uncertainties associated with that evidence. When this evidence and its
associated uncertainties were taken together, the ISA concluded that
the results of epidemiologic and experimental studies form a plausible
and coherent data set that supports a relationship between
SO2 exposures and respiratory endpoints, including
respiratory symptoms and ED visits, at ambient concentrations that are
present in areas that meet the current 24-hour SO2 NAAQS
(ISA, section 5.5). Thus, taking into consideration the evidence
discussed above, particularly the epidemiologic studies reporting
SO2-associated health effects in locations that meet the
current 24-hour standard, the REA concluded that the epidemiologic
evidence calls into question the adequacy of the current 24-hour
standard to protect public health (REA, section 10.3.4).
b. Air quality, exposure, and risk-based considerations
    As previously mentioned, the ISA found the evidence for an
association between respiratory morbidity and SO2 exposure
to be ``sufficient to infer a causal relationship'' (ISA, section 5.2)
and that the ``definitive evidence'' for this conclusion comes from the
results of controlled human exposure studies demonstrating decrements
in lung function and/or respiratory symptoms in exercising asthmatics
(ISA, section 5.2). Accordingly, the exposure and risk analyses
presented in the REA focused on exposures and risks associated with 5-
minute peaks of SO2 in excess of the potential health effect
benchmark values of 100, 200, 300, and 400 ppb SO2. In
considering the results presented in these analyses, the REA
particularly noted exceedances or exposures with respect to the 200 and
400 ppb 5-minute benchmark levels. These benchmark levels were
highlighted in the REA because (1) 400 ppb represents the lowest
concentration in controlled human exposure studies where moderate or
greater lung function decrements which were often statistically
significant at the group mean level, were frequently accompanied by
respiratory symptoms; and (2) 200 ppb is the lowest level at which
moderate or greater decrements in lung function in free-breathing human
exposure studies have been observed (notably, 200 ppb is also the
lowest level that has been tested). The REA also recognized that there
was very limited evidence demonstrating small decrements in lung
function at 100 ppb from two mouthpiece exposure studies. However, as
previously noted (see section II.B.1.b), the results of these studies
are not directly comparable to free-breathing chamber studies, and
thus, the REA primarily considered exceedences of the 200 ppb and 400
ppb benchmark levels in its evaluation of the adequacy of the current
24-hour (as well

as the annual; see section II.E.2) standard.
    A key output of the air quality analysis was the predicted number
of statistically estimated 5-minute daily maximum SO2
concentrations above benchmark levels given air quality simulated to
just meet the level of the current 24-hour or annual SO2
standard, whichever was controlling for a given county. Under this
scenario, in 40 counties selected for detailed analysis, the REA found
that the predicted yearly mean number of statistically estimated 5-
minute daily maximum concentrations > 400 ppb ranges from 1-102 days
per year,\14\ with most counties in this analysis experiencing a mean
of at least 20 days per year when statistically estimated 5-minute
daily SO2 concentrations exceed 400 ppb (REA, Table 7-14).
In addition, the predicted yearly mean number of statistically
estimated 5-minute daily maximum concentrations > 200 ppb ranged from
21-171 days per year, with about half of the counties in this analysis
experiencing >= 70 days per year when 5-minute daily maximum
SO2 concentrations exceed 200 ppb (REA, Table 7-12).
---------------------------------------------------------------------------

    \14\ Air quality estimates presented in this section represent
the mean number of days per year when 5-minute daily maximum
SO2 concentrations exceed a particular benchmark level
given 2001-2006 air quality adjusted to just meet the current
standards (see REA, Tables 7-11 to 7-14).
---------------------------------------------------------------------------

    The REA also generated exposure and risk estimates for two study
areas in Missouri (i.e., Greene County and several counties
representing the St. Louis urban area) which had significant emission
sources of SO2. As noted in REA section 8.10, there were
differences in the number of exposures above benchmark values when the
results of the Greene County and St. Louis exposure assessments were
compared. In addition, given that the results of the exposure
assessment were used as inputs into the quantitative risk assessment,
it was not surprising that there were also differences in the number of
asthmatics at elevated ventilation rates estimated to have a moderate
or greater lung function response in Greene County when compared to St.
Louis. The REA noted that the differences in the St. Louis and Greene
County exposure and quantitative risk results are likely indicative of
the different types of locations they represent (see section 8.10).
Greene County is a rural county with much lower population and emission
densities, compared to the St. Louis study area which has population
and emissions density similar to other urban areas in the U.S. It
therefore follows that there would be greater exposures, and hence
greater numbers and percentages of asthmatics at elevated ventilation
rates experiencing moderate or greater lung function responses in the
St. Louis study area. Thus, when considering the risk and exposure
results as they relate to the adequacy of the current standards, the
REA concluded that the St. Louis results were more informative in terms
of ascertaining the extent to which the current standards protect
against effects linked to the various benchmarks (linked in turn to 5-
minute exposures). The results in fact suggested that the current
standards may not adequately protect public health (REA, section
10.3.3). Moreover, the REA judged that the exposure and risk estimates
for the St. Louis study area provided useful insights into exposures
and risks for other urban areas in the U.S. with similar population and
SO2 emissions densities (REA, section 10.3.3).
    When considering the St. Louis exposure results as they relate to
the adequacy of the current standards, results discussed in the policy
chapter of the REA included the percent of asthmatic children at
moderate or greater exertion estimated to experience at least one
exceedance of either the 200 or 400 ppb benchmark given air quality
that was adjusted upward to simulate just meeting the current 24-hour
standard (i.e., the controlling standard in St. Louis).\15\ Given this
scenario, the REA found that approximately 24% of asthmatic children in
that city would be estimated to experience at least one SO2
exposure concentration greater than or equal to the 400 ppb benchmark
level per year while at moderate or greater exertion (e.g., while
exercising; REA, Figure 8-19). Similarly, the REA found that
approximately 73% of asthmatic children would be expected to experience
at least one SO2 exposure greater than or equal to a 200 ppb
benchmark level while at moderate or greater exertion (REA, Figure 8-
19).
---------------------------------------------------------------------------

    \15\ Exposure and risk results presented in this notice are with
respect to asthmatic children, results for all asthmatics are
presented in REA chapters, 8, 9, and 10.
---------------------------------------------------------------------------

    When considering the St. Louis risk results as they relate to the
adequacy of the current 24-hour standard, the policy assessment chapter
of the REA included the percent of asthmatic children at elevated
ventilation rates likely to experience at least one lung function
response given air quality that is adjusted upward to simulate just
meeting the current standards. Under this scenario, 19.1% to 19.2% of
exposed asthmatic children at elevated ventilation rates were estimated
to experience at least one moderate lung function response per year
(defined as an increase in sRaw >= 100% (REA, Table 9-
8)).\16\ \17\ Furthermore, 7.9% to 8.1% of exposed asthmatic
children at moderate or greater exertion were estimated to experience
at least one large lung function response per year (defined as an
increase in sRaw >= 200% (REA, Table 9-8)).
---------------------------------------------------------------------------

    \16\ The risk results presented represent the median estimate of
exposed asthmatics expected to experience moderate or greater lung
function decrements. Results are presented for both the probit and
2-parameter logistic functional forms. The full range of estimates
can be found in chapter 9 of the REA, and in all instances the
smaller estimate is a result of using the probit function to
estimate the exposure-response relationship.
    \17\ In this notice, risk results with respect to moderate or
greater lung function responses are presented in terms of sRaw
(i.e., >= 100% increases in sRaw). Risk results with respect to
decrements in lung function defined in terms of FEV1 can
be found in chapter 9 of the REA.
---------------------------------------------------------------------------

c. Summary of considerations from the REA regarding the 24-hour
standard
    As noted above, the policy chapter of the REA considered several
lines of scientific evidence when evaluating the adequacy of the
current 24-hour standard to protect the public health. These included
causality judgments made in the ISA, as well as the human exposure and
epidemiologic evidence supporting those judgments. In particular, the
REA concluded that numerous epidemiologic studies reporting positive
associations between ambient SO2 and respiratory morbidity
endpoints were conducted in locations that met, or were below the
current 24-hour standard (REA, section 10.3.4). The REA concluded that
to the extent that these considerations are emphasized, the adequacy of
the current 24-hour standard to protect the public health would clearly
be called into question (REA, section 10.3.4). The REA found this
suggested consideration of a revised 24-hour standard and/or that an
additional shorter-averaging time standard may be needed to provide
additional health protection for sensitive groups, including asthmatics
and individuals who spend time outdoors at elevated ventilation rates
(REA, section 10.3.4). This also suggested that an alternative
SO2 standard(s) should protect against health effects
ranging from lung function responses and increased respiratory symptoms
following 5-10 minute peak SO2 exposures, to increased
respiratory symptoms and respiratory-related ED visits and hospital
admissions associated with 1-hour daily maximum or 24-hour average

