of data to determine whether the monitor met the standard at the
locations for which the commenter provided data, it is not possible to
evaluate the protectiveness of the current standard or the suggested
alternatives at these monitoring locations. Further, the commenter
focused their statistics only on hours (or 3-hour, 8-hour or 24-hour
periods) for which the average concentrations were at or below 75 ppb.
Yet given the form for the current standard, a 3-year period at a
location that meets the current standard (or the commenter's
alternatives) could also include hours (or 3-hour, 8-hour or 24-hour
periods) above 75 ppb, along with the associated 5-minute
concentrations. Lastly, the commenter's analysis summarizes the
occurrences of 5-minute concentrations at or above 200 ppb in terms of
percentages (of hours at or below 75 ppb), rather than the number of
occurrences during a year or the full 3-year period. This framing of
their analysis precludes a consideration of the frequency of such peak
concentrations at monitors meeting the standard. The frequency is an
appropriate consideration because increasing frequency would directly
relate to increasing potential for exposure to such peak
concentrations, while percentage of a subset of the hours cannot be
interpreted with regard to such a relevant consideration.
    Accordingly, in considering the commenter's view that an
alternative averaging time would still be protective of exposures to 5-
minute concentrations at or above 200 ppb, the EPA conducted an
analysis that, like the commenter's analysis, focused on SO2
monitoring sites located within 1 km of emissions sources with greater
than 4,000 tons per year of reported SO2 emissions according
to the 2014 NEI, but that also included three complete years of data
for each site, consistent with the form of the current standard
(Solomon et al., 2019).\97\ Further, the EPA analysis summarizes the
frequency of occurrences of 5-minute concentrations at or above 200 ppb
and does this for those monitoring locations that meet the current
standard, and also at those that would meet an alternative 3-hour, 8-
hour, or 24-hour standard (with a level of 75 ppb) \98\ (Solomon et
al., 2019, Tables 5 through 8). At sites that would meet standards with
such alternative averaging times, there were many more 5-minute daily
maximum SO2 concentrations at or above 200 ppb than at sites
that meet the current standard, in many instances 20 to 200 times more.
(Solomon et al., 2019, Tables 5 through 8). This relates in part to the
fact that more sites meet the alternative standards than the current
standard due to the lesser stringency of a standard with a longer
averaging time that has the same level as the current standard.
Additionally, however, when evaluating 5-minute concentrations on a
per-monitor basis, it can also be seen that as many as 15, 29, and 144
times more 5-minute daily maximum SO2 concentrations at or
above 200 ppb are allowed to occur at monitors that would meet an
alternative standard with a 3-hour, 8-hour or 24-hour averaging time,
respectively, compared with only two at the monitor meeting the current
standard (Solomon et al., 2019, Table 9). Thus, it can be seen even
from this analysis of the small number of sites near very large
emissions sources (>4,000 tons per year in 2014 NEI), that a standard
with a longer averaging time (and the level of 75 ppb) would provide
less public health protection than that provided by the current 1-hour
standard. We additionally note that the focus for the commenter
analysis on monitors near sources emitting 4,000 or more tons per year
as of 2014 yields an analysis focused on a small percentage

of all monitors in the U.S. Although this may capture monitors near
(within 1 km of) the largest sources in the U.S., it does not
necessarily capture areas with the highest SO2
concentrations that still meet the current (and the commenter's
alternative) standard. For example, an analysis in the PA of all the
monitors meeting the current standard documents a monitor with as many
as 32 days per year having a 5-minute concentration at or above 200 ppb
(PA, p. 2-12 and Appendix C, Figure C-2). Thus, we find the commenter's
analysis to be insufficient to examine the implications for public
health protection of a revised averaging time. Based on the more
complete analyses we have conducted with recent air quality data from
across the U.S., which is focused on the locations near large sources
consistent with the commenter analysis and where peak concentrations
would be expected to be more frequent, we find that a longer averaging
time, as advocated by the comment, would be appreciably less effective
at limiting 5-minute ambient air concentrations at and above 200 ppb,
and also at and above 400 ppb, and, consequently, would be expected to
provide a lesser level of protection of at-risk populations from
exposure to such concentrations.
---------------------------------------------------------------------------

    \97\ The resulting set of 3-year data included six monitoring
sites, with five of these also included in the commenter's 1-year
dataset (Solomon et al., 2019). Three years of data were not
available for any of the other monitors in the commenter's dataset.
    \98\ The Solomon et al. (2019) analysis derived DVs at each
monitoring site based on the three alternative averaging times cited
by the commenter. Then it sorted and binned the sites based on
whether the design value was above or below a level of 75 ppb (which
commenters stated to be the level for their preferred alternative
standard).
---------------------------------------------------------------------------

    Three commenters recommended revising the form of the standard to
remove the focus on daily maximum 1-hour concentrations. They
recommended revising the form of the standard to one based on all 1-
hour average concentrations (versus the daily maximum 1-hour average
concentrations). They claimed that a standard with such a revised form,
yet otherwise identical to the existing standard, would still be
protective against short-term SO2 exposures at or above 200
ppb. These commenters stated that a standard with such a form would be
preferable to the current standard as it would consider the
concentrations of all hours in a year (including multiple hours in any
day) in judging attainment with the standard rather than considering
only the highest 1-hour concentrations per day within the year. In
supporting materials for this comment, the commenters provide an
example in which the fourth highest daily maximum 1-hour concentration
\99\ in 2 years of the 3-year evaluation period for the standard is
above 75 ppb, while this concentration in the third year is well below
75 ppb such that the current standard might be met. In the two high
years in the example, the commenters note that if all hours in the 4
days are above 75 ppb, then 96 hours (24 hours in each of the 4 days)
would be above 75 ppb. Yet they claim that their example would only
allow 88 hours above 75 ppb for their preferred alternative form. As
the premise of their example is that there may be much higher
concentrations in two of the three years, however, it is unclear why
they claim only 88 hours above 75 ppb would be allowed by their
preferred alternative. If the 3rd year is suitable low, there could be
many more than 88 hours above 75 ppb and still meet their alternative
standard. The commenters additionally provided observations related to
ambient air monitoring data for 2011-2013 at monitors within the three
REA study areas, and observations from a year of ambient air monitoring
data at two monitors near aluminum smelters, stating that such
observations supported their view regarding the protectiveness of a
standard with a 99th percentile hourly form.
---------------------------------------------------------------------------

    \99\ When measurements are available for all hours in a year,
the 99th percentile of the 8760 hours in a year is 88, while the
99th percentile of 365 days in a year is four (and there are 96
hours in 4 days).
---------------------------------------------------------------------------

    We disagree with these commenters' claims. As an initial matter, we
find the commenters' example to be incorrect given its dependence on
the specific scenario created by the commenter. We note that there are
many other distributions of hourly concentrations across 3 years that
could meet a design value of 75 ppb in which the total number of hours
greater than 75 ppb is greater for the commenter's preferred
alternative standard. Given the 3-year average aspect of the current
form, the simplest example is one based on the average year. In order
to meet the current standard in an average year, only 3 days (and at
most the associated 72 hours) can have a daily maximum 1-hour
concentration above 75 ppb because the 4th daily maximum 1-hour
concentration could be no higher than 75 ppb. If the average year has a
99th percentile equal to 75 ppb (and consequently just meets the
current standard), there could be no more than 72 hours above 75 ppb in
each of the 3 years (3 days times 24 hours per day). Yet as the 99th
percentile of the 8760 hours in a year is 88, an alternative standard
with a 99th percentile hourly form could be met with 87 1-hour average
concentrations above 75 ppb--15 more hours than that allowed by the
current standard. Further, if the hours above 75 ppb in the average
year all occurred on separate days, the commenter's alternative
standard would allow there to be 87 days with a 1-hour concentration
above 75 ppb, while the current standard allows there to be only 3 such
days. Thus, a standard with a 99th percentile hourly form (rather than
a form based on the 99th percentile of daily maximum 1-hour
concentrations) would allow there to be many more days with an hour
above the level of the standard (87 compared to 3). Given the
variability in 1-hour SO2 concentrations that is common near
sources (e.g., 95th percent confidence intervals on mean hourly
concentrations at six locations indicate hourly variation can be a
factor of two and greater [ISA, Figure 2-23]), such a consideration is
relevant. Additionally, the health effects evidence indicates a greater
response associated with exposures that are separated in time compared
to those that are close in time.\100\ Together, these observations
based both in the air quality data and in the health effects evidence
increase the importance of exposures on separate days versus those in
consecutive hours. Further, presentations in the PA of recent air
quality data demonstrate the control of peak 5-minute concentrations
exerted by a standard based on daily maximum 1-hour concentrations (PA,
Appendix B).
---------------------------------------------------------------------------

    \100\ As noted in section II.C.2 of the proposal (83 FR 26771,
June 8, 2018) and section II.A.2 above, 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 a similar effect as that of the initial
exposure event (REA, Table 6-3).
---------------------------------------------------------------------------

