emissions inventory information (2014), which is also characterized in
the ISA (ISA, section 2.2.2).\63\ This information indicates that there
are more than 300,000 and 60,000 children living within 1 km of
facilities emitting at least 1,000 and 2,000 tpy of SO2,
respectively. Within 5 km of such sources, the numbers are
approximately 1.4 million and 700,000, respectively (PA, Table 3-5).
While information on SO2 concentrations in locations of
maximum impact of such sources is not available for all these areas,
and SO2 concentrations vary appreciably near sources, simply
considering the 2015 national estimate of asthma prevalence of
approximately 8% (noted above), this information would suggest there
may be as many as 24,000 to more than 100,000 children with asthma that
live in areas near substantially sized sources of SO2
emissions to ambient air (PA, section <IP_ADDRESS>; Table 3-5).
---------------------------------------------------------------------------

    \60\ Core-based statistical area (CBSA) is a geographic area
defined by the U.S. Office of Management and Budget to consist of an
urban area of at least 10,000 people in combination with its
surrounding or adjacent counties (or equivalents) with which there
are socioeconomic ties through commuting (https://www.census.gov/geo/reference/gtc/gtc_cbsa.html). Populations in the 15 CBSAs
referred to in the body of the text range from approximately 30,000
to more than a million (based on 2016 U.S. Census Bureau estimates).
    \61\ Table 5c. Monitoring Site Listing for Sulfur Dioxide 1-Hour
NAAQS in the Excel file labeled
So2_designvalues_20142016_final_07_19_16.xlsx downloaded from
https://www.epa.gov/air-trends/air-quality-design-values on January
26, 2018.
    \62\ As state and local air agencies have the flexibility to
characterize air quality using either modeling of actual source
emissions or using appropriately sited ambient air monitors for
designation purposes, both types of information have been used to
support designations of areas not meeting the standard. To date, 42
areas have been designated as nonattainment areas, although air
quality improvements in two of these 42 areas has led to the areas
meeting the standard and being redesignated. The population residing
in the remaining 40 nonattainment areas is approximately 3.3 million
people (see https://www3.epa.gov/airquality/greenbook/tnsum.html).
Detailed information about source types in these areas can be found
in the technical support documents for individual nonattainment
areas, available via https://www.epa.gov/sulfur-dioxide-designations/sulfur-dioxide-designations-regulatory-actions. These
areas generally had significant SO2 point sources, with
the majority of these point sources being electric generating units.
    \63\ Although source characteristics and meteorological
conditions--in addition to magnitude of emissions--influence the
distribution of concentrations in ambient air, these estimates are
based on proximity to large sources, rather than ambient
concentrations, due to limitations in the available information with
regard to spatial (and temporal) patterns of SO2
concentrations in the proximity of such sources in urban areas (ISA,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    The information discussed in this section indicates the potential
for exposures to SO2 in ambient air to be of public health
importance. Such considerations contributed to the basis for the 2010
decision to appreciably strengthen the primary SO2 NAAQS and
to establish a 1-hour standard to provide the requisite public health
protection for at-risk populations from short-term exposures of
concern.

C. Summary of Risk and Exposure Information

    Our consideration of the scientific evidence available in the
current review (summarized in section II.B above), as at the time of
the last review, is informed by results from a quantitative analysis of
estimated population exposure and associated risk of
bronchoconstriction-related effects that the evidence indicates to be
elicited in some portion of exercising people with asthma by short
exposures to elevated SO2 concentrations, e.g., such
exposures lasting 5 or 10 minutes. This analysis, for the air quality
scenario of just meeting the current standard, estimates two types of
risk metrics in terms of percentages of the simulated at-risk
populations of adults with asthma and children with asthma (REA,
section 4.6). The first of the two risk metrics is based on comparison
of the estimated 5-minute exposure concentrations for individuals
breathing at elevated rates to 5-minute exposure concentrations of
potential concern (benchmark concentrations), and the second utilizes
exposure-response (E-R) information from studies in which subjects
experienced moderate or greater lung function decrements (specifically
a doubling or more in sRaw) to estimate the portion of the simulated
at-risk population likely to experience one or more days with an
SO2-related increase in sRaw of at least 100% (REA, sections
4.6.1 and 4.6.2). Both of these metrics are used in the REA to
characterize health risk associated with 5-minute peak SO2
exposures among simulated at-risk populations during periods of
elevated breathing rates. These risk metrics were also derived in the
REA for the last review and the associated estimates informed the
Administrator's 2010 decision to establish the current standard (75 FR
35546-35547, June 22, 2010).
    The following subsections summarize key aspects of the design and
methods of the quantitative assessment (section II.C.1) and the
important uncertainties associated with these analyses (section
II.C.2). The results of the analyses are summarized in section II.C.3.
1. Key Design Aspects
    In this section, we provide an overview of key aspects of the
quantitative exposure and risk assessment conducted for this review,
including the study areas, air quality adjustment approach, modeling
tools, at-risk populations simulated, and benchmark concentrations
assessed. The assessment is described in detail in the REA and
summarized in section 3.2.2 of the PA.
    Given the primary overarching consideration in this review of
whether the currently available information calls into question the
adequacy of protection provided by the current standard, the air
quality scenario analyzed in the REA focuses on air quality conditions
that just meet the current standard. With this focus, the analyses
estimate exposure and risk for at-risk populations in three urban study
areas in: (1) Fall River, MA; (2) Indianapolis, IN; and (3) Tulsa, OK.
The three study areas present a variety of circumstances related to
population exposure to short-term peak concentrations of SO2
in ambient air. These study areas range in total population size from
approximately 180,000 to 540,000 and reflect different mixtures of
SO2 emissions sources, including electric utilities using
fossil fuels, as well as sources such as petroleum refineries and
secondary lead smelting (REA, section 3.1). The three study areas--in
Massachusetts, Indiana and Oklahoma--are in three different climate
regions of the U.S.: The Northeast, Ohio River Valley (Central), and
South (Karl and Koss, 1984). The latter two regions comprising the part
of the U.S. with generally the greatest prevalence of elevated
SO2 concentrations and large emissions sources (PA, Figure
2-7 and Appendix F).\64\ Accordingly, the three study areas illustrate
three different patterns of exposure to SO2 concentrations
in a populated area in the U.S. (REA, section 5.1). While the same air
quality scenario

is simulated in all three study areas (conditions that just meet the
current standard), study-area-specific source and population
characteristics contribute to variation in the estimated magnitude of
exposure and associated risk across study areas.
---------------------------------------------------------------------------

    \64\ Additionally, continuous 5-minute ambient air monitoring
data (i.e., all 5-minute values for each hour) are available in all
three study areas (REA, section 3.2).
---------------------------------------------------------------------------

    As indicated by this case study approach to assessing exposure and
risk, the analyses in the REA are intended to provide assessments of an
air quality scenario just meeting the current standard for a small,
diverse set of study areas and associated exposed at-risk populations
that will be informative to the EPA's consideration of potential
exposures and risks that may be associated with the air quality
conditions occurring under the current SO2 standard. The REA
analyses are not designed to provide a comprehensive national
assessment of such conditions (REA, section 2.2). The objective of the
REA is not to present an exhaustive analysis of exposure and risk in
areas of the U.S. that currently just meet the standard and/or of
exposure and risk associated with air quality adjusted to just meet the
standard in areas that currently do not meet the standard.\65\ Rather,
the purpose is to assess, based on current tools and information, the
potential for exposures and risks beyond those indicated by the
information available at the time the current standard was established.
Accordingly, capturing an appropriate diversity in study areas and air
quality conditions (that reflect the current standard scenario) is
important to the role of the REA in informing the EPA's conclusions on
the public health protection afforded by the current standard (PA,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \65\ Nor is the objective of the REA to provide a comprehensive
assessment of current air quality across the U.S.
---------------------------------------------------------------------------

    A broad variety of spatial and temporal patterns of SO2
concentrations can exist when ambient air concentrations just meet the
current standard. These patterns will vary due to many factors
including the types of emissions sources in a study area and several
characteristics of those sources, such as magnitude of emissions and
facility age, use of various control technologies, patterns of
operation, and local factors, as well as local meteorology. Estimates
derived by the particular analytical approaches and methodologies used
to describe the study area-specific air quality provide an indication
of this variability in the spatial and temporal patterns of
SO2 concentrations associated with air quality conditions
just meeting the current standard, while recognizing the associated
uncertainty in these concentration estimates.
    In this regard, the REA presents results from two different
approaches to adjusting air quality. The first approach uses the
highest design value across all modeled air quality receptors to adjust
the air quality concentrations in each area to just meet the standard
(REA, section 3.4). This is done by estimating the amount of
SO2 concentration reduction needed for concentrations at
this highest receptor to be adjusted to just meet the current standard.
Based on this amount, all other receptors impacted by the highest
source(s) are adjusted proportionately. The second approach is included
in the REA as a sensitivity analysis in recognition of the potential
uncertainty associated with the estimated concentrations across the
modeling domain, particularly the very highest concentrations.
Accordingly, the second approach uses the air quality receptor having
the 99th percentile of the distribution of design values (instead of
the receptor having the maximum design value) to estimate the
SO2 concentration reductions needed to adjust the air
quality to just meet the standard (REA, section <IP_ADDRESS>).
    Consistent with the health effects evidence summarized in section
II.B above, the focus of the REA is on short-term (5-minute) exposures
of individuals in the population with asthma during times when they are
breathing at an elevated rate. Five-minute concentrations in ambient
air were estimated for the current standard scenario using a
combination of 1-hour concentrations from the EPA's preferred near-
field dispersion model, the American Meteorological Society/EPA
regulatory model (AERMOD), with adjustment such that they just meet the
current standard, and relationships between 1-hour and 5-minute
concentrations occurring in the local ambient air monitoring data. Air
quality modeling with AERMOD is used to capture the spatial variation
in ambient SO2 concentrations across an urban area, which
can be relatively high in areas affected by large point sources, and
which the limited number of monitoring locations in each area is
unlikely to capture. This provides 1-hour concentrations at model
receptor sites across the modeling domain across the 3-year modeling
period (consistent with the 3-year form of the standard). These
concentrations were adjusted such that the air quality modeling
receptor location with the highest concentrations just met the current
standard.\66\ Relationships between 1-hour and 5-minute concentrations
at local monitors were then used to estimate 5-minute concentrations
associated with the adjusted 1-hour concentrations across the 3-year
period at all model receptor locations in each of the three study areas
(REA, section 3.5). In this way, available continuous 5-minute ambient
air monitoring data (datasets with all twelve 5-minute concentrations
in each hour) were used to reflect the fine-scale temporal variation in
SO2 concentrations documented by these data and for which
air quality modeling is limited, e.g., by limitations in the time steps
of currently available model input data such as for emissions
estimates.
---------------------------------------------------------------------------

