthma) are estimated to

[[Page 49861]]

experience 15% (or larger) and 20% (or larger) FEV1 
decrements, respectively, for two or more days, and at most, about 2.5% 
of children are estimated to experience two or more days with a 10% 
FEV1 decrement.

     Table 3--Percent of Simulated Children and Children With Asthma Estimated To Experience at Least One or More Days per Year With a Lung Function
                   Decrement at or Above 10, 15 or 20% While Breathing at an Elevated Rate in Areas Just Meeting the Current Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                 One or more days                Two or more days                Four or more days
                                                         -----------------------------------------------------------------------------------------------
               Lung function decrement \A\                  Average per    Highest in a     Average per    Highest in a     Average per    Highest in a
                                                               year         single year        year         single year        year         single year
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                      E-R Function
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                      Percent of Simulated Children With Asthma \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=20%...................................................         0.2-0.3             0.4         0.1-0.2             0.2    <0.1 \B\-0.1             0.1
>=15%...................................................         0.5-0.9             1.0         0.3-0.6             0.6         0.2-0.4             0.4
>=10%...................................................         2.3-3.3             3.6         1.5-2.4             2.6         0.9-1.7             1.8
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                          Percent of All Simulated Children \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=20%...................................................         0.2-0.3             0.4         0.1-0.2             0.2        <0.1-0.1             0.1
>=15%...................................................         0.5-0.8             0.9         0.3-0.5             0.6         0.2-0.4             0.4
>=10%...................................................         2.2-3.1             3.3         1.3-2.2             2.4         0.8-1.6             1.7
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each urban case study area were averaged across the 3-year assessment period. Ranges reflect the ranges across urban study area
  averages.
\B\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).

D. Proposed Conclusions on the Primary Standard

    In reaching proposed conclusions on the current O3 
primary standard (presented in section II.D.3), the Administrator has 
taken into account the current evidence and associated conclusions in 
the ISA, in light of the policy-relevant evidence-based and exposure- 
and risk-based considerations discussed in the PA (summarized in 
section II.D.1), as well as advice from the CASAC, and public comment 
received on the standard thus far in the review (section II.D.2). In 
general, the role of the PA is to help ``bridge the gap'' between the 
Agency's assessment of the current evidence and quantitative analyses 
(of air quality, exposure and risk), and the judgments required of the 
Administrator in determining whether it is appropriate to retain or 
revise the NAAQS. Evidence-based considerations draw upon the EPA's 
integrated assessment of the scientific evidence of health effects 
related to O3 exposure presented in the ISA (summarized in 
section II.B above) to address key policy-relevant questions in the 
review. Similarly, the exposure- and risk-based considerations draw 
upon our assessment of population exposure and associated risk 
(summarized in section II.C above) in addressing policy-relevant 
questions focused on the potential for O3 exposures 
associated with respiratory effects under air quality conditions 
meeting the current standard.
    The approach to reviewing the primary standard is consistent with 
requirements of the provisions of the CAA related to the review of the 
NAAQS and with how the EPA and the courts have historically interpreted 
the CAA. As discussed in section I.A above, these provisions require 
the Administrator to establish primary standards that, in the 
Administrator's judgment, are requisite (i.e., neither more nor less 
stringent than necessary) to protect public health with an adequate 
margin of safety. Consistent with the Agency's approach across all 
NAAQS reviews, the EPA's approach to informing these judgments is based 
on a recognition that the available health effects evidence generally 
reflects a continuum that includes ambient air exposures for which 
scientists generally agree that health effects are likely to occur 
through lower levels at which the likelihood and magnitude of response 
become increasingly uncertain. The CAA does not require the 
Administrator to establish a primary standard at a zero-risk level or 
at background concentration levels, but rather at a level that reduces 
risk sufficiently so as to protect public health, including the health 
of sensitive groups, with an adequate margin of safety.
    The proposed decision on the adequacy of the current primary 
standard described below is a public health policy judgment by the 
Administrator that draws on the scientific evidence for health effects, 
quantitative analyses of population exposures and/or health risks, and 
judgments about how to consider the uncertainties and limitations that 
are inherent in the scientific evidence and quantitative analyses. The 
four basic elements of the NAAQS (i.e., indicator, averaging time, 
form, and level) have been considered collectively in evaluating the 
health protection afforded by the current standard. The Administrator's 
final decision will additionally consider public comments received on 
this proposed decision.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy 
Assessment
    The main focus of the policy-relevant considerations in the PA is 
consideration of the question: Does the currently available scientific 
evidence- and exposure/risk-based information support or call into 
question the adequacy of the protection afforded by the current primary 
O3 standard? The PA response to this overarching question 
takes into account discussions that address the specific policy-
relevant questions for this review, focusing first on consideration of 
the evidence, as evaluated in the ISA, including that newly available 
in this review, and the extent to which it alters key conclusions 
supporting the current standard. The PA also considers the quantitative 
exposure and risk estimates drawn from the exposure/risk analyses 
(presented in detail in Appendices 3C and 3D of the PA), including 
associated limitations and uncertainties, and the extent to which they 
may indicate different conclusions from those in the last review 
regarding the magnitude of risk,

[[Page 49862]]

as well as level of protection from adverse effects, associated with 
the current standard. The PA additionally considers the key aspects of 
the evidence and exposure/risk estimates that were emphasized in 
establishing the current standard, as well as the associated public 
health policy judgments and judgments about the uncertainties inherent 
in the scientific evidence and quantitative analyses that are integral 
to consideration of whether the currently available information 
supports or calls into question the adequacy of the current primary 
O3 standard (PA, section 3.5).
    With regard to the support in the current evidence for 
O3 as the indicator for photochemical oxidants, no newly 
available evidence has been identified in this review regarding the 
importance of photochemical oxidants other than O3 with 
regard to abundance in ambient air, and potential for health 
effects.\90\ As summarized in section 2.1 of the PA, O3 is 
one of a group of photochemical oxidants formed by atmospheric 
photochemical reactions of hydrocarbons with NOX in the 
presence of sunlight, with O3 being the only photochemical 
oxidant other than nitrogen dioxide that is routinely monitored in 
ambient air. Data for other photochemical oxidants are generally 
derived from a few focused field studies such that national-scale data 
for these other oxidants are scarce (ISA, Appendix 1, section 1.1; 2013 
ISA, sections 3.1 and 3.6). Moreover, few studies of the health impacts 
of other photochemical oxidants beyond O3 have been 
identified by literature searches conducted for the 2013 ISA or 2006 
AQCD (ISA, Appendix 1, section 1.1). As stated in the ISA, ``the 
primary literature evaluating the health . . . effects of photochemical 
oxidants includes ozone almost exclusively as an indicator of 
photochemical oxidants'' (ISA, section IS.1.1, p. IS-3). Thus, as was 
the case for previous reviews, the PA finds that the evidence base for 
health effects of photochemical oxidants does not indicate an 
importance of any other photochemical oxidants such that O3 
continues to be appropriately considered for the primary standard's 
indicator.
---------------------------------------------------------------------------

    \90\ Close agreement between past O3 measurements and 
photochemical oxidant measurements indicated the very minor 
contribution of other oxidant species in comparison to O3 
(U.S. DHEW, 1970).
---------------------------------------------------------------------------

    The currently available evidence on the health effects of 
O3, including that newly available in this review, is 
largely consistent with the conclusions reached in the last review 
regarding health effects causally related to O3 exposures 
(i.e., respiratory effects). Specifically, as in the last review, 
respiratory effects are concluded to be causally related to short-term 
exposures to O3. Also, as in the last review, the evidence 
is sufficient to conclude that the relationship between longer-term 
O3 exposures and respiratory effects is likely to be causal 
(ISA, section IS.1.3.1, Appendix 3). Further, while a causal 
determination was not made in the last review regarding metabolic 
effects, the ISA for this review finds there to be sufficient evidence 
to conclude there to likely be a causal relationship of short-term 
O3 exposures and metabolic effects and finds the evidence to 
be suggestive of, but not sufficient to infer, such a relationship 
between long-term O3 exposure and metabolic effects (ISA, 
section IS.1.3.1). These new determinations are based on evidence on 
this category of effects, largely from experimental animal studies, 
that is newly available in this review (ISA, Appendix 5). Additionally, 
conclusions reached in the current review differ with regard to 
cardiovascular effects and mortality, based on newly available evidence 
in combination with uncertainties in the previously available evidence 
that had been identified in the last review (ISA, Appendix 4, section 
4.1.17 and Appendix 6, section 6.1.8). The current evidence base is 
concluded to be suggestive of, but not sufficient to infer, causal 
relationships between O3 exposures (short- and long-term) 
and cardiovascular effects, mortality, reproductive and developmental 
effects, and nervous system effects (ISA, section IS.1.3.1). As in the 
last review, the strongest evidence, including with regard to 
characterization of relationships between O3 exposure and 
occurrence and magnitude of effects, is for respiratory effects, and 
particularly for effects such as lung function decrements, respiratory 
symptoms, airway responsiveness, and respiratory inflammation.
    The current evidence does not alter our understanding of 
populations at increased risk from health effects of O3 
exposures. As in the last review, people with asthma, and particularly 
children, are the at-risk population groups for which the evidence is 
strongest. In addition to populations with asthma, groups with 
relatively greater exposures, particularly those who spend more time 
outdoors during times when ambient air concentrations of O3 
are highest and while engaged in activities that result in elevated 
ventilation, are recognized as at increased risk. Such groups include 
outdoor workers and children. Other groups identified as at risk, and 
for which the recent evidence is less clear, include older adults (in 
light of changes in causality determinations, as discussed in section 
II.B.2 above), and recent evidence regarding individuals with reduced 
intake of certain nutrients and individuals with certain genetic 
variants does not provide additional information for these groups 
beyond the evidence available at the time of the last review (ISA, 
section IS.4.4).
    As in the last review, the most certain evidence of health effects 
in humans elicited by specific O3 exposure concentrations is 
provided by controlled human exposure studies (largely with generally 
healthy adults). This category of short-term studies includes an 
extensive evidence base of 1- to 3-hour studies, conducted with 
continuous or intermittent exercise and generally involving relatively 
higher exposure concentrations, e.g., greater than 120 ppb (as 
summarized in the PA, Appendix 3A, Table 3A-3, based on assessments of 
the studies in the 1996 and 2006 AQCDs, as well as the 2013 and current 
ISA). Given the lack of ambient air concentrations of this magnitude in 
areas meeting the current standard (as documented in section 2.4.1 of 
the PA), the focus in reviewing the current standard continues to 
primarily be on a second group of somewhat longer-duration studies of 
much lower exposure concentrations. These studies employ a 6.6-hour 
protocol that includes six 50-minute periods of exercise at moderate or 
greater exertion.
    Respiratory effects continue to be the effects for which the 
experimental information regarding exposure concentrations eliciting 
effects is well established, as summarized here and in section II.B.3 
above. Such information allows for characterization of potential 
population risk associated with O3 in ambient air under 
conditions allowed by the current standard. The respiratory effects 
evidence includes support from a large number of epidemiologic studies 
that report positive associations of O3 with severe 
respiratory health outcomes, such as asthma-related hospital admissions 
and emergency department visits, coherent with findings from the 
controlled human exposure and experimental animal studies. However, as 
summarized in section II.B.3 above, all but a few of these short- and 
long-term studies (and all U.S. studies) include areas and periods in 
which O3 exceeds the current standard, making them less 
useful with regard to indication of effects of exposures that would 
occur with air quality allowed by the current standard.

[[Page 49863]]

    Within the evidence base for the newly identified category of 
metabolic effects, the evidence derives largely from experimental 
animal studies of exposures appreciably higher than those for the 6.6-
hour human exposure studies along with a small number of epidemiologic 
studies. The PA notes that, as discussed in section II.B.3 above, these 
studies do not prove to be informative to our consideration of exposure 
circumstances likely to elicit health effects.
    Thus, the PA finds that the currently available evidence regarding 
O3 exposures associated with health effects is largely 
similar to that available at the time of the last review and does not 
indicate effects attributable to exposures of shorter duration or lower 
concentrations than previously understood. The 6.6-hour controlled 
human exposure studies of respiratory effects remain the focus for our 
consideration of exposure circumstances associated with O3 
health effects. Based on these studies, the exposure concentrations 
investigated range from as low as approximately 40 ppb to 120 ppb. This 
information on concentrations that have been found to elicit effects 
for 6.6-hour exposures while exercising is unchanged from what was 
available in the last review. The lowest concentration for which lung 
function decrements have been found to be statistically significantly 
increased over responses to filtered air remains approximately 60 ppb 
\91\ (target concentration, as average across exercise periods), at 
which group mean O3-related FEV1 decrements on 
the order of 2% to 3.5% have been reported (with decrements on the 
order of 2% to 3% of statistically significance), with associated 
individual study subject variability in decrement size; these results 
were not accompanied by a statistically significant increase in 
respiratory symptoms (Table 1).\92\ In the single study assessing the 
next highest exposure concentration (73 ppb as the 6.6-hour average 
based on study-reported measurements), the group mean FEV1 
decrement was higher (6%) and was also statistically significant, as 
were respiratory symptom scores, as summarized in section II.B.3 above. 
At still higher exposure concentrations (80 ppb and above), the 
reported incidence of both respiratory symptom scores and 
O3-related lung function decrements in the study subjects is 
increased and the incidence of decrements at or above 15% is larger. 
Other respiratory effects, such as inflammatory response and airway 
resistance, are also increased at higher exposures (ISA; 2013 ISA).
---------------------------------------------------------------------------

    \91\ Two studies have assessed exposure concentrations at the 
lower concentration of 40 ppb, with no statistically significant 
finding of O3-related FEV1 decrement for the 
group mean in either study, which is just above 1% in one study and 
well below 1% in the second (Table 1).
    \92\ A statistically significant, small increase in a marker of 
airway inflammation was observed in one controlled human exposure 
study following 6.6-hour exposures to 60 ppb (Table 1). An increase 
in respiratory symptoms has not been reported with this exposure 
level.
---------------------------------------------------------------------------

    The PA concludes that important uncertainties identified in the 
health effects evidence at the time of the last review generally remain 
in the current evidence. Although the evidence clearly demonstrates 
that short-term O3 exposures cause respiratory effects, as 
was the case in the last review, uncertainties remain in several 
aspects of our understanding of these effects. These include 
uncertainties related to exposures likely to elicit effects (and the 
associated severity and extent) in population groups not studied, or 
less well studied (including individuals with asthma and children) and 
also the severity and prevalence of responses to short (e.g., 6.6- to 
8-hour) O3 exposures at and below 60 ppb. The PA 
additionally recognizes uncertainties associated with the epidemiologic 
studies concerning the potential influence of exposure history and co-
exposure to other pollutants (including complications of prior 
population exposures) on the relationship between short-term 
O3 exposure and respiratory effects. In so doing, however, 
the PA notes the appreciably greater strength in the epidemiologic 
evidence in its support for determination of a causal relationship for 
respiratory effects than that related to other categories, such as 
metabolic effects, for the current ISA newly determines there likely to 
be a causal relationship with short-term O3 exposures (as 
summarized in section II.B.3 above), and recognizes the greater 
uncertainty with regard relationships between O3 exposures 
and health effects other than respiratory effects. The array of 
important areas of uncertainty related to the current health evidence, 
including the evidence newly available in this review, is summarized 
below.
    With regard to less well studied population groups, the PA notes 
that the majority of the available studies have generally involved 
healthy young adult subjects, although there are some studies involving 
subjects with asthma, and a limited number of studies, generally of 
very short durations (i.e., less than four hours), involving 
adolescents and adults older than 50 years. For example, the only 
controlled human exposure study of 6.6- to 8-hour duration (7.6 hours 
with quasi-continuous light exercise) conducted in people with asthma 
was for an exposure concentration of 160 ppb (PA, Appendix 3A, Table 
3A-2). Given a general lack of studies using subjects that have asthma, 
particularly those at exposure concentrations likely to occur under 
conditions meeting the current standard, uncertainties remain with 
regard to characterizing the response in people with asthma while at 
elevated ventilation to lower exposure concentrations, e.g., below 80 
ppb. The extent to which the epidemiologic evidence, including that 
newly available, can inform this specific area of uncertainty also may 
be limited.\93\ As discussed in section II.B.2 above, given the effects 
of asthma on the respiratory system, exposures associated with 
significant respiratory responses in healthy people may pose an 
increased risk of more severe responses, including asthma exacerbation, 
in people with asthma. Thus, uncertainty remains with regard to the 
responses of the populations, such as children with asthma, that may be 
most at risk of O3-related respiratory effects (e.g., 
through an increased likelihood of severe responses, or greatest 
likelihood of response) to short-term (e.g., 6.6 hr) exposures with 
exercise to concentrations at or below 80 ppb.
---------------------------------------------------------------------------

    \93\ Associations of health effects with O3 that are 
reported in the epidemiologic analyses are based on air quality 
concentration metrics used as surrogates for the actual pattern of 
O3 exposures experienced by study population individuals 
over the period of a particular study. Accordingly, the studies are 
limited in what they can convey regarding the specific patterns of 
exposure circumstances (e.g., magnitude of concentrations over 
specific duration and frequency) that might be eliciting reported 
health outcomes.
---------------------------------------------------------------------------

    Other areas of uncertainty concerning the potential influence of 
O3 exposure history and co-exposure to other pollutants on 
the relationship between O3 exposures and respiratory 
effects in epidemiologic studies also remain from the last review. As 
in the epidemiologic evidence in the last review, there is a limited 
number of studies that include copollutant analyses for a small set of 
pollutants (e.g., PM or NO2). Recent studies with such 
analyses suggest that observed associations between O3 
concentrations and respiratory effects are independent of co-exposures 
to correlated pollutants or aeroallergens (ISA, sections IS.4.3.1 and 
IS.6.1; Appendix 3, sections 3.1.10.1 and 3.1.10.2). Despite the 
increased prevalence of copollutant modeling in recent epidemiologic 
studies, uncertainty still exists with regard to the independent effect 
of O3 given the high correlations observed for some 
copollutants in some studies and the small fraction of all atmospheric

[[Page 49864]]

pollutants included in these analyses (ISA, section IS.4.3.1; Appendix 
2, section 2.5).
    Further, although there remains uncertainty in the evidence with 
regard to the potential role of exposures to O3 in eliciting 
health effects other than respiratory effects, the evidence has been 
strengthened since the last review with regard to metabolic effects. As 
noted in section II.B.1 above, the ISA newly identifies metabolic 
effects as likely to be causally related to short-term O3 
exposures. The evidence supporting this relationship is limited and not 
without its own uncertainties, such as the fact that the conclusion for 
this relationship is based primarily on animal toxicological studies 
conducted at much higher O3 concentrations than those common 
in ambient air in the U.S. Only a handful of epidemiologic studies of 
short-term O3 exposure and metabolic effects, with some 
inconsistencies, are available, ``many of these did not control for 
copollutant confounding,'' and the two U.S. studies in the group did 
not find a statistically significant association (ISA, p. 5-29 and 
Appendix 5, section 5.1; PA, section 3.3).
    With regard to the evidence for other categories of health effects, 
its support for a causal relationship with O3 in ambient air 
is appreciably more uncertain. For example, as noted in section II.B.1 
above, the ISA has determined the evidence to be suggestive of, but not 
sufficient to infer, a causal relationship between long-term 
O3 exposures and metabolic effects, and between 
O3 exposures and several other categories of health effects, 
including effects on the cardiovascular, reproductive and nervous 
systems, and mortality (ISA, section IS.4.3).\94\ Additionally, the ISA 
finds the evidence to be inadequate to determine if a causal 
relationship exists with O3 and cancer (ISA, section 
IS.4.3).
---------------------------------------------------------------------------

    \94\ An evidence base determined to be ``suggestive of, but not 
sufficient to infer, a causal relationship'' is described as 
``limited, and chance, confounding, and other biases cannot be ruled 
out'' (U.S. EPA, 2015, p. 23).
---------------------------------------------------------------------------

    As at the time of the last review, consideration of the scientific 
evidence in the current review is informed by results from a newly 
performed quantitative analysis of estimated population exposure and 
associated risk. The overarching PA consideration regarding these 
results is whether they alter the overall conclusions from the previous 
review regarding health risk associated with exposure to O3 
in ambient air and associated judgments on the adequacy of public 
health protection provided by the now-current standard. The 
quantitative exposure and risk analyses completed in this review update 
and in many ways improve upon analyses completed in the last review (as 
summarized in section II.C.1 above).
    The exposure and risk analyses conducted for this review, as was 
true for those conducted for the last review, develop exposure and risk 
estimates for study area populations of children with asthma, as well 
as the populations of all children in each study area. The primary 
analyses focus on exposure and risk associated with air quality that 
might occur in an area under conditions that just meet the current 
standard. These study areas reflect different combinations of different 
types of sources of O3 precursor emissions, and also 
illustrate different patterns of exposure to O3 
concentrations in a populated area in the U.S. (PA, Appendix 3C, 
section 3C.2). While the same conceptual air quality scenario is 
simulated in all eight study areas (i.e., conditions that just meet the 
existing standard), variability in emissions patterns of O3 
precursors, meteorological conditions, and population characteristics 
in the study areas contribute to variability in the estimated magnitude 
of exposure and associated risk across study areas. In this way, the 
eight areas provide a variety of examples of exposure patterns that can 
be informative to the Administrator's consideration of potential 
exposures and risks that may be associated with air quality conditions 
occurring under the current O3 standard.
    In considering the exposure and risk analyses available in this 
review, the PA notes that there are a number of ways in which the 
current analyses update and improve upon those available in the last 
review. These include a number of improvements to input data and 
modeling approaches summarized in section II.C.1 above. As in prior 
reviews, exposure and risk are estimated from air quality scenarios 
designed to just meet an O3 standard in all its elements. 
That is, the air quality scenarios are defined by the highest design 
value in the study area, which is the monitor location with the highest 
3-year average of annual fourth highest daily maximum 8-hour 
O3 concentrations (e.g., equal to 70 ppb for the current 
standard scenario). The current risk and exposure analyses include air 
quality simulations based on more recent ambient air quality data that 
include O3 concentrations closer to the current standard 
than was the case for the development of the air quality scenarios in 
the last review. As a result of this and the use of updated 
photochemical modeling, there is reduced uncertainty associated with 
the spatial and temporal patterns of O3 concentrations that 
define these scenarios across all eight study areas. Additionally, the 
approach for deriving population exposure estimates, both for 
comparison to benchmark concentrations and for use in deriving lung 
function risk using the E-R function approach, has been modified to 
provide for a better match of the simulated population exposure 
estimates with the 6.6-hour duration of the controlled human exposure 
studies and with the study subject ventilation rates. Together, these 
differences, as well as a variety of updates to model inputs, are 
believed to reduce uncertainty associated with interpretation of the 
analysis results.
    The PA also notes the array of air quality and exposure 
circumstances represented by the eight study areas. As summarized in 
section II.C.1 above, the areas fall into seven of the nine climate 
regions in the continental U.S. The population sizes of the associated 
metropolitan areas range in size from approximately 2.4 to 8 million 
and vary in population demographic characteristics. While there are 
uncertainties and limitations associated with the exposure and risk 
estimates, as noted in II.C.2, the PA considers the factors recognized 
here to contribute to their usefulness in informing the current review.
    The PA gives primary attention to results for the comparison-to-
benchmarks analysis in recognition of the relatively lesser uncertainty 
of these results (than the lung function risk estimates), and also of 
the broader characterization of respiratory effects that they can 
inform, as noted in section II.C above. Similarly, the results for this 
risk metric also received greater emphasis in the last review and were 
a focus in establishing the current standard in 2015. The estimates 
across all study areas from the current review are generally similar to 
those reported across all study areas assessed in the last review, 
particularly for estimates for two or more occurrences at or above a 
benchmark, and for the 80 ppb benchmark (Table 4). For consistency with 
the estimates highlighted in the 2015 review (e.g., 80 FR 65313-65315, 
October 26, 2015), the PA comparison, summarized in Table 4 below, 
focuses on the simulated population of all children. We additionally 
note, however, the similarity of the estimates for all children to the 
estimates for the simulated population of children with asthma (Table 
2). For example, for urban study areas with air quality that just meets 
the current standard, as many as 0.7% of children with asthma, on

[[Page 49865]]

average across the 3-year period, and up to 1.0% in a single year might 
be expected to experience, while at elevated exertion, at least one day 
with a 7-hour average O3 exposure concentration at or above 
70 ppb (Table 2). The corresponding estimates for the simulated 
population of all children are as many as 0.6% of all children, on 
average across the 3-year period, and up to 0.9% in a single year 
(Table 2). For the benchmark concentration of 80 ppb (which reflects 
the potential for more severe effects), a much lower percentage (0.1%) 
of children with asthma, on average across the 3-year period or in any 
single year (compared to less than 0.1% on average and as many as 0.1% 
in a single year for all children), might be expected to experience, 
while at elevated exertion, at least one day with such a concentration 
(Table 2). Regarding estimates for multiple days, the percent of 
children with asthma (as well as the percent of all children) estimated 
to experience two or more days with an exposure at or above 70 ppb is 
less than 0.1%, on average across three years, and up to 0.1% in a 
single year period. There are no children estimated to experience more 
than a single day per year with a 7-hour average O3 
concentration at or above 80 ppb. With regard to the lowest benchmark 
concentration of 60 ppb, the percentages for the simulated population 
of children with asthma for more than a single day occurrence are 3%, 
on average across the three years, and just below 5% in a single year 
period, with just slightly lower percentages (2.9 and 4.3%) for the 
population of all children (Table 2).
    The PA additionally compares the estimates derived in the current 
analyses with those from the 2014 HREA in the last review, finding them 
to be quite similar.\95\ For example, with regard to the 80 ppb 
benchmark and air quality conditions just meeting the current standard, 
the percentage of children estimated to experience a day or more with 
such an exposure, ranges from zero (in both assessments) up to 0.1% 
(2014 HREA) and a nonzero value less than 0.1% (current assessment), on 
average across the three year period (Table 4). The estimates for the 
highest year (0.2 and 0.1%, for the 2014 and current assessments, 
respectively) are within 0.1% of each other. Both assessments estimate 
zero children to experience two or more days with an exposure at or 
above 80 ppb. The differences observed, which are particularly evident 
for the lower benchmarks and in the estimates for the highest year, are 
generally slight. Much larger differences are seen in comparing 
different air quality scenario results for the same benchmark. For 
example, for the 70 ppb benchmark, the differences between the 75 ppb 
scenario and the current standard (or between the 65 ppb scenario and 
the current standard) in either assessment are appreciably larger than 
are the slight differences observed between the two assessments for any 
air quality scenario. The factors likely contributing to the slight 
differences, e.g., for the lowest benchmark, include greater variation 
in ambient air concentrations in some of the study areas in the 2014 
HREA, as well as the lesser air quality adjustments required in study 
areas for the current assessment due to closer proximity of conditions 
to meeting the current standard (70 ppb).\96\ Other important 
differences between the two assessments are the updates made to the 
ventilation rates used for identifying when a simulated individual is 
at moderate or greater exertion and the use of 7 hours for the exposure 
duration. Both of these changes were made to provide closer linkages to 
the conditions of the controlled human exposure studies which are the 
basis for the benchmark concentrations. Thus, the PA recognizes there 
to be reduced uncertainty associated with the current estimates.
---------------------------------------------------------------------------

    \95\ In this comparison, the PA focuses on the full array of 
study areas assessed in each analysis given the purpose of each in 
providing estimates across a range of study areas to inform decision 
making with regard to the exposures and risks that may occur across 
the U.S. in areas that just meet the current standard.
    \96\ The 2014 HREA air quality scenarios involved adjusting 
2006-2010 ambient air concentrations, and some study areas had 
design values in that time period that were well above the then-
existing standard (and more so for the current standard). Study 
areas included the current exposure analysis had 2015-2017 design 
values close to the current standard, requiring less of an 
adjustment for the current standard (70 ppb) air quality scenario.

 Table 4--Comparison of Current Assessment and 2014 HREA (All Study Areas) for Percent of Children Estimated To
   Experience at Least One, or Two, Days With an Exposure at or Above Benchmarks While at Moderate or Greater
                                                    Exertion
----------------------------------------------------------------------------------------------------------------
                                           Estimated average % of simulated    Estimated average % of simulated
                                          children with at least one day per    children with at least two days
                                              year at or above benchmark        per year at or above benchmark
     Air quality scenario (DV, ppb)           (highest in single season)          (highest in single season)
                                         -----------------------------------------------------------------------
                                           Current PA \A\     2014 HREA \B\    Current PA \A\     2014 HREA \B\
----------------------------------------------------------------------------------------------------------------
                                   Benchmark Exposure Concentration of 80 ppb
----------------------------------------------------------------------------------------------------------------
75......................................      <0.1 \A\-0.3       0-0.3 (1.1)     0-<0.1 (<0.1)           0 (0.1)
                                                     (0.6)
70......................................      0-<0.1 (0.1)       0-0.1 (0.2)             0 (0)             0 (0)
65......................................     0-<0.1 (<0.1)             0 (0)             0 (0)             0 (0)
----------------------------------------------------------------------------------------------------------------
                                   Benchmark Exposure Concentration of 70 ppb
----------------------------------------------------------------------------------------------------------------
75......................................     1.1-2.0 (3.4)     0.6-3.3 (8.1)     0.1-0.3 (0.7)     0.1-0.6 (2.2)
70......................................     0.2-0.6 (0.9)     0.1-1.2 (3.2)        <0.1 (0.1)       0-0.1 (0.4)
65......................................       0-0.2 (0.2)       0-0.2 (0.5)     0-<0.1 (<0.1)             0 (0)
----------------------------------------------------------------------------------------------------------------
                                   Benchmark Exposure Concentration of 60 ppb
----------------------------------------------------------------------------------------------------------------
75......................................   6.6-15.7 (17.9)   9.5-17.0 (25.8)     1.7-8.0 (9.9)    3.1-7.6 (14.4)
70......................................    3.2-8.2 (10.6)   3.3-10.2 (18.9)     0.6-2.9 (4.3)     0.5-3.5 (9.2)

[[Page 49866]]

 
65......................................     0.4-2.3 (3.7)       0-4.2 (9.5)    <0.1-0.3 (0.5)       0-0.8 (2.8)
----------------------------------------------------------------------------------------------------------------
\A\ For the current analysis, calculated percent is rounded to the nearest tenth decimal using conventional
  rounding. Values equal to zero are designated by ``0'' (there are no individuals exposed at that level).
  Small, non-zero values that do not round upwards to 0.1 (i.e., <0.05) are given a value of ``<0.1''.
\B\ For the 2014 HREA. calculated percent was rounded to the nearest tenth decimal using conventional rounding.
  Values that did not round upwards to 0.1 (i.e., <0.05) were given a value of ``0''.

    Overall, the comparison-to-benchmarks estimates are generally 
similar to those which were the focus in the 2015 decision on 
establishing the current standard. For example, in the 2015 decision to 
set the standard level at 70 ppb, the Administrator took note of 
several findings for the air quality scenarios for this level, noting 
that ``a revised standard with a level of 70 ppb is estimated to 
eliminate the occurrence of two or more exposures of concern to 
O3 concentrations at or above 80 ppb and to virtually 
eliminate the occurrence of two or more exposures of concern to 
O3 concentrations at or above 70 ppb for all children and 
children with asthma, even in the worst-case year and location 
evaluated'' (80 FR 65363, October 26, 2015). This statement remains 
true for the results of the current assessment (Table 4). With regard 
to the 60 ppb benchmark, for which the 2015 decision placed relatively 
greater weight on multiple (versus single) occurrences of exposures at 
or above it, the Administrator at that time noted the 2014 HREA 
estimates for the 70 ppb air quality scenario that estimated 0.5 to 
3.5% of children to experience multiple such occurrences on average 
across the study areas, stating that the now-current standard ``is 
estimated to protect the vast majority of children in urban study areas 
. . . from experiencing two or more exposures of concern at or above 60 
ppb'' (80 FR 65364, October 26, 2015). The corresponding estimates, on 
average across the 3-year period in the current assessments, are 
remarkably similar at 0.6 to 2.9% (Table 4).
    In considering the public health implications of the estimated 
occurrence of exposures of different magnitudes, the PA considers the 
magnitude or severity of the effects associated with the estimated 
exposures as well as their adversity, the size of the population 
estimated to experience exposures associated with such effects, as well 
as consideration for such implications in previous NAAQS decisions and 
ATS policy statements (as summarized in section II.B.2 above). As an 
initial matter, the PA considers the severity of responses associated 
with the exposure and risk estimates, taking note of the health effects 
evidence for the different benchmark concentrations and judgments made 
with regard to the severity of these effects in the last review. As in 
the last review, the PA recognizes the greater prevalence of more 
severe lung function decrements among study subjects exposed to 80 ppb 
or higher concentrations compared to 60 or 70 ppb exposure 
concentrations, as well as the prevalence of other effects such as 
respiratory symptoms. In so doing, the PA notes that such exposures are 
appropriately considered to be associated with adverse respiratory 
effects consistent with past and recent ATS position statements. 
Studies of 6.6-hour controlled human exposures, with quasi-continuous 
exercise, to the lowest benchmark concentration of 60 ppb have found 
small but statistically significant O3-related decrements in 
lung function (specifically reduced FEV1) and airway 
inflammation. Somewhat above 70 ppb,\97\ statistically significant 
increases in lung function decrements, of a somewhat greater magnitude 
(e.g., approximately 6% increase, as study group average, versus 2 to 
3% [Table 1]), and respiratory symptoms have been reported, which has 
led to characterization of these exposure conditions as also being 
associated with adverse responses, consistent with past ATS statements 
as summarized in section II.B.1 above (e.g., 80 FR 65343, 65345, 
October 26, 2015).
---------------------------------------------------------------------------

    \97\ As noted in sections II.A.1 and II.B.3 above, the 70 ppb 
target exposure concentration comes from Schelegle et al. (2009). 
That study reported, based on O3 measurements during the 
six 50-minute exercise periods, that the mean O3 
concentration during the exercise portion of the study protocol was 
72 ppb. Based on the measurements for the six exercise periods, the 
time weighted average concentration across the full 6.6-hour 
exposure was 73 ppb (Schelegle et al., 2009).
---------------------------------------------------------------------------

    The PA additionally takes note of the greater significance of 
estimates for multiple occurrences of exposures at or above these 
benchmarks consistent with the evidence, as has been recognized in 
multiple past O3 NAAQS reviews. The role of such a 
consideration has also differed across the three benchmarks. More 
specifically, while estimates of one or more exposures at or above the 
higher benchmark concentrations (70 ppb and 80 ppb) was an important 
consideration in the decision on the current standard, estimates of 
multiple exposures at or above the lowest benchmark concentration of 60 
ppb were given greater weight than estimates for one or more such 
exposures. More specifically, in the 2015 decision leading to 
establishment of the current standard, a greater emphasis on protection 
against multiple (versus single) occurrences of exposures at or above 
60 ppb last was based in part on a recognition of the lesser severity 
of the effects at this exposure level in combination with the 
recognition that for effects such as inflammation (even when occurring 
to a small extent). This greater emphasis reflected a recognition that, 
while isolated occurrences can resolve entirely, repeated occurrences 
from repeated exposure could potentially result in more severe effects 
(2013 ISA, section 6.2.3 and p. 6-76). Additionally, while even 
multiple occurrences of such effects of lesser severity to otherwise 
healthy individuals may not result in severe effects, they may 
contribute to more important effects in individuals with compromised 
respiratory function, such as those with asthma. The ascribing of 
greater significance to repeated occurrences of exposures of potential 
concern is also consistent with public

[[Page 49867]]

health judgments in NAAQS reviews for other pollutants, such as sulfur 
oxides and CO (84 FR 9900, March 18, 2019; 76 FR 54307, August 31, 
2011).
    As in the last review, while the exposure-based analyses include 
two types of metrics, the quantitative exposure and risk analyses 
results in which the PA expresses the greatest confidence are estimates 
from the comparison-to-benchmarks analysis, as discussed in section 
II.C above. In light of the conclusions that people with asthma and 
children are at-risk populations for O3-related health 
effects (summarized in section II.B.2 above) and the exposure and risk 
analysis findings of higher exposures and risks for children (in terms 
of percent of that population), the PA focused its consideration of the 
analysis results on children (and also specifically children with 
asthma). The exposure and risk estimates indicate that in some areas of 
the U.S. where O3 concentrations just meet the current 
standard, on average across the 3-year period simulated, less than 1%, 
and less than 0.1% of the simulated population of children with asthma 
might be expected to experience a single day per year with a maximum 7-
hour exposure at or above 70 ppb and 80 ppb, respectively, while 
breathing at an elevated rate (Table 2). With regard to the lowest 
benchmark considered (60 ppb), the corresponding percentage is less 
than approximately 9%, on average across the 3-year period (Table 2). 
The corresponding estimates for the 75 ppb air quality scenario are 
notably higher, e.g., 1.1 to 2.1% of children with asthma, on average 
across the 3-year design period, for the 70 ppb benchmark, with as many 
as 3.9% in a single year (PA, Table 3-5). The estimates for the 65 ppb 
scenario are appreciably lower (PA, Table 3-5).
    While recognizing greater uncertainty and accordingly less 
confidence in the lung function risk estimates, the PA noted the 
results based on the E-R model that estimated 0.2 to 0.3% of children 
with asthma, on average across the 3-year design period are estimated 
to experience one or more days with a lung function decrement at or 
above 20%, and 0.5 to 0.9% to experience one or more days with a 
decrement at or above 15% (Table 3). In a single year, the highest 
estimate is 1.0% of this at-risk population expected to experience one 
or more days with a decrement at or above 15%. The corresponding 
estimate for two or more days is 0.6% (Table 3).
    As summarized in section II.B.2 above, the size of the at-risk 
population (people with asthma, particularly children) in the U.S. is 
substantial. Nearly 8% of the total U.S. population and 8.4% of U.S. 
children have asthma.\98\ The asthma prevalence in U.S. child 
populations (younger than 18 years) of different races or ethnicities 
ranges from 6.2% for Hispanic, Mexican or Mexican-American children to 
12.6% for black non-Hispanic children (PA, Table 3-1). This is well 
reflected in the exposure and risk analysis study areas in which the 
asthma prevalence ranged from 7.7% to 11.2% of the total populations 
and 9.2% to 12.3% of the children. In each study area, the prevalence 
varies among census tracts, with the highest tract having a prevalence 
in boys of 25.5% and a prevalence in girls of 17.1% (PA, Appendix 3D, 
Table 3D-3).
---------------------------------------------------------------------------

    \98\ The number of people in the US with asthma is estimated to 
be about 25 million. As shown in the PA, Table 3-1 the estimated 
number of people with asthma was 25,191,000 in 2017. The updated 
estimate from the 2018 National Health Interview Survey is 
24,753,000 (CDC, 2020). For children (younger than 18 years), the 
2017 estimate is approximately 6,182,000, while the estimate for 
2018 is slightly lower at 5,530,131 (PA, Table 3-1).
---------------------------------------------------------------------------

    The exposure and risk analyses inherently recognize that 
variability in human activity patterns (where people go and what they 
do) is key to understanding the magnitude, duration, pattern, and 
frequency of population exposures. For O3 in particular, the 
amount and frequency of afternoon time outdoors at moderate or greater 
exertion is an important factor for understanding the fraction of the 
population that might experience O3 exposures that have 
elicited respiratory effects in experimental studies (2014 HREA, 
section 5.4.2). In considering the available information regarding 
prevalence of behavior (time outdoors and exertion levels) and daily 
temporal pattern of O3 concentrations, the PA notes the 
findings of evaluations of the data in the CHAD. Based on these 
evaluations of human activity pattern data, it appears that children 
and adults both, for days having some time spent outdoors spend, on 
average, about 2 hours of afternoon time outdoors per day, but differ 
substantially in their participation in these events at elevated 
exertion levels (rates of about 80% versus 60%, respectively) (2014 
HREA, section 5.4.1.5), indicating children are more likely to 
experience exposures that may be of concern. This is one basis for 
their identification as an at-risk population for O3-related 
health effects. The human activity pattern evaluations have also shown 
there is little to no difference in the amount or frequency of 
afternoon time outdoors at moderate or greater exertion for people with 
asthma compared with those who do not have asthma (2014 HREA, section 
5.4.1.5). Further, recent CHAD analyses indicate that while 46-73% of 
people do not spend any afternoon time outdoors at moderate or greater 
exertion, a fraction of the population (i.e., between 5.5-6.8% of 
children) spend more than 4 hours per day outdoors at moderate or 
greater exertion and may have greater potential to experience exposure 
events of concern than adults (PA, Appendix 3D, section 3D.2.5.3 and 
Figure 3D-9). It is this potential that contributes importance to 
consideration of the exposure and risk estimates.
    In considering the public health implications of the exposure and 
risk estimates across the eight study areas, the PA notes that the 
purpose for the study areas is to illustrate exposure circumstances 
that may occur in areas that just meet the current standard, and not to 
estimate exposure and risk associated with conditions occurring in 
those specific locations today. To the extent that concentrations in 
the specific areas simulated may differ from others across the U.S., 
the exposure and risk estimates for these areas are informative to 
consideration of potential exposures and risks in areas existing across 
the U.S. that have air quality and population characteristics similar 
to the study areas assessed, and that have ambient concentrations of 
O3 that just meet the current standard today or that will be 
reduced to do so at some period in the future. We note that numerous 
areas across the U.S. have air quality for O3 that is near 
or above the existing standard.\99\ Thus, the air quality and exposure 
circumstances assessed in the eight study areas are of particular 
importance in considering whether the currently available information 
calls into question the adequacy of public health protection afforded 
by the current standard.
---------------------------------------------------------------------------

    \99\ Based on the most recently available data from 2016-2018, 
142 counties have O3 concentrations that exceed the 
current standard. Population size in these counties ranges from 
approximately 20,000 to more than ten million, with a total 
population of over 112 million living in counties that exceed the 
current standard. Air quality data are from Table 4. Monitor Status 
in the Excel file named 
ozone_designvalues_20162018_final_06_28_19.xlsx downloaded from 
https://www.epa.gov/air-trends/air-quality-design-values. Population 
sizes are based on 2017 estimates from the U.S. Census Bureau 
(https://www.census.gov/programs-surveys/popest.html).
---------------------------------------------------------------------------

    The exposure and risk estimates for the study areas assessed for 
this review reflect differences in exposure circumstances among those 
areas and illustrate the exposures and risks that might be expected to 
occur in other areas with such circumstances under air quality 
conditions that just meet the current standard (or the alternate

