[Federal Register Volume 83, Number 111 (Friday, June 8, 2018)]
[Proposed Rules]
[Pages 26752-26785]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2018-12061]



[[Page 26751]]

Vol. 83

Friday,

No. 111

June 8, 2018

Part II





 Environmental Protection Agency





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40 CFR Part 50





Review of the Primary National Ambient Air Quality Standards for Sulfur 
Oxides; Proposed Rule

  Federal Register / Vol. 83 , No. 111 / Friday, June 8, 2018 / 
Proposed Rules  

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2013-0566; FRL-9979-00-OAR]
RIN 2060-AT68


Review of the Primary National Ambient Air Quality Standards for 
Sulfur Oxides

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed action.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review 
of the air quality criteria addressing human health effects and the 
primary national ambient air quality standard (NAAQS) for sulfur oxides 
(SOX), the EPA is proposing to retain the current standard, 
without revision.

DATES: Comments must be received on or before July 23, 2018.
    If, by June 15, 2018, the EPA receives a request from a member of 
the public to speak at a public hearing concerning the proposed 
decision (see SUPPLEMENTARY INFORMATION below), we will hold a public 
hearing, with information about the hearing provided in a subsequent 
notice in the Federal Register.

ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2013-0566, to the Federal eRulemaking Portal: http://www.regulations.gov.
    Instructions: Follow the online instructions for submitting 
comments. Once submitted to the Federal eRulemaking Portal, comments 
cannot be edited or withdrawn. The EPA may publish any comment received 
to its public docket. Do not submit electronically any information you 
consider to be Confidential Business Information (CBI) or other 
information whose disclosure is restricted by statute. Multimedia 
submissions (audio, video, etc.) must be accompanied by a written 
comment. The written comment is considered the official comment and 
should include discussion of all points you wish to make. The EPA will 
generally not consider comments or comment contents located outside of 
the primary submission (i.e., on the web, the cloud, or other file 
sharing system). For additional submission methods, the full EPA public 
comment policy, information about CBI or multimedia submissions, and 
general guidance on making effective comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets.
    If a public hearing is to be held on this proposed action (see 
SUPPLEMENTARY INFORMATION below), in addition to publishing a Federal 
Register notice, the EPA will post information regarding it, including 
date and time, online at https://www.epa.gov/so2-pollution/primary-national-ambient-air-quality-standard-naaqs-sulfur-dioxide.
    Docket: All documents in the dockets pertaining to this action are 
listed on the www.regulations.gov website. This includes documents in 
the docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2013-
0566) and a separate docket, established for the Integrated Science 
Assessment (ISA) for this review (Docket ID No. EPA-HQ-ORD-2013-0357) 
that has been incorporated by reference into the docket for this 
proposed decision. Although listed in the index, some information is 
not publicly available, e.g., CBI or other information whose disclosure 
is restricted by statute. Certain other material, such as copyrighted 
material, is not placed on the internet and may be viewed, with prior 
arrangement, at the EPA Docket Center. Publicly available docket 
materials are available either electronically in www.regulations.gov or 
in hard copy at the Air and Radiation Docket Information Center, EPA/
DC, WJC West Building, Room 3334, 1301 Constitution Ave. NW, 
Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 
p.m., Monday through Friday, excluding legal holidays. The telephone 
number for the Public Reading Room is (202) 566-1744 and the telephone 
number for the Air and Radiation Docket Information Center is (202) 
566-1742.

FOR FURTHER INFORMATION CONTACT: Dr. Nicole Hagan, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail Code C504-06, 
Research Triangle Park, NC 27711; telephone: (919) 541-3153; fax: (919) 
541-0237; email: [email protected].

SUPPLEMENTARY INFORMATION: 

General Information

Preparing Comments for the EPA

1. Submitting CBI
    Do not submit this information to the EPA through 
www.regulations.gov or email. Clearly mark the part or all of the 
information that you claim to be CBI. For CBI information in a disk or 
CD-ROM that you mail to the EPA, mark the outside of the disk or CD-ROM 
as CBI and then identify electronically within the disk or CD-ROM the 
specific information that is claimed as CBI. In addition to one 
complete version of the comment that includes information claimed as 
CBI, a copy of the comment that does not contain the information 
claimed as CBI must be submitted for inclusion in the public docket. 
Information so marked will not be disclosed except in accordance with 
procedures set forth in 40 Code of Federal Regulations (CFR) part 2.
2. Tips for Preparing Your Comments
    When submitting comments, remember to:
     Identify the action by docket number and other identifying 
information (subject heading, Federal Register date and page number).
     Follow directions--the agency may ask you to respond to 
specific questions or organize comments by referencing a CFR part or 
section number.
     Explain why you agree or disagree, suggest alternatives, 
and substitute language for your requested changes.
     Describe any assumptions and provide any technical 
information and/or data that you used.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.
    Public Hearing: If, by June 15, 2018, the EPA receives a request 
from a member of the public to speak at a public hearing concerning the 
proposed decision, we will hold a public hearing, with information 
about the hearing provided in a subsequent notice in the Federal 
Register. To request a hearing, to register to speak at a hearing or to 
inquire if a hearing will be held, please contact Ms. Regina Chappell 
at (919) 541-3650 or by email at [email protected]. If a public 
hearing is to be held on this proposed action, the EPA will also post 
information regarding it, including, date and time, online at https://www.epa.gov/so2-pollution/primary-national-ambient-air-quality-standard-naaqs-sulfur-dioxide.

Availability of Information Related to This Action

    A number of the documents that are relevant to this proposed 
decision are available through the EPA's website at https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards. 
These documents include the Integrated Review Plan for the Primary

[[Page 26753]]

National Ambient Air Quality Standard for Sulfur Dioxide (U.S. EPA, 
2014a), available at https://www3.epa.gov/ttn/naaqs/standards/so2/data/20141028so2reviewplan.pdf, the Integrated Science Assessment for Sulfur 
Oxides--Health Criteria (U.S. EPA, 2017a), available at https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=338596, the Risk and 
Exposure Assessment for the Review of the National Ambient Air Quality 
Standard for Sulfur Oxides (U.S. EPA, 2018a), available at https://www.epa.gov/naaqs/sulfur-dioxide-so2-standards-risk-and-exposure-assessments-current-review and the Policy Assessment for the Review of 
the Primary National Ambient Air Quality Standard for Sulfur Oxides 
(U.S. EPA, 2018b), available at https://www.epa.gov/naaqs/sulfur-dioxide-so2-standards-policy-assessments-current-review. These and 
other related documents are also available for inspection and copying 
in the EPA docket identified above.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related SO2 Control Programs
    C. Review of the Air Quality Criteria and Standard for Sulfur 
Oxides
    D. Air Quality Information
    1. Sources and Emissions of Sulfur Oxides
    2. Ambient Concentrations
II. Rationale for Proposed Decision
    A. General Approach
    1. Approach in the Last Review
    2. Approach for the Current Review
    B. Health Effects Information
    1. Nature of Effects
    2. At-Risk Populations
    3. Exposure Concentrations Associated With Health Effects
    4. Potential Impacts on Public Health
    C. Summary of Risk and Exposure Information
    1. Key Design Aspects
    2. Key Limitations and Uncertainties
    3. Summary of Exposure and Risk Estimates
    D. Proposed Conclusions on the Current Standard
    1. Evidence- and Exposure and Risk-Based Considerations in the 
Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Conclusions on the Current Standard
III. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination with 
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From 
Environmental Health and Safety Risks
    I. Executive Order 13211: Actions that Significantly Affect 
Energy Supply, Distribution or Use
    J. National Technology Transfer and Advancement Act
    K. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    L. Determination Under Section 307(d)
References

Executive Summary

    This document presents the Administrator's proposed decision in the 
current review of the primary (health-based) NAAQS for SOX, 
a group of closely related gaseous compounds that include sulfur 
dioxide (SO2). Of these compounds, SO2 (the 
indicator for the current standard) is the most prevalent in the 
atmosphere and the one for which there is a large body of scientific 
evidence on health effects. The current primary standard is set at a 
level of 75 ppb, as the 99th percentile of daily maximum 1-hour 
SO2 concentrations, averaged over 3 years. This document 
summarizes the background and rationale for the Administrator's 
proposed decision to retain the current standard, without revision, and 
solicits comment on this proposed decision and on the array of issues 
associated with review of this standard, including public health and 
science policy judgments inherent in the proposed decision. The EPA 
solicits comment on the four basic elements of the current NAAQS 
(indicator, averaging time, level, and form), including whether there 
are appropriate alternative approaches for the averaging time or 
statistical form that provide comparable public health protection, and 
the rationale upon which such views are based.
    This review of the primary SO2 standard is required by 
the Clean Air Act (CAA) on a periodic basis. The schedule for 
completing this review is established by a consent decree, which 
established May 25, 2018 as the deadline for signature of a notice 
setting forth the proposed decision in this review and January 28, 2019 
as the deadline for signature on a final decision notice.
    The last review of the primary SO2 NAAQS was completed 
in 2010 (75 FR 35520, June 22, 2010). In that review, the EPA 
significantly strengthened the primary standard, establishing a 1-hour 
standard and revoking the 24-hour and annual standards. The 1-hour 
standard was established to provide protection from respiratory effects 
associated with exposures as short as a few minutes based on evidence 
from health studies that documented respiratory effects in people with 
asthma exposed to SO2 for 5 to 10 minutes while breathing at 
elevated rates. Revisions to the NAAQS were accompanied by revisions to 
the ambient air monitoring and reporting regulations, requiring the 
reporting of hourly maximum 5-minute SO2 concentrations, in 
addition to the hourly concentrations.
    Emissions of SO2 and associated concentrations in 
ambient air have declined appreciably since 2010 and over the longer 
term. For example, emissions nationally are estimated to have declined 
by 82% over the period from 2000 to 2016, with a 64% decline from 2010 
to 2016 (PA, Figure 2-2; 2014 NEI). Such declines in SO2 
emissions are likely related to the implementation of national control 
programs developed under the Clean Air Act Amendments of 1990, as well 
as changes in market conditions, e.g., reduction in energy generation 
by coal (PA, section 2.1, Figure 2-2; U.S. EIA, 2017). One-hour 
concentrations of SO2 in ambient air the U.S. declined more 
than 82% from 1980 to 2016 at locations continuously monitored over 
this period (PA, Figure 2-4). The decline since 2000 has been 69% at a 
larger number of locations continuously monitored since that time (PA, 
Figure 2-5). Daily maximum 5-minute concentrations have also 
consistently declined from 2011 to 2016 (PA, Figure 2-6).
    In this review, as in past reviews of the primary NAAQS for 
SOX, the health effects evidence evaluated in the ISA is 
focused on SO2. The health effects of particulate 
atmospheric transformation products of SOX, such as 
sulfates, are addressed in the review of the NAAQS for particulate 
matter (PM). Additionally, the welfare effects of sulfur oxides and the 
ecological effects of particulate atmospheric transformation products 
are being considered in the review of the secondary NAAQS for oxides of 
nitrogen, oxides of sulfur, and PM, while the visibility, climate, and 
materials damage-related welfare effects of particulate sulfur 
compounds are being evaluated in the review of the secondary NAAQS for 
PM.
    The proposed decision to retain the current primary NAAQS for 
SOX, without revision, has been informed by careful 
consideration of the key aspects

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of the currently available health effects evidence and conclusions 
contained in the ISA, quantitative risk and exposure information 
presented in the REA, considerations of this evidence and information 
discussed in the Policy Assessment, advice from the Clean Air 
Scientific Advisory Committee (CASAC), and public input received as 
part of the ongoing review of the primary NAAQS for SOX.
    The health effects evidence newly available in this review, as 
critically assessed in the ISA in conjunction with the full body of 
evidence, reaffirms the conclusions from the last review. The health 
effects evidence continues to support the conclusion that respiratory 
effects are causally related to short-term SO2 exposures, 
including effects related to asthma exacerbation in people with asthma, 
particularly children with asthma. The clearest evidence for this 
conclusion comes from controlled human exposure studies, available at 
the time of the last review, that show that people with asthma 
experience respiratory effects following very short (e.g., 5-10 minute) 
exposures to SO2 while breathing at elevated rates. 
Epidemiologic evidence, including studies not available in the last 
review, also supports this conclusion, primarily due to studies 
reporting positive associations between ambient air concentrations and 
emergency department visits and hospital admissions, specifically for 
children.
    The quantitative analyses of population exposure and risk also 
inform the proposed decision. These analyses expand and improve upon 
the quantitative analyses available in the last review. Unlike the REA 
available in the last review, which analyzed single-year air quality 
scenarios for potential standard levels bracketing the now current 
level, the current REA assesses an air quality scenario for three years 
of air quality conditions that just meet the now-current standard, 
considering all of its elements, including its 3-year form. Other ways 
in which the current REA analyses are improved and expanded include 
improvements to models, model inputs and underlying databases, 
including the vastly expanded ambient air monitoring dataset for 5-
minute concentrations, available as a result of changes in the last 
review to data reporting requirements.
    Based on this evidence and quantitative information, as well as 
CASAC advice and public comment thus far in this review, the 
Administrator proposes to conclude that the current primary 
SO2 standard is requisite to protect public health, with an 
adequate margin of safety, from effects of SOX in ambient 
air and should be retained, without revision. These proposed 
conclusions are consistent with CASAC recommendations. In its advice to 
the Administrator, the CASAC concurred with the preliminary conclusions 
in the draft PA that ``the current scientific literature does not 
support revision of the primary NAAQS for SO2'' (Cox and 
Diez Roux, 2018b, p. 1 of letter). The CASAC further stated that it 
``supports retaining the current standard, and specifically recommends 
that all four elements (indicator, averaging time, form, and level) 
should remain the same'' (Cox and Diez Roux, 2018b, p. 1 of letter). 
The Administrator solicits comment on the proposed conclusion that the 
current standard is requisite to protect public health, with an 
adequate margin of safety, and on the proposed decision to retain the 
standard, without revision. The Administrator also solicits comment on 
the array of issues associated with review of this standard, including 
public health and science policy judgments inherent in the proposed 
decision, as discussed in detail in section II below. The EPA solicits 
comment on the four basic elements of the current NAAQS (indicator, 
averaging time, level, and form), including whether there are 
appropriate alternative approaches for the averaging time or 
statistical form that provide comparable public health protection, and 
the rationale upon which such views are based.

I. Background

    This review focuses on the presence in ambient air of 
SOX, a group of closely related gaseous compounds that 
includes SO2 and sulfur trioxide and of which SO2 
(the indicator for the current standard) is the most prevalent in the 
atmosphere and the one for which there is a large body of scientific 
evidence on health effects. The health effects of particulate 
atmospheric transformation products of SOX, such as 
sulfates, are addressed in the review of the NAAQS for PM (U.S. EPA 
2014a, 2016a). Additionally, the ecological welfare effects of sulfur 
oxides and particulate atmospheric transformation products are being 
considered in the review of the secondary NAAQS for oxides of nitrogen, 
oxides of sulfur, and PM (U.S. EPA, 2014a, 2017b), while the 
visibility, climate, and materials damage-related welfare effects of 
particulate sulfur compounds are being evaluated in the review of the 
secondary NAAQS for PM.\1\
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    \1\ Additional information on the review of secondary NAAQS for 
oxides of nitrogen, oxides of sulfur, and PM with regard to 
ecological welfare effects is available at: https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards. Additional information on the review of the PM 
NAAQS is available at: https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards.
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A. Legislative Requirements

    Two sections of the Clean Air Act (CAA or the Act) 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 air 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 . . . [the Administrator] 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(b). 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. Section 109(b)(1) defines a primary 
standard as one ``the attainment and maintenance of which in the 
judgment of the Administrator, based on such criteria and allowing an 
adequate margin of safety, [is] requisite to protect the public 
health.'' \2\ A secondary standard, as defined in section 109(b)(2), 
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.'' \3\
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    \2\ 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.'' 
See S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970). See also 
Lead Industries Association v. EPA, 647 F.2d 1130, 1152 (D.C. Cir 
1980); American Lung Association v. EPA, 134 F.3d 388, 389 (D.C. 
Cir. 1998) (``NAAQS must protect not only average healthy 
individuals, but also `sensitive citizens'--children, for example, 
or people with asthma, emphysema, or other conditions rendering them 
particularly vulnerable to air pollution.'').
    \3\ As specified in 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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    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); American Petroleum Institute v. Costle, 665 F.2d 
1176, 1186 (D.C. Cir. 1981); American Farm Bureau Federation v. EPA, 
559 F.3d 512, 533 (D.C. Cir. 2009); Association of Battery Recyclers v. 
EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of 
uncertainties 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 provide 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. However, the CAA does not 
require the Administrator to establish a primary NAAQS at a zero-risk 
level or at background concentrations, see Lead Industries Association 
v. EPA, 647 F.2d at 1156 n.51, 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 sensitive population(s) at risk,\4\ and 
the kind and degree of the uncertainties that must be addressed. 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.
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    \4\ As used here and similarly throughout this notice, the term 
population (or group) refers to persons having a quality or 
characteristic in common, such as a specific pre-existing illness or 
a specific age or lifestage. Section II.B.2 below describes the 
identification of sensitive groups (called at-risk groups or at-risk 
populations) in this review.
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    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 for these purposes. In 
so doing, the EPA may not consider the costs of implementing the 
standards. See generally Whitman v. American Trucking Associations, 531 
U.S. 457, 465-472, 475-76 (2001). Likewise, ``[a]ttainability and 
technological feasibility are not relevant considerations in the 
promulgation of national ambient air quality standards.'' American 
Petroleum Institute v. Costle, 665 F.2d at 1185.
    Section 109(d)(1) requires that ``not later than December 31, 1980, 
and at 5-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 Clean 
Air Scientific Advisory Committee (CASAC).\5\
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    \5\ Lists of CASAC members and members of the CASAC Sulfur 
Oxides Review Panel are available at: https://yosemite.epa.gov/sab/sabpeople.nsf/WebCommitteesSubcommittees/CASAC%20Sulfur%20Oxides%20Panel.
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B. Related SO2 Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under section 110 of the Act, 42 U.S.C. 7410, and 
related provisions, states are to submit, for EPA approval, state 
implementation plans (SIPs) that provide for the attainment and 
maintenance of such standards through control programs directed to 
sources of the pollutants involved. The states, in conjunction with the 
EPA, also administer the prevention of significant deterioration 
program that covers these pollutants. See 42 U.S.C. 7470-7479. In 
addition, federal programs provide for nationwide reductions in 
emissions of these and other air pollutants under Title II of the Act, 
42 U.S.C. 7521-7574, which involves controls for automobile, truck, 
bus, motorcycle, nonroad engine and equipment, and aircraft emissions; 
the new source performance standards under section 111 of the Act, 42 
U.S.C. 7411; and the national emission standards for hazardous air 
pollutants under section 112 of the Act, 42 U.S.C. 7412.

