behavior were discussed in the proposal within the context of the
discussion of uncertainties in the HREA assessment of exposures of
concern (II.C.2.b in the proposal) and, contrary to the claims of some
commenters, they were not used to support the proposed decision.
    Some industry groups also claimed that the time-location-activity
diaries used by APEX to estimate exposures are out-of-date, and do not
represent activity patterns in the current population. These commenters
asserted that the use of out-of-date diary information leads to
overestimates in exposures of concern. This issue was explicitly
addressed in the HREA and the EPA disagrees with commenters'
conclusions. In particular, diary data was updated in this review to
include data from studies published as late as 2010, directly in
response to CASAC concerns. In their review of this data, CASAC stated
that ``[t]he addition of more recent time activity pattern data
addresses a concern raised previously by the CASAC concerning how
activity pattern information should be brought up to date'' (Frey,
2014a, p. 8). As indicated in the HREA (U.S. EPA, 2014a, Appendix 5G,
Figures 5G-7 and Figure 5G-8), the majority of diary days used in
exposure simulations of children originate from the most recently
conducted activity pattern studies (U.S. EPA, 2014a, Table 5-3). In
addition, evaluations included in the HREA indicated that there were
not major systematic differences in time-location-activity patterns
based on information from older diaries versus those collected more
recently (U.S. EPA,

2014a, Appendix 5G, Figures 5G-1 and 5G-2). Given all of the above, the
EPA does not agree with commenters who claimed that the time-location-
activity diaries used by APEX are out-of-date, and result in
overestimates of exposures of concern.
ii. Risk of O3-Induced FEV1 Decrements
    The EPA also received a large number of comments on the
FEV1 risk assessment presented in chapter 6 of the HREA
(U.S. EPA, 2014a) and summarized in the proposal (II.C.3.a in the
proposal). Commenters representing medical, public health, and
environmental groups generally expressed the view that these risk
estimates support the need to revise the current primary O3
standard in order to increase public health protection, though these
groups also questioned some of the assumptions inherent in the EPA's
interpretation of those risk estimates. For example, ALA et al. (p.
127) stated that ``[t]he HREA uses a risk function derived from a
controlled human exposure study of healthy young adults to estimate
lung function decrements in children, including children with asthma.
This assumption could result in an underestimate of risk.'' On this
same issue, commenters representing industry groups opposed to revising
the standard also asserted that assumptions about children's responses
to O3 exposures are highly uncertain. In contrast to medical
and public health groups, these commenters concluded that this
uncertainty, along with others discussed below, call into question the
use of FEV1 risk estimates to support a decision to revise
the current primary O3 standard.
    The EPA agrees that an important source of uncertainty is the
approach to estimating the risk of FEV1 decrements in
children and in children with asthma based on data from healthy adults.
However, this issue is discussed at length in the HREA and the PA, and
was considered carefully by CASAC in its review of draft versions of
these documents. The conclusions of the HREA and PA, and the advice of
CASAC, were reflected in the Administrator's interpretation of
FEV1 risk estimates in the proposal, as described below.
Commenters have not provided additional information that changes the
EPA's views on this issue.
    As discussed in the proposal (II.C.3.a.ii in the proposal), in the
near absence of controlled human exposure data for children, risk
estimates are based on the assumption that children exhibit the same
lung function response following O3 exposures as healthy 18-
year olds (i.e., the youngest age for which sufficient controlled human
exposure data is available) (U.S. EPA, 2014a, section 6.5.3). As noted
by CASAC (Frey, 2014a, p. 8), this assumption is justified in part by
the findings of McDonnell et al. (1985), who reported that children (8-
11 years old) experienced FEV1 responses similar to those
observed in adults (18-35 years old). The HREA concludes that this
approach could result in either over- or underestimates of
O3-induced lung function decrements in children, depending
on how children compare to the adults used in controlled human exposure
studies (U.S. EPA, 2014a, section 6.5.3). With regard to people with
asthma, although the evidence has been mixed (U.S. EPA, 2013, section
<IP_ADDRESS>), several studies have reported statistically larger, or a
tendency for larger, O3-induced lung function decrements in
asthmatics than in non-asthmatics (Kreit et al., 1989; Horstman et al.,
1995; Jorres et al., 1996; Alexis et al., 2000). On this issue, CASAC
noted that ``[a]sthmatic subjects appear to be at least as sensitive,
if not more sensitive, than non-asthmatic subjects in manifesting
O3-induced pulmonary function decrements'' (Frey, 2014c, p.
4). To the extent asthmatics experience larger O3-induced
lung function decrements than the healthy adults used to develop
exposure-response relationships, the HREA could underestimate the
impacts of O3 exposures on lung function in asthmatics,
including asthmatic children (U.S. EPA, 2014a, section 6.5.4). As noted
above, these uncertainties have been considered carefully by the EPA
and by CASAC during the development of the HREA and PA. In addition,
the Administrator has appropriately considered these and other
uncertainties in her interpretation of risk estimates, as discussed
further below (II.B.3, II.C.4.b, II.C.4.c).
    Some commenters additionally asserted that the HREA does not
appropriately characterize the uncertainty in risk estimates for
O3-induced lung function decrements. Commenters pointed out
that there is statistical uncertainty in model coefficients that is not
accounted for in risk estimates. One commenter presented an analysis of
this uncertainty, and concluded that there is considerable overlap
between risk estimates for standard levels of 75, 70, and 65 ppb,
undercutting the confidence in estimated risk reductions for standard
levels below 75 ppb.
    The Agency recognizes that there are important sources of
uncertainty in the FEV1 risk assessment. In some cases,
these sources of uncertainty can contribute to substantial variability
in risk estimates, complicating the interpretation of those estimates.
For example, as discussed in the proposal, the variability in
FEV1 risk estimates across urban study areas is often
greater than the differences in risk estimates between various standard
levels (Table 2, above and 79 FR 75306 n. 164). Given this, and the
resulting considerable overlap between the ranges of FEV1
risk estimates for different standard levels, in the proposal the
Administrator viewed these risk estimates as providing a more limited
basis than exposures of concern for distinguishing between the degree
of public health protection provided by alternative standard levels.
Thus, although the EPA does not agree with the overall conclusions of
industry commenters, their analysis of statistical uncertainty in risk
estimates, and the resulting overlap between risk estimates for
standard levels of 75, 70, and 65 ppb, tends to reinforce the
Administrator's approach, which places greater weight on estimates of
O3 exposures of concern than on risk estimates for
O3-induced FEV1 decrements.
iii. Risk of O3-Associated Mortality and Morbidity
    In the proposal, the Administrator placed the greatest emphasis on
the results of controlled human exposure studies and on quantitative
analyses based on information from these studies, and less weight on
mortality and morbidity risk assessments based on information from
epidemiology studies. The EPA received a number of comments on its
consideration of epidemiology-based risks, with some commenters
expressing support for the Agency's approach and others expressing
opposition.
    In general, commenters representing industry organizations or
states opposed to revising the current primary O3 standard
agreed with the Administrator's approach in the proposal to viewing
epidemiology-based risk estimates, though these commenters reached a
different conclusion than the EPA regarding the adequacy of the current
standard. In supporting their views, these commenters highlighted a
number of uncertainties in the underlying epidemiologic studies, and
concluded that risk estimates based on information from such studies do
not provide an appropriate basis for revising the current standard. For
example, commenters noted considerable spatial heterogeneity in health
effect associations; the potential for co-occurring pollutants (e.g.,
PM2.5) to confound O3 health effect associations;

and the lack of statistically significant O3 health effect
associations in many of the individual cities evaluated as part of
multicity analyses. In contrast, some commenters representing medical,
public health, or environmental organizations placed greater emphasis
than the EPA on epidemiology-based risk estimates. These commenters
asserted that risk estimates provide strong support for a lower
standard level, and pointed to CASAC advice to support their position.
    As in the proposal, the EPA continues to place the greatest weight
on the results of controlled human exposure studies and on quantitative
analyses based on information from these studies (particularly
exposures of concern, as discussed below in II.B.3 and II.C.4), and
less weight on risk analyses based on information from epidemiologic
studies. In doing so, the Agency continues to note that controlled
human exposure studies provide the most certain evidence indicating the
occurrence of health effects in humans following specific O3
exposures. In addition, the effects reported in these studies are due
solely to O3 exposures, and interpretation of study results
is not complicated by the presence of co-occurring pollutants or
pollutant mixtures (as is the case in epidemiologic studies). The
Agency further notes the CASAC judgment that ``the scientific evidence
supporting the finding that the current standard is inadequate to
protect public health is strongest based on the controlled human
exposure studies of respiratory effects'' (Frey, 2014c, p. 5).
Consistent with this emphasis, the HREA conclusions reflect relatively
greater confidence in the results of the exposure and risk analyses
based on information from controlled human exposure studies than the
results of epidemiology-based risk analyses. As discussed in the HREA
(U.S. EPA, 2014a, section 9.6), several key uncertainties complicate
the interpretation of these epidemiology-based risk estimates,
including the heterogeneity in O3 effect estimates between
locations, the potential for exposure measurement errors in these
epidemiologic studies, and uncertainty in the interpretation of the
shape of concentration-response functions at lower O3
concentrations. Commenters who opposed the EPA's approach in the
proposal to viewing the results of quantitative analyses tended to
highlight aspects of the evidence and CASAC advice that were considered
by the EPA at the time of proposal and nothing in these commenters'
views has changed those considerations. Therefore, the EPA continues to
place the most emphasis on using the information from controlled human
exposure studies to inform consideration of the adequacy of the primary
O3 standard.
    However, while the EPA agrees that there are important
uncertainties in the O3 epidemiology-based risk estimates,
the Agency disagrees with industry commenters that these uncertainties
support a conclusion to retain the current standard. As discussed
below, the decision to revise the current primary O3
standard is based on the EPA's consideration of the broad body of
scientific evidence, quantitative analyses of O3 exposures
and risks, CASAC advice, and public comments. While recognizing
uncertainties in the epidemiology-based risk estimates here, and giving
these uncertainties appropriate consideration, the Agency continues to
conclude that these risk estimates contribute to the broader body of
evidence and information supporting the need to revise the primary
O3 standard.
    Some commenters opposed to revising the current O3
standard highlighted the fact that, in a few urban study locations,
larger risks are estimated for standard levels below 75 ppb than for
the current standard with its level of 75 ppb. For example, TCEQ (p. 3)
states that ``differential effects on ozone in urban areas also lead to
the EPA's modeled increases in mortality in Houston and Los Angeles
with decreasing ozone standards.'' These commenters cited such
increases in estimated risk as part of the basis for their conclusion
that the current standard should be retained.
    For communities across the U.S. (including in the Houston and Los
Angeles areas), exposure and risk analyses indicate that reducing
emissions of O3 precursors (NOX, VOCs) to meet a
revised standard with a level of 70 ppb will substantially reduce the
occurrence of adverse respiratory effects and mortality risk
attributable to high O3 concentrations (U.S. EPA, 2014a,
Appendix 9A; U.S. EPA, 2014c, sections <IP_ADDRESS> to <IP_ADDRESS>). However,
because of the complex chemistry governing the formation and
destruction of O3, some NOX control strategies
designed to reduce the highest ambient O3 concentrations can
also result in increases in relatively low ambient O3
concentrations. As a result of the way the EPA's epidemiology-based
risk assessments were conducted (U.S. EPA, 2014a, Chapter 7), increases
estimated in low O3 concentrations impacted mortality and
morbidity risks, leading to the estimated risk increases highlighted by
some commenters. However, while the EPA is confident that reducing the
highest ambient O3 concentrations will result in substantial
improvements in public health, including reducing the risk of
O3-associated mortality, the Agency is far less certain
about the public health implications of the changes in relatively low
ambient O3 concentrations (79 FR at 75278/3, 75291/1, and
75308/2). Therefore, reducing precursor emissions to meet a lower
O3 standard is expected to result in important reductions in
O3 concentrations from the part of the air quality
distribution where the evidence provides the strongest support for
adverse health effects.
    Specifically, for area-wide O3 concentrations at or
above 40 ppb,\101\ a revised standard with a level of 70 ppb is
estimated to reduce the number of premature deaths associated with
short-term O3 concentrations by about 10%, compared to the
current standard. In addition, for area-wide concentrations at or above
60 ppb, a revised standard with a level of 70 ppb is estimated to
reduce O3-associated premature deaths by about 50% to
70%.\102\ The EPA views these results, which focus on the portion of
the air quality distribution where the evidence indicates the most
certainty regarding the occurrence of adverse O3-
attributable health effects, not only as supportive of the need to
revise the current standard (II.B.3, below), but also as showing the
benefits of reducing the peak O3 concentrations associated
with air quality distributions meeting the current standard (II.C.4,
below).
---------------------------------------------------------------------------

