[Federal Register Volume 59, Number 219 (Tuesday, November 15, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-27646]
[[Page Unknown]]
[Federal Register: November 15, 1994]
_______________________________________________________________________
Part III
Environmental Protection Agency
_______________________________________________________________________
40 CFR Parts 50 and 53
National Ambient Air Quality Standards for Sulfur Oxides (Sulfur
Dioxide)--Reproposal; Proposed Rule
ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50 and 53
[AD-FDL-5103-1]
RIN 2060-AA61
National Ambient Air Quality Standards for Sulfur Oxides (Sulfur
Dioxide)--Reproposal
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
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SUMMARY: The EPA today is proposing not to revise the current 24-hour
and annual primary standards but is also soliciting comment on the
possible need to adopt additional regulatory measures to address short-
term peak (SO2) exposures and thereby further reduce the health
risk to exercising asthmatic individuals. The alternatives under
consideration include: revising the existing national ambient air
quality standards (NAAQS) by adding a new 5-minute standard of 0.60
ppm, 1 expected exceedance; establishing a new regulatory program under
section 303 of the Clean Air Act to supplement the protection provided
by the existing NAAQS; and augmenting implementation of the existing
standards by focusing on those sources or source types likely to
produce high 5-minute peak SO2 concentrations.
Included in this document are proposals to incorporate certain
associated technical changes to the requirements for Ambient Air
Monitoring Reference and Equivalent Methods (40 CFR part 53) and other
minor technical changes regarding the 40 CFR part 50 regulations.
A related document will be published shortly in the Federal
Register that proposes for comment the requirements for implementing
the alternative regulatory measures. Included in that document are
technical revisions to 40 CFR parts 51 and 58.
DATES: Written comments on this proposal must be received by February
13, 1995. The EPA will hold a public hearing on this notice in
approximately 30 days. The time and place will be announced in a
subsequent Federal Register document.
ADDRESSES: Submit comments on the proposed action on the NAAQS (40 CFR
part 50) (duplicate copies are preferred) to: Air & Radiation Docket
Information Center (6102), Room M-1500, Environmental Protection
Agency, Attn: Docket No. A-84-25, 401 M Street, SW., Washington, DC
20460. Comments on the proposed revisions to the Ambient Air Monitoring
Reference and Equivalent Methods (40 CFR part 53) should be separated
from those pertaining to the standards and sent to the same address,
Attn: Docket No. A-94-42. These dockets are located in the Central
Docket Section of the U.S. Environmental Protection Agency, South
Conference Center, Room M-1500, 401 M St., SW., Washington, DC. The
docket may be inspected between 8 a.m. and 5:30 p.m. on weekdays, and a
reasonable fee may be charged for copying. For the availability of
related information, see the Supplementary Information section.
FOR FURTHER INFORMATION CONTACT: Part 50 Notice--Mr. John H. Haines,
Air Quality Strategies and Standards Division (MD-12), U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711,
telephone (919) 541-5533. Part 53 Notice--Mr. Frank McElroy,
Atmospheric Research and Exposure Assessment Laboratory (MD-77), U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711,
telephone (919) 541-2622.
SUPPLEMENTARY INFORMATION:
Background
In 1971, the EPA promulgated primary and secondary NAAQS for sulfur
oxides (measured as SO2). The primary standards were set at 365
micrograms per cubic meter (g/m\3\) (0.14 part per million
(ppm)), averaged over a 24-hour period and not to be exceeded more than
once per year, and 80 g/m\3\ (0.030 ppm) annual arithmetic
mean. The secondary standard was set at 1300 g/m\3\ (0.5 ppm)
averaged over a period of 3 hours and not to be exceeded more than once
per year. In accordance with sections 108 and 109 of the Act, EPA
reviewed and revised the health and welfare criteria upon which these
primary and secondary SO2 standards were based.
On April 26, 1988 (53 FR 14926), the EPA announced its proposed
decision not to revise these standards. In that notice, the
Administrator also solicited comment on an alternative of adding a 1-
hour primary standard of 0.4 ppm. The EPA also sought comment on
additional revisions in the event a 1-hour standard was promulgated. At
that time, the EPA also proposed to revise the significant harm levels,
associated episode contingency plan guidance (40 CFR part 51), and the
Pollutant Standard Index for SO2 (40 CFR part 58). The EPA also
proposed revisions to certain monitoring and reporting requirements (40
CFR part 58).
On April 21, 1993, the EPA announced its final decision that
revision of the secondary standard was not appropriate (58 FR 21351).
Availability of Related Information
The revised criteria document, Air Quality Criteria for Particulate
Matter and Sulfur Oxides (three volumes, EPA-600/8-82-029af-cf,
December 1982; Volume I, NTIS # PB-84-120401, $36.50 paper copy and
$9.00 microfiche; Volume II, NTIS # PB-84-120419, $77.00 paper copy and
$9.00 microfiche; Volume III, NTIS # PB-84-120427, $77.00 paper copy
and $20.50 microfiche); the criteria document addendum, Second Addendum
to Air Quality Criteria for Particulate Matter and Sulfur Oxides
(1982): Assessment of Newly Available Health Effects Information (EPA/
600/8-86-020-F, NTIS # PB-87-176574, $36.50 paper copy and $9.00
microfiche); the criteria document supplement, Supplement to the Second
Addendum (1986) to Air Quality Criteria for Particulate Matter and
Sulfur Oxides (1982): Assessment of New Findings on Sulfur Dioxide
Acute Exposure Health Effects in Asthmatic Individuals (1994) (EPA-600/
FP-93/002); the 1982 staff paper, Review of the National Ambient Air
Quality Standards for Sulfur Oxides: Assessment of Scientific and
Technical Information (EPA-450/5-82-007, November 1982; NTIS # PB-84-
102920, $36.50 paper copy and $9.00 microfiche); the staff paper
addendum, Review of the National Ambient Air Quality Standards for
Sulfur Oxides: Updated Assessment of Scientific and Technical
Information (EPA-450/05-86-013, December 1986; NTIS # PB-87-200259,
$19.50 paper copy and $9.00 microfiche) and the staff paper supplement,
Review of the National Ambient Air Quality Standards For Sulfur Oxides:
Updated Assessment of Scientific and Technical Information, Supplement
to the 1986 OAQPS Staff Paper Addendum (1994) (EPA-452/R-94-013) are
available from: U.S. Department of Commerce, National Technical
Information Service, 5285 Port Royal Road, Springfield, Virginia 22161,
or call 1-800-553-NTIS. (Add $3.00 handling charge per order.) A
limited number of copies of other documents generated in connection
with this standard review, such as the control techniques document, can
be obtained from: U.S. Environmental Protection Agency Library (MD-35),
Research Triangle Park, NC 27711, telephone (919) 541-2777. These and
other related documents are also available in the EPA dockets
identified above.
Table of Contents
I. Background
A. Legislative Requirements Affecting This Rule
1. The Primary Standards
2. Related Control Requirements
B. Sulfur Oxides and Existing Standards for SO2
C. Development of Revised Air Quality Criteria for Sulfur Oxides
and Review of the Standards: Development of the Staff Paper
D. Rulemaking Docket
II. Summary of the 1988 Proposed Decision Not to Revise the Current
Standards
III. Post-Proposal Developments
A. Opportunities for Public Comment
B. Legislative Activity
C. Litigation on Secondary Standard
D. Decision on Secondary Standard
E. Litigation on Primary Standard
F. Supplementation of the Criteria Document and the Staff Paper
IV. Summary of Public Comments as to Primary Standards and
Associated Technical Changes
A. Current 24-Hour and Annual Standards
B. Averaging Convention for the Current Standards
C. 1-Hour Standard Alternative
D. Other Changes to Standards
E. Technical Revisions to 40 CFR 50.4 and 50.5
V. Rationale for Proposed Decisions
A. Basis for the Current 24-Hour and Annual Standards
B. Consideration of Short-Term Peak SO2 Exposures
1. Assessment of Health Effects Associated With Short-Term
SO2 Exposures
2. Air Quality and Exposure Considerations
C. Regulatory Considerations
1. 5-Minute Standard Alternative
2. Section 303 Program
3. Retain Current Standards
D. Averaging Convention for the Current Standards
E. Form of the Current Standards
F. Other Technical Changes
VI. Federal Reference Methods and Equivalent Methods
VII. Regulatory Impacts
A. Regulatory Impacts Administrative Requirements
B. Impact on Small Entities
C. Reduction of Governmental Burden
D. Environmental Justice
E. Impact on Reporting Requirements
References
Appendix I
Appendix II
I. Background
A. Legislative Requirements Affecting This Rule
1. The Primary Standards
Two sections of the Act govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify pollutants which ``may reasonably be anticipated to endanger
public health or welfare'' and to issue air quality criteria for them.
These air quality criteria are to ``reflect the latest scientific
knowledge useful in indicating the kind and extent of all identifiable
effects on public health or welfare which may be expected from the
presence of (a) pollutant in the ambient air. * * *''
Section 109 (42 U.S.C. 7409) directs the Administrator to propose
and promulgate ``primary'' NAAQS for pollutants identified under
section 108. Section 109(b)(1) defines a primary standard as one ``the
attainment and maintenance of which, in the judgment of the
Administrator, based on the criteria and allowing an adequate margin of
safety, (is) requisite to protect the public health.''
