[Federal Register Volume 61, Number 100 (Wednesday, May 22, 1996)]
[Rules and Regulations]
[Pages 25566-25580]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-12863]



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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[AD-FRL-5508-5]
RIN 2060-AA61


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

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final decision.

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SUMMARY: In accordance with sections 108 and 109 of the Clean Air Act 
(Act), EPA has reviewed and revised the air quality criteria upon which 
the existing national ambient air quality standards (NAAQS) for sulfur 
oxides are based. Based on that review, this document announces EPA's 
final decision under section 109(d)(1) that revisions of the NAAQS for 
sulfur oxides are not appropriate at this time, aside from several 
minor technical changes.
    In lieu of the two alternatives to short-term NAAQS proposed on 
November 15, 1994, EPA will shortly propose revisions to 40 CFR part 51 
to establish concern and intervention levels under section 303 of the 
Act and associated guidance to assist States in addressing short-term 
peaks of sulfur dioxide (SO2). Final action will be taken on 
proposed changes to 40 CFR parts 53 and 58 when final action is taken 
on the 40 CFR part 51 proposal and associated guidance.

EFFECTIVE DATE: May 22, 1996.

ADDRESSES: A docket containing information relating to EPA's review of 
the SO2 NAAQS (Docket No. A-84-25) is available for public 
inspection in the Air & Radiation Docket Information Center, U.S. 
Environmental Protection Agency, South Conference Center, Room M-1500, 
401 M Street, SW, Washington, DC, telephone (202) 260-7548. The docket 
may be inspected between 8 a.m. and 5:30 p.m. on weekdays, and a 
reasonable fee may be charged for

[[Page 25567]]

copying. For the availability of related information, see SUPPLEMENTARY 
INFORMATION.

FOR FURTHER INFORMATION CONTACT: Ms. Susan Lyon Stone, Air Quality 
Strategies and Standards Division (MD-15), U.S. Environmental 
Protection Agency, Research Triangle Park, NC 27711, telephone (919) 
541-1146.

SUPPLEMENTARY INFORMATION:

Availability of Related Information

    The 1982 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 1986 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 1994 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 1986 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 
1994 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, September 1994; NTIS # PB-95-124160, 
$27.00 paper copy and $12.50 microfiche) 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.) Other documents generated in 
connection with this standard review are also available in the EPA 
docket identified above.

Table of Contents

I. Background
    A. Legislative Requirements Affecting this Decision
    1. Primary Standards
    2. Related Control Requirements
    B. Sulfur Oxides and Existing Standards for SO2
    C. 1988 Proposal
    D. 1994 Reproposal
    E. Rulemaking Docket
II. Summary of Public Comments
    A. Current 24-hour and Annual Standards
    B. Regulatory Alternatives to Address Short-term Peak SO2 
Exposures
III. Rationale for Final Decision
    A. Current 24-hour and Annual Standards
    B. Short-term Peak SO2 Exposures
    1. Assessment of Health Effects Associated with Short-term 
SO2 Exposures
    2. Air Quality and Exposure Considerations
    3. Conclusions
    C. Final Decision on Primary Standards
    D. Technical Changes
IV. Regulatory Impacts
    A. Executive Order 12866
    B. Regulatory Flexibility Analysis
    C. Impact on Reporting Requirements
    D. Unfunded Mandates Reform Act
    E. Environmental Justice

References

Appendix I--1987 Clean Air Scientific Advisory Committee (CASAC) 
Closure Letter
Appendix II--1994 CASAC Closure Letter

I. Background

A. Legislative Requirements Affecting This Decision

1. Primary Standards
    Two sections of the Act govern the establishment and revision of 
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.'' For a discussion 
of the margin of safety requirement, see the November 15, 1994 proposed 
rule (59 FR 58958).
    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 EPA has reviewed the 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 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 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 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. 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. At elevated concentrations, SO2 can adversely affect human 
health.
    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. Information 
on the effects of the principal atmospheric transformation products of 
SO2 (i.e., sulfuric acid and sulfates) was

[[Page 25568]]

considered in the review of the particulate matter standards that 
culminated in revision of the standards 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).
    On April 30, 1971, EPA promulgated primary and secondary NAAQS for 
sulfur oxides, measured as SO2, under section 109 of the Act (36 
FR 8186). The existing primary standards for 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 secondary standard was set at 
1300 g/m3 (0.50 ppm) averaged over a period of 3 hours 
and not to be exceeded more than once per year. The scientific and 
technical bases for the current standards are contained in the original 
criteria document, Air Quality Criteria for Sulfur Oxides (DHEW, 1970). 
For a history of the effects of SO2 regulations on trends in 
SO2 emissions and ambient concentrations, see the November 15, 
1994 proposed rule (59 FR 58958).
    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 greater detail in the revised 
criteria document (EPA, 1982a), in the staff paper (EPA, 1982b), in the 
criteria document addendum (EPA, 1986a), the staff paper addendum (EPA, 
1986b), the criteria document supplement (EPA, 1994a), and the staff 
paper supplement (EPA, 1994b).

C. 1988 Proposal

    Based on reviews of the original air quality criteria and standards 
for sulfur oxides, EPA published a proposed decision not to revise the 
existing primary and secondary standards on April 26, 1988 (53 FR 
14926).1 In reaching the provisional conclusion that the current 
standards provided adequate protection against the health and welfare 
effects associated with SO2, EPA was mindful of uncertainties in 
the available evidence concerning the risk that elevated short-term (< 
1-hour) SO2 concentrations might pose to asthmatic individuals 
exercising in ambient air. 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 (0.50 ppm)) with a 1-hour secondary standard set 
equal to the primary standard, and adopting an expected-exceedance form 
for all of the standards.2
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    \1\ The proposal notice contains a detailed history of the 
process leading to the 1988 proposal.
    \2\ EPA also concluded that it was not appropriate at that time 
to propose a separate secondary SOx standard to provide increased 
protection against acidic deposition-related effects of SOX.
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    In the same notice, 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. In addition, EPA 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, EPA also 
proposed to revise the significant harm levels for SO2 and the 
associated example air pollution episode levels (40 CFR part 51). 
Finally, EPA proposed some minor modifications to the ambient air 
quality surveillance requirements (40 CFR part 58).

