Primary National Ambient Air Quality Standard for Sulfur Dioxide, 64810-64881 [E9-28058]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 74, Number 234 (Tuesday, December 8, 2009)]
[Proposed Rules]
[Pages 64810-64881]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: E9-28058]



[[Page 64809]]

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

Part II





Environmental Protection Agency





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



40 CFR Parts 50, 53, and 58



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

Federal Register / Vol. 74, No. 234 / Tuesday, December 8, 2009 / 
Proposed Rules

[[Page 64810]]


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

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

 [EPA-HQ-OAR-2007-0352; FRL-8984-3]
RIN 2060-A048


Primary National Ambient Air Quality Standard for Sulfur Dioxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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

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 proposing to revise the primary SO2 NAAQS to provide 
requisite protection of public health with an adequate margin of 
safety. Specifically, EPA proposes to establish a new 1-hour 
SO2 standard within the range of 50-100 parts per billion 
(ppb), based on the 3-year average of the annual 99th percentile (or 
4th highest) of 1-hour daily maximum concentrations. The EPA also 
proposes to revoke both the existing 24-hour and annual primary 
SO2 standards.

DATES: Comments must be received on or before February 8, 2010. Under 
the Paperwork Reduction Act, comments on the information collection 
provisions must be received by OMB on or before January 7, 2010.
    Public Hearings: A public hearing is scheduled for this proposed 
rule. The public hearing will be held on January 5, 2010 in Atlanta, 
Georgia.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2007-0352 by one of the following methods:
     https://www.regulations.gov: Follow the on-line 
instructions for submitting comments.
     E-mail: a-and-r-Docket@epa.gov.
     Fax: 202-566-9744.
     Mail: Docket No. EPA-HQ-OAR-2007-0352, Environmental 
Protection Agency, Mail Code 6102T, 1200 Pennsylvania Ave., NW., 
Washington, DC 20460. Please include a total of two copies.
     Hand Delivery: Docket No. EPA-HQ-OAR-2007-0352, 
Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution 
Ave., NW, Washington, DC. Such deliveries are only accepted during the 
Docket's normal hours of operation, and special arrangements should be 
made for deliveries of boxed information.
    Public Hearings: A public hearing is scheduled for this proposed 
rule. The public hearing will be held on January 5, 2010 in Atlanta, 
Georgia. The hearing will be held at the following location: Sam Nunn 
Atlanta Federal Center, Conference Rooms B and C, 61 Forsyth Street, 
SW., Atlanta, GA 30303, Telephone: (404) 562-9077.

    Note:  All persons entering the Atlanta Federal Center must have 
a valid picture ID such as a Driver's License and go through Federal 
security procedures. All persons must go through a magnetometer and 
all personal items must go through x-ray equipment, similar to 
airport security procedures. After passing through the equipment, 
all persons must sign in at the guard station and show their picture 
ID.

    See the SUPPLEMENTARY INFORMATION under ``Public Hearing'' for 
further information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2007-0352. EPA's policy is that all comments received will be included 
in the public docket without change and may be made available online at 
www.regulations.gov, including any personal information provided, 
unless the comment includes information claimed to be Confidential 
Business Information (CBI) or other information whose disclosure is 
restricted by statute. Do not submit information that you consider to 
be CBI or otherwise protected through www.regulations.gov or e-mail. 
The www.regulations.gov Web site is an ``anonymous access'' system, 
which means EPA will not know your identity or contact information 
unless you provide it in the body of your comment. If you send an e-
mail comment directly to EPA without going through www.regulations.gov 
your e-mail address will be automatically captured and included as part 
of the comment that is placed in the public docket and made available 
on the Internet. If you submit an electronic comment, EPA recommends 
that you include your name and other contact information in the body of 
your comment and with any disk or CD-ROM you submit. If EPA cannot read 
your comment due to technical difficulties and cannot contact you for 
clarification, EPA may not be able to consider your comment. Electronic 
files should avoid the use of special characters, any form of 
encryption, and be free of any defects or viruses. For additional 
information about EPA's public docket visit the EPA Docket Center 
homepage at https://www.epa.gov/epahome/dockets.htm.
    Docket: All documents in the docket are listed in the 
www.regulations.gov index. Although listed in the index, some 
information is not publicly available, e.g., CBI or other information 
whose disclosure is restricted by statute. Certain other material, such 
as copyrighted material, will be publicly available only in hard copy. 
Publicly available docket materials are available either electronically 
in 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:

General Information

What Should I Consider as I Prepare My Comments for EPA?