The U.S. Court of Appeals for the D.C. Circuit has held that the
requirement for an adequate margin of safety for primary standards was
intended to address uncertainties associated with inconclusive
scientific and technical information available at the time of standard
setting. It was also intended to provide a reasonable degree of
protection against hazards that research has not yet identified. Lead
Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir. 1980),
cert. denied, 101 S. Ct. 621 (1980); American Petroleum Institute v.
Costle, 665 F.2d 1176, 1177 (D.C. Cir. 1981), cert. denied, 102 S. Ct.
1737 (1982). Both kinds of uncertainties are components of the risk
associated with pollution at levels below those at which human health
effects can be said to occur with reasonable scientific certainty.
Thus, by selecting primary standards that provide an adequate margin of
safety, the Administrator is seeking not only to prevent pollution
levels that have been demonstrated to be harmful, but also to prevent
lower pollutant levels that she finds pose an unacceptable risk of
harm, even if that risk is not precisely identified as to nature or
degree.
In selecting a margin of safety, the EPA has considered such
factors as the nature and severity of the health effects involved, the
size of the sensitive population(s) at risk, and the kind and degree of
the uncertainties that must be addressed. Given that the ``margin of
safety'' requirement by definition only comes into play where no
conclusive showing of harm exists, such factors, which involve unknown
or only partially quantified risks, have their inherent limits as
guides to action. The selection of any particular approach to providing
an adequate margin of safety is a policy choice left specifically to
the Administrator's judgment. Lead Industries Association v. EPA,
supra, 647 F.2d at 1161-62.
Section 109(d) of the Act (42 U.S.C. 7409(d)) requires periodic
review and, if appropriate, revision of existing criteria and
standards. The process by which the EPA has reviewed the original
criteria and standards for sulfur oxides under section 109(d) is
described in a later section of this notice.
2. Related Control Requirements
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under section 110 (42 U.S.C. 7410) and part D of
title I of the Act (42 U.S.C. 7501-7515), States are to submit, for EPA
approval, State implementation plans (SIP's) that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved. The States, in
conjunction with the EPA, also administer the prevention of significant
deterioration program (42 U.S.C. 7470-7479) for these pollutants. In
addition, Federal programs provide for nationwide reductions in
emissions of these and other air pollutants through the Federal motor
vehicle control program under title II of the Act (42 U.S.C. 7521-
7574), which involves controls for automobile, truck, bus, motorcycle,
and aircraft emissions; new source performance standards under section
111 (42 U.S.C. 7411); National Emission Standards for Hazardous Air
Pollutants under section 112 (42 U.S.C. 7412); and title IV of the
Clean Air Act Amendments of 1990 (42 U.S.C. 7651-76510), which
specifically provides for major reductions in SO2 emissions.
B. Sulfur Oxides and Existing Standards for SO2
The principal focus of this standard review is on the health
effects of SO2, alone and in combination with other pollutants.
Other sulfur oxide (SOX) vapors (e.g., sulfur trioxide, SO3)
are not commonly found in the atmosphere. Information on the effects of
the principal atmospheric transformation products of SO2 (i.e.,
sulfuric acid and sulfates) was considered in the review of the
particulate matter standards and addressed in the revisions to these
standards promulgated on July 1, 1987 (52 FR 24634); it will be
considered again in the next review of the particulate matter
standards, the commencement of which was announced on April 12, 1994
(59 FR 17375).
Sulfur dioxide is a rapidly diffusing reactive gas that is very
soluble in water. It is emitted principally from combustion or
processing of sulfur-containing fossil fuels and ores. Sulfur dioxide
occurs in the atmosphere with a variety of particles and other gases,
and undergoes chemical and physical interactions with them forming
sulfates and other transformation products. At elevated concentrations,
SO2 can adversely affect human health. Annual average SO2
levels range from less than 0.004 ppm in remote rural sites to over
0.03 ppm in the most polluted urban industrial areas. The highest
short-term values are found in the vicinity (<20 km)="" of="" major="" point="" sources.="" in="" the="" absence="" of="" adequate="" controls,="" maximum="" levels="" at="" such="" sites="" for="" 24-hour,="" 3-hour,="" and="" 1-hour="" averages="" can="" reach="" or="" exceed="" 0.4="" ppm,="" 1.4="" ppm,="" and="" 2.3="" ppm,="" respectively.="" the="" origins,="" relevant="" concentrations="" and="" potential="" effects="" of="">2 are discussed in more
detail in the revised criteria document (EPA, 1982a), in the staff
paper (EPA, 1982b), in the criteria document addendum (EPA, 1986a), and
the staff paper addendum (EPA, 1986b).
On April 30, 1971, the EPA promulgated the primary NAAQS for
SO2 under section 109 of the Act (36 FR 8186). The existing
primary standards for sulfur oxides, measured as SO2, are 365
g/m3 (0.14 ppm), averaged over a period of 24 hours and
not to be exceeded more than once per year, and 80 g/m3
(0.030 ppm) annual arithmetic mean. The scientific and technical bases
for the current standards are contained in the original criteria
document, Air Quality Criteria for Sulfur Oxides (DHEW, 1970).
Implementation of SO2 air quality standards by the States and
the EPA, together with fuel use shifts and siting decisions motivated
by changing economic conditions, have resulted in substantial
improvements in ground level air quality. Annual emissions decreased
significantly between 1975 and 1982, from 25.7 to 21.4 million metric
tons/year. During the mid to late eighties and early nineties, however,
annual emissions of SO2 have remained basically the same, at
approximately 20.6 million metric tons/year (EPA, 1993a).
Title IV of the Act, the acid rain program, requires that electric
utilities reduce annual SO2 emissions by 10 million short tons (9
million metric tons) per year from the 1980 baseline of 23.3 million
metric tons. This reduction will be implemented in two phases. The
phase 1 reductions are to be accomplished by 1995, and the bulk of the
phase 2 reductions are to be accomplished by the year 2000, with an
expected annual emission rate of 16.38 million metric tons that year.
Total expected reductions from title IV will result in an annual
emission rate of 14.22 metric tons in the year 2015.
Ambient air SO2 trends over the decade from 1983 to 1992 show
a definite downward trend, though the rate of decline has slowed over
the last few years. Annual mean SO2 decreased at a median rate of
approximately 2 percent per year, resulting in a total drop of 23
percent. The annual second highest 24-hour values over this same time
period decreased 31 percent, at an average rate of 4 percent per year
(EPA, 1993a). The most recent trends of SO2 measured in the
ambient air have continued to show improvement. Annual mean
concentrations decreased a total of 11 percent between 1990 to 1992.
Over the last 2 years, the average annual mean SO2 decrease was 7
percent. Second maximum 24-hour SO2 concentrations declined 12
percent between 1990 and 1992 and 4 percent between 1991 and 1992 (EPA,
1993a).
C. Development of Revised Air Quality Criteria for Sulfur Oxides and
Review of the Standards: Development of the Staff Paper
On October 2, 1979, the EPA announced it was revising the original
criteria document for sulfur oxides concurrently with that for
particulate matter to produce a combined particulate matter/sulfur
oxides (PM/SOx) criteria document (44 FR 56731). A more complete
history of the revisions and addenda to the criteria document and staff
paper, as well as the text of all CASAC closure letters, is presented
in the 1988 proposal (53 FR 14926, April 26, 1988). A brief synopsis
appears below.
The EPA provided a number of opportunities for review and comment
on the revised criteria document by organizations and individuals
outside the Agency. Three drafts of the revised criteria document,
prepared by the EPA's Environmental Criteria and Assessment Office
(ECAO), were made available for external review (45 FR 24913, April 11,
1980; 46 FR 9746, Jan. 29, 1981; 46 FR 53210, Oct. 28, 1981). The EPA
received and considered numerous and often extensive comments on each
of these drafts, and CASAC has held three public meetings (August 20-
22, 1980; July 7-9, 1981; November 16-18, 1981) to review successive
drafts of the document. Transcripts of these meetings have been placed
in the docket for the criteria document (ECAO CD 79-1). In addition,
five public workshops were held at which the EPA, its consulting
authors and reviewers, and other scientifically and technically
qualified experts selected by the EPA discussed the various chapters of
the draft document and suggested ways of resolving outstanding issues
(45 FR 74047, Nov. 7, 1980; 45 FR 76790, Nov. 20, 1980; 45 FR 78224,
Nov. 26, 1980; 45 FR 80350, Dec. 4, 1980; 46 FR 1775, Jan. 7, 1981).
The comments received were considered in the preparation of the final
document. A CASAC ``closure'' memorandum indicating the Committee's
satisfaction with the final draft of the criteria document and
outlining key issues and recommendations was issued in December 1981.
Following closure, a number of scientific articles were published,
or accepted for publication, that appeared to be of sufficient
importance to the development of criteria for the primary standards for
SO2 to necessitate an addendum to the criteria document. Two
drafts of the addendum were reviewed by CASAC and members of the public
in two public meetings (April 26-27, 1982; August 30-31, 1982), and
transcripts of the meetings have been placed in the docket. The
addendum was included as Appendix A to Volume I of the criteria
document (EPA, 1982a) when the document was issued on March 20, 1984
with the proposed revisions to the ambient air quality standards for
particulate matter (49 FR 10408, Mar. 20, 1984).
