1

6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

[EPA-HQ-OAR-2007-0352; FRL-]

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 [insert date 60 days after date of 

publication in the Federal Register]. Under the Paperwork Reduction Act, comments on the 

information collection provisions must be received by OMB on or before {insert date thirty 

days after date of publication in the Federal Register.}

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 
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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.

The public hearing 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 10:00 a.m. and continue until 7:00 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 7:00 p.m. A lunch 

break is scheduled from 12:30 p.m. until 2:00 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.
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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 website for the 

proposal at http://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.

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 website 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, 
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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 http://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; email: stewart.michael@epa.gov.

SUPPLEMENTARY INFORMATION: 

General Information
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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 email. Clearly mark the part or all of the information that you claim to 

be CBI. For CBI information in a disk or CD ROM that you mail to 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.

 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
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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 http://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 

http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_isa.html, and the Risk and Exposure 

Assessment (REA), available at http://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. 
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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
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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 
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IV. Proposed Appendix T--Interpretation of the Primary NAAQS for Oxides of Sulfur 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 
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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 

 

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)
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such pollutant in the ambient air.”2 Section 109(b)(2) This proposal concerns exclusively the 

primary NAAQS for oxides of sulfur.

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 (D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); American 

Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert. denied, 455 U.S. 1034 

(1982). 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 

 

2 EPA is currently conducting a separate review of the secondary SO2 NAAQS jointly with a review of the 

secondary NO2 NAAQS (see http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html for more information).
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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 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 
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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 
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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”, 
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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 (D.C. 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 
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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 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 
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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 
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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 
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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 
20

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 

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 

 

3A 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. 
21

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).

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 
22

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 
23

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 SO2 exposures alone and/or in combination with cooccurring 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 
24

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 
25

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-hour) 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 
26

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 
27

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, 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
28

exposure4. 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. 

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 studies6, approximately 5-

30% percent 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 

 

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.

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) 
29

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.

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 
30

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

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 

 

7 These transcripts can be found in Docket ID No. EPA-HQ-ORD-2006-0260. Available at www.regulations.gov.
31

that are frequently accompanied by respiratory symptoms. Being mindful that the ATS 

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 

exercising asthmatics exposed to SO2 concentrations as low as 200 ppb would have diminished 

reserve lung function and would be at greater risk if affected by another respiratory agent (e.g., 

viral infection). Importantly, diminished reserve lung function in a population that is attributable 

to air pollution is an adverse effect under ATS guidance. In addition to the 2000 ATS 

guidelines, the REA was also mindful of: 1) 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; and 2) subjects 

participating in these controlled human exposure studies not likely including the most severe 

asthmatics. 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.

Importantly, the final REA noted that this conclusion was in agreement with CASAC 

comments following the first draft SO2 REA (REA section 4.3). The first draft SO2 REA 

focused its analyses on exposures and risk associated with 5-minute SO2 concentrations ≥ 400 
32

ppb. However, CASAC strongly advised the Administrator that effects to exercising asthmatics 

at levels as at least as low as 200 ppb can be adverse, and thus, should be considered in the 

second draft and final REAs (Henderson 2008). 

2. Respiratory effects and 1- to 24-hour exposure to SO2

In addition to the controlled human exposure evidence described above, the ISA based its 

causal finding of an association between short-term (5-minutes to 24-hour) exposure to SO2 and 

respiratory morbidity on results from epidemiologic studies of respiratory symptoms, as well as 

ED visits and hospital admissions for all respiratory causes and asthma. More specifically, the 

ISA describes the results from these epidemiologic studies as providing “supporting evidence” 

for its determination of causality (ISA section 5.2). Key epidemiologic studies of respiratory

symptoms, as well as ED visits and hospital admissions are discussed below. 

a. Respiratory symptoms

The ISA found that the strongest epidemiologic evidence of an association between shortterm SO2 concentrations and respiratory symptoms was in children. Studies conducted in North 

America and abroad generally reported positive associations between ambient SO2

concentrations and respiratory symptoms in children. U.S. studies of respiratory symptoms in 

children (identified from Table 5-4 of the ISA), including three large multi-city studies, are 

described briefly below and in more detail in section 3.1.4.1 of the ISA. 

The National Cooperative Inner-City Asthma Study (NCICAS, Mortimer et al. 2002) 

included asthmatic children (n = 846) from eight U.S. urban areas and examined the relationship 

between respiratory symptoms and summertime air pollution levels. The strongest associations 

were found between morning symptoms (e.g. morning cough) and the median 3-hour average 

SO2 concentrations during morning hours (8 a.m. to 11 a.m.)- following a 1- to 2-day lag (ISA, 
33

Figure 3-2). Three hour average concentrations in the morning hours ranged from 17 ppb in 

Detroit to 37 ppb in East Harlem, NY. This relationship remained robust and statistically

significant in multi-pollutant models with ozone (O3), and nitrogen dioxide (NO2). When PM10

was also added to the model, the effect estimate remained relatively unchanged, although was no 

longer statistically significant (ISA, Figure 3-2). However, the ISA noted that the loss of 

statistical significance could have been the result of reduced statistical power since only three of 

the eight cities were included in the multi-pollutant analysis with PM (ISA, section 3.1.4.1).

The Childhood Asthma Management Program (CAMP, Schildcrout et al. 2006) examined 

the association between ambient air pollution and asthma exacerbations in children (n = 990) 

from eight North American cities. The median 24-hour average SO2 concentrations (collected in 

seven of the eight study locations) ranged from 2.2 ppb in San Diego to 7.4 ppb in St. Louis. 

Positive associations with an increased risk of asthma symptoms were observed at all lags, but 

only the association at the 3-day moving average was statistically significant (ISA, Figure 3-3). 

In joint-pollutant models with carbon monoxide (CO) and NO2, the 3-day moving average effect 

estimates remained robust and statistically significant. In a joint-pollutant model with PM10, the 

3-day moving average effect estimate remained relatively unchanged, but was no longer 

statistically significant (ISA Figure 3-3). 

A longitudinal study of schoolchildren (n = 1,844) during the summer months from the 

Harvard Six Cities Study suggested that the association between SO2 and respiratory symptoms 

may potentially be confounded by PM10 (Schwartz et al., 1994). It should be noted that unlike 

the NCICAS and CAMP studies, this study was not limited to asthmatic children. The median 

24-hour average SO2 concentration during this period was 4.1 ppb. SO2 concentrations were 

found to be statistically significantly associated with cough incidence and lower respiratory
34

symptoms in single pollutant models. However, the effect of SO2 was substantially reduced and 

no longer statistically significant after adjustment for PM10 in a co-pollutant model. The ISA 

noted that because PM10 concentrations were correlated strongly to SO2-derived sulfate particles 

(r = 0.80), the reduced SO2 effect estimate may indicate that for PM10 dominated by fine sulfate 

particles, PM10 has a slightly stronger association than SO2 to cough incidence and lower 

respiratory symptoms (ISA, section 3.1.4.1.1).

In addition to the three U.S. multi-city studies mentioned above, evidence of an 

association between ambient SO2 and respiratory symptoms in children was found in two 

additional U.S. respiratory symptom studies. Delfino et al., (2003) reported a statistically

significant positive association between 1-hour daily maximum SO2 concentrations in Los 

Angeles and respiratory symptoms in Hispanic children with asthma (n = 22). Similarly, Neas et 

al., (1995) reported a positive association between 12-hour average SO2 concentrations in 

Uniontown, PA and incidence of evening cough in 4

th and 5th graders (n = 83; ISA section 

3.1.4.1). Neither of these single city studies employed multi-pollutant models, but given the 

consistency of results with other epidemiologic evidence, they nonetheless support the 

association between ambient SO2 concentrations and respiratory symptoms in children

b. Emergency department visits and hospitalizations

Respiratory causes for ED and hospitalization visits typically include asthma, pneumonia, 

Chronic Obstructive Pulmonary Disorder (COPD), upper and lower respiratory infections, as 

well as other minor categories. Since the last review, there have been more than 50 peer 

reviewed epidemiologic studies published worldwide and overall, the ISA concluded that these 

studies provide evidence to support an association between ambient SO2 concentrations and ED 

visits and hospitalizations for all respiratory causes and asthma (ISA, section 3.1.4.6). Notably, 
35

the ISA also found that when analyses of ED visit and hospitalizations for all respiratory causes 

were restricted by age, the results among children (0-14 years) and older adults (65+ years) were

mainly positive, but not always statistically significant (ISA, section 3.1.4.6). In these same 

studies, when all age groups were combined, the ISA found that the results were mainly positive; 

however, the excess risk estimates were generally smaller compared to children and older adults 

(ISA, Figure 3-6). Results from key ED visit and hospital admission studies conducted in the 

U.S. are described in general below, and a more detailed discussion of both the U.S. and 

international literature can be found in the ISA (ISA, section 3.1.4.6).

Of the respiratory ED visit and hospital admission studies reviewed in the ISA, 10 key

studies were conducted in the United States (ISA, Table 5-5). Of these 10 studies, three

evaluated associations with SO2 using multi-pollutant models (Schwartz et al., (1995) in 

Tacoma, WA and New Haven CT; New York Department of Health (NYDOH), (2006) in Bronx 

and Manhattan, NY; and Ito et al., (2007) in New York City), while seven studies evaluated the 

SO2 effect using only single pollutant models (Wilson et al., (2005) in Manchester, NH and 

Portland, ME; Peel et al., (2005) in Atlanta, GA; Tolbert et al., (2007) in Atlanta GA; Jaffe et al., 

(2003) in Cleveland, Cincinnati and Columbus, OH; Schwartz et al., (1996) in Cleveland OH; 

Sheppard et al., (2003) in Seattle, WA; and Lin et al., (2004) in Bronx, NY ). Taken together, 

these studies generally reported positive, but frequently not statistically significant associations 

between ambient SO2 and ED visits and hospital admissions for all respiratory causes and for 

asthma. With regard to U.S. studies employing multi-pollutant models, results reported in 

Bronx, NY (NYDOH 2006) and New York City, NY (Ito et., al 2007) remained robust and 

statistically significant in the presence of PM2.5, [10% (4, 16) and 29.6% (14.3, 46.8), 

respectively] while in New Haven, CT (Schwartz et al., 1995) results remained robust and 
36

statistically significant in the presence of PM10 [2% (1, 3)]. However, in Manhattan, NY 

(NYDOH 2006) results reported from single, and multi-pollutant models were negative

(although not statistically significantly negative), and in Tacoma, WA (Schwartz et al., 1995) the 

SO2 effect estimate [3% (1,6)] was reduced and no longer statistically significant in a multipollutant model with PM10 [-1% (-4, 3)]. In models including gaseous co-pollutants, the SO2

effect estimate in the Bronx, NY (NYDOH 2006) remained statistically significant in the 

presence of NO2 [10% (4,15)], while in NYC (Ito et al., 2007) the SO2 effect estimate remained 

statistically significant in the presence O3 [26.8% (13.7, 41.5)] and CO [31.1% (16.7, 47.2)], but 

not in the presence of NO2 [-1.6% (-16.7, 16.1)]. 

3. ISA conclusions regarding short-term (5-minutes to 24-hour) SO2 exposures

As noted above, the ISA found that moderate or greater decrements in lung function 

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., ≥ 200% increase in sRaw, 

and/or a ≥ 20% decrease in FEV1) (ISA, Table 3-1). Moreover, at SO2 concentrations ≥ 400 ppb

(5 - 10 minute exposures), moderate or greater decrements in lung function were frequently

accompanied by respiratory symptoms. 
37

In addition, the ISA concluded that epidemiologic studies of respiratory symptoms in 

children, as well as emergency department visits and hospitalizations for all respiratory causes 

and asthma 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). 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 provided a 

basis for a progression of respiratory morbidity that could lead to the ED visits and 

hospitalizations observed in epidemiologic studies (ISA, section 5.2). In addition, the ISA 

concluded that U.S. and international epidemiologic studies employing multi-pollutant models 

suggested that SO2 had a generally independent effect on respiratory morbidity outcomes (ISA, 

section 5.2).

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 human exposure studies (ISA, 

section 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 bronchoconstriction and 

respiratory symptoms that are often followed by rapid shallow breathing (ISA, section 5.2). The 

ISA noted that asthmatics are likely more sensitive to the respiratory effects of SO2 due to preexisting inflammation associated with the disease. For example, pre-existing inflammation may

lead to enhanced release of inflammatory mediators, and/or enhanced sensitization of the 
38

chemosensitive receptors (ISA, section 5.2).

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. 

4. 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). 

5. SO2-related impacts on public health

Interindividual variation in human responses to air pollutants indicates that some 

subpopulations 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 general populations and 

sensitive subpopulations, or those subgroups potentially at increased risk for health effects in 

response to ambient air pollution. To facilitate the identification of subpopulations at the 

greatest risk for SO2-related health effects, studies have identified factors that contribute to the 

susceptibility and/or vulnerability of an individual to SO2. Susceptible individuals are broadly

defined as those with a greater likelihood of an adverse outcome given a specific exposure in 

comparison with the general population (American Lung Association, 2001). The susceptibility
39

of an individual to SO2 can encompass a multitude of factors which represent normal 

developmental phases (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, subpopulations may

be vulnerable to SO2 in response to an increase in their exposure during certain windows of life 

(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. 

It should be noted that in some cases specific factors may affect both the susceptibility and 

vulnerability of a subpopulation to SO2. For example, a subpopulation 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, but they may also reside in a location that results in 

exposure to higher concentrations of SO2, increasing their vulnerability to SO2. 

 To examine whether SO2 differentially affects certain subpopulations, 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

subpopulations that are more susceptible and/or vulnerable to SO2. These analyses require the 

proper identification of confounders and their subsequent adjustment 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 and 
40

vulnerability of specific subpopulations to SO2. Those groups identified in the ISA to be 

potentially at greater risk of experiencing an adverse health effect from SO2 exposure are 

described in more detail below. 

a. Pre-existing respiratory disease

In human clinical studies, asthmatics have been shown to be more responsive to the 

respiratory effects of SO2 exposure than healthy non-asthmatics. Although SO2-attributable 

decrements in lung function have generally not been demonstrated at concentrations ≤ 1000 ppb 

in non-asthmatics, statistically significant increases in respiratory symptoms and decreases in 

lung function have consistently been observed in exercising asthmatics following 5 - 10 minute 

SO2 exposures at concentrations ranging from 400-600 ppb (ISA, section 4.2.1.1). Moderate or 

greater SO2-induced decrements in lung function have also consistently been observed at SO2

concentrations ranging from 200-300 ppb in some asthmatics. The ISA also noted that a number 

of epidemiologic studies have reported respiratory morbidity in asthmatics associated with 

ambient SO2 concentrations (ISA 4.2.1.1). For example, numerous epidemiologic studies have 

observed positive associations between ambient SO2 concentrations and ED visits and 

hospitalizations for asthma (ISA section 4.2.1.1). Overall, the ISA concluded that epidemiologic 

and controlled human exposure studies indicated that individuals with pre-existing respiratory

diseases, particularly asthma, are at greater risk than the general population of experiencing SO2-

associated health effects (ISA, section 4.2.1.1).

b. Genetics

The ISA noted that a consensus now exists among scientists that the potential for genetic 

factors to increase the risk of experiencing adverse health effects due to ambient air pollution 

merits serious consideration. Several criteria must be satisfied in selecting and establishing 
41

useful links between polymorphisms in candidate genes and adverse respiratory effects. First, 

the product of the candidate gene must be significantly involved in the pathogenesis of the effect 

of interest, which is often a complex trait with many determinants. Second, polymorphisms in 

the gene must produce a functional change in either the protein product or in the level of 

expression of the protein. Third, in epidemiologic studies, the issue of effect modification by

other genes or environmental exposures must be carefully considered (ISA section 4.2.2). 

Although many studies have examined the association between genetic polymorphisms 

and susceptibility to air pollution in general, only one study has specifically examined the effects 

of SO2 exposure on genetically distinct subpopulations. Winterton et al. (2001) found a 

significant association between SO2-induced decrements in FEV1 and the homozygous wild-type 

allele in the promoter region of Tumor Necrosis Factor-α (TNF- α; AA, position -308). 

However, the ISA concluded that the overall body of evidence was too limited to reach a 

conclusion regarding the effects of SO2 exposure on genetically distinct subpopulations at this 

time.

c. Age

The ISA identified children (i.e., <18 years of age) and older adults (i.e., >65 years of 

age) as groups that are potentially at greater risk of experiencing SO2-associated adverse health 

effects. In children, the developing lung is prone to damage from environmental toxicants as it 

continues to develop through adolescence. The biological basis for increased risk in the elderly 

is unknown, but one hypothesis is that it may be related to changes in antioxidant defenses in the 

fluid lining the respiratory tract. The ISA found a number of epidemiologic studies that observed 

increased respiratory symptoms in children associated with increasing SO2 concentrations. In

addition, several studies have reported that the excess risk estimates for ED visits and 
42

hospitalizations for all respiratory causes, and to a lesser extent asthma, associated with a 10-ppb 

increase in 24-hour average SO2 concentrations were higher for children and older adults than for 

all ages together (ISA, section 4.2.3). However, the ISA also noted that the evidence from 

controlled human exposure studies does not suggest that adolescents are either more or less at 

risk than adults to the respiratory effects of SO2, but rather adolescents may experience similar 

respiratory effects at a given exposure concentration (ISA, sections 3.1.3.5 and 4.2.3).9 Overall, 

the ISA found that compared to the general population, there was limited evidence to suggest

that children and older adults are at greater risk of experiencing SO2-associated health effects 

(ISA, section 4.2.3).

d. Time spent outdoors

Outdoor SO2 concentrations are generally much higher than indoor concentrations. Thus, 

the ISA noted that individuals who spend a significant amount of time outdoors are likely at 

greater risk of experiencing SO2-associated health effects than those who spend most of their 

time indoors (ISA section 4.2.5). 

e. Ventilation rate

Controlled human exposure studies have demonstrated that decrements in lung function 

and respiratory symptoms occur at significantly lower SO2 exposure levels in exercising subjects 

compared to resting subjects. As ventilation rate increases, breathing shifts from nasal to 

oronasal, thus resulting in greater uptake of SO2 in the tracheobronchial airways due to the 

diminished absorption of SO2 in the nasal passages. Therefore, individuals who spend a 

significant amount of time at elevated ventilation rates (e.g. while playing, exercising, or 

working) are expected to be at greater risk of experiencing SO2-associated health effects (ISA 

 

9 Very young children are not included in 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. 
43

section 4.2.5).

f. Socioeconomic status

There is limited evidence that increased risk to SO2 exposure is associated with lower 

SES (ISA section 4.2.5). Finkelstein et al. (2003) found that among people with below-median 

income, the relative risk for above-median exposure to SO2 was 1.18 (95% CI: 1.11, 1.26); the 

corresponding relative risk among subjects with above-median income was 1.03 (95% CI: 0.83, 

1.28). However, the ISA concluded that there was insufficient evidence to reach a conclusion 

regarding SES and exposure to SO2 at this time (ISA section 4.2.5).

g. Size of at-risk populations

Considering the size of the groups mentioned above, large proportions of the U.S. 

population are likely to have a relatively high 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. 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 
44

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, discussed below. These 

assessments provide estimates of the likelihood that asthmatics at moderate or greater exertion 

(e.g. while exercising) would experience SO2 exposures of potential concern as well as an 

estimate of the number and percent of exposed asthmatic individuals 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 (e.g., just meeting the current or 

alternative standards). These assessments also characterize the kind and degree of uncertainties 

inherent in such estimates. 

This section describes the approach taken in the REA to characterize SO2-related 

exposures and health risks. Goals of the REA included estimating short-term exposures and 

potential human health risks associated with (1) recent levels of ambient SO2; 2) SO2 levels 

adjusted to simulate just meeting the current standards; and 3) SO2 levels adjusted to simulate

just meeting potential alternative 1-hour standards. This section discusses the scientific evidence 

from the ISA that was used as the basis for the risk characterization (II.C.1), the approaches used 

in characterizing exposures and risks (II.C.2), and important uncertainties associated with these 

analyses (II.C.3). The results of the exposure and risk analyses, as they relate to the current and 

potential alternative standards, are discussed in subsequent sections of this proposal (sections 
45

II.E and II.F, respectively). 

1. Evidence base for the risk characterization

As previously mentioned, the ISA concluded that the evidence for an association between 

respiratory morbidity and 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 REA 

concluded that quantitative exposure and risk analyses should focus on 5-minute levels of SO2 in 

excess of potential health effect benchmark values derived from the controlled human exposure 

literature (REA, section 6.2). These benchmark levels are not potential standards, but rather are 

concentrations which represent “exposures of potential concern” which are used in the analyses 

to estimate potential exposures and risks associated with 5-minute concentrations of SO2. In

addition, although the REA concluded that the epidemiologic evidence was not appropriate for 

use in quantitative risk analyses (REA, section 6.3), these studies were considered in the 

selection of potential alternative standards for use in the air quality, exposure and risk analyses

(REA, chapter 5), as well as in the REA’s assessment of the adequacy of the current and 

potential alternative primary standards (REA, sections 10.3; 10.4; and 10.5). 

As mentioned above, the health effect benchmark values used in the REA were derived 

primarily from the ISA’s evaluation of the 5 - 10 minute controlled human exposure literature. 

The ISA concluded that moderate or greater decrements in lung function occurred in 

approximately 5 - 30% of exercising asthmatics following exposure to 200 - 300 ppb SO2 for 5 -

10 minutes. As explained in section II.B.1.b, the ISA concluded that moderate or greater 

decrements in lung function occurred in approximately 20 - 60% of exercising asthmatics 
46

following exposure to 400 - 600 ppb SO2 for 5 - 10 minutes. The ISA also concluded that at SO2

concentrations ≥ 400 ppb, statistically significant moderate or greater decrements in lung 

function at the group mean level have often been reported and are frequently accompanied by 

respiratory symptoms (ISA, section 3.1.3.5). 

In addition to the health evidence from the ISA presented above, when considering 

potential health effect benchmark levels, the REA noted: (1) subjects participating in human 

exposure studies typically do not include individuals who may be most susceptible to the 

respiratory effects of SO2, (e.g., the most severe asthmatics given the obvious ethical issues of 

subjecting such persons to the clinical tests) and (2) given that approximately 5 - 30% of 

exercising asthmatics experienced moderate or greater decrements in lung function following 

exposure to 200 - 300 ppb SO2 (the lowest levels tested in free-breathing chamber studies), it is

likely that a percentage of exercising asthmatics would also experience similar decrements in 

lung function following exposure to levels lower than 200 ppb (REA, section 6.2). That is, the 

REA concluded that there was no evidence to suggest that 200 ppb represented a threshold level 

below which no adverse respiratory effects would occur (REA, section 6.2). Moreover, the REA 

considered that small SO2-induced lung function decrements have been observed in exercising 

asthmatics at concentrations as low as 100 ppb when SO2 is administered via mouthpiece (ISA, 

section 3.1.3).

Taken together, the REA concluded it appropriate to examine potential 5-minute 

benchmark values in the range of 100 - 400 ppb (REA, section 6.2). The lower end of the range 

considered the factors mentioned above, while the upper end of the range recognized that 400 

ppb represents the lowest concentration at which moderate or greater decrements in lung

function are frequently accompanied by respiratory symptoms (REA, section 6.2): a combination 
47

of effects which would clearly be considered adverse under ATS guidelines (ATS, 1985). 

Although the analysis of exposures of potential concern were conducted using discrete

benchmark levels (i.e., 100, 200, 300, 400 ppb), EPA recognizes that there is no sharp breakpoint 

within the continuum ranging from at and above 400 ppb down to 100 ppb. In considering the 

concept of exposures of potential concern, it is important to balance concerns about the potential 

for health effects and their severity with the increasing uncertainty associated with our 

understanding of the likelihood of such effects at lower SO2 levels. Within the context of this 

continuum, estimates of exposures of potential concern at discrete benchmark levels provide 

some perspective on the potential public health impacts of SO2-related health effects that have 

been demonstrated in controlled human exposure studies. They also help in understanding the 

extent to which such impacts could change by just meeting the current and potential alternative 

standards. However, estimates of the number of asthmatics likely to experience exposures of 

potential concern cannot be translated directly into quantitative estimates of the number of 

people likely to experience specific health effects. Due to individual variability in 

responsiveness, only a subset of asthmatics exposed at and above a specific benchmark level can 

be expected to experience health effects. The amount of weight to place on the estimates of

exposures of potential concern at any of these benchmark levels depends in part on the weight of 

the scientific evidence concerning health effects associated with SO2 exposures at and above that

benchmark level. Such public health policy judgments are embodied in the NAAQS standard 

setting criteria (i.e., standards that, in the judgment of the Administrator, are requisite to protect

public health with an adequate margin of safety).

Since exposures of potential concern cannot be directly translated into quantitative 

estimates of the number of individuals likely to experience specific health effects, the REA not 
48

only characterizes exposure and risks utilizing exposures of potential concern, but also uses

information from the controlled human exposure literature to conduct a quantitative risk 

assessment. The quantitative risk assessment estimated the number and percentage of exposed 

asthmatics at moderate or greater exertion expected to experience a moderate or greater lung 

function response (in terms of a ≥ 100% increase in sRaw and/or a ≥ 15% decline in FEV1; see 

section II.C.2).

2. Overview of approaches

As noted above, the purpose of the assessments described in the REA was to characterize 

air quality, exposures, and health risks associated with recent ambient levels of SO2, with SO2

levels that could be associated with just meeting the current SO2 NAAQS, and with SO2 levels 

that could be associated with just meeting potential alternative standards. The REA utilizes three 

approaches to characterize health risks In the first approach, for each air quality scenario, 

statistically estimated10 and measured ambient 5-minute SO2 concentrations were compared to

the 5-minute potential health effect benchmark levels discussed above which (as noted) were 

derived from the controlled human exposure literature (REA, chapter 7). In the second 

approach, modeled estimates of 5-minute exposures in asthmatics at moderate or greater exertion 

(e.g. while exercising) were compared to these 5-minute potential health effect benchmark 

levels. In the third approach, exposure-response relationships from individual level data from

controlled human exposure studies were used in conjunction with the outputs of the exposure 

analysis to estimate health impacts under the air quality scenarios mentioned above. A brief 

 

10 Benchmark values derived from the controlled human exposure literature were associated with a 5-minute 

averaging time. However, only 98 ambient monitors located in 13 states from 1997-2007 reported measured 5-

minute SO2 concentrations since such monitoring is not required (see section III). In contrast, 809 monitors in 48 

states, DC, Puerto Rico, and the Virgin Islands reported 1-hour SO2 concentrations over a similar time period. 

Therefore, to broaden analyses to areas where measured 5-minute SO2 concentrations were not available, the REA 

utilized a statistical relationship to estimate the highest 5-minute level in an hour, given a reported 1-hour average 

SO2 concentration (REA, section 6.4). Then, similar to measured 5-minute SO2 levels, statistically estimated 5-

minute SO2 concentrations were compared to 5-minute potential health effect benchmark values.
49

description of these approaches is provided below and each approach is described in detail in 

chapters 7 through 9 of the REA. 

In the first approach, statistically estimated and actual measured 5-minute ambient SO2

concentrations were compared to 5-minute potential health effect benchmark levels(REA, 

chapter 7). The results generated from the air quality analysis were 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 uncertainty associated with the assumption that SO2 air quality can serve as an 

adequate surrogate for total exposure to ambient SO2. Actual exposures might be influenced by 

factors not considered by this approach, including small scale spatial variability in ambient SO2

concentrations (which might not be captured by the network of fixed-site ambient monitors) and 

spatial/temporal variability in human activity patterns. 

In the second approach, an inhalation exposure model was used to generate more realistic 

estimates of personal exposures in asthmatics (REA, chapter 8). This analysis estimated

temporally and spatially variable ambient 5-minute SO2 concentrations and simulated asthmatics 

contact with these pollutant concentrations while at moderate or greater exertion (i.e., while at 

elevated ventilation rates). The approach was designed to estimate exposures that are not 

necessarily represented by the existing ambient monitoring data. AERMOD, an EPA dispersion 

model, was used to estimate 1-hour ambient SO2 concentrations using emissions estimates from 

stationary, non-point, and port sources. The Air Pollutants Exposure (APEX) model, an EPA 

human exposure model, was then used to estimate population exposures using the estimated 

hourly census block level SO2 concentrations. From these 1-hour census block concentrations, 

5-minute maximum SO2 concentrations within each hour were estimated using the statistical 
50

relationship mentioned above. A probabilistic approach was then used to model asthmatics’

exposures considering: 1) time spent in different microenvironments; 2) time spent at moderate 

or greater exertion; and 3) the variable SO2 concentrations that occur within these 

microenvironments across time, space, and microenvironment type. Estimates of personal 

exposure to 5-minute SO2 levels were then compared to the 5-minute potential health benchmark 

levels (i.e., 5-minute benchmark levels of 100, 200, 300, and 400 ppb). This approach to 

assessing exposures was more resource intensive than using ambient levels as an indicator of

exposure; therefore, the final REA included the analysis of two locations: St Louis and Greene 

County, MO. Although the geographic scope of this analysis was limited, the approach provided

estimates of SO2 exposures in asthmatics and asthmatic children in St. Louis and Greene 

Counties and thus, served to complement the broader air quality characterization. 

For the characterization of risks in both the air quality analysis and the exposure 

modeling analysis described above, the REA used a range of 5-minute potential health effect 

benchmarks: 100, 200, 300, and 400 ppb. These benchmark values were compared to both SO2

air quality levels and to estimates of SO2 exposure in asthmatics. When SO2 air quality was used 

as an indicator of exposure, a key output of the analysis was an estimate of the number of days 

per year specific locations experienced statistically estimated 5-minute daily maximum levels of 

SO2 that exceeded one of these 5-minute potential health effect benchmarks. When personal 

exposures were simulated, the output of the analysis was an estimate of the number and percent 

of asthmatics and asthmatic children at risk for experiencing, at least once per year, a statistically 

estimated 5-minute daily maximum level of SO2 of ambient origin in excess of one of these

benchmarks. An advantage of using the benchmark approach to characterize health risks is that 

the effects observed in the controlled human exposure studies clearly result from SO2 exposure, 
51

so the benchmarks are reliable levels at which effects to asthmatics from exposure to SO2 can 

occur. A limitation of this approach is that the magnitude of the SO2 effect on decrements in 

lung function and respiratory symptoms can vary considerably from individual to individual and 

thus, not all asthmatics would be expected to respond to the same levels of SO2 exposure. 

Therefore, numbers of exposures can be quantified more readily than the number of individuals 

experiencing SO2-induced lung function decrements and/or respiratory symptoms.

The third approach was a quantitative risk assessment. This approach combined results

from the exposure analysis (i.e., the number of exposed total asthmatics or asthmatic children 

while at moderate or greater exertion) with exposure-response functions derived from individual 

level data from controlled human exposure studies (see ISA, Table 3-1 and Johns (2009)11) to 

estimate the percentage and number of exposed asthmatics and asthmatic children likely to 

experience a moderate or greater lung function response (i.e., decrements in lung function 

defined in terms of FEV1 and sRaw) under the air quality scenarios mentioned above (REA, 

chapter 9). The advantage of this approach is that it recognizes that not all exposed asthmatics at 

moderate or greater exertion will have a lung function response. Moreover, it is advantageous in 

that rather than considering discrete potential health effect benchmark levels, it quantitatively

estimates the number and percent of asthmatics and asthmatic children likely to experience a 

moderate or greater lung function response considering the entire distribution of personal 

exposures.

3. Key limitations and uncertainties

The way in which air quality, exposure, and risk results will inform ultimate decisions 

regarding the current and potential alternative SO2 standards will depend upon the weight placed 

 

11 EPA recently conducted a complete quality assurance review of all individual subject data. 

The results of this review did not substantively change any of the entries in ISA, Table 3-1, and did not in anyway 

affect the conclusions of the ISA (see Johns and Simmons, 2009).
52

on each of the analyses when uncertainties associated with those analyses are taken into 

consideration. Sources of uncertainty associated with each of the analyses (air quality, exposure, 

and quantitative risk) are briefly presented below and are described in more detail in chapters 7-9 

of the REA. 

In the air quality analysis, the REA used ambient SO2 data from both the limited number 

of monitors reporting 5-minute concentrations and the broader network of monitors reporting 1-

hour concentrations of SO2 to characterize U.S. air quality. There was general agreement in the 

monitor site attributes and emissions sources potentially influencing ambient monitoring 

concentrations for each set of data analyzed. However, the REA noted that the greatest relative 

uncertainty was in the spatial representativeness of both the overall monitoring network and the 

subsets of monitors chosen for detailed analyses (REA, section 7.4.2.4).

An additional source of uncertainty in the air quality analysis is associated with the 

statistical model used to estimate 5-minute maximum SO2 concentrations at monitors that 

reported only 1-hour SO2 concentrations (REA, section 7.4.2.6). Cross-validation of statistically

estimated 5-minute concentrations with the limited number of reported 5-minute SO2

measurements indicated that the greatest difference in the predicted versus observed numbers of 

benchmark exceedances occurred at the lower and upper tails of the distribution. However, the

REA noted that overall, the results of the cross-validation analysis indicated reasonable model 

performance (REA, sections 10.3.3.1 and 10.5.2).

The air quality characterization assumes that the ambient monitoring data and the 

estimated days per year with exceedances of the specified benchmark levels can serve as an 

indicator of exposure. Longer-term personal SO2 exposure (i.e., days to weeks) concentrations 

are correlated with and are a fraction of ambient SO2 concentrations. However, uncertainty 
53

remains in this relationship when considering short-term (i.e., 5-minute) averaging times because 

of the lack of comparable measurement data (REA, section 7.4.2.7).

The St. Louis and Greene county exposure assessments were also associated with a

number of key uncertainties that should be considered when interpreting the results with regard 

to decisions on the standard. Such uncertainties are highlighted below, and these, as well as 

other sources of uncertainty, are also discussed in greater depth in section 8.11 of the REA.

In the exposure analyses, it was necessary to derive an area source emission profile rather 

than use a default profile to improve the agreement between ambient measurements and model 

predicted 1-hour SO2 concentrations. The improved model performance reduces uncertainty in 

the 1-hour SO2 concentrations predictions, but nonetheless remains as an important uncertainty

in the absence of actual local source emission profiles (REA, section 8.11.2).

The St. Louis and Greene county exposure assessments were performed to better reflect 

both the temporal and spatial representation of ambient concentrations and to estimate the rate of 

contact of asthmatic individuals with 5-minute SO2 concentrations while engaged in moderate or 

greater exertion. Estimated annual average SO2 exposures in the two exposure modeling 

domains are consistent with long-term personal exposures (i.e., days to weeks) measured in other 

U.S. locations (REA, chapter 8). However, uncertainty remains in the estimated number of 

persons with 5-minute SO2 concentrations above benchmark levels because of the lack of 

comparable measurement data, particularly considering both the short-term averaging time and 

geographic location (REA, section 8.11.2).

In addition, although all 5-minute ambient SO2 concentrations in the exposure analyses 

were estimated by the exposure model, each hour was comprised of the maximum 5-minute SO2

concentration and eleven other 5-minute SO2 concentrations normalized to the 1-hour mean 
54

concentration. The REA assumed that this approach would reasonably estimate the number of 

individuals exposed to peak concentrations. Sensitivity analyses revealed that both the number 

of persons exposed and where peak exposures occur can vary when considering an actual 5-

minute temporal profile (REA, Section 8.11.2)

A number of key uncertainties should also be considered when interpreting the results of 

the St. Louis and Greene County risk assessment with regard to decisions on the standard. Such 

uncertainties associated with the St Louis and Greene County risk assessment are discussed 

briefly below and in greater depth in section 9.4 of the REA. 

In the quantitative risk assessment, it was necessary to estimate responses at SO2 levels 

below the lowest exposure levels used in the free-breathing controlled human exposure studies 

(i.e., below 200 ppb). Probabilistic exposure-response relationships were derived in the REA 

using two different functional forms (i.e., probit and 2-parameter logistic), but nonetheless there 

remains greater uncertainty in responses below 200 ppb because of the lack of comparable 

experimental data. Moreover, because the controlled human exposure studies used in the risk 

assessment involved only SO2 exposures, it was assumed in the REA that estimates of SO2-

induced health responses are not affected by the presence of other pollutants (e.g., PM2.5, O3, 

NO2; REA, section 9.4).

The risk assessment assumes that the SO2-induced responses for individuals are 

reproducible. The REA noted that this assumption had some support in that one study (Linn et 

al., 1987) exposed the same subjects on two occasions to 600 ppb and the authors reported a high 

degree of correlation while observing a much lower correlation for the lung function response 

observed in the clean air with exercise exposure (REA, section 9.4).

Because the vast majority of controlled human exposure studies investigating lung 
55

function responses were conducted with adult subjects, the risk assessment relies on data from 

adult asthmatic subjects to estimate exposure-response relationships that have been applied to all 

asthmatic individuals, including children. The ISA (section 3.1.3.5) indicates that there is a 

strong body of evidence that suggests adolescents may experience many of the same respiratory

effects at similar SO2 levels, but recognizes that these studies administered SO2 via inhalation 

through a mouthpiece (which can result in an increase in lung SO2 uptake) rather than in an 

exposure chamber. Therefore, the uncertainty is greater in the risk estimates for asthmatic 

children (REA, section 9.4)12. 

D. Considerations in review of the standards

This section presents the integrative synthesis of the evidence and information contained 

in the ISA and the REA with regard to the current and potential alternative standards. EPA notes 

that the final decision on retaining or revising the current primary SO2 standards is a public 

health policy judgment to be made by the Administrator. The Administrator’s final decision will 

draw upon scientific information and analyses related to health effects, population exposures, 

and risks; as well as judgments about the appropriate response to the range of uncertainties that 

are inherent in the scientific evidence and analyses; and comments received in response to this 

proposal. 

1. Background on the current standards

There are currently two SO2 primary standards. The 24-hour average standard is 0.14 

ppm not to be exceeded more than once per year and the annual average standard is 0.03 ppm. 

In the last review of the SO2 NAAQS, both the 24-hour and annual standards were retained. The 

rationale for the retention of these standards is discussed briefly below.

 

12 Very young children were not included in the controlled human exposure data which served as the basis for the 

exposure-response relationships used in the risk assessment. This absence of data on what is likely to be a sensitive 

life stage is an additional source of uncertainty in the risk assessment. 
56

In the last review, retention of the 24-hour standard was based largely on epidemiologic 

studies conducted in London in the 1950’s and 1960’s. The results of those studies suggested an 

association between 24-hour average levels of SO2 and increased daily mortality and aggravation 

of bronchitis when in the presence of elevated levels of PM (53 FR 14927). Additional 

epidemiologic evidence suggested that elevated SO2 levels were associated with the possibility

of small, reversible declines in children’s lung function (53 FR 14927). 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. It was also noted that rather than 24-

hour 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 

14927). 

Retention of the annual standard in the last review was largely based on an assessment of 

qualitative evidence gathered from a limited number of epidemiologic studies. The strongest 

evidence for an association between annual SO2 concentrations and adverse health effects in the 

1982 AQCD was from a study conducted by Lunn et al (1967). The authors found that among 

children, a likely association existed between chronic upper and lower respiratory tract illnesses 

and annual SO2 levels of 70 -100 ppb in the presence of 230-301 μg/m3black smoke. Three 

additional studies described in the 1986 Second Addendum also suggested that long-term 

exposure to SO2 was associated with adverse respiratory effects. Notably, studies conducted by 

Chapman et al. (1985) and Dodge et al. (1985) found associations between long-term SO2

concentrations (with or without high particle concentrations) and cough in children and young 

adults. However, it was noted that there was considerable uncertainty associated with these 
57

studies because they were conducted in locations subject to high, short-term peak SO2

concentrations (i.e., locations near point sources); therefore it was difficult to discern whether 

this increase in cough was the result of long-term, low level SO2 exposure, or repeated shortterm peak SO2 exposures. 

It was concluded in the last review that there was no quantitative rationale to support a 

specific range for an annual standard (EPA, 1994b). However, it was also found that although

no single epidemiologic study provided clear quantitative conclusions, there appeared to be some 

consistency across studies indicating the possibility of respiratory effects associated with longterm exposure to SO2 just above the level of the existing annual standard (EPA, 1994b). In 

addition, air quality analyses conducted during the last review indicated that the short-term 

standards being considered (1-hour and/or 24-hour) could not by themselves prevent long-term 

concentrations of SO2 from exceeding the level of the existing annual standard in several large 

urban areas. Ultimately, both the scientific evidence and the air quality analyses were used by 

the Administrator to conclude that retaining the existing annual standard was requisite to protect 

human health. 13

2. Approach for reviewing the need to retain or revise the current standards

The decision in the present review on whether the current 24-hour and/or annual 

standards are requisite to protect public health with an adequate margin of safety will be 

informed by a number of scientific studies and analyses that were not available in the 1996 

review. Specifically, as discussed above (section II.B), a large number of epidemiologic studies 

have been published since the 1996 review. Many of these studies evaluated associations 

between SO2 and adverse respiratory endpoints (e.g., respiratory symptoms, emergency

 

13 Section I.C above discusses potential standards considered but not adopted in the last review, notably some type 

of standard to deal with effects of 5 to 10 minute exposures.
58

department visits, hospital admissions) in locations where 24-hour and annual average SO2

concentrations were below the levels allowed by the current standards. In addition, with respect 

to adverse health effects associated with 5-minute SO2 concentrations, the REA described 

estimates of SO2-associated health risks that could be present in counties that just meet the 

current 24-hour or annual standards, whichever was controlling in a given county.

14

 The 

approach for considering this scientific evidence and exposure/risk information is discussed 

below. 

To evaluate whether the current primary SO2 standards are adequate or whether 

consideration of revisions is appropriate, EPA is using an approach in this review described in 

chapter 10 of the REA which builds upon the approaches used in reviews of other criteria 

pollutants, including the most recent reviews of the NO2, Pb, O3, and PM NAAQS (EPA, 2008c; 

EPA, 2007c; EPA, 2007d; EPA, 2005), and reflects the body of evidence and information that is 

currently available. As in other recent reviews, EPA's considerations will include the 

implications of placing more or less weight or emphasis on different aspects of the scientific 

evidence and the exposure/risk-based information, recognizing that the weight to be given to 

various elements of the evidence and exposure/risk information is part of the public health policy

judgments that the Administrator will make in reaching decisions on the standard. 

