Document ID: EPA-HQ-OAR-2013-0566-0221
Agency: epa
Document Type: Rule
Title: Review of the Primary National Ambient Air Quality Standards for Sulfur Oxides
Posted Date: 2019-03-18T04:00Z

[Federal Register Volume 84, Number 52 (Monday, March 18, 2019)]
[Rules and Regulations]
[Pages 9866-9907]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2019-03855]

[[Page 9865]]

Vol. 84

Monday,

No. 52

March 18, 2019

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; Final Rule

  Federal Register / Vol. 84 , No. 52 / Monday, March 18, 2019 / Rules 
and Regulations  

[[Page 9866]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2013-0566; FRL-9990-28-OAR]
RIN 2060-AT68

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

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final 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 retaining the current standard, without 
revision.

DATES: This final action is effective on April 17, 2019.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2013-0566. Incorporated into this docket is a 
separate docket established for the Integrated Science Assessment for 
this review (Docket ID No. EPA-HQ-ORD-2013-0357). All documents in 
these dockets are listed on the www.regulations.gov website. Although 
listed in the index, some information is not publicly available, e.g., 
Confidential Business Information (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.

Availability of Information Related to This Action

    A number of the documents that are relevant to this action 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 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 (ISA [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 (REA [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 (PA [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.

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: hagan.nicole@epa.gov.

SUPPLEMENTARY INFORMATION: 

Table of Contents

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 Decision
    A. Introduction
    1. Background on the Current Standard
    2. Overview of Health Effects Evidence
    3. Overview of Risk and Exposure Information
    B. Conclusions on Standard
    1. Basis for Proposed Decision
    2. CASAC Advice in This Review
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Primary 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 Risks and Safety Risks
    I. Executive Order 13211: Actions Concerning Regulations 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)
    M. Congressional Review Act
References

Executive Summary

    The EPA has completed its 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 parts per billion (ppb), as 
the 99th percentile of daily maximum 1-hour SO2 
concentrations, averaged over 3 years. Based on the EPA's review of key 
aspects of the currently available health effects evidence, 
quantitative risk and exposure information, advice from the Clean Air 
Scientific Advisory Committee (CASAC), and public comments, the EPA is 
retaining the current standard, without revision.
    Reviews of the NAAQS are required by the Clean Air Act (CAA) on a 
periodic basis. 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 in 2010 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.

[[Page 9867]]

    Emissions of SO2 and associated concentrations in 
ambient air have declined appreciably since 2010 and over the longer 
term. For example, as summarized in the PA, emissions nationally are 
estimated to have declined by 82% over the period from 2000 to 2016, 
with a 64% decline from 2010 to 2016. 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. One-hour concentrations of SO2 in ambient air in 
the U.S. declined more than 82% from 1980 to 2016 at locations 
continuously monitored over this period. The decline since 2000 has 
been 69% at a larger number of locations continuously monitored since 
that time. Daily maximum 5-minute concentrations have also consistently 
declined from 2011 to 2016.
    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 SOX 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 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 that from 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.
    Quantitative analyses of population exposure and risk also inform 
the final 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 3 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 consideration of public comment, the Administrator has 
concluded 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. Therefore, the EPA is retaining the current 1-hour 
primary SO2 standard, without revision. This decision is 
consistent with CASAC recommendations.

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 (SO3) 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, as well as visibility, climate, and materials damage-
related welfare effects of such particulate sulfur compounds are being 
addressed in the review of the NAAQS for particulate matter (PM) (U.S. 
EPA, 2014a, 2016a, 2018c). Additionally, the ecological welfare effects 
of SOX and their 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).\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(a)(2). Section 109 
(42 U.S.C. 7409) directs the Administrator to propose and promulgate 
``primary'' and ``secondary'' NAAQS for pollutants for which air 
quality criteria are issued. 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\ As provided in section 109(b)(2), a secondary standard 
must ``specify a level of air quality the attainment and maintenance of 
which, in the judgment of the Administrator, based on such criteria, is 
requisite to protect the public welfare from any known or anticipated 
adverse effects associated with the presence of [the] pollutant in the 
ambient air.'' \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.'' 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) of the CAA (42 U.S.C. 
7602(h)) effects on welfare include, but are not limited to, 
``effects on soils, water, crops, vegetation, manmade 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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[[Page 9868]]

    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 document, 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.A.2.b 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, 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 
CASAC.

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 
permitting program that covers these and other air 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. Furthermore, the EPA establishes emission standards 
for stationary sources under other provisions of the CAA; these 
standards, which include 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) may also contribute to SO2 emissions 
controls and reductions, including through controls aimed at reducing 
other pollutants.

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

    The initial air quality criteria for SOX were issued in 
1967 and reevaluated in 1969 (34 FR 1988, February 11, 1969; U.S. DHEW, 
1967, 1969). Based on the 1969 criteria, the EPA, in initially 
promulgating NAAQS for SOX in 1971, established the 
indicator as SO2. SOX are a group of closely 
related gaseous compounds that include SO2 and 
SO3 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 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 
in the U.S. Court of Appeals for the District of Columbia Circuit (D.C. 
Circuit), which 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 last review of the air quality 
criteria and primary standards for SOX, which was completed 
in 2010. In that review, the

[[Page 9869]]

EPA promulgated a new 1-hour standard and also promulgated provisions 
for the revocation of the then-existing 24-hour and annual primary 
standards.\5\ 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 average SO2 
concentrations, and 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; \6\ 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 and the EPA's denial of several petitions for administrative 
reconsideration were challenged in the D.C. Circuit, and the court 
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) (``NEDA/
CAP'').
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    \5\ Timing and related requirements for the implementation of 
the revocation are specified in 40 CFR 50.4(e).
    \6\ 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 primary standard (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) \7\ 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).
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    \7\ The ISA for this review provides a comprehensive assessment 
of the current scientific literature useful in indicating the kind 
of and extent of all identifiable effects on public health 
associated with the presence of the pollutant in the ambient air, as 
described in section 108 of the CAA, emphasizing information that 
has become available since the last air quality criteria review in 
order to reflect the current state of knowledge. As such, the ISA 
forms the scientific foundation for this NAAQS review and is 
intended to provide information useful in forming policy relevant 
judgments about air quality indicator(s), form(s), averaging time(s) 
and level(s) for the NAAQS. The ISA functions in the current NAAQS 
review process as the Air Quality Criteria Document (AQCD) did in 
reviews completed prior to 2009.
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    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 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 proposed decision (henceforth ``proposal'') to retain the 
primary SO2 NAAQS was signed on May 25, 2018, and published 
in the Federal Register on June 8, 2018 (83 FR 26752). The EPA held a 
public hearing in Washington, DC on July 10, 2018 (83 FR 28843, June 
21, 2018). At the public hearing, the EPA heard testimony from three 
individuals representing specific interested organizations. The 
transcript from this hearing and written testimony provided at the 
hearing are in the docket for this review. The EPA extended the 45-day 
comment period by 17 days, until August 9, 2018 (83 FR 28843, June 21, 
2018), and comments were received from various government, industry, 
and environmental groups, as well as members of the general public.
    The schedule for completion of this review is governed by a consent 
decree resolving a lawsuit filed in July 2016 that included a claim 
that the EPA had failed to complete its review of the primary 
SO2 NAAQS within 5 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, a notice setting 
forth the final decision concerning its review of the primary NAAQS for 
SOX no later than January

[[Page 9870]]

28, 2019, with such date to be extended automatically one day for each 
day of a lapse in appropriations if such a lapse were to occur within 
120 days of this deadline.\9\ The EPA experienced such a lapse in 
appropriations in late December 2018 and January 2019, which led to the 
automatic extension of the January 28, 2019 deadline to February 25, 
2019.\10\
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    \8\ See Complaint, Center for Biological Diversity et al. v. 
Wheeler, 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. Wheeler, No. 3:16-cv-03796-VC (N.D. Cal., entered April 28, 
2017), Doc. No. 37.
    \10\ Joint Notice of Automatic Deadline Extension in Light of 
Lapse in Appropriations, Center for Biological Diversity et al. v. 
Wheeler, No. 3:16-cv-03796-VC (N.D. Cal., filed February 15, 2019), 
Doc. No. 39.
---------------------------------------------------------------------------

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 SOX sources and emissions (section I.D.1) and ambient 
concentrations (section I.D.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.\11\ Sulfur oxides known to occur in the troposphere include 
SO2 and 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).\12\
---------------------------------------------------------------------------

    \11\ 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, p. 2-24).
    \12\ 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, 
2018c).
---------------------------------------------------------------------------

    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).\13\ 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).
---------------------------------------------------------------------------

    \13\ 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 National Emissions Inventory 
(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,\14\ 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).\15\ 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.\16\ 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).
---------------------------------------------------------------------------

    \14\ When established, the MATS Rule 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).
    \15\ 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).
    \16\ 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 3-year average of annual 99th 
percentile daily maximum 1-hour concentrations) at locations 
continuously monitored over this period (PA, Figure 2-4).\17\ The 
decline since 2000 has been 69% at the larger number of locations 
continuously monitored since that time (PA, Figure 2-5).\18\
---------------------------------------------------------------------------

    \17\ This decline is the average of observations at 24 
monitoring sites that have been continuously operating from 1980-
2016.
    \18\ This decline is the average of observations at 193 
monitoring sites that have been continuously operating across 2000-
2016.
---------------------------------------------------------------------------

    As a result of changes to the monitoring data reporting 
requirements promulgated in 2010 (as summarized in section I.C above) 
maximum hourly 5-minute concentrations of SO2 in ambient air 
are available at SO2 NAAQS compliance monitoring sites (PA, 
Figure 2-3; 75 FR 35554, June 22, 2010).\19\ 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 at SO2 sites 
continuously monitored during this period declined approximately 53% 
(PA, Figure 2-6, Appendix B).
---------------------------------------------------------------------------

    \19\ 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

[[Page 9871]]

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 appreciably higher concentrations in the air, which may or may not 
impact large portions of the surrounding populated areas depending on 
specific source characteristics, meteorological conditions and terrain.
    Analyses in the ISA of ambient air monitoring 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).\20\ 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, section 
2.5.3.3; 2008 ISA [U.S. EPA 2008a], section 2.5.1).
---------------------------------------------------------------------------

    \20\ The six ``focus areas'' evaluated in the ISA are: 
Cleveland, OH; Pittsburgh, PA; New York City, NY; St. Louis, MO (and 
neighboring areas in 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 Decision

    This section presents the rationale for the Administrator's 
decision to retain the existing primary SO2 standard. This 
decision is based on a thorough review in the ISA of the latest 
scientific information, published through August 2016 (ISA, p. xlii), 
on human health effects associated with SOX in ambient air. 
This decision also accounts for analyses in the PA of policy-relevant 
information from the ISA and the REA, as well as information on air 
quality; the analyses of human exposure and health risks in the REA; 
CASAC advice; and consideration of public comments received on the 
proposal.
    Section II.A provides background on the general approach for this 
review and the basis for the existing standard, and also presents brief 
summaries of key aspects of the currently available health effects and 
exposure/risk information. Section II.B summarizes the proposed 
conclusions and CASAC advice, addresses public comments received on the 
proposal and presents the Administrator's conclusions on the adequacy 
of the current standard, drawing on consideration of this information, 
advice from the CASAC, and comments from the public. Section II.C 
summarizes the Administrator's decision on the primary standard.

A. Introduction

    As in prior reviews, the general approach to reviewing the current 
primary standard is based, most fundamentally, on using the EPA's 
assessment of current scientific evidence and associated quantitative 
analyses to inform the Administrator's judgment regarding a primary 
SO2 standard that protects public health with an adequate 
margin of safety. 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 draws 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 exposure/risk analyses. The approach to informing these judgments, 
discussed more fully below, 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 his judgment, are 
requisite to protect public health with an adequate margin of safety. 
In so doing, the Administrator seeks to establish standards that are 
neither more nor less stringent than necessary for this purpose. The 
Act does not require that primary standards be set at a zero-risk 
level, but rather at a level that avoids unacceptable risks to public 
health including the health of sensitive groups.\21\ The four basic 
elements of the NAAQS (indicator, averaging time, level, and form) are 
considered collectively in evaluating the health protection afforded by 
a standard.
---------------------------------------------------------------------------

    \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]).
---------------------------------------------------------------------------

    In evaluating the appropriateness of retaining or revising the 
current primary SO2 standard, the EPA has adopted an 
approach that builds upon the general approach used in the last review 
and reflects the body of evidence and information now available. As 
summarized in section II.A.1 below, the Administrator's decisions in 
the prior review were based on an integration of information on health 
effects associated with exposure to SO2 with information on 
the public health significance of key health effects, as well as on 
policy judgments as to when the standard is requisite to protect public 
health with an adequate margin of safety and on consideration of advice 
from the CASAC and public comments. These decisions were also informed 
by air quality and related analyses and quantitative exposure and risk 
information.
    Similarly, in this review, as described in the PA, the proposal, 
and elsewhere in this document, we draw on the current evidence and 
quantitative assessments of exposure and risk pertaining to the public 
health risk of SO2 in ambient air. The past and current 
approaches are both based, most fundamentally, on the EPA's assessments 
of the current scientific evidence and associated quantitative 
analyses. 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; 80 FR 79330, December 21, 2015; 81 FR 
89097, December 9, 2016; 82 FR 11356, February 22, 2017; 82 FR 11449, 
February 23, 2017; 82 FR 23563, May 23, 2017; 82 FR 37123, August 9, 
2017; 82 FR 43756, September 19, 2017; 83 FR 14638, April 5, 2018). To 
bridge 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

[[Page 9872]]

public health with an adequate margin of safety, the PA evaluates the 
policy implications of the current evidence in the ISA and of the 
quantitative analyses in the REA.
    In considering the scientific and technical information, we 
consider both the information available at the time of the last review 
and information newly available since the last review, including most 
particularly that which has been critically analyzed and characterized 
in the current ISA. We additionally consider the quantitative exposure 
and risk information described in the REA that estimated 
SO2-related exposures and lung function decrements 
associated with air quality conditions just meeting the current 
standard in simulated at-risk populations in multiple case study areas 
(REA, chapter 5). The evidence-based discussions presented below (and 
summarized more fully in the proposal) draw upon evidence from studies 
evaluating health effects related to exposures to SO2, as 
discussed in the ISA. The exposure/risk-based discussions also 
presented below (and summarized more fully in the proposal) have been 
drawn from the quantitative analyses for SO2, as discussed 
in the REA. Sections II.A.2 and II.A.3 below provide an overview of the 
current health effects and quantitative exposure and risk information 
with a focus on the specific policy-relevant questions identified for 
these categories of information in the PA (PA, chapter 3).
1. Background on the Current Standard
    The current primary standard was established in the last review of 
the primary NAAQS for SOX, which was completed in 2010 (75 
FR 35520, June 22, 2010). The decision in that review to revise the 
primary standards (establishing a 1-hour standard and providing for 
revocation of the 24-hour and annual standards) reflected the extensive 
body of evidence of respiratory effects in people with asthma, which 
has expanded over the four decades since the first SO2 
standards were established in 1971 (U.S. EPA, 1982, 1986, 1994, 2008a). 
This evidence was assessed in the 2008 ISA.
    A key element of the expanded evidence base was a series of 
controlled human exposure studies documenting effects on lung function 
associated with bronchoconstriction in people with asthma exposed while 
breathing at elevated rates \22\ for periods as short as minutes (U.S. 
EPA, 1982, 1986, 1994, 2008a). Another aspect of the information 
available in the 2010 review was the air quality database, which had 
expanded since the previous review (completed in 1996), and which 
provided data on the pattern of peak 5-minute SO2 
concentrations occurring at that time. The EPA used these data in the 
2009 quantitative exposure and risk assessments to provide an up-to-
date ambient air quality context for interpreting the health effects 
evidence. In addition to providing support for decisions in the 2010 
review, these aspects of that review provided support to the EPA in 
addressing the issues raised in the court remand of the Agency's 1996 
decision not to revise the standards to specifically address 5-minute 
exposures with that decision (75 FR 35523, June 22, 2010). Together, 
the evidence characterized in the 2008 ISA, which included 
epidemiologic and animal toxicologic studies as well as the extensive 
set of controlled human exposure studies, and the quantitative 
assessments in the 2009 REA, as well as advice from the CASAC and 
public comment, formed the basis for the EPA's 2010 action to 
strengthen the primary NAAQS for SOX to provide the 
requisite protection of public health with an adequate margin of 
safety, and to provide increased protection for at-risk populations, 
such as people with asthma (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 
notifications in the last review to refer to activity levels in 
adults that 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'' in place of those phrases.
---------------------------------------------------------------------------

