Primary National Ambient Air Quality Standard for Sulfur Dioxide, 35520-35603 [2010-13947]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 75, Number 119 (Tuesday, June 22, 2010)]
[Rules and Regulations]
[Pages 35520-35603]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2010-13947]



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Part II





Environmental Protection Agency





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40 CFR Parts 50, 53, and 58



Primary National Ambient Air Quality Standard for Sulfur Dioxide; Final 
Rule

Federal Register / Vol. 75 , No. 119 / Tuesday, June 22, 2010 / Rules 
and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

[EPA-HQ-OAR-2007-0352; 9160-4]
RIN 2060-A048


Primary National Ambient Air Quality Standard for Sulfur Dioxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Based on its review of the air quality criteria for oxides of 
sulfur and the primary national ambient air quality standard (NAAQS) 
for oxides of sulfur as measured by sulfur dioxide (SO2), 
EPA is revising the primary SO2 NAAQS to provide requisite 
protection of public health with an adequate margin of safety. 
Specifically, EPA is establishing a new 1-hour SO2 standard 
at a level of 75 parts per billion (ppb), based on the 3-year average 
of the annual 99th percentile of 1-hour daily maximum concentrations. 
The EPA is also revoking both the existing 24-hour and annual primary 
SO2 standards.

DATES: This final rule is effective on August 23, 2010.

ADDRESSES: EPA has established a docket for this action under Docket ID 
No. EPA-HQ-OAR-2007-0352. All documents in the docket are listed on the 
https://www.regulations.gov Web site. Although listed in the index, some 
information is not publicly available, e.g., confidential business 
information or other information whose disclosure is restricted by 
statute. Certain other material, such as copyrighted material, will be 
publicly available only in hard copy form. Publicly available docket 
materials are available either electronically through https://www.regulations.gov or in hard copy at the Air and Radiation Docket and 
Information Center, EPA/DC, EPA West, 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 and Information 
Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Dr. Michael J. Stewart, 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: 919-541-7524; fax: 919-
541-0237; e-mail: stewart.michael@epa.gov.

SUPPLEMENTARY INFORMATION:

Table of Contents

    The following topics are discussed in this preamble:

I. Background
    A. Summary of Revisions to the SO2 Primary NAAQS
    B. Statutory Requirements
    C. Related SO2 Control Programs
    D. History of Reviews of the Primary NAAQS for Sulfur Oxides
    E. Summary of Proposed Revisions to the SO2 Primary 
NAAQS
    F. Organization and Approach to Final SO2 Primary 
NAAQS Decisions
II. Rationale for Decisions on the Primary Standards
    A. Characterization of SO2 Air Quality
    1. Anthropogenic Sources and Current Patterns of SO2 
Air Quality
    2. SO2 Monitoring
    B. Health Effects Information
    1. Short-Term (5-Minute to 24-Hour) SO2 Exposure and 
Respiratory Morbidity Effects
    a. Adversity of Short-Term Respiratory Morbidity Effects
    2. Health Effects and Long-Term Exposures to SO2
    3. SO2-Related Impacts on Public Health
    C. Human Exposure and Health Risk Characterization
    D. Approach for Determining Whether To Retain or Revise the 
Current Standards
    E. Adequacy of the Current Standards
    1. Rationale for Proposed Decision
    2. Comments on the Adequacy of the Current Standards
    a. Comments on EPA's Interpretation of the Epidemiologic 
Evidence
    b. Comments on EPA's Interpretation of the Controlled Human 
Exposure Evidence
    c. Comments on EPA's Characterization of SO2-
Associated Exposures and Health Risks
    3. Conclusions Regarding the Adequacy of the Current 24-Hour and 
Annual Standards
    F. Conclusions on the Elements of a New Short-Term Standard
    1. Indicator
    a. Rationale for Proposed Decision
    b. Comments on Indicator
    c. Conclusions on Indicator
    2. Averaging Time
    a. Rationale for Proposed Decision
    b. Comments on Averaging Time
    c. Conclusions on Averaging Time
    3. Form
    a. Rationale for Proposed Decision
    b. Comments on Form
    c. Conclusions on Form
    4. Level
    a. Rationale for Proposed Decision
    b. Comments on Level
    c. Conclusions on Level
    5. Retaining or Revoking the Current 24-Hour and Annual 
Standards
    a. Rationale for Proposed Decision
    b. Comments on Retaining or Revoking the Current 24-Hour and 
Annual Standards
    c. Conclusions on Retaining or Revoking the Current 24-Hour and 
Annual Standards
    G. Summary of Decisions on Primary Standards
III. Overview of the Approach for Monitoring and Implementation
IV. Amendments to Ambient Monitoring and Reporting Requirements
    A. Monitoring Methods
    1. Requirements for SO2 Federal Reference Method 
(FRM)
    a. Proposed Ultraviolet Fluorescence SO2 FRM and 
Implementation
    b. Public Comments
    c. Conclusions on Ultraviolet Fluorescence SO2 FRM 
and Implementation
    2. Requirements for Automated SO2 Methods
    a. Proposed Performance Specifications for Automated Methods
    b. Public Comments
    c. Conclusions for Performance Specifications for SO2 
Automated Methods
    B. Network Design
    1. Approach for Network Design
    a. Proposed Approach for Network Design
    b. Alternative Network Design
    c. Public Comments
    2. Modeling Ambient SO2 Concentrations
    3. Monitoring Objectives
    a. Proposed Monitoring Objectives
    b. Public Comments
    c. Conclusions on Monitoring Objectives
    4. Final Monitoring Network Design
    5. Population Weighted Emissions Index
    a. Proposed Use of the Population Weighted Emissions Index
    b. Public Comments
    c. Conclusions on the Use of the Population Weighted Emissions 
Index
    6. Regional Administrator Authority
    a. Proposed Regional Administrator Authority
    b. Public Comments
    c. Conclusions on Regional Administrator Authority
    7. Monitoring Network Implementation
    a. Proposed Monitoring Network Implementation
    b. Public Comments
    c. Conclusions on Monitoring Network Implementation
    C. Data Reporting
    1. Proposed Data Reporting
    2. Public Comments
    3. Conclusions on Data Reporting
V. Initial Designation of Areas for the 1-Hour SO2 NAAQS
    A. Clean Air Act Requirements
    1. Approach Described in Proposal
    2. Public Comments
    B. Expected Designations Process
VI. Clean Air Act Implementation Requirements
    A. How This Rule Applies to Tribes
    B. Nonattainment Area Attainment Dates
    1. Attaining the NAAQS
    2. Consequences of a Nonattainment Area Failing To Attain by the 
Statutory Attainment Date
    C. Section 110(a)(1) and (2) NAAQS Maintenance/Infrastructure 
Requirements
    1. Section 110(a)(1)-(2) Submission

