60 to 70%, in a year with relatively higher O3 levels
(2002), there are estimated to be about 500,000 occurrences in all
school children across the entire 12 urban areas, and about 40,000
occurrences in asthmatic children across just 5 urban areas. As noted
above, it is clear that even when the current standard is met over a
three-year period, O3 levels in each year can vary
considerably, as evidenced by relatively large differences between risk
estimates based on 2002 to 2004 air quality. The Staff Paper expressed
the view that it was appropriate to consider this yearly variation in
O3 levels allowed by the current standard in judging the
extent to which impacts on members of at-risk groups in a year with
relatively higher O3 levels remains of concern from a public
health perspective.
    With regard to other O3-related health effects, as shown
in Tables 6-4 through 6-6 of the Staff Paper, the estimated risks of
respiratory symptom days in moderate to severe asthmatic children,
respiratory-related hospital admissions, and non-accidental and
cardiorespiratory mortality, respectively, are not reduced to as great
an extent by meeting the current standard as are lung function
decrements. For example, just meeting the current standard reduces the
estimated average incidence of chest tightness in moderate to severe
asthmatic children living in the Boston urban area by 11 to 15%, based
on 2002 and 2004 simulations, respectively, resulting in an estimated
incidence of about 23,000 to 31,000 per 100,000 children attributable
to O3 exposure (Table 6-4). Just meeting the current
standard is estimated to reduce the incidence of respiratory-related
hospital admissions in the New York City urban area by about 16 to 18%,
based on 2002 and 2004 simulations, respectively, resulting in an
estimated incidence per 100,000 population of 4.6 to 6.4, respectively
(Table 6-5). Across the 12 urban areas, the estimates of non-accidental
mortality incidence per 100,000 relevant population range from 0.4 to
2.6 (for 2002) and 0.5 to 1.5 (for 2004) (Table 6-6). Meeting the
current standard results in a reduction of the estimated incidence per
100,000 population to a range of 0.3 to 2.4 based on the 2002
simulation and a range of 0.3 to 1.2 based on the 2004 simulation.
Estimates for cardiorespiratory mortality show similar patterns.
    In considering the estimates of the proportion of population
affected and the number of occurrences of the health effects that are
included in the risk assessment, the Staff Paper notes that these
limited estimates are indicative of a much broader array of
O3-related health endpoints that are part of a ``pyramid of
effects'' that include various indicators of morbidity that could not
be included in the risk assessment (e.g., school absences, increased
medication use, emergency department visits) and which primarily affect
members of at-risk groups. While the Staff Paper had sufficient
information to estimate and consider the number of symptom days in
children with moderate to severe asthma, it recognized that there are
many other effects that may be associated with symptom days, such as
increased medication use, school and work absences, or visits to
doctors' offices, for which there was not sufficient information to
estimate risks but which are important to consider in assessing the
adequacy of the current standard. The same is true for more serious,
but less frequent effects. The Staff Paper estimated hospital
admissions, but there was not sufficient information to estimate
emergency department visits in a quantitative risk assessment.
Consideration of such unquantified risks in the Staff Paper reinforces
the Staff Paper conclusion that consideration should be given to
revising the standard so as to provide increased public health
protection, especially for at-risk groups such as people with asthma or
other lung diseases, as well as children and older adults, particularly
those active outdoors, and outdoor workers.
c. Summary
    Based on the available information and taking into account the
views of CASAC and public comments, the Staff Paper initially notes
that all parties commenting on the NAAQS review agree that the standard
should be at least as protective as the current standard, as no party
suggested it should be revised to provide less protection. The Staff
Paper concludes that the overall body of evidence clearly calls into
question the adequacy of the current standard in protecting at-risk
groups, notably including asthmatic children and other people with lung
disease, as well as all children and older adults, especially those
active outdoors, and outdoor workers,\48\ against an array of adverse
health effects that range from decreased lung function to serious
indicators of respiratory morbidity including emergency department
visits and hospital admissions for respiratory causes, nonaccidental
mortality, and possibly cardiovascular effects. The available
information provides strong support for consideration of an
O3 standard that would provide increased health protection
for these at-risk groups. The Staff Paper also concludes that risks
projected to remain upon meeting the current standard, based on the
exposure and risk estimates discussed above and in more detail in the
Staff Paper, are indicative of risks to at-risk groups that can be
judged to be important from a public health perspective, which
reinforces the Staff Paper conclusion that consideration should be
given to revising the level of the standard so as to provide increased

public health protection (EPA, 2007, section 6.3.6).
---------------------------------------------------------------------------

    \48\ In defining at-risk groups this way we are including both
groups with greater inherent sensitivity and those more likely to be
exposed.
---------------------------------------------------------------------------

3. CASAC Views
    In its letter to the Administrator, the CASAC O3 Panel,
with full endorsement of the chartered CASAC, unanimously concluded
that there is ``no scientific justification for retaining'' the current
primary O3 standard, and the current standard ``needs to be
substantially reduced to protect human health, particularly in
sensitive subpopulations'' (Henderson, 2006c, pp. 1-2). In its
rationale for this conclusion, the CASAC Panel concluded that ``new
evidence supports and build-upon key, health-related conclusions drawn
in the 1997 Ozone NAAQS review'' (id., p. 3). The Panel points to
studies discussed in chapter 3 and Appendix 3B of the Staff Paper in
noting that several new single-city studies and large multi-city
studies have provided more evidence for adverse health effects at
concentrations lower than the current standard, and that these
epidemiological studies are backed-up by evidence from controlled human
exposure studies. The Panel specifically noted evidence from the recent
Adams (2006) study that reported statistically significant decrements
in the lung function of healthy, moderately exercising adults at a
0.080 ppm exposure level, and importantly, also reported adverse lung
function effects in some healthy individuals at 0.060 ppm. The Panel
concluded that these results indicate that the current standard ``is
not sufficiently health-protective with an adequate margin of safety,''
noting that that while similar studies in sensitive groups such as
asthmatics have yet to be conducted, ``people with asthma, and
particularly children, have been found to be more sensitive and to
experience larger decrements in lung function in response to
O3 exposures than would healthy volunteers (Mortimer et al.,
2002)'' (Henderson, 2006c, p. 4).
    The CASAC Panel also highlighted a number of O3-related
adverse health effects, that are associated with exposure to ambient
O3, below the level of the current standard, based on a
broad range of epidemiological studies (Henderson, 2006c). These
adverse health effects include increases in school absenteeism,
respiratory hospital emergency department visits among asthmatics and
patients with other respiratory diseases, hospitalizations for
respiratory illnesses, symptoms associated with adverse health effects
(including chest tightness and medication usage), and premature
mortality (nonaccidental, cardiorespiratory deaths) reported at
exposure levels well below the current standard. ``The CASAC considers
each of these findings to be an important indicator of adverse health
effects'' (Henderson, 2006c).
    The CASAC Panel expressed the view that more emphasis should be
placed on the subjects in controlled human exposure studies with
FEV1 decrements greater than 10 percent, which can be
clinically significant, rather than on the relatively small average
decrements. The Panel also emphasized significant O3-related
inflammatory responses and markers of injury to the epithelial lining
of the lung that are independent of spirometric responses. Further, the
Panel expressed the view that the Staff Paper did not place enough
emphasis on serious morbidity (e.g., hospital admissions) and mortality
observed in epidemiology studies. On the basis of the large amount of
recent data evaluating adverse health effects at levels at and below
the current O3 standard, it was the unanimous opinion of the
CASAC Panel that the current primary O3 standard is not
adequate to protect human health, that the relevant scientific data do
not support consideration of retaining the current standard, and that
the current standard needs to be substantially reduced to be protective
of human health, particularly in sensitive subpopulations (Henderson,
2006c, pp. 4-5).
    Further, the CASAC letter noted that ``there is no longer
significant scientific uncertainty regarding the CASAC's conclusion
that the current 8-hour primary NAAQS must be lowered'' (Henderson,
2006c, p. 5). The Panel noted that a ``large body of data clearly
demonstrates adverse human health effects at the current level'' of the
standard, such that ``[R]etaining this standard would continue to put
large numbers of individuals at risk for respiratory effects and/or
significant impact on quality of life including asthma exacerbations,
emergency room visits, hospital admissions and mortality'' (Henderson,
2006c). The Panel also noted that ``scientific uncertainty does exist
with regard to the lower level of O3 exposure that would be
fully protective of human health,'' concluding that ``it is possible
that there is no threshold for an O3-induced impact on human
health and that some adverse events may occur at policy-relevant
background'' (Henderson, 2006c, p.5).
4. Administrator's Proposed Conclusions Concerning Adequacy of Current
Standard
    Based on the large body of evidence concerning the public health
impacts of O3 pollution, including significant new evidence
concerning effects at O3 concentrations below the level of
the current standard, the Administrator proposes that the current
standard does not protect public health with an adequate margin of
safety and should be revised to provide additional public health
protection. In considering whether the primary standard should be
revised, the Administrator has carefully considered the conclusions
contained in the Criteria Document, the rationale and recommendations
contained in the Staff Paper, the advice and recommendations from the
CASAC, and public comments to date. The Administrator notes that
evidence of a range of respiratory-related morbidity effects seen in
the last review has been considerably strengthened, both through
toxicological and controlled human exposure studies as well as through
many new panel and epidemiological studies.
    In addition, new evidence from controlled human exposure and
epidemiological studies identifies people with asthma as an important
susceptible population for which estimates of respiratory effects in
the general population likely underestimate the magnitude or importance
of these effects. New evidence about mechanisms of toxicity more
completely explains the biological plausibility of O3-
induced respiratory effects and is beginning to suggest mechanisms that
may link O3 exposure to cardiovascular effects. Further,
there is now relatively strong evidence for associations between
O3 and total nonaccidental and cardiopulmonary mortality,
even after adjustment for the influence of season and PM. Relative to
the information that was available to inform the Agency's 1997 decision
to set the current standard, the newly available evidence increases the
Administrator's confidence that respiratory morbidity effects such as
lung function decrements and respiratory symptoms are causally related
to O3 exposures, that indicators of respiratory morbidity
such as emergency department visits and hospital admissions are
causally related to O3 exposures, and that the evidence is
highly suggestive that O3 exposures during the O3
season contribute to premature mortality.
    The Administrator judges that there is important new evidence
demonstrating that exposures to O3 at levels below the level
of the current standard are associated with a broad array of adverse
health effects, especially in at-risk populations. These at-risk
populations include people with asthma or other lung diseases who are
likely to experience more serious effects from

exposure to O3. As discussed in section II.A.4 above, these
groups also include children and older adults with increased
susceptibility, as well as those who are likely to be vulnerable as a
result of spending a lot of time outdoors engaged in physical activity,
especially active children and outdoor workers.
    Examples of this important new evidence include demonstration of
O3-induced lung function effects and respiratory symptoms in
some healthy individuals down to the previously observed exposure level
of 0.080 ppm, as well as very limited new evidence at exposure levels
well below the level of the current standard. In addition, there is now
epidemiological evidence of statistically significant O3-
related associations with lung function and respiratory symptom
effects, respiratory-related emergency department visits and hospital
admissions, and increased mortality, in areas that likely would have
met the current standard. There are also many epidemiological studies
done in areas that likely would not have met the current standard but
which nonetheless report statistically significant associations that
generally extend down to ambient O3 concentrations that are
below the level of the current standard. Further, there are a few
studies that have examined subsets of data that include only days with
ambient O3 concentrations below the level of the current
standard, or below even much lower O3 concentrations, and
continue to report statistically significant associations with
respiratory morbidity outcomes and mortality. The Administrator
recognizes that the evidence from controlled human exposure studies,
together with animal toxicological studies, provides considerable
support for the biological plausibility of the respiratory morbidity
associations observed in the epidemiological studies and for concluding
that the associations extend below the level of the current standard.
    Based on the strength of the currently available evidence of
adverse health effects, and on the extent to which the evidence
indicates that such effects result from exposures to ambient
O3 concentrations below the level of the current standard,
the Administrator judges that the current standard does not protect
public health with an adequate margin of safety and that the standard
should be revised to provide such protection, especially for at-risk
groups, against a broad array of adverse health effects.
    In reaching this judgment, the Administrator has also considered
the results of both the exposure and risk assessments conducted for
this review, to provide some perspective on the extent to which at-risk
groups would likely experience ``exposures of concern'' \49\ and on the
potential magnitude of the risk of experiencing various adverse health
effects when recent air quality data (from 2002 to 2004) are used to
simulate meeting the current standard and alternative standards in a
number of urban areas in the U.S.\50\ In considering the exposure
assessment results, the Administrator is relying on analyses that
define exposures of concern by three benchmark exposure levels: 0.080,
0.070, and 0.060 ppm. Estimates of exposures of concern in at-risk
groups at and above these benchmark levels, using O3 air
quality data in 2002 and 2004, provide some indication of the potential
magnitude of the incidence of health outcomes that cannot currently be
evaluated in a quantitative risk assessment, such as increased airway
responsiveness, increased pulmonary inflammation, including increased
cellular permeability, and decreased pulmonary defense mechanisms.
These physiological effects have been demonstrated to occur in healthy
people at O3 exposures as low as 0.080 ppm, the lowest level
tested. They are associated with aggravation of asthma, increased
medication use, increased school and work absences, increased
susceptibility to respiratory infection, increased visits to doctors'
offices and emergency departments, increased admissions to hospitals,
and possibly to cardiovascular system effects and chronic effects such
as chronic bronchitis or long-term damage to the lungs that can lead to
reduced quality of life.
---------------------------------------------------------------------------

