[Federal Register Volume 85, Number 251 (Thursday, December 31, 2020)]
[Rules and Regulations]
[Pages 87256-87351]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-28871]
[[Page 87255]]
Vol. 85
Thursday,
No. 251
December 31, 2020
Part IV
Environmental Protection Agency
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40 CFR Part 50
Review of the Ozone National Ambient Air Quality Standards; Final Rule
Federal Register / Vol. 85 , No. 251 / Thursday, December 31, 2020 /
Rules and Regulations
[[Page 87256]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[EPA-HQ-OAR-2018-0279; FRL-10019-04-OAR]
RIN 2060-AU40
Review of the Ozone National Ambient Air Quality Standards
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final action.
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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review
of the air quality criteria and the national ambient air quality
standards (NAAQS) for photochemical oxidants including ozone
(O3), the EPA is retaining the current standards, without
revision.
DATES: This final action is effective December 31, 2020.
ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2018-0279. Incorporated into this docket is a
separate docket established for the Integrated Science Assessment for
this review (Docket ID No. EPA-HQ-ORD-2018-0274). All documents in
these dockets are listed on the www.regulations.gov website. Although
listed in the index, some information is not publicly available, e.g.,
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Certain other material, such as
copyrighted material, is not placed on the internet and will be
publicly available only in hard copy form. With the exception of such
material, publicly available docket materials are available
electronically through https://www.regulations.gov/. Out of an
abundance of caution for members of the public and our staff, the EPA
Docket Center and Reading Room are closed to the public, with limited
exceptions, to reduce the risk of transmitting COVID-19. Our Docket
Center staff will continue to provide remote customer service via
email, phone, and webform. For further information on EPA Docket Center
services and the current status, please visit us online at https://www.epa.gov/dockets.
Availability of Information Related to This Action
A number of the documents that are relevant to this action are
available through the EPA's website at https://www.epa.gov/naaqs/ozone-o3-air-quality-standards. These documents include the Integrated Review
Plan for the Ozone National Ambient Air Quality Standards (IRP [U.S.
EPA, 2019b]), available at https://www.epa.gov/naaqs/ozone-o3-standards-planning-documents-current-review, the Integrated Science
Assessment for Ozone and Related Photochemical Oxidants (ISA [U.S. EPA,
2020a]), available at https://www.epa.gov/naaqs/ozone-o3-standards-integrated-science-assessments-current-review, the Policy Assessment
for the Review of the Ozone National Ambient Air Quality Standards (PA
[U.S. EPA, 2020b]), available at https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-current-review. These and other related
documents are also available for inspection and copying in the EPA
docket identified above.
FOR FURTHER INFORMATION CONTACT: Dr. Deirdre Murphy, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail Code C504-06,
Research Triangle Park, NC 27711; telephone: (919) 541-0729; fax: (919)
541-0237; email: [email protected].
SUPPLEMENTARY INFORMATION:
Basis for Immediate Effective Date
In accordance with section 307(d)(1)(V), the Administrator has
designated this action as being subject to the rulemaking procedures in
section 307(d) of the Clean Air Act (CAA). Section 307(d)(1) of the CAA
states that: ``The provisions of section 553 through 557 * * * of Title
5 shall not, except as expressly provided in this subsection, apply to
actions to which this subsection applies.'' Thus, section 553(d) of the
Administrative Procedure Act (APA), which requires publication of a
substantive rule to be made ``not less than 30 days before its
effective date'' subject to limited exceptions, does not apply to this
action. In the alternative, the EPA concludes that it is consistent
with APA section 553(d) to make this action effective December 31,
2020.
Section 553(d)(3) of the APA, 5 U.S.C. 553(d)(3), provides that
final rules shall not become effective until 30 days after publication
in the Federal Register ``except . . . as otherwise provided by the
agency for good cause found and published with the rule.'' ``In
determining whether good cause exists, an agency should `balance the
necessity for immediate implementation against principles of
fundamental fairness which require that all affected persons be
afforded a reasonable amount of time to prepare for the effective date
of its ruling.'' Omnipoint Corp. v. Fed. Commc'n Comm'n, 78 F.3d 620,
630 (D.C. Cir. 1996) (quoting United States v. Gavrilovic, 551 F.2d
1099, 1105 (8th Cir. 1977)). The purpose of this provision is to ``give
affected parties a reasonable time to adjust their behavior before the
final rule takes effect.'' Id.; see also Gavrilovic, 551 F.2d at 1104
(quoting legislative history).
The EPA is determining that in light of the nature of this action,
good cause exists to make this final action effective immediately
because the Agency seeks to provide regulatory certainty as soon as
possible and the Administrator's decision to retain the current NAAQS
does not change the status quo or impose new obligations on any person
or entity. As a result, there is no need to provide parties additional
time to adjust their behavior, and no person will be harmed by making
the action immediately effective as opposed to delaying the effective
date by 30 days. Accordingly, the EPA is making this action effective
immediately upon publication.
Table of Contents
The following topics are discussed in this preamble:
Executive Summary
I. Background
A. Legislative Requirements
B. Related O3 Control Programs
C. History of the Air Quality Criteria and O3
Standards
D. Current Review of the Air Quality Criteria and O3
Standards
E. Air Quality Information
II. Rationale for Decision on the Primary Standard
A. Introduction
1. Background on the Current Standard
2. Overview of Health Effects Information
3. Overview of Exposure and Risk Information
B. Conclusions on the Primary Standard
1. Basis for the Proposed Decision
2. Comments on the Proposed Decision
3. Administrator's Conclusions
C. Decision on the Primary Standard
III. Rationale for Decision on the Secondary Standard
A. Introduction
1. Background on the Current Standard
2. Overview of Welfare Effects Information
3. Overview of Air Quality and Exposure Information
B. Conclusions on the Secondary Standard
1. Basis for the Proposed Decision
2. Comments on the Proposed Decision
3. Administrator's Conclusions
C. Decision on the Secondary Standard
IV. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs
[[Page 87257]]
C. Paperwork Reduction Act (PRA)
D. Regulatory Flexibility Act (RFA)
E. Unfunded Mandates Reform Act (UMRA)
F. Executive Order 13132: Federalism
G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
H. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
I. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
J. National Technology Transfer and Advancement Act
K. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
L. Determination Under Section 307(d)
M. Congressional Review Act
V. References
Executive Summary
This document presents the Administrator's decisions in the current
review of the primary (health-based) and secondary (welfare-based)
O3 NAAQS, to retain the current standards, without revision.
In reaching these decisions, the Administrator has considered the
currently available scientific evidence in the ISA, quantitative and
policy analyses presented in the PA, advice from the Clean Air
Scientific Advisory Committee (CASAC), and public comments on the
proposed decision. This document provides background and summarizes the
rationale for these decisions.
This review of the O3 standards, required by the Clean
Air Act (CAA) on a periodic basis, was initiated in 2018. In the last
review, completed in 2015, the EPA significantly strengthened the
primary and secondary O3 standards by revising the level of
both standards from 75 parts per billion (ppb) to 70 ppb and retaining
their indicators (O3), forms (annual fourth-highest daily
maximum, averaged across three consecutive years) and averaging times
(eight hours) (80 FR 65291, October 26, 2015). In subsequent litigation
on the 2015 decisions, the U.S. Court of Appeals for the District of
Columbia Circuit (D.C. Circuit) upheld the 2015 primary standard but
remanded the 2015 secondary standard to the EPA for further
justification or reconsideration. The court's remand of the secondary
standard has been considered in reaching the decision described in this
document on this standard, and in associated conclusions and judgments,
also described here. Accordingly, this decision incorporates the EPA's
response to the judicial remand of the 2015 secondary standard.
In this review as in past reviews of the air quality criteria and
NAAQS for O3 and related photochemical oxidants, the health
and welfare effects evidence evaluated in the ISA is focused on
O3. Ozone is the most prevalent photochemical oxidant in the
atmosphere and the one for which there is a large body of scientific
evidence on health and welfare effects. A component of smog,
O3 in ambient air is a mixture of mostly tropospheric
O3 and some stratospheric O3. Tropospheric
O3 forms in the atmosphere when emissions of precursor
pollutants, such as nitrogen oxides and volatile organic compounds
(VOCs), interact with solar radiation. Such emissions result from man-
made sources (e.g. motor vehicles and power plants) and natural sources
(e.g. vegetation and wildfires). In addition, O3 that is
created naturally in the stratosphere also mixes with tropospheric
O3 near the tropopause, and, less frequently can mix nearer
the earth's surface.
Based on the current health effects evidence and quantitative
information, as well as consideration of CASAC advice and public
comment, the Administrator concludes that the current primary standard
is requisite to protect public health, including the health of at-risk
populations, with an adequate margin of safety, and should be retained,
without revision. This decision has been informed by key aspects of the
health effects evidence newly available in this review, in conjunction
with the full body of evidence critically evaluated in the ISA, that
continues to support prior conclusions that short-term O3
exposure causes and long-term O3 exposure is likely to cause
respiratory effects. The strongest evidence continues to come from
studies of short- and long-term O3 exposure and an array of
respiratory health effects, including effects related to asthma
exacerbation in people with asthma, particularly children with asthma.
The clearest evidence comes from controlled human exposure studies,
available at the time of the last review, of individuals exposed for
6.6 hours during quasi-continuous exercise, that report an array of
respiratory responses including lung function decrements and
respiratory symptoms. Epidemiologic studies additionally describe
consistent, positive associations between O3 exposures and
hospital admissions and emergency department visits, particularly for
asthma exacerbation in children. Populations and lifestages at risk
include people with asthma, children, the elderly, and outdoor workers.
The quantitative analyses of population exposure and risk, as well as
policy considerations in the PA, summarized in this document and
described in detail in the PA, also inform the decision on the primary
standard. The general approach and methodology used for the exposure-
based assessment is similar to that used in the last review, although a
number of updates and improvements have been implemented. These include
a more recent period (2015-2017) of ambient air monitoring data in
which O3 concentrations in the areas assessed are at or near
the current standard, as well as improvements and updates to models,
model inputs and underlying databases.
In its advice to the Administrator, the CASAC stated that the newly
available health effects evidence does not differ substantially from
that available in the last review when the standard was set. Part of
CASAC concluded that the primary standard should be retained. Another
part of CASAC expressed concern regarding the margin of safety provided
by the current standard, pointing to comments from the 2014 CASAC, who
while agreeing that the evidence supported a standard level of 70 ppb,
additionally provided policy advice expressing support for a lower
standard. In summary, the current evidence and quantitative analyses,
advice from the CASAC and consideration of public comments have
informed the Administrator's judgments in reaching his decision that
the current primary standard of 70 ppb O3, as the annual
fourth-highest daily maximum 8-hour concentration averaged across three
consecutive years, provides the requisite public health protection,
with an adequate margin of safety.
Based on the current welfare effects evidence and quantitative
information, as well as consideration of CASAC advice and public
comment, the Administrator concludes that the current secondary
standard is requisite to protect the public welfare from known or
anticipated adverse effects of O3 and related photochemical
oxidants in ambient air, and should be retained, without revision. This
decision has been informed by key aspects of the welfare effects
evidence newly available in this review, in conjunction with the full
body of evidence critically evaluated in the ISA, that supports,
sharpens and expands somewhat on the conclusions reached in the last
review. The currently available evidence describes an array of
O3 effects on vegetation and related ecosystem effects, as
well as the role of O3 in radiative forcing and subsequent
climate-related effects. The ISA includes findings of causal or likely
causal
[[Page 87258]]
relationships for a number of such effects with O3 in the
ambient air. As in the last review, the strongest evidence, including
quantitative characterizations of relationships between O3
exposure and occurrence and magnitude of effects, is for vegetation
effects. The scales of these effects range from the individual plant
scale to the ecosystem scale, with potential for impacts on the public
welfare.
While the welfare effects of O3 vary widely with regard
to the extent and level of detail of the available information that
describes the exposure circumstances that may elicit them, such
information is most advanced for plant growth-related effects. For
example, the information on exposure metric and relationships for these
effects with the cumulative, concentration-weighted exposure index,
W126, is long-standing, having been first described in the 1997 review.
Utilizing this information in reviewing the public welfare protection
provided by the current secondary standard, reduced growth has been
considered as proxy or surrogate for a broad array of related
vegetation effects. Quantitative analyses of air quality and vegetation
exposure, including in terms of the W126 index, as well as policy-
relevant considerations discussed in the PA, have also informed the
Administrator's decision on the secondary standard. These include
analyses of air quality monitoring data in areas meeting the current
standard across the U.S., as well as in Class I areas, updated and
expanded from analyses conducted in the last review. Lastly, in its
advice to the Administrator on the secondary standard, the full CASAC
found the current evidence to support the current standard and
concurred with the draft PA that it should be retained without
revision. In summary, the current evidence and quantitative analyses,
advice from the CASAC and consideration of public comments have
informed the Administrator's judgments in reaching his decision that
the current secondary standard of 70 ppb O3, as the annual
fourth-highest daily maximum 8-hour concentration averaged across three
consecutive years, provides the requisite public welfare protection.
I. Background
A. Legislative Requirements
Two sections of the CAA govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify and list certain air pollutants and then to issue air quality
criteria for those pollutants. The Administrator is to list those
pollutants ``emissions of which, in his judgment, cause or contribute
to air pollution which may reasonably be anticipated to endanger public
health or welfare''; ``the presence of which in the ambient air results
from numerous or diverse mobile or stationary sources''; and for which
he ``plans to issue air quality criteria . . . .'' (42 U.S.C.
7408(a)(1)). Air quality criteria are intended to ``accurately reflect
the latest scientific knowledge useful in indicating the kind and
extent of all identifiable effects on public health or welfare which
may be expected from the presence of [a] pollutant in the ambient air .
. . .'' (42 U.S.C. 7408(a)(2)).
Section 109 (42 U.S.C. 7409) directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for
which air quality criteria are issued (42 U.S.C. 7409(a)). Section
109(b)(1) defines primary standards as ones ``the attainment and
maintenance of which in the judgment of the Administrator, based on
such criteria and allowing an adequate margin of safety, are requisite
to protect the public health.'' \1\ Under section 109(b)(2), a
secondary standard must ``specify a level of air quality the attainment
and maintenance of which, in the judgment of the Administrator, based
on such criteria, is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
[the] pollutant in the ambient air.'' \2\
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
\2\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on
welfare include, but are not limited to, ``effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife, weather,
visibility, and climate, damage to and deterioration of property,
and hazards to transportation, as well as effects on economic values
and on personal comfort and well-being.''
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In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary. In so doing, the EPA
may not consider the costs of implementing the standards. See
generally, Whitman v. American Trucking Ass'ns, 531 U.S. 457, 465-472,
475-76 (2001). Likewise, ``[a]ttainability and technological
feasibility are not relevant considerations in the promulgation of
national ambient air quality standards.'' See American Petroleum
Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord
Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). At
the same time, courts have clarified the EPA may consider ``relative
proximity to peak background . . . concentrations'' as a factor in
deciding how to revise the NAAQS in the context of considering standard
levels within the range of reasonable values supported by the air
quality criteria and judgments of the Administrator. See American
Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 (D.C. Cir. 2002), hereafter
referred to as ``ATA III.''
The requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. See Lead Industries Ass'n v. EPA, 647 F.2d 1130, 1154 (D.C.
Cir. 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186;
Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C.
Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013).
Both kinds of uncertainties are components of the risk associated with
pollution at levels below those at which human health effects can be
said to occur with reasonable scientific certainty. Thus, in selecting
primary standards that include an adequate margin of safety, the
Administrator is seeking not only to prevent pollution levels that have
been demonstrated to be harmful but also to prevent lower pollutant
levels that may pose an unacceptable risk of harm, even if the risk is
not precisely identified as to nature or degree. The CAA does not
require the Administrator to establish a primary NAAQS at a zero-risk
level or at background concentration levels (see Lead Industries Ass'n
v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351),
but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the
EPA considers such factors as the nature and severity of the health
effects involved, the size of the sensitive population(s),\3\
[[Page 87259]]
and the kind and degree of uncertainties. The selection of any
particular approach to providing an adequate margin of safety is a
policy choice left specifically to the Administrator's judgment. See
Lead Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA,
744 F.3d at 1353.
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\3\ As used here and similarly throughout this document, the
term population (or group) refers to persons having a quality or
characteristic in common, such as a specific pre-existing illness or
a specific age or life stage. As summarized in section II.A.2.c
below, the identification of sensitive groups (called at-risk groups
or at-risk populations) involves consideration of susceptibility and
vulnerability.
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Section 109(d)(1) of the Act requires periodic review and, if
appropriate, revision of existing air quality criteria to reflect
advances in scientific knowledge concerning the effects of the
pollutant on public health and welfare. Under the same provision, the
EPA is also to periodically review and, if appropriate, revise the
NAAQS, based on the revised air quality criteria.\4\
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\4\ This section of the Act requires the Administrator to
complete these reviews and make any revisions that may be
appropriate ``at five-year intervals.''
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Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate . . .'' Since the
early 1980s, this independent review function has been performed by the
CASAC of the EPA's Science Advisory Board. A number of other advisory
functions are also identified for the committee by section
109(d)(2)(C), which reads:
Such committee shall also (i) advise the Administrator of areas
in which additional knowledge is required to appraise the adequacy
and basis of existing, new, or revised national ambient air quality
standards, (ii) describe the research efforts necessary to provide
the required information, (iii) advise the Administrator on the
relative contribution to air pollution concentrations of natural as
well as anthropogenic activity, and (iv) advise the Administrator of
any adverse public health, welfare, social, economic, or energy
effects which may result from various strategies for attainment and
maintenance of such national ambient air quality standards.
As previously noted, the Supreme Court has held that section 109(b)
``unambiguously bars cost considerations from the NAAQS-setting
process,'' in Whitman v. American Trucking Ass'ns, 531 U.S. 457, 471
(2001). Accordingly, while some of the issues listed in section
109(d)(2)(C) as those on which Congress has directed the CASAC to
advise the Administrator, are ones that are relevant to the standard
setting process, others are not. Issues that are not relevant to
standard setting may be relevant to implementation of the NAAQS once
they are established.\5\
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\5\ Because some of these issues are not relevant to standard
setting, some aspects of CASAC advice may not be relevant to EPA's
process of setting primary and secondary standards that are
requisite to protect public health and welfare. Indeed, were the EPA
to consider costs of implementation when reviewing and revising the
standards ``it would be grounds for vacating the NAAQS.'' Whitman v.
American Trucking Ass'ns, 531 U.S. 457, 471 n.4 (2001). At the same
time, the CAA directs CASAC to provide advice on ``any adverse
public health, welfare, social, economic, or energy effects which
may result from various strategies for attainment and maintenance''
of the NAAQS to the Administrator under section 109(d)(2)(C)(iv). In
Whitman, the Court clarified that most of that advice would be
relevant to implementation but not standard setting, as it
``enable[s] the Administrator to assist the States in carrying out
their statutory role as primary implementers of the NAAQS'' (id. at
470 [emphasis in original]). However, the Court also noted that
CASAC's ``advice concerning certain aspects of `adverse public
health . . . effects' from various attainment strategies is
unquestionably pertinent'' to the NAAQS rulemaking record and
relevant to the standard setting process (id. at 470 n.2).
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B. Related O3 Control Programs
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under sections 110 and 171 through 185 of the CAA,
and related provisions and regulations, states are to submit, for the
EPA's approval, state implementation plans (SIPs) that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved. The states, in
conjunction with the EPA, also administer the prevention of significant
deterioration of air quality program that covers these pollutants. See
42 U.S.C. 7470-7479. In addition, federal programs provide for
nationwide reductions in emissions of O3 precursors and
other air pollutants under Title II of the Act, 42 U.S.C. 7521-7574,
which involves controls for automobile, truck, bus, motorcycle, nonroad
engine and equipment, and aircraft emissions; the new source
performance standards under section 111 of the Act, 42 U.S.C. 7411; and
the national emissions standards for hazardous air pollutants under
section 112 of the Act, 42 U.S.C. 7412.
C. History of the Air Quality Criteria and Standards
Primary and secondary NAAQS were first established for
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971) based on
the air quality criteria developed in 1970 (U.S. DHEW, 1970; 35 FR
4768, March 19, 1970). The EPA set both primary and secondary standards
at 0.08 parts per million (ppm), as a 1-hour average of total
photochemical oxidants, not to be exceeded more than one hour per year.
Since that time, the EPA has reviewed the air quality criteria and
standards a number of times, with the most recent review being
completed in 2015.
The EPA initiated the first periodic review of the NAAQS for
photochemical oxidants in 1977. Based on the 1978 air quality criteria
document (AQCD [U.S. EPA, 1978]), the EPA proposed revisions to the
original NAAQS in 1978 (43 FR 26962, June 22, 1978) and adopted
revisions in 1979 (44 FR 8202, February 8, 1979). At that time, the EPA
changed the indicator from photochemical oxidants to O3,
revised the level of the primary and secondary standards from 0.08 to
0.12 ppm and revised the form of both standards from a deterministic
(i.e., not to be exceeded more than one hour per year) to a statistical
form. With these changes, attainment of the standards was defined to
occur when the average number of days per calendar year (across a 3-
year period) with maximum hourly average O3 concentration
greater than 0.12 ppm equaled one or less (44 FR 8202, February 8,
1979; 43 FR 26962, June 22, 1978). Several petitioners challenged the
1979 decision. Among those, one claimed natural O3
concentrations and other physical phenomena made the standard
unattainable in the Houston area.\6\ The U.S. Court of Appeals for the
District of Columbia Circuit (D.C. Circuit) rejected this argument,
holding (as noted in section I.A above) that attainability and
technological feasibility are not relevant considerations in the
promulgation of the NAAQS (American Petroleum Institute v. Costle, 665
F.2d at 1185). The court also noted that the EPA need not tailor the
NAAQS to fit each region or locale, pointing out that Congress was
aware of the difficulty in meeting standards in some locations and had
addressed it through various compliance-related provisions in the CAA
(id. at 1184-86).
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\6\ The EPA has determined that air quality in the area
including Houston has attained the 1979 1-hour standard (85 FR 8411,
February 14, 2020).
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The next periodic reviews of the criteria and standards for
O3 and other
[[Page 87260]]
photochemical oxidants began in 1982 and 1983, respectively (47 FR
11561, March 17, 1982; 48 FR 38009, August 22, 1983). As part of these
reviews, the EPA published an AQCD, a Staff Paper, and a supplement to
the AQCD (U.S. EPA, 1986; U.S. EPA, 1989; U.S. EPA, 1992). The schedule
for completion of this review was governed by court order. In August of
1992, the EPA proposed to retain the existing primary and secondary
standards (57 FR 35542, August 10, 1992). In March 1993, the EPA
concluded this review by finalizing its proposed decision to retain the
standards, without revision (58 FR 13008, March 9, 1993).
In the next review of the air quality criteria and standards for
O3 and other photochemical oxidants, for which the EPA had
announced in August 1992 its intention to proceed rapidly, the EPA
developed an AQCD and Staff Paper (57 FR 35542, August 10, 1992; U.S.
EPA, 1996a; U.S. EPA, 1996b). Based on consideration of these
assessments, the EPA proposed revisions to both the primary and
secondary standards (61 FR 65716, December 13, 1996). The EPA completed
this review in 1997 by revising both standards to 0.08 ppm, as the
annual fourth-highest daily maximum 8-hour average concentration,
averaged over three years (62 FR 38856, July 18, 1997).
In response to challenges to the EPA's 1997 decision revising the
standards, the D.C. Circuit remanded the standards to the EPA, finding
that section 109 of the CAA, as interpreted by the EPA, effected an
unconstitutional delegation of legislative authority. See American
Trucking Ass'ns v. EPA, 175 F.3d 1027, 1034-1040 (D.C. Cir. 1999). The
court also directed that, in responding to the remand, the EPA should
consider the potential beneficial health effects of O3
pollution in shielding the public from the effects of solar ultraviolet
(UV) radiation, as well as adverse health effects (id. at 1051-53). See
American Trucking Ass'ns v. EPA, 195 F.3d 4, 10 (D.C. Cir. 1999)
(granting panel rehearing in part but declining to review the ruling on
consideration of the potential beneficial effects of O3
pollution). After granting petitions for certiorari, the U.S. Supreme
Court unanimously reversed the judgment of the D.C. Circuit on the
constitutional issue, holding that section 109 of the CAA does not
unconstitutionally delegate legislative power to the EPA. See Whitman
v. American Trucking Ass'ns, 531 U.S. 457, 472-74 (2001). The Court
remanded the case to the D.C. Circuit to consider challenges to the
1997 O3 NAAQS that had not yet been addressed. On remand,
the D.C. Circuit found the 1997 O3 NAAQS to be ``neither
arbitrary nor capricious,'' and so denied the remaining petitions for
review. See ATA III, 283 F.3d at 379.
Coincident with the continued litigation of the other issues, the
EPA responded to the court's 1999 remand to consider the potential
beneficial health effects of O3 pollution in shielding the
public from effects of UV radiation (66 FR 57268, Nov. 14, 2001; 68 FR
614, January 6, 2003). In 2001, the EPA proposed to leave the 1997
primary standard unchanged (66 FR 57268, Nov. 14, 2001). After
considering public comment on the proposed decision, the EPA published
its final response to this remand in 2003, re-affirming the 8-hour
primary standard set in 1997 (68 FR 614, January 6, 2003).
The EPA initiated the fourth periodic review of the air quality
criteria and standards for O3 and other photochemical
oxidants with a call for information in September 2000 (65 FR 57810,
September 26, 2000). In this review, the EPA developed an AQCD, Staff
Paper and related technical support documents and proposed revisions to
the primary and secondary standards (U.S. EPA, 2006; U.S. EPA, 2007; 72
FR 37818, July 11, 2007). The review was completed in March 2008 with
revision of the levels of both the primary and secondary standards from
0.08 ppm to 0.075 ppm, and retention of the other elements of the prior
standards (73 FR 16436, March 27, 2008). A number of petitioners filed
suit challenging this decision.
In September 2009, the EPA announced its intention to reconsider
the 2008 O3 standards,\7\ and initiated a rulemaking to do
so. At the EPA's request, the court held the consolidated cases in
abeyance pending the EPA's reconsideration of the 2008 decision. In
January 2010, the EPA issued a notice of proposed rulemaking to
reconsider the 2008 final decision (75 FR 2938, January 19, 2010).
Later that year, in view of the need for further consideration and the
fact that the Agency's next periodic review of the O3 NAAQS
required under CAA section 109 had already begun (as announced in
September 2008),\8\ the EPA consolidated the reconsideration with its
statutorily required periodic review.\9\
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\7\ The press release of this announcement is available at:
https://archive.epa.gov/epapages/newsroom_archive/newsreleases/85f90b7711acb0c88525763300617d0d.html.
\8\ A ``Call for Information'' initiated the review (73 FR
56581, September 29, 2008).
\9\ This rulemaking, completed in 2015, concluded the
reconsideration process.
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In light of the EPA's decision to consolidate the reconsideration
with the review then ongoing, the D.C. Circuit proceeded with the
litigation on the 2008 O3 NAAQS decision. On July 23, 2013,
the court upheld the EPA's 2008 primary standard, but remanded the 2008
secondary standard to the EPA. See Mississippi v. EPA, 744 F.3d 1334
(D.C. Cir. 2013). With respect to the secondary standard, the court
held that the EPA's explanation for the setting of the secondary
standard identical to the revised 8-hour primary standard was
inadequate under the CAA because the EPA had not adequately explained
how that standard provided the required public welfare protection.
At the time of the court's decision, the EPA had already completed
significant portions of its next statutorily required periodic review
of the O3 NAAQS, which had been formally initiated in 2008,
as summarized above. The documents developed for this review included
the ISA,\10\ Risk and Exposure Assessments (REAs) for health and
welfare, and PA (Frey, 2014a, Frey, 2014b, Frey, 2014c, U.S. EPA, 2013,
U.S. EPA, 2014a, U.S. EPA, 2014b, U.S. EPA, 2014c).\11\ In late 2014,
the EPA proposed to revise the 2008 primary and secondary standards (79
FR 75234, December 17, 2014). The EPA's final decision in this review
established the now-current standards (80 FR 65292, October 26, 2015;
40 CFR 50.19). In this decision, based on consideration of the health
effects evidence on respiratory effects of O3 in at-risk
populations, the EPA revised the primary standard from a level of 0.075
ppm to a level of 0.070 ppm, while retaining all other elements of the
standard (80 FR 65292, October 26, 2015). The EPA's decision on the
level for the standard was based on the weight of the scientific
evidence and quantitative exposure/risk information. The level of the
secondary standard was also revised from 0.075 ppm to 0.070 ppm based
on the scientific evidence of O3 effects on welfare,
particularly the evidence of O3 impacts on vegetation, and
quantitative analyses available in the review. The other elements of
the standard were retained. This decision on the secondary standard
also incorporated the EPA's response to the
[[Page 87261]]
D.C. Circuit's remand of the 2008 secondary standard in Mississippi v.
EPA, 744 F.3d 1344 (D.C. Cir. 2013).\12\
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\10\ The ISA, as the AQCD in prior reviews, serves the purpose
of reviewing the air quality criteria.
\11\ The PA presents an evaluation, for consideration by the
Administrator, of the policy implications of the currently available
scientific information, assessed in the ISA; the quantitative air
quality, exposure or risk analyses presented in the PA and developed
in light of the ISA findings; and related limitations and
uncertainties. The role of the PA is to help ``bridge the gap''
between the Agency's scientific assessment and quantitative
technical analyses, and the judgments required of the Administrator
in his decisions in the NAAQS review.
\12\ The 2015 revisions to the NAAQS were accompanied by
revisions to the data handling procedures, ambient air monitoring
requirements, the air quality index and several provisions related
to implementation (80 FR 65292, October 26, 2015).
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After publication of the final rule, a number of industry groups,
environmental and health organizations, and certain states filed
petitions for judicial review in the D.C. Circuit. The industry and
state petitioners argued that the revised standards were too stringent,
while the environmental and health petitioners argued that the revised
standards were not stringent enough to protect public health and
welfare as the Act requires. On August 23, 2019, the court issued an
opinion that denied all the petitions for review with respect to the
2015 primary standard while also concluding that the EPA had not
provided a sufficient rationale for aspects of its decision on the 2015
secondary standard and remanding that standard to the EPA. See Murray
Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019). The court's
decision on the secondary standard focused on challenges to particular
aspects of EPA's decision. The court concluded that EPA's
identification of particular benchmarks for evaluating the protection
the standard provided against welfare effects associated with tree
growth loss was reasonable and consistent with CASAC's advice. However,
the court held that EPA had not adequately explained its decision to
focus on a 3-year average for consideration of the cumulative exposure,
in terms of W126, identified as providing requisite public welfare
protection, or its decision to not identify a specific level of air
quality related to visible foliar injury. The EPA's decision not to use
a seasonal W126 index as the form and averaging time of the secondary
standard was also challenged, but the court did not reach a decision on
that issue, concluding that it lacked a basis to assess the EPA's
rationale because the EPA had not yet fully explained its focus on a 3-
year average W126 in its consideration of the standard. See Murray
Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019). Accordingly,
the court remanded the secondary standard to EPA for further
justification or reconsideration. The court's remand of the secondary
standard has been considered in reaching the decision, and associated
conclusions and judgments, described in section III.B.3 below.
In the August 2019 decision, the court additionally addressed
arguments regarding considerations of background O3
concentrations, and socioeconomic and energy impacts. With regard to
the former, the court rejected the argument that the EPA was required
to take background O3 concentrations into account when
setting the NAAQS, holding that the text of CAA section 109(b)
precluded this interpretation because it would mean that if background
O3 levels in any part of the country exceeded the level of
O3 that is requisite to protect public health, the EPA would
be obliged to set the standard at the higher nonprotective level (id.
at 622-23). Thus, the court concluded that the EPA did not act
unlawfully or arbitrarily or capriciously in setting the 2015 NAAQS
without regard for background O3 (id. at 624). Additionally,
the court denied arguments that the EPA was required to consider
adverse economic, social, and energy impacts in determining whether a
revision of the NAAQS was ``appropriate'' under section 109(d)(1) of
the CAA (id. at 621-22). The court reasoned that consideration of such
impacts was precluded by Whitman's holding that the CAA ``unambiguously
bars cost considerations from the NAAQS-setting process'' (531 U.S. at
471, summarized in section I.A above). Further, the court explained
that section 109(d)(2)(C)'s requirement that CASAC advise the EPA ``of
any adverse public health, welfare, social, economic, or energy effects
which may result from various strategies for attainment and
maintenance'' of revised NAAQS had no bearing on whether costs are to
be considered in setting the NAAQS (Murray Energy Corp. v. EPA, 936
F.3d at 622). Rather, as described in Whitman and discussed further in
section I.A above, most of that advice would be relevant to
implementation but not standard setting (id.).
D. Current Review of the Air Quality Criteria and Standards
In May 2018, the Administrator directed his Assistant
Administrators to initiate this current review of the O3
NAAQS (Pruitt, 2018). In conveying this direction, the Administrator
further directed the EPA staff to expedite the review, implementing an
accelerated schedule aimed at completion of the review within the
statutorily required period (Pruitt, 2018). Accordingly, the EPA took
immediate steps to proceed with the review. In June 2018, the EPA
announced the initiation of the periodic reviews of the air quality
criteria for photochemical oxidants and of the O3 NAAQS and
issued a call for information in the Federal Register (83 FR 29785,
June 26, 2018). Two types of information were called for: Information
regarding significant new O3 research to be considered for
the ISA for the review, and policy-relevant issues for consideration in
this NAAQS review. Based in part on the information received in
response to the call for information, the EPA developed a draft IRP,
which was made available for consultation with the CASAC and for public
comment (83 FR 55163, November 2, 2018; 83 FR 55528, November 6, 2018).
Comments from the CASAC (Cox, 2018) and the public were considered in
preparing the final IRP (U.S. EPA, 2019b).
Under the plan outlined in the IRP and consistent with revisions to
the process identified by the Administrator in his 2018 memo directing
initiation of the review, the current review of the O3 NAAQS
has progressed on an accelerated schedule (Pruitt, 2018). The EPA has
incorporated a number of efficiencies in various aspects of the review
process, as summarized in the IRP, to support the accelerated schedule
(Pruitt, 2018). As one example of such an efficiency, rather than
produce separate documents for the PA and associated quantitative
analyses, the human exposure and health risk analyses (that inform the
decision on the primary standard) and the air quality and exposure
analyses (that inform the decision on the secondary standard) are
included as appendices in the PA, along with other technical appendices
that inform these standards decisions. The draft PA (including these
analyses as appendices) was reviewed by the CASAC and made available
for public comment while the draft ISA was also being reviewed by the
CASAC and was available for public comment (84 FR 50836, September 26,
2019; 84 FR 58711, November 1, 2019).\13\ The CASAC was assisted in its
review by a pool of consultants with expertise in a number of fields
(84 FR 38625, August 7, 2019). The approach employed by the CASAC in
utilizing outside technical expertise represents an additional
modification of the process from past reviews. Rather than join with
some or all of the CASAC members in a CASAC review panel as has been
common in other NAAQS reviews in the past, in this O3 NAAQS
review (and also in the recent CASAC review of the PA for the
[[Page 87262]]
particulate matter NAAQS), the consultants comprised a pool of
expertise that CASAC members drew on through the use of specific
questions, posed in writing prior to the public meeting, regarding
aspects of the documents being reviewed, obtaining subject matter
expertise for their review in a focused, efficient and transparent
manner.
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\13\ The draft ISA and draft PA were released for public comment
and CASAC review on September 26, 2019 and October 31, 2019,
respectively. The charges for the CASAC review summarized the
overarching context for the document review (including reference to
Pruitt [2018], and the CASAC's functions under section 109(d)(2)(B)
and (C) of the Act), as well as specific charge questions for review
of each of the documents.
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The CASAC discussed its review of both the draft ISA and the draft
PA over three days at a public meeting in December 2019 (84 FR 58713,
November 1, 2019).\14\ The CASAC discussed its draft letters describing
its advice and comments on the documents in a public teleconference in
early February 2020 (85 FR 4656; January 27, 2020). The letters to the
Administrator conveying the CASAC advice and comments on the draft PA
and draft ISA were released later that month (Cox, 2020a; Cox, 2020b).
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\14\ While simultaneous review of first drafts of both documents
has not been usual in past reviews, there have been occurrences of
the CASAC review of a draft PA (or draft REA when the process
involved a policy assessment being included within the REA document)
simultaneous with review of a second (or later) draft ISA (e.g., 73
FR 19835, April 11, 2008; 73 FR 34739, June 18, 2008; 77 FR 64335,
October 19, 2012; 78 FR 938, January 7, 2013).
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The letters from the CASAC and public comment on the draft ISA and
draft PA informed completion of the final documents and further
informed development of the Administrator's proposed and final
decisions in this review. Comments from the CASAC on the draft ISA were
considered by the EPA and led to a number of revisions in developing
the final document. The CASAC review of the draft ISA and the EPA's
consideration of CASAC comments are described in Appendix 10, section
10.4.5 of the final ISA. In his reply to the CASAC letter conveying its
review, ``Administrator Wheeler noted, `for those comments and
recommendations that are more significant or cross-cutting and which
were not fully addressed, the Agency will develop a plan to incorporate
these changes into future O3 ISAs as well as ISAs for other
criteria pollutant reviews' '' (ISA, p. 10-28; Wheeler, 2020). The ISA
was completed and made available to the public in April 2020 (85 FR
21849, April 20, 2020).\15\ Based on the rigorous scientific approach
utilized in its development, summarized in Appendix 10 of the final
ISA, the EPA considers the final ISA to ``accurately reflect the latest
scientific knowledge useful in indicating the kind and extent of all
identifiable effects on public health or welfare which may be expected
from the presence of [O3] in the ambient air, in varying
quantities'' as required by the CAA (42 U.S.C. 7408(a)(2)).
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\15\ The ISA builds on evidence and conclusions from previous
assessments, focusing on synthesizing and integrating the newly
available evidence (ISA, section IS.1.1). Past assessments are
generally cited when providing further, still relevant, details that
informed the current assessment but are not repeated in the latest
assessment.
---------------------------------------------------------------------------
The CASAC comments additionally provided advice with regard to the
primary and secondary standards, as well as a number of comments
intended to improve the PA. These comments were considered in
completing that document (85 FR 31182, May 22, 2020). The CASAC advice
to the Administrator regarding the O3 standards has also
been described and considered in the PA, and in sections II and III
below. The CASAC advice on the primary standard is summarized in II.B.2
below and its advice on the secondary standard is summarized in section
III.B.1.b.
Materials upon which this proposed decision is based, including the
documents described above, are available to the public in the docket
for the review.\16\ As in prior NAAQS reviews, the EPA is basing its
decision in this review on studies and related information included in
the air quality criteria, which have undergone CASAC and public review.
The studies assessed in the ISA \17\ and PA, and the integration of the
scientific evidence presented in them, have undergone extensive
critical review by the EPA, the CASAC, and the public. The rigor of
that review makes these studies, and their integrative assessment, the
most reliable source of scientific information on which to base
decisions on the NAAQS, decisions that all parties recognize as of
great import. Decisions on the NAAQS can have profound impacts on
public health and welfare, and NAAQS decisions should be based on
studies that have been rigorously assessed in an integrative manner not
only by the EPA but also by the statutorily mandated independent
scientific advisory committee, as well as the public review that
accompanies this process. Some commenters have referred to and
discussed individual scientific studies on the health effects of
O3 that were not included in the ISA (`` `new' studies'')
and that have not gone through this comprehensive review process. In
considering and responding to comments for which such ``new'' studies
were cited in support, the EPA has provisionally considered the cited
studies in the context of the findings of the ISA. The EPA's
provisional consideration of these studies did not and could not
provide the kind of in-depth critical review described above, but
rather was focused on determining whether they warranted reopening the
review of the air quality criteria to enable the EPA, the CASAC and the
public to consider them further.
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\16\ The docket for this review, EPA-HQ-OAR-2018-0279, has
incorporated the ISA docket (EPA-HQ-ORD-2018-0274) by reference.
Both are publicly accessible at www.regulations.gov.
\17\ In addition to the review's opening ``Call for
Information'' (83 FR 29785, June 26, 2018), systematic review
methodologies were applied to identify relevant scientific findings
that have emerged since the 2013 ISA, which included peer reviewed
literature published through July 2011. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2011 (providing some overlap with the
cutoff date for the last ISA) and March 30, 2018. Studies published
after the literature cutoff date for this ISA were also considered
if they were submitted in response to the Call for Information or
identified in subsequent phases of ISA development, particularly to
the extent that they provide new information that affects key
scientific conclusions (ISA, Appendix 10, section 10.2). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737.
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This approach, and the decision to rely on studies and related
information included in the air quality criteria, which have undergone
CASAC and public review, is consistent with the EPA's practice in prior
NAAQS reviews and its interpretation of the requirements of the CAA.
Since the 1970 amendments, the EPA has taken the view that NAAQS
decisions are to be based on scientific studies and related information
that have been assessed as a part of the pertinent air quality
criteria, and the EPA has consistently followed this approach. This
longstanding interpretation was strengthened by new legislative
requirements enacted in 1977, which added section 109(d)(2) of the Act
concerning CASAC review of air quality criteria. See 71 FR 61144, 61148
(October 17, 2006, final decision on review of NAAQS for particulate
matter) for a detailed discussion of this issue and the EPA's past
practice.
As discussed in the EPA's 1993 decision not to revise the
O3 NAAQS, ``new'' studies may sometimes be of such
significance that it is appropriate to delay a decision in a NAAQS
review and to supplement the pertinent air quality criteria so the
studies can be taken into account (58 FR at 13013-13014, March 9,
1993). In the present case, the EPA's provisional consideration of
``new'' studies concludes that, taken in context, the ``new''
information and findings do not materially change any of the broad
scientific conclusions regarding the health and welfare effects of
O3 in
[[Page 87263]]
ambient air made in the air quality criteria. For this reason,
reopening the air quality criteria review would not be warranted.
Accordingly, the EPA is basing the final decisions in this review
on the studies and related information included in the O3
air quality criteria that have undergone rigorous review by the EPA,
the CASAC and the public. The EPA will consider these ``new'' studies
for inclusion in the air quality criteria for the next O3
NAAQS review, which the EPA expects to begin soon after the conclusion
of this review and which will provide the opportunity to fully assess
these studies through a more rigorous review process involving the EPA,
the CASAC, and the public.
E. Air Quality Information
Ground level O3 concentrations are a mix of mostly
tropospheric O3 and some stratospheric O3.
Tropospheric O3 is formed due to chemical interactions
involving solar radiation and precursor pollutants including VOCs and
nitrogen oxides (NOX). Methane (CH4) and carbon
monoxide (CO) are also important precursors, particularly at the
regional to global scale. The precursor emissions leading to
tropospheric O3 formation can result from both man-made
sources (e.g., motor vehicles and electric power generation) and
natural sources (e.g., vegetation and wildfires). In addition,
O3 that is created naturally in the stratosphere also
contributes to O3 in the troposphere. The stratosphere
routinely mixes with the troposphere high above the earth's surface
and, less frequently, there are intrusions of stratospheric air that
reach deep into the troposphere and even to the surface. Once formed,
O3 near the surface can be transported by winds before
eventually being removed from the atmosphere via chemical reactions or
deposition to surfaces. In sum, O3 concentrations are
influenced by complex interactions between precursor emissions,
meteorological conditions, and topographical characteristics (PA,
section 2.1; ISA, Appendix 1).
For compliance and other purposes, state and local environmental
agencies operate O3 monitors across the U.S. and submit the
data to the EPA. At present, there are approximately 1,300 monitors
across the U.S. reporting hourly O3 averages during the
times of the year when local O3 pollution can be important
(PA, section 2.3.1).\18\ Most of this monitoring is focused on urban
areas where precursor emissions tend to be largest, as well as
locations directly downwind of these areas. There are also over 100
routine monitoring sites in rural areas, including sites in the Clean
Air Status and Trends Network (CASTNET) which is specifically focused
on characterizing conditions in rural areas. Based on the monitoring
data for the three year period from 2016 to 2018, the EPA identified
142 counties, in which together approximately 106 million Americans
reside where O3 design values \19\ were above 0.070 ppm, the
level of the existing NAAQS (PA, section 2.4.1). Across these areas,
the highest design values are typically observed in California, Texas,
Denver, around Lake Michigan and along the Northeast Corridor,
locations with some of the most densely populated areas in the country
(e.g., PA, Figure 2-8).
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\18\ O3 monitoring seasons in each state vary from
five months (May to September in Oregon and Washington) to year
round (in 11 states), with March to October being most common (27
states).
\19\ A design value is a statistic that summarizes the air
quality data for a given area in terms of the indicator, averaging
time, and form of the standard. Design values can be compared to the
level of the standard and are typically used to designate areas as
meeting or not meeting the standard and assess progress towards
meeting the NAAQS.
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From a temporal perspective, the highest O3
concentrations tend to occur during the afternoon and within the warmer
months of the year due to higher levels of solar radiation and other
conducive meteorological conditions during these times. The exceptions
to this general rule include (1) some rural sites where transport of
O3 from upwind urban areas can occasionally result in high
nighttime levels of O3, (2) high-elevation sites which can
be episodically influenced by stratospheric intrusions in other months
of the year, and (3) mountain basins in the western U.S. where large
quantities of O3 precursors emissions associated with oil
and gas development can be trapped in a shallow inversion layer and
form O3 under clear, calm skies with snow cover during the
colder months (PA, section 2.1; ISA, Appendix 1).
Monitoring data indicate long-term reductions in peak O3
concentrations. For example, monitoring sites operating since 1980
indicate a 32% reduction in the national average annual fourth highest
daily maximum 8-hour concentration from 1980 to 2018. (PA, Figure 2-
10). This has been accompanied by appreciable reductions in peak 1-hour
concentrations, as seen by reductions in annual second highest daily
maximum 1-hour concentrations (PA, Figure 2-17).
Concentrations of O3 in ambient air that result from
natural and non-U.S. anthropogenic sources are collectively referred to
as U.S. background O3 (USB; PA, section 2.5). As in the last
review, we generally characterize O3 concentrations that
would exist in the absence of U.S. anthropogenic emissions (as USB).
Findings from air quality modeling analyses performed for this review
to investigate patterns of USB in the U.S. are largely consistent with
conclusions reached in the last review (PA, section 2.5.4). The current
modeling analysis indicates spatial variation in USB O3
concentrations that is related to geography, topography and proximity
to international borders and is also influenced by seasonal variation,
with long-range international anthropogenic transport contributions
peaking in the spring while U.S. anthropogenic contributions tend to
peak in summer. The West is predicted to have higher USB concentrations
than the East, with higher contributions from natural and international
anthropogenic sources that exert influences in western high-elevation
and near-border areas. The modeling predicts that for both the West and
the East, days with the highest 8-hour concentrations of O3
generally occur in summer and are likely to have substantially greater
concentrations due to U.S. anthropogenic sources. While the USB
contributions to O3 concentrations on days with the highest
8-hour concentrations are generally predicted to come largely from
natural sources, the modeling also indicates that some areas near the
Mexico border may receive appreciable contributions from a combination
of natural and international anthropogenic sources on these days. In
such locations, the modeling suggests the potential for relatively
infrequent events with substantial background contributions where daily
maximum 8-hour O3 concentrations approach or exceed the
level of the current NAAQS (i.e., 70 ppb). This contrasts with most
monitor locations in the U.S. for which international contributions are
predicted to be the lowest during the season with the most frequent
occurrence of daily maximum 8-hour O3 concentrations above
70 ppb. This is generally because, except for in near-border areas,
larger international contributions are associated with long-distance
transport and that is most efficient in the springtime (PA, section
2.5.4).
II. Rationale for Decision on the Primary Standard
This section presents the rationale for the Administrator's
decision to retain the current primary O3 standard. This
rationale is based on the scientific information presented in the ISA,
on human health effects associated with
[[Page 87264]]
photochemical oxidants including O3 and pertaining to the
presence of these pollutants in ambient air. As summarized in section
I.D above, the ISA was developed based on a thorough review of the
latest scientific information generally published between January 2011
and March 2018, as well as more recent studies identified during peer
review, submitted in response to the Call for Information, or public
comments on the draft ISA, integrated with the information and
conclusions from previous assessments (ISA, section IS.1.2 and Appendix
10, section 10.2). The Administrator's rationale also takes into
account: (1) The PA evaluation of the policy-relevant information in
the ISA and presentation of quantitative analyses of air quality, human
exposure and health risks; (2) CASAC advice and recommendations, as
reflected in discussions of drafts of the ISA and PA at public meetings
and in the CASAC's letters to the Administrator; and (3) public
comments on the proposed decision.
Within this section, introductory and background information is
presented in section II.A. Section II.A.1 summarizes the 2015
establishment of the existing standard, as background for this review.
Section II.A.2 provides an overview of the currently available health
effects evidence, and section II.A.3 provides an overview of the
current exposure and risk information, drawing on the quantitative
analyses presented in the PA. Section II.B summarizes the basis for the
proposed decision (II.B.1), discusses public comments on the proposed
decision (II.B.2), and presents the Administrator's considerations,
conclusions and decision in this review of the primary standard
(II.B.3). The decision on the current primary standard is summarized in
section II.C.
A. Introduction
As in prior reviews, the general approach to reviewing the current
primary standard is based, most fundamentally, on using the Agency's
assessments of the current scientific evidence and associated
quantitative analyses to inform the Administrator's judgment regarding
a primary standard for photochemical oxidants that is requisite to
protect the public health with an adequate margin of safety. The EPA's
assessments are primarily documented in the ISA and PA, both of which
have received CASAC review and public comment (84 FR 50836, September
26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019;
85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging
the gap between the scientific assessments of the ISA and the judgments
required of the Administrator in his decisions on the current standard,
the PA evaluates policy implications of the assessment of the current
evidence in ISA and the quantitative exposure and risk analyses
documented extensively in appendices of the PA. In evaluating the
public health protection afforded by the current standard, the four
basic elements of the NAAQS (indicator, averaging time, level, and
form) are considered collectively.
The final decision on the adequacy of the current primary standard
is a public health policy judgment to be made by the Administrator. In
reaching conclusions on the standard, the decision draws on the
scientific information and analyses about health effects, population
exposure and risks, as well as judgments about how to consider the
range and magnitude of uncertainties that are inherent in the
scientific evidence and analyses. This approach is based on the
recognition that the available health effects evidence generally
reflects a continuum, consisting of levels at which scientists
generally agree that health effects are likely to occur, through lower
levels at which the likelihood and magnitude of the response become
increasingly uncertain. This approach is consistent with the
requirements of the NAAQS provisions of the Clean Air Act and with how
the EPA and the courts have historically interpreted the Act
(summarized in section I.A. above). These provisions require the
Administrator to establish primary standards that, in the judgment of
the Administrator, are requisite to protect public health with an
adequate margin of safety. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that primary
standards be set at a zero-risk level, but rather at a level that
avoids unacceptable risks to public health, including the health of
sensitive groups.\20\
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\20\ As noted in section I.A above, consideration of such
protection is focused on the sensitive group of individuals and not
a single person in the sensitive group (see S. Rep. No. 91-1196,
91st Cong., 2d Sess. 10 [1970]).
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1. Background on the Current Standard
As a result of the last O3 NAAQS review, completed in
2015, the level of the primary standard was revised from 0.075 to 0.070
ppm,\21\ in conjunction with retaining the existing indicator,
averaging time, and form. This revision, establishing the current
standard, was based on the scientific evidence and quantitative
exposure and risk analyses available at that time, as well as the
Administrator's judgments regarding the available health effects
evidence, the appropriate degree of public health protection for the
revised standard, and the available exposure and risk information
regarding the exposures and risk that may be allowed by such a standard
(80 FR 65292, October 26, 2015). In establishing this standard, the
Administrator considered the extensive body of evidence spanning
several decades documenting the causal relationship between
O3 exposure and a broad range of respiratory effects (80 FR
65292, October 26, 2015; 2013 ISA, p. 1-14),\22\ that had been
augmented by evidence available since the prior review was completed in
2008. Such effects range from small, reversible changes in pulmonary
function and pulmonary inflammation (documented in controlled human
exposure studies involving exposures ranging from 1 to 8 hours) \23\ to
more serious health outcomes such as asthma-related emergency
department visits and hospital admissions, which have been associated
with ambient air concentrations of O3 in epidemiologic
studies (2013 ISA, section 6.2).\24\ The 2015 decision, which provided
increased protection for at-risk populations,\25\ such as children and
[[Page 87265]]
people with asthma, against an array of adverse health effects, drew
upon the available scientific evidence assessed in the 2013 ISA, the
exposure and risk information presented and assessed in the 2014 health
REA (HREA), the consideration of that evidence and information in the
2014 PA, the advice and recommendations of the CASAC, and public
comments on the proposed decision (79 FR 75234, December 17, 2014).
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\21\ Although ppm are the units in which the level of the
standard is defined, the units, ppb, are more commonly used
throughout this document for greater consistency with the more
recent literature. The level of the current primary standard, 0.070
ppm, is equivalent to 70 ppb.
\22\ In addition to concluding there to be a causal relationship
between short-term O3 exposures and respiratory effects,
and that the relationship between longer-term exposure and
respiratory effects was likely to be causal, the 2013 ISA also
concluded there likely to be a causal relationship between short-
term exposure and mortality, as well as short-term exposure and
cardiovascular effects, including related mortality, and that the
evidence was suggestive of causal relationships between long-term
exposures and total mortality, cardiovascular effects and
reproductive, developmental effects, and between short- and long-
term exposure and nervous system effects (2013 ISA, p. 1-14, section
2.5.2).
\23\ Study subjects in most of the controlled human exposure
studies are generally healthy adults.
\24\ The evidence base also includes experimental animal studies
that provide insight into potential modes of action, contributing to
the coherence and robust nature of the evidence.
\25\ As used here and similarly throughout the document, the
term population refers to persons having a quality or characteristic
in common, such as, and including, a specific pre-existing illness
or a specific age or lifestage. A lifestage refers to a
distinguishable time frame in an individual's life characterized by
unique and relatively stable behavioral and/or physiological
characteristics that are associated with development and growth.
Identifying at-risk populations includes consideration of intrinsic
(e.g., genetic or developmental aspects) or acquired (e.g., disease
or smoking status) factors that increase the risk of health effects
occurring with exposure to a substance (such as O3) as
well as extrinsic, nonbiological factors, such as those related to
socioeconomic status, reduced access to health care, or exposure.
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Across the different study types, the controlled human exposure
studies, which were recognized to provide the most certain evidence
indicating the occurrence of health effects in humans following
specific O3 exposures, additionally document the roles of
ventilation rate \26\ and exposure duration, in addition to exposure
concentration, in eliciting responses to O3 exposure (80 FR
65343, October 26, 2015; 2014 PA, section 3.4).\27\ These aspects of
the evidence were represented in exposure-based analyses developed to
inform the NAAQS decision with estimates of exposure and risk
associated with air quality conditions just meeting the then-existing
standard, and also for air quality conditions just meeting potential
alternative standards (U.S. EPA, 2014a, hereafter 2014 HREA). The
exposure-based analyses given greatest weight in the Administrator's
consideration of the HREA estimates involved comparison of estimates
for study area populations of children of exposure at elevated exertion
to exposure benchmark concentrations (exposures of concern). The
benchmark concentrations (60, 70 and 80 ppb) were identified from
controlled human exposure studies (conducted with generally healthy
adults).
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\26\ Ventilation rate (VE) is a specific technical
term referring to breathing rate in terms of volume of air taken
into the body per unit of time. A person engaged in different
activities will exert themselves at different levels and experience
different ventilation rates.
\27\ For example, the exposure concentrations eliciting a given
level of response in subjects at rest are higher than those
eliciting such response in subjects exposed while at elevated
ventilation, such as while exercising (2013 ISA, section 6.2.1.1).
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In weighing the health effects evidence and making judgments
regarding the public health significance of the quantitative estimates
of exposures and risks allowed by the then-existing standard and
potential alternative standards considered, as well as judgments
regarding margin of safety, the Administrator's 2015 decision
considered the currently available information and commonly accepted
guidelines or criteria within the public health community, including
statements of the American Thoracic Society (ATS), an organization of
respiratory disease specialists, advice from the CASAC, and public
comments. In so doing, she recognized that the determination of what
constitutes an adequate margin of safety is expressly left to the
judgment of the EPA Administrator. See Lead Industries Ass'n v. EPA,
647 F.2d 1130, 1161-62 (D.C. Cir 1980); Mississippi v. EPA, 744 F.3d
1334, 1353 (D.C. Cir. 2013). In NAAQS reviews generally, evaluations of
how particular primary standards address the requirement to provide an
adequate margin of safety include consideration of such factors as the
nature and severity of the health effects, the size of the sensitive
population(s) at risk, and the kind and degree of the uncertainties
present. Consistent with past practice and long-standing judicial
precedent, the Administrator took the need for an adequate margin of
safety into account as an integral part of her decision-making.
In the decisions regarding adequacy of protection provided by the
then-existing primary standard and on alternatives for a new revised
standard, primary consideration was given to the evidence of
respiratory effects from controlled human exposure studies, including
those newly available in the review, and for which the exposure
concentrations were at the lower end of those studied (80 FR 65342-47
and 65362-66, October 26, 2015). This emphasis was consistent with
comments on the strength of this evidence from the CASAC at that time
(Frey, 2014b, p. 5). In placing weight on these studies, the
Administrator at that time took note of the variety of respiratory
effects reported from the studies of healthy adults engaged in quasi-
continuous exercise within a 6.6-hour exposure to O3
concentrations of 60 ppb and higher.\28\ The lowest exposure
concentration in such studies for which a combination of statistically
significant reduction in lung function and increase in respiratory
symptoms was somewhat above 70 ppb,\29\ while reduced lung function and
increased pulmonary inflammation were reported following such exposures
to O3 concentrations as low as 60 ppb. In considering these
findings, the Administrator noted that the combination of
O3-induced lung function decrements and respiratory symptoms
met ATS criteria for an adverse response,\30\ and noted CASAC comments,
which included a caution regarding the potential for effects in some
groups of people, such as people with asthma, at exposure
concentrations below those affecting healthy subjects (Frey, 2014b, pp.
5-6; 80 FR 65343, October 26, 2015). With regard to the epidemiologic
evidence, the Administrator noted the ISA finding that the pattern of
effects observed across the range of exposures assessed in the
controlled human exposure studies, increasing in severity at higher
exposures, is coherent with (i.e., reasonably related to) the health
outcomes reported to be associated with ambient air concentrations in
epidemiologic studies. Additionally, while recognizing that most
O3 epidemiologic studies reported health outcome
associations with O3 concentrations in ambient air that
violated the then-existing standard, the Administrator took note of a
study that reported associations between short-term O3
concentrations and asthma emergency department visits in children and
adults in a U.S. location that would have met the then-existing
standard over the entire 5-year study period (80 FR 65344, October 26,
2015; Mar and Koenig, 2009).\31\ Taken together, the Administrator
concluded that the scientific evidence from controlled human exposure
and epidemiologic studies called into question the adequacy of the
public health protection provided by the 75 ppb standard that had been
set in 2008.
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\28\ The studies given primary focus were those in which
O3 exposures occurred over the course of 6.6 hours during
which the subjects engaged in six 50-minute exercise periods
separated by 10-minute rest periods, with a 35-minute lunch period
occurring after the third hour (e.g., Folinsbee et al., 1988 and
Schelegle et al., 2009). Responses after O3 exposure were
compared to those after filtered air exposure.
\29\ For the 70 ppb target exposure, Schelegle et al. (2009)
reported, based on O3 measurements during the six 50-
minute exercise periods, that the mean O3 concentration
during the exercise portion of the study protocol was 72 ppb. Based
on the six exercise period measurements, the time weighted average
concentration across the full 6.6-hour exposure was 73 ppb
(Schelegle et al., 2009).
\30\ The most recent statement from the ATS available at the
time of the 2015 decision stated that ``[i]n drawing the distinction
between adverse and nonadverse reversible effects, this committee
recommended that reversible loss of lung function in combination
with the presence of symptoms should be considered as adverse''
(ATS, 2000).
\31\ The design values in this location during the study period
were at or somewhat below 75 ppb (Wells, 2012).
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In considering the exposure and risk information, the
Administrator's 2015 decision gave particular attention to the
exposure-based comparison-to-benchmarks analysis, focusing on the
estimates of exposures of concern for
[[Page 87266]]
children \32\ in 15 urban study areas for air quality conditions just
meeting the then-current standard. Consistent with the finding that
larger percentages of children than adults were estimated to experience
exposures at or above benchmarks, the Administrator focused on the
results for all children and for children with asthma, noting that the
results for these two groups, in terms of percent of the population
group, are virtually indistinguishable (2014 HREA, sections 5.3.2,
5.4.1.5 and section 5F-1). The Administrator placed the greatest weight
on estimates of two or more days with occurrences of exposures at or
above the benchmarks, in light of her increased concern about the
potential for adverse responses with repeated occurrences of such
exposures, noting that the types of effects shown to occur following
exposures to O3 concentrations from 60 ppb to 80 ppb, such
as inflammation, if occurring repeatedly as a result of repeated
exposure, could potentially result in more severe effects (80 FR 65343,
65345, October 26, 2015; 2013 ISA, section 6.2.3).\33\ The
Administrator also considered estimates for single exposures at or
above the higher benchmarks of 70 and 80 ppb (80 FR 65345, October 26,
2015). With regard to the 60 ppb benchmark, while the Administrator
recognized the effects reported from controlled human exposure studies
of 60 ppb to be less severe than those for higher O3
concentrations, she also recognized there were limitations and
uncertainties in the evidence base with regard to unstudied population
groups. As a result, she judged it appropriate for the standard, in
providing an adequate margin of safety, to provide some control of
exposures at or above the 60 ppb benchmark (80 FR 65345-65346, October
26, 2015).
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\32\ Consideration focused on estimates for children, reflecting
the finding that the estimates for percent of children experiencing
an exposure at or above the benchmarks were higher than percent of
adults due to the greater time children spend outdoors engaged in
activities at elevated exertion (2014 HREA, section 5.3.2).
\33\ In addition to recognizing the potential for continued
inflammation to evolve into other outcomes, the 2013 ISA also
recognized that inflammation induced by a single exposure (or
several exposures over the course of a summer) can resolve entirely
(2013 ISA, p. 6-76; 80 FR 65331, October 26, 2015).
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In considering public health implications of the exposure and risk
information, the Administrator concluded that the exposures and risks
projected to remain upon meeting the then-current (75 ppb) standard
were reasonably judged important from a public health perspective. This
conclusion was particularly based on her judgment that it is
appropriate to set a standard that would be expected to eliminate, or
almost eliminate, the occurrence of exposures, while at moderate
exertion, at or above 70 and 80 ppb (80 FR 65346, October 26, 2015). In
addition, given that in the air quality scenario for the existing
standard, the average percent of children estimated to experience two
or more days with exposures at or above the 60 ppb benchmark approached
10% in some urban study areas (on average across the analysis years),
the Administrator concluded that the existing standard did not
incorporate an adequate margin of safety against the potentially
adverse effects that could occur following repeated exposures at or
above 60 ppb (80 FR 65345-46, October 26, 2015). Thus, the exposure and
risk estimates \34\ were judged to support a conclusion that the
existing standard was not sufficiently protective and did not
incorporate an adequate margin of safety. In consideration of all of
the above, as well as the CASAC advice, which included the unanimous
recommendation ``that the Administrator revise the current primary
ozone standard to protect public health'' (Frey, 2014b, p. 5),\35\ the
Administrator concluded that the then-current primary O3
standard (with its level of 75 ppb) was not requisite to protect public
health with an adequate margin of safety, and should be revised to
provide increased public health protection (80 FR 65346, October 26,
2015).
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\34\ Although the Administrator recognized increased uncertainty
in and placed less weight on the other types of HREA risk estimates,
she found they supported her conclusion of public health importance
on a broad national scale (80 FR 65347).
\35\ The Administrator also noted that the CASAC for the prior
review (2008) likewise recommended the standard level be revised
below 75 ppb based on the evidence and information in the record for
the 2008 decision (Samet, 2011; Frey and Samet, 2012).
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With regard to the most appropriate indicator for the revised
standard, key considerations included the finding that O3 is
the only photochemical oxidant (other than nitrogen dioxide) that is
routinely monitored and for which a comprehensive database exists, and
the consideration that, since the precursor emissions that lead to the
formation of O3 also generally lead to the formation of
other photochemical oxidants, measures leading to reductions in
population exposures to O3 can generally be expected to lead
to reductions in other photochemical oxidants (2013 ISA, section 3.6;
80 FR 65347, October 26, 2015). The CASAC also indicated O3
to be the appropriate indicator ``based on its causal or likely causal
associations with multiple adverse health outcomes and its
representation of a class of pollutants known as photochemical
oxidants'' (Frey, 2014b, p. ii). Based on all of these considerations
and public comments, the Administrator retained O3 as the
indicator for the primary standard (80 FR 65347, October 26, 2015).
With regard to averaging time, eight hours was the duration
established in 1997 with the replacement of the then-existing 1-hour
standard (62 FR 38856, July 18, 1997). The decision at that time was
based on evidence from numerous controlled human exposure studies
reporting adverse respiratory effects resulting from 6- to 8-hour
exposures, as well as quantitative analyses indicating the control
provided by an 8-hour averaging time of both 8-hour and 1-hour peak
exposures and associated health risk (62 FR 38861, July 18, 1997; U.S.
EPA, 1996b). The 1997 decision was also consistent with CASAC advice at
that time (62 FR 38861, July 18, 1997; 61 FR 65727, December 13, 1996).
For similar reasons, the 8-hour averaging time was retained in the
subsequent 2008 review (73 FR 16436, March 27, 2008). In 2015, the
decision, based on then-available health effects information, was to
again retain the 8-hour averaging time, as appropriate for addressing
health effects associated with short-term exposures to ambient air
O3, and based on the conclusion that it could effectively
limit health effects attributable to both short- and long-term
O3 exposures (80 FR 65348, 65350, October 26, 2015).
With regard to the form for the standard, the existing nth-high
metric form had been established in the 1997 review, when the form was
revised from an expected exceedance form. At that time, it was
recognized that a concentration-based form, by giving proportionally
more weight to years when 8-hour O3 concentrations are well
above the level of the standard than years when concentrations are just
above the level, better reflects the continuum of health effects
associated with increasing O3 concentrations than does an
expected exceedance form (80 FR 65350-65352, October 26, 2015).\36\ The
subsequent 2008 review also
[[Page 87267]]
considered the potential value of a percentile-based form, but the EPA
concluded that, because of the differing lengths of the monitoring
season for O3 across the U.S., such a form would not be
effective in ensuring the same degree of public health protection
across the country (73 FR 16474-75, March 27, 2008). Additionally, the
EPA recognized the importance of a form that provides stability to
ongoing control programs and insulation from the impacts of extreme
meteorological events that are conducive to O3 occurrence
(73 FR 16474-16475, March 27, 2008). In the 2015 decision, based on all
of these considerations, and including advice from the CASAC, which
stated that this form ``provides health protection while allowing for
atypical meteorological conditions that can lead to abnormally high
ambient ozone concentrations which, in turn, provides programmatic
stability'' (Frey, 2014b, p. 6), the existing form (the annual fourth-
highest daily maximum 8-hour O3 average concentration,
averaged over three consecutive years) was retained (80 FR 65352,
October 26, 2015).
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\36\ With regard to a specific concentration-based form, the
fourth-highest daily maximum was selected in 1997, recognizing that
a less restrictive form (e.g., fifth highest) would allow a larger
percentage of sites to experience O3 peaks above the
level of the standard, and would allow more days on which the level
of the standard may be exceeded when the site attains the standard
(62 FR 38868-38873, July 18, 1997), and there was no basis
identified for selection of a more restrictive form (62 FR 38856,
July 18, 1997).
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As for the decision on adequacy of protection provided by the
combination of all elements of the existing standard, the 2015 decision
to set the level of the revised standard at 70 ppb placed the greatest
weight on the results of controlled human exposure studies and on
quantitative analyses based on information from these studies,
particularly analyses of O3 exposures of concern, consistent
with CASAC advice and interpretation of the scientific evidence (80 FR
65362, October 26, 2015; Frey, 2014b).\37\ This weighting reflected the
recognition that controlled human exposure studies provide the most
certain evidence indicating the occurrence of health effects in humans
following specific O3 exposures, and, in particular, that
the effects reported in the controlled human exposure studies are due
solely to O3 exposures, and are not complicated by the
presence of co-occurring pollutants or pollutant mixtures (as is the
case in epidemiologic studies) (80 FR 65362-65363, October 26, 2015).
With regard to this evidence, the Administrator at that time recognized
that: (1) The largest respiratory effects, and the broadest range of
effects, have been studied and reported following exposures to 80 ppb
O3 or higher (i.e., decreased lung function, increased
airway inflammation, increased respiratory symptoms, airway
hyperresponsiveness, and decreased lung host defense); (2) exposures to
O3 concentrations somewhat above 70 ppb have been shown to
both decrease lung function and to result in respiratory symptoms; and
(3) exposures to O3 concentrations as low as 60 ppb have
been shown to decrease lung function and to increase airway
inflammation (80 FR 65363, October 26, 2015). The Administrator also
noted that 70 ppb was well below the O3 exposure
concentration documented to result in the widest range of respiratory
effects (i.e., 80 ppb), and below the lowest O3 exposure
concentration shown in 6.6 hour exposures with quasi-continuous
exercise to result in the combination of lung function decrements and
respiratory symptoms (80 FR 65363, October 26, 2015).
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\37\ The Administrator viewed the results of other quantitative
analyses in this review--the lung function risk assessment, analyses
of O3 air quality in locations of epidemiologic studies,
and epidemiologic-study-based quantitative health risk assessment--
as being of less utility for selecting a particular standard level
among a range of options (80 FR 65362, October 26, 2015).
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In considering the degree of protection to be provided by a revised
standard, and the extent to which that standard would be expected to
limit population exposures to the broad range of O3
exposures shown to result in health effects, the Administrator focused
particularly on the HREA estimates of two or more exposures of concern.
