65           0-0.2  14,000 [2,000]......              95              0.5
                                       60         \134\ 0  1,400 [200] \135\...             100              0.1
>=60 ppb.................              70        3.3-10.2  1,176,000 [126,000].              46             18.9
                                       65           0-4.2  392,000 [42,000]....              80              9.5

                                       60           0-1.2  70,000 [8,000]......              96              2.2
----------------------------------------------------------------------------------------------------------------
                                   Two or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>=70 ppb.................              70           0-0.1  5,400 [600].........              95              0.4
                                       65               0  300 [100]...........             100                0
                                       60               0  0 [0]...............             100                0
>=60 ppb.................              70         0.5-3.5  320,000 [35,000]....              61              9.2
                                       65           0-0.8  67,000 [7,500]......              92              2.8
                                       60           0-0.2  5,100 [700].........             100              0.3
----------------------------------------------------------------------------------------------------------------

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

    \136\ Estimates in each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas.
    \137\ Numbers of children estimated to experience decrements in
each urban case study area were averaged over 2006 to 2010. These
averages were then summed across urban study areas. Numbers are
rounded to nearest thousand unless otherwise indicated.
    \138\ As noted in section II.C.3.a.ii, the responsiveness of
asthmatics to O3 exposures could depend on factors that
have not been well-evaluated such as asthma severity, the
effectiveness of asthma control, or the prevalence of medication
use.
    \139\ As discussed in section 4.3.3 of the HREA (U.S. EPA,
2014a), the model-based air quality adjustment approach used to
estimate risks associated with the current and alternative standards
was unable to estimate the distribution of ambient O3
concentrations in New York City upon just meeting an alternative
standard with a level of 60 ppb. Therefore, for the 60 ppb standard
level the numbers of children and asthmatic children reflect all of
the urban study areas except New York.

 Table 5--Summary of Estimated Lung Function Decrements for Potential Alternative O3 Standard Levels of 70, 65,
                                      and 60 ppb in Urban Case Study Areas
----------------------------------------------------------------------------------------------------------------
                                                             Number of children
                                                               (5 to 18 years)       Average %      % Children
  Lung function decrement     Alternative      Average %    [number of asthmatic  reduction from  worst year and
                            standard level  children \136\     children] \137\        current          area
                                                                    \138\            standard
----------------------------------------------------------------------------------------------------------------
                                        One or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%.....................              70           11-17  2,527,000 [261,000].              15              20
                                        65            3-15  1,896,000 [191,000].              31              18
                                        60            5-11  1,404,000 [139,000]               45              13
                                                             \139\.
>=15%.....................              70             2-4  562,000 [58,000]....              26               5
                                        65             0-3  356,000 [36,000]....              50               4
                                        60             1-2  225,000 [22,000]....              67               3
>=20%.....................              70             1-2  189,000 [20,000]....              32             2.1
                                        65             0-1  106,000 [11,000]....              59             1.4
                                        60             0-1  57,000 [6,000]......              77             0.7
----------------------------------------------------------------------------------------------------------------
                                        Two or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%.....................              70          5.5-11  1,414,000 [145,000].              17              13
                                        65         1.3-8.8  1,023,000 [102,000].              37              11
                                        60         2.1-6.4  741,000 [73,000]....              51             7.3
>=15%.....................              70         0.9-2.4  276,000 [28,000]....              29             3.1
                                        65         0.1-1.8  168,000 [17,000]....              54             2.3
                                        60         0.2-1.0  101,000 [10,000]....              71             1.4
>=20%.....................              70         0.3-0.8  81,000 [8,000]......              34             1.1
                                        65           0-0.5  43,000 [4,000]......              66             0.8
                                        60           0-0.2  21,000 [2,000]......              83             0.4
----------------------------------------------------------------------------------------------------------------

i. PA Consideration of an O3 Standard Level of 70 ppb
    The PA notes that a level of 70 ppb is below the lowest
O3 exposure concentration that has been reported to elicit a
range of respiratory effects that includes AHR and decreased lung host
defense, in addition to lung function decrements, airway inflammation,
and respiratory symptoms (i.e., 80 ppb). A level of 70 ppb is also
below the lowest exposure concentration at which the combined
occurrence of respiratory symptoms and lung function decrements have
been reported (i.e., 72 ppb), a combination judged adverse by the ATS
(U.S. EPA, 2014c, section 3.1.3). A level of 70 ppb is above the lowest
exposure concentration demonstrated to result in lung function
decrements large enough to be judged an abnormal response by ATS and
above the lowest exposure concentration demonstrated to result in
pulmonary inflammation (i.e., 60 ppb).
    Compared to the current standard, the HREA estimates that a revised
O3

standard with a level of 70 ppb would reduce exposures of concern to
O3 concentrations of 60, 70, and 80 ppb in urban study
areas, with such a standard level estimated to be most effective at
limiting exposures at or above the higher health benchmark
concentrations and at limiting multiple occurrences of such exposures.
On average over the years 2006 to 2010, for a standard with a level of
70 ppb, up to about 1% of children (i.e., ages 5 to 18) are estimated
to experience exposures of concern at or above 70 ppb (73% reduction,
compared to current standard), and far less than 1% are estimated to
experience two or more such exposures (95% reduction, compared to
current standard). In the worst-case location and year (i.e., location
and year with the largest exposure estimate), about 3% of children are
estimated to experience one or more exposures of concern at or above 70
ppb, and less than 1% are estimated to experience two or more. Far less
than 1% of children are estimated to experience exposures of concern at
or above the 80 ppb benchmark concentration, even in the worst-case
year (Table 4, above).\140\
---------------------------------------------------------------------------

    \140\ As noted above, due to interindividual variability,
children (or adults) exposed at these levels will not necessarily
experience health effects; the information available for some health
effects is not sufficient to quantify the numbers of children in the
urban study areas who might experience these effects.
---------------------------------------------------------------------------

    As noted above, CASAC advised the EPA that 60 ppb is an appropriate
exposure of concern with respect to adverse effects on people with
asthma, including children (Frey, 2014c, pp. 6 and 8). For an
O3 standard with a level of 70 ppb, about 3 to 10% of
children, including asthmatic children, are estimated to experience one
or more exposures of concern at or above 60 ppb in a single
O3 season. Compared to the current standard, this reflects
about a 46% reduction, on average across the urban study areas. About
1% to 4% of children are estimated to experience two or more exposures
of concern at or above 60 ppb (approximately 60% reduction, compared to
current standard). In the worst-case location and year, for a standard
set at 70 ppb, about 19% of children are estimated to experience one or
more exposures of concern at or above 60 ppb, and 9% are estimated to
experience two or more such exposures (Table 4, above).
    Compared to the current standard, the HREA estimates that a revised
O3 standard with a level of 70 ppb would also reduce
O3-induced lung function decrements in children. A level of
70 ppb is estimated to be most effective at limiting the occurrences of
moderate and large lung function decrements (i.e., FEV1
decrements >=15% and >=20%, respectively), and at limiting multiple
occurrences of O3-induced decrements. On average over the
years 2006 to 2010, for a standard with a level of 70 ppb, about 2 to
4% of children in the urban study areas are estimated to experience one
or more moderate O3-induced lung function decrements (i.e.,
FEV1 decrement >=15%), which would be of concern for healthy
people, and about 1 to 2.5% of children are estimated to experience two
or more such decrements (approximately 30% reduction, compared to the
current standard). In the worst-case location and year, up to 5% of
children are estimated to experience one or more O3-induced
lung function decrements >=15%, and up to 3% are estimated to
experience two or more such decrements. For a standard set at 70 ppb,
about 2% or fewer children are estimated to experience large
O3-induced lung function decrements (i.e., FEV1
decrement >=20%), and about 1% or fewer children are estimated to
experience two or more such decrements, even in the worst-case years
and locations (Table 5, above).
    On average over the years 2006 to 2010, for an O3
standard set at 70 ppb, about 11 to 17% of children in the urban study
areas are estimated to experience one or more moderate O3-
induced lung function decrements (i.e., FEV1 decrement
>=10%), which could be adverse for people with lung disease. This
reflects an average reduction of about 15%, compared to the current
standard. About 6 to 11% of children are estimated to experience two or
more such decrements (17% reduction, compared to current standard). In
the worst-case location and year, for a standard set at 70 ppb, about
20% of children in the urban study areas are estimated to experience
one or more O3-induced lung function decrements >=10%, and
13% are estimated to experience two or more such decrements (Table 5,
above).
    Compared to the current standard, a revised standard with a level
of 70 ppb would also more effectively maintain short-term ambient
O3 concentrations below those present in the epidemiologic
studies that reported significant O3 health effect
associations in locations likely to have met the current standard. In
particular, the single-city study by Mar and Koenig (2009) reported
positive and statistically significant associations with respiratory
emergency department visits in children and adults in a location that
likely would have met the current O3 standard over the
entire study period but violated a revised standard with a level of 70
ppb or below. None of the single-city studies evaluated in section
4.4.1 of the PA (U.S. EPA, 2014c) provide evidence for O3
health effect associations in locations meeting a standard with a level
of 70 ppb or below. While this analysis does not provide information on
the extent to which the reported O3-associated emergency
department visits would persist upon meeting an O3 standard
with a level of 70 ppb, or on the extent to which standard levels below
70 ppb could further reduce the incidence of such emergency department
visits,\141\ it suggests that a revised O3 standard with a
level at or below 70 ppb would require reductions in the ambient
O3 concentrations that provided the basis for the health
effect associations reported by Mar and Koenig (2009).
---------------------------------------------------------------------------

    \141\ Put another way, one cannot infer from this analysis the
extent to which effects would occur at O3 concentrations
below those observed in the study.
---------------------------------------------------------------------------

    As discussed above, compared to single-city studies, there is
greater uncertainty in linking air quality concentrations from
individual study cities to multicity effect estimates. With regard to
the multicity studies in this review, the PA notes that Dales et al.
(2006) reported significant associations with respiratory hospital
admissions based on air quality in 11 Canadian cities, most of which
would likely have met the current standard over the entire study
period, but violated a revised standard with a level of 70 ppb or below
over at least part of that period (Table 4-1). This analysis suggests
that although the current standard would allow the ambient
O3 concentrations in most of the study locations that
provided the basis for the association with hospital admissions, a
revised O3 standard with a level at or below 70 ppb would
require reductions in those ambient O3 concentrations. As
with the study by Mar and Koenig (2009), this analysis does not provide
information on the extent to which the reported O3-
associated hospital admissions would persist upon meeting an
O3 standard with a level of 70 ppb, or on the extent to
which standard levels below 70 ppb could further reduce the incidence
of such hospital admissions.\142\
---------------------------------------------------------------------------

    \142\ In addition, for the other multicity studies identified in
Table 4-1 of the PA (Cakmak et al., 2006; Stieb et al., 2009;
Katsouyanni et al., 2009), and for the study by Bell et al. (2006)
(for the 30 ppb cut point) (Table 4-2 of the PA), the majority of
study locations would likely have met a standard with a level of 70
ppb (U.S. EPA, 2014c).
---------------------------------------------------------------------------

    With regard to long-term O3 concentrations, the PA
evaluates the long-term O3 metrics reported to be associated
with mortality or morbidity in recent epidemiologic studies (e.g.,

seasonal averages of 1-hour or 8-hour daily max concentrations).
Compared to the current standard, a revised standard with a level of 70
ppb would be expected to reduce the risk of respiratory mortality
associated with long-term O3 concentrations, based on
information from the study by Jerrett et al. (2009), though the PA
notes the HREA conclusion, discussed above, that lower confidence
should be placed in respiratory mortality risk estimates based on this
study (U.S. EPA, 2014a, section 9.6). In addition, a standard with a
level of 70 ppb would be expected to more effectively maintain long-
term O3 concentrations below those where the study by
Jerrett et al. (2009) indicates the most confidence in the reported
association with respiratory mortality.\143\ Specifically, air quality
analyses indicate this to be the case in 9 out of the 12 urban study
areas for a level of 70 ppb, compared to 6 out of 12 areas for the
current standard. Finally, a revised standard with a level of 70 ppb
would be expected to reduce long-term O3 concentrations
based on the types of metrics that have been reported in recent
epidemiologic studies to be associated with respiratory morbidity
(i.e., seasonal averages of daily maximum 8-hour concentrations).
---------------------------------------------------------------------------