[[Page 49868]]

conditions assessed). Thus, the exposure and risk estimates indicate 
the magnitude of exposure and risk that might be expected in many areas 
of the U.S. with O3 concentrations at or near the current 
standard. Although the methodologies and data used to estimate 
population exposure and lung function risk in this review differ in 
several ways from what was used in the last review, the findings and 
considerations summarized here present a pattern of exposure and risk 
that is generally similar to that considered in the last review (as 
described above), and indicate a level of protection from respiratory 
effects that is generally consistent with that described in the 2015 
decision.
    Collectively, the PA finds that the evidence and exposure and risk-
based considerations provide the basis for its conclusion that 
consideration should be given to retaining the current primary 
standard, without revision (PA, section 3.5.4). Accordingly, and in 
light of this conclusion that it is appropriate to consider the current 
primary standard to be adequate, the PA did not identify any potential 
alternative primary standards for consideration in this review (PA, 
section 3.5.4). In reaching these conclusions, the PA additionally 
notes that considerations raised in the PA are important to conclusions 
and judgments to be made by the Administrator concerning the public 
health significance of the evidence and of the exposure and risk 
estimates. Such judgments that are common to NAAQS decisions include 
those related to public health implications of effects of differing 
severity (75 FR 355260 and 35536, June 22, 2010; 76 FR 54308, August 
31, 2011; 80 FR 65292, October 26, 2015). Such judgments also include 
those concerning the public health significance of effects at exposures 
for which evidence is limited or lacking, such as effects at the lower 
benchmark concentrations considered and lung function risk estimates 
associated with exposure concentrations lower than those tested or for 
population groups not included in the controlled exposure studies. The 
PA recognizes that such public health policy judgments will weigh in 
the Administrator's decision in this review with regard to the adequacy 
of protection afforded by the current standard.
2. CASAC Advice
    The CASAC has provided advice on the adequacy of the current 
primary O3 standard in the context of its review of the 
draft PA.\100\ In this context, the CASAC agreed with the draft PA 
findings that the evidence newly available in this review does not 
substantially differ from that available in the 2015 review, stating 
that, ``[t]he CASAC agrees that the evidence newly available in this 
review that is relevant to setting the ozone standard does not 
substantially differ from that of the 2015 Ozone NAAQS review'' (Cox, 
2020a, p. 12 of the Consensus Responses). With regard to the adequacy 
of the current standard, views of individual CASAC members differed. 
Part of the CASAC ``agree with the EPA that the available evidence does 
not call into question the adequacy of protection provided by the 
current standard, and thus support retaining the current primary 
standard'' (Cox, 2020a, p. 1 of letter). Another part of the CASAC 
indicated its agreement with the previous CASAC's advice, based on 
review of the 2014 draft PA, that a primary standard with a level of 70 
ppb may not be protective of public health with an adequate margin of 
safety, including for children with asthma (Cox, 2020a, p. 1 of letter 
and p. 12 of the enclosed Consensus Responses).\101\ Additional 
comments from the CASAC in the ``Consensus Responses to Charge 
Questions'' on the draft PA attached to the CASAC letter provide 
recommendations on improving the presentation of the information on 
health effects and exposure and risk estimates in completing the final 
PA. The EPA considered these comments in completing the PA and in 
presentations of the information in prior sections of this proposal 
document.
---------------------------------------------------------------------------

    \100\ A limited number of public comments have also been 
received in this review to date, including comments focused on the 
draft IRP or draft PA. Of the public comment that addressed adequacy 
of the current primary O3 standard, some expressed 
agreement with staff conclusions in the draft PA, while others 
expressed the view that the standard should be more restrictive. In 
support of this latter view, commenters largely cited advice from, 
and considerations raised by, the previous CASAC in the last review 
regarding adequacy of the margin of safety.
    \101\ In the last review, the advice from the prior CASAC 
included a range of recommended levels for the standard, with the 
CASAC concluding that ``there is adequate scientific evidence to 
recommend a range of levels for a revised primary ozone standard 
from 70 ppb to 60 ppb'' (Frey, 2014, p. ii). In so doing, the prior 
CASAC noted that ``[i]n reaching its scientific judgment regarding a 
recommended range of levels for a revised ozone primary standard, 
the CASAC focused on the scientific evidence that identifies the 
type and extent of adverse effects on public health'' and further 
acknowledged ``that the choice of a level within the range 
recommended based on scientific evidence is a policy judgment under 
the statutory mandate of the Clean Air Act'' (Frey, 2014, p. ii). 
The prior CASAC then described that its ``policy advice [emphasis 
added] is to set the level of the standard lower than 70 ppb within 
a range down to 60 ppb, taking into account [the Administrator's] 
judgment regarding the desired margin of safety to protect public 
health, and taking into account that lower levels will provide 
incrementally greater margins of safety'' (Frey, 2014, p. ii).
---------------------------------------------------------------------------

    The comments from the CASAC also took note of uncertainties that 
remain in this review of the primary standard and identified a number 
of additional areas for future research and data gathering that would 
inform the next review of the primary O3 NAAQS (Cox, 2020a, 
p. 14 of the Consensus Responses).
3. Administrator's Proposed Conclusions
    Based on the large body of evidence concerning the health effects 
and potential public health impacts of exposure to O3 in 
ambient air, and taking into consideration the attendant uncertainties 
and limitations of the evidence, the Administrator proposes to conclude 
that the current primary O3 standard provides the requisite 
protection of public health, including an adequate margin of safety, 
and should therefore be retained, without revision. In reaching these 
proposed conclusions, the Administrator has carefully considered the 
assessment of the available health effects evidence and conclusions 
contained in the ISA; the evaluation of policy-relevant aspects of the 
evidence and quantitative analyses in the PA (summarized in section 
II.D.1 above); the advice and recommendations from the CASAC 
(summarized in section II.D.2 above); and public comments received to 
date in this review.
    In the discussion below, the Administrator considers first the 
evidence base on health effects associated with exposure to 
photochemical oxidants, including O3, in ambient air. In so 
doing, he considers that health effects evidence newly available in 
this review, and the extent to which it alters key scientific 
conclusions in the last review. The Administrator additionally 
considers the quantitative exposure and risk estimates developed in 
this review, including associated limitations and uncertainties, and 
what they indicate regarding the magnitude of risk, as well as level of 
protection from adverse effects, associated with the current standard. 
Further, the Administrator considers the key aspects of the evidence 
and exposure/risk estimates emphasized in establishing the current 
standard. He additionally considers uncertainties in the evidence and 
the exposure/risk information, as a part of public health judgments 
that are essential and integral to his decision on the adequacy of 
protection provided by the standard, similar to the judgments made in 
establishing the current

[[Page 49869]]

standard. Such judgments include public health policy judgments and 
judgments about the uncertainties inherent in the scientific evidence 
and quantitative analyses. The Administrator draws on the PA 
considerations, and PA conclusions in the current review, taking note 
of key aspects of the rationale presented for those conclusions. 
Further, the Administrator considers the advice and conclusions of the 
CASAC, including particularly its overall agreement that the currently 
available evidence does not substantially differ from that which was 
available in the 2015 review when the current standard was established. 
With attention to such factors as these, the Administrator considers 
the information currently available in this review with regard to the 
adequacy and appropriateness of the protection provided by the current 
standard.
    As an initial matter, the Administrator recognizes the continued 
support in the current evidence for O3 as the indicator for 
photochemical oxidants (as recognized in section II.D.1 above). He 
takes note of the PA conclusion that no newly available evidence has 
been identified in this review regarding the importance of 
photochemical oxidants other than O3 with regard to 
abundance in ambient air, and potential for health effects, and of the 
ISA observation that ``the primary literature evaluating the health and 
ecological effects of photochemical oxidants includes ozone almost 
exclusively as an indicator of photochemical oxidants'' (ISA, p. IS-3). 
Accordingly, the information relating health effects to photochemical 
oxidants in ambient air is also focused on O3. Thus, he 
proposes to conclude it is appropriate for O3 to continue to 
be the indicator for the primary standard for photochemical oxidants.
    With regard to the extensive evidence base for health effects of 
O3, the Administrator gives particular attention to the 
longstanding evidence of respiratory effects causally related to short-
term O3 exposures. This array of effects, and the underlying 
evidence base, was integral to the basis for setting the current 
standard. The Administrator takes note of the ISA conclusion that this 
evidence base of studies on O3 exposure and respiratory 
health is the ``strongest evidence for health effects due to ozone 
exposure'' (ISA p. IS-8). While the overall health effects evidence 
base has been augmented somewhat since the time of the last review, the 
Administrator notes that, as summarized in section II.B.1 above, the 
newly available evidence does not lead to different conclusions 
regarding the respiratory effects of O3 in ambient air or 
regarding exposure concentrations associated with those effects; nor 
does it identify different populations at risk of O3-related 
effects, than in the last review.
    The Administrator recognizes that this strong evidence base 
continues to demonstrate a causal relationship between short-term 
O3 exposures and respiratory effects, including in people 
with asthma. He also recognizes that the strongest and most certain 
evidence for this conclusion, as in the last review, is that from 
controlled human exposure studies that report an array of respiratory 
effects in study subjects (largely generally healthy adults) engaged in 
quasi-continuous or intermittent exercise. He additionally notes the 
supporting experimental animal and epidemiologic evidence, including 
the epidemiologic studies reporting positive associations for asthma-
related hospital admissions and emergency department visits, which are 
strongest for children, with short-term O3 exposures. The 
Administrator also notes the ISA conclusion that the relationship 
between long-term exposures and respiratory effects is likely to be 
causal, a conclusion that is consistent with the conclusion in the last 
review and that reflects a general similarity in the underlying 
evidence base.
    With regard to populations at increased risk of O3-
related health effects, the Administrator notes the populations and 
lifestages identified in the ISA and summarized in section II.B.2 
above. In so doing, he takes note of the longstanding and robust 
evidence that supports identification of people with asthma as being at 
increased risk of O3 related respiratory effects, including 
specifically asthma exacerbation and associated health outcomes, and 
also children, particularly due to their generally greater time 
outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections 
IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). This 
tendency of children to spend more time outdoors while at elevated 
exertion than other age groups, including in the summer when 
O3 levels may be higher, makes them more likely to be 
exposed to O3 in ambient air under conditions contributing 
to increased dose due to greater air volumes taken into the lungs (2013 
ISA, section 5.2.2.7). These factors and the strong evidence (briefly 
summarized in section II.B.2 above, and section 3.3.2 of the PA, based 
on evidence described in detail in the ISA), indicate people with 
asthma, including children, to be at increased risk of O3 
related respiratory effects, including specifically asthma exacerbation 
and associated health outcomes. Based on these considerations, the 
Administrator proposes to conclude it is appropriate to give particular 
focus to people with asthma and children, population groups for which 
the evidence of increased risk is strongest, in evaluating whether the 
current standard provides requisite protection. He proposes to judge 
that such a focus will also provide protection of other population 
groups, identified in the ISA, for which the current evidence is less 
robust and clear as to the extent and type of any increased risk, and 
the exposure circumstances that may contribute to it.
    With regard to ISA conclusions that differ from those in the last 
review, the Administrator recognizes the new conclusions regarding 
metabolic effects, cardiovascular effects and mortality (as summarized 
in section II.B.1 above; ISA, Table ES-1). As an initial matter, he 
takes note of the fact that while the 2013 ISA considered the evidence 
available in the last review sufficient to conclude that the 
relationships for short-term O3 exposure with cardiovascular 
effects and mortality were likely to be causal, that conclusion is not 
supported by the now more expansive evidence base which the ISA now 
determines to be suggestive of, but not sufficient to infer, a causal 
relationship for these health effect categories. Further, the 
Administrator recognizes the new ISA determination that the 
relationship between short-term O3 exposure and metabolic 
effects is likely to be causal. In so doing, he takes note that the 
basis for this conclusion is largely experimental animal studies in 
which the exposure concentrations were well above those in the 
controlled human exposure studies for respiratory effects as well as 
above those likely to occur in areas of the U.S. that meet the current 
standard (as summarized in section II.B.3 and II.D.1 above). Thus, 
while recognizing the ISA's conclusion regarding this potential hazard 
of O3, he also recognizes that the evidence base is largely 
focused on circumstances of elevated concentrations above those 
occurring in areas that meet the current standard. In light of these 
considerations, he proposes to judge the current standard to be 
protective of such circumstances leading him to continue to focus on 
respiratory effects in evaluating whether the current standard provides 
requisite protection.
    With regard to exposures of interest for respiratory effects, the 
Administrator notes the 6.6 hour controlled human exposure studies 
involving exposure,

[[Page 49870]]

with quasi-continuous exercise,\102\ to concentrations ranging from as 
low as approximately 40 ppb to 120 ppb (as considered in the PA, and 
summarized in sections II.B.3 and II.D.1 above). He also notes that, as 
in the last review, these studies, and particularly those that examine 
exposures from 60 to 80 ppb, are the primary focus of the PA 
consideration of exposure circumstances associated with O3 
health effects important to Administrator judgments regarding the 
adequacy of the current standard. The Administrator further recognizes 
that this information on exposure concentrations that have been found 
to elicit effects in exercising study subjects is unchanged from what 
was available in the last review. With regard to the epidemiologic 
studies, the Administrator recognizes that while, as a whole, these 
investigations of associations between O3 and respiratory 
effects and health outcomes (e.g., asthma-related hospital admission 
and emergency department visits) provide strong support for the 
conclusions of causality (as summarized in section II.B.1 above), these 
studies are less useful for his consideration of the potential for 
O3 exposures associated with air quality conditions allowed 
by the current standard to contribute to such health outcomes. The 
Administrator takes note of the PA conclusions in this regard, 
including the scarcity of U.S. studies conducted in locations in which 
and during time periods when the current standard would have been met 
(as summarized in sections II.B.3 and II.D.1 above).\103\ He also 
recognizes the additional considerations raised in the PA and 
summarized in section II.B.3 above regarding information on exposure 
concentrations in these studies during times and locations that would 
not have met the current standard, and also including considerations 
such as complications in disentangling specific O3 exposures 
that may be eliciting effects (PA, section 3.3.3; ISA, p. IS-86 to IS-
88). While he notes that such considerations do not lessen their 
importance in the evidence base documenting the causal relationship 
between O3 and respiratory effects, he concurs with the PA 
that these studies are less informative in considering O3 
exposure concentrations occurring under air quality conditions allowed 
by the current standard. Thus, the Administrator does not find the 
available epidemiologic studies to provide insights regarding exposure 
concentrations associated with health outcomes that might be expected 
under air quality conditions that meet the current standard. In 
consideration of this evidence from controlled human exposure and 
epidemiologic studies, as assessed in the ISA and summarized in the PA, 
the Administrator notes that the evidence base in this review does not 
include new evidence of respiratory effects associated with appreciably 
different exposure circumstances than the evidence available in the 
last review, including particularly any circumstances that would also 
be expected to be associated with air quality conditions likely to 
occur under the current standard. In light of these considerations, he 
finds it appropriate to give particular focus to the studies of 6.6-
hour exposures with quasi-continuous exercise to concentrations 
generally ranging from 60 to 80 ppb.
---------------------------------------------------------------------------

    \102\ These studies employ a 6.6-hour protocol that includes six 
50-minute periods of exercise at moderate or greater exertion.
    \103\ Among the epidemiologic studies finding a statistically 
significant positive relationship of short- or long-term 
O3 concentrations with respiratory effects, there are no 
single-city studies conducted in the U.S. in locations with ambient 
air O3 concentrations that would have met the current 
standard for the entire duration of the study. Nor is there a U.S. 
multicity study for which all cities met the standard for the entire 
study period. The extent to which reported associations with health 
outcomes in the resident populations in these studies are influenced 
by the periods of higher concentrations during times that did not 
meet the current standard is unknown. These and additional 
considerations are summarized in section II.B.3 above and in the PA.
---------------------------------------------------------------------------

    With regard to these 6.6-hour controlled human exposure studies, 
although two such studies have assessed exposures at the lower 
concentration of 40 ppb, statistically significant responses have not 
been reported from those exposures. Studies at the next highest 
concentration studied (a 60 ppb target) have reported decrements in 
lung function (assessed by FEV1) that are statistically 
significantly increased over the decrements occurring with filtered 
air, with group mean O3-related decrements on the order of 2 
to 3% (and associated individual study subject variability in decrement 
size). A statistically significant, small increase in a marker of 
airway inflammation has also been reported in one of these 60 ppb 
studies. Exposure with the same study protocol to a concentration 
slightly above 70 ppb (73 ppb as the 6.6-hour average and 72 ppb as the 
exercise period average, based on study-reported measurements) has been 
reported to elicit statistically significant increases in both lung 
function decrements (group mean of 6%) and respiratory symptom scores, 
as summarized in section II.B.3 above. Further increases in 
O3-related lung function decrements and respiratory symptom 
scores, as well as inflammatory response and airway responsiveness, are 
reported for exposure concentrations of 80 ppb and higher (ISA; 2013 
ISA; 2006 AQCD).
    In this review, as in the last review, the Administrator recognizes 
some uncertainty, reflecting limitations in the evidence base, with 
regard to the exposure levels eliciting effects (as well as the 
severity of the effects) in some population groups not included in the 
available controlled human exposure studies, such as children and 
individuals with asthma. In so doing, the Administrator recognizes that 
the controlled human exposure studies, primarily conducted in healthy 
adults, on which the depth of our understanding of O3-
related health effects is based, provide limited, but nonetheless 
important information with regard to responses in people with asthma or 
in children. Additionally, some aspects of our understanding continue 
to be limited; among these aspects are the risk posed to these less 
studied population groups by 7-hour exposures with exercise to 
concentrations as low as 60 ppb that are estimated in the exposure 
analyses. Collectively, these aspects of the evidence and associated 
uncertainties contribute to a recognition that for O3, as 
for other pollutants, the available evidence base in a NAAQS review 
generally reflects a continuum, consisting of ambient levels at which 
scientists generally agree that health effects are likely to occur, 
through lower levels at which the likelihood and magnitude of the 
response become increasingly uncertain.
    In light of these uncertainties, as well as those associated with 
the exposure and risk analyses, the Administrator notes that, as is the 
case in NAAQS reviews in general, the extent to which the current 
primary O3 standard is judged to be adequate will depend on 
a variety of factors, including his science policy judgments and public 
health policy judgments. These factors include judgments regarding 
aspects of the evidence and exposure/risk estimates, such as judgments 
concerning the appropriate benchmark concentrations on which to place 
weight, in light of the available evidence and of associated 
uncertainties, as well as judgments on the public health significance 
of the effects that have been observed at the exposures evaluated in 
the health effects evidence. The factors relevant to judging the 
adequacy of the standards also include the interpretation of, and 
decisions as to the weight to place on, different aspects of the 
results of the exposure and risk assessment for the eight areas studied 
and the associated

[[Page 49871]]

uncertainties. Together, these and related factors will inform the 
Administrator's judgment about the degree of protection that is 
requisite to protect public health with an adequate margin of safety, 
and, accordingly, his conclusion regarding the adequacy of the current 
standard.
    As at the time of the last review, the exposure and risk estimates 
developed from modeling exposures to O3 in ambient air are 
critically important to consideration of the potential for exposures 
and risks of concern under air quality conditions of interest, and 
consequently are critically important to judgments on the adequacy of 
public health protection provided by the current standard. In 
considering the public health implications of estimated occurrences of 
exposures, while at increased exertion, to the three benchmark 
concentrations, the Administrator considers the effects reported in 
controlled human exposure studies of this range of concentrations 
during quasi-continuous exercise. In so doing, he notes the statements 
from the ATS, as well as judgments made by the EPA in considering 
similar effects in previous NAAQS reviews and the extent to which they 
may be adverse to health (80 FR 65343, October 26, 2015). In 
considering the ATS statements, including the most recent one which is 
newly available in the current review (Thurston et al., 2017), the 
Administrator recognizes the role of such statements, as described by 
the ATS, and as summarized in section II.B.2 above, as providing 
principles or considerations for weighing the evidence rather than 
offering ``strict rules or numerical criteria'' (ATS, 2000, Thurston et 
al., 2017). The more recent statement is generally consistent with the 
prior statement (that was considered in the last O3 NAAQS 
review) and the attention of that statement to at-risk or vulnerable 
population groups, while also broadening the discussion of effects, 
responses and biomarkers to reflect the expansion of scientific 
research in these areas, as summarized in section II.B.2 above. In this 
way, the most recent statement updates the prior statement, while 
retaining previously identified considerations, including, for example, 
its emphasis on consideration of vulnerable populations, thus expanding 
upon (e.g., with some increased specificity), while retaining core 
consistency with, the earlier ATS statement. In considering these 
statements, the Administrator notes that, in keeping with the intent of 
avoiding specific criteria, the statements do not provide specific 
descriptions of responses, such as with regard to magnitude, duration 
or frequency of small pollutant-related changes in lung function, and 
also takes note of the broader ATS emphasis on consideration of 
individuals with pre-existing compromised function, such as that 
resulting from asthma, recognizing such a focus to be important in his 
judgment on the adequacy of protection provided by the current standard 
for at-risk populations.
    In this review of the 2015 standard, the Administrator takes note 
of several aspects of the rationale by which it was established. As 
summarized in section II.A.1 above, the decision in the last review 
considered the breadth of the O3 respiratory effects 
evidence, recognizing the relatively greater significance of effects 
reported for exposures while at elevated exertion to average 
O3 concentrations at and above 80 ppb, as well as to the 
greater array of effects elicited. The decision also recognized the 
significance of effects observed at the next lower studied exposures 
(slightly above 70 ppb) that included both lung function decrements and 
respiratory symptoms. The standard level was set to provide a high 
level of protection from such exposures. The decision additionally 
emphasized consideration of lower exposures down to 60 ppb, 
particularly with regard to consideration of a margin of safety in 
setting the standard. In this context, the decision identified the 
appropriateness of a standard that provided a degree of control of 
multiple or repeated occurrences of exposures, while at elevated 
exertion, at or above 60 ppb (80 FR 65365, October 26, 2015).\104\ The 
controlled human exposure study evidence as a whole provided context 
for consideration of the 2014 HREA results for the exposures of 
concern, i.e., the comparison-to-benchmarks analysis (80 FR 65363, 
October 26, 2015). The Administrator proposes to similarly consider the 
exposure and risk analyses for this review.
---------------------------------------------------------------------------

    \104\ With the 2015 decision, the prior Administrator judged 
there to be uncertainty in the adversity of the effects shown to 
occur following exposures to 60 ppb O3, including the 
inflammation reported by the single study at the level, and 
accordingly placed greater weight on estimates of multiple exposures 
for the 60 ppb benchmark, particularly when considering the extent 
to which the current and revised standards incorporate a margin of 
safety (80 FR 65344-45, October 26, 2015). She based this, at least 
in part, on consideration of effects at this exposure level, the 
evidence for which remains the same in the current review. In one 
such consideration in 2015, the EPA noted that ``inflammation 
induced by a single exposure (or several exposures over the course 
of a summer) can resolve entirely. Thus, the inflammatory response 
observed following the single exposure to 60 ppb in the study by Kim 
et al. (2011) is not necessarily a concern. However, the EPA notes 
that it is also important to consider the potential for continued 
acute inflammatory responses to evolve into a chronic inflammatory 
state and to affect the structure and function of the lung'' (80 FR 
65344, October 26, 2015; 2013 ISA, p. 6-76). The prior Administrator 
considered this information in judgments regarding the 2014 HREA 
estimates for the 60 ppb benchmark.
---------------------------------------------------------------------------

    As recognized above, people with asthma, and children, are key 
populations at increased risk of respiratory effects related to 
O3 in ambient air. Children with asthma, which number 
approximately six million in the U.S., may be particularly at risk. 
While there are more adults in the U.S. with asthma than children with 
asthma, the exposure and risk analysis results in terms of percent of 
the simulated at-risk populations, indicate higher frequency of 
exposures of potential concern and risks for children as compared to 
adults. This finding relates to children's greater frequency and 
duration of outdoor activity, as well as their greater activity level 
while outdoors (PA, section 3.4.3). In light of these factors and those 
recognized above, the Administrator is focusing his consideration of 
the exposure and risk analyses here on children and children with 
asthma.
    In considering the exposure and risk analyses available in this 
review, the Administrator first notes that there are a number of ways 
in which the current analyses update and improve upon those available 
in the last review (as summarized in sections II.C.1 and II.D.1 above). 
For example, the Administrator notes that the air quality scenarios in 
the current assessment are based on the combination of updated 
photochemical modeling with more recent air quality data that include 
O3 concentrations closer to the current standard than was 
the case for the development of the air quality scenarios in the last 
review. As a result of this and the use of updated photochemical 
modeling, there is reduced uncertainty with the resulting exposure and 
risk estimates. Additionally, two modifications have been made to the 
exposure and risk analysis in light of comments received in past 
reviews that provide for a better match of the exposure modeling 
estimates with the 6.6-hour duration of the controlled human exposure 
studies and with the study subject ventilation rates. The Administrator 
notes, as summarized in section II.C.2 above, that these and other 
updates have reduced the uncertainty associated with interpretation of 
the analysis results from that associated with results in the last 
review (PA, sections 3.4 through 3.6).
    While the Administrator notes reduced uncertainty in several 
aspects

[[Page 49872]]

of the exposure and risk analysis approach as compared to the analyses 
in the last review, he recognizes the relatively greater uncertainty 
associated with the lung function risk estimates compared to the 
results of the comparison-to-benchmarks analysis. In so doing, he notes 
the PA analyses of uncertainty associated with the lung function risk 
estimates (and relatively greater uncertainty with estimates derived 
using the MSS model, versus the E-R models approach), as summarized in 
section II.C.2 above. In light of these uncertainties, as well as the 
recognition that the comparison-to-benchmarks analysis provides for 
characterization of risk for the broad array of respiratory effects 
compared to a narrower focus limited to lung function decrements, the 
Administrator focuses primarily on the estimates of exposures at or 
above different benchmark concentrations that represent different 
levels of significance of O3-related effects, both with 
regard to the array of effects and severity of individual effects.
    In considering the exposure and risk estimates, the Administrator 
also notes that the eight study areas assessed represent an array of 
air quality and exposure circumstances reflecting such variation that 
occurs across the U.S. The areas fall into seven of the nine climate 
regions represented in the continental U.S., with populations of the 
associated metropolitan areas ranging in size from approximately 2.4 to 
8 million and varying in demographic characteristics. The Administrator 
considers such factors as those identified here to contribute to their 
usefulness in informing the current review. As a result of such 
variation in exposure-related factors, the eight study areas represent 
an array of exposure circumstances, and accordingly, illustrate the 
magnitude of exposures and risks that may be expected in areas of the 
U.S. that just meet the current standard but that may differ in ways 
affecting population exposures of interest. The Administrator finds the 
estimates from these analyses to be informative to consideration of 
potential exposures and risks associated with the current standard and 
to his judgment on the adequacy of protection provided by the current 
standard.
    Taking into consideration related information, limitations and 
uncertainties, such as those recognized above, the Administrator 
considers the exposure estimates across the eight study areas (with 
their array of exposure conditions) for air quality conditions just 
meeting the current standard. Given the greater severity of responses 
reported in controlled human exposures, with quasi-continuous exercise, 
at and above 73 ppb, the Administrator finds it appropriate to focus 
first on the higher two benchmark concentrations (which at 70 and 80 
ppb are, respectively, slightly below and above this level) and the 
estimates for one-or-more-day occurrences. In so doing, he notes that 
across all eight study areas, less than 1% of children with asthma (and 
also of all children) are estimated to experience, while breathing at 
an elevated rate, a daily maximum 7-hour exposure per year at or above 
70 ppb, on average across the 3-year period, with a maximum of about 1% 
for the study area with the highest estimates in the highest single 
year (Table 2). Further, the percentage (for both population groups) 
for at least one day with such an exposure at or above 80 ppb is less 
than 0.1%, as an average across the 3-year period (and 0.1% or less in 
each of the three years simulated across the eight study areas). No 
simulated children were estimated to experience more than a single such 
day with an exposure at or above the 80 ppb benchmark (Table 2). The 
Administrator recognizes these estimates to indicate a very high level 
of protection from exposures that been found in controlled human 
exposure studies to elicit lung function decrements of notable 
magnitude (e.g., 6% at the study group mean for exposure to 73 ppb) 
accompanied by increases in respiratory symptom scores, as summarized 
in section II.B.3.
    The Administrator additionally considers the estimated occurrences 
of days that include lower 7-hour exposures, while at elevated exertion 
(i.e., daily maximum exposures at or above 60 ppb). In so doing, the 
Administrator takes note of the lesser severity of effects observed in 
controlled human exposure studies to 60 ppb (while at increased 
exertion) compared to the effects at the higher concentrations that 
have been studied (e.g., statistically significant O3-
related decrements on the order of 2 to 3% at the study group mean 
compared to 6%). He notes the finding of statistically significant 
increased respiratory symptom scores with exposures targeted at an 
exposure concentration of 70 ppb (and averaging 73 ppb across the 
exposure period), and the lack of such finding for any lower exposure 
concentrations that have been studied. In light of these 
considerations, he finds occurrences of exposures at or above the 
lowest benchmark of 60 ppb to be of lesser concern than occurrences for 
the next higher benchmark of 70 ppb. As described above for the higher 
exposure concentrations, he additionally recognizes that the studies of 
60 ppb were of generally healthy adults. While he notes the uncertainty 
regarding the risk that may be posed by this exposure concentration to 
at-risk populations, such as people with asthma, he additionally notes 
that the limited evidence available at higher exposure concentrations 
indicates lung function responses for this group that are similar to 
those for the generally healthy subjects, as well as the evidence of 
the transience of the responses in controlled human exposure studies. 
Further, he considers that due to the inherent characteristics of 
asthma as a disease, there is a potential, as summarized in section 
II.B.2 above, for O3 exposures to trigger asthmatic 
responses, such as through causing an increase in airway 
responsiveness. In this context, he additionally recognizes the 
potential for such a response to be greater, in general, at relatively 
higher, versus lower, exposure concentrations, noting 80 ppb to be the 
lowest exposure concentration at which increased airway responsiveness 
has been reported in generally healthy adults. In recognizing that the 
finding for this exposure concentration is for generally healthy adults 
and does not directly relate to people with asthma, he finds it 
appropriate to give additional consideration to the two lower 
benchmarks. In so doing, he judges that a high level of protection is 
desirable against one or more occurrences of days with exposures while 
breathing at an elevated rate to concentrations at or above 70 ppb. 
Additionally, he takes note of the lesser severity of responses 
observed in studies of the lowest benchmark concentration of 60 ppb, 
while considering the exposure analysis estimates of occurrences of 
daily maximum exposures at or above this benchmark, while also 
recognizing there to be greater risk for occurrence of a more serious 
effect with greater frequency of such exposure occurrence. Thus, based 
on the considerations recognized here, including potential risks for 
at-risk populations, the Administrator considers it appropriate to give 
greater weight to the exposure analysis estimates of occurrences of two 
or more days (rather than one or more) with an exposure at or above the 
60 ppb benchmark.
    The exposure analysis estimates indicate fewer than 1% to just over 
3% of children with asthma (just under 3% of all children), on average 
across the 3-year period to be expected to experience two or more days 
with an exposure at

[[Page 49873]]

or above 60 ppb, while at elevated ventilation. The Administrator notes 
this to indicate that some 97% to more than 99% of children, on 
average, and more than 95% in the single highest year, are protected 
from experiencing two or more days with exposures at or above 60 ppb 
while at elevated exertion. He also considers this in combination with 
the high level of protection indicated by the exposure estimates for 
the higher benchmark concentration of 70 ppb, which is slightly below 
the exposure level at which increases in FEV1 decrement (6% 
at the study group mean) accompanied by respiratory symptoms have been 
demonstrated. The current exposure analysis, with reduced uncertainty 
compared to the analysis available in the last review for air quality 
conditions in areas that just meet the current standard, indicates more 
than 99% of children with asthma (and of all children), on average per 
year, to be protected from a day or more with an exposure at or above 
70 ppb. In light of all of the considerations summarized above, the 
Administrator proposes to judge that protection from these exposures, 
as described here, provides a strong degree of protection to at-risk 
populations such as children with asthma. In light of all of the above, 
the Administrator finds the updated exposure and risk analyses based on 
updated and improved information, including air quality concentrations 
closer to the current standard, to continue to support a conclusion of 
a high level of protection, including for at-risk populations, from 
O3-related effects of exposures that might be expected with 
air quality conditions that just meet the current standard.
    In reaching his proposed conclusion, the Administrator additionally 
takes note of the comments and advice from the CASAC, including the 
CASAC conclusion that the newly available evidence does not 
substantially differ from that available in the last review, and the 
associated conclusion expressed by part of the CASAC, that the current 
evidence supports retaining the current standard. He also notes that 
another part of the CASAC indicated its agreement with the prior CASAC 
comments on the 2014 draft PA, in which the prior CASAC opined that a 
standard set at 70 ppb may not provide an adequate margin of safety 
(Cox, 2020, p. 1). With regard to the latter view (that referenced 2014 
comments from the prior CASAC), the Administrator additionally notes 
that the 2014 advice from the prior CASAC also concluded that the 
scientific evidence supported a range of standard levels that included 
70 ppb and recognized the choice of a level within its recommended 
range to be ``a policy judgment under the statutory mandate of the 
Clean Air Act'' (Frey, 2014, p. ii). The Administrator considers these 
points to provide additional context for the comments of the prior 
CASAC that were cited by part of the current CASAC in its review of the 
draft PA in this review, as noted above.\105\
---------------------------------------------------------------------------

    \105\ This 2014 advice was considered in the last review's 
decision to establish the current standard with a level of 70 ppb 
(80 FR 65362, October 26, 2015).
---------------------------------------------------------------------------

    In reflecting on all of the information currently available, the 
Administrator considers the extent to which the currently available 
information might indicate support for a less stringent standard. He 
recognizes the advice from the CASAC, which generally indicates support 
for retaining the current standard without revision or for revision to 
a more stringent level based on additional consideration of the margin 
of safety for at-risk populations. He notes that the CASAC advice did 
not convey support for a less stringent standard. He additionally 
considers the current exposure and risk estimates for the air quality 
scenario for a design value just above the level of the current 
standard (at 75 ppb), in comparison to the scenario for the current 
standard, as summarized in section II.D.1 above. In so doing, he finds 
the markedly increased estimates of exposures to the higher benchmarks 
under air quality for a higher standard level to be of concern and 
indicative of less than the requisite protection (Table 2). Thus, in 
light of the considerations raised here, including the need for an 
adequate margin of safety, the Administrator proposes to judge that a 
less stringent standard would not be appropriate to consider.
    The Administrator additionally considers whether it would be 
appropriate to consider a more stringent standard that might be 
expected to result in reduced O3 exposures. As an initial 
matter, he considers the advice from the CASAC. With regard to the 
CASAC advice, while part of the Committee concluded the evidence 
supported retaining the current standard without revision, another part 
of the Committee reiterated advice from the prior CASAC, which while 
including the current standard level among the range of recommended 
standard levels, also provided policy advice to set the standard at a 
lower level. In considering this advice now in this review, the 
Administrator notes the slight differences of the current exposure and 
risk estimates from the 2014 HREA estimates for the lowest benchmark, 
which were those considered by the prior CASAC (Table 4). For example, 
while the 2014 HREA estimated 3.3 to 10.2% of children, on average, to 
experience one or more days with an exposures at or above 60 ppb (and 
as many as 18.9% in a single year), the comparable estimates for the 
current analyses are lower, particularly at the upper end (3.2 to 8.2% 
and 10.6%). While the estimates for two or more days with occurrences 
at or above 60 ppb, on average across the assessment period, are more 
similar between the two assessments, the current estimate for the 
single highest year is much lower (9.2 versus 4.3%). The Administrator 
additionally recognizes the PA finding (summarized in section II.D.1 
above) that the factors contributing to these differences, which 
includes the use of air quality data reflecting concentrations much 
closer to the now-current standard than was the case in the 2015 
review, also contribute to a reduced uncertainty in the estimates. 
Thus, he notes that the current exposure analysis estimates indicate 
the current standard to provide appreciable protection against multiple 
days with a maximum exposure at or above 60 ppb. He considers this in 
the context of his consideration of the adequacy of protection provided 
by the standard and of the CAA requirement that the standard protect 
public health, including the health of at-risk populations, with an 
adequate margin of safety, and proposes to conclude, in light of all of 
the considerations raised here, that the current standard provides an 
adequate margin of safety, and that a more stringent standard is not 
needed.
    In light of all of the above, including advice from the CASAC, the 
Administrator finds the current exposure and risk analysis results to 
describe appropriately strong protection of at-risk populations from 
O3-related health effects. Thus, based on his consideration 
of the evidence and exposure/risk information, including that related 
to the lowest exposures studied and the associated uncertainties, the 
Administrator proposes to judge that the current standard provides the 
requisite protection, including an adequate margin of safety, and thus 
should be retained, without revision.
    As recognized above, the protection afforded by the current 
standard can only be assessed by considering its elements collectively, 
including the standard level of 70 ppb, the averaging time of eight 
hours and the form of the annual fourth-highest daily maximum

[[Page 49874]]

concentration averaged across three years. The Administrator finds that 
the current evidence presented in the ISA and considered in the PA, as 
well as the current air quality, exposure and risk information 
presented and considered in the PA provide continued support to these 
elements, as well as to the current indicator, as discussed above. In 
summary, the Administrator recognizes the newly available health 
effects evidence, critically assessed in the ISA as part of the full 
body of evidence, to reaffirm conclusions on the respiratory effects 
recognized for O3 in the last review. He additionally notes 
that the evidence newly available in this review, such as that related 
to metabolic effects, does not include information indicating a basis 
for concern for exposure conditions associated with air quality 
conditions meeting the current standard. Further, the Administrator 
notes the quantitative exposure and risk estimates for conditions just 
meeting the current standard that indicate a high level of protection 
for at-risk populations from respiratory effects. Collectively, these 
considerations (including those discussed above) provide the basis for 
the Administrator's judgments regarding the public health protection 
provided by the current primary standard of 0.070 ppm O3, as 
the fourth-highest daily maximum 8-hour concentration averaged across 
three years. On this basis, the Administrator proposes to conclude that 
the current standard is requisite to protect the public health with an 
adequate margin of safety, and that it is appropriate to retain the 
standard without revision. The Administrator solicits comment on these 
proposed conclusions.
    Having reached the proposed decision described here based on 
interpretation of the health effects evidence, as assessed in the ISA, 
and the quantitative analyses presented in the PA; the evaluation of 
policy-relevant aspects of the evidence and quantitative analyses in 
the PA; the advice and recommendations from the CASAC; public comments 
received to date in this review; and the public health policy judgments 
described above, the Administrator recognizes that other 
interpretations, assessments and judgments might be possible. 
Therefore, the Administrator solicits comment on the array of issues 
associated with review of this standard, including public health and 
science policy judgments inherent in the proposed decision, as 
described above, and the rationales upon which such views are based.

III. Rationale for Proposed Decision on the Secondary Standard

    This section presents the rationale for the Administrator's 
proposed decision to retain the current secondary O3 
standard. This rationale is based on a thorough review of the latest 
scientific information generally published between January 2011 and 
March 2018, as well as more recent studies identified during peer 
review or by public comments (ISA, section IS.1.2),\106\ integrated 
with the information and conclusions from previous assessments and 
presented in the ISA on welfare effects associated with photochemical 
oxidants including O3 and pertaining to their presence in 
ambient air. The Administrator's rationale also takes into account: (1) 
The PA evaluation of the policy-relevant information in the ISA and 
presentation of quantitative analyses of air quality, exposure, and 
risk; (2) CASAC advice and recommendations, as reflected in discussions 
of drafts of the ISA and PA at public meetings and in the CASAC's 
letters to the Administrator; (3) public comments received during the 
development of these documents; and also (4) the August 2019 decision 
of the D.C. Circuit remanding the secondary standard established in the 
last review to the EPA for further justification or reconsideration. 
See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019).
---------------------------------------------------------------------------

    \106\ 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.
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's proposed 
decision and its foundations, section III.A provides background and 
introductory information for this review of the secondary O3 
standard. It includes background on the establishment of the current 
standard in 2015 (section III.A.1) and also describes the general 
approach for its current review (section III.A.2). Section III.B 
summarizes the currently available welfare effects evidence, focusing 
on consideration of key policy-relevant aspects. Section III.C 
summarizes current air quality and environmental exposure information, 
drawing on the quantitative analyses presented in the PA. Section III.D 
presents the Administrator's proposed conclusions on the current 
standard (section III.D.3), drawing on both evidence-based and air 
quality, exposure and risk-based considerations (section III.D.1) and 
advice from the CASAC (section III.D.2).