Placing the most emphasis on a standard that limits repeated
occurrences of exposures at or above the 70 and 80 ppb benchmarks,
while at elevated ventilation, the Administrator noted that a revised
standard with a level of 70 ppb was estimated to eliminate the
occurrence of two or more days with exposures at or above 80 ppb and to
virtually eliminate the occurrence of two or more days with exposures
at or above 70 ppb for all children and children with asthma, even in
the worst-case year and location evaluated (80 FR 65363-65364, October
26, 2015).\38\ The Administrator's consideration of exposure estimates
at or above the 60 ppb benchmark (focused most particularly on multiple
occurrences), an exposure to which the Administrator was less confident
would result in adverse effects,\39\ as discussed above, was primarily
in the context of considering the extent to which the health protection
provided by a revised standard included a margin of safety against the
occurrence of adverse O3-induced effects (80 FR 65364,
October 26, 2015). In this context, the Administrator noted that a
revised standard with a level of 70 ppb was estimated to protect the
vast majority of children in urban study areas (i.e., about 96% to more
than 99% of children in individual areas) from experiencing two or more
days with exposures at or above 60 ppb (while at moderate or greater
exertion). This represented a more than 60% reduction in repeated
exposures over the estimates for the then-existing standard, with its
level of 75 ppb.
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\38\ Under conditions just meeting an alternative standard with
a level of 70 ppb across the 15 urban study areas, the estimate for
two or more days with exposures at or above 70 ppb was 0.4% of
children, in the worst year and worst area (80 FR 65313, Table 1,
October 26, 2015).
\39\ The Administrator was ``notably less confident in the
adversity to public health of the respiratory effects that have been
observed following exposures to O3 concentrations as low
as 60 ppb,'' based on her consideration of the ATS statement on
judging adversity from transient lung function decrements alone, the
uncertainty in the potential for such decrements to increase the
risk of other, more serious respiratory effects in a population (per
ATS recommendations on population-level risk), and the less clear
CASAC advice regarding potential adversity of effects at 60 ppb
compared to higher concentrations studied (80 FR 65363, October 26,
2015).
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Given the considerable protection provided against repeated
exposures of concern for all three benchmarks, including the 60 ppb
benchmark, the Administrator judged that a standard with a level of 70
ppb would incorporate a margin of safety against the adverse
O3-induced effects shown to occur in the controlled human
exposure studies following exposures (while at moderate or greater
exertion) to a concentration somewhat higher than 70 ppb (80 FR 65364,
October 26, 2015).\40\ The Administrator also judged the HREA estimates
of one or more exposures (while at moderate or greater exertion) at or
above 60 ppb to also provide support for her somewhat broader
conclusion that ``a standard with a level of 70 ppb would incorporate
an adequate margin of safety against the occurrence of O3
exposures that can result in effects that are adverse to public
health'' (80 FR 65364, October 26, 2015).\41\ Although she placed less
[[Page 87268]]
weight on the other HREA risk estimates and epidemiologic evidence for
considering the standard level, in light of associated uncertainties,
the Administrator judged that a standard with a level of 70 ppb would
be expected to result in important reductions in the population-level
risk of endpoints on which these types of information are focused and
provide associated additional public health protection, beyond that
provided by the then-existing standard (80 FR 65364, October 26, 2015).
In summary, based on the evidence, exposure and risk information,
advice from the CASAC, and public comments, the 2015 decision was to
revise the primary standard to be 70 ppb, in terms of the 3-year
average of annual fourth-highest daily maximum 8-hour average
O3 concentrations, to provide the requisite protection of
public health, including the health of at-risk populations, with an
adequate margin of safety (80 FR 65365, October 26, 2015).
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\40\ In so judging, she noted that the CASAC had recognized the
choice of a standard level within the range it recommended based on
the scientific evidence (which was inclusive of 70 ppb) to be a
policy judgment (80 FR 65355, October 26, 2015; Frey, 2014b).
\41\ While the Administrator was less concerned about single
exposures, especially for the 60 ppb benchmark, she judged the HREA
one-or-more estimates informative to margin of safety
considerations. In this regard, she noted that ``a standard with a
level of 70 ppb is estimated to (1) virtually eliminate all
occurrences of exposures of concern at or above 80 ppb; (2) protect
the vast majority of children in urban study areas from experiencing
any exposures of concern at or above 70 ppb (i.e., >= about 99%,
based on mean estimates; Table 1); and (3) to achieve substantial
reductions, compared to the [then-]current standard, in the
occurrence of one or more exposures of concern at or above 60 ppb
(i.e., about a 50% reduction; Table 1)'' (80 FR 65364, October 26,
2015).
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2. Overview of Health Effects Information
The information summarized in this section is an overview of the
scientific assessment of the health effects evidence available in this
review; the assessment is documented in the ISA and its policy
implications are further discussed in the PA. In this review, as in
past reviews, the health effects evidence evaluated in the ISA for
O3 and related photochemical oxidants is focused on
O3 (ISA, section IS.1.1). Ozone is concluded to be the most
prevalent photochemical oxidant present in the atmosphere and the one
for which there is a very large, well-established evidence base of its
health and welfare effects (ISA, section IS.1.1). Thus, the current
health effects evidence and the Agency's review of the evidence,
including the evidence newly available in this review,\42\ continues to
focus on O3. The subsections below briefly summarize the
following aspects of the evidence: The nature of O3-related
health effects, the potential public health implications and
populations at risk, and exposure concentrations associated with health
effects.
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\42\ More than 1600 studies are newly available and considered
in the ISA, including more than 1000 health studies (ISA, Appendix
10, Figure 10-2).
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a. Nature of Effects
The evidence base available in the current review includes decades
of extensive evidence that clearly describes the role of O3
in eliciting an array of respiratory effects and recent evidence
indicates the potential for relationships between O3
exposure and metabolic effects. As was established in prior reviews,
the effects for which the evidence is strongest are transient
decrements in pulmonary function and respiratory symptoms, such as
coughing and pain on deep inspiration, as a result of short-term
exposures particularly when breathing at elevated rates (ISA, section
IS.4.3.1; 2013 ISA, p. 2-26). These effects are demonstrated in the
large, long-standing evidence base of controlled human exposure studies
\43\ (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, ISA). The
epidemiologic evidence base documents consistent, positive associations
of O3 concentrations in ambient air with lung function
effects in panel studies (2013 ISA, section 6.2.1.2; ISA, Appendix 3,
section 3.1.4.1.3), and with more severe health outcomes, including
asthma-related emergency department visits and hospital admissions
(2013 ISA, section 6.2.7; ISA, Appendix 3, sections 3.1.5.1 and
3.1.5.2). Extensive experimental animal evidence informs a detailed
understanding of mechanisms underlying the short-term respiratory
effects, and studies in animal models describe effects of longer-term
O3 exposure on the developing lung (ISA, Appendix 3,
sections 3.1.11 and 3.2.6).
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\43\ The vast majority of the controlled human exposure studies
(and all of the studies conducted at the lowest exposures) involved
young healthy adults (typically 18-35 years old) as study subjects
(2013 ISA, section 6.2.1.1). There are also some controlled human
exposure studies of one to eight hours duration in older adults and
adults with asthma, and there are still fewer controlled human
exposure studies in healthy children (i.e., individuals aged younger
than 18 years) or children with asthma (See, for example, PA,
Appendix 3A, Table 3A-3).
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The full body of evidence continues to support the conclusions of a
causal relationship of respiratory effects with short-term
O3 exposures and of a relationship of respiratory effects
with longer-term exposures that is likely to be causal (ISA, sections
IS.4.3.1 and IS.4.3.2). Further, the ISA determines that the
relationship between short-term O3 exposure and metabolic
effects \44\ is likely to be causal, based primarily on newly available
experimental animal evidence (ISA, section IS.4.3.3). The newly
available evidence, particularly from controlled human exposure studies
of cardiovascular endpoints, has altered conclusions from the last
review with regard to relationships between short-term O3
exposures and cardiovascular effects and mortality, such that the
evidence no longer supports conclusions that the relationships are
likely to be causal.\45\
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\44\ The term metabolic effects is used in the ISA to refer
metabolic syndrome (a collection of risk factors including
alterations in glucose and insulin homeostasis, high blood pressure,
adiposity, elevated triglycerides and low high density lipoprotein
cholesterol), diabetes, metabolic disease mortality, and indicators
of metabolic syndrome that include peripheral inflammation, liver
function, neuroendocrine signaling, and serum lipids (ISA, section
IS.4.3.3).
\45\ The currently available evidence for cardiovascular,
reproductive and nervous system effects, as well as mortality, is
``suggestive of, but not sufficient to infer'' a causal relationship
with short- or long-term O3 exposures (ISA, Table IS-1).
The evidence is inadequate to infer the presence or absence of a
causal relationship between long-term O3 exposure and
cancer (ISA, section IS.4.3.6.6).
---------------------------------------------------------------------------
With regard to respiratory effects from short-term O3
exposure, the strongest evidence comes from controlled human exposure
studies, also available in the last review, demonstrating
O3-related respiratory effects in generally healthy adults
(ISA, section IS.1.3.1).\46\ As in the last review, the key evidence
comes from the body of controlled human exposure studies that document
respiratory effects in people exposed for short periods (6.6 to 8
hours) during quasi-continuous exercise.\47\ The potential for
O3 exposure to elicit health outcomes more serious than
those assessed in the controlled human exposure studies continues to be
indicated by the epidemiologic evidence of associations of
O3 concentrations in ambient air with increased incidence of
hospital admissions and emergency department visits for an array of
health outcomes, including asthma exacerbation, chronic obstructive
pulmonary disease (COPD) exacerbation, respiratory infection, and
combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5
and 3.1.6). The strongest such evidence is for asthma-related outcomes
and specifically asthma-related outcomes for children, indicating an
increased risk for people with asthma and particularly children with
asthma (ISA, Appendix 3, section 3.1.5.7).
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\46\ The phrases ``healthy adults'' or ``healthy subjects'' are
used to distinguish from subjects with asthma or other respiratory
diseases because the ``the study design generally precludes
inclusion of subjects with serious health conditions,'' such as
individuals with severe respiratory diseases (2013 ISA, p. lx).
\47\ A quasi-continuous exercise protocol is common to these
controlled exposure studies where study subjects complete six 50-
minute periods of exercise, each followed by 10-minute periods of
rest (e.g., ISA, Appendix 3, section 3.1.4.1.1, and p. 3-11; 2013
ISA, section 6.2.1.1).
---------------------------------------------------------------------------
Respiratory responses observed in human subjects exposed to
O3 for periods of 8 hours or less, while intermittently or
quasi-continuously exercising, include lung function decrements (e.g.,
based on forced expiratory volume in one second [FEV1]
measurements),\48\ respiratory symptoms,
[[Page 87269]]
increased airway responsiveness, mild bronchoconstriction (measured as
an increase in specific airway resistance [sRaw]), and pulmonary
inflammation, with associated injury and oxidative stress (ISA,
Appendix 3, section 3.1.4; 2013 ISA, sections 6.2.1 through 6.2.4). The
available mechanistic evidence, discussed in greater detail in the ISA,
describes pathways involving the respiratory and nervous systems by
which O3 results in pain-related respiratory symptoms and
reflex inhibition of maximal inspiration (inhaling a full, deep
breath), commonly quantified by decreases in forced vital capacity
(FVC) and total lung capacity. This reflex inhibition of inspiration
combined with mild bronchoconstriction contributes to the observed
decrease in FEV1, the most common metric used to assess
O3-related lung function effects. The evidence also
indicates that the additionally observed inflammatory response is
correlated with mild airway obstruction, generally measured as an
increase in sRaw (ISA, Appendix 3, section 3.1.3). As described below,
the prevalence and severity of respiratory effects in controlled human
exposure studies, including symptoms (e.g., pain on deep inspiration,
shortness of breath, and cough), increases with increasing
O3 concentration, exposure duration, and ventilation rate of
exposed subjects (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).
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\48\ In summarizing FEV1 responses from controlled
human exposure studies as ``decrements'', an O3-induced
change in FEV1 is typically the difference between the
change observed with O3 exposure ([post-exposure
FEV1 minus pre-exposure FEV1] divided by pre-
exposure FEV1) and what is generally an improvement
observed with filtered air (FA) exposure ([post-exposure
FEV1 minus pre-exposure FEV1] divided by pre-
exposure FEV1).
---------------------------------------------------------------------------
Within the evidence base from controlled human exposure studies,
the majority of studies involve healthy adult subjects (generally 18 to
35 years), although there are studies involving subjects with asthma,
and a limited number of studies, generally of durations shorter than
four hours, involving adolescents and adults older than 50 years. A
summary of salient observations of O3 effects on lung
function, based on the controlled human exposure study evidence
reviewed in the 1996 and 2006 AQCDs, and recognized in the 2013 ISA,
continues to pertain to this evidence base as it exists today: ``(1)
young healthy adults exposed to >=80 ppb ozone develop significant
reversible, transient decrements in pulmonary function and symptoms of
breathing discomfort if minute ventilation (Ve) or duration of exposure
is increased sufficiently; (2) relative to young adults, children
experience similar spirometric responses [i.e., as measured by
FEV1 and/or FVC] but lower incidence of symptoms from
O3 exposure; (3) relative to young adults, ozone-induced
spirometric responses are decreased in older individuals; (4) there is
a large degree of inter-subject variability in physiologic and
symptomatic responses to O3, but responses tend to be
reproducible within a given individual over a period of several months;
and (5) subjects exposed repeatedly to O3 for several days
experience an attenuation of spirometric and symptomatic responses on
successive exposures, which is lost after about a week without
exposure'' (ISA, Appendix 3, section 3.1.4.1.1, p. 3-11).\49\ Repeated
daily exposure studies at higher concentrations, such as 300 ppb, have
found FEV1 responses to be enhanced on the second day of
exposure. This enhanced response is absent, however, with repeated
exposure at lower concentrations, perhaps as a result of a more
complete recovery or less damage to pulmonary tissues (2013 ISA,
section pp. 6-13 to 6-14; Folinsbee et al., 1994).
---------------------------------------------------------------------------
\49\ A spirometric response refers to a change in the amount of
air breathed out of the body (forced expiratory volumes) and the
associated time to do so (e.g., FEV1).
---------------------------------------------------------------------------
With regard to airway inflammation and the potential for repeated
occurrences to contribute to further effects, O3-induced
respiratory tract inflammation ``can have several potential outcomes:
(1) Inflammation induced by a single exposure (or several exposures
over the course of a summer) can resolve entirely; (2) continued acute
inflammation can evolve into a chronic inflammatory state; (3)
continued inflammation can alter the structure and function of other
pulmonary tissue, leading to diseases such as fibrosis; (4)
inflammation can alter the body's host defense response to inhaled
microorganisms, particularly in potentially at-risk populations such as
the very young and old; and (5) inflammation can alter the lung's
response to other agents such as allergens or toxins'' (2013 ISA, p. 6-
76; ISA Appendix 3, section 3.1.5.6). With regard to O3-
induced increases in airway responsiveness, the controlled human
exposure study evidence for healthy adults generally indicates
resolution within 18 to 24 hours after exposure, with slightly longer
persistence in some individuals (ISA, Appendix 3, section 3.1.4.3.1;
2013 ISA, p. 6-74; Folinsbee and Hazucha, 2000).
The array of O3-associated respiratory effects,
including reduced lung function, respiratory symptoms, increased airway
responsiveness, and inflammation are of increased significance to
people with asthma given aspects of the disease that contribute to a
baseline status that includes chronic airway inflammation and greater
airway responsiveness than people without asthma (ISA, section 3.1.5).
For example, O3 exposure of a magnitude that increases
airway responsiveness may put such people at potential increased risk
for prolonged bronchoconstriction in response to asthma triggers (ISA,
Appendix 3, p. 3-7, 3-28; 2013 ISA, section 6.2.9; 2006 AQCD, section
8.4.2). The increased significance of effects in people with asthma and
risk of increased exposure for children (from greater frequency of
outdoor exercise) \50\ is illustrated by the epidemiologic findings of
positive associations between O3 exposure and asthma-related
emergency department visits and hospital admissions for children with
asthma. Thus, the evidence indicates O3 exposure to increase
the risk of asthma exacerbation, and associated outcomes, in children
with asthma.
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\50\ Children are the age group most likely to be outdoors at
activity levels corresponding to those that have been associated
with respiratory effects in the human exposure studies (PA, Appendix
3D, section 3D.2.5.3), as recognized in section II.A.2.b below.
---------------------------------------------------------------------------
With regard to an increased susceptibility to infectious diseases,
the experimental animal evidence continues to indicate, as described in
the 2013 ISA and past AQCDs, the potential role for O3
exposures through effects on defense mechanisms of the respiratory
tract (ISA, section 3.1.7.3; 2013 ISA, section 6.2.5). The evidence
base regarding respiratory infections and associated effects has been
augmented in this review by a number of epidemiologic studies reporting
positive associations between short-term O3 concentrations
and emergency department visits for a variety of respiratory infection
endpoints (ISA, Appendix 3, section 3.1.7).
Although the long-term exposure conditions that may contribute to
further respiratory effects are less well understood, experimental
studies, including with nonhuman infant primates, have provided
evidence relating O3 exposure to asthma-like effects, and
epidemiologic cohort studies have reported associations of
O3 concentrations in ambient air with asthma development in
children (ISA, IS.4.3.2 and Appendix 3, sections 3.2.4.1.3 and 3.2.6).
The biological plausibility of such a role for O3 has been
indicated by animal toxicological
[[Page 87270]]
evidence on biological mechanisms (ISA, Appendix 3, sections 3.2.3 and
3.2.4.1.2).
Overall, the respiratory effects evidence newly available in this
review is consistent with the evidence base in the last review,
supporting a generally similar understanding of the respiratory effects
of O3 (ISA, Appendix 3, section 3.1.4). A few recent studies
provide insights in previously unexamined areas, both with regard to
human study groups and animal models for different effects, while other
studies confirm and provide depth to prior findings with updated
protocols and techniques (ISA, Appendix 3, sections 3.1.11 and 3.2.6).
Newly available epidemiologic studies of hospital admissions and
emergency department visits for a variety of respiratory outcomes
supplement the previously available evidence with additional findings
of consistent associations with O3 concentrations across a
number of study locations (ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5,
3.1.6.1.1, 3.1.7.1 and 3.1.8). These studies include a number that
report positive associations for asthma-related outcomes, as well as a
few for COPD-related outcomes. Together these epidemiologic studies
continue to indicate the potential for O3 exposures to
contribute to such serious health outcomes, particularly for people
with asthma.
As was the case for the evidence available in the last review, the
currently available evidence for health effects other than those of
O3 exposures on the respiratory system is more uncertain
than that for respiratory effects.\51\ Further, the evidence now
available has contributed to changes in conclusions for some of these
effects. For example, the current evidence for cardiovascular effects
and mortality, expanded from that in the last review, is no longer
considered sufficient to conclude that the relationships of short-term
exposure with these effects are likely to be causal (ISA, sections
IS.4.3.4 and IS.4.3.5). These changes stem from newly available
evidence in combination with the uncertainties recognized for the
evidence available in the last review.\52\ Although there exists
largely consistent evidence for a limited number of O3-
induced cardiovascular endpoints in animal toxicological studies and
cardiovascular mortality in epidemiologic studies, there is a general
lack of coherence between these results and findings in controlled
human exposure and epidemiologic studies of cardiovascular health
outcomes (ISA, section IS.1.3.1, Appendix 6, section 6.1.8). The
relationships are now characterized as suggestive of, but not
sufficient to infer, a causal relationship (ISA, Appendix 4, section
4.1.17; Appendix 6, section 6.1.8).
---------------------------------------------------------------------------
\51\ For example, the available evidence for reproductive and
developmental effects, as well as for effects on the nervous system,
is suggestive of, but not sufficient to infer, a causal relationship
(ISA, section IS.4.3.6.5 and Table IS-1).
\52\ These aspects of the current evidence base include: (1) A
now-larger body of controlled human exposure studies providing
evidence that is not consistent with a cardiovascular effect in
response to short-term O3 exposure; (2) a paucity of
epidemiologic evidence indicating more severe cardiovascular
morbidity endpoints (e.g., emergency department visits and hospital
visits for cardiovascular endpoints including myocardial
infarctions, heart failure or stroke) that could connect the
evidence for impaired vascular and cardiac function from animal
toxicological studies with the evidence from epidemiologic studies
of cardiovascular mortality; and (3) the remaining uncertainties and
limitations recognized in the 2013 ISA (e.g., lack of control for
potential confounding by copollutants in epidemiologic studies)
still remain.
---------------------------------------------------------------------------
With regard to metabolic effects of short-term O3
exposures, the evidence comes primarily from experimental animal study
findings, with a limited number of epidemiologic studies (ISA, section
IS.4.3.3 and Appendix 5, section 5.1.8 and Table 5-3). The exposure
conditions from the animal studies generally involve much higher
O3 concentrations (e.g., 4-hour concentrations of 400 to 800
ppb [ISA, Appendix 5, Tables 5-8 and 5-10]) than those commonly
occurring in areas of the U.S. where the current standard is met, and
the concentration in the available controlled human exposure study is
similarly high, at 300 ppb (ISA, sections 5.1.3, 5.1.5 and 5.1.8, Table
5-3). The evidence for metabolic effects and long-term exposures is
concluded to be suggestive of, but not sufficient to infer, a causal
relationship (ISA, section IS.4.3.6.2).
b. Public Health Implications and At-Risk Populations
The public health implications of the evidence regarding
O3-related health effects, as for other effects, are
dependent on the type and severity of the effects, as well as the size
of the population affected. Judgments or interpretative statements
developed by public health experts, particularly experts in respiratory
health, also inform consideration of public health implications.
With regard to O3 in ambient air, the potential public
health impacts relate most importantly to respiratory effects.
Controlled human exposure studies have documented reduced lung
function, respiratory symptoms, increased airway responsiveness, and
inflammation, among other effects, in healthy adults exposed while at
elevated ventilation, such as while exercising. Ozone effects in
individuals with compromised respiratory function, such as individuals
with asthma, are plausibly related to emergency department visits and
hospital admissions for asthma which have been associated with ambient
air concentrations of O3 in epidemiologic studies (as
summarized in section II.A.2.a above; 2013 ISA, section 6.2.7; ISA,
Appendix 3, sections 3.1.5.1 and 3.1.5.2).
The clinical significance of individual responses to O3
exposure depends on the health status of the individual, the magnitude
of the responses, the severity of respiratory symptoms, and the
duration of the response. While a particular reduction in
FEV1 or increase in inflammation or airway responsiveness
may not be of concern for a healthy group, it may increase the risk of
a more severe effect in a group with asthma. As a more specific
example, the same increase in inflammation or airway responsiveness in
individuals with asthma could predispose them to an asthma exacerbation
event triggered by an allergen to which they may be sensitized (e.g.,
ISA, Appendix 3, section 3.1.5.6.1; 2013 ISA, sections 6.2.3 and
6.2.6). Duration and frequency of documented effects is also reasonably
expected to influence potential adversity and interference with normal
activity.\53\ In summary, consideration of differences in magnitude or
severity, and also the relative transience or persistence of the
responses (e.g., FEV1 changes) and respiratory symptoms, as
well as pre-existing sensitivity to effects on the respiratory system,
and other factors, are important to characterizing implications for
public health effects of an air pollutant such as O3 (ATS,
2000; Thurston et al., 2017).
---------------------------------------------------------------------------
\53\ For example, for most healthy individuals moderate effects
on pulmonary function, such as transient FEV1 decrements
smaller than 20% or transient respiratory symptoms, such as cough or
discomfort on exercise or deep breath, would not be expected to
interfere with normal activity, while larger effects on pulmonary
function (e.g., FEV1 decrements of 20% or larger lasting
longer than 24 hours) and/or more severe respiratory symptoms are
more likely to interfere with normal activity (e.g., PA, p. 3-30;
2006 AQCD, Table 8-2).
---------------------------------------------------------------------------
Decisions made in past reviews of the O3 primary
standard and associated judgments regarding adversity or health
significance of measurable physiological responses to air pollutants
have been informed by guidance, criteria or interpretative statements
developed within the public health community, including the ATS, an
organization of respiratory disease specialists, as well as
[[Page 87271]]
the advice from the CASAC. The ATS released its initial statement
(titled Guidelines as to What Constitutes an Adverse Respiratory Health
Effect, with Special Reference to Epidemiologic Studies of Air
Pollution) in 1985 and updated it in 2000 (ATS, 1985; ATS, 2000). The
ATS described its 2000 statement, considered in the last review of the
O3 standard, as being intended to ``provide guidance to
policy makers and others who interpret the scientific evidence on the
health effects of air pollution for the purposes of risk management''
(ATS, 2000). The recent statement further notes that it does not offer
``strict rules or numerical criteria, but rather proposes
considerations to be weighed in setting boundaries between adverse and
nonadverse health effects,'' providing a general framework for
interpreting evidence that proposes a ``set of considerations that can
be applied in forming judgments'' for this context (Thurston et al.,
2017). Similarly, in the 2000 statement, the ATS describes it as
proposing ``principles to be used in weighing the evidence and setting
boundaries'' and states that ``the placement of dividing lines should
be a societal judgment'' (ATS, 2000). The ATS explicitly states that it
does ``not attempt to provide an exact definition or fixed list of
health impacts that are, or are not, adverse,'' providing instead ``a
number of generalizable `considerations''' (ATS, 2000). The ATS state
there ``cannot be precise numerical criteria, as broad clinical
knowledge and scientific judgments, which can change over time, must be
factors in determining adversity'' (ATS, 2000).
With regard to pulmonary function decrements, the earlier ATS
statement concluded that ``small transient changes in forced expiratory
volume in 1 s[econd] (FEV1) alone were not necessarily
adverse in healthy individuals but should be considered adverse when
accompanied by symptoms'' (ATS, 2000). The more recent ATS statement
continues to support this conclusion and also gives weight to findings
of small lung function changes in the absence of respiratory symptoms
in individuals with pre-existing compromised function, such as that
resulting from asthma (Thurston et al., 2017). In keeping with the
intent of these statements to avoid specific criteria, neither
statement provides more specific descriptions of such responses, such
as with regard to magnitude, duration or frequency, for consideration
of such conclusions. The earlier ATS statement, in addition to
emphasizing clinically relevant effects, also emphasized both the need
to consider changes in ``the risk profile of the exposed population,''
and effects on the portion of the population that may have a diminished
reserve that puts its members at potentially increased risk if affected
by another agent (ATS, 2000). These concepts, including the
consideration of the magnitude of effects occurring in just a subset of
study subjects, continue to be recognized as important in the more
recent ATS statement (Thurston et al., 2017) and continue to be
relevant to the evidence base for O3.
The information newly available in this review regarding
O3 exposure and health effects among sensitive populations,
thoroughly evaluated in the ISA, has not altered our understanding of
human populations at particular risk of health effects from
O3 exposures (ISA, section IS.4.4). The respiratory effects
evidence, extending decades into the past and augmented by new studies
in this review, supports the conclusion that ``individuals with pre-
existing asthma are at greater risk of ozone-related health effects
based on the substantial and consistent evidence within epidemiologic
studies and the coherence with toxicological studies'' (ISA, p. IS-57).
Numerous epidemiologic studies document associations of O3
with asthma exacerbation. Such studies indicate the associations to be
strongest for populations of children which is consistent with their
generally greater time outdoors while at elevated exertion. Together,
these considerations indicate people with asthma, including
particularly children with asthma, to be at relatively greater risk of
O3-related effects than other members of the general
population (ISA, section IS.4.4.2 and Appendix 3).\54\
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\54\ Populations or lifestages can be at increased risk of an
air pollutant-related health effect due to one or more of a number
of factors. These factors can be intrinsic, such as physiological
factors that may influence the internal dose or toxicity of a
pollutant, or extrinsic, such as sociodemographic, or behavioral
factors.
---------------------------------------------------------------------------
With respect to people with asthma, the limited evidence from
controlled human exposure studies (which are primarily in adult
subjects) indicates similar magnitude of FEV1 decrements as
in people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Across
studies of other respiratory effects of O3 (e.g., increased
respiratory symptoms, increased airway responsiveness and increased
lung inflammation), the responses observed in study subjects generally
do not differ due to the presence of asthma, although the evidence base
is more limited with regard to study subjects with asthma (ISA,
Appendix 3, section 3.1.5.7). However, the features of asthma (e.g.,
increased airway responsiveness) contribute to a risk of asthma-related
responses, such as asthma exacerbation in response to asthma triggers,
which may increase the risk of more severe health outcomes (ISA,
section 3.1.5). For example, a particularly strong and consistent
component of the epidemiologic evidence is the appreciable number of
epidemiologic studies that demonstrate associations between ambient
O3 concentrations and hospital admissions and emergency
department visits for asthma (ISA, section IS.4.4.3.1). The strongest
associations (e.g., highest effect estimates) or associations more
likely to be statistically significant are those for childhood age
groups, which are age groups most likely to spend time outdoors during
afternoon periods (when O3 may be highest) and at activity
levels corresponding to those that have been associated with
respiratory effects in the human exposure studies (ISA, Appendix 3,
sections 3.1.4.1 and 3.1.4.2).\55\ The epidemiologic studies of
hospital admissions and emergency department visits are augmented by a
large body of individual-level epidemiologic panel studies that
demonstrated associations of short-term ozone concentrations with
respiratory symptoms in children with asthma. Additional support comes
from epidemiologic studies that observed O3-associated
increases in indicators of airway inflammation and oxidative stress in
children with asthma (ISA, section IS.4.3.1). Together, this evidence
continues to indicate the increased risk of population groups with
asthma,
[[Page 87272]]
including particularly, children (ISA, Appendix 3, section 3.1.5.7).
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\55\ Evaluations of activity pattern data in current and last
review indicate children to more frequently spend time outdoors
during afternoon and early evening hours, while at moderate or
greater exertion level, than other age groups (PA, Appendix 3D,
section 3D.2.5.3, including Figure 3D-9; 2014 HREA, section 5.4.1.5
and Appendix 5G, section 5G-1.4). For example, for days with some
time spent outdoors, children spend, on average, approximately 2\1/
4\ hours of afternoon time outdoors, 80% of which is at a moderate
or greater exertion level, regardless of their asthma status (PA,
Appendix 3D, section 3D.2.5.3). Adults, for days having some time
spent outdoors, also spend approximately 2\1/4\ hours of afternoon
time outdoors regardless of their asthma status but the percent of
afternoon time at moderate or greater exertion levels for adults
(about 55%) is lower than that observed for children. Such analyses
also note greater participation in outdoor events during the
afternoon, compared to other times of day, for children ages 6
through 19 years old during the warm season months (ISA, Appendix 2,
section 2.4.1, Table 2-1). Analyses of the limited activity pattern
data by health status do not indicate asthma status to have
appreciable impact (PA, Appendix 3D, section 3D.2.5.3; 2014 HREA,
section 5.4.1.5).
---------------------------------------------------------------------------
Children, and also outdoor adult workers, are at increased risk
largely due to their generally greater time spent outdoors while at
elevated exertion rates (including in summer afternoons and early
evenings when O3 levels may be higher). This behavior makes
them more likely to be exposed to O3 in ambient air, under
conditions contributing to increased dose, e.g., elevated ventilation
taking greater air volumes into the lungs \56\ (2013 ISA, section
5.2.2.7). In light of the evidence summarized in the prior paragraph,
children and outdoor workers with asthma may be at increased risk of
more severe outcomes, such as asthma exacerbation. Further, there is
experimental evidence from early life exposures of nonhuman primates
that indicates potential for effects in childhood when human
respiratory systems are under development \57\ (ISA, section
IS.4.4.4.1). Overall, the evidence available in the current review,
while not increasing our knowledge about susceptibility or at-risk
status of these population groups, is consistent with that in the last
review (ISA, section IS.4.4).\58\
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\56\ Additionally, compared to adults, children have higher
ventilation rates relative to their lung volume which tends to
increase the dose normalized to lung surface area. (ISA, p. IS-60).
\57\ Human lung development begins during the fetal period and
continues into early adulthood. This continued development comprises
an extended window of potential vulnerability to O3 (ISA,
p. 3-99).
\58\ Evidence available in the current review for older adults,
a population identified as at risk in the last review, adds little
to the evidence previously available (ISA, sections IS.4.4.2 and
IS.4.4.4.2). The ISA notes, however, that ``[t]he majority of
evidence for older adults being at increased risk of health effects
related to ozone exposure comes from studies of short-term ozone
exposure and mortality evaluated in the 2013 Ozone ISA'' (ISA, p.
IS-52). Such studies are part of the larger evidence base that is
now concluded to be suggestive, but not sufficient to infer a causal
relationship of O3 with mortality (ISA, sections IS.4.3.5
and IS.4.4.4.2, Appendix 4, section 4.1.16.1 and 4.1.17).
---------------------------------------------------------------------------
The ISA also expressly considered the evidence regarding
O3 exposure and health effects among populations with
several other potential risk factors. As in the last review, the
evidence for low income and minority populations, remains
``suggestive'' of increased risk, and includes several inconsistencies
(ISA, Tables IS-9 and IS-10).\59\ The ISA in the last review
additionally identified a role for dietary anti-oxidants such as
vitamins C and E in influencing risk of O3-related effects,
such as inflammation, as well as a role for genetic factors to also
confer either an increased or decreased risk (2013 ISA, sections 8.1
and 8.4.1). No newly available evidence has been evaluated that would
inform or change these prior conclusions (ISA, section IS.4.4 and Table
IS-10).
---------------------------------------------------------------------------
\59\ The 2013 ISA concluded that the overall evidence is
suggestive of socioeconomic economic status (SES) as a factor
affecting risk of O3-related health outcomes ``based on
collective evidence from epidemiologic studies of respiratory
hospital admissions but inconsistency among epidemiologic studies of
mortality and reproductive outcomes,'' additionally stating that
``[f]urther studies are needed to confirm this relationship,
especially in populations within the U.S.'' (2013 ISA, p. 8-28). The
evidence available in the current review adds little to the evidence
available at the time of the last review in this area (ISA, section
IS.4.4.2 and Table IS-10). Other factors for which the evidence
remains suggestive of an influence on risk status are being male or
being female and pre-existing obesity (ISA, Table IS-10).
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The magnitude and characterization of a public health impact is
dependent upon the size and characteristics of the populations
affected, as well as the type or severity of the effects. As summarized
above, a population most at risk of health effects associated with
O3 in ambient air is people with asthma. The National Center
for Health Statistics data for 2017 indicate that approximately 7.9% of
the U.S. populations has asthma (CDC, 2019; PA, Table 3-1) and this is
one of the principal populations that the primary O3 NAAQS
is designed to protect (80 FR 65294, October 26, 2015). Children under
the age of 18 account for 16.7% of the total U.S. population, with 6.2%
of the total population being children under 5 years of age (U.S.
Census Bureau, 2019). Another at-risk population group, also due to
time and activity outdoors, is outdoor workers.\60\ Population groups
with relatively greater asthma prevalence, such as populations in
poverty and children \61\ (CDC, 2019, Tables 3-1 and 4-1; PA, Table 3-
1), might be expected to have a relatively greater potential for
O3-related health impacts.\62\
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\60\ For example, jobs in construction and extraction
occupations and protective service occupations, as well as
installation, maintenance and repair occupations and building and
grounds cleaning and maintenance operations, had high percentages of
employees who spent part of their workday outdoors (Bureau of Labor
Statistics, 2017). Such jobs often include physically demanding
tasks and involve increased ventilation rates, increasing the
potential for exposure to O3.
\61\ In 2017 and 2018, the prevalence of asthma in children 0 to
17 years old was 8.4% and 7.5% respectively (CDC, 2019).
\62\ As the current standard was set to protect at-risk
populations, such as people with asthma, populations with asthma
living in areas not meeting the standard would be expected to be at
greater risk of effects than others in those areas.
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c. Exposure Concentrations Associated With Effects
The extensive evidence base for O3 health effects,
compiled over several decades, continues to indicate respiratory
responses to short-term exposures as the most sensitive effects. As at
the time of the last review, our conclusions regarding O3
exposure concentrations associated with respiratory effects reflect the
extensive longstanding evidence base of controlled human exposure
studies of short-term exposures of people with and without asthma (ISA,
Appendix 3). As summarized in section II.A.2.a above, these studies
have documented an array of respiratory effects, including reduced lung
function, respiratory symptoms, increased airway responsiveness, and
inflammation, in study subjects following 1- to 8-hour exposures,
primarily while exercising.\63\
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\63\ The risk of more severe health outcomes associated with
such effects is increased in people with asthma as illustrated by
the epidemiologic findings of positive associations between
O3 exposure and asthma-related ED visits and hospital
admissions.
---------------------------------------------------------------------------
The current evidence, including that newly available in this
review, does not alter the scientific conclusions reached in the last
review on exposure duration and concentrations associated with
O3-related health effects. These conclusions were largely
based on the body of evidence from the controlled human exposure
studies. A limited number of controlled human exposure studies are
newly available in the current review, with none involving lower
exposure concentrations than those previously studied or finding
effects not previously reported (ISA, Appendix 3, section 3.1.4).\64\
---------------------------------------------------------------------------
\64\ The newly available 3-hour controlled human exposure
studies (involving intermittent exercise) reported statistically
significant respiratory response at 120 ppb in adults 55 to 70 years
old (ISA, Appendix 3, section 3.1.4; PA, Appendix 3A, Table 3A-3).
---------------------------------------------------------------------------
The severity of observed responses, the percentage of individuals
responding, and strength of statistical significance at the study group
level have been found to increase with increasing exposure (ISA; 2013
ISA; 2006 AQCD). For example, the magnitude of respiratory response
(e.g., size of lung function reductions and magnitude of symptom
scores) documented in the controlled human exposure studies is
influenced by ventilation rate, exposure duration, and exposure
concentration. When performing physical activities requiring elevated
exertion, ventilation rate is increased, leading to greater potential
for health effects due to an increased internal dose (2013 ISA, section
6.2.1.1, pp. 6-5 to 6-11). Accordingly, the exposure concentrations
eliciting a given level of response after a given exposure duration is
lower for subjects exposed while at elevated ventilation, such as while
exercising (2013 ISA, pp.
[[Page 87273]]
6-5 to 6-6; ISA Appendix 3, section 3.1.4.2). For example, in studies
of healthy young adults exposed while at rest for 2 hours, 500 ppb is
the lowest concentration eliciting a statistically significant
O3-induced group mean lung function decrement, while a 1- to
2-hour exposure to 120 ppb produces a statistically significant
response in lung function when the ventilation rate of the group of
study subjects is sufficiently increased with exercise (2013 ISA, pp.
6-5 to 6-6).\65\
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\65\ The lowest exposure concentration that has elicited a
statistically significant O3-induced reduction in group
mean lung function in an exposure of 2 hours or less is 120 ppb,
occurring in trained cyclists after a 1-hour exposure during
continuous, very heavy exercise (2013 ISA, section 6.2.1.1; Gong et
al., 1986) and in young healthy adults after a 2-hour exposure
during intermittent heavy exercise (2013 ISA, section 6.2.1.1;
McDonnell et al., 1983).
---------------------------------------------------------------------------
The exposure conditions (e.g., duration and exercise) given primary
focus in the past several O3 NAAQS reviews are those of the
6.6-hour study design, which involves six 50-minute exercise periods
during which subjects maintain a moderate level of exertion to achieve
a ventilation rate of approximately 20 L/min per m\2\ body surface area
while exercising.\66\ The 6.6 hours of exposure in these quasi-
continuous exercise studies has generally occurred in an enclosed
chamber and the study design includes three hours in each of which is a
50-minute exercise period and a 10-minute rest period, followed by a
35-minute lunch (rest) period, which is followed by three more hours of
exercise and rest, as before lunch.\67\ Most of these studies performed
to date involve exposure maintained at a constant (unchanging)
concentration for the full duration, although a subset of studies have
concentrations that vary (generally in a stepwise manner) across the
exposure period and are selected so as to achieve a specific target
concentration as the exposure average.\68\
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\66\ Ventilation rate (VE) is a specific technical
term referring to breathing rate in terms of volume of air taken
into the body per unit of time. The units for VE are
usually liters (L) per minute (min). Another related term is
equivalent ventilation rate (EVR), which refers to VE
normalized by a person's body surface area in square meters (m\2\).
Accordingly, the units for EVR are generally L/min per m\2\.
\67\ A few studies have involved exposures by facemask rather
than freely breathing in a chamber. To date, there is little
research differentiating between exposures conducted with a facemask
and in a chamber since the pulmonary responses of interest do not
seem to be influenced by the exposure mechanism. However, similar
responses have been seen in studies using both exposure methods at
higher O3 concentrations (Adams, 2002; Adams, 2003). In
the facemask designs, there is a short period of zero O3
exposure, such that the total period of exposure is closer to 6
hours than 6.6 (Adams, 2000; Adams, 2002; Adams, 2003).
\68\ In these studies, the exposure concentration changes for
each of the six hours in which there is exercise and the
concentration during the 35-minute lunch is the same as in the prior
(third) hour with exercise. For example, in the study by Adams
(2006), the protocol for the 6.6-hour period is as follows: 60
minutes at 40 ppb, 60 minutes at 70 ppb, 95 minutes at 90 ppb, 60
minutes at 70 ppb, 60 minutes at 50 ppb and 60 minutes at 40 ppb.
---------------------------------------------------------------------------
Evidence from studies with similar duration and quasi-continuous
exercise aspects (6.6-hour duration with six 50-minute exercise
periods) demonstrates an exposure-response (E-R) relationship for
O3-induced reduction in lung function (Table 1; ISA,
Appendix 3, Figure 3-3 PA, Figure 3-2).\69\ No studies of the 6.6-hour
design are newly available in this review. The previously available
studies of this design document statistically significant
O3-induced reduction in lung function (FEV1) and
increased pulmonary inflammation in young healthy adults exposed to
O3 concentrations as low as 60 ppb. Statistically
significant group mean changes in FEV1, also often
accompanied by statistically significant increases in respiratory
symptoms, become more consistent across such studies of exposures to
higher O3 concentrations, such as somewhat above 70 ppb (73
ppb),\70\ and 80 ppb (Table 1 and Appendix 3A, Table 3A-1). The lowest
exposures concentration for which these studies document a
statistically significant increase in respiratory symptoms is somewhat
above 70 ppb, at 73 ppb (Schelegle et al., 2009). In the 6.6-hour
studies, the group means of O3-induced \71\ FEV1
reductions for target exposure concentrations at or below 70 ppb are
approximately 6% or lower (Table 1). For example, the group means of
O3-induced FEV1 decrements reported in these
studies that are statistically significantly different from the
responses in filtered air are 6.1% for 70 ppb and 1.7% to 3.5% for 60
ppb (Table 1).
---------------------------------------------------------------------------
\69\ The relationship also exists for size of FEV1
decrement with alternative exposure or dose metrics, including total
inhaled O3 and intake volume averaged concentration (ISA,
Appendix 3).
\70\ The design for the study on which the 70 ppb benchmark
concentration is based, Schelegle et al. (2009), involved varying
concentrations across the full exposure period, with a 35-minute
lunch period following the third exposure hour during which the
exposure concentration remains the same as in the third hour. The
study reported the average O3 concentration measured
during each of the six exercise periods. The mean concentration
across these six values is 72 ppb. The time weighted average for the
full 6.6-hour exposure period, based on the six reported
measurements and the study design, is 73 ppb (Schelegle et al.,
2009). Other 6.6-hour studies have not reported measured
concentrations for each exposure, but have generally reported an
exposure concentration precision at or tighter than 3 ppb (e.g.,
Adams 2006).
\71\ Consistent with the ISA and 2013 ISA, the phrase
``O3-induced'' decrement or reduction in lung function or
FEV1 refers to the percent change from pre-exposure
measurement of the O3 exposure minus the percent change
from pre-exposure measurement of the filtered air exposure (2013
ISA, p. 6-4).
---------------------------------------------------------------------------
The group mean O3-induced FEV1 decrements
generally increase with increasing O3 exposures, reflecting
increases in both the number of the individuals experiencing
FEV1 reductions and the magnitude of the FEV1
reduction (Table 1; ISA, Appendix 3, Figure 3-3; PA, Figure 3-2). For
example, following 6.6-hour exposures to a lower concentration (40
ppb), for which decrements were not statistically significant at the
group mean level, none of 60 subjects across two separate studies
experienced an O3-induced FEV1 reduction as large
as 15% or more (Table 1; PA, Appendix 3D, Table 3D-19). The group mean
O3-induced FEV1 decrements generally increase
with increasing O3 exposures, reflecting increases in both
the number of the individuals experiencing FEV1 reductions
and the magnitude of the FEV1 reduction (Table 1; ISA,
Appendix 3, Figure 3-3; PA, Figure 3-2). For example, following 6.6-
hour exposures to a lower concentration (40 ppb), for which decrements
were not statistically significant at the group mean level, none of 60
subjects across two separate studies experienced an O3-
induced FEV1 reduction as large as 15% or more (Table 1; PA,
Appendix 3D, Table 3D-19). Across the four experiments (with number of
subjects ranging from 30 to 59) that have reported results for a 60 ppb
target exposure,\72\ the number of subjects experiencing this magnitude
of FEV1 reduction (at or above 15%) varied (zero of 30, one
of 59, two of 31 and two of 30 exposed subjects), while, together, they
represent 3% of all 150 subjects. This percentage of subjects (with
reductions of 15% or more) increased to 10% (three of 31 subjects) for
the study at 73 ppb (70 ppb target) (PA, Appendix 3D, Table 3D-19;
Schelegle et al., 2009), and is higher still (16%) in a variable
exposure study at 80 ppb (PA, Appendix 3D, Table 3D-20; Schelegle et
al., 2009). In addition to illustrating the E-R relationship, these
findings also illustrate the considerable variability in magnitude of
responses observed among study subjects (ISA, Appendix 3, section
3.1.4.1.1; 2013 ISA, p. 6-13).\73\
---------------------------------------------------------------------------
\72\ For these four experiments, the average concentration
across the 6.6 hour period ranged from 60 to 63 ppb (PA, Appendix
3A, Table 3A-2).
\73\ With regard to decrements at or above 10%, the percentages
of study subjects with such a response increased from 7% of the 150
subjects of the four studies with target exposures of 60 ppb
(average exposure ranged from 60 to 63) to 19% for the study at 73
ppb to more than 32% in one variable exposure study of 80 ppb (PA,
Appendix 3D, Table 3D-20).
[[Page 87274]]
Table 1--Summary of 6.6-Hour Controlled Human Exposure Study-Findings, Healthy Adults
--------------------------------------------------------------------------------------------------------------------------------------------------------
O3 target exposure concentration Statistically O3-induced group mean
Endpoint \A\ significant effect \B\ response \B\ Study
--------------------------------------------------------------------------------------------------------------------------------------------------------
FEV1 Reduction...................... 120 ppb........................... Yes..................... -10.3% to -15.9% \C\... Horstman et al. 1990; Adams
2002; Folinsbee et al.
(1988); Folinsbee et al.
(1994); Adams, 2002; Adams
2000; Adams and Ollison
1997.\D\
100 ppb........................... Yes..................... -8.5% to -13.9% \C\.... Horstman et al., 1990;
McDonnell et al., 1991.\D\
87 ppb............................ Yes..................... -12.2%................. Schelegle et al., 2009.
80 ppb............................ Yes..................... -7.5%.................. Horstman et al., 1990.
-7.7%.................. McDonnell et al., 1991.
-6.5%.................. Adams, 2002.
-6.2% to -5.5% \C\..... Adams, 2003.
-7.0% to -6.1% \C\..... Adams, 2006.
-7.8%.................. Schelegle et al., 2009.
ND \E\.................. -3.5%.................. Kim et al., 2011.\F\
70 ppb............................ Yes..................... -6.1%.................. Schelegle et al., 2009.
60 ppb............................ Yes \G\................. -2.9%.................. Adams, 2006; Brown et al.,
-2.8%.................. 2008.
Yes..................... -1.7%.................. Kim et al., 2011.
No...................... -3.5%.................. Schelegle et al., 2009.
40 ppb............................ No...................... -1.2%.................. Adams, 2002.
No...................... -0.2%.................. Adams, 2006.
Increased Respiratory Symptoms...... 120 ppb........................... Yes..................... Increased symptom Horstman et al. 1990; Adams
scores. 2002; Folinsbee et al.
1988; Folinsbee et al.
1994; Adams, 2002; Adams
2000; Adams and Ollison
1997; Horstman et al.,
1990; McDonnell et al.,
1991; Schelegle et al.,
2009; Adams, 2003; Adams,
2006.\H\
100 ppb........................... Yes.....................
87 ppb............................ Yes.....................
80 ppb............................ Yes.....................
70 ppb............................ Yes.....................
60 ppb............................ No...................... ....................... Adams, 2006; Kim et al.,
40 ppb............................ No...................... 2011; Schelegle et al.,
2009; Adams, 2002.\H\
Airway Inflammation................. 80 ppb............................ Yes..................... Multiple indicators \I\ Devlin et al., 1991; Alexis
et al., 2010.
60 ppb............................ Yes..................... Increased neutrophils.. Kim et al., 2011.
Increased Airway Resistance and 120 ppb........................... Yes..................... Increased.............. Horstman et al., 1990;
Responsiveness. Folinsbee et al., 1994 (O3
induced sRaw not
reported).
100 ppb........................... Yes..................... Horstman et al., 1990.
80 ppb............................ Yes..................... Horstman et al., 1990.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ This refers to the average concentration across the six exercise periods as targeted by authors. This differs from the time-weighted average
concentration for the full exposure periods (targeted or actual). For example, as shown in Appendix 3A, Table 3A-2, in chamber studies implementing a
varying concentration protocol with targets of 0.03, 0.07, 0.10, 0.15, 0.08 and 0.05 ppm, the exercise period average concentration is 0.08 ppm while
the time weighted average for the full exposure period (based on targets) is 0.082 ppm due to the 0.6 hour lunchtime exposure between periods 3 and 4.
In some cases this also differs from the exposure period average based on study measurements. For example, based on measurements reported in Schelegle
et al., (2009), the full exposure period average concentration for the 70 ppb target exposure is 73 ppb, and the average concentration during exercise
is 72 ppb.
\B\ Statistical significance based on the O3 compared to filtered air response at the study group mean (rounded here to decimal).
\C\ Ranges reflect the minimum to maximum FEV1 decrements across multiple exposure designs and studies. Study-specific values and exposure details
provided in the PA, Appendix 3A, Tables 3A-1 and 3A-2, respectively.
\D\ Citations for specific FEV1 findings for exposures above 70 ppb are provided in PA, Appendix 3A, Table 3A-1.
\E\ ND (not determined) indicates these data have not been subjected to statistical testing.
\F\ The data for 30 subjects exposed to 80 ppb by Kim et al. (2011) are presented in Figure 5 of McDonnell et al. (2012).
\G\ Adams (2006) reported FEV1 data for 60 ppb exposure by both constant and varying concentration designs. Subsequent analysis of the FEV1 data from
the former found the group mean O3 response to be statistically significant (p < 0.002) (Brown et al., 2008; 2013 ISA, section 6.2.1.1). The varying-
concentration design data were not analyzed by Brown et al., 2008.
\H\ Citations for study-specific respiratory symptoms findings are provided in the PA, Appendix 3A, Table 3A-1.
\I\ Increased numbers of bronchoalveolar neutrophils, permeability of respiratory tract epithelial lining, cell damage, production of proinflammatory
cytokines and prostaglandins (ISA, Appendix 3, section 3.1.4.4.1; 2013 ISA, section 6.2.3.1).
For shorter exposure periods (e.g., one to two hours), with heavy
intermittent or very heavy continuous exercise, higher exposure
concentrations, ranging up from 80 ppb up to 400 ppb, have been studied
(ISA, section 3.1; 2013 ISA, section 6.2.1.1; 2006 AQCD, chapter 6; PA,
Appendix 3A, Table 3A-3). Across these shorter-duration studies (which
involved ventilation rates 2-3 times greater than in the prolonged
[6.6- or 8-hour] exposure studies) the lowest exposure concentration
for which statistically significant respiratory effects were reported
is 120 ppb, for a 1-hour exposure combined with continuous very heavy
exercise and a 2-hour exposure with intermittent heavy exercise. As
recognized above, the increased ventilation rate associated with
increased exertion increases the
[[Page 87275]]
amount of O3 entering the lung, where depending on dose and
the individual's susceptibility, it may cause respiratory effects (2013
ISA, section 6.2.1.1). Thus, for exposures involving a lower exertion
level, a comparable response would not be expected to occur without a
longer exposure duration (ISA, Appendix 3, Figure 3-3; PA, Appendix 3A,
Table 3A-1).
With regard to the epidemiologic studies reporting associations
between O3 and respiratory health outcomes such as asthma-
related emergency department visits and hospitalizations, these studies
are generally focused on investigating the existence of a relationship
between O3 occurring in ambient air and specific health
outcomes. Accordingly, while as a whole, this evidence base of
epidemiologic studies provides strong support for the conclusions of
causality,\74\ these studies provide less information on details of the
specific O3 exposure circumstances that may be eliciting
health effects associated with such outcomes, and whether these occur
under air quality conditions that meet the current standard.\75\
Further, the vast majority of these studies were conducted in locations
and during time periods that would not have met the current
standard.\76\ The extent to which reported associations with health
outcomes in the resident populations in these studies are influenced by
the periods of higher concentrations during times that did not meet the
current standard is unknown. While this does not lessen their
importance in the evidence base documenting the causal relationship
between O3 and respiratory effects, it means they are less
informative in considering O3 exposure concentrations
occurring under air quality conditions allowed by the current standard.
---------------------------------------------------------------------------
\74\ Combined with the coherent evidence from experimental
studies, the epidemiologic studies ``can support and strengthen
determinations of the causal nature of the relationship between
health effects and exposure to ozone at relevant ambient air
concentrations'' (ISA, p. ES-17).
\75\ For example, these studies generally do not measure
personal exposures of the study population or track individuals in
the population with a defined exposure to O3 alone.
\76\ Consistent with the evaluation of the epidemiologic
evidence of associations between O3 exposure and
respiratory health effects in the ISA, this focuses on those studies
conducted in the U.S. and Canada as including populations and air
quality characteristics that may be most relevant to circumstances
in the U.S. (ISA, Appendix 3, section 3.1.2). Among the
epidemiologic studies finding a statistically significant positive
relationship of short- or long-term O3 concentrations
with respiratory effects, there are no single-city studies conducted
in the U.S. in locations with ambient air O3
concentrations that would have met the current standard for the
entire duration of the study (ISA, Appendix 3, Tables 3-13, 3-14, 3-
39, 3-41, 3-42 and Appendix 6, Tables 6-5 and 6-8; PA, Appendix 3B,
Table 3B-1). There are two single city studies conducted in Canada
that include locations for which the highest-monitor design values
calculated in the PA fell below 70 ppb, at 65 and 69 ppb (PA,
Appendix 3B, Table 3B-1; Kousha and Rowe, 2014; Villeneuve et al.,
2007). These studies did not include analysis of correlations with
other co-occurring pollutants or of the strength of the associations
when accounting for effects of copollutants in copollutant models
(ISA, Appendix 3, Tables 3-14 and 3-39).
---------------------------------------------------------------------------
With regard to the experimental animal evidence (largely in
rodents) and exposure conditions associated with respiratory effects,
the exposure concentrations are generally much greater than those
examined in the controlled human exposure studies (summarized above),
and higher than concentrations commonly occurring in ambient air in
areas of the U.S. where the current standard is met. This is also true
for the small number of early life studies in nonhuman primates that
reported O3 to contribute to asthma-like effects in infant
primates.\77\ The exposures eliciting the effects in these studies
included multiple 5-day periods with O3 concentrations of
500 ppb over 8-hours per day (ISA, Appendix 3, section 3.2.4.1.2).
---------------------------------------------------------------------------
\77\ These studies indicate that sufficient early-life
O3 exposure can cause structural and functional changes
that could potentially contribute to airway obstruction and
increased airway responsiveness (ISA, Table IS-10, p. 3-92 and p.3-
113).
---------------------------------------------------------------------------
Thus, as in the last review the exposures given greatest attention
in this review, particularly with regard to considering O3
exposures expected under air quality conditions that meet the current
standard, are those informed by the controlled human exposure studies.
The full body of evidence continues to indicate respiratory effects as
the effects associated with lowest exposures, with conditions of
exposure (duration, ventilation rate, as well as concentration)
influencing dose and associated response. Evidence for other categories
of effects does not indicate effects at comparably low exposures.\78\
---------------------------------------------------------------------------
\78\ For example, the evidence base for metabolic effects is
comprised primarily of experimental animal studies, and generally
involve much higher O3 concentrations (400-800 ppb, [ISA,
Appendix 5, Table 5-87]) than those examined in the controlled human
exposure studies of respiratory effects (and much higher than
concentrations commonly occurring in ambient air in areas of the
U.S. where the current standard is met). There are only two
epidemiologic studies reporting statistically significant positive
associations of O3 with metabolic effects (e.g., changes
in glucose, insulin, metabolic clearance), both based in Asian
countries, in which there is a potential for appreciable differences
from the U.S. in air quality patterns, limiting their usefulness for
informing our understanding of exposure concentrations and
conditions eliciting such effects in the U.S. (ISA, Appendix 5,
section 5.1).
---------------------------------------------------------------------------
3. Overview of Exposure and Risk Information
Consideration of the scientific evidence available in the current
review, as at the time of the last review, is informed by results from
quantitative analyses of estimated population exposure and consequent
risk of respiratory effects. These analyses in this review have focused
on exposure-based risk analyses, producing two types of risk metrics.
The first metric estimates population occurrences of daily maximum 7-
hour average exposure concentrations (during periods of elevated
breathing rates) at or above concentrations of potential concern
(benchmark concentrations). The second metric (lung function risk) uses
E-R information for O3 exposures and FEV1
decrements to estimate the portion of the simulated at-risk population
expected to experience one or more days with an O3-related
FEV1 decrement of at least 10%, 15% or 20%. Both of these
metrics were used to characterize health risk associated with
O3 exposures among the simulated population during periods
of elevated breathing rates. Similar risk metrics were also derived in
the 2014 HREA for the last review and the associated estimates informed
the Administrator's 2015 decision on the current standard (80 FR 65292,
October 26, 2015).
The currently available evidence in this review continues to
demonstrate a causal relationship between short-term O3
exposures and respiratory effects, with the current evidence base for
respiratory effects largely consistent with that for the last review,
as summarized in section II.A.2 above. Accordingly, the exposure-based
analyses performed in this review, summarized below, are conceptually
similar to those in the last review while also incorporating a number
of updates that contribute to reduced uncertainty. Drawing on the
summary in section II.C of the proposal, while giving relatively
greater focus on the comparison-to-benchmarks analysis, the short
sections below provide an overview of key aspects of the assessment
design (II.A.3.a), key limitations and uncertainties (II.A.3.b), and
exposure/risk estimates (II.A.3.c).
a. Key Design Aspects
Exposure and risk estimates were derived for air quality conditions
just meeting the current primary O3 standard, and for two
additional scenarios reflecting conditions just meeting design values
just lower and just higher than the level of the current
[[Page 87276]]
standard (65 and 75 ppb).\79\ The analyses estimated population
exposure and risk for simulated populations in eight urban study areas
which represent a variety of circumstances with regard to population
exposure to short-term concentrations of O3 in ambient air.
The areas (Atlanta, Boston, Dallas, Detroit, Philadelphia, Phoenix,
Sacramento and St. Louis) range in total population size from
approximately two to eight million and are distributed across seven
regions of the U.S.: Northeast, Southeast, Central, East North Central,
South, Southwest and West (PA, Appendix 3D, Table 3D-1). Study-area-
specific characteristics contribute to variation in the estimated
magnitude of exposure and associated risk across the urban study areas
that reflect an array of air quality, meteorological, and population
exposure conditions. The current set of study areas, streamlined
compared to the 15-area set in the last review, was chosen to ensure it
reflects the full range of air quality and exposure variation expected
in major urban areas in the U.S. with air quality that just meets the
current standard. Seven of the eight study areas were also included in
the 2014 HREA; the eighth study area (Phoenix) is newly added in the
current assessment to insure representation of a large city in the
southwest. Additionally, the O3 concentrations simulated in
these areas are somewhat nearer the current standard than was the case
for the 2014 HREA (PA, Appendix 3C, Table 3C and 2014 HREA, Table 4-1).
This contributes to a reduction in the uncertainty associated with
development of the air quality scenarios of interest, particularly the
one reflecting air quality conditions that just meet the current
standard.
---------------------------------------------------------------------------
\79\ All analyses are summarized more fully in the PA section
3.4 and Appendices 3C and 3D.
---------------------------------------------------------------------------
With regard to the objectives for the analysis approach, the
analyses and the use of a case study approach are intended to provide
assessments of air quality scenarios, including particularly one just
meeting the current standard, for a diverse set of areas and associated
exposed populations. These analyses are not intended to provide a
comprehensive national assessment (PA, section 3.4.1). Nor is the
objective to present an exhaustive analysis of exposure and risk in the
areas that currently just meet the current standard and/or of exposure
and risk associated with air quality adjusted to just meet the current
standard in areas that currently do not meet the standard. Rather, the
purpose is to assess, based on current tools and information, the
potential for exposures and risks beyond those indicated by the
information available at the time the standard was established.
Accordingly, use of this approach recognizes that capturing an
appropriate diversity in study areas and air quality conditions \80\ is
an important aspect of the role of the exposure and risk analyses in
informing the Administrator's conclusions on the public health
protection afforded by the current standard.
---------------------------------------------------------------------------
\80\ A broad variety of spatial and temporal patterns of
O3 concentrations can exist when ambient air
concentrations just meet the current standard. These patterns will
vary due to many factors including the types, magnitude, and timing
of emissions in a study area, as well as local factors, such as
meteorology and topography. We focused our current assessment on
specific study areas having ambient air concentrations close to
conditions that reflect air quality that just meets the current
standard. Accordingly, assessment of these study areas is more
informative to evaluating the health protection provided by the
current standard than would be an assessment that included areas
with much higher and much lower concentrations.
---------------------------------------------------------------------------
Consistent with the health effects evidence in this review
(summarized in section II.A.2 above), the focus of the quantitative
assessment is on short-term exposures of individuals in the population
during times when they are breathing at an elevated rate. Exposure and
risk are characterized for four population groups. Two are populations
of school-aged children, aged 5 to 18 years: All children and children
with asthma; two are populations of adults: All adults and adults with
asthma. Estimates for adults, in terms of percentages, are generally
lower due to the lesser amount and frequency of time spent outdoors at
elevated exertion (PA, Appendix 3D, section 3D.3.2). The exception is
outdoor workers who, due to the requirements of their job, spend more
time outdoors at elevated exertion. For a number of reasons, including
the appreciable data limitations (e.g., related to specific durations
of time spent outdoors and activity data), and associated uncertainties
summarized in Table 3D-64 of Appendix 3D of the PA, the group was not
simulated in these analyses, a decision also made for past exposure
assessments.\81\ Asthma prevalence estimates for the full populations
in the eight study areas range from 7.7 to 11.2%; the rates for
children in these areas range from 9.2 to 12.3% (PA, Appendix 3D,
section 3D.3.1).
---------------------------------------------------------------------------
\81\ Limited exploratory analyses of a hypothetical outdoor
worker population in the 2014 HREA (single study area, single year)
for the 75 ppb air quality scenario estimated an appreciably greater
portion of this population to experience exposures at or above
benchmark concentrations than the full adult or child populations
simulated, although there are a number of uncertainties associated
with the estimates due to appreciable limitations in the data
underlying the analyses (2014 HREA, section 5.4.3.2). It is expected
that if an approach similar to that used in the 2014 HREA had been
used for this assessment a generally similar pattern might be
observed, although with somewhat lower overall percentages based on
the comparison of current estimates with estimates from the 2014
HREA (PA, Appendix 3D, section 3D.3.2.4).
---------------------------------------------------------------------------
The approach for this analysis incorporates an array of models and
data (PA, section 3.4.1). Ambient air O3 concentrations were
estimated in each study area for the air quality conditions of interest
by adjusting hourly ambient air concentrations, from monitoring data
for the years 2015-2017, using a photochemical model-based approach and
then applying a spatial interpolation technique to produce air quality
surfaces with high spatial and temporal resolution (PA, Appendix 3C).
The final products were datasets of ambient air O3
concentration estimates with high temporal and spatial resolution
(hourly concentrations in 500 to 1,700 census tracts) for each of the
eight study areas (PA, section 3.4.1 and Appendix 3C, section 3C.7)
representing the three air quality scenarios assessed.
Population exposures were estimated using the EPA's Air Pollutant
Exposure model (APEX) version 5, which probabilistically generates a
large sample of hypothetical individuals from population demographic
and activity pattern databases and simulates each individual's
movements through time and space to estimate their time series of
O3 exposures occurring within indoor, outdoor, and in-
vehicle microenvironments (PA, Appendix 3D, section 3D.2).\82\ The APEX
model accounts for the most important factors that contribute to human
exposure to O3 from ambient air, including the temporal and
spatial distributions of people and ambient air O3
concentrations throughout a study area, the variation of ambient air-
related O3 concentrations within various microenvironments
in which people conduct their daily activities, and the effects of
activities involving different levels of exertion on breathing rate (or
ventilation rate) for the exposed individuals of different sex, age,
and body mass in the study area (PA, Appendix 3D, section 3D.2).\83\ By
incorporating individual activity
[[Page 87277]]
patterns, and estimating physical exertion for each exposure event, the
model addresses an important determinant of their exposure (2013 ISA,
section 4.4.1).\84\ For each exposure event, the APEX model tracks
activity performed, ventilation rate, exposure concentration, and
duration for all simulated individuals throughout the assessment
period, and then utilizes the time-series of exposure events in
derivation of the exposure and risk estimates.
---------------------------------------------------------------------------
\82\ The APEX model has a history of application, evaluation,
and progressive model development in estimating human exposure,
dose, and risk for reviews of NAAQS for gaseous pollutants,
including the last review of the O3 NAAQS (U.S. EPA,
2008; U.S. EPA, 2009; U.S. EPA, 2010; U.S. EPA, 2014a; U.S. EPA,
2018).
\83\ The APEX model generates each simulated person or profile
by probabilistically selecting values for a set of profile
variables, including demographic variables, health status and
physical attributes (e.g., residence with air conditioning, height,
weight, body surface area), and activity-specific ventilation rate
(PA, Appendix 3D, section 3D.2).
\84\ To represent personal time-location-activity patterns of
simulated individuals, the APEX model draws from the consolidated
human activity database (CHAD) developed and maintained by the EPA
(McCurdy, 2000; U.S. EPA, 2019a). The CHAD provides data on human
activities through a database system of human diaries or daily time
series or daily time location activity logs collected in surveys at
city, state, and national levels. Included are personal attributes
of survey participants (e.g., age, sex), along with the locations
they visited, activities performed throughout a day, time-of-day the
activities occurred and activity duration (PA, Appendix 3D, section
3D.2.5.1).
---------------------------------------------------------------------------
The general approach and methodology for the exposure-based
assessment used in this review is similar to that used in the last
review, although a number of updates and improvements, related to the
air quality, exposure, and risk aspects of the assessment, have been
implemented (Appendices 3C and 3D). These include (1) a more recent
period (2015-2017) of ambient air monitoring data in which
O3 concentrations in the eight study areas are at or near
the current standard; (2) the most recent version of the photochemical
air quality model, CAMx (comprehensive air quality model with
extensions), with updates to the treatment of atmospheric chemistry and
physics within the model; (3) a significantly expanded CHAD, that now
has nearly 180,000 diaries, with over 25,000 school aged children; (4)
updated National Health and Nutrition Examination Survey data (2009-
2014), which are the basis for the age- and sex-specific body weight
distributions used to specify the individuals in the modeled
populations; (5) updated algorithms used to estimate age- and sex-
specific resting metabolic rate, a key input to estimating a simulated
individual's activity-specific ventilation (or breathing) rate; (6)
updates to the ventilation rate algorithm itself; and (7) an approach
that better matches the simulated exposure estimates with the 6.6-hour
duration of the controlled human exposure studies and with the study
subject ventilation rates. Further, the current APEX model uses the
most recent U.S. Census demographic and commuting data (2010), NOAA
Integrated Surface Hourly meteorological data to reflect the assessment
years studied (2015-2017), and updated estimates of asthma prevalence
for all census tracts in all study areas based on 2013-2017 National
Health Interview Survey and Behavioral Risk Factor Surveillance System
data. Additional details are described in the PA (e.g., PA, section
3.4.1, Appendices 3C and 3D).
The comparison-to-benchmarks analysis characterizes the extent to
which individuals in at-risk populations could experience O3
exposures, while engaging in their daily activities, with the potential
to elicit the effects reported in controlled human exposure studies for
concentrations at or above specific benchmark concentrations. Results
are characterized through comparison of exposure concentrations to
three benchmark concentrations of O3: 60, 70, and 80 ppb.
These are based on the three lowest concentrations targeted in studies
of 6- to 6.6-hour exposures, with quasi-continuous exercise, and that
yielded different occurrences, of statistical significance, and
severity of respiratory effects, as summarized in section II.A.2.c
above and section II.C.1 of the proposal (PA, section 3.3.3; PA,
Appendix 3A, section 3A.1; PA, Appendix 3D, section 3D.2.8.1). The
lowest benchmark, 60 ppb, represents the lowest exposure concentration
for which controlled human exposure studies have reported statistically
significant respiratory effects, as summarized in section II.A.2.c
above. Exposure to approximately 70 ppb averaged over 6.6 hours
resulted in a larger group mean lung function decrement, as well as a
statistically significant increase in prevalence of respiratory
symptoms (Table 1; ISA, Appendix 3, Figure 3-3 and section 3.1.4.1.1;
Schelegle et al., 2009). Studies of exposures to approximately 80 ppb
have reported larger lung function decrements at the study group mean
than following exposures to 60 or 70 ppb, in addition to an increase in
airway inflammation, increased respiratory symptoms, increased airway
responsiveness, and decreased resistance to other respiratory effects
(ISA, Appendix 3, sections 3.1.4.1--3.1.4.4; PA, Figure 3-2 and section
3.3.3).
The APEX-generated exposure concentrations for comparison to these
benchmark concentrations is the average of concentrations encountered
by an individual while at an activity level that elicits the specified
elevated ventilation rate. The incidence of such exposures above the
benchmark concentrations are summarized for each simulated population,
study area, and air quality scenario in Appendix 3D of the PA.
The lung function risk analysis estimates the extent to which
individuals in exposed populations could experience O3-
induced lung function decrements of different sizes in two different
ways. The population-based E-R function approach uses quantitative
descriptions of the E-R relationships for study group incidence of
different magnitudes of lung function decrements based on individual
study subject observations (PA, Appendix 3D, section 3D.2.8.2.1). The
individual-based McDonnell-Smith-Stewart (MSS) model uses quantitative
estimates of biological processes identified as important in eliciting
the different sizes of decrements at the individual level, with a
factor that also provides a representation of intra- and inter-
individual response variability (PA, Appendix 3D, section 3D.2.8.2.2;
McDonnell et al., 2013). The two approaches, summarized in sections
II.C and II.D.1 of the proposal and described in detail in Appendix 3D
of the PA, utilize evidence from the 6.6-hour controlled human exposure
studies in different ways, and accordingly, differ in strengths,
limitations and uncertainties.
