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.<IP_ADDRESS>, and IS.<IP_ADDRESS>, 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

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

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

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).
---------------------------------------------------------------------------

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

    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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

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

    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

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\
---------------------------------------------------------------------------

    \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]).
---------------------------------------------------------------------------

    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

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

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.
---------------------------------------------------------------------------

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

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.
---------------------------------------------------------------------------

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

    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

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).
---------------------------------------------------------------------------

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

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

    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\
---------------------------------------------------------------------------

    \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\
---------------------------------------------------------------------------

    \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).

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

    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

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

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.
---------------------------------------------------------------------------

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

    \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 <IP_ADDRESS>).
    \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 <IP_ADDRESS>).
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).
---------------------------------------------------------------------------

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

    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

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.<IP_ADDRESS>; 2013 ISA, Figure 9-1, sections <IP_ADDRESS> and
<IP_ADDRESS>). 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.<IP_ADDRESS>; 2013 ISA, sections
9.4.3 and <IP_ADDRESS>). 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
<IP_ADDRESS> and <IP_ADDRESS>). 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 <IP_ADDRESS> 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

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 <IP_ADDRESS>). 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.
---------------------------------------------------------------------------

    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 <IP_ADDRESS>). 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).
---------------------------------------------------------------------------

    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 <IP_ADDRESS>, 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).
---------------------------------------------------------------------------

    \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 <IP_ADDRESS>.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

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
<IP_ADDRESS> 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.
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    \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).
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    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).
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    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 <IP_ADDRESS>;
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

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).
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    \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).
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    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 <IP_ADDRESS>). 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).
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    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).

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