A. General Approach

    As is the case for all such reviews, this review of the current 
secondary O3 standard is based, most fundamentally, on using 
the EPA's assessments of the current scientific evidence and associated 
quantitative analyses to inform the Administrator's judgment regarding 
a secondary standard that is requisite to protect the public welfare 
from known or anticipated adverse effects associated with the 
pollutant's presence in the ambient air. The EPA's assessments are 
primarily documented in the ISA and PA, both of which have received 
CASAC review and public comment (84 FR 50836, September 26, 2019; 84 FR 
58711, November 1, 2019; 84 FR 58713, November 1, 2019; 85 FR 21849, 
April 20, 2020; 85 FR 31182, May 22, 2020). In bridging the gap between 
the scientific assessments of the ISA and the judgments required of the 
Administrator in determining whether the current standard provides the 
requisite public welfare protection, the PA evaluates policy 
implications of the evaluation of the current evidence in the ISA and 
the quantitative air quality, exposure and risk analyses and 
information documented in the PA. In evaluating the public welfare 
protection afforded by the current standard, the four basic elements of 
the NAAQS (indicator, averaging time, level, and form) are considered 
collectively.
    The final decision on the adequacy of the current secondary 
standard is a public welfare policy judgment to be made by the 
Administrator. In reaching conclusions with regard to the standard, the 
decision will draw on the scientific information and analyses about 
welfare effects, environmental exposure and risks, and associated 
public welfare significance, as well as judgments about how to consider 
the range and magnitude of uncertainties that are inherent in the 
scientific evidence and analyses. This approach is based on the 
recognition that the available evidence generally reflects a continuum 
that includes ambient air exposures at which scientists generally agree 
that effects are

[[Page 49875]]

likely to occur through lower levels at which the likelihood and 
magnitude of responses become increasingly uncertain. This approach is 
consistent with the requirements of the provisions of the Clean Air Act 
related to the review of NAAQS and with how the EPA and the courts have 
historically interpreted the Act. These provisions require the 
Administrator to establish secondary standards that, in the judgment of 
the Administrator, are requisite to protect the public welfare from 
known or anticipated adverse effects associated with the presence of 
the pollutant in the ambient air. In so doing, the Administrator seeks 
to establish standards that are neither more nor less stringent than 
necessary for this purpose. The Act does not require that standards be 
set at a zero-risk level, but rather at a level that reduces risk 
sufficiently so as to protect the public welfare from known or 
anticipated adverse effects.
    The subsections below provide background and introductory 
information. Background on the establishment of the current standard in 
2015, including the rationale for that decision, is summarized in 
section III.A.1. This is followed, in section III.A.2, by an overview 
of the general approach for the current review of the 2015 standard. 
Following this introductory section and subsections, the subsequent 
sections summarize current information and analyses, including that 
newly available in this review. The Administrator's proposed 
conclusions on the standard set in 2015, based on the current 
information, are provided in section III.D.3
1. Background on the Current Standard
    The current standard was set in 2015 based on the scientific and 
technical information available at that time, as well as the 
Administrator's judgments regarding the available welfare effects 
evidence, the appropriate degree of public welfare protection for the 
revised standard, and available air quality information on seasonal 
cumulative exposures that may be allowed by such a standard (80 FR 
65292, October 26, 2015). With the 2015 decision, the Administrator 
revised the level of the secondary standard for photochemical oxidants, 
including O3, to 0.070 ppm, in conjunction with retaining 
the indicator (O3), averaging time (8 hours) and form 
(fourth-highest annual daily maximum 8-hour average concentration, 
averaged across three years).
    The welfare effects evidence base available in the 2015 review 
included more than fifty years of extensive research on the phytotoxic 
effects of O3, conducted both in and outside of the U.S. 
that documents the impacts of O3 on plants and their 
associated ecosystems (U.S. EPA, 1978, 1986, 1996, 2006, 2013). As was 
established in prior reviews, O3 can interfere with carbon 
gain (photosynthesis) and allocation of carbon within the plant, making 
fewer carbohydrates available for plant growth, reproduction, and/or 
yield (U.S. EPA, 1996, pp. 5-28 and 5-29). The strongest evidence for 
effects from O3 exposure on vegetation is from controlled 
exposure studies, which ``have clearly shown that exposure to 
O3 is causally linked to visible foliar injury, decreased 
photosynthesis, changes in reproduction, and decreased growth'' in many 
species of vegetation (2013 ISA, p. 1-15).\107\ Such effects at the 
plant scale can also be linked to an array of effects at larger 
organizational (e.g., population, community, system) and spatial 
scales, with the evidence available in the last review supporting 
conclusions of causal relationships between O3 and 
alteration of below-ground biogeochemical cycles, in addition to likely 
to be a causal relationships between O3 and reduced carbon 
sequestration in terrestrial ecosystems, alteration of terrestrial 
ecosystem water cycling and alteration of terrestrial community 
composition (2013 ISA, p. lxviii and Table 9-19). Further, the 2013 ISA 
also found there to be a causal relationship between changes in 
tropospheric O3 concentrations and radiative forcing, and 
likely to be a causal relationship between tropospheric O3 
concentrations and effects on climate as quantified through surface 
temperature response (2013 ISA, section 10.5).
---------------------------------------------------------------------------

    \107\ Visible foliar injury includes leaf or needle changes such 
as small dots or bleaching (2013 ISA, p. 9-38).
---------------------------------------------------------------------------

    The 2015 decision was a public welfare policy judgment made by the 
Administrator, which drew upon the available scientific evidence for 
O3-attributable welfare effects and on quantitative analyses 
of exposures and public welfare risks, as well as judgments about the 
appropriate weight to place on the range of uncertainties inherent in 
the evidence and analyses. The analyses utilized cumulative, 
concentration-weighted exposure indices for O3. Use of this 
metric was based on conclusions in the 2013 ISA that exposure indices 
that cumulate hourly O3 concentrations, giving greater 
weight to the higher concentrations (such as the W126 index), perform 
well in describing exposure-response relationships documented in crop 
and tree seedling studies (2013 ISA, section 9.5). Included in this 
decision were judgments on the weight to place on the evidence of 
specific vegetation-related effects estimated to result across a range 
of cumulative seasonal concentration-weighted O3 exposures; 
on the weight to give associated uncertainties, including uncertainties 
of predicted environmental responses (based on experimental study 
data); variability in occurrence of the specific effects in areas of 
the U.S., especially in areas of particular public welfare 
significance; and on the extent to which such effects in such areas may 
be considered adverse to public welfare.
    The decision was based on a thorough review in the 2013 ISA of the 
scientific information on O3-induced environmental effects. 
The decision also took into account: (1) Assessments in the 2014 PA of 
the most policy-relevant information in the 2013 ISA regarding evidence 
of adverse effects of O3 to vegetation and ecosystems, 
information on biologically-relevant exposure metrics, 2014 welfare REA 
(WREA) analyses of air quality, exposure, and ecological risks and 
associated ecosystem services, and staff analyses of relationships 
between levels of a W126-based exposure index \108\ and potential 
alternative standard levels in combination with the form and averaging 
time of the then-current standard; (2) additional air quality analyses 
of the W126 index and design values based on the form and averaging 
time of the then-current standard; (3) CASAC advice and 
recommendations; and (4) public comments received during the 
development of these documents and on the proposal document. In 
addition to reviewing the most recent scientific information as 
required by the CAA, the 2015 rulemaking also incorporated the EPA's 
response to the judicial remand of the 2008 secondary O3 
standard in Mississippi v. EPA, 744 F.3d 1334 (D.C. Cir. 2013) and, in 
light of the court's decision in that case, explained the 
Administrator's conclusions as to the level of air quality judged to 
provide the requisite protection of public welfare from known or 
anticipated adverse effects.
---------------------------------------------------------------------------

    \108\ The W126 index is a cumulative seasonal metric described 
as the sigmoidally weighted sum of all hourly O3 
concentrations observed during a specified daily and seasonal time 
window, where each hourly O3 concentration is given a 
weight that increases from zero to one with increasing concentration 
(80 FR 65373-74, October 26, 2015). Accordingly, W126 index values 
are in the units of ppm-hours (ppm-hrs).
---------------------------------------------------------------------------

    Consistent with the general approach routinely employed in NAAQS 
reviews, the initial consideration in the 2015 review of the secondary 
standard was

[[Page 49876]]

with regard to the adequacy of protection provided by the existing 
standard, that was set in 2008 (0.075 ppm, as annual fourth-highest 
daily maximum 8-hour average concentration averaged over three 
consecutive years). In her decision making, the Administrator 
considered the effects of O3 on tree seedling growth, as 
suggested by the CASAC, as a surrogate or proxy for the broader array 
of vegetation-related effects of O3, ranging from effects on 
sensitive species to broader ecosystem-level effects (80 FR 65369, 
65406, October 26, 2015). The metric used for quantifying effects on 
tree seedling growth in the review was relative biomass loss (RBL), 
with the evidence base providing robust and established exposure-
response (E-R) functions for seedlings of 11 tree species (80 FR 65391-
92, October 26, 2015; 2014 PA, Appendix 5C).\109\ The Administrator 
used this surrogate or proxy in making her judgments on O3 
effects to the public welfare. In this context, exposure was evaluated 
in terms of the W126 cumulative seasonal exposure index, an index 
supported by the evidence in the 2013 ISA for this purpose and that was 
consistent with advice from the CASAC (2013 ISA, section 9.5.3, p. 9-
99; 80 FR 65375, October 26, 2015).
---------------------------------------------------------------------------

    \109\ These functions for RBL estimate the reduction in a year's 
growth as a percentage of that expected in the absence of 
O3 (2013 ISA, section 9.6.2; 2014 WREA, section 6.2).
---------------------------------------------------------------------------

    In considering the public welfare protection provided by the then-
current standard, the Administrator gave primary consideration to an 
analysis of cumulative seasonal exposures in or near Class I areas 
\110\ during periods when the then-current standard was met, and the 
associated estimates of growth effects in well-studied species of tree 
seedlings, in terms of the O3 attributable reductions in RBL 
in the median species for which E-R functions have been established (80 
FR 65385-65386, 65389-65390, October 26, 2015).\111\ The Administrator 
noted the occurrence of exposures for which the associated median 
estimates of growth effects across the species with E-R functions 
extend above a magnitude considered to be ``unacceptably high'' by the 
CASAC.\112\ This analysis estimated cumulative exposures, in terms of 
3-year average W126 index values, at and above 19 ppm-hrs, occurring 
under the then-current standard for nearly a dozen areas, distributed 
across two NOAA climatic regions of the U.S. (80 FR 65385-86, October 
26, 2015). The Administrator gave particular weight to this analysis 
because of its focus on exposures in Class I areas, which are lands 
that Congress set aside for specific uses intended to provide benefits 
to the public welfare, including lands that are to be protected so as 
to conserve the scenic value and the natural vegetation and wildlife 
within such areas, and to leave them unimpaired for the enjoyment of 
future generations. This emphasis on lands afforded special government 
protections, such as national parks and forests, wildlife refuges, and 
wilderness areas, some of which are designated Class I areas under the 
CAA, was consistent with a similar emphasis in the 2008 review of the 
standard (73 FR 16485, March 27, 2008). The Administrator additionally 
recognized that states, tribes and public interest groups also set 
aside areas that are intended to provide similar benefits to the public 
welfare for residents on those lands, as well as for visitors to those 
areas (80 FR 65390, October 26, 2015).
---------------------------------------------------------------------------

    \110\ Areas designated as Class I include all international 
parks, national wilderness areas which exceed 5,000 acres in size, 
national memorial parks which exceed 5,000 acres in size, and 
national parks which exceed 6,000 acres in size, provided the park 
or wilderness area was in existence on August 7, 1977. Other areas 
may also be Class I if designated as Class I consistent with the 
CAA.
    \111\ In specifically evaluating exposure levels in terms of the 
W126 index as to potential for impacts on vegetation, the 
Administrator focused on the median RBL estimate across the eleven 
tree species for which robust established E-R functions were 
available. The presentation of these E-R functions for growth 
effects on tree seedlings (and crops) included estimates of RBL (and 
relative yield loss [RYL]) at a range of W126-based exposure levels 
(2014 PA, Tables 5C-1 and 5C-2). The median tree species RBL or crop 
RYL was presented for each W126 level (2014 PA, Table 5C-3; 80 FR 
65391 [Table 4], October 26, 2015). The Administrator focused on RBL 
as a surrogate or proxy for the broader array of vegetation-related 
effects of potential public welfare significance, which include 
effects on growth of individual sensitive species and extend to 
ecosystem-level effects, such as community composition in natural 
forests, particularly in protected public lands, as well as forest 
productivity (80 FR 65406, October 26, 2015).
    \112\ In the CASAC's consideration of RBL estimates presented in 
the 2014 draft PA, it characterized an estimate of 6% RBL in the 
median studied species as being ``unacceptably high,'' (Frey, 
2014b).
---------------------------------------------------------------------------

    As noted across past reviews of O3 secondary standards, 
the Administrator's judgments regarding effects that are adverse to 
public welfare consider the intended use of the ecological receptors, 
resources and ecosystems affected (80 FR 65389, October 26, 2015; 73 FR 
16496, March 27, 2008). Thus, in the 2015 review, the Administrator 
utilized the median RBL estimate for the studied species as a 
quantitative tool within a larger framework of considerations 
pertaining to the public welfare significance of O3 effects. 
She recognized such considerations to include effects that are 
associated with effects on growth and that the 2013 ISA determined to 
be causally or likely causally related to O3 in ambient air, 
yet for which there are greater uncertainties affecting estimates of 
impacts on public welfare. These other effects included reduced 
productivity in terrestrial ecosystems, reduced carbon sequestration in 
terrestrial ecosystems, alteration of terrestrial community 
composition, alteration of below-ground biogeochemical cycles, and 
alteration of terrestrial ecosystem water cycles. Thus, in giving 
attention to the CASAC's characterization of a 6% estimate for tree 
seedling RBL in the median studied species as ``unacceptably high'', 
the Administrator, while mindful of uncertainties with regard to the 
magnitude of growth impact that might be expected in the field and in 
mature trees, was also mindful of related, broader, ecosystem-level 
effects for which the available tools for quantitative estimates are 
more uncertain and those for which the policy foundation for 
consideration of public welfare impacts is less well established. As a 
result, the Administrator considered tree growth effects of 
O3, in terms of RBL ``as a surrogate for the broader array 
of O3 effects at the plant and ecosystem levels'' (80 FR 
65389, October 26, 2015).
    Based on all of these considerations, and taking into consideration 
CASAC advice and public comment, the Administrator concluded that the 
protection afforded by the then-current standard was not sufficient and 
that the standard needed to be revised to provide additional protection 
from known and anticipated adverse effects to public welfare, related 
to effects on sensitive vegetation and ecosystems, most particularly 
those occurring in Class I areas, and also in other areas set aside by 
states, tribes and public interest groups to provide similar benefits 
to the public welfare for residents on those lands, as well as for 
visitors to those areas. In so doing, she further noted that a revised 
standard would provide increased protection for other growth-related 
effects, including relative yield loss (RYL) of crops, reduced carbon 
storage, and types of effects for which it is more difficult to 
determine public welfare significance, as well as other welfare effects 
of O3, such as visible foliar injury (80 FR 65390, October 
26, 2015).
    Consistent with the approach employed for considering the adequacy 
of the then-current secondary standard, the approach for considering 
revisions

[[Page 49877]]

that would result in a standard providing the requisite protection 
under the Act also focused on growth-related effects of O3, 
using RBL as a surrogate for the broader array of vegetation-related 
effects and included judgments on the magnitude of such effects that 
would contribute to public welfare impacts of concern. In considering 
the adequacy of potential alternative standards to provide protection 
from such effects, the approach also focused on considering the 
cumulative seasonal O3 exposures likely to occur with 
different alternative standards.
    In light of the judicial remand of the 2008 secondary O3 
standard referenced above, the 2015 decision on selection of a revised 
secondary standard first considered the available evidence and 
quantitative analyses in the context of an approach for considering and 
identifying public welfare objectives for such a standard (80 FR 65403-
65408, October 26, 2015). In light of the extensive evidence base of 
O3 effects on vegetation and associated terrestrial 
ecosystems, the Administrator focused on protection against adverse 
public welfare effects of O3-related effects on vegetation, 
giving particular attention to such effects in natural ecosystems, such 
as those in areas with protection designated by Congress for current 
and future generations, as well as areas similarly set aside by states, 
tribes and public interest groups with the intention of providing 
similar benefits to the public welfare. The Administrator additionally 
recognized that providing protection for this purpose will also provide 
a level of protection for other vegetation that is used by the public 
and potentially affected by O3 including timber, produce 
grown for consumption and horticultural plants used for landscaping (80 
FR 65403, October 26, 2015).
    As mentioned above, the Administrator considered the use of a 
cumulative seasonal exposure index (the W126 index) for purposes of 
assessing potential public welfare risks, and similarly, for assessing 
potential protection achieved against such risks on a national scale. 
In consideration of conclusions of the 2013 ISA and 2014 PA, as well as 
advice from the CASAC and public comments, this W126 index was defined 
as a maximum, seasonal (3-month), 12-hour index (80 FR 65404, October 
26, 2015).\113\ While recognizing that no one definition of an exposure 
metric used for the assessment of protection for multiple effects at a 
national scale will be exactly tailored to every species or each 
vegetation type, ecosystem and region of the country, the Administrator 
judged that on balance, a W126 index derived in this way, and averaged 
over three years would be appropriate for such purposes (80 FR 65403, 
October 26, 2015).
---------------------------------------------------------------------------

    \113\ As also described in section III.B.3.a below, this index 
is defined by the 3-consecutive-month period within the 
O3 season with the maximum sum of W126-weighted hourly 
O3 concentrations during the period from 8:00 a.m. to 
8:00 p.m. each day.
---------------------------------------------------------------------------

    Based on a number of considerations, the Administrator recognized 
greater confidence in judgments related to public welfare impacts based 
on a 3-year average metric than a single-year metric, and consequently 
concluded it to be appropriate to use a seasonal W126 index averaged 
across three years for judging public welfare protection afforded by a 
revised secondary standard (80 FR 65404, October 26, 2015). For 
example, the Administrator was mindful of both the strengths and 
limitations of the evidence and of the information on which to base her 
judgments with regard to adversity of effects on the public 
welfare.\114\ While the Administrator recognized the scientific 
information and interpretations, as well as CASAC advice, with regard 
to a single-year exposure index, she also took note of uncertainties 
associated with judging the degree of vegetation impacts for single-
year effects that would be adverse to public welfare. The Administrator 
was also mindful of the variability in ambient air O3 
concentrations from year to year, as well as year-to-year variability 
in environmental factors, including rainfall and other meteorological 
factors, that influence the occurrence and magnitude of O3-
related effects in any year, and contribute uncertainties to 
interpretation of the potential for harm to public welfare over the 
longer term (80 FR 65404, October 26, 2015).
---------------------------------------------------------------------------

    \114\ In this regard, she recognized uncertainties associated 
with interpretation of the public welfare significance of effects 
resulting from a single-year exposure, and that the public welfare 
significance of effects associated with multiple years of critical 
exposures are potentially greater than those associated with a 
single year of such exposure. The Administrator concluded that use 
of a 3-year average metric could address the potential for adverse 
effects to public welfare that may relate to shorter exposure 
periods, including a single year (80 FR 65404, October 26, 2015).
---------------------------------------------------------------------------

    In reaching a conclusion on the amount of public welfare protection 
from the presence of O3 in ambient air that is appropriate 
to be afforded by a revised secondary standard, the Administrator gave 
particular consideration to the following: (1) The nature and degree of 
effects of O3 on vegetation, including her judgments as to 
what constitutes an adverse effect to the public welfare; (2) the 
strengths and limitations of the available and relevant information; 
(3) comments from the public on the Administrator's proposed decision, 
including comments related to identification of a target level of 
protection; and (4) the CASAC's views regarding the strength of the 
evidence and its adequacy to inform judgments on public welfare 
protection. The Administrator recognized that such judgments should 
neither overstate nor understate the strengths and limitations of the 
evidence and information nor the appropriate inferences to be drawn as 
to risks to public welfare, and that the choice of the appropriate 
level of protection is a public welfare policy judgment entrusted to 
the Administrator under the CAA taking into account both the available 
evidence and the uncertainties (80 FR 65404-05, October 26, 2015).\115\
---------------------------------------------------------------------------

    \115\ The CAA does not require that a secondary standard be 
protective of all effects associated with a pollutant in the ambient 
air but rather those known or anticipated effects judged ``adverse 
to the public welfare'' (CAA section 109).
---------------------------------------------------------------------------

    With regard to the extensive evidence of welfare effects of 
O3, including visible foliar injury and crop RYL, the 
information available for tree species was judged to be more useful in 
informing judgments regarding the nature and severity of effects 
associated with different air quality conditions and associated public 
welfare significance. Accordingly, the Administrator gave particular 
attention to the effects related to native tree growth and 
productivity, including forest and forest community composition, 
recognizing the relationship of tree growth and productivity to a range 
of ecosystem services, (80 FR 65405-06, October 26, 2015). In making 
this judgment, the Administrator recognized that among the broad array 
of O3-induced vegetation effects were the occurrence of 
visible foliar injury and growth and/or yield loss in O3-
sensitive species, including crops and other commercial species (80 FR 
65405, October 26, 2015). In regard to visible foliar injury, the 
Administrator recognized the potential for this effect to affect the 
public welfare in the context of affecting value ascribed to natural 
forests, particularly those afforded special government protection, 
with the significance of O3-induced visible foliar injury 
depending on the extent and severity of the injury (80 FR 65407, 
October 26, 2015). In so doing, however, the Administrator also took 
note of limitations in the available visible foliar injury information, 
including the lack of established E-R functions that would allow 
prediction of

[[Page 49878]]

visible foliar injury severity and incidence under varying air quality 
and environmental conditions, a lack of consistent quantitative 
relationships linking visible foliar injury with other O3-
induced vegetation effects, such as growth or related ecosystem 
effects, and a lack of established criteria or objectives that might 
inform consideration of potential public welfare impacts related to 
this vegetation effect (80 FR 65407, October 26, 2015). Similarly, 
while O3-related growth effects on agricultural and 
commodity crops had been extensively studied and robust E-R functions 
developed for a number of species, the Administrator found this 
information less useful in informing her judgments regarding an 
appropriate level of public welfare protection (80 FR 65405, October 
26, 2015).\116\
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    \116\ With respect to commercial production of commodities, the 
Administrator noted that judgments about the extent to which 
O3-related effects on commercially managed vegetation are 
adverse from a public welfare perspective are particularly difficult 
to reach, given that the extensive management of such vegetation 
(which, as the CASAC noted, may reduce yield variability) may also 
to some degree mitigate potential O3-related effects. The 
management practices used on such vegetation are highly variable and 
are designed to achieve optimal yields, taking into consideration 
various environmental conditions. In addition, changes in yield of 
commercial crops and commercial commodities, such as timber, may 
affect producers and consumers differently, further complicating the 
question of assessing overall public welfare impacts (80 FR 65405, 
October 26, 2015).
---------------------------------------------------------------------------

    Thus, and in light of the extensive evidence base in this regard, 
the Administrator focused on trees and associated ecosystems in 
identifying the appropriate level of protection for the secondary 
standard. Accordingly, the Administrator found the estimates of tree 
seedling growth impacts (in terms of RBL) associated with a range of 
W126-based index values developed from the E-R functions for 11 tree 
species (referenced above) to be appropriate and useful for considering 
the appropriate public welfare protection objective for a revised 
standard (80 FR 65391-92, Table 4, October 26, 2015). The Administrator 
also incorporated into her considerations the broader evidence base 
associated with forest tree seedling biomass loss, including other less 
quantifiable effects of potentially greater public welfare 
significance. That is, in drawing on these RBL estimates, the 
Administrator recognized she was not simply making judgments about a 
specific magnitude of growth effect in seedlings that would be 
acceptable or unacceptable in the natural environment. Rather, though 
mindful of associated uncertainties, the Administrator used the RBL 
estimates as a surrogate or proxy for consideration of the broader 
array of related vegetation and ecosystem effects of potential public 
welfare significance that include effects on growth of individual 
sensitive species and extend to ecosystem-level effects, such as 
community composition in natural forests, particularly in protected 
public lands, as well as forest productivity (80 FR 65406, October 26, 
2015). This broader array of vegetation-related effects included those 
for which public welfare implications are more significant but for 
which the tools for quantitative estimates were more uncertain.
    In using the RBL estimates as a proxy, and in consideration of 
CASAC advice; strengths, limitations and uncertainties in the evidence; 
and the linkages of growth effects to larger population, community and 
ecosystem impacts, the Administrator considered it appropriate to focus 
on a standard that would generally limit cumulative exposures to those 
for which the median RBL estimate for seedlings of the 11 species with 
robust and established E-R functions would be somewhat below 6% (80 FR 
65406-07, October 26, 2015). In focusing on cumulative exposures 
associated with a median RBL estimate somewhat below 6%, the 
Administrator considered the relationships between W126-based exposure 
and RBL in the studied species (presented in the final PA and proposal 
document), noting that the median RBL estimate was 6% for a cumulative 
seasonal W126 exposure index of 19 ppm-hrs (80 FR 65391-92, Table 4, 
October 26, 2015).\117\ Given the information on median RBL at 
different W126 exposure levels, using a 3-year cumulative exposure 
index for assessing vegetation effects, the potential for single-season 
effects of concern, and CASAC comments on the appropriateness of a 
lower value for a 3-year average W126 index, the Administrator 
concluded it was appropriate to identify a standard that would restrict 
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances (80 FR 65407, October 26, 
2015). Based on such information, available at that time, to inform 
consideration of vegetation effects and their potential adversity to 
public welfare, the Administrator additionally judged that the RBL 
estimates associated with marginally higher exposures in isolated, rare 
instances are not indicative of effects that would be adverse to the 
public welfare, particularly in light of variability in the array of 
environmental factors that can influence O3 effects in 
different systems and uncertainties associated with estimates of 
effects associated with this magnitude of cumulative exposure in the 
natural environment (80 FR 65407, October 26, 2015).
---------------------------------------------------------------------------

    \117\ When stated to the first decimal place, the median RBL was 
6.0% for a cumulative seasonal W126 exposure index of 19 ppm-hrs. 
For 18 ppm-hrs, the median RBL estimate was 5.7%, which rounds to 
6%, and for 17 ppm-hrs, the median RBL estimate was 5.3%, which 
rounds to 5% (80 FR 65407, October 26, 2015).
---------------------------------------------------------------------------

    The Administrator's decisions regarding the revisions to the then-
current standard that would appropriately achieve these public welfare 
protection objectives were based on extensive air quality analyses that 
extended from the then most recently available data (monitoring year 
2013) back more than a decade (80 FR 65408, October 26, 2015; Wells, 
2015). These analyses evaluated the cumulative seasonal exposure levels 
in locations meeting different alternative levels for a standard of the 
existing form and averaging time, indicating reductions in cumulative 
exposures associated with air quality meeting lower levels of a 
standard of the existing form and averaging time. Based on these 
analyses, the Administrator judged that the desired level of public 
welfare protection could be achieved with a secondary standard having a 
revised level in combination with the existing form and averaging time 
(80 FR 65408, October 26, 2015).
    The air quality analyses described the occurrences of 3-year W126 
index values of various magnitudes at monitor locations where 
O3 concentrations met potential alternative standards; the 
alternative standards were different levels for the current form and 
averaging time (annual fourth-highest daily maximum 8-hour average 
concentration, averaged over three consecutive years) (Wells, 2015). In 
the then-most recent period, 2011-2013, across the more than 800 
monitor locations meeting the then-current standard (with a level of 75 
ppb), the 3-year W126 index values were above 17 ppm-hrs in 25 sites 
distributed across different NOAA climatic regions, and above 19 ppm-
hrs at nearly half of these sites, with some well above. In comparison, 
among sites meeting an alternative standard of 70 ppb, there were no 
occurrences of a W126 value above 17 ppm-hrs and fewer than a handful 
of occurrences that equaled 17 ppm-hrs.\118\ For the longer

[[Page 49879]]

time period (extending back to 2001), among the nearly 4000 instances 
where a monitoring site met a standard level of 70 ppb, the 
Administrator noted that there was only ``a handful of isolated 
occurrences'' of 3-year W126 index values above 17 ppm-hrs, ``all but 
one of which were below 19 ppm-hrs'' (80 FR 65409, October 26, 2015). 
The Administrator concluded that that single value of 19.1 ppm-hrs 
(just equaling 19, when rounded), observed at a monitor for the 3-year 
period of 2006-2008, was reasonably regarded as an extremely rare and 
isolated occurrence, and, as such, it was unclear whether it would 
recur, particularly as areas across the U.S. took further steps to 
reduce O3 to meet revised primary and secondary standards. 
Further, based on all of the then available information, as noted 
above, the Administrator did not judge RBL estimates associated with 
marginally higher exposures in isolated, rare instances to be 
indicative of adverse effects to the public welfare. The Administrator 
concluded that a standard with a level of 70 ppb and the existing form 
and averaging time would be expected to limit cumulative exposures, in 
terms of a 3-year average W126 exposure index, to values at or below 17 
ppm-hrs, in nearly all instances, and accordingly, to eliminate or 
virtually eliminate cumulative exposures associated with a median RBL 
of 6% or greater (80 FR 65409, October 26, 2015). Thus, using RBL as a 
proxy in judging effects to public welfare, the Administrator judged 
that such a standard with a level of 70 ppb would provide the requisite 
protection from adverse effects to public welfare by limiting 
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances.
---------------------------------------------------------------------------

    \118\ The more than 500 monitors that would meet an alternative 
standard of 70 ppb during the 2011-2013 period were distributed 
across all nine NOAA climatic regions and 46 of the 50 states 
(Wells, 2015 and associated dataset in the docket [document 
identifier, EPA-HQ-OAR-2008-0699-4325]).
---------------------------------------------------------------------------

    In summary, the Administrator judged that the revised standard 
would protect natural forests in Class I and other similarly protected 
areas against an array of adverse vegetation effects, most notably 
including those related to effects on growth and productivity in 
sensitive tree species. The Administrator additionally judged that the 
revised standard would be sufficient to protect public welfare from 
known or anticipated adverse effects. This judgment by the 
Administrator appropriately recognized that the CAA does not require 
that standards be set at a zero-risk level, but rather at a level that 
reduces risk sufficiently so as to protect the public welfare from 
known or anticipated adverse effects. Thus, based on the conclusions 
drawn from the air quality analyses which demonstrated a strong, 
positive relationship between the 8-hour and W126 metrics and the 
findings that indicated the significant amount of control provided by 
the fourth-high metric, the evidence base of O3 effects on 
vegetation and her public welfare policy judgments, as well as public 
comments and CASAC advice, the Administrator decided to retain the 
existing form and averaging time and revise the level to 0.070 ppm, 
judging that such a standard would provide the requisite protection to 
the public welfare from any known or anticipated adverse effects 
associated with the presence of O3 in ambient air (80 FR 
65409-10, October 26, 2015).
2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the now 
current secondary O3 standard, or whether consideration of 
revision is appropriate, the EPA has adopted an approach in this review 
that builds upon the general approach used in the last review and 
reflects the body of evidence and information now available. 
Accordingly the approach in this review takes into consideration the 
approach used in the last review, including the substantial assessments 
and evaluations performed over the course of that review, and also 
taking into account the more recent scientific information and air 
quality data now available to inform understanding of the key policy-
relevant issues in the current review. As summarized above, the 
Administrator's decisions in the prior review were based on an 
integration of O3 welfare effects information with judgments 
on the public welfare significance of key effects, policy judgments as 
to when the standard is requisite, consideration of CASAC advice, and 
consideration of public comments.
    Similarly, in this review we draw on the current evidence and 
quantitative analyses of air quality and exposure pertaining to the 
welfare effects of O3 in ambient air. In so doing, we 
consider both the information available at the time of the last review 
and information more recently available, including that which has been 
critically analyzed and characterized in the current ISA. The 
evaluations in the PA, of the potential implications of various aspects 
of the scientific evidence assessed in the ISA (building on prior such 
assessments), augmented by the quantitative air quality, exposure or 
risk-based information, are also considered along with the associated 
uncertainties and limitations.
    This review of the secondary O3 standard also considers 
the August 2019 decision by the D.C. Circuit on the secondary standard 
established in 2015 and issues raised by the court in its remand of 
that standard to the EPA such that the decision in this review will 
incorporate the EPA's response to this remand. The opinion issued by 
the court concluded, in relevant part, that EPA had not provided a 
sufficient rationale for aspects of its decision on the 2015 secondary 
standard. See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 
2019). Accordingly, the court remanded the secondary standard to EPA 
for further justification or reconsideration, particularly in relation 
to its decision to focus on a 3-year average for consideration of the 
cumulative exposure, in terms of W126, identified as providing 
requisite public welfare protection, and its decision to not identify a 
specific level of air quality related to visible foliar injury.\119\ 
Thus, in addition to considering the currently available welfare 
effects evidence and quantitative air quality, exposure and risk 
information, this proposed decision on the secondary standard that was 
established in 2015, and the associated proposed conclusions and 
judgments, also consider the court's remand. In so doing, we have, for 
example, expanded certain analyses in this review compared with those 
conducted in the last review, included discussion on issues raised in 
the remand, and provided additional explanation of rationales for 
proposed conclusions on these points in this review. Together, the 
information, evaluations and considerations recognized here inform the 
Administrator's public welfare policy judgments and conclusions, 
including his decision as to whether to retain or revise this standard.
---------------------------------------------------------------------------

    \119\ The EPA's decision not to use a seasonal W126 index as the 
form and averaging time of the secondary standard was also 
challenged in this case, but the court did not reach that issue, 
concluding that it lacked a basis to assess the EPA's rationale on 
this point because the EPA had not yet fully explained its focus on 
a 3-year average W126 in its consideration of the standard. See 
Murray Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019).
---------------------------------------------------------------------------

B. Welfare Effects Information

    The information summarized here is based on our scientific 
assessment of the welfare effects evidence available in this review; 
this assessment is documented in the ISA \120\ and its policy

[[Page 49880]]

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, p. IS-3). Ozone is concluded to be the most 
prevalent photochemical oxidant present in the atmosphere and the one 
for which there is a very large, well-established evidence base of its 
health and welfare effects. Further, ``the primary literature 
evaluating the health and ecological effects of photochemical oxidants 
includes ozone almost exclusively as an indicator of photochemical 
oxidants'' (ISA, section IS.1.1). Thus, the current welfare effects 
evidence and the Agency's review of the evidence, including the 
evidence newly available in this review, continues to focus on 
O3.
---------------------------------------------------------------------------

    \120\ 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 cited 
when providing further details not repeated in newer assessments.
---------------------------------------------------------------------------

    More than 1600 studies are newly available and considered in the 
ISA, including more than 500 studies on welfare effects (ISA, Appendix 
10, Figure 10-2). While expanding the evidence for some effect 
categories, studies on growth-related effects, a key group of effects 
from the last review, are largely consistent with the evidence that was 
previously available. Policy implications of the currently available 
evidence are discussed in the PA (as summarized in section III.D.1 
below). The subsections below briefly summarize the following aspects 
of the evidence: The nature of O3-related welfare effects 
(section III.B.1), the potential public welfare implications (section 
III.B.2), and exposure concentrations associated with effects (section 
III.B.3).
1. Nature of Effects
    The welfare effects evidence base available in the current review 
includes more than fifty years of extensive research on the phytotoxic 
effects of O3, conducted both in and outside of the U.S., 
that documents the impacts of O3 on plants and their 
associated ecosystems (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 
ISA, 2020 ISA). As was established in prior reviews, O3 can 
interfere with carbon gain (photosynthesis) and allocation of carbon 
within the plant, making fewer carbohydrates available for plant 
growth, reproduction, and/or yield (1996 AQCD, pp. 5-28 and 5-29). For 
seed-bearing plants, reproductive effects can include reduced seed or 
fruit production or yield. The strongest evidence for effects from 
O3 exposure on vegetation was recognized at the time of the 
last review to be from controlled exposure studies, which ``have 
clearly shown that exposure to O3 is causally linked to 
visible foliar injury, decreased photosynthesis, changes in 
reproduction, and decreased growth'' in many species of vegetation 
(2013 ISA, p. 1-15). Such effects at the plant scale can also be linked 
to an array of effects at larger spatial scales (and higher levels of 
biological organization), with the evidence available in the last 
review indicating that ``O3 exposures can affect ecosystem 
productivity, crop yield, water cycling, and ecosystem community 
composition'' (2013 ISA, p. 1-15, Chapter 9, section 9.4). Beyond its 
effects on plants, the evidence in the last review also recognized 
O3 in the troposphere as a major greenhouse gas (ranking 
behind carbon dioxide and methane in importance), with associated 
radiative forcing and effects on climate, and recognized the 
accompanying ``large uncertainties in the magnitude of the radiative 
forcing estimate . . . making the impact of tropospheric O3 
on climate more uncertain than the effect of the longer-lived 
greenhouse gases'' (2013 ISA, sections 10.3.4 and 10.5.1 [p. 10-30]).
    The evidence newly available in this review supports, sharpens and 
expands somewhat on the conclusions reached in the last review (ISA, 
Appendices 8 and 9). Consistent with the evidence in the last review, 
the currently available evidence describes an array of O3 
effects on vegetation and related ecosystem effects, as well as the 
role of O3 in radiative forcing and subsequent climate-
related effects. Evidence newly available in this review augments more 
limited previously available evidence related to insect interactions 
with vegetation, contributing to conclusions regarding O3 
effects on plant-insect signaling (ISA, Appendix 8, section 8.7) and on 
insect herbivores (ISA, Appendix 8, section 8.6), as well as for ozone 
effects on tree mortality (Appendix 8, section 8.4). Thus, conclusions 
reached in the last review are supported by the current evidence base 
and conclusions are also reached in a few new areas based on the now 
expanded evidence.
    The current evidence base, including a wealth of longstanding 
evidence, supports the conclusion of causal relationships between 
O3 and visible foliar injury, reduced vegetation growth and 
reduced plant reproduction,\121\ as well as reduced yield and quality 
of agricultural crops, reduced productivity in terrestrial ecosystems, 
alteration of terrestrial community composition,\122\ and alteration of 
belowground biogeochemical cycles (ISA, section IS.5). Based on the 
current evidence base, the ISA also concluded there likely to be a 
causal relationship between O3 and alteration of ecosystem 
water cycling, reduced carbon sequestration in terrestrial ecosystems, 
and with increased tree mortality (ISA, section IS.5). Additional 
evidence newly available in this review is concluded by the ISA to 
support conclusions on two additional plant-related effects: The body 
of evidence is concluded to be sufficient to infer that there is likely 
to be a causal relationship between O3 exposure and 
alteration of plant-insect signaling, and to infer that there is likely 
to be a causal relationship between O3 exposure and altered 
insect herbivore growth and reproduction (ISA, Table IS-12).
---------------------------------------------------------------------------

    \121\ The 2013 ISA did not include a separate causality 
determination for reduced plant reproduction. Rather, it was 
included with the conclusion of a causal relationship with reduced 
vegetation growth (ISA, Table IS-12).
    \122\ The 2013 ISA concluded alteration of terrestrial community 
composition to be likely causally related to O3 based on 
the then available information (ISA, Table IS-12).
---------------------------------------------------------------------------

    As in the last review, the strongest evidence and the associated 
findings of causal or likely causal relationships with O3 in 
ambient air, and the quantitative characterizations of relationships 
between O3 exposure and occurrence and magnitude of effects 
are for vegetation effects. The scales of these effects range from the 
individual plant scale to the ecosystem scale, with potential for 
impacts on the public welfare (as discussed in section III.B.2 below). 
The following summary addresses the identified vegetation-related 
effects of O3 across these scales.
    The current evidence, consistent with the decades of previously 
available evidence, documents and characterizes visible foliar injury 
in many tree, shrub, herbaceous, and crop species as an effect of 
exposure to O3 (ISA, Appendix 8, section 8.2; 2013 ISA, 
section 9.4.2; 2006 AQCD, 1996 AQCD, 1986 AQCD, 1978 AQCD). As was also 
stated in the last scientific assessment, ``[r]ecent experimental 
evidence continues to show a consistent association between visible 
injury and ozone exposure'' (ISA, Appendix 8, section 8.2, p. 8-13; 
2013 ISA, section 9.4.2, p. 9-41). Ozone-induced visible foliar injury 
symptoms on certain tree and herbaceous species, such as black cherry, 
yellow-poplar and common milkweed, have long been considered diagnostic 
of exposure to elevated O3 based on the consistent 
association established with experimental evidence (ISA, Appendix 8, 
section 8.2; 2013 ISA, p. 1-10).\123\
---------------------------------------------------------------------------

    \123\ As described in the ISA, ``[t]ypical types of visible 
injury to broadleaf plants include stippling, flecking, surface 
bleaching, bifacial necrosis, pigmentation (e.g., bronzing), and 
chlorosis or premature senescence'' and ``[t]ypical visible injury 
symptoms for conifers include chlorotic banding, tip burn, flecking, 
chlorotic mottling, and premature senescence of needles'' (ISA, 
Appendix 8, p. 8-13).