C. Review of the Air Quality Criteria and Standard for Sulfur Oxides

    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 SOX are a group of 
closely related gaseous compounds that include sulfur dioxide and 
sulfur trioxide and of which sulfur dioxide (the indicator for the 
current standard) is the most prevalent in the atmosphere and the one 
for which there is a large body of scientific evidence on health 
effects. 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 (36 FR 
8186, April 30, 1971).
    The first review of the air quality criteria and primary standards 
for SOX was initiated in the early 1980s and concluded in 
1996 with the decision to retain the standards without revision (61 FR 
25566, May 22, 1996). In reaching this decision, the Administrator 
considered the evidence newly available since the standards were set 
that documented asthma-related respiratory effects in people with 
asthma exposed for very short periods, such as 5 to 10 minutes. Based 
on his consideration of an exposure analysis using the then-limited 
monitoring data and early exposure modeling methods, the Administrator 
judged that revisions to the standards were not needed to provide 
requisite public health protection from SOX in ambient air 
at that time (61 FR 25566, May 22, 1996). This decision was challenged 
and the U.S. Court of Appeals for the District of Columbia Circuit 
(D.C. Circuit) found that the EPA had failed to adequately explain its 
determination that no revision to the primary SO2 standards 
was appropriate and remanded the determination back to the EPA for 
further explanation (American Lung Association v. EPA, 134 F.3d 388 
[D.C. Cir. 1998]).
    This remand was addressed in the most recent review, which was 
completed in 2010. In that review, the EPA promulgated a new 1-hour 
standard and also promulgated

[[Page 26756]]

provisions for the revocation of the then-existing 24-hour and annual 
primary standards.\6\ The new 1-hour standard was set with a level of 
75 parts per billion (ppb), a form of the 3-year average of the annual 
99th percentile of daily maximum 1-hour SO2 concentrations, 
and with SO2 as the indicator. The Administrator judged that 
such a standard would provide the requisite protection for at-risk 
populations, such as people with asthma, against the array of adverse 
respiratory health effects related to short-term SO2 
exposures, including those as short as 5 minutes. With regard to 
longer-term exposures, the new standard was expected to maintain 24-
hour and annual concentrations generally well below the levels of the 
previous standards, and the available evidence did not indicate the 
need for separate standards designed to protect against longer-term 
exposures (75 FR 35520, June 22, 2010). The EPA also 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; \7\ agencies 
may report all twelve 5-minute concentrations for each hour, including 
the maximum, although it is not required (75 FR 35568, June 22, 2010). 
This rule was challenged in court, and the D.C. Circuit denied or 
dismissed on jurisdictional grounds all the claims in the petitions for 
review. National Environmental Development Association's Clean Air 
Project v. EPA, 686 F.3d 803, 805 (D.C. Cir. 2012).
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    \6\ Timing and related requirements for the implementation of 
the revocation are specified in 40 CFR 50.4(e).
    \7\ The rationale for this requirement was described as 
providing additional monitoring data for use in subsequent reviews 
of the primary standard, particularly for use in considering the 
extent of protection provided by the 1-hour standard against 5-
minute peak SO2 concentrations of concern (75 FR 35568, 
June 22, 2010). In establishing this requirement, the EPA described 
such data as being ``of high value to inform future health studies 
and, subsequently, future SO2 NAAQS reviews'' (75 FR 
35568, June 22, 2010).
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    In May 2013, the EPA initiated the current review by issuing a call 
for information in the Federal Register and also announcing a public 
workshop to inform the review (78 FR 27387, May 10, 2013). As was the 
case for the prior review, this review is focused on health effects 
associated with SOX and the public health protection 
afforded by the existing standard. Participants in the kickoff workshop 
included 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, and biology). Workshop discussions focused on key policy-
relevant issues around which the Agency would structure the review and 
the newly available scientific information related to these issues. 
Based in part on the workshop discussions, the EPA developed the draft 
integrated review plan (IRP) outlining the schedule, process, and key 
policy-relevant questions to guide this review of the SOX 
air quality criteria and standards (U.S. EPA, 2014b). The draft IRP was 
released for public comment and was reviewed by the CASAC at a public 
teleconference on April 22, 2014 (79 FR 14035, March 12, 2014; Frey and 
Diez Roux, 2014). The final IRP was developed with consideration of 
comments from the CASAC and the public (U.S. EPA, 2014a; 79 FR 16325, 
March 25, 2014; 79 FR 66721, November 10, 2014).
    As an early step in development of the Integrated Science 
Assessment (ISA) for this review, the EPA's National Center for 
Environmental Assessment (NCEA) hosted a public workshop at which 
preliminary drafts of key ISA chapters were reviewed by subject matter 
experts (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 first draft ISA (U.S. EPA, 
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 second draft ISA in December 2016 (U.S. EPA, 
2016b; 81 FR 89097, December 9, 2016), 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; Diez 
Roux, 2017a). The final ISA was released in December 2017 (U.S. EPA, 
2017a; 82 FR 58600, December 13, 2017).
    In considering the need for quantitative exposure and risk analyses 
in this review, the EPA completed the Risk and Exposure Assessment 
(REA) Planning Document in February 2017 (U.S. EPA, 2017c; 82 FR 11356, 
February 22, 2017), and held a consultation with the CASAC at a public 
meeting in March 2017 (82 FR 11449, February 23, 2017; Diez Roux, 
2017b). In consideration of the CASAC's comments at that consultation 
and public comments, the EPA developed the draft REA and draft Policy 
Assessment (PA), which were released on August 24, 2017 (U.S. EPA, 
2017d,e; 82 FR 43756, September 19, 2017). The draft REA and draft PA 
were reviewed by the CASAC on September 18-19, 2017 (82 FR 37213, 
August 9, 2017; Cox and Diez Roux, 2018a,b). The EPA considered the 
advice and comments from the CASAC on the draft REA and draft PA as 
well as public comments, in developing the final REA and final PA, 
which were released in early May 2018 (U.S. EPA, 2018a,b).
    The schedule for completion of this review is governed by a consent 
decree resolving a lawsuit filed in July 2016 by a group of plaintiffs 
which included a claim that the EPA had failed to complete its review 
of the primary SO2 NAAQS within five years, as required by 
the CAA.\8\ The consent decree, which was entered by the court on April 
28, 2017, provides that the EPA will sign, for publication, notices 
setting forth proposed and final decisions concerning its review of the 
primary NAAQS for SOX no later than May 25, 2018 and January 
28, 2019, respectively.\9\
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    \8\ See Complaint, Center for Biological Diversity et al. v. 
McCarthy, No. 3:16-cv-03796-VC, (N.D. Cal., filed July 7, 2016), 
Doc. No. 1.
    \9\ Consent Judgment at 4, Center for Biological Diversity et 
al. v. McCarthy, No. 3:16-cv-03796-VC (N.D. Cal., entered April 28, 
2017), Doc. No. 37.
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D. Air Quality Information

    This section presents information on sources and emissions of 
SO2 and ambient concentrations, with a focus on information 
that is most relevant for the review of the primary SO2 
standard. This section is drawn from the more detailed discussion of 
SO2 air quality in the PA and the ISA. It presents a summary 
of SO2 sources and emissions (II.B.1) and ambient 
concentrations (II.B.2).
1. Sources and Emissions of Sulfur Oxides
    Sulfur oxides are emitted into air from specific sources (e.g., 
fuel combustion processes) and are 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.\10\ Sulfur oxides known to occur

[[Page 26757]]

in the troposphere include SO2 and sulfur trioxide 
(SO3) (ISA, section 2.3). With regard to SO3, it 
``is known to be present in the emissions of coal-fired power plants, 
factories, and refineries, but it reacts with water vapor in the stacks 
or immediately after release into the atmosphere to form 
H2SO4'' and ``gas-phase 
H2SO4. . . . quickly condenses onto existing 
atmospheric particles or participates in new particle formation'' (ISA, 
section 2.3). Thus, as a result of rapid atmospheric chemical reactions 
involving SO3, the most prevalent sulfur oxide in the 
atmosphere is SO2 (ISA, section 2.3).\11\
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    \10\ Some sulfur compounds formed from or emitted with 
SOX are very short-lived (ISA, pp. 2-23 to 2-24). For 
example, studies in the 1970s and 1980s identified particle-phase 
sulfur compounds, including inorganic SO3-2 complexed 
with Fe(III) in the particles emitted by a smelter near Salt Lake 
City, UT. Subsequent studies reported rapid oxidation of such 
compounds, ``on the order of seconds to minutes'' and ``further 
accelerated by low pH'' (ISA, p. 2-24). Thus, ``[t]he highly acidic 
aqueous conditions that arise once smelter plume particles 
equilibrate with the ambient atmosphere ensure that S(IV)-Fe(III) 
complexes have a small probability of persisting and becoming a 
matter of concern for human exposure'' (ISA, 2-24).
    \11\ The health effects of particulate atmospheric 
transformation products of SOX, such as sulfates, are 
addressed in the review of the NAAQS for PM (U.S. EPA 2014a, 2016a).
---------------------------------------------------------------------------

    Fossil fuel combustion is the main anthropogenic source of 
SO2 emissions, while volcanoes and landscape fires 
(wildfires as well as controlled burns) are the main natural sources 
(ISA, section 2.1).\12\ 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 (ISA, 
section 2.1). Anthropogenic SO2 emissions originate 
primarily from point sources, including coal-fired electricity 
generating units (EGUs) and other industrial facilities (ISA, section 
2.2.1). The largest SO2-emitting sector within the U.S. is 
electricity generation, and 97% of SO2 from electricity 
generation is from coal combustion. Other anthropogenic sources of 
SO2 emissions include industrial fuel combustion and process 
emissions, industrial processing, commercial marine activity, and the 
use of fire in landscape management and agriculture (ISA, section 
2.2.1).
---------------------------------------------------------------------------

    \12\ A modeling analysis estimated annual mean SO2 
concentrations for 2001 in the absence of any U.S. anthropogenic 
emissions of SO2 (2008 ISA, section 2.5.3; ISA, section 
2.5.5). Such concentrations are referred to as U.S 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 (ISA, section 2.5.5).
---------------------------------------------------------------------------

    National average SO2 emissions are estimated to have 
declined by 82% over the period from 2000 to 2016, with a 64% decline 
from 2010 to 2016 (PA, Figure 2-2; 2014 NEI). Such declines in 
SO2 emissions are likely related to the implementation of 
national control programs developed under the Clean Air Act Amendments 
of 1990, including Phase I and II of the Acid Rain Program, the Clean 
Air Interstate Rule, the Cross-State Air Pollution Rule, and the 
Mercury Air Toxic Standards,\13\ as well as changes in market 
conditions, e.g., reduction in energy generation by coal (PA, section 
2.1, Figure 2-2; U.S. EIA, 2017).\14\ Regulations on sulfur content of 
diesel fuel, both fuel for onroad vehicles and nonroad engines and 
equipment, may also contribute to declining trends in SO2 
emissions.\15\ Declines in emissions from all sources between 1971, 
when SOX NAAQS were first established, and 1990, when the 
Amendments were adopted, were on the order of 5,000 tpy deriving 
primarily from reductions in emissions from the metals processing 
sector (ISA, Figure 2-5).
---------------------------------------------------------------------------

    \13\ When established, the MATS Rules was estimated to reduce 
SO2 emissions from power plants by 41% beyond the 
reductions expected from the Cross State Air Pollution Rule (U.S. 
EPA, 2011).
    \14\ In 2014, the EPA promulgated Tier 3 Motor Vehicle Emission 
and Fuel Standards that set emissions standards for new vehicles and 
lowered the sulfur content of gasoline. Reductions in SO2 
emissions resulting from these standards are expected to be more 
than 14,000 tons in 2018 (U.S. EPA, 2014c).
    \15\ See https://www.epa.gov/diesel-fuel-standards/diesel-fuel-standards-and-rulemakings#nonroad-diesel.
---------------------------------------------------------------------------

2. Ambient Concentrations
    Ambient air concentrations of SO2 in the U.S. have 
declined substantially from 1980 to 2016, more than 82% in terms of the 
form of the current standard (the 99th percentile daily maximum 1-hour 
concentrations averaged over three years) at locations continuously 
monitored over this period (PA, Figure 2-4).\16\ The decline since 2000 
has been 69% at the larger number of locations continuously monitored 
since that time (PA, Figure 2-5).\17\
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    \16\ This decline is the average of observations at 24 
monitoring sites that have been continuously operating from 1980-
2016.
    \17\ This decline is the average of observations at 193 
monitoring sites that have been continuously operating across 2000-
2016.
---------------------------------------------------------------------------

    As a result of the reporting requirements promulgated in 2010 (as 
summarized in section I.C above) maximum hourly five-minute 
concentrations of SO2 in ambient air are available at 
SO2 NAAQS compliance monitoring sites (PA, Figure 2-3; FR 75 
35554, June 22, 2010).\18\ These newly available data document 
reductions in peak 5-minute concentrations across the U.S. For example, 
over the period from 2011 to 2016, the 99th percentile 5-minute 
SO2 concentrations declined approximately 53% (PA, Figure 2-
6, Appendix B).
---------------------------------------------------------------------------

    \18\ Such measurements were available for fewer than 10% of 
monitoring sites at the time of the last review. 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 (PA, section 2.2). The expanded dataset has provided a 
more robust foundation for the quantitative analyses in the REA for 
this review.
---------------------------------------------------------------------------

    Concentrations of SO2 vary across the U.S. and tend to 
be higher in areas with sources having relatively higher SO2 
emissions (e.g., locations influenced by emissions from 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 (PA, Figure 2-7). The point source nature 
of SO2 emissions contributes to the relatively high spatial 
variability of SO2 concentrations compared with pollutants 
such as ozone (ISA, section 3.2.3). Another factor in the spatial 
variability is the dispersion and oxidation of SO2 in the 
atmosphere, processes that contribute to decreasing concentrations with 
increasing distance from the source. Point source emissions of sulfur 
oxides create a plume of higher concentrations, which may or may not 
impact large portions of surrounding populated areas depending on 
meteorological conditions and terrain.
    Analyses in the ISA of data for 2013-2015 in six areas 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 
(ISA, section 2.5.3.2).\19\ This seasonal variation as well as month-
to-month variations are generally consistent with month-to-month 
emissions patterns and the expected atmospheric chemistry of 
SO2 for a given season. 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 
areas evaluated were generally low during nighttime and approached 
maxima values during daytime hours (ISA, section 2.5.3.3, Figures 2-23 
and 2-24). The timing and duration of daytime maxima in the six sites 
evaluated in the ISA were likely related to a combination of source 
emissions and meteorological parameters (ISA,

[[Page 26758]]

section 2.5.3.3; U.S. EPA 2008a, section 2.5.1).
---------------------------------------------------------------------------

    \19\ The six ``focus areas'' evaluated in the ISA are: 
Cleveland, OH; Pittsburgh, PA; New York City, NY; St. Louis, MO-IL; 
Houston, TX; and Gila County, AZ (ISA, section 2.5.2.2). These six 
locations were selected 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 SO2 sources within a given focus area boundary.
---------------------------------------------------------------------------

II. Rationale for Proposed Decision

    This section presents the rationale for the Administrator's 
proposed decision to retain the current primary SO2 
standard. This rationale is based on a thorough review of the latest 
scientific information generally published through August 2016,\20\ as 
presented in the ISA, on human health effects associated with 
SO2 and pertaining to the presence of SOX in 
ambient air. The Administrator's rationale also takes into account: (1) 
The PA evaluation of the policy-relevant information in the ISA and 
quantitative analyses of air quality, human exposure and health risks 
in the REA; (2) CASAC advice and recommendations, as reflected in 
discussions of drafts of the ISA, REA, and PA at public meetings and in 
the CASAC's letters to the Administrator; and (3) public comments 
received during the development of these documents.
---------------------------------------------------------------------------

    \20\ In addition to the review's opening ``call for 
information'' (78 FR 27387, May 10, 2013), ``the U.S. EPA routinely 
conducted literature searches to identify relevant peer-reviewed 
studies published since the previous ISA (i.e., from January 2008 
through August 2016)'' (ISA, p. 1-3). References that are cited in 
the ISA, the references that were considered for inclusion but not 
cited, and electronic links to bibliographic information and 
abstracts can be found at: https://hero.epa.gov/hero/sulfur-oxides.
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's proposed 
decision and its foundations, section II.A provides background on the 
general approach for review of the primary SO2 standard, 
including a summary of the approach used in the last review (section 
II.A.1) and the general approach for the current review (section 
II.A.2). Section II.B summarizes the currently available health effects 
evidence, focusing on consideration of key policy-relevant aspects. 
Section II.C summarizes the exposure and risk information for this 
review, drawing on the quantitative analyses for SO2, 
presented in the REA. Section II.D presents the Administrator's 
proposed conclusions on the current standard (section II.D.3), drawing 
on both evidence-based and exposure/risk-based considerations (section 
II.D.1) and advice from the CASAC (section II.D.2).

A. General Approach

    The past and current approaches described below are both based, 
most fundamentally, on using the EPA's assessments of the current 
scientific evidence and associated quantitative analyses to inform the 
Administrator's judgment regarding a primary standard for 
SOX that protects public health with an adequate margin of 
safety. The EPA's assessments are primarily documented in the ISA, REA 
and PA, all of which have received CASAC review and public comment (80 
FR 73183, November 24, 2015; 81 FR 89097, December 9, 2016; 82 FR 
11356, February 22, 2017; 82 FR 43756, September 19, 2017). In bridging 
the gap between the scientific assessments of the ISA and REA and the 
judgments required of the Administrator in determining whether the 
current standard remains requisite to protect public health with an 
adequate margin of safety, the PA evaluates policy implications of the 
evaluation of the current evidence in ISA and the quantitative analyses 
in the REA. In evaluating the health protection afforded by the current 
standard, the four basic elements of the NAAQS (indicator, averaging 
time, level, and form) are considered collectively.
    We note that in drawing conclusions with regard to the primary 
standard, the final decision on the adequacy of the current standard is 
largely a public health policy judgment to be made by the 
Administrator. The Administrator's final decision will draw upon 
scientific information and analyses about health effects, population 
exposure and risks, as well as judgments about how to consider the 
range and magnitude of uncertainties that are inherent in the 
scientific evidence and analyses. This approach is based on the 
recognition that the available health effects evidence generally 
reflects a continuum, consisting of levels at which scientists 
generally agree that health effects are likely to occur, through lower 
levels at which the likelihood and magnitude of the response become 
increasingly uncertain. This approach is consistent with the 
requirements of the NAAQS provisions of the Clean Air Act and with how 
the EPA and the courts have historically interpreted the Act. These 
provisions require the Administrator to establish primary standards 
that, in the judgment of the Administrator, 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 or 
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 avoids unacceptable risks to public health, including 
the health of sensitive groups.\21\
---------------------------------------------------------------------------

    \21\ As noted in section I.A above, such protection is specified 
for the sensitive group of individuals and not to a single person in 
the sensitive group (see S. Rep. No. 91-1196, 91st Cong., 2d Sess. 
10 [1970]).
---------------------------------------------------------------------------

1. Approach in the Last Review
    The last review of the primary NAAQS for SOX was 
completed in 2010 (75 FR 35520, June 22, 2010). The decision in that 
review to substantially revise the standards (establishing a 1-hour 
standard and revoking the 24-hour and annual standards) was based on 
the extensive body of evidence of respiratory effects in people with 
asthma that has expanded in this area over the four decades since the 
first SO2 standards were set in 1971 (U.S. EPA 1982, 1986, 
1994, 2008a). In so doing, the 2010 decision considered the full body 
of evidence, as assessed in the 2008 ISA; the 2009 REA, which included 
the staff assessment of the policy-relevant information contained in 
the ISA and analyses of air quality, exposure and risk; the advice and 
recommendations of the CASAC; and public comment. In addition to 
epidemiologic evidence linking respiratory outcomes in people with 
asthma to short-term SO2 air quality metrics, a key element 
of the expanded evidence base in the 2010 review was a series of 
controlled human exposure studies which document bronchoconstriction-
related effects on lung function in people with asthma exposed while 
breathing at elevated rates \22\ for periods as short as five minutes. 
Another key element was the air quality database, expanded since the 
previous review (completed in 1996), which documented the then-recent 
pattern of peak 5-minute SO2 concentrations. The EPA used 
these data in the quantitative exposure and risk assessments to provide 
an up-to-date ambient air quality context for interpreting the health 
effects evidence in the 2010 review. Together these aspects of the 2010 
review additionally addressed the issues raised in the court remand to 
the EPA of the Agency's 1996 decision not to revise the standards at 
that time to specifically address 5-minute exposures (75 FR 35523, June 
22, 2010). In so doing, the EPA strengthened the primary NAAQS for

[[Page 26759]]

SOX to provide the requisite protection of public health 
with an adequate margin of safety and to specifically afford increased 
protection for at-risk populations, such as people with asthma, against 
adverse respiratory health effects related to short-term SO2 
exposures (75 FR 35550, June 22, 2010).
---------------------------------------------------------------------------

    \22\ 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 rates for the range of ages and 
sizes of the simulated populations (U.S. EPA, 2009, section 
4.1.4.4). Accordingly, these phrases are used in the current review 
when referring to REA analyses from the last review. Otherwise, 
however, the documents for this review generally use the phrase 
``elevated breathing rates'' to refer to the same situation.
---------------------------------------------------------------------------