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    D. Attainment Planning Requirements
    1. SO2 Nonattainment Area SIP Requirements
    2. New Source Review and Prevention of Significant Deterioration 
Requirements
    3. General Conformity
    E. Transition From the Existing SO2 NAAQS to a 
Revised SO2 NAAQS
VII. Appendix T--Interpretation of the Primary NAAQS for Oxides of 
Sulfur and Revisions to the Exceptional Events Rule
    A. Interpretation of the NAAQS for Oxides of Sulfur
    1. Proposed Interpretation of the Standard
    2. Comments on Interpretation of the Standard
    3. Conclusions on Interpretation of the Standard
    B. Exceptional Events Information Submission Schedule
VIII. Communication of Public Health Information
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health & Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect 
Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
References

I. Background

A. Summary of Revisions to the SO2 Primary NAAQS

    Based on its review of the air quality criteria for oxides of 
sulfur and the primary national ambient air quality standard (NAAQS) 
for oxides of sulfur as measured by sulfur dioxide (SO2), 
EPA is making revisions to the primary SO2 NAAQS so the 
standards are requisite to protect public health with an adequate 
margin of safety, as appropriate under section 109 of the Clean Air Act 
(Act or CAA). Specifically, EPA is replacing the current 24-hour and 
annual standards with a new short-term standard based on the 3-year 
average of the 99th percentile of the yearly distribution of 1-hour 
daily maximum SO2 concentrations. EPA is setting the level 
of this new standard at 75 ppb. EPA is adding data handling conventions 
for SO2 by adding provisions for this new 1-hour primary 
standard. EPA is also establishing requirements for an SO2 
monitoring network. These new provisions require monitors in areas 
where there is an increased coincidence of population and 
SO2 emissions. EPA is also making conforming changes to the 
Air Quality Index (AQI).

B. Statutory Requirements

    Two sections of the Clean Air Act (Act or CAA) govern the 
establishment and revision of National Ambient Air Quality Standards 
NAAQS. Section 108 of the Act directs the Administrator to identify and 
list air pollutants that meet certain criteria, including that the air 
pollutant ``in his judgment, cause[s] or contribute[s] to air pollution 
which may reasonably be anticipated to endanger public health and 
welfare'' and ``the presence of which in the ambient air results from 
numerous or diverse mobile or stationary sources.'' CAA section 
108(a)(1)(A) and (B). For those air pollutants listed, section 108 
requires the Administrator to issue air quality criteria that 
``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 ambient air * * *'' Section 108(a)(2).
    Section 109(a) of the Act directs the Administrator to promulgate 
``primary'' and ``secondary'' NAAQS for pollutants for which air 
quality criteria have been 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 [the air quality] criteria and 
allowing an adequate margin of safety, are requisite to protect the 
public health.'' \1\ Section 109(b)(1). A secondary standard, in turn, 
must ``specify a level of air quality the attainment and maintenance of 
which, in the judgment of the Administrator, based on [the air quality] 
criteria, is requisite to protect the public welfare from any known or 
anticipated adverse effects associated with the presence of such 
pollutant in the ambient air.'' \2\ Section 109(b)(2) This rule 
concerns exclusively the primary NAAQS for oxides of sulfur.
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    \1\ 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 American 
Lung Ass'n v. EPA, 134 F. 3d 388, 389 (DC 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. If a pollutant adversely affects the health of these 
sensitive individuals, EPA must strengthen the entire national 
standard.''); Coalition of Battery Recyclers Ass'n v. EPA, No. 09-
1011 (DC Cir. May 14, 2010) slip op. at 7 (same).
    \2\ EPA is currently conducting a separate review of the 
secondary SO2 NAAQS jointly with a review of the 
secondary NO2 NAAQS (see https://www.epa.gov/ttn/naaqs/standards/no2so2sec/ for more information).
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    The requirement that primary standards include an adequate margin 
of safety is intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It is also intended to provide a reasonable degree 
of protection against hazards that research has not yet identified. 
Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (DC Cir 1980), 
cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v. 
Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied, 455 U.S. 1034 
(1982). 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 include an adequate margin of 
safety, the Administrator is seeking not only to prevent pollution 
levels that have been demonstrated to be harmful but also to prevent 
lower pollutant levels that may pose an unacceptable risk of harm, even 
if the risk is not precisely identified as to nature or degree. The CAA 
does not require the Administrator to establish a primary NAAQS at a 
zero-risk level or at background concentration levels, see Lead 
Industries 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 a margin of safety, EPA considers 
such factors as the nature and severity of the health effects involved, 
the size of the at-risk population(s), 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. Lead Industries 
Association v. EPA, 647 F.2d at 1161-62.
    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. In so doing, EPA may not consider the 
costs of implementing the standards. Whitman v. American Trucking

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Associations, 531 U.S. 457, 471, 475-76 (2001).
    Section 109(d)(1) of the Act requires the Administrator to 
periodically undertake a thorough review of the air quality criteria 
published under section 108 and the NAAQS and to revise the criteria 
and standards as may be appropriate. The Act also requires the 
Administrator to appoint an independent scientific review committee 
composed of seven members, including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies, to review the air 
quality criteria and NAAQS and to ``recommend to the Administrator any 
new * * * standards and revisions of existing criteria and standards as 
may be appropriate under section 108 and subsection (b) of this 
section.'' CAA section 109(d)(2). This independent review function is 
performed by the Clean Air Scientific Advisory Committee (CASAC) of 
EPA's Science Advisory Board.

C. Related SO2 Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once EPA has established 
them. Under section 110 of the Act, 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 EPA, also administer the prevention of 
significant deterioration program that covers these pollutants. See CAA 
sections 160-169. In addition, Federal programs provide for nationwide 
reductions in emissions of these and other air pollutants through the 
Federal motor vehicle and motor vehicle fuel control program under 
title II of the Act (CAA sections 202-250) which involves controls for 
emissions from all moving sources and controls for the fuels used by 
these sources; new source performance standards under section 111; and 
title IV of the Act (CAA sections 402-416), which specifically provides 
for major reductions in SO2 emissions. EPA has also 
promulgated the Clean Air Interstate Rule (CAIR) to require additional 
SO2 emission reductions needed in the eastern half of the 
United States to address emissions which contribute significantly to 
nonattainment with, or interfere with maintenance of, the PM NAAQS by 
downwind States in the CAIR region. This rule was remanded by the DC 
Circuit, and although it remains in effect, EPA is reevaluating it 
pursuant to the court remand.
    Currently, there are several areas designated as being in 
nonattainment of the primary SO2 NAAQS (see section VI). 
Moreover, as a result of this final rule, additional areas could be 
classified as non-attainment. Certain States would then be required to 
develop SIPs that identify and implement specific air pollution control 
measures to reduce ambient SO2 concentrations to attain and 
maintain the revised SO2 NAAQS, most likely by requiring air 
pollution controls on sources that emit oxides of sulfur 
(SOx).