    1. Submitting CBI. Do not submit this information to EPA through 
www.regulations.gov or e-mail. Clearly mark the part or all of the 
information that you claim to be CBI. For CBI information in a disk or 
CD-ROM that you mail to EPA, mark the outside of the disk or CD-ROM as 
CBI and then identify electronically within the disk or CD-ROM the 
specific information that is claimed as CBI. In addition to one 
complete version of the comment that includes information claimed as 
CBI, a copy of the comment that does not contain the information 
claimed as CBI must be submitted for inclusion in the public docket. 
Information so marked will not be disclosed except in accordance with 
procedures set forth in 40 CFR part 2.
    2. Tips for Preparing Your Comments. When submitting comments, 
remember to:
     Identify the rulemaking by docket number and other 
identifying information (subject heading, Federal Register date and 
page number).
     Follow directions--the agency may ask you to respond to 
specific questions or organize comments by referencing a Code of 
Federal Regulations (CFR) part or section number.
     Explain why you agree or disagree, suggest alternatives, 
and substitute language for your requested changes.

[[Page 64811]]

     Describe any assumptions and provide any technical 
information and/or data that you used.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Related Information

    A number of the documents that are relevant to this rulemaking are 
available through EPA's Office of Air Quality Planning and Standards 
(OAQPS) Technology Transfer Network (TTN) Web site at https://www.epa.gov/ttn/naaqs/standards/so2/s_so2_index.html. These documents 
include the Integrated Review Plan and the Health Assessment Plan, 
available at, the Integrated Science Assessment (ISA), available at 
https://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_isa.html, and 
the Risk and Exposure Assessment (REA), available at https://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_rea.html. These and 
other related documents are also available for inspection and copying 
in the EPA docket identified above.

Public Hearing

    The public hearing on January 5, 2010 will provide interested 
parties the opportunity to present data, views, or arguments concerning 
the proposed rule. The EPA may ask clarifying questions during the oral 
presentations, but will not respond to the presentations at that time. 
Written statements and supporting information submitted during the 
comment period will be considered with the same weight as any oral 
comments and supporting information presented at the public hearing. 
Written comments must be received by the last day of the comment 
period, as specified in this proposed rulemaking.
    The public hearing will begin at 10 a.m. and continue until 7 p.m. 
(local time) or later, if necessary, depending on the number of 
speakers wishing to participate. The EPA will make every effort to 
accommodate all speakers that arrive and register before 7 p.m. A lunch 
break is scheduled from 12:30 p.m. until 2 p.m.
    If you would like to present oral testimony at the hearing, please 
notify Ms. Tricia Crabtree (C504-02), U.S. EPA, Research Triangle Park, 
NC 27711. The preferred method for registering is by e-mail 
(crabtree.tricia@epa.gov). Ms. Crabtree may be reached by telephone at 
(919) 541-5688. She will arrange a general time slot for you to speak. 
The EPA will make every effort to follow the schedule as closely as 
possible on the day of the hearing.
    Oral testimony will be limited to five (5) minutes for each 
commenter to address the proposal. We will not be providing equipment 
for commenters to show overhead slides or make computerized slide 
presentations unless we receive special requests in advance. Commenters 
should notify Ms. Crabtree if they will need specific audiovisual (AV) 
equipment. Commenters should also notify Ms. Crabtree if they need 
specific translation services for non-English speaking commenters. The 
EPA encourages commenters to provide written versions of their oral 
testimonies either electronically on computer disk, CD-ROM, or in paper 
copy.
    The hearing schedule, including lists of speakers, will be posted 
on EPA's Web site for the proposal at https://www.epa.gov/ttn/naaqs/standards/so2/s_so2_index.html prior to the hearing. Verbatim 
transcripts of the hearing and written statements will be included in 
the rulemaking docket.