    Thus, the 2010 decision focused on the effects most pertinent to 
SOX in ambient air and recognized the long-standing evidence 
regarding the sensitivity of some people with asthma to brief 
SO2 exposures experienced while breathing at elevated rates. 
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 
5.3). 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 
copollutants, and other areas (75 FR 35535-36, June 22, 2010; 2008 ISA, 
section 5.3).
    The conclusions reached in the last review were based primarily on 
interpretation of the short-term health effects evidence, particularly 
the interpretation of the evidence from controlled human exposure 
studies within the context of the quantitative exposure and risk 
analyses. The epidemiologic evidence also provided support for various 
aspects of the decision. In making judgments on the public health 
significance of health effects related to ambient air-related 
SO2 exposures, the Administrator considered statements from 
the American Thoracic Society (ATS) regarding adverse effects of air 
pollution,\23\ the CASAC's written advice and recommendations,\24\ and 
judgments made by the EPA in considering similar effects in previous 
NAAQS reviews (75 FR 35526 and 35536, June 22, 2010; ATS, 1985, 2000). 
Based on these considerations, the Administrator, in reaching decisions 
in the last review, gave weight to the findings of respiratory effects 
in exercising people with asthma after 5- to 10-minute exposures as low 
as 200 ppb. With regard to higher exposures, at or above 400 ppb, she 
noted their association with respiratory symptoms as indication of 
their clear adversity, as well as the greater number of study subjects 
responding with lung function decrements. Moreover, she took note of 
the greater severity of the response, recognizing effects associated 
with exposures as low as 200 ppb to be less severe (75 FR 35547, June 
22, 2010).
---------------------------------------------------------------------------

    \23\ The 1999 statement of the ATS (published in 2000) on ``What 
Constitutes an Adverse Health Effect of Air Pollution?'' is 
``intended to provide guidance to policy makers and others who 
interpret the scientific evidence on the health effects of air 
pollution for the purpose of risk management'' and describes 
``principles to be used in weighing the evidence'' when considering 
what may be adverse and nonadverse effects on health (ATS, 2000).
    \24\ For example, the CASAC letter on the first draft 
SO2 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).
---------------------------------------------------------------------------

    In reaching her conclusion on the adequacy of the then-existing 
primary standards, the Administrator gave particular attention to the 
exposure and risk estimates from the 2009 REA for air quality 
conditions just meeting the then-existing (24-hour and annual) 
standards. In so doing, the Administrator also noted epidemiologic 
study findings of associations with respiratory outcomes in studies of 
locations where maximum 24-hour average SO2 concentrations 
were below the level of the then existing 24-hour standard. The 2009 
REA estimated that substantial percentages of children with asthma 
might be expected to experience at least once annually, exposures that 
had been associated with moderate or greater lung function decrements 
\25\ in the controlled human exposure studies (75 FR 35536, June 22, 
2010). The Administrator judged that such exposures can result in 
adverse health effects in people with asthma and found that the 
estimated population frequencies for such exposures (24% of at-risk 
population with at least one occurrence per year at or above 400 ppb 
and 73% with at least one occurrence per year at or above 200 ppb) were 
significant from a public health perspective and that the then-existing 
primary standards did not adequately protect public health (75 FR 
35536, June 22, 2010).
---------------------------------------------------------------------------

    \25\ In assessments for NAAQS reviews, the magnitude of lung 
function responses described as indicative of a moderate response 
include increases in specific airway resistance (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 forced expiratory volume in 1 second (FEV1) 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 FEV1 was also consistent with 
the relationship observed between sRaw and FEV1 responses 
in the Linn et al. studies (1987, 1990) for which ``a 100% increase 
in sRaw roughly corresponds to a 12 to 15% decrease in 
FEV1'' (U.S. EPA, 1994, p. 20). Thus, in the 2008 review, 
moderate or greater SO2-related bronchoconstriction or 
decrements in lung function referred to the occurrence of at least a 
doubling in sRaw or at least a 15% reduction in FEV1 
(2008 ISA, p. 3-5).
---------------------------------------------------------------------------

    Based on consideration of the entire body of evidence and 
information available in the review, as well as the advice from the 
CASAC and public comments, the Administrator concluded that the 
appropriate approach to revising the standards was to replace the then-
existing 24-hour standard with a new, short-term standard set to 
provide requisite protection with an adequate margin of safety to 
people with asthma and afford protection from the adverse health 
effects of 5-minute to 24-hour SO2 exposures (75 FR 35536, 
June 22, 2010). Accordingly, the available information was then 
considered in reaching conclusions on the four elements of such a new 
standard: indicator, averaging time, form, and level. Further, upon 
reviewing the evidence with regard to the potential for effects from 
long-term exposures, the Administrator revoked the annual standard. In 
so doing, she recognized the lack of sufficient health evidence to 
support a long-term standard and that the new short-term standard would 
have the effect of generally maintaining the annual SO2 
concentrations well below the level of the revoked annual standard (75 
FR 35550, June 22, 2010).
    With regard to the indicator for the new short-term standard, the 
EPA continued to focus on SO2 as the most appropriate 
indicator for SOX because the available scientific 
information regarding health effects was overwhelmingly indexed by 
SO2. Furthermore, although the presence of SOX 
species other than SO2 in ambient air had been recognized, 
no alternative to SO2 had been advanced as a more 
appropriate surrogate for 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 analyzed 
associations of health outcomes with concentrations of SO2. 
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

[[Page 26760]]

the indicator for a standard that was intended to address effects 
associated with exposure to SO2, alone or in combination 
with other SOX (75 FR 35536, 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 
SOX (75 FR 35536, June 22, 2010).
    With regard to the averaging time for the new standard, 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 further 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).\26\ Accordingly, she considered longer 
averaging times.
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    \26\ 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).
---------------------------------------------------------------------------

    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 same analyses suggested that a 1-hour 
averaging time would be more efficient and would be 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 
found 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 means of controlling short-term 
exposures to SO2 ranging from 5 minutes to 24 hours (75 FR 
35539, June 22, 2010).
    With regard to the statistical form for the new 1-hour 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 ``[t]here 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, 
exposure, and risk 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 observed in controlled human 
exposure studies and associated with ambient air SO2 
concentrations in epidemiologic studies (75 FR 35541, June 22, 2010). 
The Administrator further noted that, based on results of air quality 
and exposure analyses in the REA, a 99th percentile form was likely to 
be appreciably more effective at achieving the desired control of 5-
minute peak exposures 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).
    Lastly, based on the body of scientific evidence and information 
available, as well as CASAC recommendations and public comment, the 
Administrator decided on a standard level that, in combination with the 
specified choice of indicator, averaging time and form, would be 
requisite to protect public health, including the health of at-risk 
populations, with an adequate margin of safety. In reaching the 
decision on a level for the 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 (75 FR 35546, June 22, 2010). The Administrator drew 
on evidence from these studies in reaching judgments on the magnitude 
of adverse respiratory effects and associated potential public health 
significance for the air quality exposure and risk analysis results of 
air quality scenarios for conditions just meeting alternative levels 
for a new 1-hour standard (described in the 2009 REA).
    In light of judgments regarding the health effects evidence, the 
Administrator considered what the findings of the 2009 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 exposures to concentrations of 200 ppb and 400 ppb, given the 
specified choice of indicator, averaging time, and form.\27\ For 
example, the single-year exposure assessment for St. Louis \28\ 
estimated that a 1-hour standard at 100 ppb would likely protect more 
than 99% of children with asthma in that city from

[[Page 26761]]

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 35546-47, June 22, 2010). Results 
for the air quality scenario for a 1-hour standard level of 50 ppb 
suggested that such a standard would further limit exposures, such that 
more than 99% \29\ 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, June 22, 2010). In considering the implications of these 
estimates, and the substantial reduction in 5-minute exposures at or 
above 200 ppb, the Administrator did not judge that a standard level as 
low as 50 ppb \30\ was warranted (75 FR 35547, June 22, 2010). Before 
reaching her conclusion with regard to level for the 1-hour standard, 
the Administrator additionally considered the epidemiologic evidence, 
placing relatively more weight on 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 or 
hospital admissions related to asthma or other respiratory symptoms, 
and noting 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 PM (75 FR 35547-
48, June 22, 2010).\31\
---------------------------------------------------------------------------

    \27\ The Administrator additionally noted the results of the 
analysis of the limited available air quality data for 5-minute 
SO2 concentrations with regard to prevalence of higher 5-
minute concentrations at monitor sites when data were adjusted to 
just meet a standard level of 100 ppb. This 40-county analysis 
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 (75 FR 
35546, June 22, 2010).
    \28\ With regard to the results for the two study areas assessed 
in the 2009 REA, the EPA considered the St. Louis results to be more 
informative to consideration of the adequacy of protection 
associated with the then-current and alternative standards (75 FR 
35528, June 22, 2010; 74 FR 64840, December 8, 2009). The St. Louis 
study area included several counties and had population size and 
magnitudes of emissions density (on a spatial scale) similar to 
other urban areas in the U.S., while the second study area (Greene 
County, Missouri) was a rural county with much lower population and 
emissions density.
    \29\ The 2009 REA indicated this percentage to be 99.9% (2009 
REA, Appendix B, p. B-62).
    \30\ In the 2009 REA results for the St. Louis single year 
scenario with a level of 50 ppb (the only level below 100 ppb that 
was analyzed), 99.9% of children with asthma would be expected to be 
protected from a day with a 5-minute exposure at or above 200 ppb, 
and 100% from a day with a 5-minute exposure at or above 400 ppb 
(2009 REA).
    \31\ 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).
---------------------------------------------------------------------------

    Given the above considerations and the comments received on the 
proposal, the Administrator judged, based on the entire body of 
evidence and information available in that review (concluded in 2010), 
and the related uncertainties,\32\ that a standard level of 75 ppb was 
appropriate. 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 then-existing 
standards and would be expected to provide protection, with an adequate 
margin of safety, against the respiratory effects elicited by 
SO2 exposures in controlled human exposure studies and 
associated with ambient air concentrations in epidemiologic studies (75 
FR 35548, June 22, 2010). The Administrator found that ``a 1-hour 
standard at a level of 75 ppb is expected to substantially limit 
asthmatics' exposure to 5-10 minute SO2 concentrations >=200 
ppb, thereby substantially limiting the adverse health effects 
associated with such exposures'' (75 FR 35548, June 22, 2010). Such a 
standard was also considered likely ``to maintain SO2 
concentrations below those in locations where key U.S. epidemiologic 
studies have reported that ambient SO2 is associated with 
clearly adverse respiratory health effects, as indicated by increased 
hospital admissions and emergency department visits'' (75 FR 35548, 
June 22, 2010). Lastly, the Administrator noted ``that a standard level 
of 75 ppb is consistent with the consensus recommendation of CASAC'' 
(75 FR 35548, June 22, 2010). 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).
---------------------------------------------------------------------------

    \32\ Such uncertainties included both those with regard to the 
epidemiologic evidence, including potential confounding and exposure 
error, and also those with regard to the information from controlled 
human exposure studies for at-risk groups, including the extent to 
which the results would be expected to be similar for individuals 
with more severe asthma than that in study subjects (75 FR 35546, 
June 22, 2010).
---------------------------------------------------------------------------

2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the now 
current primary SO2 standard, or whether consideration of 
revision is appropriate, the EPA has adopted an approach in this review 
that builds upon the general approach used in the last review and 
reflects the body of evidence and information now available. 
Accordingly, the approach in this review takes into consideration the 
approach used in the last review, addressing key policy-relevant 
questions in light of currently available scientific and technical 
information. As summarized above, the Administrator's decisions in the 
prior review were based on an integration of SO2 health 
effects information with judgments on the adversity and public health 
significance of key health effects, policy judgments as to when the 
standard is requisite to protect against public health with an adequate 
margin of safety, consideration of CASAC advice, and consideration of 
public comments.
    Similarly, in this review, 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 here, we consider both the information 
available at the time of the last review and information newly 
available since the last review, including that which has been 
critically analyzed and characterized in the current ISA. The 
quantitative exposure and risk analyses provide a context for 
interpreting the evidence of lung function decrements in people with 
asthma breathing at elevated rates and the potential public health 
significance of exposures associated with air quality conditions that 
just meet the current standard.

B. Health Effects Information

    The information summarized here is based on our scientific 
assessment of the health effects evidence available in this review; 
this assessment is documented in the ISA and its policy implications 
are further discussed in the PA. More than 400 studies are newly 
available and considered in the ISA, including more than 200 health 
studies. They are consistent with the evidence that was available in 
the last review. As in the last review, the key evidence comes from the 
body of controlled human exposure studies that document effects in 
people with asthma. Policy implications of the currently available 
evidence are discussed in the PA (as summarized in section II.D.1 
below). The subsections below briefly summarize the following aspects 
of the evidence: The nature of SO2-related health effects 
(section II.B.1), the populations at risk (section II.B.2), exposure 
concentrations associated with health effects (section II.B.3), and 
potential public health implications (section II.B.4).
1. Nature of Effects
    In this review, as in past reviews, the health effects evidence 
evaluated in the ISA for SOX is focused on SO2 
(ISA, p. 5-1). As summarized in section I.D.1 above, atmospheric 
chemistry as well as emissions contribute to SO2 being the 
most prevalent sulfur oxide in the atmosphere. As concluded in the ISA, 
``[o]f the sulfur oxides, SO2 is the most

[[Page 26762]]

abundant in the atmosphere, the most important in atmospheric 
chemistry, and the one most clearly linked to human health effects'' 
(ISA, p. 2-1). Accordingly, the ISA states that ``only SO2 
is present at concentrations in the gas phase that are relevant for 
chemistry in the atmospheric boundary layer and troposphere, and for 
human exposures'' (ISA, p. 2-18). Thus, the current health effects 
evidence and the Agency's review of the evidence, including the 
evidence newly available in this review, continues to focus on 
SO2.
    Sulfur dioxide is a highly reactive and water-soluble gas that once 
inhaled is absorbed almost entirely in the upper respiratory tract \33\ 
(ISA, sections 4.2 and 4.3). Short exposures to SO2 can 
elicit respiratory effects, particularly in individuals with asthma 
(ISA, p. 1-17). Under conditions of elevated breathing rates (e.g., 
while exercising), SO2 penetrates into the tracheobronchial 
region,\34\ where, in sufficient concentration, it results in responses 
linked to asthma exacerbation in individuals with asthma (ISA, sections 
4.2, 4.3, and 5.2). More specifically, bronchoconstriction,\35\ which 
is characteristic of an asthma attack, is the most sensitive indicator 
of SO2-induced lung function effects (ISA, p. 5-8). 
Associated with this bronchoconstriction response is an increase in 
airway resistance which is an index of airway hyperresponsiveness 
(AHR).\36\ Exercising individuals without asthma have also been found 
to exhibit such responses, but at much higher SO2 exposure 
concentrations (ISA, section 5.2.1.7). For example, the ISA finds that 
``healthy adults are relatively insensitive to the respiratory effects 
of SO2 below 1 ppm'' (ISA, p. 5-9).
---------------------------------------------------------------------------

    \33\ The term ``upper respiratory tract'' refers to the portion 
of the respiratory tract, including the nose, mouth and larynx, that 
precedes the tracheobronchial region (ISA, sections 4.2 and 4.3).
    \34\ 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.
    \35\ The term bronchoconstriction refers to constriction or 
narrowing of the airways in the respiratory tract.
    \36\ 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 (ISA, section 5.2.1.2).
---------------------------------------------------------------------------

    Based on assessment of the currently available evidence, as in the 
last review, the ISA concludes that there is a causal relationship 
between short-term SO2 exposures (as short as a few minutes) 
and respiratory effects (ISA, section 5.2.1). The clearest evidence for 
this causal relationship comes from the long-standing evidence base of 
controlled human exposure studies (U.S. EPA, 1994; 2008 ISA). These 
studies demonstrate asthma exacerbation-related lung function 
decrements \37\ and respiratory symptoms (e.g., cough, chest tightness 
and wheeze) in people with asthma exposed to SO2 for 5 to 10 
minutes at elevated breathing rates (ISA, section 5.2.1). 
Bronchoconstriction, evidenced by decrements in lung function, that are 
sometimes accompanied by respiratory symptoms (e.g., cough, wheeze, 
chest tightening and shortness of breath), is observed to occur in 
these studies at SO2 concentrations as low as 200 ppb in 
some people with asthma exposed while breathing at elevated rates, such 
as during exercise (ISA, section 5.2.1.2).\38\ In contrast, respiratory 
effects are not generally observed in other people with asthma 
(nonresponders) and healthy adults exposed, while exercising, to 
SO2 concentrations below 1000 ppb (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 elevated 
breathing rates, such as exercise or with mouthpiece exposures that 
involve laboratory-facilitated rapid, deep breathing.\39\ With these 
conditions, breathing shifts from nasal breathing to oral/nasal 
breathing, which increases the concentrations of SO2 
reaching the tracheobronchial region of lower airways, where, depending 
on dose and the exposed individual's susceptibility, it may cause 
bronchoconstriction (ISA, sections 4.1.2.2, 4.2.2, and 5.2.1.2).
---------------------------------------------------------------------------

    \37\ 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 (FEV1) (ISA, section 5.2.1.2).
    \38\ The data from controlled human exposure studies of people 
with asthma indicate that there are two subpopulations that differ 
in their airway responsiveness to SO2, with the second 
subpopulation being insensitive to SO2 
bronchoconstrictive effects at concentrations as high as 1000 ppb 
(ISA, pp. 5-14 to 5-21; Johns et al., 2010).
    \39\ Laboratory-facilitated rapid deep breathing involves rapid, 
deep breathing through a mouthpiece that provides a mixture of 
oxygen with enough carbon dioxide to prevent an imbalance of gases 
in the blood usually resulting from hyperventilation. Breathing in 
the laboratory with this technique is referred to as eucapnic 
hypernea.
---------------------------------------------------------------------------

    The evidence base of controlled human exposure studies for people 
with asthma \40\ 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, as well as section 
5.2.1.2 of the ISA. The main responses observed include increases in 
specific airway resistance (sRaw) and reductions in forced expiratory 
volume in one second (FEV1) after 5- to 10-minute exposures. 
As recognized in the last review, the results of these studies indicate 
that among individuals with asthma, some individuals have a greater 
response to SO2 than others or a measurable response at 
lower exposure concentrations (ISA, p. 5-14). The SO2-
induced bronchoconstriction in these studies occurs rapidly, in as 
little as two minutes from exposure start, and is transient, with 
recovery occurring upon cessation of exposure (ISA, p. 5-14; Table 5-
2).
---------------------------------------------------------------------------

    \40\ The subjects in these studies have primarily been adults. 
The exception has been a few studies conducted in adolescents aged 
12 to 18 years of age (ISA, pp. 5-22 to 5-23; PA, sections 3.2.1.3 
and 3.2.1.4).
---------------------------------------------------------------------------

    The epidemiologic evidence, some of which is newly available since 
the time of the last review, includes studies reporting positive 
associations for asthma-related hospital admissions of children or 
emergency department visits by children with short-term SO2 
exposures (ISA, section 5.2.1). These findings provide evidence 
supportive of the EPA's conclusion of a causal relationship between 
short-term SO2 exposures and respiratory effects, for which 
the controlled human exposure studies are the primary basis (ISA, 
section 5.2.1.9). With regard to newly available epidemiologic studies, 
there are a limited number of such studies that have investigated 
SO2 effects related to asthma exacerbation, with the most 
supportive evidence coming from studies on asthma-related emergency 
department visits by children and hospital admissions of children (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 
\41\ or other copollutants (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 (ISA, p. 5-
75).
---------------------------------------------------------------------------

    \41\ The potential for confounding by PM is of particular 
interest given that SO2 is a precursor to PM (ISA, p. 1-
7).
---------------------------------------------------------------------------

    The evidence base for long-term \42\ SO2 exposure and 
respiratory effects is somewhat augmented since the last review such 
that the ISA in the current review concludes it to be suggestive of,

[[Page 26763]]

but not sufficient to infer, a causal relationship (ISA, section 
5.2.2). The support for this conclusion comes mainly from the limited 
epidemiologic 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 (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 (ISA, 
section 1.6.1.2). Further, there are uncertainties that apply to the 
epidemiologic evidence, including newly available evidence, across the 
respiratory effects examined for long-term exposure (ISA, section 
5.2.2.7).
---------------------------------------------------------------------------

    \42\ In evaluating the health effects studies in the ISA, the 
EPA has generally categorized exposures of durations longer than a 
month as ``long-term'' (ISA, p. 1-2).
---------------------------------------------------------------------------

    For effects other than respiratory effects, the current evidence is 
generally similar to the evidence available in the last review, and 
leads to similar conclusions. With regard to a relationship between 
short-term SO2 exposure and total mortality, the ISA reaches 
the same conclusion as the previous review that the evidence is 
suggestive of, but not sufficient to infer, a causal relationship (ISA, 
section 5.5.1). This conclusion is based on the evidence of previously 
and newly available multicity epidemiologic studies that provide 
consistent evidence of positive associations coupled with 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 
SO2-mortality associations in copollutant models with 
nitrogen dioxide and particulate matter with mass median aerodynamic 
diameter below 10 microns, and the lack of a potential biological 
mechanism for mortality following short-term SO2 exposures 
(ISA, section 1.6.2.4).
    For other categories of health effects,\43\ the currently available 
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 (ISA, sections 
5.3.1, 5.3.2, 5.4, 5.5.2, 5.6). These conclusions are consistent with 
those made in the previous review (ISA, p. xlviii).
---------------------------------------------------------------------------

