asthma prevalence in the exposure modeling domain is estimated based
on national prevalence information and study area demographic
information related to age, sex and poverty status.
---------------------------------------------------------------------------

    The REA for this review, consistent with the analyses in the last
review, uses the APEX model estimates of 5-minute exposure
concentrations for simulated individuals with asthma while breathing at
elevated rates to

characterize health risk in two ways (REA, section 4.5). The first is
the percentage of the simulated at-risk populations expected to
experience days with 5-minute exposures, while breathing at elevated
rates, that are at or above a range of benchmark levels. The second is
the percentage of these populations expected to experience days with an
occurrence of a doubling or tripling of sRaw.
    The benchmark concentrations used in the comparison-to-benchmarks
analysis (400, 300, 200 and 100 ppb) were identified based on
consideration of the evidence discussed in section II.A.2 above. In
particular, benchmark concentrations of 400 ppb, 300 ppb, and 200 ppb
were based on concentrations included in the well-documented controlled
human exposure studies summarized in section II.A.2 above, and the 100
ppb benchmark was selected in consideration of uncertainties with
regard to lower concentrations and population groups with more limited
data (REA, section 4.5.1). At the upper end of this range, 400 ppb
represents the lowest concentration in free-breathing controlled human
exposure studies of exercising people with asthma where moderate or
greater lung function decrements occurred that were often statistically
significant at the group mean level and were frequently accompanied by
respiratory symptoms, with some increases in these symptoms also being
statistically significant at the group level (ISA, Section <IP_ADDRESS> and
Table 5-2). At 300 ppb, statistically significant increases in lung
function decrements (specifically reductions in FEV1) have
been documented in analyses of the subset of controlled human exposure
study subjects with asthma that are responsive to SO2 at
concentrations below 600 or 1000 ppb (ISA, pp. 5-85 and 5-153 and Table
5-21; Johns et al., 2010). The 200 ppb benchmark concentration
represents the lowest level for which studies are available that have
assessed the SO2 effect versus the effect of exercise in
clean air and for which individual study subject data are available to
summarize percent changes in sRaw and FEV1; moderate or
greater lung function decrements were documented in some of these study
subjects (ISA, Table 5-2 and Figure 5-1; PA, Table 3-1; REA, section
4.6.1). With regard to exposure concentrations below 200 ppb, limited
data are available for exposures at 100 ppb that, while not directly
comparable to the data at higher concentrations because of differences
in methodology and metrics reported,\63\ do not indicate that study
subjects experienced responses of a magnitude as high as a doubling in
sRaw. However, in consideration of some study subjects with asthma
experiencing moderate or greater decrements in lung function at the 200
ppb exposure concentration (approximately 8 to 9% of the study group)
and of the paucity or lack of any specific study data for some groups
of individuals with asthma, such as primary-school-age children and
those with more severe asthma (described in sections II.B.3 and II.C.1
of the proposal), a benchmark concentration of 100 ppb (one half the
200 ppb exposure concentration) was also included in the analyses.
---------------------------------------------------------------------------

    \63\ As explained in section II.B.3 of the proposal, these
studies involved exposures via mouthpiece, and only a few of these
studies included an exposure to clean air while exercising that
would have allowed for determining the effect of SO2
versus that of exercise in causing bronchoconstriction and
associated lung function decrements (ISA, section <IP_ADDRESS>; PA,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    The E-R function for estimating risk of lung function decrements
was developed from the individual subject results for sRaw from the
controlled exposure studies of exercising, freely breathing people with
asthma exposed to SO2 concentrations from 1000 ppb down to
as low as 200 ppb (REA, Table 4-11). In addition to the assessment of
these studies and their results in past NAAQS reviews, there has been
extensive evaluation of the individual subject results, including a
data quality review in the 2010 review of the primary SO2
standard (Johns and Simmons, 2009) and detailed analysis in two
subsequent publications (Johns et al., 2010; Johns and Linn, 2011). The
E-R function was derived from the sRaw responses reported in the
controlled exposure studies as summarized in the ISA in terms of
percent of study subjects experiencing responses of a magnitude equal
to a doubling or tripling or more (e.g., ISA, Table 5-2; Long and
Brown, 2018; REA, section 4.6.2). Across the exposure range from 200 to
1000 ppb, the percentage of exercising study subjects with asthma
having at least a doubling of sRaw increases from about 8-9% (at
exposures of 200 ppb) up to approximately 50-60% (at exposures of 1000
ppb) (REA, Table 4-11).
b. Key Limitations and Uncertainties
    While the general approach and methodology for the exposure-based
assessment in this review is similar to that used in the last review,
there are a number of ways in which the current analyses are different;
some differences reflect improvements and, in some cases, reflect
improvements that may address limitations of the 2009 assessment. For
example, the number and type of study areas assessed has been expanded
since the last review, and input data and modeling approaches have
improved in a number of ways, including the availability of continuous
5-minute air monitoring data at monitors within the three study areas.
In addition, the REA for the current review extends the time period of
simulation to a 3-year simulation period, consistent with the form
established for the now-current standard. Further, the years simulated
reflect more recent emissions and circumstances subsequent to the 2010
decision.
    In characterizing uncertainty associated with the risk and exposure
estimates in this review, the REA used a qualitative uncertainty
characterization approach adapted from the World Health Organization
(WHO) approach for characterizing uncertainty in exposure assessment
(WHO, 2008) accompanied by quantitative sensitivity analyses of key
aspects of the assessment approach (REA, chapter 6).\64\ The approach
used in the REA places a greater focus on evaluating the direction and
the magnitude of the uncertainty (i.e., qualitatively rating how the
source of uncertainty, in the presence of alternative information, may
affect the estimates of exposure and risk). The evaluation considers
the limitations and uncertainties underlying the analysis inputs and
approaches and the relative impact that these uncertainties may have on
the resultant exposure/risk estimates. Consistent with the WHO (2008)
approach, the overall impact of the uncertainty is then characterized
by the extent or magnitude of the impact of the uncertainty (e.g.,
high, moderate, low) as implied by the relationship between the source
of the uncertainty and the exposure/risk output. The REA also evaluated
the direction of influence, indicating how the source of uncertainty
was judged to affect the exposure and risk estimates (e.g., likely to
produce over- or under-estimates).
---------------------------------------------------------------------------

    \64\ The approach used has been applied in REAs for past NAAQS
review for nitrogen oxides, carbon monoxide, and ozone (U.S. EPA,
2008b; 2010; 2014d), as well as SOX (U.S. EPA, 2009).
---------------------------------------------------------------------------

    Several areas of uncertainty are identified as particularly
important, with some similarities to those recognized in the last
review. Generally, these areas of uncertainty include estimation of the
spatial distribution of SO2 concentrations across each study

area under air quality conditions just meeting the current standard,
including the fine-scale temporal pattern of 5-minute concentrations.
They also include uncertainty with regard to population groups and
exposure concentrations for which the health effects evidence base is
limited or lacking (PA, section <IP_ADDRESS>).
    With regard to the spatial distribution of SO2
concentrations, there is some uncertainty associated with the ambient
air concentration estimates in the air quality scenarios assessed. A
more detailed characterization of contributors to this uncertainty is
presented in section 6.2 of the REA, with a brief overview provided
here. Some aspects of the assessment approach contributing to this
uncertainty include estimation of the 1-hour concentrations and the
approach employed to adjust the air quality surface to concentrations
just meeting the current standard (REA, section <IP_ADDRESS>; PA, section
<IP_ADDRESS>), as well as the estimation of 1-hour ambient air
concentrations resulting from emissions sources not explicitly modeled.
All of these assessment approaches influence the resultant temporal and
spatial pattern of concentrations and associated exposure circumstances
represented in the study areas (REA, sections 6.2.1 and 6.2.2). There
is also uncertainty in the estimates of 5-minute concentrations in
ambient air across the modeling receptors in each study area. The
ambient air monitoring dataset available to inform the 5-minute
estimates, much expanded in this review over the dataset available in
the last review, is used to draw on relationships occurring at one
location and over one range of concentrations to estimate the fine-
scale temporal pattern in concentrations at the other locations. While
this is an important area of uncertainty in the REA results, because
the ambient air 5-minute concentrations are integral to the 5-minute
estimates of exposure, the approach used to represent fine-scale
temporal variability in the three study areas is strongly based in the
available information and has been evaluated in the REA (REA, Table 6-
3; sections 3.5.2 and 3.5.3).
    Another important area of uncertainty in the REA is particular to
the lung function risk estimates derived for exposure concentrations
below those represented in the evidence base (REA, Table 6-3). The E-R
function on which the risk estimates are based generates non-zero
predictions of the percentage of the at-risk population expected to
experience a day with the occurrence of at least a doubling of sRaw for
all 5-minute exposure concentrations each simulated individual
encounters while breathing at an elevated rate. The uncertainty in the
response estimates increases substantially with decreasing exposure
concentrations below those well represented in the data from the
controlled human exposure studies (i.e., below 200 ppb).
    Additionally, the assessment focuses on the daily maximum 5-minute
exposure during a period of elevated breathing rate, summarizing
results in terms of the days on which the magnitude of such exposure
exceeds a benchmark or contributes to a doubling or tripling of sRaw.
Although there is some uncertainty associated with the potential for
additional, uncounted events in the same day, the health effects
evidence indicates a lack of a cumulative effect of multiple exposures
over several hours or a day (ISA, section <IP_ADDRESS>) and a reduced
response to repeated exercising exposure events over an hour (Kehrl et
al., 1987). Further, information is somewhat limited with regard to the
length of time after recovery from one exposure by which a repeat
exposure would elicit an effect similar to that of the initial exposure
event (REA, Table 6-3). In addition, there is uncertainty regarding the
potential influence of co-occurring pollutants on the relationship
between short-term SO2 exposures and respiratory effects.
For example, there is some limited evidence regarding the potential for
an increased response to SO2 exposures occurring in the
presence of other common pollutants such as PM (potentially including
particulate sulfur compounds), nitrogen dioxide and ozone, although the
studies are limited (e.g., with regard to their relevance to ambient
exposures) and/or provide inconsistent results (ISA, pp. 5-23 to 5-26,
pp. 5-143 to 5-144; 2008 ISA, section <IP_ADDRESS>).\65\
---------------------------------------------------------------------------

    \65\ For example, ``studies of mixtures of particles and sulfur
oxides indicate some enhanced effects on lung function parameters,
airway responsiveness, and host defense''; however, ``some of these
studies lack appropriate controls and others involve [sulfur-
containing species] that may not be representative of ambient
exposures'' (ISA, p. 5-144). These toxicological studies in
laboratory animals, which were newly available in the last review,
were discussed in greater detail in the 2008 ISA. That ISA stated
that ``[r]espiratory responses observed in these experiments were in
some cases attributed to the formation of particular sulfur-
containing species'' yet, ``the relevance of these animal
toxicological studies has been called into question because
concentrations of both PM (1 mg/m\3\ and higher) and SO2
(1 ppm and higher) utilized in these studies are much higher than
ambient levels'' (2008 ISA, p. 3-30).
---------------------------------------------------------------------------

    Another area of uncertainty, which remains from the last review and
is important to our consideration of the REA results, concerns the
extent to which the quantitative results represent the populations at
greatest risk of effects associated with exposures to SO2 in
ambient air. As recognized in section II.A.2, the evidence base of
controlled human exposure studies does not include studies of children
younger than 12 years old and is limited with regard to studies of
people with more severe asthma.\66\ The limited evidence that informs
our understanding of potential risk to these groups indicates the
potential for them to experience greater impacts than other population
groups with asthma under similar exposure circumstances or, in the case
of people with severe asthma, to have a more limited reserve for
addressing this risk (ISA, section <IP_ADDRESS>). Further, we note the lack
of information on the factors contributing to increased susceptibility
to SO2-induced bronchoconstriction among some people with
asthma compared to others (ISA, pp. 5-19 to 5-21). These data
limitations contribute uncertainty to the exposure/risk estimates with
regard to the extent to which they represent the populations at
greatest risk of SO2-related respiratory effects.
---------------------------------------------------------------------------

    \66\ We additionally recognize that limitations in the activity
pattern information for children younger than 5 years old precluded
their inclusion in the populations of children simulated in the REA
(REA, section 4.1.2).
---------------------------------------------------------------------------