    \49\ As discussed in section II.B.1.c above, ``exposures of
concern'' are estimates of personal exposures while at moderate or
greater exertion to 8-hour average ambient O3 levels at
and above specific benchmark levels which represent exposure levels
at which O3-related health effects are known or can with
varying degrees of certainty be inferred to occur in some
individuals. Estimates of exposures of concern provide some
perspective on the public health impacts of health effects that may
occur in some individuals at recent air quality levels but cannot be
evaluated in quantitative risk assessments, and the extent to which
such impacts might be reduced by meeting the current and alternative
standards.
    \50\ As described in the Staff Paper (section 4.5.8) and
discussed above, recent O3 air quality distributions have
been statistically adjusted to simulate just meeting the current and
selected alternative standards. These simulations do not represent
predictions of when, whether, or how areas might meet the specified
standards.
---------------------------------------------------------------------------

    In considering these various benchmark levels for exposures of
concern, the Administrator has focused primarily on estimated exposures
at and above the 0.070 ppm benchmark level as an important surrogate
measure for potentially more serious health effects in at-risk groups
such as people with asthma. This judgment is based on the strong
evidence of effects in healthy people at the 0.080 ppm exposure level
and the new evidence that people with asthma are likely to experience
larger and more serious effects than healthy people at the same level
of exposure. In the Administrator's view, this evidence does not
support a focus on exposures at and above the benchmark level of 0.080
ppm O3, as it would not adequately account for the increased
risk of harm from exposure for members of at-risk groups, especially
people with asthma. The Administrator also judges that the evidence of
demonstrated effects is too limited to support a primary focus on
exposures down to the lowest benchmark level considered of 0.060 ppm.
The Administrator particularly notes that although the analysis of
``exposures of concern'' was conducted to estimate exposures at and
above three discrete benchmark levels (0.080, 0.070, and 0.060 ppm),
the concept is appropriately viewed as a continuum. As discussed at the
outset in section II.A above, the Administrator strives to balance
concern about the potential for health effects and their severity with
the increasing uncertainty associated with our understanding of the
likelihood of such effects at lower O3 exposure levels.
    The Administrator observes that based on the aggregate exposure
estimates for the 2002 simulation summarized above in Table 1 (section
II.B.1) and in the Staff Paper (EPA, 2007b, Table 6-7) for the 12 U.S.
urban areas included in the exposure analysis, upon just meeting the
current standard up to about 20 percent of asthmatic or all school age
children are likely to experience one or more exposures of concern at
and above the 0.070 ppm benchmark level; the 2004 simulation yielded an
estimate of about 1 percent of such children. The Administrator notes
from this comparison that there is substantial year-to-year
variability, ranging up to an order of magnitude or more in estimates
of the number of people and the number of occurrences of exposures of
concern at and above this benchmark level. Moreover, within any given
year, the exposure assessment indicates that there is substantial city-
to-city variability in the estimates of the children exposed or the
number of occurrences of exposure at and above this benchmark level.
For example, city-specific estimates of the percent of asthmatic or all
school age children likely to experience exposures at and above the
benchmark level of 0.070 ppm

ranges from about 1 percent up to about 40 percent across the 12 urban
areas upon just meeting the current standard based on the 2002
simulation; the 2004 simulation yielded estimates that range from about
0 up to about 7 percent. The Administrator judges it is important to
recognize the substantial year-to-year and city-to-city variability in
considering these estimates.
    With regard to the results of the risk assessment, as discussed
above, the Administrator recognizes that a simulation of just meeting
the current standard in the cities included in the assessment indicate
that the estimated risk is lower for all of the health endpoints
evaluated. In considering the adequacy of the current standard, the
Administrator has focused on the risks estimated to remain upon just
meeting the current standard. Based on the aggregate risk estimates
summarized above in Table 2 (section II.B.2 of this notice), the
Administrator observes that upon just meeting the current standard
based on the 2002 simulation, approximately 8 percent of asthmatic
school age children across 5 urban areas (ranging up to about 11
percent in the city that receives relatively less protection) and
approximately 3 percent of all school age children across 12 urban
areas (ranging up to over 5 percent in the city that receives
relatively less protection) would still be estimated to experience
moderate or greater lung function decrements one or more times within
an O3 season. The Administrator recognizes that, as with the
estimates of exposures of concern, there is substantial year-to-year
and city-to-city variability in these risk estimates.
    In addition to the percentage of asthmatic or all children
estimated to experience 1 or more occurrences of an effect, the
Administrator recognizes that some individuals are estimated to have
multiple occurrences. For example, across all the cities in the
assessment, approximately 6 to 7 occurrences of moderate or greater
lung function decrements per child are estimated to occur in all
children and approximately 8 to 10 occurrences are estimated to occur
in asthmatic children in an O3 season, even upon just
meeting the current standard. In the last review, a general consensus
view of the adversity of such responses emerged as the frequency of
occurrences increases, with the judgment that repeated occurrences of
moderate responses, even in otherwise healthy individuals, may be
considered adverse since they may well set the stage for more serious
illness. The Administrator continues to support this view.
    Large lung function decrements (i.e., >=20 percent FEV1
decrement) would likely interfere with normal activities in many
healthy individuals, therefore single occurrences would be considered
to be adverse. In people with asthma, large lung function responses
(i.e., >= 20 percent FEV1 decrement), would likely interfere
with normal activities for most individuals and would also increase the
likelihood that these individuals would use additional medication or
seek medical treatment. Not only would single occurrences be considered
to be adverse to asthmatic individuals under the ATS definition, but
they also would be cause for medical concern for some individuals. Upon
just meeting the current standard based on the 2002 simulation, close
to 1 percent of asthmatic and all school age children are estimated to
experience one or more occurrences of large lung function decrements in
the aggregate across 5 and 12 urban areas, respectively, with close to
2 percent of both asthmatic and all school age children estimated to
experience such effects in the city that receives relatively less
protection from this standard. These estimates translate into
approximately 500,000 occurrences of large lung function decrements in
all children across 12 urban areas, and about 40,000 occurrences in
asthmatic children across just 5 urban areas upon just meeting the
current standard based on the 2002 simulation; the 2004 simulation
yielded estimates that translate into approximately 160,000 and 10,000
such occurrences in all children and asthmatic children, respectively.
    Upon just meeting the current standard based on the 2002
simulation, the estimate of the O3-related risk of
respiratory symptom days in moderate to severe asthmatic children in
the Boston area is about 8,000 symptom days; the 2004 simulation
yielded an estimate of about 6,000 such symptoms days. These estimates
translate into as many as one symptom day in 6, and one symptom day in
8, respectively, that are attributable to O3 exposure during
the O3 season of the total number of symptom days associated
with all causes of respiratory symptoms in asthmatic children during
those years.
    The estimated O3-related risk of respiratory-related
hospital admissions upon just meeting the current standard based on the
2002 simulation is greater than 500 hospital admissions in the New York
City area alone, or about 1.5 percent of the total incidence of
respiratory-related admissions associated with all causes; the 2004
simulation yielded an estimate of approximately 400 such hospital
admissions. For nonaccidental mortality, just meeting the current
standard based on the 2002 simulation results in an estimated incidence
of from 0.3 to 2.4 per 100,000 population; the 2004 simulation resulted
in an estimated incidence of from 0.3 to 1.2 per 100,000 population.
Estimates for cardiorespiratory mortality show similar patterns. (Abt
Associates, 2007, Table 4-26).
    The Administrator recognizes that in considering the estimates of
the proportion of population affected and the number of occurrences of
those specific health effects that are included in the risk assessment,
these limited estimates based on 2002 and 2004 simulations are
indicative of a much broader array of O3-related health
endpoints that are part of a ``pyramid of effects'' (discussed above in
section II.A.4.d) that include various indicators of morbidity that
could not be included in the risk assessment (e.g., school absences,
increased medication use, emergency department visits) and which
primarily affect members of at-risk groups. Moreover, the Administrator
notes that the CASAC Panel supported a qualitative consideration of the
much broader array of O3-related health endpoints, and
specifically referred to respiratory emergency department visits in
asthmatics and people with other lung diseases, increased medication
use, and increased respiratory symptoms reported at exposure levels
well below the current standard.
    The Administrator believes the exposure and risk estimates
discussed in the Staff Paper and summarized above are important from a
public health perspective and are indicative of potential exposures and
risks to at-risk groups. In reaching this proposed judgment, the
Administrator considered the following factors: (1) The estimates of
numbers of persons exposed at and above the 0.070 ppm benchmark level;
(2) the risk estimates of the proportion of the population and number
of occurrences of various health effects in areas upon just meeting the
current standard; (3) the year-to-year and city-to-city variability in
both the exposure and risk estimates; (4) the uncertainties in these
estimates; and (5) recognition that there is a broader array of
O3-related adverse health outcomes for which risk estimates
could not be quantified (that are part of a broader ``pyramid of
effects'') and that the scope of the assessment was limited to just a
sample of urban areas and to some but not all at-risk populations,
leading to an incomplete estimation of public health impacts associated
with O3 exposures

across the country. The Administrator also notes that it was the
unanimous conclusion of the CASAC Panel that there is no scientific
justification for retaining the current primary O3 standard,
that the current standard is not sufficiently health-protective with an
adequate margin of safety, and that the standard needs to be
substantially reduced to protect human health, particularly in at-risk
subpopulations.
    Based on all of these considerations, the Administrator proposes
that the current O3 standard is not requisite to protect
public health with an adequate margin of safety because it does not
provide sufficient protection and that revision would result in
increased public health protection, especially for members of at-risk
groups.