A series of general questions frames this approach to considering the scientific evidence 

and exposure-/risk-based information. First, EPA’s consideration of the scientific evidence and 

exposure/risk information with regard to the adequacy of the current standards is framed by the 

 

14 As noted in the REA, the controlling standard by definition would be the standard that allows air quality to just 

meet either the annual concentration level of 30.4 ppb (i.e., the annual standard is the controlling standard) or the 2nd

highest 24-hour concentration level of 144 ppb (i.e., the 24-hour standard is the controlling standard). The factor 

selected is derived from a single monitor within each county (even if there is more than one monitor in the county) 

for a given year. A different (or the same) monitor in each county could be used to derive the factor for other years; 

the only requirement for selection is that it be the lowest factor, whether derived from the annual or 24-hour 

standard level. "
59

following questions: 

 To what extent does evidence that has become available since the last review reinforce or 

call into question evidence for SO2-associated effects that were identified in the last 

review?

 To what extent has evidence for different health effects and/or sensitive populations 

become available since the last review?

 To what extent have uncertainties identified in the last review been reduced and/or have 

new uncertainties emerged?

 To what extent does evidence and exposure-/risk-based information that has become 

available since the last review reinforce or call into question any of the basic elements of 

the current standard?

To the extent that the available evidence and exposure-/risk-based information suggests it 

may be appropriate to consider revision of the current standards, EPA considers that evidence 

and information with regard to its support for consideration of a standard that is either more or 

less stringent than the current standards. This evaluation is framed by the following questions: 

 Is there evidence that associations, especially causal or likely causal associations, extend 

to ambient SO2 concentrations as low as, or lower than, the concentrations that have 

previously been associated with health effects? If so, what are the important uncertainties 

associated with that evidence?

 Are exposures above benchmark levels and/or health risks estimated to occur in areas that 

meet the current standard? If so, are the estimated exposures and health risks important 

from a public health perspective? What are the important uncertainties associated with 

the estimated risks?

To the extent that there is support for consideration of a revised standard, EPA then 

considers the specific elements of the standard (indicator, averaging time, form, and level) within 

the context of the currently available information. In so doing, the Agency addresses the 

following questions regarding the elements of the standard: 

 Does the evidence provide support for considering a different indicator for gaseous SOx?
60

 Does the evidence provide support for considering different, or additional averaging 

times?

 What ranges of levels and forms of alternative standards are supported by the evidence, 

and what are the associated uncertainties and limitations?

 To what extent do specific averaging times, levels, and forms of alternative standards 

reduce the estimated exposures above benchmark levels and risks attributable to exposure 

to ambient SO2, and what are the uncertainties associated with the estimated exposure 

and risk reductions?

The questions outlined above have been addressed in the REA. The following sections 

present considerations regarding the adequacy of the current standards and potential alternative 

standards, as discussed in chapter 10 of the REA, in terms of indicator, averaging time, form, and 

level. 

E. Adequacy of the current standards

In considering the adequacy of the current standards, the policy assessment chapter of the 

REA considered the scientific evidence assessed in the ISA, as well as the air quality, exposure, 

and risk-based information presented in the REA. A summary of this evidence and information 

as well as CASAC recommendations and the Administrator’s conclusions regarding the 

adequacy of the current standards are presented below. Section II.E.1 will discuss the adequacy

of the current 24-hour standard and Section II.E.2 will then discuss adequacy of the current 

annual standard. Section II.E.3 will discuss CASAC views and finally, section II.E.4 discusses 

the Administrator’s conclusions regarding the adequacy of the current 24-hour and annual 

standards.
61

1. Adequacy of the current 24-hour standard

a. Evidence-based considerations 

In considering the SO2 epidemiologic studies as they relate to the adequacy of the 

current 24-hour standard, the REA noted that 24-hour average SO2 concentrations were below 

the current 24-hour average SO2 NAAQS in many locations where positive and sometimes 

statistically significant associations were observed (REA, section 10.3). As discussed previously 

(see section II.B.3), the ISA characterized the epidemiologic evidence for respiratory effects as 

being consistent and coherent (ISA, section 5.2). The evidence is consistent in that positive 

associations are reported in studies conducted in numerous locations and with a variety of 

methodological approaches (ISA, section 5.2). It is coherent in the sense that respiratory

symptom results from epidemiologic studies predominantly using 1-hour daily maximum or 24-

hour average SO2 concentrations are generally in agreement with the respiratory symptom results 

from controlled human exposure studies of 5-10 minutes. These results are also coherent in that 

the respiratory effects observed in controlled human exposure studies of 5-10 minutes provide a 

basis for a progression of respiratory morbidity that could lead to the ED visits and 

hospitalizations observed in epidemiologic studies (ISA, section 5.2). The ISA also noted that 

when the epidemiologic literature is considered as a whole, there are generally positive 

associations between SO2 and respiratory symptoms in children, hospital admissions, and 

emergency department visits. Moreover, some of these associations were statistically significant, 

particularly the more precise effect estimates (ISA, section 5.2). 

The interpretation of these SO2 epidemiologic studies is complicated by the fact that SO2

is but one component of a complex mixture of pollutants present in the ambient air. In order to 

provide some perspective on this uncertainty, the ISA evaluates epidemiologic studies that 
62

employ multi-pollutant models. Specifically, the ISA noted that a number of SO2 epidemiologic 

studies have attempted to disentangle the effects of SO2 from those of co-occurring pollutants by

utilizing multi-pollutant models. When evaluated as a whole, SO2 effect estimates in these 

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 5.2; p. 5-9).

In drawing broad conclusions regarding the evidence, the ISA considered the 

epidemiologic and experimental evidence as well as the uncertainties associated with that 

evidence. When this evidence and its associated uncertainties were taken together, the ISA 

concluded that the results of epidemiologic and experimental studies form a plausible and 

coherent data set that supports a relationship between SO2 exposures and respiratory endpoints, 

including respiratory symptoms and ED visits, at ambient concentrations that are present in areas 

that meet the current 24-hour SO2 NAAQS (ISA, section 5.5). Thus, taking into consideration

the evidence discussed above, particularly the epidemiologic studies reporting SO2-associated 

health effects in locations that meet the current 24-hour standard, the REA concluded that the 

epidemiologic evidence calls into question the adequacy of the current 24-hour standard to 

protect public health (REA, section 10.3.4). 

b. Air quality, exposure, and risk-based considerations 

As previously mentioned, the ISA found the evidence for an association between 

respiratory morbidity and SO2 exposure to be “sufficient to infer a causal relationship” (ISA, 
63

section 5.2) and that the “definitive evidence” for this conclusion comes from the results of 

controlled human exposure studies demonstrating decrements in lung function and/or respiratory

symptoms in exercising asthmatics (ISA, section 5.2). Accordingly, the exposure and risk 

analyses presented in the REA focused on exposures and risks associated with 5-minute peaks of 

SO2 in excess of the potential health effect benchmark values of 100, 200, 300, and 400 ppb SO2. 

In considering the results presented in these analyses, the REA particularly noted exceedances or 

exposures with respect to the 200 and 400 ppb 5-minute benchmark levels. These benchmark 

levels were highlighted in the REA because (1) 400 ppb represents the lowest concentration in 

controlled human exposure studies where moderate or greater lung function decrements which 

were often statistically significant at the group mean level, 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 human exposure studies have been observed

(notably, 200 ppb is also the lowest level that has been tested). The REA also recognized that 

there was very limited evidence demonstrating small decrements in lung function at 100 ppb 

from two mouthpiece exposure studies. However, as previously noted (see section II.B.1.b), the 

results of these studies are not directly comparable to free-breathing chamber studies, and thus, 

the REA primarily considered exceedences of the 200 ppb and 400 ppb benchmark levels in its 

evaluation of the adequacy of the current 24-hour (as well as the annual; see section II.E.2 )

standard. 

A key output of the air quality analysis was the predicted number of statistically

estimated 5-minute daily maximum SO2 concentrations above benchmark levels given air quality 

simulated to just meet the level of the current 24-hour or annual SO2 standard, whichever was 

controlling for a given county. Under this scenario, in 40 counties selected for detailed analysis, 
64

the REA found that the predicted yearly mean number of statistically estimated 5-minute daily

maximum concentrations > 400 ppb ranges from 1-102 days per year15, with most counties in 

this analysis experiencing a mean of at least 20 days per year when statistically estimated 5-

minute daily SO2 concentrations exceed 400 ppb (REA, Table 7-14). In addition, the predicted 

yearly mean number of statistically estimated 5-minute daily maximum concentrations >200 ppb 

ranged from 21-171 days per year, with about half of the counties in this analysis experiencing ≥ 

70 days per year when 5-minute daily maximum SO2 concentrations exceed 200 ppb (REA, 

Table 7-12).

The REA also generated exposure and risk estimates for two study areas in Missouri (i.e., 

Greene County and several counties representing the St. Louis urban area) which had significant 

emission sources of SO2. As noted in REA section 8.10, there were differences in the number of 

exposures above benchmark values when the results of the Greene County and St. Louis 

exposure assessments were compared. In addition, given that the results of the exposure 

assessment were used as inputs into the quantitative risk assessment, it was not surprising that 

there were also differences in the number of asthmatics at elevated ventilation rates estimated to 

have a moderate or greater lung function response in Greene County when compared to St. 

Louis. The REA noted that the differences in the St. Louis and Greene County exposure and 

quantitative risk results are likely indicative of the different types of locations they represent (see 

section 8.10). Greene County is a rural county with much lower population and emission 

densities, compared to the St. Louis study area which has population and emissions density

similar to other urban areas in the U.S. It therefore follows that there would be greater 

exposures, and hence greater numbers and percentages of asthmatics at elevated ventilation rates 

 

15 Air quality estimates presented in this section represent the mean number of days per year when 5-minute daily 

maximum SO2 concentrations exceed a particular benchmark level given 2001-2006 air quality adjusted to just meet 

the current standards (see REA, Tables 7-11 to 7-14).
65

experiencing moderate or greater lung function responses in the St. Louis study area. Thus, 

when considering the risk and exposure results as they relate to the adequacy of the current 

standards, the REA concluded that the St. Louis results were more informative in terms of 

ascertaining the extent to which the current standards protect against effects linked to the various 

benchmarks (linked in turn to 5-minute exposures). The results in fact suggested that the current 

standards may not adequately protect public health (REA, section 10.3.3). Moreover, the REA 

judged that the exposure and risk estimates for the St. Louis study area provided useful insights 

into exposures and risks for other urban areas in the U.S. with similar population and SO2

emissions densities (REA, section 10.3.3). 

When considering the St. Louis exposure results as they relate to the adequacy of the 

current standards, results discussed in the policy chapter of the REA included the percent of 

asthmatic children at moderate or greater exertion estimated to experience at least one 

exceedance of either the 200 or 400 ppb benchmark given air quality that was adjusted upward to 

simulate just meeting the current 24-hour standard (i.e., the controlling standard in St. Louis).

16

 

Given this scenario, the REA found that approximately 24% of asthmatic children in that city

would be estimated to experience at least one SO2 exposure concentration greater than or equal 

to the 400 ppb benchmark level per year while at moderate or greater exertion (e.g., while 

exercising; REA, Figure 8-19). Similarly, the REA found that approximately 73% of asthmatic 

children would be expected to experience at least one SO2 exposure greater than or equal to a 

200 ppb benchmark level while at moderate or greater exertion (REA, Figure 8-19). 

When considering the St. Louis risk results as they relate to the adequacy of the current 

24-hour standard, the policy assessment chapter of the REA included the percent of asthmatic 

 

16 Exposure and risk results presented in this notice are with respect to asthmatic children, results for all asthmatics 

are presented in REA chapters, 8, 9, and 10.
66

children at elevated ventilation rates likely to experience at least one lung function response 

given air quality that is adjusted upward to simulate just meeting the current standards. Under 

this scenario, 19.1% to 19.2% of exposed asthmatic children at elevated ventilation rates were

estimated to experience at least one moderate lung function response per year (defined as an 

increase in sRaw ≥ 100% (REA, Table 9-8)).

18,19

 Furthermore, 7.9% to 8.1% of exposed 

asthmatic children at moderate or greater exertion were estimated to experience at least one large 

lung function response per year (defined as an increase in sRaw ≥ 200% (REA, Table 9-8)). 

c. Summary of considerations from the REA regarding the 24-hour standard

As noted above, the policy chapter of the REA considered several lines of scientific 

evidence when evaluating the adequacy of the current 24-hour standard to protect the public 

health. These included causality judgments made in the ISA, as well as the human exposure and 

epidemiologic evidence supporting those judgments. In particular, the REA concluded that 

numerous epidemiologic studies reporting positive associations between ambient SO2 and 

respiratory morbidity endpoints were conducted in locations that met, or were below the current 

24-hour standard (REA, section 10.3.4). The REA concluded that to the extent that these 

considerations are emphasized, the adequacy of the current 24-hour standard to protect the public 

health would clearly be called into question (REA, section 10.3.4). The REA found this 

suggested consideration of a revised 24-hour standard and/or that an additional shorter-averaging 

time standard may be needed to provide additional health protection for sensitive groups, 

including asthmatics and individuals who spend time outdoors at elevated ventilation rates

 

18

 The risk results presented represent the median estimate of exposed asthmatics expected to experience moderate 

or greater lung function decrements. Results are presented for both the probit and 2-parameter logistic functional 

forms. The full range of estimates can be found in chapter 9 of the REA, and in all instances the smaller estimate is a 

result of using the probit function to estimate the exposure-response relationship. 

19 In this notice, risk results with respect to moderate or greater lung function responses are presented in terms of 

sRaw (i.e., ≥ 100% increases in sRaw). Risk results with respect to decrements in lung function defined in terms of 

FEV1 can be found in chapter 9 of the REA.
67

(REA, section 10.3.4). This also suggested that an alternative SO2 standard(s) should protect 

against health effects ranging from lung function responses and increased respiratory symptoms 

following 5 -10 minute peak SO2 exposures, to increased respiratory symptoms and respiratoryrelated ED visits and hospital admissions associated with 1-hour daily maximum or 24-hour 

average SO2 concentrations (REA, section 10.3.4). 

In examining the air quality, exposure, and risk-based information with regard to the 

adequacy of the current 24-hour SO2 standard to protect the public health, the REA found that 

the results described above (and in more detail in chapters 7-9 of the REA) indicated that 5-

minute exposures that could reasonably be judged important from a public health perspective

(see section II.B.1.c) were associated with air quality adjusted upward to simulate just meeting 

the current 24-hour standard. These exposures were judged in the REA to be significant from a 

public health perspective due to their frequency: approximately 24% of child asthmatics at 

moderate or greater exertion in St. Louis are estimated to be exposed at least once per year to air 

quality exceeding the 5-minute 400 ppb benchmark, a level associated with lung function 

decrements in the presence of respiratory symptoms. Additionally, approximately 73% of child 

asthmatics in St. Louis would be expected to be exposed at least once per year to air quality

exceeding the 5-minute 200 ppb benchmark. Moreover, slightly over 19% of exposed child 

asthmatics in St. Louis would be expected to experience at least one adverse lung function 

response (defined in terms of a ≥ 100% increase in sRaw) each year. Therefore, the REA 

concluded that the air quality, exposure, and risk-based considerations reinforced the 

epidemiologic evidence in supporting the conclusion that consideration should be given to 

revising the current 24-hour standard and/or setting a new shorter averaging time standard (e.g., 

1-hour or less) to provide increased public health protection, especially for sensitive groups (e.g., 
68

asthmatics), from SO2-related adverse health effects (REA, section 10.3.4). 

2. Adequacy of the current annual standard 

In considering the adequacy of the current annual standard, the policy assessment chapter 

of the REA considered the scientific evidence assessed in the ISA and the air quality, exposure, 

and risk-based information presented in the REA. A summary of this evidence and information 

is presented below.

a. Evidence-based considerations 

As an initial consideration with regard to the adequacy of the current annual standard, the 

REA noted that evidence relating long-term (weeks to years) SO2 exposure to adverse health 

effects (respiratory morbidity, carcinogenesis, adverse prenatal and neonatal outcomes, and 

mortality) was judged by the ISA to be “inadequate to infer the presence or absence of a causal 

relationship” (ISA, Table 5-3). That is, the ISA found the health evidence to be of insufficient 

quantity, quality, consistency, or statistical power to make a determination as to whether SO2 is 

truly associated with these health endpoints (ISA, Table 1-2). With respect specifically to 

respiratory morbidity in children (in part, the basis for the current annual standard; see section 

II.D.1), the ISA presented recent epidemiologic evidence of an association with long-term 

exposure to SO2 (ISA, section 3.4.2). However, the ISA found the strength of these 

epidemiologic studies to be limited because of 1) variability in results across studies with respect 

to specific respiratory morbidity endpoints; 2) high correlations between long-term average SO2

and co-pollutant concentrations, particularly PM; and 3) a lack of evaluation of potential 

confounding (ISA, section 3.4.2.1). 

The REA also noted that many epidemiologic studies demonstrating positive associations 

between 1-hour daily maximum or 24-hour average SO2 concentrations and respiratory
69

symptoms, ED visits, and hospitalizations were conducted in areas where ambient SO2

concentrations were well below the level of the current annual NAAQS (REA, section 10.4.2). 

The REA noted that this evidence suggested that the current annual standard was not providing 

adequate protection against health effects associated with shorter-term SO2 concentrations found 

in epidemiologic studies (REA, section 10.4.2). 

b. Air quality, exposure, and risk-based considerations 

Results of the risk characterization based on the air quality assessment provided

additional insight into whether there is a need to revise the current annual standard, focusing 

again on the extent to which the annual standard may be providing protection against effects 

associated with short-term exposures. In general, analyses presented in the REA described the 

extent to which the current annual standard provided protection against 5-minute peaks of SO2 in 

excess of potential health effect benchmark levels (REA, chapter 7). The REA found that many

of the monitors where frequent 5-minute exceedances were reported had annual average SO2

concentrations well below the level of the current annual standard. Moreover, the REA found 

that there was little to no correlation between the annual average SO2 concentration and the 

number of 5-minute daily maximum concentrations above potential health effect benchmark 

levels at these monitors (REA section 7.3.1). Thus, the REA concluded that the annual standard 

adds little in the way of protection against 5-minute peaks of SO2 (REA, section 10.4.4).

c. Summary of considerations from the REA regarding the annual standard

As noted above, the ISA concluded that the evidence relating long-term (weeks to years) 

SO2 exposure to adverse health effects (respiratory morbidity, carcinogenesis, adverse prenatal 

and neonatal outcomes, and mortality) was “inadequate to infer the presence or absence of a 

causal relationship” (ISA, Table 5-3). The ISA also reported that many epidemiologic studies 
70

demonstrating positive associations between short-term (e.g., 1-hour daily maximum, 24-hour 

average) SO2 concentrations and respiratory symptoms, as well as ED visits and hospitalizations, 

were conducted in areas where annual ambient SO2 concentrations were well below the level of 

the current annual NAAQS. In addition, analyses conducted in the REA suggested that the 

current annual standard is not providing protection against 5 - 10 minute peaks of SO2. Thus, the 

scientific evidence and the risk and exposure information suggest that the current annual SO2

standard: 1) is likely not needed to protect against health risks associated with long term 

exposure to SO2; and 2) does not provide adequate protection from the health effects associated 

with shorter-term (i.e. ≤ 24-hours) SO2 exposures. Thus, the policy chapter of the REA 

accordingly concluded that consideration should be given to either revoking the annual standard 

or retaining it without revision, in conjunction with setting an appropriate short-term standard(s)

(REA, section 10.4.4).

3. CASAC views regarding the adequacy of the current 24-hour and annual standards

With regard to the adequacy of the current standards, CASAC conclusions were 

consistent with the views expressed in the policy assessment chapter of the REA.

20

 CASAC 

agreed that the primary concern in this review is to protect against health effects that have been 

associated with short-term SO2 exposures, particularly those of 5 - 10 minutes (Samet 2009). 

CASAC also agreed that the current 24-hour and annual standards are not sufficient to protect 

public health against the types of exposures that could lead to these health effects. Given these 

considerations, and as noted in their letter to the EPA Administrator, CASAC agreed “that the 

current 24-hour and annual standards are not adequate to protect public health, especially in 

 

20 CASAC views with respect to the current 24-hour and annual standards, as well as with respect to potential 

alternative standards are those following their review of the second draft SO2 REA, which contained a staff policy 

assessment chapter. EPA did not solicit, nor did it receive CASAC comments on the final policy assessment chapter 

contained in the final REA.
71

relation to short term exposures to SO2 (5 - 10 minutes) by exercising asthmatics” (Samet, 2009, 

p. 15). CASAC also noted: “assuming that EPA adopts a one hour standard in the range 

suggested, and if there is evidence showing that the short-term standard provides equivalent 

protection of public health in the long-term as the annual standard, the panel is supportive of the 

REA discussion of discontinuing the annual standard” (Samet 2009, p. 15). With regard to the 

current 24-hour standard, CASAC was generally supportive of using the air quality analyses in 

the REA as a means of determining whether the current 24-hour standard was needed in addition 

to a new 1-hour standard to protect public health. CASAC stated: “the evidence presented [in 

REA Table 10-3] was convincing that some of the alternative one-hour standards could also 

adequately protect against exceedences of the current 24-hour standard” (Samet 2009, p. 15) 

Discussion regarding CASAC’s views on how the standard should be revised is provided below 

within the context of discussions on the elements (i.e., indicator, averaging time, form, level) of a 

new short-term standard. 

4. The Administrator’s conclusions regarding adequacy of the current 24-hour and annual 

standards

Based on the epidemiologic evidence, the risk and exposure data set out in this section, as 

well as CASAC’s advice and recommendations, the Administrator concludes (subject to 

consideration of public comment) that the current standards are not adequate to protect public 

health with an adequate margin of safety. The basis for this conclusion is as follows. First, the 

Administrator accepts and agrees with the ISA’s conclusion that the results of controlled human 

exposure and epidemiologic studies form a plausible and coherent data set that supports a causal 

relationship between short-term (5-minutes to 24-hours) SO2 exposures and adverse respiratory

effects. The Administrator further agrees that the epidemiologic evidence (buttressed by the 
72

clinical evidence) indicates that the effects seen in the epidemiologic studies are attributable to 

exposure to SO2. She also accepts and agrees with the conclusion of the ISA that “[i]n the 

epidemiologic studies, respiratory effects were observed in areas where the maximum ambient 

24-h avg SO2 concentration was below the current 24-h avg NAAQS level .....” (ISA, section 

5.2, p. 5-2.) and so would occur at ambient SO2 concentrations that are present in locations 

meeting the current 24-hour NAAQS. The Administrator also notes that these effects occurred 

in areas with annual air quality levels considerably lower than those allowed by the current 

annual standard, indicating that the annual standard also is not providing protection against such 

effects. Existence of epidemiologic studies showing adverse effects occurring at levels allowed 

by the current standards is an accepted justification for finding that it is appropriate to revise the 

existing standards. See, e.g. American Trucking Ass’n v. EPA, 283 F. 3d 355, 370 (D.C. Cir. 

2002).

With regard to the exposure and risk results, the Administrator notes and agrees with the

analyses in the REA supporting that 5-minute exposures, reasonably judged important from a 

public health perspective, were associated with air quality adjusted upward to simulate just 

meeting the current standards. The Administrator especially notes the results of the St. Louis 

exposure analysis which, as summarized above, indicates that substantial percentages of 

asthmatic children at moderate or greater exertion would be exposed, at least once annually, to 

air quality exceeding the 400 and 200 ppb benchmarks. Moreover, in addition to the health 

evidence and risk-based information, the Administrator agrees with CASAC’s conclusion that 

the current SO2 standards do not adequately protect the public’s health.

In considering approaches to revising the current standards, the Administrator is 

proposing that it is appropriate to consider setting a new short-term standard. The Administrator 
73

initially notes that a 1-hour standard could provide increased public health protection, especially

for members of at-risk groups, from health effects described in both controlled human exposure 

and epidemiologic studies, and hence, health effects associated with 5-minute to 24-hour

exposures to SO2. As discussed in section II.F.5 below, depending on the degree of protection 

afforded by such a standard, it may be appropriate to replace, and not retain, the current 24-hour 

and annual standards in conjunction with setting a new short-term standard.

F. Conclusions on the elements of a proposed new short-term standard

In considering alternative SO2 primary NAAQS, the Administrator notes the need to 

protect at-risk populations from: 1) 1-hour daily maximum and 24-hour average exposures to 

SO2 that could cause the types of respiratory morbidity effects reported in epidemiologic studies;

and 2) 5 - 10 minute SO2 exposure concentrations reported in controlled human exposure studies 

to result in moderate or greater lung function responses and/or respiratory symptoms. 

Considerations with regard to potential alternative standards and the specific options being

proposed are discussed in the following sections in terms of indicator, averaging time, form, and 

level (sections II.F.1 to II.F.4).

1. Indicator

In the last review, EPA focused on SO2 as the most appropriate indicator for ambient 

SOx. In making a decision in the current review on the most appropriate indicator, the 

Administrator has considered the conclusions of the ISA and REA as well as the views expressed 

by CASAC. The REA noted that, although the presence of gaseous SOx species other than SO2

has been recognized, no alternative to SO2 has been advanced as being a more appropriate 

surrogate for ambient gaseous SOx. Controlled human exposure studies and animal toxicology

studies provide specific evidence for health effects following exposure to SO2. Epidemiologic 
74

studies also typically report levels of SO2, as opposed to other gaseous SOx. Because emissions 

that lead to the formation of SO2 generally also lead to the formation of other SOx oxidation 

products, measures leading to reductions in population exposures to SO2 can generally be 

expected to lead to reductions in population exposures to other gaseous SOx. Therefore, meeting 

an SO2 standard that protects the public health can also be expected to provide protection against 

potential health effects that may be independently associated with other gaseous SOx even 

though such effects are not discernable from currently available studies indexed by SO2 alone. 

See American Petroleum Institute v. EPA, 665 F, 2d 1176, 1186 (D.C. Cir. 1981) (reasonable for 

EPA to use ozone as the indicator for all photochemical oxidants even though health information 

on the other photochemical oxidants is unknown; regulating ozone alone is reasonable since it 

presents a “predictable danger” and in doing so EPA did not abandon its responsibility to 

regulate other photochemical oxidants encompassed by the determination that photochemical 

oxidants as a class may be reasonably anticipated to endanger public health or welfare). Given 

these key points, the REA concluded that the available evidence supports the retention of SO2 as 

the indicator in the current review (REA, section 10.5.1). Consistent with this conclusion, 

CASAC stated in a letter to the EPA Administrator that “for indicator, SO2 is clearly the 

preferred choice” (Samet 2009, p. 14). The Administrator agrees with this consensus, and 

therefore proposes to retain SO2 as the indicator for oxides of sulfur in the current review.

2. Averaging time

In considering whether it is appropriate to revise the averaging times of the current 

standards, the first consideration is what health effects the standard is addressing, and 

specifically whether those effects are associated with short-term (i.e., 5-minutes to 24-hours), 

and/or long-term (i.e. weeks to years) exposure to SO2. There are distinct differences in the 
75

causality judgments in the ISA as to short-term versus long-term health effects of SO2. The ISA 

found evidence relating long-term (weeks to years) SO2 exposures to adverse health effects to be 

“inadequate to infer the presence or absence of a causal relationship” (ISA, Table 5-3). In 

contrast, the ISA judged evidence relating short-term (5-minutes to 24-hours) SO2 exposure to 

respiratory morbidity to be “sufficient to infer a causal relationship” (the strongest possible 

conclusion as to causality) and short-term exposure to SO2 and mortality to be “suggestive of a 

causal relationship” (ISA, Table 5-3). Taken together, the REA concluded that these judgments 

most directly supported standard averaging time(s) that focus protection on SO2 exposures from 

5-minutes to 24-hours (REA, section, 10.5.2). 

a. Evidence and air quality, exposure, and risk-based considerations

In considering the level of support available for specific short-term averaging times, the 

REA noted the strength of evidence from human exposure and epidemiologic studies evaluated 

in the ISA. As previously mentioned, controlled human exposure studies exposed exercising 

asthmatics to 5 - 10 minute peak concentrations of SO2 and consistently found decrements in 

lung function and/or respiratory symptoms. Importantly, the ISA described the controlled human 

exposure studies as being the “definitive evidence” for its conclusion that there exists a causal 

association between short-term (5-minutes to 24-hours) SO2 exposure and respiratory morbidity

(ISA, section 5.2). In addition to the controlled human exposure evidence, there is a relatively 

small body of epidemiologic studies describing positive associations between 1-hour daily

maximum SO2 levels and respiratory symptoms as well as hospital admissions and ED visits for 

all respiratory causes and asthma (ISA Tables 5.4 and 5.5). In addition to the evidence from 

these 1-hour daily maximum epidemiologic studies, there is a considerably larger body of 

epidemiologic studies reporting positive associations between 24-hour average SO2 levels and 
76

respiratory symptoms, as well as hospitalizations and ED visits for all respiratory causes and 

asthma. Moreover, with respect to these epidemiologic studies, there is support that adverse 

respiratory effects are more likely to occur at the upper end of the distribution of ambient SO2

concentrations (see section II.F.3 on Form). In addition, when describing epidemiologic studies 

observing positive associations between ambient SO2 and respiratory symptoms, the ISA stated

“that it is possible that these associations are determined in large part by peak exposures within a 

24-hour period” (ISA, section 5.2 at p. 5-5). Similarly, the ISA stated that: “the effects of SO2

on respiratory symptoms, lung function, and airway inflammation observed in the human clinical 

studies using peak exposures further provides a basis for a progression of respiratory morbidity

resulting in increased ED visits and hospital admissions” and makes the associations observed in 

the epidemiologic studies “biologica[lly] plausib[le]” (ISA, section 5.2 at p. 5-5). 

The controlled human exposure evidence described above provided support for an 

averaging time that protects against 5-10 minute peak SO2 exposures (REA, section 10.5.2). In 

addition, the REA found that results from epidemiologic studies provided support for both 1-

hour and 24-hour averaging times (REA, section 10.5.2). In addition, both the epidemiologic and 

controlled human exposure evidence suggests that a new short-term standard should be focused

on limiting peak SO2 exposures. Thus, it can reasonably be concluded from the ISA and REA 

that it would be appropriate to consider the degree of protection potential alternative standards 

with averaging times under consideration provide against peak 5-minute to 24- hour SO2

exposures. Moreover, as fully discussed in section II.F.3, this same information makes it 

reasonable that the form of a new short-term standard reflect a strategy to limit peak SO2

exposures. Thus, with respect to the analyses presented below regarding averaging time, a 99th

percentile form will be considered. See American Petroleum Institute, 665 F. 2d at 1186 
77

(selection of highest average ozone level in one hour to determine compliance with ozone 

NAAQS is reasonable “because it is calculated to measure the maximum exposure, which has 

been found to be a relevant factor in determining the likely consequences of ozone exposure”). 

In considering the level of support available for specific short-term averaging times, the 

policy assessment chapter of the REA also took into account air quality considerations. More 

specifically, since the shortest averaging time for the current primary SO2 standard is 24-hours, 

the REA evaluated the potential for a standard based on 24-hour average SO2 concentrations to 

limit 5-minute peak SO2 exposures (REA, section 10.5.2). The REA evaluated ratios between 

99th percentile 5-minute daily maximum and 99th percentile 24-hour average SO2 concentrations 

for 42 monitors reporting measured 5-minute data for any year between 2004-2006 (REA, Table 

10-1). Across this set of monitors, ratios of 99th percentile 5-minute daily maximum to 99th

percentile 24-hour average SO2 concentrations spanned a range of 2.0 to 14.1 (REA, Table 10-1). 

These results suggested a standard based on 24-hour average SO2 concentrations would not 

likely be an effective or efficient approach for addressing 5-minute peak SO2 concentrations. 

That is, the REA concluded using a 24-hour average standard to address 5-minute peaks would 

likely result in over- controlling in some areas, while under-controlling in others (REA, section 

10.5.2). This analysis also suggested that a 5-minute standard would not likely be an effective or 

efficient means for controlling 24-hour average SO2 concentrations (REA, section 10.5.2).

The REA also reported ratios between 99th percentile 5-minute daily maximum and 99th

percentile 1-hour daily maximum SO2 levels from this set of monitors. Compared to the ratios 

discussed above (5-minute daily maximum to 24-hour average), there was far less variability

between 5-minute daily maximum and 1-hour daily maximum ratios. More specifically, 39 of 

the 42 monitors had 99th percentile 5-minute daily maximum to 99th percentile 1-hour daily
78

maximum ratios in the range of 1.2 to 2.5 (REA, Table 10-1). The remaining three monitors had 

ratios of 3.6, 4.2 and 4.6 respectively. Overall, the REA found that this relatively narrow range 

of ratios (compared to the range of ratios presented above with respect to 5-minute daily

maximum to 24-hour average) suggested that a standard with a 1-hour averaging time would be 

more efficient and effective at limiting 5-minute peaks of SO2 than a standard with a 24-hour 

averaging time (REA, section 10.5.2.2). This analysis also suggested that a 5-minute standard 

could be a relatively effective means of controlling 1-hour daily maximum SO2 concentrations.

21

 

The REA further evaluated the potential of the 1-hour daily maximum standards analyzed 

in the air quality, exposure, and risk analyses to limit peak 24-hour average SO2 exposures 

(REA, section 10.5.2) since there is epidemiologic evidence to suggest that adverse respiratory

effects are more likely to occur at the upper end of the distribution of ambient SO2

concentrations. The 99th percentile 24-hour average SO2 concentrations in cities where U.S. ED 

visit and hospitalization studies (for all respiratory causes and asthma; identified from Table 5-5 

of the ISA) were conducted ranged from 16 ppb to 115 ppb (Thompson and Stewart, 2009). 

Moreover, effect estimates that remained statistically significant in multi-pollutant models with 

PM were found in cities with 99th percentile 24-hour average SO2 concentrations ranging from 

approximately 36 ppb to 64 ppb. The REA found that a 99th percentile 1-hour daily maximum 

standard set at a level of 50- 100 ppb would generally limit 99th percentile 24-hour average SO2

concentrations in locations where epidemiologic studies reported statistically significant results 

in multi-pollutant models with PM (Table 1). That is, for 2004, given air quality adjusted to just 

meet a 50 ppb 99th percentile 1-hour daily maximum standard, the REA found that no county

 

21 The analysis of peak to mean ratios was used as an initial screen to evaluate which averaging times could be 

suited to control 5-minute peaks of SO2. The more sophisticated analysis for ultimately determining that a one-hour 

averaging time set at an appropriate level could effectively limit these 5-minute peaks was the air quality, exposure, 

and risk analyses discussed in section II.F.4. 
79

included in this analysis was estimated to have 24-hour average SO2 concentrations ≥ 36 ppb

(Table 1). In addition, given air quality adjusted to just meet a 100 ppb 99th percentile 1-hour 

daily maximum standard, only 6 of the 39 counties (Linn, Union, Bronx, Fairfax, Hudson, and 

Wayne) included in this 2004 analysis were estimated to have 99th percentile 24-hour average 

SO2 concentrations ≥ 36 ppb (Table 1). The REA repeated this analysis for the years 2005 and 

2006 and found similar results (REA, Appendix Tables D1 and D2)

22

. 

 

22 In 2005, given a 99th percentile 1-hour daily maximum standard at 50 ppb, Wayne County, West Virginia would 

have an estimated 99th percentile 24-hour average SO2 concentration >36 ppb (43 ppb; REA Appendix Table D-1) 
80

Table 1. 99th percentile 24-hour average SO2 concentrations for 

2004 given just meeting the alternative 1-hour daily maximum 99th

and 98th percentile potential standards analyzed in the air 

quality assessment (SOURCE: REA, Table 10-2).

23

State County

1-hour daily maximum standards

99th percentile 98th percentile

50 100 150 200 250 100 200

AZ Gila 6 12 18 25 31 16 32

DE New Castle 12 23 35 47 59 28 56

FL Hillsborough 10 20 30 40 50 28 55

IL Madison 12 24 36 48 60 28 56

IL Wabash 7 13 20 27 33 19 38

IN Floyd 8 15 23 31 39 20 41

IN Gibson 9 18 27 36 45 20 41

IN Lake 12 24 36 48 60 31 62

IN Vigo 10 19 29 39 48 24 48

IA Linn 21 42 64 85 106 49 98

IA Muscatine 17 34 51 68 85 38 76

MI Wayne 17 33 50 66 83 37 74

MO Greene 12 24 36 48 60 31 62

MO Jefferson 9 18 27 36 45 25 51

NH Merrimack 17 33 50 66 83 39 79

NJ Hudson 19 38 57 76 95 48 96

NJ Union 18 36 54 72 90 44 89

NY Bronx 23 47 70 93 117 54 107

NY Chautauqua 13 27 40 54 67 32 65

NY Erie 14 27 41 54 68 30 61

OH Cuyahoga 17 34 51 67 84 40 80

OH Lake 10 19 29 39 48 23 47

OH Summit 12 24 36 48 61 27 55

OK Tulsa 16 32 47 63 79 36 72

PA Allegheny 12 23 35 47 59 30 60

PA Beaver 10 20 30 40 51 25 49

PA Northampton 11 23 34 45 56 36 72

PA Warren 11 22 33 44 56 28 56

PA Washington 15 31 46 62 77 36 71

TN Blount 15 31 46 61 77 35 71

TN Shelby 17 34 51 68 85 41 81

TN Sullivan 8 16 24 32 39 23 46

TX Jefferson 9 17 26 35 44 21 41

VA Fairfax 23 46 69 92 116 52 103

WV Brooke 12 24 37 49 61 31 62

WV Hancock 15 29 44 58 73 35 69

WV Monongalia 10 20 30 40 50 25 51

WV Wayne 30 59 89 119 149 67 133

VI St Croix 14 27 41 54 68 51 101

 

23 99th or 98th percentile 1-hour daily maximum concentrations were determined for each monitor in a given county 

for the years complete data were available from 2004-2006. These concentrations were averaged, and the monitor 

with the highest average in a given county was determined. Based on this highest average, all monitors in a given 

county were adjusted to just meet the potential alternative standards defined above, and for each of the years, the 

99th percentile 24-hour average SO2 concentration was identified. Results for the years 2005 and 2006 are presented 

in the REA, Appendix D. 
81

The air quality information presented above strongly support the likelihood that an 

alternative 99th percentile (see discussion of form below in II.F.3) 1-hour daily maximum 

standard set at an appropriate level (see discussion of level in II.F.4) can substantially reduce the 

upper end of the distribution of SO2 levels more likely to be associated with adverse respiratory

effects; that is: (1) 99th percentile 1-hour daily maximum air quality concentrations in cities 

observing positive effect estimates in epidemiologic studies of hospital admissions and ED visits 

for all respiratory causes and asthma; and (2) 99th percentile 24-hour average air quality 

concentrations found in U.S. cities where ED visit and hospitalization studies (for all respiratory

causes and asthma) observed statistically significant associations in multi-pollutant models with 

PM (i.e., 99th percentile 24-hour average SO2 concentration ≥ 36 ppb). In addition, based on the 

air quality and exposure analyses presented in chapters 7 and 8 of the REA, there is also a strong 

likelihood that a 99th percentile 1-hour daily maximum standard will limit 5 - 10 minute peaks of 

SO2 shown in human exposure studies to result in decrements in lung function and/or respiratory

symptoms in exercising asthmatics (see especially: REA Tables 7-11 to 7-14 and Figure 8-19). 

Such analyses are also summarized in section II.F.4 of this notice. Taken together, these results 

support that a 1-hour daily maximum standard, with an appropriate form and level, can provide 

adequate protection against the range of health outcomes associated with averaging times from 

5-minutes to 24-hours (REA, section 10.5.2.3). 

The REA also considered the possibility of a 5-minute averaging time based solely on the 

controlled human exposure evidence. However, the REA did not favor such an approach (REA 

10.5.2.3). As in past NAAQS reviews, the stability of the design of pollution control programs 

in considering the elements of a NAAQS was considered, since more stable programs are more 

effective, and hence result in enhanced public safety. American Trucking Associations v. EPA, 
82

283 F. 3d 355, 375 (D.C. Cir. 2002) (choice of 98th percentile form for 24-hour PM NAAQS, 

which allows a number of high exposure days per year to escape regulation under the NAAQS, 

justifiable as “promot[ing] development of more ‘effective [pollution] control programs’”, since 

such programs would otherwise be “less ‘stable’ – and hence... less effective – than programs 

designed to address longer-term average conditions”, and there are other means (viz. emergency

episode plans) to control those high exposure days). In this review, there were concerns about 

the stability of a standard using a 5-minute averaging time. Specifically, there was concern that 

compared to longer averaging times (e.g., 1-hour, 24-hour), year-to-year variation in 5-minute 

SO2 concentrations were likely to be substantially more temporally and spatially diverse. Thus, 

it is likely that locations would frequently shift in and out of attainment thereby reducing public 

health protection by disrupting an area’s ongoing implementation plans and associated control 

programs. Consequently, the REA concluded that a 5-minute averaging time would not provide 

a stable regulatory target and therefore would not be the preferred approach to provide adequate 

public health protection. However, as noted above, analyses in the REA support that a 1-hour 

averaging time, given an appropriate form and level (discussed below in sections II.F.3 and 

II.F.4, respectively) can adequately limit 5-minute SO2 exposures and provide a more stable 

regulatory target than setting a 5-minute standard.

b. CASAC views

CASAC agreed with the conclusions of the policy assessment chapter of the REA that a 

primary consideration of the SO2 NAAQS should be the protection provided against health 

effects associated with short-term exposures. In their letter to the EPA Administrator, CASAC 

stated that they were “in agreement with having a short-term standard and finds that the REA 

supports a one-hour standard as protective of public health” (Samet 2009, p. 1). Furthermore, 
83

CASAC agreed with the REA that a “one-hour standard is the preferred averaging time” (Samet 

2009, p.15).”

c. Administrator’s conclusions on averaging time 

In considering the most appropriate averaging time(s) for the SO2 primary NAAQS, the 

Administrator notes the conclusions and judgments made in the ISA about the available 

scientific evidence, conclusions from the REA, and CASAC recommendations discussed above. 

Based on these considerations, the Administrator proposes to set a new standard based on 1-hour 

daily maximum SO2 concentrations to provide increased protection against effects associated 

with short-term (5-minutes to 24-hour) exposures. First, the Administrator agrees with the 

REA’s conclusion that the standard should focus protection on short-term SO2 exposures from 5-

minutes to 24-hours. As noted above, CASAC’s strong recommendation supports this approach 

as well. Second, the Administrator agrees that the standard must provide requisite protection 

from 5-10 minute exposure events (the critical issue in the previous review), but believes 

(subject to consideration of public comment) that this can be done without having a standard 

with a 5-minute averaging time. The Administrator agrees with the REA conclusion that it is 

likely a 1-hour standard -- with the appropriate form and level -- can substantially reduce 5 - 10 

minute peaks of SO2 shown in controlled human exposure studies to result in respiratory

symptoms and/or decrements in lung function in exercising asthmatics. The Administrator 

further believes that a 5-minute averaging time would result in significant and unnecessary

instability and is undesirable for that reason. The Administrator also notes the statements from 

CASAC addressing whether a one-hour averaging time can adequately control 5-10 minute peak 

exposures and whether there should be a 5-minute averaging time. CASAC stated that the REA 

had presented a “convincing rationale” for a one-hour standard, and that “a 1-hour standard is the 
84

preferred averaging time” (Samet 2009, p. 16).

