Policy Assessment for the Review of
the Primary National Ambient Air
Quality Standard for Sulfur Oxides,
External Review Draft

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EPA-452/P-17-003
August 2017
Policy Assessment for the Review of
the Primary National Ambient Air
Quality Standard for Sulfur Oxides,
External Review Draft
U. S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
Research Triangle Park, North Carolina

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DISCLAIMER
This document has been prepared by staff in the Health and Environmental Impacts
Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency
(EPA). Any findings and conclusions are those of the authors and do not necessarily reflect the
views of the Agency. This document is being circulated to facilitated discussion with the Clean
Air Scientific Advisory Committee and for public comment to inform the EPA's consideration of
the primary national ambient air quality standard for sulfur oxides. This information is
distributed for purposes of pre-dissemination peer review under applicable information quality
guidelines. It does not represent and should not be construed to represent any Agency
determination or policy.
Questions or comments related to this document should be addressed to Dr. Nicole
Hagan, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards,
C504-06, Research Triangle Park, North Carolina 27711 (email: hagan.nicole@epa.gov).
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TABLE OF CONTENTS
LIST OF APPENDICES	iii
LIST OF FIGURES	iv
LIST OF TABLES	v
LIST 01 ACRONYMS AND ABBREVIATIONS	vi
1	INTRODUCTION	1-1
1.1	Purpose	1-1
1.2	Background	1-3
1.2.1	Legislative Requirements	1-3
1.2.2	History of the Reviews of the Primary NAAQS for SOx	1-5
1.2.3	Current SO2 NAAQS Review	1-8
1.3	General Approach and Organization of this Document	1-10
REFERENCES	1-11
2	CURRENT AIR QUALITY	2-1
2.1	Sources to Ambient Air	2-1
2.2	Ambient Air Monitoring Methods and Network	2-3
2.3	Ambient Air Monitoring Concentrations	2-5
2.3.1	Trends	2-5
2.3.2	Current Concentrations	2-7
REFERENCES	2-12
3	REVIEW OF THE PRIMARY STANDARD FOR SULFUR OXIDES	3-1
3.1	Approach	3-1
3.1.1	Approach in the Previous Review	3-2
3.1.2	Approach for the Current Revi ew	3-11
3.2	Adequacy of the Current Standard	3-14
3.2.1	Evidence-based Considerations	3-14
3.2.2	Exposure/Risk-based Considerations	3-35
3.2.3	Preliminary Staff Conclusions on the Current Standard	3-51
3.3	Key Uncertainties and Areas for Future Research and Data Collection	3-58
REFERENCES	3-60
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LIST OF APPENDICES
A.	Preparation of data files for generation of figures in Chapter 2
B.	Additional information on datasets presented in Figure 2-8
C.	Occurrences of 5-minute SO2 concentrations of interest in the recent ambient air
monitoring data (2013-2015)
D.	Air quality information for geographical areas of three selected U.S.
epidemiological studies
E.	Derivation of design values presented in Appendix D
F.	Geographic distribution of continental U.S. facilities emitting more than 1,000 tpy
SO2 and population density based on U.S. census tracts
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LIST OF FIGURES
Figure 2-1.	Percent contribution of SO2 emissions by sector	2-2
Figure 2-2.	National SO2 emission trends by sector	2-3
Figure 2-3.	Temporal trend in number of monitors with 5-minute data	2-4
Figure 2-4.	National temporal trend in SO2 concentrations: 1980-2015 (45 sites)	2-5
Figure 2-5.	Temporal trend in SO2 concentrations: 2000-2015 (227 sites)	2-6
Figure 2-6.	Temporal trend in daily maximum 5-minute SO2 concentrations: 2011-2015	2-6
Figure 2-7. Concentrations of SO2 in terms of the current standard (3-year average of annual
99th percentile daily maximum 1-hour concentrations) at sites with datasets meeting
completeness requirements for 2013-2015	2-7
Figure 2-8. Distributions of daily maximum 5-minute concentrations during 2013-2015	2-11
Figure 3-1. Overview of the approach for review of the current primary standard	3-13
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LIST OF TABLES
Table 1-1. History of the primary national ambient air quality standard(s) for sulfur oxides
since 1971	1-7
Table 3-1. Percentage of adults with asthma in controlled human exposure studies experiencing
sulfur dioxide-induced decrements in lung function and respiratory symptoms
(adapted from Table 5-2 in the second draft ISA)	3-24
Table 3-2. 2015 National Asthma Prevalence	3-34
Table 3-3. 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 at elevated ventilation	3-42
Table 3-4. 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 SCh-related increase in sRaw of 100% or more	3-43
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LIST OF ACRONYMS AND ABBREVIATIONS
AHR	airway hyperresponsiveness
APEX	Air Pollutants Exposure model
AQCD	Air Quality Criteria Document
AQS	Air Quality System
CAA	Clean Air Act
CASAC	Clean Air Scientific Advisory Committee
CHAD	Consolidated Human Activity Database
DV	design value
ED	emergency department
EGU	Electricity generating unit
EPA	Environmental Protection Agency
FEM	federal equivalent method
FEVi	forced expiratory volume in one minute
FRM	federal reference method
IRP	Integrated Review Plan
ISA	Integrated Science Assessment
ME	microenvironment
NAAQS	National Ambient Air Quality Standard
NCEA	National Center for Environmental Assessment
NEI	National Emissions Inventory
NO2	nitrogen dioxide
O3	ozone
OAQPS	Office of Air Quality Planning and Standards
ppb	parts per billion
ppm	parts per million
PA	Policy Assessment
PM	particulate matter
REA	Risk and Exposure Assessment
SLAMS	State and Local Air Monitoring Stations
502	sulfur dioxide
503	sulfur tri oxide
SOx	oxides of sulfur
sRaw	specific airway resistance
USB	United States background
UVF	ultraviolet fluorescence
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1 INTRODUCTION
1.1 PURPOSE
This document, Policy Assessment for the Review of the Primary National Ambient Air
Quality Standard for Sulfur Oxides, External Review Draft (hereafter referred to as Draft PA),
presents the draft policy assessment for the U.S. Environmental Protection Agency's (EPA's)
current review of the primary (health-based)1 national ambient air quality standard (NAAQS) for
sulfur oxides (SOx).2 The overall plan and schedule for this review were presented in the
Integrated Review Plan for the Primary National Ambient Air Quality Standardfor Sulfur
Dioxide (IRP; U.S. EPA, 2014). The IRP also identified key policy-relevant issues to be
addressed in this review and discussed the key documents that generally inform NAAQS
reviews, including an Integrated Science Assessment (ISA), a Risk and Exposure Assessment
(REA), and a Policy Assessment (PA).
The PA presents a staff evaluation of the policy implications of the key scientific and
technical information in the ISA and REA for consideration by the Administrator.3 Ultimately, a
final decision on the primary standard for SOx will reflect the judgments of the Administrator.
The role of the PA is to help "bridge the gap" between the Agency's scientific assessments
presented in the ISA and REA, and the judgments required of the EPA Administrator in
determining whether it is appropriate to retain or revise the NAAQS.
In evaluating the adequacy of the current standard and whether it is appropriate to
consider alternative standards, the PA focuses on information that is most pertinent to evaluating
the basic elements of the NAAQS: indicator, averaging time, form, and level.4 These elements,
1	The EPA is separately reviewing the welfare effects associated with sulfur oxides and the public welfare protection
provided by the secondary S02 standard, in conjunction with a review of the secondary standards for nitrogen
oxides and particulate matter with respect to their protection of the public welfare from adverse effects related to
ecological effects (U.S. EPA, 2017a).
2	This review focuses on the presence in ambient air of sulfur oxides, a group of closely related gaseous compounds
that include sulfur dioxide and sulfur trioxide and of which sulfur dioxide is the most prevalent. Particulate
atmospheric transformation products of SOx, such as sulfates, are addressed in the review of the NAAQS for
particulate matter.
3	The terms "staff," "we," and "our" throughout this document refer to the staff in the EPA's Office of Air Quality
Planning and Standards (OAQPS).
4	The basic elements of a standard include the indicator, averaging time, form, and level. The indicator defines the
chemical species or mixture to be measured in the ambient air for the purpose of determining whether an area
attains the standard. The averaging time defines the period over which air quality measurements are to be
obtained and averaged or cumulated. The form of a standard defines the air quality statistic that is to be compared
to the level of the standard in determining whether an area attains the standard. The level of the standard defines
the air quality concentration used for that purpose (i.e., an ambient air concentration of the indicator).
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which together serve to define each standard, must be considered collectively in evaluating the
health protection afforded by the primary standard for SOx.
The development of the PA is also intended to facilitate advice to the Agency and
recommendations to the Administrator from an independent scientific review committee, the
Clean Air Scientific Advisory Committee (CASAC), as provided for in the Clean Air Act
(CAA). As discussed below in section 1.2.1, the CASAC is to advise not only on the Agency's
assessment of the relevant scientific information, but also on the adequacy of the current
standards, and to make recommendations as to any revisions of the standards that may be
appropriate. The EPA makes available to the CASAC and the public one or more drafts of the
PA for CASAC review and public comment.
The decision whether to prepare one or more drafts of the PA is influenced by
preliminary staff conclusions and associated CASAC advice and public comments, among other
factors. Typically, a second draft PA has been prepared in cases where the available information
calls into question the adequacy of the current standard and analyses of potential alternative
standards are developed taking into consideration CASAC advice and public comment. In such
cases, a second draft PA includes preliminary staff conclusions regarding potential alternative
standards and undergoes CASAC review and public comment prior to preparation of the final
PA. When such analyses are not undertaken, a second draft PA may not be warranted.
In this draft PA, we take into account the available scientific and technical information,
as assessed in the second draft Integrated Science Assessment for Sulfur Oxides - Health Criteria
(second draft ISA [U.S. EPA, 2016]) and the draft Risk and Exposure Assessment for the Review
of the Primary National Ambient Air Quality Standardfor Sulfur Oxides, External Review Draft
(draft REA [U.S. EPA, 2017b]). The evaluation and preliminary staff conclusions presented in
this draft PA for the primary NAAQS for SOx have been informed by comments and advice
received from the CASAC in their reviews of the other draft Agency documents prepared thus
far in this NAAQS review. Review and comments from the CASAC, and public comment, on
this draft of the PA will inform the final evaluation and staff conclusions in the final PA. The
final PA will inform the Administrator's decision in this review of the primary SO2 NAAQS.
Beyond informing the EPA Administrator and facilitating the advice and
recommendations of the CASAC, the PA is also intended to be a useful reference to all parties
interested in the review of the primary NAAQS for SOx. In these roles, it is intended to serve as
a source of policy-relevant information that is informing the Agency's review of the primary
NAAQS for SOx, and it is written to be understandable to a broad audience.
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1.2 BACKGROUND
1.2.1 Legislative Requirements
Two sections of the CAA govern the establishment and revision of the NAAQS. Section
108 [42 U.S.C. 7408] directs the Administrator to identify and list certain air pollutants and then
to issue air quality criteria for those pollutants. The Administrator is to list those pollutants that
in his "judgment, cause or contribute to air pollution which may reasonably be anticipated to
endanger public health or welfare," "the presence of which in the ambient air results from
numerous or diverse mobile or stationary sources"; and for which he "plans to issue air quality
criteria... " Air quality criteria are intended to "accurately reflect the latest scientific knowledge
useful in indicating the kind and extent of all identifiable effects on public health or welfare
which may be expected from the presence of [a] pollutant in the ambient air...." 42 U.S.C. §
7408(a)(2).
Section 109 [42 U.S.C. 7409] directs the Administrator to propose and promulgate
"primary" and "secondary" NAAQS for pollutants for which air quality criteria are issued [42
U.S.C. § 7409(a)], Section 109(b)(1) defines primary standards as ones "the attainment and
maintenance of which in the judgment of the Administrator, based on such criteria and allowing
an adequate margin of safety, are requisite to protect the public health."5 Under section
109(b)(2), a secondary standard must "specify a level of air quality the attainment and
maintenance of which, in the judgment of the Administrator, based on such criteria, is requisite
to protect the public welfare from any known or anticipated adverse effects associated with the
presence of [the] pollutant in the ambient air."6
The requirement that primary standards provide an adequate margin of safety was
intended to address uncertainties associated with inconclusive scientific and technical
information available at the time of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet identified. See Lead Industries
Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980), cert, denied. 449 U.S. 1042 (1980);
American Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert, denied, 455
U.S. 1034 (1982); Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir.
2010); Mississippi v. EPA, 744 F. 3d 1334, 1353 (D.C. Cir. 2013). Both kinds of uncertainties
5	The legislative history of section 109 indicates that a primary standard is to be set at "the maximum permissible
ambient air level.. . which will protect the health of any [sensitive] group of the population," and that for this
purpose "reference should be made to a representative sample of persons comprising the sensitive group rather
than to a single person in such a group." S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
6	Under CAA section 302(h) (42 U.S.C. § 7602(h)), effects on welfare include, but are not limited to, "effects on
soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to
and deterioration of property, and hazards to transportation, as well as effects on economic values and on personal
comfort and well-being."
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are components of the risk associated with pollution at levels below those at which human health
effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary
standards that include an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but also to prevent lower
pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely
identified as to nature or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentration levels, see Lead Industries
v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F. 3d at 1351, but rather at a level that
reduces risk sufficiently so as to protect public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the EPA considers such
factors as the nature and severity of the health effects involved, the size of the sensitive
population(s), and the kind and degree of uncertainties. The selection of any particular approach
to providing an adequate margin of safety is a policy choice left specifically to the
Administrator's judgment. See Lead Industries Association v. EPA, 647 F.2d at 1161-62;
Mississippi v. EPA, 744 F. 3d at 1353.
In setting primary and secondary standards that are "requisite" to protect public health
and welfare, respectively, as provided in section 109(b), the EPA's task is to establish standards
that are neither more nor less stringent than necessary. In so doing, the EPA may not consider the
costs of implementing the standards. See generally, Whitman v. American Trucking Associations,
531 U.S. 457, 465-472, 475-76 (2001). Likewise, "[attainability and technological feasibility are
not relevant considerations in the promulgation of national ambient air quality standards."
American Petroleum Institute v. Costle, 665 F. 2d at 1185.
Section 109(d)(1) requires that "[n]ot later than December 31, 1980, and at five-year
intervals thereafter, the Administrator shall complete a thorough review of the criteria published
under section [108] and the national ambient air quality standards.. .and shall make such
revisions in such criteria and standards and promulgate such new standards as may be
appropriate... " Section 109(d)(2) requires that an independent scientific review committee
"shall complete a review of the criteria.. .and the national primary and secondary ambient air
quality standards... and shall recommend to the Administrator any new... standards and revisions
of existing criteria and standards as may be appropriate...." Since the early 1980s, this
independent review function has been performed by the CAS AC of the EPA's Science Advisory
Board.7
7 Lists of the CASAC members and of members of the CASAC Sulfur Oxides Panel are available at:
https://vosemite.epa.gov/sab/sabpeople.nsfAVebCommitteesSubcommittees/CASAC%20Sulfur%20Qxides%20P
anel
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1.2.2 History of the Reviews of the Primary NAAQS for SOx
The initial air quality criteria for SOx were issued in 1969 (34 FR 1988, February 11,
1969). Based on these criteria, the EPA, in initially promulgating NAAQS for SOx in 1971,
established the indicator as SO2. The two primary standards set in 1971 were 0.14 parts per
million (ppm) averaged over a 24-hour period, not to be exceeded more than once per year, and
0.03 ppm, as an annual arithmetic mean.
The first review of the air quality criteria and standards for SOx was completed in several
stages. In the first stage, the EPA released the Air Quality Criteria Document (AQCD) for PM
and SOx in December 1981, and an addendum presenting information from subsequently
available controlled human exposure studies in 1982 (U.S. EPA, 1982a, 1982b). The policy
aspects of the air quality criteria, and preliminary exposure analyses were evaluated by OAQPS
staff in the 1982 Staff Paper (U.S. EPA, 1982c).
In 1986, the EPA published a second addendum to the 1982 AQCD, presenting newly
available evidence from epidemiologic and controlled human exposure studies (U.S. EPA,
1986a). Policy-relevant aspects of the new evidence and staff findings from a companion
population exposure assessment were evaluated in a 1986 Addendum to the 1982 Staff Paper
(U.S. EPA, 1986b, 1986c). The CASAC reviewed all of these documents and provided advice
and recommendations with regard to decisions for the review of the standards. Based on the
evidence in the 1982 and 1986 documents, staff evaluations and CASAC recommendations, in
1988, the EPA proposed to retain the existing standards and solicited comment on the alternative
of retaining the existing standards while additionally establishing a 1-hour standard of 0.4 ppm to
protect against short-term exposures (53 FR 14926, April 26, 1988). In 1992, the American Lung
association brought a lawsuit to compel the EPA to review and, if appropriate, revise the primary
standards for SOx, and the remainder of the review was then completed under court order (59 FR
58962, November 15, 1994; 61 FR 25566, May 22, 1996).
In response to publication of additional relevant controlled human studies on health
effects of short-term SO2 concentrations, the EPA prepared a supplement to the second
addendum to the 1982 AQCD (1994 AQCD supplement [U.S. EPA, 1994a]). Policy-relevant
aspects of the full body of evidence, including that newly available, along with the 1986
exposure analysis were evaluated in the 1994 Supplement to the 1982 Staff Paper (U.S. EPA,
1994b). In 1994, based on the available evidence, staff evaluations, CASAC advice, and public
comment on the 1988 proposal, the EPA re-proposed to retain the existing standards and also
solicited comment on retaining the existing standards in combination with one of three policy
options to further reduce the health risk posed by exposure to high 5-minute peaks of SO2 if
additional protection were judged to be necessary (59 FR 58958, November 15, 1994). The three
alternatives were: (1) Revising the existing primary SO2 NAAQS by adding a new 5-minute
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standard of 0.60 ppm SO2, not to be exceeded more than once per calendar year; (2) establishing
a new regulatory program under section 303 of the CAA to supplement protection provided by
the existing NAAQS, with a trigger level of 0.60 ppm SO2, not to be exceeded more than once
per calendar year; and (3) augmenting implementation of existing standards by focusing on those
sources or source types likely to produce high 5-minute peak concentrations of SO2.
This review was completed in 1996 with the EPA's decision to retain without revision the
existing standards (61 FR 25566, May 22, 1996). In reaching this decision, the Administrator
concluded, based on the staff exposure analysis, that exposure of individuals with asthma to SO2
levels that can reliably elicit adverse health effects was likely a rare event when viewed in the
context of the entire population of people with asthma. As a result, the Administrator judged that
5-minute peaks of SO2 did not pose a broad public health problem when viewed from a national
perspective, and a 5-minute standard was not promulgated (61 FR 25566, May 22, 1996).
In 1996, the American Lung Association and the Environmental Defense Fund
challenged the EPA's decision not to establish a 5-minute standard. On January 30, 1998, the
Court of Appeals for the District of Columbia ("D.C. Circuit") found that the EPA had failed to
adequately explain its determination that no revision to the SO2 NAAQS was appropriate and
remanded the decision back to EPA for further explanation. Specifically, the court determined
that the EPA had not provided adequate rationale to support the judgment that 5-minute peaks of
SO2 do not pose a public health problem from a national perspective even though these peaks
will likely cause adverse health impacts in a subset of individuals with asthma. American Lung
Ass'n v. EPA, 134 F. 3d 388, 392-393 (D.C. Cir. 1998). Following the remand, the EPA
requested that states voluntarily submit 5-minute SO2 monitoring data for the EPA to use to gain
a better understanding of the magnitude and frequency of high, 5-minute peak SO2
concentrations.
The next and most recent review of the air quality criteria and primary standards for SOx
was completed in 2010 (75 FR 35520, June 22, 2010). As a result of this review, the EPA
promulgated a new 1-hour standard to provide the requisite protection for at-risk populations
such as people with asthma against an array of adverse respiratory health effects related to short-
term SO2 exposures. The 1-hour standard was set with SO2 as the indicator based on its common
occurrence in the atmosphere and the predominance of SO2 studies in the health effects
information for SOx. The standard was set at a level of 75 parts per billion (ppb), based on the 3-
year average of the annual 99th percentile of 1-hour daily maximum SO2 concentrations. The
EPA also revoked the then-existing 24-hour and annual primary standards based largely on the
conclusion that the 1-hour standard would also control longer-term average concentrations,
maintaining 24-hour and annual concentrations generally well below the levels of those
standards, and on the lack of evidence indicating the need for such longer-term standards. The
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2010 action also addressed the remand by the D.C. Circuit in 1998. The 2010 and prior standards
are summarized in Table 1-1.
Table 1-1. History of the primary national ambient air quality standard(s) for sulfur
oxides since 1971.
Final
Rule/Decision
Indicator
Averaging
Time
Level
Form
April 30, 1971
(36 FR 8186)
S02
24 hours
140 ppba
one allowable exceedance
1 year
30 ppba
arithmetic average
May 22, 1996
(61 FR 25566)
Both the 24-hour and annual average standards retained without revision
June 22, 2010
(75 FR 35520)
S02
1 hour
75 ppb
99th percentile of yearly distribution of 1-hour
daily maximums, averaged over 3 years
24-hour and annual standards revoked
a Although the levels were set in terms of ppm (0.14 ppm for the 24-hour standard and 0.03 ppm for the annual standard),
they are shown here in ppb for consistency with units of current standard.
In conjunction with the 2010 revisions to the standards, the EPA revised the SO2 ambient
air monitoring regulations to require that monitoring agencies using continuous SO2 methods
report the highest 5-minute concentration for each hour of the day; many agencies additionally
report all twelve 5-minute concentrations for each hour of the day (75 FR 35554, June 22, 2010;
40 CFR 58.16). The rationale for this requirement was to provide additional monitoring data for
use in subsequent reviews of the primary standard, particularly in considering the extent of
protection provided by the 1-hour standard against 5-minute peak SO2 concentrations of concern
(75 FR 35554, June 22, 2010).
After publication of the final rule, a number of industry groups and states filed petitions
for review arguing (1) that the EPA failed to follow notice-and-comment rulemaking procedures
because the proposal did not indicate that EPA was considering changing its method of
determining attainment from an air-monitoring approach to a hybrid approach using computer
modeling in combination with air monitoring, and (2) that the decision to establish a 1 -hour SO2
NAAQS at 75 ppb was arbitrary and capricious because it was lower than statutorily authorized.
The D.C. Circuit rejected these challenges, dismissing the first argument for lack of jurisdiction
and denying the petitions with respect to the second argument, explaining that the EPA did not
act arbitrarily in setting the 2010 standard. National Environmental Developmental Association's
Clean Air Project v. EPA, 686 F. 3d 803 (D.C. Cir. 2012). Accordingly, the 2010 standard was
upheld. Id.
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1.2.3 Current SO2 NAAQS Review
In May 2013, the EPA announced the initiation of the current periodic review of the air
quality criteria for sulfur oxides and the primary NAAQS for sulfur oxides and issued a call for
information in the Federal Register (78 FR 27387, May 10, 2013). A wide range of external
experts as well as EPA staff representing a variety of areas of expertise (e.g., epidemiology,
human and animal toxicology, statistics, risk/exposure analysis, atmospheric science)
participated in a workshop, held by the EPA on June 12-13, 2013 in Research Triangle Park, NC.
The workshop provided for a public discussion of the key policy-relevant issues around which
the EPA has structured the review and of the most meaningful new scientific information that
would be available in this review to inform our understanding of these issues.
Building from the workshop discussions, the EPA developed the draft Integrated Review
Plan for the Primary National Ambient Air Quality Standards for Sulfur Dioxide (IRP, U.S.
EPA, 2014) outlining the schedule, process, and key policy-relevant questions that would guide
the evaluation of the air quality criteria for SO2 and the review of the primary NAAQS for SOx.
The draft IRP was released in March 2014 (79 FR 14035, March 12, 2014) and was the subject
of a consultation with the CASAC on April 22, 2014 (79 FR 16325, March 25, 2014). Comments
received from the CASAC and the public were considered in the preparation of the final IRP,
which was released in October 2014 (U.S. EPA, 2014).
The process for development of the first draft ISA included review of preliminary drafts
of key ISA chapters by subject matter experts at a public workshop hosted by the EPA's National
Center for Environmental Assessment (NCEA) on June 23-24, 2014 (79 FR 33750, June 12,
2014). Comments received from this review as well as comments from the public and the
CASAC on the draft IRP were considered in preparation of the Integrated Science Assessment
for Sulfur Oxides - Health Criteria (External Review Draft - November 2015), released in
November 2015 (80 FR 73183, November 24, 2015). The first draft ISA was reviewed by the
CASAC at a public meeting in January 2016 and a public teleconference in April 2016 (80 FR
79330, December 21, 2015; 80 FR 79330, December 21, 2015; Diez Roux, 2016).
The EPA released the Integrated Assessment for Sulfur Oxides - Health Criteria (Second
External Review Draft - December 2016) in December 2016 (81 FR 89097), which was
reviewed by the CASAC at a public meeting in March 2017 and a public teleconference in June
2017 (82 FR 11449, February 23, 2017; 82 FR 23563, May 23, 2017). Completion of the final
ISA is expected in December 2017.
As part of the planning process for development of the REA, the EPA completed the
Review of the Primary National Ambient Air Quality Standardfor Sulfur Oxides: Risk and
Exposure Assessment Planning Document (REA Planning Document, U.S. EPA, 2017c) in
February 2017 (82 FR 11356), and held a consultation with the CASAC at a public meeting in
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March 2017 (82 FR 11449). In consideration of CASAC comments at that consultation and
public comments, the EPA developed the draft REA (U.S. EPA, 2017b). The draft REA and this
draft PA are being provided to the CASAC for their review and released to the public for
comment. The CASAC advice and public comments will be considered in completing these
documents.
The schedule for completion of this review is governed by a consent decree entered by
the court, which, in relevant part, specifies that the appropriate EPA official issue a final
Integrated Science Assessment addressing human health effects of SOx no later than December
14, 2017; sign a notice setting forth its proposed decision concerning its review of the primary
NAAQS for SOx no later than May 25, 2018; and sign a notice setting forth its final decision
concerning its review of the primary NAAQS for SOx no later than January 28, 2019 (Consent
Decree at 4, Center for Biological Diversity etal. v. Pruitt, Case No. 3:16-cv-03796-VC (N.D.
Cal. April 28, 2017), Document No. 37).
1.3 GENERAL APPROACH AND ORGANIZATION OF THIS
DOCUMENT
This draft PA draws on the policy-relevant aspects of the scientific evidence and
quantitative air quality, exposure and risk analyses. With regard to the health effects evidence,
we consider the nature of the key effects associated with SO2 in ambient air, the types and
magnitudes of exposures associated with effects, and populations at greatest risk, as well as the
uncertainties. Based on this information, we summarize associated potential public health
impacts of SO2 in ambient air. We additionally consider the magnitude of exposures and risks
estimated in the REA, along with the associated uncertainties. This evaluation supports
preliminary staff conclusions with regard to the key policy-relevant questions for the review,
including whether the currently available information appears to call into question the adequacy
of public health protection afforded by the current standard.
Following this introductory chapter, chapter 2 focuses on current air quality, including
sources of SO2 to ambient air, the ambient monitoring network for SO2, and trends and current
conditions. Chapter 3 focuses on the review of the primary NAAQS for SOx presenting
background information on the rationale for previous reviews and the approach followed in the
current review. Chapter 3 further discusses the adequacy of the current standards, taking into
account evidence- and exposure-/risk-based considerations, and includes preliminary staff
conclusions. Chapter 3 also identifies key uncertainties and areas for future research.
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REFERENCES
Diez Roux. (2016). Letter to EPA Administrator Gina McCarthy: CASAC Review of the EPA's
Integrated Science Assessment for Sulfur Oxides - Health Criteria (External Review
Draft - November 2015). Available at:
https://vosemite.epa.eov/sab/sabprodiict.nsf/264cbl227d5Se02'•'i0200T446a4/8DE
C3 6 A.7E2 A.54B A.48525 7F9600667D81 /$Fil	'2+Unsigned.pdf
U.S. EPA. (1982a). Air quality criteria for particulate matter and sulfur oxides (final, 1982).
(EPA 600/8-82/029a). Washington, DC: Environmental Criteria and Assessment Office.
Available at: https://www3.epa.gOv/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA. (1982b). Air quality criteria for particulate matter and sulfur oxides, volume I
addendum. (EPA-600/8-82-029a). Research Triangle Park. NC: Environmental Criteria
and Assessment Office. Available at:
https://www3.epa. gov/ttn/naaq s/standards/so2/s so2 pr.html
U.S. EPA. (1982c). Review of the National Ambient Air Quality Standards for Sulfure Oxides:
Assessment of Scientific and Technical Information, OAQPS Staff Paper. (EPA-450-5-
82-007). Research Triangle Park, NC: Office of Air Quality Planning and Standards.
Available at: https://www3.epa.gOv/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA. (1986a). Air quality criteria for particulate matter and sulfur oxides (1982):
assessment of newly available health effects information, 2nd addendum. (EPA/600/8-
86/020F). Washington, DC: Office of Health and Environmental Assessment. Available
at: https://www3.epa.gov/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA. (1986b). Second addendum to air quality criteria for particulate matter and sulfur
oxides (1982): Assessment of newly available health effects information. (EPA/600/8-
86/020F). Research Triangle Park, NC: Environmental Criteria and Assessment Office.
Available at: https://www3.epa.gOv/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA. (1986c). An Analysis of Short-Term Sulfur Dioxide Population Exposures in the
Vicinity of Utility Power Plants. Research Triangle Park, NC: Office of Air Quality
Planning and Standards. Available at:
https://www3.epa. gov/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA. (1994a). Supplement to the second addendum (1986) to air quality criteria for
particulate matter and sulfur oxides (1982): Assessment of new findings on sulfur dioxide
acute exposure health effects in asthmatic individuals. (EPA/600/FP-93/002). Research
Triangle Park, NC: Environmental Criteria and Assessment Office. Available at:
https://www3.epa. gov/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA (1994b). Supplement to the Second Addendum (1986) to Air Quality Criteria for
Particulate Matter and Sulfur Oxides (1982): Assessment of New Findings on Sulfur
Dioxide and Acute Exposure Health Effects in Asthmatic Individuals. (EPA/600/FP-
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93/002). Research Triangle Park, NC: Environmental Criteria and Assessment Office.
Available at: https://www3.epa.gOv/ttn/naaqs/standards/so2/s so2 pr.html
U.S. EPA. (2014). Integrated Review Plan for the Primary National Ambient Air Quality
Standard for Sulfur Dioxide. EPA-452/P-14-005, October 2014. Available at:
https://www3.epa.gov/ttn/naaqs/standards/so2/data/20141028so2reviewplan.pdf
U.S. EPA. (2016). Integrated Science Assessment (ISA) for Sulfur Oxides - Health Criteria
(Second External Review Draft). EPA/600/R-16/351, December 2016. Available at:
https://cfpub.epa.gOv/n.cea/i sa/recordisplav.cfm?deid=326450
U.S. EPA. (2017a). Integrated Review Plan for the Secondary National Ambient Air Quality
Standard for Ecological Effects of Oxides of Nitrogen, Oxides of Sulfur and Particulate
Matter. EPA-452/R-17-002, January 2017. Available at:
https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-
secondarvstandards-planning-documents-current
U.S. EPA. (2017b). Risk and Exposure Assessment for the Review of the Primary National
Ambient Air Quality Standard for Sulfur Oxides, External Review Draft. EPA-452/P-17-
002, August 2017. Available at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-
air-qualitv-standards
U.S. EPA. (2017c). Review of the Primary National Ambient Air Quality Standard for Sulfur
Oxides: Risk and Exposure Assessment Planning Document. EPA-452/P-17-001,
February 2017. Available at:
https://www3.epa.gov/ttn/naaqs/stan.dards/so2/data/20170216so2rea.pdf
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2 CURRENT AIR QUALITY
This chapter presents a summary of our current understanding of SOx in ambient air
largely drawn from the more detailed discussion provided in the second draft ISA (second draft
ISA, chapter 2). Section 2.1 summarizes the current information on sources and emissions and
section 2.2 summarizes current ambient air monitoring methods and networks. Recent
concentrations of SO2 in ambient air are summarized in section 2.3.
2.1 SOURCES TO AMBIENT AIR
In this section, we describe the most recently available information on sources and
emissions of SOx into the ambient air. The section does not provide a comprehensive list of all
sources, nor does it provide estimates of emission rates or emission factors for all source
categories. Rather, the discussion here is intended to identify the larger source categories, either
on a national or local scale, and generally describe their emissions and distribution within the
U.S. based on the U.S. EPA 2014 National Emissions Inventory (NEI).
Sulfur oxides are emitted into air from specific sources (e.g., fuel combustion processes)
and also formed in the atmosphere from other atmospheric compounds (e.g., as an oxidation
product of reduced sulfur compounds, such as sulfides). Sulfur oxides are also transformed in the
atmosphere to particulate sulfur compounds, such as sulfates. Sulfur oxides known to occur in
the troposphere include SO2 and sulfur trioxide (SO3) (second draft ISA, section 2.3). As a result
of rapid atmospheric chemical reactions involving SO3, the most prevalent sulfur oxide in the
atmosphere is SO2 (second draft ISA, section 2.3).
Fossil fuel combustion is the main anthropogenic source of SO2 emissions, while
volcanos and landscape fires (wildfires as well as controlled burns) are the main natural sources
(second draft ISA, section 2.1).1 Industrial chemical production, pulp and paper production,
natural biological activity (plants, fungi, and prokaryotes), and volcanoes are among many
sources of reduced sulfur compounds that contribute, through various oxidation reactions in the
atmosphere, to the formation of SO2 in the atmosphere (second draft ISA, section 2.1).
Anthropogenic SO2 emissions originate primarily from point sources, including coal-fired
electricity generating units (EGUs) and other industrial facilities (second draft ISA, section
1 The 2008 ISA (U.S. EPA, 2008) described a modeling analysis that estimated SO2 concentrations for 2001 in the
absence of any U.S. anthropogenic emissions of SO2 (2008 ISA, section 2.5.3). Such concentrations are referred
to as United States background or USB. The 2008 ISA analysis estimated USB concentrations of SO2 to be below
0.01 ppb over much of the U.S., ranging up to a maximum of 0.03 ppb. In the U.S. Northwest, geothermal
sources (e.g., volcanoes) were estimated to be responsible for up to 80% of the ambient air concentrations
resulting solely from natural sources and sources outside of the U.S. (second draft ISA, section 2.5.5).
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2.2.1). The largest S02-emitting sector within the U.S. is electricity generation, of which 97% of
SO2 from electricity generation is from coal combustion, as shown in Figure 2-1. Other
anthropogenic sources of SO2 emissions include industrial fuel combustion and process
emissions, industrial processing, commercial marine activity, and fire used in landscape
management and agriculture (second draft ISA, section 2.2.1).
Commercial Marine
Vessels, 3.5
Electricity Generator!
65.9
Industrial Fuel
Combustion, 11.2
Industrial
Mobile Sources
(except CMV), 1.2
Commercial and
Residential Fuel
Combustion, 2.9
Fires (Wildfires,
Prescribed,
Agricultural), 3.0
Figure 2-1. Percent contribution of SO2 emissions by sector (Source: 2014 NEI).2
Figure 2-2 illustrates the emissions trends from 1990 to 2015. Declines in SO2 emissions
are likely related to the implementation of Clean Air Act national control programs including
Phase I and II of the Acid Rain Program, the Clean Air Interstate Rule, and the Cross-State Air
Pollution Rule. An additional factor is changes in market conditions, e.g., reduction in energy
generation by coal (U.S. EIA, 2017).3
2	Total SO2 emissions from the 2014 NEI were 4,942,063 tons.
3	The reduction in energy generation by coal resulted in the use of fuels that emit less SO2 in energy generation (U.S.
