margin of safety to a susceptible population (i.e., asthmatics at
elevated ventilation), and that the standard must afford protection
from short-term exposures to SO2 in order to prevent the
adverse health effects reported in both the controlled human exposure
and epidemiologic studies. The Administrator notes that a 1-hour
standard could provide increased public health protection, especially
for members of at-risk groups, from health effects described in both
controlled human exposure and epidemiologic studies, and hence, health
effects associated with 5-minute to 24-hour exposures to
SO2.\14\ As discussed in section II.F.5 below, given the
degree of protection afforded by such a standard, it may be appropriate
to replace, and not retain, the current 24-hour and annual standards in
conjunction with setting a new short-term standard.
---------------------------------------------------------------------------

    \14\ We also note that such a standard would, among other
things, address the deficiency in the current NAAQS which occasioned
the remand of that standard for failing to adequately explain the
absence of protection from short-term SO2 bursts which
could cause adverse health effects in hundreds of thousands of
heavily breathing asthmatics. American Lung Ass'n v. EPA, 134 F. 3d
at 392-93.
---------------------------------------------------------------------------

F. Conclusions on the Elements of a New Short-Term Standard

    In considering a revised SO2 primary NAAQS, the
Administrator notes the need to protect at-risk populations from: (1)
1-hour daily maximum and 24-hour average exposures to SO2
that could cause the types of respiratory morbidity effects reported in
epidemiologic studies; and (2) 5-10 minute SO2 exposure
concentrations reported in controlled human exposure studies to result
in moderate or greater decrements in lung function and/or respiratory
symptoms. Considerations with regard to potential alternative standards
and the specific conclusions of the Administrator are discussed in the
following sections in terms of indicator, averaging time, form, and
level (sections II.F.1 to II.F.4 below).
1. Indicator
a. Rationale for Proposed Decision
    In the last review, EPA focused on SO2 as the most
appropriate indicator for ambient SOX. In making a decision
in the current review on the most appropriate indicator, the
Administrator has considered the conclusions of the ISA and REA as well
as the views expressed by CASAC and the public. The REA noted that,
although the presence of gaseous SOX species other than
SO2 has been recognized, no alternative to SO2
has been advanced as being a more appropriate surrogate for ambient
gaseous SOX. Controlled human exposure studies and animal
toxicology studies provide specific evidence for health effects
following exposure to SO2. Epidemiologic studies also
typically report levels of SO2, as opposed to other gaseous
SOX. Because emissions that lead to the formation of
SO2 generally also lead to the formation of other
SOX oxidation products, measures leading to reductions in
population exposures to SO2 can generally be expected to
lead to reductions in population exposures to other gaseous
SOX. Therefore, as noted in the proposal, meeting an
SO2 standard that protects the public health can also be
expected to provide protection against potential health effects that
may be independently associated with other gaseous SOX even
though such effects are not discernable from currently available
studies indexed by SO2 alone. See American Petroleum
Institute v. EPA, 665 F, 2d 1176, 1186 (DC Cir. 1981) (reasonable for
EPA to use ozone as the indicator for all photochemical oxidants even
though health information on the other photochemical oxidants is
unknown; regulating ozone alone is reasonable since it presents a
``predictable danger'' and in doing so EPA did not abandon its
responsibility to regulate other photochemical oxidants encompassed by
the determination that photochemical oxidants as a class may be
reasonably anticipated to endanger public health or welfare). Given
these key points, the REA concluded that the available evidence
supports the retention of SO2 as the indicator in the
current review (REA, section 10.5.1). Consistent with this conclusion,
CASAC stated in a letter to the EPA Administrator that: ``for
indicator, SO2 is clearly the preferred choice'' (Samet
2009, p. 14).
b. Comments on Indicator
    A small number of commenters directly addressed the issue of the
indicator for the standard. These

commenters generally endorsed the proposal to continue to use
SO2 as the indicator for ambient SOX.
c. Conclusions on Indicator
    Based on the available information discussed above, and consistent
with the views of CASAC and other commenters, the Administrator
concludes that it is appropriate to continue to use SO2 as
the indicator for a standard that is intended to address effects
associated with exposure to SO2, alone or in combination
with other gaseous SOX. In so doing, the Administrator
recognizes that measures leading to reductions in population exposures
to SO2 will also reduce population exposures to other oxides
of sulfur.
2. Averaging Time
    This section discusses considerations related to the averaging time
of the SO2 primary NAAQS. Specifically, this section
summarizes the rationale for the Administrator's proposed decision
regarding averaging time (II.F.2.a below; see section II.F.2 of the
proposal for more detail at 74 FR 64832-64833), discusses public
comments and EPA responses related to averaging time (II.F.2.b), and
presents the Administrator's final conclusions regarding averaging time
(II.F.2.c). Notably, public comments and the Administrator's
conclusions on whether to retain or revoke the current 24-hour and/or
annual standards given a new 1-hour standard are discussed in section
II.F.5.
a. Rationale for Proposed Decision
    In considering the most appropriate averaging time for the
SO2 primary NAAQS, the Administrator noted in the proposal
the conclusions and judgments made in the ISA about the available
scientific evidence, air quality correlations discussed in the REA,
conclusions of the policy assessment chapter of the REA, and CASAC
recommendations (section II.F.2 in the proposal). Specifically, she
noted the following:
     The REA conclusion that an appropriate averaging time
should focus protection on SO2 exposures from 5-minutes to
24-hours (REA, section, 10.5.2).
     Air quality, exposure, and risk analyses from the REA
indicating it is likely a 1-hour standard--with the appropriate form
and level--can substantially reduce 5-10 minute peaks of SO2
shown in controlled human exposure studies to result in respiratory
symptoms and/or decrements in lung function in exercising asthmatics
(i.e. 5-minute SO2 concentrations >= 200 and 400 ppb).
     Air quality analyses indicating that a 1-hour standard--
with the appropriate form and level--can substantially reduce the upper
end of the distribution of SO2 levels more likely to be
associated with adverse respiratory effects (see section II.F.3 below);
that is: (1) 99th percentile 1-hour daily maximum air quality
concentrations in U.S. cities where positive effect estimates in
epidemiologic studies of hospital admissions and emergency department
visits for all respiratory causes and asthma were observed; and (2)
99th percentile 24-hour average air quality concentrations found in
U.S. cities where emergency department visit and hospitalization
studies (for all respiratory causes and asthma) reported statistically
significant associations in multi-pollutant models with PM.
     The REA conclusion that a 5-minute averaging time is
undesirable because it would result in significant and unnecessary
instability due to the likelihood that locations would frequently shift
in and out of attainment--thereby reducing public health protection by
disrupting an area's ongoing implementation plans and associated
control programs.
     CASAC statement addressing whether a 1-hour averaging time
can adequately control 5-10 minute peak exposures and whether there
should be a 5-minute averaging time. CASAC stated that the REA's
rationale for a one-hour standard was ``convincing'' (Samet 2009, p.
16), and that ``a one-hour standard is the preferred averaging time''
(Samet 2009, p. 15).
     CASAC's statement that they were ``in agreement with
having a short-term standard and finds that the REA supports a 1-hour
standard as protective of public health'' (Samet 2009, p. 1).
b. Comments on Averaging Time
    A large number of public commenters also endorsed the establishment
of a new standard with a 1-hour averaging time (although some groups'
support hinged on the accompanying level). These included a number of
State organizations (e.g., NACAA, NESCAUM); State environmental
agencies (e.g., such agencies in IA, IL, NY, MI, NM, OH, PA, TX, VT);
public health and environmental organizations (e.g., ALA, ATS, New York
Department of Health (NYDOH), Sierra Club, EDF); the Fond du Lac Tribe;
local groups (e.g., Houston-Galveston Area Council, New York City); and
almost all of the individual commenters (13,000). The supporting
rationales offered by these commenters often acknowledged the
recommendations of CASAC and the Administrator's rationale as discussed
in the proposal.
    Though many industry commenters did not support the proposed
revisions to the SO2 primary NAAQS (as discussed above in
section II.E.2), a few of these groups did express that if a short-term
standard were to be set, a 1-hour averaging time could be appropriate,
depending on the level and form selected (e.g., ExxonMobil, Kean
Miller). Other industry commenters (e.g., ASARCO, RIO Tinto Alcan,
Association of Battery Recyclers (ABR)) and the South Dakota Department
of Environment and Natural Resources (SD DENR) expressed that EPA
should have considered longer averaging times (e.g., 3 hours). In
addition, although health and environmental groups were supportive of
setting a new 1-hour standard to protect against short-term exposures
to SO2 (again, depending on the level of the 1-hour standard
selected), these groups also commented that a 5-minute standard to
protect susceptible populations from health effects associated with 5-
minute peaks of SO2 would be optimal (e.g., ALA, ATS, Sierra
Club, EDF). These comments, and EPA's responses, are discussed in more
detail below.
    As discussed above, industry commenters who disagreed with setting
a new 1-hour standard generally based this conclusion on their
interpretation of the scientific evidence and their conclusion that
this evidence does not support the proposed revisions to the current
SO2 NAAQS. EPA's responses to these commenters were
presented above in section II.E.2.a and II.E.2.b.
    Also noted above, some industry commenters (e.g., ASARCO, RIO Tinto
Alcan, ABR) and the SD DENR expressed that EPA should have considered
longer averaging times (e.g., 3-hour, 8-hour, 24-hour). In general,
these groups concluded that a standard with a longer averaging time
could potentially provide the same public health protection as a 1-hour
standard, while also providing a more stable regulatory target. For
example, in its comments, the SD DENR states: ``DENR recommends EPA
evaluate a 3-hour or 8-hour standard to determine if these averaging
periods are also protective of the public health. If they are, EPA
should propose a 3-hour or 8-hour sulfur dioxide standard instead of a
1-hour standard. A longer averaging period would smooth out the
variability of the upper range measurements and provide a more stable
standard.'' Similarly, Rio Tinto Alcan stated in its comments: ``the
short-term averaging period defined by EPA (i.e., 5 minutes

to 24 hours) is not limited to only 5-minute, 1-hour and 24-hour
averaging periods. EPA could explain in more detail why these three
averaging periods were examined when considering appropriate averaging
periods to limit short-term peaks of SO2 * * * a longer term
average could provide additional stability to the standard while at the
same time effectively protecting public health.''
    Although we agree that alternative averaging times could
potentially provide similar public health protection (assuming an
appropriate form and level), we believe that a 1-hour averaging time is
reasonably justified by the scientific evidence presented in the ISA
and by the air quality information presented in the REA. As described
in detail in the proposal (see section II.F.2), the controlled human
exposure evidence presented in the ISA provided support for an
averaging time that protects against 5-10 minute peak SO2
exposures (REA, section 10.5.2, pp. 371-372), and results from
epidemiologic studies most directly provided support for both 1-hour
and 24-hour averaging times (REA, section 10.5.2, p. 372). Thus, we
found it most reasonable to consider these averaging times for a
revised SO2 NAAQS given that there is very little basis in
the health evidence presented in the ISA to consider other averaging
times (e.g., 3-hour or 8-hour). In so doing, we first noted the
likelihood that averaging times of 1 and 24 hours could provide
protection against 5-minute peak SO2 exposures. As described
in detail in the proposal (see section II.F.2, 74 FR at 64830-64833),
it was initially concluded that a 1-hour averaging time, rather than a
24-hour averaging time, would be more appropriate for limiting 5-minute
peaks of SO2. Similarly, we concluded that a 1-hour
standard, given the appropriate form and level, could likely limit 99th
percentile 24-hour average air quality concentrations found in U.S.
locations where emergency department visit and hospitalization studies
(for all respiratory causes and asthma) observed statistically
significant associations in multi-pollutant models with PM (i.e., 99th
percentile 24-hour average SO2 concentration >= 36 ppb).
Taken together, we reasonably concluded that a 1-hour standard, with an
appropriate form and level, can provide adequate protection against the
range of health outcomes associated with averaging times from 5 minutes
to 24 hours (proposal section II.F.2 and REA, section <IP_ADDRESS>). We
also note that our conclusion is in agreement with CASAC comments on
the second draft REA. CASAC stated that they were ``in agreement with
having a short-term standard and finds that the REA supports a one-hour
standard as protective of public health'' (Samet 2009, p. 1). In
addition, as discussed in more detail below in section II.F.3, we found
that a 1-hour standard in combination with the selected form, will
provide a stable regulatory target.
    As noted above, although health and environmental groups were
supportive of setting a new 1-hour standard to protect against short-
term exposures to SO2 (again, depending on the level of the
1-hour standard selected), these groups generally commented that a 5-
minute standard to protect against health effects associated with 5-
minute peaks would be optimal (e.g., ALA, Sierra Club, EDF). For
example, in their combined comments ALA, EDF, NRDC, and Sierra Club
(ALA et al.,) stated: ``We need a short-term SO2 standard,
optimally a 5-minute standard, to protect against bursts of pollution
that can result from start-up, shutdown, upset, malfunction, downwash,
complex terrain, atmospheric inversion conditions, and other
situations'' and that ``EPA has over emphasized a concern about the
stability of a 5-minute standard * * * The record does not show that
any alleged instability of a 5-minute standard has any relevance to
whether such a standard is requisite to protect public health.''
    We agree that there needs to be a short-term standard to protect
against 5-minute peaks of SO2. However, we do not believe
setting a 5-minute standard to be the best way of accomplishing that
objective. As in past NAAQS reviews, EPA properly considered the
stability of the design of pollution control programs in its review of
the elements of a NAAQS, since more stable programs are more effective,
and hence result in enhanced public safety. American Trucking
Associations v. EPA, 283 F. 3d at 375 (choice of 98th percentile form
for 24-hour PM NAAQS, which allows a number of high exposure days per
year to escape regulation under the NAAQS, justifiable as ``promot[ing]
development of more `effective [pollution] control programs' '', since
such programs would otherwise be ``less `stable'--and hence * * * less
effective--than programs designed to address longer-term average
conditions'', and there are other means (viz. emergency episode plans)
to control those high exposure days). In this review, there were
legitimate concerns about the stability of a standard using a 5-minute
averaging time. Specifically, there was concern that compared to longer
averaging times (e.g., 1-hour, 24-hour), year-to-year variation in 5-
minute SO2 concentrations were likely to be substantially
more temporally and spatially diverse. Thus, it is more likely that
locations would frequently shift in and out of attainment thereby
reducing public health protection by disrupting an area's ongoing
implementation plans and associated control programs. Consequently, the
REA concluded that a 5-minute averaging time would not provide a stable
regulatory target and therefore would not be the preferred approach to
provide adequate public health protection. A 1-hour averaging time does
not have these drawbacks. As noted in the REA and the proposal (see
proposal sections II.F.2.a and II.F.2.c), air quality, exposure, and
risk analyses support that a 1-hour averaging time, given an
appropriate form and level can adequately limit 5-minute SO2
exposures and provide a more stable regulatory target than setting a 5-
minute standard. More specifically, based on the air quality and
exposure analyses presented in chapters 7 and 8 of the REA, there is
also a strong likelihood that a 99th percentile 1-hour daily maximum
standard will limit 5-10 minute peaks of SO2 shown in
controlled human exposure studies to result in decrements in lung
function and/or respiratory symptoms in exercising asthmatics (see
especially REA Tables 7-11 to 7-14 and Figure 8-19).
    We also note that a 1-hour standard to protect against 5-minute
exposures is in agreement with CASAC advice and recommendations. That
is, CASAC stated that they were ``in agreement with having a short-term
standard and finds that the REA supports a 1-hour standard as
protective of public health'' (Samet 2009, p. 1). Similarly, in a CASAC
statement addressing whether a 1-hour averaging time can adequately
control 5-10 minute peak exposures and whether there should be a 5-
minute averaging time, CASAC stated that the REA had presented a
``convincing rationale'' (Samet 2009, p. 16) for a 1-hour standard, and
that ``a one-hour standard is the preferred averaging time'' (Samet
2009, p. 15).
c. Conclusions on Averaging Time
    In considering the most appropriate averaging time(s) for the
SO2 primary NAAQS, the Administrator notes the conclusions
and judgments made in the ISA about the available scientific evidence,
air quality considerations from the REA, CASAC advice and
recommendations, and public comments received. Based on these
considerations, the Administrator concludes that a new standard based
on