    \43\ 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 exposures (ISA, section 1.6.2 and Table 1-
1).
---------------------------------------------------------------------------

    Thus, the current health effects evidence supports the primary 
conclusion that short-term exposure to SO2 in ambient air 
causes respiratory effects, in particular, asthma exacerbation in 
individuals with asthma; this evidence and these conclusions are also 
consistent with that available in the last review. The focus in this 
review, as in prior reviews, is on such effects.
2. At-Risk Populations
    In this document, we use the term ``at-risk populations'' \44\ to 
recognize populations that have a greater likelihood of experiencing 
SO2-related health effects, i.e. groups with characteristics 
that contribute to an increased risk of SO2-related health 
effects. In identifying factors that increase risk of SO2-
related health effects, we have considered evidence regarding factors 
contributing to increased susceptibility, which generally include 
intrinsic factors, such as physiological factors that may influence the 
internal dose or toxicity of a pollutant, or extrinsic factors, such as 
sociodemographic or behavioral factors (ISA, p. 6-1).
---------------------------------------------------------------------------

    \44\ As noted in section I above, we use the term ``at-risk 
populations'' to refer to persons having a quality or characteristic 
in common, such as a specific pre-existing illness or a specific age 
or lifestage for which there is an increased risk of SO2-
related health effects.
---------------------------------------------------------------------------

    The information newly available in this review has not 
substantially altered our previous understanding of at-risk populations 
for SO2 in ambient air. As in the last review, people with 
asthma are at increased risk for SO2-related health effects, 
specifically for respiratory effects, and specifically asthma 
exacerbation elicited by short-term exposures while breathing at 
elevated rates (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 with additional support from epidemiologic studies (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 ISA (relying on a framework for evaluating the 
evidence for risk factors that has been developed since the last 
review) \45\ indicates the evidence to be suggestive of increased risk 
for these groups, with some limitations and inconsistencies (ISA, 
sections 6.5.1.1 and 6.5.1.2).\46\
---------------------------------------------------------------------------

    \45\ Since the 2010 review of the primary SO2 NAAQS, 
the EPA has developed a formal framework to transparently 
characterize the strength of the evidence that can inform the 
identification of populations and lifestages at increased risk of a 
health effect related to exposure to a pollutant. This framework is 
part of the systematic approach taken in the ISA for this review 
(ISA, section 6.2).
    \46\ The current evidence for risk to older adults relative to 
other lifestages comes from epidemiologic studies, for which 
findings are somewhat inconsistent, and studies with which there are 
uncertainties in the association with the health outcome (ISA, 
section 6.5.1.2).
---------------------------------------------------------------------------

    Children with asthma, however, may be particularly at risk compared 
to adults with asthma (ISA, section 6.3.1). This conclusion reflects 
several characteristics of children as compared to adults, which 
include their greater responsiveness to methacholine,\47\ a chemical 
that can elicit bronchoconstriction in people with asthma, as well as 
their greater use of oral breathing, particularly by boys (ISA, 
sections 5.2.1.2 and 4.1.2). Oral breathing (vs. nasal breathing) and 
increased breathing rate 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 (ISA, sections 4.2.2, 5.2.1.2, and 6.3.1). 
Although the epidemiological evidence includes a number of studies 
focused on health outcomes in children that are supportive of the 
qualitative conclusions of causality (ISA, section 5.2.1.2), there are 
few controlled human exposure studies to inform our

[[Page 26764]]

understanding of exposure concentrations associated with effects in 
this population group. Those studies have not included subjects younger 
than 12 years (ISA, p. 5-22). Some characteristics particular to 
school-age children younger than 12 years, such as increased propensity 
for mouth breathing (ISA, p. 4-5), however, suggest that this age group 
of children with asthma might be expected to experience larger lung 
function decrements than adults with asthma (ISA, p. 5-25).\48\
---------------------------------------------------------------------------

    \47\ The ISA concluded that potential differences in airway 
responsiveness of children to SO2 relative to adolescents 
and adults may be inferred by differences in responses to 
methacholine (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. However, results of studies of the extent to 
which airway response to methacholine is predictive of 
SO2 responsiveness have varied somewhat. For example, 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 that were hyperresponsive to methacholine were also 
hyperresponsive to SO2 (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 SO2 and methacholine (ISA, 
section 5.2.1.2; Horstman et al., 1986).
    \48\ The ISA does not find the evidence to be adequate to 
conclude differential risk status for subgroups of children with 
asthma (ISA, Chapter 6). In consideration of the limited information 
regarding factors related to breathing habit, however, and 
recognizing the lack of evidence from controlled human exposure 
studies of SO2-induced lung function decrements in 
children, approximately 5 to 11 years of age, with asthma, the ISA 
suggests that this age group of children and ``particularly boys and 
perhaps obese children, might be expected to experience greater 
responsiveness (i.e., larger decrements in lung function) following 
exposure to SO2 than normal-weight adolescents and 
adults'' (ISA, p. 4-7 and 5-36).
---------------------------------------------------------------------------

    Additionally, some individuals with asthma have a greater response 
to SO2 than others with similar disease status (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 examined differences in lung function response using individual 
subject data available from five studies of individuals with asthma 
exposed to multiple concentrations of SO2 for 5 to 10 
minutes while breathing at elevated rates (Johns et al., 2010). As 
noted in the 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'' (ISA, p. 
5-20). While such information provides documentation 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 (ISA, pp. 5-14 to 5-21).
    The current evidence for factors evaluated in the ISA other than 
asthma status and lifestage is inadequate to determine whether they 
(e.g., sex and SES) might have an influence on risk of SO2-
related effects (ISA, section 6.6).
3. Exposure Concentrations Associated With Health Effects
    Our understanding of exposure duration and concentrations 
associated with SO2-related health effects is largely based, 
as it was in the last review, on the longstanding evidence base of 
controlled human exposure studies. These studies demonstrate a dose-
response relationship between 5- and 10-minute SO2 exposure 
concentrations and decrements in lung function (e.g., increased sRaw 
and reduced FEV1) and occurrence of respiratory symptoms in 
individuals with asthma exposed while breathing at elevated rates (ISA, 
section 1.6.1.1). Clear and consistent increases in these effects occur 
with increasing SO2 exposure (ISA, Table 5-2 and pp. 5-35, 
5-39). Further, the SO2-induced bronchoconstriction occurs 
rapidly; 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 
(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 (ISA, p. 5-14; Linn 
et al., 1984). Thus, the controlled human exposure evidence base 
demonstrates the occurrence of SO2-related effects as a 
result of peak exposures on the order of minutes.\49\
---------------------------------------------------------------------------

    \49\ As the air quality metrics in the epidemiologic studies are 
for time periods longer than the 5- to 10-minute exposures eliciting 
effects in the controlled human exposure studies, these studies may 
not adequately capture the spatial and temporal variation in 
SO2 concentrations and cannot address whether observed 
associations of asthma-related emergency room visits or hospital 
admissions with 1-hour to 24-hour ambient air concentration metrics 
are indicative of a potential response to exposure on the order of 
hours or much shorter-term exposure to peaks in SO2 
concentration (ISA, pp. 5-49, 5-59, 5-25).
---------------------------------------------------------------------------

    The controlled human exposure study findings \50\ demonstrate that 
SO2 concentrations as low as 200 to 300 ppb for 5 to 10 
minutes elicited moderate or greater lung function decrements, measured 
as a decrease in FEV1 of at least 15% or an increase in sRaw 
of at least 100%, in the study subjects (ISA, sections 1.6.1.1 and 
5.2.1). The percent of individuals affected, the severity of response, 
and the accompanying occurrence of respiratory symptoms increased with 
increasing SO2 exposure concentrations (ISA, section 5.2.1). 
At concentrations ranging from 200 to 300 ppb, the lowest levels for 
which the ISA describes SO2-related lung function decrements 
(in terms of 15% reductions in FEV1 or doubling or tripling 
of sRaw), as many as 33% of exercising study subjects with asthma 
experienced moderate or greater decrements in lung function (ISA, 
section 5.2.1, Table 5-2). Analyses focused on subjects with asthma in 
multiple studies that are responsive to SO2 at exposure 
concentrations below 1000 ppb found there to be statistically 
significant increases in lung function decrements occurring at 300 ppb 
(ISA, p. 153; Johns et al., 2010). At concentrations at or above 400 
ppb, moderate or greater decrements in lung function occurred in 20 to 
60% of exercising individuals with asthma and a larger percentage of 
individuals with asthma experienced more severe decrements in lung 
function (i.e., an increase in sRaw of at least 200%, and/or a 20% or 
more decrease in FEV1), compared to exposures at 200 to 300 
ppb (ISA, section 5.2.1.2, p. 5-9 and Table 5-2). Additionally, at 
concentrations at or above 400 ppb, moderate or greater decrements in 
lung function were frequently accompanied by respiratory symptoms, such 
as cough, wheeze, chest tightness, or shortness of breath, with some of 
these findings reaching statistical significance at the study group 
level (ISA, Table 5-2 and section 5.2.1).
---------------------------------------------------------------------------

    \50\ The findings summarized in Table 5-2 of the ISA and in 
Table 3-1 of the PA are based on results that have been adjusted for 
effects of exercise in clean air so that they have separated out any 
effect of exercise in causing bronchoconstriction and reflect only 
the SO2-specific effect.
---------------------------------------------------------------------------

    The lowest exposure concentration for which individual study 
subject data are available in terms of the sRaw and FEV1 
from studies that have assessed the SO2 effect versus the 
effect of exercise in clean air is 200 ppb (ISA, Table 5-2 and Figure 
5-1). In nearly all of these studies (and all of the studies for 
concentrations below 500 ppb), study subjects breathed freely (e.g., 
without using a mouthpiece).\51\ In 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% (ISA, Table 5-2 and Figure 5-2; PA, 
Table 3-1; Linn et al., 1983a; Linn et al., 1987).
---------------------------------------------------------------------------

    \51\ 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).
---------------------------------------------------------------------------

    With regard to exposure concentrations below 200 ppb, the very 
limited available evidence is for

[[Page 26765]]

exposure as low as 100 ppb. Some differences in methodology and the 
reporting of results complicate comparisons of the studies of 100 ppb 
exposure with studies of higher concentrations. In the studies testing 
this concentration, 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; ISA, section 5.2.1.2; PA, section 3.2.1.3). Additionally, 
only a few of these studies included an exposure to clean air while 
exercising that would have allowed for determining the effect of 
SO2 versus the effect of exercise in causing 
bronchoconstriction (Sheppard et al., 1981, 1984; Koenig et al., 1989). 
In those cases, a limited number of adult and adolescent study subjects 
were reported to experience small changes in sRaw, with the magnitudes 
of change appearing to be smaller than responses reported from studies 
at exposure concentrations of 200 ppb or more.52 53 Thus, 
the set of studies for the 100 ppb exposure concentration, while 
limited and complicated by differences from studies of higher 
concentrations with regard to reporting of results and exposure method, 
does not indicate this exposure concentration to result in as much as a 
doubling in sRaw, based on the extremely few adults and adolescents 
tested (Sheppard et al., 1981, 1984; Koenig et al., 1989).
---------------------------------------------------------------------------

    \52\ 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 slightly less than half the response of these subjects at 
250 ppb, and the results for the study by Sheppard et al. (1984) 
indicate that none of the eight study subjects experienced as much 
as a doubling in sRaw in response to the mouthpiece exposure to 125 
ppb while exercising. In the study of adolescents (aged 12 to 18 
years), among the three individual study subjects for which 
respiratory resistance appears to have increased with SO2 
exposure, the magnitude of any increase after consideration of the 
response to exercise appears to be less than 100% in each subject 
(Koenig et al., 1989).
    \53\ In a mouthpiece exposure system, 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; ISA, section 4.1.2.2). This allowance of greater 
penetration of SO2 into the tracheobronchial airways, as 
well as limited evidence comparing responses by mouthpiece and 
chamber exposures, leads to the expectation that SO2-
responsive people with asthma breathing SO2 using a 
mouthpiece, particularly while breathing at elevated rates, would 
experience greater lung function responses than if exposed to the 
same test concentration while freely breathing in an exposure 
chamber (ISA, p. 5-23; Linn et al., 1983b).
---------------------------------------------------------------------------

    Specific exposure concentrations that may be eliciting respiratory 
responses are not available from the epidemiological studies that find 
associations with outcomes such as asthma-related emergency department 
visits and hospitalizations. For example, in noting limitations of 
epidemiologic studies with regard to uncertainties in SO2 
exposure estimates, the ISA recognized that ``[it] is unclear whether 
SO2 concentrations at the available fixed site monitors 
adequately represent variation in personal exposures especially if peak 
exposures are as important as indicated by the controlled human 
exposure studies'' (ISA, p. 5-37). This extends the observation of the 
2008 ISA that ``it is possible that these epidemiologic associations 
are determined in large part by peak exposures within a 24-h[our] 
period'' (2008 ISA, p. 5-5). Given the important role of SO2 
as a precursor to PM in ambient air, however, a key uncertainty in the 
epidemiologic evidence available in this review, as in the last review, 
is potential confounding by copollutants, particularly PM (ISA, p. 5-
5). Among the U.S. epidemiologic studies reporting mostly positive and 
sometimes statistically significant associations between ambient 
SO2 concentrations and emergency department visits or 
hospital admissions (some conducted in multiple locations), few studies 
have attempted to address this uncertainty, e.g., through the use of 
copollutant models. 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.\54\ No additional such studies have been newly identified in this 
review that might inform this issue. Thus, such uncertainties regarding 
copollutant confounding, as well as exposure measurement error, remain 
in the currently available epidemiologic evidence base (ISA, p. 5-6).
---------------------------------------------------------------------------

    \54\ 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; PA, Appendix D).
---------------------------------------------------------------------------

4. Potential Impacts on Public Health
    In general, the magnitude and implications of potential impacts on 
public health are dependent upon the type and severity of the effect, 
as well as the size and other features of the population affected (ISA, 
section 1.7.4; PA, 3.2.1.5). 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 above in section II.B.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 breathing rate; asthma-related health outcomes such as 
emergency department visits and hospital admissions have also been 
statistically associated with ambient air SO2 concentration 
metrics in epidemiologic studies (ISA, section 5.2.1.9).
    As summarized in section II.B.2 above, people with asthma are the 
population at risk for SO2-related effects and children with 
asthma are considered to be at relatively greater risk than other age 
groups within this at-risk population (ISA, section 6.3.1). The 
evidence supporting this conclusion comes primarily from studies of 
individuals with mild to moderate asthma,\55\ with very little evidence 
available for individuals with severe asthma. The evidence base of 
controlled human exposure studies of exercising people with asthma 
provides very limited information indicating that there are similar 
responses (in terms of relative decrements in lung function in response 
to SO2 exposures) of individuals with differences in 
severity of their asthma.\56\ However, the two 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'' (ISA, p. 5-22; Linn et al., 1987; Trenga et al., 1999). 
Consideration

[[Page 26766]]

of such baseline differences among members of at-risk populations and 
of the relative transience or persistence of these responses (e.g., as 
noted in section II.B.3 above), as well as other factors, is important 
to characterizing implications for public health, as recognized by the 
ATS in their recent statement on evaluating adverse health effects of 
air pollution (Thurston et al., 2017).
---------------------------------------------------------------------------

    \55\ These studies categorized 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 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 their ability to engage in moderate to 
heavy levels of exercise (ISA, p. 5-22; Johns et al., 2010; Reddel, 
2009).
    \56\ The 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). The ISA 
states, ``[h]owever, both studies suggest that adults with moderate/
severe asthma may have more limited reserve to deal with an insult 
compared with individuals with mild asthma'' (ISA, p. 5-22). 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'' (ISA, p. 5-22; Johns 
et al., 2010; Reddel, 2009).
---------------------------------------------------------------------------

    The Administrator's judgment is informed by statements by the ATS 
on what constitutes an adverse health effect of air pollution. Building 
on the earlier statement by the ATS that was considered in the last 
review (ATS, 2000), the 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). The earlier ATS statement, in addition 
to emphasizing clinically relevant effects (e.g., the adversity of 
small transient changes in lung function metrics in combination with 
respiratory symptoms), 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 could put its members at potentially increased risk of effects 
from another agent (ATS, 2000). The consideration of effects on 
individuals with preexisting diminished lung function continues to be 
recognized as important in the more recent ATS statement (Thurston et 
al., 2017). For example, in adding emphasis in this area, this 
statement conveys the view that ``small lung function changes'' in 
individuals with compromised function, such as that resulting from 
asthma, should be considered adverse, even without accompanying 
respiratory symptoms (Thurston et al., 2017). All of these concepts, 
including the consideration of the magnitude of effects occurring in 
just a subset of study subjects, are recognized as important in the 
more recent ATS statement (Thurston et al., 2017) and continue to be 
relevant to consideration of the evidence base for SO2.
    Such concepts are routinely considered by the Agency in weighing 
public health implications for decisions on primary NAAQS, as 
summarized in section I.A above. For example, in deliberations on a 
standard that provides the requisite public health protection under the 
Act, the EPA traditionally recognizes the nature and severity of the 
health effects involved, recognizing the greater public health 
significance of more severe health effects, including, for example, 
effects that have been documented to be accompanied by symptoms, and of 
the risk of repeated occurrences of effects (76 FR 54308, August 31, 
2011; 80 FR 65292, October 26, 2015). Another area of consideration is 
characterization of the population at risk, including its size and, as 
pertinent, the exposure/risk estimates in this regard. Such factors 
related to public health significance, and the kind and degree of 
associated uncertainties, are considered by the EPA in addressing the 
CAA requirement that the primary NAAQS are requisite to protect public 
health, including a margin of safety, as summarized in section I.A 
above.
    Ambient air concentrations of SO2 vary considerably in 
areas near sources, but concentrations in the vast majority of the U.S. 
are well below the current standard (PA, Figure 2-7). Thus, while the 
population counts discussed below may convey information and context 
regarding the size of populations living near sizeable sources in some 
areas, the concentrations in most areas of the U.S. are well below the 
conditions assessed in the REA.
    With regard to the size of the U.S. population at risk of 
SO2-related effects, the National Center for Health 
Statistics data from the 2015 National Health Interview Survey (NHIS) 
\57\ indicate that approximately 8% of the U.S. population has asthma 
(PA, Table 3-2; CDC, 2017). Among all U.S. adults, the prevalence is 
estimated to be 7.6%, with women having a higher estimate (9.7%) than 
men (5.4%). The estimated prevalence is greater in children (8.4% for 
children less than 18 years of age) than adults (7.6%) (PA, Table 3-2; 
CDC, 2017). Asthma was the leading chronic illness affecting children 
in 2012, the most recent year for which such an evaluation is available 
(Bloom et al., 2013). As noted in the PA, there are more than 24 
million people with asthma currently in the U.S., including more than 6 
million children (PA, sections 3.2.2.4 and 3.2.4).
---------------------------------------------------------------------------

    \57\ The NHIS is conducted annually by the U.S. Centers for 
Disease Control and Prevention. The NHIS collects health information 
from a nationally representative sample of the noninstitutionalized 
U.S. civilian population through personal interviews. Participants 
(or parents of participants if the survey participant is a child) 
who have ever been told by a doctor or other health professional 
that the participant had asthma and reported that they still have 
asthma were considered to have current asthma. Data are weighted to 
produce nationally representative estimates using sample weights; 
estimates with a relative standard error greater than or equal to 
30% are generally not reported (Mazurek and Syamlal, 2018). The NHIS 
estimates described here are drawn from the 2015 NHIS, Table 4-1 
(https://www.cdc.gov/asthma/nhis/2015/table4-1.htm).
---------------------------------------------------------------------------