    In the commenters' analysis of data from monitors in the three REA
study areas, they failed to recognize that all but one of these
monitors had design values based on the current standard that were at
or below 75 ppb (i.e., the data for only one monitor violated the
NAAQS). While the commenters emphasized the few 5-minute concentrations
above benchmarks across all of these monitors (five occurrences above
200 ppb across these seven monitors), we note that such a low number of
elevated peak concentrations would be expected at monitors meeting the
current standard. We additionally note that as shown in the commenters'
submission there were seven occurrences of 5-minute concentrations
above 200 ppb at the single monitor location for which the 2011-2013
data did not meet the standard. Together, we find this dataset,
although very limited, documents a degree of control of peak
concentrations by the current standard.
    In order to more thoroughly assess the commenter's assertion that
their preferred alternative hourly form would provide similar
protection from 5-minute exposures at or above 200 ppb

as the current standard, we performed two analyses, the first focused
on the REA study areas and the second involving air quality data at
monitors nationwide. As the exposure and risk estimates for the three
REA study areas indicate the level of protection in these areas for the
air quality scenario just meeting the current standard,\101\ we
analyzed the estimated concentrations in this scenario for each study
area to determine what the design value for a standard with the
commenters' preferred alternative form (the 99th percentile of all
hours in a year, averaged over 3 years). We found that such a design
value in each study area would be below 75 ppb, with variation from 31
ppb to 65 ppb across the three areas related to the different temporal
and spatial patterns of concentrations in those areas (Solomon et al.,
2019, Table 10). This finding of lower design values (e.g., as low as
31 ppb) for a standard with such an alternative form indicates that
such a form is less stringent and that to achieve similar protection
against peak SO2 exposures in the three areas, such an
alternative SO2 standard would require a standard level
lower than 75 ppb. Additionally, looking at unadjusted concentrations
across all U.S. monitoring sites in 2014-2016, the relationship between
design values for the current standard and design values for an
alternative standard with an hourly-based form (versus one based on
daily maximum 1-hour concentrations) is seen to be approximately two to
one, indicating that the SO2 level associated with U.S. air
quality summarized in terms of the commenter's preferred alternative
form is one half the level for air quality summarized in terms of the
current standard (Solomon et al., 2019, Figure 1). Thus, these
additional analyses of adjusted air quality in the REA study areas and
of the recent unadjusted ambient air monitoring data indicate that to
achieve comparable protection of 5-minute exposures of concern, an
alternative standard with a form based on the 99th percentile of all 1-
hour concentrations in each year of the 3-year period (rather than the
99th percentile of daily maximum 1-hour concentrations) would need to
have a level appreciably lower than 75 ppb (Solomon et al., 2019).
---------------------------------------------------------------------------

    \101\ This scenario was developed through adjustments of the
hourly air quality data as described in section II.A.3.a above and
described in detail in sections 3.4 and <IP_ADDRESS> of the REA.
---------------------------------------------------------------------------

    One of these commenters provided an analysis of ambient air
monitoring data to demonstrate that an alternative standard that
retains the level of 75 ppb yet revises the form to be based on the
99th percentile of all 1-hour concentrations in each year of the 3-year
period would be protective of short-term exposures to 200 ppb
SO2. We find the commenter's analysis to be inadequate to
support this position. This analysis is limited to just two monitors at
the fenceline of an aluminum smelter facility. The NAAQS are national
standards and must provide protection across all sites in the U.S.
Moreover, the current standard is averaged over 3 years, but the
commenter's analysis only includes 1 year of data. Thus, to consider
the commenter's position using a more comprehensive dataset, we
analyzed ambient air monitoring data for SO2 at the 337
monitoring sites that met the completeness criteria for the recent 3-
year period, 2014-2016. For monitors meeting the current standard and
then for monitors meeting an alternative standard with an hourly form,
we counted the number of 5-minute daily maximum concentrations at or
above 200 ppb in each year. Across the 3-year period, for the 318
monitors meeting the current standard, there were 93 5-minute daily
maximum concentrations at or above 200 ppb (Solomon et al., 2019, Table
1). There were more than six times as many such 5-minute concentrations
across the same 3-year period at the 335 monitors meeting an
alternative hourly standard (Solomon et al., 2019, Table 3). These
results demonstrate that revision of the form to establish an
alternative hourly standard, contrary to the assertion by the
commenter, would result in a substantial reduction in control of 5-
minute concentrations at or above 200 ppb and an associated reduction
in protection from exposures to such concentrations.
    One of the commenters that recommended consideration of a revised
standard with a form based on the 99th percentile of all 1-hour
concentrations in each year of the 3-year period additionally
recommended that, if the EPA does not revise the form of the standard
in such a way, the EPA should instead include a second level of
evaluation of monitoring data in judging attainment of the standard.
The commenter explained that, under this second level of evaluation,
the EPA would not judge a monitoring site to exceed the NAAQS if the 5-
minute data for that site do not include concentrations at or above 200
ppb. The framework recommended by the commenter provides that only
those hours in which there is at least one 5-minute average
concentration above 200 ppb (or the subset for which the 1-hour
concentration is also above 75 ppb) would be used to determine whether
a monitoring site exceeded the NAAQS.\102\ The commenter claimed that
data for monitors included in the REA study areas, and their limited
analysis of 12 months of data at two monitoring locations, provided
support for their position by indicating few or no 5-minute
concentrations above 200 ppb during hours with average concentrations
above 75 ppb. The commenter concluded, based on their analysis, that
the current standard ``is more stringent than is requisite to protect
public health'' since their limited dataset includes hours with 1-hour
concentrations above 75 ppb and in which there are not any 5-minute
concentrations at or above 200 ppb. The commenter further suggests that
areas may be found in non-attainment of the 2010 NAAQS even if there is
not a single 5-minute concentration at or above 200 ppb.
---------------------------------------------------------------------------

    \102\ This comment submission includes inconsistent criteria for
inclusion of data for judging compliance with the standard. In one
place, the commenter suggests that only those hours with an average
concentration at or above 75 ppb which also have a 5-minute
concentration at or above 200 ppb would be included. Elsewhere, the
commenter suggests that any hour--regardless of the average 1-hour
concentration--that has a 5-minute concentration at or above 200 ppb
would be included. Further, the commenter does not then make clear
how the data included in this more limited dataset would be
evaluated when judging attainment of the standard. For example, the
current requirements for deriving design values for judging whether
a site violates the standard specify completeness criteria for the
dataset (see appendix T to part 50).
---------------------------------------------------------------------------

    We disagree with the commenter's assertion that the absence of 5-
minute SO2 concentrations at or above 200 ppb at the two
monitoring locations in their 12-month dataset shows that the current
standard is more stringent than necessary. Examining a more extensive
dataset demonstrates issues in the commenter's premise: Monitors
exceeding the current standard also have 5-minute SO2
concentrations at or above 200 ppb (Solomon et. al, 2019, Table 1).
Given the insufficiency of the commenter's dataset for reaching
conclusions with regard to air quality nationally under the current
standard, we investigated the frequency of 5-minute concentrations at
or above 200 ppb at monitoring sites nationally. In this analysis, we
reviewed the data for all 337 monitoring sites meeting completeness
criteria for a recent three-year period, 2014-2016 (documented in the
PA, Appendix A). The data across these 3 years at all 19 monitors that
do not meet the current standard include occurrences of 5-minute
SO2 concentrations at or above 200 ppb (Solomon et al.,
2019, Table 4). Further