1-hour daily maximum SO2 concentrations will provide
increased protection against effects associated with short-term (5
minutes to 24 hours) exposures. The rationale for this decision is
described below.
    Similar to the proposal (see section II.F.2.c), the Administrator
first agrees with the REA's conclusion that the standard should focus
protection on short-term SO2 exposures from 5 minutes to 24
hours. As noted above, CASAC's strong recommendation supports this
approach as well.\15\ The Administrator further agrees that the
standard must provide requisite protection from 5-10 minute exposure
events, but believes that this can be provided without having a
standard with a 5-minute averaging time. The Administrator agrees with
the REA conclusion that it is likely a 1-hour standard--with the
appropriate form and level--can substantially reduce 5-10 minute peaks
of SO2 shown in controlled human exposure studies to result
in respiratory symptoms and/or decrements in lung function in
exercising asthmatics. The Administrator further believes that a 5-
minute averaging time would result in significant and unnecessary
instability and is undesirable for that reason. The Administrator also
notes the statements from CASAC mentioned above addressing whether a 1-
hour averaging time can adequately control 5-10 minute peak exposures
and whether there should be a 5-minute averaging time. As noted above,
addressing this question, CASAC stated that the REA had presented a
``convincing rationale'' (Samet 2009, p. 16) for a 1-hour standard, and
that ``a one-hour standard is the preferred averaging time'' (Samet
2009, p. 15).
---------------------------------------------------------------------------

    \15\ As noted above, such a standard also satisfactorily
addresses the issue raised by the reviewing court in the litigation
that followed the last review of the SO2 NAAQS: Why was
no protection afforded in the standard for a susceptible
subpopulation known to experience repeated adverse effects from
exposure to 5-10 minute SO2 bursts. American Lung Ass'n,
134 F. 3d at 392-93.
---------------------------------------------------------------------------

    Second, as in the proposal the Administrator agrees that a 1-hour
averaging time (again, with the appropriate form and level) would
provide protection against the range of health outcomes associated with
averaging times of 1 hour to 24 hours. Specifically, the Administrator
finds that a 1-hour standard can substantially reduce the upper end of
the distribution of SO2 levels more likely to be associated
with adverse respiratory effects (see discussion on Form, section
II.F.3); that is: (1) 99th percentile 1-hour daily maximum
SO2 air quality concentrations in U.S. locations where
positive SO2 effect estimates were reported in epidemiologic
studies of emergency department visits and hospital admissions for all
respiratory causes and asthma; and (2) 99th percentile 24-hour average
SO2 air quality concentrations found in U.S. locations where
emergency department visit and hospital admission studies using multi-
pollutant models with PM reported statistically significant
associations (for all respiratory causes or asthma) with ambient
SO2 (see REA, section <IP_ADDRESS> and proposal section II.F.2,
74 FR at 64831). Finally, the Administrator again notes that
establishing a new 1-hour averaging time is in agreement with CASAC
recommendations. As noted above, CASAC stated that they were ``in
agreement with having a short-term standard and finds that the REA
supports a one-hour standard as protective of public health'' (Samet
2009, p. 1). Moreover, CASAC agreed with the REA that a ``one-hour
standard is the preferred averaging time'' (Samet 2009, p.15).
3. Form
    This section discusses considerations related to the form of the 1-
hour SO2 primary NAAQS. Specifically, this section
summarizes the rationale for the Administrator's proposed decision
regarding form (II.F.3.a; see proposal section II.F.3, 74 FR at 64833-
64834 of the proposal for more detail), discusses comments related to
form (II.F.3.b), and presents the Administrator's final conclusions
regarding form (II.F.3.c).
a. Rationale for Proposed Decision
    In considering the most appropriate form for the SO2
primary NAAQS, the Administrator noted in the proposal the conclusions
and judgments made in the ISA about available scientific evidence, air
quality information discussed in the REA, conclusions of the policy
assessment chapter of the REA, and CASAC recommendations (see section
II.F.3, 74 FR at 64833-64834 in the proposal). Specifically, the
proposal referenced the following:
     Information in the ISA that suggested that adverse
respiratory effects are more likely to occur at the upper end of the
distribution of ambient SO2 concentrations. That is, the ISA
describes a few studies that reported an increase in SO2-
related respiratory health effects at the upper end of the distribution
of SO2 concentrations (ISA, section 5.3, p. 5-9).
     The REA conclusion that a concentration-based form
averaged over three years would better reflect the continuum of health
risks posed by increasing SO2 concentrations (i.e. the
percentage of asthmatics affected and the severity of the response
increases with increasing SO2 concentrations; REA, section
10.5.3) by giving proportionally greater weight to years when 1-hour
daily maximum SO2 concentrations are well above the level of
the standard, than just above the level of the standard.
     Analyses in the REA that suggested for a given
SO2 standard level, a 99th percentile form is appreciably
more effective at limiting 5-minute peak SO2 concentrations
than a 98th percentile form (REA, section 10.5.3 and REA, Figures 7-27
and 7-28).
     Analyses in the REA indicating that over the last 10 years
and for the vast majority of the sites examined, there appears to be
little difference in 98th and 99th percentile design value stability
(REA, section 10.5.3).
     The REA conclusion that taken together, the evidence and
air quality information indicate that consideration should be given
primarily to a 1-hour daily maximum standard with a 99th percentile or
4th highest daily maximum form (REA, section <IP_ADDRESS>).
     CASAC indications that: ``there is adequate information to
justify the use of a concentration-based form averaged over 3 years''
(Samet 2009, p. 16).
     CASAC recommendations that when evaluating 98th vs. 99th
percentile forms, EPA should consider the number of days per year 98th
vs. 99th percentile forms would allow SO2 concentrations to
exceed the selected standard level. Similarly, CASAC recommendations to
consider the number of exceedences of 5-minute benchmarks given 98th
vs. 99th percentile forms at a given standard level (Samet 2009).
b. Comments on Form
    Most all State organizations and agencies (e.g., NAACA, NESCAUM and
agencies in FL, NM, PA, SC, TX, VT) supported a 99th percentile or 4th
highest form. Similarly, public health (e.g., ALA, ATS) and
environmental organizations (e.g., CBD, WEACT for Environmental
Justice) and the Alexandria Department of Transportation and
Environmental Services preferred either a 99th percentile or a more
stringent form (e.g., no exceedence) to further limit the occurrence of
SO2 concentrations that exceed the standard level in
locations that attain the standard. In contrast, many industry groups
(e.g., UARG, NAM, LEC, RRI Energy, AirQuality Research & Logistics
(AQRL)), and the SD DENR conditionally supported a

98th percentile form if EPA were to set a 1-hour standard.\16\ EPA
responses to specific comments on the form of the standard can be found
below and in the RTC document (EPA 2010).
---------------------------------------------------------------------------

    \16\ EPA did not propose or seek comment on a 98th percentile
form or a more restrictive form (e.g., an exceedence based form).
EPA also considered a 4th highest form, which is generally
equivalent to the 99th percentile. However, a percentile based form
is preferred since it results in a sampling from the same part of
the annual distribution of 1-hour daily maximum SO2
concentrations regardless of the number of 1-hour daily maximum
concentrations reported in a given year for a particular location.
---------------------------------------------------------------------------