SO2 concentrations (REA, section 10.3.4).
    In examining the air quality, exposure, and risk-based information
with regard to the adequacy of the current 24-hour SO2
standard to protect the public health, the REA found that the results
described above (and in more detail in chapters 7-9 of the REA)
indicated that 5-minute exposures that could reasonably be judged
important from a public health perspective (see section II.B.1.c) were
associated with air quality adjusted upward to simulate just meeting
the current 24-hour standard. These exposures were judged in the REA to
be significant from a public health perspective due to their frequency:
approximately 24% of child asthmatics at moderate or greater exertion
in St. Louis are estimated to be exposed at least once per year to air
quality exceeding the 5-minute 400 ppb benchmark, a level associated
with lung function decrements in the presence of respiratory symptoms.
Additionally, approximately 73% of child asthmatics in St. Louis would
be expected to be exposed at least once per year to air quality
exceeding the 5-minute 200 ppb benchmark. Moreover, slightly over 19%
of exposed child asthmatics in St. Louis would be expected to
experience at least one adverse lung function response (defined in
terms of a >= 100% increase in sRaw) each year. Therefore, the REA
concluded that the air quality, exposure, and risk-based considerations
reinforced the epidemiologic evidence in supporting the conclusion that
consideration should be given to revising the current 24-hour standard
and/or setting a new shorter averaging time standard (e.g., 1-hour or
less) to provide increased public health protection, especially for
sensitive groups (e.g., asthmatics), from SO2-related
adverse health effects (REA, section 10.3.4).
2. Adequacy of the current annual standard
    In considering the adequacy of the current annual standard, the
policy assessment chapter of the REA considered the scientific evidence
assessed in the ISA and the air quality, exposure, and risk-based
information presented in the REA. A summary of this evidence and
information is presented below.
a. Evidence-based considerations
    As an initial consideration with regard to the adequacy of the
current annual standard, the REA noted that evidence relating long-term
(weeks to years) SO2 exposure to adverse health effects
(respiratory morbidity, carcinogenesis, adverse prenatal and neonatal
outcomes, and mortality) was judged by the ISA to be ``inadequate to
infer the presence or absence of a causal relationship'' (ISA, Table 5-
3). That is, the ISA found the health evidence to be of insufficient
quantity, quality, consistency, or statistical power to make a
determination as to whether SO2 is truly associated with
these health endpoints (ISA, Table 1-2). With respect specifically to
respiratory morbidity in children (in part, the basis for the current
annual standard; see section II.D.1), the ISA presented recent
epidemiologic evidence of an association with long-term exposure to
SO2 (ISA, section 3.4.2). However, the ISA found the
strength of these epidemiologic studies to be limited because of (1)
variability in results across studies with respect to specific
respiratory morbidity endpoints; (2) high correlations between long-
term average SO2 and co-pollutant concentrations,
particularly PM; and (3) a lack of evaluation of potential confounding
(ISA, section <IP_ADDRESS>).
    The REA also noted that many epidemiologic studies demonstrating
positive associations between 1-hour daily maximum or 24-hour average
SO2 concentrations and respiratory symptoms, ED visits, and
hospitalizations were conducted in areas where ambient SO2
concentrations were well below the level of the current annual NAAQS
(REA, section 10.4.2). The REA noted that this evidence suggested that
the current annual standard was not providing adequate protection
against health effects associated with shorter-term SO2
concentrations found in epidemiologic studies (REA, section 10.4.2).
b. Air quality, exposure, and risk-based considerations
    Results of the risk characterization based on the air quality
assessment provided additional insight into whether there is a need to
revise the current annual standard, focusing again on the extent to
which the annual standard may be providing protection against effects
associated with short-term exposures. In general, analyses presented in
the REA described the extent to which the current annual standard
provided protection against 5-minute peaks of SO2 in excess
of potential health effect benchmark levels (REA, chapter 7). The REA
found that many of the monitors where frequent 5-minute exceedances
were reported had annual average SO2 concentrations well
below the level of the current annual standard. Moreover, the REA found
that there was little to no correlation between the annual average
SO2 concentration and the number of 5-minute daily maximum
concentrations above potential health effect benchmark levels at these
monitors (REA section 7.3.1). Thus, the REA concluded that the annual
standard adds little in the way of protection against 5-minute peaks of
SO2 (REA, section 10.4.4).
c. Summary of considerations from the REA regarding the annual standard
    As noted above, the ISA concluded that the evidence relating long-
term (weeks to years) SO2 exposure to adverse health effects
(respiratory morbidity, carcinogenesis, adverse prenatal and neonatal
outcomes, and mortality) was ``inadequate to infer the presence or
absence of a causal relationship'' (ISA, Table 5-3). The ISA also
reported that many epidemiologic studies demonstrating positive
associations between short-term (e.g., 1-hour daily maximum, 24-hour
average) SO2 concentrations and respiratory symptoms, as
well as ED visits and hospitalizations, were conducted in areas where
annual ambient SO2 concentrations were well below the level
of the current annual NAAQS. In addition, analyses conducted in the REA
suggested that the current annual standard is not providing protection
against 5-10 minute peaks of SO2. Thus, the scientific
evidence and the risk and exposure information suggest that the current
annual SO2 standard: (1) Is likely not needed to protect
against health risks associated with long term exposure to
SO2; and 2) does not provide adequate protection from the
health effects associated with shorter-term (i.e. <= 24-hours)
SO2 exposures. Thus, the policy chapter of the REA
accordingly concluded that consideration should be given to either
revoking the annual standard or retaining it without revision, in
conjunction with setting an appropriate short-term standard(s) (REA,
section 10.4.4).
3. CASAC views regarding the adequacy of the current 24-hour and annual
standards
    With regard to the adequacy of the current standards, CASAC
conclusions were consistent with the views expressed in the policy
assessment chapter of the REA.\18\ CASAC agreed

that the primary concern in this review is to protect against health
effects that have been associated with short-term SO2
exposures, particularly those of 5-10 minutes (Samet 2009). CASAC also
agreed that the current 24-hour and annual standards are not sufficient
to protect public health against the types of exposures that could lead
to these health effects. Given these considerations, and as noted in
their letter to the EPA Administrator, CASAC agreed ``that the current
24-hour and annual standards are not adequate to protect public health,
especially in relation to short term exposures to SO2 (5-10
minutes) by exercising asthmatics'' (Samet, 2009, p. 15). CASAC also
noted: ``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
exceedences of the current 24-hour standard'' (Samet 2009, p. 15)
Discussion regarding CASAC's views on how the standard should be
revised is provided below within the context of discussions on the
elements (i.e., indicator, averaging time, form, level) of a new short-
term standard.
---------------------------------------------------------------------------

    \18\ CASAC views with respect to the current 24-hour and annual
standards, as well as with respect to potential alternative
standards are those following their review of the second draft
SO2 REA, which contained a staff policy assessment
chapter. EPA did not solicit, nor did it receive CASAC comments on
the final policy assessment chapter contained in the final REA.
---------------------------------------------------------------------------

4. The Administrator's conclusions regarding adequacy of the current
24-hour and annual standards
    Based on the epidemiologic evidence, the risk and exposure data set
out in this section, as well as CASAC's advice and recommendations, the
Administrator concludes (subject to consideration of public comment)
that the current standards are not adequate to protect public health
with an adequate margin of safety. The basis for this conclusion is as
follows. First, the Administrator accepts and agrees with the ISA's
conclusion 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. The
Administrator further agrees that the epidemiologic evidence
(buttressed by the clinical evidence) indicates that the effects seen
in the epidemiologic studies are attributable to exposure to
SO2. She also accepts and agrees with the conclusion of the
ISA that ``[i]n the epidemiologic studies, respiratory effects were
observed in areas where the maximum ambient 24-h avg SO2
concentration was below the current 24-h avg NAAQS level * * *'' (ISA,
section 5.2, p. 5-2.) and so would occur at ambient SO2
concentrations that are present in locations meeting the current 24-
hour NAAQS. The Administrator also notes that these effects occurred in
areas with annual air quality levels considerably lower than those
allowed by the current annual standard, indicating that the annual
standard also is not providing protection against such effects.
Existence of epidemiologic studies showing adverse effects occurring at
levels allowed by the current standards is an accepted justification
for finding that it is appropriate to revise the existing standards.
See, e.g. American Trucking Ass'n v. EPA, 283 F. 3d 355, 370 (DC Cir.
2002).
    With regard to the exposure and risk results, the Administrator
notes and agrees with the analyses in the REA supporting that 5-minute
exposures, reasonably judged important from a public health
perspective, were associated with air quality adjusted upward to
simulate just meeting the current standards. The Administrator
especially notes the results of the St. Louis exposure analysis which,
as summarized above, indicates that substantial percentages of
asthmatic children at moderate or greater exertion would be exposed, at
least once annually, to air quality exceeding the 400 and 200 ppb
benchmarks. Moreover, in addition to the health evidence and risk-based
information, the Administrator agrees with CASAC's conclusion that the
current SO2 standards do not adequately protect the public's
health.
    In considering approaches to revising the current standards, the
Administrator is proposing that it is appropriate to consider setting a
new short-term standard. The Administrator initially 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. As discussed in section II.F.5 below, depending on 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.

F. Conclusions on the elements of a proposed new short-term standard

    In considering alternative 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 lung function responses and/or respiratory
symptoms. Considerations with regard to potential alternative standards
and the specific options being proposed are discussed in the following
sections in terms of indicator, averaging time, form, and level
(sections II.F.1 to II.F.4).
1. Indicator
    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. 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,
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). The Administrator agrees
with this consensus, and therefore proposes to retain SO2 as
the indicator for oxides of sulfur in the current review.
2. Averaging time
    In considering whether it is appropriate to revise the averaging
times of the current standards, the first consideration is what health
effects the standard is addressing, and specifically whether those
effects are associated with short-term (i.e., 5-minutes to 24-hours),
and/or long-term (i.e. weeks to years) exposure to SO2.
There are distinct differences in the causality judgments in the ISA as
to short-term versus long-term health effects of SO2. The
ISA found evidence relating long-term (weeks to years) SO2
exposures to adverse health effects to be ``inadequate to infer the
presence or absence of a causal relationship'' (ISA, Table 5-3). In
contrast, the ISA judged evidence relating short-term (5-minutes to 24-
hours) SO2 exposure to respiratory morbidity to be
``sufficient to infer a causal relationship'' (the strongest possible
conclusion as to causality) and short-term exposure to SO2
and mortality to be ``suggestive of a causal relationship'' (ISA, Table
5-3). Taken together, the REA concluded that these judgments most
directly supported standard averaging time(s) that focus protection on
SO2 exposures from 5-minutes to 24-hours (REA, section,
10.5.2).
a. Evidence and air quality, exposure, and risk-based considerations
    In considering the level of support available for specific short-
term averaging times, the REA noted the strength of evidence from human
exposure and epidemiologic studies evaluated in the ISA. As previously
mentioned, controlled human exposure studies exposed exercising
asthmatics to 5-10 minute peak concentrations of SO2 and
consistently found decrements in lung function and/or respiratory
symptoms. Importantly, the ISA described the controlled human exposure
studies as being the ``definitive evidence'' for its conclusion that
there exists a causal association between short-term (5-minutes to 24-
hours) SO2 exposure and respiratory morbidity (ISA, section
5.2). In addition to the controlled human exposure evidence, there is a
relatively small body of epidemiologic studies describing positive
associations between 1-hour daily maximum SO2 levels and
respiratory symptoms as well as hospital admissions and ED visits for
all respiratory causes and asthma (ISA Tables 5.4 and 5.5). In addition
to the evidence from these 1-hour daily maximum epidemiologic studies,
there is a considerably larger body of epidemiologic studies reporting
positive associations between 24-hour average SO2 levels and
respiratory symptoms, as well as hospitalizations and ED visits for all
respiratory causes and asthma. Moreover, with respect to these
epidemiologic studies, there is support that adverse respiratory
effects are more likely to occur at the upper end of the distribution
of ambient SO2 concentrations (see section II.F.3 on Form).
In addition, 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 ED visits and hospital admissions''
and makes the associations observed in the epidemiologic studies
``biologica[lly] plausib[le]'' (ISA, section 5.2 at p. 5-5).
    The controlled human exposure evidence described above provided
support for an averaging time that protects against 5-10 minute peak
SO2 exposures (REA, section 10.5.2). In addition, the REA
found that results from epidemiologic studies provided support for both
1-hour and 24-hour averaging times (REA, section 10.5.2). In addition,
both the epidemiologic and controlled human exposure evidence suggests
that a new short-term standard should be focused on limiting peak
SO2 exposures. Thus, it can reasonably be concluded from the
ISA and REA that it would be appropriate to consider the degree of
protection potential alternative standards with averaging times under
consideration provide against peak 5-minute to 24-hour SO2
exposures. Moreover, as fully discussed in section II.F.3, this same
information makes it reasonable that the form of a new short-term
standard reflect a strategy to limit peak SO2 exposures.
Thus, with respect to the analyses presented below regarding averaging
time, a 99th percentile form will be considered. See American Petroleum
Institute, 665 F. 2d at 1186 (selection of highest average ozone level
in one hour to determine compliance with ozone NAAQS is reasonable
``because it is calculated to measure the maximum exposure, which has
been found to be a relevant factor in determining the likely
consequences of ozone exposure'').
    In considering the level of support available for specific short-
term averaging times, the policy assessment chapter of the REA also
took into account air quality considerations. More specifically, since
the shortest averaging time for the current primary SO2
standard is 24-hours, the REA evaluated the potential for a standard
based on 24-hour average SO2 concentrations to limit 5-
minute peak SO2 exposures (REA, section 10.5.2). The REA
evaluated ratios between 99th percentile 5-minute daily maximum and
99th percentile 24-hour average SO2 concentrations for 42
monitors reporting measured 5-minute data for any year between 2004-
2006 (REA, Table 10-1). Across this set of monitors, ratios of 99th
percentile 5-minute daily maximum to 99th percentile 24-hour average
SO2 concentrations spanned a range of 2.0 to 14.1 (REA,
Table 10-1). These results suggested a standard based on 24-hour
average SO2 concentrations would not likely be an effective
or efficient approach for addressing 5-minute peak SO2
concentrations. That is, the REA concluded using a 24-hour average
standard to address 5-minute peaks would likely result in over-
controlling in some areas, while under-controlling in others (REA,
section 10.5.2). This analysis also suggested that a 5-minute standard
would not likely be an effective or efficient means for controlling 24-
hour average SO2 concentrations (REA, section 10.5.2).
    The REA also reported ratios between 99th percentile 5-minute daily
maximum and 99th percentile 1-hour daily maximum SO2 levels
from this set of monitors. Compared to the ratios discussed above (5-
minute daily maximum to 24-hour average), there was far less
variability between 5-