Table of Contents

    The following topics are discussed in this preamble:

I. Background
    A. Legislative requirements
    B. Related SO2 control programs
    C. History of reviews of the primary NAAQS for sulfur oxides
II. Rationale for proposed 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. Respiratory effects and 5-10 minute exposure to 
SO2
    a. Respiratory symptoms
    b. Lung function decrements
    c. Adversity of 5-10 minute respiratory effects
    2. Respiratory effects and 1 to 24-hour exposures to 
SO2
    a. Respiratory symptoms
    b. Emergency department visits and hospitalizations
    3. ISA conclusions regarding short-term (5-minutes to 24-hour) 
SO2 exposures
    4. Health effects and long-term exposures to SO2
    5. SO2-related impacts on public health
    a. Pre-existing respiratory disease
    b. Genetics
    c. Age
    d. Time spent outdoors
    e. Ventilation rate
    f. Socioeconomic status
    g. Size of at-risk population
    C. Human exposure and health risk characterization
    1. Evidence base for the risk characterization
    2. Overview of approaches
    3. Key limitations and uncertainties
    D. Considerations in review of the standards
    1. Background on the current standards
    2. Approach for reviewing the need to retain or revise the 
current standards
    E. Adequacy of the current standards
    1. Adequacy of the current 24-hour standard
    a. Evidence-based considerations
    b. Air quality, exposure, and risk-based considerations
    c. Summary of considerations from the REA regarding the 24-hour 
standard
    2. Adequacy of the current annual standard
    a. Evidence-based considerations
    b. Air quality, exposure, and risk-based considerations
    c. Summary of considerations from the REA regarding the annual 
standard
    3. CASAC views regarding adequacy of the current 24-hour and 
annual standards
    4. Administrator's conclusions regarding adequacy of the current 
24-hour and annual standards
    F. Conclusions on the elements of a proposed new short-term 
standard
    1. Indicator
    2. Averaging time
    a. Evidence and air quality, exposure, and risk-based 
considerations
    b. CASAC views
    c. Administrator's conclusions on averaging time
    3. Form
    a. Evidence, air quality, and risk-based considerations
    b. CASAC views
    c. Administrator's conclusions on form
    4. Level
    a. Evidence-based considerations
    b. Air quality, exposure and risk-based considerations
    c. Observations based on evidence and risk-based considerations
    d. CASAC views
    e. Administrator's conclusions on level for a 1-hour standard
    5. Implications for retaining or revoking current standards
    G. Summary of proposed decisions on primary standards
III. Proposed Amendments to Ambient Monitoring and Reporting 
Requirements
    A. Monitoring methods
    1. Background
    2. Proposed new FRM measurement technique
    3. Technical description of the proposed UVF FRM
    4. Implications to air monitoring networks
    5. Proposed revisions to 40 CFR Part 53
    B. Network design
    1. Background
    2. Proposed changes
    a. Population Weighted Emissions Index (PWEI) Triggered 
Monitoring
    b. State-level emissions triggered monitoring
    c. Monitor placement and siting
    d. Monitoring required by the Regional Administrator
    e. Alternative Network Design
    C. Data Reporting
IV. Proposed Appendix T--Interpretation of the Primary NAAQS for 
Oxides of Sulfur

[[Page 64812]]

and Proposed Revisions to the Exceptional Events Rule
    A. Background
    B. Interpretation of the NAAQS for Oxides of Sulfur
    1. 1-hour standard based on the annual 4th highest daily value 
form
    2. 1-hour primary standard based on the 99th percentile value 
form
    C. Exceptional events information submission schedule
V. Designations for the SO2 NAAQS
VI. Clean Air Act Implementation Requirements
    A. How this rule applies to tribes
    B. Attainment dates
    1. Attaining the NAAQS
    2. Consequences of failing to attain by the Statutory Attainment 
Date
    C. Section 110(a)(2) NAAQS Infrastructure Requirements
    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. Communication of public health information
VIII. 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. Legislative 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) & (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 proposal 
concerns exclusively the primary NAAQS for oxides of sulfur.
---------------------------------------------------------------------------

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

    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.
    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 
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.