we note that these concentrations occur in some 1-hour periods with
average concentrations above 75 ppb and also in some 1-hour periods
with average concentrations below 75 ppb, while the commenter appears
to limit their focus only to hours with average concentrations above 75
ppb. Further, analyses of these data in the PA demonstrate the
reduction of 5-minute concentrations above 200 ppb and higher
benchmarks achieved by the current standard (PA, section <IP_ADDRESS> and
Figure C-5). These analyses do not indicate overcontrol of 5-minute
concentrations; for example, among sites meeting the current standard,
as many as 32 days per year were recorded with a 5-minute concentration
at or above 200 ppb, and as many as 7 days per year with a 5-minute
concentration at or above 400 ppb (PA, section <IP_ADDRESS> and Figure C-5).
Thus, the commenter's position that the current approach to judging
attainment (based on a valid design value at or below 75 ppb) is overly
stringent in its control of 5-minute concentrations at and above 200
ppb is not supported by a comprehensive analysis of the available data
across the U.S.
    Although the comments do not make clear the exact inclusion
criteria for data or the exact calculations they are advocating be
applied in the second level of evaluation for judging attainment, such
a second level evaluation would appear to allow the designation of
areas as attaining the current standard when the areas do not meet the
standard. As specified under the Clean Air Act, primary ambient air
quality standards are those the attainment and maintenance of which are
judged requisite to protect public health with an adequate margin of
safety. The elements of the current standard include the highest daily
1-hour concentrations, not the highest 5-minute concentrations. To
apply a second level of data evaluation for purposes of determining
attainment that is based on consideration of 5-minute concentrations
would have the effect of changing the standard itself rather than
evaluating attainment with the existing standard. Thus, we disagree
with the commenter that such an evaluation could be adopted for judging
attainment without effecting a change to the standard itself.
d. Other Comments
    Comments on topics not directly related to consideration of the
current primary standard included recommendations for addressing data
gaps and uncertainties to inform future reviews. We agree with many of
these suggestions and note that the PA highlighted key uncertainties
and data gaps associated with reviewing and establishing NAAQS for
SO2 and also areas for future health-related research, model
development, and data gathering. We encourage research in these areas,
although we note that research planning and priority setting are beyond
the scope of this action.
    The EPA also received several comments related to implementation of
the primary SO2 NAAQS, including comments concerning the use
of AERMOD for estimating 1-hour concentrations versus concentrations
over longer time periods, and comments citing facilities' difficulty
demonstrating compliance with the 1-hour SO2 standard. We
are not addressing those comments here because, as described in section
I.A above, this action is being taken pursuant to CAA section 109(d)(1)
and relevant case law. Additionally, consistent with this case law, the
EPA has not considered costs associated with attaining the standard as
a part of this review, including the costs or economic impacts related
to permitting or other implementation concerns, in this action
(Whitman, 531 U.S. at 471 & n.4). Under CAA section 109(d)(1) the EPA
has the obligation to periodically review the air quality criteria and
the existing primary NAAQS and make sure revisions as may be
appropriate. Accordingly, the scope of this action is to satisfy that
obligation; it is not to address concerns related to implementation of
the existing standard. State and federal SO2 control
programs, such as those discussed in section I.D, may provide an
opportunity for permitting and other implementation concerns to be
addressed. For example, in light of public comments suggesting
potential unintended consequences for areas with low peak-to-mean
SO2 concentrations, the EPA intends to continue to work
closely with the relevant air agencies for these areas in implementing
the standard, building upon its 2014 Guidance for 1-Hour SO2
Nonattainment Area SIP Submissions.\103\
---------------------------------------------------------------------------

    \103\ Available at: https://www.epa.gov/sites/production/files/2016-06/documents/20140423guidance_nonattainment_sip.pdf.
---------------------------------------------------------------------------

4. Administrator's Conclusions
    Having carefully considered the public comments, as discussed
above, the Administrator believes that the fundamental scientific
conclusions on effects of SO2 in ambient air that were
reached in the ISA and summarized in the PA, the air quality analyses
summarized in the PA, and estimates of potential SO2
exposures and risks described in the REA and PA, and summarized above
and in sections II.B and II.C of the proposal, remain valid.
Additionally, the Administrator believes the judgments he proposed to
reach in the proposal (section II.D) with regard to the evidence and
the quantitative exposure/risk information remain appropriate. Thus, as
described below, the Administrator concludes that the current primary
SO2 standard provides the requisite protection of public
health with an adequate margin of safety, including for at-risk
populations, and should be retained.
    In considering the adequacy of the current primary SO2
standard in this review, the Administrator has carefully considered the
policy-relevant evidence and conclusions contained in the ISA; the
exposure/risk information presented and assessed in the REA; the
evaluation of this evidence, the exposure/risk information and air
quality analyses, and the rationale and conclusions presented in the
PA; the advice and recommendations from the CASAC; and public comments,
as addressed in section II.B.3 above. In the discussion below, the
Administrator gives weight to the PA conclusions, with which the CASAC
has concurred, as summarized in section II.D of the proposal, and takes
note of key aspects of the rationale for those conclusions that
contribute to his decision in this review.
    In considering the PA evaluations and conclusions, the
Administrator specifically takes note of the overall conclusions that
the health effects evidence and exposure/risk information are generally
consistent with what was considered in the last review when the current
standard was established (PA, section 3.2.4). In so doing, he
additionally notes the CASAC conclusion that, as the new scientific
information in the current review does not lead to different
conclusions from the last review, the CASAC supports retaining the
current standard (Cox and Roux, 2018b, p. 3 of letter). As noted below,
the newly available health effects evidence, critically assessed in the
ISA as part of the full body of current evidence, reaffirms conclusions
on the respiratory effects recognized in the last review, including
with regard to key aspects on which the current standard is based.
Further, the quantitative exposure and risk estimates for conditions
just meeting the current standard indicate a similar level of
protection, for at-risk populations, as that described in the last
review for the now-current standard. The Administrator also recognizes
limitations and uncertainties that

continue to be associated with the available information.
    With regard to the current evidence, as summarized in the PA and
discussed in detail in the ISA, the Administrator takes note of the
long-standing evidence that has established key health effects
associated with short-term exposure to SO2. This evidence,
largely drawn 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 supports identification of people
with asthma as the population at risk from short-term peak
concentrations in ambient air (ISA; 2008 ISA; U.S. EPA, 1994).\104\ The
available epidemiologic evidence, generally consistent with that in the
last review, provides support for 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. The epidemiologic studies report positive
associations of short-term (i.e., hourly or daily) concentrations of
SO2 in ambient air with asthma-related health outcomes,
including hospital admissions and emergency department visits. In
considering these epidemiologic studies in the context of the larger
evidence base, the Administrator recognizes that, as described in the
ISA, while these studies analyze hourly or daily metrics, there is the
potential for shorter-term peak concentrations within the study area to
be playing a role in such associations. The Administrator further takes
note of the associated uncertainties identified in the ISA related to
potential confounding from co-occurring pollutants such as PM, a
chemical mixture including some components for which SO2 is
a precursor,\105\ and also related to the ability of available fixed-
site monitors to adequately represent variations in personal
SO2 exposure, particularly with regard to peak exposures
(ISA, p. 5-37; PA, section <IP_ADDRESS>; 83 FR 26764, June 8, 2018).
---------------------------------------------------------------------------

    \104\ For people without asthma, such effects have only been
observed in studies of exposure concentrations at or above 1000 ppb
(ISA, section <IP_ADDRESS>).
    \105\ Sulfur dioxide is a precursor to sulfate, which commonly
occurs in particulate form (ISA, section 2.3; U.S. EPA, 2009,
section 3.3.2 and Table 3-2).
---------------------------------------------------------------------------

    With regard to health effects evidence newly available in this
review, the Administrator takes note of the PA finding that, while the
health effects evidence, as assessed in the ISA, has been augmented
with additional studies since the time of the last review, the newly
available evidence 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 recognizes that, as in the
last review, the health effects evidence 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
recognizes that the ISA conclusion on the respiratory effects caused by
short-term exposures is based primarily on evidence from controlled
human exposure studies, also available at the time of the last review,
that document moderate or greater lung function decrements and
respiratory symptoms in people with asthma exposed to SO2
for 5 to 10 minutes while breathing at an elevated rate, and that the
current 1-hour standard was established to provide protection from
effects such as these (ISA, section <IP_ADDRESS>; 75 FR 35520, June 22,
2010).
    With regard to exposure concentrations of interest in this review,
the Administrator particularly takes note of the evidence assessed in
the ISA from controlled human exposure studies that demonstrate the
occurrence of moderate or greater lung function decrements, at times
accompanied by respiratory symptoms, in subjects with asthma exposed
for very short periods of time while breathing at elevated rates,
focusing primarily on the ISA analysis of findings from such studies
for which respiratory response measurements are available to the EPA
for individual study subjects (ISA, Table 5-2 and Figure 5-1; PA, Table
3-1).\106\ These data demonstrate respiratory effects in a percentage
of people with asthma exposed while exercising to SO2
concentrations as low as 200 ppb. Nearly 10% of the study subjects
experienced moderate or greater lung function decrements at this
exposure level and respiratory symptoms were also reported to occur in
some subjects in some studies at the study group level (ISA, Table 5-2;
Linn et al., 1983; Linn et al., 1987). In weighing this evidence, the
Administrator notes the statements from the ATS which continue to
emphasize the importance of the consideration of effects on individuals
with preexisting diminished lung function (ATS, 2000a; Thurston et al.,
2017). Consistent with the ATS characterization of their most recent
statement as ``providing a set of considerations that can be applied in
forming judgments,'' the Administrator notes the importance of
considering whether effects occur in people with diminished reserve,
such as people with asthma, as well as consideration of the magnitude
or severity of effects, the persistence or transience of the effects,
and the potential for repeated occurrences (Thurston et al., 2017).
Thus, as in the last review, when the current standard was set, the
Administrator judges it appropriate to consider the protection provided
by the current standard to the at-risk population of people with asthma
from exposures to peak concentrations as low as 200 ppb while breathing
at elevated rates, while also recognizing the reduced severity of
effects at this exposure level, as was recognized by the Administrator
in the last review.
---------------------------------------------------------------------------

    \106\ The availability of individual study subject data allowed
for the comparison of results in a consistent manner across studies
(ISA, Table-2; Long and Brown, 2018).
---------------------------------------------------------------------------

