factor that could obscure the presence of thresholds, should they
exist, in multicity studies (U.S. EPA, 2013a, sections <IP_ADDRESS> and
<IP_ADDRESS>). The ISA notes that community characteristics (e.g., activity
patterns, housing type, age distribution, prevalence of air
conditioning) could be important contributors to reported regional
heterogeneity (U.S. EPA, 2013a, section <IP_ADDRESS>). Given this
heterogeneity, the ISA concludes that ``a national or combined analysis
may not be appropriate to identify whether a threshold exists in the
O3-mortality [concentration-response] relationship'' (U.S.
EPA, 2013a, p. 2-33). This represents an important source of
uncertainty when characterizing confidence in reported concentration-
response relationships over distributions of ambient O3
concentrations, based on multicity studies. The PA notes that this
uncertainty becomes increasingly important when interpreting
concentration-response relationships at lower ambient O3
concentrations, particularly those concentrations corresponding to
portions of distributions where data density decreases notably (U.S.
EPA, 2014c, section <IP_ADDRESS>).
    Another important uncertainty, related specifically to the PA
analysis of cut points by Bell et al. (2006), is that EPA staff was
unable to obtain the air quality data used to generate the cut-point
analyses in the published study (U.S. EPA, 2014c, section <IP_ADDRESS>).
Therefore, the analyses in the PA identified 2-day averages of 24-hour
O3 concentrations in study locations using the air quality
data available in AQS, combined with the published description of study
area definitions. An important uncertainty in this approach is the
extent to which the PA appropriately recreated the cut-point analyses
in the published study (U.S. EPA, 2014c, section <IP_ADDRESS>).
    An uncertainty that applies to epidemiologic studies in general is
the extent to which reported health effects are caused by exposures to
O3 itself, as opposed to other factors such as co-occurring
pollutants or pollutant mixtures. The PA notes that this uncertainty
becomes an increasingly important consideration as health effect
associations are evaluated at lower ambient O3
concentrations. In particular, there is increasing uncertainty as to
whether the observed associations remain plausibly related to exposures
to ambient O3, rather than to the broader mix of air
pollutants present in the ambient air. In considering the potential
importance of this uncertainty at the relatively low ambient
O3 concentrations that are the focus of the PA analyses, the
PA notes that Silverman and Ito (2010) and Strickland (2010) reported
O3 health effect associations in co-pollutant models,\97\
providing support for associations with O3 itself (U.S. EPA,
2014c, section <IP_ADDRESS>). The PA also concludes that air quality
analyses indicate coherence with the results of experimental studies
(i.e., in which the study design dictates that exposures to
O3 itself are responsible for reported effects), and are
consistent with the occurrence of O3-attributable
respiratory hospital admissions and emergency department visits, even
when virtually all monitored concentrations were below the level of the
current standard (U.S. EPA, 2014c, section <IP_ADDRESS>, Tables 3-4, 3-5).
---------------------------------------------------------------------------

    \97\ In addition, Bell et al. (2006) reported that, based on a
previous study (Bell et al., 2004), associations with mortality were
robust to the inclusion of PM10 in the model.
---------------------------------------------------------------------------

c. Concentrations in Epidemiologic Studies--Long-Term
    The PA also considers the extent to which epidemiologic studies
employing longer-term ambient O3 concentration metrics
inform our understanding of the air quality conditions associated with
O3-attributable health effects, and specifically inform
consideration of the extent to which such effects could occur under air
quality conditions meeting the current standard (U.S. EPA, 2014c,
section <IP_ADDRESS>). Unlike for the studies of short-term O3
discussed above, the available U.S. and Canadian epidemiologic studies
evaluating long-term ambient O3 concentration metrics have
not been conducted in locations likely to have met the current 8-hour
O3 standard during the study period, and have not reported
concentration-response functions that indicate confidence in health
effect associations at O3 concentrations meeting the current
standard (U.S. EPA, 2014c, section <IP_ADDRESS>). Therefore, although these
studies contribute to understanding of health effects associated with
long-term or repeated exposures to ambient O3, consideration
of study area air quality does not inform consideration of the extent
to which those health effects may be occurring in locations that meet
the current standard.
d. PA Conclusions Based on Consideration of the Evidence
    As discussed above (II.D.1.a to II.D.1.c), in considering the
available scientific evidence, including associated uncertainties, as
it relates to the degree of public health protection provided by the
current primary O3 standard, the PA evaluates the extent to
which health effects have been reported for the O3 exposure
concentrations evaluated in controlled human exposure studies and over
the distributions of ambient O3 concentrations in locations
where epidemiologic studies have been conducted. The PA concludes that
(1) the evidence from controlled human exposure studies provides strong
support for the occurrence of adverse respiratory effects following
exposures to O3 concentrations below the level of the
current standard and that (2) epidemiologic studies provide support for
the occurrence of adverse respiratory effects and mortality under air
quality conditions that would likely meet the current standard. In
further considering the public health protection provided by the
current standard, the PA next considers the results of exposure and
health risk assessments.

2. Summary of Exposure- and Risk-Based Considerations in the PA
    In order to further inform judgments about the potential public
health implications of the current O3 NAAQS, the PA
considers the exposure and risk assessments presented in the HREA (U.S.
EPA, 2014c, section 3.2). Overviews of these exposure and risk
assessments, including summaries of key results and uncertainties, are
provided in section II.C above. This section summarizes key
observations from the PA related to the adequacy of the current
O3 NAAQS, based on consideration of the HREA exposure
assessment (II.D.2.a), lung function risk assessment (II.D.2.b), and
mortality/morbidity risk assessments (II.D.2.c).
a. Exposure Assessment--Key Observations
    As discussed above (II.C.2), the exposure assessment provides
estimates of the number and percent of people who would experience
exposures of concern at or above benchmark concentrations of 60, 70,
and 80 ppb. Benchmarks reflect exposure concentrations at which
O3-induced respiratory effects are known to occur in some
healthy adults engaged in moderate, intermittent exertion, based on
evidence from controlled human exposure studies (U.S. EPA, 2014c,
section <IP_ADDRESS>; U.S. EPA, 2013a, section 6.2).
    The PA focuses on exposure estimates in children. Compared to
recent (i.e., unadjusted) air quality, the PA notes that adjusting air
quality to just meet the current O3 NAAQS consistently
reduces the estimated occurrence of exposures of concern in children
(U.S. EPA, 2014a, Appendix 5F). When averaged over the years evaluated
in the HREA, reductions of up to about 70% were estimated. These
reductions in estimated exposures of concern, relative to unadjusted
air quality, reflect the consistent reductions in the highest ambient
O3 concentrations upon model adjustment to just meet the
current standard (U.S. EPA, 2014c, section 3.2.1; U.S. EPA, 2014a,
Chapter 4). Such reductions in estimated exposures of concern are
evident throughout urban study areas, including in urban cores and in
surrounding areas (U.S. EPA, 2014a, Appendix 9A).
    Based on Figures 3-7 to 3-10 in the PA (U.S. EPA, 2014c), and the
associated details described in the HREA (U.S. EPA, 2014a, Chapter 5),
the PA further highlights key observations with regard to exposures of
concern in children that are estimated to be allowed by the current
standard. These key observations are summarized below for exposures of
concern >=60, 70, and 80 ppb.
    For exposures of concern at or above 60 ppb, the PA highlights the
following key observations for air quality adjusted to just meet the
current standard:
    (1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 10 to 18% of children in urban study
areas to experience one or more exposures of concern at or above 60
ppb. Summing across urban study areas, these percentages correspond to
almost 2.5 million children experiencing approximately 4 million
exposures of concern at or above 60 ppb during a single O3
season. Of these children, almost 250,000 are asthmatics.\98\
---------------------------------------------------------------------------

    \98\ As discussed above (II.C.2.b), due to variability in
responsiveness, only a subset of individuals who experience
exposures at or above a benchmark concentration can be expected to
experience adverse health effects.
---------------------------------------------------------------------------

    (2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 3 to 8% of children in urban study
areas to experience two or more exposures of concern to O3
concentrations at or above 60 ppb. Summing across the urban study
areas, these percentages correspond to almost 900,000 children
(including almost 90,000 asthmatic children) estimated to experience at
least two O3 exposure concentrations at or above 60 ppb
during a single O3 season.
    (3) In the worst-case years (i.e., those with the largest exposure
estimates), the current standard is estimated to allow approximately 10
to 25% of children to experience one or more exposures of concern at or
above 60 ppb, and approximately 4 to 14% to experience two or more
exposures of concern at or above 60 ppb.
    For exposures of concern at or above 70 ppb, the PA highlights the
following key observations for air quality adjusted to just meet the
current standard:
    (1) On average over the years 2006 to 2010, the current standard is
estimated to allow up to approximately 3% of children in urban study
areas to experience one or more exposures of concern at or above 70
ppb. Summing across urban study areas, almost 400,000 children
(including almost 40,000 asthmatic children) are estimated to
experience O3 exposure concentrations at or above 70 ppb
during a single O3 season.\99\
---------------------------------------------------------------------------

    \99\ As discussed above (II.C.2.b), due to variability in
responsiveness, only a subset of individuals who experience
exposures at or above a benchmark concentration can be expected to
experience adverse health effects.
---------------------------------------------------------------------------