D. Conclusions on the Elements of the Primary Standard

1. Indicator
    In the last review EPA focused on a standard for O3 as
the most appropriate surrogate for ambient photochemical oxidants. In
this review, while the complex atmospheric chemistry in which
O3 plays a key role has been highlighted, no alternative to
O3 has been advanced as being a more appropriate surrogate
for ambient photochemical oxidants.
    The Staff Paper (section 2.2.2) notes that it is generally
recognized that control of ambient O3 levels provides the
best means of controlling photochemical oxidants. Among the
photochemical oxidants, the acute exposure chamber, panel, and field
epidemiological human health database provides specific evidence for
O3 at levels commonly reported in the ambient air, in part
because few other photochemical oxidants are routinely measured.
However, recent investigations on copollutant interactions have used
simulated urban photochemical oxidant mixes. These investigations
suggest the need for similar studies to help in understanding the
biological basis for effects observed in epidemiological studies that
are associated with air pollutant mixtures, where O3 is used
as the surrogate for the mix of photochemical oxidants. Meeting the
O3 standard can be expected to provide some degree of
protection against potential health effects that may be independently
associated with other photochemical oxidants but which are not
discernable from currently available studies indexed by O3
alone. 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.
    The Staff Paper notes that while the new body of time-series
epidemiological evidence cannot resolve questions about the relative
contribution of other photochemical oxidant species to the range of
morbidity and mortality effects associated with O3 in these
types of studies, control of ambient O3 levels is generally
understood to provide the best means of controlling photochemical
oxidants in general, and thus of protecting against effects that may be
associated with individual species and/or the broader mix of
photochemical oxidants, independent of effects specifically related to
O3.
    In its letter to the Administrator, the CASAC O3 Panel
noted that O3 is ``the key indicator of the extent of
oxidative chemistry and serves to integrate multiple pollutants.''
CASAC also stated that ``although O3 itself has direct
effects on human health and ecosystems, it can also be considered as an
indicator of the mixture of photochemical oxidants and of the oxidizing
potency of the atmosphere'' (Henderson, 2006c, p. 9).
    Based on the available information, and consistent with the views
of EPA staff and the CASAC, the Administrator proposes to continue to
use O3 as the indicator for a standard that is intended to
address effects associated with exposure to O3, alone or in
combination with related photochemical oxidants. In so doing, the
Administrator recognizes that measures leading to reductions in
population exposures to O3 will also reduce exposures to
other photochemical oxidants.
2. Averaging Time
a. Short-Term and Prolonged (1 to 8 Hours)
    The current 8-hour averaging time for the primary O3
NAAQS was set in 1997. At that time, the decision to revise the
averaging time of the primary standard from 1 to 8 hours was supported
by the following key observations and conclusions:
    (1) The 1-hour averaging time of the previous NAAQS was originally
selected primarily on the basis of health effects associated with
short-term (i.e., 1- to 3-hour) exposures.
    (2) Substantial health effects information was available for the
1997 review that demonstrated associations between a wide range of
health effects (e.g., moderate to large lung function decrements,
moderate to severe symptoms and pulmonary inflammation) and prolonged
(i.e., 6- to 8-hour) exposures below the level of the then current 1-
hour NAAQS.
    (3) Results of the quantitative risk analyses showed that
reductions in risks from both short-term and prolonged exposures could
be achieved through a primary standard with an averaging period of
either 1 or 8 hours. Thus establishing both a 1-hour and an 8-hour
standard would not be necessary to reduce risks associated with the
full range of observed health effects.
    (4) The 8-hour averaging time is more directly associated with
health effects of concern at lower O3 concentrations than
the 1-hour averaging time. It was thus the consensus of CASAC ``that an
8-hour standard was more appropriate for a human health-based standard
than a 1-hour standard.'' (Wolff, 1995)
    (5) An 8-hour averaging results in a significantly more uniformly
protective national standard than the then current 1-hour standard.
    (6) An 8-hour averaging time effectively limits both 1- and 8-hour
exposures of concern.
    In looking at the new information that is discussed in section
7.6.2 of the current Criteria Document, the Staff Paper noted that
epidemiological studies have used various averaging periods for
O3 concentrations, most commonly 1-hour, 8-hour and 24-hour
averages. As described more specifically in sections 3.3 and 3.4 of the
Staff Paper, in general the results presented from U.S. and Canadian
studies show no consistent difference for various averaging times in
different studies. Because the 8-hour averaging time continues to be
more directly associated with health effects of concern from controlled
human exposure studies at lower concentrations than do shorter
averaging periods, the Staff Paper did not evaluate alternative
averaging times in this review and did not conduct exposure or risk
assessments for standards with averaging times other than 8 hours.
    The Staff Paper discusses an analysis of a recent three-year period
of air quality data (2002 to 2004) which was conducted to determine
whether the comparative 1- and 8-hour air quality patterns that were
observed in the last review continue to be observed based on more
recent air quality data. This updated air quality analysis (McCluney,
2007) is very consistent with the analysis done in the last review in
that it indicates that only two urban areas of the U.S. have such
``peaky'' air quality patterns such that the ratio of 1-hour to 8-hour
design values is greater than 1.5. This suggests that, based on recent
air quality data, it is reasonable to again conclude that an 8-hour
average

standard at or below the current level would generally be expected to
provide protection equal to or greater than the previous 1-hour
standard of 0.12 ppm in almost all urban areas. Thus, the Staff Paper
again concluded that setting a standard with an 8-hour averaging time
can effectively limit both 1- and 8-hour exposures of concern and is
appropriate to provide adequate and more uniform protection of public
health from both short-term and prolonged exposures to O3 in
the ambient air.
    In its letter to the Administrator, the CASAC O3 Panel
supported the continued use of an 8-hour averaging time for the primary
O3 standard (Henderson, 2006c, p. 2), as did many
commenters. Some other commenters expressed the view that consideration
should be given to setting or reinstating a 1-hour standard, in
addition to maintaining the use of an 8-hour averaging time, to protect
people in those parts of the country with relatively more ``peaky''
exposure profiles. These commenters point out that when controlled
exposure studies using triangular exposure patterns (with relatively
higher 1-hour peaks) have been compared to constant exposure patterns
with the same aggregate O3 dose (in terms of concentration x
time), ``peaky'' exposure patterns are seen to lead to higher risks.
The California Air Resources Board made particular note of this point,
expressing the view that a 1-hour standard would more closely represent
actual exposures, in that many people spend only 1 to 2 hours a day
outdoors, and that it would be better matched to O3
concentration profiles along the coasts where O3 levels are
typically high for shorter averaging periods than 8 hours.
b. Long-term
    During the last review, there was a large animal toxicological
database for consideration that provided clear evidence of associations
between long-term (e.g., from several months to years) exposures and
lung tissue damage, with additional evidence of reduced lung elasticity
and accelerated loss of lung function. However, there was no
corresponding evidence for humans, and the state of the science had not
progressed sufficiently to allow quantitative extrapolation of the
animal study findings to humans. For these reasons, consideration of a
separate long-term primary O3 standard was not judged to be
appropriate at that time, recognizing that the 8-hour standard would
act to limit long-term exposures as well as short-term and prolonged
exposures.
    Taking into consideration the currently available evidence on long-
term O3 exposures, discussed above in section II.A.2.a.ii,
the Staff Paper concludes that a health-based standard with a longer-
term averaging time than 8 hours is not warranted at this time. The
Staff Paper notes that, while potentially more serious health effects
have been identified as being associated with longer-term exposure
studies of laboratory animals and in epidemiology studies, there
remains substantial uncertainty regarding how these data could be used
quantitatively to develop a basis for setting a long-term health
standard. Because long-term air quality patterns would be improved in
areas coming into attainment with an 8-hour standard, the potential
risk of health effects associated with long-term exposures would be
reduced in any area meeting an 8-hour standard. Thus, the Staff Paper
did not recommend consideration of a long-term, health-based standard
at this time.
    In its final letter to the Administrator, the CASAC O3
Panel offered no views on the long-term exposure evidence, nor did it
suggest that consideration of a primary O3 standard with a
long-term averaging time was appropriate. In fact, the CASAC
O3 Panel agreed with the choice of an 8-hour averaging time
for the primary O3 NAAQS suggested by Agency staff
(Henderson, 2007). Similarly, no commenters expressed support for
considering such a long-term standard.
c. Administrator's Conclusions on Averaging Time
    In considering the information discussed above, CASAC views and
public comments, the Administrator concludes that a standard with an 8-
hour averaging time can effectively limit both 1- and 8-hour exposures
of concern and that an 8-hour averaging time is appropriate to provide
adequate and more uniform protection of public health from both short-
term (1- to 3-hour) and prolonged (6- to 8-hour) exposures to
O3 in the ambient air. This conclusion is based on the
observations summarized above, particularly: (1) The fact that the 8-
hour averaging time is more directly associated with health effects of
concern at lower O3 concentrations than are averaging times
of shorter duration and (2) results from quantitative risk analyses
showing that attaining an 8-hour standard reduces the risk of
experiencing health effects associated with both 8-hour and shorter
duration exposures. Furthermore, the Administrator observes that the
CASAC O3 Panel agreed with the choice of averaging time
(Henderson, 2007). Therefore, the Administrator proposes to retain the
8-hour averaging time and is not proposing a separate 1-hour standard.
The Administrator also concludes that a standard with a long-term
averaging time is not warranted at this time.
3. Form
    In 1997, the primary O3 NAAQS was changed from a ``1-
expected-exceedance'' form per year over three years \51\ to a
concentration-based statistic, specifically the 3-year average of the
annual fourth-highest daily maximum 8-hour concentrations. The
principal advantage of the concentration-based form is that it is more
directly related to the ambient O3 concentrations that are
associated with the health effects. With a concentration-based form,
days on which higher O3 concentrations occur would weigh
proportionally more than days with lower concentrations, since the
actual concentrations are used in determining whether the standard is
attained. That is, given that there is a continuum of effects
associated with exposures to varying levels of O3, 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.
---------------------------------------------------------------------------

    \51\ The 1-expected-exceedance form essentially requires that
the fourth-highest air quality value in 3 years, based on
adjustments for missing data, be less than or equal to the level of
the standard for the standard to be met at an air quality monitoring
site.
---------------------------------------------------------------------------

    During the 1997 review, consideration was given to a range of
alternative forms, including the second-, third-, fourth- and fifth-
highest daily maximum 8-hour concentrations in an O3 season,
recognizing that the public health risks associated with exposure to a
pollutant without a clear, discernable threshold can be appropriately
addressed through a standard that allows for multiple exceedances to
provide increased stability, but that also significantly limits the
number of days on which the level may be exceeded and the magnitude of
such exceedances. Consideration was given to setting a standard with a
form that would provide a margin of safety against possible, but
uncertain chronic effects, and would also provide greater stability to
ongoing control programs. The fourth-highest daily maximum was selected
because it was decided that the differences in the degree of protection
against potential chronic effects afforded by the alternatives within
the range were not well enough understood to use any such differences
as a basis for