    In summary, among the multiple uncertainties and limitations in
data and tools that affect the quantitative estimates of exposure and
risk and their interpretation in the context of considering the current
standard, several are particularly important. These include
uncertainties related to the following: Estimation of 5-minute
concentrations in ambient air; the lack of information from controlled
human exposure studies for the lower, more prevalent concentrations of
SO2 and limited information regarding multiple exposure
episodes within a day; the prevalence of different exposure
circumstances represented by the three study areas; and
characterization of particular subgroups of people with asthma that may
be at greater risk.
c. Summary of Exposure and Risk Estimates
    The REA provides estimates for two simulated at-risk populations:
Adults with asthma and school-aged children \67\

with asthma (REA, section 2.2). This summary focuses on the population
of children with asthma given that the ISA describes children as
``particularly at risk'' and the REA generally yields higher exposure
and risk estimates for children than adults (in terms of percentage of
the population group). Summarized here are two sets of exposure and
risk estimates for the 3-year simulation in each study area: (1) The
number (and percent) of simulated persons experiencing exposures at or
above the particular benchmark concentrations of interest while
breathing at elevated rates; and (2) the number and percent of people
estimated to experience at least one SO2-related lung
function decrement in a year and the number and percent of people
experiencing multiple lung function decrements associated with
SO2 exposures (detailed results are presented in chapter 5
of the REA). Both types of estimates are lower for adults with asthma
compared to children with asthma, generally due to the lesser amount
and frequency of time spent outdoors while breathing at elevated rates
(REA, section 5.2). As summarized in section II.A.3.b above, the REA
provides results for two different approaches to adjusting air quality.
The estimates summarized here are drawn from the results for both
approaches, as presented in Tables 1 and 2 of the proposal (83 FR
26772, June 8, 2018).
---------------------------------------------------------------------------

    \67\ The adult population group is comprised of individuals
older than 18 years of age and school-aged children are individuals
aged 5 to 18 years old. As in other NAAQS reviews, this REA does not
estimate exposures and risk for children younger than 5 years old
due to the more limited information contributing relatively greater
uncertainty in modeling their activity patterns and physiological
processes compared to children between the ages of 5 to 18 (REA, p.
2-8).
---------------------------------------------------------------------------

    This summary focuses first on the results for the benchmark-based
risk metric in terms of the percent of the simulated populations of
children with asthma estimated to experience at least one daily maximum
5-minute exposure per year at or above the different benchmark
concentrations while breathing at elevated rates under air quality
conditions just meeting the current standard (REA, Tables 6-8 and 6-9).
In two of the three study areas, approximately 20% to just over 25% of
a study area's simulated children with asthma, on average across the 3-
year period, are estimated to experience one or more days per year with
a 5-minute exposure at or above 100 ppb while breathing at elevated
rates (83 FR 26772 [Table 1], June 8, 2018).\68\ With regard to the 200
ppb benchmark concentration, these two study areas' estimates are as
high as 0.7%, on average across the 3-year period, and range up to as
high as 2.2% in a single year. Less than 0.1% of either area's
simulated children with asthma were estimated to experience multiple
days with such an exposure at or above 200 ppb (REA, Tables 6-8 and 6-
9). Additionally, in the study area with the highest estimates for
exposures at or above 200 ppb, approximately a quarter of a percent of
simulated children with asthma also were estimated to experience a day
with a 5-minute exposure at or above 300 ppb across the 3-year period
(the percentage for the 400 ppb benchmark was 0.1% or lower). Across
all three areas, no children were estimated to experience multiple days
with a daily maximum 5-minute exposure (while breathing at an elevated
rate) at or above 300 ppb (REA, Table 6-9).
---------------------------------------------------------------------------

    \68\ These estimates for the third area (Tulsa) are much lower
than those for the other two areas. No individuals of the simulated
at-risk population in the third study area were estimated to
experience exposures at or above 200 ppb and less than 0.5% are
estimated to experience an exposure at or above the 100 ppb
benchmark.
---------------------------------------------------------------------------

    With regard to lung function risk, in the two study areas for which
the exposure estimates are highest, as many as 1.3% and 1.1%,
respectively, of children with asthma, on average across the 3-year
period (and as many as 1.9% in a single year), were estimated to
experience at least 1 day per year with a SO2-related
doubling in sRaw (83 FR 26772 [Table 2], June 8, 2018; REA, Tables 6-10
and 6-11).\69\ The corresponding percentage estimates for experiencing
two or more such days ranged as high as 0.7%, on average across the 3-
year simulation period (REA, Table 6-11). Additionally, as much as 0.2%
and 0.3%, in Fall River and Indianapolis, respectively, of the
simulated populations of children with asthma, on average across the 3-
year period, was estimated to experience a single day with a
SO2-related tripling in sRaw (83 FR 26772 [Table 2], June 8,
2018).
---------------------------------------------------------------------------

    \69\ As with the comparison-to-benchmark results, the estimates
for risk of lung function decrements in terms of a doubling or more
in sRaw are also lower in the Tulsa study area than the other two
areas (83 FR 26772 [Table 2], June 8, 2018; REA, Tables 6-10 and 6-
11).
---------------------------------------------------------------------------

B. Conclusions on Standard

    In drawing conclusions on the adequacy of the current primary
SO2 standard, in view of the advances in scientific
knowledge and additional information now available, the Administrator
has considered the evidence base, information, and policy judgments
that were the foundation of the last review and reflects upon the body
of evidence and information newly available in this review. In so
doing, the Administrator has taken into account both evidence-based and
exposure- and risk-based considerations, as well as advice from the
CASAC and public comments. Evidence-based considerations draw upon the
EPA's assessment and integrated synthesis of the scientific evidence
from controlled human exposure studies and epidemiologic studies
evaluating health effects related to exposures of SO2 as
presented in the ISA, with a focus on policy-relevant considerations as
discussed in the PA (summarized in sections II.B and II.D.1 of the
proposal and section II.A.2 above). The exposure- and risk-based
considerations draw from the results of the quantitative analyses
presented in the REA (as summarized in section II.C of the proposal and
section II.A.3 above) and consideration of these results in the PA.
    Consideration of the evidence and exposure/risk information in the
PA and by the Administrator is framed by consideration of a series of
key policy-relevant questions. Section II.B.1 below summarizes the
rationale for the Administrator's proposed decision, drawing from
section II.D.3 of the proposal. The advice and recommendations of the
CASAC and public comments on the proposed decision are addressed below
in sections II.B.2 and II.B.3, respectively. The Administrator's
conclusions in this review regarding the adequacy of the current
primary standard and whether any revisions are appropriate are
described in section II.B.4.
1. Basis for Proposed Decision
    At the time of the proposal, the Administrator carefully considered
the assessment of the current evidence and conclusions reached in the
ISA; the currently available exposure and risk information, including
associated limitations and uncertainties, described in detail in the
REA and characterized in the PA; considerations and staff conclusions
and associated rationales presented in the PA, including consideration
of commonly accepted guidelines or criteria within the public health
community, including the ATS, an organization of respiratory disease
specialists; the advice and recommendations from the CASAC; and public
comments that had been offered up to that point (83 FR 26778, June 8,
2018). In reaching his proposed decision on the primary SO2
standard, the Administrator first recognized the long-standing evidence
that has established the key aspects of the harmful effects of very
short SO2 exposures on people with asthma. This evidence,
drawn largely from the controlled human exposure studies, demonstrates
that very short exposures (for as short as a few minutes) to less than
1000 ppb SO2, while breathing at an elevated rate (such as
while exercising), induces bronchoconstriction and related

respiratory effects in people with asthma and provides support for
identification of this group as the population at risk from short-term
peak concentrations in ambient air (ISA; 2008 ISA; U.S. EPA, 1994).\70\
Within this evidence base, there is a relative lack of such information
for some subgroups of this population, including young children and
people with severe asthma. The evidence base additionally includes
epidemiologic evidence that supports the conclusion of a causal
relationship between short-term SO2 exposures and
respiratory effects, for which the controlled human exposure studies
are the primary evidence.
---------------------------------------------------------------------------

    \70\ For people without asthma, such effects have only been
observed in studies of exposure concentrations at or above 1000 ppb
(ISA, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    With regard to the health effects evidence newly available in this
review, in the proposal the Administrator noted that, while the health
effects evidence, as assessed in the ISA, has been augmented with
additional studies since the time of the last review, including more
than 200 new health studies, it does not lead to different conclusions
regarding the primary health effects of SO2 in ambient air
or regarding exposure concentrations associated with those effects. Nor
does it identify different or additional populations at risk of
SO2-related effects. Thus, the Administrator recognized that
the health effects evidence available in this review and addressed in
the ISA is consistent with evidence available in the last review when
the current standard was established and that this strong evidence base
continues to demonstrate a causal relationship between relevant short-
term exposures to SO2 and respiratory effects, particularly
with regard to effects related to asthma exacerbation in people with
asthma. He also recognized that the ISA conclusion on the respiratory
effects caused by short-term exposures is based primarily on the
evidence from controlled human exposure studies that reported effects
in people with asthma exposed to SO2 for 5 to 10 minutes
while breathing at an elevated rate (ISA, section <IP_ADDRESS>), and that
the current 1-hour standard was established to provide protection from
effects such as these (75 FR 35520, June 22, 2010; 83 FR 26778, June 8,
2018).
    In considering exposure concentrations of interest in this review,
the Administrator particularly noted the evidence from controlled human
exposure studies, also available in the last review, that demonstrate
the occurrence of moderate or greater lung function decrements in some
people with asthma exposed to SO2 concentrations as low as
200 ppb for very short periods of time while breathing at elevated
rates (ISA, Table 5-2 \71\ and Figure 5-1, summarized in Table 3-1 of
the PA).\72\ He recognized that the data for the 200 ppb exposures
include limited evidence of respiratory symptoms accompanying the lung
function effects observed, and that the severity and number of
individuals affected is found to increase with increasing exposure
levels, as is the frequency of accompaniment by respiratory symptoms,
such that, at concentrations at or above 400 ppb, the moderate or
greater decrements in lung function were frequently accompanied by
respiratory symptoms, with some of these findings reaching statistical
significance at the study group level (ISA, Table 5-2 and section
5.2.1; PA, section <IP_ADDRESS>; 83 FR 26779, June 8, 2018).
---------------------------------------------------------------------------

    \71\ The availability of individual subject data from these
studies allowed for the comparison of results in a consistent manner
across studies (ISA, Table 5-2; Long and Brown, 2018).
    \72\ The Administrator additionally considered the very limited
evidence for exposure concentrations below 200 ppb, for which
relatively less severe effects are indicated, while noting the
limitations of this dataset (83 FR 26781, June 8, 2018).
---------------------------------------------------------------------------

    In considering the potential public health significance of these
effects associated with SO2 exposures, the Administrator's
proposed decision recognized both the greater significance of larger
lung function decrements, which are more frequently documented at
exposures above 200 ppb, and the potential for greater impacts of
SO2-induced decrements in people with more severe asthma, as
recognized in the ISA and by the CASAC (as summarized in section II.D.2
of the proposal).\73\ Thus, the Administrator recognized that health
effects resulting from exposures at and above 400 ppb are appreciably
more severe than those elicited by exposure to SO2
concentrations at 200 ppb, and that health impacts of short-term
SO2 exposures (including those occurring at concentrations
below 400 ppb) have the potential to be more significant in the
subgroup of people with asthma that have more severe disease and for
which the study data are more limited (83 FR 26779, June 8, 2018).
---------------------------------------------------------------------------

    \73\ The ISA notes that while extremely limited evidence for
adults with moderate to severe asthma indicates such groups may have
similar relative lung function decrements in response to
SO2 as adults with less severe asthma, individuals with
severe asthma may have greater absolute decrements that may relate
to the role of exercise (ISA, pp. 1-17 and 5-22). The ISA concluded
that individuals with severe asthma may have ``less reserve capacity
to deal with an insult compared with individuals with mild asthma''
(ISA, pp. 1-17 and 5-22).
---------------------------------------------------------------------------