    (2) On average over the years 2006 to 2010, the current standard is
estimated to allow less than 1% of children in urban study areas to
experience two or more exposures of concern to O3
concentrations at or above 70 ppb.
    (3) In the worst-case years, the current standard is estimated to
allow approximately 1 to 8% of children to experience one or more
exposures of concern at or above 70 ppb, and up to approximately 2% to
experience two or more exposures of concern, at or above 70 ppb.
    For exposures of concern at or above 80 ppb, the PA highlights the
observation that the current standard is estimated to allow about 1% or
fewer children in urban study areas to experience exposures of concern
at or above 80 ppb, even in years with the highest exposure estimates.
b. Lung Function Risk Assessment--Key Observations
    As discussed above (II.C.3.a), the HREA estimates risks of moderate
to large lung function decrements (i.e., FEV1 decrements
>=10%, 15%, or 20%) in school-aged children (ages 5 to 18), asthmatic
school-aged children, and the general adult population for 15 urban
study areas. As for exposures of concern, the PA focuses on lung
function risk estimates in children (including children with asthma).
    Compared to risks associated with recent air quality, risk
estimates for air quality just meeting the current standard are
consistently smaller across urban study areas (U.S. EPA, 2014a,
Appendix 6B). When averaged over the years evaluated in the HREA, risk
reductions of up to about 40% were estimated compared to recent air
quality. These reductions reflect the consistent decreases in
relatively high ambient O3 concentrations upon adjustment to
just meet the current standard (U.S. EPA, 2014a, Chapter 4). Such
reductions in estimated lung function risks are evident throughout
urban study areas, including in urban cores and in surrounding areas
(U.S. EPA, 2014, Appendix 9A).
    Based on Figures 3-11 to 3-14 in the PA (U.S. EPA, 2014c), and the
associated details described in the HREA (U.S. EPA, 2014a, chapter 6),
the PA highlights key observations with regard to lung function risks
estimated in children for air quality adjusted to just meet the current
standard. These key observations are presented below for
FEV1 decrements >=10, 15, and 20%.
    With regard to decrements >=10%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:

    (1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 14 to 19% of children in urban study
areas to experience one or more lung function decrements >=10%. Summing
across urban study areas, this corresponds to approximately 3 million
children experiencing 15 million O3-induced lung function
decrements >=10% during a single O3 season. Of these
children, about 300,000 are asthmatics.
    (2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 7 to 12% of children in urban study
areas to experience two or more O3-induced lung function
decrements >=10%. Summing across the urban study areas, this
corresponds to almost 2 million children (including almost 200,000
asthmatic children) estimated to experience two or more O3-
induced lung function decrements greater than 10% during a single
O3 season.
    (3) In the worst-case years, the current standard is estimated to
allow approximately 17 to 23% of children in urban study areas to
experience one or more lung function decrements >=10%, and
approximately 10 to 14% to experience two or more O3-induced
lung function decrements >=10%.
    With regard to decrements >=15%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
    (1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 3 to 5% of children in urban study
areas to experience one or more lung function decrements >=15%. Summing
across urban study areas, this corresponds to approximately 800,000
children (including approximately 80,000 asthmatic children) estimated
to experience at least one O3-induced lung function
decrement >=15% during a single O3 season.
    (2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 2 to 3% of children in urban study
areas to experience two or more O3-induced lung function
decrements >=15%.
    (3) In the worst-case years, the current standard is estimated to
allow approximately 4 to 6% of children in urban study areas to
experience one or more lung function decrements >=15%, and
approximately 2 to 4% to experience two or more O3-induced
lung function decrements >=15%.
    With regard to decrements >=20%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
    (1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 1 to 2% of children in urban study
areas to experience one or more lung function decrements >=20%. Summing
across urban study areas, this corresponds to approximately 300,000
children (including approximately 30,000 asthmatic children) estimated
to experience at least one O3-induced lung function
decrement >=20% during a single O3 season.
    (2) On average over the years 2006 to 2010, the current standard is
estimated to allow less than 1% of children in urban study areas to
experience two or more O3-induced lung function decrements
>=20%.
    (3) In the worst-case years, the current standard is estimated to
allow approximately 2 to 3% of children to experience one or more lung
function decrements >=20%, and less than 2% to experience two or more
O3-induced lung function decrements >=20%.
c. Mortality and Morbidity Risk Assessments--Key Observations
    As discussed above (II.C.3.b), risk estimates based on
epidemiologic studies can provide perspective on the most serious
O3-associated public health outcomes (e.g., mortality,
hospital admissions, emergency department visits) in populations that
often include at-risk groups. The HREA estimates such O3-
associated risks in 12 urban study areas \100\ using concentration-
response relationships drawn from epidemiologic studies. These
concentration-response relationships are based on ``area-wide'' average
O3 concentrations.\101\ The HREA estimates risks for the
years 2007 and 2009 in order to provide estimates of risk for a year
with generally higher O3 concentrations (2007) and a year
with generally lower O3 concentrations (2009) (U.S. EPA,
2014a, section 7.1.1).
---------------------------------------------------------------------------

    \100\ The 12 urban areas evaluated are Atlanta, Baltimore,
Boston, Cleveland, Denver, Detroit, Houston, Los Angeles, New York,
Philadelphia, Sacramento, and St. Louis. Morbidity endpoints were
evaluated in subsets of these areas, based on availability of
appropriate studies (U.S. EPA, 2014a, Chapter 7).
    \101\ In the epidemiologic studies that provide the health basis
for HREA risk assessments, concentration-response relationships are
based on daytime O3 concentrations, averaged across
multiple monitors within study areas. These daily averages are used
as surrogates for the spatial and temporal patterns of exposures in
study populations. Consistent with this approach, the HREA
epidemiologic-based risk estimates also utilize daytime
O3 concentrations, averaged across monitors, as
surrogates for population exposures. In this notice, these averaged
concentrations are referred to as ``area-wide'' O3
concentrations. Area-wide concentrations are discussed in more
detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
---------------------------------------------------------------------------

    In considering these estimates, the PA notes that HREA conclusions
reflect somewhat lower confidence in epidemiologic-based risk estimates
than in estimates of O3 exposures of concern and
O3-induced lung function decrements (U.S. EPA, 2014a,
section 9.6). In particular, the HREA highlights the unexplained
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations (U.S. EPA, 2014a, section 9.6). The HREA also concludes
that lower confidence should be placed in the results of the assessment
of respiratory mortality risks associated with long-term O3
exposures, primarily because that analysis is based on only one study,
though that study is well-designed, and because of the uncertainty in
that study about the existence and identification of a potential
threshold in the concentration-response function (U.S. EPA, 2014a,
section 9.6). These and other uncertainties are considered in the PA in
reaching conclusions on the current and alternative standards (U.S.
EPA, 2014c, sections 3.4, 4.6).
    Key observations from the PA are summarized below for mortality and
morbidity risks associated with air quality adjusted to simulate just
meeting the current O3 NAAQS. These include key observations
for estimates of total (nonaccidental) mortality associated with short-
term O3 concentrations, respiratory morbidity associated
with short-term O3 concentrations, and respiratory mortality
associated with long-term O3 concentrations (U.S. EPA,
2014c, section <IP_ADDRESS>).
    With regard to total mortality or morbidity associated with short-
term O3, the PA notes the following for air quality adjusted
to just meet the current standard:

    (1) When air quality was adjusted to the current standard for
the 2007 model year (the year with generally ``higher''
O3-associated risks), 10 of 12 urban study areas
exhibited either decreases or virtually no change in estimates of
the number of O3-associated deaths (U.S. EPA, 2014a,
Appendix 7B). Increases were estimated in two of the urban

study areas (Houston, Los Angeles) \102\ (U.S. EPA, 2014a, Appendix
7B).\103\
---------------------------------------------------------------------------

    \102\ As discussed above (II.C.1), in locations and time periods
when NOX is predominantly contributing to O3
formation (e.g., downwind of important NOX sources, where
the highest O3 concentrations often occur), model-based
adjustment to the current and alternative standards decreases
estimated ambient O3 concentrations compared to recent
monitored concentrations (U.S. EPA, 2014a, section <IP_ADDRESS>). In
contrast, in locations and time periods when NOX is
predominantly contributing to O3 titration (e.g., in
urban centers with high concentrations of NOX emissions,
where ambient O3 concentrations are often suppressed and
thus relatively low), model-based adjustment increases ambient
O3 concentrations compared to recent monitored
concentrations (U.S. EPA, 2014a, section <IP_ADDRESS>). Changes in
epidemiology-based risk estimates depend on the balance between the
daily decreases in high O3 concentrations and increases
in low O3 concentrations following the model-based air
quality adjustment. Commenting on this issue, CASAC noted that
``controls designed to reduce the peak levels of ozone (e.g., the
4th highest annual MDA8) may not be effective at reducing lower
levels of ozone on more typical days and may actually increase ozone
levels on days where ozone concentrations are low'' (Frey 2014a, p.
2). CASAC further noted that risk results ``suggest that the ozone-
related health risks in the urban cores can increase for some of the
cities as ozone NAAQS alternatives become more stringent. This is
because reductions in nitrogen oxides emissions can lead to less
scavenging of ozone and free radicals, resulting in locally higher
levels of ozone'' (Frey 2014c, p. 10).
    \103\ For the 2009 adjusted year (i.e., the year with generally
lower O3 concentrations), changes in risk were generally
smaller than in 2007 (i.e., most changes about 2% or smaller).
Increases were estimated for Houston, Los Angeles, and New York
City.
---------------------------------------------------------------------------

    (2) In focusing on total risk, the current standard is estimated
to allow thousands of O3-associated deaths per year in
the urban study areas. In focusing on the risks associated with the
upper portions of distributions of ambient concentrations (area-wide
concentrations >=40, 60 ppb), the current standard is estimated to
allow hundreds to thousands of O3-associated deaths per
year in the urban study areas.
    (3) The current standard is estimated to allow tens to thousands
of O3-associated morbidity events per year (i.e.,
respiratory-related hospital admissions, emergency department
visits, and asthma exacerbations).