B. 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

[[Page 64813]]

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 define additional SO2 emission reductions needed 
in the Eastern United States to address the interstate impact 
provisions of CAA section 110(a)(2)(D), a rule which EPA is 
reevaluating pursuant to court remand.
    Currently, there are several areas designated as being in 
nonattainment of the primary SO2 NAAQS (see section VI). If 
the SO2 NAAQS is revised as a result of this review; 
however, some 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).

C. 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 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 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). 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 such 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 peak 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 
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, 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, 
but that a NAAQS was found not be appropriate. 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: 
``[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 will help inform the current review, which will 
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

[[Page 64814]]

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 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).
    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, notices of proposed and final rulemaking concerning the 
review of the primary SO2 NAAQS no later than November 16, 
2009 and June 2, 2010, respectively.
    This action presents the Administrator's proposed decisions on the 
current primary SO2 standards. Throughout this preamble a 
number of conclusions, findings, and determinations proposed by the 
Administrator are noted. Although they identify the reasoning that 
supports this proposal, they are not intended to be final or 
conclusive. EPA invites general, specific, and/or technical comments on 
all issues involved with this proposal, including all such proposed 
judgments, conclusions, findings, and determinations. In addition to 
requesting comment on the overall approach, EPA invites 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.

II. Rationale for proposed decisions on the primary standards

    This section presents the rationale for the Administrator's 
proposed decision to revise the existing SO2 primary 
standards by replacing the current 24-hour and annual standards with a 
1-hour standard and to specify this 1-hour standard to the nearest 
parts per billion (ppb). 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 CASAC letters to the 
Administrator (Henderson 2008; Samet, 2009); and (3) public comments 
received at CASAC meetings during the development of the ISA and the 
REA.
    In developing this rationale, 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 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 in particular on those health endpoints 
for which the ISA finds associations with SO2 to be causal 
or likely causal (see section II.B below). Thus, the focus of this 
proposal will be 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. The newly available research studies evaluated in the ISA 
have undergone intensive scrutiny through multiple layers of peer 
review and opportunities for public review and comment. Although 
important uncertainties remain in the qualitative and quantitative 
characterizations of health effects attributable to exposure to ambient 
SO2, the review of this information has been extensive and 
deliberate.
    The remainder of this section discusses the Administrator's 
rationale for the proposed decisions on the primary standard. Section 
II.A presents a discussion of the principal emitting sources and 
current patterns of SO2 air quality, as well as the current 
SO2 monitoring network from which those air quality patterns 
are obtained. 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 discusses the 
approaches taken by EPA to assess exposures and health risks associated 
with exposure to ambient SO2, including a discussion of key 
uncertainties associated with the analyses. Section II.D presents the 
approach that is being 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 the scientific evidence and the air quality, 
exposure, and risk-based results specifically as they relate to the 
current and potential alternative standards, including discussion of 
the Administrator's proposed decisions on the standards. Section II.G 
summarizes the Administrator's proposed 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

[[Page 64815]]

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 human clinical studies have demonstrated 
that exposure to these peaks can result in adverse respiratory effects 
in exercising asthmatics (see section II.B). 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). The highest reported 
concentration was 921 ppb, but the 99th percentile values for 
aggregated within-state data were all below 90 ppb (ISA, section 
2.5.2).
---------------------------------------------------------------------------

    \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 the SO2 standard was established in 1971, 
uniform minimum monitoring requirements for SO2 monitoring 
did not appear until May 1979. From the time of the implementation of 
the 1979 monitoring rule through 2008, the SO2 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 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, based on the fact that there were no SO2 
nonattainment areas at that time, coupled with trends evidence showing 
an increasing gap between national average SO2 
concentrations and the current 24-hour and annual standards. 
Additionally, the minimum requirements were removed to provide State, 
local, and tribal air monitoring agencies flexibility in meeting higher 
priority monitoring needs for pollutants such as ozone and 
PM2.5, or implementing the new multi-pollutant sites (NCore 
network) required by the 2006 rule revisions, by allowing them to 
discontinue lower priority monitoring. More information on 
SO2 monitoring can be found in section III.