    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 robust evidence base, comprised of findings from controlled human 
exposure, epidemiologic, and animal toxicological studies, was 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 
associations of a broader range of health outcomes with ambient air 
concentrations of SO2, with uncertainty noted about the 
magnitude of the study effect estimates, quantification of the 
concentration-response relationship, potential confounding by 
copollutants, and other aspects (75 FR 35535-36, June 22, 2010; 2008 
ISA, section 5.3).
    Accordingly, conclusions reached in the last review were based 
primarily on consideration of the health effects evidence for short-
term exposures, and particularly on 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 short-term 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 comments,\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, 2000a). Based on these considerations, 
the Administrator, in 2010, gave weight to the findings of respiratory 
effects in exercising people with asthma after 5- to 10-minute 
exposures as low as 200 ppb, and further recognized that higher 
exposures (at or above 400 ppb) were associated with respiratory 
symptoms and with a greater number of study subjects experiencing lung 
function decrements. Moreover, she took note of the greater severity of 
the response at and above 400 ppb, recognizing effects associated

[[Page 9873]]

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, 2000a). 
For example, the ATS statements recognized a distinction between 
reversible and irreversible effects, recommending that reversible 
loss of lung function in combination with the presence of symptoms 
be considered adverse (ATS 1985, 2000a; 75 FR 35526, June 22, 2010).
    \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).
---------------------------------------------------------------------------

    As a result and based on consideration of the entire body of 
evidence and information available in the review, with particular 
attention to the exposure and risk estimates from the 2009 REA, as well 
as the advice from the CASAC and public comments, the Administrator 
concluded that the then-existing 24-hour standard did not adequately 
protect public health (75 FR 35536, June 22, 2010). The 2009 REA 
estimated that substantial percentages of children with asthma might be 
expected to experience exposures at least once annually 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 the 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).\26\ In order to provide the requisite protection to people with 
asthma from the adverse health effects of 5-minute to 24-hour 
SO2 exposures, she replaced the 24-hour standard with a new, 
1-hour standard (75 FR 35536, June 22, 2010). Further, upon reviewing 
the evidence with regard to the potential for effects from long-term 
exposures,\27\ the Administrator revoked the annual standard based on 
her recognition of the lack of sufficient health evidence to support a 
long-term standard and on air quality information indicating that the 
new short-term standard would have the effect of generally maintaining 
annual SO2 concentrations well below the level of the 
revoked annual standard (75 FR 35550, June 22, 2010).
---------------------------------------------------------------------------

    \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).
    \26\ In giving particular attention to the exposure and risk 
estimates from the 2009 REA for air quality just meeting the then-
existing standards, the Administrator also noted epidemiologic study 
findings of associations with respiratory-related health outcomes in 
studies of locations where maximum 24-hour average SO2 
concentrations were below the level of the then-existing 24-hour 
standard, while also recognizing uncertainties associated with the 
epidemiologic evidence (75 FR 35535-36, June 22, 2010).
    \27\ In evaluating the health effects studies in the ISA, the 
EPA has generally categorized exposures of durations longer than a 
month to be ``long-term'' (ISA, p. 1-2; 2008 ISA, p. 3-1).
---------------------------------------------------------------------------

    The Administrator selected a 1-hour averaging time for the new 
standard based on available air quality analyses in the REA that 
indicated that a 1-hour averaging time would be effective in addressing 
5-minute peak SO2 concentrations such 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).\28\ The analyses suggested that, compared to a 24-hour 
averaging time, a 1-hour averaging time would more efficiently and 
effectively limit 5-minute peak concentrations 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 (2009 REA, section 10.5.2.2; 
75 FR 35539, June 22, 2010). The analyses 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 effects, while the longer averaging time 
was shown to lack effectiveness and efficiency in addressing 5-minute 
peak SO2 concentrations, likely over-controlling in some 
areas while under-controlling in others (75 FR 35539, June 22, 2010; 
2009 REA, section 10.5.2.2). The CASAC additionally advised that ``a 
one-hour standard is the preferred averaging time'' (Samet, 2009, pp. 
15, 16), finding the REA to provide a ``convincing rationale'' that 
supported ``a one-hour standard as protective of public health'' 
(Samet, 2009, pp. 1, 15 and 16). Thus, in consideration of the 
available information summarized here and CASAC advice, the 
Administrator judged that a 1-hour standard (given the appropriate 
level and form) was the appropriate means for controlling short-term 
exposures to SO2 ranging from 5 minutes to 24 hours (75 FR 
35539, June 22, 2010).
---------------------------------------------------------------------------

    \28\ The Administrator 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). 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).
---------------------------------------------------------------------------

    The statistical form for the 1-hour standard, the 99th percentile 
daily maximum 1-hour average concentrations averaged over 3 years, is 
based on consideration of the health effects evidence, stability in the 
public health protection provided by the programs implementing the 
standard, and advice from the CASAC, as well as results of the 2009 REA 
for alternative standard forms (75 FR 35541, June 22, 2010). With 
regard to stability, the concentration-based form averaged over 3 years 
was concluded to 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 Administrator's objective in selecting 
the specific concentration-based form was for the form of the new 
standard to 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). Based on results of air quality and exposure analyses 
in the REA which indicated the 99th percentile form likely to be 
appreciably more effective at achieving the desired control of 5-minute 
peak exposures than a 98th percentile form, the Administrator decided 
the form should be the 99th percentile of daily maximum 1-hour 
concentrations averaged over 3 years (75 FR 35541, June 22, 2010).
    The level for the new standard was set primarily based on 
consideration of the findings of the 2009 REA exposure analyses with 
regard to the varying degrees of protection that different levels of a 
1-hour daily maximum SO2 standard might be expected to 
provide against 5-minute exposures to concentrations of 200 ppb and 400 
ppb.\29\ For example, the single-year

[[Page 9874]]

exposure assessment for St. Louis \30\ estimated that a 1-hour standard 
at 100 ppb would likely protect more than 99% of children with asthma 
in that city from experiencing any days in a year with at least one 5-
minute exposure at or above 400 ppb while at moderate or greater 
exertion, and approximately 97% of those children with asthma from 
experiencing any days in a year with at least one exposure at or above 
200 ppb while at moderate or greater exertion (75 FR 35546-47, June 22, 
2010). The St. Louis study area results for the air quality scenario 
representing a 1-hour standard level of 50 ppb suggested that such a 
standard would further limit exposures, such that more than 99% of 
children at moderate or greater exertion would likely be protected from 
experiencing any days in a year with a 5-minute exposure at or above 
the 200 ppb benchmark concentration (75 FR 35542, 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 \31\ 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 those 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).\32\
---------------------------------------------------------------------------

    \29\ 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, 
which compared 5-minute concentrations estimated to occur in these 
air quality scenarios to benchmark levels, indicated for a 1-hour 
standard level of 100 ppb, there would be a maximum annual average 
of 2 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).
    \30\ Of the two study areas assessed in the 2009 REA (St. Louis 
and Greene County, Missouri), 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.
    \31\ 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, Appendix, p. B-62).
    \32\ 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).
---------------------------------------------------------------------------

    Based on the above considerations and the comments received on the 
proposal, advice from the CASAC, the entire body of evidence and 
information available in that review, and the related 
uncertainties,\33\ the Administrator selected a standard level of 75 
ppb. She concluded that such a standard, with a 1-hour averaging time 
and 99th percentile form, would provide an increase in public health 
protection compared to the then-existing standards and would be 
expected to provide the desired degree of protection 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).\34\ The 
Administrator emphasized the latter in judging that the level of 75 ppb 
provided an adequate margin of safety (75 FR 35548, June 22, 2010). 
Thus, she concluded that a NAAQS for SOX of 75 ppb, as the 
99th percentile of daily maximum 1-hour average SO2 
concentrations averaged over 3 years, would provide the requisite 
protection of public health with an adequate margin of safety (75 FR 
35547-35548, June 22, 2010).
---------------------------------------------------------------------------

    \33\ Such uncertainties included both those with regard to the 
epidemiologic evidence, including potential confounding and exposure 
measurement 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).
    \34\ For example, such a standard was 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'' and also was ``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).
---------------------------------------------------------------------------

2. Overview of Health Effects Evidence
    In this section, we provide an overview of the policy-relevant 
aspects of the health effects evidence available for consideration in 
this review. Section II.B of the proposal provides a detailed summary 
of key information contained in the ISA and in the PA on the health 
effects associated with SO2 exposures, and the related 
public health implications, focusing particularly on the information 
most relevant to consideration of effects associated with the presence 
of SO2 in ambient air (83 FR 26761, June 8, 2018). The 
subsections below briefly outline this information in the four topic 
areas addressed in section II.B of the proposal.
a. Nature of Effects
    Sulfur dioxide is a highly reactive and water-soluble gas that once 
inhaled is absorbed almost entirely in the upper respiratory tract \35\ 
(ISA, sections 4.2 and 4.3). Brief exposures to SO2 can 
elicit respiratory effects, particularly in individuals with asthma 
when breathing at elevated rates (ISA, p. 1-17). Under conditions of 
elevated breathing rates (e.g., while exercising), SO2 
penetrates the upper respiratory tract, entering the tracheobronchial 
region,\36\ 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). People with asthma have an increased propensity for 
the airways to narrow in response to certain inhaled stimuli, as 
compared to people without asthma or allergies (ISA, section 
5.2.1.2).\37\ This narrowing or constriction of the airways in the 
respiratory tract, termed bronchoconstriction, is characteristic of an 
asthma attack and is the most sensitive indicator of SO2-
induced lung function effects (ISA, p. 5-8). Bronchoconstriction causes 
an increase in airway resistance, often assessed by measurement of 
specific airway resistance (sRaw). Exercising individuals without 
asthma have also been found to exhibit increased sRaw or related 
responses, such as reduced forced expiratory volume in 1 second 
(FEV1), but at much higher SO2

[[Page 9875]]

exposure concentrations than exercising individuals with asthma (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).
---------------------------------------------------------------------------

    \35\ 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).
    \36\ 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, bronchi, 
and bronchioles.
    \37\ The propensity for airways to narrow following inhalation 
of some stimuli is termed bronchial or airway responsiveness (ISA, 
section 5.2.1.2, p. 5-8). In clinical situations where airway 
responsiveness to methacholine or histamine is assessed and the 
concentration resulting in a specific reduction in lung function 
(the provocative concentration) meets the ATS criteria for 
classification of the subject as hyperresponsive, the terms airway 
hyperresponsiveness (AHR) or bronchial hyperresponsiveness (BHR) are 
used (ATS, 2000b). Along with symptoms, variable airway obstruction, 
and airway inflammation, AHR (or BHR) is a primary feature in the 
clinical definition and characterization of asthma severity (ISA, 
section 5.2.1.2; Reddel et al., 2009).
---------------------------------------------------------------------------

    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 
comes from the long-standing evidence base of controlled human exposure 
studies demonstrating effects related to asthma exacerbation including 
lung function decrements \38\ and respiratory symptoms (e.g., cough, 
shortness of breath, chest tightness and wheeze) in people with asthma 
exposed to SO2 for 5 to 10 minutes at elevated breathing 
rates (U.S. EPA, 1994; 2008 ISA; ISA, section 5.2.1). 
Bronchoconstriction, evidenced by decrements in lung function, that are 
sometimes accompanied by respiratory symptoms, occurs 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). In contrast, respiratory effects are 
not generally observed in other people with asthma (nonresponders \39\) 
and healthy adults exposed to SO2 concentrations below 1000 
ppb while exercising (ISA, sections 5.2.1.2 and 5.2.1.7). Across 
studies, bronchoconstriction in response to SO2 exposure is 
seen during respiratory conditions of elevated breathing rates, such as 
exercise, or with mouthpiece exposures that involve laboratory-
facilitated rapid, deep breathing.\40\ With these breathing conditions, 
breathing shifts from nasal breathing to oral (with mouthpiece) or 
oronasal breathing, which increases the concentrations of 
SO2 reaching the tracheobronchial 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).
---------------------------------------------------------------------------

    \38\ The specific responses reported in the evidence base that 
are described in the ISA as lung function decrements are increased 
sRaw and FEV1 (ISA, section 5.2.1.2).
    \39\ 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 (non-responders) being insensitive to SO2 
bronchoconstrictive effects at concentrations as high as 1000 ppb 
(ISA, pp. 5-14 to 5-21; Johns et al., 2010).
    \40\ 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 (ISA, p. 5-6).
---------------------------------------------------------------------------

    The current evidence base of controlled human exposure studies of 
individuals with asthma,\41\ is consistent with the evidence base from 
the last review, and is summarized in the ISA (ISA, section 5.2.1.2, 
Tables 5-1 and 5-2). With regard to effects related to asthma 
exacerbation, 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 (e.g., responders) 
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 just a few minutes) when individuals are exposed while 
breathing at an elevated rate, and is transient, with recovery 
occurring with a return to resting breathing rate or cessation of 
exposure, generally within an hour (ISA, p. 5-14, Table 5-2; Linn et 
al., 1984; Johns et al., 2010).
---------------------------------------------------------------------------

    \41\ 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 currently available epidemiologic evidence includes studies 
reporting positive associations with short-term SO2 
exposures for asthma-related hospital admissions of children or 
emergency department visits by children (ISA, section 5.2.1). These 
findings provide supporting evidence 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). Among the epidemiologic 
studies newly available in this review, there are a limited number that 
have investigated SO2 effects related to asthma 
exacerbation, with the most supportive evidence coming from studies of 
asthma-related hospital admissions of children or emergency department 
visits by children (ISA, section 5.2.1.2). As in the last review, areas 
of uncertainty in the epidemiologic evidence are related to the 
characterization of exposure based on the use of ambient air 
concentrations at fixed site monitors 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 \42\ 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).
---------------------------------------------------------------------------

    \42\ The potential for confounding by PM is of particular 
interest given that SO2 is a precursor to PM (ISA, p. 1-
7).
---------------------------------------------------------------------------

    For long-term SO2 exposure and respiratory effects, the 
evidence base is somewhat augmented since the last review such that the 
current ISA concludes it to be suggestive of, but not sufficient to 
infer, a causal relationship (ISA, section 5.2.2). The support for this 
conclusion comes mainly from the limited epidemiologic 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, sections 1.6.1.2 and 5.2.2.7). Further, there 
are uncertainties associated with the epidemiologic evidence across the 
respiratory effects examined for long-term exposure (ISA, section 
5.2.2.7).
    For effects other than those involving the respiratory system, the 
current evidence is generally similar to the evidence available in the 
last review and leads to similar conclusions about the totality of 
adverse health effects. 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 findings of previously and 
newly available multicity epidemiologic studies that report positive 
associations, accompanied by uncertainty with respect to the potential 
for SO2 to have an independent effect on mortality. While 
recent studies have analyzed some key uncertainties and addressed data 
gaps from the previous review, uncertainties still exist. These 
uncertainties include that: The number of studies that examined 
copollutant confounding is limited; there is evidence of a reduction in 
the SO2-mortality effect estimates (i.e., relative risks) in 
copollutant models with