    The Administrator recognizes that both the percent of individuals
experiencing lung function decrements and the severity of the
decrements, as well as the frequency with which they are accompanied by
symptoms, increase with increasing SO2 concentrations across
the range of exposure levels studied (ISA, Table 5-2; PA, section
<IP_ADDRESS>). For example, while almost 10% of study subjects experienced
moderate or greater lung function decrements at 200 ppb, as noted
above, at exposures of 300 to 400 ppb, as many as approximately 30% of
subjects in some studies experienced moderate or greater decrements (as
defined in section II.A above). Also, while less than 5% of study
subjects exposed to 200 ppb experienced decrements that were greater
than moderate, the percentage experiencing such larger decrements was
nearly 15% and higher in some studies of 300 and 400 ppb (ISA, Table 5-
2). Further, at concentrations at or above 400 ppb, moderate or greater
lung function decrements were frequently accompanied by respiratory
symptoms, such as cough, wheeze, chest tightness, or shortness of
breath, with some of these findings reaching statistical significance
at the study group level (ISA, Table 5-2 and section 5.2.1).
    In considering the potential public health significance of these
effects associated with SO2 exposures, and documented in
studies of individuals with asthma, the Administrator recognizes there
to be greater significance associated with lung

function decrements accompanied by respiratory symptoms and with larger
decrements, both of which are more frequently documented to occur at
exposures above 200 ppb, and also with the potential for greater
impacts of SO2-induced decrements in the much less well
studied population of people with more severe asthma or young children
with asthma, as recognized by the CASAC and summarized in sections
II.A.2.d and II.B.2 above.\107\ For example, he recognizes that health
effects resulting from exposures at and above 400 ppb are appreciably
more severe than those elicited by exposure to SO2
concentrations of 200 ppb (or lower), 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. He also notes that
controlled human exposure studies may be limited or lacking in other
population subgroups identified by the CASAC. Thus, the Administrator
finds it important to consider the protection afforded from
concentrations as low as 200 ppb, particularly in light of limitations
in the evidence base for some population groups, as in the last review
when the standard was set, and also judges it particularly important to
provide a high degree of protection against exposures at and above 400
ppb given the increased prevalence and severity of effects in study
subjects at such exposures.
---------------------------------------------------------------------------

    \107\ The ISA notes that while the 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, p. 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, p. 1-17 and 5-22).
---------------------------------------------------------------------------

    In judging the level of protection afforded by the current
standard, the Administrator turns to the REA, recognizing that health
effects in people with asthma are linked to exposures during periods of
elevated breathing rates, such as while exercising. Accordingly, the
Administrator finds that, as was the case at the time of the last
review, population exposure modeling that takes human activity levels
into account is integral to consideration of population exposures
compared to SO2 benchmark concentrations and of population
lung function risk, and that such consideration is integral to judging
whether the protection afforded by the primary SO2 standard
is requisite. He additionally notes that the populations modeled in the
REA, children and adults with asthma, are those identified as at risk
from SO2 related effects.
    In his consideration of the REA estimates available in this review,
the Administrator recognizes a number of improvements of the current
REA compared to the REA in the last review, including that the current
REA assesses an air quality scenario for 3 years of air quality
conditions adjusted to just meet the current standard.\108\ The current
REA is additionally expanded from the prior one with regard to the
number of study areas in that it now includes three urban areas, each
with populations of more than 100,000 people.\109\ The Administrator
also notes that 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>). The other ways in which the current REA analyses
are improved and expanded from those in the REA for the last review
relate to improvements that have been made to models, model inputs and
underlying databases. These improvements include the database, vastly
expanded since the last review, of ambient air monitoring data for 5-
minute concentrations, as summarized in section II.A.3 above.\110\
While recognizing the differences between the current REA analyses and
the 2009 REA analyses, the Administrator notes the PA finding of a
rough consistency of the associated estimates when considering the
array of study areas in both reviews. He additionally notes the PA
findings that the newly available quantitative analyses comport with
the conclusions reached in the last review regarding the control
expected to be exerted by the now-current 1-hour standard on 5-minute
exposures of concern (83 FR 26775-26776, June 8, 2018).
---------------------------------------------------------------------------

    \108\ In the 2009 REA, the exposure and risk estimates were
analyzed for single-year air quality scenarios for potential
standard levels (50 ppb and 100 ppb) bracketing the now current
level of 75 ppb.
    \109\ In the 2009 REA, there was only one urban study area
included in the analysis.
    \110\ Additional 5-minute monitoring data are available in this
review as a result of the monitoring data reporting requirement
established in the last review to inform subsequent primary NAAQS
reviews for SOX and the associated assessments (75 FR
25567-68, June 22, 2010).
---------------------------------------------------------------------------

    As at the time of proposal, the Administrator finds that when
taking the REA estimates of exposure and risk together, and while
recognizing the uncertainties associated with developing such estimates
for air quality conditions adjusted to just meet the current standard,
the current standard provides a very high degree of protection to at-
risk populations from SO2 exposures associated with health
effects of more clear public health concern, as indicated by extremely
low estimates of occurrences of exposures at or above 400 ppb \111\ and
of lung function risk for multiple days with moderate or greater
decrement as well as for single days with the occurrence of a larger
decrement, such as a tripling in sRaw. In reaching this judgment, the
Administrator notes that the REA results for the three REA study areas
under air quality conditions that just meet the current standard
indicate 99.9% or more of children with asthma, on average across the 3
year period, to be protected from experiencing as much as a single day
per year with an exposure, while breathing at an elevated rate, that is
at or above the benchmark concentration of 400 ppb, an exposure level
frequently associated with respiratory symptoms in controlled human
exposure studies. In so noting, he recognizes the limitations and
uncertainties associated with the REA modeling, including those
associated with simulating temporal and spatial patterns of 5-minute
concentrations in areas near large sources. Moreover, he finds it
important that the REA results do not estimate any children in any of
the three study areas to experience more than one such exposure in a
year for the assessed conditions of air quality that just meets the
current standard. Given the very transient nature of the effects
associated with such short SO2 exposures (as summarized in
section II.A.2.a above), the Administrator gives greater attention to
such findings regarding the potential for multiple (versus single) days
with occurrences of such exposures which he considers an additional
indication of the strength of protection against the occurrence of the
potential for SO2-related health effects. The Administrator
judges these REA estimates for population exposures compared to the 400
ppb benchmark to represent a very high level of protection (at least
99.7% protected from a single occurrence in the highest year and 100%
protected from multiple occurrences) from the risk of respiratory
effects that have been

observed to occur in as many as approximately 25% of controlled human
exposure study subjects with asthma exposed to 400 ppb while breathing
at elevated rates, and that have been accompanied by respiratory
symptoms (PA, Table 3-3; ISA, Table 5-2 and section 5.2.1).\112\ He
additionally notes the similarity of such findings to those considered
by the Administrator in establishing the standard in 2010 in the last
review (as summarized in section II.D.1. of the proposal).
---------------------------------------------------------------------------

    \111\ REA estimates are also extremely low for occurrences of
exposures at or above 300 ppb, the exposure concentration at which
an analysis that is newly available in this review finds
statistically significant differences in response among groups of
individuals with asthma that are responsive to SO2
exposures at or below 1000 ppb (PA, Table 3-3; ISA, p. 5-153).
    \112\ The ISA finds controlled human exposure studies of
exposures at 400 ppb to include stronger evidence (than at lower
concentrations) of the occurrence of respiratory symptoms, with
statistical significance (ISA, Table 5-2).
---------------------------------------------------------------------------

    The Administrator additionally finds the REA estimates for risk of
moderate or greater lung function decrements, in terms of doubling and
tripling of sRaw, to also indicate the current standard to provide a
high level of protection for the simulated at-risk populations,
including specifically the population of children with asthma. With
regard to a doubling of sRaw, the REA results indicate nearly 99% or
more of the at-risk population to be protected from experiencing a
single day per year with this estimated magnitude of SO2-
related response, based on average estimates across the 3-year period,
and 99% or more of this population to be protected from multiple such
days. The REA results indicate still greater protection from a more
severe tripling in sRaw, e.g., more than 99.7% of children with asthma
protected from experiencing a day per year with a SO2-
related tripling of sRaw, based on average estimates across the 3-year
period, and at least 99.8% from experiencing multiple such days per
year in areas with air quality just meeting the current standard. As
with his consideration of the REA estimates for multiple days with
exposures at or above benchmarks and recognizing somewhat lesser
uncertainty in the comparison-to-benchmarks estimates,\113\ the
Administrator finds these lung function risk estimates for multiple
occurrences and for occurrences of days with a tripling of sRaw to also
be informative to his judgment on the appropriateness of the protection
provided by the current standard. Together, the Administrator judges
both sets of REA estimates to indicate that the current standard
provides an appropriately high level of protection from the more severe
and well characterized effects from very short exposures to
SO2, such as those at and above 400 ppb on people with
asthma breathing at elevated rates.
---------------------------------------------------------------------------