While the lung function risk analysis focuses only on the specific
O3 effect of FEV1 reduction, the comparison-to-
benchmark analysis, with its use of multiple benchmark concentrations,
provides for risk characterization of the array of respiratory effects
elicited by O3 exposure, the type and severity of which
increase with increased exposure concentration. In this way, the
comparison-to-benchmark analysis (involving comparison of daily maximum
7-hour average exposure concentrations that coincide with 7-hour
average elevated ventilation rates at or above the target rate to
benchmark concentrations) provides perspective on the extent to which
the air quality being assessed could be associated with discrete
exposures to O3 concentrations reported to result in an
array of respiratory effects. For example, estimates of such exposures
can indicate the potential for O3-related effects in the
exposed population, including effects for which we do not have E-R
functions that could be used in quantitative risk analyses. Thus, the
comparison-to-benchmark analysis provides for a broader risk
characterization with consideration of the array of O3-
related respiratory effects.
b. Key Limitations and Uncertainties
Uncertainty in the exposure and risk analyses was characterized
using a
[[Page 87278]]
largely qualitative approach adapted from the World Health Organization
approach for characterizing uncertainty in exposure assessment (WHO,
2008) augmented by several quantitative sensitivity analyses for key
aspects of the assessment approach (PA, section 3.4.4 and Appendix 3D,
section 3D.3.4). This characterization and associated analyses build on
information generated from a previously conducted quantitative
uncertainty analysis of population-based O3 exposure
modeling (Langstaff, 2007), considering the various types of data,
algorithms, and models that together yield exposure and risk estimates
for the eight study areas. In this way, we considered the limitations
and uncertainties underlying these data, algorithms, and models and the
extent of their influence on the resultant exposure/risk estimates
using the general approach applied in past risk and exposure
assessments for O3, nitrogen dioxide, carbon monoxide, and
sulfur dioxide (U.S. EPA, 2008; U.S. EPA, 2010; U.S. EPA, 2014a; U.S.
EPA, 2018).
Key uncertainties and limitations in data and tools that affect the
quantitative estimates of exposure and risk and their interpretation in
the context of considering the current standard are summarized here.
These include uncertainty related to estimation of the concentrations
in ambient air for the current standard and the additional air quality
scenarios; lung function risk approaches that rely, to varying extents,
on extrapolating from controlled human exposure study conditions to
lower exposure concentrations, lower ventilation rates, and shorter
durations; and characterization of risk for particular population
groups that may be at greatest risk, particularly for people with
asthma, and particularly children with asthma. Areas in which
uncertainty has been reduced by new or updated information or methods
include the use of updated air quality modeling, with a more recent
model version and model inputs, applied to study areas with design
values near the current standard, as well as updates to several inputs
to the exposure model, including changes to the exposure duration to
better match those in the controlled human exposure studies and an
alternate approach to characterizing periods of activity while at
moderate or greater exertion for simulated individuals.
With regard to the analysis approach overall, two updates since the
2014 HREA reduce uncertainty in the results. The first relates to
identifying when simulated individuals may be at moderate or greater
exertion, with the new approach reducing the potential for
overestimation of the number of people achieving the associated
ventilation rate, which was an important uncertainty in the 2014 HREA.
Additionally, the current analysis focus on exposures of 7 hours
duration better represents the 6.6-hour exposures from the controlled
human exposure studies (than the 8-hour exposure durations used for the
2014 HREA and prior assessments).
Additional aspects of the analytical design pertaining to both
exposure-based risk metrics include the estimation of ambient air
O3 concentrations for the air quality scenarios, and main
components of the exposure modeling. Uncertainties include the modeling
approach used to adjust ambient air concentrations to meet the air
quality scenarios of interest and the method used to interpolate
monitor concentrations to census tracts. While the adjustment to
conditions near, just above, or just below the current standard is an
important area of uncertainty, the size of the adjustment needed to
meet a given air quality scenario is minimized with the selection of
study areas for which recent O3 design values were near the
level of the current standard. Also, more recent data are used as
inputs for the air quality modeling, such as more recent O3
concentration data (2015-2017), meteorological data (2016) and
emissions data (2016), as well as a recently updated air quality
photochemical model which includes state-of-the-science atmospheric
chemistry and physics (PA, Appendix 3C). Further, the number of ambient
monitors sited in each of the eight study areas provides a reasonable
representation of spatial and temporal variability for the air quality
conditions simulated in those areas. Among other key aspects, there is
uncertainty associated with the simulation of study area populations
(and at-risk populations), including those with particular physical and
personal attributes. As also recognized in the 2014 HREA, exposures
could be underestimated for some population groups that are frequently
and routinely outdoors during the summer (e.g., outdoor workers,
children). In addition, longitudinal activity patterns do not exist for
these and other potentially important population groups (e.g., those
having respiratory conditions other than asthma), limiting the extent
to which the exposure model outputs reflect information that may be
particular to these groups. Important uncertainties in the approach
used to estimate energy expenditure (i.e., metabolic equivalents of
work or METs used to estimate ventilation rates), include the use of
longer-term average MET distributions to derive short-term estimates,
along with extrapolating adult observations to children. Both of these
approaches are reasonable based on the availability of relevant data
and appropriate evaluations conducted to date, and uncertainties
associated with these steps are somewhat reduced in the current
analyses (compared to the 2014 HREA) because of the added specificity,
and use of redeveloped METs distributions (based on newly available
information), which is expected to more realistically estimate
activity-specific energy expenditure.
There are some uncertainties that apply to the estimation of lung
function risk and not to the comparison-to-benchmarks analysis. For
example, both lung function risk approaches utilized in the risk
analyses incorporate some degree of extrapolation beyond the exposure
circumstances evaluated in the controlled human exposure studies.
Accordingly, the uncertainty in the lung function risk estimates
increases with decreasing exposure concentration and is particularly
increased for concentrations below those evaluated in controlled human
exposure studies (85 FR 49857-49859, PA, section 3.4.4 and Appendix 3D,
section 3D.3.4). The two lung function risk approaches differ in how
they extrapolate beyond the controlled human exposure study conditions
and in the impact on the estimates. The E-R function approach generates
nonzero predictions from the full range of nonzero concentrations for
7-hour average durations in which the average exertion levels meets or
exceeds the target. The MSS model, which draws on evidence-based
concepts of how human physiological processes respond to O3,
extrapolates beyond the controlled experimental conditions with regard
to exposure concentration, duration and ventilation rate (both
magnitude and duration). Differences in percent of the risk estimates
for days for which the highest 7-hour average concentration is below
the lowest 6.6-hour exposure concentration tested, as presented in the
PA, Tables 3-6 and 3-7, illustrate the impact.
An overarching area of uncertainty, remaining from the last review
and important to consideration of the exposure and risk analysis
results, relates to the underlying health effects evidence base.
Although the quantitative analysis focuses on the evidence providing
the ``strongest evidence'' of O3 respiratory effects (ISA,
[[Page 87279]]
p. IS-1), the controlled human exposure studies, and on the array of
respiratory responses documented in those studies, evidence is lacking
from controlled human exposure studies at the lower concentrations
(e.g., 60, 70 and 80 ppb) for children and for people of any age with
asthma. While the limited evidence informing our understanding of
potential risk to people with asthma is uncertain, it indicates the
potential for this group, given their disease status, to be at great
risk, as summarized in section II.A.2 above. Such a conclusion is
consistent with the epidemiologic study findings of positive
associations of O3 concentrations with asthma-related ED
visits and hospital admissions (and the higher effect estimates from
these studies).
c. Summary of Exposure and Risk Estimates
The benchmark-based risk metric results are summarized in terms of
the percent of the simulated populations of all children and children
with asthma estimated to experience at least one day per year \85\ with
a 7-hour average exposure concentration at or above the different
benchmark concentrations while breathing at elevated rates under air
quality conditions just meeting the current standard (Table 2). Given
the recognition of people with asthma as an at-risk population and the
relatively greater amount and frequency of time spent outdoors at
elevated exertion of children, this summary focuses on the estimates
from the comparison-to-benchmarks analysis for children, including
children with asthma, which were the focus of the Administrator's
proposed decision. Under air quality conditions just meeting the
current standard, less than 0.1% of any study area's children with
asthma, on average, were estimated to experience any days per year with
a 7-hour average exposure at or above 80 ppb, while breathing at
elevated rates (Table 3; PA, section 3.4 and Appendix 3D). With regard
to the 70 ppb benchmark, the study areas' estimates for children with
asthma range up to 0.7 percent (0.6% for all children), on average
across the 3-year period, and range up to 1.0% in a single year.
Approximately 3% to nearly 9% of each study area's simulated children
with asthma, on average across the 3-year period, are estimated to
experience one or more days per year with a 7-hour average exposure at
or above 60 ppb. This range is very similar for the populations of all
children.
---------------------------------------------------------------------------
\85\ While the duration of an O3 season for each year
may vary across the study areas, for the purposes of the exposure
and risk analyses, the O3 season in each study area is
considered synonymous with a year. These seasons capture the times
during the year when concentrations are elevated (80 FR 65419-65420,
October 26, 2015).
---------------------------------------------------------------------------
Regarding multiday occurrences, the analyses indicate that no
children would be expected to experience more than a single day with a
7-hour average exposure at or above 80 ppb in any year simulated in any
location (Table 2). For the 70 ppb benchmark, the estimate is less than
0.1% of any area's children (on average across 3-year period), both
those with asthma and all children. The estimates for the 60 ppb
benchmark are slightly higher, with up to 3% of children estimated to
experience more than a single day with a 7-hour average exposure at or
above 60 ppb, on average (and more than 4% in the highest year across
all eight study area locations).
Framed from the perspective of estimated protection provided by the
current standard, these results indicate that, in the single year with
the highest concentrations across the 3-year period, 99% of the
population of children with asthma would not be expected to experience
such a day with an exposure at or above the 70 ppb benchmark; 99.9%
would not be expected to experience such a day with exposure at or
above the 80 ppb benchmark. The estimates, on average across the 3-year
period, indicate that over 99.9%, 99.3% and 91.2% of the population of
children with asthma would not be expected to experience a day with a
7-hour average exposure while at elevated ventilation that is at or
above 80 ppb, 70 ppb and 60 ppb, respectively (Table 1). Further, more
than approximately 97% of all children or children with asthma are
estimated to be protected against multiple days of exposures at or
above 60 ppb.
Table 2--Percent and Number of Simulated Children and Children With Asthma Estimated To Experience at Least One or More Days per Year With a 7-Hour
Average Exposure At or Above Indicated Concentration While Breathing at an Elevated Rate in Areas Just Meeting the Current Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
One or more days Two or more days Four or more days
-----------------------------------------------------------------------------------------------
Exposure concentration (ppb) Average per Highest in a Average per Highest in a Average per Highest in a
year single year year single year year single year
--------------------------------------------------------------------------------------------------------------------------------------------------------
Children with asthma--percent of simulated population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0 \B\-<0.1 \C\ 0.1 0 0 0 0
>=70.................................................... 0.2-0.7 1.0 <0.1 0.1 0 0
>=60.................................................... 3.3-8.8 11.2 0.6-3.2 4.9 <0.1-0.8 1.3
--------------------------------------------------------------------------------------------------------------------------------------------------------
--number of individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0-67 202 0 0 0 0
>=70.................................................... 93-1145 1616 3-39 118 0 0
>=60.................................................... 1517-8544 11776 282-2609 3977 23-637 1033
--------------------------------------------------------------------------------------------------------------------------------------------------------
All children--percent of simulated population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0 \B\-<0.1 0.1 0 0 0 0
>=70.................................................... 0.2-0.6 0.9 <0.1 0.1 0-<0.1 <0.1
>=60.................................................... 3.2-8.2 10.6 0.6-2.9 4.3 <0.1-0.7 1.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
--number of individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0-464 1211 0 0 0 0
>=70.................................................... 727-8305 11923 16-341 660 0-5 14
[[Page 87280]]
>=60.................................................... 14928-69794 96261 2601-24952 36643 158-5997 9554
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each study area were averaged across the 3-year assessment period. Ranges reflect the ranges of averages.
\B\ A value of zero (0) means that there were no individuals estimated to have the selected exposure in any year.
\C\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).
These estimates are of generally similar magnitude to those which
were the focus in the 2015 decision establishing the current standard
(Table 3; PA, sections 3.1 and 3.4, Appendix 3D, section 3D.3.2.4,
Table 3D-38).\86\ The differences observed are generally slight, likely
reflecting influences of a number of the differences in the
quantitative modeling and analyses performed in the current assessment
from those for the 2014 HREA, summarized in section II.A.3.a above
(e.g., 2015-2017 vs. 2006-2010 distribution of ambient air
O3 concentrations, better matching of simulated exposure
estimates with the 6.6-hour duration of the controlled human exposure
studies and with the study subject ventilation rates). Much larger
differences are seen between different air quality scenario results for
the same benchmark. For example, for the 70 ppb benchmark, the
differences between the 75 ppb and current standard scenario (or
between the 65 ppb and current standard scenarios) in either assessment
are appreciably larger than the slight differences between the two
assessments for any one air quality scenario.
---------------------------------------------------------------------------
\86\ For example, the 2015 decision to set the standard level at
70 ppb noted that ``a revised standard with a level of 70 ppb is
estimated to eliminate the occurrence of two or more exposures of
concern to O3 concentrations at or above 80 ppb and to
virtually eliminate the occurrence of two or more exposures of
concern to O3 concentrations at or above 70 ppb for all
children and children with asthma, even in the worst-case year and
location evaluated'' (80 FR 65363, October 26, 2015). This statement
remains true for the current assessment (Table 3). For the 60 ppb
benchmark, on which the 2015 decision placed relatively greater
weight for multiple (versus single) occurrences of exposures at or
above it, the Administrator at that time noted the 2014 HREA
estimates for the 70 ppb air quality scenario that estimated 0.5 to
3.5% of children to experience multiple such occurrences on average
across the study areas, stating that the now-current standard ``is
estimated to protect the vast majority of children in urban study
areas . . . from experiencing two or more exposures of concern at or
above 60 ppb'' (80 FR 65364, October 26, 2015). The corresponding
estimates, on average across the 3-year period in the current
assessments, are remarkably similar at 0.6 to 2.9% (Table 3).
Table 3--Comparison of Current Assessment and 2014 HREA (All Study Areas) for Percent of Children Estimated To
Experience at Least One, or Two, Days With an Exposure At or Above Benchmarks While at Moderate or Greater
Exertion
----------------------------------------------------------------------------------------------------------------
Estimated average % of simulated Estimated average % of simulated
children with at least one day per children with at least two days
year at or above benchmark per year at or above benchmark
Air Quality Scenario (DV,\C\ ppb) (highest in single season) (highest in single season)
-----------------------------------------------------------------------
Current PA \A\ 2014 HREA \B\ Current PA \A\ 2014 HREA \B\
----------------------------------------------------------------------------------------------------------------
Benchmark Exposure Concentration of 80 ppb
----------------------------------------------------------------------------------------------------------------
75...................................... <0.1 \A\-0.3 0-0.3 (1.1) 0-<0.1 (<0.1) 0 (0.1)
(0.6)
70...................................... 0-<0.1 (0.1) 0-0.1 (0.2) 0 (0) 0 (0)
65...................................... 0-<0.1 (<0.1) 0 (0) 0 (0) 0 (0)
----------------------------------------------------------------------------------------------------------------
Benchmark Exposure Concentration of 70 ppb
----------------------------------------------------------------------------------------------------------------
75...................................... 1.1-2.0 (3.4) 0.6-3.3 (8.1) 0.1-0.3 (0.7) 0.1-0.6 (2.2)
70...................................... 0.2-0.6 (0.9) 0.1-1.2 (3.2) <0.1 (0.1) 0-0.1 (0.4)
65...................................... 0-0.2 (0.2) 0-0.2 (0.5) 0-<0.1 (<0.1) 0 (0)
----------------------------------------------------------------------------------------------------------------
Benchmark Exposure Concentration of 60 ppb
----------------------------------------------------------------------------------------------------------------
75...................................... 6.6-15.7 (17.9) 9.5-17.0 (25.8) 1.7-8.0 (9.9) 3.1-7.6 (14.4)
70...................................... 3.2-8.2 (10.6) 3.3-10.2 (18.9) 0.6-2.9 (4.3) 0.5-3.5 (9.2)
65...................................... 0.4-2.3 (3.7) 0-4.2 (9.5) <0.1-0.3 (0.5) 0-0.8 (2.8)
----------------------------------------------------------------------------------------------------------------
\A\ For the current analysis, calculated percent is rounded to the nearest tenth decimal using conventional
rounding. Values equal to zero are designated by ``0'' (there are no individuals exposed at that level).
Small, non-zero values that do not round upwards to 0.1 (i.e., <0.05) are given a value of ``<0.1''.
\B\ For the 2014 HREA. calculated percent was rounded to the nearest tenth decimal using conventional rounding.
Values that did not round upwards to 0.1 (i.e., <0.05) were given a value of ``0''.
\C\ The monitor location with the highest concentrations in each area had a design value just equal to the
indicated value.
[[Page 87281]]
B. Conclusions on the Primary Standard
In drawing conclusions on the adequacy of the current primary
standard, in view of the advances in scientific knowledge and
additional information now available, the Administrator has considered
the currently available health effects evidence and exposure/risk
information. He additionally has considered the evidence base,
information, and policy judgments that were the foundation of the last
review, to the extent they remain relevant in light of the currently
available information. The Administrator has taken into account both
evidence-based and exposure- and risk-based considerations discussed in
the PA, as well as advice from the CASAC and public comments. Evidence-
based considerations draw upon the EPA's assessment and integrated
synthesis of the scientific evidence, particularly that from controlled
human exposure studies and epidemiologic studies evaluating health
effects related to O3 exposures as presented in the ISA,
with a focus on policy-relevant considerations as discussed in the PA
(summarized in sections II.B and II.D.1 of the proposal and section
II.A.2 above). The exposure- and risk-based considerations draw from
the results of the quantitative analyses presented and considered in
the PA (as summarized in section II.C of the proposal and section
II.A.3 above).
The consideration of the evidence and exposure/risk information in
the PA informed the Administrator's proposed conclusions and judgments
in this review, and his associated proposed decision. Section II.B.1
below briefly summarizes the basis for the Administrator's proposed
decision, drawing from section II.D of the proposal. Section II.B.1.a
provides a brief overview of key aspects of the policy evaluations
presented in the PA, and the advice and recommendations of the CASAC
are summarized in section II.B.1.b. An overview of the Administrator's
proposed conclusions is presented in section II.B.1.c. Public comments
on the proposed decision are addressed in section II.B.2, and the
Administrator's conclusions and decision in this review regarding the
adequacy of the current primary standard and whether any revisions are
appropriate are described in section II.B.3.
1. Basis for Proposed Decision
a. Policy-Relevant Evaluations in the PA
The main focus of the policy-relevant considerations in the PA is
consideration of the question: Does the currently available scientific
evidence- and exposure/risk-based information support or call into
question the adequacy of the protection afforded by the current primary
O3 standard? The PA response to this overarching question
takes into account discussions that address the specific policy-
relevant questions for this review, focusing first on consideration of
the evidence, as evaluated in the ISA, including that newly available
in this review, and the extent to which it alters key conclusions
supporting the current standard. The PA also considers the quantitative
exposure and risk estimates drawn from the exposure/risk analyses
(presented in detail in Appendices 3C and 3D of the PA), including
associated limitations and uncertainties, and the extent to which they
may indicate different conclusions from those in the last review
regarding the magnitude of risk, as well as level of protection from
adverse effects, associated with the current standard. The PA
additionally considers the key aspects of the evidence and exposure/
risk estimates that were emphasized in establishing the current
standard, as well as the associated public health policy judgments and
judgments about the uncertainties inherent in the scientific evidence
and quantitative analyses that are integral to the Administrator's
consideration of whether the currently available information supports
or calls into question the adequacy of the current primary
O3 standard (PA, section 3.5).
As summarized in section II.D.1 of the proposal, based on the
evidence in the ISA, the PA concludes that the respiratory effects
evidence newly available in this review is consistent with the evidence
base in the last review, supporting a generally similar understanding
of the respiratory effects of O3 (PA, section 3.5.4; ISA,
Appendix 3). As was the case for the evidence available in the last
review, the currently available evidence for health effects other than
those of O3 exposures on the respiratory system is more
uncertain than that for respiratory effects. Such effects include
metabolic effects, for which the evidence available in this review is
sufficient to conclude there to likely be a causal relationship with
short-term O3 exposures and suggestive of, but not
sufficient to infer, such a relationship between long-term
O3 exposure (ISA, section IS.1.3.1). These new
determinations are based on evidence largely from experimental animal
studies, that is newly available in this review (ISA, Appendix 5).
Additionally, newly available evidence regarding cardiovascular effects
and mortality, in combination with uncertainties in the previously
available evidence that had been identified in the last review,
contributes to conclusions that the evidence is suggestive of, but not
sufficient to infer, causal relationships with O3 exposures
(ISA, Appendix 4, section 4.1.17 and Appendix 6, section 6.1.8). As in
the last review, the evidence is also suggestive of such relationships
for reproductive and developmental effects, and nervous system effects
(ISA, section IS.1.3.1).
In evaluating the policy implications of the current evidence, the
PA observes that within the respiratory effects evidence base, the most
certain evidence comes from controlled human exposure studies, the
majority of which involve healthy adult subjects (generally 18 to 35
years), although there are studies (generally not at the lowest studied
exposures) involving subjects with asthma, and a limited number of
studies, generally of durations shorter than four hours, involving
adolescents and adults older than 50 years. Respiratory responses
observed in human subjects exposed to O3 for periods of 8
hours or less, while intermittently or quasi-continuously exercising,
include lung function decrements (e.g., based on FEV1
measurements), respiratory symptoms, increased airway responsiveness,
mild bronchoconstriction (measured as an increase in sRaw), and
pulmonary inflammation, with associated injury and oxidative stress
(ISA, Appendix 3, section 3.1.4; 2013 ISA, sections 6.2.1 through
6.2.4). Newly available epidemiologic studies of hospital admissions
and emergency department visits for a variety of respiratory outcomes
supplement the previously available evidence with additional findings
of consistent associations with O3 concentrations across a
number of study locations (ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5,
3.1.6.1.1, 3.1.7.1 and 3.1.8). Together, the clinical and
epidemiological bodies of evidence, in combination with the insights
gained from the experimental animal evidence, continue to indicate the
potential for O3 exposures to contribute to serious health
outcomes and to indicate the increased risk of population groups with
asthma, including particularly, children (ISA, Appendix 3, section
3.1.5.7).
The PA concludes that the newly available evidence in this review
does not alter conclusions from the last review on exposure duration
and concentrations associated with O3-related effects,
observing that the 6.6-hour controlled human exposure studies
[[Page 87282]]
of respiratory effects remain the focus for our consideration of
exposure circumstances associated with O3 health effects.
The PA additionally recognizes that while the evidence clearly
demonstrates that short-term O3 exposures cause respiratory
effects, as was the case in the last review, uncertainties remain in
several aspects of our understanding of these effects. These include
uncertainties related to exposures likely to elicit effects (and the
associated severity and extent) in population groups not studied, or
less well studied (including individuals with asthma and children) and
also the severity and prevalence of responses to short (e.g., 6.6- to
8-hour) O3 exposures, at and below 60 ppb, while at
increased exertion levels.
The PA additionally includes exposure/risk analyses of air quality
scenarios in eight study areas, with a focus on the scenario for air
quality that just meets the current standard, as described in section
II.C of the proposal and summarized in section II.A.3 above. In
considering the results of these analyses, the PA gives particular
emphasis to the comparison-to-benchmarks analysis, which provides a
characterization of the extent to which population exposures to
O3 concentrations, similar to those evaluated in controlled
human exposure studies, have the potential to occur in areas of the
U.S. when air quality just meets the current standard (PA, section
3.4). The policy evaluations of the exposure/risk analyses focus on
children and children with asthma as key at-risk populations, and
consideration of the potential for one versus multiple exposures to
occur. The PA recognizes that consideration of differences in magnitude
or severity of responses (e.g., FEV1 changes) including the
relative transience or persistence of the responses and respiratory
symptoms, as well as pre-existing sensitivity to effects on the
respiratory system, and other factors, are important to characterizing
implications for public health effects of an air pollutant such as
O3 (PA, sections 3.3.2, 3.4.5 and 3.5).
In summary, the PA concludes that the newly available health
effects evidence, critically assessed in the ISA as part of the full
body of evidence, reaffirms conclusions on the respiratory effects
recognized for O3 in the last review on which the current
standard is based. The PA additionally draws on the quantitative
exposure and risk estimates for conditions just meeting the current
standard (PA, sections 3.4 and 3.5.2). Limitations and uncertainties
associated with the available information remain (PA, sections 3.5.1
and 3.5.2). The PA recognizes that the newly available quantitative
exposure/risk estimates for conditions just meeting the current
standard indicate a generally similar level of protection for at-risk
populations from respiratory effects, as that described in the last
review for the now-current standard (section II.A.3, Table 3, above;
PA, sections 3.1 and 3.4, Appendix 3D, section 3D.3.2.4, Table 3D-38).
Collectively, in consideration of the evidence and quantitative
exposure/risk information available in the current review, as well as
advice from the CASAC, the PA concludes that it is appropriate to
consider retaining the current primary standard of 0.070 ppm
O3, as the fourth-highest daily maximum 8-hour concentration
averaged across three years, without revision.
b. CASAC Advice in This Review
In comments on the draft PA, the CASAC agreed with the draft PA
findings that the health effects evidence newly available in this
review does not substantially differ from that available in the 2015
review, stating that, ``[t]he CASAC agrees that the evidence newly
available in this review that is relevant to setting the ozone standard
does not substantially differ from that of the 2015 Ozone NAAQS
review'' (Cox, 2020a, Consensus Responses to Charge Questions p. 12).
With regard to the adequacy of the current standard, views of
individual CASAC members differed. Part of the CASAC ``agree with the
EPA that the available evidence does not call into question the
adequacy of protection provided by the current standard, and thus
support retaining the current primary standard'' (Cox, 2020a, p. 1).
Another part of the CASAC indicated its agreement with the previous
CASAC's advice, based on review of the 2014 draft PA, that a primary
standard with a level of 70 ppb may not be protective of public health
with an adequate margin of safety, including for children with asthma
(Cox, 2020a, p. 1 and Consensus Responses to Charge Questions p.
12).\87\ Additional comments from the CASAC in the ``Consensus
Responses to Charge Questions'' on the draft PA attached to the CASAC
letter provide recommendations on improving the presentation of the
information on health effects and exposure and risk estimates in
completing the final PA. The EPA considered these comments, making a
number of revisions to address them in completing the PA. The comments
from the CASAC also took note of uncertainties that remain in this
review of the primary standard and identified a number of additional
areas for future research and data gathering that would inform the next
review of the primary O3 NAAQS (Cox, 2020a, Consensus
Responses to Charge Questions p. 14). The recommendations from the
CASAC were considered in the proposed decision and have been considered
by the Administrator in his decision in this review, summarized in
section II.B.3 below.
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\87\ In the last review, the advice from the prior CASAC
included a range of recommended levels for the standard, with the
CASAC concluding that ``there is adequate scientific evidence to
recommend a range of levels for a revised primary ozone standard
from 70 ppb to 60 ppb'' (Frey, 2014b, p. ii). In so doing, the prior
CASAC noted that ``[i]n reaching its scientific judgment regarding a
recommended range of levels for a revised ozone primary standard,
the CASAC focused on the scientific evidence that identifies the
type and extent of adverse effects on public health'' and further
acknowledged ``that the choice of a level within the range
recommended based on scientific evidence is a policy judgment under
the statutory mandate of the Clean Air Act'' (Frey, 2014b, p. ii).
The prior CASAC then described that its ``policy advice [emphasis
added] is to set the level of the standard lower than 70 ppb within
a range down to 60 ppb, taking into account [the Administrator's]
judgment regarding the desired margin of safety to protect public
health, and taking into account that lower levels will provide
incrementally greater margins of safety'' (Frey, 2014b, p. ii).
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c. Administrator's Proposed Conclusions
In reaching conclusions on the adequacy and appropriateness of
protection provided by the current primary standard and his proposed
decision to retain the standard, the Administrator carefully
considered: (1) The assessment of the current evidence and conclusions
reached in the ISA; (2) the currently available exposure and risk
information, including associated limitations and uncertainties,
described in detail in the PA; (3) the considerations and staff
conclusions and associated rationales presented in the PA, including
consideration of commonly accepted guidelines or criteria within the
public health community, including the ATS, an organization of
respiratory disease specialists; (4) the advice and recommendations
from the CASAC; and (5) public comments that had been offered up to
that point (85 FR 49830, August 14, 2020). In so doing, he considered
the evidence base on health effects associated with exposure to
photochemical oxidants, including O3, in ambient air, noting
the health effects evidence newly available in this review, and the
extent to which it alters key scientific conclusions in the last
review. He additionally considered the quantitative exposure and risk
estimates
[[Page 87283]]
developed in this review, including associated limitations and
uncertainties, and what they indicate regarding the magnitude of risk,
as well as level of protection from adverse effects, associated with
the current standard. The Administrator also considered the key aspects
of the evidence and exposure/risk estimates from the 2015 review that
were emphasized in establishing the standard at that time. Further, he
considered uncertainties in the current evidence and the exposure/risk
information, as a part of public health judgments that are essential
and integral to his decision on the adequacy of protection provided by
the standard, similar to the judgments made in establishing the current
standard. Such judgments include public health policy judgments and
judgments about the uncertainties inherent in the scientific evidence
and quantitative analyses. The Administrator drew on the considerations
and conclusions in the current PA, taking note of key aspects of the
associated rationale, and he considered the advice and conclusions of
the CASAC, including particularly its overall agreement that the
currently available evidence does not substantially differ from that
which was available in the 2015 review when the current standard was
established.
As an initial matter, the Administrator recognized the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants, taking note that no newly available evidence
has been identified in this review regarding the importance of
photochemical oxidants other than O3 with regard to
abundance in ambient air, and potential for health effects. For such
reasons, described with more specificity in the ISA and PA and
summarized in the proposal, he proposed to conclude it is appropriate
for O3 to continue to be the indicator for the primary
standard for photochemical oxidants and focused on the current
information for O3 (85 FR 49830, August 14, 2020).
With regard to O3 health effects, the Administrator
recognized the long-standing evidence that has established there to be
a causal relationship between respiratory effects and short-term
O3 exposures. He recognized that the strongest and most
certain evidence for this conclusion, as in the last review, is that
from controlled human exposure studies that report an array of
respiratory effects in study subjects (which are largely generally
healthy adults) engaged in quasi-continuous or intermittent exercise.
He also recognized the supporting experimental animal and epidemiologic
evidence, including the epidemiologic studies reporting positive
associations for asthma-related hospital admissions and emergency
department visits, which are strongest for children, with short-term
O3 exposures (85 FR 49830, August 14, 2020).
Regarding the current evidence and EPA conclusions for populations
at increased risk of O3-related health effects (ISA, section
4.4), the Administrator took particular note of the robust evidence
that continues to identify people with asthma as being at increased
risk of O3 related respiratory effects, including
specifically asthma exacerbation and associated health outcomes, and
also children, particularly due to their generally greater time
outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections
IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11).
Based on this evidence and related factors, the Administrator proposed
to conclude it appropriate to give particular focus to people with
asthma and children (population groups for which the evidence of
increased risk is strongest) in evaluating whether the current standard
provides requisite protection based on the judgment that such a focus
will also provide protection of other population groups, identified in
the ISA, for which the current evidence is less robust and clear as to
the extent and type of any increased risk, and the exposure
circumstances that may contribute to it.
The Administrator additionally recognized newly available evidence
and conclusions regarding O3 exposures and metabolic
effects. In so doing, he also noted that the basis for the conclusions
is largely experimental animal studies in which the exposure
concentrations were well above those in the controlled human exposure
studies for respiratory effects, and also above those likely to occur
in areas of the U.S. that meet the current standard. In light of these
considerations, he further proposed to judge the current standard to be
protective of such circumstances, leading him to continue to focus on
respiratory effects in evaluating whether the current standard provides
requisite protection (85 FR 49830, August 14, 2020).
With regard to exposure circumstances of interest for respiratory
effects, the Administrator focused particularly on the 6.6-hour
controlled human exposure studies involving exposure, with quasi-
continuous exercise, that examine exposures from 60 to 80 ppb. In so
doing, he recognized that this information on exposure concentrations
that have been found to elicit effects in exercising study subjects is
unchanged from what was available in the last review. He additionally
recognized that while, as a whole, the epidemiologic studies of
associations between O3 and respiratory effects and health
outcomes (e.g., asthma-related hospital admission and emergency
department visits) provide strong support for the conclusions of
causality, they are less useful for his consideration of the potential
for O3 exposures associated with air quality conditions
allowed by the current standard to contribute to such health outcomes,
taking note of the scarcity of U.S. studies conducted in locations in
which and during time periods when the current standard would have been
met (85 FR 49830, August 14, 2020).
In reaching his proposed decision to retain the 2015 standard, the
Administrator took note of several aspects of the rationale by which it
was established, giving weight to the considerations summarized here.
The 2015 decision considered the breadth of the O3
respiratory effects evidence, recognizing the relatively greater
significance of effects reported for exposures while at elevated
exertion to average O3 concentrations at and above 80 ppb,
as well as to the greater array of effects elicited. The decision also
recognized the significance of effects observed at the next lower
studied exposures (slightly above 70 ppb) that included both lung
function decrements and respiratory symptoms. The standard level was
set to provide a high level of protection from such exposures. The
decision additionally emphasized consideration of lower exposures down
to 60 ppb, particularly with regard to consideration of a margin of
safety in setting the standard. In this context, the 2015 decision
identified the appropriateness of a standard that provided a degree of
control of multiple or repeated occurrences of exposures, while at
elevated exertion, at or above 60 ppb (80 FR 65365, October 26,
2015).\88\ The controlled human exposure study evidence as a whole
provided context for consideration of the 2014 HREA estimates for the
[[Page 87284]]
comparison-to-benchmarks analysis (80 FR 65363, October 26, 2015). The
current Administrator proposed to similarly consider the currently
available exposure and risk analyses in this review (85 FR 49830,
August 14, 2020).
---------------------------------------------------------------------------
\88\ With the 2015 decision, the prior Administrator judged
there to be uncertainty in the adversity of the effects shown to
occur following exposures to 60 ppb O3, including the
inflammation reported by the single study at the level, and
accordingly placed greater weight on estimates of multiple, versus
single, exposures for the 60 ppb benchmark, particularly when
considering the extent to which the current and revised standards
incorporate a margin of safety (80 FR 65344-45, October 26, 2015).
She based this, at least in part, on consideration of effects at
this exposure level, the evidence for which remains the same in the
current review, and she considered this information in judgments
regarding the 2014 HREA estimates for the 60 ppb benchmark.
---------------------------------------------------------------------------
The Administrator also recognized some uncertainty, reflecting
limitations in the evidence base, with regard to the exposure levels
eliciting effects (as well as the severity of the effects) in some
population groups not well represented in the available controlled
human exposure studies, such as children and individuals with asthma.
In so doing, the Administrator recognizes that the controlled human
exposure studies, primarily conducted in healthy adults, on which the
depth of our understanding of O3-related health effects is
based, provide limited, but nonetheless important information with
regard to responses in people with asthma or in children. Additionally,
some aspects of our understanding continue to be limited, as in the
2015 review; among these aspects are the risk posed to these less
studied population groups by 7-hour exposures with exercise to
concentrations as low as 60 ppb that are estimated in the exposure
analyses. Collectively, these aspects of the evidence and associated
uncertainties contribute to a recognition that for O3, as
for other pollutants, the available evidence base in a NAAQS review
generally reflects a continuum, consisting of ambient levels at which
scientists generally agree that health effects are likely to occur,
through lower levels at which the likelihood and magnitude of the
response become increasingly uncertain.
As in the 2015 decision, the Administrator's proposed decision in
this review recognized that the exposure and risk estimates developed
from modeling exposures to O3 in ambient air are critically
important to consideration of the potential for exposures and risks of
concern under air quality conditions of interest, and consequently are
critically important to judgments on the adequacy of public health
protection provided by the current standard. Thus taking into
consideration related information, limitations and uncertainties
recognized in the proposal, the Administrator considered the exposure
and risk estimates across the eight study areas (with their array of
exposure conditions) for air quality conditions just meeting the
current standard. In light of factors recognized above and summarized
in section II.D.4 of the proposal, the Administrator, in his
consideration of the exposure and risk analyses, focused in the
proposal on the results for children and children with asthma. In
considering the public health implications of estimated occurrences of
exposures, while at increased exertion, at or above the three benchmark
concentrations (60, 70, and 80 ppb), the Administrator considered the
effects reported in controlled human exposure studies of this range of
concentrations during 6.6 hours of quasi-continuous exercise. While the
Administrator noted reduced uncertainty in several aspects of the
exposure and risk approaches as compared to the analyses in the last
review, he recognized the relatively greater uncertainty associated
with the lung function risk estimates compared to the results of the
comparison-to-benchmarks analysis. In light of these uncertainties, as
well as the recognition that the comparison-to-benchmarks analysis
provides for characterization of risk for the broad array of
respiratory effects compared to a narrower focus limited to lung
function decrements, the Administrator focused in the proposal
primarily on the estimates of exposures at or above different benchmark
concentrations that represent different levels of significance of
O3-related effects, both with regard to the array of effects
and severity of individual effects (85 FR 49830, August 14, 2020).
In his consideration of the exposure analysis estimates for
exposures at or above the different benchmark concentrations (with
reduced associated uncertainty compared to the analysis available in
2015) and based on the greater severity of responses reported in
controlled human exposures, with quasi-continuous exercise, at and
above 73 ppb, the Administrator focused in the proposal first on the
higher two benchmark concentrations (which at 70 and 80 ppb are,
respectively, slightly below and above this level) and the estimates
for one-or-more-day occurrences. In this context, he proposed to judge
it desirable that the standard provide a high level of protection
against one or more occurrences of days with exposures, while breathing
at an elevated rate, to concentrations at or above 70 ppb. With regard
to the 60 ppb benchmark, the Administrator gave greater weight to
estimates of occurrences of two or more (rather than one or more) days
with an exposure at or above that benchmark, taking note of the lesser
severity of responses observed in studies of the lowest benchmark
concentration of 60 ppb and other considerations summarized in the
proposal, including potential risks for at-risk populations. Based on
this weighting of the exposure analysis results for the eight urban
study areas, the Administrator noted what was indicated by the exposure
estimates for air quality conditions just meeting the current standard
with regard to protection for the simulated at-risk populations. Some
97% to more than 99% of all children (including those with asthma), on
average, and more than 95% in the single highest year, are estimated to
be protected from experiencing two or more days with exposures at or
above 60 ppb while at elevated exertion. More than 99% of children with
asthma (and of all children), on average per year, are estimated to be
protected from a day or more with an exposure at or above 70 ppb.
Lastly, the percentage (for both population groups) for at least one
day with such an exposure at or above 80 ppb is 99.9% or more in each
of the three years simulated, with no simulated children estimated to
experience more than a single such day. The Administrator proposed to
judge that protection from this set of exposures provides a strong
degree of protection to at-risk populations, such as children with
asthma. In so doing, he found that the updated exposure and risk
analyses continue to support a conclusion of a high level of
protection, including for at-risk populations, from O3-
related effects of exposures that might be expected with air quality
conditions that just meet the current standard (85 FR 49830, August 14,
2020).
In reaching his proposed conclusion, the Administrator additionally
took note of the comments and advice from the CASAC, including the
CASAC conclusion that the newly available evidence does not
substantially differ from that available in the last review, and the
conclusion expressed by part of the CASAC, that the currently available
evidence supports retaining the current standard (85 FR 49873, August
14, 2020). He also noted that another part of the CASAC indicated its
agreement with the prior CASAC comments on the 2014 draft PA, in which
the prior CASAC opined that a standard set at 70 ppb may not provide an
adequate margin of safety (Cox, 2020a, p. 1). With regard to the latter
view (that referenced 2014 comments from the prior CASAC), the
Administrator additionally noted that the 2014 advice from the prior
CASAC also concluded that the scientific evidence supported a range of
standard levels that included 70 ppb and recognized the choice of a
level within its recommended range to be ``a policy judgment under the
statutory mandate
[[Page 87285]]
of the Clean Air Act'' (Frey, 2014b, p. ii).\89\
---------------------------------------------------------------------------
\89\ This 2014 advice was considered in the last review's
decision to establish the current standard with a level of 70 ppb
(80 FR 65362, October 26, 2015).
---------------------------------------------------------------------------
In reflecting on all of the information currently available, the
Administrator also considered the extent to which the currently
available information might indicate support for a less stringent
standard, noting that the CASAC advice did not convey support for such
a standard. He additionally considered the current exposure and risk
estimates for the air quality scenario for a design value just above
the level of the current standard (at 75 ppb), in comparison to the
scenario for the current standard, with its level of 70 ppb. In so
doing, he found the markedly increased estimates of exposures to the
higher benchmarks under air quality for a higher standard level to be
of concern and indicative of less than the requisite protection. Thus,
in light of considerations raised in the proposal, including the need
for an adequate margin of safety, the Administrator proposed to judge
that a less stringent standard would not be appropriate to consider (85
FR 49830, August 14, 2020).
Similarly, the Administrator also considered whether it would be
appropriate to consider a more stringent standard that might be
expected to result in reduced O3 exposures. As an initial
matter in this regard, he considered the advice from the CASAC
(summarized in section II.B.1.b above). With regard to the CASAC
advice, he noted that while part of the Committee concluded that the
evidence supported retaining the current standard without revision,
another part of the Committee reiterated advice from the prior CASAC,
which while including the current standard level among the range of
recommended standard levels, also provided policy advice to set the
standard at a lower level (85 FR 49873, August 14, 2020). In
considering the reference to the 2014 CASAC advice, the Administrator
noted the slight differences of the current exposure and risk estimates
from the 2014 HREA estimates considered by the prior CASAC. The
Administrator additionally recognized the PA finding that the factors
contributing to these differences, which include the use of air quality
data reflecting concentrations much closer to the now-current standard
than was the case in the 2015 review, also contribute to a reduced
uncertainty in the estimates. Thus, he noted that the current exposure
analysis estimates indicate the current standard to provide appreciable
protection against multiple days with a maximum exposure at or above 60
ppb. He considered this in the context of the adequacy of protection
provided by the standard and of the CAA requirement that the standard
protect public health, including the health of at-risk populations,
with an adequate margin of safety, and proposed to conclude that the
current standard provides an adequate margin of safety, and that a more
stringent standard is not needed (85 FR 49873, August 14, 2020).
In light of all of the above, including advice from the CASAC, the
Administrator proposed to judge the current exposure and risk analysis
results to describe appropriately strong protection of at-risk
populations from O3-related health effects. Thus, based on
his consideration of the evidence and exposure/risk information,
including that related to the lowest exposures studied and the
associated uncertainties, the Administrator proposed to judge that the
current standard provides the requisite protection, including an
adequate margin of safety, and thus should be retained, without
revision (85 FR 49874, August 14, 2020). In so doing, he recognized
that the protection afforded by the current standard can only be
assessed by considering its elements collectively, including the
standard level of 70 ppb, the averaging time of eight hours and the
form of the annual fourth-highest daily maximum concentration averaged
across three years. The Administrator proposed to judge that the
current evidence presented in the ISA and considered in the PA, as well
as the current air quality, exposure and risk information presented and
considered in the PA, provide continued support to these elements, as
well as to the current indicator.
In summary, in the proposal the Administrator recognized that the
ISA found the newly available health effects evidence, critically
assessed in the ISA as part of the full body of evidence, consistent
with the conclusions on the respiratory effects recognized for
O3 in the last review. He additionally noted that the
evidence newly available in this review, such as that related to
metabolic effects, does not include information indicating a basis for
concern for exposure conditions associated with air quality conditions
meeting the current standard. Further, the Administrator noted the
quantitative exposure and risk estimates for conditions just meeting
the current standard that indicate a high level of protection for at-
risk populations from respiratory effects. Collectively, these
considerations (including those discussed more completely in the
proposal) provided the basis for the Administrator's proposed judgments
regarding the public health protection provided by the current primary
standard of 0.070 ppm O3, as the fourth-highest daily
maximum 8-hour concentration averaged across three years. On this
basis, the Administrator proposed to conclude that the current standard
is requisite to protect the public health with an adequate margin of
safety, and that it is appropriate to retain the standard without
revision (85 FR 49874, August 14, 2020).
2. Comments on the Proposed Decision
Over 50,000 individuals and organizations indicated their views in
public comments on the proposed decision. Most of these are associated
with mass mail campaigns or petitions. Approximately 40 separate
submissions were also received from individuals, and 75 from
organizations and groups of organizations; forty elected officials also
submitted comments. Among the organizations commenting were state and
local agencies and organizations of state agencies, organizations of
health professionals and scientists, environmental and health
protection advocacy organizations, industry organizations and
regulatory policy-focused organizations. The comments on the proposed
decision to retain the current primary standard are addressed here.
Those in support of the proposed decision are addressed in section
II.B.2.a and those in disagreement are addressed in section II.B.2.b.
Comments related to aspects of the process followed in this review of
the O3 NAAQS (described in section I.D above), as well as
comments related to other legal, procedural or administrative issues,
and those related to issues not germane to this review are addressed in
the separate Response to Comments document.
a. Comments in Support of Proposed Decision
Of the commenters supporting the Administrator's proposed decision
to retain the current primary standard, without revision, all generally
note their agreement with the rationale provided in the proposal, with
the CASAC conclusion that the current evidence is generally consistent
with that available in the last review, and with the CASAC members that
conclude the evidence does not call into question the adequacy of the
current standard. Some commenters further remarked that the primary
standard was upheld in the litigation following its 2015
[[Page 87286]]
establishment (Murray Energy Corp. v. EPA, 936 F.3d 597 [D.C. Cir.
2019]) and that this review is based largely on the same body of
respiratory effects evidence. These commenters all find the process for
the review to conform to Clean Air Act requirements and the proposed
decision to retain the current standard to be well supported, noting
that the there are no new controlled human exposure studies (of the
type given primary focus in the establishment of the current standard)
and concurring with the proposed judgment that at-risk populations are
protected with an adequate margin of safety. Some commenters also
variously cited EPA statements that the recent metabolic studies, as
well as the epidemiologic and toxicological studies newly available in
this review for other health endpoints, do not demonstrate effects of
O3 when the current standard is met and thus do not call
into question the protection provided by the standard. The EPA agrees
with these commenters' conclusion on the current standard.
Further, these comments concur with the EPA's consideration of
epidemiologic and toxicological studies of respiratory effects, and
with the weight the proposed decision placed on the evidence for other
effects, including metabolic and cardiovascular effects, and total
mortality. Some of these comments also express the view that health
benefits of a more restrictive O3 standard are highly
uncertain, while such a standard would likely cause an increase in
nonattainment areas and socioeconomic impacts that the EPA should
consider and find to outweigh the uncertain benefits. While, as
discussed in section II.B.3 below, the Administrator does not find a
more stringent standard necessary to provide requisite public health
protection, he does not consider the number of nonattainment areas or
economic impacts of alternate standards in reaching this judgment.\90\
As summarized in section I.A. above, in setting primary and secondary
standards that are ``requisite'' to protect public health and welfare,
respectively, as provided in section 109(b), the EPA may not consider
the costs of implementing the standards. See generally, Whitman v.
American Trucking Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001).
Likewise, ``[a]ttainability and technological feasibility are not
relevant considerations in the promulgation of national ambient air
quality standards'' (American Petroleum Institute v. Costle, 665 F.2d
1176, 1185 [D.C. Cir. 1981]; accord Murray Energy Corp. v. EPA, 936
F.3d 597, 623-24 [D.C. Cir. 2019]). Arguments such as the views on
socioeconomic impacts expressed by these commenters have been rejected
by the courts, as summarized in section I.A above, including in Murray
Energy, with the reasoning that consideration of such impacts was
precluded by Whitman's holding that the ``plain text of the Act
`unambiguously bars cost considerations from the NAAQS-setting process'
'' (Murray Energy Corp. v. EPA, 936 F.3d at 621, quoting Whitman, 531
U.S. at 471).
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\90\ Comments related to implementation programs are not
addressed here because, as described in section I.A above, this
action is being taken pursuant to CAA section 109(d)(1) and relevant
case law. Accordingly, concerns related to implementation of the
existing or an alternate standard are outside the scope of this
action.
---------------------------------------------------------------------------
We also note that some commenters that stated their support for
retaining the current standard without revision additionally claimed
that, based on the results of the exposure and risk analyses in this
review, the current standard provides somewhat more public health
protection than the EPA recognized in the 2015 decision establishing
it. As support for this view, these commenters cite conclusions
(including those in the PA) that the exposure and risk estimates are
equivalent or slightly lower than those from the 2014 HREA. In
generally agreeing with the commenters' observation with regard to the
differences in exposure/risk estimates from analyses in this review
compared to those from 2014, we note that the current exposure/risk
estimates, while based on conceptually similar approaches to those used
in the 2014 HREA, reflect a number of improvements to input data and
modeling approaches, summarized in section II.A.3 above, which have
reduced uncertainties. These updated analyses inform the
Administrator's judgments in this review.
b. Comments in Disagreement With Proposed Decision
Of the commenters that disagreed with the proposal to retain the
current standard, some recommend tightening the standard, while one
submission recommends a less stringent standard. The commenters
supporting a less stringent standard generally assert that the current
standard is overprotective, stating that information they provide
supports returning to the pre-2015 standard of 75 ppb and/or revising
the form from the 4th highest daily maximum to the seventh highest
daily maximum. The commenters that recommended a more stringent
standard describe a need for revision to provide greater public health
protection, generally claiming that the current standard is inadequate
and does not provide an adequate margin of safety for potentially
vulnerable groups. We address these sets of comments in turn below.
(i) Comments in Disagreement With Proposed Decisions--Calling for Less
Stringent Standard
The commenters recommending revision to a less stringent standard
generally expressed the view that the current standard is more
stringent than necessary to protect public health. In support of this
view the commenters argue (1) that in this review the EPA
``discredited'' a cardiovascular mortality study on which commenters
assert the 2015 decision had placed especially heavy weight; (2) that
in light of limitations they assert for the exposure and risk estimate
analyses conducted in this review, a 75 ppb standard would meet 2015
objectives; and, (3) that additional factors they identify indicate
that the current standard of 70 ppb is too close to background levels
while a standard of 75 ppb or one with a form that uses the seventh
(versus fourth) highest daily maximum 8-hour O3
concentration would not be.
With regard to the first argument, the EPA knows of no
cardiovascular mortality study, much less any health study, that was
relied on in the 2015 review that has been discredited, and the
commenters provide no citation for such a study. To the extent that the
commenter may be intending to refer to the difference of the current
review from the 2015 review with regard to the Agency's causality
determinations for cardiovascular effects and all-cause mortality, we
note that these changes did not involve ``discrediting'' of any studies
in the 2013 ISA. Rather, as summarized in section II.A.2.a above, since
the time of the last review the controlled human exposure study
evidence base has been appreciably expanded from one study to several,
none of which report O3-induced cardiovascular endpoints.
This update to the evidence base for cardiovascular effects, which also
includes epidemiologic studies, has contributed to a change in the
weight of evidence that supports the Agency's causality determinations
for both cardiovascular effects and mortality. To the extent that the
commenters intend to suggest that these changes in causality
determinations indicate that the current standard is more stringent
than necessary to protect public health, the Agency disagrees. The
Administrator's reasons for concluding that the current standard
provides the requisite public
[[Page 87287]]
health protection are explained in section II.B.3 below.
With regard to the risk and exposure analyses, the comment argues
that 2019 O3 ambient air monitoring data for locations
meeting a design value of 75 ppb indicate that a 75 ppb standard could
achieve comparable exposure estimates to those derived for air quality
just meeting the current standard by the EPA's exposure/risk analyses.
The comment also asserts that uncertainty in the controlled human
exposure evidence base with regard to children with asthma suggests
``some latitude'' is needed in the risk calculations. The analysis
provided in the comment appears to focus on counties in designated
nonattainment areas with 2019 design values ranging from 71 to 75. For
these counties, the commenters' analysis appears to sum the population
of the subset of these counties with at least one daily maximum 8-hour
average concentration in 2019 falling in the range from 73 to 79 ppb
(and, separately, the population of counties with at least one such
value above 80 ppb). From these population counts, the analysis derives
estimates of the subpopulations of children with asthma spending
afternoons outdoors (using national estimates for representation of
children in the total population, of children with asthma in the total
child population, and of children in asthma spending afternoons
outdoors using analysis of CHAD diaries for children). The analysis
divides the two values by the commenters' estimate of children with
asthma in the U.S. (304 million [total population of the U.S.] x 10.5%
[percentage representing children] x 9.7% [percentage representing
children with asthma]).
There are many aspects of the analysis submitted with the comment
that are not focused on the objective of estimating exposures of
concern that might be expected to be experienced by at-risk populations
in U.S. areas that just meet a standard with an alternative level of 75
ppb. As just one example of these aspects, the denominator in the final
step of the commenters' calculation is inflated by population counts
for areas of the U.S. excluded from the commenters' analysis (with this
larger population multiplied by a national estimates of percent that
are children, 10.5%, and a national estimate of percent of children
that have asthma, 9.7%), yielding a percentage of unclear relevance to
consideration of exposures occurring in areas just meeting an
alternative standards of 75 ppb. If the population of the nonattainment
areas on which the commenters' focus is substituted in the calculation
for the total population of the U.S. as the denominator (29.5 million x
10.5% x 9.7% = 146,664), with the commenters' estimates of children in
those areas that may experience an exposure at or above 80 ppb (4,788)
or below 80 ppb and at or above 73 ppb (12,641), the percentages are
3.3% and 8.6%, respectively (and the percentage for at or above 73 ppb
would be 5.8%).\91\ Thus, contrary to the commenters' assertion, their
analytical approach, with use of a denominator that reflects the
commenters' focus areas, results in higher estimates of the percentage
of at-risk children that may experience particular exposures of concern
in areas meeting a 75 ppb standard than does the EPA's analysis, which
takes into account a number of factors in much greater detail (e.g.,
through the use of exposure modeling and human activity data to
estimate time series contributing to 7-hour exposure periods with
average O3 concentrations at or above benchmarks), and
focuses on temporal and spatial patterns of air quality in areas just
meeting a standard of 75 ppb. The commenters analysis is not focused on
the factors that are key determinants of population exposures of
concern, leading to results that are inconsistent with and less
informative than the findings of EPA's more detailed, extensive and
technically sound exposure and risk analyses (summarized in section
II.A.3 above and Appendices 3C and 3D of the PA). Based on
consideration of these analyses, among other factors, as described in
section II.B.3 below, the EPA disagrees that the available evidence and
quantitative analyses supports the conclusion that the current standard
is overprotective and that a standard of 75 ppb would protect public
health with an adequate margin of safety.
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\91\ The EPA's exposure and risk analyses estimate <.1 to 0.3%
of children with asthma might be expected to experience at least one
exposure, while at increased exertion, at or above 80 ppb, on
average across a 3-year period in areas just meeting a potential
alternative standard of 75 ppb (85 FR 49865,Table 4, August 14,
2020). For the 70 ppb benchmark, these percentages are 1.1 to 2.0%.
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In support of the commenters' additional argument that the current
standard is too close to background and that a 75 ppb standard (or a
standard using the seventh highest form) would not be, the commenters
(1) state that just because a D.C. Circuit decision has stated that EPA
is not required to take U.S. background O3 (USB) into
consideration in NAAQS decisions does not mean that such considerations
are precluded; (2) cite the lower number of counties (and associated
population) that would be in nonattainment for a 75 ppb standard as
compared to the current standard (while also suggesting that revision
of the form to a seventh highest would appropriately allow for
additional high O3 days due to wildfires); and (3) suggest
that the EPA is underestimating USB by a factor of three.
With regard to the legal point, the EPA agrees that while it is not
required to take USB into account in NAAQS decisions, it may do so when
such consideration is consistent with the Clean Air Act and prior court
decisions. The EPA is not relying on consideration of background
O3 levels to support its decision in this review. Moreover,
given the differences in public health protection, as noted in the
Administrator's proposed conclusions and described in his conclusions
in section II.B.3 below, we do not believe that we could use proximity
to background concentrations as a basis for revising the current 70 ppb
standard to a potential 75 ppb standard.\92\ On the commenters' second
point, the EPA notes that the number of counties that would or would
not be in nonattainment, the size of population living in them, and the
increasing number of days for high O3 due to wildfires are
not relevant factors in judging whether a particular standard is
requisite under the Clean Air Act. Regardless of such implications of a
decision to retain or revise a NAAQS, the key consideration for the
review of a primary standard is whether the standard is judged to
provide the requisite protection of public health with an adequate
margin of safety.\93\ The commenters have provided no evidence
suggesting that the current standard provides more than the requisite
public health protection under the CAA or indicating that an alternate
standard
[[Page 87288]]
with a level of 75 ppb or with a seventh highest form would provide
requisite protection. For these reasons, we do not find these comments
persuasive in supporting consideration of revising the current standard
to an alternate standard with a level of 75 ppb or with a seventh
highest form.
---------------------------------------------------------------------------
\92\ Taken together, the EPA generally understands prior court
decisions addressing consideration of background O3 in
NAAQS reviews to hold that while the Agency may not establish a
NAAQS that is outside the range of reasonable values supported by
the air quality criteria and the judgments of the Administrator
because of proximity to background concentrations, it is not
precluded from considering relative proximity to background
O3 as one factor in selecting among standards that are
within that range (American Trucking Ass'ns v. EPA, 283 F.3d 355,
379 [D.C. Cir. 2002]; Murray Energy v. EPA, 936 F.3d at 622-624;
American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 [D.C.
Cir. 1982]).
\93\ Comments related to implementation programs are not
addressed here because, as described in section I.A above, this
action is being taken pursuant to CAA section 109(d)(1) and relevant
case law. Furthermore, leaving the NAAQS unaltered will not require
the EPA to make new air quality designations, nor require States or
authorized tribes to undertake new planning or control efforts.
Accordingly, concerns related to implementation of the existing or
an alternate standard are outside the scope of this action.
---------------------------------------------------------------------------
With regard to USB, the commenters present an argument focused on
an urban/``rural'' comparison and one focused on a 1-month analysis of
O3 concentrations in response to population mobility changes
attributed to restrictions placed to manage infections of Corona virus
19 disease (COVID-19). We find there to be limitations in both
arguments that undercut the conclusions reached by the commenter. As a
result, we disagree that the observations made by the commenters
support their statements regarding USB and with the implication that
they contradict the EPA's findings from the detailed and extensive
analyses presented in the PA (PA, section 2.5 and Appendix 2B).
With regard to the urban/``rural'' comparison, the commenters'
first cite EPA's analysis in the PA which indicated, based on daily
maximum 8-hour (MDA8) concentrations for the nation as a whole, that
from one quarter (10 out of 42 ppb) to one third (14 out of 45 ppb) of
average MDA8 concentrations in spring and summer, respectively, are
derived from anthropogenic sources. They then state that differences in
monthly mean MDA8 concentration between two sets of monitoring sites in
the Philadelphia metropolitan area that they identify as the three
highest and the three most rural was 3.3 ppb in April 2020. The
commenters suggest that this amount is much smaller than the 10 to 14
ppb that EPA estimated to be from anthropogenic sources. Based on these
two statements, they contend that USB is being underestimated by a
factor of three.
We find the commenters' analysis to have several flaws that
undercut their conclusion. First, the difference between the two sets
of sites, all of which fall in the Philadelphia metropolitan area, are
not indicative of either USA (i.e., U.S. anthropogenic) or USB
contributions. There is no evidence that this difference is indicative
of either USB or USA, and it is especially anomalous given that the
commenters' analysis is based on 2020 data (affected by reduced
emissions during the reduced travel during the initial months of the
COVID-19 epidemic in the U.S.) while EPA's is based on 2016 data.
Second, the authors cite a country-wide seasonal average despite the
fact that the U.S. anthropogenic contributions are clearly higher in
the nonattainment area (than a U.S. average) being referenced. Further,
the conclusions about USB underestimation appear quantitatively
incorrect and to perhaps confuse USA and USB in the calculations. Even
if all USA anthropogenic contributions cited (10 USA and 30 USB of
total 40 ppb) in spring of 2016 were actually USB, the underestimation
of USB would be 25% at most (0 USA and 40 USB of total 40 ppb; (40-30)/
40 = 25%), thus it is unclear how the commenter concluded a factor of
three (300%) under-estimation of USB. In addition, the commenter's
dataset is for the Philadelphia-Wilmington-Atlantic City CSA, where
O3 more frequently exceeds the level of the standard in May
through September (e.g., PA, Appendix 3C, Figure 3C-79), months that
have lower USB and higher US anthropogenic than month of April, which
the commenters analyzed. Finally, the commenter has focused on low
concentration days (averaging ~40-45ppb) that the PA shows tend to be
different than high days (PA, section 2.5 and Appendix 2B).
The second argument is based on data on Apple Mobility data \94\
and O3 and NO2 concentrations for the period from
3/22/2020 to 4/20/2020 (when transportation activity was affected by
the behavioral changes in response to COVID-19) and differences from
the same period in prior years. Based on the differences, the
commenters conclude that O3 concentrations were less
responsive to the 40 to 60% reduction in mobility than were
NO2 concentrations (7% vs 22% difference), indicating to the
commenters that society is reaching a period of diminishing returns of
actions to control O3 concentrations. We note, however, that
the period of the commenters' analysis is April, while the majority of
days with MDA8 greater than 70 ppb in the Philadelphia nonattainment
area occur in May to September. In the mid to late summer period, local
production of O3 is increased (see PA section 2.5.3.2) and
MDA8 concentrations in the Philadelphia nonattainment area more
frequently are above the level of the standard. Thus, the analysis does
not support the commenters' argument for a less stringent standard.\95\
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\94\ https://covid19.apple.com/mobility.
\95\ We also note, contrary to the commenters' premise,
NO2 and/or NOX are not conserved over a day.
Rather, the overall lifetime of NOX is on the order of
six hours. Further, while the commenter describes the ``local''
nature of O3, it is well established that O3
has a large transport component. The diurnal pattern of
O3 concentrations highlighted on this point is likely
illustrating O3 concentrations subject to local
NOX-titration rather than purely local formation as
suggested by the commenters.
---------------------------------------------------------------------------
(ii) Comments in Disagreement With Proposed Decision and Calling for
More Stringent Standard
Among the commenters that disagree with the proposed decision and
call for a more stringent standard, most express concerns regarding the
process for reviewing the criteria and standards in this review and
assert that the proposal must be withdrawn, and a new review conducted.
The commenters expressing the view that a more stringent standard is
needed variously cite a number of concerns. Some state that EPA cannot,
as some commenters imply it does, simply base its decision on a
judgment that the available evidence is similar to that when the
standard was established in a prior review, and some argue that the
available health effects evidence indicates that adverse health effects
occur from exposures allowed by the current standard. Further some
commenters express their views that the combined consideration of the
complete evidence base indicates that sensitive or vulnerable
populations are not protected by the current standard; and/or that the
standard does not provide an adequate margin of safety. Additionally,
in support of their view that the standard should be made more
stringent, some commenters disagree with the conclusions of the
exposure and risk analyses, characterizing the analyses as deficient,
and contending that other quantitative analyses they cite indicate
health impacts that would be avoided by a lower standard level. Most of
the commenters advocating a more stringent standard recommend revision
of the level to a value at or below 60 ppb and others support a level
at or below 65 ppb. Some of these commenters additionally note they had
raised similar concerns during the 2015 review.\96\ Some commenters
also express the view that the EPA should establish a separate long-
term standard.
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\96\ We note that comments raised in the prior review were fully
considered in reaching the decision in that review. Such comments
are addressed in the decision and associated Response to Comments
(80 FR 65292, October 26, 2020; U.S. EPA, 2015). To the extent that
commenters are raising similar issues in support of their comments
on the proposed decision in this review, we have addressed them in
the current decision, based on the information now available.
---------------------------------------------------------------------------
With regard to the process by which this review has been conducted,
we disagree with the commenters that it is arbitrary and capricious or
that it does not comport with legislative requirements. The review
process, summarized in section I.D, implemented a number of features,
some of which have been employed in past reviews and others which have
not,
[[Page 87289]]
and several which represent efficiencies in consideration of the
statutorily required time frame for completion of the review. The
comments that raise concerns regarding specific aspects of the process
are addressed in the separate Response to Comments document. As
indicated there, the EPA disagrees with these comments. The EPA finds
the review to have been lawfully conducted, the process reasonably
explained, and thus finds no reason to withdraw the proposal.
We disagree with some commenters' contention that the EPA based its
proposed decision simply on the similarity of the health effects
evidence to that available in the last review. While the health effects
information is generally similar to that available in the last review,
particularly with regard to respiratory effects (the effects causally
related to O3 exposure), the current health effects evidence
base includes hundreds of new health studies. Based on consideration of
the full evidence base, including that the newly available in the
current review, the EPA has reached different conclusions regarding
some categories of effects (as summarized in II.A.2.a above). The EPA's
observation that the nature of the evidence has not substantially
changed with regard to effects causally related to O3
exposure, was not, as implied by the comment, the primary consideration
in the Administrator's proposed decision. The Administrator considered
a number of factors in reaching his proposed decision, including the
full extent of the currently available health effects evidence, and the
details in which it is, and is not, similar to the last review, which
has led to conclusions similar to prior conclusions for some categories
of O3 effects and resulted in changes to others (85 FR
49868-49874, August 14, 2020).\97\ Further, in reaching his final
decision in this review, as described in section II.B.3 below, he has
again considered the currently available information, now in light of
the public comments received on the proposal, among other factors.\98\
In sum, while we have noted the similarities in the health effects
information between this review and the last review (particularly for
respiratory effects), we have engaged in independent analysis and
assessment of the health effects information in this review, and the
Administrator has exercised his independent judgment based on the
current health effects assessment, in combination with current
exposure/risk information, advice from the CASAC and public comment.
Thus, contrary to the suggestion by these commenters, the decision on
the primary standard has been made in consideration of the current
health effects evidence, current analyses of air quality, exposure and
risk, advice from the CASAC, and public comments, consistent with
requirements under the CAA.
---------------------------------------------------------------------------
\97\ As just one example, the causal determinations for
cardiovascular effects and total mortality in this review differ
from those made in the last review, as described in section
II.A.2.a.
\98\ In so doing, to the extent the current evidence before the
Administrator continues to support or reinforce conclusions reached
in prior reviews, he may reasonably reach those same conclusions.
---------------------------------------------------------------------------
In support of their position that the available health effects
evidence indicates that O3 exposures occurring in areas that
meet the current standard are causing adverse effects, some commenters
cite studies that investigate associations of O3
concentrations and effects, such as respiratory effects, mortality, and
preterm birth.\99\ These studies include some already evaluated in the
air quality criteria,100 101 some published subsequent to
the literature cutoff date for the ISA, and some which some commenters
claim the EPA arbitrarily dismissed or inconsistently weighed in
reaching the proposed decision.\102\ As discussed in I.D above, we have
provisionally considered these ``new'' studies that have not already
been evaluated in the air quality criteria and that were cited by
commenters in support of their comments on the proposed decision (Luben
et al., 2020). Based on this consideration, we conclude that these
studies do not materially change the broad conclusions of the ISA with
regard to these health effects, including the conclusions that there is
a causal relationship of short-term respiratory effects with
O3 exposures; a relationship of long-term respiratory
effects with O3 exposure that is likely to be causal;
evidence that is suggestive of, but not sufficient to infer, causal
relationships of cardiovascular effects and total mortality with short-
or long-term O3 exposure; evidence that is suggestive of,
but not sufficient to infer, causal relationships of central nervous
system effects with short- or long-term O3 exposure; and,
evidence that is suggestive of, but not sufficient to infer, causal
relationships of reproductive and developmental effects with long-term
O3 exposure (ISA, section IS.1.3.1). Nor do we find that
these studies warrant reopening the air quality criteria for further
review (Luben et al., 2020). Thus, we do not find these publications to
be contrary to the discussions and associated conclusions in the PA and
proposal or to indicate the current standard to be inadequate. We
disagree that studies cited by commenters show these categories of
effects to be caused by O3 exposures associated with
O3 air quality that meets the current standard. We continue
to focus on the studies of respiratory effects as most important to the
Administrator's judgments concerning the public health protection
provided by the current standard.
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\99\ With regard to effects other than respiratory effects,
studies cited by these commenters include studies of cardiovascular
effects (Day et al., 2017; Shin et al., 2019; Wang et al., 2019b),
all-cause mortality (Bell et al., 2014; Cohen et al., 2017; Di et
al., 2017a, b), neurological effects (Cleary et al., 2018), and
reproductive and developmental effects (Wallace et al., 2016;
Lavigne et al., 2016; Salam et al., 2005; Steib et al, 2019;
Morello-Frosch et al., 2010).
\100\ In updating the air quality criteria in the current
review, the current ISA evaluates relevant scientific literature
published since the 2013 ISA, integrating with key information and
judgments contained in the 2013 Ozone ISA and previous assessments
(ISA, p. lxix; 2013 ISA; U.S. EPA, 2006; U.S. EPA, 1996a; U.S. EPA,
1982; U.S. EPA, 1986; U.S. EPA 1978; NAPCA, 1969).
\101\ This commenter cited a epidemiologic study (Day et al.,
2017) that had been among studies of short term O3 and
cardiovascular effects excluded from the draft ISA due to location,
however this study was considered by the EPA in response to advice
from the CASAC on the draft ISA (Luben, 2020). This consideration of
these studies did not change EPA's analysis of the weight of
evidence from that described in the draft ISA, thus supporting the
causality determination for cardiovascular effects described in the
final ISA (ISA, section IS.4.3).
\102\ We note that one study identified by a commenter to
support their view that O3 concentrations allowed by the
current standard is causing health effects does not include
O3 among the pollutants it examines (Gan et al., 2014).
Accordingly, we do not find the study to provide support to the
commenter's point.