    As mentioned above, a number of industry groups and the SD DENR
preferred a 98th percentile form. In general, their preference for a
98th percentile form was based on their conclusion that a form based on
the 98th percentile would be more stable than a form based on the 99th
percentile, and that a 98th percentile form is consistent with the
forms selected in recent NAAQS reviews (i.e. PM2.5 and
NO2). For example AQRL stated: ``The Administrator should
reconsider her proposal and choose instead the 98th percentile (or
equivalent nth highest value) form of the standard for the added
reliability and stability it offers in determining compliance or
progress towards attainment. This approach has been promulgated for
recent revisions of the PM2.5 and NO2 standards
and this consistency should be maintained with SO2.''
    We agree with the commenters that it is important that a 1-hour
standard have a form that is reasonably stable, but we disagree that a
98th percentile form is significantly more stable than a 99th
percentile form. We note that the REA discussed analyses (also briefly
described in the proposal; see section II.F.3, 74 FR at 64834)
comparing trends in 98th and 99th percentile design values from 54
sites located in the 40 counties selected for the detailed air quality
analysis (REA section 10.5.3 and Thompson, 2009). These results
suggested that at the vast majority of sites, there would have been
similar changes in 98th and 99th percentile design values over the last
ten years (i.e. based on evaluating overlapping three year intervals
over the last ten years; see REA, Figure 10-1 and Thompson, 2009). As
part of this analysis, all of the design values over this ten year
period for all 54 sites were aggregated and the standard deviation
calculated (REA, Figure 10-2 and Thompson, 2009). Results demonstrated
similar standard deviations--i.e. similar stability--based on
aggregated 98th or aggregated 99th percentile design values over the
ten year period (see REA, Figure 10-2 and Thompson 2009). Thus, we
believe that in most locations, there will not be a substantial
difference in stability between 98th and 99th percentile forms.
    We also disagree with the commenters that the forms of NAAQS
standards should be consistent across different NAAQS pollutants. This
is almost like advocating consistent levels or averaging times for
different NAAQS pollutants. Each pollutant is manifestly different from
another, and the decision as to an appropriate standard for each, and
appropriate elements (including form) of each standard and the
interaction of these elements, necessarily is fact-specific. Cf. Sierra
Club v. EPA, 353 F. 3d 976, 986 (DC Cir. 2004) (``This court has
adopted an `every tub on its own bottom' approach to EPA's setting of
standards pursuant to the CAA, under which the adequacy of the
underlying justification offered by the agency is the pertinent
factor--not what the agency did on a different record concerning a
different industry'') (Roberts J.). There is thus no basis to say a
priori that any element of one NAAQS should be consistent with another,
although if all other things are equal, selecting stable forms for each
NAAQS is a legitimate objective.
    A 99th percentile form, rather than a 98th percentile form, is also
needed for the standard to provide requisite public health protection.
In this review of the primary SO2 NAAQS, we considered
information in the ISA suggesting that adverse respiratory effects are
more likely to occur at the upper end of the distribution of ambient
SO2 concentrations. That is, the ISA described a few studies
that reported an increase in SO2-related respiratory health
effects at the upper end of the distribution of SO2
concentrations (i.e., above 90th percentile SO2
concentrations; ISA, section 5.3, p. 5-9). Moreover, we considered the
extent to which different percentile forms, given the same standard
level, limit 5-minute concentrations of SO2 above benchmark
levels. As noted above in section II.F.3.a, and in more detail in the
proposal (see section II.F.3.a, 74 FR at 64834), air quality analyses
presented in the REA suggested that at a given SO2 standard
level, a 99th percentile form is appreciably more effective at limiting
5-minute peak SO2 concentrations than a 98th percentile form
(REA, section 10.5.3, and REA, Figures 7-27 and 7-28). Taken together
with the analyses suggesting that 98th and 99th percentile forms have
similar stabilities, we reasonably concluded that a 99th percentile
form was most appropriate for a 1-hour SO2 standard.
    As mentioned above, a number of health and environmental groups
supported a 99th percentile form, but expressed that they would prefer
a more restrictive form, such as a no-exceedence based form. In
addition, the Alexandria Department of Transportation and Environmental
Services only recommended a no, or one exceedence based form. In
general, these groups concluded that a more restrictive form would
further limit the: (1) Number of days an area could exceed the standard
level and still attain the standard; and (2) the occurrence of 5-minute
peaks of SO2 above benchmark levels.
    It is important that the particular form selected for a 1-hour
daily maximum standard reflect the nature of the health risks posed by
increasing SO2 concentrations. The REA and proposal (see
section II.F.3, 74 FR at 64833) noted that the form of the standard
should reflect results from controlled human exposure studies
demonstrating that the percentage of asthmatics affected, and the
severity of the respiratory response (i.e. decrements in lung function,
respiratory symptoms) increases as SO2 concentrations
increase. Taking this into consideration, EPA staff concluded that a
concentration-based form, averaged over three years, is more
appropriate than an exceedance-based form (REA, section 10.5.3). This
is because a concentration-based form averaged over three years gives
proportionally greater weight to years when 1-hour daily maximum
SO2 concentrations are well above the level of the standard,
as it gives to years when 1-hour daily maximum SO2
concentrations are just above the level of the standard. In contrast,
an expected exceedance form gives the same weight to years when 1-hour
daily maximum SO2 concentrations are just above the level of
the standard as it gives to years when 1-hour daily maximum
SO2 concentrations are well above the level of the standard.
Therefore, we concluded that a concentration-based form, averaged over
three years (which also increases the stability of the standard) better
reflects the continuum of health risks posed by increasing
SO2 concentrations (i.e. the percentage of asthmatics
affected and the severity of the response increases with increasing
SO2 concentrations; REA, section 10.5.3). Moreover, we note
that analyses in the REA indicate that in most locations analyzed, a
99th percentile form would correspond to the 4th highest daily maximum
concentration in a year, and that the 99th percentile, combined with
the standard level

selected, will substantially limit 5-minute peaks of SO2
above the 200 ppb and higher benchmark levels (see below, section
II.F.4). Finally, we note that a concentration based form is in
agreement with CASAC advice that: ``there is adequate information to
justify the use of a concentration-based form averaged over 3 years''
(Samet 2009, p. 16).
c. Conclusions on Form
    The Administrator agrees that the form of the standard should
reflect the health evidence presented in the ISA indicating that the
percentage of asthmatics affected and the severity of the response
increases with increasing SO2 concentrations. The
Administrator also agrees that it is reasonable to consider the
standard's stability as part of consideration of the form of the
standard. Thus, the Administrator agrees that the standard should use a
concentration-based form averaged over three years in order to give due
weight to years when 1-hour SO2 concentrations are well
above the level of the standard, than to years when 1-hour
SO2 concentrations are just above the level of the standard.
She also notes that a concentration-based form averaged over 3 years
would likely be appreciably more stable than a no-exceedence based
form.
    In selecting a specific concentration based form, the Administrator
first notes that a few epidemiologic studies described in the ISA
reported an increase in SO2-related respiratory health
effects at the upper end of the distribution of ambient SO2
concentrations (i.e., above 90th percentile SO2
concentrations; see ISA, section 5.3, p. 5-9). The Administrator notes
further that numerous controlled human exposure studies have reported
decrements in lung function and/or respiratory symptoms in exercising
asthmatics exposed to peak 5-10 minute SO2 concentrations.
The Administrator therefore concludes that the form of a new 1-hour
standard should be especially focused on limiting the upper end of the
distribution of ambient SO2 concentrations (i.e., above 90th
percentile SO2 concentrations) in order to provide
protection with an adequate margin of safety against effects reported
in both epidemiologic and controlled human exposure studies.
    In further considering specific concentration based forms, the
Administrator notes as outlined above in section II.F.3.b, and
discussed in more detail in the REA (REA, section 10.5.3) and proposal
(see section II.F.3, 74 FR at 64834), that a 99th percentile form is
likely to be appreciably more effective at limiting 5-minute benchmark
exposures of concern compared to a 98th percentile form. Taken together
with the considerations just discussed above, the Administrator has
selected a 99th percentile form, averaged over 3 years. The
Administrator concludes that a 99th percentile form, given the level
selected (see section II.F.4 immediately below), will limit both the
upper end of the distribution of ambient SO2 concentrations
reported in some epidemiologic studies to be associated with increased
risk of SO2-related respiratory morbidity effects (e.g.,
emergency department visits), as well as 5-minute peak SO2
concentrations resulting in decrements in lung function and/or
respiratory symptoms in exercising asthmatics participating in
controlled human exposure studies.
4. Level
    As discussed below and in more detail in the proposal (section
II.F.4, 74 FR at 64834), the Administrator proposed to set a 1-hour
standard with a 99th percentile form (averaged over three years), with
a level in the range of 50 to 100 ppb. The Administrator also solicited
comment on standard levels greater than 100 ppb up to 150 ppb. This
section summarizes the rationale for the Administrator's proposed range
of standard levels (II.F.3.a), discusses comments related to the range
of standard levels (II.F.3.b), and presents the Administrator's final
conclusions regarding the level of a new 1-hour SO2 standard
(II.F.3.c).
a. Rationale for Proposed Decision
    In assessing the level of a 1-hour standard with a 99th percentile
form (averaged over three years), the Administrator considered the
broad range of scientific evidence assessed in the ISA, including the
epidemiologic studies and controlled human exposure studies, as well as
the results of air quality, exposure, and risk analyses presented in
the REA. In light of this body of evidence and analyses, the
Administrator found it is necessary to provide increased public health
protection for at-risk populations against an array of adverse
respiratory health effects related to short-term (i.e., 5 minutes to 24
hours) exposures to ambient SO2. In considering the most
appropriate way to provide this protection, the Administrator was
mindful of the extent to which the available evidence and analyses
could inform a decision on the level of a standard. The Administrator's
proposed decisions on level, as discussed in detail in the proposal
(see section II.F.4.e), are outlined below.
    Given the above considerations, the Administrator proposed to set a
level for a new 99th percentile 1-hour daily maximum primary
SO2 standard within the range from 50 to 100 ppb and took
comment on levels above 100 ppb, up to 150 ppb. In reaching this
proposed decision, the Administrator considered: (1) The evidence-based
considerations from the final ISA and the final REA; (2) the results of
the air quality, exposure, and risk assessments discussed above and in
the final REA; (3) CASAC advice and recommendations on both the ISA and
REA discussed above and provided in CASAC's letters to the
Administrator; and (4) public comments received on the first and second
drafts of the ISA and REA. In considering what level of a 1-hour
SO2 standard is requisite to protect public health with an
adequate margin of safety, the Administrator was mindful that this
choice requires judgments based on an interpretation of the evidence
and other information that neither overstates nor understates the
strength and limitations of that evidence and information.
    As noted above, the Administrator selected an upper end of a range
of levels to propose at 100 ppb. The selection of this level focused on
the results of the controlled human exposure studies and is primarily
based on the results of the air quality and exposure analyses which
suggest that a 1-hour standard should be at or below 100 ppb to
appreciably limit 5-minute SO2 benchmark concentrations >=
200 ppb (see proposal Tables 2-4, and proposal sections II.F.4.a and
II.F.4.b). That is, as described in the proposal (see section
II.F.4.e), the 40-county air quality analysis estimates that a 100 ppb
1-hour standard would allow at most 2 days per year on average when
estimated 5-minute daily maximum SO2 concentrations exceed
the 400 ppb benchmark, and at most 13 days per year on average when 5-
minute daily maximum SO2 concentrations exceed the 200 ppb
benchmark (see proposal Table 2). Furthermore, given a simulated 1-hour
100 ppb standard level, most counties in the air quality analysis were
estimated to experience 0 days per year on average when 5-minute daily
maximum SO2 concentrations exceed the 400 ppb benchmark and
<= 3 days per year on average when 5-minute daily maximum
SO2 concentrations were estimated to exceed the 200 ppb
benchmark (see REA, Tables 7-14 and 7-12). The Administrator also noted
that the St. Louis exposure analysis indicated that a 1-hour standard
at

100 ppb would still be estimated to protect > 99% of asthmatic children
at moderate or greater exertion from experiencing at least one 5-minute
SO2 exposure >= 400 ppb per year, and about 97% of these
children from exposures >= 200 ppb. In contrast, as described in the
proposal (see section II.F.4.b), the St. Louis exposure analysis
estimated that a 1-hour standard at 150 ppb would likely only protect
about 88% of asthmatic children at moderate or greater exertion from
experiencing at least one 5-minute SO2 exposure >= 200 ppb
per year.
    As noted above and described in detail in the proposal (see section
II.F.4.e), the Administrator selected 50 ppb as the lower end of a
range of levels to propose, which is consistent with CASAC's advice.
The selection of this level focused in part on the U.S. epidemiologic
evidence described in detail in the proposal (see sections II.B.2,
II.F.4.a, and II.F.4.e). With respect to these epidemiologic studies,
seven of ten U.S. emergency department visit and hospital admission
studies reporting generally positive associations with ambient
SO2 were conducted in locations where 99th percentile 1-hour
daily maximum SO2 levels were about 75-150 ppb, and three of
these studies observed statistically significant positive associations
between ambient SO2 and respiratory-related emergency
department visits and hospitalizations in multi-pollutant models with
PM (NYDOH (2006), Ito et al., (2007), and Schwartz et. al, (1995)).
Thus, the Administrator noted that a 99th percentile 1-hour daily
maximum standard set at a level of 50 ppb is well below the 99th
percentile 1-hour daily maximum SO2 concentrations reported
in locations where these three studies were conducted (i.e. well below
99th percentile 1-hour daily maximum SO2 levels of 78-150
ppb seen in NYDOH (2006), Ito et al., (2007), and Schwartz et. al,
(1995)). Finally, the Administrator noted that two epidemiologic
studies reported generally positive associations between ambient
SO2 and emergency department visits in cities when 99th
percentile 1-hour daily maximum SO2 concentrations were
approximately 50 ppb, but did not consider that evidence strong enough
to propose setting a standard level lower than 50 ppb.
    In considering the results of the air quality and exposure
analyses, the Administrator also noted that the 40-county air quality
analysis estimates that a 99th percentile 1-hour daily maximum standard
set at a level of 50 ppb would result in zero days per year when
estimated 5-minute SO2 concentrations exceed the 400 ppb 5-
minute benchmark level and at most 2 days per year when modeled 5-
minute SO2 concentrations exceed the 200 ppb 5-minute
benchmark level (see proposal section II.F.4.b and proposal Table 2).
In addition, the St. Louis exposure analysis estimates that a 99th
percentile 1-hour daily maximum standard set at a level of 50 ppb would
likely protect > 99% of asthmatic children at moderate or greater
exertion from experiencing at least one 5-minute exposure both >= 400
and  200 ppb per year (see proposal section II.F.4.b and
Table 3). In addition, although not directly analyzed in the REA, the
proposal (section II.F.4.b) noted that a 1-hour daily maximum standard
at a level of 75 ppb would be bound by the exposure estimates from air
quality adjusted to just meet 99th percentile 1-hour daily maximum
standards at 50 and 100 ppb. Thus, a 1-hour daily maximum standard at a
level of 75 ppb would be estimated to protect > 99% of asthmatic
children at moderate or greater exertion in St. Louis from experiencing
at least one exposure >= 400 ppb per year, and about 97% to > 99% of
these children from experiencing at least one exposure >= 200 ppb per
year.
    The Administrator thus proposed to set the level of a new 1-hour
standard that would protect public health with an adequate margin of
safety between 50 ppb and 100 ppb. In so doing, the Administrator
relied on reported findings from both epidemiologic and controlled
human exposure studies, as well as the results of air quality and
exposure analyses. The Administrator noted that the lower end of the
proposed range was consistent with CASAC advice that there is clearly
sufficient evidence for consideration of standard levels starting at 50
ppb (Samet 2009, p. 16). With respect to the upper end of the proposed
range, the Administrator noted that CASAC concluded that standards up
to 150 ppb ``could be justified under some interpretations of weight of
evidence, uncertainties, and policy choices regarding margin of
safety'' (id.), although the letter did not provide any indication of
what interpretations, uncertainties, or policy choices might support
selection of a level as high as 150 ppb.
    In light of the range of levels included in CASAC's advice, the
Administrator also solicited comment on setting a standard level above
100 ppb and up to 150 ppb. In so doing, the Administrator recognized
that there are uncertainties with the scientific evidence, such as
attributing effects reported in epidemiologic studies specifically to
SO2 given the presence of co-occurring pollutants,
especially PM, and the uncertainties associated with using ambient
SO2 concentrations as a surrogate for exposure. However, the
Administrator noted that compared to the proposed range of 50-100 ppb,
a standard level as high as 150 ppb would not comparably limit 5-minute
SO2 exposures >= 200 ppb. That is, she noted that the St.
Louis exposure analysis estimated that a 150 ppb standard would protect
approximately 88% of asthmatic children at moderate or greater exertion
from experiencing at least one SO2 exposure >= 200 ppb per
year (compared to > 99% and approximately 97% given standards at 50 and
100 ppb respectively; see proposal Table 3 at 74FR at 64841).
b. Comments on Level
    Most State and local agencies and organizations that commented on
this issue expressed support for setting the level of a 1-hour
SO2 standard somewhere within the proposed range of 50 to
100 ppb. More specifically, State environmental organizations (i.e.,
NACAA and NESCAUM); State environmental agencies (e.g., such agencies
in DE, IL, MI, NY, NM, PA, VT), the Fond du Lac Tribe, and local groups
(e.g., NYDOH, City of Houston, New York City, Houston-Galveston Area
Council) supported a level of a 1-hour SO2 standard in the
range of 50 to 100 ppb. In addition, State environmental agencies in IA
and TX specifically supported a standard level of 100 ppb. In general,
these groups cited the conclusions of CASAC and the Administrator's
rationale as stated in the proposal as a basis for their
recommendations, though State environmental agencies in IA and TX
generally recommended placing more weight on the controlled human
exposure evidence rather than on the epidemiology.
    A number of environmental and medical/public health organizations
(e.g., ALA, ATS, EDF, Sierra Club, WEACT for Environmental Justice,
NRDC, CBD) and some local organizations (e.g., Alexandria Department of
Transportation and Environmental Services, and Harris County (TX)
Public Health & Environmental Services) supported setting a standard
level at or near 50 ppb. This recommendation was typically based on the
commenters' interpretation of the controlled human exposure and
epidemiologic evidence, as described below.
    With regard to the controlled human exposure evidence, health and
environmental groups generally concluded that a 1-hour SO2
standard