    \101\ The ISA concludes that there is less certainty in the
shape of concentration-response functions for area-wide
O3 concentrations at the lower ends of warm season
distributions (i.e., below about 20 to 40 ppb) (U.S. EPA, 2013,
section <IP_ADDRESS>).
    \102\ Available experimental studies provide the strongest
evidence for O3-induced effects following exposures to
O3 concentrations corresponding to the upper portions of
typical ambient distributions. In particular, as discussed above,
controlled human exposure studies showing respiratory effects
following exposures to O3 concentrations at or above 60
ppb.
---------------------------------------------------------------------------

    In addition, even considering risk estimates based on the full
distribution of ambient O3 concentrations (i.e., estimates
influenced by decreases in higher concentrations and increases in lower
concentrations), the EPA notes that, compared to the current standard,
standards with lower levels are estimated to result in overall
reductions in mortality risk across the urban study areas evaluated
(U.S. EPA, 2014c, Figure 4-10). As discussed above (II.A.2.a,
II.A.2.c), analyses in the HREA indicate that these overall risk
reductions could understate the actual reductions that

would be experienced by the U.S. population as a whole.
    For example, the HREA's national air quality modeling analyses
indicate that the HREA urban study areas tend to underrepresent the
populations living in areas where reducing NOX emissions
would be expected to result in decreases in warm season averages of
daily maximum 8-hour ambient O3 concentrations.\103\ Given
the strong connection between these warm season average O3
concentrations and risk, risk estimates for the urban study areas are
likely to understate the average reductions in O3-associated
mortality and morbidity risks that would be experienced across the U.S.
population as a whole upon reducing NOX emissions (U.S. EPA,
2014a, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \103\ Specifically, the HREA urban study areas tend to
underrepresent populations living in suburban, smaller urban, and
rural areas, where reducing NOX emissions would be
expected to result in decreases in warm season averages of daily
maximum 8-hour ambient O3 concentrations (U.S. EPA,
2014a, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    In addition, in recognizing that the reductions in modeled
NOX emissions used in the HREA's core analyses are meant to
be illustrative, rather than to imply a particular control strategy for
meeting a revised O3 NAAQS, the HREA also conducted
sensitivity analyses in which both NOX and VOC emissions
reductions were evaluated. In all of the urban study areas evaluated in
these analyses, the increases in low O3 concentrations were
smaller for the NOX/VOC emission reduction scenarios than
the NOX only emission reduction scenario (U.S. EPA, 2014a,
Appendix 4D, section 4.7). This was most apparent for Denver, Houston,
Los Angeles, New York, and Philadelphia. These results suggest that in
some locations, optimized emissions reduction strategies could result
in larger reductions in O3-associated mortality and
morbidity than indicated by HREA's core estimates.
    Thus, the patterns of estimated mortality and morbidity risks
across various air quality scenarios and locations have been evaluated
and considered extensively in the HREA and the PA, as well as in the
proposal. Epidemiology-based risk estimates have also been considered
by CASAC, and those considerations are reflected in CASAC's advice.
Specifically, in considering epidemiology-based risk estimates in its
review of the REA, CASAC stated that ``[a]lthough these estimates for
short-term exposure impacts are subject to uncertainty, the CASAC is
confident that that the evidence of health effects of O3
presented in the ISA and Second Draft HREA in its totality, indicates
that there are meaningful reductions in mean, absolute, and relative
premature mortality associated with short-term exposures to
O3 levels lower than the current standard'' (Frey, 2014a, p.
3). Commenters' views on this issue are not based on new information,
but on an interpretation of the analyses presented in the HREA that is
different from the EPA's, and CASAC's, interpretation. Given this, the
EPA's considerations and conclusions related to this issue, as
described in the proposal and as summarized briefly above, remain
valid. Therefore, the EPA does not agree with commenters who cited
increases in estimated risk in some locations as supporting a
conclusion that the current standard should be retained.
    For risk estimates of respiratory mortality associated with long-
term O3, several industry commenters supported placing more
emphasis on threshold models, and including these models as part of the
core analyses rather than as sensitivity analyses. The EPA agrees with
these commenters that an important uncertainty in risk estimates of
respiratory mortality associated with long-term O3 stems
from the potential for the existence of a threshold. Based on
sensitivity analyses included in the HREA in response to CASAC advice,
the existence of a threshold could substantially reduce estimated
risks. CASAC discussed this issue at length during its review of the
REA and supported the EPA's approach to including a range of threshold
models as sensitivity analyses (Frey, 2014a p. 3). Based in part on
uncertainty in the existence and identification of a threshold, the
HREA concluded that lower confidence should be placed in risk estimates
for respiratory mortality associated with long-term O3
exposures (U.S. EPA, 2014a, section 9.6). This uncertainty was also a
key part of the Administrator's rationale for placing only limited
emphasis on risk estimates for long-term O3 exposures. In
her final decisions, discussed below (II.B.3, II.C.4.b, II.C.4.c), the
Administrator continues to place only limited emphasis on these
estimates. The EPA views this approach to considering risk estimates
for respiratory mortality as generally consistent with the approach
supported by the commenters noted above.
3. Administrator's Conclusions on the Need for Revision
    This section discusses the Administrator's conclusions related to
the adequacy of the public health protection provided by the current
primary O3 standard, and her final decision that the current
standard is not requisite to protect public health with an adequate
margin of safety. These conclusions, and her final decision, are based
on the Administrator's consideration of the available scientific
evidence assessed in the ISA (U.S. EPA, 2013), the exposure/risk
information presented and assessed in the HREA (U.S. EPA, 2014a), the
consideration of that evidence and information in the PA (U.S. EPA,
2014c), the advice of CASAC, and public comments received on the
proposal.
    As an initial matter, the Administrator concludes that reducing
precursor emissions to achieve O3 concentrations that meet
the current primary O3 standard will provide important
improvements in public health protection, compared to recent air
quality. In reaching this conclusion, she notes the discussion in
section 3.4 of the PA (U.S. EPA, 2014c). In particular, the
Administrator notes that this conclusion is supported by (1) the strong
body of scientific evidence indicating a wide range of adverse health
outcomes attributable to exposures to O3 at concentrations
commonly found in the ambient air and (2) estimates indicating
decreased occurrences of O3 exposures of concern and
decreased O3-associated health risks upon meeting the
current standard, compared to recent air quality. Thus, she concludes
that it would not be appropriate in this review to consider a standard
that is less protective than the current standard.
    After reaching the conclusion that meeting the current primary
O3 standard will provide important improvements in public
health protection, and that it is not appropriate to consider a
standard that is less protective than the current standard, the
Administrator next considers the adequacy of the public health
protection that is provided by the current standard. In doing so, the
Administrator first notes that studies evaluated since the completion
of the 2006 AQCD support and expand upon the strong body of evidence
that, in the last review, indicated a causal relationship between
short-term O3 exposures and respiratory morbidity outcomes
(U.S. EPA, 2013, section 2.5). This is the strongest causality finding
possible under the ISA's hierarchical system for classifying weight of
evidence for causation. In addition, the Administrator notes that the
evidence for respiratory health effects attributable to long-term
O3 exposures, including the development of asthma in
children, is much stronger than in previous reviews, and the ISA
concludes that there is ``likely to be'' a causal relationship

between such O3 exposures and adverse respiratory health
effects (the second strongest causality finding).
    Together, experimental and epidemiologic studies support
conclusions regarding a continuum of O3 respiratory effects
ranging from small, reversible changes in pulmonary function, and
pulmonary inflammation, to more serious effects that can result in
respiratory-related emergency department visits, hospital admissions,
and premature mortality. Recent animal toxicology studies support
descriptions of modes of action for these respiratory effects and
augment support for biological plausibility for the role of
O3 in reported effects. With regard to mode of action,
evidence indicates that the initial key event is the formation of
secondary oxidation products in the respiratory tract, that antioxidant
capacity may modify the risk of respiratory morbidity associated with
O3 exposure, and that the inherent capacity to quench (based
on individual antioxidant capacity) can be overwhelmed, especially with
exposure to elevated concentrations of O3.
    In addition, based on the consistency of findings across studies
and the coherence of results from different scientific disciplines, the
available evidence indicates that certain populations are at increased
risk of experiencing O3-related effects, including the most
severe effects. These include populations and lifestages identified in
previous reviews (i.e., people with asthma, children, older adults,
outdoor workers) and populations identified since the last review
(i.e., people with certain genotypes related to antioxidant and/or
anti-inflammatory status; people with reduced intake of certain
antioxidant nutrients, such as Vitamins C and E).
    In considering the O3 exposure concentrations reported
to elicit respiratory effects, as in the proposal, the Administrator
agrees with the conclusions of the PA that controlled human exposure
studies provide the most certain evidence indicating the occurrence of
health effects in humans following specific O3 exposures. In
particular, she notes that the effects reported in controlled human
exposure studies are due solely to O3 exposures, and
interpretation of study results is not complicated by the presence of
co-occurring pollutants or pollutant mixtures (as is the case in
epidemiologic studies). Therefore, consistent with CASAC advice (Frey,
2014c), she places the most weight on information from controlled human
exposure studies in reaching conclusions on the adequacy of the current
primary O3 standard.
    In considering the evidence from controlled human exposure studies,
the Administrator first notes that these studies have reported a
variety of respiratory effects in healthy adults following exposures to
O3 concentrations of 60, 63,\104\ 72,\105\ or 80 ppb, and
higher. The largest respiratory effects, and the broadest range of
effects, have been studied and reported following exposures of healthy
adults to 80 ppb O3 or higher, with most exposure studies
conducted at these higher concentrations. As discussed above (II.A.1),
the Administrator further notes that recent evidence includes
controlled human exposure studies reporting the combination of lung
function decrements and respiratory symptoms in healthy adults engaged
in moderate exertion following 6.6-hour exposures to concentrations as
low as 72 ppb, and lung function decrements and pulmonary inflammation
following exposures to O3 concentrations as low as 60 ppb.
---------------------------------------------------------------------------

    \104\ For a 60 ppb target exposure concentration, Schelegle et
al. (2009) reported that the actual 6.6-hour mean exposure
concentration was 63 ppb.
    \105\ For a 70 ppb target exposure concentration, Schelegle et
al. (2009) reported that the actual 6.6-hour mean exposure
concentration was 72 ppb.
---------------------------------------------------------------------------

    As discussed in her response to public comments above (II.B.2.b.i),
and in detail below (II.C.4.b, II.C.4.c), the Administrator concludes
that these controlled human exposure studies indicate that adverse
effects are likely to occur following exposures to O3
concentrations below the level of the current standard. The effects
observed following such exposures are coherent with the serious health
outcomes that have been reported in O3 epidemiologic studies
(e.g., respiratory-related hospital admissions, emergency department
visits), and the Administrator judges that such effects have the
potential to be important from a public health perspective.
    In reaching these conclusions, she particularly notes that the
combination of lung function decrements and respiratory symptoms
reported to occur in healthy adults following exposures to 72 ppb
O3 meets ATS criteria for an adverse response (II.B.2.b.i,
above). In specifically considering the 72 ppb exposure concentration,
CASAC noted that ``the combination of decrements in FEV1
together with the statistically significant alterations in symptoms in
human subjects exposed to 72 ppb ozone meets the American Thoracic
Society's definition of an adverse health effect'' (Frey, 2014c, p. 5).
In addition, given that the controlled human exposure study reporting
these results was conducted in healthy adults, CASAC judged that the
adverse combination of lung function decrements and respiratory
symptoms ``almost certainly occur in some people'' (e.g., people with
asthma) following exposures to lower O3 concentrations
(Frey, 2014c, p. 6).
    While the Administrator is less certain regarding the adversity of
the lung function decrements and airway inflammation that have been
observed following exposures as low as 60 ppb, as discussed in more
detail elsewhere in this preamble (II.B.2.b.i, II.C.4.b, II.C.4.c), she
judges that these effects also have the potential to be adverse, and to
be of public health importance, particularly if they are experienced
repeatedly. With regard to this judgment, she specifically notes the
ISA conclusion that, while the airway inflammation induced by a single
exposure (or several exposures over the course of a summer) can resolve
entirely, continued inflammation could potentially result in adverse
effects, including the induction of a chronic inflammatory state;
altered pulmonary structure and function, leading to diseases such as
asthma; altered lung host defense response to inhaled microorganisms;
and altered lung response to other agents such as allergens or toxins
(U.S. EPA, 2013, section 6.2.3). Thus, the Administrator becomes
increasingly concerned about the potential for adverse effects at 60
ppb O3 as the number of exposures increases, though she
notes that the available evidence does not indicate a particular number
of occurrences of such exposures that would be required to achieve an
adverse respiratory effect, and that this number is likely to vary
across the population.
    In addition to controlled human exposure studies, the Administrator
also considers what the available epidemiologic evidence indicates with
regard to the adequacy of the public health protection provided by the
current primary O3 standard. She notes that recent
epidemiologic studies provide support, beyond that available in the
last review, for associations between short-term O3
exposures and a wide range of adverse respiratory outcomes (including
respiratory-related hospital admissions, emergency department visits,
and mortality) and with total mortality. As discussed above in the EPA
responses to public comments (II.B.2.b.ii), associations with morbidity
and mortality are stronger during the warm or summer months, and remain
robust after adjustment for copollutants (U.S. EPA, 2013, Chapter 6).