    \66\ The air quality adjustments were implemented with a focus
on reducing emissions from the source(s) contributing most to the
standard exceedances until the areas just met the standard. This
approach focuses on the concentrations associated with the primary
contributing source(s), identifying the amount by which they need to
be adjusted in order for the highest design value across all air
quality receptors to just meet the current standard (REA, section
3.4). Based on this amount, all other receptors impacted by the
highest source(s) are adjusted accordingly. In recognition of the
potential uncertainty associated with this approach, particularly
for the highest estimated concentrations, a second approach was also
evaluated that bases the adjustments on the air quality receptor
having the 99th percentile of the distribution of design values
instead of the receptor having the maximum design value (REA,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    The estimated 5-minute concentrations in ambient air across each
study area were then used together with the Air Pollutants Exposure
(APEX) model, a probabilistic human exposure model that simulates the
activity of individuals in the population, including their exertion
levels and movement through time and space, to estimate concentrations
of 5-minute exposure events of the individuals in indoor, outdoor, and
in-vehicle microenvironments. The use of APEX for estimating exposures
allows for consideration of factors that affect exposures that are not
addressed by consideration of ambient air concentrations alone. These
factors include: (1) Attenuation in SO2 concentrations
expected to occur in some indoor microenvironments; (2) the influence
of human activity patterns on the time series of exposure
concentrations; and (3) accounting for human physiology and the
occurrence of elevated breathing rates concurrent with SO2
exposures. These factors are all key to appropriately characterizing
health risk for SO2.
    The APEX model has a history of application, evaluation, and
progressive model development in estimating human exposure and dose for
review of

NAAQS for gaseous pollutants (see, e.g., U.S. EPA, 2008b; 2010; 2014d).
This general exposure modeling approach was also used in the 2009 REA
for the last review of the primary standard for SOX,
although a number of updates have been made to the model and various
datasets used with it (2009 REA; REA Planning Document, section 3.4).
For example, exposure modeling in the current REA includes reliance on
updates to several key inputs of the model, including: (1) A
significantly expanded Consolidated Human Activity Database (CHAD),
that now has over 55,000 diaries, with over 25,000 school-aged
children; (2) updated National Health and Nutrition Examination Survey
(NHANES) data (2009-2014), which are the basis for the age- and sex-
specific body weight distributions that APEX samples to specify the
individuals in the modeled populations; (3) the algorithms used to
estimate age- and sex-specific resting metabolic rate, a key input to
estimating a simulated individual's activity-specific ventilation (or
breathing) rate; and (4) the ventilation rate algorithm itself.
Further, the current model uses updated population demographic data
based on the most recent Census.
    As used in the current assessment, the APEX model probabilistically
generates a sample of hypothetical individuals based on sampling from
an actual population database, and simulates each individual's
movements through time and space (e.g., indoors at home, inside
vehicles) to estimate his or her exposure to a pollutant. Population
characteristics are taken into account to represent the demographic
profile of the population in each study area. Age and gender
demographics for the simulated at-risk population (adults and children
with asthma) were drawn from the prevalence estimates provided by the
2011-2015 NHIS.\67\ The APEX model generates each simulated person or
profile by probabilistically selecting values for a set of profile
variables, including demographic variables, status and physical
attributes (e.g., residence with air conditioning, height, weight, body
surface area) and ventilation rate.
---------------------------------------------------------------------------

    \67\ Data for these years were obtained from the NHIS, available
at https://www.cdc.gov/nchs/nhis/data-questionnaires-documentation.htm.
---------------------------------------------------------------------------

    Based on minute-by-minute activity levels and physiological
characteristics of the simulated person, APEX estimates an equivalent
ventilation rate (EVR) based on normalizing the simulated individuals'
activity-specific ventilation rate to their body surface area; the EVR
is used to identify exposure periods during which an individual is at
or above a specified ventilation level (REA, section 4.1). The level
specified is based on the ventilation rates of subjects in the
controlled human exposure studies of exercising people with asthma
(ISA, Table 5-2). The APEX simulations performed for this review have
focused on exposures to SO2 emitted into ambient air that
occurs in microenvironments \68\ without additional contribution from
indoor SO2 emissions sources.\69\
---------------------------------------------------------------------------

    \68\ Five microenvironments (MEs) are modeled in the REA as
representative of a larger number of MEs. The 2009 REA results
indicated that the majority of peak SO2 exposures
occurred while individuals were within outdoor MEs (2009 REA, Figure
8-21). Based on that finding and the objective (i.e., understanding
how often and where short-term peak SO2 exposures occur),
some MEs that were used in the 2009 REA were aggregated to address
exposures of ambient origin that occur within a core group of
indoor, outdoor, and vehicle MEs (REA, section 4.2).
    \69\ Indoor sources of SO2 are generally minor in
comparison to SO2 from ambient air (ISA, p. 3-6; REA,
section 2.1.1 and 2.1.2).
---------------------------------------------------------------------------

    The at-risk populations for which exposure and risk are estimated
(people with asthma) comprise 8.0 to 8.7% of the populations in the
exposure modeling domains for the three study areas (REA, section 5.1).
The percent of children with asthma in the simulated populations ranges
from 9.7 to 11.2% across the three study areas (REA, section 5.1).
Within each study area the percent varies with age, sex and whether
family income is above or below the poverty level (REA, section 4.1.2,
Appendix E).\70\ This variation is greatest in the Fall River study
area, with census block level, age-specific asthma prevalence estimates
ranging from 7.9 to 18.6% for girls and from 10.7 to 21.5% for boys
(REA, Table 4-1).
---------------------------------------------------------------------------

    \70\ As described in section 4.1.2 and Appendix E of the REA,
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.
---------------------------------------------------------------------------

    As in the last review, the REA for this 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 were
identified based on consideration of the evidence discussed in section
II.B above.
    For the benchmark metric, the REA uses benchmark concentrations of
400 ppb, 300 ppb, 200 ppb based on concentrations included in the well-
documented controlled human exposure studies summarized in section II.B
above, and also 100 ppb in consideration of uncertainties with regard
to lower concentrations and population groups with more limited data,
as discussed in section II.B above (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 reduced
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
individual study subject data are available in terms of the sRaw and
FEV1 from studies that have assessed the SO2
effect versus the effect of exercise in clean air; 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). For 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,\71\ 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,\72\ a benchmark concentration of 100 ppb (one
half the lowest exposure concentration tested in free breathing
exposure studies that assessed the SO2 effect versus the
effect of exercise in clean air) is also included.
---------------------------------------------------------------------------

    \71\ As explained in section II.B.3 above, 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 (ISA, section <IP_ADDRESS>; PA,
section <IP_ADDRESS>).
    \72\ As summarized in section II.B.3 above, recognizing that
even the study subjects described as ``moderate/severe'' group (had
well-controlled asthma, were generally able to withhold medication,
were not dependent on corticosteroids, and were able to engage in
moderate to heavy levels of exercise) would likely be classified as
moderate by today's classification standards (ISA, p. 5-22; Johns et
al., 2010; Reddel, 2009), we have considered the evidence with
regard to the response of individuals with severe asthma that are
not generally represented in the full set of controlled human
exposure studies. There is no evidence to indicate such individuals
would experience moderate or greater SO2-related lung
function decrements at lower SO2 exposure concentrations
than individuals with moderate asthma. With regard to the severity
of response, 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 (e.g., 5-12 years) indicates a potential for greater
response (ISA, pp. 5-22 to 5.25).
---------------------------------------------------------------------------

    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). Beyond the assessment of these
studies and their results in past reviews, there has been extensive
evaluation of the individual subject results, including a data quality
review in the last primary SO2 NAAQS review (Johns and
Simmons, 2009), and detailed analysis in two subsequent publications
(Johns et al., 2010; Johns and Linn, 2011). The sRaw responses reported
in the controlled exposure studies have been 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). 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). The E-R function was derived from these data using a probit
function (REA, section 4.6.2).
2. 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 differ and
incorporate improvements. For example, in addition to an expansion in
the number and type of study areas assessed, 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. 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 an approach intended to identify
and compare the relative impact that important sources of uncertainty
may have (REA, section 6.2). This approach is 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).73 74
The REA considers the limitations and uncertainties underlying the
analysis inputs and approaches and the extent of their influence on the
resultant exposure/risk estimates. Consistent with the WHO (2008)
guidance, the overall impact of the uncertainty is scaled by
considering the extent or magnitude of the impact of the uncertainty 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).
---------------------------------------------------------------------------