[[Page 9876]]

nitrogen dioxide and PM with mass median aerodynamic diameter nominally 
below 10 microns (PM10); and a potential biological 
mechanism for mortality following short-term SO2 exposures 
is lacking (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, the 
potential for 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, given the strength of the evidence supporting the conclusion 
of a causal relationship between short-term exposure to SO2 
in ambient air and respiratory effects, in particular, asthma 
exacerbation in individuals with asthma, the focus in this review, as 
in prior reviews, is on such effects.
b. At-Risk Populations
    In this review, we use the term ``at-risk populations'' to 
recognize populations with a quality or characteristic in common (e.g., 
a specific pre-existing illness or specific age or lifestage) that 
contributes to them having a greater likelihood of experiencing 
SO2-related health effects. 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, as was the case in 2010, is based on the well-
established and well-characterized evidence from controlled human 
exposure studies, supported by the evidence related to mode of action 
for SO2 and evidence from epidemiologic studies (ISA, 
sections 5.2.1.2 and 6.3.1). Further, 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). The ISA also finds the evidence to be suggestive of increased 
risk for children and older adults, while noting some limitations and 
inconsistencies (ISA, sections 6.5.1.1 and 6.5.1.2).\44\ 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, as summarized in 
section II.B of the proposal, that may put children with asthma at 
greater risk of SO2-related bronchoconstrictive effects than 
adults with asthma.\45\
---------------------------------------------------------------------------

    \44\ The current evidence for risk to older adults relative to 
other lifestages comes from epidemiologic studies, for which the 
findings are somewhat inconsistent, and studies with which there are 
uncertainties in the association with the health outcome (ISA, 
section 6.5.1.2).
    \45\ There are few controlled human exposure studies to inform 
our understanding of any differences in exposure concentrations 
associated with bronchoconstrictive effects in young children as 
compared to adults or adolescents as those studies have not included 
subjects younger than 12 years (ISA, p. 5-22). The ISA does not find 
the evidence to be adequate to conclude differential risk status for 
subgroups of children with asthma (ISA, sections 6.5.1.1 and 6.6). 
In consideration of the limited information regarding factors 
related to breathing habit, however, the ISA suggests that children 
with asthma approximately 5 to 11 years of age, 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. 5-36 and 4-7).
---------------------------------------------------------------------------

    The finding that some individuals with asthma have a greater 
response to SO2 than others with similar disease status is 
quantitatively analyzed in a study, newly available in this review, 
that 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). In analyses focused on the more sensitive subpopulation, the 
study demonstrated statistically significant increases in 
bronchoconstriction with exposures as low as 0.3 ppm (Johns et al., 
2010). While such information provides documentation that some 
individuals with asthma have a greater response to SO2 than 
others, the factors contributing to this greater susceptibility are not 
yet known (ISA, pp. 5-14 to 5-21).
c. 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 in individuals with 
asthma exposed to SO2 for 5 to 10 minutes while breathing at 
elevated rates demonstrate clear and consistent increases in magnitude 
and occurrence of decrements in lung function (e.g., increased sRaw and 
reduced FEV1) and in occurrence of respiratory symptoms with 
increasing SO2 exposure (ISA, section 1.6.1.1, Table 5-2 and 
pp. 5-35, 5-39). Further, the evidence base demonstrates the occurrence 
of SO2-related effects resulting from peak exposures on the 
order of minutes \46\ and other short-term exposures have been found to 
elicit a similar bronchoconstrictive response for somewhat longer 
(e.g., 30-minute) exposure durations (ISA, p. 5-14; Kehrl et al., 
1987).
---------------------------------------------------------------------------

    \46\ While 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 studies of people with asthma further 
demonstrate \47\ that SO2 concentrations as low as 200 to 
300 ppb for 5 to 10 minutes elicited moderate or greater lung function 
decrements (a decrease in FEV1 of at least 15% or an 
increase in sRaw of at least 100%) in a subset of 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 the 
occurrence of moderate or greater SO2-related lung function 
decrements, as many as 33% of exercising study subjects with asthma 
experienced such decrements in lung function (ISA,

[[Page 9877]]

section 5.2.1, Table 5-2).\48\ At concentrations at or above 400 ppb, 
moderate or greater decrements in lung function occurred in as many as 
approximately 30 to 60% of exercising individuals with asthma, and 
compared to the results for exposures at 200 to 300 ppb, a larger 
percentage of individuals with asthma experienced the 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) at these higher 
concentrations (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).
---------------------------------------------------------------------------

    \47\ 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.
    \48\ Additionally, analyses of data from the full set of these 
studies that focused only on the results for the study subjects 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).
---------------------------------------------------------------------------

    Two hundred ppb is the lowest exposure concentration for which 
individual study subject data for percent changes in sRaw and 
FEV1 are available from studies that have assessed the 
SO2 effect versus the effect of exercise in clean air (ISA, 
Table 5-2 and Figure 5-1). In nearly all of these studies (and all of 
these studies with such data for concentrations from 200 to 400 ppb), 
study subjects breathed freely (e.g., without using a mouthpiece).\49\ 
In studies that tested 200 ppb exposures, 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).
---------------------------------------------------------------------------

    \49\ 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, very limited 
evidence is available for concentrations as low as 100 ppb. Some 
differences in methodology and the reporting of results complicate 
comparison of the studies with 100 ppb exposure to studies using higher 
exposures. In the studies evaluating the 100 ppb concentration level, 
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 distinguishing the effect of SO2 from the 
effect of exercise in causing bronchoconstriction (Sheppard et al., 
1981; Sheppard et al., 1984; Koenig et al., 1989). In those few 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 
exposures of 200 ppb or more.50 51 Thus, while the studies 
evaluating 100 ppb exposures are limited and their interpretation is 
complicated by the use of different reporting of results and exposure 
methods that differ from those used in studies of higher 
concentrations, the 100 ppb studies do not indicate that exposure at 
100 ppb results in as much as a doubling in sRaw, based on the 
extremely few adults and adolescents tested (Sheppard et al., 1981; 
Sheppard et al., 1984; Koenig et al., 1989).
---------------------------------------------------------------------------

    \50\ For example, although individual study subject data for 
SO2-attributable changes in sRaw in these studies are not 
available in the terms needed to summarize the responses consistent 
with the study result summaries in the ISA, Table 5-2 (e.g., percent 
change), 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 Table 2 of Sheppard et al., 1984, 
concentrations calculated to cause a doubling of sRaw in all 
subjects are higher than 125 ppb, the lowest exposure 
concentration). In the study of adolescents (aged 12 to 18 years), 
among the three individual study subjects for which total 
respiratory resistance appears to have increased with SO2 
exposure, the magnitude of increase in that metric after 
consideration of the response to exercise appears to be less than 
100% in each subject (Koenig et al., 1989).
    \51\ 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 the tracheobronchial airways (2008 ISA, 
p. 3-4; ISA, section 4.1.2.2). This allowance of deeper 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 epidemiologic evidence base, which 
includes studies that find associations with outcomes such as asthma-
related emergency department visits and hospital admissions. 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). Another key 
uncertainty in the epidemiologic evidence available in this review, as 
in the last review, is potential confounding by copollutants, 
particularly PM, given the important role of SO2 as a 
precursor to PM in ambient air (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 the uncertainty of potential copollutant confounding. 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. No additional such studies 
have been newly identified in this review that might inform this issue 
(83 FR 26765, June 8, 2018). Thus, such uncertainties regarding 
copollutant confounding, as well as exposure measurement error, remain 
in the currently available epidemiologic evidence base (ISA, p. 5-6).
d. 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). 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.
    The potential public health impacts of SO2 
concentrations in ambient air relate to respiratory effects of short-
term exposures and particularly those effects

[[Page 9878]]

associated with asthma exacerbation in people with asthma. As 
summarized above in section II.A.2.a, these effects include 
bronchoconstriction resulting in decrements in lung function and 
elicited by short-term exposures during periods of elevated breathing 
rate. Consistent with these SO2-related effects, asthma-
related health outcomes such as emergency department visits and 
hospital admissions have been positively associated with ambient air 
concentrations of SO2 in epidemiologic studies (ISA, section 
5.2.1.9).
    As summarized in section II.A.2.b 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 (ISA, section 6.3.1). The evidence supporting this conclusion 
comes primarily from studies of individuals with mild to moderate 
asthma,\52\ 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) 
across individuals with asthma of differing severity.\53\ 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 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.A.2.a 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).
---------------------------------------------------------------------------

    \52\ 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).
    \53\ 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). Based 
on the criteria used in the study by Linn et al. (1987) for placing 
individuals in the ``moderate/severe'' group, however, 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).
---------------------------------------------------------------------------

    Multiple statements by the ATS on what constitutes an adverse 
health effect of air pollution inform the Administrator's judgment on 
the public health significance of SO2-related effects, 
particularly those with the potential to occur under air quality 
conditions allowed by the current standard. Building on the earlier 
statement by the ATS that was considered in the last review (ATS, 
2000a), the recent policy statement by the ATS 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, 2000a). 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). All of these concepts, including the consideration of the 
magnitude or severity of effects occurring in just a subset of study 
subjects, as well as the consideration of persistence or transience of 
effects,\54\ are recognized as important considerations in the more 
recent ATS statement (Thurston et al., 2017) and continue to be 
relevant to consideration of the evidence base for SO2.
---------------------------------------------------------------------------

    \54\ In speaking of transient effects, the recent statement 
refers to effects lasting on the order of hours (Thurston et al., 
2017).
---------------------------------------------------------------------------

    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, 
responses that have been documented to be accompanied by respiratory 
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 be requisite to 
protect public health, including an adequate 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 of 
SO2 emissions 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) 
\55\ indicate that approximately 8% of the U.S. population has asthma 
(PA, Table 3-2; CDC, 2017). 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). Among populations of different races or ethnicities, black non-
Hispanic and Puerto Rican Hispanic children are estimated to have the 
highest

[[Page 9879]]

prevalence, 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 (CDC, 2017).
---------------------------------------------------------------------------

    \55\ 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).
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    With regard to the potential for exposure of the populations at 
risk from exposures to SO2 in ambient air, 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 PA noted that the air quality monitoring data for the 
2014-2016 period indicated there to be 15 core-based statistical areas 
\56\ with air quality exceeding the primary SO2 standard 
(design values \57\ were above the existing standard level of 75 ppb), 
of which a number have sizeable populations (PA, section 3.2.2.4). 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). In recognition of these limitations, we also examined the 
proximity of populations to sizeable SO2 point sources using 
the recently available emissions inventory information (2014 NEI), 
which is also characterized in the ISA (PA, section 3.2.2.4, Appendix 
F; ISA, section 2.2.2). This information indicates that there are more 
than 300,000 and 60,000 children living within 1 km of facilities 
emitting at least 1000 and 2000 tpy of SO2, respectively 
(PA, section 3.2.2.4). 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).
---------------------------------------------------------------------------

    \56\ 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).
    \57\ A design value is a statistic that describes the air 
quality status of a given area relative to the level of the 
standard, taking into account the averaging time and form (as well 
as indicator). Thus, design values for the SO2 NAAQS are 
in terms of 3-year averages of annual 99th percentile 1-hour daily 
maximum concentrations of SO2. Design values are 
typically used to assess whether the NAAQS is violated, to classify 
nonattainment areas, to track air quality trends and progress toward 
meeting the NAAQS and to develop control strategies.
---------------------------------------------------------------------------

3. Overview of Risk and Exposure Information
    Our consideration of the scientific evidence available in the 
current review (summarized in section II.A.2 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 respiratory 
effects that the evidence indicates to be elicited in some portion of 
exercising people with asthma by short-term exposures to elevated 
SO2 concentrations, e.g., such as 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). The second risk metric 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 a SO2-related 
increase in sRaw of at least 100% (REA, sections 4.6.1 and 4.6.2). Both 
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 2010 decision that established the 
current standard (75 FR 35546-35547, June 22, 2010).
    The following subsections provide brief overviews of the key 
aspects of the design and methods of the quantitative assessment in 
this review (section II.A.3.a) and the important uncertainties 
associated with these analyses (section II.A.3.b). The results of the 
analyses are summarized in section II.A.3.c. These overviews are drawn 
from the summary presented in section II.C of the proposal (83 FR 
26767, June 8, 2018).
a. Key Design Aspects
    In this section, we provide a brief overview of key aspects of the 
quantitative exposure and risk assessment conducted for this review and 
summarized in more detail in section II.C.1 of the proposal (83 FR 
26767, June 8, 2018), 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.
    The REA focuses on air quality conditions that just meet the 
current standard, and 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, including a range 
in total population size, different mixtures of SO2 
emissions sources, and three different climate regions of the U.S.: The 
Northeast, Ohio River Valley (Central), and South (REA, section 3.1; 
Karl and Koss, 1984).\58\ The latter two regions comprise 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). Accordingly, the three study areas illustrate 
three different patterns of exposure to SO2 concentrations 
in a populated area in the U.S. (REA, section 5.1). While the same air 
quality scenario is simulated in all three study areas (conditions that 
just meet the current standard), study-area-specific characteristics 
related to sources, meteorology, topography and population contribute 
to variation in the estimated magnitude of exposure and associated risk 
across study areas.
---------------------------------------------------------------------------

    \58\ 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

[[Page 9880]]

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 or an analysis 
of exposure and risk associated with air quality adjusted down to just 
meet the standard in areas that currently do not meet the standard.\59\ 
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 level of 
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 understanding of, and conclusions on, the public 
health protection afforded by the current standard (PA, section 
3.2.2.2).
---------------------------------------------------------------------------

    \59\ 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 using the particular analytical approaches and methodologies 
for characterizing the study area-specific air quality provide an 
indication of this variability in the spatial and temporal patterns of 
SO2 concentrations occurring under air quality conditions 
just meeting the current standard. In light of the uncertainty 
associated with these concentration estimates, the REA presents results 
from two different approaches to adjusting air quality to just meet the 
current standard (described in more detail in sections 3.4 and 6.2.2.2 
of the REA).\60\
---------------------------------------------------------------------------

    \60\ The first approach uses the highest design value across all 
modeled air quality receptors to estimate the amount of 
SO2 concentration reduction needed to adjust the air 
quality concentrations in each area to just meet the standard (REA, 
section 3.4). In recognition of potential uncertainty in the first 
approach, the second approach uses the air quality receptor having 
the 99th percentile of the distribution of design values (instead of 
the receptor with the maximum design value) to estimate the 
SO2 concentration reductions needed to adjust the air 
quality to just meet the standard, setting all receptors at or above 
the 99th percentile to just meet the standard (REA, section 
6.2.2.2).
---------------------------------------------------------------------------