    With regard to respiratory mortality associated with long-term
O3, the PA notes the following for air quality adjusted to
just meet the current standard:

    (1) Based on a linear concentration-response function, the
current standard is estimated to allow thousands of O3-
associated respiratory deaths per year in the urban study areas.
    (2) Based on threshold models, HREA sensitivity analyses
indicate that the number of respiratory deaths associated with long-
term O3 concentrations could potentially be considerably
lower (i.e., by more than 75% if a threshold exists at 40 ppb, and
by about 98% if a threshold exists at 56 ppb) (U.S. EPA, 2014a,
Figure 7-9).\104\
---------------------------------------------------------------------------

    \104\ Risk estimates for respiratory mortality associated with
long-term O3 exposures are based on the study by Jerrett
et al. (2009) (U.S. EPA, 2014a, Chapter 7). As discussed above
(II.B.2.b.iv) and in the PA (U.S. EPA, 2014c, section <IP_ADDRESS>),
Jerrett et al. (2009) reported that when seasonal averages of 1-hour
daily maximum O3 concentrations ranged from 33 to 104
ppb, there was no statistical deviation from a linear concentration-
response relationship between O3 and respiratory
mortality across 96 U.S. cities (U.S. EPA, 2013a, section 7.7).
However, the authors reported ``limited evidence'' for an effect
threshold at an O3 concentration of 56 ppb (p=0.06). In
communications with EPA staff (Sasser, 2014), the study authors
indicated that it is not clear whether a threshold model is a better
predictor of respiratory mortality than the linear model, and that
``considerable caution should be exercised in accepting any specific
threshold.''
---------------------------------------------------------------------------

3. Policy Assessment Conclusions on the Current Standard
    As an initial matter, the PA concludes that reducing precursor
emissions to achieve O3 concentrations that meet the current
standard will provide important improvements in public health
protection. This initial conclusion is based on (1) the strong body of
scientific evidence indicating a wide range of adverse health outcomes
attributable to exposures to O3 concentrations commonly
found in the ambient air and (2) estimates indicating decreased
occurrences of O3 exposures of concern and decreased health
risks upon meeting the current standard, compared to recent air
quality.
    In particular, the PA concludes that strong support for this
initial conclusion is provided by controlled human exposure studies of
respiratory effects, and by quantitative estimates of exposures of
concern and lung function decrements based on information in these
studies. Analyses in the HREA estimate that the percentages of children
(i.e., all children and children with asthma) in urban study areas
experiencing exposures of concern, or experiencing abnormal and
potentially adverse lung function decrements, are consistently lower
for air quality that just meets the current O3 standard than
for recent air quality. The HREA estimates such reductions consistently
across the urban study areas evaluated and throughout various portions
of individual urban study areas, including in urban cores and the
portions of urban study areas surrounding urban cores. These reductions
in exposures of concern and O3-induced lung function
decrements reflect the consistent decreases in the highest
O3 concentrations following reductions in precursor
emissions to meet the current standard. Thus, populations in both urban
and non-urban areas would be expected to experience important
reductions in O3 exposures and O3-induced lung
function risks upon meeting the current standard.\105\
---------------------------------------------------------------------------

    \105\ As discussed above (II.C.1), CASAC recommended that the
EPA evaluate how health risks in urban centers, as well as outside
urban centers, change upon reducing NOX emissions, given
the varying impacts of NOX emissions reductions on
ambient O3 concentrations.
---------------------------------------------------------------------------

    The PA further concludes that support for this initial conclusion
is also provided by estimates of O3-associated mortality and
morbidity based on application of concentration-response relationships
from epidemiologic studies to air quality adjusted to just meet the
current standard. These estimates, which are based on the assumption
that concentration-response relationships are linear over entire
distributions of ambient O3 concentrations, are associated
with uncertainties that complicate their interpretation (II.C.3).
However, risk estimates for effects associated with short- and long-
term O3 exposures, combined with the HREA's national
analysis of O3 responsiveness to reductions in precursor
emissions and the consistent reductions estimated for the highest
ambient O3 concentrations, suggest that O3-
associated mortality and morbidity would be expected to decrease
nationwide following reductions in precursor emissions to meet the
current O3 standard.
    Reductions in O3 precursor emissions (i.e.,
NOX) could also increase public health protection by
reducing the ambient concentrations of pollutants other than
O3. For example, in their advice on the second draft HREA
CASAC acknowledged the potential for ambient NO2
concentrations to be affected by changes in NOX emissions
(Frey, 2014a, p. 10). Consistent with this, the PA notes that
NOX emissions contribute to ambient NO2, and that
NOX and VOCs can contribute to secondary formation of
PM2.5 constituents, including ammonium sulfate
(NH4SO4), ammonium nitrate
(NH4NO3), and organic carbon (OC). Therefore, at
some times and in some locations, control strategies that would reduce
NOX emissions (i.e., to meet an O3 standard)
could reduce ambient concentrations of NO2 and
PM2.5, resulting in health benefits beyond those directly
associated with reducing ambient O3 concentrations.
    After reaching the initial 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 PA
considers the adequacy of the public health protection that is provided
by the

current standard. In considering the available scientific evidence,
exposure/risk information, advice from CASAC (II.D.4, below), and input
from the public, the PA reaches the conclusion that the available
evidence and information clearly call into question the adequacy of
public health protection provided by the current primary standard. In
reaching this conclusion, the PA notes that evidence from controlled
human exposure studies provides strong support for the occurrence of
adverse respiratory effects following exposures to O3
concentrations below the level of the current standard. Epidemiologic
studies provide support for the occurrence of adverse respiratory
effects and mortality under air quality conditions that would likely
meet the current standard. In addition, based on the analyses in the
HREA, the PA concludes that the exposures and risks projected to remain
upon meeting the current standard are indicative of risks that can
reasonably be judged to be important from a public health perspective.
Thus, the PA concludes that the evidence and information provide strong
support for giving consideration to revising the current primary
standard in order to provide increased public health protection against
an array of adverse health effects that range from decreased lung
function and respiratory symptoms to more serious indicators of
morbidity (e.g., including emergency department visits and hospital
admissions), and mortality. In consideration of all of the above, the
PA draws the conclusion that it is appropriate for the Administrator to
consider revision of the current primary O3 standard to
provide increased public health protection.
4. CASAC Advice
    Following the 2008 decision to revise the primary O3
standard by setting the level at 0.075 ppm (75 ppb), CASAC strongly
questioned whether the standard met the requirements of the CAA. In
September 2009, the EPA announced its intention to reconsider the 2008
standards, issuing a notice of proposed rulemaking in January 2010 (75
FR 2938). Soon after, the EPA solicited CASAC review of that proposed
rule and in January 2011, solicited additional advice. This proposal
was based on the scientific and technical record from the 2008
rulemaking, including public comments and CASAC advice and
recommendations. As further described above (I.C), in the fall of 2011,
the EPA did not revise the standard as part of the reconsideration
process but decided to defer decisions on revisions to the
O3 standards to the next periodic review, which was already
underway. Accordingly, in this section we describe CASAC's advice
related to the 2008 final decision and the subsequent reconsideration,
as well as its advice on this current review of the O3 NAAQS
that was initiated in September 2008.
    In April 2008, the members of the CASAC Ozone Review Panel sent a
letter to EPA stating ``[I]n our most-recent letters to you on this
subject--dated October 2006 and March 2007--the CASAC unanimously
recommended selection of an 8-hour average Ozone NAAQS within the range
of 0.060 to 0.070 parts per million [60 to 70 ppb] for the primary
(human health-based) Ozone NAAQS'' (Henderson, 2008). The letter
continued:

The CASAC now wishes to convey, by means of this letter, its
additional, unsolicited advice with regard to the primary and
secondary Ozone NAAQS. In doing so, the participating members of the
CASAC Ozone Review Panel are unanimous in strongly urging you or
your successor as EPA Administrator to ensure that these
recommendations be considered during the next review cycle for the
Ozone NAAQS that will begin next year . . . numerous medical
organizations and public health groups have also expressed their
support of these CASAC recommendations' . . . [The CASAC did] not
endorse the new primary ozone standard as being sufficiently
protective of public health. The CASAC--as the EPA's statutorily-
established science advisory committee for advising you on the
national ambient air quality standards--unanimously recommended
decreasing the primary standard to within the range of 0.060-0.070
ppm [60 to 70 ppb]. It is the Committee's consensus scientific
opinion that your decision to set the primary ozone standard above
this range fails to satisfy the explicit stipulations of the Clean
Air Act that you ensure an adequate margin of safety for all
individuals, including sensitive populations.

    In response to the EPA's solicitation of advice on the EPA's
proposed rulemaking as part of the reconsideration, CASAC conveyed
support (Samet, 2010).

CASAC fully supports EPA's proposed range of 0.060-0.070 parts per
million (ppm) for the 8-hour primary ozone standard. CASAC considers
this range to be justified by the scientific evidence as presented
in the Air Quality Criteria for Ozone and Related Photochemical
Oxidants (March 2006) and Review of the National Ambient Air Quality
Standards for Ozone: Policy Assessment of Scientific and Technical
Information, OAQPS Staff Paper (July 2007). As stated in our letters
of October 24, 2006, March 26, 2007 and April 7, 2008 to former
Administrator Stephen L. Johnson, CASAC unanimously recommended
selection of an 8-hour average ozone NAAQS within the range proposed
by EPA (0.060 to 0.070 ppm). In proposing this range, EPA has
recognized the large body of data and risk analyses demonstrating
that retention of the current standard would leave large numbers of
individuals at risk for respiratory effects and/or other significant
health impacts including asthma exacerbations, emergency room
visits, hospital admissions and mortality.