choosing the most restrictive forms. On the other hand, the relatively
large percentage of sites that would experience O3 peaks
well above 0.080 ppm and the number of days on which the level of the
standard may be exceeded, even when attaining a fifth-highest 0.080 ppm
concentration-based standard, argued against choosing that form.
    As an initial matter, the Staff Paper considered whether it is
appropriate to continue to specify the level of the O3
standard to the nearest hundredth (two decimal places) ppm, or whether
the precision with which ambient O3 concentrations are
measured supports specifying the standard level to the thousandth ppm
(i.e., to the part per billion (ppb)). The Staff Paper discusses an
analysis conducted by EPA staff to determine the impact of ambient
O3 measurement error on calculated 8-hour average
O3 design value concentrations, which are compared to the
level of the standard to determine whether the standard is attained
(Cox and Camalier, 2006). The results of this analysis suggest that
instrument measurement error, or possible instrument bias, contribute
very little to the uncertainty in design values. More specifically,
measurement imprecision was determined to contribute less than 1 ppb to
design value uncertainty, and a simulation study indicated that
randomly occurring instrument bias could contribute approximately 1
ppb. EPA staff interpreted this analysis as being supportive of
specifying the level of the standard to the nearest thousandth ppm. If
the current standard were to be specified to this degree of precision,
the current standard would effectively be at a level of 0.084 ppm,
reflecting the data rounding conventions that are part of the
definition of the current 0.080 ppm 8-hour standard. This information
was provided to the CASAC O3 Panel and made available to the
public.
    In evaluating alternative forms for the primary standard in
conjunction with specific standard levels, the Staff Paper considered
the adequacy of the public health protection provided by the
combination of the level and form to be the foremost consideration. In
addition, the Staff Paper recognized that it is important to have a
form of the standard that is stable and insulated from the impacts of
extreme meteorological events that are conducive to O3
formation. Such instability can have the effect of reducing public
health protection, because frequent shifting in and out of attainment
due to meteorological conditions can disrupt an area's ongoing
implementation plans and associated control programs. Providing more
stability is one of the reasons that EPA moved to a concentration-based
form in 1997.
    The Staff Paper considered two concentration-based forms of the
standard: the nth-highest maximum concentration and a percentile-based
form. A percentile-based 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, 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. An nth-highest maximum concentration form would more
effectively ensure that people who live in areas with different length
O3 seasons receive the same degree of public health
protection. For this reason, the exposure and risk analyses were based
on a form specified in terms of an nth-highest concentration, with n
ranging from 3 to 5.
    The results of some of these analyses are shown in the Staff Paper
(Figures 6-1 through 6-4) and specifically discussed in chapter 6.
These figures illustrate the estimated percent change in risk estimates
for the incidence of moderate or greater decrements in lung function
(>=15 percent FEV1) in all school age children and moderate
or greater lung function decrements (>=10 percent FEV1) in
asthmatic school age children, associated with going from meeting the
current standard to meeting alternative standards with alternative
forms based on the 2002 and 2004 simulations. Figures 6-5 and 6-6
illustrate the estimated percent of change in the estimated incidence
of non-accidental mortality, associated with going from meeting the
current standard to meeting alternative standards, based on the 2002
and 2004 simulations. These results are generally representative of the
patterns found in all of the analyses. The estimated reductions in risk
associated with different forms of the standard, ranging from third- to
fourth-highest daily maximum concentrations at 0.084 ppm, and from
third- to fifth-highest daily maximum concentrations at 0.074 ppm, are
generally less than the estimated reductions associated with the
different levels that were analyzed. As seen in these figures, there is
much city-to-city variability, particularly in the percent changes
associated with going from a fourth-highest to third-highest form at
the current level of 0.084 ppm, and with estimated reductions
associated with the fifth-highest form at a 0.074 ppm level. In most
cities, there are generally only small differences in the estimated
reductions in risks associated with the third- to fifth-highest forms
at a level of 0.074 ppm simulated using 2002 and 2004 O3
monitoring data.
    The Staff Paper noted that there is not a clear health-based
threshold for selecting a particular nth-highest daily maximum form of
the standard from among the ones analyzed. It also noted that the
changes in the form considered in the analyses result in only small
differences in the estimated reductions in risks in most cities,
although in some cities larger differences are estimated. The Staff
Paper concluded that a range of concentration-based forms from the
third- to the fifth-highest daily maximum 8-hour average concentration
is appropriate for consideration in setting the standard. Given that
there is a continuum of effects associated with exposures to varying
levels of O3, 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. The principal advantage of a concentration-
based form is that it is more directly related to the ambient
O3 concentrations that are associated with health effects.
Robust, concentration-based forms, in the range of the third- to fifth-
highest daily maximum 8-hour average concentration, including the
current 4th-highest daily maximum form, minimize the inherent lack of
year-to-year stability of exceedance-based forms and provide insulation
from the impacts of extreme meteorological events. Such instability can
have the effect of reducing public health protection by disrupting
ongoing implementation plans and associated control programs.
    With regard to the precision of the standard, in their letter to
the Administrator, the CASAC concluded that current monitoring
technology ``allows accurate measurement of O3
concentrations with a precision of parts per billion'' (Henderson,
2006c). The CASAC recommended that the specification of the level of
the O3 standard should reflect this degree of precision
(Henderson, 2006c). Some public comments supported specifying the
standard in terms of parts per billion, or to three decimal places if
specified in terms of parts per million.\52\ Other public commenters
stated that the

basis for changing the current rounding procedures is not supported by
a complete analysis of the O3 compliance monitoring
procedures, including consideration of uncertainty related to humidity
effects and interferences from aromatic compounds in the monitoring of
O3 levels.
---------------------------------------------------------------------------

    \52\ The Staff Paper notes that the 8-hour O3
standard adopted by the State of California in 2006 is specified to
the nearest thousandth part per million (at a level of 0.070 ppm)
(http://www.arb.ca.gov/research/aaqs/ozone-rs/ozone-rs.htm).

---------------------------------------------------------------------------

    With regard to the form of the standard, in their letter to the
Administrator, CASAC recommended that ``a range of concentration-based
forms from the third-to the fifth-highest daily maximum 8-hour average
concentration'' be considered (Henderson, 2006c, p. 5). Some public
commenters that expressed the view that the current primary
O3 standard is not adequate also submitted comments that
supported a more health-protective form of the standard than the
current form (e.g., a second-or third-highest daily maximum form).
Commenters who expressed the view that the current standard is adequate
did not provide any views on alternative forms that would be
appropriate for consideration should the Administrator consider
revisions to the standard.
    The Administrator proposes that the level of the standard be
specified to the nearest thousandth ppm, based on the staff's analysis
and conclusions discussed in the Staff Paper that current monitoring
technology allows accurate measurement of O3 to support
specifying the 8-hour standard to this degree of precision, and on
CASAC's recommendation with respect to this aspect of the standard. The
Administrator invites comment on this proposal to specify the standard
to the thousandth ppm.
    The Administrator recognizes that there is not a clear health-based
threshold for selecting a particular nth-highest daily maximum form of
the standard from among the ones analyzed in the Staff Paper, and that
the current form of the standard provides a stable target for
implementing programs to improve air quality. The Administrator also
agrees that the adequacy of the public health protection provided by
the combination of the level and form is a foremost consideration.
Based on this, the Administrator proposes to retain the form of the
current standard, 4th-highest daily maximum 8-hour average
concentration, recognizing that the public health protection that would
be provided by the standard is based on combining this form with the
level discussed below. Mindful of the recommendation of the
O3 CASAC Panel and the view expressed by commenters, the
Administrator also invites comment on two alternative forms of the
standard, the third- and the fifth-highest daily maximum 8-hour average
concentrations.
4. Level
a. Evidence and Exposure/Risk Based Considerations in the Staff Paper
    The approach used in the Staff Paper as a basis for staff
recommendations on standard levels builds upon and broadens the general
approach used by EPA in the last review. This approach reflects the
more extensive and stronger body of evidence now available on a broader
range of health effects associated with exposure to O3,
including: (1) Additional respiratory-related endpoints; (2) new
information about the mechanisms underlying respiratory morbidity
effects supporting a judgment that the link between O3
exposure and these effects is causal; (3) newly identified
cardiovascular-related health endpoints from animal toxicology, and
controlled human exposures studies that are highly suggestive that
O3 can directly or indirectly contribute to cardiovascular
morbidity, and (4) new U.S. multi-city time series studies, single city
studies, and several meta-analyses of these studies that provide
relatively strong evidence for associations between short-term
O3 exposures and all-cause (nonaccidental) mortality, at
levels below the current primary standard: as well as (5) evidence of
increased susceptibility in people with asthma and other lung diseases.
In evaluating evidence-based and exposure/risk-based considerations,
the Staff Paper considered: (1) The ranges of levels of alternative
standards that are supported by the evidence, and the uncertainties and
limitations in that evidence and (2) the extent to which specific
levels of alternative standards reduce the estimated exposures of
concern and risks attributable to O3 and other photochemical
oxidants, and the uncertainties associated with the estimated exposure
and risk reductions.
    In taking into account evidence-based considerations, the Staff
Paper evaluated available evidence from controlled human exposure
studies and epidemiological studies, as well as the uncertainties and
limitations in that evidence. In particular, it focused on the extent
to which controlled human exposure studies provide evidence of lowest-
observed-effects levels and the extent to which epidemiological studies
provide evidence of associations that extend down to the lower levels
of O3 concentrations observed in the studies or some
indication of potential effect thresholds in terms of 8-hour average
O3 concentrations.
    In considering the available controlled human exposure studies, as
discussed above in section II.A.2.a.i(a)(i), two new studies are
notable in that they are the only controlled human exposure studies
that examined respiratory effects, including lung function decrements
and respiratory symptoms, in healthy adults at lower exposure levels
than had previously been examined. EPA's reanalysis of the data from
the most recent study shows small group mean decrements in lung
function responses to be statistically significant at the 0.060 ppm
exposure level, while the author's analysis did not yield statistically
significant lung function responses (but did yield some statistically
significant respiratory symptom responses toward the end of the
exposure period). Notably, these studies report a small percentage of
subjects experiencing lung function decrements (> 10 percent) at the
0.060 ppm exposure level. These studies provide very limited evidence
of O3-related lung function decrements and respiratory
symptoms at this lower exposure level.
    In considering controlled human exposure studies of pulmonary
inflammation, airway responsiveness, and impaired host defense
capabilities, the Staff Paper notes that these studies provide evidence
of a lowest-observed-effects level for such effects in healthy adults
at prolonged moderate exertion of 0.080 ppm. As discussed above, 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. Further, pulmonary inflammation is
related to increased cellular permeability in the lung, which may be a
mechanism by which O3 exposure can lead to cardiovascular
system effects, and to potential chronic effects such as chronic
bronchitis or long-term damage to the lungs that can lead to reduced
quality of life. These are all indicators of adverse O3-
related morbidity effects, which are consistent with and lend
plausibility to the adverse morbidity effects and mortality effects
observed in epidemiological studies.
    In considering epidemiological studies, the Staff Paper first
recognizes that the available evidence neither supports nor refutes the
existence of effect thresholds at the population level for morbidity
and mortality effects and that if a population threshold level does
exist, it would likely be well below the level of the current
O3 standard and

possibly within the range of background levels. As discussed above (and
more fully in the Staff Paper in chapter 3 and the Criteria Document in
chapter 7), a number of studies reported some suggestive evidence of
possible thresholds for morbidity and mortality outcomes in terms of
24-hour, 8-hour, and 1-hour averaging times. These results, taken
together, provide some indication of possible 8-hour average threshold
levels from below about 0.025 to 0.035 ppm up to approximately 0.050
ppm. Other studies, however, observe linear concentration-response
functions suggesting no effect threshold. The Staff Paper concludes
that the statistically significant associations between ambient
O3 concentrations and lung function decrements, respiratory
symptoms, indicators of respiratory morbidity including increased
emergency department visits and hospitals admissions, and possibly
mortality reported in a large number of studies likely extend down to
ambient O3 concentrations that are well below the level of
the current standard (EPA, 2007, p. 6-60). Toward the lower end of the
range of O3 concentrations observed in such studies, ranging
down to background levels, however, the Staff paper states that 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
atmosphere.
    The Staff Paper also considered studies that did subset analyses
that include only days with ambient O3 concentrations below
the level of the current standard, or below even lower O3
concentrations, and continue to report statistically significant
associations. Notably, as discussed above, Bell et al. (2006) conducted
a subset analysis that continued to show statistically significant
associations even when only days with a maximum 8-hour average
O3 concentration below a value of approximately 0.061 ppm
were included.\53\ Also of note is the large multi-city NCICAS
(Mortimer et al., 2002) that reported statistically significant
associations between ambient O3 concentrations and lung
function decrements even when days with 8-hour average O3
levels greater than 0.080 ppm were excluded (which consisted of less
than 5 percent of the days in the eight urban areas in the study).
---------------------------------------------------------------------------

    \53\ Bell et al. (2006) referred to this level as being
approximately equivalent to 120 [mu]g/m3, daily 8-hour
maximum, the World Health Organization guideline and European
Commission target value for O3.
---------------------------------------------------------------------------

    Being mindful of the uncertainties and limitations inherent in
interpreting the available evidence, the Staff Paper states the view
that the range of alternative O3 standards for consideration
should take into account information on lowest-observed-effects levels
in controlled human exposure studies as well as indications of possible
effects thresholds reported in some epidemiological studies and
questions of biological plausibility in attributing associations
observed down to background levels to O3 exposures alone.
Based on the evidence and these considerations, it concluded that the
upper end of the range of consideration should be somewhat below 0.080
ppm, the lowest-observed-effects level for effects such as pulmonary
inflammation, increased airway responsiveness and impaired host-defense
capabilities in healthy adults while at prolonged moderate exertion.
The Staff Paper also concludes that the lower end to the range of
alternative O3 standards appropriate for consideration
should be the lowest-observed-effects level for potentially adverse
lung function decrements and respiratory symptoms in some healthy
adults, 0.060 ppm.
    In addition to the evidence-based considerations informing staff
recommendations on alternative levels, the Staff Paper also evaluated
quantitative exposures and health risks estimated to occur upon meeting
the current and alternative standards.\54\ In so doing, it presented
the important uncertainties and limitations associated with these
exposure and risk assessments. For example, the Staff Paper noted
important uncertainties affecting the exposure estimates are related to
modeling human activity patterns over an O3 season
(especially repetitive exposures), modeling ambient concentrations near
roadways and modeling building air exchange rates which impact
estimates of indoor O3 concentrations. With regard to the
risk assessment, important uncertainties include, for example, those
related to exposure estimates for children engaged in moderate or
greater exertion, as well as those related to estimation of
concentration-response functions, specification of concentration-
response models, the possible role of copollutants in interpreting
reported associations with O3, and inferences of a likely
causal relationship between O3 exposure and nonaccidental
mortality (for risk estimates based on epidemiological studies).
---------------------------------------------------------------------------