B. Health effects information

    During the last review, EPA retained the current 24-hour and annual 
averaging times for the primary SO2 NAAQS. The 24-hour NAAQS 
was largely based on epidemiologic studies that observed associations 
between 24-hour average SO2 levels and adverse respiratory 
effects and daily mortality (EPA 1982, 1994a, 1994b). The annual 
standard was supported by a few epidemiologic studies that found an 
association between adverse respiratory effects and annual average 
SO2 concentrations (EPA 1982, 1994a, 1994b). However, it was 
noted that in the locations where these epidemiologic studies were 
conducted, high SO2 levels were usually accompanied by high 
levels of PM, thus making it difficult to disentangle the individual 
contribution each pollutant had on these health outcomes. Moreover, EPA 
noted that rather than 24-hour or annual average SO2 levels, 
the health effects observed in these studies may have been related, at 
least in part, to the occurrence of shorter-term peaks of 
SO2 within a 24-hour period (53 FR 14930; April 26, 1988).
    In the current review, the ISA along with its associated annexes, 
provided a comprehensive review and assessment of the scientific 
evidence related to the health effects associated with SO2 
exposures. For these health effects, the ISA characterized judgments 
about causality with a hierarchy that contains five levels (ISA, 
section 1-3): sufficient to infer a causal relationship, sufficient to 
infer a likely causal relationship (i.e., more likely than not), 
suggestive but not sufficient to infer a causal relationship, 
inadequate to infer the presence or absence of a causal relationship, 
and suggestive of no causal relationship. Judgments about causality 
were informed by a series of aspects that are based on those set forth 
by Sir Austin Bradford Hill in 1965 (ISA, Table 1-1). These aspects 
include strength of the observed association, availability of 
experimental evidence, consistency of the observed association, 
biological plausibility, coherence of the evidence, temporal 
relationship of the observed association, and the presence of an 
exposure-response relationship.
    Judgments made in the ISA about the extent to which relationships 
between various health endpoints and exposure to SO2 are 
likely causal have been informed by several factors. As discussed in 
the ISA in section 1.3, these factors include the nature of the 
evidence (i.e., controlled human exposure, epidemiologic, and/or 
toxicological studies) and the weight of evidence. The weight of 
evidence takes into account such considerations as biological 
plausibility, coherence of the evidence, strength of associations, and 
consistency of the evidence. Controlled human exposure studies provide 
directly applicable information for determining causality because these 
studies are not limited by differences in dosimetry and species 
sensitivity, which would need to be addressed in extrapolating animal 
toxicology data to human health effects, and because they provide data 
relating health effects specifically to SO2 exposures, in 
the absence of the co-occurring pollutants present in ambient air. 
Epidemiologic studies provide evidence of associations between 
SO2 concentrations and more serious health endpoints (e.g., 
hospital admissions and emergency department visits) that cannot be 
assessed in controlled human exposure studies. For these studies the 
degree of uncertainty introduced by confounding variables (e.g., other 
pollutants) affects the level of confidence that the health effects 
being investigated are attributable to

[[Page 64816]]