D. History of Reviews of the Primary NAAQS for Sulfur Oxides

    On April 30, 1971, the EPA promulgated primary SO2 NAAQS 
(36 FR 8187). These primary standards, which were based on the findings 
outlined in the original 1969 Air Quality Criteria for Sulfur Oxides, 
were set at 0.14 parts per million (ppm) averaged over a 24-hour 
period, not to be exceeded more than once per year, and 0.030 ppm 
annual arithmetic mean. In 1982, EPA published the Air Quality Criteria 
for Particulate Matter and Sulfur Oxides (EPA, 1982) along with an 
addendum of newly published controlled human exposure studies, which 
updated the scientific criteria upon which the initial standards were 
based (EPA, 1982). In 1986, EPA published a second addendum presenting 
newly available evidence from epidemiologic and controlled human 
exposure studies (EPA, 1986). In 1988, EPA published a proposed 
decision not to revise the existing standards (53 FR 14926) (April 26, 
1988). However, EPA specifically requested public comment on the 
alternative of revising the current standards and adding a new 1-hour 
primary standard of 0.4 ppm (400 ppb) to protect asthmatics against 5-
10 minute peak SO2 concentrations.
    As a result of public comments on the 1988 proposal and other post-
proposal developments, EPA published a second proposal on November 15, 
1994 (59 FR 58958). The 1994 re-proposal was based in part on a 
supplement to the second addendum of the criteria document, which 
evaluated new findings on 5-10 minute SO2 exposures in 
asthmatics (EPA, 1994a; EPA, 1994b). As in the 1988 proposal, EPA 
proposed to retain the existing 24-hour and annual standards. EPA also 
solicited comment on three regulatory alternatives to further reduce 
the health risk posed by exposure to high 5-minute peaks of 
SO2 if additional protection were judged to be necessary. 
The three alternatives were: (1) Revising the existing primary 
SO2 NAAQS by adding a new 5-minute standard of 0.6 ppm (600 
ppb) SO2; (2) establishing a new regulatory program under 
section 303 of the Act to supplement protection provided by the 
existing NAAQS, with a trigger level of 0.6 ppm (600 ppb) 
SO2, one expected exceedance; and (3) augmenting 
implementation of existing standards by focusing on those sources or 
source types likely to produce high 5-minute peak concentrations of 
SO2.
    On May 22, 1996, EPA announced its final decision not to revise the 
NAAQS for SOx (61 FR 25566). EPA found that asthmatics--a 
susceptible population group--could be exposed to short-term 
SO2 bursts resulting in repeated `exposure events' such that 
tens or hundreds of thousands of asthmatics could be exposed annually 
to lung function effects ``distinctly exceeding * * * [the] typical 
daily variation in lung function'' that asthmatics routinely 
experience, and found further that repeated occurrences should be 
regarded as significant from a public health standpoint. 61 FR at 
25572, 25573. Nonetheless, the agency concluded that ``the likelihood 
that asthmatic individuals will be exposed * * * is very low when 
viewed from a national perspective'', that ``5-minute peak 
SO[2] levels do not pose a broad public health problem when 
viewed from a national perspective'', and that ``short-term peak 
concentrations of SO[2] do not constitute the type of 
ubiquitous public health problem for which establishing a NAAQS would 
be appropriate.'' Id. at 25575. EPA concluded, therefore, that it would 
not revise the existing standards or add a standard to specifically 
address 5-minute exposures. EPA also announced an intention to propose 
guidance, under section 303 of the Act, to assist States in responding 
to short-term peaks of SO2 and later initiated a rulemaking 
to do so (62 FR 210 (Jan. 2, 1997).
    The American Lung Association and the Environmental Defense Fund 
challenged EPA's decision not to establish a 5-minute standard. On 
January 30, 1998, the Court of Appeals for the District of Columbia 
Circuit found that EPA had failed to adequately explain its 
determination that no revision to the SO2 NAAQS was 
appropriate and remanded the determination back to EPA for further 
explanation. American Lung Ass'n v. EPA, 134 F. 3d 388 (DC Cir. 1998). 
Specifically, the court held that EPA had failed to adequately explain 
the basis for its conclusion that short-term SO2 exposures 
to asthmatics do not constitute a public health problem,