EIA, 2016).
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25,000
¦1 ~
1990	1995	2000	2005	2010	2015
~ Stationary fuel combustion ~ Industrial and other processes ~ Transportation ¦ Miscellaneous
Figure 2-2. National SO2 emission trends by sector.
2.2 AMBIENT AIR MONITORING METHODS AND NETWORK
To promote uniform enforcement of the air quality standards set forth under the CAA, the
EPA has established federal reference methods (FRMs) and federal equivalent methods (FEMs)
for ambient air sample collection and analysis. Measurements for determinations of NAAQS
compliance must be made with FRMs 01* FEMs. The current SO2 monitoring network relies on
the automated pulsed ultraviolet fluorescence (UVF) FRM (40 CFR Appendix A-l to Part 50; 40
CFR Appendix A-2 to Part 50). The UVF method is a continuous automated method that
quantifies SO2 concentrations, providing averages across desired time periods, such as 5-minute
and 1-hour averages.
Measurements of SO2 concentrations in ambient air are collected by networks of FRM
monitors, primarily operated by state and local monitoring agencies in the U.S. The main
network providing ambient data for NAAQS surveillance monitoring purposes is the State and
Local Air Monitoring Stations (SLAMS) network. In 2016, there were 363 SLAMS sites
reporting SO2 concentrations to the Air Quality System (AQS), the EPA's repository for detailed
air pollution data. For each SO2 monitoring site, the SLAMS monitoring agencies report hourly
concentrations and either the maximum 5-minute concentration in the hour (one of twelve 5-
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minute periods within an hour) or all twelve 5-minute average SO2 concentrations within the
hour.
Five minute SO2 data have become much more widely available due to regulatory
requirements promulgated in 2010 (Figure 2-3).4 Although 5-minute data were available for
fewer than 10% of monitoring sites at the time of the last review, such data (either all 12 values
in each hour or just the maximum 5-minute concentrations) are currently available for nearly all
of the operating SO2 sites nationwide, providing a more robust foundation for characterization of
5-minute ambient air concentrations in this review. Further, the newly available monitoring data
also include more monitors reporting the 12 consecutive 5-minute concentrations for each hour
than were available in the last review (Figure 2-3). Of the monitors reporting 5-minute data in
2016, almost 40% are reporting all twelve 5-minute SO2 measurements in each hour while about
60% are reporting the maximum 5-minute SO2 concentration in each hour.5
600
• •
* • •
500	•
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016
0 5-min values (max/hr only) ~ 5-min values (all 12/hr) • 1-hr values
Figure 2-3. Temporal trend in number of monitors with 5-minute data.
4	At SO2 NAAQS compliance monitoring sites, air monitoring agencies are now required to report, for every hour of
the day, the hourly average and either the maximum 5-minute value (one of twelve 5-minute periods) in the hour
or all twelve 5-minute averages within the hour (75 FR 35554, June 22, 2010).
5	In 2016, three of the sites reported both the continuous 5-minute data and the maximum 5-minute data separately.
Therefore, these monitors are included in the count for each of the types of 5-minute monitors.
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2.3 AMBIENT AIR MONITORING CONCENTRATIONS
This section briefly summarizes trends and current conditions based on recent ambient air
monitoring data.
2.3.1 Trends
Ambient air concentrations of SO2 in the U.S. have declined substantially from 1980 to
2015. Figure 2-4 illustrates this decline in terms of the distribution of 3-year average of annual
99th percentile daily maximum 1-hour concentrations6 at 45 monitoring sites that have been
operating across this period. The average of this dataset has declined by more than 83% over the
35-year period. Over the past 15 years, a larger dataset of 227 sites operating from 2000-2015
also indicates a decline, which is on the order of 67% for the average of that dataset (Figure 2-
5).7 Daily maximum 5-minute SO2 concentrations have also consistently declined over time from
2011 to 2015 (Figure 2-6).8
400
c:
2 200-
% 150-
c 100-
o
U 50-I
1 1 1 1 1 1 111111 1 1 1 1 1 1 1 12222222222222222
999999999999999999990000000000000000
8888888388999999999900000000001 1 1 111
0 1 2345678901 2345678901 234567390 1 2345
Figure 2-4. National temporal trend in SO2 concentrations: 1980-2015 (45 sites).
Three-year average of annual 99th percentile of daily maximum 1-hour
concentrations. (Note: Dashed line indicates the current standard [75 ppb].)
6	The form of the current 1-hour SO2 NAAQS is the 99th percentile of yearly distribution of 1-hour daily maximums,
averaged over 3 years.
7	In Figures 2-4 and 2-5, the year on the x-axis represents the last year of the 3 year average (e.g., 2015 represents
the average of 2013-2015).
8	In Figure 2-6, the number of sites with monitors for 2011, 2012, 2013, 2014, and 2015 were 301, 321, 366, 359,
and 352, respectively.
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Figure 2-5. Temporal trend in SO2 concentrations: 2000-2015 (227 sites). Three-year
average of annual 99th percentile of daily maximum 1-hour concentrations.
(Note: Dashed line indicates the current standard [75 ppb]).
150
¦§.130 -
a.
cs>
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cu
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-•- Mean —•—75th percentile —95th percentile	99th percentile
Figure 2-6. Temporal trend in daily maximum 5-minute SO2 concentrations: 2011-
2015. (N = number of measurements)
102666
108918
123853
121012
122902
2011
2012
2013
2014
2015
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2.3.2 Current Concentrations
2.3.2.1 Geographic Variation in Concentrations
Concentrations of SO2 vary across the U.S. and tend to be higher in areas with sources
having relatively higher SO2 emissions (e.g., EGUs). Consistent with the locations of larger SO2
sources, higher concentrations are primarily located in the eastern half of the continental U.S.,
especially in the Ohio River valley, upper Midwest, and along the Atlantic coast (Figure 2-7).
The point source nature of SO2 emissions contribute to the relatively high spatial variability of
SO2 concentrations compared with pollutants such as ozone (O3) (second draft ISA, section
3.2.3). Another contributing factor to the spatial variability is the dispersion and oxidation of SO2
in the atmosphere, resulting in decreasing SO2 concentrations with increasing distance from the
source. Sulfur oxides emitted from point sources tends to travel away from the emissions source
as a plume, which may or may not impact large portions of surrounding populated areas
depending on meteorological conditions and terrain.
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S02 2015 Site Level DVs
DV2015 (ppb)
1
2	Figure 2-7. Concentrations of SO2 in terms of the current standard (3-year average of annual 99th percentile daily
3	maximum 1-hour concentrations) at sites with datasets meeting completeness requirements for 2013-2015.
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2.3.2.2	Seasonal and Diel Variability in Concentrations
Recent (2013-2015) data indicate that 1-hour daily maximum SO2 concentrations vary
across seasons, with the greatest variations seen in the upper percentile concentrations (versus
average or lower percentiles) for each season (second draft ISA, section 2.5.3.2). This is seen in
the data presented for six urban areas in the draft ISA9 (second draft ISA, section 2.5.3.2). This
variation, along with month-to-month variations in 1-hour daily maximum SO2 concentrations
also presented in the ISA, were generally consistent with month-to-month emissions patterns and
the expected atmospheric chemistry of SO2 for a given season. For example, "summertime
minima, observed in the New York City, NY and Houston, TX, sites may correspond to
enhanced oxidation of SO2 to SO42" by photochemically derived atmospheric oxidants that are
more prevalent during the humid summer (Khoder, 2002)" (second draft ISA, p. 2-55). The
differences in seasonal pattern (as well as magnitude) of concentrations among areas studied
indicate the variability of SO2 concentrations across local and regional scales (second draft ISA,
section 2.5.3.2).
Consistent with the nationwide diel patterns reported in the last review, 1-hour average
and 5-minute hourly maximum SO2 concentrations for 2013-2015 in all six urban areas
evaluated were generally low during nighttime and approached maxima values during daytime
hours (second draft ISA, section 2.5.3.3, Figures 2-23 and 2-24). The timing and duration of
daytime maxima in the six sites evaluated were likely related to a combination of source
emissions and meteorological parameters (second draft ISA, section 2.5.3.3; U.S. EPA 2008,
section 2.5.1). For example, SO2 emitted from elevated point sources (e.g., power plants and
industrial sources) may be entrained into the mixed boundary layer, which expands during the
day with rising surface temperatures (U.S. EPA 2008, section 2.5.1, Figures 2-23 and 2-24).
2.3.2.3	Relationship Between 1-hour and 5-minute Concentrations
Peak concentrations within a plume of SO2 downwind from, but relatively nearby to, a
source can greatly exceed mean concentrations across the plume (second draft ISA, section
2.5.4). Further, measured 5-minute concentrations at a particular location can be much higher
than the average concentration at the same location for the associated hour. However, as
emissions travel further downwind and experience ever increasing dispersion, these differences
lessen both spatially and temporally. This can contribute to greater spatial and temporal
9 The six locations evaluated are: Cleveland, OH, Pittsburgh, PA, New York City, NY, St. Louis, MO-IL, Houston,
TX, and Gila County, AZ (second draft ISA, section 2.5.2.2). These six locations were based on (1) their
relevance to current health studies (i.e., areas with peer-reviewed, epidemiologic analysis), (2) the existence of
four or more monitoring sites located within the area boundaries, and (3) the presence of several diverse S02
sources within a given focus area boundary.
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variability in 5-minute than in 1-hour concentrations, as is seen in the six urban locations
evaluated in the ISA (second draft ISA, p. 2-52).
Using monitoring data from 2013-2015, Figure 2-8 illustrates the general pattern of lower
5-minute concentrations with lower 1-hour concentrations. The left panel of Figure 2-8 shows
that across the monitors meeting data completeness criteria, on days when the maximum 1-hour
concentrations are relatively low, the maximum 5-minute concentrations are also relatively low.
Similarly, as shown in the right panel of Figure 2-8, at sites with relatively lower design values,10
the distribution of maximum 5-minute concentrations is also lower. This is documented by the
distinct reduction in 99th percentile daily maximum 5-minute concentrations at lower design
values. For example, in areas with design values at or below the current standard (75 ppb), 99.9
percent of daily maximum 5-minute concentrations are at or below approximately 150 ppb.11
This contrasts with the much higher distribution of daily maximum 5-minute concentrations at
sites with design values exceeding the current standard. The 99th percentile of these daily
maximum 5-minute concentrations is 365 ppb, meaning that one percent of the days at these sites
has a maximum 5-minute concentration above 365 ppb (i.e., 210 occurrences).
10	The design value (DV) for the standard is the metric used to determine whether areas meet or exceed the NAAQS.
A design value is a statistic that describes the air quality status of a given area relative to the NAAQS. Design
values are considered to be valid if the monitoring data used to calculate them meet the regulatory completeness
criteria which for SO2 require four quarters of all three years of the period to have data for at least 75 percent of
the sampling days (40 CFR 50.17 and appendix T to Part 50).
11	Additional information related to data in Figure 2-8 is presented in Appendix B.
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N 315964
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500-
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N 225863
59011
18504
21028
1000-
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Daily Maximum 1-Hour S02 Concentration (ppb)
>25-50
>50-75
>75-735
e/5
o
<=25
>25-50	>50-75
Design Value (ppb)
>75-246
Figure 2-8. Distributions of daily maximum 5-minute concentrations during 2013-
2015. Left panel presents varying distributions with varying daily maximum
1-hour concentrations. Right panel presents varying distributions with varying
design values; the last bin (>75 ppb) presents data for sites not meeting the
current standard. (Note: The values represented in the boxplots are the 25th
percentile, the median, and the 75 percentile. The asterisk represents the 99th
percentile.)
Analyses of the current monitoring dataset, expanded since the last review, provide
information on the occurrence of daily maximum 5-minute concentrations of interest at monitors
having differing design values. For example, analysis of these data for the years 2013 to 2015
indicates that among monitors with design values meeting the current standard (i.e., at or below
75 ppb), the vast majority have zero days with a daily maximum 5-minute concentration above
100 ppb or even 400 ppb. Among the few monitors with any days recording a 5-minute
concentrations above 400 ppb, the maximum number of such days in a year was five; for
monitors with any days recording 5-minute concentrations above 200 ppb, the maximum number
of such days/year was 22 (Appendix C, Figures C-2 and C-4, lower panel).
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REFERENCES
Khoder, MI. (2002). Atmospheric conversion of sulfur dioxide to particulate sulfate and nitrogen
dioxide to particulate nitrate and gaseous nitric acid in an urban area. Chemosphere 49:
675-684. http://dx.doi.sy liM 6/80045-6535(0:I 0
U.S. EIA (U.S. Energy Information Administration). (2016). Electric Power Annual 2015.
November 2016. Available at: https://www.eia.eov/electricitv/aimual/pdf/epa.pdf
U.S. EIA (U.S. Energy Information Administration). (2017). Monthly Energy Review July 2017.
DOE/EIA-0035(2017-07), July 2017. Available at:
https://www.eia.gov/totalenergy/data/monthlv/pdf/mer.pdf
U.S. EPA. (2008). Integrated science assessment for sulfur oxides: Health criteria [EPA Report],
EPA/600/R-08/047F. Research Triangle Park, NC: U.S. Environmental Protection
Agency, Office of Research and Development, National Center for Environmental
Assessment- RTP. http://cfpub.epa.gov/ncea/cfm/recordisplav.cfm?deid=l98843
U.S. EPA. (2016). Integrated Science Assessment (ISA) for Sulfur Oxides - Health Criteria
(Second External Review Draft). EPA/600/R-16/351, December 2016. Available at:
https://cfpub. epa.gov/ncea/isa/recordisplav. cfm?deid=326450
U.S. EPA. (2017). Integrated Review Plan for the Secondary National Ambient Air Quality
Standard for Ecological Effects of Oxides of Nitrogen, Oxides of Sulfur and Particulate
Matter. EPA-452/R-17-002, January 2017. Available at:
https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-
secondarystandards-planning-documents-current
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3 REVIEW OF THE PRIMARY STANDARD FOR
SULFUR OXIDES
This chapter evaluates the policy implications of the key scientific and technical
information in the second draft ISA and draft REA. This evaluation is based on consideration of
the available body of evidence assessed in the second draft ISA and of quantitative analyses of
SO2 air quality, exposures and risks presented in the draft REA and in this document. Based on
this information, the staff offer preliminary conclusions regarding each of the critical elements of
the standard, including indicator, averaging time, form, and level. The final PA will also be
informed by the advice and recommendations received from the CASAC during its review of the
draft PA, and by public comments received on the draft document. The final PA is designed to
help the Administrator in considering the currently available scientific and risk information and
formulating judgments regarding the adequacy of the current primary standard.1
3.1 APPROACH
Staffs approach in this review of the primary standard for SOx takes into consideration
the approaches used in the previous review. The past and current approaches described below are
both based, most fundamentally, on using the EPA's assessment of the current scientific
evidence and associated quantitative analyses to inform the Administrator's judgment regarding
a primary standard for SOx that is requisite to protect public health with an adequate margin of
safety. In reaching conclusions on options for the Administrator's consideration, we note that the
final decision to retain or revise the current SO2 primary standard is a public health policy
judgment to be made by the Administrator.
The final decision by the Administrator will draw upon the available scientific evidence
for S02-attributable health effects, and on quantitative analyses of population exposures and
health risks, including judgments about the appropriate weight to assign the range of
uncertainties inherent in the evidence and analyses. Therefore, in developing conclusions in this
draft PA, we are mindful that the Administrator's judgments on the standard will reflect an
interpretation of the available scientific evidence and exposure/risk information in consideration
of the strengths and limitations of that evidence and information. Our general approach to
1 The basic elements of a standard include the indicator, averaging time, form, and level. The indicator defines the
chemical species or mixture to be measured in the ambient air for the purpose of determining whether an area
attains the standard. The averaging time defines the period over which air quality measurements are to be
obtained and averaged or cumulated. The form of a standard defines the air quality statistic that is to be compared
to the level of the standard in determining whether an area attains the standard. The level of the standard defines
the air quality concentration used for that purpose (i.e., an ambient air concentration of the indicator).
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informing these judgments, discussed more fully below, recognizes that the available health
effects evidence reflects a continuum from relatively higher SO2 concentrations, at which
scientists generally agree that health effects are likely to occur, through lower concentrations at
which the likelihood and magnitude of a response become increasingly uncertain. This approach
is consistent with the requirements of sections 108 and 109 of the CAA, as well as with how the
EPA and the courts have historically interpreted the Act. These provisions require the
Administrator to establish primary standards that in the Administrator's judgment are requisite to
protect public health with an adequate margin of safety. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than necessary for this purpose. The
Act does not require that primary standards be set at a zero-risk level, but rather at a level that
reduces risk sufficiently so as to protect public health with an adequate margin of safety.2
Section 3.1.1 below summarizes the approach used in the last review of the primary
NAAQS for SOx and section 3.1.2 summarizes the general approach for the current review.
3.1.1 Approach in the Previous Review
The last review of the primary NAAQS for SOx was completed in 2010 and resulted in
substantial revisions to the standards (75 FR 35520, June 22, 2010). In consideration of the
evidence of respiratory effects in people with asthma in response to exposures as short as five
minutes, the EPA established a new short-term standard to provide increased protection for this
at-risk group and other potentially at-risk populations3 against an array of adverse respiratory
effects that have been linked to short-term SO2 exposures in both controlled human exposure and
epidemiologic studies (75 FR 35525, June 22, 2010; 2008 ISA, section 5.5). Specifically, the
EPA replaced the then-existing 24-hour standard with a short-term standard defined by the 3-
year average of the 99th percentile of the yearly distribution of 1-hour daily maximum SO2
concentrations, with a level of 75 ppb. In addition to replacing the 24-hour standard with a new
1-hour standard, the EPA revoked the then-existing annual standard, based largely on the lack of
sufficient health evidence to support a long-term standard and a recognition that a 1-hour
standard set at 75 ppb would have the effect of generally maintaining annual SO2 concentrations
well below the level of the revoked annual standard (75 FR 35550, June 22, 2010).
2	The four basic elements of the NAAQS (indicator, averaging time, level and form) are considered collectively in
evaluating the health protection afforded by the current standard.
3	As used here and similarly throughout the document, the term population refers to persons having a quality or
characteristic in common, such as a specific pre-existing illness or a specific age or lifestage. A lifestage refers to
a distinguishable time frame in an individual's life characterized by unique and relatively stable behavioral and/or
physiological characteristics that are associated with development and growth. Identifying at-risk populations
includes consideration of intrinsic (e.g., genetic or developmental aspects) or acquired (e.g., disease or smoking
status) factors that increase the risk of health effects occurring with exposure to sulfur oxides as well as extrinsic,
nonbiological factors, such as those related to socioeconomic status, reduced access to health care, or exposure.
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The emphasis in the 2010 review on short-term exposures of people with asthma
reflected the health effects evidence that has expanded in this area over the four decades since
the then-existing 24-hour and annual standards were set in 1971. A key element of the expanded
evidence base was a series of controlled human exposure studies which documented effects on
lung function in people with asthma exposed, while at moderate or greater levels of exertion, for
periods as short as five minutes. In the 2010 review, the EPA also conducted quantitative
analyses of air quality data, including 5-minute ambient air measurements, and of potential
exposures for people with asthma. Consideration of these key aspects of the evidence and
quantitative analyses informed the decision in the 2010 review, which additionally addressed the
court remand4 to the EPA of the EPA's 1996 decision to retain the 1971 standards without
revision.
The evidence-based and quantitative assessments performed for the 2010 review focused
particularly on the issue of exposures to SO2 in ambient air of a duration as short as five minutes
(2008 ISA; 2009 REA). The quantitative analyses documented in the REA included
characterizations of the likelihood of people with asthma being exposed (while they were at
elevated exertion5) to concentrations of SO2 from ambient air of a magnitude documented to
elicit decrements in lung function (2009 REA). These analyses were performed both for air
quality conditions associated with just meeting the then-existing standards and for conditions
associated with just meeting potential alternative standards. The REA additionally included air
quality analyses that explored the extent to which potential alternative standards with 1-hour, 24-
hour, and annual averaging times might be expected to control 5-minute ambient air
concentrations (2009 REA, section 7.3). Together the evidence-based and quantitative
assessments informed the policy options considered by the Administrator in that review.
Considerations, conclusions and judgments by the Administrator that provided the basis for her
decisions in the 2010 review are summarized below.
3.1.1.1 Considering the Need for Revision
The conclusions reached by the Administrator in the last review were based on the
extensive body of scientific evidence on SCh-related health effects and quantitative analyses of
air quality, exposure and risk. In her conclusion on the adequacy of the then-existing standards,
4	See Am. Lung Ass'n v. EPA, 134 F.3d 388 (D.C. Cir. 1998) (remanding the 1996 decision to EPA).
5	The phrase "elevated ventilation" (or "moderate or greater exertion") was used in the 2009 REA and Federal
Register notices in the last review to refer to activity levels that in adults would be associated with ventilation
rates at or above 40 liters per minute; an equivalent ventilation rate was derived in order to identify corresponding
rate for the range of ages and sizes of the simulated populations (2009 REA, section 4.1.4.4). Accordingly, this
phrase is used in this draft PA when referring to the REA from the last review. Otherwise, however, the draft
REA and draft PA for this review generally uses the phrase "elevated ventilation" to refer to the same occurrence.
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which were set in 1971, the Administrator emphasized the evidence and quantitative analyses
concerning 5-minute exposures. The Administrator gave particular attention to the robust
evidence base, comprised of findings from controlled human exposure, epidemiologic, and
animal toxicological studies that collectively were judged "sufficient to infer a causal
relationship" between short-term SO2 exposures ranging from 5 minutes to 24 hours and
respiratory morbidity (75 FR 35535, June 22, 2010). The "definitive evidence" for this
conclusion came from studies of 5- to 10-minute controlled exposures that reported respiratory
symptoms and decreased lung function in exercising individuals with asthma (2008 ISA, section
5.2). Supporting evidence was provided by epidemiologic studies of a broader range of
respiratory outcomes, with uncertainty noted about the magnitude of the study effect estimates,
quantification of the exposure concentration-response relationship, potential confounding by co-
pollutants, and other areas (75 FR 35535-35536, June 22, 2010; 2008 ISA, section 5.3).
In the controlled human exposure studies of exercising individuals with asthma, a dose-
response relationship was documented, with both the percentage of individuals affected and the
severity of the response increasing with increasing SO2 concentrations (75 FR 35525, June 22,
2010). The evidence from these studies documents the occurrence of SCh-related decrements in
lung function based on reductions in forced expiratory volume (FEVi) and increases in specific
resistance of the airways (sRaw). Moderate6 or greater decrements in lung function were reported
in response to short (5- to 10-minute) exposures to concentrations as low as 200 to 300 ppb in
approximately 5-30% of exercising individuals with asthma. In response to exposures at or above
400 ppb, approximately 20-60% experienced such decrements, frequently accompanied by
respiratory symptoms; in many studies, responses at these concentrations were often statistically
significant at the group mean level7 (75 FR 35525-35526, June 22, 2010).
In reaching conclusions regarding the significance of the reported responses to the 5- to
10-minute controlled exposures, the Administrator considered guidelines from the American
Thoracic Society (ATS), the CASAC's written advice and recommendations, and judgments
made by the EPA in considering similar effects in previous NAAQS reviews (75 FR 35526 and
6	In assessments for NAAQS reviews, the lung function responses described as indicative of a moderate functional
response include increases in sRaw of at least 100% (e.g., 2008 ISA; U.S. EPA, 1994, Table 8; U.S. EPA, 1996,
Table 8-3). The moderate category has also generally included reductions in FEVi of 10 to 20% (e.g., U.S. EPA,
1996, Table 8). For the 2008 ISA, the midpoint of that range (15%) was used to indicate a moderate response. A
focus on 15% reduction in FEVi is also consistent with the relationship observed between sRaw and FEVi
responses in the Linn et al. studies for which "a 100% increase in sRaw roughly corresponds to a 12 to 15%
decrease in FEVi" (U.S. EPA, 1994, p. 20). Thus, in the 2008 review, moderate or greater SCh-related
bronchoconstriction or decrements in lung function referred to the occurrence of at least a doubling in sRaw or at
least a 15% reduction in FEVi (2008 ISA, p. 3-5).
7	At concentrations of 400 to 500 ppb, the 2008 ISA notes that the evidence shows "stronger evidence with some
statistically significant increases in respiratory symptoms," and at 600 ppb to 1 ppm, the 2008 ISA notes the
evidence to show "clear and consistent increases in SO2 induced respiratory symptoms" (2008 ISA, Table 3-1).
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35536, June 22, 2010). Based on these considerations, the Administrator judged that the effects
reported in exercising people with asthma following 5- to 10-minute SO2 exposures at or above
200 ppb, especially at or above 400 ppb, can result in adverse health effects (75 FR 35536, June
22, 2010). In so doing, she recognized that effects reported for exposures below 400 ppb are
appreciably less severe than those at and above 400 ppb (75 FR 35547, June 22, 2010).
In applying the health effects evidence to her consideration of the adequacy of the then-
existing standards, the Administrator gave particular attention to the quantitative analyses that
evaluated the potential for exercising individuals with asthma to experience exposures of a
magnitude associated with adverse effects under air quality conditions that just met the then-
existing standards. In addition to comparison of 5-minute air concentrations in 40 U.S. counties
to 5-minute concentrations of potential concern (benchmark concentrations ranging from 100-
400 ppb), the analyses included a population exposure-based assessment in two study areas, St.
Louis, MO and Greene County, MO. Five-minute exposure concentrations were estimated for
people with asthma while at elevated exertion levels. The 5-minute exposure concentrations were
compared to benchmark concentrations, and also used to estimate the risk of lung function
decrements in simulated at-risk populations. Among these analyses, the Administrator
emphasized those that utilized the 5-minute benchmark concentrations that were derived from
the controlled human exposure evidence and ranged from 100 ppb to 400 ppb. Based on her
judgments regarding the significance of effects associated with 5-minute concentrations at or
above 200 ppb and 400 ppb, the Administrator considered results of comparisons of exposure
estimates to those benchmark concentrations to be particularly important, giving greater
emphasis to those at or above 400 ppb (75 FR 35547, June 22, 2010).
The exposure-based assessment estimated the portion of the population with asthma in
these two areas that would be expected to experience 5-minute exposures at or above 400 ppb
and 200 ppb while engaged in activities causing them to be at elevated exertion levels. The
Administrator particularly noted the exposure analysis results for the St. Louis case study. This
analysis estimated that for air quality simulated to just meet the then-existing standards,
substantial percentages of children with asthma at moderate or greater exertion8 would be
exposed, at least once annually, to air quality exceeding the 200 ppb and 400 ppb 5-minute
benchmarks (75 FR 35536, June 22, 2010). The Administrator judged these 5-minute exposures
to be significant from a public health perspective due to their estimated frequency:
approximately 24% of children with asthma in St. Louis were estimated to be exposed while at
moderate or greater exertion at least once per year to air quality exceeding the 5-minute 400 ppb
8 In the 2009 REA, an equivalent ventilation rate of 22 L/min-m2 was identified as the minimum value to reflect
"moderate" or greater exertion that would correspond to the elevated ventilation rate for the exercising subjects in
the controlled human exposure studies, which was 40-50 L/min (2009 REA, p. 236).
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benchmark. Additionally, approximately 73% of children with asthma in St. Louis at moderate or
greater exertion were estimated to be exposed at least once per year to air quality exceeding the
5-minute 200 ppb benchmark (75 FR 35536, June 22, 2010).
The Administrator also took note of the CASAC conclusion that the then-existing
standards did not adequately protect public health. Specifically, the CASAC advised that: "the
current 24-hour and annual standards are not adequate to protect public health, especially in
relation to short-term exposures to SO2 (5-10 minutes) by exercising asthmatics" (Samet, 2009,
p. 15). Based on all of the considerations summarized above, the Administrator concluded that
the then-existing 24-hour and annual primary standards were not providing the requisite
protection of public health with an adequate margin of safety. In considering approaches to
revising the standards, the Administrator concluded it to be appropriate to set a new standard that
would provide requisite protection with an adequate margin of safety to people with asthma at
elevated ventilation and that would afford protection from the adverse health effects of 5-minute
to 24-hour SO2 exposures (75 FR 35536, June 22, 2010).
3.1.1.2 Approach for Considering Revisions to the Standards
With regard to revisions to provide requisite public health protection, the Administrator
concluded it was appropriate to set a 1-hour SO2 standard at a level of 75 ppb based on the 3-
year average of the 99th percentile of the yearly distribution of 1-hour daily maximum
concentrations. The rationale and approach for selecting the 1-hour standard is presented below
in terms of the individual elements of a NAAQS: indicator, averaging time, form, and level.
3.1.1.2.1 Indicator
In reaching her decision on the indicator for the new standard, the Administrator
considered the conclusions of the ISA and REA, as well as advice from the CASAC and public
comments (75 FR 35536, June 22, 2010). The EPA continued to focus on SO2 as the most
appropriate indicator for gaseous sulfur oxides because the available scientific information
regarding health effects was overwhelmingly indexed by SO2. Although the presence of gaseous
SOx species other than SO2 in ambient air had been recognized, no alternative to SO2 had been
advanced as a more appropriate surrogate for ambient gaseous SOx (75 FR 35536, June 22,
2010). Controlled human exposure studies and animal toxicological studies provided specific
evidence for health effects following exposures to SO2, and epidemiologic studies typically
reported effects associated with SO2 concentrations. Based on the information available in the
last review and consistent with the views of the CASAC that "for indicator, SO2 is clearly the
preferred choice" (Samet, 2009, p. 14), the Administrator concluded it was appropriate to
continue to use SO2 as the indicator for a standard that was intended to address effects associated
with exposure to SO2, alone or in combination with other gaseous sulfur oxides (75 FR 35536,
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June 22, 2010). In so doing, the EPA recognized that measures leading to reductions in
population exposures to SO2 will also likely reduce exposures to other sulfur oxides (75 FR
35536, June 22, 2010).
3.1.1.2.2 Averaging Time
With regard to the setting of the new standard, the Administrator agreed with the staff
conclusion, based on conclusions in the ISA, advice from the CASAC, and air quality analyses,
that the standard should be set to provide protection from short-term exposures of 5 minutes to
24 hours (75 FR 35539, June 22, 2010). Based on air quality analyses presented in the REA, the
Administrator judged that the requisite protection from 5- to 10-minute exposure events could be
provided without having a standard with a 5-minute averaging time (75 FR 35539, June 22,
2010). She judged that a standard with a 5-minute averaging time would result in significant and
unnecessary instability in public health protection (75 FR 35539, June 22, 2010).9 Accordingly,
she considered other averaging times.
Results of air quality analyses in the REA suggested that a standard based on 24-hour
average SO2 concentrations would not likely be an effective or efficient approach for addressing
5-minute peak SO2 concentrations, likely over-controlling in some areas, while under-controlling
in others (2009 REA, section 10.5.2.2). In contrast, these analyses suggested that a 1-hour
averaging time would be more efficient and effective at limiting 5-minute peaks of SO2 (2009
REA, section 10.5.2.2.). Drawing on this information, the Administrator concluded that a 1-hour
standard, with the appropriate form and level, would be likely to substantially reduce 5- to 10-
minute peaks of SO2 that had been shown in controlled human exposure studies to result in
increased prevalence of respiratory symptoms and/or decrements in lung function in exercising
people with asthma (75 FR 35539, June 22, 2010). Further she found that a 1-hour standard
could substantially reduce the upper end of the distribution of SO2 concentrations in ambient air
that were more likely to be associated with respiratory outcomes (75 FR 35539, June 22, 2010).
The Administrator additionally took note of advice from the CASAC. The CASAC stated
that the REA had presented a "convincing rationale" for a 1-hour standard, and that "a one-hour
standard is the preferred averaging time" (Samet, 2009, pp. 15, 16). The CASAC further stated
that it was "in agreement with having a short-term standard and finds that the REA supports a
one-hour standard as protective of public health" (Samet, 2009, p. 1). Thus, in consideration of
the available information summarized here and the CASAC's advice, the Administrator
concluded that a 1-hour standard (given the appropriate level and form) was an appropriate
9 Such instability could reduce public health protection by disrupting an area's ongoing implementation plans and
associated control programs (75 FR 35537, June 22, 2010).
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means of controlling short-term exposures to SO2 ranging from 5 minutes to 24 hours (75 FR
35539, June 22, 2010).
3.1.1.2.3	Form
In considering the statistical form for the new short-term standard, the Administrator
judged that the form of the standard should reflect the health effects evidence presented in the
ISA that indicated that the percentage of people with asthma affected and the severity of the
response increased with increasing SO2 concentrations (75 FR 35541, June 22, 2010). She
additionally found it reasonable to consider stability (e.g., to avoid disruption of programs
implementing the standard and the related public health protections from those programs) as part
of her consideration of the form for the standard (75 FR 35541, June 22, 2010). In so doing, she
noted that a concentration-based form averaged over three years would likely be appreciably
more stable than a no-exceedance based form, which had been the form of the then-existing 24-
hour standard (75 FR 35541, June 22, 2010). The CASAC additionally stated that "[tjhere is
adequate information to justify the use of a concentration-based form averaged over 3 years"
(Samet, 2009, p. 16). In consideration of this information, the Administrator judged a
concentration-based form averaged over three years to be most appropriate (75 FR 35541, June
22, 2010).
In selecting a specific concentration-based form, the Administrator considered health
evidence from the ISA as well as air quality and exposure information from the REA. In so
doing, the Administrator concluded that the form of a new 1-hour standard should be especially
focused on limiting the upper end of the distribution of ambient SO2 concentrations (i.e., above
90th percentile SO2 concentrations) in order to provide protection with an adequate margin of
safety against effects reported in epidemiologic and controlled human exposure studies (75 FR
35541, June 22, 2010). The Administrator further noted, based on results of air quality and
exposure analyses in the REA, that a 99th percentile form was likely to be appreciably more
effective at limiting 5-minute peak exposures of concern than a 98th percentile form (75 FR
35541, June 22, 2010). Thus, the Administrator selected a 99th percentile form averaged over
three years (75 FR 35541, June 22, 2010).
3.1.1.2.4	Level
In selecting the level of a new 1-hour standard, the Administrator gave primary emphasis
to the body of health effects evidence assessed in the ISA. In so doing, she noted that the
controlled human exposure studies provided the most direct evidence of respiratory effects from
exposure to SO2. The Administrator drew on evidence from these studies in reaching judgments
on the magnitude of adverse respiratory effects and associated potential public health
