metrics, with one being a 3-month W126 index and a second in terms of

SUM06.\227\ For each metric, three ranges of ``thresholds'' are
presented (for different purposes). The ranges for SUM06 come from a
1996 workshop report (Heck and Cowling, 1997). The ranges for W126
index are based on a W126 index conversion of the SUM06 ranges. One of
the ranges is labeled as pertaining to foliar injury as a response,
yet, the publication cited does not provide data on foliar injury in
relation to that range, nor do publications cited by the former
publication. As we can best discern based on cited and related
publications, it appears to at the lower end relate to a benchmark
derived for growth effects (10% RBL) in the highly sensitive species,
black cherry, rather than visible foliar injury (Kohut, 2007b; Lefohn
et al., 1997; 80 FR 65378, October 26, 2015). Thus, contrary to the
commenters' assertion, the range for W126 index (labeled as pertaining
to foliar injury) does not appear to provide a threshold based on
evidence for visible foliar injury.
---------------------------------------------------------------------------

    \226\ For example, valid design values include: (1) 73 (2002)
and 72 (2003) at monitoring site 450190046, (2) 91 (2002), 94
(2003), and 88 (2004) at 230090102; (3) 77 ppb (2004) at 261530001,
and (4) 90 (2002 and 2003) at 340010005.
    \227\ We note that the third assessment approach utilizes a
combination of a W126 index metric with the N100 metric,
illustrating the consideration by the National Park Service of the
role of peak concentrations in posing risk of visible foliar injury
(Kohut, 2020).
---------------------------------------------------------------------------

    Some commenters (citing page 4C-18 of the PA), express confusion
over how EPA can state there to be an incomplete understanding of the
relationships influencing severity of visible foliar injury while also
using the USFS scores to inform the Administrator's judgments regarding
conditions that may be adverse to the public welfare. We see no
contradiction in this. Rather, it is this recognition of an incomplete
understanding, including the recognition of uncertainty in ``specific
aspects of [the influences of environmental/genetic factors] on the
relationship between O3 exposures, the most appropriate
exposure metrics, and the occurrence or severity of visible foliar
injury'' (PA, Appendix 4C, p. 4C-18), that leads the EPA to place
greatest weight on the most clear findings from the USFS data. With
regard to the PA presentation, with its recognized uncertainties and
limitations, such a finding is the obviously increased prevalence and
severity of visible foliar injury for records with W126 index estimates
above 25 ppm-hrs.
    Further, in considering public welfare implications of
O3 related visible foliar injury, the EPA continues to
recognize that the occurrence of visible foliar injury has the
potential to be adverse to the public welfare (e.g., as summarized in
section III.A.2.b above and section III.B.2 of the proposal). However,
as noted in the proposal, the EPA does not find that any small
discoloring on a single leaf of a plant (which might yield a quite low,
nonzero BI score in the USFS system) is reasonably considered adverse
to the public welfare. Thus, findings such as those raised by
commenters of injury on individual plants in controlled conditions,
while providing support to the conclusion of a causal relationship
between O3 exposure and visible foliar injury (ISA, Appendix
8, Table 8-3), are less informative to the Administrator's judgment on
adequacy of the protection provided by the current standard from
adverse effects to the public welfare. Rather, the USFS biosite
monitoring data provide information that is more useful for such a
judgment because this monitoring program, as summarized in section
III.A.2.b above (and III.B.3.b of the proposal), and the scale of its
objectives which focus on natural settings in the U.S. and forests as
opposed to individual plants is better suited for the Administrator's
consideration with regard to the public welfare protection afforded by
the current standard. In this context, as described in section III.B.3
below, the Administrator judges that very low BI scores, such as those
less than 5, described by the USFS scheme as ``little or no foliar
injury'' do not pose concern for the public welfare.\228\
---------------------------------------------------------------------------

    \228\ Studies that consider such data for purposes of
identifying areas of potential impact to the forest resource suggest
this category corresponds to ``none'' with regard to ``assumption of
risk'' (Smith et al., 2007; Smith et al., 2012).
---------------------------------------------------------------------------

    Lastly, we disagree with the comment that the Act requires the EPA
to specify ``a level'' of injury that is adverse. The Court of Appeals
for the D.C. Circuit has held that ``the Agency may sometimes need to
articulate the level of threat to the population it considers
tolerable; but there is no separate methodological requirement under
Sec.  109 that the Administrator establish a measure of the risk to
safety it considers adequate to protect public health every time it
establishes a standard pursuant to Sec.  109.'' See Nat. Res. Def.
Council, Inc. v. EPA, 902 F.2d 962, 973 (D.C. Cir. 1990), opinion
vacated in part on other grounds, 921 F.2d 326 (D.C. Cir. 1991). The
same principle applies for consideration of the protection of public
welfare in the context of establishing or reviewing secondary
standards. The court later confirmed that it ``expressly rejected the
notion that the Agency must `establish a measure of the risk to safety
it considers adequate to protect public health every time it
establishes a [NAAQS].'' See ATA III, 283 F.3d at 369 (D.C. Cir. 2002)
(quoting Natural Res. Def. Council, Inc. v. EPA, 902 F.2d 962, 973
[D.C. Cir. 1990]). As is recognized by the courts and by EPA and CASAC
across NAAQS reviews, the judgment of the Administrator, in addition to
being based on the scientific evidence, depends on a variety of
factors, including science policy judgments and public welfare policy
judgments. As noted by the case law and also in section III.B.2.b(iv)
below, the EPA is not required under the Act to identify individual
levels of adversity or set separate standards for every type of effect
that may be caused by a pollutant in ambient air, as long as it has
engaged in reasoned decision making in determining that a particular
standard provides the requisite protection. Thus, it is common for one
NAAQS to provide protection for multiple effects, with the most
sensitive effect influencing the stringency of the standard and
accordingly leading to protection that is adequate for other, less
sensitive effects. Given the significant uncertainties which are
present in every NAAQS review, it is enough for the Administrator to
set standards that specify a level of air quality that will be
``tolerable,'' (NRDC, 902 F.2d at 973), and ``qualitatively to describe
the standard governing its selection of particular NAAQS'' (ATA III,
283 F.3d at 369). In reviewing each standard, the EPA gives due
consideration to each of the effects that are relevant for that
standard in considering whether the standard provides adequate
protection from the type, magnitude or extent of such effects known or
anticipated to be adverse to the public welfare. In the case of visible
foliar injury, as discussed in section III.B.3 below, the Administrator
has considered the available scientific evidence, with associated
uncertainties and limitations, in reaching his decision that the
current secondary standard provides adequate public welfare protection
for this effect.
(iv) Crop Yield Effects
    Some commenters object to the proposed conclusions with regard to
the protection provided by the existing secondary standard from adverse
effects on the public welfare related to O3 effects on crop
yield, expressing the view that the EPA must specify ``a level'' to
protect the public welfare against crop yield reductions and that not
doing so is unlawful and arbitrary. These commenters' additionally
object to the Administrator's proposed judgment that a decision based
on RBL as a proxy for other vegetation-related effects will also
provide adequate protection against crop related effects, indicating
their view that EPA does not

adequately explain the basis for this judgment. These commenters
additionally claim that the prior CASAC described 5.1% RYL as
constituting an adverse welfare effect and express the view that the
EPA arbitrarily and unlawfully does not ``give effect to'' the prior
CASAC's recommendation.
    We disagree with the implication of these commenters that, in
judging adequacy of protection provided by the current standard for a
particular effect, it is per se unlawful to conclude that the air
quality achieved by the current standard provides adequate protection
for that particular effect, even if the greater attention in reviewing
the current standard is on another effect. The EPA is not precluded
from reaching such a conclusion as long as the Agency has engaged in
reasoned decision-making in doing so.\229\ In reaching his proposed
conclusions regarding the extent to which the current standard provides
appropriate protection from O3 effects on crop yield that
may be adverse to the public welfare, as in his conclusions described
in section III.B.3 below, the Administrator recognizes the long-
standing evidence of O3 effects on crop yield and the
established E-R functions for which RYL estimates for the median crop
species are presented in the PA (PA, Appendix 3A). He also considers
factors that might be important to his judgments related to the
requisite protection for a secondary standard that protects against
adverse effects to the public welfare. In this context he judges that
the median RYL estimated for air quality that achieves his RBL-related
objectives for the current standard does not constitute an adverse
effect on public welfare and thus concludes that the current standard
also provides adequate protection for crop yield-related effects. Given
that the decision on adequacy of protection is a judgment of the
Administrator and that the Clean Air Act does not require a particular
approach for reaching such judgments, we disagree with the commenters
to the extent that they suggest that it is per se unlawful for the
Administrator to use such an approach. The circumstances for his use of
this approach include particular aspects of the information available
on O3-related crop yield effects and other factors important
to judgments on public welfare effects related to crop yield effects.
---------------------------------------------------------------------------

    \229\ Section 109(b)(2) of the CAA provides only that any
secondary standard ``shall specify a level of air quality the
attainment and maintenance of which in the judgment of the
Administrator, based on [the air quality ] criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of such air pollutant in the
ambient air.''
---------------------------------------------------------------------------

    In reaching his decision in this review, as described in section
III.B.3 below, the Administrator has also considered public comments on
these issues, including that regarding a prior CASAC statement. The
comment regarding the prior CASAC appears to draw on a judgment of the
prior CASAC that a median RYL of 5% ``represents an adverse impact''
(Frey, 2014b, p. 14). The prior CASAC provided no clear scientific
foundation for this judgment. While we infer this judgment to draw on
discussion at a 1996 workshop,\230\ neither the prior CASAC nor the
workshop summary provides any explicit rationale for identification of
5% (with regard to RYL), or any description of a connection of an
estimated 5% RYL to broader impacts of a specific magnitude or type, or
to judgments on significance of a 5% RYL to the public welfare. Thus
the EPA disagrees with the commenters regarding the weight to give the
prior CASAC statement and, as described below, respectfully disagrees
with the prior CASAC on this statement.
---------------------------------------------------------------------------

    \230\ The first reference to 5% RYL by the prior CASAC (in the
2015 O3 NAAQS review) appears to be in its letter on the
first draft PA (Frey and Samet, 2012). In that letter, the prior
CASAC identifies 5% RYL as a factor on which levels for a W126 index
secondary standard should be based, although no rationale is
provided for this recommendation. In a letter attachment, comments
from an individual member point to a 1996 workshop (2014 PA, pp. 6-
15 through 6-17; Heck and Cowling, 1997). As summarized in the 2015
O3 decision, the 1996 workshop participants (16 leading
scientists, discussing their views for a secondary O3
standard) indicated an interest in protecting against crop yield
reductions of 5% yet noted uncertainties surrounding such a
percentage which led them to identify 10% RYL (80 FR 65378, October
26, 2015). In their emphasis on 5%, the 2012 comments from the
individual prior CASAC member expressed the view that the ability to
estimate 5% RYL has improved (Frey and Samet, 2012, p. A-54).
Neither the individual prior CASAC member nor the 1997 workshop
report provide any explicit rationale for the percentages identified
or any description of their connection to ecosystem impacts of a
specific magnitude or type, or to judgments on significance of the
identified effects for public welfare (80 FR 65378, October 26,
2015; Heck and Cowling, 1997).
---------------------------------------------------------------------------