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noting that the agency had failed to explain the link between its 
finding that repeated short-term exposures were significant, and that 
there would be tens to hundreds of thousands of such exposures annually 
to a susceptible subpopulation. 134 F. 3d at 392. The court also 
rejected the explanation that short-term SO2 bursts were 
``localized, infrequent, and site-specific'' as a rational basis for 
the conclusion that no public health problem existed for purposes of 
section 109: ``[N]othing in the Final Decision explains why 
`localized', `site-specific', or even `infrequent' events might 
nevertheless create a public health problem, particularly since, in 
some sense, all pollution is local and site-specific * * *''. Id. The 
court accordingly remanded the case to EPA to adequately explain its 
determination or otherwise take action in accordance with the opinion. 
In response, EPA has collected and analyzed additional air quality data 
focused on 5-minute concentrations of SO2. These air quality 
analyses conducted since the last review helped inform the current 
review, which (among other things) address the issues raised in the 
court's remand of the Agency's last decision.
    EPA formally initiated the current review of the air quality 
criteria for oxides of sulfur and the SO2 primary NAAQS on 
May 15, 2006 (71 FR 28023) with a general call for information. EPA's 
draft Integrated Review Plan for the Primary National Ambient Air 
Quality Standards for Sulfur Dioxide (EPA, 2007a) was made available in 
April 2007 for public comment and was discussed by the CASAC via a 
publicly accessible teleconference on May 11, 2007. As noted in that 
plan, SOX includes multiple gaseous (e.g., SO3) 
and particulate (e.g., sulfate) species. Because the health effects 
associated with particulate species of SOX have been 
considered within the context of the health effects of ambient 
particles in the Agency's review of the NAAQS for particulate matter 
(PM), the current review of the primary SO2 NAAQS is focused 
on the gaseous species of SOX and does not consider health 
effects directly associated with particulate species.
    The first draft of the Integrated Science Assessment for Oxides of 
Sulfur-Health Criteria (ISA) and the Sulfur Dioxide Health Assessment 
Plan: Scope and Methods for Exposure and Risk Assessment (EPA, 2007b) 
were reviewed by CASAC at a public meeting held on December 5-6, 2007. 
Based on comments received from CASAC and from the public, EPA 
developed the second draft of the ISA and the first draft of the Risk 
and Exposure Assessment to Support the Review of the SO2 
Primary National Ambient Air Quality Standard (Risk and Exposure 
Assessment (REA)). These documents were reviewed by CASAC at a public 
meeting held on July 30-31, 2008. Based on comments received from CASAC 
and the public at this meeting, EPA released the final ISA in September 
of 2008 (EPA, 2008a; henceforth referred to as ISA). In addition, 
comments received were considered in developing the second draft of the 
REA. Importantly, the second draft of the REA contained a draft staff 
policy assessment that considered the evidence presented in the final 
ISA and the air quality, exposure, and risk characterization results 
presented in the second draft REA, as they related to the adequacy of 
the current SO2 NAAQS and potential alternative primary 
SO2 standards. This document was reviewed by CASAC at a 
public meeting held on April 16-17, 2009. In preparing the final REA 
report, which included the final staff policy assessment, EPA 
considered comments received from CASAC and the public at and 
subsequent to that meeting. The final REA containing the final staff 
policy assessment was completed in August 2009 (EPA 2009a; henceforth 
referred to as REA)).
    On December 8, 2009 EPA published its proposed revisions to the 
primary SO2 NAAQS. 74 FR 64810 presented a number of 
conclusions, findings, and determinations proposed by the 
Administrator. EPA invited general, specific, and/or technical comments 
on all issues involved with this proposal, including all such proposed 
judgments, conclusions, findings, and determinations. EPA invited 
specific comment on the level, or range of levels, appropriate for such 
a standard, as well as on the rationale that would support that level 
or range of levels. These comments were carefully considered by the 
Administrator as she made her final decisions, as described in this 
notice, on the primary SO2 NAAQS
    The schedule for completion of this review is governed by a 
judicial order resolving a lawsuit filed in September 2005, concerning 
the timing of the current review. Center for Biologic Diversity v. 
Johnson (Civ. No. 05-1814) (D.D.C. 2007). The order that now governs 
this review, entered by the court in August 2007 and amended in 
December 2008, provides that the Administrator will sign, for 
publication, a final rulemaking concerning the review of the primary 
SO2 NAAQS no later than June 2, 2010.

E. Summary of Proposed Revisions to the SO2 Primary NAAQS

    For the reasons discussed in the preamble of the proposal for the 
SO2 primary NAAQS, EPA proposed to make revisions to the 
primary SO2 NAAQS (and to add SO2 data handling 
conventions) so the standards provide requisite protection of public 
health with an adequate margin of safety. Specifically, EPA proposed to 
replace the current 24-hour and annual standards with a new short-term 
SO2 standard. EPA proposed that this new short-term standard 
would be based on the 3-year average of the 99th percentile (or 4th 
highest) of the yearly distribution of 1-hour daily maximum 
SO2 concentrations. EPA proposed to set the level of this 
new 1-hour standard within the range of 50 to 100 ppb and solicited 
comment on standard levels as high as 150 ppb. EPA also proposed to 
establish requirements for an SO2 monitoring network at 
locations where maximum SO2 concentrations are expected to 
occur and to add a new Federal Reference Method (FRM) for measuring 
SO2 in the ambient air. Finally, EPA proposed to make 
corresponding changes to the Air Quality Index for SO2.

F. Organization and Approach to Final SO2 Primary NAAQS Decisions

    This action presents the Administrator's final decisions regarding 
the need to revise the current SO2 primary NAAQS, and what 
those revisions should be. Revisions to the primary NAAQS for 
SO2, and the rationale supporting those revisions, are 
described below in section II.
    An overview of the approach for monitoring and implementation is 
presented in section III. Requirements for the SO2 ambient 
monitoring network and for a new, additional FRM for measuring 
SO2 in the ambient air are described in section IV. EPA's 
current plans for designations and for implementing the revised 
SO2 primary NAAQS are discussed in sections V and VI 
respectively. Related requirements for data completeness, data 
handling, data reporting, rounding conventions, and exceptional events 
are described in section VII. Communication of public health 
information through the AQI is discussed in section VIII. A recitation 
of statutory authority and a discussion of those executive order 
reviews which are relevant are provided in section IX.
    Today's final decisions are based on a thorough review in the ISA 
of scientific information on known and potential human health effects 
associated with exposure to SO2 in the

[[Page 35524]]

air. These final decisions also take into account: (1) Assessments in 
the REA of the most policy-relevant information in the ISA as well as 
quantitative exposure and risk analyses based on that information; (2) 
CASAC Panel advice and recommendations, as reflected in its letters to 
the Administrator and its public discussions of the ISA and REA; (3) 
public comments received during the development of the ISA and REA; and 
(4) public comments received on EPA's notice of proposed rulemaking.