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significance for the air quality exposure and risk analysis results of air quality scenarios
representing just meeting alternative levels for a new 1-hour standard.
In particular, the Administrator considered effects in exercising people with asthma after
5- to 10-minute exposures as low as 200 ppb to be adverse in light of the CAS AC advice on
relevance of these effects, conclusions on similar effects in prior NAAQS reviews, and ATS
guidelines (ATS, 1985, 2000). This judgment was based on several findings from the controlled
human exposures studies. Five-to 10-minute exposures to 400 ppb or greater resulted in
moderate or greater decrements in lung function in 20-60% of exercising individuals with
asthma. These decrements are often statistically significant at the group mean level and
frequently accompanied by respiratory symptoms. Thus, exposures to SO2 concentrations at or
above 400 ppb were concluded to clearly result in adverse respiratory effects based on the ATS
guidelines (ATS, 1985). Further, 5- to 10-minute exposures to 200 to 300 ppb resulted in
moderate or greater decrements in lung function in 5-30% of exercising individuals with asthma
(75 FR 35546, June 22, 2010). Although such effects have not been shown to be statistically
significant at the full study group mean level,10 or to be frequently accompanied by respiratory
symptoms, the Administrator considered effects associated with exposures as low as 200 ppb to
be adverse in light of the CASAC's advice11 and similar conclusions in prior reviews as well as
the ATS guidelines (ATS, 1985, 2000).
The Administrator then considered what the findings of the REA exposure analyses
indicated with regard to varying degrees of protection that different 1-hour standard levels might
be expected to provide against 5-minute benchmark concentrations of 200 ppb and 400 ppb.12
For example, the exposure assessment for St. Louis13 estimated that a 1-hour standard at 100 ppb
would likely protect more than 99% of children with asthma in that city from experiencing any
days in a year with at least one 5-minute exposure at or above 400 ppb while at moderate or
greater exertion, and approximately 97% of those children with asthma from experiencing any
days in a year with at least one exposure at or above 200 ppb while at moderate or greater
exertion (75 FR 35547, June 22, 2010). Results for the air quality scenario for a 1-hour standard
10	As summarized in section 3.2.1.1 below and described more fully in the second draft ISA for this review, study
subjects have since been characterized as falling into two subpopulations that differ in susceptibility to SO2.
11	The CAS AC letter on the first draft SO2 REA to the Administrator stated: "CAS AC believes strongly that the
weight of clinical and epidemiology evidence indicates there are detectable clinically relevant health effects in
sensitive subpopulations down to a level at least as low as 0.2 ppm SO2" (Henderson, 2008).
12	The Administrator additionally noted the results of the 40-county analysis of limited available 5-minute
concentration data that indicated for a 1-hour standard level of 100 ppb a maximum annual average of two days
per year with 5-minute concentrations above 400 ppb and 13 days with 5-minute concentrations above 200 ppb
(76 FR 35546, June 22, 2010).
13	St. Louis was one of two study areas assessed in the REA (2009 REA).
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level of 50 ppb suggested that such a standard would somewhat further limit exposures, such that
more than 99% of children at moderate or greater exertion would likely be protected from
experiencing any days in a year with a 5-minute exposure at or above the 200 ppb benchmark
concentration (75 FR 35542-47, June 22, 2010).
In considering the implications of the exposure assessment results the Administrator
noted that although she considered the health effects resulting from 5-minute SO2 exposures as
low as 200 ppb to be adverse, she also recognized that such effects are appreciably less severe
than those at SO2 concentrations at or above 400 ppb and found little difference between the
results for standard levels of 50 and 100 ppb with regard to 5-minute exposures at or above 400
ppb. She recognized that a standard level below 100 ppb may somewhat further limit 5-minute
SO2 ambient air concentrations and exposures above 200 ppb, although she did not judge that a
standard level of 50 ppb was warranted.
Before reaching her conclusion with regard to level for the 1-hour standard, the
Administrator additionally considered the epidemiological evidence among the U.S.
epidemiologic studies (some conducted in multiple locations) reporting mostly positive and
sometimes statistically significant associations between ambient SO2 concentrations and
emergency department visits and hospital admissions. She noted there was a cluster of three
studies for which 99th percentile 1-hour daily maximum concentrations were estimated to be
between 78-150 ppb and for which the SO2 effect estimate remained positive and statistically
significant in copollutant models with particulate matter (PM) (75 FR 35547, June 22, 2010).14
Given the above considerations and the comments received on the proposal, the
Administrator determined that the appropriate judgment, based on the entire body of evidence
and information available in this review, and the related uncertainties,15 was a standard level of
75 ppb. She concluded that such a standard, with a 1-hour averaging time and 99th percentile
form, would provide a significant increase in public health protection compared to the current
standards and would be expected to provide protection, with an adequate margin of safety,
against the respiratory effects that have been linked with SO2 exposures in both controlled
human exposure and epidemiologic studies. Specifically, she concluded that such a standard
would limit 1-hour exposures at and above 75 ppb. (75 FR 35548, June 22, 2010). Such a
standard was also considered likely "to maintain S02 concentrations below those in locations
14	Regarding the monitor concentrations in these studies, the EPA noted that although they may be a reasonable
approximation of concentrations occurring in the areas, the monitored concentrations were likely somewhat lower
than the absolute highest 99th percentile 1-hour daily maximum SO2 concentrations occurring across these areas
(75 FR 35547, June 22, 2010).
15	Such uncertainties included both those with regard to the epidemiologic evidence and also those with regard to the
information from controlled human exposure studies for at-risk groups, including representation of individuals
with more severe asthma than that in study subjects (75 FR 35546, June 22, 2010).
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where key U.S. epidemiologic studies have reported that ambient S02 is associated with clearly
adverse respiratory health effects, as indicated by increased hospital admissions and emergency
department visits." The Administrator also found that "a 1-hour standard at a level of 75 ppb is
expected to substantially limit asthmatics' exposure to 5-10 minute S02 concentrations > 200
ppb, thereby substantially limiting the adverse health effects associated with such exposures."
Lastly, the Administrator noted "that a standard level of 75 ppb is consistent with the consensus
recommendation of CAS AC". The Administrator also considered the likelihood of public health
benefits at lower standard levels, and judged a 1-hour standard at 75 ppb to be sufficient to
protect public health with an adequate margin of safety (75 FR 35547-35548, June 22, 2010).
This judgment included consideration of the appropriate degree of protection with an
adequate margin of safety for populations at increased risk for adverse respiratory effects from
short-term exposures to SO2 for which the evidence supports a causal relationship with SO2
exposures. In reaching these conclusions, the Administrator considered the requirement for a
standard that is neither more nor less stringent than necessary for this purpose and recognized
that the CAA does not require that primary NAAQS be set at a zero-risk level or to protect the
most susceptible individual, but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety (75 FR 35548, June 22, 2010).
3.1.1.2.5 Revoking the Then-Existing 24-Hour and Annual Standards
In addition to setting a new 1-hour standard at 75 ppb, the then-current 24-hour and
annual standards were revoked in the last review based largely on the recognition that a 1-hour
standard set at 75 ppb would have the effect of generally maintaining 24-hour and annual SO2
concentrations well below the levels of those standards (75 FR 35550, June 22, 2010). In
addition, the annual standard was also revoked because of the lack of evidence supporting a
relationship between long-term SO2 exposures and adverse health effects. That is, the 2008 ISA
judged the health evidence linking long-term SO2 exposure to adverse health effects to be
"inadequate" to infer the presence or absence of a causal relationship (75 FR 35550, June 22,
2010; 2008 ISA, section 5.5).
3.1.2 Approach for the Current Review
To evaluate whether it is appropriate to consider retaining the current SO2 primary
standard, or whether consideration of revision is appropriate, we have adopted an approach in
this review that builds on the general approach used in the last review and reflects the body of
evidence and information now available. As summarized above, the Administrator's decisions in
the prior review were based on an integration of information on health effects associated with
exposure to SO2, expert judgments on the adversity and public health significance of key health
effects, air quality and related analyses and quantitative exposure and risk assessments, and
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policy judgments as to when the standard is requisite to protect public health with an adequate
margin of safety.
In conducting this assessment, we draw on the current evidence and quantitative
assessments of exposure pertaining to the public health risk of SO2 in ambient air. In considering
the scientific and technical information, we consider both the information available at the time of
the last review and information newly available since the last review, including the second draft
ISA and draft REA for this review. Figure 3-1 below illustrates the basic construct of our two-
part approach in developing preliminary conclusions regarding options to consider with regard to
the adequacy of the current primary standard and, as appropriate, potential alternative standards.
In the boxes of Figure 3-1, the range of questions that we consider in sections 3.2.1 and 3.2.2
below are represented by a summary of policy-relevant questions that frame our consideration of
the scientific evidence and quantitative analyses.
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Adequacy of Current Standard
Does information
call into question
adequacy of current
standard?
Consider Retaining
Current Standard
NO
YES
Consider Potential Alternative Standards
Elements of Potential Alternative Standards
--Indicator
^Averaging Time
>*Form
> Level
Exposure- and Risk-Based Considerations
Mature, magnitude, and importance of
estimated exposures and risks associated
with just meeting the current standard?
> Uncertainties in the exposure and risk
estimates?
/'Does currently available evidence and related
uncertainties strengthen or call into question prior
conclusions?
¦	Evidence of health effects not previously
identified?
¦	Newly identified at-risk populations?
•	Evidence of health effects at lower levels or for
different exposure durations?
*	Uncertainties identified in the last review
reduced or new uncertainties emerged?
^Does newly available information call into question
arty of the basic elements of the standard?	
Evidence-Based Considerations
2
3
(^Potential Alternative Standards for Consideration^)
4 Figure 3-1. Overview of the approach for review of the current primary standard.
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3.2 ADEQUACY OF THE CURRENT STANDARD
In considering the adequacy of the current SO2 primary standard, the overarching
question we consider is:
•	Does the currently available scientific evidence- and exposure/risk-based
information, as reflected in the ISA and REA, support or call into question the
adequacy of the protection afforded by the current SO2 primary standard?
To assist us in interpreting the currently available scientific evidence and the results of
recent quantitative exposure/risk analyses to address this question, we have focused on a series
of more specific questions, as detailed in sections 3.2.1 and 3.2.2 below. In considering the
scientific and technical information, we consider both the information available at the time of the
last review and information newly available since the last review which have been critically
analyzed and characterized in the 2008 ISA for the last review and the second draft ISA for the
current review.
3.2.1 Evidence-based Considerations
In considering the evidence with regard to the overarching question posed above
regarding the adequacy of the current standard, we address a series of more specific questions
that focus on policy-relevant aspects of the evidence. These questions begin with consideration
of the available evidence regarding the health effects associated with exposure to SOx, and
particularly SO2 (section 3.2.1.1). The subsequent questions consider identification of
populations at-risk of SCh-related health effects (section 3.2.1.2), and the exposure durations and
levels of SO2 associated with health effects (section 3.2.1.3). Important uncertainties associated
with the evidence are considered in section 3.2.1.4.
3.2.1.1 Health Effects Associated with Exposure to SOx
Among the species of SOx (a group of closely related gaseous compounds including SO2
and SO3), SO2 is the most commonly occurring in the atmosphere. Accordingly, the large body
of scientific evidence has over the past reviews been predominantly focused on exposures to
SO2.
•	Is there newly available evidence that indicates the importance of SOx other than
SO2 with regard to abundance in ambient air, and potential for human exposures
and health effects?
As in the last review, the health effects evidence evaluated in the second draft ISA for
SOx is focused on SO2 (second draft ISA, p. 5-1). This is consistent with the conclusion that
"[o]f the sulfur oxides, SO2 is the most abundant in the atmosphere, the most important in
atmospheric chemistry, and the one most clearly linked to human health effects" (second draft
ISA, p. 2-1). While "SO3 can be emitted by some sources, it reacts within seconds with water in
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the stacks or immediately after release into the atmosphere to form H2SO4 and gas-phase sulfuric
acid quickly condenses or contributes to particle formation" (second draft ISA, section 2.3).
Thus, the second draft ISA states that "only SO2 is present at concentrations relevant for
chemistry in the troposphere, boundary layer, and for human exposures" (second draft ISA, p. 2-
17), and also that the available health evidence for SOx is focused on SO2 (second draft ISA, p.
5-1). Thus, we conclude 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.
• Does the current evidence alter our conclusions from the previous review regarding
the health effects associated with exposure to SO2?
Rather than altering our conclusions from the last review, the current evidence continues
to support our prior conclusions regarding the key health effects associated with SO2 exposure.
Specifically, the full body of evidence continues to support the conclusion that short-term SO2
exposures of durations as short as a few minutes are causally related to respiratory effects in at-
risk individuals (second draft ISA, section 5.2.1.9). With regard to respiratory effects and long-
term exposures, as well as total mortality and short-term exposures, the evidence available in this
review is "suggestive of, but not sufficient to infer," a causal relationship (second draft ISA,
sections 5.2.2.7 and 5.5.1.6). The evidence is inadequate for reaching conclusions regarding
causality for other categories of effects (second draft ISA, section 1.6.2).16
Respiratory Effects
As in the last review, the currently available evidence in this review supports the
conclusion that there is a causal relationship between short-term SO2 exposure and respiratory
effects, particularly in individuals with asthma (second draft ISA, p. 1-16).17 The clearest
evidence for this conclusion comes from controlled human exposure studies available at the time
of the previous review and included in the 2008 ISA. These studies demonstrate lung function
16	Based on the currently available evidence, the ISA concluded that the evidence was inadequate to infer the
presence or absence of a causal relationship between SO2 exposures and reproductive and developmental effects;
between long-term SO2 exposures and mortality or cancer; and, between short- or long-term SO2 exposures and
cardiovascular effects (second draft ISA, sectionl.6.2).
17	While effects have been documented for short (5 to 10 minutes) exposures lower than 1.0 ppm in controlled
exposure studies of individuals with asthma, the exposure concentrations consistently eliciting effects in study
subjects without asthma are higher. Such exposures are generally above 1.0 or 5.0 ppm, with most studies
reporting no respiratory symptoms at concentrations up to 2.0 ppm (second draft ISA, section 5.2.1.7).
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decrements18 and respiratory symptoms in people with asthma exposed to SO2 for 5 to 10
minutes at elevated breathing rates (second draft ISA, section 5.2.1). The epidemiologic
evidence, including recent evidence not available at the time of the previous review, includes
studies reporting positive associations for asthma-related hospital admissions and emergency
department visits with short-term SO2 exposures (second draft ISA, section 5.2.1). These
findings are generally supportive of the causal relationship conclusion for which the controlled
exposure studies are the primary basis (second draft ISA, section 5.2.1.9).
Sulfur dioxide is a highly reactive and water-soluble gas that once inhaled is absorbed
almost entirely in the upper respiratory tract19 (second draft ISA, sections 4.2 and 4.3). Under
conditions of elevated ventilation (e.g., while exercising), SO2 penetrates into the
tracheobronchial region,20 where it may contribute to responses linked to asthma exacerbation in
individuals with asthma (second draft ISA, sections 4.2, 4.3 and 5.2). More specifically,
bronchoconstriction, which is characteristic of an asthma attack, is the most sensitive indicator of
S02-induced lung function effects. Associated with this bronchoconstriction response is an
increase in airway resistance which is an index of airway hyperresponsiveness (AHR),21
Exercising individuals without asthma have also been found to exhibit such responses, but at
much higher SO2 exposure concentrations, above 1000 ppb (second draft ISA, section 1.5.2).
Bronchoconstriction, evidenced by decrements in lung function, is observed in controlled
human exposure studies after approximately 5- to 10-minute exposures and can occur at SO2
concentrations as low as 200 ppb in some people with asthma exposed while breathing at
elevated ventilation, such as during exercise (second draft ISA, section 5.2.1.2).22 More
consistent decrements in lung function are seen in such individuals with asthma following
exposures to 400 ppb and greater (second draft ISA, section 5.2.1.2). In contrast, respiratory
effects are not observed in other people with asthma (nonresponders) and healthy adults exposed
18	The specific responses reported in the evidence base that are described in the ISA as lung function decrements are
increased specific airway resistance (sRaw) and reduced forced expiratory volume in 1 second (FEVi) (second
draft ISA, section 5.2.1.2).
19	The term "upper respiratory tract" refers to the portion of the respiratory tract, including the nose, mouth and
larynx, that precedes the tracheobronchial region (second draft ISA, sections 4.2 and 4.3).
20	The term "tracheobronchial region" refers to the region of the respiratory tract subsequent to the larynx and
preceding the deep lung (or alveoli). This region includes the trachea and bronchii.
21	Airway hyperresponsiveness, which is an increased propensity of the airways to narrow in response to
bronchoconstrictive stimuli, is a characteristic feature of people with asthma (second draft ISA, section 5.2.1.2).
22	The data from controlled human exposure studies of people with asthma indicate there to be two subpopulations
that differ in their airway responsiveness to S02, with the second subpopulation being insensitive to S02
bronchoconstrictive effects at concentrations as high as 1.0 ppm (second draft ISA, pp. 5-14 to 5-20; Johns et al.,
2010).
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while exercising to SO2 concentrations below 1000 ppb (second draft ISA, sections 5.2.1.2 and
5.2.1.7). Across studies, bronchoconstriction in response to SO2 exposure is mainly seen during
conditions of increased ventilation rates, such as exercise or laboratory-facilitated rapid, deep
breathing.23 These conditions lead to a shift from nasal breathing to oral/nasal breathing, which
increases the concentration of SO2 reaching the tracheobronchial region of lower airways, where
depending on dose and the exposed individual's susceptibility, it may cause bronchoconstriction
(second draft ISA, sections 4.1.2.2, 4.2.2 and 5.2.1.2).
The evidence base of controlled human exposure studies for people with asthma is the
same in this review as in the last review. Such studies reporting asthma exacerbation-related
effects for individuals with asthma are summarized in Tables 5-1 and 5-2, and section 5.2.1.2 of
the second draft ISA. The main responses observed include increases in specific airway
resistance (sRaw) and reductions in forced expiratory volume in one second (FEVi) after 5- to
10-minute exposures. As in the last review, the second draft ISA in this review quantifies the
percentage of exposed study subjects with at least 100%, 200% or 300% increases in sRaw (i.e.,
a doubling, tripling or greater increase) and also those with at least 15%, 20% or 30% reduction
in FEVi. As recognized in the last review, the results of these studies indicate that among
individuals with similar disease status, some individuals have a greater response to SO2 than
others (second draft ISA, p. 5-14). The SCh-induced bronchoconstriction in these studies occurs
rapidly, in as little as two minutes from exposure start, and is transient, with recovery following
cessation of exposure (second draft ISA, p. 5-41).
The studies of subjects with asthma breathing at elevated ventilation have found effects
to become more pronounced with increased exposure concentrations. Among individuals with
asthma, both the percentage of individuals affected and the severity of the response increases
with increasing SO2 concentrations. For example, at concentrations ranging from 200 to 300 ppb,
as many as 5 to 30% of exercising study subjects with asthma experienced moderate24 or greater
decrements in lung function (second draft ISA, Table 5-2). At concentrations at or above 400
ppb, moderate or greater decrements in lung function occurred in 20 to 60% of exercising study
subjects with asthma, and compared to exposures at 200 to 300 ppb, a larger percentage of
subjects experienced severe decrements in lung function (i.e., an increase in sRaw of at least
200%), and/or a reduction in FEVi of at least 20%) (second draft ISA, Table 5-2). Moreover, at
23	In the laboratory, study subjects perform this rapid, deep breathing through a mouthpiece that provides a mixture
of oxygen with enough carbon dioxide to prevent the imbalance of gases in the blood usually resulting from
hyperventilation. Breathing in the laboratory with this technique is referred to as eucapnic hyperpnea.
24	As in the last review (described in section 3.1.1.1 above), the second draft ISA describes moderate or greater lung
function decrements as the occurrence of at least a doubling in sRaw or at least a 15% reduction in FEVi (second
draft ISA, section 1.6.1.1).
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the higher SO2 concentrations, moderate or greater decrements in lung function were frequently
accompanied by respiratory symptoms, such as cough, wheeze, chest tightness, or shortness of
breath (second draft ISA, Table 5-2).
With regard to newly available epidemiological studies, there are a limited number of
such studies that have investigated SO2 effects related to asthma exacerbation, with the most
cohesive evidence coming from studies on asthma-related emergency department (ED) visits
(second draft ISA, section 5.2.1.2). As in the last review, areas of uncertainty in the
epidemiologic evidence relate to the characterization of exposure through the use of fixed site
monitor concentrations as surrogates for population exposure (often over a substantially sized
area and for durations greater than an hour) and the potential for confounding by PM or other
copollutants (second draft ISA, section 5.2.1). In general, the pattern of associations across the
newly available studies is consistent with the studies available in the last review (second draft
ISA, p. 5-70).
As in the last review, the evidence base for short-term SO2 exposures and respiratory
effects other than asthma exacerbation is limited and inconsistent. The second draft ISA finds the
evidence for an effect of SO2 exposure on allergy exacerbation, COPD exacerbation, respiratory
infection, respiratory effects in healthy populations, and respiratory mortality to be inconsistent
within and across disciplines and outcomes, and finds there to be uncertainty associated with the
epidemiological evidence for these endpoints that is related to potential confounding by
copollutants (second draft ISA, section 5.2.1.9).
The evidence base for long-term SO2 exposure and respiratory effects is somewhat
augmented since the last review such that the second draft ISA in the current review concludes it
to be suggestive of, but not sufficient to infer, a causal relationship (second draft ISA, section
5.2.2). The support for this conclusion comes mainly from the limited epidemiological study
findings of associations between long-term SO2 concentrations and increases in asthma incidence
combined with findings of laboratory animal studies involving newborn rodents that indicate a
potential for SO2 exposure to contribute to the development of asthma, especially allergic
asthma, in children (second draft ISA, section 1.6.1.2). The evidence showing increases in
asthma incidence is coherent with results of animal toxicological studies that provide a
pathophysiologic basis for the development of asthma. The overall body of evidence, however,
lacks consistency (second draft ISA, section 1.6.1.2). Further there are uncertainties, discussed in
section 3.2.1.4 below, that apply to the epidemiologic evidence, including that newly available,
across the respiratory effects examined for long-term SO2 exposure (second draft ISA, section
5.2.2.7).
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Other Health Effects
For effects other than respiratory effects, the current evidence is generally similar to the
evidence available in the last review, and is supportive of similar conclusions. With regard to a
relationship between short-term SO2 exposure and total mortality, the second draft ISA reaches
the same conclusion as in the previous review that the evidence is suggestive of, but not
sufficient to infer, a causal relationship (second draft ISA, section 5.5.1). This conclusion is
based on previous and recent multicity epidemiologic studies providing consistent evidence of
positive associations, although there is uncertainty regarding the potential for SO2 to have an
independent effect on mortality. While recent studies have analyzed some key uncertainties and
data gaps from the previous review, uncertainties still exist, given the limited number of studies
that examined copollutant confounding, the evidence for a decrease in the size of SCh-mortality
associations in copollutant models with NO2 and PM10, and the lack of a potential biological
mechanism for mortality following short-term SO2 exposures (second draft ISA, section 1.6.2.4).
For other categories of health effects,25 the evidence is inadequate to infer the presence or
absence of a causal relationship, mainly due to inconsistent evidence across specific outcomes
and uncertainties regarding exposure measurement error, copollutant confounding, and potential
modes of action (second draft ISA, sections 5.3.1, 5.3.2, 5.4, 5.5.1, 5.5.2, 5.6). These conclusions
are consistent with those made in the previous review.
In summary, rather than altering our conclusions from the previous review, the current
evidence provides continued support for our previous conclusions regarding the health effects
associated with exposure to SO2 and most particularly respiratory effects following short-term
SO2 exposure, particularly in individuals with asthma. Accordingly, as in prior reviews, this
review gives primary focus to those effects most pertinent to exposures related to current
concentrations in ambient air, in particular, asthma exacerbation in individuals with asthma.
3.2.1.2 Populations At-Risk of SCh-Related Health Effects
Populations or lifestages can be at increased risk of an air pollutant-related health effect
due to one or more of a number of factors. These factors can be intrinsic, such as physiological
factors that may influence the internal dose or toxicity of a pollutant, or extrinsic, such as
sociodemographic, or behavioral factors. The questions considered in this section address what
the currently available evidence indicates regarding which populations are particularly at risk of
health effects related to exposure to SO2 in ambient air.
25 The other categories evaluated in the ISA include cardiovascular effects with short or long term exposures;
reproductive and developmental effects; and cancer and total mortality with long-term exposure (second draft
ISA, Table 1-1).
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• Does the current evidence alter our understanding of populations that are
particularly at-risk from SO2 exposures? Is there new evidence that suggests
additional at-risk populations that should be given increased focus in this review?
The currently available evidence continues to support our primary conclusions from the
previous review that people with asthma are at increased risk for SCh-related health effects,
specifically for respiratory effects, and specifically asthma exacerbation, associated with short-
term exposures while at elevated ventilation (second draft ISA, sections 5.2.1.2 and 6.3.1). This
conclusion of the at-risk status of people with asthma is based on the well-established, and well-
characterized, evidence from controlled human exposure studies, supported by the evidence on
mode of action for SO2 and with limited additional support from epidemiologic studies (second
draft ISA, sections 5.2.1.2 and 6.3.1). Somewhat similar to the conclusion in the last review that
children and older adults are potentially susceptible populations, the second draft ISA (relying on
a more systematic approach for evaluating the evidence than in the last review) indicates the
evidence to be suggestive of increased risk for these groups, with some limitations and
inconsistencies (second draft ISA, sections 6.5.1.1 and 6.5.1.2).26
Further, the second draft ISA finds that children with asthma may be particularly at risk
compared to adults with asthma (second draft ISA, section 6.3.1). This conclusion reflects
several characteristics of children as compared to adults, which include their greater
responsiveness to methacholine,27 a chemical that can elicit bronchoconstriction in people with
asthma, as well as their greater use of oral breathing, particularly by boys (second draft ISA,
sections 5.2.1.2 and 4.1.2). Oral breathing (vs nasal breathing) and increased ventilation are
factors that allow for greater SO2 penetration into the tracheobronchial region of the lower
airways, and reflect conditions of individuals with asthma in which bronchoconstriction-related
responses have been observed in the controlled exposure studies (second draft ISA, sections
4.2.2, 5.2.1.2 and 6.3.1).
26	The current evidence for risk to older adults relative to other lifestages comes from epidemiological studies, for
which the findings are somewhat inconsistent, and studies with which there are uncertainties in the association
with the health outcome (second draft ISA, section 6.5.1.2).
27	The second draft ISA concluded that potential differences in airway responsiveness of children to SO2 relative to
adolescents and adults may be inferred by the responses to methacholine (second draft ISA, section 5.2.1.2).
Methacholine is a chemical that can elicit bronchoconstriction through its action on airway smooth muscle
receptors. It is commonly used to identify people with asthma and accordingly has been used to screen subjects
for studies of SO2 effects. An analysis of the extent to which airway responsiveness to methacholine, a history of
respiratory symptoms, and atopy were significant predictors of airway responsiveness to SO2, found that about 20
to 25% of subjects ranging in age from 20 to 44 years were hyperresponsive to methacholine (second draft ISA,
section 5.2.1.2; Nowak et al., 1997). Another study focused on individuals with airway responsiveness to
methacholine found only a weak correlation between airway responsiveness to S02 and methacholine (second
draft ISA, section 5.2.1.2; Horstman et al., 1986).
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We additionally recognize the well-documented finding that some individuals with
asthma have a greater response to SO2 than others with similar disease status (second draft ISA,
section 5.2.1.2; Horstman et al., 1986; Johns et al., 2010). This occurrence is quantitatively
analyzed in a study newly available in this review. This study uses the available individual
subject data from five studies involving exposure of individuals with asthma to multiple
concentrations of SO2 for 5 to 10 minutes while at elevated ventilation to examine the
differences in lung function response (Johns et al., 2010). As noted in the second draft ISA,
"these data demonstrate a bimodal distribution of airway responsiveness to SO2 in individuals
with asthma, with one subpopulation that is insensitive to the bronchoconstrictive effects of SO2
even at concentrations as high as 1.0 ppm, and another subpopulation that has an increased risk
for bronchoconstriction at low concentrations of SO2" (second draft ISA, p. 5-17). To date, the
characteristics that may define the subpopulation of responders have not been identified. The
current evidence for factors other than those discussed above (asthma status and lifestage) is
inadequate to determine whether they might contribute to an increased risk of SCh-related effects
(second draft ISA, section 6.6).
3.2.1.3 Exposure Concentrations Associated with Health Effects
At the time of the last review, the EPA's conclusions regarding concentrations of SO2
associated with respiratory effects were based primarily on the strong evidence base of
controlled human exposure studies of individuals with asthma. These studies have documented
bronchoconstriction-related moderate or greater decrements in lung function following 5- to 10-
minute exposures during exercise. The severity of observed responses, the percentage of
individuals responding, statistical significance at the study group level and the accompanying
occurrence of respiratory symptoms have been found to increase with increasing exposure
concentration (75 FR 35526, June 22, 2010). This information was critical in the REA analyses
in the last review, the results of which were a primary consideration in reaching a conclusion on
the level for the 2010 standard.
• Does the current evidence alter our conclusions from the previous review regarding
the exposure duration and concentrations associated with health effects?
The current evidence, including that newly available in this review, supports conclusions
from the last review on exposure duration and concentrations associated with SCh-related health
effects. These conclusions were largely based on the longstanding evidence base of controlled
human exposure studies that demonstrates a relationship between 5- and 10-minute SO2 exposure
concentrations and decrements in lung function (e.g., increased sRaw and reduced FEVi) in
individuals with asthma exposed while at elevated ventilation rate (second draft ISA, section
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1.6.1.1). At the higher concentrations, there are clear and consistent increases in SCh-induced
respiratory symptoms (second draft ISA, Table 5-2).
The available and well characterized evidence documents an effect of short-term
exposures on the respiratory system. As summarized in section 3.2.1.1, SCh-induced
bronchonstriction occurs rapidly in responding study subjects with asthma exposed for just a few
minutes while breathing at elevated ventilation rates (second draft ISA, section 5.2.1.2).
Additionally, exposures as short as 5 minutes have been found to elicit a similar
bronchoconstrictive response as somewhat longer exposures. For example, during exposure to
SO2 over a 30-minute period with continuous exercise, the response to SO2 has been found to
develop rapidly and is maintained throughout the 30-minute exposure (second draft ISA, p. 5-
14). In a study involving short exercise periods within a 6-hour exposure period, the effects
observed following exercise were documented to return to baseline levels within one hour after
the cessation of exercise, even with continued exposure (Linn et al., 1984). In considering the
epidemiological evidence with regard to the question of exposure duration, while we note the
associations of asthma-related emergency room visits and hospital admissions with 1-hour to 24-
hour ambient air concentration metrics, we recognize that current methods are not able to address
whether these associations are due to exposure on the order of hours or much shorter-term
exposure to peaks in SO2 concentration. As noted in the second draft ISA, the air quality metrics
in the epidemiological studies are for time periods longer than the 5- to 10-minute exposures
eliciting effects in the controlled human exposure studies and may not adequately capture the
spatial and temporal variation in SO2 concentrations (second draft ISA, pp. 5-47, 5-55).
With regard to the evidence for exposure concentrations eliciting effects, we focus
primarily on the controlled human exposure study findings for which data are available to the
EPA for individual subjects with asthma that were exposed during elevated ventilation,
summarized in Table 3-1 (second draft ISA, Table 5-2).28 These data demonstrate that SO2
concentrations as low as 200 to 300 ppb for 5 to 10 minutes elicited moderate or greater
bronchoconstriction, measured as a decrease in FEVi of at least 15% or an increase in sRaw of at
least 100% in a subset of the subjects (second draft ISA, section 5.2.1). Both the percent of
individuals affected and the severity of response increased with increasing SO2 concentrations.
At concentrations ranging from 200 to 300 ppb, the lowest levels tested in free breathing
chamber studies, 5 to 30% of exercising individuals with asthma experienced moderate or greater
decrements in lung function (second draft ISA, section 5.2.1). At concentrations at or above 400
ppb, moderate or greater decrements in lung function occurred in 20 to 60% of exercising
28 The findings summarized in Table 5-2 of the second draft ISA and in Table 3-1 of this draft PA are based on
results that have been adjusted for effects at exercise in clean air so that they have separated out any effect of
exercise in causing bronchoconstriction and reflect the SCh-specific effect.
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1	individuals with asthma and a larger percentage of individuals with asthma experienced more
2	severe decrements in lung function (i.e., an increase in sRaw of at least 200%, and/or a 20% or
3	more decrease in FEVi), compared to exposures at 200 to 300 ppb (second draft ISA, section
4	5.2.1). Additionally, at concentrations at or above 400 ppb, moderate or greater decrements in
5	lung function were frequently accompanied by respiratory symptoms, with some of these
6	findings reaching statistical significance (second draft ISA, section 5.2.1).
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Table 3-1. Percentage of adults with asthma in controlled human exposure studies experiencing sulfur dioxide-induced
decrements in lung function and respiratory symptoms (adapted from Table 5-2 in the second draft ISA).
Cumulative Percentage of Responders
(Number of Subjects)3
so2
Cone
Exposure
Duration