    Consistent with the health effects evidence summarized in section 
II.A.2 above, the focus of the REA is on short-term (5-minute) 
exposures of individuals with asthma in the simulated populations 
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. The air quality modeling step was taken to capture the spatial 
variation in ambient SO2 concentrations across each urban 
study area. Such variation can be relatively high in areas affected by 
large point sources and is unlikely to be captured by the limited 
number of monitoring locations in each area. The modeling step yields 
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(s) with the highest 
concentrations just met the current standard. Rather than applying the 
same adjustment to concentrations at all receptors in a study area, the 
adjustment was derived by focusing on reducing emissions from the 
source(s) contributing the most to the standard exceedances (REA, 
section 3.4 and 6.2.2.1). 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. This approach was used in recognition of the limitations 
associated with air quality modeling at this fine temporal scale, e.g., 
limitations in the time steps of currently available model input data 
such as for emissions estimates.
    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 SO2 exposure events 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 (REA, section 2.2). These factors are all key to 
appropriately characterizing exposure and associated health risk for 
SO2.\61\
---------------------------------------------------------------------------

    \61\ The exposure modeling performed for this review, including 
ways in which it has been updated since the 2009 REA are summarized 
in section II.C of the proposal and described in detail in the REA 
(e.g., REA, Chapter 4 and Appendices E through I).
---------------------------------------------------------------------------

    The at-risk populations for which exposure and risk are estimated 
(children and adults with asthma) ranges from 8.0 to 8.7% of the total 
populations (ages 5-95) 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).\62\ 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).
---------------------------------------------------------------------------

    \62\ 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.
---------------------------------------------------------------------------

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

[[Page 9881]]

characterize health risk in two ways (REA, section 4.5). The first is 
the percentage of the simulated at-risk populations expected to 
experience days with 5-minute exposures, while breathing at elevated 
rates, that are at or above a range of benchmark levels. The second is 
the percentage of these populations expected to experience days with an 
occurrence of a doubling or tripling of sRaw.
    The benchmark concentrations used in the comparison-to-benchmarks 
analysis (400, 300, 200 and 100 ppb) were identified based on 
consideration of the evidence discussed in section II.A.2 above. In 
particular, benchmark concentrations of 400 ppb, 300 ppb, and 200 ppb 
were based on concentrations included in the well-documented controlled 
human exposure studies summarized in section II.A.2 above, and the 100 
ppb benchmark was selected in consideration of uncertainties with 
regard to lower concentrations and population groups with more limited 
data (REA, section 4.5.1). At the upper end of this range, 400 ppb 
represents the lowest concentration in free-breathing controlled human 
exposure studies of exercising people with asthma where moderate or 
greater lung function decrements occurred that were often statistically 
significant at the group mean level and were frequently accompanied by 
respiratory symptoms, with some increases in these symptoms also being 
statistically significant at the group level (ISA, Section 5.2.1.2 and 
Table 5-2). At 300 ppb, statistically significant increases in lung 
function decrements (specifically reductions in FEV1) have 
been documented in analyses of the subset of controlled human exposure 
study subjects with asthma that are responsive to SO2 at 
concentrations below 600 or 1000 ppb (ISA, pp. 5-85 and 5-153 and Table 
5-21; Johns et al., 2010). The 200 ppb benchmark concentration 
represents the lowest level for which studies are available that have 
assessed the SO2 effect versus the effect of exercise in 
clean air and for which individual study subject data are available to 
summarize percent changes in sRaw and FEV1; moderate or 
greater lung function decrements were documented in some of these study 
subjects (ISA, Table 5-2 and Figure 5-1; PA, Table 3-1; REA, section 
4.6.1). With regard to exposure concentrations below 200 ppb, limited 
data are available for exposures at 100 ppb that, while not directly 
comparable to the data at higher concentrations because of differences 
in methodology and metrics reported,\63\ do not indicate that study 
subjects experienced responses of a magnitude as high as a doubling in 
sRaw. However, in consideration of some study subjects with asthma 
experiencing moderate or greater decrements in lung function at the 200 
ppb exposure concentration (approximately 8 to 9% of the study group) 
and of the paucity or lack of any specific study data for some groups 
of individuals with asthma, such as primary-school-age children and 
those with more severe asthma (described in sections II.B.3 and II.C.1 
of the proposal), a benchmark concentration of 100 ppb (one half the 
200 ppb exposure concentration) was also included in the analyses.
---------------------------------------------------------------------------

    \63\ As explained in section II.B.3 of the proposal, these 
studies involved exposures via mouthpiece, and only a few of these 
studies included an exposure to clean air while exercising that 
would have allowed for determining the effect of SO2 
versus that of exercise in causing bronchoconstriction and 
associated lung function decrements (ISA, section 5.2.1.2; PA, 
section 3.2.1.3).
---------------------------------------------------------------------------

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

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

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

[[Page 9882]]

area under air quality conditions just meeting the current standard, 
including the fine-scale temporal pattern of 5-minute concentrations. 
They also include uncertainty with regard to population groups and 
exposure concentrations for which the health effects evidence base is 
limited or lacking (PA, section 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 section 6.2 of the REA, with a brief overview provided 
here. Some aspects of the assessment approach contributing to this 
uncertainty include estimation of the 1-hour concentrations and the 
approach employed to adjust the air quality surface to concentrations 
just meeting the current standard (REA, section 6.2.2.2; PA, section 
3.2.2.2), as well as the estimation of 1-hour ambient air 
concentrations resulting from emissions sources not explicitly modeled. 
All of these assessment approaches influence the resultant temporal and 
spatial pattern of concentrations and associated exposure circumstances 
represented in the study areas (REA, sections 6.2.1 and 6.2.2). There 
is also uncertainty in the estimates of 5-minute concentrations in 
ambient air across the modeling receptors in each study area. The 
ambient air monitoring dataset available to inform the 5-minute 
estimates, much expanded in this review over the dataset available in 
the last review, is used to draw on relationships occurring at one 
location and over one range of concentrations to estimate the fine-
scale temporal pattern in concentrations at the other locations. While 
this is an important area of uncertainty in the REA results, because 
the ambient air 5-minute concentrations are integral to the 5-minute 
estimates of exposure, the approach used to represent fine-scale 
temporal variability in the three study areas is strongly based in the 
available information and has been evaluated in the REA (REA, Table 6-
3; sections 3.5.2 and 3.5.3).
    Another important area of uncertainty in the REA is particular to 
the lung function risk estimates derived for exposure concentrations 
below those represented in the evidence base (REA, Table 6-3). The E-R 
function on which the risk estimates are based generates non-zero 
predictions of the percentage of the at-risk population expected to 
experience a day with the occurrence of at least a doubling of sRaw for 
all 5-minute exposure concentrations each simulated individual 
encounters while breathing at an elevated rate. The uncertainty in the 
response estimates increases substantially with decreasing exposure 
concentrations below those well represented in the data from the 
controlled human exposure studies (i.e., below 200 ppb).
    Additionally, the assessment focuses on the daily maximum 5-minute 
exposure during a period of elevated breathing rate, summarizing 
results in terms of the days on which the magnitude of such exposure 
exceeds a benchmark or contributes to a doubling or tripling of sRaw. 
Although there is some uncertainty associated with the potential for 
additional, uncounted events in the same day, the health effects 
evidence indicates a lack of a cumulative effect of multiple exposures 
over several hours or a day (ISA, section 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 an effect similar to that of the initial exposure 
event (REA, Table 6-3). In addition, there is uncertainty regarding the 
potential influence of co-occurring pollutants on the relationship 
between short-term SO2 exposures and respiratory effects. 
For example, there is some limited evidence regarding the potential for 
an increased response to SO2 exposures occurring in the 
presence of other common pollutants such as PM (potentially including 
particulate sulfur compounds), nitrogen dioxide and ozone, although the 
studies are limited (e.g., with regard to their relevance to ambient 
exposures) and/or provide inconsistent results (ISA, pp. 5-23 to 5-26, 
pp. 5-143 to 5-144; 2008 ISA, section 3.1.4.7).\65\
---------------------------------------------------------------------------

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

    Another area of uncertainty, which remains from the last review and 
is important to our consideration of the REA results, concerns the 
extent to which the quantitative results represent the populations at 
greatest risk of effects associated with exposures to SO2 in 
ambient air. As recognized in section II.A.2, the evidence base of 
controlled human exposure studies does not include studies of children 
younger than 12 years old and is limited with regard to studies of 
people with more severe asthma.\66\ The limited evidence that informs 
our understanding of potential risk to these groups indicates the 
potential for them to experience greater impacts than other population 
groups with asthma under similar exposure circumstances or, in the case 
of people with severe asthma, to have a more limited reserve for 
addressing this risk (ISA, section 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.
---------------------------------------------------------------------------

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

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

[[Page 9883]]

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

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

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

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

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

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

B. Conclusions on Standard

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

[[Page 9884]]

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

    \70\ For people without asthma, such effects have only been 
observed in studies of exposure concentrations at or above 1000 ppb 
(ISA, section 5.2.1.7).
---------------------------------------------------------------------------

    With regard to the health effects evidence newly available in this 
review, in the proposal the Administrator noted that, while the health 
effects evidence, as assessed in the ISA, has been augmented with 
additional studies since the time of the last review, including more 
than 200 new health studies, it does not lead to different conclusions 
regarding the primary health effects of SO2 in ambient air 
or regarding exposure concentrations associated with those effects. Nor 
does it identify different or additional populations at risk of 
SO2-related effects. Thus, the Administrator recognized that 
the health effects evidence available in this review and addressed in 
the ISA is consistent with evidence available in the last review when 
the current standard was established and that this strong evidence base 
continues to demonstrate a causal relationship between relevant short-
term exposures to SO2 and respiratory effects, particularly 
with regard to effects related to asthma exacerbation in people with 
asthma. He also recognized that the ISA conclusion on the respiratory 
effects caused by short-term exposures is based primarily on the 
evidence from controlled human exposure studies that reported effects 
in people with asthma exposed to SO2 for 5 to 10 minutes 
while breathing at an elevated rate (ISA, section 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; 83 FR 26778, June 8, 
2018).
    In considering exposure concentrations of interest in this review, 
the Administrator particularly noted the evidence from controlled human 
exposure studies, also available in the last review, that demonstrate 
the occurrence of moderate or greater lung function decrements in some 
people with asthma exposed to SO2 concentrations as low as 
200 ppb for very short periods of time while breathing at elevated 
rates (ISA, Table 5-2 \71\ and Figure 5-1, summarized in Table 3-1 of 
the PA).\72\ He recognized that the data for the 200 ppb exposures 
include limited evidence of respiratory symptoms accompanying the lung 
function effects observed, and that the severity and number of 
individuals affected is found to increase with increasing exposure 
levels, as is the frequency of accompaniment by respiratory symptoms, 
such that, at concentrations at or above 400 ppb, the moderate or 
greater decrements in lung function were frequently accompanied by 
respiratory symptoms, with some of these findings reaching statistical 
significance at the study group level (ISA, Table 5-2 and section 
5.2.1; PA, section 3.2.1.3; 83 FR 26779, June 8, 2018).
---------------------------------------------------------------------------

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

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

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

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

[[Page 9885]]

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

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

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

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

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

[[Page 9886]]

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

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

3. Comments on the Proposed Decision
    During the public comment period for the proposed decision, we 
received 24 comments.
a. Comments in Support of Proposed Decision
    Of the comments addressing the proposed decision, the majority 
supported the Administrator's proposed decision to retain the current 
primary standard, without revision. This group includes an association 
of state and local air agencies, all of the state agencies that 
submitted comments, more than half of the industry organizations that 
submitted comments, and a couple of comments from individuals. All of 
these commenters generally note their agreement with the rationale 
provided in the proposal and the CASAC concurrence with the PA 
conclusion that the current evidence does not support revision to the 
standard. Most also cite the EPA and CASAC statements that information 
newly available in this review has not substantially altered our 
previous understanding of effects from exposures lower than what was 
previously examined or of the at-risk populations and does not call 
into question the adequacy of the current standard. They all find the 
proposed decision to retain the current standard to be well supported. 
The EPA agrees with these comments and with the CASAC advice regarding 
the adequacy of the current primary standard and the lack of support 
for revision of the standard.
    We additionally note that some of the industry commenters that 
stated their support for retaining the current standard without 
revision additionally stated that in their view the current standard 
provides more public health protection than the EPA has recognized in 
the proposal. As support for this view, these comments variously state 
that concentrations in most of the U.S. are well below those evaluated 
in the REA; that the studies in the ISA do not demonstrate 
statistically significant response to SO2 concentrations 
below 300 ppb; and, that a large percentage of the REA estimates of 
lung function risk is attributable to exposures below 200 ppb. The 
commenters also claim that in the 2010 decision that established the 
current standard (75 FR 33547, June 22, 2010), the EPA had determined 
that a standard protecting about 97-98% of exposed children with asthma 
from a doubling of sRaw would be appropriate, but that the estimates in 
the current REA indicate that over 99% of exercising children with 
asthma receive such protection from the current NAAQS.
    As an initial matter, while we agree with the commenters that most 
of the U.S. has SO2 concentrations below those assessed in 
the REA, we disagree that this indicates the standard is overly 
protective. Rather, this simply indicates the lack of large 
SO2 emissions sources in many parts of the country (although 
their presence in other parts of the country contributes to ambient air 
concentrations of SO2 similar to or higher than those in the 
REA). As recognized in section II.A.3 above, the REA is designed to 
inform our understanding of exposure and risk in areas of the U.S. 
where SO2 emissions contribute to airborne concentrations 
such that the current standard is just met because the REA is intended 
to inform the Agency's decision regarding the public health protection 
provided by the current standard, rather than to describe exposure and 
risk in areas with SO2 concentrations well below the current 
standard (e.g., such that they that would meet alternative more 
restrictive standards). This approach is consistent with section 109 of 
the CAA, which requires the EPA to review whether the current primary 
standard--not current air quality--is requisite to protect public 
health with an adequate margin of safety (CAA section 109(b)(1) and 
109(d)(1); see also NEDA/CAP, 686 F.3d at 813 [rejecting the notion 
that it would be inappropriate for the EPA to revise a NAAQS if current 
air quality does not warrant revision, stating ``[n]othing in the CAA 
requires EPA to give the current air quality such a controlling role in 
setting NAAQS'']). Thus, the EPA disagrees with the commenters that the 
public health protection provided by the standard is indicated by 
exposure and risk associated with air quality in parts of the U.S. with 
concentrations well below the standard, and finds the REA appropriately 
designed for purposes of informing consideration of the adequacy of the 
public health protection provided by the current standard.
    With regard to the characterization of risk in the REA, it is true 
as the commenters state that the lung function risk estimates include 
estimates of risk based on 5-minute exposures below 200 ppb and that 
the evidence from controlled human exposure studies is very limited for 
concentrations below 200 ppb. We recognize this as an uncertainty in 
the estimates (e.g., PA, section 3.2.2.3).\75\ In considering the 
uncertainties in and any associated implications of these estimates, we 
also recognize, however, that we lack information for some population 
groups, including young children with asthma and individuals with 
severe asthma who might exhibit responses at lower exposures than those 
already studied. And, as is noted in section II.A.2 above and by the 
CASAC in their advice (summarized in section II.B.2 above), there is 
the potential for responses in these populations to exposure 
concentrations lower than those that have been tested in the controlled 
human exposure studies. Thus, while we recognize the uncertainty in the 
estimates noted by the commenters, we have considered the methodology 
(which derived risk estimates based on

[[Page 9887]]

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

    \75\ For example, the PA recognizes the uncertainty in the lung 
function risk estimates increases substantially with decreasing 
exposure concentrations below those examined in the controlled human 
exposure studies (PA, section 3.2.2.3; REA, Table 6-3).
---------------------------------------------------------------------------