As part of this process, the EPA's Office of Air Quality Planning
and Standards (OAQPS) in the spring of 1982 prepared the first draft of
a staff paper, ``Review of the National Ambient Air Quality Standards
for Sulfur Oxides: Assessment of Scientific and Technical Information-
OAQPS Staff Paper.'' The first draft and a second draft of the staff
paper were reviewed at CASAC meetings on April 26-27, 1982 (47 FR
16885, April 20, 1982), and August 30-31, 1982 (47 FR 34855, Aug. 10,
1982), respectively, and transcripts of these meetings have been placed
in the docket (Docket No. A-79-28). Numerous written and oral comments
were received on the drafts from CASAC, representatives of
organizations, individual scientists, and other interested members of
the public, and some revisions engendered by these comments are
discussed in an August 5, 1982 letter to CASAC (Padgett, 1982), as well
as the executive summary of the staff paper. The EPA released the final
OAQPS staff paper (EPA, 1982b), upon receipt of the formal CASAC
closure letter in August 1983 (Goldstein, 1983), accompanied by a
minority statement by one member (Higgins, 1983).
In 1984, the Administrator reviewed the standards in light of the
above information and decided, at that time, not to propose any
revision of the standards.
In 1986, in response to the publication in the scientific
literature of a number of additional studies on the health effects of
SO2 (as well as some new particulate matter studies), ECAO
commenced a second addendum to the PM/SOX criteria document (51 FR
11058, Apr. 1, 1986). An external review draft was made available for
public comment (51 FR 24392, Jul. 3, 1986) and CASAC held a public
meeting on October 15-16, 1986 to review the criteria document addendum
(transcript in public docket No. A-82-37). When development of a second
addendum of the criteria document was initiated in 1986, OAQPS decided
to simultaneously commence an addendum to the staff paper as well (51
FR 24392, Jul. 3, 1986). An external review draft of the addendum to
the staff paper was also issued, and the staff paper was reviewed at
the same public CASAC meeting at which the second addendum to the
criteria document was considered.
The CASAC sent a closure letter on the criteria document addendum
to the Administrator dated December 15, 1986, and another on the staff
paper, dated February 1987. The closure letter on the staff paper
addendum, which also discusses major issues addressed by the CASAC and
the Committee's recommendations, is reprinted in Appendix 1 to this
notice. The final addenda to the criteria document (EPA, 1986a) and the
staff paper (EPA, 1986b), are available from the address listed above.
Where there are differences between the 1982 criteria document and
staff paper and the more recent addenda, the addenda supersede the
earlier documents.
D. Rulemaking Docket
The EPA established a standard review docket for the sulfur oxides
review in July 1979. The EPA also established a rulemaking docket
(Docket No. A-84-25) for the April 26, 1988 proposal as required by
section 307(d) of the Act. The standard review docket (Docket No. A-79-
28) and a separate docket established for criteria document revision
(Docket No. ECAO-CD-79-1) have been incorporated into the rulemaking
docket.
II. Summary of the 1988 Proposed Decision Not To Revise the Current
Standards
On April 26, 1988 (53 FR 14926), the EPA announced its proposed
decision not to revise the existing primary and secondary SOX
standards (measured as SO2). In reaching the provisional
conclusion that the current standards provided adequate protection
against the health and welfare effects associated with SO2, the
EPA was mindful of uncertainties in the available evidence concerning
the risk that elevated short-term (<1-hour)>2 concentrations
pose to asthmatic individuals exercising in ambient air. Therefore, the
EPA specifically requested broad public comment on the alternative of
revising the current standards and adding a new 1-hour primary standard
of 0.4 ppm. The notice also announced that if a 1-hour primary standard
were adopted, consideration would be given to replacing the current 3-
hour secondary standard (1,300 g/m3 (0.5 ppm)) with a 1-
hour secondary standard set equal to the primary standard, and adopting
an expected-exceedance form for all of the standards.
The EPA also concluded in the April 26, 1988 notice, based upon the
then-current scientific understanding of the acidic deposition problem,
that it would not be appropriate, at that time, to propose a separate
secondary SOX standard to provide increased protection against the
acidic deposition-related effects of SOX. The notice added that
when the fundamental scientific uncertainties had been reduced through
ongoing research activities, the EPA would draft and support an
appropriate set of control measures.
The EPA also proposed minor technical revisions to the standards,
including restating the levels for the primary and secondary standards
in terms of ppm rather than g/m3, adding explicit
rounding conventions, and specifying data completeness and handling
conventions. The EPA also announced its intention to retain the block
averaging convention for the 24-hour, annual, and 3-hour standards and
proposed to eliminate any future questions in this regard by adding
clarifying language to 40 CFR 50.4 and 50.5. Based on its assessment of
the SO2 health effects information, the EPA also proposed to
revise the significant harm levels for SO2 and the associated
example air pollution episode levels (40 CFR part 51). Finally, the EPA
proposed some minor modifications to the ambient air quality
surveillance requirements (40 CFR part 58).
The April 26, 1988 (53 FR 14926) notice sets forth in detail the
rationale for the proposals discussed above and provides other
background information.
III. Post-Proposal Developments
A. Opportunities for Public Comment
Following the publication of the proposal, the EPA held a public
meeting in Washington on June 10, 1988 to receive comment on the April
26, 1988 proposal. A transcript of the meeting has been placed in the
public docket (Docket No. A-84-25). On July 20, 1988, the EPA announced
an extension of the public comment period from July 25, 1988 to
September 23, 1988 (53 FR 27362). The EPA issued a second notice on
September 21, 1988 (53 FR 36587) to clarify that issues concerning
block versus running averaging conventions should be fully aired in the
sulfur dioxide rulemaking initiated by the April 26, 1988 notice (53 FR
14926). At the same time, the EPA extended the comment period until
November 22, 1988 to provide ample opportunity for the public to
comment.
B. Legislative Activity
In July 1989, legislative proposals for amending the Act were
submitted to Congress. This initiative included a comprehensive program
to address the acidic deposition problem. After extensive deliberation,
the 1990 Amendments, including the title IV acid rain provisions, were
passed by Congress and signed into law by the President on November 15,
1990. As discussed earlier in section I.B., and below, title IV of the
1990 Amendments was developed specifically to address the acidic
deposition problem but will have an attendant benefit of reducing
SO2-related health effects.
C. Litigation on Secondary Standard
Prior to the 1988 proposal, the Environmental Defense Fund and
other plaintiffs had sued the EPA under section 304 of the Act to
compel review and revision of the NAAQS for SOX under section
109(d)(1) of the Act, Environmental Defense Fund v. Reilly, No. 85 C.V.
9507 (S.D.N.Y.). In response to a decision of the U.S. Court of Appeals
for the Second Circuit in 1989, Environmental Defense Fund v. Thomas,
870 F.2d 892 (2d Cir. 1989), the EPA and the plaintiffs ultimately
entered into a consent decree as an alternative to further litigation.
The decree required the EPA to take final action by April 15, 1993 on
the secondary standard portion of the 1988 proposed rulemaking.
D. Decision on Secondary Standard
A final decision under section 109(d)(1) of the Act that revision
of the secondary standard was not appropriate was signed on April 15,
1993 and was published in the Federal Register on April 21, 1993 (58 FR
21351). The rationale for the decision is set forth in the April 21,
1993 notice. At that time it was also announced that when action was
completed on the primary standards portion of the 1988 proposal, the
EPA would decide whether to adopt minor technical changes discussed in
the 1988 proposal.
E. Litigation on Primary Standard
In 1992, the American Lung Association sued the EPA to compel
review and, if appropriate, revision of the primary standards for
SOX, American Lung Association v. Browner, No. 92-CV-5316 (ERK)
(E.D.N.Y.). The U.S. District Court for the Eastern District of New
York subsequently issued an order requiring that the EPA by November 1,
1994: take final action on the 1988 proposed decision not to revise the
primary standards, or repropose and take final action on the reproposal
within 1 year after the close of the public comment period.
F. Supplementation of the Criteria Document and the Staff Paper
In response to the more recent publication of controlled human
studies on the health effects of short-term peaks of SO2 on
asthmatic individuals, the ECAO commenced preparation of a supplement
to the second addendum to the PM/SO2 criteria document in 1992.
The OAQPS prepared a draft of a supplement to the staff paper addendum
to update its assessment of the new information contained in the
Criteria Document Supplement and to take into account more recent air
quality and exposure information. Initial drafts of these documents
were completed in June, 1993. The EPA announced the availability of an
external review draft of both documents for public comment on July 30,
1993 (58 FR 40818), and the documents were reviewed by the CASAC at a
public meeting on August 19, 1993. Recommended changes were made, and
revised drafts of both documents were made available for public comment
(59 FR 11985, March 15, 1994). Both documents were reviewed at a public
CASAC meeting on April 12, 1994. The CASAC provided its advice and
recommendations to the Administrator in a letter dated June 1, 1994
that is reprinted in Appendix 2.
IV. Summary of Public Comments as to Primary Standards and Associated
Technical Changes
The following discussion summarizes in general terms the comments
received from the public regarding the key aspects of the April 26,
1988 notice as they pertain to the primary standards and associated
technical changes. The individual comments have been entered into the
public docket (Docket No. A-84-25). For a summary of public comments on
the secondary standard, see 58 FR 21354, Apr. 21, 1993.