    In considering information from epidemiologic studies within the
context of her conclusions on the adequacy of the current standard, the
Administrator specifically considers analyses in the PA that evaluate
the extent to which O3 health effect associations have been
reported for air quality concentrations likely to be allowed by the
current standard. She notes that such analyses can provide insight into
the extent to which the current standard would allow the distributions
of ambient O3 concentrations that provided the basis for
these health effect associations. While the majority of O3
epidemiologic studies evaluated in the PA were conducted in areas that
would have violated the current standard during study periods, as
discussed above (II.B.2.b.ii), the Administrator observes that the
study by Mar and Koenig (2009) reported associations between short-term
O3 concentrations and asthma emergency department visits in
children and adults in a U.S. location that would have met the current
O3 standard over the entire study period.\106\ Based on
this, she notes the conclusion from the PA that the current primary
O3 standard would have allowed the distribution of ambient
O3 concentrations that provided the basis for the
associations with asthma emergency department visits reported by Mar
and Koenig (2009) (U.S. EPA, 2014c, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \106\ The large majority of locations evaluated in U.S.
epidemiologic studies of long-term O3 would have violated
the current standard during study periods, thus providing limited
insight into the adequacy of the current standard (U.S. EPA, 2014c,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    In addition, even in some single-city study locations where the
current standard was violated (i.e., those evaluated in Silverman and
Ito, 2010; Strickland et al., 2010), the Administrator notes that PA
analyses of reported concentration-response functions and available air
quality data support the occurrence of O3-attributable
hospital admissions and emergency department visits on subsets of days
with virtually all ambient O3 concentrations below the level
of the current standard. PA analyses of study area air quality further
support the conclusion that exposures to the ambient O3
concentrations present in the locations evaluated by Strickland et al.
(2010) and Silverman and Ito (2010) could have plausibly resulted in
the respiratory-related emergency department visits and hospital
admissions reported in these studies (U.S. EPA, 2014c, section
<IP_ADDRESS>). The Administrator agrees with the PA conclusion that these
analyses indicate a relatively high degree of confidence in reported
statistical associations with respiratory health outcomes on days when
virtually all monitored 8-hour O3 concentrations were 75 ppb
or below. She further agrees with the PA conclusion that although these
analyses do not identify true design values, the presence of
O3-associated respiratory effects on such days provides
insight into the types of health effects that could occur in locations
with maximum ambient O3 concentrations below the level of
the current standard.
    Compared to the single-city epidemiologic studies discussed above,
the Administrator notes additional uncertainty in interpreting the
relationships between short-term O3 air quality in
individual study cities and reported O3 multicity effect
estimates. In particular, she judges that the available multicity
effect estimates in studies of short-term O3 do not provide
a basis for considering the extent to which reported O3
health effect associations are influenced by individual locations with
ambient O3 concentrations low enough to meet the current
O3 standard, versus locations with O3
concentrations that violate this standard.\107\ While such
uncertainties limit the extent to which the Administrator bases her
conclusions on air quality in locations of multicity epidemiologic
studies, she does note that O3 associations with respiratory
morbidity or premature mortality have been reported in several
multicity studies when the majority of study locations (though not all
study locations) would have met the current O3 standard
(U.S. EPA, 2014c, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \107\ As noted in the proposal (II.E.4.d), this uncertainty
applies specifically to interpreting air quality analyses within the
context of multicity effect estimates for short-term O3
concentrations, where effect estimates for individual study cities
are not presented (as is the case for the key O3 studies
analyzed in the PA, with the exception of the study by Stieb et al.
(2009) where none of the city-specific effect estimates for asthma
emergency department visits were statistically significant). This
specific uncertainty does not apply to multicity epidemiologic
studies of long-term O3 concentrations, where multicity
effect estimates are based on comparisons across cities. For
example, see discussion of study by Jerrett et al. (2009) in the PA
(U.S. EPA, 2014c, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    Looking across the body of epidemiologic evidence, the
Administrator thus reaches the conclusion that analyses of air quality
in study locations support the occurrence of adverse O3-
associated effects at ambient O3 concentrations that met, or
are likely to have met, the current standard. She further concludes
that the strongest support for this conclusion comes from single-city
studies of respiratory-related hospital admissions and emergency
department visits associated with short-term O3
concentrations, with some support also from multicity studies of
morbidity or mortality.
    Taken together, the Administrator concludes that the scientific
evidence from controlled human exposure and epidemiologic studies calls
into question the adequacy of the public health protection provided by
the current standard. In reaching this conclusion, she particularly
notes that the current standard level is higher than the lowest
O3 exposure concentration shown to result in the adverse
combination of lung function decrements and respiratory symptoms (i.e.,
72 ppb), and that CASAC concluded that such effects ``almost certainly
occur in some people'' following exposures to O3
concentrations below 72 ppb (Frey, 2014c, p. 6). While she also notes
that the current standard level is well-above the lowest O3
exposure concentration shown to cause respiratory effects (i.e., 60
ppb), she has less confidence that the effects observed at 60 ppb are
adverse (discussed in II.B.2.b.i, II.C.4.b, II.C.4.c). She further
considers these effects, and the extent to which the current primary
O3 standard could protect against them, within the context
of quantitative analyses of O3 exposures (discussed below).
With regard to the available epidemiologic evidence, the Administrator
notes PA analyses of O3 air quality indicating that, while
most O3 epidemiologic studies reported health effect
associations with ambient O3 concentrations that violated
the current standard, a small number of single-city U.S. studies
support the occurrence of asthma-related hospital admissions and
emergency department visits at ambient O3 concentrations
below the level of the current standard, including one study with air
quality that would have met the current standard during the study
period. Some support for such O3 associations is also
provided by multicity studies of morbidity or mortality. The
Administrator further judges that the biological plausibility of
associations with clearly adverse morbidity effects is supported by the
evidence noted above from controlled human exposure studies conducted
at, or in some cases below, typical warm-season ambient O3
concentrations.
    Beyond her consideration of the scientific evidence, the
Administrator also considers the results of the HREA exposure and risk
analyses in reaching final conclusions regarding the adequacy of the
current primary O3 standard. In doing so, consistent with

her consideration of the evidence, she focuses primarily on
quantitative analyses based on information from controlled human
exposure studies (i.e., exposures of concern and risk of O3-
induced FEV1 decrements). Consistent with the considerations
in the PA, and with CASAC advice (Frey, 2014c), she particularly
focuses on exposure and risk estimates in children.\108\ As discussed
in the HREA and PA (and II.B, above), the patterns of exposure and risk
estimates across urban study areas, across years, and across air
quality scenarios are similar in children and adults though, because
children spend more time being physically active outdoors and are more
likely to experience the types of O3 exposures shown to
cause respiratory effects, larger percentages of children are estimated
to experience exposures of concern and O3-induced
FEV1 decrements. Children also have intrinsic risk factors
that make them particularly susceptible to O3-related
effects (e.g., higher ventilation rates relative to lung volume) (U.S.
EPA, 2013, section <IP_ADDRESS>; see section II.A.1.d above). In focusing on
exposure and risk estimates in children, the Administrator recognizes
that the exposure patterns for children across years, urban study
areas, and air quality scenarios are indicative of the exposure
patterns in a broader group of at-risk populations that also includes
asthmatic adults and older adults. She judges that, to the extent the
primary O3 standard provides appropriate protection for
children, it will also do so for adult populations,\109\ given the
larger exposures and intrinsic risk factors in children.
---------------------------------------------------------------------------

    \108\ She focuses on estimates for all children and estimates
for children with asthma, noting that exposure and risk estimates
for these groups are virtually indistinguishable in terms of the
percent estimated to experience exposures of concern or
O3-induced FEV1 decrements (U.S. EPA, 2014c,
sections 3.2 and 4.4.2).
    \109\ As noted below (II.C.4.2), this includes populations of
highly active adults, such as outdoor workers. Limited sensitivity
analyses in the HREA indicate that when diaries were selected to
mimic exposures that could be experienced by outdoor workers, the
percentages of modeled individuals estimated to experience exposures
of concern were generally similar to the percentages estimated for
children (i.e., using the full database of diary profiles) in the
urban study areas and years with the largest exposure estimates
(U.S. EPA, 2014, section <IP_ADDRESS>, Figure 5-14).
---------------------------------------------------------------------------

    In first considering estimates of exposures of concern, the
Administrator considers the extent to which estimates indicate that the
current standard limits population exposures to the broader range of
O3 concentrations shown in controlled human exposure studies
to cause respiratory effects. In doing so, she focuses on estimates of
O3 exposures of concern at or above the benchmark
concentrations of 60, 70, and 80 ppb. She notes that the current
O3 standard can provide some protection against exposures of
concern to a range of O3 concentrations, including
concentrations below the standard level, given that (1) with the
current fourth-high form, most days will have concentrations below the
standard level and that (2) exposures of concern depend on both the
presence of relatively high ambient O3 concentrations and on
activity patterns in the population that result in exposures to such
high concentrations while at an elevated ventilation rate (discussed in
detail below, II.C.4.b and II.C.4.c).
    In considering estimates of O3 exposures of concern
allowed by the current standard, she notes that while single exposures
of concern could be adverse for some people, particularly for the
higher benchmark concentrations (70, 80 ppb) where there is stronger
evidence for the occurrence of adverse effects (II.B.2.b.i, II.C.4.b,
II.C.4.c, below), she becomes increasingly concerned about the
potential for adverse responses as the number of occurrences
increases.\110\ In particular, as discussed above with regard to
inflammation, she notes that the types of lung injury shown to occur
following exposures to O3 concentrations from 60 to 80 ppb,
particularly if experienced repeatedly, provide a mode of action by
which O3 may cause other more serious effects (e.g., asthma
exacerbations). Therefore, the Administrator places the most weight on
estimates of two or more exposures of concern (i.e., as a surrogate for
the occurrence of repeated exposures), though she also considers
estimates of one or more exposures for the 70 and 80 ppb benchmarks.
---------------------------------------------------------------------------

    \110\ Not all people who experience an exposure of concern will
experience an adverse effect (even members of at-risk populations).
For the endpoints evaluated in controlled human exposure studies,
the number of those experiencing exposures of concern who will
experience adverse effects cannot be reliably quantified.
---------------------------------------------------------------------------

    In considering estimates of exposures of concern, the Administrator
first notes that if the 15 urban study areas evaluated in the HREA were
to just meet the current O3 standard, fewer than 1% of
children in those areas would be estimated to experience two or more
exposures of concern at or above 70 ppb, based on exposure estimates
averaged over the years of analysis, though up to about 2% would be
estimated to experience such exposures in the worst-case year and
location (i.e., year and location with the largest exposure
estimates).\111\ Although the Administrator is less concerned about
single occurrences of exposures of concern, she notes that even single
occurrences could cause adverse effects in some people, particularly
for the 70 and 80 ppb benchmarks.\112\ As illustrated in Table 1
(above), the current standard could allow up to about 3% of children to
experience one or more exposures of concern at or above 70 ppb,
averaged over the years of analysis, and up to about 8% in the worst-
case year and location. In addition, in the worst-case year and
location, the current standard could allow about 1% of children to
experience at least one exposure of concern at or above 80 ppb, the
highest benchmark evaluated.
---------------------------------------------------------------------------

    \111\ Virtually no children in those areas would be estimated to
experience two or more exposures of concern at or above 80 ppb.
    \112\ That is, adverse effects are a possible outcome of single
exposures of concern at/above 70 or 80 ppb, though the available
information is not sufficient to estimate the likelihood of such
effects.
---------------------------------------------------------------------------