minute daily maximum and 1-hour daily maximum ratios. More
specifically, 39 of the 42 monitors had 99th percentile 5-minute daily
maximum to 99th percentile 1-hour daily maximum ratios in the range of
1.2 to 2.5 (REA, Table 10-1). The remaining three monitors had ratios
of 3.6, 4.2 and 4.6 respectively. Overall, the REA found that this
relatively narrow range of ratios (compared to the range of ratios
presented above with respect to 5-minute daily maximum to 24-hour
average) suggested that a standard with a 1-hour averaging time would
be more efficient and effective at limiting 5-minute peaks of
SO2 than a standard with a 24-hour averaging time (REA,
section <IP_ADDRESS>). This analysis also suggested that a 5-minute
standard could be a relatively effective means of controlling 1-hour
daily maximum SO2 concentrations.\19\
---------------------------------------------------------------------------

    \19\ The analysis of peak to mean ratios was used as an initial
screen to evaluate which averaging times could be suited to control
5-minute peaks of SO2. The more sophisticated analysis
for ultimately determining that a one-hour averaging time set at an
appropriate level could effectively limit these 5-minute peaks was
the air quality, exposure, and risk analyses discussed in section
II.F.4.
---------------------------------------------------------------------------

    The REA further evaluated the potential of the 1-hour daily maximum
standards analyzed in the air quality, exposure, and risk analyses to
limit peak 24-hour average SO2 exposures (REA, section
10.5.2) since there is epidemiologic evidence to suggest that adverse
respiratory effects are more likely to occur at the upper end of the
distribution of ambient SO2 concentrations. The 99th
percentile 24-hour average SO2 concentrations in cities
where U.S. ED visit and hospitalization 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,
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. The REA found that a 99th percentile 1-hour daily
maximum standard set at a level of 50-100 ppb would generally limit
99th percentile 24-hour average SO2 concentrations in
locations where epidemiologic studies reported statistically
significant results in multi-pollutant models with PM (Table 1). That
is, for 2004, given air quality adjusted to just meet a 50 ppb 99th
percentile 1-hour daily maximum standard, the REA found that no county
included in this analysis was estimated to have 24-hour average
SO2 concentrations >= 36 ppb (Table 1). In addition, given
air quality adjusted to just meet a 100 ppb 99th percentile 1-hour
daily maximum standard, only 6 of the 39 counties (Linn, Union, Bronx,
Fairfax, Hudson, and Wayne) included in this 2004 analysis were
estimated to have 99th percentile 24-hour average SO2
concentrations >= 36 ppb (Table 1). The REA repeated this analysis for
the years 2005 and 2006 and found similar results (REA, Appendix Tables
D1 and D2).\20\
---------------------------------------------------------------------------

    \20\ In 2005, given a 99th percentile 1-hour daily maximum
standard at 50 ppb, Wayne County, West Virginia would have an
estimated 99th percentile 24-hour average SO2
concentration > 36 ppb (43 ppb; REA Appendix Table D-1).

 Table 1--99th Percentile 24-Hour Average SO2 Concentrations for 2004 Given Just Meeting the Alternative 1-Hour
        Daily Maximum 99th and 98th Percentile Potential Standards Analyzed in the Air Quality Assessment
                                         [Source: REA, Table 10-2].\21\
----------------------------------------------------------------------------------------------------------------
                                                            1-hour daily maximum standards
                                    ----------------------------------------------------------------------------
     State             County                           99th percentile                        98th percentile
                                    ----------------------------------------------------------------------------
                                         50        100        150        200        250        100        200
----------------------------------------------------------------------------------------------------------------
AZ............  Gila...............          6         12         18         25         31         16         32
DE............  New Castle.........         12         23         35         47         59         28         56
FL............  Hillsborough.......         10         20         30         40         50         28         55
IL............  Madison............         12         24         36         48         60         28         56
IL............  Wabash.............          7         13         20         27         33         19         38
IN............  Floyd..............          8         15         23         31         39         20         41
IN............  Gibson.............          9         18         27         36         45         20         41
IN............  Lake...............         12         24         36         48         60         31         62
IN............  Vigo...............         10         19         29         39         48         24         48
IA............  Linn...............         21         42         64         85        106         49         98
IA............  Muscatine..........         17         34         51         68         85         38         76
MI............  Wayne..............         17         33         50         66         83         37         74
MO............  Greene.............         12         24         36         48         60         31         62
MO............  Jefferson..........          9         18         27         36         45         25         51
NH............  Merrimack..........         17         33         50         66         83         39         79
NJ............  Hudson.............         19         38         57         76         95         48         96
NJ............  Union..............         18         36         54         72         90         44         89
NY............  Bronx..............         23         47         70         93        117         54        107
NY............  Chautauqua.........         13         27         40         54         67         32         65
NY............  Erie...............         14         27         41         54         68         30         61
OH............  Cuyahoga...........         17         34         51         67         84         40         80
OH............  Lake...............         10         19         29         39         48         23         47
OH............  Summit.............         12         24         36         48         61         27         55
OK............  Tulsa..............         16         32         47         63         79         36         72
PA............  Allegheny..........         12         23         35         47         59         30         60
PA............  Beaver.............         10         20         30         40         51         25         49
PA............  Northampton........         11         23         34         45         56         36         72
PA............  Warren.............         11         22         33         44         56         28         56
PA............  Washington.........         15         31         46         62         77         36         71
TN............  Blount.............         15         31         46         61         77         35         71

TN............  Shelby.............         17         34         51         68         85         41         81
TN............  Sullivan...........          8         16         24         32         39         23         46
TX............  Jefferson..........          9         17         26         35         44         21         41
VA............  Fairfax............         23         46         69         92        116         52        103
WV............  Brooke.............         12         24         37         49         61         31         62
WV............  Hancock............         15         29         44         58         73         35         69
WV............  Monongalia.........         10         20         30         40         50         25         51
WV............  Wayne..............         30         59         89        119        149         67        133
VI............  St Croix...........         14         27         41         54         68         51        101
----------------------------------------------------------------------------------------------------------------

    The air quality information presented above strongly support the
likelihood that an alternative 99th percentile (see discussion of form
below in II.F.3) 1-hour daily maximum standard set at an appropriate
level (see discussion of level in II.F.4) can substantially reduce the
upper end of the distribution of SO2 levels more likely to
be associated with adverse respiratory effects; that is: (1) 99th
percentile 1-hour daily maximum air quality concentrations in cities
observing positive effect estimates in epidemiologic studies of
hospital admissions and ED visits for all respiratory causes and
asthma; and (2) 99th percentile 24-hour average air quality
concentrations found in U.S. cities where ED 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). In
addition, 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 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).
Such analyses are also summarized in section II.F.4 of this notice.
Taken together, these results support that a 1-hour daily maximum
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 (REA, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \21\ 99th or 98th percentile 1-hour daily maximum concentrations
were determined for each monitor in a given county for the years
complete data were available from 2004-2006. These concentrations
were averaged, and the monitor with the highest average in a given
county was determined. Based on this highest average, all monitors
in a given county were adjusted to just meet the potential
alternative standards defined above, and for each of the years, the
99th percentile 24-hour average SO2 concentration was
identified. Results for the years 2005 and 2006 are presented in the
REA, Appendix D.
---------------------------------------------------------------------------