    \54\ As described in the Staff Paper (section 4.5.8) and
discussed above, recent O3 air quality distributions have
been statistically adjusted to simulate just meeting the current and
selected alternative standards. These simulations do not represent
predictions of when, whether, or how areas might meet the specified
standards. Modeling that projects whether and how areas might attain
alternative standards in a future year is presented in the
Regulatory Impact Analysis being prepared in connection with this
rulemaking.
---------------------------------------------------------------------------

    Beyond these uncertainties, the Staff Paper also recognized
important limitations to the exposure and risk analyses. For example,
the Staff Paper did not have sufficient information to evaluate all
relevant at-risk groups (e.g., outdoor workers) or all O3-
related health outcomes (e.g., increased medication use, school
absences, emergency department visits), and the scope of the Staff
Paper analyses was generally limited to estimating exposures and risks
in 12 urban areas across the U.S., and to only five or just one area
for some risk analyses. The Staff Paper notes that it is clear that
national-scale public health impacts of ambient O3 exposures
are much larger than the quantitative estimates of O3-
related incidences of adverse health effects and the numbers of
children likely to experience exposures of concern associated with
meeting the current or alternative standards. On the other hand, due to
individual variability in responsiveness, only a subset of individuals
who are estimated to experience exposures of concern at and above a
specific benchmark level can be expected to experience certain adverse
health effects, although susceptible subpopulations such as those with
asthma are expected to be affected more by such exposures than healthy
individuals. In taking these limitations into account, the Staff Paper
reflected CASAC's advice not to rely solely on the results of the
exposure and risk assessments in considering alternative standards, but
also to place significant weight on the body of evidence of
O3-related health effects in drawing conclusions about an
appropriate range of levels for consideration.
    The Staff Paper focused on alternative standards with the same form
as the current O3 standard (i.e. the 0.074/4, 0.070/4 and
0.064/4 scenarios).\55\ Having concluded in the Staff Paper that it was
appropriate to consider a range of standard levels from somewhat below
0.080 ppm down to as low as 0.060 ppm, the Staff Paper looked to
results of the analyses of exposure and risk for the 0.074/4 scenario
to represent the public health impacts of selecting a standard in

the upper part of the range, the results of analyses of the 0.070/4
scenario to represent the impacts in the middle part of the range, and
the results of the analyses of the 0.064/4 scenario to represent the
lower part of the range.
---------------------------------------------------------------------------

    \55\ The abbreviated notation used to identify the current and
alternative standards in this section and in the risk assessment
section of the Staff Paper is in terms of ppm and the nth highest
daily maximum 8-hour average. For example, the current standard is
identified as ``0.084/4.''
---------------------------------------------------------------------------

    As discussed in section II.B.1 of this notice, the exposure
estimates presented in the Staff Paper are for the number and percent
of all children and asthmatic children exposed, and the number of
person-days (occurrences) of exposures, with daily 8-hour maximum
exposures at or above several benchmark levels while at intermittent
moderate or greater exertion. For reasons discussed above in section
II.C.2, the Staff Paper focused on exposures of concern at the 0.070
and 0.060 ppm benchmark levels for the purpose of evaluating
alternative standard levels. As shown in the Table 1 in this notice,
the percent of population exposed at any given level is very similar
for all and asthmatic school age children. Substantial year-to-year
variability in exposure estimates is observed, ranging to over an order
of magnitude at the higher alternative standard levels, in estimates of
the number of children and the number of occurrences of exposures of
concern at both of these benchmark levels. The Staff Paper also notes
that there is substantial city-to-city variability in these estimates,
and notes that it is appropriate to consider not just the aggregate
estimates across all cities, but also to consider the public health
impacts in cities that receive relatively less protection from the
alternative standards.
    As discussed in the Staff Paper, a standard set at the upper part
of the range recommended by EPA staff (e.g., the 0.074/4 scenario)
would result in an aggregate estimate of about 4 percent of all or
asthmatic school age children likely to experience exposures of concern
at the >=0.070 ppm benchmark level based on the 2002 simulation, a year
with relatively high O3 levels, while the estimates range up
to 12 percent of all or asthmatic school age children in the single
city with the least degree of protection from this standard. Using the
2004 simulation, a year with relatively low O3 levels,
exposures of concern at this level are essentially eliminated. At the
benchmark level of >=0.060 ppm, in aggregate using the 2002 simulation
about 22 percent of all or asthmatic school age children are estimated
to experience exposures of concern; this estimate ranges up to about 46
percent of all or asthmatic school age children in the single city with
the least degree of protection from this standard. Using the 2004
simulation, exposures of concern at this level are estimated to be
substantially lower. A standard set at this level is estimated to
reduce the number of all and asthmatic school age children estimated to
experience one or more moderate lung function decrements by about 30 to
50 percent relative to the current standard, with city-to-city
differences accounting for most of the variability in estimates. A
standard set at this level is estimated to reduce non-accidental
mortality by about 10 to 40 percent, with most of the variability
occurring across the 12 city estimates.
    Using the 2002 simulation, a standard set at this level (the 0.074/
4 scenario) is estimated to reduce the incidence of symptom days in
children with moderate to severe asthma in the Boston area by about
1,000 days, a 15 percent reduction relative to the current standard.
With this reduction, it is estimated that about 1 respiratory symptom
day in 8 during the O3 season would be attributable to
O3 exposure. Estimated incidence of respiratory-related
hospital admissions was reduced by 14 to 17 percent by a standard set
at this level relative to the current standard, in the year with
relatively high and relatively low O3 air quality levels
respectively.
    The Staff Paper notes that a standard set at the middle part of the
staff-recommended range, as indicated by the estimates for the 0.070/4
scenario, would reduce the exposures of concern at the 0.070 ppm level
substantially over the current standard, resulting in an aggregate
estimate of about 1.5 to nearly 2 percent of all or asthmatic school
age children likely to experience exposures of concern even using the
2002 simulation, and leaving approximately 5 percent or less of
children likely to experience exposures of concern in the city with the
least degree of protection. Using the 2004 simulation, it essentially
eliminates exposures of concern at this level. It reduces exposures of
concern at the 0.060 ppm benchmark level less so, leaving larger
percentages of all school age children unprotected using the 2002
simulation (about 15 percent in aggregate) or in the city with the
least protection from this standard (about 33 percent). However, using
the 2004 simulation, it is estimated to reduce exposures of concern at
this benchmark level to approximately 5 percent or less of children
even in the city with the least degree of protection. It provides
considerable additional protection for members of at-risk groups, over
the current O3 standard, against respiratory morbidity
effects such as lung function decrements, respiratory symptom days and
hospital admissions, as well as non-accidental mortality.
    A standard set at lower part of the staff-recommended range (e.g.,
the 0.064/4 scenario), would result in an aggregate estimate of less
than 0.5 percent of all and asthmatic school age children likely to
experience exposures of concern at the 0.070 ppm benchmark level using
the 2002 simulation and only about 1 percent of all and asthmatic
school age children in the city with the least degree of protection
from this standard. At the benchmark level of 0.060 ppm, in aggregate
using the 2002 simulation about 5 percent of all and asthmatic school
age children are estimated to experience exposures of concern; this
number ranges up to 15 percent of all and asthmatic school age children
in the city with the least degree of protection from this standard. A
standard set at this level is estimated to reduce the number of all and
asthmatic school age children estimated to experience one or more
moderate lung function decrements by about 40 to 75 percent over the
current standard, and non-accidental mortality by about 25 to 75
percent, with most of the variability occurring across the 12 city
estimates.
    A standard set at the 0.064/4 scenario is estimated, based on the
2002 simulation, to reduce the incidence of symptom days in children
with moderate to severe asthma in the Boston area by about 1,900 days,
about a 25 to 30 percent reduction over the current standard. But even
with this reduction, it is estimated that 1 respiratory symptom day in
10 during the O3 season is attributable to O3
exposure. Estimated incidence of respiratory-related hospital
admissions would be reduced by 30 to 35 percent over the current
standard, a reduction of 125 to 150 hospital admissions in the New York
City area alone, using the 2002 and 2004 simulations, respectively.
b. CASAC Views
    As stated in its letter to the Administrator, ``the CASAC
unanimously recommends that the current primary ozone NAAQS be revised
and that the level that should be considered for the revised standard
be from 0.060 to 0.070 ppm'' (Henderson, 2006c, p. 5). This
recommendation follows from its more general recommendation, discussed
above, that the current standard of 0.084 ppm needs to be substantially
reduced to be protective of human health, particularly in at-risk
subpopulations. The CASAC Panel noted that beneficial reductions in
some adverse health effects were estimated to occur upon meeting the
lowest standard level (0.064 ppm)