    As was the case for the 2010 decision, the Administrator's proposed
decision in this review recognized the importance of considering the
health effects evidence in the context of the exposure and risk
modeling performed for this review. The Administrator recognized that
such a context is critical for SO2, a chemical for which the
associated health effects that occur in people with asthma are linked
to exposures during periods of elevated breathing rates, such as while
exercising. Accordingly, in considering the adequacy of public health
protection provided by the current standard, the Administrator
considered the evidence in this context. In so doing, he found the PA
considerations regarding the REA results and the associated
uncertainties, as well as the nature and magnitude of the uncertainties
inherent in the scientific evidence upon which the REA is based, to be
important to judgments such as the extent to which the exposure and
risk estimates for air quality conditions that just meet the current
standard in the three study areas indicate exposures and risks that are
important from a public health perspective.
    Thus, in considering whether the current standard provides the
requisite protection of public health in the proposal, the
Administrator took note of: (1) The PA consideration of a sizeable
number of at-risk individuals living in locations near large
SO2 emissions sources that may contribute to increased
concentrations in ambient air, and associated exposures and risk; (2)
the REA estimates of children with asthma estimated to experience
single or multiple days across the 3-year assessment period, as well as
in a single year, with a 5-minute exposure at or above 200 ppb, while
breathing at elevated rates; and (3) limitations and associated
uncertainties with regard to population groups at potentially greater
risk but for which the evidence is lacking, recognizing that the CAA
requirement that primary standards provide an adequate margin of safety
is intended to address uncertainties associated with inconclusive
scientific and technical information, as well as to provide a
reasonable degree of protection against hazards that research has not
yet identified (83 FR 26780, June 8, 2018). Further, the proposed
decision recognized advice received from the CASAC, including its
conclusion that the current evidence and exposure/risk information
supports retaining the current standard, as well as its statement that
it did not

recommend reconsideration of the level of the standard to provide a
greater margin of safety (83 FR 26780, June 8, 2018). Based on all of
these considerations, the Administrator proposed to conclude that a
less stringent standard would not provide the requisite protection of
public health, including an adequate margin of safety (83 FR 26780,
June 8, 2018).
    The Administrator also considered the adequacy of protection
provided by the current standard from effects associated with lower
short-term exposures, including those at or below 200 ppb. In so doing,
he considered the REA estimates for such effects, and the significance
of estimates for single (versus multiple) occurrences of exposures at
or above the lower benchmark concentrations and associated lung
function decrements, and the nature and magnitude of the various
uncertainties that are inherent in the underlying scientific evidence
and REA analyses. Based on these, he placed little weight on the
significance of estimates of occurrences of short-term exposures below
200 ppb and focused on the REA results for exposures at and above 200
ppb in light of his considerations, noted above, regarding the health
significance of findings from the controlled human exposure studies. He
further placed relatively less weight on the significance of infrequent
or rare occurrences of exposures at or just above 200 ppb, and more
weight on the significance of repeated such occurrences, as well as
occurrences of higher exposures. With this weighing of the REA
estimates and recognizing the uncertainties associated with such
estimates for the scenarios of air quality developed to represent
conditions just meeting the current standard, the Administrator
considered the current standard to provide a high degree of protection
to at-risk populations from SO2 exposures associated with
the more severe health effects, which are more clearly of public health
concern, as indicated by the extremely low estimates of occurrences of
exposures at or above 400 ppb (and at or above 300 ppb); and to
additionally provide a slightly lower, but still high, degree of
protection for the appreciably less severe effects associated with
lower exposures (i.e., at and below 200 ppb), for which public health
implications are less clear. The Administrator further observed that
although the CASAC stated that there is uncertainty in the adequacy of
the margin of safety provided by the current standard for less well
studied yet potentially susceptible population groups, it concluded
that ``the CASAC does not recommend reconsideration of the level in
order to provide a greater margin of safety'' (Cox and Diez Roux,
2018b, Consensus Responses, p. 5; 83 FR 26780, June 8, 2018). Based on
these and all of the above considerations, the Administrator proposed
to conclude that a more stringent standard is not needed to provide
requisite protection and that the current standard provides the
requisite protection of public health under the Act (83 FR 26781, June
8, 2018).
    In summary, the Administrator considered the specific elements of
the existing standard and proposed to retain the existing standard, in
all of its elements. With regard to SO2 as the indicator, he
recognized the support for retaining this indicator in the current
evidence base, noting the ISA conclusion that SO2 is the
most abundant of the SOX in the atmosphere and the one most
clearly linked to human health effects. The Administrator additionally
recognized the control exerted by the 1-hour averaging time on 5-minute
ambient air concentrations of SO2 and the associated
exposures of particular importance for SO2-related health
effects. Lastly, with regard to form and level of the standard, the
Administrator noted the REA results and the level of protection that
they indicate the elements of the current standard to collectively
provide. The Administrator additionally noted CASAC support for
retaining the current standard and the CASAC's specific recommendation
that all four elements should remain the same.
    Thus, based on consideration of the evidence and exposure/risk
information available in this review, with its attendant uncertainties
and limitations, and information that might inform public health policy
judgments, as well as consideration of advice from the CASAC, including
their concurrence with the PA conclusions that the current evidence
does not support revision of the primary SO2 standard, the
Administrator proposed to conclude that it is appropriate to retain the
current standard without revision based on his judgment that the
current primary SO2 standard provides an adequate margin of
safety against adverse effects associated with short-term exposures to
SOX in ambient air. For these reasons, and all of the
reasons discussed above, and recognizing the CASAC conclusion that the
current evidence and REA results provide support for retaining the
current standard, the Administrator proposed to conclude that the
current primary SO2 standard is requisite to protect public
health with an adequate margin of safety from effects of SOX
in ambient air and should be retained, without revision.
2. CASAC Advice in This Review
    In comments on the draft PA, the CASAC concurred with staff's
overall preliminary conclusions that ``the current scientific
literature does not support revision of the primary NAAQS for
SO2,'' additionally stating the following (Cox and Diez
Roux, 2018b, p. 3 of letter):

    The CASAC notes that the new scientific information in the
current review does not lead to different conclusions from the
previous review. Thus, based on review of the current state of the
science, the CASAC supports retaining the current standard, and
specifically notes that all four elements (indicator, averaging
time, form, and level) should remain the same.

The CASAC further stated the following (Cox and Diez Roux, 2018b, p. 3
of letter):

    With regard to indicator, SO2 is the most abundant of
the gaseous SOX species. Because, as the PA states, ``the
available scientific information regarding health effects was
overwhelmingly indexed by SO2,'' it is the most
appropriate indicator. The CASAC affirms that the one-hour averaging
time will protect against high 5-minute exposures and reduce the
number of instances where the 5-minute concentration poses risks to
susceptible individuals. The CASAC concurs that the 99th percentile
form is preferable to a 98th percentile form to limit the upper end
of the distribution of 5-minute concentrations. Furthermore, the
CASAC concurs that a three-year averaging time for the form is
appropriate.
    The choice of level is driven by scientific evidence from the
controlled human exposure studies used in the previous NAAQS review,
which show a causal effect of SO2 exposure on asthma
exacerbations. Specifically, controlled five-minute average
exposures as low as 200 ppb lead to adverse health effects. Although
there is no definitive experimental evidence below 200 ppb, the
monotonic dose-response suggests that susceptible individuals could
be affected below 200 ppb. Furthermore, short-term epidemiology
studies provide supporting evidence even though these studies cannot
rule out the effects of co-exposures and are limited by the
available monitoring sites, which do not adequately capture
population exposures to SO2. Thus, the CASAC concludes
that the 75 ppb average level, based on the three-year average of
99th percentile daily maximum one-hour concentrations, is protective
and that levels above 75 ppb do not provide the same level of
protection.

The comments from the CASAC also took note of the uncertainties that
remain in this review. In so doing, it stated that the ``CASAC notes
that there are many susceptible subpopulations

that have not been studied and which could plausibly be more affected
by SO2 exposures than adults with mild to moderate asthma,''
providing as examples people with severe asthma and obese children with
asthma, and citing physiologic and clinical understanding (Cox and Diez
Roux, 2018b, p. 3 of letter). The CASAC stated that ``[i]t is plausible
that the current 75 ppb level does not provide an adequate margin of
safety in these groups[, h]owever because there is considerable
uncertainty in quantifying the sizes of these higher risk
subpopulations and the effect of SO2 on them, the CASAC does
not recommend reconsideration of the level at this time'' (Cox and Diez
Roux, 2018b, p. 3 of letter).
    The CASAC additionally noted that the draft PA ``clearly identifies
most of the key uncertainties, including uncertainties in dose-
response'' and that ``[t]here are also some additional uncertainties
that should be mentioned'' (Cox and Diez Roux, 2018b, pp. 6-7 of
Consensus Response to Charge Questions). These are in a variety of
areas including risk for various population groups, personal exposures
to SO2, and estimating short-term ambient air
concentrations.\74\ The CASAC additionally recommended attention to
assessment of the impact of relatively lower levels of SO2
in persons who may be at increased risk, including those referenced
above (Cox and Diez Roux, 2018b, p. 3 of letter). The CASAC suggested
research and data gathering in these and other areas that would inform
the next primary SO2 standard review (Cox and Diez Roux,
2018b, p. 6 of Consensus Responses to Charge Questions).
---------------------------------------------------------------------------

    \74\ These and other comments from the CASAC on the draft PA and
REA were considered in preparing the final PA and REA, as well as in
developing the proposed and final decisions in this review.
---------------------------------------------------------------------------

3. Comments on the Proposed Decision
    During the public comment period for the proposed decision, we
received 24 comments.
a. Comments in Support of Proposed Decision
    Of the comments addressing the proposed decision, the majority
supported the Administrator's proposed decision to retain the current
primary standard, without revision. This group includes an association
of state and local air agencies, all of the state agencies that
submitted comments, more than half of the industry organizations that
submitted comments, and a couple of comments from individuals. All of
these commenters generally note their agreement with the rationale
provided in the proposal and the CASAC concurrence with the PA
conclusion that the current evidence does not support revision to the
standard. Most also cite the EPA and CASAC statements that information
newly available in this review has not substantially altered our
previous understanding of effects from exposures lower than what was
previously examined or of the at-risk populations and does not call
into question the adequacy of the current standard. They all find the
proposed decision to retain the current standard to be well supported.
The EPA agrees with these comments and with the CASAC advice regarding
the adequacy of the current primary standard and the lack of support
for revision of the standard.
    We additionally note that some of the industry commenters that
stated their support for retaining the current standard without
revision additionally stated that in their view the current standard
provides more public health protection than the EPA has recognized in
the proposal. As support for this view, these comments variously state
that concentrations in most of the U.S. are well below those evaluated
in the REA; that the studies in the ISA do not demonstrate
statistically significant response to SO2 concentrations
below 300 ppb; and, that a large percentage of the REA estimates of
lung function risk is attributable to exposures below 200 ppb. The
commenters also claim that in the 2010 decision that established the
current standard (75 FR 33547, June 22, 2010), the EPA had determined
that a standard protecting about 97-98% of exposed children with asthma
from a doubling of sRaw would be appropriate, but that the estimates in
the current REA indicate that over 99% of exercising children with
asthma receive such protection from the current NAAQS.
    As an initial matter, while we agree with the commenters that most
of the U.S. has SO2 concentrations below those assessed in
the REA, we disagree that this indicates the standard is overly
protective. Rather, this simply indicates the lack of large
SO2 emissions sources in many parts of the country (although
their presence in other parts of the country contributes to ambient air
concentrations of SO2 similar to or higher than those in the
REA). As recognized in section II.A.3 above, the REA is designed to
inform our understanding of exposure and risk in areas of the U.S.
where SO2 emissions contribute to airborne concentrations
such that the current standard is just met because the REA is intended
to inform the Agency's decision regarding the public health protection
provided by the current standard, rather than to describe exposure and
risk in areas with SO2 concentrations well below the current
standard (e.g., such that they that would meet alternative more
restrictive standards). This approach is consistent with section 109 of
the CAA, which requires the EPA to review whether the current primary
standard--not current air quality--is requisite to protect public
health with an adequate margin of safety (CAA section 109(b)(1) and
109(d)(1); see also NEDA/CAP, 686 F.3d at 813 [rejecting the notion
that it would be inappropriate for the EPA to revise a NAAQS if current
air quality does not warrant revision, stating ``[n]othing in the CAA
requires EPA to give the current air quality such a controlling role in
setting NAAQS'']). Thus, the EPA disagrees with the commenters that the
public health protection provided by the standard is indicated by
exposure and risk associated with air quality in parts of the U.S. with
concentrations well below the standard, and finds the REA appropriately
designed for purposes of informing consideration of the adequacy of the
public health protection provided by the current standard.
    With regard to the characterization of risk in the REA, it is true
as the commenters state that the lung function risk estimates include
estimates of risk based on 5-minute exposures below 200 ppb and that
the evidence from controlled human exposure studies is very limited for
concentrations below 200 ppb. We recognize this as an uncertainty in
the estimates (e.g., PA, section <IP_ADDRESS>).\75\ In considering the
uncertainties in and any associated implications of these estimates, we
also recognize, however, that we lack information for some population
groups, including young children with asthma and individuals with
severe asthma who might exhibit responses at lower exposures than those
already studied. And, as is noted in section II.A.2 above and by the
CASAC in their advice (summarized in section II.B.2 above), there is
the potential for responses in these populations to exposure
concentrations lower than those that have been tested in the controlled
human exposure studies. Thus, while we recognize the uncertainty in the
estimates noted by the commenters, we have considered the methodology
(which derived risk estimates based on

the lower exposure concentrations) to be appropriate in light of the
potential for the estimates to inform our consideration of the
protection afforded to these unstudied populations. Further, in
considering the risk estimates with regard to the level of protection
provided to at-risk populations in reaching a conclusion about the
adequacy of the current standard, the Administrator has recognized them
to be associated with somewhat greater uncertainty than the comparison-
to-benchmark estimates (see section II.B.4 below).
---------------------------------------------------------------------------