D. 1994 Reproposal

    As a result of public comments on the 1988 proposal and other post-
proposal developments, EPA published a second proposal regarding 
revision of the primary standards for sulfur oxides on November 15, 
1994 (59 FR 58958).3 The 1994 reproposal was based in part on 
supplements to the criteria document (EPA, 1994a) and staff paper (EPA, 
1994b) that were prepared to take into account recent health studies. 
Drafts of these documents were made available for review by the public 
and by the Clean Air Scientific Advisory Committee (CASAC) of EPA's 
Science Advisory Board, which provided its advice and recommendations 
in a letter dated June 1, 1994 (reprinted as Appendix II to this 
preamble). These and other aspects of the administrative process 
leading to the 1994 reproposal are described more fully in the 
reproposal notice.
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    \3\ A final decision that revision of the secondary standard was 
not appropriate was signed on April 15, 1993 and published in the 
Federal Register on April 21, 1993 (58 FR 21351).
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    As in the 1988 proposal, EPA proposed to retain the existing 24-
hour and annual standards. The EPA also solicited comment on three 
regulatory alternatives to further reduce the health risk posed by 
exposure to high 5-minute peaks of SO2 if additional protection 
were judged to be necessary. The three alternatives included: 1) 
Revising the existing primary SO2 NAAQS by adding a new 5-minute 
standard of 0.60 ppm SO2, 1 expected exceedance; 2) establishing a 
new regulatory program under section 303 of the Act to supplement 
protection provided by the existing NAAQS, with a trigger level of 0.60 
ppm SO2, 1 expected exceedance; and 3) augmenting implementation 
of existing standards by focusing on those sources or source types 
likely to produce high 5-minute peak concentrations of SO2.4
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    \4\ In a subsequent notice, EPA solicited comment on proposed 
requirements for implementing each of the alternatives (59 FR 12492, 
March 7, 1995).
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    In the reproposal notice, EPA specifically requested public comment 
in several key areas. First, EPA requested the submittal of 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 events that are most likely to give rise 
to such peak SO2 levels. Second, EPA requested the submission of 
data that would allow better characterization of the asthmatic 
population at risk and the frequency that an asthmatic individual would 
likely be exposed to peak concentrations of 0.60 ppm SO2 and 
above, while at elevated ventilation rates. Third, EPA requested that 
asthma specialists in the medical community submit their views on the 
medical significance of the reported SO2 effects, and on whether a 
numerical value below or above 0.60 ppm SO2 would be more 
appropriate to protect asthmatic individuals.
    The technical changes to the SO2 NAAQS that were first 
proposed in 1988, including formally adopting the block averaging 
convention, stating the standards in ppm rather than g/
m3, adopting explicit rounding and data completeness conventions 
and other technical changes, were reproposed in this notice. Comments 
on this reproposal were to be received by February 13, 1995.
    On December 29, 1994 (59 FR 67255), EPA announced that a public 
hearing on the reproposal would be held on February 8, 1995, and that 
the public

[[Page 25569]]

comment period was being extended to March 15, 1995. The public hearing 
was held at the U.S. Environmental Protection Agency's Environmental 
Research Center Auditorium in Research Triangle Park, NC.
    On March 14, 1995 (60 FR 13663), the public comment period was 
extended again, to April 14, 1995, to allow additional time for 
commenters to review the proposed requirements for implementing the 
three regulatory alternatives (59 FR 12492, March 7, 1995) before 
submitting comments on the 1994 reproposal.

E. Rulemaking Docket

    The EPA established a standard review docket (Docket No. A-79-28) 
for the sulfur oxides review in July 1979. The EPA also established a 
rulemaking docket (Docket No. A-84-25) for the 1988 proposal as 
required by section 307(d) of the Act. The standard review docket 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 Public Comments

    There were 95 written comments received prior to the end of the 
comment period on April 14, 1995. An additional 10 written comments 
were received after the close of the comment period. Of the 105 
submissions, 53 were provided by individual industrial companies or 
industrial associations, 16 by Federal, State and local government 
agencies, 7 by environmental and public interest groups, and 5 by 
interested individuals, including one neighborhood association. 
Comments also were received from physicians and other independent 
experts knowledgeable about the health effects described in the 
reproposal. Along with its written comments, one environmental group 
submitted videotaped testimony.
    In addition, 14 persons presented testimony at the February 8, 1995 
public hearing. The written text of the comments presented, as well as 
a transcript of the hearing, may be found in Docket No. A-84-25, 
Category VIII-F, located in the Air and Radiation Docket Information 
Center (see the Addresses section above).
    A general summary of the public comments follows. Some of the most 
significant comments are addressed, explicitly or implicitly, in other 
sections of this preamble. A more detailed summary of the comments 
received and EPA's responses to them has been placed in Docket No. A-
84-25, Category IX-C.

A. Current 24-hour and Annual Standards

    Most commenters concurred with EPA's conclusion that the existing 
24-hour and annual standards provide adequate protection against 
SO2-induced health effects associated with those averaging 
periods.

B. Regulatory Alternatives To Address Short-term Peak SO2 
Exposures

    Almost all commenters agreed on the basic nature of the health 
effects associated with short-term exposure to SO2 in controlled 
human exposure studies; that is, that brief (5-minute) exposures to 0.5 
to 1.0 ppm SO2 caused a proportion of asthmatic subjects at 
elevated ventilation rates to develop measurable and statistically 
significant bronchoconstriction, producing a range of symptoms from 
barely perceptible to severe enough to cause cessation of activity and 
medication use. In contrast, the comments were sharply divided on 
whether the existing standards should be supplemented by one of the 
three regulatory alternatives identified in the 1994 reproposal.
    In general, industry commenters and affiliated physicians argued 
that additional regulatory protection from health effects associated 
with exposure to short-term peaks of SO2 was unnecessary. Two 
broad arguments were made for this position. First, these commenters 
typically argued that the health effects associated with 5-minute peaks 
in the range of 0.6 to 1.0 ppm SO2 are not significant because the 
effects are transient, subsiding within 1 to 2 hours without 
medication, do not include a late-phase inflammatory response, can be 
avoided or ameliorated with medication, and are similar qualitatively 
and quantitatively to the kinds of effects that asthmatic individuals 
experience on an almost daily basis as a result of exposure to common 
stimuli. Second, these commenters argued that exposures to 5-minute 
peaks of SO2 are currently rare and, with the advent of title IV 
reductions in SO2 emissions, likely to become even rarer. In this 
regard, some commenters agreed with EPA's conclusion that the existing 
standards markedly limit the occurrence of short-term peaks of 
SO2.
    Conversely, environmental and public interest groups and affiliated 
physicians, citizens and physicians living in SO2-impacted areas, 
and independent experts argued that health effects that cause cessation 
of activity and medication use are adverse health effects, even if 
transient and preventable or reversible with medication. Citizens and 
physicians living in SO2-impacted areas also argued that asthmatic 
individuals living around industrial sources of SO2 are repeatedly 
exposed to short-term peaks of SO2, and that such repeated 
exposures affect their health adversely through exacerbation of their 
asthma and reduction in their quality of life. Some of these commenters 
disagreed with EPA's conclusion that the existing standards limit the 
occurrence of short-term peaks of SO2.
    In general, Federal, State and local government agencies focused on 
the same two broad issues as the other commenters (significance of the 
health effects and likelihood of exposure) as a basis for supporting or 
not supporting adoption of one of the three proposed regulatory 
alternatives to address short-term peaks of SO2. In addition, most 
governmental agencies submitted comments on implementation of the 
alternatives and tended to favor one or another based on the 
anticipated efficiency and effectiveness of implementing them. Of the 
11 State agencies that commented, four favored adopting either the 
proposed 5-minute NAAQS or the section 303 program. One State agency 
recommended that EPA not adopt any of the proposed alternatives at this 
time but continue to study the problem, adding that the proposed level 
of the standard, 0.60 ppm SO2, might not be low enough to include 
an adequate margin of safety. Another State agency was not in favor of 
adopting any of the proposed regulatory alternatives because it already 
had adequate authority to eliminate short-term peaks of SO2 in 
problem areas. The remaining five State agencies opposed adoption of 
any of the three proposed regulatory alternatives. Of the two local 
agencies that commented, one opposed any new regulations. The other did 
not comment on the need for new SO2 regulations but provided 5-
minute SO2 data from the local SO2 surveillance network and 
relevant information about the causes and temporal distribution of 5-
minute peaks 0.60 ppm SO2. Of the three Federal 
agencies that commented, all supported adoption of a 5-minute NAAQS or 
the section 303 program alternative.