    Relatively greater population-level SO2 impacts (i.e., 
greater numbers of individuals affected) might be expected in 
population groups with relatively greater asthma prevalence (i.e., 
groups with relatively higher percentages of individuals that have 
asthma). Among all U.S. children, the asthma prevalence estimate is 
greater for boys than girls (CDC, 2017). Asthma prevalence estimates 
from the 2015 NHIS vary for children of different races or ethnicities 
and household income, among other factors (CDC, 2017). Among 
populations of different races or ethnicities, black non-Hispanic and 
Puerto Rican Hispanic children are estimated to have the highest 
prevalences, at 13.4% and 13.9%, respectively. Asthma prevalence is 
also increased among populations in poverty, with the prevalence 
estimated to be 11.1% among people living in households below the 
poverty level compared to 7.2% of those living above it.
    The information on which to base estimates of asthma prevalence in 
other subgroups of children is much more limited (e.g., as discussed in 
the REA, section 4.1.2). For example, the more limited information from 
the NHIS for 2011-2015 indicates there to be a greater prevalence of 
asthma in children that are obese \58\ compared to those that are not 
(REA, section 4.1.2, Figure 4-2).\59\
---------------------------------------------------------------------------

    \58\ Although the CDC does not report NHIS estimates for the 
percent of obese adults or children that have asthma, they do report 
that that more adults with asthma are obese than adults without 
asthma. As discussed in the REA, the NHIS sample size for children 
with asthma identified as obese is very limited (REA, section 
4.1.2).
    \59\ In consideration of the limited information regarding 
factors related to breathing habit (whether one is breathing through 
their nose or mouth) and recognizing the lack of evidence from 
controlled human exposure studies of SO2-induced lung 
function decrements in children, approximately 5 to 11 years of age, 
with asthma, the ISA suggests that this age group of children and 
``particularly boys and perhaps obese children, might be expected to 
experience greater responsiveness (i.e., larger decrements in lung 
function) following exposure to SO2 than normal-weight 
adolescents and adults'' (ISA, pp. 4-7 and 5-36). However, the ISA 
does not find the evidence to be adequate to conclude differential 
risk status for subgroups of children with asthma (ISA, Chapter 6).
---------------------------------------------------------------------------

    With regard to the potential for exposure of the populations at 
risk from exposures to SO2 in ambient air, the PA recognizes 
that while SO2 concentrations have generally declined across 
the U.S. since 2010 when the current standard was set (PA, Figures 2-5 
and 2-6), there are numerous areas where SO2 concentrations 
still contribute to air quality that is near or above the standard. For 
example, the

[[Page 26767]]

most recently available design values for the primary SO2 
standard (those based on monitoring data for the 2014-2016 period) 
indicate there to be 15 core-based statistical areas \60\ with design 
values above the existing standard level of 75 ppb, of which a number 
have sizeable populations.\61\ In addition to this evidence of elevated 
ambient air SO2 concentrations, there are limitations in the 
monitoring network with regard to the extent that it might be expected 
to capture all areas with the potential to exceed the standard (e.g., 
75 FR 35551; June 22, 2010).\62\ In recognition of these limitations, 
the PA also examined the proximity of populations to sizeable 
SO2 point sources using the most recently available 
emissions inventory information (2014), which is also characterized in 
the ISA (ISA, section 2.2.2).\63\ This information indicates that there 
are more than 300,000 and 60,000 children living within 1 km of 
facilities emitting at least 1,000 and 2,000 tpy of SO2, 
respectively. Within 5 km of such sources, the numbers are 
approximately 1.4 million and 700,000, respectively (PA, Table 3-5). 
While information on SO2 concentrations in locations of 
maximum impact of such sources is not available for all these areas, 
and SO2 concentrations vary appreciably near sources, simply 
considering the 2015 national estimate of asthma prevalence of 
approximately 8% (noted above), this information would suggest there 
may be as many as 24,000 to more than 100,000 children with asthma that 
live in areas near substantially sized sources of SO2 
emissions to ambient air (PA, section 3.2.1.5; Table 3-5).
---------------------------------------------------------------------------

    \60\ Core-based statistical area (CBSA) is a geographic area 
defined by the U.S. Office of Management and Budget to consist of an 
urban area of at least 10,000 people in combination with its 
surrounding or adjacent counties (or equivalents) with which there 
are socioeconomic ties through commuting (https://www.census.gov/geo/reference/gtc/gtc_cbsa.html). Populations in the 15 CBSAs 
referred to in the body of the text range from approximately 30,000 
to more than a million (based on 2016 U.S. Census Bureau estimates).
    \61\ Table 5c. Monitoring Site Listing for Sulfur Dioxide 1-Hour 
NAAQS in the Excel file labeled 
So2_designvalues_20142016_final_07_19_16.xlsx downloaded from 
https://www.epa.gov/air-trends/air-quality-design-values on January 
26, 2018.
    \62\ As state and local air agencies have the flexibility to 
characterize air quality using either modeling of actual source 
emissions or using appropriately sited ambient air monitors for 
designation purposes, both types of information have been used to 
support designations of areas not meeting the standard. To date, 42 
areas have been designated as nonattainment areas, although air 
quality improvements in two of these 42 areas has led to the areas 
meeting the standard and being redesignated. The population residing 
in the remaining 40 nonattainment areas is approximately 3.3 million 
people (see https://www3.epa.gov/airquality/greenbook/tnsum.html). 
Detailed information about source types in these areas can be found 
in the technical support documents for individual nonattainment 
areas, available via https://www.epa.gov/sulfur-dioxide-designations/sulfur-dioxide-designations-regulatory-actions. These 
areas generally had significant SO2 point sources, with 
the majority of these point sources being electric generating units.
    \63\ Although source characteristics and meteorological 
conditions--in addition to magnitude of emissions--influence the 
distribution of concentrations in ambient air, these estimates are 
based on proximity to large sources, rather than ambient 
concentrations, due to limitations in the available information with 
regard to spatial (and temporal) patterns of SO2 
concentrations in the proximity of such sources in urban areas (ISA, 
section 2.5.2.2).
---------------------------------------------------------------------------

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

C. Summary of Risk and Exposure Information

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

[[Page 26768]]

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

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

    As indicated by this case study approach to assessing exposure and 
risk, the analyses in the REA are intended to provide assessments of an 
air quality scenario just meeting the current standard for a small, 
diverse set of study areas and associated exposed at-risk populations 
that will be informative to the EPA's consideration of potential 
exposures and risks that may be associated with the air quality 
conditions occurring under the current SO2 standard. The REA 
analyses are not designed to provide a comprehensive national 
assessment of such conditions (REA, section 2.2). The objective of the 
REA is not to present an exhaustive analysis of exposure and risk in 
areas of the U.S. that currently just meet the standard and/or of 
exposure and risk associated with air quality adjusted to just meet the 
standard in areas that currently do not meet the standard.\65\ Rather, 
the purpose is to assess, based on current tools and information, the 
potential for exposures and risks beyond those indicated by the 
information available at the time the current standard was established. 
Accordingly, capturing an appropriate diversity in study areas and air 
quality conditions (that reflect the current standard scenario) is 
important to the role of the REA in informing the EPA's conclusions on 
the public health protection afforded by the current standard (PA, 
section 3.2.2.2).
---------------------------------------------------------------------------

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

    A broad variety of spatial and temporal patterns of SO2 
concentrations can exist when ambient air concentrations just meet the 
current standard. These patterns will vary due to many factors 
including the types of emissions sources in a study area and several 
characteristics of those sources, such as magnitude of emissions and 
facility age, use of various control technologies, patterns of 
operation, and local factors, as well as local meteorology. Estimates 
derived by the particular analytical approaches and methodologies used 
to describe the study area-specific air quality provide an indication 
of this variability in the spatial and temporal patterns of 
SO2 concentrations associated with air quality conditions 
just meeting the current standard, while recognizing the associated 
uncertainty in these concentration estimates.
    In this regard, the REA presents results from two different 
approaches to adjusting air quality. The first approach uses the 
highest design value across all modeled air quality receptors to adjust 
the air quality concentrations in each area to just meet the standard 
(REA, section 3.4). This is done by estimating the amount of 
SO2 concentration reduction needed for concentrations at 
this highest receptor to be adjusted to just meet the current standard. 
Based on this amount, all other receptors impacted by the highest 
source(s) are adjusted proportionately. The second approach is included 
in the REA as a sensitivity analysis in recognition of the potential 
uncertainty associated with the estimated concentrations across the 
modeling domain, particularly the very highest concentrations. 
Accordingly, the second approach uses the air quality receptor having 
the 99th percentile of the distribution of design values (instead of 
the receptor having the maximum design value) to estimate the 
SO2 concentration reductions needed to adjust the air 
quality to just meet the standard (REA, section 6.2.2.2).
    Consistent with the health effects evidence summarized in section 
II.B above, the focus of the REA is on short-term (5-minute) exposures 
of individuals in the population with asthma during times when they are 
breathing at an elevated rate. Five-minute concentrations in ambient 
air were estimated for the current standard scenario using a 
combination of 1-hour concentrations from the EPA's preferred near-
field dispersion model, the American Meteorological Society/EPA 
regulatory model (AERMOD), with adjustment such that they just meet the 
current standard, and relationships between 1-hour and 5-minute 
concentrations occurring in the local ambient air monitoring data. Air 
quality modeling with AERMOD is used to capture the spatial variation 
in ambient SO2 concentrations across an urban area, which 
can be relatively high in areas affected by large point sources, and 
which the limited number of monitoring locations in each area is 
unlikely to capture. This provides 1-hour concentrations at model 
receptor sites across the modeling domain across the 3-year modeling 
period (consistent with the 3-year form of the standard). These 
concentrations were adjusted such that the air quality modeling 
receptor location with the highest concentrations just met the current 
standard.\66\ Relationships between 1-hour and 5-minute concentrations 
at local monitors were then used to estimate 5-minute concentrations 
associated with the adjusted 1-hour concentrations across the 3-year 
period at all model receptor locations in each of the three study areas 
(REA, section 3.5). In this way, available continuous 5-minute ambient 
air monitoring data (datasets with all twelve 5-minute concentrations 
in each hour) were used to reflect the fine-scale temporal variation in 
SO2 concentrations documented by these data and for which 
air quality modeling is limited, e.g., by limitations in the time steps 
of currently available model input data such as for emissions 
estimates.
---------------------------------------------------------------------------

    \66\ The air quality adjustments were implemented with a focus 
on reducing emissions from the source(s) contributing most to the 
standard exceedances until the areas just met the standard. This 
approach focuses on the concentrations associated with the primary 
contributing source(s), identifying the amount by which they need to 
be adjusted in order for the highest design value across all air 
quality receptors to just meet the current standard (REA, section 
3.4). Based on this amount, all other receptors impacted by the 
highest source(s) are adjusted accordingly. In recognition of the 
potential uncertainty associated with this approach, particularly 
for the highest estimated concentrations, a second approach was also 
evaluated that bases the adjustments on the air quality receptor 
having the 99th percentile of the distribution of design values 
instead of the receptor having the maximum design value (REA, 
section 6.2.2.1).
---------------------------------------------------------------------------

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

[[Page 26769]]

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

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

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

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

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

    \70\ As described in section 4.1.2 and Appendix E of the REA, 
asthma prevalence in the exposure modeling domain is estimated based 
on national prevalence information and study area demographic 
information related to age, sex and poverty status.
---------------------------------------------------------------------------

    As in the last review, the REA for this review uses the APEX model 
estimates of 5-minute exposure concentrations for simulated individuals 
with asthma while breathing at elevated rates to characterize health 
risk in two ways (REA, section 4.5). The first is the percentage of the 
simulated at-risk populations expected to experience days with 5-minute 
exposures, while breathing at elevated rates, that are at or above a 
range of benchmark levels. The second is the percentage of these 
populations expected to experience days with an occurrence of a 
doubling or tripling of sRaw. The benchmark concentrations were 
identified based on consideration of the evidence discussed in section 
II.B above.
    For the benchmark metric, the REA uses benchmark concentrations of 
400 ppb, 300 ppb, 200 ppb based on concentrations included in the well-
documented controlled human exposure studies summarized in section II.B 
above, and also 100 ppb in consideration of uncertainties with regard 
to lower concentrations and population groups with more limited data, 
as discussed in section II.B above (REA, section 4.5.1). At the upper 
end of this range, 400 ppb represents the lowest concentration in free-
breathing controlled human exposure studies of exercising people with 
asthma where moderate or greater lung function decrements occurred that 
were often statistically significant at the group mean level and were 
frequently accompanied by respiratory symptoms, with some increases in 
these symptoms also being statistically significant at the group level 
(ISA, Section 5.2.1.2 and Table 5-2). At 300 ppb, statistically 
significant increases in lung function decrements (specifically reduced 
FEV1) have been documented in analyses of the subset of 
controlled human exposure study subjects with asthma that are 
responsive to SO2 at concentrations below 600 or 1000 ppb 
(ISA, pp. 5-85 and 5-153 and Table 5-21; Johns et al., 2010). The 200 
ppb benchmark concentration represents the lowest level for which 
individual study subject data are available in terms of the sRaw and 
FEV1 from studies that have assessed the SO2 
effect versus the effect of exercise in clean air; moderate or greater 
lung function decrements were documented in some of these study 
subjects (ISA, Table 5-2 and Figure 5-1; PA, Table 3-1; REA, section 
4.6.1). For exposure concentrations below 200 ppb, limited data are 
available for exposures at 100 ppb that, while not directly comparable 
to the data at higher concentrations because of differences in 
methodology and metrics reported,\71\ do not indicate that study 
subjects experienced responses of a magnitude as high as a doubling in 
sRaw. However, in consideration of some study subjects with asthma 
experiencing moderate or greater decrements in lung function at the 200 
ppb exposure concentration (approximately 8 to 9% of the study group) 
and of the paucity or lack of any specific study data for some groups 
of individuals with asthma, such as primary-school-age children and 
those

[[Page 26770]]

with more severe asthma,\72\ a benchmark concentration of 100 ppb (one 
half the lowest exposure concentration tested in free breathing 
exposure studies that assessed the SO2 effect versus the 
effect of exercise in clean air) is also included.
---------------------------------------------------------------------------

    \71\ As explained in section II.B.3 above, these studies 
involved exposures via mouthpiece, and only a few of these studies 
included an exposure to clean air while exercising that would have 
allowed for determining the effect of SO2 versus that of 
exercise in causing bronchoconstriction (ISA, section 5.2.1.2; PA, 
section 3.2.1.3).
    \72\ As summarized in section II.B.3 above, recognizing that 
even the study subjects described as ``moderate/severe'' group (had 
well-controlled asthma, were generally able to withhold medication, 
were not dependent on corticosteroids, and were able to engage in 
moderate to heavy levels of exercise) would likely be classified as 
moderate by today's classification standards (ISA, p. 5-22; Johns et 
al., 2010; Reddel, 2009), we have considered the evidence with 
regard to the response of individuals with severe asthma that are 
not generally represented in the full set of controlled human 
exposure studies. There is no evidence to indicate such individuals 
would experience moderate or greater SO2-related lung 
function decrements at lower SO2 exposure concentrations 
than individuals with moderate asthma. With regard to the severity 
of response, the limited data that are available indicate a similar 
magnitude of relative lung function decrements in response to 
SO2 as that for individuals with less severe asthma, 
although the individuals with more severe asthma are indicated to 
have a larger absolute response and a greater response to exercise 
prior to SO2 exposure, indicating uncertainty in the role 
of exercise versus SO2 and that those individuals ``may 
have more limited reserve to deal with an insult compared with 
individuals with mild asthma'' (ISA, p. 5-22). As noted previously, 
evidence from controlled human exposure studies are not available 
for children younger than 12 years old, and the ISA indicates that 
the information regarding breathing habit and methacholine 
responsiveness for the subset of this age group that is of primary 
school age (e.g., 5-12 years) indicates a potential for greater 
response (ISA, pp. 5-22 to 5.25).
---------------------------------------------------------------------------

    The E-R function for estimating risk of lung function decrements 
was developed from the individual subject results for sRaw from the 
controlled exposure studies of exercising freely breathing people with 
asthma exposed to SO2 concentrations from 1000 ppb down to 
as low as 200 ppb (REA, Table 4-11). Beyond the assessment of these 
studies and their results in past reviews, there has been extensive 
evaluation of the individual subject results, including a data quality 
review in the last primary SO2 NAAQS review (Johns and 
Simmons, 2009), and detailed analysis in two subsequent publications 
(Johns et al., 2010; Johns and Linn, 2011). The sRaw responses reported 
in the controlled exposure studies have been summarized in the ISA in 
terms of percent of study subjects experiencing responses of a 
magnitude equal to a doubling or tripling or more (e.g., ISA, Table 5-
2; Long and Brown, 2018). Across the exposure range from 200 to 1000 
ppb, the percentage of exercising study subjects with asthma having at 
least a doubling of sRaw increases from about 8-9% (at exposures of 200 
ppb) up to approximately 50-60% (at exposures of 1000 ppb) (REA, Table 
4-11). The E-R function was derived from these data using a probit 
function (REA, section 4.6.2).
2. Key Limitations and Uncertainties
    While the general approach and methodology for the exposure-based 
assessment in this review is similar to that used in the last review, 
there are a number of ways in which the current analyses differ and 
incorporate improvements. For example, in addition to an expansion in 
the number and type of study areas assessed, input data and modeling 
approaches have improved in a number of ways, including the 
availability of continuous 5-minute air monitoring data at monitors 
within the three study areas. The REA for the current review extends 
the time period of simulation to a 3-year simulation period, consistent 
with the form established for the now-current standard. Further, the 
years simulated reflect more recent emissions and circumstances 
subsequent to the 2010 decision.
    In characterizing uncertainty associated with the risk and exposure 
estimates in this review, the REA used an approach intended to identify 
and compare the relative impact that important sources of uncertainty 
may have (REA, section 6.2). This approach is a qualitative uncertainty 
characterization approach adapted from the World Health Organization 
(WHO) approach for characterizing uncertainty in exposure assessment 
(WHO, 2008) accompanied by quantitative sensitivity analyses of key 
aspects of the assessment approach (REA, chapter 6).73 74 
The REA considers the limitations and uncertainties underlying the 
analysis inputs and approaches and the extent of their influence on the 
resultant exposure/risk estimates. Consistent with the WHO (2008) 
guidance, the overall impact of the uncertainty is scaled by 
considering the extent or magnitude of the impact of the uncertainty as 
implied by the relationship between the source of the uncertainty and 
the exposure/risk output. The REA also evaluated the direction of 
influence, indicating how the source of uncertainty was judged to 
affect the exposure and risk estimates (e.g., likely to produce over- 
or under-estimates).
---------------------------------------------------------------------------

    \73\ The approach used has been applied in REAs for past NAAQS 
review for nitrogen oxides, carbon monoxide, ozone (U.S. EPA, 2008b; 
2010; 2014d), and SOX (U.S. EPA, 2009).
    \74\ The approach used varies from that of WHO (2008) in that 
the REA approach placed a greater focus on evaluating the direction 
and the magnitude of the uncertainty (i.e., qualitatively rating how 
the source of uncertainty, in the presence of alternative 
information, may affect the estimated exposures and health risk 
results).
---------------------------------------------------------------------------

    Several areas of uncertainty are identified as particularly 
important, with some similarities to those in the last review. 
Generally, these areas of uncertainty include estimation of the spatial 
distribution of SO2 concentrations across each study area 
under air quality conditions just meeting the current standard, 
including the fine-scale temporal pattern of 5-minute concentrations. 
Among other areas, there is also uncertainty with regard to population 
groups and exposure concentrations for which the health effects 
evidence base is limited or lacking (PA, section 3.2.2.3).
    With regard to the spatial distribution of SO2 
concentrations, there is some uncertainty associated with the ambient 
air concentration estimates in the air quality scenarios assessed. A 
more detailed characterization of contributors to this uncertainty is 
presented in the REA (REA, section 6.2), with a general summary 
provided here. Assessment approach-related aspects contributing to this 
uncertainty include the model estimates of 1-hour concentrations and 
the approach employed to adjust the air quality surface to 
concentrations just meeting the current standard,\75\ as well as the 
estimation of 1-hour ambient air concentrations resulting from 
emissions sources not explicitly modeled, all of which influence the 
temporal and spatial pattern of concentrations and associated exposure 
circumstances represented in the study areas (REA, sections 6.2.1 and 
6.2.2). There is also uncertainty in the estimates of 5-minute 
concentrations in ambient air across the modeling receptors in each 
study area. The ambient air monitoring dataset available to inform the 
5-minute estimates, much expanded in this review over the dataset 
available in the last review, is used to draw on relationships 
occurring at one location and over one range of concentrations to 
estimate the fine-scale temporal pattern in concentrations at the other 
locations. While this is an important area of uncertainty in the REA 
results because the ambient air 5-minute concentrations