---------------------------------------------------------------------------
The epidemiologic studies of respiratory effects identified by the
commenters include some investigating associations of O3
exposure with hospital admissions or emergency department visits for
respiratory outcomes, or with various respiratory effects for selected
population groups. Studies of O3 and respiratory effects
cited by these commenters in support of their comment include studies
that have already been evaluated in the air quality criteria (Goodman
et al., 2017; O'Lenick et al., 2017; Jerrett et al., 2009; Lin et al.,
2008; Islam et al., 2009; Galizia et al., 1999; Peters et al., 1999;
Wendt et al., 2014), and also several ``new'' studies, including four
that investigate a relationship between O3 and COVID-19
(Ware et al., 2016; Strosnider et al., 2019; Wang et al., 2019a;
Adhikari and Yin, 2020; Zhu et al., 2020; Zoran et al., 2020; Petroni
et al., 2020).\103\ We do not
[[Page 87290]]
find these studies to contradict any of the scientific conclusions on
respiratory effects described in the ISA.
---------------------------------------------------------------------------
\103\ As discussed in section I.D above, the ``new'' studies
identified by commenters have not been through the comprehensive
CASAC and public review process that the air quality criteria went
through. To address these comments, we have provisionally considered
these studies, as discussed in I.D above, and found they do not
materially change the broad scientific conclusions of the ISA with
regard to respiratory effects, or warrant re-opening the air quality
criteria for further review (Luben et al., 2020).
---------------------------------------------------------------------------
With regard to the four studies on COVID-19, we disagree with the
commenters that they provide evidence that O3 exposure
contributes to COVID-19 incidence, much less that they indicate that
O3 concentrations occurring when the current standard is met
would do so. These studies investigate an association between
O3 and COVID-19 cases or deaths. We note, however, that the
time-series study design used in three of these studies (Zhu et al.,
2020 [incorrectly cited by some commenters as Yongiian et al, 2020];
Adhikari and Yin, 2020; Zoran et al., 2020) is not appropriate for
infectious disease cases, which do not follow a Poisson distribution,
as they increase exponentially with community spread. The fourth study,
an ecological study (Petroni et al. 2020), is also limited by its study
design, which is susceptible to confounding or other biases related to
ecologic fallacy,\104\ as well as its manner of assigning exposure to
the population.\105\ Further, the time periods in none of the four
studies is long enough to rule out a coincidental increase in the
community spread of COVID-19 with the increased O3
concentrations expected with the beginning of O3 season in
these areas (e.g., March-April). Lastly, the biological basis by which
a gaseous pollutant such as O3 would be expected to
contribute to incidence of this disease is unclear.\106\ Thus, we do
not find these studies to support a conclusion that O3
exposure causes COVID-19 morbidity or mortality.\107\
---------------------------------------------------------------------------
\104\ Ecologic fallacy is a specific type of bias that results
when group- or population-level data are used to estimate
individual-level risks in an epidemiologic study
\105\ This study uses 2016 summertime average O3 as a
surrogate for O3 from 3/1/2020 to 7/11/2020 (Petroni et
al., 2020). Yet COVID-19 cases did not surge in many parts of the
U.S. until late summer or fall 2020. To the extent these areas
(e.g., rural upper midwest) have lower O3 concentrations
than areas of the country where COVID-19 cases surged earlier (e.g.,
New York City), a correlation between O3 concentrations
and COVID-19 deaths would be overestimated.
\106\ While there may be correlations between O3
concentrations and COVID-19 cases and deaths, they could be
explained by coincidental timing of the COVID-19 community
transmission period in New York City and Milan with the early part
of the O3 seasons in those areas, and neither the
investigators or commenters provide evidence supporting an
alternative plausible basis (Adhikari and Yin, 2020; Zoran et al.,
2020).
\107\ While the full evidence base indicates the potential for
O3 to increase susceptibility to some respiratory infections, the
studies cited by commenters do not provide evidence that short-term
or long-term O3 exposure increases susceptibility to
COVID-19.
---------------------------------------------------------------------------
With regard to the commenters' claims that effects other than
respiratory effects (see above) are occurring as a result of
O3 concentrations allowed by the current standard, we note
that the standard is exceeded in nearly all of the locations and time
periods analyzed in these studies.\108\ Although some studies analyzed
multiple cities or locations in which the current standard was met
during some time periods, air quality during other time periods or
locations in the dataset does not meet the current standard. As noted
in past reviews, compared to single-city studies, there is additional
uncertainty in interpreting relationships between O3 air
quality in individual study cities and reported O3 multicity
effect estimates. Specifically, as recognized in section II.A.2.c
above, the available multicity effect estimates in studies of short-
term O3 do not provide a basis for considering the extent to
which O3 health effect associations are influenced by
individual locations with ambient O3 concentrations low
enough to meet the current O3 standards versus locations
with O3 concentrations that violate this standard (85 FR
49853, August 14, 2020; 80 FR 65344, October 26, 2015).\109\ Thus,
based on this information and the full health effects evidence base for
O3, we disagree with commenters about the implications of
the cited epidemiologic studies regarding health risks of O3
exposures resulting from the O3 concentrations in ambient
air allowed by the current standard.
---------------------------------------------------------------------------
\108\ Locations and time periods analyzed in these studies
include three large metropolitan areas in Texas before 2012 (Goodman
et al., 2017); Atlanta, Dallas and St. Louis from 2002 to 2008
(O'Lenick et al., 2017); large cities across the U.S. from late
1970s through 2000 (Jerrett et al., 2009); New York State, primarily
during the 1990s (Lin et al., 2008); U.S. location chosen for O3
concentrations not meeting the standard (Galizia and Kinney, 1999);
a set of southern California communities during period (1990s)
recognized to be exceeding the NAAQS (Peters et al., 1999; Islam et
al., 2009); Houston metropolitan area during 2005 to 2007 (Wendt et
al., 2014); multiple locations including St. Louis, Memphis and
Atlanta 2003 through 2012 (Ware et al., 2016); six U.S. metropolitan
areas, including Los Angeles, Baltimore and New York City, from 1999
thru 2018 (Wang et al., 2019a); and 894 U.S. counties, including
those for New York City and Los Angeles, 2001 to 2014 (Strosnider et
al., 2019). Air quality data and design values derived by the U.S.
indicate that the current 70 ppb standard was not met throughout the
study period, or, for multicity studies for which single-city
analyses not performed, was not met in all cities throughout the
study (PA, Appendix 3B and Excell files available at: https://www.epa.gov/air-trends/air-quality-design-values).
\109\ This uncertainty applies specifically to interpreting air
quality analyses within the context of multicity effect estimates
for short-term O3 concentrations, where effect estimates
for individual study cities are not presented, as is the case for
some of the multicity studies identified by commenters (85 FR 49870,
August 14, 2020).
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Protection of Sensitive Groups: Commenters expressing the view that
the current standard does not protect sensitive or at-risk populations,
variously state that the EPA does not consider risks to a number of
population groups the commenters identify as at higher risk for
O3-related health effects, and that retaining the current
standard ``creates additional and unacceptable risks'' for Black and
low-income communities. Further, some commenters express the views that
together the evidence from controlled human exposure studies and from
epidemiologic studies indicates adverse effects associated with
exposures allowed by the current standard; and that the EPA has not
appropriately considered a number of aspects of the evidence related to
risks to people with asthma.
Some commenters, in addition to contending that the current
standard will not protect populations for which the EPA has concluded
there is adequate evidence for identification of increased risk (e.g.,
people with asthma, children, and outdoor workers), additionally assert
that the current standard will not protect populations of color,
American Indian/American Native groups, low SES communities, people of
any age with respiratory issues other than asthma, diabetes or atrial
fibrillation and pregnant women. As described in section I.A. above,
primary NAAQS are intended to protect the public health, including at-
risk populations, with an adequate margin of safety. Accordingly, in
reviewing the air quality criteria, the EPA evaluates the evidence with
regard to factors that place some populations at increased risk of harm
from the subject pollutant. In this review, the populations for which
the evidence indicates increased risk include people with asthma,
children and outdoor workers, among other groups, as summarized in
section II.A.2.b above (ISA, section IS.4.4).
In support of their argument that individuals with atrial
fibrillation are at increased risk of O3-related health
effects, the commenter cited a study of O3 exposure and
total mortality that has been evaluated in the ISA (Medina-Ramon and
Schwartz, 2008). It was initially evaluated in the last review and
explicitly discussed again as part of the evidence base available in
the current review (ISA, section 6.1.5.2 and Table IS-10; 2013 ISA,
sections 6.6.2.2 and 8.2.4). Based on consideration of that study and
others investigating a potential for increased risk among populations
with cardiovascular disease
[[Page 87291]]
(CVD), the 2013 ISA concluded that the evidence was ``inadequate to
classify pre-existing CVD as a potential at-risk factor for
O3-related health effects'' (2013 ISA, sections 8.2.4). In
the current review, while a limited number of recent studies add to the
evidence available in the 2013 ISA,\110\ collectively the evidence
remains inadequate to conclude whether individuals with pre-existing
CVD are at greater risk of O3-related health effects (ISA,
Table IS-10, section IS.4.4.3.5). Thus, the evidence does not support
the commenters assertion that populations with atrial fibrillation are
at increased risk of O3-related effects and that the current
standard does not protect these groups.
---------------------------------------------------------------------------
\110\ Further, we note that a more recent study than that cited
by the commenters investigated the potential for an association of
O3-related mortality risk with individuals with atrial
fibrillation and observed no evidence of an association (ISA,
Appendix 6, p. 6-11).
---------------------------------------------------------------------------
The commenters who contend pregnant women are at increased risk do
not provide supporting evidence, and the ISA does not reach such a
conclusion based on the currently available evidence. Further, the ISA
determined the evidence to be suggestive of, but not sufficient to
infer, a causal relationship between O3 exposure and
reproductive effects (ISA, section IS.4.3.6.3). Thus, we disagree with
the commenters that pregnant women may be at increased risk of
O3-related effects and disagree that the current standard
does not protect these groups.
With regard to a potential for increased risk of O3-
related health effects based upon race or ethnicity, including American
Indians or Native Americans), the available evidence is inadequate to
make such a determination (ISA, section IS.4.4, Tables IS-9 and IS-
10).\111\ Additionally, the evidence of increased O3 risk
based on SES has been evaluated in the ISA and concluded to be
``suggestive,'' but the evidence is limited by inconsistencies (ISA,
section IS.4.4). Thus, contrary to the view expressed by some
commenters, the EPA has considered this factor in this review and the
evidence was not adequate to identify SES as a risk factor for
O3 related health effects. As noted by the commenters, the
evidence for low SES populations is ``suggestive'' of increased risk
(ISA, section IS.4.4), in part because it includes several
inconsistencies (as summarized in section II.A.2.b above), including
studies that did not find O3-related risk to be higher in
lower SES communities.\112\ While we agree with the commenters that
populations of some particular races or ethnic backgrounds or with low
SES have higher rates of some health conditions, including asthma,\113\
the available evidence is not adequate to conclude an increased risk
status based solely on racial, ethnic or income variables alone (ISA,
section IS.4.4). Thus, we disagree with commenters that EPA has
arbitrarily not considered such factors in reaching the decision on the
primary standard.
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\111\ In support of their view that O3-related risk
is increased in Black populations, some commenters cite a study
published after the ISA (Gharibi et al., 2019). We have
provisionally considered this study, as described in section I.D.
above, and found that it does not materially affect the broad
conclusions in the ISA, including those regarding the adequacy of
evidence for finding an influence on O3-related risk of
different categories of population status, or warrant reopening the
air quality criteria for further review (Luben et al., 2020).
\112\ We note that two studies described by one commenter as
indicating that those with low SES or who live in low SES
communities face higher risk of hospital admissions and emergency
department visits related to O3 pollution have been
evaluated by the EPA and found not to report such findings (2013
ISA, section 8.3.3; ISA, Table IS-10). In the first, a study of
O3 exposure and respiratory hospital admissions in 10
Canadian cities (Cakmak et al., 2006) ``no consistent trend in the
effect was seen across quartiles of income,'' and the second, a
study of O3 exposure and asthma hospital admissions and
emergency visits (Burra et al., 2009), ``reported inverse effects
for all levels of SES'' (2013 ISA, p. 8-27; ISA, Table IS-10).
\113\ This is noted in the PA and proposal with regard to Black
non-Hispanic and several Hispanic population groups (PA, Table 3-1).
As some commenters note, this is also the case for American Indian
and Native American population groups. Based on the recently
available, 2016-2018 National Health Interview Survey, while just
under 8% of the U.S. population is estimated to have asthma, the
estimate is more than 10% for American Indian or Native American
populations in the U.S. (https://www.cdc.gov/asthma/most_recent_national_asthma_data.htm; document identifier EPA-HQ-
OAR-2018-0279-0086).
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Some commenters further claim that tribal populations and
communities of color are at increased risk of O3-related
health effects due to increased impacts of COVID-19. We disagree with
commenters that the studies they cite provide support for the role of
O3 exposure in the observed increase in prevalence. The
studies cited simply describe greater prevalence of COVID-19 among such
communities and do not investigate and therefore do not provide
evidence for a role for O3 exposure.\114\ An additional
study cited by one commenter in support of their statement that people
with COVID-19 are more susceptible to effects of O3, does
not include any analyses with O3 among its analyses (Wu et
al, 2020). With regard to diabetes, we note that the evidence related
to a potential for this to affect risk of O3-related effects
has been explicitly evaluated and found to be inadequate, thus
indicating a lack of basis in the evidence for the statement by some
commenters that diabetes prevalence in a community increases the risk
of O3-related effects (ISA, Table IS-10).
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\114\ The commenter cites Price-Haywood et al. (2020), Stokes et
al. (2020), Millett et al. (2020), Killerby et al. (2020), and Gold
et al. (2020). These studies present information regarding COVID-19
cases, hospitalizations and/or deaths among various population
groups, but they do not investigate association of those occurrences
with O3.
---------------------------------------------------------------------------
Additionally, commenters that contend that retaining the current
standard ``creates additional and unacceptable risks'' for minority and
low-income populations variously cite higher rates of asthma and other
preexisting conditions in these populations and higher levels of
pollution.\115\ In making this claim, these commenters state that non-
Hispanic Blacks have been found to be more likely to live in counties
with higher O3 pollution. To the extent that such patterns
in the distribution of certain population groups and O3
concentrations result in these populations residing in areas that do
not currently meet the current standard, we note that they are at
greater risk than populations residing in areas that meet the current
standard, and implementing the standard will reduce their risks. But we
disagree with the commenters' conclusion that retaining the current
standard, without any change, creates additional risks for these
populations.
---------------------------------------------------------------------------
\115\ In making their argument, these commenters do not provide
any explanation for why retaining the existing standard (i.e.,
making no regulatory change) would create additional risk for these
populations. Rather, these commenters seem to be describing
differences in predicted risk or mortality of air quality associated
with a lower standard level and that of the current standard. In
that way, they are claiming that retaining the current standard
``creates'' additional risk. We address comments advocating a lower
standard based on commenter-cited risk estimates (e.g., mortality)
further below.
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Thus, contrary to statements by some commenters, the EPA's proposed
decision to retain the current standard did consider evidence regarding
risk to and thus protection of specific populations, such as those of
particular races or ethnicities or low-income populations. The proposed
decision, and the Administrator's decision described in section II.B.3
below, are based on consideration of the currently available evidence,
particularly that with regard to populations that may be at greater
risk of O3-related health effects than the general
population. As described in section II.B.3 below, the Administrator
judges that by basing his decision on consideration of these
populations, including adults and children with asthma, the at-risk
population groups for which the
[[Page 87292]]
evidence is strongest and most extensive, will also provide protection
for other at-risk populations for which the evidence is less certain
and less complete.
The commenters who express the view that the current standard does
not provide sufficient protection of people with asthma raise concerns
with the EPA's consideration of this group and O3-related
effects. Further, some commenters state that the EPA has not adequately
explained how its approach for decision-making in this review protects
at-risk populations, such as people with asthma. Such commenters state
that the EPA does not explain how the proposed decision accounts for
the greater vulnerability of people with asthma, given the attention to
evidence from controlled human exposure studies of largely healthy
subjects. Some commenters contend that the EPA arbitrarily focuses on
lung function decrements and respiratory symptoms ahead of lung
inflammation, and/or that the EPA has not rationally considered the
most recent ATS statement with regard to consideration of effects in
people with respiratory disease, such as asthma (which the commenters
describe as a difference from past reviews).
We disagree with these commenters. In this review, as in past
reviews, the EPA has fully considered the health effects evidence in
this review, including for sensitive populations, such as people with
asthma, and explained its conclusions regarding the adequacy of public
health protection offered by the current standard, including for such
populations. Thus, the decision in this review, as described in section
II.B.3 below, is based on the current scientific information. Further,
our approach in this review does not differ appreciably from our
approach in the last review. This approach is consistent with the
applicable legal requirements for this review, including with
provisions of the CAA related to the review of the NAAQS, and with how
the EPA and the courts have historically interpreted the CAA. The
approach is based fundamentally on the current health effects evidence
in the ISA and quantitative analyses of exposure and risk in the PA.
The policy implications of this information, along with guidance,
criteria or interpretive statements developed within the public health
community, including, also, statements from the ATS, in addition to
advice from the CASAC are evaluated in the PA for consideration by the
Administrator. The PA evaluations inform the Administrator's public
health policy judgments and conclusions. Thus, as in past reviews, the
Administrator's decision on the adequacy of the current primary
standard draws upon the scientific evidence for health effects,
quantitative analyses of population exposures and health risks, CASAC
advice, and judgments about how to consider the uncertainties and
limitations that are inherent in the scientific evidence and
quantitative analyses, as well as public comments on the proposed
decision.
As described in section II.B.3 below, key aspects of the evidence
informing the Administrator's decision-making in this review include:
(1) The causal relationship of O3 with respiratory effects,
based on the full health effects evidence base, including both the
controlled human exposure studies conducted primarily in largely
healthy adult subjects, and the epidemiologic studies of health
outcomes for people with asthma, and particularly children with asthma;
(2) the increased risk to children and people with asthma, among other
groups (3) the respiratory effects reported at the lowest exposures in
the controlled human exposure studies; and (4) features of asthma that
contribute to the susceptibility of people with asthma to
O3-related effects. As a whole, the evidence base in this
NAAQS review generally reflects a continuum, consisting of exposure
levels at which scientists generally agree that health effects are
likely to occur, through lower levels at which the likelihood and
magnitude of the response become increasingly uncertain. As summarized
in section I.A above, the CAA does not require the Administrator to
establish a primary NAAQS at a zero-risk level or at background
concentration levels (see Lead Industries Ass'n v. EPA, 647 F.2d at
1156 n.51, Mississippi v. EPA, 744 F.3d at 1351), but rather at a level
that reduces risk sufficiently so as to protect public health with an
adequate margin of safety. The Administrator's consideration of the
scientific evidence is informed by the quantitative estimates of
exposure and risk for air quality allowed by the current standard, and
associated judgments on the adequacy of public health protection
provided by the current standard are informed by advice from the CASAC
and statements from ATS on adversity.
With regard to the most recent ATS statement, the commenters' claim
that the EPA does not adequately consider the implications of the
sentence that ``small lung function changes should be considered
adverse in individuals with extant compromised function, such as that
resulting from asthma, even without accompanying respiratory symptoms''
and to consider the importance of examining effects in susceptible
subsets of broader populations (Thurston et al., 2017). We disagree.
The ATS statements (from the initial statement in 1985 to the recent
2017 statement) and their role in primary O3 standard
reviews, summarized in section II.A.2.b above, occupy a prominent role
in consideration of public health implications in the PA and the
proposal (PA, section 3.3.2; 85 FR 49848, 49866, 49871, August 14,
2020), and the Administrator considers them in his decision, as
described in section II.B.3 below. The PA presentation includes
summaries of the purpose and intentions articulated by the ATS, and of
the evolution and areas of consistency across the statements. The PA
gave particular attention to the ATS emphasis on consideration of the
significance or adversity of effects, particularly for more susceptible
individuals. It recognized both the 2000 ATS statement concluding that
``small transient changes in forced expiratory volume in 1 s[econd]
(FEV1) alone were not necessarily adverse in healthy
individuals, but should be considered adverse when accompanied by
symptoms'' (ATS, 2000), and also the more recent statement that also
gives weight to findings of such lung function changes in the absence
of respiratory symptoms in individuals with pre-existing compromised
function, such as that resulting from asthma (Thurston et al., 2017).
With regard to population risk (another aspect of the ATS statement
cited by commenters), the PA and proposal summarized the 2000 and 2017
ATS statements, recognizing that the 2017 statement references and
further describes concepts described in the 2000 statement, such as its
discussion of considering effects on the portion of the population that
may have a diminished reserve that puts its members at potentially
increased risk if affected by another agent (ATS, 2000).\116\ As
described in section II.B.3 below, the Administrator considers the ATS
statements in reaching his conclusions in this review.
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\116\ The sentence in the 2017 statement of which one commenter
quoted only a part, ``As discussed in the previous ATS statement, a
small but statistically significant mean reduction in
FEV1 in a population means that some people had larger
reductions, with the likelihood that reductions in a subset of
susceptible subjects can have passed a threshold for clinical
importance'' This paragraph goes on to note that a study in which
the mean decrement is about 3%, included two subjects with
decrements greater than 10% (Thurston et al., 2017).
---------------------------------------------------------------------------
In support of their claim that the EPA has not appropriately
considered the ATS statements, some commenters
[[Page 87293]]
additionally take issue with the EPA's use of the number of subjects
experiencing at least a 15% FEV1 decrement in its
description in the proposal of the increased response evident by
comparing from the lowest exposure levels studied (40 ppb) up to 70 ppb
(85 FR 49851, August 14, 2020). These comments also state that EPA did
not discuss the clinical significance of FEV1 decrements of
10% or higher for people with existing lung disease, while stating that
the ATS statement mentions this magnitude of decrement. The ATS
statement references decrements at or above 10% in illustrating a point
about variation of subject responses beyond a group mean, noting that
while the mean of an exposed group of study subjects may be small, some
group members have larger reductions and can have passed a threshold
for clinical importance. It does not provide a discussion of thresholds
of clinical importance.\117\ In claiming that EPA's discussion on this
represents a difference from the last review, the commenters cite the
2014 HREA and state that we have not considered FEV1
decrements at or above 10% in the current review, however this is not
the case.\118\ Furthermore, the PA states that the mid- to upper-end of
the range of moderate levels of functional responses and higher (i.e.,
FEV1 decrements >=15% and >=20%) are included to generally
represent potentially adverse lung function decrements in active
healthy adults, while for people with asthma or lung disease, a focus
on moderate functional responses (FEV1 decrements down to
10%) may be appropriate (PA, Appendix 3D, p. 3D-76).
---------------------------------------------------------------------------
\117\ With regard to 10% as a magnitude decrement, the prior ATS
statement noted that the EPA had graded this ``mild'' in a prior
review, while noting that such a grading has not been evaluated
against other measures (ATS, 2000). In this review, as in past
reviews, the EPA has summarized study results with regard to
multiple magnitudes of lung function decrement, including 10%,
recognizing that 10% has been used in clinical settings to detect a
FEV1 change likely indicative of a response rather than
intrasubject variability, e.g., for purposes of identifying subjects
with responses to increased ventilation (Dryden, 2010). For example,
the PA in the current review provides such a summary (PA, Appendix
3D, p. 3D-77).
\118\ Contrary to this claim, the lung function risk analysis in
the current review (which is an update of the very same analysis in
the 2014 HREA to which the commenters cite) presents the results for
exactly the same categories of lung function decrement (at/above
10%, at/above 15% and at/above 20%) as in the 2014 HREA (e.g., PA,
Table 3-4).
---------------------------------------------------------------------------
In objecting to the EPA's approach to considering the ATS
statement, these commenters cite a reference to the ATS statement in
CASAC's advice as additional evidence that the EPA approach to
considering the ATS statement is arbitrary.\119\ This comment was made
within the context of the CASAC comments on the draft PA that
emphasized the need to improve discussion of the susceptibility of
people with asthma, including giving attention to the occurrence of
lung function decrements in susceptible groups, specifically children
with asthma. This section of the CASAC letter also cautions against too
great a focus on lung function decrements and emphasizes the need for
fuller consideration of respiratory effects that are likely to be
important in people with asthma due to features of that disease. In
consideration of these comments, the final PA includes an improved
discussion on the unique vulnerability of people with asthma (PA,
sections 3.3.1.1, 3.3.2, 3.3.4, and 3.5.1) that contributes to due
consideration of this population group in decision-making on the
primary O3 standard. Further, in considering the exposure
and risk analysis results, we recognize the comparison-to-benchmarks
analysis as providing a more robust consideration of risk to sensitive
groups as it provides the ability to consider O3 effects
more broadly, with each benchmark representing the array of effects, at
different severities, associated with that exposure level. This is one
of the reasons (consistent with the CASAC advice) that this analysis
(rather than the lung function risk analysis) receives greater emphasis
in the PA, consistent with the CASAC advice in this area.
---------------------------------------------------------------------------
\119\ The citation provided by the commenters is the CASAC
letter on the draft PA; in this letter the CASAC cites the ATS
statement in making a comment on the draft PA indicating that the
concept that lung function decrements in the absence of symptoms do
not represent an adverse health effect should not apply to the
susceptible group of children with asthma (Cox, 2020a, Consensus
Responses to Charge Questions, pp. 8-9).
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In light of the above discussion, we note that the PA, the
proposal, and the decision described in section II.B.3 below, focus
specifically on consideration of people with asthma, and particularly
children with asthma. While the evidence regarding the susceptibility
of people with asthma to the effects of O3 is robust, our
understanding of the exposures at which various effects (of varying
severity) would be elicited is less defined. For example, the inherent
characteristics of asthma contribute to a risk of asthma-related
responses, such as asthma exacerbation in response to asthma triggers,
which may increase the risk of more severe health outcomes (ISA,
section 3.1.5). This is supported by the strong and consistent
epidemiologic evidence that demonstrates associations between ambient
O3 concentrations and hospital admissions and emergency
department visits for asthma (ISA, section IS.4.4.3.1). In moving to
consideration of the potential specific exposure scenarios (e.g.,
multiple-hour exposures to 60 to 80 ppb O3 during quasi-
continuous exercise), we note that the evidence is for largely healthy
adult subjects. With regard to lung function decrements, the limited
evidence from controlled human exposure studies (primarily at higher
exposures and in adult subjects) indicates similar magnitude of
O3-related FEV1 decrements for people with as for
people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Further,
across other respiratory effects of O3 (e.g., increased
respiratory symptoms, increased airway responsiveness and increased
lung inflammation), the evidence has also found the observed responses
to generally not differ due to the presence of asthma, although the
evidence base is more limited with regard to study subjects with asthma
(ISA, Appendix 3, section 3.1.5.7). Thus, in light of the uncertainties
in the evidence base with respect to people with asthma and exposures
eliciting effects and the severity of those effects, other aspects of
the evidence are informative to the necessary judgments. Accordingly,
the advice from the CASAC and the statements from the ATS are important
to the judgments made by the Administrator in basing his decision on
the current evidence and ensuring a primary standard that protects at-
risk populations, such as people with asthma.
Contrary to the claim from some commenters, our consideration of
effects in people with asthma did not focus solely on lung function
responses. As noted above, we recognize that the inherent
characteristics of asthma as a disease provide the potential for
O3 exposures to trigger asthmatic responses, such as through
causing an increase in airway responsiveness. Based on the available
evidence, we consider the potential for such a response to be greater,
in general, at relatively higher, versus lower, exposure
concentrations, noting 80 ppb to be the lowest exposure concentration
at which increased airway responsiveness has been reported in generally
healthy adults. We recognize that this evidence and the evidence
represented by the three benchmark concentrations used in the exposure/
risk analyses (60, 70 and 80 ppb) is for largely healthy adults and
does not include data for people with
[[Page 87294]]
asthma. In reaching his decision in this review, the Administrator
gives additional consideration to the effects of particular concern for
people with asthma, such as asthma exacerbation, in light of the
limitations of the evidence represented by the benchmarks in this
regard, as discussed in section II.B.3 below.
In support of their view that the EPA gives too little weight to
effects reported in studies of 60 ppb, some commenters assert that the
EPA arbitrarily focused on the evidence for lung function decrements
and respiratory symptoms, and does not explain how the proposed
decision protects against the harm posed by inflammatory responses to
O3. In making this statement they cite the study by Kim et
al. (2011) and discussions in the ISA regarding studies documenting the
role of O3 in eliciting inflammatory responses and regarding
possible conceptual mechanisms by which inflammatory responses can
contribute to other effects (including cardiovascular effects). In so
doing, they contend that exposures lower than those for which the
current standard is intended can cause inflammation resulting in
permanent lung damage and the development of severe lung disease. They
additionally state that airway inflammation of O3 is of
particular concern for people with asthma as airway inflammation is a
feature in the definition of asthma.
Contrary to the view of some commenters, the Administrator has
given significant consideration (in the proposal and in section II.B.3
below) to the exposure estimates for the 60 ppb benchmark. In
considering the O3 inflammatory response, we note that
inflammation induced by a single exposure (or several exposures over
the course of a summer) can resolve entirely (2013 ISA, p. 6-76). Thus,
the inflammatory response observed following the single exposure to 60
ppb in the study by Kim et al. (2011) of largely healthy subjects is
not necessarily an adverse response.\120\ We further consider the
comments from the CASAC regarding airway inflammation as an important
aspect of asthma, including the CASAC's description of increased airway
inflammation in people with asthma as having the potential to increase
the risk of an asthma exacerbation. As described in section II.B.3
below, the Administrator also considers this, while noting also the
lack of evidence from studies of people with asthma at 60 ppb. In so
doing, he recognizes that, due to interindividual variability in
responsiveness, both in regard to O3 and in regard to asthma
exacerbation triggering events, not every occurrence of an exposure
considered to have the potential to increase airway inflammation will
result in such an adverse effect. We find it important to note,
however, that continued acute inflammation can contribute to a chronic
inflammatory state, with the potential to affect the structure and
function of the lung (2013 ISA, p. 6-76; ISA, sections 3.1.4.4.2 and
3.1.5.6.2).\121\ In light of this evidence, the Administrator, in his
consideration of the exposure/risk estimates of exposures at or above
the 60 ppb benchmark (described in section II.B.3 below), is less
concerned about such estimates representing a single occurrence, and
gives weight to estimates of multiple occurrences and their associated
greater risk. Thus, rather than a sole focus on a single exposure level
or type of effect (such as lung function decrements), the Administrator
considers the quantitative estimates for all three benchmarks (with
regard to single and multiple occurrences), recognizing that they
represent differing levels of significance and severity of
O3-related effects, both with regard to the array of effects
and severity of each type of effects, as well as the implications for
the at-risk populations, including people with asthma. The comparison-
to-benchmarks analysis provides for this full characterization of risk
for the broad array of respiratory effects, including inflammation and
airway responsiveness, thus avoiding an inadequate and narrower focus,
e.g., limited to lung function decrements (85 FR 49872, August 14,
2020).
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\120\ One commenter contends that inflammation is apparent from
short-term O3 exposures ranging from 12 to 35 ppb, based
on air quality metrics reported in some epidemiologic studies, such
as mean 24-hour averages or monthly averages of 8-hour
concentrations (ISA, Table 4-28). The commenter implies that such
values for these metrics are lower than the level of the standard
(70 ppb) means that exposures allowed by the standard are causing
outcomes analyzed in the study. However, none of the metrics for
which values are cited by the commenter are in terms of design
values for the current standard, such that a direct comparison of
the values is not meaningful.
\121\ The currently available evidence does not support the
implication of the commenters that the inflammatory response
reported in some individuals after a 6.6-hour exposure to 60 ppb,
during quasi-continuous exercise (as in Kim et al., 2011), causes
permanent lung damage or development of severe lung disease. While
the experimental animal evidence indicates the potential for
repeated exposures to elevated concentrations (e.g., at or above 500
ppb over multiple days) can contribute to other effects in animal
models or to other asthmatic responses in animal models of asthma,
the full evidence base for single exposures to lower concentrations
does not provide such a finding (ISA, sections 3.1.4.4, 3.1.4.4.2
and 3.1.5.6.2; 2013 ISA, section 6.2.3). Thus, the potential for
effects reported from 6.6-hour exposures to 60 ppb O3,
during quasi-continuous exercise, including the inflammation
reported by Kim et al. (2011) to contribute to adverse health
effects is uncertain. Newly available evidence in this review does
not reduce this uncertainty or provide a contradiction to conclusion
regarding the implications of inflammation induced by single or
isolated exposures (ISA, Appendix 3).
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Contrary to the commenters' claims, the Administrator, in reaching
his proposed decision, and in his final decision, as described in
section II.B.3 below, placed primary focus on what the evidence
indicates with regard to health effects in the at-risk population of
people with asthma, particularly children with asthma, and on results
of the exposure and risk analysis for this population. In so doing, he
recognizes key aspects of the evidence, as summarized in section
II.A.2.a above, that indicate the array of O3-associated
respiratory effects to be of increased significance to people with
asthma given aspects of the disease that may put such peoples at
increased risk for prolonged bronchoconstriction in response to asthma
triggers. The increased significance of effects in people with asthma
and risk of increased exposure for children (from greater frequency of
outdoor exercise) is illustrated by the epidemiologic findings of
positive associations between O3 exposure and asthma-related
emergency department visits and hospital admissions for children with
asthma. In this context, the Administrator focuses on the breadth of
O3 respiratory effects evidence at the lowest exposures
tested in the controlled human exposure studies which provide the most
certain evidence, considering this in light of the fuller evidence base
which provides a foundation for necessary judgments in light of
uncertainties.
Thus, we disagree with commenters that we have not considered the
full body of evidence and quantitative information available in this
review with regard to exposures that might be expected to elicit
effects in at-risk populations. In so doing, as summarized in section
II.A.2.a above, section II.B.1.a of the proposal, and the PA, we
recognize that the currently available evidence supports the conclusion
of a causal relationship between short-term O3 exposure and
respiratory effects, with the strongest evidence coming from controlled
human exposure studies that document subtle reversible effects in 6.6-
hour exposures of largely healthy adult subjects, engaged in quasi-
continuous exercise, to average concentrations as low as 60 ppb. The
epidemiologic evidence of associations of O3 concentrations
in ambient air with
[[Page 87295]]
increased incidence of hospital admissions and emergency department
visits for an array of respiratory health outcomes further indicates
the potential for O3 exposures to elicit health outcomes
more serious than those assessed in the experimental studies,
particularly for children with asthma, and the evidence base of such
epidemiologic studies as a whole provides strong support for the
conclusion of causality for respiratory effects.\122\ Further as
described in the PA and the proposal and summarized in section II.A.2.a
above, very few of these studies were conducted in locations during
periods when the current standard was met. While some commenters cite
the low values of some of the air quality metrics analyzed in such
studies, those metrics are not in the form of the design value for the
current standard and so, contrary to commenters' assertion, cannot show
that serious health effects are occurring under air quality conditions
allowed by the current 70 ppb standard.
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\122\ As described in section II.A.2.c above and in the PA,
these studies generally do not detail the specific exposure
circumstances eliciting such effects.
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Protection With an Adequate Margin of Safety: Some commenters
expressed the view that the current standard does not provide an
adequate margin of safety variously argue that the EPA is ignoring
precedent and CAA requirements for considering scientific uncertainty
in its judgments regarding an adequate margin of safety, and that
statements from the prior CASAC and new evidence suggests that the
current 70 ppb standard provides little margin of safety for protection
of sensitive subpopulations from harm. These commenters generally
advocate revision to a 60 ppb standard to address this concern. In
support of their views, some state that the EPA is ignoring findings of
a statistically significant lung function response to 6.6-hour exposure
to 60 ppb during quasi-continuous exercise while others cite the EPA
consideration of epidemiologic evidence, claiming that the EPA is
inappropriately using identified uncertainties as a basis for not
revising the standard. We disagree with these characterizations.
As an initial matter, we note that, contrary to the statements made
by these commenters, the Administrator, in reaching his proposed
decision, as in reaching his final decision, has considered legal
precedent and CAA requirements for a primary standard that protects
public health, including the health of sensitive groups, with an
adequate margin of safety. With regard to scientific uncertainty, as
summarized in section I.A above, the CAA requirement that primary
standards provide an adequate margin of safety was intended to address
uncertainties associated with inconclusive scientific and technical
information available at the time of standard setting. It was also
intended to provide a reasonable degree of protection against hazards
that research has not yet identified. See Lead Industries Ass'n v. EPA,
647 F.2d 1130, 1154 (D.C. Cir. 1980); American Petroleum Institute v.
Costle, 665 F.2d at 1186; Coalition of Battery Recyclers Ass'n v. EPA,
604 F.3d 613, 617-18 (D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d
1334, 1353 (D.C. Cir. 2013). Thus, in considering whether the primary
standard includes an adequate margin of safety, the Administrator is
seeking to ensure that the standard not only prevents pollution levels
that have been demonstrated to be harmful but also prevents lower
pollutant levels that may pose an unacceptable risk of harm, even if
the risk is not precisely identified as to nature or degree. In so
doing, however, the CAA does not require the Administrator to establish
a primary NAAQS at a zero-risk level or at background concentration
levels (see Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51,
Mississippi v. EPA, 744 F.3d at 1351), but rather at a level that
reduces risk sufficiently so as to protect public health with an
adequate margin of safety.
In the proposed decision, as in the decision described in section
II.B.3 below, the Administrator's consideration of the kind and degree
of uncertainties associated with the current information (as some of
the factors the EPA considers in addressing the requirement for an
adequate margin of safety) involved a number of judgments. With regard
to his consideration of the epidemiologic evidence, for example, the
Administrator recognizes that, as a whole, investigations of
associations between O3 and respiratory effects and health
outcomes (e.g., asthma-related hospital admission and emergency
department visits) provide strong support for the overarching
conclusion of that O3 causes respiratory effects, and its
risks to people with asthma. In his consideration of O3
exposures of concern, the Administrator, agrees with staff evaluations
in the PA, that such studies available in this review are less
informative to his judgments related to air quality conditions allowed
by the current standard (85 FR 49870, August 14, 2020). For example, as
summarized in section II.A.2.c above, none of the U.S. studies that
show associations between O3 and the clearly adverse health
outcomes of hospital admissions or emergency department visits for
respiratory causes were based in locations during time periods when the
current standard was always met (PA, section 3.3.3). While there were
two such studies based in single cities in Canada, as discussed above,
the interpretation of individual single-city results is complicated by
the presence of co-occurring pollutants or pollutant mixtures (PA,
section 3.3.3).\123\ Thus, as in reaching his decision in this review,
the Administrator has fully considered conclusions reached in the ISA
regarding the epidemiologic evidence and the policy evaluations in the
PA, and does not find the currently available epidemiologic studies to
provide insights regarding exposure concentrations associated with
health outcomes that might be expected under air quality conditions
that meet the current standard (85 FR 49870, August 14, 2020). Thus,
the EPA's decision on the standard in this review fully and
appropriately considers the full evidence base, including the
epidemiologic evidence, and associated uncertainties and limitations.
---------------------------------------------------------------------------
\123\ Accordingly, uncertainties remain with regard to the
independent role of O3 exposures in eliciting the
reported health outcomes analyzed, and in the absence of analyses
that might reduce such uncertainties (e.g., analyses of the presence
and effects of co-occurring pollutants).
---------------------------------------------------------------------------
With regard to the controlled human exposure studies, and the
nature and degree of effects that might be expected at exposures lower
than those studied or in unstudied population groups, the Administrator
has considered first what the evidence base indicates with regard to
the lowest exposures as well as differences and similarities between
the studied populations and the less well studied population groups
recognized to be at increased risk. In so doing, he considers the
findings of statistically significant respiratory responses in the
studies of 60 ppb exposures in largely healthy subjects, particularly
in his consideration of the exposure and risk estimates. For example,
in reaching his decision in section II.B.3 below, as for his proposed
decision, he finds it appropriate to consider the level of protection
provided by the current standard from single exposures, but to give
greater weight to multiple exposures, in judging adequacy of the margin
of safety provided by the current standard.\124\ Such considerations
[[Page 87296]]
contributed to the Administrator's proposed judgments with regard to
the requisite level of protection needed to protect at-risk populations
with an adequate margin of safety, as required by the Act and
consistent with the factors recognized in the relevant case law. Thus,
consistent with the CAA requirements and prior judicial decisions, the
Administrator based his proposed decision, and bases his final decision
(as summarized in II.B.4 below) on the scientific evidence, our current
understanding of it, and his judgments concerning associated
uncertainties, both those associated with inconclusive scientific and
technical information, and those associated with hazards that research
has not yet identified. These judgments, along with the factors
recognized above that the EPA generally considers in each NAAQS review,
contribute to his reasoned decision making in this review, as described
in section II.B.3 below.
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\124\ Contrary to implications of some commenters, this judgment
by the current Administrator is consistent with that made by the
prior Administrator in establishing the current standard, as seen
from the summary of the prior Administrator's judgment in that
regard that was summarized in the proposal and that these commenters
cite:
Further, while the Administrator recognized the effects
documented in the controlled human exposure studies for exposures to
60 ppb to be less severe than those associated with exposures to
higher O3 concentrations, she also recognized there to be
limitations and uncertainties in the evidence base with regard to
unstudied population groups. As a result, she judged it appropriate
for the standard, in providing an adequate margin of safety, to
provide some control of exposures at or above the 60 ppb benchmark
(80 FR 65345-65346, October 26, 2015). [85 FR 49841, August 14,
2020]
---------------------------------------------------------------------------
With regard to advice provide by CASAC in the last review as a
general matter, we disagree with the commenters' presumption that it is
necessary for EPA to address in this review each statement a prior
CASAC made in a prior review. The Clean Air Act does not impose such a
requirement. We further note that a prior CASAC's advice would be based
on review of the prior air quality criteria, exposure/risk analyses and
standard, as well as considerations pertinent in the prior review
(which may, depending on the issue, differ from the pertinent evidence,
information and considerations before a current CASAC). We note,
however, that this specific advice from the prior CASAC on the adequacy
of the margin of safety was cited by part of the CASAC in the current
review. As summarized in the proposal and in section II.B.1.b above,
while the prior CASAC advised that the size of the margin of safety
provided varied across different standard levels within the range from
70 to 60 ppb that the prior CASAC recommended, it found a level of 70
ppb could be supported by the scientific evidence. Further, the prior
CASAC recognized, as summarized in section II.B.1.b above, that with
regard to the ``size'' of the margin of safety, the selection of any
particular approach to providing an adequate margin of safety is a
policy choice left specifically to the Administrator's judgment (Lead
Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744
F.3d at 1353; section I.A above). Thus, in reaching his proposed
decision, the Administrator explicitly considered the advice provided
by the prior CASAC to the extent it was represented in advice from the
current CASAC as emphasized by part of the current CASAC (85 FR 49873,
August 14, 2020), and he has similarly again considered it reaching his
final decision, as described in section II.B.3 below, in light of
public comments raising it.
Some commenters also express the view that EPA is using limited
data in sensitive population groups as an excuse for not establishing a
level at which there is ``an absence of adverse effect'' in sensitive
groups. In support of their view, some commenters claim that the EPA
has not addressed a statement of the prior CASAC regarding the
potential for lung function decrements and respiratory symptoms to
occur in people with asthma with exposure to 70 ppb (while at elevated
exertion). Contrary to this claim,\125\ the EPA considered in the last
review the point made by the prior CASAC in the statement highlighted
by the commenters. The statement from the prior CASAC that the
commenters reference was provided in the CASAC review of a draft PA in
the last review, fully considered in completing the final 2014 PA, and,
along with the totality of the prior CASAC advice, taken into account
in establishing the current primary standard (80 FR 65292, October 26,
2020). It is not necessary for the EPA to address in this review each
statement a prior CASAC made in a prior review.
---------------------------------------------------------------------------
\125\ The context for this statement is in considering the
benchmark concentrations utilized in the exposure-to-benchmarks
analysis of the 2014 HREA and reflecting on responses reported in
controlled human exposure studies of healthy subjects exposed for
6.6 hours with quasi-continuous exercise. With regard to the
responses reported from exposure to 72 ppb, on average across the
exercise periods, the prior CASAC stated its view ``that these
effects almost certainly occur in some people, including asthmatics
and others with low lung function . . . at levels of 70 ppb and
below'' (Frey, 2014b, p. 6).
---------------------------------------------------------------------------
We agree with commenters who express the view that to protect
sensitive populations from effects reported in some largely healthy
subjects from the 6.6 hour exposure to 73 ppb (with quasi-continuous
exercise), the standard should provide protection against somewhat
lower exposures. As summarized in section II.B.3 below, this is an
objective the Administrator identifies for the standard and, based on
the exposure/risk estimates, he finds the standard to provide strong
protection from such exposures (and associated risk of such effects).
In addition, in highlighting this isolated statement from the last
review, the commenters fail to distinguish CASAC advice on the primary
standard from consideration of the exposure benchmark for comparison to
a multi-hour exposure while engaged in quasi-continuous exercise.
With regard to the prior CASAC's scientific and policy advice on
the primary standard,\126\ the prior CASAC concluded that the
scientific evidence supported a range of standard levels that included
70 ppb, and also recognized the choice of a level within that range to
be ``a policy judgment under the statutory mandate of the Clean Air
Act'' (85 FR 49873).\127\ We further note that the current CASAC
concludes in this review that newly available evidence relevant to
standard setting does not substantially differ from that available in
the last review (Cox, 2020a, Consensus Responses to Charge Questions p.
12; 85 FR 49873, August 14, 2020). As discussed further below, we note
that the CAA does not require the Administrator to establish a primary
NAAQS at a zero-risk level or at background concentration levels (see
Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v.
EPA, 744 F.3d at 1351), but rather at a level that reduces risk
sufficiently so as to protect public health with an adequate margin of
safety.
---------------------------------------------------------------------------
\126\ In their 2014 advice, the prior CASAC concluded by
explicitly stating ``our policy advice is to set the level of the
standard lower than 70 ppb within a range down to 60 ppb, taking
into account your judgment regarding the desired margin of safety to
protect public health.''
\127\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.''
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Some commenters also state that the primary NAAQS must be set at a
level at which there is an absence of adverse effects in sensitive
populations. While the EPA agrees that the NAAQS must be set to protect
sensitive populations with an adequate margin of safety, it is well
established that the NAAQS are not
[[Page 87297]]
meant to be zero risk standards. See Lead Industries v. EPA, 647 F.2d
at 1156 n.51; ATA III, 283 F. 3d at 360 (``[t]he lack of a threshold
concentration below which these pollutants are known to be harmless
makes the task of setting primary NAAQS difficult, as EPA must select
standard levels that reduce risks sufficiently to protect public health
even while recognizing that a zero-risk standard is not possible'');
Mississippi, 744 F. 3d at 1351 (same); see also id. at 1343
(``[d]etermining what is `requisite' to protect the `public health'
with an `adequate' margin of safety may indeed require a contextual
assessment of acceptable risk. See Whitman, 531 U.S. at 494-95 (Breyer
J. concurring)''). As the Court of Appeals for the D.C. Circuit said in
reviewing the 2015 O3 NAAQS, ``the primary standard for a
non-threshold pollutant like ozone is not required to produce zero
risk, and `[t]he task of determining what standard is `requisite' to
protect the qualitative value of public health or what margin of safety
is `adequate' to protect sensitive subpopulations necessarily requires
the exercise of policy judgment.' '' Murray Energy, 936 F.3d at 610
(quoting Mississippi v. EPA).\128\ The Administrator's judgments in
this review are rooted in his evaluation of the evidence, which
reflects the scientific uncertainty as to the O3
concentrations at which sensitive subpopulations would experience
adverse health effects, and his judgments weigh both the risks and the
uncertainties. This is a legitimate, and well recognized, exercise of
``reasoned decision-making.'' ATA III. 283 F. 3d at 370; see also id.
at 370 (``EPA's inability to guarantee the accuracy or increase the
precision of the . . . NAAQS in no way undermines the standards'
validity. Rather, these limitations indicate only that significant
scientific uncertainty remains about the health effects of fine
particulate matter at low atmospheric concentration. . . .'');
Mississippi, 744 F. 3d at 1352-53 (appropriate for EPA to balance
scientific uncertainties in determining level of revised O3
NAAQS).
---------------------------------------------------------------------------
\128\ The legislative history of the Clean Air Act provides
further support for these holdings, as do the statutory deadlines
for attainment. See H. Rep. 95-294, 95th Cong. 1st sess. 127, 123
Cong. Rec. S9423 (daily ed. June 10, 1977) (statement of Senator
Muskie during the floor debates on the 1977 Amendments that ``there
is no such thing as a threshold for health effects. Even at the
national primary standard level, which is the health standard, there
are health effects that are not protected against.''
---------------------------------------------------------------------------
Exposure/risk Analyses: In expressing the view that the standard
should be made more stringent, some commenters disagree with EPA
conclusions based on the exposure/risk analyses, and point to other
analyses that they state show that a lower standard level (e.g. 65 ppb
or lower) would avoid important health effects. These commenters'
claims of deficiencies with the exposure/risk analyses include claims
that the study area selection is not explained, that population size of
the study areas analyzed is too small to support conclusions and does
not include particular areas; that the analysis does not include
adults, and other groups of interest, and that selection of study areas
with air quality close to the current standard contributed to
underestimates of population exposures. We disagree with these
commenters' claims.
With regard to study area selection and population size for the
analysis, we note that an exposure and risk analysis based on eight
study areas, all of which are major metropolitan areas provides a
robust foundation for population exposure estimates. The eight study
areas included reflect the full range of air quality and exposure
variation expected across major urban areas in the U.S and seven
different NOAA climate regions (PA, section 3.4.1).\129\ This number of
areas (8) and combined population size (more than 45 million in the
combined metropolitan areas [PA, Appendix 3D, Table 3D-1]) are much
larger than similar analyses in recent NAAQS reviews for other
pollutants (e.g., sulfur dioxide [U.S. 2018]), and not that dissimilar
to similar analyses in past O3 NAAQS reviews.\130\ Some
commenters claim that the exclusion of specific urban areas in which
O3 concentrations are much higher than those analyzed
resulted in underestimates of exposure. We disagree with this claim as
the air quality analyzed across all study areas was adjusted to just
meet the current standard (or alternative scenarios). Thus, an urban
area that currently has O3 concentrations well in exceedance
of the current standard would not necessarily have been found to have
higher exposure estimates if it were simulated to have air quality just
meeting the current standard. Such estimates would, however, have
greater uncertainty, which is the reason such study areas as those
identified by commenters (e.g., Los Angeles) were excluded. Areas
included were those for which only small adjustments were required for
the air quality to just meet the current standard (and alternative
scenarios), yielding reduced uncertainty (e.g., given the need for
larger air quality adjustments to achieve conditions that just meet the
current standard) in these estimates compared to those from the 2014
HREA (PA, sections 3.4.1 and 3.4.4, and Appendix 3D). In selecting such
areas, however, we considered a number of other characteristics in
order to achieve a varied set of study areas, including with regard to
air quality patterns.\131\ This variation contributed to variation in
exposure estimates, even for the same air quality scenario (PA,
Appendix 3D, Tables 3D-26, 3D-28 and 3D-30). Thus, in addition to
focusing on study areas with ambient air concentrations close to
conditions that reflect air quality that just meets the current
standard that would be more informative to evaluating the health
protection provided by the current standard than areas with much higher
(or much lower) concentrations, the approach employed recognizes that
capturing an appropriate diversity in study areas and air quality
conditions (that reflect the current standard scenario) is an important
aspect of the role of the exposure and risk analyses in informing the
Administrator's conclusions on the public health protection afforded by
the current standard.
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\129\ Contrary to the commenters' assertion of a lack of
explanation for the study areas included in the analyses, the PA
describes the study area selection criteria and process, including
steps taken to include adequate representation of diverse
conditions. As observed in the PA, seven of the eight study areas
were also included in the 2014 HREA, and the eighth study area
(Sacramento) was newly added in the current review to insure
representation of a large city in the southwest (PA, section 3.4.1
and Appendix 3D, section 3D.2.1). Clarification on this point in the
final PA was responsive to the only CASAC comment on completeness of
the description of study area selection (Cox, 2020a). We disagree
with the implication by some commenters that each review's analyses
must focus on the same areas. There is no such requirement under the
Act, and such a view ignores the need to consider the current
information in each review in planning appropriate analyses.
\130\ For example, the exposure assessment for the 1997
O3 NAAQS review included nine urban study areas, for
which the combined population simulated was 41.7 million. The
exposure assessment for the current review included eight urban
study areas with a combined simulated population size of
approximately 39 million (PA, p. 3D-96; U.S. EPA, 1996b, p. 76). We
additionally note the focus on analysis results in terms of
population percentages rather than population counts.
\131\ A broad variety of spatial and temporal patterns of O3
concentrations can exist when ambient air concentrations just meet
the current standard. These patterns will vary due to many factors
including the types, magnitude, and timing of emissions in a study
area, as well as local factors, such as meteorology and topography.
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Contrary to one commenter's assertion that adults were not included
in the exposure assessment, the populations assessed included two adult
populations groups: All adults and
[[Page 87298]]
adults with asthma.\132\ The results for these groups and all of the
populations assessed are presented in detail in the PA (PA, Appendix
3D). As described in the PA and proposal, the estimates for adults as a
percentage of the study populations were generally lower than those for
children. Thus, we focused discussion on the estimates for children,
including particularly children with asthma. As recognized by the
Administrator in section II.B.3 below, his judgments on the adequacy of
protection provided by the current standard takes into account
protection provided to the U.S. population, including those population
groups at increased risk, which includes children and people of all
ages with asthma, among other groups.
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\132\ Further, contrary to the implication of one comment, the
exposure/risk analyses did not exclude athletes, hikers and others
who exercise outdoors, using their full lung capacity, a group the
commenter characterizes as at increased risk. In fact, it is just
such individuals who are most likely, depending on their locations,
to experience exposures of concern due to their high exertion
levels. As described in the PA, the comparison to benchmarks
analysis identifies the portion of the exposed population whose 7-
hour average concentration, while at moderate or greater exertion,
is at or above the benchmarks (PA, section 3.4 and Appendix 3D).
---------------------------------------------------------------------------
Some commenters claimed that the EPA should have separately
assessed exposure for certain additional population subgroups, such as
children at summer camps, adults with lung impairments other than
asthma or outdoor workers. As an initial matter, we recognize
appreciably increased uncertainty regarding key aspects of the
information necessary for such simulations for all of these groups. Of
the three groups, only outdoor workers are identified as an at-risk
population in this review (ISA, Table IS-10), and accordingly this
group was explicitly considered in designing the exposure
analyses.\133\ The information available, however, was considered to be
too uncertain to produce estimates for this population, as a separate
group, with confidence. As described in the PA, important uncertainties
exist in generating the simulated activity patterns for this group,
including the limited number of CHAD diary days available for outdoor
workers, assignment of diaries to proper occupation categories, and in
approximating number of days/week and hours/day outdoors, among other
pertinent aspects (PA, Appendix 3D, Table 3D-64). We note that these
appreciable data limitations and associated uncertainties were also
recognized in the 2014 HREA in which a limited sensitivity analyses was
conducted for this subgroup. Those limited analyses, conducted for a
single area with air quality just meeting the prior 75 ppb standard,
indicated that when diaries were selected to mimic exposures that could
be experienced by outdoor workers, the percentages of modeled
individuals estimated to experience exposures of concern were generally
similar to the percentages estimated for children (i.e., using the full
database of diary profiles) in the urban study areas and years with the
largest exposure estimates (2014 HREA, section 5.4.3.2, Figure 5-
14).\134\ Accordingly, in this review, in recognition of the data
limitations that remain in the current review,\135\ outdoor workers
were not assessed as a separate population group, and in light of our
consideration of conclusions from the sensitivity analyses in the last
review, we have generally given primary focus to the estimates for the
populations of children.
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\133\ With regard to the other two groups, we note the ISA
explicitly evaluated evidence for people with the lung disease,
COPD, and concluded the evidence was inadequate to determine whether
this lung impairment confers increased risk of O3 related
effects (ISA, Table IS-10). With regard to children at summer camp,
we note that to the extent that the behaviors of such children
(e.g., exercising outdoors) are represented in the CHAD, they are
represented among the at-risk populations of children and children
with asthma that were simulated in the exposure/risk analyses.
\134\ Similarly, the EPA also did not conduct an exposure
analysis for outdoor workers in the 2008 review and instead focused
on children since it was judged that school aged children presented
the greatest likelihood of being outdoors and exposed under moderate
exertion averaged over the critical time period based on prior
analysis findings. Thus, while as recognized in multiple reviews,
outdoor workers are also at risk, the EPA has focused, in past
reviews as in the current one, on children, the population group for
which the analysis estimates in terms of percentage of population
are greatest (PA, section 3.4.2). Accordingly, providing protection
for this population group will provide protection for other at-risk
populations as well.
\135\ In support of their view that estimates should have been
derived for outdoor workers, one group of commenters cites a study
on research priorities for assessing climate change impacts on
outdoor workers (Moda et al., 2019). We note, that other than being
focused on outdoor workers and recognizing there to be significant
research needed for impacts assessment, this paper has little
relevance in this review. The paper is focused on climate change
impacts in tropical developing countries with a focus on sub-Saharan
Africa and does not discuss exposure modeling of outdoor workers or
O3.
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In summary, we disagree with comments stating that the exposure/
risk analyses were deficient and do not provide support for their
conclusions. As summarized above, in planning and conducting the
exposure/risk analyses, we have appropriately considered issues raised
by the commenters, such that the analyses reasonably reflect current
understanding, information, tools and methodologies. Further, in
presenting the analyses in the PA, we have recognized any associated
limitations and uncertainties in an uncertainty characterization that
utilized a largely qualitative approach adapted from the World Health
Organization approach (and commonly utilized in NAAQS exposure/risk
assessments, as discussed in the PA and proposal [85 FR 49857, August
14, 2020]), accompanied by a number of quantitative sensitivity
analyses. This characterization and accompanying analyses build on
previously conducted work in the 2014 HREA and provide a transparent
and explicit recognition of strengths, limitations and uncertainties of
the current exposure/risk analysis that were described the PA,
considered in the proposal and also in the Administrator's decision
described in section II.B.3 below. Thus, the exposure/risk analyses
conducted for this review appropriately and soundly reflect current
information and methodologies; and we have interpreted their results
appropriately in light of any associated limitations and uncertainties.
With regard to other quantitative analyses identified by some
commenters and described as showing health impacts that would be
avoided by a more stringent standard (e.g. with a level of 65 ppb or
lower), we note that these analyses utilize epidemiologic study effect
estimates as concentration-response functions to generate predictions
of the occurrence of health outcomes, primarily mortality, under
different air quality conditions (characterized by the metric used in
the epidemiologic study).\136\ As an initial matter, we note that our
understanding of the relationship between O3 exposures and
total mortality is different in this review than it was in the last
review, based on the more extensive evidence base now available. As
summarized in section II.A.2.a above, and noted earlier in this
section, while our conclusion in the last review was that the
relationship of O3 exposure with mortality was likely to be
causal, the current evidence base does not support that conclusion
because of limited evidence for cardiovascular mortality, which is by
far the largest contributor to total mortality. Rather, the EPA has
concluded the evidence in
[[Page 87299]]
this review to be suggestive of, but not sufficient to infer, causal
relationships of total mortality with short- or long-term O3
exposure, as summarized in section II.A.2.a above (ISA, Appendix 6).
Thus, we do not find the weight the commenter is suggesting we place on
predictions of total mortality from the epidemiologic study based risk
analyses cited by commenters to appropriately reflect the current
evidence base for O3 and mortality, or the evidence base for
O3 and cardiovascular effects (the primary contributor to
mortality in the U.S.).
---------------------------------------------------------------------------
\136\ The analyses cited by these commenters include Cromar et
al (2019) and OTC (2020). To address these comments, we have
provisionally considered the documents, as discussed in I.D above,
and found they do not materially change the broad scientific
conclusions of the ISA with regard to respiratory effects, or
warrant re-opening the air quality criteria for further review
(Luben et al., 2020). Further, some of these commenters reference
epidemiologic study based risk, analyses in the 2014 HREA.
---------------------------------------------------------------------------
With regard to estimates of avoided respiratory mortality from the
analyses cited by these commenters, we note that, while the
epidemiologic studies that are inputs to the quantitative analyses
cited by the commenters are part of the evidence base that supports our
conclusion of a causal relationship between short-term O3
exposure and respiratory effects, there are uncertainties inherent in
the derivation of estimates of mortality ascribed to O3
exposures using effect estimates from these studies. For example, in
planning for analyses in this review, the IRP recognized several
important uncertainties associated with aspects of the O3
epidemiologic study-based approach used in the 2014 HREA (one of the
analyses cited by commenters), and similar to the approach used in
other analyses cited by commenters, that the EPA concluded to have a
moderate or greater impact on risk estimates (IRP, Appendix 5A). Such
uncertainties include complications posed by the presence of co-
occurring pollutants or pollutant mixtures, as well as those involving
the correlation of population O3 exposures and ambient air
monitor concentrations (including the use of area wide average
O3 concentrations) and uncertainties in the derived
concentration-response functions (IRP, Appendix 5A; PA, Appendix 34D,
section 3D.1.4). Specifically with regard to the 2014 HREA estimates of
respiratory mortality, the EPA has recognized uncertainty about the
extent to which mortality estimates based on the long-term metric in
Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily maximum
concentrations) reflect associations with long-term average
O3 versus repeated occurrences of elevated short-term
concentrations; and given potential nonlinearity of the C-R function to
reflect a threshold of the mortality response, these estimates should
be viewed with caution (IRP, Appendix 5A). Accordingly, the 2014 HREA
concluded that lower confidence should be placed in the results of the
assessment of respiratory mortality risks associated with long-term
O3, primarily because that analysis relies on just one study
(Jerrett et al., 2009), and because of the uncertainty in that study
about the existence and identification of a potential threshold in the
concentration-response function (2014 HREA, section 9.6; 80 FR 65316,
October 26, 2015). The other analysis cited by the commenters for
predictions of respiratory mortality is also based its estimates on
Jerrett et al. (2009). Thus, we find the conclusions regarding
uncertainty and low confidence recognized for the 2014 HREA estimates
to also apply to the other analysis by commenters and disagree with the
conclusions reached by these commenters on this analysis. Further, we
do not find that the 2014 HREA or other analyses cited by the
commenters, in combination with the full body of evidence currently
available, support a conclusion of significant health outcomes for
O3 air quality that meets the current standard.
Long-term Standard Consideration: In support of their view that EPA
should establish an additional primary standard that targets long-term
exposure, some commenters stated that recent epidemiologic studies
indicate causal linkages between long-term exposures and adverse health
outcomes, while also suggesting there was support for such a standard
in a statement made by the CASAC in reviewing the draft PA. With regard
to the epidemiologic studies, these commenters cite several studies
published after the literature cut-off date for the ISA \137\ that they
describe as showing linkages of long-term O3 exposure to a
number of outcomes, including mortality, smokers progression to COPD,
hospital admissions for acute respiratory disease syndrome and
emergency department visits (Dominici et al., 2019; Seltzer et al.,
2020; Limaye and Knowlton, 2020; Dedoussi et al., 2020; Lim et al.,
2019; Paulin et al,. 2020; \138\ Rhee et al., 2019; Strosnider et al.,
2019).\139\ We have provisionally considered these ``new'' studies in
addressing these comments consistent with section I.D above (Luben et
al., 2020). Of the studies focused on mortality that these commenters
cite as providing new information in support of a long-term standard,
just three represent new evidence related to investigation of
associations of long-term O3 exposure with mortality (Lim et
al., 2019) or respiratory morbidity (Paulin et al., 2020 and Rhee et
al., 2019).\140\ The study by Lim et al. (2019) analyzes associations
between long-term O3 exposure and respiratory mortality in a
U.S. population of older adults in the U.S., reporting a positive
association with an effect estimate lower than a previously published
study included in the ISA. These results contribute to the evidence
base for respiratory effects, e.g., with an additional high-quality
study of a previously studied population group (Lim et al., 2019) or
with studies investigating additional populations and respiratory
outcomes (Paulin et al., 2020; Rhee et al., 2019), albeit with
limitations,\141\ without reducing uncertainties in the evidence base
as a whole. These studies are
[[Page 87300]]
generally consistent with the evidence assessed in the ISA, and they do
not materially change the broad conclusions in the ISA regarding the
scientific evidence.
---------------------------------------------------------------------------
\137\ These commenters also assert that some other studies
published after the ISA cut-off date were arbitrarily included in
the ISA, citing just a single study (Garcia et al., 2019). Contrary
to implication by the commenters, such an occurrence is clearly
described in the ISA, which states ``[s]tudies published after the
literature cutoff date for this review were also considered if they
were submitted in response to the Call for Information or identified
in subsequent phases of ISA development . . ., particularly to the
extent that they provide new information that affects key scientific
conclusions'' (ISA, Appendix 10, p. 10-1).
\138\ Although the commenters submitted a document that appears
to be an unpublished draft of an earlier manuscript of this paper,
to which they assigned a 2019 publication date and a very slightly
different title (rather than the published paper, it is the
published study, Paulin et al., (2020) that we have provisionally
considered (Luben et al., 2020).
\139\ Some commenters imply that projections of increasing
O3 concentrations in response to climate change in the
future will ``heighten'' long-term O3 concentrations and
chronic exposures and indicate a need for a long-term standard. In
making this claim, they cite an analysis of air quality projected in
2045 through 2055 (Nassikas et al., 2020) and an evaluation of the
effects of climate change on air quality including O3
concentrations. (Archer et al., 2019). The former ``new'' study has
been provisionally considered and found not to materially affect the
broad scientific conclusions regarding the air quality criteria
documented in the ISA or to warrant reopening the air quality
criteria (Luben et al., 2020) As neither is evaluating health
effects associated with air quality under the current standard, we
do not find these studies informative to consideration of a need for
a long-term standard to protect public health.
\140\ Two others (Dedoussi et al 2020; Seltzer et al, 2020) are
quantitative assessments that estimate O3 impacts based
on use of effect estimates from previously published studies that
are included in the ISA, another (Dominici et al., 2019) is the full
technical report from the Health Effects Institute, the main results
of which were previously published in studies that are included in
the ISA, and a fourth (Limaye and Knowlton., 2020) is commentary on
a previously published study that is included in the ISA. One other
study cited by the commenters is focused on short-term O3
exposures, not long-term O3 exposure as indicated by the
commenters (Strosnider et al., 2019)
\141\ While studies by Paulin et al. (2020) and Rhee et al.
(2019) provide evidence for a novel population sub-group (smokers)
or endpoint (e.g., acute respiratory distress syndrome, ARDS), each
study has limitations. For example, the cross-sectional design of
Paulin et al. (2020) is a major limitation, while limitations
associated with Rhee et al. (2019) relate to linking long-term
exposure with hospital admissions for ARDS based on exposure timing
and the mechanism for acute vs. chronic development of disease, and
to power in the study (e.g., very low hospital admission counts per
year per ZIP code [Rhee et al., 2019, Table 2]).
---------------------------------------------------------------------------
We additionally note that the O3 concentrations did not
meet the current standard in all locations and time periods analyzed in
these three multicity studies. Although two of these studies include
some locations across the U.S. in which the current standard was likely
met during some portions of the study period, air quality during other
time periods of locations in the dataset did not meet the current
standard.\142\ Further, the multicity effect estimates in these studies
do not provide a basis for considering the extent to which
O3 health effect associations are influenced by individual
locations with ambient O3 concentrations low enough to meet
the current O3 standards versus locations with O3
concentrations that violate this standard. Thus, while these more
recent studies may be consistent with the existing evidence base
evaluated in the ISA, they do not provide a basis for conclusions
regarding whether the O3 exposures occurring under air
quality conditions allowed by the current standard may be eliciting the
effects analyzed.
---------------------------------------------------------------------------
\142\ The studies by Lim et al (2019) and Rhee et al (2019)
include zip codes across the entire U.S., while Paulin et al (2020)
includes the cities of Baltimore, Maryland, New York City, New York,
Los Angeles and San Francisco, California, Ann Arbor, Michigan, Salt
Lake City, Utah and Winston-Salem, North Carolina. The study time
periods include ten or more years extending from the early 2000s to
the late 2010s; a period within which the design values for most of
those identified cities and many other U.S. metropolitan areas
exceeded the level of the current standard (as seen by the design
values presented for those areas during those time periods at
https://www.epa.gov/air-trends/air-quality-design-values).
---------------------------------------------------------------------------
We additionally note that while epidemiologic studies evaluate the
relationship between health effects and specific O3
concentrations during a defined study period and the generally
consistent and coherent associations observed in these epidemiologic
studies contribute to the causality determinations and conclusions
regarding the causal nature of the effect of O3 exposure on
health effects, ``they do not provide information about which averaging
times or exposure metrics may be eliciting the health effects under
study'' (ISA, section IS.6.1). As noted in the ISA, ``disentangling the
effects of short-term ozone exposure from those of long-term ozone
exposure (and vice-versa) is an inherent uncertainty in the evidence
base,'' as ``the populations included in epidemiologic studies have
long-term, variable, and uncharacterized exposures to ozone and other
ambient pollutants'' (ISA, section IS.6.1). As summarized in the
proposal, however, we have also considered the toxicological studies of
effects associated with long-term exposures and note that they involve
much higher exposures than those occurring at the current standard (85
FR, 49853, August 14, 2020).
Lastly, we disagree with the commenters' implication that the EPA
has not addressed a CASAC recommendation. The commenters appear to be
asserting that the CASAC recommended that EPA consider a long-term
standard. However, the CASAC did not make such a recommendation (Cox,
2020a). In making this assertion, the commenter cites a comment the
CASAC makes with regard to a sentence in the draft PA that is drawn
from the Administrator's conclusions section of the 2015 decision.
Rather than ignoring this CASAC comment, as asserted by the commenters,
we made a revision to that section of the PA (moving the statement from
the draft PA to a footnote in the final PA with the objective of
retaining an accurate description of a consideration related to that
2015 decision, while lessening the potential for confusion of a 2015
consideration with considerations in the current review).\143\
Notwithstanding sentences pertaining to the last review, we note the PA
evaluates the information in the current review with regard to the
protection offered by the current standard (and that the Administrator
considered the PA evaluation, as well as the CASAC advice in his
proposed decision [summarized in section II.B.1.c above] as in his
final decision below). We further note that the description of the
Administrator's conclusion in the last review, which is also summarized
in the proposal (and in section II.A.1 above), does not describe health
effects associated with long-term average concentrations likely under
the current standard.