    While the Administrator has less confidence in the adversity of the
effects observed following exposures to 60 ppb O3
(II.B.2.b.i, II.C.4.b, II.C.4.c), particularly for single exposures,
she judges that the potential for adverse effects increases as the
number of exposures of concern increases. With regard to the 60 ppb
benchmark, she particularly notes that the current standard is
estimated to allow approximately 3 to 8% of children in urban study
areas, including approximately 3 to 8% of asthmatic children, to
experience two or more exposures of concern to O3
concentrations at or above 60 ppb, based on estimates averaged over the
years of analysis. To provide some perspective on the average
percentages estimated, the Administrator notes that they correspond to
almost 900,000 children in urban study areas, including about 90,000
asthmatic children. Nationally, if the current standard were to be just
met, the number of children experiencing such exposures would be
larger.
    Based on her consideration of these estimates within the context of
her judgments on adversity, as discussed in her responses to public
comments (II.B.2.b.i, II.C.4.b), the Administrator concludes that the
exposures projected to remain upon meeting the current standard can
reasonably be judged to be important from a public health perspective.
In particular, given that the average percent of children estimated to
experience two or more exposures of concern for the 60 ppb benchmark
approaches 10% in some areas, even based on estimates averaged over the

years of the analysis, she concludes that the current standard does not
incorporate an adequate margin of safety against the potentially
adverse effects that can occur following repeated exposures at or above
60 ppb. Although she has less confidence that the effects observed at
60 ppb are adverse, compared to the effects at and above 72 ppb, she
judges that this approach to considering the results for the 60 ppb
benchmark is appropriate given CASAC advice, which clearly focuses the
EPA on considering the effects observed at 60 ppb (Frey, 2014c)
(II.C.4.b, II.C.4.c below).\113\ This approach to considering estimated
exposures of concern is consistent with setting standards that provide
some safeguard against dangers to human health that are not fully
certain (i.e., standards that incorporate an adequate margin of safety)
(See, e.g., State of Mississippi, 744 F. 3d at 1353).
---------------------------------------------------------------------------

    \113\ Though this advice is less clear regarding the adversity
of effects at 60 ppb than CASAC's advice regarding the adversity of
effects at 72 ppb (II.C.4.b, II.C.4.c).
---------------------------------------------------------------------------

    In addition to estimated exposures of concern, the Administrator
also considers HREA estimates of the risk of O3-induced
FEV1 decrements >=10 and 15%. In doing so, she particularly
notes CASAC advice that ``estimation of FEV1 decrements of
>=15% is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults, whereas an FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3). The Administrator notes that while single occurrences of
O3-induced lung function decrements could be adverse for
some people, as discussed above (II.B.1), she agrees with the judgment
in past reviews that a more general consensus view of the potential
adversity of such decrements emerges as the frequency of occurrences
increases. Therefore, as in the proposal, the Administrator focuses
primarily on the estimates of two or more O3-induced lung
function decrements. When averaged over the years evaluated in the
HREA, the Administrator notes that the current standard is estimated to
allow about 1 to 3% of children in the 15 urban study areas
(corresponding to almost 400,000 children) to experience two or more
O3-induced lung function decrements >=15%, and to allow
about 8 to 12% of children (corresponding to about 180,000 asthmatic
children) to experience two or more O3-induced lung function
decrements >=10%.
    In further considering the HREA results, the Administrator
considers the epidemiology-based risk estimates. As discussed in the
proposal, compared to the weight given to HREA estimates of exposures
of concern and lung function risks, she places relatively less weight
on epidemiology-based risk estimates. In giving some consideration to
these risk estimates, as discussed in the proposal and above in the
EPA's responses to public comments (II.B.2.b.iii), the Administrator
focuses on the risks associated with O3 concentrations in
the upper portions of ambient distributions. In doing so, she notes the
increasing uncertainty associated with the shapes of concentration-
response curves for O3 concentrations in the lower portions
of ambient distributions and the evidence from controlled human
exposure studies, which provide the strongest support for
O3-induced effects following exposures to O3
concentrations corresponding to the upper portions of typical ambient
distributions (i.e., 60 ppb and above). Even when considering only
area-wide O3 concentrations from the upper portions of
seasonal distributions (i.e., >=40, 60 ppb, Table 3 in the proposal),
the Administrator notes that the general magnitude of mortality risk
estimates suggests the potential for a substantial number of
O3-associated deaths and adverse respiratory events to occur
nationally, even when the current standard is met (79 FR 75277 and
II.B.2.c.iii above).
    In addition to the evidence and exposure/risk information discussed
above, the Administrator also takes note of the CASAC advice in the
current review, in the 2008 review and decision establishing the
current standard, and in the 2010 reconsideration of the 2008 decision.
As discussed in more detail above, the current CASAC ``finds that the
current NAAQS for ozone is not protective of human health'' and
``unanimously recommends that the Administrator revise the current
primary ozone standard to protect public health'' (Frey, 2014c, p. 5).
The prior CASAC O3 Panel likewise recommended revision of
the current standard to one with a lower level due to the lack of
protectiveness of the current standard. This earlier recommendation was
based entirely on the evidence and information in the record for the
2008 standard decision, which, as discussed above, has been
substantially strengthened in the current review (Samet, 2011; Frey and
Samet, 2012).
    In consideration of all of the above, the Administrator concludes
that the current primary O3 standard is not requisite to
protect public health with an adequate margin of safety, and that it
should be revised to provide increased public health protection. This
decision is based on the Administrator's conclusions that the available
evidence and exposure and risk information clearly call into question
the adequacy of public health protection provided by the current
primary standard such that it is not appropriate, within the meaning of
section 109(d)(1) of the CAA, to retain the current standard. With
regard to the evidence, she particularly notes that the current
standard level is higher than the lowest O3 exposure
concentration shown to result in the adverse combination of lung
function decrements and respiratory symptoms (i.e., 72 ppb), and also
notes CASAC's advice that at-risk groups (e.g., people with asthma)
could experience adverse effects following exposure to lower
concentrations. In addition, while the Administrator is less certain
about the adversity of the effects that occur following lower exposure
concentrations, she judges that recent controlled human exposure
studies at 60 ppb provide support for a level below 75 ppb in order to
provide an increased margin of safety, compared to the current
standard, against effects with the potential to be adverse,
particularly if they are experienced repeatedly. With regard to
O3 epidemiologic studies, she notes that while most
available studies reported health effect associations with ambient
O3 concentrations that violated the current standard, a
small number provide support for the occurrence of adverse respiratory
effects at ambient O3 concentrations below the level of the
current standard.\114\
---------------------------------------------------------------------------

    \114\ Courts have repeatedly held that this type of evidence
justifies an Administrator's conclusion that it is ``appropriate''
(within the meaning of section 109 (d)(1) of the CAA) to revise a
primary NAAQS to provide further protection of public health. See
e.g. Mississippi, 744 F. 3d at 1345; American Farm Bureau, 559 F. 3d
at 525-26.
---------------------------------------------------------------------------

    Based on the analyses in the HREA, the Administrator concludes that
the exposures and risks projected to remain upon meeting the current
standard can reasonably be judged to be important from a public health
perspective. In particular, this conclusion is based on her judgment
that it is appropriate to set a standard that would be expected to
eliminate, or almost eliminate, exposures of concern at or above 70 and
80 ppb. In addition, given that the average percent of children
estimated to experience two or more exposures of concern for the 60 ppb
benchmark approaches 10% in some urban study areas, the Administrator
concludes that the current standard does not incorporate an adequate
margin of safety

against the potentially adverse effects that could occur following
repeated exposures at or above 60 ppb. Beyond estimated exposures of
concern, the Administrator concludes that the HREA risk estimates
(FEV1 risk estimates, mortality risk estimates) further
support a conclusion that the O3-associated health effects
estimated to remain upon just meeting the current standard are an issue
of public health importance on a broad national scale. Thus, she
concludes that O3 exposure and risk estimates, when taken
together, support a conclusion that the exposures and health risks
associated with just meeting the current standard can reasonably be
judged important from a public health perspective, such that the
current standard is not sufficiently protective and does not
incorporate an adequate margin of safety.
    In the next section, the Administrator considers what revisions are
appropriate in order to set a standard that is requisite to protect
public health with an adequate margin of safety.

C. Conclusions on the Elements of a Revised Primary Standard

    Having reached the conclusion that the current O3
standard is not requisite to protect public health with an adequate
margin of safety, based on the currently available scientific evidence
and exposure/risk information, the Administrator next considers the
range of alternative standards supported by that evidence and
information. Consistent with her consideration of the adequacy of the
current standard, the Administrator's conclusions on the elements of
the primary standard are informed by the available scientific evidence
assessed in the ISA, exposure/risk information presented and assessed
in the HREA, the evidence-based and exposure-/risk-based considerations
and conclusions in the PA, CASAC advice, and public comments. The
sections below discuss the evidence and exposure/risk information,
CASAC advice and public input, and the Administrator's proposed
conclusions, for the major elements of the NAAQS: Indicator (II.C.1),
averaging time (II.C.2), form (II.C.3), and level (II.C.4).
1. Indicator
    In the 2008 review, the EPA focused on O3 as the most
appropriate indicator for a standard meant to provide protection
against ambient photochemical oxidants. In this review, while the
complex atmospheric chemistry in which O3 plays a key role
has been highlighted, no alternatives to O3 have been
advanced as being a more appropriate indicator for ambient
photochemical oxidants. More specifically, the ISA noted that
O3 is the only photochemical oxidant (other than
NO2) that is routinely monitored and for which a
comprehensive database exists (U.S. EPA, 2013, section 3.6). Data for
other photochemical oxidants (e.g., peroxyacetyl nitrate, hydrogen
peroxide, etc.) typically have been obtained only as part of special
field studies. Consequently, no data on nationwide patterns of
occurrence are available for these other oxidants; nor are extensive
data available on the relationships of concentrations and patterns of
these oxidants to those of O3 (U.S. EPA, 2013, section 3.6).
In its review of the second draft PA, CASAC stated ``The indicator of
ozone is appropriate based on its causal or likely causal associations
with multiple adverse health outcomes and its representation of a class
of pollutants known as photochemical oxidants'' (Frey, 2014c, p. ii).
    In addition, the PA notes that meeting an O3 standard
can be expected to provide some degree of protection against potential
health effects that may be independently associated with other
photochemical oxidants, even though such effects are not discernible
from currently available studies indexed by O3 alone (U.S.
EPA, 2014c, section 4.1). That is, since the precursor emissions that
lead to the formation of O3 generally also lead to the
formation of other photochemical oxidants, measures leading to
reductions in population exposures to O3 can generally be
expected to lead to reductions in population exposures to other
photochemical oxidants. In considering this information, and CASAC's
advice, the Administrator reached the proposed conclusion that
O3 remains the most appropriate indicator for a standard
meant to provide protection against photochemical oxidants.\115\
---------------------------------------------------------------------------

    \115\ The DC Circuit upheld the use of O3 as the
indicator for photochemical oxidants based on these same
considerations. American Petroleum Inst. v. Costle, 665 F. 2d 1176,
1186 (D.C. Cir. 1981).
---------------------------------------------------------------------------

    The EPA received very few comments on the indicator of the primary
standard. Those who did comment supported the proposed decision to
retain O3 as the indicator, noting the rationale put forward
in the preamble to the proposed rule. These commenters generally
expressed support for retaining the current indicator in conjunction
with retaining other elements of the current standard, such as the
averaging time and form. After considering the available evidence,
CASAC advice, and public comments, the Administrator concludes that
O3 remains the most appropriate indicator for a standard
meant to provide protection against photochemical oxidants. Therefore,
she is retaining O3 as the indicator for the primary
standard in this final rule.
2. Averaging Time
    The EPA established the current 8-hour averaging time \116\ for the
primary O3 NAAQS in 1997 (62 FR 38856). The decision on
averaging time in that review was based on numerous controlled human
exposure and epidemiologic studies reporting associations between
adverse respiratory effects and 6- to 8-hour O3
concentrations (62 FR 38861). The EPA also noted that a standard with a
maximum 8-hour averaging time is likely to provide substantial
protection against respiratory effects associated with 1-hour peak
O3 concentrations. The EPA reached similar conclusions in
the last O3 NAAQS review and thus, the EPA retained the 8-
hour averaging time in 2008.
---------------------------------------------------------------------------

    \116\ This 8-hour averaging time reflects daily maximum 8-hour
average O3 concentrations.
---------------------------------------------------------------------------