Ventil-
ation
sRaw
>100% *
>200% *
>300% «t«

(ppm)
(min)
N
(L/min)
FEV1
>15% *
>20% *
>30% >1/
Study
0.2
5
23
-48
sRaw
9% (2)b
0
0
Linn etal. (1983b)

10
40
-40
sRaw
7.5% (3)c
2.5% (1)c
oc
Linn etal. (1987)c

10
40
-40
FEV1
9% (3.5)c
2.5% (1)c
1% (0.5)c
Linn etal. (1987)c
0.25
5
19
-50-60
sRaw
32% (6)
16% (3)
0
Bethel etal. (1985)

5
9
-80-90
sRaw
22% (2)
0
0
" Bethel et al. (1985)

10
27
-42
sRaw
0
0
0
Horstman et al. (1986)

10
28
-40
sRaw
4% (1)
0
0
Roger et al. (1985)
0.3
10
20
-50
sRaw
10% (2)
5% (1)
5% (1)
Linn et al. (1988)d

10
21
-50
sRaw
33% (7)
10% (2)
0
Linn etal. (1990)d

10
20
-50
FEV1
15% (3)
0
0
Linn etal. (1988)

10
21
-50
FEV1
24% (5)
14% (3)
10% (2)
Linn etal. (1990)
0.4
5
23
-48
sRaw
13% (3)
4% (1)
0
Linn et al. (1983b)

10
40
-40
sRaw
24% (9.5)c
9% (3.5)c
4% (1.5)c
Linn etal. (1987)c

10
40
-40
FEV1
27.5% (11)c
17.5% (7)c
10% (4)c
Linn etal. (1987)c
0.5
5
10
-50-60
sRaw
60% (6)
40% (4)
20% (2)
Bethel etal. (1983)

10
27
-42
sRaw
22.2% (6)
7.4% (1)
3.7% (1)
Horstman et al. (1986)