Third, the Administrator agrees that a one-hour averaging time (again, with the 

appropriate form and level) would provide protection against the range of health outcomes 

associated with averaging times of one hour to 24 hours. Specifically, the Administrator finds 

that a 1-hour standard can substantially reduce the upper end of the distribution of SO2 levels 

more likely to be associated with adverse respiratory effects; that is: 1) 99th percentile 1-hour 

daily maximum air quality concentrations in U.S. cities where positive effect estimates in 

epidemiologic studies of hospital admissions and ED visits for all respiratory causes and asthma

were observed; and 2) 99th percentile 24-hour average air quality concentrations found in U.S. 

cities where ED visit and hospitalization studies (for all respiratory causes and asthma) observed 

statistically significant associations in multi-pollutant models with PM. Finally, the 

Administrator notes that the proposal to establish a new 1-hour averaging time is in agreement 

with CASAC recommendations. As noted above, CASAC stated that they were “in agreement 

with having a short-term standard and finds that the REA supports a one-hour standard as 

protective of public health” (Samet, 2009, p. 1).

3. Form

When evaluating alternative forms in conjunction with specific levels, the REA 

considered the adequacy of the public health protection provided by the combination of level and 

form to be the foremost consideration. In addition, the REA recognized that it is important that 

the standard have a form that is reasonably stable. As just explained in the context of a fiveminute averaging time, a standard set with a high degree of instability could have the effect of 

reducing public health protection because shifting in and out of attainment could disrupt an 

area’s ongoing implementation plans and associated control programs.
85

a. Evidence, air quality, and risk- based considerations

As previously mentioned, the policy chapter of the REA (chapter 10) recognized that the 

adequacy of the public health protection provided by a 1-hour daily maximum potential 

alternative standard will be dependent on the combination of form and level. It is therefore 

important that the particular form selected for a 1-hour daily maximum potential alternative 

standard reflect the nature of the health risks posed by increasing SO2 concentrations. That is, 

the REA noted that the form of the standard should reflect results from controlled human 

exposure studies demonstrating that the percentage of asthmatics affected, and the severity of the 

respiratory response (i.e. decrements in lung function, respiratory symptoms) increases as SO2

concentrations increase. Taking this into consideration, the REA concluded that a concentrationbased form, averaged over three years, is more appropriate than an exceedance-based form

(REA, section 10.5.3). This is because a concentration-based form averaged over three years 

would give proportionally greater weight to years when 1-hour daily maximum SO2

concentrations are well above the level of the standard, than to years when 1-hour daily 

maximum SO2 concentrations are just above the level of the standard. In contrast, an expected 

exceedance form would give the same weight to years when 1-hour daily maximum SO2

concentrations are just above the level of the standard, as to years when 1-hour daily maximum 

SO2 concentrations are well above the level of the standard. Therefore, the REA concluded that 

a concentration-based form, averaged over three years (which also increases the stability of the 

standard) better reflects the continuum of health risks posed by increasing SO2 concentrations 

(i.e. the percentage of asthmatics affected and the severity of the response increases with 

increasing SO2 concentrations; REA, section 10.5.3).

The form of the standard should also reflect health information in the ISA that suggests 
86

that adverse respiratory effects are more likely to occur at the upper end of the distribution of 

ambient SO2 concentrations. Specifically, a few studies found that the increase in SO2-related 

respiratory health effects was observed at the upper end of the distribution of SO2 concentrations 

(ISA, section 5.3, p. 5-9). For example, an epidemiologic study conducted in Bronx, NY 

suggested an increased risk of asthma hospitalizations on the days with the highest SO2

concentrations (Lin et al., 2004). More specifically, the authors observed an increasing linear 

trend with respect to asthma hospitalizations across the range of SO2 concentrations, with more 

marked effects observed at SO2 concentrations somewhere between the 90th and 95th percentiles 

(ISA, section 4.1.2 and ISA, Figure 4-4). 

The epidemiologic evidence is consistent with the large body of controlled human 

exposure studies of exercising asthmatics exposed to short-term peak concentrations of SO2; 

these controlled human exposure studies provide the “definitive evidence” that short term peak 

SO2 exposure is associated with respiratory morbidity (SOx ISA, Section 5.3, page 5-2). These 

studies consistently found moderate or greater decrements in lung function (i.e. ≥ 100% increase 

in sRaw and/or ≥ 15% decline in FEV1)

24 and/or respiratory symptoms in exercising asthmatics 

following 5 - 10 minute peak exposures to SO2. Moreover, as noted in the discussion on 

averaging time (section II.F.2), when discussing the possible relationship between effects 

observed in controlled human exposure studies and associations reported in epidemiologic 

analyses, the ISA stated with respect to epidemiologic studies of respiratory symptoms: “it is 

possible that these associations are determined in large part by peak exposures within a 24-hour 

period” (ISA, section 5.2 at p. 5-5). Similarly, the ISA stated that: “the effects of SO2 on 

respiratory symptoms, lung function, and airway inflammation observed in the human clinical 

studies using peak exposures further provides a basis for a progression of respiratory morbidity

 

24 See section II.B.1.b above explaining sRaw and FEV1.
87

resulting in increased ED visits and hospital admissions” and makes the associations observed in 

the epidemiologic studies “biologica[lly] plausib[le]” (ISA, section 5.2 at p. 5-5). Thus, both the 

epidemiologic and controlled human exposure evidence suggests that the form of the standard 

should be focused on limiting peak SO2 exposures. 

In considering specific concentration-based forms, the REA recognized the importance 

of: 1) minimizing the number of days per year that an area could exceed the level of the standard 

and still attain the standard and thus, limiting the upper end of the distribution of SO2 levels most 

likely associated with adverse respiratory effects 2) limiting the prevalence of 5-minute peaks of 

SO2; and 3) providing a stable regulatory target to prevent areas from frequently shifting in and 

out of attainment. The REA focused on 98th and 99th percentile forms averaged over 3 years. 

The REA first noted that in most locations analyzed, the 99th percentile form of a 1-hour daily

maximum standard would correspond to the 4th highest daily maximum concentration in a year, 

while a 98th percentile form would correspond approximately to the 7th to 8th highest daily

maximum concentration in a year (REA, Table 10-5 and Thompson, 2009). In addition, results 

from the REA air quality analysis suggested that at a given SO2 standard level, a 99th percentile 

form is appreciably more effective at limiting 5-minute peak SO2 concentrations than a 98th

percentile form (REA, section 10.5.3 and REA, Figures 7-27 and 7-28). For example, the REA 

reported that compared to the same standard with a 99th percentile form, a 98thpercentile 1-hour 

daily maximum standard set at a level of 100 ppb allows for on average, an estimated 90 and 

74% more days per year when SO2 concentrations would likely exceed the 200 and 400 ppb 

benchmark values respectively (REA, section 10.5.3 and REA, Figure 7-28). Moreover, in the 

counties selected for analysis in the REA air quality assessment, the estimated number of 

benchmark exceedances using a 98th percentile 1-hour daily maximum standard set at a level of 
88

200 ppb was similar to the corresponding 99th percentile standard set at a level of 250 ppb (REA,

section 10.5.3 and REA, Tables 7-11 through 7-14). Similarly, the estimated number of 

benchmark exceedances considering a 98th percentile standard set at a level of 100 ppb fell 

within the range of benchmark exceedances estimated for 99th percentile standards set at levels 

of 100 and 150 ppb (id).

As an additional matter, the REA compared trends in 98th and 99th percentile design 

values, as well as design values based on the 4th highest daily maximum from 54 sites located in 

the 40 counties selected for the detailed air quality analysis (REA section 10.5.3 and Thompson, 

2009). These results suggested that at the vast majority of sites, there would have been similar 

changes in 98th and 99th percentile design values over the last ten years (i.e. based evaluating 

overlapping three year intervals over the last ten years; see REA, Figure 10-1 and Thompson, 

2009). These results also demonstrated that design values based on the 4th highest daily 

maximum are virtually indistinguishable from design values based on the 99th percentile (REA,

Figure 10-1 and Thompson, 2009). As part of this analysis, all of the design values over this ten 

year period for all 54 sites were aggregated and the standard deviation calculated (REA, Figure 

10-2 and Thompson, 2009). Results demonstrated similar standard deviations – i.e. similar 

stability -- based on aggregated 98th or aggregated 99th percentile design values over the ten year 

period (see REA, Figure 10-2 and Thompson 2009). 

Considering the evidence and air quality analyses presented above, the REA concluded

that a concentration-based form provides the best protection against the health risks posed by

increasing SO2 concentrations (REA, section 10.5.3). Moreover, the REA found that at a given 

standard level, a 99th percentile or 4th highest daily maximum form provides appreciably more 

public health protection against 5-minute peaks than a 98th percentile or 7th- 8

th highest daily
89

maximum form (REA, section 10.5.3). In addition, over the last 10 years and for the vast 

majority of the sites examined, there appears to be little difference in 98th and 99th percentile 

design value stability (REA, section 10.5.3). Thus, the REA ultimately concluded that 

consideration should be given primarily to a 1-hour daily maximum standard with a 99th

percentile or 4th highest daily maximum form (REA, section 10.5.3.3).

b. CASAC views 

CASAC agreed with the importance of considering the public health protection provided 

by the combination of form and level. Moreover, CASAC was in general agreement with the 

forms being considered. In a letter to the Administrator, CASAC stated: “there is adequate 

information to justify the use of a concentration-based form averaged over 3 years” (Samet 2009, 

p. 16). Moreover, when considering 98th vs. 99th percentile forms, CASAC encouraged EPA to 

consider analyses in the REA (and perhaps additional analyses) with respect to the number of 

days per year 98th vs. 99th percentile forms would allow SO2 concentrations to exceed the 

selected level. CASAC also encouraged EPA to consider analyses such as those presented above 

with respect to the number exceedences of 5-minute benchmarks given 98th vs. 99th percentile 

forms at a given standard level (Samet 2009).

c. Administrator’s conclusions on form

When considering alternative forms, the Administrator notes and agrees with the views 

expressed in the REA and the recommendations from CASAC, as described above. In particular, 

she agrees that the standard should use a concentration-based form averaged over three years in 

order to give due weight to years when 1-hour SO2 concentrations are well above the level of the 

standard, than to years when 1-hour SO2 concentrations are just above the level of the standard. 

The Administrator agrees further, for the reasons given above, that a 99th percentile (or 4th
90

highest) form could be appreciably more protective than a 98th (or 7th or 8th highest) form, and 

thus, should be utilized. Given these considerations, and in light of the specific range proposed 

for level below, the Administrator proposes to adopt either a 99th percentile or a 4th highest form, 

averaged over 3 years. 

4. Level

In assessing the level of a one-hour standard with either a 99th percentile or 4th highest 

average form (averaged over three years in either case) to propose, the Administrator has 

considered the broad range of scientific evidence assessed in the ISA, including the 

epidemiologic studies and controlled human exposure studies, as well as the results of air quality, 

exposure, and risk analyses presented in the REA. In light of this body of evidence and 

analyses, the Administrator reiterates that it is necessary to provide increased public health 

protection for at-risk populations against an array of adverse respiratory health effects related to 

short-term (i.e., 5 minutes to 24 hours) exposures to ambient SO2. In considering the most 

appropriate way to provide this protection, the Administrator is mindful of the extent to which 

the available evidence and analyses can inform a decision on the level of a standard. 

Specifically, the range of proposed standard levels discussed below is informed by

epidemiologic and controlled human exposure studies. 

a. Evidence-based considerations

Evidence-based considerations take into account the full body of scientific evidence 

assessed in the ISA. When considering the extent to which this scientific evidence can inform a 

decision on the level of a 1-hour standard, it is important to note that SO2 concentrations 

represent different measures of exposure when drawn from experimental versus epidemiologic 

studies. Concentrations of SO2 tested in experimental studies, such as controlled human 
91

exposure studies, represent exposure concentrations in the breathing zone of the individual test 

subjects. In cases where controlled human exposure studies report effects, those effects are 

caused directly by exposure to a specified concentration of SO2. In contrast, concentrations of 

SO2 drawn from epidemiologic studies are often based on ambient monitoring data. SO2

concentrations recorded at these ambient monitors are used as surrogates for the distribution of 

SO2 exposures across the study area and over the time period of the study. 

Since the last review, there have been more than 50 peer reviewed epidemiologic studies 

published worldwide dealing with SO2 exposure and effects (see ISA Tables 5-4 and 5-5). 

Overall, the ISA concluded that these studies provide evidence of an association between 

ambient SO2 concentrations and respiratory symptoms, as well as ED visits and hospitalizations 

for all respiratory causes and asthma (ISA, section 3.1.4). Moreover, the ISA indicates that 

many of these epidemiologic studies have reported that children and older adults may be at 

increased risk for SO2- associated adverse respiratory effects (ISA, section 5.2). In assessing the 

extent to which these studies and their associated air quality information can inform the level of a 

new 99th percentile (see sections II.F.2 and II.F. 3) 1-hour daily maximum standard for the U.S., 

the REA considered U.S. and Canadian air quality information to be most relevant. EPA sent a 

request to the authors of U.S. and Canadian epidemiologic studies (studies were identified from 

Tables 5-4 and 5-5 of the ISA) for 99th (and 98th) percentile 1-hour daily maximum SO2

concentrations from the monitor recording the highest SO2 level in the location and time period

corresponding to their studies (see Thompson and Stewart (2009)). Air quality information was 

received from authors of both U.S. and Canadian studies; however, as noted in the REA (REA,

section 5.5), SO2 concentrations reported for Canadian studies are not directly comparable to

those reported for studies in the U.S. because SO2 levels reported for Canadian analyses
92

represent the average 1-hour daily maximum level across multiple monitors in a given city (see 

REA Figure 5-5), rather than the concentration from the single monitor that recorded the highest 

SO2 concentration (see Thompson and Stewart, 2009). Thus, the REA noted that SO2

concentrations associated with Canadian studies would be relatively lower (potentially

significantly lower) than those levels presented for U.S. epidemiologic studies, and therefore the 

REA focused on 99th percentile air quality information from U.S. studies for informing potential 

1-hour standard levels.

Figures 1 to 4 present 99th (and 98th) percentile 1-hour daily maximum SO2

concentrations from ten U.S. epidemiologic studies (some of which were conducted in multiple 

cities) of ED visits and hospital admissions25 (Figures 5-1 to 5-4 in the REA). The REA noted 

that this information provides evidence for effects in cities with particular 99th percentile 1-hour 

SO2 levels, and hence, was of particular relevance for identifying standard levels that could 

protect against the SO2 concentrations observed in these studies. The air quality information 

presented in these figures generally shows that positive associations between ambient SO2

concentrations and ED visit and hospitalizations have been reported in cities where 99th

percentile 1-hour daily maximum SO2 concentrations ranged from approximately 50 – 460 ppb. 

More specifically, seven of these studies were in cities where 99th percentile 1-hour daily 

maximum SO2 concentrations ranged from approximately 75 -150 ppb. Among these

epidemiologic studies in the range of 75 – 150 ppb, there is a cluster of three studies reporting

statistically significant results in multi-pollutant models with PM. Specifically, in epidemiologic 

studies conducted in the Bronx, NY (NYDOH 2006,) and in NYC, NY (Ito et al., 2007), the SO2

 

25 In some cases, U.S. authors provided the AQS monitor IDs used in their studies and the statistics from the highest 

reporting monitor were calculated by EPA. In cases where U.S. authors were unable to provide the requested data 

(Schwartz 1995, Schwartz 1996, and Jaffe 2003), EPA identified the maximum reporting monitor from all monitors 

located in the study area and calculated the 98th and 99th percentile statistics (see Thompson and Stewart 2009). 

Results presented from study locations for which effect estimates were reported. 
93

effect estimate remained positive and statistically significant in multi-pollutant models with 

PM2.5 in these locations when 99th percentile 1-hour daily maximum SO2 levels were 78 and 82 

ppb respectively. (ISA, Table 5-5). Moreover, in an epidemiologic study conducted in New 

Haven, CT (Schwartz et al., 1995), the SO2 effect estimate remained positive and statistically

significant in a multi-pollutant model with PM10 in this location when the 99th percentile 1-hour 

daily maximum SO2 concentration was 150 ppb. The REA noted that although statistical 

significance in co-pollutant models is an important consideration, it is not necessary for 

appropriate consideration of and reliance on such epidemiologic evidence.26 However, as noted 

earlier, there is special sensitivity in this review in disentangling PM-related effects (especially 

sulfate PM) from SO2-related effects in interpreting the epidemiologic studies; thus, these studies 

are of particular relevance here, lending strong support both to the conclusion that SO2 effects 

are generally independent of PM (ISA, section 5.2) and that these independent adverse effects of 

SO2 have occurred in cities with 1-hour daily maximum, 99th percentile concentrations in the 

range of 78 - 150 ppb. 

In addition to the study locations where SO2 concentrations ranged from 75 - 150 ppb, the 

REA noted that two epidemiologic studies included cities reporting positive associations 

between ambient SO2 levels and ED visits when 99th percentile 1-hour daily maximum SO2

concentrations were approximately 50 ppb (Wilson et al., (2005) in Portland, ME and Jaffe et al., 

(2003) in Columbus, OH). These studies reported generally positive and sometimes statistically

significant results using single pollutant models (Figures 1 and 2), and did not evaluate potential 

confounding through the use of multi-pollutant models. Nonetheless, these studies provide 

 

26 For example, evidence of a pattern of results from a group of studies that find effect estimates similar in direction 

and magnitude would warrant consideration of and reliance on such studies even if the studies did not all report 

statistically significant associations in single- or multi-pollutant models. The SO2 epidemiologic studies fit this 

pattern, and are buttressed further by the results of the clinical studies. ISA, section 5.2.
94

limited evidence of an association between ED visits and 99th percentile 1-hour daily maximum 

SO2 concentrations in locations where SO2 levels were approximately 50 ppb. Finally, the REA 

noted that studies conducted in Cleveland and Cincinnati, OH (Schwartz et., al. 1996 and Jaffe 

et., al. 2003) reported positive associations between ambient SO2 levels and ED visits and 

hospital admissions when 99th percentile 1-hour daily maximum SO2 concentrations in these 

cities ranged from 170 – 457 ppb (REA, section 5.5). The REA found the SO2 level in

Cincinnati (Jaffe et., al. 2003; REA section 5.5) to be of particular concern. The 99th percentile 

1-hour daily maximum SO2 level in Cincinnati was > 400 ppb (Figure 2), which in 5-10 minute 

controlled human exposure studies, was an SO2 concentration range consistently shown to result 

in clearly adverse health effects in exercising asthmatics (i.e., decrements in lung function 

accompanied by respiratory symptoms). 

Taken together, the epidemiologic evidence described above suggests that standard levels 

at and below 75 ppb should be considered to limit SO2 concentrations such that the upper end of

the distribution of daily maximum hourly concentrations would likely be below that observed in 

most of these U.S. studies. Notably, a standard at or below 75 ppb would be lower than the SO2

air quality levels found in the cluster of three epidemiologic studies finding statistically

significant effects in multi-pollutant models with PM (i.e., 99th percentile 1-hour daily maximum 

SO2 concentrations ≥ 78 ppb). Moreover, standard levels at or below 75 ppb recognize the 

limited evidence from two epidemiologic studies reporting mostly positive and sometimes 

statistically significant associations in single pollutant models when 99th percentile 1-hour daily

maximum SO2 concentrations were approximately 50 ppb (Wilson et al., (2005) in Portland, ME 

and Jaffe et al., (2003) in Columbus, OH; see Figures 1 and 2). Judgments about the weight to 

place on uncertainties inherent in such studies should also inform selection of a specific standard 
95

level.

-15

-10

-5

0

5

10

15

20

25

30

Percent Excess Risk

Wilson 2005

Manchester

1-hr 99: 69

1-hr 98: 59

Wilson 2005

Portland

1-hr 99: 47

1-hr 98: 36

Peel 2005

Atlanta

1-hr 99: 81

1-hr 98: 70

Tolbert 2007

Atlanta

1-hr 99: 76

1-hr 98: 62

EDRA

EDRA

EDRA

EDRA

EDRC

EDRC

EDRW

EDRW

EDRE

EDRE

24-hour effect estimates 1-hour effect estimates

Legend:

EDRA: ED visits for all respiratory causes- all ages 

EDRC: ED visits for all respiratory causes- children

EDRW: ED visits for all respiratory causes- ages 15-64

EDRE: ED visits for all respiratory causes- ages 65+

-15

-10

-5

0

5

10

15

20

25

30

Percent Excess Risk

Wilson 2005

Manchester

1-hr 99: 69

1-hr 98: 59

Wilson 2005

Portland

1-hr 99: 47

1-hr 98: 36

Peel 2005

Atlanta

1-hr 99: 81

1-hr 98: 70

Tolbert 2007

Atlanta

1-hr 99: 76

1-hr 98: 62

EDRA

EDRA

EDRA

EDRA

EDRC

EDRC

EDRW

EDRW

EDRE

EDRE

24-hour effect estimates 1-hour effect estimates

Legend:

EDRA: ED visits for all respiratory causes- all ages 

EDRC: ED visits for all respiratory causes- children

EDRW: ED visits for all respiratory causes- ages 15-64

EDRE: ED visits for all respiratory causes- ages 65+

Figure 1. Effect estimates for U.S. all respiratory ED visit studies and associated 98th and 99th

percentile 1-hour daily maximum SO2 levels. 
96

Legend:

EDAA: ED visits for asthma- all ages 

EDAC: ED visits for asthma- children

EDAW: ED visits for asthma- ages 15-64

EDAE: ED visits for asthma- ages 65+

EDAJ: ED visits for asthma- ages 5-34

-40

-20

0

20

40

60

80

100

Percent Excess Risk

Manchester

1-hr 99: 69

1-hr 98: 59

Portland

1-hr 99: 47

1-hr 98: 36

NYDOH 2006

Manhattan

1-hr 99: 80

1-hr 98: 62

Bronx

1-hr 99: 78 

1-hr 98: 65

Jaffe 2003

New York

1-hr 99: 82

1-hr 98: 71

Columbus

1-hr 99: 51

1-hr 98: 42

Cleveland

1-hr 99: 211

1-hr 98: 175

Cincinnati

1-hr 99: 457

1-hr 98: 401

EDAA

EDAA

EDAA

EDAA

EDAC

EDAC

EDAA

EDAW

EDAW

EDAE

EDAE EDAJ

EDAJ

EDAJ

Wilson 2005 Wilson 2005 Ito 2007

24-hour effect estimates

Figure 2. 24-hour effect estimates for U.S. asthma ED visit studies and associated 98th and 99th

percentile 1-hour daily maximum SO2 levels. 
97

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

Percent Excess Risk

NYDOH 2006

Manhattan

1-hr 99:80

1-hr 98: 62

Bronx

1-hr 99: 78 

1-hr 98: 65

Peel 2005

Atlanta

1-hr 99: 81

1-hr 98: 70

Legend:

EDAA: ED visits for asthma- all ages 

EDAA

EDAA

EDAA

1-hour effect estimates

Figure 3. 1-hour effect estimates for U.S. asthma ED visit studies and associated 98th and 99th

percentile 1-hour daily maximum SO2 levels.
98

-10

-5

0

5

10

15

20

25

30

35

Percent Excess Risk

Schwartz 1995 Schwartz 1996 Sheppard 2003 Lin 2004

Tacoma

1-hr 99: 100

1-hr 98: 89

New Haven

1-hr 99: 150

1-hr 98: 126

Cleveland

1-hr 99: 170

1-hr 98: 150

Seattle

1-hr 99: 84

1-hr 98: 70

Bronx

1-hr 99:119

1-hr 98: 93

Legend:

HARE: Hospital admissions for all respiratory causes- ages 65+ 

HAAS: Hospital admissions for asthma- ages <65

HAAL: Hospital admissions for asthma- ages 0-14

HAAS

HARE

HARE

HARE

HAAL

24-hour effect estimates

-10

-5

0

5

10

15

20

25

30

35

Percent Excess Risk

Schwartz 1995 Schwartz 1996 Sheppard 2003 Lin 2004

Tacoma

1-hr 99: 100

1-hr 98: 89

New Haven

1-hr 99: 150

1-hr 98: 126

Cleveland

1-hr 99: 170

1-hr 98: 150

Seattle

1-hr 99: 84

1-hr 98: 70

Bronx

1-hr 99:119

1-hr 98: 93

Legend:

HARE: Hospital admissions for all respiratory causes- ages 65+ 

HAAS: Hospital admissions for asthma- ages <65

HAAL: Hospital admissions for asthma- ages 0-14

HAAS

HARE

HARE

HARE

HAAL

24-hour effect estimates

Figure 4. 24-hour effect estimates for U.S. hospitalization studies and associated 98th and 99th

percentile 1-hour daily maximum SO2 levels.

27

The REA also considered findings from controlled human exposure studies when 

evaluating potential alternative standard levels. The ISA found that the most consistent evidence 

of decrements in lung function and/or respiratory symptoms was from controlled human 

exposure studies exposing exercising asthmatics to SO2 concentrations ≥ 400 ppb for 5 - 10 

minute durations (ISA, section 3.1.3.5). As previously mentioned, at SO2 concentrations ranging 

from 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 subjects experience severe decrements in lung function. Moreover, at concentrations ≥ 400 

 

27 There were no U.S. hospitalization studies with 1-hour effect estimates identified in Table 5-5 of the ISA
99

ppb, decrements in lung function are often statistically significant at the group mean level, and

are frequently accompanied by respiratory symptoms (ISA, Table 5-1).

Controlled human exposure studies have also demonstrated decrements in lung function 

in exercising asthmatics following 5 - 10 minute SO2 exposures starting as low as 200 - 300 ppb 

in free-breathing chamber studies. At concentrations ranging from 200 - 300 ppb, the lowest 

levels tested in free breathing chamber studies, approximately 5 - 30% percent of exercising 

asthmatics are likely to experience moderate or greater decrements in lung function in these 

studies. Moreover, although these individuals experienced lung function decrements, they were 

not frequently accompanied by respiratory symptoms and at these SO2 concentrations, group 

mean changes in lung function have not been shown to be statistically significant. However, the 

ISA and REA noted that for evident ethical reasons, the subjects participating in the controlled 

human exposure studies described above do not include the most severe asthmatics. Thus, the 

REA found it is reasonable to anticipate that individuals who are more sensitive to SO2 would 

have a greater response at 200 - 300 ppb SO2, and/or would respond to SO2 concentrations even 

lower than 200 ppb (REA, section 10.5.4). Similarly, the REA noted that there is no evidence to 

suggest that 200 ppb represents a threshold below which no adverse respiratory effects occur

(REA, section 10.5.4). In fact, limited evidence from two mouthpiece exposure studies suggests 

that exposure to 100 ppb SO2 can result in small decrements in lung function.

28

 

Considering the controlled human exposure evidence presented above, the ISA concluded 

that as SO2 concentrations increase the percentage of asthmatics affected increases as does the 

severity of the response. Moreover, as previously noted, effects associated with SO2

concentrations ≥ 400 ppb are clearly considered adverse effects of air pollution under ATS 

 

28 Although not directly comparable to free-breathing chamber studies, findings from these mouthpiece studies may 

be particularly relevant to those asthmatics who breathe oronasally even at rest (EPA, 1994b).
100

guidelines, while effects at 200- 300 ppb were considered adverse in the REA based on 

interpretation of ATS guidelines, CASAC recommendations, and previous conclusions from 

comparable situations in other NAAQS reviews (see section II.B.1.c). Taken together, the REA 

concluded that the level of a new 99th percentile 1-hour daily maximum standard should provide 

substantial protection against SO2 concentrations ≥ 400 ppb, and appreciable protection against 

5-minute SO2 concentrations ≥ 200 ppb (REA, section 10.5.4).

b. Air quality, exposure and risk-based considerations

In evaluating the extent to which 99th percentile 1-hour daily maximum alternative 

standard levels limit 5-minute SO2 concentrations ≥ 400 and ≥ 200 ppb, the REA first considered 

key results of the air quality analysis. As previously noted, the results generated from the air 

quality analysis were from 40 counties and considered a broad characterization of national air 

quality and human exposures that might be associated with these 5-minute SO2 concentrations

(see section II.C). However, there is uncertainty associated with the assumption that SO2 air 

quality measured at fixed site monitors can serve as an adequate surrogate for total exposure to 

ambient SO2. Actual exposures might be influenced by factors not considered in this analysis

including small scale spatial variability in ambient SO2 concentrations (which might not be 

captured by the network of fixed-site ambient monitors) and spatial/temporal variability in 

human activity patterns. 

 Table 2 reports the maximum mean number of days per year 5-minute daily maximum 

SO2 levels would be expected to exceed a given 5-minute potential health effect benchmark level 

in any of the 40 counties included in the air quality analysis, given air quality simulated to just

meet the current, and potential alternative 99th percentile 1-hour daily maximum standards 

analyzed in the REA. In addition, although not directly analyzed in the REA, these tables 
101

include air quality results given a 99th percentile 1-hour daily maximum standard at 75 ppb; this 

concentration was included in these tables because as mentioned above, the epidemiologic 

evidence suggested consideration of a standard level at or below 75 ppb.

29

 Table 2 shows that at 

standard levels ranging from 50 – 100 ppb, there would be at most two days per year when 

statistically estimated 5-minute SO2 concentrations in these counties exceed the 400 ppb 

benchmark, while at standard levels of 150 ppb and above there is a marked increase in the 

maximum number of days per year the 400 ppb benchmark is exceeded. Similar trends are seen 

with respect to the 300 ppb benchmark level. With respect to the 200 and 100 ppb benchmarks, 

the 50 ppb standard is clearly the most effective at limiting these 5-minute SO2 concentrations. 

However, compared to standards at 150 ppb and above, standards in the range of 75 - 100 ppb 

would allow considerably less exceedence of the 200 and 100 ppb benchmarks. Additional and 

more detailed results from the air quality analysis can be found in chapter 7 of the REA.

Table 2. Maximum mean number of days per year in any of the 

counties included in the air quality analysis when 5-minute 

daily maximum SO2 concentrations exceed the 100, 200, 300, and 

400 ppb potential health effect benchmark values given air 

quality adjusted to just meet the current standards, or 

alternative 99th percentile 1-hour daily maximum standards.

Exposure 

Benchmarks

(5-minute 

exposures)

Air Quality Scenarios

Just 

Meeting 

Current 

Standards

99th percentile 1-hour Daily Maximum Standards

50 ppb 75 ppb 100 ppb 150 ppb 200 ppb 250 ppb

400 ppb 102 0 (0 - 2) 2 7 13 18

300 ppb 130 0 (0 - 5) 5 13 20 27

200 ppb 171 2 (2 - 13) 13 24 42 69

100 ppb 234 13 (13- 43) 43 93 133 180

While the air quality analysis results presented in Table 2 used estimated 5-minute SO2

concentrations as a surrogate for exposure, the results from the exposure analysis considered the 

 

29

 Air quality, exposure, and risk numbers reported in Chapter 10 of the REA for a 75 ppb standard were bound by 

the estimates from air quality adjusted to just meet 99th percentile 1-hour daily maximum standards at 50 and 100 

ppb. 
102

likelihood that an asthmatic at elevated ventilation rate would come into contact with a 5-minute 

SO2 concentration at or above a given benchmark level one or more times per year. As 

previously noted, this resource intensive analysis was performed for St. Louis and Greene 

County, MO, but results from the St. Louis analysis were found to be more informative with 

respect to informing standard levels given that the St. Louis results: 1) suggested that the current 

standards were not adequate to protect public health; and 2) likely provide useful insights into 

exposures and risk for other urban areas in the U.S. with similar population and SO2 emissions 

density (i.e., areas where SO2 exposures are more likely). 

Table 3 reports the estimated percent of asthmatic children at moderate or greater 

exertion in St. Louis, that would be expected to experience at least one SO2 exposure per year, at 

or above a health effect benchmark level in scenarios in which air quality was adjusted to meet 

the current, and alternative 99th percentile 1-hour daily maximum standards. This analysis 

estimates that standard levels ranging from 50 - 100 ppb would protect > 99% of asthmatic 

children, at moderate or greater exertion, from experiencing at least one SO2 exposure ≥ 400 ppb

per year30. Similarly, a standard at 150 ppb is estimated to protect ~ 99% of asthmatic children 

at moderate or greater exertion from experiencing at least one SO2 exposure ≥ 400 ppb. 

Compared to standards ranging from 50 -150 ppb, standards at 200 and 250 ppb are estimated to 

allow appreciably more exposures ≥ 400 ppb (Table 3). With respect to the 300 ppb benchmark, 

standards at 50, 75, and 100 ppb provide similar protection, while there is a marked increase in 

exposures of asthmatic children at moderate or greater exertion at standard levels ≥ 150 ppb

(Table 3). Considering the 200 ppb benchmark level, it is estimated that 1-hour standard levels 

 

30 Table 3 reports that given a 99th percentile 1-hour daily maximum standard in the range of 50 – 100 ppb, <1% of 

asthmatic children at moderate or greater exertion would be estimated to experience an SO2 exposure ≥ 400 ppb, 

hence it can be stated that this range of levels would protect > 99% of asthmatic children at moderate or greater 

exertion from experiencing at least one SO2 exposure ≥ 400 ppb per year. 
103

ranging from 50 – 100 ppb limit 5-minute SO2 exposures ≥ 200 ppb considerably more than 1-

hour standard levels ≥ 150 ppb. More specifically, standards in the range of 50 – 100 ppb are 

estimated to protect approximately 97 to > 99% of asthmatic children at moderate or greater 

exertion from experiencing at least one 5-minute exposure ≥ 200 ppb per year, while standards 

ranging from 150 – 250 ppb are estimated to protect approximately 60 to 88% of these children

from experiencing at least one 5-minute SO2 exposure ≥ 200 ppb per year. Finally, similar to the 

air quality analysis, a standard at 50 ppb is clearly most effective at limiting 5-minute SO2

exposures ≥ 100 ppb. Additional and more detailed results from the exposure assessment can be 

found in chapter 8 of the REA.

Table 3. Estimated percent of asthmatic children in St. Louis 

at moderate or greater exertion expected to experience at least 

one 5-minute exposure above the 100, 200, 300, and 400 ppb 

potential health effect benchmark levels given air quality 

adjusted to just meet the current standards, or alternative 99th

percentile 1-hour daily maximum standards.

Exposure 

Benchmarks

(5-minute 

exposures)

Air Quality Scenarios

Just 

Meeting 

Current 

Standards

99th percentile 1-hour Daily Maximum Standards

50 ppb 75 ppb 100 ppb 150 ppb 200 ppb 250 ppb

400 ppb 24% < 1% < 1% < 1% ~1% 2.7% 6.3%

300 ppb 43.8% < 1% < 1% < 1% 2.7% 8% 16%

200 ppb 73.1% < 1% (~1 to 

2.7%) 2.7% 11.6% 24.5% 40%

100 ppb 96.7% 2.7% (2.7 to 

24.5%) 24.5% 54.5% 73.6% 84.8%

In evaluating the extent to which alternative standard levels provide protection against the 

health effects associated with 5-minute SO2 exposures, the REA also considered key results from 

the quantitative risk assessment (REA, chapter 9). Table 4 presents the percent of exposed 

asthmatic children at moderate or greater exertion in St. Louis expected to experience at least one 

moderate or greater lung function response per year, in terms of sRaw, given the 99th percentile 
104

1-hour daily maximum standards analyzed in the REA. Results presented in Table 4 show that 

standard levels in the range of 100 to 150 ppb would generally be expected to protect 

approximately 95 to 98% of exposed asthmatic children at moderate or greater exertion from 

experiencing at least one ≥ 100% increase in sRaw per year, while standards around and below 

75 ppb would be estimated to provide exposed asthmatic children with protection approaching 

99% or greater. Additional and more detailed risk analyses can be found in chapter 9 of the 

REA.

Table 4. Estimated percent of asthmatic children in St. Louis at 

moderate or greater exertion expected to experience a ≥ 100% 

increase in sRaw given air quality adjusted to just meet either 

the current standards, or alternative 99th percentile 1-hour 

daily maximum standards. 

Air Quality Scenarios

Just 

Meeting 

Current 

Standards

99th percentile 1-hour Daily Maximum Standards

50 ppb 75 ppb 100 ppb 150 ppb 200 ppb 250 ppb

19.1 -

19.2%

0.4 -

0.9%

(0.4 -

2.9%)

2.1 -

2.9%

4.6 -

5.4%

7.4 –

 8.1%

10.4 -

10.9%

c. Observations based on evidence and risk-based considerations

The policy assessment chapter of the REA considered the scientific evidence and the air 

quality, exposure, and risk information as they relate to considering alternative 1-hour SO2

standards that could be judged to be requisite to protect public health with an adequate margin of 

safety. This evidence and information supports the following conclusions:

 Given the U.S. epidemiologic evidence and their associated air quality levels (see 

Figures 1-4), 99th percentile 1-hour standard levels at and below 75 ppb should be 

considered to limit SO2 concentrations such that the upper end of the distribution 

of daily maximum hourly concentrations would likely be below that observed in 

most of the U.S. studies. Judgments about the weight to place on uncertainties 
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inherent in such studies should also inform selection of a specific standard level.

 Based on the air quality and exposure results, 1-hour standard levels in the range 

of 50 - 100 ppb should be considered to substantially limit 5-minute SO2

concentrations ≥ 400 ppb and appreciably limit 5-minute SO2 concentrations ≥ 

200 ppb.

 Based on the air quality and exposure results, compared to a 1-hour standard in 

the range of 50 - 100 ppb, a 1-hour standard level at 150 ppb would be expected 

similarly limit 5-minute SO2 concentrations ≥ 400 ppb, but would limit 5-minute 

SO2 concentrations ≥ 200 ppb considerably less.

 If relatively more weight is placed on certain types of uncertainties in the 

epidemiologic and controlled human exposure evidence, levels up to 150 ppb 

could be considered, recognizing the questions as to the adequacy of protection 

that would be raised by levels at the higher end of this range.

 Placing relatively more weight on the consideration that participants in controlled 

human exposure studies do not include the most severe asthmatics would add 

support to considering standard levels down to 50 ppb. 

d. CASAC views

CASAC expressed their views on potential levels for a standard in a letter to the EPA 

Administrator (Samet, 2009) within the context of their review of the 2nd draft REA, which also 

contained the draft policy assessment chapter. In drawing conclusions regarding the level of a 

short-term standard, CASAC considered the scientific evidence evaluated in the ISA, the air 

quality, exposure, and risk results presented in the 2

nd draft REA, and the evidence- and riskbased considerations presented in the policy assessment chapter of the 2

nd draft REA. CASAC 
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concurred with the conclusion from the policy assessment chapter for a range of standard levels 

beginning at 50 ppb: “[that chapter 10] clearly provides sufficient rationale for the range of 

levels beginning at a lower limit of 50 ppb” (Samet 2009, p. 16). For instance, CASAC has 

previously indicated that EPA should consider in its analyses the uncertainty that asthmatics 

participating in controlled human exposure studies do not represent the most SO2 sensitive 

asthmatics (Henderson 2008 p. 6). With respect to the upper end of the range, CASAC stated, 

“an upper limit of 150 ppb posited in Chapter 10 could be justified under some interpretations of 

weight of evidence, uncertainties, and policy choices regarding margin of safety,” (Samet 2009, 

p. 16) although the letter did not provide any indication of what interpretations, uncertainties, or 

policy choices might support selection of a level as high as 150 ppb. Further, CASAC stated that 

“the draft REA appropriately implies that levels greater than 150 ppb are not adequately 

supported” (id). Moreover, CASAC stated that: “the panel agrees that the posited range of 50 to 

150 ppb and the exposition of factors to consider when comparing values within the range are 

appropriately conveyed (Samet 2009, p. 16).”

e. Administrator’s conclusions on level for a 1-hour standard

As discussed above, in sections II.F.2 and II.F.3, the Administrator has proposed setting a 

1-hour standard with a 99th percentile form. For the reasons discussed below, the Administrator 

proposes to set a level for a new 99th percentile 1-hour daily maximum primary SO2 standard 

within the range from 50 to 100 ppb. In reaching this proposed decision, the Administrator has 

considered: 1) the evidence-based considerations from the final ISA and the final REA; 2) the 

results of the air quality, exposure, and risk assessments discussed above and in the final REA; 3) 

CASAC advice and recommendations on both the ISA and REA discussed above and provided 

in CASAC’s letters to the Administrator; and 4) public comments received on the first and 
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second drafts of the ISA and REA. In considering what level of a 1-hour SO2 standard is 

requisite to protect public health with an adequate margin of safety, the Administrator is mindful 

that this choice requires judgments based on an interpretation of the evidence and other 

information that neither overstates nor understates the strength and limitations of that evidence 

and information.

The Administrator notes that the most direct evidence of respiratory effects from 

exposure to SO2 comes from the controlled human exposure studies. These studies exposed 

groups of exercising asthmatics to defined concentrations of SO2 for 5 - 10 minutes and found 

adverse respiratory effects. As discussed above, SO2 exposure levels which resulted in 

respiratory effects in controlled human exposure studies were used in the REA as 5-minute 

benchmark exposures of potential concern. With respect to these 5-minute benchmarks, the 

Administrator focused on exceedences of the 400 and 200 ppb benchmarks. She notes that under 

ATS guidelines (ATS 1985, 2000) exposure to 5-10 minute SO2 concentrations ≥ 400 ppb results 

in health effects which are clearly adverse: moderate or greater decrements in lung function (in

terms of FEV1 or sRaw31) that are frequently accompanied by respiratory symptoms32. 

The Administrator also focused on exceedences of the 200 ppb benchmark, the lowest 

SO2 concentration tested in free-breathing chamber studies. In these studies, moderate or greater 

decrements in lung function occurred in approximately 5 to 30% of exercising asthmatics, 

depending on the study. The Administrator further notes that while concentrations as low as 200 

ppb have not been frequently accompanied by respiratory symptoms, she considers these effects 

 

31 Decreases of 10 – 20% in FEV1 (forced expiratory volume) and/or 100 - 200% increases in sRaw (specific airway 

resistance) are defined as moderate decrements in lung function. 