    \143\ As discussed in section <IP_ADDRESS> of the PA (U.S. EPA,
2014c), the study by Jerrett et al. (2009) suggests notably
decreased confidence in the reported linear concentration-response
function for long-term O3 concentrations in the first
quartile (i.e., at or below about 53 ppb), given the widening in
confidence intervals for lower concentrations; the fact that most
study cities contributing to the linear function had O3
concentrations in the highest three quartiles, accounting for
approximately 72% of the respiratory deaths in the cohort (based on
Table 2 in the published study); and the limited evidence presented
in the published study for a threshold at or near 56 ppb.
---------------------------------------------------------------------------

    In further considering the potential implications of epidemiology
studies for alternative standard levels, the PA notes estimates of
total mortality associated with short-term O3
concentrations.\144\ As discussed above, the PA considers estimates of
total risk (i.e., based on the full distributions of ambient
O3 concentrations) and estimates of risk associated with
O3 concentrations in the upper portions of ambient
distributions. With regard to total risk the PA notes that, when summed
across urban study areas, a standard with a level of 70 ppb is
estimated to reduce the number of deaths associated with short-term
O3 concentrations by about 4% (2007) and 2% (2009), compared
to the current standard.\145\ Based on a national modeling analysis,
the majority of the U.S. population would be expected to experience
reductions in such risks upon reducing precursor emissions.
---------------------------------------------------------------------------

    \144\ As discussed above, compared to the weight given to the
evidence and to HREA estimates of exposures of concern and lung
function risks, the PA places relatively less weight on
epidemiologic-based risk estimates.
    \145\ A standard with a level of 70 ppb is also estimated to
reduce respiratory mortality associated with long-term O3
concentrations in urban study areas. However, given uncertainties
associated with these risk estimates, as discussed above, the PA
gives them limited weight.
---------------------------------------------------------------------------

    Compared to the total risk estimates noted above, an O3
standard with a level of 70 ppb is estimated to be more effective at
reducing the number of deaths associated with short-term O3
concentrations at the upper ends of ambient distributions.
Specifically, for area-wide O3 concentrations at or above 40
ppb, a standard with a level of 70 ppb is estimated to reduce the
number of deaths associated with short-term O3
concentrations by about 10% compared to the current standard. In
addition, for area-wide concentrations at or above 60 ppb, a standard
with a level of 70 ppb is estimated to reduce O3-associated
deaths by about 50% to 70% (U.S. EPA, 2014c, Figure 4-13).
    The PA noted that in providing the advice that 70 ppb is an
appropriate upper bound for consideration, CASAC advised that a level
of 70 ppb would provide little margin of safety for protection of
public health, particularly for sensitive subpopulations (Frey, 2014c,
p. 8). In particular, CASAC stated that:

At 70 ppb, there is substantial scientific certainty of a variety of
adverse effects, including decrease in lung function, increase in
respiratory symptoms, and increase in airway inflammation. Although
a level of 70 ppb is more protective of public health than the
current standard, it may not meet the statutory requirement to
protect public health with an adequate margin of safety (Frey,
2014c, p. 8).\146\
---------------------------------------------------------------------------

    \146\ Also see Frey (2014c, p. ii).

However, the committee also acknowledged that ``the choice of a level
within the range recommended based on scientific evidence [i.e., 70 to
60 ppb] is a policy judgment under the statutory mandate of the Clean
Air Act'' (Frey, 2014c, pp. ii and 8).
    In summary, compared to the current standard, the PA concludes that
a revised O3 standard with a level of 70 ppb would be
expected to (1) reduce the occurrence of exposures of concern to
O3 concentrations that result in respiratory effects in
healthy adults (at or above 60 and 70 ppb) by about 45 to 95%, almost
eliminating the occurrence of multiple exposures at or above 70 ppb;
(2) reduce the occurrence of moderate-to-large O3-induced
lung function decrements (FEV1 decrements >=10, 15, 20%) by
about 15 to 35%, most effectively limiting the occurrence of multiple
decrements and decrements >=15, 20%; (3) more effectively maintain
short- and long-term O3 concentrations below those present
in the epidemiologic studies that reported significant O3
health effect associations in locations likely to have met the current
standard; \147\ and (4) reduce the risk of O3-associated
mortality and morbidity, particularly the risk associated with the
upper portions of the distributions of ambient O3
concentrations.
---------------------------------------------------------------------------

    \147\ Epidemiologic studies also provide some evidence for
O3 health effect associations in locations likely to have
met a standard with a level of 70 ppb, as discussed below for lower
standard levels.
---------------------------------------------------------------------------

ii. PA Consideration of an O3 Standard Level of 65 ppb
    The PA also considers a standard with a level of 65 ppb. A level of
65 ppb is well below 80 ppb, an O3 exposure concentration
that has been reported to elicit a range of respiratory effects that
includes airway hyperresponsiveness and decreased lung host defense, in
addition to lung function decrements, airway inflammation, and
respiratory symptoms. A standard level of 65 ppb is also below the
lowest exposure concentration at which the combined occurrence of
respiratory symptoms and lung function decrements has been reported
(i.e., 72 ppb), a combination judged adverse by the ATS (U.S. EPA,
2014c, section 3.1.3). A level of 65 ppb is above the lowest exposure
concentration demonstrated to result in lung function decrements large
enough to be judged an abnormal response by ATS, where statistically
significant changes in group mean responses would be judged to be
adverse by ATS, and which the CASAC has indicated could be adverse in
people with lung disease (i.e., 60 ppb). A level of 65 ppb is also
above the lowest exposure concentration at which pulmonary inflammation
has been reported in healthy adults (i.e., 60 ppb).
    Compared to the current standard and a revised standard with a
level of 70 ppb, the HREA estimates that a standard with a level of 65
ppb would reduce exposures of concern to the range of O3
benchmark concentrations analyzed (i.e., 60, 70, and 80 ppb). The HREA
estimates that meeting a standard with a level of 65 ppb would
eliminate exposures of concern at or above 80 ppb in the urban study
areas. Such a standard is estimated to allow far less than 1% of
children in the urban study areas to experience one or more exposures
of concern at or above the 70

ppb benchmark level, even in the worst-case years and locations, and is
estimated to eliminate the occurrence of two or more exposures at or
above 70 ppb (Table 4, above).
    In addition, for a standard with a level of 65 ppb, between 0 and
about 4% of children (including asthmatic children) in urban study
areas are estimated to experience exposures of concern at or above 60
ppb, which CASAC has indicated is an appropriate exposure of concern
for people with asthma, including children. This reflects an 80%
reduction (on average across areas), relative to the current standard.
Less than 1% of children are estimated to experience two or more
exposures of concern at or above 60 ppb (> 90% reduction, compared to
current standard). In the worst-case location and year, about 10% of
children are estimated to experience one or more exposures of concern
at or above 60 ppb, with about 3% estimated to experience two or more
such exposures (Table 4, above).
    Compared to the current standard and a revised standard with a
level of 70 ppb, the HREA estimates that a standard with a level of 65
ppb would also further reduce the occurrence of O3-induced
lung function decrements. For a level of 65 ppb, about 4% of children,
or less, are estimated to experience moderate O3-induced
FEV1 decrements >=15% (50% reduction, compared to current
standard), even considering the worst-case location and year. About 2%
of children, or less, are estimated to experience two or more such
decrements. Only about 1% of children, or less, are estimated to
experience large O3-induced lung function decrements (i.e.,
FEV1 decrement >=20%), even in the worst-case year and
location.
    In addition, for a standard with a level of 65 ppb, about 3 to 15%
of children are estimated to experience one or more moderate
O3-induced lung function decrements (i.e., FEV1
decrement >=10%), which CASAC has indicated could be adverse for people
with lung disease. This reflects an average reduction of about 30%,
relative to the current standard. About 1 to 9% of children in the
urban study areas are estimated to experience two or more such
decrements (37% reduction, compared to current standard). In the worst-
case location and year, for a standard set at 65 ppb, up to about 18%
of these children are estimated to experience one or more moderate
O3-induced lung function decrements >=10%, and up to 11% are
estimated to experience two or more such decrements.
    With regard to O3 epidemiologic studies, the PA notes
that a revised standard with a level of 65 ppb would be expected to
maintain short-term ambient O3 concentrations below those
present in some of the study locations that provided the basis for
reported O3 health effect associations and that were likely
to have met a revised standard with a level of 70 ppb. In particular,
Katsouyanni et al. (2009) reported statistically significant
associations with mortality based on air quality in 12 Canadian cities,
most of which would likely have met a standard with a level of 70 ppb
over the entire study period but violated a revised standard with a
level of 65 ppb or below over at least part of that period (U.S. EPA,
2014c, Table 4-1). This analysis suggests that although the current
standard or a standard with a level of 70 ppb would allow the ambient
O3 concentrations in most of the study locations that
provided the basis for the association with mortality in this study, a
revised O3 standard with a level at or below 65 ppb would
require reductions in those ambient O3 concentrations. As
discussed above for a level of 70 ppb, this analysis does not provide
information on the extent to which O3-associated mortality
would persist upon meeting an O3 standard with a level of 65
ppb, or on the extent to which standard levels below 65 ppb could
further reduce the incidence of this mortality.\148\
---------------------------------------------------------------------------

    \148\ For the other multicity studies identified in Table 4-1 of
the PA (Cakmak et al., 2006; Stieb et al., 2009; Katsouyanni et al.,
2009 (for hospital admissions)), and for the study by Bell et al.
(2006) (for the 30 ppb cut point) (Table 4-2 of the PA), the
majority of study locations would have met a standard with a level
of 65 ppb (U.S. EPA, 2014c).
---------------------------------------------------------------------------

    With regard to long-term O3 concentrations, as for 70
ppb (above) the PA evaluates the long-term O3 metrics
reported to be associated with mortality or morbidity in recent
epidemiologic studies (e.g., seasonal averages of 1-hour or 8-hour
daily max concentrations). Compared to the current standard or a
revised O3 standard with a level of 70 ppb, a revised
standard with a level of 65 ppb would be expected to further reduce the
risk of respiratory mortality associated with long-term O3
concentrations, based on information from the study by Jerrett et al.
(2009).\149\ In addition, a standard with a level of 65 ppb would be
expected to more effectively maintain long-term O3
concentrations below those where the study by Jerrett et al. (2009)
indicates the most confidence in the reported association with
respiratory mortality. Specifically, air quality analyses indicate this
to be the case in 10 out of the 12 urban study areas for a level of 65
ppb, compared to 6 out of 12 areas for the current standard and 9 out
of 12 for a standard with a level of 70 ppb (U.S. EPA, 2014c, Table 4-
3). Finally, a revised standard with a level of 65 ppb would be
expected to further reduce long-term O3 concentrations based
on the types of metrics that have been reported in recent epidemiologic
studies to be associated with respiratory morbidity (i.e., seasonal
averages of daily maximum 8-hour concentrations).
---------------------------------------------------------------------------

    \149\ Though as discussed above, the PA notes the lower
confidence placed in these risk results (U.S. EPA, 2014a, section
9.6).
---------------------------------------------------------------------------

    In further considering the potential implications of epidemiology
studies for alternative standard levels, the PA notes estimates of
total mortality associated with short-term O3.\150\ As
discussed above, the PA considers estimates of total risk (i.e., based
on the full distributions of ambient O3 concentrations) and
estimates of risk associated with O3 concentrations in the
upper portions of ambient distributions. With regard to total risk the
PA notes that, when summed across urban study areas, a standard with a
level of 65 ppb is estimated to reduce the number of deaths associated
with short-term O3 exposures by about 13% (2007) and 9%
(2009), compared to the current standard.\151\ For area-wide
concentrations at or above 40 ppb, a standard level of 65 ppb is
estimated to reduce O3-associated deaths by almost 50%
compared to the current standard, when summed across urban study areas.
For area-wide concentrations at or above 60 ppb, a standard level of 65
ppb is estimated to reduce O3-associated deaths by more than
80% (U.S. EPA, 2014c, Figure 4-13).
---------------------------------------------------------------------------

    \150\ As discussed above, compared to the weight given to the
evidence and to HREA estimates of exposures of concern and lung
function risks, the PA places relatively less weight on
epidemiologic-based risk estimates.
    \151\ A standard with a level of 65 ppb is also estimated to
reduce respiratory mortality associated with long-term O3
concentrations in urban study areas. However, given uncertainties
associated with these risk estimates, as discussed above, we give
them limited weight.
---------------------------------------------------------------------------