    In reaching his judgment regarding whether the current standard
provides the requisite public welfare protection, as described in
section III.B.3 below, the Administrator considers the extent to which
a specific estimate of RYL may be indicative of adverse effects to the
public welfare. In so doing, he notes that the secondary standard is
not intended to protect against all known or anticipated O3-
related effects, but rather those that are judged to be adverse to the
public welfare, and that a bright-line determination of adversity is
not required in judging what is requisite. In his decision described
below, the Administrator also notes that the determination of the
extent of RYL estimated from experimental O3 exposures that
should be judged adverse to the public welfare is not clear, in light
of the extensive management of agricultural crops that occurs to elicit
optimum yields (e.g., through irrigation and usage of soil amendments,
such as fertilizer). Further, in considering effects on the public
welfare that may relate to agricultural markets, we note that
detrimental impacts on crops, as well as beneficial impacts, can be
unevenly distributed between producers and consumers, complicating
conclusions with regard to relative adversity. In light of such
considerations, the Administrator, while finding consideration of the
RYL estimate for the median crop species informative to his judgment on
the adequacy of the protection provided by the current standard for
this effect, does not find such an RYL estimate of 5% to represent an
adverse effect to the public welfare, as described more fully in
section III.B.3 below. For these reasons, and for the reasons discussed
earlier in this section, including those regarding advice from a prior
CASAC, the EPA also disagrees with the commenters' assertion that the
Administrator is arbitrarily and unlawfully failing to ``give effect
to'' the prior CASAC's recommendation.
    Further, we disagree with the comment that the Act requires the EPA
to specify ``a level'' to protect the public welfare against crop yield
reductions. As discussed in greater detail in section III.B.2.b(iii)
above, the EPA is not required under the Act to set separate standards
for every type of effect that may be caused by a pollutant in ambient
air, as long as it has engaged in reasoned decision making in
determining that a particular standard provides the requisite
protection. Thus, it is common for one NAAQS to provide protection for
multiple effects, with the most sensitive effect influencing the
stringency of the standard and accordingly leading to protection that
is adequate for other less sensitive effects. As discussed further in
section III.B.2.b(iii) above, in reviewing each standard, the EPA gives
due consideration to each effect relevant for that standard in
considering whether the standard provides adequate protection from the
type, magnitude or extent of such effects known or anticipated to be
adverse to the public welfare. In the case of crop yield loss, as
discussed in section III.B.3 below, the Administrator has considered
the magnitude of RYL that may be

associated with W126 index values that occur under the current standard
and, based on the current information with regard to the RYL estimates,
notes that these estimates are generally no higher than 5.1% and
predominantly well below that. In so doing, he has also considered
factors such as those raised above, and in light of all of these
considerations, he judges that a RYL of 5.1% does not represent an
adverse effect to the public welfare. Thus, the Administrator judges
that the current standard provides adequate protection of the public
welfare for crop yield loss related effects.
(v) RBL
    In objecting to the EPA's proposed decision, some commenters
disagree with the target level of protection identified based on use of
RBL. In so doing, such commenters variously claim that a 3-year average
of 17 ppm-hrs is ``ill-suited'' to protect against adverse impacts to
the public welfare; that 6% RBL is too high to protect the public
welfare; that use of a 3-year average instead of a single year W126
index is needed; and, that EPA must focus a target on exposures that
would avoid 2% RBL, citing comments from the prior CASAC on the second
draft PA in the 2015 review, and claiming that a focus on a W126 index
of 7 ppm-hrs is needed for that. With regard to the EPA's use of 6% in
considering the adequacy of protection related to RBL, these commenters
recognize that Murray Energy rejected an argument that EPA's prior
reliance on 6% (in the 2015 decision) was arbitrary based on the record
in that case (Murray Energy, 936 F.3d at 615-16). In pressing their
views, however, the commenters state that nothing in Murray Energy
prevents EPA from revising its prior determination based on the
scientific evidence and CASAC advice.
    With respect to the latter point, the EPA agrees that the
Administrator's decision in this review must take into account the
currently available scientific evidence and advice from the CASAC, and
that the Agency is not bound by the Administrator's conclusions in the
prior review. As summarized in the proposal for the current review, in
the proposal, the Administrator took the currently available scientific
evidence and advice from the CASAC into account, while also choosing to
consider the judgments and decision made by the prior Administrator in
that Administrator's consideration of RBL related targets for
cumulative seasonal exposure. He did so, in light of the welfare
effects evidence and air quality information now available, as well as
the advice from the current CASAC reflecting its concurrence that
implementation of the prior Administrator's approach or framework is
``still effective'' in protecting the public welfare from vegetation
effects of O3 (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). As described in section III.B.3 below, after
considering the public comments on this point, he is taking a similar
approach in reaching his decision in this review.
    With regard to the commenters' objection to the EPA's use of a 3-
year average in assessing RBL, we note, as an initial matter, that the
EPA's focus on a 3-year average of 17 ppm-hrs as a target level relates
to an RBL estimate of 5.3%, a value that was also chosen in 2015 in
recognition of the prior CASAC advice both with regard to 6% RBL and
about considering a lower W126 index target for a 3-year average due to
the prior CASAC's concern about ``unusually damaging years.'' In the
current review, the CASAC has explicitly considered the EPA's
interpretation of 6% in identifying a target of 17 ppm-hrs as a 3-year
average, and expressed its view that this target ``is still effective
in particularly protecting the public welfare in light of vegetation
impacts from ozone'' (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). Accordingly, the EPA disagrees with the comments that
6% RBL and a 3-year average W126 index target of 17 ppm-hrs are too
high to inform the Administrator's judgments on O3 air
quality that protects the public welfare; rather, the Administrator
continues to find this useful in informing his judgments regarding the
public welfare protection provided by the standard, together with a
broader consideration of air quality patterns associated with meeting
the current standard, such as control of peak hourly concentrations, as
described in section III.B.3 below. Further, we refer to the discussion
above of how the existing standard, with its current averaging time and
form provides the protection from the occurrence of elevated hourly
concentrations that may characterize what the prior CASAC described as
``unusually damaging years.'' As discussed above, the available air
quality data demonstrate the strong protection provided by the current
standard from elevated concentrations that may occur in some years. As
noted above, these analyses indicate that while the current form and
averaging time of the existing standard provides control of these
concentrations and the associated peak exposures, reliance solely on a
standard in the form of the W126 index based standard, as advocated by
the commenters, even with a level as low as 7 ppm-hrs cannot be relied
on to provide it.
    In support of their view that the EPA must focus on avoiding 2% RBL
with a W126 index of 7 ppm-hrs, these commenters provide little
rationale beyond citing a comment by the prior CASAC made in the last
review. In so doing, the commenters assert that because the prior CASAC
had noted that 7 ppm-hrs was the only W126 index level for which the E-
R functions yielded a RBL for the median tree species that was less
than or equal to 2%, the EPA must protect against 2% RBL and adopt a
W126 index level of 7 ppm-hrs. We disagree. As an initial matter, we
note our discussion above regarding the EPA's consideration in this
review of advice from a prior CASAC, including prior CASAC statements
that are raised by commenters, such as those noted here. Further, in
making the statement that the commenters' cite, the prior CASAC did not
reach the same conclusion as the commenters with regard to the extent
to which a revised secondary standard should limit cumulative exposures
and associated estimates of RBL, such that the prior CASAC did not
recommend that the EPA consider only W126 index levels associated with
median RBL estimates at or below 2%.\231\ See Murray Energy, 936 F.36
at 615-16 (noting that ``CASAC did not identify 2% growth loss as the
only sufficiently protective level'' but merely recommended ``2% as the
lower end of a range of permissible target levels'' to be considered).
In fact, seven of the nine W126 index levels in the range recommended
by the prior CASAC (7 to 15 ppm-hrs [Frey, 2014b]) are associated with
RBL estimates higher than 2% (PA, Appendix 4A). As a basis for their
assertion that the secondary standard should protect against a median
RBL of 2%, these commenters additionally oddly declare that after three
years, a 2% RBL per year ``becomes 6%.'' There is no evidence in the
record, and the commenter provides no evidence, that would support
their declaration that without a tripling in exposure, the
O3-attributable reduction in annual growth (the RBL) would
triple.\232\ Nor is there

evidence that would support an alternative interpretation of the
commenters' statement as a claim that a tree experiencing a 2% RBL per
year is reduced in absolute biomass by 6% after three years.\233\
---------------------------------------------------------------------------

    \231\ Additionally, an explicit scientific rationale for 2% is
not provided by the former CASAC. Nor is it provided in the workshop
report referenced by the prior CASAC in its discussion, as further
discussed in the 2015 decision (80 FR 65394, October 26, 2015; Frey,
2014b, p. 14).
    \232\ It is unclear by what logic the commenters conclude that
RBL, a metric describing the effect of the O3 exposure in
a single year, can be modified by the RBL in a prior year.
    \233\ The fallacy of such interpretations can be seen in the
presentation of above-ground biomass from a multiyear study of
O3 exposure of aspen that varies little over six years.
Across the six years, the above-ground biomass of the trees
receiving elevated O3 exposure is 25%, 30%, 29%, 29%, 31%
and 29% lower than the reference trees (2013 ISA, Table 9-14; 2020
ISA, Figure 8-17).
---------------------------------------------------------------------------

    Some commenters who disagree with the proposed decision also
express the view that the EPA has ``proposed'' to use RBL functions for
trees as a proxy for all vegetation effects. Based on this view, these
commenters variously assert that the EPA is failing to comply with its
obligation under the Clean Air Act that a secondary standard protect
the public welfare from ``any known or anticipated adverse effects'';
that the EPA's approach is not the same as the prior CASAC's discussion
of RBL as a surrogate; that the EPA is contravening its statutory
obligation by using one adverse effect as a surrogate for another
without showing that prevention of the former will prevent the latter;
and that, based on the commenters' interpretation of a statement made
by the prior CASAC, a standard that allows tree growth loss above 2%
cannot protect against visible foliar injury. As an initial matter, we
note that the citation provided by the commenters for their statement
that the ``EPA proposes'' to use RBL functions as a proxy for the broad
array of O3 vegetation-related effects does not include such
a ``proposal.'' Rather the commenters' citation points to the
background section of the proposal which simply summarizes the concept
of RBL as a proxy or surrogate which was employed in the last review
and which was described by the prior CASAC (85 FR 49899, August 14,
2020). In describing use of RBL as a proxy or surrogate, the proposal
(and the PA) use several phrases, ranging from ``for consideration of
the broader array of vegetation-related effects of potential public
welfare significance, that included effects on growth of individual
sensitive species and extended to ecosystem-level effects, such as
community composition in natural forests, particularly in protected
public lands, as well as forest productivity'' (85 FR 49878, August 14,
2020), to shorter phrases, such as ``for the broad array of vegetation
related effects that extend to the ecosystem scale'' (85 FR 49911,
August 14, 2020).
    We disagree with these commenters that the way the EPA uses RBL as
a ``proxy'' or ``surrogate'' is contrary to law, and with their
contention that the EPA uses one adverse effect as a surrogate for
another without showing that prevention of the former will prevent the
latter. As described in the Administrator's decision below, the most
precise use of RBL as a surrogate or proxy is in the target level of
protection for cumulative seasonal exposure (17 ppm-hrs as a 3-year
average W126 index). This use relates specifically to public welfare
effects related to O3 effects on growth of individual
sensitive species and related effects, including ecosystem-level
effects, such as community composition in natural forests, particularly
in protected public lands, as well as forest productivity (as discussed
in the PA, section <IP_ADDRESS>). In fact, the ISA describes (or relies on)
conceptual relationships among such effects in considering causality
determinations for ecosystem-scale effects such as altered terrestrial
community composition and reduced productivity, as well as reduced
carbon sequestration, in terrestrial ecosystems (ISA, Appendix 8,
sections 8.8 and 8.10). Beyond these relationships of plant-level
effects and ecosystem-level effects,\234\ RBL can be appropriately
described as a scientifically valid surrogate of a variety of welfare
effects based on consideration of ecosystem services and the potential
for adverse impacts on public welfare, as well as conceptual
relationships between vegetation growth-related effects (including
carbon allocation) and ecosystem-scale effects (PA, pp. 4-75 and 4-76).
Both the prior CASAC and the current CASAC recognized this (Frey,
2014b, pp. iii, 9-10; \235\ Cox, 2020a, Consensus Responses to Charge
Questions pp. 18 and 21 \236\). As was discussed in the proposal, the
information available in this review provides continued support for the
use of tree seedling RBL as a proxy for the broad array of vegetation-
related effects conceptually related to growth effects, a conclusion
with which the CASAC agreed (85 FR 49899,49906, August 14, 20202).\237\
---------------------------------------------------------------------------

    \234\ As summarized in the ISA, O3 can mediate
changes in plant carbon budgets (affecting carbon allocation to
leaves, stems, roots and other biomass pools) contributing to growth
impacts, and altering ecosystem properties such as productivity,
carbon sequestration and biogeochemical cycling. In this way,
O3 mediated changes in carbon allocation can ``scale up''
to population, community and ecosystem-level effects including
changes in soil biogeochemical cycling, increased tree mortality,
shifts in community composition, changes in species interactions,
declines in ecosystem productivity and carbon sequestration and
alteration of ecosystem water cycling (ISA, section 8.1.3).
    \235\ The prior CASAC 2014 letter on the second draft PA in that
review stated the following (Frey, 2014b, p. 9-10):
    For example, CASAC concurs that trees are important from a
public welfare perspective because they provide valued services to
humans, including aesthetic value, food, fiber, timber, other forest
products, habitat, recreational opportunities, climate regulation,
erosion control, air pollution removal, and hydrologic and fire
regime stabilization. Damage effects to trees that are adverse to
public welfare occur in such locations as national parks, national
refuges, and other protected areas, as well as to timber for
commercial use. The CASAC concurs that biomass loss in trees is a
relevant surrogate for damage to tree growth that affects ecosystem
services such as habitat provision for wildlife, carbon storage,
provision of food and fiber, and pollution removal. Biomass loss may
also have indirect process-related effects such as on nutrient and
hydrologic cycles. Therefore, biomass loss is a scientifically valid
surrogate of a variety of adverse effects to public welfare.
    \236\ The CASAC letter on the draft PA in the current review
stated the following (Cox, 2020a, Consensus Responses to Charge
Questions p. 18):
    The RBL appears to be appropriately considered as a surrogate
for an array of adverse welfare effects and based on consideration
of ecosystem services and potential for impacts to the public as
well as conceptual relationships between vegetation growth effects
and ecosystem scale effects. Biomass loss is a scientifically sound
surrogate of a variety of adverse effects that could be exerted to
public welfare. . . . In the previous review, the Administrator used
RBL as a surrogate for consideration of the broader array of
vegetation related effects of potential welfare significance that
included effects of growth of individual sensitive species and
extended to ecosystem level effects such as community composition in
natural forests, particularly in protected public lands (80 FR
65406, October 26, 2015). The EPA believes, and the CASAC concurs,
that information available in the present review does not call into
question this approach, indicating there continues to be support for
the use of tree seedling RBL as a proxy for the broader array of
vegetation-related effects, most particularly those related to
growth.
    \237\ Further, the EPA lacks sufficient information in the air
quality criteria to identify requisite air quality for these
effects.
---------------------------------------------------------------------------