Extensive written comments were received on the 1988 proposal. Of
some 90 written submissions, 33 were provided by individual industrial
concerns or industry groups, 14 by State, local and Federal government
agencies and organizations, 14 by environmental and public interest
groups, and 29 by individual private citizens.\1\ The comments on the
key aspects of the April 26, 1988 notice pertaining to the primary
standard and associated part 50 technical changes are summarized below.
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\1\The numerical distribution of comments in each category
should be viewed with caution. Industry groups typically submit
comments on behalf of their member companies in lieu of having each
of their member companies sending separate comments. Similarly,
comments from environmental or other interest groups represent the
views of a number of individuals.
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A. Current 24-Hour and Annual Standards
Virtually all of the comments that specifically addressed the
adequacy of the current standards supported the Administrator's 1988
finding that the current primary SO2 standards are adequate to
protect the public health from the effects associated with 24-hour and
annual average SO2 concentrations in the atmosphere. As discussed
below, the principal exceptions were the comments submitted on the
issue of the averaging convention of the standards. These commenters
maintained that the current primary standards would not provide
adequate protection against adverse health effects if measurements of
the currently prescribed concentration levels were restricted to the
block averaging convention.
B. Averaging Convention for the Current Standards
Comments on the Administrator's decision to retain the block
averaging convention for the 3-hour, 24-hour, and annual standards were
sharply divided. The industry comments on this issue strongly supported
the proposed decision to retain the block averaging convention as the
appropriate method for determining compliance with the current
standards. In support of this position, these commenters typically took
note of the text of the 1971 promulgation notice, the Air Quality
Criteria for Sulfur Oxides (DHEW, 1970), contemporaneous papers that
discussed how the measurements were to be collected and analyzed, and
the fact that implementation of the standards for the most part has
been based on block averaging. The environmental groups maintained,
however, that the wording of the original standards clearly did not
preclude the use of the running averaging convention; that the EPA's
monitoring capabilities, guidance, and implementation practice
demonstrated that the standards were not restricted to block averaging;
and accordingly that the use of running averaging would not represent a
tightening of the standards. Several State agencies supported the
adoption of a running interpretation or requested that the EPA remain
silent so as not to undercut the States' use of running averages, while
other States and municipalities supported the EPA's proposed decision.
C. 1-Hour Standard Alternative
Discussion on this subject was highly polarized. Industry groups
and their representatives uniformly opposed a short-term standard,
while environmental groups, private citizens, and most State and local
agencies that commented strongly favored the adoption of such a
standard. Industry maintained that the clinical studies of asthmatics
used to support the possible need for a short-term standard failed to
show effects that were of such medical significance as to be considered
``adverse'' under the Act. Environmental groups argued that the effects
seen were medically significant and ``adverse'' at concentrations below
0.5 ppm and called for a standard to be set at levels considerably
below the 0.4 ppm, 1-hour alternative that was presented for comment.
The nature of the comments were such that there was virtually no
consensus over the significance of effects among industry,
environmental groups, and the different medical experts that commented
on the issue.
In support of their position that a short-term standard was not
needed, industry groups placed great weight on the results of the
exposure analysis presented in the April 26, 1988 notice. They
maintained that the analysis demonstrated that the current standards
provided considerable protection against short-term peak exposures and
that the remaining risk did not pose a significant public health
problem. Some environmental groups took exception to the EPA's use of
the exposure analysis. They maintained that a large under-counting of
exposures occurred because the analysis did not address potential
exposures from nonutility sources such as nonferrous smelters, paper
mills, and petroleum refineries. Some also argued that the EPA's
reliance on the exposure analysis as a basis for retaining the existing
standards was without legal authority. These commenters were also
critical of the Agency's use of typical activity patterns and
maintained that other aspects of the analysis were deficient. Industry
groups generally supported the use of exposure analyses in the standard
setting process and maintained that the EPA's focus on utilities was
appropriate given that they are the largest emitters of SO2.
Environmental groups and private citizens also expressed concern
that the significance of asthma episodes were being downplayed and
raised concerns about exposures of children, who were dependent on
adults for medication and care. They were also highly critical of the
EPA's characterization of the number of asthmatics (up to 100,000)
potentially at risk to SO2 peak exposures as small.
State and local agencies that commented mostly supported the
adoption of a short-term 1-hour standard.
Finally, environmental groups maintained that the 1-hour
alternative would not protect against short-term 2- to 10-minute peak
SO2 concentrations. In support of their position, data were
submitted showing that certain types of SO2 sources may have very
high 5-minute peaks (>1 ppm) and still have hourly averages below 0.4
ppm even when the current standards are being attained. One of the
industry commenters also noted that an averaging time shorter than 1
hour would be needed to protect against very high 3- to 5-minute peak
SO2 levels and cited an instance where a 3- to 5-minute peak of
3.7 ppm SO2 occurred, yet the 1-hour average was only 0.29 ppm.
This commenter went on to suggest, however, that such problems would be
better addressed through a properly designed program under the
authority of section 303 of the Act rather than through the adoption of
a new short-term ambient air quality standard.
D. Other Changes to Standards
While a number of commenters favored the adoption of a new 1-hour
standard, little, if any, support was voiced for the associated
revisions that the EPA indicated it was considering if a 1-hour
standard was adopted. Few, if any, commenters supported the adoption of
an expected exceedance form for all of the standards. While several
commenters recognized that a statistical form had certain technical
advantages, they expressed concern that its adoption would reduce the
protection afforded by the current 3-hour, 24-hour and annual
standards.
E. Technical Revisions to 40 CFR 50.4 and 50.5
There was general support for the EPA's proposal to restate the
levels of the standards in terms of ppm rather than g/m\3\ and
for adding explicit rounding conventions and data completeness and
handling conventions to the regulations.
V. Rationale for Proposed Decisions
A. Basis for the Current 24-Hour and Annual Standards
The rationale for retaining the current 24-hour and annual primary
standards was presented in some detail in the 1988 proposal (53 FR
14930, Apr. 26, 1988) and remains unchanged. At that time, the EPA
concluded that the current 24-hour and annual standards appeared to be
both necessary and adequate to protect human health against SO2
concentrations associated with those averaging periods. The EPA also
concluded that retaining the current 24-hour and annual standards was
consistent with the scientific data assessed in the criteria document
and staff paper and their addenda and with the advice and
recommendations of the staff and the CASAC.
The EPA again provisionally concludes, based on the information
assessed in the criteria document and staff paper and their addenda,
that the current 24-hour and annual primary standards provide adequate
health protection against the effects associated with those averaging
periods. In reaching this proposed decision, the EPA takes note that
the health effects information on 24-hour and annual SO2 exposures
remains largely unchanged since 1988. When newer information becomes
available and has undergone the rigorous and comprehensive assessment,
including CASAC review, necessary for incorporation into a new criteria
document, it will provide the basis for the next periodic review of the
24-hour and annual primary standards.
B. Consideration of Short-Term Peak SO2 Exposures
A number of new studies have become available since 1988 that
examine the potential health effects on asthmatic individuals
associated with short-term (1-hour) exposures to SO2.
In view of these new studies and other relevant new information, the
EPA prepared a ``Supplement to the Second Addendum (1986) to Air
Quality Criteria for Particulate Matter and Sulfur Oxides (1982):
Assessment of New Findings on Sulfur-Dioxide Acute Exposure Health
Effects in Asthmatic Individuals'' (``Criteria Document Supplement'')
(EPA, 1994a) and an associated staff paper supplement ``Review of the
National Ambient Air Quality Standards for Sulfur Oxides: Updated
Assessment of Scientific and Technical Information--Supplement to the
1986 OAQPS Staff Paper Addendum'' (``Staff Paper Supplement'') (EPA,
1994b). These two documents, together with the 1986 addenda, provide
the primary basis for the EPA's present assessment of the health
effects and related information on short-term SO2 exposures and
the Administrator's consideration of appropriate regulatory responses.
The discussion below summarizes the basis for considering alternative
regulatory responses to address the potential effects associated with
short-term peak SO2 exposures.
1. Assessment of Health Effects Associated With Short-Term SO2
Exposures
a. Sensitive Populations. It is clear that healthy nonasthmatic
individuals are essentially unaffected by acute exposures to SO2
at concentrations below 2 ppm and do not constitute a population of
concern for short-term, acute SO2 exposure effects.
Based on the assessment in the Criteria Document Supplement (EPA,
1994a), the EPA concludes that mild and moderate asthmatic children,
adolescents, and adults that are physically active outdoors represent
the population segments at most risk for acute SO2 induced
respiratory effects. Individuals with more severe asthmatic conditions
have poor exercise tolerances; as a result, they are very unlikely to
engage in sufficiently intense outdoor activity to achieve the
requisite breathing rates for SO2-induced respiratory effects to
occur and therefore maybe at somewhat lower risk. While current studies
are suggestive of greater SO2 responsiveness among those asthmatic
patients with more severe disease, this issue cannot be unequivocally
resolved. However, because of the lower baseline function in moderate
and severe asthmatic persons, especially those lacking optimal
medication, any effect of SO2 would further reduce their lung
function toward levels that may become cause for medical concern (EPA,
1994a, p. 44).
