[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
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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
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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 SO