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

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

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

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

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

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

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

IV. Statutory and Executive Order Reviews

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

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

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

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

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

C. Paperwork Reduction Act (PRA)

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

D. Regulatory Flexibility Act (RFA)

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

E. Unfunded Mandates Reform Act (UMRA)

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

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

F. Executive Order 13132: Federalism

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

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

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

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

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

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

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

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

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

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

L. Determination Under Section 307(d)

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

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List of Subjects in 40 CFR Part 50

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

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