    \73\ The approach used has been applied in REAs for past NAAQS
review for nitrogen oxides, carbon monoxide, ozone (U.S. EPA, 2008b;
2010; 2014d), and SOX (U.S. EPA, 2009).
    \74\ The approach used varies from that of WHO (2008) in that
the REA approach placed 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 estimated exposures and health risk
results).
---------------------------------------------------------------------------

    Several areas of uncertainty are identified as particularly
important, with some similarities to those 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.
Among other areas, there is also 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 the REA (REA, section 6.2), with a general summary
provided here. Assessment approach-related aspects contributing to this
uncertainty include the model estimates of 1-hour concentrations and
the approach employed to adjust the air quality surface to
concentrations just meeting the current standard,\75\ as well as the
estimation of 1-hour ambient air concentrations resulting from
emissions sources not explicitly modeled, all of which influence the
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).
---------------------------------------------------------------------------

    \75\ In study areas in which estimated SO2
concentrations at a very small number of receptors are substantially
higher than those at all other air quality receptors, the two
different adjustment approaches investigated in the REA (described
in section II.C.1 above) can result in very different concentrations
across the area. In areas with this characteristic, the first
approach (which involves determining adjustments based on
concentrations at the very highest receptor locations) generally
results in appreciably lower concentrations than those associated
with the second approach at receptor locations beyond the small
group with the very highest concentrations in the area. This is
discussed in greater detail in the REA, section <IP_ADDRESS>.
---------------------------------------------------------------------------

    Another important area of uncertainty, particular to interpretation
of the lung function risk estimates, concerns 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 at least a doubling of
sRaw for all exposures experienced 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 a similar effect as that of the initial exposure
event (REA, Table 6-3). Another area of uncertainty concerns 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>).\76\
---------------------------------------------------------------------------

    \76\ 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.B, the controlled human
exposure study evidence base does not include studies of children
younger than 12 years old and is limited with regard to studies of
people with more severe asthma.\77\ 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.
---------------------------------------------------------------------------

    \77\ We additionally recognize that limitations in the activity
pattern information for children younger than five 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 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 circumstance represented by
the three study areas; and characterization of particular subgroups of
people with asthma that may be at greater risk.
3. Summary of Exposure and Risk Estimates
    The REA provides estimates for two simulated at-risk populations:
Adults with asthma and school-aged children \78\ with asthma (REA,
section 2.2). Focusing on the at-risk population of children with
asthma, 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 the REA). Both types of estimates for adults with
asthma are lower, generally due to the lesser amount and frequency of
time spent outdoors (REA, section 5.2). As described in section II.C.1
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.
---------------------------------------------------------------------------

    \78\ 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 than children between the ages of 5 to 18 (REA, p. 2-8).
---------------------------------------------------------------------------

    Table 1 presents 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). These estimates
for the Tulsa study area are much lower than those for the other two
areas (Table 1). No individuals of the simulated at-risk population in
that 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.
    In the other two study areas (Indianapolis and Fall River),
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 (Table 1). 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 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 200 ppb
(Indianapolis), 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).

 Table 1--Air Quality Conditions Adjusted To Just Meet the Current Standard: Percent of Simulated Populations of
 Children With Asthma Estimated To Experience at Least One Daily Maximum 5-Minute Exposure per Year at or Above
                          Indicated Concentrations While Breathing at an Elevated Rate
----------------------------------------------------------------------------------------------------------------
                               Percent (%) of population of children (5-18 years) with asthma  average per year
      5-Minute exposure                                               \A\
    concentration  (ppb)     -----------------------------------------------------------------------------------
                                    Fall River, MA             Indianapolis, IN                Tulsa, OK
----------------------------------------------------------------------------------------------------------------
>=100.......................  19.4-26.7                   22.4-23.0                   0.1-0.4
>=200.......................  <0.1 \B\-0.7 \C\            0.6-0.7                     0
>=300.......................  0                           0.2-0.3 \D\                 0
>=400.......................  ..........................  <0.1-0.1 \D\                ..........................
----------------------------------------------------------------------------------------------------------------
\A\ The values presented in each cell are the averages of the results for the three years simulated for the two
  approaches to air quality adjustment (drawn from Table 6-8 of the REA).
\B\ <0.1 is used to represent nonzero estimates below 0.1%. A value of zero (0) indicates there were no
  individuals estimated to have the selected exposure in any year.
\C\ The highest single year result for 200 ppb was for Fall River where the estimate ranged up to 2.2% (for the
  second air quality adjustment approach in REA, Table 6-8).
\D\ The highest single year results for 300 and 400 ppb were for Indianapolis where the estimates ranged up to
  0.8% and 0.3%, respectively (REA, Table 6-8).

    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 (Table 2;
REA, Tables 6-10 and 6-11). Under conditions just meeting the current
standard in the Indianapolis and Fall River study areas, as many as
1.3% and 1.1%, respectively, of children with asthma, on average across
the 3-year period, were estimated to experience at least one day per
year with a SO2-related doubling in sRaw (Table 2). 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 (Table 2).

 Table 2--Air Quality Conditions Adjusted to Just Meet the Current Standard: Percent of Simulated Population of
  Children With Asthma Estimated To Experience at Least One Day per Year With a SO2-Related Increase in sRaw of
                                                  100% or More
----------------------------------------------------------------------------------------------------------------
                               Percent (%) of population of children (5-18 years) with asthma  average per year
   Lung function decrement                                            \A\
     (increase in sRaw)      -----------------------------------------------------------------------------------
                                    Fall River, MA             Indianapolis, IN                Tulsa, OK
----------------------------------------------------------------------------------------------------------------
>=100%......................  0.9-1.1 \C\                 1.3-1.3                     <0.1 \B\-<0.1
>=200%......................  0.1-0.2 \D\                 0.3-0.3 \D\                 0
----------------------------------------------------------------------------------------------------------------
\A\ The values presented in each cell are the averages of the results for the three years simulated for the two
  approaches to air quality adjustment (drawn from Table 6-10 of the REA).
\B\ <0.1 is used to represent nonzero estimates below 0.1%. A value of zero (0) indicates there were no
  individuals estimated to have the selected decrement in any year.
\C\ The highest single year result for at least 100% increase in sRaw was for Fall River where the estimate
  ranged up to 1.9% (for the second air quality adjustment approach in REA, Table 6-10).
\D\ The highest single year results for at least 200% increase in sRaw were for Indianapolis and Fall River
  where the estimates ranged up to 0.4% (REA, Table 6-10).

D. Proposed Conclusions on the Current Standard

    In reaching proposed conclusions on the current SO2
primary standard, the Administrator has taken into account policy-
relevant evidence-based and quantitative exposure- and risk-based
considerations, as well as advice from the CASAC, and public comment
received thus far in the review. Evidence-based considerations draw
upon the EPA's assessment and integrated synthesis of the scientific
evidence in the ISA of health effects related to SO2
exposure, with a focus on policy-relevant considerations. Exposure- and
risk-based considerations draw upon the EPA's assessment of population
exposure and associated risk in the REA, with a focus on effects
related to asthma exacerbation in the at-risk population of people with
asthma, exposed while breathing at elevated