no higher than 50 ppb is needed to protect against 5-minute
SO2 benchmark exposures as low as 100 ppb identified from
mouthpiece exposure studies, rather than the 200 ppb 5-minute
SO2 benchmark identified from ``free breathing'' controlled
human exposure studies. More specifically, ALA et al., stated:

    In its analysis of data from chamber studies in the ISA and in
the REA, EPA focuses on studies of ``free breathing'' exposure. In
doing so, EPA improperly and arbitrarily downplays important
evidence that reported increased airway resistance, a measure of
bronchoconstriction, in subjects with mild asthma at concentrations
of 100 ppb. Regrettably, EPA does not rely on the mouthpiece studies
in formulating its proposed standards * * * In downplaying the
mouthpiece studies, EPA ignores the large segment of people who rely
on oral or oronasal breathing some or all of the time.

    The Administrator disagrees with the assertion that results from
mouthpiece studies were improperly downplayed. These studies are
discussed in the ISA, REA, and proposed rule as demonstrating
respiratory effects of SO2 at concentrations of 100 ppb, the
lowest concentration tested using a mouthpiece exposure system.
Nonetheless, these mouthpiece studies are not a reasonable proxy for
actual exposure. In these studies, SO2 is delivered directly
through the mouth, typically in conjunction with nasal occlusion. This
allows a greater fraction of the inhaled SO2 to reach the
tracheobronchial airways. Although we agree with commenters that some
individuals do breathe oronasally both while at rest and during
exercise, nasal ventilation still constitutes a significant percentage
of total ventilation. The consequence is that individuals exposed to
SO2 through a mouthpiece are likely to experience greater
respiratory effects from a given SO2 exposure than they
would in real life. Thus, as noted in the REA (REA, section 6.2) and in
the proposal preamble (see section II.B.1.b), these mouthpiece studies
only provide very limited evidence of decrements in lung function
following exposure to 100 ppb SO2. Therefore, the
Administrator did not place great weight on these mouthpiece studies
when considering the appropriate level of a 1-hour SO2
standard.
    In addition to their interpretation of the controlled human
exposure evidence, health and environmental groups (e.g., ALA, ATS,
EDF, NRDC, Sierra Club, CBD) and the Alexandria Department of
Transportation and Environmental Services generally concluded that the
epidemiologic evidence indicates that a standard no higher than 50 ppb
is required to protect public health. For example, it its comments the
CBD stated:

    Epidemiologic studies referenced in the Proposed Rule showed
positive, and in many cases statistically significant, relationships
between ambient SO2 concentrations and hospital
admissions where 99th percentile 1-hour concentrations ranged from
50-460 ppb. Of these studies, two showed positive and sometimes
statistically significant relationships in single-pollutant models
at 50 ppb, and three studies showed statistically significant
correlations at 78-150 ppb in multi-pollutant models. These three
multipollutant studies, moreover, ``lend[] strong support * * * to
the conclusion that SO2 effects are generally
independent'' of those of co-pollutants like particulate matter.
Giving these studies their proper weight, and allowing for an
adequate margin of safety, EPA should set a one-hour NAAQS at a
level no higher than the lowest concentration at which positive,
adverse relationships have been demonstrated: 50 ppb (note that
footnotes were omitted).

    The Administrator agrees that the epidemiologic studies referenced
in the proposal need to be considered in judging the appropriate level
for a new 99th percentile 1-hour SO2 standard. However, she
disagrees that when considered in total, these studies strongly support
an SO2 standard no higher than 50 ppb. The Administrator
notes that selecting a standard level of 50 ppb would place
considerable weight on the two U.S. emergency department visit studies
conducted in locations where 99th percentile 1-hour SO2
concentrations were approximately 50 ppb (i.e., Wilson et al., (2005)
in Portland, ME and Jaffe et al., (2003) in Columbus, OH). However, the
Administrator does not find this appropriate given that, importantly,
neither of these studies evaluated the potential for confounding by co-
pollutants through the use of multipollutant models and thus, left
unaddressed the issue of whether the effects seen in the studies were
partially or totally attributable to exposure to sulfate PM. In
addition, the Administrator notes that the overall results reported in
these studies are mixed. It is important to note that mixed results do
not automatically disqualify studies from being used as part of the
evidence base for setting levels in NAAQS reviews. However, in this
review the Administrator judges that the lack of mutipollutant model
evaluation for potential confounding by PM in two locations with the
lowest SO2 levels combined with the presence of mixed
emergency department visit results renders these two studies
inappropriate to serve as the primary basis for the selection of the
level of the SO2 NAAQS. As an additional matter, the
suggestion in some of the comments that EPA should necessarily base the
level of a NAAQS on the lowest level seen in epidemiologic studies has
been rejected repeatedly. See, e.g. American Petroleum Inst. v. EPA,
665 F. 2d at 1187 (``In so arguing NRDC essentially ignores the mixed
results of the medical studies evident in the record, choosing instead
to rely only on the studies that favor its position. The Administrator,
however, was required to take into account all the relevant studies
revealed in the record. Because he did so in a rational manner, we will
not overrule his judgment as to the margin of safety.'') Thus, although
the Administrator finds that these two studies provide limited evidence
of emergency department visits in cities where 99th percentile 1-hour
daily maximum SO2 concentrations are approximately 50 ppb,
she also concludes that these studies do not provide enough evidence to
warrant a standard at this level.
    As discussed above in section, II.E.2, a number of industry groups
(e.g., ACC, UARG) did not support setting a new 1-hour SO2
standard. However, several of these groups (e.g., UARG, API) and the SC
Chamber of Commerce concluded that, if EPA does choose to set a new 1-
hour standard, the level of that standard should be >= 150 ppb. In
addition, State environmental agencies in SD (SD DENR) and OH
recommended standard levels at 150 ppb. As a basis for this
recommendation, these groups generally emphasized uncertainties in the
scientific evidence. Specifically, as discussed in more detail above
(section II.E.2.a), these commenters typically concluded that the
available epidemiologic studies do not support the conclusion that
SO2 causes the reported health effects. This was based on
their assertion that the presence of co-pollutants in the ambient air
precludes the identification of a specific SO2 contribution
to reported effects. Thus, these groups generally concluded that weight
should not be placed on the cluster of three epidemiologic studies
reporting statistically significant effects in multipollutant models
with PM (i.e., NYDOH 2006; Ito 2007; and Schwartz 1995). That is, these
groups contend that these studies do not demonstrate an independent
effect of SO2. In addition, as noted in section II.E.2.b,
many of these groups also disagreed with the Agency's judgment that
adverse respiratory effects occur following 5-minute exposures to
SO2 concentrations as low as 200 ppb. These comments and
EPA's responses are discussed below

and in section II of the RTC document (EPA 2010).
    As described in more detail in section II.E.2.a, we agree that the
interpretation of SO2 epidemiologic studies is complicated
by the fact that SO2 is but one component of a complex
mixture of pollutants present in the ambient air. However, the ISA
concluded that when U.S. and international epidemiologic literature is
evaluated as a whole, SO2 effect estimates generally
remained positive and relatively unchanged in multi-pollutant models
with gaseous or particulate co-pollutants. Thus, although recognizing
the uncertainties associated with separating the effects of
SO2 from those of co-occurring pollutants, the ISA concluded
that the limited available evidence from studies employing multi-
pollutant models indicates that the effect of SO2 on
respiratory health outcomes appears to be generally robust and
independent of the effects of gaseous co-pollutants, including
NO2 and O3, as well as particulate co-pollutants,
particularly PM2.5 (ISA, section 5.2; p. 5-9).
    In addition, as described in detail above in section II.E.2.a, the
ISA emphasized that controlled human exposure studies provide support
for the plausibility of the associations reported in epidemiologic
studies. The ISA noted that the results of controlled human exposure
and epidemiologic studies form a plausible and coherent data set that
supports a causal relationship between short-term (5-minutes to 24-
hours) SO2 exposures and adverse respiratory effects, and
that the epidemiologic evidence (buttressed by the clinical evidence)
indicates that the effects seen in the epidemiologic studies are
attributable to exposure to SO2 (ISA, section 5.2). The ISA
in fact made the strongest finding possible regarding causality:
``[e]valuation of the health evidence, with consideration of issues
related to atmospheric sciences, exposure assessment, and dosimetry,
led to the conclusion that there is a causal relationship between
respiratory morbidity and short-term exposure to SO2. This
conclusion is supported by the consistency, coherence, and plausibility
of findings observed in the human clinical, epidemiologic, and animal
toxicological studies.'' ISA p. 5-2 (emphasis original).
    As mentioned above, many groups dispute the ISA conclusion that
taken together, results from U.S. and international epidemiologic
studies employing multipollutant models indicate that SO2
has an independent effect on the respiratory health outcomes reported
in these studies. Thus, these groups contend that the Administrator
should not place weight on epidemiologic studies and their associated
air quality information in general, and more specifically, the
Administrator should not place weight on air quality information from
the three U.S. epidemiologic studies reporting statistically
significant effects in multipollutant models with PM (i.e., NYDOH 2006;
Ito 2007; and Schwartz 1995). Specific comments on these three
epidemiologic studies reporting statistically significant effects in
multi-pollutant models with PM, and EPA responses are presented below
and in the RTC document (EPA 2010).
    Industry groups (e.g., API) had several comments with respect to
the study conducted by the NYDOH (NYDOH, 2006). First, these groups
generally concluded that the results of this study are mixed. That is,
while SO2 effect estimates were positive and statistically
significant even in multipollutant models with PM2.5 or
NO2 in the Bronx, SO2 effect estimates were
actually negative in Manhattan in both single and multipollutant
models. These groups also contend that this report was not peer-
reviewed and that the authors of this study indicated that high
correlations among pollutants in the Bronx made it difficult to
confidently identify which pollutants are actually increasing risks.
For these reasons, industry groups generally concluded that this study
should not be relied upon by the Administrator.
    We acknowledge that the results of the NYDOH analysis are mixed
when comparing the Bronx and Manhattan study areas. However, we
disagree that the presence of mixed results renders this study
unreliable. We note that the mixed results reported in this study are
likely to reflect greater statistical power for identifying effects in
the Bronx, where the average daily emergency department visits differed
substantially from those in Manhattan. Specifically, daily asthma
emergency department visits were six times higher in the Bronx study
area (43 per day) than in the Manhattan study area (7.2 per day). Thus,
the more prominent effects in the Bronx likely at least partially
reflect greater statistical power for identifying effects there. To put
these numbers in perspective, the crude daily rates of asthma emergency
department visits can be estimated by dividing the daily asthma counts
by the population. The mean daily crude rates of asthma emergency
department visits were over eight-fold higher in the Bronx study area
(16.9 per 100,000 persons) than in the Manhattan area (2.02 per 100,000
persons). Population age structures were quite different in the two
communities, with larger proportions of younger persons in the Bronx
versus Manhattan. There are likely additional differences in population
structures of the two communities, including differences in SES, race/
ethnicity, and access to primary asthma care. These differences in the
two communities may explain the differences in the results, and do not
prevent EPA from legitimately relying on this study.
    As mentioned above, these groups also contend that the NYDOH
epidemiologic study should not be relied upon because it was not peer-
reviewed. We disagree with this assertion. The NYDOH study was subject
to multiple peer-review processes. This included reviews by the Agency
for Toxic Substances and Disease Registry (ATSDR), EPA, and CASAC.
    Finally, as also mentioned above, these groups contend that the
NYDOH epidemiologic study is unreliable because the study authors
indicated that high correlations among pollutants in the Bronx make it
difficult to confidently identify which pollutants are actually
increasing risks. In response, we note that high correlations among
ambient air pollutant concentrations are not specific to the NYDOH
study, and may contribute to uncertainty in the interpretation of many
epidemiologic studies of air pollution. The approach most commonly
utilized to disentangle the effects of correlated pollutants in air
pollution epidemiology is the copollutant model. The NYDOH uses
copollutant models and finds that the results for SO2 remain
significant in models considering the simultaneous effects of
NO2, O3, and PM2.5. This indicates an
independent effect of SO2 on the asthma emergency department
visits reported in this study.
    With respect to Ito et al., (2007), industry groups generally
commented that since the SO2 effect estimate did not remain
statistically significant in multipollutant models with NO2,
this study does not indicate an independent effect of SO2 on
emergency department visits in the NYC study area. API specifically
commented:

    The RR for an increase of 6 ppb SO2 was statistically
significant (1.20; 95% CI: 1.13, 1.28) and remained so when
PM2.5, O3, or CO was included in the model,
but became nonsignificant when NO2 was included in the
model (RR not provided, 95% CI: 0.9, 1.1). Because associations with
SO2 could be attributable to NO2, this study
cannot be used to assess the effects of SO2 on health
effects with small incremental increases in exposure.

    We disagree with the commenters. We believe that this study does
demonstrate an independent effect of SO2 on emergency
department visits in NYC. We note that evidence from controlled human
exposure studies has demonstrated effects of NO2 (EPA,
2008b) and SO2 independently on respiratory morbidity. Since
each of these criteria pollutants has an independent effect on the
respiratory system, it is logical that each may be responsible for an
increase in emergency department visits for asthma in epidemiologic
studies. In addition, the authors note that the attenuation of the
SO2 effect estimate when NO2 is included in the
model is ``consistent with the result of monitor-to-monitor
correlations, suggesting that NO2 has less exposure error
than CO or SO2 in this data set.'' Thus, it appears as
though the high spatial heterogeneity of SO2 compared to
NO2, leading to increased exposure error, may be causing the
attenuation of the SO2 effect estimate when NO2
is included in the model in this study--not that the effects seen in
the study are attributable to NO2. Overall, the results from
this study are consistent with the SO2 effect on respiratory
emergency department visits and hospital admissions across studies and
are coherent with the respiratory effects observed in controlled human
exposure studies. This study thus provides persuasive evidence of an
independent effect of short-term SO2 exposure on respiratory
morbidity.
    With respect to Schwartz et al., (1995), industry groups generally
commented that the results of this study are mixed, and therefore
should not be considered by the Administrator. More specifically, these
commenters noted that although the results in New Haven remained
statistically significant in the presence of PM10, the
SO2 effect estimate in Tacoma was reduced and no longer
statistically significant in the presence of PM10.
Commenters also noted that in both cities, the SO2 effect
estimate was reduced and no longer statistically significant in the
presence of O3.
    We disagree that the results of this study of hospital admissions
should not be considered by the Administrator. As noted by the
commenters, this study was conducted in two cities, New Haven, CT and
Tacoma, WA. These cities were chosen because they differ in several
important aspects and the author expected the results from the two
cities to be different due to the inherent nature of the study design
and study locations. ``New Haven has almost twice the mean
SO2 concentration of Tacoma, almost two and a half times the
SO2 concentration in the peak winter season, and a much
larger summer ozone peak than Tacoma (Schwartz 1995).'' Since the study
was designed to examine the differences in these two cities, the fact
that the results differed in the two cities does not invalidate those
results. In addition, EPA considers the SO2 effect to be
robust to inclusion of O3 in New Haven. The central effect
estimate for SO2 changed from 1.03 to 1.02 after the
addition of O3 as a copollutant and likely lost statistical
significance due to a greater than 40% reduction in the number of days
included because O3 was only measured during the warm
months. This reduction likely led to model instability and a loss of
statistical significance. To be consistent with how results of other
studies were interpreted in the ISA, and as supported by the CASAC, the
effect of SO2 is considered robust to the inclusion of
O3 in New Haven.
    In addition to generally concluding that the epidemiology is too
uncertain to demonstrate that SO2 has an independent effect
on the respiratory effects reported in those studies, many industry
groups (e.g., API, ACC, Progress Energy, EEI, CIBO) also commented that
adverse health effects do not occur following 5-10 minute
SO2 exposures < 400 ppb in controlled human exposure studies
(an issue also discussed above in section II.E.2.b). Thus, these groups
generally maintained that the level of a 1-hour standard should not
take into account limiting 5-minute peaks as low as 200 ppb. From this
argument, many of these groups further maintained that 1-hour standard
levels >= 150 ppb are requisite to protect public health with an
adequate margin of safety.
    As first discussed in section II.E.2.b above, we disagree with the
commenters that adverse respiratory effects do not occur following 5-
minute SO2 exposures as low as 200 ppb. The ISA reported
that exposure to SO2 concentrations as low as 200-300 ppb
for 5-10 minutes results in approximately 5-30% of exercising
asthmatics experiencing moderate or greater decrements in lung function
(defined in terms of a >= 15% decline in FEV1 or 100%
increase in sRaw; ISA, Table 3-1). Considering the 2000 ATS guidelines
described in section II.E.2.b, we determined that these results could
reasonably indicate an SO2-induced shift in these lung
function measurements for this sub-population. Under this scenario, an
appreciable percentage of exercising asthmatics exposed to
SO2 concentrations as low as 200 ppb would likely have
diminished reserve lung function and thus would likely be at greater
risk if affected by another respiratory agent (e.g., viral infection).
Importantly, diminished reserve lung function in a population that is
attributable to air pollution is considered an adverse effect under ATS
guidance.\17\ Also noted in section II.E.2.b, we were mindful of
CASAC's pointed comments. The second draft ISA placed relatively little
weight on health effects associated with SO2 exposures at
200-300 ppb. CASAC strongly disagreed with this characterization of the
health evidence. Their consensus letter following the second draft ISA
states:
---------------------------------------------------------------------------

    \17\ See Coalition of Battery Recyclers Association v. EPA, No.
09-1011 (DC Cir., May 14, 2010), slip opinion at 9, holding that it
was reasonable for EPA to conclude that a two IQ point mean
population loss is an adverse effect based in part on consideration
of comments from the American Academy of Pediatrics that such a loss
should be prevented.

    Our major concern is the conclusions in the ISA regarding the
weight of the evidence for health effects for short-term exposure to
low levels of SO2. Although the ISA presents evidence
from both clinical and epidemiological studies that indicate health
effects occur at 0.2 ppm or lower, the final chapter emphasizes
health effects at 0.4 ppm and above * * * CASAC believes the
clinical and epidemiological evidence warrants stronger conclusions
in the ISA regarding the available evidence of health effects at 0.2
ppm or lower concentrations of SO2. The selection of a
lower bound concentration for health effects is very important
because the ISA sets the stage for EPA's risk assessment decisions.
In its draft Risk and Exposure Assessment (REA) to Support the
Review of the SO2 Primary National Ambient Air Quality Standards
(July 2008), EPA chose a range of 0.4 ppm--0.6 ppm SO2
concentrations for its benchmark analysis. As CASAC will emphasize
in a forthcoming letter on the REA, we recommend that a lower bound
---------------------------------------------------------------------------
be set at least as low as 0.2 ppm (Henderson 2008a).

    Similarly, we were also mindful of CASAC comments on the first
draft of the REA. The consensus CASAC letter following the 1st draft
REA states:

    The CASAC believes strongly that the weight of clinical and
epidemiology evidence indicates there are detectable clinically
relevant health effects in sensitive subpopulations down to a level
at least as low as 0.2 ppm SO2. These sensitive
subpopulations represent a substantial segment of the at-risk
population (Henderson 2008b).

    As noted in section II.E.2.b, we were also mindful of: (1) Previous
CASAC recommendations (Henderson 2006) and NAAQS review conclusions
(EPA 2006, EPA 2007d) indicating that moderate decrements in lung
function can be clinically significant in some asthmatics (see section
II.E.2.b for more detail) and

(2) controlled human exposure studies not including severe asthmatics
and thus, that it is reasonable to assume that persons with more severe
asthma than the study participants would have a more serious health
effect from short-term exposure to 200 ppb SO2. CASAC echoed
this concern in its comments on the policy assessment chapter of the
REA:

    Chapter 10 should better address uncertainty in identifying
alternative NAAQS for SO2. In particular, the
uncertainties discussed in the health risk characterization should
be considered in specifying a NAAQS that provides adequate margin of
safety. One particular source of uncertainty needing acknowledgment
is the characteristics of persons included in the clinical studies.
The draft REA acknowledges that clinical studies are unlikely to
have included severe asthmatics that are likely to be potentially at
greater risk than those persons included in the clinical studies
(Samet 2009; p. 15).

    Taken together, the Administrator concluded that exposure to
SO2 concentrations as low as 200 ppb can result in adverse
health effects in asthmatics. Consequently the Administrator also
concluded that a 1-hour standard of 150 ppb is not requisite to protect
public health with an adequate margin of safety, even with a 99th
percentile form. This conclusion takes into account the St. Louis
exposure analysis estimating that only 88% of asthmatic children at
moderate or greater exertion would be protected from at least one 5-
minute SO2 exposure >= 200 ppb per year at a 1-hour standard
level of 150 ppb, and appropriate weight placed on the epidemiologic
evidence (see section II.F.4.c for a discussion of the epidemiologic
evidence with respect to level).
c. Conclusions on Standard Level
    Having carefully considered the public comments on the appropriate
level for a 1-hour SO2 standard, as discussed above, the
Administrator believes the fundamental conclusions reached in the ISA
and REA remain valid. In considering the level at which the 1-hour
primary SO2 standard should be set, the Administrator
continues to place primary emphasis on the body of controlled human
exposure and epidemiologic evidence assessed in the ISA, as summarized
above in section II.B. In addition, the Administrator continues to view
the results of exposure and risk analyses, discussed above in section
II.C, as providing supporting information for her decision.
    In considering the level of a 1-hour SO2 standard, the
Administrator notes that there is no bright line clearly mandating the
choice of level within the reasonable range proposed. Rather, the
choice of what is appropriate within this reasonable range is a public
health policy judgment entrusted to the Administrator. This judgment
must include consideration of the strengths and limitations of the
evidence and the appropriate inferences to be drawn from the evidence
and the exposure and risk assessments. These considerations and the
Administrator's final decision with regard to the level of a new 1-hour
SO2 standard are discussed below.
    In considering the controlled human exposure studies, the
Administrator notes that these studies provide the most direct evidence
of respiratory effects from exposure to SO2. These studies
exposed groups of exercising asthmatics to defined concentrations of
SO2 for 5-10 minutes and found adverse respiratory effects.
As noted above (see section II.C), SO2 exposure levels which
resulted in respiratory effects in these studies were considered 5-
minute benchmark exposures of potential concern in the analyses found
in the REA. With respect to this evidence, the Administrator notes the
following key points:
     Exposure of exercising asthmatics to 5-10 minute
SO2 concentrations >= 400 ppb results in moderate or greater
decrements in lung function (in terms of FEV1 or sRaw) in
20-60% of tested individuals in these studies. Moreover, these
decrements in lung function are often statistically significant at the
group mean level and are frequently accompanied by respiratory
symptoms.\18\ Based on ATS guidelines, exposure to SO2
concentrations >= 400 ppb clearly result in adverse respiratory effects
(i.e., decrements in lung function in the presence of respiratory
symptoms). Therefore, the Administrator has concluded it appropriate to
place weight on the 400 ppb 5-minute SO2 benchmark
concentration of concern.
---------------------------------------------------------------------------

    \18\ The ISA concluded that collective evidence from key
controlled human exposure studies considered in the previous review,
along with a limited number of new controlled human exposure
studies, consistently indicates that with elevated ventilation rates
a large percentage of asthmatic individuals tested in a given
chamber study (up to 60%, depending on the study) experience
moderate or greater decrements in lung function, frequently
accompanied by respiratory symptoms, following peak exposures to
SO2 at concentrations of 0.4-0.6 ppm. (ISA, p. 3-9).
---------------------------------------------------------------------------