II. Rationale for Decisions on the Primary Standards

    This section presents the rationale for the Administrator's 
decision to revise the existing SO2 primary standards by 
replacing the current 24-hour and annual standards with a new 1-hour 
SO2 standard at a level of 75 ppb, based on the 3-year 
average of the annual 99th percentile of 1-hour daily maximum 
concentrations. As discussed more fully below, this rationale takes 
into account: (1) Judgments and conclusions presented in the ISA and 
the REA; (2) CASAC advice and recommendations as reflected in the CASAC 
panel's discussions of drafts of the ISA and REA at public meetings, in 
separate written comments, and in letters to the Administrator 
(Henderson 2008a; Henderson 2008b; Samet, 2009); (3) public comments 
received at CASAC meetings during the development of the ISA and the 
REA; and (4) public comments received on the notice of proposed 
rulemaking.
    In reaching this decision, EPA has drawn upon an integrative 
synthesis of the entire body of evidence on human health effects 
associated with the presence of SO2 in the ambient air, and 
upon the results of the quantitative exposure and risk assessments 
reflecting this evidence. As discussed below, this body of evidence 
addresses a broad range of health endpoints associated with exposure to 
SO2 in the ambient air. In considering this entire body of 
evidence, EPA chose to focus most on those health endpoints for which 
the ISA found the strongest evidence of an association with 
SO2 (see section II.B below). Thus, the rationale for this 
final decision on the SO2 NAAQS focused primarily on 
respiratory morbidity following short-term (5-minutes to 24-hours) 
exposure to SO2, for which the ISA found a causal 
relationship.
    As discussed below, a substantial amount of new research has been 
conducted since EPA's last review of the SO2 NAAQS, with 
important new information coming from epidemiologic studies in 
particular. In addition to the substantial amount of new epidemiologic 
research, the ISA considered a limited number of new controlled human 
exposure studies and re-evaluated key older controlled human exposure 
studies. In evaluating both the new and key older controlled human 
exposure studies, the ISA utilized updated guidelines published by the 
American Thoracic Society (ATS) on what constitutes an adverse effect 
of air pollution (see ISA, section 3.1.3; p. 3-4). Importantly, all 
controlled human exposure and epidemiologic studies evaluated in the 
ISA have undergone intensive scrutiny through multiple layers of peer 
review and opportunities for public review and comment. Thus, the 
review of this information has been extensive and deliberate.
    After a background discussion of the principal emitting sources and 
current patterns of SO2 air quality and a description of the 
current SO2 monitoring network from which those air quality 
patterns are obtained (section II.A), the remainder of this section 
discusses the Administrator's rationale for her final decisions on the 
primary standards. Section II.B includes an overview of the scientific 
evidence related to the respiratory effects associated with ambient 
SO2 exposure. This overview includes a discussion of the at-
risk populations considered in the ISA. Section II.C summarizes the key 
approaches taken by EPA to assess exposures and health risks associated 
with exposure to ambient SO2. Section II.D summarizes the 
approach that was used in the current review of the SO2 
NAAQS with regard to consideration of the scientific evidence and the 
air quality, exposure, and risk-based results related to the adequacy 
of the current standards and potential alternative standards. Sections 
II.E and II.F discuss, respectively, the Administrator's decisions 
regarding the adequacy of the current standards and the elements of a 
new short-term standard, taking into consideration public comments on 
the proposed decisions. Section II.G summarizes the Administrator's 
decisions with regard to the SO2 primary NAAQS.

A. Characterization of SO2 Air Quality

1. Anthropogenic Sources and Current Patterns of SO2 Air 
Quality
    Anthropogenic SO2 emissions originate chiefly from point 
sources, with fossil fuel combustion at electric utilities (~66%) and 
other industrial facilities (~29%) accounting for the majority of total 
emissions (ISA, section 2.1). Other anthropogenic sources of 
SO2 include both the extraction of metal from ore as well as 
the burning of high sulfur-containing fuels by locomotives, large 
ships, and equipment utilizing diesel engines. SO2 emissions 
and ambient concentrations follow a strong east to west gradient due to 
the large numbers of coal-fired electric generating units in the Ohio 
River Valley and upper Southeast regions. In the 12 Consolidated 
Metropolitan Statistical Areas (CMSAs) that had at least four 
SO2 regulatory monitors from 2003-2005, 24-hour average 
concentrations in the continental U.S. ranged from a reported low of ~1 
ppb in Riverside, CA and San Francisco, CA to a high of ~12 ppb in 
Pittsburgh, PA and Steubenville, OH (ISA, section 2.5.1). In addition, 
outside or inside all CMSAs from 2003-2005, the annual average 
SO2 concentration was 4 ppb (ISA, Table 2-8). However, 
spikes in hourly concentrations occurred. The mean 1-hour maximum 
concentration outside or inside CMSAs was 13 ppb, with a maximum value 
of greater than 600 ppb outside CMSAs and greater than 700 ppb inside 
CMSAs (ISA, Table 2-8).
    Temporal and spatial patterns of 5-minute peaks of SO2 
are also important given that controlled human exposure studies have 
demonstrated that exposure to these peaks can result in adverse 
respiratory effects in exercising asthmatics (see section II.B below). 
For those monitors which voluntarily reported 5-minute block average 
data,\3\ when maximum 5-minute concentrations were reported, the 
absolute highest concentration over the ten-year period exceeded 4000 
ppb, but for all individual monitors, the 99th percentile was below 200 
ppb (ISA, section 2.5.2 Table 2-10). Median concentrations from these 
monitors reporting 5-minute data ranged from 1 ppb to 8 ppb, and the 
average for each maximum 5-minute level ranged from 3 ppb to 17 ppb. 
Delaware, Pennsylvania, Louisiana, and West Virginia had mean values 
for maximum 5-minute data exceeding 10 ppb. Among aggregated within-
State data for the 16 monitors from which all 5-minute average 
intervals were reported, the median values ranged from 1 ppb to 5 ppb, 
and the means ranged from 3 ppb to 11 ppb (ISA, section 2.5.2 at 2-43). 
The highest reported concentration was 921 ppb, but the 99th percentile 
values

[[Page 35525]]

for aggregated within-State data were all below 90 ppb (id).
---------------------------------------------------------------------------

    \3\ A small number of sites, 98 total from 1997 to 2007 of the 
approximately 500 SO2 monitors, and not the same sites in 
all years, voluntarily reported 5-minute block average data to AQS 
(ISA, section 2.5.2). Of these, 16 reported all twelve 5-minute 
averages in each hour for at least part of the time between 1997 and 
2007. The remainder reported only the maximum 5-minute average in 
each hour.
---------------------------------------------------------------------------

2. SO2 Monitoring
    Although EPA established the SO2 standards in 1971, 
uniform minimum monitoring network requirements for SO2 
monitoring were only adopted in May 1979. From the time of the 
implementation of the 1979 monitoring rule through 2008, the 
SO2 monitoring network has steadily decreased in size from 
approximately 1496 sites in 1980 to the approximately 488 sites 
operating in 2008. At present, except for SO2 monitoring 
required at National Core Monitoring Stations (NCore stations), there 
are no minimum monitoring requirements for SO2 in 40 CFR 
part 58 Appendix D, other than a requirement for EPA Regional 
Administrator approval before removing any existing monitors and a 
requirement that any ongoing SO2 monitoring must have at 
least one monitor sited to measure the maximum concentration of 
SO2 in that area. EPA removed the specific minimum 
monitoring requirements for SO2 in the 2006 monitoring rule 
revisions, except for monitoring at NCore stations, based on the fact 
that there were no SO2 nonattainment areas at that time, 
coupled with trends showing an increasing gap between national average 
SO2 concentrations and the current 24-hour and annual 
standards. The rule was also intended to provide State, local, and 
Tribal air monitoring agencies flexibility in meeting perceived higher 
priority monitoring needs for other pollutants, or to implement the new 
multi-pollutant sites (NCore network) required by the 2006 rule 
revisions (71 FR 61236, (October 6, 2006)). More information on 
SO2 monitoring can be found in section IV.