    The REA also considered the possibility of a 5-minute averaging
time based solely on the controlled human exposure evidence. However,
the REA did not favor such an approach (REA <IP_ADDRESS>). As in past NAAQS
reviews, the stability of the design of pollution control programs in
considering the elements of a NAAQS was considered, since more stable
programs are more effective, and hence result in enhanced public
safety. American Trucking Associations v. EPA, 283 F. 3d 355, 375 (DC
Cir. 2002) (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 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 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. However, as noted above, analyses in the REA
support that a 1-hour averaging time, given an appropriate form and
level (discussed below in sections II.F.3 and II.F.4, respectively) can
adequately limit 5-minute SO2 exposures and provide a more
stable regulatory target than setting a 5-minute standard.
b. CASAC views
    CASAC agreed with the conclusions of the policy assessment chapter
of the REA that a primary consideration of the SO2 NAAQS
should be the protection provided against health effects associated
with short-term exposures. In their letter to the EPA Administrator,
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). Furthermore, CASAC
agreed with the REA that a ``one-hour standard is the preferred
averaging time'' (Samet 2009, p.15).''
c. Administrator's 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,
conclusions from the REA, and CASAC recommendations discussed above.
Based on these considerations, the Administrator proposes to set a new
standard based on 1-hour daily maximum SO2

concentrations to provide increased protection against effects
associated with short-term (5-minutes to 24-hours) exposures. First,
the Administrator 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. Second, the Administrator agrees that
the standard must provide requisite protection from 5-10 minute
exposure events (the critical issue in the previous review), but
believes (subject to consideration of public comment) that this can be
done 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 addressing whether a
one-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'' for a one-
hour standard, and that ``a 1-hour standard is the preferred averaging
time'' (Samet 2009, p. 16).
    Third, the Administrator agrees that a one-hour averaging time
(again, with the appropriate form and level) would provide protection
against the range of health outcomes associated with averaging times of
one 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; 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 ED 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 ED visit and hospitalization studies (for all respiratory
causes and asthma) observed statistically significant associations in
multi-pollutant models with PM. Finally, the Administrator notes that
the proposal to establish 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).
3. Form
    When evaluating alternative forms in conjunction with specific
levels, the REA considered the adequacy of the public health protection
provided by the combination of level and form to be the foremost
consideration. In addition, the REA recognized that it is important
that the standard have a form that is reasonably stable. As just
explained in the context of a five-minute averaging time, a standard
set with a high degree of instability could have the effect of reducing
public health protection because shifting in and out of attainment
could disrupt an area's ongoing implementation plans and associated
control programs.
a. Evidence, air quality, and risk-based considerations
    As previously mentioned, the policy chapter of the REA (chapter 10)
recognized that the adequacy of the public health protection provided
by a 1-hour daily maximum potential alternative standard will be
dependent on the combination of form and level. It is therefore
important that the particular form selected for a 1-hour daily maximum
potential alternative standard reflect the nature of the health risks
posed by increasing SO2 concentrations. That is, the REA
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,
the REA 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 would give proportionally greater weight to years when 1-hour
daily maximum SO2 concentrations are well above the level of
the standard, than to years when 1-hour daily maximum SO2
concentrations are just above the level of the standard. In contrast,
an expected exceedance form would give the same weight to years when 1-
hour daily maximum SO2 concentrations are just above the
level of the standard, as to years when 1-hour daily maximum
SO2 concentrations are well above the level of the standard.
Therefore, the REA 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).
    The form of the standard should also reflect health information in
the ISA that suggests that adverse respiratory effects are more likely
to occur at the upper end of the distribution of ambient SO2
concentrations. Specifically, a few studies found that the increase in
SO2-related respiratory health effects was observed at the
upper end of the distribution of SO2 concentrations (ISA,
section 5.3, p. 5-9). For example, an epidemiologic study conducted in
Bronx, NY suggested an increased risk of asthma hospitalizations on the
days with the highest SO2 concentrations (Lin et al., 2004).
More specifically, the authors observed an increasing linear trend with
respect to asthma hospitalizations across the range of SO2
concentrations, with more marked effects observed at SO2
concentrations somewhere between the 90th and 95th percentiles (ISA,
section 4.1.2 and ISA, Figure 4-4).
    The epidemiologic evidence is consistent with the large body of
controlled human exposure studies of exercising asthmatics exposed to
short-term peak concentrations of SO2; these controlled
human exposure studies provide the ``definitive evidence'' that short
term peak SO2 exposure is associated with respiratory
morbidity (SOx ISA, Section 5.3, page 5-2). These studies
consistently found moderate or greater decrements in lung function
(i.e. >= 100% increase in sRaw and/or >= 15% decline in
FEV1)\22\ and/or respiratory symptoms in exercising
asthmatics following 5-10 minute peak exposures to SO2.
Moreover, as noted in the discussion on averaging time (section
II.F.2), when discussing the possible relationship between effects
observed in controlled human exposure studies and associations reported
in epidemiologic analyses, the ISA stated with respect to epidemiologic
studies of respiratory symptoms: ``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 ED visits and hospital
admissions'' and makes the associations observed in the epidemiologic
studies ``biologica[lly] plausib[le]'' (ISA, section 5.2 at p. 5-5).
Thus, both the epidemiologic and controlled human exposure evidence
suggests that the form of the standard should be focused on limiting
peak SO2 exposures.
---------------------------------------------------------------------------

    \22\ See section II.B.1.b above explaining sRaw and FEV1.
---------------------------------------------------------------------------

    In considering specific concentration-based forms, the REA
recognized the importance of: (1) Minimizing the number of days per
year that an area could exceed the level of the standard and still
attain the standard and thus, limiting the upper end of the
distribution of SO2 levels most likely associated with
adverse respiratory effects (2) limiting the prevalence of 5-minute
peaks of SO2; and (3) providing a stable regulatory target
to prevent areas from frequently shifting in and out of attainment. The
REA focused on 98th and 99th percentile forms averaged over 3 years.
The REA first noted that in most locations analyzed, the 99th
percentile form of a 1-hour daily maximum standard would correspond to
the 4th highest daily maximum concentration in a year, while a 98th
percentile form would correspond approximately to the 7th to 8th
highest daily maximum concentration in a year (REA, Table 10-5 and
Thompson, 2009). In addition, results from the REA air quality analysis
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). For example, the REA reported
that compared to the same standard with a 99th percentile form, a 98th
percentile 1-hour daily maximum standard set at a level of 100 ppb
allows for on average, an estimated 90 and 74% more days per year when
SO2 concentrations would likely exceed the 200 and 400 ppb
benchmark values respectively (REA, section 10.5.3 and REA, Figure 7-
28). Moreover, in the counties selected for analysis in the REA air
quality assessment, the estimated number of benchmark exceedances using
a 98th percentile 1-hour daily maximum standard set at a level of 200
ppb was similar to the corresponding 99th percentile standard set at a
level of 250 ppb (REA, section 10.5.3 and REA, Tables 7-11 through 7-
14). Similarly, the estimated number of benchmark exceedances
considering a 98th percentile standard set at a level of 100 ppb fell
within the range of benchmark exceedances estimated for 99th percentile
standards set at levels of 100 and 150 ppb (id.).
    As an additional matter, the REA compared trends in 98th and 99th
percentile design values, as well as design values based on the 4th
highest daily maximum 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 evaluating
overlapping three year intervals over the last ten years; see REA,
Figure 10-1 and Thompson, 2009). These results also demonstrated that
design values based on the 4th highest daily maximum are virtually
indistinguishable from design values based on the 99th percentile (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).
    Considering the evidence and air quality analyses presented above,
the REA concluded that a concentration-based form provides the best
protection against the health risks posed by increasing SO2
concentrations (REA, section 10.5.3). Moreover, the REA found that at a
given standard level, a 99th percentile or 4th highest daily maximum
form provides appreciably more public health protection against 5-
minute peaks than a 98th percentile or 7th--8th highest daily maximum
form (REA, section 10.5.3). In addition, 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). Thus, the REA ultimately concluded 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>).
b. CASAC views
    CASAC agreed with the importance of considering the public health
protection provided by the combination of form and level. Moreover,
CASAC was in general agreement with the forms being considered. In a
letter to the Administrator, CASAC stated: ``there is adequate
information to justify the use of a concentration-based form averaged
over 3 years'' (Samet 2009, p. 16). Moreover, when considering 98th vs.
99th percentile forms, CASAC encouraged EPA to consider analyses in the
REA (and perhaps additional analyses) with respect to the number of
days per year 98th vs. 99th percentile forms would allow SO2
concentrations to exceed the selected level. CASAC also encouraged EPA
to consider analyses such as those presented above with respect to the
number exceedences of 5-minute benchmarks given 98th vs. 99th
percentile forms at a given standard level (Samet 2009).
c. Administrator's conclusions on form
    When considering alternative forms, the Administrator notes and
agrees with the views expressed in the REA and the recommendations from
CASAC, as described above. In particular, she 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. The Administrator agrees further, for the reasons given
above, that a 99th percentile (or 4th highest) form could be
appreciably more protective than a 98th (or 7th or 8th highest) form,
and thus, should be utilized. Given these considerations, and in light
of the specific range proposed for level below, the Administrator
proposes to adopt either a 99th percentile or a 4th highest form,
averaged over 3 years.
4. Level
    In assessing the level of a one-hour standard with either a 99th
percentile or 4th highest average form (averaged over three years in
either case) to propose, the Administrator has 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 reiterates that 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 is mindful