    \75\ For example, the PA recognizes the uncertainty in the lung
function risk estimates increases substantially with decreasing
exposure concentrations below those examined in the controlled human
exposure studies (PA, section <IP_ADDRESS>; REA, Table 6-3).
---------------------------------------------------------------------------

    Lastly, we do not agree with the comment that the estimates of
children protected from exposures of concern by the now-current
standard were appreciably lower when the standard was established.
While there are a number of differences between the 2009 REA and the
quantitative modeling and analyses performed in the current REA (as
described in PA, section 3.2.2 and summarized in section II.A.3 above),
the percentage of children with asthma that are estimated in the
current REA to experience at least a doubling in sRaw ranges up to
98.7% as a 3-year average across the three study areas.\76\ Although
the REA in the last review did not estimate risk for a 1-hour standard
with a level of 75 ppb, the estimate from the current REA falls
squarely between the 2009 REA estimates for the two air quality
scenarios most similar to a scenario just meeting the current standard:
99.5% for a level of 50 ppb and 97.1% for a level of 100 ppb (PA,
section <IP_ADDRESS>; 74 FR 64841, Table 4, December 8, 2009). In making
their comment, the commenters claim that the 2010 decision conveyed
that the selected standard of 75 ppb would protect 97 to 98 percent of
exposed children from a doubling of sRaw. Given the lack of 2009 REA
estimates for the level of 75 ppb, it might be presumed that the
commenter's two percentages represent the results for the 50 ppb and
100 ppb scenarios, thus providing a range within which results for 75
ppb might be expected to fall. However, that is not the case; rather,
the percentages cited by the commenter (97-98%) pertain to the 2009 REA
sRaw risk estimates for the air quality scenario with a standard level
of 100 ppb (75 FR 35547, June 22, 2010; 74 FR 64841 and Table 4,
December 8, 2009). Thus, the comment's statement is not borne out by
the risk estimates relevant to the current standard. Further, while we
recognize distinctions between the methodology and scenarios for the
two REAs, we find the estimates for lung function risk based on sRaw
and the similar estimates for exposures at or above the 200 ppb
benchmark to be of a magnitude roughly consistent between the two REAs
(as summarized in PA, section <IP_ADDRESS> and <IP_ADDRESS>.4). Accordingly,
while we agree there are uncertainties in the evidence and in the
exposure and risk estimates, the currently available information
indicates a level of protection to be afforded by the current standard
that is generally similar to what was indicated by the evidence
available when the standard was set in 2010. For these reasons, we
disagree that the current standard provides more public health
protection than recognized in the proposal.
---------------------------------------------------------------------------

    \76\ We note that in claiming that the current REA indicates
``over 99%'' of exercising asthmatic children to be protected from a
doubling of sRaw, the commenter erroneously cites the percentage for
multiple occurrences of a doubling of sRaw (83 FR 26781/3, June 8,
2018). In multiple other locations in the proposal, the percentage
for one or more occurrences is given as up to 98.7% across the three
study areas as a 3-year average (83 FR 26772, Table 2 and text,
26775/2, 26777/1, 26779/3, June 8, 2018).
---------------------------------------------------------------------------

b. Comments in Disagreement With Proposed Decision
    Of the commenters that disagreed with the proposal to retain the
current standard, three recommend a tightening of the standard, while
five recommend a less stringent standard. The commenters that
recommended a tighter standard state their support for revisions to
provide greater public health protection, generally claiming that the
current standard is inadequate and does not provide an adequate margin
of safety for potentially vulnerable groups. Commenters supporting a
less stringent standard assert that the current standard is
overprotective, with some of these commenters stating that the EPA is
inappropriately concerned about respiratory effects from exposures as
low as 200 ppb. We address these comments in turn below.
(i) Comments in Disagreement With Proposed Decision and Calling for
More Stringent Standard
    The commenters advocating for a more stringent standard variously
recommend that the level of the existing standard be revised to a value
no higher than 50 ppb, the form should be revised to allow the
occurrence of fewer hours with average concentrations above 75 ppb,
and/or that a new 24-hour standard be established. These three points
are addressed below.
    With regard to a standard level of 50 ppb, two of the commenters
supporting this view note that they also expressed this view in
comments they submitted during the 2010 review. In the comment in the
current review, these commenters cite asthma prevalence estimates for
children and other population groups, noting that asthma attacks may
contribute to missed school days, potentially affecting children's
education. These commenters additionally suggest that the current
standard does not adequately protect all population groups or provide
an adequate margin of safety given uncertainties in the health effects
evidence base, including those associated with the lack of controlled
human exposure studies that have investigated effects in particular at-
risk populations, such as young children with asthma, or at
concentrations below 100 ppb, as well as their view that available
studies did not address multiple exposures in the same day.
    One of the commenters quoted from the comment they submitted in the
last review which supported revisions to the then-current standards
(different from the revisions in the 2009 proposal).\77\ The quoted
text stated that epidemiologic studies (available in the decade prior
to the 2010 decision) include associations of health outcomes with 24-
hour SO2 concentrations that are below the level of the
then-current 24-hour standard (140 ppb) and that these studies indicate
SO2 effects at concentrations below the then-current
standards. The commenter then expressed the view that the science
accumulated in the intervening years has strengthened and reaffirmed
this. As the 2010 decision concluded that the then-existing 24-hour
standard did not provide adequate public health protection from short-
term SO2 concentrations (and consequently established a new
standard expressly for that purpose), we find that the commenter's
statements regarding the then-current 24-hour standard do not pertain
to the issue at hand in the current review, i.e., the adequacy of
protection provided by the current 1-hour standard. Moreover,
assessments in the last review supported the Administrator's conclusion
at that time that the then-existing 24-hour standard

did not provide adequate protection from the short-term concentrations
of most concern. As a result, the decision in the last review was to
provide for revocation of the 24-hour standard and to establish the now
current 1-hour standard to provide the needed protection of at-risk
populations with asthma from respiratory effects of SO2 (75
FR 35550, June 22, 2010). To the extent that these comments on the
proposal in the current review are intended to imply that the
epidemiologic studies briefly mentioned in the quotation from the
comment in the last review or studies that have become available in the
intervening years indicate that the current standard is inadequate, the
comments do not provide any explanation or analysis to support such an
assertion. With regard to the current standard and the epidemiologic
evidence, we further note that such evidence was considered by the
Administrator in 2010 (as were the comments submitted at that time) in
the setting of the now-current standard, and that the EPA has again
considered the complete body of evidence in this review and found no
newly available studies that might support alternative conclusions (75
FR 35548, June 22, 2010; 83 FR 26765, June 8, 2018). While the pattern
of associations across the newly available epidemiologic studies is
consistent with the studies available in the last review, key
uncertainties remain, including the potential for confounding by PM or
other copollutants (as summarized in section II.A.2 above). Among the
U.S. epidemiologic studies reporting mostly positive and sometimes
statistically significant associations between ambient SO2
concentrations and emergency department visits or hospital admissions
(some conducted in multiple locations), few studies have attempted to
address this uncertainty, e.g., through the use of copollutant models
(83 FR 26765, June 8, 2018; ISA, section <IP_ADDRESS>). In the last review,
there were three U.S. studies for which the SO2 effect
estimate remained positive and statistically significant in copollutant
models with PM.\78\ As noted in the proposal, no additional such
studies have been newly identified in this review (83 FR 26765, June 8,
2018). The conclusions of these studies and the air quality of the
study areas were given consideration by the Administrator in 2010 in
setting the current standard (83 FR 26761, June 8, 2018), and they do
not call into question the adequacy of the current standard in this
review.
---------------------------------------------------------------------------

    \77\ As part of the comments they submitted in the current
review, this commenter incorporated by reference their comments on
the 2009 proposal. Given the different framing of the current
proposal (to retain the now-existing 1-hour standard) from the
proposal in the last review (to significantly revise the then-
existing standards including the establishment of a new 1-hour
standard) and that this review relies on the current record, which
differs in a number of ways from that in the last review (e.g., the
updated analyses in the REA), we do not believe that merely
incorporating 2009 comments by reference is sufficient to raise a
significant comment with reasonable specificity in this review,
without further description of why the issues presented in the prior
comment are still relevant to the proposal in the current review.
    \78\ Based on data available for specific time periods at some
monitors in the areas of these studies, the 99th percentile 1-hour
daily maximum concentrations were estimated in the last review to be
between 78-150 ppb (83 FR 26765, June 8, 2018).
---------------------------------------------------------------------------

    Another comment in support of revising the standard level to 50 ppb
cites information on the impact of asthma and asthma attacks on
children and other population groups as a basis for their view that
many people are being harmed under the current standard with its level
of 75 ppb. While this comment described some of the health effects of
SO2 exposures for people with asthma and opined that
SO2-induced asthma attacks interfere with children's health,
school attendance and education, the commenter did not provide evidence
that such effects were allowed by and occurring under the current
standard. While we agree with the commenter regarding the important
impact of asthma on public health in the U.S., including impacts on the
health of children and population groups for which asthma prevalence
may be higher than the national average, and we agree that people with
asthma, and particularly children with asthma, are at greatest risk of
SO2-related effects, we do not find the information
currently available in this review to provide evidence of
SO2-induced asthma attacks or other harm to public health in
areas of the U.S. that meet the current standard.\79\ Thus, we disagree
with the comment that the current standard fails to address the need to
provide protection from asthma-related effects of SOX in
ambient air.
---------------------------------------------------------------------------

    \79\ An overview of the evidence available in this review, and
the ISA and PA conclusions regarding it, is provided in section
II.A.2 above and summarized in the proposal. These conclusions did
not find the currently available evidence to indicate that air
quality conditions allowed by the current standard allow
SO2-induced asthma attacks that interfere with children's
health, school attendance and education. The CASAC has concurred
with the ISA conclusions regarding the evidence, which also support
the overarching conclusion in the PA that the currently available
evidence and exposure/risk information does not call into question
the adequacy of public health protection provided by the current
standard, a conclusion with which the CASAC also concurred, as
summarized in section II.B.2 above.
---------------------------------------------------------------------------