10
28
-40
sRaw
18% (5)
4% (1)
4% (1)
Roger et al. (1985)
Respiratory Symptoms:
Supporting Studies
Limited evidence of SC>2-induced
increases in respiratory
symptoms in some people with
asthma: Bethel etal., 1985;
Horstman et al., 1986; Linn et al.,
1983b; Linn et al., 1987; Linn et
al., 1988; Linn et al. 1990;
Schachter et al., 1984
Stronger evidence with some
statistically significant increases
in respiratory symptoms: Balmes
et al., 1987;f Gong etal., 1995 ;
Linn et al., 1983b; Linn et al.,
1987 ; Roger et al., 1985
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so2
Cone
(ppm)
Exposure
Duration
(min)
Ventil-
ation
(L/min)
Cumulative Percentage of Responders
(Number of Subjects)3
sRaw >100% * >200% * >300%
FEVi >15% ^ >20% ^ >30% >1/ study
Respiratory Symptoms:
Supporting Studies
10
45
-30 sRaw
36% (16) 16% (7) 13% (6) Magnussen et al. (1990)f
Clear and consistent increases in
S02-induced respiratory
symptoms: Gong et al., 1995;
Horstman et al., 1988; Linn et al.,
1983b; Linn et al., 1987; Linn et
al., 1990
10
40
-40
FEVi
47.5% (19)c
39% (15.5)c
17.5% (7)c
Linn et al. (1987)c
10
20
-50
FEVi
55% (11)
55% (11)
5% (1)
Linn et al. (1988)
10
21
-50
FEVi
43% (9)
38% (8)
14% (3)
Linn et al. (1990)
10
28
-40
sRaw
50% (14)
25% (7)
14% (4)
Roger et al. (1985)e
10
10
-40
sRaw
60% (6)
20% (2)
0
Kehrl et al. (1987)
10
27
-42
sRaw
55.6% (15)
25.9% (7)
11% (1)
Horstman et al. (1986)
0.6	5 23 -48 sRaw 39% (9) 26% (6) 17% (4) Linn et al. (1983b)
10
40
-40 sRaw 34% (13.5)c 24% (9.5)c
19% (7.5)c Linn et al. (1987)°
10
20
-50 sRaw 60% (12) 35% (7)
10% (2) Linn et al. (1988)
10 21 -50 sRaw 62% (13) 29% (6) 14% (3) Linn et al. (1990)
Cone = concentration; FEVi = forced expiratory volume in 1 sec; sRaw = specific airway resistance; S02 = sulfur dioxide.
aData presented from all references from which individual data were available in the published paper or were provided to EPA (Johns, 2009; Johns and Simmons, 2009; Smith,
1993). Percentage of individuals who experienced greater than or equal to a 100, 200, or 300% increase in specific airway resistance, or a 15, 20, or 30% decrease in FEVi. Lung
function decrements are adjusted for the effects of exercise in clean air (calculated as the difference between the percent change relative to baseline with exercise/S02 and the
percent change relative to baseline with exercise/clean air).
bNumbers in parenthesis represent the number of subjects experiencing the indicated effect.
°Responses of people with mild and moderate asthma reported in Linn et al. (1987) have been combined. Data are the average of the first and second round exposure responses
following the first 10 min period of exercise.
dAnalysis includes data from only people with mild Linn et al. (1988) and moderate Linn et al. (1990) asthma who were not receiving supplemental medication.
eOne subject was not exposed to 1 ppm due to excessive wheezing and chest tightness experienced at 0.5 ppm. For this subject, the values used for 0.5 ppm were also used for
1.0 ppm under the assumption that the response at 1.0 ppm would be equal to or greater than the response at 0.5 ppm.
'Indicates studies in which exposures were conducted using a mouthpiece rather than a chamber.
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The lowest exposure concentration in Table 3-1 is 200 ppb. This is the lowest exposure
concentration tested by studies in which study subjects breathed freely (e.g., without using a
mouthpiece).29 In such studies that tested 200 ppb, a portion of the exercising study subjects with
asthma (approximately 8 to 9%) responded with at least a doubling in sRaw or an increase in
FEV1 of at least 15% (Table 3-1; Linn et al., 1983b; Linn et al., 1987).
With regard to exposure concentrations below 200 ppb, the available evidence is very
limited. This information is derived from studies in which the subjects were exposed by
mouthpiece rather than freely breathing in an exposure chamber (Sheppard et al., 1981; Sheppard
et al., 1984; Koenig et al., 1989; Koenig et al., 1990; Trenga et al., 2001).30 Additionally, some
of these studies did not include an exposure to clean air while exercising that would have
allowed for determining the effect of SO2 versus that of exercise in causing bronchoconstriction.
In those cases, the lung function measurements (e.g., sRaw, FEVi) following SO2 exposure are
assessed relative to measurements taken prior to exposure (baseline), rather than being assessed
relative to measurements for a control exposure to clean air while exercising. The studies cited
here, of a limited number of adults and adolescents, reported small changes in FEVi or sRaw in
the individual study subjects, with magnitudes of change appearing to be smaller than responses
reported from studies at exposure concentrations of 200 ppb or more. For example, the increase
in sRaw reported for two young adult subjects exposed to 100 ppb in the study by Sheppard et al.
(1981) was approximately half the response of these subjects at 250 ppb. In the two studies of
adolescents (aged 12 to 18 years), among the individual study subjects for which respiratory
resistance increased with SO2 exposure, the magnitude of increase was less than 100% in each
subject (Koenig et al., 1990; Koenig et al., 1989).
In considering what can be gleaned from these mouthpiece studies of 100 ppb, we note
that the results of studies that utilize a mouthpiece exposure system cannot be directly compared
to results from studies involving freely breathing subjects because when a mouthpiece is used,
the inhaled breath completely bypasses the nasal passages where SO2 is efficiently removed, thus
allowing more of the inhaled SO2 to penetrate into the tracheobronchial airways (2008 ISA, p. 3-
4; second draft ISA, section 4.1.2.2). This occurrence as well as limited evidence comparing
29	Studies of free-breathing subjects generally make use of small rooms in which the atmosphere is experimentally
controlled such that study subjects are exposed by freely breathing the surrounding air (e.g., Linn et al., 1987).
30	A subset of these studies are cited in the second draft ISA; additionally, three of them (Sheppard et al., 1981;
Koenig et al., 1990; Trenga et al., 2001) are cited in the 2008 ISA and a fourth (Sheppard et al., 1984) is cited in
the 1986 Addendum and 1994 Supplement to the 1982 AQCD. The fifth study (Koenig et al., 1989) is not cited in
the prior AQCDs, the 2008 ISA, or the second draft ISA. This study is an investigation involving nine adolescent
subjects with allergic asthma (positive response to a methacholine challenge test at or below 20 mg/mL) exposed
by mouthpiece to 0.1 ppm during exercise. Measurements of FEVi and RT were taken at baseline and subsequent
to SO2 and air only exposures during exercise (Koenig et al., 1989).
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responses by mouthpiece and chamber exposures leads to the expectation that SCh-responsive
people with asthma breathing SO2 using a mouthpiece, particularly while at elevated ventilation,
would experience greater lung function responses than if exposed to the same test concentration
while freely breathing in an exposure chamber (second draft ISA, p. 5-22; Linn et al., 1983a).
We have also considered what can be gleaned from the epidemiological studies regarding
exposure concentrations associated with health effects. Although exposure concentrations
eliciting respiratory responses are not available from such studies, the ambient air concentrations
occurring in studies that find associations with outcomes such as asthma-related ED visits and
hospitalizations have the potential to indicate ambient air concentrations that may contribute to
exposures that may be eliciting effects. For example, in recognizing the general coherence of
epidemiological study findings for 24-hour ambient air concentrations with the findings of the
controlled human exposure studies for exercising study subjects with asthma exposed for 5 to 10
minutes, the 2008 ISA recognized that "it is possible that these epidemiologic associations are
determined in large part by peak exposures within a 24-h period" (2008 ISA, p. 5-5). In
considering the epidemiological studies in this light, however, we note that given the important
role of SO2 as a precursor to PM in ambient air, a key uncertainty in the epidemiological
evidence available in the last review was potential confounding by PM (second draft ISA, p. 5-
5). Among the U.S. epidemiologic studies (some conducted in multiple locations) reporting
mostly positive and sometimes statistically significant associations between ambient SO2
concentrations and emergency department visits and hospital admissions, few studies have
attempted to address this uncertainty, e.g., through the use of copollutant models. For example,
as in the last review, there are three U.S. studies for which the SO2 effect estimate remained
positive and statistically significant in copollutant models with PM (Appendix D).31 No
additional such studies have been newly identified in this review. Further, the second draft ISA
states that uncertainty with regard to potential confounding by PM remains in the currently
available epidemiologic evidence base (second draft ISA, p. 5-144).
3.2.1.4 Uncertainties in the Health Effects Evidence
A number of key uncertainties and limitations were identified in the previous review with
respect to the health effects evidence, as described in the 2009 REA. This section considers the
currently available information, including that newly available in this review, with regard to such
areas of uncertainty.
31 Based on data available for specific time periods at some monitors in the areas of these studies, the 99th percentile
1-hour daily maximum concentrations were estimated in the last review to be between 78-150 ppb (Thompson
and Stewart, 2009).
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• To what extent have important uncertainties identified in the last review been
reduced and/or have new uncertainties emerged?
We have not identified any new uncertainties since the last review. However, we
continue to recognize important uncertainties that also existed in the last review. This array of
important areas of uncertainty related to the current health evidence, including that newly
available in this review, is summarized below.
Although the evidence clearly demonstrates that short-term SO2 exposures cause
respiratory effects, particularly asthma exacerbation in exercising individuals with asthma, as in
the previous review, we continue to recognize uncertainties that remain in several aspects of our
understanding of these effects. Such uncertainties include those associated with severity and
prevalence of responses to very short (5- to 10-minute) SO2 exposures below 200 ppb and
responses of some population groups not included in the controlled exposure studies (e.g., those
with more severe asthma and children). There are also uncertainties concerning the potential
influence of exposure history and co-exposure to other pollutants on the relationship between
short-term SO2 exposures and respiratory effects. With regard to the evidence base, we also
recognize a complication associated with interpreting the epidemiologic evidence related to
uncertainty in the exposure estimates. The following discussion touches on each of these types of
uncertainty.
With regard to the potential for and magnitude of these effects in at-risk populations
exposed to 5- to 10-minute concentrations below 200 ppb, there is very limited evidence from
small mouthpiece studies of exposure concentrations as low as 100 ppb, as discussed in section
3.2.1.3 above. These studies indicate the likelihood of an appreciable reduction in SCh-induced
response in exercising people with asthma from that observed from exposures at 200 ppb. Given
the limited size of these studies and their differences from free breathing chamber studies,
however, uncertainties remain with regard to a complete characterization of the extent of
response in exercising individuals with asthma exposed through natural or free breathing to
exposure concentrations below 200 ppb. The extent to which the epidemiological evidence,
including that newly available, can inform this area of uncertainty also may be limited.32
Accordingly, this remains an area of uncertainty in this review.
Some uncertainty also remains with regard to the extent to which the controlled human
exposure study evidence describes the responses of the populations most at risk of SCh-related
32 As associations reported in the epidemiologic analyses are associated with air quality concentration metrics as
surrogates for the actual pattern of exposures experienced by study population individuals over the period of a
particular study, the studies are limited in what they can convey regarding the specific patterns of exposure
circumstances (e.g., magnitude of concentrations over specific durations and frequency) that might be eliciting
reported health outcomes.
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respiratory effects (e.g., those with the most severe responses, or greatest likelihood of response).
For example, the available studies have generally involved subjects with mild or moderate
asthma, such that the response of individuals with more severe asthma is unknown.33 Further,
while it is well documented that some individuals have a greater response to SO2 than others
with the same disease status, the factors contributing to this greater susceptibility are not yet
known (second draft ISA, pp. 5-14 to 5-20).
Uncertainty also remains related to the responses for children with asthma. Although the
epidemiological evidence includes a number of studies focused on health outcomes in children
that are supportive of the qualitative conclusions of causality (second draft ISA, section 5.2.1.2),
there are few controlled human exposure studies to inform our understanding of concentrations
associated with effects. Those studies have not included subjects younger than 12 years (second
draft ISA, p. 5-21). Some characteristics particular to school age children younger than 12 years,
such as increased propensity for mouth breathing (second draft ISA, section 4.1.2.2), however,
suggest that this age group of children with asthma might be expected to experience larger lung
function decrements than adults with asthma (second draft ISA, p. 5-24).
Other areas of uncertainty concerning the potential influence of SO2 exposure history and
co-exposure to other pollutants on the relationship between short-term SO2 exposures and
respiratory effects also remain from the last review. There is some limited evidence regarding the
potential for an increased response to SO2 exposures occurring in the presence of other common
pollutants (e.g., PM, nitrogen dioxide and ozone), although the studies are limited (e.g., with
regard to their relevance to ambient exposures) and/or provide inconsistent results (second draft
ISA, p. 5-24; 2008 ISA, section 3.1.4.7). There is also some evidence suggestive of a potential
for SO2 exposure to contribute to an increased sensitivity to allergens, however the studies are
very few and are limited to experimental animal models (second draft ISA, section 5.2.1.9).
There are additional complications associated with interpretation of epidemiologic
studies of SO2 in ambient air that pertain to exposure measurement error and copollutant
confounding (second draft ISA, sections 3.4, 5.2.1.1 and 5.2.1.2). With regard to the former, a
key uncertainty in the epidemiologic evidence is whether study findings reflect an independent
association for SO2 given that the studies assigned exposure from fixed site monitors while SO2
concentrations in ambient air tend to show high spatiotemporal variability within a city, and
33 The second draft ISA identifies two studies that have investigated the influence of asthma severity on
responsiveness to SO2, with one finding that a larger change in lung function observed in the moderate/severe
asthma group was attributable to the exercise component of the study protocol while the other did not assess the
role of exercise in differences across individuals with asthma of differing severity (Linn et al., 1987; Trenga et al.,
1999). Based on the criteria used in the study by Linn et al (1987) for placing individuals in the
"moderate/severe" group, the ISA concluded that the asthma of these individuals "would likely be classified as
moderate by today's classification standards" (second draft ISA, p. 5-20; Johns et al., 2010; Reddel, 2009).
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correlations with personal exposure are poorly characterized. Accordingly, there is uncertainty
regarding the extent to which measurements at the study monitors, and the associated air quality
concentration metric for the study, adequately represent the spatiotemporal variability in ambient
SO2 concentrations in the study area (second draft ISA, sections 5.2.1.2 and 3.4.1.3).
Further, not only is SO2 but one component of a complex mixture of pollutants present in
the ambient air, an issue not unique to SO2 epidemiological studies, but SO2 is also a precursor to
sulfate, which can be a principal component of PM, an air pollutant commonly occurring across
the U.S. This uncertainty affects the extent to which effect estimates from epidemiologic studies
reflect the independent contribution of SO2 to the adverse respiratory outcomes assessed in these
studies. This area of uncertainty was recognized in the last review and remains in the current
review. In first summarizing the epidemiological evidence from the last review, the second draft
ISA indicated that it was strongest for increased respiratory symptoms and respiratory-related
hospital admissions and ED visits, especially in children, while noting that "a key uncertainty
was potential confounding by copollutants, particularly PM" (second draft ISA, p. 5-5). With
regard to the newly available evidence, the second draft ISA states that "[t]he caution expressed
in the 2008 SOx ISA (U.S. EPA, 2008d) related to the limitation of attributing an independent
effect to SO2 (due to the relationship of SO2 levels to PM levels) is still a concern" (second draft
ISA, p. 5-144).34
There remains uncertainty in the evidence with regard to the potential role of long-term
exposure to SO2 in eliciting SCh-related respiratory effects. As noted in section 3.2.1.1 above,
the ISA has determined the evidence to be suggestive of this being a causal relationship. The
strongest evidence supporting this conclusion is provided by epidemiological study findings of
associations between long-term SO2 concentrations and increases in asthma incidence combined
with findings of laboratory animal studies involving newborn rodents that indicate a potential for
SO2 exposure to contribute to the development of asthma, especially allergic asthma, in children.
However, uncertainties that relate to the limitations of the animal toxicological evidence,
particularly for long-term exposure, and the potential for confounding by other pollutants is
unexamined, and largely unavailable, for epidemiologic studies of asthma among children
(second draft ISA, section 5.2.2.7).
Another area of uncertainty recognized by the ISA, is that contributing to conclusions
regarding the potential for SO2 in ambient air to contribute to health effects other than respiratory
34 A few recent epidemiologic studies add evidence for SO2 in copollutant models with PM, NO2, or O3, although
the pollutants are measured at central site monitors (second draft ISA, p. 5-8). Across the full epidemiologic
evidence base, some associations were relatively unchanged in magnitude after adjustment for a copollutant,
while others did not persist. However, the second draft concludes that "inference from copollutant models is
limited given potential differences in exposure measurement error for S02 compared to N02, CO, PM, and 03 and
in many cases, high copollutant correlations" (second draft ISA, p. 5-139).
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effects. As noted in section 3.2.1.1 above, the ISA has determined the evidence to be suggestive
of, but insufficient to infer, a causal relationship between short-term SO2 exposure and mortality
and to be inadequate to infer the presence or absence of a causal relationship for other types of
exposures and health effects for which there are studies available.
In summary, a variety of uncertainties from the last review remain, including those
related to the extent of effects at concentrations below those evaluated in controlled human
exposure studies of exercising individuals with asthma, and the potential for greater impacts in
individuals with more severe asthma and in children with asthma (second draft ISA, section
5.2.1.9).
3.2.1.5 Public Health Implications
Implications and the magnitude of potential impacts on public health are dependent upon
the type and severity of the effect, as well as the size of population affected. With regard to SO2
concentrations in ambient air, the public health implications and potential public health impacts
relate to the effects causally related to SO2 exposures of interest in this review. These are
respiratory effects of short-term exposures, and particularly those effects associated with asthma
exacerbation in people with asthma. As summarized in section 3.2.1.1, the most strongly
demonstrated effects are bronchoconstriction-related effects resulting in decrements in lung
function elicited by short term exposures during periods of elevated ventilation, while asthma-
related health outcomes such as ED visits and hospital admissions have also been statistically
associated with ambient air SO2 concentration metrics in epidemiological studies (second draft
ISA, section 5.2.1.9).
In considering public health implications, in addition to the difference in severity of
different effects, it is important to consider aspects of the same effect with regard to its impact on
population groups of differing susceptibility. For example, with regard to bronchoconstriction-
related effects, the same percentage increase in sRaw or reduction in FEVi for two groups of
individuals that differ in their baseline sRaw or FEVi may result in the two groups being affected
differently with regard to increased susceptibility to other physiological threats or challenges.
Accordingly, consideration of such baseline differences and also the relative transience or
persistence of such sRaw or FEVi changes, as well as other factors, is important to
characterizing implications for public health, as recognized by the American Thoracic Society in
their statements on evaluating adverse health effects of air pollution (ATS, 2000; Thurston et al.,
2017).
The most recent policy statement by the ATS on what constitutes an adverse health effect
of air pollution provides a general framework for interpreting evidence that proposes a "set of
considerations that can be applied in forming judgments" for this context (Thurston et al., 2017).
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The earlier ATS statement, in addition to emphasizing clinically relevant effects, also
emphasized both the need to consider changes in "the risk profile of the exposed population,"
and effects on the portion of the population that may have a diminished reserve that puts its
members at potentially increased risk if affected by another agent (ATS, 2000). These concepts,
including the consideration of the magnitude of effects occurring in just a subset of study
subjects, continue to be recognized as important in the more recent ATS statement (Thurston et
al., 2017) and continue to be relevant to the evidence base for SO2.35
As summarized in section 3.2.1.3 above, people with asthma are the key population at
risk for SCh-related effects and children with asthma are considered to be at relatively greater
risk than other age groups within this at-risk population (second draft ISA, section 6.3.1). In
recognizing that asthma as a disease can vary in its severity, we take note of the lack of evidence
for individuals with the most severe asthma. The evidence base of controlled exposure studies of
exercising people with asthma provides limited information that indicates there to be similar
relative responses of individuals with differences in severity of their asthma,36 although the
evidence indicates that the absolute changes in lung function are larger for individuals with more
severe asthma compared to those characterized as having mild asthma. It is uncertain whether a
greater response to the exercise itself (vs the SO2 exposure) played a role in such findings,
however the available studies "suggest that adults with moderate/severe asthma may have more
limited reserve to deal with an insult compared with individuals with mild asthma" (second draft
ISA, p. 5-20; Linn et al., 1987; Trenga et al., 1999).
The information below characterizes the size and other features of the populations in the
U.S. concluded to be at risk of SCh-related effects, when under elevated ventilation conditions.
As a whole, the discussion in this section indicates the potential for exposures to SO2 in ambient
air to be of appreciable 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 protect the at-risk populations from short term exposures of concern. Such
considerations remain relevant in the current review.
35	In the Administrator's judgments on the then-existing standard in the last review, as well as on the appropriate
level for the new 1-hour standard, the Administrator considered the 2000 ATS policy statement, as well as advice
from CAS AC and recommendations and judgments made by EPA in previous NAAQS reviews (section 3.1.1
above).
36	These studies categorized with regard to asthma severity based mainly on the individual's use of medication to
control asthma, such that individuals not regularly using medication were classified as minimal/mild, and those
regularly using medication as moderate/severe (Linn et al., 1987). The second draft ISA indicates that the
moderate/severe grouping would likely be classified as moderate by today's asthma classification standards due
to the level to which their asthma was controlled and ability to engage in moderate to heavy levels of exercise
(second draft ISA, p. 5-20 to 5-21; Johns et al., 2010; Reddel, 2009).
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• What does the information available in this review indicate with regard to the size of
at-risk populations and their distribution in the U.S.?
The magnitude and characterization of a public health impact is dependent upon the size
and characterization of the populations affected, as well as the type or severity of the effect. As
summarized above, the population group most at risk of health effects associated with exposure
to SO2 in ambient air is people with asthma. The National Center for Health Statistics data for
2015 indicate that approximately 8.0% of the U.S. population has asthma (Table 3-2; CDC,
2017). These data indicate the size of the key at-risk population for SO2 in ambient air. It is this
population that the primary NAAQS for SO2 is intended to protect. Table 3-2 below considers
the currently available information that helps to characterize key features of this population.
The age group for which the prevalence documented by these data is greatest is children
aged five to 19, with 10.2% of children aged 15 to 19 years having asthma. In 2012 (the most
recent year for which such an evaluation is available), it was the leading chronic illness affecting
children (Bloom et al., 2013). The prevalence is greater for boys than girls. Among populations
of different races or ethnicities, black non-Hispanic children have the highest prevalence, at
13.4%. Asthma prevalence is also increased among populations in poverty (e.g., 11.1% among
people living in households below the poverty level compared to 7.2% of those living above it).
Population groups with relatively greater asthma prevalence might be expected to have a
relatively greater potential for SO2 impacts.
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Table 3-2.
2015 National Asthma Prevalence.