32 The ISA concluded that collective evidence from controlled human exposure studies considered in the previous 

review, along with a limited number of new controlled human exposure studies, consistently indicates that with 

elevated ventilation rates a large percentage of asthmatic individuals tested in a given chamber study (up to 60%, 

depending on the study) experience moderate or greater decrements in lung function, frequently accompanied by 

respiratory symptoms, following peak exposures to SO2 at concentrations of 0.4-0.6 ppm. (ISA, p 3-9). 
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to be adverse in light of CASAC advice and ATS guidelines. The REA concluded that these 

controlled human exposure studies could reasonably be interpreted to indicate an SO2-induced 

shift in lung function for this population of asthmatics (REA, section 4.3), such that asthmatics 

would have diminished reserve lung function and would be at greater risk if affected by another 

respiratory agent (e.g., viral infection). Importantly, diminished reserve lung function in a 

population that is attributable to air pollution is an adverse effect under ATS guidelines as 

discussed in section II.B.1.c.

As discussed below, the Administrator also considered the results of the air quality, 

exposure, and risk analyses, as they serve to estimate the extent to which a given 1-hour standard 

limits peaks of SO2 above the 5-minute benchmark concentrations derived from controlled 

human exposure studies. In considering these results as they relate to limiting 5-minute SO2

concentrations ≥ 400 ppb and ≥ 200 ppb, and being mindful that more severe effects occur 

following 5-minute SO2 exposures ≥ 400 ppb, the Administrator finds the most support for 99th

percentile 1-hour daily maximum standard levels up to 100 ppb to protect against 5-minute SO2

exposures ≥ 200 ppb. She notes that the 40-county air quality analysis estimates that a 100 ppb 

1-hour standard would allow at most 2 days per year on average when estimated 5-minute daily 

maximum SO2 concentrations exceed the 400 ppb benchmark, and at most 13 days per year on 

average when 5-minute SO2 concentrations exceed the 200 ppb benchmark (Table 2). 

Furthermore, given a simulated 1-hour 100 ppb standard level, most counties in the air quality

analysis were estimated to experience 0 days per year on average when 5-minute SO2

concentrations exceed the 400 ppb benchmark and ≤ 3 days per year on average when 5-minute 

SO2 concentrations were estimated to exceed the 200 ppb benchmark (see REA, Tables 7-14 and 

7-12). 
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In addition, the St. Louis exposure analysis estimates that a 99th percentile 1-hour 

standard at a level of 100 ppb would likely protect > 99% of asthmatic children at moderate or 

greater exertion from experiencing at least one 5-minute exposure ≥ 400 ppb per year, and 

approximately 97% of asthmatic children at moderate or greater exertion from experiencing at 

least one exposure ≥ 200 ppb per year. In contrast, the Administrator notes that the St. Louis 

exposure analysis estimates a 99th percentile 1-hour daily maximum standard at a level of 150 

ppb would likely protect only about 88% of asthmatic children at moderate or greater exertion 

from experiencing at least one 5-minute exposure ≥ 200 ppb per year. Finally, the Administrator 

notes that the St. Louis risk assessment estimates that a 99th percentile 1-hour standard level at 

100 ppb would likely protect about 97- 98% of exposed asthmatic children from experiencing at 

least one moderate or greater lung function response (defined as a ≥ 100% increase in sRaw). 

Based on these considerations, she concludes that there is support for a 99th percentile 1-hour 

daily maximum standard level at or below 100 ppb to appreciably limit 5-minute exposures to 

SO2 above the 200 ppb benchmark level.

Turning to the epidemiologic evidence, the Administrator notes that epidemiologic 

studies have reported associations between more serious health outcomes (i.e. respiratory-related 

ED visits and hospitalizations) and ambient SO2 concentrations. Unlike the controlled human 

exposure studies however, results from epidemiologic studies can be complicated by the fact that 

SO2 is but one component of a complex mixture of pollutants in the ambient air. This 

uncertainty is addressed by the ISA which 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 5.2; p. 5-9).
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The Administrator also notes that in general, associations reported in epidemiologic 

analyses are not associated with a defined exposure level of a pollutant (unlike the controlled 

human exposure studies), but represent concentrations of a pollutant taken from ambient

monitoring data during the study period. These concentrations are used as surrogates for the 

distribution of pollutant exposures across the study area over the time period of the study. This 

introduces a degree of uncertainty in the interpretation of epidemiologic results in that it can be 

difficult to discern what part of the distribution of pollutant levels are likely most linked to the 

associations reported in epidemiologic analyses. 

With respect to SO2 specifically, the Administrator notes that adverse respiratory effects 

in epidemiologic studies are especially likely to occur at the upper end of the distribution of 

ambient SO2 concentrations. Although some epidemiologic studies reported a linear relationship 

across the entire range of SO2 concentrations, a few other studies found that the increase in SO2-

related respiratory health effects was observed at the upper end of the distribution of SO2

concentrations (ISA, section 5.3, p. 5-9). For example, an epidemiologic study conducted in 

Bronx, NY suggested an increased risk of asthma hospitalizations on the days with the highest 

SO2 concentrations (Lin et al., 2004). More specifically, these authors observed increased risk of 

asthma hospitalizations at SO2 concentrations somewhere between the 90th and 95th percentiles 

(ISA, section 4.1.2 and ISA, Figure 4-4). 

This epidemiologic evidence, though not independently sufficient to draw conclusions 

regarding causation, is consistent with, and informed by, the large body of controlled human 

exposure studies of exercising asthmatics exposed to short-term peak concentrations of SO2; 

these controlled human exposure studies provide the “definitive evidence” that short-term peak 

SO2 exposure is associated with respiratory morbidity (ISA, Section 5.3, page 5-8). These 
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studies consistently found moderate or greater decrements in lung function (i.e. ≥ 100% increase 

in sRaw and/or ≥ 15% decline in FEV1) and/or respiratory symptoms in exercising asthmatics

following 5 - 10 minute peak exposures to SO2. Discussing the possible relationship between 

effects observed in these controlled human exposure studies and the associations reported in the 

epidemiologic studies, the ISA stated: “it is possible that these associations [in the epidemiologic 

studies] are determined in large part by peak exposures within a 24-hour period” (ISA, section 

5.2 at p. 5-5). Similarly, the ISA stated that: “the effects of SO2 on respiratory symptoms, lung 

function, and airway inflammation observed in the human clinical studies using peak exposures 

further provides a basis for a progression of respiratory morbidity resulting in increased ED visits 

and hospital admissions” and makes the associations observed in the epidemiologic studies 

“biologica[lly] plausib[le]” (ISA, section 5.2 at p. 5-5). Thus, considered together, the 

epidemiologic and controlled human exposure evidence suggest that it is a reasonable approach

to move the air quality distribution lower in a manner that targets control of both hourly and 5-10 

minute peak SO2 exposures. 

For the reasons discussed above in section II.F.3, the Administrator has proposed a 99th percentile of the 1-

hour daily maximum concentration as an appropriate form.33 Moreover, as just discussed, there is support for the 

Agency’s view that adverse respiratory effects in epidemiologic studies are especially likely to occur at the upper 

end of the distribution of ambient SO2 concentrations. Therefore, the Administrator finds it reasonable to focus on 

limiting the 99th percentile SO2 levels reported in locations where positive associations were found in key 

epidemiologic studies. Adjusting the distribution of SO2 levels in this manner will target control of those hourly and 

5-10 minute peak SO2 concentrations that are of most concern. 

In considering the epidemiologic evidence with regard to level, the Administrator notes 

that there have been more than 50 peer reviewed epidemiologic studies evaluating SO2 published 

 

33 As previously discussed in section II.F.3, a 99th percentile form was proposed to: 1) minimize the number of days 

per year that an area could exceed the level of the standard and still attain the standard; 2) limit the prevalence of 5-

minute peaks of SO2; and 3) provide a stable regulatory target to prevent areas from frequently shifting in and out of 

attainment.
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worldwide (ISA, Tables 5-4 and 5-5). The Administrator finds that in assessing the extent to 

which these studies and their associated air quality information can inform the level of a new 99th

percentile 1-hour daily maximum standard, U.S. and Canadian air quality information is most 

relevant. As described in section II.F.4.a, SO2 concentrations reported for Canadian studies are

not directly comparable to those reported for U.S. studies. That is, concentrations reported for

Canadian analyses represent the average 99th percentile 1-hour daily maximum level across 

multiple monitors in a given city (REA Figure 5-5), rather than the concentration from the single 

monitor that recorded the highest SO2 level (see Thompson and Stewart, 2009). Thus, the 

Administrator focused on 99th percentile air quality information from U.S. studies for informing

potential 1-hour standard levels.

The Administrator notes that Figures 1 to 4 include 99th percentile 1-hour daily

maximum SO2 concentrations from ten U.S. epidemiologic studies of ED visits and hospital 

admissions (Figures 5-1 to 5-4 in the REA). The Administrator agrees with the REA finding that 

this information provides evidence of associations between ambient SO2 and ED visits and 

hospital admissions in cities with particular 99th percentile 1-hour SO2 levels. This information 

is relevant for identifying standard levels that could significantly limit SO2 concentrations so that 

the upper end of the distribution of daily maximum hourly concentrations would likely be below 

that observed in most of these studies. These figures report mostly positive, and sometimes 

statistically significant, associations between ambient SO2 concentrations and ED visit and 

hospital admissions in locations where 99th percentile 1-hour daily maximum SO2 levels ranged 

from 50 - 460 ppb. Moreover, within this broader range of SO2 concentrations, seven of these 

studies were in locations where the 99th percentile of the 1-hour daily maximum SO2

concentrations were in the range of 75 -150 ppb. The Administrator particularly notes the cluster 
113

of three epidemiologic studies between 78 - 150 ppb (for the 99th percentile of the 1-hour SO2

concentrations) where the SO2 effect estimate remained positive and statistically significant in 

multi-pollutant models with PM (NYDOH (2006), Ito et al., (2007), and Schwartz et. al, (1995)). 

The Administrator also notes the limited evidence from two epidemiologic studies employing 

single pollutant models that found mostly positive, and sometimes statistically significant,

associations between ambient SO2 and ED visits in locations where 1-hour SO2 concentrations 

were approximately 50 ppb (Figures 1 and 2). Based on the interpretation of the epidemiologic 

evidence discussed above, the Administrator concludes that this evidence provides support for 

consideration of a 99th percentile 1-hour daily maximum standard level at or below 75 ppb to 

limit SO2 concentrations such that the upper end of the distribution of daily maximum hourly

concentrations would likely be below that observed in most of the U.S. studies. The 

Administrator also recognizes that judgments about the weight to place on uncertainties inherent 

in such studies should inform selection of a specific standard level.

Based on the epidemiologic and controlled human exposure information presented above, 

the Administrator considered what range of standard levels would be requisite to protect public 

health, including the health of at-risk groups, with an adequate margin of safety that is sufficient 

but not more than necessary to achieve that result. The assessment of a standard level calls for 

consideration of both the degree of risk to public health at alternative levels of the standard as 

well as the certainty that such risk will occur at any specific level. Based on the information 

available in the ISA, there is no evidence-based bright line that indicates a single appropriate 

level. Moreover, given that a 1-hour averaging time is being used to control 5-minute peaks of 

SO2, the Administrator also recognizes that the results of the air quality, exposure, and risk 

analyses will have to be considered given that these analyses indicate the extent to which a 
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particular 99th percentile 1-hour daily maximum standard will likely limit 5-minute SO2 peaks of 

a given concentration. Thus, the combination of scientific evidence and air quality, exposure, 

and risk-based information needs to be considered as a whole in making this public health policy

judgment.

In selecting a level that would serve as an appropriate upper end for a range of levels to 

propose, the Administrator has considered a cautious approach to interpreting the available 

evidence and exposure/risk-based information – that is, an approach that places relatively more 

weight on those types of uncertainties and limitations in the information that would lead to 

placing less reliance on the results of the epidemiologic studies. This approach would tend to 

avoid potentially overestimating public health risks and the degree of protection likely to be 

associated with just meeting a particular standard level. This approach would place more weight 

in particular on uncertainties in epidemiologic evidence such as concerns related to exposure 

measurement error, the possible role of co-pollutants and effects modifiers, and interindividual 

differences in susceptibility to SO2-related effects.

In applying this approach, the Administrator has selected an upper end of a range of 

levels to propose at 100 ppb. The selection of this level focuses on the results of the controlled 

human exposure studies and is primarily based on the results of the air quality and exposure 

analyses which suggest that a 1-hour standard should be at or below 100 ppb to appreciably limit 

5-minute SO2 benchmark concentrations ≥ 200 ppb. That is, as mentioned above, the St. Louis 

exposure analysis indicates that a 1-hour standard at 100 ppb would still be estimated to protect 

about 97% of asthmatic children at moderate or greater exertion from experiencing at least one 

5-minute SO2 exposure ≥ 200 ppb. In contrast, the St. Louis exposure analysis estimates that a 

1-hour standard at 150 ppb would likely only protect about 88% of asthmatic children at 
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moderate or greater exertion from experiencing at least one 5-minute SO2 exposure ≥ 200 ppb.

In selecting a level that would serve as an appropriate lower end for a range of levels to 

propose, the Administrator has considered a precautionary approach to interpreting the available 

evidence and exposure/risk-based information – that is, an approach that places relatively more 

weight on the results of the epidemiological studies, as well as more weight on those types of 

uncertainties that may be associated with potentially underestimating health effects in the most 

sensitive populations. This approach would tend to avoid potentially underestimating public 

health risks and the degree of protection likely to be associated with just meeting a particular 

standard level. This approach would place more weight on the consideration that the participants 

in controlled human exposure studies did not include individuals with severe asthma.

In applying this approach, she has selected 50 ppb as the lower end of a range of levels to 

propose, which is consistent with CASAC’s advice. The selection of this level focuses in part on 

the epidemiologic evidence. With respect to the epidemiologic studies, seven of ten U.S. ED visit

and hospital admission studies reporting generally positive associations with ambient SO2 were 

conducted in locations where 99th percentile 1-hour daily maximum SO2 levels were about 75-

150 ppb, and three of these studies observed statistically significant positive associations 

between ambient SO2 and respiratory-related ED visits and hospitalizations in multi-pollutant 

models with PM (NYDOH (2006), Ito et al., (2007), and Schwartz et. al, (1995)). Further, the 

Administrator notes that a 99th percentile 1-hour daily maximum standard set at a level of 50 ppb 

is well below the 99th percentile 1-hour daily maximum SO2 concentrations reported in locations 

where these studies were conducted (i.e. well below 99th percentile 1-hour daily maximum SO2

levels of 78- 150 ppb). Finally, the Administrator notes that two epidemiologic studies reported 

generally positive associations between ambient SO2 and ED visits in cities when 99th percentile 
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1-hour daily maximum SO2 concentrations were approximately 50 ppb, but does not consider 

that evidence strong enough to set a lower standard level. 

In considering the results of the air quality and exposure analyses, the Administrator also 

notes that the 40-county air quality analysis estimates that a 99th percentile 1-hour daily 

maximum standard set at a level of 50 ppb would result in zero days per year when estimated 5-

minute SO2 concentrations exceed the 400 ppb 5-minute benchmark level and at most 2 days per 

year when modeled 5-minute SO2 concentrations exceed the 200 ppb 5-minute benchmark level. 

In addition, the St. Louis exposure analysis estimates that a 99th percentile 1-hour daily 

maximum standard set at a level of 50 ppb would likely protect > 99% of asthmatic children at 

moderate or greater exertion from experiencing at least one 5-minute exposure both ≥ 400 and >

200 ppb per year.

The Administrator thus proposes to set the level of a new 1-hour standard that would protect 

public health with an adequate margin of safety between 50 ppb and 100 ppb. In so doing, the 

Administrator is relying on reported findings from both epidemiologic and controlled human 

exposure studies, as well as the results of air quality and exposure analyses. The Administrator 

solicits comment on this proposed range of standard levels as well as on the approach she has 

used to identify the range. Specifically, the Administrator solicits comment on the following: 

 The weight she has placed on the epidemiologic evidence, the controlled human exposure 

evidence, and the air quality, exposure, and risk information, the benchmark used to 

select the proposed range, and the uncertainties associated with each of these. 

 The most appropriate level within this proposed range given the available scientific 

evidence, and air quality, exposure, and risk information, and the uncertainties associated 

with each.
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With regard to the proposed range of standard levels, the Administrator notes that the 

lower end of the proposed range is consistent with CASAC advice that there is clearly sufficient 

evidence for consideration of standard levels starting at 50 ppb (Samet 2009). With respect to 

the upper end of the proposed range, the Administrator notes that CASAC concluded that 

standards up to 150 ppb “could be justified under some interpretations of weight of evidence, 

uncertainties, and policy choices regarding margin of safety” (Samet 2009, p. 16), although the 

letter did not provide any indication of what interpretations, uncertainties, or policy choices 

might support selection of a level as high as 150 ppb.

In light of the range of levels included in CASAC’s advice, the Administrator solicits 

comment on setting a standard level above 100 ppb and up to 150 ppb. In so doing, the 

Administrator again recognizes that there are uncertainties with the scientific evidence, such as 

attributing effects reported in epidemiologic studies specifically to SO2 given the presence of cooccurring pollutants, especially PM, and the uncertainties associated with using ambient SO2

concentrations as a surrogate for exposure. Any comments should specifically address the 

cluster of epidemiologic studies that remained statistically significant in co-pollutant models 

with PM, two of which had 99th percentile levels appreciably lower than 150 ppb. Commenters

should also address the conclusion in the ISA that the respiratory effects seen in the 

epidemiologic studies are generally robust and independent of co-pollutants. In addition, the 

Administrator notes that compared to the proposed range of 50 - 100 ppb, a standard level as

high as 150 ppb would not comparably limit 5-minute SO2 exposures ≥ 200 ppb. She notes that 

the St. Louis exposure analysis estimates that a 150 ppb standard would protect approximately

88% of asthmatic children at moderate or greater exertion from experiencing at least one SO2

exposure ≥ 200 ppb per year (compared to > 99% and approximately 97% given standards at 50 
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and 100 ppb respectively; see Table 3). There are also questions as to whether a standard set at 

this level would provide an adequate margin of safety. Thus, with respect to considering 

whether it would be appropriate to set a standard level as high as 150 ppb, the Administrator 

invites comment on the extent to which it is appropriate to emphasize uncertainties with respect 

to the epidemiologic evidence. She also invites comment on the implications such 

considerations would have on limiting 5-minute SO2 exposures ≥ 200 ppb. 

5. Implications for retaining or revoking current standards

The REA recognized that the particular level selected for a new 1-hour daily maximum standard 

would have implications for reaching decisions on whether to retain or revoke the current 24-

hour and annual standards. That is, with respect to SO2-induced respiratory morbidity, the lower 

the level selected for a 99th percentile 1-hour daily maximum standard, the less additional public 

health protection the current standards would be expected to provide. As previously mentioned 

(see section II.E.3), CASAC expressed a similar view following their review of the 2nd draft 

REA: “assuming that EPA adopts a one hour standard in the range suggested, and if there is 

evidence showing that the short-term standard provides equivalent protection of public health in 

the long-term as the annual standard, the panel is supportive of the REA discussion of 

discontinuing the annual standard” (Samet 2009, p. 15). With regard to the current 24-hour

standard, CASAC was generally supportive of using the air quality analyses in the REA as a 

means of determining whether the current 24-hour standard was needed in addition to a new 1-

hour standard to protect public health. CASAC stated: “the evidence presented [in REA Table 

10-3] was convincing that some of the alternative one-hour standards could also adequately 

protect against exceedances of the current 24-hour standard” (Samet 2009, p. 15).

In accordance with the REA findings and CASAC recommendations mentioned above, the 
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Administrator notes that the 1-hour standards being proposed (i.e., 99th percentile 1-hour daily 

maximum SO2 standards at 50 – 100 ppb) would have the effect of maintaining 24-hour and 

annual SO2 concentrations generally well below the levels of the current 24-hour and annual 

NAAQS (see REA Tables 10-3 and 10-4 and REA Appendix Tables D-3 to D-6). Thus, if a new 

99th percentile 1-hour daily maximum standard is set in the proposed range of 50 – 100 ppb, than 

the Administrator proposes to revoke the current 24-hour and annual standards. However, if a 

standard is set at a level >100 ppb and up to 150 ppb, then the Administrator proposes to retain 

the existing 24-hour standard, recognizing that a 99th percentile 1-hour daily maximum standard 

at 150 ppb would not have the effect of maintaining 24-hour average SO2 concentrations below 

the level of the current 24-hour standard in all locations analyzed (see REA Appendix Table D4). However, the Administrator would revoke the current annual standard recognizing: 1) 99th

percentile 1-hour daily maximum standards in the range of 50 -150 ppb would maintain annual 

average SO2 concentrations below the level of the current annual standard (see REA Table 10-4 

and REA Appendix tables D-5 and D-6); and 2) the lack of sufficient evidence linking long-term 

SO2 exposure to adverse health effects. 
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G. Summary of proposed decisions on the primary standard 

For the reasons discussed above, and taking into account information and assessments 

presented in the ISA and REA as well as the advice and recommendations of CASAC, the 

Administrator proposes that the current 24-hour and annual standards are not requisite to protect 

public health with an adequate margin of safety. The Administrator proposes to establish a new 

1-hour standard that will afford increased protection for asthmatics and other at-risk populations 

against an array of adverse respiratory health effects related to short-term (5-minutes to 24-

hours) SO2 exposure. These effects include increased decrements in lung function (defined in 

terms of sRaw and FEV1), increases in respiratory symptoms, and related serious indicators of 

respiratory morbidity including emergency department visits and hospital admissions for 

respiratory causes.

Specifically, the Administrator proposes to set a new short-term primary SO2 standard

with a 1-hour (daily maximum) averaging time and a form defined as the 3-year average of the 

99th percentile or the 4th highest daily maximum concentration. The level for the new standard is 

proposed to be within the range of 50 - 100 ppb. The Administrator also solicits comment on 

levels as high as 150 ppb. In addition to setting a new 1-hour standard in the proposed rage of 50 

– 100 ppb, the Administrator proposes to revoke the current 24-hour and annual standards 

recognizing that a 1-hour standard set in the proposed range of 50 - 100 ppb will have the effect 

of generally maintaining 24-hour and annual SO2 concentrations well below the levels of the 

current 24-hour and annual standards. Moreover, the Administrator notes that there is little 

health evidence to support an annual standard for the purpose of protecting against health effects 

associated with long-term SO2 exposures.
121

III.Proposed Amendments to Ambient Monitoring and Reporting Requirements

EPA is proposing changes to the ambient air monitoring, reporting, and network design 

requirements for the SO2 NAAQS. This section discusses the changes we are proposing that are 

intended to support the proposed 1-hour NAAQS, and the possible retention of the existing 24-

hour NAAQS depending on the selected level of the 1-hour NAAQS, as described in Section II 

above. Ambient SO2 monitoring data are used to determine whether an area is in violation of the 

SO2 NAAQS. Ambient SO2 monitoring data are collected by state, local, and tribal monitoring 

agencies (“monitoring agencies”) in accordance with the monitoring requirements contained in 

40 CFR parts 50, 53, and 58. 

A. Monitoring Methods

To be used in a determination of compliance with the SO2 NAAQS, SO2 data must be 

collected using either a Federal Reference Method (FRM) or a Federal Equivalent Method 

(FEM) as defined in 40 CFR Parts 50 and 53. The current monitoring methods in use by most 

State and local monitoring agencies are FEM analyzers based on the ultraviolet fluorescence 

(UVF) measurement principle. These continuous analyzers were implemented into the SO2

monitoring networks in the early 1980s, and the current manual FRM for SO2 is no longer used 

for field monitoring. The current list of all approved FRMs and FEMs capable of providing 

ambient SO2 data for use in attainment designations may be found on the EPA website 

http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf. 

For reasons explained subsequently, EPA proposes to establish a new FRM for 

measuring SO2 in the ambient air. This proposed new FRM for SO2 would be an automated 

method based on UVF (the same type of analyzers now in widespread use), and it would be 
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specified in the form of a reference measurement principle and a calibration procedure. It would 

be in a new Appendix A-1 to 40 CFR Part 50. Analyzers approved as FRMs for SO2 after the 

effective date of the final rule would be subject to performance specifications and other 

requirements set forth in 40 CFR Part 53, under associated amendments proposed for Part 53. 

The existing FRM for SO2 (a wet-chemical, manual method) would be retained for some period 

of time, thereby permitting continued use of currently designated FEMs to avoid any disruption 

to existing SO2 monitoring networks. 

1. Background

FRMs, as set forth in several appendices to 40 CFR Part 50, serve either or both of two 

primary purposes. The first is to provide a specified, definitive methodology for routinely 

measuring concentrations of various ambient air pollutants for comparison to the NAAQS in Part 

50 and for other air monitoring objectives. The second is to provide a standard of comparison 

for determining equivalence to the specified reference method of alternative and perhaps more 

practical pollutant measurement methods (FEMs) that can be used in lieu of the FRM for routine 

monitoring. 

Some of the FRMs contained in appendices to Part 50 (such as the current SO2 FRM) are 

manual methods that are completely specified within their respective appendices. Others (such 

as the ozone FRM) are in the form of a measurement principle and associated calibration 

procedure that must be implemented in a commercial FRM analyzer model. Such FRM 

analyzers must be tested and shown to meet explicit performance and other requirements that are 

set forth in 40 CFR Part 53 (Ambient Air Monitoring Reference and Equivalent Methods). Each 

of these analyzer models is considered to be an FRM only upon specific designation as such by 

EPA under the provisions of Part 53. 
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From time to time, as pollutant measurement technology advances, the reference methods 

in these Part 50 appendices need to be assessed to determine if improved or more suitable 

measurement technology has become available to better meet current FRM needs as well as 

potential future FRM requirements. Such new technology can either be presented to EPA for 

evaluation by an FEM applicant under §53.16, or (as in this case) EPA can originate the process 

itself as provided in §53.7. If, after reviewing a new methodology, the Administrator determines 

that the new methodology is substantially superior, §53.16 of Part 53 provides for supersession 

of FRMs under these circumstances.

The FRM for measuring SO2 in the ambient air was promulgated on April 30, 1971 (36 

FR 8186), in conjunction with EPA’s establishment (originally as 42 CFR Part 410) of the first 

national ambient air quality standards (NAAQS) for six pollutants (including sulfur dioxide) as 

now set forth in 40 CFR Part 50. This SO2 FRM is specified in Appendix A of Part 50 and 

identified as the pararosaniline method. It is a manual, wet-chemical method requiring sample 

air to be bubbled through an absorbing reagent (tetrachloromecurate), which is then returned to a 

laboratory for chemical analysis. At the time of its promulgation, the method was considered the 

best available method and was in considerable use for monitoring SO2 in the air. However, 

newly developed automated continuous analyzers approved as FEMs rapidly supplanted use of 

this manual method for air monitoring in the U.S. By the 1990’s, the FRM was no longer used at 

all in domestic air monitoring (EPA, 2009b), and since then the method has been used mainly as 

a comparison reference method for the testing and designation of candidate FEMs for SO2 in 

accordance with 40 CFR Part 53.

The pararosaniline manual FRM has served its role for many years, but now a better 

method is needed that more fully meets the needs of contemporary SO2 monitoring. The existing 
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FRM is primarily a 24-hour integrated method, whereas a 1-hour SO2 FRM measurement 

capability would be needed to implement the proposed 1-hour SO2 NAAQS. Existing FEM 

analyzers can and do provide 1-hour measurement capability, but EPA wishes to facilitate the 

approval of new monitoring technologies as well. While the existing manual reference method 

can produce 1-hour averages, it is clearly impractical for routine use in making 1-hour SO2

measurements. Also, the 1-hour mode of the manual method is not a good standard for 

approving new FEMs with 1-hour measurement capability, because scores of 1-hour 

measurements would be needed during equivalency testing. Further, the existing FRM is 

cumbersome to use and requires a mercury-containing reagent that is potentially hazardous to 

operators or to the environment if it is mishandled.

These operational shortcomings suggest that the existing FRM should be replaced with a 

more suitable methodology. Fortunately, the existing SO2 instrumental measurement technique

based on the UVF measurement principle offers superior performance and substantial 

operational advantages, as reported in an FRM evaluation for EPA produced by Research 

Triangle Institute (Rickman, 1987). Analyzers using this technique can well provide the needed 

detection limits, precision, and accuracy and fulfill other purposes of an FRM, including use as 

an appropriate standard of reference for testing and designation of new FEM analyzers. After 

reviewing these factors, EPA has determined that a new, automated FRM for SO2 based on the 

UVF measurement principle should be adopted. EPA is proposing to add the new FRM in a new 

Appendix A-1 to Part 50. 

In association with the proposed new FRM, EPA is also proposing to update the 

performance-based requirements for FEM SO2 analyzers currently in 40 CFR Part 53. These 

requirements were established in the 1970’s, based primarily on the wet-chemical measurement 
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technology available at that time. Those initial requirements have become significantly outdated 

and should be modified to match current technology, particularly because they would apply to 

new FRM analyzers under the proposed new FRM. The better instrumental performance 

available with the proposed new UVF reference method technique allows the performance 

requirements for SO2 in 40 CFR Part 53 to be made more stringent for both FRM and FEM 

analyzers (EPA, 2009c). 

2. Proposed New FRM Measurement Technique

Since the 1970’s, a variety of measurement principles have been successfully used to 

produce continuous analyzers for SO2, some of which have qualified for EPA designation as 

equivalent methods(found at http://www.epa.gov/ttn/amtic/files/ambient/criteria/referenceequivalent-methods-list.pdf). These include methods based on ultraviolet fluorescence, flame 

photometry, differential optical absorption spectroscopy, coulometric and conductometric 

techniques, and second derivative ultraviolet absorption spectrometry. Although some of these 

techniques saw considerable utilization in the 1970’s, their use dwindled after the introduction of 

UVF analyzers because of various shortcomings such as non-specificity for SO2, susceptibility to 

interferences, marginal performance, or operational disadvantages (e.g. requiring hydrogen gas 

or wet-chemical reagents). Consequently, the UVF technique has emerged as the clearly 

dominant measurement technique for SO2, providing a majority of the domestic air monitoring 

data obtained over the last three decades, and virtually 100% of the current monitoring data 

(EPA, 2009b). As the proposed new reference method, the UVF technique would be specified in 

performance-based form, with a generic reference measurement principle and associated 

calibration procedure in a new Appendix A-1 to 40 CFR Part 50. Associated performance 

requirements applicable to candidate UVF FRM analyzers would be specified in 40 CFR Part 53. 
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This form of the FRM is consistent with that specified for FRMs for CO, O3, and NO2 in 

Appendices C, D, and F (respectively) to 40 CFR Part 50.

Reasonable commercial availability of high quality analyzers utilizing the reference 

measurement principle that can be offered by multiple manufacturers, ideally over many years, is 

an important aspect of any new reference measurement principle. EPA has designated more than 

a dozen UVF analyzers as equivalent to the current reference method over the last 30 years. 

Although most of the early model UVF analyzers are no longer in production, many have been 

replaced by redesigned and improved models, and entirely new models continue to become 

designated as FEMs. Currently, more than a half-dozen designated FEM models offered by 

multiple manufacturers are commercially available. The widespread use of the method has three 

important technical advantages for an FRM: 1) a variety of analyzer models are available and 

will likely continue to be available from multiple manufacturers for many years to come, 2) 

analyzer manufacturers have had (and continue to have) a strong marketing incentive to improve, 

refine, perfect, and continue to market such analyzers, and 3) the number of accumulated UVF 

field monitoring datasets (including related QC data) provide an extensive, available 

performance track record that can be evaluated to assess the performance of the analyzers in 

actual monitoring use.

The only other equivalent method measurement technique that has even a small 

representation among currently available FEM analyzers is the differential optical absorption 

spectrometric method. The open-path nature of this method (measurement of pollutants in the 

open air without a closed measurement cell) is not suitable for many of the purposes of a 

reference method. Further, this method is only available as two product models from two 

manufacturers, and very few State and local monitoring agencies are using such analyzers. 
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The UVF technique is not without some imperfections as a reference method. Analyzers 

utilizing the technique are, to a limited degree, susceptible to interference from aromatic 

hydrocarbon species and potentially other compounds at existing levels or levels that may occur 

at many monitoring sites. However, analyzer manufacturers have developed very effective ways 

to reduce these potential limitations, including careful selection of wavelengths, optimum optical 

design, and sample air scrubbers, such that typical interferences are minimal.

All UVF analyzers that have been designated as SO2 FEMs have been tested and shown 

to meet the existing performance requirements of 40 CFR Part 53. These include required 

testing for both positive and negative potential interferents, minimum level of measurement, zero 

and span drift, and precision. The results of these tests have been submitted to EPA and are in 

the archived FEM applications for these analyzers. Many newer models substantially exceed 

those requirements, with sensitivities down to less than 1 ppb, and typically commensurate levels 

of signal noise, precision, and zero drift (EPA, 2009c). In addition, UVF analyzers can 

accommodate a wide range of concentration measurement ranges. They are quite well suited to 

measure high, short-term SO2 concentrations near sources, and they can also be used to measure 

trace-level concentrations in clean areas.

For these reasons, EPA has decided to propose a new automated SO2 FRM based on the 

UVF measurement technology. EPA is confident that commercially available UVF instrument 

models would provide capability to serve not only current monitoring and FRM applications but 

anticipated monitoring and FRM needs well into future years. EPA solicits comment on the 

proposal to promulgate an FRM for SO2 that would be an automated method based on ultraviolet 

fluorescence, which would be specified in the form of a reference measurement principle and 

calibration procedure, as stated here, and contained in a new Appendix A-1 to 40 CFR Part 50.
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3. Technical Description of the Proposed UVF FRM

The proposed new reference method is based on automated measurement of the intensity

of the characteristic fluorescence released by SO2 in an ambient air sample when irradiated by 

ultraviolet light. The SO2 fluorescence produced is also in the ultraviolet range, but is measured 

at a longer wavelength. An analyzer implementing this measurement principle would include a 

measurement cell, an ultraviolet light source of appropriate wavelength, an ultraviolet detector 

system with appropriate wavelength sensitivity, and a pump and flow control system for 

sampling the ambient air. Generally, the analyzer also requires a means to reduce concentrations 

of aromatic hydrocarbons and possibly other compounds (depending on target wavelengths and 

other parameters used) in the air sample to control for potential measurement interferences. The 

analyzer is calibrated by referencing the instrumental fluorescence measurements to SO2

standard concentrations traceable to a NIST (National Institute of Standards and Technology) 

primary standard for SO2. This generic description of the FRM would be contained in Appendix 

A-1 to 40 CFR Part 50 and would be coupled with explicit analyzer performance requirements 

specified in Subpart B of 40 CFR Part 53. To qualify as an FRM, an analyzer model based on 

this principle would have to be tested in accordance with test procedures in Subpart B Part 53 

and shown to meet the performance requirements specified in that Subpart. EPA could then 

designate the analyzer model as an FRM analyzer, as provided in Part 53.

4. Implications to Air Monitoring Networks

Under §53.16, EPA must consider the benefits of a proposed supersession of an existing 

reference method, the potential economic consequences of such action for State and local 

monitoring agencies, and any disruption of State and local air quality monitoring programs that 

might result from such action. Supersession of an existing reference method, as described in 
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§53.16, presumes that the existing FRM would be deleted from Part 50 and replaced with a new 

FRM, and that all equivalent methods based on the old FRM would be cancelled. In the case of 

SO2, essentially all current domestic air monitoring activity is carried out using FEM UVF 

analyzers. Cancellation of the FEM designations of all these analyzers now would be potentially

very disruptive to State, local, and other monitoring networks, even though §53.16 alludes to a 

possible transition period to allow monitoring agencies some period of time to replace cancelled 

FEM analyzers.

EPA recognizes that these existing SO2 FEMs are providing monitoring data that are 

adequate for the current and the proposed SO2 NAAQS and for many other purposes, and there 

appears to be no need or purpose served by their withdrawal. Therefore, in this case, EPA 

proposes instead to retain the existing manual FRM for SO2 and to promulgate an entirely new 

automated FRM for SO2. The new FRM description would be contained in a new Appendix A-1 

to 40 CFR Part 50, and the existing FRM would be re-codified as Appendix A-2 to 40 CFR Part 

50, with both reference methods coexisting. Following adoption of the new Appendix A-1, new 

language proposed for §53.2(a) and (b) would provide that new FRM and FEM analyzers for 

SO2 be designated only with reference to the proposed new Appendix A-1. At the same time, 

retention of the existing SO2 reference method will preclude the need to cancel the designations 

of all existing FEMs for SO2. 

Under this proposal, no monitoring agencies would be required to change their SO2

monitoring procedures as a result of the proposed changes, so it would have no economic costs 

for implementation and no disruptive effects on state, local, or tribal air quality monitoring 

programs. Further, since UVF FEM analyzers have been in dominant use for many years, no 

bias or discontinuity in any aspect of the monitoring data obtained subsequently would result 
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from the proposed change in the SO2 reference methodology. 

In conjunction with the proposed new FRM, EPA is also proposing to adopt updated 

performance requirements in 40 CFR Part 53, applicable to both FRM and FEM analyzers, 

consistent with the automated methods and in anticipation of future NAAQS needs. This would 

ensure that, going forward, all new SO2 monitors would have improved performance. EPA 

believes that the proposal to retain the existing FRM while adding the new FRM would provide 

for a smooth, evolutionary transition from the older, manual FRM to the new, modern, 

automated FRM and FEM technology and the associated better performance requirements, with 

no immediate impact to current monitoring activities. For purposes of comparing SO2

monitoring data to the SO2 NAAQS, the EPA believes that the UVF FEMs are appropriate for 

continued use under the current standards and under the option being considered for a new 1-

hour averaged primary SO2 NAAQS. After several years, at a time when either a new SO2

NAAQS would require higher monitoring data quality or there would be no further potential for 

disruption to monitoring agencies, EPA would plan to withdraw the older reference method and 

it’s associated FEMs.

5. Proposed Revisions to 40 CFR Part 53

Several amendments associated with the proposed new SO2 reference measurement 

principle are proposed to 40 CFR Part 53. The most significant of these would update the 

performance requirements for both new FRM and new FEM analyzers for SO2, as set forth in 

proposed revised Table B-1. Based on typical performance capabilities available for UVF 

analyzers, EPA is proposing to reduce the allowable noise from 5 ppb to 1 ppb, the lower 

detectable limit from 10 ppb to 2 ppb, and the allowable interference equivalent limits from ±20 

ppb to ±5 ppb for each interferent and from 60 ppb to 20 ppb for the total of all interferents. 
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Also, EPA proposes to change the allowable zero drift limits from ±20 ppb to ±4 ppb, and to 

delete the specified limits for span drift at 20% of the upper range limit (URL) for SO2 analyzers. 

Review of FEM analyzer performance test results has shown that the 20% URL span limit 

requirements are unnecessary because drift performance requirements are adequately covered by 

the zero drift and 80% URL span drift limits. EPA proposes to change the lag time allowed from 

20 to 2 minutes and change the rise and fall time limits from 15 to 2 minutes. For precision, 

EPA proposes to change the form of the precision limit specifications from ppm to percent (of 

the URL) for SO2 analyzers and to set the limit at 2 percent for both 20% and 80% of the URL. 

Two percent is equivalent to 10 ppb for the standard (500 ppb) range, which is equivalent to the 

existing limit value for precision at 20% of the URL, but would be a reduction from 15 ppb to 10 

ppb for the limit value at 80% of the URL. This change in units from ppm (or ppb as given here) 

to percent makes the requirement responsive to higher and lower measurement ranges. Also, a 

new footnote is proposed to be added to Table B-1 to clarify how noise tests are to be carried out 

for candidate analyzers having an adjustable or automatic time constant capability.

EPA recognizes that SO2 monitoring needs can vary widely, from monitoring background 

levels in pristine areas to measuring short-term (1-hour) or even very short-term (less than 1-

hour) high-level averages in the vicinity of substantial sources of SO2. To address the need for 

more sensitive, lower measurement ranges for SO2 analyzers, EPA is proposing a separate set of 

performance requirements that would apply specifically to narrower measurement ranges, i.e.

ranges extending from zero to concentrations less than 0.5 ppm. These additional requirements 

are listed in the proposed revised Table B-1. A candidate analyzer that meets the Table B-1 

requirements for the standard measurement range (0 to 0.5 ppm) could optionally have one or 

more narrower ranges included in its FRM or FEM designation by further testing to show that it 
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meets these supplemental, narrower-range requirements.

At the other (high) end of the concentration measurement spectrum, another related 

change proposed for §53.20 would allow optional designation of measurement ranges for SO2 up 

to 2 ppm rather than 1 ppm as is now permitted, and designation of these higher ranges would be 

applicable to both FRM and FEM analyzers. Such higher ranges are often needed for 

measurement of short-interval SO2 averages. Finally, EPA is proposing to clarify in §53.20 that 

optional testing for auxiliary higher or lower measurement ranges (for all gaseous pollutants) 

may include tests for only some of the performance parameters, since the test results for the other 

performance parameters carried out for the standard measurement range would be technically 

applicable and adequate for the higher and/or lower ranges as well.

EPA believes that these changes in performance requirements are appropriate, based on 

analyzer performance data available from analyzer manuals and recent FEM applications. EPA 

solicits comments especially from UVF instrument users and manufacturers on these proposed 

changes, particularly in regard to whether they are reasonable, appropriate, of significant benefit, 

and achievable without undue cost. Comments are also requested on such issues as the trade off 

between a high measurement range and the need for adequate resolution at concentrations near 

the annual NAAQS, a similar trade off between noise level and response time (some analyzers 

allow these parameters to be adjusted by the operator or may adjust them automatically based on 

the rate of change of the concentration level), and whether such performance parameters should 

be addressed in more detail in 40 CFR Part 53. In particular, should SO2 analyzer requirements 

address the potential need for faster measurement response time to permit more accurate 

monitoring of short-term intervals such as 5-minute or 10-minute averages, and are the special 

performance requirements EPA is proposing for measuring very low levels (trace levels) of SO2
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appropriate and effective?

Another significant change proposed to 40 CFR Part 53 would add some low and 

medium level 1-hour comparability tests to the Subpart C comparability test requirements, as 

specified in Table C-1. These would help to ensure that the 1-hour measurement performance of 

candidate FEMs are adequate, relative to the FRM. Also, EPA proposes to amend Table A-1 in 

Subpart A to reflect the new FRM description in proposed new Appendix A-1 of 40 CFR Part 

50. This table would also be amended to correct some printing errors in the current table as well 

as to add new entries related to the new FRM for lead in PM10 that was recently promulgated. 