    As recognized in the proposal (and PA) there are two other
vegetation effect categories with extensive evidence bases (which
include analyses that assess the influence of cumulative seasonal
exposure); these are crop yield loss and visible foliar injury. As
discussed above, the consideration of protection provided by the
current standard for the former goes beyond the target focused on RBL
and includes aspects of the evidence specific to those effects. As
described above and in section III.B.3 below, the EPA is concluding
that the level of protection is adequate to protect the public welfare
from effects related to crop yield loss. With regard to the latter,
contrary to the commenter's assertion, the EPA is not claiming that
protection focused on RBL provides protection for visible foliar
injury. The EPA's consideration of visible foliar injury is described
earlier in this section and in section III.B.3 below.

    With regard to the two newly identified categories of insect-
related effects, the Administrator finds there to be insufficient
information to judge the current standard inadequate based on these
effects, as discussed in section III.B.3 below. He does not claim that
RBL provides a surrogate for these effects. However, he notes that the
available information in the air quality criteria does not indicate a
greater sensitivity of such effects as compared to O3
effects on vegetation growth, and that he lacks sufficient information
in the air quality criteria to identify requisite air quality for these
effects.
(vi) W126 Index in Areas Meeting Current Standard
    In objecting to the proposed decision, one group of commenters
disagree with EPA's findings regarding the W126 index levels in areas
that meet the current standard. In so doing, these commenters claim
that the EPA is mistaken to claim that in virtually all design value
periods and locations at which the current standard was met across the
period covered by the historical dataset the 3-year W126 index was at
or below 17 ppm-hrs because they variously assert there are either 25
or 21 such occurrences, and they further assert there to be either 50
occurrences of a single-year W126 index at or above 19 ppm-hrs or 52
occurrences of a single-year W126 index above 19 ppm-hrs. These counts
are in disagreement with the air quality analyses documented in
Appendix 4D of the PA. For example, out of 8,292 values across nearly
20 years for U.S. ambient air monitoring sites, distributed across all
nine climate regions, with air quality that meets the current standard,
there are just eight occurrences of a 3-year W126 index value above 17
ppm-hrs (PA, Appendix 4D, Tables 4D-10 and 4D-7). This means that 99.9%
of the records (virtually all) were at or below 17 ppm-hrs. While the
details of each step of the analyses in the PA are extensively
documented, including data handling, rounding conventions and data
acceptability criteria (PA, Appendix 4D, section4D.2), the lack of
documentation provided by the commenters and their conflicting claims
(indicated above) leave the EPA to hypothesize that the reason for the
disagreements include differences with regard to these details, such as
those regarding rounding conventions. As described in the PA, W126
values ``were rounded to the nearest unit ppm-hr for applications
requiring direct comparison to a W126 level,'' a convention intended to
provide consistency in the precision of the comparison as the W126
levels for comparison were also in whole numbers (PA, Appendix 4D,
section 4D.2.2). With the rounding conventions applied in the PA, there
are eight 3-year W126 index values greater than 17 ppm-hrs (i.e., equal
to 18 or 19). It may be that the commenters counted unrounded 3-year
W126 index values as low as 17.01 as being greater than 17 ppm-hrs,
although the reason for them providing two conflicting counts is
unclear. Similarly with regard to the counts for single-year W126 index
values above 19 ppm-hrs, the commenters may have counted unrounded
single-year index values as low as 19.01 ppm-hrs as being greater than
19 ppm-hrs. Thus, we find the commenters criticism of the EPA's
characterization of the findings of the air quality analyses, as well
as the commenters' counts, to be unfounded.
    Some commenters claim EPA pays inadequate attention to the
relatively few occurrences of single-year W126 index values at or above
19 ppm-hrs that have occurred at sites meeting the current standard
since 2002 and that the standard must be set to avoid such occurrences.
The EPA disagrees with these commenters, as described below, after
carefully considering the relatively few occurrences of W126 index
values at or above 19 ppm-hrs, including single-year values. In so
doing, we have given particular focus on Class I areas, recognizing the
attention given to such areas by the Administrator in judging the
potential for effects adverse to the public welfare, a focus recognized
by the CASAC and with which the prior CASAC explicitly concurred (Cox,
2020a; Frey, 2014b, p. 9).
    Among the nearly 500 values for monitoring sites in or near Federal
Class I areas across the U.S., during periods from 2000 through 2018
when the current standard was met, there are no occurrences of a 3-year
average W126 index above 19 ppm-hrs (PA, Table 4-1). Across this same
period in the same Class I locations, there are just 15 occurrences of
single-year W126 index values above 19 ppm-hrs, all of which date prior
to 2013 (PA, Appendix 4D, section 4D.3.2.4). All of these occurrences
are below 25 ppm-hrs. Thus, in addition to their being relatively few
occurrences of a single-year W126 above 19 ppm-hrs in/near a Class I
area in the 19-year dataset, none of them (the most recent of which was
in 2012) is higher than 25 ppm-hrs; in fact, the highest is 23 ppm-hrs
(PA, Appendix 4D, section 4D.3.2.4).
    We have also considered the full 19-year dataset for locations
beyond those in or near Class I areas, noting that, at other sites
across the U.S., occurrences of single-year W126 index above 19 ppm-hrs
(22) were predominantly in urban areas, including Los Angeles, and the
highest values were just equal to 25 ppm-hrs, or, in one instance, just
equal to 26 ppm-hrs, when rounded (85 FR 49895, 49904, August 14, 2020;
PA, sections 4.4 and 4.5, Appendix 4D). In considering the potential
risk posed by these scattered occurrences, largely in urban areas, with
none since 2012 in or near a Class I areas, we additionally consider
the data on peak hourly concentrations also discussed above (Wells,
2020). Together, these data indicate the control provided by the
current standard in areas that are of particular focus in protecting
the public welfare, on the extent and frequency of occurrence of
cumulative exposures in terms of the W126 index (and of peak
concentrations) of a magnitude of potential concern. As discussed in
section III.B.3 below, the Administrator does not find the air quality
patterns allowed by the current standard, as indicated by these
analyses, to pose a risk of adverse effects to the public welfare.
    In their criticism of the EPA's air quality analyses, one commenter
claims that the analyses are difficult to evaluate for California and
other West region states and suggest that California sites brought into
compliance with the existing standard would still have elevated W126
index values, similar to sites in the Southwest region. We disagree
with the commenter's claim that the air quality analyses suggest that
compliance with the existing standard would not reduce the W126 index
values at California sites. In making their claim, the commenters cite
Figures 4D-4 and 4D-5 of the PA. These figures, however, simply
document W126 index at sites with various design values at one point in
time (2016-2018). They do not describe analyses of trends over time,
with changes in air quality. Yet, that very issue was the subject of
separate regression analyses in the PA (PA, Appendix 4D, section
4D.3.2.3). These analyses show that the Southwest region, which had
highest W126 index values at sites meeting the current standard, also
exhibited the greatest improvement in the W126 metric values per unit
decrease in their design value (slope of 0.93) over the nearly 20 year
period analyzed. The pattern is very similar for the West region (with
a slope of 0.80), with the exception of three sites (in downtown LA);
however, the design values for these sites are above 100 ppb, making
such projections quite uncertain (PA, Appendix 4D, section 4D.3.2.3).

(vii) Climate Effects
    In support of their disagreement with the EPA's proposed decision,
some commenters claim that EPA needs to establish a standard to protect
from radiative forcing and related climate effects. In so doing, they
stated that EPA cannot rely on uncertainty by retaining the existing
standard and instead, given the uncertainties recognized in the ISA,
which they suggest could mean current information underestimates
O3 climate related impacts, the Administrator should
strengthen the existing standard or establish an additional standard.
Some commenters additionally assert that the EPA has failed to address
a recommendation from CASAC regarding a quantitative analysis, while
also criticizing EPA conclusions regarding a carbon storage analysis in
the last review. The EPA disagrees with the commenters that the
available information is sufficient to identify such a standard that
could be judged to provide the requisite protection under the Act, and
notes that the commenters do not submit or describe such information;
nor do the commenters identify a standard that they claim would provide
such protection.
    With regard to the CASAC recommendation cited by some commenters,
we note in its review of the draft PA, the CASAC recommended changes to
the PA to ``more thoroughly address effects of ozone on climate
change,'' that would include some quantitation, such as estimates of
climate change related to a change in O3 (Cox 2020a,
Consensus Responses to Charge Questions p. 22). In consideration of
this advice, the final PA included additional discussion on the
available information and tools related to such estimates. As discussed
below, we conclude that this information, as documented in the ISA,
does not provide a foundation with which to derive such estimates as
might pertain to O3 and public health and welfare
considerations relevant to decisions on the NAAQS.\238\
---------------------------------------------------------------------------

    \238\ In raising EPA's conclusions on a carbon storage analysis
in the last review, some commenters repeat their comments in the
last review that claimed that the relatively lesser weight the EPA
placed on the 2014 WREA estimates of carbon storage (in terms of
CO2) was inconsistent with the emphasis the EPA placed on
CO2 emissions reductions estimated for another regulatory
action. The commenters overlook, however, key distinctions between
the two types of estimates in the two different analyses which
appropriately led the EPA to recognize much greater uncertainty in
the WREA estimates and accordingly give them less weight. While the
WREA estimates were for amounts of CO2 removed from the
air and stored in vegetation as a result of plant photosynthesis
occurring across the U.S., the estimates for the other action were
for reductions in CO2 produced and emitted from power
plants (79 FR 34830, 34931-33). The potentially transient nature of
carbon storage in vegetation makes a ton of additional carbon uptake
by plants in the former arguably unequal to a ton of reduced
emissions from fossil fuels. Further, there are appreciably larger
uncertainties involved in attempting to quantify the additional
carbon uptake by plants which requires complex modeling of
biological and ecological processes and their associated sources of
uncertainty, and there is no new information available in the
current review that would reduce such uncertainties in quantitative
estimates of carbon storage benefits to climate. In recognizing the
public welfare value of ecosystem carbon storage, we additionally
note, however, that protection provided by the current standard from
vegetation effects (and RBL) also provides a degree of protection in
terms of carbon storage.
---------------------------------------------------------------------------