[[Page 26771]]

are integral to the 5-minute estimates of exposure, the approach used 
to represent fine-scale temporal variability in the three study areas 
is strongly based in the available information and has been evaluated 
in the REA (REA, Table 6-3; sections 3.5.2 and 3.5.3).
---------------------------------------------------------------------------

    \75\ In study areas in which estimated SO2 
concentrations at a very small number of receptors are substantially 
higher than those at all other air quality receptors, the two 
different adjustment approaches investigated in the REA (described 
in section II.C.1 above) can result in very different concentrations 
across the area. In areas with this characteristic, the first 
approach (which involves determining adjustments based on 
concentrations at the very highest receptor locations) generally 
results in appreciably lower concentrations than those associated 
with the second approach at receptor locations beyond the small 
group with the very highest concentrations in the area. This is 
discussed in greater detail in the REA, section 6.2.2.2.
---------------------------------------------------------------------------

    Another important area of uncertainty, particular to interpretation 
of the lung function risk estimates, concerns estimates derived for 
exposure concentrations below those represented in the evidence base 
(REA, Table 6-3). The E-R function on which the risk estimates are 
based generates non-zero predictions of the percentage of the at-risk 
population expected to experience a day with at least a doubling of 
sRaw for all exposures experienced while breathing at an elevated rate. 
The uncertainty in the response estimates increases substantially with 
decreasing exposure concentrations below those well represented in the 
data from the controlled human exposure studies (i.e., below 200 ppb).
    Additionally, the assessment focuses on the daily maximum 5-minute 
exposure during a period of elevated breathing rate, summarizing 
results in terms of the days on which the magnitude of such exposure 
exceeds a benchmark or contributes to a doubling or tripling of sRaw. 
Although there is some uncertainty associated with the potential for 
additional, uncounted events in the same day, the health effects 
evidence indicates a lack of a cumulative effect of multiple exposures 
over several hours or a day (ISA, section 5.2.1.2) and a reduced 
response to repeated exercising exposure events over an hour (Kehrl et 
al., 1987). Further, information is somewhat limited with regard to the 
length of time after recovery from one exposure by which a repeat 
exposure would elicit a similar effect as that of the initial exposure 
event (REA, Table 6-3). Another area of uncertainty concerns the 
potential influence of co-occurring pollutants on the relationship 
between short-term SO2 exposures and respiratory effects. 
For example, there is some limited evidence regarding the potential for 
an increased response to SO2 exposures occurring in the 
presence of other common pollutants such as PM (potentially including 
particulate sulfur compounds), nitrogen dioxide and ozone, although the 
studies are limited (e.g., with regard to their relevance to ambient 
exposures) and/or provide inconsistent results (ISA, pp. 5-23 to 5-26, 
pp. 5-143 to 5-144; 2008 ISA, section 3.1.4.7).\76\
---------------------------------------------------------------------------

    \76\ For example, ``studies of mixtures of particles and sulfur 
oxides indicate some enhanced effects on lung function parameters, 
airway responsiveness, and host defense,'' however, ``some of these 
studies lack appropriate controls and others involve [sulfur-
containing species] that may not be representative of ambient 
exposures'' (ISA, p. 5-144). These toxicological studies in 
laboratory animals, which were newly available in the last review, 
were discussed in greater detail in the 2008 ISA. That ISA stated 
that ``[r]espiratory responses observed in these experiments were in 
some cases attributed to the formation of particular sulfur-
containing species'' yet, ``the relevance of these animal 
toxicological studies has been called into question because 
concentrations of both PM (1 mg/m\3\ and higher) and SO2 
(1 ppm and higher) utilized in these studies are much higher than 
ambient levels'' (2008 ISA, p. 3-30).
---------------------------------------------------------------------------

    Another area of uncertainty, which remains from the last review and 
is important to our consideration of the REA results, concerns the 
extent to which the quantitative results represent the populations at 
greatest risk of effects associated with exposures to SO2 in 
ambient air. As recognized in section II.B, the controlled human 
exposure study evidence base does not include studies of children 
younger than 12 years old and is limited with regard to studies of 
people with more severe asthma.\77\ The limited evidence that informs 
our understanding of potential risk to these groups indicates the 
potential for them to experience greater impacts than other population 
groups with asthma under similar exposure circumstances or, in the case 
of people with severe asthma, to have a more limited reserve for 
addressing this risk (ISA, section 5.2.1.2). Further, we note the lack 
of information on the factors contributing to increased susceptibility 
to SO2-induced bronchoconstriction among some people with 
asthma compared to others (ISA, pp. 5-19 to 5-21). These data 
limitations contribute uncertainty to the exposure/risk estimates with 
regard to the extent to which they represent the populations at 
greatest risk of SO2-related respiratory effects.
---------------------------------------------------------------------------

    \77\ We additionally recognize that limitations in the activity 
pattern information for children younger than five years old 
precluded their inclusion in the populations of children simulated 
in the REA (REA, section 4.1.2).
---------------------------------------------------------------------------

    In summary, among the multiple uncertainties and limitations in 
data and tools that affect the quantitative estimates of exposure and 
risk and their interpretation in the context of considering the current 
standard, several are particularly important. These include 
uncertainties related to estimation of 5-minute concentrations in 
ambient air; the lack of information from controlled human exposure 
studies for the lower, more prevalent, concentrations of SO2 
and limited information regarding multiple exposure episodes within a 
day; the prevalence of different exposure circumstance represented by 
the three study areas; and characterization of particular subgroups of 
people with asthma that may be at greater risk.
3. Summary of Exposure and Risk Estimates
    The REA provides estimates for two simulated at-risk populations: 
Adults with asthma and school-aged children \78\ with asthma (REA, 
section 2.2). Focusing on the at-risk population of children with 
asthma, summarized here are two sets of exposure and risk estimates for 
the 3-year simulation in each study area: (1) The number (and percent) 
of simulated persons experiencing exposures at or above the particular 
benchmark concentrations of interest while breathing at elevated rates; 
and (2) the number and percent of people estimated to experience at 
least one SO2-related lung function decrement in a year and 
the number and percent of people experiencing multiple lung function 
decrements associated with SO2 exposures (detailed results 
are presented in the REA). Both types of estimates for adults with 
asthma are lower, generally due to the lesser amount and frequency of 
time spent outdoors (REA, section 5.2). As described in section II.C.1 
above, the REA provides results for two different approaches to 
adjusting air quality. The estimates summarized here are drawn from the 
results for both approaches.
---------------------------------------------------------------------------

    \78\ The adult population group is comprised of individuals 
older than 18 years of age and school-aged children are individuals 
aged 5 to 18 years old. As in other NAAQS reviews, this REA does not 
estimate exposures and risk for children younger than 5 years old 
due to the more limited information contributing relatively greater 
uncertainty in modeling their activity patterns and physiological 
processes than children between the ages of 5 to 18 (REA, p. 2-8).
---------------------------------------------------------------------------

    Table 1 presents the results for the benchmark-based risk metric in 
terms of the percent of the simulated populations of children with 
asthma estimated to experience at least one daily maximum 5-minute 
exposure per year at or above the different benchmark concentrations 
while breathing at elevated rates under air quality conditions just 
meeting the current standard (REA, Tables 6-8 and 6-9). These estimates 
for the Tulsa study area are much lower than those for the other two 
areas (Table 1). No individuals of the simulated at-risk population in 
that study area were estimated to experience exposures at or above 200 
ppb and less than 0.5% are estimated to experience an exposure at or 
above the 100 ppb benchmark.
    In the other two study areas (Indianapolis and Fall River), 
approximately 20% to just over 25% of a study area's simulated children 
with

[[Page 26772]]

asthma, on average across the 3-year period, are estimated to 
experience one or more days per year with a 5-minute exposure at or 
above 100 ppb while breathing at elevated rates (Table 1). With regard 
to the 200 ppb benchmark concentration, these two study areas' 
estimates are as high as 0.7%, on average across the 3-year period, and 
range up to as high as 2.2% in a single year. Less than 0.1% of either 
area's children with asthma were estimated to experience multiple days 
with such an exposure at or above 200 ppb (REA, Tables 6-8 and 6-9). 
Additionally, in the study area with the highest estimates for 200 ppb 
(Indianapolis), approximately a quarter of a percent of simulated 
children with asthma also were estimated to experience a day with a 5-
minute exposure at or above 300 ppb across the 3-year period (the 
percentage for the 400 ppb benchmark was 0.1% or lower). Across all 
three areas, no children were estimated to experience multiple days 
with a daily maximum 5-minute exposure (while breathing at an elevated 
rate) at or above 300 ppb (REA, Table 6-9).

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

    As with the comparison-to-benchmark results, the estimates for risk 
of lung function decrements in terms of a doubling or more in sRaw are 
also lower in the Tulsa study area than the other two areas (Table 2; 
REA, Tables 6-10 and 6-11). Under conditions just meeting the current 
standard in the Indianapolis and Fall River study areas, as many as 
1.3% and 1.1%, respectively, of children with asthma, on average across 
the 3-year period, were estimated to experience at least one day per 
year with a SO2-related doubling in sRaw (Table 2). The 
corresponding percentage estimates for experiencing two or more such 
days ranged as high as 0.7%, on average across the 3-year simulation 
period (REA, Table 6-11). Additionally, as much as 0.2% and 0.3%, in 
Fall River and Indianapolis, respectively, of the simulated populations 
of children with asthma, on average across the 3-year period, was 
estimated to experience a single day with a SO2-related 
tripling in sRaw (Table 2).

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

D. Proposed Conclusions on the Current Standard

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

[[Page 26773]]

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

[[Page 26774]]

    In considering the currently available health effects evidence 
base, augmented in some aspects since the last review, that provides 
the foundation of our understanding of the health effects of 
SO2 in ambient air, the PA gives particular attention to the 
evidence from controlled human exposure studies that (1) demonstrates 
that very short exposures (as short as a few minutes) to 
SO2, while breathing at an elevated rate, induces 
bronchoconstriction and associated decrements in lung function, which 
can be accompanied by symptoms, among individuals with asthma; and, (2) 
supports the identification of people with asthma as the population at 
risk from short-term peak concentrations in ambient air (ISA, sections 
1.6, 1.7, 1.8, 5.2, 6.6; 2008 ISA; U.S. EPA, 1994). While the evidence 
base has been augmented since the time of the last review, the newly 
available evidence does not lead to different conclusions regarding the 
primary health effects of SO2 in ambient air or regarding 
exposure concentrations associated with those effects; nor does it 
identify different populations at risk of SO2-related 
effects (PA, section 3.2.1). In this way, the health effects evidence 
available in this review is consistent with evidence available in the 
last review when the current standard was established (ISA; 2008 ISA; 
U.S. EPA, 1994).
    This strong evidence base continues to demonstrate a causal 
relationship between short-term SO2 exposures and 
respiratory effects, particularly in people with asthma (ISA, p. xlix 
and section 5.2.1.2). This conclusion is primarily based on evidence 
from controlled human exposure studies, also available at the time of 
the last review, that reported lung function decrements and respiratory 
symptoms in people with asthma exposed to SO2 for 5 to 10 
minutes while breathing at an elevated rate. Support is also provided 
by the epidemiologic evidence that is coherent with the controlled 
human exposure studies. As in the last review, the currently available 
epidemiologic evidence, including that newly available in this review, 
includes studies reporting positive associations for asthma-related 
hospital admissions and emergency department visits (of individuals of 
all ages, including adults and children) with short-term SO2 
exposures (ISA, section 5.2.1.2).\79\
---------------------------------------------------------------------------

    \79\ While uncertainties remain related to the potential for 
confounding by PM or other copollutants and the representation of 
fine-scale temporal variation in personal exposures, the findings of 
the epidemiologic studies continue to provide supporting evidence 
for the conclusion on the causal relationship (ISA, section 
5.2.1.2).
---------------------------------------------------------------------------

    The health effects evidence newly available in this review also 
does not extend our understanding of the range of 5-minute exposure 
concentrations that elicit effects in people with asthma exposed while 
breathing at an elevated rate beyond what was understood in the last 
review (PA, section 3.2.1.3). As in the last review, 200 ppb remains 
the lowest concentration tested in exposure studies where study 
subjects are freely breathing in exposure chambers (ISA, section 
5.2.1.2). At that exposure concentration, approximately 8 to 9% of 
study subjects with asthma, breathing at an elevated rate, experienced 
moderate or greater lung function decrements following 5- to 10-minute 
controlled exposures (ISA, Table 5-2). The limited information 
available for exposure concentrations below 200 ppb is from mouthpiece 
exposure studies in which subjects were exposed to a concentration of 
100 ppb, with only a few of these studies including an exposure to 
clean air while exercising that would have allowed for determining the 
effect of SO2 versus the effect of exercise alone (ISA, 
section 5.2.1.2; PA, section 3.2.1.3). While, for these reasons, these 
data are not amenable to direct quantitative comparisons with the data 
for higher exposure concentrations, they generally indicate a somewhat 
lesser response. In considering what may be indicated by the 
epidemiologic evidence with regard to exposure concentrations eliciting 
effects, we recognize complications associated with interpretation of 
epidemiologic studies of SO2 in ambient air that relate to 
whether measurements at the study monitors adequately represent the 
spatiotemporal variability in ambient SO2 concentrations in 
the study areas and associated population exposures (ISA, section 
5.2.1.9).
    In this review, as in the last review, there is uncertainty with 
regard to exposure levels eliciting effects in some population groups 
for which data are limited or not available from the controlled human 
exposure studies, such as individuals with severe asthma and children 
younger than 12 years old, as well as uncertainty in the extent of 
effects at exposure levels below those studied (PA, section 3.2.1; ISA, 
p. 5-22). Collectively, these aspects of the evidence and associated 
uncertainties contribute to a recognition that for SO2, as 
for other pollutants, the available evidence base in this NAAQS review 
generally reflects a continuum, consisting of ambient levels at which 
scientists generally agree that health effects are likely to occur, 
through lower levels at which the likelihood and magnitude of the 
response become increasingly uncertain.
    As at the time of the last review, the exposure and risk estimates 
developed from modeling exposures to SO2 emitted into 
ambient air are critically important to consideration of the potential 
for exposures and risks of concern under air quality conditions of 
interest, and consequently they are critically important to judgments 
on the adequacy of public health protection provided by the current 
standard. In considering the REA analyses available in this review, the 
PA notes the various ways in which these analyses differ and improve 
upon those available in the last review. In addition to an expansion in 
the number and type of study areas assessed, there are a number of 
improvements to input data and modeling approaches, including the 
availability of continuous 5-minute air monitoring data at monitors 
within the three study areas (PA, section 3.2.2; REA, section 1.3.1). 
The current REA extends the time period of simulation by including a 3-
year simulation period consistent with the form established for the 
now-current standard (PA, section 3.2.2; REA, section 1.3.1). Further, 
the years simulated reflect more recent patterns of emissions and 
associated exposure circumstances subsequent to the 2010 decision (PA, 
section 3.2.2; REA, section 1.3.1).
    As at the time of the last review, people with asthma are the 
population at risk of respiratory effects related to SO2 in 
ambient air. Children with asthma may be particularly at risk (PA 
section 3.2.1.2; ISA, section 6.5.1.1). While in the U.S. there are 
more adults with asthma than children with asthma, the REA results, in 
terms of percent of the simulated at-risk populations, indicate higher 
exposures and risks for children with asthma as compared to adults. 
This finding relates to children's greater frequency and duration of 
outdoor activity (REA, sections 2.1.2, 4.3.3, 4.4, 5.2, and 5.3). In 
light of these conclusions and findings, we have focused our 
consideration of the REA results here on the results for children with 
asthma.
    As can be seen by the variation in exposure estimates, the three 
study areas in the REA represent an array of emissions sources and 
associated exposure circumstances, including those contributing to 
relatively higher and relatively lower exposures and associated risk 
(PA, section 3.2.2; REA, section 5.4).\80\ As recognized in the

[[Page 26775]]

REA, the analyses there are not intended to provide a comprehensive 
national assessment. Rather, the analyses for this array of study areas 
are intended to indicate the magnitude of exposures and risks that may 
be expected in areas of the U.S. that just meet the current standard 
but may differ in ways affecting population exposures of interest. In 
that way, the REA is intended to be informative to the EPA's 
consideration of potential exposures and risks associated with the 
current standard and the Administrator's judgments regarding the 
protection provided by the current standard. For example, the PA 
considered locations within areas that just meet the current standard 
where the areas' locations of relatively higher ambient air 
concentrations coincide with locations of higher population density. In 
so doing, the PA recognized that consideration of such exposures is 
particularly important to consideration of the public health protection 
afforded by the current standard, and particularly to the overarching 
question concerning the availability of information that calls into 
question the adequacy of the current standard (PA, sections 3.2.2.2 and 
3.2.2.4).
---------------------------------------------------------------------------

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

    With regard to the REA representation of air quality conditions 
associated with just meeting the current standard, the PA notes reduced 
uncertainty (compared to the 2009 REA) in a few aspects of the approach 
for developing this air quality scenario, while additionally 
recognizing the uncertainty associated with the application of air 
quality adjustments to estimate conditions just meeting the current 
standard (PA, sections 3.2.2.2 and 3.2.2.3; REA, section 6.2.2). Given 
the importance of this aspect of the REA to consideration of the level 
of protection provided by the current standard, the PA considers the 
results for each study area in terms of a range that reflects variation 
associated with the two different methodologies for the first air 
quality adjustment approach (REA, section 6.2.2.2).
    In this context, the PA notes that across all three study areas, 
which provide an array of SO2 emissions and exposure 
situations, the percent of children with asthma estimated to experience 
at least one day with as much as a doubling in sRaw (attributable to 
SO2), on average across the 3-year period, ranges from <0.1% 
to 1.3%; the highest study area estimate is just under 2% for the 
highest single year (PA, section 3.2.4; PA, Table 3-4; REA, Table 6-
10). Accordingly, results for the three case study areas indicate at 
least 98.7% or more of the at-risk population of children with asthma 
to be protected from experiencing a SO2-related doubling in 
sRaw, as an average across the 3-year period, and approximately 98% or 
more protected from as much as a single occurrence in the single 
highest year. Greater protection (e.g., 99% or more) is indicated for 
multiple days with a doubling in sRaw and also for single occurrences 
of as much as a tripling in sRaw (PA, section 3.2.4; REA, Table 6-11).
    With regard to exposures compared to benchmark concentrations, the 
PA notes that less than 1% of children with asthma are estimated to 
experience, while breathing at an elevated rate, a daily maximum 5-
minute exposure per year at or above 200 ppb, on average across the 3-
year period, with a maximum for the study area with the highest 
estimates just over 2% in the highest single year (PA, section 3.2.4; 
PA, Table 3-3; REA, Table 6-8). Further, the percentage for at least 
one day with such an exposure at or above 400 ppb is 0.1% or less, as 
an average across the 3-year period, and 0.3% or less in each of the 
three years simulated across the three study areas (PA, section 3.2.4; 
PA, Table 3-3; REA, Table 6-8). No simulated at-risk individuals were 
estimated to experience multiple such days (PA, section 3.2.4; REA, 
Table 6-9).
    In considering the public health implications of the REA estimated 
occurrences of exposures of different magnitudes, the PA takes note of 
guidance from the ATS (Thurston et al., 2017; ATS, 2000),\81\ CASAC 
advice, and judgments made by the EPA in considering the public health 
implications of similar effects in previous NAAQS reviews.\82\
---------------------------------------------------------------------------

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

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

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

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

[[Page 26776]]

including the 2009 REA's lack of an air quality scenario specific to 
the now-current standard in the last review, as well as uncertainties 
associated with such analyses, the PA notes a rough consistency of the 
associated estimates when considering the array of study areas in both 
reviews (PA, section 3.2.4). Overall, the PA finds the newly available 
quantitative analyses to comport with the conclusions reached in the 
last review regarding the control expected to be exerted by the now-
current 1-hour standard on 5-minute exposures of concern (PA, section 
3.2.4). With regard to the results for the REA in the last review 
(which were for a single-year simulation), the 2010 decision recognized 
those results for the area with the highest estimates and largest 
population (St. Louis) to indicate that a 1-hour standard of a 
magnitude between the two levels assessed in the 2009 REA (50 and 100 
ppb) might be expected to protect more than 97% of children with asthma 
(and somewhat less than 100%) from experiencing exposures at or above a 
200 ppb benchmark concentration and more than 99% of that population 
group from experiencing exposures at or above a 400 ppb benchmark (75 
FR 35546-47, June 22, 2010; 2009 REA, pp. B-62 and B-63). Single-year 
results in the current REA for the two study areas with the highest 
estimates (including the area with the most sizeable population, 
Indianapolis) indicate protection for the now-current standard of 75 
ppb of approximately 98 to 99% of the populations of children with 
asthma from experiencing exposures at or above a 200 ppb benchmark 
concentration and 99.7% or more of the study area at-risk populations 
from exposures at or above 400 ppb (PA, sections 3.2.2.2 and 3.2.4; 
REA, Table 6-8). These and the similar estimates for a doubling or more 
in sRaw are of a magnitude roughly consistent with the level of 
protection that was described in establishing the now-current standard 
in 2010 (PA, section 3.1.1.2.4).\84\
---------------------------------------------------------------------------

