>=80....................................................  0 \B\-<0.1 \C\            0.1%               0               0               0               0
>=70....................................................         0.2-0.7            1.0%            <0.1             0.1               0               0
>=60....................................................         3.3-8.8            11.2         0.6-3.2             4.9        <0.1-0.8             1.3
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                     Children With Asthma--Number of Individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................            0-67             202               0               0               0               0
>=70....................................................        93-1,145           1,616            3-39             118               0               0
>=60....................................................     1,517-8,544          11,776      282--2,609           3,977          23-637           1,033
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                    All Children--Percent of Simulated Population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................      0 \B\-<0.1             0.1               0               0               0               0
>=70....................................................         0.2-0.6             0.9            <0.1             0.1          0-<0.1            <0.1
>=60....................................................         3.2-8.2            10.6         0.6-2.9             4.3        <0.1-0.7             1.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                         All Children--Number of Individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80....................................................           0-464           1,211               0               0               0               0
>=70....................................................       727-8,305          11,923          16-341             660             0-5              14
>=60....................................................   14,928-69,794          96,261    2,601-24,952          36,643       158-5,997           9,554
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each study area were averaged across the 3-year assessment period. Ranges reflect the ranges of averages.
\B\ A value of zero (0) means that there were no individuals estimated to have the selected exposure in any year.
\C\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).

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

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

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

D. Proposed Conclusions on the Primary Standard

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

III. Rationale for Proposed Decision on the Secondary Standard

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

    \106\ In addition to the review's opening ``Call for
Information'' (83 FR 29785, June 26, 2018), systematic review
methodologies were applied to identify relevant scientific findings
that have emerged since the 2013 ISA, which included peer reviewed
literature published through July 2011. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2011 (providing some overlap with the
cutoff date for the last ISA) and March 30, 2018. Studies published
after the literature cutoff date for this ISA were also considered
if they were submitted in response to the Call for Information or
identified in subsequent phases of ISA development, particularly to
the extent that they provide new information that affects key
scientific conclusions (ISA, Appendix 10, section 10.2). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737.
---------------------------------------------------------------------------

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

A. General Approach

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

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

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

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

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

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

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

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

    In considering the public welfare protection provided by the then-
current standard, the Administrator gave primary consideration to an
analysis of cumulative seasonal exposures in or near Class I areas
\110\ during periods when the then-current standard was met, and the
associated estimates of growth effects in well-studied species of tree
seedlings, in terms of the O3 attributable reductions in RBL
in the median species for which E-R functions have been established (80
FR 65385-65386, 65389-65390, October 26, 2015).\111\ The Administrator