    Commenters in support of a lower level for the standard
additionally express concern that populations living in communities
near large sources of SO2 emissions, including children in
population groups with relatively higher asthma prevalence, may not be
adequately protected by the current standard. In considering this
comment, we note that while the REA did not categorize simulated
children with asthma with regard to specific demographic subgroups,
such as those mentioned by the commenter or discussed in section
II.A.2.d above, the estimates are for children with asthma in areas
with large sources of SO2 emissions and with air quality
just meeting the current standard. As noted in section II.A.3 above,
the asthma prevalence across census tracts in the three REA study areas
ranged from 8.0 to 8.7% for all ages (REA, section 5.1) and from 9.7 to
11.2% for children (REA, section 5.1), which reflects some of the
higher prevalence rates in the U.S. today (PA, sections <IP_ADDRESS> and
<IP_ADDRESS>). Thus, in considering these results to inform his decision
regarding the adequacy of protection provided by the current standard,
the Administrator is focused on the patterns of exposure and
populations with elevated rates of asthma stated to be the situation of
concern to these commenters.
    In two of the three REA study areas, each of which include large
emissions sources and air quality adjusted to just meet the current
standard, no children with asthma were estimated to experience a day
with an exposure while breathing at elevated rates to a 5-minute
SO2 concentration at or above 400 ppb, the concentration at
which moderate or greater lung function decrements have been documented
in 20-60% of study subjects, with decrements frequently accompanied by
respiratory symptoms. In the third area the estimate was less than
0.1%, on average across the 3-year period. Further, fewer than 1% of
children with asthma, on average across the 3-year assessment period,
were estimated to experience any days with exposures at or above 200
ppb in two of the areas, and no children were estimated to experience
such days in the third area (PA, Table 3-3; 83 FR 26775, June 8, 2018).
Thus, the REA exposure and risk estimates for the current review
indicate that the current standard is likely to provide a very high
level of protection from SO2-related effects documented at
higher concentrations and a high level of protection from the transient
lung-function decrements documented in individuals with asthma in
controlled human exposure study concentrations as low as 200 ppb.
    The comment claiming that the current standard does not provide an
adequate margin of safety emphasized limitations in the evidence base
of controlled human exposure studies, noting the very limited available
studies that examined 5-minute SO2 exposures as low as 100
ppb; the lack of studies in young children with asthma and people of
any age with severe asthma; and that the studies did not examine the
impact of multiple exposures in the same day. While we agree that the

evidence base is limited with regard to examination of potential
effects at lower concentrations and in some population groups, we
disagree with the latter statement that the currently available studies
have not investigated multiple exposures within the same day. In fact,
there are some studies that inform our understanding of responses to
repeated occurrences of exposure during exercise within the same day
(REA, Table 6-3; ISA, section <IP_ADDRESS>). For example, there are studies
that have investigated the magnitude of lung function response from
separate exercise events within the same 1-hour or 6-hour exposure, and
from exposures with exercise occurring on subsequent days (Linn et al.,
1984; Kehrl et al., 1987). As an initial matter, we note that the
evidence shows lung function decrements that occur with short
SO2 exposures are resolved with the cessation of either the
exposure or exercise, with lung function returning to baseline in
either situation (ISA, section <IP_ADDRESS>). Further, responses to repeated
exercise events occurring within the same 1-hour or 6-hour exposure are
diminished in comparison to the response to the initial event (Kehrl et
al., 1987; Linn et al., 1984; Linn et al., 1987). Even responses to
exposures while exercising that are separated by a day are still very
slightly diminished from the initial response (Linn et al., 1984).
Thus, we disagree with the commenter's statement that the available
controlled human exposure studies have not examined the impact of
multiple exposures in the same day. While the studies involve single
continuous exposure periods shorter than a day, the discontinuous
nature of the exercise component of the exposure design provides the
relevant circumstances for assessing the impact of multiple exposure-
with-exercise events in a single day. The evidence from these studies
documents the transient nature of the lung function response, even to
the high concentrations studied (600 to 1000 ppb), as well as a
lessening of decrements in response to subsequent occurrences within a
day.
    We agree with this comment that the evidence base is limited with
regard to examination of potential effects at lower concentrations and
in some population groups. As summarized in I.A.2 above, the health
effects evidence newly available in this review does not extend our
understanding of the range of exposure concentrations that elicit
effects in people with asthma exposed while breathing at an elevated
rate beyond what was understood in the last review. As in the last
review, 200 ppb remains the lowest concentration tested in controlled
human exposure studies where study subjects are freely breathing in
exposure chambers. The limited information available for exposure
concentrations below 200 ppb, including exposure concentrations of 100
ppb, while not amenable to direct quantitative comparisons with
information from studies at higher concentrations, generally indicates
a lesser response. Further, as discussed in section II.A.2 above, we
recognize that evidence for some at-risk population groups, including
young children with asthma and individuals with severe asthma, is
limited or lacking at any exposure concentration. As discussed in
section II.B.4 below, the Administrator has explicitly recognized this
in reaching conclusions regarding the adequacy of the public health
protection provided by the current standard, including considerations
of margin of safety for the health of at-risk populations.
    One commenter advocating a more stringent standard additionally
notes that evidence from controlled human exposure studies is also
lacking for adults older than 75 years, an age group for which the
commenter states there is new research placing this age group at
increased risk. While some recent epidemiologic studies have examined
associations of SO2 with the occurrence of various health
outcomes in older adults (typically ages 65 years and older), such
studies have not consistently found stronger associations for this
group compared to younger adults (ISA, sections <IP_ADDRESS> and 6.6). As a
result, the ISA concluded that the evidence was only suggestive of the
older age group being at increased risk of SO2-related
health effects. Such a characterization indicates that ``the evidence
is limited due to some inconsistency within a discipline or, where
applicable, a lack of coherence across disciplines'' (ISA, Table 6-1),
and in this case, the ISA indicates that the study results were
concluded to be ``mixed'' or ``generally inconsistent'' (ISA, Table 6-
7). Further, there is no evidence indicating that the individuals in
this group would be affected at lower exposure concentrations than
other population groups or that they would be inadequately protected by
the current standard. As noted by the CASAC more broadly, ``there are
many susceptible subpopulations that have not been studied and which
could plausibly be more affected by SO2 exposures than
adults with mild to moderate asthma'' (Cox and Diez Roux, 2018b, p. 3
of letter).
    With that recognition in mind, the CASAC explicitly considered the
issue of margin of safety provided by the current standard. While
noting that ``[i]t is plausible that the current 75 ppb level does not
provide an adequate margin of safety in these groups,'' the CASAC
additionally stated that ``because there is considerable uncertainty in
quantifying the sizes of these higher risk subpopulations and the
effect of SO2 on them, the CASAC does not recommend
reconsideration of the level at this time'' (Cox and Diez Roux, 2018b,
p. 3 of letter). The CASAC additionally concluded that the 75 ppb level
of the standard ``is protective'' and that the current scientific
evidence ``does not support revision of the primary NAAQS for
SO2'' (Cox and Diez Roux, 2018b, pp. 1 and 3 of letter). In
addition, we note that the D.C. Circuit has concluded that the
selection of any particular approach for providing an adequate margin
of safety is a policy choice left specifically to the Administrator's
judgment (Lead Industries Association v. EPA, 647 F.2d at 1161-62;
Mississippi, 744 F.3d at 1353). In light of such considerations, as
discussed in section II.B.4 below, the Administrator does not agree
with commenters that the current standard fails to include an adequate
margin of safety or otherwise insufficiently protects older adults or
other population groups, including those that are recognized as being
most at risk of SO2-related effects in this review, i.e.,
people with asthma, in particular children with asthma.
    As additional support for their view that the standard level should
be revised to 50 ppb, one of the commenters states that any new
standard would have to be more protective to make up for the lack of
progress on implementation of the 2010 standard. Such a rationale lacks
a basis in the CAA. The requirements in sections 108 and 109 of the CAA
for establishing and reviewing the NAAQS are separate and distinct from
the CAA requirements for implementing the NAAQS (e.g., CAA sections
107, 110, and 172), and the time it takes to attain a standard under
those requirements is not evidence pertaining to the adequacy of that
standard with regard to public health protection under section 109. In
setting primary and secondary standards that are ``requisite'' to
protect public health and public welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary for these purposes.\80\

Moreover, section 109(d)(1), the statutory provision that governs the
review and revision of the NAAQS, provides that the Administrator shall
periodically review the NAAQS and the air quality criteria and ``shall
make such revisions . . . as may be appropriate in accordance'' with
sections 108 and 109(b), but does not mention any of the sections of
the Act related to NAAQS implementation as relevant to that review. In
addition, the Act contains specific provisions addressing the timing of
NAAQS implementation, such as promulgating area designations under
section 107(d) and adoption of state implementation plans for NAAQS
implementation and enforcement under sections 110(a)(1) and 172(c), and
these provisions establish their own requirements for timing and
substantive decisions that are, likewise, not governed by the deadlines
and criteria that govern the EPA's review under section 109. Each of
these sections--109, 107, 110 and 172--govern EPA action independently
of each other, and the EPA's performance of its duties under each
provision is independently and fully reviewable without regard to the
timeliness of its actions under the other provisions. Thus, there is no
reason to think that Congress intended to require the Agency to address
issues of the timing of NAAQS implementation through the NAAQS review
process, including in the consideration of whether a specific standard
provides the requisite protection.
---------------------------------------------------------------------------

    \80\ In so doing, the EPA may not consider the costs of
implementing the standards. See generally, Whitman v. American
Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001).
Likewise, ``[a]ttainability and technological feasibility are not
relevant considerations in the promulgation of national ambient air
quality standards.'' American Petroleum Institute, 665 F.2d at 1185.
---------------------------------------------------------------------------

    One of the comments submitted in support of a lower standard level
also recommended that the form of the standard be revised to one that
would allow fewer daily maximum 1-hour concentrations above 75 ppb.
This commenter stated that if the level of the current standard is
retained, a more restrictive form of the standard should be adopted. In
support of this position, this commenter stated that the current 99th
percentile form allows for ``multiple days a year of dangerous levels
of SO2.'' The commenter does not provide a basis for their
characterization of any 1-hour SO2 concentration above 75
ppb as dangerous and does not explain their view of what ``dangerous''
encompasses with respect to potential exposures and health risk,
estimates of which are provided by the REA for air quality scenarios
that just meet the current standard and would allow no more than 4 days
per year (on average across a 3-year period) with 1-hour concentrations
above 75 ppb. We do not consider the exposures allowed by the current
standard and characterized in the REA to be dangerous to public health.
Thus, we disagree with the commenter's view that the small number of
days that may have 1-hour concentrations above 75 ppb under conditions
meeting the current standard create ``dangerous'' circumstances. The
evidence base of controlled human exposure studies, which provides the
most detailed information about human health effects resulting from
exposure to SO2, does not include exposure concentrations
below 100 ppb. While the data are limited at that concentration, they
indicate a lesser response than that at the 200 ppb level. The results
for exposures at 200 ppb indicate that, which includes less than 10% of
study subjects with asthma, exposed while exercising, experiencing a
moderate or greater lung function decrement, with the response ceasing
with cessation of exposure or exertion. Nor do we agree that a more
restrictive form of the standard is necessary to protect at-risk
populations from adverse effects associated with short (e.g., 5-minute)
peak SO2 exposures which was an explicit consideration in
the establishment of the current standard (75 FR 35539, June 22, 2010).
Section II.A.2 above summarizes the current health effects evidence
regarding concentrations associated with effects of such exposures and
the severity of such effects. As noted there, the current evidence is
consistent with that available in the last review when the standard was
set. Further, as recognized in sections II.A.1 and II.B.1 above, the
protection afforded by the current standard stems from its elements
collectively, including the level of 75 ppb, in combination with the
averaging time of one hour and the form of the 3-year average of annual
99th percentile daily maximum concentrations. The REA analyses of
exposure and risk for air quality conditions just meeting the current
standard (in all its elements) indicate a high level of protection of
children with asthma from days with an exposure, while exercising, to
peak concentrations as low as 200 ppb, the lowest concentration at
which moderate or greater lung function decrements have been
documented, and a very high level of protection against 400 ppb
exposures.\81\ We additionally note that analyses of air quality at the
308 monitors across the U.S. at which the current standard was met
during the recent 3-year period analyzed in the PA (2014-2016),
indicate that peak SO2 concentrations in ambient air at or
above 200 ppb are quite rare (PA, Figure C-5). Lastly, we note that in
explicitly considering the elements of the standard the CASAC advised
that ``all four elements (indicator, averaging time, form, and level)
should remain the same'' (Cox and Diez Roux, 2018b, p. 3 of letter).
Considerations such as these from the CASAC inform the Administrator's
conclusion (discussed in section II.B.4 below) that no revisions to the
current standard, including its form, are needed.
---------------------------------------------------------------------------

    \81\ Although aspects of the studies of concentrations below 200
ppb complicate comparisons with the studies at 200 ppb, the limited
evidence available does not indicate a response in any of the few
subjects studied as severe as a doubling in sRaw (83 FR 26764, June
8, 2018).
---------------------------------------------------------------------------