    In summarizing CASAC's advice regarding a standard with a level of
65, the PA noted CASAC's conclusion that an alternative standard with a
level of 65 ppb would further reduce, though not eliminate, the
frequency of lung function decrements >=15% and would lead to lower
frequency of short-term premature mortality (i.e., compared to a
standard with a level of 70 ppb) (Frey, 2014c, p. 8).
    In summary, compared to a standard with a level of 70 ppb, the PA
concludes that a revised standard with a level of

65 ppb would be expected to further reduce O3 exposures and
health risks. In particular, a standard with a level of 65 ppb is
estimated to (1) reduce the occurrence of exposures of concern by about
80 to 100%, compared to the current standard, decreasing exposures at
or above 60 ppb and almost eliminating exposures at or above 70 and 80
ppb; (2) reduce the occurrence of FEV1 decrements >=10, 15,
and 20% by about 30 to 65%, compared to the current standard; (3) more
effectively maintain short- and long-term O3 concentrations
below those present in the epidemiologic studies that reported
significant O3 health effect associations in locations
likely to have met the current standard; \152\ and (4) further reduce
the risk of O3-associated mortality and morbidity,
particularly the risk associated with the upper portion of the
distribution of ambient O3 concentrations.
---------------------------------------------------------------------------

    \152\ Though epidemiologic studies also provide evidence for
O3 health effect associations in locations likely to have
met a standard with a level of 65 ppb, as discussed below for a
level of 60 ppb.
---------------------------------------------------------------------------

iii. PA Consideration of an O3 Standard Level of 60 ppb
    The PA also considers a standard with a level of 60 ppb. A level of
60 ppb is well below the O3 exposure concentration that has
been reported to elicit a wide range of potentially adverse respiratory
effects in healthy adults (i.e., 80 ppb). A level of 60 ppb is also
below the lowest concentration where the combined occurrence of
respiratory symptoms and lung function decrements was observed, a
combination judged adverse by the ATS (i.e., 72 ppb). A level of 60 ppb
corresponds to the lowest exposure concentration demonstrated to result
in lung function decrements that are large enough to be judged an
abnormal response by ATS, that meet ATS criteria for adversity based on
a downward shift in the distribution of FEV1, and that the CASAC
indicated could be adverse in people with lung disease. A level of 60
ppb also corresponds to the lowest exposure concentration at which
pulmonary inflammation has been reported in a single controlled human
exposure study.
    Based on the HREA analyses of O3 exposures of concern, a
standard with a level of 60 ppb is estimated to eliminate exposures of
concern at or above the 70 and 80 ppb benchmark concentrations and to
be more effective than the higher standard levels at limiting exposures
of concern at or above 60 ppb. On average over the years 2006 to 2010,
for a standard with a level of 60 ppb, between 0 and about 1% of
children, including asthmatic children, in urban study areas are
estimated to experience exposures of concern at or above 60 ppb, which
CASAC indicated is an appropriate exposure of concern for asthmatic
children. This reflects a 96% reduction (on average across areas),
compared to the current standard. Virtually no children are estimated
to experience two or more exposures of concern at or above 60 ppb. In
the worst-case location and year, about 2% of children are estimated to
experience exposures of concern at or above 60 ppb, with far less than
1% estimated to experience two or more such exposures (Table 4, above).
    Based on the HREA analyses of O3-induced lung function
decrements, a standard with a level of 60 ppb would be expected to be
more effective than a level of 65 or 70 ppb at limiting the occurrence
of O3-induced lung function decrements. For a standard with
a level of 60 ppb, about 2% of children, or less, in the urban study
areas are estimated to experience one or more moderate O3-
induced FEV1 decrements >=15% (almost 70% reduction,
compared to current standard), and about 1% or less are estimated to
experience two or more such decrements (3% in the location and year
with the largest estimates). About 1% of children, or less, are
estimated to experience large O3-induced lung function
decrements (i.e., FEV1 decrement >=20%), even in the worst-
case locations and year (Table 5, above).
    In addition, for a standard with a level of 60 ppb, about 5 to 11%
of children in the urban study areas are estimated to experience one or
more moderate O3-induced lung function decrements that CASAC
indicated could be adverse for people with lung disease (i.e.,
FEV1 decrements >=10%). This reflects an average reduction
of about 45%, compared to the current standard. About 2 to 6% of
children in these areas are estimated to experience two or more such
decrements (51% reduction, compared to current standard). In the worst-
case location and year, for a standard set at 60 ppb, up to about 13%
of children are estimated to experience one or more moderate
O3-induced FEV1 decrements >=10%, and 7% are
estimated to experience two or more such decrements (Table 5, above).
    With regard to O3 epidemiologic studies, the PA notes
that a revised standard with a level of 60 ppb would be expected to
maintain short-term ambient O3 concentrations below those
present in some of the study locations that provided the basis for
reported O3 health effect associations and that were likely
to have met a revised standard with a level of 70 or 65 ppb.
Specifically, in all of the U.S. and Canadian epidemiologic studies
evaluated, the majority of study cities had ambient O3
concentrations that would likely have violated a standard with a level
of 60 ppb. Thus, none of the U.S. and Canadian epidemiologic studies
analyzed provide evidence for O3 health effect associations
when the majority of study locations would likely have met a standard
with a level of 60 ppb (U.S. EPA, 2014c, Tables 4-1 and 4-2). As
discussed above, while this analysis does not provide information on
the extent to which the O3-associated morbidity or mortality
would persist upon meeting an O3 standard with a level of 60
ppb, it suggests that a revised O3 standard with a level of
60 ppb would require reductions in the ambient O3
concentrations that provided the basis for those health effect
associations.
    With regard to long-term O3 concentrations, compared to
the current standard or a revised O3 standard with a level
of 65 or 70 ppb, a revised standard with a level of 60 ppb would be
expected to further reduce the risk of respiratory mortality associated
with long-term O3 concentrations, based on information from
the study by Jerrett et al. (2009).\153\ In addition, a standard with a
level of 60 ppb would be expected to more effectively maintain long-
term O3 concentrations below those where the study by
Jerrett et al. (2009) indicates the most confidence in the reported
association with respiratory mortality. Specifically, air quality
analyses indicate this to be the case in all of the urban study areas
evaluated at a level of 60 ppb, compared to 6 out of 12 areas for the
current standard, 9 out of 12 for a standard with a level of 70 ppb,
and 10 out of 12 for a standard with a level of 65 ppb (U.S. EPA,
2014c, Table 4-3). Finally, a revised standard with a level of 60 ppb
would be expected to further reduce long-term O3
concentrations based on the types of metrics that have been reported in
recent epidemiologic studies to be associated with respiratory
morbidity (i.e., seasonal averages of daily maximum 8-hour
concentrations).
---------------------------------------------------------------------------

    \153\ Though as discussed above, the PA notes the lower
confidence we place in these risk results (U.S. EPA, 2014a, section
9.6).
---------------------------------------------------------------------------

    In further considering the potential implications of epidemiology
studies for alternative standard levels, the PA notes estimates of
total mortality associated with short-term O3
concentrations.\154\

As discussed above, the PA considers estimates of total risk (i.e.,
based on the full distributions of ambient O3
concentrations) and estimates of risk associated with O3
concentrations in the upper portions of ambient distributions. With
regard to total risk the PA notes that, when summed across urban study
areas, a standard with a level of 60 ppb is estimated to reduce the
number of deaths associated with short-term O3 exposures by
about 15% (2007) and 11% (2009), compared to the current standard (U.S.
EPA, 2014c, Figure 4-13).\155\ For area-wide concentrations at or above
40 ppb, a standard with a level set at 60 ppb is estimated to reduce
O3-associated deaths by almost 60% compared to the current
standard. For area-wide concentrations at or above 60 ppb, a standard
level of 60 ppb is estimated to reduce O3-associated deaths
by over 95% compared to the current standard.
---------------------------------------------------------------------------

    \154\ As discussed above, compared to the weight given to the
evidence and to HREA estimates of exposures of concern and lung
function risks, we place relatively less weight on epidemiologic-
based risk estimates.
    \155\ A standard with a level of 60 ppb is also estimated to
reduce respiratory mortality associated with long-term O3
concentrations in urban study areas. However, given uncertainties
associated with these risk estimates, as discussed above, the PA
gives them limited weight.
---------------------------------------------------------------------------

    In summary, compared to a standard with a level of 65 or 70 ppb,
the PA concludes that a revised standard with a level of 60 ppb would
be expected to further reduce O3 exposures and health risks.
In particular, a standard with a level of 60 ppb is estimated to (1)
reduce the occurrence of exposures of concern by about 95 to 100%,
compared to the current standard, almost eliminating exposures at or
above 60 ppb; (2) reduce the occurrence of FEV1 decrements
>=10, 15, and 20% by about 45 to 85%, compared to the current standard;
(3) more effectively maintain short- and long-term O3
concentrations below those present in the epidemiologic studies that
reported significant O3 health effect associations in
locations likely to have met the current standard; \156\ and (4)
further reduce the risk of O3-associated mortality and
morbidity, particularly the risk associated with the upper portion of
the distribution of ambient O3 concentrations.
---------------------------------------------------------------------------

    \156\ As discussed above, these studies do not provide
information on the extent to which O3 health effect
associations would persist following reductions in ambient
O3 concentrations in order to meet a standard with a
level of 60 ppb.
---------------------------------------------------------------------------

c. CASAC Advice
    The PA recognizes that decisions regarding the weight to place on
various types of evidence, exposure/risk information, and associated
uncertainties reflect public health policy judgments that are
ultimately left to the Administrator. To help inform those judgments
with regard to the range of alternative primary O3 standard
levels appropriate for consideration, CASAC has provided advice to the
Administrator based on their reviews of draft versions of the
O3 ISA, HREA, and PA. This section summarizes the advice
provided by CASAC regarding alternative standard levels, as well as the
views expressed at the CASAC meetings by public commenters. This
section includes CASAC advice from the reconsideration of the 2008
final decision on the level of the standard, as well as CASAC advice
received during the current review as it pertains to alternative
standards.
    Consistent with its advice in 2008, CASAC reiterated during the
reconsideration its support for an 8-hour primary O3
standard with a level ranging from 60 to 70 ppb, combined with the
current indicator, averaging time, and form. Specifically, in response
to the EPA's solicitation of CASAC advice during the reconsideration,
the CASAC letter (Samet, 2010) to the Administrator stated:

CASAC fully supports EPA's proposed range of 0.060-0.070 parts per
million (ppm) for the 8-hour primary ozone standard. CASAC considers
this range to be justified by the scientific evidence as presented
in the Air Quality Criteria for Ozone and Related Photochemical
Oxidants (March 2006) and Review of the National Ambient Air Quality
Standards for Ozone: Policy Assessment of Scientific and Technical
Information, OAQPS Staff Paper (July 2007).

    Similarly, in response to the EPA's request for additional advice
on the reconsideration in 2011, CASAC reaffirmed its conclusion that
``the evidence from controlled human and epidemiologic studies strongly
supports the selection of a new primary ozone standard within the 60-70
ppb range for an 8-hour averaging time'' (Samet, 2011). CASAC further
concluded that this range ``would provide little margin of safety at
its upper end'' (Samet, 2011, p. 2).
    In the current review of the Second Draft PA, CASAC concurred with
staff's conclusions that it is appropriate to consider retaining the
current indicator (O3), averaging time (8-hour average) and
form (3-year average of the 4th highest maximum daily 8-hour average.
With regard to level, CASAC stated the following (Frey, 2014c, pp. ii
to iii):

The CASAC further concludes that there is adequate scientific
evidence to recommend a range of levels for a revised primary ozone
standard from 70 ppb to 60 ppb. The CASAC reached this conclusion
based on the scientific evidence from clinical studies,
epidemiologic studies, and animal toxicology studies, as summarized
in the Integrated Science Assessment (ISA), the findings from the
exposure and risk assessments as summarized in the HREA, and the
interpretation of the implications of these sources of information
as given in the Second Draft PA.
    The CASAC acknowledges that the choice of a level within the
range recommended based on scientific evidence [i.e., 70 to 60 ppb]
is a policy judgment under the statutory mandate of the Clean Air
Act. The CASAC advises that, based on the scientific evidence, a
level of 70 ppb provides little margin of safety for the protection
of public health, particularly for sensitive subpopulations.
    Thus, our policy advice is to set the level of the standard
lower than 70 ppb within a range down to 60 ppb, taking into account
your judgment regarding the desired margin of safety to protect
public health, and taking into account that lower levels will
provide incrementally greater margins of safety.