While it has been suggested that nonasthmatic atopic individuals
may also represent a broader population group at increased risk (White,
1994; 53 FR 14931-14932, Apr. 26, 1988), other assessments have not
found evidence establishing the atopic group to be particularly
responsive to SO2 (EPA, 1994a, p. 52; EPA, 1994b, p. 10; Linn et
al., 1987).
b. Asthma. About 10 million people or 4 percent of the population
of the United States are estimated to have asthma (NIH, 1991). The true
prevalence may be as high as 7 to 10 percent of the population (Evans
et al., 1987), because some individuals with mild asthma may be unaware
that they have the disease and thus go unreported. The prevalence is
higher among African-Americans, older (8- to 11-year-old) children, and
urban residents (Schwartz et al., 1990).
The Expert Panel Report from the National Asthma Education Program
of the National Heart, Lung and Blood Institute (NIH, 1991) has
recently defined asthma as ``a lung disease with the following
characteristics: (1) Airway obstruction that is reversible (but not
completely so in some patients) either spontaneously or with treatment,
(2) airway inflammation, and (3) increased airway responsiveness to a
variety of stimuli.'' Common symptoms include cough, wheezing,
shortness of breath, chest tightness, and sputum production. Asthma is
characterized by an exaggerated bronchoconstrictor response to many
physical challenges (e.g., cold or dry air, exercise) and chemical and
pharmacologic agents (e.g., histamine or methacholine).
Daily variability in lung function measurements is a typical
feature of asthma, with the poorest function (i.e., lowest forced
expiratory volume in 1 second (FEV1) and highest specific airway
resistance (SRaw) being experienced in the early morning hours and the
best function (i.e., highest FEV1 and lowest SRaw) occurring in
the mid- afternoon.
The degree of exercise tolerance varies with the severity of
disease. Mild asthmatic individuals have good exercise tolerance but
may not tolerate vigorous exercise such as prolonged running. Moderate
asthmatic individuals have diminished exercise tolerance and
individuals with severe disease have very poor exercise tolerance that
markedly limits physical activity.
Exercise-induced bronchoconstriction is followed by a refractory
period of several hours during which an asthmatic individual is less
susceptible to bronchoconstriction (Edmunds et al., 1978). This
refractory period may alter an asthmatic individual's responsiveness to
SO2 or other inhaled substances.
Data from the United Kingdom and United States suggest an incidence
rate of asthma attacks requiring medical attention of <1 asthmatic="" patient-year.="" it="" is="" estimated="" that="" the="" incidence="" rate="" of="" hospitalization="" due="" to="" asthma="" for="" all="" asthmatic="" individuals="" in="" the="" united="" states="" is="" about="" 45="" per="" 1,000="" asthmatics="" per="" year="" (nih,="" 1991).="" death="" due="" to="" asthma="" is="" a="" rare="" event:="" about="" one="" per="" 10,000="" asthmatic="" individuals="" per="" year.="" mortality="" rates="" are="" higher="" among="" males="" and="" about="" 100="" percent="" higher="" among="" nonwhites="" (epa,="" 1994a).="" in="" assessing="" the="" results="" from="" the="" controlled="" human="" exposure="" studies,="" it="" should="" be="" noted="" that="" the="" individuals="" who="" participate="" in="" such="" studies="" typically="" have="" mild="" allergic="" asthma="" and="" can="" go="" without="" medication="" altogether="" or="" can="" discontinue="" medication="" for="" brief="" periods="" of="" time="" if="" exposures="" are="" conducted="" outside="" their="" normal="" allergy="" season.="" in="" addition,="" african-american="" and="" hispanic="" adolescents="" and="" young="" adults="" have="" not="" been="" studied="" systematically.="" finally,="" subjects="" who="" participate="" in="" controlled="" exposure="" studies="" are="" also="" generally="" self-selected="" and="" this="" may="" introduce="" some="" bias.="" thus,="" the="" extent="" to="" which="" the="" participants="" in="" the="" studies="" reflect="" the="" characteristics="" of="" the="" asthmatic="" population="" at="" large="" is="" not="" known.="" nevertheless,="" the="" high="" degree="" of="" consistency="" among="" studies="" suggests="" that="" the="" subjects="" are="" generally="" representative="" of="" the="" population="" at="" risk="" or="" that="" any="" selection="" bias="" is="" consistently="" present="" across="" a="" diverse="" group="" of="" laboratories="" (epa,="" 1994a).="" c.="" short-term="" health="" effects.="" the="" basis="" for="" considering="" whether="" additional="" regulatory="" measures="" are="" needed="" to="" reduce="" the="" occurrence="" of="" short-term="" peaks="" of="">2 rests primarily on the extensive
literature involving brief (2- to 10-min) controlled exposures of
persons with mild (and in some cases more moderate) asthma to
concentrations of SO2 in the range of 0.1 ppm to 2 ppm while at
elevated ventilation. The major effect of SO2 on sensitive
asthmatic individuals is bronchoconstriction, usually evidenced in
these studies by increased specific airway resistance (SRaw) or
decreased forced expiratory volume (FEV1), and the occurrence of
clinical symptoms such as wheezing, chest tightness, and shortness of
breath. The magnitude of the response and likely occurrence of symptoms
increase at higher SO2 concentrations and ventilation levels and
are relatively brief in duration. Numerous studies have shown that lung
function typically returns to normal for most subjects within an hour
of exposure. No substantial ``late phase'' responses have been noted
for SO2, unlike the case for more specific stimuli (e.g., pollen,
dust mites, or other allergens) in which ``late phase'' inflammatory
responses often occur 4-8 hours after exposure and are often much more
severe and dangerous than earlier immediate responses.
In a summary of the literature up to 1986 in the Staff Paper
Addendum (EPA, 1986b), the staff concluded that changes in lung
function ( SRaw 70 percent) accompanied by symptoms could be
observed in some free-breathing asthmatics at 0.4 ppm at ``moderate-
heavy exercise.'' At 0.5 ppm, slightly larger functional changes on
individual and group basis were seen at moderate exercise (
SRaw 50--100 percent), while at 0.6-0.75 ppm SO2 functional
changes and symptoms could be observed at light-moderate exercise
( SRaw 120-260 percent), with the effects being judged
``indicative of clinical significance.'' Effects at 1-2 ppm SO2
were seen as even more pronounced, ranging from ``moderate'' to
``incapacitating'' for some individuals (53 FR 14948, April 26, 1988).
As the concentration increases within the range studied, effects are
more pronounced and the fraction of asthmatic subjects who respond
increases (53 FR 14947, April 26, 1988).
Since 1986 several new studies have been published providing
pertinent information on: (1) The response of individuals with more
moderate asthma to SO2, (2) the duration of exposure necessary to
provoke a response to SO2, and (3) the effects of medication on
the SO2 response. Much of these data also provide a more thorough
picture of the magnitude of responses in the range of 0.4 to 1.0 ppm,
the range previously identified as being of interest (53 FR 14948,
April 26, 1988). Data from several of these recent large-scale chamber
studies were reexamined to provide a better understanding of the
response observed in more sensitive subjects. Forced expiratory volume
in one second was used as a measure of lung function, in addition to
specific airway resistance, and other endpoints examined included
symptoms, alteration of workload, and medication usage occurring as a
consequence of these exposures.
Table B-1 of the Criteria Document Supplement (EPA, 1994a)
summarizes the lung function changes in response to SO2
concentrations in the range of 0.6-1.0 ppm from controlled human
exposure studies. Because different studies used different measures of
lung function (FEV1 or SRaw), and different concentrations of
SO2, the discussion that follows will describe group mean changes
first for the studies that used the measure SRaw, then group mean
changes for studies that used FEV1, and then finally the
individual responses.
The data indicate that, in terms of group mean changes, total SRaw
changes\2\ were approximately twice as great at 0.6 ppm and above as at
0.5 ppm and below. The differences were even more pronounced when the
changes in airway resistance due to SO2 alone (i.e., after
correction for the effects of exercise) were considered.
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\2\Since elevated ventilation sufficient for oronasal breathing
to occur is a requirement for most asthmatic persons to respond to
SO2, and because many asthmatic individuals experience
bronchoconstriction responses to exercise alone, it is useful to
distinguish between the two different effects. Any measure of lung
function such as FEV1 or SRaw can be expressed as the ``Total
FEV1 or SRaw,'' which is the total change in lung function
experienced by the subject as a result of an exposure to SO2
while at exercise, or broken down to ``the effect of changes due to
SO2 alone,'' which represents the total lung function change
observed minus the change seen for that subject from a control
exposure at exercise in clean air. Both measures have their utility:
total FEV1 or SRaw indicates the magnitude of overall lung
function change actually experienced by the subject, while the
change due to SO2 alone indicates how much of this total change
is attributable to the pollutant itself.
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For FEV1, the difference in responses between 0.4 ppm and 0.6
ppm SO2 were not as pronounced. At 0.6 ppm SO2, group mean
decreases in total FEV1 of approximately 20 percent were observed
in the mild and moderate asthmatics studied. The changes in FEV1
due to SO2 alone resulted in decreases in FEV1 of
approximately 15 percent (EPA, 1994a, Table B-1).
In addition, at 0.6 ppm SO2, 25 percent or more of the
subjects had pronounced individual responses (either a 200 percent or
greater increase in SRaw or a 20 percent or greater decrease in
FEV1) due to SO2 alone (total changes in lung function for
these individuals would be expected to be even greater). In contrast,
at 0.5 ppm SO2 these more pronounced individual
responses were less frequent, occurring in fewer than 25 percent of the
subjects for both measures of lung function for all but one group
studied (EPA, 1994a, p. B-2).