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

[[Page 49881]]

    The currently available evidence, consistent with that in past 
reviews, indicates that ``visible foliar injury usually occurs when 
sensitive plants are exposed to elevated ozone concentrations in a 
predisposing environment,'' with a major factor for such an environment 
being the amount of soil moisture available to the plant (ISA, Appendix 
8, p. 8-23; 2013 ISA, section 9.4.2). Further, the significance of 
O3 injury at the leaf and whole plant levels also depends on 
an array of factors that include the amount of total leaf area 
affected, age of plant, size, developmental stage, and degree of 
functional redundancy among the existing leaf area (ISA, Appendix 8, 
section 8.2; 2013 ISA, section 9.4.2). In this review, as in the past, 
such modifying factors contribute to the difficulty in quantitatively 
relating visible foliar injury to other vegetation effects (e.g., 
individual tree growth, or effects at population or ecosystem levels), 
such that visible foliar injury ``is not always a reliable indicator of 
other negative effects on vegetation'' (ISA, Appendix 8, section 8.2, 
p. 8-24; 2013 ISA, p. 9-39).\124\
---------------------------------------------------------------------------

    \124\ Similar to the 2013 ISA, the ISA for the current review 
states the following (ISA, pp. 8-24).
    Although visible injury is a valuable indicator of the presence 
of phytotoxic concentrations of ozone in ambient air, it is not 
always a reliable indicator of other negative effects on vegetation 
[e.g., growth, reproduction; U.S. EPA (2013)]. The significance of 
ozone injury at the leaf and whole-plant levels depends on how much 
of the total leaf area of the plant has been affected, as well as 
the plant's age, size, developmental stage, and degree of functional 
redundancy among the existing leaf area (U.S. EPA, 2013). Previous 
ozone AQCDs have noted the difficulty in relating visible foliar 
injury symptoms to other vegetation effects, such as individual 
plant growth, stand growth, or ecosystem characteristics (U.S. EPA, 
2006, 1996). Thus, it is not presently possible to determine, with 
consistency across species and environments, what degree of injury 
at the leaf level has significance to the vigor of the whole plant.
---------------------------------------------------------------------------

    Consistent with conclusions in past reviews, the evidence, 
extending back several decades, continues to document the detrimental 
effects of O3 on plant growth and reproduction (ISA, 
Appendix 8, sections 8.3 and 8.4; 2013 ISA, p. 9-42). The available 
studies come from a variety of different study types that cover an 
array of different species, effects endpoints, and exposure methods and 
durations. In addition to studies on scores of plant species that have 
found O3 to reduce plant growth, the evidence accumulated 
over the past several decades documents O3 alteration of 
allocation of biomass within the plant and plant reproduction (ISA, 
Appendix 8, sections 8.3 and 8.4; 2013 ISA, p. 1-10). The biological 
mechanisms underlying the effect of O3 on plant reproduction 
include ``both direct negative effects on reproductive tissues and 
indirect negative effects that result from decreased photosynthesis and 
other whole plant physiological changes'' (ISA, p. IS-71). A newly 
available meta-analysis of more than 100 studies published between 1968 
and 2010 summarizes effects of O3 on multiple measures of 
reproduction (ISA, Appendix 8, section 8.4.1).
    Studies involving experimental field sites have also reported 
effects on measures of plant reproduction, such as effects on seeds 
(reduced weight, germination, and starch levels) that could lead to a 
negative impact on species regeneration in subsequent years, and bud 
size that might relate to a delay in spring leaf development (ISA, 
Appendix 8, section 8.4; 2013 ISA, section 9.4.3; Darbah et al., 2007, 
Darbah et al., 2008). A more recent laboratory study reported 6-hour 
daily O3 exposures of flowering mustard plants to 100 ppb 
during different developmental stages to have mixed effects on 
reproductive metrics. While flowers exposed early versus later in 
development produced shorter fruits, the number of mature seeds per 
fruit was not significantly affected by flower developmental stage of 
exposure (ISA, Appendix 8, section 8.4.1; Black et al., 2012). Another 
study assessed seed viability for a flowering plant in laboratory and 
field conditions, finding effects on seed viability of O3 
exposures (90 and 120 ppb) under laboratory conditions but less clear 
effects under more field-like conditions (ISA, Appendix 8, section 
8.4.1; Landesmann et al., 2013).
    With regard to agricultural crops, the current evidence base, as in 
the last review, is sufficient to infer a causal relationship between 
O3 exposure and reduced yield and quality (ISA, section 
IS.5.1.2). The current evidence is augmented by new research in a 
number of areas, including studies on soybean, wheat and other nonsoy 
legumes. The new information assessed in the ISA remains consistent 
with the conclusions reached in the 2013 ISA (ISA, section IS.5.1.2).
    The evidence base for trees includes a number of studies conducted 
at the Aspen free-air carbon-dioxide and ozone enrichment (FACE) 
experiment site in Wisconsin (that operated from 1998 through 2011) and 
also available in the last review (ISA, IS.5.1 and Appendix 8, section 
8.1.2.1; 2013 ISA, section 9.2.4). These studies, which occurred in a 
field setting (more similar to natural forest stands than open-top-
chamber studies), reported reduced tree growth when grown in single or 
three species stands within 30-m diameter rings and exposed over one or 
more years to elevated O3 concentrations (hourly 
concentrations 1.5 times concentrations in ambient air at the site) 
compared to unadjusted ambient air concentrations (2013 ISA, section 
9.4.3; Kubiske et al., 2006, Kubiske et al., 2007).\125\
---------------------------------------------------------------------------

    \125\ Seasonal (90-day) W126 index values for unadjusted 
O3 concentrations over six years of the Aspen FACE 
experiments ranged from 2 to 3 ppm-hrs, while the elevated exposure 
concentrations (reflecting addition of O3 to ambient air 
concentrations) ranged from somewhat above 20 to somewhat above 35 
ppm-hrs (ISA, Appendix 8, Figure 8-17).
---------------------------------------------------------------------------

    With regard to tree mortality, the 2013 ISA did not include a 
determination of causality (ISA, Appendix 8, section 8.4). While the 
then-available evidence included studies identifying ozone as a 
contributor to tree mortality, which contributed to the 2013 conclusion 
regarding O3 and alteration of community composition (2013 
ISA, section 9.4.7.4), a separate causality determination regarding 
O3 and tree mortality was not assessed (ISA, Appendix 8, 
section 8.4; 2013 ISA, Table 9-19). The evidence assessed in the 2013 
ISA (and 2006 AQCD) was largely observational, including studies that 
reported declines in conifer forests for which elevated O3 
was identified as contributor but in which a variety of environmental 
factors may have also played a role (2013 ISA, section 9.4.7.1; 2006 
AQCD, sections AX9.6.2.1, AX9.6.2.2, AX9.6.2.6, AX9.6.4.1 and 
AX9.6.4.2). Since the last review, three additional studies are 
available (ISA, Appendix 8, Table 8-9). Two of these are analyses of 
field observations, one of which is set in the Spanish Pyrenees.\126\ A 
second study is a large-scale empirical statistical analysis of factors 
potentially contributing to tree mortality in eastern and central U.S. 
forests during the 1971-2005 period, which reported O3 
(county-level 11-year [1996-2006] average 8 hour metric) \127\ to be 
ninth among the 13 potential factors assessed \128\ and to have a

[[Page 49882]]

significant positive correlation with tree mortality (ISA, section 
IS.5.1.2, Appendix 8, section 8.4.3; Dietze and Moorcroft, 2011). A 
newly available experimental study also reported increased mortality in 
two of five aspen genotypes grown in mixed stands under elevated 
O3 concentrations (ISA, section IS.5.1.2; Moran and Kubiske, 
2013). Coupled with the plant-level evidence of phytotoxicity discussed 
above, as well as consideration of community composition effects, this 
evidence was concluded to indicate the potential for elevated 
O3 concentrations to contribute to tree mortality (ISA, 
section IS.5.1.2 and Appendix 8, sections 8.4.3 and 8.4.4). Based on 
the current evidence, the ISA concludes there is likely to be a causal 
relationship between O3 and increased tree mortality (ISA, 
Table IS-2, Appendix 8, section 8.4.4). A variety of factors in natural 
environments can either mitigate or exacerbate predicted O3-
plant interactions and are recognized sources of uncertainty and 
variability. Such factors at the plant level include multiple 
genetically influenced determinants of O3 sensitivity, 
changing sensitivity to O3 across vegetative growth stages, 
co-occurring stressors and/or modifying environmental factors (ISA, 
Appendix 8, section 8.12).
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    \126\ The concentration gradient with altitude in the Spanish 
study, includes--at the highest site--annual average April-to-
September O3 concentrations for the 2004 to 2007 period 
that range up to 74 ppb (Diaz-de-Quijano et al., 2016).
    \127\ Annual fourth highest daily maximum 8-hour O3 
concentrations in these regions were above 80 ppb in the early 2000s 
and median design values at national trend sites were nearly 85 ppb 
(PA, Figures 2-11 and 2-12).
    \128\ This statistical analysis, which utilized datasets from 
within the 1971-2005 period, included an examination of the 
sensitivity of predicted mortality rate to 13 different covariates. 
On average across the predictions for 10 groups of trees (based on 
functional type and major representative species), the order of 
mortality rate sensitivity to the covariates, from highest to 
lowest, was: Sulfate deposition, tree diameter, nitrate deposition, 
summer temperature, tree age, elevation, winter temperature, 
precipitation, O3 concentration, tree basal area, 
topographic moisture index, slope and topographic radiation index 
(Dietze and Moorcroft, 2011).
---------------------------------------------------------------------------

    Ozone-induced effects at the scale of the whole plant have the 
potential to translate to effects at the ecosystem scale, such as 
reduced terrestrial productivity and carbon storage, and altered 
terrestrial community composition, as well as impacts on ecosystem 
functions, such as belowground biogeochemical cycles and ecosystem 
water cycling. For example, under the relevant exposure conditions, 
O3-related reduced tree growth and reproduction, as well as 
increased mortality, could lead to reduced ecosystem productivity. 
Recent studies from the Aspen FACE experiment and modeling simulations 
indicate that O3-related negative effects on ecosystem 
productivity may be temporary or may be limited in some systems (ISA, 
Appendix 8, section 8.8.1). Previously available studies had reported 
impacts on productivity in some forest types and locations, such as 
ponderosa pine in southern California and other forest types in the 
mid-Atlantic region (2013 ISA, section 9.4.3.4). Through reductions in 
sensitive species growth, and related ecosystem productivity, 
O3 could lead to reduced ecosystem carbon storage (ISA, 
IS.5.1.4; 2013 ISA, section 9.4.3). With regard to forest community 
composition, available studies have reported changes in tree 
communities composed of species with relatively greater and relatively 
lesser sensitivity to O3 (ISA, section IS.5.1.8.1, Appendix 
8, section 8.10; 2013 ISA, section 9.4.3; Kubiske et al., 2007). As the 
ISA concludes, ``[t]he extent to which ozone affects terrestrial 
productivity will depend on more than just community composition, but 
other factors, which both directly influence [net primary productivity] 
(i.e., availability of N and water) and modify the effect of ozone on 
plant growth'' (ISA, Appendix 8, section 8.8.1). Thus, the magnitude of 
O3 impact on ecosystem productivity, as on forest 
composition, can vary among plant communities based on several factors, 
including the type of stand or community in which the sensitive species 
occurs (e.g., single species versus mixed canopy), the role or position 
of the species in the stand (e.g., dominant, sub-dominant, canopy, 
understory), and the sensitivity of co-occurring species and 
environmental factors (e.g., drought and other factors).
    The effects of O3 on plants and plant populations have 
implications for ecosystem functions. Two such functions, effects with 
which O3 is concluded to be likely causally or causally 
related, are ecosystem water cycling and belowground biogeochemical 
cycles, respectively (ISA, Appendix 8, sections 8.11 and 8.9). With 
regard to the former, the effects of O3 on plants (e.g., via 
stomatal control, as well as leaf and root growth and changes in wood 
anatomy associated with water transport) can affect ecosystem water 
cycling through impacts on root uptake of soil moisture and groundwater 
as well as transpiration through leaf stomata to the atmosphere (ISA, 
Appendix 8, section 8.11.1). These ``impacts may in turn affect the 
amount of water moving through the soil, running over land or through 
groundwater and flowing through streams'' (ISA, Appendix 8, p. 8-161). 
Evidence newly available in this review is supportive of previously 
available evidence in this regard (ISA, Appendix 8, section 8.11.6). 
The current evidence, including that newly available, indicates the 
extent to which the effects of O3 on plant leaves and roots 
(e.g., through effects on chemical composition and biomass) can impact 
belowground biogeochemical cycles involving root growth, soil food web 
structure, soil decomposer activities, soil microbial respiration, soil 
carbon turnover, soil water cycling and soil nutrient cycling (ISA, 
Appendix 8, section 8.9).
    Additional vegetation-related effects with implications beyond 
individual plants include the effects of O3 on insect 
herbivore growth and reproduction and plant-insect signaling (ISA, 
Table IS-12, Appendix 8, sections 8.6 and 8.7). With regard to insect 
herbivore growth and reproduction, the evidence includes multiple 
effects in an array of insect species, although without a consistent 
pattern of response for most endpoints (ISA, Appendix 8, Table 8-11). 
As was also the case with the studies available at the time of the last 
review,\129\ in the newly available studies individual-level responses 
are highly context- and species-specific and not all species tested 
showed a response (ISA, section IS.5.1.3 and Appendix 8, section 8.6). 
Evidence on plant-insect signaling that is newly available in this 
review comes from laboratory, greenhouse, open top chambers (OTC) and 
FACE experiments (ISA, section IS.5.1.3 and Appendix 8, section 8.7). 
The available evidence indicates a role for elevated O3 in 
altering and degrading emissions of chemical signals from plants and 
reducing detection of volatile plant signaling compounds (VPSCs) by 
insects, including pollinators. Elevated O3 concentrations 
degrade some VPSCs released by plants, potentially affecting ecological 
processes including pollination and plant defenses against herbivory. 
Further, the available studies report elevated O3 conditions 
to be associated with plant VPSC emissions that may make a plant either 
more attractive or more repellant to herbivorous insects, and to 
predators and parasitoids that target phytophagous (plant-eating) 
insects (ISA, section IS.5.1.3 and Appendix 8, section 8.7).
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    \129\ During the last review, the 2013 ISA stated with regard to 
O3 effects on insects and other wildlife that ``there is 
no consensus on how these organisms respond to elevated 
O3'' (2013 ISA, section 9.4.9.4, p. 9-98).
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    Ozone welfare effects also extend beyond effects on vegetation and 
associated biota due to it being a major greenhouse gas and radiative 
forcing agent.\130\ As in the last review, the

[[Page 49883]]

current evidence, augmented since the 2013 ISA, continues to support a 
causal relationship between the global abundance of O3 in 
the troposphere and radiative forcing, and a likely causal relationship 
between the global abundance of O3 in the troposphere and 
effects on temperature, precipitation, and related climate variables 
\131\ (ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013). As was 
also true at the time of the last review, tropospheric O3 
has been ranked third in importance for global radiative forcing, after 
carbon dioxide and methane, with the radiative forcing of O3 
since pre-industrial times estimated to be about 25 to 40% of the total 
warming effects of anthropogenic carbon dioxide and about 75% of the 
effects of anthropogenic methane (ISA, Appendix 9, section 9.1.3.3). 
Uncertainty in the magnitude of radiative forcing estimated to be 
attributed to tropospheric O3 is a contributor to the 
relatively greater uncertainty associated with climate effects of 
tropospheric O3 compared to such effects of the well mixed 
greenhouse gases, such as carbon dioxide and methane (ISA, section 
IS.6.2.2).
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    \130\ Radiative forcing is a metric used to quantify the change 
in balance between radiation coming into and going out of the 
atmosphere caused by the presence of a particular substance. The ISA 
describes it more specifically as ``a perturbation in net radiative 
flux at the tropopause (or top of the atmosphere) caused by a change 
in radiatively active forcing agent(s) after stratospheric 
temperatures have readjusted to radiative equilibrium 
(stratospherically adjusted RF)'' (ISA, Appendix 9, section 
9.1.3.3).
    \131\ Effects on temperature, precipitation, and related climate 
variables were referred to as ``climate change'' or ``effects on 
climate'' in the 2013 ISA (ISA, p. IS-82; 2013 ISA, pp. 1-14 and 10-
31).
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    Lastly, the evidence regarding tropospheric O3 and UV-B 
shielding (shielding of ultraviolet radiation at wavelengths of 280 to 
320 nanometers) was evaluated in the 2013 ISA and determined to be 
inadequate to draw a causal conclusion (2013 ISA, section 10.5.2). The 
current ISA concludes there to be no new evidence since the 2013 ISA 
relevant to the question of UV-B shielding by tropospheric 
O3 (ISA, IS.1.2.1 and Appendix 9, section 9.1.3.4).
2. Public Welfare Implications
    The secondary standard is to ``specify a level of air quality the 
attainment and maintenance of which in the judgment of the 
Administrator . . . is requisite to protect the public welfare from any 
known or anticipated adverse effects associated with the presence of 
such air pollutant in the ambient air'' (CAA, section 109(b)(2)). As 
recognized in prior reviews, the secondary standard is not meant to 
protect against all known or anticipated O3-related welfare 
effects, but rather those that are judged to be adverse to the public 
welfare, and a bright-line determination of adversity is not required 
in judging what is requisite (78 FR 3212, January 15, 2013; 80 FR 
65376, October 26, 2015; see also 73 FR 16496, March 27, 2008). Thus, 
the level of protection from known or anticipated adverse effects to 
public welfare that is requisite for the secondary standard is a public 
welfare policy judgment to be made by the Administrator. In each 
review, the Administrator's judgment regarding the currently available 
information and adequacy of protection provided by the current standard 
is generally informed by considerations in prior reviews and associated 
conclusions.
    The categories of effects identified in the CAA to be included 
among welfare effects are quite diverse,\132\ and among these 
categories, any single category includes many different types of 
effects that are of broadly varying specificity and level of 
resolution. For example, effects on vegetation, is a category 
identified in CAA section 302(h), and the ISA recognizes numerous 
vegetation-related effects of O3 at the organism, 
population, community and ecosystem level, as summarized in section 
III.B.1 above (ISA, Appendix 8). The significance of each type of 
vegetation-related effect with regard to potential effects on the 
public welfare depends on the type and severity of effects, as well as 
the extent of such effects on the affected environmental entity, and on 
the societal use of the affected entity and the entity's significance 
to the public welfare. Such factors are generally considered in light 
of judgments and conclusions made in prior reviews regarding effects on 
the public welfare. For example, a key consideration with regard to 
public welfare implications in prior reviews of the O3 
secondary standard was the intended use of the affected or sensitive 
vegetation and the significance of the vegetation to the public welfare 
(73 FR 16496, March 27, 2008; 80 FR 65292, October 26, 2015).
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    \132\ Section 302(h) of the CAA states that language referring 
to ``effects on welfare'' in the CAA ``includes, but is 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.''
---------------------------------------------------------------------------

    More specifically, judgments regarding public welfare significance 
in the last two O3 NAAQS decisions gave particular attention 
to O3 effects in areas with special federal protections, and 
lands set aside by states, tribes and public interest groups to provide 
similar benefits to the public welfare (73 FR 16496, March 27, 2008; 80 
FR 65292, October 26, 2015). For example, in the decision to revise the 
secondary standard in the 2008 review, the Administrator took note of 
``a number of actions taken by Congress to establish public lands that 
are set aside for specific uses that are intended to provide benefits 
to the public welfare, including lands that are to be protected so as 
to conserve the scenic value and the natural vegetation and wildlife 
within such areas, and to leave them unimpaired for the enjoyment of 
future generations'' (73 FR 16496, March 27, 2008).\133\ Such areas 
include Class I areas \134\ which are federally mandated to preserve 
certain air quality related values. Additionally, as the Administrator 
recognized, ``States, Tribes and public interest groups also set aside 
areas that are intended to provide similar benefits to the public 
welfare, for residents on State and Tribal lands, as well as for 
visitors to those areas'' (73 FR 16496, March 27, 2008). The 
Administrator took note of the ``clear public interest in and value of 
maintaining these areas in a condition that does not impair their 
intended use and the fact that many of these lands contain 
O3-sensitive species'' (73 FR 16496, March 27, 2008). 
Similarly, in the 2015 review, the Administrator indicated particular 
concern for O3-related effects on plant function and 
productivity and associated ecosystem effects in natural ecosystems 
``such as those in areas with protection designated by Congress for 
current and future generations, as well

[[Page 49884]]

as areas similarly set aside by states, tribes and public interest 
groups with the intention of providing similar benefits to the public 
welfare'' (80 FR 65403, October 26, 2015).
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    \133\ For example, the fundamental purpose of parks in the 
National Park System ``is to conserve the scenery, natural and 
historic objects, and wild life in the System units and to provide 
for the enjoyment of the scenery, natural and historic objects, and 
wild life in such manner and by such means as will leave them 
unimpaired for the enjoyment of future generations'' (54 U.S.C. 
100101). Additionally, the Wilderness Act of 1964 defines designated 
``wilderness areas'' in part as areas ``protected and managed so as 
to preserve [their] natural conditions'' and requires that these 
areas ``shall be administered for the use and enjoyment of the 
American people in such manner as will leave them unimpaired for 
future use and enjoyment as wilderness, and so as to provide for the 
protection of these areas, [and] the preservation of their 
wilderness character . . .'' (16 U.S.C. 1131 (a) and (c)). Other 
lands that benefit the public welfare include national forests which 
are managed for multiple uses including sustained yield management 
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3); 
16 U.S.C. 1601(d)(1)).
    \134\ Areas designated as Class I include all international 
parks, national wilderness areas which exceed 5,000 acres in size, 
national memorial parks which exceed 5,000 acres in size, and 
national parks which exceed 6,000 acres in size, provided the park 
or wilderness area was in existence on August 7, 1977. Other areas 
may also be Class I if designated as Class I consistent with the CAA 
(as described in the PA, Appendix 4D, section 4D.2.4).
---------------------------------------------------------------------------

    The 2008 and 2015 decisions recognized that the degree to which 
effects on vegetation in specially protected areas, such as those 
identified above, may be judged adverse involves considerations from 
the species level to the ecosystem level, such that judgments can 
depend on the intended use for, or service (and value) of, the affected 
vegetation, ecological receptors, ecosystems and resources and the 
significance of that use to the public welfare (73 FR 16496, March 27, 
2008; 80 FR 65377, October 26, 2015). Uses or services provided by 
areas that have been afforded special protection can flow in part or 
entirely from the vegetation that grows there. For example, ecosystem 
services are the ``benefits that people derive from functioning 
ecosystems'' (Costanza et al., 2017; ISA, section IS.5.1).\135\ 
Ecosystem services range from those directly related to the natural 
functioning of the ecosystem to ecosystem uses for human recreation or 
profit, such as through the production of lumber or fuel (Costanza et 
al., 2017). Aesthetic value and outdoor recreation depend, at least in 
part, on the perceived scenic beauty of the environment. Further, there 
have been analyses that report the American public values--in monetary 
as well as nonmonetary ways--the protection of forests from air 
pollution damage (Haefele et al., 1991). In fact, public surveys have 
indicated that Americans rank as very important the existence of 
resources, the option or availability of the resource and the ability 
to bequest or pass it on to future generations (Cordell et al., 2008). 
The spatial, temporal and social dimensions of public welfare impacts 
are also influenced by the type of service affected. For example, a 
national park can provide direct recreational services to the thousands 
of visitors that come each year, but also provide an indirect value to 
the millions who may not visit but receive satisfaction from knowing 
that it exists and is preserved for the future (80 FR 65377, October 
26, 2015).
---------------------------------------------------------------------------

    \135\ Ecosystem services analyses were one of the tools used in 
the last review of the secondary standards for oxides of nitrogen 
and sulfur to inform the decisions made with regard to adequacy of 
protection provided by the standards and as such, were used in 
conjunction with other considerations in the discussion of adversity 
to public welfare (77 FR 20241, April 3, 2012).
---------------------------------------------------------------------------

    The different types of effects on vegetation discussed in section 
III.B.1 above differ with regard to aspects important to judging their 
public welfare significance. In the case of crop yield loss, such 
judgments depend on considerations related to the heavy management of 
agriculture in the U.S. Judgments for other categories of effects may 
generally relate to considerations regarding forested areas, including 
specifically those forested areas that are not managed for harvest. For 
example, effects on tree growth and reproduction, and also visible 
foliar injury, have the potential to be significant to the public 
welfare through impacts in Class I and other areas given special 
protection in their natural/existing state, although they differ in how 
they might be significant. Additionally, as described in section 
III.B.1 above, O3 effects on tree growth and reproduction 
could, depending on severity, extent and other factors, lead to effects 
on a larger scale including reduced productivity, altered forest and 
forest community (plant, insect and microbe) composition, reduced 
carbon storage and altered ecosystem water cycling (ISA, section 
IS.5.1.8.1; 2013 ISA, Figure 9-1, sections 9.4.1.1 and 9.4.1.2). For 
example, forest or forest community composition can be affected through 
O3 effects on growth and reproductive success of sensitive 
species in the community, with the extent of compositional changes 
dependent on factors such as competitive interactions (ISA, section 
IS.5.1.8.1; 2013 ISA, sections 9.4.3 and 9.4.3.1). Impacts on some of 
these characteristics (e.g., forest or forest community composition) 
may be considered of greater public welfare significance when occurring 
in Class I or other protected areas, due to value for particular 
services that the public places on such areas.
    Depending on the type and location of the affected ecosystem, 
however, a broader array of services benefitting the public can be 
affected in a broader array of areas as well. For example, other 
services valued by people that can be affected by reduced tree growth, 
productivity and associated forest effects include aesthetic value, 
food, fiber, timber, other forest products, habitat, recreational 
opportunities, climate and water regulation, erosion control, air 
pollution removal, and desired fire regimes (PA, Figure 4-2; ISA, 
section IS.5.1; 2013 ISA, sections 9.4.1.1 and 9.4.1.2). In considering 
such services in past reviews, the Agency has given particular 
attention to effects in natural ecosystems, indicating that a 
protective standard, based on consideration of effects in natural 
ecosystems in areas afforded special protection, would also ``provide a 
level of protection for other vegetation that is used by the public and 
potentially affected by O3 including timber, produce grown 
for consumption and horticultural plants used for landscaping'' (80 FR 
65403, October 26, 2015). For example, locations potentially vulnerable 
to O3-related impacts might include forested lands, both 
public and private, where trees are grown for timber production. 
Forests in urbanized areas also provide a number of services that are 
important to the public in those areas, such as air pollution removal, 
cooling, and beautification. There are also many other tree species, 
such as various ornamental and agricultural species (e.g., Christmas 
trees, fruit and nut trees), that provide ecosystem services that may 
be judged important to the public welfare.
    Depending on its severity and spatial extent, visible foliar 
injury, which affects the physical appearance of the plant, also has 
the potential to be significant to the public welfare through impacts 
in Class I and other similarly protected areas. In cases of widespread 
and severe injury during the growing season (particularly when 
sustained across multiple years, and accompanied by obvious impacts on 
the plant canopy), O3-induced visible foliar injury might be 
expected to have the potential to impact the public welfare in scenic 
and/or recreational areas, particularly in areas with special 
protection, such as Class I areas.\136\ The ecosystem services most 
likely to be affected by O3-induced visible foliar injury 
(some of which are also recognized above for tree growth-related 
effects) are cultural services, including aesthetic value and outdoor 
recreation.
---------------------------------------------------------------------------

    \136\ For example, although analyses specific to visible foliar 
injury are of limited availability, there have been analyses 
developing estimates of recreation value damages of severe impacts 
related to other types of forest effects, such as tree mortality due 
to bark beetle outbreaks (e.g., Rosenberger et al., 2013). Such 
analyses estimate reductions in recreational use when the damage is 
severe (e.g., reductions in the density of live, robust trees). Such 
damage would reasonably be expected to also reflect damage 
indicative of injury with which a relationship with other plant 
effects (e.g., growth and reproduction) would be also expected. 
Similarly, a couple of studies from the 1970s and 1980s indicated 
likelihood for reduced recreational use in areas with stands of pine 
in which moderate to severe injury was apparent from 30 or 40 feet 
(PA, section 4.3.2).
---------------------------------------------------------------------------

    The geographic extent of protected areas that may be vulnerable to 
public welfare effects of O3, such as impacts to outdoor 
recreation, is potentially appreciable. For example, biomonitoring 
surveys that were routinely administered by the U.S.

[[Page 49885]]

Forest Service (USFS) as far back as 1994 in the eastern U.S. and 1998 
in the western U.S. include many field sites at which there are plants 
sensitive to O3-related visible foliar injury; there are 450 
field sites across 24 states in the North East and North Central 
regions (Smith, 2012).\137\ Since visible foliar injury is a visible 
indication of O3 exposure in species sensitive to this 
effect, a number of such species have been established as bioindicator 
species, and such surveys have been used by federal land managers as 
tools in assessing potential air quality impacts in Class I areas (U.S. 
Forest Service, 2010). Additionally, the USFS has developed categories 
for the scoring system that it uses for purposes of describing and 
comparing injury severity at biomonitoring sites. The sites are termed 
biosites and the scoring system involves deriving biosite index (BI) 
scores that may be described with regard to one of several categories 
ranging from little or no foliar injury to severe injury (e.g., Smith 
et al., 2003; Campbell et al., 2007; Smith et al., 2007; Smith, 
2012).\138\ As noted in section III.B.1 above, there is not an 
established quantitative relationship between visible foliar injury and 
other effects, such as reduced growth and productivity as visible 
foliar injury ``is not always a reliable indicator of other negative 
effects'' (ISA, Appendix 8, section 8.2).
---------------------------------------------------------------------------

    \137\ This aspect of the USFS biomonitoring surveys has 
apparently been suspended, with the most recent surveys conducted in 
2011 (USFS, 2013, USFS, 2017).
    \138\ Studies presenting USFS biomonitoring program data have 
suggested what might be ``assumptions of risk'' related to scores in 
these categories, e.g., none, low, moderate and high for BI scores 
of zero to five, five to 15, 15 to 25 and above 25, respectively 
(e.g., Smith et al., 2003; Smith et al., 2012).
---------------------------------------------------------------------------

    Public welfare implications associated with visible foliar injury 
might further be considered to relate largely to effects on scenic and 
aesthetic values. The available information does not yet address or 
describe the relationships expected to exist between some level of 
injury severity (e.g., little, low/light, moderate or severe) and/or 
spatial extent affected and scenic or aesthetic values. This gap 
impedes consideration of the public welfare implications of different 
injury severities, and accordingly judgments on the potential for 
public welfare significance. That notwithstanding, while minor spotting 
on a few leaves of a plant may easily be concluded to be of little 
public welfare significance, some level of severity and widespread 
occurrence of visible foliar injury, particularly if occurring in 
specially protected areas, such as Class I areas, where the public can 
be expected to place value (e.g., for recreational uses), might 
reasonably be concluded to impact the public welfare. Accordingly, key 
considerations for public welfare significance of this endpoint would 
relate to qualitative consideration of the potential for such effects 
to affect the aesthetic value of plants in protected areas, such as 
Class I areas (73 FR 16490, March 27, 2008).
    While, as noted above, public welfare benefits of forested lands 
can be particular to the type of area in which the forest occurs, some 
of the potential public welfare benefits associated with forest 
ecosystems are not location dependent. A potentially extremely valuable 
ecosystem service provided by forested lands is carbon sequestration or 
storage (ISA, section IS.5.1.4 and Appendix 8, section 8.8.3; 2013 ISA, 
section 2.6.2.1 and p. 9-37).\139\ As noted above, the EPA has 
concluded that effects on this ecosystem service are likely causally 
related to O3 in ambient air (ISA, Table IS-12). The 
importance of carbon sequestration to the public welfare relates to its 
role in counteracting the impact of greenhouse gases on radiative 
forcing and related climate effects. As summarized in section III.B.1 
above, O3 is also a greenhouse gas and O3 
abundance in the troposphere is causally related to radiative forcing 
and likely causally related to subsequent effects on temperature, 
precipitation and related climate variables (ISA, section IS.6.2.2). 
Accordingly, such effects also have important public welfare 
implications, although their quantitative evaluation in response to 
O3 concentrations in the U.S. is complicated by ``[c]urrent 
limitations in climate modeling tools, variation across models, and the 
need for more comprehensive observational data on these effects'' (ISA, 
section IS.6.2.2). The service of carbon storage is of paramount 
importance to the public welfare no matter in what location the trees 
are growing or what their intended current or future use (e.g., 2013 
ISA, section 9.4.1.2). In other words, the benefit exists as long as 
the trees are growing, regardless of what additional functions and 
services it provides.
---------------------------------------------------------------------------

    \139\ While carbon sequestration or storage also occurs for 
vegetated ecosystems other than forests, it is relatively larger in 
forests given the relatively greater biomass for trees compared to 
other plants.
---------------------------------------------------------------------------

    With regard to agriculture-related effects, the EPA has recognized 
other complexities related to areas and plant species that are heavily 
managed to obtain a particular output (such as commodity crops or 
commercial timber production). For example, the EPA has recognized that 
the degree to which O3 impacts on vegetation that could 
occur in such areas and on such species would impair the intended use 
at a level that might be judged adverse to the public welfare has been 
less clear (80 FR 65379, October 26, 2015; 73 FR 16497, March 27, 
2008). While having sufficient crop yields is of high public welfare 
value, important commodity crops are typically heavily managed to 
produce optimum yields. Moreover, based on the economic theory of 
supply and demand, increases in crop yields would be expected to result 
in lower prices for affected crops and their associated goods, which 
would primarily benefit consumers. These competing impacts on producers 
and consumers complicate consideration of these effects in terms of 
potential adversity to the public welfare (2014 WREA, sections 5.3.2 
and 5.7). When agricultural impacts or vegetation effects in other 
areas are contrasted with the emphasis on ecosystem effects in Class I 
and similarly protected areas, the EPA most recently has judged the 
significance to the public welfare of O3-induced effects on 
sensitive vegetation growing within the U.S. to differ depending on the 
nature of the effect, the intended use of the sensitive plants or 
ecosystems, and the types of environments in which the sensitive 
vegetation and ecosystems are located, with greater significance 
ascribed to areas identified for specific uses and benefits to the 
public welfare, such as Class I areas, than to areas for which such 
uses have not been established (80 FR 65292, October 26, 2015; FR 73 
16496-16497, March 27, 2008).
    Categories of effects newly identified as likely causally related 
to O3 in ambient air, such as alteration of plant-insect 
signaling and insect herbivore growth and reproduction, also have 
potential public welfare implications. For example, given the role of 
plant-insect signaling in such important ecological processes as insect 
herbivore growth and reproduction. The potential to contribute to 
adverse effects to the public welfare, e.g., given the role of the 
plant-insect signaling process in pollination and seed dispersal, as 
well as natural plant defenses against predation and parasitism, 
particular effects on particular signaling processes can be seen to 
have the potential for adverse effects on the public welfare (ISA, 
section IS.5.1.3). However, uncertainties and limitations in the 
current evidence (e.g., summarized in sections III.B.3 and III.D.1 
below) preclude an assessment of the extent

[[Page 49886]]

and magnitude of O3 effects on these endpoints, which thus 
also precludes an evaluation of the potential for associated public 
welfare implications, particularly under exposure conditions expected 
to occur in areas meeting the current standard.
    In summary, several considerations are recognized as important to 
judgments on the public welfare significance of the array of welfare 
effects of different O3 exposure conditions. There are 
uncertainties and limitations associated with the consideration of the 
magnitude of key welfare effects that might be concluded to be adverse 
to ecosystems and associated services. There are numerous locations 
where the presence of O3-sensitive tree species may 
contribute to a vulnerability to impacts from O3 on tree 
growth, productivity and carbon storage and their associated ecosystems 
and services. Exposures that may elicit effects and the significance of 
the effects in specific situations can vary due to differences in 
exposed species sensitivity, the severity and associated significance 
of the observed or predicted O3-induced effect, the role 
that the species plays in the ecosystem, the intended use of the 
affected species and its associated ecosystem and services, the 
presence of other co-occurring predisposing or mitigating factors, and 
associated uncertainties and limitations.
3. Exposures Associated With Effects
    The welfare effects identified in section III.B.1 above vary widely 
with regard to the extent and level of detail of the available 
information that describes the O3 exposure circumstances 
that may elicit them. As recognized in the 2013 ISA and in the ISA for 
this review, such information is most advanced for growth-related 
effects such as growth and yield. For example, the information on 
exposure metric and E-R relationships for these effects is long-
standing, having been first described in the 1997 review. The current 
information regarding exposure metrics and relationships between 
exposure and the occurrence and severity of visible foliar injury, 
summarized in section III.B.3.b below, is much less advanced or well 
established. The evidence base for other categories of effects is still 
more lacking in information that might support characterization of 
potential impacts related to these effects of changes in O3 
concentrations.
a. Growth-Related Effects
(i) Exposure Metric
    The long-standing body of vegetation effects evidence includes a 
wealth of information on aspects of O3 exposure that are 
important in influencing effects on plant growth and yield that has 
been described in the scientific assessments across the last several 
decades (1996 AQCD; 2006 AQCD; 2013 ISA; 2020 ISA). A variety of 
factors have been investigated, including ``concentration, time of day, 
respite time, frequency of peak occurrence, plant phenology, 
predisposition, etc.'' (2013 ISA, section 9.5.2), and the importance of 
the duration of the exposure as well as the relatively greater 
importance of higher concentrations over lower concentrations have been 
consistently well documented (2013 ISA, section 9.5.3). Based on the 
associated improved understanding of the biological basis for plant 
response to O3 exposure, a number of mathematical approaches 
have been developed for summarizing O3 exposure for the 
purpose of assessing effects on vegetation, including those that 
cumulate exposures over some specified period while weighting higher 
concentrations more than lower (2013 ISA, sections 9.5.2 and 9.5.3; 
ISA, Appendix 8, section 8.2.2.2).
    In the last several reviews, based on the then-available evidence, 
as well as advice from the CASAC, the EPA's scientific assessments have 
focused on the use of a cumulative, seasonal \140\ concentration-
weighted index for considering the growth-related effects evidence and 
in quantitative exposure analyses for purposes of reaching conclusions 
on the secondary standard. More specifically, the Agency used the W126-
based cumulative, seasonal metric (80 FR 65404, October 26, 2015; ISA, 
section IS.3.2, Appendix 8, section 8.13). This metric, commonly called 
the W126 index, is a non-threshold approach described as the 
sigmoidally weighted sum of all hourly O3 concentrations 
observed during a specified daily and seasonal time window, where each 
hourly O3 concentration is given a weight that increases 
from zero to one with increasing concentration (2013 ISA, pp. 9-101, 9-
104).
---------------------------------------------------------------------------

    \140\ The ``seasonal'' descriptor refers to the duration of the 
period quantified (3 months) rather than a specific season of the 
year.
---------------------------------------------------------------------------

    Across the last several decades, several different exposure metrics 
have been evaluated, primarily for their ability to summarize ambient 
air O3 concentrations into a metric that best describes 
quantitatively the relationship of O3 in ambient air with 
the occurrence and/or extent of effects on vegetation, particularly 
growth-related effects. More specifically, an important objective has 
been to identify the metric that summarizes O3 exposure in a 
way that is most predictive of the effect of interest (e.g., reduced 
growth). Along with the continuous weighted, W126 index, the two other 
cumulative indices that have received greatest attention across the 
past several O3 NAAQS reviews are the threshold weighted 
indices, AOT60 \141\ and SUM06.\142\ Accordingly, some studies of 
O3 vegetation effects have reported exposures using these 
metrics. Alternative methods for characterizing O3 exposure 
to predict various plant responses (particularly those related to 
photosynthesis, growth and productivity) have, in recent years, also 
included flux models (models that are based on the amount of 
O3 that enters the leaf). However, as was the case in the 
last review, there remain a variety of complications, limitations and 
uncertainties associated with this approach. For example, ``[w]hile 
some efforts have been made in the U.S. to calculate ozone flux into 
leaves and canopies, little information has been published relating 
these fluxes to effects on vegetation'' (ISA, section IS.3.2). Further, 
as flux of O3 into the plant under different conditions of 
O3 in ambient air is affected by several factors including 
temperature, vapor pressure deficit, light, soil moisture, and plant 
growth stage, use of this approach to quantify the vegetation impact of 
O3 would require information on these various types of 
factors (ISA, section IS.3.2). In addition to these data requirements, 
each species has different amounts of internal detoxification potential 
that may protect species to differing degrees. The lack of detailed 
species- and site-specific data required for flux modeling in the U.S. 
and the lack of understanding of detoxification

[[Page 49887]]

processes continues to make this technique less viable for use in risk 
assessments in the U.S. (ISA, section IS.3.2).
---------------------------------------------------------------------------

    \141\ The AOT60 index is the seasonal sum of the difference 
between an hourly concentration above 60 ppb, minus 60 ppb (2006 
AQCD, p. AX9-161). More recently, some studies have also reported 
O3 exposures in terms of AOT40, which is conceptually 
similar but with 40 substituted for 60 in its derivation (ISA, 
Appendix 8, section 8.13.1).
    \142\ The SUM06 index is the seasonal sum of hourly 
concentrations at or above 0.06 ppm during a specified daily time 
window (2006 AQCD, p. AX9-161; 2013 ISA, section 9.5.2). This may 
sometimes be referred to as SUM60, e.g., when concentrations are in 
terms of ppb. There are also variations on this metric that utilize 
alternative reference points above which hourly concentrations are 
summed. For example, SUM08 is the seasonal sum of hourly 
concentrations at or above 0.08 ppm and SUM0 is the seasonal sum of 
all hourly concentrations.
---------------------------------------------------------------------------

    Based on extensive review of the published literature on different 
types of E-R metrics, including comparisons between metrics, the EPA 
has generally focused on cumulative, concentration-weighted indices of 
exposure, recognizing them as the most appropriate biologically based 
metrics to consider in this context (1996 AQCD; 2006 AQCD; 2013 ISA). 
Quantifying exposure in this way has been found to improve the 
explanatory power of E-R models for growth and yield over using indices 
based only on mean and peak exposure values (2013 ISA, section 2.6.6.1, 
p. 2-44). The most well-analyzed datasets in such evaluations are two 
detailed datasets established two decades ago, one for seedlings of 11 
tree species and one for 10 crops, described further in section 
III.B.3.a(ii) below (e.g., Lee and Hogsett, 1996, Hogsett et al., 
1997). These datasets, which include species-specific seedling growth 
and crop yield response information across multiple seasonal cumulative 
exposures, were used to develop robust quantitative E-R functions to 
predict growth reduction relative to a zero-O3 setting 
(termed relative biomass loss or RBL) in seedlings of the tree species 
and E-R functions for RYL for a set of common crops (ISA, Appendix 8, 
section 8.13.2; 2013 ISA, section 9.6.2).
    Among the studies newly available in this review, no new exposure 
indices for assessing effects on vegetation growth or other 
physiological process parameters have been identified. The SUM06, AOTx 
(e.g., AOT60) and W126 exposure metrics remain the cumulative metrics 
that are most commonly discussed (ISA, Appendix 8, section 8.13.1). The 
ISA notes that ``[c]umulative indices of exposure that differentially 
weight hourly concentrations [which would include the W126 index] have 
been found to be best suited to characterize vegetation exposure to 
ozone with regard to reductions in vegetation growth and yield'' (ISA, 
section ES.3). Accordingly, in this review, as in the last two reviews, 
the seasonal W126-based cumulative, concentration-weighted metric 
receives primary attention in considering the effects evidence and 
exposure analyses, particularly related to growth effects (e.g., in 
sections III.C and III.D below).
    The first step in calculating the seasonal W126 index for a 
specific year, as described and considered in this review, is to sum 
the weighted hourly O3 concentrations in ambient air during 
daylight hours (defined as 8:00 a.m. to 8:00 p.m. local standard time) 
within each calendar month, resulting in monthly index values. The 
monthly W126 index values are calculated from hourly O3 
concentrations as follows.\143\
---------------------------------------------------------------------------

    \143\ In situations where data are missing, an adjustment is 
factored into the monthly index (PA, Appendix 4D, section 4D.2.2).
[GRAPHIC] [TIFF OMITTED] TP14AU20.001

---------------------------------------------------------------------------
where,

N is the number of days in the month
d is the day of the month (d = 1, 2, . . ., N)
h is the hour of the day (h = 0, 1, . . ., 23)
Cdh is the hourly O3 concentration observed on 
day d, hour h, in parts per million