    As recognized in the proposal and summarized in section III.A.2
above, there are a number of limitations and uncertainties that affect
our ability to characterize the extent of any relationships between
O3 concentrations in ambient air in the U.S. and climate-
related effects, thus precluding a quantitative characterization of
climate responses to changes in O3 concentrations in ambient
air at regional (vs global) scales. While evidence supports a causal
relationship between the global abundance of O3 in the
troposphere and radiative forcing, and a likely causal relationship
between the global abundance of O3 in the troposphere and
effects on temperature, precipitation, and related climate variables
(ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013), the non-
uniform distribution of O3 (spatially and temporally) makes
the development of quantitative relationships between the magnitude of
such effects and differing O3 concentrations in the U.S.
challenging (ISA, Appendix 9). Additionally, ``the heterogeneous
distribution of ozone in the troposphere complicates the direct
attribution of spatial patterns of temperature change to ozone induced
[radiative forcing]'' and there are ``ozone climate feedbacks that
further alter the relationship between ozone [radiative forcing] and
temperature (and other climate variables) in complex ways'' (ISA,
Appendix 9, section 9.3.1, p. 9-19). Thus, various uncertainties
``render the precise magnitude of the overall effect of tropospheric
ozone on climate more uncertain than that of the well-mixed GHGs'' and
``[c]urrent limitations in climate modeling tools, variation across
models, and the need for more comprehensive observational data on these
effects represent sources of uncertainty in quantifying the precise
magnitude of climate responses to ozone changes, particularly at
regional scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p.
9-22). For example, current limitations in modeling tools include
``uncertainties associated with simulating trends in upper tropospheric
ozone concentrations'' (ISA, section 9.3.1, p. 9-19), and uncertainties
such as ``the magnitude of [radiative forcing] estimated to be
attributed to tropospheric ozone'' (ISA, section 9.3.3, p. 9-22).
Further, ``precisely quantifying the change in surface temperature (and
other climate variables) due to tropospheric ozone changes requires
complex climate simulations that include all relevant feedbacks and
interactions'' (ISA, section 9.3.3, p. 9-22). For example, an important
limitation in current climate modeling capabilities for O3
is representation of important urban- or regional-scale physical and
chemical processes, such as O3 enhancement in high-
temperature urban situations or O3 chemistry in city centers
where NOx is abundant. Because of such limitations we cannot quantify
or judge the impact of incremental changes in O3
concentrations in the U.S. on radiative forcing and subsequent climate
effects.
    Thus, as discussed in section III.B.3 below, the significant
limitations and uncertainties summarized here together preclude
identification of an O3 standard that could be judged to
provide requisite protection of the public welfare from adverse effects
linked to O3 influence on radiative forcing, and related
climate effects. Contrary to the commenters' charge that the lack of a
quantitative analysis of climate-related effects due to recognition of
such limitations and uncertainties is unlawful and arbitrary, the
information available in this review is insufficient to judge the
existing standard inadequate or to identify an appropriate revision
based on O3-related climate effects. In the face of
insufficient evidence for such conclusions, it might, on the contrary,
be judged unlawful and arbitrary for the Agency to perform guesswork to
assert a particular new standard provided requisite protection for this
category of effects. The EPA agrees with the commenters that ``perfect
information'' is not required. However, information that provides for
assessment of how the current and potential alternative or additional
standards would affect O3-related climate impacts is
lacking. As noted in the ISA, few studies have even attempted to
estimate ``climate response to changes in tropospheric ozone
concentrations alone in the future atmosphere,'' and effects of
O3 on radiative forcing and climate depend on many factors
other than tropospheric ozone concentrations, including ``changes in a
suite of climate-sensitive

factors, such as the water vapor content of the atmosphere'' (ISA, p.
9-20; Myhre et al., 2013). Thus, as discussed in section III.B.3 below,
while the Administrator recognizes that the evidence supports a
relationship of tropospheric O3 with climate effects, he
judges the quantitative uncertainties to be too great to support
identification of a standard specific to such effects.
4. Administrator's Conclusions
    Based on the large body of evidence concerning the welfare effects,
and potential for public welfare impacts, of exposure to O3
in ambient air, and taking into consideration the attendant
uncertainties and limitations of the evidence, the Administrator
concludes that the current secondary O3 standard provides
the requisite protection against known or anticipated adverse effects
to the public welfare, and should therefore be retained, without
revision. In reaching these conclusions, the Administrator has
carefully considered the assessment of the available welfare effects
evidence and conclusions contained in the ISA, with supporting details
in the 2013 ISA and past AQCDs; the evaluation of policy-relevant
aspects of the evidence and quantitative analyses in the PA (summarized
in sections III.A.2 and III.A.3 above); the advice and recommendations
from the CASAC (summarized in section III.B.1.b above); and public
comments (as discussed in section III.B.2 above and the separate
Response to Comments document), as well as the August 2019 decision of
the D.C. Circuit remanding the secondary standard established in the
last review to the EPA for further justification or reconsideration.
    In considering the currently available information in this review
of the O3 secondary standard, the Administrator recognizes
the longstanding evidence base for vegetation-related effects,
augmented in some aspects since the last review, described in section
III.A.2.a above. The currently available evidence describes an array of
effects on vegetation and related ecosystem effects causally or likely
to be causally related to O3 in ambient air, as well as the
causal relationship of tropospheric O3 in radiative forcing
and subsequent likely causally related effects on temperature,
precipitation and related climate variables. The Administrator also
takes note of the quantitative analyses and policy evaluations
documented in the PA that, with CASAC advice and consideration of
public comment, inform the judgments required of him in reaching his
decision on a secondary standard that provides the requisite protection
under the Act.
    As an initial matter, the Administrator recognizes the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants (as recognized in section III.B.1.c above). In
so doing, he notes that no newly available evidence has been identified
in this review regarding the importance of photochemical oxidants other
than O3 with regard to abundance in ambient air, and
potential for welfare effects, and that, as stated in the current ISA,
``the primary literature evaluating the health and ecological effects
of photochemical oxidants includes ozone almost exclusively as an
indicator of photochemical oxidants'' (ISA, section IS.1.1). Thus, the
Administrator recognizes that, as was the case for previous reviews,
the evidence base for welfare effects of photochemical oxidants does
not indicate an importance of any other photochemical oxidants. For
these reasons, described with more specificity in the ISA and PA, he
proposes to conclude it is appropriate to retain O3 as the
indicator for the secondary NAAQS for photochemical oxidants (85 FR
49896, August 14, 2020).
    In his review of the existing secondary O3 standard, in
light of the evidence base and quantitative analyses available today,
the Administrator has given particular attention to consideration of
the issues raised by the August 2019 court remand, and related issues
raised in public comment, as well as analyses that were conducted or
updated in this review in consideration of the remand and related
public comment. In so doing, he has also given careful consideration of
the form and averaging time of the current standard and its ability to
control the patterns of O3 concentrations that contribute to
environmental exposures of potential concern to the public welfare.
Further, he has considered what is indicated by the information
currently available with regard to exposure metrics, supported by the
current evidence, for assessing potential risks posed to vegetation,
and protection provided from such exposures. Additionally, with regard
to visible foliar injury, he has considered the current evidence in the
ISA in combination with quantitative information and policy evaluations
in the PA, advice from the CASAC and public comment, in making
judgments regarding adequacy of the protection provided by the current
standard from adverse effects to the public welfare related to this
effect. Before turning to these issues, discussed, in turn, below in
the context of the EPA's understanding of the information now available
in the current review, he addresses two endpoints newly identified in
this review, as well as tropospheric O3 effects related to
climate.
    With regard to the two insect-related categories of effects with
new ISA determinations in this review, the Administrator takes note of
the conclusions that the current evidence is sufficient to infer likely
causal relationships of O3 with alterations of plant-insect
signaling and insect herbivore growth and reproduction (as summarized
in section III.A.2.a above). He additionally recognizes the PA finding
that uncertainties in the current evidence, as summarized in section
III.A.2 above, preclude a full understanding of such effects, the air
quality conditions that might elicit them, the potential for impacts in
a natural ecosystem. Accordingly, the Administrator notes a lack of
clarity in the characterization of these effects, and a lack of
important quantitative information to consider such effects in this
context such that it is not feasible to relate different patterns of
O3 concentrations with specific risks of such alterations.
As a result, the Administrator concludes there is insufficient
information to judge how particular ambient air concentrations of
O3 relate to the degree of impacts on public welfare related
to these effects. Thus, he concludes there is insufficient information
to judge the current standard inadequate or to identify an appropriate
revision based on these effects.
    Before focusing further on the key vegetation-related effects
identified above, the Administrator first considers the strong evidence
documenting tropospheric O3 as a greenhouse gas causally
related to radiative forcing, and likely causally related to subsequent
effects on variables such as temperature and precipitation. In so
doing, he takes note of the limitations and uncertainties in the
evidence base that affect characterization of the extent of any
relationships between O3 concentrations in ambient air in
the U.S. and climate-related effects, and preclude quantitative
characterization of climate responses to changes in O3
concentrations in ambient air at regional or national (vs global)
scales, as summarized in sections III.A.2 above. As a result, he
recognizes the lack of important quantitative tools with which to
consider such effects in this context such that it is not feasible to
relate different patterns of O3 concentrations at the
regional (or national) scale in the U.S. with specific risks of
alterations in temperature, precipitation and other

climate-related variables. The Administrator finds that these
significant limitations and uncertainties together preclude his
identification of an O3 standard reasonably judged to
provide requisite protection of the public welfare from adverse effects
linked to O3 influence on radiative forcing, and related
climate effects. Thus, the Administrator concludes that the information
available in this review is insufficient to judge the existing standard
inadequate or to identify an appropriate revision based on
O3-related climate effects.
    The Administrator turns now to vegetation-related effects, the
evidence for which as a whole is extensive, spans several decades, and
supports the Agency's conclusions of causal or likely to be causal
relationship for O3 in ambient air with an array of effect
categories. These categories include reduced vegetation growth,
reproduction, crop yield, productivity and carbon sequestration in
terrestrial systems; increased tree mortality; alteration of
terrestrial community composition, belowground biogeochemical cycles
and ecosystem water cycling; and visible foliar injury (ISA, Appendix
8). As an initial matter, the Administrator notes the new ISA
determination that the current evidence is sufficient to infer likely
causal relationships of O3 with increased tree mortality.
With regard to the current evidence for this effect, the Administrator
notes that the evidence does not indicate a potential for O3
concentrations that occur in locations that meet the current standard
to cause increased tree mortality, as summarized in section III.A.2.a
above (PA, section 4.3.1). Accordingly, he finds it appropriate to
focus on more sensitive effects, such as tree seedling growth, in his
review of the standard. Thus, in considering the adequacy of protection
provided by the current standard from adverse effects to the public
welfare related to these effects, the Administrator begins by
considering vegetation growth and conceptually related effects with a
focus on RBL (described in section III.B.2 above), then turns to a
specific consideration of crop yield loss and lastly, to consideration
of visible foliar injury.
    With regard to vegetation growth and related effects, the
Administrator has considered discussions in the PA and in response to
public comments in section III.B.2 above, and finds it appropriate for
identification of the requisite protection to extend beyond
consideration of a magnitude of growth effects, per se, that he may
judge adverse to the public welfare. Rather, the Administrator extends
his consideration beyond that, judging it appropriate to consider
reduced growth (i.e., RBL) as a proxy for an array of other vegetation-
related effects to the public welfare. As discussed in section III.B.2
above, these categories of effects include reduced vegetation growth,
reproduction, productivity and carbon sequestration in terrestrial
systems, and also alteration of terrestrial community composition,
belowground biogeochemical cycles and ecosystem water cycling. In
adopting RBL as a proxy for this array of effects, the Administrator
notes that such a use is consistent with advice from CASAC, and that
RBL was also adopted as a proxy for this array of effects by the prior
Administrator, in consideration of advice from the prior CASAC.
    In assessments of RBL estimated from O3 exposure, the
Administrator takes note of the PA consideration of the established E-R
relationships for RBL in tree seedlings of 11 species with
O3 exposures in terms of W126 index (PA, Appendix 4A). In so
doing, he agrees with the PA conclusion regarding 6% RBL, with which
the CASAC concurred, as described in sections III.B.1.b and III.B.2
above), and judges that for his use of RBL as a proxy, maintaining
O3 concentrations such that associated estimates of RBL fall
below 6%, as a median across the 11 species represented by the
established E-R relationships would assure the appropriate protection.
In making these judgments, he observes that they were also adopted by
the prior Administrator, with consideration of advice from the prior
CASAC.
    Further, based on considerations discussed in the PA, advice from
CASAC and discussion in section III.B.2 above, Administrator has
considered the use of RBL in his judgment of the public welfare
protection provided by the secondary standard. Based on those
considerations, including uncertainties in the E-R relationships and
their use in the way described here, the Administrator judges it
appropriate for the standard to protect against W126 index values
associated with a median RBL at or above 6% (while also controlling
peak hourly concentrations, as discussed below). Based on this
judgment, in addition to a recognition of uncertainty in these
estimates (in light of the discussion in section III.B.2.b(ii) above
regarding the appropriate duration or averaging for the W126 index
metric) he concludes it appropriate for the standard to generally
control exposures in terms of W126 index to a level of 17 ppm-hrs,
recognizing that the RBL estimated for such a W126 index value is 5.3%,
a value appreciably below 6%.
    With regard to the appropriate O3 exposure metric to
employ in assessing adequacy of air quality control in protecting
against RBL, the Administrator has considered the discussions in the
PA, and in response to public comments in section III.B.2 above
regarding the available evidence and air quality analyses. He has also
considered this in the context of the court remand with regard to the
EPA's use of a 3-year average W126 index to assess protection from RBL
and the court's reference to advice from the prior CASAC on protection
against ``unusually damaging years'' (described in section III.B.2
above). In so doing, the Administrator considers below the extent of
conceptual similarities of the 3-year average W126 index with some
aspects of the derivation approach for the established E-R functions,
the context of RBL as a proxy (as recognized above), and limitations
associated with a reliance solely on W126 index as a metric to control
exposures that might be termed ``unusually damaging.''
    With regard to the established E-R functions used to describe the
relationship of RBL with O3 in terms of a seasonal W126
index, the Administrator recognizes that the E-R functions were derived
mathematically from studies of different exposure durations (varying
from shorter than one to multiple growing seasons) by applying
adjustments so that they would yield estimates normalized to the same
period of time (season), such that the estimates may represent average
impact for a season, as summarized in section III.A.1.c(ii) above (PA,
section <IP_ADDRESS>, Appendix 4A, Attachment 1). He notes the compatibility
of W126 index averaged over multiple growing seasons or years with
these adjustments. He also notes the exposure levels represented in the
data underlying the E-R functions are somewhat limited with regard to
the relatively lower cumulative exposure levels most commonly
associated with the current standard (e.g., at or below 17 ppm-hrs),
indicating additional uncertainty for application to such levels.
Further, he notes the PA observation that some of the underlying
studies did not find statistically significant effects of O3
at the lower exposure levels, indicating some uncertainty in
predictions of an O3-related RBL at those levels, as
summarized in section III.A.1.c(ii) above (PA, section <IP_ADDRESS>). He
additionally notes the differing patterns of hourly concentrations of
the elevated exposure levels in the datasets from which the E-R
functions from the patterns in ambient