Number with Current
Percent with Current
unaracierisnc1
Asthma (in thousands)2
Asthma
Total
24,633
7.8
Child (Age <18)
6,188
8.4
Adult (Age 18+)
18,445
7.6
All Age Groups
0-4 years
935
4.7
5-14 years
4,033
9.8
15-19 years
2,107
10.2
20-24 years
1,655
7.6
25-34 years
2,916
6.8
35-64 years
9,907
8.0
65+ years
3,079
6.6
Child Age Group
0-4 years
935
4.7
5-11 years
2,761
9.6
12-17 years
2,492
10.3
Sex
Males
9,998
6.5
Boys (Age <18)
3,705
9.9
Men (Age 18+)
6,293
5.4
Females
14,634
9.1
Girls (Age <18)
2,483
6.9
Women (Age 18+)
12,151
9.7
Race/Ethnicity
White NH3
15,244
7.8
Child (Age <18)
2,810
7.4
Adult (Age 18+)
12,435
7.9
Black NH
3,931
10.3
Child (Age <18)
1,336
13.4
Adult (Age 18+)
2,595
9.1
Other NH
1,793
6.9
Child (Age <18)
605
8.4
Adult (Age 18+)
1,188
6.3
Hispanic
3,665
6.6
Child (Age <18)
1,438
8.0
Adult (Age 18+)
2,227
5.9
Federal Poverty Threshold
Below 100% of poverty level
5,086
11.1
100% to less than 250% of poverty level
7,664
8.4
250% to less than 450% of poverty level
4,989
6.3
450% of poverty level or higher
6,894
6.9
1	Numbers within selected characteristics may not sum to total due to rounding
2	Includes persons who answered "yes" to the questions "Have you EVER been told by a doctor or other health
professional that you had asthma" and "Do you still have asthma?"
3NH = non-Hispanic
Adapted from https://www.cdc.gov/asthma/most_recent_data.htm	
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3.2.2 Exposure/Risk-based Considerations
Our consideration of the scientific evidence available in the current review, as at the time
of the last review (summarized in section 3.1 above), is informed by results from a quantitative
analysis of estimated population exposure and associated risk. The overarching consideration is
whether the current exposure/risk information alters our overall conclusions from the previous
review regarding health risk associated with exposure to SO2 in ambient air. As in our
consideration of the evidence in section 3.2.1 above, we have organized the discussion regarding
the exposure/risk information around a set of key questions to assist us in considering the
exposure/risk analyses of at-risk populations living in three urban areas under air quality
conditions simulated to just meet the existing SO2 primary standard.
Prior to addressing the individual exposure/risk questions, we provide a summary of key
aspects of the assessment, including the study areas, populations simulated, modeling tools and
exposure and risk metrics derived (section 3.2.2.1). We then consider aspects of the questions
beginning with the magnitude of exposure and risk estimated for the simulated at-risk
populations (section 3.2.2.2), followed by the key uncertainties associated with the quantitative
analyses with regard to drawing conclusions as to the adequacy of protection afforded by the
current SO2 standard (section 3.2.2.3). Lastly, we consider the exposure and risk estimates from
the quantitative assessment with regard to the extent to which such estimates may be judged to
be important from a public health perspective (section 3.2.2.4).
3.2.2.1 Exposure/risk analyses
In the assessment conducted for this review, described in detail in the draft REA, we have
estimated SO2 exposure and risk associated with air quality conditions that just meet the current
standard. These analyses inform our understanding of the protection provided by the current SO2
standard from effects that the health effects evidence indicates to be elicited in some portion of
exercising people with asthma by short (e.g., 5to 10 minutes) elevations in SO2 exposure
concentrations. 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 with regard to population
exposure to short-term peak concentrations of SO2 in ambient air. This set of study areas and the
associated exposed populations are intended to be informative to the EPA's consideration of
potential exposures and risks that may be associated with the air quality conditions that meet the
current SO2 standard. As discussed further in sections 3.2.2.2 and 3.2.2.4 below, the Fall River
study area is found to present particularly informative exposure circumstances given that it
provides an example of an area in the U.S. in which there is substantial overlap between
locations that are relatively more populated and where SO2 concentrations are relatively higher
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(draft REA, section 5.4). As such, this area represents places in the U.S. with the potential for
exposures of greatest concern, making it important in considering the protection provided by the
current standard.
The three study areas range in total population size from approximately 180,000 to
540,000 and reflect different mixtures of SO2 emissions sources, including utilities using fossil
fuel and non-utility sources, such as petroleum refineries and secondary lead smelting (draft
REA, section 3.1). They include locations in New England, Ohio River Valley and the Midwest,
the latter two regions comprising the part of the U.S. with generally the greatest prevalence of
elevated SO2 concentrations and large emissions sources (Figure 2-7, Appendix F). Additionally,
continuous 5-minute ambient air monitoring data (i.e., all 12 5-minute values for each hour) are
available in two of the three study areas, with hourly maximum 5-minute concentration data
available in the third (draft REA, section 3.2).
Asthma prevalence estimates for the populations simulated in the three study areas ranges
from 8.0 to 8.7% (draft REA, section 5.1). For children, the study area prevalence rates range
from 9.7 to 11.2% (draft REA, section 5.1). Variation within each studyrelated to age, sex and
whether family income is above or below the poverty level was also accounted for (section 4.1.2
and Appendix E of draft REA,).37 This variation is greatest in the Fall River study area, with
census block level, age-specific prevalence estimates ranging from 7.9 to 18.6% for girls and
from 10.7 to 21.5% for boys (draft REA, Table 4-1).
In the draft REA, 1-hour SO2 concentrations were estimated across a 3-year period
(consistent with the period represented by the form of the standard) using air quality modeling of
SO2 emissions sources in each area, and were adjusted, as described in the draft REA, such that
the air quality modeling receptor location with the highest concentrations just met the current
standard.38 In addition, sensitivity analyses were performed using an alternative adjustment
approach and are summarized in section 3.2.2.2. 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 receptor locations in each
area (draft REA, section 3.5).
The exposure modeling, presented in detail in the draft REA, relied on the EPA's Air
Pollutant Exposure model (APEX), which estimates human exposure using a stochastic, event-
based microenvironmental approach. This model has a history of application, evaluation, and
37	As described in section 4.1.2 and Appendix E of the draft 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.
38	As described in more detail in section 3.4 of the draft REA, the adjustments were implemented with a focus on
reducing emissions from the source contributing to the standard exceedances until the areas just met the standard.
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progressive model development in estimating human exposure and dose for reviews of NAAQS
for gaseous pollutants (U.S. EPA, 2008; 2010; 2014). 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;
U.S. EPA, 2017b, section 3.4). For example, exposure modeling for the draft REA includes
reliance on updates to several key inputs to the model including (1) a significantly expanded
Consolidated Human Activity Database (CHAD), that now has over 55,000 diaries, with over
25,000 for school-aged children; (2) the updated NHANES data (2009-2014), which are the basis
for the age- and sex-specific body mass distributions from which APEX samples to specify the
individuals in the modeled population; (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 rate; and (4) the ventilation rate algorithm itself. Further, the current model uses
updated population demographic data based on the most recent Census.
The APEX model probabilistically generates a sample of hypothetical individuals 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 National Health Interview Survey.39 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.
Based on minute-by-minute activity levels, and physiological characteristics of the
simulated person (see draft REA, section 4.1), 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 (draft REA, section 4.1.4.4). The level specified is based on
the ventilation rates of subjects in the controlled human exposure studies of exercising people
with asthma (Table 3-1). The APEX simulations performed for this review have focused on
39 Information about the National Health Interview Survey is available at http://www.cdc.gov/nchs/nhis.htm.
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exposures to SO2 emitted into ambient air that occurs in microenvironments,40 without additional
contribution from indoor SO2 emissions sources.41
As in the last review, the draft REA for this review uses the APEX model estimates of 5-
minute exposure concentrations for simulated individuals with asthma at elevated ventilation to
characterize health risk in two ways based on information from the controlled human exposure
studies on the occurrence of bronchoconstriction-related effects in some study subjects with
asthma who are exposed during exercise (draft REA, section 4.5). In drawing on this evidence
base for this purpose, the draft REA has given primary focus to the well-documented studies
summarized in Table 5-2 and Figure 5-1 of the second draft ISA for 5- to 10-minute exposure
concentrations ranging from 200 ppb to 600 ppb (Table 3-1 of this document). The first risk
metric is based on comparison of the estimated 5-minute exposure concentrations individuals at
elevated ventilation to 5-minute concentrations of potential concern (benchmark concentrations),
and the second utilizes exposure-response information for study subjects experiencing
bronchoconstriction-related effects on lung function (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 SCh-related increase in sRaw of at least 100%. Both of these metrics are used in the draft
REA to characterize health risk associated with 5-minute peak SO2 exposures among the
simulated at-risk population during periods of elevated ventilation. These risk metrics were also
derived in the REA for the last review and the associated estimates informed the Administrator's
2010 decision on the new standard (75 FR 35546-35547, June 22, 2010).
For the benchmarks metric, the draft REA for this review, like the 2009 REA in the last
review, uses benchmark concentrations that range from 400 ppb down to 100 ppb (draft 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. The 200 ppb
benchmark concentration represents the lowest level tested in studies where subjects were freely
breathing in exposure chambers (moderate or greater lung function decrements in some of these
40	Five microenvironments (MEs) are modeled in the draft REA as representative of a larger number of
microenvironments. The 2009 REA results indicated that the majority of peak SO2 exposures occurred while
individuals were within outdoor microenvironments (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), the approach
implemented in the draft REA recognizes the added efficiency of minimizing the number of MEs, particularly
indoor MEs, that are parameterized and included in the modeling. Accordingly, the number of MEs was
aggregated to address exposures of ambient origin that occur within a core group of indoor, outdoor, and vehicle
MEs (draft REA, section 4.2).
41	Indoor sources are generally minor in comparison to S02 from ambient air (draft REA, sections 2.1.1 and 2.1.2).
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subjects) (75 FR 35527, June 22, 2010). The lowest benchmark concentration (100 ppb), which
is one half the lowest exposure concentration tested in free breathing exposure studies, has been
included in consideration of the nonzero percentage of subjects with asthma experiencing
moderate transient decrements in lung function at the 200 ppb exposure concentration
(approximately 8 to 9%) and the lack of specific study data for some groups of individuals with
asthma, such as primary-school-age children and those with more severe asthma.42
The exposure-response (E-R) function for the 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 as low as 200 ppb (draft REA, Table 4-9). 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 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 these studies have been summarized in the second draft ISA, as in
the last review, in terms of percent of study subjects experiencing responses of a magnitude
equal to a doubling or tripling or more. 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) (draft REA, Table 4-9). The E-R function used in the main analysis of the draft REA was
derived from these data using a probit function (draft REA, section 4.5.2).
In summary, while the general approach and methodology for the exposure-based
assessment in this review is similar to that in the last review, there are a number of ways in
which these analyses differ (see 2009 REA and draft REA for this review). In addition to the
expansion in the number and type of study areas assessed, we note the number of improvements
to input data and modeling approaches, including the availability of continuous 5-minute air
42 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
(second draft ISA, pp. 5-20 to 5-21; 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 lung function decrements at lower SO2 exposure concentrations than individuals with moderate asthma.
With regard to the severity of the response, the limited data that are available indicate a similar magnitude SO2-
specific response (in sRaw) as that for individuals with less severe asthma, although the individuals with more
severe asthma are indicated to have a greater response to exercise prior to SO2 exposure, indicating that those
individuals "may have more limited reserve to deal with an insult compared with individuals with mild asthma"
(second draft ISA, p. 5-21). As noted in sections 3.2.1.3 and 3.2.1.4 above, evidence from controlled human
exposure studies are not available for children younger than 12 years old, and the second draft ISA indicates that
the information regarding behavior and methacholine responsiveness for the subset of this age group that is of
school age (e.g., 5-12 years) indicates a potential for greater response (second draft ISA, pp. 5-21 to 5-24).
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monitoring data at monitors within two of the three study areas. The current draft REA 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.
3.2.2.2 At-risk population exposures and risk
In this section, we summarize the exposure and risk estimates from the draft REA and
consider the following question.
• What is the magnitude of population exposure and risk in at-risk populations in
areas simulated to just meet the current SO2 standard? What portion of the at-risk
populations are estimated to experience exposures of concern or lung function
decrements at levels of potential health concern?
In addressing these questions, we consider the population estimates provided by the draft
REA simulations of exposure to SO2 emitted into ambient air (draft REA, Chapters 5 and 6). In
considering these REA estimates for air quality conditions just meeting the current standard, we
particularly focus on the extent of protection provided by the standard from SO2 exposures of
potential concern. As described in the prior section, the draft REA presents two sets of 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 at elevated ventilation; and (2) the number and percent of people estimated to experience
at least one SCh-related lung function decrement in a year and the number and percent of people
experiencing multiple lung function decrements associated with SO2 exposures.
In presenting the exposure and risk estimates, the draft REA recognizes that the approach
applied to adjust air quality to conditions just meeting the current standard can have important
impacts on the risk and exposures estimates (draft REA, section 6.2.2). Because of this, the draft
REA presents results for 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 (draft REA, section 3.4). This is done by
estimating the amount of SO2 concentration reduction needed for this highest receptor to be
adjusted to the current SO2 standard, and based on this amount, all other receptors impacted by
the highest source(s) are adjusted accordingly. The second approach is included as a sensitivity
analysis that recognizes the potential uncertainty associated with the modeled concentrations,
particularly the very highest modeled 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 (draft REA, section 6.2.2.1). In study areas in
which modeled concentrations at a very small number of receptors are substantially higher than
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those at all other air quality receptors, these two different approaches can result in very different
SO2 concentrations across an area. In such study areas, in particular, the first approach generally
results in much more significant reductions being applied to reduce SO2 concentrations at the
small group of highest modeled receptors such that concentrations at those receptors are just at or
just below the standard and concentrations at the other receptors across the area are appreciably
lower. Given that these two approaches to adjusting air quality can result in important
differences in the magnitude of risk to at-risk populations in areas simulated to just meet the
current standard, the tables below present estimates based on both approaches.43
Of the two types of risk metrics derived in the draft REA, we turn first to the results for
the benchmark-based risk metric with regard to the percent of the study area populations with
asthma estimated to experience at least one daily maximum 5-minute exposure per year at or
above the different benchmark concentrations while at elevated ventilation (Table 3-3). Under air
quality conditions just meeting the current standard across the three study areas, approximately
20 to 25% of 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 ventilation rates (Table 3-3). With regard to the 200 ppb benchmark, as
many as 0.7 percent of the simulated population of children with asthma, on average across the
3-year period, was estimated to experience a single day with a 5-minute exposure at or above
200 ppb while breathing at elevated ventilation rates (Table 3-3). The percentage in a single year
ranged up to 2.2% for a single day, while less than 0.1% of children with asthma were estimated
to experience more than a single day with an exposure at or above 200 ppb while at elevated
ventilation (draft REA, Tables 6-5 and 6-6). No simulated children with asthma were estimated
to experience a day with a 5-minute exposure at or above 300 or 400 ppb. The estimates for
adults are lower, generally due to the lesser amount and frequency of time spent outdoors (draft
REA, section 5.2).
43 Details regarding these sensitivity analyses focused on the impact of the adjustment approach are presented in the
draft REA, section 6.2.2.1.
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Table 3-3. 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 at elevated ventilation.
5-minute
Exposure
Concentration
(ppb)
Percent (%) of Population of Children (5-18 years) with Asthma
Average per yearA
Fall River, MA
Indianapolis, IN
Tulsa, OK
>100
19.4-26.7
O
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CD
o
V
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>200
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>300
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AThe values presented in each cell are the average of the results for the three years simulated based on the two
approaches to air quality adjustment (drawn from Table 6-5 of the draft REA).
B <0.1 is used to represent nonzero estimates below 0.1%. A value of zero (0) indicates there were no individuals
having the selected exposure in any year.
We next consider the estimates for risk of lung function decrements in terms of a
doubling or more in sRaw (Table 3-4). Under conditions just meeting the current standard in the
three study areas, as many as 1.1% of children with asthma, on average across the 3-year period,
were estimated to experience at least one day per year with a SCh-related increase in sRaw of
100% or more in the study area with the highest estimates (Table 3-4, Fall River). The
corresponding percent estimated to experience two or more such days ranged as high as 0.6%, on
average across the 3-year simulation period (draft REA, Table 6-8). Additionally, in the same
study area, as much as 0.2% of the simulated populations of children with asthma, on average
across the 3-year period, was estimated to experience a single day with a SCh-related increase in
sRaw of 200% or more. The estimates for adults are very slightly lower, again, generally due to
the lesser time spent outdoors (draft REA, section 5.3).
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Table 3-4. 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 SC>2-related increase in sRaw of 100% or more.
Lung function
decrement
(increase in sRaw)
Percent (%) of Population of Children (5-18 years) with Asthma*
Average per year
Fall River, MA
Indianapolis, IN
Tulsa, OK
>100%
0.9-1.1
0
<0.1B - <0.1
>200%
CM
O
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0
AThe values presented in each cell are the average of the results for the three years simulated based on two
approaches to air quality adjustment (drawn from Table 6-7 of the draft 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.
In understanding these results, we note that the three study areas selected provide a
variety of circumstances with regard to population exposure to short-term peak concentrations of
SO2 in ambient air. These three study areas reflect different combinations of different types of
SO2 emissions sources, including utilities using fossil fuels and non-utility sources, and provide
three different patterns of exposure to SO2 concentrations in a populated area in the U.S. In this
way, the three areas provide a variety of examples of exposure patterns that can be informative to
the EPA's consideration of potential exposures and risks that may be associated with air quality
conditions occurring under the current SO2 standard. As indicated by the discussion above and
also recognized in section 3.2.2.3 below, there is variability in the estimated magnitude of
exposure and associated risk across study areas and uncertainties associated with these estimates.
In developing the air quality scenarios for the current standard in the three study areas,
the draft REA recognizes that these scenarios of adjusted air quality provide representations of
the pattern of air quality that might occur in each study area under conditions that just meet the
current standard. Where such conditions include relatively large spatial extents of higher
concentrations - i.e., areas with design values in proximity to the level of the standard - that
overlap with the more populated parts of the study area, exposure and risk results are relatively
higher (draft REA, section 5.4). Among the three study areas, this best describes the Fall River
study area, which is an area where source characteristics contribute to a sizeable spread of
source-influenced relatively higher concentrations that coincide or overlap with locations where
people reside and/or frequent. This association between concentrations and population in Fall
River is illustrated in Figure 5-4 of the draft REA. Inclusion of an area with these characteristics
in the REA provides some insight into the potential exposure and risk associated with other areas
across the U.S. with similar characteristics and is therefore particularly informative to evaluation
of the level of protection provided by the standard.
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The other two study areas (Indianapolis and Tulsa) provide examples of areas where the
higher SO2 concentrations that result from the sizeable SO2 sources in the study area do not
strongly coincide with parts of the area in which people reside and/or frequent (draft REA,
section 5.4). This relationship between SO2 concentrations and population is these two areas is
illustrated in Figures 5-5 and 5-6 of the draft REA. Accordingly, the corresponding exposure and
risk estimates for these area are lower than those estimated for the Fall River study area, even
though the populations are larger (draft REA, sections 5.1 and 5.4).
As discussed above, among the three study areas, the Fall River study area presents the
exposure circumstances associated with highest SCh-related exposures and risk for the current
standard air quality scenario. Because of this, we recognize that the Fall River study area is of
particular importance in considering the adequacy of the protection afforded by the current
standard. Recognizing this, we note that the draft REA indicates that the percent of children with
asthma that might be expected to experience 5-minute SO2 concentrations at or above the 200
ppb benchmark concentration, in an urban area that just meets the current standard, may be as
high as 0.7%, on average across the three years, and 2.2% in a single-year period. With regard to
the lung function risk, the draft REA indicates the percent of children that might be expected to
experience at least a doubling of specific airway resistance, under conditions just meeting the
current standard, may be as high as 1.1%, on average across the three-year period, and 1.9% in a
single year. Thus, these results indicate that, in the single year with the highest concentrations
across the 3-year period, nearly 98% of the population of children with asthma in the Fall River
study area, would not be expected to experience a day with a 5-minute exposure at or above the
200 ppb and 400 ppb benchmarks and would not be expected to experience as much as a
doubling in sRaw, a magnitude roughly consistent with the level of protection that was described
in establishing the now-current standard in 2010 (as summarized in section 3.1.1.2.4 above)44
On average across the 3-year period, the corresponding percentage is nearly 99% (Tables 3-3 and
3-4, above).
44 Although the 2009 REA did not include an air quality scenario representing the now-current standard, among the
scenarios it did include were single-year air quality scenarios representing standard levels of 100 and 50 ppb. For
the single-year scenario representing a standard level of 100 ppb in the study area with the highest population
exposure and risk, the 2009 REA estimated 2.7% of children with asthma to experience at least one day with
exposure at or above 200 ppb, while at elevated ventilation (2.1-2.9% to experience one or more S02-attributable
increases in sRaw of at least 100%); this estimate was 0.09% for the scenario representing a standard level of 50
ppb (0.4-0.9% to experience one or more SCh-attributable increases in sRaw of at least 100%) (2009 REA, Table
9-8 and Appendix B). While we recognize a number of differences between the 2009 REA and the quantitative
modeling and analyses performed in the current draft REA, we note that the single year estimates for the Fall
River study area in the current draft REA fall between the estimates for the two most similar air quality scenarios
assessed in the last review.
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3.2.2.3 Uncertainties
In this section, we consider the uncertainties associated with the quantitative estimates of
exposure and risk, including those recognized by the characterization of uncertainty in the draft
REA (draft REA, section 6.2). The characterization in the draft REA is based on an approach
intended to identify and compare the relative impact that important sources of uncertainty may
have on the exposure and risk estimates. The approach used has been applied in REAs for past
NAAQS reviews for ozone, nitrogen oxides, carbon monoxide (U.S. EPA, 2008; 2010; 2014)
and SOx (U.S. EPA, 2009). In the characterization of uncertainty for the current analysis, the
draft REA utilized a qualitative uncertainty characterization approach adapted from the WHO
approach for characterizing uncertainty in exposure assessment (WHO, 2008) accompanied by
quantitative sensitivity analyses of key aspects of the assessment approach. This characterization
and analyses are described in detail in chapter 6 of the draft REA. The approach used in the draft
REA varies from that of WHO (2008) in that the draft 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).
The characterization and analyses in the draft REA involve consideration of the various
types of inputs and approaches that together result in the exposure and risk estimates for the three
study areas. In so doing, the draft REA considers the limitations and uncertainties underlying
these 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 draft REA
also evaluated the direction of influence, indicating how the source of uncertainty was judged to
affect the exposure/risk estimates (e.g., likely to over- or under-estimation).
• What are the key uncertainties associated with the exposure and risk estimates,
including those of particular significance with regard to drawing conclusions as to
the adequacy of the protection afforded by the current SO2 standard?
Based on the uncertainty characterization and associated analyses in the draft REA and
consideration of associated policy implications, we recognize several areas of uncertainty as
particular important in our consideration of the exposure and risk estimates, as was also the case
in the last review. Generally, these areas include estimation of the spatial distribution of SO2
concentrations across each study area under air quality conditions just meeting the existing
standard, including the fine-scale temporal pattern of 5-minute concentrations. We additionally
recognize the uncertainty with regard to population groups and exposure concentrations for
which the health effects evidence base is limited or lacking.
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With regard to the spatial distribution of SO2 concentrations, the draft REA recognizes
some uncertainty associated with the approach used to adjust the air quality surface to
concentrations just meeting the current standard. Accordingly, the draft REA has investigated the
potential quantitative impact of this uncertainty on the exposure and risk estimates by deriving
estimates based on an alternative adjustment approach (described in section 6.2.2.1 of the draft
REA). Given the results of this sensitivity analysis, we have considered estimates from both
approaches in summarizing the draft REA estimates in section 3.2.2.2 above. Additionally, we
recognize uncertainty in the estimates of 5-minute concentrations in ambient air across the
modeling receptors in each study area. While the ambient air monitoring dataset available to
inform these estimates is much expanded in this review over the dataset available in the last
review, we are still drawing on relationships occurring at one location and over one range of
concentrations to estimate the fine-scale temporal pattern in concentrations at other locations.
This is an important area of uncertainty in the draft REA results because the ambient air 5-
minute concentrations are integral to the 5-minute estimates of exposure. While we recognize
this as an important area of uncertainty, the approach used has taken into account the currently
available information and is considered to provide a reasonable representation of fine-scale
temporal variability in the three study areas.
An additional area of uncertainty affecting our interpretation of the exposure and risk
results for the set of study areas assessed concerns our understanding of, and the prevalence
across the U.S. of, the different exposure circumstances they represent. As noted in section
3.2.2.2, the circumstances particularly pertinent to consideration of the adequacy of protection of
the current standard include those in which areas where populations reside and/or exercise
overlap with concentrations of SO2 that are near, albeit just under, the level of the standard. Such
circumstances are influenced by source characteristics and meteorological conditions, as well as
housing and recreational area patterns in urban areas. While there is some uncertainty in our
understanding of the prevalence of the exposure circumstances represented by the three study
areas, including those for the Fall River study area, the available information indicates there to
be many densely-populated areas in the U.S. in which there are facilities with sizeable SO2
emissions (e.g., Appendix F) 45
We also recognize an important area of uncertainty that is particular to our interpretation
of the lung function risk estimates. This area concerns estimates of lung function risk derived for
exposure concentrations below those represented in the evidence base. The exposure-response
45 Although source characteristics and meteorological conditions - in addition to magnitude of emissions - influence
the distribution of concentrations in ambient air, Appendix F focuses on the distribution of 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 (second draft ISA, section 2.5.2.2).
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function on which the primary risk estimates are based generates non-zero predictions of a
percent of the at-risk population exposure expected to experience a day with at least a doubling
of sRaw for all exposures experienced while at elevated ventilation. In considering these
estimates, we recognize that the uncertainty in the response estimates increases substantially with
decreasing exposure concentration below those supported by study data. In so doing, we note the
contribution to the risk estimates of exposure concentrations below 200 ppb. Additionally, we
note that the assessment focuses on the daily maximum 5-minute exposure during elevated
ventilation, summarizing results in terms of the days on which the magnitude of such exposure
exceeds a benchmark or contributes to increased sRaw. While the health effects evidence
indicates the lack of a cumulative effect of multiple exposures over several hours or a day
(second draft ISA, section 5.2.1.2), and a reduced response to repeated exercising exposure
events over an hour (second draft ISA, section 5.2.1.2; Kehrl et al., 1987), 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 event.
Another area of uncertainty, which remains from the last review and is important to our
consideration of the draft 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 sections 3.2.1.1 and 3.2.1.4, the controlled human exposure study evidence
base does not include studies of children younger than 12 years old or studies of people with
more severe asthma.46 The limited evidence that informs our understanding of potential risk to
these groups indicates the potential for them to experience greater effects than other population
groups with asthma under similar exposure circumstances, as summarized in section 3.2.1.4
above. Further we note the lack of information on the factors contributing to increased
susceptibility to SCh-induced bronchoconstriction among some people with asthma. Thus, there
is uncertainty associated with our interpretation of the exposure/risk estimates with regard to the
extent to which they represent the populations at greatest risk of SCh-related respiratory effects
that is important to consideration of the exposure and risk results with regard to the adequacy of
protection provided by the current standard.
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, we recognize several here as 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,
46 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 draft REA.
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concentrations of SO2 and limited information regarding multiple exposure episodes within a
day; the prevalence of different exposure circumstances represented by the three study areas;
and, characterization of particular subgroups of people with asthma that may be at greater risk.
3.2.2.4 Potential public health implications
In considering public health implications of the quantitative exposure and risk estimates
that may inform the Administrator's judgments in this area, this section discusses the information
pertaining to the following question.
• To what extent are the estimates of exposures and risks to at-risk populations that
remain under conditions just meeting the current SO2 standard important from a
public health perspective?
Several factors are important to consideration of public health implications. These
include the magnitude or severity of the effects associated with the exposures estimated in the
draft REA, as well as their adversity at the individual and population scale. Other important
considerations include the size of the population estimated to experience such effects or to
experience exposures associated with such effects. These considerations are discussed below.
Based on the currently available evidence which is largely consistent with that available
in the last review (as summarized in section 3.2.1 above), the quantitative exposure and risk
analyses focus on the potential for lung function decrements in people with asthma exposed to
SO2 while at elevated ventilation. Additionally, we have again focused on estimates for two
types of risk metrics, one involving comparison to benchmark concentrations and the second
involving estimates of lung function risk with regard to moderate or greater increases in sRaw. In
considering these estimates, we recognize that although the lung function decrements, which are
related to bronchoconstriction, are expected to be transient, we additionally recognize that such
decrements, while occurring, may contribute to a diminished reserve in lung function (second
draft ISA, p. 1-17, section 5.2.1.2). For population groups already at diminished reserve, such as
those with more severe asthma, this may be particularly important. Thus, the discussion here
reflects consideration of the health evidence, and exposure and risk estimates, as well as the
consideration of potential public health implications in previous NAAQS decisions and ATS
policy statements (as also discussed in section 3.2.1.5).
In light of the conclusion that among all people with asthma, children may be particularly
at risk (summarized in section 3.2.1.2 above) and the draft REA findings of higher exposures and
risks for children (in terms of percent of that population), we have focused the discussion here on
children. We recognize that the draft REA results for air quality conditions just meeting the
current standards indicate that, on average across the 3-year period simulated (consistent with the
form of the current standard), less than 1% of the simulated population of children with asthma
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might be expected to experience a single day with a 5-minute exposure at or above 200 ppb
while breathing at elevated ventilation rates. The draft REA simulations also estimated no
children with asthma to experience any days in a 3-year simulation period with a 5-minute
exposure at or above 300 or 400 ppb. With regard to the lowest benchmark considered (100 ppb),
the draft REA also indicates that in some areas of the U.S., approximately 25% of children with
asthma, on average across the 3-year period, might be expected to experience one or more days
per year with a 5-minute exposure at or above 100 ppb while breathing at elevated ventilation
rates, with higher percentages in some years. With regard to estimates of lung function
decrements, the draft REA indicates that in some areas, approximately 1% of children with
asthma, on average across a 3-year period, might be expected to experience at least one day per
year with a SCh-related increase in sRaw of 100% or more; the estimate for two or more days is
appreciably lower, at 0.4% (draft REA, Table 6-8). Additionally, under such conditions (just
meeting the current standard), the estimated percent of children with asthma that might be
expected to experience a single day per year with a SCh-related increase in sRaw of 200% or
more, on average across the 3-year period, is 0.2% (Table 3-4).
In considering the severity of responses associated with the REA estimates, we take note
of the health effects evidence for the different benchmark concentrations and judgments made
with regard to the severity of these effects in the last review. As in the last review, we recognize
that the responses documented for exposures of 400 ppb are frequently accompanied by
respiratory symptoms and thus are appropriately considered to be adverse respiratory effects
consistent with past and recent ATS position statements. With regard to the lower benchmark
concentration of 200 ppb, we recognize that, while the responses documented in studies of
exercising subjects with asthma are not consistently accompanied by respiratory symptoms,
conclusions in past NAAQS reviews recognized that moderate decrements in lung function can
be clinically significant in some individuals with asthma (75 FR 35526, June 22, 2010).
Accordingly, the Administrator in the last review considered effects associated with exposures as
low as 200 ppb to be adverse in light of CASAC advice,47 ATS statements and conclusions in
past NAAQS reviews. While noting the lack of information for some populations groups with
asthma, including primary-school-age children and people with more severe asthma, we
additionally recognize the uncertainty with regard to effects that might be associated with
exposures as low as 100 ppb (as discussed in section 3.2.1.3 and 3.2.1.4 above).
As indicated in section 3.1 above, the at-risk population in this review is people with
asthma, as was the case in the last review. Further, children with asthma are identified as
47 The CASAC letter on the first draft S02 REA to the Administrator stated: "CASAC believes strongly that the
weight of clinical and epidemiology evidence indicates there are detectable clinically relevant health effects in
sensitive subpopulations down to a level at least as low as 0.2 ppm SO2" (Henderson, 2008).
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particularly at risk. The size of the at-risk population in the U.S. is substantial. As summarized in
section 3.2.1.5, nearly eight percent of the U.S. population (more than 24 million people) and
8.4% of U.S. children have asthma. The prevalence in U.S. child populations of different races or
ethnicities ranges from 7.4% to 13.4% (Table 3-2 above). This is well reflected in the draft REA
study areas in which the asthma prevalence ranged from 8% to 8.7% of the total populations and
9.7%) to 11.2% of the children, with the highest prevalence represented in the Fall River study
area. In Fall River, the prevalence varies among census tracts, with the highest tract having a
prevalence in boys of 21.5% (draft REA, Table 4-1).
In considering the three study areas and the variation in exposure/risk estimates among
them, we recognize the Fall River study area to be particularly informative to consideration of
public health risk associated with and public health protection provided by the current standard.
This is because, as summarized in section 2.2.2.2 above, the source characteristics and
population distribution in the area cause the locations of relatively higher SO2 concentrations in
ambient air across the area (i.e., those closest to just meeting the standard) to overlap with
locations of higher population density. These exposure circumstances contribute to higher
exposure and risk estimates than in the other study areas (draft REA, section 5.4), making
estimates for Fall River important in considering the adequacy of protection provided by the
current standard. Thus, we have given particular attention to estimates for this study area.
Although exposure and risk estimates were not available in the last review for air quality
conditions just meeting the now-current standard, the findings and considerations summarized
here are generally similar to those considered in the last review, and indicate a level of protection
consistent with that described in the 2010 decision. The exposure and risk estimates for the three
study areas assessed in the draft REA for this review reflect differences in exposure
circumstances among those areas and illustrate the exposures and risk that might be expected to
occur in other areas with such circumstances under air quality conditions that just meet the
current standard. Thus, the draft REA estimates indicate the magnitude of exposure and risk that
might be expected in some areas and illustrate the importance to consideration of the public
health protection afforded by the current standard of those areas where locations of relatively
higher SO2 concentrations in ambient air across the area coincide with the locations of higher
population density. These considerations, and others raised above, are important to conclusions
regarding the public health significance of the draft REA results. We recognize that such
conclusions also depend in part on public health policy judgments that will weigh in the
Administrator's decision in this review with regard to the adequacy of protection afforded by the
current standard. Such judgments include those concerning the public health significance of
effects at exposures for which evidence is limited or lacking, such as effects at the lower
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benchmark concentrations considered and lung function risk estimates associated with exposure
concentrations lower than those tested in the controlled exposure studies.
3.2.3 Preliminary Staff Conclusions on the Current Standard
This section describes preliminary staff conclusions regarding the adequacy of the current
primary SO2 standard. These preliminary conclusions are based on considerations described
above, and in the discussion below regarding the currently available scientific evidence (as
summarized in the second draft ISA, and the ISA and AQCDs from prior reviews), and the risk
and exposure information drawn from the draft REA. Conclusions in the final PA will draw upon
the final ISA, developed in consideration of CASAC review and public comment on the second
draft ISA, and on the final REA, developed in consideration of CASAC review and public
comment on the draft REA. Further, staff conclusions presented in the final PA will take into
account advice from the CASAC and public comment on the draft PA and on these preliminary
conclusions.
Taking into consideration the discussions responding to specific questions above in this
and the prior chapter, this section addresses the following overarching policy question.
• Does the currently available scientific evidence- and exposure/risk-based
information, as reflected in the ISA and REA, support or call into question the
adequacy of the protection afforded by the current SO2 standard?
In considering this question, we recognize as an initial matter that, as is the case in
NAAQS reviews in general, the extent to which the current primary SO2 standard is judged to be
adequate will depend on a variety of factors inclusive of science policy judgments and public
health policy judgments. 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. The factors relevant to judging the adequacy of the standards also
include the interpretation of, and decisions as to the weight to place on, different aspects of the
results of the exposure assessment for the three areas studied and the associated uncertainties.
Thus, we recognize that the Administrator's conclusions regarding the adequacy of the current
standard will depend in part on public health policy judgments, science policy judgments
regarding aspects of the evidence and exposure/risk estimates, as well as, judgments about the
level of public health protection with an adequate margin of safety that is requisite under the
Clean Air Act.
Our response to this question takes into consideration the discussions that address the
specific policy-relevant questions in prior sections of this document (see sections 3.2.1-3.2.2)
and the approach described in section 3.1 that builds on the approach from the last review. We
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focus first on consideration of the evidence, including that newly available in this review, and the
extent to which it alters key conclusions supporting the current standard. We then turn to
consideration of the quantitative exposure and risk estimates drawn from the draft REA,
including associated limitations and uncertainties, and the extent to which they indicate differing
conclusions regarding the magnitude of risk, as well as level of protection from adverse effects,
associated with the current standard. We additionally consider the key aspects of the evidence
and exposure/risk estimates emphasized in establishing the now-current standard, and the
associated public health policy judgments and judgments about the uncertainties inherent in the
scientific evidence and quantitative analyses that are integral to decisions on the adequacy of the
current primary SO2 standard.
In considering the currently available evidence, staff gives great weight to the long-
standing body of health effects evidence for SO2, augmented in some aspects since the last
review, that provides the foundation of our understanding of the health effects of SO2 in ambient
air. In so doing, we give particular attention to the evidence from controlled human exposure
studies that demonstrates that very short exposures to less than 1000 ppb SO2, while breathing at
elevated ventilation rates, induces bronchoconstriction in some people with asthma; and,
supports the identification of people with asthma as the population at risk from short-term peak
concentrations in ambient air (second draft ISA; 2008 ISA; 1994 AQCD supplement).
Further, while the evidence base has been augmented since the time of the last review, we
note that 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. 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. This strong evidence base continues to
demonstrate a causal relationship between short-term SO2 exposures and respiratory effects,
particularly in people with asthma. This conclusion is primarily based on evidence from
controlled human exposure studies 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 elevated ventilation. Support is provided by the epidemiological
evidence that is coherent with the controlled exposure studies. The epidemiological evidence,
including that recently available, includes studies reporting positive associations for asthma-
related hospital admissions and emergency department visits (of individuals of all ages,
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including adults and children) with short-term SO2 exposures (second draft ISA, section
5.2.1.2).48
The health effects evidence newly available in this review also does not extend our
understanding of the range of 5-minute exposure concentrations eliciting effects in people with
asthma exposed while breathing at elevated ventilation rates beyond what was understood in the
last review. As in the last review, 200 ppb remains the lowest concentration tested in exposure
studies where study subjects are freely breathing in exposure chambers. At that exposure
concentration, approximately eight to nine percent of study subjects with asthma, breathing at
elevated ventilation rates, experienced moderate or greater lung function decrements following
5- to 10-minute controlled exposures. The limited information available for lower exposure
concentrations, while not amenable to direct quantitative comparisons, generally indicates
somewhat lesser response. In considering what may be gleaned from the epidemiological
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 (second
draft ISA, section 5.2.1.9).
In this review, as in the last review, we recognize some uncertainty with regard to
exposure levels eliciting effects in some population groups not studied, such as individuals with
severe asthma, as well as uncertainty in the extent of effects at exposure levels below those
studied. 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 a 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 are critically important to judgments on the adequacy of public health protection
provided by the current standard. In considering the public health implications of estimated
occurrences of exposures of different magnitudes, we take note of guidance from the ATS, the
CASAC's written advice and recommendations in past reviews, and judgments made by the EPA
in considering similar effects in previous NAAQS reviews (75 FR 35526 and 35536, June 22,
48 While uncertainties remain related to the potential for confounding by PM or other co-pollutants and the
representation of fine-scale temporal variation in personal exposures, the findings of the epidemiological evidence
continue to provide support for the conclusion on the causal relationship (second draft ISA, section 5.2.1.2).
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2010). As recognized in section 3.2.1.5, an additional publication by the ATS that further
addresses judgments on what constitutes an adverse health effect of air pollution is newly
available in this review (Thurston et al., 2017). The more recent statement expands upon the
2000 statement, that was considered in the last SO2 NAAQS review, and recognizes additional
considerations with regard to such judgments that remain consistent with the EPA's judgments in
the 2010 review. 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, and especially
at or above 400 ppb (often accompanied by respiratory symptoms and for which the evidence is
stronger), 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 (75 FR 35547, June 22, 2010).
In considering the draft REA analyses available in this review, we are aware of a number
of ways in which these analyses differ from those available in the last review. In addition to the
expansion in the number and type of study areas assessed, we note the number of improvements
to input data and modeling approaches, including the availability of continuous 5-minute air
monitoring data at monitors within two of the three study areas. The current draft REA extends
the time period of simulation by including 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 considering the draft REA
results, we also take note of the array of emissions and exposure circumstances represented by
the three study areas. As summarized in section 3.2.2 above, the areas fall into three different
geographic regions of the U.S. They range in total population size from approximately 180,000
to approximately one half million, and vary in population 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.
As at the time of the last review, people with asthma are the population at risk of S02-
related respiratory effects. Children with asthma may be particularly at risk (section 3.2.1.2
above). While there are more adults in the U.S. with asthma than children with asthma, the draft
REA results in terms of percent of the simulated at-risk populations, indicates higher exposures
and risks for children with asthma as compared to adults. This finding relates to children's
greater frequency and duration of occasions outdoors (section 3.2.2.2 above). In light of these
conclusions and findings, we have focused our consideration of the draft REA results here on
children.
As can be seen by the variation in exposure estimates, the three study areas in the draft
REA represent an array of exposure circumstances, including those contributing to relatively
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higher and relatively lower exposures and associated risk. As recognized in the draft 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 that may
differ in ways affecting population exposures of interest. In that way, the draft 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 decision on the adequacy of protection provided by the
current standard. As discussed in sections 3.2.2.2 and 3.2.2.4 above, consideration of exposures
occurring in those areas where locations of relatively higher SO2 concentrations in ambient air
across an area that just meets the current standard coincide with the locations of higher
population density are particularly important to consideration of the public health protection
afforded by the current standard.
With regard to the draft REA representation of air quality conditions associated with just
meeting the current standard, while we note reduced uncertainty in a few aspects of the approach
for developing this air quality scenario, we recognize the uncertainty associated with the
application of adjustments to the highest model receptor in the study area. As summarized in
sections 3.2.2.2 and 3.2.2.3 above, sensitivity analyses described in section 6.2.2 of the draft
REA indicate the quantitative impact potentially associated with area of uncertainty. Given the
importance of this aspect of the REA to consideration of the level of protection provided by the
current standard, we have considered the results for each study area in terms of a range bounded
on the low end by the results for the main analysis and on the upper end by those based on the
alternative adjustment approach used in the sensitivity analysis. In this context, we note 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 0.9 to
1.1%; the highest estimate is just under 2% for the highest single year. Less than 1% of children
with asthma are estimated to experience, while at elevated ventilation, a daily maximum 5-
minute exposure per year at or above 200 ppb, on average across the 3-year period, with a
maximum of approximately 2% in the highest single year. Further, no child (or adult) with
asthma is estimated to experience, while at elevated ventilation, a daily maximum 5-minute
exposure per year at or above 400 ppb (in any of the three years simulated across the three study
areas). Thus, in light of current ATS guidance, as well as conclusions and CASAC advice in
prior NAAQS reviews, the draft REA exposure and risk estimates for the current review indicate
that the current standard is likely to provide effective protection from SCh-related health effects
to at-risk populations of children and adults with asthma.
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In summarizing the information discussed thus far, we reflect on the key aspects of the
2010 decision that established the current standard. As an initial matter, effects associated with
5- to 10-minute exposures as low as 200 ppb of people with asthma while breathing at elevated
ventilation were considered to be adverse; this judgment was based on consideration of CASAC
advice and EPA decisions in prior NAAQS reviews, as well as ATS guidance. We note that the
newly available information in this review includes an additional statement from ATS on
adversity which is generally consistent with the earlier statement (available at the time of the
2010 decision). While recognizing the differences between the current and past analyses,
including the lack of an air quality scenario specific to the now-current standard in the last
review, as well as uncertainties associated with such analyses, we note a rough consistency of the
associated estimates when considering the array of study areas in both reviews. Overall, the
newly available quantitative analyses appear to comport with the conclusions reached in the last
review regarding control expected to be exerted by the now-current 1-hour standard on 5-minute
exposures of concern. 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 to indicate that a one-hour
standard of 75 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. Single-year results for study areas assessed in the current draft REA
indicate protection of approximately 98 to more than 99% of the populations of children with
asthma from experiencing exposures at or above a 200 ppb benchmark concentration and of all
of the study area at-risk populations from exposures at or above 400 ppb. Additionally, the 2010
decision also took note of the magnitude of the ambient air SO2 concentrations in U.S.
epidemiological studies of associations between ambient air concentrations and emergency
department visits and hospital admissions, for which the effect estimate remained positive and
statistically significant in copollutant models with PM. In considering these studies, the
Administrator judged that the level chosen for the new 1-hour standard provided an adequate
margin of safety. No additional such studies are available in the current review. Thus, in
considering the key aspects of the decision in the last review, we find the currently available
information to be consistent with that on which the decision establishing the current standard was
based.
Based on all of the above, and taking into consideration related information, limitations
and uncertainties, such as those recognized above, we draw preliminary conclusions regarding
the extent to which the newly available information in this review supports or calls into question
the adequacy of protection afforded by the current standard. In considering the conclusions that
may be supported by the exposure and risk estimates, we take note of the more than 24 million
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people with asthma in the U.S., including more than 6 million children. We additionally note 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 so doing, we recognize
that the controlled human exposure studies, on which the depth of our understanding of SO2-
related health effects is based, do not provide information with regard to responses in people
with more severe asthma or in children younger than 12 years. Additionally, some aspects of our
understanding continue to be limited; among these aspects are 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. In light of this we note the draft REA results for the
lowest benchmark that indicate that in some areas of the U.S. with air quality conditions that just
meet the current standard, appproximately 20 to 25% of children with asthma may experience
one or more exposures, on average across a 3-year period, to concentrations at or above 100 ppb
while at elevated ventilation. Thus, the evidence and exposure/risk information related to the
lowest exposures studied lead us to conclude that the combined consideration of the body of
evidence and the quantitative exposure estimates continue to provide support for a standard as
protective as the current one.
Further, we recognize that conclusions regarding the adequacy of the current standard
depend in part on public health policy judgments identified above and judgments about the level
of public health protection with an adequate margin of safety. In so doing, we take note of the
long-standing health effects evidence that documents the effects of SO2 exposures as short as a
few minutes on people with asthma that are exposed while breathing at elevated ventilation rates
and recognize that such effects have been documented in the lowest concentration studied in
exposure chambers with appropriate clean-air controls (200 ppb). In so doing, we recognize the
limitations, and associated uncertainty, in the evidence available for lower exposure
concentrations (e.g., 100 ppb), as was the case in the last review, and we note the lower
responses reported. Thus, in focusing on the potential for 5-minute exposures at and above 200
ppb, that have been previously recognized as adverse (June 22, 2010; 75 FR 35547), we take
note of the draft REA results that indicate the current standard may be expected to protect
approximately 98% to nearly 99% of at-risk populations with asthma from experiencing any
days with such exposures, in a single- and 3-year period, respectively. We additionally note the
draft REA finding of no children (or adults) estimated to experience any days with a 5-minute
exposure of 300 ppb or higher. In light of ATS guidance, CASAC advice and EPA conclusions
in past NAAQS reviews, these results indicate effective protection of at-risk populations from
S02-related health effects that we note is consistent with the level of protection specified when
the standard was set. Thus, we reach the preliminary conclusion that the currently available
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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.
In summary, 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. Further, we observe the general consistency of the current evidence with
the evidence that was available in the last review with regard to key aspects on which the current
standard is based. We additionally note the quantitative exposure and risk estimates for
conditions just meeting the current standard that indicate a similar level of protection, for at-risk
populations from respiratory effects considered to be adverse, as that described in the last review
for the now-current standard. We also recognize, as in the last review, the limitations and
uncertainties associated with the available information. Collectively, these considerations
(including those discussed above) provide the basis for the preliminary staff conclusion that
consideration should be given to retaining the current standard, without revision. Accordingly,
and in light of this preliminary staff conclusion that it is appropriate to consider the current
standard to be adequate, we have not identified any potential alternative standards for
consideration in this review.
3.3 KEY UNCERTAINTIES AND AREAS FOR FUTURE RESEARCH
AND DATA COLLECTION
In this section, we highlight key uncertainties associated with reviewing and establishing
the primary NAAQS for sulfur oxides. Such key uncertainties and areas for future research,
model development, and data gathering are outlined below. In some cases, research in these
areas can go beyond aiding standard setting to aiding in the development of more efficient and
effective control strategies. We note, however, that a full set of research recommendations to
meet standards implementation and strategy development needs is beyond the scope of this
discussion. Rather, listed below are key uncertainties, research questions and data gaps that have
been thus far highlighted in this review of the primary standard.
• A critical aspect of our consideration of the evidence and the quantitative dose estimates
is our understanding of SO2 effects below the lowest concentrations studied in controlled
human exposure studies. Additional information related in several areas would reduce
uncertainty in our interpretation of the available information for purposes of risk
characterization. These areas include the following.
- Our understanding of whether and to what extent some population groups,
including children or people with severe asthma, are more responsive to peak SO2
exposures (or responsive to lower concentrations), while breathing at elevated
ventilation rates, than the groups that have been studied.
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-	A better understanding of the effects and the shape of the exposure-response
relationship at lower 5-minute exposure concentrations (i.e., below 200 ppb)
would help to reduced uncertainty in our estimates of lung function effects and,
accordingly, in characterizing S02-related health effects.
-	Little information is available on the factors contributing to the susceptibility to
lower concentrations of SO2 of a subgroup of people with asthma, termed
"responders" in the second draft ISA (second draft ISA, section 5.2.1.2; Johns and
Linn, 2011).
-	There is also only very limited evidence regarding the potential influence of
history of exposure and co-occurring exposure to other air pollutants, including
particulate matter. Further research is needed in this area to better inform our
characterization of health risk related to SO2.
•	An understanding of the fine-scale spatial and temporal gradients of ambient air SO2
concentrations in residential areas, as well as near sources of SO2 emissions, is a key
element in our assessment of exposure and risk. Additional information in this area is
needed. Current limitations in this area additionally contribute to uncertainty in
characterization of ambient air SO2 levels in the risk assessment and the resulting
exposure and risk estimates. Further characterization of the fine-scale spatial and
temporal variation in ambient air SO2 concentrations in different environments and
related to different sources, as well as different air quality conditions that just meet the
existing standard, would help to reduce this uncertainty.
•	Uncertainties with regard to other aspects of the health effects evidence include that
regarding what can be gleaned from the epidemiologic studies showing an association
between short-term SO2 exposures and asthma-related hospital admission and emergency
department visits. Uncertainty remains regarding the extent of copollutant confounding in
these studies, particularly by PM. Additionally, there is uncertainty related to the
representation of exposure through fixed site monitors and capturing peak SO2
concentrations that limits the informativeness of the ambient air concentrations analyzed
in the studies to standards reviews.
•	While the CHAD is much expanded over the last review, limited information and
associated uncertainty remain in several aspects of the available data. Additional
information would reduce uncertainty in these aspects of our exposure and risk estimates.
-	Collection and analysis of multiday activity patterns that consider the attributes
most influential to determining long-term activity patterns. Further research
would assist in better evaluating and improving existing approaches used to
generate longitudinal activity profiles (as discussed in the draft REA, section
4.1.5.1).
-	Activity data for some population subgroups, such as people with severe asthma
and very young children.
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National Ambient Air Quality Standards (First Draft, July 2008). EPA-CASAC-08-019.
Available at: https://www3.epa.eov/ttn/naaqs/standards/so2/s so2 cr.html
Horstman, D; Roger, LJ; Kehrl, H; Hazucha, M. (1986). Airway sensitivity of asthmatics to
sulfur dioxide. Toxicol Ind Health 2: 289-298.
Horstman, DH; Seal, E, Jr; Folinsbee, LJ; Ives, P; Roger, LJ. (1988). The relationship between
exposure duration and sulfur dioxide-induced bronchoconstriction in asthmatic subjects.
AIHA J 49: 38-47. http://dx.doi.org/10.1080/15298668891379341
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Johns, D. (2009). Presentation and analysis of controlled human exposure data described in
Table 3-1 of the 2008 Integrated Science Assessment (ISA) for Sulfur Oxides.
Memorandum to Sulfur Oxides NAAQS Review Docket. April 29, 2009. Available in
Regulations.gov, EPA-HQ-OAR-2007-0352-0043.
Johns, D; Simmons, K. (2009). Quality assurance review of individual subject data presented in
Table 3-1 of the 2008 Integrated Science Assessment (ISA) for Sulfur Oxides.
Memorandum to Sulfur Oxides NAAQS Review Docket. October 2, 2009. Available in
Regulations.gov, EPA-HQ-ORD-2006-0260-0036.
Johns, DO; Svendsgaard, D; Linn, WS. (2010). Analysis of the concentration-respiratory
response among asthmatics following controlled short-term exposures to sulfur dioxide.
Inhal Toxicol 22: 1184-1193.
Kehrl, HR; Roger, LJ; Hazucha, MJ; Horstman, DH. (1987). Differing response of asthmatics to
sulfur dioxide exposure with continuous and intermittent exercise. Am J Respir Crit Care
Med 135: 350-355.
Koenig, JQ; DS Covert; Pierson, WE. (1989). Effects of inhalation of acidic compounds on
pulmonary function in allergic adolescent subjects. Env Health Persp 79: 173-178.
Koenig, JQ; Covert, DS; Hanley, QS; Van Belle, G; Pierson, WE. (1990). Prior exposure to
ozone potentiates subsequent response to sulfur dioxide in adolescent asthmatic subjects.
Am J Respir Crit Care Med 141: 377-380.
Linn, WS; Shamoo, DA; Spier, CE; Valencia, LM; Anzar, UT; Venet, TG; Hackney, JD.
(1983a). Respiratory effects of 0.75 ppm sulfur dioxide in exercising asthmatics:
Influence of upper-respiratory defenses. Environ Res 30: 340-348.
Linn, WS; Venet, TG; Shamoo, DA; Valencia, LM; Anzar, UT; Spier, CE; Hackney, JD.
(1983b). Respiratory effects of sulfur dioxide in heavily exercising asthmatics: A dose-
response study. Am Rev Respir Dis 127: 278-283.
Linn, WS; Avol, EL; Shamoo, DA; Venet, TG; Anderson, KR; Whynot, JD; Hackney, JD.
(1984). Asthmatics' responses to 6-hr sulfur dioxide exposures on two successive days.
Arch Environ Health 39: 313-319. http://dx.doi.org/10.1080/00039896.1S 45856
Linn, WS; Avol, EL; Peng, RC; Shamoo, DA; Hackney, JD. (1987). Replicated dose-response
study of sulfur dioxide effects in normal, atopic, and asthmatic volunteers. Am Rev
Respir Dis 136: 1127-1134. http://dx.doi.Org/10.1164/ajrccm/136.5.1127
Linn, WS; Avol, EL; Shamoo, DA; Peng, RC; Spier, CE; Smith, MN; Hackney, JD. (1988).
Effect of metaproterenol sulfate on mild asthmatics' response to sulfur dioxide exposure
and exercise. Arch Environ Occup Health 43: 399-406.
http://dx.doi.org/10.1080/00039896.1988.9935858
Linn, WS; Shamoo, DA; Peng, RC; Clark, KW; Avol, EL; Hackney, JD. (1990). Responses to
sulfur dioxide and exercise by medication-dependent asthmatics: Effect of varying
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medication levels. Arch Environ Occup Health 45: 24-30.
http://dx.doi.org/10.1080/00039896.1990.9935920
Magnussen, H; Jorres, R; Wagner, HM; von Nieding, G. (1990). Relationship between the
airway response to inhaled sulfur dioxide, isocapnic hyperventilation, and histamine in
asthmatic subjects. Int Arch Occup Environ Health 62: 485-491.
http://dx.doi.org/10.1007/BF00381178
Nowak, D; Jorres, R; Berger, J; Claussen, M; Magnussen, H. (1997). Airway responsiveness to
sulfur dioxide in an adult population sample. Am J Respir Crit Care Med 156: 1151-
1156. http://dx.doi.ore/ 4/airccm. 156.4.9607025
Reddel, HK. (2009). Characterizing asthma phenotypes: Predictors and outcomes at the extremes
of asthma severity [Editorial], Respirology 14: 778-780.
Roger, LJ; Kehrl, HR; Hazucha, M; Horstman, DH. (1985). Bronchoconstriction in asthmatics
exposed to sulfur dioxide during repeated exercise. J Appl Physiol 59: 784-791.
Samet J. (2009). Clean Air Scientific Advisory Committee's (CASAC) Review of EPA's Risk
and Exposure Assessment (REA) to Support the Review of the SO2 Primary National
Ambient Air Quality Standards: Second Draft. EPA-CASAC-09-007. Available at:
https://www3. epa. gov/ttn/naaqs/ standards/ so2/s so2 cr.html
Sheppard, D; Saisho, A; Nadel, JA; Boushey, HA. (1981). Exercise increases sulfur dioxide-
induced bronchoconstriction in asthmatic subjects. Am Rev Respir Dis 123: 486-491.
Sheppard, D; Eschenbacher, WL; Boushey, HA; Bethel, RA. (1984). Magnitude of the
interaction between the bronchomotor effects of sulfur dioxide and those of dry (cold) air.
Am Rev Respir Dis 130: 52-55.
Smith, E. (1993). Subject Data Supplied by the Researchers for the Recent Controlled Human
Studies Analyzed in the Staff Paper Supplement and Accompanying Memorandum.
Memorandum to SO2 Doeket (A-84-25). August 11, 1993.Strickland, MJ; Darrow, LA;
Klein, M; Flanders, WD; Sarnat, JA; Waller, LA; Sarnat, SE; Mulholland, JA; Tolbert,
PE. (2010). Short-term associations between ambient air pollutants and pediatric asthma
emergency department visits. Am J Respir Crit Care Med 182: 307-316.
http://dx.doi.ore h* 1 1 I rccm.200908-1 JO 1<„H
Thurston, GD; Kipen, H; Annesi-Maesano, I; et al. (2017). A joint ERS/ATS policy statement:
what constitutes an adverse health effect of air pollution? An analytical framework. Eur
Respir J 2017; 49: 1600419 [https://doi.org/10.1183/13993003.00419-2016],
Trenga, CA; Koenig, JQ; Williams, PV. (1999). Sulphur dioxide sensitivity and plasma
antioxidants in adult subjects with asthma. Occup Environ Med 56: 544-547.
Trenga, CA; Koenig, JQ; Williams, PV. (2001). Dietary antioxidants and ozone-induced
bronchial hyperresponsiveness in adults with asthma. Arch Environ Occup Health 56:
242-249.
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U.S. EPA. (1994). Supplement to the Second Addendum (1986) to Air Quality Criteria for
Particulate Matter and Sulfur Oxides (1982). Research Triangle Park, NC: Office of
Health and Environmental Assessment, Environmental Criteria and Assessment Office.
EPA-600/FP-93/002.
U.S. EPA. (1996). Air Quality Criteria for Ozone and Related Photochemical Oxidants.
Washington, DC: Office of Research and Development. EPA/600/P-93/004aF
U.S. EPA. (2008). Integrated Science Assessment (ISA) for Sulfur Oxides - Health Criteria
(Final Report). EPA-600/R-08/047F. Available at:
http://cfpub.epa. gov/ncea/cfm/recordisplay.cfm?deid=l 98843
U.S. EPA. (2009). Risk and Exposure Assessment to Support the Review of the SO2 Primary
National Ambient Air Quality Standard. EPA-452/R-09-007, July 2009. Available at:
https://www3.epa.gov/ttn/naaqs/standards/so2/data/200908S02REAFinalReport.pdf
U.S. EPA. (2010). Quantitative Risk and Exposure Assessment for Carbon Monoxide -
Amended. Office of Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Research Triangle Park, NC, 27711. EPA-452/R-10-006. Available at:
https://www.epa.gov/naaas/carbon-rnonoxide-co-standards-risk-and-exposure-
as ses sm ents-current-re vi ew
U.S. EPA. (2014). Health Risk and Exposure Assessment for Ozone. Office of Air Quality
Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park,
NC, 27711. EPA-452/R-14-004a. Available at: https://www.epa.gov/naaqs/ozome-o3-
standards-risk-and-exposure-assessments-current-review
U.S. EPA. (2016). Integrated Science Assessment (ISA) for Sulfur Oxides - Health Criteria
(Second External Review Draft). EPA/600/R-16/351, December 2016. Available at:
https://cfpub.epa.gOv/ncea/i sa/recordisplav.cfrn?deid=326450
U.S. EPA. (2017a). Risk and Exposure Assessment for the Review of the Primary National
Ambient Air Quality Standard for Sulfur Oxides, External Review Draft. EPA-452/P-17-
002, August 2017. Available at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-
air-quality-standards
U.S. EPA. (2017b). Review of the Primary National Ambient Air Quality Standard for Sulfur
Oxides: Risk and Exposure Assessment Planning Document. EPA-452/P-17-001,
February 2017. Available at:
https://www3.epa.gov/ttn/naaqs/standards/so2/data/20170216so2rea.pdf
WHO. (2008). WHO/IPCS Harmonization Project Document No. 6. Part 1: Guidance Document
on Characterizing and Communicating Uncertainty in Exposure Assessment. Geneva,
World Health Organization, International Programme on Chemical Safety. Available at:
http://www.who.int/ipcs/methods/harmonization/areas/exposure/en/
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APPENDIX A
PREPARATION OF DATA FILES FOR GENERATION OF FIGURES IN CHAPTER 2
The raw data came from pre-generated AQS extract files. Files are located at
http://www.epa.gov/airqualitv/airdata/ad data.html. Documentation of files is located at
http://aqsdr 1.epa.gov/aqsweb/aqstmp/airdata/FileFormats.html. Hourly Data Files were used. A
separate Hourly Data File for each parameter and year combination was run. The type of SO2
data is determined by the parameter code and duration code and is coded as follows:
•	1-hour values data - parameter code = 42401 and duration code = 1
•	5-minute data (12 observations per hour) - parameter code = 42401 and duration code = H
•	5-minute data (hourly max) - parameter code = 42406 and duration code = 1
For the 1-hour data at a Site/POC to be used, it must have met the following
completeness criteria:
•	75% or more of the hourly observations in a day (18 or more) must be present.
•	75% or more of the days in a quarter must be present and complete:
•	1st Quarter - 68 observations or 69 observations in leap year
-	2nd Quarter - 69 observations
-	3rd Quarter - 69 observations
-	4th Quarter - 69 observations
•	4 quarters for each of at least 3 of the 5 years (2011-2015) must be present and complete.
For this analytical purpose, the three years do not have to be consecutive. This dataset
was prepared in June 2016.
After completeness criteria were applied, the following data screens were also performed
to account for some outliers in the 5-minute data:
•	Only 5 minute data with a corresponding hourly value in AQS (parameter 42401 and
duration code 1) were kept.
•	Only 5 minute values with an hourly mean value under 120% of the hourly value in AQS
(parameter 42401 and duration code 1) were kept.
•	Only hours where a 5-minute max hourly value (AQS parameter 42406 and duration code
1) was reported and fell between 1 and 12 times the AQS hourly value (parameter 42401
and duration code 1) were kept.
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1	APPENDIX B
2	ADDITIONAL INFORMATION ON DATASETS PRESENTED IN FIGURE 2-8
3
4	Table B-l. Summary statistics (in ppb) for distributions of daily maximum 5-minute
5	SO2 concentrations on days with differing daily maximum 1-hour S02
6	concentrations.
Daily Maximum 1-hour Concentration (ppb)
<=25 >25-50 >50-75	>75
N
315964
5486
1521
1435
25th percentile
1
46.6
95
170.1
Median
2.0
62
122.6
220
Mean
5
74
140
259
75th percentile
5.2
88
167.2
294.25
95th percentile
22.0
152.5
259
502.9
99th percentile
44
225.6
389
822.5
When the three data sets for sites with DVs at or below 75 ppb are combined, the
99th percentile is 58.3 ppb and the 99.9th percentile is 150 ppb.	
7
8
9	Table B-2. Summary statistics (in ppb) for distributions of daily maximum 5-minute
10 SO2 concentrations at sites with differing design values.
Bin