    \113\ In considering these estimates, the Administrator
recognizes the quantitative uncertainty discussed in the REA, noted
in section II.A.3.b above and cited in some public comments with
regard to risk estimates associated with exposure concentrations
below those assessed in the controlled human exposure studies.
Accordingly, he recognizes somewhat greater uncertainty associated
with the lung function risk estimates than the comparison-to-
benchmark estimates, and in considering the lung function risk
estimates, places relatively greater weight on the estimates for
occurrences of days with larger decrements (associated with
relatively higher exposure concentrations).
---------------------------------------------------------------------------

    In making this judgment, the Administrator also considers whether
this level of protection is more than what is requisite and whether a
less stringent standard would be appropriate to consider. In so doing,
he first recognizes that a less stringent standard would allow the
occurrence of higher peak SO2 concentrations and a greater
frequency of concentrations above benchmarks of interest, likely
contributing to higher exposures and risks than those estimated by the
REA. That is, a less stringent standard, with its lesser control on
peak SO2 concentrations, would be expected to allow a higher
frequency of ambient air SO2 concentrations at or above
benchmarks of interest, including the 400 ppb benchmark, at which
controlled human exposure studies of exercising people with asthma have
reported nearly 25% of study subjects to experience a moderate or
greater lung function decrement and nearly 10% of subjects to
experience greater than moderate lung function decrements (e.g., a
tripling of sRaw). Such air quality patterns would likely contribute to
higher exposures and risks than those estimated by the REA, and
accordingly relatively lesser protection of people with asthma from
exposures at or above benchmarks of interest.
    Additionally, in considering potential ramifications of a less
stringent standard, the Administrator recognizes that through its
control of SO2 concentrations at or above the lowest
benchmark of 200 ppb, the current standard provides a margin of safety
for less well studied exposure levels and population groups for which
the evidence is limited or lacking. In so doing, he recognizes that our
understanding of the relationships between the presence of a pollutant
in ambient air and associated health effects is based on a broad body
of information encompassing not only more established aspects of the
evidence, such as the conclusion that exposure to higher SO2
concentrations results in more severe lung function decrements, but
also aspects with which there may be substantial uncertainty. For
example, in the case of this review, he notes there to be increased
uncertainty associated with characterization of the risk of lung
function decrements (including their magnitude and prevalence, and the
associated public health significance) at exposure levels below 400
ppb, and indeed below those represented in the controlled human
exposure studies. In this regard, the Administrator notes the
uncertainty regarding characterization of the risk of respiratory
effects in populations at risk but for which the evidence base is
limited or lacking, such as children with asthma or individuals with
more severe asthma (PA, section <IP_ADDRESS>; REA, section 5.3). He also
takes note of the CASAC comments on these uncertainties, and on
consideration of these groups in assuring the standard's adequate
margin of safety. Further, he considers the epidemiologic evidence,
taking note of the uncertainties associated with exposure measurement
error and copollutant confounding in the evidence. In considering the
uncertainties in both the controlled human exposure and epidemiologic
of studies, he recognizes that collectively, the health effects
evidence generally reflects a continuum, consisting of levels at which
scientists generally agree that health effects are likely to occur,
through lower levels at which the likelihood and magnitude of the
response become increasingly uncertain. In light of these
uncertainties, the Administrator recognizes that the CAA requirement
that primary standards provide an adequate margin of safety, as
summarized in section I.A above, 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. Based on all of the
considerations noted here, and considering the current body of
evidence, including the associated limitations and uncertainties, in
combination with the exposure/risk information, the Administrator
concludes that a less stringent standard than the current standard
would not provide the requisite protection of public health, including
an adequate margin of safety.
    Having concluded that a less stringent standard would not provide
the requisite protection of public health, based in part on his
judgment that the evidence and exposure/risk information indicates that
the current standard provides an appropriately high level of protection
from the more severe and well characterized effects on people with
asthma from very short exposures to SO2 while breathing at
elevated rates

(e.g., those associated with exposures at or above 400 ppb), and in
part on his judgment that a less stringent standard would not provide
the appropriate margin of safety in consideration of uncertainties
regarding population groups at risk or potentially at risk but for
which the evidence is limited or lacking, the Administrator also judges
it appropriate to consider whether the level of protection associated
with the current standard is less than what is requisite and whether a
more stringent standard would be appropriate to consider. In this
context, he first takes note of the very high level of protection that
the REA results indicate to be provided by the current standard,
including 99.9% or more of the simulated at-risk population with
asthma, on average across the 3-year period, to be protected from
experiencing a single day with an exposure at or above 400 ppb, while
breathing at an elevated rate (as well as at least 99.7% with such
protection in the highest year and 100% protected from multiple
occurrences).\114\ He finds such findings to indicate an appropriate
level of protection from such exposures.
---------------------------------------------------------------------------

    \114\ The REA estimates further indicate 99.7% or more of the
simulated at-risk population with asthma, on average across the 3-
year period, to be protected from experiencing a single day with an
exposure at or above 300 ppb, while exercising (as well as at least
99.2% with such protection in the highest year and 100% protected
from multiple such occurrences).
---------------------------------------------------------------------------

    The Administrator additionally considers, as raised above, the
level of protection offered by the current standard from exposures for
which public health implications are less clear. In so doing, he again
notes that information is lacking on concentrations associated with
effects in populations such as young children with asthma and that
information is limited for individuals of any age with severe asthma.
With this in mind, he first considers the REA results for air quality
adjusted to just meet the current standard across the 3-year period
analyzed in each of the three study areas that indicate 0.7% or fewer
of children with asthma to experience a single day per year (on average
across the 3-year period) with a 5-minute exposure at or above 200 ppb
in a single year, while breathing at elevated rates. Somewhat less than
0.1% of children with asthma are estimated to experience multiple such
days, in any 1 year (see section II.A.3 above and section II.C.3 in the
proposal). Based on the information that is available for studied
individuals with asthma, summarized in section II.A.2 above, the
Administrator recognizes exposures to 200 ppb to be associated with
less severe effects than those associated with higher exposures (i.e.,
at or above 300 or 400 ppb). In recognition of the limitations in the
available evidence that contribute uncertainty to our understanding of
the magnitude or severity of lung function decrements in young children
with asthma and in individuals of any age with severe asthma exposed to
SO2 at such lower levels, the Administrator next considers
the findings of the epidemiologic studies that document positive
associations of short-term concentrations of SO2 in ambient
air with asthma-related health outcomes for children, including
hospital admissions and emergency department visits. Yet, in so doing,
he recognizes complications in our ability to discern the exposure
concentrations that may be contributing to such outcomes, noting the
conclusions of the current ISA and the ISA for the last review
regarding the lack of clarity in the evidence regarding the
concentrations that may be eliciting the associated outcomes (83 FR
26765, June 8, 2018).\115\ \116\
---------------------------------------------------------------------------

    \115\ The ISA in the current review concluded that ``[i]t is
unclear whether SO2 concentrations at the available fixed
site monitors adequately represent variation in personal exposures
especially if peak exposures are as important as indicated by the
controlled human exposure studies'' (ISA, p. 5-37). This extends the
observation of the 2008 ISA that ``it is possible that these
epidemiologic associations are determined in large part by peak
exposures within a 24-h[our] period'' (2008 ISA, p. 5-5).
    \116\ Notwithstanding such complications, the Administrator
notes the lack of newly available epidemiologic studies for these
health outcomes for children that include copollutant models for PM,
and he also observes that based on data available for specific time
periods at some monitors in the areas of the three such U.S. studies
that are available from the last review and for which the
SO2 effect estimate remains positive and statistically
significant in copollutant models with PM, the 99th percentile 1-
hour daily maximum concentrations were estimated in the last review
to be between 78 and 150 ppb, i.e., higher than the level of the
now-current 1-hour standard (83 FR 26765, June 8, 2018).
---------------------------------------------------------------------------