air meeting the current standard across the U.S. today, as summarized
in section III.B.2.b(ii). With these considerations regarding the E-R
functions and their underlying datasets in mind, he also takes note of
year-to-year variability of factors other than O3 exposures
that affect tree growth in the natural environment (e.g., related to
variability in soil moisture, meteorological, plant-related and other
factors), that have the potential to affect O3 E-R
relationships (ISA, Appendix 8, section 3.12; 2013 ISA section <IP_ADDRESS>;
PA, sections 4.3 and 4.5). Based on these considerations, the
Administrator finds there to be a consistency of his use of the W126
index averaged over multiple years with the approach used in deriving
the E-R function, and with other factors that may affect growth in the
natural environment.
    In light of such considerations, the Administrator agrees with the
PA finding that several factors contribute uncertainty and some
resulting imprecision or inexactitude to RBL estimated from single-year
seasonal W126 index values, as discussed in section III.D.1.b(ii) of
the proposal (85 FR 49900-01, August 14, 2020; PA sections <IP_ADDRESS> and
4.5.3). The Administrator additionally recognizes the qualitative and
conceptual nature of our understanding, in many cases, of relationships
of O3 effects on plant growth and productivity with larger-
scale impacts, such as those on populations, communities and
ecosystems. From these considerations, he judges that use of a seasonal
RBL averaged over multiple years, such as a 3-year average, is
reasonable, and provides a more stable and well-founded RBL estimate
for his purposes as a proxy to represent the array of vegetation-
related effects identified above. The Administrator additionally takes
note of the CASAC advice agreeing with the EPA's focus on a 3-year
average W126 for this purpose, concluding such a focus to be reasonable
and scientifically sound, as summarized in section III.B.1.b above. In
light of these considerations, the Administrator finds there to be
support for use of an average seasonal W126 index derived from multiple
years (with their representation of variability in environmental
factors), concluding the use of such averaging to provide an
appropriate representation of the evidence and attention to
considerations summarized above. In so doing, he finds that sole
reliance on single year W126 estimates for reaching judgments with
regard to magnitude of O3 related RBL and associated
judgments of public welfare protection would ascribe a greater
specificity and certainty to such estimates than supported by the
current evidence. Rather, he finds it appropriate, for purposes of
considering public welfare protection from effects for which RBL is
used as a proxy, to primarily consider W126 index in terms of a 3-year
average metric.
    In his consideration of the appropriateness of using a 3-year
average W126 metric, the Administrator additionally takes note of the
discussion in section III.B.2 above with regard to protection against
``unusually damaging years,'' a caution raised by the prior CASAC in
considering a secondary standard in terms of a 3-year average W126
index (and an issue raised in the court remand). With regard to this
caution, the Administrator finds informative the discussion in section
III.B.2 above regarding the extent to which a standard in terms of a
W126 metric might be expected to control exposure circumstances of
concern (e.g., for growth effects, among others). This discussion and
its focus on air quality analyses in the PA and additional analyses
conducted in consideration of public comment investigate the annual
occurrence of elevated hourly O3 concentrations which may
contribute to vegetation exposures of concern (PA, Appendix 2A, section
2A.2; Wells, 2020).\239\
---------------------------------------------------------------------------

    \239\ The ISA references the longstanding recognition of the
risk posed to vegetation of peak hourly O3 concentrations
(e.g., ``[h]igher concentrations appear to be more important than
lower concentrations in eliciting a response'' [ISA, p. 8-180];
``higher hourly concentrations have greater effects on vegetation
than lower concentrations'' [2013 ISA, p. 91-4] ``studies published
since the 2006 O3 AQCD do not change earlier conclusions,
including the importance of peak concentrations, . . . in altering
plant growth and yield'' [2013 ISA, p. 9-117]).
---------------------------------------------------------------------------

    These air quality analyses illustrate limitations of the W126 index
for purposes of controlling peak concentrations, and also the strengths
of the current standard in this regard. As discussed more fully in
section III.B.2.b(ii) above, the W126 index cannot, by virtue of its
definition, always differentiate between air quality patterns with high
peak concentrations and those without such concentrations. This is
demonstrated in the air quality analyses which show that the form and
averaging time of the existing standard is much more effective than the
W126 index in limiting peak concentrations (e.g., hourly O3
concentrations at or above 100 ppb) and in limiting number of days with
any such hours (Wells, 2020, e.g., Figures 4, 5, 8, 9 compared to
Figures 6, 7, 10 and 11). A similar finding is evidence in the
historical data extending back to 2000. These data show the appreciable
reductions in peak concentrations that have been achieved in the U.S.
as air quality has improved under O3 standards of the
existing form and averaging time (Wells, 2020, Figures 12 and 13). From
these analyses, the Administrator concludes that the form and averaging
time of the current standard is effective in controlling peak hourly
concentrations and that a W126 index based standard would be much less
effective in providing the needed protection against years with such
elevated and potentially damaging hourly concentrations. Thus, in light
of the current evidence and quantitative air quality analyses, the
Administrator notes that the W126 index, by its very definition, does
not provide specificity with regard to year-to-year variability in
elevated hourly O3 concentrations with the potential to
contribute to the ``unusually damaging years'' that the prior CASAC
identified for increased concern. In so doing, he disagrees with the
statement of the prior CASAC that a single-year W126 index would
necessarily provide protection from such years. Further, he judges that
a standard based on either a 3-year or a single-year W126 index would
not be expected to provide effective control of the peak concentrations
that may contribute to ``unusually damaging years'' for vegetation.
    Thus, in considering the extent of protection provided by the
current standard, in addition to considering seasonal W126 averaged
over a 3-year period to estimate median RBL using the established E-R
functions, the Administrator finds it appropriate to also consider
other metrics, including peak hourly concentrations. While he
recognizes that the evidence does not indicate a particular threshold
number of hours at or above 100 ppb (or another reference point for
elevated concentrations), he takes particular note of the evidence of
greater impacts from higher concentrations (particularly with increased
frequency) and of the air quality analyses that document variability in
such concentrations for the same W126 index value. In light of these
considerations, he judges such a multipronged approach to be needed to
ensure appropriate consideration of exposures of concern and the
associated protection from them afforded by the secondary standard.
Thus, the Administrator concludes that use of a seasonal W126 averaged
over a 3-year period, which is the design value period for the current
standard, to estimate median RBL using the established E-R functions,
in combination with a

broader consideration of air quality patterns, such as peak hourly
concentrations, is appropriate for considering the public welfare
protection provided by the standard.
    In the discussion above, the Administrator recognizes a number of
public welfare policy judgments important to his review of the current
standard that include the appropriateness of the W126 index, averaged
across a 3-year period, for assessing the extent of protection afforded
by the standard from cumulative seasonal O3 exposures. In
reflecting on these judgments, the current evidence presented in the
ISA and the associated evaluations in the PA, the Administrator
concludes that the currently available information supports such
judgments, additionally noting the CASAC concurrence with regard to the
scientific support for these judgments (Cox 2020a, Consensus Responses
to Charge Questions p. 21). Accordingly, the Administrator concludes
that the current evidence base and available information (qualitative
and quantitative) continues to support consideration of the potential
for O3-related vegetation impacts in terms of the RBL
estimates from established E-R functions as a quantitative tool within
a larger framework of considerations pertaining to the public welfare
significance of O3 effects. Such consideration includes
effects that are associated with effects on vegetation, and
particularly those that conceptually relate to growth, and that are
causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting estimates of
impacts on public welfare. The Administrator additionally notes that
this approach to weighing the available information in reaching
judgments regarding the secondary standard additionally takes into
account uncertainties regarding the magnitude of growth impact that
might be expected in mature trees (e.g., compared to seedlings), and of
related, broader, ecosystem-level effects for which the available tools
for quantitative estimates are more uncertain and those for which the
policy foundation for consideration of public welfare impacts is less
well established.
    In his consideration of the adequacy of protection provided by the
current standard, the Administrator does not consider every possible
instance of an effect on vegetation growth from O3 to be
adverse to public welfare, although he recognizes that, depending on
factors including extent and severity, such vegetation-related effects
have the potential to be adverse to public welfare. Comments from the
current CASAC, in the context of its review of the draft PA, expressed
the view that the strategy described by the prior Administrator for the
secondary standard established in 2015 with its focus on limiting 3-
year average W126 index values somewhat below those associated with a
6% RBL in the median species and associated W126 index target of 17
ppm-hrs (in terms of a 3-year average), at or below which the 2015
standard was expected to generally restrict cumulative seasonal
exposure, is ``still scientifically reasonable'' and ``still effective
in particularly protecting the public welfare in light of vegetation
impacts from ozone'' (Cox, 2020a, Consensus Responses to Charge
Questions p. 21). In light of this advice and based on the current
evidence as evaluated in the PA, the Administrator judges that this
approach or framework, with its focus on controlling cumulative
seasonal exposures associated with an RBL of 6% or greater, by limiting
air quality in terms of a 3-year average W126 index, to or below a
target of 17 ppm-hrs, in combination with a broader consideration of
air quality patterns, such as control of peak hourly concentrations,
associated with meeting the current standard, is appropriate for his
use in this review. In so doing, he additional notes the isolated, rare
occurrences in locations meeting the current standard of such exposures
at 19 ppm-hrs. Based on the current information to inform consideration
of vegetation effects and their potential adversity to public welfare,
he additionally judges that the RBL estimates associated with such
marginally higher exposures in isolated, rare instances are not
indicative of effects that would be adverse to the public welfare,
particularly in light of variability in the array of environmental
factors that can influence O3 effects in different systems
and uncertainties associated with estimates of effects associated with
this magnitude of cumulative exposure in the natural environment.
    With regard to O3 effects on crop yield, the
Administrator, as an initial matter, takes note of the long-standing
evidence, qualitative and quantitative, of the reducing effect of
O3 on the yield of many crops, as summarized in the PA and
current ISA and characterized in detail in past reviews (e.g., 2013
ISA, 2006 AQCD, 1997 AQCD, 2014 WREA). He additionally notes the
established E-R functions for 10 crops and the estimates of RYL derived
from them, as presented in the PA (PA, Appendix 4A, section 4A.1, Table
4A-4), and the potential public welfare significance of reductions in
crop yield, as summarized in section III.A.2.b above. In so doing,
however, he additionally recognizes that not every effect on crop yield
will be adverse to public welfare. In the case of crops in particular
there are a number of complexities related to the heavy management of
many crops to obtain a particular output for commercial purposes, and
related to other factors, that the Administrator takes into
consideration in evaluating potential O3-related public
welfare impacts, as summarized in section III.B.2.b(iv) above (PA,
sections <IP_ADDRESS> and 4.5.3).
    Similarly, the Administrator concludes that the extensive
management of agricultural crops that occurs to elicit optimum yields
(e.g., through irrigation and usage of soil amendments, such as
fertilizer) is relevant in evaluating the extent of RYL estimated from
experimental O3 exposures that should be judged adverse to
the public welfare. He considers these opportunities in crop management
for market objectives, as well as complications in judging relative
adversity that relate to market responses and their effects on
producers and consumers in evaluating the potential impact on public
welfare of estimated crop yield losses. Further, the Administrator
takes note of the conclusion of the CASAC that the available evidence
does not call into question the adequacy of the current secondary
standard and that it should be retained (Cox 2020a, p.1).
    The Administrator also considered the public comments, discussed in
section III.B.2.b(iv) above, suggesting that the proposed decision was
not giving adequate consideration to crop yield effects and that his
decision should consider a statement by the prior CASAC, raised in
public comments, that a 5% RYL estimate, as the median based on the 10
E-R functions, ``represents an adverse impact.'' With regard to the
prior CASAC statement, he notes the discussion in section III.B.2.b(iv)
above regarding the unclear basis for the prior CASAC judgment, both
with regard to a connection of an estimated 5% RYL to broader impacts
and to judgments on significance of a 5% RYL to the public welfare. In
considering the adequacy of protection of the public welfare from
effects related to crop yield loss, the Administrator considers the air
quality analyses and the W126 index levels commonly occurring in areas
that meet the current standard. In so doing, he notes that W126 index
values (3-year average) were at or below 17 ppm-hrs in virtually all
monitoring sites with air quality meeting the current standard.