    The public commenters who expressed the view that the current
primary O3 standard is not adequate (II.D.3) also submitted
comments that supported revising the level of the primary O3
standard. Several of these commenters expressed the view that the level
should be revised to the lower end of the range of 70 to 60 ppb, or in
some cases to a level below 60 ppb. These commenters often placed a
large amount of emphasis on evidence from controlled human exposure
studies for respiratory effects following exposures to 60 ppb
O3.
    In addition, as discussed above (II.D.3), some public commenters
expressed the view that revision of the current standard is not
necessary. Consistent with their view that it would not be appropriate
to revise the current standard, these commenters did not provide any
provisional views on alternative levels below 75 ppb that would be
appropriate for consideration.
d. Administrator's Proposed Conclusions on Level
    This section discusses the Administrator's proposed conclusions on
the level of the primary O3 standard. In conjunction with
her proposed decisions to retain the current indicator, averaging time,
and form (II.E.1 to II.E.3, above), the Administrator proposes to
revise the level of the primary O3 standard to within the
range of 65 to 70 ppb. In doing so, she is mindful that the selection
of a primary O3 standard that is requisite to protect public
health with an adequate margin of safety requires judgments based on an
interpretation of the scientific evidence and exposure/risk information
that neither overstates nor understates the strengths and limitations
of that evidence and information, nor the appropriate

inferences to be drawn therefrom.\157\ The rationale supporting the
Administrator's proposed conclusions on alternative standard levels is
discussed below.
---------------------------------------------------------------------------

    \157\ As discussed above (I.B), in addressing the requirement
for an adequate margin of safety the EPA considers such factors as
the nature and severity of the health effects, the size of sensitive
population(s) at risk, and the kind and degree of the uncertainties
that must be addressed. The selection of any particular approach for
providing an adequate margin of safety is a policy choice left
specifically to the Administrator's judgment. See Lead Industries
Association v. EPA, 647 F. 2d at 1161-62; State of Mississippi, 744
F. 3d at 1353.
---------------------------------------------------------------------------

    The Administrator's proposed conclusions on alternative standard
levels build upon her proposed conclusion that the overall body of
scientific evidence and exposure/risk information call into question
the adequacy of public health protection afforded by the current
primary O3 standard, particularly for at-risk populations
and lifestages (II.D.5). These proposed conclusions are based on
consideration of the scientific evidence assessed in the ISA (U.S. EPA,
2013a); the results of the exposure and risk assessments in the HREA
(U.S. EPA, 2014a); the evidence-based and exposure-/risk-based
considerations and conclusions in the PA (U.S. EPA, 2014c); CASAC
advice and recommendations, as reflected in CASAC's letters to the
Administrator and in public discussions of drafts of the ISA, HREA, and
PA; and public input received during the development of these
documents.
    In reaching proposed conclusions on alternative levels for the
primary O3 standard, the Administrator considers the extent
to which various alternatives would be expected to protect the public,
including at-risk populations, against the wide range of adverse health
effects that have been linked with short- or long-term O3
exposures. At-risk populations include people with asthma; children and
older adults; people who are active outdoors, including outdoor
workers; people with certain genetic variants; and people with reduced
intake of certain nutrients.
    As was the case for her consideration of the adequacy of the
current primary O3 standard (II.D.5), the Administrator
places the greatest weight on the results of controlled human exposure
studies and on exposure and risk analyses based on information from
these studies. In doing so, she notes that controlled human exposure
studies provide the most certain evidence indicating the occurrence of
health effects in humans following exposures to specific O3
concentrations. The effects reported in these studies are due solely to
O3 exposures, and interpretation of study results is not
complicated by the presence of co-occurring pollutants or pollutant
mixtures (as is the case in epidemiologic studies). She further notes
the CASAC judgment that ``the scientific evidence supporting the
finding that the current standard is inadequate to protect public
health is strongest based on the controlled human exposure studies of
respiratory effects'' (Frey, 2014c, p. 5). Consistent with this
emphasis, the HREA conclusions reflect relatively greater confidence in
the results of the exposure and risk analyses based on information from
controlled human exposure studies (i.e., exposures of concern and risk
of lung function decrements) than the results of epidemiology-based
risk analyses, given the greater uncertainties in the epidemiology-
based risk estimates (U.S. EPA, 2014a, section 9.6). For all of these
reasons, the Administrator has the most confidence in using the
information from controlled human exposure studies to reach proposed
conclusions on alternative standard levels.
    In considering the evidence from controlled human exposure studies,
the Administrator first notes that these studies have reported a
variety of respiratory effects in healthy adults following exposures to
O3 concentrations of 60,\158\ 72,\159\ or 80 ppb, and
higher. The largest respiratory effects, and the broadest range of
effects, have been studied and reported following exposures of healthy
adults to 80 ppb O3 or higher, with most exposure studies
conducted at these higher concentrations. Exposures of healthy adults
to O3 concentrations of 80 ppb or higher have been reported
to decrease lung function, increase airway inflammation, increase
respiratory symptoms, result in airway hyperresponsiveness, and
decrease lung host defenses (II.B.2).
---------------------------------------------------------------------------

    \158\ As discussed above (II.B.2), exposures to 60 ppb
O3 have been evaluated in studies by Adams (2002, 2006),
Schelegle et al. (2009), and Kim et al. (2011). In the study by
Schelegle, for the 60 ppb target exposure concentration, study
authors reported that the actual mean exposure concentration was 63
ppb.
    \159\ As noted above, for the 70 ppb target exposure
concentration, Schelegle et al. (2009) reported that the actual mean
exposure concentration was 72 ppb.
---------------------------------------------------------------------------

    The Administrator notes that O3 exposure concentrations
as low as 72 ppb have been shown to both decrease lung function and
increase respiratory symptoms (Schelegle et al., 2009), a combination
that meets the ATS criteria for an adverse response. In considering
effects at 72 ppb, CASAC likewise noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5).
    With regard to lower exposure concentrations, the Administrator
notes that the combination of statistically significant increases in
respiratory symptoms and decrements in lung function has not been
reported. More specifically, she notes that respiratory symptoms have
been evaluated following 6.6-hour exposures to average O3
concentrations of 60 ppb (Adams, 2006; Kim et al., 2011) and 63 ppb
(Schelegle et al., 2009) and that none of these studies reported a
statistically significant increase in respiratory symptoms, compared to
filtered air controls.\160\
---------------------------------------------------------------------------

    \160\ However, following exposures to 60 ppb O3,
several studies have observed decreases in lung function and one
study (Kim et al., 2011) observed an increase in airway inflammation
(II.B.2).
---------------------------------------------------------------------------

    Based on this evidence, the Administrator reaches the initial
conclusion that the results of controlled human exposure studies
strongly support setting the level of a revised O3 standard
no higher than 70 ppb. In reaching this initial conclusion, the
Administrator places a large amount of weight on the importance of
setting the level of the standard well below 80 ppb, the O3
exposure concentration shown in healthy adults to result in the
broadest range of respiratory effects, and below 72 ppb, the lowest
O3 exposure concentration shown in healthy adults to result
in the adverse combination of respiratory symptoms and lung function
decrements.
    In further considering the potential public health implications of
a standard with a level of 70 ppb, the Administrator also considers the
extent to which such a standard would be expected to limit population
exposures to the broader range of O3 concentrations reported
in controlled human exposure studies to cause respiratory effects.
Given the range of effects reported following exposures to 80 ppb
O3, and the evidence for the adverse combination of lung
function decrements and respiratory symptoms in healthy adults
following exposures as low as 72 ppb, the Administrator concludes that
the evidence in this review supports the occurrence of adverse
respiratory effects for exposures to O3 concentrations at or
above 72 ppb.
    The Administrator has decreasing confidence that adverse effects
will occur following exposures to O3 concentrations below 72
ppb. In particular, compared to O3 exposure

concentrations at or above 72 ppb, she has less confidence that adverse
effects will occur following exposures to O3 concentrations
as low as 60 ppb. In reaching this conclusion, she notes that, as
discussed above, statistically significant increases in respiratory
symptoms, combined with lung function decrements, have not been
reported following exposures to 60 or 63 ppb O3, though
several studies have evaluated the potential for such effects.
    Although she has decreasing confidence in the occurrence of adverse
effects following exposures to O3 concentrations below 72
ppb, the Administrator notes the CASAC judgment that the adverse
combination of lung function decrements and respiratory symptoms
``almost certainly occur in some people'' following exposures to lower
concentrations (Frey, 2014c, p. 6). In particular, when commenting on
the extent to which the study by Schelegle et al. (2009) suggests the
potential for adverse effects following O3 exposures below
72 ppb, CASAC judged that:

[I]f subjects had been exposed to ozone using the 8-hour averaging
period used in the standard [i.e., rather than the 6.6 hour
exposures evaluated in the study], adverse effects could have
occurred at lower concentration. Further, in our judgment, the level
at which adverse effects might be observed would likely be lower for
more sensitive subgroups, such as those with asthma [i.e., compared
to the healthy adults evaluated in the study] (Frey, 2014c, p. 5).

Though CASAC did not provide advice as to how far below 72 ppb adverse
effects would likely occur, the Administrator agrees that such effects
could occur following exposures at least somewhat below 72 ppb.
    Based on the evidence and CASAC advice noted above, when
considering the extent to which a standard with a level of 70 ppb would
be expected to limit population exposures to the broader range of
O3 concentrations shown to cause respiratory effects, the
Administrator considers the extent to which such a standard would be
expected to limit the occurrence of O3 exposures of concern
at or above 60, 70, and 80 ppb.\161\ In doing so, she notes that an
O3 standard established at a particular level can provide
protection against a range of exposure concentrations, including
concentrations below the standard level. This is because the degree of
protection provided by any NAAQS is due to the combination of all of
the elements of the standard (i.e., indicator, averaging time, form,
level). In the case of the 4th maximum form of the O3 NAAQS,
which the Administrator is proposing to retain in the current review
(II.E.3), the large majority of days in areas that meet the standard
will have 8-hour O3 concentrations below the level of the
standard.
---------------------------------------------------------------------------

    \161\ As with her consideration of the current standard
(II.D.5), the Administrator focuses on estimated exposures of
concern in children, including asthmatic children, noting the HREA
analyses indicating that exposures of concern occur in a larger
percentage of children than adults (given that a larger percentage
of children are estimated to spend an extended period of time being
physically active outdoors when O3 concentrations are
elevated) (II.C.2). To the extent alternative standards provide an
appropriate degree of protection for children, she judges that those
standards will also protect adult populations (including at-risk
adult populations).
---------------------------------------------------------------------------

    In considering exposures of concern at or above 60, 70, and 80 ppb,
the Administrator judges that the evidence supporting the occurrence of
adverse respiratory effects is strongest for exposures at or above the
70 and 80 ppb benchmarks. While the Administrator has less confidence
that adverse effects will occur following exposures to O3
concentrations as low as 60 ppb, she notes the possibility for adverse
effects following such exposures given that (1) CASAC has indicated the
moderate lung function decrements (i.e., FEV1 decrements
>=10%) that occur in some healthy adults following exposures to 60 ppb
O3, which are large enough to be judged an abnormal response
by ATS, could be adverse to people with lung disease (II.B.3), and that
(2) airway inflammation has been reported following exposures as low as
60 ppb O3. She also takes note of CASAC advice that the
occurrence of exposures of concern at or above 60 ppb is an appropriate
consideration for people (including children) with asthma (Frey, 2014c,
p. 6).
    Due to interindividual variability in responsiveness, the
Administrator further notes that not every occurrence of an exposure of
concern will result in an adverse effect.\162\ Repeated occurrences of
some of the effects demonstrated following exposures of concern could
increase the likelihood of adversity. For example, as discussed in the
ISA (U.S. EPA, 2013a, Section 6.2.3), repeated occurrences of airway
inflammation could lead to the induction of a chronic inflammatory
state; altered pulmonary structure and function, leading to diseases
such as asthma; altered lung host defense response to inhaled
microorganisms, particularly in potentially at-risk populations such as
the very young and old; and altered lung response to other agents such
as allergens or toxins. The Administrator notes that the types of lung
injury that can occur following exposures of concern, particularly if
experienced repeatedly, provide a plausible mode of action by which
O3 may cause other more serious effects. Therefore, the
Administrator is most concerned about protecting at-risk populations
against repeated occurrences of exposures of concern.
---------------------------------------------------------------------------