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

    Additionally, the 2010 decision also took note of the magnitude of 
the SO2 concentrations in ambient air in U.S. epidemiologic 
studies of associations between ambient air concentrations and 
emergency department visits or hospital admissions, for which the 
effect estimate remained positive and statistically significant in 
copollutant models with PM (PA, sections 3.1.1.2.4 and 3.2.4).\85\ No 
additional such studies are available in the current review, as 
summarized in section II.B.3 above (PA, section 3.2.1.3). Accordingly, 
in considering the main aspects of the decision in the last review, the 
PA finds the currently available information to be consistent with that 
on which the decision establishing the current standard was based (PA, 
section 3.2.4).
---------------------------------------------------------------------------

    \85\ In considering these studies and information regarding 
SO2 concentrations in the areas studied, as well as 
associated uncertainties, the Administrator concluded that the level 
of 75 ppb chosen for the new 1-hour standard provided an adequate 
margin of safety (PA, section 3.1.1.2.4; 75 FR 35548, June 22, 
2010).
---------------------------------------------------------------------------

    In considering potential public health implications of the current 
REA exposure and risk estimates for the three case studies, the PA 
recognizes the importance of these estimates to consideration of 
whether the currently available information calls into question the 
adequacy of public health protection afforded by the current standard. 
In so doing, the PA notes that the REA estimates for conditions 
associated with just meeting the current standard, are of particular 
importance to consideration of exposures and risks in areas still 
existing across the U.S. that have source and population 
characteristics similar to the study areas assessed, and with ambient 
concentrations of SO2 that just meet the current standard 
today or that will be reduced to do so at some period in the future. In 
this context, the PA takes note of the more than 24 million people with 
asthma currently in the U.S., including more than 6 million children, 
with potentially somewhat more than 100,000 living within 5 km of large 
\86\ sources of SO2 emissions (PA, sections 3.2.2.4 and 
3.2.4).
---------------------------------------------------------------------------

    \86\ As also summarized in section II.D.1 above, these estimates 
are drawn from the PA presentation of estimates of the number of 
children living near SO2 emissions sources emitting 1,000 
tpy based on the 2014 NEI and the 2015 national estimates of asthma 
prevalence (PA, section 3.2.2.4 and Table 3-5).
---------------------------------------------------------------------------

    The PA additionally takes note of the uncertainties or limitations 
of the current evidence base with regard to the exposure levels at 
which effects may be elicited in some population groups (e.g., children 
with asthma and individuals with severe asthma), as well as the 
severity of the effects in those groups (PA, sections 3.2.1.4 and 
3.2.4; ISA, pp. 5-22 to 5-25). In so doing, the PA recognizes that the 
controlled human exposure studies, on which the depth of the general 
understanding of SO2-related health effects is based, are 
limited or lacking in providing information with regard to responses in 
people with more severe asthma or in children younger than 12 years 
(PA, sections 3.2.1.4 and 3.2.4; ISA, pp. 5-22 to 5.25). Additional 
limitations in understanding relate to the potential for effects in 
some people with asthma exposed to concentrations below 200 ppb, as 
well as the potential for other air pollutants to affect responses to 
SO2 (PA, sections 3.2.1.4 and 3.2.4; ISA, pp. 5-22 to 5-26). 
In light of these uncertainties, the PA additionally takes note of the 
REA results for the lowest benchmark concentration (100 ppb) that 
indicate that in some areas of the U.S. under air quality conditions 
that just meet the current standard, approximately 20% to just over 25% 
of children with asthma may experience one or more days per year, on 
average across a 3-year period, with a 5-minute exposure to 
concentrations at or above this benchmark while breathing at an 
elevated rate (PA, section 3.2.4 and Table 3-3; REA, Table 6-8). Based 
on such consideration of the evidence across the exposure 
concentrations studied and the exposure/risk information related to the 
lowest benchmark concentration, the PA finds that the combined 
consideration of the body of evidence and the quantitative exposure 
estimates continues to provide support for a standard as protective as 
the current one (PA, section 3.2.4).
    The PA further recognizes that the EPA's conclusions regarding the 
adequacy of the current standard depend in part on public health policy 
judgments identified above and judgments by the Administrator about the 
level of public health protection that is appropriate, allowing for an 
adequate margin of safety. In so doing, the PA takes note of the long-
standing health effects evidence that documents the effects of 
SO2 exposures as short as a few minutes in people with 
asthma that are exposed while breathing at elevated rates and 
recognizes that such effects have been documented at the lowest 
concentration studied in exposure chambers with appropriate clean-air 
controls (PA, section 3.2.4). The PA additionally notes that it was 
recognized in the last review that such exposures can result in adverse 
health effects in people with asthma (75 FR 35546-47, June 22, 2010), 
and that there are limitations, and associated uncertainty, in the 
evidence available for the lower exposure concentration of 100 ppb 
(summarized in section II.B.3 above), as was the case in the last 
review. The PA further notes the indication of an appreciable reduction 
in the magnitude of the SO2-induced response in exercising 
people with asthma at this

[[Page 26777]]

lower exposure concentration compared with responses observed for 
exposures at 200 ppb (PA, sections 3.2.1.3, 3.2.1.4 and 3.2.4). Thus, 
in focusing on the potential for 5-minute exposures at and above 200 
ppb, the PA takes note of the REA results that indicate the current 
standard may be expected to protect approximately 98% and nearly 99% of 
populations of children with asthma from experiencing any days with 
such exposures in the highest year and on average each year in a 3-year 
period, respectively (PA, sections 3.2.2.4 and 3.2.4; REA, Table 6-8). 
The PA additionally notes that the REA estimates indicate the current 
standard may be expected to protect more than 99% of children from 
experiencing any days with a 5-minute exposure of 300 ppb or higher, 
with the estimates for the 400 ppb benchmark indicating protection of 
at least 99.7% and 99.9% of children with asthma from experiencing any 
days with a 5-minute exposure of 400 ppb or higher in the highest year 
and in each year on average for a 3-year period, respectively (PA, 
sections 3.2.2.4 and 3.2.4; REA, Table 6-8). In considering these 
results, the PA notes the lesser severity of effects reported for 
exposures below 400 ppb than those at and above 400 ppb, which include 
larger decrements in lung function that are frequently accompanied by 
respiratory symptoms, facts given weight in establishing the current 
standard in 2010 (75 FR 35547, June 22, 2010).\87\ With regard to the 
potential for children to experience SO2-related lung 
function decrements in terms of at least a doubling in sRaw, the PA 
takes note of the REA results that indicate the current standard may be 
expected to protect approximately 98.1% and nearly 98.7% from 
experiencing any days with such decrements, in the highest year of the 
3-year period and in each year on average for the period, respectively 
(PA, sections 3.2.2.4 and 3.2.4; REA, Table 6-10). In light of ATS 
guidance, CASAC advice and EPA judgments in past NAAQS reviews, the PA 
finds these results to indicate a high level of protection of at-risk 
populations from SO2-related health effects. The PA further 
notes that this protection is also consistent with the level of 
protection indicated by the information considered when the standard 
was set (PA, section 3.2.4). Accordingly, the PA finds that the 
currently available evidence and quantitative information, including 
the associated uncertainties, do not call into question the adequacy of 
protection provided by the current standard and thus support 
consideration of retaining the current standard, without revision (PA, 
section 3.2.4).
---------------------------------------------------------------------------

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

    Overall, the PA recognizes that the newly available health effects 
evidence, critically assessed in the ISA as part of the full body of 
evidence, reaffirms conclusions on the respiratory effects recognized 
for SO2 in the last review (PA, sections 3.2.1 and 3.2.4). 
Further, there is a general consistency of the currently available 
evidence with the evidence that was available in the last review, 
including with regard to key aspects on which the current standard is 
based (PA, sections 3.2.1 and 3.2.4). The quantitative exposure and 
risk estimates for conditions just meeting the current standard 
indicate a similar level of protection, for at-risk populations from 
respiratory effects considered to be adverse, as that indicated by the 
information considered in the decision for the 2010 review in 
establishing the now-current standard (PA, sections 3.2.2 and 3.2.4.). 
As in the last review, limitations and uncertainties are associated 
with the available information, as summarized in section 3.2.4 of the 
PA.
    Collectively, the PA finds that the evidence and exposure/risk 
based considerations provide the basis for its conclusion that 
consideration should be given to retaining the current standard, 
without revision (PA, section 3.2.4). Accordingly, and in light of this 
conclusion that it is appropriate to consider the current standard to 
be adequate, the PA did not identify any potential alternative 
standards for consideration in this review (PA, section 3.2.4).
2. CASAC Advice
    In the current review of the primary standard for SOX, 
the CASAC has provided advice and recommendations in their review of 
drafts of the IRP, ISA, REA and PA, and of the REA Planning Document.
    In their comments on the draft PA, the CASAC concurred with staff's 
overall preliminary conclusions that ``the current scientific 
literature does not support revision of the primary NAAQS for 
SO2,'' additionally stating the following (Cox and Diez 
Roux, 2018b, p. 3 of letter).

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

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

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

    The comments from the CASAC also took note of the uncertainties 
that remain in this review. In so doing, it stated that the ``CASAC 
notes that there are many susceptible subpopulations that have not been 
studied and which could plausibly be more affected by SO2 
exposures than adults with mild to moderate asthma,'' providing as 
examples people with severe asthma and obese children with asthma, and 
citing physiologic and clinical understanding (Cox and Diez Roux, 
2018b, p. 3 of letter). The CASAC stated that ``[i]t is plausible that 
the current 75 ppb level does not provide an adequate margin of safety 
in these groups[, h]owever because there is considerable uncertainty in 
quantifying the sizes of these higher risk subpopulations and the 
effect of SO2 on them, the CASAC

[[Page 26778]]

does not recommend reconsideration of the level at this time'' (Cox and 
Diez Roux, 2018b, p. 3 of letter).
    The CASAC comments additionally state that the draft PA ``clearly 
identifies most of the key uncertainties, including uncertainties in 
dose-response'' and that ``[t]here are also some additional 
uncertainties that should be mentioned'' (Cox and Diez Roux, 2018b, pp. 
6-7 of Consensus Responses to Charge Questions). These are in a variety 
of areas including risk for various population groups, personal 
exposures to SO2, and estimating short-term ambient air 
concentrations.\88\ The CASAC suggested research and data gathering in 
these and other areas that would inform the next SO2 primary 
standard review (Cox and Diez Roux, 2018b, p. 6 of the Consensus 
Responses to Charge Questions).
---------------------------------------------------------------------------

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

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

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

    In the discussion below, the Administrator considers first the 
evidence base on health effects associated with short-term exposure to 
SO2, including the controlled human exposure studies that 
document respiratory effects in people with asthma exposed for as short 
as a few minutes while breathing at elevated rates and the relative 
lack of such information for some subgroups of this population, 
including young children and people with severe asthma. He additionally 
notes the available epidemiologic evidence that documents associations 
between short-term concentrations of SO2 in ambient air and 
asthma-related health outcomes, particularly in children. Further, the 
Administrator considers the estimates of SO2 exposures and 
risk in multiple study areas under air quality conditions just meeting 
the current standard (summarized in sections II.C and II.D.1 above), 
and the public health implications of those results. The Administrator 
additionally considers uncertainties in the evidence and the exposure/
risk information, as a part of public health policy judgments essential 
to decisions regarding the adequacy of the protection provided by the 
standard, similar to the judgements made in establishing the current 
standard. He draws on the PA considerations, and PA conclusions in the 
current review, with which the CASAC has concurred, taking note of key 
aspects of the rationale presented for those conclusions. Further, the 
Administrator considers the advice of the CASAC, including particularly 
its overall agreement with the PA conclusion that the current evidence 
and quantitative exposure and risk estimates provide support for 
retaining the current standard and the CASAC's recommendation to retain 
all elements of the standard without revision (Cox and Diez Roux, 
2018b).
    With regard to the evidence base for SO2, the 
Administrator first recognizes the long-standing evidence that has 
established the key aspects of the harmful effects of very short 
SO2 exposures on people with asthma that are relevant to 
this review as they were relevant in 2010 when the current short-term 
standard was established. This evidence, drawn largely from the 
controlled human exposure studies, demonstrates that very short 
exposures (for as short as a few minutes) to less than 1000 ppb 
SO2, while breathing at an elevated rate (such as while 
exercising), induces bronchoconstriction and related respiratory 
effects in people with asthma and supports identification of people 
with asthma as the population at risk from short-term peak 
concentrations in ambient air (ISA; 2008 ISA; U.S. EPA, 1994).\90\ The 
evidence base additionally includes epidemiologic studies that provide 
support for the conclusion of a causal relationship between short-term 
SO2 exposures and respiratory effects for which the 
controlled human exposure studies are the primary evidence. The 
epidemiologic studies report positive associations of short-term (i.e., 
hourly or daily) concentrations of SO2 in ambient air with 
asthma-related health outcomes, including hospital admissions and 
emergency department visits. In considering these epidemiologic studies 
in the context of the larger evidence base, the ISA recognizes that 
while these studies analyze hourly or daily metrics, there is the 
potential for shorter-term concentrations within the study areas to be 
playing a role in such associations. The ISA also notes associated 
uncertainties related to potential confounding from co-occurring 
pollutants such as PM, a chemical mixture including some components for 
which SO2 is a precursor, and also related to exposure 
estimates and the ability of fixed-site monitors to adequately 
represent variations in personal exposure, particularly with regard to 
peak exposures, as summarized in section II.B.3 above (ISA, p. 5-37; 
PA, section 3.2.1.4).\91\
---------------------------------------------------------------------------

    \90\ For people without asthma, such effects have only been 
observed in studies of exposure concentrations at or above 1000 ppb 
(ISA, section 5.2.1.7).
    \91\ Sulfur dioxide is a precursor to sulfate, which commonly 
occurs in particulate form (ISA, section 2.3; U.S. EPA, 2009, 
section 3.3.2 and Table 3-2).
---------------------------------------------------------------------------

    With regard to the health effects evidence newly available in this 
review, the Administrator takes note of the PA finding that, while the 
health effects evidence, as assessed in the ISA, has been augmented 
with additional studies since the time of the last review, including 
more than 200 new health studies, the newly available evidence does not 
lead to different conclusions regarding the primary health effects of 
SO2 in ambient air or regarding exposure concentrations 
associated with those effects. Nor does it identify different or 
additional populations at risk of SO2-related effects. Thus, 
the Administrator recognizes that the health effects evidence available 
in this review is consistent with evidence available in the last review 
when the current standard was established and that this strong evidence 
base continues to demonstrate a causal relationship between relevant 
short-term exposures to SO2 and respiratory effects, 
particularly with regard to effects related to asthma exacerbation in 
people with asthma. He also recognizes that the ISA conclusion on the 
respiratory

[[Page 26779]]

effects caused by short-term exposures is based primarily on evidence 
from controlled human exposure studies, available at the time of the 
last review, that reported moderate or greater lung function decrements 
and respiratory symptoms in people with asthma exposed to 
SO2 for 5 to 10 minutes while breathing at an elevated rate 
(ISA, section 5.2.1.9), and that the current 1-hour standard was 
established to provide protection from effects such as these (75 FR 
35520, June 22, 2010). The Administrator further notes the control of 
peak 5-minute exposures that is provided by the current 1-hour 
standard, as indicated by the exposure analysis in the REA and air 
quality analyses in the PA (PA, chapter 2 and Appendix B).
    With regard to exposure concentrations of interest in this review, 
the Administrator takes particular note of the evidence from controlled 
human exposure studies that demonstrate the occurrence of lung function 
decrements, at times accompanied by respiratory symptoms, in subjects 
with asthma exposed for very short periods of time while breathing at 
elevated rates, focusing primarily on such study findings for which 
exposure concentration-specific data are available to the EPA for 
individual subjects (ISA, Table 5-2 and Figure 5-1, summarized in Table 
3-1 of the PA).\92\ These data demonstrate such effects related to 
asthma exacerbation in sensitive people with asthma exposed to 
SO2 concentrations as low as 200 ppb. These data include 
limited evidence of respiratory symptoms accompanying the lung function 
effects at this exposure level (ISA, Table 5-2). The Administrator 
recognizes that both the percent of individuals experiencing lung 
function decrements and the severity of the decrements, as well as the 
frequency with which they are accompanied by symptoms, increase with 
increasing SO2 concentrations across the range of exposure 
levels studied (ISA, Table 5-2; PA, section 3.2.1.3). For example, 
approximately 10% of study subjects experienced moderate or greater 
lung function decrements at 200 ppb, while at 300-400 ppb, as many as 
approximately 30% of subjects in some studies experienced such 
decrements. Further, at concentrations at or above 400 ppb, the 
moderate or greater decrements in lung function were frequently 
accompanied by respiratory symptoms, such as cough, wheeze, chest 
tightness, or shortness of breath, with some of these findings reaching 
statistical significance at the study group level (ISA, Table 5-2 and 
section 5.2.1).
---------------------------------------------------------------------------

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

    In considering the potential public health significance of effects 
associated with SO2 exposures, the Administrator further 
recognizes the greater significance accorded both to larger lung 
function decrements, which are more frequently documented at exposures 
above 200 ppb, and the potential for greater impacts of SO2-
induced decrements in people with more severe asthma, as recognized in 
the ISA and by the CASAC (as summarized in section II.D.2 above).\93\ 
For example, he notes that the ATS indicated it to be appropriate to 
consider small lung function changes as adverse when they occur in 
individuals with pre-existing compromised function, ``such as resulting 
from asthma, even without accompanying respiratory symptoms'' (Thurston 
et al., 2017). Thus, with regard to the health effects evidence for 
SO2, the Administrator recognizes that health effects 
resulting from exposures at and above 400 ppb are appreciably more 
severe than those elicited by exposure to SO2 concentrations 
as low as 200 ppb (and lower), and that health impacts of short-term 
SO2 exposures (including those occurring at concentrations 
below 400 ppb) have the potential to be more significant in the 
subgroup of people with asthma that have more severe disease and for 
which the study data are more limited.
---------------------------------------------------------------------------

    \93\ The ISA notes that while the extremely limited evidence for 
adults with moderate to severe asthma indicates such groups may have 
similar relative lung function decrements in response to 
SO2 as adults with less severe asthma, individuals with 
severe asthma may have greater absolute decrements that may relate 
to the role of exercise (ISA, p. 1-17 and 5-22). The ISA concluded 
that individuals with severe asthma may have ``less reserve capacity 
to deal with an insult compared with individuals with mild asthma'' 
(ISA, p. 1-17 and 5-22).
---------------------------------------------------------------------------