    In response to EPA's request for additional advice on the
reconsideration in 2011, CASAC reaffirmed their conclusion that ``the
evidence from controlled human and epidemiological studies strongly
supports the selection of a new primary ozone standard within the 60-70
ppb range for an 8-hour averaging time'' (Samet, 2011, p ii). As
requested by the EPA, CASAC's advice and recommendations were based on
the scientific and technical record from the 2008 rulemaking. In
considering the record for the 2008 rulemaking, CASAC stated the
following to summarize the basis for their conclusions (Samet, 2011,
pp. ii to iii).
    (1) The evidence available on dose-response for effects of
O3 shows associations extending to levels within the range
of concentrations currently experienced in the United States.
    (2) There is scientific certainty that 6.6-hour exposures with
exercise of young, healthy, non-smoking adult volunteers to
concentrations >=80 ppb cause clinically relevant decrements of lung
function.
    (3) Some healthy individuals have been shown to have clinically
relevant responses, even at 60 ppb.
    (4) Since the majority of clinical studies involve young, healthy
adult populations, less is known about health effects in such
potentially ozone sensitive populations as the elderly, children and
those with cardiopulmonary disease. For these susceptible groups,
decrements in lung function may be greater than in healthy volunteers
and are likely to have a greater clinical significance.
    (5) Children and adults with asthma are at increased risk of acute
exacerbations on or shortly after days when elevated O3
concentrations occur, even when exposures do not exceed the NAAQS
concentration of 75 ppb.
    (6) Large segments of the population fall into what the EPA terms a
``sensitive population group,'' i.e., those at increased risk because
they are more intrinsically susceptible (children, the elderly, and
individuals with chronic lung disease) and those who are more
vulnerable due to increased exposure because they work outside or live
in areas that are more polluted than the mean levels in their
communities.
    With respect to evidence from epidemiologic studies, CASAC stated
``while epidemiological studies are

inherently more uncertain as exposures and risk estimates decrease (due
to the greater potential for biases to dominate small effect
estimates), specific evidence in the literature does not suggest that
our confidence on the specific attribution of the estimated effects of
ozone on health outcomes differs over the proposed range of 60-70 ppb''
(Samet, 2011, p. 10).
    Following its review of the second draft PA in the current review,
which considers an updated scientific and technical record since the
2008 rulemaking, CASAC concluded that ``there is clear scientific
support for the need to revise the standard'' (Frey, 2014c, p. ii). In
particular, CASAC noted the following (Frey, 2014c, p. 5):

[T]he scientific evidence provides strong support for the occurrence
of a range of adverse respiratory effects and mortality under air
quality conditions that would meet the current standard. Therefore,
CASAC unanimously recommends that the Administrator revise the
current primary ozone standard to protect public health.\106\
---------------------------------------------------------------------------

    \106\ CASAC provided similar advice in their letter to the
Administrator on the REA, stating that ``The CASAC finds that the
current primary NAAQS for ozone is not protective of human health
and needs to be revised'' (Frey, 2014a, p. 15).

    In supporting these conclusions, CASAC judged that the strongest
evidence comes from controlled human exposure studies of respiratory
effects. The Committee specifically 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). CASAC further judged that ``if
subjects had been exposed to ozone using the 8-hour averaging period
used in the standard, adverse effects could have occurred at lower
concentration'' and that ``the level at which adverse effects might be
observed would likely be lower for more sensitive subgroups, such as
those with asthma'' (Frey, 2014c, p. 5).
    With regard to lung function risk estimates based on information
from controlled human exposure studies, CASAC concluded 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 Committee further concluded
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). In considering estimates of the occurrence of these decrements in
urban study areas, CASAC specifically noted that the current standard
is estimated to allow 11 to 22% of school age children to experience at
least one day with an FEV1 decrement >=10% (Frey, 2014c, p.
7).
    Although CASAC judged that controlled human exposure studies of
respiratory effects provide the strongest evidence supporting their
conclusion on the current standard, the Committee judged that there is
also ``sufficient scientific evidence based on epidemiologic studies
for mortality and morbidity associated with short-term exposure to
ozone at the level of the current standard'' (Frey, 2014c, p. 5). In
support of the biological plausibility of the associations reported in
these epidemiologic studies, CASAC noted that ``[r]ecent animal
toxicological studies support identification of modes of action and,
therefore, the biological plausibility associated with the
epidemiological findings'' (Frey, 2014c, p. 5).
    Consistent with the advice of CASAC, several public commenters
supported revising the primary O3 standard to provide
increased public health protection. In considering the available
evidence as a basis for their views, these commenters generally noted
that the health evidence is stronger in the current review than in past
reviews, with new evidence for effects attributable to short- and long-
term exposures, and new evidence for effects at lower O3
exposure concentrations.
    Other public commenters opposed considering revised standards.
These commenters discussed a variety of reasons for their views. A
number of commenters expressed the view that the EPA should not lower
the level of the standard because a lower level would be closer to
background O3 concentrations. In addition, several
commenters challenged the interpretation of the evidence presented in
the ISA. With respect to the risk assessment, several commenters
expressed the view that the EPA should only estimate risks above
O3 background concentrations, or above threshold
concentrations. Some commenters also expressed the view that, based on
the mortality and morbidity risk estimates in the HREA, there is little
to no difference between the risks estimated for the current
O3 standard and the risks estimated for revised standards
with lower levels. These commenters concluded that the HREA and PA have
not shown that the public health improvements likely to be achieved by
a revised O3 standard would be greater than the improvements
likely to be achieved by the current standard.
5. Administrator's Proposed Conclusions Concerning the Adequacy of the
Current Standard
    This section discusses the Administrator's proposed conclusions
related to the adequacy of the public health protection provided by the
current primary O3 standard, resulting in her proposed
decision to revise that standard. These proposed conclusions, and her
proposed decision, are based on the Administrator's consideration of
the available scientific evidence, exposure/risk information, the
comments and advice of CASAC, and public input received thus far, as
summarized below.
    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 initial conclusion, she notes the discussion
in section 3.4 of the PA (U.S. EPA, 2014c), summarized above (II.D.3).
In particular, the Administrator notes that this initial 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.\107\
---------------------------------------------------------------------------

    \107\ While not analyzed quantitatively, consistent with CASAC
advice (Frey, 2014a, p. 10), the Administrator notes that reductions
in O3 precursor emissions (e.g., NOX; VOC) to
achieve O3 concentrations that meet the current standard
could also increase public health protection by reducing the ambient
concentrations of pollutants other than O3 (i.e.,
PM2.5, NO2).
---------------------------------------------------------------------------

    After reaching the initial 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 O3 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 health effects. This is the
strongest causality finding possible under the ISA's hierarchical
system for classifying weight of evidence for causation. 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
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 evidence for the coherence of results from different
scientific disciplines, 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).
    The Administrator further notes that evidence for adverse
respiratory health effects attributable to long-term, or repeated
short-term, O3 exposures 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).
Uncertainties related to the extrapolation of data generated by rodent
toxicology studies to the understanding of health effects in humans
have been reduced by studies in non-human primates and by recent
epidemiologic studies. The evidence available in this review includes
new epidemiologic studies using a variety of designs and analysis
methods, conducted by different research groups in different locations,
evaluating the relationships between long-term O3 exposures
and measures of respiratory morbidity and mortality. New evidence
supports associations between long-term O3 exposures and the
development of asthma in children, with several studies reporting
interactions between genetic variants and such O3 exposures.
Studies also report associations between long-term O3
exposures and asthma prevalence, asthma severity and control,
respiratory symptoms among asthmatics, and respiratory mortality.
    In considering the O3 exposure concentrations reported
to elicit respiratory effects, the Administrator agrees with the
conclusions of the PA and with the advice of CASAC (Frey, 2014c) that
controlled human exposure studies provide the most certain evidence
indicating the occurrence of health effects in humans following
exposures to specific O3 concentrations. In particular, as
discussed further in section II.E.4.d below, 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, she
places the most weight on information from these controlled human
exposure studies.
    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, 72,\108\ 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. She 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 intermittent, 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. As discussed below, compared to the
evidence available in the last review, these studies have strengthened
support for the occurrence of abnormal and adverse respiratory effects
attributable to short-term exposures to O3 concentrations
below the level of the current standard.\109\ The Administrator
concludes that such exposures to O3 concentrations below the
level of the current standard are potentially important from a public
health perspective, given the following:
---------------------------------------------------------------------------

    \108\ As noted above, for the 70 ppb target exposure
concentration, Schelegle et al. (2009) reported that the actual mean
exposure concentration was 72 ppb.
    \109\ Cf. State of Misisssippi. 744 F.3d 1350 (``Perhaps more
studies like the Adams studies will yet reveal that the 0.060 ppm
level produces significant adverse decrements that simply cannot be
attributed to normal variation in lung function.'').
---------------------------------------------------------------------------