     Exposure of exercising asthmatics to 5-10 minute
SO2 concentrations at 200-300 ppb results in moderate or
greater decrements in lung function in 5-30% of the tested individuals
in these studies. The Administrator notes that although these
decrements in lung function have not been shown to be statistically
significant at the group mean level, or to be frequently accompanied by
respiratory symptoms, she considers effects associated with exposures
as low as 200 ppb to be adverse in light of CASAC advice, similar
conclusions in prior NAAQS reviews, and the ATS guidelines described in
detail above (see section II.E.2.b and II.F.4.b). Therefore, she has
concluded it appropriate to place weight on the 200 ppb 5-minute
benchmark concentration.
     There is very limited evidence from two mouthpiece
exposure studies suggesting respiratory effects in exercising
asthmatics following SO2 exposures at 100 ppb. However,
given the uncertainties and potential unrepresentativeness associated
with mouthpiece studies (see section II.F.4.b above), the Administrator
found it appropriate not to place weight on this 5-minute
SO2 benchmark concentration.
    The Administrator also considered the results of the air quality,
exposure, and risk analyses, as they serve to estimate the extent to
which a given 1-hour standard limits the 5-minute benchmark
concentrations of concern identified from controlled human exposure
studies (see REA chapters 7-9, proposal section II.F.4.b, and proposal
Tables 2-4). In considering these results as they relate to limiting 5-
minute SO2 benchmark concentrations >= 200 and 400 ppb, the
Administrator notes the following key points:
     The 40-county air quality analysis estimates that a 100
ppb 1-hour daily maximum standard would allow at most 2 days per year
on average in any county when estimated 5-minute daily maximum
SO2 concentrations exceed the 400 ppb benchmark, and at most
13 days per year on average when 5-minute daily maximum SO2
concentrations exceed the 200 ppb benchmark (see proposal, Table 2, 74
FR at 64840). Furthermore, given a simulated 1-hour 100 ppb standard
level, most of the counties in that air quality analysis were estimated
to experience 0 days per year on average when 5-minute daily maximum
SO2 concentrations exceed the 400 ppb benchmark and <= 3
days per year on average when 5-minute daily maximum SO2
concentrations were estimated to exceed the 200 ppb benchmark (see REA,
Tables 7-14 and 7-12).
     The St. Louis exposure analysis estimates that a 99th
percentile 1-hour daily maximum standard at a level of 100 ppb would
likely protect > 99% of asthmatic children in that city at moderate or
greater exertion from experiencing at least one 5-minute exposure >=
400 ppb per year, and

approximately 97% of those asthmatic children at moderate or greater
exertion from experiencing at least one exposure >= 200 ppb per year
(see proposal, section II.F.4.b).
     The St. Louis risk assessment estimates that a 99th
percentile 1-hour standard level at 100 ppb would likely protect about
97-98% of exposed asthmatic children in that city at moderate or
greater exertion from experiencing at least one moderate or greater
lung function response (defined as a >= 100% increase in sRaw; see
proposal, section II.F.4.b).
    Given the above considerations, the Administrator concludes that a
1-hour standard at a level of 100 ppb would appropriately limit 5-
minute SO2 benchmark concentrations >= 200 or 400 ppb.
Moreover, although the Administrator acknowledges that the air quality
and exposure analyses mentioned above suggest that a 50 ppb standard
may somewhat further limit 5-minute SO2 concentrations/
exposures in excess of the 200 ppb benchmark (see proposal section
II.F.4.b), she does not believe this information alone warrants a
standard level lower than 100 ppb. More specifically, although she
considers the health effects resulting from 5-minute SO2
exposures as low as 200 ppb to be adverse, she also recognizes that
such effects are appreciably less severe than those at SO2
concentrations >= 400 ppb. Thus, she concludes that there is little
difference in limiting 5-minute concentrations/exposures >= 400 ppb
given 1-hour standard levels in the range of 50 to 100 ppb.
    In considering the epidemiologic evidence with regard to level, the
Administrator notes that there have been more than 50 peer reviewed
epidemiologic studies published worldwide evaluating SO2
(ISA, Tables 5-4 and 5-5). These studies have generally reported
positive, although not always statistically significant associations
between more serious health outcomes (i.e. respiratory-related
emergency department visits and hospitalizations) and ambient
SO2 concentrations and have generally included populations
potentially at increased risk for SO2-related respiratory
effects (e.g, children, older adults, and those with pre-existing
respiratory disease). The Administrator finds that in assessing the
extent to which these studies and their associated air quality
information can inform the level of a new 99th percentile 1-hour daily
maximum standard for the U.S., air quality information from the U.S.
and Canada is most relevant since these areas have similar monitor
network designs and patterns of air quality. However, as described in
proposal section II.F.4.a, SO2 concentrations reported for
Canadian studies were not directly comparable to those reported for
U.S. studies due to use of different monitoring protocols in those
studies. Thus, the Administrator focused on 99th percentile air quality
information from U.S. studies for informing potential 1-hour standard
levels. She concludes that this information provides evidence of
associations between ambient SO2 and emergency department
visits and hospital admissions in U.S. cities with particular 99th
percentile 1-hour SO2 levels, and thus provides information
that is particularly relevant for setting the level of a 1-hour
SO2 standard. With regard to these studies she notes the
following key points:
     Ten studies (some conducted in multiple locations)
reported mostly positive, and sometimes statistically significant,
associations between ambient SO2 concentrations and
emergency department visit and hospital admissions in locations where
99th percentile 1-hour daily maximum SO2 levels ranged from
approximately 50-460 ppb.
     Within this broader range of SO2
concentrations, there is a cluster of three epidemiologic studies
between 78-150 ppb (for the 99th percentile of the 1-hour
SO2 concentrations) where the SO2 effect estimate
remained positive and statistically significant in multi-pollutant
models with PM (NYDOH (2006), Ito et al., (2007), and Schwartz et al.,
(1995)). Notably, although statistical significance in multi-pollutant
models is an important consideration, it is not the only consideration
when relying on such epidemiologic evidence.\19\ However, as noted
earlier, there is special sensitivity in this review in disentangling
PM-related effects (especially sulfate PM) from SO2-related
effects in interpreting the epidemiologic studies; thus, these studies
are of particular relevance here, lending strong support both to the
conclusion that SO2 effects are generally independent of PM
(ISA, section 5.2) and that these independent adverse effects of
SO2 have occurred in cities with 1-hour daily maximum, 99th
percentile concentrations in the range of 78-150 ppb. Nor did EPA find
the comments criticizing these studies persuasive, as explained above
in section II.F.4.b and in the RTC document (EPA 2010). The
Administrator therefore judges it appropriate to place substantial
weight on this cluster of three U.S. epidemiologic studies in selecting
a standard level, as they are a group of studies that reported positive
and statistically significant associations between ambient
SO2 and emergency department visits or hospital admissions
even when potential confounding by PM was considered.
---------------------------------------------------------------------------

    \19\ For example, as noted in the proposal (proposal, section
II.F.4, 74 FR at 64835) evidence of a pattern of results from a
group of studies that find effect estimates similar in direction and
magnitude would warrant consideration of and reliance on such
studies even if the studies did not all report statistically
significant associations in single- or multi-pollutant models. The
SO2 epidemiologic studies fit this pattern, and are
buttressed further by the results of the clinical studies. ISA,
section 5.2.
---------------------------------------------------------------------------

     The Administrator agrees with the finding in the ISA that
the controlled human exposure evidence lends biological plausibility to
the effects reported in epidemiologic studies (ISA, p. 5-9).
     There is limited evidence from two epidemiologic studies
employing single pollutant models that found generally positive
associations between ambient SO2 and emergency department
visits in locations where 99th percentile 1-hour SO2
concentrations were approximately 50 ppb (see proposal, Figures 1 and
2). However, considering that the results of these studies were mixed,
and importantly, that neither of these two studies evaluated the
potential for confounding by co-pollutants through the use of
multipollutant models (particularly with PM), the Administrator judges
it appropriate to place limited weight on these studies.
     With regard to the cluster of three studies conducted in
the Bronx (NYDOH 2006), NYC (Ito et al., 2007), and New Haven (Scwartz
et al., 1995), there is a degree of uncertainty as to whether the 99th
percentile 1-hour daily maximum SO2 concentrations reported
from monitors in these three study areas reflect the highest 99th
percentile 1-hour daily maximum SO2 concentration. Our
limited qualitative analysis suggests that 99th percentile 1-hour daily
maximum SO2 concentrations reported by monitors in these
study areas are reasonable approximations for the absolute highest 99th
percentile 1-hour daily maximum SO2 concentration that can
occur across the entire area in these studies (including the areas
where monitors were not located) (see Brode, 2010). However, although a
reasonable approximation, it is still likely that these monitored
concentrations are somewhat lower than the absolute highest 99th
percentile 1-hour daily maximum SO2 concentrations occurring
across these epidemiologic study areas.

    Weighing all of this evidence, the Administrator concludes that the
epidemiologic studies provide strong support for setting a standard
that limits the 99th percentile of the distribution of 1-hour daily
maximum SO2 concentrations to 75 ppb. This judgment takes
into account the strong determinations in the ISA (and endorsed by
CASAC), based on a much broader body of evidence, that there is a
causal association between exposure to SO2 and the types of
respiratory morbidity effects reported in these studies. The
Administrator further judges that it is not necessary based on existing
epidemiologic evidence, to set a standard below 75 ppb. That is, the
Administrator concludes that a standard level of 75 ppb is sufficiently
below the SO2 levels in three cities where epidemiologic
studies found statistically significant effects in multipollutant
models with PM (i.e., 78, 82, and 150 ppb) to provide an adequate
margin of safety given the uncertainty as to whether monitors in these
study locations reflected the highest 1-hour daily maximum
SO2 concentration across the entire study area. Thus, a
standard set at a level of 75 ppb is likely further below the 99th
percentile 1-hour daily maximum concentrations in these three study
areas than the bare comparison of levels would otherwise indicate.
Finally, the Administrator again notes that epidemiologic evidence
below 75 ppb is more uncertain because studies below 75 ppb did not
evaluate potential confounding of results in multipollutant models, and
because these studies reported mixed results.
    Given the above considerations and the comments received on the
proposal, the Administrator determines that the appropriate judgment,
based on the entire body of evidence and information available in this
review, and the related uncertainties, is a standard level of 75 ppb.
She concludes that such a standard, with a 1-hour averaging time and
99th percentile form, will provide a significant increase in public
health protection compared to the current standards and would be
expected to protect against the respiratory effects that have been
linked with SO2 exposures in both controlled human exposure
and epidemiologic studies. Specifically, she concludes that such a
standard will limit 1-hour exposures at and above 75 ppb for those in
susceptible populations that are at-risk of experiencing adverse health
effects from short-term exposure to SO2. Such a standard
will also maintain SO2 concentrations below those in
locations where key U.S. epidemiologic studies have reported that
ambient SO2 is associated with clearly adverse respiratory
health effects, as indicated by increased hospital admissions and
emergency department visits. She also notes that a 1-hour standard at a
level of 75 ppb is expected to substantially limit asthmatics' exposure
to 5-10 minute SO2 concentrations >= 200 ppb, thereby
substantially limiting the adverse health effects associated with such
exposures. Finally, the Administrator notes that a standard level of 75
ppb is consistent with the consensus recommendation of CASAC.
    In setting the standard level at 75 ppb rather than at a lower
level, the Administrator notes that a 1-hour standard with a level
lower than 75 ppb would only result in significant further public
health protection if, in fact, there is a continuum of serious, adverse
health risks caused by exposure to SO2 concentrations below
75 ppb. Based on the available evidence, the Administrator does not
believe that such assumptions are warranted. Taking into account the
uncertainties that remain in interpreting the evidence from available
controlled human exposure and epidemiologic studies, the Administrator
notes that the likelihood of obtaining benefits to public health with a
standard set below 75 ppb decreases, while the likelihood of requiring
reductions in ambient concentrations that go beyond those that are
needed to protect public health increases.
    Therefore, the Administrator judges that a 1-hour SO2
standard at 75 ppb is sufficient to protect public health with an
adequate margin of safety. This includes protection with an adequate
margin of safety for susceptible populations at increased risk for
adverse respiratory effects from short-term exposures to SO2
for which the evidence supports a causal relationship with
SO2 exposures. The Administrator does not believe that a
lower standard level is needed to provide this degree of protection.
These conclusions by the Administrator appropriately consider the
requirement for a standard that is neither more nor less stringent than
necessary for this purpose and recognizes that the CAA does not require
that primary NAAQS be set at a zero-risk level or to protect the most
susceptible individual, but rather at a level that reduces risk
sufficiently so as to protect the public health with an adequate margin
of safety.
5. Retaining or Revoking the Current 24-Hour and Annual Standards
    This section discusses considerations related to retaining or
revoking the current 24-hour and annual SO2 primary NAAQS.
Specifically, this section summarizes the rationale for the
Administrator's proposed decision regarding whether to retain or revoke
the current standards (section II.F.5.a), discusses public comments
related to whether to retain or revoke the current standards
(II.F.5.b), and presents the Administrator's final conclusions
regarding whether to retain or revoke the current standards (II.F.5.c).
a. Rationale for Proposed Decision
    As noted in the proposal (see section II.F.5), the REA recognized
that the particular level selected for a new 99th percentile 1-hour
daily maximum standard would have implications for deciding whether to
retain or revoke the current 24-hour and annual standards. That is,
with respect to SO2-induced respiratory morbidity, the lower
the level selected for a 99th percentile 1-hour daily maximum standard,
the less additional public health protection the current standards
would be expected to provide. CASAC expressed a similar view following
their review of the 2nd draft REA: ``Assuming that EPA adopts a one
hour standard in the range suggested, and if there is evidence showing
that the short-term standard provides equivalent protection of public
health in the long-term as the annual standard, the panel is supportive
of the REA discussion of discontinuing the annual standard'' (Samet
2009, p. 15). With regard to the current 24-hour standard, CASAC was
generally supportive of using the air quality analyses in the REA as a
means of determining whether the current 24-hour standard was needed in
addition to a new 1-hour standard to protect public health. CASAC
stated: ``The evidence presented [in REA Table 10-3] was convincing
that some of the alternative one-hour standards could also adequately
protect against exceedances of the current 24-hour standard'' (Samet
2009, p. 15).
    In accordance with the REA findings and CASAC recommendations
mentioned above, the Administrator noted that 1-hour standards in the
range of 50-100 ppb would have the effect of maintaining 24-hour and
annual SO2 concentrations generally well below the levels of
the current 24-hour and annual NAAQS (see REA Tables 10-3 and 10-4 and
REA Appendix Tables D-3 to D-6). Thus, if a new 99th percentile 1-hour
daily maximum standard was set in the proposed range of 50-100 ppb,
then the Administrator proposed to revoke the current 24-hour and
annual standards. However, as noted in the proposal, if a standard was
set at a level >100 ppb and