rates, expected to occur under air quality conditions just meeting the
current standard.
    Building on the discussions of the scientific and technical
assessments presented in the ISA and the REA, and summarized in
sections II.B and II.C above, section II.D.1 below summarizes evidence-
and exposure/risk-based considerations discussed in the PA and
associated conclusions reached in the PA. Section II.D.2 describes
advice received from the CASAC. The Administrator's proposed
conclusions on the current standard are presented in section II.D.3.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy
Assessment
    As in previous NAAQS reviews, the role of the PA in this review is
to help ``bridge the gap'' between the Agency's scientific and
quantitative assessments presented in the ISA and REA, and the
judgments required of the Administrator in determining whether it is
appropriate to retain or revise the NAAQS. Evaluations in the PA focus
on the policy-relevant aspects of the assessment and integrative
synthesis of the currently available health effects evidence in the
ISA, the exposure and risk assessments in the REA, and comments and
advice of the CASAC, with consideration of public comment on drafts of
the ISA, REA, and PA. The PA describes evidence- and exposure/risk-
based considerations and presents conclusions for consideration by the
Administrator in reaching his proposed decision on the current
standard. The main focus of the PA conclusions is consideration of the
question: Does the currently available scientific evidence and
exposure/risk information, as reflected in the ISA and REA, support or
call into question the adequacy of the protection afforded by the
current standard?
    In considering this question, the PA recognizes as an initial
matter that, as is the case in NAAQS reviews in general, the
Administrator's conclusions regarding whether the current primary
SO2 standard provides the requisite public health protection
under the Act will depend on a variety of factors, including science
policy judgments and public health policy judgments. Accordingly, these
factors include public health policy judgments concerning the
appropriate benchmark concentrations on which to place weight, as well
as judgments on the public health significance of the effects that have
been observed at the exposures evaluated in the health effects
evidence. Such judgments, in turn, rely on the interpretation of, and
decisions as to the weight to place on, different aspects of the
results of the REA for the three types of urban exposure circumstances
assessed and associated uncertainties. Accordingly, the Administrator's
conclusions regarding the current standard will depend in part on
judgments regarding aspects of the evidence and exposure/risk
estimates, as well as judgments about the public health protection,
including an adequate margin of safety, that is requisite under the
Clean Air Act.
    The PA response to the overarching question above takes into
consideration the discussions that address the specific policy-relevant
questions for this review, focusing first on consideration of the
evidence, as evaluated in the ISA, including that newly available in
this review, and the extent to which it alters key conclusions
supporting the current standard. The PA also considers the quantitative
exposure and risk estimates drawn from the REA, including associated
limitations and uncertainties, and the extent to which they may
indicate different conclusions from those in the last review regarding
the magnitude of risk, as well as level of protection from adverse
effects, associated with the current standard. The PA additionally
considers the key aspects of the evidence and exposure/risk estimates
that were emphasized in establishing the now-current standard, as well
as the associated public health policy judgments and judgments about
the uncertainties inherent in the scientific evidence and quantitative
analyses that are integral to consideration of whether the currently
available information supports or calls into question the adequacy of
the current primary SO2 standard.
    With regard to the support in the current evidence for
SO2 as the indicator for SOX, the ISA concludes
that of the SOX, ``only SO2 is present at
concentrations in the gas phase that are relevant for chemistry in the
atmospheric boundary layer and troposphere, and for human exposures''
(ISA, p. 2-18), and also that the available health evidence for
SOX is focused on SO2 (ISA, p. 5-1). Thus, the PA
concludes that the current evidence, including that newly available in
this review, continues to support a focus on SO2 in
considering the adequacy of public health protection provided by the
primary NAAQS for SOX.
    As described in the PA and summarized in section II.A.1 above,
selection of the averaging time for the current standard was based on
the need for control of peak SO2 concentrations that have
the potential to contribute to exposures that pose health risks to
people with asthma (for which the current evidence is described in
section II.B above and considered below). When the standard was set in
2010, the Administrator considered a 5-minute averaging time,
concluding that such a standard would result in significant and
unnecessary instability in public health protection, and that the
requisite protection from 5- to 10-minute exposure events could be
provided with a longer, 1-hour averaging time. A 1-hour averaging time
was supported by analyses at that time and by CASAC advice. In
considering pertinent information newly available in this review, the
PA additionally describes analyses of newly available 5-minute and 1-
hour concentrations. The PA finds these newly available quantitative
analyses to demonstrate the current 1-hour standard to exert control on
5-minute exposures of potential concern that is similar to expectations
for such control when the standard was set (PA, section 3.2.4).
    With regard to form and level of the standard, as described in the
PA and summarized in section II.A.1 above, the 99th percentile daily
maximum 1-hour concentration and the level of 75 ppb were chosen for
the new standard in 2010 as providing the appropriate degree of public
health protection from adverse effects associated with short-term
SO2 exposures. These selections were also consistent with
CASAC advice at the time. Newly available in this review are analyses
in the REA focused on assessment of exposure and risk for air quality
conditions just meeting the current standard in all its elements. In
particular, simulation of these conditions includes use of a 3-year
period consistent with the form established for the current standard
(PA, section 3.2.2; REA, section 1.3.1). The resultant exposure and
risk estimates are presented in the REA and considered in the PA, as
summarized below. Based on such considerations, the PA concluded that
it is appropriate to consider retaining the current standard, without
revision in any of its elements. The CASAC concurred, specifically
stating ``that all four elements (indicator, averaging time, form, and
level) should remain the same'' (Cox and Diez Roux, 2018b, p. 3 of
letter). As summarized below, the PA considers the information
pertaining to the four elements of the standard (indicator, averaging
time, level, and form) collectively in evaluating the health protection
afforded by the current standard, consistent with the general approach
summarized in section II.A above.

    In considering the currently available health effects evidence
base, augmented in some aspects since the last review, that provides
the foundation of our understanding of the health effects of
SO2 in ambient air, the PA gives particular attention to the
evidence from controlled human exposure studies that (1) demonstrates
that very short exposures (as short as a few minutes) to
SO2, while breathing at an elevated rate, induces
bronchoconstriction and associated decrements in lung function, which
can be accompanied by symptoms, among individuals with asthma; and, (2)
supports the identification of people with asthma as the population at
risk from short-term peak concentrations in ambient air (ISA, sections
1.6, 1.7, 1.8, 5.2, 6.6; 2008 ISA; U.S. EPA, 1994). While the evidence
base has been augmented 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 populations at risk of SO2-related
effects (PA, section 3.2.1). In this way, the health effects evidence
available in this review is consistent with evidence available in the
last review when the current standard was established (ISA; 2008 ISA;
U.S. EPA, 1994).
    This strong evidence base continues to demonstrate a causal
relationship between short-term SO2 exposures and
respiratory effects, particularly in people with asthma (ISA, p. xlix
and section <IP_ADDRESS>). This conclusion is primarily based on evidence
from controlled human exposure studies, also available at the time of
the last review, that reported lung function decrements and respiratory
symptoms in people with asthma exposed to SO2 for 5 to 10
minutes while breathing at an elevated rate. Support is also provided
by the epidemiologic evidence that is coherent with the controlled
human exposure studies. As in the last review, the currently available
epidemiologic evidence, including that newly available in this review,
includes studies reporting positive associations for asthma-related
hospital admissions and emergency department visits (of individuals of
all ages, including adults and children) with short-term SO2
exposures (ISA, section <IP_ADDRESS>).\79\
---------------------------------------------------------------------------

    \79\ While uncertainties remain related to the potential for
confounding by PM or other copollutants and the representation of
fine-scale temporal variation in personal exposures, the findings of
the epidemiologic studies continue to provide supporting evidence
for the conclusion on the causal relationship (ISA, section
<IP_ADDRESS>).
---------------------------------------------------------------------------

    The health effects evidence newly available in this review also
does not extend our understanding of the range of 5-minute exposure
concentrations that elicit effects in people with asthma exposed while
breathing at an elevated rate beyond what was understood in the last
review (PA, section <IP_ADDRESS>). As in the last review, 200 ppb remains
the lowest concentration tested in exposure studies where study
subjects are freely breathing in exposure chambers (ISA, section
<IP_ADDRESS>). At that exposure concentration, approximately 8 to 9% of
study subjects with asthma, breathing at an elevated rate, experienced
moderate or greater lung function decrements following 5- to 10-minute
controlled exposures (ISA, Table 5-2). The limited information
available for exposure concentrations below 200 ppb is from mouthpiece
exposure studies in which subjects were exposed to a concentration of
100 ppb, with only a few of these studies including an exposure to
clean air while exercising that would have allowed for determining the
effect of SO2 versus the effect of exercise alone (ISA,
section <IP_ADDRESS>; PA, section <IP_ADDRESS>). While, for these reasons, these
data are not amenable to direct quantitative comparisons with the data
for higher exposure concentrations, they generally indicate a somewhat
lesser response. In considering what may be indicated by the
epidemiologic evidence with regard to exposure concentrations eliciting
effects, we recognize complications associated with interpretation of
epidemiologic studies of SO2 in ambient air that relate to
whether measurements at the study monitors adequately represent the
spatiotemporal variability in ambient SO2 concentrations in
the study areas and associated population exposures (ISA, section
<IP_ADDRESS>).
    In this review, as in the last review, there is uncertainty with
regard to exposure levels eliciting effects in some population groups
for which data are limited or not available from the controlled human
exposure studies, such as individuals with severe asthma and children
younger than 12 years old, as well as uncertainty in the extent of
effects at exposure levels below those studied (PA, section 3.2.1; ISA,
p. 5-22). Collectively, these aspects of the evidence and associated
uncertainties contribute to a recognition that for SO2, as
for other pollutants, the available evidence base in this NAAQS review
generally reflects a continuum, consisting of ambient 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.
    As at the time of the last review, the exposure and risk estimates
developed from modeling exposures to SO2 emitted into
ambient air are critically important to consideration of the potential
for exposures and risks of concern under air quality conditions of
interest, and consequently they are critically important to judgments
on the adequacy of public health protection provided by the current
standard. In considering the REA analyses available in this review, the
PA notes the various ways in which these analyses differ and improve
upon those available in the last review. In addition to an expansion in
the number and type of study areas assessed, there are a number of
improvements to input data and modeling approaches, including the
availability of continuous 5-minute air monitoring data at monitors
within the three study areas (PA, section 3.2.2; REA, section 1.3.1).
The current REA extends the time period of simulation by including a 3-
year simulation period consistent with the form established for the
now-current standard (PA, section 3.2.2; REA, section 1.3.1). Further,
the years simulated reflect more recent patterns of emissions and
associated exposure circumstances subsequent to the 2010 decision (PA,
section 3.2.2; REA, section 1.3.1).
    As at the time of the last review, people with asthma are the
population at risk of respiratory effects related to SO2 in
ambient air. Children with asthma may be particularly at risk (PA
section <IP_ADDRESS>; ISA, section <IP_ADDRESS>). While in the U.S. there are
more adults with asthma than children with asthma, the REA results, in
terms of percent of the simulated at-risk populations, indicate higher
exposures and risks for children with asthma as compared to adults.
This finding relates to children's greater frequency and duration of
outdoor activity (REA, sections 2.1.2, 4.3.3, 4.4, 5.2, and 5.3). In
light of these conclusions and findings, we have focused our
consideration of the REA results here on the results for children with
asthma.
    As can be seen by the variation in exposure estimates, the three
study areas in the REA represent an array of emissions sources and
associated exposure circumstances, including those contributing to
relatively higher and relatively lower exposures and associated risk
(PA, section 3.2.2; REA, section 5.4).\80\ As recognized in the