Other minor changes would be necessary in the wording of a few sections of Subparts A and B 

due to the proposed change in the nature of the SO2 FRM from a manual to an automated method 

or to update the language. These changes are reflected in the proposed regulatory text section of 

this notice.

EPA proposes additional minor revisions to Tables B-2 and B-3 of Subpart B. The 

changes proposed to Table B-2 would update some of the analytical methods for generation or 

verification of SO2 and interferent test concentrations and their associated references. Similarly, 

Table B-3 would be updated to add a specific listing for ultraviolet fluorescent methods and to 

add a few additional interferent test species for some other measurement techniques that have 

been found from experience to be needed. 

B. Network Design

1. Background

The basic objectives of an ambient monitoring network, as noted in 40 CFR Part 58

Appendix D, include 1) providing air pollution data to the general public in a timely manner, 2) 

supporting compliance with ambient air quality standards and emissions strategy development, 
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and 3) providing support for air pollution research. The SO2 network was originally deployed to 

support implementation of the SO2 NAAQS established in 1971. Although the SO2 standard was 

established in 1971, EPA did not establish uniform minimum monitoring requirements for SO2

monitoring 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 (Watkins and Thompson, 2009). The 

reduction in network size is due in part to the change in the source sector contributions to the 

overall SO2 inventory and the general decline of ambient SO2 levels over time. In the early 

decades of the SO2 network, particularly the 1970’s, there was a wider variety of more 

ubiquitous SO2 sources in urban areas, including residential coal and oil furnaces, when 

compared to the stationary source, electric generating unit (EGU)-dominated inventories of today

(see below). The situation in the 1970s led to a network design keyed on population, an 

appropriate approach at the time considering the close proximity of sources and people, 

particularly in urban, residential settings (Watkins and Thompson, 2009). 

An analysis of the approximately 488 monitoring sites comprising the current (2008) SO2

monitoring network indicates that just under half (~46%) of the sites in the current SO2 network 

are reported to be for the assessment of concentrations for general population exposure. As for 

the present day inventory, the 2005 NEI (http://www.epa.gov/ttn/chief/net/2005inventory.html) 

indicates that SO2 emissions from EGUs contribute approximately 70% of the anthropogenic 

SO2 emissions in the U.S. However, only approximately one third (~35%) of the network is 

reported to be addressing locations of maximum (highest) concentrations, likely linked to a 

specific source or group of sources such as EGUs.
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The current network supports the reporting of 1-hour data to EPA’s Air Quality System 

(AQS) database, as required in §58.12 of 40 CFR Part 58, since the network utilizes the 

continuous UVF FEM, which can provide time-resolved data averaged over periods as short as 

several minutes. The routine submittal of hourly data by state, local, and tribal air monitoring 

agencies to AQS is suitable for use in comparison to both of the current primary 24-hour and 

annual NAAQS. There are a few monitoring agencies who also report 5-minute data voluntarily 

to AQS. 

The current network is sited at a variety of spatial scales; however a majority of the 

network, just over sixty percent, is sited at the neighborhood spatial scale34 (Watkins and 

Thompson, 2009). Although there are 488 SO2 monitors operating in the network, there are 

currently no minimum monitoring requirements for SO2 in 40 CFR part 58 Appendix D, other 

than the following three: (1) SO2 must be monitored at National Core (NCore) monitoring sites 

(discussed below), (2) the EPA Regional Administrator must approve the removal of any 

 

34 Spatial scales are defined in 40 CFR Part 58 Appendix D, Section 1.2, where the scales of representativeness include:

1. Microscale – Defines the concentration in air volumes associated with area dimensions ranging from several meters up to about 100 

meters.

2. Middle scale – Defines the concentration typical of areas up to several city blocks in size, with dimensions ranging from about 100 

meters to 0.5 kilometers.

3. Neighborhood scale – Defines concentrations within some extended area of the city that has relatively uniform land use with 

dimensions in the 0.5 to 4.0 kilometers range. 

4. Urban scale – Defines concentrations within an area of city-like dimensions, on the order of 4 to 50 kilometers. Within a city, the 

geographic placement of sources may result in there being no single site that can be said to represent air quality on an urban scale. The 

neighborhood and urban scales have the potential to overlap in applications that concern secondarily formed or homogeneously distributed air 

pollutants.

5. Regional scale – Defines usually a rural area of reasonably homogeneous geography without large sources, and extends from tens to 

hundreds of kilometers. 
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existing monitors, and (3) any ongoing SO2 monitoring must have at least one monitor sited to 

measure the maximum concentration of SO2 in that area.

The SO2 monitors that are required as part of the National Core monitoring network 

(NCore) were not required solely for providing direct support of the SO2 NAAQS. The 

monitoring rule promulgated in 2006 (71 FR 61236) and codified at 40 CFR Part 58 and its 

Appendices established the NCore multi-pollutant network requirement to support integrated air 

quality management data needs. Further, NCore is intended to establish long-term sites 

providing data for air quality trends analysis, model evaluation, and, for urban sites, tracking 

metropolitan air quality statistics. To do this, NCore sites are required to measure various 

pollutants, including SO2, but are not sited to monitor maximum concentrations of SO2. NCore 

sites provide data representing concentrations at the broader neighborhood and urban spatial 

scales. The data from the NCore sites will be compared to the NAAQS although, as noted 

earlier, NAAQS comparisons are not the primary objective of NCore. The NCore network, 

which will be fully implemented by January 1, 2011, will result in approximately 83 sites, each 

with an SO2 monitor, with approximately 60 sites being located in urban areas. 

As set out in detail in section II.B of this notice, there is a causal relationship between 

short-term SO2 exposure and respiratory morbidity, with ”short-term” meaning exposures from 

5-10 minutes up to and including 24-hours. This finding is based primarily on results from 

controlled human exposure studies of 5-10 minutes as well as epidemiologic studies using 

mostly 1-hour daily maximum and 24-hour average SO2 concentrations. Importantly, the ISA 

described the controlled human exposure studies of 5-10 minutes as being the “definitive 

evidence” for this conclusion (ISA, section 5.2). In addition, when describing epidemiologic 

studies observing positive associations between ambient SO2 and respiratory symptoms, the ISA 
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stated “that it is possible that these associations are determined in large part by peak exposures 

within a 24-hour period” (ISA, section 5.2 at p. 5-5). The ISA also stated that the respiratory

effects following 5 - 10 minute SO2 exposures in controlled human exposure studies provide a 

basis for a progression of respiratory morbidity that could result in increased ED visits and 

hospital admissions (ISA, section 5.2). Thus, the monitoring network to support the proposed 

NAAQS should be focused on identifying the expected maximum short-term concentrations in 

any particular area.

The ISA (Section 2.1) indicates that point (i.e., stationary) sources account for 

approximately 95% of the total anthropogenic SO2 emissions in the U.S. According to the 2005 

National Emissions Inventory (http://www.epa.gov/ttn/chief/net/2005inventory.html), electrical 

generating units (EGUs) emit approximately 70% of the anthropogenic SO2 emissions in the 

U.S. The 2005 NEI indicates that the total anthropogenic emission inventory of SO2 is 

approximately 14,742 thousand tons per year. Of those 14,742 thousand tons per year of emitted 

SO2, approximately 85% were emitted by stationary sources that emit 100 or more tons per year 

(comprising approximately 1,928 of the 32,988 facilities listed in the 2005 NEI). This 

information indicates that a relatively small number (6 %) of all SO2 emitting stationary sources

are responsible for a large majority of the total anthropogenic emissions inventory (85%) in the 

U.S. Therefore, monitors sited to reflect locations of expected maximum concentrations should 

be primarily oriented towards locations influenced by one or a cluster of high SO2 emitting 

sources.

As noted in the key observations of the exposure analysis of the REA (REA, Section 

8.12), there are a variety of factors that influence overall population exposure to ground-level 

concentrations in a given area, including population density and proximity to sources, emissions 
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density in an area, and source specific emission parameters such as stack height, among other 

factors. In general, however, it is expected that any short-term peaks that may occur in an area 

are more likely to occur nearer to a source or sources, or in an area where multiple sources are 

significantly contributing to increased ground level concentrations (an area with high emissions 

density)

35. Given that maximum ground-level concentrations of SO2 are usually directly

traceable to specific sources, or a cluster of sources, the network design should support 

implementation of the proposed 1-hour SO2 NAAQS by targeting maximum ground-level 

concentrations in areas of both higher population and higher emissions.

2. Proposed Changes

In conjunction with the proposed 1-hour primary NAAQS and (if EPA should adopt a 

standard at the upper end of the range of levels for which the Agency is soliciting comment) the 

potential retention of the current 24-hour NAAQS, we are proposing a number of changes to the 

SO2 monitoring network. As just noted, there are currently minimum monitoring requirements 

for SO2 only at NCore sites. The proposal for a new 1-hour NAAQS necessitates the reintroduction of minimum monitoring requirements. An analysis of the approximately 488 

monitoring sites comprising the current (2008) SO2 monitoring network indicates that just under 

half (~46%) of the sites in the current SO2 network are reported to be for the assessment of 

concentrations for general population exposure. The current network was not originally

deployed to address current short-term, peak concentrations, such as those locations nearer to 

stationary sources or in areas of higher emissions densities, where maximum hourly and 5-10 

minute concentrations are likely to occur. The Agency has data indicating that only about one 

 

35 There is inherent variability in where peak ground level concentrations may occur in space and time from an 

individual source or group of sources, due to multiple factors including tons emitted, stack height, meteorology, 

among others. These factors are discussed further in the Monitor Placement and Siting section of this chapter. 
139

third of the existing SO2 network may be source-oriented monitors and/or sited in locations of 

expected maximum concentrations (Watkins and Thompson, 2009). 

To fully support the proposed SO2 NAAQS, the monitoring network needs to identify

where short-term, peak ground- level concentrations – i.e. concentrations from 5 minutes to one 

hour (or potentially up to 24 hours) -- may occur. Due to the multiple variables that affect ground 

level SO2 concentrations caused by one or more stationary sources, it is difficult to specify a 

source specific threshold, algorithm, or metric by which to require monitoring in a rule such as 

this. To achieve this goal, therefore, EPA is proposing a two-pronged network design to ensure 

that States perform a sufficient amount of monitoring of ambient concentrations of SO2 to 

determine attainment of the proposed SO2 NAAQS that intends to prevent exposure to peak 

concentrations. EPA anticipates this two-pronged network would require approximately 345 

monitors nationwide, providing data for comparison with both the proposed 1-hour and the 24-

hour standard if retained. The network would be wholly comprised of monitors sited at locations 

of expected maximum hourly concentrations. EPA is proposing that the two prongs of this SO2

network design would be distributed based on: (1) a Population Weighted Emissions Index 

(PWEI) and (2) the state-level contribution to the national, SO2 emissions inventory. EPA notes 

that although we propose that the network include a minimum number of required monitors, 

State, local, and tribal agencies may conduct additional monitoring above the minimum 

requirements. If those additional monitors satisfy all applicable requirements in 40 CFR Part 58, 

the data from those monitors would be comparable to the NAAQS. EPA estimates that one-half 

to two-thirds of the monitors in the existing network (excluding any currently operating NCore 

sites) may have to be moved in order to be counted towards the requirement for monitors sited at 

locations of expected maximum short-term concentrations of SO2. 
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We solicit comment on whether the estimated 348 monitors required by this proposal, 

distributed based on the two network design components presented below, are too few, too 

many, or suitable to establish a minimum network sufficient to meet the monitoring objectives 

noted above, including supporting compliance with the proposed 1-hour SO2 NAAQS. 

We propose that state and, where appropriate, local air monitoring agencies submit a plan 

for deploying SO2 monitors in accordance with the proposed requirements presented below by 

July 1, 2011. We also propose that the SO2 network being proposed be physically established no 

later than January 1, 2013. Considering the proposed timeline and criteria presented in the 

network design, we solicit comment on whether alternative dates would be more appropriate as 

deadlines for state and local monitoring agencies to submit a monitoring plan. We also solicit 

comments on whether alternative dates would be more appropriate as deadlines for state and 

local monitoring agencies to physically deploy monitors.

a. Population Weighted Emissions Index (PWEI) Triggered Monitoring

The EPA proposes that the first prong of the ambient SO2 monitoring network account 

for SO2 exposure by requiring monitors in locations where population and emissions may lead to 

higher potential for population exposure to peak hourly SO2 concentrations. In order to do this, 

EPA has developed a Population Weighted Emissions Index (PWEI) that uses population and 

emissions inventory data at the CBSA36 level to assign required monitoring for a given CBSA 

(population and emissions being obvious relevant factors in prioritizing numbers of required 

monitors). The PWEI for a particular CBSA is calculated by multiplying the population (using 

the latest Census Bureau estimates) of a CBSA by the total amount of SO2 emissions in that 

CBSA. The CBSA emission value is in tons per year, and is calculated by aggregating the county

 

36 CBSAs are defined by the U.S. Census Bureau, and are comprised of both Metropolitan Statistical Areas and 

Micropolitan Statistical Areas (http://www.census.gov). 
141

level emissions for each county in a CBSA. We then normalize by dividing the resulting product 

of CBSA population and CBSA SO2 emissions by 1,000,000 to provide a PWEI value, the units 

of which are millions of people-tons per year. This calculation has been performed for each 

CBSA and has been posted in the docket as “CBSA PWEI Calculation, 2009”. EPA believes 

that using this PWEI metric to inform where monitoring is required is more appropriate for the 

SO2 network design than utilizing a population-only type of approach, so that we may focus 

monitoring resources in areas of the country where people and emission sources are in greater 

proximity. In addition, EPA’s initial view is that this PWEI concept is appropriate for SO2 but is 

not necessarily transferrable to the other criteria pollutants. From a very broad vantage point, 

SO2 is exclusively a primarily emitted pollutant (i.e. unlike PM2.5 and ozone there is no 

secondary formation of SO2), is almost exclusively emitted by stationary sources (unlike NO2,

CO, PM2.5, thoracic coarse PM, and ozone), and is a gaseous pollutant which is somewhat more 

subject to transport (unlike Pb in the Total Suspended Particulate (TSP) and PM10 size fractions). 

We propose that the first prong of the SO2 network design require monitors in CBSAs, 

according to the following criteria. For any CBSA with a calculated PWEI value equal to or 

greater than 1,000,000, a minimum of three SO2 monitors are required within that CBSA. For 

any CBSA with a calculated PWEI value equal to or greater than 10,000, but less than 1,000,000, 

a minimum of two SO2 monitors are required within that CBSA. For any CBSA with a 

calculated PWEI value equal to or greater than 5,000, but less than 10,000, a minimum of one 

SO2 monitor is required within that CBSA. EPA believes that the monitors required within these 

breakpoints provide a reasonable minimum number of monitors in a CBSA that considers the 

combination of population and emissions that exist in a given CBSA. This proposed requirement 

is based on factors that will ensure highly populated areas will receive monitoring even if the 
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emissions in that area are moderate, which is appropriate given the fact that the greater 

population creates increased potential for exposure to those moderate sources. Additionally, this 

proposed requirement also ensures that those areas with higher emissions or emission densities, 

with moderate or modest populations will receive monitoring since those increased emissions are 

likely to have a significant impact on whatever population may exist nearby.

EPA estimates that these criteria will result in 231 required sites in 132 CBSAs. 

We propose that monitors triggered in this first prong of the network design must be sited in 

locations of expected maximum 1-hour concentrations, at the appropriate spatial scale37, within 

the boundaries of a given CBSA. EPA also proposes that when state or local agencies make 

selections for monitoring sites from a pool of similar candidate site locations, they shall prioritize 

monitoring where the maximum expected hourly concentrations occur in relative greater 

proximity to populations. EPA believes that states will likely need to use some form of 

quantitative analysis, such as modeling, data analysis, or saturation studies to aid in determining 

where ground-level SO2 maxima may occur in a given CBSA. The selection of these sites shall 

be documented in the Annual Monitoring Network Plan per §58.10, which includes a 

requirement for public inspection or comment, and approval by the EPA Regional Administrator. 

EPA solicits comment on 1) the use of the Population Weighted Emissions Index 

(PWEI), 2) the PWEI calculation method, 3) the PWEI breakpoints that correlate to a number of 

required monitors, 4) the requirement that the monitors shall be sited in locations of expected 

maximum 1-hour concentration, and (5) that state or local agencies making selections for 

 

37 Due to the variability in where maximum ground-level concentrations may occur (discussed in the Monitor Siting 

and Placement section of this chapter), the appropriate spatial scales within which an SO2 monitor might be placed 

include the microscale, middle, and neighborhood scales, which are defined in 40 CFR Part 58 Appendix D. [could 

also refer to the fn above where these are described]
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monitoring sites from a pool of similar candidate site locations shall prioritize monitoring where

the maximum expected hourly concentrations occur in relative greater proximity to populations. 

EPA recognizes that CBSA populations and emissions inventories change over time, 

suggesting a need for periodic review of the monitoring network. At the same time, EPA 

recognizes the advantages of a stable monitoring network. Therefore, while EPA currently 

provides for updates of the NEI every 3 years, EPA believes that the current network review 

requirements per §58.10 which requires an annual network plan and recurring 5-year 

assessments provide a suitable schedule for planning and assessing the monitoring network. 

Through the 5-year assessments, states will be in a position to review emissions distributions 

from updated NEIs to calculate PWEI values for each CBSA and subsequently assess whether 

the operational monitoring network remains appropriate. EPA proposes that the number of sites 

required to operate as a result of the PWEI values calculated for each CBSA be reviewed and 

revised for each CBSA through the 5-year network assessment cycle required in §58.10. EPA 

solicits comment on whether such adjustments to the network should be required on a 5-year 

cycle that matches the general frequency of network assessments or some other frequency.

b. State-level Emissions Triggered Monitoring 

As the second prong of the SO2 network, we are proposing to require a monitor or 

monitors in each state, allocated by state-level SO2 emissions. In this prong, EPA proposes to 

distribute approximately 117 sites, based on the corresponding percent contribution of each 

individual state to the national anthropogenic SO2 emission inventory. This prong of the 

network design is intended to allow a portion of the overall required monitors to be placed where 

needed, independent of the PWEI, inside or outside of CBSAs. EPA proposes to require 

monitors, using state boundaries as the geographic unit for allocation purposes, in proportion to a 
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state’s SO2 emissions, i.e., a state with higher emissions will be required to have a proportionally

higher number of monitors. The proposed percent contribution of individual states is based on 

the most recent NEI, with SO2 emissions being aggregated by state. Each one percent (after 

rounding) would correspond to one required monitor. For example, according to the 2005 NEI, 

the State of Ohio contributes 8.66% of the total anthropogenic SO2 inventory, which would 

correspond to requiring nine monitors to be distributed within Ohio. Further, EPA proposes that 

each state have at least one monitor required as part of this second prong, even if a particular 

state contributes less than 0.5% of the total anthropogenic national emissions inventory. As a 

result, approximately 117 monitoring sites would be required and distributed based on state-level 

SO2 emissions in the most recent NEI, which in this case, is the 2005 NEI. EPA solicits 

comment on the use of state-level emission inventories based on the most recent NEI to 

proportionally distribute approximately one third (~117 sites) of the required monitoring 

network. 

According to the most recent NEI, for this proposed second prong, we estimate the 

state/percent contribution to the national inventory/required monitor distribution to be: 

State or Territory

Percent Contribution to 

the National SO2

Inventory

Proposed

Number of Required 

Monitors

Alabama 4.02% 4

Alaska 0.46% 1

American Samoa N/A 1

Arizona 0.60% 1

Arkansas 0.77% 1

California 1.48% 1

Colorado 0.54% 1

Connecticut 0.23% 1

Delaware 0.58% 1

District of Columbia 0.03% 1

Florida 4.40% 4

Georgia 5.07% 5

Guam N/A 1
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Hawaii 0.08%1

Idaho 0.16%1

Illinois 3.51%4

Indiana 7.10%7

Iowa 1.50%2

Kansas 1.33%1

Kentucky 3.88%4

Louisiana 2.40%2

Maine 0.25%1

Maryland 2.58%3

Massachusetts 1.07%1

Michigan 3.32%3

Minnesota 1.05%1

Mississippi 0.81%1

Missouri 2.8%3

Montana 0.26%1

Nebraska 0.82%1

Nevada 0.49%1

New Hampshire 0.43%1

New Jersey 0.69%1

New Mexico 0.32%1

New York 2.65%3

North Carolina 4.40%4

North Dakota 1.08%1

Northern Mariana Islands N/A1

Ohio 8.66%9

Oklahoma 1.12%1

Oregon 0.32%1

Pennsylvania 7.96%8

Puerto Rico N/A1

Rhode Island 0.06%1

South Carolina 2.06%2

South Dakota 0.19%1

Tennessee 2.63%3

Texas 6.34%6

Utah 0.35%1

Vermont 0.05%1

Virgin Islands N/A1

Virginia 2.34%2

Washington 0.45%1

West Virginia 3.63%4

Wisconsin 1.79%2

Wyoming 0.83%1
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Table 5. State-level Emission Triggered Monitors. This table shows state and territory level contributions to the 

national SO2 inventory and the corresponding number of monitors required for each state as proposed in prong 2 of 

the proposed network design. 

EPA proposes siting requirements for this second prong of required monitors to be the 

same as those in the first prong: siting in locations of expected maximum 1-hour concentrations, 

at the appropriate spatial scale, within the boundaries of a given state, and prioritizing the 

selection of candidate sites where the maximum expected hourly concentrations occur in greater 

proximity to populations. This again would need to be determined case-by-case using 

quantitative analysis, such as modeling, data analysis, or saturation studies to aid in determining 

where ground-level SO2 maxima may occur in a given state. We propose that these monitors can 

be located inside or outside of CBSA boundaries. However, if a monitor required by the second 

prong is placed inside a CBSA that already has a requirement for monitoring due to the first 

prong of this network design, that monitor would not be allowed to count towards satisfying the 

first prong requirements. As noted for the first prong of required monitors, the selection of these 

sites shall be documented in the Annual Monitoring Network Plan per §58.10, which includes a 

requirement for public inspection or comment, and approval by the EPA Regional Administrator. 

The EPA solicits comment on 1) the use of state-level emission inventories to 

proportionally distribute required monitors, 2) requiring each state to have at least one monitor 

under this prong of the network design, and 3) requiring all monitors to be sited in locations of 

expected maximum1-hour concentration inside or outside of CBSAs

EPA recognizes that emissions inventories change over time, suggesting a need for 

periodic review of the monitoring network. At the same time, EPA recognizes the advantages of 

a stable monitoring network. Therefore, while EPA currently provides for updates of the NEI 

every 3 years, EPA believes that the current network review requirements per §58.10 which 

requires an annual network plan and recurring 5-year assessments provide a suitable schedule for 
147

planning and assessing the monitoring network. Through the 5-year assessments, states will be 

in a position to review emissions distributions from updated NEIs to assess whether the 

monitoring requirements remain appropriate. EPA proposes that the number of sites required to 

operate as a result of state-level emissions be reviewed and revised for each state through the 5-

year network assessment cycle required §58.10. EPA solicits comment on whether such 

adjustments to the network should be required on a 5-year cycle that matches the general 

frequency of network assessments or some other frequency.

c. Monitor Placement and Siting

Sites that are to be placed in locations of expected maximum 1-hour concentrations, will 

also likely discern 5-minute peaks as well. EPA expects that in general, these locations will be 

in proximity to larger emitting sources (in tons per year) and/or areas of relatively high emissions 

densities where multiple sources may be contributing to peak ground-level concentrations. The 

variability in where such locations exist relative to the responsible emission source(s) depends on 

multiple factors including the tonnage emitted by a source (or group of sources), stack height, 

stack diameter, emission exit velocity, emission temperature, terrain, and meteorology. 

Depending on these variables, plumes may heavily fumigate areas immediately downwind of a 

source, or may never truly touch down at all, dispersing into ambient air where SO2

concentrations continually decrease with increasing distance away from the source. This is 

illustrated in an example where a relatively large source with a tall stack height may not produce 

exceedingly high ground level concentrations anywhere along its plume trajectory while a 

smaller source with a relatively short stack may cause relatively higher ground level 

concentrations under the same meteorological conditions at the same location. The primary

reason for this variability is because the peak impacts of sources with higher stacks will 
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generally be farther downwind and may be more variably located than is the case for sources 

with shorter stacks. Further, depending on meteorology, an emission plume from an individual 

source may cause increased ground-level concentrations at any heading, relative to the parent 

source, corresponding to the prevailing winds.

When analyzing a particular source, a state may find multiple locations where peak 

ground-level concentrations may occur around an individual source. EPA does not intend for 

multiple monitors to be sited around or in proximity to one source. Not siting multiple monitors 

around, or in proximity, to one source ensures that more individual sources or groups of sources 

will receive attention by the monitoring network. States always have the discretion to perform 

additional monitoring above the minimum requirements to increase monitoring around a 

particular source or group of sources. 

Due to the variability of how, when, where, and to what degree a source or group of 

sources can contribute to peak, ground-level SO2 concentrations, EPA expects that State and 

local monitoring agencies will need to analyze all relevant information, including available 

ambient and emissions data, and potentially use air quality modeling or saturation studies to 

select appropriate monitoring site locations. Further, due to the variability in where maximum 

ground-level concentrations may occur, the appropriate spatial scales within which a monitor 

might be placed include the microscale, middle, and neighborhood scales, which are defined in 

40 CFR Part 58 Appendix D. EPA believes that states, in evaluating a source (or group of 

sources) that contribute to a peak ground-level SO2 concentration that varies with space and 

time, should identify where the highest concentrations are expected to occur in developing 

candidate site locations. EPA proposes that when state and local agencies make selections for 

monitoring sites from candidate site locations, they shall prioritize monitoring where the 
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maximum expected hourly concentrations occur in greater proximity to populations. EPA 

solicits comment on the role of population exposure in the site selection process.

d. Monitoring required by the Regional Administrator

In addition to the two prongs of the proposed SO2 network design, we propose that the 

Regional Administrator will have discretion to require monitoring above these minimum 

requirements under prongs 1 and 2, as necessary to address situations where the minimum 

monitoring requirements are not sufficient to meet monitoring objectives noted above. EPA 

recognizes that the minimum required monitors in the proposed network design under the two 

prongs described above are based on indicators that may not provide for all the monitoring that 

may be necessary in an area. An example where EPA envisions requiring an additional monitor 

might be a case where a source having modest emissions still has high potential to cause a 

violation of the NAAQS in a community or neighborhood. This situation might occur where a 

modest SO2 source has, for example, a low emission stack and/or is in an area where 

meteorological conditions cause situations, such as inversions or stagnation, that might lead to 

high ground-level concentrations of SO2. In this example, such a monitor might be needed even 

though a state is fulfilling its monitoring requirements under the first and second prongs of the 

proposed network design. The purpose of this provision is to monitor in and provide data for 

otherwise non-monitored locations that have the potential to exceed the level of the NAAQS or 

that are perceived to have higher exposure risks due to proximity to a source or sources. In such 

an example, the Regional Administrators may make use of any available data including existing 

model data, existing data analyses, or screening tools such as AERSCREEN or SCREEN3, to 

inform a decision of whether or not a monitor should be required for a given area or location. 

Any monitor required through the Regional Administrator and selected by the state or local 
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agency would be included in the Annual Monitoring Network Plan per §58.10, which includes a 

requirement for public inspection or comment, and approval by the EPA Regional Administrator. 

In any case, EPA encourages state, local, and tribal monitoring agencies to provide input and 

information to the appropriate Regional Administrators in determining whether additional 

monitors are needed and the locations of such monitors. We solicit comment on the proposal to 

allow Regional Administrators the discretion to require monitoring above the requirements under 

prongs 1 and 2 for any area or location where those monitoring requirements are not sufficient to 

meet monitoring objectives. 

EPA notes that existing requirements detailed in §58.14(c) address certain conditions 

where existing monitors can be shut down, with EPA Regional Administrator approval. EPA is 

not reopening or otherwise reconsidering this provision. However, this requirement is noted here 

so that state or local agency requests to potentially relocate SO2 monitors to meet the proposed 

requirements of prongs 1 or 2 will be considered with the specific provisions of §58.14 (c) in 

mind.

e. Alternative Network Design

EPA solicits comments on alternative network designs, including alternative methods to 

determine the minimum number of monitors per state. We are particularly interested in whether 

a screening approach for assessing the likelihood of a NAAQS exceedance could be developed 

and serve as a basis for determining the number and location of required monitors.

More specifically, EPA requests comment on whether it should utilize existing screening 

tools such as AERSCREEN or SCREEN3, which use parameters such as effective stack height 

and emissions levels to identify facilities with the potential to cause an exceedance of the 

proposed standard. For that set of sources, EPA could then require states to conduct more 
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refined modeling (likely using the American Meteorological Society (AMS)/EPA Regulatory

Model (AERMOD)) to determine locations where monitoring should be conducted. Any 

screening or modeling would likely be carried out by states by using EPA recommended models 

and techniques referenced by 40 CFR Part 51, Appendix W, which provides guidance on air 

quality modeling. Such screening or modeling uses facility emission tonnage, stack heights, 

stack diameters, emission temperatures, emission velocities, and accounts for local terrain and 

meteorology in determining where expected maximum hourly concentrations may occur. In 

using this approach, EPA would then require states to locate monitors at the point of maximum 

concentration around sources identified as likely causing NAAQS exceedances. 

This approach could lead to monitors being required at a significantly larger number of 

locations than under the proposed approach. For example, the NEI shows that 2,407 sources 

emit 50 tons per year or more of SO2, while 1,928 sources emit 100 tons per year or more of 

SO2. If, for example, the state screening approach found that a substantial fraction of those 50 or 

100 ton per year sources had a significant probability of violating the NAAQS, states could be 

required to model, evaluate, and potentially monitor a corresponding number of sources. EPA 

also notes that this alternative approach would not distinctly use population as a factor for where 

monitors should be placed. EPA solicits comment on the resource implications for state and local

agencies associated with this approach. 

If EPA selects a standard level near the lower end of the proposed range, it is likely that a 

greater number of areas would exceed the NAAQS, leading to the need for additional monitors. 

A facility screening approach, as described above would explicitly account for the specific 

parameters of a facility, air quality information, and the stringency of the standard for 

determining the number of monitors, in contrast to the proposed approach. EPA solicits comment 
152

on how, in the absence of a facility screening approach, the number of monitors required 

nationwide could be adjusted if EPA finalizes a standard near the lower end of the proposed 

range. 

C. Data Reporting

SO2 UV fluorescence FEMs are continuous gas analyzers, producing updated data values on 

the order of every 20 seconds. Data values are typically aggregated into minute averages and 

then compiled into hourly averages for reporting purposes. EPA proposes to retain the existing 

requirement that State and local monitoring agencies report hourly SO2 data to AQS within 90 

days of the end of each calendar quarter. EPA encourages monitoring agencies to voluntarily

report their pre-validated data on an hourly basis to EPA’s real time AIRNow data system.

The definitive evidence for the ISA’s conclusion of causal association between shortterm SO2 exposure and respiratory morbidity is from controlled human exposure studies of 5-10 

minutes in exercising asthmatics (ISA, section 5.2). The REA therefore assessed exposure and 

risks associated with 5-minute SO2 concentrations above 5-minute health effect benchmark 

levels derived from these controlled human exposure studies. In performing these analyses, the 

REA noted that: 1) the majority of the current SO2 monitoring network reported 1-hour SO2

concentrations (REA section 7.2.3); 2) very few state and local agencies in the U.S. voluntary

reported ambient 5-minute SO2 concentrations, as such reporting is not required (REA, section 

10.3.3.2); and 3) the lack of 5-minute monitoring data necessitated the use of statistically

estimated 5-minute SO2 concentrations derived from reported 1-hour SO2 levels (see REA 

section 7.2.3) in order to expand the geographic scope of the exposure and risk analyses. Thus 

given the demonstrated importance of 5-minute SO2 concentrations, EPA proposes that State and 

local agencies shall report to AQS the maximum 5-minute block average of the twelve 5-minute 
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block averages of SO2 for each hour, in addition to the existing requirement to report the 1-hour

average. 

EPA solicits comment on the proposed requirement for state and local monitoring 

agencies to report both hourly average and the maximum 5-minute block average out of the 

twelve 5-minute block averages of SO2 for each hour. EPA also solicits comment on the 

advantages and disadvantages of alternatively requiring state and local agencies to report all 

twelve 5-minute SO2 values for each hour. Having all twelve 5-minute SO2 values for each hour 

would provide more detailed information for health research purposes and provide additional 

information to help inform the next review of the SO2 standard. We also solicit comment on 

alternatively requiring state and local agencies to report the maximum 5-minute concentration in 

an hour based on a moving 5-minute averaging period rather than time block averaging.

EPA notes the potential resource burden with the proposed requirement to report 5-

minute average values in addition to 1-hour average values, as is currently required. 

Accordingly, we solicit comment on the magnitude and importance of this resource burden, 

recognizing that monitoring agencies utilize a variety of automated data acquisition and 

management programs, and that the resulting burden of validating and reporting 5-minute data 

may vary from a relatively trivial matter to an issue of greater importance, depending on the 

procedures utilized within each agency’s data reporting process. 

As a part of the larger data quality performance requirements of the ambient monitoring 

program, we are proposing data quality objectives (DQOs) for the proposed SO2 network. The 

DQOs are meant to identify measurement uncertainty for a given pollutant method. We propose 

a goal for acceptable measurement uncertainty for SO2 methods to be defined for precision as an 

upper 90 percent confidence limit for the coefficient of variation (CV) of 15 percent and for bias 
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as an upper 95 percent confidence limit for the absolute bias of 15 percent. We solicit comment 

on the proposed DQOs and on what the acceptable measurement uncertainty should be.

IV. Proposed Appendix T--Interpretation of the Primary NAAQS for Oxides of Sulfur 

and Proposed Revisions to the Exceptional Events Rule

The EPA is proposing to add Appendix T, Interpretation of the Primary National 

Ambient Air Quality Standards for Oxides of Sulfur, to 40 CFR Part 50 in order to provide data 

handling procedures for the proposed SO2 1-hour primary standard. The proposed section 50.11 

which sets the averaging period, level, indicator and form of the NAAQS refers to this Appendix 

T. The proposed Appendix T would detail the computations necessary for determining when the 

proposed 1-hour primary SO2 NAAQS is met. The proposed Appendix T also would address 

data reporting, data completeness considerations, and rounding conventions.

Two versions of the proposed Appendix T are printed at the end of this notice. The first 

applies to a 1-hour primary standard based on the annual 4th high value form, while the second 

applies to a 1-hour primary standard based on the 99th percentile daily value form. (As 

explained in section II.F. 3 above, EPA is proposing alternative forms here based on technical 

analysis that they are equally effective.) The discussion here addresses the first of these 

versions, followed by a brief description of the differences found in the second version.

For the proposed 1-hour primary standard, EPA is proposing data handling procedures, 

a proposed addition of a cross-reference to the Exceptional Events Rule, a proposed addition to 

allow the Administrator discretion to consider otherwise incomplete data to be complete, and a 

proposed provision addressing the possibility of there being multiple SO2 monitors at one site. 

The EPA is also proposing SO2-specific changes to the deadlines in 40 CFR 50.14, by

which states must flag ambient air data that they believe have been affected by exceptional 
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events and submit initial descriptions of those events, and to the deadlines by which states must 

submit detailed justifications to support the exclusion of that data from EPA determinations of 

attainment or nonattainment with the NAAQS. The deadlines now contained in 40 CFR 50.14 

are generic, and are not always appropriate for SO2 given the anticipated schedule for the 

designations of areas under the proposed SO2 NAAQS.

A. Background

The general purpose of a data interpretation appendix is to provide the practical details 

on how to make a comparison between multi-day and possibly multi-monitor ambient air 

concentration data and the level of the NAAQS, so that determinations of attainment and 

nonattainment are as objective as possible. Data interpretation guidelines also provide criteria for 

determining whether there are sufficient data to make a NAAQS level comparison at all.

The regulatory language for the current SO2 NAAQS, originally adopted in 1977, 

contains data interpretation instructions only for the issue of data completeness. This situation 

contrasts with the situations for ozone, PM2.5, PM10, and most recently Pb for which there are 

detailed data interpretation appendices in 40 CFR Part 50 addressing issues that can arise in 

comparing monitoring data to the NAAQS. EPA has used its experience developing and 

applying these other data interpretation appendices to develop the proposed text for Appendix T.

An exceptional event is defined in 40 CFR 50.1 as an event that affects air quality, is 

not reasonably controllable or preventable, is an event caused by human activity that is unlikely 

to recur at a particular location or is a natural event, and is determined by the Administrator in 

accordance with 40 CFR 50.14 to be an exceptional event. Air quality data that is determined, 

under the procedural steps and substantive criteria specified in section 50.14, to have been 

affected by an exceptional event may be excluded from consideration when EPA makes a 
156

determination that an area is meeting or not meeting the associated NAAQS. The key procedural 

deadlines in section 50.14 are that a State must notify EPA that data have been affected by an 

event, i.e., “flag” the data in the Air Quality Systems (AQS) database, and provide an initial 

description of the event by July 1 of the year after the data are collected, and that the State must 

submit the full justification for exclusion within 3 years after the quarter in which the data were 

collected. However, if a regulatory decision based on the data, for example a designation action, 

is anticipated, the schedule is shortened and all information must be submitted to EPA no later 

than a year before the decision is to be made. This generic schedule presents problems when a 

NAAQS has been recently revised, as discussed below.

B. Interpretation of the primary NAAQS for Oxides of Sulfur 

The purpose of a data interpretation rule for the SO2 NAAQS is to give effect to the 

form, level, averaging time, and indicator specified in the proposed regulatory text at 40 CFR 

50.11, anticipating and resolving in advance various future situations that could occur. The 

proposed Appendix T provides definitions and requirements that apply to the proposed 1-hour 

primary standard for SO2. The requirements concern how ambient data are to be reported, what 

ambient data are to be considered (including the issue of which of multiple monitors’ data sets 

will be used when more than one monitor has operated at a site), and the applicability of the 

Exceptional Events Rule to the primary SO2 NAAQS. 

1. 1-hour primary standard based on the annual 4th high value form

With regard to data completeness for the proposed 1-hour primary standard, the 

proposed Appendix follows past EPA practice for other NAAQS pollutants by requiring that in 

general at least 75% of the monitoring data that should have resulted from following the planned 

monitoring schedule in a period must be available for the key air quality statistic from that period 
157

to be considered valid. For the proposed 1-hour primary SO2 NAAQS, the key air quality 

statistics are the daily maximum 1-hour concentrations in three successive years. It is important 

that sampling within a day encompass the period when concentrations are likely to be highest 

and that all seasons of the year are well represented. Hence, the 75% requirement is proposed to 

be applied at the daily and quarterly levels. EPA invites comment on the proposed completeness 

requirements.

Recognizing that there may be years with incomplete data, the proposed text provides 

that a design value derived from incomplete data will nevertheless be considered valid in either 

of two situations. 

First, if the design value calculated from at least four days of monitoring observations 

in each of these years exceeds the level of the 1-hour primary standard, it would be valid. This 

situation could arise if monitoring was intermittent but high SO2 levels were measured on 

enough hours and days for the mean of the three annual 4th highest values to exceed the standard. 

In this situation, more complete monitoring could not possibly have indicated that the standard 

was actually met. 

Second, we are proposing a diagnostic data substitution test which is intended to 

identify those cases with incomplete data in which it nevertheless is very likely, if not virtually 

certain, that the daily 1-hour design value would have been observed to be below the level of the 

NAAQS if monitoring data had been minimally complete. 

The diagnostic test would be applied only if there is at least 50% data capture in each 

quarter of each year and if the 3-year mean of the observed annual 4th highest maximum hourly

values in the incomplete data is below the NAAQS level. The test would substitute a high 

hypothetical concentration for as much of the missing data as needed to meet the 100% 
158

requirement in each quarter. The value that is substituted for the missing values is the highest 

daily maximum 1-hour observed in the same quarter, looking across all three years under 

evaluation. If the resulting 3-year design value is below the NAAQS, it is highly likely that the 

design value calculated from complete data would also have been below the NAAQS, so the 

original design value indicating compliance would be considered valid. 

It should be noted that one possible outcome of applying the proposed substitution test 

is that a year with incomplete data may nevertheless be determined to not have a valid design 

value and thus to be unusable in making 1-hour primary NAAQS compliance determinations for 

that 3-year period. EPA invites comment on incorporating the proposed substitution test into the 

final rule.

EPA is proposing that the Administrator have general discretion to use incomplete data 

to calculate design values that would be treated as valid for comparison to the NAAQS despite 

the incompleteness, either at the request of a state or at her own initiative. Similar provisions 

exist already for the PM2.5 and lead NAAQS, and EPA has recently proposed such provisions to 

accompany the proposed 1-hour NO2 and SO2 NAAQS. The Administrator would consider 

monitoring site closures/moves, monitoring diligence, and nearby concentrations in determining 

whether to use such data.

2. 1-hour primary standard based on the annual 99th percentile daily value form 

The second version of the proposed Appendix T appearing at the end of this notice 

contains proposed interpretation procedures for a 1-hour primary standard based on the 99th

percentile daily value form. The 4th high daily value form and the 99th percentile daily value 

form would yield the same design value in a situation in which every hour and day of the year 

has reported monitoring data, since the 99th percentile of 365 daily values is the 4th highest value. 
159

However, the two forms diverge if data completeness is 82% or less, because in that case the 99th

percentile value is the 3rd highest (or higher) value, to compensate for the lack of monitoring data 

on days when concentrations could also have been high.

Logically, provisions to address possible data incompleteness under the 99th percentile 

daily value form should be somewhat different from those for the 4th highest form. With a 4th

highest form, incompleteness should not invalidate a design value that exceeds the standard, for 

reasons explained above. With the 99th percentile form, however, a design value exceeding the 

standard stemming from incomplete data should not automatically be considered valid, because 

concentrations on the unmonitored days could have been relatively low, such that the actual 99th

percentile value for the year could have been lower, and the design value could have been below 

the standard. The second proposed version of Appendix T accordingly has somewhat different 

provisions for dealing with data incompleteness. One difference is the addition of another 

diagnostic test based on data substitution, which in some cases can validate a design value based 

on incomplete data that exceeds the standard.

The second version of the proposed Appendix T provides a table for determining which 

day’s maximum 1-hour concentration will be used as the 99th percentile concentration for the 

year. The proposed table is similar to one used now for the 24-hour PM2.5 NAAQS, which is 

based on a 98th percentile form, but adjusted to reflect a 99th percentile form for the 1-hour 

primary SO2 standard. The proposed Appendix T also provides instructions for rounding (not 

truncating) the average of three annual 99th percentile hourly concentrations before comparison 

to the level of the primary NAAQS.