B. Health Effects Information

    The ISA concluded that there was sufficient evidence to infer a 
``causal relationship'' between respiratory morbidity and short-term 
(5-minutes to 24-hours) exposure to SO2 (ISA, section 5.2). 
Importantly, we note that a ``causal relationship'' is the strongest 
finding the ISA can make.\4\ This conclusion was based on the 
consistency, coherence, and plausibility of findings observed in 
controlled human exposure studies of 5-10 minutes, epidemiologic 
studies mostly using 1-hour daily maximum and 24-hour average 
SO2 concentrations, and animal toxicological studies using 
exposures of minutes to hours (ISA, section 5.2). This evidence is 
briefly summarized below and discussed in more detail in the proposal 
(see sections II.B.1 to II.B.5, see 74 FR at 64815-821). We also note 
that the ISA judged evidence of an association between SO2 
exposure and other health categories to be less convincing; other 
associations were judged to be suggestive but not sufficient to infer a 
causal relationship (i.e., short-term exposure to SO2 and 
mortality) or inadequate to infer the presence or absence of a causal 
relationship (i.e., short-term exposure to SO2 and 
cardiovascular morbidity, and long-term exposure to SO2 and 
respiratory morbidity, other morbidity, and mortality). Key conclusions 
from the ISA are described in greater detail in Table 5-3 of the ISA.
---------------------------------------------------------------------------

    \4\ A causal relationship is based on ``[e]vidence [that] is 
sufficient to conclude that there is a causal relationship between 
relevant pollutant exposures and the health outcome. That is, a 
positive association has been observed between the pollutant and the 
outcome in studies in which chance, bias, and confounding could be 
ruled out with reasonable confidence. Evidence includes, for 
example, controlled human exposure studies; or observational studies 
that cannot be explain by plausible alternatives or are supported by 
other lines of evidence (e.g. animal studies or mechanism of action 
information). Evidence includes replicated and consistent high-
quality studies by multiple investigators.'' ISA Table 1-2, at 1-11.
---------------------------------------------------------------------------

1. Short-Term (5-minute to 24-hour) SO2 Exposure and 
Respiratory Morbidity Effects
    The ISA examined numerous controlled human exposure studies and 
found that moderate or greater decrements in lung function (i.e., 
[gteqt] 15% decline in Forced Expiratory Volume (FEV1) and/
or [gteqt] 100% increase in specific airway resistance (sRaw)) occur in 
some exercising asthmatics exposed to SO2 concentrations as 
low as 200-300 ppb for 5-10 minutes. The ISA also found that among 
asthmatics, both the percentage of individuals affected, and the 
severity of the response increased with increasing SO2 
concentrations. That is, at 5-10 minute concentrations ranging from 
200-300 ppb, the lowest levels tested in free breathing chamber 
studies, approximately 5-30% percent of exercising asthmatics 
experienced moderate or greater decrements in lung function (ISA, Table 
3-1). At concentrations of 400-600 ppb, moderate or greater decrements 
in lung function occurred in approximately 20-60% of exercising 
asthmatics, and compared to exposures at 200-300 ppb, a larger 
percentage of asthmatics experienced severe decrements in lung function 
(i.e., [gteqt] 20% decrease in FEV1 and/or [gteqt] 200% 
increase in sRaw; ISA, Table 3-1). Moreover, at SO2 
concentrations [gteqt] 400 ppb (5-10 minute exposures), moderate or 
greater decrements in lung function were often statistically 
significant at the group mean level and frequently accompanied by 
respiratory symptoms. Id.
    The ISA also found that in locations meeting the current 
SO2 NAAQS, numerous epidemiologic studies reported positive 
associations between ambient SO2 concentrations and 
respiratory symptoms in children, as well as emergency department 
visits and hospitalizations for all respiratory causes and asthma 
across multiple age groups. Moreover, the ISA concluded that these 
epidemiologic studies were consistent and coherent. This evidence was 
consistent in that associations were reported in studies conducted in 
numerous locations and with a variety of methodological approaches 
(ISA, section 5.2; p. 5-5). It was coherent in that respiratory symptom 
results from epidemiologic studies of short-term (predominantly 1-hour 
daily maximum or 24-hour average) SO2 concentrations were 
generally in agreement with respiratory symptom results from controlled 
human exposure studies of 5-10 minutes. These results were also 
coherent in that the respiratory effects observed in controlled human 
exposure studies of 5-10 minutes further provided a basis for a 
progression of respiratory morbidity that could lead to the increased 
emergency department visits and hospital admissions observed in 
epidemiologic studies (ISA, section 5.2; p. 5-5). In addition, the ISA 
found that when evaluated as a whole, SO2 effect estimates 
in multi-pollutant models generally remained positive and relatively 
unchanged when co-pollutants were included. Therefore, although 
recognizing the uncertainties associated with separating the effects of 
SO2 from those of co-occurring pollutants, the ISA concluded 
that ``the limited available evidence indicates that the effect of 
SO2 on respiratory health outcomes appears to be generally 
robust and independent of the effects of gaseous co-pollutants, 
including NO2 and O3, as well as particulate co-
pollutants, particularly PM2.5'' (ISA, section 5.3; p. 5-9).
    The ISA also found that the respiratory effects of SO2 
were consistent with the mode of action as it is currently understood 
from animal toxicological and controlled human exposure studies (ISA, 
section 5.2; p. 5-2). The immediate effect of SO2 on the 
respiratory system is bronchoconstriction. This response is mediated by 
chemosensitive receptors in the tracheobronchial tree. Activation of 
these receptors triggers central nervous system reflexes that result in

[[Page 35526]]