    The W126 index value for a specific year is the maximum sum of the 
monthly index values for three consecutive months within a calendar 
year (i.e., January to March, February to April, . . . October to 
December). Three-year average W126 index values are calculated by 
taking the average of seasonal W126 index values for three consecutive 
years (e.g., as described in the PA, Appendix 4D, section 4D.2.2).
(ii) Relationships Between Exposure Levels and Effects
    Across the array of O3-related welfare effects, 
consistent and systematically evaluated information on E-R 
relationships across multiple exposure levels is limited. Most 
prominent is the information on E-R relationships for growth effects on 
tree seedlings and crops,\144\ which has been available for the past 
several reviews. The information on which these functions are based 
comes primarily from the U.S. EPA's National Crop Loss Assessment 
Network (NCLAN) \145\ project for crops and the NHEERL-WED project for 
tree seedlings, projects implemented primarily to define E-R 
relationships for major agricultural crops and tree species, thus 
advancing understanding of responses to O3 exposures (ISA, 
Appendix 8, section 8.13.2). These projects included a series of 
experiments that used OTCs to investigate tree seedling growth response 
and crop yield over a growing season under a variety of O3 
exposures and growing conditions (2013 ISA, section 9.6.2; Lee and 
Hogsett, 1996). These experiments have produced multiple studies that 
document O3 effects on tree seedling growth and crop yield 
across multiple levels of exposure. Importantly, the information on 
exposure includes hourly concentrations across the season-long (or 
longer) exposure period which can then be summarized in terms of the 
various seasonal metrics.\146\ In the initial analyses of these data, 
exposure was characterized in terms of several metrics, including 
seasonal SUM06 and W126 indices (Lee and Hogsett, 1996; 1997 Staff 
Paper, sections IV.D.2 and IV.D.3; 2007 Staff Paper, section 7.6), 
while use of these functions more recently has focused on their 
implementation in terms of seasonal W126 index (2013 ISA, section 9.6; 
80 FR 65391-92, October 26, 2015).
---------------------------------------------------------------------------

    \144\ The E-R functions estimate O3-related reduction 
in a year's tree seedling growth or crop yield as a percentage of 
that expected in the absence of O3 (ISA, Appendix 8, 
section 8.13.2).
    \145\ The NCLAN program, which was undertaken in the early to 
mid-1980s, assessed multiple U.S. crops, locations, and 
O3 exposure levels, using consistent methods, to provide 
the largest, most uniform database on the effects of O3 
on agricultural crop yields (1996 AQCD, 2006 AQCD, 2013 ISA, 
sections 9.2, 9.4, and 9.6; ISA, Appendix 8, section 8.13.2).
    \146\ This underlying database for the exposure is a key 
characteristic that sets this set of studies (and their associated 
E-R analyses) apart from other available studies.
---------------------------------------------------------------------------

    The 11 tree species for which robust and well-established E-R 
functions for RBL are available are black cherry, Douglas fir, loblolly 
pine, ponderosa pine, quaking aspen, red alder, red maple, sugar maple, 
tulip poplar, Virginia pine, and white pine (PA, Appendix 4A; 2013 ISA, 
section 9.6).\147\ While these 11 species represent only a small 
fraction of the total number of native tree species in the contiguous 
U.S., this small subset includes eastern and western species, deciduous 
and coniferous species, and species that

[[Page 49888]]

grow in a variety of ecosystems and represent a range of tolerance to 
O3 (PA, Appendix 4B; 2013 ISA, section 9.6.2). The 
established E-R functions for most of the 11 species were derived using 
data from multiple studies or experiments involving a wide range of 
exposure and/or growing conditions. From the available data, separate 
E-R functions were developed for each combination of species and 
experiment (2013 ISA, section 9.6.1; Lee and Hogsett, 1996). From these 
separate species-experiment-specific E-R functions, species-specific 
composite E-R functions were developed (PA, Appendix 4A).
---------------------------------------------------------------------------

    \147\ A quantitative analysis of E-R information for an 
additional species was considered in the 2014 WREA. But the 
underlying study, rather than being a controlled exposure study, 
involves exposure to ambient air along an existing gradient of 
O3 concentrations in the New York City metropolitan area, 
such that O3 and climate conditions were not controlled 
(2013 ISA, section 9.6.3.3). Based on recognition that this dataset 
is not as strong as those for the 11 species for which E-R functions 
are based on controlled ozone exposure, this study is not included 
with the established E-R functions for the 11 species (PA, section 
4.3.3).
---------------------------------------------------------------------------

    In total, the 11 species-specific composite E-R functions are based 
on 51 tree seedling studies or experiments (PA, Appendix 4A, section 
4A.1.1). For six of the 11 species, this function is based on just one 
or two studies (e.g., red maple and black cherry), while for other 
species there were as many as 11 studies available (e.g., ponderosa 
pine). A stochastic analysis drawing on the experiment-specific 
functions provides a sense of the variability and uncertainty 
associated with the estimated E-R relationships among and within 
species (PA, Appendix 4A, section 4A.1.1, Figure 4A-13). Based on the 
species-specific E-R functions, growth of the studied tree species at 
the seedling stage appears to vary widely in sensitivity to 
O3 exposure (PA, Appendix 4A, section 4A.1.1). Since the 
initial set of studies were completed, several additional studies, 
focused on aspen, have been published based on the Aspen FACE 
experiment in a planted forest in Wisconsin; the findings were 
consistent with many of the earlier OTC studies (ISA, Appendix 8, 
section 8.13.2).
    With regard to crops, established E-R functions are available for 
10 crops: Barley, field corn, cotton, kidney bean, lettuce, peanut, 
potato, grain sorghum, soybean and winter wheat (PA, Appendix 4A, 
section 4A.1; ISA, Appendix 8, section 8.13.2). Studies available since 
the last review for seven soybean cultivars support conclusions from 
prior studies that of similarity of current soybean cultivar 
sensitivity compared to the earlier genotypes from which the soybean E-
R functions were (ISA, Appendix 8, section 8.13.2).
    Newly available studies that investigated growth effects of 
O3 exposures are also consistent with the existing evidence 
base, and generally involve particular aspects of the effect rather 
than expanding the conditions under which plant species, particularly 
trees, have been assessed (ISA, section IS.5.1.2). These include a 
compilation of previously available studies on plant biomass response 
to O3 (in terms of AOT40); the compilation reports linear 
regressions conducted on the associated varying datasets (ISA, Appendix 
8, section 8.13.2; van Goethem et al., 2013). Based on these 
regressions, this study describes distributions of sensitivity to 
O3 effects on biomass across nearly 100 plant species (trees 
and grasslands) including 17 species native to the U.S. and 65 
additional species that have been introduced to the U.S. (ISA, Appendix 
8, section 8.13.2; van Goethem et al., 2013). Additional information is 
needed to more completely describe O3 exposure response 
relationships for these species in the U.S.\148\
---------------------------------------------------------------------------

    \148\ The set of studies included in this compilation were 
described as meeting a set of criteria, such as: Including 
O3 only exposures in conditions described as ``close to 
field'' exposures (which were expressed as AOT40); including at 
least 21 days exposure above 40 ppb O3; and having a 
maximum hourly concentration that was no higher than 100 ppb (van 
Goethem et al., 2013). The publication does not report exposure 
duration for each study or details of biomass response measurements, 
making it less useful for the purpose of describing E-R 
relationships that might provide for estimation of specific impacts 
associated with air quality conditions meeting the current standard 
(e.g., 2013 ISA, p. 9-118).
---------------------------------------------------------------------------

b. Visible Foliar Injury
    With regard to visible foliar injury, as with the evidence 
available in the last review, the current evidence ``continues to show 
a consistent association between visible injury and ozone exposure,'' 
while also recognizing the role of modifying factors such as soil 
moisture and time of day (ISA, section IS.5.1.1). The current ISA, in 
concluding that the newly available information is consistent with 
conclusions of the 2013 ISA, also summarizes several recently available 
studies that continue to document that O3 elicits visible 
foliar injury in many plant species. These include a synthesis of 
previously published studies that categorizes studied species (and 
their associated taxonomic classifications) as to whether or not 
O3-related foliar injury has been reported to occur in the 
presence of elevated O3,\149\ while not providing 
quantitative information regarding specific exposure conditions or 
analyses of E-R relationships (ISA, Appendix 8, section 8.2). The 
evidence in the current review, as was the case in the last review, 
while documenting that elevated O3 conditions in ambient air 
generally results in visible foliar injury in sensitive species (when 
in a predisposing environment),\150\ does not include a quantitative 
description of the relationship of incidence or severity of visible 
foliar injury in sensitive species in natural locations in the U.S. 
with specific metrics of O3 exposure.
---------------------------------------------------------------------------

    \149\ The publication identifies 245 species across 28 plant 
genera, many native to the U.S., in which O3-related 
visible foliar injury has been reported (ISA, Appendix 8, Table 8-
3).
    \150\ As noted in the 2013 ISA and the ISA for the current 
review, visible foliar injury usually occurs when sensitive plants 
are exposed to elevated ozone concentrations in a predisposing 
environment, with a major modifying factor being the amount of soil 
moisture available to a plant. Accordingly, dry periods are 
concluded to decrease the incidence and severity of ozone-induced 
visible foliar injury, such that the incidence of visible foliar 
injury is not always higher in years and areas with higher ozone, 
especially with co-occurring drought (ISA, Appendix 8, p. 8-23; 
Smith, 2012; Smith et al., 2003).
---------------------------------------------------------------------------

    Several studies of the extensive USFS field-based dataset of 
visible foliar injury incidence in forests across the U.S.\151\ 
illustrate the extent to which our current understanding of this 
relationship is limited. For example, a study that was available in the 
last review presents a trend analysis of these data for sites located 
in 24 states of the northeast and north central U.S. for the 16-year 
period from 1994 through 2009 that provides some insight into the 
influence of changes in air quality and soil moisture on visible foliar 
injury and the difficulty inherent in predicting foliar injury response 
under different air quality and soil moisture scenarios (Smith, 2012, 
Smith et al., 2012; ISA, Appendix 8, section 8.2). This study, like 
prior analyses of such data, shows the dependence of foliar injury 
incidence and severity on local site conditions for soil moisture 
availability and O3 exposure. For example, while the authors 
characterize the ambient air O3 concentrations to be the 
``driving force'' behind incidence of injury and its severity, they 
state that ``site moisture conditions are also a very strong influence 
on the biomonitoring data'' (Smith et al., 2003). In general, the USFS 
data analyses have found foliar injury prevalence and severity to be 
higher during seasons and sites that have experienced the highest 
O3 than during other periods (e.g., Campbell et al., 2007; 
Smith, 2012).
---------------------------------------------------------------------------

    \151\ These data were collected as part of the U.S. Forest 
Service Forest Health Monitoring/Forest Inventory and Analysis (USFS 
FHM/FIA) biomonitoring network program (2013 ISA, section 9.4.2.1; 
Campbell et al., 2007, Smith et al., 2012).
---------------------------------------------------------------------------

    Although studies of the incidence of visible foliar injury in 
national forests, wildlife refuges, and similar areas have often used 
cumulative indices (e.g., SUM06) to investigate variations in incidence 
of foliar injury, studies also suggest an additional role for metrics 
focused on peak concentrations (ISA; 2013 ISA; 2006 AQCD; Hildebrand et 
al., 1996; Smith, 2012). For example, a

[[Page 49889]]

study of six years of USFS biosite \152\ data (2000-2006) for three 
western states found that the biosites with the highest O3 
exposure (SUM06 at or above 25 ppm-hrs) had the highest percentage of 
biosites with injury and the highest mean BI, with little discernable 
difference among the lower exposure categories; this study also 
identified ``better linkage between air levels and visible injury'' as 
an O3 research need (Campbell et al., 2007).\153\ More 
recent studies of the complete 16 years of data in 24 northeast and 
north central states have suggested that a cumulative exposure index 
alone may not completely describe the O3-related risk of 
this effect at USFS sites (Smith et al., 2012; Smith, 2012). For 
example, Smith (2012) observed there to be a declining trend in the 16-
year dataset, ``especially after 2002 when peak ozone concentrations 
declined across the entire region'' thus suggesting a role for peak 
concentrations.
---------------------------------------------------------------------------

    \152\ As described in section III.B.2 above, biosites are 
biomonitoring sites where the USFS applies a scoring system for 
purposes of categorizing areas with regard to severity of visible 
foliar injury occurrence (U.S. Forest Service, 2010).
    \153\ In considering their findings, the authors expressed the 
view that ``[a]lthough the number of sites or species with injury is 
informative, the average biosite injury index (which takes into 
account both severity and amount of injury on multiple species at a 
site) provides a more meaningful measure of injury'' for their 
assessment at a statewide scale (Campbell et al., 2007).
---------------------------------------------------------------------------

    Some studies of visible foliar injury incidence data have 
investigated the role of peak concentrations quantified by an 
O3 exposure index that is a count of hourly concentrations 
(e.g., in a growing season) above a threshold 1-hour concentration of 
100 ppb, N100 (e.g., Smith, 2012; Smith et al., 2012). For example, the 
study by Smith (2012) discussed injury patterns at biosites in 24 
states in the Northeast and North Central regions in the context of the 
SUM06 index and N100 metrics (although not via a statistical 
model).\154\ That study of 16 years of biomonitoring data from these 
sites suggested that there may be a threshold exposure needed for 
injury to occur, and the number of hours of elevated O3 
concentrations during the growing season (such as what is captured by a 
metric like N100) may be more important than cumulative exposure in 
determining the occurrence of foliar injury (Smith, 2012).\155\ The 
study's authors noted this finding to be consistent with findings 
reported by a study of statistical analyses of seven years of visible 
foliar injury data from a wildlife refuge in the mid-Atlantic (Davis 
and Orendovici, 2006, Smith et al., 2012). The latter study 
investigated the fit of multiple models that included various metrics 
of cumulative O3 (SUM06, SUM0, SUM08), alone and in 
combination with some other variables (Davis and Orendovici, 2006). 
Among the statistical models investigated (which did not include one 
with either W126 index or N100 alone), the model with the best fit to 
the visible foliar injury incidence data was found to be one that 
included the cumulative metric, W126, and the N100 index, as well as 
drought index (Davis and Orendovici, 2006).\156\
---------------------------------------------------------------------------

    \154\ The current ISA, 2013 ISA and prior AQCDs have not 
described extensive evaluation of specific peak-concentration 
metrics such as the N100 that might assist in identifying the one 
best suited for such purposes.
    \155\ In summarizing this study in the last review, the ISA 
observed that ``[o]verall, there was a declining trend in the 
incidence of foliar injury as peak O3 concentrations 
declined'' (2013 ISA, p. 9-40).
    \156\ The models evaluated included several with cumulative 
exposure indices alone. These included SUM60, SUM0, and SUM80, but 
not W126. They did not include a model with W126 that did not also 
include N100. Across all of the models evaluated, the model with the 
best fit to the data was found to be the one that included N100 and 
W126, along with the drought index (Davis and Orendovici, 2006).
---------------------------------------------------------------------------

    The established significant role of higher or peak O3 
concentrations, as well as pattern of their occurrence, in plant 
responses has been noted in prior ISAs or AQCDs. In identifying support 
with regard to foliar injury as the response, the 2013 ISA and 2006 
AQCD both cite studies that support the ``important role that peak 
concentrations, as well as the pattern of occurrence, plays in plant 
response to O3'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-
169). For example, a study of European white birch saplings reported 
that peak concentrations and the duration of the exposure event were 
important determinants of foliar injury (2013 ISA, section 9.5.3.1; 
Oksanen and Holopainen, 2001). This study also evaluated tree growth, 
which was found to be more related to cumulative exposure (2013 ISA, p. 
9-105).\157\ A second study that was cited by both assessments that 
focused on aspen, reported that ``the variable peak exposures were 
important in causing injury, and that the different exposure 
treatments, although having the same SUM06, resulted in very different 
patterns of foliar injury'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-169; 
Yun and Laurence, 1999). As noted in the 2006 AQCD, the cumulative 
exposure indices (e.g., SUM06, W126) were ``originally developed and 
tested using only growth/yield data, not foliar injury'' and ``[t]his 
distinction is critical in comparing the efficacy of one index to 
another'' (2006 AQCD, p. AX9-173). It is also recognized that where 
cumulative indices are highly correlated with the frequency or 
occurrence of higher hourly average concentrations, they could be good 
predictors of such effects (2006 AQCD, section AX9.4.4.3).
---------------------------------------------------------------------------

    \157\ The study authors concluded that ``high peak 
concentrations were important for visible injuries and stomatal 
conductance, but less important for determining growth responses'' 
(Oksanen and Holopainen, 2001).
---------------------------------------------------------------------------

    In a more recent study (by Wang et al. [2012]) that is cited in the 
current ISA, a statistical modeling analysis was performed on a subset 
of the years of data that were described in Smith (2012). This 
analysis, which involved 5,940 data records from 1997 through 2007 from 
the 24 northeast and north central states, tested a number of models 
for their ability to predict the presence of visible foliar injury (a 
nonzero biosite score), regardless of severity, and generally found 
that the type of O3 exposure metric (e.g., SUM06 versus 
N100) made only a small difference, although the models that included 
both a cumulative index (SUM06) and N100 had a just slightly better fit 
(Wang et al., 2012). Based on their investigation of 15 different 
models, using differing combination of several types of potential 
predictors, the study authors concluded that they were not able to 
identify environmental conditions under which they ``could reliably 
expect plants to be damaged'' (Wang et al., 2012). This is indicative 
of the current state of knowledge, in which there remains a lack of 
established quantitative functions describing E-R relationships that 
would allow prediction of visible foliar injury severity and incidence 
under varying air quality and environmental conditions.
    The available information related to O3 exposures 
associated with visible foliar injury of varying severity also includes 
the dataset developed by the EPA in the last review from USFS BI 
scores, collected during the years 2006 through 2010 at locations in 37 
states. In developing this dataset, the BI scores were combined with 
estimates of soil moisture \158\ and estimates of seasonal cumulative 
O3 exposure in terms of

[[Page 49890]]

W126 index \159\ (Smith and Murphy, 2015; PA, Appendix 4C). This 
dataset includes more than 5,000 records of which more than 80 percent 
have a BI score of zero (indicating a lack of visible foliar injury). 
While the estimated W126 index assigned to records in this dataset 
ranges from zero to somewhat above 50 ppm-hrs, more than a third of all 
the records (and also of records with BI scores above zero or five) 
\160\ are at sites with W126 index estimates below 7 ppm-hrs.
---------------------------------------------------------------------------

    \158\ Soil moisture categories (dry, wet or normal) were 
assigned to each biosite record based on the NOAA Palmer Z drought 
index values obtained from the NCDC website for the April-through-
August periods, averaged for the relevant year; details are provided 
in the PA, Appendix 4C, section 4C.2. There are inherent 
uncertainties in this assignment, including the substantial spatial 
variation in soil moisture and large size of NOAA climate divisions 
(hundreds of miles). This dataset, including associated 
uncertainties and limitations, is described in the PA, Appendix 4C, 
section 4C.5.
    \159\ The W126 index values assigned to the biosite locations 
are estimates developed for 12 kilometer (km) by 12 km cells in a 
national-scale spatial grid for each year. The grid cell estimates 
were derived from applying a spatial interpolation technique to 
annual W126 values derived from O3 measurements at 
ambient air monitoring locations for the years corresponding to the 
biosite surveys (details in the PA, Appendix 4C, sections 4.C.2 and 
4C.5).
    \160\ One third (33%) of scores above 15 are at sites with W126 
below 7 ppm-hrs (PA, Appendix 4C, Table 4C-3).
---------------------------------------------------------------------------

    In an extension of analyses of this dataset developed in the last 
review, the presentation in the PA \161\ describes the BI scores for 
the records in the dataset in relation to the W126 index estimate for 
each record, using bins of increasing W126 index values. The PA 
presentation utilizes the BI score breakpoints in the scheme used by 
the USFS to categorize severity. The lowest USFS category encompasses 
BI scores from zero to just below 5; scores of this magnitude are 
described as ``little or no foliar injury'' (Smith et al., 2012). The 
next highest category encompasses scores from five to just below 15 and 
is described as ``light to moderate foliar injury,'' BI scores of 15 up 
to 25 are described as ``moderate'' and above 25 is described as 
``severe'' (Smith et al., 2012). The PA presentation indicates that 
across the W126 bins, there is variation in both the incidence of 
particular magnitude BI scores and in the average score per bin. In 
general, however the greatest incidence of records with BI scores above 
zero, five, or higher--and the highest average BI score--occurs with 
the highest W126 bin, i.e., the bin for W126 index estimates greater 
than 25 ppm-hrs (PA, Appendix 4C, Table 4C-6).
---------------------------------------------------------------------------

    \161\ Beyond the presentation of a statistical analysis 
developed in the last review, the PA presentations are primarily 
descriptive (as compared to statistical) in recognition of the 
limitations and uncertainties of the dataset (PA, Appendix 4C, 
section 4C.5).
---------------------------------------------------------------------------

    While recognizing limitations in the dataset,\162\ the PA makes 
several observations, focusing particularly on records in the normal 
soil category (PA, section 4.5.1). For records categorized as wet soil 
moisture, the sample size for the W126 bins above 13 ppm-hrs is quite 
small (including only 18 of the 1,189 records in that soil moisture 
category), precluding meaningful interpretation.\163\ For the normal 
soil category, the percentages of records in the greater than 25 ppm-
hrs bin that have BI scores above 15 (``moderate'' and ``severe'' 
injury) or above 5 (``little,'' ``moderate'' and ``severe'' injury) are 
both more than three times greater than such percentages in any of the 
lower W126 bins.\164\ For example, the proportion of records with BI 
above five fluctuates between 5% and 13% across all but the highest 
W126 bin (>25 ppm-hrs) for which the proportion is 41% (PA, Appendix 
4C, Table 4C-6). The same pattern is observed for BI scores above 15 at 
sites with normal and dry soil moisture conditions, albeit with lower 
incidences. For example, the incidence of normal soil moisture records 
with BI score above 15 in the bin for W126 index values above 25 ppm-
hrs was 20% but fluctuates between 1% and 4% in the bin for W126 index 
values at or below 25 ppm-hrs (PA, Appendix 4C, Table 4C-6). The 
average BI of 7.9 in the greater-then-25-ppm-hrs bin is more than three 
times the next highest W126 bin average. The average BI in each of the 
next two lower W126 bins is just slightly higher than average BIs for 
the rest of the bins, and the average BI for all bins at or below 25 
ppm-hrs are well below 5 (PA, Appendix 4C).
---------------------------------------------------------------------------

    \162\ For example, the majority of records have W126 index 
estimates at or below 9 ppm-hrs, and fewer than 10% have W126 
estimates above 15 ppm-hrs. Further, the BI scores are quite 
variable across the range of W126 bins, with even the lowest W126 
bin (estimates below 7 ppm-hrs) including BI scores well above 15 
(PA, Appendix 4C, section 4C.4.2). The records for the wet soil 
moisture category in the higher W126 bins are more limited that the 
other categories, with nearly 90% of the wet soil moisture records 
falling into the bins for W126 index at or below 9 ppm-hrs, limiting 
interpretations for higher W126 bins (PA, Appendix 4C, Table 4C.4 
and section 4C.6). Accordingly, the PA observations focused 
primarily on the records for the normal or dry soil moisture 
categories, for which W126 index above 13 ppm-hrs is better 
represented.
    \163\ The full database includes only 18 records at sites in the 
wet soil moisture category with estimated W126 index above 13 ppm-
hrs, with 9 or fewer (less than 1%) in each of the W126 bins above 
13 ppm-hrs (PA, Appendix 4C, Table 4C-3). Among the bins for W126 at 
or below 13 ppm-hrs, the average BI score is less than 2 (PA, 
Appendix 4C, Table 4C-5).
    \164\ When scores characterized as ``little injury'' by the USFS 
classification scheme are also included (i.e., when considering all 
scores above zero), there is a suggestion of increased frequency of 
records for the W126 bins above 19 or 17 ppm-hrs, although 
difference from lower bins is less than a factor of two (PA, 
Appendix 4C).
---------------------------------------------------------------------------

    Overall, the dataset described in the PA generally indicates the 
risk of injury, and particularly injury considered at least light, 
moderate or severe, to be higher at the highest W126 index values, with 
appreciable variability in the data for the lower bins (PA, Appendix 
4C). This appears to be consistent with the conclusions of the studies 
of detailed quantitative analyses, summarized above, that the pattern 
is stronger at higher O3 concentrations. A number of factors 
may contribute to the observed variability in BI scores and lack of a 
clear pattern with W126 index bin; among others, these may include 
uncertainties in assignment of W126 estimates and soil moisture 
categories to biosite locations, variability in biological response 
among the sensitive species monitored, and the potential role of other 
aspects of O3 air quality not captured by the W126 index. 
Thus, the dataset has limitations affecting associated conclusions and 
uncertainty remains regarding the tools for and the appropriate metric 
(or metrics) for quantifying O3 exposures, as well as 
perhaps soil moisture conditions, with regard to their influence on 
extent and/or severity of injury in sensitive species in natural areas 
(Davis and Orendovici, 2006, Smith et al., 2012; Wang et al., 2012).
    Dose modeling or flux models (referenced in section III.B.3.a(i) 
above, have also been considered for quantifying O3 dose 
that may be related to plant leaf injury. Among the newly available 
evidence is a study examining relationships between short-term flux and 
leaf injury on cotton plants that described a sensitivity parameter 
that might characterize the influence on the flux-injury relationship 
of diel and seasonal variability in plant defenses (among other 
factors) and suggested additional research might provide for such a 
sensitivity parameter to ``function well in combination with a 
sigmoidal weighting of flux, analogous to the W126 weighting of 
concentration'', and perhaps an additional parameter (Grantz et al., 
2013, p. 1710; ISA, Appendix 8, section 8.13.1). However, the ISA 
recognizes there is ``much unknown'' with regard to the relationship 
between O3 uptake and leaf injury, and relationships with 
detoxification processes (ISA, Appendix 8, section 8.13.1 and p. 8-
184). These uncertainties have made this technique less viable for 
assessments in the U.S., precluding use of a flux-based approach at 
this time (ISA, Appendix 8, section 8.13.1 and p. 8-184).
c. Other Effects
    With regard to radiative forcing and subsequent climate effects 
associated with the global tropospheric abundance of O3, the 
newly available evidence in this review does not provide more detailed 
quantitative information

[[Page 49891]]

regarding O3 concentrations at the national scale. For 
example, tropospheric O3 continues to be recognized as 
having a causal relationship with radiative forcing, although 
``uncertainty in the magnitude of radiative forcing estimated to be 
attributed to tropospheric ozone is a contributor to the relatively 
greater uncertainty associated with climate effects of tropospheric 
ozone compared to such effects of the well mixed greenhouse gases 
(e.g., carbon dioxide and methane)'' (ISA, section IS.6.2.2).
    While tropospheric O3 also continues to be recognized as 
having a likely causal relationship with subsequent effects on 
temperature, precipitation and related climate variables, the non-
uniform distribution of O3 within the troposphere (spatially 
and temporally) makes the development of quantitative relationships 
between the magnitude of such effects and differing O3 
concentrations in the U.S. challenging (ISA, Appendix 9). Additionally, 
``the heterogeneous distribution of ozone in the troposphere 
complicates the direct attribution of spatial patterns of temperature 
change to ozone induced [radiative forcing]'' and there are ``ozone 
climate feedbacks that further alter the relationship between ozone 
[radiative forcing] and temperature (and other climate variables) in 
complex ways'' (ISA, Appendix 9, section 9.3.1, p. 9-19). Thus, various 
uncertainties ``render the precise magnitude of the overall effect of 
tropospheric ozone on climate more uncertain than that of the well-
mixed GHGs'' and ``[c]urrent limitations in climate modeling tools, 
variation across models, and the need for more comprehensive 
observational data on these effects represent sources of uncertainty in 
quantifying the precise magnitude of climate responses to ozone 
changes, particularly at regional scales'' (ISA, section IS.6.2.2, 
Appendix 9, section 9.3.3, p. 9-22). For example, current limitations 
in modeling tools include ``uncertainties associated with simulating 
trends in upper tropospheric ozone concentrations'' (ISA, section 
9.3.1, p. 9-19), and uncertainties such as ``the magnitude of 
[radiative forcing] estimated to be attributed to tropospheric ozone'' 
(ISA, section 9.3.3, p. 9-22). Further, ``precisely quantifying the 
change in surface temperature (and other climate variables) due to 
tropospheric ozone changes requires complex climate simulations that 
include all relevant feedbacks and interactions'' (ISA, section 9.3.3, 
p. 9-22). For example, an important limitation in current climate 
modeling capabilities for O3 is representation of important 
urban- or regional-scale physical and chemical processes, such as 
O3 enhancement in high-temperature urban situations or 
O3 chemistry in city centers where NOx is abundant. Such 
limitations impede our ability to quantify the impact of incremental 
changes in O3 concentrations in the U.S. on radiative 
forcing and subsequent climate effects.
    With regard to tree mortality (the evidence for which the 2013 ISA 
did not assess with regard to its support for inference of a causal 
relationship with O3 exposure), the evidence available in 
the last several reviews included field studies of pollution gradients 
that concluded O3 damage to be an important contributor to 
tree mortality although several confounding factors such as drought, 
insect outbreak and forest management were identified as potential 
contributors (2013 ISA, section 9.4.7.1). Although three newly 
available studies contribute to the ISA conclusion of sufficient 
evidence to infer a likely causal relationship for O3 with 
tree mortality (ISA, Appendix 8, section 8.4), there is only limited 
experimental evidence that isolates the effect of O3 on tree 
mortality and might be informative regarding O3 
concentrations of interest in the review. This evidence, primarily from 
an Aspen FACE study of aspen survival, involves cumulative seasonal 
exposure to W126 index levels above 30 ppm-hrs during the first half of 
the 11-year study period (ISA, Appendix 8, Tables 8-8 and 8-9). 
Evidence is lacking regarding exposure conditions closer to those 
occurring under the current standard and any contribution to tree 
mortality.
    With regard to the two categories of welfare effects involving 
insects (for which there are new causal determinations in this review), 
there are multiple limitations and uncertainties regarding 
characterization of exposure conditions that might elicit effects and 
the comprehensive characterization of the effects (ISA, p. IS-91, 
Appendix 8, section 8.6.3). For example, with regard to alteration of 
herbivore growth and reproduction, although ``[t]here are multiple 
studies demonstrating ozone effects on fecundity and growth in insects 
that feed on ozone-exposed vegetation'', ``no consistent directionality 
of response is observed across studies and uncertainties remain in 
regard to different plant consumption methods across species and the 
exposure conditions associated with particular severities of effects '' 
(ISA, pp. ES-18). The ISA also notes the variation in study designs and 
endpoints used to assess O3 response (ISA, IS.6.2.1 and 
Appendix 8, section 8.6). Thus, while the evidence describes changes in 
nutrient content and leaf chemistry following O3 exposure 
(ISA, p. IS-73), the effect of these changes on herbivores consuming 
the leaves is not well characterized, and factors such as identified 
here preclude broader characterization, as well as quantitative 
analysis related to air quality conditions meeting the O3 
standard.
    The evidence for the second category, alteration of plant-insect 
signaling, draws on new research that has provided clear evidence of 
O3 modification of VPSCs and behavioral responses of insects 
to these modified chemical signals (ISA, section IS.6.2.1). The 
available evidence involves a relatively small number of plant species 
and plant-insect associations. While the evidence documents effects on 
plant production of signaling chemicals and on the atmospheric 
persistence of signaling chemicals, as well as on the behaviors of 
signal-responsive insects, it is limited with regard to 
characterization of mechanisms and the consequences of any modification 
of VPSCs by O3 (ISA, p. ES-18; sections ES.5.1.3 and 
IS.6.2.1). Further, the available studies vary with regard to the 
experimental exposure circumstances in which the different types of 
effects have been reported (most of the studies have been carried out 
in laboratory conditions rather than in natural environments), and many 
of the studies involve quite short controlled exposures (hours to days) 
to elevated concentrations, posing limitations for our purposes of 
considering the potential for impacts associated with the studied 
effects to be elicited by air quality conditions that meet the current 
standard (ISA, section IS.6.2.1 and Appendix 8, section 8.7).
    With regard to previously recognized categories of vegetation-
related effects, other than growth and visible foliar injury, such as 
reduced plant reproduction, reduced productivity in terrestrial 
ecosystems, alteration of terrestrial community composition and 
alteration of below-ground biogeochemical cycles, the newly available 
evidence includes a variety of studies, as identified in the ISA (ISA, 
Appendix 8, sections 8.4, 8.8 and 8.10). Across the studies, a variety 
of metrics (including AOT40, 4- to 12-hour mean concentrations, and 
others) are used to quantify exposure over varying durations and 
various countries. The ISA additionally describes publications that 
summarize previously published studies in several ways. For example, a 
meta-analysis of reproduction studies categorized the reported 
O3 exposures into bins of differing magnitude,

[[Page 49892]]

grouping differing concentration metrics and exposure durations 
together, and performed statistical analyses to reach conclusions 
regarding the presence of an O3-related effect (ISA, 
Appendix 8, section 8.4.1). While such studies continue to support 
conclusions of the ecological hazards of O3, they do not 
improve capabilities for characterizing the likelihood of such effects 
under varying patterns of environmental O3 concentrations 
that occur with air quality conditions that meet the current standard.
    As at the time of the last review, growth impacts, most 
specifically as evaluated by RBL for tree seedlings and RYL for crops, 
remain the type of vegetation-related effects for which we have the 
best understanding of exposure conditions likely to elicit them. Thus, 
as was the case in the decision for the last review, the quantitative 
analyses of exposures occurring under air quality that meets the 
current standard, summarized below, are focused primarily on the W126 
index, given its established relationship with growth effects.

C. Summary of Air Quality and Exposure Information

    The air quality and exposure analyses developed in this review, 
like those in the last review, are of two types: (1) W126-based 
cumulative exposure estimates in Class I areas; and (2) analyses of 
W126-based exposures and their relationship with the current standard 
for all U.S. monitoring locations (PA, Appendix 4D). As summarized in 
the IRP, we identified these analyses to be updated in this review in 
recognition of the relatively reduced uncertainty associated with the 
use of these types of analyses (compared to the national or regional-
scale modeling analyses performed in the last review) to inform a 
characterization of cumulative O3 exposure (in terms of the 
W126 index) associated with air quality just meeting the current 
standard (IRP, section 5.2.2). As in the last review, the lesser 
uncertainty of these air quality monitoring-based analyses contributes 
to their value in informing the current review. The sections below 
present findings of the updated analyses that have been performed in 
the current review using recently available information.
    As in the last review, the analyses focus on both the most recent 
3-year period (2016 to 2018) for which data were available when the 
analyses were performed, and also across the full historical period 
back to 2000, which is now expanded from that available in the last 
review.\165\ Design values (3-year average annual fourth-highest 8-hour 
daily maximum concentration, also termed ``4th max metric'' in this 
analysis) and W126 index values (in terms of the 3-year average) were 
calculated at each site where sufficient data were available.\166\ 
Across the seventeen 3-year periods from 2000-2002 to 2016-2018, the 
number of monitoring sites with sufficient data for calculation of 
valid design values and W126 index values (across the 3-year design 
value period) ranged from a low of 992 in 2000-2002 to a high of 1119 
in 2015-2017. The specific monitoring sites differed somewhat across 
the 19 years. There were 1,557 sites with sufficient data for 
calculation of valid design values and W126 index values for at least 
one 3-year period between 2000 and 2018, and 543 sites had such data 
for all seventeen 3-year periods. Analyses in the current review are 
based on the expanded set of air monitoring data now available \167\ 
(PA, Appendix 4D, section 4D.2.2).
---------------------------------------------------------------------------

    \165\ In the last review, the dataset analyzed included data 
from 2000 through 2013, with the most recent period being 2011 to 
2013 (Wells, 2015).
    \166\ Data adequacy requirements and methods for these 
calculations are described in Appendix 4D, section 4D.2 of the PA.
    \167\ In addition to being expanded with regard to data for more 
recent time periods than were available during the last review, the 
current dataset also includes a small amount of newly available 
older data for some rural monitoring sites that are now available in 
the AQS.
---------------------------------------------------------------------------

    These analyses are based primarily on the hourly air monitoring 
data that were reported to EPA from O3 monitoring sites 
nationwide. In the recent and historical datasets, the O3 
monitors (more than 1000 in the most recent period) are distributed 
across the U.S., covering all nine NOAA climate regions and all 50 
states (PA, Figure 4-6 and Appendix 4D, Table 4D-1). Some geographical 
areas within these regions and states are more densely covered and well 
represented by monitoring sites, while others may have sparse or no 
data. Given that there has been a longstanding emphasis on urban areas 
in the EPA's monitoring regulations, urban areas are generally well 
represented in the U.S. dataset, with the effect being that the current 
dataset is more representative of locations where people live than of 
complete spatial coverage for all areas in the U.S., (i.e., the current 
dataset is more population weighted than geographically weighted). As 
O3 precursor sources are also generally more associated with 
urban areas, one impact of this may be a greater representation of 
relatively higher concentration sites (PA, section 4.4.3 and Appendix 
4D, section 4D.4).
    With regard to Class I areas, of the 158 mandated federal Class I 
areas, 65 (just over 40%) have or have had O3 monitors 
within 15 km with valid design values, thus allowing inclusion in the 
Class I area analysis. Even so, the Class I areas dataset includes 
monitoring sites in 27 states distributed across all nine NOAA climatic 
regions across the contiguous U.S, as well as Hawaii and Alaska. Some 
NOAA regions have far fewer numbers of Class I areas with monitors than 
others. For instance, the Central, Northeast, East North Central, and 
South regions all have three or fewer Class I areas in the dataset. 
However, these areas also have appreciably fewer Class I areas in 
general when compared to the Southwest, Southeast, West, and West North 
Central regions, which are more well represented in the dataset. The 
West and Southwest regions are identified as having the largest number 
of Class I areas, and they have approximately one third of those areas 
represented with monitors, which include locations where W126 index 
values are generally higher, thus playing a prominent role in the 
analysis (PA, section 4.4.3 and Appendix 4D, section 4D.4).
    These updated air quality analyses, and what they indicate 
regarding environmental exposures of interest in this review, are 
summarized in the following two subsections which differ in their areas 
of focus. The first subsection (section III.C.1) summarizes information 
regarding relationships between air quality in terms of the form and 
averaging time of the current standard and environmental exposures in 
terms of the W126 index. The second subsection (section III.C.2) 
summarizes findings of the analyses of the currently available 
monitoring data with regard to the magnitude of environmental 
exposures, in terms of the W126 index, in areas across the U.S., and 
particularly in Class I areas, during periods in which air quality met 
the current standard.
1. Influence of Form and Averaging Time of Current Standard on 
Environmental Exposure
    In revising the standard in 2015 to the now-current standard, the 
Administrator concluded that, with revision of the standard level, the 
existing form and averaging time provided the control of cumulative 
seasonal exposure circumstances needed for the public welfare 
protection desired (80 FR 65408, October 26, 2015). The focus on 
cumulative seasonal exposure as the type of exposure metric of interest 
primarily reflects the

[[Page 49893]]

evidence on E-R relationships for plant growth (summarized in section 
III.B.3 above). The 2015 conclusion was based on the air quality data 
analyzed at that time (80 FR 65408, October 26, 2015). Analyses in the 
current review of the now expanded set of air monitoring data, which 
now span 19 years and 17 3-year periods, document similar findings as 
from the analysis of data from 2000-2013 described in the last review 
(PA, Appendix 4D, section 4D.2.2).
    Among the analyses performed is an evaluation of the variability in 
the annual W126 index values across a 3-year period (PA, Appendix 4D, 
section 4D.3.1.2). This evaluation was performed for all U.S. 
monitoring sites with sufficient data available in the most recent 3-
year period, 2016 to 2018. This analysis indicates the extent to which 
the three single-year W126 index values within a 3-year period deviate 
from the average for the period. Across the full set of sites, 
regardless of W126 index magnitude (or whether or not the current 
standard is met), single-year W126 index values differ less than 15 
ppm-hrs from the average for the 3-year period (PA, Appendix 4D, Figure 
4D-6). Focusing on the approximately 850 sites meeting the current 
standard (i.e., sites with a design value at or below 70 ppb), over 99% 
of single-year W126 index values in this subset differ from the 3-year 
average by no more than 5 ppm-hrs, and 87% by no more than 2 ppm-hrs 
(PA, Appendix 4D, Figure 4D-7).
    Another air quality analysis performed for the current review 
documents the positive nonlinear relationship that is observed between 
cumulative seasonal exposure, quantified using the W126 index, and 
design values, based on the form and averaging time of the current 
standard. This relationship is shown for both the average W126 index 
across the 3-year design value period and for W126 index values for 
individual years within the period (PA, Figure 4-7). From this 
presentation, it is clear that cumulative seasonal exposures, assessed 
in terms of W126 index (in a year or averaged across years), are lower 
at monitoring sites with lower design values. This is seen both for 
design values above the level of the current standard (70 ppb), where 
the slope is steeper (due to the sigmoidal weighting of higher 
concentrations by the W126 index function), as well as for lower design 
values that meet the current standard (PA, Figure 4-7). This 
presentation also indicates some regional differences in the 
relationship. For example, for the 2016-2018 period, at sites meeting 
the current standard in the regions outside of the West and Southwest 
regions, all 3-year average W126 index values are at or below 12 ppm-
hrs and all single-year values are at or below 16 ppm-hrs (PA, Figures 
4-6 and 4-7). The W126 index values are generally higher in the West 
and Southwest regions. However, the positive relationship between the 
W126 index and the design value is evident in all nine regions (PA, 
Figure 4-7).
    An additional analysis assesses the relationship between long-term 
changes in design value and long-term changes in the W126 index. This 
analysis is presented in detail in the PA and focuses on the 
relationship between changes (at each monitoring site) in the 3-year 
design value across the 16 design value periods from 2000-2002 to 2016-
2018 and changes in the W126 index over the same period (PA, Appendix 
4D, section 4D.3.2.3).\168\ This analysis, performed using either the 
3-year average W126 index or values for individual years, shows there 
to be a positive, linear relationship between the changes in the W126 
index and the changes in the design value at monitoring sites across 
the U.S. (PA, Appendix 4D, Figure 4D-11). The existence of this 
relationship means that a change in the design value at a monitoring 
site was generally accompanied by a similar change in the W126 index. 
Nationally, the W126 index (in terms of 3-year average) decreased by 
approximately 0.62 ppm-hrs per ppb decrease in design value over the 
full period from 2000 to 2018 (PA, Appendix 4D, Table 4D-12). This 
relationship varies across the NOAA climate regions, with the greatest 
change in the W126 index per unit change in design value observed in 
the Southwest and West regions. Thus, the regions which had the highest 
W126 index values at sites meeting the current standard (PA, Figure 4D-
6) also showed the greatest improvement in the W126 index per unit 
decrease in their design values over the past 19 years (PA, Appendix 
4D, Table 4D-12 and Figure 4D-14).
---------------------------------------------------------------------------