Based on the established E-R functions, the median RYL estimate
corresponding to 17 ppm-hrs is 5.1%. In considering single-year index
values, as discussed in section III.B.2.b(vi), the vast majority are
similarly low (with more than 99% less than or equal to 17 ppm-hrs),
and the higher values predominantly occur in urban areas. The
Administrator additionally takes note of the discussion in section
III.B.2.b(ii) above regarding the role of elevated hourly
concentrations in effects on vegetation growth and yield. In so doing,
in addition to his consideration of W126 index occurring in areas that
meet the current standard, he also takes note of the control of
elevated hourly O3 concentrations that is exerted by the
current standard.
    In light of all of the above, in reaching his judgment regarding
public welfare implications of the W126 index values summarized here
(and associated estimated RYL), including the isolated and rare
occurrence of somewhat higher values, the Administrator notes that the
secondary standard is not intended to protect against all known or
anticipated O3-related effects, but rather those that are
judged to be adverse to the public welfare. He also takes into
consideration the extensive management of agricultural crops, and the
complexities associated with identifying adverse public welfare effects
for market-traded goods (where producers and consumers may be impacted
differently). Based on all of these factors, the Administrator
disagrees with the prior CASAC statement that an estimated median RYL
of 5% represents an adverse impact and further judges that an estimated
median RYL of 5.1%, based on experimental exposures, would not
constitute an adverse effect on public welfare. Accordingly, the
Administrator notes that the current standard generally maintains air
quality at a W126 index below 17 ppm-hrs, with few exceptions, and
accordingly would limit the estimated RYL (based on experimental
O3 exposures) to this degree. Therefore, he concludes that
the current standard provides adequate protection of public welfare
related to crop yield loss and does not need to be revised to provide
additional protection against this effect. In so doing, the
Administrator notes the conclusions by the current CASAC that the
evidence supports retaining the current standard, without revision.
    Turning to consideration of visible foliar injury and protection
afforded by the secondary standard from associated impacts to the
public welfare, the Administrator takes note of the long-standing and
well-established evidence base, updated in the ISA for this review, and
of policy-relevant analyses presented in the PA to inform his judgments
regarding a secondary standard that provides appropriate protection of
the public welfare from this effect. In so doing, he has also taken
into account issues raised by public comments, both with regard to our
understanding of relationships between O3 exposure
circumstances and extent and severity of injury in natural areas across
the U.S., and with regard to the extent of our understanding of the
relationship of injury extent and severity to public welfare effects
anticipated to be adverse, and the Murray Energy remand.
    In considering public welfare implications of this effect, he notes
the potential for this effect, when of a significant extent and
severity, to reduce aesthetic and recreational values, such as the
aesthetic value of scenic vistas in protected natural areas including
national parks and wilderness areas, as well as other areas similarly
protected by state and local governments for similar public uses. Based
on these considerations, the Administrator recognizes that, depending
on its severity and spatial extent, as well as the location(s) and the
associated intended use, the impact of visible foliar injury on the
physical appearance of plants has the potential to be significant to
the public welfare. In this regard, he agrees with the PA statement
that cases of widespread and relatively severe injury during the
growing season (particularly when sustained across multiple years and
accompanied by obvious impacts on the plant canopy) might reasonably be
expected to have the potential to adversely impact the public welfare
in scenic and/or recreational areas, particularly in areas with special
protection, such as Class I areas (PA, sections 4.3.2 and 4.5.1). In so
doing, the Administrator notes that the secondary standard is not meant
to protect against all known or anticipated O3-related
welfare effects, but rather those that are judged to be adverse to the
public welfare, and further notes that there are not established
measures for when such welfare effects should be judged adverse to the
public welfare. Rather, the level of protection from known or
anticipated adverse effects to public welfare that is requisite for the
secondary standard is a public welfare policy judgment to be made by
the Administrator.
    While recognizing there to be a paucity of established approaches
for interpreting specific levels of severity and extent of foliar
injury in natural areas with regard to impacts on the public welfare
(e.g., related to recreational services), the Administrator recognizes
that injury to whole stands of trees of a severity apparent to the
casual observer (e.g., when viewed as a whole from a distance) would
reasonably be expected to affect recreational values and thus pose a
risk of adverse effects to the public welfare. He further notes that
the available information does not provide for specific
characterization of the incidence and severity that would not be
expected to be apparent to the casual observer, nor for clear
identification of the pattern of O3 concentrations that
would provide for such a situation.
    In this context, the Administrator takes note of the system
developed by the USFS for its monitoring program \240\ to categorize BI
scores of visible foliar injury at biosites (sites with O3-
sensitive vegetation assessed for visible foliar injury) in natural
vegetated areas by severity levels (described in section III.A.2.c(ii)
above). While recognizing that quantitative analyses and evidence are
lacking that might support a more precise conclusion with regard to a
magnitude of BI score coupled with an extent of occurrence that might
be specifically identified as adverse to the public welfare, the
Administrator also takes note of the D.C. Circuit's holding that
substantial uncertainty about the level at which visible foliar injury
may become adverse to public welfare does not necessarily provide a
basis for declining to evaluate whether the existing standard provides
requisite protection against such effects. See Murray Energy Corp. v.
EPA, 936 F.3d 597, 619-20 (D.C. Cir. 2019). In this context, he
considers the discussion in the PA and in sections III.A.2.b, III.A.2.c
and III.B.2 above regarding the USFS biosite monitoring program. He
finds the scale of this program's objectives, which focus on natural
settings in the U.S. and forests as opposed to individual plants, to be
suited for his consideration with regard to the public welfare
protection afforded by the current standard, and consequently, he finds
the data and analyses generated by the program informative in such
considerations.
---------------------------------------------------------------------------

    \240\ During the period from 1994 (beginning in eastern U.S.)
through 2011, the USFS conducted surveys of the occurrence and
severity of visible foliar injury on sensitive species at sites
across most of the U.S. following a national protocol.
---------------------------------------------------------------------------

    In this context, he takes note of the USFS system, including its
descriptors for BI scores of differing magnitude intended for that
Agency's consideration in identifying areas of potential impact to
forest resources. As described in section III.A.2.b(iii) above, very
low BI scores (at or below 5) are

described by the USFS scheme as ``little or no foliar injury'' (Smith
et al., 2007; Smith et al., 2012).\241\ The Administrator notes that BI
scores above 15 are categorized as moderate to severe (and scores above
25 as severe). In so doing, in light of considerations raised in the PA
and consideration of public comment, he recognizes the lower categories
of BI scores as indicative of injury of generally lesser risk to the
natural area or to public enjoyment, which he judges unlikely to be
indicative of injury of such a magnitude or extent as to pose risk of
adverse effects to the public welfare. Thus, the Administrator reaches
the conclusion that occurrence of the lower categories of BI scores
does not pose concern for the public welfare, but that findings of BI
scores categorized as ``moderate to severe'' injury by the USFS scheme
would be an indication of visible foliar injury occurrence that,
depending on extent and severity, may raise public welfare concerns. In
this framework, the Administrator considers the PA evaluations of the
currently available information and what it indicates with regard to
patterns of air quality of concern for such an occurrence, and the
extent to which they are expected to occur in areas that meet the
current standard.
---------------------------------------------------------------------------

    \241\ Studies that consider such data for purposes of
identifying areas of potential impact to the forest resource suggest
this category corresponds to ``none'' with regard to ``assumption of
risk'' (Smith et al., 2007; Smith et al., 2012).
---------------------------------------------------------------------------

    In so doing, the Administrator takes particular note of the USFS
biosite monitoring program studies of the occurrence, extent and
severity of visible foliar injury in indicator species in defined plots
or biosites in natural areas across the U.S. These studies of data for
USFS biosites (sites with O3-sensitive vegetation assessed
for visible foliar injury) have often summarized O3
concentrations in terms of cumulative exposure metrics (e.g., SUM06 or
W126 index). Some of these studies, particularly those examining such
data across multiple years and multiple regions of the U.S., have
reported that variation in cumulative O3 exposure, in terms
of such metrics, does not completely explain the patterns of occurrence
and severity of injury observed. Although the availability of detailed
analyses that have explored multiple exposure metrics and other
influential variables is limited, multiple studies have indicated a
potential role for an additional metric, one related to the occurrence
of days with relatively high concentrations (e.g., number of days with
a 1-hour concentration at or above 100 ppb), as summarized in section
III.A.2.c above (PA, section <IP_ADDRESS>). Thus, the Administrator takes
note of this evidence indicating an influence of peak concentrations on
BI scores (beyond an influence of cumulative exposure). He also finds
noteworthy the extensive evidence of trends across the past nearly 20
years that indicate reductions in severity of visible foliar injury
that parallel reductions in peak concentrations that have been
suggested to be influential in the severity of visible foliar
injury.\242\
---------------------------------------------------------------------------

    \242\ For example, the PA describes findings from USFS studies
that have concluded a ``declining risk of probable impact'' over the
16-year period of the program, especially after 2002 (e.g., Smith,
2012), and the parallel national reductions in O3
concentrations from 2000 through 2018 in terms of cumulative
seasonal exposures, as well as in peak O3 concentrations
such as the annual fourth highest daily maximum 8-hour concentration
and also the occurrence of 1-hour concentrations above 100 ppb (PA,
Figure 2-11, Appendix 2A, Tables 2A-2 to 2A-4, and Appendix 4D,
Figure 4D-9).
---------------------------------------------------------------------------

    Further, the Administrator considers the PA analysis of a dataset
developed from USFS biosite index scores, combined with W126 estimates
and soil moisture categories, summarized in section III.A.2.c above. In
so doing, he takes note of the PA observation that important
uncertainties remain in the understanding of the O3 exposure
conditions that will elicit visible foliar injury of varying severity
and extent in natural areas, and particularly in light of the other
environmental variables that influence its occurrence, and of the
recognition by the CASAC that ``uncertainties continue to hamper
efforts to quantitatively characterize the relationship of [visible
foliar injury] occurrence and relative severity with ozone exposures''
(Cox 2020a, Consensus Responses to Charge Questions, p. 20).
Notwithstanding, and while being mindful of, such uncertainties with
regard to predictive O3 metric or metrics and a quantitative
function relating them to incidence and severity of visible foliar
injury in natural areas (as also noted in the USFS studies referenced
above), the Administrator takes note of the PA finding that the
incidence of nonzero BI scores, and, particularly of relatively higher
scores (such as scores above 15 which are indicative of ``moderate to
severe'' injury in the USFS scheme) appears to markedly increase only
with W126 index values above 25 ppm-hrs, as summarized in section
III.B.2.b above (PA, section 4.3.3 and Appendix 4C).
    In light of these observations, the Administrator finds the current
evidence to be incomplete with regard to information to support a
quantitative characterization of air quality that would be anticipated
to result in visible foliar injury of an extent and severity to cause
adverse effects to the public welfare. The Administrator also considers
discussion in the court's remand of the 2015 standard with regard to
visible foliar injury (Murray Energy Corp. v EPA, 936 F.3d at 619-20).
The court concluded that the EPA had failed to offer a reasoned
explanation for deciding not to specify a level of air quality to
protect against adverse effects related to visible foliar injury. In
particular, the court stated that the EPA had not explained why it was
unable to choose such a level although the prior CASAC had provided
advice with regard to a specific level. The EPA's disagreement with the
prior CASAC on its identified level is explained in section III.B.2
above, as is the reason why the EPA did not find the analysis on which
the prior CASAC based its advice to be appropriate for such a
conclusion.\243\ This and other associated issues raised by the court
have been raised in public comments on the proposal for this action and
are addressed in section III.B.2 above.
---------------------------------------------------------------------------

    \243\ As discussed in section III.B.2.b, the cumulative
frequency graph relied on by the CASAC does not present biosite
scores for comparison at different cumulative exposure levels.
Accordingly, it does not provide the type of analysis that is needed
for the EPA to reach a conclusion about the extent of protection
that different patterns of O3 concentrations would
provide against visible foliar injury of an extent and severity as
to pose risk of adverse effects to the public welfare.
---------------------------------------------------------------------------