Design Value (ppb)

<=25
>25-50
>50-75
>75
N
225863
59011
18504
21028
25th percentile
0.8
2
2
2.3
Median
1.7
5
7
8
Mean
3
10
20
41
75th percentile
3.8
13
26
40
95th percentile
12.2
37
82.6
199.6
99th percentile
28.3
68
146.1
365
11
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Figure B-l. Monitoring data for sites meeting the current standard: Frequency of daily
maximum 5-minute values on days with differing daily maximum 1-hour concentrations
(2013-2015).
350
300
250
d 200
o
0	150
tm.
25-50	>50-75
Maximum Hourly Concentrations (ppb)
>75
#>100 ppb R# >200 ppb mi# >300 ppb «# >400 ppb
Figure B-2. Monitoring data for sites not meeting the current standard: Frequency of daily
maximum 5-minute values on days with differing daily maximum 1-hour concentrations
(2013-2015).
1400
1200
1000
o 800
o
O
o 600
Q>
E 400
200
0
<=25
>25-50	>50-75
Maximum Hourly Concentrations (ppb)
>75
#>100 ppb R# >200 ppb mi# >300 ppb «# >400 ppb
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APPENDIX C
OCCURRENCES OF 5-MINUTE S02 CONCENTRATIONS OF INTEREST
IN THE RECENT AMBIENT AIR MONITORING DATA (2013-2015)
Figure C-l. As is (unadjusted) SO2 monitoring data (2013-2015). Mean number of days/year
(top panel) and maximum number of days/year (bottom panel) with daily maximum 5-minute
concentrations of SO2 above 100 ppb.
As Is Air Quality (2013-2015): Mean Days Above 100 ppb
<
c
I
" 30











•











•



•






•





•
•
•


• .