    In reaching a proposed conclusion on averaging time in the current
review, the Administrator considered the extent to which the available
evidence continues to support the appropriateness of a standard with an
8-hour averaging time (79 FR 75292). Specifically, the Administrator
considered the extent to which the available information indicates that
a standard with the current 8-hour averaging time provides appropriate
protection against short- and long-term O3 exposures. These
considerations from the proposal are summarized below in sections
II.C.2.a (short-term) and II.C.2.b (long-term). Section II.C.2.c
summarizes the Administrator's proposed decision on averaging time.
Section II.C.2.d discusses comments received on averaging time. Section
II.C.2.e presents the Administrator's final decision regarding
averaging time.
a. Short-Term
    As an initial consideration with respect to the most appropriate
averaging time for the O3 NAAQS, in the proposal the
Administrator noted that the strongest evidence for O3-
associated health effects is for respiratory effects following short-
term exposures. More specifically, the Administrator noted the ISA
conclusion that the evidence is ``sufficient to infer a causal
relationship'' between short-term O3 exposures and
respiratory effects. The ISA also judges that for short-term
O3 exposures, the evidence indicates ``likely to be causal''
relationships with

both cardiovascular effects and mortality (U.S. EPA, 2013, section
2.5.2). Therefore, as in past reviews, the Administrator noted that the
strength of the available scientific evidence provides strong support
for a standard that protects the public health against short-term
exposures to O3.
    In first considering the level of support available for specific
short-term averaging times, the Administrator noted in the proposal the
evidence available from controlled human exposure studies. As discussed
in more detail in Chapter 3 of the PA, substantial health effects
evidence from controlled human exposure studies demonstrates that a
wide range of respiratory effects (e.g., pulmonary function decrements,
increases in respiratory symptoms, lung inflammation, lung
permeability, decreased lung host defense, and airway
hyperresponsiveness) occur in healthy adults following 6.6-hour
exposures to O3 (U.S. EPA, 2013, section <IP_ADDRESS>). Compared
to studies evaluating shorter exposure durations (e.g., 1-hour),
studies evaluating 6.6-hour exposures in healthy adults have reported
respiratory effects at lower O3 exposure concentrations and
at more moderate levels of exertion.
    The Administrator also noted in the proposal the strength of
evidence from epidemiologic studies that evaluated a wide variety of
populations (e.g., including at-risk lifestages and populations, such
as children and people with asthma, respectively). A number of
different averaging times have been used in O3 epidemiologic
studies, with the most common being the max 1-hour concentration within
a 24-hour period (1-hour max), the max 8-hour average concentration
within a 24-hour period (8-hour max), and the 24-hour average. These
studies are assessed in detail in Chapter 6 of the ISA (U.S. EPA,
2013). Limited evidence from time-series and panel epidemiologic
studies comparing risk estimates across averaging times does not
indicate that one exposure metric is more consistently or strongly
associated with respiratory health effects or mortality, though the ISA
notes some evidence for ``smaller O3 risk estimates when
using a 24-hour average exposure metric'' (U.S. EPA, 2013, section
<IP_ADDRESS>; p. 2-31). For single- and multi-day average O3
concentrations, lung function decrements were associated with 1-hour
max, 8-hour max, and 24-hour average ambient O3
concentrations, with no strong difference in the consistency or
magnitude of association among the averaging times (U.S. EPA, 2013, p.
6-71). Similarly, in studies of short-term exposure to O3
and mortality, Smith et al. (2009) and Darrow et al. (2011) have
reported high correlations between risk estimates calculated using 24-
hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA,
2013, p. 6-253). Thus, the Administrator noted that the epidemiologic
evidence alone does not provide a strong basis for distinguishing
between the appropriateness of 1-hour, 8-hour, and 24-hour averaging
times.
    Considering the health information discussed above, in the proposal
the Administrator concluded that an 8-hour averaging time remains
appropriate for addressing health effects associated with short-term
exposures to ambient O3. An 8-hour averaging time is similar
to the exposure periods evaluated in controlled human exposure studies,
including recent studies that provide evidence for respiratory effects
following exposures to O3 concentrations below the level of
the current standard. In addition, epidemiologic studies provide
evidence for health effect associations with 8-hour O3
concentrations, as well as with 1-hour and 24-hour concentrations. As
in previous reviews, the Administrator noted that a standard with an 8-
hour averaging time (combined with an appropriate standard form and
level) would also be expected to provide substantial protection against
health effects attributable to 1-hour and 24-hour exposures (e.g., 62
FR 38861, July 18, 1997). This conclusion is consistent with the advice
received from CASAC that ``the current 8-hour averaging time is
justified by the combined evidence from epidemiologic and clinical
studies'' (Frey, 2014c, p. 6).
b. Long-Term
    The ISA concludes that the evidence for long-term O3
exposures indicates that there is ``likely to be a causal
relationship'' with respiratory effects (U.S. EPA, 2013, chapter 7).
Thus, in this review the Administrator also considers the extent to
which currently available evidence and exposure/risk information
suggests that a standard with an 8-hour averaging time can provide
protection against respiratory effects associated with longer term
exposures to ambient O3.
    In considering this issue in the 2008 review of the O3
NAAQS, the Staff Paper noted that ``because long-term air quality
patterns would be improved in areas coming into attainment with an 8-hr
standard, the potential risk of health effects associated with long-
term exposures would be reduced in any area meeting an 8-hr standard''
(U.S. EPA, 2007, p. 6-57). In the current review, the PA further
evaluates this issue, with a focus on the long-term O3
metrics reported to be associated with mortality or morbidity in recent
epidemiologic studies. As discussed in section 3.1.3 of the PA (U.S.
EPA, 2014c, section 4.2), much of the recent evidence for such
associations is based on studies that defined long-term O3
in terms of seasonal averages of daily maximum 1-hour or 8-hour
concentrations.
    As an initial consideration, in the proposal the Administrator
noted the risk results from the HREA for respiratory mortality
associated with long-term O3 concentrations. These HREA
analyses indicate that as air quality is adjusted to just meet the
current 8-hour standard, most urban study areas are estimated to
experience reductions in respiratory mortality associated with long-
term O3 concentrations based on the seasonal averages of 1-
hour daily maximum O3 concentrations evaluated in the study
by Jerrett et al. (2009) (U.S. EPA, 2014a, chapter 7).\117\ As air
quality is adjusted to meet lower alternative standard levels, for
standards based on 3-year averages of the annual fourth-highest daily
maximum 8-hour O3 concentrations, respiratory mortality
risks are estimated to be reduced further in urban study areas. This
analysis indicates that an O3 standard with an 8-hour
averaging time, when coupled with an appropriate form and level, can
reduce respiratory mortality reported to be associated with long-term
O3 concentrations.
---------------------------------------------------------------------------

    \117\ Though the Administrator also notes important
uncertainties associated with these risk estimates, as discussed in
section II.C.3.b of the proposal.
---------------------------------------------------------------------------

    In further considering the study by Jerrett et al. (2009), in the
proposal the Administrator noted the PA comparison of long-term
O3 concentrations following model adjustment in urban study
areas (i.e., adjusted to meet the current and alternative 8-hour
standards) to the concentrations present in study cities that provided
the basis for the positive and statistically significant association
with respiratory mortality. As indicated in Table 4-3 of the PA (U.S.
EPA, 2014c, section 4.2), this comparison suggests that a standard with
an 8-hour averaging time can decrease seasonal averages of 1-hour daily
maximum O3 concentrations, and can maintain those
O3 concentrations below the seasonal average concentration
where the study indicates the most confidence in the reported
concentration-response relationship with respiratory mortality (U.S.
EPA, 2014c, sections 4.2 and 4.4.1).

    The Administrator also noted in the proposal that the HREA
conducted analyses evaluating the impacts of reducing regional
NOX emissions on the seasonal averages of daily maximum 8-
hour O3 concentrations. Seasonal averages of 8-hour daily
max O3 concentrations reflect long-term metrics that have
been reported to be associated with respiratory morbidity effects in
several recent O3 epidemiologic studies (e.g., Islam et al.,
2008; Lin et al., 2008a, 2008b; Salam et al., 2009). The HREA analyses
indicate that the large majority of the U.S. population lives in
locations where reducing NOX emissions would be expected to
result in decreases in seasonal averages of daily max 8-hour ambient
O3 concentrations (U.S. EPA, 2014a, chapter 8). Thus,
consistent with the respiratory mortality risk estimates noted above,
these analyses suggest that reductions in O3 precursor
emissions in order to meet a standard with an 8-hour averaging time
would also be expected to reduce the long-term O3
concentrations that have been reported in recent epidemiologic studies
to be associated with respiratory morbidity.
c. Administrator's Proposed Conclusion on Averaging Time
    In the proposal the Administrator noted that, when taken together,
the analyses summarized above indicate that a standard with an 8-hour
averaging time, coupled with the current fourth-high form and an
appropriate level, would be expected to provide appropriate protection
against the short- and long-term O3 concentrations that have
been reported to be associated with respiratory morbidity and
mortality. The CASAC agreed with this conclusion, stating that ``[t]he
current 8-hour averaging time is justified by the combined evidence
from epidemiologic and clinical studies'' and that ``[t]he 8-hour
averaging window also provides protection against the adverse impacts
of long-term ozone exposures, which were found to be ``likely causal''
for respiratory effects and premature mortality'' (Frey, 2014c, p. 6).
Therefore, considering the available evidence and exposure risk
information, and CASAC's advice, the Administrator proposed to retain
the current 8-hour averaging time, and not to set an additional
standard with a different averaging time.
d. Comments on Averaging Time
    Most public commenters did not address the issue of whether the EPA
should consider additional or alternative averaging times. Of those who
did address this issue, some commenters representing state agencies or
industry groups agreed with the proposed decision to retain the current
8-hour averaging time, generally noting the supportive evidence
discussed in the preamble to the proposed rule. In contrast, several
medical organizations and environmental groups questioned the degree of
health protection provided by a standard based on an 8-hour averaging
time. For example, one group asserted that ``[a]veraging over any time
period, such as 8 hours, is capable of hiding peaks that may be very
substantial if they are brief enough.''
    The EPA agrees with these commenters that an important issue in the
current review is the appropriateness of using a standard with an 8-
hour averaging time to protect against adverse health effects that are
attributable to a wide range of O3 exposure durations,
including those shorter and longer than 8 hours. This is an issue that
has been thoroughly evaluated by the EPA in past reviews, as well as in
the current review.
    The 8-hour O3 NAAQS was originally set in 1997, as part
of revising the then-existing standard with its 1-hour averaging time,
and was retained in the review completed in 2008 (73 FR 16472). In both
of these reviews, several lines of evidence and information provided
support for an 8-hour averaging time rather than a shorter averaging
time. For example, substantial health evidence demonstrated
associations between a wide range of respiratory effects and 6- to 8-
hour exposures to relatively low O3 concentrations (i.e.,
below the level of the 1-hour O3 NAAQS in place prior to the
review completed in 1997). A standard with an 8-hour averaging time was
determined to be more directly associated with health effects of
concern at lower O3 concentrations than a standard with a 1-
hour averaging time. In addition, results of quantitative analyses
showed that a standard with an 8-hour averaging time can effectively
limit both 1- and 8-hour exposures of concern, and that an 8-hour
averaging time results in a more uniformly protective national standard
than a 1-hour averaging time. In past reviews, CASAC has agreed that an
8-hour averaging time is appropriate.
    In reaching her proposed decision to retain the 8-hour averaging
time in the current review, the Administrator again considered the body
of evidence for adverse effects attributable to a wide range of
O3 exposure durations, including studies specifically
referenced by public commenters who questioned the protectiveness of a
standard with an 8-hour averaging time. For example, as noted above a
substantial body of health effects evidence from controlled human
exposure studies demonstrates that a wide range of respiratory effects
occur in healthy adults following 6.6-hour exposures to O3
(U.S. EPA, 2013, section <IP_ADDRESS>). Compared to studies evaluating
shorter exposure durations (e.g., 1-hour), studies evaluating 6.6-hour
exposures in healthy adults have reported respiratory effects at lower
O3 exposure concentrations and at more moderate levels of
exertion. The Administrator also noted the strength of evidence from
epidemiologic studies that evaluated a number of different averaging
times, with the most common being the maximum 1-hour concentration
within a 24-hour period (1-hour max), the maximum 8-hour average
concentration within a 24-hour period (8-hour max), and the 24-hour
average. Evidence from time-series and panel epidemiologic studies
comparing risk estimates across averaging times does not indicate that
one exposure metric is more consistently or strongly associated with
respiratory health effects or mortality (U.S. EPA, 2013, section
<IP_ADDRESS>; p. 2-31). For single- and multi-day average O3
concentrations, lung function decrements were associated with 1-hour
max, 8-hour max, and 24-hour average ambient O3
concentrations, with no strong difference in the consistency or
magnitude of association among the averaging times (U.S. EPA, 2013, p.
6-71). Similarly, in studies of short-term exposure to O3
and mortality, Smith et al. (2009) and Darrow et al. (2011) have
reported high correlations between risk estimates calculated using 24-
hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA,
2013, p. 6-253). Thus, the epidemiologic evidence does not provide a
strong basis for distinguishing between the appropriateness of 1-hour,
8-hour, and 24-hour averaging times.
    In addition, quantitative exposure and risk analyses in the HREA
are based on an air quality adjustment approach that estimates hourly
O3 concentrations, and on scientific studies that evaluated
health effects attributable to a wide range of O3 exposure
durations. For example, the risk of lung function decrements is
estimated using a model based on controlled human exposure studies with
exposure durations ranging from 2 to 7.6 hours (U.S. EPA, 2013, section
<IP_ADDRESS>). Epidemiology-based risk estimates are based on studies that
reported health effect associations with short-term ambient
O3 concentrations ranging from 1-hour to 24-hours and with
long-term seasonal average concentrations (U.S. EPA, 2014a, Table 7-2).
Thus, the HREA estimated health

risks associated with a wide range of O3 exposure durations
and the Administrator's conclusions on averaging time in the current
review are based, in part, on consideration of these estimates.
    When taken together, the evidence and analyses indicate that a
standard with an 8-hour averaging time, coupled with the current
fourth-high form and an appropriate level, would be expected to provide
appropriate protection against the short- and long-term O3
concentrations that have been reported to be associated with
respiratory morbidity and mortality. The CASAC agreed with this,
stating the following (Frey, 2014c, p. 6):