REA, the analyses there are not intended to provide a comprehensive
national assessment. Rather, the analyses for this array of study areas
are intended to indicate the magnitude of exposures and risks that may
be expected in areas of the U.S. that just meet the current standard
but may differ in ways affecting population exposures of interest. In
that way, the REA is intended to be informative to the EPA's
consideration of potential exposures and risks associated with the
current standard and the Administrator's judgments regarding the
protection provided by the current standard. For example, the PA
considered locations within areas that just meet the current standard
where the areas' locations of relatively higher ambient air
concentrations coincide with locations of higher population density. In
so doing, the PA recognized that consideration of such exposures is
particularly important to consideration of the public health protection
afforded by the current standard, and particularly to the overarching
question concerning the availability of information that calls into
question the adequacy of the current standard (PA, sections <IP_ADDRESS> and
<IP_ADDRESS>).
---------------------------------------------------------------------------

    \80\ More specifically, the three areas fall into three
different geographic regions of the U.S. They range from
approximately 180,000 to approximately one half million in total
population, and their populations vary in demographic
characteristics. Additionally, the types of large sources of
SO2 emissions represented in the three study areas vary
with regard to emissions characteristics and include EGUs, petroleum
refineries, glass-making facilities, secondary lead smelters (from
battery recycling), and chemical manufacturing (REA, section 3.1).
---------------------------------------------------------------------------

    With regard to the REA representation of air quality conditions
associated with just meeting the current standard, the PA notes reduced
uncertainty (compared to the 2009 REA) in a few aspects of the approach
for developing this air quality scenario, while additionally
recognizing the uncertainty associated with the application of air
quality adjustments to estimate conditions just meeting the current
standard (PA, sections <IP_ADDRESS> and <IP_ADDRESS>; REA, section 6.2.2). Given
the importance of this aspect of the REA to consideration of the level
of protection provided by the current standard, the PA considers the
results for each study area in terms of a range that reflects variation
associated with the two different methodologies for the first air
quality adjustment approach (REA, section <IP_ADDRESS>).
    In this context, the PA notes that across all three study areas,
which provide an array of SO2 emissions and exposure
situations, the percent of children with asthma estimated to experience
at least one day with as much as a doubling in sRaw (attributable to
SO2), on average across the 3-year period, ranges from <0.1%
to 1.3%; the highest study area estimate is just under 2% for the
highest single year (PA, section 3.2.4; PA, Table 3-4; REA, Table 6-
10). Accordingly, results for the three case study areas indicate at
least 98.7% or more of the at-risk population of children with asthma
to be protected from experiencing a SO2-related doubling in
sRaw, as an average across the 3-year period, and approximately 98% or
more protected from as much as a single occurrence in the single
highest year. Greater protection (e.g., 99% or more) is indicated for
multiple days with a doubling in sRaw and also for single occurrences
of as much as a tripling in sRaw (PA, section 3.2.4; REA, Table 6-11).
    With regard to exposures compared to benchmark concentrations, the
PA notes that less than 1% of children with asthma are estimated to
experience, while breathing at an elevated rate, a daily maximum 5-
minute exposure per year at or above 200 ppb, on average across the 3-
year period, with a maximum for the study area with the highest
estimates just over 2% in the highest single year (PA, section 3.2.4;
PA, Table 3-3; REA, Table 6-8). Further, the percentage for at least
one day with such an exposure at or above 400 ppb is 0.1% or less, as
an average across the 3-year period, and 0.3% or less in each of the
three years simulated across the three study areas (PA, section 3.2.4;
PA, Table 3-3; REA, Table 6-8). No simulated at-risk individuals were
estimated to experience multiple such days (PA, section 3.2.4; REA,
Table 6-9).
    In considering the public health implications of the REA estimated
occurrences of exposures of different magnitudes, the PA takes note of
guidance from the ATS (Thurston et al., 2017; ATS, 2000),\81\ CASAC
advice, and judgments made by the EPA in considering the public health
implications of similar effects in previous NAAQS reviews.\82\
---------------------------------------------------------------------------

    \81\ As recognized in section II.B.4 above, a recent publication
by the ATS provides an updated statement on what constitutes an
adverse health effect of air pollution (Thurston et al., 2017). The
recent ATS statement, while expanding upon the 2000 ATS statement
that was considered in the last review, is generally consistent with
it with regard to aspects pertaining to SO2-related
effects. In that review, the Administrator judged that the effects
reported in exercising people with asthma following 5- to 10-minute
SO2 exposures at or above 200 ppb can result in adverse
health effects (75 FR 35536, June 22, 2010). In so doing, she also
recognized that effects reported for exposures below 400 ppb are
less severe than those at and above 400 ppb, which include larger
decrements in lung function that are frequently accompanied by
respiratory symptoms (75 FR 35547, June 22, 2010).
    \82\ Judgments by the EPA across NAAQS reviews for various
pollutants have particularly emphasized the protection of at-risk
population members from multiple occurrences of exposures or effects
of concern and from such effects of greater severity or that have
been documented to be accompanied by symptoms (75 FR 35520, June 22,
2010; 76 FR 54308, August 31, 2011; 80 FR 65292, October 26, 2015).
---------------------------------------------------------------------------

    In so doing, the PA finds the REA exposure and risk estimates to
indicate that the current standard is likely to provide a high level of
protection from SO2-related health effects to at-risk
populations of children and adults with asthma (PA, section 3.2.4). In
summarizing these findings, the PA also notes the uncertainties in the
REA results (summarized in section II.C.2 above) associated with the
limited or lacking evidence from the controlled human exposure studies
for some subgroups in these populations such as people with severe
asthma and children younger than 12 years old (PA, section 3.2.4).
    The PA additionally reflects on the key aspects of the 2010
decision that established the current standard, such as considerations
of adversity of SO2-related effects to health, and also the
public health implications of associated exposure and risk estimates
for simulated at-risk populations. As an initial matter, the 2010
decision recognized that 5 to 10 minutes ``exposure to SO2
concentrations as low as 200 ppb can result in adverse health effects
in [people with asthma]'' (75 FR 35546, June 22, 2010); \83\ this
judgment was based on consideration of CASAC advice and EPA judgments
in prior NAAQS reviews, as well as ATS guidance. Since the last review,
the ATS has released an additional statement on adversity of air
pollution, which is generally consistent with and supportive of the
earlier statement (available at the time of the 2010 decision) and the
2010 judgments. Additionally, the CASAC has provided advice in the
context of this SO2 NAAQS review, which is summarized in
section II.D.2 below.
---------------------------------------------------------------------------

    \83\ The decision notice additionally stated that ``[t]he
Administrator notes that although these decrements in lung function
have not been shown to be statistically significant at the group
mean level, or to be frequently accompanied by respiratory symptoms,
she considers effects associated with exposures as low as 200 ppb to
be adverse in light of CASAC advice, similar conclusions in prior
NAAQS reviews, and the ATS guidelines described in detail above''
and that ``[t]herefore, she has concluded it appropriate to place
weight on the 200 ppb 5-minute benchmark concentration'' (75 FR
35546, June 22, 2010).
---------------------------------------------------------------------------

    Further, while recognizing the differences between the current REA
analyses and the 2009 REA analyses,

including the 2009 REA's lack of an air quality scenario specific to
the now-current standard in the last review, as well as uncertainties
associated with such analyses, the PA notes a rough consistency of the
associated estimates when considering the array of study areas in both
reviews (PA, section 3.2.4). Overall, the PA finds the newly available
quantitative analyses to 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 (PA, section
3.2.4). With regard to the results for the REA in the last review
(which were for a single-year simulation), the 2010 decision recognized
those results for the area with the highest estimates and largest
population (St. Louis) to indicate that a 1-hour standard of a
magnitude between the two levels assessed in the 2009 REA (50 and 100
ppb) might be expected to protect more than 97% of children with asthma
(and somewhat less than 100%) from experiencing exposures at or above a
200 ppb benchmark concentration and more than 99% of that population
group from experiencing exposures at or above a 400 ppb benchmark (75
FR 35546-47, June 22, 2010; 2009 REA, pp. B-62 and B-63). Single-year
results in the current REA for the two study areas with the highest
estimates (including the area with the most sizeable population,
Indianapolis) indicate protection for the now-current standard of 75
ppb of approximately 98 to 99% of the populations of children with
asthma from experiencing exposures at or above a 200 ppb benchmark
concentration and 99.7% or more of the study area at-risk populations
from exposures at or above 400 ppb (PA, sections <IP_ADDRESS> and 3.2.4;
REA, Table 6-8). These and the similar estimates for a doubling or more
in sRaw are of a magnitude roughly consistent with the level of
protection that was described in establishing the now-current standard
in 2010 (PA, section <IP_ADDRESS>.4).\84\
---------------------------------------------------------------------------

    \84\ For the single-year scenario representing a standard level
of 100 ppb in the study area with the highest population exposure
and risk (St. Louis), the 2009 REA estimated 2.1-2.9% of children
with asthma to experience at least one day with an SO2-
attributable increase in sRaw of at least 100%; the comparable
estimates for a level of 50 ppb were 0.4-0.9% (2009 REA, Table 9-8
and Appendix B).
---------------------------------------------------------------------------