While not examined in as much detail as lung function, other
indicators of severity also tend to increase with increasing SO2
concentration. For instance, in one study, four of 24 moderate/severe
asthmatic subjects were required to reduce their exercise level because
of asthma symptoms at 0.6 ppm SO2. This occurred only once at each
of the lower concentrations (EPA, 1994a). Two recent studies which
considered medication used to mitigate the effects of SO2 as a
health endpoint and which followed the subjects' medication use in
detail, found approximately twice as many subjects took medication
immediately after exposure to 0.6 ppm SO2 than after exposure to
0.3 ppm SO2 (EPA, 1994a, Table 7, p. 40).
Considering the variety of endpoints for which information is
available, clearly the effects beginning at 0.6 ppm and up to 1.0 ppm
are more pronounced than those at lower concentrations. This is in
agreement with the conclusions reached in the Staff Paper Addendum
(EPA, 1986b), which stated that there were ``clearer indications of
clinically or physiologically significant effects at 0.6 to 0.75 ppm
SO2 and above'' (53 FR 14947, Apr. 26, 1988).
d. Significance of Effects. Opinions on the significance of the
effect expressed by CASAC and others have been widely divergent. Some
CASAC members and outside commenters feel that the responses reported
in the range of 0.6 to 1.0 ppm SO2 are not significant, especially
when viewed in the context of the frequency with which asthmatics
ordinarily experience similar effects in the course of their daily
lives. Other CASAC members and commenters strongly felt that
bronchoconstriction of the degree reported in this range of exposure is
of medical significance and likely to place an exposed asthmatic at an
unacceptable risk of harm.
The frequency of SO2 induced asthmatic episodes relative to
those provided by other stimuli (such as cold/dry air or moderate
exercise) would be expected to vary from one asthmatic individual to
another and from one location to another. As such, the relative
contribution of SO2 to acute episodes of asthma cannot be
precisely assessed. However, staff did compare the effects of SO2
observed in the recent controlled human exposure studies to the effects
of moderate exercise, typical daily variation in lung function, and the
severity of frequently experienced asthma symptoms. The effects of 0.6
ppm SO2 exposure at moderate exercise, as measured by FEV1,
exceeded either the typical effect of exercise alone or typical daily
variations in FEV1 (EPA, 1994a, sections 4.3 and 5.3). For
symptomatic responses, two to eight times as many subjects after
exposure at exercise to 0.6 ppm SO2 experienced symptoms of at
least moderate severity (13-62 percent of subjects) than after exercise
in clean air alone (4-19 percent of subjects) (EPA, 1994a, p. B-12). In
addition, a significant portion of subjects (approximately 15 to 60
percent, depending on asthma status) participating in certain
controlled human exposure studies seemed to experience symptoms more
frequently in response to 0.6 ppm SO2 than reported at any other
time during the majority of the weeks during which they participated in
the study (EPA, 1994a, p. B-12).
Furthermore, the response seen in the most sensitive 25 percent of
responders at 0.6 ppm equaled or exceeded approximately a 30 percent
decline in FEV1 for mild asthmatic subjects and approximately a 40
percent decline for moderate asthmatic individuals. By comparison,
during clinical bronchoprovocation testing changes are not usually
induced beyond a 20 percent decrease in FEV1.
In addition, while at least some subjects can experience such a 20
percent decline without experiencing symptoms, in recent studies
focusing on effects at 0.6 ppm SO2, from 33 percent to 43 percent
of moderate asthmatics and from 6 percent to 35 percent of mild
asthmatics experienced at least a 20 percent decrease in total
FEV1 in conjunction with symptoms rated as being of moderate
severity or worse. Also deserving consideration is the fact that
moderate/severe asthmatic subjects start an exposure with compromised
lung function compared to mild asthmatic subjects. Thus, it is not
clear that similar functional declines beginning from a different
baseline have the same biological importance (EPA, 1994a, pp. 21-25).
In the Staff Paper Addendum, ``bronchoconstriction . . .
accompanied by at least noticeable symptoms,'' was seen as an
appropriate measure of concern (EPA, 1986b, p. 37). However, a
substantial proportion of the subjects in these more recent studies are
experiencing greater effects, bronchoconstriction with at least
moderate symptoms, beginning at 0.6 ppm SO2 (EPA, 1994a).
Considering the recent body of evidence along with previous
studies, the Criteria Document Supplement (EPA, 1994a) concluded that
substantial percentages (25 percent) of mild or moderate
asthmatic individuals exposed to 0.6 to 1.0 ppm SO2 during
moderate exercise would be expected to have respiratory function
changes and severity of symptoms that distinctly exceed those
experienced as typical daily variation in lung function or in response
to other stimuli, such as moderate exercise. The severity of effects
for many of the responders is likely to be of sufficient concern to
cause disruption of ongoing activities, use of bronchodilator
medication, and/or possible seeking of medical attention. At most, only
10 to 20 percent of mild or moderate asthmatic individuals are likely
to exhibit lung function decrements in response to SO2 exposures
of 0.2 to 0.5 ppm that would be of distinctly larger magnitude than
typical diurnal variation in lung function or changes in lung function
experienced by them in response to other often encountered stimuli.
Furthermore, it appears likely that only the most sensitive responders
might experience sufficiently large lung function changes and/or
respiratory symptoms of such severity as to be of potential health
concern, that is leading to the disruption of ongoing activities, the
need for bronchodilator medication, or seeking of medical attention.
Based on the staff's assessment, a number of additional factors are
important in assessing the significance of effects resulting from
SO2 exposures and determining appropriate concentrations of
concern.
Time Course of Response. If an asthmatic individual is at elevated
ventilation and encounters a brief SO2 peak concentration, the
onset of the effect can be very rapid although the response does not
typically approach maximal levels until 5 minutes of exposure. For
example, the total lung function response from a 2-minute exposure was
reported to be only 50 percent of that observed after 5 minutes of
exposure (Horstman et al., 1988). Balmes (1987) reported (in a
mouthpiece exposure study) the response after 3 minutes of exposure was
67 percent of that observed after 5 minutes. After 5 minutes of
exposure the magnitude of the response does not appear to significantly
increase based on comparisons of lung function changes after 5-minute
and 10-minute exposures (Linn, 1983b; EPA, 1986b, p. A-1).
The response is also generally brief in duration; numerous studies
have shown that lung function typically returns to normal for most
subjects within an hour of exposure. This duration is similar to that
experienced in response to exercise and somewhat less than experienced
in response to allergens (EPA, 1994b, p. 18). Even if exposure
continues beyond the initial 5-10 minutes, lung function may still
return to normal as long as the subject ceases to exercise and their
ventilation rate decreases to resting levels (Hackney et al., 1984;
Schachter et al., 1984).
Effect of Varying Temperature and Humidity. Broncho-constriction in
response to SO2 and exercise is: (a) Reduced by warm or humid
conditions, and (b) exacerbated by cold or dry conditions. Thus, the
observed effects such as those described above could be either more
pronounced, less pronounced, or similar depending on the ambient
conditions present during exposure at elevated ventilation.
Effect of Varying Ventilation Rate and Breathing Mode. Another
factor that can affect the magnitude of the SO2 induced response
is ventilation rate. At higher ventilation rates the responses are
likely to be more pronounced at any given SO2 concentration than
those observed at lower ventilation rates. The effects of SO2
increase with both increased overall ventilation rates and an increased
proportion of oral ventilation in relation to total ventilation (EPA,
1986a, p. 11). Oral ventilation is thought to accentuate the response
because the scrubbing of SO2 by the nasal passageways is bypassed.
Based on its assessment of the available data, the staff concluded that
the ventilation rates of concern begin at 35-50 L/min, when most
individuals generally switch to oronasal breathing.
Ventilation rates in the range of 35-40 L/min are comparable to
ventilation rates induced by climbing three flights of stairs, light
cycling, shoveling snow, light jogging, or playing tennis, and can be
induced in a laboratory by walking at 3.5 mph up a 4 percent grade.
Ventilation rates in the range of 45-50 L/min are equivalent to
moderate cycling, chopping wood, light uphill running, and can be
induced by walking at 3.5 mph up an 8 percent grade (EPA, 1994b, p.
20).
While the SO2 effects reported for mild or moderate asthmatic
individual are likely to be more pronounced if an individual asthmatic
is at a ventilation rate higher than 35-50 L/min (EPA, 1994b, p. 19),
the available activity and ventilation data indicate that individuals
engage in outdoor activities that induce ventilation rates of 35-50 L/
min only a small percentage of the time (EPA, 1994b, p. 20). Thus, it
is unlikely that asthmatic individuals in general would attain
sufficiently high ventilation rates (i.e., greater than 35-50 L/min)
frequently enough to markedly increase the health risk posed by peak
SO2 exposures.
Use of Medication. The extent to which an asthmatic individual is
already medicated for protection against other bronchoconstriction
inducing stimuli (e.g., cold dry air, allergens, etc.) and thus would
be protected against SO2, has been considered relevant in
assessing (a) the likelihood of experiencing a bronchoconstriction
response to SO2 and, by extension, (b) the significance of these
effects (53 FR 14932, Apr. 26, 1988). The available data now indicate
that most types of regularly administered asthma medications are not
very effective in blocking the SO2 response. The exception,
however, is the most commonly used class of asthma medications, the
-sympathomimetic drugs (beta-agonist bronchodilator), which
are usually highly effective in preventing the SO2 response from
developing if taken shortly before exposure.