    Based on the evidence and quantitative analyses available in the
present review, and advice from the current CASAC, the Administrator
considers the question of a level of air quality that would provide
protection against visible foliar injury related effects known or
anticipated to cause adverse effects to the public welfare. Based on
the evidence and associated quantitative analyses in this review, the
Administrator's judgment reflects his recognition of less confidence
and greater uncertainty in the existence of adverse public welfare
effects with lower O3 exposures. In this context, the
Administrator judges that W126 index values at or below 25 ppm-hrs,
when in combination with infrequent occurrences of hourly
concentrations at or above 100 ppb, would not be anticipated to pose
risk of visible foliar injury of an extent and severity so as to be
adverse to the public welfare.
    With these conclusions in mind, the Administrator considers the air
quality analyses presented in the PA and the additional analyses
developed in response to public comment. In so doing, he notes that a
W126 index above

25 ppm-hrs (either as a 3-year average or in a single year) is not seen
to occur at monitoring locations (including in or near Class I areas)
where the current standard is met, and that, in fact, values above 17
or 19 ppm-hrs are rare, as summarized in section III.A.3 above (PA,
Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3). Further, the
Administrator takes note of the PA consideration of the USFS
publications that identify an influence of peak concentrations on BI
scores (beyond an influence of cumulative exposure) and the PA
observation of the appreciable control of peak concentrations exerted
by the form and averaging time of the current standard, as evidenced by
the air quality analyses which document reductions in 1-hour daily
maximum concentrations with declining design values. He also notes, as
discussed above, the uncommonness of days with any hours at or above
100 ppb at monitoring sites that meet the current standard, as well as
the minimal number of hours on any such days (Wells, 2020). Based on
these considerations, the Administrator concludes that the current
standard provides control of air quality conditions that contribute to
increased BI scores and to scores of a magnitude indicative of
``moderate to severe'' foliar injury.
    The Administrator further takes note of the PA finding that the
current information, particularly in locations meeting the current
standard or with W126 index estimates likely to occur under the current
standard, does not indicate a significant extent and degree of injury
(e.g., based on analyses of BI scores in the PA, Appendix 4C) or
specific impacts on recreational or related services for areas, such as
wilderness areas or national parks. Thus, he gives credence to the
associated PA conclusion that the evidence indicates that areas that
meet the current standard are unlikely to have BI scores reasonably
considered to be impacts of public welfare significance. Based on all
of the considerations raised here, the Administrator concludes that the
current standard provides sufficient protection of natural areas,
including particularly protected areas such as Class I areas, from
O3 concentrations in the ambient air that might be expected
to elicit visible foliar injury of such an incidence and severity as
would reasonably be judged adverse to the public welfare.
    With a primary focus on RBL in its role as proxy, the Administrator
further considers the analyses available in this review of recent air
quality at sites across the U.S., particularly including those sites in
or near Class I areas, and also the analyses of historical air quality.
In so doing, the Administrator recognizes that these analyses are
distributed across all nine NOAA climate regions and 50 states,
although some geographic areas within specific regions and states may
be more densely covered and represented by monitors than others, as
summarized in section III.C of the proposal (PA, Appendix 4D). The
Administrator notes that the findings from both the analysis of the air
quality data from the most recent period and from the larger analysis
of historical air quality data extending back to 2000, as presented in
the PA and summarized in section III.A.3 above, are consistent with the
air quality analyses available in the last review. That is, in
virtually all design value periods and all locations at which the
current standard was met across the 19 years and 17 design value
periods (in more than 99.9% of such observations), the 3-year average
W126 metric was at or below 17 ppm-hrs. Further, in all such design
value periods and locations the 3-year average W126 index was at or
below 19 ppm-hrs. The Administrator additionally considers the
protection provided by the current standard from the occurrence of
O3 exposures within a single year with potentially damaging
consequences, such as a significantly increased incidence of areas with
visible foliar injury that might be judged moderate to severe, as
discussed in section III.B.2 above. In so doing, he takes notes of the
PA analyses, summarized in section III.A.2.c above, of USFS BI scores,
giving particular focus to scores above 15, termed ``moderate to severe
injury'' by the USFS categorization scheme, as described in section
III.A.2.b above (PA, sections <IP_ADDRESS>, <IP_ADDRESS> and Appendix 4C). He
notes the PA finding that incidence of sites with BI scores above 15
markedly increases with W126 index estimates above 25 ppm-hrs. In this
context, he additionally takes note of the air quality analysis finding
of a scarcity of single-year W126 index values above 25 ppm-hrs at
sites that meet the current standard, with just a single occurrence
across all U.S. sites with design values meeting the current standard
in the 19-year historical dataset dating back to 2000 (PA, section 4.4
and Appendix 4D). Further, in light of the evidence indicating that
peak short-term concentrations (e.g., of durations as short as one
hour) may also play a role in the occurrence of visible foliar injury,
the Administrator additionally takes note of the air quality analyses
in the PA and in the additional analysis documented in Wells (2020).
These analyses of data from the past 20 years show a declining trend in
1-hour daily maximum concentrations mirroring the declining trend in
design values, supporting the PA conclusion that the form and averaging
time of the current standard provides appreciable control of peak 1-
hour concentrations. Furthermore, these analyses indicate there to be
only a few days among sites meeting the current standard, with hourly
concentrations at or above 100 ppb (just seven in the period from 2000
through 2018) (Wells, 2020). In light of these findings from the air
quality analyses and considerations in the PA, both with regard to 3-
year average W126 index values at sites meeting the current standard
and the rarity of such values at or above 19 ppm-hrs, and with regard
to single-year W126 index values at sites meeting the current standard,
and the rarity of such values above 25 ppm-hrs, as well as with regard
to the appreciable control of 1-hour daily maximum concentrations, the
Administrator judges that the current standard provides adequate
protection from air quality conditions with the potential to be adverse
to the public welfare.
    In reaching his conclusion on the current secondary O3
standard, the Administrator recognizes, as is the case in NAAQS reviews
in general, his decision depends on a variety of factors, including
science policy judgments and public welfare policy judgments, as well
as the currently available information. With regard to the current
review, the Administrator gives primary attention to the principal
effects of O3 as recognized in the current ISA, the 2013 ISA
and past AQCDs, and for which the evidence is strongest (e.g., growth,
reproduction, and related larger-scale effects, as well as, visible
foliar injury). With regard to growth and the categories of effects
identified above for which RBL has been identified for use as a proxy,
based on all of the considerations above, including the discussion of
air quality immediately above, the Administrator judges the current
standard to provide adequate protection for air quality conditions with
the potential to be adverse to the public welfare. Further, with regard
to visible foliar injury, the Administrator concludes that the
currently available information on visible foliar injury and with
regard to air quality analyses that may be informative with regard to
air quality conditions associated with appreciably increased incidence
and severity of BI scores at USFS biomonitoring sites, and with
particular attention to Class I and other areas afforded special
protection,

indicates the current standard to provide adequate protection from
visible foliar injury of an extent or severity that might be
anticipated to be adverse to the public welfare.
    In summary, the Administrator has based his decision on the public
welfare protection afforded by the secondary O3 standard
from identified O3-related welfare effects, and from their
potential to present adverse effects to the public welfare, on
judgments regarding what the available evidence, quantitative
information, and associated uncertainties and limitations (such as
those identified above) indicate with regard to the protection provided
from the array of O3 welfare effects. He finds that, as a
whole, this information, as summarized above, and presented in detail
in the ISA and PA, does not indicate the current standard to allow air
quality conditions with implications of concern for the public welfare.
He has additionally considered the advice from the CASAC in this
review, including its finding ``that the available evidence does not
reasonably call into question the adequacy of the current secondary
ozone standard and concurs that it should be retained'' (Cox, 2020a, p.
1), and well as public comment on the proposed decision. Based on all
of the above considerations, including his consideration of the
currently available evidence and quantitative exposure/risk
information, the Administrator concludes that the current secondary
standard is requisite to protect the public welfare from known or
anticipated adverse effects of O3 and related photochemical
oxidants in ambient air, and thus that the current standard should be
retained, without revision.

C. Decision on the Secondary Standard

    For the reasons discussed above and taking into account information
and assessments presented in the ISA and PA, the advice from the CASAC,
and consideration of public comments, the Administrator concludes that
the current secondary O3 standard is requisite to protect
the public welfare from known or anticipated adverse effects, and is
retaining the current standard without revision.

IV. Statutory and Executive Order Reviews

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

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

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

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

    This action is not an Executive Order 13771 regulatory action.
There are no quantified cost estimates for this action because EPA is
retaining the current standards.

C. Paperwork Reduction Act (PRA)

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

D. Regulatory Flexibility Act (RFA)

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

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely
affect small governments. This action imposes no enforceable duty on
any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

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

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

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

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

    This action is not subject to Executive Order 13045 because it is
not economically significant as defined in Executive Order 12866. The
health effects evidence and risk assessment information for this
action, which focuses on children and people (of all ages) with asthma
as key at-risk populations, is summarized in section II.A.2 and II.A.3
above and described in the ISA and PA, copies of which are in the
public docket for this action.

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

    This action is not a ``significant energy action'' for purposes of
Executive Order 13211. The action is not likely to have a significant
adverse effect on the supply, distribution, or use of energy. This
action retains the current O3 NAAQS. This decision does not
change existing requirements. The Administrator of the Office of
Information and Regulatory Affairs has not otherwise designated this
action as a significant energy action. Thus, this decision does not
constitute a significant energy action as defined in Executive Order
13211.

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

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

    The EPA believes that this action does not have disproportionately
high and

adverse human health or environmental effects on minority, low-income
populations and/or indigenous peoples, as specified in Executive Order
12898 (59 FR 7629, February 16, 1994). The action described in this
document is to retain without revision the existing O3 NAAQS
based on the Administrator's conclusions that the existing primary
standard protects public health, including the health of sensitive
groups, with an adequate margin of safety, and that the existing
secondary standard protects public welfare from known or anticipated
adverse effects. As discussed in section II above, the EPA expressly
considered the available information regarding health effects among at-
risk populations in reaching the decision that the existing standard is
requisite.

L. Determination Under Section 307(d)

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

M. Congressional Review Act

    The EPA will submit a rule report to each House of the Congress and
to the Comptroller General of the United States. This action is not a
``major rule'' as defined by 5 U.S.C. 804(2).