    The Administrator additionally considers comments from the CASAC,
including those regarding uncertainties that remain in this review
(summarized in section II.B.2 above). In these comments, the CASAC
noted 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 one example, people with severe asthma, and also citing
physiologic and clinical understanding (Cox and Diez Roux, 2018, p. 3
of letter). In considering these comments, in which the CASAC
additionally stated that ``[i]t is plausible that the current 75 ppb
level does not provide an adequate margin of safety in these groups,''
the Administrator takes note of the CASAC consideration of uncertainty
related to this issue and its conclusion that ``the CASAC does not
recommend reconsideration of the level at this time'' (Cox and Diez
Roux, 2018, p. 3 of letter). The Administrator further notes the CASAC
overall conclusion in this review that the current evidence and
exposure/risk information supports retaining the current standard.
    Thus, in light of the currently available information, including
uncertainties and limitations of the evidence base available to inform
his judgments regarding protection for the at-risk population groups,
as referenced above, as well as CASAC advice, the Administrator does
not find it appropriate to increase the stringency of the standard in
order to provide the requisite public health protection. Rather, he
judges it appropriate to maintain the high level of protection provided
by the current standard for people with asthma of different subgroups
that may be exposed to such levels while breathing at elevated rates
and he does not judge the available information and the associated
uncertainties to indicate the need for a greater level of public health
protection.
    With regard to the uncertainties raised above, the Administrator
notes that his final decision in this review is a public health policy
judgment that draws upon scientific information and analyses about
health effects and risks, as well as judgments about how to consider
the range and magnitude of uncertainties that are inherent in the
information and analyses. Accordingly, he recognizes that his decision
requires judgments based on an interpretation of the evidence and other
information that neither overstates nor understates the strength and
limitations of the evidence and information nor the appropriate
inferences to be drawn. He recognizes, as described in section I.A
above, that the Act does not require that primary standards be set at a
zero-risk level; rather, the NAAQS must be sufficient but not more
stringent than necessary to protect public health, including the health
of sensitive groups, with an adequate margin of safety.
    Recognizing and building upon all of the above considerations and
judgments, the Administrator has reached his conclusions in the current
review. As an initial matter, he recognizes the control exerted by the
current standard on short-term peak concentrations of SO2 in
ambient air, as indicated by the PA analyses of recent air quality data
that examined the occurrence of 5-minute concentrations above
benchmarks of interest (PA,

chapter 2 and Appendix B). Taking the REA estimates of exposure and
risk for air quality conditions just meeting the current standard
together (summarized in section II.A.3 above), while recognizing the
uncertainties associated with such estimates, the Administrator judges
the current standard to provide an appropriately high degree of
protection to at-risk populations (and specifically people with asthma)
from SO2 exposures associated with health effects of more
clear 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). He further judges the current standard to additionally
provide a slightly lower, but still appropriately high degree of
protection for the appreciably less severe effects associated with
lower exposures (i.e., at or below 200 ppb while breathing at elevated
rates), for which public health implications are less clear. In
considering the adequacy of protection afforded by the current standard
from these lower exposure concentrations, the Administrator recognizes,
as noted above, that the effects reported at such concentrations are
less severe than at the higher exposure levels. However, considering
the array of limitations in the evidence with regard to characterizing
the potential response of at-risk individuals to exposures below 200
ppb, as well as the limitations in the evidence for population groups
at risk or potentially at risk but for which the evidence is lacking,
the Administrator finds it appropriate to provide protection from these
exposures in light of the CAA requirements for an adequate margin of
safety to address uncertainties generally associated with limitations
in the scientific and technical information and hazards that research
has not yet identified. In this light, he judges the current standard
to provide the appropriate protection from peak SO2
concentrations in ambient air. Based on these and all of the above
considerations, the Administrator concludes 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, and accordingly concludes that the current standard
provides the requisite protection of public health under the Act.
    With regard to key aspects of the specific elements of the
standard, the Administrator recognizes the support in the current
evidence base for SO2 as the indicator for SOX,
as summarized in section II.B.1 of the proposal. In so doing, he notes
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. He additionally recognizes the control exerted by
the 1-hour averaging time on 5-minute ambient air concentrations of
SO2 (including, particularly, concentrations at and above
200 to 400 ppb) and the associated exposures of particular importance
for SO2-related health effects (e.g., as indicated by the
REA estimates). After consideration of the public comments advocating
revision of the averaging time, as addressed in section II.B.3 above,
the Administrator continues to find that the current standard as
defined by the existing 1-hour averaging time along with the other
elements, is requisite. Similarly, with regard to form and level of the
standard, the Administrator takes note of the REA results as discussed
above and the level of protection that they indicate the elements of
the current standard collectively to provide. He has additionally
considered the public comments regarding revisions to these elements of
the standard, as addressed in section II.B.3 above, and continues to
judge that the existing level and the existing form, in all its
aspects, together with the other elements of the existing standard
provide the appropriate level of public health protection.
    The Administrator additionally takes note of the CASAC support for
retaining the current standard and the CASAC's specific recommendation
that all four elements should remain the same. Beyond his recognition
of this support in the available information and in CASAC advice for
the elements of the current standard, the Administrator has considered
the elements collectively in evaluating the health protection afforded
by the current standard. For 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 concludes that the current primary SO2
standard (in all of its elements) is requisite to protect public health
with an adequate margin of safety from effects of SOX in
ambient air, including the health of at-risk populations, and should be
retained, without revision.

C. Decision on the Primary Standard

    For the reasons discussed above and taking into account information
and assessments presented in the ISA, REA, and PA, the advice from the
CASAC, and consideration of public comments, the Administrator
concludes that the current primary standard for SOX is
requisite to protect public health with an adequate margin of safety,
including the health of at-risk populations, and is retaining the
current standard without revision.

III. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review

    This action is not a significant regulatory action and was,
therefore, not submitted to the Office of Management and Budget (OMB)
for review.

B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs

    This action is not an Executive Order 13771 regulatory action
because this action is not significant under Executive Order 12866.

C. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under
the PRA. There are no information collection requirements directly
associated with a decision to retain a NAAQS without any revision under
section 109 of the CAA. This action retains the current primary
SO2 NAAQS without any revisions.

D. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. Rather, this
action retains, without revision, the existing national standard for
allowable concentrations of SO2 in ambient air as required
by section 109 of the CAA. See also American Trucking Associations v.
EPA, 175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have
significant impacts upon small entities because NAAQS themselves impose
no regulations upon small entities), rev'd in part on other grounds,
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely
affect small governments. This action imposes no enforceable duty on
any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.

G. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

    This action does not have tribal implications, as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian tribes. This action does not change existing
regulations; it retains the current primary SO2 NAAQS,
without revision. The primary NAAQS protects public health, including
the health of at-risk or sensitive groups, with an adequate margin of
safety. Thus, Executive Order 13175 does not apply to this action.

H. Executive Order 13045: Protection of Children From Environmental
Health Risks and Safety Risks

    This action is not subject to Executive Order 13045 because it is
not economically significant as defined in Executive Order 12866. The
health effects evidence and risk assessment information for this
action, which focuses on children with asthma as a key at-risk
population, is summarized in sections II.A.2 and II.A.3 above and
described in the ISA and PA, copies of which are in the public docket
for this action.

I. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use

    This action is not subject to Executive Order 13211, because it is
not a significant regulatory action under Executive Order 12866.

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

K. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations

    The EPA believes that this action does not have disproportionately
high and adverse human health or environmental effects on minority
populations, low-income populations and/or indigenous peoples, as
specified in Executive Order 12898 (59 FR 7629, February 16, 1994). The
documentation related to this is summarized in section II above and
presented in detail in the ISA for the review. The action in this
notification is to retain without revision the existing primary
SO2 NAAQS based on the Administrator's conclusion that the
existing standard protects public health, including the health of
sensitive groups, with an adequate margin of safety. As discussed in
section II, the EPA expressly considered the available information
regarding health effects among at-risk populations in reaching the
decision that the existing standard is requisite.

L. Determination Under Section 307(d)

    Section 307(d)(1)(V) of the CAA provides that the provisions of
section 307(d) apply to ``such other actions as the Administrator may
determine.'' Pursuant to section 307(d)(1)(V), the Administrator
determines that this action is subject to the provisions of section
307(d).

M. Congressional Review Act

    The EPA will submit a rule report to each House of the Congress and
to the Comptroller General of the United States. This action is not a
``major rule'' as defined by 5 U.S.C. 804(2).