---------------------------------------------------------------------------
\143\ In its comments regarding the 2015 statement, the CASAC
and its consultants stated that controls that reduce peak
O3 concentrations will not consistently reduce mean
O3 concentrations. We don't disagree with this statement,
and we note that we did not make a statement to the contrary in
either the proposal or this final decision document.
---------------------------------------------------------------------------
Further, in considering an implication of the commenters' claim
that a ``long-term standard'' is needed in order to provide protection
from health effects that may be elicited by long-term exposures to
O3, we note that the impact of standards with short
averaging times, such as eight hours, is not limited to reducing short-
term exposures. This is because a reduction in magnitude of short-term
exposure concentrations (e.g., daily maximum 8-hour concentrations) is
also a reduction in exposure to such concentrations over the longer
term. For example, a standard, such as the current one, that limits
daily maximum 8-hour concentrations to not exceed 70 ppb as a 3-year
average of the annual fourth highest value, in addition to limiting the
magnitude of concentrations to which a population is exposed in eight
hour periods, also limits the frequency to which the population is
exposed to such concentrations over the long term. That is, the
reduction in frequency of the higher concentrations reduces exposures
to such concentrations over the short and long term. Thus, given that,
as indicated by the current and established evidence, the O3
concentrations most likely to contribute to health effects are the
higher concentrations, the current standard provides control of
exposures to such concentrations over both the short and long term. In
light of all of the considerations raised here, we disagree with the
commenters assertion of the need for a long-term O3
standard.
4. Administrator's Conclusions
Having carefully considered the public comments, as discussed
above, the Administrator believes that the fundamental scientific
conclusions on effects of photochemical oxidants, including
O3, in ambient air that were reached in the ISA and
summarized in the PA, and estimates of potential O3
exposures and risks described in the PA, and summarized above and in
sections II.B and II.C of the proposal, remain valid. Additionally, the
Administrator believes the judgments he proposed to reach in the
proposal (section II.D.3) with regard to the evidence and the
quantitative exposure/risk information remain appropriate. Thus, as
described below, the Administrator concludes that the current primary
O3 standard provides the requisite protection of the public
health with an adequate margin of safety, including for at-risk
populations, and should be retained. In considering the adequacy of the
current primary O3 standard, the Administrator has carefully
considered the assessment of the available health effects evidence and
conclusions contained in the ISA; the evaluation of policy-relevant
aspects of the evidence and quantitative analyses in the PA (summarized
in sections II.A.2 and II.A.3 above); the advice and recommendations
from the CASAC (summarized in section II.B.1.b above); and public
comments (discussed in section II.B.2 above and in the separate RTC
document).
[[Page 87301]]
In the discussion below, the Administrator considers the key
aspects of the evidence and exposure/risk estimates important to his
judgment regarding the adequacy of protection provided by the current
standard. First, the Administrator considers the evidence base on
health effects associated with exposure to photochemical oxidants,
including O3, in ambient air. He additionally considers the
quantitative exposure and risk estimates developed in this review,
including associated limitations and uncertainties, and what they
indicate regarding the magnitude of risk, as well as degree of
protection from adverse health effects, associated with the current
standard. He additionally considers uncertainties in the evidence and
the exposure/risk information, as a part of public health judgments
that are essential and integral to his decision on the adequacy of
protection provided by the standard. Such judgments include public
health policy judgments and judgments about the implications of the
uncertainties inherent in the scientific evidence and quantitative
analyses. The Administrator draws on the PA considerations, and PA
conclusions in the current review, taking note of key aspects of the
rationale presented for those conclusions. Further, the Administrator
considers the advice and conclusions of the CASAC, including
particularly its overall agreement that the currently available
evidence does not substantially differ from that which was available in
the 2015 review when the current standard was established.
As an initial matter, the Administrator recognizes the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants (as recognized in section II.D.1 of the
proposal). As recognized in the proposal, no newly available evidence
has been identified in this review regarding the importance of
photochemical oxidants other than O3 with regard to
abundance in ambient air, and potential for health effects, and the
``the primary literature evaluating the health and ecological effects
of photochemical oxidants includes ozone almost exclusively as an
indicator of photochemical oxidants'' (ISA, p. IS-3). Accordingly, the
information relating health effects to photochemical oxidants in
ambient air is also focused on O3. Thus, the Administrator
concludes it is appropriate for O3 to continue to be the
indicator for the primary standard for photochemical oxidants.
With regard to the extensive evidence base for health effects of
O3, the Administrator gives particular attention to the
longstanding evidence of respiratory effects causally related to short-
term O3 exposures (summarized in section II.A.2.a above). He
recognizes that the strongest and most certain evidence for this
conclusion, as in the last review, is that from controlled human
exposure studies that report an array of respiratory effects in study
subjects (largely generally healthy adults) engaged in quasi-continuous
or intermittent exercise. He additionally recognizes the supporting
experimental animal and epidemiologic evidence. In so doing, he takes
note of the epidemiologic evidence of positive associations for
increased incidence of hospital admissions and emergency department
visits for an array of respiratory outcomes, with the strongest such
evidence being for asthma-related outcomes and specifically asthma-
related outcomes for children, with short-term O3 exposures.
As a whole, this strong evidence base continues to demonstrate a causal
relationship between short-term O3 exposures and respiratory
effects, including in people with asthma. The Administrator also notes
the ISA conclusion that the relationship between long-term exposures
and respiratory effects is likely to be causal. These conclusions are
also consistent with the conclusions in the last review and reflect a
general similarity in the underlying evidence base for such effects.
With regard to conclusions regarding the health effects evidence
that differ from those in the last review, the Administrator recognizes
the new conclusions regarding metabolic effects, cardiovascular effects
and mortality (as summarized in section II.A.2.a above; ISA, Table ES-
1). As an initial matter, he takes note of the fact that while the 2013
ISA considered the evidence available in the last review sufficient to
conclude that the relationships for short-term O3 exposure
with cardiovascular health effects and mortality were likely to be
causal, that conclusion is no longer supported by the now more
expansive evidence base which the current ISA determines to be
suggestive of, but not sufficient to infer, a causal relationship for
these health effect categories (ISA, Appendix 4, section 4.1.17;
Appendix 6, section 6.1.8). Further, the Administrator recognizes the
new ISA determination that the relationship between short-term
O3 exposure and metabolic effects is likely to be causal
(ISA, section IS.4.3.3). In so doing, he takes note that the basis for
this conclusion is largely experimental animal studies in which the
exposure concentrations were well above those in the controlled human
exposure studies for respiratory effects as well as above those likely
to occur in areas of the U.S. that meet the current standard (as
summarized in section II.A.2.c above). Thus, while recognizing the
ISA's conclusion regarding this potential hazard of O3, he
also recognizes that the evidence base is largely focused on
circumstances of elevated concentrations above those occurring in areas
that meet the current standard. In light of these considerations, he
judges the current standard to be protective of such circumstances
leading him to continue to focus on respiratory effects in his
evaluation of whether the current standard provides requisite
protection.
With regard to populations at increased risk of O3-
related health effects, the Administrator notes the populations and
lifestages identified in the ISA and summarized in section II.A.2.b
above. In so doing, he takes note of the longstanding and robust
evidence that supports identification of people with asthma as being at
increased risk of O3-related respiratory effects, including
specifically asthma exacerbation and associated health outcomes, and
also children, particularly due to their generally greater time
outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections
IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). This
tendency of children to spend more time outdoors while at elevated
exertion than other age groups, including in the summer when
O3 levels may be higher, makes them more likely to be
exposed to O3 in ambient air under conditions contributing
to increased dose due to greater air volumes taken into the lungs.
Based on these considerations, the Administrator concludes it is
appropriate to give particular focus to people with asthma and
children, population groups for which the evidence of increased risk is
strongest, in evaluating whether the current standard provides
requisite protection. He judges that such a focus will also provide
protection of other potentially at-risk population groups, identified
in the ISA, for which the current evidence is less robust and clear as
to the extent and type of any increased risk, and the exposure
circumstances that may contribute to it.
With regard to exposures of interest for respiratory effects, the
Administrator refers to the controlled human exposure studies of 6.6-
hour exposures, with
[[Page 87302]]
quasi-continuous exercise,\144\ to concentrations ranging from as low
as approximately 40 ppb to 120 ppb (as considered in the PA, and
summarized in sections II.A.2.c above). He also notes that, as in the
last review, these studies, and particularly those that examine
exposures from 60 to 80 ppb, are the primary focus of the PA
consideration of exposure circumstances associated with O3
health effects important to the Administrator's judgments regarding the
adequacy of the current standard. The Administrator further recognizes
that this information on exposure concentrations that have been found
to elicit effects in exercising study subjects is unchanged from what
was available in the last review.
---------------------------------------------------------------------------
\144\ These studies employ a 6.6-hour protocol that includes six
50-minute periods of exercise at moderate or greater exertion.
---------------------------------------------------------------------------
With regard to the epidemiologic studies of respiratory effects,
the Administrator recognizes that, as a whole, these investigations of
associations between O3 and respiratory effects and health
outcomes (e.g., asthma-related hospital admission and emergency
department visits) provide strong support for the conclusions of
causality (as summarized in section II.A.2.a above). He additionally
takes note of the PA observation that these studies are generally
focused on investigating the existence of relationships between
O3 in ambient air and specific health outcomes and not on
detailing the specific exposure circumstances eliciting such effects
(PA, section 3.3.3). In so doing, he takes note of the PA conclusions
in this regard, including the scarcity of U.S. studies \145\ conducted
in locations in which and during time periods when the current standard
would have been met (as summarized in sections II.A.2.c above).\146\ He
also recognizes the additional considerations raised in the PA and
summarized in section II.A.2.c above regarding information on exposure
concentrations in these studies during times and locations that would
not have met the current standard, including considerations such as
complications in disentangling specific O3 exposures that
may be eliciting effects (PA, section 3.3.3; ISA, p. IS-86 to IS-88).
He takes note that such considerations do not lessen the importance of
these studies in the evidence base documenting the causal relationship
between O3 and respiratory effects. With regard to his
consideration of exposure concentrations associated with O3
air quality conditions that meet the current standard, based on
information cited here and discussed in the PA and section II.B.2.b(ii)
above, he judges these studies that are available in the current review
to be less informative. Thus, the Administrator agrees with the PA
conclusions in consideration of this evidence from controlled human
exposure and epidemiologic studies (as assessed in the ISA and
summarized in the PA), and in consideration of public comments in
section II.B.2.b(ii) above, that the evidence base in this review does
not include new evidence of respiratory effects associated with
appreciably different exposure circumstances than the evidence
available in the last review, including particularly any circumstances
that would also be expected to be associated with air quality
conditions likely to occur under the current standard. In light of
these considerations, he finds it appropriate to focus on the studies
of 6.6-hour exposures with quasi-continuous exercise, and particularly
on study results for concentrations ranging from 60 to 80 ppb.
---------------------------------------------------------------------------
\145\ Consistent with the evaluation of the epidemiologic
evidence of associations between short-term O3 exposure
and respiratory health effects in the ISA, we focus on those studies
conducted in the U.S. and Canada, and most particularly in the U.S.,
to provide a focus on study populations and air quality
characteristics that are most relevant to circumstances in the U.S.
(PA, p. 3-45).
\146\ Among the epidemiologic studies finding a statistically
significant positive relationship of short- or long-term
O3 concentrations with respiratory effects, there are no
single-city studies conducted in the U.S. in locations with ambient
air O3 concentrations that would have met the current
standard for the entire duration of the study. Nor is there a U.S.
multicity study for which all cities met the standard for the entire
study period. The extent to which reported associations with health
outcomes in the resident populations in these studies are influenced
by the periods of higher concentrations during times that did not
meet the current standard is unknown. These and additional
considerations are summarized in section II.A.2.c above and in the
PA.
---------------------------------------------------------------------------
In considering the significance of responses documented in these
studies and the full evidence base for his purposes in judging
implications of the current information on public health protection
provided by the current standard, notes that the responses reported
from exposures ranging from 60 to 80 ppb are transient and reversible
in the study subjects. In so doing, he also notes that these studies
are in largely healthy adult subjects, that such data are lacking at
these exposure levels for children and people with asthma, and that the
evidence indicates that such responses, if repeated or sustained,
particularly in people with asthma, pose risks of effects of greater
concern, including asthma exacerbation, as cautioned by the CASAC.\147\
The Administrator also takes note of statements from the ATS
(summarized in section II.A.2.b above), as well as judgments made by
the EPA in considering similar effects (and ATS statements) in previous
NAAQS reviews (80 FR 65343, October 26, 2015). With regard to the ATS
statements, including the one newly available in this review (Thurston
et al., 2017), the Administrator recognizes the role of such
statements, as described by the ATS, as proposing principles or
considerations for weighing the evidence rather than offering ``strict
rules or numerical criteria'' (ATS, 2000, Thurston et al., 2017).
---------------------------------------------------------------------------
\147\ The CASAC noted that ``[a]rguably the most important
potential adverse effect of acute ozone exposure in a child with
asthma is not whether it causes a transient decrement in lung
function, but whether it causes an asthma exacerbation'' and that
increases in airway inflammation also have the potential to increase
the risk for an asthma exacerbation. The CASAC further cautioned
with regard to repeated episodes of such responses, e.g., airway
inflammation, indicating that they have the potential to contribute
to irreversible reductions in lung function (Cox, 2020a, Consensus
Responses to Charge Questions pp. 7-8).
---------------------------------------------------------------------------
The more recent ATS statement is generally consistent with the
prior statement (that was considered in the last O3 NAAQS
review) and the attention that statement gives to at-risk or vulnerable
population groups, while also broadening the discussion of effects,
responses and biomarkers to reflect the expansion of scientific
research in these areas. In this way, the most recent statement updates
the prior statement, while retaining previously identified
considerations, including, for example, its emphasis on consideration
of vulnerable populations, thus expanding upon (e.g., with some
increased specificity), while retaining core consistency with, the
earlier ATS statement (Thurston et al., 2017; ATS, 2000). One example
of this increased specificity that was raised in public comments and
discussed in section II.B.2 above, is in the discussion of small
changes in lung function (in terms of FEV1) in people with
compromised function, such as people with asthma (Thurston et al.,
2017). In considering these statements, the Administrator notes that,
in keeping with the intent of these statements to avoid specific
criteria, the statements, in discussing what constitutes an adverse
health effect, do not comprehensively describe all the biological
responses raised, e.g., with regard to magnitude, duration or frequency
of small pollutant-related changes in lung function. In so doing, he
also recognizes the limitations in the current evidence base with
regard to our
[[Page 87303]]
understanding of these aspects of such changes that may be associated
with exposure concentrations of interest. Notwithstanding these
limitations and associated uncertainties, he takes note of the emphasis
of the ATS statement on consideration of individuals with pre-existing
compromised function, such as that resulting from asthma (an emphasis
which is reiterated and strengthened in the current statement), and
agrees that these are important considerations in his judgment on the
adequacy of protection provided by the current standard for at-risk
populations, as recognized below.
The Administrator recognizes some uncertainty, reflecting
limitations in the evidence base, with regard to the exposure levels
eliciting effects (as well as the severity of the effects) in some
population groups not included in the available controlled human
exposure studies, such as children and individuals with asthma. In so
doing, the Administrator recognizes that the controlled human exposure
studies, primarily conducted in healthy adults, on which the depth of
our understanding of O3-related health effects is based, in
combination with the larger evidence base, informs our conceptual
understanding of O3 responses in people with asthma and in
children. Aspects of our understanding continue to be limited, however,
including with regard to the risk of particular effects and associated
severity for these less studied population groups that may be posed by
7-hour exposures with exercise to concentrations as low as 60 ppb that
are estimated in the exposure analyses. Collectively, these aspects of
the evidence and associated uncertainties contribute to the
Administrator's recognition that for O3, as for other
pollutants, the available evidence base in a NAAQS review generally
reflects a continuum, consisting of levels at which scientists
generally agree that health effects are likely to occur, through lower
levels at which the likelihood and magnitude of the response become
increasingly uncertain.
In light of these uncertainties in the evidence, as well as those
associated with the exposure and risk analyses, the Administrator notes
that, as is the case in NAAQS reviews in general, his decision
regarding the primary O3 standard in this review depends on
a variety of factors, including his science policy judgments and public
health policy judgments. These factors include judgments regarding
aspects of the evidence and exposure/risk estimates, such as judgments
concerning his interpretation of the different benchmark
concentrations, in light of the available evidence and of associated
uncertainties, as well as judgments on the public health significance
of the effects that have been observed at the exposures evaluated in
the health effects evidence. These judgments are rooted in his
interpretation of the evidence, which reflects a continuum of health-
relevant exposures, with less confidence and greater uncertainty in the
existence of adverse health effects as one considers lower
O3 exposures. The factors relevant to judging the adequacy
of the standards also include the interpretation of, and decisions as
to the relative weight to place on, different aspects of the results of
the exposure and risk assessment for the eight areas studied and the
associated uncertainties. Together, factors described here inform the
Administrator's judgment about the degree of protection that is
requisite to protect public health with an adequate margin of safety,
including the health of sensitive groups, and, accordingly, his
conclusion that the current standard is requisite to protect public
health with an adequate margin of safety.
As in prior O3 NAAQS reviews, the Administrator
considers the exposure estimates developed from modeling exposures to
O3 in ambient air in this review to be critically important
to consideration of the potential for exposures and risks of concern
under air quality conditions of interest, and consequently important to
his judgments on the adequacy of public health protection provided by
the current standard. The exposure/risk analysis provides a framework
within which to consider implications of the health effects evidence
with regard to protection afforded by the current standard. In his
consideration of the exposure/risk estimates, the Administrator places
greater weight and gives primary attention to the comparison-to-
benchmarks analysis. This focus reflects his recognition of multiple
factors, including the relatively greater uncertainty associated with
the lung function risk estimates compared to the results of the
comparison-to-benchmarks analysis. Additionally, he recognizes that, as
noted in the PA, the comparison-to-benchmarks analysis provides for
characterization of risk for the broad array of respiratory effects
documented in the controlled human exposure studies. Accordingly, this
analysis facilitates consideration of an array of respiratory effects,
including but not limited to lung function decrements. Accordingly, the
Administrator focuses primarily on the estimates of exposures at or
above different benchmark concentrations that represent different
levels of significance of O3-related effects, both with
regard to the array of effects and severity of individual effects. In
so doing, he notes that this assures his consideration of the
protection provided by the standard from the array of respiratory
effects documented in the currently available evidence base.
In considering the public health implications of estimated
occurrences of exposures (while at increased exertion) to the three
benchmark concentrations (60, 70 and 80 ppb), the Administrator
considers the respiratory effects reported in controlled human exposure
studies of this range of concentrations (during quasi-continuous
exercise). Accordingly, the controlled human exposure study evidence
base, as a whole, provides context for consideration of the exposure/
risk estimates. The Administrator recognizes the three benchmarks to
represent exposure conditions associated with different levels of
respiratory response in the subjects studied and to inform his
judgments on different levels of risk that might be posed to unstudied
members of at-risk populations. The highest benchmark concentration (80
ppb) represents an exposure where multiple controlled human exposure
studies involving 6.6-hour exposures during quasi-continuous exercise
demonstrate a range of O3-related respiratory effects
including inflammation and airway responsiveness, as well as
respiratory symptoms and lung function decrements in healthy adult
subjects. Findings for this O3 exposure include: A
statistically significant increase in multiple types of respiratory
inflammation indicators in multiple studies; statistically
significantly increased airway resistance and responsiveness;
statistically significant FEV1 decrements; and statistically
significant increases in respiratory symptoms (Table 1). In one
variable exposure study for which this (80 ppb) was the exposure period
average concentration, the study subject mean FEV1 decrement
was nearly 8%, with individual decrements of 15% or greater (moderate
or greater) in 16% of subjects and decrements of 10% or greater in 32%
of subjects (Schelegle et al 2009); the percentages of individual
subjects with decrements great than 10 or 15% were lower in other
studies for this exposure. The second benchmark (70 ppb) represents an
exposure level below the lowest exposures that have reported both
statistically significant FEV1
[[Page 87304]]
decrements \148\ and increased respiratory symptoms (reported at 73
ppb, Schelegle et al 2009) or statistically significant increases in
airway resistance and responsiveness (reported at 80 ppb, Horstman et
al., 1990). The lowest benchmark (60 ppb) represents still lower
exposure, and a level for which findings from controlled human exposure
studies of largely healthy subjects have included: Statistically
significant decrements in lung function (with mean decrements ranging
from 1.7% to 3.5% across the four studies with average exposures of 60
to 63 ppb \149\), but not respiratory symptoms; and, a statistically
significant increase in a biomarker of airway inflammatory response
relative to filtered air exposures in one study (Kim et al, 2011).
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\148\ The study group mean lung function decrement for the 73
ppb exposure was 6%, with individual decrements of 15% or greater
(moderate or greater) in about 10% of subjects and decrements of 10%
or greater in 19% of subjects. Decrements of 20% or greater were
reported in 6.5% of subjects (Schelegle et al., 2009; PA, Table 3-2
and Appendix 3D, Table 3D-20). In studies of 80 ppb exposure, the
percent of study subjects with individual FEV1 decrements
of this size ranged up to nearly double this (PA, Appendix 3D, Table
3D-20).
\149\ Among subjects in all four of these studies, individual
FEV1 decrements of at least 15% were reported in 3% of
subjects, with 7% of subjects reported to have decrements at or
above a lower value of 10% (PA, Appendix 3D, Table 3D-20).
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In turning to the exposure/risk analysis results, the Administrator
considers the evidence represented by these benchmarks noting that due
to differences among individuals in responsiveness, not all people
experiencing such exposures experience a response, such as a lung
function decrement, as illustrated by the percentages cited above.
Further, among those that experience a response, not all will
experience an adverse effect. Accordingly, the Administrator notes that
not all people estimated to experience an exposure of 7-hour duration
while at elevated exertion above even the highest benchmark would be
expected to experience an adverse effect, even members of at-risk
populations. With these considerations in mind, he notes that while
single occurrences could be adverse for some people, particularly for
the higher benchmark concentration where the evidence base is stronger,
the potential for adverse response increases with repeated occurrences
(as cautioned by the CASAC).\150\ In so doing, he also notes that while
the exposure/risk analyses provide estimates of exposures of the at-
risk population to concentrations of potential concern, they do not
provide information on how many of such populations will have an
adverse health outcome. Accordingly, in considering the exposure/risk
analysis results, while giving due consideration to occurrences of one
or more days with an exposure at or above a benchmark, particularly the
higher benchmarks, he judges multiple occurrences to be of greater
concern than single occurrences.
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\150\ For example, for people with asthma, the risk of an asthma
exacerbation event may be expected to increase with repeated
occurrences of lung function decrements of 10% or 15% as compared to
a single occurrence.
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In this context, the Administrator considers the exposure risk
estimates, focusing first on the results for the highest benchmark
concentration (80 ppb), which represents an exposure well established
to elicit an array of responses in sensitive individuals among study
groups of largely healthy adult subjects, exposed while at elevated
exertion. Similar to judgments of past Administrators, the current
Administrator judges these effects in combination and severity to
represent adverse effects for individuals in the population group
studied, and to pose risk of adverse effects for individuals in at-risk
populations, most particularly people with asthma, as noted above.
Accordingly, he judges that the primary standard should provide
protection from such exposures. In considering the exposure/risk
estimates, he focuses on the results for children, and children with
asthma, given the higher frequency of exposures of potential concern
for children compared to adults, in terms of percent of the population
groups.\151\ The exposure/risk estimates indicate more than 99.9% to
100% of children and children with asthma, on average across the three
years, to be protected from one or more occasions of exposure at or
above this level; the estimate is 99.9% of children with asthma and of
all children for the highest year and study area (Table 2). Further, no
children in the simulated populations (zero percent) are estimated to
be exposed more than once (two or more occasions) in the 3-year
simulation to 7-hr concentrations, while at elevated exertion, at or
above 80 ppb (Table 2). These estimates indicate strong protection
against exposures of at-risk populations that have been demonstrated to
elicit a wide array of respiratory responses in multiple studies.
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\151\ This finding relates to children's greater frequency and
duration of outdoor activity, as well as their greater activity
level while outdoors (PA, section 3.4.3).
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The Administrator next considers the results for the second
benchmark concentration (70 ppb), which is just below the lowest
exposure concentration (73 ppb) for which a study has reported a
combination of a statistically significant increase in respiratory
symptoms and statistically significant lung function decrements in
sensitive individuals in a study group of largely healthy adult
subjects, exposed while at elevated exertion (Schelegle et al., 2009).
Recognizing the lack of evidence for people with asthma from studies at
80 ppb and 73 ppb, as well as the emphasis in the ATS statement on the
vulnerability of people with compromised respiratory function, such as
people with asthma, the Administrator judges it appropriate that the
standard protect against exposure, particularly multiple occurrences of
exposure, to somewhat lower levels. In so doing, he notes that the
exposure/risk estimates indicate more than 99% of children with asthma,
and of all children, to be protected from one or more occasions in a
year, on average, of 7-hour exposures to concentrations at or above 70
ppb, while at elevated exertion (Table 2). The estimate is 99% of
children with asthma for the highest year and study area (Table 2).
Further, he notes that 99.9% of these groups are estimated to be
protected from two or more such occasions, and 100% from still more
occasions. These estimates also indicate strong protection of at-risk
populations against exposures similar to those demonstrated to elicit
lung function decrements and increased respiratory symptoms in healthy
subjects, a response described as adverse by the ATS.
In consideration of the exposure/risk results for the lowest
benchmark (60 ppb), the Administrator notes that the lung function
decrements in controlled human exposure studies of largely healthy
adult subjects exposed while at elevated exertion to concentrations of
60 ppb, although statistically significant, are much reduced from that
observed in the next higher studied concentration (73 ppb), both at the
mean and individual level, and are not reported to be associated with
increased respiratory symptoms in healthy subjects.\152\ In light of
these results and the transient nature of the responses, the
Administrator does not judge these responses to represent adverse
effects for generally healthy individuals. However, he further
considers these findings specifically with regard to protection of at-
risk populations, such as people with asthma. In so doing, he notes
that such data are lacking for at-risk groups, such as people with
asthma, and considers the evidence and
[[Page 87305]]
comments from the CASAC regarding the need to consider endpoints of
particular importance for this population group, such as risk of asthma
exacerbation and prolonged inflammation. He takes note of comments from
the CASAC (and also noted in the ATS statement) that small lung
function decrements in this at-risk group may contribute to a risk of
asthma exacerbation, an outcome described by the CASAC as ``arguably
the most important potential adverse effect'' of O3 exposure
for a child with asthma. Thus, he judges it important for the standard
to provide protection that reduces such risks. However, he recognizes
gaps in our ability to predict risk of such events at the low
concentrations such as those represented by the lowest benchmark in the
exposure/risk analysis. With regard to the inflammatory response he
notes the evidence, discussed in section II.B.2 above, indicating the
role of repeated occurrences of inflammation in contributing to
severity of response. Thus, he finds repeated occurrences of exposure
events of potential concern to pose greater risk than single events,
leading him to place greater weight to exposure/risk estimates for
multiple occurrences.
---------------------------------------------------------------------------
\152\ The response for the 60 ppb studies is also somewhat lower
than that for the 63 ppb study (Table 1; PA, Appendix 3D, Table 3D-
20).
---------------------------------------------------------------------------
In light of the uncertainties associated with the lack of
controlled human exposure data for people with asthma, particularly
with regard to the extent to which the lower exposure concentrations
studied in generally healthy adults might be expected to elicit
asthmatic responses in this at-risk population, the Administrator notes
that the CASAC also recognized this, describing the gap in clinical
studies to be a ``key knowledge gap'' important to considerations of
margin of safety for the standard. The Administrator further notes that
the CAA requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. This approach is consistent with the requirements
of the NAAQS provisions of the CAA and with how the EPA and the courts
have historically interpreted the Act (summarized in section I.A.
above). These provisions require the Administrator to establish primary
standards that, in the judgment of the Administrator, are requisite to
protect public health with an adequate margin of safety. In so doing,
the Administrator seeks to establish standards that are neither more
nor less stringent than necessary for this purpose. The Act does not
require that primary standards be set at a zero-risk level, but rather
at a level that avoids unacceptable risks to public health, including
the health of sensitive groups.\153\
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\153\ As noted in section I.A above, consideration of such
protection is focused on the sensitive group of individuals and not
a single person in the sensitive group (see S. Rep. No. 91-1196,
91st Cong., 2d Sess. 10 [1970]).
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Thus, in this context, and given that the 70 ppb benchmark
represents an exposure level somewhat below the lowest exposure
concentration for which both statistically significant lung function
decrements and increased respiratory symptoms have been reported in
largely healthy adult subjects, the Administrator considers the
exposure/risk estimates for the third benchmark of 60 ppb to be
informative most particularly to his judgments on an adequate margin of
safety. In that context, the Administrator turns to the third benchmark
concentration (60 ppb). In so doing, he takes note that these estimates
indicate more than 96% to more than 99% of children with asthma to be
protected from more than one occasion in a year (two or more), on
average, of 7-hour exposures to concentrations at or above this level,
while at elevated exertion (Table 2). Additionally, the analysis
estimates more than 90% of all children, on average across the three
years, to be protected from one or more occasions of exposure at or
above this level. The Administrator finds this to indicate an
appropriate degree of protection from such exposures.
The Administrator additionally takes note of the new finding in
this review of evidence of a likely to be causal relationship between
O3 and metabolic effects. In so doing, he notes the lack of
evidence that would suggest such effects to be associated with
exposures likely to occur with air quality conditions meeting the
current standard, as discussed in section II.A.2.c above. Thus, he
judges the current standard to provide protection from effects other
than respiratory effects, for which the evidence is less certain.
Accordingly, the Administrator concludes that the standard does not
need to be revised to provide additional protection from such effects.
In reflecting on all of the information currently available, the
Administrator considers the extent to which the currently available
information might indicate support for a less stringent standard. He
recognizes the advice from the CASAC, which generally indicates support
for retaining the current standard without revision or for revision to
a more stringent level based on additional consideration of the margin
of safety for at-risk populations. He notes that the CASAC advice did
not convey support for a less stringent standard. He additionally
considers the current exposure and risk estimates for the air quality
scenario for a design value just above the level of the current
standard (at 75 ppb), in comparison to the scenario for the current
standard, as summarized in section II.A.3 above. In so doing, he finds
the markedly increased estimates of exposures to the higher benchmarks
under air quality for a higher standard level to be of concern and
indicative of less than the requisite protection (Table 2). Thus, in
light of the considerations raised here, including the need for an
adequate margin of safety, the Administrator judges that a less
stringent standard would not be appropriate.
The Administrator additionally considers whether it would be
appropriate to consider a more stringent standard that might be
expected to result in reduced O3 exposures. As an initial
matter, he considers the advice from the CASAC. With regard to the
CASAC advice, while part of the Committee concluded the evidence
supported retaining the current standard without revision, another part
of the Committee reiterated advice from the prior CASAC, which while
including the current standard level among the range of recommended
standard levels, also provided policy advice to set the standard at a
lower level. In considering this advice now in this review, as it was
raised by part of the current CASAC, the Administrator notes the slight
differences of the current exposure and risk estimates from the 2014
HREA estimates for the lowest benchmark, which were those considered by
the prior CASAC (Table 4). For example, while the 2014 HREA estimated
3.3 to 10.2% of children, on average, to experience one or more days
with an exposures at or above 60 ppb (and as many as 18.9% in a single
year), the comparable estimates for the current analyses are lower,
particularly at the upper end (3.2 to 8.2% and 10.6%). While the
estimates for two or more days with occurrences at or above 60 ppb, on
average across the assessment period, are more similar between the two
assessments, the current estimate for the single highest year is much
lower (9.2 versus 4.3%). The Administrator additionally recognizes the
PA finding that the factors contributing to these differences, which
includes the use of air quality data reflecting concentrations much
closer to the now-current standard than was the case in the 2015
review, also contribute to a reduced
[[Page 87306]]
uncertainty in the current estimates, as summarized in section II.A.3
above (PA, sections 3.4 and 3.5). Thus, he notes that the current
exposure analysis estimates indicate the current standard to provide
appreciable protection against multiple days with a maximum exposure at
or above 60 ppb. In the context of his consideration of the adequacy of
protection provided by the standard and of the CAA requirement that the
standard protect public health, including the health of at-risk
populations, with an adequate margin of safety, the Administrator
concludes, in light of all of the considerations raised here, that the
current standard provides appropriate protection, and that a more
stringent standard would be more than requisite to protect public
health.
In light of all of the above, including advice from the CASAC, the
Administrator finds the current exposure and risk analysis results to
describe appropriately strong protection of at-risk populations from
exposures associated with O3-related health effects.
Therefore, based on his consideration of the evidence and exposure/risk
information, including that related to the lowest exposures studied in
controlled human exposure studies, and the associated uncertainties,
the Administrator judges that the current standard provides the
requisite protection of public health, including an adequate margin of
safety, and thus should be retained, without revision. Accordingly, he
concludes that a more stringent standard is not needed to provide
requisite protection and that the current standard provides the
requisite protection of public health under the Act. With regard to key
aspects of the specific elements of the standard, the Administrator
recognizes the support in the current evidence base for O3
as the indicator for photochemical oxidants. In so doing, he notes the
ISA conclusion that O3 is the most abundant of the
photochemical oxidants in the atmosphere and the one most clearly
linked to human health effects. He additionally recognizes the control
exerted by the 8-hour averaging time on associated exposures of
importance for O3-related health effects. Lastly, with
regard to form and level of the standard, the Administrator takes note
of the exposure and risk results as discussed above and the level of
protection that they indicate the elements of the current standard to
provide. Beyond his recognition of this support in the available
information for the elements of the current standard, the Administrator
has considered the elements collectively in evaluating the health
protection afforded by the current standard. For all of the reasons
discussed above, the Administrator concludes that the current primary
O3 standard (in all of its elements) is requisite to protect
public health with an adequate margin of safety, including the health
of at-risk populations, and thus should be retained, without revision.
C. Decision on the Primary Standard
For the reasons discussed above and taking into account information
and assessments presented in the ISA and PA, the advice from the CASAC,
and consideration of public comments, the Administrator concludes that
the current primary O3 standard is requisite to protect
public health with an adequate margin of safety, including the health
of at-risk populations, and is retaining the current standard without
revision.
III. Rationale for Decision on the Secondary Standard
This section presents the rationale for the Administrator's
decision to retain the current secondary O3 standard. This
rationale is based on the scientific information presented in the ISA,
on welfare effects associated with photochemical oxidants including
O3 and pertaining to the presence of these pollutants in
ambient air. As summarized in section I.D above, the ISA was developed
based on a thorough review of the latest scientific information
generally published between January 2011 and March 2018, as well as
more recent studies identified during peer review or by public comments
on the draft ISA integrated with the information and conclusions from
previous assessments (ISA, section IS.1.2 and Appendix 10, section
10.2). The Administrator's rationale also takes into account: (1) The
PA evaluation of the policy-relevant information in the ISA and
presentation of quantitative analyses of air quality, exposure, and
risk; (2) CASAC advice and recommendations, as reflected in discussions
of drafts of the ISA and PA at public meetings, and in the CASAC's
letters to the Administrator; (3) public comments on the proposed
decision; and also (4) the August 2019 decision of the D.C. Circuit
remanding the secondary standard established in the last review to the
EPA for further justification or reconsideration. See Murray Energy
Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019).
Within this section, introductory and background information is
presented in section III.A. Section III.A.1 summarizes the 2015
establishment of the existing standard, as background for this review.
Sections III.A.2 and III.A.3 provide overviews of the currently
available welfare effects evidence and current air quality and
environmental exposure information, respectively. Section III.B
summarizes the basis for the proposed decision (III.B.1), including
CASAC advice, discusses public comments on the proposed decision
(III.B.2), and presents the Administrator's considerations, conclusions
and decision in this review of the secondary standard (III.B.3). The
decision is summarized in section III.C.
A. Introduction
As in prior reviews, the general approach to reviewing the current
secondary standard is based, most fundamentally, on using the Agency's
assessments of the current scientific evidence and associated
quantitative analyses to inform the Administrator's judgment regarding
a secondary standard for photochemical oxidants that is requisite to
protect the public welfare from known or anticipated adverse effects
associated with the pollutant's presence in the ambient air. The EPA's
assessments are primarily documented in the ISA and PA, both of which
have received CASAC review and public comment (84 FR 50836, September
26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019;
85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging
the gap between the scientific assessments of the ISA and the judgments
required of the Administrator in his decisions on the current standard,
the PA evaluates policy implications of the assessment of the current
evidence in the ISA and the quantitative air quality, exposure and risk
analyses and information documented in the PA. In evaluating the public
welfare protection afforded by the current standard, the four basic
elements of the NAAQS (indicator, averaging time, level, and form) are
considered collectively.
The final decision on the adequacy of the current secondary
standard is a public welfare policy judgment to be made by the
Administrator. In reaching conclusions on the standard, the decision
draws on the scientific information and analyses about welfare effects,
environmental exposure and risks, and associated public welfare
significance, as well as judgments about how to consider the range and
magnitude of uncertainties that are inherent in the scientific evidence
and analyses. This approach is based on the recognition that the
available evidence generally reflects a continuum that includes ambient
air exposures at which
[[Page 87307]]
scientists generally agree that effects are likely to occur through
lower levels at which the likelihood and magnitude of responses become
increasingly uncertain. This approach is consistent with the
requirements of the provisions of the Clean Air Act related to the
review of NAAQS and with how the EPA and the courts have historically
interpreted the Act. These provisions require the Administrator to
establish secondary standards that, in the judgment of the
Administrator, are requisite to protect the public welfare from known
or anticipated adverse effects associated with the presence of the
pollutant in the ambient air. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that standards be
set at a zero-risk level, but rather at a level that reduces risk
sufficiently so as to protect the public welfare from known or
anticipated adverse effects.
This decision on the secondary O3 standard also
considers the August 2019 decision by the D.C. Circuit and issues
raised by the court in its remand of the 2015 standard to the EPA such
that the decision in this review incorporates the EPA's response to the
court's remand. The opinion issued by the court concluded, in relevant
part, that EPA had not provided a sufficient rationale for aspects of
its 2015 decision on the secondary standard. See Murray Energy Corp. v.
EPA, 936 F.3d 597 (D.C. Cir. 2019). Accordingly, the court remanded
that standard to EPA for further justification or reconsideration,
particularly in relation to its decision to focus on a 3-year average
for consideration of the cumulative exposure for vegetation, in terms
of W126, identified as providing requisite public welfare protection,
and its decision to not identify a specific level of air quality
related to visible foliar injury.\154\ Thus, in addition to considering
the currently available welfare effects evidence and quantitative air
quality, exposure and risk information, the decision described here,
and the associated conclusions and judgments, also consider the court's
remand. In consideration of the court remand, for example, certain
analyses in this review are expanded compared with those conducted in
the last review, issues raised in the remand have been discussed, and
additional explanation of rationales for conclusions on these points is
provided in this review.
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\154\ The EPA's decision not to use a seasonal W126 index as the
form and averaging time of the secondary standard was also
challenged in this case, but the court did not reach a decision on
that issue, concluding that it lacked a basis to assess the EPA's
rationale on this point because the EPA had not yet fully explained
its focus on a 3-year average W126 in its consideration of the
standard. See Murray Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C.
Cir. 2019).
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1. Background on the Current Standard
As a result of the last O3 review, completed in 2015,
the level of the secondary standard was revised to 0.070 ppm, in
conjunction with retaining the indicator, averaging time and form. This
revision, establishing the current standard, was based on the
scientific evidence and technical analyses available at that time, as
well as the Administrator's judgments regarding the available welfare
effects evidence, the appropriate degree of public welfare protection
for the revised standard, and available air quality information on
seasonal cumulative exposures that may be allowed by such a standard
(80 FR 65292, October 26, 2015). In establishing this standard, the
Administrator considered the extensive welfare effects evidence base
compiled from more than fifty years of extensive research on the
phytotoxic effects of O3, conducted both in and outside of
the U.S., that documents the impacts of O3 on plants and
their associated ecosystems (U.S. EPA, 1978, 1986, 1996, 2006, 2013).
As was established in prior reviews, O3 can interfere with
carbon gain (photosynthesis) and allocation of carbon within the plant,
making fewer carbohydrates available for plant growth, reproduction,
and/or yield (U.S. EPA, 1996b, pp. 5-28 and 5-29).\155\ The 2015
decision drew upon: (1) The available scientific evidence assessed in
the 2013 ISA; (2) assessments in the 2014 PA of the most policy-
relevant information in the 2013 ISA regarding evidence of adverse
effects of O3 to vegetation and ecosystems, information on
biologically-relevant exposure metrics, 2014 welfare REA (WREA)
analyses of air quality, exposure, and ecological risks and associated
ecosystem services, and staff analyses of relationships between levels
of a W126-based exposure index \156\ and potential alternative standard
levels in combination with the form and averaging time of the existing
standard; (3) additional air quality analyses of the W126 index and
design values based on the form and averaging time of the existing
standard; (4) CASAC advice and recommendations; and (5) public comments
received during the development of these documents and on the 2014
proposal (80 FR 65292, October 26, 2015). In addition to reviewing the
most recent scientific information as required by the CAA, the 2015
rulemaking also incorporated the EPA's response to the judicial remand
of the 2008 secondary O3 standard in Mississippi v. EPA, 744
F.3d 1334 (D.C. Cir. 2013) and, in light of the court's decision in
that case, explained the Administrator's conclusions as to the level of
air quality judged to provide the requisite protection of public
welfare from known or anticipated adverse effects.
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\155\ In addition to concluding there to be causal relationships
between O3 and visible foliar injury, reduced vegetation
growth, reduced productivity, reduced growth and yield of
agricultural crops, and alteration of below-ground biogeochemical
cycles, the 2013 ISA also concluded there likely to be a causal
relationships between O3 and reduced carbon sequestration
in terrestrial ecosystems, alteration of terrestrial ecosystem water
cycling and alteration of terrestrial community composition (2013
ISA, p. lxviii and Table 9-19). The 2013 ISA also found there to be
a causal relationship between changes in tropospheric O3
concentrations and radiative forcing, and likely to be a causal
relationship between tropospheric O3 concentrations and
effects on climate as quantified through surface temperature
response (2013 ISA, section 10.5).
\156\ The W126 index is a cumulative seasonal metric described
as the sigmoidally weighted sum of all hourly O3
concentrations during a specified daily and seasonal time window,
with each hourly O3 concentration given a weight that
increases from zero to one with increasing concentration (80 FR
65373-74, October 26, 2015). The units for W126 index values are
ppm-hours (ppm-hrs).
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Across the different types of studies, the strongest evidence for
effects from O3 exposure on vegetation was from controlled
exposure studies of many species of vegetation (2013 ISA, p. 1-15).
Primary consideration in the decision was given to the studies of
O3 exposures that reduced growth in tree seedlings from
which E-R functions of seasonal relative biomass loss (RBL) have been
established (80 FR 65385-86, 65389-90, October 26, 2015). The
Administrator considered the effects of O3 on tree seedling
growth, as suggested by the CASAC, as a surrogate or proxy for the
broader array of vegetation-related effects of O3, ranging
from effects on sensitive species to broader ecosystem-level effects
(80 FR 65369, 65406, October 26, 2015). The metric used for quantifying
effects on tree seedling growth in the review was RBL, with the
evidence base providing robust and established E-R functions for
seedlings of 11 tree species (80 FR 65391-92, October 26, 2015; 2014
PA, Appendix 5C).\157\ The Administrator used this metric in her
judgments on O3 effects on the public welfare. In this
context, exposure was evaluated in terms of the W126 cumulative
seasonal
[[Page 87308]]
exposure index, an index supported by the evidence in the 2013 ISA for
this purpose and that was consistent with advice from the CASAC (2013
ISA, section 9.5.3, p. 9-99; 80 FR 65375, October 26, 2015).
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\157\ These functions for RBL estimate the reduction in a year's
growth as a percentage of that expected in the absence of
O3 (2013 ISA, section 9.6.2; 2014 WREA, section 6.2).
---------------------------------------------------------------------------
The 2015 decision was a public welfare policy judgment made by the
Administrator, that drew upon the available scientific evidence for
O3-attributable welfare effects and on quantitative analyses
of exposures and public welfare risks, as well as judgments about the
appropriate weight to place on the range of uncertainties inherent in
the evidence and analyses. Included in this decision were judgments on
the weight to place on the evidence of specific vegetation-related
effects estimated to result across a range of cumulative seasonal
concentration-weighted O3 exposures; on the weight to give
associated uncertainties, including uncertainties of predicted
environmental responses (based on experimental study data); variability
in occurrence of the specific effects in areas of the U.S., especially
in areas of particular public welfare significance; and on the extent
to which such effects in such areas may be considered adverse to public
welfare. For example, in considering the public welfare protection
provided by the then-existing standard, the Administrator gave primary
consideration to an analysis of cumulative seasonal exposures in or
near Class I areas,\158\ which are lands that Congress set aside for
specific uses intended to provide benefits to the public welfare,
including lands that are to be protected so as to conserve the scenic
value and the natural vegetation and wildlife within such areas, and to
leave them unimpaired for the enjoyment of future generations.\159\ The
decision additionally recognized that states, tribes and public
interest groups also set aside areas that are intended to provide
similar benefits to the public welfare for residents on those lands, as
well as for visitors to those areas (80 FR 65390, October 26, 2015). In
recognizing that her judgments regarding effects that are adverse to
the public welfare consider the intended use of the natural resources
and ecosystems affected, the Administrator utilized the RBL as a
quantitative tool within a larger framework of considerations
pertaining to the public welfare significance of O3 effects
(80 FR 65389, October 26, 2015; 73 FR 16496, March 27, 2008).
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\158\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed 6,000 acres in size, provided the park
or wilderness area was in existence on August 7, 1977. Other areas
may also be Class I if designated as Class I consistent with the
CAA.
\159\ This emphasis on such lands was consistent with a similar
emphasis in the 2008 review of the standard (73 FR 16485, March 27,
2008).
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In the Administrator's consideration of the adequacy of public
welfare protection afforded by the existing standard, she gave
particular attention to the air quality analysis for Class I areas that
estimated cumulative exposures, in terms of 3-year average W126 index
values, at and above 19 ppm-hrs, to have occurred under the standard in
nearly a dozen areas distributed across two NOAA climatic regions of
the U.S (80 FR 65385-86, October 26, 2015). The Administrator took note
of these occurrences of exposures in Class I areas during periods when
the existing standard was met, for which the associated estimates of
growth effects across the species with E-R functions extend above a
magnitude considered to be ``unacceptably high'' by the CASAC (80 FR
65385-65386, 65389-65390, October 26, 2015).\160\ Based on this
analysis and the considerations summarized above, including
consideration of CASAC advice and public comment, the Administrator
concluded that the protection afforded by the then-existing standard
was not sufficient and that the standard needed to be revised to
provide additional protection from known and anticipated adverse
effects to public welfare, related to effects on sensitive vegetation
and ecosystems, most particularly those occurring in Class I areas, and
also in other areas set aside by states, tribes and public interest
groups to provide similar benefits to the public welfare. In so doing,
she further noted that a revised standard would provide increased
protection for other growth-related effects, including relative yield
loss (RYL) of crops, reduced carbon storage, and types of effects for
which it is more difficult to determine public welfare significance, as
well as other welfare effects of O3, such as visible foliar
injury \161\ (80 FR 65390, October 26, 2015).
---------------------------------------------------------------------------
\160\ The Administrator focused on the median RBL estimate
across the eleven tree species for which robust established E-R
functions were available and took note of the CASAC's consideration
of RBL estimates presented in the 2014 draft PA, in which it
characterized an estimate of 6% RBL in the median studied species as
being ``unacceptably high,'' (Frey, 2014b).
\161\ As described in the ISA, ``[t]ypical types of visible
injury to broadleaf plants include stippling, flecking, surface
bleaching, bifacial necrosis, pigmentation (e.g., bronzing), and
chlorosis or premature senescence'' and ``[t]ypical visible injury
symptoms for conifers include chlorotic banding, tip burn, flecking,
chlorotic mottling, and premature senescence of needles'' (ISA,
Appendix 8, p. 8-13).
---------------------------------------------------------------------------
In light of the judicial remand of the 2008 secondary O3
standard referenced above, the 2015 decision on selection of a revised
secondary standard first considered the available evidence and
quantitative analyses in the context of an approach for considering and
identifying public welfare objectives for the revised standard (80 FR
65403-65408, October 26, 2015). In light of the extensive evidence base
of O3 effects on vegetation and associated terrestrial
ecosystems, the Administrator focused on protection against adverse
public welfare effects of O3-related effects on vegetation,
giving particular attention to such effects in natural ecosystems, such
as those in areas with protection designated by Congress, and areas
similarly set aside by states, tribes and public interest groups, with
the intention of providing benefits to the public welfare for current
and future generations.\162\
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\162\ The Administrator additionally recognized that providing
protection for this purpose will also provide a level of protection
for other vegetation that is used by the public and potentially
affected by O3 including timber, produce grown for
consumption, and horticultural plants used for landscaping (80 FR
65403, October 26, 2015).
---------------------------------------------------------------------------
In reaching a conclusion on the amount of public welfare protection
from the presence of O3 in ambient air that is appropriate
to be afforded by a revised secondary standard, the Administrator gave
particular consideration to the following: (1) The nature and degree of
effects of O3 on vegetation, including her judgments as to
what constitutes an adverse effect to the public welfare; (2) the
strengths and limitations of the available and relevant information;
(3) comments from the public on the Administrator's proposed decision,
including comments related to identification of a target level of
protection; and (4) the CASAC's views regarding the strength of the
evidence and its adequacy to inform judgments on public welfare
protection. The Administrator recognized that such judgments should
neither overstate nor understate the strengths and limitations of the
evidence and information nor the appropriate inferences to be drawn as
to risks to public welfare, and that the choice of the appropriate
level of protection is a public welfare policy judgment entrusted to
the Administrator under the CAA taking into account both the available
evidence and the uncertainties (80 FR 65404-05, October 26, 2015).\163\
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\163\ The CAA does not require that a secondary standard be
protective of all effects associated with a pollutant in the ambient
air but rather those known or anticipated effects judged adverse to
the public welfare (CAA section 109).
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[[Page 87309]]
With regard to the extensive evidence of welfare effects of
O3, including visible foliar injury and crop RYL, the RBL
information available for seedlings of a set of 11 tree species was
judged to be more useful (particularly in a role as surrogate for the
broader array of vegetation-related effects) in informing judgments
regarding the nature and severity of effects associated with different
air quality conditions and associated public welfare significance (80
FR 65405-06, October 26, 2015). With regard to visible foliar injury,
while the Administrator recognized the potential for this effect to
affect the public welfare in the context of affecting value ascribed to
natural forests, particularly those afforded special government
protection, she also recognized limitations in the available
information that might inform consideration of potential public welfare
impacts related to this vegetation effect noting the significant
challenges in judging the specific extent and severity at which such
effects should be considered adverse to public welfare (80 FR 65407,
October 26, 2015).\164\ Similarly, while O3-related growth
effects on agricultural and commodity crops had been extensively
studied and robust E-R functions developed for a number of species, the
Administrator found this information less useful in informing her
judgments regarding an appropriate level of public welfare protection
(80 FR 65405, October 26, 2015).\165\ Thus, and in light of the
extensive evidence base in this regard, the Administrator focused on
the information related to trees and growth impacts in identifying the
public welfare objectives for the revised secondary standard.
---------------------------------------------------------------------------
\164\ These limitations included the lack of established E-R
functions that would allow prediction of visible foliar injury
severity and incidence under varying air quality and environmental
conditions, a lack of consistent quantitative relationships linking
visible foliar injury with other O3-induced vegetation
effects, such as growth or related ecosystem effects, and a lack of
established criteria or objectives relating reports of foliar injury
with public welfare impacts (80 FR 65407, October 26, 2015).
\165\ With respect to commercial production of commodities, the
Administrator noted the difficulty in discerning the extent to which
O3-related effects on commercially managed vegetation are
adverse from a public welfare perspective, given that the extensive
management of such vegetation (which, as the CASAC noted, may reduce
yield variability) may also to some degree mitigate potential
O3-related effects. Management practices are highly
variable and are designed to achieve optimal yields, taking into
consideration various environmental conditions. Further, changes in
yield of commercial crops and commercial commodities, such as
timber, may affect producers and consumers differently, complicating
the assessment of overall public welfare effects still further (80
FR 65405, October 26, 2015).
---------------------------------------------------------------------------
Accordingly, consideration of the appropriate public welfare
protection objective for a revised standard focused on the estimates of
tree seedling growth impacts (in terms of RBL) for a range of W126
index values, developed from the E-R functions for 11 tree species (80
FR 65391-92, Table 4, October 26, 2015). The Administrator also
incorporated into her considerations the broader evidence base
associated with forest tree seedling biomass loss, including other less
quantifiable effects of potentially greater public welfare
significance. That is, in drawing on these RBL estimates, the
Administrator was not simply making judgments about a specific
magnitude of growth effect in seedlings that would be acceptable or
unacceptable in the natural environment. Rather, though mindful of
associated uncertainties, the Administrator used the RBL estimates as a
surrogate or proxy for consideration of the broader array of related
vegetation-related effects of potential public welfare significance,
which included effects on individual species and extending to
ecosystem-level effects (80 FR 65406, October 26, 2015). This broader
array of vegetation-related effects included those for which public
welfare implications are more significant but for which the tools for
quantitative estimates were more uncertain.
In using the RBL estimates as a proxy, the Administrator focused
her attention on a revised standard that would generally limit
cumulative exposures to those for which the median RBL estimate for
seedlings of the 11 species with robust and established E-R functions
would be somewhat below 6% (80 FR 65406-07, October 26, 2015). In so
doing, she noted that the median RBL estimate was 6% for a cumulative
seasonal W126 exposure index of 19 ppm-hrs (80 FR 65391-92, Table 4,
October 26, 2015).\166\ Given the information on median RBL at
different W126 exposure levels, using a 3-year cumulative exposure
index for assessing vegetation effects,\167\ the potential for single-
season effects of concern, and CASAC comments on the appropriateness of
a lower value for a 3-year average W126 index, the Administrator
concluded it was appropriate to identify a standard that would restrict
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances (80 FR 65407, October 26,
2015). Based on such information, available at that time, to inform
consideration of vegetation effects and their potential adversity to
public welfare, the Administrator additionally judged that the RBL
estimates associated with marginally higher exposures in isolated, rare
instances were not indicative of effects that would be adverse to the
public welfare, particularly in light of variability in the array of
environmental factors that can influence O3 effects in
different systems and uncertainties associated with estimates of
effects associated with this magnitude of cumulative exposure in the
natural environment (80 FR 65407, October 26, 2015).
---------------------------------------------------------------------------
\166\ When stated to the first decimal place, the median RBL was
6.0% for a cumulative seasonal W126 exposure index of 19 ppm-hrs.
For 18 ppm-hrs, the median RBL estimate was 5.7%, which rounds to
6%, and for 17 ppm-hrs, the median RBL estimate was 5.3%, which
rounds to 5% (80 FR 65407, October 26, 2015).
\167\ Based on a number of considerations, the Administrator
recognized greater confidence in judgments related to public welfare
impacts based on a 3-year average metric than a single-year metric,
and consequently concluded it to be appropriate to use a seasonal
W126 index averaged across three years for judging public welfare
protection afforded by a revised secondary standard. For example,
she recognized uncertainties associated with interpretation of the
public welfare significance of effects resulting from a single-year
exposure, and that the public welfare significance of effects
associated with multiple years of critical exposures are potentially
greater than those associated with a single year of such exposure.
She additionally concluded that use of a 3-year average metric could
address the potential for adverse effects to public welfare that may
relate to shorter exposure periods, including a single year (80 FR
65404, October 26, 2015).
---------------------------------------------------------------------------
Using these objectives, the Administrator's decision regarding a
revised standard was based on extensive air quality analyses that
included the most recently available data (monitoring year 2013) and
extended back more than a decade (80 FR 65408, October 26, 2015; Wells,
2015). These analyses evaluated the cumulative seasonal exposure levels
in locations meeting different alternative levels for a standard of the
existing form and averaging time. Based on these analyses, the
Administrator judged that the desired level of public welfare
protection, considered in terms of cumulative exposure (quantified as
the W126 index), could be achieved by a standard with a revised level
in combination with the existing form and averaging time (80 FR 65408,
October 26, 2015).
In the most recent period of air quality data (2011-2013), across
the more than 800 monitor locations meeting the existing standard (with
its level of 75 ppb), the 3-year average W126 index values were above
17 ppm-hrs in 25 sites distributed across different NOAA climatic
regions, and
[[Page 87310]]
above 19 ppm-hrs at nearly half of these sites, with some well above
(Wells, 2015). In comparison, among the more than 500 sites meeting an
alternative standard of 70 ppb across 46 of the 50 states, there were
no occurrences of a W126 value above 17 ppm-hrs and fewer than five
occurrences that equaled 17 ppm-hrs (Wells, 2015 and associated dataset
[document identifier, EPA-HQ-OAR-2008-0699-4325]). For the full air
quality dataset (extending back to 2001), among the nearly 4000
instances where a monitoring site met a standard level of 70 ppb, the
Administrator noted that there was only ``a handful of isolated
occurrences'' of 3-year W126 index values above 17 ppm-hrs, ``all but
one of which were below 19 ppm-hrs'' (80 FR 65409, October 26, 2015).
The Administrator concluded that that single value of 19.1 ppm-hrs
(just equaling 19, when rounded), observed at a monitor for the 3-year
period of 2006-2008, was reasonably regarded as an extremely rare and
isolated occurrence, and, as such, it was unclear whether it would
recur, particularly as areas across the U.S. took further steps to
reduce O3 to meet revised primary and secondary standards.
Further, based on all of the then available information, as noted
above, the Administrator did not judge RBL estimates associated with
marginally higher exposures in isolated, rare instances to be
indicative of adverse effects to the public welfare. The Administrator
concluded that a standard with a level of 70 ppb and the existing form
and averaging time would be expected to limit cumulative exposures, in
terms of a 3-year average W126 exposure index, to values at or below 17
ppm-hrs, in nearly all instances, and accordingly, to eliminate or
virtually eliminate cumulative exposures associated with a median RBL
of 6% or greater (80 FR 65409, October 26, 2015). Thus, using RBL as a
proxy in judging effects to public welfare, the Administrator judged
that such a standard with a level of 70 ppb would provide the requisite
protection from adverse effects to public welfare by limiting
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances, and decided to revise the
standard level to 70 ppb.
In summary, the Administrator judged that the revised standard
would protect natural forests in Class I and other similarly protected
areas against an array of adverse vegetation effects, most notably
including those related to effects on growth and productivity in
sensitive tree species. The Administrator additionally judged that the
revised standard would be sufficient to protect public welfare from
known or anticipated adverse effects. This judgment by the
Administrator appropriately recognized that the CAA does not require
that standards be set at a zero-risk level, but rather at a level that
reduces risk sufficiently so as to protect the public welfare from
known or anticipated adverse effects. Thus, based on the conclusions
drawn from the air quality analyses which demonstrated a strong,
positive relationship between the 8-hour and W126 metrics and the
findings that indicated the significant amount of control provided by
the fourth-high metric, the evidence base of O3 effects on
vegetation and her public welfare policy judgments, as well as public
comments and CASAC advice, the Administrator decided to retain the
existing form and averaging time and revise the level to 0.070 ppm,
judging that such a standard would provide the requisite protection to
the public welfare from any known or anticipated adverse effects
associated with the presence of O3 in ambient air (80 FR
65409-10, October 26, 2015).
2. Overview of Welfare Effects Information
The information summarized here is an overview of the scientific
assessment of the welfare effects evidence available in this review;
this assessment is documented in the ISA and its policy implications
are further discussed in the PA. As in past reviews, the welfare
effects evidence evaluated in the ISA for O3 and related
photochemical oxidants is focused on O3 (ISA, p. IS-3).
Ozone is the most prevalent photochemical oxidant present in the
atmosphere and the one for which there is a very large, well-
established evidence base of its health and welfare effects (ISA, p.
IS-3). Thus, the current welfare effects evidence and the Agency's
review of the evidence, including the evidence newly available in this
review,\168\ continues to focus on O3. The subsections below
briefly summarize the following aspects of the evidence: the nature of
O3-related welfare effects, the potential public welfare
implications, and exposure concentrations associated with effects.
---------------------------------------------------------------------------
\168\ More than 1600 studies are newly available and considered
in the ISA, including nearly 600 studies on welfare effects (ISA,
Appendix 10, Figure 10-2).
---------------------------------------------------------------------------
a. Nature of Effects
The welfare effects evidence base available in the current review
includes more than sixty years of extensive research on the phytotoxic
effects of O3 and subsequent effects on associated
ecosystems (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, 2020
ISA). As described in past reviews, O3 can interfere with
carbon gain (photosynthesis) and allocation of carbon within the plant,
making fewer carbohydrates available for plant growth, reproduction,
and/or yield (2013 ISA, p. 1-10; 1996 AQCD, pp. 5-28 and 5-29). As
described in the 2013 ISA, the strongest evidence for effects from
O3 exposure on vegetation is from controlled exposure
studies, which ``have clearly shown that exposure to O3 is
causally linked to visible foliar injury, decreased photosynthesis,
changes in reproduction, and decreased growth'' in many species of
vegetation (2013 ISA, p. 1-15). Such effects at the plant scale can
also be linked to an array of effects at larger spatial scales (and
higher levels of biological organization), with the evidence available
in the last review indicating that ``O3 exposures can affect
ecosystem productivity, crop yield, water cycling, and ecosystem
community composition'' (2013 ISA, p. 1-15, Chapter 9, section 9.4).
Beyond its effects on plants, the 2013 ISA also recognized
O3 in the troposphere as a major greenhouse gas (ranking
behind carbon dioxide and methane in importance), with associated
radiative forcing and effects on climate, and recognized the
accompanying ``large uncertainties in the magnitude of the radiative
forcing estimate . . . making the impact of tropospheric O3
on climate more uncertain than the effect of the longer-lived
greenhouse gases'' (2013 ISA, sections 10.3.4 and 10.5.1 [p. 10-30]).
The evidence newly available in this review supports, sharpens and
expands somewhat on the conclusions reached in the last review (ISA,
Appendices 8 and 9). Consistent with the evidence in the last review,
the currently available evidence describes an array of O3
effects on vegetation and related ecosystem effects, as well as the
role of O3 in radiative forcing and subsequent climate-
related effects. The ISA concludes there to be causal relationships
between O3 and visible foliar injury,\169\ reduced
vegetation growth and reduced plant reproduction,\170\ as well as
reduced
[[Page 87311]]
yield and quality of agricultural crops, reduced productivity in
terrestrial ecosystems, alteration of terrestrial community
composition,\171\ and alteration of belowground biogeochemical cycles
(ISA, section IS.5). The current ISA also concludes there likely to be
a causal relationship between O3 and alteration of ecosystem
water cycling, reduced carbon sequestration in terrestrial ecosystems,
and with increased tree mortality (ISA, section IS.5). Additionally,
evidence newly available in this review augments more limited
previously available evidence related to insect interactions with
vegetation, contributing to the ISA conclusion that the evidence is
sufficient to infer that there are likely to be causal relationships
between O3 exposure and alteration of plant-insect signaling
(ISA, Appendix 8, section 8.7) and of insect herbivore growth and
reproduction (ISA, Appendix 8, section 8.6). Thus, conclusions reached
in the last review continue to be supported by the current evidence
base and conclusions are also reached in a few new areas based on the
now expanded evidence.
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\169\ Evidence continues to indicate that ``visible foliar
injury usually occurs when sensitive plants are exposed to elevated
ozone concentrations in a predisposing environment,'' with a major
factor for such an environment being the amount of soil moisture
available to the plant (ISA, Appendix 8, p. 8-23; 2013 ISA, section
9.4.2).
\170\ The 2013 ISA did not include a separate causality
determination for reduced plant reproduction. Rather, it was
included with the conclusion of a causal relationship with reduced
vegetation growth (ISA, Table IS-12).
\171\ The 2013 ISA had concluded alteration of terrestrial
community composition to be likely causally related to O3
based on the then available information (ISA, Table IS-12).
---------------------------------------------------------------------------
As in the last review, the strongest evidence and the associated
findings of causal or likely causal relationships with O3 in
ambient air, and the quantitative characterizations of relationships
between O3 exposure and occurrence and magnitude of effects
are for vegetation effects. Visible foliar injury has long been used as
a bioindicator of O3 exposure, although it is not always a
reliable indicator of other negative effects on vegetation (ISA,
sections IS.5.1.2 and 8.2). Effects of O3 on physiology of
individual plants at the cellular level, such as through photosynthesis
and carbon allocation, can impact plant growth and reproduction (ISA,
section IS.5.1.2). The scales of these effects range from the
individual plant scale to the ecosystem scale, with potential for
impacts on the public welfare (as discussed in section III.A.2.b
below). The effects of O3 on plants and plant populations
have implications for ecosystems. Effects at the ecosystem scale
include reduced terrestrial productivity and carbon storage, and
altered terrestrial community composition, as well as impacts on
ecosystem functions, such as belowground biogeochemical cycles and
ecosystem water cycling (ISA, Appendix 8, sections 8.11 and 8.9).
Ozone welfare effects also extend beyond effects on vegetation and
associated biota due to it being a major greenhouse gas and radiative
forcing agent.\172\ The current evidence, augmented since the 2013 ISA,
continues to support a causal relationship between the global abundance
of O3 in the troposphere and radiative forcing, and a likely
causal relationship between the global abundance of O3 in
the troposphere and effects on temperature, precipitation, and related
climate variables \173\ (ISA, section IS.5.2 and Appendix 9; Myhre et
al., 2013). Uncertainty in the magnitude of radiative forcing estimated
to be attributed to tropospheric O3 contributes to the
relatively greater uncertainty associated with climate effects of
tropospheric O3 compared to such effects of the well mixed
greenhouse gases, such as carbon dioxide and methane (ISA, section
IS.6.2.2).
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\172\ Radiative forcing is a metric used to quantify the change
in balance between radiation coming into and going out of the
atmosphere caused by the presence of a particular substance (ISA,
Appendix 9, section 9.1.3.3).
\173\ Effects on temperature, precipitation, and related climate
variables were referred to as ``climate change'' or ``effects on
climate'' in the 2013 ISA (ISA, p. IS-82; 2013 ISA, pp. 1-14 and 10-
31).
---------------------------------------------------------------------------
Lastly, the evidence regarding tropospheric O3 and UV-B
shielding (shielding of ultraviolet radiation at wavelengths of 280 to
320 nanometers) was evaluated in the 2013 ISA and determined to be
inadequate to draw a causal conclusion (2013 ISA, section 10.5.2). The
current ISA concludes there to be no new evidence since the 2013 ISA
relevant to the question of UV-B shielding by tropospheric
O3 (ISA, IS.1.2.1 and Appendix 9, section 9.1.3.4).
b. Public Welfare Implications
The secondary standard is to ``specify a level of air quality the
attainment and maintenance of which in the judgment of the
Administrator . . . is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
such air pollutant in the ambient air'' (CAA, section 109(b)(2)). As
recognized in prior reviews of secondary standards, the secondary
standard is not meant to protect against all known or anticipated
O3-related welfare effects, but rather those that are judged
to be adverse to the public welfare, and a bright line determination of
adversity is not required in judging what is requisite (78 FR 3212,
January 15, 2013; 80 FR 65376, October 26, 2015; see also 73 FR 16496,
March 27, 2008). The significance of each type of welfare effect with
regard to potential effects on the public welfare depends on the type
and severity of effects, as well as the extent of such effects on the
affected environmental entity, and on the societal use of the affected
entity and the entity's significance to the public welfare. Such
factors have been considered in the context of judgments and
conclusions made in some prior reviews regarding public welfare
effects. For example, judgments regarding public welfare significance
in the last two O3 NAAQS decisions gave particular attention
to O3 effects in areas with special federal protections
(such as Class I areas), and lands set aside by states, tribes and
public interest groups to provide similar benefits to the public
welfare (73 FR 16496, March 27, 2008; 80 FR 65292, October 26,
2015).\174\ In the 2015 review, the EPA recognized the ``clear public
interest in and value of maintaining these areas in a condition that
does not impair their intended use and the fact that many of these
lands contain O3-sensitive species'' (73 FR 16496, March 27,
2008).
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\174\ For example, the fundamental purpose of parks in the
National Park System ``is to conserve the scenery, natural and
historic objects, and wild life in the System units and to provide
for the enjoyment of the scenery, natural and historic objects, and
wild life in such manner and by such means as will leave them
unimpaired for the enjoyment of future generations'' (54 U.S.C.
100101). Additionally, the Wilderness Act of 1964 defines designated
``wilderness areas'' in part as areas ``protected and managed so as
to preserve [their] natural conditions'' and requires that these
areas ``shall be administered for the use and enjoyment of the
American people in such manner as will leave them unimpaired for
future use and enjoyment as wilderness, and so as to provide for the
protection of these areas, [and] the preservation of their
wilderness character . . .'' (16 U.S.C. 1131(a) and (c)). Other
lands that benefit the public welfare include national forests which
are managed for multiple uses including sustained yield management
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3);
16 U.S.C. 1601(d)(1)).
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Judgments regarding effects on the public welfare can depend on the
intended use for, or service (and value) of, the affected vegetation,
ecological receptors, ecosystems and resources and the importance of
that use to the public welfare (73 FR 16496, March 27, 2008; 80 FR
65377, October 26, 2015). Uses or services provided by areas that have
been afforded special protection can flow in part or entirely from the
vegetation that grows there. Ecosystem services range from those
directly related to the natural functioning of the ecosystem to
ecosystem uses for human recreation or profit, such as through the
production of lumber or fuel (Costanza et al., 2017; ISA, section
IS.5.1). Services of aesthetic value and outdoor recreation depend, at
least in part, on
[[Page 87312]]
the perceived scenic beauty of the environment. Additionally, public
surveys have indicated that Americans rank as very important the
existence of resources, the option or availability of the resource and
the ability to bequest or pass it on to future generations (Cordell et
al., 2008).