    The commenter that recommended establishment of a 24-hour standard,
with a level of 40 ppb, stated that epidemiologic studies support the
need for an additional 24-hour standard and note their position in the
2010 review for revision of the level of the then-existing 24-hr
standard to 40 ppb, matching the level of California's current 24-hour
standard. In terms of support for their advocacy of a 24-hour standard,
the commenter cited three epidemiologic studies of associations of
short-term SO2 concentrations with premature death from
respiratory causes in Chinese cities and two studies of associations of
longer-term SO2 concentrations with the development of
asthma (conducted in the U.S. and Canada). We disagree that these
studies indicate an inadequacy of the existing standard or indicate a
need for an additional standard. As an initial matter, we note that the
ISA for this review has assessed the current evidence regarding
SO2 and mortality, including the evidence provided by the
three studies in Chinese cities. We agree with the comment that these
three studies include analyses that controlled for some co-occurring
pollutants, although we note that those analyses were limited to
investigation of just two co-occurring pollutants, PM10 and
NO2. We additionally note that while the copollutant
analyses found associations with SO2 that generally remain
positive and statistically significant after adjustment for
PM10, those after-adjustment associations are somewhat
attenuated, indicating potential contributions to the association from
PM10 (ISA, section <IP_ADDRESS>, p. 5-145).\82\ Moreover, these
analyses show that after

adjustment for NO2, the associations are much more
attenuated and lose statistical significance (ISA, section <IP_ADDRESS>, p.
5-145). Further, none of the studies adjusted for PM2.5 (PM
with mass median aerodynamic diameter nominally below 2.5 microns), a
pollutant of particular importance with regard to potential confounding
of epidemiologic analyses for SO2 because of the fact that
SO2 is a precursor of PM2.5 (ISA, section
<IP_ADDRESS>; PA, section <IP_ADDRESS>). Additionally, these studies are limited
in that they were conducted in Asian cities where the air pollution
mixture and concentrations are different from the U.S., e.g.,
SO2 concentrations are much higher than concentrations in
the U.S., which limits generalizability and ``complicates the
interpretation of independent association for SO2'' (ISA,
Table 5-21; section <IP_ADDRESS>) at lower concentrations where there are no
studies that have controlled for relevant copollutants. In
consideration of the full evidence base in this review, including these
studies, the ISA concludes that the evidence regarding short-term
SO2 concentrations and respiratory mortality ``is
inconsistent within and across disciplines and outcomes, and there is
uncertainty related to potential confounding by copollutants'' (ISA, p.
5-155). Accordingly, as noted in the ISA, this limited and inconsistent
evidence for associations with premature mortality does not
substantially contribute to the determination that short-term
SO2 exposure is causally related to respiratory effects, a
determination supported primarily by evidence from controlled human
exposure studies (ISA, p. 5-153).
---------------------------------------------------------------------------

    \82\ When adjusted for PM10 concentrations in the
analyses, the magnitude of effect in the relationship between
SO2 and mortality was lower, compared to when
PM10 was not controlled for.
---------------------------------------------------------------------------

    Further, with regard to the commenter's suggestion concerning a 24-
hour standard and their reference to the current 24-hour standard in
the state of California, the commenter simply states that they
advocated such a standard in comments on the 2009 proposal in the 2010
review. We first note that as a general matter, we do not believe that
merely stating that that was their position in the 2010 review is
sufficient to raise a significant comment with reasonable specificity
in this review. Moreover, we note that the California 24-hour standard
was adopted in 1991, nearly 20 years prior to the EPA's last review of
the primary SO2 NAAQS in which we reviewed the then-
currently available health effects evidence.\83\ Since that time, the
body of evidence has been expanded, including the epidemiologic studies
raised by the commenter. As summarized in section II.A above, the 24-
hour standard that had existed prior to the last review of the
SO2 NAAQS, was revoked based on the determination in the
last review that the new 1-hour daily maximum standard would control
SO2 concentrations and protect public health from the
associated short-term exposures (ranging from 5 minutes to 24 hours)
with an adequate margin of safety (75 FR 35548, June 22, 2010). As
summarized above and in the proposal, the evidence in this review is
not substantively changed from that in the last review. Thus, based on
the consistency of the currently available epidemiologic evidence (as
well as the evidence from controlled human exposure studies) with that
available in the last review, we continue to conclude that an
additional standard with a 24-hour averaging time is not needed to
provide the protection required of the NAAQS. Accordingly, we find the
comment regarding a 24-hour standard and the rationale provided by the
commenter to lack a foundation in the currently available health
effects evidence. Furthermore, as explained in section I.A above, under
section 109(b)(1) of the CAA the EPA Administrator is to set primary
standards for criteria pollutants that are, in his judgment, requisite
to protect public health with an adequate margin of safety, and these
standards are to be based on the current air quality criteria for that
pollutant. Under this framework, the mere fact that a different agency
has previously established a different standard for a pollutant has no
bearing on the Administrator's conclusions. As discussed in section
II.B.4 below, the Administrator judges the current standard, based on
the currently available evidence and exposure/risk information, to
protect public health with an adequate margin of safety. Thus, we
disagree with the commenter that the existing primary standard provides
inadequate public health protection or that a 24-hour standard is
needed to provide the appropriate protection.\84\
---------------------------------------------------------------------------

    \83\ https://www.arb.ca.gov/research/aaqs/caaqs/hist1/hist1.htm.
    \84\ We additionally note that in addition to the 24-hour
standard of 40 ppb, the California 1-hour air quality standard for
SO2 is set at a level of 250 ppb, more than three times
the level of the current primary SO2 NAAQS that was set
in 2010. The 1-hour NAAQS of 75 ppb was established to protect
against short-term exposures of a few minutes up to 24 hours, and
was concluded in 2010 to provide the requisite protection of public
health with an adequate margin of safety that was lacking under the
prior 24-hour and annual standards.
---------------------------------------------------------------------------

    With regard to the epidemiologic studies of associations between
long-term SO2 concentrations and respiratory effects,
including development of asthma, the ISA concluded that, for long-term
exposure and respiratory effects, the complete evidence base, including
those studies cited by the commenter, was suggestive of, but not
sufficient to infer, the presence of a causal relationship (ISA,
Section 5.2.2, Table 5-24). While limited animal toxicological evidence
suggests biological plausibility for such effects of SO2,
the overall body of evidence across disciplines lacks consistency and
there are uncertainties that apply to the epidemiologic evidence,
including that newly available in this review, across the respiratory
effects examined for long-term exposure (ISA, sections <IP_ADDRESS> and
<IP_ADDRESS>). In this light, the ISA concludes that there is uncertainty
remaining regarding potential copollutant confounding and an
independent effect of long-term SO2 exposure, so that
chance, confounding, and other biases cannot be ruled out (ISA, Table
1-1). Thus, we disagree with the commenter that the current evidence
base supports their concern regarding long-term exposure or a need for
longer-term standard. In so doing, we additionally note the conclusion
reached in the last review that a standard based on 1-hour daily
maximum SO2 concentrations will afford requisite increased
protection for people with asthma and other at-risk populations against
an array of adverse respiratory health effects \85\ related to short-
term SO2 exposures ranging from 5 minutes to 24 hours. As
described in section II.B.4 below, the Administrator also concludes,
based on the current review of the available scientific evidence
documented in the ISA (which includes the studies cited by the
commenter) and the REA estimates, that the current standard continues
to provide the requisite protection of public health from health
effects of sulfur oxides in ambient air.
---------------------------------------------------------------------------

    \85\ The effects were recognized to include decrements in lung
function, increases in respiratory symptoms, and related serious
indicators of respiratory morbidity that had been investigated in
epidemiologic studies, including emergency department visits and
hospital admissions for respiratory causes (75 FR 35550, June 22,
2010).
---------------------------------------------------------------------------

(ii) Comments in Disagreement With Proposed Decision and Calling for
Less Stringent Standard
    Among the five commenters recommending revision to a less stringent
standard, most generally expressed the view that the current standard
is more stringent than necessary to protect public health. In support
of this view some of these commenters claimed that the EPA was

inappropriately concerned with limiting 5-minute exposures of 200 ppb
and higher, rather than focusing only on exposures at or above 300 ppb
or 400 ppb. Based on their view that the standard should focus only on
limiting population exposures to these higher concentrations, these
commenters variously recommended raising the level of the standard to
150 ppb or to just below 110 ppb, or, revising the percentile aspect of
the form from a 99th to a 98th percentile. Other commenters stated that
even for a focus on limiting 5-minute exposures at and above 200 ppb,
the current standard is overly protective. These commenters recommended
either revision of the averaging time or of the form, each claiming
that such a revision, accompanied by no change to any other element of
the standard, would still achieve adequate protection from exposures at
or above 200 ppb.
    The commenters in whose view the standard did not need to limit 5-
minute exposures as low as 200 ppb stated that the studies of this
exposure level did not find a statistically significant lung function
response across the full group of study subjects and that the EPA
should focus on a higher concentration, one at which the study subject
group response was statistically significant. These commenters
variously state that the controlled human exposure studies do not
demonstrate statistically significant responses in lung function at
SO2 exposure concentrations less than 300 ppb or 400 ppb,
respectively.
    The EPA disagrees with the premise of these comments that the
Agency's consideration of the adequacy of protection provided by the
current standard is focused solely, and inappropriately, on limiting
exposures to peak SO2 concentrations at or above 200 ppb.
Both the proposed decision and the Administrator's final decision,
discussed in section II.B.4 below, consider the evidence from
controlled human exposure studies and what it indicates regarding the
severity and prevalence of lung function decrements in people with
asthma exposed to the range of concentrations from 200 ppb through 400
ppb, and above, while breathing at elevated rates. The decision also
considers what can be discerned from the extremely limited evidence at
100 ppb and also what the available evidence does not address, such as
the concentrations at which a moderate or greater lung function
decrement \86\ might be expected to be elicited in exposed young
children with asthma or people of any age that have severe asthma.
Given the more severe response observed in some of the study subjects
exposed to 400 ppb, the greater percentage of the study subjects with
at least a moderate lung function decrement at this exposure, and the
frequent association of these findings with respiratory symptoms, such
as cough, wheeze, chest tightness, or shortness of breath, as well as
the findings of statistical significance in various studies (ISA, Table
5-2 and section 5.2.1), the Administrator recognizes the importance of
the standard providing a high degree of protection from exposures at
and above 400 ppb, as discussed in section II.B.4 below. Thus, we agree
with commenters that it is important to consider the level of
protection provided by the current standard against 5-minute exposures
to 400 ppb.
---------------------------------------------------------------------------

    \86\ As described in section II.A.2.c and consistent with the
ISA in the last review, moderate or greater SO2-related
bronchoconstriction or decrements in lung function referred to the
occurrence of at least a doubling in sRaw or at least a 15%
reduction in FEV1 (ISA, section <IP_ADDRESS> and Table 5-2).
---------------------------------------------------------------------------

    We disagree, however, with commenters who claim that it is not
important to also consider the protection afforded by the standard
against exposures below 400 ppb (including those at 200 ppb). As
discussed in section II.B.4 below, in reaching a judgment on the
adequacy of the current standard, the Administrator has considered the
evidence of effects from exposures below 400 ppb. In so doing, the
Administrator has taken note of the findings of a statistically
significant decrement in lung function at 300 ppb at the study group
level for a group of more SO2-responsive study subjects
(ISA, p. 5-153; Johns et al., 2010),\87\ and of the percentage of
subjects (as many as nearly 10%) experiencing a moderate or greater
lung function decrement in controlled exposure studies of 200 ppb (ISA,
Table 3-2). In considering the public health importance of effects
associated with exposure to levels of SO2 below 400 ppb, the
Administrator gives weight to these findings, particularly in light of
limitations in the evidence base, as well as to the ATS statement with
regard to respiratory effects in people with asthma. Based on the
findings, and in light of the fact that the evidence base is lacking or
extremely limited for some population groups, including particularly
young children with asthma, a group which the ISA concludes to be at
greater risk than other individuals with asthma, and individuals of any
age with severe asthma, a group for which the ISA suggests a potential
for greater sensitivity,\88\ the Administrator judges it important that
the standard provide appropriate protection from peak SO2
concentrations as low as 200 ppb, as discussed in section II.B.4 below.
We also note that in the decision that established the current
standard, weight was given to ensuring the new standard provided some
level of protection from short exposures of people with asthma,
breathing at elevated rates, to concentrations as low as 200 ppb (75 FR
35546, June 22, 2010). In denying the petitions for review of that
decision, the D.C. Circuit concluded that the EPA acted reasonably, and
within its discretion, in considering results from the controlled human
exposure studies at concentrations as low as 200 ppb (NEDA/CAP, 686
F.3d at 812-13). In its conclusion that the standard was neither
unreasonable nor unsupported by the record, the D.C. Circuit, noted the
EPA's recognition that statistical significance was not reported for
lung function decrements at that exposure level, and it also cited the
EPA's conclusion that some groups, such as people with severe asthma,
were not included among those studied and could suffer more serious
health consequences from short-term exposures to 200 ppb SO2
(NEDA/CAP, 686 F.3d at 812-13).
---------------------------------------------------------------------------