References

ATS (American Thoracic Society). (1985). Guidelines as to what
constitutes an adverse respiratory health effect, with special
reference to epidemiological studies of air pollution. Am. Rev.
Respir. Dis. 131: 666-668.
ATS (American Thoracic Society). (2000a). What constitutes an
adverse health effect of air pollution? Am. J. Respir. Crit. Care
Med. 161: 665-673.
ATS (American Thoracic Society). (2000b). Guidelines for
methacholine and exercise challenge testing-1999, Am. J. Respir.
Crit. Care Med. 161: 309-329.
Bloom, B; Jones, LI; Freeman, G. (2013). Summary health statistics
for U.S. children: National health interview survey, 2012. In Vital
and health statistics. National Center for Health Statistics,
Centers for Disease Control and Prevention, Hyattsville, MD,
December 2013. http://www.cdc.gov/nchs/data/series/sr_10/sr10_258.pdf.
CDC. (2017). National Health Interview Survey, 2015. National Center
for Health Statistics, Centers for Disease Control and Prevention,
Washington, DC. Available at: https://www.cdc.gov/asthma/most_recent_data.htm and https://www.cdc.gov/asthma/nhis/2015/data.htm.
Cox, LA; Diez Roux, A. (2018a). Letter from Louis Anthony Cox,
Chair, Clean Air Scientific Advisory Committee, and Ana Diez Roux,
Immediate Past Chair, Clean Air Scientific Advisory Committee, to
Administrator E. Scott Pruitt. Re: CASAC Review of the EPA's Risk
and Exposure Assessment for the Review of the Primary National
Ambient Air Quality Standard for Sulfur Oxides (External Review
Draft--August 2017). April 30, 2018.
Cox, LA; Diez Roux, A. (2018b). Letter from Louis Anthony Cox,
Chair, Clean Air Scientific Advisory Committee, and Ana Diez Roux,
Immediate Past Chair, Clean Air Scientific Advisory Committee, to
Administrator E. Scott Pruitt. Re: CASAC Review of the EPA's Policy
Assessment for the Review of the Primary National Ambient Air
Quality Standard for Sulfur Oxides (External Review Draft--August
2017). April 30, 2018.
Frey, HC; Diez Roux, A. (2014). Letter from H. Christopher Frey,
Chair, Clean Air Scientific Advisory Committee, and Ana Diez Roux,
Past Chair, Clean Air Scientific Advisory Committee, to
Administrator Gina McCarthy. Re: CASAC Review of the EPA's
Integrated Review Plan for the Primary National Ambient Air Quality
Standard for Sulfur Dioxide (External Review Draft--March 2014).
July 24, 2014.
Diez Roux, A. (2016). Letter from Ana Diez Roux, Chair, Clean Air
Scientific Advisory Committee, to Administrator Gina McCarthy. Re:
CASAC Review of the EPA's Integrated Science Assessment for Sulfur
Oxides--Health Criteria (External Review Draft--November 2015).
April 15, 2016.
Diez Roux, A. (2017a). Letter from Ana Diez Roux, Chair, Clean Air
Scientific Advisory Committee, to Administrator Gina McCarthy. Re:
CASAC Review of the EPA's Integrated Science Assessment for Sulfur
Oxides--Health Criteria (Second External Review Draft--December
2016). June 30, 2017.
Diez Roux, A. (2017b). Letter from Ana Diez Roux, Chair, Clean Air
Scientific Advisory Committee, to Administrator Gina McCarthy. Re:
Consultation on the EPA's Review of the Primary National Ambient Air
Quality Standard for Sulfur Oxides: Risk and Exposure Assessment
Planning Document (External Review Draft--February 2017). April 18,
2017.
Henderson R. (2008). Letter from Rogene Henderson, Chair, Clean Air
Scientific Advisory Committee to Administrator Stephen L. Johnson.
Re: Clean Air Scientific Advisory Committee's (CASAC) Peer Review of
EPA's Risk and Exposure Assessment to Support the Review of the
SO2 Primary National Ambient Air Quality Standards (First
Draft, July 2008). August 22, 2008.
Horstman, D; Roger, LJ; Kehrl, H; Hazucha, M. (1986). Airway
sensitivity of asthmatics to sulfur dioxide. Toxicol Ind Health 2:
289-298.
Johns, D; Simmons, K. (2009). Memorandum to Sulfur Oxides NAAQS
Review Docket (EPA-HQ-ORD-2006-0260). Quality assurance review of
individual subject data presented in Table 3-1 of the 2008
Integrated Science Assessment (ISA) for Sulfur Oxides. Docket ID No.
EPA-HQ-ORD-2006-0260-0036.
Johns, DO; Svendsgaard, D; Linn, WS. (2010). Analysis of the
concentration-respiratory response among asthmatics following

controlled short-term exposures to sulfur dioxide. Inhal Toxicol 22:
1184-1193.
Johns, DO; Linn, WS. (2011). A review of controlled human
SO2 exposure studies contributing to the US EPA
integrated science assessment for sulfur oxides. Inhal Toxicol 23:
33-43.
Karl, TR; Koss, WJ. (1984). Regional and national monthly, seasonal
and annual temperature weighted by area, 1895-1983. National Oceanic
and Atmospheric Administrations, National Environmental Satellite,
Data, and Information Service, National Climatic Data Center,
Asheville, NC, June 1984. Available at: https://repository.library.noaa.gov/view/noaa/10238.
Kehrl, HR; Roger, LJ; Hazucha, MJ; Horstman, DH. (1987). Differing
response of asthmatics to sulfur dioxide exposure with continuous
and intermittent exercise. Am J Respir Crit Care Med 135: 350-355.
Koenig, JQ; DS Covert; Pierson, WE. (1989). Effects of inhalation of
acidic compounds on pulmonary function in allergic adolescent
subjects. Env Health Persp 79: 173-178.
Koenig, JQ; Covert, DS; Hanley, QS; Van Belle, G; Pierson, WE.
(1990). Prior exposure to ozone potentiates subsequent response to
sulfur dioxide in adolescent asthmatic subjects. Am J Respir Crit
Care Med 141: 377-380.
Linn, WS; Venet, TG; Shamoo, DA; Valencia, LM; Anzar, UT; Spier, CE;
Hackney, JD. (1983a). Respiratory effects of sulfur dioxide in
heavily exercising asthmatics: A dose-response study. Am Rev Respir
Dis 127: 278-283.
Linn, WS; Shamoo, DA; Spier, CE; Valencia, LM; Anzar, UT; Venet, TG;
Hackney, JD. (1983b). Respiratory effects of 0.75 ppm sulfur dioxide
in exercising asthmatics: Influence of upper-respiratory defenses.
Environ Res 30: 340-348.
Linn, WS; Avol, EL; Shamoo, DA; Venet, TG; Anderson, KR; Whynot, JD;
Hackney, JD. (1984). Asthmatics' responses to 6-hr sulfur dioxide
exposures on two successive days. Arch Environ Health 39: 313-319.
Linn, WS; Avol, EL; Peng, RC; Shamoo, DA; Hackney, JD. (1987).
Replicated dose-response study of sulfur dioxide effects in normal,
atopic, and asthmatic volunteers. Am Rev Respir Dis 136: 1127-1134.
Linn, WS; Shamoo, DA; Peng, RC; Clark, KW; Avol. EL; Hackney, JD.
(1990). Responses to sulfur dioxide and exercise medication-
dependent asthmatics: effect of varying medication levels. Arch
Environ Health 45: 24-30.
Long, TC; Brown, J. (2018). Memorandum to Integrated Science
Assessment (ISA) for Sulfur Oxides--Health Criteria Docket (EPA-HQ-
ORD-2013-0357). Description of controlled human exposure data
sources for Table 5-2 of the 2017 Integrated Science Assessment
(ISA) for Sulfur Oxides--Health Criteria. Docket ID No. EPA-HQ-ORD-
2013-0357-0034.
Mazurek JM; Syamlal G. (2018). Prevalence of Asthma, Asthma Attacks,
and Emergency Department Visits for Asthma Among Working Adults--
National Health Interview Survey, 2011-2016. MMWR Morb Mortal Wkly
Rep 67:377-386.
Reddel, HK. (2009). Characterizing asthma phenotypes: Predictors and
outcomes at the extremes of asthma severity [Editorial]. Respirology
14: 778-780.
Samet J. (2009). Letter from Johnathan M. Samet, Chair, Clean Air
Scientific Advisory Committee, to Administrator Lisa P. Jackson. Re:
Clean Air Scientific Advisory Committee's (CASAC) Review of EPA's
Risk and Exposure Assessment (REA) to Support the Review of the
SO2 Primary National Ambient Air Quality Standards:
Second Draft. May 18, 2009.
Sheppard, D; Saisho, A; Nadel, JA; Boushey, HA. (1981). Exercise
increases sulfur dioxide-induced bronchoconstriction in asthmatic
subjects. Am Rev Respir Dis 123: 486-491.
Sheppard, D; Eschenbacher, WL; Boushey, HA; Bethel, RA. (1984).
Magnitude of the interaction between the bronchomotor effects of
sulfur dioxide and those of dry (cold) air. Am Rev Respir Dis 130:
52-55.
Solomon, D; Hagan, N; Graham, S. (2019). Memorandum to Sulfur
Dioxide NAAQS Docket (EPA-HQ-OAR-2013-0566). Additional
SO2 air quality analyses.
Thurston, GD; Kipen, H; Annesi-Maesano, I; Balmes, J; Brook, RD;,
Cromar, K; De Matteis, S; Forastiere, F; Forsberg, B; Frampton, MW;
Grigg, J; Heederik, D; Kelly, FJ; Kuenzli, N; Laumbach, R; Peters,
A; Rajagopalan, ST; Rich, D; Ritz, B; Samet, JM; Sandstrom, T;
Sigsgaard, T; Sunyer, J; Brunekreef, B. (2017). A joint ERS/ATS
policy statement: what constitutes an adverse health effect of air
pollution? An analytical framework. Eur Respir J 11: 1600419.
Trenga, CA; Koenig, JQ; Williams, PV. (1999). Sulphur dioxide
sensitivity and plasma antioxidants in adult subjects with asthma.
Occup Environ Med 56: 544-547.
Trenga, CA; Koenig, JQ; Williams, PV. (2001). Dietary antioxidants
and ozone-induced bronchial hyperresponsiveness in adults with
asthma. Arch Environ Occup Health 56: 242-249.
U.S. DHEW. (1967). Air quality criteria for sulfur oxides. National
Center for Air Pollution Control, Bureau of Disease Prevention and
Environmental Control, Public Health Service, U.S. Department of
Health, Education, and Welfare, Washington, DC, Public Health
Service Publication No. 1619, March 1967. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/s_so2_pr.html.
U.S. DHEW. (1969). Air quality criteria for sulfur oxides. National
Center for Air Pollution Control, Consumer Protection and
Environmental Health Service, Public Health Service, U.S. Department
of Health, Education, and Welfare, Washington, DC, National Air
Pollution Control Administration Publication No. AP-50, January
1969. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/s_so2_pr.html.
U.S. EIA (U.S. Energy Information Administration). (2017). Monthly
Energy Review July 2017. U.S. Department of Energy, Washington, DC,
DOE/EIA-0035(2017-07), July 2017. Available at: https://www.eia.gov/totalenergy/data/monthly/pdf/mer.pdf.
U.S. EPA. (1982). Air quality criteria for particulate matter and
sulfur oxides. Environmental Criteria and Assessment Office, Office
of Research and Development, Research Triangle Park, NC, EPA 600/8-
82/029a, December 1982. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/s_so2_pr.html.
U.S. EPA. (1986). Air quality criteria for particulate matter and
sulfur oxides (1982): assessment of newly available health effects
information, 2nd addendum. Environmental Criteria and Assessment
Office, Office of Health and Environmental Assessment, Office of
Research and Development, Research Triangle Park, NC, EPA/600/8-86/
020F, December 1986. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/s_so2_pr.html.
U.S. EPA. (1994). Supplement to the second addendum (1986) to air
quality criteria for particulate matter and sulfur oxides (1982):
Assessment of new findings on sulfur dioxide acute exposure health
effects in asthmatic individuals. Environmental Criteria and
Assessment Office, Office of Health and Environmental Assessment,
Office of Research and Development, Research Triangle Park, NC, EPA/
600/FP-93/002, August 1994. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/s_so2_pr.html.
U.S. EPA. (1996). Air Quality Criteria for Ozone and Related
Photochemical Oxidants. National Center for Environmental
Assessment, Office of Research and Development, Research Triangle
Park, NC, EPA/600/P-93/004aF, July 1996. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=300026GN.PDF.
U.S. EPA. (2008a). Integrated Science Assessment (ISA) for Sulfur
Oxides--Health Criteria (Final Report). National Center for
Environmental Assessment-RTP Division, Office of Research and
Development, Research Triangle Park, NC, EPA-600/R-08/047F,
September 2008. Available at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=198843.
U.S. EPA. (2008b). Risk and Exposure Assessment to Support the
Review of the NO2 Primary National Ambient Air Quality
Standard. Office of Air Quality Planning and Standards, Research
Triangle Park, NC, EPA-452/R-08-008a, November 2008. Available at:
https://www3.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_rea.html.
U.S. EPA. (2009). Risk and Exposure Assessment to Support the Review
of the SO2 Primary National Ambient Air Quality Standard.
Office of Air Quality Planning and Standards, Research Triangle
Park, NC, EPA-452/R-09-007, July 2009. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/data/200908SO2REAFinalReport.pdf.