•



••
•


' - . •
•
• 	 ~	
• •


25	50	75	100
Ambient Monitor 1-hr Design Value (2013-2015)
As Is Air Quality (2013-2015): Max Days Above 100 ppb
<
c
I





~
Observation: For DV < 75, are monitors in the dataset with
as manv as 70 davs w. a 5-min concentration >100 Dob.





•





•




•





•
•


•
•
•
•
•



•
• •
•



•

•
•



* •
• " it*
LiWj	
~* V %t •
• • • • •
• •
•


50	75	100
Ambient Monitor 1-hr Design Value (2013-2015)
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Figure C-2. As is (unadjusted) SO2 monitoring data (2013-2015). Mean number of days/year
(top panel) and maximum number of days/year (bottom panel) with daily maximum 5-minute
concentrations of SO2 above 200 ppb.
As Is Air Quality (2013-2015): Mean Days Above 200 ppb









































•









•
•
• • •

•
* *• • * • '
• •• ••
•
•

0	25	50	75	100	125	150
Ambient Monitor 1-hr Design Value (2013-2015)
As is Air Quality (2013-2015): Max Days Above 200 ppb
100
90
80
c
-Q
<
>-
a;
Q.


















UDservation: i-oruvwo
as many as
, are monitors in tne aataset witn
22 days with a 5-min concentration >200 ppb.





















«

•
•
•




•


•
• * *
• • «
r t.f. t
. .*
•

50	75	100
Ambient Monitor 1-hr Design Value (2013-2015)
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II
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Figure C-3. As is (unadjusted) SO2 monitoring data (2013-2015). Mean number of days/year
(top panel) and maximum number of days/year (bottom panel) with daily maximum 5-minute
concentrations of SO2 above 300 ppb.
As Is Air Quality (2013-2015): Mean Days Above 300 ppb
-Q
CL
Q- 90
> 80
o
-Q
<
C
c 70
60
50
>
£ 40
30























































•
»-~ Vl *H — t	4
•
*
	
• *
• •
50	75	100
Ambient Monitor 1-hr Design Value (2013-2015)
As Is Air Quality (2013-2015): Max Days Above 300 ppb






























Observation: For DV < 7b, are monitors in the dataset with
as many as 8 days w. a 5-imin concentration >300 ppb.
















•


m 11 n i
•
i i i
•
•
•
. M •
•
•
# •
•
0	25	50	75	100	125	150
Ambient Monitor 1-hr Design Value (2013-2015)
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Figure C-4. As is (unadjusted) SO2 monitoring data (2013-2015). Mean number of days/year
(top panel) and maximum number of days/year (bottom panel) with daily maximum 5-minute
concentrations of SO2 above 400 ppb.
100
-Q
a
Q,
o
?
as
g
J2
<
c
90
80
70
<5
i
D
60
50^
40
30
£
20
As Is Air Quality (2013-2015): Mean Days Above 400 ppb
50	75	100
Ambient Monitor 1-hr Design Value (2013-2015)
J2
CL
Q.
O
9
.£ 70
4A
1
2
-Q
e
60
Q 50
40
O 30
20
10
As Is Air Quality (2013-2015); Max Days Above 400 ppb
Observation: For DV < 75, are monitors in the dataset with
as many as 5 days w. a 5-min concentration >400 ppb.
25	SO	75	100
Ambient Monitor 1-hr Design Value (2013-20IS)
125
ISO
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1	Figure C-5. Monitoring data (2013-2015), unadjusted. Total number of days across 3-year
2	period with daily maximum 5-minute concentrations of SO2 above 100, 200, 300 and 400 ppb
3	across monitors grouped by design value.
3000
2500
(/>
o 2000
E
Q)
i—
i—
3
O
O 1500
0
25-50	DV >50-75
: # >100 ppb ff# >200 ppb mi #>300 ppb ¦# >400 ppb
DV >75

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22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Figure C-6. Monitoring data (2013-2015) adjusted to just meet the current standard (75
ppb as a 3-year average of annual 99th percentile 1-hour daily maximum concentrations).
Mean number of days/year (top panel) and maximum number of days/year (bottom panel) with
daily maximum 5-minute concentrations of SO2 above 100, 200, 300 and 400 ppb.
	100 ppb
	200 ppb
300 ppb
—400 ppb
100 ppb
200 ppb
—300 ppb
400 ppb
All Ambient Monitors Adjusted to Meet 1-hr 75 ppb std (2013-2015): Mean Exceedances
5	10	15	20
Mean Number of Days per Year Daily Max 5-min Above Benchmark
All Ambient Monitors Adjusted to Meet 1-hr 75 ppb std (2013-2015): Max Exceedances
0	10	20	30	40	50	60	70
Max Number of Days per Year Daily Max 5-min Above Benchmark
10% of monitors in dataset:
¦	average 1 to 5 days per year when the maximum
5-minute concentration is above 200 ppb.
¦	average 1 to 3 days per year when the maximum
5-minute concentration is above 300 ppb.
* average <1 to 2 days per year when maximum
5-minute concentration is above 400 ppb.
At 10% of monitors in the dataset:
¦	there are as many as 3-15 days per year when
the maximum 5-minute concentration is above 200 ppb.
¦	there are as many as 1 -6 days per year when
the maximum 5-minute concentration is above 300 ppb.
¦	there are as many as 1 -5 days per year when
the maximum 5-minute concentration is above 400 ppb.
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APPENDIX D
AIR QUALITY INFORMATION FOR GEOGRAPHICAL AREAS
OF THREE SELECTED U.S. EPIDEMIOLOGICAL STUDIES
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Table D-l. Air quality information for geographical areas of the three U.S. epidemiological studies for which the SO2 effect
estimates for hospital admissions or emergency department visits (for asthma or other respiratory disease) and
areawide 24-hour average SO2 concentrations remained positive and statistically significant in copollutant models with
particulate matter.
Study Information
Ambient Air Quality A
Study Area
Study Time
Period
Study
Reference
SO2
Concentration
Metric
Associated
with Health
Outcome
Assignment of
Monitors to
Study Subjects
for Study
Analyses
Study-reported SO2
Concentrations,B
24-hour average
(ppm)
99th
percentile of
daily
maximum
1-hour
concentratio
ns across
study period
at highest
monitor in
study
dataset (ppb)
Annual 99th
percentile of
daily maximum
1-hour
concentrations
at monitor
yielding
highest design
Design Value
for Current NAAQS
(3-year average of annual
99th percentile daily
maximum 1-hour
concentrations),
ppm





Mean
Upper
Percentiles
value (ppb)
(monitor ID)
Bronx County,
Jan 1999-Dec 2000
ATSDR 2006 °
24-hr ave
2 monitors collecting
12

78°
1999
-

E
NY

data in series

2000











1999
78
1999-2001
73
New York City,
Jan 1999-Dec 2002
Itoetal 2007
24-hr ave
Average across all
7.8
75th=10
82 F
2000
71
(36-061-0056)
NY
(19) monitors

95th=17
2001
71
2000-2002
69








2002
65
(36-061-0056)




Average across all
(6) monitors

75th =38.2
90th=60.7

1988
159

147
(09-009-1123)
New Haven, CT
Jan 1988-Dec 1990
Schwartz, 1995
24-hr ave
29.8
150 G
1989
167
1988-1990






1990
116

A Air quality information provided here is drawn from monitors reporting to AQS, as documented in Appendix E). Design values are SO2 concentrations for the study area in the statistical form of
the standard, derived in accordance with 40 CFR, Part 50, Appendix T. Presented is the highest valid design value at a monitor reporting to AQS for specified 3-year period.
B Ambient SO2 concentrations in terms of study metric that are reported in the second draft ISA Table 5-9 (for ATSDR, 2006 and Itoetal., 2007) and Table 5-14 (for Schwartz, 1995). Where
multiple monitors contribute data, these are the arithmetic mean and percentiles of the dataset of daily multi-monitor average concentrations for the full study period.
C This study was cited as NY DOH, 2006 in the 2008 ISA.
D This statistic is for combined dataset of 2 monitoring sites due to construction at the initial site (Thompson and Stewart, 2009). Data are from the first monitor (36-005-0073) for the period Jan 1 to
July 14,1999. Data are from the second monitor (36-005-0110), approximately 1/2 mile northeast of first, for the period Sept 2,1999 to Nov 22,2000.
E Due to incomplete quarters or years, there is not a valid design value for a monitor in the Bronx any of the 3-year periods that include the study period.
F This statistic is based on monitor 36-061-0080 (Thompson and Stewart, 2009), for which five quarters of data are available during the study period (from 1999 through first quarter of 2000).
G This statistic is based on monitor 09-009-1123 (Thompson and Stewart, 2009), for which 12 quarters of data are available during the study period (1988 through 1990).
August 24, 2017
D-2
External Review Draft - Do Not Quote or Cite

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REFERENCES
ATSDR (Agency for Toxic Substances and Disease Registry). (2006). A study of ambient air
contaminants and asthma in New York City: Part A and B. Atlanta, GA: U.S. Department
of Health and Human Services.
http://permanent.access.gpo.gov/lps88357/ASTHMA_BRONX_FINAL_REPORT.pdf
Ito, K; Thurston, GD; Silverman, RA. (2007). Characterization of PM2.5, gaseous pollutants,
and meteorological interactions in the context of time-series health effects models. J
Expo Sci Environ Epidemiol 17: S45-S60. http://dx.doi.org/10.1038/sj.jes.7500627
Schwartz, J; Morris, R. (1995). Air pollution and hospital admissions for cardiovascular disease
in Detroit, Michigan. Am J Epidemiol 142: 23-35.
Thompson, R; Stewart, M. (2009). Air Quality Statistics for Cities Referenced in Key U.S. and
Canadian Epidemiological Studies of Hospital Admissions and Emergency Department
Visits for All Respiratory Causes and Asthma. OAQPS Staff Memorandum to Ozone
NAAQS Review, March 25. Washington, DC: Office of Air Radiation. (EPA docket
number EPA-HQ-OAR-2007-0352). Available at:
https://www3.epa.gov/ttn/naaqs/standards/so2/data/2009 03 Thompson AirQualitv.pdf
August 24, 2017
D-3
External Review Draft - Do Not Quote or Cite

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APPENDIX E
DERIVAITON OF DESIGN VALUES PRESENTED IN APPENDIX D
August 24, 2017
E-l External Review Draft - Do Not Quote or Cite

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
User ID: DST	DESIGN VALUE REPORT
Report Request ID: 1565153	Report Code:	AMP480	Jun. 20, 2017

GEOGRAPHIC SELECTIONS




Tribal




EPA
Code State County Site
Parameter POC City AQCR
UAR
CBSA
CSA
Region
09	009
PROTOCOL SELECTIONS
Parameter
Classification Parameter Method Duration
DESIGN VALUE	424 01
SELECTED OPTIONS

Option Type
Option Value
SINGLE EVENT PROCESSING	EXCLUDE REGIONALLY CONCURRED EVENTS
WORKFILE DELIMITER
USER SITE METADATA	STREET ADDRESS
MERGE PDF FILES	YES
QUARTERLY DATA IN WORKFILE	NO
AGENCY ROLE	PQAO
DATE CRITERIA
Start Date
1990
End Date
1990
APPLICABLE STANDARDS
Standard Description
S02 1-hour 2010
Selection Criteria Page 1

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Report Date: Jun. 20, 2017
Notes: 1. Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
2.	Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
3.	Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 1 of 3

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY	Report Date: Jun. 20, 2 017
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Pollutant: Sulfur dioxide(42401)
Standard Units: Parts per billion(008)
NAAQS Standard: S02 1-hour 2 010
Statistic: Annual 99th Percentile
Level: 75
Design Value Year: 1990
REPORT EXCLUDES MEASUREMENTS WITH REGIONALLY CONCURRED EVENT FLAGS.
State Name:
Connecticut
Site ID
09-009-0010
09-009-0017
09-009-1003
09-009-1123
09-009-2123
09-009-3008
STREET ADDRESS
EGAN CENTER, MATHEW ST
LOMBARD STREET
ANIMAL SHELTER, COMMERCE ST
715 STATE STREET
Bank St at Meadow St (see c
LYDIA STREET EXTENTION

1990

1
1989

1
1988

1 3_
Year
Comp.
99th
Cert&
Comp.
99th
Cert&
Comp.
99th
Cert&
Design
Valid
Qrtrs
Percentile
Eval
| Qrtrs
Percentile
Eval
| Qrtrs
Percentile
Eval
Value
Ind.
3
114 *
Y
4
113

3
118 *

115
N



3
112 *

4
113

113
N
4
68
Y
4
99

4
95

87
Y
4
116
Y
4
167

4
159

147
Y
4
83
Y
4
97

4
85

88
Y
3
93 *
Y
4
110

4
100

101
Y
Notes: 1. Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
2.	Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
3.	Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 2 of 3

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Report Date: Jun. 20, 2017
CERTIFICATION EVALUATION AND CONCURRENCE FLAG MEANINGS
FLAG	MEANING
M	The monitoring organization has revised data from this monitor since the
most recent certification letter received from the state.
N	The certifying agency has submitted the certification letter and required
summary reports, but the certifying agency and/or EPA has determined
that issues regarding the quality of the ambient concentration data cannot
be resolved due to data completeness, the lack of performed quality
assurance checks or the results of uncertainty statistics shown in the
AMP255 report or the certification and quality assurance report.
S	The certifying agency has submitted the certification letter and required
summary reports. A value of "S" conveys no Regional assessment regarding
data quality per se. This flag will remain until the Region provides an "N" or
"Y" concurrence flag.
U	Uncertified. The certifying agency did not submit a required certification
letter and summary reports for this monitor even though the due date has
passed, or the state's certification letter specifically did not apply the
certification to this monitor.
X	Certification is not required by 40 CFR 58.15 and no conditions apply to be
the basis for assigning another flag value
Y	The certifying agency has submitted a certification letter, and EPA has no
unresolved reservations about data quality (after reviewing the letter, the
attached summary reports, the amount of quality assurance data
submitted to AQS, the quality statistics, and the highest reported
concentrations).
Notes: 1.
2	.
3	.
Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 3 of 3

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
User ID: DST	DESIGN VALUE REPORT
Report Request ID: 1565370	Report Code:	AMP480	Jun. 21, 2017

GEOGRAPHIC SELECTIONS




Tribal




EPA
Code State County Site
Parameter POC City AQCR
UAR
CBSA
CSA
Region
36	005
36	047
36	061
36	081
36	085
PROTOCOL SELECTIONS
Parameter
Classification Parameter Method Duration
DESIGN VALUE	424 01
SELECTED OPTIONS

Option Type
Option Value
SINGLE EVENT PROCESSING
EXCLUDE REGIONALLY CONCURRED EVENTS
WORKFILE DELIMITER
,
USER SITE METADATA
STREET ADDRESS
MERGE PDF FILES
YES
QUARTERLY DATA IN WORKFILE
NO
AGENCY ROLE
PQAO
APPLICABLE STANDARDS
Standard Description
S02 1-hour 2010
DATE CRITERIA
Start Date	End Date
2000	2002
Selection Criteria Page 1

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Report Date: Jun. 21, 2017
Notes: 1. Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
2.	Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
3.	Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 1 of 5

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY	Report Date: Jun. 21, 2 017
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Pollutant: Sulfur dioxide(42401)
Standard Units: Parts per billion(008)
NAAQS Standard: S02 1-hour 2 010
Statistic: Annual 99th Percentile
Level: 75
Design Value Year: 2000
REPORT EXCLUDES MEASUREMENTS WITH REGIONALLY CONCURRED EVENT FLAGS.
State Name:
New York
2000
Comp. 99th
Certfc
Comp.
1999
99th
Certfc
Comp.
1998
99th
Certfc
3-Year
Design Valid
Site ID
STREET ADDRESS 1
Qrtrs
Percentile Eval
I Qrtrs
Percentile
Eval
| Qrtrs
Percentile
Eval
i Value
Ind
36-005-0073
1
IS 155, 470 JACKSON AV.



1
2
68
*
Y
4
70
Y
1
69
N
36-005-0080
MORRISANIA CENTER, 1225-57
1
94
*
4
77

Y
4
69
Y
80
N
36-005-0083
2 0 0TH STREET AND SOUTHERN B
2
62
*







62
N
36-005-0110
IS 52 681 KELLY ST
4
86

1
98
*
Y



92
N
36-047-0011
3 01 GREENPOINT AVENUE



3
51
*
Y
4
42
Y
47
N
36-047-0076
PS 321 180 7TH AV,
0
36
*
4
54

Y
3
59 *
Y
50
N
36-061-0010
MABEL DEAN HIGH SCH.ANNEX,
3
72
*
4
79

Y
3
64 *
Y
72
N
36-061-0056
PS 59, 228 E. 57TH STREET,
4
71

4
78

Y
4
69
Y
73
Y
36-081-0097
56TH AVE AT SPRINGFIELD BLV
4
50

4
53

Y
2
52 *
Y
52
N
36-085-0067
SUSAN WAGNER HS, 12 0 0 MAN
1
54
*
4
46

Y
4
46
Y
49
N
Notes: 1. Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
2.	Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
3.	Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 2 of 5

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY	Report Date: Jun. 21, 2 017
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Pollutant: Sulfur dioxide(42401)
Standard Units: Parts per billion(008)
NAAQS Standard: S02 1-hour 2 010
Statistic: Annual 99th Percentile
Level: 75
Design Value Year: 2001
REPORT EXCLUDES MEASUREMENTS WITH REGIONALLY CONCURRED EVENT FLAGS.
State Name:
New York
2001
Comp. 99th
Certfc
Site ID
36-005-0073
36-005-0080
36-005-0083
36-005-0110
36-047-0011
36-047-0076
36-061-0010
36-061-0056
36-081-0097
36-081-0124
36-085-0067
STREET ADDRESS
IS 155, 470 JACKSON AV.
MORRISANIA CENTER, 1225-57
2	0 0TH STREET AND SOUTHERN B
IS 52 681 KELLY ST
3	01 GREENPOINT AVENUE
PS 321 180 7TH AV,
MABEL DEAN HIGH SCH.ANNEX,
PS 59, 228 E. 57TH STREET,
56TH AVE AT SPRINGFIELD BLV
Queens College 65-30 Kiss
SUSAN WAGNER HS, 12 0 0 MAN
Qrtrs Percentile Eval
71
81
69
71
50
57
Y
Y
Y
Y
Y
Y
2000
Comp. 99th	Cert&
Qrtrs Percentile Eval
94 *
62 *
86
36 *
72 *
71
50
54 *
Comp.
Qrtrs
2
1999
99th	Cert&
Percentile Eval
68 *
77
98 *
51 *
54
79
78
53
46
Y
Y
Y
Y
Y
Y
Y
Y
3-Year
Design Valid
Value Ind.
68
86
67
88
51
45
73
73
51
57
50
N
N
N
N
N
N
N
Y
Y
N
N
Notes: 1.
2	.
3	.
Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 3 of 5

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY	Report Date: Jun. 21, 2 017
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Pollutant: Sulfur dioxide(42401)
Standard Units: Parts per billion(008)
NAAQS Standard: S02 1-hour 2 010
Statistic: Annual 99th Percentile
Level: 75
Design Value Year: 2002
REPORT EXCLUDES MEASUREMENTS WITH REGIONALLY CONCURRED EVENT FLAGS.
State Name:
New York
Site ID
36-005-0080
36-005-0083
36-005-0110
36-047-0076
36-061-0010
36-061-0056
36-081-0097
36-081-0124
36-085-0067
STREET ADDRESS
MORRISANIA CENTER, 1225-57
2 0 0TH STREET AND SOUTHERN B
IS 52 681 KELLY ST
PS 321 180 7TH AV,
MABEL DEAN HIGH SCH.ANNEX,
PS 59, 228 E. 57TH STREET,
56TH AVE AT SPRINGFIELD BLV
Queens College 65-30 Kiss
SUSAN WAGNER HS, 12 0 0 MAN
2002
Comp. 99th	Certfc
Qrtrs Percentile Eval
62
67
65
57
Y
Y
2001
Comp. 99th	Cert&
Qrtrs Percentile Eval
71
81
69
71
50
57
Y
Y
Y
Y
Y
Y
2000
Comp.	99 th Cert&
Qrtrs	Percentile Eval
1	94 *
2	62 *
4	86
0	36 *
3	72 *
4	71
4	50
1	54 *
3-Year
Design Valid
Value Ind.
94
65
78
36
71
69
50
57
54
N
N
N
N
N
Y
N
N
N
Notes: 1. Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
2.	Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
3.	Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 4 of 5

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
AIR QUALITY SYSTEM
PRELIMINARY DESIGN VALUE REPORT
Report Date: Jun. 21, 2017
CERTIFICATION EVALUATION AND CONCURRENCE FLAG MEANINGS
FLAG	MEANING
M	The monitoring organization has revised data from this monitor since the
most recent certification letter received from the state.
N	The certifying agency has submitted the certification letter and required
summary reports, but the certifying agency and/or EPA has determined
that issues regarding the quality of the ambient concentration data cannot
be resolved due to data completeness, the lack of performed quality
assurance checks or the results of uncertainty statistics shown in the
AMP255 report or the certification and quality assurance report.
S	The certifying agency has submitted the certification letter and required
summary reports. A value of "S" conveys no Regional assessment regarding
data quality per se. This flag will remain until the Region provides an "N" or
"Y" concurrence flag.
U	Uncertified. The certifying agency did not submit a required certification
letter and summary reports for this monitor even though the due date has
passed, or the state's certification letter specifically did not apply the
certification to this monitor.
X	Certification is not required by 40 CFR 58.15 and no conditions apply to be
the basis for assigning another flag value
Y	The certifying agency has submitted a certification letter, and EPA has no
unresolved reservations about data quality (after reviewing the letter, the
attached summary reports, the amount of quality assurance data
submitted to AQS, the quality statistics, and the highest reported
concentrations).
Notes: 1.
2	.
3	.
Computed design values are a snapshot of the data at the time the report was run (may not be all data for year).
Some PM2.5 24-hour DVs for incomplete data that are marked invalid here may be marked valid in the Official report due to additional analysis.
Annual Values not meeting completeness criteria are marked with an asterisk ('*') .
Page 5 of 5

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APPENDIX F
GEOGRAPHIC DISTRIBUTION OF CONTINENTAL
U.S. FACILITIES EMITTING MORE THAN 1,000 TPY S02
AND POPULATION DENSITY BASED ON U.S. CENSUS TRACTS
August 24, 2017
F-l
External Review Draft - Do Not Quote or Cite

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NEI 2011
• Sources emitting 1,000 tpy or more
Population Density 2010 (People/Sq. Mile)
0 to 389
390 to 2,612
2\ 2,613 to 5,548
| 5,549 to 9,977
¦ 9,978 to 191,050
Figure F-l. Continental U.S.: Facilities emitting more than 1,000 tpy SO2 (11=619 in 2011 NEI) and population density.
August 24, 2017	F-2	External Review Draft - Do Not Quote or Cite

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• Sources emitting 1,000 tpy or more
Population Density 2010 (People/Sq. Mile)
0 to 389
390 to 2,612
I I 2,613 to 5,548
5 549 t0 9 977
| 9,978 to 191,050
Figure F-2. Northeast U.S.: Facilities emitting more than 1,000 tpy SO2 and population density.
August 24, 2017	F-3	External Review Draft - Do Not Quote or Cite

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NEI 2011
• Sources emitting 1,000 tpy or more
Population Density 2010 (People/Sq. Mile)
0 to 389
390 to 2,612
_ 2,613 to 5,548
5,549 to 9,977
¦ 9,978 to 191,050
Figure F-3. Southeast U.S.: Facilities emitting more than 1,000 tpy SO2 and population density.
August 24, 2017	F-4	External Review Draft - Do Not Quote or Cite

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i
*
"ts
NEI 2011
• Sources emitting 1,000 tpy or more
Population Density 2010 (People/Sq. Mile)
0 to 389
390 to 2,612
2,613 to 5,548
5,549 to 9,977
¦ 9,978 to 191,050
Figure F-4. Northwest U.S.: Facilities emitting more than 1,000 tpy SO2 and population density.
August 24, 2017	F-5	External Review Draft - Do Not Quote or Cite

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NEI 2011
• Sources emitting 1,000 tpy or more
Population Density 2010 (People/Sq. Mile)
0 to 389
390 to 2,612
I 2,613 to 5,548
| 5,549 to 9,977
¦ 9,978 to 191,050
Figure F-5. Southwest U.S.: Facilities emitting more than 1,000 tpy SO2 and population density.
August 24, 2017	F-6	External Review Draft - Do Not Quote or Cite

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United States	Office of Air Quality Planning and Standards	Publication No. EPA-452/P-17-003
Environmental Protection	Health and Environmental Impacts Division	August, 2017
Agency	Research Triangle Park, NC

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