    \162\ For most of the effects demonstrated in controlled human
exposure studies (e.g., airway inflammation, AHR, decreased lung
host defense, respiratory symptoms) the available data are not
sufficient to quantify the number of people who would experience
adverse effects due to O3 exposures.
---------------------------------------------------------------------------

    Based on the above considerations, the Administrator focuses on the
extent to which a revised standard would be expected to protect
populations from experiencing two or more O3 exposures of
concern (i.e., as a surrogate for repeated exposures). While she
emphasizes the importance of limiting two or more exposures and
reducing their occurrence, compared to the current standard, she
balances this emphasis by noting that (1) not all exposures of concern
will result in adverse effects; (2) she has less confidence in the
occurrence of adverse effects at the 60 ppb benchmark than at the 70 or
80 ppb benchmarks; and (3) the NAAQS are not meant to be zero-risk
standards.\163\ Therefore, in using estimates of exposures of concern
to inform her decisions on alternative standard levels, the
Administrator judges that it would not be appropriate to set a standard
intended to eliminate all exposures of concern for all benchmarks,
particularly the 60 ppb benchmark. Her consideration of specific
estimates of exposures of concern is discussed below.
---------------------------------------------------------------------------

    \163\ State of Mississippi, 744 F. 3d at 1343.
---------------------------------------------------------------------------

    As illustrated in Table 1 (above), the Administrator notes that, in
urban study areas, a revised standard with a level of 70 ppb would be
expected to eliminate the occurrence of two or more exposures of
concern to O3 concentrations at and above 80 ppb and to
virtually eliminate the occurrence of two or more exposures of concern
to O3 concentrations at and above 70 ppb, even in the worst-
case urban study area and year. For the 70 ppb benchmark, this reflects
about a 95% reduction in the occurrence of two or more exposures of
concern, compared to the current standard (Table 4).
    Though the Administrator acknowledges greater uncertainty with
regard to the occurrence of adverse effects following exposures of
concern at or above 60 ppb, she notes that a revised standard with a
level of 70 ppb would also be expected to protect the large majority of
children in the urban study areas (i.e., about 96% to more

than 99% of children in individual urban study areas) from experiencing
two or more exposures of concern at or above 60 ppb. Compared to the
current standard, this represents a reduction of more than 60% in the
occurrence of two or more exposures of concern (Tables 1 and 4).
    Based on the above information, the Administrator concludes that a
revised O3 standard with a level of 70 ppb would be expected
to virtually eliminate the occurrence of two or more O3
exposures of concern for the 70 and 80 ppb benchmarks, and to
substantially reduce the occurrence of two or more O3
exposures of concern for the 60 ppb benchmark, compared to the current
standard.
    Although the Administrator is less concerned about single
occurrences of exposures of concern, she acknowledges that even single
exposures to O3 concentrations at or above benchmark
concentrations (particularly for the 70 and 80 ppb benchmarks) could
potentially result in adverse effects. To the extent this may be the
case, the Administrator notes that a standard with a level of 70 ppb
would also be expected to (1) virtually eliminate all occurrences of
exposures of concern at or above 80 ppb, even in the worst-case year
and location and (2) achieve important reductions, compared to the
current standard, in the occurrence of one or more exposures of concern
at or above 70 and 60 ppb (i.e., about a 70% reduction for the 70 ppb
benchmark and almost a 50% reduction for the 60 ppb benchmark) (Tables
1 and 4).
    In further evaluating the potential public health impacts of a
standard with a level of 70 ppb, the Administrator also considers the
HREA estimates of O3-induced lung function decrements. To
inform her consideration of these decrements, the Administrator takes
note of CASAC advice that ``estimation of FEV1 decrements of
>=15% is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults, whereas an FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3). Consistent with this advice, she considers estimates of the
occurrence of O3-induced FEV1 decrements >=10 and
15% as surrogates for the occurrence of adverse health outcomes.
    While these surrogates provide perspective on the potential for the
occurrence of adverse respiratory effects following O3
exposures, the Administrator agrees with the conclusion in past reviews
that a more general consensus view of the adversity of moderate
responses emerges as the frequency of occurrence increases (61 FR
65722-3) (Dec. 13, 1996). Specifically, she concludes that not every
estimated occurrence of an O3-induced FEV1
decrement will be adverse and that repeated occurrences of moderate
responses, even in otherwise healthy individuals, may be considered to
be adverse since they could set the stage for more serious illness.
Therefore, the Administrator becomes increasingly concerned about the
potential for adversity as the frequency of occurrences increases and,
as a result, she focuses primarily on estimates of two or more
O3-induced FEV1 decrements (i.e., as a surrogate
for repeated exposures).
    Given the above considerations, the Administrator does not believe
it would be appropriate to set a standard that is intended to eliminate
all O3-induced FEV1 decrements. She notes that
this is consistent with CASAC advice, which did not include a
recommendation to set the standard level low enough to eliminate all
O3-induced FEV1 decrements >=10 or 15% (Frey,
2014c). Rather, the Administrator considers the extent to which a
standard with a level of 70 ppb would be expected to protect the
population from experiencing O3-induced FEV1
decrements >=10 and 15%, including the extent to which such a standard
would be expected to achieve reductions in the occurrence of
O3-induced FEV1 decrements, relative to the
current standard.\164\
---------------------------------------------------------------------------

    \164\ The Administrator additionally notes that, unlike
exposures of concern, the variability in lung function risk
estimates across urban study areas is often greater than the
differences in risk estimates between various standard levels (Table
2, above). Given this, and the resulting considerable overlap
between the ranges of lung function risk estimates for different
standard levels, although the Administrator has confidence in the
lung function risk estimates themselves, she views them as providing
a more limited basis than exposures of concern for distinguishing
between the degree of public health protection provided by
alternative standard levels.
---------------------------------------------------------------------------

    The Administrator notes that a revised O3 standard with
a level of 70 ppb is estimated to protect about 98 to 99% of children
in urban study areas from experiencing two or more O3-
induced FEV1 decrements >=15%, and about 89 to 94% from
experiencing two or more decrements >=10%.\165\ Compared to the current
standard, these estimates represent decreases in the occurrence of two
or more O3-induced decrements of about 29 and 17%,
respectively (Tables 2 and 5). Although the Administrator is less
concerned about the public health implications of single O3-
induced lung function decrements, she also gives some consideration to
estimates of one or more O3-induced FEV1
decrements. In particular, she notes that a revised standard with a
level of 70 ppb is estimated to reduce the occurrence of one or more
O3-induced decrements, compared to the current standard, by
about 26% (for decrements >=15%) and 15% (for decrements >=10%) (Tables
2 and 5).
---------------------------------------------------------------------------

    \165\ In the worst-case year and location, a standard with a
level of 70 ppb is estimated to protect about 97% of children in
urban study areas from experiencing two or more O3-
induced FEV1 decrements >=15%, and about 87% from
experiencing two or more decrements >=10%.
---------------------------------------------------------------------------

    Given all of the above information, the Administrator concludes
that a revised standard with a level of 70 ppb would be expected to
provide substantial protection against O3 exposures of
concern (for benchmark concentrations of 60, 70, 80 ppb) and
O3-induced lung function decrements, and would be expected
to result in important reductions in the occurrence of such exposures
and decrements, compared to the current standard. This is particularly
the case for estimates of two or more occurrences of exposures of
concern and lung function decrements.
    In next considering the additional protection that would be
expected from standard levels below 70 ppb, the Administrator evaluates
the extent to which a standard with a level of 65 ppb would be expected
to further limit O3 exposures of concern and O3-
induced lung function decrements.
    In addition to eliminating almost all exposures of concern to
O3 concentrations at or above 80 and 70 ppb, even in the
worst-case years and locations, the Administrator notes that a revised
standard with a level of 65 ppb would be expected to protect more than
99% of children in urban study areas (and 100% of children in some
urban study areas) from experiencing two or more exposures of concern
at or above 60 ppb. Compared to the current standard, this represents
about a 95% reduction in the occurrence of two or more exposures of
concern for the 60 ppb benchmark (Tables 1 and 4). In addition, the
Administrator notes that a revised standard with a level of 65 ppb is
estimated to reduce the occurrence of one or more exposures of concern
for the 60 ppb benchmark by about 80%, compared to the current standard
(Tables 1 and 4).
    With regard to O3-induced lung function decrements, the
Administrator notes that an O3 standard with a level of 65
ppb is estimated to protect about 98% to more than 99% of children from
experiencing two or more O3-induced FEV1
decrements >=15%, even considering the worst-case year and location,
and about 91 to 99% from

experiencing two or more decrements >=10% (89% in worst-case year and
location). These estimates reflect reductions, compared to the current
standard, of about 54 and 37%, respectively. A revised standard with a
level of 65 ppb is also estimated to reduce the occurrence of one or
more lung function decrements >=15 and 10%, compared to the current
standard, by about 50 and 31%, respectively.
    Taken together, the Administrator initially concludes that the
evidence from controlled human exposure studies, and the information
from quantitative analyses that draw upon these studies (i.e.,
exposures of concern, O3-induced FEV1
decrements), provide strong support for standard levels from 65 to 70
ppb. In particular, she bases this conclusion on the fact that such
standard levels would be well below the O3 exposure
concentration shown to result in the widest range of respiratory
effects (i.e., 80 ppb), and below the lowest O3 exposure
concentration shown to result in the adverse combination of lung
function decrements and respiratory symptoms (i.e., 72 ppb). A standard
with a level from 65 to 70 ppb would also be expected to result in
important reductions, compared to the current standard, in the
occurrence of O3 exposures of concern for all of the
benchmarks evaluated (i.e., 60, 70, and 80 ppb) and in the risk of
O3-induced lung function decrements >=10 and 15%.
    In further considering the evidence and exposure/risk information,
the Administrator considers the extent to which the epidemiologic
evidence, and the quantitative risk estimates based on information from
epidemiologic studies, also provide support for standard levels from 65
to 70 ppb. In doing so, as in her consideration of the adequacy of the
current O3 standard, the Administrator focuses on
epidemiologic studies of respiratory-related hospital admissions,
emergency department visits, and mortality. These considerations are
discussed below.
    The Administrator first considers the extent to which available
epidemiologic studies have reported associations between short-term
O3 concentrations and emergency department visits, hospital
admissions, and/or mortality in locations that would likely have met
alternative standards with levels from 65 to 70 ppb (U.S. EPA, 2014c,
section 4.4.1). In evaluating the epidemiologic evidence in this way,
the Administrator places the most weight on single-city studies of
short-term O3 concentrations, recognizing that there were no
multicity studies for which air quality data indicated that all cities
included in the analyses would likely have met alternative standard
levels. In particular, she notes that while single-city studies are
more limited than multicity studies in terms of statistical power and
geographic coverage, conclusions linking air quality in a given city
with health effect associations in that same city can be made with
greater certainty for single-city studies of short-term O3,
compared to health effect associations aggregated across multiple
cities in multicity studies. In particular, the Administrator notes
considerable uncertainty in linking multicity effect estimates
(aggregated across multiple cities) for short-term O3 with
the air quality for subsets of study locations (rather than all
locations) likely to have met an alternative standard.\166\
---------------------------------------------------------------------------

    \166\ In recognizing that multicity studies are often emphasized
over single-city studies for purposes of making weight of evidence
judgments (U.S. EPA, 2013a), the Administrator's judgment in this
case applies specifically to interpreting air quality analyses for
epidemiologic studies of short-term O3 concentrations
where multicity effect estimates are aggregated across cities, and
where individual city effect estimates are not presented (as is the
case for the key O3 studies analyzed in the PA, with the
exception of the study by Stieb et al. (2009) where none of the
city-specific effect estimates for asthma emergency department
visits were statistically significant). Because reported multicity
effect estimates do not allow health effect associations to be
disaggregated by individual city, it is not possible to assign the
multicity health effect association to the air quality in any one
study location, or to the air quality in a particular subset of
locations. In contrast, for epidemiologic studies of long-term
concentrations, where multicity effect estimates are based on
comparisons across cities, different judgments have been made by EPA
with regard to the utility of multicity studies (see, e.g. 78 FR
3086 at 3103/2, January 15, 2013) (and see discussion below of study
by Jerrett et al., 2009).
---------------------------------------------------------------------------