The different types of O3 effects on vegetation
recognized in section III.A.2.a above differ with regard to aspects
important to judging their public welfare significance. For example, in
the case of effects on crop yield, such judgments may consider aspects
such as the heavy management of agriculture in the U.S., while
judgments for other categories of effects may generally relate to
considerations regarding natural areas, including specifically those
areas that are not managed for harvest. In this context, it may be
important to consider that O3 effects on tree growth and
reproduction could, depending on severity, extent and other factors,
lead to effects on a larger scale including reduced productivity,
altered forest and forest community (plant, insect and microbe)
composition, reduced carbon storage and altered ecosystem water cycling
(ISA, section IS.5.1.8.1; 2013 ISA, Figure 9-1, sections 9.4.1.1 and
9.4.1.2). For example, the composition of vegetation or of terrestrial
community composition can be affected through O3 effects on
growth and reproductive success of sensitive species in the community,
with the extent of compositional changes dependent on factors such as
competitive interactions (ISA, section IS.5.1.8.1; 2013 ISA, sections
9.4.3 and 9.4.3.1). Impacts on some of these characteristics (e.g.,
forest or forest community composition) may be considered of greater
public welfare significance when occurring in Class I or other
protected areas, due to value for particular services that the public
places on such areas.
Agriculture and silviculture provide ecosystem services with clear
public welfare benefits. With regard to agriculture-related effects of
O3, however, there are complexities in this consideration
related to areas and plant species that are heavily managed to obtain a
particular output (such as commodity crops or commercial timber
production). In light of this, the degree to which O3
impacts on agriculturally important vegetation would impair the
intended use at a level that might be judged adverse to the public
welfare has been less clear (80 FR 65379, October 26, 2015; 73 FR
16497, March 27, 2008). While having sufficient crop yields is of high
public welfare value, important commodity crops are typically heavily
managed to produce optimum yields. Moreover, based on the economic
theory of supply and demand, increases in crop yields would be expected
to result in lower prices for affected crops and their associated
goods, which would primarily benefit consumers. Analyses in past
reviews have described how these competing impacts on producers and
consumers complicate consideration of these effects in terms of
potential adversity to the public welfare (2014 WREA, sections 5.3.2
and 5.7).
Other ecosystem services valued by people that can be affected by
reduced tree growth, productivity and associated forest effects include
aesthetic value; provision of food, fiber, timber, other forest
products, habitat, and recreational opportunities; climate and water
regulation; erosion control; air pollution removal, and desired fire
regimes (PA, Figure 4-2; ISA, section IS.5.1; 2013 ISA, sections
9.4.1.1 and 9.4.1.2). In considering such services in past reviews, the
Agency has given particular attention to effects in natural ecosystems,
indicating that a protective standard, based on consideration of
effects in natural ecosystems in areas afforded special protection,
would also ``provide a level of protection for other vegetation that is
used by the public and potentially affected by O3 including
timber, produce grown for consumption and horticultural plants used for
landscaping'' (80 FR 65403, October 26, 2015). For example, locations
potentially vulnerable to O3-related impacts might include
forested lands, both public and private, where trees are grown for
timber production. Forests in urbanized areas also provide a number of
services that are important to the public in those areas, such as air
pollution removal, cooling, and beautification. There are also many
other tree species, such as various ornamental and agricultural species
(e.g., Christmas trees, fruit and nut trees), that provide ecosystem
services that may be judged important to the public welfare.
With its effect on the physical appearance of plants, visible
foliar injury has the potential to be significant to the public
welfare, depending on its severity and spatial extent, by impacting
aesthetic or scenic values and outdoor recreation in Class I and other
similarly protected areas valued by the public. To assess evidence of
injury to plants in forested areas on national and regional scales, the
U.S. Forest Service (USFS) conducted surveys of the occurrence and
severity of visible foliar injury on sensitive (bioindicator) species
at biomonitoring sites across most of the U.S., beginning in 1994 (in
eastern U.S.) and extending through 2011 (Smith et al., 2003; Coulston
et al., 2003). At these sites (biosites), a national protocol,
including verification and quality assurance procedures and a scoring
system, was implemented. The resultant biosite index (BI) scores may be
described with regard to one of several categories ranging from little
or no foliar injury to severe injury (e.g., Smith et al., 2003;
Campbell et al., 2007; Smith et al., 2007; Smith, 2012).\175\ However,
the available information does not yet address or describe the
relationships expected to exist between some level of injury severity
(e.g., little, low/light, moderate or severe) and/or spatial extent
affected and scenic or aesthetic values. This gap impedes consideration
of the public welfare implications of different injury severities, and
accordingly judgments on the potential for public welfare significance.
That notwithstanding, while minor spotting on a few leaves of a plant
may easily be concluded to be of little public welfare significance,
some level of severity and widespread occurrence of visible foliar
injury, particularly if occurring in specially protected areas, where
the public can be expected to place value (e.g., for recreational
uses), might reasonably be concluded to impact the public welfare.
---------------------------------------------------------------------------
\175\ Authors of studies presenting USFS biomonitoring program
data have suggested what might be ``assumptions of risk'' (e.g., for
the forest resource) related to scores in these categories, e.g.,
none, low, moderate and high for BI scores of zero to five, five to
15, 15 to 25 and above 25, respectively (e.g., Smith et al., 2003;
Smith et al., 2012. For example, maps of localized moderate to high
risk areas may be used to identify areas where more detailed
evaluations are warranted (Smith et al., 2012).
---------------------------------------------------------------------------
The tropospheric O3-related effects of radiative forcing
and subsequent effects on temperature, precipitation and related
climate also have important public welfare implications, although their
quantitative evaluation in response to O3 concentrations in
the U.S. is complicated by ``[c]urrent limitations in climate modeling
tools, variation across models, and the need for more comprehensive
observational data on these effects'' (ISA, section IS.6.2.2). An
ecosystem service provided by forested lands is carbon sequestration or
storage (ISA, section IS.5.1.4 and Appendix 8, section 8.8.3; 2013 ISA,
section 2.6.2.1 and p. 9-37) \176\, which has an extremely valuable
role in counteracting the impact of greenhouse gases on radiative
forcing and related climate effects on the public welfare. Accordingly,
the
[[Page 87313]]
service of carbon storage can be of paramount importance to the public
welfare no matter in what location the trees are growing or what their
intended current or future use (e.g., 2013 ISA, section 9.4.1.2). This
benefit exists as long as the trees are growing, regardless of what
additional functions and services it provides.
---------------------------------------------------------------------------
\176\ While carbon sequestration or storage also occurs for
vegetated ecosystems other than forests, it is relatively larger in
forests given the relatively greater biomass for trees compared to
other plants.
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Categories of effects newly identified as likely causally related
to O3 in ambient air, such as alteration of plant-insect
signaling and insect herbivore growth and reproduction, also have
potential public welfare implications (e.g., given the role of the
plant-insect signaling process in pollination and seed dispersal).
Uncertainties and limitations in the current evidence (e.g., summarized
in sections III.B.3.c and III.D.1 of the proposal) preclude an
assessment of the extent and magnitude of O3 effects on
these endpoints, which thus also precludes an evaluation of the
potential for associated public welfare implications.
In summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of welfare
effects of different O3 exposure conditions. These include
uncertainties and limitations associated with the magnitude of key
welfare effects that might be concluded to be adverse to ecosystems and
associated services. Additionally, the presence of O3-
sensitive tree species may contribute to a vulnerability of numerous
locations to public welfare impacts from O3 related to tree
growth, productivity and carbon storage and their associated ecosystems
and services. Other important considerations include the exposure
circumstances that may elicit effects and the potential for the
significance of the effects to vary in specific situations due to
differences in sensitivity of the exposed species, the severity and
associated significance of the observed or predicted O3-
induced effect, the role that the species plays in the ecosystem, the
intended use of the affected species and its associated ecosystem and
services, the presence of other co-occurring predisposing or mitigating
factors, and associated uncertainties and limitations.
c. Exposures Associated With Effects
The welfare effects identified in section III.A.2.a above vary
widely with regard to the extent and level of detail of the available
information that describes the O3 exposure circumstances
that may elicit the effects. The information on exposure metric and E-R
relationships for effects related to vegetation growth is long-
standing, having been first described in the 1997 review, while such
information is much less established for visible foliar injury. The
evidence base for other categories of effects is also lacking in
information that might support characterization of potential impacts of
changes in O3 concentrations.
(i) Growth-Related Effects
The long-standing body of vegetation effects evidence includes a
wealth of information on aspects of O3 exposure that
influence its effects on plant growth and yield, and that has been
described in the scientific assessments across the last several decades
(1996 AQCD; 2006 AQCD; 2013 ISA; 2020 ISA). A variety of factors have
been investigated, and a number of mathematical approaches have been
developed for summarizing O3 exposure for the purpose of
assessing effects on vegetation, including several that cumulate
exposures over some specified period while weighting higher more than
lower concentrations (2013 ISA, sections 9.5.2 and 9.5.3; ISA, Appendix
8, section 8.2.2.2). Over this period, the EPA's scientific assessments
have focused on the use of a cumulative, seasonal \177\ concentration-
weighted index when considering the growth-related effects evidence and
when analyzing exposures for purposes of reaching conclusions on the
secondary standard. Such metrics have included SUM06,\178\ in the past,
and more recently (since the 2008 review), the focus has been on the
W126-based, seasonal metric, termed the ``W126 index'' \179\ (ISA,
section IS.3.2, Appendix 8, sections 8.1 and 8.13).
---------------------------------------------------------------------------
\177\ The ``seasonal'' descriptor refers to the duration of the
period quantified (3 months) rather than a specific season of the
year.
\178\ The SUM06 index received attention across past
O3 NAAQS reviews. It is the seasonal sum of hourly
concentrations at or above 0.06 ppm during a specified daily time
window (2006 AQCD, p. AX9-161; 2013 ISA, section 9.5.2).
\179\ The W126 index is described in section III.B.3.a(i) of the
proposal (85 FR 49887, August 14, 2020) and in the PA (PA, Appendix
4D, section 4D.2.2).
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Quantifying exposure using cumulative, concentration-weighted
indices of exposure, such as the W126 index, has been found to improve
the explanatory power of E-R models for growth and yield over using
indices based only on mean and peak exposure values (ISA, section
IS.5.1.9, p. IS-79; 2013 ISA, section 2.6.6.1, p. 2-44). The most well-
analyzed datasets in such evaluations are two detailed datasets
established two decades ago, one for seedlings of 11 tree species \180\
and one for 10 crops (e.g., Lee and Hogsett, 1996, Hogsett et al.,
1997). These datasets, which include species-specific seedling growth
and crop yield response information across multiple seasonal cumulative
exposures, were used to develop robust quantitative E-R functions to
predict growth reduction relative to a zero-O3 setting (RBL)
in seedlings of the tree species \181\ and similarly, E-R functions for
predicting RYL for a set of 10 common crops (ISA, Appendix 8, section
8.13.2; 2013 ISA, section 9.6.2).
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\180\ In total, the 11 species-specific composite E-R functions
are based on 51 tree seedling studies or experiments, many of which
employed open top chambers, an established experimental approach
(PA, Appendix 4A, section 4A.1.1; ISA, section 8.1.2.1.2). For six
of the 11 species, this function is based on just one or two
studies, while for other species there were as many as 11 studies
available.
\181\ While the 11 species represent only a small fraction of
the total number of native tree species in the contiguous U.S., this
subset includes eastern and western species, deciduous and
coniferous species, and species that grow in a variety of ecosystems
and represent a range of tolerance to O3 (PA, Appendix
4B; 2013 ISA, section 9.6.2).
---------------------------------------------------------------------------
The tree seedling E-R functions were derived from data for multiple
studies documenting effects on tree seedling growth under a variety of
O3 exposures \182\ and growing conditions. Importantly the
data included hourly concentrations recorded across the duration of the
exposure, which allowed for derivation of various metrics that were
analyzed for association with reduced growth (2013 ISA, section 9.6.2;
Lee and Hogsett, 1996). In producing E-R functions of consistent
duration across the experiments, the E-R functions were derived first
based on the exposure duration of the experiment \183\ and then
normalized to 3-month (seasonal) periods \184\ (see Lee and Hogsett,
1996, section I.3; PA, Appendix 4A). The species-specific composite E-R
functions developed from the experiment-specific functions indicate the
wide variation in growth
[[Page 87314]]
sensitivity of the studied tree species at the seedling stage (PA,
Appendix 4A, section 4A.1.1).
---------------------------------------------------------------------------
\182\ Across the experiments for the 11 tree species, the
exposure levels assessed are more extensive for relatively higher
seasonal exposures (e.g., at/above a SUM06 of 30 ppm-hrs). Across
these experiments, there is more limited representation of lower
cumulative exposure levels, such as SUM06 values below those that
may correspond to a W126 index of 20 ppm-hrs. These lowest levels
did not always yield a statistically significant effect (PA, section
4.5.1.2 and Appendix 4A; 85 FR 49901, August 14, 2020).
\183\ The exposure durations varied from periods of 82 to 140
days over a single year to periods of 180 to 555 days across two
years (Lee and Hogsett, 1996; PA, Appendix 4A, Table 4A-5).
\184\ Underlying the adjustment is a simplifying assumption of
uniform W126 distribution across the exposure periods and of a
linear relationship between duration of cumulative exposure in terms
of the W126 index and plant growth response (85 FR 49901; August 14,
2020; PA). Some functions for experiments that extended over two
seasons were derived by distributing responses observed at the end
of two seasons of varying exposures equally across the two seasons
(e.g., essentially applying the average to both seasons).
---------------------------------------------------------------------------
Since the initial set of tree seedling growth studies were
completed, several additional studies, focused on aspen, have been
published based on the Aspen FACE experiment in a planted forest in
Wisconsin; the findings were consistent with earlier open top chamber
(OTC) studies \185\ (ISA, Appendix 8, section 8.13.2). Newly available
studies that investigated growth effects of O3 exposures are
also consistent with the existing evidence base, and generally involve
particular aspects of the effect rather than expanding the conditions
under which plant species, particularly tree species, have been
assessed (ISA, section IS.5.1.2). These publications include a
compilation of previously available studies on plant biomass response
to O3; the compilation reports linear regressions conducted
on the associated varying datasets. Based on these regressions, this
study describes distributions of sensitivity to O3 effects
on biomass across many tree and grassland species, including 17 species
native to the U.S. and 65 introduced species (ISA, Appendix 8, section
8.13.2; van Goethem et al., 2013). Additional information is needed to
more completely describe O3 exposure response relationships
for these species in the U.S.\186\
---------------------------------------------------------------------------
\185\ These studies included experiments that used OTCs to
investigate tree seedling growth response and crop yield over a
growing season under a variety of O3 exposures and
growing conditions (2013 ISA, section 9.6.2; Lee and Hogsett, 1996).
\186\ The studies compiled in this publication included at least
21 days exposure above 40 ppb O3 (expressed as AOT40
[seasonal sum of the difference between an hourly concentration
above 40 ppb and 40 ppb]); and had a maximum hourly concentration
that was no higher than 100 ppb (van Goethem et al., 2013). The
publication does not report study-specific exposure durations,
details of biomass response measurements or hourly O3
concentrations, making it less useful for describing E-R
relationships that might support estimation of specific impacts
associated with air quality conditions meeting the current standard
(e.g., 2013 ISA, p. 9-118).
---------------------------------------------------------------------------
(ii) Visible Foliar Injury
Current evidence ``continues to show a consistent association
between visible injury and ozone exposure,'' while also recognizing the
role of modifying factors such as soil moisture and time of day (ISA,
section IS.5.1.1). The ISA summarizes several recently available
studies that continue to document that O3 elicits visible
foliar injury in many plant species. As in the prior review, the
evidence in the current review, while documenting that elevated
O3 conditions in ambient air generally results in visible
foliar injury in sensitive species (when in a predisposing
environment),\187\ does not include a quantitative description of the
relationship of incidence or severity of visible foliar injury in
natural areas of the U.S. with specific metrics of seasonal
O3 exposure.
---------------------------------------------------------------------------
\187\ As a major modifying factor is the amount of soil moisture
available to a plant, dry periods decrease the incidence and
severity of ozone-induced visible foliar injury, such that the
incidence of visible foliar injury is not always higher in years and
areas with higher ozone, especially with co-occurring drought (ISA,
Appendix 8, p. 8-23; Smith, 2012; Smith et al., 2003).
---------------------------------------------------------------------------
Although studies of the incidence of visible foliar injury in
national forests, wildlife refuges, and similar areas have often used
cumulative indices (e.g., SUM06) to investigate variations in incidence
of foliar injury, studies also suggest an additional role for metrics
focused on peak concentrations (ISA; 2013 ISA; 2006 AQCD; Hildebrand et
al., 1996; Smith, 2012). Other studies have indicated this uncertainty
regarding the influential metric(s), e.g., by recognizing the need for
research to help develop a ``better linkage between air levels and
visible injury'' (Campbell et al., 2007).\188\ Some studies of visible
foliar injury incidence data have investigated such a role for peak
concentrations quantified by an O3 exposure index that is a
count of hourly concentrations (e.g., in a growing season) above a
threshold concentration of 100 ppb, N100 (e.g., Smith, 2012; Smith et
al., 2012). For example, a study describing injury patterns over 16
years at USFS biosites in 24 states in the Northeast and North Central
regions, in the context of the SUM06 index and N100 metrics, suggested
that there may be a threshold exposure needed for injury to occur, and
that the number of hours of elevated O3 concentrations
during the growing season (such as what is captured by a metric like
N100) may be more important than cumulative exposure in determining the
occurrence of foliar injury (Smith, 2012).\189\ This finding is
consistent with statistical analyses of seven years of visible foliar
injury data from a wildlife refuge in the mid-Atlantic area (Davis and
Orendovici, 2006).\190\
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\188\ In considering their findings, the authors expressed the
view that ``[a]lthough the number of sites or species with injury is
informative, the average biosite injury index (which takes into
account both severity and amount of injury on multiple species at a
site) provides a more meaningful measure of injury'' for their
assessment at a statewide scale (Campbell et al., 2007).
\189\ Although the ISA and past assessments have not described
extensive evaluations of specific peak concentration metrics such as
the N100, in summarizing this study in the last review, the ISA
observed that ``[o]verall, there was a declining trend in the
incidence of foliar injury as peak O3 concentrations declined''
(2013 ISA, p. 9-40).
\190\ The models evaluated included several with cumulative
exposure indices alone. These included SUM60 (i.e., SUM06 in ppb),
SUM0, and SUM80 (SUM08 in ppb), but not W126. They did not include a
model with W126 that did not also include N100. Across all of the
models evaluated, the model with the best fit to the data was found
to be the one that included N100 and W126, along with the drought
index (Davis and Orendovici, 2006).
---------------------------------------------------------------------------
The established significant role of higher or peak O3
concentrations, as well as pattern of their occurrence, in plant
responses has also been noted in prior ISAs or AQCDs. The evidence has
included studies that use indices to summarize the incidence of injury
on bioindicator species present at specific monitored sites, as well as
experimental studies that assess varying O3 treatments on
cultured stands of different tree species (2013 ISA, section 9.5.3.1;
2006 AQCD, p. AX9-169; Oksanen and Holopainen, 2001; Yun and Laurence,
1999). In identifying support for such O3 metrics with
regard to foliar injury as the response, the 2013 ISA and 2006 AQCD
both cite studies that support the ``important role that peak
concentrations, as well as the pattern of occurrence, plays in plant
response to O3'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-
169).
A recent study (by Wang et al. [2012]) involved a statistical
modeling analysis on a subset of the years of USFS BI data that were
described in Smith (2012). This analysis tested a number of models for
their ability to predict the presence of visible foliar injury (a
nonzero biosite score), regardless of severity, and generally found
that the type of O3 exposure metric (e.g., SUM06 versus
N100) made only a small difference, although the models that included
both a cumulative index (SUM06) and N100 had a just slightly better fit
(Wang et al., 2012). Based on their investigation of 15 different
models, using differing combinations of several types of potential
predictors, the study authors concluded that they were not able to
identify environmental conditions under which they ``could reliably
expect plants to be damaged'' (Wang et al., 2012). This is indicative
of the current state of knowledge, in which there remains a lack of
established quantitative functions describing E-R relationships that
would allow prediction of visible foliar injury severity and incidence
under varying air quality and other environmental conditions.
The information related to O3 exposures associated with
visible foliar injury of varying severity available in this review also
includes quantitative
[[Page 87315]]
presentations of the dataset (developed by the EPA in the last review)
of USFS BI scores, collected during the years 2006 through 2010 at
locations in 37 states. In developing this dataset, the BI scores were
combined with estimates of soil moisture \191\ and estimates of
seasonal cumulative O3 exposure in terms of W126 index \192\
(PA, Appendix 4C). This dataset includes more than 5,000 records of
which more than 80 percent have a BI score of zero (indicating a lack
of visible foliar injury). While the estimated W126 index assigned to
records in this dataset ranges from zero to somewhat above 50 ppm-hrs,
more than a third of all the records (and also of records with BI
scores above zero or five) \193\ are at sites with W126 index estimates
below 7 ppm-hrs. In an extension of analyses developed in the last
review, the presentation in the PA \194\ describes the BI scores for
the records in this dataset in relation to the W126 index estimate for
each record, using ``bins'' of increasing W126 index values. The PA
presentation utilizes the BI score breakpoints in the scheme used by
the USFS to categorize severity. This presentation indicates that,
across the W126 bins, there is variation in both the incidence of
particular magnitude BI scores and in the average score per bin. In
general, however, the greatest incidence of records with BI scores
above zero, five, or higher--and the highest average BI score--occurs
with the highest W126 bin, i.e., the bin for W126 index estimates
greater than 25 ppm-hrs (PA, Appendix 4C, Table 4C-6).
---------------------------------------------------------------------------
\191\ This dataset, including associated uncertainties and
limitations in the assignment of soil moisture categories (dry, wet
or normal), such as the substantial spatial variation in soil
moisture and large size of NOAA climate divisions, is described in
the PA, Appendix 4C.
\192\ The W126 index estimates assigned to the biosite locations
were developed for 12 kilometer (km) by 12 km cells in a national-
scale spatial grid for each year. A spatial interpolation technique
was applied to annual W126 values derived from O3
measurements at ambient air monitoring locations for the years of
the BI data (PA, Appendix 4C, sections 4.C.2 and 4C.5).
\193\ One third (33%) of scores above 15 are at sites with W126
below 7 ppm-hrs (PA, Appendix 4C, Table 4C-3).
\194\ Beyond the presentation of a statistical analysis
developed in the last review, the PA presentations are primarily
descriptive (as compared to statistical) in recognition of the
limitations and uncertainties of the dataset (PA, Appendix 4C,
section 4C.5).
---------------------------------------------------------------------------
Overall, the dataset described in the PA generally indicates the
risk of injury, and particularly injury considered at least light,
moderate or severe, to be higher at the highest W126 index values, with
appreciable variability in the data for the lower bins (PA, Appendix
4C). This appears to be consistent with the conclusions of the detailed
quantitative analysis studies, summarized above, that the pattern is
stronger at higher O3 concentrations. A number of factors
may contribute to the observed variability in BI scores and lack of a
clear pattern with W126 index bin; among other factors, these may
include uncertainties in assignment of W126 estimates and soil moisture
categories to biosite locations, variability in biological response
among the sensitive species monitored, and the potential role of other
aspects of O3 air quality not captured by the W126 index.
Thus, the dataset has limitations affecting associated conclusions, and
uncertainty remains regarding the tools for and the appropriate metric
(or metrics) for quantifying O3 exposures, as well as
perhaps for quantifying soil moisture conditions, with regard to their
influence on extent and/or severity of injury in sensitive species in
natural areas, as quantified via BI scores (Davis and Orendovici, 2006,
Smith et al., 2012; Wang et al., 2012).
(iii) Other Effects
With regard to radiative forcing and subsequent climate effects
associated with the global tropospheric abundance of O3, the
newly available evidence in this review does not provide more detailed
quantitative information regarding response to O3
concentrations at the national scale. Rather, it is noted that ``the
heterogeneous distribution of ozone in the troposphere complicates the
direct attribution of spatial patterns of temperature change to ozone
induced [radiative forcing]'' and there are ``ozone climate feedbacks
that further alter the relationship between ozone [radiative forcing]
and temperature (and other climate variables) in complex ways'' (ISA,
Appendix 9, section 9.3.1, p. 9-19). Further, ``precisely quantifying
the change in surface temperature (and other climate variables) due to
tropospheric ozone changes requires complex climate simulations that
include all relevant feedbacks and interactions'' (ISA, section 9.3.3,
p. 9-22). Yet, there are limitations in current climate modeling
capabilities for O3; an important one is representation of
important urban- or regional-scale physical and chemical processes,
such as O3 enhancement in high-temperature urban situations
or O3 chemistry in city centers where NOX is
abundant. Such limitations impede our ability to quantify the impact of
incremental changes in O3 concentrations in the U.S. on
radiative forcing and subsequent climate effects.
With regard to tree mortality, the evidence available in the last
several reviews included field studies of pollution gradients that
concluded O3 damage to be an important contributor to tree
mortality although ``several confounding factors such as drought,
insect outbreak and forest management'' were identified as potential
contributors (2013 ISA, p. 9-81, section 9.4.7.1). Among the newly
available studies, there is only limited experimental evidence that
isolates the effect of O3 on tree mortality \195\ and might
be informative regarding O3 concentrations of interest in
the review, and evidence is lacking regarding exposure conditions
closer to those occurring under the current standard and any
contribution to tree mortality.
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\195\ Of the three new studies on tree mortality described in
the ISA is another field study of a pollution gradient that, like
such studies in prior reviews, recognizes O3 exposures as
one of several contributing environmental and anthropogenic
stressors (ISA, p. 8-55).
---------------------------------------------------------------------------
With regard to alteration of herbivore growth and reproduction,
although ``[t]here are multiple studies demonstrating ozone effects on
fecundity and growth in insects that feed on ozone-exposed
vegetation'', ``no consistent directionality of response is observed
across studies and uncertainties remain in regard to different plant
consumption methods across species and the exposure conditions
associated with particular severities of effects'' (ISA, pp. ES-18).
The evidence for alteration of plant-insect signaling draws on new
research yielding clear evidence of O3 modification of
volatile plant signaling compounds and behavioral responses of insects
to the modified chemical signals (ISA, section IS.6.2.1). The evidence
includes a relatively small number of plant species and plant-insect
associations and is limited to short controlled exposures, posing
limitations for consideration of the potential for associated impacts
to be elicited by air quality conditions that meet the current standard
(ISA, section IS.6.2.1 and Appendix 8, section 8.7).
For categories of vegetation-related effects that were recognized
in past reviews, other than growth and visible foliar injury (e.g.,
reduced plant reproduction, reduced productivity in terrestrial
ecosystems, alteration of terrestrial community composition and
alteration of below-ground biogeochemical cycles), the newly available
evidence includes a variety of studies that quantify exposure of
varying duration in various countries using a variety of metrics (ISA,
Appendix 8, sections 8.4, 8.8 and 8.10).
[[Page 87316]]
The ISA also describes publications that analyze and summarize
previously published studies. For example, a meta-analysis of
reproduction studies categorized the reported O3 exposures
into bins of differing magnitude, grouping differing concentration
metrics and exposure durations together, and performed statistical
analyses investigating associations with an O3-related
effect (ISA, Appendix 8, section 8.4.1). While such studies continue to
support conclusions of O3 ecological hazards, they do not
improve capabilities for characterizing the likelihood of such effects
under patterns of environmental O3 concentrations occuring
with air quality conditions that meet the current standard (e.g.,
factors such as variation in exposure assessments and limitations in
response information preclude detailed analysis for such conditions),
as discussed further in the PA.
As at the time of the last review, growth impacts, most
specifically as evaluated by RBL for tree seedlings and RYL for crops,
remain the type of vegetation-related effects for which we have the
best understanding of exposure conditions likely to elicit them.
Accordingly, as was the case in the last review, the quantitative
analyses of exposures occurring under air quality that meets the
current standard, summarized below, are focused primarily on the W126
index, given its established relationship with growth effects.
3. Overview of Air Quality and Exposure Information
The air quality and exposure analyses developed in this review,
like those in the last review, are of two types: (1) W126-based
cumulative exposure estimates in Class I areas; and (2) analyses of
W126-based exposures and their relationship with the current standard
for all U.S. monitoring locations (PA, Appendix 4D). We recognize
relatively lower uncertainty associated with the use of these types of
analyses (compared to the national or regional-scale modeling analyses
performed in the last review) to inform a characterization of
cumulative O3 exposure (in terms of the W126 index)
associated with air quality just meeting the current standard (IRP,
section 5.2.2). As in the last review, the lower uncertainty of these
air quality monitoring-based analyses contributes to their value in
informing the current review.
The analyses conducted in this review focus on design values (3-
year average annual fourth-highest 8-hour daily maximum concentration,
also termed the ``4th max metric'') and W126 index values (in terms of
the 3-year average) for the recent 2016 to 2018 period and across the
historical record back to 2000 (PA, Section 4.3). These analyses are
based primarily on the hourly air monitoring data that were reported to
EPA from O3 monitoring sites nationwide and in or near Class
1 areas.\196\
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\196\ Across the seventeen 3-year periods from 2000-2002 to
2016-2018, the number of monitoring sites with sufficient data for
calculation of valid design values and W126 index values (across the
3-year design value period) ranged from a low of 992 in 2000-2002 to
a high of 1119 in 2015-2017 (PA, Section 4.3).
---------------------------------------------------------------------------
a. Influence of Form and Averaging Time of Current Standard on
Environmental Exposure
The findings of the quantitative analyses in this review of
relationships between air quality in terms of the form and averaging
time of the current standard and environmental exposures in terms of
the W126 index are similar to those based on the data available during
the last review (PA, Appendix 4D, section 4D.2.2).\197\ As previously,
the current analysis of data spanning 19 years and including seventeen
3-year periods documented a positive nonlinear relationship between
cumulative seasonal exposure (quantified using the W126 index) and
design values (based on the form and averaging time of the current
standard). In the current analysis, which revealed the variability in
the annual W126 index values across a 3-year period to be relatively
low,\198\ the positive nonlinear relationship is shown for both the
average W126 index across the 3-year design value period and for W126
index values for individual years within the period (PA, Figure 4-7;
Appendix 4D, section 4D.3.1.2). That is, W126 index values (in a single
year or averaged across years) are lower at monitoring sites with lower
design values. This is seen both for design values above the standard
and across lower design values, indicating the effectiveness of the
averaging time and form of the current standard at controlling W126-
based cumulative exposures.
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\197\ In 2015 the Administrator concluded that, with revision of
the standard level, the existing form and averaging time provided
the control of cumulative seasonal exposure circumstances needed for
the public welfare protection desired (80 FR 65408, October 26,
2015).
\198\ This evaluation, performed for all U.S. monitoring sites
with sufficient data available in the most recent 3-year period,
2016 to 2018, indicates the extent to which the three single-year
W126 index values within a 3-year period deviate from the average
for the period. Across the full set of sites, regardless of W126
index magnitude (or whether or not the current standard is met),
single-year W126 index values differ less than 15 ppm-hrs from the
average for the 3-year period (PA, Appendix 4D, Figure 4D-6). For
the approximately 850 sites meeting the current standard, over 99%
of single-year W126 index values differ from the 3-year average by
no more than 5 ppm-hrs, and 87% by no more than 2 ppm-hrs (PA,
Appendix 4D, Figure 4D-7).
---------------------------------------------------------------------------
Further, analysis of the relationship between trends or long-term
changes in design value and long-term changes in W126 index shows there
to be a positive, linear relationship at monitoring sites across the
U.S. (PA, Appendix 4D, section 4D.3.2.3). The existence of this
relationship means that a change in the design value at a monitoring
site was generally accompanied by a similar change in the W126 index.
The relationship varies across the NOAA climate regions, with the
greatest change in W126 index per unit change in design value observed
in the Southwest and West regions, the regions which had the highest
W126 index values at sites meeting the current standard (PA, Appendix
4D, Figures 4D-6 and 4D-14, Table 4D-12). Thus, this analysis indicates
that going forward, as design values are reduced in areas that are
presently not meeting the current standard, the W126 index in those
areas would also be expected to decline and the greatest improvement in
W126 index per unit decrease in design value would be expected in the
Southwest and West regions (PA, Appendix 4D, section 4D.3.2.3 and
4D.5). The overall trend showing reductions in the W126 index
concurrent with reductions in the design value metric for the current
standard is positive whether the W126 index is expressed in terms of
the average across the 3-year design value period or the annual value
(PA, Appendix 4D, section 4D.3.2.3).
The available air quality information also indicates that the
current standard's form and averaging time exerts control on other
vegetation exposures of potential concern, such as days with
particularly high O3 concentrations that may contribute to
visible foliar injury. The current form and averaging time, by their
very definition, limit occurrences of such concentrations. This is
demonstrated by reductions in daily maximum 8-hour concentrations, as
well as in the frequency of elevated 1-hour concentrations, including
concentrations at or above 100 ppb, with decreasing design values (PA,
Figure 2-11, Appendix 2A, section 2A.2). As the form and averaging time
of the secondary standard have not changed since 1997, the analyses
have been able to assess the amount of control exerted by these aspects
of the standard, in combination with reductions in the standard level
(i.e.,
[[Page 87317]]
from 0.08 ppm in 1997 to 0.075 ppm in 2008 to 0.070 ppm in 2015), on
cumulative seasonal exposures in terms of W126 index, and on the
magnitude of short-term peak concentrations. These analyses indicate
that the long-term reductions in the design values, presumably
associated with implementation of the revised standards, were
accompanied by reductions in W126 index, as well as in short-term peak
concentrations.
b. Environmental Exposures in Terms of W126 Index
The analyses summarized here describe the nature and magnitude of
vegetation exposures associated with conditions meeting the current
standard at sites across the U.S., particularly in specially protected
areas, such as Class I areas. Given the evidence indicating the W126
index to be strongly related to growth effects and its use in the E-R
functions for tree seedling RBL (as summarized in section III.A.2.c
above), exposure is quantified using the W126 metric. These analyses
include a particular focus on monitoring sites in or near Class I
areas,\199\ in light of the greater public welfare significance of many
O3 related impacts in such areas, as described in section
III.A.2.b above, and consider both recent air quality (2016-2018) and
the air quality record since 2000 (PA, Appendix 4D). As was the case in
the last review, the currently available quantitative information
continues to indicate appreciable control of seasonal W126 index-based
cumulative exposure at all sites with air quality meeting the current
standard.
---------------------------------------------------------------------------
\199\ This includes monitors sited within Class I areas or the
closest monitoring site within 15 km of the area boundary.
---------------------------------------------------------------------------
Among sites nationwide meeting the current standard in the recent
period of 2016 to 2018, there are none with a W126 index, based on the
3-year average, above 19 ppm-hrs, and just one with such a value above
17 ppm-hrs (Table 4).\200\ Additionally, the full historical dataset
includes no occurrences of a 3-year average W126 index above 19 ppm-hrs
for sites meeting the current standard, and just eight occurrences of a
W126 index above 17 ppm-hrs (less than 0.1% of the dataset), with the
highest such occurrence just equaling 19 ppm-hrs (Table 4; PA, Appendix
4D, section 4D.3.2.1).
---------------------------------------------------------------------------
\200\ Rounding conventions are described in detail in the PA,
Appendix 4D, section 4D.2.2.
---------------------------------------------------------------------------
With regard to Class I areas, the updated air quality analyses
include data from sites in or near 65 Class I areas. The findings for
these sites, which are distributed across all nine NOAA climate regions
in the contiguous U.S., as well as Alaska and Hawaii, mirror the
findings for the analysis of all U.S. sites in the dataset. Among the
Class I area sites meeting the current standard (i.e., having a design
value at or below 70 ppb) in the most recent period of 2016 to 2018,
there are none with a W126 index (averaged over the design value
period) above 17 ppm-hrs (Table 4). The historical dataset includes
just seven occurrences (all dating from the 2000-2010 period) of a
Class I area site meeting the current standard and having a 3-year
average W126 index above 17 ppm-hrs, and no such occurrences above 19
ppm-hrs (Table 4).
The W126 index values at sites that do not meet the current
standard are much higher, with values at such sites ranging as high as
approximately 60 ppm-hrs (PA, Appendix 4D, Figure 4D-3). Among all
sites across the U.S. that do not meet the current standard in the 2016
to 2018 period, more than a quarter have average W126 index values
above 19 ppm-hrs and a third exceed 17 ppm-hrs (Table 4). A similar
situation exists for Class I area sites (Table 4). For example, out of
the 11 Class I area sites with design values above 70 ppb during the
most recent period, eight sites had a 3-year average W126 index above
19 ppm-hrs (with a maximum value of 47 ppm-hrs) and for nine, it was
above 17 ppm-hrs (Table 4; PA, Appendix 4D, Table 4D-17).
Table 4--Distribution of 3-Yr Average Seasonal W126 Index for Sites in Class I Areas and Across U.S. That Meet the Current Standard and For Those That
Do Not
--------------------------------------------------------------------------------------------------------------------------------------------------------
Number of occurrences or site-DVs \A\
-------------------------------------------------------------------------------------------------------
In Class I areas Across all monitoring sites (urban and rural)
3-year periods -------------------------------------------------------------------------------------------------------
W126 (ppm-hrs) W126 (ppm-hrs)
Total --------------------------------------- Total --------------------------------------
>19 >17 <=17 >19 >17 <=17
--------------------------------------------------------------------------------------------------------------------------------------------------------
At sites that meet the current standard (design value at or below 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018....................................... 47 0 0 47 849 0 1 848
All from 2000 to 2018........................... 498 0 7 491 8,292 0 8 8,284
--------------------------------------------------------------------------------------------------------------------------------------------------------
At sites that exceed the current standard (design value above 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018....................................... 11 8 9 2 273 78 91 182
All from 2000 to 2018........................... 362 159 197 165 10,695 2,317 3,174 7,521
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Counts presented here are drawn from the PA, Appendix D, Tables 4D-1, 4D-4, 4D-5, 4D-6, 4D-9, 4D-10 and 4D-13 through 16.
B. Conclusions on the Secondary Standard
In drawing conclusions on the adequacy of the current secondary
standard, in view of the advances in scientific knowledge and
additional information now available, the Administrator has considered
the currently available welfare effects evidence and air quality and
ecological exposure information. He additionally has considered the
evidence base, information, and policy judgments that were the
foundation of the last review, to the extent they remain relevant in
light of the currently available information. The Administrator has
taken into account both evidence-based and air quality and exposure-
based considerations discussed in the PA, as well as advice from the
CASAC and
[[Page 87318]]
public comments. Evidence-based considerations draw upon the EPA's
assessment and integrated synthesis of the scientific evidence as
presented in the ISA, with a focus on policy-relevant considerations as
discussed in the PA (summarized in sections III.B and III.D.1 of the
proposal and section III.A.2 above). The air quality and exposure-based
considerations draw from the results of the quantitative air quality
analyses presented and considered in the PA (as summarized in section
III.C of the proposal and section III.A.3 above). The Administrator
additionally considered the August 2019 decision of the D.C. Circuit
remanding the 2015 secondary standard for further justification or
reconsideration.
The consideration of the evidence and air quality/exposure
information in the PA informed the Administrator's proposed conclusions
and judgments in this review, and his associated proposed decision.
Section III.B.1 below briefly summarizes the basis for the
Administrator's proposed decision, drawing from section III.D of the
proposal. Section III.B.1.a provides a brief overview of key aspects of
the policy evaluations presented in the PA, and the advice and
recommendations of the CASAC are summarized in III.B.1.b. An overview
of the Administrator's proposed conclusions is presented in section
III.B.1.c. Public comments on the proposed decision are addressed below
in sections III.B.2 and the Administrator's conclusions and decision in
this review regarding the adequacy of the current secondary standard
and whether any revisions are appropriate are described in section
III.B.3.
1. Basis for Proposed Decision
a. Policy-Relevant Evaluations in the PA
The main focus of the policy-relevant considerations in the PA is
consideration of the question: Does the currently available scientific
evidence- and air quality and environmental exposure-based information
support or call into question the adequacy of the protection afforded
by the current secondary O3 standard? The PA response to
this overarching question takes into account discussions that address
the specific policy-relevant questions for this review, focusing first
on consideration of the evidence, as evaluated in the ISA, including
that newly available in this review. The PA also considers the
quantitative information available in this review that relates
O3 environmental exposures to vegetation responses
(presented in Appendices 4A and 4C of the PA) and the air quality
analyses that investigate relationships between air quality that meets
the current standard and cumulative and peak exposures (presented in
detail in Appendix 4D of the PA). The PA additionally discusses the key
aspects of the evidence and exposure/risk estimates that were
emphasized in establishing the current standard, and key aspects of the
2019 court remand on the standard. In so doing, the PA also considers
associated public welfare policy judgments and judgments about the
uncertainties inherent in the scientific evidence and quantitative
analyses that are integral to the Administrator's consideration of
whether the currently available information supports or calls into
question the adequacy of the current secondary O3 standard
(PA, section 3.5).
Key policy-relevant considerations identified by the PA included
the following. The new information available is consistent with that
available in the last review for the principal effects for which the
evidence is strongest (e.g., growth, reproduction, and related larger-
scale effects, as well as, visible foliar injury) and for key aspects
of the decision in that review. The currently available information
does not provide established quantitative relationships and tools for
estimating incidence and severity of visible foliar injury in protected
areas across the U.S. or provide information linking extent and
severity of injury to aesthetic values that might be useful for
considering public welfare implications. Further, the currently
available evidence for forested locations across the U.S., such as
studies of USFS biosites, does not indicate widespread incidence of
significant visible foliar injury. Additionally, the evidence regarding
RBL and air quality in areas meeting the current standard does not
appear to call into question the adequacy of protection. For other
vegetation-related effects that the ISA newly concludes likely to be
causally related to O3, the new information does not provide
us an indication of the extent to which such effects might be
anticipated to occur in areas that meet the current standard of a
significance reasonably judged significant to public welfare. Thus, the
PA does not find the current information for these newly identified
categories to call into question the adequacy of the current standard.
Similarly, the current information regarding O3 contribution
to radiative forcing or effects on temperature, precipitation and
related climate variables is not strengthened from that available in
the last review, including with regard to uncertainties that limit
quantitative evaluations. Based on such considerations, discussed in
detail in the PA, it concludes that the currently available evidence
and quantitative exposure/risk information does not call into question
the adequacy of the current secondary standard such that it is
appropriate to consider retaining the current standard without
revision. In so doing, it recognized that, as is the case in NAAQS
reviews in general, the extent to which the Administrator judges the
current secondary O3 standard to be adequate will depend on
a variety of factors, including science policy judgments and public
welfare policy judgments.
b. CASAC Advice in This Review
In comments on the draft PA, the CASAC concurred with the PA
conclusions, stating that it ``finds, in agreement with the EPA, that
the available evidence does not reasonably call into question the
adequacy of the current secondary ozone standard and concurs that it
should be retained'' (Cox, 2020a, p. 1). The CASAC additionally stated
that it ``commends the EPA for the thorough discussion and rationale
for the secondary standard'' (Cox, 2020a, p. 2). The CASAC also
provided comments particular to the consideration of climate and
growth-related effects.
With regard to O3 effects on climate, the CASAC
recommended quantitative uncertainty and variability analyses, with
associated discussion (Cox, 2020a, p. 2 and Consensus Responses to
Charge Questions p. 22).\201\ With regard to growth-related effects and
consideration of the evidence in quantitative exposure analyses, it
stated that the W126 index ``appears reasonable and scientifically
sound,'' ``particularly [as] related to growth effects'' (Cox, 2020a,
Consensus Responses to Charge Questions p. 16). Additionally, with
regard to the prior Administrator's expression of greater confidence in
judgments related to public welfare impacts based on a seasonal W126
index estimated by a three-year average and accordingly relying on that
metric the CASAC expressed the view that this ``appears of reasonable
thought and scientifically
[[Page 87319]]
sound'' (Cox, 2020a, Consensus Responses to Charge Questions p. 19).
Further, the CASAC stated that ``RBL appears to be appropriately
considered as a surrogate for an array of adverse welfare effects and
based on consideration of ecosystem services and potential for impact
to the public as well as conceptual relationships between vegetation
growth effects and ecosystem scale effects'' and that it agrees ``that
biomass loss, as reported in RBL, is a scientifically-sound surrogate
of a variety of adverse effects that could be exerted to public
welfare,'' concurring that this approach is not called into question by
the current evidence which continues to support ``the use of tree
seedling RBL as a proxy for the broader array of vegetation related
effects, most particularly those related to growth that could be
impacted by ozone'' (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). The CASAC additionally concurred that the strategy of
a secondary standard that generally limits 3-year average W126 index
values somewhat below those associated with a 6% RBL in the median
species is ``scientifically reasonable'' and that, accordingly, a W126
index target value of 17 ppm-hrs for generally restricting cumulative
exposures ``is still effective in particularly protecting the public
welfare in light of vegetation impacts from ozone'' (Cox, 2020a,
Consensus Responses to Charge Questions p. 21).
---------------------------------------------------------------------------
\201\ As recognized in the ISA, ``[c]urrent limitations in
climate modeling tools, variation across models, and the need for
more comprehensive observational data on these effects represent
sources of uncertainty in quantifying the precise magnitude of
climate responses to ozone changes, particularly at regional
scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p. 9-
22). These complexities impede our ability to consider specific
O3 concentrations in the U.S. with regard to specific
magnitudes of impact on radiative forcing and subsequent climate
effects.
---------------------------------------------------------------------------
With regard to the court's remand of the 2015 secondary standard to
the EPA for further justification or reconsideration (``particularly in
relation to its decision to focus on a 3-year average for consideration
of the cumulative exposure, in terms of W126, identified as providing
requisite public welfare protection, and its decision to not identify a
specific level of air quality related to visible foliar injury''),
while the CASAC stated that it was not clear whether the draft PA had
fully addressed this concern (Cox, 2020a, Consensus Responses to Charge
Questions p. 21), it described there to be a solid scientific
foundation for the current secondary standard and also commented on
areas related to the remand. With regard to support in the information
available in the current review for the focus on the 3-year average
W126 index in assessing different patterns of air quality using median
tree seedling RBL, in addition to the comments summarized above, the
CASAC concluded, in considering the approach used in the last review,
that reliance on the 3-year average and associated judgments in doing
so ``appears of reasonable thought and scientifically sound'' (Cox,
2020a, Consensus Responses to Charge Questions p. 19). Further, while
recognizing the existence of established E-R functions that relate
cumulative seasonal exposure of varying magnitudes to various
incremental reductions in expected tree seedling growth (in terms of
RBL) and in expected crop yield, the CASAC letter also noted that while
decades of research also recognizes visible foliar injury as an effect
of O3, ``uncertainties continue to hamper efforts to
quantitatively characterize the relationship of its occurrence and
relative severity with ozone exposures'' (Cox, 2020a, Consensus
Responses to Charge Questions p. 20). In summary, the CASAC stated that
the approach described in the draft PA to considering the evidence for
welfare effects ``is laid out very clearly, thoroughly discussed and
documented, and provided a solid scientific underpinning for the EPA
conclusion leaving the current secondary standard in place'' (Cox,
2020a, Consensus Responses to Charge Questions p. 22).
c. Administrator's Proposed Conclusions
In reaching conclusions on the adequacy and appropriateness of
protection provided by the current secondary standard and his proposed
decision to retain the standard, the Administrator carefully
considered: (1) The assessment of the available welfare effects
evidence and conclusions contained in the ISA, with supporting details
in the 2013 ISA and past AQCDs; (2) the evaluation of policy-relevant
aspects of the evidence and quantitative analyses in the PA; (3) the
advice and recommendations from the CASAC; (4) the August 2019 decision
of the D.C. Circuit remanding the secondary standard established in the
last review to the EPA for further justification or reconsideration;
and (5) public comments that had been received up to that point (85 FR
49830, August 14, 2020). In considering the evidence base on welfare
effects associated with exposure to photochemical oxidants, including
O3, in ambient air, he noted the newly available evidence,
and the extent to which it alters key scientific conclusions from the
last review. He additionally considered the quantitative analyses
developed in this review, and their associated limitations and
uncertainties, with regard to what they indicate regarding the
protection provided by the current standard. Key aspects of the
evidence and air quality and exposure information emphasized in
establishing the current standard were also considered. Further, he
considered uncertainties in the evidence and quantitative information
as a part of public welfare policy judgments that are essential and
integral to his decision on the adequacy of protection provided by the
standard. In considering the CASAC advice, he noted the CASAC
characterization of the ``thorough discussion and rationale for the
secondary standard'' presented in the PA (Cox, 2020a, p. 2), and also
considered the Committee's overall agreement that the currently
available evidence does not call into question the adequacy of the
current standard and that it should be retained (Cox, 2020a, p. 1).
As an initial matter, the Administrator recognized the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants, noting that no newly available evidence has
been identified in this review on the importance of photochemical
oxidants other than O3 with regard to abundance in ambient
air and potential for welfare effects. For such reasons, described with
more specificity in the ISA and PA and summarized in the proposal, he
proposed to conclude it to be appropriate to retain O3 as
the indicator for the secondary NAAQS for photochemical oxidants and he
focused on the current information for O3.
With regard to the currently available welfare effects evidence,
the Administrator recognized that, consistent with the evidence in the
last review, the currently available evidence describes an array of
effects on vegetation and related ecosystem effects causally or likely
to be causally related to O3 in ambient air, as well as the
causal relationship of tropospheric O3 in radiative forcing
and subsequent likely causally related effects on temperature,
precipitation and related climate variables. The evidence for three
additional categories of effects was newly determined in this review to
be sufficient to infer likely causal relationships with O3.
However, the Administrator did not find the evidence for these effects
to be informative to his proposed decision in review of the standard.
For example, the Administrator noted the PA did not find the current
evidence to indicate air quality under the current standard to cause
increased tree mortality, and, accordingly, he found it appropriate to
focus on more sensitive effects, such as tree seedling growth, in his
review of the standard. With regard to the two insect-related
categories of effects with new ISA determinations (alteration of plant-
insect signaling and alteration of
[[Page 87320]]
insect herbivore growth and reproduction), the Administrator noted the
associated uncertainties in the evidence that preclude a full
understanding of key aspects of the effects and indicate there to be
insufficient information to judge the current standard inadequate based
on these effects as described in the proposal.
In considering the evidence documenting tropospheric O3
as a greenhouse gas causally related to radiative forcing, and likely
causally related to subsequent effects on variables such as temperature
and precipitation, the Administrator took note of the limitations and
uncertainties in the evidence base that affect characterization of the
extent of any relationships between O3 concentrations in
ambient air in the U.S. and climate-related effects. He found this to
preclude quantitative characterization of climate responses to changes
in O3 concentrations in ambient air at regional (versus
global) scales. This lack of quantitative tools precluding important
analyses and the resulting uncertainty led the Administrator to
conclude there to be insufficient information available for these
effects in the current review to support judging the existing standard
inadequate or to identify an appropriate revision.
With regard to visible foliar injury, the Administrator recognized
that, depending on its severity and spatial extent, as well as the
location(s) and intended use(s), the impact of visible foliar injury on
the physical appearance of plants has the potential to be significant
to the public welfare. For example, depending on its extent and
severity, its occurrence in specially protected natural areas may
affect aesthetic and recreational values, such as the aesthetic value
of scenic vistas in protected natural areas (e.g., national parks and
wilderness areas). While recognizing there to be a paucity of
information that relates incidence or severity of injury on vegetation
in public lands to impacts on the public welfare (e.g., related to
recreational services), the Administrator noted the USFS BI scoring
scheme, and proposed to judge that occurrence of the lower categories
of BI scores does not pose concern for the public welfare, but that
findings of BI scores categorized as ``moderate to severe'' injury by
the USFS scheme would be an indication of visible foliar injury
occurrence that, depending on extend and severity, may raise public
welfare concerns.
While recognizing that important uncertainties remain in the
understanding of the O3 exposure conditions that will elicit
visible foliar injury of varying severity and extent in natural areas,
the Administrator took note of the evidence indicating a general
association of injury incidence and severity with cumulative exposure
metrics, including the W126 index, and also an influence of peak
concentrations, as well as the quantitative analyses in the PA of USFS
biosite data and of air quality monitoring data. In the PA analysis of
biosite scores, the incidence of nonzero BI scores, and particularly of
relatively higher scores, such as those indicative of ``moderate to
severe'' injury in the USFS scheme, appear to markedly increase only
with W126 index values above 25 ppm-hrs. The Administrator noted that
such a magnitude of W126 index (either as a 3-year average or in a
single year) is not seen to occur at monitoring locations in or near
Class I areas where the current standard is met (and such a W126 index,
in a single year, has occurred only once in 19 years of monitoring data
at sites across the U.S.), and that values above 17 or 19 ppm-hrs are
rare (PA, Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3; 85
FR 49911, August 14, 2020). The Administrator further took note of the
PA consideration of the USFS publications that identify an influence of
peak concentrations on BI scores (beyond an influence of cumulative
exposure) and the PA observation of the appreciable control of peak
concentrations exerted by the form and averaging time of the current
standard, as evidenced by the air quality analyses which document
reductions in 1-hour daily maximum concentrations with declining design
values. Based on these considerations, the Administrator agreed with
the PA finding that the current standard provides control of air
quality conditions that contribute to increased BI scores and to scores
of a magnitude indicative of ``moderate to severe'' foliar injury.
Based on his consideration of PA findings that areas that meet the
current standard are unlikely to have BI scores reasonably considered
to be impacts of public welfare significance, the Administrator further
proposed to conclude that the current standard provides sufficient
protection of natural areas, including particularly protected areas
such as Class I areas, from O3 concentrations in the ambient
air that might be expected to elicit visible foliar injury of such an
incidence and severity as would reasonably be judged adverse to the
public welfare.
With regard to the welfare effects of reduced plant growth or
yield, the Administrator recognized that the evidence base continues to
indicate growth-related effects as sensitive welfare effects, with the
potential for ecosystem-scale ramifications. While recognizing
associated uncertainties, the Administrator took note of the PA
conclusion and CASAC advice that the approach taken in the last review
of using estimates of O3 impacts on tree seedling growth (in
terms of RBL) as a surrogate for comparable information on other
species and lifestages, as well as a proxy or surrogate for other
vegetation-related effects, including larger-scale effects, continues
to appear to be a reasonable judgment in the current review (85 FR
49910, August 14, 2020; PA, section 4.5.3). These estimates were
medians based on the established E-R functions for 11 tree species. In
light of this and the lack of an alternative metric or approach being
indicated by the current evidence, the Administrator found it
appropriate to adopt this approach in the current review.
The Administrator additionally took note of considerations in the
PA regarding aspects of the derivation of the tree seedling E-R
functions that he found informative to his consideration of issues
discussed in the court's remand of the 2015 secondary standard with
respect to use of a 3-year average W126. In this context, the
Administrator considered whether aspects of this evidence support
making judgments using the E-R functions with W126 index derived as an
average across multiple years. He noted that such averaging would have
some conceptual similarity to the assumptions underlying the adjustment
made to develop seasonal W126 E-R functions from exposures that
extended over multiple seasons (or less than a single season).\202\ The
Administrator also noted uncertainties in regard to estimated RBL at
lower cumulative exposure levels, given the more limited data and fewer
findings of statistical significance supporting the functions at the
relatively lower cumulative exposure levels most commonly
[[Page 87321]]
associated with the current standard (e.g., at or below 17 ppm-hrs).
The Administrator additionally took note of the PA summary of different
comparisons that had been performed in the 2013 ISA and the current ISA
of RBL estimated via the aspen E-R function using either a cumulative
average multi-year W126 index (2013 ISA) or a single-year W126 index
(current ISA) with RBL estimates derived directly from aspen growth
information in a multi-year O3 exposure study. In this
context, he noted the PA finding that consideration of these two
different comparisons illustrate the variability inherent in the
magnitude of growth impacts of O3 and in the quantitative
relationship of O3 exposure and RBL,\203\ while also
providing general agreement of predictions (based on either metric)
with observations. In light of these considerations, the Administrator
recognized that such factors as identified in the proposal, including
the currently available evidence and its recognized limitations,
variability and uncertainties, support a conclusion that it is
reasonable to use a seasonal RBL averaged over multiple years, such as
a 3-year average (85 FR 49910, August 14, 2020). The Administrator
additionally took note of the CASAC advice reaffirming the EPA's focus
on a 3-year average W126, concluding such a focus to be reasonable and
scientifically sound. In light of these considerations, the
Administrator found there to be support for use of an average seasonal
W126 index derived from multiple years (with their representation of
variability in environmental factors), concluding the use of such
averaging to provide an appropriate representation of the evidence and
attention to considerations summarized above. In so doing, he found
that a reliance on single year W126 estimates for reaching judgments
with regard to magnitude of O3-related RBL and associated
judgments of public welfare protection would ascribe a greater
specificity and certainty to such estimates than supported by the
current evidence. Thus, he proposed to conclude that it is appropriate
to use a seasonal W126 averaged over a 3-year period, which is the
design value period for the current standard, to estimate median RBL
using the established E-R functions for purposes in this review of
considering the public welfare protection provided by the standard.
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\202\ The E-R functions for the 11 species were derived in terms
of a seasonal W126 index from experiments that varied in duration
from less than three months to many more. Underlying the adjustments
made to derive the functions for a 3-month season duration are
simplifying assumptions of uniform W126 distribution over the
exposure period and linear relationship between cumulative exposure
duration and response. Averaging of seasonal W126 across three
years, with its reduction of the influence of annual variations in
seasonal W126, would give less influence to RBL estimates derived
from such potentially variable representations of W126, thus
providing an estimate of W126 considered more suitably paired with
the E-R functions.
\203\ For example, there is variability associated with tree
growth in the natural environment (e.g., related to variability in
plant, soil, meteorological and other factors), as well as
variability associated with plant responses to O3
exposures in the natural environment (85 FR 49910, August 14, 2020).
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In reaching his proposed conclusions and judgments related to the
use of RBL as a surrogate for the broad array of vegetation-related
effects, the Administrator recognized a number of important public
welfare policy judgments. The Administrator proposed to conclude that
the current evidence base and available information (qualitative and
quantitative) continue to support consideration of the potential for
O3-related vegetation impacts in terms of the RBL estimates
from established E-R functions as a quantitative tool within a larger
framework of considerations pertaining to the public welfare
significance of O3 effects. He judged the framework to
include consideration of effects that are associated with effects on
vegetation, and particularly those that conceptually relate to growth,
and that are causally or likely causally related to O3 in
ambient air, yet for which there are greater uncertainties affecting
estimates of impacts on public welfare. In his consideration of the
adequacy of protection provided by the current standard, the
Administrator also noted judgments of the prior Administrator in
considering the public welfare significance of small magnitude
estimates of RBL and associated unquantified potential for larger-scale
related effects. In light of CASAC advice and based on the current
evidence as evaluated in the PA, the Administrator proposed to conclude
that the approach or framework initially described with the 2015
decision, with its focus on controlling air quality such that
cumulative exposures at or above 19 ppm-hrs, in terms of a 3-year
average W126 index, are isolated and rare, is appropriate for a
secondary standard that provides the requisite public welfare
protection and proposed to use such an approach in this review (85 FR
49911, August 14, 2020).
With this approach and protection target in mind, the Administrator
considered the analyses of air quality at sites across the U.S.,
particularly including those sites in or near Class I areas. In
virtually all design value periods and all locations at which the
current standard was met (i.e., in more than 99.9% of such instances)
across the 19 years of the data analyzed, the 3-year average W126
metric was at or below 17 ppm-hrs. Further, in all such design value
periods and locations the 3-year average W126 index was at or below 19
ppm-hrs. The Administrator additionally considered the protection
provided by the current standard from the occurrence of O3
exposures within a single year with potentially damaging consequences,
such as a significantly increased incidence of areas with visible
foliar injury that might be judged moderate to severe. In so doing, he
noted the PA findings that incidence of sites with BI scores above 15
(termed ``moderate to severe injury'' by the USFS categorization
scheme) markedly increases with W126 index estimates above 25 ppm-hrs,
and the scarcity of single-year W126 index values above 25 ppm-hrs at
sites that meet the current standard, with just a single occurrence
across all U.S. sites with design values meeting the current standard
in the 19-year historical dataset dating back to 2000 (PA, section 4.4
and Appendix 4D). In light of the evidence indicating that peak short-
term concentrations (e.g., of durations as short as one hour) may also
play a role in the occurrence of visible foliar injury, the
Administrator additionally recognized the control of peak 1-hour
concentrations provided by the form and averaging time of the current
standard and noted there to be less than one day per site with a
maximum hourly concentration at or above 100 ppb (PA, Appendix 2A,
section 2A.2). In consideration of these findings, the Administrator
proposed to judge that the current standard provides adequate
protection from air quality conditions with the potential to be adverse
to the public welfare (85 FR 49912, August 14, 2020).
In reaching his proposed decision, the Administrator gave primary
attention to the principal effects of O3 as recognized in
the current ISA, the 2013 ISA and past AQCDs, and for which the
evidence is strongest (e.g., growth, reproduction, and related larger-
scale effects, as well as visible foliar injury). With respect to the
currently available information related to O3-related
visible foliar injury, the Administrator considered air quality
analyses that may be informative with regard to air quality conditions
associated with appreciably increased incidence and severity of BI
scores at USFS biomonitoring sites, noting that this information does
not indicate a potential for public welfare impacts of concern under
air quality conditions that meet the current standard. In light of
these and other considerations discussed more completely in the
proposal, and with particular attention to Class I and other areas
afforded special protection, the Administrator proposed to conclude
that the evidence regarding visible foliar injury and air quality in
areas meeting the current standard indicates that the current standard
provides adequate protection for this effect.
[[Page 87322]]
The Administrator additionally considered O3 effects on
crop yield, taking note of the long-standing evidence, qualitative and
quantitative, of the reducing effect of O3 on the yield of
many crops, as summarized in the PA and current ISA and characterized
in detail in past reviews (e.g., 2013 ISA, 2006 AQCD, 1997 AQCD, 2014
WREA). In so doing, he recognized that not every effect on crop yield
will be adverse to public welfare and in the case of crop yield effects
in particular there are a number of complexities related to the heavy
management of many crops to obtain a particular output for commercial
purposes, and to other factors, that contribute uncertainty to
predictions of potential O3-related public welfare impacts,
as summarized in sections III.B.2 and III.D.1 of the proposal (PA,
sections 4.5.1.3 and 4.5.3). Thus, in judging the extent to which the
median RYL estimated for the W126 index values generally occurring in
areas meeting the current standard would be expected to be of public
welfare significance, he recognized the potential for a much larger
influence of extensive management of such crops, and also considered
other factors recognized in the PA and proposal, including similarities
in median estimates of RYL and RBL (PA, sections 4.5.1.3 and 4.5.3).
With this context, the information for crop yield effects did not lead
the Administrator to identify this endpoint as requiring separate
consideration or to provide a more appropriate focus for the standard
than RBL, in its role as a proxy or surrogate for the broader array of
vegetation-related effects, as discussed above. Rather, in light of
these considerations, he proposed to judge that a decision based on RBL
as a proxy for other vegetation-related effects will provide adequate
protection against crop related effects. In light of the current
information and considerations discussed more completely in the
proposal, the Administrator further proposed to conclude that the
evidence regarding RBL, and its use as a proxy or surrogate for the
broader array of vegetation-related effects, in combination with air
quality in areas meeting the current standard, provide adequate
protection for these effects (85 FR 49912, August 14, 2020).
In reaching his proposed conclusion on the current standard, the
Administrator also considered the extent to which the current
information may provide support for an alternative standard, proposing
to conclude that the appreciably greater occurrence of higher levels of
cumulative exposure, in terms of the W126 index, as well as an
appreciably greater occurrence of peak concentrations (both hourly and
8-hour average concentrations) in areas that do not meet the current
standard (e.g., areas meeting a higher standard level), would not
provide the appropriate protection of public welfare in light of the
potential for adverse effects on the public welfare. The Administrator
also considered an alternative based solely on the W126 metric, as was
considered in the last review, based on such a concentration-weighted,
cumulative exposure metric having been identified as quantifying
exposure in a way that relates to reduced plant growth (ISA, Appendix
8, section 8.13.1). While recognizing a role for W126 index in
quantifying exposure to develop estimates of RBL that the Administrator
considers appropriately used as a proxy or surrogate for the broader
array of vegetation-related effects, he notes that the evidence
indicates there to be aspects of O3 air quality not captured
by measures of cumulative exposure like W126 index that may pose a risk
of harm to the public welfare (e.g., risk of visible foliar injury
related to peak concentrations). Thus, in light of the information
available in this review, the Administrator proposed to conclude that
such an alternative standard in terms of a W126 index would be less
likely to provide sufficient protection against such occurrences and
accordingly would not provide the requisite control of aspects of air
quality that pose risk to the public welfare.
In summary, the Administrator recognized that his proposed decision
on the public welfare protection afforded by the current secondary
O3 standard from identified O3-related welfare
effects, and from their potential to present adverse effects to the
public welfare, is based in part on judgments regarding uncertainties
and limitations in the available information, such as those identified
above. In this context, he considered what the available evidence and
quantitative information indicated with regard to the protection
provided from the array of O3 welfare effects, finding it to
not indicate the current standard to allow air quality conditions with
implications of concern for the public welfare. He additionally took
note of the advice from the CASAC in this review. Based on all of the
above considerations, described in more detail in the proposal,
including his consideration of the currently available evidence and
quantitative exposure/risk information, the Administrator proposed to
conclude that the current secondary standard provides the requisite
protection against known or anticipated effects to the public welfare,
and thus that the current standard should be retained, without
revision.
3. Comments on the Proposed Decision
Over 50,000 individuals and organizations indicated their views in
public comments on the proposed decision. Most of these are associated
with mass mail campaigns or petitions. Approximately 40 separate
submissions were also received from individuals, and 75 from
organizations and groups of organizations; 40 elected officials also
submitted comments. Among the organizations commenting were state and
local agencies and organizations of state agencies, organizations of
health professionals and scientists, environmental and health
protection advocacy organizations, industry organizations and
regulatory policy-focused organizations. The comments on the proposed
decision to retain the current secondary standard are addressed here.
Those in support of the proposed decision are addressed in section
III.B.2.a and those in disagreement are addressed in section III.B.2.b.
Comments related to aspects of the process followed in this review of
the O3 NAAQS (described in section I.D above), as well as
comments related to other legal, procedural or administrative issues,
and those related to issues not germane to this review are addressed in
the separate Response to Comments document.
a. Comments in Support of Proposed Decision
Of the comments supporting the Administrator's proposed decision to
retain the current secondary standard without revision, all generally
state that the record supports the proposed decision, and note the
CASAC conclusion that the current evidence is generally consistent with
that available in the last review, and the CASAC conclusion that the
evidence does not call into question the adequacy of the current
standard and should be retained. In support of their views, some of
these commenters state that new evidence is lacking that might call
into question the objective for the standard to generally protect
against cumulative exposures associated with median RBL estimates above
6%. They additionally state that the proposed decision appropriately
addresses the Murray Energy remand issues. Further, these commenters
conclude that the available evidence with regard to areas meeting the
current standard does not call into question the adequacy of protection
provided by the current standard from
[[Page 87323]]
the array of vegetation effects, including in Class I areas. Lastly,
these commenters find the EPA's proposed judgments regarding the
uncertainties associated with predicting responses of climate-related
effects to changes in O3 concentrations across the U.S., as
well as the limitations in the availability of tools for such purposes,
to be appropriate and well supported. The EPA agrees with these
comments.
Some of these comments also express the view that welfare benefits
of a more restrictive O3 standard are highly uncertain,
while such a standard would likely cause socioeconomic impacts that the
EPA should consider and find to outweigh the uncertain benefits. While
as discussed in section III.B.3 below, the Administrator does not find
a more stringent secondary standard requisite to protect the public
welfare, he does not consider economic impacts of alternate standards
in reaching this judgment. As summarized in section I.A. above, in
setting primary and secondary standards that are ``requisite'' to
protect public health and welfare, respectively, as provided in section
109(b), the EPA may not consider the costs of implementing the
standards. See generally Whitman v. American Trucking Ass'ns, 531 U.S.
457, 465-472, 475-76 (2001). Likewise, ``[a]ttainability and
technological feasibility are not relevant considerations in the
promulgation of national ambient air quality standards.'' See American
Petroleum Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981);
accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir.
2019). Arguments such as the views on socioeconomic impacts expressed
by these commenters have been rejected by the courts, as summarized in
section I.A above, including in the recent Murray Energy decision, with
the reasoning that consideration of such impacts was precluded by
Whitman's holding that the ``plain text of the Act unambiguously bars
cost considerations from the NAAQS-setting process'' (Murray Energy
Corp. v. EPA, 936 F.3d at 621, quoting Whitman [531 U.S. at 471]).
b. Comments in Disagreement With Proposed Decision
Among those submitting comments that disagreed with the proposed
decision to retain the current secondary standard, or that raised
concerns with the basis for the decision, most of these commenters
expressed concerns regarding the process for reviewing the criteria and
standards and state that the proposal must be withdrawn, and a new
review conducted. Most of these commenters also disagree with the EPA's
proposed conclusion that the current standard, with its current
averaging time and form, provides the requisite public welfare
protection from known or anticipated adverse public welfare effects
associated with the array of O3-related effects, and
generally state that the standard should be revised to be in terms of a
single-year W126 index. Among the claims made in describing the basis
for their view, these commenters claim that EPA failed to describe the
basis for its proposed conclusion; to explain why a standard using the
W126 index was not proposed, consistent with 2014 advice from the
former CASAC, and to address the issues raised by court remand of the
2015 standard. Some commenters expressing the view that the standard
should be revised also express the view that an additional standard
should be established to protect from O3 effects on climate.
With regard to the process by which this review has been conducted,
we disagree with the commenters that claim that it is arbitrary and
capricious or that it does not comport with legislative requirements.
The review process, summarized in section I.D, implemented a number of
features, some of which have been employed in past reviews and others
which have not, and several which represent efficiencies in
consideration of the statutorily required time frame for completion of
the review. The comments received that raise concerns regarding
specific aspects of the process are addressed in the separate Response
to Comments document. As indicated there, the EPA disagrees with these
comments. The EPA finds the review to have been lawfully conducted and
the process reasonably explained. Accordingly, the EPA is not
withdrawing the proposal and restarting the review.
(i) Metric for Standard
The premise of many of the comments expressing disagreement with
the proposed decision is that the secondary standard must be a
``biologically relevant'' metric, which they identify to be the W126
index. Similarly, some commenters assert that EPA cannot lawfully or
rationally set a secondary standard using the metric of the current
standard, which is also the metric used for the primary standard,
claiming that this contradicts EPA's recognition of the relevance of
the W126 index as an exposure metric for assessing the level of
protection from welfare effects, such as RBL. These commenters also
claim that this approach arbitrarily disregards the recommendations of
the prior CASAC, and, in doing so, imply that EPA must establish a W126
based standard because of prior CASAC advice.