    Lastly, we do not agree with the comment that the estimates of 
children protected from exposures of concern by the now-current 
standard were appreciably lower when the standard was established. 
While there are a number of differences between the 2009 REA and the 
quantitative modeling and analyses performed in the current REA (as 
described in PA, section 3.2.2 and summarized in section II.A.3 above), 
the percentage of children with asthma that are estimated in the 
current REA to experience at least a doubling in sRaw ranges up to 
98.7% as a 3-year average across the three study areas.\76\ Although 
the REA in the last review did not estimate risk for a 1-hour standard 
with a level of 75 ppb, the estimate from the current REA falls 
squarely between the 2009 REA estimates for the two air quality 
scenarios most similar to a scenario just meeting the current standard: 
99.5% for a level of 50 ppb and 97.1% for a level of 100 ppb (PA, 
section 3.2.2.2; 74 FR 64841, Table 4, December 8, 2009). In making 
their comment, the commenters claim that the 2010 decision conveyed 
that the selected standard of 75 ppb would protect 97 to 98 percent of 
exposed children from a doubling of sRaw. Given the lack of 2009 REA 
estimates for the level of 75 ppb, it might be presumed that the 
commenter's two percentages represent the results for the 50 ppb and 
100 ppb scenarios, thus providing a range within which results for 75 
ppb might be expected to fall. However, that is not the case; rather, 
the percentages cited by the commenter (97-98%) pertain to the 2009 REA 
sRaw risk estimates for the air quality scenario with a standard level 
of 100 ppb (75 FR 35547, June 22, 2010; 74 FR 64841 and Table 4, 
December 8, 2009). Thus, the comment's statement is not borne out by 
the risk estimates relevant to the current standard. Further, while we 
recognize distinctions between the methodology and scenarios for the 
two REAs, we find the estimates for lung function risk based on sRaw 
and the similar estimates for exposures at or above the 200 ppb 
benchmark to be of a magnitude roughly consistent between the two REAs 
(as summarized in PA, section 3.2.2.2 and 3.1.1.2.4). Accordingly, 
while we agree there are uncertainties in the evidence and in the 
exposure and risk estimates, the currently available information 
indicates a level of protection to be afforded by the current standard 
that is generally similar to what was indicated by the evidence 
available when the standard was set in 2010. For these reasons, we 
disagree that the current standard provides more public health 
protection than recognized in the proposal.
---------------------------------------------------------------------------

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

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

[[Page 9888]]

did not provide adequate protection from the short-term concentrations 
of most concern. As a result, the decision in the last review was to 
provide for revocation of the 24-hour standard and to establish the now 
current 1-hour standard to provide the needed protection of at-risk 
populations with asthma from respiratory effects of SO2 (75 
FR 35550, June 22, 2010). To the extent that these comments on the 
proposal in the current review are intended to imply that the 
epidemiologic studies briefly mentioned in the quotation from the 
comment in the last review or studies that have become available in the 
intervening years indicate that the current standard is inadequate, the 
comments do not provide any explanation or analysis to support such an 
assertion. With regard to the current standard and the epidemiologic 
evidence, we further note that such evidence was considered by the 
Administrator in 2010 (as were the comments submitted at that time) in 
the setting of the now-current standard, and that the EPA has again 
considered the complete body of evidence in this review and found no 
newly available studies that might support alternative conclusions (75 
FR 35548, June 22, 2010; 83 FR 26765, June 8, 2018). While the pattern 
of associations across the newly available epidemiologic studies is 
consistent with the studies available in the last review, key 
uncertainties remain, including the potential for confounding by PM or 
other copollutants (as summarized in section II.A.2 above). Among the 
U.S. epidemiologic studies reporting mostly positive and sometimes 
statistically significant associations between ambient SO2 
concentrations and emergency department visits or hospital admissions 
(some conducted in multiple locations), few studies have attempted to 
address this uncertainty, e.g., through the use of copollutant models 
(83 FR 26765, June 8, 2018; ISA, section 5.2.1.2). In the last review, 
there were three U.S. studies for which the SO2 effect 
estimate remained positive and statistically significant in copollutant 
models with PM.\78\ As noted in the proposal, no additional such 
studies have been newly identified in this review (83 FR 26765, June 8, 
2018). The conclusions of these studies and the air quality of the 
study areas were given consideration by the Administrator in 2010 in 
setting the current standard (83 FR 26761, June 8, 2018), and they do 
not call into question the adequacy of the current standard in this 
review.
---------------------------------------------------------------------------

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

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

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

    Commenters in support of a lower level for the standard 
additionally express concern that populations living in communities 
near large sources of SO2 emissions, including children in 
population groups with relatively higher asthma prevalence, may not be 
adequately protected by the current standard. In considering this 
comment, we note that while the REA did not categorize simulated 
children with asthma with regard to specific demographic subgroups, 
such as those mentioned by the commenter or discussed in section 
II.A.2.d above, the estimates are for children with asthma in areas 
with large sources of SO2 emissions and with air quality 
just meeting the current standard. As noted in section II.A.3 above, 
the asthma prevalence across census tracts in the three REA study areas 
ranged from 8.0 to 8.7% for all ages (REA, section 5.1) and from 9.7 to 
11.2% for children (REA, section 5.1), which reflects some of the 
higher prevalence rates in the U.S. today (PA, sections 3.2.1.5 and 
3.2.2.1). Thus, in considering these results to inform his decision 
regarding the adequacy of protection provided by the current standard, 
the Administrator is focused on the patterns of exposure and 
populations with elevated rates of asthma stated to be the situation of 
concern to these commenters.
    In two of the three REA study areas, each of which include large 
emissions sources and air quality adjusted to just meet the current 
standard, no children with asthma were estimated to experience a day 
with an exposure while breathing at elevated rates to a 5-minute 
SO2 concentration at or above 400 ppb, the concentration at 
which moderate or greater lung function decrements have been documented 
in 20-60% of study subjects, with decrements frequently accompanied by 
respiratory symptoms. In the third area the estimate was less than 
0.1%, on average across the 3-year period. Further, fewer than 1% of 
children with asthma, on average across the 3-year assessment period, 
were estimated to experience any days with exposures at or above 200 
ppb in two of the areas, and no children were estimated to experience 
such days in the third area (PA, Table 3-3; 83 FR 26775, June 8, 2018). 
Thus, the REA exposure and risk estimates for the current review 
indicate that the current standard is likely to provide a very high 
level of protection from SO2-related effects documented at 
higher concentrations and a high level of protection from the transient 
lung-function decrements documented in individuals with asthma in 
controlled human exposure study concentrations as low as 200 ppb.
    The comment claiming that the current standard does not provide an 
adequate margin of safety emphasized limitations in the evidence base 
of controlled human exposure studies, noting the very limited available 
studies that examined 5-minute SO2 exposures as low as 100 
ppb; the lack of studies in young children with asthma and people of 
any age with severe asthma; and that the studies did not examine the 
impact of multiple exposures in the same day. While we agree that the

[[Page 9889]]

evidence base is limited with regard to examination of potential 
effects at lower concentrations and in some population groups, we 
disagree with the latter statement that the currently available studies 
have not investigated multiple exposures within the same day. In fact, 
there are some studies that inform our understanding of responses to 
repeated occurrences of exposure during exercise within the same day 
(REA, Table 6-3; ISA, section 5.2.1.2). For example, there are studies 
that have investigated the magnitude of lung function response from 
separate exercise events within the same 1-hour or 6-hour exposure, and 
from exposures with exercise occurring on subsequent days (Linn et al., 
1984; Kehrl et al., 1987). As an initial matter, we note that the 
evidence shows lung function decrements that occur with short 
SO2 exposures are resolved with the cessation of either the 
exposure or exercise, with lung function returning to baseline in 
either situation (ISA, section 5.2.1.2). Further, responses to repeated 
exercise events occurring within the same 1-hour or 6-hour exposure are 
diminished in comparison to the response to the initial event (Kehrl et 
al., 1987; Linn et al., 1984; Linn et al., 1987). Even responses to 
exposures while exercising that are separated by a day are still very 
slightly diminished from the initial response (Linn et al., 1984). 
Thus, we disagree with the commenter's statement that the available 
controlled human exposure studies have not examined the impact of 
multiple exposures in the same day. While the studies involve single 
continuous exposure periods shorter than a day, the discontinuous 
nature of the exercise component of the exposure design provides the 
relevant circumstances for assessing the impact of multiple exposure-
with-exercise events in a single day. The evidence from these studies 
documents the transient nature of the lung function response, even to 
the high concentrations studied (600 to 1000 ppb), as well as a 
lessening of decrements in response to subsequent occurrences within a 
day.
    We agree with this comment that the evidence base is limited with 
regard to examination of potential effects at lower concentrations and 
in some population groups. As summarized in I.A.2 above, the health 
effects evidence newly available in this review does not extend our 
understanding of the range of exposure concentrations that elicit 
effects in people with asthma exposed while breathing at an elevated 
rate beyond what was understood in the last review. As in the last 
review, 200 ppb remains the lowest concentration tested in controlled 
human exposure studies where study subjects are freely breathing in 
exposure chambers. The limited information available for exposure 
concentrations below 200 ppb, including exposure concentrations of 100 
ppb, while not amenable to direct quantitative comparisons with 
information from studies at higher concentrations, generally indicates 
a lesser response. Further, as discussed in section II.A.2 above, we 
recognize that evidence for some at-risk population groups, including 
young children with asthma and individuals with severe asthma, is 
limited or lacking at any exposure concentration. As discussed in 
section II.B.4 below, the Administrator has explicitly recognized this 
in reaching conclusions regarding the adequacy of the public health 
protection provided by the current standard, including considerations 
of margin of safety for the health of at-risk populations.
    One commenter advocating a more stringent standard additionally 
notes that evidence from controlled human exposure studies is also 
lacking for adults older than 75 years, an age group for which the 
commenter states there is new research placing this age group at 
increased risk. While some recent epidemiologic studies have examined 
associations of SO2 with the occurrence of various health 
outcomes in older adults (typically ages 65 years and older), such 
studies have not consistently found stronger associations for this 
group compared to younger adults (ISA, sections 6.5.1.2 and 6.6). As a 
result, the ISA concluded that the evidence was only suggestive of the 
older age group being at increased risk of SO2-related 
health effects. Such a characterization indicates that ``the evidence 
is limited due to some inconsistency within a discipline or, where 
applicable, a lack of coherence across disciplines'' (ISA, Table 6-1), 
and in this case, the ISA indicates that the study results were 
concluded to be ``mixed'' or ``generally inconsistent'' (ISA, Table 6-
7). Further, there is no evidence indicating that the individuals in 
this group would be affected at lower exposure concentrations than 
other population groups or that they would be inadequately protected by 
the current standard. As noted by the CASAC more broadly, ``there are 
many susceptible subpopulations that have not been studied and which 
could plausibly be more affected by SO2 exposures than 
adults with mild to moderate asthma'' (Cox and Diez Roux, 2018b, p. 3 
of letter).
    With that recognition in mind, the CASAC explicitly considered the 
issue of margin of safety provided by the current standard. While 
noting that ``[i]t is plausible that the current 75 ppb level does not 
provide an adequate margin of safety in these groups,'' the CASAC 
additionally stated that ``because there is considerable uncertainty in 
quantifying the sizes of these higher risk subpopulations and the 
effect of SO2 on them, the CASAC does not recommend 
reconsideration of the level at this time'' (Cox and Diez Roux, 2018b, 
p. 3 of letter). The CASAC additionally concluded that the 75 ppb level 
of the standard ``is protective'' and that the current scientific 
evidence ``does not support revision of the primary NAAQS for 
SO2'' (Cox and Diez Roux, 2018b, pp. 1 and 3 of letter). In 
addition, we note that the D.C. Circuit has concluded that the 
selection of any particular approach for providing an adequate margin 
of safety is a policy choice left specifically to the Administrator's 
judgment (Lead Industries Association v. EPA, 647 F.2d at 1161-62; 
Mississippi, 744 F.3d at 1353). In light of such considerations, as 
discussed in section II.B.4 below, the Administrator does not agree 
with commenters that the current standard fails to include an adequate 
margin of safety or otherwise insufficiently protects older adults or 
other population groups, including those that are recognized as being 
most at risk of SO2-related effects in this review, i.e., 
people with asthma, in particular children with asthma.
    As additional support for their view that the standard level should 
be revised to 50 ppb, one of the commenters states that any new 
standard would have to be more protective to make up for the lack of 
progress on implementation of the 2010 standard. Such a rationale lacks 
a basis in the CAA. The requirements in sections 108 and 109 of the CAA 
for establishing and reviewing the NAAQS are separate and distinct from 
the CAA requirements for implementing the NAAQS (e.g., CAA sections 
107, 110, and 172), and the time it takes to attain a standard under 
those requirements is not evidence pertaining to the adequacy of that 
standard with regard to public health protection under section 109. In 
setting primary and secondary standards that are ``requisite'' to 
protect public health and public welfare, respectively, as provided in 
section 109(b), the EPA's task is to establish standards that are 
neither more nor less stringent than necessary for these purposes.\80\

[[Page 9890]]

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

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

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

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

    The commenter that recommended establishment of a 24-hour standard, 
with a level of 40 ppb, stated that epidemiologic studies support the 
need for an additional 24-hour standard and note their position in the 
2010 review for revision of the level of the then-existing 24-hr 
standard to 40 ppb, matching the level of California's current 24-hour 
standard. In terms of support for their advocacy of a 24-hour standard, 
the commenter cited three epidemiologic studies of associations of 
short-term SO2 concentrations with premature death from 
respiratory causes in Chinese cities and two studies of associations of 
longer-term SO2 concentrations with the development of 
asthma (conducted in the U.S. and Canada). We disagree that these 
studies indicate an inadequacy of the existing standard or indicate a 
need for an additional standard. As an initial matter, we note that the 
ISA for this review has assessed the current evidence regarding 
SO2 and mortality, including the evidence provided by the 
three studies in Chinese cities. We agree with the comment that these 
three studies include analyses that controlled for some co-occurring 
pollutants, although we note that those analyses were limited to 
investigation of just two co-occurring pollutants, PM10 and 
NO2. We additionally note that while the copollutant 
analyses found associations with SO2 that generally remain 
positive and statistically significant after adjustment for 
PM10, those after-adjustment associations are somewhat 
attenuated, indicating potential contributions to the association from 
PM10 (ISA, section 5.2.1.2, p. 5-145).\82\ Moreover, these 
analyses show that after

[[Page 9891]]

adjustment for NO2, the associations are much more 
attenuated and lose statistical significance (ISA, section 5.2.1.2, p. 
5-145). Further, none of the studies adjusted for PM2.5 (PM 
with mass median aerodynamic diameter nominally below 2.5 microns), a 
pollutant of particular importance with regard to potential confounding 
of epidemiologic analyses for SO2 because of the fact that 
SO2 is a precursor of PM2.5 (ISA, section 
1.6.2.4; PA, section 3.2.1.1). Additionally, these studies are limited 
in that they were conducted in Asian cities where the air pollution 
mixture and concentrations are different from the U.S., e.g., 
SO2 concentrations are much higher than concentrations in 
the U.S., which limits generalizability and ``complicates the 
interpretation of independent association for SO2'' (ISA, 
Table 5-21; section 5.2.1.8) at lower concentrations where there are no 
studies that have controlled for relevant copollutants. In 
consideration of the full evidence base in this review, including these 
studies, the ISA concludes that the evidence regarding short-term 
SO2 concentrations and respiratory mortality ``is 
inconsistent within and across disciplines and outcomes, and there is 
uncertainty related to potential confounding by copollutants'' (ISA, p. 
5-155). Accordingly, as noted in the ISA, this limited and inconsistent 
evidence for associations with premature mortality does not 
substantially contribute to the determination that short-term 
SO2 exposure is causally related to respiratory effects, a 
determination supported primarily by evidence from controlled human 
exposure studies (ISA, p. 5-153).
---------------------------------------------------------------------------