    \168\ At each site, the trend in values of a metric (W126 or 
design value), in terms of a per-year change in metric value, is 
calculated using the Theil-Sen estimator, a type of linear 
regression method that chooses the median slope among all lines 
through pairs of sample points. For example, if applying this method 
to a dataset with metric values for four consecutive years (e.g., 
W1261, W1262, W1263, 
W1264), the trend would be the median of the different 
per-year changes observed in the six possible pairs of values 
([W1264-W1263]/1, [W1263-
W1262]/1, [W1262-W1261]/1, 
[W1264-W1262]/2, [W1263-
W1261]/2, [W1264-W1261]/3).
---------------------------------------------------------------------------

    The trends analyses indicate that going forward as design values 
are reduced in areas that are presently not meeting the current 
standard, the W126 index in those areas would also be expected to 
decline (PA, Appendix 4D, section 4D.3.2.3 and 4D.5). The overall trend 
showing reductions in the W126 index concurrent with reductions in the 
design value metric for the current standard is positive whether the 
W126 index is expressed in terms of the average across the 3-year 
design value period or the annual value (PA, Appendix 4D, section 
4D.3.2.3). This similarity is consistent with the strong positive 
relationship that exists between the W126 index and the design value 
metric for the current standard summarized above.
    With regard to the control of the current form and averaging time 
on vegetation exposures of potential concern, the PA also describes air 
quality information pertinent to the evidence discussed in section 
III.B.3 above regarding the potential for days with particularly high 
O3 concentrations to play a contributing role in visible 
foliar injury. In so doing, the PA notes that the current standard's 
form and averaging time, by their very definition, limit occurrences of 
such concentrations. For example, the peak 8-hour average 
concentrations are lower at sites with lower design values, as 
illustrated by the declining trends in annual fourth highest MDA8 
concentrations that accompany the declining trend in design values (PA, 
Figure 2-11). Additionally, the frequency of elevated 1-hour 
concentrations, including concentrations at or above 100 ppb, decrease 
with decreasing design values (PA, Appendix 2A, section 2A.2). For 
example, in the most recent design value period (2016-2018) across all 
sites with adequate data to derive design values, the mean number of 
daily maximum 1-hour observations per site at or above 100 ppb was well 
below one (0.19) for sites that meet the current standard, compared to 
well above one (8.09) for sites not meeting the current (PA, Appendix 
2A, Table 2A-2).
    In summary, monitoring sites with lower O3 
concentrations as measured by the design value metric (based on the 
current form and averaging time of the secondary standard) have lower 
cumulative seasonal exposures, as quantified by the W126 index, as well 
as lower short-term peak concentrations. As the form and averaging time 
of the secondary standard have not changed since 1997, the analyses 
performed have been able to assess the amount of control exerted by 
these aspects of the standard, in combination with reductions in the 
standard level (i.e., from 0.08 ppm in 1997 to 0.075 ppm in 2008 to 
0.070 ppm in 2015) on

[[Page 49894]]

cumulative seasonal exposures in terms of W126 index (and on the 
magnitude of short-term peak concentrations). The analyses have found 
that the long-term reductions in the design values, presumably 
associated with implementation of the revised standards, have been 
accompanied by reductions in cumulative seasonal exposures in terms of 
W126 index, as well as reductions in short-term peak concentrations.
2. Environmental Exposures in Terms of W126 Index
    The following presentation is framed by the question: What are the 
nature and magnitude of vegetation exposures associated with conditions 
meeting the current standard at sites across the U.S., particularly in 
specially protected areas, such as Class I areas, and what do they 
indicate regarding the potential for O3-related vegetation 
impacts? Given the evidence indicating the W126 index to be strongly 
related to growth effects and its use in the E-R functions for tree 
seedling RBL (as summarized in section III.B above), exposure is 
quantified using the W126 metric. The potential for impacts of interest 
is assessed through considering the magnitude of estimated exposure, in 
light of current information and, in comparison to levels given 
particular focus in the 2015 decision on the current standard (80 FR 
65292; October 26, 2015). The updated analyses summarized here, while 
including assessment of all monitoring sites nationally, include a 
particular focus on monitoring sites in or near Class I areas \169\, in 
light of the greater public welfare significance of many O3 
related impacts in such areas, as described in section III.B.2 above.
---------------------------------------------------------------------------

    \169\ This includes monitors sited within Class I areas or the 
closest monitoring site within 15 km of the area boundary.
---------------------------------------------------------------------------

    The analyses summarized here consider both recent air quality 
(2016-2018) and air quality since 2000 (PA, Appendix 4D). These air 
quality analyses of cumulative seasonal exposures associated with 
conditions meeting the current standard nationally provide conclusions 
generally similar to those based on the data available at the time of 
the last review when the current standard was set, when the most recent 
data were available for 2011 to 2013 (Wells, 2015). Such conclusions 
are with regard to regional differences as well as the rarity of W126 
index values at or above 19 ppm-hrs in areas with air quality meeting 
the current standard.\170\
---------------------------------------------------------------------------

    \170\ Rounding conventions are described in detail in the PA, 
Appendix 4D, section 4D.2.2.
---------------------------------------------------------------------------

    Cumulative exposures vary across the U.S. with the highest W126 
index values for sites that met the current standard being located 
exclusively in Southwest and West climate regions (PA, Figure 4-6). At 
sites meeting the current standard in all other NOAA climate regions, 
W126 index values, averaged over the 3-year design value period are at 
or below 13 ppm-hrs (PA, Figure 4-6 and Appendix 4D, Figure 4D-2). At 
Southwest and West region sites that met the current standard, W126 
index values, averaged across the 3-year design value period, are at or 
below 17 ppm-hrs in virtually all cases in the most recent 3-year 
period and across all of the seventeen 3-year periods in the full 
dataset evaluated (i.e., all but one site out of 147 for recent period 
and all but eight out of over 1,800 cases across full dataset). Across 
all U.S. sites with valid design values at or below 70 ppb in the full 
2000 to 2018 dataset, the W126 index, averaged over three years, was at 
or below 17 ppm-hrs on 99.9% of all occasions, and at or below 13 ppm-
hrs on 97% of all occasions. All but one of the eight occasions when 
the 3-year W126 index was above 17 ppm-hrs (including the highest 
occasion at 19 ppm-hrs) occurred in the Southwest region during a 
period before 2011. The most recent occasion occurred in 2018 at a site 
in the West region when the 3-year average W126 index value was 18 ppm-
hrs (PA, Appendix 4D, section 4D.3.2).
    In summary, among sites meeting the current standard in the most 
recent period of 2016 to 2018, there are none with a W126 index, based 
on the 3-year average, above 19 ppm-hrs, and just one with such a value 
above 17 ppm-hrs (Table 5). Additionally, the full historical dataset 
includes no occurrences of a 3-year average W126 index above 19 ppm-hrs 
for sites meeting the current standard, and just eight occurrences of a 
W126 index above 17 ppm-hrs, with the highest such occurrence just 
equaling 19 ppm-hrs (Table 5; PA, Appendix 4D, section 4D.3.2.1).
    With regard to Class I areas, the updated air quality analyses 
include data at sites in or near 65 Class I areas. The findings for 
these sites, which are distributed across all nine NOAA climate regions 
in the contiguous U.S., as well as Alaska and Hawaii, mirror the 
findings for the analysis of all U.S. sites. Among the Class I area 
sites meeting the current standard (i.e., having a design value at or 
below 70 ppb) in the most recent period of 2016 to 2018, there are none 
with a W126 index (as average over design value period) above 17 ppm-
hrs (Table 5). The historical dataset includes just seven occurrences 
(all dating from the 2000-2010 period) of a Class I area site meeting 
the current standard and having a 3-year average W126 index above 17 
ppm-hrs, and no such occurrences above 19 ppm-hrs (Table 5).
    The W126 exposures at sites with design values above 70 ppb range 
up to approximately 60 ppm-hrs (Table 5). Among all sites across the 
U.S. that do not meet the current standard in the 2016 to 2018 period, 
more than a quarter have average W126 index values above 19 ppm-hrs and 
a third exceed 17 ppm-hrs (Table 5). A similar situation exists for 
Class I area sites (Table 5). Thus, as was the case in the last review, 
the currently available quantitative information continues to indicate 
appreciable control of seasonal W126 index-based cumulative exposure at 
all sites with air quality meeting the current standard.

[[Page 49895]]

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

    As summarized above, the information available in this review 
continues to indicate that average cumulative seasonal exposure levels 
at virtually all sites and 3-year periods with air quality meeting the 
current standard fall at or below the level of 17 ppm-hrs that was 
identified when the current standard was established (80 FR 65393; 
October 26, 2015). Additionally, the full dataset indicates that at 
sites meeting the current standard, annual W126 index values were less 
than or equal to 19 ppm-hrs well over 99% of the time (PA, Appendix 4D, 
section 4D.3.2.1). Additionally, the average W126 index in Class I 
areas that meet the current standard for the most recent 3-year period 
is below 17 ppm-hrs at all areas which have a monitor within or near 
their borders (PA, Appendix 4D, Table 4D-16). Further, with the 
exception of seven values that occurred prior to 2011, cumulative 
seasonal exposures, in terms of average 3-year W126, in all Class I 
areas during periods that met the current standard were no higher than 
17 ppm-hrs. This contrasts with the occurrence of much higher W126 
index values at sites when the current standard was not met. For 
example, out of the 11 Class I area sites with design values above 70 
ppb during the most recent period, eight sites had a 3-year average 
W126 index above 19 ppm-hrs (ranging up to 47 ppm-hrs) and for nine, it 
was above 17 ppm-hrs (Table 5; PA, Appendix 4D, Table 4D-17).

D. Proposed Conclusions on the Secondary Standard

    In reaching proposed conclusions on the current secondary 
O3 standard (presented in section III.D.3), the 
Administrator has taken into account policy-relevant evidence-based and 
air quality-, exposure- and risk-based considerations discussed in the 
PA (summarized in section III.D.1), as well as advice from the CASAC, 
and public comment on the standard received thus far in the review 
(section III.D.2). In general, the role of the PA is to help ``bridge 
the gap'' between the Agency's assessment of the current evidence and 
quantitative analyses (of air quality, exposure and risk), and the 
judgments required of the Administrator in determining whether it is 
appropriate to retain or revise the NAAQS. Evidence-based 
considerations draw upon the EPA's integrated assessment of the 
scientific evidence of welfare effects related to O3 
exposure presented in the ISA (summarized in section III.B above) to 
address key policy-relevant questions in the review. Similarly, the air 
quality-, exposure- and risk-based considerations draw upon our 
assessment of air quality, exposure and associated risk (summarized in 
section III.C above) in addressing policy-relevant questions focused on 
the potential for O3 exposures associated with welfare 
effects under air quality conditions meeting the current standard.
    This approach to reviewing the secondary standard is consistent 
with requirements of the provisions of the CAA related to the review of 
the NAAQS and with how the EPA and the courts have historically 
interpreted the CAA. As discussed in section I.A above, these 
provisions require the Administrator to establish secondary standards 
that, in the Administrator's judgment, are requisite (i.e., neither 
more nor less stringent than necessary) to protect the public welfare 
from known or anticipated adverse effects associated with the presence 
of the pollutant in the ambient air. Consistent with the Agency's 
approach across all NAAQS reviews, the EPA's approach to informing 
these judgments is based on a recognition that the available welfare 
effects evidence generally reflects a continuum that includes ambient 
air exposures for which scientists generally agree that effects are 
likely to occur through lower levels at which the likelihood and 
magnitude of response become increasingly uncertain. The CAA does not 
require the Administrator to establish a secondary standard at a zero-
risk level, but rather at a level that reduces risk sufficiently so as 
to protect the public welfare from known or anticipated adverse 
effects.
    The proposed decision on the adequacy of the current secondary 
standard described below is a public welfare policy judgment by the 
Administrator that draws upon the scientific evidence for welfare 
effects, quantitative analyses of air quality, exposure and risks, as 
available, and judgments about how to consider the uncertainties and 
limitations that are inherent in the scientific evidence and 
quantitative analyses. This proposed decision has additionally 
considered the August 2019 remand of the secondary standard. The four 
basic elements of the NAAQS (i.e., indicator, averaging time, form, and 
level) have been considered collectively in evaluating the public 
welfare protection afforded by the current standard. The 
Administrator's final decision will additionally consider public 
comments received on this proposed decision.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy 
Assessment
    Based on its evaluation of the evidence and quantitative analyses 
of

[[Page 49896]]

air quality, exposure and potential risk, the PA for this review 
reaches the conclusion that consideration should be given to retaining 
the current secondary standard, without revision (PA, section 4.5.3). 
Accordingly, and in light of this conclusion that it is appropriate to 
consider the current secondary standard to be adequate, the PA did not 
identify any potential alternative secondary standards for 
consideration in this review (PA, section 4.5.3). The PA additionally 
recognized that, as is the case in NAAQS reviews in general, the extent 
to which the Administrator judges the current secondary O3 
standard to be adequate will depend on a variety of factors, including 
science policy judgments and public welfare policy judgments. These 
factors include public welfare policy judgments concerning the 
appropriate benchmarks on which to place weight, as well as judgments 
on the public welfare significance of the effects that have been 
observed at the exposures evaluated in the welfare effects evidence. 
The factors relevant to judging the adequacy of the standard also 
include the interpretation of, and decisions as to the weight to place 
on, different aspects of the quantitative analyses of air quality and 
cumulative O3 exposure and any associated uncertainties. 
Thus, the Administrator's conclusions regarding the adequacy of the 
current standard will depend in part on public welfare policy 
judgments, science policy judgments regarding aspects of the evidence 
and exposure/risk estimates, as well as judgments about the level of 
public welfare protection that is requisite under the Clean Air Act.
    The subsections below summarize key considerations and conclusions 
from the PA. The main focus of the policy-relevant considerations in 
the PA is the question: Does the currently available scientific 
evidence- and exposure/risk-based information support or call into 
question the adequacy of the protection afforded by the current 
secondary O3 standard? In addressing this overarching 
question, the PA focuses first on consideration of the evidence, as 
evaluated in the ISA (and supported by the prior ISA and AQCDs), 
including that newly available in this review, and the extent to which 
it alters the EPA's overall conclusions regarding welfare effects 
associated with photochemical oxidants, including O3, in 
ambient air. The PA also considers questions related to the general 
approach or framework in which to evaluate public welfare protection of 
the standard. Additionally, the PA considers the currently available 
quantitative information regarding environmental exposures likely to 
occur in areas of the U.S. where the standard is met, including 
associated limitations and uncertainties, and the significance of these 
exposures with regard to the potential for O3-related 
vegetation effects, their potential severity and any associated public 
welfare implications and judgments about the uncertainties inherent in 
the scientific evidence and quantitative analyses that are integral to 
consideration of whether the currently available information supports 
or calls into question the adequacy of the current secondary 
O3 standard.
a. Welfare Effects Evidence
    With regard to the support in the current evidence for 
O3 as the indicator for photochemical oxidants, no newly 
available evidence has been identified in this review regarding the 
importance of photochemical oxidants other than O3 with 
regard to abundance in ambient air, and potential for welfare 
effects.\171\ Data for photochemical oxidants other than O3 
are generally derived from a few special field studies; such that 
national-scale data for these other oxidants are scarce (ISA, Appendix 
1, section 1.1; 2013 ISA, sections 3.1 and 3.6). Moreover, few studies 
of the welfare effects of other photochemical oxidants beyond 
O3 have been identified by literature searches conducted for 
the 2013 ISA and prior AQCDs, such that ``the primary literature 
evaluating the . . . ecological effects of photochemical oxidants 
includes ozone almost exclusively as an indicator of photochemical 
oxidants'' (ISA, section IS.1.1, Appendix 1, section 1.1). Thus, as was 
the case for previous reviews, the PA finds that the evidence base for 
welfare effects of photochemical oxidants does not indicate an 
importance of any other photochemical oxidants such that O3 
continues to be appropriately considered for the secondary standard's 
indicator.
---------------------------------------------------------------------------

    \171\ Close agreement between past ozone measurements and the 
photochemical oxidant measurements upon which the early NAAQS (for 
photochemical oxidants including O3) was based indicated 
the very minor contribution of other oxidant species in comparison 
to O3 (U.S. DHEW, 1970).
---------------------------------------------------------------------------

(i) Nature of Effects
    Across the full array of welfare effects, summarized in section 
III.B.1 above, the evidence newly available in this review strengthens 
previous conclusions, provides further mechanistic insights and 
augments current understanding of varying effects of O3 
among species, communities and ecosystems (ISA, sections IS.1.3.2, IS.5 
and IS.6.2, and Appendices 8 and 9). The current evidence, including 
the wealth of long-standing evidence, continues to support conclusions 
of causal relationships between O3 and visible foliar 
injury, reduced yield and quality of agricultural crops, reduced 
vegetation growth and plant reproduction, reduced productivity in 
terrestrial ecosystems, and alteration of belowground biogeochemical 
cycles. The current evidence additionally continues to support 
conclusions of likely causal relationships between O3 and 
reduced carbon sequestration in terrestrial systems, and alteration of 
terrestrial ecosystem water cycling (ISA, section IS.I.3.2). Also as in 
the last review, the current ISA determines there to be a causal 
relationship between tropospheric O3 and radiative forcing 
and a likely causal relationship between tropospheric O3 and 
temperature, precipitation and related climate variables (ISA, section 
IS.1.3.3). The current evidence has led to an updated conclusion on the 
relationship of O3 with alteration of terrestrial community 
composition to causal (ISA, sections IS.I.3.2). Lastly, the current ISA 
concludes the current evidence sufficient to infer likely causal 
relationships of O3 with three additional categories of 
effects (ISA, sections IS.I.3.2). For example, while previous 
recognition of O3 as a contributor to tree mortality in a 
number of field studies was a factor in the 2013 conclusion of a likely 
causal relationship between O3 and alterations in community 
composition, tree mortality has been separately assessed in this 
review. Additionally, newly available evidence on two additional plant 
related effects augments more limited previously available evidence 
related to insect interactions with vegetation, contributing to 
additional conclusions that the body of evidence is sufficient to infer 
likely causal relationships between O3 and alterations of 
plant-insect signaling and insect herbivore growth and reproduction 
(ISA, Appendix 8, sections 8.6 and 8.7).\172\
---------------------------------------------------------------------------

    \172\ As in the last review, the ISA again concludes that the 
evidence is inadequate to determine if a causal relationship exists 
between changes in tropospheric ozone concentrations and UV-B 
effects (ISA, Appendix 9, section 9.1.3.4; 2013 ISA, section 
10.5.2).
---------------------------------------------------------------------------

    As in the last review, the strongest evidence and the associated 
findings of causal or likely causal relationships with O3 in 
ambient air, and quantitative characterizations of relationships 
between O3 exposure and occurrence and magnitude of effects 
are for vegetation-related effects. With regard to uncertainties and 
limitations associated with the current welfare effects

[[Page 49897]]

evidence, the PA recognized that the type of uncertainties for each 
category of effects tends to vary, generally in relation to the 
maturity of the associated evidence base, from those associated with 
overarching characterizations of the effects to those associated with 
quantification of the cause and effect relationships. For example, 
given the longstanding nature of the evidence for many of the 
vegetation effects identified in the ISA as causally or likely causally 
related to O3 in ambient air, the key uncertainties and 
limitations in our understanding of these effects relate largely to the 
implications or specific aspects of the evidence, as well as to current 
understanding of the quantitative relationships between O3 
concentrations in the environment and the occurrence and severity (or 
relative magnitude) of such effects or understanding of key influences 
on these relationships. For more newly identified categories of 
effects, the evidence may be less extensive, and accordingly, the areas 
of uncertainty greater, thus precluding consideration of quantitative 
details related to risk of such effects under varying air quality 
conditions that would inform review of the current standard.
    The evidence bases for the three newly identified categories 
provide examples of such gaps in relevant information. For example, the 
evidence for increased tree mortality includes previously available 
studies with field observations from locations and periods of 
O3 concentrations higher than are common today and three 
more recently available publications assessing O3 exposures 
not expected under conditions meeting the current standard, as 
summarized in section III.B.1 above. The information available 
regarding the newly identified categories of plant-insect signaling and 
insect herbivore growth and reproduction additionally does not provide 
for a clear understanding of the specific environmental effects that 
may occur in the natural environment under specific exposure 
conditions, as summarized in sections III.B.1 and III.B.3 above (PA, 
section 4.5.1.1). Accordingly, the PA does not find the current 
evidence for these newly identified categories to call into question 
the adequacy of the current standard.
    With regard to tropospheric O3 as a greenhouse gas at 
the global scale, and associated effects on climate, the PA notes that 
while additional characterizations of tropospheric O3 and 
climate have been completed since the last review, uncertainties and 
limitations in the evidence that were also recognized in the last 
review remain (PA, section 4.5.1.1). As summarized in section III.B.3 
above, there is appreciable uncertainty associated with understanding 
quantitative relationships involving regional O3 
concentrations near the earth's surface and climate effects of 
tropospheric O3 on a global scale. Further, there are 
limitations in our modeling tools and associated uncertainties in 
interpretations related to capabilities for quantitatively estimating 
effects of regional-scale lower tropospheric O3 
concentrations on climate. These uncertainties and limitations affect 
our ability to make a quantitative characterization of the potential 
magnitude of climate response to changes in O3 
concentrations in ambient air, particularly at regional (vs global) 
scales, and thus our ability to assess the impact of changes in ambient 
air O3 concentrations in regions of the U.S. on global 
radiative forcing or temperature, precipitation and related climate 
variables. Consequently, the PA finds that current evidence in this 
area is not informative to consideration of the adequacy of public 
welfare protection of the current standard (PA, section 4.5.1.1).
(ii) E-R Information
    The category of O3 welfare effects for which current 
understanding of quantitative relationships is strongest continues to 
be reduced plant growth. While the ISA describes studies of welfare 
effects associated with O3 exposures newly identified since 
the last review, the established E-R functions for tree seedling growth 
and crop yield that have been available in the last several reviews 
continue to be the most robust descriptions of E-R relationships for 
welfare effects. These well-established E-R functions for seedling 
growth reduction in 11 tree species and yield loss in 10 crop species 
are based on response information across multiple levels of cumulative 
seasonal exposure (estimated from extensive records of hourly 
O3 concentrations across the exposure periods). Studies of 
some of the same species, conducted since the derivation of these 
functions, provide supporting information (ISA, Appendix 8, section 
8.13.2; 2013 ISA, sections 9.6.3.1 and 9.6.3.2). The E-R functions 
provide for estimation of the growth-related effect, RBL, for a range 
of cumulative seasonal exposures.
    The evidence newly available in this review does not include 
studies that assessed reductions in tree growth or crop yield responses 
across multiple O3 exposures and for which sufficient data 
are available for analyses of the shape of the E-R relationship across 
a range of cumulative exposure levels (e.g., in terms of W126 index) 
relevant to conditions associated with the current standard. While 
there are several newly available studies that summarize previously 
available studies or draw from them, such as for linear regression 
analyses, these do not provide robust E-R functions or cumulative 
seasonal exposure levels associated with important vegetation effects, 
such as reduced growth, that define the associated exposure 
circumstances in a consistent manner (as summarized in section III.B.3 
above).\173\ This limits their usefulness for considering the potential 
for occurrence of welfare effects in air quality conditions that meet 
the current standard. Thus, the PA concludes that robust E-R functions 
are not available for growth or yield effects on any additional tree 
species or crops in this review.
---------------------------------------------------------------------------

    \173\ For example, among the newly available publications cited 
in the ISA is a study that compiles EC10 values 
(estimated concentration at which 10% lower biomass [compared to 
zero O3] is predicted) derived for trees and grassland 
species (including 17 native to the U.S. [ISA, Table 8-26]) using 
linear regression of previously published data on plant growth 
response and O3 concentration quantified as AOT40. The 
data were from studies of various experimental designs, that 
involved various durations ranging up from 21 days, and involving 
various concentrations no higher than 100 ppb as a daily maximum 
hourly concentration. More detailed analyses of exposure and 
response information across a relevant range of seasonal exposure 
levels (e.g., accompanied by detailed records of O3 
concentrations) that would support derivation of robust E-R 
functions for purposes discussed here are not available.
---------------------------------------------------------------------------

    In considering the E-R functions and their use in informing 
judgments regarding such effects in areas with air quality of interest, 
the PA additionally recognized a number of limitations, and associated 
uncertainties, that remain in the current evidence base, and that 
affect characterization of the magnitude of cumulative exposure 
conditions eliciting growth reductions in U.S. forests (PA, section 
4.3.4). For example, there are uncertainties in the extent to which the 
11 tree species for which there are established E-R functions encompass 
the range of O3 sensitive species in the U.S., and also the 
extent to which they represent U.S. vegetation as a whole. These 11 
species include both deciduous and coniferous trees with a wide range 
of sensitivities and species native to every NOAA climate region across 
the U.S. and in most cases are resident across multiple states and 
regions. Thus, they may provide a range that encompasses species 
without E-R

[[Page 49898]]

functions.\174\ The PA additionally recognizes important uncertainties 
in the extent to which the E-R functions for reduced growth in tree 
seedlings are also descriptive of such relationships during later 
lifestages, for which there is a paucity of established E-R 
relationships. Although such information is limited with regard to 
mature trees, analyses in the 2013 ISA indicated that reported growth 
response of young aspen over six years was similar to the reported 
growth response of seedlings (ISA, Appendix 8, section 8.13.2; 2013 
ISA, section 9.6.3.2). Additionally, there are uncertainties with 
regard to the extent to which various factors in natural environments 
can either mitigate or exacerbate predicted O3-plant 
interactions and contribute variability in vegetation-related effects, 
including reduced growth. Such factors include multiple genetically 
influenced determinants of O3 sensitivity, changing 
sensitivity to O3 across vegetative growth stages, co-
occurring stressors and/or modifying environmental factors (PA, section 
4.3.4).
---------------------------------------------------------------------------

    \174\ This was the view of the CASAC in the 2015 review (Frey, 
2014b, p. 11).
---------------------------------------------------------------------------

    The PA additionally considered the quantitative information for 
other long-recognized effects of O3 (PA, section 4.3.4). For 
example, with regard to crop yield effects, as at the time of the last 
review, the PA recognized the potential for greater uncertainty in 
estimating the impacts of O3 exposure on agricultural crop 
production than that associated with O3 impacts on 
vegetation in natural forests. This relates to uncertainty in the 
extent to which agricultural management methods influence potential for 
O3-related effects and accordingly, the applicability of the 
established E-R functions for RYL in current agricultural areas (PA, 
section 4.3.4).
    With regard to visible foliar injury, the PA finds that, as in the 
last review, there remains a lack of established E-R functions that 
would quantitatively describe relationships between the occurrence and 
severity of visible foliar injury and O3 exposure, as well 
as factors influential in those relationships, such as soil moisture 
conditions (PA, section 4.5.1.1). While the currently available 
information continues to include studies that document foliar injury in 
sensitive plant species in response to specific O3 
exposures, investigations of a quantitative relationship between 
environmental O3 exposures and visible foliar injury 
occurrence/severity have not yielded a predictive result. In addition 
to experimental studies, the evidence includes multiple studies that 
have analyzed data collected as part of the USFS biosite biomonitoring 
program (e.g., Smith, 2012). These analyses continue to indicate the 
limitations in capabilities for predicting the exposure circumstances 
under which visible foliar injury would be expected to occur, as well 
as the circumstances contributing to increased injury severity. As 
noted in section III.B.3.b above, expanded summaries of the dataset 
compiled in the 2015 review from several years of USFS biosite records 
also does not clearly and consistently describe a relationship between 
incidence of foliar injury or severity (based on individual site 
scores) and W126 index estimates across the range of exposures. 
Overall, however, the dataset indicates that the proportion of records 
having different levels of severity score is generally highest in the 
records at sites with the highest W126 index (e.g., greater than 25 
ppm-hrs for the normal and dry soil moisture categories). This analysis 
does not provide for identification of air quality conditions, in terms 
of O3 concentrations associated with the relatively lower 
environmental exposures most common in the USFS dataset that would 
correspond to a specific magnitude of injury incidence or severity 
scores across locations.
    As discussed in section III.B.3 above, a number of analyses of the 
USFS biosite data (as well as several experimental studies), while 
often using cumulative exposure metrics to quantify O3 
exposures have additionally reported there to be a role for a metric 
that quantifies the incidence of ``high'' O3 days (2013 ISA, 
p. 9-10; Smith, 2012; Wang et al., 2012). Such analyses have not, 
however, established specific air quality metrics and associated 
quantitative functions for describing the influence of ambient air 
O3 on incidence and severity of visible foliar injury. As a 
result, the PA concludes that limitations recognized in the last review 
remain in our ability to quantitatively estimate incidence and severity 
of visible foliar injury likely to occur in areas across the U.S. under 
different air quality conditions over a year, or over a multi-year 
period.
    In looking across the full array of O3 welfare effects, 
the PA recognizes that the E-R functions for growth-related effects 
that were available in the last review continue to be the most robust 
E-R information available. The currently available evidence for growth-
related effects, including that newly available in this review, does 
not indicate the occurrence of growth-related responses attributable to 
cumulative O3 exposures lower than was established at the 
time of the last review. With regard to visible foliar injury, the 
available information that would support estimates of occurrence and 
severity across a range of air quality conditions continues to be 
limited, affecting the nature of conclusions that may be reached 
related to potential occurrence and/or severity for conditions. The 
quantitative information for other effects is more limited, as 
recognized earlier in this section and in section III.B.3 above. Thus, 
the PA concludes that the newly available evidence does not appreciably 
address key limitations or uncertainties as would be needed to expand 
capabilities for estimating welfare impacts that might be expected as a 
result of differing patterns of O3 concentrations in the 
U.S.
(iii) W126 Index as Exposure Metric
    With regard to exposure metric the currently available evidence 
continues to support a cumulative, seasonal exposure index as a 
biologically relevant and appropriate metric for assessment of the 
evidence of exposure/risk information for vegetation, most particularly 
for growth-related effects. The most commonly used such metrics are the 
SUM06, AOT40 (or AOT60) and W126 indices (ISA, section IS.3.2).\175\ 
The evidence for growth-related effects continues to support important 
roles for cumulative exposure and for weighting higher concentrations 
over lower concentrations. Thus, among the various such indices 
considered in the literature, the cumulative, concentration-weighted 
metric, defined by the W126 function, continues to be best supported 
for purposes of relating O3 air quality to growth-related 
effects. Accordingly, the PA continues to find the W126 index 
appropriate for consideration of the potential for vegetation-related 
effects to occur under air quality conditions (PA, section 4.5.1.1). 
The PA also recognizes, as recognized in the past, the lack of support 
for E-R functions for incidence and severity of visible foliar injury 
with W126 index as the descriptor of exposure, particularly in 
environmental settings where exposures are below a

[[Page 49899]]

W126 index of 25 ppm-hrs. While the PA analysis of the dataset of USFS 
biosite scores indicates appreciable increases in incidence and 
severity at and above 25 ppm-hrs, a pattern is unclear at lower W126 
index estimates across which the dataset does not support a predictive 
relationship. As summarized in section III.3.b above, while the overall 
evidence also indicates an important role for peak concentrations 
(e.g., N100) in influencing the occurrence and severity of visible 
foliar injury, the current evidence does not include an established 
predictive relationship based on such an additional metric (PA, section 
4.5.1.1).
---------------------------------------------------------------------------

    \175\ The evidence includes some studies reporting 
O3-reduced soybean yield and perennial plant biomass loss 
using AOT40 (as well as W126) as the exposure metric, however, no 
newly available analyses are available that compare AOT40 to W126 in 
terms of the strength of association with such responses. Nor are 
studies available that provide analyses of E-R relationships for AOT 
with reduced growth or RBL with such extensiveness as the analyses 
supporting the established E-R functions for W126 with RBL and RYL.
---------------------------------------------------------------------------

b. General Approach for Considering Public Welfare Protection
    This section summarizes PA consideration of the current evidence 
and air quality information with regard to key aspects of the general 
approach and risk management framework for making judgments and 
reaching conclusions regarding the adequacy of public welfare 
protection provided by the secondary standard that was applied in 2015 
(summarized in section III.A.1 above). Key aspects of the approach 
include the use of RBL as a proxy for the broad array of O3 
vegetation-related effects, E-R relationships for this endpoint with 
the W126 index, and the focus on this index averaged across a 3-year 
period.
(i) RBL as Proxy or Surrogate
    In the last review, the Administrator used RBL as a proxy or 
surrogate for an array of adverse welfare effects based on 
consideration of ecosystem services and potential for impacts to the 
public, as well as conceptual relationships between vegetation growth 
effects and ecosystem-scale effects. Such a use was supported by the 
CASAC at that time (80 FR 65406, October 26, 2015; Frey, 2014b, pp. 
iii, 9-10).\176\ In consideration of the broader evidence base and 
public welfare implications, including associated strengths, 
limitations and uncertainties, the Administrator focused on RBL, not 
simply in making judgments specific to a magnitude of growth effect in 
seedlings that would be acceptable or unacceptable in the natural 
environment, but as a surrogate or proxy for consideration of the 
broader array of vegetation-related effects of potential public welfare 
significance, that included effects on growth of individual sensitive 
species and extended to ecosystem-level effects, such as community 
composition in natural forests, particularly in protected public lands 
(80 FR 65406, October 26, 2015).
---------------------------------------------------------------------------

    \176\ The CASAC letter on the second draft PA in that review 
stated the following (Frey, 2014b, p. 9-10):
    For example, CASAC concurs that trees are important from a 
public welfare perspective because they provide valued services to 
humans, including aesthetic value, food, fiber, timber, other forest 
products, habitat, recreational opportunities, climate regulation, 
erosion control, air pollution removal, and hydrologic and fire 
regime stabilization. Damage effects to trees that are adverse to 
public welfare occur in such locations as national parks, national 
refuges, and other protected areas, as well as to timber for 
commercial use. The CASAC concurs that biomass loss in trees is a 
relevant surrogate for damage to tree growth that affects ecosystem 
services such as habitat provision for wildlife, carbon storage, 
provision of food and fiber, and pollution removal. Biomass loss may 
also have indirect process-related effects such as on nutrient and 
hydrologic cycles. Therefore, biomass loss is a scientifically valid 
surrogate of a variety of adverse effects to public welfare.
---------------------------------------------------------------------------

    The currently available evidence related to conceptual 
relationships between plant growth impacts and the broader array of 
vegetation effects (e.g., that supported the use of RBL as a surrogate 
or proxy) is largely consistent with that available in the last review. 
In fact, the ISA for the current review describes (or relies on) such 
relationships in considering causality determinations for ecosystem-
scale effects such as altered terrestrial community composition and 
reduced productivity, as well as reduced carbon sequestration, in 
terrestrial ecosystems (ISA, Appendix 8, sections 8.8 and 8.10). Thus, 
the PA concludes that the current evidence does not call into question 
conceptual relationships between plant growth impacts and the broader 
array of vegetation effects. Rather, the current evidence continues to 
support the use of tree seedling RBL as a proxy for the broad array of 
vegetation-related effects, most particularly those conceptually 
related to growth (PA, sections 4.5.1.2 and 4.5.3).
    Beyond tree seedling growth, on which RBL is specifically based, 
two other vegetation effect categories with extensive evidence bases, 
crop yield and visible foliar injury, were also given attention in 
considering the public welfare protection provided by the standard in 
2015. Based on the available information for these endpoints, along 
with associated limitations and uncertainties, the Administrator at 
that time concluded there was not support for giving a primary focus, 
in selecting a revised secondary standard, to these two types of 
effects. With regard to crop yield, the Administrator recognized the 
significant role of agricultural management practices in agricultural 
productivity, as well as market variability, concluding that, in 
describing her public welfare protection objectives, additional 
attention to this endpoint was not necessary. The rough similarities in 
estimated W126 levels of median crops and tree species are also 
noteworthy. With regard to foliar injury, the lack of clear 
quantitative relationships that would support predictive E-R functions 
was recognized. In light of such considerations, the Administrator 
focused on RBL estimates in identifying the requisite standard, and 
judged that a standard set based on public welfare protection 
objectives described in terms of cumulative exposures and relationships 
with tree seedling RBL was an appropriate means to, and would, provide 
appropriate protection for the array of vegetation-related effects. 
With regard to the information available in the current review, the PA 
concludes it does not call into question the basis for such judgments 
and continues to be supportive of the use of tree seedling RBL as a 
proxy for the broad array of vegetation-related effects (PA, section 
4.5.1.2).
    In considering the magnitude of estimated RBL on which to focus in 
its role as a surrogate or proxy for the full array of vegetation 
effects in the last review, the Administrator endeavored to identify a 
secondary standard that would limit 3-year average O3 
exposures somewhat below W126 index values associated with a 6% RBL 
median estimate from the established species-specific E-R functions. 
This led to identification of a seasonal W126 index value of 17 ppm-hrs 
that the Administrator concluded appropriate as a target at or below 
which the new standard would generally restrict cumulative seasonal 
exposures (80 FR 65407, October 26, 2015). In identifying this exposure 
level as a target, the Administrator, recognizing limitations and 
uncertainties in the evidence and variability in biota and ecosystems 
in the natural environment, additionally judged that RBL estimates 
associated with isolated rare instances of marginally higher cumulative 
exposures (in terms of a 3-year average W126 index), e.g., those that 
round to 19 ppm-hrs (which corresponds to 6% RBL as median from 11 
established E-R functions), were not indicative of adverse effects to 
the public welfare (80 FR 65409, October 26, 2015).
    The PA concludes that the information newly available in this 
review does not differ from that available in the last review with 
regard to a magnitude of RBL in the median species appropriately 
considered a reference for judgments concerning

[[Page 49900]]

potential vegetation-related impacts to the public welfare (PA, section 
4.5.1.2). The currently available evidence continues to indicate 
conceptual relationships between reduced growth and the broader array 
of vegetation-related effects, and limitations and uncertainties remain 
with regard to quantitation. The PA notes that consideration of the 
magnitude of tree growth effects that might cause or contribute to 
adverse effects for trees, forests, forested ecosystems or the public 
welfare is complicated by various uncertainties or limitations in the 
evidence base, including those associated with relating magnitude of 
tree seedling growth reduction to larger-scale forest ecosystem 
impacts. Further, other factors can influence the degree to which 
O3-induced growth effects in a sensitive species affect 
forest and forest community composition and other ecosystem service 
flows (e.g., productivity, belowground biogeochemical cycles and 
terrestrial ecosystem water cycling) from forested ecosystems. These 
include (1) the type of stand or community in which the sensitive 
species is found (i.e., single species versus mixed canopy); (2) the 
role or position the species has in the stand (i.e., dominant, sub-
dominant, canopy, understory); (3) the O3 sensitivity of the 
other co-occurring species (O3 sensitive or tolerant); and 
(4) environmental factors, such as soil moisture and others. The lack 
of such established relationships with O3 complicates 
consideration of the extent to which different estimates of impacts on 
tree seedling growth would indicate significance to the public welfare. 
Further, efforts to estimate O3 effects on carbon 
sequestration are handicapped by the large uncertainties involved in 
attempting to quantify the additional carbon uptake by plants as a 
result of avoided O3-related growth reductions. Such 
analyses require complex modeling of biological and ecological 
processes with their associated sources of uncertainty.
    Quantitative representations of such relationships have been used 
to study potential impacts of tree growth effects on such larger-scale 
effects as community composition and productivity with the results 
indicating the array of complexities involved (e.g., ISA, Appendix 8, 
section 8.8.4). Given their purpose in exploring complex ecological 
relationships and their responses to environmental variables, as well 
as limitations of the information available for such work, these 
analyses commonly utilize somewhat general representations. The PA 
notes that this work indicates how established the existence of such 
relationships is, while also identifying complexities inherent in 
quantitative aspects of such relationships and interpretation of 
estimated responses. Thus, the PA finds the currently available 
evidence to be little changed from the last review with regard to 
informing identification of an RBL reference point reflecting 
ecosystem-scale effects with public welfare impacts elicited through 
such linkages (PA, section 4.5.1.2).
(ii) Focus on 3-Year Average W126 Index
    In setting the current standard, as described in section III.A.1 
above, the Administrator focused on control of seasonal cumulative 
exposures in terms of a 3-year average W126 index. The evaluations in 
the PA for that review recognized there to be limited information to 
discern differences in the level of protection afforded for cumulative 
growth-related effects by a standard focused on a single-year W126 
index as compared to a 3-year W126 index (80 FR 65390, October 26, 
2015). Accordingly, 3-year average was identified for considering the 
seasonal W126 index based on the recognition that there was year-to-
year variability not just in O3 concentrations, but also in 
environmental factors, including rainfall and other meteorological 
factors, that influence the occurrence and magnitude of O3-
related effects in any year (e.g., through changes in soil moisture), 
contributing uncertainties to projections of the potential for harm to 
public welfare (80 FR 65404 October 26, 2015). Given this recognition, 
as well as other considerations, the Administrator expressed greater 
confidence in judgments related to projections of public welfare 
impacts based on seasonal W126 index estimated by a 3-year average and 
accordingly, relied on that metric.
    A general area of uncertainty that remains in the current evidence 
continues to affect interpretation of the potential for harm to public 
welfare over multi-year periods of air quality that meet the current 
standard (PA, section 4.3.4). As recognized in the last review, there 
is variability in ambient air O3 concentrations from year to 
year, as well as year-to-year variability in environmental factors, 
including rainfall and other meteorological factors that affect plant 
growth and reproduction, such as through changes in soil moisture. 
Accordingly, these variabilities contribute uncertainties to estimates 
of the occurrence and magnitude of O3-related effects in any 
year, and to such estimates over multi-year periods. The PA recognizes 
that limitations in our ability to estimate the effects on growth over 
tree lifetimes of year-to-year variation in O3 
concentrations, particularly those associated with conditions meeting 
the current standard, contribute uncertainty to estimates of cumulative 
growth (biomass) effects over multi-year periods in the life of 
individual trees and associated populations, as well as related effects 
in associated communities and ecosystems (PA, section 4.3.4).
    As summarized in section III.B.3 above, the longstanding evidence 
on O3 effects on plant growth includes the established and 
robust E-R functions for 11 species of tree seedlings (ISA, Appendix 8, 
Table 8-24; PA, Appendix 4A, Table 4A-1,). The PA recognized the 
strength of these functions in describing tree seedling response across 
a broad range of W126 index values, concluding that the evidence 
continues to support their use in estimating the median RBL across 
species in this review. In considering the appropriate representation 
of seasonal W126 for use of these functions with air quality data, the 
PA additionally considered the available information underlying the E-R 
functions and the extent to which the information is specific to a 
single seasonal exposure, e.g., as compared to providing representation 
for an average W126 index across multiple seasons (PA, section 
4.5.1.2). In so doing, the PA took note of aspects of the evidence that 
reflect variability in organism response under different experimental 
conditions and the extent to which this variability is represented in 
the available data. This might indicate an appropriateness of assessing 
environmental conditions using a mean across seasons in recognition of 
the existence of such year-to-year variability in conditions and 
responses. An additional aspect of the information underlying the E-R 
functions that was identified as relevant to consider is the extent to 
which the exposure conditions represented include those associated with 
O3 concentrations that meet the current standard, and the 
extent to which tree seedling growth responses to such conditions may 
have been found to not be significantly different from responses to the 
control (e.g., zero O3) conditions. The extent to which E-R 
predictions are extrapolated beyond the tested exposure conditions also 
contributes to uncertainty which the PA indicated may argue for a less 
precise interpretation, such as an average across multiple seasons.
    The experiments from which the functions were derived vary in 
duration