of the extent to which the available evidence and analyses can inform a
decision on the level of a standard. Specifically, the range of
proposed standard levels discussed below is informed by epidemiologic
and controlled human exposure studies.
a. Evidence-based considerations
    Evidence-based considerations take into account the full body of
scientific evidence assessed in the ISA. When considering the extent to
which this scientific evidence can inform a decision on the level of a
1-hour standard, it is important to note that SO2
concentrations represent different measures of exposure when drawn from
experimental versus epidemiologic studies. Concentrations of
SO2 tested in experimental studies, such as controlled human
exposure studies, represent exposure concentrations in the breathing
zone of the individual test subjects. In cases where controlled human
exposure studies report effects, those effects are caused directly by
exposure to a specified concentration of SO2. In contrast,
concentrations of SO2 drawn from epidemiologic studies are
often based on ambient monitoring data. SO2 concentrations
recorded at these ambient monitors are used as surrogates for the
distribution of SO2 exposures across the study area and over
the time period of the study.
    Since the last review, there have been more than 50 peer reviewed
epidemiologic studies published worldwide dealing with SO2
exposure and effects (see ISA Tables 5-4 and 5-5). Overall, the ISA
concluded that these studies provide evidence of an association between
ambient SO2 concentrations and respiratory symptoms, as well
as ED visits and hospitalizations for all respiratory causes and asthma
(ISA, section 3.1.4). Moreover, the ISA indicates that many of these
epidemiologic studies have reported that children and older adults may
be at increased risk for SO2- associated adverse respiratory
effects (ISA, section 5.2). In assessing the extent to which these
studies and their associated air quality information can inform the
level of a new 99th percentile (see sections II.F.2 and II.F.3) 1-hour
daily maximum standard for the U.S., the REA considered U.S. and
Canadian air quality information to be most relevant. EPA sent a
request to the authors of U.S. and Canadian epidemiologic studies
(studies were identified from Tables 5-4 and 5-5 of the ISA) for 99th
(and 98th) percentile 1-hour daily maximum SO2
concentrations from the monitor recording the highest SO2
level in the location and time period corresponding to their studies
(see Thompson and Stewart (2009)). Air quality information was received
from authors of both U.S. and Canadian studies; however, as noted in
the REA (REA, section 5.5), SO2 concentrations reported for
Canadian studies are not directly comparable to those reported for
studies in the U.S. because SO2 levels reported for Canadian
analyses represent the average 1-hour daily maximum level across
multiple monitors in a given city (see REA Figure 5-5), rather than the
concentration from the single monitor that recorded the highest
SO2 concentration (see Thompson and Stewart, 2009). Thus,
the REA noted that SO2 concentrations associated with
Canadian studies would be relatively lower (potentially significantly
lower) than those levels presented for U.S. epidemiologic studies, and
therefore the REA focused on 99th percentile air quality information
from U.S. studies for informing potential 1-hour standard levels.
    Figures 1 to 4 present 99th (and 98th) percentile 1-hour daily
maximum SO2 concentrations from ten U.S. epidemiologic
studies (some of which were conducted in multiple cities) of ED visits
and hospital admissions \23\ (Figures 5-1 to 5-4 in the REA). The REA
noted that this information provides evidence for effects in cities
with particular 99th percentile 1-hour SO2 levels, and
hence, was of particular relevance for identifying standard levels that
could protect against the SO2 concentrations observed in
these studies. The air quality information presented in these figures
generally shows that positive associations between ambient
SO2 concentrations and ED visit and hospitalizations have
been reported in cities where 99th percentile 1-hour daily maximum
SO2 concentrations ranged from approximately 50-460 ppb.
More specifically, seven of these studies were in cities where 99th
percentile 1-hour daily maximum SO2 concentrations ranged
from approximately 75-150 ppb. Among these epidemiologic studies in the
range of 75-150 ppb, there is a cluster of three studies reporting
statistically significant results in multi-pollutant models with PM.
Specifically, in epidemiologic studies conducted in the Bronx, NY
(NYDOH 2006), and in NYC, NY (Ito et al., 2007), the SO2
effect estimate remained positive and statistically significant in
multi-pollutant models with PM2.5 in these locations when
99th percentile 1-hour daily maximum SO2 levels were 78 and
82 ppb respectively. (ISA, Table 5-5). Moreover, in an epidemiologic
study conducted in New Haven, CT (Schwartz et al., 1995), the
SO2 effect estimate remained positive and statistically
significant in a multi-pollutant model with PM10 in this
location when the 99th percentile 1-hour daily maximum SO2
concentration was 150 ppb. The REA noted that although
statistical significance in co-pollutant models is an important
consideration, it is not necessary for appropriate consideration of and
reliance on such epidemiologic evidence.\24\ 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.
---------------------------------------------------------------------------

    \23\ In some cases, U.S. authors provided the AQS monitor IDs
used in their studies and the statistics from the highest reporting
monitor were calculated by EPA. In cases where U.S. authors were
unable to provide the requested data (Schwartz 1995, Schwartz 1996,
and Jaffe 2003), EPA identified the maximum reporting monitor from
all monitors located in the study area and calculated the 98th and
99th percentile statistics (see Thompson and Stewart 2009). Results
presented from study locations for which effect estimates were
reported.
    \24\ For example, 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.
---------------------------------------------------------------------------

    In addition to the study locations where SO2
concentrations ranged from 75-150 ppb, the REA noted that two
epidemiologic studies included cities reporting positive associations
between ambient SO2 levels and ED visits when 99th
percentile 1-hour daily maximum SO2 concentrations were
approximately 50 ppb (Wilson et al., (2005) in Portland, ME and Jaffe
et al., (2003) in Columbus, OH). These studies reported generally
positive and sometimes statistically significant results using single
pollutant models (Figures 1 and 2), and did not evaluate potential
confounding through the use of multi-pollutant models. Nonetheless,
these studies provide limited evidence of an association between ED
visits and 99th percentile 1-hour daily maximum SO2
concentrations in locations where SO2 levels were
approximately 50 ppb. Finally, the REA noted that studies

conducted in Cleveland and Cincinnati, OH (Schwartz et al. 1996 and
Jaffe et al. 2003) reported positive associations between ambient
SO2 levels and ED visits and hospital admissions when 99th
percentile 1-hour daily maximum SO2 concentrations in these
cities ranged from 170-457 ppb (REA, section 5.5). The REA found the
SO2 level in Cincinnati (Jaffe et al., 2003; REA section
5.5) to be of particular concern. The 99th percentile 1-hour daily
maximum SO2 level in Cincinnati was > 400 ppb (Figure 2),
which in 5-10 minute controlled human exposure studies, was an
SO2 concentration range consistently shown to result in
clearly adverse health effects in exercising asthmatics (i.e.,
decrements in lung function accompanied by respiratory symptoms).
    Taken together, the epidemiologic evidence described above suggests
that standard levels at and below 75 ppb should be considered to limit
SO2 concentrations such that the upper end of the
distribution of daily maximum hourly concentrations would likely be
below that observed in most of these U.S. studies. Notably, a standard
at or below 75 ppb would be lower than the SO2 air quality
levels found in the cluster of three epidemiologic studies finding
statistically significant effects in multi-pollutant models with PM
(i.e., 99th percentile 1-hour daily maximum SO2
concentrations >= 78 ppb). Moreover, standard levels at or below 75 ppb
recognize the limited evidence from two epidemiologic studies reporting
mostly positive and sometimes statistically significant associations in
single pollutant models when 99th percentile 1-hour daily maximum
SO2 concentrations were approximately 50 ppb (Wilson et al.,
(2005) in Portland, ME and Jaffe et al., (2003) in Columbus, OH; see
Figures 1 and 2). Judgments about the weight to place on uncertainties
inherent in such studies should also inform selection of a specific
standard level.
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    The REA also considered findings from controlled human exposure
studies when evaluating potential alternative standard levels. The ISA
found that the most consistent evidence of decrements in lung function
and/or respiratory symptoms was from controlled human exposure studies
exposing exercising asthmatics to SO2 concentrations >= 400
ppb for 5-10 minute durations (ISA, section <IP_ADDRESS>). As previously
mentioned, at SO2 concentrations ranging from 400-600 ppb,
moderate or greater decrements in lung function occur in approximately
20-60% of exercising asthmatics, and compared to exposures at 200-300
ppb, a larger percentage of subjects experience severe decrements in
lung function. Moreover, at concentrations >= 400 ppb, decrements in
lung function are often statistically significant at the group mean
level, and are frequently accompanied by respiratory symptoms (ISA,
Table 5-1).
---------------------------------------------------------------------------

    \25\ There were no U.S. hospitalization studies with 1-hour
effect estimates identified in Table 5-5 of the ISA.
---------------------------------------------------------------------------

    Controlled human exposure studies have also demonstrated decrements
in lung function in exercising asthmatics following 5-10 minute
SO2 exposures starting as low as 200-300 ppb in free-
breathing chamber studies. At concentrations ranging from 200-300 ppb,
the lowest levels tested in free breathing chamber studies,
approximately 5-30% percent of exercising asthmatics are likely to
experience moderate or greater decrements in lung function in these
studies. Moreover, although these individuals experienced lung function
decrements, they were not frequently accompanied by respiratory
symptoms and at these SO2 concentrations, group mean changes
in lung function have not been shown to be statistically significant.
However, the ISA and REA noted that for evident ethical reasons, the
subjects participating in the controlled human exposure studies
described above do not include the most severe asthmatics. Thus, the
REA found it is reasonable to anticipate that individuals who are more
sensitive to SO2 would have a greater response at 200-300
ppb SO2, and/or would respond to SO2
concentrations even lower than 200 ppb (REA, section 10.5.4).
Similarly, the REA noted that there is no evidence to suggest that 200
ppb represents a threshold below which no adverse respiratory effects
occur (REA, section 10.5.4). In fact, limited evidence from two
mouthpiece exposure studies suggests that exposure to 100 ppb
SO2 can result in small decrements in lung function.\26\
---------------------------------------------------------------------------

    \26\ Although not directly comparable to free-breathing chamber
studies, findings from these mouthpiece studies may be particularly
relevant to those asthmatics who breathe oronasally even at rest
(EPA, 1994b).
---------------------------------------------------------------------------

    Considering the controlled human exposure evidence presented above,
the ISA concluded that as SO2 concentrations increase the
percentage of asthmatics affected increases as does the severity of the
response. Moreover, as previously noted, effects associated with
SO2 concentrations >= 400 ppb are clearly considered adverse
effects of air pollution under ATS guidelines, while effects at 200-300
ppb were considered adverse in the REA based on interpretation of ATS
guidelines, CASAC recommendations, and previous conclusions from
comparable situations in other NAAQS reviews (see section II.B.1.c).
Taken together, the REA concluded that the level of a new 99th
percentile 1-hour daily maximum

standard should provide substantial protection against SO2
concentrations >= 400 ppb, and appreciable protection against 5-minute
SO2 concentrations >= 200 ppb (REA, section 10.5.4).
b. Air quality, exposure and risk-based considerations
    In evaluating the extent to which 99th percentile 1-hour daily
maximum alternative standard levels limit 5-minute SO2
concentrations >= 400 and >= 200 ppb, the REA first considered key
results of the air quality analysis. As previously noted, the results
generated from the air quality analysis were from 40 counties and
considered a broad characterization of national air quality and human
exposures that might be associated with these 5-minute SO2
concentrations (see section II.C). However, there is uncertainty
associated with the assumption that SO2 air quality measured
at fixed site monitors can serve as an adequate surrogate for total
exposure to ambient SO2. Actual exposures might be
influenced by factors not considered in this analysis including small
scale spatial variability in ambient SO2 concentrations
(which might not be captured by the network of fixed-site ambient
monitors) and spatial/temporal variability in human activity patterns.
    Table 2 reports the maximum mean number of days per year 5-minute
daily maximum SO2 levels would be expected to exceed a given
5-minute potential health effect benchmark level in any of the 40
counties included in the air quality analysis, given air quality
simulated to just meet the current, and potential alternative 99th
percentile 1-hour daily maximum standards analyzed in the REA. In
addition, although not directly analyzed in the REA, these tables
include air quality results given a 99th percentile 1-hour daily
maximum standard at 75 ppb; this concentration was included in these
tables because as mentioned above, the epidemiologic evidence suggested
consideration of a standard level at or below 75 ppb.\27\ Table 2 shows
that at standard levels ranging from 50-100 ppb, there would be at most
two days per year when statistically estimated 5-minute SO2
concentrations in these counties exceed the 400 ppb benchmark, while at
standard levels of 150 ppb and above there is a marked increase in the
maximum number of days per year the 400 ppb benchmark is exceeded.
Similar trends are seen with respect to the 300 ppb benchmark level.
With respect to the 200 and 100 ppb benchmarks, the 50 ppb standard is
clearly the most effective at limiting these 5-minute SO2
concentrations. However, compared to standards at 150 ppb and above,
standards in the range of 75-100 ppb would allow considerably less
exceedence of the 200 and 100 ppb benchmarks. Additional and more
detailed results from the air quality analysis can be found in chapter
7 of the REA.
---------------------------------------------------------------------------