    As at the time of the last review, the Administrator considers the 
health effects evidence in the context of the exposure and risk 
modeling, including key limitations and uncertainties, as summarized in 
the PA and section II.C.1 above (described in detail in the REA). In so 
doing, he recognizes such a context to be critical for SO2, 
for which health effects in people with asthma are linked to exposures 
during periods of elevated breathing rates, such as while exercising. 
Thus, population exposure modeling that takes activity levels into 
account is integral to consideration of population exposures compared 
to benchmark concentrations and of population risk of lung function 
decrements.
    In considering the exposure and risk estimates, the Administrator 
recognizes that unlike the REA available in the last review, which 
analyzed single-year air quality scenarios for potential standard 
levels bracketing the now current level, the current REA assesses an 
air quality scenario for three years of air quality conditions that 
just meet the current standard, including its 3-year form. The other 
ways in which the current REA analyses are improved and expanded from 
those in the REA for the last review relate to improvements that have 
been made to models, model inputs and underlying databases. These 
improvements include the database, vastly expanded since the last 
review, of ambient air monitoring data for 5-minute concentrations. 
These data are available as a result of the monitoring data reporting 
requirement established in the last review to inform subsequent primary 
NAAQS reviews for SOX and the associated assessments of the 
protection provided from elevated short-term (5- to 10-minute exposure) 
SO2 concentrations for people with asthma breathing at 
elevated rates (75 FR 35567-68, June 22, 2010). The current REA is 
additionally expanded from the prior one with regard to the number of 
study areas in that it now includes three urban areas, each with 
populations of more than 100,000 people, as contrasted to the single 
such area in the 2009 REA.
    In considering the REA results for the benchmark comparisons for 
the three years analyzed in each of the three study areas, the 
Administrator notes the estimates of as many as 0.7% of children with 
asthma to experience a single day per year (on average across the 3-
year period) with a 5-minute exposure at or above 200 ppb in a single 
year, while breathing at elevated rates, and as many as 2.2% in a 
single year. He additionally takes note of the REA findings that also 
estimate somewhat less than 0.1% of children with asthma to experience 
multiple days with such exposures in any one year. In turning to 
consideration of the REA estimates of lung function risk, the 
Administrator notes that as many as 1.9% of children with asthma are 
estimated to experience a day in a single year with an SO2-
related doubling of sRaw, and as many as 1.3% per year on average 
across three years. He further takes note that as many as 1% of 
children with asthma may be estimated to experience multiple days in a 
single year (0.7% on average across multiple years) with a lung 
function decrement of such a magnitude, and as many as 0.3% (on average 
across multiple years) may be estimated to

[[Page 26780]]

experience a day with at least a tripling in sRaw (as summarized in 
section II.C.3 above).
    In considering the level of protection indicated by these estimates 
of exposure and risk under air quality conditions that just meet the 
current standard, the Administrator additionally recognizes the 
limitations in the available evidence base that contribute to 
uncertainties with regard to the risk estimates for lung function 
decrements in young children with asthma and in individuals of any age 
with severe asthma. While health effects study data are limited or 
lacking for these population groups, the ISA indicates a potential for 
these groups to experience somewhat greater health impacts than the 
populations studied (as summarized in section II.B above). In light of 
these limitations of the evidence and the potential articulated in the 
ISA for the risk to be greater for these groups for which the evidence 
is limited or lacking, the Administrator notes that the CAA requirement 
that primary standards provide an adequate margin of safety, as 
summarized in section I.A above, is intended to address uncertainties 
associated with inconclusive scientific and technical information, as 
well as to provide a reasonable degree of protection against hazards 
that research has not yet identified.
    The Administrator additionally notes the PA consideration of the 
sizeable number of at-risk individuals living in locations near large 
SO2 emissions sources that may contribute to increased 
SO2 concentrations in ambient air. The information 
concerning population exposure characteristics such as the co-
occurrence of elevated ambient air concentrations with areas of 
relatively higher population density is not available for all of these 
locations. Consideration of the population sizes in these areas and the 
potential for similarity of exposure characteristics in some of these 
areas to the study areas assessed in the REA (as summarized in section 
II.D.1 above) confirms the public health relevance of the REA results 
to this review of the current standard.
    In considering the adequacy of the protection provided by the 
current standard, the Administrator notes the findings of the REA in 
light of considerations recognized above regarding the significance 
associated with different exposure benchmark concentrations and 
severity of lung function decrements, as well as the estimated 
frequency of occurrence of such concentrations and decrements under air 
quality conditions just meeting the current standard. Given the clear 
concentration-response relationship documented in the evidence for the 
key effects in people with asthma across the range of exposure 
concentrations studied, higher SO2 concentrations would be 
expected to contribute to greater severity and frequency in occurrence 
of responses in at-risk groups. Other considerations summarized above, 
include the strong evidence for lung function decrements in people with 
asthma exposed for just a few minutes while breathing at elevated rates 
(e.g., while exercising) to SO2 concentrations as low as 200 
ppb, the public health implications of such exposures, and related 
considerations raised by the ATS in its statement on adverse effects of 
air pollution. Further, advice from the CASAC included its conclusion 
that the current evidence and exposure/risk information supports 
retaining the current standard and its associated caution as to 
uncertainty in the adequacy of the margin of safety provided by the 
current standard for less well studied yet potentially susceptible 
population groups.\94\ Based on all of these considerations, the 
Administrator gives weight to the PA findings, summarized in section 
II.D.1 above, that the current body of evidence, in combination with 
the exposure/risk information, does not support a primary standard that 
is less protective than the current standard. Thus, he proposes to 
conclude that a less stringent standard would not provide the requisite 
protection of public health, including an adequate margin of safety.
---------------------------------------------------------------------------

    \94\ In conveying this caution related to such population 
groups, the CASAC additionally recognized there to be ``considerable 
uncertainty'' and concluded that ``the CASAC does not recommend 
reconsideration of the level in order to provide a greater margin of 
safety'' (Cox and Diez Roux, 2018, Consensus Responses, p. 5).
---------------------------------------------------------------------------

    Turning to consideration of the adequacy of protection provided by 
the current standard from effects associated with lower exposures, 
including those at or below 200 ppb, the Administrator considers the 
public health significance of the REA estimates for such effects, and 
of single (versus multiple) occurrences of exposures at or above the 
lower benchmark concentrations and associated lung function decrements, 
and the nature and magnitude of the various uncertainties that are 
inherent in the underlying scientific evidence and REA analyses. In so 
doing, the Administrator recognizes that our understanding of the 
relationships between the presence of a pollutant in ambient air and 
associated health effects is based on a broad body of information 
encompassing not only more established aspects of the evidence, but 
also aspects with which there may be substantial uncertainty. In the 
case of the primary SO2 standard review, he considers the 
increased uncertainty recognized in the PA with regard to 
characterization of the risk of lung function decrements (including 
their magnitude and prevalence, and the associated health significance) 
at exposure levels below those represented in the controlled human 
exposure studies and in populations potentially at risk \95\ but for 
which the evidence base is limited or lacking (PA, section 3.2.2.3; 
REA, section 5.3). He additionally considers the uncertainties 
recognized in the PA, and summarized in section II.B and II.D.1 above, 
regarding exposure measurement error and copollutant confounding in the 
epidemiologic evidence. In so doing, the Administrator recognizes that 
collectively, these aspects of the evidence and associated 
uncertainties support an acknowledgment that for SO2, as for 
other pollutants, the available health effects evidence generally 
reflects a continuum, consisting of levels at which scientists 
generally agree that health effects are likely to occur, through lower 
levels at which the likelihood and magnitude of the response become 
increasingly uncertain.
---------------------------------------------------------------------------

    \95\ Such populations include those for which the CASAC 
described there to be ``considerable uncertainty'' (Cox and Diez 
Roux, 2018, Consensus Responses, p. 5).
---------------------------------------------------------------------------

    In considering the point at which health effects associated with 
lower levels of SO2 exposure become important from a public 
health perspective, the Administrator takes note of the PA 
consideration of the CASAC advice and EPA judgments in establishing the 
current standard in 2010, as well as the currently available 
information and commonly accepted guidelines or criteria within the 
public health community, including the ATS, an organization of 
respiratory disease specialists,\96\ for interpreting public health 
significance of moderate or greater lung function decrements, 
particularly when accompanied by respiratory symptoms, and their 
occurrence in a portion of the at-risk populations. In so doing, the 
Administrator additionally notes that the most recent ATS statement on 
adversity of air pollution is generally consistent with its prior 
statement that was referenced when the current standard was set (PA, 
section 3.2.1.5.). He also takes note of EPA judgments in prior NAAQS 
decisions for SOX and

[[Page 26781]]

other pollutants that, consistent with these statements, have 
particularly emphasized the protection of at-risk population members 
from multiple occurrences of exposures or effects of concern and from 
such effects of greater severity or that have been documented to be 
accompanied by symptoms (75 FR 35520, June 22, 2010; 76 FR 54308, 
August 31, 2011; 80 FR 65292, October 26, 2015). Together these factors 
inform the Administrator's consideration in this review of public 
health implications of the exposure and risk estimates for air quality 
conditions just meeting the current primary SO2 standard.
---------------------------------------------------------------------------

    \96\ With regard to commonly accepted guidelines or criteria 
within the public health community, the PA considered statements 
issued by the ATS (as summarized in section II.D.1 above).
---------------------------------------------------------------------------

    Thus, in considering the evidence and quantitative exposure and 
risk estimates available in this review with regard to the adequacy of 
public health protection provided by the current primary standard from 
respiratory effects associated with the lowest SO2 exposure 
concentrations represented in the health effects evidence, the 
Administrator recognizes that, as noted in section II.A above, the 
final decision on such judgments is largely a public health policy 
judgment that draws upon scientific information and analyses about 
health effects and risks, as well as judgments about how to consider 
the range and magnitude of uncertainties that are inherent in the 
information and analyses. These judgments are informed by the 
recognition, noted just above, that the available health effects 
evidence generally reflects a continuum, consisting of ambient levels 
at which scientists generally agree that health effects are likely to 
occur, through lower levels at which the likelihood and magnitude of 
the response become increasingly uncertain. Accordingly, the 
Administrator's final decision requires judgments based on an 
interpretation of the evidence and other information that neither 
overstates nor understates the strength and limitations of the evidence 
and information nor the appropriate inferences to be drawn. As 
described in section I.A above, the Act does not require that primary 
standards be set at a zero-risk level; the NAAQS must be sufficient but 
not more stringent than necessary to protect public health, including 
the health of sensitive groups, with an adequate margin of safety.
    In this light, the Administrator takes note of PA considerations 
regarding the REA results and the associated uncertainties (summarized 
in section II.C above), as well as the nature and magnitude of the 
uncertainties inherent in the scientific evidence upon which the REA is 
based. The Administrator finds such considerations collectively to be 
important to judgments such as the extent to which the exposure and 
risk estimates for air quality conditions that just meet the current 
standard in the three study areas indicate exposures and risks that are 
important from a public health perspective.\97\ In turning first to the 
REA estimates of the percent of children with asthma estimated to 
experience a day with a 5-minute SO2 exposure, while 
breathing at elevated rates, above benchmark concentrations, the 
Administrator notes the very small percentage (no more than 0.3% in a 
the highest year) of children with asthma estimated to experience a 
single day per year at/above the benchmark concentration of 400 ppb, an 
exposure level frequently associated with respiratory symptoms in 
controlled human exposure studies. In particular, he takes note of the 
fact that the REA results do not estimate any children in any of the 
three study areas to experience more than one such exposure in a year. 
The Administrator considers these results to represent a very high 
level of protection (at least 99.7% protected from a single occurrence 
in the highest year and 100% protected from multiple occurrences) from 
the risk of respiratory effects that have been observed to occur in as 
many as approximately 25% of controlled human exposure study subjects 
with asthma exposed to 400 ppb while breathing at elevated rates, and 
that have frequently been accompanied by respiratory symptoms. The 
Administrator additionally notes the small percentage (no more than 
approximately 2% in the highest year) of children with asthma estimated 
to experience a single day with a 5-minute exposure at or above the 
lower exposure concentration of 200 ppb, and that less than 0.1% of 
that population group is estimated to experience more than a single 
such day in the highest year. In so doing, he recognizes, as did the 
Administrator in the last review, that effects resulting from this 
lower exposure concentration are appreciably less severe (e.g., in 
terms of prevalence of study subjects experiencing a tripling or more 
in sRaw as well as a 20% reduction in FEV1) than those 
elicited by exposures at or above 400 ppb, and that they are less 
frequently accompanied by respiratory symptoms (ISA, Table 5-2 and 
Figure 5-1; PA, Table 3-1 and section 3.2.1.3).
---------------------------------------------------------------------------

    \97\ Such judgments are among those important to decisions on 
the adequacy of the margin of safety allowed by the current 
standard.
---------------------------------------------------------------------------

    The Administrator additionally considers the PA findings regarding 
the REA estimates of lung function risk in terms of lung function 
decrements as assessed using doubling and tripling of sRaw. The 
Administrator finds the REA estimates to indicate a high level of 
protection for children with asthma against the risk of lung function 
decrements, and particularly against the larger decrements (e.g., 
tripling in sRaw) and against multiple occurrences. The REA results for 
air quality conditions that just meet the current standard indicate, 
based on average estimates across the 3-year period, protection of more 
than 99.7% of children with asthma from experiencing a day per year 
with a SO2-related tripling of sRaw and at least 99.8% from 
experiencing multiple such days per year. The results further indicate 
99% or more of children with asthma to be protected from multiple days 
with a SO2-related doubling of sRaw.
    Taking the REA estimates of exposure and risk together, while 
recognizing the uncertainties associated with such estimates for the 
scenarios of air quality developed to represent conditions just meeting 
the current standard, the Administrator considers the current standard 
to provide a high degree of protection to at-risk populations from 
SO2 exposures associated with health effects of public 
health concern, as indicated by the extremely low estimates of 
occurrences of exposures at or above 400 ppb (and at or above 300 ppb). 
He further considers the current standard to additionally provide a 
slightly lower, but still high, degree of protection for the 
appreciably less severe effects associated with lower exposures (i.e., 
at and below 200 ppb), for which public health implications are less 
clear. In considering the adequacy of protection provided by the 
current standard from these lower exposure concentrations, the 
Administrator additionally takes note of the array of limitations in 
the evidence summarized above with regard to characterizing the 
potential response of at-risk individuals to exposures below 200 ppb, 
which the PA indicates to be much reduced. He also notes the 
limitations in the evidence for population groups potentially at risk 
but for which the evidence of risk is limited (PA, section 3.2.2.3; 
REA, section 5.3). Based on these and all of the above considerations, 
the Administrator proposes to conclude that a more stringent standard 
is not needed to provide requisite protection and that the current 
standard provides the requisite protection of public health under the 
Act.
    With regard to key aspects of the specific elements of the 
standard, the Administrator recognizes first the support in the current 
evidence base for

[[Page 26782]]

SO2 as the indicator for SOX. In so doing, he 
notes the ISA conclusion that SO2 is the most abundant of 
the SOX in the atmosphere and the one most clearly linked to 
human health effects, as described in the PA and summarized in sections 
II.B.1 and II.D.1 above. He additionally recognizes the control exerted 
by the 1-hour averaging time on 5-minute ambient air concentrations of 
SO2 and the associated exposures of particular importance 
for SO2-related health effects. Lastly, with regard to form 
and level of the standard, the Administrator takes note of the REA 
results as discussed above and the level of protection that they 
indicate the elements of the current standard to provide. The 
Administrator additionally takes note of the CASAC support for 
retaining the current standard and the CASAC's specific recommendation 
that all four elements should remain the same. Beyond his recognition 
of this support in the available information and in CASAC advice for 
the elements of the current standard, the Administrator has considered 
the elements collectively in evaluating the health protection afforded 
by the current standard, as described above.
    Thus, based on consideration of the evidence and exposure/risk 
information available in this review with its attendant uncertainties 
and limitations and information that might inform public health policy 
judgments, as well as advice from the CASAC, including their 
concurrence with the PA conclusions that the current evidence does not 
support revision of the primary SO2 standard, the 
Administrator further proposes to conclude that it is appropriate to 
retain the current standard without revision. The Administrator bases 
these proposed conclusions on consideration of the health effects 
evidence, including consideration of this evidence in the context of 
the quantitative exposure and risk analyses, recognizing the 
uncertainties associated with both. Inherent in the Administrator's 
proposed conclusions are public health policy judgments, including 
those regarding the public health significance of the SO2-
related effects estimated to occur in small portions of the at-risk 
populations under air quality conditions adjusted to just meet the 
current standard. In reaching his proposed conclusion on the adequacy 
of public health protection afforded by the existing primary standard, 
the Administrator recognizes that the Act requires primary standards to 
be requisite to protect public health with an adequate margin of 
safety, and neither more nor less stringent than necessary for this 
purpose (see generally, Whitman v. American Trucking Associations, 531 
U.S. 457, 465-472, 475-76 [2001]). The Administrator also recognizes 
that the Act does not require that primary standards be set at a zero-
risk level or to protect the most sensitive individual, but rather at a 
level that avoids unacceptable risks to public health, even if the risk 
is not precisely identified as to nature or degree. The Administrator 
finds the current standard to provide such a level of public health 
protection. Thus, the Administrator proposes to conclude that the 
current primary SO2 standard provides an adequate margin of 
safety against adverse effects associated with short-term exposures to 
SOX in ambient air. For these reasons, and all of the 
reasons discussed above, and recognizing the CASAC conclusion that the 
current evidence and REA results provide support for retaining the 
current standard, the Administrator proposes to conclude that the 
current primary SO2 standard is requisite to protect public 
health with an adequate margin of safety from effects of SOX 
in ambient air and should be retained, without revision. The 
Administrator solicits comment on this proposed conclusion.
    Having reached the proposed decision described here based on 
interpretation of the health effects evidence, as assessed in the ISA, 
and the quantitative analyses in the REA; the evaluation of policy-
relevant aspects of the evidence and quantitative analyses in the PA; 
the advice and recommendations from the CASAC; public comments received 
to date in this review; and the public health policy judgments 
described above, the Administrator recognizes that other 
interpretations, assessments and judgments might be possible. 
Therefore, the Administrator solicits comment on the array of issues 
associated with review of this standard, including public health and 
science policy judgments inherent in the proposed decision, as 
described above. The EPA also solicits comment on the four basic 
elements of the current NAAQS (indicator, averaging time, level, and 
form), including whether there are appropriate alternative approaches 
for the averaging time or statistical form that provide comparable 
public health protection, and the rationale upon which such views are 
based.

III. Statutory and Executive Order Reviews

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

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

    The Office of Management and Budget (OMB) determined that this 
action is a significant regulatory action and it was submitted to OMB 
for review. Any changes made in response to OMB recommendations have 
been documented in the docket. Because this action does not propose to 
change the existing primary NAAQS for SOX, it does not 
impose costs or benefits relative to the baseline of continuing with 
the current NAAQS in effect. EPA has thus not prepared a Regulatory 
Impact Analysis for this action.

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

    This action is not expected to be an E.O. 13771 regulatory action. 
There are no quantified cost estimates for this proposed action because 
EPA is proposing to retain the current standard.

C. Paperwork Reduction Act (PRA)

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

D. Regulatory Flexibility Act (RFA)

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

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely 
affect small

[[Page 26783]]

governments. This action imposes no enforceable duty on any state, 
local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

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

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

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

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

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

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

    This action is not subject to Executive Order 13211, because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this document is to 
propose to retain the current primary SO2 NAAQS. This 
proposal does not change existing requirements. Thus, the EPA concludes 
that this proposal does not constitute a significant energy action as 
defined in Executive Order 13211.

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

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

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

L. Determination Under Section 307(d)

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

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Quality Planning and Standards, Research Triangle Park, NC, EPA-452/
R-17-002, January 2017. Available at: https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondarystandards-planning-documents-current.
U.S. EPA. (2017c). Review of the Primary National Ambient Air 
Quality Standard for Sulfur Oxides: Risk and Exposure Assessment 
Planning Document. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC, EPA-452/P-17-001, February 2017. 
Available at: https://www3.epa.gov/ttn/naaqs/standards/so2/data/20170216so2rea.pdf.
U.S. EPA. (2017d). Risk and Exposure Assessment for the Review of 
the Primary National Ambient Air Quality Standard for Sulfur Oxides, 
External Review Draft. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC, EPA-452/P-17-002, August 2017. Available 
at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2017e). Policy Assessment for the Review of the Primary 
National Ambient Air Quality Standard for Sulfur Oxides, External 
Review Draft. Office of Air Quality Planning and Standards, Research 
Triangle Park, NC, EPA-452/P-17-003, August 2017. Available at: 
https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2018a). Risk and Exposure Assessment for the Review of 
the Primary National Ambient Air Quality Standard for Sulfur Oxides, 
Final. Office of Air Quality Planning and Standards, Research 
Triangle Park, NC, EPA-452/R-18-003, May 2018. Available at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
U.S. EPA. (2018b). Policy Assessment for the Review of the Primary 
National Ambient Air Quality Standard for Sulfur Oxides, Final. 
Office of Air Quality Planning and Standards, Research Triangle 
Park, NC, EPA-452/R-18-002, May 2018. Available at: https://www.epa.gov/naaqs/sulfur-dioxide-so2-primary-air-quality-standards.
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List of Subjects in 40 CFR Part 50

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

    Dated: May 25, 2018.
E. Scott Pruitt,
Administrator.
[FR Doc. 2018-12061 Filed 6-7-18; 8:45 am]
 BILLING CODE 6560-50-P