    (1) The combination of lung function decrements and respiratory
symptoms reported to occur in healthy adults following exposures to 72
ppb O3 or higher, while at moderate exertion, meet ATS
criteria for an adverse response. 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).
    (2) With regard to 60 ppb O3, CASAC agreed that ``a
level of 60 ppb corresponds to the lowest exposure concentration
demonstrated to result in lung function decrements large enough to be
judged an abnormal response by ATS and that could be adverse in
individuals with lung disease'' (Frey, 2014c, p. 7). CASAC further
noted that ``a level of 60 ppb also corresponds to the lowest exposure
concentration at which pulmonary inflammation has been reported''
(Frey, 2014c, p. 7).
    (3) The controlled human exposure studies reporting these
respiratory effects were conducted in healthy adults, while at-risk
groups (e.g., children, people with asthma) could experience larger
and/or more serious effects. In their advice to the Administrator,
CASAC concurred with this reasoning (Frey, 2014a, p. 14; Frey, 2014c,
p. 5).
    (4) These respiratory effects are coherent with the serious health
outcomes that have been reported in epidemiologic studies evaluating
exposure to O3 (e.g., respiratory-related hospital
admissions, emergency department visits, and mortality).
    As noted above, the Administrator's proposed conclusions regarding
the adequacy of the current primary O3 standard place a
large amount of weight on the results of controlled human exposure
studies. In particular, given the combination of lung function

decrements and respiratory symptoms following 6.6 hour exposures to
O3 concentrations as low as 72 ppb, and given CASAC advice
regarding effects at 72 ppb along with ATS adversity criteria, she
concludes that the evidence in this review supports the occurrence of
adverse respiratory effects following exposures to O3
concentrations lower than the level of the current standard.\110\ As
discussed below, the Administrator further considers information from
the broader body of controlled human exposure studies within the
context of quantitative estimates of exposures of concern and
O3-induced FEV1 decrements.
---------------------------------------------------------------------------

    \110\ The use of evidence from controlled human exposure studies
conducted in healthy adults to characterize the potential for
adverse effects, including in at-risk groups such as children and
asthmatics, is discussed in sections II.C.2 and II.C.3.a, above.
CASAC advice on this issue is discussed in sections II.D.4 and
II.E.4.c.
---------------------------------------------------------------------------

    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.\111\ 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. Associations with morbidity
and mortality are stronger during the warm or summer months, and remain
robust after adjustment for copollutants.
---------------------------------------------------------------------------

    \111\ As noted above, she places less weight on information from
epidemiologic studies than on information from controlled human
exposure studies.
---------------------------------------------------------------------------

    In considering information from epidemiologic studies within the
context of her conclusions on the adequacy of the current standard, the
Administrator considers the extent to which available studies support
the occurrence of O3 health effect associations with air
quality likely to be allowed by the current standard. In doing so, she
places the most weight on air quality analyses in locations of single-
city studies of short-term O3, as discussed in more detail
in section II.E.4.d below.\112\ In particular, she notes that a U.S.
single-city study reported associations with respiratory emergency
department visits in children and adults in a location that would
likely have met the current O3 standard over the entire
study period (Mar and Koenig, 2009). In addition, even in some single-
city study locations where the current standard was likely not met
(i.e., Silverman and Ito, 2010; Strickland et al., 2010), the
Administrator notes PA analyses indicating that reported concentration-
response functions and available air quality data support the
occurrence of O3-health effect associations on subsets of
days with ambient O3 concentrations below the level of the
current standard (II.D.1). Compared to single-city studies, the
Administrator notes additional uncertainty in interpreting the
relationships between air quality in individual study cities and health
effects based on multicity analyses (discussed further in sections
II.D.1 and II.E.4.d). 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 mortality have
been reported in several multicity studies when the majority of study
locations (though not all study locations) would likely have met the
current O3 standard. When taken together, the Administrator
reaches the conclusion that single-city epidemiologic studies and
associated air quality information support the occurrence of
O3-associated hospital admissions and emergency department
visits for ambient O3 concentrations likely to have met the
current standard, and that air quality analyses in locations of
multicity studies provide some support for this conclusion for a
broader range of effects (i.e., including mortality).
---------------------------------------------------------------------------

    \112\ As discussed in section II.E.4.d of this preamble, this
judgment applies specifically to epidemiologic studies of short-term
O3 concentrations where multicity effect estimates are
presented, based on combining the effect estimates from multiple
individual cities, and where individual city effect estimates are
not presented (as is the case for key multicity studies analyzed in
the PA). Because these reported multicity effect estimates do not
allow health effect associations to be disaggregated by individual
city, it is not possible to assign the health effect association to
the air quality in any one study location, or to the air quality in
a subset of locations. In contrast, for epidemiologic studies of
long-term concentrations, where multicity effect estimates are based
on comparisons across cities, different judgments have been made
with regard to the utility of multicity studies (see, e.g. 78 FR
3086 at 3103/2) (January 15, 2013) (and see discussion below of
study by Jerrett et al., (2009)).
---------------------------------------------------------------------------

    Beyond her consideration of the scientific evidence, the
Administrator also considers the results of the HREA exposure and risk
analyses in reaching initial conclusions regarding the adequacy of the
current primary O3 standard. In doing so, as noted above,
she focuses primarily on exposure and risk estimates based on
information from controlled human exposure studies (i.e., exposures of
concern and O3-induced lung function decrements). She places
relatively less weight on epidemiologic-based risk estimates, noting
that the overall conclusions from the HREA likewise reflect less
confidence in estimates of epidemiologic-based risks than in estimates
of exposures and lung function risks (U.S. EPA, 2014, section 9.6).
Consistent with the conclusions in the PA, her determination to attach
less weight to the epidemiologic-based risk estimates reflects her
consideration of key uncertainties, including the heterogeneity in
effect estimates between locations, the potential for exposure
measurement errors, and uncertainty in the interpretation of the shape
of concentration-response functions for O3 concentrations in
the lower portions of ambient distributions (U.S. EPA, 2014, section
9.6) (II.D.2). In particular, she concludes that lower confidence
should be placed in the results of the assessment of respiratory
mortality risks associated with long-term O3 exposures,
primarily because that analysis is based on only one study (even though
that study is well-designed) and because of the uncertainty in that
study about the existence and level of a potential threshold in the
concentration-response function (U.S. EPA, 2014a, section 9.6)
(II.D.2).\113\
---------------------------------------------------------------------------

    \113\ CASAC also called into question the extent to which it is
appropriate to place confidence in risk estimates for respiratory
mortality (Frey, 2014a, p. 11).
---------------------------------------------------------------------------

    With regard to estimates of exposures of concern, the Administrator
considers the extent to which the current standard provides protection
against exposures to O3 concentrations at or above 60, 70,
and 80 ppb, noting CASAC advice that 60 ppb ``is an appropriate
exposure of concern for asthmatic children'' (Frey, 2014c, p. 8). She
further 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 (discussed further in II.E.4.d, below), she becomes
increasingly concerned about the potential for adverse responses as the
frequency of occurrences increases.\114\ In particular,

she notes that repeated occurrences of the types of effects shown to
occur following exposures of concern can have potentially adverse
outcomes. For example, repeated occurrences of airway inflammation
could potentially result in 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, 2013a, section 6.2.3). Thus, the
Administrator 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, particularly for the 70 and 80 ppb
benchmarks.
---------------------------------------------------------------------------

    \114\ Not all people who experience an exposure of concern will
experience an adverse effect (even members of at-risk populations).
For most of the endpoints evaluated in controlled human exposure
studies (with the exception of O3-induced FEV1
decrements, as discussed below), the number of those experiencing
exposures of concern who will experience adverse effects cannot be
reliably quantified.
---------------------------------------------------------------------------

    Consistent with CASAC advice (Frey, 2014c), the Administrator
focuses on children in these analyses of O3 exposures,
noting that estimates for all children and asthmatic children are
virtually indistinguishable (in terms of the percent estimated to
experience exposures of concern). Though she focuses on children, she
also recognizes that exposures to O3 concentrations at or
above 60 or 70 ppb could be of concern for adults. As discussed in the
HREA and PA (and II.C.2.a, above), the patterns of exposure estimates
across urban study areas, across years, and across air quality
scenarios are similar in adults with asthma, older adults, all
children, and children with asthma, though smaller percentages of adult
populations are estimated to experience exposures of concern than
children and children with asthma. Thus, 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.
    As illustrated in Table 1 (above), the Administrator 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, though approximately 3 to 8% of children, including
approximately 3 to 8% of asthmatic children, would be estimated to
experience two or more exposures of concern to O3
concentrations at or above 60 ppb \115\ (based on estimates averaged
over the years of analysis). To provide some perspective on these
percentages, the Administrator notes that they correspond to almost
900,000 children in urban study areas, including about 90,000 asthmatic
children, estimated to experience two or more exposures of concern at
or above 60 ppb. Nationally, if the current standard were to be just
met the number of children experiencing such exposures would be larger.
In the worst-case year and location (i.e., year and location with the
largest exposure estimates), the Administrator notes that over 2% of
children are estimated to experience two or more exposures of concern
at or above 70 ppb and over 14% are estimated to experience two or more
exposures of concern at or above 60 ppb.
---------------------------------------------------------------------------

    \115\ Almost no children in those areas would be estimated to
experience two or more exposures of concern at or above 80 ppb.
---------------------------------------------------------------------------