    Given the above, the Administrator notes analyses in the PA (U.S.
EPA, 2014c, section 4.4.1) indicating that a revised standard with a
level of 65 or 70 ppb would be expected to maintain short-term ambient
O3 concentrations below those present in the locations of
all of the single-city studies analyzed. As discussed in the PA (U.S.
EPA, 2014c, section 4.4.1), this includes several single-city studies
conducted in locations that would have violated the current standard,
and the single-city study by Mar and Koenig (2009) that reported
positive and statistically significant associations with respiratory
emergency department visits with children and adults in a location that
likely would have met the current standard over the entire study period
but that would likely not have met a revised standard with a level of
70 ppb or below. Thus, the Administrator notes that, while the current
standard would allow the ambient O3 concentrations that
provided the basis for the health effect associations reported by Mar
and Koenig (2009), a revised O3 standard with a level at or
below 70 ppb would require reductions in those ambient O3
concentrations. While the Administrator acknowledges uncertainty in the
extent to which the reported O3-associated emergency
department visits could be further reduced by standard levels below 65
or 70 ppb, she concludes that this analysis indicates that a revised
standard with a level at least as low as 70 ppb would result in
improvements in public health, beyond the protection provided by the
current standard, in the locations of the single-city epidemiologic
studies that reported significant health effect associations.
    As discussed above, the Administrator notes the greater uncertainty
in interpreting air quality in locations of multicity epidemiologic
studies of short-term O3 for the purpose of evaluating
alternative standard levels (II.D.1 and U.S. EPA, 2014c, section
4.4.1). Therefore, she places less weight on these studies than on the
single-city studies noted above. Despite this uncertainty, she notes
that PA analyses suggest that standard levels of 65 or 70 ppb would
require additional reductions, beyond those required by the current
standard, in ambient O3 concentrations in several of the
epidemiologic study locations that provided the basis for statistically
significant O3 health effect associations. For example, she
notes that Dales et al. (2006) reported significant associations with
respiratory hospital admissions based on air quality in 11 Canadian
cities, most of which would likely have met the current standard over
the entire study period (i.e., seven cities) but would have violated a
standard with a level of 70 ppb or below over at least part of that
period (U.S. EPA, 2014c, Table 4-1). She further notes that Katsouyanni
et al. (2009) reported statistically significant associations with
mortality based on air quality in 12 Canadian cities, most of which
would likely have met the current standard (i.e., eight study cities)
and a standard with a level of 70 ppb (i.e., seven study cities) over
the entire study period, but would have violated a standard with a
level of 65 ppb over at least part of that period (U.S. EPA, 2014c,
Table 4-1). While most of the other multicity epidemiologic studies
evaluated also suggest that a level from 65 to 70 ppb would result in
public health improvements, compared to the current standard, the
Administrator acknowledges that several multicity epidemiologic studies
reported O3 health effect associations when the majority of
study cities would likely

have met standards with levels from 65 to 70 ppb. However, given the
important uncertainties in interpreting the air quality in these
multicity studies, the Administrator places limited weight on them
overall, relative to the single-city studies noted above (and relative
to the information based on controlled human exposure studies).
    With regard to long-term O3 concentrations, the
Administrator considers the long-term O3 metrics reported to
be associated with mortality or morbidity in recent epidemiologic
studies (e.g., seasonal averages of 1-hour or 8-hour daily max
concentrations). Compared to the current standard, she notes that
analyses in the PA (U.S. EPA, 2014c, section 4.4.1) suggest a revised
standard with a level of 65 or 70 ppb would more effectively maintain
long-term O3 concentrations below those where the multicity
study by Jerrett et al. (2009) indicates the most confidence in the
reported association with respiratory mortality (II.B.2, II.D.1). Based
on additional information from the study by Jerrett et al. (2009), the
Administrator also notes HREA analyses indicating that a revised
standard with a level of 65 or 70 ppb would be expected to reduce the
risk of respiratory mortality associated with long-term O3
concentrations (though she also notes important uncertainties with
these risk estimates, as described below). Finally, she notes analyses
in the HREA suggesting that a revised standard with a level of 65 or 70
ppb would be expected to reduce long-term O3 concentrations,
defined in terms of O3 metrics similar to the long-term
metrics that have been reported in recent epidemiologic studies to be
associated with respiratory morbidity (i.e., seasonal averages of daily
maximum 8-hour concentrations). Given the above evidence and
information, the Administrator concludes that a revised 8-hour standard
with a level from 70 to 65 ppb could increase public health protection,
compared to the current standard, against effects associated with long-
term O3 exposures.
    In further evaluating information from epidemiologic studies, the
Administrator also considers the HREA's epidemiology-based risk
estimates of morbidity and mortality associated with short-term
O3 (U.S. EPA, 2014a). Compared to the weight given to the
evidence from controlled human exposure studies, and to HREA estimates
of exposures of concern and lung function risks, she places relatively
less weight on epidemiology-based risk estimates. In doing so, she
notes that the overall conclusions from the HREA likewise reflect
relatively less confidence in estimates of epidemiology-based risks
than in estimates of exposures of concern and lung function risks. As
discussed above (II.C.3.b), this is based on the greater uncertainties
associated with mortality and morbidity risk estimates, including the
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations. The Administrator further notes the HREA conclusion
that lower confidence should be placed in the results of the assessment
of respiratory mortality risks associated with long-term O3
exposures, primarily because that analysis is based on only one study
(even though that study is well-designed) and because of the
uncertainty in that study regarding the existence and identification of
a potential threshold in the concentration-response function (U.S. EPA,
2014a, section 9.6).
    In considering epidemiology-based risk estimates, the Administrator
focuses on the extent to which potential alternative O3
standards are estimated to reduce the risk of mortality associated with
short-term exposures to O3, noting the similar patterns of
risk across urban study areas and air quality scenarios for respiratory
morbidity endpoints (II.C.3). Given the uncertainties in epidemiology-
based risk estimates, the Administrator focuses on the general
magnitudes of risk changes estimated for standard levels of 65 and 70
ppb, compared to the current standard, rather than placing a large
amount of weight on the absolute estimates of O3-associated
deaths. In doing so, she notes the CASAC conclusion that ``[a]lthough
the estimates for short-term exposure impacts are subject to
uncertainty, the data supports a conclusion that there are meaningful
reductions in mean premature mortality associated with ozone levels
lower than the current standard'' (Frey, 2014a, p. 10). She further
notes that, as discussed above (II.C.3.b), the HREA risk estimates for
urban study areas are likely to understate the average reductions in
O3-associated mortality and morbidity risks that would be
experienced across the U.S. population as a whole upon meeting
standards with lower levels.
    The Administrator's primary focus is on risks associated with
O3 concentrations in the upper portions of ambient
distributions, given the greater uncertainty associated with the shapes
of concentration-response curves for O3 concentrations in
the lower portions of ambient distributions.\167\ The Administrator
further notes that experimental studies provide the strongest evidence
for O3-induced effects following exposures to O3
concentrations corresponding to the upper portions of typical ambient
distributions. In particular, as discussed above, she notes controlled
human exposure studies showing respiratory effects following exposures
to O3 concentrations at or above 60 ppb (II.B).
---------------------------------------------------------------------------

    \167\ The ISA concludes that there is less certainty in the
shape of concentration-response functions for area-wide
O3 concentrations at the lower ends of warm season
distributions (i.e., below about 20 to 40 ppb) (U.S. EPA, 2013a,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    In considering risks associated with O3 concentrations
in the upper portions of ambient distributions, the Administrator
focuses on area-wide O3 concentrations at or above 40 ppb
and 60 ppb. For area-wide O3 concentrations at or above 40
ppb, the Administrator notes that revised standards with levels of 70
or 65 ppb are estimated to reduce the number of premature deaths
associated with short-term O3 concentrations by about 10%
and almost 50%, respectively, compared to the current standard.\168\ In
addition, for area-wide concentrations at or above 60 ppb, revised
standards are estimated to reduce O3-associated premature
deaths by about 50% to 70% for a standard level of 70 ppb, and by more
than 80% for a standard level of 65 ppb.\169\ Risk reductions are
smaller when total risks are considered (II.C.3.b).
---------------------------------------------------------------------------

    \168\ For area-wide O3 concentrations at or above 40
ppb, reductions in estimated premature deaths are disproportionately
larger with the 65 ppb standard level than with the 70 ppb standard
level. This results from the larger air quality adjustments required
to meet the 65 ppb level. Across urban study areas, the additional
reductions required to meet 65 ppb result in many fewer days with
area-wide O3 concentrations at or above 40 ppb and,
therefore, many fewer O3-associated deaths for area-wide
concentrations at or above 40 ppb (U.S. EPA, 2014a, Figures 7-2 and
7-3).
    \169\ Though only a relatively small number of days in urban
study areas had area-wide O3 concentrations at or above
60 ppb.
---------------------------------------------------------------------------

    Given all of the above evidence, exposure/risk information, and
advice from CASAC, the Administrator proposes to revise the level of
the current primary O3 standard to within the range of 65 to
70 ppb. She concludes that a standard with a level from within this
range could reasonably be judged to be requisite to protect public
health with an adequate margin of safety, based on her consideration of
the evidence and information discussed above. In reaching this
conclusion, she particularly notes that a level from anywhere within
this range would be below the lowest O3 exposure
concentration shown to result in the

adverse combination of respiratory symptoms and lung function
decrements (i.e., 72 ppb), would be expected to maintain ambient
O3 concentrations below those in locations where single-city
studies assessed in the ISA have reported statistically significant
O3 health effect associations, and would be expected to
result in important reductions in O3 exposures and health
risks, compared to the current standard.
    The Administrator notes that the determination of what constitutes
an adequate margin of safety is expressly left to the judgment of the
EPA Administrator. She further notes that in evaluating how particular
standards address the requirement to provide an adequate margin of
safety, the Administrator must consider such factors as the nature and
severity of the health effects, the size of sensitive population(s) at
risk, and the kind and degree of the uncertainties that must be
addressed (I.B, above). Consistent with past practice and long-standing
judicial precedent, she takes the need for an adequate margin of safety
into account as an integral part of her decision-making on the
appropriate level, averaging time, form, and indicator of the
standard.\170\
---------------------------------------------------------------------------

    \170\ See, e.g., NRDC v. EPA, 902 F. 2d 962, 973-74 (D.C. Cir.
1990).
---------------------------------------------------------------------------

    The Administrator notes that the NAAQS are not designed to be zero-
risk or background standards, and that the sizeable risk reductions
that are estimated in the HREA to be associated with standard levels of
65 or 70 ppb represent substantial improvements in public health for
important segments of the population, including at-risk groups such as
children and people with asthma. Although any rationale supporting a
decision to set a specific level within the range of 65 to 70 ppb would
discuss the full body of evidence and information, the Administrator
notes that certain aspects of this evidence and information could be
particularly important in distinguishing between the appropriateness of
a level closer to 65 ppb versus a level closer to 70 ppb.\171\
---------------------------------------------------------------------------

    \171\ Although this discussion refers to supporting rationale
for a level of 65 ppb or 70 ppb, the Administrator is proposing the
entire range between 65 and 70 ppb. The Administrator notes that
although neither the PA nor CASAC reached conclusions or provided
advice on a standard set at a specific level between 65 ppb and 70
ppb, there is nothing in either the evidence, exposure/risk
information, or CASAC advice that would preclude such a standard
level.
---------------------------------------------------------------------------

