National Ambient Air Quality Standards for Sulfur Oxides (Sulfur Dioxide)Reproposal; Proposed Rule ENVIRONMENTAL PROTECTION AGENCY

SUMMARY: The EPA today is proposing not to revise the current 24-hour

and annual primary standards but is also soliciting comment on the

possible need to adopt additional regulatory measures to address short-

term peak (SO2) exposures and thereby further reduce the health

risk to exercising asthmatic individuals. The alternatives under

consideration include: revising the existing national ambient air

quality standards (NAAQS) by adding a new 5-minute standard of 0.60

ppm, 1 expected exceedance; establishing a new regulatory program under

section 303 of the Clean Air Act to supplement the protection provided

by the existing NAAQS; and augmenting implementation of the existing

standards by focusing on those sources or source types likely to

produce high 5-minute peak SO2 concentrations.

Included in this document are proposals to incorporate certain

associated technical changes to the requirements for Ambient Air

Monitoring Reference and Equivalent Methods (40 CFR part 53) and other

minor technical changes regarding the 40 CFR part 50 regulations.

A related document will be published shortly in the Federal

Register that proposes for comment the requirements for implementing

the alternative regulatory measures. Included in that document are

technical revisions to 40 CFR parts 51 and 58.

DATES: Written comments on this proposal must be received by February

13, 1995. The EPA will hold a public hearing on this notice in

approximately 30 days. The time and place will be announced in a

subsequent Federal Register document.

ADDRESSES: Submit comments on the proposed action on the NAAQS (40 CFR

part 50) (duplicate copies are preferred) to: Air & Radiation Docket

Information Center (6102), Room M-1500, Environmental Protection

Agency, Attn: Docket No. A-84-25, 401 M Street, SW., Washington, DC

20460. Comments on the proposed revisions to the Ambient Air Monitoring

Reference and Equivalent Methods (40 CFR part 53) should be separated

from those pertaining to the standards and sent to the same address,

Attn: Docket No. A-94-42. These dockets are located in the Central

Docket Section of the U.S. Environmental Protection Agency, South

Conference Center, Room M-1500, 401 M St., SW., Washington, DC. The

docket may be inspected between 8 a.m. and 5:30 p.m. on weekdays, and a

reasonable fee may be charged for copying. For the availability of

related information, see the Supplementary Information section.

FOR FURTHER INFORMATION CONTACT: Part 50 Notice--Mr. John H. Haines,

Air Quality Strategies and Standards Division (MD-12), U.S.

Environmental Protection Agency, Research Triangle Park, NC 27711,

telephone (919) 541-5533. Part 53 Notice--Mr. Frank McElroy,

Atmospheric Research and Exposure Assessment Laboratory (MD-77), U.S.

Environmental Protection Agency, Research Triangle Park, NC 27711,

telephone (919) 541-2622.

SUPPLEMENTARY INFORMATION:

Background

In 1971, the EPA promulgated primary and secondary NAAQS for sulfur

oxides (measured as SO2). The primary standards were set at 365

micrograms per cubic meter (g/m\3\) (0.14 part per million

(ppm)), averaged over a 24-hour period and not to be exceeded more than

once per year, and 80 g/m\3\ (0.030 ppm) annual arithmetic

mean. The secondary standard was set at 1300 g/m\3\ (0.5 ppm)

averaged over a period of 3 hours and not to be exceeded more than once

per year. In accordance with sections 108 and 109 of the Act, EPA

reviewed and revised the health and welfare criteria upon which these

primary and secondary SO2 standards were based.

On April 26, 1988 (53 FR 14926), the EPA announced its proposed

decision not to revise these standards. In that notice, the

Administrator also solicited comment on an alternative of adding a 1-

hour primary standard of 0.4 ppm. The EPA also sought comment on

additional revisions in the event a 1-hour standard was promulgated. At

that time, the EPA also proposed to revise the significant harm levels,

associated episode contingency plan guidance (40 CFR part 51), and the

Pollutant Standard Index for SO2 (40 CFR part 58). The EPA also

proposed revisions to certain monitoring and reporting requirements (40

CFR part 58).

On April 21, 1993, the EPA announced its final decision that

revision of the secondary standard was not appropriate (58 FR 21351).

Availability of Related Information

The revised criteria document, Air Quality Criteria for Particulate

Matter and Sulfur Oxides (three volumes, EPA-600/8-82-029af-cf,

December 1982; Volume I, NTIS # PB-84-120401, $36.50 paper copy and

$9.00 microfiche; Volume II, NTIS # PB-84-120419, $77.00 paper copy and

$9.00 microfiche; Volume III, NTIS # PB-84-120427, $77.00 paper copy

and $20.50 microfiche); the criteria document addendum, Second Addendum

to Air Quality Criteria for Particulate Matter and Sulfur Oxides

(1982): Assessment of Newly Available Health Effects Information (EPA/

600/8-86-020-F, NTIS # PB-87-176574, $36.50 paper copy and $9.00

microfiche); the criteria document supplement, Supplement to the Second

Addendum (1986) to Air Quality Criteria for Particulate Matter and

Sulfur Oxides (1982): Assessment of New Findings on Sulfur Dioxide

Acute Exposure Health Effects in Asthmatic Individuals (1994) (EPA-600/

FP-93/002); the 1982 staff paper, Review of the National Ambient Air

Quality Standards for Sulfur Oxides: Assessment of Scientific and

Technical Information (EPA-450/5-82-007, November 1982; NTIS # PB-84-

102920, $36.50 paper copy and $9.00 microfiche); the staff paper

addendum, Review of the National Ambient Air Quality Standards for

Sulfur Oxides: Updated Assessment of Scientific and Technical

Information (EPA-450/05-86-013, December 1986; NTIS # PB-87-200259,

$19.50 paper copy and $9.00 microfiche) and the staff paper supplement,

Review of the National Ambient Air Quality Standards For Sulfur Oxides:

Updated Assessment of Scientific and Technical Information, Supplement

to the 1986 OAQPS Staff Paper Addendum (1994) (EPA-452/R-94-013) are

available from: U.S. Department of Commerce, National Technical

Information Service, 5285 Port Royal Road, Springfield, Virginia 22161,

or call 1-800-553-NTIS. (Add $3.00 handling charge per order.) A

limited number of copies of other documents generated in connection

with this standard review, such as the control techniques document, can

be obtained from: U.S. Environmental Protection Agency Library (MD-35),

Research Triangle Park, NC 27711, telephone (919) 541-2777. These and

other related documents are also available in the EPA dockets

identified above.

Table of Contents

I. Background

A. Legislative Requirements Affecting This Rule

1. The Primary Standards

2. Related Control Requirements

B. Sulfur Oxides and Existing Standards for SO2

C. Development of Revised Air Quality Criteria for Sulfur Oxides

and Review of the Standards: Development of the Staff Paper

D. Rulemaking Docket

II. Summary of the 1988 Proposed Decision Not to Revise the Current

Standards

III. Post-Proposal Developments

A. Opportunities for Public Comment

B. Legislative Activity

C. Litigation on Secondary Standard

D. Decision on Secondary Standard

E. Litigation on Primary Standard

F. Supplementation of the Criteria Document and the Staff Paper

IV. Summary of Public Comments as to Primary Standards and

Associated Technical Changes

A. Current 24-Hour and Annual Standards

B. Averaging Convention for the Current Standards

C. 1-Hour Standard Alternative

D. Other Changes to Standards

E. Technical Revisions to 40 CFR 50.4 and 50.5

V. Rationale for Proposed Decisions

A. Basis for the Current 24-Hour and Annual Standards

B. Consideration of Short-Term Peak SO2 Exposures

1. Assessment of Health Effects Associated With Short-Term

SO2 Exposures

2. Air Quality and Exposure Considerations

C. Regulatory Considerations

1. 5-Minute Standard Alternative

2. Section 303 Program

3. Retain Current Standards

D. Averaging Convention for the Current Standards

E. Form of the Current Standards

F. Other Technical Changes

VI. Federal Reference Methods and Equivalent Methods

VII. Regulatory Impacts

A. Regulatory Impacts Administrative Requirements

B. Impact on Small Entities

C. Reduction of Governmental Burden

D. Environmental Justice

E. Impact on Reporting Requirements

References

Appendix I

Appendix II

I. Background

A. Legislative Requirements Affecting This Rule

1. The Primary Standards

Two sections of the Act govern the establishment and revision of

the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to

identify pollutants which ``may reasonably be anticipated to endanger

public health or welfare'' and to issue air quality criteria for them.

These air quality criteria are to ``reflect the latest scientific

knowledge useful in indicating the kind and extent of all identifiable

effects on public health or welfare which may be expected from the

presence of (a) pollutant in the ambient air. * * *''

Section 109 (42 U.S.C. 7409) directs the Administrator to propose

and promulgate ``primary'' NAAQS for pollutants identified under

section 108. 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 criteria and allowing an adequate margin of

safety, (is) requisite to protect the public health.''

The U.S. Court of Appeals for the D.C. Circuit has held that the

requirement for an adequate margin of safety for primary standards was

intended to address uncertainties associated with inconclusive

scientific and technical information available at the time of standard

setting. It was also intended to provide a reasonable degree of

protection against hazards that research has not yet identified. Lead

Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir. 1980),

cert. denied, 101 S. Ct. 621 (1980); American Petroleum Institute v.

Costle, 665 F.2d 1176, 1177 (D.C. Cir. 1981), cert. denied, 102 S. Ct.

1737 (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, by selecting primary standards that provide an adequate margin of

safety, the Administrator is seeking not only to prevent pollution

levels that have been demonstrated to be harmful, but also to prevent

lower pollutant levels that she finds pose an unacceptable risk of

harm, even if that risk is not precisely identified as to nature or

degree.

In selecting a margin of safety, the EPA has considered such

factors as the nature and severity of the health effects involved, the

size of the sensitive population(s) at risk, and the kind and degree of

the uncertainties that must be addressed. Given that the ``margin of

safety'' requirement by definition only comes into play where no

conclusive showing of harm exists, such factors, which involve unknown

or only partially quantified risks, have their inherent limits as

guides to action. 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,

supra, 647 F.2d at 1161-62.

Section 109(d) of the Act (42 U.S.C. 7409(d)) requires periodic

review and, if appropriate, revision of existing criteria and

standards. The process by which the EPA has reviewed the original

criteria and standards for sulfur oxides under section 109(d) is

described in a later section of this notice.

2. Related Control Requirements

States are primarily responsible for ensuring attainment and

maintenance of ambient air quality standards once the EPA has

established them. Under section 110 (42 U.S.C. 7410) and part D of

title I of the Act (42 U.S.C. 7501-7515), States are to submit, for EPA

approval, State implementation plans (SIP's) that provide for the

attainment and maintenance of such standards through control programs

directed to sources of the pollutants involved. The States, in

conjunction with the EPA, also administer the prevention of significant

deterioration program (42 U.S.C. 7470-7479) for these pollutants. In

addition, Federal programs provide for nationwide reductions in

emissions of these and other air pollutants through the Federal motor

vehicle control program under title II of the Act (42 U.S.C. 7521-

7574), which involves controls for automobile, truck, bus, motorcycle,

and aircraft emissions; new source performance standards under section

111 (42 U.S.C. 7411); National Emission Standards for Hazardous Air

Pollutants under section 112 (42 U.S.C. 7412); and title IV of the

Clean Air Act Amendments of 1990 (42 U.S.C. 7651-76510), which

specifically provides for major reductions in SO2 emissions.

B. Sulfur Oxides and Existing Standards for SO2

The principal focus of this standard review is on the health

effects of SO2, alone and in combination with other pollutants.

Other sulfur oxide (SOX) vapors (e.g., sulfur trioxide, SO3)

are not commonly found in the atmosphere. Information on the effects of

the principal atmospheric transformation products of SO2 (i.e.,

sulfuric acid and sulfates) was considered in the review of the

particulate matter standards and addressed in the revisions to these

standards promulgated on July 1, 1987 (52 FR 24634); it will be

considered again in the next review of the particulate matter

standards, the commencement of which was announced on April 12, 1994

(59 FR 17375).

Sulfur dioxide is a rapidly diffusing reactive gas that is very

soluble in water. It is emitted principally from combustion or

processing of sulfur-containing fossil fuels and ores. Sulfur dioxide

occurs in the atmosphere with a variety of particles and other gases,

and undergoes chemical and physical interactions with them forming

sulfates and other transformation products. At elevated concentrations,

SO2 can adversely affect human health. Annual average SO2

levels range from less than 0.004 ppm in remote rural sites to over

0.03 ppm in the most polluted urban industrial areas. The highest

short-term values are found in the vicinity (2 are discussed in more

detail in the revised criteria document (EPA, 1982a), in the staff

paper (EPA, 1982b), in the criteria document addendum (EPA, 1986a), and

the staff paper addendum (EPA, 1986b).

On April 30, 1971, the EPA promulgated the primary NAAQS for

SO2 under section 109 of the Act (36 FR 8186). The existing

primary standards for sulfur oxides, measured as SO2, are 365

g/m3 (0.14 ppm), averaged over a period of 24 hours and

not to be exceeded more than once per year, and 80 g/m3

(0.030 ppm) annual arithmetic mean. The scientific and technical bases

for the current standards are contained in the original criteria

document, Air Quality Criteria for Sulfur Oxides (DHEW, 1970).

Implementation of SO2 air quality standards by the States and

the EPA, together with fuel use shifts and siting decisions motivated

by changing economic conditions, have resulted in substantial

improvements in ground level air quality. Annual emissions decreased

significantly between 1975 and 1982, from 25.7 to 21.4 million metric

tons/year. During the mid to late eighties and early nineties, however,

annual emissions of SO2 have remained basically the same, at

approximately 20.6 million metric tons/year (EPA, 1993a).

Title IV of the Act, the acid rain program, requires that electric

utilities reduce annual SO2 emissions by 10 million short tons (9

million metric tons) per year from the 1980 baseline of 23.3 million

metric tons. This reduction will be implemented in two phases. The

phase 1 reductions are to be accomplished by 1995, and the bulk of the

phase 2 reductions are to be accomplished by the year 2000, with an

expected annual emission rate of 16.38 million metric tons that year.

Total expected reductions from title IV will result in an annual

emission rate of 14.22 metric tons in the year 2015.

Ambient air SO2 trends over the decade from 1983 to 1992 show

a definite downward trend, though the rate of decline has slowed over

the last few years. Annual mean SO2 decreased at a median rate of

approximately 2 percent per year, resulting in a total drop of 23

percent. The annual second highest 24-hour values over this same time

period decreased 31 percent, at an average rate of 4 percent per year

(EPA, 1993a). The most recent trends of SO2 measured in the

ambient air have continued to show improvement. Annual mean

concentrations decreased a total of 11 percent between 1990 to 1992.

Over the last 2 years, the average annual mean SO2 decrease was 7

percent. Second maximum 24-hour SO2 concentrations declined 12

percent between 1990 and 1992 and 4 percent between 1991 and 1992 (EPA,

1993a).

C. Development of Revised Air Quality Criteria for Sulfur Oxides and

Review of the Standards: Development of the Staff Paper

On October 2, 1979, the EPA announced it was revising the original

criteria document for sulfur oxides concurrently with that for

particulate matter to produce a combined particulate matter/sulfur

oxides (PM/SOx) criteria document (44 FR 56731). A more complete

history of the revisions and addenda to the criteria document and staff

paper, as well as the text of all CASAC closure letters, is presented

in the 1988 proposal (53 FR 14926, April 26, 1988). A brief synopsis

appears below.

The EPA provided a number of opportunities for review and comment

on the revised criteria document by organizations and individuals

outside the Agency. Three drafts of the revised criteria document,

prepared by the EPA's Environmental Criteria and Assessment Office

(ECAO), were made available for external review (45 FR 24913, April 11,

1980; 46 FR 9746, Jan. 29, 1981; 46 FR 53210, Oct. 28, 1981). The EPA

received and considered numerous and often extensive comments on each

of these drafts, and CASAC has held three public meetings (August 20-

22, 1980; July 7-9, 1981; November 16-18, 1981) to review successive

drafts of the document. Transcripts of these meetings have been placed

in the docket for the criteria document (ECAO CD 79-1). In addition,

five public workshops were held at which the EPA, its consulting

authors and reviewers, and other scientifically and technically

qualified experts selected by the EPA discussed the various chapters of

the draft document and suggested ways of resolving outstanding issues

(45 FR 74047, Nov. 7, 1980; 45 FR 76790, Nov. 20, 1980; 45 FR 78224,

Nov. 26, 1980; 45 FR 80350, Dec. 4, 1980; 46 FR 1775, Jan. 7, 1981).

The comments received were considered in the preparation of the final

document. A CASAC ``closure'' memorandum indicating the Committee's

satisfaction with the final draft of the criteria document and

outlining key issues and recommendations was issued in December 1981.

Following closure, a number of scientific articles were published,

or accepted for publication, that appeared to be of sufficient

importance to the development of criteria for the primary standards for

SO2 to necessitate an addendum to the criteria document. Two

drafts of the addendum were reviewed by CASAC and members of the public

in two public meetings (April 26-27, 1982; August 30-31, 1982), and

transcripts of the meetings have been placed in the docket. The

addendum was included as Appendix A to Volume I of the criteria

document (EPA, 1982a) when the document was issued on March 20, 1984

with the proposed revisions to the ambient air quality standards for

particulate matter (49 FR 10408, Mar. 20, 1984).

As part of this process, the EPA's Office of Air Quality Planning

and Standards (OAQPS) in the spring of 1982 prepared the first draft of

a staff paper, ``Review of the National Ambient Air Quality Standards

for Sulfur Oxides: Assessment of Scientific and Technical Information-

OAQPS Staff Paper.'' The first draft and a second draft of the staff

paper were reviewed at CASAC meetings on April 26-27, 1982 (47 FR

16885, April 20, 1982), and August 30-31, 1982 (47 FR 34855, Aug. 10,

1982), respectively, and transcripts of these meetings have been placed

in the docket (Docket No. A-79-28). Numerous written and oral comments

were received on the drafts from CASAC, representatives of

organizations, individual scientists, and other interested members of

the public, and some revisions engendered by these comments are

discussed in an August 5, 1982 letter to CASAC (Padgett, 1982), as well

as the executive summary of the staff paper. The EPA released the final

OAQPS staff paper (EPA, 1982b), upon receipt of the formal CASAC

closure letter in August 1983 (Goldstein, 1983), accompanied by a

minority statement by one member (Higgins, 1983).

In 1984, the Administrator reviewed the standards in light of the

above information and decided, at that time, not to propose any

revision of the standards.

In 1986, in response to the publication in the scientific

literature of a number of additional studies on the health effects of

SO2 (as well as some new particulate matter studies), ECAO

commenced a second addendum to the PM/SOX criteria document (51 FR

11058, Apr. 1, 1986). An external review draft was made available for

public comment (51 FR 24392, Jul. 3, 1986) and CASAC held a public

meeting on October 15-16, 1986 to review the criteria document addendum

(transcript in public docket No. A-82-37). When development of a second

addendum of the criteria document was initiated in 1986, OAQPS decided

to simultaneously commence an addendum to the staff paper as well (51

FR 24392, Jul. 3, 1986). An external review draft of the addendum to

the staff paper was also issued, and the staff paper was reviewed at

the same public CASAC meeting at which the second addendum to the

criteria document was considered.

The CASAC sent a closure letter on the criteria document addendum

to the Administrator dated December 15, 1986, and another on the staff

paper, dated February 1987. The closure letter on the staff paper

addendum, which also discusses major issues addressed by the CASAC and

the Committee's recommendations, is reprinted in Appendix 1 to this

notice. The final addenda to the criteria document (EPA, 1986a) and the

staff paper (EPA, 1986b), are available from the address listed above.

Where there are differences between the 1982 criteria document and

staff paper and the more recent addenda, the addenda supersede the

earlier documents.

D. Rulemaking Docket

The EPA established a standard review docket for the sulfur oxides

review in July 1979. The EPA also established a rulemaking docket

(Docket No. A-84-25) for the April 26, 1988 proposal as required by

section 307(d) of the Act. The standard review docket (Docket No. A-79-

28) and a separate docket established for criteria document revision

(Docket No. ECAO-CD-79-1) have been incorporated into the rulemaking

docket.

II. Summary of the 1988 Proposed Decision Not To Revise the Current

Standards

On April 26, 1988 (53 FR 14926), the EPA announced its proposed

decision not to revise the existing primary and secondary SOX

standards (measured as SO2). In reaching the provisional

conclusion that the current standards provided adequate protection

against the health and welfare effects associated with SO2, the

EPA was mindful of uncertainties in the available evidence concerning

the risk that elevated short-term (2 concentrations

pose to asthmatic individuals exercising in ambient air. Therefore, the

EPA specifically requested broad public comment on the alternative of

revising the current standards and adding a new 1-hour primary standard

of 0.4 ppm. The notice also announced that if a 1-hour primary standard

were adopted, consideration would be given to replacing the current 3-

hour secondary standard (1,300 g/m3 (0.5 ppm)) with a 1-

hour secondary standard set equal to the primary standard, and adopting

an expected-exceedance form for all of the standards.

The EPA also concluded in the April 26, 1988 notice, based upon the

then-current scientific understanding of the acidic deposition problem,

that it would not be appropriate, at that time, to propose a separate

secondary SOX standard to provide increased protection against the

acidic deposition-related effects of SOX. The notice added that

when the fundamental scientific uncertainties had been reduced through

ongoing research activities, the EPA would draft and support an

appropriate set of control measures.

The EPA also proposed minor technical revisions to the standards,

including restating the levels for the primary and secondary standards

in terms of ppm rather than g/m3, adding explicit

rounding conventions, and specifying data completeness and handling

conventions. The EPA also announced its intention to retain the block

averaging convention for the 24-hour, annual, and 3-hour standards and

proposed to eliminate any future questions in this regard by adding

clarifying language to 40 CFR 50.4 and 50.5. Based on its assessment of

the SO2 health effects information, the EPA also proposed to

revise the significant harm levels for SO2 and the associated

example air pollution episode levels (40 CFR part 51). Finally, the EPA

proposed some minor modifications to the ambient air quality

surveillance requirements (40 CFR part 58).

The April 26, 1988 (53 FR 14926) notice sets forth in detail the

rationale for the proposals discussed above and provides other

background information.

III. Post-Proposal Developments

A. Opportunities for Public Comment

Following the publication of the proposal, the EPA held a public

meeting in Washington on June 10, 1988 to receive comment on the April

26, 1988 proposal. A transcript of the meeting has been placed in the

public docket (Docket No. A-84-25). On July 20, 1988, the EPA announced

an extension of the public comment period from July 25, 1988 to

September 23, 1988 (53 FR 27362). The EPA issued a second notice on

September 21, 1988 (53 FR 36587) to clarify that issues concerning

block versus running averaging conventions should be fully aired in the

sulfur dioxide rulemaking initiated by the April 26, 1988 notice (53 FR

14926). At the same time, the EPA extended the comment period until

November 22, 1988 to provide ample opportunity for the public to

comment.

B. Legislative Activity

In July 1989, legislative proposals for amending the Act were

submitted to Congress. This initiative included a comprehensive program

to address the acidic deposition problem. After extensive deliberation,

the 1990 Amendments, including the title IV acid rain provisions, were

passed by Congress and signed into law by the President on November 15,

1990. As discussed earlier in section I.B., and below, title IV of the

1990 Amendments was developed specifically to address the acidic

deposition problem but will have an attendant benefit of reducing

SO2-related health effects.

C. Litigation on Secondary Standard

Prior to the 1988 proposal, the Environmental Defense Fund and

other plaintiffs had sued the EPA under section 304 of the Act to

compel review and revision of the NAAQS for SOX under section

109(d)(1) of the Act, Environmental Defense Fund v. Reilly, No. 85 C.V.

9507 (S.D.N.Y.). In response to a decision of the U.S. Court of Appeals

for the Second Circuit in 1989, Environmental Defense Fund v. Thomas,

870 F.2d 892 (2d Cir. 1989), the EPA and the plaintiffs ultimately

entered into a consent decree as an alternative to further litigation.

The decree required the EPA to take final action by April 15, 1993 on

the secondary standard portion of the 1988 proposed rulemaking.

D. Decision on Secondary Standard

A final decision under section 109(d)(1) of the Act that revision

of the secondary standard was not appropriate was signed on April 15,

1993 and was published in the Federal Register on April 21, 1993 (58 FR

21351). The rationale for the decision is set forth in the April 21,

1993 notice. At that time it was also announced that when action was

completed on the primary standards portion of the 1988 proposal, the

EPA would decide whether to adopt minor technical changes discussed in

the 1988 proposal.

E. Litigation on Primary Standard

In 1992, the American Lung Association sued the EPA to compel

review and, if appropriate, revision of the primary standards for

SOX, American Lung Association v. Browner, No. 92-CV-5316 (ERK)

(E.D.N.Y.). The U.S. District Court for the Eastern District of New

York subsequently issued an order requiring that the EPA by November 1,

1994: take final action on the 1988 proposed decision not to revise the

primary standards, or repropose and take final action on the reproposal

within 1 year after the close of the public comment period.

F. Supplementation of the Criteria Document and the Staff Paper

In response to the more recent publication of controlled human

studies on the health effects of short-term peaks of SO2 on

asthmatic individuals, the ECAO commenced preparation of a supplement

to the second addendum to the PM/SO2 criteria document in 1992.

The OAQPS prepared a draft of a supplement to the staff paper addendum

to update its assessment of the new information contained in the

Criteria Document Supplement and to take into account more recent air

quality and exposure information. Initial drafts of these documents

were completed in June, 1993. The EPA announced the availability of an

external review draft of both documents for public comment on July 30,

1993 (58 FR 40818), and the documents were reviewed by the CASAC at a

public meeting on August 19, 1993. Recommended changes were made, and

revised drafts of both documents were made available for public comment

(59 FR 11985, March 15, 1994). Both documents were reviewed at a public

CASAC meeting on April 12, 1994. The CASAC provided its advice and

recommendations to the Administrator in a letter dated June 1, 1994

that is reprinted in Appendix 2.

IV. Summary of Public Comments as to Primary Standards and Associated

Technical Changes

The following discussion summarizes in general terms the comments

received from the public regarding the key aspects of the April 26,

1988 notice as they pertain to the primary standards and associated

technical changes. The individual comments have been entered into the

public docket (Docket No. A-84-25). For a summary of public comments on

the secondary standard, see 58 FR 21354, Apr. 21, 1993.

Extensive written comments were received on the 1988 proposal. Of

some 90 written submissions, 33 were provided by individual industrial

concerns or industry groups, 14 by State, local and Federal government

agencies and organizations, 14 by environmental and public interest

groups, and 29 by individual private citizens.\1\ The comments on the

key aspects of the April 26, 1988 notice pertaining to the primary

standard and associated part 50 technical changes are summarized below.

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\1\The numerical distribution of comments in each category

should be viewed with caution. Industry groups typically submit

comments on behalf of their member companies in lieu of having each

of their member companies sending separate comments. Similarly,

comments from environmental or other interest groups represent the

views of a number of individuals.

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A. Current 24-Hour and Annual Standards

Virtually all of the comments that specifically addressed the

adequacy of the current standards supported the Administrator's 1988

finding that the current primary SO2 standards are adequate to

protect the public health from the effects associated with 24-hour and

annual average SO2 concentrations in the atmosphere. As discussed

below, the principal exceptions were the comments submitted on the

issue of the averaging convention of the standards. These commenters

maintained that the current primary standards would not provide

adequate protection against adverse health effects if measurements of

the currently prescribed concentration levels were restricted to the

block averaging convention.

B. Averaging Convention for the Current Standards

Comments on the Administrator's decision to retain the block

averaging convention for the 3-hour, 24-hour, and annual standards were

sharply divided. The industry comments on this issue strongly supported

the proposed decision to retain the block averaging convention as the

appropriate method for determining compliance with the current

standards. In support of this position, these commenters typically took

note of the text of the 1971 promulgation notice, the Air Quality

Criteria for Sulfur Oxides (DHEW, 1970), contemporaneous papers that

discussed how the measurements were to be collected and analyzed, and

the fact that implementation of the standards for the most part has

been based on block averaging. The environmental groups maintained,

however, that the wording of the original standards clearly did not

preclude the use of the running averaging convention; that the EPA's

monitoring capabilities, guidance, and implementation practice

demonstrated that the standards were not restricted to block averaging;

and accordingly that the use of running averaging would not represent a

tightening of the standards. Several State agencies supported the

adoption of a running interpretation or requested that the EPA remain

silent so as not to undercut the States' use of running averages, while

other States and municipalities supported the EPA's proposed decision.

C. 1-Hour Standard Alternative

Discussion on this subject was highly polarized. Industry groups

and their representatives uniformly opposed a short-term standard,

while environmental groups, private citizens, and most State and local

agencies that commented strongly favored the adoption of such a

standard. Industry maintained that the clinical studies of asthmatics

used to support the possible need for a short-term standard failed to

show effects that were of such medical significance as to be considered

``adverse'' under the Act. Environmental groups argued that the effects

seen were medically significant and ``adverse'' at concentrations below

0.5 ppm and called for a standard to be set at levels considerably

below the 0.4 ppm, 1-hour alternative that was presented for comment.

The nature of the comments were such that there was virtually no

consensus over the significance of effects among industry,

environmental groups, and the different medical experts that commented

on the issue.

In support of their position that a short-term standard was not

needed, industry groups placed great weight on the results of the

exposure analysis presented in the April 26, 1988 notice. They

maintained that the analysis demonstrated that the current standards

provided considerable protection against short-term peak exposures and

that the remaining risk did not pose a significant public health

problem. Some environmental groups took exception to the EPA's use of

the exposure analysis. They maintained that a large under-counting of

exposures occurred because the analysis did not address potential

exposures from nonutility sources such as nonferrous smelters, paper

mills, and petroleum refineries. Some also argued that the EPA's

reliance on the exposure analysis as a basis for retaining the existing

standards was without legal authority. These commenters were also

critical of the Agency's use of typical activity patterns and

maintained that other aspects of the analysis were deficient. Industry

groups generally supported the use of exposure analyses in the standard

setting process and maintained that the EPA's focus on utilities was

appropriate given that they are the largest emitters of SO2.

Environmental groups and private citizens also expressed concern

that the significance of asthma episodes were being downplayed and

raised concerns about exposures of children, who were dependent on

adults for medication and care. They were also highly critical of the

EPA's characterization of the number of asthmatics (up to 100,000)

potentially at risk to SO2 peak exposures as small.

State and local agencies that commented mostly supported the

adoption of a short-term 1-hour standard.

Finally, environmental groups maintained that the 1-hour

alternative would not protect against short-term 2- to 10-minute peak

SO2 concentrations. In support of their position, data were

submitted showing that certain types of SO2 sources may have very

high 5-minute peaks (>1 ppm) and still have hourly averages below 0.4

ppm even when the current standards are being attained. One of the

industry commenters also noted that an averaging time shorter than 1

hour would be needed to protect against very high 3- to 5-minute peak

SO2 levels and cited an instance where a 3- to 5-minute peak of

3.7 ppm SO2 occurred, yet the 1-hour average was only 0.29 ppm.

This commenter went on to suggest, however, that such problems would be

better addressed through a properly designed program under the

authority of section 303 of the Act rather than through the adoption of

a new short-term ambient air quality standard.

D. Other Changes to Standards

While a number of commenters favored the adoption of a new 1-hour

standard, little, if any, support was voiced for the associated

revisions that the EPA indicated it was considering if a 1-hour

standard was adopted. Few, if any, commenters supported the adoption of

an expected exceedance form for all of the standards. While several

commenters recognized that a statistical form had certain technical

advantages, they expressed concern that its adoption would reduce the

protection afforded by the current 3-hour, 24-hour and annual

standards.

E. Technical Revisions to 40 CFR 50.4 and 50.5

There was general support for the EPA's proposal to restate the

levels of the standards in terms of ppm rather than g/m\3\ and

for adding explicit rounding conventions and data completeness and

handling conventions to the regulations.

V. Rationale for Proposed Decisions

A. Basis for the Current 24-Hour and Annual Standards

The rationale for retaining the current 24-hour and annual primary

standards was presented in some detail in the 1988 proposal (53 FR

14930, Apr. 26, 1988) and remains unchanged. At that time, the EPA

concluded that the current 24-hour and annual standards appeared to be

both necessary and adequate to protect human health against SO2

concentrations associated with those averaging periods. The EPA also

concluded that retaining the current 24-hour and annual standards was

consistent with the scientific data assessed in the criteria document

and staff paper and their addenda and with the advice and

recommendations of the staff and the CASAC.

The EPA again provisionally concludes, based on the information

assessed in the criteria document and staff paper and their addenda,

that the current 24-hour and annual primary standards provide adequate

health protection against the effects associated with those averaging

periods. In reaching this proposed decision, the EPA takes note that

the health effects information on 24-hour and annual SO2 exposures

remains largely unchanged since 1988. When newer information becomes

available and has undergone the rigorous and comprehensive assessment,

including CASAC review, necessary for incorporation into a new criteria

document, it will provide the basis for the next periodic review of the

24-hour and annual primary standards.

B. Consideration of Short-Term Peak SO2 Exposures

A number of new studies have become available since 1988 that

examine the potential health effects on asthmatic individuals

associated with short-term (1-hour) exposures to SO2.

In view of these new studies and other relevant new information, the

EPA prepared a ``Supplement to the Second Addendum (1986) to Air

Quality Criteria for Particulate Matter and Sulfur Oxides (1982):

Assessment of New Findings on Sulfur-Dioxide Acute Exposure Health

Effects in Asthmatic Individuals'' (``Criteria Document Supplement'')

(EPA, 1994a) and an associated staff paper supplement ``Review of the

National Ambient Air Quality Standards for Sulfur Oxides: Updated

Assessment of Scientific and Technical Information--Supplement to the

1986 OAQPS Staff Paper Addendum'' (``Staff Paper Supplement'') (EPA,

1994b). These two documents, together with the 1986 addenda, provide

the primary basis for the EPA's present assessment of the health

effects and related information on short-term SO2 exposures and

the Administrator's consideration of appropriate regulatory responses.

The discussion below summarizes the basis for considering alternative

regulatory responses to address the potential effects associated with

short-term peak SO2 exposures.

1. Assessment of Health Effects Associated With Short-Term SO2

Exposures

a. Sensitive Populations. It is clear that healthy nonasthmatic

individuals are essentially unaffected by acute exposures to SO2

at concentrations below 2 ppm and do not constitute a population of

concern for short-term, acute SO2 exposure effects.

Based on the assessment in the Criteria Document Supplement (EPA,

1994a), the EPA concludes that mild and moderate asthmatic children,

adolescents, and adults that are physically active outdoors represent

the population segments at most risk for acute SO2 induced

respiratory effects. Individuals with more severe asthmatic conditions

have poor exercise tolerances; as a result, they are very unlikely to

engage in sufficiently intense outdoor activity to achieve the

requisite breathing rates for SO2-induced respiratory effects to

occur and therefore maybe at somewhat lower risk. While current studies

are suggestive of greater SO2 responsiveness among those asthmatic

patients with more severe disease, this issue cannot be unequivocally

resolved. However, because of the lower baseline function in moderate

and severe asthmatic persons, especially those lacking optimal

medication, any effect of SO2 would further reduce their lung

function toward levels that may become cause for medical concern (EPA,

1994a, p. 44).

While it has been suggested that nonasthmatic atopic individuals

may also represent a broader population group at increased risk (White,

1994; 53 FR 14931-14932, Apr. 26, 1988), other assessments have not

found evidence establishing the atopic group to be particularly

responsive to SO2 (EPA, 1994a, p. 52; EPA, 1994b, p. 10; Linn et

al., 1987).

b. Asthma. About 10 million people or 4 percent of the population

of the United States are estimated to have asthma (NIH, 1991). The true

prevalence may be as high as 7 to 10 percent of the population (Evans

et al., 1987), because some individuals with mild asthma may be unaware

that they have the disease and thus go unreported. The prevalence is

higher among African-Americans, older (8- to 11-year-old) children, and

urban residents (Schwartz et al., 1990).

The Expert Panel Report from the National Asthma Education Program

of the National Heart, Lung and Blood Institute (NIH, 1991) has

recently defined asthma as ``a lung disease with the following

characteristics: (1) Airway obstruction that is reversible (but not

completely so in some patients) either spontaneously or with treatment,

(2) airway inflammation, and (3) increased airway responsiveness to a

variety of stimuli.'' Common symptoms include cough, wheezing,

shortness of breath, chest tightness, and sputum production. Asthma is

characterized by an exaggerated bronchoconstrictor response to many

physical challenges (e.g., cold or dry air, exercise) and chemical and

pharmacologic agents (e.g., histamine or methacholine).

Daily variability in lung function measurements is a typical

feature of asthma, with the poorest function (i.e., lowest forced

expiratory volume in 1 second (FEV1) and highest specific airway

resistance (SRaw) being experienced in the early morning hours and the

best function (i.e., highest FEV1 and lowest SRaw) occurring in

the mid- afternoon.

The degree of exercise tolerance varies with the severity of

disease. Mild asthmatic individuals have good exercise tolerance but

may not tolerate vigorous exercise such as prolonged running. Moderate

asthmatic individuals have diminished exercise tolerance and

individuals with severe disease have very poor exercise tolerance that

markedly limits physical activity.

Exercise-induced bronchoconstriction is followed by a refractory

period of several hours during which an asthmatic individual is less

susceptible to bronchoconstriction (Edmunds et al., 1978). This

refractory period may alter an asthmatic individual's responsiveness to

SO2 or other inhaled substances.

Data from the United Kingdom and United States suggest an incidence

rate of asthma attacks requiring medical attention of 2 rests primarily on the extensive

literature involving brief (2- to 10-min) controlled exposures of

persons with mild (and in some cases more moderate) asthma to

concentrations of SO2 in the range of 0.1 ppm to 2 ppm while at

elevated ventilation. The major effect of SO2 on sensitive

asthmatic individuals is bronchoconstriction, usually evidenced in

these studies by increased specific airway resistance (SRaw) or

decreased forced expiratory volume (FEV1), and the occurrence of

clinical symptoms such as wheezing, chest tightness, and shortness of

breath. The magnitude of the response and likely occurrence of symptoms

increase at higher SO2 concentrations and ventilation levels and

are relatively brief in duration. Numerous studies have shown that lung

function typically returns to normal for most subjects within an hour

of exposure. No substantial ``late phase'' responses have been noted

for SO2, unlike the case for more specific stimuli (e.g., pollen,

dust mites, or other allergens) in which ``late phase'' inflammatory

responses often occur 4-8 hours after exposure and are often much more

severe and dangerous than earlier immediate responses.

In a summary of the literature up to 1986 in the Staff Paper

Addendum (EPA, 1986b), the staff concluded that changes in lung

function ( SRaw 70 percent) accompanied by symptoms could be

observed in some free-breathing asthmatics at 0.4 ppm at ``moderate-

heavy exercise.'' At 0.5 ppm, slightly larger functional changes on

individual and group basis were seen at moderate exercise (

SRaw 50--100 percent), while at 0.6-0.75 ppm SO2 functional

changes and symptoms could be observed at light-moderate exercise

( SRaw 120-260 percent), with the effects being judged

``indicative of clinical significance.'' Effects at 1-2 ppm SO2

were seen as even more pronounced, ranging from ``moderate'' to

``incapacitating'' for some individuals (53 FR 14948, April 26, 1988).

As the concentration increases within the range studied, effects are

more pronounced and the fraction of asthmatic subjects who respond

increases (53 FR 14947, April 26, 1988).

Since 1986 several new studies have been published providing

pertinent information on: (1) The response of individuals with more

moderate asthma to SO2, (2) the duration of exposure necessary to

provoke a response to SO2, and (3) the effects of medication on

the SO2 response. Much of these data also provide a more thorough

picture of the magnitude of responses in the range of 0.4 to 1.0 ppm,

the range previously identified as being of interest (53 FR 14948,

April 26, 1988). Data from several of these recent large-scale chamber

studies were reexamined to provide a better understanding of the

response observed in more sensitive subjects. Forced expiratory volume

in one second was used as a measure of lung function, in addition to

specific airway resistance, and other endpoints examined included

symptoms, alteration of workload, and medication usage occurring as a

consequence of these exposures.

Table B-1 of the Criteria Document Supplement (EPA, 1994a)

summarizes the lung function changes in response to SO2

concentrations in the range of 0.6-1.0 ppm from controlled human

exposure studies. Because different studies used different measures of

lung function (FEV1 or SRaw), and different concentrations of

SO2, the discussion that follows will describe group mean changes

first for the studies that used the measure SRaw, then group mean

changes for studies that used FEV1, and then finally the

individual responses.

The data indicate that, in terms of group mean changes, total SRaw

changes\2\ were approximately twice as great at 0.6 ppm and above as at

0.5 ppm and below. The differences were even more pronounced when the

changes in airway resistance due to SO2 alone (i.e., after

correction for the effects of exercise) were considered.

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

\2\Since elevated ventilation sufficient for oronasal breathing

to occur is a requirement for most asthmatic persons to respond to

SO2, and because many asthmatic individuals experience

bronchoconstriction responses to exercise alone, it is useful to

distinguish between the two different effects. Any measure of lung

function such as FEV1 or SRaw can be expressed as the ``Total

FEV1 or SRaw,'' which is the total change in lung function

experienced by the subject as a result of an exposure to SO2

while at exercise, or broken down to ``the effect of changes due to

SO2 alone,'' which represents the total lung function change

observed minus the change seen for that subject from a control

exposure at exercise in clean air. Both measures have their utility:

total FEV1 or SRaw indicates the magnitude of overall lung

function change actually experienced by the subject, while the

change due to SO2 alone indicates how much of this total change

is attributable to the pollutant itself.

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

For FEV1, the difference in responses between 0.4 ppm and 0.6

ppm SO2 were not as pronounced. At 0.6 ppm SO2, group mean

decreases in total FEV1 of approximately 20 percent were observed

in the mild and moderate asthmatics studied. The changes in FEV1

due to SO2 alone resulted in decreases in FEV1 of

approximately 15 percent (EPA, 1994a, Table B-1).

In addition, at 0.6 ppm SO2, 25 percent or more of the

subjects had pronounced individual responses (either a 200 percent or

greater increase in SRaw or a 20 percent or greater decrease in

FEV1) due to SO2 alone (total changes in lung function for

these individuals would be expected to be even greater). In contrast,

at 0.5 ppm SO2 these more pronounced individual

responses were less frequent, occurring in fewer than 25 percent of the

subjects for both measures of lung function for all but one group

studied (EPA, 1994a, p. B-2).

While not examined in as much detail as lung function, other

indicators of severity also tend to increase with increasing SO2

concentration. For instance, in one study, four of 24 moderate/severe

asthmatic subjects were required to reduce their exercise level because

of asthma symptoms at 0.6 ppm SO2. This occurred only once at each

of the lower concentrations (EPA, 1994a). Two recent studies which

considered medication used to mitigate the effects of SO2 as a

health endpoint and which followed the subjects' medication use in

detail, found approximately twice as many subjects took medication

immediately after exposure to 0.6 ppm SO2 than after exposure to

0.3 ppm SO2 (EPA, 1994a, Table 7, p. 40).

Considering the variety of endpoints for which information is

available, clearly the effects beginning at 0.6 ppm and up to 1.0 ppm

are more pronounced than those at lower concentrations. This is in

agreement with the conclusions reached in the Staff Paper Addendum

(EPA, 1986b), which stated that there were ``clearer indications of

clinically or physiologically significant effects at 0.6 to 0.75 ppm

SO2 and above'' (53 FR 14947, Apr. 26, 1988).

d. Significance of Effects. Opinions on the significance of the

effect expressed by CASAC and others have been widely divergent. Some

CASAC members and outside commenters feel that the responses reported

in the range of 0.6 to 1.0 ppm SO2 are not significant, especially

when viewed in the context of the frequency with which asthmatics

ordinarily experience similar effects in the course of their daily

lives. Other CASAC members and commenters strongly felt that

bronchoconstriction of the degree reported in this range of exposure is

of medical significance and likely to place an exposed asthmatic at an

unacceptable risk of harm.

The frequency of SO2 induced asthmatic episodes relative to

those provided by other stimuli (such as cold/dry air or moderate

exercise) would be expected to vary from one asthmatic individual to

another and from one location to another. As such, the relative

contribution of SO2 to acute episodes of asthma cannot be

precisely assessed. However, staff did compare the effects of SO2

observed in the recent controlled human exposure studies to the effects

of moderate exercise, typical daily variation in lung function, and the

severity of frequently experienced asthma symptoms. The effects of 0.6

ppm SO2 exposure at moderate exercise, as measured by FEV1,

exceeded either the typical effect of exercise alone or typical daily

variations in FEV1 (EPA, 1994a, sections 4.3 and 5.3). For

symptomatic responses, two to eight times as many subjects after

exposure at exercise to 0.6 ppm SO2 experienced symptoms of at

least moderate severity (13-62 percent of subjects) than after exercise

in clean air alone (4-19 percent of subjects) (EPA, 1994a, p. B-12). In

addition, a significant portion of subjects (approximately 15 to 60

percent, depending on asthma status) participating in certain

controlled human exposure studies seemed to experience symptoms more

frequently in response to 0.6 ppm SO2 than reported at any other

time during the majority of the weeks during which they participated in

the study (EPA, 1994a, p. B-12).

Furthermore, the response seen in the most sensitive 25 percent of

responders at 0.6 ppm equaled or exceeded approximately a 30 percent

decline in FEV1 for mild asthmatic subjects and approximately a 40

percent decline for moderate asthmatic individuals. By comparison,

during clinical bronchoprovocation testing changes are not usually

induced beyond a 20 percent decrease in FEV1.

In addition, while at least some subjects can experience such a 20

percent decline without experiencing symptoms, in recent studies

focusing on effects at 0.6 ppm SO2, from 33 percent to 43 percent

of moderate asthmatics and from 6 percent to 35 percent of mild

asthmatics experienced at least a 20 percent decrease in total

FEV1 in conjunction with symptoms rated as being of moderate

severity or worse. Also deserving consideration is the fact that

moderate/severe asthmatic subjects start an exposure with compromised

lung function compared to mild asthmatic subjects. Thus, it is not

clear that similar functional declines beginning from a different

baseline have the same biological importance (EPA, 1994a, pp. 21-25).

In the Staff Paper Addendum, ``bronchoconstriction . . .

accompanied by at least noticeable symptoms,'' was seen as an

appropriate measure of concern (EPA, 1986b, p. 37). However, a

substantial proportion of the subjects in these more recent studies are

experiencing greater effects, bronchoconstriction with at least

moderate symptoms, beginning at 0.6 ppm SO2 (EPA, 1994a).

Considering the recent body of evidence along with previous

studies, the Criteria Document Supplement (EPA, 1994a) concluded that

substantial percentages (25 percent) of mild or moderate

asthmatic individuals exposed to 0.6 to 1.0 ppm SO2 during

moderate exercise would be expected to have respiratory function

changes and severity of symptoms that distinctly exceed those

experienced as typical daily variation in lung function or in response

to other stimuli, such as moderate exercise. The severity of effects

for many of the responders is likely to be of sufficient concern to

cause disruption of ongoing activities, use of bronchodilator

medication, and/or possible seeking of medical attention. At most, only

10 to 20 percent of mild or moderate asthmatic individuals are likely

to exhibit lung function decrements in response to SO2 exposures

of 0.2 to 0.5 ppm that would be of distinctly larger magnitude than

typical diurnal variation in lung function or changes in lung function

experienced by them in response to other often encountered stimuli.

Furthermore, it appears likely that only the most sensitive responders

might experience sufficiently large lung function changes and/or

respiratory symptoms of such severity as to be of potential health

concern, that is leading to the disruption of ongoing activities, the

need for bronchodilator medication, or seeking of medical attention.

Based on the staff's assessment, a number of additional factors are

important in assessing the significance of effects resulting from

SO2 exposures and determining appropriate concentrations of

concern.

Time Course of Response. If an asthmatic individual is at elevated

ventilation and encounters a brief SO2 peak concentration, the

onset of the effect can be very rapid although the response does not

typically approach maximal levels until 5 minutes of exposure. For

example, the total lung function response from a 2-minute exposure was

reported to be only 50 percent of that observed after 5 minutes of

exposure (Horstman et al., 1988). Balmes (1987) reported (in a

mouthpiece exposure study) the response after 3 minutes of exposure was

67 percent of that observed after 5 minutes. After 5 minutes of

exposure the magnitude of the response does not appear to significantly

increase based on comparisons of lung function changes after 5-minute

and 10-minute exposures (Linn, 1983b; EPA, 1986b, p. A-1).

The response is also generally brief in duration; numerous studies

have shown that lung function typically returns to normal for most

subjects within an hour of exposure. This duration is similar to that

experienced in response to exercise and somewhat less than experienced

in response to allergens (EPA, 1994b, p. 18). Even if exposure

continues beyond the initial 5-10 minutes, lung function may still

return to normal as long as the subject ceases to exercise and their

ventilation rate decreases to resting levels (Hackney et al., 1984;

Schachter et al., 1984).

Effect of Varying Temperature and Humidity. Broncho-constriction in

response to SO2 and exercise is: (a) Reduced by warm or humid

conditions, and (b) exacerbated by cold or dry conditions. Thus, the

observed effects such as those described above could be either more

pronounced, less pronounced, or similar depending on the ambient

conditions present during exposure at elevated ventilation.

Effect of Varying Ventilation Rate and Breathing Mode. Another

factor that can affect the magnitude of the SO2 induced response

is ventilation rate. At higher ventilation rates the responses are

likely to be more pronounced at any given SO2 concentration than

those observed at lower ventilation rates. The effects of SO2

increase with both increased overall ventilation rates and an increased

proportion of oral ventilation in relation to total ventilation (EPA,

1986a, p. 11). Oral ventilation is thought to accentuate the response

because the scrubbing of SO2 by the nasal passageways is bypassed.

Based on its assessment of the available data, the staff concluded that

the ventilation rates of concern begin at 35-50 L/min, when most

individuals generally switch to oronasal breathing.

Ventilation rates in the range of 35-40 L/min are comparable to

ventilation rates induced by climbing three flights of stairs, light

cycling, shoveling snow, light jogging, or playing tennis, and can be

induced in a laboratory by walking at 3.5 mph up a 4 percent grade.

Ventilation rates in the range of 45-50 L/min are equivalent to

moderate cycling, chopping wood, light uphill running, and can be

induced by walking at 3.5 mph up an 8 percent grade (EPA, 1994b, p.

20).

While the SO2 effects reported for mild or moderate asthmatic

individual are likely to be more pronounced if an individual asthmatic

is at a ventilation rate higher than 35-50 L/min (EPA, 1994b, p. 19),

the available activity and ventilation data indicate that individuals

engage in outdoor activities that induce ventilation rates of 35-50 L/

min only a small percentage of the time (EPA, 1994b, p. 20). Thus, it

is unlikely that asthmatic individuals in general would attain

sufficiently high ventilation rates (i.e., greater than 35-50 L/min)

frequently enough to markedly increase the health risk posed by peak

SO2 exposures.

Use of Medication. The extent to which an asthmatic individual is

already medicated for protection against other bronchoconstriction

inducing stimuli (e.g., cold dry air, allergens, etc.) and thus would

be protected against SO2, has been considered relevant in

assessing (a) the likelihood of experiencing a bronchoconstriction

response to SO2 and, by extension, (b) the significance of these

effects (53 FR 14932, Apr. 26, 1988). The available data now indicate

that most types of regularly administered asthma medications are not

very effective in blocking the SO2 response. The exception,

however, is the most commonly used class of asthma medications, the

-sympathomimetic drugs (beta-agonist bronchodilator), which

are usually highly effective in preventing the SO2 response from

developing if taken shortly before exposure.

Prophylactic use of beta-agonist bronchodilators to prevent the

effects of SO2 requires either anticipation of exposure or routine

use prior to engaging in vigorous outdoor activities. While some

asthmatic persons do premedicate before exercise, available published

data suggest infrequent bronchodilator use in general among mild

asthmatic persons and a wide range of compliance rates (from very low

to full) among regularly medicated asthmatic persons as a whole (EPA,

1994a, section 2.2). The staff's assessment of this also found low use

of beta-agonist bronchodilators among asthmatic subjects participating

in some of the clinical studies evaluating SO2 effects, as well as

the relative absence of routine medication use before exercise among

such subjects (EPA, 1994a). Given the infrequent use of medication by

many mild asthmatic individuals and the poor medication compliance of

30 to 50 percent of the ``regularly medicated'' asthmatic patients, it

appears that a substantial proportion of asthmatic subjects would not

likely be ``protected'' by medication use from impacts of environmental

factors on their respiratory health. However, the frequency of use of

medication (bronchodilators) specifically prior to engaging in outdoor

activity cannot be confidently extrapolated from epidemiologic data on

medication compliance. Thus, the relative number of persons who may be

protected by medication prior to exercise is unclear (EPA, 1994a, pp.

9-10).

It also should be noted that beta-agonist bronchodilators are

effective in ameliorating SO2-induced bronchoconstriction if an

asthmatic individual has immediate access to such medication after

exposure.

Effect of Other Pollutants. It has been suggested by one study

(Koenig et al., 1990) that prior exposure to ozone may result in

greater SO2 effects, at any given SO2 concentration, than

those reported in the controlled human exposure studies that examined

the effects of SO2 alone. In the ambient situation, however,

potential ozone (O3)-induced increases in SO2 effects may be

at least partially attenuated by the hot humid weather that is often

associated with elevated O3 concentrations.

Data on whether prior nitrogen dioxide exposure produces an

increased response to SO2 are unclear, with a mouthpiece study

showing positive effects (Jorres et al., 1990), while a chamber study

of younger subjects showed no effects of NO2 on responsiveness to

SO2 (Rubenstein et al., 1990). It appears that a pollutant that

increases nonspecific bronchial responsiveness may also increase airway

responses to SO2 (EPA, 1994a, p. 48).

Epidemiological Evidence. Available epidemiological studies show no

evidence of significant associations between either 24-hour or 1-hour

average ambient air SO2 concentrations above 0.1 ppm and increased

visits to hospital emergency rooms for asthma (EPA, 1994a, p. 52).

However, it is not clear to what extent epidemiologic studies could

detect possible associations between very brief (10-minute),

geographically localized, peak SO2 exposures and respiratory

effects in asthmatic individuals. In the absence of such data, it is

not possible to associate peak ambient SO2 concentrations with

excess asthma mortality rates reported to be observed among nonwhite

population groups in large urban areas.

Frequency of Exposure Considerations. Based on this assessment of

the available health effects information, the authors of the Criteria

Document Supplement (EPA, 1994a) concluded that an important

consideration in determining the public health significance of the

reported SO2 induced effects is the likely frequency that an

asthmatic individual would be exposed to a 5-minute peak SO2

concentration 0.6 ppm. Because asthmatic individuals must be

at elevated ventilation in order to experience significant

bronchoconstriction in response to peak SO2 concentrations, any

analysis undertaken to estimate the size of the asthmatic population

potentially at risk from such exposures must account for both the

likelihood that an asthmatic individual will be outdoors at sufficient

ventilation and the likelihood that he or she will encounter an

SO2 concentration of concern.

2. Air Quality and Exposure Considerations

A central issue raised during the comment period on the 1988

proposal concerned whether a 1-hour standard of 0.4 ppm, based on a

typical peak-to-mean ratio of approximately 2 to 1, would provide

adequate protection from high 5-minute peak SO2 levels near all

sources. Based on examination of more recent data, the staff concluded

(EPA, 1994b) that no typical peak-to-mean ratio exists that can be used

to determine a uniformly-applicable hourly standard. Given the broad

range of hourly values associated with 5-minute peaks of SO2 (EPA,

1994b, Table 3-2), it was concluded that reliance on any hourly peak-

to-mean ratio would risk over-controlling some sources (if a high peak-

to-mean ratio is assumed and a low hourly standard chosen) or under-

controlling other sources (if a low peak-to-mean ratio is assumed and a

high hourly standard chosen).

The available 5-minute SO2 data examined in the staff paper

supplement (EPA, 1994b, pp. 34-37) clearly indicate that high 5-minute

peak SO2 concentrations can occur with some frequency near some

sources. Absent comprehensive data on 5-minute peak SO2 levels,

the staff used hourly data to estimate the likely nationwide prevalence

of high short-term SO2 peaks. The staff examined all hourly

averages reported in the AIRS database for the year 1992 and applied

different peak-to-mean ratios to produce upper and lower bound

estimates of 5-minute peaks 0.25 ppm. The method used for

calculating the incidence of short-term peaks is given in the Staff

Paper Supplement (EPA, 1994b). The lower bound estimate of the number

of 5-minute peaks 0.75 ppm SO2 indicated that 50

monitors, in 38 counties which contained 18 urban areas, would register

at least one 5-minute peak of SO2 0.75 ppm. The upper

bound estimate was that 132 monitors, in 91 counties with 65 urban

areas might experience a short-term peak of SO2 0.75

ppm. The same analysis indicated that 132 monitors, in 91 counties

containing 65 urban areas, would be the lower bound estimate of the

occurrence of at least one 5-minute peak of SO2 0.50

ppm. The upper bound estimate was that 247 monitors in 148 counties

with 124 urban areas might record at least one 5-minute peak of

SO2 0.50 ppm. This analysis also suggests that the

number of monitoring sites likely to record multiple high 5-minute

peaks in a single year, or over several years, can vary considerably

(EPA, 1994b, pgs. 41-42).

The use of existing hourly data to assess the potential prevalence

of 5-minute peak SO2 levels has other limitations beyond those

introduced by the use of peak-to-mean ratios. The existing monitoring

network is designed to accurately characterize ambient air quality

associated with 3-hour, 24-hour, and annual SO2 concentrations

rather than to detect short-term peaks SO2 levels. As a result,

the EPA's monitoring guidance on siting criteria, the spanning of

SO2 instruments, and instrument response time could lead to

underestimates of high 5-minute peaks and thus the 1-hour averages for

hours containing those peaks. Of these factors, monitoring siting may

be the largest potential source of underestimation of SO2 peaks

and therefore changes in monitoring siting and density near SO2

sources most likely to produce high 5-minute peaks should increase the

number of high 5-minute peaks and associated 1-hour averages recorded.

In addition to estimating the occurrence of peak SO2 levels in

the ambient air, an important consideration in assessing the public

health significance of SO2-induced effects is determining the

likely frequency that an asthmatic individual will be exposed (EPA,

1994a, p. 51). To address this issue, exposure analyses have been

conducted that predict both the frequency of high SO2 peaks

(through air quality modeling) and the probability that an asthmatic

individual will be outdoors at sufficient ventilation (>35 L/min) to

experience an SO2-induced effect. The methodologies employed in

these analyses, together with the associated uncertainties, are

discussed in some detail in the Staff Paper Supplement (EPA, 1994b, pp.

46-47, appendix B).

These analyses indicate that 68,000 to 166,000 asthmatic

individuals (or 0.7 to 1.8 percent of the total asthmatic population)

potentially could be exposed one or more times, while outdoors at

exercise, to 5 minute peaks of SO2 0.5 ppm. Fewer

asthmatic individuals are likely to be exposed to 0.6 ppm

SO2 under the same conditions. The estimated number of asthmatic

individuals exposed one or more times results in an estimate of 180,000

to 395,000 total exposure events of which the utility sector accounts

for about 68,000. After full implementation of the title IV program of

the Act, in the year 2015, the number of exposure events at

0.5 ppm SO2 attributable to the utility sector is

estimated to drop to 40,000, contingent on trading decisions.

Based on the available air quality and exposure data assessed in

the Staff Paper Supplement (EPA, 1994b) and summarized above, the

Administrator concurs with the staff and CASAC's views that the

likelihood that asthmatic individuals will be exposed to 5- to 10-

minute peak SO2 concentration of concern, while outdoors and at

exercise, is relatively low when viewed from a national perspective.

The Administrator takes note, however, as did the staff, that the data

also indicate high peak SO2 concentrations can occur around

certain sources or source types (EPA, 1994b, p. 37) with some

frequency, suggesting that asthmatic individuals who reside in the

vicinity of such sources or source types may be at greater health risk

than indicated for the asthmatic population as a whole.

C. Regulatory Considerations

Taking into account the staff's assessments and the advice and

recommendations of the CASAC, the Administrator has considered whether

additional regulatory measures are needed to protect asthmatic

individuals against short-term (5- to 10-minute) peak SO2

exposures. In her judgment, the current 3-hour, 24-hour, and annual

standards appear to provide substantial protection against the health

effects associated with short-term SO2 exposures. As indicated by

the air quality analyses described above, the current standards,

together with implementation of title IV of the Act, markedly limit the

frequency and extent of short-term concentrations of concern. The

exposure analyses that take into account normal day-to-day activity

patterns further suggest that the risk is relatively low that

individuals with mild or moderate asthma will experience exposure

conditions approximating those that produced effects of concern in

controlled human studies. In view of those analyses, the nature of the

reported effects, the effectiveness of bronchodilator medication to

prevent or ameliorate SO2 effects if available and properly used,

and the fact that similar events can be provoked more frequently by

other stimuli, the Administrator concurs with the staff's and the

CASAC's assessment that the public health risk posed by short-term peak

SO2 levels is limited when viewed from a national perspective and

does not constitute a broad national public health problem.

The Administrator is mindful, however, that the available data

indicate that those asthmatic individuals who reside in proximity to

certain individual sources or source types will be at higher risk of

being exposed to short-term peak SO2 levels than the asthmatic

population as a whole. While some asthma specialists question the

health significance of the reported health effects, the Administrator

notes that others believe the effects are significant and that

additional protection is warranted. This information, combined with

uncertainties regarding the use of bronchodilator medication prior to

exercise, particularly among asthmatic children and asthmatic

individuals who may not perceive a need to medicate regularly prior to

engaging in outdoor activities, suggests to the Administrator that

additional regulatory measures may be needed.

In their assessment of the available scientific and technical

information, the EPA staff recommended a range of concern for the

Administrator's consideration when examining the potential need for new

regulatory measures to provide additional public health protection

beyond that provided by the existing set of standards (EPA, 1994b).

This range, based on the most recent assessments presented in the

criteria document and staff paper supplements and summarized above, is

0.6 to 1.0 ppm SO2. The staff's assessment concluded that a

substantial percentage (20 percent or more) of mild to moderate

asthmatic individuals exposed to 0.6 to 1.0 ppm SO2 for 5 to 10

minutes during moderate exercise would be expected to have respiratory

function changes and severity of respiratory symptoms that clearly

exceed those experienced from typical daily variation in lung function

or in response to other stimuli (e.g., moderate exercise or cold/dry

air). For many of the responders the effects are likely to be both

perceptible and thought to be of some immediate health concern, i.e.,

to cause disruption of ongoing activities, use of bronchodilator

medication, and/or possibly seeking of medical attention. At SO2

concentrations at or below 0.5 ppm, the staff concluded that at most

only 10 to 20 percent of mild and moderate asthmatic individuals

exposed to 0.2 to 0.5 ppm SO2 during moderate exercise are likely

to experience lung function changes distinctly larger than those

typically experienced and that, compared to the response at 0.6 to 1.0

ppm SO2, the response at or below 0.5 ppm SO2 is less likely

to be perceptible and of immediate health concern.

In considering the staff's most recent assessment of the available

health information, the Administrator found it to be generally

consistent with the staff's 1986 review. During both reviews there has

been divergent opinion as to the appropriate level for the lower bound

for the range of concern. Both assessments, however, concluded that 1.0

ppm SO2 is the appropriate upper bound. At that level there is

clear concern that if an asthmatic individual is exposed while at

exercise to 1.0 ppm SO2 for 5 minutes the risk of significant

functional and symptomatic responses will be high. This finding in 1986

led several CASAC members to recommend a 1-hour standard level that

would restrict the concentration of 5-minute SO2 peaks to 0.6 to

0.8 ppm in order to preclude 5-minute peaks of 1.0 ppm SO2

(Lippmann, 1987). The Administrator finds the staff's present

recommendations consistent with that point of view.

The Administrator also took note that the current CASAC review

panel, while acknowledging the existence of a wide spectrum of views

among asthma specialists regarding the clinical and public health

significance of the reported effects, did not comment on the range of

concern or present the individual panel members' views as to the

significance of the reported effects in its ``closure'' letter. At the

April 12, 1994 ``closure'' meeting, however, the panel found that the

range recommended by the staff was consistent with the available

scientific information. Three members of the panel who addressed the

public health significance of the reported effects in their written

comments concluded that segments of the asthmatic population exposed to

peak SO2 concentrations while at elevated ventilation were at risk

of incurring clinically significant effects if not properly medicated.

While the basis for their judgments differed, their views as to the 5-

minute concentrations of concern overlapped (0.4 to 0.8 ppm SO2;

above 0.6 ppm SO2; and 0.6 to 1.0 ppm SO2) and are in general

agreement with both the 1986 and 1994 staff assessments. On the other

hand, another panel member who addressed the general issue, while

recognizing that SO2 can cause bronchoconstriction, questioned the

public health significance of short-term peak SO2 exposures, based

in part on his judgment that the likelihood of an asthmatic individual

being exposed while at exercise is exceedingly low given the protection

afforded by the existing standards. In its closure letter, the CASAC

expressed the view that such exposures are rare events and that the

likelihood of such exposures should be considered in selecting an

appropriate regulatory response.

Based on its assessment of the available data, the staff

recommended consideration of three regulatory alternatives: (1)

Revising the existing NAAQS by adding a new 5-minute standard

implemented through a risk-based targeted strategy, (2) establishing a

new regulatory program under section 303 of the Act, or (3) augmenting

the implementation of current NAAQS by focusing on those sources likely

to cause high 5-minute peaks. In considering these alternatives, the

Administrator has taken into account the divergent views expressed by

the public, asthma specialists, and the CASAC with respect to the

public health significance of short-term SO2 exposures and the

appropriate degree of protection needed. In doing so she is mindful

that in the absence of conclusive scientific and technical information,

the Act requires that the Administrator make a judgmental determination

as to whether the reported effects endanger public health and pose an

unacceptable risk of harm. At the April 12, 1994 CASAC meeting and in

written comment, individual members of the 1994 CASAC panel recognized

that choosing among the regulatory alternatives presented in the staff

paper supplement must be guided by legal and policy considerations,

given the nature of the available scientific and technical information

and the divergent views as to the health significance of the reported

effect and the pollution level of concern.

The Administrator therefore is proposing for public comment three

alternative regulatory approaches for supplementing the protection

provided by the current standards if additional protection is judged to

be necessary. In so doing, the Administrator has carefully considered

the 1994 CASAC review panel's strong recommendation that any additional

regulatory measures be implemented through a risk-based, targeted

strategy. Consistent with this recommendation, all three regulatory

alternatives under consideration, as described below, are based upon

such a strategy. The Administrator believes it is important to air the

key issues and uncertainties fully and specifically requests broad

public comment and deliberation on these alternatives.

1. 5-Minute NAAQS Alternative

After considering the staff's recommendations and the views of the

1986 and 1994 CASAC review panels, the Administrator believes that it

is both appropriate and necessary to solicit public comment on a 5-

minute NAAQS of 0.60 ppm SO2. Based on the staff's assessments of

the available scientific and technical information, the Administrator

is concerned that 5-minute peak SO2 levels beginning at 0.60 ppm

and above may present an unacceptable risk of harm to asthmatic

individuals who have not premedicated with beta-agonist bronchodilators

and are exposed at elevated ventilation. In proposing a 5-minute NAAQS,

the Administrator is particularly concerned that asthmatic individuals

in the proximity of sources with a high potential to cause or

contribute to a 5-minute peak SO2 concentration greater than 0.60

ppm may be at substantially greater risk of experiencing an exposure

event, which triggers bronchoconstriction, than the asthmatic

population as a whole. Adoption and implementation of a 5-minute NAAQS

of 0.60 ppm SO2 would prevent such exposures and further reduce

the likelihood that an asthmatic individual would be exposed at

elevated ventilation to lesser concentrations. Therefore, it is the

Administrator's provisional judgment that a 5-minute NAAQS of 0.60 ppm

SO2 would adequately protect the public health.

In assessing the possible need for additional protection against

peak SO2 exposures, the Administrator has considered the specific

issue of medication usage. While it is clear from the available data

that the use of beta- agonist bronchodilators to prevent the effects of

other stimuli (e.g., exercise, cold/dry air) will also prevent or

ameliorate the effects of SO2, there is considerable debate as to

compliance rates and therefore the degree of protection provided. As

one CASAC panel member noted, ``many moderate asthmatics, particularly

those from urban areas and lower economic status, may have less than

ideal medical follow-up and are prone to irregular medication use and

frequent deterioration'' (Schachter, 1994). In public comment on the

1988 proposal, a number of individuals made the point that asthmatic

children, who are dependent on adults for their medication and care,

are more likely to be unprotected and therefore at particular risk from

SO2 exposures of concern. Other commenters on the criteria

document and staff paper supplements noted that asthmatic individuals

who do not perceive the need to medicate prior to engaging in strenuous

outdoor activities would also be at increased risk from SO2

exposures. While the Administrator believes these are important

considerations, the overriding issue is whether the availability of,

and reliance on, prophylactic medications should be viewed as an

alternative to further regulatory action to reduce the risk posed by

high peak SO2 concentrations in the ambient air. In this regard,

the Administrator is concerned whether reliance on medications, even if

taken to prevent the effects caused by other stimuli, as an alternative

to environmental controls would be an appropriate public policy choice,

particularly given the potential environmental equity issues involved.

In seeking comment on a possible 5-minute NAAQS of 0.60 ppm

SO2, to further reduce the risk posed by high peak SO2

concentrations, the Administrator concurs with the staff's

recommendation that such a standard be implemented through a risk-based

targeted approach. By focusing on those sources or source types that

are most likely to cause or contribute to high 5-minute SO2

concentrations and thus pose the greatest risk to asthmatic

individuals, such a program would be effective in reducing peak

SO2 concentrations of concern. In response to questions raised by

the 1994 CASAC review panel, the Agency continues to believe that such

a program would be enforceable, based on its longstanding enforcement

experience.

The Administrator recognizes, however, as did the 1994 CASAC review

panel,\3\ that the adoption of a 5-minute NAAQS might not be

appropriate given the nature of the problem or the most efficient means

of achieving the desired reductions. Under sections 108 through 110 of

the Act, NAAQS and State plans to implement them are designed to

address air pollution problems that emanate from numerous and diverse

sources whose collective emissions contribute to unacceptable pollution

levels, rather than from a limited number of discrete point sources

that cause only very localized pollution problems. Moreover, the

implementation process for a 5-minute NAAQS (described in detail in the

40 CFR part 51 document to be published shortly in the Federal

Register) could impose significant planning and other requirements on

the States and the regulated community that are neither very efficient

nor necessary for addressing the limited number of point sources that

the EPA believes may produce high 5-minute peak SO2 levels. While

the targeting strategy presented in the part 51 notice is designed to

reduce such burdens to the extent practicable under the Act, the

implementation process includes a number of time-consuming steps (e.g.,

area designations) that are not particularly germane, given the nature

of the problem, and could significantly delay effective remediation.

With these factors in mind and in view of her desire to provide such

additional protection (beyond the existing NAAQS) as may be appropriate

in the most efficient manner, the Administrator is also advancing for

public comment the alternative of establishing a new control program

based on sections 303, 110(a)(2)(G), and 301(a) of the Act.

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\3\In its ``closure letter'', the 1994 CASAC panel stated, ``It

was the consensus of CASAC that any regulatory strategy to

ameliorate such exposure be risk-based--targeted on the most likely

sources of short-term sulfur dioxide spikes rather than imposing

short-term standards on all sources. All of the nine CASAC Panel

members recommended that Option 1, the establishment of a new 5-

minutes standard, not be adopted. Reasons cited for this

recommendation included: the clinical experiences of many ozone

experts which suggest that the effects are short-term, readily

reversible, and typical of response seen with other stimuli.

Further, the committee viewed such exposures as rare events which

will even become rarer as sulfur dioxide emissions are further

reduced as the 1990 amendments are implemented. In addition, the

committee pointed out that enforcement of a short-term NAAQS would

require substantial technical resources. Furthermore, the committee

did not think that such a standard would be enforceable . . .'' To

the extent CASAC comments about enforcement of a short-term NAAQS

took into account such factors as cost and technological

feasibility, the courts have held that such factors are not

appropriate considerations in the establishment or revision of

NAAQS. The extent to which these factors influenced the CASAC

recommendation regarding a 5-minute NAAQS is unclear.

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2. Section 303 Program

As an alternative to a new 5-minute NAAQS, the staff recommended in

the staff paper supplement that consideration be given to establishing

a new regulatory program under section 303 to supplement the protection

provided by the existing NAAQS. The staff recommended that the new

program establish a target level for control in the range of 0.60 to

1.0 ppm SO2, expressed as the maximum 5-minute block average in 1

hour, and that the program be implemented through a risk-based,

targeted strategy. This approach would supplement the existing NAAQS

by, in effect, placing a cap on ambient short-term peak SO2

levels. Exceedance of this cap would lead to source-specific control

efforts designed to prevent recurrence of such peak levels, thus

providing additional protection to asthmatic individuals in proximity

to the source(s) involved.

Section 303 authorizes the Administrator to bring suits for

injunctive relief or to issue appropriate administrative orders if air

pollution levels in an area pose ``an imminent and substantial

endangerment to public health or welfare, or the environment.''

Although section 303 is probably best known in connection with EPA

regulations for the prevention of ``emergency episodes'' involving high

concentrations of criteria pollutants (40 CFR part 51, subpart H), the

Agency interprets it as providing authority to act in a variety of

circumstances, including situations involving pollution concentrations

lower than ``emergency'' levels and incidents involving industrial

accidents or malfunctions (EPA, 1983b, pp. 1-2, 5).\4\ Section

110(a)(2)(G) of the Act requires State implementation plans (SIP's) to

contain authority comparable to section 303 and adequate contingency

plans to implement that authority. As indicated above, the program

proposed in this notice would be based on both of these provisions, as

well as section 301(a) of the Act, which grants general authority to

prescribe regulations necessary to carry out the functions of the

Administrator.

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\4\Similar provisions in other EPA statutes have been similarly

construed (see, e.g., EPA 1993b (section 504 of the Clean Water

Act); EPA 1991 (section 1431 of the Safe Drinking Water Act); EPA

1983a (section 106(a) of the Comprehensive Environmental Response,

Compensation, and Liability Act)).

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Although the proposed program would differ in some respects from

the approach adopted in the Agency's ``emergency episodes'' program, it

would be based on some of the same fundamental concepts. The emergency

episodes program was designed to supplement the NAAQS by providing

additional protection in situations not effectively addressed by them,

i.e., in periods of air stagnation when air pollution levels can build

up to levels well in excess of the NAAQS. Under the program, SIP's are

required to include contingency plans that specify two or more stages

of episode criteria--such as the alert, warning, and emergency levels

specified in example regulations issued by the EPA--and progressively

more stringent abatement actions, including shutting down entire

industries to the extent necessary, as pollution levels advance from

one stage to another (see 40 CFR part 51, subpart H and appendix L).

The episode criteria and associated abatement actions are preventive

measures designed to ensure that certain pollution concentrations--

referred to as significant harm levels (SHL's)--are never achieved.\5\

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\5\This preventive approach--combining elements of rulemaking

and advance planning--helps to avoid some of the practical problems

associated with attempting to address emergency episodes by seeking

injunctive relief on an ad hoc basis.

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Although the Agency established SHL's for these purposes at

concentrations associated with relatively severe health effects, the

use of section 303 to protect public health is not limited to

situations involving such extreme conditions. By design, the SHL's are

levels that should never be reached, and relatively drastic measures to

prevent their occurrence, including court actions for injunctive

relief, are authorized at a lower level, usually the ``emergency''

level (EPA, 1993b, pp. 4-5). Indeed, abatement measures may be required

at even lower levels (id.), both to prevent air quality levels from

deteriorating further (36 FR 20513, Oct. 23, 1971), and to avoid less

serious health effects that can occur at those levels (39 FR 9672,

9673, Mar. 13, 1974).

Even where there is uncertainty about a threatened harm, the EPA

interprets section 303 as authorizing action where there is a

``reasonable medical concern'' about public health (EPA, 1983b, p. 4).

More generally, the courts have construed similar provisions in other

EPA statutes liberally, indicating that action under them is not

limited to extreme, extraordinary, or ``crisis'' situations but may be

based on circumstances posing a ``reasonable cause for concern that

someone or something may be exposed to a risk of harm'' if remedial

action is not taken (see, e.g., U.S. versus Conservation Chemical Co.,

619 F. Supp. 162, 194 (W.D.Mo. 1985); EPA, 1993b, pp. 10-13 (CWA

section 504); EPA, 1991, pp. 5-7 (SDWA section 1431); EPA, 1983b, pp.

2-5 (CAA section 303); EPA, 1983a, pp. 8-9 (CERCLA section 106(a))).

For these and other reasons, the Agency believes that its authority to

address threats to public health or welfare or the environment under

section 303 is not limited to situations involving pollutant

concentrations associated with severe effects.\6\

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\6\This conclusion is consistent with the legislative history of

section 303, as well as that of similar provisions in other EPA

statutes (see, e.g., S. Rep. No. 91-1196, 91st Cong., 2d Sess. 35-36

(1970) (section 303 authority applies not only in situations

involving incapacitating body damage, irreversible body damage, and

increases in mortality but also ``whenever air pollution agents

reach levels of concentration that are associated with . . . the

production of significant health effects . . . in any significant

portion of the general population''). It is also consistent with the

steady pattern of broadening and strengthening of section 303

evident in all amendments to the Act since 1967 see, e.g., S. Rep.

No. 101-228, 101st Cong., 1st Sess. 370-71 (1989)).

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Like the emergency episodes program, the new section 303 program

would attempt to avoid the need for ad hoc court actions by

establishing a framework for remedial efforts in advance through the

Agency's rulemaking authority. However, because 5-minute peak SO2

concentrations of concern can occur rapidly, with little or no prior

build-up of SO2 levels, and because such peak concentrations are

relatively quickly dispersed, the Agency believes that a section 303

program modeled closely on the emergency episodes program would not

provide an effective response. Instead, the Administrator concurs with

the staff recommendation that a health-based, ambient-air target or

trigger level be established if this alternative is selected, and that

sources that cause or contribute to exceedances of the trigger level be

identified and regulated on a case-by-case, source-specific basis to

prevent 5-minute peaks of concern from recurring. Given the nature of

the problem being addressed, the trigger level would need to be

preventive in nature; that is, it would need to be set at a level

designed to ensure that pollution levels that might pose a significant

risk to the public health would not occur in the ambient air.

If this alternative is selected, it is the Administrator's

provisional judgment, based on her assessment of available health

information and for the reasons discussed above, that the appropriate

trigger level for the section 303 program would be 0.60 ppm SO2 as

measured in the ambient air, so as to provide the same level and degree

of protection as would be afforded by a possible new 5-minute NAAQS. As

discussed earlier, the Administrator is concerned that 5-minute peak

SO2 concentrations of 0.60 ppm and above may present an

unacceptable risk of harm to asthmatic individuals who have not

premedicated with beta- agonist bronchodilators and are exposed at

elevated ventilation.

The details of the proposed section 303 program will be described

in the Federal Register in the document concerning implementation

issues. Like the emergency episodes program, the proposed program would

require States to adopt SIP provisions containing necessary legal

authority and contingency plans. Once a violation of the trigger level

proposed in today's notice was detected, the State and the pertinent

emission source(s) would need to take steps to determine the cause of

the violation, and the source(s) would need to implement appropriate

remedial actions to prevent recurrences of such emissions. The EPA

would also be able to take action, either by enforcing the SIP

provisions or directly under its section 303 authority.

The proposed section 303 program would offer several distinct

advantages. It would provide an enforceable, health-based target to

guide the actions of the regulated community, and it could be focused

specifically on those sources most likely to cause or contribute to

high 5-minute peak SO2 exposures. Once information became

available that a source had caused or contributed to an exceedance of

the trigger level, appropriate actions could be initiated quickly.

While some SIP revisions would be necessary for States to implement

this program, more time-consuming aspects of the SIP process such as

designations could be avoided. The EPA would also be able to take

action directly if necessary. The likelihood that this program could

bring about prompt and effective remediation of problems causing high

5-minute peak SO2 levels is a factor of considerable importance to

the Administrator.

3. Retain Current Standards

The Administrator has also considered the staff's third alternative

of retaining the current set of standards but augmenting their

implementation by focusing on those sources that are most likely to

produce high 5-minute peak SO2 levels. The targeting strategy and

implementation plan will be discussed more specifically in the Federal

Register document on implementation issues. This approach would be

aimed at assuring that the existing standards were met through more

targeted monitoring, including the routine collection and reporting of

5-minute data, and more vigorous enforcement of existing regulatory

provisions governing good operating practices, upsets, and

malfunctions. The Administrator believes that additional risk

reductions can be achieved by these means, and the EPA is presently

taking steps to initiate such activities. In summary, the EPA is

requesting public comment on three alternative approaches for

supplementing the protection provided by the current standards against

the health risk posed by short-term peak SO2 levels if additional

protection is judged to be necessary. Given the available scientific

and analytical data, the final selection of the most appropriate course

of action will be based in large part on policy and legal

considerations. To better inform the Administrator's final

determination, the EPA specifically requests public comment in several

key areas. First, the EPA requests the submittal of additional factual

information on the frequency of occurrence of 5-minute peak SO2

levels in the ambient air, as well as information on the source or

source types and the nature of the events that are most likely to give

rise to such peak SO2 levels. Such information would assist in

determining the most effective regulatory response. Second, throughout

the review there has been considerable debate as to the adequacy of the

available exposure analyses. In light of the uncertainties in these

analyses, the EPA requests the submission of data that would allow for

better characterization of the asthmatic population at risk and of the

frequency that an asthmatic individual would likely be exposed to peak

SO2 concentrations, particularly at levels of 0.60 ppm and above,

while at elevated ventilation. Third, of particular interest to the

Administrator is the issue of the medical significance of the reported

SO2 induced effects. Given the broad diversity of opinion of the

asthma specialists that have participated in the review to date, the

EPA specifically requests other members of the medical community who

are experts in this area to submit their views on this important issue.

Finally, the EPA requests comment on the appropriateness of the 0.60

ppm level for 5-minute NAAQS and the section 303 program, and whether a

numerical value below or above 0.60 ppm would be more appropriate to

protect asthmatic individuals.

D. Averaging Convention for the Standards

The averaging convention specifies the interpretation of standards

for a particular averaging time (in this case, 3-hour, 24-hour, annual)

with respect to when (time and day) the averaging period(s) begins and

ends. The two major alternative averaging conventions are known as

``block'' and ``running.'' Under the block convention, periods such as

24 hours and 3 hours are measured sequentially and do not overlap; when

one averaging period ends, the next begins. For example, one 24-hour

measurement would be taken from midnight on day one to midnight on day

two; the next would begin at midnight on day two. Under the running

convention, measurements are allowed to overlap. Thus, if one 24-hour

period were measured from midnight to midnight, the next might be

measured from 1 a.m. to 1 a.m. or from 12:01 a.m. to 12:01 a.m. Given a

fixed standard level, running averages would produce a somewhat more

restrictive standard (Faoro, 1983; Possiel, 1985).

Although the wording of the original 24-hour, 3-hour, and annual

SO2 standards was ambiguous on the matter, the earliest actions of

the EPA signify that the block averaging convention was intended for

these standards (OAQPS, l986), and block averages have generally been

used in implementing the standards.\7\ The use of running averages

would therefore represent a tightening of the standards. Because the

Administrator has determined, for the reasons explained in this notice

and in the April 21, 1993 notice on the secondary NAAQS (58 FR 21351),

that protection of the public health and welfare does not require

tightening the existing standards, the Administrator proposes to retain

the block averaging convention for the 24-hour, 3-hour, and annual

standards. To eliminate any future questions on this aspect of the

standards, clarifying language is being proposed in the regulation (40

CFR 50.4 and 50.5).

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\7\Although EPA generally does not specify use of a running

average in evaluating SO2 SIP's for attainment and maintenance

of the NAAQS, running averages have been used in a limited number of

instances. In the enforcement context, in cases where supplementary

control systems (SCS) were used as an interim measure to protect the

NAAQS at primary copper smelters, consent decrees for such

facilities specified running average requirements see, e.g., U.S. v.

Phelps Dodge Corp. Civil No. 81-088-TUC-MAR (D. Ariz. filed October

20, 1986)).

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E. Form of the Current Standards

In revising the standards for ozone and particulate matter, the EPA

concluded that it would be appropriate to make technical improvements

to the form in which the standards were expressed (44 FR 8202, Feb. 8,

1979; 52 FR 24653, July 1, 1987). These improvements were embodied in a

revised statistical form for the standards, which was intended to

maintain desired health protection while improving ease of

implementation. The decisions on the statistical form were made in

conjunction with decisions on the level of the standard. The EPA has

also considered the alternative of expressing the SO2 standards in

a similar statistical form, with one expected exceedance per year for

the 24-hour and 3-hour standards and expressing the annual standard as

an expected annual mean. The EPA examined the relative protection

afforded by the current standards if they were expressed in statistical

form (EPA, 1984a; Frank, 1987). These analyses found that the standards

expressed in a statistical form would afford reduced protection against

the 24-hour, annual, and 3-hour health and welfare effects associated

with these averaging periods and, in addition, would significantly

reduce the degree of protection the existing set of standards provides

against 5-minute peak SO2 exposures. Thus, adopting a statistical

form would necessitate revisions to the levels of the existing 24-hour,

3-hour, and annual standards to maintain the requisite level of

protection needed. In the judgment of the Administrator, the limited

technical advantages of adopting a statistical form for these standards

are not sufficient to warrant the administrative burden associated with

such a change.

In advancing the new alternatives of a 5-minute NAAQS and a section

303 program for public comment, however, the Administrator believes it

is appropriate to propose that they take a statistical form as

recommended by the staff. In reaching a judgment that a new 5-minute

NAAQS of 0.60 ppm SO2 or a new section 303 trigger level of 0.60

ppm SO2 may be needed to provide additional public health

protection, the Administrator was cognizant of and took into account

that these measures would be expressed in the statistical form when

determining the level to be proposed for each alternative. The EPA is,

however, requesting comment on whether more than one expected

exceedance should be allowed as suggested by the staff (EPA 1994b, pp.

60-62). In seeking comment on this question, the EPA is concerned that

a single upset or malfunction during a day could cause multiple

exceedances of the proposed 5-minute standard level or the alternative

section 303 trigger level despite a source operator's good faith and

willingness to take prompt and effective abatement action.

F. Other Technical Changes

The EPA is proposing to make some minor technical changes in the

part 50 regulations concerning the SO2 standards (Frank, 1988).

First, the levels for the primary and secondary NAAQS would be restated

in ppm rather than g/m\3\ (40 CFR 50.4 and 50.5). This would

be done to make the SO2 NAAQS consistent with other pollutants and

to improve understanding by the public. The levels would be restated as

follows: (a) The level of the annual standard is 0.030 parts per

million (ppm) (approximately 80 g/m\3\), (b) the level of the

24-hour standard is 0.14 ppm (approximately 365 g/m\3\), and

(c) the level of the 3-hour standard is 0.5 ppm (approximately 1300

g/m\3\). Secondly, explicit rounding conventions would be

added (40 CFR 50.4 and 50.5). This would aid State and local air

pollution control agencies in interpreting the standard. Finally, data

completeness and handling conventions would be specified (40 CFR 50.4

and 50.5). These conventions would be consistent with the definitions

used with ozone and would ensure that omission or deletion of some

hourly or 5-minute data will not negate obvious exceedances (see 40 CFR

part 50, appendix H for the equivalent ozone language).

VI. Federal Reference Methods and Equivalent Methods

The Federal Reference Method for measuring ambient concentrations

of SO2 set forth in appendix A of part 50 is not capable of

providing 5-minute average concentration measurements. Even if it

could, such a manual method would not be practical for 5-minute

measurements because of the large number of individual samples that

would have to be obtained and analyzed. Clearly, an automated,

continuous monitoring method (equivalent method) is required for 5-

minute monitoring. This requirement is innocuous, however, since the

reference method is now rarely used for routine field monitoring, even

for 3-hour or 24-hour measurements, having already been replaced with

use of continuous, instrumental equivalent methods. Thus, no revisions

are proposed to the reference method.

Although most of these instrumental equivalent methods provide

nominally continuous SO2 concentration measurements, these

measurements are almost universally reduced to standardized hourly

averages (block averages, by convention, as opposed to running or

overlapping averages) for purposes of recording, validation, storage,

interpretation, and use. (Longer-term averages are computed from the

hourly averages.) Accordingly, the performance of the instruments is

usually optimized by the manufacturer toward production of hourly

averages. Specifically, the response of the analyzers may be

intentionally slowed to provide concentration measurements that change

more slowly than the actual input concentration. This ``smoothing''

filters random fluctuations (noise), provides more stable readings for

instrument operators, aids calibration accuracy, and facilitates more

accurate integration of the readings into hourly averages.

When such instruments are used to obtain 5-minute average

concentration measurements, however, the slowed response often causes

the measurements to underestimate the actual peak concentration of

short-duration concentration peaks (Eaton et al., 1991; Eaton et al.,

1993). The degree of error is estimated to be from a few percent to as

much as 20 or 25 percent, depending on the response time of the

instrument and the sharpness (height to duration ratio) of the

concentration peak. (The smoothed measurements correspondingly

overestimate the duration of the peak such that the peak is correctly

integrated for longer averaging periods such as 1 hour.)

Fortunately, more accurate 5-minute average concentration

measurements can be obtained from most of the equivalent method

analyzers available currently by relatively minor modifications to

increase their response times. These modifications may include minor

electronic adjustments, substitution of modified circuit cards or

software programs, or increased flow rates, and the modifications could

also likely be made available for existing analyzers through either

user or manufacturer retrofitting. Prior to promulgation of one of the

regulatory alternatives, SO2 analyzer manufacturers would be

informed of the new requirements for faster response time for both new

and existing analyzers as may be appropriate.

Based on this assessment, the EPA is proposing to establish

special, supplemental performance specifications that would be

applicable to equivalent method analyzers used for 5-minute SO2

monitoring. These new performance specifications would be added to 40

CFR part 53, which sets forth the provisions under which the EPA

designates reference and equivalent methods for air monitoring to

determine attainment of the NAAQS. Part 53 gives the quantitative

performance specifications and other requirements that a candidate

method must meet to be designated as a reference or equivalent method,

as well as the detailed test procedures by which the various

performance parameters are to be measured.

Capability for accurate 5-minute monitoring requires more stringent

specifications for certain performance parameters than are required for

1-hour average measurements. The primary performance specifications

that must be changed are those having to do with the response time of

the analyzer. These are the ``rise time'' and ``fall time''

specifications of part 53, which describe the time required for the

output measurement or signal of the analyzer to respond to increases or

decreases, respectively, in the input concentration. More specifically,

these times are defined as the time required for the instrument

measurement to reach 95 percent of the final, stable reading after a

step increase or decrease (respectively) in the input concentration.

For 1-hour average SO2 measurements, analyzer response can be

relatively slow; the specifications in part 53 for rise and fall time

are both 15 minutes. Typical rise and fall times of several widely used

designated SO2 equivalent method analyzers are between 2 and 5

minutes.

However, as noted previously, such an analyzer may underestimate

the actual 5-minute average concentration of a short-term concentration

peak by as much as 20 or 25 percent, depending on the response time of

the instrument and the nature (shape) of the concentration peak. To

provide more accurate 5-minute measurements, the maximum rise and fall

time specifications must be reduced to 2 minutes or less. Accordingly,

part 53 is proposed to be amended by adding supplemental maximum rise

and fall time specifications of 2 minutes to be applicable to

designated equivalent methods for SO2 that would be used for 5-

minute monitoring.

Another performance parameter that is associated with rise and fall

time (and sometimes included in the generic term ``response time'') is

``lag time,'' which describes the time between the presentation of a

step change in the input concentration and the first indication of the

change in the measurement readings. Although the lag time represents a

delay in the presentation of concentration measurement readings by the

analyzer, technically it does not affect the ultimate accuracy or

precision of 5-minute measurements relative to the accuracy or

precision of 1-hour measurements. Therefore, no supplemental lag time

specification is needed for 5-minute monitoring.

The only other performance specification that is of special concern

for 5-minute monitoring is the measurement range of the analyzer.

Measurements of 5-minute SO2 concentrations in source-targeted

areas where high short-term concentrations may occur would likely

require a higher measurement range than for monitoring in other areas.

It is expected that a 1.0 ppm measurement range would be adequate for

most 5-minute monitoring sites. However, accurate measurements require

that the measured concentration not exceed the measurement range during

any portion of the 5-minute averaging period. Therefore, measurement

ranges higher than 1.0 ppm may be needed at some monitoring sites.

Part 53 specifies a base measurement range of 0.5 ppm and permits

alternative ranges up to 1.0 ppm. All designated equivalent methods for

SO2 in wide use today have 1.0 ppm measurement ranges that are

approved for use under their equivalent method designations. Further,

if a higher range is needed at a particular monitoring site, provisions

in 40 CFR part 58, appendix C, section 2.6 allow individual approval of

ranges higher than 1.0 ppm at sites where such a higher range is

justified. Accordingly, only a minor change is proposed to part 53--to

require a 1.0 ppm range for equivalent methods for SO2 that would

be used for 5-minute monitoring.

The currently existing rise and fall time and range specifications

in 40 CFR part 53 (for 1-hour average measurements) are not proposed to

be changed. Hence, there would be no change in the base requirements in

40 CFR part 53 for designation of equivalent methods for SO2. The

new, supplemental rise and fall time and range specifications being

proposed would be applicable only to designated equivalent methods used

for 5-minute monitoring and would create a subset of SO2

equivalent methods that would be additionally approved for 5-minute

monitoring. Methods that meet all of the existing performance

specifications but not the supplemental specifications for rise and

fall time and range would be acceptable for all NAAQS monitoring other

than 5-minute monitoring. This situation would be similar to that for

other performance parameters where, for example, some designated

equivalent methods are approved for use on multiple measurement ranges

or over a wider operating temperature range than the minimum range

specified. In all such cases, the additional performance

qualifications, over the minimum requirements of 40 CFR part 53, are

clearly identified and indicated in the equivalent method description.

This description appears in both the notice of designation published in

the Federal Register and in the List of Reference and Equivalent

Methods maintained in accordance with Sec. 53.8(c) and distributed to

the EPA Regional Offices and to others upon request.

Manufacturers of new SO2 analyzers may redesign their

analyzers to provide for additional ranges, faster response, or

capability for user-selection of these parameters. The test procedures

to show that an analyzer meets the new supplemental range and rise and

fall time specifications for 5-minute monitoring are the same range and

rise and fall time test procedures currently described in 40 CFR part

53. Test results from these tests would be submitted along with the

results from the other tests in an application for an equivalent method

determination under 40 CFR part 53. A manufacturer of an existing

analyzer that is currently designated as an equivalent method for

SO2 but does not meet the new supplemental specifications for

range and rise and fall time would be encouraged to develop

modifications to the analyzer that would allow it to meet the new

specifications. The manufacturer should then carry out appropriate

tests to demonstrate that the modified analyzer meets the new

specifications and apply for approval of the modifications under

Sec. 53.14 (modification of a reference or equivalent method).

Manufacturers should note, however, that tests other than the range and

rise and fall time tests may have to be carried out, since increasing

the range or response time could have a possible adverse effect on

other performance parameters, such as noise and lower detectable limit.

Ideally, such analyzer modifications should be made available to users

in the form of a retrofit kit for user installation, if possible.

Alternatively, the analyzer may have to be returned to the factory for

the modifications to meet the new 5-minute monitoring specifications.

No other changes to 40 CFR part 53 are deemed necessary to support

the 5-minute monitoring requirement.

VII. Regulatory Impacts

A. Regulatory Impacts Administrative Requirements

Under Executive Order 12866 (58 FR 51713, Oct. 4, 1993), the EPA

must determine whether a regulatory action is ``significant'' and

therefore subject to OMB review and the requirements of the Executive

Order. The Order defines a ``significant regulatory action'' as one

that is likely to result in a rule that may:

(1) Have an annual effect on the economy of $100 million or more or

adversely affect in a material way the economy, a sector of the

economy, productivity, competition, jobs, the environment, public

health or safety, or State, local, or tribal governments or

communities;

(2) Create a serious inconsistency or otherwise interfere with an

action taken or planned by another Agency;

(3) Materially alter the budgetary impact of entitlement, grants,

user fees, or loan programs or the rights and obligations of recipients

thereof; or

(4) Raise novel legal or policy issues arising out of legal

mandates, the President's priorities, or the principles set forth in

the Executive Order.

Pursuant to the terms of Executive Order 12866, it has been

determined that this notice is a significant regulatory action because

of its potential to have an annual effect on the economy of $100

million or more. As such, this action was submitted to OMB for review.

Changes made in response to OMB suggestions or recommendations will be

documented in the public record.

Summary of Regulatory Impacts

The EPA has prepared and entered into the docket a draft regulatory

impact analysis (RIA) entitled ``Regulatory Impact Analysis for the

Proposed Regulatory Options to Address Short-Term Peak Sulfur Dioxide

Exposures (June 1994).'' This draft RIA includes estimates of costs,

economic impacts, and net benefits associated with implementation of

the regulatory alternatives discussed above. The proposed regulatory

action is intended to be implemented through a risk-based, targeted

monitoring strategy given the localized nature of the short-term

SO2 problem. Absent specific information on which sources would be

impacted under this implementation strategy, modeling is used to

identify SO2 sources likely to cause exceedances of either the

0.60 ppm SO2, 1 or 5 expected exceedance forms of the standard.

Although there are large uncertainties associated with the modeling

analysis, such analyses are currently the only available tools for

predicting sources of short-term SO2 peaks and estimating

associated control costs for reducing peak, ambient concentrations.

Given the modeling uncertainties, as well as that the modeling analyses

are not reflective of the specific sources to be targeted by States

under a risk-based, targeted implementation strategy, the following

estimated impacts should be viewed with caution.

Short-term SO2 NAAQS Regulatory Alternative

The cost estimates for the short-term SO2 NAAQS regulatory

alternative represent a snapshot of the estimated total industry costs

that could be incurred at some unspecified time in the future following

full implementation of a short-term SO2 NAAQS. The costs are based

on the use of add-on control devices and fuel switching to lower-sulfur

fuels. Given that EPA believes that many sources will be able to reduce

their peaks through other, nontechnological means, this assumption may

result in overstating costs. With this caveat in mind, nonutility

annualized costs are estimated to be approximately $250 million for an

ambient SO2 concentration level of 0.60 ppm, 1 expected

exceedance. Annualized costs for a 0.60 ppm, 5 annual exceedance

concentration level are estimated to be approximately $160 million. It

is estimated that SO2 will be reduced by approximately 910

thousand tons, and 560 thousand tons for the 1 and 5 exceedance cases,

respectively. Incremental to the title IV requirements and attainment

of the existing SO2 NAAQS, total utility annualized costs in 2005

are estimated to be an additional $1.5 billion for the 0.60 ppm, 1

expected exceedance case, and $400 million for the 5 expected

exceedance case. Estimated total utility SO2 emissions in 2005 are

not expected to change given the title IV emissions trading program.

Administrative costs are estimated to be approximately $18 million

for the short-term NAAQS regulatory alternative. Monitoring costs are

estimated to be minimal.

Section 303 Regulatory Alternative

The section 303 regulatory alternative may provide for lower

control costs at the national level relative to the cost estimates for

the short-term SO2 NAAQS. First, under the section 303 program,

sources would be allowed to use intermittent controls and other

practices normally barred by section 123 of the Act (e.g., supplemental

control systems, stack height in excess of GEP) to prevent exceedances

of a 5-minute trigger level. These types of controls are generally less

costly to employ relative to add-on controls. Secondly, given the

timetables in the Act regarding SIP development and attainment of the

NAAQS, it is probable that emission reductions from a section 303

program could be achieved in a more timely fashion. While some SIP

revisions would be necessary for States to implement the section 303

program, more time-consuming aspects of the SIP process such as

designations could be avoided. There is a greater likelihood that the

section 303 program could bring about more prompt and effective

remediation of high 5-minute SO2 concentration relative to the

short-term NAAQS alternative. In respect to total annual emission

reductions, it is likely that the section 303 program would achieve

less emission reductions than a short-term NAAQS program.

Administrative costs are expected to be minimal as some resource-

intensive components of the SIP process could be bypassed under a

section 303 program. Likewise, monitoring costs are estimated to be

minimal.

Analysis of Potential Benefits

A quantitative analysis of the benefits of reducing short-term

SO2 peaks through implementation of the regulatory options under

consideration in this RIA is not possible at this time. Results of a

staff paper exposure analysis conducted on a subset of SO2 sources

potentially affected by this rulemaking indicate that as many as

180,000-395,000 exposure events above 0.5 ppm SO2 may occur among

68,000-166,000 exercising asthmatics nationally every year. Moreover,

this analysis shows that there is a clustering of risk of exposure

around a subset of those SO2 sources analyzed. It is expected that

reductions in short-term SO2 peaks resulting from this rulemaking

could reduce potential risks of adverse respiratory effects (e.g.,

bronchoconstriction, wheezing, chest tightness, shortness of breath)

among exercising asthmatic individuals that are potentially exposed to

these high 5-minute SO2 ambient concentrations. Additionally,

reductions in adverse welfare effects due to SO2 such as

improvements in visual air quality and reductions in ecosystem impacts,

odors, and materials damage, and reductions in adverse health and

welfare effects due to particulate matter may be achieved as a result

of implementing the regulatory alternatives considered in this document

today.

A final RIA will be issued at the time of promulgation of final

standards. This draft RIA has not been considered in issuing this

proposal. In accordance with Executive Order 12866, this proposed rule

was submitted to OMB for review. Written comments from OMB and the EPA

written responses to these comments are available for public inspection

at the EPA's Central Docket Section (Docket No. A-84-25), South

Conference Center, Room 4, Waterside Mall, 401 M Street, SW.,

Washington, DC.

B. Impact on Small Entities

Pursuant to the EPA guidelines issued in response to the Regulatory

Flexibility Act, 5 U.S.C., 600 et seq., a regulatory flexibility

analysis has been prepared and is discussed in the draft RIA cited

above. The analysis examined industry-wide cost and economic impacts

for nonutility and utility sources of SO2 emissions likely to be

impacted by the regulatory alternatives discussed in this notice. The

EPA also analyzed various industries for the existence of small

entities. Given data limitations and because the regulatory

alternatives would be implemented through a risk-based targeted

strategy described in the Federal Register document on implementation

issues, it was not feasible to quantitatively ascertain whether small

entities within a given industry category would be differentially

impacted when compared to the industry category as a whole.

C. Reduction of Governmental Burden

Executive Order 12875 (``Enhancing the Intergovernmental

Partnership'') is designed to reduce the burden to State, local, and

tribal governments of the cumulative effect of unfunded Federal

mandates, and recognizes the need for these entities to be free from

unnecessary Federal regulation to enhance their ability to address

problems they face and provides for Federal agencies to grant waivers

to these entities from discretionary Federal requirements. In

accordance with the purposes of Executive Order 12875, the EPA will

consult with representatives of State, local, and tribal governments to

inform them of the requirements for implementing the alternative

regulatory measures being proposed to address short-term peak SO2

exposures. The EPA will summarize the concerns of the governmental

entities and respond to their comments prior to taking final action.

D. Environmental Justice

Executive Order 12898 requires that each Federal Agency shall make

achieving environmental justice part of its mission by identifying and

addressing, as appropriate, disproportionately high and adverse human

health or environmental effects of its programs, policies, and

activities on minority and low-income populations. The requirements of

Executive Order 12898 have been addressed in the draft RIA cited above.

On average, approximately 25 percent of the total population and 14

percent of total households residing in geographic areas that are

potentially impacted by short-term SO2 peaks of 0.60 ppm or

greater are nonwhite and below the poverty level, respectively. These

estimates exceed the national averages of 19.7 percent and 12.7

percent, respectively. It also follows that, on average, 25 percent of

the asthmatics potentially exposed to short-term SO2 peaks of 0.60

ppm or greater are nonwhite. Upon closer examination, 44 percent of

these potentially SO2-impacted areas have a nonwhite population

greater than the national average with 24 percent between 1 and 2 times

greater, 10 percent between 2 and 3 times greater, 7 percent between 3

and 4 times greater, and 3 percent between 4 and 5 times greater.

E. Impact on Reporting Requirements

Air quality monitoring activities that would occur as a result of

this proposed rule would increase the costs and man-hour burdens to

State and local agencies for conducting ambient SO2 surveillance

required by 40 CFR part 58 and currently approved under OMB Control

Number 2060-0084. Increased costs would result from the relocation of

some monitors currently operated as part of the State and Local Air

Monitoring Stations (SLAMS) networks and from the purchase and

operation of additional monitors in a small number of agencies (see the

related document to be published shortly in the Federal Register

revising 40 CFR parts 51 and 58 for information on compliance with

Paperwork Reduction Act requirements).

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Linn, W.S.; Avol, E.L.; Peng, R.-C.; Shamoo, D.A.; Hackney, J.D.

(1987), Replicated dose-response study of sulfur dioxide effects in

normal, atopic, and asthmatic volunteers, Am. Rev. Respir. Dis. 136:

1127-1134.

Lippmann, M. (1987), Letter from Morton Lippmann, CASAC Chairman, to

EPA Administrator Lee M. Thomas, February 19, 1987, Docket No. 1 A-

79-28, II-D-83.

National Institutes of Health (1991), Guidelines for the diagnosis

and management of asthma, Bethesda, MD: U.S. Department of Health

and Human Services, National Heart, Lung, and Blood Institute,

National Asthma Education Program; publication no. 91-3042.

OAQPS [Office of Air Quality Planning and Standards] (1986), Proper

Interpretation of the Averaging Convention for the National Ambient

Air Quality Standards for Sulfur Oxides, OAQPS Staff Position Paper,

March 1986, Docket No. 1 A-79-28, II-A-15.

Padgett, J., U.S. EPA, Strategies and Air Standards Division (1982),

Letter to Dr. Sheldon Friedlander, Chairman, Clean Air Scientific

Advisory Committee, August 5, 1982, Docket No. 1 A-79-28, II-C-3.

Possiel, N.C., U.S. EPA, Model Applications Section (1985), Analysis

of Running Versus Block SO2 Model Estimates, Memorandum to

Henry C. Thomas, Standards Development Section, August 27, 1985,

Docket No. 1 A-79-28, II-B-12.

Rubinstein, I.; Bigby, B.G.; Reiss, T.F.; Boushey, H.A., Jr. (1990),

Short-term exposure to 0.3 ppm nitrogen dioxide does not potentiate

airway responsiveness to sulfur dioxide in asthmatic subjects, Am.

Rev. Respir. Dis. 141: 381-385.

Schachter, E.N. (1994), Letter to Randall C. Bond, U.S. EPA from E.

Neil Schachter, M.D., CASAC Member, re: comments on SO2 NAAQS

Staff Paper Addendum, May 2, 1994, Docket No. A-84-25, V-D-30.

Schachter, E.N.; Witek, T.J., Jr.; Beck, G.J.; Hosein, H.R.; Colice,

G.; Leaderer, B.P.; Cain, W. (1984) Airway effects of low

concentrations of sulfur dioxide: dose-response characteristics.

Arch. Environ. Health 39: 34-42.

Schwartz, J.; Gold, D.; Dockery, D.W.; Weiss, S.T.; Speizer, F.E.

(1990), Predictors of asthma and persistent wheeze in a national

sample of children in the United States: association with social

class, perinatal events, and race, Am. Rev. Respir. Dis. 142: 555-

562.

White, R.H. (1994), Letter to John Haines, U.S. EPA from Ronald H.

White, American Lung Association, re: comments on SO2 NAAQS

Staff Paper Addendum Supplement, July 1, 1994, Docket No. A-84-25,

V-D-37.

Appendix I to the Preamble

February 19, 1987.

The Honorable Lee M. Thomas,

Administrator, U.S. Environmental Protection Agency, Washington, DC

20460.

Dear Mr. Thomas: The Clean Air Scientific Advisory Committee

(CASAC) has completed its review of the 1986 Addendum to the 1982

Staff Paper on Sulfur Oxides (Review of the National Ambient Air

Quality Standards for Sulfur Oxides: Updated Assessment of

Scientific and Technical Information) prepared by the Agency's

Office of Air Quality Planning and Standards (OAQPS).

The Committee unanimously concludes that this document is

consistent in all significant respects with the scientific evidence

presented and interpreted in the combined Air Quality Criteria

Document for Particulate Matter/Sulfur Oxides (1982) and its 1986

Addendum, on which CASAC issued its closure letter on December 15,

1986. The Committee believes that the 1986 Addendum to the 1982

Staff Paper on Sulfur Oxides provides you with the kind and amount

of technical guidance that will be needed to make appropriate

decisions with respect to the standards. The Committee's major

findings and conclusions concerning the various scientific issues

and studies discussed in the Staff Paper Addendum are contained in

the attached report.

Thank you for the opportunity to present the Committee's views

on this important public health and welfare issue.

Sincerely,

Morton Lippmann, Ph.D.,

Chairman, Clean Air Scientific Advisory Committee.

cc: A. James Barnes

Gerald Emison

Lester Grant

Vaun Newill

John O'Connor

Craig Potter

Terry Yosie

Summary of Major Scientific Issues and CASAC Conclusions on the 1986

Draft Addendum to the 1982 Sulfur Oxides Staff Paper

The Committee found the technical discussions contained in the

Staff Paper Addendum to be scientifically thorough and acceptable,

subject to minor editorial revisions. This document is consistent in

all significant respects with the scientific evidence presented in the

1982 combined Air Quality Criteria Document for Particulate Matter/

Sulfur Oxides and its 1986 Addendum, on which the Committee issued its

closure letter on December 15, 1986.

Scientific Basis for Primary Standards

The Committee addressed the scientific basis for a 1-hour, 24-hour,

and annual primary standards at some length in its August 26, 1983

closure letter on the 1982 Sulfur Oxides Staff Paper. That letter was

based on the scientific literature which had been published up to 1982.

The present review has examined the more recently published studies.

It is clear that no single study of SO2 can fully address the

range of public health issues that arise during the standard setting

process. The Agency has completed a thorough analysis of the strengths

and weaknesses of various studies and has derived its recommended

ranges of interest by evaluating the weight of the evidence. The

Committee endorses this approach.

The Committee wishes to comment on several major issues concerning

the scientific data that are available. These issues include:

Recent studies more clearly implicate particulate matter

than SO2 as a longer-term public health concern at low exposure

levels.

A majority of Committee members believe that the effects

reported in the clinical studies of asthmatics represent effects of

significant public health concern.

The exposure uncertainties associated with a 1-hour

standard are quite large. The relationship between the frequency of

short-term peak exposures and various scenarios of asthmatic responses

is not well understood. Both EPA and the electric power industry are

conducting further analyses of a series of exposure assessment issues.

Such analyses have the potential to increase the collective

understanding of the relationship between SO2 exposures and

responses observed in subgroups of the general population.

The number of asthmatics vulnerable to peak exposures near

electric power plants, given the protection afforded by the current

standards, represents a small number of people. Although the Clean Air

Act requires that sensitive population groups receive protection, the

size of such groups has not been defined. CASAC believes that this

issue represents a legal/policy matter and has no specific scientific

advice to provide on it.

CASAC's advice on primary standards for three averaging times is

presented below:

1-Hour Standard--It is our conclusion that a large, consistent data

base exists to document the bronchoconstrictive response in mild to

moderate asthmatics subjected in clinical chambers to short- term, low

levels of sulfur dioxide while exercising. There is, however, no

scientific basis at present to support or dispute the hypothesis that

individuals participating in the SO2 clinical studies are

surrogates for more sensitive asthmatics. Estimates of the size of the

asthmatic population that experience exposures to short-term peaks of

SO2 (0.2-0.5 parts per million (ppm) SO2 for 5-10 minutes)

during light to moderate exercise, and that can be expected to exhibit

a bronchoconstrictive response, varies from 5,000 to 50,000.

The majority of the Committee believes that the scientific evidence

supporting the establishment of a new 1-hour standard is stronger than

it was in 1983. As a result, and in view of the significance of the

effects reported in these clinical studies, there is strong, but not

unanimous support for the recommendation that the Administrator

consider establishing a new 1-hour standard for SO2 exposures. The

Committee agrees that the range suggested by EPA staff (0.2-0.5 ppm) is

appropriate, with several members of the Committee suggesting a

standard from the middle of this range. The Committee concludes that

there is not a scientifically demonstrated need for a wide margin of

safety for a 1-hour standard.

24-Hour Standard--The more recent studies presented and analyzed in

the 1986 Staff Paper Addendum, in particular, the episodic lung

function studies in children (Dockery et al., and Dassen et al.) serve

to strengthen our previous conclusion that the rationale for

reaffirming the 24-hour standard is appropriate.

Annual Standard--The Committee reaffirms its conclusion, voiced in

its 1983 closure letter, that there is no quantitative basis for

retaining the current annual standard. However, a decision to abolish

the annual standard must be considered in the light of the total

protection that is to be offered by the suite of standards that will be

established.

The above recommendations reflect the consensus position of CASAC.

Not all CASAC reviewers agree with each position adopted because of the

uncertainties associated with the existing scientific data. However, a

strong majority supports each of the specific recommendations presented

above, and the entire Committee agrees that this letter represents the

consensus position.

Secondary Standards

The 3-hour secondary standard was not addressed at this review.

APPENDIX II to the Preamble

June 1, 1994.

Honorable Carol M. Browner,

Administrator, U.S. Environmental Protection Agency, 401 M St.,

S.W., Washington, D.C. 20460.

Subject: Clean Air Scientific Advisory Committee Closure on the

Supplements to Criteria Document and Staff Position Papers for

SO2

Dear Ms. Browner: The Clean Air Scientific Advisory Committee

(CASAC) at a meeting on April 12, 1994, completed its review of the

documents: Supplement to the Second Addendum (1986) to Air Quality

Criteria for Particulate Matter and Sulfur Oxides; Assessment of New

Findings on Sulfur Dioxide and Acute Exposure Health Effects in

Asthmatics; and Review of the National Ambient Air Quality Standards

for Sulfur Oxides: Updated Assessment of Scientific and Technical

Information, Supplement to the 1986 OAQPS Staff Paper Addendum. The

Committee notes, with satisfaction, the improvements made in the

scientific quality and completeness of the documents.

With the changes recommended at our March 12 session, written

comments submitted to the Agency subsequent to the meeting, and the

major points provided below, the documents are consistent with the

scientific evidence available for sulfur dioxide. They have been

organized in a logical fashion and should provide an adequate basis

for a regulatory decision. Nevertheless, there are four major points

which should be called to your attention while reviewing these

materials:

1. A wide spectrum of views exists among the asthma specialists

regarding the clinical and public health significance of the effects

of 5 to 10 minute concentrations of sulfur dioxide on asthmatics

engaged in exercise. On one end of the spectrum is the view that

spirometric test responses can be observed following such short-term

exposures and they are a surrogate for significant health effects.

Also, there is some concern that the effects are underestimated

because moderate asthmatics, not severe asthmatics, were used in the

clinical tests.

At the other end of the spectrum, the significance of the

spirometric test results are questioned because the response is

similar to that evoked by other commonly encountered, non-specific

stimuli such as exercise alone, cold, dry air inhalation, vigorous

coughing, psychological stress, or even fatigue. Typically, the

bronchoconstriction reverses itself within one or two hours, is not

accompanied by a late-phase response (often more severe and

potentially dangerous than the immediate response), and shows no

evidence of cumulative or long-term effects. Instead, it is

characterized by a short-term period of bronchoconstriction, and can

be prevented or ameliorated by beta-agonist aerosol inhalation.

2. It was the consensus of CASAC that the exposure scenario of

concern is a rare event. The sensitive population in this case is an

unmedicated asthmatic engaged in moderate exercise who happens to be

near one of the several hundred sulfur dioxide sources that have the

potential to produce high ground-level sulfur dioxide concentrations

over a small geographical area under rare adverse meteorological

conditions. In addition, CASAC pointed out that sulfur dioxide

emissions have been significantly reduced since EPA conducted its

exposure analysis and emissions will be further reduced as the 1990

Clean Air Act Amendments are implemented. Consequently, such

exposures will become even rarer in the future.

3. It was the consensus of CASAC that any regulatory strategy to

ameliorate such exposures be risk-based--targeted on the most likely

sources of short-term sulfur dioxide spikes rather than imposing

short-term standards on all sources. All of the nine CASAC Panel

members recommended that Option 1, the establishment of a new 5-

minutes standard, not be adopted. Reasons cited for this

recommendation included: the clinical experiences of many ozone

experts which suggest that the effects are short-term, readily

reversible, and typical of response seen with other stimuli.

Further, the committee viewed such exposures as rare events which

will even become rarer as sulfur dioxide emissions are further

reduced as the 1990 amendments are implemented. In addition, the

committee pointed out that enforcement of a short-term NAAQS would

require substantial technical resources. Furthermore, the committee

did not think that such a standard would be enforceable (see below).

4. CASAC questioned the enforceability of a 5-minute NAAQS or

``target level.'' Although the Agency has not proposed an air

monitoring strategy, to ensure that such a standard or ``target

level'' would not be exceeded, we infer that potential sources would

have to be surrounded by concentric circles of monitors. The

operation and maintenance of such monitoring networks would be

extremely resource intensive. Furthermore, current instrumentation

used to routinely monitor sulfur dioxide does not respond quickly

enough to accurately characterize 5-minute spikes.

The Committee appreciates the opportunity to participate in this

review and looks forward to receiving notice of your decision on the

standard. Please do not hesitate to contact me if CASAC can be of

further assistance on this matter.

Sincerely,

George T. Wolff, Ph.D.,

Chair, Clean Air Scientific Advisory Committee.

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, Carbon monoxide, Lead, Nitrogen dioxide, Ozone,

Particulate matter, Reporting and recordkeeping requirements.

Dated: November 1, 1994.

Carol M. Browner,

Administrator.

For the reasons set forth in the preamble, chapter I of title 40 of

the Code of Federal Regulations is proposed to be amended as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY

STANDARDS

1. The authority citation for part 50 continues to read as follows:

Authority: Secs. 109 and 301(a), Clean Air Act, as amended (42

U.S.C. 7409, 7601(a)).

2. Section 50.4 is revised to read as follows:

Sec. 50.4 National primary ambient air quality standards for sulfur

oxides (sulfur dioxide).

(a) The level of the annual standard is 0.030 parts per million

(ppm), not to be exceeded in a calendar year. The annual arithmetic

mean shall be rounded to three decimal places (fractional parts equal

to or greater than 0.0005 ppm must be rounded up).

(b) The level of the 24-hour standard is 0.14 parts per million

(ppm), not to be exceeded more than once per calendar year. The 24-hour

averages shall be determined from successive nonoverlapping 24-hour

blocks starting at midnight each calendar day and shall be rounded to

two decimal places (fractional parts equal to or greater than 0.005 ppm

must be rounded up).

(c) The level of the 5-minute standard is 0.60 parts per million

(ppm), not to be exceeded more than once per calendar year, as

determined in accordance with appendix I of this part.

(d) Sulfur oxides shall be measured in the ambient air as sulfur

dioxide by the reference method described in appendix A of this part or

by an equivalent method designated in accordance with part 53 of this

chapter.

(e) To demonstrate attainment, the annual arithmetic mean and the

second-highest 24-hour averages must be based upon hourly data that are

at least 75 percent complete in each calendar quarter. A 24-hour block

average shall be considered valid if at least 75 percent of the hourly

averages for the 24-hour period are available. In the event that only

18, 19, 20, 21, 22, or 23 hourly averages are available, the 24-hour

block average shall be computed as the sum of the available hourly

averages using 18, 19, etc. as the divisor. If less than 18 hourly

averages are available, but the 24-hour average would exceed the level

of the standard when zeros are substituted for the missing values,

subject to the rounding rule of paragraph (b) of this section, then

this shall be considered a valid 24-hour average. In this case, the 24-

hour block average shall be computed as the sum of the available hourly

averages divided by 24.

3. Section 50.5 is revised to read as follows:

Sec. 50.5 National secondary ambient air quality standard for sulfur

oxides (sulfur dioxide).

(a) The level of the 3-hour standard is 0.5 parts per million

(ppm), not to be exceeded more than once per calendar year. The 3-hour

averages shall be determined from successive nonoverlapping 3-hour

blocks starting at midnight each calendar day and shall be rounded to 1

decimal place (fractional parts equal to or greater than 0.05 ppm must

be rounded up).

(b) Sulfur oxides shall be measured in the ambient air as sulfur

dioxide by the reference method described in appendix A of this part or

by an equivalent method designated in accordance with Part 53 of this

chapter.

(c) To demonstrate attainment, the second-highest 3-hour average

must be based upon hourly data that are at least 75 percent complete in

each calendar quarter. A 3-hour block average shall be considered valid

only if all three hourly averages for the 3-hour period are available.

If only one or two hourly averages are available, but the 3-hour

average would exceed the level of the standard when zeros are

substituted for the missing values, subject to the rounding rule of

paragraph (a) of this section, then this shall be considered a valid 3-

hour average. In all cases, the 3-hour block average shall be computed

as the sum of the hourly averages divided by 3.

4. Appendix I is added to part 50 to read as follows:

Appendix I to Part 50--Interpretation of the 5-Minute National Ambient

Air Quality Standard for Sulfur Dioxide

1.0 General.

1.1 This appendix explains the computations necessary for

analyzing sulfur dioxide data to determine attainment of the 5-

minute standard specified in 40 CFR 50.4. Sulfur dioxide is measured

in the ambient air by the reference method specified in Appendix A

of this part or an equivalent method designated in accordance with

part 53 of this chapter.

1.2 Several terms used in this appendix must be defined. A ``5-

minute hourly maximum'' for SO2 refers to the highest of the 12

possible nonoverlapping 5-minute SO2 averages calculated or

measured during a clock hour. The term ``exceedance'' of the 5-

minute standard means a 5-minute hourly maximum that is greater than

the level of the 5-minute standard after rounding to the nearest

hundredth ppm (i.e. values ending in or greater than 0.005 ppm are

rounded up; e.g., a value of 0.605 would be rounded to 0.61, which

is the smallest value for an exceedance). The term ``year'' refers

to a calendar year. The term ``quarter'' refers to a calendar

quarter. The 5-minute SO2 standard is expressed in terms of the

number of exceedances per year after adjusting for missing data (if

required) and after averaging over a two year period.

2.0 Attainment Determination.

2.1 Under 40 CFR 50.4(c) the 5-minute standard is attained when

the number of exceedances per year is less than or equal to one. In

general, this determination is to be made by recording the number of

5-minute hourly maximum exceedances at a monitoring site for each

year, using the calculations in section 3.2 to compensate for

missing data (if required), averaging the number of exceedances over

a two year period, and comparing the number of exceedances (rounded

to the nearest integer) to the number of allowable exceedances.

2.2 There are less stringent requirements for showing that a

monitor has failed an attainment test and thus has recorded a

violation of the sulfur dioxide standards. Although it is necessary

to meet the minimum data completeness requirements to use the

computational formula described in section 3.2, this criterion does

not apply when there are obvious nonattainment situations. For

example, when a site fails to meet the completeness criteria,

nonattainment of the 5-minute standard can still be established on

the basis of the observed number of exceedances in a year (e.g.

three observed exceedances in a single year).

3.0 Calculations for the 5-Minute Standard

3.1 Calculating a 5-Minute hourly maximum. A 5-minute hourly

maximum value for SO2 is the highest of the 5-minute averages

from the twelve possible nonoverlapping periods during a clock hour.

These 5-minute values shall be rounded to the nearest hundredth ppm

(fractional values equal to or greater than 0.005 ppm are rounded

up). A 5-minute maximum shall be considered valid if (1) 5-minute

averages were available for at least 9 of the twelve five-minute

periods during the clock hour or (2) the value of the 5-minute

average exceeds the level of the 5-minute standard.

3.2 Calculating estimated exceedances for a year.

3.2 Because of practical considerations, a 5-minute maximum

SO2 value may not be available for each hour of the year. To

account for the possible effect of incomplete data, an adjustment

must be made to the data collected at a particular monitoring

location to estimate the number of exceedances in a year. The

adjustment is made on a quarterly basis to ensure that the entire

year is adequately represented. In this adjustment, the assumption

is made that the fraction of missing values that would have exceeded

the standard level is identical to the fraction of measured values

above this level.

3.2.2 The computation for incomplete data is to be made for all

NAMS and SLAMS sites with 50 percent to 90 percent complete data in

each quarter. If a site has more than 90 percent complete data in a

quarter, no adjustment for missing data is required. If a site has

less than 50 percent complete data in a quarter, no adjust

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