    \27\ Air quality, exposure, and risk numbers reported in Chapter
10 of the REA for a 75 ppb standard were bound by the estimates from
air quality adjusted to just meet 99th percentile 1-hour daily
maximum standards at 50 and 100 ppb.

Table 2--Maximum Mean Number of Days Per Year in Any of the Counties Included in the Air Quality Analysis When 5-Minute Daily Maximum SO2 Concentrations
    Exceed the 100, 200, 300, and 400 ppb Potential Health Effect Benchmark Values Given Air Quality Adjusted To Just Meet the Current Standards, or
                                               Alternative 99th Percentile 1-Hour Daily Maximum Standards
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Air quality scenarios
                                                              ------------------------------------------------------------------------------------------
                                                                   Just                    99th percentile 1-hour daily maximum standards
           Exposure benchmarks (5-minute exposures)              meeting   -----------------------------------------------------------------------------
                                                                 current
                                                                standards      50 ppb       75 ppb      100 ppb      150 ppb      200 ppb      250 ppb
--------------------------------------------------------------------------------------------------------------------------------------------------------
400 ppb......................................................          102            0        (0-2)            2            7           13           18
300 ppb......................................................          130            0        (0-5)            5           13           20           27
200 ppb......................................................          171            2       (2-13)           13           24           42           69
100 ppb......................................................          234           13      (13-43)           43           93          133          180
--------------------------------------------------------------------------------------------------------------------------------------------------------

    While the air quality analysis results presented in Table 2 used
estimated 5-minute SO2 concentrations as a surrogate for
exposure, the results from the exposure analysis considered the
likelihood that an asthmatic at elevated ventilation rate would come
into contact with a 5-minute SO2 concentration at or above a
given benchmark level one or more times per year. As previously noted,
this resource intensive analysis was performed for St. Louis and Greene
County, MO, but results from the St. Louis analysis were found to be
more informative with respect to informing standard levels given that
the St. Louis results: (1) Suggested that the current standards were
not adequate to protect public health; and (2) likely provide useful
insights into exposures and risk for other urban areas in the U.S. with
similar population and SO2 emissions density (i.e., areas
where SO2 exposures are more likely).
    Table 3 reports the estimated percent of asthmatic children at
moderate or greater exertion in St. Louis, that would be expected to
experience at least one SO2 exposure per year, at or above a
health effect benchmark level in scenarios in which air quality was
adjusted to meet the current, and alternative 99th percentile 1-hour
daily maximum standards. This analysis estimates that standard levels
ranging from 50-100 ppb would protect > 99% of asthmatic children, at
moderate or greater exertion, from experiencing at least one
SO2 exposure >= 400 ppb per year.\28\ Similarly, a standard
at 150 ppb is estimated to protect ~ 99% of asthmatic children at
moderate or greater exertion from experiencing at least one
SO2 exposure >= 400 ppb. Compared to standards ranging from
50-150 ppb, standards at 200 and 250 ppb are estimated to allow
appreciably more exposures >= 400 ppb (Table 3). With respect to the
300 ppb benchmark, standards at 50, 75, and 100 ppb provide similar
protection, while there is a marked increase in exposures of asthmatic
children at moderate or greater exertion at standard levels >= 150 ppb
(Table 3). Considering the 200 ppb benchmark level, it is estimated
that 1-hour standard levels ranging from 50-100 ppb limit 5-minute
SO2 exposures >= 200 ppb considerably more than 1-hour
standard levels >= 150 ppb. More

specifically, standards in the range of 50-100 ppb are estimated to
protect approximately 97 to > 99% of asthmatic children at moderate or
greater exertion from experiencing at least one 5-minute exposure >=
200 ppb per year, while standards ranging from 150-250 ppb are
estimated to protect approximately 60 to 88% of these children from
experiencing at least one 5-minute SO2 exposure >= 200 ppb
per year. Finally, similar to the air quality analysis, a standard at
50 ppb is clearly most effective at limiting 5-minute SO2
exposures >= 100 ppb. Additional and more detailed results from the
exposure assessment can be found in chapter 8 of the REA.
---------------------------------------------------------------------------

    \28\ Table 3 reports that given a 99th percentile 1-hour daily
maximum standard in the range of 50-100 ppb, < 1% of asthmatic
children at moderate or greater exertion would be estimated to
experience an SO2 exposure >= 400 ppb, hence it can be
stated that this range of levels would protect > 99% of asthmatic
children at moderate or greater exertion from experiencing at least
one SO2 exposure >= 400 ppb per year.

   Table 3--Estimated Percent of Asthmatic Children in St. Louis at Moderate or Greater Exertion Expected to Experience at Least One 5-Minute Exposure
     Above the 100, 200, 300, and 400 ppb Potential Health Effect Benchmark Levels Given Air Quality Adjusted To Just Meet the Current Standards, or
                                               Alternative 99th Percentile 1-Hour Daily Maximum Standards
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Air quality scenarios
                                            ------------------------------------------------------------------------------------------------------------
                                                 Just                             99th Percentile 1-hour daily maximum standards
  Exposure benchmarks (5-minute exposures)     meeting   -----------------------------------------------------------------------------------------------
                                               current
                                              standards      50 ppb                75 ppb               100 ppb      150 ppb      200 ppb      250 ppb
--------------------------------------------------------------------------------------------------------------------------------------------------------
400 ppb....................................          24%         < 1%  < 1%.........................         < 1%          ~1%         2.7%         6.3%
300 ppb....................................        43.8%         < 1%  < 1%.........................         < 1%         2.7%           8%          16%
200 ppb....................................        73.1%         < 1%  (~1 to 2.7%).................         2.7%        11.6%        24.5%          40%
100 ppb....................................        96.7%         2.7%  (2.7 to 24.5%)...............        24.5%        54.5%        73.6%        84.8%
--------------------------------------------------------------------------------------------------------------------------------------------------------

    In evaluating the extent to which alternative standard levels
provide protection against the health effects associated with 5-minute
SO2 exposures, the REA also considered key results from the
quantitative risk assessment (REA, chapter 9). Table 4 presents the
percent of exposed asthmatic children at moderate or greater exertion
in St. Louis expected to experience at least one moderate or greater
lung function response per year, in terms of sRaw, given the 99th
percentile 1-hour daily maximum standards analyzed in the REA. Results
presented in Table 4 show that standard levels in the range of 100 to
150 ppb would generally be expected to protect approximately 95 to 98%
of exposed asthmatic children at moderate or greater exertion from
experiencing at least one >= 100% increase in sRaw per year, while
standards around and below 75 ppb would be estimated to provide exposed
asthmatic children with protection approaching 99% or greater.
Additional and more detailed risk analyses can be found in chapter 9 of
the REA.

    Table 4--Estimated Percent of Asthmatic Children in St. Louis at Moderate or Greater Exertion Expected To
 Experience a >= 100% Increase in sRaw Given Air Quality Adjusted To Just Meet Either the Current Standards, or
                           Alternative 99th Percentile 1-Hour Daily Maximum Standards
----------------------------------------------------------------------------------------------------------------
                                              Air quality scenarios
-----------------------------------------------------------------------------------------------------------------
                                                   99th Percentile 1-hour daily maximum standards
  Just meeting current standards   -----------------------------------------------------------------------------
                                       50 ppb       75 ppb      100 ppb      150 ppb      200 ppb      250 ppb
----------------------------------------------------------------------------------------------------------------
19.1-19.2%........................     0.4-0.9%   (0.4-2.9%)     2.1-2.9%     4.6-5.4%     7.4-8.1%   10.4-10.9%
----------------------------------------------------------------------------------------------------------------

c. Observations based on evidence and risk-based considerations
    The policy assessment chapter of the REA considered the scientific
evidence and the air quality, exposure, and risk information as they
relate to considering alternative 1-hour SO2 standards that
could be judged to be requisite to protect public health with an
adequate margin of safety. This evidence and information supports the
following conclusions:
     Given the U.S. epidemiologic evidence and their associated
air quality levels (see Figures 1-4), 99th percentile 1-hour standard
levels at and below 75 ppb should be considered to limit SO2
concentrations such that the upper end of the distribution of daily
maximum hourly concentrations would likely be below that observed in
most of the U.S. studies. Judgments about the weight to place on
uncertainties inherent in such studies should also inform selection of
a specific standard level.
     Based on the air quality and exposure results, 1-hour
standard levels in the range of 50-100 ppb should be considered to
substantially limit 5-minute SO2 concentrations >= 400 ppb
and appreciably limit 5-minute SO2 concentrations >= 200
ppb.
     Based on the air quality and exposure results, compared to
a 1-hour standard in the range of 50-100 ppb, a 1-hour standard level
at 150 ppb would be expected similarly limit 5-minute SO2
concentrations >= 400 ppb, but would limit 5-minute SO2
concentrations >= 200 ppb considerably less.
     If relatively more weight is placed on certain types of
uncertainties in the epidemiologic and controlled human exposure
evidence, levels up to 150 ppb could be considered, recognizing the
questions as to the adequacy of protection that would be raised by
levels at the higher end of this range.
     Placing relatively more weight on the consideration that
participants in controlled human exposure studies do not include the
most severe asthmatics would add support to considering standard levels
down to 50 ppb.
d. CASAC views
    CASAC expressed their views on potential levels for a standard in a
letter to the EPA Administrator (Samet, 2009) within the context of
their review of the 2nd draft REA, which also contained the draft
policy assessment chapter. In drawing conclusions regarding the level
of a short-term standard, CASAC considered the scientific evidence