    The current 8-hour averaging time is justified by the combined
evidence from epidemiologic and clinical studies referenced in
Chapter 4. Results from clinical studies, for example, show a wide
range of respiratory effects in healthy adults following 6.6 hours
of exposure to ozone, including pulmonary function decrements,
increases in respiratory symptoms, lung inflammation, lung
permeability, decreased lung host defense, and airway
hyperresponsiveness. These findings are supported by evidence from
epidemiological studies that show causal associations between short-
term exposures of 1, 8 and 24-hours and respiratory effects and
``likely to be causal'' associations for cardiovascular effects and
premature mortality. The 8-hour averaging window also provides
protection against the adverse impacts of long-term ozone exposures,
which were found to be ``likely causal'' for respiratory effects and
premature mortality.

Given all of the above, the EPA disagrees with commenters who question
the protectiveness of an O3 standard with an 8-hour
averaging time, particularly for an 8-hour standard with the revised
level of 70 ppb that is being established in this review, as discussed
below (II.C.4).
e. Administrator's Final Decision Regarding Averaging Time
    In considering the evidence and information summarized in the
proposal and discussed in detail in the ISA, HREA, and PA; CASAC's
views; and public comments, the Administrator concludes that a standard
with an 8-hour averaging time can effectively limit health effects
attributable to both short- and long-term O3 exposures. As
was the case in the proposal, this final conclusion is based on (1) the
strong evidence that continues to support the importance of protecting
public health against short-term O3 exposures (e.g., <= 1-
hour to 24-hour) and (2) analyses in the HREA and PA supporting the
conclusion that the current 8-hour averaging time can effectively limit
long-term O3 exposures. Furthermore, the Administrator
observes that the CASAC Panel agreed with the choice of averaging time
(Frey, 2014c). Therefore, in the current review, the Administrator
concludes that it is appropriate to retain the 8-hour averaging time
and to not set a separate standard with a different averaging time in
this final rule.
3. Form
    The ``form'' of a standard defines the air quality statistic that
is to be compared to the level of the standard in determining whether
an area attains that standard. The foremost consideration in selecting
a form is the adequacy of the public health protection provided by the
combination of the form and the other elements of the standard. In this
review, the Administrator considers the extent to which the available
evidence and/or information continue to support the appropriateness of
a standard with the current form, defined by the 3-year average of
annual fourth-highest 8-hour daily maximum O3
concentrations. Section II.C.3.a below summarizes the basis for the
current form. Section II.C.3.b discusses the Administrator's proposed
decision to retain the current form. Section II.C.3.c discusses public
comments received on the form of the primary standard. Section II.C.3.d
discusses the Administrator's final decision on form.
a. Basis for the Current Form
    The EPA established the current form of the primary O3
NAAQS in 1997 (62 FR 38856). Prior to that time, the standard had a
``1-expected-exceedance'' form.\118\ An advantage of the current
concentration-based form recognized in the 1997 review is that such a
form better reflects the continuum of health effects associated with
increasing ambient O3 concentrations. Unlike an expected
exceedance form, a concentration-based form gives proportionally more
weight to years when 8-hour O3 concentrations are well above
the level of the standard than years when 8-hour O3
concentrations are just above the level of the standard.\119\ The EPA
judged it appropriate to give more weight to higher O3
concentrations, given that available health evidence indicated a
continuum of effects associated with exposures to varying
concentrations of O3, and given that the extent to which
public health is affected by exposure to ambient O3 is
related to the actual magnitude of the O3 concentration, not
just whether the concentration is above a specified level.
---------------------------------------------------------------------------

    \118\ For a standard with a 1-expected-exceedance form to be met
at an air quality monitoring site, the fourth-highest air quality
value in 3 years, given adjustments for missing data, must be less
than or equal to the level of the standard.
    \119\ As discussed (61 FR 65731), this is because with an
exceedance-based form, days on which the ambient O3
concentration is well above the level of the standard are given
equal weight to those days on which the O3 concentration
is just above the standard (i.e., each day is counted as one
exceedance), even though the public health impact of such days would
be very different. With a concentration-based form, days on which
higher O3 concentrations occur would weigh proportionally
more than days with lower O3 concentrations since the
actual concentrations are used directly to calculate whether the
standard is met or violated.
---------------------------------------------------------------------------

    During the 1997 review, the EPA considered a range of alternative
``concentration-based'' forms, including the second-, third-, fourth-
and fifth-highest daily maximum 8-hour concentrations in an
O3 season. The fourth-highest daily maximum was selected,
recognizing that a less restrictive form (e.g., fifth-highest) would
allow a larger percentage of sites to experience O3 peaks
above the level of the standard, and would allow more days on which the
level of the standard may be exceeded when the site attains the
standard (62 FR 38856). The EPA also considered setting a standard with
a form that would provide a margin of safety against possible but
uncertain chronic effects, and would provide greater stability to
ongoing control programs.\120\ A more restrictive form was not
selected, recognizing that the differences in the degree of protection
afforded by the alternatives were not well enough understood to use any
such differences as a basis for choosing the most restrictive forms (62
FR 38856).
---------------------------------------------------------------------------

    \120\ See American Trucking Assn's v. EPA, 283 F. 3d at 374-75
(less stable implementation programs may be less effective and would
thereby provide less public health protection; EPA may therefore
legitimately consider programmatic stability in determining the form
of a NAAQS).
---------------------------------------------------------------------------

    In the 2008 review, the EPA additionally considered the potential
value of a percentile-based form. In doing so, the EPA recognized that
such a statistic is useful for comparing datasets of varying length
because it samples approximately the same place in the distribution of
air quality values, whether the dataset is several months or several
years long. However, the EPA concluded that a percentile-based
statistic would not be effective in ensuring the same degree of public
health protection across the country. Specifically, a percentile-based
form would allow more days with higher air quality values in locations
with longer O3 seasons relative to locations with shorter
O3 seasons. Thus, in the 2008 review, the EPA concluded that
a form based on the nth-highest maximum O3 concentration
would more effectively ensure that people who live in areas

with different length O3 seasons receive the same degree of
public health protection.
    Based on analyses of forms specified in terms of an nth-highest
concentration (n ranged from 3 to 5), advice from CASAC, and public
comment, the Administrator concluded that a fourth-highest daily
maximum should be retained (73 FR 16465, March 27, 2008). In reaching
this decision, the Administrator recognized that ``there is not a clear
health-based threshold for selecting a particular nth-highest daily
maximum form of the standard'' and that ``the adequacy of the public
health protection provided by the combination of the level and form is
a foremost consideration'' (73 FR 16475, March 27, 2008). Based on
this, the Administrator judged that the existing form (fourth-highest
daily maximum 8-hour average concentration) should be retained,
recognizing the increase in public health protection provided by
combining this form with a lower standard level (i.e., 75 ppb).
    The Administrator also recognized that it is important to have a
form that provides stability with regard to implementation of the
standard. In the case of O3, for example, he noted the
importance of a form insulated from the impacts of extreme
meteorological events that are conducive to O3 formation.
Such events could have the effect of reducing public health protection,
to the extent they result in frequent shifts in and out of attainment
due to meteorological conditions. The Administrator noted that such
frequent shifting could disrupt an area's ongoing implementation plans
and associated control programs (73 FR 16474, March 27, 2008). In his
final decision, the Administrator judged that a fourth-high form
``provides a stable target for implementing programs to improve air
quality'' (id. at 16475).
b. Proposed Decision on Form
    In the proposal for the current review, the Administrator
considered the extent to which newly available information provides
support for the current form (79 FR 75293). In so doing, she took note
of the conclusions of prior reviews summarized above. She recognized
the value of an nth-high statistic over that of an expected exceedance
or percentile-based form in the case of the O3 standard, for
the reasons summarized above. The Administrator additionally took note
of the importance of stability in implementation to achieving the level
of protection specified by the NAAQS. Specifically, she noted that to
the extent areas engaged in implementing the O3 NAAQS
frequently shift from meeting the standard to violating the standard,
it is possible that ongoing implementation plans and associated control
programs could be disrupted, thereby reducing public health protection.
    In light of this, while giving foremost consideration to the
adequacy of public health protection provided by the combination of all
elements of the standard, including the form, the Administrator
considered particularly the findings from prior reviews with regard to
the use of the nth-high metric. As noted above, the EPA selected the
fourth-highest daily maximum, recognizing the public health protection
provided by this form, when coupled with an appropriate averaging time
and level, and recognizing that such a form can provide stability for
implementation programs. In the proposal the Administrator concluded
that the currently available evidence and information do not call into
question these conclusions from previous reviews. In reaching this
initial conclusion, the Administrator noted that CASAC concurred that
the O3 standard should be based on the fourth-highest, daily
maximum 8-hour average value (averaged over 3 years), stating that this
form ``provides health protection while allowing for atypical
meteorological conditions that can lead to abnormally high ambient
ozone concentrations which, in turn, provides programmatic stability''
(Frey, 2014c, p. 6). Thus, a standard with the current fourth-high
form, coupled with a level lower than 75 ppb as discussed below, would
be expected to increase public health protection relative to the
current standard while continuing to provide stability for
implementation programs. Therefore, the Administrator proposed to
retain the current fourth-highest daily maximum form for an
O3 standard with an 8-hour averaging time and a revised
level.
c. Public Comments on Form
    Several commenters focused on the stability of the standard to
support their positions regarding form. Some industry associations and
state agencies support changing to a form that would allow a larger
number of exceedances of the standard level than are allowed by the
current fourth-high form. In some cases, these commenters argued that a
standard allowing a greater number of exceedances would provide the
same degree of public health protection as the current standard. Some
commenters advocated a percentile-based form, such as the 98th
percentile. These commenters cited a desire for consistency with short-
term standards for other criteria pollutants (e.g., PM2.5,
NO2), as well as a desire to allow a greater number of
exceedances of the standard level, thus making the standard less
sensitive to fluctuations in background O3 concentrations
and to extreme meteorological events.
    Other commenters submitted analyses purporting to indicate that a
fourth-high form provides only a small increase in stability, relative
to forms that allow fewer exceedances of the standard level (i.e.,
first-high, second-high). These commenters also called into question
the degree of health protection achieved by a standard with a fourth-
high form and a level in the proposed range (i.e., 65 to 70 ppb). They
pointed out that a fourth-high form will, by definition, allow 3 days
per year, on average, with 8-hour O3 concentrations above
the level of the standard. Commenters further stated that ``[i]f ozone
levels on these peak days are appreciably higher than on the fourth-
highest day, given EPA's acknowledged concerns regarding single or
multiple (defined by EPA as 2 or more) exposures to elevated ozone
concentrations, EPA must account for the degree of under-protection in
setting the level of the NAAQS'' (e.g., ALA et al., p. 138).
    For the reasons discussed in the proposal, and summarized above,
the EPA disagrees with commenters who supported a percentile-based
form, such as the 98th percentile, for the O3 NAAQS. As
noted above, a percentile-based statistic would not be effective in
ensuring the same degree of public health protection across the
country. Rather, a percentile-based form would allow more days with
higher air quality values in locations with longer O3
seasons relative to locations with shorter O3 seasons. Thus,
as in the 2008 review, in the current review the EPA concludes that a
form based on the nth-highest maximum O3 concentration would
more effectively ensure that people who live in areas with different
length O3 seasons receive the same degree of public health
protection.
    In considering various nth-high values, as in past reviews (e.g.,
73 FR 16475, March 27, 2008), the EPA recognizes that there is not a
clear health-based threshold for selecting a particular nth-highest
daily maximum form. Rather, the primary consideration is the adequacy
of the public health protection provided by the combination of all of
the elements of the standard, including the form. Environmental and
public health commenters are correct that a standard with the current
fourth-high form will allow 3 days per year, on average, with 8-hour
O3 concentrations higher than the standard level. However,
the EPA disagrees with these