up to 150 ppb, then the Administrator indicated that she would retain
the existing 24-hour standard, recognizing that a 99th percentile 1-
hour daily maximum standard at 150 ppb would not have the effect of
maintaining 24-hour average SO2 concentrations below the
level of the current 24-hour standard in all locations analyzed (see
REA Appendix Table D-4). Under this scenario, the Administrator would
still revoke the current annual standard recognizing: (1) 99th
percentile 1-hour daily maximum standards in the range of 50-150 ppb
would maintain annual average SO2 concentrations below the
level of the current annual standard (see REA Table 10-4 and REA
Appendix tables D-5 and D-6); and (2) the lack of sufficient evidence
linking long-term SO2 exposure to adverse health effects.
b. Comments on Retaining or Revoking the Current 24-Hour and Annual
Standards
    As noted above, most industry groups were opposed to the proposed
revisions to the SO2 NAAQS. However, some of these groups
noted that if a 1-hour standard was adopted, then they would support
revoking the current 24-hour and annual standards. State agencies
generally supported revoking the current standards if a 1-hour standard
was set in the proposed range, although NAACA, NESCAUM, and VT, while
supportive of revoking the existing standards, also suggested that EPA
explore setting a new 24-hour standard to minimize the potential that
multiple hours within a day would exceed a 1-hour standard (see RTC
document (EPA 2010), section IV). Groups which supported revoking the
current 24-hour and annual standards (if a 1-hour standard was set in
the proposed ranged) generally referenced the Administrator's rationale
and CASAC advice described in the proposal (see section II.F.5).
    Public health (e.g., ALA, ATS) and environmental organizations
(e.g., CBD, WEACT for Environmental Justice) were generally opposed to
revoking the current 24-hour and annual standards. These groups
generally concluded that the 24-hour standard should be revised while
the annual standard should be retained. In support of this position,
ALA et al., cited air quality information from the REA indicating that
if air quality was simulated to just meet a 99th percentile 1-hour
daily maximum standard in the proposed range of 50-100 ppb, then in
some locations analyzed, 99th percentile 24-hour average SO2
concentrations would be above concentrations (i.e., above 99th
percentile 24-hour average concentrations) in cities where U.S.
emergency department visit and hospital admission studies reported
positive associations with SO2. In addition, many of these
groups were opposed to revoking the current annual standard. In
general, these groups concluded that given the uncertainties associated
with SO2 exposure and long-term health effects, EPA should
err on the side of being health protective and retain the existing
annual standard. EPA responses to comments on whether the current
standards should be retained or revoked are presented below as well as
in section IV of the RTC document (EPA 2010).
    As stated in the REA and proposal, 99th percentile 24-hour average
SO2 concentrations in cities where U.S. emergency department
visit and hospital admission studies (for all respiratory causes and
asthma; identified from Table 5-5 of the ISA) were conducted ranged
from 16 ppb to 115 ppb (Thompson and Stewart, 2009). Moreover, as
stated in the REA and proposal (see section II.F.2), effect estimates
that remained statistically significant in multi-pollutant models with
PM were found in cities with 99th percentile 24-hour average
SO2 concentrations ranging from approximately 36 ppb to 64
ppb. In its comments, ALA et al., stated (based on the air quality
information in REA Appendix Table D-2) ``with a 1-hour 50 ppb 99th
percentile standard, 7 counties would experience a 99th percentile 24-
hour concentration of 16 ppb or greater, the range found to be harmful
in epidemiological studies. With an hourly standard of 100 ppb, 24 of
30 counties would have 99th percentile 24-hour concentrations above 16
ppb, with 1 county exceeding 36 ppb.'' Thus, these commenters generally
maintained that a lowered 24-hour standard is needed to protect against
these 24-hour SO2 concentrations.
    We disagree that a lowered 24-hour standard is needed to protect
against 24-hour average SO2 concentrations of concern
identified from cities where U.S. emergency department visit and
hospital admission studies were conducted. As noted in detail in the
REA, there is uncertainty as to whether the health effects reported in
epidemiologic studies using 24-hour average SO2
concentrations are in fact due to 24-hour average SO2
exposures (REA, section 10.5.2). That is, when describing epidemiologic
studies observing positive associations between ambient SO2
and respiratory symptoms, the ISA stated ``that it is possible that
these associations are determined in large part by peak exposures
within a 24-hour period'' (ISA, section 5.2 at p. 5-5). Similarly, the
ISA stated that: ``The effects of SO2 on respiratory
symptoms, lung function, and airway inflammation observed in the human
clinical studies using peak exposures further provides a basis for a
progression of respiratory morbidity resulting in increased emergency
department visits and hospital admissions'' and makes the associations
observed in the epidemiologic studies ``biologica[lly] plausib[le]''
(id.). In contrast, evidence from controlled human exposure studies of
5-10 minutes and epidemiologic studies using 1-hour daily maximum
SO2 concentrations provided appreciably stronger evidence of
respiratory morbidity effects following SO2 exposures <= 1-
hour.
    Given that respiratory morbidity effects following SO2
exposure may be most related to averaging times <=1-hour, EPA found it
most reasonable to consider the extent to which a 1-hour averaging
time, given an appropriate form and level (which as discussed above,
also substantially limits 5-minute benchmark exposures of concern; see
sections II.F.2 and II.F.4), limited 99th percentile 24-hour average
concentrations of SO2 in locations where emergency
department visit/hospitalization studies reported that the
SO2 effect estimate remained statistically significant in
multi-pollutant models with PM (i.e., locations with 99th percentile
24-hour average SO2 concentrations >=36 ppb). Considering
this, we note that ALA et al., identified only one county with 99th
percentile 24-hour average SO2 concentrations >=36 ppb given
a 99th percentile 1-hour daily maximum standard at 100 ppb, and no
counties >=36 ppb given a 99th percentile 1-hour daily maximum standard
at 50 ppb. Thus, given a 99th percentile 1-hour daily maximum standard
level at 75 ppb (i.e., the form and level selected for a new 1-hour
SO2 standard), it is possible that no county in the ALA et
al., analysis would have had a 99th percentile 24-hour average
SO2 concentration >=36 ppb.
    With regard to the annual standard, we also disagree that this
standard needs to be retained. First, the ISA found that ``[t]he
evidence linking short-term SO2 exposure and cardiovascular
effects, and morbidity and mortality with long-term exposures to
SO2 is inadequate to infer a causal relationship.'' ISA, p.
5-10. Thus, an annual standard is unnecessary to prevent long-term
health effects. The remaining issue is whether such a standard provides
further protection

against short-term effects, given the new one hour standard. We
conclude that it does not. As noted in the proposal, our air quality
information indicates that 1-hour standard levels in the range of 50-
100 ppb are estimated to generally keep annual SO2
concentrations well below the level of the current annual standard.
CASAC agreed. The panel stated: ``Assuming that EPA adopts a one hour
standard in the range suggested, and if there is evidence showing that
the short-term standard provides equivalent protection of public health
in the long-term as the annual standard, the panel is supportive of the
REA discussion of discontinuing the annual standard'' (Samet 2009, p.
15). Taken together, this information indicates that retaining the
annual standard would add no additional public health protection.
c. Administrator's Conclusions on Retaining or Revoking the Current 24-
Hour and Annual Standards
    In accordance with the REA findings and CASAC recommendations
mentioned above, the Administrator concludes that a 1-hour standard at
level of 75 ppb would have the effect of maintaining 24-hour and annual
SO2 concentrations generally well below the levels of the
current 24-hour and annual NAAQS (see REA Tables 10-3 and 10-4 and REA
Appendix Tables D-3 to D-6). She also concludes that, as noted above in
section II.F.2, a 1-hour standard at 75 ppb will likely limit 99th
percentile 24-hour SO2 concentrations in U.S. locations
where emergency department visit and hospital admission studies
reported statistically significant associations in multi-pollutant
models with PM. Finally, she notes the lack of sufficient health
evidence to support an annual standard to protect against health
effects associated with long-term SO2 exposure. Taken
together, the Administrator concludes it appropriate to revoke the
current 24-hour and annual standards.

G. Summary of Decisions on the Primary Standards

    For the reasons discussed above, and taking into account
information and assessments presented in the ISA and REA as well as the
advice and recommendations of CASAC, the Administrator concludes that
the current 24-hour and annual primary standards are not requisite to
protect public health with an adequate margin of safety. The
Administrator also concludes that establishing a new 1-hour standard
will appropriately protect public health with an adequate margin of
safety, and specifically will afford requisite increased protection for
asthmatics and other at-risk populations against an array of adverse
respiratory health effects related to short-term (5 minutes to 24
hours) SO2 exposure. These effects include decrements in
lung function (defined in terms of sRaw and FEV1), increases
in respiratory symptoms, and related serious indicators of respiratory
morbidity including emergency department visits and hospital admissions
for respiratory causes.
    Specifically, the Administrator is establishing a new short-term
primary SO2 standard with a 1-hour (daily maximum) averaging
time and a form defined as the 3-year average of the 99th percentile of
the yearly distribution of 1-hour daily maximum SO2
concentrations, and a level of 75 ppb. In addition to setting a new 1-
hour standard at 75 ppb, the Administrator is revoking the current 24-
hour and annual standards recognizing that a 1-hour standard set at 75
ppb will have the effect of generally maintaining 24-hour and annual
SO2 concentrations well below the levels of the current 24-
hour and annual standards.

III. Overview of the Approach for Monitoring and Implementation

    We received several comments regarding the approaches discussed in
the proposal for monitoring and modeling for comparison to the proposed
new 1-hour SO2 NAAQS, designations of areas as either
attaining or not attaining the NAAQS, and implementation of the new
NAAQS in State implementation plans (SIPs) that would ensure ultimate
attainment of the new NAAQS in transitioning from the annual and 24-
hour NAAQS in a timely manner. These comments raised fundamental
questions regarding our contemplated approaches in all three areas, and
caused us to re-examine them and review their consistency with past
practice under the SO2 NAAQS implementation program. After
conducting that review, and in response to the public comments we are
revising our general anticipated approach toward implementation of the
new 1-hour NAAQS. This revised approach would better address: (1) The
unique source-specific impacts of SO2 emissions; (2) the
special challenges SO2 emissions present in terms of
monitoring short-term SO2 levels for comparison with the
NAAQS in many situations; (3) the superior utility that modeling offers
for assessing SO2 concentrations; and (4) the most
appropriate method for ensuring that areas attain and maintain the new
1-hour SO2 NAAQS in a manner that is as expeditious as
practicable, taking into account the potential for substantial
SO2 emissions reductions from forthcoming national and
regional rules that are currently underway.
    Below, we provide an overview of our revised approach to
monitoring, and of our expected approaches to designations of areas,
and implementation of the NAAQS. Due to the unique challenges presented
by SO2, we do not expect that the anticipated approaches
discussed below would be necessarily transferable to other NAAQS
pollutant situations. For NAAQS pollutants other than SO2,
air quality monitoring is more appropriate for determining whether all
areas are attaining the NAAQS, and there is comparatively less
dependence upon conducting refined modeling. Each of these subjects
(i.e., our revised approach to monitoring, and our expected approaches
to designations of areas, and implementation of the NAAQS) is further
addressed later in the preamble, in sections IV, V and VI,
respectively. Where specific public comments on the proposal are
addressed and responded to, further details of the specific revised
approaches are explained. In many respects, both the overview
discussion below and the subsequent more detailed discussions explain
our expected and intended future action in implementing the new 1-hour
NAAQS--in other words, they constitute guidance, rather than final
agency action--and it is possible that our approaches may continue to
evolve as we, States, and other stakeholders proceed with actual
implementation. In other respects, such as in the final regulatory
provisions regarding the promulgated monitoring network, we are
explaining EPA's final conclusions regarding what is required by this
rule. We expect to issue further guidance regarding implementation,
particularly concerning issues that may arise regarding the application
of refined dispersion modeling under this revised approach for
monitoring and implementation, and issues that States and other
stakeholders may also ask us to address as we proceed toward various
stages of ensuring attainment. EPA intends to solicit public comment
prior to finalizing this guidance.
    The main necessary elements of implementing the new 1-hour NAAQS
are: (1) An approach for assessing ambient concentrations to determine
compliance with the NAAQS; (2) a process for using these assessments to
designate areas relative to the new standard; and (3) the development
of State plans that include control measures sufficient for ensuring
the NAAQS is attained everywhere as expeditiously as possible, which we