    \87\ As discussed in the ISA and summarized in the PA, and
recognized in the last review, among individuals with asthma, some
individuals have a greater response to SO2 than other
individuals with asthma or a measurable response at lower exposure
concentrations (ISA, p. 5-14). Data from a study newly available in
this review ``demonstrate a bimodal distribution of airway
responsiveness to SO2 in individuals with asthma, with
one subpopulation that is insensitive to the bronchoconstrictive
effects of SO2 even at concentrations as high as 1.0 ppm,
and another subpopulation that has an increased risk for
bronchoconstriction at low concentrations of SO2'' (ISA,
p. 5-20).
    \88\ Even the study subjects described as having ``moderate/
severe'' asthma would likely be classified as moderate by today's
classification standards (83 FR 26765, June 8, 2018; ISA, p. 5-22;
Johns et al., 2010; Reddel, 2009). The limited data that are
available indicate a similar magnitude of relative lung function
decrements in response to SO2 as that for individuals
with less severe asthma, although the individuals with more severe
asthma are indicated to have a larger absolute response and a
greater response to exercise prior to SO2 exposure,
indicating uncertainty in the role of exercise versus SO2
and that those individuals ``may have more limited reserve to deal
with an insult compared with individuals with mild asthma'' (ISA, p.
5-22). As noted previously, evidence from controlled human exposure
studies are not available for children younger than 12 years old,
and the ISA indicates that the information regarding breathing habit
and methacholine responsiveness for the subset of this age group
that is of primary school age (i.e., 5-12 years) indicates a
potential for greater response (ISA, pp. 5-22 to 5.25).
---------------------------------------------------------------------------

    Three of the commenters, in whose views 400 ppb or 300 ppb is the
lowest SO2 exposure level that the standard

should protect against, stated that the standard of 75 ppb is more
stringent than necessary and advocate revision of the level to a value
no lower than 150 ppb, or a level just below 110 ppb.
    The commenters advocating a level no lower than 150 ppb emphasize
their view that the current standard is more stringent than necessary
because it considers protection against 5-minute SO2
concentrations of 200 ppb and higher rather than only 400 ppb and
higher. They claim that adjusting the focus to one aimed at
concentrations of 400 ppb and higher provides support for a revised
level of 150 ppb and point, without further elaboration, to their
comment submission during the public comment period for the 2010
rulemaking as providing supporting analysis. Similar to the cited
submission from the 2010 rulemaking, the core argument of their current
comments appears to be that the standard does not need to protect
against exposures lower than 400 ppb, and that the EPA should not
consider information about exposures as low as 200 ppb, which they
claim was EPA's focus in its 2009 proposal to set the level for the new
1-hour standard within the range of 50 to 100 ppb. Rather, the
commenters claimed that the EPA should focus only on 400 ppb and that
based on results of analyses presented in the 2009 REA, a standard no
lower than 150 ppb provides comparable protection for the 400 ppb
benchmark as a standard between 50 and 100 ppb was estimated to provide
for the 200 ppb benchmark. For example, the cited 2010 comment
submission stated that the air quality analyses presented in the 2009
REA (based on air quality data for 40 U.S. counties from the late 1990s
through 2007 and an estimated relationship between 1-hour and 5-minute
concentrations, and involving the adjustment of the 1-hour
concentrations to just meet different 99th percentile daily maximum 1-
hour standards) indicates that the range of maximum annual mean number
of days estimated to have 5-minute concentrations at or above 400 ppb
at monitors adjusted to just meet 99th percentile daily maximum 1-hour
standard levels of 150 and 200 ppb (7 to 13 days) was similar to the
number of such days estimated to have 5-minute concentrations at or
above 200 ppb at monitors adjusted to just meet 99th percentile daily
maximum 1-hour standard levels of 50 and 100 ppb (2 to 13 days).
    As an initial matter, as noted above, we do not believe that merely
pointing to a comment or analysis offered during the last review, on
the 2009 proposal, is sufficient to raise a significant comment in this
review, without further description of why the issues raised in the
2010 review are still relevant to the proposal in the current review,
which the commenter has not provided. Additionally, as explained above,
the EPA continues to disagree with the view that the Agency should not
consider the amount of protection provided by the primary
SO2 standard against 5-minute exposures to 200 ppb
SO2 in evaluating the current standard. Further we disagree
with the commenter that the air quality and exposure analyses for
different standard levels presented in the 2009 REA provide an
appropriate basis for considering potential exposures allowed by the
current standard. This is because the air quality and exposures
analyses presented in the 2009 REA are appreciably limited compared to
those available in the current review. The exposure analyses for this
review are extensively improved and expanded over the 2009 analyses, as
summarized in section II.A.3 above, including the fact that they
address the full 3-year period of the standard rather than a single
year of air quality and that they assess the existing standard rather
than standard levels above and below the existing level. Additionally,
the air quality data available in this review are appreciably expanded
since the dataset used in the 2009 REA, such that the current dataset
is much more robust. As just one example of this, the analyses of
frequency of 5-minute concentrations above specific benchmarks at
monitors meeting the current standard have been able to be conducted
with 5-minute measurements rather than 5-minute concentration estimates
as was the case in the last review. These analyses of recent air
quality data indicate that at monitors with concentrations that meet
the current standard, the maximum annual mean number of days with a 5-
minute concentration above 400 ppb was seven (PA, section <IP_ADDRESS>,
Appendix C), a value falling within the range that the 2010 comment had
found acceptable for the what was to be a new 1-hour standard (based on
the then-available data). Thus, putting aside the commenter's view that
no weight should be given to 5-minute SO2 concentrations
below 400 ppb (a view with which we disagree as discussed above), we
note that the air quality analyses available in this review, which
provide a more robust characterization of 5-minute concentrations
occurring in locations meeting the current standard than that estimated
in the 2009 REA, indicate the control of 5-minute 400 ppb
concentrations provided by the current standard to be within with the
commenter's target range. Thus, even if we accepted the premise that
the current standard should be evaluated based solely on the degree of
control of 5-minute 400 ppb concentrations, the basis for the
commenter's concern that the current standard is overly stringent is
not found in the current air quality analyses.
    The comment that advocated revision of the level to a value just
below 110 ppb provides little explanation for this specific alternative
level. Given this commenter's emphasis on 300 ppb as the relevant
benchmark from the controlled human exposure studies (and their view
that EPA inappropriately considered 200 ppb), we interpret this comment
as relating to application of a factor to the existing standard level,
with the factor being derived by dividing 300 ppb (the exposure the
commenter claims should be the focus for the standard) by 200 ppb (the
concentration the commenter claims is the focus of the existing
standard).\89\ This commenter additionally cites several court
decisions in support of EPA standard-setting decisions, two of which
related to the EPA's setting of the level for the PM standard (a
standard established with primary consideration of epidemiologic rather
than controlled human exposure studies) at a concentration which the
commenter describes as ``just below'' concentrations in areas and study
periods for which epidemiologic studies observed a statistical
association with health outcomes.\90\ Thus, we interpret the comment to
suggest that the standard level should be set slightly below the value
resulting from application of the factor of 300 ppb divided by 200 ppb
to the existing standard level of 75 ppb, i.e., the level should be
revised to just below about 110 ppb.
---------------------------------------------------------------------------

    \89\ Multiplying 75 times 300 and dividing by 200 yields a value
of 112.5 which rounds to 110 ppb.
    \90\ We agree with the comment states that an approach of
setting standard levels below concentrations associated with
statistically significant associations with negative health effects,
such as in prior PM NAAQS reviews, has been upheld on judicial
review. We additionally note, however, that caselaw, including that
associated with challenges to the current SO2 standard,
makes clear that EPA has discretion in the approach it uses to set
standard levels, provided it has presented a reasonable rationale
that is supported by the record (NEDA/CAP, 686 F.3d at 813).
---------------------------------------------------------------------------

    The EPA disagrees with the implication of the comment that the
relevant basis for the primary standard level stems or should stem from
a simple proportional relationship between the level of the 1-hour
standard and the magnitude of the 5-minute concentration for which
protection should be provided. Rather, consistent

with the requirements of CAA sections 108 and 109 and the caselaw
interpreting these provisions, as discussed in detail in section I.A
above, the level of the standard, and the standard itself (as a
reflection of its elements collectively), should be firmly based on the
evidence in the review and other relevant considerations, such as
consideration of the strengths and limitations of the evidence
base.\91\ The commenter provides no explicit rationale for why they
consider such a proportional relationship to be appropriate and have
not provided a clear explanation, based on health effects evidence or
exposure/risk information, for the value of 110 ppb. Further, even if
the commenter intends to imply that if the relevant 5-minute benchmark
of concern is increased by a factor (e.g., 150%), then the appropriate
level for the 1-hour standard should also be increased by the same
factor, the commenter provides no evidence for this assumption and the
EPA is aware of none. Thus, the EPA disagrees with these comments that
the level of the standard should be raised to 110 (or just below that
value) or 150 ppb.
---------------------------------------------------------------------------

    \91\ For example, in Mississippi, 744 F.3d at 1352-53, the D.C.
Circuit concluded that EPA had reasonably explained the limitations
of the scientific evidence in determining the level of the 2008
ozone NAAQS.
---------------------------------------------------------------------------

    As summarized in section II.A.1 above, the existing standard, with
its level of 75 ppb, was established in 2010 based on consideration of
the level of protection provided from short exposures to peak
concentrations of SO2, as indicated from the REA results
available at that time for standard levels above and below 75 ppb, as
well as judgments of an adequate margin of safety in light of
concentrations in a set of epidemiologic studies that found
statistically significant associations of SO2 concentrations
with respiratory health outcomes when using copollutant models with PM.
Review of the current standard is based on the health effects evidence
and exposure and risk information now available, including the exposure
and risk estimates for air quality scenarios in which the current
standard is just met (which were not available at the time the standard
was set). Based on all of the currently available information, the
Administrator has concluded that the current standard (in all of its
elements) remains requisite to protect public health with an adequate
margin of safety (as discussed in section II.B.4, below), and that a
less stringent standard would not provide adequate protection.
    The commenters who stated that the percentile aspect of the form of
the standard should be revised to be the 98th percentile rather than
the current 99th percentile based their rationale primarily on their
views that either 300 ppb or 400 ppb is the lowest exposure level that
should be considered in evaluating the protection provided by the
standard. These commenters state that the EPA's 2010 selection of the
99th percentile was based on the Agency's conclusion regarding the
greater effectiveness of a 99th percentile form than a 98th percentile
form with regard to controlling 5-minute concentrations at and above
200 ppb. These commenters generally state that with a change in focus
to one that considers only the protection provided from exposures at
and above either 300 ppb or 400 ppb (a change that they advocate), a
98th percentile form would provide effective control of the relevant 5-
minute concentrations. Additionally, beyond the disagreement with the
EPA about the need to protect at-risk populations from exposures below
300 ppb or 400 ppb (addressed above), the commenters variously cite the
following reasons for such a revision in form: (1) The view that a 98th
percentile would provide greater regulatory stability than a 99th
percentile form; and (2) a claim that EPA's choice of a 99th percentile
form in 2010 was inappropriately based in part on concentrations in
three U.S. epidemiologic studies and in part on EPA's air quality
analyses of the effectiveness of control of 5-minute
concentrations.\92\
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    \92\ The commenter making this claim additionally states that
the EPA has not to date provided an explanation of why a 99th
percentile form would be more effective than a 98th percentile form
in providing such control.
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    With regard to the first reason, the issue of regulatory stability
was considered by the EPA in selecting the 99th percentile form when
the standard was established in 2010. As described in the last review,
analyses in the 2009 REA indicated that over a 10-year period, there
appeared to be little difference in the stability of design values
based on a 98th or 99th percentile form, leading the EPA to conclude at
that time that there would ``not be a substantial difference in
stability between 98th and 99th percentile forms'' (75 FR 35540, June
22, 2010; 2009 REA, section 10.5.3). Further, the commenter provides no
alternative analysis to support their view that the 98th percentile is
more stable; nor do they provide any reasoning or analysis that would
demonstrate a flaw in the EPA analysis or conclusions. Thus, we are not
aware of any basis for the view that a 98th percentile form would offer
greater stability.
    With regard to the second reason, as an initial matter, we note
that the question of whether the 99th percentile form was appropriately
adopted in 2010 is a question that the EPA resolved in the last review,
and one that is not before us in this review.\93\ However, to the
extent that the comment is intended to suggest that we should not
retain the 99th percentile form in this review based on the objections
raised in the comments, we respond as follows. First, we find the
commenter to be mistaken in their assertion that the EPA's choice of
the 99th percentile for the percentile aspect of the form in setting
the current standard relied on specific concentrations in three U.S.
epidemiologic studies. In making this assertion, the commenter
incompletely paraphrases a statement in the proposal for this review
regarding the elements of the 2010 standard and the Administrator's
judgment that this standard would provide the requisite protection for
at-risk populations against the array of adverse respiratory health
effects related to short-term SO2 exposures, including those
as short as 5 minutes (83 FR 26756, June 8, 2018) and then incorrectly
relates the EPA's 2010 judgment on form for the standard to a statement
in the proposal in the current review that summarized 99th percentile
daily maximum 1-hour concentrations \94\ in a set of U.S. studies for
which the SO2 effect estimates remain positive and
statistically significant in copollutant models with PM (83 FR 26765,
June 8, 2018). The disconnected statements cited by the commenter do
not refer to the EPA's rationale in setting the form for the current
standard or its rationale in the proposal in this review to retain the
current standard without revision. Rather, the basis for the form for
the current standard, and rationale in this review, is summarized in
sections II.A.1 and II.B.3 of the proposal (83 FR 26760, 26782, June 8,
2018) \95\ and in sections