C. Exceptional events information submission schedule

The Exceptional Events Rule at 40 CFR 50.14 contains generic deadlines for a state to 
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submit to EPA specified information about exceptional events and associated air pollutant 

concentration data. A state must initially notify EPA that data have been affected by an event by

July 1 of the calendar year following the year in which the event occurred; this is done by

flagging the data in AQS and providing an initial event description. The state must also, after 

notice and opportunity for public comment, submit a demonstration to justify any claim within 3 

years after the quarter in which the data were collected. However, if a regulatory decision based 

on the data (for example, a designation action) is anticipated, the schedule to flag data in AQS 

and submit complete documentation to EPA for review is shortened, and all information must be 

submitted to EPA no later than one year before the decision is to be made. 

These generic deadlines are suitable for the period after initial designations have been 

made under a NAAQS, when the decision that may depend on data exclusion is a redesignation 

from attainment to nonattainment or from nonattainment to attainment. However, these 

deadlines present problems with respect to initial designations under a newly revised NAAQS. 

One problem is that some of the deadlines, especially the deadlines for flagging some relevant 

data, may have already passed by the time the revised NAAQS is promulgated. Until the level 

and form of the NAAQS have been promulgated a state does not know whether the criteria for 

excluding data (which are tied to the level and form of the NAAQS) were met on a given day. 

Another problem is that it may not be feasible for information on some exceptional events that 

may affect final designations to be collected and submitted to EPA at least one year in advance 

of the final designation decision. This could have the unintended consequence of EPA 

designating an area nonattainment because of uncontrollable natural or other qualified 

exceptional events. 

The Exceptional Events Rule at section 50.14(c)(2)(v) indicates “when EPA sets a 
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NAAQS for a new pollutant, or revises the NAAQS for an existing pollutant, it may revise or set 

a new schedule for flagging data for initial designation of areas for those NAAQS.”

For the specific case of SO2, EPA anticipates that the signature date for the revised SO2

NAAQS will be June 2, 2010 (a date specified by Consent Decree), that state/tribal designations 

recommendations will be due by June 2, 2011, and that initial designations under the revised 

NAAQS will be made by June 1, 2012 (since June 2, 2012 would be on a Saturday) and will be 

based on air quality data from the years 2008-2010 or 2009-2011 if there is sufficient data for 

these data years. (See Section VI below for more detailed discussion of the designation schedule 

and what data EPA intends to use.) Under the current rule, because final designations would be 

made by June 1, 2012, all events to be considered during the designations process would have to 

be flagged and fully documented by states one year prior to designations, by June 1, 2011. A 

state would not be able to flag and submit documentation regarding events that occurred between 

June to December 2011 by one year before designations are made in June 2012. 

EPA is proposing revisions to 40 CFR 50.14 only to change submission dates for 

information supporting claimed exceptional events affecting SO2 data. The proposed rule text at 

the end of this notice shows the changes that would apply if a revised SO2 NAAQS is 

promulgated by June 2, 2010, and designations are made two years after such promulgation. 

For air quality data collected in 2008, we propose to extend the generic July 1, 2009 deadline for 

flagging data (and providing a brief initial description of the event) to October 1, 2010. EPA 

believes this extension would provide adequate time for states to review the impact of 

exceptional events from 2008 on the revised standard and notify EPA by flagging the relevant 

data in AQS. EPA is not proposing to change the foreshortened deadline of June 1, 2011 for 

submitting documentation to justify an SO2-related exceptional event from 2008. We believe the 
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generic deadline provides adequate time for states to develop and submit proper documentation.

For data collected in 2009, EPA proposes to extend generic deadline of July 1, 2010 for 

flagging data and providing initial event descriptions to October 1, 2010. EPA is retaining the 

deadline of June 1, 2011 for states to submit documentation to justify an SO2-related exceptional 

event from 2009. EPA plans to assist the states by providing at the time of signature our 

assessment of which monitoring sites and days have exceeded the NAAQS in 2008 and 2009.

For data collected in 2010, EPA is proposing a deadline of June 1, 2011 for flagging data and 

providing initial event descriptions and for submitting documentation to justify exclusion of the 

flagged data. EPA believes that this deadline provides states with adequate time to review and 

identify potential exceptional events that occur in calendar year 2010, even for those events that 

might occur late in the year. EPA believes these deadlines will be feasible because experience 

suggest that exceptional events affecting SO2 data are few in number and easily assessed, so no 

state is likely to have a large workload.

If a state intends 2011 data to be considered in SO2 designations, 2011 data must be 

flagged and detailed event documentation submitted 60 days after the end of the calendar quarter 

in which the event occurred or by March 31, 2011, whichever date occurs first. Again, EPA 

believes these deadlines will be feasible because experience suggest that exceptional events 

affecting SO2 data are few in number and easily assessed, so no state is likely to have a large 

workload.

Table 6 summarizes the proposed designation deadlines discussed in this section and 

provides designation schedule information from recent, pending or prior NAAQS revisions for 

other pollutants. If the promulgation date for a revised SO2 NAAQS occurs on a different date 

than June 1, 2010 (i.e. if the consent decree should be amended – which EPA does not presently 
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anticipate), EPA will revise the final SO2 exceptional event flagging and documentation 

submission deadlines accordingly, consistent with this proposal, to provide states with 

reasonably adequate opportunity to review, identify, and document exceptional events that may

affect an area designation under a revised NAAQS. EPA invites comment on these proposed 

changes in the exceptional event flagging and documentation submission deadlines for the 

revised SO2 NAAQS shown in Table 6.
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Table 6. Schedule for Exceptional Event Flagging and Documentation 

Submission for Data to be Used in Designations Decisions for New or Revised 

NAAQS

NAAQS Pollutant/

Standard/(Level)/

Promulgation Date

Air Quality 

Data Collected 

for Calendar 

Year

Event Flagging & 

Initial Description 

Deadline

Detailed 

Documentation 

Submission 

Deadline

PM2.5/24-Hr 

Standard (35 

μg/m3) 

Promulgated 

October 17, 2006

2004-2006 October 1, 2007a April 15, 2008a

Ozone/8-Hr 

Standard (0.075 

ppm)Promulgated 

March 12, 2008

2005-2007 June 18, 2009a June 18, 2009a

2008 June 18, 2009a June 18, 2009a

2009 60 Days after the 

end of the calendar 

quarter in which 

the event occurred 

or February 5, 

2010, whichever 

date occurs firstb

60 Days after the 

end of the 

calendar quarter 

in which the event 

occurred or 

February 5, 2010, 

whichever date 

occurs firstb

NO2/1-Hour 

Standard(80-100 

PPB, final level 

TBD)

2008 July 1, 2010a January 22, 2011a

2009 July 1, 2010a January 22, 2011a

2010 April 1, 2011a July 1, 2011 a

SO2/1-Hour

Standard (50-100 

PPB, final level 

TBD)

2008 October 1, 2010b June 1, 2011b

2009 October 1, 2010b June 1, 2011 b

2010 June 1, 2011 b June 1, 2011 b

2011 60 Days after the 

end of the calendar 

quarter in which 

the event occurred 

or March 31, 2011, 

whichever date 

occurs firstb

60 Days after the 

end of the 

calendar quarter 

in which the event 

occurred or March 

31, 2011, 

whichever date 

occurs firstb

aThese dates are unchanged from those published in the original 

rulemaking, or are being proposed elsewhere and are shown in this table 

for informational purposes—the Agency is not opening these dates for 

comment under this rulemaking.

bIndicates change from general schedule in 40 CFR 50.14.

Note: EPA notes that the table of revised deadlines only applies to data 

EPA will use to establish the final initial designations for new or 

revised NAAQS. The general schedule applies for all other purposes, most 

notably, for data used by EPA for redesignations to attainment.
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V. Designations for the SO2 NAAQS

After EPA establishes or revises a NAAQS, the CAA directs EPA and the states to begin 

taking steps to ensure that the new or revised NAAQS is met. The first step is to identify areas 

of the country that do not meet the new or revised NAAQS. This step is known as the initial area 

designations. 

Section 107(d)(1)(A) of the CAA provides that, "By such date as the Administrator may

reasonably require, but not later than 1 year after promulgation of a new or revised NAAQS for 

any pollutant under section 109, the Governor of each state shall * * * submit to the 

Administrator a list of all areas (or portions thereof) in the state" that designates those areas as 

nonattainment, attainment, or unclassifiable. The CAA section 107(d)(1)(A)(i) defines an area 

as nonattainment if it is violating the NAAQS or if it is contributing to a violation in a nearby

area. 

Section 107(d)(1)(B)(i) further provides, "Upon promulgation or revision of a NAAQS, 

the Administrator shall promulgate the designations of all areas (or portions thereof) * * * as 

expeditiously as practicable, but in no case later than 2 years from the date of promulgation. 

Such period may be extended for up to one year in the event the Administrator has insufficient 

information to promulgate the designations within 2 years. By no later than 120 days prior to 

promulgating designations, EPA is required to notify states of any intended modifications to their 

boundaries as EPA may deem necessary. States then have an opportunity to comment on EPA's 

intended decisions. (See section 107(d)(1)(B)(ii).) Whether or not a state provides a 

recommendation, EPA must promulgate the designation that the Agency deems appropriate. 

Therefore, following promulgation of any revised SO2 NAAQS in June 2010, EPA must 

promulgate initial designations by June 2012, or, by June 2013 in the event that the 
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Administrator has insufficient information to promulgate initial designations within 2 years. 

Along with the proposal to set a new 1-hour primary SO2 NAAQS, elsewhere in this action, EPA 

is proposing new SO2 ambient air monitoring network requirements. As proposed, any new 

monitors would be deployed no later than January 1, 2013. Compliance with the proposed 1-

hour SO2 NAAQS would be determined based on 3 years of complete, quality assured, certified 

monitoring data. We do not expect newly sited monitors for the proposed new network to 

generate sufficient monitoring data for EPA to use in determining whether areas are in 

compliance with the revised SO2 NAAQS by the statutory deadline for EPA to complete initial 

designations, even if EPA were to take an additional third year. Therefore, EPA intends to 

complete the designations on a 2-year schedule, by June 2012, based on 3 years of complete, 

quality assured, certified air quality monitoring data from the current monitoring network.

EPA expects to base designations on air quality data from the years 2008-2010 or 2009-

2011. Because the new monitoring network requirements would not apply until January 1, 2013, 

EPA expects that many SO2 monitors now operating will continue in operation at their current 

locations at least through the end of 2011.39 The SO2 monitors in the current network were 

generally sited to measure the highest 24-hour and annual average SO2 concentrations. 

However, all of the monitors report hourly data. EPA estimates that around 488 monitors 

operated in 2008. EPA believes at least one third of the monitors meet the proposed network 

design requirements and therefore would not need to be moved. Additional monitors may be 

retained in their current locations if they are measuring high levels of SO2. If a monitor in the 

existing network indicates a violation of the 1-hour SO2 NAAQS, EPA intends to designate the 

 

39 EPA Regional Administrator approval will be required for any state to discontinue an existing 

monitoring site, and EPA does not expect that it will before 2011 approve discontinuation of 

monitoring at any site which appears to have a substantial likelihood of violating the 1-hour 

NAAQS.
167

area nonattainment, regardless of whether or not the monitor is located such that it could be 

counted towards meeting the proposed new network requirements. However, if the monitor 

indicates that the monitoring site meets the 1-hour SO2 NAAQS, EPA’s decision on the 

designation of the area would be made on a case-by-case basis. One possible outcome is that the 

area may be designated as unclassifiable because EPA would be unable to determine whether the 

area is violating the 1-hour SO2 NAAQS, or contributing to a violation in a nearby area, because 

of a lack of a complete monitoring network meeting the new network requirements.

Accordingly, state Governors would need to submit their initial designation 

recommendations to EPA no later than June 2011. If the Administrator intends to modify any 

state recommendation, EPA would notify the state's Governor no later than February 2012, 120 

days prior to promulgating the final designations. States would then have an opportunity to 

comment on EPA's tentative decisions before EPA promulgates the final designations in June 

2012. 

While CAA section 107 specifically addresses states, EPA intends to follow the same 

process for tribes to the extent practicable, pursuant to section 301(d) of the CAA regarding 

tribal authority, and the Tribal Authority Rule (63 FR 7254; February 12, 1998). Pursuant to the 

Tribal Authority Rule, Tribes are not subject to the schedule requirements that apply to states. 

However, EPA intends to promulgate designations for Tribal land as well as state land according 

to the schedule mandated for state land, so EPA encourages Tribes that wish to provide input on 

EPA’s designations to provide this input on the schedule mandated for states.

VI. Clean Air Act Implementation Requirements

This section of the preamble discusses the Clean Air Act (CAA) requirements that states 

and emissions sources would need to address when implementing new or revised SO2 NAAQS 
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based on the structure outlined in the CAA and existing rules.40 The EPA believes that there are 

sufficient guidance documents and regulations currently in place to fully implement the proposed 

revision to the SO2 NAAQS.

41

 However, EPA may provide additional guidance in the future, as

necessary, to assist states and emissions sources to comply with the CAA provisions for 

implementing a new or revised SO2 NAAQS. 

The CAA assigns important roles to EPA, states and tribal governments to achieve the 

NAAQS. States have the primary responsibility for developing and implementing state 

implementation plans (SIPs) that contain state measures necessary to achieve the air quality 

standards in each area once EPA has established the NAAQS. EPA provides assistance to states 

and tribes by providing technical tools, assistance, and guidance, including information on the 

potential control measures that may assist in helping areas attain the standards. 

Under section 110 of the CAA, 42 U.S.C. § 7410, and related provisions, states are 

directed to submit, for EPA approval, SIPs that provide for the attainment and maintenance of 

such standards through control programs directed at sources of SO2 emissions. If a state fails to 

adopt and implement the required SIPs by the time periods provided in the CAA, EPA has the 

responsibility under the CAA to adopt a federal implementation plan (FIP) to assure that areas 

attain the NAAQS in an expeditious manner. The states, in conjunction with EPA, also 

administer the prevention of significant deterioration (PSD) program for SO2. See sections 160-

169 of the CAA, 42 U.S.C. §§ 7470-7479. In addition, federal programs provide for nationwide 

reductions in emissions of SO2 and other air pollutants under Title II of the Act, 42 U.S.C. 

§§7521–7574. These programs involve limits on the sulfur content of the fuel used by 

 

40

 Since EPA is proposing to take comments on retaining the current 24-hr standards without revision if the 1-hr 

standard is set at 100-150 ppb, the discussion in this section relates to implementation of the proposed 1-hour 

standard and the possible retention or revocation of the current 24-hr standard.

41 See SO2 Guideline Document, Office of Air Quality Planning and Standards, Research Triangle Park, NC 27711, 

EPA-452/R-94-008, February 1994. 
169

automobiles, trucks, buses, motorcycles, non-road engines and equipment, marine vessels and 

locomotives. EPA is also in the process of establishing limits on the sulfur content of the fuel 

used by ocean going vessels. Emissions reductions for SO2 are also obtained from 

implementation of the new source performance standards (NSPS) for stationary sources under 

sections 111 and 129 of the CAA, 42 §§ U.S.C. 7411 and 7429; and the national emission 

standards for hazardous air pollutants (NESHAP) for stationary sources under section 112 of the 

CAA, 42 U.S.C. § 7412.

A. How this rule applies to tribes 

CAA section 301(d) authorizes EPA to treat eligible Indian tribes in the same manner as 

states (TAS) under the CAA and requires EPA to promulgate regulations specifying the 

provisions of the statute for which such treatment is appropriate. EPA has promulgated these 

regulations – known as the Tribal Authority Rule or TAR – at 40 C.F.R. Part 49. See 63 Fed. 

Reg. 7254 (February 12, 1998). The TAR establishes the process for Indian tribes to seek TAS 

eligibility and sets forth the CAA functions for which TAS will be available. Under the TAR, 

eligible tribes may seek approval for all CAA and regulatory purposes other than a small number 

of functions enumerated at section 49.4. Implementation plans under section 110 are included 

within the scope of CAA functions for which eligible tribes may obtain approval. Section 110(o) 

also specifically describes tribal roles in submitting implementation plans. Eligible Indian tribes 

may thus submit implementation plans covering their reservations and other areas under their 

jurisdiction.

The CAA and TAR do not, however, direct tribes to apply for TAS or implement any

CAA program. In promulgating the TAR EPA explicitly determined that it was not appropriate 

to treat tribes similarly to states for purposes of, among other things, specific plan submittal and 
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implementation deadlines for NAAQS-related requirements. 40 C.F.R. § 49.4(a). In addition, 

where tribes do seek approval of CAA programs, including section 110 implementation plans, 

the TAR provides flexibility and allows them to submit partial program elements, so long as such 

elements are reasonably severable – i.e., “not integrally related to program elements that are not 

included in the plan submittal, and are consistent with applicable statutory and regulatory

requirements”. 40 C.F.R. § 49.7.

To date, very few tribes have sought TAS for purposes of section 110 implementation 

plans. However, some tribes may be interested in pursuing such plans to implement today’s 

proposed standard, once it is promulgated. In several sections of this preamble, EPA describes 

the various roles and requirements states will address in implementing today’s proposed 

standard. Such references to states generally include eligible Indian tribes to the extent 

consistent with the flexibility provided to tribes under the TAR. Where tribes do not seek TAS 

for section 110 implementation plans, EPA under its discretionary authority will promulgate 

FIPs as “necessary or appropriate to protect air quality.” 40 C.F.R. § 49.11(a). EPA also notes 

that some tribes operate air quality monitoring networks in their areas. For such monitors to be 

used to measure attainment with the proposed revised primary NAAQS for SO2, the criteria and 

procedures identified in this proposed rule would apply.

B. Attainment dates

The latest date by which an area is required to attain the SO2 NAAQS is determined from 

the effective date of the nonattainment designation for the affected area. For areas designated 

nonattainment for the revised SO2 NAAQS, SIPs must provide for attainment of the NAAQS as 

expeditiously as practicable, but no later than 5 years from the effective date of the 

nonattainment designation for the area. See section 192(a) of the CAA. The EPA will determine 
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whether an area has demonstrated attainment of the SO2 NAAQS by evaluating air quality

monitoring data consistent with the form of the NAAQS for SO2, if revised, which will be 

codified at 40 CFR part 50, Appendix T. 

1. Attaining the NAAQS

In order for an area to be redesignated as attainment, it must meet five conditions 

provided under section 107(d)(3)(E) of the CAA. This section requires that:

 EPA must have determined that the area has met the SO2 NAAQS; 

 EPA has fully approved the state’s implementation plan; 

 the improvement in air quality in the affected area is due to permanent and enforceable 

reductions in emissions;

 EPA has fully approved a maintenance plan for the area; and

 the state(s) containing the area have met all applicable requirements under section 110 

and part D. 

2. Consequences of failing to attain by the statutory attainment date 

Any SO2 nonattainment area that fails to attain by its statutory attainment date would be 

subject to the requirements of sections 179(c) and (d) of the CAA. EPA is required to make a 

finding of failure to attain no later than 6 months after the specified attainment date and publish a 

notice in the Federal Register. The state would then need to submit an implementation plan 

revision no later than one year following the effective date of the Federal Register notice making 

the determination of the area’s failure to attain. This submission must demonstrate that the 

standard will be attained as expeditiously as practicable, but no later than 5 years from the 

effective date of EPA’s finding that the area failed to attain. In addition, section 179(d)(2) 

provides that the SIP revision must include any specific additional measures as may be 
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reasonably prescribed by EPA, including “all measures that can be feasibly implemented in the 

area in light of technological achievability, costs, and any nonair quality and other air qualityrelated health and environmental impacts.” 

C. Section 110(a)(1) and (2) NAAQS infrastructure requirements 

Section 110(a)(2) of the CAA directs all states to develop and maintain a solid air quality

management infrastructure, including enforceable emission limitations, an ambient monitoring 

program, an enforcement program, air quality modeling capabilities, and adequate personnel, 

resources, and legal authority. Section 110(a)(2)(D) also requires state plans to prohibit 

emissions from within the state which contribute significantly to nonattainment or maintenance 

areas in any other state, or which interfere with programs under part C of the CAA to prevent 

significant deterioration of air quality or to achieve reasonable progress toward the national 

visibility goal for Federal class I areas (national parks and wilderness areas).

Under sections 110(a)(1) and (2) of the CAA, all states are directed to submit SIPs to 

EPA which demonstrate that basic program elements have been addressed within 3 years of the 

promulgation of any new or revised NAAQS. Subsections (A) through (M) of section 110(a)(2) 

set forth the elements that a state’s program must contain in the SIP.42 The list of section 

110(a)(2) NAAQS implementation requirements are the following: 

 Ambient air quality monitoring/data system: Section 110(a)(2)(B) requires SIPs to 

provide for setting up and operating ambient air quality monitors, collecting and 

analyzing data and making these data available to EPA upon request.

 Program for enforcement of control measures: Section 110(a)(2)(C) requires SIPs to 

 

42

 Two elements identified in section 110(a)(2) are not listed below because, as EPA interprets the CAA, SIPs 

incorporating any necessary local nonattainment area controls would not be due within 3 years, but rather are due at 

the time the nonattainment area planning requirements are due. These elements are: (1) Emission limits and other 

control measures, section 110(a)(2)(A), and (2) Provisions for meeting part D, section 110(a)(2)(I), which requires 

areas designated as nonattainment to meet the applicable nonattainment planning requirements of part D, title I of 

the CAA. 
173

include a program providing for enforcement of SIP measures and the regulation and 

permitting of new/modified sources.

 Interstate transport: Section 110(a)(2)(D) requires SIPs to include provisions prohibiting 

any source or other type of emissions activity in the state from contributing significantly 

to nonattainment or interfering with maintenance of the NAAQS in another state, or from 

interfering with measures required to prevent significant deterioration of air quality or to 

protect visibility. 

 Adequate resources: Section 110(a)(2)(E) directs states to provide assurances of 

adequate funding, personnel and legal authority to implement their SIPs.

 Stationary source monitoring system: Section 110(a)(2)(F) directs states to establish a 

system to monitor emissions from stationary sources and to submit periodic emissions 

reports to EPA.

 Emergency power: Section 110(a)(2)(G) directs states to include contingency plans, and 

adequate authority to implement them, for emergency episodes in their SIPs. 

 Provisions for SIP revision due to NAAQS changes or findings of inadequacies: Section 

110(a)(2)(H) directs states to provide for revisions of their SIPs in response to changes in 

the NAAQS, availability of improved methods for attaining the NAAQS, or in response 

to an EPA finding that the SIP is inadequate.

 Consultation with local and Federal government officials: Section 110(a)(2)(J) directs 

states to meet applicable local and Federal government consultation requirements when 

developing SIPs and reviewing preconstruction permits.

 Public notification of NAAQS exceedances: Section 110(a)(2)(J) directs states to adopt 

measures to notify the public of instances or areas in which a NAAQS is exceeded.
174

 PSD and visibility protection: Section 110(a)(2)(J) also directs states to adopt emissions 

limitations, and such other measures, as may be necessary to prevent significant 

deterioration of air quality in attainment areas and protect visibility in Federal Class I 

areas in accordance with the requirements of CAA Title I, part C. 

 Air quality modeling/data: Section 110(a)(2)(K) requires that SIPs provide for 

performing air quality modeling for predicting effects on air quality of emissions of any 

NAAQS pollutant and submission of data to EPA upon request.

 Permitting fees: Section 110(a)(2)(L) requires the SIP to include requirements for each 

major stationary source to pay permitting fees to cover the cost of reviewing, approving, 

implementing and enforcing a permit.

 Consultation/participation by affected local government: Section 110(a)(2)(M) directs 

states to provide for consultation and participation by local political subdivisions affected 

by the SIP.

D. Attainment planning requirements 

1. SO2 nonattainment area SIP requirements

Any state containing an area designated as nonattainment with respect to the SO2

NAAQS would need to develop for submission to EPA a SIP meeting the requirements of part 

D, Title I, of the CAA, providing for attainment by the applicable statutory attainment date. See 

sections 191(a) and 192(a) of the CAA. As indicated in section 191(a), all components of the 

SO2 part D SIP must be submitted within 18 months of the effective date of an area’s designation 

as nonattainment. 

Section 172 of the CAA addresses the general requirements for areas designated as 

nonattainment. Section 172(c) directs states with nonattainment areas to submit a SIP which 
175

contains an attainment demonstration showing that the affected area will attain the standard by 

the applicable statutory attainment date. The SIP must show that the area will attain the standard 

as expeditiously as practicable, and must “provide for the implementation of all Reasonably 

Available Control Measures (RACM) as expeditiously as practicable (including such reductions 

in emissions from existing sources in the area as may be obtained through the adoption, at a 

minimum, of Reasonably Available Control Technology (RACT)).” 

SIPs required under Part D of the CAA must also provide for reasonable further progress 

(RFP). See section 172(c)(2) of the CAA. The CAA defines RFP as “such annual incremental 

reductions in emissions of the relevant air pollution as are required by part D, or may reasonably 

be required by the Administrator for the purpose of ensuring attainment of the applicable 

NAAQS by the applicable attainment date.” See section 171 of the CAA. Historically, for some 

pollutants, RFP has been met by showing annual incremental emission reductions sufficient to 

maintain generally linear progress toward attainment by the applicable attainment date. 

All SO2 nonattainment area SIPs must include contingency measures which must be 

implemented in the event that an area fails to meet RFP or fails to attain the standards by its 

attainment date. See section 172(c)(9) of the CAA. These contingency measures must be fully

adopted rules or control measures that take effect without further action by the state or the 

Administrator. The EPA interprets this requirement to mean that the contingency measures must 

be implemented with only minimal further action by the state or the affected sources with no 

additional rulemaking actions such as public hearings or legislative review. 

Emission inventories are also critical for the efforts of state, local, and federal agencies to 

attain and maintain the NAAQS that EPA has established for criteria pollutants including SO2. 

Section 191(a) in conjunction with section 172(c) requires that areas designated as nonattainment 
176

for SO2 submit an emission inventory to EPA no later than 18 months after designation as 

nonattainment. In the case of SO2, sections 191(a) and 172(c) also direct states to submit 

periodic emission inventories for nonattainment areas. The periodic inventory must include 

emissions of SO2 for point, nonpoint, mobile, and area sources. 

2. New source review and prevention of significant deterioration requirements 

The Prevention of Significant Deterioration (PSD) and nonattainment New Source 

Review (NSR) programs contained in parts C and D of Title I of the CAA govern 

preconstruction review of any new or modified major stationary sources of air pollutants 

regulated under the CAA as well as any precursors to the formation of that pollutant when 

identified for regulation by the Administrator.43 The EPA rules addressing these programs can 

be found at 40 CFR 51.165, 51.166, 52.21, 52.24, and Part 51, appendix S. 

The PSD program applies when a major source located in an area that is designated as 

attainment or unclassifiable for any criteria pollutant is constructed or undergoes a major 

modification.44 The nonattainment NSR program applies on a pollutant-specific basis when a 

major source constructs or modifies in an area that is designated as nonattainment for that 

pollutant. The minor NSR program addresses major and minor sources that undergo 

construction or modification activities that do not qualify as major, and it applies, as necessary to 

assure attainment, regardless of the designation of the area in which a source is located. 

PSD permit requirements are effective on the promulgation date of a new or revised 

standard. SIPs that address the PSD requirements related to attainment areas are due no later 

 

43 The terms “major” and “minor” define the size of a stationary source, for applicability purposes, in terms of an 

annual emissions rate (tons per year, tpy) for a pollutant. Generally, a minor source is any source that is not 

“major.” “Major” is defined by the applicable regulations—PSD or nonattainment NSR.

44 In addition, the PSD program applies to non-criteria pollutants subject to regulation under the Act, except those 

pollutants regulated under section 112 and pollutants subject to regulation only under section 211(o).
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than 3 years after the promulgation of a revised NAAQS for SO2. The PSD requirements include 

but are not limited to the following:

 installation of Best Available Control Technology (BACT);

 air quality monitoring and modeling analyses to ensure that a project’s emissions will not 

cause or contribute to a violation of any NAAQS or maximum allowable pollutant increase 

(PSD increment);

 notification of Federal Land Manager of nearby Class I areas; and

public comment on the permit.

If EPA establishes a 1-hour NAAQS for SO2, the owner or operator of any major 

stationary source or major modification locating in an attainment or unclassifiable area for SO2

will be required, as a prerequisite for a PSD permit, to demonstrate that the emissions increases 

from the new or modified source will not cause or contribute to a violation of the that new 

NAAQS. The EPA does not anticipate that this will pose a technical problem, since the 

modeling capability and SO2 emissions input data already exist. Depending on the final form of 

the 1-hour NAAQS, it may be necessary to make adjustments to the AERMOD modeling system 

to accommodate the form of the standard; however, EPA anticipates that any such adjustments 

can be readily accomplished in coordination with the promulgation of any new NAAQS for SO2

in time to enable states to implement such standard via the PSD program. The analyses for the 

1-hour NAAQS will be in addition to the existing demonstration of compliance for the annual 

and 24-hour SO2 NAAQS, which will continue to be required unless EPA revokes these 

standards in conjunction with its promulgation of a new 1-hour NAAQS for SO2. 
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The owner of operator of a new or modified source will still be required to demonstrate 

compliance with the annual and 24-hour SO2 increments, even if their counterpart NAAQS are 

revoked. The annual and 24-hour increments are established in the CAA and will need to remain 

in the PSD regulations because EPA does not interpret the Clean Air Act to authorize EPA to 

remove them. It appears necessary for Congress to amend the Act to make appropriate changes 

to the statutory SO2 increments, perhaps similar to the way the Act was amended to 

accommodate PM10 increments in lieu of the statutory TSP increments. If we establish a new 1-

hour SO2 NAAQS, EPA will consider the need to adopt new 1-hour SO2 increments. 

In association with the requirement to demonstrate compliance with the NAAQS and 

increments, the owner or operator of a new or modified source must submit for review and 

approval a source impact analysis and an air quality analysis. The source impact analysis, 

primarily a modeling analysis, must demonstrate that allowable emissions increases from the 

proposed source or modification, in conjunction with emissions from other existing sources will 

not cause or contribute to either a NAAQS or increment violation. The air quality analysis must 

assess the ambient air quality in the area that the proposed source or modification would affect. 

For the air quality analysis, the owner or operator must submit in its permit application 

air quality monitoring data that shall have been gathered over a period of one year and is 

representative of air quality in the area of the proposed project. If existing data representative of 

the area of the proposed project is not available, new data may need to be collected by the owner 

or operator of the source or modification. Where data is already available, it might be necessary

to evaluate the location of the monitoring sites from which the SO2 data were collected in 

comparison to any new siting requirements associated with the 1-hour NAAQS. If existing sites 

are inappropriate for providing the necessary representative data, then new monitoring data will 
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need to be collected by the owner or operator of the proposed project. 

Historically, EPA has allowed the use of several screening tools to help facilitate the 

implementation of the new source review program by reducing the permit applicant’s burden, 

and streamlining the permitting process for de minimis circumstances. These screening tools 

include a significant emissions rate (SER), significant impact levels (SILs), and a significant 

monitoring concentration (SMC). The SER, as defined in tons per year for each regulated 

pollutant, is used to determine whether any proposed source or modification will emit sufficient 

amounts of a particular pollutant to require the review of that pollutant under the NSR permit 

program. EPA will consider whether to evaluate the existing significant emissions rate (SER) 

for SO2 to see if it would change substantially based on the NAAQS levels for the 1-hour 

averaging period. Historically, we have defined a de minimis pollutant impact as one that results 

in a modeled ambient impact of less than approximately 4% of the short-term NAAQS. The 

current SER for SO2 (40 tpy) is based on the impact on the 24-hour SO2 NAAQS. See, 45 FR 

52676, 52707 (August 7, 1980). We have typically used the most sensitive averaging period to 

calculate the SER, and we may want to evaluate the new 1-hour period for SO2 because it is 

likely to represent most sensitive averaging period for SO2. 

The SIL, expressed as an ambient pollutant concentration (ug/m3), is used to determine 

whether the impact of a particular pollutant is significant enough to warrant a complete air 

quality impact analysis for any applicable NAAQS and increments. EPA has promulgated 

regulations under 40 CFR 51.165(b) which include SILs for SO2 to determine whether a source’s 

impact would be considered to cause or contribute to a NAAQS violation for either the 3-hour, 

24-hour or annual averaging periods. These SILs were originally developed in 1978 to limit the 

application of air quality dispersion models to a downwind distance of no more than 50 
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kilometers or to “insignificant levels.” See, 43 FR 26398, June 19, 1978. Through guidance, 

EPA has also allowed the use of SILs to determine whether or not it is necessary for a source to 

carry out a comprehensive source impact analysis and to determine the extent of the impact area 

in which the analysis will be carried out. The existing SILs for SO2 were not developed on the 

basis of specific SO2 NAAQS levels, so if the existing NAAQS are not being revised, there is 

probably no need to revise the existing SILs. Even if we decide to revoke any of the existing 

NAAQS, the corresponding SIL should still be useful for increment assessment. A SIL for the 1-

hour averaging period does not exist, and would need to be developed for use with modeling for 

1-hour SO2 NAAQS and increments (if and when developed). 

Finally, the SMC, also measured as an ambient pollutant concentration (ug/m3), is used 

to determine whether it may be appropriate to exempt a proposed project from the requirement to 

collect ambient monitoring data for a particular pollutant as part of a complete permit 

application. EPA first defined SMCs for regulated pollutants under the PSD program in 1980. 

See, 45 FR 52676, 52709-10 (August 7, 1980). The existing SMC for SO2, based on a 24-hour 

averaging period, may need to be re-evaluated to consider the effect of basing the SMC on the 1-

hour averaging period, especially in light of the fact that we may revoke the NAAQS for the 24-

hour averaging period. Third, even if the 1-hour averaging period does not indicate the need for 

a revised SMC for SO2, the fact that the original SMC for SO2 is based on 1980 monitoring data 

(Lowest Detectable Level, correction factor of “5”), could be a basis for revising the existing 

value. More up-to-date monitoring data and statistical analyses of monitoring accuracy may

yield a different—possibly lower—correction factor today. A new 1-hour NAAQS would not 

necessarily cause this result, but may provide a “window of opportunity” to re-evaluate the SMC 

for SO2. See sections II. E. 2 and II.F. 2 above.
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As a means of reducing the permit applicant’s burden, and to streamline permitting, 

permit authorities use screening tools referred to as significant impact levels (SILs) and a 

significant monitoring concentration (SMC). EPA issued unofficial SO2 SILs for the 3-hour 

(secondary standard), 24-hour and annual averaging periods. These SILs were developed in 

1978 to limit the application of air quality dispersion models to a downwind distance of no more 

than 50 kilometers or to “insignificant levels.” See, 43 FR 263--, 26398, (June 19, 1978). These 

values were not developed on the basis of specific SO2 NAAQS levels, so if the existing 

NAAQS are not being revised, there is probably no need to revise the existing SILs. Even if we 

decide to revoke any of the existing NAAQS, the corresponding SIL should still be useful for 

increment assessment. A SIL for the 1-hour averaging period does not exist, and would need to 

be developed for use with modeling for the 1-hour SO2 NAAQS and increments (if and when 

developed). 

States which have areas designated as nonattainment for the SO2 NAAQS are directed to 

submit, as a part of the SIP due 18 months after an area is designated as nonattainment, 

provisions requiring permits for the construction and operation of new or modified stationary 

sources anywhere in the nonattainment area. Prior to adoption of the SIP revision addressing 

major source nonattainment NSR for SO2 nonattainment areas, the requirements of 40 CFR part 

51, appendix S will apply. Nonattainment NSR requirements include but are not limited to:

 installation of Lowest Achievable Emissions Rate (LAER) control technology;

 offsetting new emissions with creditable emissions reductions;

 a certification that all major sources owned and operated in the state by the same owner 

are in compliance with all applicable requirements under the CAA;
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 an alternative siting analysis demonstrating that the benefits of a proposed source 

significantly outweigh the environmental and social costs imposed as a result of its 

location, construction, or modification; and

 public comment on the permit.

Minor NSR programs must meet the statutory requirements in section 110(a)(2)(C) of the 

CAA which requires “...regulation of the modification and construction of any stationary source 

...as necessary to assure that the [NAAQS] are achieved.” These programs must be established 

in each state within 3 years of the promulgation of a new or revised NAAQS. 

3. General conformity

Section 176(c) of the CAA requires that all federal actions conform to an applicable 

implementation plan developed pursuant to section 110 and part D of the CAA. The EPA rules 

developed under section 176(c) prescribe the criteria and procedures for demonstrating and 

assuring conformity of federal actions to a SIP. Each federal agency must determine that any 

actions covered by the general conformity rule conform to the applicable SIP before the action is 

taken. The criteria and procedures for conformity apply only in nonattainment areas and those 

areas redesignated attainment since 1990 (“maintenance areas”) with respect to the criteria 

pollutants under the CAA45: carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone 

(O3), particulate matter (PM2.5 and PM10), and sulfur dioxide (SO2). The general conformity 

rules apply one year following the effective date of designations for any new or revised

NAAQS.46

 

45 Criteria pollutants are those pollutants for which EPA has established a NAAQS under section 109 of the CAA.

46 Transportation conformity is required under CAA section 176(c) (42 U.S.C. 7506(c) to ensure that federally 

supported highway and transit project activities are consistent with (“conform to”) the purpose of the SIP. 

Transportation conformity applies to areas that are designated nonattainment, and those areas redesignated to 

attainment after 1990 (“maintenance areas” with plans developed under CAA section 175A) for transportation-
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The general conformity determination examines the impacts of direct and indirect 

emissions related to federal actions. The general conformity rule provides several options to 

satisfy air quality criteria, such as modeling or offsets, and requires the federal action to also 

meet any applicable SIP requirements and emissions milestones. The general conformity rule 

also requires that notices of draft and final general conformity determinations be provided 

directly to air quality regulatory agencies and to the public by publication in a local newspaper.

E. Transition from the existing SO2 NAAQS to a revised SO2 NAAQS 

As stated in section II.F.5 of this notice, in addition to proposing a short-term 1-hour SO2

NAAQS, EPA is proposing to revoke the current annual and 24-hour standards, (annual 0.03 

ppm and 24-hour 0.14 ppm). Specifically, EPA is proposing that the level for the 1-hour 

standard for SO2 be a range between 50-100 ppb, and is taking comment on setting the level of 

the standard up to 150 ppb. If the Administrator sets the 1-hour standard at 100 ppb or lower, 

EPA is proposing to revoke the current 24-hour standard. If the Administrator sets the level of 

the 1-hour standard between a range of 100-150 ppb, then EPA would retain the current 24-hour 

standard. 

If EPA revises the SO2 NAAQS and revokes either the current annual or 24-hour 

standard, EPA would need to promulgate adequate anti-backsliding provisions. The CAA 

establishes anti-backsliding requirements where EPA relaxes a NAAQS. Here, if EPA were to 

replace the annual and/or 24-hour standard with a short term 1-hour standard, EPA would need 

to address the section 172(e) anti-backsliding provision of the CAA and determine whether it 

applies on its face or by analogy, and what provisions would be appropriate to provide for 

 

related criteria pollutants. Due to the relatively small amounts of sulfur in gasoline and on-road diesel fuel, 

transportation conformity does not apply to the SO2 NAAQS. 40 CFR 93.102(b)(1).
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transition to the new standard. States would need to insure that the health protection provided 

under the existing SO2 NAAQS continues to be achieved as well as maintained as states begin to 

implement a revised NAAQS. This means that states would be directed to continue 

implementing attainment and maintenance SIPs associated with the existing SO2 NAAQS until 

such time as they are subsumed by any new planning and control requirements associated with a 

revised NAAQS.

Whether or not section 172(e) directly applies to EPA’s final action on the SO2 NAAQS, 

EPA has previously looked to other provisions of the CAA to determine how to address antibacksliding. The CAA contains a number of provisions that indicate Congress’s intent to not 

allow provisions from implementation plans to be altered or removed if the plan revision would 

jeopardize the air quality protection being provided by the existing plan when EPA revises a 

NAAQS to make it more stringent. For example, section 110(l) provides that EPA may not 

approve a SIP revision if it interferes with any applicable requirement concerning attainment and 

RFP, or any other applicable requirement under the CAA. In addition, section 193 of the CAA 

prohibits the modification of a control, or a control requirement, in effect or required to be 

adopted as of November 15, 1990 (i.e., prior to the promulgation of the Clean Air Act 

Amendments of 1990), unless such a modification would ensure equivalent or greater emissions 

reductions. Further, section 172(e) of the CAA specifies that if EPA revises a NAAQS to make 

it less stringent than a previous NAAQS, control obligations that apply in nonattainment area 

SIPs may not be relaxed, and adopting those controls that have not yet been adopted as needed

may not be avoided. The intent of Congress, concerning the aforementioned sections of the 

CAA, was confirmed in a recent D.C. Circuit Court opinion on the Phase I ozone implementation 

rule. See South Coast Air Quality Management Dist. v. EPA, 472 F.3d 882 (D.C. Cir. 2006).
185

To ensure that the antibacksliding provisions and principles of section 172(e) are met and 

applied if EPA revokes the current standards, EPA is proposing that the current SO2 NAAQS 

would remain in effect for one year following the effective date of the initial designations under 

section 107(d)(1) for the revised SO2 NAAQS before the current NAAQS are revoked in most 

attainment areas. However, any existing SIP provisions under CAA sections 110, 191 and 192 

associated with the existing annual and 24-hour SO2 NAAQS would remain in effect, including 

all currently implemented planning and emissions control obligations, including both those in the 

state’s SIP and that have been promulgated by EPA in FIPs. This would ensure that both the 

new nonattainment NSR requirements and the general conformity requirements for a revised 

standard are in place so that there will be no gap in the public health protections provided by

these two programs. It will also insure that all nonattainment areas under the current NAAQS 

and all areas for which SIP calls have been issued would continue to be protected by currently 

required control measures. 

EPA is also proposing that the existing NAAQS remain in place for any current 

nonattainment area, or any area for which a state has not fulfilled the requirements of a SIP call, 

until the affected area submits, and EPA approves, a SIP with an attainment demonstration which 

fully addresses the attainment requirements of the revised SO2 NAAQS. This, in combination 

with the CAA mechanisms provided in sections 110(l), 193, and 172(e) will help to ensure that 

continued progress is made toward timely attainment of the SO2 NAAQS. Also, in light of the 

nature of the proposed revision of the SO2 NAAQS, the lack of classifications (and mandatory

controls associated with such classifications pursuant to the CAA), and the small number of 

current nonattainment areas, and areas subject to SIP calls, EPA believes (subject to 

consideration of public comment) that retaining the current standard for a limited period of time 
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until attainment SIPs are approved for the new standard in current nonattainment areas and SIP 

call areas, and one year after designations in other areas, will adequately serve the antibacksliding requirements and goals of the CAA.47

VII. Communication of Public Health Information

Information on the public health implications of ambient concentrations of criteria 

pollutants is currently made available primarily through EPA's Air Quality Index (AQI) program. 