U.S. EPA. (2010). Quantitative Risk and Exposure Assessment for
Carbon Monoxide--Amended. Office of Air Quality Planning and
Standards, Research Triangle Park, NC, EPA-452/R-10-006, July 2010.
Available at: https://www.epa.gov/naaqs/carbon-monoxide-co-standards-risk-and-exposure-assessments-current-review.
U.S. EPA. (2011). Regulatory Impact Analysis for the Final Mercury
and Air Toxic Standards. Office of Air Quality Planning and
Standards, Research Triangle Park, NC, EPA-452/R-11-011, December
2011. Available at: https://www.epa.gov/sites/production/files/2015-11/documents/matsriafinal.pdf.
U.S. EPA. (2014a). Integrated Review Plan for the Primary National
Ambient Air Quality Standard for Sulfur Dioxide, Final. Office of
Air Quality Planning and Standards, Research Triangle Park, NC, EPA-
452/P-14-007, October 2014. Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/data/20141028so2reviewplan.pdf.
U.S. EPA. (2014b). Integrated Review Plan for the Primary National
Ambient Air Quality Standard for Sulfur Dioxide, External Review
Draft. Office of Air Quality Planning and Standards, Research
Triangle Park, NC, EPA-452/P-14-005, March 2014. Available at:
https://www3.epa.gov/ttn/naaqs/standards/so2/data/20140318so2reviewplan.pdf.
U.S. EPA. (2014c). EPA Sets Tier 3 Motor Vehicle Emission and Fuel
Standards. Office of Transportation and Air Quality, Washington, DC,
EPA-420-F-14-009, March 2014. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi/P100HVZV.PDF?Dockey=P100HVZV.PDF.
U.S. EPA. (2014d). Health Risk and Exposure Assessment for Ozone.
Office of Air Quality Planning and Standards, Research Triangle
Park, NC, EPA-452/R-14-004a, August 2014. Available at: https://www.epa.gov/naaqs/ozone-o3-standards-risk-and-exposure-assessments-current-review.
U.S. EPA. (2015). Integrated Science Assessment (ISA) for Sulfur
Oxides--Health Criteria (External Review Draft, Nov 2015). National
Center for Environmental Assessment-RTP Division, Office of Research
and Development, Research Triangle Park, NC, EPA/600/R-15/066,
November 2015. Available at: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310044.
U.S. EPA. (2016a). Integrated Review Plan for the Secondary National
Ambient Air Quality Standards for Particulate Matter. Office of Air
Quality Planning and Standards, Research Triangle Park, NC, EPA-452/
R-16-005, December 2016. Available at: https://www.epa.gov/naaqs/particulate-matter-pm-standards-planning-documents-current-review.
U.S. EPA. (2016b). Integrated Science Assessment (ISA) for Sulfur
Oxides--Health Criteria (Second External Review Draft). National
Center for Environmental Assessment-RTP Division, Office of Research
and Development, Research Triangle Park, NC, EPA/600/R-16/351,
December 2016. Available at: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=326450.
U.S. EPA. (2017a). Integrated Science Assessment (ISA) for Sulfur
Oxides--Health Criteria (Final). National Center for Environmental
Assessment-RTP Division, Office of Research and Development,
Research Triangle Park, NC, EPA/600/R-17/451, December 2017.
Available at: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=338596.
U.S. EPA. (2017b). Integrated Review Plan for the Secondary National
Ambient Air Quality Standard for Ecological Effects of Oxides of
Nitrogen, Oxides of Sulfur and Particulate Matter. Office of Air
Quality Planning and Standards, Research Triangle Park, NC, EPA-452/
R-17-002, January 2017. Available at: https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondarystandards-planning-documents-current.
U.S. EPA. (2017c). Review of the Primary National Ambient Air
Quality Standard for Sulfur Oxides: Risk and Exposure Assessment
Planning Document. Office of Air Quality Planning and Standards,
Research Triangle Park, NC, EPA-452/P-17-001, February 2017.
Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/data/20170216so2rea.pdf.
U.S. EPA. (2017d). Risk and Exposure Assessment for the Review of
the Primary National Ambient Air Quality Standard for Sulfur Oxides,
External Review Draft. Office of Air Quality Planning and Standards,
Research Triangle Park, NC, EPA-452/P-17-002, August 2017. Available
at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2017e). Policy Assessment for the Review of the Primary
National Ambient Air Quality Standard for Sulfur Oxides, External
Review Draft. Office of Air Quality Planning and Standards, Research
Triangle Park, NC, EPA-452/P-17-003, August 2017. Available at:
https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2018a). Risk and Exposure Assessment for the Review of
the Primary National Ambient Air Quality Standard for Sulfur Oxides,
Final. Office of Air Quality Planning and Standards, Research
Triangle Park, NC, EPA-452/R-18-003, May 2018. Available at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2018b). Policy Assessment for the Review of the Primary
National Ambient Air Quality Standard for Sulfur Oxides, Final.
Office of Air Quality Planning and Standards, Research Triangle
Park, NC, EPA-452/R-18-002, May 2018. Available at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2018c). Integrated Science Assessment (ISA) for
Particulate Matter (External Review Draft). National Center for
Environmental Assessment-RTP Division, Office of Research and
Development, Research Triangle Park, NC, EPA/600/R-18/179, October
2018. Available at: http://cfint.rtpnc.epa.gov/ncea/prod/recordisplay.cfm?deid=341593.
WHO. (2008). WHO/IPCS Harmonization Project Document No. 6. Part 1:
Guidance Document on Characterizing and Communicating Uncertainty in
Exposure Assessment. International Programme on Chemical Safety,
World Health Organization, Geneva, Switzerland. Available at: http://www.who.int/ipcs/methods/harmonization/areas/exposure/en/.

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: February 25, 2019.
Andrew Wheeler,
Acting Administrator.
[FR Doc. 2019-03855 Filed 3-15-19; 8:45 am]
 BILLING CODE 6560-50-P