    Additionally, the 2010 decision also took note of the magnitude of
the SO2 concentrations in ambient air in U.S. epidemiologic
studies of associations between ambient air concentrations and
emergency department visits or hospital admissions, for which the
effect estimate remained positive and statistically significant in
copollutant models with PM (PA, sections <IP_ADDRESS>.4 and 3.2.4).\85\ No
additional such studies are available in the current review, as
summarized in section II.B.3 above (PA, section <IP_ADDRESS>). Accordingly,
in considering the main aspects of the decision in the last review, the
PA finds the currently available information to be consistent with that
on which the decision establishing the current standard was based (PA,
section 3.2.4).
---------------------------------------------------------------------------

    \85\ In considering these studies and information regarding
SO2 concentrations in the areas studied, as well as
associated uncertainties, the Administrator concluded that the level
of 75 ppb chosen for the new 1-hour standard provided an adequate
margin of safety (PA, section <IP_ADDRESS>.4; 75 FR 35548, June 22,
2010).
---------------------------------------------------------------------------

    In considering potential public health implications of the current
REA exposure and risk estimates for the three case studies, the PA
recognizes the importance of these estimates to consideration of
whether the currently available information calls into question the
adequacy of public health protection afforded by the current standard.
In so doing, the PA notes that the REA estimates for conditions
associated with just meeting the current standard, are of particular
importance to consideration of exposures and risks in areas still
existing across the U.S. that have source and population
characteristics similar to the study areas assessed, and with ambient
concentrations of SO2 that just meet the current standard
today or that will be reduced to do so at some period in the future. In
this context, the PA takes note of the more than 24 million people with
asthma currently in the U.S., including more than 6 million children,
with potentially somewhat more than 100,000 living within 5 km of large
\86\ sources of SO2 emissions (PA, sections <IP_ADDRESS> and
3.2.4).
---------------------------------------------------------------------------

    \86\ As also summarized in section II.D.1 above, these estimates
are drawn from the PA presentation of estimates of the number of
children living near SO2 emissions sources emitting 1,000
tpy based on the 2014 NEI and the 2015 national estimates of asthma
prevalence (PA, section <IP_ADDRESS> and Table 3-5).
---------------------------------------------------------------------------

    The PA additionally takes note of the uncertainties or limitations
of the current evidence base with regard to the exposure levels at
which effects may be elicited in some population groups (e.g., children
with asthma and individuals with severe asthma), as well as the
severity of the effects in those groups (PA, sections <IP_ADDRESS> and
3.2.4; ISA, pp. 5-22 to 5-25). In so doing, the PA recognizes that the
controlled human exposure studies, on which the depth of the general
understanding of SO2-related health effects is based, are
limited or lacking in providing information with regard to responses in
people with more severe asthma or in children younger than 12 years
(PA, sections <IP_ADDRESS> and 3.2.4; ISA, pp. 5-22 to 5.25). Additional
limitations in understanding relate to the potential for effects in
some people with asthma exposed to concentrations below 200 ppb, as
well as the potential for other air pollutants to affect responses to
SO2 (PA, sections <IP_ADDRESS> and 3.2.4; ISA, pp. 5-22 to 5-26).
In light of these uncertainties, the PA additionally takes note of the
REA results for the lowest benchmark concentration (100 ppb) that
indicate that in some areas of the U.S. under air quality conditions
that just meet the current standard, approximately 20% to just over 25%
of children with asthma may experience one or more days per year, on
average across a 3-year period, with a 5-minute exposure to
concentrations at or above this benchmark while breathing at an
elevated rate (PA, section 3.2.4 and Table 3-3; REA, Table 6-8). Based
on such consideration of the evidence across the exposure
concentrations studied and the exposure/risk information related to the
lowest benchmark concentration, the PA finds that the combined
consideration of the body of evidence and the quantitative exposure
estimates continues to provide support for a standard as protective as
the current one (PA, section 3.2.4).
    The PA further recognizes that the EPA's conclusions regarding the
adequacy of the current standard depend in part on public health policy
judgments identified above and judgments by the Administrator about the
level of public health protection that is appropriate, allowing for an
adequate margin of safety. In so doing, the PA takes note of the long-
standing health effects evidence that documents the effects of
SO2 exposures as short as a few minutes in people with
asthma that are exposed while breathing at elevated rates and
recognizes that such effects have been documented at the lowest
concentration studied in exposure chambers with appropriate clean-air
controls (PA, section 3.2.4). The PA additionally notes that it was
recognized in the last review that such exposures can result in adverse
health effects in people with asthma (75 FR 35546-47, June 22, 2010),
and that there are limitations, and associated uncertainty, in the
evidence available for the lower exposure concentration of 100 ppb
(summarized in section II.B.3 above), as was the case in the last
review. The PA further notes the indication of an appreciable reduction
in the magnitude of the SO2-induced response in exercising
people with asthma at this

lower exposure concentration compared with responses observed for
exposures at 200 ppb (PA, sections <IP_ADDRESS>, <IP_ADDRESS> and 3.2.4). Thus,
in focusing on the potential for 5-minute exposures at and above 200
ppb, the PA takes note of the REA results that indicate the current
standard may be expected to protect approximately 98% and nearly 99% of
populations of children with asthma from experiencing any days with
such exposures in the highest year and on average each year in a 3-year
period, respectively (PA, sections <IP_ADDRESS> and 3.2.4; REA, Table 6-8).
The PA additionally notes that the REA estimates indicate the current
standard may be expected to protect more than 99% of children from
experiencing any days with a 5-minute exposure of 300 ppb or higher,
with the estimates for the 400 ppb benchmark indicating protection of
at least 99.7% and 99.9% of children with asthma from experiencing any
days with a 5-minute exposure of 400 ppb or higher in the highest year
and in each year on average for a 3-year period, respectively (PA,
sections <IP_ADDRESS> and 3.2.4; REA, Table 6-8). In considering these
results, the PA notes the lesser severity of effects reported for
exposures below 400 ppb than those at and above 400 ppb, which include
larger decrements in lung function that are frequently accompanied by
respiratory symptoms, facts given weight in establishing the current
standard in 2010 (75 FR 35547, June 22, 2010).\87\ With regard to the
potential for children to experience SO2-related lung
function decrements in terms of at least a doubling in sRaw, the PA
takes note of the REA results that indicate the current standard may be
expected to protect approximately 98.1% and nearly 98.7% from
experiencing any days with such decrements, in the highest year of the
3-year period and in each year on average for the period, respectively
(PA, sections <IP_ADDRESS> and 3.2.4; REA, Table 6-10). In light of ATS
guidance, CASAC advice and EPA judgments in past NAAQS reviews, the PA
finds these results to indicate a high level of protection of at-risk
populations from SO2-related health effects. The PA further
notes that this protection is also consistent with the level of
protection indicated by the information considered when the standard
was set (PA, section 3.2.4). Accordingly, the PA finds that the
currently available evidence and quantitative information, including
the associated uncertainties, do not call into question the adequacy of
protection provided by the current standard and thus support
consideration of retaining the current standard, without revision (PA,
section 3.2.4).
---------------------------------------------------------------------------

    \87\ In that review, the Administrator judged that the effects
reported in exercising people with asthma following 5- to 10-minute
SO2 exposures at or above 200 ppb can result in adverse
health effects (75 FR 35536, June 22, 2010). In so doing, she also
recognized that effects reported for exposures below 400 ppb are
less severe than those at and above 400 ppb, which include larger
decrements in lung function that are frequently accompanied by
respiratory symptoms (75 FR 35547, June 22, 2010).
---------------------------------------------------------------------------

    Overall, the PA recognizes that the newly available health effects
evidence, critically assessed in the ISA as part of the full body of
evidence, reaffirms conclusions on the respiratory effects recognized
for SO2 in the last review (PA, sections 3.2.1 and 3.2.4).
Further, there is a general consistency of the currently available
evidence with the evidence that was available in the last review,
including with regard to key aspects on which the current standard is
based (PA, sections 3.2.1 and 3.2.4). The quantitative exposure and
risk estimates for conditions just meeting the current standard
indicate a similar level of protection, for at-risk populations from
respiratory effects considered to be adverse, as that indicated by the
information considered in the decision for the 2010 review in
establishing the now-current standard (PA, sections 3.2.2 and 3.2.4.).
As in the last review, limitations and uncertainties are associated
with the available information, as summarized in section 3.2.4 of the
PA.
    Collectively, the PA finds that the evidence and exposure/risk
based considerations provide the basis for its conclusion that
consideration should be given to retaining the current standard,
without revision (PA, section 3.2.4). Accordingly, and in light of this
conclusion that it is appropriate to consider the current standard to
be adequate, the PA did not identify any potential alternative
standards for consideration in this review (PA, section 3.2.4).
2. CASAC Advice
    In the current review of the primary standard for SOX,
the CASAC has provided advice and recommendations in their review of
drafts of the IRP, ISA, REA and PA, and of the REA Planning Document.
    In their 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 comments additionally state 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 Responses 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.\88\ The CASAC suggested research and data gathering in
these and other areas that would inform the next SO2 primary
standard review (Cox and Diez Roux, 2018b, p. 6 of the Consensus
Responses to Charge Questions).
---------------------------------------------------------------------------