SO2 exposures alone and/or in combination with co-occurring 
pollutants.
    In using a weight of evidence approach to inform judgments about 
the degree of confidence that various health effects are likely to be 
caused by exposure to SO2, confidence increases with the 
number of studies consistently reporting a particular health endpoint, 
with increasing support for the biological plausibility of the health 
effects, and with the strength and coherence of the evidence. 
Conclusions regarding biological plausibility, consistency, and 
coherence of evidence of SO2-related health effects are 
drawn from the integration of epidemiologic studies with controlled 
human exposure studies and with mechanistic information from animal 
toxicological studies. As discussed below, the weight of evidence is 
strongest for respiratory morbidity endpoints (e.g., lung function 
decrements, respiratory symptoms, hospital admissions, and emergency 
department visits) associated with short-term (5-minutes to 24-hours) 
exposure to ambient SO2.
    For epidemiologic studies, strength of association refers to the 
magnitude of the association and its statistical strength, which 
includes assessment of both effect estimate size and precision. In 
general, when associations yield large relative risk estimates, it is 
less likely that the association could be completely accounted for by a 
potential confounder or some other bias. Consistency refers to the 
persistent finding of an association between exposure and outcome in 
multiple studies of adequate power in different persons, places, 
circumstances and times.
    Being mindful of the considerations discussed above, 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). The ISA based 
this conclusion 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). 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.
    As summarized above, the ISA found a ``causal'' association between 
short-term (5 minutes to 24 hour) exposure to SO2 and 
respiratory morbidity. The evidence leading to this conclusion will be 
discussed throughout this section as well as in the context of the 
adequacy of the current and proposed alternative standards (see section 
II.E and II.F) The ISA also found ``suggestive but not sufficient'' 
evidence to infer a causal relationship between short-term 
SO2 exposure and mortality. EPA considered this suggestive 
evidence within the context of proposing a new 1-hour averaging time 
(see section II.F.2). The association between short- and long-term 
SO2 exposure and other health categories was found to be 
inadequate to infer the presence or absence of a causal relationship 
and thus, will not be discussed in detail in this notice.
    Section II.B.1 discusses the results of controlled human exposure 
studies demonstrating respiratory effects in exercising asthmatics 
following 5-10 minute exposures to SO2, and conclusions in 
the REA regarding the adversity of such effects. Section II.B.2 
discusses the respiratory effects reported in U.S. epidemiologic 
studies of respiratory symptoms, as well as emergency department visits 
and hospital admissions for all respiratory causes and asthma. Section 
II.B.3 discusses ISA conclusions regarding short-term (5 minutes to 24-
hours) exposure to SO2 and respiratory effects, and section 
II.B.4 discusses long-term SO2 exposure and potentially 
adverse health effects. Finally, section II.B.5 discusses 
SO2-related impacts on public health.
1. Respiratory effects and 5-10 minute exposure to SO2
    As noted above, 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). This determination was primarily based on controlled 
human exposure studies demonstrating a relationship between 5-10 minute 
peak SO2 exposures and adverse effects on the respiratory 
system in exercising asthmatics. The ISA described the controlled human 
exposure results as being the ``definitive evidence'' for its causal 
finding (ISA, section 5.2; p. 5-2).
    Since the last review, several additional controlled human exposure 
studies have been published that provide supportive evidence of 
SO2-induced decrements in lung function and increases in 
respiratory symptoms among exercising asthmatics (see ISA, Annex Table 
D-2). However, based in part on recent guidance from the American 
Thoracic Society (ATS) regarding what constitutes an adverse health 
effect of air pollution (ATS, 2000), a much larger body of key older 
studies described in the prior review were analyzed in the ISA along 
with studies published since the last review. In their official 
statement, the ATS concluded that an air pollution-induced shift in a 
population distribution of a given health-related endpoint (e.g., lung 
function) should be considered adverse, even if this shift does not 
result in the immediate occurrence of illness in any one individual in 
the population (ATS 2000). The ATS also recommended that transient loss 
in lung function with accompanying respiratory symptoms attributable to 
air pollution should be considered adverse. However, it was noted in 
the ISA that symptom perception is highly variable among asthmatics 
even during severe episodes of asthmatic bronchoconstriction, and that 
an asymptomatic decrease in lung function may pose a significant health 
risk to asthmatic individuals as it is less likely that these 
individuals will seek treatment (ISA, section 3.1.3). Therefore, 
whereas the conclusions in the prior review of the SO2 NAAQS 
were based on SO2 exposure concentrations which resulted in 
large decrements in lung function and moderate to severe respiratory 
symptoms, the ISA's current review of data from controlled human 
exposure studies focused on moderate to large SO2-induced 
decrements in lung function and/or respiratory symptoms ranging from 
mild (perceptible wheeze or chest tightness) to severe (breathing 
distress requiring the use of a bronchodilator). See also section 
II.B.1.c below discussing adversity of effects. Key controlled human 
exposure studies of respiratory symptoms and lung function are 
described briefly below and in more detail in section 3.1.3 of the ISA.
a. Respiratory symptoms
    Numerous free-breathing controlled human exposure studies have 
evaluated respiratory symptoms (e.g. cough, wheeze, or chest tightness) 
in exercising asthmatic following 5-10 minute SO2 exposures. 
Linn et al. (1983) reported that 5-minute exposures to SO2 
levels as low as 400 ppb resulted in exercising asthmatics experiencing 
statistically significant increases in respiratory symptoms (e.g., 
wheeze, chest tightness,