We disagree with these commenters. The Clean Air Act includes no
requirements with respect to what metrics should be used to establish
the secondary standards. As is clear from the text of Section 109(b)(2)
of the CAA, the critical test for NAAQS is whether they achieve the
requisite protection. In so doing, it is not uncommon for the form and
averaging time of a NAAQS to differ from exposure metrics most relevant
to assessment of particular effects. These exposure metrics are based
on the health or welfare effects evidence for the specific pollutant
and commonly, in assessments for primary standards, on established
exposure-response relationships or health-based benchmarks (doses or
exposures of concern) for effects associated with specific exposure
circumstances. Evidence for this is found in the common use, in
assessments conducted for NAAQS reviews, of exposure metrics that
differ in a variety of ways from the ambient air concentration metrics
of those standards.\204\ Across reviews for the various NAAQS
pollutants over the years, the EPA has used a variety of exposure
metrics to evaluate the protection afforded by the standards (see
examples identified at 80 FR 65399-65400, October 26, 2015). Further, a
single standard may provide protection from multiple different effects,
the protection for which may be assessed using different exposure
metrics. One standard may also provide protection from multiple
pathways of exposure. Both the primary and secondary Pb standards
provide examples of this. While these standards are expressed in terms
of the concentration of lead in particles suspended in air, different
exposure metrics have been used to evaluate the protection provided by
the Pb standards. The salient exposure metric for assessment of
protection provided by the primary standard has been blood Pb, while
for the secondary standard, concentrations of lead in soil, surface
water and sediment are pertinent, and have been evaluated to assess the
potential for welfare effects related to lead deposition from air (73
FR 67009, November 12, 2008). In somewhat similar manner, the exposure
metric used to evaluate health impacts in the primary sulfur dioxide
standard review includes a 5-minute exposure
[[Page 87324]]
concentration. In contrast, the health-based standard for this
pollutant is the average across three years of the 99th percentile of
1-hour daily maximum concentration of sulfur dioxide in ambient air (75
FR 35520, June 22, 2010; 84 FR 9866, March 18, 2019).
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\204\ The term design value, defined above, is used in this
discussion to refer to the metric for the standard.
---------------------------------------------------------------------------
We disagree with the comment that a secondary standard with the
same form and averaging time as the primary standard does not comply
with the CAA. The CAA does not require that the secondary standard be
established in a specific form or averaging time. The Act, at Section
109(b)(2), provides only that any secondary NAAQS ``shall specify a
level of air quality the attainment and maintenance of which in the
judgment of the Administrator, based on [the air quality] criteria, is
requisite to protect the public welfare from any known or anticipated
adverse effects associated with the presence of such air pollutant in
the ambient air. . . . [S]econdary standards may be revised in the same
manner as promulgated.'' The EPA interprets this provision to leave it
considerable discretion to determine whether a particular form and
averaging time are appropriate, in combination with the other aspects
of the standard (level and indicator), for specifying the air quality
that provides the requisite protection, and to determine whether, once
a standard has been established in a particular form, that form must be
revised. Moreover, nothing in the Act or the relevant case law
precludes the EPA from establishing a secondary standard equivalent to
the primary standard in some or all respects, as long as the Agency has
engaged in reasoned decision-making.\205\
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\205\ In fact, the D.C. Circuit has upheld secondary NAAQS that
were identical to the corresponding primary standard for the
pollutant (e.g., ATA III, 283 F.3d at 375, 380 [D.C. Cir. 2002,
upholding secondary standards for PM2.5 and O3
that were identical to primary standards]).
---------------------------------------------------------------------------
Thus, we note that particular metrics may logically, reasonably,
and for technically or scientifically sound reasons, be used in
assessing exposures of concern or characterizing risk. The purpose, and
use, of exposure metrics is different from the purpose, and use, of
metrics for the standard, and as a result the metrics may differ from
one use to the other. Exposure metrics are used to assess the likely
occurrence and/or frequency and extent of effects under different air
quality conditions, while the air quality standards are intended to
control air quality to the extent requisite to protect from the
occurrence of public health or welfare effects judged to be adverse. In
this review of the O3 secondary standard, the EPA agrees
that based on evidence summarized in section III.A above, metrics such
as the W126 index are appropriate for assessing exposures of concern
for vegetation, characterizing risk to public welfare, and evaluating
what air quality conditions might provide the appropriate degree of
public welfare protection. We disagree, however, that the secondary
standard must be established using those same metrics. Rather, when the
Administrator judges that a standard using a different metric provides
the requisite protection, in light of his consideration of all the
elements of the standard together, he may reasonably establish or
retain such a standard.
With regard to the commenter's emphasis on recommendations from the
CASAC on the form of the secondary standard, the EPA generally agrees
with the importance of giving such recommendations careful
consideration. However, it is not necessary for EPA to address in this
review each statement a prior CASAC made in a prior review. In
addition, if a recommendation of a prior CASAC is raised in a
subsequent review (e.g., in public comments or as a focus in court
decision being addressed), it is reasonable for the Agency to consider
it in the context both of the current review and of consideration of
all the other now available scientific, technical and policy-relevant
information, including advice from the current CASAC. We note that in
this review of the secondary standard, the current CASAC, based on its
review of the information and analyses available in the current review,
concurs with retention of a secondary standard with a metric that
differs from commonly used vegetation exposure metrics, such as the
W126 index (Cox, 2020a). We further note, under the relevant provisions
of the CAA and case law interpreting them, the Administrator is never
bound by the CASAC's conclusions but rather may depart from them when
he has provided an explanation of the reasons for such
differences.\206\ While the EPA does not interpret the requirements of
CAA sections 307(d)(3) and 307(d)(6)(A) to apply to every
recommendation it has received from a prior CASAC, even assuming there
are some circumstances in which EPA were required to comply with the
requirements of CAA section 307(d)(3) and (6)(A) with respect to
particular recommendations from a prior CASAC, these same principles
would apply. Thus, the Administrator would not be bound to follow those
recommendations, but rather could depart from them when he had
explained his reasons for doing so. Accordingly, in reaching
conclusions on the revised secondary standard in this review, the
Administrator has given careful consideration to the current CASAC
advice in this review and to issues raised by the prior CASAC that are
subject to the Murray Energy remand. When he has differed from those
CASAC recommendations, the reasons and judgments that led to a
different conclusion are explained, as summarized in this section and
in section III.B.3 below. Consistent with his consideration of all
significant issues raised in public comments, the Administrator has
also considered the issues raised by commenters that have also been
raised by a prior CASAC, together with the Agency's responses to those
comments, as summarized in this section and in section III.B.3 below.
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\206\ See CAA sections 307(d)(3) and 307(d)(6)(A); see also
Mississippi v. EPA, 744 F.3d 1334, 1354 (D.C. Cir. 2013) (``Although
EPA is not bound by CASAC's recommendations, it must fully explain
its reasons for any departure from them''); id. at 1358 (noting
CASAC, like EPA, exercises both scientific judgment and public
health policy judgment). Selection of a metric for the standard is a
public health or public welfare policy judgment about what standards
will control air quality to the extent judged requisite to protect
from adverse public health or welfare effects.
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The current air quality analyses demonstrate the successfulness of
the current form and averaging time in controlling cumulative
exposures, in terms of W126. These extensive air quality analyses,
presented in the PA and summarized in the proposal, are based on data
collected across the U.S. over a time span of nearly 20 years (85 FR
49892-49895, 49903-49904, August 14, 2020). One of these analyses
describes the positive, linear relationship between long-term changes
in the O3 design value and long-term changes in the W126
index at monitoring sites across the U.S.\207\ This positive, linear
relationship exists for the O3 design value with both a 3-
year average and single-year W126 index (PA, Appendix 4D, Figure 4D-
11). The existence of this relationship means that a change (e.g.,
reduction) in the design value at a monitoring site was generally
accompanied by a similar change (e.g., reduction) in the W126 index,
both in the 3-year average and in the single-year values. As the form
and averaging time of the secondary standard have not changed since
1997, the analyses performed have been able to assess the amount of
control exerted by these aspects of the standard, in combination
[[Page 87325]]
with reductions in the level (i.e., from 80 ppb in 1997 to 75 ppb in
2008 to 70 ppb in 2015) on cumulative seasonal exposures in terms of
W126 index. The analyses have found that the reductions in design
value, presumably associated with implementation of the revised
standards, have been accompanied by reductions in cumulative seasonal
exposures in terms of W126 index (PA, section 4.4.1). Further, while
the formulation of the W126 metric gives more weight to higher
concentrations (in the context of its focus on cumulative exposure), it
is much less effective at curbing elevated hourly concentrations (that
can be important in altering plant growth and yield) than the current
design value metric, as discussed in section III.B.2.b(ii) below.
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\207\ This analysis focuses on the relationship between changes
(at each monitoring site) in the 3-year design value across the 17
design value periods from 2000-2002 to 2016-2018 and changes in the
W126 index over the same period (PA, Appendix 4D, section 4D.3.2.3).
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In expressing the view that the secondary standard should be in
terms of a W126 index, some commenters describe the EPA's statements
regarding the protection from cumulative exposures that is provided by
the current form and averaging time to be ``incidental'' and
``happenstance,'' which leads them to claim the EPA's findings of
protection to be arbitrary. In support of their view, the commenters
quote a statement of the prior CASAC cautioning against interpreting
the W126 index levels in the W126 index scenario created for the 2014
WREA, by first adjusting air quality to meet the then-existing fourth
maximum standard of 75 ppb, to be representative of implementation of a
W126 index standard. The issue described by the prior CASAC related to
the application to all monitoring sites of the precursor reduction
necessary for the highest monitoring site in a region to just meet the
scenario target; the prior CASAC's concern was that actual
implementation of the target as a standard would not necessarily yield
such reductions. We disagree with the commenters that this is relevant
to the air quality analysis in the current review, in which we simply
observe the W126 index values that exist in reality at sites that have
met the existing secondary standard. Contrary to the context for the
prior CASAC's caution, the analysis in the current review is not
showing the results of a theoretical scenario created by modeling
theoretical precursor reductions estimated for attaining a particular
W126-based or fourth high standard. Rather, we are observing what the
W126-based cumulative exposure is at ambient air monitoring sites that
meet the current secondary standard. Thus, regardless of the labels
assigned by the commenter to the findings of the air quality analyses
in the current review, these analyses clearly document the success of
the existing standard (with its fourth maximum form and 8-hour
averaging time) in controlling exposure in terms of the W126 index.
Thus, in light of this evidence, the EPA disagrees with the
commenters who express the view that to provide the requisite
protection the secondary standard must be a W126 index standard. In
assessing the air quality necessary to provide the requisite degree of
protection, particularly for growth and related vegetation and
ecosystem effects, the Agency has recognized the importance of
cumulative exposures, but also the significance of higher peak
exposures (as summarized in section III.B.2.b(ii) below) that can be
characterized through other metrics (e.g., N100). As a result, in
assessing the protection provided by the current standard, the Agency
has focused on the W126 index, expressed in terms of the average of
three consecutive years (in light of considerations discussed below),
as a metric for cumulative exposure, but has also considered the
frequency and magnitude of elevated single-year W126 index values, and
of elevated hourly O3 concentrations (as discussed further
below).
(ii) Protection Against Unusually Damaging Years
In the last review, the Administrator relied on the 70 ppb standard
(as the fourth highest daily maximum 8-hour average concentration
averaged over three consecutive years) to achieve a level of air
quality that would restrict cumulative seasonal exposures to 17 ppm-hrs
or lower, in terms of a 3-year average W126 value, in nearly all
instances. The Murray Energy court found in relevant part that the EPA
had not explained why that level of protection was requisite, in light
of certain comments from the CASAC in 2014 recommending that EPA base a
standard on a one-year W126 metric, in part to limit exposures in
single unusually damaging years.\208\ In responding to the remand,\209\
we are explaining in this document that the EPA is looking to prevent
the damaging effects of O3 on tree growth as a proxy for
public welfare effects related to the broad array of O3's
vegetation-related effects conceptually related to growth effects,
including ecosystem-level effects (as discussed in section III.B.2.b(v)
below). In this review, in assessing the air quality requisite to
prevent adverse effects on public welfare from these effects, the EPA
is not relying solely on maintaining a particular 3-year W126 value.
Rather, we are considering air quality patterns that are associated
with meeting the current standard, including control of peak hourly
concentrations, and the exposures that would be expected under the
current standard, including in terms of W126 values, particularly those
averaged over a 3-year period. The EPA is explaining the grounds for
our conclusion that use of the 3-year average W126 index is a
reasonable basis for assessing protection from RBL, but also that the
Administrator is using other exposure information in reaching the
conclusion that retention of the existing standard (with its form and
averaging time of the fourth highest annual daily maximum 8-hour
average concentration, averaged over three years) provides the needed
protection of RBL, including from what the Murray Energy court noted
that the prior CASAC termed ``unusually damaging years.''
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\208\ The prior CASAC comments on this matter were in the
context of its recommendation for a secondary standard in the form
of a single-year W126 index, which as discussed below would be
expected to provide relatively less control against high-
concentration years compared with the current secondary standard.
The prior CASAC additionally commented that it ``favor[ed] a single-
year period'' which it stated would ``provide more protection for
annual crops and for the anticipated cumulative effects on perennial
species.'' The prior CASAC continued on to state that if the
Administrator preferred, instead, to establish a secondary standard
as a 3-year average W126 index, as a policy matter, the level should
be revised downward (Frey, 2014b, p. iii). The prior CASAC stated
the purpose for this step would be to be protecting ``against single
unusually damaging years that will be obscured in the average''
(Frey, 2014b, p. 13).
\209\ The Agency intends this decision, associated analyses
conducted for this review in consideration of issues raised by the
court's remand, and the discussions herein to constitute its
response to the Murray Energy remand on this issue.
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In disagreeing with the EPA's proposed decision, some commenters
object to the EPA's use of a 3-year average W126 index in assessing
different patterns of air quality using median tree seedling RBL as a
surrogate for an array of vegetation-related effects, particularly
those related to growth and productivity. In so doing, these commenters
variously claim that this use of a 3-year average W126 index (rather
than a single-year W126 index) is inconsistent with recommendations
from the prior CASAC, does not address the court remand on this point,
and that it is inadequate to protect vegetation from high years or
years with hourly O3 concentrations that can be most
important in eliciting adverse effects.
The EPA disagrees with these commenters and notes that it has taken
such concerns, as well as the court's remand, into account in the final
decision. In evaluating the air quality
[[Page 87326]]
conditions allowed by the current standard, the EPA has focused on the
W126 metric averaged over 3 years as the most appropriate measure of
cumulative exposure for consideration of adverse effects on public
welfare, but EPA has also considered other relevant exposure
information, including higher exposures that might be expected to occur
in an ``unusually damaging year.'' The Administrator's decision on the
adequacy of protection provided by the current standards is based on
the full scope of exposure information he has considered.
The EPA concludes that the 3-year average W126 index is a
reasonable metric for assessing the level of protection provided by the
current standard from cumulative seasonal exposures related to RBL,
while noting that our evaluation for the protection provided by the
current standard has also been informed by our consideration of other
metrics (as described further below). In reaching this conclusion, we
have taken into account the available evidence base and air quality
analyses, with a focus on two types of considerations, as well as
consideration of the context for RBL as a proxy for an array of other
vegetation effects (discussed in section III.B.2.b(v) below). The first
of the two consideration types concerns the E-R functions and their use
with a 3-year average W126 index, and the second concerns the control
by the W126 index metric of exposures that might be termed ``unusually
damaging.'' With regard to the first, we find our use of the 3-year
average W126 index appropriate in light of the approach used in
deriving the E-R functions from the underlying data (from exposures of
varying durations, including of multiple years), and the evidence
available for evaluating these functions across multiyear
exposures.\210\ Additionally, with regard to the second consideration,
we recognize limitations associated with a reliance solely on W126
index as a metric to control exposures that might be termed ``unusually
damaging.'' For example, two different air quality patterns for which
the associated W126 index is the same may have very different incidence
of elevated O3 concentrations, and accordingly pose
different risks to vegetation. As discussed below, however, the
occurrence of such concentrations (and any associated risk of damage)
are controlled by the current secondary standard.
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\210\ Additionally, as described in section III.B.1.c above and
III.B.2.b(v) below, the EPA's identification of 17 ppm-hrs for a
target W126 index of 17 ppm-hrs (e.g., versus 18 ppm-hrs) was in
consideration of the prior CASAC recommendation for considering a
``lower'' level ppm-hrs.
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In light of this evidence, and recognizing the role for both peak
and cumulative exposures in eliciting growth and related vegetation and
ecosystem effects, the EPA concludes that focusing solely on W126 index
(either in terms of a single year or 3-year average) in considering the
public welfare protection provided by the current standard would not be
considering all the relevant scientific information. To the extent that
the prior CASAC advised that the EPA should focus solely on single-year
W126 index values in evaluating the protection provided by the
secondary standard, the EPA disagrees that this would provide the
needed protection, for the reasons explained more fully below. In this
regard, we additionally note that the current CASAC concluded that
focusing on three-year average W126 index values in considering the
public welfare protection offered by the secondary standard ``appears
of reasonable thought and scientifically sound'' (Cox, 2020a, p. 19).
With regard to the established tree seedling E-R functions, we note
there are aspects of the datasets and methodology on which the E-R
functions are based which provide support for a 3-year average
approach. As summarized in section III.A.2.c(i) above, in deriving the
E-R functions from studies of durations that varied from shorter than
90 days to multiple years or growing seasons, the results were
normalized to the duration of a single 90-day seasonal period (PA,
section 4.5.1.2 and Appendix 4A, pp. 4A-28 to 4A-29 and footnote 17).
Inherent in this approach is an assumption that the growth impacts
relate generally to the cumulative O3 exposure across the
multiple growing seasons, i.e., with little additional influence
related to any year to year differences in the exposures. As discussed
in the proposal, the use of a 3-year average in assessing RBL using the
established tree seedling E-R functions is compatible with the
normalization step taken to derive functions for a seasonal 90-day
period from the underlying data with its varying exposure durations (85
FR 49901, August 14, 2020).
This concept of the importance of cumulative multiyear
O3 exposure to multiyear impacts, and its representation as
an average, is also reflected in the evaluation of the predicted growth
impacts compared to observations from the multiyear study of
O3 impacts on aspen by King et al (2005), as presented in
the 2013 and 2020 ISAs and summarized in the PA (PA, Section 4.5.1.2).
The ISAs considered the 6-year experimental dataset of O3
exposures and aspen growth effects with regard to correspondence of E-R
function predictions with study observations (2020 ISA, Appendix 8,
section 8.13.2 and Figure 8-17; 2013 ISA, section 9.6.3.2, Table 9-15,
Figure 9-20). The analysis in the 2013 ISA compared observed reductions
in growth for each of the six years to those predicted by applying the
established E-R function for Aspen to cumulative multi-year average
W126 index values (2013 ISA, section 9.6.3.2).211 212 The
evaluation in the 2020 ISA applied the E-R functions to the single-year
W126 index for each year rather than the cumulative multi-year W126
(2020 ISA, Appendix 8, Figure 8-17), with this approach indicating a
somewhat less tight fit to the experimental observations (2020 ISA,
Appendix 8, p. 8-192),\213\ Both ISAs reach similar conclusions
regarding general support for the E-R functions across a multiyear
study of trees in naturalistic settings (ISA, Appendix 8, section
8.13.3 and p. 8-192; 2013 ISA, p. 9-135).
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\211\ For example, the growth impact estimate for year 1 used
the W126 index for year 1; the estimate for year 2 used the average
of W126 index in year 1 and W126 index in year 2; the estimate for
year 3 used the average of W126 index in years 1, 2 and 3; and so
on.
\212\ One finding of this evaluation was that ``the function
based on one year of growth was shown to be applicable to subsequent
years'' (2013 ISA, p. 9-135).
\213\ Based on information drawn from Figure 8-17 in the 2020
ISA, the correlation metric (r\2\) for the percent difference
(estimated vs observed biomass) and year of growth can be estimated
to be approximately 0.7, while using values reported in Table 9-15
of the 2013 ISA (which are plotted in Figure 9-20), the r\2\ for
predicted O3 impact versus observed impact is 0.99 and
for the percent difference versus year is approximately 0.85.
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Based on all of the above considerations, the EPA finds the
evidence to support a 3-year average W126 index for use in assessing
the level of protection provided by the current standard from
cumulative seasonal exposures related to RBL of concern based on the
established E-R functions. As discussed in section III.B.3 below, the
EPA additionally finds the 3-year average metric to be reasonable in
the context of the use of RBL as a proxy to represent an array of
vegetation-related effects. In the discussion immediately below, we
additionally and specifically address the issue of protection from
``unusually damaging years'' of vegetation exposure.
With regard to the comment that cited a recommendation from the
prior CASAC on protection of vegetation
[[Page 87327]]
against ``unusually damaging years'' and the part of the court remand
referencing that CASAC recommendation, we have considered the CASAC
discussion using this term, in the context of the court remand. Use of
this term by the prior CASAC occurs in the 2014 letter on the second
draft PA in the 2015 review (Frey, 2014b). Most prominently, the prior
CASAC defined as damage ``injury effects that reach sufficient
magnitude as to reduce or impair the intended use or value of the plant
to the public, and thus are adverse to public welfare'' (Frey, 2014b,
p. 9). The prior CASAC additionally provided advice with regard to
surrogate metrics for judging such ``damage,'' e.g., use of RBL for
judging effects on trees and their related functions and ecosystem
services, use of crop RYL for judging public welfare effects of crop
effects (Frey, 2014b, p. 10). We also note that the context for the
prior CASAC's use of the phrase ``unusually damaging years'' is in
considering the form and averaging time for a revised secondary
standard in terms of a W126 index (Frey, 2014b, p. 13), which as
discussed below is relatively less controlling of high-concentration
years, rather than in the context of the current secondary standard and
its fourth highest daily maximum 8-hour metric.
While the prior CASAC did not provide any specificity or details as
to the exposure circumstances and damage intended by its more general
phrasing, nor did it cite to specific evidence in scientific
publications, we agree with the general concept that particular air
quality patterns in a year may pose particular risk of vegetation
damage, in terms of both or either growth-related effects or visible
foliar injury (discussed in section III.B.2(iii) below). Across past
O3 NAAQS reviews, the air quality criteria for vegetation
effects have emphasized the risk posed to vegetation from higher hourly
average O3 concentrations (e.g., ``[h]igher concentrations
appear to be more important than lower concentrations in eliciting a
response'' [ISA, p. 8-180]; ``higher hourly concentrations have greater
effects on vegetation than lower concentrations'' [2013 ISA, p. 91-4]
``studies published since the 2006 O3 AQCD do not change
earlier conclusions, including the importance of peak concentrations, .
. . in altering plant growth and yield'' [2013 ISA, p. 9-117]). In
fact, the EPA has recognized the W126 index for E-R models for growth
and yield (in the current and prior ISA and prior AQCD) in part due to
its preferential weighting of higher concentrations (ISA, p. 8-130).
We note, however, that while the W126 index weights higher hourly
concentrations, it cannot, given its definition as an index that sums
three months of weighted hourly concentrations into a single value,
always differentiate between air quality patterns with high peak
concentrations and those without such concentrations. This is
illustrated by the following two hypothetical examples. In the first
example, two air quality monitors have a similar pattern of generally
lower average hourly concentrations, but differ in the occurrence of
higher concentrations (e.g., hourly concentrations at or above 100
ppb). The W126 index describing these two monitors would differ. In the
second example, one monitor has appreciably more hourly concentrations
above 100 ppb compared to a second monitor; but the second monitor has
higher average hourly concentrations than the first. In the second
example, the two monitors may have the same W126 index, even though the
air quality patterns observed at those monitors are quite different,
particularly with regard to the higher concentrations, which have been
recognized to be important in eliciting responses (as noted above).
Thus, the EPA disagrees with a view implied by many of the
commenters (who object to the EPA's proposed decision) that the sole
focus for assessing public welfare protection, related to vegetation
damage, and air quality control provided by the secondary standard
should be on the W126 index. This view ignores both the limitations of
the W126 index itself in distinguishing among different patterns of
hourly O3 concentrations and the fact that the current
secondary standard has, by virtue of its form, a metric that does. With
regard to these limitations of the W126 index, as described above, two
different locations or years may have different patterns of hourly
concentrations but the same W126 index value. This was recognized in
the study by Lefohn et al. (1997), which observed the appreciable
differences between the prevalence of hourly concentrations at or above
100 ppb in exposures on which the E-R functions are based and those
common in ambient air.\214\
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\214\ For example, many of the experimental exposures of
elevated O3 on which the established E-R functions for
the 11 tree seedling species are based, had hundreds of hours of
O3 concentrations above 100 ppb, far more than are common
in (unadjusted) ambient air, including in areas that meet the
current standard (Lefohn et al. 1997; PA, Appendix 2A, section 2A.2;
Wells, 2020). Similarly, the experimental exposures in studies
supporting some of the established E-R functions for 10 crop species
also include many hours with hourly O3 concentrations at
or above 100 ppb (Lefohn and Foley, 1992).
---------------------------------------------------------------------------
This potential for such a difference in peak concentrations between
two different locations with the same W126 index was noted by one
commenter who objected to the EPA's focus on a 3-year average W126
index in assessing RBL and advocated use of a single-year W126 index.
This commenter stated that the same 3-year average could be maintained
in two different locations in which the annual exposure may differ due
to ``variability of the higher hourly average concentrations associated
with vegetation effects.'' In emphasizing the higher hourly average
concentrations associated with effects, the commenter cited the support
provided by the evidence for the San Bernardino National Forest,
described in the 2013 ISA and prior CDs (e.g., 2013 ISA, section
9.5.3.1). We agree with this point and additionally note that this
point also applies to two locations with the same single-year W126
index, given its definition (as noted above).
Given the mathematics inherent in calculation of the W126 index,
while the metric is useful for comparing cumulative exposures, it can
conceal peak concentrations that can be of concern (as described
above). More specifically, one year or location could have few, or even
no, hourly concentrations above 100 ppb \215\ and the second could have
many such concentrations; yet each of the two years or locations could
have the identical W126 index (e.g., equal to 25 or 17 or 10 ppm-hrs,
or some other value). However, as can be seen by the historical ambient
air monitoring dataset of O3 concentrations, the form of the
current standard limits the occurrence of such elevated concentrations,
e.g., at or above 100 ppb (PA, Appendix 2A, section 2A.2; Wells, 2020).
---------------------------------------------------------------------------
\215\ The value of 100 ppb is used here as it has been in some
studies focused on O3 effects on vegetation, simply as an
indicator of elevated or peak hourly O3 concentrations
(e.g., Lefohn et al. 1997, Smith, 2012; Davis and Orendovici, 2006;
Kohut, 2007a). Values of 95 ppb and 110 ppb have also been
considered in this way (2013 ISA, section 9.5.3.1).
---------------------------------------------------------------------------
Analyses of hourly concentrations for different air quality
scenarios developed in consideration of the remand and such comments
(and documented in a technical memorandum to the docket) show the form
and averaging time of the existing standard to be much more effective
than the W126 index in limiting the number of hours with O3
concentrations at or above 100 ppb (N100) and in limiting the number of
days with any such hours (Wells,
[[Page 87328]]
2020).\216\ For example, during the recent design value period (2016-
2018), across all sites that met the current standard, few sites had
any hours at or above 100 ppb in a year (6% in the highest year, Wells,
2020, Table 2).\217\ Among the sites with any such hours, the vast
majority had fewer than five such hours (99.5% in the highest year,
Wells, 2020, Table 2), with none having more than ten such hours,\218\
and no site having more than three days in any one year with any such
concentrations (Wells, 2020, Figures 4 and 5). In comparison, sites
with an annual W126 index below 15 ppm-hrs recorded nearly 40 hourly
concentrations at or above 100 ppb, and as many as seven days with such
a concentration (Wells, 2020, e.g., Figures 10 and 11).\219\ A similar
pattern is seen using the historical dataset extending back to 2000.
This historical dataset also shows the appreciable reductions in peak
concentrations (via either the N100 or D100 metric) that have been
achieved in the U.S. as air quality has improved under O3
standards of the existing form and averaging time (Wells, 2020, Figures
12 and 13). Thus, based on the findings of both the analyses in the PA
(PA, Appendix 2A) and the additional analyses (Wells, 2020), the EPA
disagrees with the commenter that the proposed decision ignores the
importance of elevated hourly O3 concentrations in eliciting
effects on vegetation. Rather, the proposed decision, and final
decision to retain the existing standard, which controls peak
concentrations and also cumulative seasonal exposure in terms of W126
index, explicitly considers this importance and address it in a way
that is more effective than a standard expressed in terms of the W126
index would be, even based on a single-year W126 well below 17 ppm-hrs
(as shown in the additional air quality analyses [Wells, 2020]).
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\216\ The impact of the current form of the standard on
occurrence of elevated hourly concentrations is also seen by a
recent study submitted with comments (Neufeld et al., 2019). For
example, the frequency of episodes defined by three consecutive
hours at or above 60 ppb, as well as the magnitude of W126 index,
has appreciably declined at locations within and immediately
adjacent to the Smoky Mountains National Park, and the periods of
respite from elevated episodes has appreciably increased (Neufeld et
al., 2019). This was found for low elevation sites, and also high
elevation Park sites, which generally have higher levels (Neufeld et
al., 2019).
\217\ In these analyses the N100 and D100 metrics are based on
counts of hourly O3 concentrations at or above 100 ppb
across the consecutive 3-month period with the highest total (Wells,
2020). The metric D100 is the count of days with an hour at or above
100 ppb.
\218\ We note that we are not intending to ascribe specific
significance to five days with an hour at or above 100 ppb or ten
hours such, per se. Rather, these are used simply as reference
points to facilitate comparison to illustrate the point that such
high concentrations, which based on toxicological principles, pose
greater risk to biota than lower concentrations (e.g., ``[h]igher
concentrations appear to be more important than lower concentrations
in eliciting a response'' [ISA, p. 8-180]; ``higher hourly
concentrations have greater effects on vegetation than lower
concentrations'' [2013 ISA, p. 91-4] ``studies published since the
2006 O3 AQCD do not change earlier conclusions, including
the importance of peak concentrations, . . . in altering plant
growth and yield'' [2013 ISA, p. 9-117]).
\219\ We also note the higher percentages of sites with an N100
above five among sites meeting a single-year W126 index of 7 ppm-hrs
than sites meeting the current standard (Wells, 2020, Table 2).
Sites with an annual W126 index of 7 ppm-hrs also record a greater
percentage of sites with more than two days with an hour at or above
100 ppb (Wells, 2020, Table 2).
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In summary, we find that a 3-year average is appropriate for use in
assessing protection for RBL based on the established tree seedling E-R
functions, in light of the discussion above, while also finding it
important to consider additional aspects of O3 air quality,
that influence vegetation exposures of potential concern, in reaching
conclusions about the adequacy of the current standard. We disagree
with the commenters and the prior CASAC that focus on a single year
W126 index is needed to protect against years with O3
concentrations with the potential to be ``unusually damaging,'' Rather,
as described here, the metric of the current standard provides strong
protection against elevated hourly concentrations that might contribute
to ``unusually damaging'' years with the potential to be adverse to the
public welfare, as well as providing protection against effects of
cumulative exposures seen in experimental studies. Accordingly, we
disagree with those commenters that express the view that the current
standard does not provide such protection.
(iii) Visible Foliar Injury
In support of their disagreement with the EPA's proposed decision,
some commenters express the view that the EPA's proposed conclusion
that the current standard provides sufficient protection from an
incidence and severity of visible foliar injury that would reasonably
be judged adverse to the public welfare is unlawful. These commenters
variously claim that EPA analyses are flawed, arbitrary, and ignore
conclusions and judgments of the prior CASAC; cite some studies that
they state indicate a threshold for foliar injury lower than 25 or 17
ppm-hrs; claim that the EPA must, yet does not, identify a level of
injury that is adverse; state that the EPA does not explain its use of
USFS biosite scores in this regard, and state that the EPA does not
adequately address the Murray Energy remand related to these effects.
With regard to the latter, the Agency intends this decision, associated
analyses conducted for this review in consideration of issues raised by
the court remand, and the discussions herein to constitute its response
to the Murray Energy remand on these effects.
With regard to EPA's analyses of the current information on
O3-related visible foliar injury, some commenters claim that
the EPA needs to and has not adequately explained why it disagrees with
the conclusions and judgments of the prior CASAC in comments on the
2014 draft PA regarding a W126 index value of 10 ppm-hrs. As an initial
matter, we note that in discussing this topic, these commenters
conflate the prior CASAC's scientific evidence-based recommendations on
the secondary standard with its judgments of scientific information in
the context of its policy recommendations. In its letter on the draft
PA, the prior CASAC explicitly separates into two separate paragraphs
its scientific judgment based recommendations to the Administrator on
the standard from its additional policy recommendations, with this
statement regarding visible foliar injury occurring in the second
paragraph (that addresses policy recommendations) (Frey, 2014b, p.
iii).\220\ Thus, we
[[Page 87329]]
reasonably interpreted the statement by the prior CASAC as simply
indicating a consideration of the prior CASAC in reaching its decision
on the recommended range of levels, stated multiple times in the same
letter and including levels higher than 10 ppm-hrs, that the Committee
thought might be useful (e.g., as a ``policy recommendation'') to the
Administrator in exercising the discretion granted him under the Act
for specifying a secondary standard (Frey, 2014b, p. iii). The prior
CASAC statement regarding a W126 index value of 10 ppm-hrs, is related
to visible foliar injury at biosites, and, more specifically, is based
on its consideration of an EPA cumulative analysis of a biomonitoring
dataset presented in the 2013 draft WREA.\221\ This analysis, the
dataset for which is further described in Appendix 3C of the PA for the
current review, does not show, as implied by the 2014 CASAC comments,
that, in considering sites with W126 index values from highest to
lowest, there is no reduction in prevalence of sites with visible
foliar injury above a W126 index of 10 ppm-hrs (i.e., there are not
differences in the occurrence of injury across higher values).\222\ The
2014 WREA analysis could not and was not addressing this issue.
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\220\ The first paragraph, conveying scientific judgment
provides a range of levels for a revised standard (Frey, 2014b, p.
iii). The second begins by noting that the ``scientific judgment''
regarding a revised secondary standard, in prior paragraph, are
based on the scientific evidence. Midway through that paragraph, as
shown below, the prior CASAC turns to its policy recommendations, in
which it relates various W126 index values in different ways to
various effect categories, including crop yield loss, foliar injury,
and relative biomass loss (Frey, 2014b, p. iii). Given that the
prior CASAC recommended multiple times in this letter a standard
level range that extends higher than 10 ppm-hrs (to 15 ppm-hrs), the
fact that the sentence regarding visible foliar injury in the
version of this second paragraph that appears within the attachment
to the letter begins with the phrase ``[b]ased on its scientific
judgment'' cannot reasonably be interpreted to be overriding the
Committee's scientific advice on the standard. Rather, the prior
CASAC appears to be implying that to the extent the Administrator
judges, as a matter of public welfare policy, it important to
consider such a focus on foliar injury, the prior CASAC's scientific
judgment is that 10 ppm-hrs is required to reduce it (Frey, 2014b,
pp. iii and 15). In relevant part, the second paragraph reads:
In reaching its scientific judgment regarding the indicator,
form, summation time, and range of levels for a revised secondary
standard, the CASAC has focused on the scientific evidence for the
identification of the kind and extent of adverse effects on public
welfare. The CASAC acknowledges that the choice of a level within
the range recommended based on scientific evidence is a policy
judgment under the statutory mandate of the Clean Air Act. . . . As
a policy recommendation, separate from its advice above regarding
scientific findings, the CASAC advises that a level . . . below 10
ppm-hrs is required to reduce foliar injury. A level of 7 ppm-hrs .
. . offers additional protection against crop yield loss and foliar
injury. . . . Thus, lower levels within the recommended range offer
a greater degree of protection of more endpoints than do higher
levels within the range. (Frey, 2014b, p. iii, [emphasis added]).
\221\ In reference to the 2013 draft WREA cumulative frequency
analysis (e.g., 2013 draft WREA, Figures 7-9 to 7-12), a 2014 CASAC
comment cited by commenters states that ``W126 values below 10 ppm-
hrs [are] required to reduce the number of sites showing visible
foliar symptoms'' (Frey, 2014b, p. 14).
\222\ We note that in light of, and subsequent to, the prior
CASAC's 2014 letter in the last review, the EPA had considered the
extensive evidence documented in the 2013 ISA, as well as analyses
of USFS data in the 2008 and 2015 reviews, including technical memos
developed after the prior CASAC provided its 2014 advice (80 FR
65376, 65395-96, October 26, 2015). In the current review, the now
expanded available data and analyses augment the support for EPA's
conclusions in this regard.
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The 2014 WREA analysis is a cumulative analysis of the proportion
of records with nonzero BI scores; each point graphed in the analysis
includes the records for the same and lower W126 index values. Not only
is the analysis silent with regard to severity of injury, but it also
does not compare the incidence of visible foliar injury for records of
differing W126 index values. Rather, each point in the cumulative
frequency figure represents all the records included in the group (thus
far), which increase by one with each new point (moving through
dataset). Where the record added to the group has the same W126 index
value as the prior included record, the point is at the same location
along the x-axis, but at a slightly higher location along the y-axis
(if it has a nonzero BI), thus contributing to an increase in the
proportion of sites (the metric assessed on the y-axis). Thus, where
there are many records with quite similar W126 index values, the points
do not appreciably move along the x-axis, yet when they have a nonzero
BI score, they are placed higher along the y-axis (as each represents
another nonzero record in the dataset, thus increasing the proportion
of records). At such a location along the x-axis, an inflection occurs
(i.e., a location along the x-axis for which each additional record had
the same or quite similar W126 index as the prior record such that the
point is at a similar location on the x-axis but contributes to
increasing values along the y-axis). As the addition of each new record
makes the dataset larger, such increases (or decreases for zero BI
records) become progressively smaller (along the y-axis), making such
changes or inflections less pronounced at higher W126 index values.
Accordingly, given the much greater representation in the dataset of
relatively lower W126 index records (some two thirds of the dataset has
W126 index values at/below 11 ppm-hrs), the prominent inflection point
noted by the prior CASAC on the cumulative frequency graph occurs
around 11 ppm-hrs, and the figure from the 2014 WREA shows only small
changes in the height of the line with increasing W126 index. This does
not mean that records with higher W126 index values have no greater
occurrence of foliar injury than values below 11 ppm-hrs; in fact, they
do, most particularly the records with W126 index values above 25 ppm-
hrs (PA, Figure 4-5). Thus, we disagree with the prior CASAC statement
that W126 index values below 10 ppm-hrs are required for any reduction
in visible foliar injury and with the suggestion that the WREA
cumulative analysis supports such a conclusion. Given that the
statement by the prior CASAC did not provide any information to
indicate another basis for its statement and because the 2014 WREA
analysis cannot and does not address this issue, we conclude that the
prior CASAC's statement lacks scientific support. Based on this
conclusion, the Administrator does not find this statement from the
prior CASAC informative to his consideration of the adequacy of the
protection provided by the current standard for adverse public welfare
effects related to visible foliar injury (discussed in section III.B.3
below).
Unlike the 2014 WREA cumulative frequency analysis, the
presentations in the PA for this review allow for comparison of injury
incidence, and severity, at distinctly different exposures. As can be
seen by graphs of the distribution of nonzero BI scores for bins of
increasing W126 index estimates, the greatest representation of nonzero
BI scores occurs in the bin with the highest W126 index estimates,
which for the normal soil moisture category is above 25 ppm-hrs (PA,
Figure 4-5). In disagreeing with the EPA's observations from this
analysis, these commenters express the view that the higher percentage
at the higher W126 index level is not meaningful because there are
fewer records for the higher W126 index levels. While we agree that
there are fewer records in the higher W126 index bins, as noted above,
we disagree that there are too few records in those bins to support
some interpretation for some soil moisture categories (such as the
normal or dry categories), although for other soil moisture categories
(i.e., wet), the small sample size does limit interpretation. Sample
size in each bin was considered in the PA analysis and was recognized
as placing a limitation on interpretation of patterns for the wet soil
moisture category. Contrary to these commenters' view that EPA provides
no reason for giving little focus to the higher W126 index bins for the
wet soil moisture category, the PA explains that interpretations of
patterns across the higher W126 bins are limited for the wet soil
moisture category, noting that the number of records in each of the
W126 bins above 13 ppm-hrs comprise less than 1% of the records
available for that soil moisture category (PA, Appendix 4C, section
4C.6). Thus, we agree with these commenters that sample size is an
important consideration in reaching conclusions from this dataset, and,
contrary to the commenters' assertion of providing no valid reasons
with regard to the EPA's lesser emphasis on the wet soil moisture
category, the proposal stated that the PA observations focused
primarily on the records for the normal or dry soil moisture categories
explicitly in recognition of those categories having adequate sample
size which the bins above 13 ppm-hrs did not for the wet soil moisture
category (85 FR 49890, August 14, 2020). While the dataset includes an
extremely small number of records in the wet soil moisture category
that fall into the higher W126 index bins
[[Page 87330]]
(just 18 distributed across the three W126 index bins above 13 ppm-
hrs),\223\ there are more than 550 records categorized as normal soil
moisture distributed across all five bins for W126 index above 13 ppm-
hrs, more than 40 in each bin (PA, Appendix 4C, Table 4C-4). To the
extent that the commenters are suggesting that the EPA is disregarding
data for sites categorized as wet soil moisture, we disagree. In
recognition of the role of soil moisture in contributing to a condition
``necessary for visible foliar injury to occur,'' the PA analysis
presents BI scores separated into groups based on categorization
related to soil moisture (ISA, Appendix 8, p. 8-13; 85 FR 49881; PA,
pp. 4-40 to 4-41). The EPA thus considered the available evidence for
all of the soil moisture categories, but with regard to any patterns
evidenced for the higher W126 index bins (above 13 ppm-hrs), the EPA
reasonably explained its focus on two of the three categories (the
normal or dry soil moisture categories), and lesser attention to the
third category (wet soil moisture) due to the extremely small number of
records in that category that fall into the higher W126 index bins.
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\223\ The records for the wet soil moisture category in the
higher W126 bins are more limited than the other categories, with
nearly 90% of the wet soil moisture records falling into the bins
for W126 index at or below 9 ppm-hrs, limiting interpretations for
higher W126 bins (PA, Appendix 4C, Table 4C.4 and section 4C.6). The
number of records in each of the W126 bins above 13 ppm-hrs (sample
size ranging from zero to 9) comprise less than 1% of the wet soil
moisture category. Accordingly, the PA observations focused
primarily on the records for the normal or dry soil moisture
categories, for which all W126 index in the analysis, including
those above 13 ppm-hrs, are better represented (85 FR 49890, August
14, 2020). For the wet soil moisture category, we agree with the
commenter's statement that ``higher percentage at higher levels
isn't necessarily meaningful, because there are fewer sites with any
data at those levels,'' however note that there is much greater
representation of the normal and dry soil moisture categories in
each of the higher bins, extending to the highest bins, than is the
case for the wet soil moisture category bins.
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Further, in addition to incidence of sites with any injury, the PA
presentations indicate that the severity of injury is also highest in
records for the highest W126 index values, appreciably higher that it
is in all of the lower W126 index bins. For normal soil moisture
category, the median BI score across the nonzero records in the highest
W126 bin (greater than 25 ppm-hrs) is just over 10 (with an average
over 15), compared to well below 5 (averages below 7) for each of the
lower W126 bins (PA, Figure 4-5, Appendix 4C, Table 4C-5). Both of
these observations are consistent with an E-R relationship of
O3 with visible foliar injury, while the variability
observed across the full dataset, in addition to perhaps indicating
limitations in some aspects of the dataset (e.g., categorization by
soil moisture, among others [PA, Appendix 4C, section 4C.5]), no doubt
also indicates the role of other factors that have not been completely
accounted for. Given the evidence from controlled experiments
documented across many years, the lack of noticeable change in
incidence or severity across lower W126 index values may, as recognized
in the PA, relate to a number of factors, including uncertainties in
the assignment of W126 index estimates to the biosite locations and the
soil moisture categorization of sites, as well as potential for
differences in individual plant responses in controlled experiments
from plant communities in natural environmental settings. Although such
factors may contribute to an unclear pattern at lower exposures,
precluding reaching conclusions regarding O3-related
response across the lower W126 index bins, the observed response for
the highest bin clearly indicates an O3-related response for
W126 index values above 25 ppm-hrs.
Some commenters question the significance EPA ascribes to its
observation that the BI scores are appreciably higher for records in
the highest W126 index bin, cryptically characterizing the observation
as describing a ``derivative of a derivative.'' Yet, this observation
is simply focused on the response (e.g., incidence of BI score greater
than 0 or 5 or 15) exhibited across the range of exposure levels
evaluated. The EPA makes this observation in assessing the dataset as
to whether an E-R relationship is exhibited and if so, at what part of
the exposure range is there a noticeable increase in response. This
assessment, in combination with related evidence, then informs the
Agency's conclusions regarding O3 exposure circumstances
that influence BI scores, as well as levels of W126 for which such an
influence is indicated.\224\ The commenters quote the prior CASAC as
characterizing the 2014 WREA analysis as ``a change in the E-R slope,''
\225\ but, as discussed in detail above, the 2014 WREA figure is
presenting a cumulative frequency analysis, which, by its design, does
not show ``a change in the E-R slope.'' Such an analysis, because
responses are not compared among distinct and discrete exposures, as
explained above, is not well described as an exposure-response
assessment (i.e., an analysis of responses occurring across a range of
different exposures). This is in contrast to the current PA
presentation of BI scores across bins of increasing W126 index, which
presents the occurrence of responses, quantified by magnitude of BI
score, associated with multiple different exposures (presented as
bins). Thus, the EPA finds the current analyses in the PA, and not the
cumulative frequency analysis in the 2014 WREA, to be informative to
the consideration of relationships between extent of visible foliar
injury and W126 index, and finds the 2014 WREA analysis to be
mistakenly interpreted by the commenters.
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\224\ Such information informs the Administrator's consideration
of the currently available evidence and the extent to which it can
inform his judgments on O3 air quality associated with
visible foliar injury of such an extent and severity in the
environment as to indicate adverse effects to the public welfare.
Such judgments, as discussed further below, rely on information on
relationships between different O3 air quality metrics
and injury incidence and severity as well as factors influencing the
public welfare significance of different incidence and severity of
foliar injury in vegetated areas valued by the public (e.g., as
summarized in section III.A.2.b).
\225\ This characterization was made in the 2014 letter
providing the prior CASAC's review of the second draft WREA. As
noted by some commenters, the letter goes on to state, ``[b]ased on
this E-R slope change, 10 ppm-hrs is a reasonable candidate level
for consideration in the WREA, along with other levels'' (Frey,
2014c, p. 7). Although the EPA did not examine the specific value of
10 ppm-hrs in the 2014 WREA, as observed by these commenters, the
EPA did consider this recommendation in the 2015 decision, contrary
to the claim of the commenters (80 FR 65395-96, October 26, 2020).
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Further some commenters, who object to the Administrator's proposed
focus on BI scores above 15 for his consideration of visible foliar
injury that may be adverse to the public welfare, additionally suggest
that EPA should give weight to all nonzero BI scores in considering the
appropriate protection against this effect for the standard. As an
initial matter, contrary to the implication of the commenters that any
amount of visible foliar injury is adverse to the affected plant, we
note the long-standing conclusions that visible foliar injury ``is not
always a reliable indicator of other negative effects on vegetation,''
such as growth and reproduction, and the ``significance of ozone injury
at the leaf and whole-plant levels depends on how much of the total
leaf area of the plant has been affected, as well as the plant's age,
size, developmental stage, and degree of functional redundancy, among
the existing leaf area'' (ISA, p. 8-24; 2013 ISA, section 9.4.2).
Further, we disagree with the further implication of these commenters
that any occurrence of a nonzero BI score in the PA dataset can be used
to identify O3 exposure conditions that are adverse to the
public
[[Page 87331]]
welfare. As discussed in section III.A.2.b above, a number of factors
influence the public welfare implications of visible foliar injury, and
as discussed further below, the Administrator has taken these into
account in his decision making regarding the protection from such
effects that should be afforded by the secondary standard.
These commenters additionally claim that the USFS dataset indicates
a clear relationship between the W126 metric and foliar injury. While
we agree that the dataset provides some support for the conclusion of a
greater incidence of nonzero BI scores and higher scores for the
highest W126 bin, a change in response is not evident across the full
range of W126 index levels (for records of similar soil moisture
category), thus suggesting a limitation of the dataset in its ability
to describe the E-R relationship of BI scores with W126 index. As
discussed in the PA, limitations in the dataset (e.g., with regard to
assignment of W126 index estimates to biosite records and the approach
for accounting for the role of soil moisture) may be contributing to
the lack of a clearly delineated E-R relationship of injury occurrence
and BI score with W126 index across a range of W126 index values, such
that a clear shape for a relationship between these variables is not
evident with this dataset, and may be contributing to uncertainties in
this regard. It is with the increase in W126 for the last bin (>25 ppm-
hrs) that the accompanying noticeable increase in response provides
increased confidence in that response (BI scores) being related to a
particular magnitude of the O3 metric. It is this
consideration which leads to the emphasis that EPA's conclusions from
this analysis place on W126 index above 25 ppm-hrs, albeit with a
recognition of some associated uncertainty.
Regarding the Administrator's judgment of the extent and severity
of visible foliar injury that may be adverse to the public welfare,
some commenters state that the EPA must, and has not, considered the
full USFS dataset, including records for which the BI scores are below
5, and they express the view that the USFS data indicate injury (i.e.,
a nonzero BI score) to be occurring at W126 index values as low as 3
ppm-hrs. In so doing, they note the occurrence of scores above 15 in
the lowest bin (W126 index below 7 ppm-hrs). These commenters note that
a third of all records with a BI above 15 are in the lowest W126 index
bin (W126 <7 ppm-hrs) and more than 500 records with nonzero BI are in
higher bins, seemingly intending this as support of their view that the
EPA should identify a W126 of 7 ppm-hrs as a target level for visible
foliar protection. However, this line of logic seems to ignore the fact
that this bin also has over a third of the records with a BI above zero
(PA, Table 4C-4), a fact which would seem contrary to these commenters'
position that 7 ppm-hrs would protect against such scores. All three of
these observations are likely due to the fact that this bin contains
42% of all records and the most records of any bin, by far (PA,
Appendix 4C, Table 4C-4). Accordingly, the more important observation
with regard to the extent of conclusions supported by the dataset on
the role of W126 index in influencing BI scores is that the proportion
of records in the lowest W126 bin that have scores above 15, 5 or 0 is
appreciably less that in the highest W126 index bin (PA, Appendix 4C,
Table 4C-6). The fact that there is not a clear pattern of increasing
proportion across the intervening (and full set of) bins indicates
there to be factors unaccounted-for in this dataset with regard to the
O3 exposure circumstances and the environmental
circumstances that together elicit increased scores in vegetated areas.
In considering the PA analyses of the biosite dataset in light of
these comments, we first note that, as described in the PA, the USFS
dataset includes a broad assortment of BI scores, extending down to
zero, occurring across the range of W126 estimates applied to the
records (PA, Appendix 4C, Figure 4C-3). Contrary to the statement by
these commenters, the EPA has considered the full dataset. The PA
documents the various ways in which this is done, and the proposal
discusses key observations from this dataset to inform the
Administrator's judgment on adversity to public welfare (PA, section
4.3.3.2, 4.5.1.2 and Appendix 4C; 85 FR 49889-90, 49903, August 14,
2020). For example, the lack of clear BI score response to W126 across
the range of lower values is consistent with findings of published
studies of the USFS biomonitoring data which find that W126 index alone
may not be sufficient to characterize the O3 conditions
contributing to injury levels that may be of interest (e.g., Smith et
al., 2012; Smith, 2012; 85 FR 49888-49889, August 14, 2020). Similar to
the discussion above, these studies suggest a role for the occurrence
of elevated hourly concentrations and a focus solely on W126 index may
miss this. This consideration of the larger evidence base for visible
foliar injury and associated USFS biomonitoring findings is important
to judging the findings of analyses of the BI dataset and their
informativeness to the Administrator's needs in judging public welfare
adversity. Based on a detailed evaluation of the currently available
record regarding such data, the EPA recognizes the need to consider
factors beyond just W126 index in considering O3 conditions
most influential in the incidence and extent of visible foliar injury.
With regard to lower ``thresholds,'' the commenters simply cite a
set of studies that describe visible foliar injury observations in
bioindicator species and for which estimates of W126 index for a prior
time period are below 25 ppm-hrs. The first group of these studies
focus on naturally occurring plants in locations during which the
current standard (with its level of 70 ppb) is not met.\226\ As
discussed above, the current standard limits the occurrence of elevated
concentrations which, as discussed above, is suggested to be important
in the occurrence of visible foliar injury in sites of the USFS biosite
monitoring program, and such elevated concentrations are much more
prevalent in areas that do not meet the current standard (e.g., PA,
Appendix 4A, section 2A.2; Wells [2020]). Thus, this group of studies
do not provide sufficient information to characterize the O3
exposure circumstances that may be eliciting the observed responses.
Nor are they informative with regard to consideration of the incidence
and extent or severity of injury that may occur under air quality
conditions allowed by the current standard. Two other examples raised
by commenters (but without complete study citations), appear to relate
to leaf injury assessed in potted plants either outdoors but watered
daily or maintained in greenhouse conditions. The injury assessed is at
the individual plant level, making implications with regard to natural
vegetation communities unclear, and the extent to which either finding
in artificial conditions might represent such plant responses in
natural environmental conditions is unknown. These commenters
additionally note what they describe as ``threshold values'' reported
in a National Park Service publication (Kohut, 2020). This publication
includes three ``injury thresholds'' in terms of three assessment
metrics, with one being a 3-month W126 index and a second in terms of
[[Page 87332]]
SUM06.\227\ For each metric, three ranges of ``thresholds'' are
presented (for different purposes). The ranges for SUM06 come from a
1996 workshop report (Heck and Cowling, 1997). The ranges for W126
index are based on a W126 index conversion of the SUM06 ranges. One of
the ranges is labeled as pertaining to foliar injury as a response,
yet, the publication cited does not provide data on foliar injury in
relation to that range, nor do publications cited by the former
publication. As we can best discern based on cited and related
publications, it appears to at the lower end relate to a benchmark
derived for growth effects (10% RBL) in the highly sensitive species,
black cherry, rather than visible foliar injury (Kohut, 2007b; Lefohn
et al., 1997; 80 FR 65378, October 26, 2015). Thus, contrary to the
commenters' assertion, the range for W126 index (labeled as pertaining
to foliar injury) does not appear to provide a threshold based on
evidence for visible foliar injury.
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\226\ For example, valid design values include: (1) 73 (2002)
and 72 (2003) at monitoring site 450190046, (2) 91 (2002), 94
(2003), and 88 (2004) at 230090102; (3) 77 ppb (2004) at 261530001,
and (4) 90 (2002 and 2003) at 340010005.
\227\ We note that the third assessment approach utilizes a
combination of a W126 index metric with the N100 metric,
illustrating the consideration by the National Park Service of the
role of peak concentrations in posing risk of visible foliar injury
(Kohut, 2020).
---------------------------------------------------------------------------
Some commenters (citing page 4C-18 of the PA), express confusion
over how EPA can state there to be an incomplete understanding of the
relationships influencing severity of visible foliar injury while also
using the USFS scores to inform the Administrator's judgments regarding
conditions that may be adverse to the public welfare. We see no
contradiction in this. Rather, it is this recognition of an incomplete
understanding, including the recognition of uncertainty in ``specific
aspects of [the influences of environmental/genetic factors] on the
relationship between O3 exposures, the most appropriate
exposure metrics, and the occurrence or severity of visible foliar
injury'' (PA, Appendix 4C, p. 4C-18), that leads the EPA to place
greatest weight on the most clear findings from the USFS data. With
regard to the PA presentation, with its recognized uncertainties and
limitations, such a finding is the obviously increased prevalence and
severity of visible foliar injury for records with W126 index estimates
above 25 ppm-hrs.
Further, in considering public welfare implications of
O3 related visible foliar injury, the EPA continues to
recognize that the occurrence of visible foliar injury has the
potential to be adverse to the public welfare (e.g., as summarized in
section III.A.2.b above and section III.B.2 of the proposal). However,
as noted in the proposal, the EPA does not find that any small
discoloring on a single leaf of a plant (which might yield a quite low,
nonzero BI score in the USFS system) is reasonably considered adverse
to the public welfare. Thus, findings such as those raised by
commenters of injury on individual plants in controlled conditions,
while providing support to the conclusion of a causal relationship
between O3 exposure and visible foliar injury (ISA, Appendix
8, Table 8-3), are less informative to the Administrator's judgment on
adequacy of the protection provided by the current standard from
adverse effects to the public welfare. Rather, the USFS biosite
monitoring data provide information that is more useful for such a
judgment because this monitoring program, as summarized in section
III.A.2.b above (and III.B.3.b of the proposal), and the scale of its
objectives which focus on natural settings in the U.S. and forests as
opposed to individual plants is better suited for the Administrator's
consideration with regard to the public welfare protection afforded by
the current standard. In this context, as described in section III.B.3
below, the Administrator judges that very low BI scores, such as those
less than 5, described by the USFS scheme as ``little or no foliar
injury'' do not pose concern for the public welfare.\228\
---------------------------------------------------------------------------
\228\ Studies that consider such data for purposes of
identifying areas of potential impact to the forest resource suggest
this category corresponds to ``none'' with regard to ``assumption of
risk'' (Smith et al., 2007; Smith et al., 2012).
---------------------------------------------------------------------------
Lastly, we disagree with the comment that the Act requires the EPA
to specify ``a level'' of injury that is adverse. The Court of Appeals
for the D.C. Circuit has held that ``the Agency may sometimes need to
articulate the level of threat to the population it considers
tolerable; but there is no separate methodological requirement under
Sec. 109 that the Administrator establish a measure of the risk to
safety it considers adequate to protect public health every time it
establishes a standard pursuant to Sec. 109.'' See Nat. Res. Def.
Council, Inc. v. EPA, 902 F.2d 962, 973 (D.C. Cir. 1990), opinion
vacated in part on other grounds, 921 F.2d 326 (D.C. Cir. 1991). The
same principle applies for consideration of the protection of public
welfare in the context of establishing or reviewing secondary
standards. The court later confirmed that it ``expressly rejected the
notion that the Agency must `establish a measure of the risk to safety
it considers adequate to protect public health every time it
establishes a [NAAQS].'' See ATA III, 283 F.3d at 369 (D.C. Cir. 2002)
(quoting Natural Res. Def. Council, Inc. v. EPA, 902 F.2d 962, 973
[D.C. Cir. 1990]). As is recognized by the courts and by EPA and CASAC
across NAAQS reviews, the judgment of the Administrator, in addition to
being based on the scientific evidence, depends on a variety of
factors, including science policy judgments and public welfare policy
judgments. As noted by the case law and also in section III.B.2.b(iv)
below, the EPA is not required under the Act to identify individual
levels of adversity or set separate standards for every type of effect
that may be caused by a pollutant in ambient air, as long as it has
engaged in reasoned decision making in determining that a particular
standard provides the requisite protection. Thus, it is common for one
NAAQS to provide protection for multiple effects, with the most
sensitive effect influencing the stringency of the standard and
accordingly leading to protection that is adequate for other, less
sensitive effects. Given the significant uncertainties which are
present in every NAAQS review, it is enough for the Administrator to
set standards that specify a level of air quality that will be
``tolerable,'' (NRDC, 902 F.2d at 973), and ``qualitatively to describe
the standard governing its selection of particular NAAQS'' (ATA III,
283 F.3d at 369). In reviewing each standard, the EPA gives due
consideration to each of the effects that are relevant for that
standard in considering whether the standard provides adequate
protection from the type, magnitude or extent of such effects known or
anticipated to be adverse to the public welfare. In the case of visible
foliar injury, as discussed in section III.B.3 below, the Administrator
has considered the available scientific evidence, with associated
uncertainties and limitations, in reaching his decision that the
current secondary standard provides adequate public welfare protection
for this effect.
(iv) Crop Yield Effects
Some commenters object to the proposed conclusions with regard to
the protection provided by the existing secondary standard from adverse
effects on the public welfare related to O3 effects on crop
yield, expressing the view that the EPA must specify ``a level'' to
protect the public welfare against crop yield reductions and that not
doing so is unlawful and arbitrary. These commenters' additionally
object to the Administrator's proposed judgment that a decision based
on RBL as a proxy for other vegetation-related effects will also
provide adequate protection against crop related effects, indicating
their view that EPA does not
[[Page 87333]]
adequately explain the basis for this judgment. These commenters
additionally claim that the prior CASAC described 5.1% RYL as
constituting an adverse welfare effect and express the view that the
EPA arbitrarily and unlawfully does not ``give effect to'' the prior
CASAC's recommendation.
We disagree with the implication of these commenters that, in
judging adequacy of protection provided by the current standard for a
particular effect, it is per se unlawful to conclude that the air
quality achieved by the current standard provides adequate protection
for that particular effect, even if the greater attention in reviewing
the current standard is on another effect. The EPA is not precluded
from reaching such a conclusion as long as the Agency has engaged in
reasoned decision-making in doing so.\229\ In reaching his proposed
conclusions regarding the extent to which the current standard provides
appropriate protection from O3 effects on crop yield that
may be adverse to the public welfare, as in his conclusions described
in section III.B.3 below, the Administrator recognizes the long-
standing evidence of O3 effects on crop yield and the
established E-R functions for which RYL estimates for the median crop
species are presented in the PA (PA, Appendix 3A). He also considers
factors that might be important to his judgments related to the
requisite protection for a secondary standard that protects against
adverse effects to the public welfare. In this context he judges that
the median RYL estimated for air quality that achieves his RBL-related
objectives for the current standard does not constitute an adverse
effect on public welfare and thus concludes that the current standard
also provides adequate protection for crop yield-related effects. Given
that the decision on adequacy of protection is a judgment of the
Administrator and that the Clean Air Act does not require a particular
approach for reaching such judgments, we disagree with the commenters
to the extent that they suggest that it is per se unlawful for the
Administrator to use such an approach. The circumstances for his use of
this approach include particular aspects of the information available
on O3-related crop yield effects and other factors important
to judgments on public welfare effects related to crop yield effects.
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\229\ Section 109(b)(2) of the CAA provides only that any
secondary standard ``shall specify a level of air quality the
attainment and maintenance of which in the judgment of the
Administrator, based on [the air quality ] criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of such air pollutant in the
ambient air.''
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In reaching his decision in this review, as described in section
III.B.3 below, the Administrator has also considered public comments on
these issues, including that regarding a prior CASAC statement. The
comment regarding the prior CASAC appears to draw on a judgment of the
prior CASAC that a median RYL of 5% ``represents an adverse impact''
(Frey, 2014b, p. 14). The prior CASAC provided no clear scientific
foundation for this judgment. While we infer this judgment to draw on
discussion at a 1996 workshop,\230\ neither the prior CASAC nor the
workshop summary provides any explicit rationale for identification of
5% (with regard to RYL), or any description of a connection of an
estimated 5% RYL to broader impacts of a specific magnitude or type, or
to judgments on significance of a 5% RYL to the public welfare. Thus
the EPA disagrees with the commenters regarding the weight to give the
prior CASAC statement and, as described below, respectfully disagrees
with the prior CASAC on this statement.
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\230\ The first reference to 5% RYL by the prior CASAC (in the
2015 O3 NAAQS review) appears to be in its letter on the
first draft PA (Frey and Samet, 2012). In that letter, the prior
CASAC identifies 5% RYL as a factor on which levels for a W126 index
secondary standard should be based, although no rationale is
provided for this recommendation. In a letter attachment, comments
from an individual member point to a 1996 workshop (2014 PA, pp. 6-
15 through 6-17; Heck and Cowling, 1997). As summarized in the 2015
O3 decision, the 1996 workshop participants (16 leading
scientists, discussing their views for a secondary O3
standard) indicated an interest in protecting against crop yield
reductions of 5% yet noted uncertainties surrounding such a
percentage which led them to identify 10% RYL (80 FR 65378, October
26, 2015). In their emphasis on 5%, the 2012 comments from the
individual prior CASAC member expressed the view that the ability to
estimate 5% RYL has improved (Frey and Samet, 2012, p. A-54).
Neither the individual prior CASAC member nor the 1997 workshop
report provide any explicit rationale for the percentages identified
or any description of their connection to ecosystem impacts of a
specific magnitude or type, or to judgments on significance of the
identified effects for public welfare (80 FR 65378, October 26,
2015; Heck and Cowling, 1997).
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In reaching his judgment regarding whether the current standard
provides the requisite public welfare protection, as described in
section III.B.3 below, the Administrator considers the extent to which
a specific estimate of RYL may be indicative of adverse effects to the
public welfare. In so doing, he notes that the secondary standard is
not intended to protect against all known or anticipated O3-
related effects, but rather those that are judged to be adverse to the
public welfare, and that a bright-line determination of adversity is
not required in judging what is requisite. In his decision described
below, the Administrator also notes that the determination of the
extent of RYL estimated from experimental O3 exposures that
should be judged adverse to the public welfare is not clear, in light
of the extensive management of agricultural crops that occurs to elicit
optimum yields (e.g., through irrigation and usage of soil amendments,
such as fertilizer). Further, in considering effects on the public
welfare that may relate to agricultural markets, we note that
detrimental impacts on crops, as well as beneficial impacts, can be
unevenly distributed between producers and consumers, complicating
conclusions with regard to relative adversity. In light of such
considerations, the Administrator, while finding consideration of the
RYL estimate for the median crop species informative to his judgment on
the adequacy of the protection provided by the current standard for
this effect, does not find such an RYL estimate of 5% to represent an
adverse effect to the public welfare, as described more fully in
section III.B.3 below. For these reasons, and for the reasons discussed
earlier in this section, including those regarding advice from a prior
CASAC, the EPA also disagrees with the commenters' assertion that the
Administrator is arbitrarily and unlawfully failing to ``give effect
to'' the prior CASAC's recommendation.
Further, we disagree with the comment that the Act requires the EPA
to specify ``a level'' to protect the public welfare against crop yield
reductions. As discussed in greater detail in section III.B.2.b(iii)
above, the EPA is not required under the Act to set separate standards
for every type of effect that may be caused by a pollutant in ambient
air, as long as it has engaged in reasoned decision making in
determining that a particular standard provides the requisite
protection. Thus, it is common for one NAAQS to provide protection for
multiple effects, with the most sensitive effect influencing the
stringency of the standard and accordingly leading to protection that
is adequate for other less sensitive effects. As discussed further in
section III.B.2.b(iii) above, in reviewing each standard, the EPA gives
due consideration to each effect relevant for that standard in
considering whether the standard provides adequate protection from the
type, magnitude or extent of such effects known or anticipated to be
adverse to the public welfare. In the case of crop yield loss, as
discussed in section III.B.3 below, the Administrator has considered
the magnitude of RYL that may be
[[Page 87334]]
associated with W126 index values that occur under the current standard
and, based on the current information with regard to the RYL estimates,
notes that these estimates are generally no higher than 5.1% and
predominantly well below that. In so doing, he has also considered
factors such as those raised above, and in light of all of these
considerations, he judges that a RYL of 5.1% does not represent an
adverse effect to the public welfare. Thus, the Administrator judges
that the current standard provides adequate protection of the public
welfare for crop yield loss related effects.
(v) RBL
In objecting to the EPA's proposed decision, some commenters
disagree with the target level of protection identified based on use of
RBL. In so doing, such commenters variously claim that a 3-year average
of 17 ppm-hrs is ``ill-suited'' to protect against adverse impacts to
the public welfare; that 6% RBL is too high to protect the public
welfare; that use of a 3-year average instead of a single year W126
index is needed; and, that EPA must focus a target on exposures that
would avoid 2% RBL, citing comments from the prior CASAC on the second
draft PA in the 2015 review, and claiming that a focus on a W126 index
of 7 ppm-hrs is needed for that. With regard to the EPA's use of 6% in
considering the adequacy of protection related to RBL, these commenters
recognize that Murray Energy rejected an argument that EPA's prior
reliance on 6% (in the 2015 decision) was arbitrary based on the record
in that case (Murray Energy, 936 F.3d at 615-16). In pressing their
views, however, the commenters state that nothing in Murray Energy
prevents EPA from revising its prior determination based on the
scientific evidence and CASAC advice.
With respect to the latter point, the EPA agrees that the
Administrator's decision in this review must take into account the
currently available scientific evidence and advice from the CASAC, and
that the Agency is not bound by the Administrator's conclusions in the
prior review. As summarized in the proposal for the current review, in
the proposal, the Administrator took the currently available scientific
evidence and advice from the CASAC into account, while also choosing to
consider the judgments and decision made by the prior Administrator in
that Administrator's consideration of RBL related targets for
cumulative seasonal exposure. He did so, in light of the welfare
effects evidence and air quality information now available, as well as
the advice from the current CASAC reflecting its concurrence that
implementation of the prior Administrator's approach or framework is
``still effective'' in protecting the public welfare from vegetation
effects of O3 (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). As described in section III.B.3 below, after
considering the public comments on this point, he is taking a similar
approach in reaching his decision in this review.
With regard to the commenters' objection to the EPA's use of a 3-
year average in assessing RBL, we note, as an initial matter, that the
EPA's focus on a 3-year average of 17 ppm-hrs as a target level relates
to an RBL estimate of 5.3%, a value that was also chosen in 2015 in
recognition of the prior CASAC advice both with regard to 6% RBL and
about considering a lower W126 index target for a 3-year average due to
the prior CASAC's concern about ``unusually damaging years.'' In the
current review, the CASAC has explicitly considered the EPA's
interpretation of 6% in identifying a target of 17 ppm-hrs as a 3-year
average, and expressed its view that this target ``is still effective
in particularly protecting the public welfare in light of vegetation
impacts from ozone'' (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). Accordingly, the EPA disagrees with the comments that
6% RBL and a 3-year average W126 index target of 17 ppm-hrs are too
high to inform the Administrator's judgments on O3 air
quality that protects the public welfare; rather, the Administrator
continues to find this useful in informing his judgments regarding the
public welfare protection provided by the standard, together with a
broader consideration of air quality patterns associated with meeting
the current standard, such as control of peak hourly concentrations, as
described in section III.B.3 below. Further, we refer to the discussion
above of how the existing standard, with its current averaging time and
form provides the protection from the occurrence of elevated hourly
concentrations that may characterize what the prior CASAC described as
``unusually damaging years.'' As discussed above, the available air
quality data demonstrate the strong protection provided by the current
standard from elevated concentrations that may occur in some years. As
noted above, these analyses indicate that while the current form and
averaging time of the existing standard provides control of these
concentrations and the associated peak exposures, reliance solely on a
standard in the form of the W126 index based standard, as advocated by
the commenters, even with a level as low as 7 ppm-hrs cannot be relied
on to provide it.