Prophylactic use of beta-agonist bronchodilators to prevent the
effects of SO2 requires either anticipation of exposure or routine
use prior to engaging in vigorous outdoor activities. While some
asthmatic persons do premedicate before exercise, available published
data suggest infrequent bronchodilator use in general among mild
asthmatic persons and a wide range of compliance rates (from very low
to full) among regularly medicated asthmatic persons as a whole (EPA,
1994a, section 2.2). The staff's assessment of this also found low use
of beta-agonist bronchodilators among asthmatic subjects participating
in some of the clinical studies evaluating SO2 effects, as well as
the relative absence of routine medication use before exercise among
such subjects (EPA, 1994a). Given the infrequent use of medication by
many mild asthmatic individuals and the poor medication compliance of
30 to 50 percent of the ``regularly medicated'' asthmatic patients, it
appears that a substantial proportion of asthmatic subjects would not
likely be ``protected'' by medication use from impacts of environmental
factors on their respiratory health. However, the frequency of use of
medication (bronchodilators) specifically prior to engaging in outdoor
activity cannot be confidently extrapolated from epidemiologic data on
medication compliance. Thus, the relative number of persons who may be
protected by medication prior to exercise is unclear (EPA, 1994a, pp.
9-10).
It also should be noted that beta-agonist bronchodilators are
effective in ameliorating SO2-induced bronchoconstriction if an
asthmatic individual has immediate access to such medication after
exposure.
Effect of Other Pollutants. It has been suggested by one study
(Koenig et al., 1990) that prior exposure to ozone may result in
greater SO2 effects, at any given SO2 concentration, than
those reported in the controlled human exposure studies that examined
the effects of SO2 alone. In the ambient situation, however,
potential ozone (O3)-induced increases in SO2 effects may be
at least partially attenuated by the hot humid weather that is often
associated with elevated O3 concentrations.
Data on whether prior nitrogen dioxide exposure produces an
increased response to SO2 are unclear, with a mouthpiece study
showing positive effects (Jorres et al., 1990), while a chamber study
of younger subjects showed no effects of NO2 on responsiveness to
SO2 (Rubenstein et al., 1990). It appears that a pollutant that
increases nonspecific bronchial responsiveness may also increase airway
responses to SO2 (EPA, 1994a, p. 48).
Epidemiological Evidence. Available epidemiological studies show no
evidence of significant associations between either 24-hour or 1-hour
average ambient air SO2 concentrations above 0.1 ppm and increased
visits to hospital emergency rooms for asthma (EPA, 1994a, p. 52).
However, it is not clear to what extent epidemiologic studies could
detect possible associations between very brief (10-minute),
geographically localized, peak SO2 exposures and respiratory
effects in asthmatic individuals. In the absence of such data, it is
not possible to associate peak ambient SO2 concentrations with
excess asthma mortality rates reported to be observed among nonwhite
population groups in large urban areas.
Frequency of Exposure Considerations. Based on this assessment of
the available health effects information, the authors of the Criteria
Document Supplement (EPA, 1994a) concluded that an important
consideration in determining the public health significance of the
reported SO2 induced effects is the likely frequency that an
asthmatic individual would be exposed to a 5-minute peak SO2
concentration 0.6 ppm. Because asthmatic individuals must be
at elevated ventilation in order to experience significant
bronchoconstriction in response to peak SO2 concentrations, any
analysis undertaken to estimate the size of the asthmatic population
potentially at risk from such exposures must account for both the
likelihood that an asthmatic individual will be outdoors at sufficient
ventilation and the likelihood that he or she will encounter an
SO2 concentration of concern.
2. Air Quality and Exposure Considerations
A central issue raised during the comment period on the 1988
proposal concerned whether a 1-hour standard of 0.4 ppm, based on a
typical peak-to-mean ratio of approximately 2 to 1, would provide
adequate protection from high 5-minute peak SO2 levels near all
sources. Based on examination of more recent data, the staff concluded
(EPA, 1994b) that no typical peak-to-mean ratio exists that can be used
to determine a uniformly-applicable hourly standard. Given the broad
range of hourly values associated with 5-minute peaks of SO2 (EPA,
1994b, Table 3-2), it was concluded that reliance on any hourly peak-
to-mean ratio would risk over-controlling some sources (if a high peak-
to-mean ratio is assumed and a low hourly standard chosen) or under-
controlling other sources (if a low peak-to-mean ratio is assumed and a
high hourly standard chosen).
The available 5-minute SO2 data examined in the staff paper
supplement (EPA, 1994b, pp. 34-37) clearly indicate that high 5-minute
peak SO2 concentrations can occur with some frequency near some
sources. Absent comprehensive data on 5-minute peak SO2 levels,
the staff used hourly data to estimate the likely nationwide prevalence
of high short-term SO2 peaks. The staff examined all hourly
averages reported in the AIRS database for the year 1992 and applied
different peak-to-mean ratios to produce upper and lower bound
estimates of 5-minute peaks 0.25 ppm. The method used for
calculating the incidence of short-term peaks is given in the Staff
Paper Supplement (EPA, 1994b). The lower bound estimate of the number
of 5-minute peaks 0.75 ppm SO2 indicated that 50
monitors, in 38 counties which contained 18 urban areas, would register
at least one 5-minute peak of SO2 0.75 ppm. The upper
bound estimate was that 132 monitors, in 91 counties with 65 urban
areas might experience a short-term peak of SO2 0.75
ppm. The same analysis indicated that 132 monitors, in 91 counties
containing 65 urban areas, would be the lower bound estimate of the
occurrence of at least one 5-minute peak of SO2 0.50
ppm. The upper bound estimate was that 247 monitors in 148 counties
with 124 urban areas might record at least one 5-minute peak of
SO2 0.50 ppm. This analysis also suggests that the
number of monitoring sites likely to record multiple high 5-minute
peaks in a single year, or over several years, can vary considerably
(EPA, 1994b, pgs. 41-42).
The use of existing hourly data to assess the potential prevalence
of 5-minute peak SO2 levels has other limitations beyond those
introduced by the use of peak-to-mean ratios. The existing monitoring
network is designed to accurately characterize ambient air quality
associated with 3-hour, 24-hour, and annual SO2 concentrations
rather than to detect short-term peaks SO2 levels. As a result,
the EPA's monitoring guidance on siting criteria, the spanning of
SO2 instruments, and instrument response time could lead to
underestimates of high 5-minute peaks and thus the 1-hour averages for
hours containing those peaks. Of these factors, monitoring siting may
be the largest potential source of underestimation of SO2 peaks
and therefore changes in monitoring siting and density near SO2
sources most likely to produce high 5-minute peaks should increase the
number of high 5-minute peaks and associated 1-hour averages recorded.
In addition to estimating the occurrence of peak SO2 levels in
the ambient air, an important consideration in assessing the public
health significance of SO2-induced effects is determining the
likely frequency that an asthmatic individual will be exposed (EPA,
1994a, p. 51). To address this issue, exposure analyses have been
conducted that predict both the frequency of high SO2 peaks
(through air quality modeling) and the probability that an asthmatic
individual will be outdoors at sufficient ventilation (>35 L/min) to
experience an SO2-induced effect. The methodologies employed in
these analyses, together with the associated uncertainties, are
discussed in some detail in the Staff Paper Supplement (EPA, 1994b, pp.
46-47, appendix B).
These analyses indicate that 68,000 to 166,000 asthmatic
individuals (or 0.7 to 1.8 percent of the total asthmatic population)
potentially could be exposed one or more times, while outdoors at
exercise, to 5 minute peaks of SO2 0.5 ppm. Fewer
asthmatic individuals are likely to be exposed to 0.6 ppm
SO2 under the same conditions. The estimated number of asthmatic
individuals exposed one or more times results in an estimate of 180,000
to 395,000 total exposure events of which the utility sector accounts
for about 68,000. After full implementation of the title IV program of
the Act, in the year 2015, the number of exposure events at
0.5 ppm SO2 attributable to the utility sector is
estimated to drop to 40,000, contingent on trading decisions.
Based on the available air quality and exposure data assessed in
the Staff Paper Supplement (EPA, 1994b) and summarized above, the
Administrator concurs with the staff and CASAC's views that the
likelihood that asthmatic individuals will be exposed to 5- to 10-
minute peak SO2 concentration of concern, while outdoors and at
exercise, is relatively low when viewed from a national perspective.
The Administrator takes note, however, as did the staff, that the data
also indicate high peak SO2 concentrations can occur around
certain sources or source types (EPA, 1994b, p. 37) with some
frequency, suggesting that asthmatic individuals who reside in the
vicinity of such sources or source types may be at greater health risk
than indicated for the asthmatic population as a whole.