evaluated in the ISA, the air quality, exposure, and risk results
presented in the 2nd draft REA, and the evidence- and risk-based
considerations presented in the policy assessment chapter of the 2nd
draft REA. CASAC concurred with the conclusion from the policy
assessment chapter for a range of standard levels beginning at 50 ppb:
``[that chapter 10] clearly provides sufficient rationale for the range
of levels beginning at a lower limit of 50 ppb'' (Samet 2009, p. 16).
For instance, CASAC has previously indicated that EPA should consider
in its analyses the uncertainty that asthmatics participating in
controlled human exposure studies do not represent the most
SO2 sensitive asthmatics (Henderson 2008 p. 6). With respect
to the upper end of the range, CASAC stated, ``an upper limit of 150
ppb posited in Chapter 10 could be justified under some interpretations
of weight of evidence, uncertainties, and policy choices regarding
margin of safety,'' (Samet 2009, p. 16) 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.
Further, CASAC stated that ``the draft REA appropriately implies that
levels greater than 150 ppb are not adequately supported'' (id).
Moreover, CASAC stated that: ``the panel agrees that the posited range
of 50 to 150 ppb and the exposition of factors to consider when
comparing values within the range are appropriately conveyed (Samet
2009, p. 16).''
e. Administrator's conclusions on level for a 1-hour standard
    As discussed above, in sections II.F.2 and II.F.3, the
Administrator has proposed setting a 1-hour standard with a 99th
percentile form. For the reasons discussed below, the Administrator
proposes to set a level for a new 99th percentile 1-hour daily maximum
primary SO2 standard within the range from 50 to 100 ppb. In
reaching this proposed decision, the Administrator has 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 is
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.
    The Administrator notes that the most direct evidence of
respiratory effects from exposure to SO2 comes from the
controlled human exposure studies. These studies exposed groups of
exercising asthmatics to defined concentrations of SO2 for
5-10 minutes and found adverse respiratory effects. As discussed above,
SO2 exposure levels which resulted in respiratory effects in
controlled human exposure studies were used in the REA as 5-minute
benchmark exposures of potential concern. With respect to these 5-
minute benchmarks, the Administrator focused on exceedences of the 400
and 200 ppb benchmarks. She notes that under ATS guidelines (ATS 1985,
2000) exposure to 5-10 minute SO2 concentrations >= 400 ppb
results in health effects which are clearly adverse: moderate or
greater decrements in lung function (in terms of FEV1 or
sRaw \29\) that are frequently accompanied by respiratory symptoms.\30\
---------------------------------------------------------------------------

    \29\ Decreases of 10-20% in FEV1 (forced expiratory
volume) and/or 100-200% increases in sRaw (specific airway
resistance) are defined as moderate decrements in lung function.
    \30\ The ISA concluded that collective evidence from 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).
---------------------------------------------------------------------------

    The Administrator also focused on exceedences of the 200 ppb
benchmark, the lowest SO2 concentration tested in free-
breathing chamber studies. In these studies, moderate or greater
decrements in lung function occurred in approximately 5 to 30% of
exercising asthmatics, depending on the study. The Administrator
further notes that while concentrations as low as 200 ppb have not been
frequently accompanied by respiratory symptoms, she considers these
effects to be adverse in light of CASAC advice and ATS guidelines. The
REA concluded that these controlled human exposure studies could
reasonably be interpreted to indicate an SO2-induced shift
in lung function for this population of asthmatics (REA, section 4.3),
such that asthmatics would have diminished reserve lung function and
would 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 an adverse
effect under ATS guidelines as discussed in section II.B.1.c.
    As discussed below, 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 peaks of
SO2 above the 5-minute benchmark concentrations derived from
controlled human exposure studies. In considering these results as they
relate to limiting 5-minute SO2 concentrations >= 400 ppb
and >= 200 ppb, and being mindful that more severe effects occur
following 5-minute SO2 exposures >= 400 ppb, the
Administrator finds the most support for 99th percentile 1-hour daily
maximum standard levels up to 100 ppb to protect against 5-minute
SO2 exposures >= 200 ppb. She notes that 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 SO2
concentrations exceed the 200 ppb benchmark (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 SO2 concentrations exceed the 400 ppb
benchmark and <= 3 days per year on average when 5-minute
SO2 concentrations were estimated to exceed the 200 ppb
benchmark (see REA, Tables 7-14 and 7-12).
    In addition, the St. Louis exposure analysis estimates that a 99th
percentile 1-hour standard at a level of 100 ppb would likely protect >
99% of asthmatic children at moderate or greater exertion from
experiencing at least one 5-minute exposure >= 400 ppb per year, and
approximately 97% of asthmatic children at moderate or greater exertion
from experiencing at least one exposure >= 200 ppb per year. In
contrast, the Administrator notes that the St. Louis exposure analysis
estimates a 99th percentile 1-hour daily maximum standard at a level of
150 ppb would likely protect only about 88% of asthmatic children at
moderate or greater exertion from experiencing at least one 5-minute
exposure >= 200 ppb per year. Finally, the Administrator notes that 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 from experiencing at least one moderate or greater
lung function response (defined as a >= 100% increase in sRaw). Based
on these

considerations, she concludes that there is support for a 99th
percentile 1-hour daily maximum standard level at or below 100 ppb to
appreciably limit 5-minute exposures to SO2 above the 200
ppb benchmark level.
    Turning to the epidemiologic evidence, the Administrator notes that
epidemiologic studies have reported associations between more serious
health outcomes (i.e. respiratory-related ED visits and
hospitalizations) and ambient SO2 concentrations. Unlike the
controlled human exposure studies however, results from epidemiologic
studies can be complicated by the fact that SO2 is but one
component of a complex mixture of pollutants in the ambient air. This
uncertainty is addressed by the ISA which concluded that the limited
available evidence 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).
    The Administrator also notes that in general, associations reported
in epidemiologic analyses are not associated with a defined exposure
level of a pollutant (unlike the controlled human exposure studies),
but represent concentrations of a pollutant taken from ambient
monitoring data during the study period. These concentrations are used
as surrogates for the distribution of pollutant exposures across the
study area over the time period of the study. This introduces a degree
of uncertainty in the interpretation of epidemiologic results in that
it can be difficult to discern what part of the distribution of
pollutant levels are likely most linked to the associations reported in
epidemiologic analyses.
    With respect to SO2 specifically, the Administrator
notes that adverse respiratory effects in epidemiologic studies are
especially likely to occur at the upper end of the distribution of
ambient SO2 concentrations. Although some epidemiologic
studies reported a linear relationship across the entire range of
SO2 concentrations, a few other studies found that the
increase in SO2-related respiratory health effects was
observed at the upper end of the distribution of SO2
concentrations (ISA, section 5.3, p. 5-9). For example, an
epidemiologic study conducted in Bronx, NY suggested an increased risk
of asthma hospitalizations on the days with the highest SO2
concentrations (Lin et al., 2004). More specifically, these authors
observed increased risk of asthma hospitalizations at SO2
concentrations somewhere between the 90th and 95th percentiles (ISA,
section 4.1.2 and ISA, Figure 4-4).
    This epidemiologic evidence, though not independently sufficient to
draw conclusions regarding causation, is consistent with, and informed
by, the large body of controlled human exposure studies of exercising
asthmatics exposed to short-term peak concentrations of SO2;
these controlled human exposure studies provide the ``definitive
evidence'' that short-term peak SO2 exposure is associated
with respiratory morbidity (ISA, Section 5.3, page 5-8). These studies
consistently found moderate or greater decrements in lung function
(i.e. >= 100% increase in sRaw and/or >= 15% decline in
FEV1) and/or respiratory symptoms in exercising asthmatics
following 5-10 minute peak exposures to SO2. Discussing the
possible relationship between effects observed in these controlled
human exposure studies and the associations reported in the
epidemiologic studies, the ISA stated: ``it is possible that these
associations [in the epidemiologic studies] 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 ED visits and hospital admissions''
and makes the associations observed in the epidemiologic studies
``biologica[lly] plausib[le]'' (ISA, section 5.2 at p. 5-5). Thus,
considered together, the epidemiologic and controlled human exposure
evidence suggest that it is a reasonable approach to move the air
quality distribution lower in a manner that targets control of both
hourly and 5-10 minute peak SO2 exposures.
    For the reasons discussed above in section II.F.3, the
Administrator has proposed a 99th percentile of the 1-hour daily
maximum concentration as an appropriate form.\31\ Moreover, as just
discussed, there is support for the Agency's view that adverse
respiratory effects in epidemiologic studies are especially likely to
occur at the upper end of the distribution of ambient SO2
concentrations. Therefore, the Administrator finds it reasonable to
focus on limiting the 99th percentile SO2 levels reported in
locations where positive associations were found in key epidemiologic
studies. Adjusting the distribution of SO2 levels in this
manner will target control of those hourly and 5-10 minute peak
SO2 concentrations that are of most concern.
---------------------------------------------------------------------------

    \31\ As previously discussed in section II.F.3, a 99th
percentile form was proposed to: (1) Minimize the number of days per
year that an area could exceed the level of the standard and still
attain the standard; (2) limit the prevalence of 5-minute peaks of
SO2; and (3) provide a stable regulatory target to
prevent areas from frequently shifting in and out of attainment.
---------------------------------------------------------------------------