III. Rationale for Final Decision

A. Current 24-hour and Annual Standards

    In the 1994 reproposal, EPA proposed to determine that revisions to 
the 24-hour and annual standards were not appropriate. As in the 1988 
proposal, EPA provisionally concluded that the current 24-hour and 
annual standards

[[Page 25570]]

were both necessary and adequate to protect public health against 
effects associated with those averaging periods. The EPA also 
provisionally 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 CASAC (Appendix I).
    Most comments on the 1994 reproposal focused on whether or not 
there was a need to adopt one of the regulatory alternatives to limit 
short-term peaks of SO2. Virtually every commenter that mentioned 
the existing primary standards agreed with EPA's conclusion that these 
standards were necessary and adequate to protect the public health 
against effects associated with those averaging periods. No commenter 
argued that the concentrations of these standards should be changed.
    After taking into account the public comments, the Administrator 
again concludes, based on the scientific data assessed in the criteria 
document and staff paper and their addenda, and consistent with the 
advice and recommendations of the staff and CASAC, that the 24-hour and 
annual standards provide adequate protection against the health effects 
associated with 24-hour and annual SO2 concentrations. 
Accordingly, the Administrator concludes that revisions to the 24-hour 
and annual standards are not appropriate at this time. In reaching this 
decision, the Administrator notes that the health effects information 
on 24-hour and annual SO2 exposures has remained largely unchanged 
since 1988. As newer information becomes available and is incorporated 
into new criteria documents, it will provide the basis for future 
reviews of the 24-hour and annual standards.

B. Short-Term Peak SO2 Exposures

    As reflected in the 1994 reproposal and in public comments on the 
reproposal, the question of whether revision of the existing NAAQS is 
appropriate to address risks that may be posed by short-term peaks of 
SO2 depends upon two factors: (1) The nature and significance of 
the health effects per se, and (2) the number of people likely to be 
exposed under conditions likely to produce such effects. The next two 
sections address these factors in turn, and the Administrator's overall 
conclusions are discussed in section III.B.3.
1. Assessment of Health Effects Associated With Short-term SO2 
Exposures
    This section focuses on the nature and significance of health 
effects that have been observed in controlled human exposure studies, 
putting aside temporarily, questions about the likelihood of such 
effects occurring under real-life conditions. Subsections a.-c. are 
adopted from the summary discussion in the 1994 reproposal of several 
important aspects of the health effects associated with short-term peak 
concentrations of SO2. Additional references on these subjects are 
provided in the reproposal notice. Public comments on the most 
important and controversial aspects of the short-term SO2 health 
effects are discussed in subsection d., with some indication of the 
Administrator's conclusions on particular issues. The last subsection 
contains the Administrator's overall conclusions regarding the 
significance of health effects associated with exposure to short-term 
peaks of SO2.
    a. Sensitive Populations. It is clear that healthy, nonasthmatic 
individuals are essentially unaffected by acute exposures to SO2 
at concentrations below 2 ppm, and that the population of concern for 
the effects of short-term SO2 exposure consists of mild and 
moderate asthmatic children, adolescents and adults that are physically 
active outdoors. This is a subset of the approximately 10 million 
people or 4 percent of the population of the United States that 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).
    b. Asthma. The Expert Panel Report from the National Asthma 
Education Program of the National Heart, Lung and Blood Institute (NIH, 
1991) has 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. Many asthmatic individuals 
experience bronchoconstriction when exercising, even in clean air. This 
response, called exercise-induced bronchoconstriction, is made worse by 
cold, dry air. 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.
    c. Short-term SO2 Health Effects. The EPA's concern about the 
potential public health consequences of exposures to short-term peaks 
of SO2 arose from 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 rates. The major effect of 
SO2 on sensitive asthmatic individuals is bronchoconstriction, 
usually evidenced in these studies by increased SRaw or decreased 
FEV1, and the occurrence of clinical symptoms such as wheezing, 
chest tightness, and shortness of breath. The proportion of asthmatic 
individuals who respond, the magnitude of the response and the 
occurrence of symptoms increase as SO2 concentrations and 
ventilation rates increase. The health effects are relatively 
transient. 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

[[Page 25571]]

often much more severe and dangerous than earlier immediate responses.
    The available data also 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, or ameliorating the effect, if taken after symptoms 
develop.
    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, the responses of African-American and Hispanic adolescents 
and young adults to short-term SO2 exposures 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).
    The criteria document supplement (EPA, 1994a) contains a summary of 
the literature on the health effects associated with brief exposures to 
SO2. Recent studies have provided useful information about the 
magnitude of responses in the range of 0.4 to 1.0 ppm SO2, the 
range of interest identified in the 1988 proposal (53 FR 14948, April 
26, 1988). Data from several of these recent large-scale chamber 
studies were reexamined in Appendix B of the criteria document 
supplement (EPA, 1994a) to provide a better understanding of the 
responses observed in more sensitive subjects. Forced expiratory volume 
in 1 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 describes 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 5 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 SRaw due to SO2 alone (i.e., 
after correction for the effects of exercise) were considered.
---------------------------------------------------------------------------

    \5\ 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. In this discussion, 
``total FEV1 (or SRaw)'' refers to the total change in 
lung function experienced by a subject as a result of an exposure to 
SO2 while at exercise, while ``the effect of changes due to 
SO2 alone'' refers to the total lung function change observed 
minus the change seen for that subject from a control exposure at 
exercise in clean air. Both measures have their utility: total 
FEV1 or SRaw indicates the magnitude of overall lung 
function change actually experienced by the subject, while the 
change due to SO2 alone indicates how much of this total change 
is attributable to the pollutant itself.
---------------------------------------------------------------------------

    For FEV1, the differences 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. In one study, for example, 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.''
    The staff also compared the effects of SO2 observed in these 
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 at any other time during their 
participation in the studies (EPA, 1994a, p. B-12).
    Furthermore, the response seen in the most sensitive 25 percent of 
responders at 0.6 ppm equalled 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 33-43 percent of moderate 
asthmatics and from 6-35 percent of mild asthmatics experienced at 
least a 20 percent

[[Page 25572]]