[[Page 49901]]

from periods of 82 to 140 days over a single year to periods of 180 to 
555 days across two years, and in whether measurements were made 
immediately following exposure period or in the subsequent season (PA, 
section 4.5.1.2, Appendix 4A, Table 4A-5; Lee and Hogsett, 1996). In 
producing E-R functions of consistent duration across the experiments, 
the E-R functions were derived first based on the exposure duration of 
the experiment and then normalized to 3-month (seasonal) periods (see 
Lee and Hogsett, 1996, section I.3; PA, Appendix 4A). Underlying the 
adjustment is a simplifying assumption of uniform W126 distribution 
across the exposure periods and of a linear relationship between 
duration of cumulative exposure in terms of the W126 index and plant 
growth response. Some functions for experiments that extended over two 
seasons were derived by distributing responses observed at the end of 
two seasons of varying exposures equally across the two seasons (e.g., 
essentially applying the average to both seasons).
    The PA additionally recognizes that the experiment-specific E-R 
functions for both aspen and ponderosa pine illustrate appreciable 
variability in response across experiments (PA, Appendix 4A, Figure 4A-
10). The PA suggested that reasons for this variability may relate to a 
number of factors, including variability in seasonal response related 
to variability in non-O3 related environmental influences on 
growth, such as rainfall, temperature and other meteorological 
variables, as well as biological variability across individual 
seedlings, in addition to potentially variability in the pattern of 
O3 concentrations contributing to similar cumulative 
exposures (PA, section 4.5.1.2). In recognition of some of the 
variability in both seasonal environmental conditions in the studies 
and the associated experimental data, the 11 species-specific E-R 
functions are based on median responses (derived from experiment-
specific functions) across an array of W126 index values (PA, Appendix 
4A; Lee and Hogsett, 1996).\177\ The number of experiments used in 
deriving the E-R functions for each species varies. For example, there 
are 7 experimental studies for wild aspen and 11 for ponderosa pine 
(PA, Appendix 4A, Table 4A-5), and only two or three for the three 
species (black cherry, sugar maple and tulip poplar) that exhibit 
greater sensitivity than aspen and ponderosa pine (PA, Appendix 4A, 
section 4A-2, Table 4A-5; 1996 AQCD, Table 5-28; Lee and Hogsett, 
1996). Regarding the extent or strength of the database underlying the 
E-R functions for cumulative exposure levels of interest in the current 
review, the PA also notes that the data generally appear to be more 
extensive for relatively higher (e.g., at/above a SUM06 of 30 ppm-hrs), 
versus lower, seasonal exposures (PA, Appendix 4A, Table 4A-6). 
Additionally, while the evidence is long-standing and robust for growth 
effects of O3, the studies available for some species appear 
to be somewhat limited in the extent to which they include cumulative 
O3 exposures commonly occuring with air quality conditions 
that meet the current standard (e.g., W126 index values below 20 ppm-
hrs).\178\ The PA concludes the factors identified here to contribute 
to uncertainty or inexactitude in estimates based on the E-R functions.
---------------------------------------------------------------------------

    \177\ This median-based approach is expected to guard against 
statistical bias in parameter values.
    \178\ The evidence is unclear on the extent to which six of the 
11 species include exposure treatments likely to correspond to W126 
index values at or below 20 ppm-hrs (PA, Appendix 4A, Table 4A-5). 
For five of the species in Table 4A-5 in Appendix 4A, SUM06 index 
values below 25 ppm-hrs range from 12 to 21.7. In considering these 
values, we note that an approach used in the 2007 Staff Paper on 
specific temporal patterns of O3 concentrations concluded 
that a SUM06 index value of 25 ppm-hrs would be estimated to 
correspond to a W126 index value of approximately 21 ppm-hrs (U.S. 
EPA, 2007, Appendix 7B, p. 7B-2). Accordingly, a SUM06 value of 21 
ppm-hrs might be expected to correspond to a W126 index value below 
20 ppm-hrs. The PA further notes that for one of the species for 
which lower exposures were studied, black cherry, the findings for 
at least one study reported statistical significance only for 
effects observed for higher exposures (PA, section 4.3.4, Appendix 
4A, Table 4A-6).
---------------------------------------------------------------------------

    The PA recognizes that the evidence that allows for specific 
evaluation of the predictability of growth impacts from single-year 
versus multiple-year average exposure estimates is quite limited. Such 
evidence would include multi-year studies reporting results for each 
year of the study, which are the most informative to the question of 
plant annual and cumulative responses to individual years (high and 
low) over multiple-year periods. The evidence is quite limited with 
regard to studies of O3 effects that report seasonal 
observations across multi-year periods and that also include detailed 
hourly O3 concentration records (to allow for derivation of 
exposure index values). Such a limitation contributes uncertainty and 
accordingly a lack of precision to our understanding of the 
quantitative impacts of seasonal O3 exposure, including its 
year-to-year variability on tree growth and annual biomass accumulation 
(PA, section 4.3.4). The PA finds this uncertainty to limit our 
understanding of the extent to which tree biomass would be expected to 
appreciably differ at the end of multi-year exposures for which the 
overall average exposure is the same, yet for which the individual year 
exposures varied in different ways (e.g., as analyzed in Appendix 4D of 
the PA). Thus, the PA notes that the extent of any differences in tree 
biomass for two multi-year scenarios with the same 3-year average W126 
index but differing single-year indices is not clear, including for 
exposures associated with O3 concentrations that would meet 
the current standard (PA, section 4.3.4).\179\
---------------------------------------------------------------------------

    \179\ Variation in annual W126 index values indicates that for 
the period, 2016-2018, the amount by which annual W126 index values 
at a site differ from the 3-year average varies is generally below 
10 ppm-hrs across all sites and generally below 5 ppm-hrs at sites 
with design values at or below 70 ppb (PA, Appendix 4D, Figure 4D-
7).
---------------------------------------------------------------------------

    One such study, which tracked exposures across six years, is 
available for aspen (King et al., 2005; 2013 ISA, section 9.6.3.2; ISA, 
Appendix 8, section 8.13.2).\180\ This study was used in a presentation 
of the 2013 ISA that compared the observed growth response to that 
predicted from the E-R function for aspen. Specifically, the observed 
aboveground biomass (and RBL) after each of the six growing seasons was 
compared to estimates derived from the aspen E-R function based on the 
cumulative multiple-year average seasonal W126 index values for each 
year \181\ (2013 ISA, section 9.6.3.2). The conclusions reached were 
that the agreement between the set of predictions and the Aspen FACE 
observations were ``very close'' and that ``the function based on one 
year of growth was shown to be applicable to subsequent years'' (2013 
ISA, p. 9-135). The PA observes that such results indicate that when 
considering O3 impacts on growing trees across multiple 
years, a multi-year average index yields predictions close to observed 
measurements across the multi-year time period (2013 ISA, section 
9.6.3.2 and Figure 9-20; PA, Appendix 4A, section 4.A.3). The PA also 
includes example analyses that use biomass measurements from the multi-
year study (King et al., 2005) to estimate aboveground aspen biomass 
over a multi-year period using the established

[[Page 49902]]

E-R function for aspen with a constant single-year W126 index, e.g., of 
17 ppm-hrs, or with varying annual W126 index values (10, 17 and 24 
ppm-hrs) for which the 3-year average is 17 ppm-hrs, and that yield 
somewhat similar total biomass estimates after multiple years (PA, 
Appendix 4A, section 4A.3).\182\
---------------------------------------------------------------------------

    \180\ A similar comparison is presented in the current ISA (ISA, 
Appendix 8).
    \181\ Although not emphasized or explained in detail in the 2013 
ISA, the W126 estimates used to generate the predicted growth 
response were cumulative average. To clarify, the cumulative average 
W126 for year 1 is simply the W126 index for that year (e.g., based 
on highest 3 months). For year 2, it is the average of the year 1 
seasonal W126 and year 2 seasonal W126, and so on. For year 6, it is 
the average of each of the six year's seasonal W126 index values.
    \182\ This example, while simplistic in nature, and with 
inherent uncertainties, including with regard to broad 
interpretation given the reliance on data available for the single 
study, quantitatively illustrates potential differences in growth 
impacts of W126 index, as a 3-year average, for which individual 
year values vary while still meeting the value specified for the 
average, from such impacts from exposure controlled to the same W126 
index value annually. The PA suggests that this example indicates 
based on the magnitude of variation documented for annual W126 index 
values occurring under the current standard, a quite small magnitude 
of differences in tree biomass between single-year and multi-year 
average approaches to controlling cumulative exposure (PA, Appendix 
4A, section 4A.3).
---------------------------------------------------------------------------

    Thus, the PA finds that, while the E-R functions are based on 
strong evidence of seasonal and cumulative seasonal O3 
exposure reducing tree growth, and while they provide for quantitative 
characterization of the extent of such effects across O3 
exposure levels of appreciable magnitude, there is uncertainty 
associated with the resulting RBL predictions. Further, the current 
evidence does not indicate single-year seasonal exposure in combination 
with the established E-R functions to be a better predictor of RBL than 
a seasonal exposure based on a multi-year average, or vice versa 
(Appendix 4A, section 4A.3.1). Rather, associated uncertainty 
contributes or implies an imprecision or inexactitude in the resulting 
predictions, particularly for the lower W126 index estimates of 
interest in this review. In light of this, the current evidence does 
not support concluding there to be an appreciable difference in the 
effect of three years of exposure held at 17 ppm-hrs compared to a 3-
year exposure that averaged 17 ppm-hrs yet varied by 5 to 10 ppm (e.g., 
7 ppm-hrs) from 17 ppm-hrs in any of the three years for tree RBL over 
such multiple-year periods. The PA considered all of the factors 
identified here, the currently available evidence and recognized 
limitations, variability and uncertainties, to contribute uncertainty 
and resulting imprecision or inexactitude to RBL estimates of single-
year seasonal W126 index values. The PA found these considerations to 
indicate there to be no lesser support for use of an average seasonal 
W126 index derived from multiple years (with their representation of 
variability in environmental factors), such as for a 3-year period, for 
estimating median RBL using the established E-R functions than for use 
of a single-year index.
(iii) Visible Foliar Injury
    In considering a public welfare protection approach related to 
visible foliar injury, the PA first notes that some level of visible 
foliar injury can impact public welfare and thus might reasonably be 
judged adverse to public welfare.\183\ As summarized in section III.B.2 
above, depending on its spatial extent and severity, there are many 
situations or locations in which visible foliar injury can adversely 
affect the public welfare. For example, significant, readily 
perceivable and widespread injury in national parks and wilderness 
areas can adversely affect the perceived scenic beauty of these areas, 
harming the aesthetic experience for both outdoor enthusiasts and the 
occasional park visitor. Such considerations have also been recognized 
by the Agency in past reviews, in which decisions to revise the 
O3 secondary standard emphasized protection of Class I 
areas, which are areas such as national wilderness areas and national 
parks given special protections by the Congress (e.g., 73 FR 16496, 
March 27, 2008, ``the Administrator concludes it is appropriate to 
revise the secondary standard, in part, to provide increased protection 
against O3-caused impairment to such protected vegetation 
and ecosystems'').\184\
---------------------------------------------------------------------------

    \183\ As stated in the 2015 decision notice: ``both tree growth-
related effects and visible foliar injury have the potential to be 
significant to the public welfare'' (80 FR 65377, October 26, 2015); 
``O3-induced visible foliar injury also has the potential 
to be significant to the public welfare through impacts in Class I 
and other similarly protected areas'' (80 FR 65378, October 26, 
2015); ``[d]epending on the extent and severity, O3-
induced visible foliar injury might be expected to have the 
potential to impact the public welfare in scenic and/or recreational 
areas during the growing season, particularly in areas with special 
protection, such as Class I areas. (80 FR 65379, October 26, 2015); 
``[t]he Administrator also recognizes the potential for this effect 
to affect the public welfare in the context of affecting values 
pertaining to natural forests, particularly those afforded special 
government protection (80 FR 65407, October 26, 2015).
    \184\ In the discussion of the need for revision of the 1997 
secondary standard, the 2008 decision noted that ``[i]n considering 
what constitutes a vegetation effect that is adverse from a public 
welfare perspective, . . . the Administrator has taken note of a 
number of actions taken by Congress to establish public lands that 
are set aside for specific uses that are intended to provide 
benefits to the public welfare, including lands that are to be 
protected so as to conserve the scenic value and the natural 
vegetation and wildlife within such areas, and to leave them 
unimpaired for the enjoyment of future generations'' (73 FR 16496, 
March 27, 2008). This passage of the 2008 decision notice clarified 
that ``[s]uch public lands that are protected areas of national 
interest include national parks and forests, wildlife refuges, and 
wilderness areas'' (73 FR 16496, March 27, 2008).
---------------------------------------------------------------------------

    In establishing the current secondary standard and describing its 
underlying public welfare protection objectives (as summarized in 
section III.A.1, above), the Administrator at that time focused 
primarily on RBL in tree seedlings as a proxy or surrogate for the full 
array of vegetation related effects of O3, while 
additionally concluding that the then-available information on visible 
foliar injury provided some support for establishing a strengthened 
standard. In so doing, she took note of the indication of the evidence 
of the association between O3 and visible foliar injury, as 
well as in the declines generally observed in USFS BI scores with 
reductions in W126 index from well above 20 ppm-hrs to lower levels (80 
FR 65407-65408, October 26, 2015). She recognized, however, that the 
evidence was not conducive to use in identifying a quantitative public 
welfare protection objective focused specifically on visible foliar 
injury (based on judgment of the specific extent and severity at which 
such effects should be considered adverse to the public welfare) due to 
uncertainties and complexities associated with the available 
information. In related manner, she specifically recognized significant 
challenges posed by the lack of clear quantitative relationships 
(including robust exposure-response functions that addressed the 
variability observed in the available data, likely associated with the 
variables creating a predisposing environment), that would allow 
prediction of visible foliar injury severity and incidence under 
varying air quality and environmental conditions, as well as the lack 
of established criteria or objectives that might inform consideration 
of potential public welfare impacts related to this vegetation effect 
(80 FR 65407, October 26, 2015).
    The PA finds that these challenges are not addressed by the 
information available in the current review. Beyond the lack of 
established descriptive quantitative relationships for O3 
concentrations or exposure metrics with incidence or severity of 
visible foliar injury, summarized in sections III.D.1.a and III.B.3 
above, there is a paucity of information clearly relating differing 
levels of severity and extent of location affected to scenic or 
aesthetic values (e.g., reflective of visitor enjoyment and likelihood 
of frequenting such areas) that might inform judgments of public 
welfare protection from adversity (PA, section 4.5.1). Thus, there 
remain appreciable limitations of the current information for the 
purpose of providing a foundation for judgments on public

[[Page 49903]]

welfare protection objectives specific to visible foliar injury.
    Notwithstanding these limitations with regard to a detailed 
approach or framework for judging public welfare protection related to 
impacts of visible foliar injury, the current evidence and analyses are 
informative to such considerations. For example, the published studies 
and EPA analyses of the USFS biosite data indicate that incidence and 
severity of injury are increased at the highest exposures. With regard 
to the dataset analyzed in the PA, while clear trends in incidence and 
severity related to increasing W126 index are not evident across the 
W126 bins below 25 ppm-hrs, the incidence of sites with the more severe 
classification of injury (e.g., BI score above 15 [``moderate'' or 
``severe''] or 5 [``light,'' ``moderate,'' or ``severe'']) is 
appreciably lower at sites with W126 index values below 25 ppm-hrs than 
at sites with higher values (e.g., PA, Appendix 4C, Figures 4C-5 and 
4C-6 and Table 4C-5). This observation is based primarily on records 
for the normal soil moisture category, for which is sufficient sample 
size across the full range of W126 and the largest differences in 
incidence and average score are observed.\185\ Based on these 
observations and the full analysis, the PA concludes that the currently 
available information does not support precise conclusions as to the 
severity and extent of such injury associated with the lower values of 
W126 index most common at USFS sites during the years of the dataset, 
2006-2010.\186\ Based on the general pattern observed, however, the PA 
suggests a reduced severity (average BI score below 5) and incidence of 
visible foliar injury, as quantified by BI scores, to be expected under 
conditions that maintain W126 index values below 25 ppm-hrs, (PA, 
section 4.5.1.3).
---------------------------------------------------------------------------

    \185\ Across W126 bins in which at least 1% of the wet soil 
moisture records are represented, differences of highest bin from 
lower bins for injury incidence or average score is less than a 
factor of two (PA, section 4.3.3).
    \186\ Factors that may contribute to the observed variability in 
BI scores and lack of a clear pattern with W126 index bin may 
include uncertainties in assignment of W126 estimates and soil 
moisture categories to biosite locations, variability in biological 
response among the sensitive species monitored, and potential role 
of other aspects of O3 air quality not captured by the 
W126 index.
---------------------------------------------------------------------------

    Given the evidence regarding the role of peak O3 
concentrations as an influence on occurrence of visible foliar injury 
separate from that of the cumulative, concentration-weighted, W126 
index (summarized in section III.B.3.b above), the PA additionally 
finds that the conditions associated with visible foliar injury in 
locations with sensitive species appear to relate to peak concentration 
as well as cumulative exposure to generally higher concentrations over 
the growing season (PA. section 4.5.1.2). Accordingly, the PA also 
considered the current information with regard to peak concentration 
metrics. Such information includes the 2007 Staff Paper comparison 
based on the less extensive USFS dataset of counties grouped by fourth 
highest annual daily maximum 8-hour concentration. This analysis found 
a smaller incidence of nonzero BI biosites in counties with a fourth-
high metric at or below 74 ppb as compared to counties limited to 
metric values at or below 84 ppb (U.S. EPA 2007, pp. 7-63 to 7-64). The 
indication of this finding that the averaging time and form of the 
current standard, which emphasizes peak concentrations through a short 
(8-hour) averaging time and a rare-occurrence form (annual fourth 
highest daily maximum), exert some control on the incidence of sites 
with visible foliar injury has a conceptual similarity to the finding 
of the most extensive study of USFS data (1994-2009) that reductions in 
peak 1-hour concentrations have influenced the declining trend observed 
in visible foliar injury since 2002 (Smith, 2012).
(iv) Climate Effects
    In considering the currently available information for the effects 
of the global tropospheric abundance of O3 on radiative 
forcing, and temperature, precipitation and related climate variables, 
the PA recognized there to be limitations and uncertainties in the 
associated evidence bases with regard to assessing potential for 
occurrence of climate-related effects as a result of varying 
O3 concentrations in ambient air of locations in the U.S (as 
summarized in III.B.3 above). The current evidence is limited with 
regard to support for such quantitative analyses that might inform 
considerations related to the current standard. For example, as stated 
in the ISA, ``[c]urrent limitations in climate modeling tools, 
variation across models, and the need for more comprehensive 
observational data on these effects represent sources of uncertainty in 
quantifying the precise magnitude of climate responses to ozone 
changes, particularly at regional scales'' (ISA, section 9.3.1). These 
are ``in addition to the key sources of uncertainty in quantifying 
ozone RF changes, such as emissions over the time period of interest 
and baseline ozone concentrations during preindustrial times'' (ISA, 
section IS.9.3.1). Together such uncertainties limit development of 
quantitative estimates of climate-related effects in response to earth 
surface O3 concentrations at the regional scale, such as in 
the U.S. While these complexities inhibit our ability to consider 
tropospheric O3 effects, such as radiative forcing, we note 
that our consideration of O3 growth-related impacts on trees 
inherently encompasses consideration of the potential for O3 
to reduce carbon sequestration in terrestrial ecosystems (e.g., through 
reduced tree biomass as a result of reduced growth). That is, limiting 
the extent of O3-related effects on growth would be expected 
to also limit reductions in carbon sequestration, a process that can 
reduce the tropospheric abundance of CO2, the greenhouse gas 
ranked highest in importance as a greenhouse gas and radiative forcing 
agent (section III.B.3 above; ISA, section 9.1.1).
c. Public Welfare Implications of Air Quality Under the Current 
Standard
    In considering the potential for effects and related public welfare 
implications of air quality conditions and associated exposures 
indicated to occur under the current standard, the PA first looked to 
the air quality analyses particular to cumulative O3 
exposures, in terms of the W126 index, given its established 
relationship with growth-related effects and specifically RBL as the 
identified proxy or surrogate for the full array of such effects (PA, 
section 4.5.1.3, Appendix 4D). In that context, the PA gave relatively 
greater emphasis to air quality in Class I areas in recognition of the 
increased significance of effects in such areas that have been accorded 
special protection, as discussed in section III.B.2 above. In 
evaluating the extent and magnitude of O3 exposures, in 
terms of W126, in such areas that meet the current standard, the PA 
also considered year to year variability in the index, while 
recognizing that, with regard to W126 index relationships with RBL, 
there was uncertainty associated with RBL predictions from a single 
year W126 estimate (PA, sections 4.3.4 and 4.5.1, Appendix 4A). As 
discussed in section III.D.1.b above, the evidence does not indicate 
estimates based on an average of seasonal W126 across three years to be 
less, or more, predictive of RBL or resulting total plant biomass (PA, 
sections 4.3.4 and 4.5.1.2). The PA considered the magnitude of W126 
index occurring in areas nationwide, and particularly in Class I areas, 
that meet the current standard, as well as the frequency of the 
relatively higher index values. Further, the PA evaluated the extent of 
control of such index values exerted by the current standard, as

[[Page 49904]]

evidence by comparisons of sites with design values at or below the 
current standard level and sites with higher design values (PA, section 
4.4). Lastly, the PA also considered what the currently available 
information indicated with regard to the incidence and severity of 
visible foliar injury that might be expected to occur under air quality 
conditions that meet the current standard, and the potential for 
impacts on public welfare (PA, sections 4.5.1.2, 4.5.1.3 and 4.5.3).
    The air quality analyses of monitoring data at sites across the 
U.S. that meet the current standard in the most recent 3-year period 
find that the seasonal W126 index, as assessed by the 3-year average, 
is at or below 17 ppm-hrs, with just one exception, among 849 
locations, where it equaled 18 ppm-hrs. No 3-year average W126 index 
values exceeded 17 ppm-hrs in or near Class I areas. Further, such W126 
exposures are generally well below 17 ppm-hrs across most of the U.S. 
These findings for sites meeting the current standard, differ 
dramatically from sites with higher design values. For example, a third 
of all U.S. sites with design values above 70 ppb in the recent period, 
and more than 80% of Class I area sites with design values above 70 
ppb, have average W126 index values above 17 ppm-hrs. Looking back 
across the 19 years covered by the full historical dataset, the 
cumulative exposure estimates, averaged over the design value periods, 
were virtually all at or below 17 ppm-hrs, with most of the W126 index 
values below 13 ppm-hrs (PA, Appendix 4D, Table 4D-9).\187\
---------------------------------------------------------------------------

    \187\ Based on the established E-R functions for tree seedlings 
of 11 species, the median RBL estimates for such W126 index values 
are 3.8% or less (PA, Appendix 4A).
---------------------------------------------------------------------------

    The PA also considered the general occurrence and distribution of 
relatively higher single-year W126 index values, finding a generally 
similar pattern to that for averages over the design value period. For 
example, fewer than two dozen of the 849 sites meeting the current 
standard in the recent period had a single-year index above 17 ppm-hrs; 
about a dozen of these sites fall above 19 ppm- hrs, the highest of 
which just reaches 25 ppm-hrs in downtown Denver, CO.\188\ The 
frequency of such occurrences is still lower for the Class I area 
monitors. For example, during the most recent three years, when the 
average seasonal W126 index is at or below 17 ppm-hrs in all Class I 
areas meeting the current standard, there were just three single-year 
W126 index values above 17 ppm-hrs and none above 19 ppm-hrs (PA, 
Appendix 4D, Table 4D-15).\189\ The PA additionally notes that single-
year W126 index values in Class I areas over the 19-year dataset 
evaluated were generally at or below 19 ppm-hrs, particularly in the 
more recent years (PA, Appendix 4D, section 4D.3.2.3).
---------------------------------------------------------------------------

    \188\ These highest W126 index values occur in the South West 
and West regions in which there are nearly 150 monitor locations 
meeting the current standard (PA, Figure 4-6, Appendix 4D, Figure 
4D-5, Table 4D-1). Across the full 19-year dataset, the downtown 
Denver site value is just one of six instances in the more than 
8,000 design value periods meeting the current standard of a single-
year W126 index value at or above 25 ppm-hrs. All but one of these 
instances were equal to 25 ppm-hrs; the single higher occurrence was 
equal to 26 ppm-hrs.
    \189\ Across the full 19-year dataset for Class I area monitors 
meeting the current standard (58 monitors with at least one such 
occurrence and approximately 500 total occurrences), there are no 
more than 15 occurrences of single-year W126 index values above 19 
ppm-hrs, all of which date prior to 2013 (PA, Appendix 4D, section 
4D.3.2.4).
---------------------------------------------------------------------------

    In reflecting on the air quality analysis findings summarized here, 
the PA additionally recognized limitations and uncertainties of the 
underlying database, noting there to be inherent limitations in any air 
monitoring network. The monitors for O3 are distributed 
across the U.S., covering all NOAA regions and all states although some 
geographical areas are more densely covered than others, which may have 
sparse or no data. For example, only about 40% of all Federal Class I 
Areas have or have had O3 monitors (with valid design 
values) within 15 km, thus allowing inclusion in the Class I area 
analysis. Even so, the dataset for that analysis includes sites in 27 
states distributed across all nine NOAA climatic regions across the 
contiguous U.S, as well as Hawaii and Alaska. While some NOAA regions 
have far fewer numbers of Class I areas with monitors than others 
(e.g., the Central, North East, East North Central, and South regions 
versus other regions), these areas also have appreciably fewer Class 1 
areas in general. Thus, the regions with relatively more Class I area 
are also more well represented in the dataset. For example, the West 
and Southwest regions (with the largest number of Class I areas) have 
approximately a third of those areas represented with monitors, which 
include locations where W126 index values are generally higher, thus 
playing a prominent role in the analysis.
    Another inherent uncertainty is with regard to the extent to which 
the results will prove to reflect conditions far out into the future as 
air quality and patterns of O3 concentrations in ambient air 
continue to change in response to changing circumstances, such as 
changes in precursor emissions to meet the current standard across the 
U.S. However, findings from these analyses in the current review are 
largely consistent with those from analyses of the data available in 
the last review. Further, the analysis of how changes in O3 
patterns in the past have affected the relationship between W126 index 
and the averaging time and form of the current standard finds a 
positive, linear relationship between trends in design values and 
trends in the W126 index (both in terms of single-year W126 index and 
averages over 3-year design value period), as was also the case for 
similar analyses conducted for the data available at the time of the 
last review (Wells, 2015). While this relationship varied across NOAA 
regions, the regions showing the greatest potential for exceeding W126 
index values of interest (e.g., with 3-year average values above 17 
and/or 19 ppm-hrs) also showed the greatest improvement in the W126 
index per unit decrease in design value over the historical period 
assessed (PA, Appendix 4D, section 4D.3.2.3). Thus, the available data 
and this analysis appear to indicate that as design values are reduced 
to meet the current standard in areas that presently do not, W126 
values in those areas would also be expected to decline (PA, Appendix 
4D, section 4D.4).
    In the last review, the Administrator focused on cumulative 
exposure estimates derived as the average W126 index over the 3-year 
design value period, concluding variations of single-year W126 index 
from the average to be of little significance in assessing public 
welfare protection. This focus generally reflected the judgment that 
estimates based on the average adequately, and appropriately reflected 
the precision of current understanding of O3-related growth 
reductions, given the various limitations and uncertainties in such 
predictions, that have been further evaluated in the current review (as 
summarized in section III.D.1.b above). Based on the information 
available in the current review, the PA concludes that, with the year-
to-year variation observed in areas meeting the current standard,\190\ 
differences in year-to-year tree growth in response to each year's 
seasonal exposure from the tree growth estimated from the 3-year 
average of the single-year values would, given the offsetting impacts 
of seasonal exposures above and below the average, reasonably be 
expected to generally be small over

[[Page 49905]]

tree lifetimes (PA, section 4.5.1.2). In so doing, the PA takes note of 
limitations in aspects of the data underlying the E-R functions that 
contribute to imprecision or inexactitude to estimates of growth 
impacts associated with multi-year exposures in the relatively lower 
W126 index values pertinent to air quality under the current standard. 
The information newly available in the current review does not 
appreciably address such limitations and uncertainties or improve the 
certainty or precision in RBL estimates for such exposures (PA, 
sections 4.3.4, 4.5.1).
---------------------------------------------------------------------------

    \190\ The current air quality data indicates single-year W126 
index values generally to vary by less than 5 ppm-hrs from the 3-
year average when the 3-year average is below 20 ppm-hrs, which is 
the case for locations meeting the current standard (PA, Appendix 
4D).
---------------------------------------------------------------------------

    Combining the findings of W126 index values (averaged over design 
value period) likely under the current standard with the established E-
R functions for reduced growth in 11 tree seedling species yields a 
median species RBL for tree seedlings at or below 5.3% for the recent 
period, with very few exceptions, with the highest estimates occurring 
in areas not near or within Class I areas. This general pattern is 
confirmed over the longer time period (2000-2018) for the vast majority 
of the data, with virtually all RBL estimates below 6%.\191\ Further, 
given the variability and uncertainty associated with the data 
underlying the E-R functions (as summarized in section III.D.1.a 
above), the few higher single-year occurrences are reasonably 
considered to be of less significance than 3-year average values. 
Judgments in the last review (in the context of the framework 
summarized in section III.D.1.b above) concluded isolated rare 
occurrences of exposures for which median RBL estimates might be at or 
just above 6% to not be indicative of conditions adverse to the public 
welfare, particularly considering the variability in the array of 
environmental factors that can influence O3 effects in 
different systems, and the uncertainties associated with estimates of 
effects in the natural environment.
---------------------------------------------------------------------------

    \191\ Although potential for effects on crop yield was not given 
particular emphasis in the last review (for reasons similar to those 
summarized earlier), we additionally note that combining the 
exposure levels summarized for areas across the U.S. where the 
current standard is met with the E-R functions established for 10 
crop species indicates a median RYL across crops to be at or below 
5.1%, on average, with very few exceptions. Further, estimates based 
on W126 index at the great majority of the areas are below 5% (PA, 
Appendices 4A and 4D).
---------------------------------------------------------------------------

    With regard to visible foliar injury, the PA observes that the 
available evidence does not include an approach for characterizing 
natural areas experiencing some severity or extent injury (e.g., via 
USFS BI score) with regard to public perception and potential impacts 
on public enjoyment; nor does it address this in combination with 
information on whether air quality conditions in sites with scores of a 
particular severity level do or do not meet the current standard (PA, 
section 4.5.1). As summarized in section III.B.2 above, public welfare 
implications relate largely to effects on scenic and aesthetic values. 
Accordingly, key considerations of this endpoint in past reviews have 
generally related to qualitative consideration of potential impacts 
related to the plant's aesthetic value in protected forested areas and 
the somewhat general, nonspecific judgment that a more restrictive 
standard is likely to provide increased protection. The currently 
available information does not yet address or describe the 
relationships expected to exist for some level of visible foliar injury 
severity (below that at which broader physiological effects on plant 
growth and survival might also be expected) and/or extent of location 
or site injury (e.g., BI) scores with values held by the public and 
associated impacts on public uses of the locations.\192\ Additionally, 
no criteria have been established regarding a level or prevalence of 
visible foliar injury considered to be adverse to the affected 
vegetation as the current evidence does not provide for determination 
of a degree of leaf injury that would have significance to the vigor of 
the whole plant (ISA, Appendix 8, p. 8-24). Nevertheless, while minor 
spotting on a few leaves of a plant may easily be concluded to be of 
little public welfare significance, it is reasonable to conclude that 
cases of widespread and relatively severe injury during the growing 
season (particularly when sustained across multiple years, and 
accompanied by obvious impacts on the plant canopy) would likely impact 
the public welfare in scenic and/or recreational areas, particularly in 
areas with special protection, such as Class I areas. However, the gaps 
in our information and tools, as summarized in prior sections, restrict 
our ability to identify air quality conditions that might be expected 
to provide a specific level of protection from public welfare effects 
of this endpoint.
---------------------------------------------------------------------------

    \192\ Information with some broadly conceptual similarity to 
this has been used for judging public welfare implications of 
visibility effects of PM in setting the PM secondary standard (78 FR 
3086, January 15, 2012).
---------------------------------------------------------------------------

    Assessment of any public welfare implications of air quality 
occurring under the current standard with regard to visible foliar 
injury is further hampered by the lack of an established quantitative 
description of the relationship between O3 concentrations 
(or exposure metrics) and injury extent or incidence, as well as 
severity, that would support estimates of potential injury for varying 
air quality and environmental conditions (e.g., moisture), most 
particularly for situations that meet the current standard. Although no 
such relationship or pertinent metrics for describing exposure are 
established, the available information, indicates a role for both a 
cumulative metric of exposure as well as the occurrence of relatively 
higher concentrations. More specifically, the PA notes the information 
indicating potential for increased incidence and severity of injury in 
locations with W126 index above 25 ppm-hrs and with increased 
occurrence of peak (1-hour) concentrations such as above 100 ppb (PA, 
section 4.5.1).
    The analyses of recent and historical air quality at monitoring 
sites where the current standard is met do not indicate a tendency for 
such occurrence of cumulative exposures or peak concentrations (PA, 
sections 2.4.5 and 4.4, Appendices 2A and 4D). In these analyses, all 
3-year average W126 index values are below 25 ppm-hrs, and values above 
17 ppm-hrs are rare. In addition, all single-year, W126 index values at 
Class I area locations meeting the current standard (and virtually all 
sites across the U.S.) are at or below 25 ppm-hr; even, and values 
above 19 ppm-hrs are rare, and mores so in more recent years (PA, 
section 4.4.2, Appendix 4D). Accordingly, while the current evidence is 
limited for the purposes of identifying public welfare protection 
objectives related to visible foliar injury in terms of specific air 
quality metrics, the PA notes that the current information indicates 
that the occurrence of injury categorized as more severe than 
``little'' by the USFS categorization (i.e., a BI scores above 5 or 
above 15) would be expected to be infrequent in areas that meet the 
current standard.
    In light of the evidence regarding a role for peak concentrations, 
the PA additionally took note of the control of peak concentrations 
exerted by the form and averaging time of the current standard. For 
example, daily maximum 1-hour, as well as 8-hour average O3 
concentrations have declined over the past 15 years, a period in which 
there have been two revisions of the level of the secondary standard, 
each providing greater stringency, while retaining the same averaging 
time and form as the current standard (e.g., PA, Figures 2-10, 2-12 and 
2-17). Further, during periods when the current standard is met, there 
is less than one day per site, on average

[[Page 49906]]

with a maximum hourly concentration at or above 100 ppb. This compares 
with roughly 40 times as many such days, on average, for sites with 
design values above the current standard level (PA, Appendix 2A, 
section 2A.2). The currently available information indicates that the 
current standard provides appreciable control of peak 1-hour 
concentrations, as well as W126 index values, and thus, to the extent 
that such metrics play a role in the occurrence and severity of visible 
foliar injury, the current standard also provides appreciable control 
of these.
    Thus, although the current information does not establish a metric 
or combination of metrics that well describes the relationship between 
occurrence and severity of visible foliar injury across a broad range 
of O3 concentration patterns from those more common in the 
past to those in areas recently meeting the current standard, the PA 
concludes that the currently available information does not indicate 
that a situation of widespread and relatively severe visible foliar 
injury, with apparent implications for the public welfare, is likely 
associated with air quality that meets the current standard. Based on 
the USFS dataset presentations as well as the air quality analyses of 
W126 index values and frequency of 1-hour observations at or above 100 
ppb, the prevalence of injury scores categorized as severe, or even 
moderate, which, depending on spatial extent, might reasonably be 
concluded to have potential to be adverse to the public welfare do not 
appear likely to occur under air quality conditions that meet the 
current standard. Thus, the PA finds, based on the current evidence and 
currently available air quality information, that the exposure 
conditions associated with air quality meeting the current standard are 
not those that might reasonably be concluded to result in the 
occurrence of significant foliar injury (with regard to severity and 
extent).
    With regard to other vegetation-related effects, including those at 
the ecosystem scale, such as alteration in community composition or 
reduced productivity in terrestrial ecosystems, as recognized in 
section III.D.1.a above, the available evidence is not clear with 
regard to the risk of such impacts (and their magnitude or severity) 
associated with the environmental O3 exposures estimated to 
occur under air quality conditions meeting the current standard, which 
primarily include W126 index at or below 17 ppm-hrs. In considering 
effects on crop yield, the air quality analyses at monitoring locations 
that meet the current standard indicate estimates of RYL for such 
conditions to be at and below 5.1%, based on the median estimate 
derived from the established E-R functions for 10 crops (PA, Appendix 
4A, Table 4A-5). We additionally recognize there to be complexities 
involved in interpreting the significance of such small RYL estimates 
in light of the factors also recognized in the last review. These 
included the extensive management of crops in agricultural areas that 
may to some degree mitigate potential O3-related effects, as 
well as the use of variable management practices to achieve optimal 
yields, while taking into consideration various environmental 
conditions. We also recognize that changes in yield of commercial crops 
and commercial commodities may affect producers and consumers 
differently, further complicating the question of assessing overall 
public welfare impacts for such RYL estimates (80 FR 65405, October 26, 
2015).
2. CASAC Advice
    The CASAC provided its advice regarding the current secondary 
standard in the context of its review of the draft PA (Cox, 
2020a).\193\ In so doing, the CASAC concurred with the PA conclusions, 
stating that it ``finds, in agreement with the EPA, that the available 
evidence does not reasonably call into question the adequacy of the 
current secondary ozone standard and concurs that it should be 
retained'' (Cox, 2020a, p. 1). The CASAC additionally stated that it 
``commends the EPA for the thorough discussion and rationale for the 
secondary standard'' (Cox, 2020, p. 2). The CASAC also provided 
comments particular to the consideration of climate and growth-related 
effects.
---------------------------------------------------------------------------

    \193\ A limited number of public comments have been received in 
this review to date, including comments focused on the draft IRP, 
draft ISA or draft PA. Of the commenters that addressed adequacy of 
the current secondary O3 standard, most expressed 
agreement with staff conclusions in the draft PA, while some 
expressed the view that the standard should be revised to a W126-
based form or that articulation of its rationale should more 
explicitly address the protection the standard provides for public 
welfare effects.
---------------------------------------------------------------------------

    With regard to O3 effects on climate, the CASAC 
recommended quantitative uncertainty and variability analyses, with 
associated discussion (Cox, 2020a, pp. 2, 22).\194\ With regard to 
growth-related effects and consideration of the evidence in 
quantitative exposure analyses, it stated that the W126 index ``appears 
reasonable and scientifically sound,'' ``particularly [as] related to 
growth effects'' (Cox, 2020a, p. 16). Additionally, with regard to the 
prior Administrator's expression of greater confidence in judgments 
related to public welfare impacts based on a seasonal W126 index 
estimated by a three-year average and accordingly relying on that 
metric the CASAC expressed the view that this ``appears of reasonable 
thought and scientifically sound'' (Cox, 2020, p. 19). Further, the 
CASAC stated that ``RBL appears to be appropriately considered as a 
surrogate for an array of adverse welfare effects and based on 
consideration of ecosystem services and potential for impact to the 
public as well as conceptual relationships between vegetation growth 
effects and ecosystem scale effects'' and that it agrees ``that biomass 
loss, as reported in RBL, is a scientifically-sound surrogate of a 
variety of adverse effects that could be exerted to public welfare,'' 
concurring that this approach is not called into question by the 
current evidence which continues to support ``the use of tree seedling 
RBL as a proxy for the broader array of vegetation related effects, 
most particularly those related to growth that could be impacted by 
ozone'' (Cox, 2020a, p. 21). The CASAC additionally concurred that the 
strategy of a secondary standard that generally limits 3-year average 
W126 index values somewhat below those associated with a 6% RBL in the 
median species is ``scientifically reasonable'' and that, accordingly, 
a W126 index target value of 17 ppm-hrs for generally restricting 
cumulative exposures ``is still effective in particularly protecting 
the public welfare in light of vegetation impacts from ozone'' (Cox, 
2020a, p 21.).
---------------------------------------------------------------------------

    \194\ As recognized in the ISA, ``[c]urrent limitations in 
climate modeling tools, variation across models, and the need for 
more comprehensive observational data on these effects represent 
sources of uncertainty in quantifying the precise magnitude of 
climate responses to ozone changes, particularly at regional 
scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p. 9-
22). These complexities impede our ability to consider specific 
O3 concentrations in the U.S. with regard to specific 
magnitudes of impact on radiative forcing and subsequent climate 
effects.
---------------------------------------------------------------------------

    With regard to the court's remand of the 2015 secondary standard to 
the EPA for further justification or reconsideration (``particularly in 
relation to its decision to focus on a 3-year average for consideration 
of the cumulative exposure, in terms of W126, identified as providing 
requisite public welfare protection, and its decision to not identify a 
specific level of air quality related to visible foliar injury''), 
while the CASAC stated that it was not clear whether the draft PA had 
fully addressed this concern (Cox, 2020a, p. 21), it described there to 
be a solid