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

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

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

    With regard to the epidemiologic studies of associations between 
long-term SO2 concentrations and respiratory effects, 
including development of asthma, the ISA concluded that, for long-term 
exposure and respiratory effects, the complete evidence base, including 
those studies cited by the commenter, was suggestive of, but not 
sufficient to infer, the presence of a causal relationship (ISA, 
Section 5.2.2, Table 5-24). While limited animal toxicological evidence 
suggests biological plausibility for such effects of SO2, 
the overall body of evidence across disciplines lacks consistency and 
there are uncertainties that apply to the epidemiologic evidence, 
including that newly available in this review, across the respiratory 
effects examined for long-term exposure (ISA, sections 1.6.1.2 and 
5.2.2.7). In this light, the ISA concludes that there is uncertainty 
remaining regarding potential copollutant confounding and an 
independent effect of long-term SO2 exposure, so that 
chance, confounding, and other biases cannot be ruled out (ISA, Table 
1-1). Thus, we disagree with the commenter that the current evidence 
base supports their concern regarding long-term exposure or a need for 
longer-term standard. In so doing, we additionally note the conclusion 
reached in the last review that a standard based on 1-hour daily 
maximum SO2 concentrations will afford requisite increased 
protection for people with asthma and other at-risk populations against 
an array of adverse respiratory health effects \85\ related to short-
term SO2 exposures ranging from 5 minutes to 24 hours. As 
described in section II.B.4 below, the Administrator also concludes, 
based on the current review of the available scientific evidence 
documented in the ISA (which includes the studies cited by the 
commenter) and the REA estimates, that the current standard continues 
to provide the requisite protection of public health from health 
effects of sulfur oxides in ambient air.
---------------------------------------------------------------------------

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

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

[[Page 9892]]

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

    \86\ As described in section II.A.2.c and consistent with the 
ISA in the last review, moderate or greater SO2-related 
bronchoconstriction or decrements in lung function referred to the 
occurrence of at least a doubling in sRaw or at least a 15% 
reduction in FEV1 (ISA, section 5.2.1.2 and Table 5-2).
---------------------------------------------------------------------------

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

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

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

[[Page 9893]]

should protect against, stated that the standard of 75 ppb is more 
stringent than necessary and advocate revision of the level to a value 
no lower than 150 ppb, or a level just below 110 ppb.
    The commenters advocating a level no lower than 150 ppb emphasize 
their view that the current standard is more stringent than necessary 
because it considers protection against 5-minute SO2 
concentrations of 200 ppb and higher rather than only 400 ppb and 
higher. They claim that adjusting the focus to one aimed at 
concentrations of 400 ppb and higher provides support for a revised 
level of 150 ppb and point, without further elaboration, to their 
comment submission during the public comment period for the 2010 
rulemaking as providing supporting analysis. Similar to the cited 
submission from the 2010 rulemaking, the core argument of their current 
comments appears to be that the standard does not need to protect 
against exposures lower than 400 ppb, and that the EPA should not 
consider information about exposures as low as 200 ppb, which they 
claim was EPA's focus in its 2009 proposal to set the level for the new 
1-hour standard within the range of 50 to 100 ppb. Rather, the 
commenters claimed that the EPA should focus only on 400 ppb and that 
based on results of analyses presented in the 2009 REA, a standard no 
lower than 150 ppb provides comparable protection for the 400 ppb 
benchmark as a standard between 50 and 100 ppb was estimated to provide 
for the 200 ppb benchmark. For example, the cited 2010 comment 
submission stated that the air quality analyses presented in the 2009 
REA (based on air quality data for 40 U.S. counties from the late 1990s 
through 2007 and an estimated relationship between 1-hour and 5-minute 
concentrations, and involving the adjustment of the 1-hour 
concentrations to just meet different 99th percentile daily maximum 1-
hour standards) indicates that the range of maximum annual mean number 
of days estimated to have 5-minute concentrations at or above 400 ppb 
at monitors adjusted to just meet 99th percentile daily maximum 1-hour 
standard levels of 150 and 200 ppb (7 to 13 days) was similar to the 
number of such days estimated to have 5-minute concentrations at or 
above 200 ppb at monitors adjusted to just meet 99th percentile daily 
maximum 1-hour standard levels of 50 and 100 ppb (2 to 13 days).
    As an initial matter, as noted above, we do not believe that merely 
pointing to a comment or analysis offered during the last review, on 
the 2009 proposal, is sufficient to raise a significant comment in this 
review, without further description of why the issues raised in the 
2010 review are still relevant to the proposal in the current review, 
which the commenter has not provided. Additionally, as explained above, 
the EPA continues to disagree with the view that the Agency should not 
consider the amount of protection provided by the primary 
SO2 standard against 5-minute exposures to 200 ppb 
SO2 in evaluating the current standard. Further we disagree 
with the commenter that the air quality and exposure analyses for 
different standard levels presented in the 2009 REA provide an 
appropriate basis for considering potential exposures allowed by the 
current standard. This is because the air quality and exposures 
analyses presented in the 2009 REA are appreciably limited compared to 
those available in the current review. The exposure analyses for this 
review are extensively improved and expanded over the 2009 analyses, as 
summarized in section II.A.3 above, including the fact that they 
address the full 3-year period of the standard rather than a single 
year of air quality and that they assess the existing standard rather 
than standard levels above and below the existing level. Additionally, 
the air quality data available in this review are appreciably expanded 
since the dataset used in the 2009 REA, such that the current dataset 
is much more robust. As just one example of this, the analyses of 
frequency of 5-minute concentrations above specific benchmarks at 
monitors meeting the current standard have been able to be conducted 
with 5-minute measurements rather than 5-minute concentration estimates 
as was the case in the last review. These analyses of recent air 
quality data indicate that at monitors with concentrations that meet 
the current standard, the maximum annual mean number of days with a 5-
minute concentration above 400 ppb was seven (PA, section 2.3.2.3, 
Appendix C), a value falling within the range that the 2010 comment had 
found acceptable for the what was to be a new 1-hour standard (based on 
the then-available data). Thus, putting aside the commenter's view that 
no weight should be given to 5-minute SO2 concentrations 
below 400 ppb (a view with which we disagree as discussed above), we 
note that the air quality analyses available in this review, which 
provide a more robust characterization of 5-minute concentrations 
occurring in locations meeting the current standard than that estimated 
in the 2009 REA, indicate the control of 5-minute 400 ppb 
concentrations provided by the current standard to be within with the 
commenter's target range. Thus, even if we accepted the premise that 
the current standard should be evaluated based solely on the degree of 
control of 5-minute 400 ppb concentrations, the basis for the 
commenter's concern that the current standard is overly stringent is 
not found in the current air quality analyses.
    The comment that advocated revision of the level to a value just 
below 110 ppb provides little explanation for this specific alternative 
level. Given this commenter's emphasis on 300 ppb as the relevant 
benchmark from the controlled human exposure studies (and their view 
that EPA inappropriately considered 200 ppb), we interpret this comment 
as relating to application of a factor to the existing standard level, 
with the factor being derived by dividing 300 ppb (the exposure the 
commenter claims should be the focus for the standard) by 200 ppb (the 
concentration the commenter claims is the focus of the existing 
standard).\89\ This commenter additionally cites several court 
decisions in support of EPA standard-setting decisions, two of which 
related to the EPA's setting of the level for the PM standard (a 
standard established with primary consideration of epidemiologic rather 
than controlled human exposure studies) at a concentration which the 
commenter describes as ``just below'' concentrations in areas and study 
periods for which epidemiologic studies observed a statistical 
association with health outcomes.\90\ Thus, we interpret the comment to 
suggest that the standard level should be set slightly below the value 
resulting from application of the factor of 300 ppb divided by 200 ppb 
to the existing standard level of 75 ppb, i.e., the level should be 
revised to just below about 110 ppb.
---------------------------------------------------------------------------

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

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

[[Page 9894]]

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

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

    As summarized in section II.A.1 above, the existing standard, with 
its level of 75 ppb, was established in 2010 based on consideration of 
the level of protection provided from short exposures to peak 
concentrations of SO2, as indicated from the REA results 
available at that time for standard levels above and below 75 ppb, as 
well as judgments of an adequate margin of safety in light of 
concentrations in a set of epidemiologic studies that found 
statistically significant associations of SO2 concentrations 
with respiratory health outcomes when using copollutant models with PM. 
Review of the current standard is based on the health effects evidence 
and exposure and risk information now available, including the exposure 
and risk estimates for air quality scenarios in which the current 
standard is just met (which were not available at the time the standard 
was set). Based on all of the currently available information, the 
Administrator has concluded that the current standard (in all of its 
elements) remains requisite to protect public health with an adequate 
margin of safety (as discussed in section II.B.4, below), and that a 
less stringent standard would not provide adequate protection.
    The commenters who stated that the percentile aspect of the form of 
the standard should be revised to be the 98th percentile rather than 
the current 99th percentile based their rationale primarily on their 
views that either 300 ppb or 400 ppb is the lowest exposure level that 
should be considered in evaluating the protection provided by the 
standard. These commenters state that the EPA's 2010 selection of the 
99th percentile was based on the Agency's conclusion regarding the 
greater effectiveness of a 99th percentile form than a 98th percentile 
form with regard to controlling 5-minute concentrations at and above 
200 ppb. These commenters generally state that with a change in focus 
to one that considers only the protection provided from exposures at 
and above either 300 ppb or 400 ppb (a change that they advocate), a 
98th percentile form would provide effective control of the relevant 5-
minute concentrations. Additionally, beyond the disagreement with the 
EPA about the need to protect at-risk populations from exposures below 
300 ppb or 400 ppb (addressed above), the commenters variously cite the 
following reasons for such a revision in form: (1) The view that a 98th 
percentile would provide greater regulatory stability than a 99th 
percentile form; and (2) a claim that EPA's choice of a 99th percentile 
form in 2010 was inappropriately based in part on concentrations in 
three U.S. epidemiologic studies and in part on EPA's air quality 
analyses of the effectiveness of control of 5-minute 
concentrations.\92\
---------------------------------------------------------------------------

    \92\ The commenter making this claim additionally states that 
the EPA has not to date provided an explanation of why a 99th 
percentile form would be more effective than a 98th percentile form 
in providing such control.
---------------------------------------------------------------------------

    With regard to the first reason, the issue of regulatory stability 
was considered by the EPA in selecting the 99th percentile form when 
the standard was established in 2010. As described in the last review, 
analyses in the 2009 REA indicated that over a 10-year period, there 
appeared to be little difference in the stability of design values 
based on a 98th or 99th percentile form, leading the EPA to conclude at 
that time that there would ``not be a substantial difference in 
stability between 98th and 99th percentile forms'' (75 FR 35540, June 
22, 2010; 2009 REA, section 10.5.3). Further, the commenter provides no 
alternative analysis to support their view that the 98th percentile is 
more stable; nor do they provide any reasoning or analysis that would 
demonstrate a flaw in the EPA analysis or conclusions. Thus, we are not 
aware of any basis for the view that a 98th percentile form would offer 
greater stability.
    With regard to the second reason, as an initial matter, we note 
that the question of whether the 99th percentile form was appropriately 
adopted in 2010 is a question that the EPA resolved in the last review, 
and one that is not before us in this review.\93\ However, to the 
extent that the comment is intended to suggest that we should not 
retain the 99th percentile form in this review based on the objections 
raised in the comments, we respond as follows. First, we find the 
commenter to be mistaken in their assertion that the EPA's choice of 
the 99th percentile for the percentile aspect of the form in setting 
the current standard relied on specific concentrations in three U.S. 
epidemiologic studies. In making this assertion, the commenter 
incompletely paraphrases a statement in the proposal for this review 
regarding the elements of the 2010 standard and the Administrator's 
judgment that this standard would provide the requisite protection for 
at-risk populations against the array of adverse respiratory health 
effects related to short-term SO2 exposures, including those 
as short as 5 minutes (83 FR 26756, June 8, 2018) and then incorrectly 
relates the EPA's 2010 judgment on form for the standard to a statement 
in the proposal in the current review that summarized 99th percentile 
daily maximum 1-hour concentrations \94\ in a set of U.S. studies for 
which the SO2 effect estimates remain positive and 
statistically significant in copollutant models with PM (83 FR 26765, 
June 8, 2018). The disconnected statements cited by the commenter do 
not refer to the EPA's rationale in setting the form for the current 
standard or its rationale in the proposal in this review to retain the 
current standard without revision. Rather, the basis for the form for 
the current standard, and rationale in this review, is summarized in 
sections II.A.1 and II.B.3 of the proposal (83 FR 26760, 26782, June 8, 
2018) \95\ and in sections

[[Page 9895]]

II.A.1 and II.B.1 above. Briefly, the statistical form of the current 
standard is based on consideration of the health effects evidence, 
stability in the public health protection provided by the programs 
implementing the standard, and advice from the CASAC, as well as 
results of air quality analyses in the 2009 REA for alternative 
standard forms (75 FR 35539-41, June 22, 2010). Because the premise of 
the comment is mistaken, it does not provide grounds to conclude in 
this review that the 99th percentile form is inappropriate.
---------------------------------------------------------------------------

    \93\ The EPA has not reopened the last review in this action.
    \94\ The commenter additionally states their view regarding 
comparison of 99th and 98th percentiles of daily maximum hourly 
concentrations in these epidemiologic studies (which variously 
differed by some 10 to 20%) that there is little if any statistical 
difference between them, although no statistical analyses were 
submitted in support of this view.
    \95\ The relevant section in the Federal Register notification 
of proposed decision for this review begins with the phrase ``[w]ith 
regard to the statistical form for the new 1-hour standard.'' This 
section is a summary of the section titled ``Conclusions on Form'' 
in the 2010 Federal Register notification of final decision (75 FR 
35541, June 22, 2010). While the Administrator's conclusion on form 
for the current standard considered the need to limit the upper end 
of the distribution of SO2 concentrations in ambient air 
to provide protection with an adequate margin of safety against 
effects reported in both epidemiologic and controlled human exposure 
studies, the choice of 99th percentile over 98th percentile was not 
based on specific epidemiologic study concentrations. Rather, in 
considering the epidemiological evidence in her decision on standard 
level, the Administrator considered SO2 concentrations in 
three specific epidemiologic studies (as summarized in II.A.1 above) 
in terms of the 99th percentile in light of her selection of that 
percentile for the standard form (75 FR 35547, June 22, 2010).
---------------------------------------------------------------------------