decrease in total FEV1 in conjunction with symptoms rated as being 
of moderate severity or worse. It should be noted that the asthmatic 
subjects with moderate/severe disease started an exposure with 
compromised lung function compared to mild asthmatic subjects. While 
the response to SO2 was similar in the mild versus the moderate/
severe asthmatic subjects, similar functional declines beginning from a 
different baseline may have different 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 experienced 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 distinctly exceeding 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.
    d. Public Comments on Significance of Health Effects. In regard to 
the measured changes in lung function (expressed as FEV1 or 
SRaw), commenters did not disagree with the EPA's summary of the 
available literature contained in the November 15, 1994 (59 FR 58958) 
reproposal. Where there continues to be a real divergence of opinion 
among asthma specialists and others is on interpretation of the 
results, or on the medical significance of the lung function changes 
that have been measured in exercising asthmatic subjects and summarized 
in the various EPA documents. At issue are not the published data about 
SO2-induced bronchoconstriction, but how they are interpreted.
    As noted in the 1994 reproposal, bronchoconstriction caused by 
brief exposure to 0.6 to 1.0 ppm SO2 is transient. Measurements of 
lung function start to improve when the exposure ceases, or when the 
subject ceases to exercise and the ventilation rate decreases to 
resting levels; after 5 minutes of exposure, the magnitude of the 
response does not worsen even if exposure and elevated ventilation rate 
continue. Most often, lung function returns to preexposure levels 
within 1 hour, occasionally taking up to 2 hours to return to normal. A 
dose of one of the most commonly used classes of medication, inhaled 
beta2-agonists, rapidly attenuates or prevents the response. The 
transient nature of the response led some commenters to argue that the 
health effects are not significant. These commenters stated that 
although they would advise an asthmatic individual to take medication, 
cease activity or avoid the stimulus, this behavior was an everyday 
part of an asthmatic individual's life and not cause for medical 
concern. Other commenters argued that any effect which may entail 
bronchoconstriction severe enough to limit activity or cause medication 
use is a significant health effect.
    Many commenters argued that the documented effects are not 
medically significant because, as one commenter put it, ``changes in 
lung function are not meaningful endpoints in themselves, but must be 
placed in the context of asthmatics' typical respiratory function, 
which is both highly variable and reactive to many stimuli and 
conditions'' (see Docket No. A-84-25, VIII-D-71). In general, these 
commenters argued that the responses to short-term peaks of SO2 in 
the range of 0.6 to 1.0 ppm are similar in nature and magnitude to the 
well-tolerated responses to a variety of non-specific stimuli (cold, 
dry air, exercise, irritants such as perfume) encountered on a daily 
basis by most asthmatic individuals and are not in themselves 
deleterious to the asthmatic individual's health. Other commenters 
argued that this fact does not justify the neglect of potential ambient 
air SO2 effects, and that unusual susceptibility to an inhaled 
pollutant does not simply constitute a problem for the susceptible 
individual.
    Despite these opposing points of view, there was some agreement 
that frequency of occurrence of SO2-induced health effects could 
make a difference in the concern that a physician feels. That is, some 
physicians felt that the documented SO2-induced health effects 
were well tolerated by asthmatic individuals; however, if the effects 
occurred frequently enough, then they would be cause for medical 
concern (public hearing transcript, 1995, p. 155). Other physicians 
felt that such effects are a cause for concern despite their transient 
and reversible nature; if exposures occurred rarely enough, however, 
these physicians would be less concerned (public hearing transcript, 
1995, p. 89-90). Several commenters also noted that cold air appears to 
act at least additively with SO2, and that the bronchoconstrictive 
effect of cold air which contains SO2 is larger than that of 
either exposure condition alone.
    Some commenters took issue with EPA's assessment of the proportion 
of asthmatic individuals who would experience meaningful symptoms or 
have any disruption of daily activities. Based on personal experience, 
one commenter stated that most asthmatics do not begin to perceive 
bronchoconstriction until FEV1 falls to about 50 percent of its normal 
value and SRaw increases about 400 percent (see Docket No. A-84-
25, VIII-D-71). Other commenters agreed that the kinds of symptomatic 
responses experienced by asthmatic subjects exposed to SO2 in the 
reviewed chamber studies are no more than brief, perceptible reactions 
that might temporarily disrupt activities, but are well tolerated and 
do not endanger the individuals' health or cause them to seek medical 
attention. On the other hand, commenters who believed the effects were 
significant argued that transient and reversible decrements in lung 
function are adverse if they cause physical discomfort, interfere with 
normal activity or impair the performance of daily activities, or 
aggravate chronic respiratory disease by increasing the frequency or 
severity of asthma attacks. Several commenters argued that measurable 
effects have occurred after brief exposures, with elevated ventilation 
rates, to concentrations as low as 0.25 to 0.28 ppm SO2, and thus 
that the proposed 5-minute standard of 0.60 ppm SO2 leaves no 
margin of safety. However, as stated above, considering a variety of 
endpoints for which information is available, clearly the effects 
beginning at 0.6 ppm and up to 1.0 ppm are more

[[Page 25573]]

pronounced than at lower concentrations.
    As noted in the criteria document supplement (EPA, 1994a), the 
staff paper supplement (EPA, 1994b) and the November 15, 1994 
reproposal (59 FR 58958), unlike the effects of allergens and viral 
infections, there is no evidence that short-term exposure to SO2 
while at an elevated ventilation rate leads to any ``late phase'' 
response. ``Late-phase'' bronchoconstriction is indicative of a more 
serious inflammatory reaction which takes much longer to resolve and 
which can lead to emergency room visits and/or hospitalization. The 
``late phase'' inflammatory response can also cause the airways to 
become more sensitive to other stimuli. Since this type of response has 
not been observed with brief exposures in the range of 0.6 to 1.0 ppm 
SO2, many commenters argued that the health of asthmatic 
individuals is not affected by such exposures.
    The ability of inhaled beta2-agonists, the most commonly 
prescribed class of asthma medications, to prevent or ameliorate the 
effects of SO2 exposure was frequently cited as one reason why 
most asthmatic individuals are unlikely to experience 
bronchoconstriction due to exposure to short-term peaks of SO2. 
These commenters argued that since most asthmatic individuals 
experience exercise-induced bronchoconstriction, they are highly likely 
to premedicate with an inhaled beta2-agonist medication prior to 
exercise and therefore be protected from SO2-induced health 
effects. Further, these commenters stated that the highly variable 
compliance rates for medicine usage cited by EPA in the criteria 
document supplement (EPA, 1994a), staff paper supplement (EPA, 1994b) 
and November 15, 1994 reproposal (59 FR 58958) do not apply to 
physically active asthmatic individuals, for whom medication compliance 
rates are significantly better.
    Conversely, many other commenters agreed with EPA that medication 
compliance rates can be very poor, even for individuals who are 
physically active, like children, and that many asthmatic individuals 
use medication only after symptoms occur. These individuals would be at 
risk for experiencing SO2-induced bronchoconstriction. Some 
commenters, including one from a State's Office of Environmental Health 
Hazard Assessment, which recently reviewed that State's 1-hour SO2 
standard (see Docket No. A-84-25, VIII-D-65), commented that an optimal 
medication regimen from the standpoint of reducing SO2-induced 
bronchoconstriction may result in undesirable side effects. Some of 
these commenters also noted that SO2 exposure could cause 
asymptomatic, exercise-induced bronchoconstriction to become 
symptomatic, thereby causing an asthmatic individual to take medicine 
that would normally not be needed. Several commenters argued that 
relying on medication use instead of regulation was poor public policy. 
Some of these commenters also argued that asthmatic individuals of 
lower socioeconomic status may not be able to afford medication or have 
limited access to health care. In the Administrator's judgment, these 
concerns about accessibility of medication and health care, and the 
variability of medication compliance rates, are legitimate ones. 
Although the use of medication may substantially reduce the incidence 
and/or severity of SO2-induced bronchoconstriction, the mere 
availability of medication does not necessarily mean that all asthmatic 
individuals will necessarily be protected from this effect. The 
Administrator therefore concludes that this factor should not be 
regarded as dispositive in assessing the appropriateness of regulatory 
action to provide additional protection against short-term SO2 
peaks.
    Many commenters argued that there are no epidemiological studies 
which show an association between short-term peaks of SO2 and 
adverse health effects such as asthma symptoms or increased visits to 
physicians or hospital emergency rooms. Some of these commenters argued 
that the changes in lung function and symptoms found in some subjects 
in controlled human exposure studies may not be indicative of what 
would occur in real-world situations. The reason that there are no 
epidemiological studies showing an association between short-term (5- 
to 10-minute) peaks of SO2 and real-world health effects is that 
apparently no studies have been conducted to examine the association or 
lack thereof of short-term SO2 peaks and adverse health effects. 
This is most likely because it would be difficult to design and conduct 
an epidemiological study that could detect possible associations 
between very brief (5- to 10-minute), geographically localized, peak 
SO2 exposures and respiratory effects in asthmatic individuals. 
Furthermore, the responses of naturally-breathing asthmatics exposed to 
SO2 under controlled conditions in an environmental chamber 
presumably reflect responses that would be observed in the ambient 
(``real-world'') environment under similar conditions of activity 
level, air temperature, and humidity. Although there is evidence that 
other inhaled materials that modify airway responsiveness can influence 
the response to SO2, there is no reason, at the present time, to 
suggest that the ambient pollutant mixture would cause either a 
suppression or an augmentation of SO2 effects through some, as yet 
unrecognized, chemical interaction.
    e. Significance of Health Effects. Taking into account the 
available health effects studies and the body of comments on the health 
effects, the Administrator agrees with the staff assessment 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 at elevated ventilation rates, such as would be expected during 
moderate exercise, would be expected to have lung 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 health concern; that is, likely to cause some disruption 
of ongoing activities, use of bronchodilator medication, and/or 
possibly seeking of medical attention. The EPA agrees with other 
commenters that the frequency with which such effects are experienced 
may affect the public health concern that is appropriate. Taking into 
account the broad range of opinions expressed by CASAC members, medical 
experts, and the public, the Administrator concludes that repeated 
occurrences of such effects should be regarded as significant from a 
public health standpoint. Accordingly, the Administrator also concurs 
with the staff judgment that the likely frequency of occurrence of such 
effects should be a consideration in assessing the overall public 
health risk in a given situation.
2. Air Quality and Exposure Considerations
    Another major basis for considering whether additional regulatory 
measures are appropriate to reduce the occurrence of short-term peaks 
of SO2 has been the estimation of the geographic extent and the 
frequency of 5-minute peaks greater than 0.60 ppm SO2 in the 
ambient air, and the likelihood that these peaks would result in 
exposure conditions that could cause significant health effects. As 
discussed in the staff paper supplement (EPA, 1994b) and the 1994 
reproposal, the occurrence of short-term peaks of SO2 is 
relatively infrequent and highly localized around point sources of