The current Air Quality Index has been in use since its inception in 1999 (64 FR 42530). It 

provides accurate, timely, and easily understandable information about daily levels of pollution 

(40 CFR 58.50). The AQI establishes a nationally uniform system of indexing pollution levels 

for NO2, carbon monoxide, ozone, particulate matter and sulfur dioxide. The AQI converts 

pollutant concentrations in a community's air to a number on a scale from 0 to 500. Reported 

AQI values enable the public to know whether air pollution levels in a particular location are 

characterized as good (0 - 50), moderate (51 - 100), unhealthy for sensitive groups (101 - 150), 

unhealthy (151 - 200), very unhealthy (201 - 300), or hazardous (300 - 500). The AQI index 

value of 100 typically corresponds to the level of the short-term primary NAAQS for each 

pollutant. An AQI value greater than 100 means that a pollutant is in one of the unhealthy 

categories (i.e., unhealthy for sensitive groups, unhealthy, very unhealthy, or hazardous) on a 

given day; an AQI value at or below 100 means that a pollutant concentration is in one of the 

satisfactory categories (i.e., moderate or good). Decisions about the pollutant concentrations at 

which to set the various AQI breakpoints, that delineate the various AQI categories, draw 

directly from the underlying health information that supports the review of the primary NAAQS.

 

47 The areas that are currently designated as nonattainment for the pre-existing SO2 primary NAAQS are Hayden, 

AZ; Armstrong, PA; Laurel, MT; Piti, GU; and Tanguisson, GU. The areas that are designated nonattainment for 

both the primary and the secondary standards are East Helena, MT, Salt Lake Co, MT, Toole Co, UT, and Warren 

Co, NJ. ( See http://www.epa.gov/oar/oaqps/greenbk/lnc.html). The Billings/Laurel, MT, area is the only area 

currently subject to a SIP call.
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The Agency recognizes the importance of revising the AQI in a timely manner to be 

consistent with any revisions to the primary NAAQS. Therefore EPA proposes to finalize 

conforming changes to the AQI, in connection with the Agency's final decision on the SO2

NAAQS if revisions to the primary standard are promulgated. If EPA promulgates a short-term 

primary SO2 NAAQS, conforming changes would include setting the 100 level of the AQI at the 

same level as the revised primary SO2 NAAQS. Conforming changes also would include setting 

the other AQI breakpoints at the lower end of the AQI scale (i.e., AQI values of 50 and 150). 

EPA does not propose to change breakpoints at the higher end of the AQI scale (from 200 to 

500), which would apply to state contingency plans or the Significant Harm Level (40 CFR 

51.16), because the information from this review does not inform decisions about breakpoints at 

those higher levels. 

With regard to an AQI value of 50, the breakpoint between the good and moderate 

categories, historically this value is set at the level of the annual NAAQS, if there is one, or onehalf the level of the short-term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 

12, 1998). Taking into consideration this practice, EPA is proposing to set the AQI value of 50 

to be between 25 and 50 ppb SO2, 1-hour average. EPA anticipates that figures towards the 

lower end of this range would be appropriate if the standard is set towards the lower end of the 

range for the proposed standard (e.g. 50 ppb), while figures towards the higher end of the range 

would be more appropriate for standards set at the higher end of the range (e.g., 100 ppb). If the 

short-term standard is set at a level above 100 ppb, and (contrary to the proposal) the annual 

standard is not revoked, then consideration could be given to setting an AQI value of 50 at the 

level of the annual standard, or 30 ppb. EPA solicits comments on this range for an AQI of 50, 

and the appropriate basis for selecting an AQI of 50 both within this range and, in light of EPA's 
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solicitation of comment on 1-hour standard levels above 100 ppb, above this range. 

With regard to an AQI value of 150, the breakpoint between the unhealthy for sensitive 

groups and unhealthy categories, historically values between the short-term standard and an AQI 

value of 500 are set at levels that are approximately equidistant between the AQI values of 100 

and 500 unless there is health evidence that suggests a specific level would be appropriate (63 

FR 67829, Dec. 12, 1998). For an AQI value of 150, the range of 175 to 200 ppb SO2, 1-hour 

average, represents the midpoint between the proposed range for the short-term standard and the 

level of an AQI value of 200 (300 ppb SO2, 1-hour average). 

VIII Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

Under section 3(f)(1) of Executive Order 12866 (58 FR 51735, October 4, 1993), this action 

is an “economically significant regulatory action” because it is likely to have an annual effect on 

the economy of $100 million or more. Accordingly, EPA submitted this action to the Office of 

Management and Budget (OMB) for review under EO 12866 and any changes made in response 

to OMB recommendations have been documented in the docket for this action. In addition, EPA 

prepared a Regulatory Impact Analysis (RIA) of the potential costs and benefits associated with 

this action. However, the CAA and judicial decisions make clear that the economic and technical 

feasibility of attaining the national ambient standards cannot be considered in setting or revising 

NAAQS, although such factors may be considered in the development of State implementation 

plans to implement the standards. Accordingly, although an RIA has been prepared, the results 

of the RIA have not been considered by EPA in developing this proposed rule.
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B. Paperwork Reduction Act

The information collection requirements in this proposed rule have been submitted for 

approval to the Office of Management and Budget (OMB) under the Paperwork Reduction Act, 

44 U.S.C. 3501 et seq. The Information Collection Request (ICR) document prepared by EPA 

for these proposed revisions to part 58 has been assigned EPA ICR number 2370.01

The information collected under 40 CFR part 53 (e.g., test results, monitoring records, 

instruction manual, and other associated information) is needed to determine whether a candidate 

method intended for use in determining attainment of the NAAQS in 40 CFR part 50 will meet 

the design, performance, and/or comparability requirements for designation as a Federal 

reference method (FRM) or Federal equivalent method (FEM). We do not expect the number of 

FRM or FEM determinations to increase over the number that is currently used to estimate 

burden associated with SO2 FRM/FEM determinations provided in the current ICR for 40 CFR 

part 53 (EPA ICR numbers 2370.01). As such, no change in the burden estimate for 40 CFR 

part 53 has been made as part of this rulemaking. 

The information collected and reported under 40 CFR part 58 is needed to determine 

compliance with the NAAQS, to characterize air quality and associated health impacts, to 

develop emissions control strategies, and to measure progress for the air pollution program. The 

proposed amendments would revise the technical requirements for SO2 monitoring sites, require 

the siting and operation of additional SO2 ambient air monitors, and the reporting of the collected 

ambient SO2 monitoring data to EPA’s Air Quality System (AQS). The annual average reporting 

burden for the collection under 40 CFR part 58 (averaged over the first 3 years of this ICR) is 

$13,863,950. Burden is defined at 5 CFR 1320.3(b). State, local, and tribal entities are eligible 
190

for State assistance grants provided by the Federal government under the CAA which can be 

used for monitors and related activities.

An agency may not conduct or sponsor, and a person is not required to respond to, a 

collection of information unless it displays a currently valid OMB control number. The OMB 

control numbers for EPA's regulations in 40 CFR are listed in 40 CFR part 9. 

To comment on the Agency's need for this information, the accuracy of the provided 

burden estimates, and any suggested methods for minimizing respondent burden, EPA has 

established a public docket for this rule, which includes this ICR, under Docket ID number EPAHQ-OAR-2007-0352. Submit any comments related to the ICR to EPA and OMB. See 

ADDRESSES section at the beginning of this notice for where to submit comments to EPA. 

Send comments to OMB at the Office of Information and Regulatory Affairs, Office of 

Management and Budget, 725 17th Street, NW, Washington, DC 20503, Attention: Desk Office 

for EPA. Since OMB is required to make a decision concerning the ICR between 30 and 60 days 

after [Insert date of publication in the Federal Register], a comment to OMB is best assured 

of having its full effect if OMB receives it by [Insert date 30 days after publication in the 

Federal Register.]. The final rule will respond to any OMB or public comments on the 

information collection requirements contained in this proposal.

C. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) generally requires an agency to prepare a 

regulatory flexibility analysis of any rule subject to notice and comment rulemaking 

requirements under the Administrative Procedure Act or any other statute unless the agency

certifies that the rule will not have a significant economic impact on a substantial number of 
191

small entities. Small entities include small businesses, small organizations, and small 

governmental jurisdictions.

For purposes of assessing the impacts of this rule on small entities, small entity is defined 

as: (1) a small business that is a small industrial entity as defined by the Small Business 

Administration’s (SBA) regulations at 13 CFR 121.201; (2) a small governmental jurisdiction 

that is a government of a city, county, town, school district or special district with a population of 

less than 50,000; and (3) a small organization that is any not-for-profit enterprise which is 

independently owned and operated and is not dominant in its field.

After considering the economic impacts of this proposed rule on small entities, I certify

that this action will not have a significant economic impact on a substantial number of small 

entities. This proposed rule will not impose any requirements on small entities. Rather, this rule 

establishes national standards for allowable concentrations of SO2 in ambient air as required by 

section 109 of the CAA. American Trucking Ass’ns v. EPA, 175 F. 3d 1027, 1044-45 (D.C. Cir. 

1999) (NAAQS do not have significant impacts upon small entities because NAAQS themselves 

impose no regulations upon small entities). Similarly, the proposed amendments to 40 CFR Part 

58 address the requirements for States to collect information and report compliance with the 

NAAQS and will not impose any requirements on small entities. We continue to be interested in 

the potential impacts of the proposed rule on small entities and welcome comments on issues 

related to such impacts.

D. Unfunded Mandates Reform Act

Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law 104-4, 

establishes requirements for Federal agencies to assess the effects of their regulatory actions on 

State, local, and tribal governments and the private sector. Unless otherwise prohibited by law, 
192

under section 202 of the UMRA, EPA generally must prepare a written statement, including a 

cost-benefit analysis, for proposed and final rules with “Federal mandates” that may result in 

expenditures to State, local, and tribal governments, in the aggregate, or to the private sector, of 

$100 million or more in any one year. Before promulgating an EPA rule for which a written 

statement is required under section 202, section 205 of the UMRA generally requires EPA to 

identify and consider a reasonable number of regulatory alternatives and to adopt the least costly, 

most cost-effective or least burdensome alternative that achieves the objectives of the rule. The 

provisions of section 205 do not apply when they are inconsistent with applicable law. 

Moreover, section 205 allows EPA to adopt an alternative other than the least costly, most costeffective or least burdensome alternative if the Administrator publishes with the final rule an 

explanation why that alternative was not adopted. Before EPA establishes any regulatory

requirements that may significantly or uniquely affect small governments, including tribal 

governments, it must have developed under section 203 of the UMRA a small government 

agency plan. The plan must provide for notifying potentially affected small governments, 

enabling officials of affected small governments to have meaningful and timely input in the 

development of EPA regulatory proposals with significant Federal intergovernmental mandates, 

and informing, educating, and advising small governments on compliance with the regulatory

requirements. 

This action is not subject to the requirements of sections 202 and 205 of the UMRA. 

EPA has determined that this proposed rule does not contain a Federal mandate that may result in 

expenditures of $100 million or more for State, local, and tribal governments, in the aggregate, 

or the private sector in any one year. The revisions to the SO2 NAAQS impose no enforceable 

duty on any State, local or Tribal governments or the private sector. The expected costs 
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associated with the monitoring requirements are described in EPA’s ICR document, but those 

costs are not expected to exceed $100 million in the aggregate for any year. Furthermore, as 

indicated previously, in setting a NAAQS, EPA cannot consider the economic or technological 

feasibility of attaining ambient air quality standards. Because the CAA prohibits EPA from 

considering the types of estimates and assessments described in section 202 when setting the 

NAAQS, the UMRA does not require EPA to prepare a written statement under section 202 for 

the revisions to the SO2 NAAQS. 

With regard to implementation guidance, the CAA imposes the obligation for States to 

submit SIPs to implement the SO2 NAAQS. In this proposed rule, EPA is merely providing an 

interpretation of those requirements. However, even if this rule did establish an independent 

obligation for States to submit SIPs, it is questionable whether an obligation to submit a SIP 

revision would constitute a Federal mandate in any case. The obligation for a State to submit a 

SIP that arises out of section 110 and section 191 of the CAA is not legally enforceable by a 

court of law, and at most is a condition for continued receipt of highway funds. Therefore, it is 

possible to view an action requiring such a submittal as not creating any enforceable duty within 

the meaning of U.S.C. 658 for purposes of the UMRA. Even if it did, the duty could be viewed 

as falling within the exception for a condition of Federal assistance under U.S.C. 658.

EPA has determined that this proposed rule contains no regulatory requirements that 

might significantly or uniquely affect small governments because it imposes no enforceable duty 

on any small governments. Therefore, this rule is not subject to the requirements of section 203 

of the UMRA.
194

E. Executive Order 13132: Federalism

Executive Order 13132, entitled “Federalism” (64 FR 43255; August 10, 1999), requires 

EPA to develop an accountable process to ensure “meaningful and timely input by State and 

local officials in the development of regulatory policies that have federalism implications.” 

“Policies that have federalism implications” is defined in the Executive Order to include 

regulations that have “substantial direct effects on the States, on the relationship between the 

national government and the States, or on the distribution of power and responsibilities among 

the various levels of government.” 

This proposed rule does not have federalism implications. It will not have substantial 

direct effects on the States, on the relationship between the national government and the States, 

or on the distribution of power and responsibilities among the various levels of government, as 

specified in Executive Order 13132. The rule does not alter the relationship between the Federal 

government and the States regarding the establishment and implementation of air quality 

improvement programs as codified in the CAA. Under section 109 of the CAA, EPA is 

mandated to establish NAAQS; however, CAA section 116 preserves the rights of States to 

establish more stringent requirements if deemed necessary by a State. Furthermore, this rule 

does not impact CAA section 107 which establishes that the States have primary responsibility

for implementation of the NAAQS. Finally, as noted in section E (above) on UMRA, this rule 

does not impose significant costs on State, local, or tribal governments or the private sector. 

Thus, Executive Order 13132 does not apply to this rule.

However, EPA recognizes that States will have a substantial interest in this rule and any 

corresponding revisions to associated air quality surveillance requirements, 40 CFR part 58. 

Therefore, in the spirit of Executive Order 13132, and consistent with EPA policy to promote 
195

communications between EPA and State and local governments, EPA specifically solicits 

comment on this proposed rule from State and local officials.

F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments

Executive Order 13175, entitled “Consultation and Coordination with Indian Tribal 

Governments” (65 FR 67249, November 9, 2000), requires EPA to develop an accountable 

process to ensure “meaningful and timely input by tribal officials in the development of 

regulatory policies that have tribal implications.” This proposed rule does not have tribal 

implications, as specified in Executive Order 13175. It does not have a substantial direct effect 

on one or more Indian tribes, on the relationship between the Federal government and Indian 

tribes, or on the distribution of power and responsibilities between the Federal government and 

tribes. The rule does not alter the relationship between the Federal government and tribes as 

established in the CAA and the TAR. Under section 109 of the CAA, EPA is mandated to 

establish NAAQS; however, this rule does not infringe existing tribal authorities to regulate air 

quality under their own programs or under programs submitted to EPA for approval. 

Furthermore, this rule does not affect the flexibility afforded to tribes in seeking to implement 

CAA programs consistent with the TAR, nor does it impose any new obligation on tribes to 

adopt or implement any NAAQS. Finally, as noted in section E (above) on UMRA, this rule 

does not impose significant costs on tribal governments. Thus, Executive Order 13175 does not 

apply to this rule. However, EPA recognizes that tribes may be interested in this rule and any 

corresponding revisions to associated air quality surveillance requirements. Therefore, in the 

spirit of Executive Order 13175, and consistent with EPA policy to promote communications

between EPA and tribes, EPA specifically solicits additional comment on this proposed rule 

from tribal officials. 
196

G. Executive Order 13045: Protection of Children from Environmental Health & Safety 

Risks

This action is subject to Executive Order (62 FR 19885, April 23, 1997) because it is an 

economically significant regulatory action as defined by Executive Order 12866, and we believe 

that the environmental health risk addressed by this action has a disproportionate effect on 

children. The proposed rule will establish uniform national ambient air quality standards for 

SO2; these standards are designed to protect public health with an adequate margin of safety, as 

required by CAA section 109. The protection offered by these standards may be especially 

important for asthmatics, including asthmatic children, because respiratory effects in asthmatics 

are among the most sensitive health endpoints for SO2 exposure. Because asthmatic children are 

considered a sensitive population, we have evaluated the potential health effects of exposure to 

SO2 pollution among asthmatic children. These effects and the size of the population affected 

are discussed in chapters 3 and 4 of the ISA; chapters 3, 4, 7, 8, 9 of the REA, and sections II.A 

through II.E of this preamble. 

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution or 

Use

This rule is not a “significant energy action” as defined in Executive Order 13211, 

“Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use” 

(66 FR 28355; May 22, 2001) because it is not likely to have a significant adverse effect on the 

supply, distribution, or use of energy. The purpose of this rule is to establish revised NAAQS for 

SO2. The rule does not prescribe specific control strategies by which these ambient standards 

will be met. Such strategies will be developed by States on a case-by-case basis, and EPA 

cannot predict whether the control options selected by States will include regulations on energy
197

suppliers, distributors, or users. Thus, EPA concludes that this rule is not likely to have any

adverse energy effects.

I. National Technology Transfer and Advancement Act

Section 12(d) of the National Technology Transfer and Advancement Act of 1995 

(NTTAA), Public Law 104-113, section 12(d) (15 U.S.C. 27) directs EPA to use voluntary

consensus standards in its regulatory activities unless to do so would be inconsistent with 

applicable law or otherwise impractical. Voluntary consensus standards are technical standards 

(e.g., materials specifications, test methods, sampling procedures, and business practices) that are 

developed or adopted by voluntary consensus standards bodies. The NTTAA directs EPA to 

provide Congress, through OMB, explanations when the Agency decides not to use available and 

applicable voluntary consensus standards.

This proposed rulemaking involves technical standards with regard to ambient 

monitoring of SO2. The use of this voluntary consensus standard would be impractical because 

the analysis method does not provide for the method detection limits necessary to adequately 

characterize ambient SO2 concentrations for the purpose of determining compliance with the 

proposed revisions to the SO2 NAAQS.

EPA welcomes comments on this aspect of the proposed rule, and specifically invites the 

public to identify potentially applicable voluntary consensus standards and to explain why such 

standards should be used in the regulation.

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations

Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes federal executive policy

on environmental justice. Its main provision directs federal agencies, to the greatest extent 
198

practicable and permitted by law, to make environmental justice part of their mission by

identifying and addressing, as appropriate, disproportionately high and adverse human health or 

environmental effects of their programs, policies, and activities on minority populations and lowincome populations in the United States. 

EPA has determined that this proposed rule will not have disproportionately high and 

adverse human health or environmental effects on minority or low-income populations because it 

increases the level of environmental protection for all affected populations without having any 

disproportionately high and adverse human health effects on any population, including any

minority or low-income population. The proposed rule will establish uniform national standards 

for SO2 in ambient air. EPA solicits comment on environmental justice issues related to the 

proposed revision of the SO2 NAAQS. 
199

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List of Subjects 

40 CFR Part 50

Environmental protection, Air pollution control, Carbon monoxide, Lead, Nitrogen 

dioxide, Ozone, Particulate matter, Sulfur oxides.

40 CFR Part 53

Environmental protection, Administrative practice and procedure, Air pollution control, 

Intergovernmental relations, Reporting and recordkeeping requirements

40 CFR Part 58

Environmental protection, Administrative practice and procedure, Air pollution control,

Intergovernmental relations, Reporting and recordkeeping requirements

Dated:

Lisa P. Jackson

Administrator
206

For the reasons stated in the preamble, title 40, chapter I of the Code of Federal Regulations is 

proposed to be amended as follows: 
207

PART 50-NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

1. The authority citation for part 50 continues to read as follows:

Authority: 42 U.S.C. 7401, et seq.

Subpart A—General Provisions

2. Section 50.4 is amended by adding paragraph (e) to read as follows:

§50.4 National primary ambient air quality standards for sulfur oxides (sulfur dioxide).

* * * * *

(e) The standards set forth in this section will remain applicable to all areas notwithstanding the 

promulgation of SO2 national ambient air quality standards (NAAQS) in §50.17. The SO2

NAAQS set forth in this section will no longer apply to an area one year after the effective date 

of the designation of that area, pursuant to section 107 of the Clean Air Act, for the SO2 NAAQS 

set forth in §50. 17; except that for areas designated nonattainment for the SO2 NAAQS set forth 

in this section as of the effective date of §50. 17, and areas not meeting the requirements of a SIP 

call with respect to requirements for the SO2 NAAQS set forth in this section, the SO2 NAAQS 

set forth in this section will apply until that area submits, pursuant to section 191 of the Clean 

Air Act, and EPA approves, an implementation plan providing for attainment of the SO2

NAAQS set forth in §50.17.

3. Section 50.14 is amended by revising paragraph (c)(2)(vi) to read as follows:

§50.14 Treatment of air quality monitoring data influenced by exceptional events.
208

* * * * *

(c) ***

(2) ***

(vi) When EPA sets a NAAQS for a new pollutant or revises the NAAQS for an existing 

pollutant, it may revise or set a new schedule for flagging exceptional event data, providing 

initial data descriptions and providing detailed data documentation in AQS for the initial 

designations of areas for those NAAQS. Table 1 provides the schedule for submission of flags 

with initial descriptions in AQS and detailed documentation. These schedules shall apply for 

those data which will or may influence the initial designation of areas for those NAAQS. EPA 

anticipates revising Table 1 as necessary to accommodate revised data submission schedules for 

new or revised NAAQS.
209

Table 1- Schedule or Exceptional Event Flagging and Documentation Submission for Data to be Used in 

Designations Decisions for New or Revised NAAQS

NAAQS Pollutant/

Standard/(Level)/

Promulgation Date

Air Quality 

Data Collected 

for Calendar 

Year

Event Flagging & 

Initial Description 

Deadline

Detailed 

Documentation 

Submission 

Deadline

PM2.5/24-Hr 

Standard (35 

μg/m3) 

Promulgated 

October 17, 2006

2004-2006 October 1, 2007a April 15, 2008a

Ozone/8-Hr 

Standard (0.075 

ppm)Promulgated 

March 12, 2008

2005-2007 June 18, 2009a June 18, 2009a

2008 June 18, 2009a June 18, 2009a

2009 60 Days after the 

end of the calendar 

quarter in which 

the event occurred 

or February 5, 

2010, whichever 

date occurs firstb

60 Days after the 

end of the 

calendar quarter 

in which the event 

occurred or 

February 5, 2010, 

whichever date 

occurs firstb

NO2/1-Hour 

Standard(80-100 

PPB, final level 

TBD)

2008 July 1, 2010a January 22, 2011a

2009 July 1, 2010a January 22, 2011a

2010 April 1, 2011a July 1, 2011 a

SO2/1-Hour 

Standard (50-100 

PPB, final level 

TBD)

2008 October 1, 2010b June 1, 2011b

2009 October 1, 2010b June 1, 2011 b

2010 June 1, 2011 b June 1, 2011 b

2011 60 Days after the 

end of the calendar 

quarter in which 

the event occurred 

or March 31, 2011, 

whichever date 

occurs firstb

60 Days after the 

end of the 

calendar quarter 

in which the event 

occurred or March 

31, 2011, 

whichever date 

occurs firstb

aThese dates are unchanged from those published in the original 

rulemaking, or are being proposed elsewhere and are shown in this table 

for informational purposes—the Agency is not opening these dates for 

comment under this rulemaking.

bIndicates change from general schedule in 40 CFR 50.14.

Note: EPA notes that the table of revised deadlines only applies to data 

EPA will use to establish the final initial designations for new or 

revised NAAQS. The general schedule applies for all other purposes, most 

notably, for data used by EPA for redesignations to attainment.

*****
210

4. A new 50.17 is added to read as follows:

§ 50.17 National primary ambient air quality standards for sulfur oxides (sulfur dioxide).

(a) The level of the national primary 1-hour annual ambient air quality standard for oxides of 

sulfur is (50-100) parts per billion (ppb, which is 1 part in 1,000,000,000), measured in the 

ambient air as sulfur dioxide (SO2). 

(b) The 1-hour primary standard is met when the three-year average of the annual (99th

percentile)(fourth highest) of the daily maximum 1-hour average concentrations is less than or 

equal to (50-100) ppb, as determined in accordance with Appendix T of this part.

5. Add Appendix A-1 to Part 50 to read as follows:

Appendix A-1 to Part 50—Reference Measurement Principle and Calibration Procedure for the 

Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet Fluorescence Method)

1.0 Applicability. 

1.1 This ultraviolet fluorescence (UVF) method provides a measurement of the concentration of 

sulfur dioxide (SO2) in ambient air for determining compliance with the national primary and 

secondary ambient air quality standards for sulfur oxides (sulfur dioxide) as specified in §50.4 

and §50.5 of this chapter. The method is applicable to the measurement of ambient SO2 

concentrations using continuous (real-time) sampling. Additional quality assurance procedures 
211

and guidance are provided in part 58, appendix A, of this chapter and in Reference 3.

2.0 Principle. 

2.1 This reference method is based on automated measurement of the intensity of the 

characteristic fluorescence released by SO2 in an ambient air sample contained in a measurement 

cell of an analyzer when the air sample is irradiated by ultraviolet (UV) light passed through the 

cell. The fluorescent light released by the SO2 is also in the ultraviolet region, but at longer 

wavelengths than the excitation light. Typically, optimum instrumental measurement of SO2

concentrations is obtained with an excitation wavelength in a band between approximately 190 

to 230 nm, and measurement of the SO2 fluorescence in a broad band around 320 nm, but these 

wavelengths are not necessarily constraints of this reference method. Generally, the 

measurement system (analyzer) also requires means to reduce the effects of aromatic 

hydrocarbon species, and possibly other compounds, in the air sample to control measurement 

interferences from these compounds, which may be present in the ambient air. References 1 and 

2 describe UVF method.

2.2. The measurement system is calibrated by referencing the instrumental fluorescence 

measurements to SO2 standard concentrations traceable to a National Institute of Science and 

Technology (NIST) primary standard for SO2 (see Calibration Procedure below).

2.3. An analyzer implementing this measurement principle is shown schematically in Figure 1. 

Designs should include a measurement cell, a UV light source of appropriate wavelength, a UV 

detector system with appropriate wave length sensitivity, a pump and flow control system for 
212

sampling the ambient air and moving it into the measurement cell, sample air conditioning 

components as necessary to minimize measurement interferences, suitable control and 

measurement processing capability, and other apparatus as may be necessary. The analyzer must 

be designed to provide accurate, repeatable, and continuous measurements of SO2 concentrations 

in ambient air, with measurement performance as specified in subpart B of part 53 of this 

chapter.

2.4. Sampling considerations: The use of a particle filter on the sample inlet line of a UVF SO2

analyzer is required to prevent interference, malfunction, or damage due to particles in the 

sampled air.

3.0 Interferences. 

3.1 The effects of the principal potential interferences may need to be mitigated to meet the 

interference equivalent requirements of part 53 of this chapter. Poly-nuclear aromatic (PNA) 

hydrocarbons such as xylene and naphthalene can fluoresce and act as strong positive 

interferences. These gases can be removed by using a permeation type scrubber (hydrocarbon 

“kicker”). Nitrogen oxide (NO) in high concentrations can also fluoresce and cause positive 

interference. Optical filtering can be employed to improve the rejection of interference from 

high NO. Ozone can absorb UV light given off by the SO2 molecule and cause a measurement 

offset. This effect can be reduced by minimizing the measurement path length between the area 

where SO2 fluorescence occurs and the photomultiplier tube detector (e.g. <5 cm). A 

hydrocarbon scrubber, optical filter and appropriate distancing of the measurement path length 

may be required method components to reduce interference.
213

4.0 Calibration Procedure. Atmospheres containing accurately known concentrations of sulfur 

dioxide are prepared using a compressed gas transfer standard diluted with accurately metered 

clean air flow rates. 

4.1 Apparatus: Figure 2 shows a typical generic system suitable for diluting a SO2 gas cylinder 

concentration standard with clean air through a mixing chamber to produce the desired 

calibration concentration standards. A valve may be used to conveniently divert the SO2 from 

the sampling manifold to provide clean zero air at the output manifold for zero adjustment. The 

system may be made up using common laboratory components, or it may be a commercially 

manufactured system. In either case, the principle components are as follows:

4.1.1 Air and standard gas flow controllers, capable of maintaining constant gas flow rates to 

within ± 2 percent.

4.1.2 Air and standard gas flow meters, capable of measuring and monitoring air or N2 (standard 

gas) flow rates to within ± 2 percent and properly calibrated to a NIST-traceable standard.

4.1.3 Mixing chamber, of an inert material such as glass and of proper design to provide 

thorough mixing of pollutant gas and diluent air streams.

4.1.4 Sampling manifold, constructed of glass, polytetrafluoroethylene (PTFE TeflonTM), or 

other suitably inert material and of sufficient diameter to insure a minimum pressure drop at the 
214

analyzer connection, with a vent designed to insure a minimum over-pressure (relative to 

ambient air pressure) at the analyzer connection and to prevent ambient air from entering the 

manifold.

4.1.5 Standard gas pressure regulator, of clean stainless steel with a stainless steel diaphragm, 

suitable for use with a high pressure SO2 gas cylinder.

4.1.6 Reagents.

4.1.6.1 SO2 gas transfer standard, in N2, with the concentration traceable to a NIST Standard 

Reference Material (SRM) such as SRM 1693a (50 μmole/mole) or SRM 1694a (100 

μmole/mole) Since UVF analyzers may be sensitive to O2-to-N2 ratios, it is important that the 

SO2 standard concentration be sufficiently high (50 to 100 ppm) such that the O2 content in the 

diluent air is not significantly changed by the added standard gas.

4.1.6.2 Clean zero air, free of contaminants that could cause a detectable response or a change in 

sensitivity of the analyzer. Since ultraviolet fluorescence analyzers may be sensitive to aromatic 

hydrocarbons and O2-to-N2 ratios, it is important that the clean zero air contains less than 0.1 

ppm aromatic hydrocarbons and O2 and N2 percentages approximately the same as in ambient 

air. A procedure for generating zero air is given in reference 1.

4.2 Procedure
215

4.2.1 Obtain a suitable calibration apparatus, such as the one shown schematically in Figure 1, 

and verify that all materials in contact with the pollutant are of glass, TeflonTM, or other suitably

inert material and completely clean.

4.2.2 Purge the SO2 standard gas lines and pressure regulator to remove any residual air.

4.2.3 Ensure that there are no leaks in the system and that the flow measuring devices are 

properly and accurately calibrated under the conditions of use against a reliable volume or flow 

rate standard such as a soap-bubble meter or a wet-test meter traceable to a NIST standard. All 

volumetric flow rates should be corrected to the same reference temperature and pressure by 

using the formula below:

760 273.15

298.15





m

m

c m

T

P

F F

where:

Fc = corrected flow rate (L/min at 25° C and 760 mm Hg),

Fm = measured flow rate, (at temperature, Tm and pressure, Pm),

Pm = measured pressure in mm Hg, (absolute), and

Tm = measured temperature in degrees Celsius.

4.2.4 Allow the SO2 analyzer under calibration to sample zero air until a stable response is 

obtained, then make the proper zero adjustment.

4.2.5 Adjust the airflow to provide an SO2 concentration of approximately 80 percent of the 

upper measurement range limit of the SO2 instrument and verify that the total air flow of the 
216

calibration system exceeds the demand of all analyzers sampling from the output manifold (with 

the excess vented).

4.2.6 Calculate the actual SO2 calibration concentration standard as:

t

p

F

F

[SO2]  C

where:

C = the concentration of the SO2 gas standard

Fp = the flow rate of SO2 gas standard 

Ft = the total air flow rate of pollutant and diluent gases

4.2.7 When the analyzer response has stabilized, adjust the SO2 span control to obtain the desired 

response equivalent to the calculated standard concentration. If substantial adjustment of the 

span control is needed, it may be necessary to re-check the zero and span adjustments by

repeating steps 4.2.4 through 4.2.7 until no further adjustments are needed.

4.2.8 Adjust the flow rate(s) to provide several other SO2 calibration concentrations over the 

analyzer's measurement range. At least five different concentrations evenly spaced throughout 

the analyzer's range are suggested.

4.2.9 Plot the analyzer response (vertical or Y-axis) versus SO2 concentration (horizontal or Xaxis). Compute the linear regression slope and intercept and plot the regression line to verify that 

no point deviates from this line by more than 2 percent of the maximum concentration tested.
217

Note: Additional information on calibration and pollutant standards is provided in Section 12 of 

Reference 3.

5.0 Frequency of calibration.

The frequency of calibration, as well as the number of points necessary to establish the 

calibration curve and the frequency of other performance checking will vary by analyzer; 

however, the minimum frequency, acceptance criteria, and subsequent actions are specified in 

Reference 3, Appendix D: Measurement Quality Objectives and Validation Template for SO2 

(page 9 of 30). The user's quality control program should provide guidelines for initial 

establishment of these variables and for subsequent alteration as operational experience is 

accumulated. Manufacturers of analyzers should include in their instruction/operation manuals 

information and guidance as to these variables and on other matters of operation, calibration, 

routine maintenance, and quality control.

6.0 References for SO 2 Method. 

1. H. Okabe, P. L. Splitstone, and J. J. Ball, "Ambient and Source SO2 Detector Based on a 

Fluorescence Method", Journal of the Air Control Pollution Association, vol. 23, p. 514 - 516 

(1973).

2. F. P. Schwarz, H. Okabe, and J. K. Whittaker, "Fluorescence Detection of Sulfur Dioxide in 

Air at the Parts per Billion Level," Analytical Chemistry, vol. 46, pp. 1024-1028 (1974).
218

3. QA Handbook for Air Pollution Measurement Systems - Volume II. Ambient Air Quality

Monitoring Programs. U. S. EPA. EPA-454/B-08-003 (2008). (Available at 

www.epa.gov/ttn/amtic/qabook.html.)

Photomultiplier 

Tube

Inlet Sample 

Chamber

UV Lamp

Particule 

Filter

Hydrocarbon

Scrubber

Pump

Exhaust

Inlet

Flow 

Sensor/

Control

Pressure 

Sensor

Optical Filter 

(330nm)

Optical Filter 

(214nm)

Figure 1. UVF SO2 analyzer schematic diagram.
219

Figure 2. Calibration system using a compressed gas standard.
220

6. Appendix A is redesignated as Appendix A-2 to read as follows:

Appendix A-2 to Part 50—Reference Method for the Determination of Sulfur Dioxide in the 

Atmosphere (Pararosaniline Method)

7. Appendix T is added to be read as follows: 

OPTION 1 for Appendix T to Part 50:

Appendix T to Part 50—Interpretation of the Primary National Ambient Air Quality 

Standards for Oxides of Sulfur (Sulfur Dioxide) [1-hour primary standard based on the 4

th

highest daily maximum value form]

1. General.

(a) This appendix explains the data handling conventions and computations necessary for 

determining when the primary national ambient air quality standards for Oxides of Sulfur as 

measured by Sulfur Dioxide (“SO2 NAAQS”) specified in § 50.4 are met. Sulfur Dioxide (SO2) 

is measured in the ambient air by a Federal reference method (FRM) based on appendix A to this 

part or by a Federal equivalent method (FEM) designated in accordance with part 53 of this 

chapter. Data handling and computation procedures to be used in making comparisons between 

reported SO2 concentrations and the levels of the SO2 NAAQS are specified in the following 

sections.

(b) Decisions to exclude, retain, or make adjustments to the data affected by exceptional events, 

including natural events, are made according to the requirements and process deadlines specified 

in §§ 50.1, 50.14 and 51.930 of this chapter.
221

(c) The terms used in this appendix are defined as follows: 

Daily maximum 1-hour values for SO2 refers to the maximum 1-hour SO2 concentration values 

measured from midnight to midnight (local standard time) that are used in NAAQS 

computations.

Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to 

determine compliance, calculated as specified in section 5 of this appendix. The design value for 

the primary NAAQS is the 3-year average of annual 4th highest daily maximum 1-hour values for 

a monitoring site (referred to as the “1-hour primary standard design value”).

Annual 4th highest daily maximum 1-hour value refers to the 4th highest daily 1-hour maximum 

value at a site in a particular year.

Quarter refers to a calendar quarter.

Year refers to a calendar year.

2. Requirements for Data Used for Comparisons with the SO2 NAAQS and Data

Reporting Considerations.

(a) All valid FRM/FEM SO2 hourly data required to be submitted to EPA’s Air Quality System 

(AQS), or otherwise available to EPA, meeting the requirements of part 58 of this chapter 

including appendices A, C, and E shall be used in design value calculations. Multi-hour average 

concentration values collected by wet chemistry methods shall not be used.

(b) When two or more SO2 monitors are operated at a site, the state may in advance designate 
222

one of them as the primary monitor. If the state has not made this designation in advance, the 

Administrator will make the designation, either in advance or retrospectively. Design values will 

be developed using only the data from the primary monitor, if this results in a valid design value. 

If data from the primary monitor do not allow the development of a valid design value, data 

solely from the other monitor(s) will be used in turn to develop a valid design value, if this 

results in a valid design value. If there are three or more monitors, the order for such comparison 

of the other monitors will be determined by the Administrator. The Administrator may combine 

data from different monitors in different years for the purpose of developing a valid 1-hour 

primary standard design value, if a valid design value cannot be developed solely with the data 

from a single monitor. However, data from two or more monitors in the same year at the same 

site will not be combined in an attempt to meet data completeness requirements, except if one 

monitor has physically replaced another instrument permanently, in which case the two 

instruments will be considered to be the same monitor, or if the state has switched the 

designation of the primary monitor from one instrument to another during the year.

(c) Hourly SO2 measurement data shall be reported to AQS in units of parts per billion (ppb), to

at most one place after the decimal, with additional digits to the right being truncated with no 

further rounding. 

3. Comparisons with the 1-hour Primary SO2 NAAQS.

(a) The 1-hour primary SO2 NAAQS is met at a site when the valid 1-hour primary standard 

design value is less than or equal to [50-150] parts per billion (ppb).
223

(b) An SO2 1-hour primary standard design value is valid if it encompasses three consecutive 

calendar years of complete data. A year meets data completeness requirements when all 4 

quarters are complete. A quarter is complete when at least 75 percent of the sampling days for 

each quarter have complete data. A sampling day has complete data if 75 percent of the hourly

concentration values are reported.

(c) In the case of one, two, or three years that do not meet the completeness requirements of 

section 3(b) of this appendix and thus would normally not be useable for the calculation of a 

valid 3-year 1-hour primary standard design value, the 3-year 1-hour primary standard design 

value shall nevertheless be considered valid if either of the following conditions is true: 

(i) If there are at least four days in each of the 3 years that have at least one reported hourly 

value, and the resulting 3-year 1-hour primary standard design value exceeds the 1-hour primary

NAAQS. In this situation, more complete data capture could not possibly have resulted in a 

design value below the 1-hour primary NAAQS:

(ii)(A) A 1-hour primary standard design value that is below the level of the NAAQS can be 

validated if the substitution test in section 3(c)(ii)(B) results in a “test design value” that is below 

the level of the NAAQS. The test substitutes actual ‘‘high’’ reported daily maximum 1-hour 

values from the same site at about the same time of the year (specifically, in the calendar quarter) 

for unknown hourly values that were not successfully measured. Note that the test is merely

diagnostic in nature, intended to confirm that there is a very high likelihood that the original 

design value (the one with less than 75 percent data capture of hours by day and of days by

quarter) reflects the true under-NAAQS-level status for that 3-year period; the result of this data 

substitution test (the ‘‘test design value”, as defined in section 3(c)(ii)(B)) is not considered the 
224

actual design value. For this test, substitution is permitted only if there are at least 200 days 

across the three matching quarters of the three years under consideration (which is about 75 

percent of all possible daily values in those three quarters) for which 75 percent of the hours in 

the day have reported concentrations. However, maximum 1-hour values from days with less

than 75 percent of the hours reported shall also be considered in identifying the high value to be 

used for substitution.

(B) The substitution test is as follows: Data substitution will be performed in all quarter periods 

that have less than 75 percent data capture but at least 50 percent data capture; if any quarter has 

less than 50 percent data capture then this substitution test cannot be used. Identify for each 

quarter (e.g., January-March) the highest reported daily maximum 1-hour value for that quarter, 

looking across those three months of all three years under consideration. All daily maximum 1-

hour values from all days in the quarter period shall be considered when identifying this highest 

value, including days with less than 75 percent data capture. If after substituting the highest 

reported daily maximum 1-hour value for a quarter for as much of the missing daily data in the 

matching deficient quarter(s) as is needed to make them 100 percent complete, the procedure in 

section 5 yields a recalculated 3-year 1-hour standard “test design value” below the level of the 

standard, then the 1-hour primary standard design value is deemed to have passed the diagnostic 

test and is valid, and the level of the standard is deemed to have been met in that 3-year period. 

As noted in section 3(c)(i), in such a case, the 3-year design value based on the data actually

reported, not the ‘‘test design value’’, shall be used as the valid design value.

(d) A 1-hour primary standard design value based on data that do not meet the completeness 

criteria stated in 3(b) and also do not satisfy section 3(c), may also be considered valid with the 
225

approval of, or at the initiative of, the Administrator, who may consider factors such as 

monitoring site closures/moves, monitoring diligence, the consistency and levels of the valid 

concentration measurements that are available, and nearby concentrations in determining 

whether to use such data.

(e) The procedures for calculating the 1-hour primary standard design values are given in section 

5 of this appendix.

4. Rounding Conventions for the 1-hour Primary SO2 NAAQS. 

(a) Hourly SO2 measurement data shall be reported to AQS in units of parts per billion (ppb), to

at most one place after the decimal, with additional digits to the right being truncated with no 

further rounding.

(b) Daily maximum 1-hour values, including the annual 4

th highest of those daily values, are not 

rounded.

(c) The 1-hour primary standard design value is calculated pursuant to section 5 and then

rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up to the 

nearest whole number, and any decimal lower than 0.5 is rounded down to the nearest whole 

number).

5. Calculation Procedures for the 1-hour Primary SO2 NAAQS.

(a) When the data for a particular site and year meet the data completeness requirements in 

section 3(b), or if one of the conditions of section 3(c) is met, or if the Administrator exercises 
226

the discretionary authority in section 3(d), calculation of the 4

th highest daily 1-hour maximum is 

accomplished as follows.