    With regard to the comment about the 2009 REA air quality analyses 
in the 2010 review, the analyses found a 99th percentile form to be 
appreciably more effective at limiting 5-minute peak SO2 
concentrations than a 98th percentile form (75 FR 35539-40, June 22, 
2010; 2009 REA, section 10.5.3, Figures 7-27 and 7-28). To the extent 
that the commenter intended to assert that it is inappropriate to 
retain the 99th percentile based on objections to this analysis or its 
consideration in establishing the form of the standard, we disagree. 
While the comment notes the findings of these air quality analyses and 
the fact that a 98th percentile form would allow appreciably more days 
per year with 5-minute concentrations above 400 ppb and 200 ppb, it 
claims that the EPA's conclusion in the last review of greater 
effectiveness was arbitrary and misplaced for four reasons, three of 
which refer to aspects of epidemiologic studies and one which appears 
to point to the controlled human exposure studies stating that 
statistically significant findings at the study group level have not 
been found for exposures to short-term SO2 concentrations 
below 300 ppb. As above, we note that any challenges to whether the EPA 
reached the appropriate conclusions in the last review are not properly 
before us in this review, as this is a new review of the current 
standard based on the current record and the EPA did not reopen the 
last review in this action. However, to the extent that the comment is 
intended to suggest that we should not retain the 99th percentile form 
in this review based on these four reasons, we respond as follows. As 
the epidemiologic studies were not identified as a factor in the EPA's 
2010 decision on the 99th percentile (versus a 98th percentile) form 
for the standard (75 FR 35541, June 22, 2010),\96\ and were not 
identified as a basis for the proposal in this review to retain the 
current standard, without revision, we find the commenter's reasons 
related to epidemiologic studies to have no relevance to our decision 
here. With regard to statistical significance of study subject 
responses below 300 ppb, putting aside our disagreement with the 
comment about the need to protect at-risk populations from exposures 
below 300 ppb (addressed above), we note that the air quality analyses 
relied on in the 2010 decision also demonstrated greater control of 5-
minute concentrations above 300 (at 400 ppb) by the 99th percentile. 
Further, the comment also does not provide any reason for why a 98th 
percentile would be a more appropriate form. Accordingly, we find the 
comment lacks a sound basis for any claim that the form of the standard 
is arbitrary and misplaced or should not be retained. Therefore, we 
conclude that this comment does not call into question the 
appropriateness of the form of the current standard.
---------------------------------------------------------------------------

    \96\ The EPA's consideration of epidemiologic studies in its 
2010 decision on the specific percentile for the form for the 
standard was with regard to the appropriateness of a percentile 
above the 90th, and not, as implied by the commenter, with regard to 
the selection of the 99th percentile (e.g., as compared to the 98th 
percentile). Specifically, the Administrator at that time noted 
that, in line with the controlled human exposure study findings of 
effects from peak concentrations, some of the epidemiologic studies 
described in the 2008 ISA reported an increase in SO2-
related respiratory health effects at the upper end of the 
distribution of ambient air concentrations (i.e., above 90th 
percentile SO2 concentrations; see ISA, section 5.3, p. 
5-9). Accordingly, the Administrator concluded that the form of a 
new 1-hour standard should be especially focused on limiting the 
upper end of the distribution of ambient SO2 
concentrations (i.e., above 90th percentile SO2 
concentrations) in order to provide protection with an adequate 
margin of safety against effects reported in both epidemiologic and 
controlled human exposure studies (75 FR 35541, June 22, 2010).
---------------------------------------------------------------------------

    We also disagree with these commenters that a 98th percentile form 
would provide effective control of short exposures to peak 
SO2 concentrations, for either exposures at and above 200 
ppb or exposures to the still higher concentrations on which the 
commenters prefer to focus (at and above 300 ppb or 400 ppb). In this 
regard, we note as an initial matter the EPA analysis on which the 2010 
conclusion is based (summarized immediately above); that analysis, 
presented in the 2009 REA ``indicated that at a given SO2 
standard level, a 99th percentile form is appreciably more effective at 
limiting 5-minute peak SO2 concentrations than a 98th 
percentile form'' (75 FR 35540, June 22, 2010; 2009 REA, section 
10.5.3, Figures 7-27 and 7-28). Further, we describe here a set of 
additional analyses of more recent air quality performed in the current 
review, the results of which support that conclusion in this review 
(Solomon et al., 2019). From these analyses of air monitoring data at 
337 monitoring sites in the U.S., it can be seen that, compared to the 
current 99th percentile standard, a standard with an alternative 98th 
percentile-based form exerts less control of 5-minute peaks. For 
example, during this recent time period (2014-2016), there were three 
times as many 5-minute daily maximum concentrations at or above 400 
ppb, 24 times as many such concentrations at or above 300 ppb, and more 
than 25 times as many such concentrations at or above 200 ppb at sites 
meeting an alternative 98th percentile standard as at sites meeting the 
current standard with its 99th percentile form (Solomon et al., 2019, 
Tables 1 and 2).
    Thus, together, the stability analyses documented in the 2010 
review and the analyses of more recent air quality demonstrate that the 
98th and 99th percentile forms have similar stability, and that a 
standard revised to have a 98th percentile form provides appreciably 
less control than the current standard, both with regard to 5-minute 
concentrations above 400 ppb and 300 ppb, and also such concentrations 
above 200 ppb. The CASAC similarly concluded that the 99th percentile 
form is preferable to a 98th percentile form to limit the upper end of 
the distribution of 5-minute concentrations (Cox and Diez Roux, 2018b, 
p. 3 of letter). Accordingly, a standard with a 98th percentile-based 
form would provide less protection than that provided by the current 
standard from peak SO2 concentrations, even from those at or 
above 400 ppb or 300 ppb, the concentrations that the commenters state 
are appropriate for the standard to provide protection from. 
Additionally, as discussed in section II.B.4 below, the Administrator 
considers it appropriate for the primary SO2 standard to 
control 5-minute concentrations at and above 200 ppb, as well as those 
at and above 400 ppb, and considers the current standard, with the 
current form, to provide requisite protection from exposures to such 
concentrations. Thus, the EPA disagrees with the commenters and, for 
the reasons described above, finds that a revised standard with a 98th

[[Page 9896]]

percentile-based form would not provide the desired control of 5-minute 
concentrations at and above either 200 ppb or 400 ppb, nor the 
appropriate protection from the exposures associated with such 
concentrations.
    Three commenters that recommended revision of the standard to be 
less stringent stated that, even when focused on limiting exposures at 
and above 200 ppb, the current standard is overly protective. These 
commenters recommended either revision of the averaging time or of the 
form, each claiming that their recommended revision, accompanied by no 
change to any other element of the standard, would still achieve 
adequate protection from exposures at or above 200 ppb. We address 
these comments in turn below.
    The commenter that recommended revising the averaging time of the 
standard, stated that a standard with an averaging time of 3 hours, 8 
hours, or 24 hours, and keeping all other elements of the current 
standard the same (including the level of 75 ppb, and the form that 
involves averaging annual 99th percentile daily maximum concentrations 
across a three consecutive period), would still be protective of a peak 
5-minute 200 ppb concentration, and would provide regulatory stability. 
In support of this position, this commenter submitted a statistical 
analysis of SO2 data from a subset of ambient air monitors 
in the U.S. The commenter's dataset was limited to 16 monitors located 
within 1 km of SO2 emissions sources with greater than 4,000 
tons per year of reported SO2 emissions in the 2014 NEI; it 
included at most only 18 months of data from these monitors, and fewer 
data from some monitors. From the limited data available for these 
monitors, most of which do not yet have 3 full years of data from which 
to calculate a valid design value for the current standard, the 
commenter identified the 1-hour, 3-hour, 8-hour, and 24-hour periods in 
which the average concentrations were less than 75 ppb, and counted the 
number of times a 5-minute concentration within those periods was at or 
above 200 ppb. The commenter then summarized the results in terms of 
the percentage of the 1-hour, 3-hour, 8-hour or 24-hour periods with 
average concentrations less than 75 ppb that included a 5-minute 
concentration at or above 200 ppb. The commenter, while noting that the 
percentages were higher for longer periods than for shorter periods, 
claimed that this limited dataset covering 18 or fewer months 
demonstrated that even a standard with a 24-hour averaging time would 
be protective of 5-minute SO2 concentrations at and above 
200 ppb.
    We disagree with the commenter that their analysis is adequate to 
judge the level of control that the existing standard exerts over 5-
minute concentrations of potential concern, much less to judge the 
protection provided by the current standard against exposures 
associated with respiratory effects in people with asthma or the 
adequacy of that protection. The commenter's analysis focuses on a 
dataset that by definition is biased to underestimate the occurrences 
of 5-minute concentrations at or above 200 ppb. First, by limiting the 
analysis to 18 months or less, the commenter's analysis did not include 
3 years of data that would allow for judgment of whether or not the 
monitors included met the current standard or any of the suggested 
alternatives. Over a timeframe longer than that provided by the 
commenter, there would be opportunity for more peak 5-minute 
concentrations at or above 200 ppb. Given the lack of three full years 
of data to determine whether the monitor met the standard at the 
locations for which the commenter provided data, it is not possible to 
evaluate the protectiveness of the current standard or the suggested 
alternatives at these monitoring locations. Further, the commenter 
focused their statistics only on hours (or 3-hour, 8-hour or 24-hour 
periods) for which the average concentrations were at or below 75 ppb. 
Yet given the form for the current standard, a 3-year period at a 
location that meets the current standard (or the commenter's 
alternatives) could also include hours (or 3-hour, 8-hour or 24-hour 
periods) above 75 ppb, along with the associated 5-minute 
concentrations. Lastly, the commenter's analysis summarizes the 
occurrences of 5-minute concentrations at or above 200 ppb in terms of 
percentages (of hours at or below 75 ppb), rather than the number of 
occurrences during a year or the full 3-year period. This framing of 
their analysis precludes a consideration of the frequency of such peak 
concentrations at monitors meeting the standard. The frequency is an 
appropriate consideration because increasing frequency would directly 
relate to increasing potential for exposure to such peak 
concentrations, while percentage of a subset of the hours cannot be 
interpreted with regard to such a relevant consideration.
    Accordingly, in considering the commenter's view that an 
alternative averaging time would still be protective of exposures to 5-
minute concentrations at or above 200 ppb, the EPA conducted an 
analysis that, like the commenter's analysis, focused on SO2 
monitoring sites located within 1 km of emissions sources with greater 
than 4,000 tons per year of reported SO2 emissions according 
to the 2014 NEI, but that also included three complete years of data 
for each site, consistent with the form of the current standard 
(Solomon et al., 2019).\97\ Further, the EPA analysis summarizes the 
frequency of occurrences of 5-minute concentrations at or above 200 ppb 
and does this for those monitoring locations that meet the current 
standard, and also at those that would meet an alternative 3-hour, 8-
hour, or 24-hour standard (with a level of 75 ppb) \98\ (Solomon et 
al., 2019, Tables 5 through 8). At sites that would meet standards with 
such alternative averaging times, there were many more 5-minute daily 
maximum SO2 concentrations at or above 200 ppb than at sites 
that meet the current standard, in many instances 20 to 200 times more. 
(Solomon et al., 2019, Tables 5 through 8). This relates in part to the 
fact that more sites meet the alternative standards than the current 
standard due to the lesser stringency of a standard with a longer 
averaging time that has the same level as the current standard. 
Additionally, however, when evaluating 5-minute concentrations on a 
per-monitor basis, it can also be seen that as many as 15, 29, and 144 
times more 5-minute daily maximum SO2 concentrations at or 
above 200 ppb are allowed to occur at monitors that would meet an 
alternative standard with a 3-hour, 8-hour or 24-hour averaging time, 
respectively, compared with only two at the monitor meeting the current 
standard (Solomon et al., 2019, Table 9). Thus, it can be seen even 
from this analysis of the small number of sites near very large 
emissions sources (>4,000 tons per year in 2014 NEI), that a standard 
with a longer averaging time (and the level of 75 ppb) would provide 
less public health protection than that provided by the current 1-hour 
standard. We additionally note that the focus for the commenter 
analysis on monitors near sources emitting 4,000 or more tons per year 
as of 2014 yields an analysis focused on a small percentage

[[Page 9897]]

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

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

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

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

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

    \100\ As noted in section II.C.2 of the proposal (83 FR 26771, 
June 8, 2018) and section II.A.2 above, the health effects evidence 
indicates a lack of a cumulative effect of multiple exposures over 
several hours or a day (ISA, section 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).
---------------------------------------------------------------------------

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

[[Page 9898]]

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

    \101\ This scenario was developed through adjustments of the 
hourly air quality data as described in section II.A.3.a above and 
described in detail in sections 3.4 and 6.2.2.2 of the REA.
---------------------------------------------------------------------------

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

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

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

[[Page 9899]]

we note that these concentrations occur in some 1-hour periods with 
average concentrations above 75 ppb and also in some 1-hour periods 
with average concentrations below 75 ppb, while the commenter appears 
to limit their focus only to hours with average concentrations above 75 
ppb. Further, analyses of these data in the PA demonstrate the 
reduction of 5-minute concentrations above 200 ppb and higher 
benchmarks achieved by the current standard (PA, section 2.3.2.3 and 
Figure C-5). These analyses do not indicate overcontrol of 5-minute 
concentrations; for example, among sites meeting the current standard, 
as many as 32 days per year were recorded with a 5-minute concentration 
at or above 200 ppb, and as many as 7 days per year with a 5-minute 
concentration at or above 400 ppb (PA, section 2.3.2.3 and Figure C-5). 
Thus, the commenter's position that the current approach to judging 
attainment (based on a valid design value at or below 75 ppb) is overly 
stringent in its control of 5-minute concentrations at and above 200 
ppb is not supported by a comprehensive analysis of the available data 
across the U.S.
    Although the comments do not make clear the exact inclusion 
criteria for data or the exact calculations they are advocating be 
applied in the second level of evaluation for judging attainment, such 
a second level evaluation would appear to allow the designation of 
areas as attaining the current standard when the areas do not meet the 
standard. As specified under the Clean Air Act, primary ambient air 
quality standards are those the attainment and maintenance of which are 
judged requisite to protect public health with an adequate margin of 
safety. The elements of the current standard include the highest daily 
1-hour concentrations, not the highest 5-minute concentrations. To 
apply a second level of data evaluation for purposes of determining 
attainment that is based on consideration of 5-minute concentrations 
would have the effect of changing the standard itself rather than 
evaluating attainment with the existing standard. Thus, we disagree 
with the commenter that such an evaluation could be adopted for judging 
attainment without effecting a change to the standard itself.
d. Other Comments
    Comments on topics not directly related to consideration of the 
current primary standard included recommendations for addressing data 
gaps and uncertainties to inform future reviews. We agree with many of 
these suggestions and note that the PA highlighted key uncertainties 
and data gaps associated with reviewing and establishing NAAQS for 
SO2 and also areas for future health-related research, model 
development, and data gathering. We encourage research in these areas, 
although we note that research planning and priority setting are beyond 
the scope of this action.
    The EPA also received several comments related to implementation of 
the primary SO2 NAAQS, including comments concerning the use 
of AERMOD for estimating 1-hour concentrations versus concentrations 
over longer time periods, and comments citing facilities' difficulty 
demonstrating compliance with the 1-hour SO2 standard. We 
are not addressing those comments here because, as described in section 
I.A above, this action is being taken pursuant to CAA section 109(d)(1) 
and relevant case law. Additionally, consistent with this case law, the 
EPA has not considered costs associated with attaining the standard as 
a part of this review, including the costs or economic impacts related 
to permitting or other implementation concerns, in this action 
(Whitman, 531 U.S. at 471 & n.4). Under CAA section 109(d)(1) the EPA 
has the obligation to periodically review the air quality criteria and 
the existing primary NAAQS and make sure revisions as may be 
appropriate. Accordingly, the scope of this action is to satisfy that 
obligation; it is not to address concerns related to implementation of 
the existing standard. State and federal SO2 control 
programs, such as those discussed in section I.D, may provide an 
opportunity for permitting and other implementation concerns to be 
addressed. For example, in light of public comments suggesting 
potential unintended consequences for areas with low peak-to-mean 
SO2 concentrations, the EPA intends to continue to work 
closely with the relevant air agencies for these areas in implementing 
the standard, building upon its 2014 Guidance for 1-Hour SO2 
Nonattainment Area SIP Submissions.\103\
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    \103\ Available at: https://www.epa.gov/sites/production/files/2016-06/documents/20140423guidance_nonattainment_sip.pdf.
---------------------------------------------------------------------------