[[Page 64817]]

cough, substernal irritation). In a separate study, exercising 
asthmatics exhibited respiratory symptoms following a 10-minute 
exposure to 400-600 ppb SO2 (Linn et al., (1987); Smith 
(1993)). Gong et al., (1995) exposed SO2-sensitive 
asthmatics to 0, 500 and 1000 ppb SO2 for 10 minutes while 
performing different levels of exercise (light, medium, or heavy) and 
reported that respiratory symptoms increased with increasing 
SO2 concentrations. The authors further reported that 
exposure to 500 ppb SO2 during light exercise evoked a more 
severe symptomatic response than heavy exercise in clean air.
    In addition to these free breathing chamber results described 
above, studies using mouthpiece exposure systems have reported 
respiratory symptoms within minutes of SO2 exposure.\4\ 
Balmes et al. (1987) reported that 7 out of 8 exercising asthmatics 
developed respiratory symptoms following a 500 ppb 3-minute exposure to 
SO2 via mouthpiece (ISA section 3.1.3.1). In an additional 
study, Trenga et al. (1999) reported increases in respiratory symptoms 
in exercising asthmatics following 10-minute exposures to 500 ppb 
SO2. Although not directly comparable to the free-breathing 
chamber results described above, these mouthpiece exposure results 
nonetheless support an association between SO2 exposure and 
respiratory symptoms.
---------------------------------------------------------------------------

    \4\ Studies utilizing a mouthpiece exposure system cannot be 
directly compared to studies involving freely breathing subjects, as 
nasal absorption of SO2 is bypassed during oral 
breathing, thus allowing a greater fraction of inhaled 
SO2 to reach the tracheobronchial airways. As a result, 
individuals exposed to SO2 through a mouthpiece are 
likely to experience greater respiratory effects from a given 
SO2 exposure.
---------------------------------------------------------------------------

b. Lung function decrements
    The ISA found that in free-breathing chamber studies, asthmatic 
individuals exposed to SO2 concentrations as low as 200-300 
ppb for 5-10 minutes during exercise have been shown to experience 
moderate or greater bronchoconstriction, measured as a decrease in 
Forced Expiratory Volume in the first second (FEV1) of >= 
15%, or an increase in specific airway resistance (sRaw) of >= 100% 
after correction for exercise-induced responses in clean air (Bethel et 
al., 1985; Linn et al., 1983, 1987; 1988; 1990; Roger et al., 1985).\5\ 
In addition, the ISA concluded that among asthmatics, both the 
percentage of individuals affected, and the severity of the response 
increases with increasing SO2 concentrations. That is, at 
concentrations ranging from 200-300 ppb, the lowest levels tested in 
free breathing chamber studies,\6\ approximately 5-30% of exercising 
asthmatics experience moderate or greater decrements in lung function 
(ISA, Table 3-1). At concentrations of 400-600 ppb, moderate or greater 
decrements in lung function occur in approximately 20-60% of exercising 
asthmatics, and compared to exposures at 200-300 ppb, a larger 
percentage of asthmatics experience severe decrements in lung function 
(i.e., >= 200% increase in sRaw, and/or a >= 20% decrease in 
FEV1) (ISA, Table 3-1). The ISA also noted that at 
SO2 concentrations >= 400 ppb, moderate or greater 
decrements in lung function are frequently accompanied by respiratory 
symptoms (e.g., cough, wheeze, chest tightness, shortness of breath) 
(ISA, Table 3-1). Further analysis and discussion of the individual 
studies presented above can be found in Sections 3.1.1 to 3.1.3.5 of 
the ISA.
---------------------------------------------------------------------------