    Although, as discussed above and in section II.E.4.d, the
Administrator is less concerned about single occurrences of exposures
of concern, she notes that even single occurrences can cause adverse
effects in some people, particularly for the 70 and 80 ppb benchmarks.
Therefore, she also considers estimates of one or more exposures of
concern. As illustrated in Table 1 (above), 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 one or more exposures of concern at or above 80 ppb
(based on estimates averaged over the years of analysis). However,
approximately 1 to 3% of children, including 1 to 3% of asthmatic
children, would be estimated to experience one or more exposures of
concern to O3 concentrations at or above 70 ppb and
approximately 10 to 17% would be estimated to experience one or more
exposures of concern to O3 concentrations at or above 60
ppb. In the worst-case year and location, the Administrator notes that
over 1% of children are estimated to experience one or more exposures
of concern at or above 80 ppb, over 8% are estimated to experience one
or more exposures of concern at or above 70 ppb, and about 26% are
estimated to experience one or more exposures of concern at or above 60
ppb.
    In addition to estimated exposures of concern, the Administrator
also considers HREA estimates of the occurrence of O3-
induced lung function decrements. 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.3), a more general consensus view
of the potential adversity of such decrements emerges as the frequency
of occurrences increases. Therefore, 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 \116\) to
experience two or more O3-induced lung function decrements
>=10%. Nationally, larger numbers of children would be expected to
experience such O3-induced decrements if the current
standard were to be just met. The current standard is also estimated to
allow about 3 to 5% of children in the urban study areas to experience
one or more decrements >=15% and about 14 to 19% of children to
experience one or more decrements >=10%. In the worst-case year and
location, the current standard is estimated to allow 4% of children in
the urban study areas to experience two or more decrements >=15% (and
7% to experience one or more such decrements) and 14% of children to
experience two or more decrements >=10% (and 22% to experience one or
more such decrements).
---------------------------------------------------------------------------

    \116\ As noted above, CASAC concluded that ``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) and that such decrements ``could be adverse for people with lung
disease'' (Frey, 2014c, p. 7).
---------------------------------------------------------------------------

    In further considering the HREA results, the Administrator
considers the epidemiology-based risk estimates. As discussed above,
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, the Administrator notes estimates of total risks (i.e.,
based on the full distributions of ambient O3
concentrations) and risks associated with O3 concentrations
in the upper portions of ambient distributions. The Administrator notes
that estimates of total risks are based on the assumption that
concentration-response relationships remain linear over the entire
distributions of ambient O3 concentrations. With regard to
total risks, she notes that the HREA estimates thousands of
O3-associated hospital admissions, emergency department
visits, and deaths per year for air quality conditions associated with
just meeting the current standard in the 12 urban study areas (II.C.3).
    However, the Administrator also notes the increasing uncertainty
associated with the shapes of concentration-response curves for
O3 concentrations in the lower portions of ambient
distributions. She particularly notes 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 depending on the
O3 metric, health endpoint, and study population) (U.S. EPA,
2013a, section <IP_ADDRESS>). The Administrator further notes 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). Therefore, the
Administrator judges it appropriate to focus on risks associated with
O3 concentrations in the upper portions of ambient
distributions. Even when considering only area-wide O3
concentrations from the upper portions of seasonal distributions, the
Administrator notes that the current standard is estimated to allow
hundreds to thousands of O3-associated deaths per year in
urban study areas (II.C.3).
    Although the Administrator notes the HREA conclusions indicating
somewhat less confidence in estimates of O3-associated
mortality and morbidity risks, compared to estimates of exposures of
concern and risk of lung function decrements, she concludes that the
general magnitude of mortality and morbidity 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. She especially notes that this is the case
based on the risks associated with the upper ends of distributions of
ambient O3 concentrations, where she has the greatest
confidence in O3-attributable effects.
    In addition to the evidence and exposure/risk information discussed
above, the Administrator also takes note of the CASAC advice in the
current review and in the 2010 proposed reconsideration of the 2008
decision establishing the current standard. 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. 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; Samet, 2012).
    In consideration of all of the above, the Administrator proposes
that the current primary O3 standard is not adequate to
protect public health, and that it should be revised to provide
increased public health protection. This proposed decision is based on
the Administrator's initial 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
and, therefore, that the current standard is not requisite to protect
public health with an adequate margin of safety. With regard to the
evidence, she specifically notes that (1) controlled human exposure
studies provide support for the occurrence of adverse respiratory
effects following exposures to O3 concentrations below the
level of the current standard (i.e., as low as 72 ppb), and that (2)
single-city epidemiologic studies provide support for the occurrence of
adverse respiratory effects under air quality conditions that would
likely meet the current standard, with multicity studies providing some
support for this conclusion for a broader range of effects (i.e.,
including mortality). 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.\117\ In addition, based on the analyses in the HREA, the
Administrator initially 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. Thus, she
reaches the proposed conclusion that the evidence and information,
together with CASAC advice based on their consideration of that
evidence and information, provide strong support for revising the
current primary standard in order to increase public health protection
against an array of adverse effects that range from decreased lung
function and respiratory symptoms to more serious indicators of
morbidity (e.g., including emergency department visits and hospital
admissions), and mortality.
---------------------------------------------------------------------------

    \117\ See e.g. State of Mississippi, 744 F. 3d at 1345; American
Farm Bureau, 559 F. 3d at 525-26.
---------------------------------------------------------------------------

    The Administrator solicits comment on her proposed decision to
revise the current primary O3 NAAQS, including on her
considerations and proposed conclusions based on the scientific
evidence, exposure/risk information, and CASAC advice. In doing so, she
recognizes that some have expressed alternative approaches to viewing
the evidence and information, including alternative approaches to
viewing, evaluating, and weighing important uncertainties. In some
cases, these alternative approaches have led some public commenters to
recommend retaining the current standard. Given these alternative
views, in addition to proposing to revise the current primary
O3 standard, the Administrator solicits comment on the
option of retaining that standard. In doing so, she also solicits
comment on the potential approaches to viewing the scientific evidence
and exposure/risk information that could support a conclusion that the
current standard is requisite to protect public health with an adequate
margin of safety.

E. Conclusions on the Elements of the Primary Standard

    Having reached the proposed conclusion that the currently available
scientific evidence and exposure/risk information call into question
the adequacy of the current O3 standard, 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 proposed
conclusions on alternative standards 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
input from members of the public. 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.E.1), averaging time (II.E.2), form (II.E.3), and
level (II.E.4).
1. Indicator
    In the last 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, 2013a, section 3.6). Data for
other photochemical oxidants (e.g., PAN, H2O2,
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, 2013a, 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 reaches the proposed conclusion that
O3 remains the most appropriate indicator for a standard
meant to provide protection against photochemical oxidants.\118\
---------------------------------------------------------------------------

    \118\ 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).
---------------------------------------------------------------------------

2. Averaging Time
    The EPA established the current 8-hour averaging time \119\ 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 6 to
8 hour O3 concentrations and adverse respiratory effects (62
FR 38861). It was also noted that a standard with a max 8-hour
averaging time is likely to provide substantial protection against
respiratory effects associated with 1-hour peak O3
concentrations. Similar conclusions were reached in the last
O3 NAAQS review and thus, the 8-hour averaging time was
retained in 2008.
---------------------------------------------------------------------------

    \119\ This 8-hour averaging time reflects daily max 8-hour
average O3 concentrations.
---------------------------------------------------------------------------

    In reaching a proposed conclusion on averaging time in the current
review, the Administrator considers the extent to which the available
evidence continues to support the appropriateness of a standard with an
8-hour averaging time. Specifically, the Administrator considers 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.
a. Short-Term
    As an initial consideration with respect to the most appropriate
averaging time for the O3 NAAQS, the Administrator notes
that the strongest evidence for O3-associated health effects
is for respiratory effects following short-term exposures. More
specifically, the Administrator notes 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, 2013a, section 2.5.2).
Therefore, as in past reviews, 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 notes 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 AHR) occur in healthy adults following 6.6 hour
exposures to O3 (U.S. EPA, 2013a, 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 notes the strength of evidence from
epidemiologic studies that have 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 are 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-hr max), and the 24-hour average. These studies are summarized above
and assessed in detail in chapter 6 of the ISA (U.S. EPA, 2013a).
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, 2013a, 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, 2013a, 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, 2013a, p. 6-253). Thus, the
Administrator notes 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, the
Administrator concludes 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 notes 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, 2013a, 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 last 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, the Administrator notes 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).\120\ 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.
---------------------------------------------------------------------------

    \120\ Though the Administrator also notes important
uncertainties associated with these risk estimates, as discussed
above (II.C.3.b).
---------------------------------------------------------------------------

    In further considering the study by Jerrett et al. (2009), the
Administrator notes 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 notes 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., 2008; 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
    Taken together, the Administrator notes that the analyses
summarized above indicate that a standard with an 8-hour averaging
time, coupled with the current 4th 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 proposes to retain the current 8-hour
averaging time, and not to set an additional standard with a different
averaging time.
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 4th-highest 8-hour daily maximum O3 concentrations.
    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.\121\ 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.\122\ It was judged 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.
---------------------------------------------------------------------------

    \121\ 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.
    \122\ 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). Consideration was also given to 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.\123\ 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).
---------------------------------------------------------------------------

    \123\ See American Trucking Assn's v. EPA, 283 F. 3d at 374-75
(less stable implementation programs may be less effective, and
therefore the EPA can 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 places 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,\124\ the Administrator concluded that a 4th-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 (4th-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).
---------------------------------------------------------------------------

    \124\ In the 2008 review, one group of commenters expressed the
view that the standard was not adequate and supported a more health-
protective form (e.g., a second- or third-highest daily max form).
Another group of commenters expressed the view that the standard was
adequate and did not provide any views on alternative forms that
would be appropriate should the Administrator consider revisions to
the standard. The Administrator considered the protection afforded
by the combination of level and form in revising the standard in
2008 to 75 ppb, as a 3-year average of the annual fourth-highest
daily max 8-hour concentrations (73 FR 16475, March 27, 2008).
---------------------------------------------------------------------------