[[Page 25574]]

SO2. None of the air quality or exposure information subsequently 
received by EPA has changed this assessment.
    In 1993 and again in 1994, EPA requested that States collect and 
submit 5-minute SO2 ambient monitoring data from source-based 
monitors. Data were submitted from both industry and State-run monitors 
and while much of this information was considered in the 1994 staff 
paper supplement (EPA 1994b) and in the 1994 reproposal, a few sites 
subsequently provided more data. Available data have been compiled and 
statistical parameters calculated in a report for EPA by Systems 
Applications International or SAI (1996).6 In general, the data 
confirm that a substantial number of short-term peaks greater than 0.60 
ppm can occur in the vicinity of certain sources.
---------------------------------------------------------------------------

    \6\ The 5-minute concentrations ranged from 0 to > 2.5 ppm 
SO2. The number of observations recorded at any monitor ranged 
from 308 to 48,795 hours, with the mean number of observations 
equalling 7,646 hours (a complete year of hourly maximum 5-minute 
averages would contain 8,760 observations). There were 63 monitors, 
located in 16 States, with continuous data sets of either the 
maximum 5-minute block average per hour or all of the 5-minute block 
averages per hour. For data sets containing all of the 5-minute 
block averages per hour, the maximum 5-minute block average for each 
hour was extracted and that parameter was used throughout the 
analysis. Of the 63 monitors, 26 (41 percent) registered 1 or more 
concentrations greater than the proposed short-term standard of 0.60 
ppm SO2 during the time periods represented for the monitors 
involved. For any given monitor, the number of such exceedances 
ranged from 0 to 139, which corresponds to 0 to 3 percent of the 
hours represented in the data. Of the 26 monitors measuring at least 
1 exceedance, 11 monitors recorded from 1 to 5 exceedances, while 8 
monitors in 4 communities recorded from 25 to 139 exceedances. While 
these data came from sourcebased monitors, the existing SO2 
monitoring network is designed to characterize ambient air quality 
associated with 3-hour, 24-hour, and annual SO2 concentrations 
rather than to detect short-term peak SO2 levels. This could 
have resulted in underestimates of the maximum 5-minute block 
averages recorded. Therefore, changes in monitor siting and density 
near SO2 sources most likely to produce high 5-minute peaks 
could increase both the number of exceedances and the concentrations 
of the maximum 5-minute block averages recorded.
---------------------------------------------------------------------------

    As indicated previously, an important consideration is whether such 
short-term peaks of SO2 are likely to cause episodes of 
bronchoconstriction in asthmatic individuals. Thus, one method of 
assessing the public health significance of SO2-induced effects is 
to estimate the likelihood that asthmatic individuals will be exposed 
to such peaks while simultaneously at elevated ventilation rates (EPA, 
1994a, p.51). It should be noted, however, that not all asthmatic 
individuals who experience such exposures will necessarily experience 
SO2-induced health effects, either because of individual 
variability or other factors.
    At the time of the 1994 reproposal, three exposure analyses were 
available that estimated the frequency of SO2 exposures that could 
result in measurable health effects. Two of the analyses estimated the 
potential frequency of exposure events resulting from operation of 
utility boilers nationwide. For these two studies, detailed information 
on actual emissions was available on a plant-by-plant basis (Burton et 
al., 1987; Rosenbaum et al., 1992) to use in estimating ambient 
SO2 concentrations and then exposures. The utility analyses 
estimated there would be 68,000 exposure events per year at  
0.5 ppm SO2, which would affect approximately 44,000 asthmatic 
individuals at elevated ventilation rates. Taking into account 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 was estimated to drop to 40,000 per 
year, contingent on trading decisions.
    The third exposure analysis available at the time of the 1994 
reproposal estimated nationwide SO2 exposures resulting from the 
operation of nonutility sources. Because actual data were not 
available, some conservative assumptions had to be made about operating 
parameters, which increased the uncertainties in the analysis 
(Stoeckenius et al., 1990). Probably the largest single source of 
uncertainty in this analysis was the emissions estimates used for the 
nonutility sources. The analysis estimated 114,000 to 326,000 exposures 
to 0.5 ppm SO2 per year around nonutility sources. These exposures 
were estimated to affect 24,000 to 122,000 asthmatic individuals at 
elevated ventilation rates, implying that exposed individuals may be 
exposed more than four times a year, on average.
    Combining the utility and nonutility exposure estimates results in 
a prediction of 180,000 to 395,000 total exposure events to 0.5 ppm 
SO2 nationwide, per year. 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. The number of asthmatic individuals likely to be exposed to 
 0.60 ppm SO2 under the same conditions, of course, 
would be smaller. 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).
    In response to the 1994 reproposal, several industry associations 
sponsored and submitted as a public comment a revised analysis of 
exposures around four types of nonutility sources (industrial/
commercial/institutional boilers, kraft and sulfite process pulp and 
paper mills, and copper smelters) by Sciences International, Inc. 
(1995). This study incorporated new data and additional analyses 
designed to eliminate the need for some of the more conservative 
assumptions employed in the Stoeckenius et al. (1990) study. A 
principal feature of the new study is the use of improved source and 
emissions data for all four source categories examined and especially 
for sulfite process pulp mills and copper smelters. The new analysis 
estimated significantly fewer expected exposure events for the four 
source categories examined. In the original study, the four categories 
were estimated to contribute a total of 73,000 to 259,000 exposure 
events (Stoeckenius et al., 1990). In the revised analysis, this range 
decreased by an order of magnitude, to between 7,892 and 23,099 events. 
The same basic procedures were used to calculate expected exposures in 
both the 1990 and 1995 studies. However, a direct comparison of the 
results of the two exposure analyses may not be possible due to 
differences in some key details between the two studies, which are 
highlighted in a technical review by Stoeckenius (1995) of the Sciences 
International, Inc. (1995) exposure analysis. In general, that review 
indicates that while the Stoeckenius et al. (1990) study utilized 
several very conservative assumptions, which most likely led to an 
overestimate of exposures for these three source categories. The 
Sciences International, Inc. (1995) reanalysis did not provide reliable 
estimates of the degree of conservatism resulting from the original 
assumptions which could then be used for the purpose of comparison. In 
contrast, the updated information and data for copper smelters used in 
the Sciences International, Inc. (1995) reanalysis most likely resulted 
in a more accurate estimate of exposures for that source category than 
did previously available estimates (Stoeckenius, 1995).
    Another industry commenter submitted an exposure analysis (see 
Docket No. A-84-25, VIII-G-08) that utilized actual SO2 ambient 
air monitoring and demographic data from a community located near a 
copper-smelting facility. The results of this analysis indicate that 
the probability of SO2-related episodes of bronchoconstriction in 
the sensitive