bronchoconstriction and respiratory symptoms that are often followed by 
rapid shallow breathing (id). The ISA noted that asthmatics are likely 
more sensitive to the respiratory effects of SO2 due to pre-
existing inflammation associated with the disease. For example, pre-
existing inflammation may lead to enhanced release of inflammatory 
mediators, and/or enhanced sensitization of the chemosensitive 
receptors (id).
    Taken together, the ISA concluded that the controlled human 
exposure, epidemiologic, and toxicological evidence supported its 
determination of a causal relationship between respiratory morbidity 
and short-term (5-minutes to 24-hours) exposure to SO2.
a. Adversity of Short-Term Respiratory Morbidity Effects
    As discussed more fully in the proposal (section II.B.1.c, 74 FR at 
64817) and in section II.E.2.b below, based on: (1) American Thoracic 
Society (ATS) guidelines; (2) advice and recommendations from CASAC 
(see specific consensus CASAC comments in sections II.E.2.b and 
II.F.4.b below); and (3) conclusions from previous NAAQS reviews, EPA 
found that 5-10 minute exposures to SO2 concentrations at 
least as low as 200 ppb can result in adverse health effects in some 
asthmatics (i.e., 5-30% of the tested individuals in controlled human 
exposure studies of 200-300 ppb). As just mentioned, at SO2 
concentrations >= 400 ppb, controlled human exposure studies have 
reported decrements in lung function that are often statistically 
significant at the group mean level, and that are frequently 
accompanied by respiratory symptoms. Being mindful that the ATS 
guidelines specifically indicate decrements in lung function with 
accompanying respiratory symptoms as being adverse (see proposal 
section II.B.1.c, 74 FR at 64817 and section II.E.2.b below), exposure 
to 5-10 minute SO2 concentrations >= 400 ppb can result in 
health effects that are clearly adverse.
    The ATS also indicated that exposure to air pollution that 
increases the risk of an adverse effect to a population is adverse, 
even though it may not increase the risk of any individual to an 
unacceptable level (ATS 2000; see proposal section II.B.1.c, 74 FR at 
64817). As an example, ATS states:

    A population of children with asthma could have a distribution 
of lung function such that no individual child has a level 
associated with significant impairment. Exposure to air pollution 
could shift the distribution toward lower levels without bringing 
any individual child to a level that is associated with clinically 
relevant consequences. Individuals within the population would, 
however, have diminished reserve function and are at potentially 
increased risk if affected by another agent, e.g., a viral 
infection. Assuming that the relationship between the risk factor 
and the disease is causal, the committee considered that such a 
shift in the risk factor distribution, and hence the risk profile of 
the exposed population, should be considered adverse, even in the 
absence of the immediate occurrence of frank illness (ATS 2000, p. 
668).

    As mentioned above, the ISA reported that exposure to 
SO2 concentrations as low as 200-300 ppb for 5-10 minutes 
results in approximately 5-30% of exercising asthmatics experiencing 
moderate or greater decrements in lung function (defined in terms of a 
>= 15% decline in FEV1 or 100% increase in sRaw; ISA, Table 
3-1). Even though these results were not statistically significant at 
the group mean level, in light of EPA's interpretation of how to apply 
the ATS guidelines for defining an adverse effect, as described above, 
the REA found that these results could reasonably indicate an 
SO2-induced shift in these lung function measurements for 
this subset of the population. As a result, an appreciable percentage 
of exercising asthmatics exposed to SO2 concentrations as 
low as 200 ppb would be expected to have diminished reserve lung 
function and would be expected to be at greater risk if affected by 
another respiratory agent, for example, viral infection. Importantly, 
as explained immediately above, diminished reserve lung function in a 
population that is attributable to air pollution is considered an 
adverse effect under ATS guidance. In addition to the 2000 ATS 
guidelines, the REA was also mindful of previous CASAC recommendations 
(Henderson 2006) and NAAQS review conclusions (EPA 2006, EPA 2007d) 
indicating that moderate decrements in lung function can be clinically 
significant in some asthmatics (discussed in detail below, see section 
II.E.2.b). The REA further considered that subjects participating in 
these controlled human exposure studies do not include severe 
asthmatics and that it was reasonable to presume that persons with more 
severe asthma than the study participants would have a more serious 
health effect from short-term exposure to 200 ppb SO2.\5\ 
Taken together, the REA concluded that exposure to SO2 
concentrations at least as low as 200 ppb can result in adverse health 
effects in asthmatics and that this conclusion was in agreement with 
consensus CASAC comments and recommendations expressed during the 
current SO2 NAAQS review (see sections II.E.2.b and II.F.4.b 
below).
---------------------------------------------------------------------------

    \5\ We also note that very young children were not included in 
the controlled human exposure studies and this absence of data on 
what is likely to be a sensitive life stage is a source of 
uncertainty for children's susceptibility to SO2.
---------------------------------------------------------------------------

    In addition to the controlled human exposure evidence, 
epidemiologic studies also indicate that adverse respiratory morbidity 
effects are associated with SO2 (REA, section 4.3). As 
mentioned above, in reaching the conclusion of a causal relationship 
between respiratory morbidity and short-term SO2 exposure, 
the ISA generally found positive associations between ambient 
SO2 concentrations and emergency department visits and 
hospitalizations for all respiratory causes and asthma. Notably, 
emergency department visits, hospitalizations, episodic respiratory 
illness, and aggravation of respiratory diseases (e.g. asthma) 
attributable to air pollution are considered adverse health effects 
under ATS guidelines.
2. Health Effects and Long-Term Exposures to SO2
    There were numerous studies published since the last review 
examining possible associations between long-term SO2 
exposure and mortality and morbidity (respiratory morbidity, 
carcinogenesis, adverse prenatal and neonatal outcomes) endpoints. 
However, the ISA concluded that the evidence relating long-term (weeks 
to years) SO2 exposure to adverse health effects was 
``inadequate to infer the presence or absence of a causal 
relationship'' (ISA, Table 5-3). That is, the ISA found the long-term 
health evidence to be of insufficient quantity, quality, consistency, 
or statistical power to make a determination as to whether 
SO2 was truly associated with these health outcomes (ISA, 
Table 1-2).
3. SO2-Related Impacts on Public Health
    Interindividual variation in human responses to air pollutants 
indicates that some populations are at increased risk for the 
detrimental effects of ambient exposure to SO2. The NAAQS 
are intended to provide an adequate margin of safety for both the 
general population and susceptible populations that are potentially at 
increased risk for health effects in response to exposure to ambient 
air pollution (see footnote 1 above). To facilitate the identification 
of populations at increased risk for SO2-related health 
effects, studies have identified factors that contribute to the 
susceptibility of individuals to SO2. Susceptible 
individuals are broadly defined as those with a greater

[[Page 35527]]

likelihood of an adverse outcome given a specific exposure in 
comparison with the general population (American Lung Association, 
2001). The susceptibility of an individual to SO2 can 
encompass a multitude of factors which represent normal developmental 
phases or life stages (e.g., age) or biologic attributes (e.g., 
gender); however, other factors (e.g., socioeconomic status (SES)) may 
influence the manifestation of disease and also increase an 
individual's susceptibility (American Lung Association, 2001). In 
addition, populations may be at increased risk to SO2 due to 
an increase in their exposure during certain life stages (e.g., 
childhood or old age) or as a result of external factors (e.g., SES) 
that contribute to an individual being disproportionately exposed to 
higher concentrations than the general population.\6\ It should be 
noted that in some cases specific populations may be affected by 
multiple susceptibility factors. For example, a population that is 
characterized as having low SES may have less access to healthcare 
resulting in the manifestation of a disease, which increases their 
susceptibility to SO2, while they may also reside in a 
location that results in disproportionately high exposure to 
SO2.
---------------------------------------------------------------------------