In support of their view that the EPA must focus on avoiding 2% RBL
with a W126 index of 7 ppm-hrs, these commenters provide little
rationale beyond citing a comment by the prior CASAC made in the last
review. In so doing, the commenters assert that because the prior CASAC
had noted that 7 ppm-hrs was the only W126 index level for which the E-
R functions yielded a RBL for the median tree species that was less
than or equal to 2%, the EPA must protect against 2% RBL and adopt a
W126 index level of 7 ppm-hrs. We disagree. As an initial matter, we
note our discussion above regarding the EPA's consideration in this
review of advice from a prior CASAC, including prior CASAC statements
that are raised by commenters, such as those noted here. Further, in
making the statement that the commenters' cite, the prior CASAC did not
reach the same conclusion as the commenters with regard to the extent
to which a revised secondary standard should limit cumulative exposures
and associated estimates of RBL, such that the prior CASAC did not
recommend that the EPA consider only W126 index levels associated with
median RBL estimates at or below 2%.\231\ See Murray Energy, 936 F.36
at 615-16 (noting that ``CASAC did not identify 2% growth loss as the
only sufficiently protective level'' but merely recommended ``2% as the
lower end of a range of permissible target levels'' to be considered).
In fact, seven of the nine W126 index levels in the range recommended
by the prior CASAC (7 to 15 ppm-hrs [Frey, 2014b]) are associated with
RBL estimates higher than 2% (PA, Appendix 4A). As a basis for their
assertion that the secondary standard should protect against a median
RBL of 2%, these commenters additionally oddly declare that after three
years, a 2% RBL per year ``becomes 6%.'' There is no evidence in the
record, and the commenter provides no evidence, that would support
their declaration that without a tripling in exposure, the
O3-attributable reduction in annual growth (the RBL) would
triple.\232\ Nor is there
[[Page 87335]]
evidence that would support an alternative interpretation of the
commenters' statement as a claim that a tree experiencing a 2% RBL per
year is reduced in absolute biomass by 6% after three years.\233\
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\231\ Additionally, an explicit scientific rationale for 2% is
not provided by the former CASAC. Nor is it provided in the workshop
report referenced by the prior CASAC in its discussion, as further
discussed in the 2015 decision (80 FR 65394, October 26, 2015; Frey,
2014b, p. 14).
\232\ It is unclear by what logic the commenters conclude that
RBL, a metric describing the effect of the O3 exposure in
a single year, can be modified by the RBL in a prior year.
\233\ The fallacy of such interpretations can be seen in the
presentation of above-ground biomass from a multiyear study of
O3 exposure of aspen that varies little over six years.
Across the six years, the above-ground biomass of the trees
receiving elevated O3 exposure is 25%, 30%, 29%, 29%, 31%
and 29% lower than the reference trees (2013 ISA, Table 9-14; 2020
ISA, Figure 8-17).
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Some commenters who disagree with the proposed decision also
express the view that the EPA has ``proposed'' to use RBL functions for
trees as a proxy for all vegetation effects. Based on this view, these
commenters variously assert that the EPA is failing to comply with its
obligation under the Clean Air Act that a secondary standard protect
the public welfare from ``any known or anticipated adverse effects'';
that the EPA's approach is not the same as the prior CASAC's discussion
of RBL as a surrogate; that the EPA is contravening its statutory
obligation by using one adverse effect as a surrogate for another
without showing that prevention of the former will prevent the latter;
and that, based on the commenters' interpretation of a statement made
by the prior CASAC, a standard that allows tree growth loss above 2%
cannot protect against visible foliar injury. As an initial matter, we
note that the citation provided by the commenters for their statement
that the ``EPA proposes'' to use RBL functions as a proxy for the broad
array of O3 vegetation-related effects does not include such
a ``proposal.'' Rather the commenters' citation points to the
background section of the proposal which simply summarizes the concept
of RBL as a proxy or surrogate which was employed in the last review
and which was described by the prior CASAC (85 FR 49899, August 14,
2020). In describing use of RBL as a proxy or surrogate, the proposal
(and the PA) use several phrases, ranging from ``for consideration of
the broader array of vegetation-related effects of potential public
welfare significance, that included effects on growth of individual
sensitive species and extended to ecosystem-level effects, such as
community composition in natural forests, particularly in protected
public lands, as well as forest productivity'' (85 FR 49878, August 14,
2020), to shorter phrases, such as ``for the broad array of vegetation
related effects that extend to the ecosystem scale'' (85 FR 49911,
August 14, 2020).
We disagree with these commenters that the way the EPA uses RBL as
a ``proxy'' or ``surrogate'' is contrary to law, and with their
contention that the EPA uses one adverse effect as a surrogate for
another without showing that prevention of the former will prevent the
latter. As described in the Administrator's decision below, the most
precise use of RBL as a surrogate or proxy is in the target level of
protection for cumulative seasonal exposure (17 ppm-hrs as a 3-year
average W126 index). This use relates specifically to public welfare
effects related to O3 effects on growth of individual
sensitive species and related effects, including ecosystem-level
effects, such as community composition in natural forests, particularly
in protected public lands, as well as forest productivity (as discussed
in the PA, section 4.5.1.2). In fact, the ISA describes (or relies on)
conceptual relationships among such effects in considering causality
determinations for ecosystem-scale effects such as altered terrestrial
community composition and reduced productivity, as well as reduced
carbon sequestration, in terrestrial ecosystems (ISA, Appendix 8,
sections 8.8 and 8.10). Beyond these relationships of plant-level
effects and ecosystem-level effects,\234\ RBL can be appropriately
described as a scientifically valid surrogate of a variety of welfare
effects based on consideration of ecosystem services and the potential
for adverse impacts on public welfare, as well as conceptual
relationships between vegetation growth-related effects (including
carbon allocation) and ecosystem-scale effects (PA, pp. 4-75 and 4-76).
Both the prior CASAC and the current CASAC recognized this (Frey,
2014b, pp. iii, 9-10; \235\ Cox, 2020a, Consensus Responses to Charge
Questions pp. 18 and 21 \236\). As was discussed in the proposal, the
information available in this review provides continued support for the
use of tree seedling RBL as a proxy for the broad array of vegetation-
related effects conceptually related to growth effects, a conclusion
with which the CASAC agreed (85 FR 49899,49906, August 14, 20202).\237\
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\234\ As summarized in the ISA, O3 can mediate
changes in plant carbon budgets (affecting carbon allocation to
leaves, stems, roots and other biomass pools) contributing to growth
impacts, and altering ecosystem properties such as productivity,
carbon sequestration and biogeochemical cycling. In this way,
O3 mediated changes in carbon allocation can ``scale up''
to population, community and ecosystem-level effects including
changes in soil biogeochemical cycling, increased tree mortality,
shifts in community composition, changes in species interactions,
declines in ecosystem productivity and carbon sequestration and
alteration of ecosystem water cycling (ISA, section 8.1.3).
\235\ The prior CASAC 2014 letter on the second draft PA in that
review stated the following (Frey, 2014b, p. 9-10):
For example, CASAC concurs that trees are important from a
public welfare perspective because they provide valued services to
humans, including aesthetic value, food, fiber, timber, other forest
products, habitat, recreational opportunities, climate regulation,
erosion control, air pollution removal, and hydrologic and fire
regime stabilization. Damage effects to trees that are adverse to
public welfare occur in such locations as national parks, national
refuges, and other protected areas, as well as to timber for
commercial use. The CASAC concurs that biomass loss in trees is a
relevant surrogate for damage to tree growth that affects ecosystem
services such as habitat provision for wildlife, carbon storage,
provision of food and fiber, and pollution removal. Biomass loss may
also have indirect process-related effects such as on nutrient and
hydrologic cycles. Therefore, biomass loss is a scientifically valid
surrogate of a variety of adverse effects to public welfare.
\236\ The CASAC letter on the draft PA in the current review
stated the following (Cox, 2020a, Consensus Responses to Charge
Questions p. 18):
The RBL appears to be appropriately considered as a surrogate
for an array of adverse welfare effects and based on consideration
of ecosystem services and potential for impacts to the public as
well as conceptual relationships between vegetation growth effects
and ecosystem scale effects. Biomass loss is a scientifically sound
surrogate of a variety of adverse effects that could be exerted to
public welfare. . . . In the previous review, the Administrator used
RBL as a surrogate for consideration of the broader array of
vegetation related effects of potential welfare significance that
included effects of growth of individual sensitive species and
extended to ecosystem level effects such as community composition in
natural forests, particularly in protected public lands (80 FR
65406, October 26, 2015). The EPA believes, and the CASAC concurs,
that information available in the present review does not call into
question this approach, indicating there continues to be support for
the use of tree seedling RBL as a proxy for the broader array of
vegetation-related effects, most particularly those related to
growth.
\237\ Further, the EPA lacks sufficient information in the air
quality criteria to identify requisite air quality for these
effects.
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As recognized in the proposal (and PA) there are two other
vegetation effect categories with extensive evidence bases (which
include analyses that assess the influence of cumulative seasonal
exposure); these are crop yield loss and visible foliar injury. As
discussed above, the consideration of protection provided by the
current standard for the former goes beyond the target focused on RBL
and includes aspects of the evidence specific to those effects. As
described above and in section III.B.3 below, the EPA is concluding
that the level of protection is adequate to protect the public welfare
from effects related to crop yield loss. With regard to the latter,
contrary to the commenter's assertion, the EPA is not claiming that
protection focused on RBL provides protection for visible foliar
injury. The EPA's consideration of visible foliar injury is described
earlier in this section and in section III.B.3 below.
[[Page 87336]]
With regard to the two newly identified categories of insect-
related effects, the Administrator finds there to be insufficient
information to judge the current standard inadequate based on these
effects, as discussed in section III.B.3 below. He does not claim that
RBL provides a surrogate for these effects. However, he notes that the
available information in the air quality criteria does not indicate a
greater sensitivity of such effects as compared to O3
effects on vegetation growth, and that he lacks sufficient information
in the air quality criteria to identify requisite air quality for these
effects.
(vi) W126 Index in Areas Meeting Current Standard
In objecting to the proposed decision, one group of commenters
disagree with EPA's findings regarding the W126 index levels in areas
that meet the current standard. In so doing, these commenters claim
that the EPA is mistaken to claim that in virtually all design value
periods and locations at which the current standard was met across the
period covered by the historical dataset the 3-year W126 index was at
or below 17 ppm-hrs because they variously assert there are either 25
or 21 such occurrences, and they further assert there to be either 50
occurrences of a single-year W126 index at or above 19 ppm-hrs or 52
occurrences of a single-year W126 index above 19 ppm-hrs. These counts
are in disagreement with the air quality analyses documented in
Appendix 4D of the PA. For example, out of 8,292 values across nearly
20 years for U.S. ambient air monitoring sites, distributed across all
nine climate regions, with air quality that meets the current standard,
there are just eight occurrences of a 3-year W126 index value above 17
ppm-hrs (PA, Appendix 4D, Tables 4D-10 and 4D-7). This means that 99.9%
of the records (virtually all) were at or below 17 ppm-hrs. While the
details of each step of the analyses in the PA are extensively
documented, including data handling, rounding conventions and data
acceptability criteria (PA, Appendix 4D, section4D.2), the lack of
documentation provided by the commenters and their conflicting claims
(indicated above) leave the EPA to hypothesize that the reason for the
disagreements include differences with regard to these details, such as
those regarding rounding conventions. As described in the PA, W126
values ``were rounded to the nearest unit ppm-hr for applications
requiring direct comparison to a W126 level,'' a convention intended to
provide consistency in the precision of the comparison as the W126
levels for comparison were also in whole numbers (PA, Appendix 4D,
section 4D.2.2). With the rounding conventions applied in the PA, there
are eight 3-year W126 index values greater than 17 ppm-hrs (i.e., equal
to 18 or 19). It may be that the commenters counted unrounded 3-year
W126 index values as low as 17.01 as being greater than 17 ppm-hrs,
although the reason for them providing two conflicting counts is
unclear. Similarly with regard to the counts for single-year W126 index
values above 19 ppm-hrs, the commenters may have counted unrounded
single-year index values as low as 19.01 ppm-hrs as being greater than
19 ppm-hrs. Thus, we find the commenters criticism of the EPA's
characterization of the findings of the air quality analyses, as well
as the commenters' counts, to be unfounded.
Some commenters claim EPA pays inadequate attention to the
relatively few occurrences of single-year W126 index values at or above
19 ppm-hrs that have occurred at sites meeting the current standard
since 2002 and that the standard must be set to avoid such occurrences.
The EPA disagrees with these commenters, as described below, after
carefully considering the relatively few occurrences of W126 index
values at or above 19 ppm-hrs, including single-year values. In so
doing, we have given particular focus on Class I areas, recognizing the
attention given to such areas by the Administrator in judging the
potential for effects adverse to the public welfare, a focus recognized
by the CASAC and with which the prior CASAC explicitly concurred (Cox,
2020a; Frey, 2014b, p. 9).
Among the nearly 500 values for monitoring sites in or near Federal
Class I areas across the U.S., during periods from 2000 through 2018
when the current standard was met, there are no occurrences of a 3-year
average W126 index above 19 ppm-hrs (PA, Table 4-1). Across this same
period in the same Class I locations, there are just 15 occurrences of
single-year W126 index values above 19 ppm-hrs, all of which date prior
to 2013 (PA, Appendix 4D, section 4D.3.2.4). All of these occurrences
are below 25 ppm-hrs. Thus, in addition to their being relatively few
occurrences of a single-year W126 above 19 ppm-hrs in/near a Class I
area in the 19-year dataset, none of them (the most recent of which was
in 2012) is higher than 25 ppm-hrs; in fact, the highest is 23 ppm-hrs
(PA, Appendix 4D, section 4D.3.2.4).
We have also considered the full 19-year dataset for locations
beyond those in or near Class I areas, noting that, at other sites
across the U.S., occurrences of single-year W126 index above 19 ppm-hrs
(22) were predominantly in urban areas, including Los Angeles, and the
highest values were just equal to 25 ppm-hrs, or, in one instance, just
equal to 26 ppm-hrs, when rounded (85 FR 49895, 49904, August 14, 2020;
PA, sections 4.4 and 4.5, Appendix 4D). In considering the potential
risk posed by these scattered occurrences, largely in urban areas, with
none since 2012 in or near a Class I areas, we additionally consider
the data on peak hourly concentrations also discussed above (Wells,
2020). Together, these data indicate the control provided by the
current standard in areas that are of particular focus in protecting
the public welfare, on the extent and frequency of occurrence of
cumulative exposures in terms of the W126 index (and of peak
concentrations) of a magnitude of potential concern. As discussed in
section III.B.3 below, the Administrator does not find the air quality
patterns allowed by the current standard, as indicated by these
analyses, to pose a risk of adverse effects to the public welfare.
In their criticism of the EPA's air quality analyses, one commenter
claims that the analyses are difficult to evaluate for California and
other West region states and suggest that California sites brought into
compliance with the existing standard would still have elevated W126
index values, similar to sites in the Southwest region. We disagree
with the commenter's claim that the air quality analyses suggest that
compliance with the existing standard would not reduce the W126 index
values at California sites. In making their claim, the commenters cite
Figures 4D-4 and 4D-5 of the PA. These figures, however, simply
document W126 index at sites with various design values at one point in
time (2016-2018). They do not describe analyses of trends over time,
with changes in air quality. Yet, that very issue was the subject of
separate regression analyses in the PA (PA, Appendix 4D, section
4D.3.2.3). These analyses show that the Southwest region, which had
highest W126 index values at sites meeting the current standard, also
exhibited the greatest improvement in the W126 metric values per unit
decrease in their design value (slope of 0.93) over the nearly 20 year
period analyzed. The pattern is very similar for the West region (with
a slope of 0.80), with the exception of three sites (in downtown LA);
however, the design values for these sites are above 100 ppb, making
such projections quite uncertain (PA, Appendix 4D, section 4D.3.2.3).
[[Page 87337]]
(vii) Climate Effects
In support of their disagreement with the EPA's proposed decision,
some commenters claim that EPA needs to establish a standard to protect
from radiative forcing and related climate effects. In so doing, they
stated that EPA cannot rely on uncertainty by retaining the existing
standard and instead, given the uncertainties recognized in the ISA,
which they suggest could mean current information underestimates
O3 climate related impacts, the Administrator should
strengthen the existing standard or establish an additional standard.
Some commenters additionally assert that the EPA has failed to address
a recommendation from CASAC regarding a quantitative analysis, while
also criticizing EPA conclusions regarding a carbon storage analysis in
the last review. The EPA disagrees with the commenters that the
available information is sufficient to identify such a standard that
could be judged to provide the requisite protection under the Act, and
notes that the commenters do not submit or describe such information;
nor do the commenters identify a standard that they claim would provide
such protection.
With regard to the CASAC recommendation cited by some commenters,
we note in its review of the draft PA, the CASAC recommended changes to
the PA to ``more thoroughly address effects of ozone on climate
change,'' that would include some quantitation, such as estimates of
climate change related to a change in O3 (Cox 2020a,
Consensus Responses to Charge Questions p. 22). In consideration of
this advice, the final PA included additional discussion on the
available information and tools related to such estimates. As discussed
below, we conclude that this information, as documented in the ISA,
does not provide a foundation with which to derive such estimates as
might pertain to O3 and public health and welfare
considerations relevant to decisions on the NAAQS.\238\
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\238\ In raising EPA's conclusions on a carbon storage analysis
in the last review, some commenters repeat their comments in the
last review that claimed that the relatively lesser weight the EPA
placed on the 2014 WREA estimates of carbon storage (in terms of
CO2) was inconsistent with the emphasis the EPA placed on
CO2 emissions reductions estimated for another regulatory
action. The commenters overlook, however, key distinctions between
the two types of estimates in the two different analyses which
appropriately led the EPA to recognize much greater uncertainty in
the WREA estimates and accordingly give them less weight. While the
WREA estimates were for amounts of CO2 removed from the
air and stored in vegetation as a result of plant photosynthesis
occurring across the U.S., the estimates for the other action were
for reductions in CO2 produced and emitted from power
plants (79 FR 34830, 34931-33). The potentially transient nature of
carbon storage in vegetation makes a ton of additional carbon uptake
by plants in the former arguably unequal to a ton of reduced
emissions from fossil fuels. Further, there are appreciably larger
uncertainties involved in attempting to quantify the additional
carbon uptake by plants which requires complex modeling of
biological and ecological processes and their associated sources of
uncertainty, and there is no new information available in the
current review that would reduce such uncertainties in quantitative
estimates of carbon storage benefits to climate. In recognizing the
public welfare value of ecosystem carbon storage, we additionally
note, however, that protection provided by the current standard from
vegetation effects (and RBL) also provides a degree of protection in
terms of carbon storage.
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As recognized in the proposal and summarized in section III.A.2
above, there are a number of limitations and uncertainties that affect
our ability to characterize the extent of any relationships between
O3 concentrations in ambient air in the U.S. and climate-
related effects, thus precluding a quantitative characterization of
climate responses to changes in O3 concentrations in ambient
air at regional (vs global) scales. While evidence supports a causal
relationship between the global abundance of O3 in the
troposphere and radiative forcing, and a likely causal relationship
between the global abundance of O3 in the troposphere and
effects on temperature, precipitation, and related climate variables
(ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013), the non-
uniform distribution of O3 (spatially and temporally) makes
the development of quantitative relationships between the magnitude of
such effects and differing O3 concentrations in the U.S.
challenging (ISA, Appendix 9). Additionally, ``the heterogeneous
distribution of ozone in the troposphere complicates the direct
attribution of spatial patterns of temperature change to ozone induced
[radiative forcing]'' and there are ``ozone climate feedbacks that
further alter the relationship between ozone [radiative forcing] and
temperature (and other climate variables) in complex ways'' (ISA,
Appendix 9, section 9.3.1, p. 9-19). Thus, various uncertainties
``render the precise magnitude of the overall effect of tropospheric
ozone on climate more uncertain than that of the well-mixed GHGs'' and
``[c]urrent limitations in climate modeling tools, variation across
models, and the need for more comprehensive observational data on these
effects represent sources of uncertainty in quantifying the precise
magnitude of climate responses to ozone changes, particularly at
regional scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p.
9-22). For example, current limitations in modeling tools include
``uncertainties associated with simulating trends in upper tropospheric
ozone concentrations'' (ISA, section 9.3.1, p. 9-19), and uncertainties
such as ``the magnitude of [radiative forcing] estimated to be
attributed to tropospheric ozone'' (ISA, section 9.3.3, p. 9-22).
Further, ``precisely quantifying the change in surface temperature (and
other climate variables) due to tropospheric ozone changes requires
complex climate simulations that include all relevant feedbacks and
interactions'' (ISA, section 9.3.3, p. 9-22). For example, an important
limitation in current climate modeling capabilities for O3
is representation of important urban- or regional-scale physical and
chemical processes, such as O3 enhancement in high-
temperature urban situations or O3 chemistry in city centers
where NOx is abundant. Because of such limitations we cannot quantify
or judge the impact of incremental changes in O3
concentrations in the U.S. on radiative forcing and subsequent climate
effects.
Thus, as discussed in section III.B.3 below, the significant
limitations and uncertainties summarized here together preclude
identification of an O3 standard that could be judged to
provide requisite protection of the public welfare from adverse effects
linked to O3 influence on radiative forcing, and related
climate effects. Contrary to the commenters' charge that the lack of a
quantitative analysis of climate-related effects due to recognition of
such limitations and uncertainties is unlawful and arbitrary, the
information available in this review is insufficient to judge the
existing standard inadequate or to identify an appropriate revision
based on O3-related climate effects. In the face of
insufficient evidence for such conclusions, it might, on the contrary,
be judged unlawful and arbitrary for the Agency to perform guesswork to
assert a particular new standard provided requisite protection for this
category of effects. The EPA agrees with the commenters that ``perfect
information'' is not required. However, information that provides for
assessment of how the current and potential alternative or additional
standards would affect O3-related climate impacts is
lacking. As noted in the ISA, few studies have even attempted to
estimate ``climate response to changes in tropospheric ozone
concentrations alone in the future atmosphere,'' and effects of
O3 on radiative forcing and climate depend on many factors
other than tropospheric ozone concentrations, including ``changes in a
suite of climate-sensitive
[[Page 87338]]
factors, such as the water vapor content of the atmosphere'' (ISA, p.
9-20; Myhre et al., 2013). Thus, as discussed in section III.B.3 below,
while the Administrator recognizes that the evidence supports a
relationship of tropospheric O3 with climate effects, he
judges the quantitative uncertainties to be too great to support
identification of a standard specific to such effects.
4. Administrator's Conclusions
Based on the large body of evidence concerning the welfare effects,
and potential for public welfare impacts, of exposure to O3
in ambient air, and taking into consideration the attendant
uncertainties and limitations of the evidence, the Administrator
concludes that the current secondary O3 standard provides
the requisite protection against known or anticipated adverse effects
to the public welfare, and should therefore be retained, without
revision. In reaching these conclusions, the Administrator has
carefully considered the assessment of the available welfare effects
evidence and conclusions contained in the ISA, with supporting details
in the 2013 ISA and past AQCDs; the evaluation of policy-relevant
aspects of the evidence and quantitative analyses in the PA (summarized
in sections III.A.2 and III.A.3 above); the advice and recommendations
from the CASAC (summarized in section III.B.1.b above); and public
comments (as discussed in section III.B.2 above and the separate
Response to Comments document), as well as the August 2019 decision of
the D.C. Circuit remanding the secondary standard established in the
last review to the EPA for further justification or reconsideration.
In considering the currently available information in this review
of the O3 secondary standard, the Administrator recognizes
the longstanding evidence base for vegetation-related effects,
augmented in some aspects since the last review, described in section
III.A.2.a above. The currently available evidence describes an array of
effects on vegetation and related ecosystem effects causally or likely
to be causally related to O3 in ambient air, as well as the
causal relationship of tropospheric O3 in radiative forcing
and subsequent likely causally related effects on temperature,
precipitation and related climate variables. The Administrator also
takes note of the quantitative analyses and policy evaluations
documented in the PA that, with CASAC advice and consideration of
public comment, inform the judgments required of him in reaching his
decision on a secondary standard that provides the requisite protection
under the Act.
As an initial matter, the Administrator recognizes the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants (as recognized in section III.B.1.c above). In
so doing, he notes that no newly available evidence has been identified
in this review regarding the importance of photochemical oxidants other
than O3 with regard to abundance in ambient air, and
potential for welfare effects, and that, as stated in the current ISA,
``the primary literature evaluating the health and ecological effects
of photochemical oxidants includes ozone almost exclusively as an
indicator of photochemical oxidants'' (ISA, section IS.1.1). Thus, the
Administrator recognizes that, as was the case for previous reviews,
the evidence base for welfare effects of photochemical oxidants does
not indicate an importance of any other photochemical oxidants. For
these reasons, described with more specificity in the ISA and PA, he
proposes to conclude it is appropriate to retain O3 as the
indicator for the secondary NAAQS for photochemical oxidants (85 FR
49896, August 14, 2020).
In his review of the existing secondary O3 standard, in
light of the evidence base and quantitative analyses available today,
the Administrator has given particular attention to consideration of
the issues raised by the August 2019 court remand, and related issues
raised in public comment, as well as analyses that were conducted or
updated in this review in consideration of the remand and related
public comment. In so doing, he has also given careful consideration of
the form and averaging time of the current standard and its ability to
control the patterns of O3 concentrations that contribute to
environmental exposures of potential concern to the public welfare.
Further, he has considered what is indicated by the information
currently available with regard to exposure metrics, supported by the
current evidence, for assessing potential risks posed to vegetation,
and protection provided from such exposures. Additionally, with regard
to visible foliar injury, he has considered the current evidence in the
ISA in combination with quantitative information and policy evaluations
in the PA, advice from the CASAC and public comment, in making
judgments regarding adequacy of the protection provided by the current
standard from adverse effects to the public welfare related to this
effect. Before turning to these issues, discussed, in turn, below in
the context of the EPA's understanding of the information now available
in the current review, he addresses two endpoints newly identified in
this review, as well as tropospheric O3 effects related to
climate.
With regard to the two insect-related categories of effects with
new ISA determinations in this review, the Administrator takes note of
the conclusions that the current evidence is sufficient to infer likely
causal relationships of O3 with alterations of plant-insect
signaling and insect herbivore growth and reproduction (as summarized
in section III.A.2.a above). He additionally recognizes the PA finding
that uncertainties in the current evidence, as summarized in section
III.A.2 above, preclude a full understanding of such effects, the air
quality conditions that might elicit them, the potential for impacts in
a natural ecosystem. Accordingly, the Administrator notes a lack of
clarity in the characterization of these effects, and a lack of
important quantitative information to consider such effects in this
context such that it is not feasible to relate different patterns of
O3 concentrations with specific risks of such alterations.
As a result, the Administrator concludes there is insufficient
information to judge how particular ambient air concentrations of
O3 relate to the degree of impacts on public welfare related
to these effects. Thus, he concludes there is insufficient information
to judge the current standard inadequate or to identify an appropriate
revision based on these effects.
Before focusing further on the key vegetation-related effects
identified above, the Administrator first considers the strong evidence
documenting tropospheric O3 as a greenhouse gas causally
related to radiative forcing, and likely causally related to subsequent
effects on variables such as temperature and precipitation. In so
doing, he takes note of the limitations and uncertainties in the
evidence base that affect characterization of the extent of any
relationships between O3 concentrations in ambient air in
the U.S. and climate-related effects, and preclude quantitative
characterization of climate responses to changes in O3
concentrations in ambient air at regional or national (vs global)
scales, as summarized in sections III.A.2 above. As a result, he
recognizes the lack of important quantitative tools with which to
consider such effects in this context such that it is not feasible to
relate different patterns of O3 concentrations at the
regional (or national) scale in the U.S. with specific risks of
alterations in temperature, precipitation and other
[[Page 87339]]
climate-related variables. The Administrator finds that these
significant limitations and uncertainties together preclude his
identification of an O3 standard reasonably judged to
provide requisite protection of the public welfare from adverse effects
linked to O3 influence on radiative forcing, and related
climate effects. Thus, the Administrator concludes that the information
available in this review is insufficient to judge the existing standard
inadequate or to identify an appropriate revision based on
O3-related climate effects.
The Administrator turns now to vegetation-related effects, the
evidence for which as a whole is extensive, spans several decades, and
supports the Agency's conclusions of causal or likely to be causal
relationship for O3 in ambient air with an array of effect
categories. These categories include reduced vegetation growth,
reproduction, crop yield, productivity and carbon sequestration in
terrestrial systems; increased tree mortality; alteration of
terrestrial community composition, belowground biogeochemical cycles
and ecosystem water cycling; and visible foliar injury (ISA, Appendix
8). As an initial matter, the Administrator notes the new ISA
determination that the current evidence is sufficient to infer likely
causal relationships of O3 with increased tree mortality.
With regard to the current evidence for this effect, the Administrator
notes that the evidence does not indicate a potential for O3
concentrations that occur in locations that meet the current standard
to cause increased tree mortality, as summarized in section III.A.2.a
above (PA, section 4.3.1). Accordingly, he finds it appropriate to
focus on more sensitive effects, such as tree seedling growth, in his
review of the standard. Thus, in considering the adequacy of protection
provided by the current standard from adverse effects to the public
welfare related to these effects, the Administrator begins by
considering vegetation growth and conceptually related effects with a
focus on RBL (described in section III.B.2 above), then turns to a
specific consideration of crop yield loss and lastly, to consideration
of visible foliar injury.
With regard to vegetation growth and related effects, the
Administrator has considered discussions in the PA and in response to
public comments in section III.B.2 above, and finds it appropriate for
identification of the requisite protection to extend beyond
consideration of a magnitude of growth effects, per se, that he may
judge adverse to the public welfare. Rather, the Administrator extends
his consideration beyond that, judging it appropriate to consider
reduced growth (i.e., RBL) as a proxy for an array of other vegetation-
related effects to the public welfare. As discussed in section III.B.2
above, these categories of effects include reduced vegetation growth,
reproduction, productivity and carbon sequestration in terrestrial
systems, and also alteration of terrestrial community composition,
belowground biogeochemical cycles and ecosystem water cycling. In
adopting RBL as a proxy for this array of effects, the Administrator
notes that such a use is consistent with advice from CASAC, and that
RBL was also adopted as a proxy for this array of effects by the prior
Administrator, in consideration of advice from the prior CASAC.
In assessments of RBL estimated from O3 exposure, the
Administrator takes note of the PA consideration of the established E-R
relationships for RBL in tree seedlings of 11 species with
O3 exposures in terms of W126 index (PA, Appendix 4A). In so
doing, he agrees with the PA conclusion regarding 6% RBL, with which
the CASAC concurred, as described in sections III.B.1.b and III.B.2
above), and judges that for his use of RBL as a proxy, maintaining
O3 concentrations such that associated estimates of RBL fall
below 6%, as a median across the 11 species represented by the
established E-R relationships would assure the appropriate protection.
In making these judgments, he observes that they were also adopted by
the prior Administrator, with consideration of advice from the prior
CASAC.
Further, based on considerations discussed in the PA, advice from
CASAC and discussion in section III.B.2 above, Administrator has
considered the use of RBL in his judgment of the public welfare
protection provided by the secondary standard. Based on those
considerations, including uncertainties in the E-R relationships and
their use in the way described here, the Administrator judges it
appropriate for the standard to protect against W126 index values
associated with a median RBL at or above 6% (while also controlling
peak hourly concentrations, as discussed below). Based on this
judgment, in addition to a recognition of uncertainty in these
estimates (in light of the discussion in section III.B.2.b(ii) above
regarding the appropriate duration or averaging for the W126 index
metric) he concludes it appropriate for the standard to generally
control exposures in terms of W126 index to a level of 17 ppm-hrs,
recognizing that the RBL estimated for such a W126 index value is 5.3%,
a value appreciably below 6%.
With regard to the appropriate O3 exposure metric to
employ in assessing adequacy of air quality control in protecting
against RBL, the Administrator has considered the discussions in the
PA, and in response to public comments in section III.B.2 above
regarding the available evidence and air quality analyses. He has also
considered this in the context of the court remand with regard to the
EPA's use of a 3-year average W126 index to assess protection from RBL
and the court's reference to advice from the prior CASAC on protection
against ``unusually damaging years'' (described in section III.B.2
above). In so doing, the Administrator considers below the extent of
conceptual similarities of the 3-year average W126 index with some
aspects of the derivation approach for the established E-R functions,
the context of RBL as a proxy (as recognized above), and limitations
associated with a reliance solely on W126 index as a metric to control
exposures that might be termed ``unusually damaging.''
With regard to the established E-R functions used to describe the
relationship of RBL with O3 in terms of a seasonal W126
index, the Administrator recognizes that the E-R functions were derived
mathematically from studies of different exposure durations (varying
from shorter than one to multiple growing seasons) by applying
adjustments so that they would yield estimates normalized to the same
period of time (season), such that the estimates may represent average
impact for a season, as summarized in section III.A.1.c(ii) above (PA,
section 4.5.1.2, Appendix 4A, Attachment 1). He notes the compatibility
of W126 index averaged over multiple growing seasons or years with
these adjustments. He also notes the exposure levels represented in the
data underlying the E-R functions are somewhat limited with regard to
the relatively lower cumulative exposure levels most commonly
associated with the current standard (e.g., at or below 17 ppm-hrs),
indicating additional uncertainty for application to such levels.
Further, he notes the PA observation that some of the underlying
studies did not find statistically significant effects of O3
at the lower exposure levels, indicating some uncertainty in
predictions of an O3-related RBL at those levels, as
summarized in section III.A.1.c(ii) above (PA, section 4.5.1.2). He
additionally notes the differing patterns of hourly concentrations of
the elevated exposure levels in the datasets from which the E-R
functions from the patterns in ambient
[[Page 87340]]
air meeting the current standard across the U.S. today, as summarized
in section III.B.2.b(ii). With these considerations regarding the E-R
functions and their underlying datasets in mind, he also takes note of
year-to-year variability of factors other than O3 exposures
that affect tree growth in the natural environment (e.g., related to
variability in soil moisture, meteorological, plant-related and other
factors), that have the potential to affect O3 E-R
relationships (ISA, Appendix 8, section 3.12; 2013 ISA section 9.4.8.3;
PA, sections 4.3 and 4.5). Based on these considerations, the
Administrator finds there to be a consistency of his use of the W126
index averaged over multiple years with the approach used in deriving
the E-R function, and with other factors that may affect growth in the
natural environment.
In light of such considerations, the Administrator agrees with the
PA finding that several factors contribute uncertainty and some
resulting imprecision or inexactitude to RBL estimated from single-year
seasonal W126 index values, as discussed in section III.D.1.b(ii) of
the proposal (85 FR 49900-01, August 14, 2020; PA sections 4.5.1.2 and
4.5.3). The Administrator additionally recognizes the qualitative and
conceptual nature of our understanding, in many cases, of relationships
of O3 effects on plant growth and productivity with larger-
scale impacts, such as those on populations, communities and
ecosystems. From these considerations, he judges that use of a seasonal
RBL averaged over multiple years, such as a 3-year average, is
reasonable, and provides a more stable and well-founded RBL estimate
for his purposes as a proxy to represent the array of vegetation-
related effects identified above. The Administrator additionally takes
note of the CASAC advice agreeing with the EPA's focus on a 3-year
average W126 for this purpose, concluding such a focus to be reasonable
and scientifically sound, as summarized in section III.B.1.b above. In
light of these considerations, the Administrator finds there to be
support for use of an average seasonal W126 index derived from multiple
years (with their representation of variability in environmental
factors), concluding the use of such averaging to provide an
appropriate representation of the evidence and attention to
considerations summarized above. In so doing, he finds that sole
reliance on single year W126 estimates for reaching judgments with
regard to magnitude of O3 related RBL and associated
judgments of public welfare protection would ascribe a greater
specificity and certainty to such estimates than supported by the
current evidence. Rather, he finds it appropriate, for purposes of
considering public welfare protection from effects for which RBL is
used as a proxy, to primarily consider W126 index in terms of a 3-year
average metric.
In his consideration of the appropriateness of using a 3-year
average W126 metric, the Administrator additionally takes note of the
discussion in section III.B.2 above with regard to protection against
``unusually damaging years,'' a caution raised by the prior CASAC in
considering a secondary standard in terms of a 3-year average W126
index (and an issue raised in the court remand). With regard to this
caution, the Administrator finds informative the discussion in section
III.B.2 above regarding the extent to which a standard in terms of a
W126 metric might be expected to control exposure circumstances of
concern (e.g., for growth effects, among others). This discussion and
its focus on air quality analyses in the PA and additional analyses
conducted in consideration of public comment investigate the annual
occurrence of elevated hourly O3 concentrations which may
contribute to vegetation exposures of concern (PA, Appendix 2A, section
2A.2; Wells, 2020).\239\
---------------------------------------------------------------------------
\239\ The ISA references the longstanding recognition of the
risk posed to vegetation of peak hourly O3 concentrations
(e.g., ``[h]igher concentrations appear to be more important than
lower concentrations in eliciting a response'' [ISA, p. 8-180];
``higher hourly concentrations have greater effects on vegetation
than lower concentrations'' [2013 ISA, p. 91-4] ``studies published
since the 2006 O3 AQCD do not change earlier conclusions,
including the importance of peak concentrations, . . . in altering
plant growth and yield'' [2013 ISA, p. 9-117]).
---------------------------------------------------------------------------
These air quality analyses illustrate limitations of the W126 index
for purposes of controlling peak concentrations, and also the strengths
of the current standard in this regard. As discussed more fully in
section III.B.2.b(ii) above, the W126 index cannot, by virtue of its
definition, always differentiate between air quality patterns with high
peak concentrations and those without such concentrations. This is
demonstrated in the air quality analyses which show that the form and
averaging time of the existing standard is much more effective than the
W126 index in limiting peak concentrations (e.g., hourly O3
concentrations at or above 100 ppb) and in limiting number of days with
any such hours (Wells, 2020, e.g., Figures 4, 5, 8, 9 compared to
Figures 6, 7, 10 and 11). A similar finding is evidence in the
historical data extending back to 2000. These data show the appreciable
reductions in peak concentrations that have been achieved in the U.S.
as air quality has improved under O3 standards of the
existing form and averaging time (Wells, 2020, Figures 12 and 13). From
these analyses, the Administrator concludes that the form and averaging
time of the current standard is effective in controlling peak hourly
concentrations and that a W126 index based standard would be much less
effective in providing the needed protection against years with such
elevated and potentially damaging hourly concentrations. Thus, in light
of the current evidence and quantitative air quality analyses, the
Administrator notes that the W126 index, by its very definition, does
not provide specificity with regard to year-to-year variability in
elevated hourly O3 concentrations with the potential to
contribute to the ``unusually damaging years'' that the prior CASAC
identified for increased concern. In so doing, he disagrees with the
statement of the prior CASAC that a single-year W126 index would
necessarily provide protection from such years. Further, he judges that
a standard based on either a 3-year or a single-year W126 index would
not be expected to provide effective control of the peak concentrations
that may contribute to ``unusually damaging years'' for vegetation.
Thus, in considering the extent of protection provided by the
current standard, in addition to considering seasonal W126 averaged
over a 3-year period to estimate median RBL using the established E-R
functions, the Administrator finds it appropriate to also consider
other metrics, including peak hourly concentrations. While he
recognizes that the evidence does not indicate a particular threshold
number of hours at or above 100 ppb (or another reference point for
elevated concentrations), he takes particular note of the evidence of
greater impacts from higher concentrations (particularly with increased
frequency) and of the air quality analyses that document variability in
such concentrations for the same W126 index value. In light of these
considerations, he judges such a multipronged approach to be needed to
ensure appropriate consideration of exposures of concern and the
associated protection from them afforded by the secondary standard.
Thus, the Administrator concludes that use of a seasonal W126 averaged
over a 3-year period, which is the design value period for the current
standard, to estimate median RBL using the established E-R functions,
in combination with a
[[Page 87341]]
broader consideration of air quality patterns, such as peak hourly
concentrations, is appropriate for considering the public welfare
protection provided by the standard.
In the discussion above, the Administrator recognizes a number of
public welfare policy judgments important to his review of the current
standard that include the appropriateness of the W126 index, averaged
across a 3-year period, for assessing the extent of protection afforded
by the standard from cumulative seasonal O3 exposures. In
reflecting on these judgments, the current evidence presented in the
ISA and the associated evaluations in the PA, the Administrator
concludes that the currently available information supports such
judgments, additionally noting the CASAC concurrence with regard to the
scientific support for these judgments (Cox 2020a, Consensus Responses
to Charge Questions p. 21). Accordingly, the Administrator concludes
that the current evidence base and available information (qualitative
and quantitative) continues to support consideration of the potential
for O3-related vegetation impacts in terms of the RBL
estimates from established E-R functions as a quantitative tool within
a larger framework of considerations pertaining to the public welfare
significance of O3 effects. Such consideration includes
effects that are associated with effects on vegetation, and
particularly those that conceptually relate to growth, and that are
causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting estimates of
impacts on public welfare. The Administrator additionally notes that
this approach to weighing the available information in reaching
judgments regarding the secondary standard additionally takes into
account uncertainties regarding the magnitude of growth impact that
might be expected in mature trees (e.g., compared to seedlings), and of
related, broader, ecosystem-level effects for which the available tools
for quantitative estimates are more uncertain and those for which the
policy foundation for consideration of public welfare impacts is less
well established.
In his consideration of the adequacy of protection provided by the
current standard, the Administrator does not consider every possible
instance of an effect on vegetation growth from O3 to be
adverse to public welfare, although he recognizes that, depending on
factors including extent and severity, such vegetation-related effects
have the potential to be adverse to public welfare. Comments from the
current CASAC, in the context of its review of the draft PA, expressed
the view that the strategy described by the prior Administrator for the
secondary standard established in 2015 with its focus on limiting 3-
year average W126 index values somewhat below those associated with a
6% RBL in the median species and associated W126 index target of 17
ppm-hrs (in terms of a 3-year average), at or below which the 2015
standard was expected to generally restrict cumulative seasonal
exposure, is ``still scientifically reasonable'' and ``still effective
in particularly protecting the public welfare in light of vegetation
impacts from ozone'' (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). In light of this advice and based on the current
evidence as evaluated in the PA, the Administrator judges that this
approach or framework, with its focus on controlling cumulative
seasonal exposures associated with an RBL of 6% or greater, by limiting
air quality in terms of a 3-year average W126 index, to or below a
target of 17 ppm-hrs, in combination with a broader consideration of
air quality patterns, such as control of peak hourly concentrations,
associated with meeting the current standard, is appropriate for his
use in this review. In so doing, he additional notes the isolated, rare
occurrences in locations meeting the current standard of such exposures
at 19 ppm-hrs. Based on the current information to inform consideration
of vegetation effects and their potential adversity to public welfare,
he additionally judges that the RBL estimates associated with such
marginally higher exposures in isolated, rare instances are not
indicative of effects that would be adverse to the public welfare,
particularly in light of variability in the array of environmental
factors that can influence O3 effects in different systems
and uncertainties associated with estimates of effects associated with
this magnitude of cumulative exposure in the natural environment.
With regard to O3 effects on crop yield, the
Administrator, as an initial matter, takes note of the long-standing
evidence, qualitative and quantitative, of the reducing effect of
O3 on the yield of many crops, as summarized in the PA and
current ISA and characterized in detail in past reviews (e.g., 2013
ISA, 2006 AQCD, 1997 AQCD, 2014 WREA). He additionally notes the
established E-R functions for 10 crops and the estimates of RYL derived
from them, as presented in the PA (PA, Appendix 4A, section 4A.1, Table
4A-4), and the potential public welfare significance of reductions in
crop yield, as summarized in section III.A.2.b above. In so doing,
however, he additionally recognizes that not every effect on crop yield
will be adverse to public welfare. In the case of crops in particular
there are a number of complexities related to the heavy management of
many crops to obtain a particular output for commercial purposes, and
related to other factors, that the Administrator takes into
consideration in evaluating potential O3-related public
welfare impacts, as summarized in section III.B.2.b(iv) above (PA,
sections 4.5.1.3 and 4.5.3).
Similarly, the Administrator concludes that the extensive
management of agricultural crops that occurs to elicit optimum yields
(e.g., through irrigation and usage of soil amendments, such as
fertilizer) is relevant in evaluating the extent of RYL estimated from
experimental O3 exposures that should be judged adverse to
the public welfare. He considers these opportunities in crop management
for market objectives, as well as complications in judging relative
adversity that relate to market responses and their effects on
producers and consumers in evaluating the potential impact on public
welfare of estimated crop yield losses. Further, the Administrator
takes note of the conclusion of the CASAC that the available evidence
does not call into question the adequacy of the current secondary
standard and that it should be retained (Cox 2020a, p.1).
The Administrator also considered the public comments, discussed in
section III.B.2.b(iv) above, suggesting that the proposed decision was
not giving adequate consideration to crop yield effects and that his
decision should consider a statement by the prior CASAC, raised in
public comments, that a 5% RYL estimate, as the median based on the 10
E-R functions, ``represents an adverse impact.'' With regard to the
prior CASAC statement, he notes the discussion in section III.B.2.b(iv)
above regarding the unclear basis for the prior CASAC judgment, both
with regard to a connection of an estimated 5% RYL to broader impacts
and to judgments on significance of a 5% RYL to the public welfare. In
considering the adequacy of protection of the public welfare from
effects related to crop yield loss, the Administrator considers the air
quality analyses and the W126 index levels commonly occurring in areas
that meet the current standard. In so doing, he notes that W126 index
values (3-year average) were at or below 17 ppm-hrs in virtually all
monitoring sites with air quality meeting the current standard.
[[Page 87342]]
Based on the established E-R functions, the median RYL estimate
corresponding to 17 ppm-hrs is 5.1%. In considering single-year index
values, as discussed in section III.B.2.b(vi), the vast majority are
similarly low (with more than 99% less than or equal to 17 ppm-hrs),
and the higher values predominantly occur in urban areas. The
Administrator additionally takes note of the discussion in section
III.B.2.b(ii) above regarding the role of elevated hourly
concentrations in effects on vegetation growth and yield. In so doing,
in addition to his consideration of W126 index occurring in areas that
meet the current standard, he also takes note of the control of
elevated hourly O3 concentrations that is exerted by the
current standard.
In light of all of the above, in reaching his judgment regarding
public welfare implications of the W126 index values summarized here
(and associated estimated RYL), including the isolated and rare
occurrence of somewhat higher values, the Administrator notes that the
secondary standard is not intended to protect against all known or
anticipated O3-related effects, but rather those that are
judged to be adverse to the public welfare. He also takes into
consideration the extensive management of agricultural crops, and the
complexities associated with identifying adverse public welfare effects
for market-traded goods (where producers and consumers may be impacted
differently). Based on all of these factors, the Administrator
disagrees with the prior CASAC statement that an estimated median RYL
of 5% represents an adverse impact and further judges that an estimated
median RYL of 5.1%, based on experimental exposures, would not
constitute an adverse effect on public welfare. Accordingly, the
Administrator notes that the current standard generally maintains air
quality at a W126 index below 17 ppm-hrs, with few exceptions, and
accordingly would limit the estimated RYL (based on experimental
O3 exposures) to this degree. Therefore, he concludes that
the current standard provides adequate protection of public welfare
related to crop yield loss and does not need to be revised to provide
additional protection against this effect. In so doing, the
Administrator notes the conclusions by the current CASAC that the
evidence supports retaining the current standard, without revision.
Turning to consideration of visible foliar injury and protection
afforded by the secondary standard from associated impacts to the
public welfare, the Administrator takes note of the long-standing and
well-established evidence base, updated in the ISA for this review, and
of policy-relevant analyses presented in the PA to inform his judgments
regarding a secondary standard that provides appropriate protection of
the public welfare from this effect. In so doing, he has also taken
into account issues raised by public comments, both with regard to our
understanding of relationships between O3 exposure
circumstances and extent and severity of injury in natural areas across
the U.S., and with regard to the extent of our understanding of the
relationship of injury extent and severity to public welfare effects
anticipated to be adverse, and the Murray Energy remand.
In considering public welfare implications of this effect, he notes
the potential for this effect, when of a significant extent and
severity, to reduce aesthetic and recreational values, such as the
aesthetic value of scenic vistas in protected natural areas including
national parks and wilderness areas, as well as other areas similarly
protected by state and local governments for similar public uses. Based
on these considerations, the Administrator recognizes that, depending
on its severity and spatial extent, as well as the location(s) and the
associated intended use, the impact of visible foliar injury on the
physical appearance of plants has the potential to be significant to
the public welfare. In this regard, he agrees with the PA statement
that cases of widespread and relatively severe injury during the
growing season (particularly when sustained across multiple years and
accompanied by obvious impacts on the plant canopy) might reasonably be
expected to have the potential to adversely impact the public welfare
in scenic and/or recreational areas, particularly in areas with special
protection, such as Class I areas (PA, sections 4.3.2 and 4.5.1). In so
doing, the Administrator notes that the secondary standard is not meant
to protect against all known or anticipated O3-related
welfare effects, but rather those that are judged to be adverse to the
public welfare, and further notes that there are not established
measures for when such welfare effects should be judged adverse to the
public welfare. Rather, the level of protection from known or
anticipated adverse effects to public welfare that is requisite for the
secondary standard is a public welfare policy judgment to be made by
the Administrator.
While recognizing there to be a paucity of established approaches
for interpreting specific levels of severity and extent of foliar
injury in natural areas with regard to impacts on the public welfare
(e.g., related to recreational services), the Administrator recognizes
that injury to whole stands of trees of a severity apparent to the
casual observer (e.g., when viewed as a whole from a distance) would
reasonably be expected to affect recreational values and thus pose a
risk of adverse effects to the public welfare. He further notes that
the available information does not provide for specific
characterization of the incidence and severity that would not be
expected to be apparent to the casual observer, nor for clear
identification of the pattern of O3 concentrations that
would provide for such a situation.
In this context, the Administrator takes note of the system
developed by the USFS for its monitoring program \240\ to categorize BI
scores of visible foliar injury at biosites (sites with O3-
sensitive vegetation assessed for visible foliar injury) in natural
vegetated areas by severity levels (described in section III.A.2.c(ii)
above). While recognizing that quantitative analyses and evidence are
lacking that might support a more precise conclusion with regard to a
magnitude of BI score coupled with an extent of occurrence that might
be specifically identified as adverse to the public welfare, the
Administrator also takes note of the D.C. Circuit's holding that
substantial uncertainty about the level at which visible foliar injury
may become adverse to public welfare does not necessarily provide a
basis for declining to evaluate whether the existing standard provides
requisite protection against such effects. See Murray Energy Corp. v.
EPA, 936 F.3d 597, 619-20 (D.C. Cir. 2019). In this context, he
considers the discussion in the PA and in sections III.A.2.b, III.A.2.c
and III.B.2 above regarding the USFS biosite monitoring program. He
finds the scale of this program's objectives, which focus on natural
settings in the U.S. and forests as opposed to individual plants, to be
suited for his consideration with regard to the public welfare
protection afforded by the current standard, and consequently, he finds
the data and analyses generated by the program informative in such
considerations.
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\240\ During the period from 1994 (beginning in eastern U.S.)
through 2011, the USFS conducted surveys of the occurrence and
severity of visible foliar injury on sensitive species at sites
across most of the U.S. following a national protocol.
---------------------------------------------------------------------------
In this context, he takes note of the USFS system, including its
descriptors for BI scores of differing magnitude intended for that
Agency's consideration in identifying areas of potential impact to
forest resources. As described in section III.A.2.b(iii) above, very
low BI scores (at or below 5) are
[[Page 87343]]
described by the USFS scheme as ``little or no foliar injury'' (Smith
et al., 2007; Smith et al., 2012).\241\ The Administrator notes that BI
scores above 15 are categorized as moderate to severe (and scores above
25 as severe). In so doing, in light of considerations raised in the PA
and consideration of public comment, he recognizes the lower categories
of BI scores as indicative of injury of generally lesser risk to the
natural area or to public enjoyment, which he judges unlikely to be
indicative of injury of such a magnitude or extent as to pose risk of
adverse effects to the public welfare. Thus, the Administrator reaches
the conclusion that occurrence of the lower categories of BI scores
does not pose concern for the public welfare, but that findings of BI
scores categorized as ``moderate to severe'' injury by the USFS scheme
would be an indication of visible foliar injury occurrence that,
depending on extent and severity, may raise public welfare concerns. In
this framework, the Administrator considers the PA evaluations of the
currently available information and what it indicates with regard to
patterns of air quality of concern for such an occurrence, and the
extent to which they are expected to occur in areas that meet the
current standard.
---------------------------------------------------------------------------
\241\ Studies that consider such data for purposes of
identifying areas of potential impact to the forest resource suggest
this category corresponds to ``none'' with regard to ``assumption of
risk'' (Smith et al., 2007; Smith et al., 2012).
---------------------------------------------------------------------------
In so doing, the Administrator takes particular note of the USFS
biosite monitoring program studies of the occurrence, extent and
severity of visible foliar injury in indicator species in defined plots
or biosites in natural areas across the U.S. These studies of data for
USFS biosites (sites with O3-sensitive vegetation assessed
for visible foliar injury) have often summarized O3
concentrations in terms of cumulative exposure metrics (e.g., SUM06 or
W126 index). Some of these studies, particularly those examining such
data across multiple years and multiple regions of the U.S., have
reported that variation in cumulative O3 exposure, in terms
of such metrics, does not completely explain the patterns of occurrence
and severity of injury observed. Although the availability of detailed
analyses that have explored multiple exposure metrics and other
influential variables is limited, multiple studies have indicated a
potential role for an additional metric, one related to the occurrence
of days with relatively high concentrations (e.g., number of days with
a 1-hour concentration at or above 100 ppb), as summarized in section
III.A.2.c above (PA, section 4.5.1.2). Thus, the Administrator takes
note of this evidence indicating an influence of peak concentrations on
BI scores (beyond an influence of cumulative exposure). He also finds
noteworthy the extensive evidence of trends across the past nearly 20
years that indicate reductions in severity of visible foliar injury
that parallel reductions in peak concentrations that have been
suggested to be influential in the severity of visible foliar
injury.\242\
---------------------------------------------------------------------------
\242\ For example, the PA describes findings from USFS studies
that have concluded a ``declining risk of probable impact'' over the
16-year period of the program, especially after 2002 (e.g., Smith,
2012), and the parallel national reductions in O3
concentrations from 2000 through 2018 in terms of cumulative
seasonal exposures, as well as in peak O3 concentrations
such as the annual fourth highest daily maximum 8-hour concentration
and also the occurrence of 1-hour concentrations above 100 ppb (PA,
Figure 2-11, Appendix 2A, Tables 2A-2 to 2A-4, and Appendix 4D,
Figure 4D-9).
---------------------------------------------------------------------------
Further, the Administrator considers the PA analysis of a dataset
developed from USFS biosite index scores, combined with W126 estimates
and soil moisture categories, summarized in section III.A.2.c above. In
so doing, he takes note of the PA observation that important
uncertainties remain in the understanding of the O3 exposure
conditions that will elicit visible foliar injury of varying severity
and extent in natural areas, and particularly in light of the other
environmental variables that influence its occurrence, and of the
recognition by the CASAC that ``uncertainties continue to hamper
efforts to quantitatively characterize the relationship of [visible
foliar injury] occurrence and relative severity with ozone exposures''
(Cox 2020a, Consensus Responses to Charge Questions, p. 20).
Notwithstanding, and while being mindful of, such uncertainties with
regard to predictive O3 metric or metrics and a quantitative
function relating them to incidence and severity of visible foliar
injury in natural areas (as also noted in the USFS studies referenced
above), the Administrator takes note of the PA finding that the
incidence of nonzero BI scores, and, particularly of relatively higher
scores (such as scores above 15 which are indicative of ``moderate to
severe'' injury in the USFS scheme) appears to markedly increase only
with W126 index values above 25 ppm-hrs, as summarized in section
III.B.2.b above (PA, section 4.3.3 and Appendix 4C).
In light of these observations, the Administrator finds the current
evidence to be incomplete with regard to information to support a
quantitative characterization of air quality that would be anticipated
to result in visible foliar injury of an extent and severity to cause
adverse effects to the public welfare. The Administrator also considers
discussion in the court's remand of the 2015 standard with regard to
visible foliar injury (Murray Energy Corp. v EPA, 936 F.3d at 619-20).
The court concluded that the EPA had failed to offer a reasoned
explanation for deciding not to specify a level of air quality to
protect against adverse effects related to visible foliar injury. In
particular, the court stated that the EPA had not explained why it was
unable to choose such a level although the prior CASAC had provided
advice with regard to a specific level. The EPA's disagreement with the
prior CASAC on its identified level is explained in section III.B.2
above, as is the reason why the EPA did not find the analysis on which
the prior CASAC based its advice to be appropriate for such a
conclusion.\243\ This and other associated issues raised by the court
have been raised in public comments on the proposal for this action and
are addressed in section III.B.2 above.
---------------------------------------------------------------------------
\243\ As discussed in section III.B.2.b, the cumulative
frequency graph relied on by the CASAC does not present biosite
scores for comparison at different cumulative exposure levels.
Accordingly, it does not provide the type of analysis that is needed
for the EPA to reach a conclusion about the extent of protection
that different patterns of O3 concentrations would
provide against visible foliar injury of an extent and severity as
to pose risk of adverse effects to the public welfare.
---------------------------------------------------------------------------
Based on the evidence and quantitative analyses available in the
present review, and advice from the current CASAC, the Administrator
considers the question of a level of air quality that would provide
protection against visible foliar injury related effects known or
anticipated to cause adverse effects to the public welfare. Based on
the evidence and associated quantitative analyses in this review, the
Administrator's judgment reflects his recognition of less confidence
and greater uncertainty in the existence of adverse public welfare
effects with lower O3 exposures. In this context, the
Administrator judges that W126 index values at or below 25 ppm-hrs,
when in combination with infrequent occurrences of hourly
concentrations at or above 100 ppb, would not be anticipated to pose
risk of visible foliar injury of an extent and severity so as to be
adverse to the public welfare.
With these conclusions in mind, the Administrator considers the air
quality analyses presented in the PA and the additional analyses
developed in response to public comment. In so doing, he notes that a
W126 index above
[[Page 87344]]
25 ppm-hrs (either as a 3-year average or in a single year) is not seen
to occur at monitoring locations (including in or near Class I areas)
where the current standard is met, and that, in fact, values above 17
or 19 ppm-hrs are rare, as summarized in section III.A.3 above (PA,
Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3). Further, the
Administrator takes note of the PA consideration of the USFS
publications that identify an influence of peak concentrations on BI
scores (beyond an influence of cumulative exposure) and the PA
observation of the appreciable control of peak concentrations exerted
by the form and averaging time of the current standard, as evidenced by
the air quality analyses which document reductions in 1-hour daily
maximum concentrations with declining design values. He also notes, as
discussed above, the uncommonness of days with any hours at or above
100 ppb at monitoring sites that meet the current standard, as well as
the minimal number of hours on any such days (Wells, 2020). Based on
these considerations, the Administrator concludes that the current
standard provides control of air quality conditions that contribute to
increased BI scores and to scores of a magnitude indicative of
``moderate to severe'' foliar injury.
The Administrator further takes note of the PA finding that the
current information, particularly in locations meeting the current
standard or with W126 index estimates likely to occur under the current
standard, does not indicate a significant extent and degree of injury
(e.g., based on analyses of BI scores in the PA, Appendix 4C) or
specific impacts on recreational or related services for areas, such as
wilderness areas or national parks. Thus, he gives credence to the
associated PA conclusion that the evidence indicates that areas that
meet the current standard are unlikely to have BI scores reasonably
considered to be impacts of public welfare significance. Based on all
of the considerations raised here, the Administrator concludes that the
current standard provides sufficient protection of natural areas,
including particularly protected areas such as Class I areas, from
O3 concentrations in the ambient air that might be expected
to elicit visible foliar injury of such an incidence and severity as
would reasonably be judged adverse to the public welfare.
With a primary focus on RBL in its role as proxy, the Administrator
further considers the analyses available in this review of recent air
quality at sites across the U.S., particularly including those sites in
or near Class I areas, and also the analyses of historical air quality.
In so doing, the Administrator recognizes that these analyses are
distributed across all nine NOAA climate regions and 50 states,
although some geographic areas within specific regions and states may
be more densely covered and represented by monitors than others, as
summarized in section III.C of the proposal (PA, Appendix 4D). The
Administrator notes that the findings from both the analysis of the air
quality data from the most recent period and from the larger analysis
of historical air quality data extending back to 2000, as presented in
the PA and summarized in section III.A.3 above, are consistent with the
air quality analyses available in the last review. That is, in
virtually all design value periods and all locations at which the
current standard was met across the 19 years and 17 design value
periods (in more than 99.9% of such observations), the 3-year average
W126 metric was at or below 17 ppm-hrs. Further, in all such design
value periods and locations the 3-year average W126 index was at or
below 19 ppm-hrs. The Administrator additionally considers the
protection provided by the current standard from the occurrence of
O3 exposures within a single year with potentially damaging
consequences, such as a significantly increased incidence of areas with
visible foliar injury that might be judged moderate to severe, as
discussed in section III.B.2 above. In so doing, he takes notes of the
PA analyses, summarized in section III.A.2.c above, of USFS BI scores,
giving particular focus to scores above 15, termed ``moderate to severe
injury'' by the USFS categorization scheme, as described in section
III.A.2.b above (PA, sections 4.3.3.2, 4.5.1.2 and Appendix 4C). He
notes the PA finding that incidence of sites with BI scores above 15
markedly increases with W126 index estimates above 25 ppm-hrs. In this
context, he additionally takes note of the air quality analysis finding
of a scarcity of single-year W126 index values above 25 ppm-hrs at
sites that meet the current standard, with just a single occurrence
across all U.S. sites with design values meeting the current standard
in the 19-year historical dataset dating back to 2000 (PA, section 4.4
and Appendix 4D). Further, in light of the evidence indicating that
peak short-term concentrations (e.g., of durations as short as one
hour) may also play a role in the occurrence of visible foliar injury,
the Administrator additionally takes note of the air quality analyses
in the PA and in the additional analysis documented in Wells (2020).
These analyses of data from the past 20 years show a declining trend in
1-hour daily maximum concentrations mirroring the declining trend in
design values, supporting the PA conclusion that the form and averaging
time of the current standard provides appreciable control of peak 1-
hour concentrations. Furthermore, these analyses indicate there to be
only a few days among sites meeting the current standard, with hourly
concentrations at or above 100 ppb (just seven in the period from 2000
through 2018) (Wells, 2020). In light of these findings from the air
quality analyses and considerations in the PA, both with regard to 3-
year average W126 index values at sites meeting the current standard
and the rarity of such values at or above 19 ppm-hrs, and with regard
to single-year W126 index values at sites meeting the current standard,
and the rarity of such values above 25 ppm-hrs, as well as with regard
to the appreciable control of 1-hour daily maximum concentrations, the
Administrator judges that the current standard provides adequate
protection from air quality conditions with the potential to be adverse
to the public welfare.
In reaching his conclusion on the current secondary O3
standard, the Administrator recognizes, as is the case in NAAQS reviews
in general, his decision depends on a variety of factors, including
science policy judgments and public welfare policy judgments, as well
as the currently available information. With regard to the current
review, the Administrator gives primary attention to the principal
effects of O3 as recognized in the current ISA, the 2013 ISA
and past AQCDs, and for which the evidence is strongest (e.g., growth,
reproduction, and related larger-scale effects, as well as, visible
foliar injury). With regard to growth and the categories of effects
identified above for which RBL has been identified for use as a proxy,
based on all of the considerations above, including the discussion of
air quality immediately above, the Administrator judges the current
standard to provide adequate protection for air quality conditions with
the potential to be adverse to the public welfare. Further, with regard
to visible foliar injury, the Administrator concludes that the
currently available information on visible foliar injury and with
regard to air quality analyses that may be informative with regard to
air quality conditions associated with appreciably increased incidence
and severity of BI scores at USFS biomonitoring sites, and with
particular attention to Class I and other areas afforded special
protection,
[[Page 87345]]
indicates the current standard to provide adequate protection from
visible foliar injury of an extent or severity that might be
anticipated to be adverse to the public welfare.
In summary, the Administrator has based his decision on the public
welfare protection afforded by the secondary O3 standard
from identified O3-related welfare effects, and from their
potential to present adverse effects to the public welfare, on
judgments regarding what the available evidence, quantitative
information, and associated uncertainties and limitations (such as
those identified above) indicate with regard to the protection provided
from the array of O3 welfare effects. He finds that, as a
whole, this information, as summarized above, and presented in detail
in the ISA and PA, does not indicate the current standard to allow air
quality conditions with implications of concern for the public welfare.
He has additionally considered the advice from the CASAC in this
review, including its finding ``that the available evidence does not
reasonably call into question the adequacy of the current secondary
ozone standard and concurs that it should be retained'' (Cox, 2020a, p.
1), and well as public comment on the proposed decision. Based on all
of the above considerations, including his consideration of the
currently available evidence and quantitative exposure/risk
information, the Administrator concludes that the current secondary
standard is requisite to protect the public welfare from known or
anticipated adverse effects of O3 and related photochemical
oxidants in ambient air, and thus that the current standard should be
retained, without revision.
C. Decision on the Secondary Standard
For the reasons discussed above and taking into account information
and assessments presented in the ISA and PA, the advice from the CASAC,
and consideration of public comments, the Administrator concludes that
the current secondary O3 standard is requisite to protect
the public welfare from known or anticipated adverse effects, and is
retaining the current standard without revision.
IV. Statutory and Executive Order Reviews
Additional information about these statutes and Executive Orders
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.
A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review
The Office of Management and Budget (OMB) has determined that this
action is a significant regulatory action and it was submitted to OMB
for review. Any changes made in response to OMB recommendations have
been documented in the docket. Because this action does not change the
existing O3 NAAQS, it does not impose costs or benefits
relative to the baseline of continuing with the current NAAQS in
effect. EPA has thus not prepared a Regulatory Impact Analysis for this
action.
B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs
This action is not an Executive Order 13771 regulatory action.
There are no quantified cost estimates for this action because EPA is
retaining the current standards.
C. Paperwork Reduction Act (PRA)
This action does not impose an information collection burden under
the PRA. There are no information collection requirements directly
associated with a decision to retain a NAAQS without any revision under
section 109 of the CAA, and this action retains the existing
O3 NAAQS without any revisions.
D. Regulatory Flexibility Act (RFA)
I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. Rather, this
action retains, without revision, existing national standards for
allowable concentrations of O3 in ambient air as required by
section 109 of the CAA. See also American Trucking Associations v. EPA,
175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant
impacts upon small entities because NAAQS themselves impose no
regulations upon small entities), rev'd in part on other grounds,
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).
E. Unfunded Mandates Reform Act (UMRA)
This action does not contain any unfunded mandate as described in
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely
affect small governments. This action imposes no enforceable duty on
any state, local, or tribal governments or the private sector.
F. Executive Order 13132: Federalism
This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.
G. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
This action does not have tribal implications, as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian Tribes. This action does not change existing
regulations; it retains the existing O3 NAAQS, without
revision. Executive Order 13175 does not apply to this action.
H. Executive Order 13045: Protection of Children From Environmental
Health and Safety Risks
This action is not subject to Executive Order 13045 because it is
not economically significant as defined in Executive Order 12866. The
health effects evidence and risk assessment information for this
action, which focuses on children and people (of all ages) with asthma
as key at-risk populations, is summarized in section II.A.2 and II.A.3
above and described in the ISA and PA, copies of which are in the
public docket for this action.
I. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution or Use
This action is not a ``significant energy action'' for purposes of
Executive Order 13211. The action is not likely to have a significant
adverse effect on the supply, distribution, or use of energy. This
action retains the current O3 NAAQS. This decision does not
change existing requirements. The Administrator of the Office of
Information and Regulatory Affairs has not otherwise designated this
action as a significant energy action. Thus, this decision does not
constitute a significant energy action as defined in Executive Order
13211.
J. National Technology Transfer and Advancement Act
This action does not involve technical standards.
K. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
The EPA believes that this action does not have disproportionately
high and
[[Page 87346]]
adverse human health or environmental effects on minority, low-income
populations and/or indigenous peoples, as specified in Executive Order
12898 (59 FR 7629, February 16, 1994). The action described in this
document is to retain without revision the existing O3 NAAQS
based on the Administrator's conclusions that the existing primary
standard protects public health, including the health of sensitive
groups, with an adequate margin of safety, and that the existing
secondary standard protects public welfare from known or anticipated
adverse effects. As discussed in section II above, the EPA expressly
considered the available information regarding health effects among at-
risk populations in reaching the decision that the existing standard is
requisite.
L. Determination Under Section 307(d)
Section 307(d)(1)(V) of the CAA provides that the provisions of
section 307(d) apply to ``such other actions as the Administrator may
determine.'' Pursuant to section 307(d)(1)(V), the Administrator
determines that this action is subject to the provisions of section
307(d).
M. Congressional Review Act
The EPA will submit a rule report to each House of the Congress and
to the Comptroller General of the United States. This action is not a
``major rule'' as defined by 5 U.S.C. 804(2).
V. References
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Adams, WC (2006). Comparison of chamber 6.6-h exposures to 0.04-0.08
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Cox, LA (2018). Letter from Dr. Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Acting Administrator Andrew R.
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of the Administrator, Science Advisory Board U.S. EPA HQ, Washington
DC. Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/
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Cox, LA (2020a). Letter from Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Administrator Andrew R.
Wheeler. Re:CASAC Review of the EPA's Policy Assessment for the
Review of the Ozone National Ambient Air Quality Standards (External
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Office of the Adminstrator, Science Advisory Board Washington, DC.
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Cox, LA (2020b). Letter from Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Administrator Andrew R.
Wheeler. Re:CASAC Review of the EPA's Integrated Science Assessment
for Ozone and Related Photochemical Oxidants (External Review
Draft--September 2019). February 19, 2020. EPA-CASAC-20-002. Office
of the Adminstrator, Science Advisory Board Washington, DC.
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Cromar, KR, Gladson, LA and Ewart, G (2019). Trends in Excess
Morbidity and Mortality Associated with Air Pollution above American
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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. 2020-28871 Filed 12-30-20; 8:45 am]
BILLING CODE 6560-50-P