[[Page 25575]]

population of asthmatic individuals in the community is very low. There 
was no evidence of an association between 5-minute concentrations of 
SO2 > 0.60 ppm and episodes of bronchoconstriction in the 
sensitive population.
    These exposure analyses and the body of 5-minute SO2 
monitoring data underscore the views of the Administrator, the staff 
and the CASAC, reflected in the 1994 reproposal, that the likelihood 
that asthmatic individuals will be exposed to 5-minute peak SO2 
concentrations of concern, while outdoors and at elevated ventilation 
rates, is very low when viewed from a national perspective. Even in 
communities where frequent 5-minute peaks have been recorded, the 
likelihood of exposure is highly variable. One county public health 
agency submitted 5-minute SO2 monitoring data (see Docket No. A-
84-25, VIII-D-15), for the years 1993-1994, from the 10 continuous 
SO2 monitors in the local surveillance network. Only monitors 
located near large industrial sources of SO2 measured exceedances 
of 0.60 ppm SO2. Of 29 exceedances measured over a 2-year period, 
approximately half of the exceedances were associated with breakdowns 
of the desulfurization equipment used to control SO2 emissions 
from coke plants in the county. The agency noted that more than 70 
percent of the hours in which exceedances were measured occurred very 
late at night or early in the morning, which would reduce the 
likelihood of the exceedances affecting the sensitive population.
    Nonetheless, the 5-minute monitoring data indicate that some 
communities in proximity to SO2 sources are repeatedly subjected 
to high short-term concentrations of SO2 in the ambient air. 
Asthmatic individuals who reside in proximity to certain individual 
sources may be at greater risk of being exposed to such peak SO2 
levels while at elevated ventilation rates, and, therefore, at greater 
risk of suffering health effects than the asthmatic population as a 
whole. This conclusion is supported by the comments of citizens and 
physicians living in areas where high 5-minute peaks of SO2 have 
been recorded. Citizens have reported, for example, that they developed 
asthma upon moving to an SO2-impacted area; that their asthma is 
better, both in terms of symptoms and indicators such as peak flow 
measurements when they leave the SO2-impacted area on vacation or 
for medical treatment; and that their peak flow measurements decrease 
when the wind is blowing from the direction of the local SO2 
source(s). These citizens express the belief that ambient SO2 
concentrations are responsible for their symptoms. Physicians have 
commented that they believe that ambient air SO2 concentrations in 
their communities are negatively affecting the health of their 
patients. Most of these comments came from two of the six communities 
for which SO2 monitoring data show repeated high 5-minute peaks 
greater than 0.60 ppm SO2.
    The data also indicate that asthmatic individuals living in 
communities in which 5-minute peaks greater than 0.60 ppm SO2 
rarely occur may be subject to much less risk of experiencing health 
effects that cause cessation of activities or increased medication use. 
Even when monitors record a substantial number of such peaks, the 
likelihood that a significant number of asthmatic individuals will be 
exposed to such peaks with some frequency while at elevated ventilation 
rates may range from nonexistent to fairly high depending upon such 
localized factors as the magnitude and frequency of the peaks, the 
times of occurrence, meteorological conditions in the area, the density 
of the population near the source(s) involved, and daily activity 
patterns. Thus, estimation of risk must be done on a case-by-case basis 
and be based on site-specific factors. In short, the data clearly show 
that 5-minute peaks greater than 0.60 ppm SO2 can occur around 
particular industrial point sources of SO2, that such peaks are 
not ubiquitous from a national perspective but instead appear to occur 
only in the vicinity of such sources, and that the risk of exposures 
that could cause significant health effects in asthmatic individuals 
cannot be estimated based solely on the number of recorded high 5-
minute peaks of SO2, but instead must be estimated using site-
specific factors.
3. Conclusions
    For reasons discussed above, based on her assessment of the 
relevant scientific and technical information and taking into account 
public comment, it is the Administrator's judgment that 5-minute peak 
SO2 levels do not pose a broad public health problem when viewed 
from a national perspective. As discussed in some detail in the 1994 
reproposal, the existing suite of SO2 standards and associated 
control strategies clearly limit both the occurrence of high 5-minute 
peak SO2 levels, and the likelihood that asthmatic individuals 
will be exposed to them while outdoors and at elevated ventilation 
rates.
    In considering the residual risk posed by such peak concentrations, 
the Administrator has taken a number of factors into account. As 
discussed in the criteria document and staff paper supplements (EPA 
1994a, p. 51, EPA 1994b, p. 59), an important consideration in 
determining the public health risk posed by 5-minute concentrations in 
the range of 0.60 to 1.0 ppm SO2 is the frequency with which an 
asthmatic individual may be exposed while at an elevated ventilation 
rate. As discussed earlier, there is some agreement that infrequent 
exposures in this range may not be a cause for significant concern. As 
the frequency of exposure increases, so does concern about the 
associated public health risk. Asthmatic individuals living in 
communities in which 5-minute peaks in the range of 0.60 to 1.0 ppm 
SO2 rarely occur may be unlikely to experience exposure events 
that would cause them to cease their activities or increase medication 
use. In particular locations, of course, the concentrations involved in 
exposure events can exceed 1.0 ppm SO2, and could induce a greater 
response in an exposed asthmatic individual than lower concentrations. 
Thus, frequency of exposure events alone is not an adequate indicator 
of the risk to public health. As discussed above, factors such as the 
magnitude of 5-minute SO2 peaks, time of day, activity patterns, 
and the size of the population exposed are also relevant. As a result, 
whether 5-minute peak SO2 concentrations will pose a significant 
public health risk depends largely on highly localized factors.
    Given the localized, infrequent and site-specific nature of the 
risk involved, the Administrator has concluded that short-term peak 
concentrations of SO2 do not constitute the type of ubiquitous 
public health problem for which establishing a NAAQS would be 
appropriate. For similar reasons, the Administrator concludes that 
adoption of a section 303 program employing a uniform, nationwide 
trigger level would not be an appropriate response. With respect to the 
third alternative identified in the 1994 reproposal (augmenting 
implementation of existing SO2 NAAQS), it has become increasingly 
clear that even full attainment of the existing SO2 standards 
would not preclude the occurrence of high 5-minute SO2 peaks in 
particular locations. Moreover, given the site-specific nature of the 
problem, States can more effectively identify for monitoring purposes, 
sources that may be causing or contributing to high 5-minute SO2 
concentrations.
    For the reasons discussed previously, the Administrator has 
concluded that repeated exposures to 5-minute peak SO2 levels of 
0.60 ppm and above could