considered in the risk assessment (Henderson, 2006c, p. 4). The lower
end of this range reflects CASAC's views that ``[W]hile data exist that
adverse health effects may occur at levels lower than 0.060 ppm, these
data are less certain and achievable gains in protecting human health
can be accomplished through lowering the ozone NAAQS to a level between
0.060 and 0.070 ppm.'' (id.). In a subsequent letter to the
Administrator, the CASAC reiterated that the Panel members ``were
unanimous in recommending that the level of the current primary ozone
standard should be lowered from 0.08 ppm to no greater than 0.070 ppm''
(Henderson, 2007, p. 2). Further, the CASAC Panel expressed the view
that the Criteria Document and Staff Paper, together with the
information in its earlier letter, provide ``overwhelming scientific
evidence for this recommendation,'' and emphasized the Clean Air Act
requirement that the primary standard must be set to protect the public
health with an adequate margin of safety (id.).
c. Administrator's Proposed Conclusions on Level
    For the reasons discussed below, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and the public
comments to date, the Administrator proposes to revise the existing 8-
hour primary O3 standard. Specifically, the Administrator
proposes to revise (1) The level of the primary O3 standard
to within a range from 0.070 to 0.075 ppm and (2) the degree of
precision to which the level of the standard is specified to the
thousandth ppm.
    However, in recognition of alternative views of the science, the
exposure and risk assessments and the uncertainties inherent in these
assessments, and the appropriate policy responses based on the
currently available information, the Administrator also solicits
comments on whether to proceed instead with: (1) Alternative levels of
the 8-hour primary O3 standard, within ranges of below 0.070
ppm down to 0.060 ppm and above 0.075 ppm up to and including retaining
the current standard; (2) alternative forms of the standard, including
the 3-year average of the annual third- and fifth-highest daily maximum
8-hour average O3 concentrations; and (3) retaining the
degree of precision of the current standard (to the nearest hundredth
ppm). Based on the comments received and the accompanying rationales,
the Administrator may adopt other standards within the range of the
alternative levels and forms identified above in lieu of the standards
he is proposing today.
    The Administrator's consideration of alternative levels of the
primary O3 standard builds on his proposal, discussed above,
that the overall body of evidence indicates that the current 8-hour
O3 standard is not requisite to protect public health with
an adequate margin of safety because it does not provide sufficient
protection and that revision would result in increased public health
protection, especially for members of at-risk groups, notably including
asthmatic children and other people with lung disease, as well as all
children and older adults, especially those active outdoors, and
outdoor workers, against an array of adverse health effects. These
effects range from health outcomes that could be quantified in the risk
assessment, including decreased lung function, respiratory symptoms,
serious indicators of respiratory morbidity such as hospital admissions
for respiratory causes, and nonaccidental mortality, to health outcomes
that could not be directly estimated, including pulmonary inflammation,
increased medication use, emergency department visits, and possibly
cardiovascular-related morbidity effects. In reaching a proposed
decision about the level of the O3 primary standard, the
Administrator has considered: the evidence-based considerations from
the Criteria Document and the Staff Paper; the results of the exposure
and risk assessments discussed above and in the Staff Paper, giving
weight to the exposure and risk assessments as judged appropriate;
CASAC advice and recommendations, as reflected in discussions of drafts
of the Criteria Document and Staff Paper at public meetings, in
separate written comments, and in CASAC's letters to the Administrator;
EPA staff recommendations; and public comments received during the
development of these documents, either in connection with CASAC
meetings or separately. In considering what 8-hour standard is
requisite to protect public health with an adequate margin of safety,
the Administrator is mindful that this choice requires judgment based
on an interpretation of the evidence and other information that neither
overstates nor understates the strength and limitations of the evidence
and information nor the appropriate inferences to be drawn.
    The Administrator notes that the most certain evidence of adverse
health effects from exposure to O3 comes from the clinical
studies, and that the large bulk of this evidence derives from studies
of exposures at levels of 0.080 and above. At those levels, there is
consistent evidence of lung function decrements and respiratory
symptoms in healthy young adults, as well as evidence of inflammation
and other medically significant airway responses. Moreover there is no
evidence that the 0.080 ppm level is a threshold for these effects.
Although the Administrator takes note of the very limited new evidence
of lung function decrements and respiratory symptoms in some healthy
individuals at the 0.060 ppm exposure level, he judges this evidence
too limited to support a primary focus at this level. The Administrator
also notes that clinical studies, supported by epidemiological studies,
provide important new evidence that people with asthma are likely to
experience larger and more serious effects than healthy people from
exposure to O3. There are also epidemiological studies that
provide evidence of statistically significant associations between
short-term O3 exposures and more serious health effects such
as emergency department visits and hospital admissions, and premature
mortality, in areas that likely would have met the current standard.
There are also many epidemiological studies done in areas that likely
would not have met the current standard but which nonetheless report
statistically significant associations that generally extend down to
ambient O3 concentrations that are below the level of the
current standard. Further, there are a few studies that have examined
subsets of data that include only days with ambient O3
concentrations below the level of the current standard, or below even
much lower O3 concentrations, and continue to report
statistically significant associations with respiratory morbidity
outcomes and mortality. In considering this evidence, the Administrator
notes that the extent to which these studies provide evidence of causal
relationships with exposures to O3 alone down to the lowest
levels observed remains uncertain. To further inform the interpretation
of this evidence, EPA seeks comment on the degree to which associations
observed in epidemiological studies reflect causal relationships
between important health endpoints and exposure to O3 alone
at ambient O3 levels below the current standard.
    Therefore, the Administrator judges that revising the current
standard to protect public health with an adequate margin of safety is
warranted, and

would reduce risk to public health, based on: (1) The strong body of
clinical evidence in healthy people at exposure levels of 0.080 and
above of lung function decrements, respiratory symptoms, pulmonary
inflammation, and other medically significant airway responses, as well
as some indication of lung function decrements and respiratory symptoms
at lower levels; (2) the substantial body of clinical and
epidemiological evidence indicating that people with asthma are likely
to experience larger and more serious effects than healthy people; and
(3) the body of epidemiological evidence indicating associations are
observed for a wide range of serious health effects, including
respiratory emergency department visits and hospital admissions, and
premature mortality, at and below 0.080 ppm. The Administrator also
judges that the estimates of exposures of concern and risks remaining
upon just meeting the current standard or a standard at the 0.080 ppm
level provide additional support for this view. For the same reasons,
and the reasons discussed above in section II.C on the adequacy of the
current standard, the Administrator judges that the standard should be
set below 0.080 ppm, a level at which the evidence provides a high
degree of certainty about the adverse effects of O3 exposure
even in healthy people.
    The Administrator next considered what standard level below 0.080
ppm would be requisite to protect public health with an adequate margin
of safety, that is sufficient but not more than necessary to achieve
that result, recognizing that such a standard would result in increased
public health protection. The assessment of a standard level calls for
consideration of both the degree of additional protection that
alternative levels of the standard might be expected to provide as well
as the certainty that any specific level will in fact provide such
protection. In the circumstances present in this review, there is no
evidence-based bright line that indicates a single appropriate level;
instead there is a combination of scientific evidence and other
information that needs to be considered holistically in making this
public health policy judgment, and selecting a standard level from a
range of reasonable values.
    The Administrator notes that at exposure levels below 0.080 ppm
there is only a very limited amount of evidence from clinical studies
indicating effects in some healthy individuals at levels as low as
0.060 ppm. The great majority of the evidence concerning effects below
0.080 ppm is from epidemiological studies. The epidemiological studies
do not identify any bright-line threshold level for effects. At the
same time, the epidemiological studies are not themselves direct
evidence of a causal link between exposure to O3 and the
occurrence of the effects. The Administrator considers these studies in
the context of all the other available evidence in evaluating the
degree of certainty that O3-related adverse health effects
would occur at various ambient levels below 0.080 ppm, including the
strong human clinical studies and the toxicological studies that
demonstrate the biological plausibility and mechanisms for the effects
of O3 on airway inflammation and increased airway
responsiveness at exposure levels of 0.080 ppm and above.
    Based on consideration of the entire body of evidence and
information available at this time, as well as the recommendations of
CASAC, the Administrator proposes that a standard within the range of
0.070 to 0.075 ppm would be requisite to protect public health with an
adequate margin of safety. A standard level within this range would
reduce the risk of a variety of health effects associated with exposure
to O3, including the respiratory symptoms and lung function
effects demonstrated in the clinical studies, and the emergency
department visits, hospital admissions and mortality effects indicated
in the epidemiological studies. All of these effects are indicative of
a much broader array of O3-related health endpoints, such as
school absences and increased medication use, that are plausibly linked
to these observed effects.
    The Administrator considered the degree of improvements in public
health that potentially could be achieved by a standard of 0.070 to
0.075 ppm, giving weight to the exposure and risk assessments as he
judged appropriate, as discussed below. In considering the results of
the exposure assessment, as discussed above (section II.C.4), the
Administrator has primarily focused on exposures at and above the 0.070
ppm benchmark level as an important surrogate measure for potentially
more serious health effects for at-risk groups, including people with
asthma. In so doing, the Administrator particularly notes that although
the analysis of ``exposures of concern'' was conducted to estimate
exposures at and above three discrete benchmark levels, the concept is
appropriately viewed as a continuum. As discussed above, the
Administrator strives to balance concern about the potential for health
effects and their severity with the increasing uncertainty associated
with our understanding of the likelihood of such effects at lower
O3 exposure levels. In focusing on this benchmark, the
Administrator notes that upon just meeting a standard within the range
of 0.070 to 0.075 ppm based on the 2002 simulation, the number of
school age children likely to experience exposures at and above this
benchmark level in aggregate (for the 12 cities in the assessment), is
estimated to be approximately 2 to 4 percent of all and asthmatic
children, and generally less than 10 percent of children even in cities
that receive the least degree of protection from such a standard in a
recent year with relatively high O3 levels. A standard
within the 0.070 to 0.075 ppm range would thus substantially reduce
exposures of concern by about 90 to 80 percent, respectively, from
those estimated to occur upon just meeting the current standard. While
placing less weight on the results of the risk assessment, in light of
the important uncertainties inherent in the assessment, the
Administrator notes that the results indicate that a standard set
within this range would likely reduce risks to at-risk groups from the
O3-related health effects considered in the risk assessment,
and by inference across the much broader array of O3-related
health effects that can only be considered qualitatively, relative to
the level of protection afforded by the current standard. This lends
support to the proposed range.
    The Administrator judges that a standard set within the range of
0.070 to 0.075 ppm would provide a degree of reduction in risk that is
important from a public health perspective, and that a standard within
this range would be requisite to protect public health, including the
health of at-risk groups, with an adequate margin of safety. EPA's
evaluation of the body of scientific evidence and quantitative
estimates of exposures and risks indicates that substantial reductions
in public health risks would occur throughout this range. Because there
is no bright line clearly directing the choice of level within this
reasonable range, the choice of what is appropriate, considering the
strengths and limitations of the evidence, and the appropriate
inferences to be drawn from the evidence and the exposure and risk
assessments, is a public health policy judgment. To further inform this
judgment, EPA seeks comment on the extent to which the epidemiological
and clinical evidence provides guidance as to the level of a standard
that would be requisite to protect public health with

an adequate margin of safety, especially for at-risk groups.
    In considering the available information, the Administrator also
judges that a standard level below 0.070 ppm would not be appropriate.
In reaching this judgment, the Administrator notes that there is only
quite limited evidence from clinical studies at exposure levels below
0.080 ppm O3. Moreover, the Administrator recognizes that in
the body of epidemiological evidence, many studies report positive and
statistically significant associations, while others report positive
results that are not statistically significant, and a few do not report
any positive O3-related associations. In addition, the
Administrator judges that evidence of a causal relationship between
adverse health outcomes and O3 exposures becomes
increasingly uncertain at lower levels of exposure.
    The Administrator also has considered the results of the exposure
assessments in reaching his judgment that a standard level below 0.070
ppm would not be appropriate. The Administrator notes that in
considering the results from the exposure assessment, a standard set at
the 0.070 ppm level, with the same form as the current standard, is
estimated to provide substantial reductions in exposures of concern
(i.e., approximately 90 to 92 percent reductions in the numbers of
school age children and 94 percent reduction in the total number of
occurrences) for both all and asthmatic school age children relative to
just meeting the current standard based on a simulation of a recent
year with relatively high O3 levels (2002). Thus, a 0.070
ppm standard would be expected to provide protection from the exposures
of concern that the Administrator has primarily focused on for over 98
percent of all and asthmatic school age children even in a year with
relatively high O3 levels, increasing to over 99.9 percent
of children in a year with relatively low O3 levels (2004).
    In considering the results of the health risk assessment, as
discussed in section II.B above, the Administrator notes that there are
important uncertainties and assumptions inherent in the risk assessment
and that this assessment is most appropriately used to simulate trends
and patterns that can be expected as well as providing informed but
still imprecise estimates of the potential magnitude of risks. The
Administrator particularly notes that as lower standard levels are
modeled, including a standard set at a level below 0.070 ppm, the risk
assessment continues to assume a causal link between O3
exposures and the occurrence of the health effects examined, such that
the assessment continues to indicate reductions in O3-
related risks upon meeting a lower standard level. As discussed above,
however, the Administrator recognizes that evidence of a causal
relationship between adverse health effects and O3 exposures
becomes increasingly uncertain at lower levels of exposure. Given all
of the information available to him at this time, the Administrator
judges that the increasing uncertainty of the existence and magnitude
of additional public health protection that standards below 0.070 ppm
might provide suggests that such lower standard levels would likely be
below what is necessary to protect public health with an adequate
margin of safety.
    In addition, the Administrator judges that a standard level higher
than 0.075 ppm would also not be appropriate. This judgment takes into
consideration the information discussed above in section II.B, and is
based on the strong body of clinical evidence in healthy people at
exposure levels of 0.080 ppm and above, the substantial body of
clinical and epidemiological evidence indicating that people with
asthma are likely to experience larger and more serious effects than
healthy people, the body of epidemiological evidence indicating that
associations are observed for a wide range of more serious health
effects at levels below 0.080 ppm, and the estimates of exposure and
risk remaining upon just meeting a standard set at 0.080 ppm. The much
greater certainty of the existence and magnitude of additional public
health protection that such levels would forego provides the basis for
judging that levels above 0.075 ppm would be higher than what is
requisite to protect public health, including the health of at-risk
groups, with an adequate margin of safety.
    For the reasons discussed above, the Administrator proposes to
revise the level of the primary O3 standard to within the
range of 0.070 to 0.075 ppm.
    Having reached this decision based on the approach to interpreting
the evidence described above, the Administrator recognizes that other
approaches to selecting a standard level have been presented to the
Agency. As described above, the CASAC has stated in two letters to the
Administrator (Henderson, 2006c; Henderson, 2007) its unanimous
recommendation that the current primary O3 NAAQS be revised
to within the range from 0.060 to 0.070 ppm. The CASAC Panel noted that
while data exist that adverse health effects may occur at levels lower
than 0.060 ppm, these data are less certain and that achievable gains
in protecting human health can be accomplished through lowering the
O3 NAAQS to a level between 0.060 and 0.070 ppm. In addition
to the views of CASAC described above, the Agency received the public
comments described below.
    One group of commenters submitted comments that supported revising
the level of the primary O3 standard from 0.070 ppm down to
or even below 0.060 ppm, consistent with or below the range recommended
by CASAC. In considering the available evidence as a basis for their
views, these commenters generally noted that the controlled human
exposure studies, showing statistically significant declines in lung
function, and increases in respiratory symptoms, airway inflammation
and airway responsiveness at a 0.080 ppm exposure level, were conducted
with healthy adults, not members of at-risk groups including people
with asthma and active children generally. Further, recognizing the
substantial variability in response between subjects, some of these
commenters felt that the number of subjects included in these studies
was too small to ascertain the full range of responses, especially for
at-risk groups. Such considerations in part were the basis for these
commenters' view that an O3 standard set at 0.080 ppm is not
protective of public health and has no margin of safety for at-risk
groups. In addition, some of these commenters also noted that the World
Health Organization's guidelines for O3 air quality are in
the range of 0.061 to 0.051 ppm.
    In considering the results of the human exposure and health risk
assessment, this group of commenters generally expressed the view that
these assessments substantially underestimate the public health impacts
of exposure to O3. For example, several commenters noted
that the assessments are done for a limited number of cities, they do
not address risks to important at-risk subpopulations (e.g., outdoor
workers, active people who spend their summers outdoors, children up to
5 years of age), and they do not include many health effects that are
important from a public health perspective (e.g., school absences,
restricted activity days). Further, some of these commenters expressed
the view that the primary O3 standard should be set to
protect the most exposed and most vulnerable groups, and the fact that
some children are frequently indoors, and thus at lower risk, should
not weigh against setting a standard to protect those children who are
active outdoors. To the extent the exposure and risk estimates