    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 the 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 4th high form ``provides a
stable target for implementing programs to improve air quality'' (73 FR
16475, March 27, 2008).
    In the current review, the Administrator considers the extent to
which newly available information provides support for the current
form. In so doing, she takes note of the conclusions of prior reviews
summarized above. She recognizes 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 takes note of the importance of
stability in implementation to achieving the level of protection
specified by the NAAQS. Specifically, she notes 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
considers particularly findings from prior reviews with regard to the
use of the nth-high metric. As noted above, the 4th-highest daily
maximum was selected 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
implementation programs. The Administrator concludes that the currently
available evidence and information do not call into question these
conclusions from previous reviews. In reaching this conclusion, the
Administrator notes 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 4th 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
proposes to retain the current 4th-highest daily maximum form for an
O3 standard with an 8-hour averaging time and a revised
level, as discussed below.
4. Level
    The Administrator next considers the extent to which alternative
levels below 75 ppb could provide greater protection than the current
primary standard against short- and long- term exposures to
O3 in ambient air, for a standard based on the 3-year
average of the annual 4th highest daily maximum 8-hour O3
concentration. In doing so, she particularly notes the evidence-based
and exposure-/risk-based considerations in the PA, which take into
account the experimental and epidemiologic evidence as assessed in the
ISA; quantitative estimates of O3 exposures and health risks
in at-risk populations provided by the HREA; uncertainties and
limitations associated with this evidence and information; CASAC
advice; and public input (U.S. EPA, 2014c, sections 4.4 and 4.5).
Section II.E.4.a below summarizes the PA's approach to considering the
scientific evidence and the exposure/risk information related to level
of the primary standard. Section II.E.4.b presents the PA's conclusions
on alternative primary O3 standard levels. Section II.E.4.c
summarizes CASAC advice on the level of the primary standard, and
public input received thus far. Section II.E.4.d presents the
Administrator's proposed conclusions on primary O3 standard
levels.
a. PA Approach to Considering the Evidence and Information Related to
Alternative Levels of the Primary Standard
    The PA's approach to reaching conclusions on alternative standard
levels focuses on the evidence from controlled human exposure and
epidemiologic studies, as assessed in the ISA (U.S. EPA, 2013a), and
the exposure and health risk analyses presented in the HREA (U.S. EPA,
2014a). This approach is discussed in detail in Chapters 1 and 4 of the
PA (U.S. EPA, 2014c, sections 1.3, 4.6), and is summarized below.
    As an initial matter, the PA notes that controlled human exposure
studies provide the most certain evidence indicating the occurrence of
health effects in humans following exposures to specific O3
concentrations. Consistent with this, CASAC concluded 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). As discussed above and in section <IP_ADDRESS> of the PA (U.S.
EPA, 2014c), controlled human exposure studies have reported a variety
of respiratory effects in healthy adults following exposures to
O3 concentrations of 60, 72,\125\ or 80 ppb, and higher.
---------------------------------------------------------------------------

    \125\ As noted above, for the 70 ppb exposure concentration
Schelegle et al. (2009) reported that the actual 6.6-hour mean
exposure concentration was 72 ppb.
---------------------------------------------------------------------------

    Given the evidence for respiratory effects from controlled human
exposure studies, the PA considers the extent to which standards with
revised levels would be estimated to protect at-risk populations
against exposures of concern to O3 concentrations at or
above the health benchmark concentrations of 60, 70, and 80 ppb (i.e.,
based on HREA estimates of one or more and two or more exposures of
concern). In doing so, the PA notes the CASAC conclusion that (Frey,
2014c, p. 6):

The 80 ppb-8hr benchmark level represents an exposure level for
which there is substantial clinical evidence demonstrating a range
of ozone-related effects including lung inflammation and airway
responsiveness in healthy individuals. The 70 ppb-8hr benchmark
level reflects the fact that in healthy subjects, decreases in lung
function and respiratory symptoms occur at concentrations as low as
72 ppb and that these effects almost certainly occur in some people,
including asthmatics and others with low lung function who are less
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which
ozone-related effects have been observed in clinical studies of
healthy individuals.

    The PA also notes that, due to individual variability in
responsiveness, only a subset of people who experience exposures at or
above the three benchmark concentrations can be expected to experience
associated health effects, and that available data are not sufficient
to quantify that subset of people for most of the endpoints that have
been evaluated in controlled human exposure studies (i.e., with the
exception of FEV1 decrements). The PA views the health
effects evidence as a continuum with greater confidence and less
uncertainty about the occurrence of adverse health effects at higher
O3 exposure concentrations, and less confidence and greater
uncertainty as one considers lower exposure concentrations (U.S. EPA,
2014c, section 3.2.2, p. 3-101).
    While there is greater uncertainty regarding the occurrence of
adverse health effects at lower concentrations, the PA also notes that
the controlled human exposure studies that provided the basis for
benchmark concentrations have not evaluated responses in populations at
the greatest risk from exposures to O3 (e.g., children,
people with asthma). Compared to the healthy people included in most
controlled human exposure studies, members of at-risk populations and
lifestages are at greater risk of experiencing adverse effects. Thus,
the effects reported in healthy adults at each of the benchmark
concentrations may underestimate effects in these at-risk groups. In
considering the health evidence within the context of drawing
conclusions on alternative standard levels, the PA balances concerns
about the potential for adverse health effects, especially in at-risk
populations, with the increasing uncertainty regarding the likelihood
of such effects following exposures to lower O3
concentrations.
    With respect to the lung function decrements that have been
evaluated in controlled human exposure studies, the PA considers the
extent to which standards with revised levels would be estimated to
protect healthy and at-risk populations against O3-induced
lung function decrements large enough to be adverse in some people
(based on quantitative risk estimates in the HREA). As discussed in
section 3.1.3 of the PA (U.S. EPA, 2014c) and section II.B.3 above,
although some experts would judge single occurrences of moderate
responses to be a nuisance, especially for healthy individuals, a more
general consensus view of the adversity of moderate lung function
decrements emerges as the frequency of occurrence increases. Repeated
occurrences of moderate responses, even in otherwise healthy
individuals, may be considered to be adverse, since they could well set
the stage for more serious illness (73 FR 16448). In reaching
conclusions on alternative standard levels, the PA considers the extent
to which standards with revised levels would be estimated to protect
healthy and at-risk populations against one or more, and two or more,
moderate (i.e., FEV1 decrements >=10% and >=15%) and large
(i.e., FEV1 decrements >=20%) lung function decrements.
    In evaluating the epidemiologic evidence within the context of
drawing conclusions on alternative standard levels, the PA considers
the extent to which available studies have reported associations with
emergency

department visits, hospital admissions, and/or mortality in locations
that would likely have met alternative standards with levels below 75
ppb. In evaluating the epidemiologic evidence in this way, the PA
considers both multicity and single-city studies, recognizing the
strengths and limitations of each. In particular, while single-city
studies are more limited than multicity studies in terms of statistical
power and geographic coverage, conclusions linking air quality in a
specific area with health effect associations in that same area can be
made with greater certainty for single-city studies (i.e., compared to
multicity studies reporting only multicity effect estimates).
    The PA also considers the epidemiologic evidence within the context
of epidemiology-based risk estimates. Compared to the weight given to
HREA estimates of exposures of concern and lung function risks, and the
weight given to the evidence, the PA places relatively less weight on
epidemiologic-based risk estimates. In doing so, the PA notes that the
overall conclusions from the HREA likewise reflect less confidence in
estimates of epidemiologic-based risks than in estimates of exposures
and lung function risks. The determination to attach less weight to the
epidemiologic-based estimates reflects the uncertainties associated
with mortality and morbidity risk estimates, including the
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations (U.S. EPA, 2014a, section 9.6). The HREA also concludes
that lower confidence should be placed in the results of the assessment
of respiratory mortality risks associated with long-term O3
exposures, primarily because that analysis is based on only one study
(even though that study is well-designed) and because of the
uncertainty in that study about the existence and level of a potential
threshold in the concentration-response function (U.S. EPA, 2014a,
section 9.6).
    In considering the epidemiology-based risk estimates, the PA
focuses on the extent to which potential alternative O3
standards with levels below 75 ppb are estimated to reduce the risk of
O3-associated mortality.\126\ As discussed for the current
standard (II.D.2.c), the PA considers estimates of total risk (i.e.,
based on the full distributions of ambient O3
concentrations) and estimates of risk associated with O3
concentrations in the upper portions of ambient distributions.
---------------------------------------------------------------------------

    \126\ Differences in estimated respiratory morbidity risks
between alternative standard levels are similar to the differences
estimated for total mortality associated with short-term
O3 concentrations.
---------------------------------------------------------------------------

b. PA Conclusions on Alternative O3 Standard Levels
    Using the approach discussed above to consider the scientific
evidence and exposure/risk information, CASAC advice (II.E.4.c, below),
and public comments, the PA reaches the conclusion that it is
appropriate for the Administrator to consider alternative primary
O3 standard levels from 70 to 60 ppb. The basis for this
conclusion is discussed in detail in sections 4.4.1 and 4.4.2 of the PA
(U.S. EPA, 2014c), and is summarized below.
    With regard to controlled human exposure studies, the PA considers
the lowest O3 exposure concentrations at which various
effects have been evaluated and statistically significant effects
reported. The PA also considers the potential for reported effects to
be adverse, including in at-risk populations and lifestages. As
discussed in section <IP_ADDRESS> of the PA (U.S. EPA, 2014c), controlled
human exposure studies provide evidence of respiratory symptoms
combined with lung function decrements (an adverse response based on
ATS criteria) in healthy adults following 6.6 hour exposures to
O3 concentrations as low as 72 ppb, and evidence of
potentially adverse lung function decrements and airway inflammation
following 6.6 hour exposures to O3 concentrations as low as
60 ppb.
    Although some studies show that respiratory symptoms also develop
during exposures to 60 ppb O3, the increase in symptoms has
not been reported to reach statistical significance by the end of the
6.6 hour exposure period (Adams, 2006; Schelegle et al., 2009). Thus,
while significant increases in respiratory symptoms combined with lung
function decrements have not been reported following exposures to 60
ppb O3, this combination of effects is likely to occur to
some degree in healthy adults with 6.6-hour exposures to concentrations
below 72 ppb, and also are more likely to occur with longer (i.e., 8-
hour) exposures.\127\ In addition, pulmonary inflammation, particularly
if experienced repeatedly, provides a mechanism by which O3
may cause other more serious respiratory morbidity effects (e.g.,
asthma exacerbations) and possibly extrapulmonary effects. As discussed
in section <IP_ADDRESS> of the PA (U.S. EPA, 2014c), the physiological
effects reported in controlled human exposure studies down to 60 ppb
O3 have been linked to aggravation of asthma and increased
susceptibility to respiratory infection, potentially leading to
increased medication use, increased school and work absences, increased
visits to doctors' offices and emergency departments, and increased
hospital admissions.
---------------------------------------------------------------------------