    For example, a level at or near 65 ppb could be judged requisite to
protect public health with an adequate margin of safety to the extent
the Administrator places greater weight on the importance of: (1)
Eliminating almost all exposures of concern (even single occurrences)
at or above 70 and 80 ppb, even in worst-case years and locations; (2)
almost eliminating the occurrence of two or more exposures of concern
at or above 60 ppb; (3) achieving additional reductions in
O3-induced FEV1 decrements, beyond those achieved
with a level of 70 ppb (4) maintaining ambient concentrations below
those in locations of single-city studies and more effectively doing so
for multicity studies (i.e., more effectively than 70 ppb); and (5)
achieving substantial reductions, compared to a standard with a level
of 70 ppb, in mortality associated with the upper portion of the
distribution of ambient O3 concentrations, despite
uncertainties in risk estimates.
    In contrast, a level at or near 70 ppb could be judged requisite to
protect public health with an adequate margin of safety to the extent
the Administrator places a greater amount of weight (i.e., greater than
for 65 ppb) on the importance of: (1) Almost eliminating the occurrence
of two or more exposures of concern at or above 70 and 80 ppb, even in
the worst-case year and location; (2) substantially reducing, but not
eliminating, the occurrence of two or more exposures of concern at or
above 60 ppb, noting conclusions regarding increasing uncertainty in
adverse effects for the 60 ppb benchmark; (3) reducing, but not
eliminating, the occurrence of one or more exposures of concern, noting
that not all exposures of concern result in adverse effects; (4)
maintaining ambient O3 concentrations below those in
locations of single-city epidemiologic studies, and uncertainties in
analyses of air quality in multicity study locations; and (5)
recognizing uncertainties in epidemiology-based risk estimates.
    In considering CASAC advice on the range of standard levels, the
Administrator first notes CASAC's conclusion that there is adequate
scientific evidence to consider a range of levels for a primary
standard that includes an upper end at 70 ppb. For the reasons
discussed above, she agrees with this advice. She also notes that while
CASAC concluded that a standard with a level of 70 ppb ``may not meet
the statutory requirement to protect public health with an adequate
margin of safety'' (Frey, 2014c, p. 8), it further acknowledged that
``the choice of a level within the range recommended based on
scientific evidence is a policy judgment under the statutory mandate of
the Clean Air Act'' (Frey, 2014c, p. ii). While she agrees with CASAC
that it is appropriate to consider levels below 70 ppb, as reflected in
her range of proposed levels from 65 to 70 ppb, for the reasons
discussed above she also concludes that a standard level as high as 70
ppb, which CASAC concluded could be supported by the scientific
evidence, could reasonably be judged to be requisite to protect public
health with an adequate margin of safety.
    The Administrator has also considered the appropriateness of
standard levels below 65 ppb. In doing so, she notes the conclusions of
the PA and the advice of CASAC that it would be appropriate for her to
consider standard levels as low as 60 ppb. In particular, she notes
that a decision to set the primary O3 standard level at 60
ppb would place a large amount of weight on the potential public health
importance of virtually eliminating even single occurrences of
exposures of concern at and above 60 ppb, though controlled human
exposure studies have not reported the adverse combination of
respiratory symptoms and decrements in lung function following
exposures to 60 ppb O3; on the potential public health
importance of further reducing the occurrence of O3-induced
lung function decrements >=10 and 15%; on analyses of ambient
O3 concentrations in locations of multicity epidemiologic
studies, despite uncertainties in linking multicity effect estimates
for short-term O3 with air quality in individual study
cities; and on epidemiology-based risk estimates, despite the important
uncertainties in those estimates. However, as discussed more fully
above, given the uncertainties associated with the adversity of
exposures to 60 ppb O3, particularly single occurrence of
such exposures; uncertainties associated with air quality analyses in
locations of multicity epidemiologic studies; and uncertainties in
epidemiology-based risk estimates, particularly uncertainties in the
shape of the concentration-response functions at lower O3
concentrations and uncertainties associated with the heterogeneity in
O3 effect estimates across locations, the Administrator does
not agree that it is appropriate to place significant weight on these
factors or to use them to support the appropriateness of standard
levels below 65 ppb. Compared to O3 standard levels from 65
to 70 ppb, the Administrator concludes that the extent to which
standard levels below 65 ppb could result in further public health
improvements becomes notably less certain. Therefore, she concludes
that it

is not appropriate to propose standard levels below 65 ppb.\172\
---------------------------------------------------------------------------

    \172\ Although, as discussed below, she solicits comment on
standard levels as low as 60 ppb.
---------------------------------------------------------------------------

    The Administrator acknowledges that her proposed range of 65 to 70
ppb does not include the lower portion of the range supported by CASAC.
In reaching the conclusion that this is appropriate, she focuses on
CASAC's rationale for levels as low as 60 ppb. In particular, she notes
the following CASAC advice (Frey, 2014c, p. 7):

    The CASAC concurs that 60 ppb is an appropriate and justifiable
scientifically based lower bound for a revised primary standard.
This is based upon findings of adverse effects, including clinically
significant lung function decrements and airway inflammation, after
exposures to 60 ppb ozone in healthy adults with moderate exertion
(Adams 2006; Schelegle et al., 2009; Brown et al., 2008; Kim et al.,
2011), with limited evidence of adverse effects below 60 ppb.

    In considering this advice, the Administrator notes that CASAC
focused on the importance of limiting exposures to O3
concentrations as low as 60 ppb. As discussed above, the Administrator
agrees with this advice. In particular, she notes that standards within
the proposed range of 65 to 70 ppb would be expected to substantially
limit the occurrence of exposures of concern to O3
concentrations at or above 60 ppb, particularly the occurrence of two
or more exposures.\173\ When she further considers that not all
exposures of concern lead to adverse effects, and that the NAAQS are
not meant to be zero-risk or background standards, the Administrator
judges that alternative standard levels below 65 ppb are not needed to
further reduce such exposures. Therefore, the Administrator's initial
conclusion is that standard levels below 65 ppb would be more than
requisite to protect public health with an adequate margin of safety.
---------------------------------------------------------------------------

    \173\ In fact, as noted above (Table 4), a standard with a level
of 70 ppb would be expected to limit multiple occurrences of
exposures of concern at or above the 60 ppb benchmark to as low as
0.5% in urban case study areas (and as low as 0% for a standard with
a level of 65 ppb).
---------------------------------------------------------------------------

    In reaching this initial conclusion, the Administrator acknowledges
that alternative approaches to viewing the available scientific
evidence and exposure/risk information, and to viewing the
uncertainties inherent in that evidence and information, could lead one
to reach a different conclusion. In particular, as noted above, she
recognizes that levels as low as 60 ppb could potentially be supported,
to the extent substantial weight is placed on the public health
importance of estimates of one or more occurrences of exposures of
concern at or above 60 ppb and O3-induced lung function
decrements >=10%; analyses of ambient O3 concentrations in
locations of multicity epidemiologic studies; and epidemiology-based
estimates of total risk. This approach would also place a large amount
of weight on the possibility that at-risk groups would experience
adverse effects at lower levels than the benchmarks derived from
clinical studies conducted using healthy adult subjects, despite the
fact that these studies have not reported a statistically significant
increase in respiratory symptoms, combined with lung function
decrements, following exposures to 60 ppb.\174\ Such an approach to
viewing the evidence and exposure/risk information would place very
little weight on the uncertainties in these estimates and analyses. In
some cases, elements of this approach have been supported by public
commenters, leading some commenters to recommend setting the level of
the primary O3 standard at least as low as 60 ppb. In
recognition of such an alternative approach to viewing the evidence and
information, in addition to proposing to set the level of the
O3 standard from 65 to 70 ppb, the Administrator solicits
comment on alternative standard levels below 65 ppb, and as low as 60
ppb. In doing so, the Administrator reiterates that the CAA does not
require the establishment of a primary NAAQS at a zero-risk level or at
background concentration levels, but rather at a level that reduces
risk sufficiently so as to protect public health with an adequate
margin of safety (I.A).
---------------------------------------------------------------------------

    \174\ More specifically, as discussed above, respiratory
symptoms have been evaluated following 6.6-hour exposures to average
O3 concentrations of 60 ppb (Adams, 2006; Kim et al.,
2011) and 63 ppb (Schelegle et al., 2009). None of these studies
reported a statistically significant increase in respiratory
symptoms, compared to filtered air controls.
---------------------------------------------------------------------------

F. Proposed Decision on the Primary Standard

    For the reasons discussed above, and taking into account
information and assessments presented in the 2013 ISA, 2014 HREA and
integration of this information and assessments into staff conclusions
in the 2014 PA, the advice and recommendations of CASAC, and public
comments received during the development of these documents, the
Administrator proposes to retain the current indicator, averaging time
and form of the primary O3 standard, and to set a new level
for the 8-hour primary O3 standard. Specifically, the
Administrator proposes to set the level of the 8-hour primary
O3 standard to within the range of 65 to 70 ppb. The
proposed 8-hour primary standard would be met at an ambient air
monitoring site when the 3-year average of the annual fourth-highest
daily maximum 8-hour average O3 concentration is less than
or equal to the level of the revised standard that is promulgated.
Thus, the Administrator proposes to set a standard with a level within
this range. For the reasons discussed above, the Administrator also
solicits comment on setting the level of the primary O3
standard below 65 ppb, and as low as 60 ppb.

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 AQI 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). It is designed to tell individual
members of the public how clean or unhealthy their air is, whether
health effects might be a concern, and, if so, measures individuals can
take to reduce their exposure to air pollution. The AQI focuses on
health effects individuals may experience within a few hours or days
after breathing unhealthy air. The AQI establishes a nationally uniform
system of indexing pollution concentrations 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 concentrations 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 (301-500). The AQI index value of 100 typically
corresponds to the level of the short-term NAAQS for each pollutant.
For the O3 NAAQS, an 8-hour average concentration of 75 ppb
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). An additional consideration in

selecting breakpoints is for each category to span at least a 15 ppb
range to allow for more accurate forecasting. 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 is proposing conforming changes to the AQI, in connection
with the Agency's proposed decision on revisions to 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 adjustments based on health information from this NAAQS review
to AQI breakpoints at the lower end of each range (i.e., AQI values of
50, 150, 200 and 300). The EPA does not propose to change the level at
the top of the index (i.e., AQI value of 500) that typically is set
equal to the Significant Harm Level (40 CFR 51.16), which would apply
to state contingency plans.
    The EPA is proposing to revise the AQI for O3 by setting
an AQI value of 100 equal to the level of the revised O3
standard (65-70 ppb). The EPA is also proposing to revise the following
breakpoints: An AQI value of 50 to within a range from 49-54 ppb; an
AQI value of 150 to 85 ppb; an AQI value of 200 to 105 ppb, and an AQI
value of 300 to 200 ppb. All these levels are averaged over 8 hours.
The EPA is proposing to set an AQI value of 50, the breakpoint between
the good and moderate categories, at 15 ppb below the value of the
proposed standard, i.e. to within a range from 49 to 54 ppb. The EPA is
taking comment on what level within this range to select, recognizing
that there is no health message for either at-risk or healthy
populations in the good category. Thus, the level selected should be
below the lowest concentration (i.e., 60 ppb) that has been shown in
controlled human exposure studies of healthy adults \175\ to cause
moderate lung function decrements (i.e., FEV1 decrements
>=10%, which could be adverse to people with lung disease), large lung
function decrements (i.e., FEV1 decrements >=20%) in a small
proportion of people, and airway inflammation.\176\ The EPA is
proposing to set an AQI value of 150, the breakpoint between the
unhealthy for sensitive groups and unhealthy categories, at 85 ppb. At
this level, controlled human exposure studies of healthy adults
indicate that up to 25% of exposed people are likely to have moderate
lung function decrements (i.e., 25% have FEV1 decrements
>=10%; 12% have FEV1 decrements >=15%) and up to 7% are
likely to have large lung function decrements (i.e., FEV1
decrements >=20%) (McDonnell et al., 2012; Figure 7). Large lung
function decrements would likely interfere with normal activity for
many healthy people. For people with lung disease, large lung function
decrements would likely interfere with normal activity for most people
and would increase the likelihood that they would seek medical
treatment (72 FR 37850, July 11, 2007). The EPA is proposing to set an
AQI value of 200, the breakpoint between the unhealthy and very
unhealthy categories, at 105 ppb. At this level, controlled human
exposure studies of healthy adults indicate that up to 38% of exposed
people are likely to have moderate lung function decrements (i.e., 38%
have FEV1 decrements >=10%; 22% have FEV1
decrements >=15%) and up to 13% are likely to have large lung function
decrements (i.e., FEV1 decrements >=20%). The EPA is
proposing to set an AQI value of 300, the breakpoint between the very
unhealthy and hazardous categories, at 200 ppb. At this level,
controlled human exposure studies of healthy adults indicate that up to
25% of exposed individuals are likely to have large lung function
decrements (i.e., FEV1 decrements >=20%), which would
interfere with daily activities for many of them. Large lung function
decrements would interfere with daily activities for most people with
lung disease, and likely cause them to seek medical attention.
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    \175\ Effects would likely be greater in people with asthma.
    \176\ Exposures to 50 ppb have not been evaluated
experimentally, but are estimated to potentially affect only a small
proportion of healthy adults and with only a half to a third of the
moderate to large lung function decrements observed at 60 ppb
(McDonnell et al., 2012; Figure 7).