    In considering the epidemiologic evidence with regard to level, the
Administrator notes that there have been more than 50 peer reviewed
epidemiologic studies evaluating SO2 published worldwide
(ISA, Tables 5-4 and 5-5). 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, U.S. and Canadian air quality information is most
relevant. As described in section II.F.4.a, SO2
concentrations reported for Canadian studies are not directly
comparable to those reported for U.S. studies. That is, concentrations
reported for Canadian analyses represent the average 99th percentile 1-
hour daily maximum level across multiple monitors in a given city (REA
Figure 5-5), rather than the concentration from the single monitor that
recorded the highest SO2 level (see Thompson and Stewart,
2009). Thus, the Administrator focused on 99th percentile air quality
information from U.S. studies for informing potential 1-hour standard
levels.
    The Administrator notes that Figures 1 to 4 include 99th percentile
1-hour daily maximum SO2 concentrations from ten U.S.
epidemiologic studies of ED visits and hospital admissions (Figures 5-1
to 5-4 in the REA). The Administrator agrees with the REA finding that
this information provides evidence of associations between ambient
SO2 and ED visits and hospital admissions in cities with
particular 99th percentile 1-hour SO2 levels. This
information is relevant for identifying standard levels that could
significantly limit SO2 concentrations so that the upper end
of the distribution of daily maximum hourly concentrations would likely
be below that observed in most of these studies. These figures report
mostly positive, and sometimes statistically significant, associations
between ambient SO2 concentrations and ED visit and hospital
admissions in locations where 99th percentile 1-hour daily maximum
SO2 levels ranged from 50-460 ppb. Moreover, within this
broader range of SO2 concentrations, seven of these studies
were in locations where the 99th percentile of the 1-hour daily maximum
SO2 concentrations were in the range of 75-150 ppb. The
Administrator particularly notes the

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)). The Administrator also notes
the limited evidence from two epidemiologic studies employing single
pollutant models that found mostly positive, and sometimes
statistically significant, associations between ambient SO2
and ED visits in locations where 1-hour SO2 concentrations
were approximately 50 ppb (Figures 1 and 2). Based on the
interpretation of the epidemiologic evidence discussed above, the
Administrator concludes that this evidence provides support for
consideration of a 99th percentile 1-hour daily maximum standard level
at or below 75 ppb to limit SO2 concentrations such that the
upper end of the distribution of daily maximum hourly concentrations
would likely be below that observed in most of the U.S. studies. The
Administrator also recognizes that judgments about the weight to place
on uncertainties inherent in such studies should inform selection of a
specific standard level.
    Based on the epidemiologic and controlled human exposure
information presented above, the Administrator considered what range of
standard levels would be requisite to protect public health, including
the health of at-risk groups, with an adequate margin of safety that is
sufficient but not more than necessary to achieve that result. The
assessment of a standard level calls for consideration of both the
degree of risk to public health at alternative levels of the standard
as well as the certainty that such risk will occur at any specific
level. Based on the information available in the ISA, there is no
evidence-based bright line that indicates a single appropriate level.
Moreover, given that a 1-hour averaging time is being used to control
5-minute peaks of SO2, the Administrator also recognizes
that the results of the air quality, exposure, and risk analyses will
have to be considered given that these analyses indicate the extent to
which a particular 99th percentile 1-hour daily maximum standard will
likely limit 5-minute SO2 peaks of a given concentration.
Thus, the combination of scientific evidence and air quality, exposure,
and risk-based information needs to be considered as a whole in making
this public health policy judgment.
    In selecting a level that would serve as an appropriate upper end
for a range of levels to propose, the Administrator has considered a
cautious approach to interpreting the available evidence and exposure/
risk-based information--that is, an approach that places relatively
more weight on those types of uncertainties and limitations in the
information that would lead to placing less reliance on the results of
the epidemiologic studies. This approach would tend to avoid
potentially overestimating public health risks and the degree of
protection likely to be associated with just meeting a particular
standard level. This approach would place more weight in particular on
uncertainties in epidemiologic evidence such as concerns related to
exposure measurement error, the possible role of co-pollutants and
effects modifiers, and interindividual differences in susceptibility to
SO2-related effects.
    In applying this approach, the Administrator has selected an upper
end of a range of levels to propose at 100 ppb. The selection of this
level focuses 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. That is, as mentioned above, the St. Louis
exposure analysis indicates that a 1-hour standard at 100 ppb would
still be estimated to protect about 97% of asthmatic children at
moderate or greater exertion from experiencing at least one 5-minute
SO2 exposure >= 200 ppb. In contrast, the St. Louis exposure
analysis estimates 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.
    In selecting a level that would serve as an appropriate lower end
for a range of levels to propose, the Administrator has considered a
precautionary approach to interpreting the available evidence and
exposure/risk-based information--that is, an approach that places
relatively more weight on the results of the epidemiological studies,
as well as more weight on those types of uncertainties that may be
associated with potentially underestimating health effects in the most
sensitive populations. This approach would tend to avoid potentially
underestimating public health risks and the degree of protection likely
to be associated with just meeting a particular standard level. This
approach would place more weight on the consideration that the
participants in controlled human exposure studies did not include
individuals with severe asthma.
    In applying this approach, she has 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 focuses in part on the
epidemiologic evidence. With respect to the epidemiologic studies,
seven of ten U.S. ED 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 ED visits and
hospitalizations in multi-pollutant models with PM (NYDOH (2006), Ito
et al., (2007), and Schwartz et al., (1995)). Further, the
Administrator notes 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 studies were conducted (i.e. well below 99th percentile 1-
hour daily maximum SO2 levels of 78-150 ppb). Finally, the
Administrator notes that two epidemiologic studies reported generally
positive associations between ambient SO2 and ED visits in
cities when 99th percentile 1-hour daily maximum SO2
concentrations were approximately 50 ppb, but does not consider that
evidence strong enough to set a lower standard level.
    In considering the results of the air quality and exposure
analyses, the Administrator also notes 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. 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.
    The Administrator thus proposes 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 is
relying on reported findings from both epidemiologic and controlled
human exposure studies, as well as the results of air quality and
exposure analyses. The Administrator

solicits comment on this proposed range of standard levels as well as
on the approach she has used to identify the range. Specifically, the
Administrator solicits comment on the following:
     The weight she has placed on the epidemiologic evidence,
the controlled human exposure evidence, and the air quality, exposure,
and risk information, the benchmark used to select the proposed range,
and the uncertainties associated with each of these.
     The most appropriate level within this proposed range
given the available scientific evidence, and air quality, exposure, and
risk information, and the uncertainties associated with each.
    With regard to the proposed range of standard levels, the
Administrator notes that the lower end of the proposed range is
consistent with CASAC advice that there is clearly sufficient evidence
for consideration of standard levels starting at 50 ppb (Samet 2009).
With respect to the upper end of the proposed range, the Administrator
notes 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'' (Samet
2009, p. 16), 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 solicits comment on setting a standard level above 100
ppb and up to 150 ppb. In so doing, the Administrator again recognizes
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. Any comments
should specifically address the cluster of epidemiologic studies that
remained statistically significant in co-pollutant models with PM, two
of which had 99th percentile levels appreciably lower than 150 ppb.
Commenters should also address the conclusion in the ISA that the
respiratory effects seen in the epidemiologic studies are generally
robust and independent of co-pollutants. In addition, the Administrator
notes 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. She notes that the St. Louis
exposure analysis estimates 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 Table 3). There are also questions as to
whether a standard set at this level would provide an adequate margin
of safety. Thus, with respect to considering whether it would be
appropriate to set a standard level as high as 150 ppb, the
Administrator invites comment on the extent to which it is appropriate
to emphasize uncertainties with respect to the epidemiologic evidence.
She also invites comment on the implications such considerations would
have on limiting 5-minute SO2 exposures >= 200 ppb.
5. Implications for retaining or revoking current standards
    The REA recognized that the particular level selected for a new 1-
hour daily maximum standard would have implications for reaching
decisions on 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. As previously
mentioned (see section II.E.3), 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 notes that the 1-hour standards
being proposed (i.e., 99th percentile 1-hour daily maximum
SO2 standards at 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 is set in the
proposed range of 50-100 ppb, than the Administrator proposes to revoke
the current 24-hour and annual standards. However, if a standard is set
at a level >100 ppb and up to 150 ppb, then the Administrator proposes
to 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). However, the Administrator would
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.

G. Summary of proposed decisions on the primary standard

    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 proposes that
the current 24-hour and annual standards are not requisite to protect
public health with an adequate margin of safety. The Administrator
proposes to establish a new 1-hour standard that will afford 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
increased 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 proposes to set 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 or
the 4th highest daily maximum concentration. The level for the new
standard is proposed to be within the range of 50-100 ppb. The
Administrator also solicits comment on levels as high as 150 ppb. In
addition to

setting a new 1-hour standard in the proposed rage of 50-100 ppb, the
Administrator proposes to revoke the current 24-hour and annual
standards recognizing that a 1-hour standard set in the proposed range
of 50-100 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. Moreover, the Administrator notes
that there is little health evidence to support an annual standard for
the purpose of protecting against health effects associated with long-
term SO2 exposures.

III. Proposed Amendments to Ambient Monitoring and Reporting
Requirements

    EPA is proposing changes to the ambient air monitoring, reporting,
and network design requirements for the SO2 NAAQS. This
section discusses the changes we are proposing that are intended to
support the proposed 1-hour NAAQS, and the possible retention of the
existing 24-hour NAAQS depending on the selected level of the 1-hour
NAAQS, as described in Section II above. Ambient SO2
monitoring data are used to determine whether an area is in violation
of the SO2 NAAQS. Ambient SO2 monitoring data are
collected by state, local, and tribal monitoring agencies (``monitoring
agencies'') in accordance with the monitoring requirements contained in
40 CFR parts 50, 53, and 58.

A. Monitoring methods

    To be used in a determination of compliance with the SO2
NAAQS, SO2 data must be collected using either a Federal
Reference Method (FRM) or a Federal Equivalent Method (FEM) as defined
in 40 CFR Parts 50 and 53. The current monitoring methods in use by
most State and local monitoring agencies are FEM analyzers based on the
ultraviolet fluorescence (UVF) measurement principle. These continuous
analyzers were implemented into the SO2 monitoring networks
in the early 1980s, and the current manual FRM for SO2 is no
longer used for field monitoring. The current list of all approved FRMs
and FEMs capable of providing ambient SO2 data for use in
attainment designations may be found on the EPA Web site http://
www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-
methods-list.pdf.
    For reasons explained subsequently, EPA proposes to establish a new
FRM for measuring SO2 in the ambient air. This proposed new
FRM for SO2 would be an automated method based on UVF (the
same type of analyzers now in widespread use), and it would be
specified in the form of a reference measurement principle and a
calibration procedure. It would be in a new Appendix A-1 to 40 CFR Part
50. Analyzers approved as FRMs for SO2 after the effective
date of the final rule would be subject to performance specifications
and other requirements set forth in 40 CFR Part 53, under associated
amendments proposed for Part 53. The existing FRM for SO2 (a
wet-chemical, manual method) would be retained for some period of time,
thereby permitting continued use of currently designated FEMs to avoid
any disruption to existing SO2 monitoring networks.
1. Background