    \6\ This aspect of susceptibility is referred to as 
vulnerability in the proposal and in the ISA.
---------------------------------------------------------------------------

    To examine whether SO2 differentially affects certain 
populations, stratified analyses are often conducted in epidemiologic 
investigations to identify the presence or absence of effect 
modification. A thorough evaluation of potential effect modifiers may 
help identify susceptible populations that are at increased risk to 
SO2 exposure. These analyses are based on the proper 
identification of confounders and subsequent adjustment for them in 
statistical models, which helps separate a spurious from a true causal 
association. Although the design of toxicological and human clinical 
studies does not allow for an extensive examination of effect 
modifiers, the use of animal models of disease and the study of 
individuals with underlying disease or genetic polymorphisms do allow 
for comparisons between subgroups. Therefore, the results from these 
studies, combined with those results obtained through stratified 
analyses in epidemiologic studies, contribute to the overall weight of 
evidence for the increased susceptibility of specific populations to 
SO2. Those populations identified in the ISA to be 
potentially at greater risk of experiencing an adverse health effect 
from SO2 were described in detail in the proposal (section 
II.B.5) and include: (1) Those with pre-existing respiratory disease; 
(2) children and older adults; (3) persons who spend increased time 
outdoors or at elevated ventilation rates; (4) persons with lower SES; 
and (5) persons with certain genetic factors.
    As discussed in the proposal (section II.B.5.g, 74 FR at 64821), 
large proportions of the U.S. population are likely to be at increased 
risk of experiencing SO2-related health effects. In the 
United States, approximately 7% of adults and 9% of children have been 
diagnosed with asthma. Notably, the prevalence and severity of asthma 
is higher among certain ethnic or racial groups such as Puerto Ricans, 
American Indians, Alaskan Natives, and African Americans (EPA 2008b). 
Furthermore, a higher prevalence of asthma among persons of lower SES 
and an excess burden of asthma hospitalizations and mortality in 
minority and inner-city communities have been observed (EPA, 2008b). In 
addition, population groups based on age comprise substantial segments 
of individuals that may be potentially at risk for SO2-
related health impacts. Based on U.S. census data from 2000, about 72.3 
million (26%) of the U.S. population are under 18 years of age, 18.3 
million (7.4%) are under 5 years of age, and 35 million (12%) are 65 
years of age or older. There is also concern for the large segment of 
the population that is potentially at risk to SO2-related 
health effects because of increased time spent outdoors at elevated 
ventilation rates (those who work or play outdoors). Overall, the 
considerable size of the population groups at risk indicates that 
exposure to ambient SO2 could have a significant impact on 
public health in the United States.

C. Human Exposure and Health Risk Characterization

    To put judgments about SO2-associated health effects 
into a broader public health context, EPA has drawn upon the results of 
the quantitative exposure and risk assessments. Judgments reflecting 
the nature of the evidence and the overall weight of the evidence are 
taken into consideration in these quantitative exposure and risk 
assessments. These assessments include estimates of the likelihood that 
asthmatic children at moderate or greater exertion (e.g. while 
exercising) in St. Louis or Greene County, Missouri would experience 
SO2 exposures of potential concern. In addition, these 
analyses include an estimate of the number and percent of exposed 
asthmatic children in these locations likely to experience 
SO2-induced lung function responses (i.e., moderate or 
greater decrements in lung function defined in terms of sRaw or 
FEV1) under varying air quality scenarios (i.e., current air 
quality and air quality simulated to just meet the current or potential 
alternative standards). These assessments also characterize the kind 
and degree of uncertainties inherent in such estimates.
    As previously mentioned, the ISA concluded that the evidence for an 
association between respiratory morbidity and short-term SO2 
exposure was ``sufficient to infer a causal relationship'' (ISA, 
section 5.2) and that the ``definitive evidence'' for this conclusion 
was from the results of 5-10 minute controlled human exposure studies 
demonstrating decrements in lung function and/or respiratory symptoms 
in exercising asthmatics (ISA, section 5.2). Accordingly, the air 
quality and exposure analyses and their associated risk 
characterizations focused on 5-minute concentrations of SO2 
in excess of potential health effect benchmark values derived from the 
controlled human exposure literature (see proposal section II.C.1, 74 
FR at 64821, and REA, section 6.2). These benchmark levels are not 
potential standards, but rather are SO2 exposure 
concentrations which represent ``exposures of potential concern'' which 
are used in these analyses to estimate potential exposures and risks 
associated with 5-minute concentrations of SO2. The REA 
considered 5-minute benchmark levels of 100, 200, 300, and 400 ppb in 
these analyses, but especially noted exceedances or exposures with 
respect to the 200 and 400 ppb 5-minute benchmark levels. These 
benchmark levels were highlighted because (1) 400 ppb represents the 
lowest concentration in free-breathing controlled human exposure 
studies where moderate or greater lung function decrements occurred 
which were often statistically significant at the group mean level and 
were frequently accompanied by respiratory symptoms; and (2) 200 ppb is 
the lowest level at which moderate or greater decrements in lung 
function in free-breathing controlled human exposure studies were found 
in some individuals, although these lung function changes were not 
statistically significant at the group mean level. Notably, 200 ppb is 
also the lowest level that has been tested in free-breathing controlled 
human exposure studies (REA, section 4.2.2).\7\
---------------------------------------------------------------------------

    \7\ The ISA cites one chamber study with intermittent exercise 
where healthy and asthmatic children were exposed to 100 ppb 
SO2 in a mixture with ozone and sulfuric acid. The ISA 
notes that compared to exposure to filtered air, exposure to the 
pollutant mix did not result in statistically significant changes in 
lung function or respiratory symptoms (ISA, section 3.1.3.4).

---------------------------------------------------------------------------

[[Page 35528]]

    The REA utilized three approaches to characterize health risks. In 
the first approach, for each air quality scenario, statistically 
estimated 5-minute SO2 concentrations \8\ and measured 
ambient 5-minute SO2 concentrations were compared to the 5-
minute potential health effect benchmark levels discussed above (REA, 
chapter 7). This air quality analysis included all available ambient 
monitoring data as well as a more detailed analysis in 40 counties. The 
air quality analysis was considered a broad characterization of 
national air quality and human exposures that might be associated with 
these 5-minute SO2 concentrations. An advantage of the air 
quality analysis is its relative simplicity; however, there is 
unc