are considered, some of these commenters felt that primary
consideration should be given to the estimates based on 2002 air
quality, for which most areas had relatively higher O3
levels than in 2004, so as to ensure public health protection even in
years with relatively worse O3 air quality levels. Some
commenters also felt that the exposure analysis should focus on a
benchmark concentration for exposures of concern at the 0.060 ppm
level, the lower end of the range of alternative standards advocated by
the CASAC Panel.
    In sharp contrast to the views discussed above, other public
commenters supported retaining the current standard. In considering the
available evidence as a basis for their views, these commenters
challenged a number of aspects of the interpretation of the evidence
presented in the Criteria Document. For example, some of these
commenters asserted that EPA generally overestimates the magnitude and
consistency of the results of short-term exposure epidemiological
studies (e.g., for respiratory symptoms, school absences, hospital
admissions, mortality), mistakenly links statistical significance and
consistency with strength of associations, and underestimates the
uncertainties in interpreting the results of such studies. Further,
these commenters generally express the view that there is significant
uncertainty related to the reliability of estimates from time-series
studies, in that ambient monitors do not provide reliable estimates of
personal exposures, such that the small reported morbidity and
mortality risks are unlikely to be attributable to people's exposures
to O3. Rather, these commenters variously attribute the
reported risks to the inability of time series studies to account for
key model specification factors such as smoothing for time-varying
parameters, meteorological factors, and removal of O3 by
building ventilation systems, and confounding by co-pollutants. In
particular, these commenters generally asserted that reported
associations between short-term O3 exposure and mortality
are not causal, in that the reported relative risks are too small to
provide a basis for inferring causality and the associations are most
likely due to confounding, inappropriately specified statistical
models, or publication bias.
    In considering the results of the human exposure and health risk
assessment, this group of commenters generally expressed the view that
these assessments are based on a number of studies that should not be
used in quantitative risk assessment. For example, some commenters
asserted that the results of time-series studies should not be used at
all in quantitative risk assessments, that risk estimates from single
city time-series studies should not be used since they are highly
heterogeneous and influenced by publication bias, and that risk
estimates from multi-city studies should not be used in estimating risk
for individual cities. This group of commenters also generally
expressed the view that the assessments generally overestimate the
public health impacts of exposure to O3. Noting that the
risk assessment used a nonlinear exposure-response function to estimate
decreased lung function risks, some commenters expressed the view that
a nonlinear approach should also be used to assess other acute
morbidity effects and mortality. This view was in part based on
judgments that it is not possible to determine if thresholds exist
using time-series analyses and that the lack of association of
O3 to mortality in the winter season is highly supportive of
the likelihood of the existence of an effect threshold. With regard to
the risk assessment based on controlled human exposure studies of lung
function decrements, some commenters expressed the view that the
assessment should not rely on what they characterized as ``outlier''
information to define exposure-response relationships, with reference
to the data in the Adams (2006) study at the 0.060 and 0.040 ppm
exposure levels, but rather should focus on group central tendency
response levels. Further, some commenters expressed the view that the
air quality rollback algorithm used introduces significant uncertainty,
especially when applied to areas requiring very large reductions in air
quality to meet the alternative standards examined, and may result in
overestimates in benefits from emission reductions. Some commenters
noted that potential beneficial effects of O3 in shielding
from UV-B radiation are not quantified in the assessment, and that the
assessment should discuss the evidence for both adverse and beneficial
effects with the same objectivity. Finally, some of these commenters
asserted that since estimates of exposures of concern (which they
defined as the benchmark concentration of 0.080 ppm) and lung function
decrements are substantially below the estimates available when the
current O3 standard was set in 1997, retaining the current
standard is the most appropriate policy alternative.
    Some commenters also have raised concerns about potential
uncertainties with regard to estimating policy-relevant background
O3 levels including: (1) Stratospheric O3
contributions to the mid- and upper troposphere, which are relatively
long-lived (1 to 2 months), and are transported downward to the surface
over time; (2) potential trends in stratospheric O3 levels
due to changes in stratospheric circulation or to reduction of
O3 depleting chemicals; (3) O3 levels due to
lightning strikes in estimating policy-relevant background
concentrations; and (4) potential uncertainty with regard to policy-
relevant background O3 levels having to do with increases in
O3 precursors elsewhere in the world. EPA asks for comments
on these issues and on how they may relate to the estimation and
consideration of policy-relevant background levels in setting the
O3 standards.
    Several Governors, State Legislators, and other local officials
have expressed concerns related to a more stringent standard. These
officials recognize that State and local governments have important
roles in developing and implementing policy that improve air quality
while at the same time achieving economic and quality of life
objectives. In addition, these officials note that States are just
beginning to implement current air quality standards and raise concern
with moving forward on revised standards without first realizing the
results from the last revision.
    As a related concern, a number of areas--including some of the
cities involved in the risk assessment--will have difficulty in
complying with the current 8-hour standard within the next decade. As a
result, the full public health gains in these areas from a more
stringent 8-hour standard are unlikely to be realized for a number of
years. In light of the fact that these public health gains may not
fully materialize within the attainment date structure set forth in the
Clean Air Act, some commenters question whether the Agency can or
should consider these projected gains as a health based criterion for
its decisionmaking. EPA requests comment on this view.
    The Administrator is mindful that the country has important goals
related to the increased production and use of renewable energy, and
that these new energy sources can have important public health,
environmental and other benefits, such as national security benefits.
In some contexts and situations, however, the use of renewable fuels
may impact compliance with a lowered ozone NAAQS standard. For example,
the Agency recently promulgated final regulations pursuant to section
211(o) of the Clean Air Act,

which was enacted as part of the Energy Policy Act of 2005. This
provision requires the use of 7.5 billion gallons of renewable fuel by
2012, a level which will be greatly exceeded in practice. In the
Regulatory Impact Analysis which accompanied the renewable fuel
regulations, the Agency recognized the impact of this program on
emissions related to ozone, toxics and greenhouse gases and otherwise
reviewed the impacts on energy security. The Administrator requests
comment on such factors and any relationship to this rulemaking,
including the extent of EPA's discretion under the Clean Air Act to
take such factors into account (see section I.A).
    In general, these commenters' concerns are consistent with the view
that adopting a more stringent 8-hour standard now, without a better
understanding of the health effects associated with O3
exposure at lower levels, would have an uncertain public health
benefit. The Administrator recognizes that commenters have raised
numerous concerns regarding various types of uncertainties in the
available information, including for example uncertainties in (1) The
assessment of exposures, (2) the estimation of concentration-response
associations in the epidemiological studies, (3) the potential role of
co-pollutants in interpreting the reported associations in
epidemiological studies, and (4) the estimation of background
concentrations. The Administrator has heard these concerns from
Governors and other commenters and invites comment on whether it would
be appropriate to retain the existing standard and delay considering
modification of the 8-hour standard until the next NAAQS review, when a
more complete body of information is expected to be available.
    Consistent with the goal of soliciting comment on a wide array of
views, the Administrator also solicits comments on these alternative
approaches and views, and on related standard levels, including levels
down to 0.060 ppm and up to retaining the level of the current 8-hour
standard (i.e., effectively 0.084 ppm with the current rounding
convention). The Administrator recognizes that these sharply divergent
views on the appropriate level of the standard are based on very
different interpretations of the science itself, including its relative
strengths and limitations, very different judgments as to how such
scientific evidence should be used in making policy decisions on
proposed standards, and very different public health policy judgments.

E. Proposed Decision on the Primary Standard

    For the reasons discussed above, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and the public
comments to date, the Administrator proposes to revise the existing 8-
hour primary O3 standard. Specifically, the Administrator
proposes to revise: (1) The level of the primary O3 standard
to within a range from 0.070 to 0.075 ppm and (2) the degree of
precision to which the level of the standard is specified to the
thousandth ppm. The proposed 8-hour primary standard, with a level in
the range of 0.070 to 0.075 ppm, would be met at an ambient air
monitoring site when the 3-year average of the annual forth-highest
daily maximum 8-hour average O3 concentration is less than
or equal to the level of the standard that is promulgated. Data
handling conventions are specified in the proposed creation of Appendix
P, as discussed in section V below.
    However, in recognition of alternative views of the science, the
exposure and risk assessments and the uncertainties inherent in these
assessments, and the appropriate policy responses based on the
currently available information, the Administrator also solicits
comments on whether to proceed instead with: (1) Alternative levels of
the 8-hour primary O3 standard, within ranges of below 0.070
ppm down to 0.060 ppm and above 0.075 ppm up to and including retaining
the current standard; (2) alternative forms of the standard, including
the 3-year average of the annual third- and fifth-highest daily maximum
8-hour average O3 concentrations; and (3) retaining the
degree of precision of the current standard (to the nearest hundredth
ppm). Based on the comments received and the accompanying rationales,
the Administrator may adopt other standards within the range of the
alternative levels and forms identified above in lieu of the standards
he is proposing today.