commenters' assertion that using a fourth-high form results in a
standard that is under-protective. The O3 exposure and risk
estimates that informed the Administrator's consideration of the degree
of public health protection provided by various standard levels were
based on air quality that ``just meets'' various standards with the
current 8-hour averaging time and fourth-high, 3-year average form
(U.S. EPA, 2014a, section 4.3.3). Therefore, air quality adjusted to
meet various levels of the standard with the current form and averaging
time will include days with concentrations above the level of the
standard, and these days contribute to exposure and risk estimates. In
this way, the Administrator has reasonably considered the public health
protection provided by the combination of all of the elements of the
standard, including the fourth-high form.
    In past reviews, EPA selected the fourth-highest daily maximum form
in recognition of the public health protection provided by this form,
when coupled with an appropriate averaging time and level, and
recognizing that such a form can provide stability for ongoing
implementation programs. As noted above, some commenters submitted
analyses suggesting that a fourth-high form provides only a small
increase in stability, relative to a first- or second-high form. The
EPA has conducted analyses of ambient O3 monitoring data to
further consider these commenters' assertions regarding stability. The
EPA's analyses of nth-high concentrations ranging from first-high to
fifth-high have been summarized in a memo to the docket (Wells, 2015a).
Consistent with commenters' analyses, Wells (2015a) indicates a
progressive decrease in the variability of O3
concentrations, and an increase in the stability of those
concentrations, as ``n'' increases. Based on these analyses, there is
no clear threshold for selecting a particular nth-high form based on
stability alone. Rather, as in past reviews, the decision on form in
this review focuses first and foremost on the Administrator's judgments
on public health protection, with judgments regarding stability of the
standard being a legitimate, but secondary consideration. The
Administrator's final decision on form is discussed below.
d. Administrator's Final Decision Regarding Form
    In reaching a final decision on the form of the primary
O3 standard, as described in the proposal and above, the
Administrator recognizes that there is not a clear health-based
rationale for selecting a particular nth-highest daily maximum form.
Her foremost consideration is the adequacy of the public health
protection provided by the combination of all of the elements of the
standard, including the form. In this regard, the Administrator
recognizes the support from analyses in previous reviews, and from the
CASAC in the current review, for the conclusion that the current
fourth-high form of the standard, when combined with a revised level as
discussed below, provides an appropriate balance between public health
protection and a stable target for implementing programs to improve air
quality. In particular, she notes that the CASAC concurred that the
O3 standard should be based on the fourth-highest, daily
maximum 8-hour average value (averaged over 3 years), stating that this
form ``provides health protection while allowing for atypical
meteorological conditions that can lead to abnormally high ambient
ozone concentrations which, in turn, provides programmatic stability''
(Frey, 2014c, p. 6). Based on these considerations, and on
consideration of public comments on form as discussed above, the
Administrator judges it appropriate to retain the current fourth-high
form (fourth-highest daily maximum 8-hour O3 concentration,
averaged over 3 years) in this final rule.
4. Level
    This section summarizes the basis for the Administrator's proposed
decision to revise the current standard level (II.C.4.a); discusses
public comments, and the EPA's responses, on that proposed decision
(II.C.4.b); and presents the Administrator's final decision regarding
the level of the primary O3 standard (II.C.4.c).
a. Basis for the Administrator's Proposed Decision on Level
    In conjunction with her proposed decisions to retain the current
indicator, averaging time, and form (II.C.1 to II.C.3, above), the
Administrator proposed to revise the level of the primary O3
standard to within the range of 65 to 70 ppb. In proposing this range
of standard levels, as discussed in section II.E.4 of the proposal, the
Administrator carefully considered the scientific evidence assessed in
the ISA (U.S. EPA, 2013); the results of the exposure and risk
assessments in the HREA (U.S. EPA, 2014a); the evidence-based and
exposure-/risk-based considerations and conclusions in the PA (U.S.
EPA, 2014c); CASAC advice and recommendations, as reflected in CASAC's
letters to the Administrator and in public discussions of drafts of the
ISA, HREA, and PA (Frey and Samet, 2012; Frey, 2014 a, c); and public
input received during the development of these documents.
    The Administrator's proposal to revise the standard level built
upon her proposed conclusion that the overall body of scientific
evidence and exposure/risk information calls into question the adequacy
of public health protection afforded by the current primary
O3 standard, particularly for at-risk populations and
lifestages. In reaching proposed conclusions on alternative levels for
the primary O3 standard, the Administrator considered the
extent to which various alternatives would be expected to protect the
public, including at-risk populations, against the wide range of
adverse health effects that have been linked with short- or long-term
O3 exposures.
    As was the case for her consideration of the adequacy of the
current primary O3 standard (II.B.3, above), the
Administrator placed the greatest weight on the results of controlled
human exposure studies and on exposure and risk analyses based on
information from these studies. In doing so, she noted that controlled
human exposure studies provide the most certain evidence indicating the
occurrence of health effects in humans following exposures to specific
O3 concentrations. The effects reported in these studies are
due solely to O3 exposures, and interpretation of study
results is not complicated by the presence of co-occurring pollutants
or pollutant mixtures (as is the case in epidemiologic studies). She
further noted the CASAC judgment that ``the scientific evidence
supporting the finding that the current standard is inadequate to
protect public health is strongest based on the controlled human
exposure studies of respiratory effects'' (Frey, 2014c, p. 5).
    In considering the evidence from controlled human exposure studies,
the Administrator first noted that the largest respiratory effects, and
the broadest range of effects, have been studied and reported following
exposures to 80 ppb O3 or higher, with most exposure studies
conducted at these higher concentrations. Exposures of healthy adults
to O3 concentrations of 80 ppb or higher have been reported
to decrease lung function, increase airway inflammation, increase
respiratory symptoms, result in airway hyperresponsiveness, and
decrease lung host defenses. The Administrator further noted that
O3 exposure concentrations as low as 72 ppb have been shown
to both decrease lung function and increase respiratory

symptoms (Schelegle et al., 2009),\121\ a combination that meets the
ATS criteria for an adverse response, and that exposures as low as 60
ppb have been reported to decrease lung function and increase airway
inflammation.
---------------------------------------------------------------------------

    \121\ As noted above, for the 70 ppb target exposure
concentration, Schelegle et al. (2009) reported that the actual mean
exposure concentration was 72 ppb.
---------------------------------------------------------------------------

    Based on this evidence, the Administrator reached the initial
conclusion that the results of controlled human exposure studies
strongly support setting the level of a revised O3 standard
no higher than 70 ppb. In reaching this conclusion, she placed a large
amount of weight on the importance of setting the level of the standard
well below 80 ppb, the exposure concentration at which the broadest
range of effects have been studied and reported, and below 72 ppb, the
lowest exposure concentration shown to result in the adverse
combination of lung function decrements and respiratory symptoms. She
placed significant weight on this combination of effects, as did CASAC,
in making judgments regarding the potential for adverse responses.
    In further considering the potential public health implications of
a standard with a level of 70 ppb, the Administrator also considered
quantitative estimates of the extent to which such a standard would be
expected to limit population exposures to the broader range of
O3 concentrations shown in controlled human exposure studies
to cause respiratory effects. In doing so, she focused on estimates of
O3 exposures of concern at or above the benchmark
concentrations of 60, 70, and 80 ppb. The Administrator judged that the
evidence supporting the occurrence of adverse respiratory effects is
strongest for exposures at or above the 70 and 80 ppb benchmarks.
Therefore, she placed a large amount of emphasis on the importance of
setting a standard that limits exposures of concern at or above these
benchmarks.
    The Administrator expressed less confidence that adverse effects
will occur following exposures to O3 concentrations as low
as 60 ppb. In reaching this conclusion, she highlighted the fact that
statistically significant increases in respiratory symptoms, combined
with lung function decrements, have not been reported following
exposures to 60 or 63 ppb O3, though several studies have
evaluated the potential for such effects (Kim et al., 2011; Schelegle
et al., 2009; Adams, 2006).\122\ The proposal specifically stated that
``[t]he Administrator has decreasing confidence that adverse effects
will occur following exposures to O3 concentrations below 72
ppb. In particular, compared to O3 exposure concentrations
at or above 72 ppb, she has less confidence that adverse effects will
occur following exposures to O3 concentrations as low as 60
ppb'' (79 FR 73304-05).
---------------------------------------------------------------------------

    \122\ In the study by Schelegle, for the 60 ppb target exposure
concentration, study authors reported that the actual mean exposure
concentration was 63 ppb.
---------------------------------------------------------------------------

    However, she noted the possibility for adverse effects following
such exposures given that: (1) CASAC judged the adverse combination of
lung function decrements and respiratory symptoms ``almost certainly
occur in some people'' following exposures to O3
concentrations below 72 ppb (though CASAC did not specify or otherwise
indicate how far below) (Frey, 2014c, p. 6); (2) CASAC indicated the
moderate lung function decrements (i.e., FEV1 decrements >=
10%) that occur in some healthy adults following exposures to 60 ppb
O3 could be adverse to people with lung disease; and (3)
airway inflammation has been reported following exposures as low as 60
ppb O3. She also took note of CASAC advice that the
occurrence of exposures of concern at or above 60 ppb is an appropriate
consideration for people with asthma (Frey, 2014c, p. 6). Therefore,
while the Administrator expressed less confidence that adverse effects
will occur following exposures to O3 concentrations as low
as 60 ppb, compared to 70 ppb and above, based on the evidence and
CASAC advice she also gave some consideration to exposures of concern
for the 60 ppb benchmark.
    Due to interindividual variability in responsiveness, the
Administrator further noted that not every occurrence of an exposure of
concern will result in an adverse effect, and that repeated occurrences
of some of the effects demonstrated following exposures of concern
could increase the likelihood of adversity (U.S. EPA, 2013, section
6.2.3). Therefore, the Administrator was most concerned about
protecting at-risk populations against repeated occurrences of
exposures of concern. Based on the above considerations, the
Administrator focused on the extent to which a revised standard with a
level of 70 ppb would be expected to protect populations from
experiencing two or more O3 exposures of concern (i.e., as a
surrogate for repeated exposures).
    As illustrated in Table 1 in the proposal (and Table 1 above), the
Administrator noted that, in urban study areas, a revised standard with
a level of 70 ppb is estimated to eliminate the occurrence of two or
more exposures of concern to O3 concentrations at and above
80 ppb and to virtually eliminate the occurrence of two or more
exposures of concern to O3 concentrations at and above 70
ppb, even in the worst-case urban study area and year evaluated. Though
the Administrator acknowledged greater uncertainty with regard to the
occurrence of adverse effects following exposures to 60 ppb, she noted
that a revised standard with a level of 70 ppb would also be expected
to protect the large majority of children in the urban study areas
(i.e., about 96% to more than 99% of children in individual urban study
areas) from experiencing two or more exposures of concern at or above
the 60 ppb benchmark. Compared to the current standard, this represents
a reduction of more than 60%.\123\
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    \123\ The Administrator judged that the evidence is less
compelling, and indicates greater uncertainty, with regard to the
potential for adverse effects following single occurrences of
O3 exposures of concern. While acknowledging this greater
uncertainty, she noted that a standard with a level of 70 ppb would
also be expected to virtually eliminate all occurrences (including
single occurrences) of exposures of concern at or above 80 ppb, even
in the worst-case year and location. She also judged that such a
standard will achieve important reductions, compared to the current
standard, in the occurrence of one or more exposures of concern at
or above 70 and 60 ppb.
---------------------------------------------------------------------------