believe should be no later than 2017. EPA's revised anticipated
approach to determining compliance with the new SO2 NAAQS is
consistent with our historical approach to SO2 designations
and implementation through permits and emissions limitations, which
involves the combined use of monitoring and modeling. The emphasis we
would place on monitoring and modeling, compared with each other, under
the revised expected approach is therefore significantly different than
that in the approach discussed in the proposal, which was less in line
with our historical practice for SO2, as the public comments
highlighted.
    In the SO2 NAAQS proposal, we recommended a monitoring-
focused approach for comparison to the new NAAQS, featuring a two-
pronged monitoring network design. This included monitors in certain
CBSAs based on a combination of population and SO2 emissions
coupled with additional monitors within a State based on that State's
contribution to national SO2 emissions. The resulting
proposed network would have required approximately 348 monitors
nationwide to be sited at the locations of maximum concentration.
Numerous State and local government commenters expressed concerns
regarding the burdens of implementing the proposed monitoring network
and the sufficiency of its scope for purposes of identifying
violations. These commenters contended that our proposed monitoring
network was too small and insufficient to cover the range of
SO2 sources, and yet too burdensome and expensive to expand
to an adequate scale. Some of these commenters (the City of Alexandria,
and the States of Delaware, North Carolina and Pennsylvania) suggested
using modeling to determine the scope of monitoring requirements, or
favored modeling over monitoring to determine compliance with the
NAAQS.
    Partly in response to these comments, and after reconsidering the
proposal's monitoring-focused approach in light of EPA's historical
approach to SO2 NAAQS implementation and area designations
decisions, we intend to use a hybrid analytic approach that would
combine the use of monitoring and modeling to assess compliance with
the new 1-hour SO2 NAAQS. We believe that some type of
hybrid approach is more consistent with our historical approach and
longstanding guidance toward SO2 than what we originally
proposed. In addition, we believe that for a short-term 1-hour standard
it is more technically appropriate, efficient, and effective to use
modeling as the principle means of assessing compliance for medium to
larger sources, and to rely more on monitoring for groups of smaller
sources and sources not as conducive to modeling. We discuss the
details of the final revised monitoring network requirements in section
IV later in the preamble, but note here the relationship that the
revised approach toward monitoring and modeling--taken partly in
response to the public comments mentioned above--has to the other two
general subject areas in implementation for which we are providing
guidance, namely initial area designations and development of
substantive implementation plans that ensure timely attainment and
maintenance of the NAAQS. Our ultimate intention is to place greater
emphasis on modeling than did the proposed rule as the most technically
appropriate, efficient, and readily available method for assessing
short-term ambient SO2 concentrations in areas with large
point sources. This projected change in approach would necessarily
result in a lesser emphasis on the less appropriate, more expensive,
and slower to establish monitoring tool than did the proposed rule.
Therefore, the minimum requirements for the SO2 monitoring
network in this final rule are of a smaller scale than proposed, and we
do not expect monitoring to become the primary method by which ambient
concentrations are compared to the new 1-hour SO2 NAAQS.
    Instead, in areas without currently operating monitors but with
sources that might have the potential to cause or contribute to
violations of the NAAQS, we anticipate that the identification of NAAQS
violations and compliance with the 1-hour SO2 NAAQS would
primarily be done through refined, source-oriented air quality
dispersion modeling analyses, supplemented with a new, limited network
of ambient air quality monitors. Historically, we have favored
dispersion modeling to support SO2 NAAQS compliance
determinations for areas with sources that have the potential to cause
an SO2 NAAQS violation, and we have explained that for an
area to be designated as ``attainment,'' dispersion modeling regarding
such sources needs to show the absence of violations even if monitoring
does not show a violation. This has been our general position
throughout the history of implementation of the SO2 NAAQS
program. See, e.g., ``Air Quality Control Regions, Criteria, and
Control techniques; Attainment Status Designations,'' 43 FR 40412,
40415-16 (Sept. 11, 1978); ``Air Quality Control Regions, Criteria, and
Control Techniques,'' 43 FR 45993, 46000-02 (Oct. 5, 1978); ``Air
Quality Implementation Plans: State Implementation Plans; General
Preamble,'' 57 FR 13498, 13545, 13547-48 (Apr. 16, 1992); ``Approval
and Promulgation of State Implementation Plans; Call for Sulfur Dioxide
SIP Revisions for Billings/Laurel, MT,'' 58 FR 41430 (Aug. 4, 1993);
``Designation of Areas for Air Quality Planning Purposes; Ohio,'' 59 FR
12886, 12887 (Mar. 18, 1994); ``Ambient Air Quality Standards, National
and Implementation Plans for Sulfur Oxides (Sulfur Dioxide),'' 60 FR
12492, 12494-95 (Mar. 7, 1995); ``Air Quality Implementation Plans;
Approval and Promulgation: Various States: Montana,'' 67 FR 22167,
22170-71, 22183-887 (May 2, 2002).
    Compared to other NAAQS pollutants, we would not consider ambient
air quality monitoring alone to be the most appropriate means of
determining whether all areas are attaining a short-term SO2
NAAQS. Due to the generally localized impacts of SO2, we
have not historically considered monitoring alone to be an adequate,
nor the most appropriate, tool to identify all maximum concentrations
of SO2. In the case of SO2, we further believe
that monitoring is not the most cost-efficient method for identifying
all areas of maximum concentrations. However, for some situations
monitoring is well suited, and we therefore will require it to some
extent, as further explained in section IV of the preamble. For
example, monitoring may appropriately be relied upon to assess
compliance with the NAAQS by groups of smaller sources and sources that
may not be as conducive to modeling as are larger SO2
sources.
    States will need to make any adjustments to the existing monitoring
network to ensure that monitors meeting today's network design
regulations for the new 1-hour NAAQS are sited and operational by
January 1, 2013. We also expect to provide additional guidance
regarding the application of refined dispersion modeling under this
revised expected approach for implementation of the new SO2
standard. Appendix A to the Guideline on Air Quality Models (Appendix W
of 40 CFR part 51), Summaries of Preferred Air Quality Models, provides
``key features of refined air quality models preferred for specific
regulatory applications'' (see Appendix A to Appendix W of Part 51 at
A.0(1)). Refined dispersion modeling, following our current Guideline
on Air Quality Models with appropriate flexibility for use in
implementation, is anticipated to better reflect and account

for source-specific SO2 impacts than the more limited
monitoring-focused proposal. As noted above, EPA intends to solicit
public comment prior to finalizing this guidance.
    Based on a revised, hybrid approach, we expect to implement the new
SO2 standard in the following manner. In accordance with CAA
section 107(d), EPA must designate areas as ``attainment,''
``nonattainment'' or ``unclassifiable'' for the new 1-hour
SO2 NAAQS by June 2012 (i.e., two years following
promulgation of the new NAAQS).\20\ State Governors are required to
submit their initial area designation recommendations to EPA no later
than June 2011. We expect that EPA's final area designation decisions
in 2012 would be based principally on data reported from SO2
monitors currently in place today, and any refined modeling the State
chooses to conduct specifically for initial area designations.\21\ For
these initial designations, we would expect to designate an area
``nonattainment'' if either monitoring data or appropriate refined
modeling results show a violation. Any area that has monitoring and
appropriate modeling data showing no violations we would expect to
designate as ``attainment.'' \22\ All other areas, absent monitoring
data and air quality modeling results showing no violations, we would
expect to initially designate as ``unclassifiable,'' as required by the
Clean Air Act. The expected presumptive boundary for any area
designated ``nonattainment'' would be the county boundary associated
with the violation unless additional information provided to EPA
demonstrates otherwise, as has been our general approach for other
NAAQS pollutants. Any area initially designated ``nonattainment'' or
``unclassifiable'' could request redesignation to ``attainment'' after
an assessment based on air quality modeling, conducted in accordance
with the new guidance, and available monitoring data indicates that the
standard has been met, as well as meeting all other requirements of the
CAA for redesignation to attainment.
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    \20\ EPA is authorized by the Clean Air Act to take up to 3
years to complete the initial area designations in the event that
insufficient information is available to complete the designations
within 2 years.
    \21\ Since three complete years of data from any newly sited
monitors meeting the new monitoring network design criteria are not
expected to be obtained until the end of 2015, any newly sited
monitors will not play a role in EPA's initial area designations.
    \22\ EPA anticipates making the determination of when monitoring
alone is ``appropriate'' for a specific area on a case-by-case
basis, informed by that area's factual record, as part of the
designations process. EPA would expect to address this issue for
such areas by examining the historic treatment of the area with
respect to prior SO2 designations as well as whether the
area is one in which monitoring would be the more technically
appropriate tool for determining compliance with the new
SO2 NAAQS. An example of a situation in which monitoring
may be the more preferred approach is a shipping port (non-point
source or ``area'' source) that is not in close proximity to other
significant stationary SO2 sources.
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    This anticipated approach toward initial area designations is a
change from the approach discussed in the proposal, and logically
follows from our general change in approach to the use and utility of
monitoring versus modeling for determining short-term SO2
ambient concentrations. As public commenters pointed out, establishment
and implementation of the proposed monitoring network would have been
both too limited and too late to inform initial area designations, and
the expense and burden of accelerating it and expanding it would have
been severe for State implementing agencies. Given the time needed to
establish monitors, it is not realistic to expect either such an
expanded monitoring network or even the more reasonable limited network
of the final rule to be the chief tool for informing initial
designations.
    That means that some other approach is needed to inform initial
designations of areas and other implementation decisions under the new
SO2 NAAQS. In addition to using any valid data generated by
existing monitors, refined dispersion modeling may inform designation
and implementation decisions regarding sources that may have the
potential to cause or contribute to a NAAQS violation. In order for
modeling to be done on the scale sufficient to identify all areas that
might violate the new 1-hour standard, EPA anticipates issuing guidance
that addresses a variety of issues, such as how to identify and
appropriately assess the air quality impacts of small SO2
sources (e.g., those emitting less than 100 tons of SO2 per
year) that may potentially cause or contribute to a violation of the
new SO2 NAAQS. EPA expects that it will take more time for
EPA to issue that guidance than is available in order to use it for the
initial round of attainment designations. In addition to any smaller
sources that might cause or contribute to NAAQS violations, States
would need to model approximately 2000 larger sources across the
country (i.e., sources that emit greater than 100 tons per year and are
collectively responsible for about 99% of all SO2 emissions
from point sources in the U.S.) to determine whether areas are
attaining or not attaining the 1-hour standard. While these sources
emitting 100 or more tons of SO2 per year represent the
significant fraction of the total emissions from point sources in the
U.S., smaller sources also have the potential to violate the new
SO2 NAAQS.
    After receiving EPA's forthcoming modeling guidance, States might
initially focus modeling assessments on these larger sources that have
been subject to permitting requirements and are generally better
characterized than smaller sources. But even this effort would entail a
substantial burden on States, under a compressed timeline following
EPA's issuance of further modeling guidance. Consequently, EPA does not
believe that for this new 1-hour SO2 NAAQS it would be
realistic or appropriate to expect States to complete such modeling and
incorporate the results in initial designation recommendations, which
under CAA section 107(d)(1)(A) must be submitted to EPA within 1 year
of the promulgation of the 1-hour standard.
    The remaining issue, then, is how to most appropriately use a
modified hybrid approach, and its constituent modeling and monitoring
tools, in the implementation plan development process in order to
ensure expeditious attainment and maintenance of the NAAQS. Under the
CAA, all States must develop and submit to EPA State implementation
plans (SIPs) to attain and maintain the new 1-hour SO2
NAAQS. CAA section 110(a)(1) requires States, regardless of designation
status, to adopt SIPs that provide for implementation, maintenance and
enforcement of each primary NAAQS. Traditionally, for areas that were
designated ``attainment'' or ``unclassifiable'', we accepted State
submissions of prevention of significant deterioration (PSD) permitting
programs and other ``infrastructure'' SIP elements contained in CAA
section 110(a)(2) as being sufficient to satisfy the section 110(a)(1)
SIP submission requirement. However, due to our recognition here that
monitoring is not generally the most appropriate or effective tool for
assessing compliance with the new 1-hour SO2 NAAQS, that
additional guidance from EPA on conducting refined modeling for the new
1-hour NAAQS is anticipated to support our expected implementation