(i) For each year, select from each day the highest hourly value. All daily maximum 1-hour 

values from all days in the quarter period shall be considered at this step, including days with 

less than 75 percent data capture.

(ii) For each year, order these daily values and take the 4th highest.

(iii) The 1-hour primary standard design value for a site is mean of the three annual 4th highest 

values, rounded according to the conventions in section 4.

OPTION 2 for Appendix T to Part 50:

Appendix T to Part 50—Interpretation of the Primary National Ambient Air Quality 

Standards for Oxides of Sulfur (Sulfur Dioxide) [1-hour primary standard based on the 

99th percentile form]

1. General.

(a) This appendix explains the data handling conventions and computations necessary for 

determining when the primary national ambient air quality standards for Oxides of Sulfur as 

measured by Sulfur Dioxide (“SO2 NAAQS”) specified in § 50.4 are met. Sulfur Dioxide (SO2) 

is measured in the ambient air by a Federal reference method (FRM) based on appendix A to this 

part or by a Federal equivalent method (FEM) designated in accordance with part 53 of this 

chapter. Data handling and computation procedures to be used in making comparisons between 

reported SO2 concentrations and the levels of the SO2 NAAQS are specified in the following 

sections.
227

(b) Decisions to exclude, retain, or make adjustments to the data affected by exceptional events, 

including natural events, are made according to the requirements and process deadlines specified 

in §§ 50.1, 50.14 and 51.930 of this chapter.

(c) The terms used in this appendix are defined as follows: 

Daily maximum 1-hour values for SO2 refers to the maximum 1-hour SO2 concentration values 

measured from midnight to midnight (local standard time) that are used in NAAQS 

computations.

Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to 

determine compliance, calculated as specified in section 5 of this appendix. The design value for 

the primary 1-hour NAAQS is the 3-year average of annual 99th percentile daily maximum 1-

hour values for a monitoring site (referred to as the “1-hour primary standard design value”).

99th percentile daily maximum 1-hour value is the value below which nominally 99 percent of 

all daily maximum 1-hour concentration values fall, using the ranking and selection method 

specified in section 5 of this appendix.

Quarter refers to a calendar quarter.

Year refers to a calendar year.

2. Requirements for Data Used for Comparisons with the SO2 NAAQS and Data

Reporting Considerations.
228

(a) All valid FRM/FEM SO2 hourly data required to be submitted to EPA’s Air Quality System 

(AQS), or otherwise available to EPA, meeting the requirements of part 58 of this chapter 

including appendices A, C, and E shall be used in design value calculations. Multi-hour average 

concentration values collected by wet chemistry methods shall not be used.

(b) When two or more SO2 monitors are operated at a site, the state may in advance designate 

one of them as the primary monitor. If the state has not made this designation, the Administrator 

will make the designation, either in advance or retrospectively. Design values will be developed 

using only the data from the primary monitor, if this results in a valid design value. If data from 

the primary monitor do not allow the development of a valid design value, data solely from the 

other monitor(s) will be used in turn to develop a valid design value, if this results in a valid 

design value. If there are three or more monitors, the order for such comparison of the other 

monitors will be determined by the Administrator. The Administrator may combine data from 

different monitors in different years for the purpose of developing a valid 1-hour primary

standard design value, if a valid design value cannot be developed solely with the data from a 

single monitor. However, data from two or more monitors in the same year at the same site will 

not be combined in an attempt to meet data completeness requirements, except if one monitor 

has physically replaced another instrument permanently, in which case the two instruments will 

be considered to be the same monitor, or if the state has switched the designation of the primary

monitor from one instrument to another during the year.

(c) Hourly SO2 measurement data shall be reported to AQS in units of parts per billion (ppb), to

at most one place after the decimal, with additional digits to the right being truncated with no 
229

further rounding.

3. Comparisons with the 1-hour Primary SO2 NAAQS.

(a) The 1-hour primary SO2 NAAQS is met at a site when the valid 1-hour primary standard 

design value is less than or equal to [50-150] parts per billion (ppb).

(b) An SO2 1-hour primary standard design value is valid if it encompasses three consecutive 

calendar years of complete data. A year meets data completeness requirements when all 4 

quarters are complete. A quarter is complete when at least 75 percent of the sampling days for 

each quarter have complete data. A sampling day has complete data if 75 percent of the hourly

concentration values are reported.

(c) In the case of one, two, or three years that do not meet the completeness requirements of 

section 3(b) of this appendix and thus would normally not be useable for the calculation of a 

valid 3-year 1-hour primary standard design value, the 3-year 1-hour primary standard design 

value shall nevertheless be considered valid if one of the following conditions is true. 

(i) At least 75 percent of the days in each quarter of each of three consecutive years have at least 

one reported hourly value, and the design value calculated according to the procedures specified 

in section 5 is above the level of the primary 1-hour standard.

(ii) (A) A 1-hour primary standard design value that is below the level of the NAAQS can be 

validated if the substitution test in section 3(c)(ii)(B) results in a “test design value” that is below 

the level of the NAAQS. The test substitutes actual ‘‘high’’ reported daily maximum 1-hour 

values from the same site at about the same time of the year (specifically, in the same calendar 
230

quarter) for unknown values that were not successfully measured. Note that the test is merely

diagnostic in nature, intended to confirm that there is a very high likelihood that the original 

design value (the one with less than 75 percent data capture of hours by day and of days by

quarter) reflects the true under-NAAQS-level status for that 3-year period; the result of this data 

substitution test (the ‘‘test design value”, as defined in section 3(c)(ii)(B)) is not considered the 

actual design value. For this test, substitution is permitted only if there are at least 200 days 

across the three matching quarters of the three years under consideration (which is about 75 

percent of all possible daily values in those three quarters) for which 75 percent of the hours in 

the day have reported concentrations. However, maximum 1-hour values from days with less 

than 75 percent of the hours reported shall also be considered in identifying the high value to be 

used for substitution.

(B) The substitution test is as follows: Data substitution will be performed in all quarter periods 

that have less than 75 percent data capture but at least 50 percent data capture; if any quarter has 

less than 50 percent data capture then this substitution test cannot be used. Identify for each 

quarter (e.g., January-March) the highest reported daily maximum 1-hour value for that quarter, 

looking across those three months of all three years under consideration. All daily maximum 1-

hour values from all days in the quarter period shall be considered when identifying this highest 

value, including days with less than 75 percent data capture. If after substituting the highest 

reported daily maximum 1-hour value for a quarter for as much of the missing daily data in the 

matching deficient quarter(s) as is needed to make them 100 percent complete, the procedure in 

section 5 yields a recalculated 3-year 1-hour standard “test design value” below the level of the 

standard, then the 1-hour primary standard design value is deemed to have passed the diagnostic 

test and is valid, and the level of the standard is deemed to have been met in that 3-year period. 
231

As noted in section 3(c)(i), in such a case, the 3-year design value based on the data actually

reported, not the ‘‘test design value’’, shall be used as the valid design value.

(iii) (A) A 1-hour primary standard design value that is above the level of the NAAQS can be 

validated if the substitution test in section 3(c)(iii)(B) results in a “test design value” that is 

above the level of the NAAQS. The test substitutes actual ‘‘low” reported daily maximum 1-

hour values from the same site at about the same time of the year (specifically, in the same three 

months of the calendar) for unknown hourly values that were not successfully measured. Note 

that the test is merely diagnostic in nature, intended to confirm that there is a very high 

likelihood that the original design value (the one with less than 75 percent data capture of hours 

by day and of days by quarter) reflects the true above-NAAQS-level status for that 3-year period; 

the result of this data substitution test (the ‘‘test design value”, as defined in section 3(c)(iii)(B))

is not considered the actual design value. For this test, substitution is permitted only if there are a 

minimum number of available daily data points from which to identify the low quarter-specific 

daily maximum 1-hour values, specifically if there are at least 200 days across the three 

matching quarters of the three years under consideration (which is about 75 percent of all 

possible daily values in those three quarters) for which 75 percent of the hours in the day have 

reported concentrations. Only days with at least 75 percent of the hours reported shall be 

considered in identifying the low value to be used for substitution.

(B) The substitution test is as follows: Data substitution will be performed in all quarter periods 

that have less than 75 percent data capture. Identify for each quarter (e.g., January-March) the 

lowest reported daily maximum 1-hour value for that quarter, looking across those three months 

of all three years under consideration. All daily maximum 1-hour values from all days with at 
232

least 75 percent capture in the quarter period shall be considered when identifying this lowest 

value. If after substituting the lowest reported daily maximum 1-hour value for a quarter for as 

much of the missing daily data in the matching deficient quarter(s) as is needed to make them 75

percent complete, the procedure in section 5 yields a recalculated 3-year 1-hour standard “test 

design value” above the level of the standard, then the 1-hour primary standard design value is 

deemed to have passed the diagnostic test and is valid, and the level of the standard is deemed to 

have been exceeded in that 3-year period. As noted in section 3(c)(i), in such a case, the 3-year 

design value based on the data actually reported, not the ‘‘test design value’’, shall be used as the 

valid design value.

(d) A 1-hour primary standard design value based on data that do not meet the completeness 

criteria stated in 3(b) and also do not satisfy section 3(c), may also be considered valid with the 

approval of, or at the initiative of, the Administrator, who may consider factors such as 

monitoring site closures/moves, monitoring diligence, the consistency and levels of the valid 

concentration measurements that are available, and nearby concentrations in determining 

whether to use such data.

(e) The procedures for calculating the 1-hour primary standard design values are given in section 

5 of this appendix.

4. Rounding Conventions for the 1-hour Primary SO2 NAAQS. 

(a) Hourly SO2 measurement data shall be reported to AQS in units of parts per billion (ppb), to

at most one place after the decimal, with additional digits to the right being truncated with no 

further rounding.
233

(b) Daily maximum 1-hour values and therefore the annual 4

th highest of those daily values are 

not rounded.

(c) The 1-hour primary standard design value is calculated pursuant to section 5 and then 

rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up to the 

nearest whole number, and any decimal lower than 0.5 is rounded down to the nearest whole 

number).

5. Calculation Procedures for the 1-hour Primary SO2 NAAQS.

(a) Procedure for identifying annual 99th percentile values. When the data for a particular site 

and year meet the data completeness requirements in section 3(b), or if one of the conditions of 

section 3(c) is met, or if the Administrator exercises the discretionary authority in section 3(d), 

identification of annual 99th percentile value is accomplished as follows.

(i) The annual 99th percentile value for a year is the higher of the two values resulting from the 

following two procedures.

(1) Procedure 1. For the year, determine the number of days with at least 75 percent of the 

hourly values reported. 

(A) For the year, from only the days with at least 75 percent of the hourly values reported, select 

from each day the maximum hourly value. 

(B) Sort all these daily maximum hourly values from a particular site and year by descending 

value. (For example: (x[1], x[2], x[3], * * *, x[n]). In this case, x[1] is the largest number and 

x[n] is the smallest value.) The 99th percentile is determined from this sorted series of daily 
234

values which is ordered from the highest to the lowest number. Using the left column of Table 1, 

determine the appropriate range (i.e., row) for the annual number of days with valid data for year 

y (cny). The corresponding “n” value in the right column identifies the rank of the annual 99th

percentile value in the descending sorted list of daily site values for year y. Thus, P0.99, y= the nth 

largest value.

(2) Procedure 2. For the year, determine the number of days with at least one hourly value 

reported. 

(A) For the year, from all the days with at least one hourly value reported, select from each day

the maximum hourly value. 

(B) Sort all these daily maximum values from a particular site and year by descending value. 

(For example: (x[1], x[2], x[3], * * *, x[n]). In this case, x[1] is the largest number and x[n] is 

the smallest value.) The 99th percentile is determined from this sorted series of daily values 

which is ordered from the highest to the lowest number. Using the left column of Table 1, 

determine the appropriate range (i.e., row) for the annual number of days with valid data for year 

y (cny). The corresponding “n” value in the right column identifies the rank of the annual 99th

percentile value in the descending sorted list of daily site values for year y. Thus, P0.99, y= the nth 

largest value.

(b) The 1-hour primary standard design value for a site is mean of the three annual 99th percentile 

values, rounded according to the conventions in section 4.
235

Table 1

Annual number of days with valid data 

for year “y” (cny)

P0.99, y is the nth maximum value of the year, where 

n is the listed number

1–100 1

101-200 2

201-300 3

301-366 4

PART 53-Ambient Air Monitoring Reference AND EQUIVALENT METHODS

8. The authority citation for part 53 continues to read as follows:

Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec. 1857g(a)), as amended by sec. 

15(c)(2) of Pub. L. 91-604, 84 Stat. 1713, unless otherwise noted.

Subpart A- [Amended]

9. Section 53.2 is amended by revising paragraphs (a)(1) and (b) to read as follows:

§ 53.2. General requirements for a reference method determination.

* * * * * 

(a) Manual methods—(1) Sulfur dioxide (SO2) and Lead. For measuring SO2 and lead, 

appendixes A-2 and G of part 50 of this chapter specify unique manual FRM for measuring those 

pollutants. After [insert effective date of Appendix A-1], a new FRM for SO2 must be an 

automated method that utilizes the measurement principle and calibration procedure specified in 

appendix A-1 to part 50 of this chapter and must meet applicable requirements of this part, as 

specified in paragraph (b) of this section. Except as provided in §53.16, other manual methods 
236

for lead will not be considered for a reference method determination under this part.

* * * * * 

(b) Automated methods. An automated FRM for measuring SO2, CO, O3, or NO2 must utilize the 

measurement principle and calibration procedure specified in the appropriate appendix to part 50 

of this chapter (appendix A-1 only for SO2 methods) and must have been shown in accordance 

with this part to meet the requirements specified in this subpart A and subpart B of this part.

10. Section 53.8 is amended by revising paragraph (c) to read as follows:

§ 53.8 Designation of reference and equivalent methods.

* * * * *

(c) The Administrator will maintain a current list of methods designated as FRM or FEM in 

accordance with this part and will send a copy of the list to any person or group upon request. A 

copy of the list will be available via the Internet and may be available from other sources.
237

11. Table A-1 of Subpart A is revised to read as follows: 

Table A–1 to Subpart A of Part 53 – Summary of Applicable Requirements for 

Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants

Pollutant

Reference or 

equivalent

Manual or 

automated

Applicable part 

50 appendix

Applicable subparts of part 53

A B C D E F

SO2 Reference Manual A-2

Automated A-1  

Equivalent Manual A-1  

Automated A-1   

CO Reference Automated C  

Equivalent Manual C  

Automated C   

O3 Reference Automated D  

Equivalent Manual D  

Automated D   

NO2 Reference Automated F  

Equivalent Manual F  

Automated F   

Pb Reference Manual G

Equivalent Manual G  

Automated G  

PM10-Pb Reference Manual Q

Equivalent Manual Q  

Automated Q  

PM10 Reference Manual J  

Equivalent Manual J   

Automated J   

PM2.5 Reference Manual L  

Equivalent Class I Manual L   

Equivalent Class II Manual L

1  

2  1, 2

Equivalent Class III Automated L

1    

1

PM10-2.5 Reference Manual L, O  

Equivalent Class I Manual L, O   

Equivalent Class II Manual L, O  

2  1, 2

Equivalent Class III Automated L

1

, O1    

1

1. Some requirements may apply, based on the nature of each particular candidate method, as determined by the 

Administrator.

2. Alternative Class III requirements may be substituted.
238

Subpart B- [Amended]

12. Section 53.20 is amended by revising paragraph (b) to read as follows:

§ 53.20 General provisions.

*****

(b) For a candidate method having more than one selectable measurement range, one range must be that specified in table B-1 (standard 

range for SO2), and a test analyzer representative of the method must pass the tests required by this subpart while operated in that range. 

The tests may be repeated for one or more broader ranges (i.e., ones extending to higher concentrations) than the range specified in table 

B-1, provided that the range does not extend to concentrations more than four times the upper range limit specified in table B-1. For 

broader ranges, only the tests for range (calibration), noise at 80% of the upper range limit, and lag, rise and fall time are required to be 

repeated. The tests may be repeated for one or more narrower ranges (ones extending to lower concentrations) than that specified in table 

B-1. For SO2 methods, table B-1 specifies special performance requirements for narrower (lower) ranges. For methods other than SO2, 

only the tests for range (calibration), noise at 0 % of the measurement range, and lower detectable limit are required to be repeated. If the 

tests are conducted or passed only for the specified range (standard range for SO2), any FRM or FEM method determination with respect 

to the method will be limited to that range. If the tests are passed for both the specified range and one or more broader ranges, any such 

determination will include the additional range(s) as well as the specified range, provided that the tests required by subpart C of this part 

(if applicable) are met for the broader range(s). If the tests are passed for both the specified range and one or more narrower ranges, any 

FRM or FEM method determination for the method will include the narrower range(s) as well as the specified range. Appropriate test 
239

data shall be submitted for each range sought to be included in a FRM or FEM method determination under this paragraph (b).

* * * * * 

13. Section 53.21 is amended by revising paragraph (a) to read as follows:

§ 53.21 Test conditions.

(a) Set-up and start-up of the test analyzer shall be in strict accordance with the operating instructions specified in the manual referred to 

in §53.4(b)(3). Allow adequate warm-up or stabilization time as indicated in the operating instructions before beginning the tests. The 

test procedures assume that the test analyzer has an analog measurement signal output that is connected to a suitable strip chart recorder 

of the servo, null-balance type. This recorder shall have a chart width of a least 25 centimeters, chart speeds up to 10 cm per hour, a 

response time of 1 second or less, a deadband of not more than 0.25 percent of full scale, and capability either of reading measurements 

at least 5 percent below zero or of offsetting the zero by at least 5 percent. If the test analyzer does not have an analog signal output, or if 

other types of measurement data output are used, an alternative measurement data recording device (or devices) may be used for the tests, 

provided it is reasonably suited to the nature and purposes of the tests and an analog representation of the analyzer measurements for 

each test can be plotted or otherwise generated that is reasonably similar to the analog measurement recordings that would be produced 

by a conventional chart recorder.

* * * * * 
240

14. Table B-1 of Subpart B is revised to read as follows:

TABLE B-1—PERFORMANCE SPECIFICATIONS FOR AUTOMATED METHODS

Performance parameter Units1

SO2

O3 CO NO2

Definitions 

and test 

procedures

Std. 

range3

Lower 

range2,3

1. Range ppm 0-0.5 <0.5 0-0.5 0-50 0-0.5 Sec. 53.23(a)

2. Noise ppm 0.001 0.0005 0.005 50 0.005 Sec. 53.23(b)

3. Lower detectable limit ppm 0.002 0.001 0.010 1.0 0.010 Sec. 53.23(c)

4. Interference equivalent

 Each interferent

 Total, all interferents

ppm

ppm

±0.005

 0.020

±0.005

 0.020

±0.02

 0.06

±1.0

 1.5

±0.02

 0.04

Sec. 53.23(d)

Sec. 53.23(d)

5. Zero drift, 12 and 24 hour ppm ±0.004 ±0.002 ±0.02 ±1.0 ±0.02 Sec. 53.23(e)

7. Span drift, 24 hour

 20 % of upper range limit

 80 % of upper range limit

Percent

Percent

---

±5.0

 ---

±5.0

±20.0

 ±5.0

±10.0

 ±2.5

±20.0

 ±5.0

Sec. 53.23(e)

Sec. 53.23(e)

8. Lag time Minutes 2 2 20 10 20 Sec. 53.23(e)

9. Rise time Minutes 2 2 15 5 15 Sec. 53.23(e)

10. Fall time Minutes 2 2 15 5 15 Sec. 53.23(e)

11. Precision

 20 % of upper range limit

 80 % of upper range limit

 

ppm

Percent

ppm

Percent

---

2

---

2

---

2

---

2

0.010

0.010

---

0.5

0.5

---

0.020

0.030

---

Sec. 53.23(e)

Sec. 53.23(e)

Sec. 53.23(e)

Sec. 53.23(e)

1. To convert from parts per million (ppm) to μg/m3at 25 °C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas. Percent means 

percent of the upper range limit.

2. Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range shows that the lower 

range specification is met for each of these test parameters.

3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct suitable tests 

to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are within specifications under all applicable 

conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct calibration, noise, lag, rise, fall times, and precision tests at 

the highest and lowest settings that are to be included in the FRM or FEM designation.
241
242

15. Table B-2 of subpart B is revised to read as follows: 

TABLE B-2—TEST ATMOSPHERES

Test gas Generation Verification

Ammonia Permeation device. Similar to system described 

in references 1 and 2

Indophenol method, reference 3.

Carbon 

dioxide

Cylinder of zero air or nitrogen containing CO2 as 

required to obtain the concentration specified in table 

B-3.

Use NIST-certified standards whenever 

possible. If NIST standards are not available, 

obtain 2 standards from independent sources 

which agree within 2 percent, or obtain one 

standard and submit it to an independent 

laboratory for analysis, which must agree 

within 2 percent of the supplier’s nominal 

analysis.

Carbon 

monoxide

Cylinder of zero air or nitrogen containing CO as 

required to obtain the concentration specified in table 

B-3.

Use a FRM CO analyzer as described in 

reference 8.

Ethane Cylinder of zero air or nitrogen containing ethane as 

required to obtain the concentration specified in table 

B-3.

Gas chromatography, ASTM D2820, 

reference 10. Use NIST-traceable gaseous 

methane or propane standards for calibration.

Ethylene Cylinder of pre-purified nitrogen containing ethylene 

as required to obtain the concentration specified in 

table B-3.

Do. 

Hydrogen 

chloride

Cylinder1of pre-purified nitrogen containing 

approximately 100 ppm of gaseous HCL. Dilute with 

zero air to concentration specified in table B-3

Collect samples in bubbler containing distilled 

water and analyze by the mercuric thiocyante 

method, ASTM (D612), p. 29, reference 4.

Hydrogen 

sulfide

Permeation device system described in references 1 

and 2.

Tentative method of analysis for H2S content 

of the atmosphere, p. 426, reference 5.

Methane Cylinder of zero air containing methane as required to 

obtain the concentration specified in table B-3.

Gas chromatography ASTM D2820, reference 

10. Use NIST-traceable methane standards 

for calibration.

Nitric oxide Cylinder1of pre-purified nitrogen containing 

approximately 100 ppm NO. Dilute with zero air to 

required concentration.

Gas phase titration as described in reference 

6, section 7.1

Nitrogen 

dioxide

1. Gas phase titration as described in reference 6.

2. Permeation device, similar to system described in 

reference 6.

1. Use an FRM NO2 analyzer calibrated with 

a gravimetrically calibrated permeation 

device.

2. Use an FRM NO2 analyzer calibrated by 

gas-phase titration as described in reference 

6.
243

Ozone Calibrated ozone generator as described in reference 

9.

Use an FEM ozone analyzer calibrated as 

described in reference 9.

Sulfur 

dioxide

1. Permeation device as described in references 1 

and 2.

2. Dynamic dilution of a cylinder containing 

approximately 100 ppm SO2 as described in 

Reference 7.

Use an SO2 FRM or FEM analyzer as 

described in reference 7.
244

TABLE B-2—TEST ATMOSPHERES (CONTINUED)

1 Use stainless steel pressure regulator dedicated 

to the pollutant measured.

Reference 1. O'Keefe, A. E., and Ortaman, G. C. 

"Primary Standards for Trace Gas Analysis," 

Anal. Chem. 38, 760 (1966).

Reference 2. Scaringelli, F. P., A. E. . Rosenberg, 

E*, and Bell, J. P., "Primary Standards for Trace 

Gas Analysis." Anal. Chem. 42, 871 (1970).

Reference 3. "Tentative Method of Analysis for 

Ammonia in the Atmosphere (Indophenol 

Method)", Health Lab Sciences, vol. 10, No. 2, 

115-118, April 1973.

Reference 4. 1973 Annual Book of ASTM 

Standards, American Society for Testing and 

Materials, 1916 Race St., Philadelphia, PA.

Reference 5. Methods for Air Sampling and 

Analysis, Intersociety Committee, 1972, 

American Public Health Association, 1015.

Reference 6. 40 CFR 50 Appendix F, 

"Measurement Principle and Calibration 

Principle for the Measurement of Nitrogen 

Dioxide in the Atmosphere (Gas Phase Chemiluminescence)."

Reference 7. 40 CFR 50 Appendix A-1, "Measurement Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet FIuorscence)."

Reference 8. 40 CFR 50 Appendix C, "Measurement Principle and Calibration Procedure for the Measurement of Carbon Monoxide in the Atmosphere" (Non-Dispersive Infrared 

Photometry)".

Reference 9. 40 CFR 50 Appendix D, "Measurement Principle and Calibration Procedure for the Measurement of Ozone in the Atmosphere".

Reference 10. "Standard Test Method for C, through C5 Hydrocarbons in the Atmosphere by Gas Chromatography", D 2820, 1987 Annual Book of Aston Standards, vol 11.03, 

American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103.

16. Table B-3 of Subpart B is revised to read as follows:

TABLE B-3—INTERFERENT TEST CONCENTRATION,

1 PARTS PER MILLION

Pollutant Analyzer Type

Hydrochloric

acid

Ammonia

Hydrogen

sulfide

Sulfur

dioxide

Nitrogen

dioxide

Nitric

oxide

Carbon

dioxide

Ethylene Ozone

Mxylene Water

vapor

Carbon

monoxide

Methane

Ethane Naphth

alene

SO2

Ultraviolet 

fluorescence 0.15

0.1440.5 0.5 0.5 0.2 20,000 0.056

SO2 Flame photometric 0.01 0.144750 20,000350

Test gas Generation Verification

Water Pass zero air through distilled water at a fixed known 

temperature between 20° and 30° C such that the air 

stream becomes saturated. Dilute with zero air to 

concentration specified in table B-3.

Measure relative humidity by means of a dewpoint indicator, calibrated electrolytic or piezo 

electric hygrometer, or wet/dry bulb 

thermometer.

Xylene Cylinder of pre-purified nitrogen containing 100 ppm 

xylene. Dilute with zero air to concentration specified 

in table B-3.

Use NIST-certified standards whenever 

possible. If NIST standards are not available, 

obtain 2 standards from independent sources 

which agree within 2 percent, or obtain one 

standard and submit it to an independent 

laboratory for analysis, which must agree 

within 2 percent of the supplier’s nominal 

analysis.

Zero air 1. Ambient air purified by appropriate scrubbers or 

other devices such that it is free of contaminants 

likely to cause a detectable response on the analyzer.

2. Cylinder of compressed zero air certified by the 

supplier or an independent laboratory to be free of 

contaminants likely to cause a detectable response 

on the analyzer.
245

Pollutant Analyzer Type

Hydrochloric

acid

Ammonia

Hydrogen

sulfide

Sulfur

dioxide

Nitrogen

dioxide

Nitric

oxide

Carbon

dioxide

Ethylene Ozone

Mxylene Water

vapor

Carbon

monoxide

Methane

Ethane Naphth

alene

SO2 Gas chromatography 0.1 0.144750 20,000350

SO2

Spectrophotometricwet chemical 

(pararosanaline)

0.2 0.1 0.1 0.1440.5 750 0.5

SO2 Electrochemical 0.2 0.1 0.1 0.1440.5 0.5 0.2 0.5 20,0003

SO2 Conductivity 0.2 0.1 0.1440.5 750

SO2

Spectrophotometricgas phase, including 

DOAS

0.1440.5 0.5 0.2

O3 Chemiluminescent 0.13750 0.08420,0003

O3 Electrochemical 0.130.5 0.5 0.084

O3

Spectrophotometricwet chemical 

(potassium iodide)

0.130.5 0.5 0.530.084

O3

Spectrophotometricgas phase, including 

ultraviolet absorption 

and DOAS)

0.5 0.5 0.5 0.0840.02 20,000

CO Infrared 750 20,000 104

CO

Gas chromatography 

with flame ionization 

detector

20,000 1040.5

CO Electrochemical 0.5 0.2 20,000 104

CO Catalytic combustionthermal detection 0.1 750 0.2 20,000 1045.0 0.5

CO IR fluorescence 750 20,000 1040.5

CO

Mercury 

replacement-UV 

photometric

0.2 1040.5
246

TABLE B-3—INTERFERANT TEST CONCENTRATION,

1 PARTS PER MILLION (CONTINUED)

Pollutant Analyzer Type

Hydrochloric

acid

Ammonia

Hydrogen

sulfide

Sulfur

dioxide

Nitrogen

dioxide

Nitric

oxide

Carbon

dioxide

Ethylene Ozone MxyleneWater

vapor

Carbon

monoxide

Methane

Ethane

Naphth

alene

NO2 Chemiluminescent 0.130.5 0.140.5 20,000

NO2

Spectrophotometricwet chemical (azodye reaction)

0.5 0.140.5 750 0.5

NO2 Electrochemical 0.2 0.130.5 0.140.5 750 0.5 20,000 50

NO2

Spectrophotometricgas phase 0.13

0.5 0.140.5 0.5 20,000 50

1. Concentrations of interferent listed must be prepared and controlled to ±10 percent of the stated value.

2. Analyzer types not listed will be considered by the Administrator as special cases.

3. Do not mix with the pollutant.

4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value.

5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.

6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the 

scrubber, and evaluate response for interference
247

Subpart C [Amended]

17. Section 53.32 is amended by revising paragraph (e)(2) to read as follows:

§ 53.32 Test procedures for methods for SO2, CO, O3, and NO2.

*****

(e) * * * 

(2) For a candidate method having more than one selectable range, one range must be that 

specified in table B–1 of subpart B of this part, and a test analyzer representative of the method 

must pass the tests required by this subpart while operated on that range. The tests may be 

repeated for one or more broader ranges (i.e., ones extending to higher concentrations) than the 

one specified in table B–1 of subpart B of this part, provided that such a range does not extend to 

concentrations more than four times the upper range limit specified in table B–1 of subpart B of 

this part and that the test analyzer has passed the tests required by subpart B of this part (if 

applicable) for the broader range. If the tests required by this subpart are conducted or passed 

only for the range specified in table B–1 of subpart B of this part, any equivalent method 

determination with respect to the method will be limited to that range. If the tests are passed for 

both the specified range and a broader range (or ranges), any such determination will include the 

broader range(s) as well as the specified range. Appropriate test data shall be submitted for each 

range sought to be included in such a determination.

* * * * * 
248

18. Table C-1 of Subpart C is revised to read as follows:

TABLE C-1 TO SUBPART C OF PART 53—TEST CONCENTRATION RANGES, NUMBER OF MEASUREMENTS

REQUIRED, AND MAXIMUM DISCREPANCY SPECIFICATIONS

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

The authority citation for part 58 continues to read as follows:

Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.

Subpart B [AMENDED]

19. Section 58.10, is amended by adding paragraph (a)(6) to read as follows:

§ 58.10 Annual monitoring network plan and periodic network assessment.

* * * * *

Pollutant

Concentration 

range, parts per 

million (ppm)

Simultaneous measurements required

Maximum 

discrepancy 

specification, 

parts per million

1-hour 24-hour

First set Second set First set Second set

Ozone

Low 0.06 to 0.10

Med. 0.15 to 0.25

High 0.35 to 0.46

5

5

4

6

6

6

--

--

--

--

--

--

0.02

0.03

0.04

Total.... 14 18 -- -- --

Carbon 

monoxide

Low 7 to 11

Med. 20 to 30

High 25 to 45

5

5

4

6

6

6

--

--

--

--

--

--

1.5

2.0

3.0

Total.... 14 18 -- -- --

Sulfur 

dioxide

Low 0.02 to 0.05

Med. 0.10 to 0.15

High 0.30 to 0.50

5

5

4

6

6

6

3

2

2

3

3

2

0.02

0.03

0.04

Total.... 14 18 7 8 --

Nitrogen 

dioxide

Low 0.02 to 0.08

Med. 0.10 to 0.20

High 0.25 to 0.35

--

--

--

--

--

--

3

2

2

3

2

2

0.02

0.02

0.03

Total.... -- -- 7 8 --
249

(a) * * * 

(6) A plan for establishing SO2 monitoring sites in accordance with the requirements of 

appendix D to this part shall be submitted to the EPA Regional Administrator by July 1, 2011 as 

part of the annual network plan required in paragraph (a) (1). The plan shall provide for all 

required SO2 monitoring sites to be operational by January 1, 2013.

* * * * *

20. Section 58.12 is amended by adding paragraph (g) to read as follows:

§ 58.12 Operating Schedules

* * * * *

(g) For continuous SO2 analyzers, the maximum 5-minute block average concentration of 

the twelve 5-minute blocks in the hour must be collected except as noted in § 58.12 (a).

* * * * *

21. Section 58.13 is amended by adding paragraph (d) to read as follows:

§ 58.13 Monitoring network completion.

* * * * *

(d) The network of SO2 monitors must be physically established no later than January 1, 

2013, and at that time, must be operating under all of the requirements of this part, including the 

requirements of appendices A, C, D, and E to this part. 
250

22. Section 58.16 is amended by adding paragraph (g) to read as follows:

§ 58.16 Data submittal and archiving requirements.

* * * * *

(g) Any State, or where applicable, local agency operating an SO2 monitor shall report 

the maximum 5-minute SO2 block average of the twelve 5-minute block averages in each hour,

in addition to the hourly SO2 average. 

23. Appendix A to Part 58 is amended as by adding paragraph 2.3.1.6 to read as follows:

Appendix A to Part 58—Quality Assurance Requirements for SLAMS, SPMs and PSD Air 

Monitoring

* * * * *

2.3.1.6 Measurement Uncertainty for SO2. The goal for acceptable measurement 

uncertainty for precision is defined as an upper 90 percent confidence limit for the coefficient of 

variation (CV) of 15 percent and for bias as an upper 95 percent confidence limit for the absolute 

bias of 15 percent.

* * * * *

24. Appendix C to Part 58 is amended as by adding paragraph 2.1.2 to read as follows:

Appendix C to Part 58—Ambient Air Quality Monitoring Methodology

* * * * *
251

2.1.2 Any SO2 FRM or FEM used for making NAAQS decisions, as prescribed in 40 

CFR Part 50 Appendix A-1, must be capable of providing 1-hour averaged and 5-minute 

averaged concentration data.

* * * * *

25. Appendix D to Part 58 is amended as by revising paragraph 4.4 to read as follows:

Appendix D to Part 58—Network Design Criteria for Ambient Air Quality Monitoring

* * * * *

4.4 Sulfur Dioxide (SO2) Design Criteria. 

4.4.1 General Requirements. (a) State and, where appropriate, local agencies must 

operate a minimum number of required SO2 monitoring sites as described below.

4.4.2 Requirement for Monitoring by the Population Weighted Emissions Index. (a) The 

population weighted emissions index (PWEI) shall be calculated by states for each CBSA they

contain or share with another state or states for use in the implementation of or adjustment to the 

SO2 monitoring network. The PWEI shall be calculated by multiplying the population of each 

CBSA, using the most current census data, by the total amount of SO2 in tons per year emitted 

within the CBSA area, using an aggregate of the most recent county level emissions data 

available in the National Emissions Inventory for each county in each CBSA. The resulting 

product shall be divided by one million, providing a PWEI value, the units of which are million 

persons-tons per year. For any CBSA with a calculated PWEI value equal to or greater than 

1,000,000, a minimum of three SO2 monitors are required within that CBSA. For any CBSA 
252

with a calculated PWEI value equal to or greater than 10,000, but less than 1,000,000, a 

minimum of two SO2 monitors are required within that CBSA. For any CBSA with a calculated 

PWEI value equal to or greater than 5,000, but less than 10,000, a minimum of one SO2 monitor 

is required within that CBSA. 

(1) The SO2 monitoring site(s) required as a result of the PWEI in each CBSA shall be 

sited by states through a process of identifying locations within the boundaries of that CBSA

where maximum ground-level 1-hour SO2 concentrations occur due to emissions that originate 

inside and/or outside of that CBSA. Where a state or local air monitoring agency identifies 

multiple acceptable candidate sites where maximum hourly SO2 concentrations are expected to 

occur, the monitoring agency shall select the location with the greater population exposure.

Where one CBSA is required to have more than one SO2 monitor, the monitoring sites shall not 

be oriented to measure maximum hourly concentrations from the same SO2 source or group of 

sources, but shall monitor a different source or group of sources. Any PWEI-triggered monitors 

shall not count toward satisfying any required monitors resulting from the state emissions 

triggered requirements described below.

(2) The number of SO2 monitors operated as a result of the PWEI shall be reviewed and 

adjusted as needed as a part of the 5-year network assessment cycle required in section §58.10 of 

this part.

4.4.3 Requirement for State Emission Triggered SO2 Monitoring. (a) Each State shall 

operate a minimum number of monitors based on that state’s contribution of SO2 emissions to 

the national, anthropogenic SO2 inventory as identified in the most recent National Emissions 

Inventory. Each state shall operate one monitor for each percent that it contributes to the NEI. 
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The percent contribution shall be rounded to the nearest whole integer value. Every state shall 

operate a minimum of one monitor under this requirement. 

(1) Each state emission triggered SO2 monitoring station shall be sited by states through a 

process of identifying locations within the boundaries of that state where maximum ground-level 

1-hour SO2 concentrations occur due to SO2 source emissions originate inside or outside the 

state. Where a state has CBSAs with PWEI-triggered monitoring, the PWEI-triggered monitors 

shall not count toward the emission-triggered monitors. State emission-triggered monitors shall 

not be sited to measure maximum hourly concentrations from the same SO2 source or group of 

sources as another SO2 monitor, but shall measure maximum hourly concentrations resulting 

from a different source or group of sources. 

(2) The number of SO2 monitors operated as a result of state-level emissions shall be 

reviewed and adjusted as needed as a part of the 5-year network assessment cycle required in 

section §58.10 of this part.

4.4.4 Regional Administrator Required Monitoring. (a) The Regional Administrator may

require additional SO2 monitoring stations above the minimum number of monitors required in 

4.4.2 and 4.4.3 of this part, where the minimum monitoring requirements are not sufficient to 

meet monitoring objectives. The Regional Administrator may require, at his/her discretion, 

additional monitors in situations where an area has the potential to have concentrations that may

violate or contribute to the violation of the NAAQS and the area is not monitored under the 

minimum monitoring provisions described above. The Regional Administrator and the 

responsible State or local air monitoring agency shall work together to design and/or maintain 

the most appropriate SO2 network to provide sufficient data to meet monitoring objectives.
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4.4.5 SO2 Monitoring Spatial Scales. (a) The appropriate spatial scales for SO2 SLAMS 

monitors are the microscale, middle, neighborhood, and possibly urban scales. Monitors sited at 

the microscale, middle, and neighborhood scales are suitable for determining maximum hourly

concentrations for SO2 and can be used for compliance actions. Monitors sited at urban scales 

are useful for identifying SO2 transport, trends, and, if sited upwind of local sources, background 

concentrations.

(1) Microscale —This scale would typify areas in close proximity to SO2 point and area 

sources. Emissions from stationary point and area sources, and non-road sources may, under 

certain plume conditions, result in high ground level concentrations at the microscale. The 

microscale typically represents an area impacted by the plume with dimensions extending up to 

approximately 100 meters. 

(2) Middle scale —This scale generally represents air quality levels in areas up to several 

city blocks in size with dimensions on the order of approximately 100 meters to 500 meters. The 

middle scale may include locations of expected maximum short-term concentrations due to 

proximity to major SO2 point, area, and/or non-road sources. 

(3) Neighborhood scale —The neighborhood scale would characterize air quality

conditions throughout some relatively uniform land use areas with dimensions in the 0.5 to 4.0 

kilometer range. Emissions from stationary point and area sources may, under certain plume 

conditions, result in high SO2 concentrations at the neighborhood scale. Where a neighborhood 

site is located away from immediate SO2 sources, the site may be useful in representing typical

air quality values for a larger residential area, and therefore suitable for population exposure and 

trends analyses.
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(4) Urban scale – Measurements in this scale would be used to estimate concentrations 

over large portions of an urban area with dimensions from 4 to 50 kilometers. Such 

measurements would be useful for assessing trends in area-wide air quality, and hence, the 

effectiveness of large scale air pollution control strategies. Urban scale sites may also support 

other monitoring objectives of the SO2 monitoring network such as identifying trends, and when 

monitors are sited upwind of local sources, background concentrations. 

4.4.6 NCore Monitoring. (a) SO2 measurements are included within the NCore 

multipollutant site requirements as described in paragraph (3)(b) of this appendix. NCore-based 

SO2 measurements are primarily used to characterize SO2 trends and assist in understanding SO2

transport across representative areas in urban or rural locations and are also used for comparison

with the SO2 NAAQS. 

* * * * *

26. Appendix G to Part 58 is amended as by revising Table 2 to read as follows:

Appendix G to Part 58 - Uniform Air Quality Index (AQI) and Daily Reporting

* * * * * 
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TABLE 2.—BREAKPOINTS FOR THE AQI

These breakpoints Equal these AQI’s

O3 (ppm)

8-hour

O3 (ppm)

1-hour1

PM2.5

(μg/m3)

PM10

(μg/m3)

CO (ppm) SO2 (ppm) 1-

hour

NO2 (ppm)

1-hour

AQI Category

0.000-0.059 .................. 0.0-15.4 0-54 0.0-4.4 0 - (0.025 – 0.050) 0 – (0.040 – 0.053) 0-50 Good.

0.060-0.075 .................. 15.5-40.4 55-154 4.5-9.4 (0.026 – 0.051) –

(0.050 – 0.100)

(0.041 – 0.054) –

(0.080 – 0.100)

51-100 Moderate.

0.076-0.095 0.125-0.164 40.5-65.4 155-254 9.5-12.4 (0.051 – 0.101) –

(.175 - .200)

(0.081 – 0.101) –

(0.360 – 0.370)

101-150 Unhealthy for Sensitive 

Groups.

0.096-0.115 0.165-0.204 365.5-150.4 255-354 12.5-15.4 (0.176 – 0.201) –

(.304)

(0.361 – 0.371) –

0.64

151-200 Unhealthy.

0.116-0.374 0.205-0.404 3150.5-250.4 355-424 15.5-30.4 0.305 – 0.604 0.65 – 1.24 201-300 Very Unhealthy.

(

2

)............... 0.405-0.504 3250.5-350.4 425-504 30.5-40.4 0.605 – 0.804 1.25 – 1.64 301-400

(

2

)............... 0.505-0.604 3350.5-500.4 505-604 40.5-50.4 0.805 – 1.004 1.65 – 2.04 401-500 Hazardous.

1 Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour ozone 

values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and 

the maximum of the two values reported.

2

8-hour O3 values do not define higher AQI values ( 301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations.

3

If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
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* * * * *