[[Page 25576]]

pose a risk of significant health effects for asthmatic individuals at 
elevated ventilation rates in some localized situations. The 
Administrator has also concluded that the residual health risks posed 
by short-term concentrations are most appropriately addressed at the 
State level. In the Administrator's judgment, the States are in a far 
better position than EPA to assess the highly localized and site-
specific factors that determine whether the occurrence of such 
concentrations in a given area poses a significant public health risk 
to the local population, and if so, to fashion an appropriate remedial 
response. This view was also advanced by some States in their comments 
on the 1994 reproposal.
    To assist the States in addressing short-term peak SO2 levels, 
EPA will publish a reproposal notice superseding the March 1995 notice 
(59 FR 12492) that proposed revisions to 40 CFR part 51 establishing a 
new program under section 303 of the Act that would differ from that 
contemplated in the 1994 reproposal. The new program would also differ 
from existing programs under section 303 that are designed to protect 
against episodic events.
    In particular, EPA plans to propose two new levels as guides to 
State action: A ``concern level'' at 0.60 ppm SO2, 5-minute block 
average; and an ``intervention level'' at 2.0 ppm SO2, 5-minute 
block average. Under the program to be proposed, the States would 
determine whether 5-minute peak SO2 levels recorded in the range 
of 0.60 to 2.0 ppm SO2 posed a significant public health risk and, 
if so, the appropriate remedial response. To assist the States in 
reaching such determinations, the proposal will identify, in the form 
of guidance, factors that EPA believes should be considered in 
assessing whether recorded peaks pose a significant health risk to the 
local population. Among other things, the factors would include the 
frequency and magnitude of observed 5-minute peaks, and the likelihood 
and frequency of exposures for asthmatic individuals at elevated 
ventilation rates. In assessing whether observed 5-minute peaks in this 
range posed a significant public health risk, thus warranting 
intervention, the States would be advised to take into account the 
above factors, as well as others they might deem appropriate. It is the 
Administrator's judgment that establishing such a program, in which the 
States would determine at the local level whether peak SO2 levels 
in the range of 0.60 to 2.0 ppm SO2 posed a significant public 
health risk and, if so, the appropriate remedial response, is the most 
effective approach for addressing this potential public health problem.

C. Final Decision on Primary Standards

    For the reasons discussed above, and in the November 15, 1994 
reproposal notice (58 FR 58958), it is the Administrator's judgment 
under section 109(d)(1) that revisions to the existing primary SO2 
NAAQS are not appropriate at this time. As provided for under the Act, 
the EPA will continue to assess the scientific information on health 
effects associated with 5-minute, 24-hour and annual SO2 exposures 
as it emerges from research and ongoing SO2 monitoring programs, 
and will update the air quality criteria for sulfur oxides accordingly. 
The revised criteria will provide the basis for the next review of the 
primary NAAQS for SO2.

D. Technical Changes

    There were relatively few comments on the proposed technical 
changes. Several environmental and public interest groups and one State 
preferred the running averaging convention, while industry comments 
supported the block averaging convention. A small number of comments 
were also received both for and against the change from mg/
m\3\ to ppm. Taking these comments into account, EPA has decided to 
promulgate the technical changes set forth in the 1994 reproposal. 
First, the block averaging convention will be retained, and language 
clarifying this point will be adopted in the regulation (40 CFR 50.4 
and 50.5). 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.
    Although the wording of the original 24-hour, 3-hour, and annual 
SO2 standards may have been 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. Given a fixed 
standard level, the use of the alternative, running averages, would 
represent a tightening of the standards (Faoro, 1983; Possiel, 1985). 
For reasons explained in this notice and in the April 21, 1993, notice 
on the secondary NAAQS (58 FR 21351), the Administrator has already 
determined that protection of the public health and welfare does not 
require tightening the existing standards. Therefore, EPA will retain 
the block averaging convention for the 24-hour, 3-hour, and annual 
standards.
    The second technical change to be adopted is that the levels for 
the primary and secondary NAAQS will be stated in ppm rather than 
g/m\3\ (40 CFR 50.4 and 50.5). This will be done to make the 
SO2 NAAQS consistent with those for other pollutants and to 
facilitate public understanding of the standards. Although the ppm 
levels are slightly less than their current g/m\3\ 
counterparts, the differences are considered negligible (Frank, 1988).
    Finally, the explicit rounding conventions and the data 
completeness and handling conventions put forth in the reproposal will 
be adopted.

IV. Regulatory Impacts

A. Executive Order 12866

    Under Executive Order 12866, the Agency must determine whether a 
regulatory action is ``significant'' and, therefore, subject to Office 
of Management and Budget (OMB) review and the requirements of the 
Executive Order. The order defines ``significant regulatory action'' as 
one 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 entitlements, grants, 
user fees, or loan programs or the rights and obligations or 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, OMB has notified 
EPA that it considers this a ``significant regulatory action'' within 
the meaning of the Executive Order. The EPA has submitted this action 
to OMB for review. Changes made in response to OMB suggestions or 
recommendations will be documented in the public docket and made 
available for public inspection at EPA's Air and Radiation Docket 
Information Center (Docket No. A-84-25).
    The EPA has judged that today's decision on the SO2 primary 
NAAQS is not an economically-significant regulatory action as defined 
by Executive Order 12866 because there are no additional costs or other 
impacts as a result of not revising the standards. The EPA, therefore, 
has deemed unnecessary the preparation of a final regulatory impact 
statement.

[[Page 25577]]

B. Regulatory Flexibility Analysis

    Pursuant to section 605(b) of the Regulatory Flexibility Act, 5 
U.S.C. 605(b), the Administrator certifies that this rule will not have 
a significant impact on a substantial number of small entities. The 
Regulatory Flexibility Act requires that all Federal agencies consider 
the impacts of final regulations on small entities, which are defined 
to be small businesses, small organizations, and small governmental 
jurisdictions (5 U.S.C. 601 et seq.). A decision not to revise the 
existing primary NAAQS for SO2 would, of course, impose no new 
requirements on small entities. In addition, the SIPs necessary to 
implement the existing primary standards have been substantially 
adopted and implemented. Additional SIP requirements will be needed 
only for those areas or sources which are designated as nonattainment 
for the existing primary standards now or in the future. Given the 
current air quality and attainment status, however, it is very unlikely 
that new SIP requirements would be required that would significantly 
affect a substantial number of small entities.