V. References

Adams, WC (2000). Ozone dose-response effects of varied equivalent
minute ventilation rates. J Expo Anal Environ Epidemiol 10(3): 217-
226.
Adams, WC (2002). Comparison of chamber and face-mask 6.6-hour
exposures to ozone on pulmonary function and symptoms responses.
Inhal Toxicol 14(7): 745-764.
Adams, WC (2003). Comparison of chamber and face mask 6.6-hour
exposure to 0.08 ppm ozone via square-wave and triangular profiles
on pulmonary responses. Inhal Toxicol 15(3): 265-281.
Adams, WC (2006). Comparison of chamber 6.6-h exposures to 0.04-0.08
PPM ozone via square-wave and triangular profiles on pulmonary
responses. Inhal Toxicol 18(2): 127-136.
Adams, WC and Ollison, WM (1997). Effects of prolonged simulated
ambient ozone dosing patterns on human pulmonary function and
symptomatology. Air & Waste Management Association, Pittsburgh, PA.
Adhikari, A and Yin, J (2020). Short-Term Effects of Ambient Ozone,
PM2.5, and Meteorological Factors on COVID-19 Confirmed
Cases and Deaths in Queens, New York. Int J Env Res Public Health
17(11): 4047.
Alexis, NE, Lay, JC, Hazucha, M, Harris, B, Hernandez, ML, Bromberg,
PA, Kehrl, H, Diaz-Sanchez, D, Kim, C, Devlin, RB and Peden, DB
(2010). Low-level ozone exposure induces airways inflammation and
modifies cell surface phenotypes in healthy humans. Inhal Toxicol
22(7): 593-600.
Archer, CL. Brodie, JF and Rauscher, SA (2019). Global Warming Will
Aggravate Ozone Pollution in the U.S. Mid-Atlantic. J Appl Meteorol
Climatol 58(6): 1267-78.
ATS (1985). Guidelines as to what constitutes an adverse respiratory
health effect, with special reference to epidemiologic studies of
air pollution. Am Rev Respir Dis 131(4): 666-668.
ATS (2000). What constitutes an adverse health effect of air
pollution? Am J Respir Crit Care Med 161(2): 665-673.
Bell, ML, Zanobetti, A and Dominici F (2014). Who Is More Affected
by Ozone Pollution? A Systematic Review and Meta-Analysis. Am J Epi
180(1): 15-28.
Brown, JS, Bateson, TF and McDonnell, WF (2008). Effects of exposure
to 0.06 ppm ozone on FEV1 in humans: a secondary analysis of
existing data. Environ Health Perspect 116(8): 1023-1026.
Bureau of Labor Statistics (2017). U.S. Department of Labor, The
Economics Daily, Over 90 percent of protective service and
construction and extraction jobs require work outdoors. Available
at: https://www.bls.gov/opub/ted/2017/over-90-percent-of-protective-service-and-construction-and-extraction-jobs-require-work-outdoors.htm (visited August 27, 2019).
Burra, TA, Moineddin, R, Agha, MM, and Glazier, RH (2009). Social
disadvantage, air pollution, and asthma physician visits in Toronto,
Canada. Environ Res 109(5):567-74.
Cakmak, S, Dales, RE and Judek, S (2006). Respiratory health effects
of air pollution gases: Modification by education and income. Arch
Environ Occup Health 61: 5-10.
Campbell, SJ, Wanek, R, and Coulston, JW (2007). Ozone injury in
west coast forests: 6 years of monitoring--Introduction. U.S.
Department of Agriculture. Portland, OR. Available at: https://www.fs.usda.gov/treesearch/pubs/27926
CDC (2019). National Health Interview Survey, 2017. National Center
for Health Statistics, CDC. Washington, DC. Available at: https://www.cdc.gov/asthma/most_recent_national_asthma_data.htm and https://www.cdc.gov/asthma/nhis/2017/data.htm. Accessed August 27, 2019.
Cleary, EG, Cifuentes, M, Grinstein, G, Brugge, D and Shea, TB
(2018). Association of low-level ozone with cognitive decline in
older adults. J Alzheimers Dis 61(1):67-78.
Cohen, AJ, Brauer, M, Burnett, R, Anderson, HR, Frostad, J, Estep,
K, Balakrishnan, K, Brunekreef, B, Dandona, L, Dandona, R, Feigin,
V, Freedman, G, Hubbell, B, Jobling, A, Kan, H, Knibbs, L, Liu, Y,
Martin, Morawska, L, Pope, CA, Shin, H, Straif, K, Shaddick, G,
Thomas, M, Van Dingenen, R, Van Dingenen, A, Vos, T, Murray, CJI and
Forouzanfart, MH (2017). Estimates and 25 year trends of the global
burden of disease Attributable to ambient air pollution: an analysis
of data from the Global Burden of Diseases Study 2015. Lancet
389(10082): 1907-18.
Cordell, H, Betz, FM, Mou, S and Green, G (2008). How do Americans
View Wilderness--A WILDERNESS Research Report in the internet
Research Information Series. National Survey on Recreation and the
Environment. This research is a collaborative effort between the
U.S. Department of Agriculture Forest Service's Southern Research
Station and its Forestry Sciences Laboratory in Athens, Georgia; the
University of Georgia in Athens; and the University in Tennessee in
Knoxville, Tennessee.
Costanza, R, De Groot, R, Braat, L, Kubiszewski, I, Fioramonti, L,
Sutton, P, Farber, S and Grasso, M (2017). Twenty years of ecosystem
services: How far have we come and how far do we still need to go?
Ecosyst Serv 28: 1-16.
Coulston, JW, Smith, GC and Smith WD (2003). Regional assessment of
ozone sensitive tree species using bioindicator plants. Environ
Monito Assess 82(2): 113-127.
Cox, LA (2018). Letter from Dr. Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Acting Administrator Andrew R.
Wheeler, Re: Consultation on the EPA's Integrated Review Plan for
the Review of the Ozone. December 10, 2018. EPA-CASAC-19-001. Office
of the Administrator, Science Advisory Board U.S. EPA HQ, Washington
DC. Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/
LookupWebReportsLastMonthCASAC/A286A0F0151DC8238525835F007D348A/
$File/EPA-CASAC-19-001.pdf.
Cox, LA (2020a). Letter from Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Administrator Andrew R.
Wheeler. Re:CASAC Review of the EPA's Policy Assessment for the
Review of the Ozone National Ambient Air Quality Standards (External
Review Draft--October 2019). February 19, 2020. EPA-CASAC-20-003.
Office of the Adminstrator, Science Advisory Board Washington, DC.
Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/
264cb1227d55e02c85257402007446a4/4713D217BC07103485258515006359BA/
$File/EPA-CASAC-20-003.pdf.
Cox, LA (2020b). Letter from Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Administrator Andrew R.
Wheeler. Re:CASAC Review of the EPA's Integrated Science Assessment
for Ozone and Related Photochemical Oxidants (External Review
Draft--September 2019). February 19, 2020. EPA-CASAC-20-002. Office
of the Adminstrator, Science Advisory Board Washington, DC.
Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/

264cb1227d55e02c85257402007446a4/F228E5D4D848BBED85258515006354D0/
$File/EPA-CASAC-20-002.pdf.
Cromar, KR, Gladson, LA and Ewart, G (2019). Trends in Excess
Morbidity and Mortality Associated with Air Pollution above American
Thoracic Society-Recommended Standards, 2008-2017. Ann Am Thorac Soc
16(7): 836-845.
Davis, DD and Orendovici, T (2006). Incidence of ozone symptoms on
vegetation within a National Wildlife Refuge in New Jersey, USA.
Environ Pollut 143(3): 555-564.
Day, DB, Xiang, J, Mo, J, Li, F, Chung, M, Gong, J, Weschler, CJ,
Ohman-Strickland, PA, Sundell, J, Weng, W, Zhang, Y and Zhang, JJ
(2017). Association of ozone exposure with cardiorespiratory
pathophysiologic mechanisms in healthy adults. JAMA Intern Med
177(9): 1344-1353.
Dedoussi, IC, Eastham, SD, Monier, E and Barrett, SRH (2020).
Premature mortality related to United States cross-state air
pollution. Nature 578: 261-265.
Devlin, RB, McDonnell, WF, Mann, R, Becker, S, House, DE,
Schreinemachers, D and Koren, HS (1991). Exposure of humans to
ambient levels of ozone for 6.6 hours causes cellular and
biochemical changes in the lung. Am J Respir Cell Mol Biol 4(1): 72-
81.
Di, Q, Dai, L, Wang, Y, Zanobetti, A, Choirat, C, Schwartz, JD and
Dominici, F (2017a). Association of short-term exposure to air
pollution with mortality in older adults. JAMA 318: 2446-2456.
Di, Q, Wang, Y, Zanobetti, A, Wang, Y, Koutrakis, P, Choirat, C,
Dominici, F and Schwartz, J (2017b). Air Pollution and Mortality in
the Medicare Population. N Engl J Med 376: 2513-2522.
Dominici, F, Schwartz, J, Di, Q, Braun, D, Choirat, C and Zanobetti,
A (2019). Assessing Adverse Health Effects of Long-Term Exposure to
Low Levels of Ambient Air Pollution: Phase 1 Health Effects
Institute. Boston, MA. Available at: https://www.healtheffects.org/system/files/dominici-rr-200-report.pdf.
Dryden, DM, Spooner, CH, Stickland, MK, Vandermeer, B, Tjosvold, L,
Bialy, L, Wong, K and Rowe, BH (2010). Exercise-induced
bronchoconstriction and asthma. (AHRQ Publication No. 10-E001).
Rockville, MD: Agency for Healthcare Research and Quality.
Folinsbee, LJ and Hazucha, MJ (2000). Time course of response to
ozone exposure in healthy adult females. Inhal Toxicol 12(3): 151-
167.
Folinsbee, LJ, Horstman, DH, Kehrl, HR, Harder, S, Abdul-Salaam, S
and Ives, PJ (1994). Respiratory responses to repeated prolonged
exposure to 0.12 ppm ozone. Am J Respir Crit Care Med 149(1): 98-
105.
Folinsbee, LJ, McDonnell, WF and Horstman, DH (1988). Pulmonary
function and symptom responses after 6.6-hour exposure to 0.12 ppm
ozone with moderate exercise. JAPCA 38(1): 28-35.
Frey, HC (2014a). Letter from Dr. H. Christopher Frey, Chair, Clean
Air Scientific Advisory Committee, to Administrator Gina McCarthy.
Re: Health Risk and Exposure Assessment for Ozone (Second External
Review Draft--February 2014) EPA-CASAC-14-005. Office of the
Administrator, Science Advisory Board Washington, DC. Available at:
https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100JR8I.txt.
Frey, HC (2014b). Letter from Dr. H. Christopher Frey, Chair, Clean
Air Scientific Advisory Committee to Honorable Gina McCarthy,
Administrator, U.S. EPA. Re: CASAC Review of the EPA's Second Draft
Policy Assessment for the Review of the Ozone National Ambient Air
Quality Standards. June 26, 2014. EPA-CASAC-14-004. Office of the
Administrator, Science Advisory Board Washington, DC. Available at:
https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100JR6F.txt.
Frey, HC (2014c). Letter from Dr. H. Christopher Frey, Chair, Clean
Air Scientific Advisory Committee, to Administrator Gina McCarthy.
Re: CASAC Review of the EPA's Welfare Risk and Exposure Assessment
for Ozone (Second External Review Draft). June 18, 2014. EPA-CASAC-
14-003. Office of the Administrator, Science Advisory Board
Washington, DC. Available at: http://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100JMSY.PDF.
Frey, HC and Samet, JM (2012). Letter from Dr. H. Christopher Frey,
Chair, Clean Air Scientific Advisory Committee and Jonathan Samet,
Immediate Past Chair, Clean Air Scientific Advisory Committee, to
Administrator Lisa Jackson. Re: CASAC Review of the EPA's Policy
Assessment for the Review of the Ozone National Ambient Air Quality
Standards (First External Review Draft--August 2012). November 26,
2012. EPA-CASAC-13-003. Office of the Administrator, Science
Advisory Board Washington DC. Available at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100J7PQ.txt.
Gan, WQ, Allen, R W, Brauer, M, Davies, HW, Mancini, GB and Lear, SA
(2014). Long-term exposure to traffic-related air pollution and
progression of carotid artery atherosclerosis: a prospective cohort
study. BMJ open, 4(4), e004743.
Galizia, A and Kinney, PL (1999). Year-round residence in areas of
high ozone: Association with respiratory health in a nationwide
sample of nonsmoking young adults. Environ Health Perspect 107: 675-
679.
Garcia, E, Berhane, KT, Islam, T, McConnell, R, Urman, R, Chen, Z
and Gilliland, FD (2019). Association of changes in air quality with
incident asthma in children in California, 1993-2014. JAMA 321:
1906-1915.
Gharibi, H, Entwistle, MR, Ha, S, Gonzalez, M, Brown, P, Schweizer,
D and Cisneros, R (2019). Ozone pollution and asthma emergency
department visits in the Central Valley, California, USA, during
June to September of 2015: a time-stratified case-crossover
analysis. J Asthma 56(10):1037-1048.
Gold, JAW, Wong, KK, Szablewski, CM, Patel, PR, Rossow, J, da Silva,
J, Natarajan, P, Morris, SB, Fanfair, RN, Rogers-Brown, J, Bruce,
BB, Browning, SD, Hernandez-Romieu, AC, Furukawa, NW, Kang, M,
Evans, ME, Oosmanally, N, Tobin-D'Angelo, M, Drenzek, C, Murphy, DJ,
Hollberg, J, Blum, JM, Jansen, R, Wright, DW, Sewell, WM III, Owens,
JD, Lefkove, B, Brown, FW, Burton, DC, Uyeki, TM, Bialek, SR and
Jackson, BR (2020). Characteristics and Clinical Outcomes of Adult
Patients Hospitalized with COVID-19--Georgia, March 2020. MMWR Morb
Mortal Wkly Rep 2020;69:545-550.
Gong, H, Jr., Bradley, PW, Simmons, MS and Tashkin, DP (1986).
Impaired exercise performance and pulmonary function in elite
cyclists during low-level ozone exposure in a hot environment. Am J
Respir Crit Care Med 134(4): 726-733.
Goodman, JE, Loftus, CT, Liu, X and Zu, K (2017). Impact of
respiratory infections, outdoor pollen, and socioeconomic status on
associations between air pollutants and pediatric asthma hospital
admissions. PLoS One 12(7): e0180522.
Jerrett, M, Burnett, RT, Pope, CA, 3rd, Ito, K, Thurston, G,
Krewski, D, Shi, Y, Calle, E and Thun, M (2009). Long-term ozone
exposure and mortality. N Engl J Med 360(11): 1085-1095.
Heck, WW and Cowling, EB (1997). The need for a long term cumulative
secondary ozone standard--An ecological perspective. EM: