[Federal Register Volume 59, Number 219 (Tuesday, November 15, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-27646]


[[Page Unknown]]

[Federal Register: November 15, 1994]


  
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Part III





Environmental Protection Agency





_______________________________________________________________________



40 CFR Parts 50 and 53




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

40 CFR Parts 50 and 53
[AD-FDL-5103-1]
RIN 2060-AA61

 
National Ambient Air Quality Standards for Sulfur Oxides (Sulfur 
Dioxide)--Reproposal

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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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 (<20 km) of major point 
sources. In the absence of adequate controls, maximum levels at such 
sites for 24-hour, 3-hour, and 1-hour averages can reach or exceed 0.4 
ppm, 1.4 ppm, and 2.3 ppm, respectively. The origins, relevant 
concentrations and potential effects of SO2 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 (<1-hour) SO2 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 <1 asthmatic 
patient-year. It is estimated that the incidence rate of 
hospitalization due to asthma for all asthmatic individuals in the 
United States is about 45 per 1,000 asthmatics per year (NIH, 1991). 
Death due to asthma is a rare event: about one per 10,000 asthmatic 
individuals per year. Mortality rates are higher among males and about 
100 percent higher among nonwhites (EPA, 1994a).
    In assessing the results from the controlled human exposure 
studies, it should be noted that the individuals who participate in 
such studies typically have mild allergic asthma and can go without 
medication altogether or can discontinue medication for brief periods 
of time if exposures are conducted outside their normal allergy season. 
In addition, African-American and Hispanic adolescents and young adults 
have not been studied systematically. Finally, subjects who participate 
in controlled exposure studies are also generally self-selected and 
this may introduce some bias. Thus, the extent to which the 
participants in the studies reflect the characteristics of the 
asthmatic population at large is not known. Nevertheless, the high 
degree of consistency among studies suggests that the subjects are 
generally representative of the population at risk or that any 
selection bias is consistently present across a diverse group of 
laboratories (EPA, 1994a).
    c. Short-Term Health Effects. The basis for considering whether 
additional regulatory measures are needed to reduce the occurrence of 
short-term peaks of SO2 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.
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    \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.
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    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.
---------------------------------------------------------------------------

    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\
---------------------------------------------------------------------------

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

    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).
---------------------------------------------------------------------------

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

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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Evans, R., III; Mullally, D.I.; Wilson, R.W.; Gergen, P.J.; 
Rosenberg, H.M.; Grauman, J.S.; Chevarley, F.M.; Feinleib, M. 
(1987), National trends in the morbidity and mortality of asthma in 
the US. Prevalence, hospitalization and death from asthma over two 
decades: 1965-1984, Chest 91(suppl.): 65S-74S.
Faoro, B., U.S. EPA, Data Analysis Section (1983), Comparison of 
Second Max Non-Overlapping and Midnight-to-Midnight (Block) Average, 
Memorandum to William F. Hunt Jr., Chief, Data Analysis Section, 
August 15, 1983, Docket No. 1 A-79-28, II-B-11.
Frank, N., U.S. EPA, Monitoring and Data Analyses Division (1987), 
Memorandum to John Bachmann, Strategies and Air Standards Division, 
February 18, 1987, Docket No. 1 A-84-25, II-B-1.
Frank, N. U.S. EPA, Technical Support Division (1988), Memorandum to 
John Haines, Air Quality Management Division, January 5, 1988, 
Docket No. 1 A-84-25, II-B-3.
Goldstein, B.G. (1983), CASAC Review and Closure of the OAQPS Staff 
Paper for Sulfur Oxides, Closure Letter to William D. Ruckelshaus, 
August 1983, Docket No. 1 A-792-28, II-B-3.
Hackney, J.D.; Linn, W.S.; Bailey, R.M.; Spier, C.E.; Valencia, L.M. 
(1984) Time course of exercise-induced bronchoconstriction in 
asthmatics exposed to sulfur dioxide. Environ. Res. 34: 321-327.
Higgins, I.T., University of Michigan, School of Public Health, 
(1983) Minority Statement, Letter to Terry F. Yosie, Executive 
Director, Clean Air Scientific Advisory Committee, October 6, 1983, 
Docket No. 1 A-79-28, II-D-51.
Horstman, D.H.; Seal, E., Jr.; Folinsbee, L.J.; Ives, P.; Roger, 
L.J. (1988), The relationship between exposure duration and sulfur 
dioxide-induced bronchoconstriction in asthmatic subjects, Am. Ind. 
Hyg. Assoc. J. 49: 38-47.
Jorres, R.; Magnussen, H. (1990), Airways response of asthmatics 
after a 30 min exposure, at resting ventilation, to 0.25 ppm 
NO2 or 0.5 ppm SO2. Eur. Respir. J. 3: 132-137.
Koenig, J.Q.; Covert, D.S.; Hanley, Q.S.; Van Belle, G.; Pierson, 
W.E. (1990), Prior exposure to ozone potentiates subsequent response 
to sulfur dioxide in adolescent asthmatic subjects, Am. Rev. Respir. 
Dis. 141: 377-380.
Linn, W.S.; Venet, T.G.; Shamoo, D.A.; Valencia, L.M.; Anzar, U.T.; 
Spier, C.E.; Hackney, J.D. (1983), Respiratory effects of sulfur 
dioxide in heavily exercising asthmatics: a dose-response study, Am. 
Rev. Respir. Dis. 127: 278-283.
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 adjustment for 
missing data is required and the observed exceedances are used. To 
demonstrate attainment, a site must have at least 75 percent 
complete data in each quarter.
    3.2.3  The estimate of the expected number of exceedances for 
the quarter is equal to the observed number of exceedances plus an 
increment associated with the missing data. The following formula 
must be used for these computations:

eq=vq+[(vq/
nq) x (Nq-nq]=vq x Nq/nq  [1]
where

eq=the estimated number of exceedances for quarter q,
vq=the observed number of exceedances for quarter q,
Nq=the number of hours in quarter q, and
nq=the number of hours in the quarter with valid 5-minute hourly 
SO2 maximums
q=the index for each quarter, q=1, 2, 3 or 4.

The estimated number of exceedances for the quarter must be rounded 
to the nearest hundredth (fractional values equal to or greater than 
0.005 are rounded up).
    3.2.4  The estimated number of exceedances for the year, e, is 
the sum of the estimates for each quarter.

TP15NO94.000

The estimated number of exceedances for a single year must be 
rounded to one decimal place (fractional values equal to or greater 
than 0.05 are rounded up).
    3.2.5  The number of exceedances is then estimated by averaging 
the individual annual estimates over a two year period, rounding to 
the nearest integer, and comparing with the allowable exceedance 
rate of one per year (fractional values equal to or greater than 0.5 
are rounded up; e.g., an estimated number of exceedances of 1.5 
would be rounded to 2, which is the lowest value for nonattainment).
    3.2.6  Example.
    i. During the most recent quarter, 1210 out of a possible 2208 
5-minute hourly maximums were recorded, with one observed exceedance 
of the 5-minute standard. Using formula [1], the estimated number of 
exceedances for the quarter is
e=1 x 2208/1210=1.825 or 1.83

    ii. If the estimated exceedances for the other four quarters 
were 0.0, then using formula [2], the estimated number of 
exceedances for the year is

1.83+0.0+0.0+0.0=1.83 or 1.8

    iii. If the estimated number of exceedances for the previous 
year was 0.0, then the expected number of exceedances is estimated 
by

(1.8+0.0)/2=0.9 or 1

    iv. Since 1 does not exceed the allowable number of exceedances, 
this monitoring site would not fail the attainment test.

PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS

    1. The authority citation for part 53 continues to read as follows:

    Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec. 
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat. 
1713, unless otherwise noted.

    2. Section 53.20 is amended by adding two sentences to the end of 
paragraph (b) and by revising the table to paragraph (c) to read as 
follows:


Sec. 53.20   General provisions.

* * * * *
    (b) * * * Candidate methods for sulfur dioxide may be additionally 
approved for use in obtaining 5-minute average concentration 
measurements by meeting all of the specified requirements for both the 
0 to 0.5 ppm and 0 to 1.0 ppm ranges and meeting the supplemental 
specifications for rise and fall time given in Table B-1. Such 
additional approval for 5-minute monitoring shall be included in any 
equivalent method designation determination for the method and shall be 
identified in the Federal Register notice of designation required under 
Sec. 53.8(a), the notice to the applicant required under Sec. 53.8(b), 
and the list of designated methods required under Sec. 53.8(c).
    (c) * * *

                          Table B-1.--Performance Specifications for Automated Methods                          
----------------------------------------------------------------------------------------------------------------
                                                  Sulfur   Photochemical    Carbon    Nitrogen   Definitions and
     Performance parameter           Units       dioxide      oxidants     monoxide   dioxide    test procedures
----------------------------------------------------------------------------------------------------------------
1. Range Supplemental, 5-        ppm\1\.......      0-0.5        0-0.5         0-50      0-0.5  Sec. 53.23(a).  
 minute\2\.                                                                                                     
                                 ppm..........      0-1.0                                                       
2. Noise.......................  ppm..........      0.005        0.005         0.50      0.005  Sec. 53.23(b).  
3. Lower detectable limit......  ppm..........       0.01         0.01          1.0       0.01  Sec. 53.23(c).  
4. Interference equivalent:                                                                                     
    Each interferant...........  ppm..........  0  0  0  2  5  g/m3 at 25  deg.C and 760 mm Hg,  
  multiply by M/0.02447, where M is the molecular weight of the gas.                                            
\2\Supplemental specifications applicable to sulfur dioxide equivalent methods to be additionally approved for  
  use for 5-minute monitoring.                                                                                  

* * * * *
[FR Doc. 94-27646 Filed 11-14-94; 8:45 am]
BILLING CODE 6560-50-P