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

[[Page 9900]]

continue to be associated with the available information.
    With regard to the current evidence, as summarized in the PA and 
discussed in detail in the ISA, the Administrator takes note of the 
long-standing evidence that has established key health effects 
associated with short-term exposure to SO2. This evidence, 
largely drawn from the controlled human exposure studies, demonstrates 
that very short exposures (for as short as a few minutes) to less than 
1000 ppb SO2, while breathing at an elevated rate (such as 
while exercising), induces bronchoconstriction and related respiratory 
effects in people with asthma and supports identification of people 
with asthma as the population at risk from short-term peak 
concentrations in ambient air (ISA; 2008 ISA; U.S. EPA, 1994).\104\ The 
available epidemiologic evidence, generally consistent with that in the 
last review, provides support for the conclusion of a causal 
relationship between short-term SO2 exposures and 
respiratory effects, for which the controlled human exposure studies 
are the primary evidence. The epidemiologic studies report positive 
associations of short-term (i.e., hourly or daily) concentrations of 
SO2 in ambient air with asthma-related health outcomes, 
including hospital admissions and emergency department visits. In 
considering these epidemiologic studies in the context of the larger 
evidence base, the Administrator recognizes that, as described in the 
ISA, while these studies analyze hourly or daily metrics, there is the 
potential for shorter-term peak concentrations within the study area to 
be playing a role in such associations. The Administrator further takes 
note of the associated uncertainties identified in the ISA related to 
potential confounding from co-occurring pollutants such as PM, a 
chemical mixture including some components for which SO2 is 
a precursor,\105\ and also related to the ability of available fixed-
site monitors to adequately represent variations in personal 
SO2 exposure, particularly with regard to peak exposures 
(ISA, p. 5-37; PA, section 3.2.1.4; 83 FR 26764, June 8, 2018).
---------------------------------------------------------------------------

    \104\ For people without asthma, such effects have only been 
observed in studies of exposure concentrations at or above 1000 ppb 
(ISA, section 5.2.1.7).
    \105\ Sulfur dioxide is a precursor to sulfate, which commonly 
occurs in particulate form (ISA, section 2.3; U.S. EPA, 2009, 
section 3.3.2 and Table 3-2).
---------------------------------------------------------------------------

    With regard to health effects evidence newly available in this 
review, the Administrator takes note of the PA finding that, while the 
health effects evidence, as assessed in the ISA, has been augmented 
with additional studies since the time of the last review, the newly 
available evidence does not lead to different conclusions regarding the 
primary health effects of SO2 in ambient air or regarding 
exposure concentrations associated with those effects. Nor does it 
identify different or additional populations at risk of SO2-
related effects. Thus, the Administrator recognizes that, as in the 
last review, the health effects evidence continues to demonstrate a 
causal relationship between relevant short-term exposures to 
SO2 and respiratory effects, particularly with regard to 
effects related to asthma exacerbation in people with asthma. He also 
recognizes that the ISA conclusion on the respiratory effects caused by 
short-term exposures is based primarily on evidence from controlled 
human exposure studies, also available at the time of the last review, 
that document moderate or greater lung function decrements and 
respiratory symptoms in people with asthma exposed to SO2 
for 5 to 10 minutes while breathing at an elevated rate, and that the 
current 1-hour standard was established to provide protection from 
effects such as these (ISA, section 5.2.1.9; 75 FR 35520, June 22, 
2010).
    With regard to exposure concentrations of interest in this review, 
the Administrator particularly takes note of the evidence assessed in 
the ISA from controlled human exposure studies that demonstrate the 
occurrence of moderate or greater lung function decrements, at times 
accompanied by respiratory symptoms, in subjects with asthma exposed 
for very short periods of time while breathing at elevated rates, 
focusing primarily on the ISA analysis of findings from such studies 
for which respiratory response measurements are available to the EPA 
for individual study subjects (ISA, Table 5-2 and Figure 5-1; PA, Table 
3-1).\106\ These data demonstrate respiratory effects in a percentage 
of people with asthma exposed while exercising to SO2 
concentrations as low as 200 ppb. Nearly 10% of the study subjects 
experienced moderate or greater lung function decrements at this 
exposure level and respiratory symptoms were also reported to occur in 
some subjects in some studies at the study group level (ISA, Table 5-2; 
Linn et al., 1983; Linn et al., 1987). In weighing this evidence, the 
Administrator notes the statements from the ATS which continue to 
emphasize the importance of the consideration of effects on individuals 
with preexisting diminished lung function (ATS, 2000a; Thurston et al., 
2017). Consistent with the ATS characterization of their most recent 
statement as ``providing a set of considerations that can be applied in 
forming judgments,'' the Administrator notes the importance of 
considering whether effects occur in people with diminished reserve, 
such as people with asthma, as well as consideration of the magnitude 
or severity of effects, the persistence or transience of the effects, 
and the potential for repeated occurrences (Thurston et al., 2017). 
Thus, as in the last review, when the current standard was set, the 
Administrator judges it appropriate to consider the protection provided 
by the current standard to the at-risk population of people with asthma 
from exposures to peak concentrations as low as 200 ppb while breathing 
at elevated rates, while also recognizing the reduced severity of 
effects at this exposure level, as was recognized by the Administrator 
in the last review.
---------------------------------------------------------------------------

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

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

[[Page 9901]]

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

    \107\ The ISA notes that while the extremely limited evidence 
for adults with moderate to severe asthma indicates such groups may 
have similar relative lung function decrements in response to 
SO2 as adults with less severe asthma, individuals with 
severe asthma may have greater absolute decrements that may relate 
to the role of exercise (ISA, p. 1-17 and 5-22). The ISA concluded 
that individuals with severe asthma may have ``less reserve capacity 
to deal with an insult compared with individuals with mild asthma'' 
(ISA, p. 1-17 and 5-22).
---------------------------------------------------------------------------

    In judging the level of protection afforded by the current 
standard, the Administrator turns to the REA, recognizing that health 
effects in people with asthma are linked to exposures during periods of 
elevated breathing rates, such as while exercising. Accordingly, the 
Administrator finds that, as was the case at the time of the last 
review, population exposure modeling that takes human activity levels 
into account is integral to consideration of population exposures 
compared to SO2 benchmark concentrations and of population 
lung function risk, and that such consideration is integral to judging 
whether the protection afforded by the primary SO2 standard 
is requisite. He additionally notes that the populations modeled in the 
REA, children and adults with asthma, are those identified as at risk 
from SO2 related effects.
    In his consideration of the REA estimates available in this review, 
the Administrator recognizes a number of improvements of the current 
REA compared to the REA in the last review, including that the current 
REA assesses an air quality scenario for 3 years of air quality 
conditions adjusted to just meet the current standard.\108\ The current 
REA is additionally expanded from the prior one with regard to the 
number of study areas in that it now includes three urban areas, each 
with populations of more than 100,000 people.\109\ The Administrator 
also notes that the asthma prevalence across census tracts in the three 
REA study areas ranged from 8.0 to 8.7% for all ages (REA, section 5.1) 
and from 9.7 to 11.2% for children (REA, section 5.1), which reflects 
some of the higher prevalence rates in the U.S. today (PA, sections 
3.2.1.5 and 3.2.2.1). The other ways in which the current REA analyses 
are improved and expanded from those in the REA for the last review 
relate to improvements that have been made to models, model inputs and 
underlying databases. These improvements include the database, vastly 
expanded since the last review, of ambient air monitoring data for 5-
minute concentrations, as summarized in section II.A.3 above.\110\ 
While recognizing the differences between the current REA analyses and 
the 2009 REA analyses, the Administrator notes the PA finding of a 
rough consistency of the associated estimates when considering the 
array of study areas in both reviews. He additionally notes the PA 
findings that the newly available quantitative analyses comport with 
the conclusions reached in the last review regarding the control 
expected to be exerted by the now-current 1-hour standard on 5-minute 
exposures of concern (83 FR 26775-26776, June 8, 2018).
---------------------------------------------------------------------------

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

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

[[Page 9902]]

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

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

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

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

    In making this judgment, the Administrator also considers whether 
this level of protection is more than what is requisite and whether a 
less stringent standard would be appropriate to consider. In so doing, 
he first recognizes that a less stringent standard would allow the 
occurrence of higher peak SO2 concentrations and a greater 
frequency of concentrations above benchmarks of interest, likely 
contributing to higher exposures and risks than those estimated by the 
REA. That is, a less stringent standard, with its lesser control on 
peak SO2 concentrations, would be expected to allow a higher 
frequency of ambient air SO2 concentrations at or above 
benchmarks of interest, including the 400 ppb benchmark, at which 
controlled human exposure studies of exercising people with asthma have 
reported nearly 25% of study subjects to experience a moderate or 
greater lung function decrement and nearly 10% of subjects to 
experience greater than moderate lung function decrements (e.g., a 
tripling of sRaw). Such air quality patterns would likely contribute to 
higher exposures and risks than those estimated by the REA, and 
accordingly relatively lesser protection of people with asthma from 
exposures at or above benchmarks of interest.
    Additionally, in considering potential ramifications of a less 
stringent standard, the Administrator recognizes that through its 
control of SO2 concentrations at or above the lowest 
benchmark of 200 ppb, the current standard provides a margin of safety 
for less well studied exposure levels and population groups for which 
the evidence is limited or lacking. In so doing, he recognizes that our 
understanding of the relationships between the presence of a pollutant 
in ambient air and associated health effects is based on a broad body 
of information encompassing not only more established aspects of the 
evidence, such as the conclusion that exposure to higher SO2 
concentrations results in more severe lung function decrements, but 
also aspects with which there may be substantial uncertainty. For 
example, in the case of this review, he notes there to be increased 
uncertainty associated with characterization of the risk of lung 
function decrements (including their magnitude and prevalence, and the 
associated public health significance) at exposure levels below 400 
ppb, and indeed below those represented in the controlled human 
exposure studies. In this regard, the Administrator notes the 
uncertainty regarding characterization of the risk of respiratory 
effects in populations at risk but for which the evidence base is 
limited or lacking, such as children with asthma or individuals with 
more severe asthma (PA, section 3.2.2.3; REA, section 5.3). He also 
takes note of the CASAC comments on these uncertainties, and on 
consideration of these groups in assuring the standard's adequate 
margin of safety. Further, he considers the epidemiologic evidence, 
taking note of the uncertainties associated with exposure measurement 
error and copollutant confounding in the evidence. In considering the 
uncertainties in both the controlled human exposure and epidemiologic 
of studies, he recognizes that collectively, the health effects 
evidence generally reflects a continuum, consisting of levels at which 
scientists generally agree that health effects are likely to occur, 
through lower levels at which the likelihood and magnitude of the 
response become increasingly uncertain. In light of these 
uncertainties, the Administrator recognizes that the CAA requirement 
that primary standards provide an adequate margin of safety, as 
summarized in section I.A above, is intended to address uncertainties 
associated with inconclusive scientific and technical information, as 
well as to provide a reasonable degree of protection against hazards 
that research has not yet identified. Based on all of the 
considerations noted here, and considering the current body of 
evidence, including the associated limitations and uncertainties, in 
combination with the exposure/risk information, the Administrator 
concludes that a less stringent standard than the current standard 
would not provide the requisite protection of public health, including 
an adequate margin of safety.
    Having concluded that a less stringent standard would not provide 
the requisite protection of public health, based in part on his 
judgment that the evidence and exposure/risk information indicates that 
the current standard provides an appropriately high level of protection 
from the more severe and well characterized effects on people with 
asthma from very short exposures to SO2 while breathing at 
elevated rates

[[Page 9903]]

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

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

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

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

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

[[Page 9904]]

chapter 2 and Appendix B). Taking the REA estimates of exposure and 
risk for air quality conditions just meeting the current standard 
together (summarized in section II.A.3 above), while recognizing the 
uncertainties associated with such estimates, the Administrator judges 
the current standard to provide an appropriately high degree of 
protection to at-risk populations (and specifically people with asthma) 
from SO2 exposures associated with health effects of more 
clear public health concern, as indicated by the extremely low 
estimates of occurrences of exposures at or above 400 ppb (and at or 
above 300 ppb). He further judges the current standard to additionally 
provide a slightly lower, but still appropriately high degree of 
protection for the appreciably less severe effects associated with 
lower exposures (i.e., at or below 200 ppb while breathing at elevated 
rates), for which public health implications are less clear. In 
considering the adequacy of protection afforded by the current standard 
from these lower exposure concentrations, the Administrator recognizes, 
as noted above, that the effects reported at such concentrations are 
less severe than at the higher exposure levels. However, considering 
the array of limitations in the evidence with regard to characterizing 
the potential response of at-risk individuals to exposures below 200 
ppb, as well as the limitations in the evidence for population groups 
at risk or potentially at risk but for which the evidence is lacking, 
the Administrator finds it appropriate to provide protection from these 
exposures in light of the CAA requirements for an adequate margin of 
safety to address uncertainties generally associated with limitations 
in the scientific and technical information and hazards that research 
has not yet identified. In this light, he judges the current standard 
to provide the appropriate protection from peak SO2 
concentrations in ambient air. Based on these and all of the above 
considerations, the Administrator concludes that the current primary 
SO2 standard provides an adequate margin of safety against 
adverse effects associated with short-term exposures to SOX 
in ambient air, and accordingly concludes that the current standard 
provides the requisite protection of public health under the Act.
    With regard to key aspects of the specific elements of the 
standard, the Administrator recognizes the support in the current 
evidence base for SO2 as the indicator for SOX, 
as summarized in section II.B.1 of the proposal. In so doing, he notes 
the ISA conclusion that SO2 is the most abundant of the 
SOX in the atmosphere and the one most clearly linked to 
human health effects. He additionally recognizes the control exerted by 
the 1-hour averaging time on 5-minute ambient air concentrations of 
SO2 (including, particularly, concentrations at and above 
200 to 400 ppb) and the associated exposures of particular importance 
for SO2-related health effects (e.g., as indicated by the 
REA estimates). After consideration of the public comments advocating 
revision of the averaging time, as addressed in section II.B.3 above, 
the Administrator continues to find that the current standard as 
defined by the existing 1-hour averaging time along with the other 
elements, is requisite. Similarly, with regard to form and level of the 
standard, the Administrator takes note of the REA results as discussed 
above and the level of protection that they indicate the elements of 
the current standard collectively to provide. He has additionally 
considered the public comments regarding revisions to these elements of 
the standard, as addressed in section II.B.3 above, and continues to 
judge that the existing level and the existing form, in all its 
aspects, together with the other elements of the existing standard 
provide the appropriate level of public health protection.
    The Administrator additionally takes note of the CASAC support for 
retaining the current standard and the CASAC's specific recommendation 
that all four elements should remain the same. Beyond his recognition 
of this support in the available information and in CASAC advice for 
the elements of the current standard, the Administrator has considered 
the elements collectively in evaluating the health protection afforded 
by the current standard. For all of the reasons discussed above, and 
recognizing the CASAC conclusion that the current evidence and REA 
results provide support for retaining the current standard, the 
Administrator concludes that the current primary SO2 
standard (in all of its elements) is requisite to protect public health 
with an adequate margin of safety from effects of SOX in 
ambient air, including the health of at-risk populations, and should be 
retained, without revision.

C. Decision on the Primary Standard

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

III. Statutory and Executive Order Reviews

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

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

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

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

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

C. Paperwork Reduction Act (PRA)

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

D. Regulatory Flexibility Act (RFA)

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

E. Unfunded Mandates Reform Act (UMRA)

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

[[Page 9905]]

F. Executive Order 13132: Federalism

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

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

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

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

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

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

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

J. National Technology Transfer and Advancement Act

    This action does not involve technical standards.

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

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

L. Determination Under Section 307(d)

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

M. Congressional Review Act

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

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