                                        Table 6--Proposed AQI Breakpoints
----------------------------------------------------------------------------------------------------------------
                                                                   Existing
                                                                  breakpoints    Proposed breakpoints  (ppb, 8-
                 AQI category                    Index values    (ppb, 8-hour             hour average)
                                                                   average)
----------------------------------------------------------------------------------------------------------------
Good..........................................            0-50            0-59  0-(49 to 54).
Moderate......................................          51-100           60-75  (50 to 55)-(65 to 70).
Unhealthy for Sensitive Groups................         101-150           76-95  (66 to 71)-85.
Unhealthy.....................................         151-200          96-115  86-105.
Very Unhealthy................................         201-300         116-374  106-200.
Hazardous.....................................         301-400            375-  201-.
                                                       401-500
----------------------------------------------------------------------------------------------------------------

    EPA believes that the proposed breakpoints reflect an appropriate
balance between reflecting the health evidence that is the basis for
the proposed primary O3 standard and providing category
ranges that are large enough to be forecasted accurately, so that the
new AQI for O3 can be implemented more easily in the public
forum for which the AQI ultimately exists. However, the EPA recognizes
that some have expressed alternative approaches to viewing the evidence
and information and solicits comment on these proposed revisions to the
AQI.
    With respect to reporting requirements (40 CFR part 58, Sec.
58.50), EPA proposes to revise 40 CFR part 58, Sec.  58.50 (c) to
require the AQI reporting requirements to be based on the latest
available census figures, rather than the most recent decennial U.S.
census. This change is consistent with our current practice of using
the latest population figures to make monitoring requirements more
responsive to changes in population.

IV. Rationale for Proposed Decision on the Secondary Standard

    This section presents the rationale for the Administrator's
proposed decisions regarding the need to revise the current secondary
O3 NAAQS and the appropriate revisions to the standard,
including her proposed decisions that the current secondary standard is
not requisite to protect public welfare and should be revised to
provide additional public welfare protection. Based on her
consideration of the full body of welfare effects evidence and related
analyses, the Administrator proposes to conclude that ambient
O3 concentrations in terms of a W126 index value, averaged
across three consecutive years, within the range from 13 ppm-hrs to 17
ppm-hrs would provide the requisite protection against known or
anticipated adverse effects to the public welfare. In considering
policy options for achieving that level of air quality, the
Administrator has further considered the full body of information,
including air quality analyses that relate ambient O3
concentrations in terms of a three-year average W-126-based metric and
in terms of the form and averaging time for the current standard. Based
on this consideration, the Administrator proposes to revise the level
of the current secondary standard to within the range of 0.065 to 0.070
ppm to achieve the appropriate air quality.
    As discussed more fully below, this proposal is based on a thorough
review, in the ISA, of the latest scientific information on
O3-induced environmental effects. This proposed decision
also takes into account: (1) Staff assessments in the PA of the most
policy-relevant information in the ISA and WREA analyses of air
quality, exposure, and ecological risks and associated ecosystem
services; (2) CASAC advice and recommendations; and, (3) public
comments received during the development of these documents, either in
connection with CASAC meetings or separately.
    This proposed decision draws on the ISA's integrative synthesis of
the entire body of evidence, published through July 2011, on
environmental effects associated with the presence of O3 and
related photochemical oxidants in the ambient air. As summarized in
section IV.B below, this body of evidence addresses the range of
environmental responses associated with exposure to ambient levels of
O3 (U.S. EPA, 2013a, ISA chapters 9-10), and includes more
than four hundred new studies that build on the extensive evidence base
from the last review. This rationale also draws upon the results of
quantitative exposure and risk assessments, summarized in section IV.C
below. Section IV.D presents the Administrator's proposed decisions on
the adequacy of the current secondary standard (section IV.D.3) drawing
on both evidence-based and exposure/risk-based considerations in the PA
(section IV.D.1) and advice from CASAC (section IV.D.2). Proposed
conclusions on alternative standards are summarized in section IV.E.

A. Approach

    In evaluating whether it is appropriate to retain or revise the
current secondary O3 standard, the Administrator adopts an
approach in this review that builds upon the general approach used in
the 2008 review \177\ and reflects the broader body of scientific
evidence now available, updated exposure/risk information, advances in
O3 air quality modeling, and air monitoring information.
This review of the standard also considers the July 2013 remand of the
secondary standard by the U.S. Court of Appeals for the D.C. Circuit,
such that the proposed decision described herein incorporates the EPA's
response to this remand.
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    \177\ The 2008 revision of the O3 secondary standard,
the proposed reconsideration of the 2008 decision, and the 2013
court decision on the 2008 revision of the secondary standard are
summarized in section I.C above.
---------------------------------------------------------------------------

    The Administrator's decisions in the 2008 review were based on an
integration of information on welfare effects associated with exposure
to O3, judgments on the adversity and public welfare
significance of key effects, and judgments as to what standard would be
requisite to protect public welfare. These considerations were informed
by air quality and related analyses, quantitative exposure and risk
assessments, and qualitative assessment of impacts that could not be
quantified. As a result of the 2008 review, the Administrator concluded
the then-current secondary standard did not provide the requisite
public welfare protection and it was revised. The current secondary
standard is 75 ppb based on the annual fourth-highest daily maximum 8-
hour average concentration, averaged over three consecutive years,
which is identical to the current primary standard. In 2008, the
Administrator considered the then-available monitoring data with regard
to relationships between the revised primary standard and degree of
protection of public welfare from cumulative seasonal O3
exposures, expressed in terms of the W126 exposure index (described in
section IV.B.1 below), and decided to revise the secondary standard to
be equal to the revised primary standard (73 FR 16499-16500, March 27,
2008). In remanding the 2008 decision on the secondary standard back to
the EPA (described in section I.C above), the U.S. Court of Appeals for
the D.C. Circuit determined that EPA did not specify what level of air
quality was requisite to protect public welfare from adverse public
welfare effects or explain why any such level would be requisite.
Mississippi, 744 F.3d at 272-73.
    In addition to reviewing the most recent scientific information as
required by the CAA, this rulemaking responds to the remand and fully
explains the Administrator's proposed conclusions as to the level of
air quality requisite to protect public welfare from known or
anticipated effects. Our general approach in considering the scientific
information available in this review involves consideration of the
integrative synthesis of the entire body of available scientific
evidence in the ISA (U.S. EPA, 2013a), including information on
biologically relevant exposure indices, exposure/risk and air quality
modeling analyses presented in the WREA (U.S. EPA, 2014b), staff
analyses in the PA; advice and recommendations from CASAC (Frey, 2014b,
c), and public comments. We note that in drawing conclusions on the
secondary standard, the final decision to retain or revise the standard
is a public welfare policy judgment to be made by the Administrator.
The Administrator's final decision will draw upon the available
scientific evidence for O3-attributable welfare effects and
on analyses of exposures and public welfare risks based on impacts to
vegetation, ecosystems and their associated services, as well as
judgments about the appropriate weight to place on the range of
uncertainties inherent in the evidence and analyses. Such judgments in
the context of this review include: The weight to place on the evidence
of specific vegetation-related effects estimated to result across a
range of cumulative seasonal concentration-weighted O3
exposures; the weight to give associated uncertainties, including those
related to the variability in occurrence of such effects in areas of
the U.S., especially areas of particular public welfare significance;
and, judgments on the extent to which such effects in such areas may be
considered adverse to public welfare.
    As provided in the CAA, section 109(b)(2), the secondary standard
is to ``specify a level of air quality the attainment and maintenance
of which in the judgment of the Administrator . . .

is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of such air
pollutant in the ambient air.'' Effects on welfare include, but are not
limited to, ``effects on soils, water, crops, vegetation, man-made
materials, animals, wildlife, weather, visibility, and climate, damage
to and deterioration of property, and hazards to transportation, as
well as effects on economic values and on personal comfort and well-
being'' (CAA section 302(h)). As recognized in the last review, the
secondary standard is not meant to protect against all known or
anticipated O3-related effects, but rather those that are
judged to be adverse to the public welfare (73 FR 16496, March 27,
2008). Thus, the level of protection from known or anticipated adverse
effects to public welfare that is requisite for the secondary standard
is a public welfare policy judgment to be made by the Administrator. In
the current review, the Administrator's judgment is informed by
conclusions drawn with regard to adversity of effects to public welfare
in decisions on secondary O3 standards in past reviews.
    In the 2008 decision, the Administrator concluded that the degree
to which O3 effects on vegetation should be considered to be
adverse to the public welfare depends on the intended use of the
vegetation and the significance of the vegetation to the public
welfare, and also applied this concept beyond the species level to the
ecosystem level (73 FR 16496, March 27, 2008). In so doing, the
Administrator took note of ``a number of actions taken by Congress to
establish public lands that are set aside for specific uses that are
intended to provide benefits to the public welfare, including lands
that are to be protected so as to conserve the scenic value and the
natural vegetation and wildlife within such areas, and to leave them
unimpaired for the enjoyment of future generations'' (73 FR 16496,
March 27, 2008). The notice for the 2008 decision further noted that
[s]uch public lands that are protected areas of national interest
include national parks and forests, wildlife refuges, and wilderness
areas'' (73 FR 16496, March 27, 2008).178 179 The
Administrator additionally recognized that ``States, Tribes and public
interest groups also set aside areas that are intended to provide
similar benefits to the public welfare, for residents on State and
Tribal lands, as well as for visitors to those areas'' (73 FR 16496,
March 27, 2008). The Administrator took note of the ``clear public
interest in and value of maintaining these areas in a condition that
does not impair their intended use and the fact that many of these
lands contain O3-sensitive species'' (73 FR 16496, March 27,
2008). Similarly, in judgments of adversity to public welfare in the
2010 proposed reconsideration, the Administrator proposed to place the
highest priority and significance on vegetation and ecosystem effects
to sensitive species that are known to or are likely to occur in
federally protected areas such as national parks and other Class I
areas,\180\ or on lands set aside by states, tribes and public interest
groups to provide similar benefits to the public welfare (75 FR 3023-
24, January 19, 2010).
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    \178\ For example, the National Park Service Organic Act of 1916
established the National Park Service (NPS) and, in describing the
role of the NPS with regard to ``Federal areas known as national
parks, monuments, and reservations'', stated that the ``fundamental
purpose'' for these Federal areas ``is to conserve the scenery and
the natural and historic objects and the wild life therein and to
provide for the enjoyment of the same in such manner and by such
means as will leave them unimpaired for the enjoyment of future
generations.'' 16 U.S.C. 1.
    \179\ As a second example, the Wilderness Act of 1964 defines
designated ``wilderness areas'' in part as areas ``protected and
managed so as to preserve [their] natural conditions'' and requires
that these areas ``shall be administered for the use and enjoyment
of the American people in such manner as will leave them unimpaired
for future use and enjoyment as wilderness, and so as to provide for
the protection of these areas, [and] the preservation of their
wilderness character . . .'' 16 U.S.C. 1131 (a).
    \180\ As defined by section 162 of the CAA, Class I areas
include all international parks, national wilderness areas which
exceed 5,000 acres in size, national memorial parks which exceed
5,000 acres in size, and national parks which exceed six thousand
acres in size, provided the park or wilderness area was in existence
on August 7, 1977, as well as other areas designated as Class I
consistent with that section of the Act. The current Class I areas
are specified at 40 CFR part 81.
---------------------------------------------------------------------------

    In the current review, our consideration of the scientific evidence
for effects on vegetation is based fundamentally on using information
from controlled chamber studies, free air methodologies, and field-
based observational, survey and gradient studies. Such evidence,