    \88\ These and other comments from the CASAC on the draft PA and
REA were considered in preparing the final PA and REA (USEPA,
2018a,b).
---------------------------------------------------------------------------

3. Administrator's Proposed Conclusions on the Current Standard
    Based on the large body of evidence concerning the health effects
and potential public health impacts of exposure to SOX in
ambient air, and taking into consideration the attendant uncertainties
and limitations of the evidence, the Administrator proposes to conclude
that the current primary SO2 standard provides the requisite
protection of public health, including an adequate margin of safety,
and should therefore be retained, without revision. In reaching these
proposed conclusions, the Administrator has carefully considered the
assessment of the available health effects evidence and conclusions
contained in the ISA; the quantitative analyses in the REA; the
evaluation of policy-relevant aspects of the evidence and quantitative
analyses in the PA; the advice and recommendations from the CASAC
(summarized in section II.D.2 above); and public comments received to
date in this review.\89\
---------------------------------------------------------------------------

    \89\ For example, of the limited public comments received in the
docket for this review to date that have addressed adequacy of the
current primary standard for SOX, two commenters, one a
state agency and one an industry organization, support retaining the
current standard without revision. Two other industry organizations
suggest that consideration be given to an increased level for the 1-
hour standard. One of these suggested a doubling in the level, while
the sole commenting environmental organization suggested reducing
the level by half.
---------------------------------------------------------------------------

    In the discussion below, the Administrator considers first the
evidence base on health effects associated with short-term exposure to
SO2, including the controlled human exposure studies that
document respiratory effects in people with asthma exposed for as short
as a few minutes while breathing at elevated rates and the relative
lack of such information for some subgroups of this population,
including young children and people with severe asthma. He additionally
notes the available epidemiologic evidence that documents associations
between short-term concentrations of SO2 in ambient air and
asthma-related health outcomes, particularly in children. Further, the
Administrator considers the estimates of SO2 exposures and
risk in multiple study areas under air quality conditions just meeting
the current standard (summarized in sections II.C and II.D.1 above),
and the public health implications of those results. The Administrator
additionally considers uncertainties in the evidence and the exposure/
risk information, as a part of public health policy judgments essential
to decisions regarding the adequacy of the protection provided by the
standard, similar to the judgements made in establishing the current
standard. He draws on the PA considerations, and PA conclusions in the
current review, with which the CASAC has concurred, taking note of key
aspects of the rationale presented for those conclusions. Further, the
Administrator considers the advice of the CASAC, including particularly
its overall agreement with the PA conclusion that the current evidence
and quantitative exposure and risk estimates provide support for
retaining the current standard and the CASAC's recommendation to retain
all elements of the standard without revision (Cox and Diez Roux,
2018b).
    With regard to the evidence base for SO2, the
Administrator first recognizes the long-standing evidence that has
established the key aspects of the harmful effects of very short
SO2 exposures on people with asthma that are relevant to
this review as they were relevant in 2010 when the current short-term
standard was established. 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 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).\90\ The
evidence base additionally includes epidemiologic studies that provide
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 ISA recognizes that
while these studies analyze hourly or daily metrics, there is the
potential for shorter-term concentrations within the study areas to be
playing a role in such associations. The ISA also notes associated
uncertainties related to potential confounding from co-occurring
pollutants such as PM, a chemical mixture including some components for
which SO2 is a precursor, and also related to exposure
estimates and the ability of fixed-site monitors to adequately
represent variations in personal exposure, particularly with regard to
peak exposures, as summarized in section II.B.3 above (ISA, p. 5-37;
PA, section <IP_ADDRESS>).\91\
---------------------------------------------------------------------------

    \90\ For people without asthma, such effects have only been
observed in studies of exposure concentrations at or above 1000 ppb
(ISA, section <IP_ADDRESS>).
    \91\ 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 the 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, including
more than 200 new health studies, 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 the health effects evidence available
in this review 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 recognizes that the ISA conclusion on the
respiratory

effects caused by short-term exposures is based primarily on evidence
from controlled human exposure studies, available at the time of the
last review, that reported 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
(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). The Administrator further notes the control of
peak 5-minute exposures that is provided by the current 1-hour
standard, as indicated by the exposure analysis in the REA and air
quality analyses in the PA (PA, chapter 2 and Appendix B).
    With regard to exposure concentrations of interest in this review,
the Administrator takes particular note of the evidence from controlled
human exposure studies that demonstrate the occurrence of 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 such study findings for which
exposure concentration-specific data are available to the EPA for
individual subjects (ISA, Table 5-2 and Figure 5-1, summarized in Table
3-1 of the PA).\92\ These data demonstrate such effects related to
asthma exacerbation in sensitive people with asthma exposed to
SO2 concentrations as low as 200 ppb. These data include
limited evidence of respiratory symptoms accompanying the lung function
effects at this exposure level (ISA, Table 5-2). 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,
approximately 10% of study subjects experienced moderate or greater
lung function decrements at 200 ppb, while at 300-400 ppb, as many as
approximately 30% of subjects in some studies experienced such
decrements. Further, at concentrations at or above 400 ppb, the
moderate or greater decrements in lung function 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).
---------------------------------------------------------------------------

    \92\ The availability of individual subject data allowed for the
comparison of results in consistent manner across studies (ISA,
Table 5-2; Long and Brown, 2018).
---------------------------------------------------------------------------

    In considering the potential public health significance of effects
associated with SO2 exposures, the Administrator further
recognizes the greater significance accorded both to 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 above).\93\
For example, he notes that the ATS indicated it to be appropriate to
consider small lung function changes as adverse when they occur in
individuals with pre-existing compromised function, ``such as resulting
from asthma, even without accompanying respiratory symptoms'' (Thurston
et al., 2017). Thus, with regard to the health effects evidence for
SO2, the Administrator recognizes that health effects
resulting from exposures at and above 400 ppb are appreciably more
severe than those elicited by exposure to SO2 concentrations
as low as 200 ppb (and 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.
---------------------------------------------------------------------------

    \93\ 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).
---------------------------------------------------------------------------

    As at the time of the last review, the Administrator considers the
health effects evidence in the context of the exposure and risk
modeling, including key limitations and uncertainties, as summarized in
the PA and section II.C.1 above (described in detail in the REA). In so
doing, he recognizes such a context to be critical for SO2,
for which health effects in people with asthma are linked to exposures
during periods of elevated breathing rates, such as while exercising.
Thus, population exposure modeling that takes activity levels into
account is integral to consideration of population exposures compared
to benchmark concentrations and of population risk of lung function
decrements.
    In considering the exposure and risk estimates, the Administrator
recognizes that unlike the REA available in the last review, which
analyzed single-year air quality scenarios for potential standard
levels bracketing the now current level, the current REA assesses an
air quality scenario for three years of air quality conditions that
just meet the current standard, including its 3-year form. 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.
These data are available 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 of the
protection provided from elevated short-term (5- to 10-minute exposure)
SO2 concentrations for people with asthma breathing at
elevated rates (75 FR 35567-68, June 22, 2010). 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, as contrasted to the single
such area in the 2009 REA.
    In considering the REA results for the benchmark comparisons for
the three years analyzed in each of the three study areas, the
Administrator notes the estimates of as many as 0.7% 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, and as many as 2.2% in a
single year. He additionally takes note of the REA findings that also
estimate somewhat less than 0.1% of children with asthma to experience
multiple days with such exposures in any one year. In turning to
consideration of the REA estimates of lung function risk, the
Administrator notes that as many as 1.9% of children with asthma are
estimated to experience a day in a single year with an SO2-
related doubling of sRaw, and as many as 1.3% per year on average
across three years. He further takes note that as many as 1% of
children with asthma may be estimated to experience multiple days in a
single year (0.7% on average across multiple years) with a lung
function decrement of such a magnitude, and as many as 0.3% (on average
across multiple years) may be estimated to