C. Regulatory Considerations
Taking into account the staff's assessments and the advice and
recommendations of the CASAC, the Administrator has considered whether
additional regulatory measures are needed to protect asthmatic
individuals against short-term (5- to 10-minute) peak SO2
exposures. In her judgment, the current 3-hour, 24-hour, and annual
standards appear to provide substantial protection against the health
effects associated with short-term SO2 exposures. As indicated by
the air quality analyses described above, the current standards,
together with implementation of title IV of the Act, markedly limit the
frequency and extent of short-term concentrations of concern. The
exposure analyses that take into account normal day-to-day activity
patterns further suggest that the risk is relatively low that
individuals with mild or moderate asthma will experience exposure
conditions approximating those that produced effects of concern in
controlled human studies. In view of those analyses, the nature of the
reported effects, the effectiveness of bronchodilator medication to
prevent or ameliorate SO2 effects if available and properly used,
and the fact that similar events can be provoked more frequently by
other stimuli, the Administrator concurs with the staff's and the
CASAC's assessment that the public health risk posed by short-term peak
SO2 levels is limited when viewed from a national perspective and
does not constitute a broad national public health problem.
The Administrator is mindful, however, that the available data
indicate that those asthmatic individuals who reside in proximity to
certain individual sources or source types will be at higher risk of
being exposed to short-term peak SO2 levels than the asthmatic
population as a whole. While some asthma specialists question the
health significance of the reported health effects, the Administrator
notes that others believe the effects are significant and that
additional protection is warranted. This information, combined with
uncertainties regarding the use of bronchodilator medication prior to
exercise, particularly among asthmatic children and asthmatic
individuals who may not perceive a need to medicate regularly prior to
engaging in outdoor activities, suggests to the Administrator that
additional regulatory measures may be needed.
In their assessment of the available scientific and technical
information, the EPA staff recommended a range of concern for the
Administrator's consideration when examining the potential need for new
regulatory measures to provide additional public health protection
beyond that provided by the existing set of standards (EPA, 1994b).
This range, based on the most recent assessments presented in the
criteria document and staff paper supplements and summarized above, is
0.6 to 1.0 ppm SO2. The staff's assessment concluded that a
substantial percentage (20 percent or more) of mild to moderate
asthmatic individuals exposed to 0.6 to 1.0 ppm SO2 for 5 to 10
minutes during moderate exercise would be expected to have respiratory
function changes and severity of respiratory symptoms that clearly
exceed those experienced from typical daily variation in lung function
or in response to other stimuli (e.g., moderate exercise or cold/dry
air). For many of the responders the effects are likely to be both
perceptible and thought to be of some immediate health concern, i.e.,
to cause disruption of ongoing activities, use of bronchodilator
medication, and/or possibly seeking of medical attention. At SO2
concentrations at or below 0.5 ppm, the staff concluded that at most
only 10 to 20 percent of mild and moderate asthmatic individuals
exposed to 0.2 to 0.5 ppm SO2 during moderate exercise are likely
to experience lung function changes distinctly larger than those
typically experienced and that, compared to the response at 0.6 to 1.0
ppm SO2, the response at or below 0.5 ppm SO2 is less likely
to be perceptible and of immediate health concern.
In considering the staff's most recent assessment of the available
health information, the Administrator found it to be generally
consistent with the staff's 1986 review. During both reviews there has
been divergent opinion as to the appropriate level for the lower bound
for the range of concern. Both assessments, however, concluded that 1.0
ppm SO2 is the appropriate upper bound. At that level there is
clear concern that if an asthmatic individual is exposed while at
exercise to 1.0 ppm SO2 for 5 minutes the risk of significant
functional and symptomatic responses will be high. This finding in 1986
led several CASAC members to recommend a 1-hour standard level that
would restrict the concentration of 5-minute SO2 peaks to 0.6 to
0.8 ppm in order to preclude 5-minute peaks of 1.0 ppm SO2
(Lippmann, 1987). The Administrator finds the staff's present
recommendations consistent with that point of view.
The Administrator also took note that the current CASAC review
panel, while acknowledging the existence of a wide spectrum of views
among asthma specialists regarding the clinical and public health
significance of the reported effects, did not comment on the range of
concern or present the individual panel members' views as to the
significance of the reported effects in its ``closure'' letter. At the
April 12, 1994 ``closure'' meeting, however, the panel found that the
range recommended by the staff was consistent with the available
scientific information. Three members of the panel who addressed the
public health significance of the reported effects in their written
comments concluded that segments of the asthmatic population exposed to
peak SO2 concentrations while at elevated ventilation were at risk
of incurring clinically significant effects if not properly medicated.
While the basis for their judgments differed, their views as to the 5-
minute concentrations of concern overlapped (0.4 to 0.8 ppm SO2;
above 0.6 ppm SO2; and 0.6 to 1.0 ppm SO2) and are in general
agreement with both the 1986 and 1994 staff assessments. On the other
hand, another panel member who addressed the general issue, while
recognizing that SO2 can cause bronchoconstriction, questioned the
public health significance of short-term peak SO2 exposures, based
in part on his judgment that the likelihood of an asthmatic individual
being exposed while at exercise is exceedingly low given the protection
afforded by the existing standards. In its closure letter, the CASAC
expressed the view that such exposures are rare events and that the
likelihood of such exposures should be considered in selecting an
appropriate regulatory response.
Based on its assessment of the available data, the staff
recommended consideration of three regulatory alternatives: (1)
Revising the existing NAAQS by adding a new 5-minute standard
implemented through a risk-based targeted strategy, (2) establishing a
new regulatory program under section 303 of the Act, or (3) augmenting
the implementation of current NAAQS by focusing on those sources likely
to cause high 5-minute peaks. In considering these alternatives, the
Administrator has taken into account the divergent views expressed by
the public, asthma specialists, and the CASAC with respect to the
public health significance of short-term SO2 exposures and the
appropriate degree of protection needed. In doing so she is mindful
that in the absence of conclusive scientific and technical information,
the Act requires that the Administrator make a judgmental determination
as to whether the reported effects endanger public health and pose an
unacceptable risk of harm. At the April 12, 1994 CASAC meeting and in
written comment, individual members of the 1994 CASAC panel recognized
that choosing among the regulatory alternatives presented in the staff
paper supplement must be guided by legal and policy considerations,
given the nature of the available scientific and technical information
and the divergent views as to the health significance of the reported
effect and the pollution level of concern.
The Administrator therefore is proposing for public comment three
alternative regulatory approaches for supplementing the protection
provided by the current standards if additional protection is judged to
be necessary. In so doing, the Administrator has carefully considered
the 1994 CASAC review panel's strong recommendation that any additional
regulatory measures be implemented through a risk-based, targeted
strategy. Consistent with this recommendation, all three regulatory
alternatives under consideration, as described below, are based upon
such a strategy. The Administrator believes it is important to air the
key issues and uncertainties fully and specifically requests broad
public comment and deliberation on these alternatives.
1. 5-Minute NAAQS Alternative
After considering the staff's recommendations and the views of the
1986 and 1994 CASAC review panels, the Administrator believes that it
is both appropriate and necessary to solicit public comment on a 5-
minute NAAQS of 0.60 ppm SO2. Based on the staff's assessments of
the available scientific and technical information, the Administrator
is concerned that 5-minute peak SO2 levels beginning at 0.60 ppm
and above may present an unacceptable risk of harm to asthmatic
individuals who have not premedicated with beta-agonist bronchodilators
and are exposed at elevated ventilation. In proposing a 5-minute NAAQS,
the Administrator is particularly concerned that asthmatic individuals
in the proximity of sources with a high potential to cause or
contribute to a 5-minute peak SO2 concentration greater than 0.60
ppm may be at substantially greater risk of experiencing an exposure
event, which triggers bronchoconstriction, than the asthmatic
population as a whole. Adoption and implementation of a 5-minute NAAQS
of 0.60 ppm SO2 would prevent such exposures and further reduce
the likelihood that an asthmatic individual would be exposed at
elevated ventilation to lesser concentrations. Therefore, it is the
Administrator's provisional judgment that a 5-minute NAAQS of 0.60 ppm
SO2 would adequately protect the public health.
In assessing the possible need for additional protection against
peak SO2 exposures, the Administrator has considered the specific
issue of medication usage. While it is clear from the available data
that the use of beta- agonist bronchodilators to prevent the effects of
other stimuli (e.g., exercise, cold/dry air) will also prevent or
ameliorate the effects of SO2, there is considerable debate as to
compliance rates and therefore the degree of protection provided. As
one CASAC panel member noted, ``many moderate asthmatics, particularly
those from urban areas and lower economic status, may have less than
ideal medical follow-up and are prone to irregular medication use and
frequent deterioration'' (Schachter, 1994). In public comment on the
1988 proposal, a number of individuals made the point that asthmatic
children, who are dependent on adults for their medication and care,
are more likely to be unprotected and therefore at particular risk from
SO2 exposures of concern. Other commenters on the criteria
document and staff paper supplements noted that asthmatic individuals
who do not perceive the need to medicate prior to engaging in strenuous
outdoor activities would also be at increased risk from SO2
exposures. While the Administrator believes these are important
considerations, the overriding issue is whether the availability of,
and reliance on, prophylactic medications should be viewed as an
alternative to further regulatory action to reduce the risk posed by
high peak SO2 concentrations in the ambient air. In this regard,
the Administrator is concerned whether reliance on medications, even if
taken to prevent the effects caused by other stimuli, as an alternative
to environmental controls would be an appropriate public policy choice,
particularly given the potential environmental equity issues involved.
In seeking comment on a possible 5-minute NAAQS of 0.60 ppm
SO2, to further reduce the risk posed by high peak SO