UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D,C, 20460
June 1, 1994
OFFICE OF TOE ADMWtSTOATOR
SCIENCE ADVISORY BOARD
EPA-SAB-CASAC-LTR-94-007
Honorable Carol M. Browner »
Administrator
U.S. Environmental Protection Agency
401 M St., S.W.
Washington, D.C, 20460
Subject: Clean Air Scientific Advisory Committee Closure on the
Supplements to Criteria Document and Staff Position
Papers for SOa
Dear Ms, Browner:
The Clean Air Scientific Advisory Committee (CASAC) at a meeting
on April 12, 1994, completed Its review of the documents; Supplement to
the Second Addendum (1986) to Air Quality Criteria for Particulate Matter
and Sulfur Oxides; Assessment of New Findings on Sulfur Dioxide and
Acute Exposure Health Effects in Asthmatics; and Review of the National
Ambient Air Quality Standards for Sulfur Oxides: Updated Assessment of
Scientific and Technical Information, Supplement to the 1986 OAQPS
Staff Paper Addendum. The Committee notes, with satisfaction, the
improvements made in the scientific quality and completeness of the
documents.
With the changes recommended at our March 12 session, written
comments submitted to the Agency subsequent to the meeting, and the
major points provided below, the documents are consistent with the
scientific evidence available for sulfur dioxide. They have been organized
in a logical fashion and should provide an adequate basis for a regulatory
decision. Nevertheless, there are four major points which should be called
to your attention while reviewing these materials;
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1. A wide spectrum of views exists among the asthma specialists
regarding the clinical and public health significance of the effects of 5 to
10 minute concentrations of sulfur dioxide on asthmatics engaged in
exercise. On one end of the spectrum is the view that spirometric test
responses can be observed following such short-term exposures and they
are a surrogate for significant health effects. Also, there is some concern
that the effects are underestimated because moderate asthmatics, not
severe asthmatics, were used in the clinical tests*
At the other end of the spectrum, the significance of the spirometric test
results are questioned because the response is similar to that evoked by
other commonly encountered, non-specific stimuli such as exercise alone,
cold, dry air inhalation, vigorous coughing, psychological stress, or even
fatigue. Typically, the bronchoconstriction reverses itself within one or
two hours, is not accompanied by a late-phase response (often more
severe and potentially dangerous than the immediate response), and shows
no evidence of cumulative or long-term effects. Instead, it is
characterized by a short-term period of bronchoconstriction, and can be
prevented or ameliorated by beta-agonist aerosol inhalation.
2, It was the consensus of CASAC that the exposure scenario of concern is
a rare event. The sensitive population in this case is an unmedicated
asthmatic engaged in moderate exercise who happens to be near one of the
several hundred sulfur dioxide sources that have the potential to produce
high ground-level sulfur dioxide concentrations over a small geographical
area under rare adverse meteorological conditions. In addition, CASAC
pointed out that sulfur dioxide emissions have been significantly reduced
since EPA conducted Its exposure analysis and emissions will be further
reduced as the 1990 Clean Air Act Amendments are implemented.
Consequently, such exposures will become even rarer in the future,
3. It was the consensus of CASAC that any regulatory strategy to
ameliorate such exposures be risk-based - targeted on the most likely
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sources of short-term sulfur dioxide spikes rather than imposing short-
term standards on all sources. All of the nine CASAC Panel members
recommended that Option 1, the establishment of a new 5-minutes
standard, not be adopted. Reasons cited for this recommendation included:
the clinical experiences of many ozone experts which suggest that the
effects are short-term, readily reversible, and typical of response seen
with other stimuli. Further, the committee viewed such exposures as rare
events which will even become rarer as sulfur dioxide emissions are
further reduced as the 1990 amendments are Implemented, In addition,
the committee pointed out that enforcement of a short-term ISIAAQS would
require substantial technical resources. Furthermore, the committee did
not think that such a standard would be enforceable (see below).
4. CASAC questioned the enforceability of a 5-minute NAAQS or "target
level." Although the Agency has not proposed an air monitoring strategy,
to ensure that such a standard or "target level" would not be exceeded, we
infer that potential sources would have to be surrounded by concentric
circles of monitors. The operation and maintenance of such monitoring
networks would be extremely resource intensive. Furthermore, current
instrumentation used to routinely monitor sulfur dioxide does not respond
quickly enough to accurately characterize 5-minute spikes.
The Committee appreciates the opportunity to participate in this
review and looks forward to receiving notice of your decision on the
standard. Please do not hesitate to contact me if CASAC can be of further
assistance on this matter.
Sincerely,
George T. Wolff, Ph.D.'
Chair, Clean Air Scientific
Advisory Committee
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