[[Page 49907]]

scientific foundation for the current secondary standard and also 
commented on areas related to the remand. With regard to the focus on 
the 3-year average W126 index, in addition to the comments summarized 
above, the CASAC concluded, as noted above, that the EPA 
Administrator's focus on the 3-year average and her judgments in doing 
so ``appears of reasonable thought and scientifically sound'' (Cox, 
2020a, p. 19). Further, while recognizing the existence of established 
E-R functions that relate cumulative seasonal exposure of varying 
magnitudes to various incremental reductions in expected tree seedling 
growth (in terms of RBL) and in expected crop yield, the CASAC letter 
also noted that while decades of research also recognizes visible 
foliar injury as an effect of O3, ``uncertainties continue 
to hamper efforts to quantitatively characterize the relationship of 
its occurrence and relative severity with ozone exposures'' (Cox, 
2020a, p 20). In summary, the CASAC stated that the approach described 
in the draft PA to considering the evidence for welfare effects ``is 
laid out very clearly, thoroughly discussed and documented, and 
provided a solid scientific underpinning for the EPA conclusion leaving 
the current secondary standard in place'' (Cox, 2020a, p. 22).
3. Administrator's Proposed Conclusions
    Based on the large body of evidence concerning the welfare effects, 
and potential for public welfare impacts, of exposure to O3 
in ambient air, and taking into consideration the attendant 
uncertainties and limitations of the evidence, the Administrator 
proposes to conclude that the current secondary O3 standard 
provides the requisite protection against known or anticipated adverse 
effects to the public welfare, and should therefore be retained, 
without revision. In reaching these proposed conclusions, the 
Administrator has carefully considered the assessment of the available 
welfare effects evidence and conclusions contained in the ISA, with 
supporting details in the 2013 ISA and past AQCDs; the evaluation of 
policy-relevant aspects of the evidence and quantitative analyses in 
the PA (summarized in section III.D.1 above); the advice and 
recommendations from the CASAC (summarized in section III.D.2 above); 
and public comments received to date in this review, as well as the 
August 2019 decision of the D.C. Circuit remanding the secondary 
standard established in the last review to the EPA for further 
justification or reconsideration.
    In the discussion below, the Administrator considers first the 
evidence base on welfare effects associated with exposure to 
photochemical oxidants, including O3, in ambient air. In so 
doing, he considers the welfare effects evidence newly available in 
this review, and the extent to which it alters key scientific 
conclusions. The Administrator additionally considers the quantitative 
analyses available in this review, including associated limitations and 
uncertainties, and the extent to which they indicate differing 
conclusions regarding level of protection indicated to be provided by 
the current standard from adverse effects to the public welfare. 
Further, the Administrator considers the key aspects of the evidence 
and air quality and exposure information emphasized in establishing the 
now-current standard. He additionally considers uncertainties in the 
evidence and quantitative information, as part of public welfare policy 
judgments that are essential and integral to his decision on the 
adequacy of protection provided by the standard. The Administrator 
draws on the considerations and conclusions in the PA, taking note of 
key aspects of the rationale presented for those conclusions. In so 
doing, he notes the CASAC characterization of the ``thorough discussion 
and rationale for the secondary standard'' presented in the PA (Cox, 
2020a, p. 2). Further, the Administrator considers the advice of the 
CASAC regarding the secondary standard, including particularly its 
overall agreement that the currently available evidence does not call 
into question the adequacy of the current standard and that it should 
be retained (Cox, 2020a, p. 1). With attention to all of the above, the 
Administrator considers the information currently available in this 
review with regard to the appropriateness of the protection provided by 
the current standard.
    As an initial matter, the Administrator recognizes the continued 
support in the current evidence for O3 as the indicator for 
photochemical oxidants (as recognized in section III.D.1 above). In so 
doing, he notes that no newly available evidence has been identified in 
this review regarding the importance of photochemical oxidants other 
than O3 with regard to abundance in ambient air, and 
potential for welfare effects, and that, as stated in the current ISA, 
``the primary literature evaluating the health and ecological effects 
of photochemical oxidants includes ozone almost exclusively as an 
indicator of photochemical oxidants'' (ISA, section IS.1.1). Thus, the 
Administrator recognizes that, as was the case for previous reviews, 
the evidence base for welfare effects of photochemical oxidants does 
not indicate an importance of any other photochemical oxidants. For 
these reasons, described with more specificity in the ISA and PA, he 
proposes to conclude it is appropriate to retain the O3 as 
the indicator for the secondary NAAQS for photochemical oxidants.
    In considering the currently available welfare effects evidence for 
O3, the Administrator recognizes the longstanding evidence 
base for vegetation-related effects, augmented in some aspects since 
the last review, described in section III.B.1 above. Consistent with 
the evidence in the last review, the currently available evidence 
describes an array of effects on vegetation and related ecosystem 
effects causally or likely to be causally related to O3 in 
ambient air, as well as the causal relationship of tropospheric 
O3 in radiative forcing and subsequent likely causally 
related effects on temperature, precipitation and related climate 
variables. The Administrator also notes the Agency conclusions on three 
categories of effects with new ISA determinations that the current 
evidence is sufficient to infer likely causal relationships of 
O3 with increased tree mortality, alteration of plant-insect 
signaling and alteration of insect herbivore growth and reproduction 
(as summarized in section III.B.1 above). With regard to the current 
evidence for increased tree mortality, the Administrator notes the PA 
finding that the evidence does not indicate a potential for 
O3 concentrations that occur in locations that meet the 
current standard to cause increased tree mortality. Accordingly, 
consistent with the approach in the PA, he finds it appropriate to 
focus on more sensitive effects, such as tree seedling growth, in his 
review of the standard. With regard to the two insect-related 
categories of effects with new ISA determinations in this review, the 
Administrator takes note of the PA finding that uncertainties in the 
current evidence, as summarized in section III.B and III.D.1 above, 
preclude a full understanding of such effects, the air quality 
conditions that might elicit them, the potential for impacts in a 
natural ecosystem and, consequently, the potential for such impacts 
under air quality conditions associated with meeting the current 
standard; thus, there is insufficient information to judge the current

[[Page 49908]]

standard inadequate based on these effects.
    In considering the evidence with regard to support for quantitative 
description of relationships between air quality conditions and 
response to inform his judgments on the current standard, the 
Administrator recognizes the supporting evidence for plant growth and 
yield. The evidence base continues to indicate growth-related effects 
as sensitive welfare effects, with the potential for ecosystem-scale 
ramifications. For this category of effects, there are established E-R 
functions that relate cumulative seasonal exposure of varying 
magnitudes to various incremental reductions in expected tree seedling 
growth (in terms of RBL) and in expected crop yield (in terms of RYL). 
Many decades of research also recognize visible foliar injury as an 
effect of O3, although uncertainties continue to hamper 
efforts to quantitatively characterize the relationship of its 
occurrence and relative severity with O3 exposures, as 
discussed further below (and summarized in sections III.B.3.b and 
III.D.1.b above).
    Before focusing further on the key vegetation-related effects 
identified above, the Administrator first considers the strong evidence 
documenting tropospheric O3 as a greenhouse gas causally 
related to radiative forcing, and likely causally related to subsequent 
effects on variables such as temperature and precipitation. In so 
doing, he takes note of the limitations and uncertainties in the 
evidence base that affect characterization of the extent of any 
relationships between O3 concentrations in ambient air in 
the U.S. and climate-related effects, and preclude quantitative 
characterization of climate responses to changes in O3 
concentrations in ambient air at regional (vs global) scales, as 
summarized in sections III.D.1 and II.B.3 above. As a result, he 
recognizes the lack of important quantitative tools with which to 
consider such effects in this context such that it is not feasible to 
relate different patterns of O3 concentrations at the 
regional scale in the U.S. with specific risks of alterations in 
temperature, precipitation and other climate-related variables. The 
resulting uncertainty leads the Administrator to conclude that, with 
respect to radiative forcing and related effects, there is insufficient 
information available in the current review to judge the existing 
standard inadequate or to identify an appropriate revision.
    The Administrator turns next to consideration of visible foliar 
injury. In so doing, he considers both the conclusions of the ISA and 
the examination and analysis in the PA of the currently available 
information as to what it indicates and supports with regard to 
adequacy of protection provided by the current standard, as summarized 
in section III.D.1 above. As an initial matter, he takes note of the 
long-standing documentation of visible foliar injury as an effect of 
O3 in ambient air under certain conditions. Further, as 
summarized in section III.B.2 above, the public welfare significance of 
visible foliar injury of vegetation in areas not closely managed for 
harvest, particularly specially protected natural areas, has generally 
been considered in the context of potential effects on aesthetic and 
recreational values, such as the aesthetic value of scenic vistas in 
protected natural areas such as national parks and wilderness areas 
(e.g., 73 FR 16496, March 27, 2008). Based on these considerations, the 
Administrator recognizes that, depending on its severity and spatial 
extent, as well as the location(s) and the associated intended use, the 
impact of visible foliar injury on the physical appearance of plants 
has the potential to be significant to the public welfare. In this 
regard, he notes the PA statement that cases of widespread and 
relatively severe injury during the growing season (particularly when 
sustained across multiple years and accompanied by obvious impacts on 
the plant canopy) might reasonably be expected to have the potential to 
adversely impact the public welfare in scenic and/or recreational 
areas, particularly in areas with special protection, such as Class I 
areas, summarized in section III.D.1 above (PA, sections 4.3.2 and 
4.5.1). Thus, he considers the PA evaluation of the currently available 
information with regard to the potential for such an occurrence with 
air quality conditions that meet the current standard.
    In considering the PA evaluations, the Administrator takes note of 
the PA observation that important uncertainties remain in the 
understanding of the O3 exposure conditions that will elicit 
visible foliar injury of varying severity and extent in natural areas, 
and particularly in light of the other environmental variables that 
influence its occurrence, as summarized in sections III.B.3 and III.D.1 
above. In so doing, he notes the recognition by the CASAC that 
``uncertainties continue to hamper efforts to quantitatively 
characterize the relationship of [visible foliar injury] occurrence and 
relative severity with ozone exposures,'' as summarized in section 
III.D.2 above.
    Notwithstanding, and while being mindful of, such uncertainties 
with regard to predictive O3 metric or metrics and a 
quantitative function relating them to incidence and severity of 
visible foliar injury in natural areas, as well as interpretation of 
such incidence and severity in the context of considering protection 
from such impacts that might reasonably be considered adverse to the 
public welfare, the Administrator takes note of several findings of the 
PA. First, he notes that the evidence for visible foliar injury, as 
well as analyses of data for USFS biosites (sites with O3-
sensitive vegetation assessed for visible foliar injury) indicate there 
to be associations with cumulative exposure metrics (e.g., SUM06 or 
W126 index), such metrics do not completely explain the occurrence and 
severity of injury. Although the availability of detailed analyses that 
have explored multiple exposure metrics and other influential variables 
is limited, multiple studies also have indicated a potential role for 
an additional metric related to the occurrence of days with relatively 
high concentrations (e.g., number of days with a 1-hour concentration 
at or above 100 ppb), as summarized in section III.B.3 above (PA, 
section 4.5.1.2).
    The Administrator also notes the PA observation that publications 
related to the evidence base for the USFS biosite monitoring program 
document reductions in the incidence of the higher BI scores over the 
16-year period of the program (1994 through 2010), especially after 
2002, leading to researcher conclusions of a ``declining risk of 
probable impact'' on the monitored forests over this period (e.g., 
Smith, 2012). The PA observes that these reductions parallel the 
O3 concentration trend information nationwide that shows 
clear reductions in cumulative seasonal exposures, as well as in peak 
O3 concentrations such as the annual fourth highest daily 
maximum 8-hour concentration, from 2000 through 2018 (PA, Figure 2-11 
and Appendix 4D, Figure 4D-9). These USFS BI score reductions also 
parallel reductions in the occurrence of 1-hour concentrations above 
100 ppb (PA, Appendix 2A, Tables 2A-2 to 2A-4). Thus, the extensive 
evidence of trends across the past nearly 20 years indicate reductions 
in severity of visible foliar injury in addition to reductions in peak 
concentrations that some studies have suggested to be influential in 
the severity of visible foliar injury, as summarized in section III.D.1 
above (PA, section 4.5.1).
    The Administrator additionally takes note of the PA recognition of 
a paucity of established approaches for

[[Page 49909]]

interpreting specific levels of severity and extent of foliar injury in 
protected forests with regard to impacts on public welfare effects, 
e.g., related to recreational services. The PA notes that injury to 
whole stands of trees of a severity apparent to the casual observer 
(e.g., when viewed as a whole from a distance) would reasonably be 
expected to affect recreational values. However, the available 
information does not provide for specific characterization of the 
incidence and severity that would not be expected to have such an 
impact, nor for clear identification of the pattern of O3 
concentrations that would provide for such a situation. In this 
context, the Administrator notes the PA description of the scheme 
developed by the USFS to categorize biosite scores of injury in natural 
vegetated areas by severity levels (as summarized in section III.B.2 
above). He notes the USFS description of scores above 15 as ``moderate 
to severe,'' as well as the USFS categorization of lower scores, such 
as those from zero to just below 5, which are described as ``little to 
no foliar injury'' and 5 to just below 10 as ``light to moderate.'' In 
so doing, he recognizes the PA consideration of such lower scores as 
being unlikely to be indicative of injury of such a magnitude or extent 
that would reasonably be considered significant risks to the public 
welfare. In light of these considerations, the Administrator takes note 
of the PA finding that quantitative analyses and evidence are lacking 
that might support a more precise conclusion with regard to a magnitude 
of BI score coupled with an extent of occurrence that might be 
specifically identified as adverse to the public welfare, but that the 
lower categories of BI scores are indicative of injury of generally 
lesser risk to the natural area or to public enjoyment. The 
Administrator also takes note of the D.C. Circuit's holding that 
substantial uncertainty about the level at which visible foliar injury 
may become adverse to public welfare does not necessarily provide a 
basis for declining to evaluate whether the existing standard provides 
requisite protection against such effects. See Murray Energy Corp. v. 
EPA, 936 F.3d 597, 619-20 (D.C. Cir. 2019). Consequently, he proposes 
to judge that occurrence of the lower categories of BI scores does not 
pose concern for the public welfare, but that findings of BI scores 
categorized as ``moderate to severe'' injury by the USFS scheme would 
be an indication of visible foliar injury occurrence that, depending on 
extent and severity, may raise public welfare concerns.
    With regard to the PA presentations of the USFS data combined with 
W126 estimates and soil moisture categories, summarized in section 
III.B.3 above, the Administrator takes note of the PA finding that the 
incidence of nonzero BI scores, and, particularly of relatively higher 
scores (such as scores above 15 which are indicative of ``moderate to 
severe'' injury in the USFS scheme) appears to markedly increase only 
with W126 index values above 25 ppm-hrs, as summarized in section 
III.B.3.b above (PA, section 4.3.3 and Appendix 4C). In so doing, he 
notes that such a magnitude of W126 index (either as a 3-year average 
or in a single year) is not seen to occur at monitoring locations 
(including in or near Class I areas) where the current standard is met, 
and that values above 17 or 19 ppm-hrs are rare, as summarized in 
section III.D.1.c above (PA, Appendix 4C, section 4C.3; Appendix 4D, 
section 4D.3.2.3). Further, the Administrator takes note of the PA 
consideration of the USFS publications that identify an influence of 
peak concentrations on BI scores (beyond an influence of cumulative 
exposure) and the PA observation of the appreciable control of peak 
concentrations exerted by the form and averaging time of the current 
standard, as evidenced by the air quality analyses which document 
reductions in 1-hour daily maximum concentrations with declining design 
values. For example, the PA finds the average number of 1-hour daily 
maximum concentrations across monitored sites to be some 40 times lower 
for sites meeting the current standards compared to sites that do not, 
as summarized in section III.D.1 above. Based on these considerations, 
the Administrator agrees with the PA finding that the current standard 
provides control of air quality conditions that contribute to increased 
BI scores and to scores of a magnitude indicative of ``moderate to 
severe'' foliar injury.
    The Administrator further takes note of the PA finding that the 
current information, particularly in locations meeting the current 
standard or with W126 index estimates likely to occur under the current 
standard, does not indicate a significant extent and degree of injury 
(e.g., based on analyses of BI scores in the PA, Appendix 4C) or 
specific impacts on recreational or related services for areas, such as 
wilderness areas or national parks. Thus, he gives credence to the 
associated PA conclusion that the evidence indicates that areas that 
meet the current standard are unlikely to have BI scores reasonably 
considered to be impacts of public welfare significance. Based on all 
of the considerations raised here, the Administrator proposes to 
conclude that the current standard provides sufficient protection of 
natural areas, including particularly protected areas such as Class I 
areas, from O3 concentrations in the ambient air that might 
be expected to elicit visible foliar injury of such an incidence and 
severity as would reasonably be judged adverse to the public welfare.
    In turning to consideration of the remaining array of vegetation-
related effects, the Administrator first takes note of uncertainties in 
the details and quantitative aspects of relationships between plant-
level effects such as growth and reproduction, and ecosystem impacts, 
the occurrence of which are influenced by many other ecosystem 
characteristics and processes. These examples illustrate the role of 
public welfare policy judgments, both with regard to the extent of 
protection that is requisite and concerning the weighing of 
uncertainties and limitations of the underlying evidence base and 
associated quantitative analyses. The Administrator notes that such 
judgments will inform his decision in the current review, as is common 
in NAAQS reviews. Public welfare policy judgments play an important 
role in each review of a secondary standard, just as public health 
policy judgments have important roles in primary standard reviews. One 
type of public welfare policy judgment focuses on how to consider the 
nature and magnitude of the array of uncertainties that are inherent in 
the scientific evidence and analyses. These judgments are traditionally 
made with a recognition that current understanding of the relationships 
between the presence of a pollutant in ambient air and associated 
welfare effects is based on a broad body of information encompassing 
not only more established aspects of the evidence but also aspects in 
which there may be substantial uncertainty. This may be true even of 
the most robust aspect of the evidence base. In the case of the 
secondary O3 standard review, as an example, while 
recognizing the strength of the established and well-founded E-R 
functions in predicting the relationship of O3 in terms of 
the W126 index cumulative exposure metric across a wide array of 
exposure levels, the Administrator additionally recognizes increased 
uncertainty, and associated imprecision or inexactitude in application 
of the E-R functions with lower cumulative exposures, and in the 
current understanding of aspects of

[[Page 49910]]

relationships of such estimated effects with larger-scale impacts, such 
as those on populations, communities and ecosystems, as discussed in 
the PA and summarized in sections III.D.1 above.
    The Administrator now turns to the welfare effects of reduced plant 
growth or yield. In so doing, he takes note of the well-established E-R 
functions for seedlings of 11 tree species that relate cumulative 
seasonal O3 exposures of varying magnitudes to various 
incremental reductions in expected tree seedling growth (in terms of 
RBL) and in expected crop yield, that have been recognized across 
multiple O3 NAAQS reviews. In so doing, he additionally 
takes note of uncertainties recognized in the PA, as summarized in 
section III.D.1.a above, that include the limited information that can 
address the extent to which the E-R functions for tree seedlings 
reflect growth impacts in mature trees, and the fact that the 11 
species represent a very small portion of the tree species across the 
U.S. (PA, sections 4.3.4 and 4.5.3). While recognizing these and other 
uncertainties, RBL estimates based on the median of the 11 species were 
used as a surrogate in the last review for comparable information on 
other species and lifestages, as well as a proxy or surrogate for other 
vegetation-related effects, including larger-scale effects. The 
Administrator takes note of the PA conclusion and CASAC advice that use 
of this approach continues to appear to be a reasonable judgment in 
this review (PA, section 4.5.3). More specifically, the PA concludes 
that the currently available information continues to support (and does 
not call into question) the use of RBL as a useful and evidence-based 
approach for consideration of the extent of protection from the broad 
array of vegetation-related effects associated with O3 in 
ambient air, as summarized in section III.D.1.b above. The 
Administrator also takes note of the PA conclusions that the currently 
available evidence, while somewhat expanded since the last review does 
not indicate an alternative metric for such a use; nor is an 
alternative approach evident. He further notes the CASAC concurrence 
that the current evidence continues to support this approach, as 
summarized in section III.D.2 above. Thus, he finds it appropriate to 
adopt this approach in the current review.
    With regard to the use of RBL and the median RBL estimate based on 
the established E-R functions for 11 species of tree seedlings, the 
Administrator takes note of considerations in the PA. For example, 
while the E-R functions for the 11 species have been derived in terms 
of a seasonal W126 index, the experiments from which they were derived 
vary in duration from less than three months to many more, such that, 
the adjustment to a 3-month season duration, with its underlying 
simplifying assumptions of uniform W126 distribution over the exposure 
period and relationship between duration and response, contributes some 
imprecision or inexactitude to the resulting functions and estimates 
derived using it, as discussed in section III.D.1.b above. 
Additionally, there is greater uncertainty with regard to estimated RBL 
at lower cumulative exposure levels, as the exposure levels represented 
in the data underlying the E-R functions are somewhat limited with 
regard to the relatively lower cumulative exposure levels, such as 
those most commonly associated with the current standard (e.g., at or 
below 17 ppm-hrs). Further, he notes the PA observation that some of 
the underlying studies did not find statistically significant effects 
of O3 at the lower exposure levels, indicating some 
uncertainty in predictions of an O3-related RBL at those 
levels. With these considerations regarding the E-R functions and their 
underlying datasets in mind, he also takes note of variability 
associated with tree growth in the natural environment (e.g., related 
to variability in plant, soil, meteorological and other factors), as 
well as variability associated with plant responses to O3 
exposures in the natural environment, as summarized in section III.D.1 
above. The Administrator also considers the issues discussed in the 
court's remand of the 2015 secondary standard with respect to use of a 
3-year average. See Murray Energy Corp. v. EPA, 936 F.3d at 617-18. In 
light of these considerations, the Administrator considers whether 
aspects of this evidence support making judgments using the E-R 
functions with W126 index derived as an average across multiple years. 
The Administrator notes that such averaging would have some conceptual 
similarity to the assumptions underlying the adjustment made to develop 
seasonal W126 E-R functions from exposures that extended over multiple 
seasons (or less than a single). Such averaging, with its reduction of 
the influence of annual variations in seasonal W126, would give less 
influence to RBL estimates derived from such potentially variable 
representations of W126, thus providing an estimate of W126 more 
suitably paired with the E-R functions. The Administrator additionally 
takes note of the PA summary of comparisons performed in the 2013 ISA 
and current ISA of RBL estimates based on either cumulative average 
multi-year W126 index or single-year W126 with estimates derived from 
information in a multi-year O3 exposure study, summarized in 
section III.D.1.b(ii) above (PA, section 4.5.1 and Appendix 4A, section 
4A.3.1). He notes the PA finding that these comparisons illustrate the 
variability inherent in the magnitude of growth impacts of 
O3 and in the quantitative relationship of O3 
exposure and RBL, while also providing general agreement of predictions 
(based on either metric) with observations. The Administrator finds 
these considerations particularly informative in considering the 
evidence with regard to the appropriateness of a focus on a multi-year 
(e.g., 3-year) average seasonal W126 index in assessing protection 
using RBL as a proxy or surrogate of the broader array of effects to 
obscure cumulative seasonal exposures of concern, a point discussed by 
the court in its 2019 remand of the 2015 secondary standard to EPA 
(Murray Energy Corp. v. EPA, 936 F.3d at 617-18).
    In light of the above considerations, the Administrator agrees with 
the PA finding that such factors as those identified here (also 
summarized in section III.D.1.b(ii) above), and discussed in the PA 
(PA, sections 4.5.1.2 and 4.5.3), including the currently available 
evidence and its recognized limitations, variability and uncertainties, 
contribute uncertainty and resulting imprecision or inexactitude to RBL 
estimates of single-year seasonal W126 index values, thus supporting a 
conclusion that it is reasonable to use a seasonal RBL averaged over 
multiple years, such as a 3-year average. The Administrator 
additionally takes note of the CASAC advice reaffirming the EPA's focus 
on a 3-year average W126, concluding such a focus to be reasonable and 
scientifically sound, as summarized in section III.D.2 above. In light 
of these considerations, the Administrator finds there to be support 
for use of an average seasonal W126 index derived from multiple years 
(with their representation of variability in environmental factors), 
concluding the use of such averaging to provide an appropriate 
representation of the evidence and attention to considerations 
summarized above. In so doing, he finds that a reliance on single year 
W126 estimates for reaching judgments with regard to magnitude of 
O3 related RBL and associated judgments of public welfare 
protection would ascribe a greater specificity and certainty to such 
estimates than supported by the current

[[Page 49911]]

evidence. Thus, the Administrator proposes to conclude that it is 
appropriate to use a seasonal W126 averaged over a 3-year period, which 
is the design value period for the current standard, to estimate median 
RBL using the established E-R functions for purposes in this review of 
considering the public welfare protection provided by the standard.
    Thus, the Administrator recognizes a number of public welfare 
policy judgments important to his review of the current standard. Those 
judgments include adoption of the median tree seedling RBL estimate for 
the studied species as a surrogate for the broad array of vegetation 
related effects that extend to the ecosystem scale, and identification 
of cumulative seasonal exposures (in terms of the average W126 index 
across the 3-year design period for the standard) for assessing 
O3 concentrations in areas that meet the standard with 
regard to the extent of protection afforded by the standard. In 
reflecting on these judgments, the current evidence presented in the 
ISA and the associated evaluations in the PA, the Administrator 
proposes to conclude that the currently available information supports 
such judgments, additionally noting the CASAC concurrence with regard 
to the scientific support for these judgments (Cox 2020, p. 21). 
Accordingly, the Administrator proposes to conclude that the current 
evidence base and available information (qualitative and quantitative) 
continues to support consideration of the potential for O3-
related vegetation impacts in terms of the RBL estimates from 
established E-R functions as a quantitative tool within a larger 
framework of considerations pertaining to the public welfare 
significance of O3 effects. Such consideration includes 
effects that are associated with effects on vegetation, and 
particularly those that conceptually relate to growth, and that are 
causally or likely causally related to O3 in ambient air, 
yet for which there are greater uncertainties affecting estimates of 
impacts on public welfare. The Administrator additionally notes that 
this approach to weighing the available information in reaching 
judgments regarding the secondary standard additionally takes into 
account uncertainties regarding the magnitude of growth impact that 
might be expected in mature trees, and of related, broader, ecosystem-
level effects for which the available tools for quantitative estimates 
are more uncertain and those for which the policy foundation for 
consideration of public welfare impacts is less well established.
    In his consideration of the adequacy of protection provided by the 
current standard, the Administrator also notes judgments of the prior 
Administrator in considering the public welfare significance of small 
magnitude estimates of RBL and associated unquantified potential for 
larger-scale related effects. As with visible foliar injury, the 
Administrator does not consider every possible instance of an effect on 
vegetation growth from O3 to be adverse to public welfare, 
although he recognizes that, depending on factors including extent and 
severity, such vegetation-related effects have the potential to be 
adverse to public welfare. In this context, the Administrator notes 
that the 2015 decision set the standard with an ``underlying objective 
of a revised secondary standard that would limit cumulative exposures 
in nearly all instances to those for which the median RBL estimate 
would be somewhat lower than 6%'' (80 FR 65407, October 26, 2015). With 
this objective, the prior Administrator did not additionally find that 
a cumulative seasonal exposure, for which such a magnitude of median 
species RBL was estimated, represented conditions that were adverse to 
the public welfare. Rather, the 2015 decision noted that ``the 
Administrator does not judge RBL estimates associated with marginal 
higher exposures [at or above 19 ppm-hrs] in isolated, rare instances 
to be indicative of adverse effects to the public welfare'' (80 FR 
65407, October 26, 2015). Comments from the current CASAC, in the 
context of its review of the draft PA, expressed the view that the 
strategy described by the prior Administrator for the secondary 
standard established in 2015 with its W126 index target of 17 ppm-hrs 
(in terms of a 3-year average), at or below which the 2015 standard was 
expected to generally restrict cumulative seasonal exposure, is ``still 
effective in particularly protecting the public welfare in light of 
vegetation impacts form ozone'' (Cox, 2020, p. 21). In light of this 
advice and based on the current evidence as evaluated in the PA, the 
Administrator proposes to conclude that this approach or framework, 
with its focus on controlling air quality such that cumulative 
exposures at or above 19 ppm-hrs, in terms of a 3-year average W126 
index, are isolated and rare, is appropriate for a secondary standard 
that provides the requisite public welfare protection and proposes to 
use such an approach in this review.
    With this approach and protection target in mind, the Administrator 
further considers the analyses available in this review of recent air 
quality at sites across the U.S., particularly including those sites in 
or near Class I areas, and also the analyses of historical air quality. 
In so doing, the Administrator recognizes that these analyses are 
distributed across all nine NOAA climate regions and 50 states, 
although some geographic areas within specific regions and states may 
be more densely covered and represented by monitors than others, as 
summarized in section III.C above. The Administrator notes that the 
findings from both the analysis of the air quality data from the most 
recent period and from the larger analysis of historical air quality 
data extending back to 2000, as presented in the PA and summarized in 
section III.C above, are consistent with the air quality analyses 
available in the last review. That is, in virtually all design value 
periods and all locations at which the current standard was met across 
the 19 years and 17 design value periods (in more than 99.9% of such 
observations), the 3-year average W126 metric was at or below 17 ppm-
hrs. Further, in all such design value periods and locations the 3-year 
average W126 index was at or below 19 ppm-hrs. The Administrator 
additionally considers the protection provided by the current standard 
from the occurrence of O3 exposures within a single year 
with potentially damaging consequences, such as a significantly 
increased incidence of areas with visible foliar injury that might be 
judged moderate to severe. In so doing, he takes notes of the PA 
analyses, summarized in section III.D.1 above, of USFS BI scores, 
giving particular focus to scores above 15 (termed ``moderate to severe 
injury'' by the USFS categorization scheme). He notes the PA finding 
that incidence of sites with BI scores above 15 markedly increases with 
W126 index estimates above 25 ppm-hrs. In this context, he additionally 
takes note of the air quality analysis finding of a scarcity of single-
year W126 index values above 25 ppm-hrs at sites that meet the current 
standard, with just a single occurrence across all U.S. sites with 
design values meeting the current standard in the 19-year historical 
dataset dating back to 2000 (PA, section 4.4 and Appendix 4D). Further, 
in light of the evidence indicating that peak short-term concentrations 
(e.g., of durations as short as one hour) may also play a role in the 
occurrence of visible foliar injury, the Administrator additionally 
takes note of the PA presentation of air quality data over the past 20 
years, as summarized in section III.D.1 above, that shows a declining 
trend in 1-hour daily maximum concentrations

[[Page 49912]]

mirroring the declining trend in design values, and the associated PA 
conclusion that the form and averaging time of the current standard 
provides appreciable control of peak 1-hour concentrations. As further 
evidence of the level of control exerted, the PA notes there to be less 
than one day per site, on average (among sites meeting the current 
standard), with a maximum hourly concentration at or above 100 ppb, 
compared to roughly 40 times as many such days, on average, for sites 
with design values above the current standard level (PA, Appendix 2A, 
section 2A.2). In light of these findings from the air quality analyses 
and considerations in the PA, summarized in section III.D.1 above, both 
with regard to 3-year average W126 index values at sites meeting the 
current standard and the rarity of such values at or above 19 ppm-hrs, 
and with regard to single-year W126 index values at sites meeting the 
current standard, and the rarity of such values above 25 ppm-hrs, as 
well as with regard to the appreciable control of 1-hour daily maximum 
concentrations, the Administrator proposes to judge that the current 
standard provides adequate protection from air quality conditions with 
the potential to be adverse to the public welfare.
    In reaching his proposed conclusion on the current secondary 
O3 standard, the Administrator recognizes, as is the case in 
NAAQS reviews in general, his decision depends on a variety of factors, 
including science policy judgments and public welfare policy judgments, 
as well as the currently available information. With regard to the 
current review, the Administrator gives primary attention to the 
principal effects of O3 as recognized in the current ISA, 
the 2013 ISA and past AQCDs, and for which the evidence is strongest 
(e.g., growth, reproduction, and related larger-scale effects, as well 
as, visible foliar injury). As discussed above, the Administrator notes 
that the currently available information on visible foliar injury and 
with regard to air quality analyses that may be informative with regard 
to air quality conditions associated with appreciably increased 
incidence and severity of BI scores at USFS biomonitoring sites 
indicates a sufficient degree of protection from such conditions. 
Further, the currently available evidence for natural areas across the 
U.S., such as studies of USFS biosites, does not indicate widespread 
incidence of significant visible foliar injury, and analyses of USFS 
biosite scores in the PA do not indicate marked increases in scores 
categorized by the USFS as ``moderate'' or ``severe'' for W126 index 
values generally occurring at sites that meet the current standard. The 
Administrator finds this information does not indicate a potential for 
public welfare impacts of concern under air quality conditions that 
meet the current standard. In light of these and other considerations 
discussed more completely above, and with particular attention to Class 
I and other areas afforded special protection, the Administrator 
proposes to conclude that the evidence regarding visible foliar injury 
and air quality in areas meeting the current standard indicates that 
the current standard provides adequate protection for this effect.
    The Administrator additionally considers O3 effects on 
crop yield. In so doing, he takes note of the long-standing evidence, 
qualitative and quantitative, of the reducing effect of O3 
on the yield of many crops, as summarized in the PA and current ISA and 
characterized in detail in past reviews (e.g., 2013 ISA, 2006 AQCD, 
1997 AQCD, 2014 WREA). He additionally notes the established E-R 
functions for 10 crops and the estimates of RYL derived from them, as 
presented in the PA (PA, Appendix 4A, section 4A.1, Table 4A-4), and 
the potential public welfare significance of reductions in crop yield, 
as summarized in section III.B.2 above. However, he additionally 
recognizes that not every effect on crop yield will be adverse to 
public welfare and in the case of crops in particular there are a 
number of complexities related to the heavy management of many crops to 
obtain a particular output for commercial purposes, and related to 
other factors, that contribute uncertainty to predictions of potential 
O3-related public welfare impacts, as summarized in sections 
III.B.2 and III.D.1 above (PA, sections 4.5.1.3 and 4.5.3). Thus, in 
judging the extent to which the median RYL estimated for the W126 index 
values generally occurring in areas meeting the current standard would 
be expected to be of public welfare significance, he recognizes the 
potential for a much larger influence of extensive management of such 
crops, and also considers other factors recognized in the PA and 
summarized in section III.D.1 above, including similarities in median 
estimates of RYL and RBL (PA, sections 4.5.1.3 and 4.5.3). With this in 
mind, the Administrator does not find that the information for crop 
yield effects leads him to identify this endpoint as requiring separate 
consideration or to provide a more appropriate focus for the standard 
than RBL, in its role as a proxy or surrogate for the broader array of 
vegetation-related effects, as discussed above. Rather, in light of 
these considerations, he proposes to judge that a decision based on RBL 
as a proxy for other vegetation-related effects will provide adequate 
protection against crop related effects. In light of the current 
information and considerations discussed more completely above, the 
Administrator further proposes to conclude that the evidence regarding 
RBL, and its use as a proxy or surrogate for the broader array of 
vegetation-related effects, in combination with air quality in areas 
meeting the current standard, provide adequate protection for these 
effects.
    In reaching his proposed conclusion on the current standard, the 
Administrator also considers the extent to which the current 
information may provide support for an alternative standard. In so 
doing, he notes the longstanding evidence documenting the array of 
welfare effects associated with O3 in ambient air, as 
summarized in section III.B.1 above. He additionally recognizes the 
robust quantitative evidence for growth-related effects and the E-R 
functions for RBL, which he considers as a proxy for the broader array 
of effects in reaching his proposed decision. He takes note of the air 
quality analyses that show an appreciably greater occurrence of higher 
levels of cumulative exposure, in terms of the W126 index, as well as 
an appreciably greater occurrence of peak concentrations (both hourly 
and 8-hour average concentrations) in areas that do not meet the 
current standard, as summarized in section III.C above for areas with 
design values above 70 ppb. He proposes to conclude that such 
occurrences contribute to air quality conditions that would not provide 
the appropriate protection of public welfare in light of the potential 
for adverse effects on the public welfare.
    Further, the Administrator recognizes that public comments thus far 
in this review have suggested that an alternative standard, such as one 
based solely on the W126 metric, is required to provide adequate 
protection of the public welfare. Such a point was raised in the 
litigation challenging the 2015 secondary standard, although the court 
did not resolve this issue in its decision. In considering this issue, 
the Administrator recognizes that, as summarized in section III.B.3.a 
above, concentration-weighted, cumulative exposure metrics, including 
the W126 index, have been identified as quantifying exposure in a way 
that relates to reduced plant growth (ISA, Appendix 8, section 8.13.1). 
The W126 index is the metric used with the 11

[[Page 49913]]

established E-R functions discussed above, which provide estimates of 
RBL that the Administrator considers appropriately used as a proxy or 
surrogate for the broader array of vegetation-related effects. The 
Administrator additionally notes, however, that the evidence indicates 
there to be aspects of O3 air quality not captured by 
measures of cumulative exposure, such as W126 index, that may pose a 
risk of harm to the public welfare. For example, as discussed above, 
the current evidence indicates a role for peak concentrations in the 
occurrence of visible foliar injury. With this in mind, the 
Administrator notes that an ambient air quality standard established in 
terms of the W126 index, while giving greater weight to generally 
higher concentrations, would not explicitly limit the occurrence of 
hourly concentrations at or above specific magnitudes. For example, two 
records of air quality may have the same W126 index while differing 
appreciably in patterns of hourly concentrations, including in the 
frequency of occurrence of peak concentrations (e.g., number of hours 
above 100 ppb). The Administrator notes, however, as discussed above, 
that the current standard, with its 8-hour averaging time and fourth-
highest daily maximum form (averaged over three years), can provide 
control of both peak concentrations and concentration-weighted 
cumulative exposures, as illustrated by the substantially limited 
occurrence of hourly concentrations of magnitudes at or above 100 ppb 
and of cumulative exposures at or above 19 ppm-hrs in areas that meet 
the current standard (PA, section 2.4.5, Appendix 2A, section 2A.2 and 
Appendix 4D). Thus, in light of the information available in this 
review, summarized in the sections above and including that related to 
a role of peak concentrations in posing risk of visible foliar injury 
to sensitive vegetation, the Administrator proposes to conclude that 
such an alternative standard in terms of a W126 index would be less 
likely to provide sufficient protection against such occurrences and 
accordingly would not provide the requisite control of aspects of air 
quality that pose risk to the public welfare. As indicated above, he 
proposes to judge that the current information indicates that the 
requisite control of such aspects of air quality is provided by the 
current standard.
    In summary, the Administrator recognizes that his proposed decision 
on the public welfare protection afforded by the secondary 
O3 standard from identified O3-related welfare 
effects, and from their potential to present adverse effects to the 
public welfare, is based in part on judgments regarding uncertainties 
and limitations in the available information, such as those identified 
above. In this context, he has considered what the available evidence 
and quantitative information indicate with regard to the protection 
provided from the array of O3 welfare effects. He finds that 
the information, as summarized above, and presented in detail in the 
ISA and PA, does not indicate the current standard to allow air quality 
conditions with implications of concern for the public welfare. He 
additionally takes note of the advice from the CASAC in this review, 
including its finding ``that the available evidence does not reasonably 
call into question the adequacy of the current secondary ozone standard 
and concurs that it should be retained'' (Cox, 2020a, p. 1). Based on 
all of the above considerations, including his consideration of the 
currently available evidence and quantitative exposure/risk 
information, the Administrator proposes to conclude that the current 
secondary standard provides the requisite protection against known or 
anticipated effects to the public welfare, and thus that the current 
standard should be retained, without revision. The Administrator 
solicits comment on this proposed conclusion.
    Having reached the proposed decision described here based on 
interpretation of the welfare effects evidence, as assessed in the ISA, 
and the quantitative analyses presented in the PA; the evaluation of 
policy-relevant aspects of the evidence and quantitative analyses in 
the PA; the advice and recommendations from the CASAC; public comments 
received to date in this review; and the public welfare policy 
judgments described above, the Administrator recognizes that other 
interpretations, assessments and judgments might be possible. 
Therefore, the Administrator solicits comment on the array of issues 
associated with review of this standard, including public welfare and 
science policy judgments inherent in the proposed decision, as 
described above, and the rationales upon which such views are based.

IV. Statutory and Executive Order Reviews

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

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

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

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

    This action is not expected to be an Executive Order 13771 
regulatory action. There are no quantified cost estimates for this 
proposed action because EPA is proposing to retain the current 
standards.

C. Paperwork Reduction Act (PRA)

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

D. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
action proposes to retain, without revision, existing national 
standards for allowable concentrations of O3 in ambient air 
as required by section 109 of the CAA. See also American Trucking 
Associations v. EPA, 175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do 
not have significant impacts upon small entities because NAAQS 
themselves impose no regulations upon small entities), rev'd in part on 
other grounds, Whitman v. American Trucking Associations, 531 U.S. 457 
(2001).

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely 
affect small governments. This action imposes no

[[Page 49914]]

enforceable duty on any state, local, or tribal governments or the 
private sector.

F. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have 
substantial direct effects on the states, on the relationship between 
the national government and the states, or on the distribution of power 
and responsibilities among the various levels of government.

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

    This action does not have tribal implications, as specified in 
Executive Order 13175. It does not have a substantial direct effect on 
one or more Indian Tribes. This action does not change existing 
regulations; it proposes to retain the current O3 NAAQS, 
without revision. Executive Order 13175 does not apply to this action.

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

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

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

    This action is not subject to Executive Order 13211, because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this document is to 
propose to retain the current O3 NAAQS. This proposal does 
not change existing requirements. Thus, the EPA concludes that this 
proposal does not constitute a significant energy action as defined in 
Executive Order 13211.

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

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

    The EPA believes that this action does not have disproportionately 
high and adverse human health or environmental effects on minority, 
low-income populations and/or indigenous peoples, as specified in 
Executive Order 12898 (59 FR 7629, February 16, 1994). The action 
proposed in this document is to retain without revision the existing 
O3 NAAQS based on the Administrator's proposed conclusions 
that the existing primary standard protects public health, including 
the health of sensitive groups, with an adequate margin of safety, and 
that the existing secondary standard protects public welfare from known 
or anticipated adverse effects. As discussed in section II above, the 
EPA expressly considered the available information regarding health 
effects among at-risk populations in reaching the proposed decision 
that the existing standard is requisite.

L. Determination Under Section 307(d)

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

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List of Subjects in 40 CFR Part 50

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

Andrew Wheeler,
Administrator.
[FR Doc. 2020-15453 Filed 8-13-20; 8:45 am]
BILLING CODE 6560-50-P