experience a day with at least a tripling in sRaw (as summarized in
section II.C.3 above).
    In considering the level of protection indicated by these estimates
of exposure and risk under air quality conditions that just meet the
current standard, the Administrator additionally recognizes the
limitations in the available evidence base that contribute to
uncertainties with regard to the risk estimates for lung function
decrements in young children with asthma and in individuals of any age
with severe asthma. While health effects study data are limited or
lacking for these population groups, the ISA indicates a potential for
these groups to experience somewhat greater health impacts than the
populations studied (as summarized in section II.B above). In light of
these limitations of the evidence and the potential articulated in the
ISA for the risk to be greater for these groups for which the evidence
is limited or lacking, the Administrator notes 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.
    The Administrator additionally notes the PA consideration of the
sizeable number of at-risk individuals living in locations near large
SO2 emissions sources that may contribute to increased
SO2 concentrations in ambient air. The information
concerning population exposure characteristics such as the co-
occurrence of elevated ambient air concentrations with areas of
relatively higher population density is not available for all of these
locations. Consideration of the population sizes in these areas and the
potential for similarity of exposure characteristics in some of these
areas to the study areas assessed in the REA (as summarized in section
II.D.1 above) confirms the public health relevance of the REA results
to this review of the current standard.
    In considering the adequacy of the protection provided by the
current standard, the Administrator notes the findings of the REA in
light of considerations recognized above regarding the significance
associated with different exposure benchmark concentrations and
severity of lung function decrements, as well as the estimated
frequency of occurrence of such concentrations and decrements under air
quality conditions just meeting the current standard. Given the clear
concentration-response relationship documented in the evidence for the
key effects in people with asthma across the range of exposure
concentrations studied, higher SO2 concentrations would be
expected to contribute to greater severity and frequency in occurrence
of responses in at-risk groups. Other considerations summarized above,
include the strong evidence for lung function decrements in people with
asthma exposed for just a few minutes while breathing at elevated rates
(e.g., while exercising) to SO2 concentrations as low as 200
ppb, the public health implications of such exposures, and related
considerations raised by the ATS in its statement on adverse effects of
air pollution. Further, advice from the CASAC included its conclusion
that the current evidence and exposure/risk information supports
retaining the current standard and its associated caution as to
uncertainty in the adequacy of the margin of safety provided by the
current standard for less well studied yet potentially susceptible
population groups.\94\ Based on all of these considerations, the
Administrator gives weight to the PA findings, summarized in section
II.D.1 above, that the current body of evidence, in combination with
the exposure/risk information, does not support a primary standard that
is less protective than the current standard. Thus, he proposes to
conclude that a less stringent standard would not provide the requisite
protection of public health, including an adequate margin of safety.
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    \94\ In conveying this caution related to such population
groups, the CASAC additionally recognized there to be ``considerable
uncertainty'' and concluded that ``the CASAC does not recommend
reconsideration of the level in order to provide a greater margin of
safety'' (Cox and Diez Roux, 2018, Consensus Responses, p. 5).
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    Turning to consideration of the adequacy of protection provided by
the current standard from effects associated with lower exposures,
including those at or below 200 ppb, the Administrator considers the
public health significance of the REA estimates for such effects, and
of 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. In so
doing, the Administrator 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, but
also aspects with which there may be substantial uncertainty. In the
case of the primary SO2 standard review, he considers the
increased uncertainty recognized in the PA with regard to
characterization of the risk of lung function decrements (including
their magnitude and prevalence, and the associated health significance)
at exposure levels below those represented in the controlled human
exposure studies and in populations potentially at risk \95\ but for
which the evidence base is limited or lacking (PA, section <IP_ADDRESS>;
REA, section 5.3). He additionally considers the uncertainties
recognized in the PA, and summarized in section II.B and II.D.1 above,
regarding exposure measurement error and copollutant confounding in the
epidemiologic evidence. In so doing, the Administrator recognizes that
collectively, these aspects of the evidence and associated
uncertainties support an acknowledgment that for SO2, as for
other pollutants, the available 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.
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    \95\ Such populations include those for which the CASAC
described there to be ``considerable uncertainty'' (Cox and Diez
Roux, 2018, Consensus Responses, p. 5).
---------------------------------------------------------------------------

    In considering the point at which health effects associated with
lower levels of SO2 exposure become important from a public
health perspective, the Administrator takes note of the PA
consideration of the CASAC advice and EPA judgments in establishing the
current standard in 2010, as well as the currently available
information and commonly accepted guidelines or criteria within the
public health community, including the ATS, an organization of
respiratory disease specialists,\96\ for interpreting public health
significance of moderate or greater lung function decrements,
particularly when accompanied by respiratory symptoms, and their
occurrence in a portion of the at-risk populations. In so doing, the
Administrator additionally notes that the most recent ATS statement on
adversity of air pollution is generally consistent with its prior
statement that was referenced when the current standard was set (PA,
section <IP_ADDRESS>.). He also takes note of EPA judgments in prior NAAQS
decisions for SOX and

other pollutants that, consistent with these statements, have
particularly emphasized the protection of at-risk population members
from multiple occurrences of exposures or effects of concern and from
such effects of greater severity or that have been documented to be
accompanied by symptoms (75 FR 35520, June 22, 2010; 76 FR 54308,
August 31, 2011; 80 FR 65292, October 26, 2015). Together these factors
inform the Administrator's consideration in this review of public
health implications of the exposure and risk estimates for air quality
conditions just meeting the current primary SO2 standard.
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    \96\ With regard to commonly accepted guidelines or criteria
within the public health community, the PA considered statements
issued by the ATS (as summarized in section II.D.1 above).
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    Thus, in considering the evidence and quantitative exposure and
risk estimates available in this review with regard to the adequacy of
public health protection provided by the current primary standard from
respiratory effects associated with the lowest SO2 exposure
concentrations represented in the health effects evidence, the
Administrator recognizes that, as noted in section II.A above, the
final decision on such judgments is largely 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. These judgments are informed by the
recognition, noted just above, that the available health effects
evidence generally reflects a continuum, consisting of ambient 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. Accordingly, the
Administrator's final 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. As
described in section I.A above, the Act does not require that primary
standards be set at a zero-risk level; 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.
    In this light, the Administrator takes note of PA considerations
regarding the REA results and the associated uncertainties (summarized
in section II.C above), as well as the nature and magnitude of the
uncertainties inherent in the scientific evidence upon which the REA is
based. The Administrator finds such considerations collectively 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.\97\ In turning first to the
REA estimates of the percent of children with asthma estimated to
experience a day with a 5-minute SO2 exposure, while
breathing at elevated rates, above benchmark concentrations, the
Administrator notes the very small percentage (no more than 0.3% in a
the highest year) of children with asthma estimated to experience a
single day per year at/above the benchmark concentration of 400 ppb, an
exposure level frequently associated with respiratory symptoms in
controlled human exposure studies. In particular, he takes note of the
fact 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.
The Administrator considers these results 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 frequently been accompanied by respiratory symptoms. The
Administrator additionally notes the small percentage (no more than
approximately 2% in the highest year) of children with asthma estimated
to experience a single day with a 5-minute exposure at or above the
lower exposure concentration of 200 ppb, and that less than 0.1% of
that population group is estimated to experience more than a single
such day in the highest year. In so doing, he recognizes, as did the
Administrator in the last review, that effects resulting from this
lower exposure concentration are appreciably less severe (e.g., in
terms of prevalence of study subjects experiencing a tripling or more
in sRaw as well as a 20% reduction in FEV1) than those
elicited by exposures at or above 400 ppb, and that they are less
frequently accompanied by respiratory symptoms (ISA, Table 5-2 and
Figure 5-1; PA, Table 3-1 and section <IP_ADDRESS>).
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    \97\ Such judgments are among those important to decisions on
the adequacy of the margin of safety allowed by the current
standard.
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    The Administrator additionally considers the PA findings regarding
the REA estimates of lung function risk in terms of lung function
decrements as assessed using doubling and tripling of sRaw. The
Administrator finds the REA estimates to indicate a high level of
protection for children with asthma against the risk of lung function
decrements, and particularly against the larger decrements (e.g.,
tripling in sRaw) and against multiple occurrences. The REA results for
air quality conditions that just meet the current standard indicate,
based on average estimates across the 3-year period, protection of more
than 99.7% of children with asthma from experiencing a day per year
with a SO2-related tripling of sRaw and at least 99.8% from
experiencing multiple such days per year. The results further indicate
99% or more of children with asthma to be protected from multiple days
with a SO2-related doubling of sRaw.
    Taking the REA estimates of exposure and risk together, while
recognizing the uncertainties associated with such estimates for the
scenarios of air quality developed to represent conditions just meeting
the current standard, the Administrator considers the current standard
to provide a high degree of protection to at-risk populations from
SO2 exposures associated with health effects 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).
He further considers the current standard 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. In considering the adequacy of protection provided by the
current standard from these lower exposure concentrations, the
Administrator additionally takes note of the array of limitations in
the evidence summarized above with regard to characterizing the
potential response of at-risk individuals to exposures below 200 ppb,
which the PA indicates to be much reduced. He also notes the
limitations in the evidence for population groups potentially at risk
but for which the evidence of risk is limited (PA, section <IP_ADDRESS>;
REA, section 5.3). Based on these and all of the above considerations,
the Administrator proposes 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.
    With regard to key aspects of the specific elements of the
standard, the Administrator recognizes first the support in the current
evidence base for

SO2 as the indicator for SOX. 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, as described in the PA and summarized in sections
II.B.1 and II.D.1 above. He additionally recognizes 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 takes note of the REA
results as discussed above and the level of protection that they
indicate the elements of the current standard to provide. 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, as described above.
    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 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 further proposes to conclude that it is appropriate to
retain the current standard without revision. The Administrator bases
these proposed conclusions on consideration of the health effects
evidence, including consideration of this evidence in the context of
the quantitative exposure and risk analyses, recognizing the
uncertainties associated with both. Inherent in the Administrator's
proposed conclusions are public health policy judgments, including
those regarding the public health significance of the SO2-
related effects estimated to occur in small portions of the at-risk
populations under air quality conditions adjusted to just meet the
current standard. In reaching his proposed conclusion on the adequacy
of public health protection afforded by the existing primary standard,
the Administrator recognizes that the Act requires primary standards to
be requisite to protect public health with an adequate margin of
safety, and neither more nor less stringent than necessary for this
purpose (see generally, Whitman v. American Trucking Associations, 531
U.S. 457, 465-472, 475-76 [2001]). The Administrator also recognizes
that the Act does not require that primary standards be set at a zero-
risk level or to protect the most sensitive individual, but rather at a
level that avoids unacceptable risks to public health, even if the risk
is not precisely identified as to nature or degree. The Administrator
finds the current standard to provide such a level of public health
protection. Thus, the Administrator proposes to conclude 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 proposes 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. The
Administrator solicits comment on this proposed conclusion.
    Having reached the proposed decision described here based on
interpretation of the health effects evidence, as assessed in the ISA,
and the quantitative analyses in the REA; the evaluation of policy-