II.A.1 and II.B.1 above. Briefly, the statistical form of the current
standard is based on consideration of the health effects evidence,
stability in the public health protection provided by the programs
implementing the standard, and advice from the CASAC, as well as
results of air quality analyses in the 2009 REA for alternative
standard forms (75 FR 35539-41, June 22, 2010). Because the premise of
the comment is mistaken, it does not provide grounds to conclude in
this review that the 99th percentile form is inappropriate.
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    \93\ The EPA has not reopened the last review in this action.
    \94\ The commenter additionally states their view regarding
comparison of 99th and 98th percentiles of daily maximum hourly
concentrations in these epidemiologic studies (which variously
differed by some 10 to 20%) that there is little if any statistical
difference between them, although no statistical analyses were
submitted in support of this view.
    \95\ The relevant section in the Federal Register notification
of proposed decision for this review begins with the phrase ``[w]ith
regard to the statistical form for the new 1-hour standard.'' This
section is a summary of the section titled ``Conclusions on Form''
in the 2010 Federal Register notification of final decision (75 FR
35541, June 22, 2010). While the Administrator's conclusion on form
for the current standard considered the need to limit the upper end
of the distribution of SO2 concentrations in ambient air
to provide protection with an adequate margin of safety against
effects reported in both epidemiologic and controlled human exposure
studies, the choice of 99th percentile over 98th percentile was not
based on specific epidemiologic study concentrations. Rather, in
considering the epidemiological evidence in her decision on standard
level, the Administrator considered SO2 concentrations in
three specific epidemiologic studies (as summarized in II.A.1 above)
in terms of the 99th percentile in light of her selection of that
percentile for the standard form (75 FR 35547, June 22, 2010).
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    With regard to the comment about the 2009 REA air quality analyses
in the 2010 review, the analyses found a 99th percentile form to be
appreciably more effective at limiting 5-minute peak SO2
concentrations than a 98th percentile form (75 FR 35539-40, June 22,
2010; 2009 REA, section 10.5.3, Figures 7-27 and 7-28). To the extent
that the commenter intended to assert that it is inappropriate to
retain the 99th percentile based on objections to this analysis or its
consideration in establishing the form of the standard, we disagree.
While the comment notes the findings of these air quality analyses and
the fact that a 98th percentile form would allow appreciably more days
per year with 5-minute concentrations above 400 ppb and 200 ppb, it
claims that the EPA's conclusion in the last review of greater
effectiveness was arbitrary and misplaced for four reasons, three of
which refer to aspects of epidemiologic studies and one which appears
to point to the controlled human exposure studies stating that
statistically significant findings at the study group level have not
been found for exposures to short-term SO2 concentrations
below 300 ppb. As above, we note that any challenges to whether the EPA
reached the appropriate conclusions in the last review are not properly
before us in this review, as this is a new review of the current
standard based on the current record and the EPA did not reopen the
last review in this action. However, to the extent that the comment is
intended to suggest that we should not retain the 99th percentile form
in this review based on these four reasons, we respond as follows. As
the epidemiologic studies were not identified as a factor in the EPA's
2010 decision on the 99th percentile (versus a 98th percentile) form
for the standard (75 FR 35541, June 22, 2010),\96\ and were not
identified as a basis for the proposal in this review to retain the
current standard, without revision, we find the commenter's reasons
related to epidemiologic studies to have no relevance to our decision
here. With regard to statistical significance of study subject
responses below 300 ppb, putting aside our disagreement with the
comment about the need to protect at-risk populations from exposures
below 300 ppb (addressed above), we note that the air quality analyses
relied on in the 2010 decision also demonstrated greater control of 5-
minute concentrations above 300 (at 400 ppb) by the 99th percentile.
Further, the comment also does not provide any reason for why a 98th
percentile would be a more appropriate form. Accordingly, we find the
comment lacks a sound basis for any claim that the form of the standard
is arbitrary and misplaced or should not be retained. Therefore, we
conclude that this comment does not call into question the
appropriateness of the form of the current standard.
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    \96\ The EPA's consideration of epidemiologic studies in its
2010 decision on the specific percentile for the form for the
standard was with regard to the appropriateness of a percentile
above the 90th, and not, as implied by the commenter, with regard to
the selection of the 99th percentile (e.g., as compared to the 98th
percentile). Specifically, the Administrator at that time noted
that, in line with the controlled human exposure study findings of
effects from peak concentrations, some of the epidemiologic studies
described in the 2008 ISA reported an increase in SO2-
related respiratory health effects at the upper end of the
distribution of ambient air concentrations (i.e., above 90th
percentile SO2 concentrations; see ISA, section 5.3, p.
5-9). Accordingly, the Administrator concluded that the form of a
new 1-hour standard should be especially focused on limiting the
upper end of the distribution of ambient SO2
concentrations (i.e., above 90th percentile SO2
concentrations) in order to provide protection with an adequate
margin of safety against effects reported in both epidemiologic and
controlled human exposure studies (75 FR 35541, June 22, 2010).
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    We also disagree with these commenters that a 98th percentile form
would provide effective control of short exposures to peak
SO2 concentrations, for either exposures at and above 200
ppb or exposures to the still higher concentrations on which the
commenters prefer to focus (at and above 300 ppb or 400 ppb). In this
regard, we note as an initial matter the EPA analysis on which the 2010
conclusion is based (summarized immediately above); that analysis,
presented in the 2009 REA ``indicated 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'' (75 FR 35540, June 22, 2010; 2009 REA, section
10.5.3, Figures 7-27 and 7-28). Further, we describe here a set of
additional analyses of more recent air quality performed in the current
review, the results of which support that conclusion in this review
(Solomon et al., 2019). From these analyses of air monitoring data at
337 monitoring sites in the U.S., it can be seen that, compared to the
current 99th percentile standard, a standard with an alternative 98th
percentile-based form exerts less control of 5-minute peaks. For
example, during this recent time period (2014-2016), there were three
times as many 5-minute daily maximum concentrations at or above 400
ppb, 24 times as many such concentrations at or above 300 ppb, and more
than 25 times as many such concentrations at or above 200 ppb at sites
meeting an alternative 98th percentile standard as at sites meeting the
current standard with its 99th percentile form (Solomon et al., 2019,
Tables 1 and 2).
    Thus, together, the stability analyses documented in the 2010
review and the analyses of more recent air quality demonstrate that the
98th and 99th percentile forms have similar stability, and that a
standard revised to have a 98th percentile form provides appreciably
less control than the current standard, both with regard to 5-minute
concentrations above 400 ppb and 300 ppb, and also such concentrations
above 200 ppb. The CASAC similarly concluded that the 99th percentile
form is preferable to a 98th percentile form to limit the upper end of
the distribution of 5-minute concentrations (Cox and Diez Roux, 2018b,
p. 3 of letter). Accordingly, a standard with a 98th percentile-based
form would provide less protection than that provided by the current
standard from peak SO2 concentrations, even from those at or
above 400 ppb or 300 ppb, the concentrations that the commenters state
are appropriate for the standard to provide protection from.
Additionally, as discussed in section II.B.4 below, the Administrator
considers it appropriate for the primary SO2 standard to
control 5-minute concentrations at and above 200 ppb, as well as those
at and above 400 ppb, and considers the current standard, with the
current form, to provide requisite protection from exposures to such
concentrations. Thus, the EPA disagrees with the commenters and, for
the reasons described above, finds that a revised standard with a 98th

percentile-based form would not provide the desired control of 5-minute
concentrations at and above either 200 ppb or 400 ppb, nor the
appropriate protection from the exposures associated with such
concentrations.
    Three commenters that recommended revision of the standard to be
less stringent stated that, even when focused on limiting exposures at
and above 200 ppb, the current standard is overly protective. These
commenters recommended either revision of the averaging time or of the
form, each claiming that their recommended revision, accompanied by no
change to any other element of the standard, would still achieve
adequate protection from exposures at or above 200 ppb. We address
these comments in turn below.
    The commenter that recommended revising the averaging time of the
standard, stated that a standard with an averaging time of 3 hours, 8
hours, or 24 hours, and keeping all other elements of the current
standard the same (including the level of 75 ppb, and the form that
involves averaging annual 99th percentile daily maximum concentrations
across a three consecutive period), would still be protective of a peak
5-minute 200 ppb concentration, and would provide regulatory stability.
In support of this position, this commenter submitted a statistical
analysis of SO2 data from a subset of ambient air monitors
in the U.S. The commenter's dataset was limited to 16 monitors located
within 1 km of SO2 emissions sources with greater than 4,000
tons per year of reported SO2 emissions in the 2014 NEI; it
included at most only 18 months of data from these monitors, and fewer
data from some monitors. From the limited data available for these
monitors, most of which do not yet have 3 full years of data from which
to calculate a valid design value for the current standard, the
commenter identified the 1-hour, 3-hour, 8-hour, and 24-hour periods in
which the average concentrations were less than 75 ppb, and counted the
number of times a 5-minute concentration within those periods was at or
above 200 ppb. The commenter then summarized the results in terms of
the percentage of the 1-hour, 3-hour, 8-hour or 24-hour periods with
average concentrations less than 75 ppb that included a 5-minute
concentration at or above 200 ppb. The commenter, while noting that the
percentages were higher for longer periods than for shorter periods,
claimed that this limited dataset covering 18 or fewer months
demonstrated that even a standard with a 24-hour averaging time would
be protective of 5-minute SO2 concentrations at and above
200 ppb.
    We disagree with the commenter that their analysis is adequate to
judge the level of control that the existing standard exerts over 5-
minute concentrations of potential concern, much less to judge the
protection provided by the current standard against exposures
associated with respiratory effects in people with asthma or the
adequacy of that protection. The commenter's analysis focuses on a
dataset that by definition is biased to underestimate the occurrences
of 5-minute concentrations at or above 200 ppb. First, by limiting the
analysis to 18 months or less, the commenter's analysis did not include
3 years of data that would allow for judgment of whether or not the
monitors included met the current standard or any of the suggested
alternatives. Over a timeframe longer than that provided by the
commenter, there would be opportunity for more peak 5-minute
concentrations at or above 200 ppb. Given the lack of three full years