    \127\ In addition, CASAC observed that, ``adverse health effects
in young healthy adults occur with exposures to 72 ppb of ozone for
6.6 hours'' and that ``[i]t is the judgment of CASAC that if
subjects had been exposed to ozone using the 8-hour averaging period
used in the standard, adverse effects could have occurred at [a]
lower concentration. Further, in our judgment, the level at which
adverse effects might be observed would likely be lower for more
sensitive subgroups, such as those with asthma'' (Frey, 2014c, p.
5).
---------------------------------------------------------------------------

    With regard to the lowest exposure concentration shown to cause
respiratory effects (i.e., 60 ppb),\128\ the PA notes that most
controlled human exposure studies have not evaluated O3
concentrations below 60 ppb. Therefore, 60 ppb does not necessarily
reflect an exposure concentration below which effects such as lung
function decrements and airway inflammation no longer occur. This is
particularly the case given that controlled human exposure studies were
conducted in healthy adults, while people with asthma, including
asthmatic children, are likely to be more sensitive to O3-
induced respiratory effects.
---------------------------------------------------------------------------

    \128\ As discussed above (II.B.2), prolonged 6.6 exposure to 40
ppb O3 has been shown to result in a small decrease in
group mean FEV1 that is not statistically different from
responses following exposure to filtered air (Adams, 2002; Adams,
2006).
---------------------------------------------------------------------------

    With regard to other O3-induced effects, the PA notes
that AHR and impaired lung host defense capabilities have been reported
in healthy adults engaged in moderate exertion following exposures to
O3 concentrations as low as 80 ppb, the lowest concentration
evaluated for these effects. As discussed in section <IP_ADDRESS> of the PA
(U.S. EPA, 2014c), these physiological effects have been linked to
aggravation of asthma and increased susceptibility to respiratory
infection, potentially leading to increased medication use, increased
school and work absences, increased visits to doctors' offices and
emergency departments, and increased hospital admissions. These are all
indicators of adverse O3-related morbidity effects, which
are consistent with, and provide plausibility for, the adverse
morbidity effects and mortality effects observed in epidemiologic
studies.
    Based on consideration of the above evidence, the PA concludes that
available controlled human exposure studies support considering
alternative O3 standard levels from 70 to 60 ppb in

the current review. In reaching this conclusion, the PA notes that 70
ppb is just below the O3 exposure concentration reported to
result in lung function decrements and respiratory symptoms in healthy
adults (i.e., 72 ppb), a combination of effects that meet ATS criteria
for an adverse response. In addition, while 70 ppb is well below the 80
ppb exposure concentration shown to cause potentially adverse
respiratory effects such as AHR and impaired host-defense capabilities,
these effects have not been evaluated at exposure concentrations below
80 ppb and there is no reason to believe that 80 ppb represents a
threshold for such effects. In addition, potentially adverse lung
function decrements and pulmonary inflammation have been demonstrated
to occur in healthy adults at 60 ppb. Thus, 60 ppb is a short-term
exposure concentration that may be reasonably concluded to elicit
adverse effects in at-risk groups.
    The PA further notes that the range of alternative levels from 70
to 60 ppb is supported by evidence from epidemiologic studies and by
exposure and risk estimates from the HREA. This evidence and exposure/
risk information indicate that a level from anywhere in the range of 70
to 60 ppb would be expected to result in important public health
improvements over the current standard. In particular, compared to the
current standard a revised standard with a level from 70 to 60 ppb
would be expected to (1) more effectively maintain short- and long-term
O3 concentrations below those present in the epidemiologic
studies that reported significant O3 health effect
associations in locations likely to have met the current standard; (2)
reduce the occurrence of exposures of concern to O3
concentrations that result in respiratory effects in healthy adults (at
or above 60, 70, and 80 ppb); (3) reduce the occurrence of moderate-to-
large O3-induced lung function decrements; and (4) reduce
the risk of O3-associated mortality and morbidity,
particularly the risk associated with the upper portions of the
distributions of ambient O3 concentrations. The PA also
notes that the range of levels from 70 to 60 ppb corresponds to the
range of levels recommended for consideration by CASAC, based on the
available evidence and information (Frey, 2014a; Frey, 2014c).
    In reaching a conclusion on whether it is appropriate to consider
alternative standard levels below 60 ppb, the PA notes the following:

    (1) While controlled human exposure studies provide evidence for
O3-induced respiratory effects following exposures to
O3 concentrations as low as 60 ppb, they do not provide
evidence for adverse effects following exposures to lower
concentrations. On this issue, CASAC concurred that 60 ppb
O3 is an appropriate and justifiable scientifically based
lower bound for a revised primary standard, based upon findings of
``adverse effects, including clinically significant lung function
decrements and airway inflammation, after exposures to 60 ppb ozone
in healthy adults with moderate exertion (Adams, 2006; Schelegle et
al., 2009; Brown et al., 2008; Kim et al., 2011), with limited
evidence of adverse effects below 60 ppb'' (Frey, 2014c, p. 7).
    (2) Based on the HREA results, meeting an O3 standard
with a level of 60 ppb would be expected to almost eliminate
exposures of concern to O3 concentrations at or above 60
ppb. To the extent lower exposure concentrations may result in
adverse health effects in some people, a standard level of 60 ppb
would be expected to also reduce exposures to O3
concentrations below 60 ppb.
    (3) U.S. and Canadian epidemiologic studies have not reported
O3 health effect associations based primarily on study
locations likely to have met a standard with a level of 60 ppb.
    (4) In all of the urban study areas evaluated, a standard with a
level of 60 ppb would be expected to maintain long-term
O3 concentrations below those where a key study indicates
the most confidence in a linear concentration-response relationship
with respiratory mortality.

    Given all of the above considerations the PA concludes that,
compared to standards with levels from 70 to 60 ppb, the extent to
which standards with levels below 60 ppb could result in further public
health improvements becomes notably less certain. Therefore, the PA
concludes that it is not appropriate in this review to consider
standard levels below 60 ppb.
    The following sections summarize the PA's consideration of the
scientific evidence and exposure/risk information specifically related
to potential alternative O3 standards with levels from the
upper (70 ppb) (II.E.4.c.i), middle (65 ppb) (II.E.4.c.ii), and lower
(60 ppb) (II.E.4.c.iii) portions of the range of 70 to 60 ppb. Key
exposure/risk information considered in the PA is summarized in Tables
4 and 5, below (from U.S. EPA, 2014c, Tables 4-4 and 4-5).
---------------------------------------------------------------------------

    \129\ All alternative standard levels evaluated in the HREA were
effective at limiting exposures of concern at or above 80 ppb (U.S.
EPA, 2014c, Figures 4-1 to 4-4). Therefore, Table 4 focuses on
exposures of concern at or above the 70 and 60 ppb benchmark
concentrations.
    \130\ Estimates for each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas.
    \131\ Numbers of children exposed in each urban case study area
were averaged over the years 2006 to 2010. These averages were then
summed across urban study areas. Numbers are rounded to nearest
thousand unless otherwise indicated.
    \132\ As noted in section II.C.3.a.ii, the responsiveness of
asthmatics to O3 exposures could depend on factors that
have not been well-evaluated such as asthma severity, the
effectiveness of asthma control, or the prevalence of medication
use.
    \133\ Percent reductions in each urban study area were
calculated and averaged across areas.
    \134\ Estimates smaller than 0.05% were rounded to zero.
    \135\ As discussed in section 4.3.3 of the HREA (U.S. EPA,
2014a), the model-based air quality adjustment approach used to
estimate risks associated with the current and alternative standards
was unable to estimate the distribution of ambient O3
concentrations in New York City upon just meeting an alternative
standard with a level of 60 ppb. Therefore, for the 60 ppb standard
level the numbers of children and asthmatic children reflect all of
the urban study areas except New York.

  Table 4--Summary of Estimated Exposures of Concern for Potential Alternative O3 Standard Levels of 70, 65, 60
                                       ppb in Urban Case Study Areas \129\
----------------------------------------------------------------------------------------------------------------
                                                            Number of children
                             Alternative      Average %       (5 to 18 years)       Average %      % Children--
     Benchmark level       standard level     children     [number of asthmatic  reduction from   worst year and
                                (ppb)       exposed \130\     children] \131\        current        worst area
                                                                   \132\         standard \133\
----------------------------------------------------------------------------------------------------------------
                                   One or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>=70 ppb.................              70         0.1-1.2  94,000 [10,000].....              73              3.2