    \5\ FEV1 and sRaw are measures of 
bronchoconstriction. Decreases in FEV1 or increases in 
sRaw can result in difficulty breathing.
    \6\ 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)
---------------------------------------------------------------------------

    In addition to the evidence from free-breathing chamber studies, 
the ISA notes very limited evidence of decrements in lung function in 
exercising asthmatics exposed to lower levels of SO2 via 
mouthpiece. That is, the ISA cites two studies where some exercising 
asthmatics had small changes in FEV1 or sRaw following 
exposure to 100 ppb SO2 via mouthpiece (Koenig et al., 1990 
and Sheppard et al., 1981).
c. Adversity of 5-10 minute respiratory effects
    The ATS has previously defined adverse respiratory health effects 
as ``medically significant physiologic changes generally evidenced by 
one or more of the following: (1) Interference with the normal activity 
of the affected person or persons, (2) episodic respiratory illness, 
(3) incapacitating illness, (4) permanent respiratory injury, and/or 
(5) progressive respiratory dysfunction'' (ATS 1985). The ATS has also 
recommended that transient loss in lung function with accompanying 
respiratory symptoms, or detectable effects of air pollution on 
clinical measures (e.g., medication use) be considered adverse (ATS 
1985). In addition, the REA noted that during the last O3 
NAAQS review, the Criteria Document (CD) and Staff Paper indicated that 
for many people with lung disease (e.g., asthma), even moderate 
decrements in lung function (e.g., FEV1 decrements > 10% but 
< 20% and/or >= 100% increases in sRaw) or respiratory symptoms would 
likely interfere with normal activities and result in additional and 
more frequent use of medication (EPA 2006, EPA 2007d). The REA also 
noted that CASAC has previously indicated that in the context of 
standard setting, a focus on the lower end of the range of moderate 
functional responses is most appropriate for estimating potentially 
adverse lung function decrements in people with lung disease (73 
FR16463). Finally, the REA noted that in the current SO2 
NAAQS review, clinicians on the CASAC Panel again advised that moderate 
or greater decrements in lung function can be clinically significant in 
some individuals with respiratory disease (hearing transcripts from 
USEPA Clean Air Scientific Advisory Committee (CASAC), July 30-31, 
2008, Sulfur Oxides-Health Criteria (part 3 of 4) pages 211-213).\7\
---------------------------------------------------------------------------

    \7\ These transcripts can be found in Docket ID No. EPA-HQ-ORD-
2006-0260. Available at www.regulations.gov.
---------------------------------------------------------------------------

    As previously mentioned, the ATS published updated guidelines on 
what constitutes an adverse health effect of air pollution in 2000 
(ATS, 2000). Among other considerations, the 2000 guidelines stated 
that measurable negative effects of air pollution on quality of life 
should be considered adverse (ATS 2000). These updated guidelines also 
indicated that exposure to air pollution that increases the risk of an 
adverse effect to the entire population is adverse, even though it may 
not increase the risk of any individual to an unacceptable level (ATS 
2000). For example, a population of asthmatics could have a 
distribution of lung function such that no individual has a level 
associated with significant impairment. Exposure to air pollution could 
shift the distribution to lower levels that still do not bring any 
individual to a level that is associated with clinically relevant 
effects. However, this would be considered adverse because individuals 
within the population would have diminished reserve function, and 
therefore would be at increased risk if affected by another agent (ATS 
2000).
    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

[[Page 64818]]

guidelines described above specifically indicate decrements in lung 
function with accompanying respiratory symptoms as being adverse, 
exposure to 5-10 minute SO2 concentrations >= 400 ppb are 
clearly adverse.
    The ISA has also 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). 
Considering the 2000 ATS guidelines mentioned above, the REA found that 
these results could reasonably indicate an SO2-induced shift 
in these lung function measurements for this population. As a result, a 
significant percentage of exercisin