C. Impact on Reporting Requirements

    There are no reporting requirements directly associated with an 
ambient air quality standard promulgated under section 109 of the Act 
(42 U.S.C. 7400). There are, however, reporting requirements associated 
with related sections of the Act, particularly sections 107, 110, 160, 
and 317 (42 U.S.C. 7407, 7410, 7460, and 7617). This final action will 
not result in any changes in these reporting requirements since it 
would retain the existing levels and averaging times for the primary 
standards. The current standards are covered under EPA Information 
Collection Request Number 940.13.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public 
Law 104-4, establishes requirements for Federal agencies to assess the 
effects of their regulatory actions on State, local, and tribal 
governments and the private sector. Under sections 202, 203, and 205, 
respectively, of the UMRA, EPA generally must: (1) Prepare a written 
statement, including a cost-benefit analysis, for proposed and final 
rules with ``Federal mandates'' that may result in expenditures to 
State, local and tribal governments, in the aggregate, or to the 
private sector, of $100 million or more in any one year; (2) develop a 
small government agency plan; and (3) identify and consider a 
reasonable number of regulatory alternatives and adopt the least 
costly, most cost-effective or least burdensome alternative that 
achieves the objectives of the rule.
    Because the Administrator has decided not to revise the existing 
primary NAAQS for SO2, this action will not impose any new 
expenditures on governments or on the private sector, or establish any 
new regulatory requirements affecting small governments. Accordingly, 
EPA has determined that the provisions of sections 202, 203, and 205 of 
the UMRA do not apply to this final decision.

E. Environmental Justice

    Executive Order 12848 requires that each Federal agency make 
achieving environmental justice part of its mission by identifying and 
addressing, as appropriate, disproportionally high and adverse human 
health or environmental effects of its programs, policies, and 
activities on minorities and low-income populations. These requirements 
were addressed in the draft Regulatory Impact Analysis (59 FR 58958; 
November 15, 1994) and taken into account by EPA in reaching its 
determination that revisions to the existing primary SO2 NAAQS are 
not appropriate at this time.

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: May 14, 1996.
Carol M. Browner,
Administrator.

References

Burton, C.S.; Stockenius, T.E.; Stocking, T.S.; Carr, E.L.; Austin, 
B.S.; Roberson, R.L. (1987) Assessment of exposures of exercising 
asthmatics to short-term SO2 levels as a result of emissions 
from U.S. fossil-fueled power plants. Systems Applications, Inc., 
San Rafael, CA. Pub. No. 87/176, September 23, 1987.
DHEW [U.S. Department of Health, Education, and Welfare] (1970), Air 
Quality Criteria for Sulfur Oxides, U.S. Government Printing Office, 
Washington, DC, AP-50.
Edmunds, A.T.; Tooley, M.; Godfrey, S. (1978) The refractory period 
after exercise-induced asthma: its duration and relation to the 
severity of exercise, Am. Rev. Resp. Dis. 117:247-254.
EPA (1982a), Air Quality Criteria for Particulate Matter and Sulfur 
Oxides, Environmental Criteria and Assessment Office, Research 
Triangle Park, NC, EPA-600/8-82-029a-c.
EPA (1982b), Review of the National Ambient Air Quality Standards 
for Sulfur Oxides: Assessment of Scientific and Technical 
Information-OAQPS Staff Paper, Office of Air Quality Planning and 
Standards, Research Triangle Park, NC, EPA-450/5-82-007.
EPA (1986a), Second Addendum to Air Quality Criteria for Particulate 
Matter and Sulfur Oxides (1982): Assessment of Newly Available 
Health Effects Information, Environmental Criteria and Assessment 
Office, Research Triangle Park, NC, EPA-450/5-86-012.
EPA (1986b), Review of the National Ambient Air Quality Standards 
for Sulfur Oxides: Updated Assessment of Scientific and Technical 
Information, Addendum to the 1982 OAQPS Staff Paper, Office of Air 
Quality Planning and Standards, Research Triangle Park, NC EPA-450/
05-86-013.
EPA (1994a), 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, Environmental Criteria and Assessment Office, 
Research Triangle Park, NC, EPA/600/FP-93/002.
EPA (1994b), Review of the Ambient Air Quality Standards for Sulfur 
Oxides: Updated Assessment of Scientific and Technical Information, 
Supplement to the 1986 OAQPS Staff Paper Addendum, Office of Air 
Quality Planning and Standards, Research Triangle Park, NC, EPA/452/
R-94-013.
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. A-79-28, II-B-11.
Frank, N. U.S. EPA, Technical Support Division (1988), Memorandum to 
John Haines, Air Quality Management Division, January 5, 1988, 
Docket No. A-84-25, II-B-3.
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. A-79-28, II-A-15.

[[Page 25578]]

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. A-79-28, II-B-12.
Public Hearing (1995), Transcript of meeting held in Research 
Triangle Park, NC on February 8, 1995, Docket No. A-84-25, VIII-F-
17.
Rosenbaum, A.S.; Hudischewskyj, A.B.; Roberson, R.L.; Burton, C.S. 
(1992) Estimates of Future Exposures of Exercising Asthmatics to 
Short-term Elevated SO2 Concentrations Resulting from Emissions 
of U.S. Fossil-fueled Power Plants: Effects of the 1990 Amendments 
to the Clean Air Act and a 5-minute Average Ambient SO2 
Standard. Pub. No. SYSAPP-92/016, April 23, 1992.
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.
Sciences International, Inc. (1995), Estimate of the Nationwide 
Exercising Asthmatic Exposure Frequency to Short-Term Peak Sulfur 
Dioxide Concentrations in the Vicinity of Non-Utility Sources. 
Alexandria, VA, Docket No. A-84-25, VIII-D-71.
Stoeckenius, T.E. (1995), Technical Review of NMA's Report on: 
`Estimate of the Nationwide Exercising Asthmatic Exposure Frequency 
to Short-Term Peak Sulfur Dioxide Concentrations in the Vicinity of 
Non-Utility Sources.' ENVIRON International Corporation, Novato, CA, 
Docket No. A-84-25, VIII-A-01.
Stoeckenius, T.E.; Garelick, B.; Austin, B.S.; O'Connor, K.; 
Pehling, J.R. (1990). Estimates of Nationwide Asthmatic Exposures to 
Short-term Sulfur Dioxide Concentrations in the Vicinity of Non-
Utility Sources. Systems Applications Inc., San Rafael, CA, Pub. No. 
SYSAPP-90/129, December 6, 1990.
Systems Applications International (1996) Summary of 1988-1995 
Ambient 5-Minute SO2 Concentration Data. Systems Applications 
International, Research Triangle Park, NC, Docket No. A-84-25, VIII-
A-02.

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

[[Page 25579]]

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 Street, 
SW., 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.

    For the reasons set forth in the preamble, chapter I of title 40 of 
the Code of Federal Regulations is 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 shall 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 
shall be rounded up).

[[Page 25580]]

    (c) Sulfur oxides shall be measured in the ambient air as sulfur 
dioxide by the reference method described in Appendix A to this part or 
by an equivalent method designated in accordance with part 53 of this 
chapter.
    (d) 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 fewer 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 shall 
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.

[FR Doc. 96-12863 Filed 5-21-96; 8:45 am]
BILLING CODE 6560-50-P