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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                                    2-
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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                                  -3-
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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