III. Communication of Public Health Information

    Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily through EPA's Air Quality Index (AQI) program. The current
Air Quality Index has been in use since its inception in 1999 (64 FR
42530). It provides accurate, timely, and easily understandable
information about daily levels of pollution (40 CFR 58.50). The AQI
establishes a nationally uniform system of indexing pollution levels
for O3, carbon monoxide, nitrogen dioxide, particulate
matter and sulfur dioxide. The AQI converts pollutant concentrations in
a community's air to a number on a scale from 0 to 500. Reported AQI
values enable the public to know whether air pollution levels in a
particular location are characterized as good (0-50), moderate (51-
100), unhealthy for sensitive groups (101-150), unhealthy (151-200),
very unhealthy (201-300), or hazardous (300-500). The AQI index value
of 100 typically corresponds to the level of the short-term NAAQS for
each pollutant. For the current O3 NAAQS, an 8-hour average
concentration of 0.084 ppm corresponds to an AQI value of 100. An AQI
value greater than 100 means that a pollutant is in one of the
unhealthy categories (i.e., unhealthy for sensitive groups, unhealthy,
very unhealthy, or hazardous) on a given day; an AQI value at or below
100 means that a pollutant concentration is in one of the satisfactory
categories (i.e., moderate or good). Decisions about the pollutant
concentrations at which to set the various AQI breakpoints, that
delineate the various AQI categories, draw directly from the underlying
health information that supports the NAAQS review.
    The Agency recognizes the importance of revising the AQI in a
timely manner to be consistent with any revisions to the NAAQS.
Therefore EPA proposes to finalize conforming changes to the AQI, in
connection with the Agency's final decision on the O3 NAAQS
if revisions to the primary standard are promulgated. These conforming
changes would include setting the 100 level of the AQI at the same
level as the revised primary O3 NAAQS, and also making
proportional adjustments to AQI breakpoints at the lower end of the
range (i.e., AQI values of 50, 150 and 200). EPA does not propose to
change breakpoints at the higher end of the range (from 300 to 500),
which would apply to state contingency plans or the Significant Harm
Level (40 CFR 51.16), because the information from this review does not
inform decisions about breakpoints at those higher levels.

IV. Rationale for Proposed Decision on the Secondary Standard

    This section presents the rationale for the Administrator's
proposed decision to revise the existing 0.08 ppm, 8-hour O3
secondary NAAQS. The Administrator proposes to revise the current
secondary standard by replacing it with one of two standard options.
One

option is to adopt a new cumulative, seasonal concentration-weighted
form, set at an annual level in the range of 7 to 21 ppm-hours. This
standard would be expressed as a sum of weighted hourly concentrations,
cumulated over the 12-hour daylight period (8 a.m. to 8 p.m.) during
the consecutive 3-month period within the O3 monitoring
season with the maximum index value. This concentration-weighted form
is commonly called W126, and is defined as the sum of sigmoidally
weighted hourly O3 concentrations over a specified period,
where the daily sigmoidal weighting function is defined as:
[GRAPHIC] [TIFF OMITTED] TP11JY07.001

The other option is to revise the current secondary standard by making
it identical to the proposed 8-hour primary standard, within the
proposed range of 0.070 to 0.075 ppm. For this option, EPA also
solicits comment on a wider range of 8-hour secondary standard levels,
including down to 0.060 ppm and up to and including retaining the
current 8-hour secondary standard of 0.08 ppm. The Administrator has
also considered and solicits comment on an alternative approach to
setting a cumulative, seasonal standard(s).
    As discussed more fully below, the rationale for these proposed
options is based on a thorough review of the latest scientific
information on vegetation effects associated with exposure to ambient
levels of O3, as assessed in the Criteria Document. This
rationale also takes into account: (1) Staff assessments of the most
policy-relevant information in the Criteria Document regarding the
evidence of adverse effects of O3 to vegetation and
ecosystems, information on biologically-relevant exposure metrics, and
staff analyses of air quality, vegetation exposure and risks, presented
in the Staff Paper and described in greater detail in the associated
Technical Report on Ozone Exposure, Risk, and Impact Assessments for
Vegetation (Abt, 2007), upon which staff recommendations for revisions
to the secondary O3 standard are based; (2) CASAC advice and
recommendations as reflected in discussion of drafts of the Criteria
Document and Staff Paper at public meetings, in separate written
comments, and in CASAC's letters to the Administrator (Henderson,
2006a, b, c; 2007); (3) public comments received during development of
these documents either in conjunction with CASAC meetings or
separately; and (4) consideration of the degree of protection to
vegetation potentially afforded by the proposed 8-hour primary
standard.
    In developing this rationale, EPA has again focused on direct
O3 effects on vegetation, specifically drawing upon an
integrative synthesis of the entire body of evidence, published through
early 2006, on the broad array of vegetation effects associated with
exposure to ambient levels of O3 (EPA, 2006a, chapter 9). In
addition, because O3 can also indirectly affect other
ecosystem components such as soils, water, and wildlife, and their
associated ecosystem goods and services, through its effects on
vegetation, a qualitative discussion of these other indirect impacts is
also included, though these effects are not quantifiable at this time.
As was concluded in the 1997 review, and based on the body of
scientific literature assessed in the current Criteria Document, the
Administrator believes that it is reasonable to conclude that a
secondary standard protecting the public welfare from known or
anticipated adverse effects to trees, native vegetation and crops would
also afford increased protection from adverse effects to other
environmental components relevant to the public welfare, including
ecosystem services and function. The peer-reviewed literature includes
studies conducted in the U.S., Canada, Europe, and many other countries
around the world. In its assessment of the evidence judged to be most
relevant to making decisions on the level of the O3
secondary standard, however, EPA has placed greater weight on U.S.
studies, due to the often species-, site- and climate-specific nature
of O3-related vegetation response.
    As with virtually any policy-relevant vegetation effects research,
there is uncertainty in the characterization of vegetation effects
attributable to exposure to ambient O3. As discussed below,
however, research conducted since the last review provides important
information coming from field-based exposure studies, including free
air, gradient and biomonitoring surveys, in addition to the more
traditional controlled open top chamber (OTC) studies. Moreover, the
newly available studies evaluated in the Criteria Document have
undergone intensive scrutiny through multiple layers of peer review and
many opportunities for public review and comment. While important
uncertainties remain, the review of the vegetation effects information
has been extensive and deliberate. In the judgment of the
Administrator, the intensive evaluation of the scientific evidence that
has occurred in this review has provided an adequate basis for
regulatory decision-making at this time. This review also provides
important input to EPA's research plan for improving our future
understanding of the effects of ambient O3 at lower levels.

A. Vegetation Effects Information

    This section outlines key information contained in the Criteria
Document (chapter 9) and in the Staff Paper (chapter 7) on known or
potential effects on public welfare which may be expected from the
presence of O3 in ambient air. The information highlighted
here summarizes: (1) New information available on potential mechanisms
for vegetation effects associated with exposure to O3; (2)
the nature of effects on vegetation that have been associated with
exposure to O3 and consequent potential impacts on
ecosystems; and (3) considerations in characterizing what constitutes
an adverse welfare impact of O3.
    Exposures to O3 have been associated quantitatively and
qualitatively with a wide range of vegetation effects. The decision in
the last review to set a more protective secondary standard primarily
reflected consideration of the quantitative information on vegetation
effects available at that time, particularly growth impairment (e.g.,
biomass loss) in sensitive forest tree species during the seedling
growth stage and yield loss in important commercial crops. This
information, derived mainly using the OTC exposure method, found
cumulative, seasonal O3 exposures were most strongly
associated with observed vegetation response. The Criteria Document
prepared for this review discusses a number of additional studies that
support and strengthen key conclusions regarding O3 effects
on vegetation and ecosystems found in the previous Criteria Document
(EPA, 1996a, 2006a), including further clarification of the underlying
mechanistic and physiological processes at the subcellular, cellular,
and whole

system levels within the plant. More importantly, however, in the
context of this review, new quantitative information is now available
across a broader array of vegetation effects (e.g., growth impairment
during seedlings, saplings and mature tree growth stages, visible
foliar injury, and yield loss in annual crops) and across a more
diverse set of exposure methods, including chamber, free air, gradient,
model, and field-based observation. These non-chambered, field-based
study results begin to address one of the key data gaps cited by the
Administrator in the last review.
    The following discussion of the policy-relevant science regarding
vegetation effects associated with cumulative, seasonal exposures to
ambient levels of O3 integrates information from the
Criteria Document (chapter 9) and the Staff Paper (chapter 7).
1. Mechanisms Governing Plant Response to Ozone
    The interpretation of predictions of risk associated with
vegetation response at ambient O3 exposure levels can be
informed by scientific understanding regarding O3 impacts at
the genetic, physiological, and mechanistic levels. In most cases, the
mechanisms of response are similar regardless of the degree of
sensitivity of the species. The evidence assessed in the 2006 Criteria
Document (EPA, 2006a) regarding the O3-induced changes in
physiology continues to support the information discussed in the last
review (EPA, 1996a, 2006a). In addition, during the last decade
understanding of the cellular processes within plants has been further
clarified and enhanced. Therefore, this section reviews the key
scientific conclusions identified in 1996 Criteria Document (EPA,
1996a), and incorporates new information from the current Criteria
Document (EPA, 2006a). This section describes: (1) Plant uptake of
O3, (2) O3-induced cellular to systemic response,
(3) plant compensation and detoxification mechanisms, (4)
O3-induced changes to plant metabolism, and (5) plant
response to chronic O3 exposures.
a. Plant Uptake of Ozone
    To cause injury, O3 must first enter the plant through
openings in the leaves called stomata. Leaves exist in a three
dimensional environment called the plant canopy, where each leaf has a
unique orientation and receives a different exposure to ambient air,
microclimatological conditions, and sunlight. In addition, a plant may
be located within a stand of other plants which further modifies
ambient air exchange with individual leaves. Not all O3
entering a plant canopy is absorbed into the leaf stomata, but may be
adsorbed to other surfaces e.g., leaf cuticles, stems, and soil (termed
non-stomatal deposition) or scavenged by reactions with intra-canopy
biogenic VOCs and naturally occurring NOX emissions from
soils. Because O3 does not penetrate the leaf's cuticle, it
must reach the stomatal openings in the leaf for absorption to occur.
The movement of O3 and other gases such as CO2
into and out of leaves is controlled by stomatal guard cells that
regulate the size of the stomatal apertures. These guard cells respond
to a variety of internal species-specific factors as well as external
site specific environmental factors such as light, temperature,
humidity, CO2 concentration, soil fertility and water
status, and in some cases the presence of air pollutants, including
O3. These modifying factors produce stomatal conductance
that vary between leaves of the same plant, individuals and genotypes
within a species and diurnally and seasonally.
b. Cellular to Systemic Response
    Once inside the leaf, O3 can react with a variety of
biochemical compounds that are exposed to the air spaces within the
leaf or it can be dissolved into the water lining the cell wall of the
air spaces. Having entered the aqueous phase, O3 can be
rapidly altered to form oxidative products that can diffuse more
readily into and through the cell and react with many biochemical
compounds. An early step in a series of O3-induced events
that leads to leaf injury seems to involve alteration in cell membrane
function, including membrane transport properties (EPA, 2006a). One
such signaling molecule is hydrogen peroxide
(H2O2). The presence of higher-than-normal levels
of H2O2 within the leaf is a potential trigger
for a set of metabolic reactions that include those typical of the well
documented ``wounding'' response or pathogen defense pathway generated
by cutting of the leaf or by pathogen/insect attack. Ethylene is
another compound produced when plants are subjected to biotic or
abiotic stressors. Increased ethylene production by plants exposed to
O3 stress was identified as a consistent marker for
O3 exposure in studies conducted decades ago (Tingey et al.,
1976).
c. Compensation and Detoxification
    Ozone injury will not occur if (1) the rate and amount of
O3 uptake is small enough for the plant to detoxify or
metabolize O3 or its metabolites or (2) the plant is able to
repair or compensate for the O3 impacts (Tingey and Taylor,
1982; U.S. EPA, 1996a). A few studies have documented direct stomatal
closure or restriction in the presence of O3 in some
species. This response may be initiated ranging from within minutes to
hours or days of exposure (Moldau et al., 1990; Dann and Pell, 1989;
Weber et al., 1993). However, exclusion of O3 simultaneously
restricts the uptake of CO2, which also limits
photosynthesis and growth. In addition, antioxidants present in plants
can effectively protect tissue against damage from low levels of
oxidants by dissipating excess oxidizing power. Since 1996, the role of
detoxification in providing a level of resistance to O3 has
been further investigated. A number of antioxidants have been found in
plants. However, the pattern of changes in the amounts of these
antioxidants varies greatly among different species and conditions.
Most recent reports indicate that ascorbate within the cell wall
provides the first significant opportunity for detoxification to occur.
In spite of the new research, however, it is still not clear as to what
extent detoxification protects against O3 injury.
Specifically, data are needed on potential rates of antioxidant
production, subcellular location(s) of antioxidants, and whether
generation of these antioxidants in response to O3-induced
stress potentially diverts resources and energy away from other vital