    In further evaluating the potential public health impacts of a
standard with a level of 70 ppb, the Administrator also considered the
HREA estimates of O3-induced lung function decrements. To
inform her consideration of these decrements, the Administrator took
note of CASAC advice that ``estimation of FEV1 decrements of
>= 15% is appropriate as a scientifically relevant surrogate for
adverse health outcomes in active healthy adults, whereas an
FEV1 decrement of >= 10% is a scientifically relevant
surrogate for adverse health outcomes for people with asthma and lung
disease'' (Frey, 2014c, p. 3).
    Although these FEV1 decrements provide perspective on
the potential for the occurrence of adverse respiratory effects
following O3 exposures, the Administrator agreed with the
conclusion in past reviews that a more general consensus view of the
adversity of moderate responses emerges as the frequency of occurrence
increases (61 FR 65722-3, Dec, 13, 1996). Specifically, she judged that
not every estimated occurrence of an O3-induced
FEV1 decrement will be adverse and

that repeated occurrences of moderate responses could lead to more
serious illness. Therefore, the Administrator noted increasing concern
about the potential for adversity as the number of occurrences
increases and, as a result, she focused primarily on estimates of two
or more O3-induced FEV1 decrements (i.e., as a
surrogate for repeated exposures).\124\
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    \124\ In the proposal, the Administrator further judged that it
would not be appropriate to set a standard that is intended to
eliminate all O3-induced FEV1 decrements. She
noted that this is consistent with CASAC advice, which did not
include a recommendation to set the standard level low enough to
eliminate all O3-induced FEV1 decrements
= 10 or 15% (Frey, 2014c).
---------------------------------------------------------------------------

    The Administrator noted that a revised O3 standard with
a level of 70 ppb is estimated to protect about 98 to 99% of children
in urban study areas from experiencing two or more O3-
induced FEV1 decrements =15%, and about 89 to 94%
from experiencing two or more decrements =10%. She judged
that these estimates reflect important risk reductions, compared to the
current standard. Given these estimates, as well as estimates of one or
more decrements per season (about which she was less concerned (79 FR
75290, December 17, 2014)), the Administrator concluded that a revised
standard with a level of 70 ppb would be expected to provide
substantial protection against the risk of O3-induced lung
function decrements, and would be expected to result in important
reductions in such risks, compared to the current standard. The
Administrator further noted, however, that the variability in lung
function risk estimates across urban study areas is often greater than
the differences in risk estimates between various standard levels
(Table 2, above). Given this, and the resulting considerable overlap
between the ranges of lung function risk estimates for different
standard levels, in the proposal the Administrator viewed lung function
risk estimates as providing a more limited basis than exposures of
concern for distinguishing between the degrees of public health
protection provided by alternative standard levels (79 FR 75306 n.
164).
    In next considering the additional protection that would be
expected from standard levels below 70 ppb, the Administrator evaluated
the extent to which a standard with a level of 65 ppb would be expected
to further limit O3 exposures of concern and O3-
induced lung function decrements. In addition to eliminating almost all
exposures of concern to O3 concentrations at or above 80 and
70 ppb, even in the worst-case years and locations, the Administrator
noted that a revised standard with a level of 65 ppb would be expected
to protect more than 99% of children in urban study areas from
experiencing two or more exposures of concern at or above 60 ppb and to
substantially reduce the occurrence of one or more such exposures,
compared to the current standard. With regard to O3-induced
lung function decrements, an O3 standard with a level of 65
ppb is estimated to protect about 98% to more than 99% of children from
experiencing two or more O3-induced FEV1
decrements =15% and about 91 to 99% from experiencing two or
more decrements =10%.\125\
---------------------------------------------------------------------------

    \125\ Although the Administrator was less concerned about the
public health implications of single O3-induced lung
function decrements, she also noted that a revised standard with a
level of 65 ppb is estimated to reduce the risk of one or more
O3-induced decrements per season, compared to the current
standard.
---------------------------------------------------------------------------

    Taken together, the Administrator concluded that the evidence from
controlled human exposure studies, and the information from
quantitative analyses that draw upon these studies, provide strong
support for standard levels from 65 to 70 ppb. In particular, she based
this conclusion on the fact that such standard levels would be well
below the O3 exposure concentration shown to result in the
widest range of respiratory effects (i.e., 80 ppb),\126\ and below the
lowest O3 exposure concentration shown to result in the
adverse combination of lung function decrements and respiratory
symptoms (i.e., 72 ppb). A standard with a level from 65 to 70 ppb
would also be expected to result in important reductions, compared to
the current standard, in the occurrence of O3 exposures of
concern for all of the benchmarks evaluated (i.e., 60, 70, and 80 ppb)
and in the risk of O3-induced lung function decrements
=10 and 15%.
---------------------------------------------------------------------------

    \126\ Although the widest range of effects have been evaluated
following exposures to 80 ppb O3, there is no evidence
that 80 ppb is a threshold for these effects.
---------------------------------------------------------------------------

    In further considering the evidence and exposure/risk information,
the Administrator considered the extent to which the epidemiologic
evidence also provides support for standard levels from 65 to 70 ppb.
In particular, the Administrator noted analyses in the PA (U.S. EPA,
2014c, section 4.4.1) indicating that a revised standard with a level
of 65 or 70 ppb would be expected to maintain distributions of short-
term ambient O3 concentrations below those present in the
locations of all the single-city epidemiologic studies of hospital
admissions or emergency department visits analyzed. She concluded that
a revised standard with a level at least as low as 70 ppb would result
in improvements in public health, beyond the protection provided by the
current standard, in the locations of the single-city epidemiologic
studies that reported significant health effect associations.\127\
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    \127\ The Administrator also concluded that analyses in the HREA
and PA indicate that a standard with an 8-hour averaging time,
coupled with the current fourth-high form and a level from 65 to 70
ppb, would be expected to provide increased protection, compared to
the current standard, against the long-term O3
concentrations that have been reported to be associated with
respiratory morbidity or mortality (79 FR 75293; 75308).
---------------------------------------------------------------------------

    The Administrator noted additional uncertainty in interpreting air
quality in locations of multicity epidemiologic studies of short-term
O3 for the purpose of evaluating alternative standard levels
(II.D.1 and U.S. EPA, 2014c, section 4.4.1). While acknowledging this
uncertainty, and therefore placing less emphasis on these analyses of
study location air quality, she noted that PA analyses suggest that
standard levels of 65 or 70 ppb would require reductions, beyond those
required by the current standard, in ambient O3
concentrations present in several of the locations that provided the
basis for statistically significant O3 health effect
associations in multicity studies.
    In further evaluating information from epidemiologic studies, the
Administrator considered the HREA's epidemiology-based risk estimates
for O3-associated morbidity or mortality (U.S. EPA, 2014a,
Chapter 7). Compared to the weight given to the evidence from
controlled human exposure studies, and to HREA estimates of exposures
of concern and lung function risks, she placed relatively less weight
on epidemiology-based risk estimates. In doing so, she noted that the
overall conclusions from the HREA likewise reflect relatively less
confidence in estimates of epidemiology-based risks than in estimates
of exposures of concern and lung function risks.
    In considering epidemiology-based risk estimates, the Administrator
focused on risks associated with O3 concentrations in the
upper portions of ambient distributions, given the greater uncertainty
associated with the shapes of concentration-response curves for
O3 concentrations in the lower portions of ambient
distributions (i.e., below about 20 to 40 ppb depending on the
O3 metric, health endpoint, and study population) (U.S. EPA,
2013, section <IP_ADDRESS>). The Administrator further noted that
experimental studies provide the strongest evidence for O3-
induced effects following exposures to O3 concentrations
corresponding to the upper portions of typical ambient

distributions. In particular, as discussed above, she noted controlled
human exposure studies showing respiratory effects following exposures
to O3 concentrations at or above 60 ppb (79 FR 75308,
December 17, 2014). Therefore, in considering risks associated with
O3 concentrations in the upper portions of ambient
distributions, the Administrator focused on the extent to which revised
standards with levels of 70 or 65 ppb are estimated to reduce the risk
of premature deaths associated with area-wide O3
concentrations at or above 40 ppb and 60 ppb.
    Given all of the above evidence, exposure/risk information, and
advice from CASAC, the Administrator proposed to revise the level of
the current primary O3 standard to within the range of 65 to
70 ppb. In considering CASAC advice on the range of standard levels,
the Administrator placed a large amount of weight on CASAC's conclusion
that there is adequate scientific evidence to consider a range of
levels for a primary standard that includes an upper end at 70 ppb. She
also noted that although CASAC expressed concern about the margin of
safety at a level of 70 ppb, it further acknowledged that the choice of
a level within the range recommended based on scientific evidence is a
policy judgment (Frey, 2014c, p. ii). While she agreed with CASAC that
it is appropriate to consider levels below 70 ppb, as reflected in her
range of proposed levels from 65 to 70 ppb, for the reasons discussed
above she also concluded that a standard level as high as 70 ppb, which
CASAC concluded could be supported by the scientific evidence, could
reasonably be judged to be requisite to protect public health with an
adequate margin of safety.
    In considering the appropriateness of standard levels below 65 ppb,
the Administrator noted the conclusions of the PA and the advice of
CASAC that it would be appropriate for her to consider standard levels
as low as 60 ppb. In making the decision to not propose levels below 65
ppb, she focused on CASAC's rationale for a level of 60 ppb, which
focused on the importance of limiting exposures to O3
concentrations as low as 60 ppb (Frey, 2014c, p. 7). As discussed
above, the Administrator agreed that it is appropriate to consider the
implications of a revised standard level for estimated exposures of
concern at or above 60 ppb. She noted that standards within the
proposed range of 65 to 70 ppb would be expected to substantially limit
the occurrence of exposures of concern to O3 concentrations
at or above 60 ppb, particularly the occurrence of two or more
exposures. When she further considered that not all exposures of
concern lead to adverse effects, and that the NAAQS are not meant to be
zero-risk or background standards, the Administrator judged that
alternative standard levels below 65 ppb are not needed to further
reduce such exposures.
b. Comments on Level
    A number of groups representing medical, public health, or
environmental organizations; some state agencies; and many individuals
submitted comments on the appropriate level of a revised primary
O3 standard.\128\ Virtually all of these commenters
supported setting the standard level within the range recommended by
CASAC (i.e., 60 to 70). Some expressed support for the overall CASAC
range, without specifying a particular level within that range, while
others expressed a preference for the lower part of the CASAC range,
often emphasizing support for a level of 60 ppb. Some of these
commenters stated that if the EPA does not set the level at 60 ppb,
then the level should be set no higher than 65 ppb (i.e., the lower
bound of the proposed range of standard levels).
---------------------------------------------------------------------------

    \128\ In general, commenters who expressed the view that the EPA
should retain the current O3 NAAQS (i.e., commenters
representing industry and business groups, and some states) did not
provide comments on alternative standard levels. As a result, this
section focuses primarily on comments from commenters who expressed
support for the proposed decision to revise the current primary
O3 standard.
---------------------------------------------------------------------------

    To support their views on the level of a revised standard, some
commenters focused on overarching issues related to the statutory
requirements for the NAAQS. For example, some commenters maintained
that the primary NAAQS must be set at a level at which there is an
absence of adverse effects in sensitive populations. While this
argument has some support in the case law and in the legislative
history to the 1970 CAA (see Lead Industries Ass'n v. EPA, 647 F. 2d
1147, 1153 (D.C. Cir. 1980)), it is well established that the NAAQS are
not meant to be zero risk standards. See Lead Industries v. EPA, 647
F.2d at 1156 n.51; Mississippi v. EPA, 744 F. 3d at 1351. From the
inception of the NAAQS standard-setting process, the EPA and the courts
have acknowledged that scientific uncertainties in general, and the
lack of clear thresholds in pollutant effects in particular, preclude
any such definitive determinations. Lead Industries, 647 F. 2d at 1156
(setting standard at a level which would remove most but not all sub-
clinical effects). Likewise, the House report to the 1977 amendments
addresses this question (H. Rep. 95-294, 95th Cong. 1st sess. 127):
\129\
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    \129\ Similarly, Senator Muskie remarked during the floor
debates on the 1977 Amendments that ``there is no such thing as a
threshold for health effects. Even at the national primary standard
level, which is the health standard, there are health effects that
are not protected against''. 123 Cong. Rec. S9423 (daily ed. June
10, 1977).

    Some have suggested that since the standards are to protect
against all known or anticipated effects and since no safe threshold
can be established, the ambient standards should be set at zero or