April 4, 1996


Honorable Carol M. Browner
U.S. Environmental Protection Agency
401 M. Street SW
Washington, DC 20460
                  RE:   Closure by the Clean Air Scientific Advisory Committee
                       (CASAC) on the Secondary Standard Portion of the Staff
                       Paper for Ozone
Dear Ms. Browner:
      A Panel of the Clean Air Scientific Advisory Committee (CASAC) of EPA's
Science Advisory Board (SAB) met on March 22, 1995, to review a draft of the primary
standard portion of the document entitled Review of National Ambient Air Quality
Standards for Ozone Assessment of Scientific and Technical Information - OAQPS Staff
Paper. At that time, a draft of the secondary standard portion of the document was not
completed.  In August, 1995, a revised Staff Paper, which included a first draft of the
secondary standard portion was sent to the CASAC panel members for review. On
September 19 and 20, 1995,  the Panel met to complete this review. The Panel
members' comments reflect their satisfaction with the improvements made in the
scientific quality and completeness of the primary standard portion of the Staff Paper
and reached closure on that part (see CASAC Letter Report: EPA-SAB-CASAC-LTR-
96-002, November 30, 1995). However, the Panel could  not come to closure on the
secondary standard portion of the Staff Paper which was a first draft. To facilitate
further development of this part of the Staff Paper, the Panel members provided
detailed comments to your staff.  The Panel felt that the suggested revisions were
extensive enough to warrant a review of the next draft.

      On March 21,  1996, a subset of the Panel, consisting of all four of the Panel
members with expertise in ozone effects on vegetation plus three additional CASAC
members, met in Research Triangle Park, NC to  review a second draft of the secondary

portion of the Staff Paper. In addition, a Panel member with expertise in economics
reviewed the Staff Paper and provided written comments. Overall, the comments from
the Panel members reflected their satisfaction that the Staff Paper was much improved;
however, the verbal and written comments provided to your staff indicated that
important,  additional modifications are still required. Nevertheless, it was the
consensus of the Panel that an additional review of the document by the Panel was not
necessary. Consequently, the majority of the Panel agreed to come to closure on the
Staff Paper assuming that the Agency would incorporate the Panel's  latest comments.
It was the opinion of six of the seven members of the Panel who were present that the
Staff Paper will provide an appropriate scientific basis for making  regulatory decisions
concerning a secondary ozone standard once the additional changes are incorporated.
The additional modifications are summarized below.

      It should be pointed out that the Panel members all agreed that damage is
occurring to vegetation and natural resources at concentrations below the present
1 -hour national ambient air quality standard (NAAQS) of 0.12 ppm. The vegetation
effects experts were in agreement that plants appear to be more sensitive to ozone
than humans. Further, it was agreed that a secondary NAAQS, more stringent than the
present primary standard, was necessary to protect vegetation from ozone.  However,
agreement on the level and form of such a standard is still elusive for a number of

      The first issue is the level of uncertainty associated with the crop loss risk
assessment presented in Tables Vll-5a-d through VII-7 of the Staff Paper. While some
of the sources of uncertainty are addressed earlier in the Staff Paper, other sources of
uncertainty are not addressed  at all. The estimates in these Tables should only be
presented as rough estimates for a number of reasons.  First, the dose-response
functions are based upon open-top chamber studies which have the advantage of
providing the least amount of environmental modification of any outdoor chamber,  but,
nevertheless, they still alter ambient microclimate conditions which will introduce
uncertainty.  In these studies, plant response to ozone has been optimized under
conditions which do not reflect the real-life ambient field  conditions.  Two  of the plant
experts said that the open-top chamber  experiments by their very design and execution
produced results that overestimated the effects of ozone on plant yield. The other two
experts agreed that the open-top chambers do alter the environment in the chamber
with respect to ambient field conditions but did not agree with there being a positive
bias.  Research has not yet provided methods that clearly are better than  open-top
chambers  for establishing ozone dose-response relationships for  a wide variety of

crops.  Second, the estimated exposures are based on a non-peer-reviewed, empirical
model which has not been subjected to any performance evaluation. In addition,
insufficient details are given either in the Staff Paper or the unpublished Agency report
for anyone to perform an evaluation. Third, the estimated exposures are then
extrapolated to hypothetical scenarios where various secondary NAAQS are attained.
Details of this extrapolation procedure are also insufficient to judge the appropriateness
of the procedure.  Fourth, the exposure estimates are then extrapolated to the entire
coterminous U.S. using a Geographic Information System (CIS) which is based on an
unpublished, non-peer-reviewed, internal EPA memorandum that contains insufficient
details to adequately evaluate the CIS. The exposure estimates and the
dose-response function estimates are then input into the economic models which
introduce additional uncertainties. Furthermore, the losses are computed from an
assumed 12-hr, background ozone concentration of 0.025 ppm which is too low  and will
over-inflate the crop loss estimates.  A more reasonable 12-hr, daylight, summertime
background is more likely closer to the 8-hr, background of 0.03-0.05 ppm. As a result,
the Panel felt that the absolute values  of the numbers in Tables VII-5a-VII-7 are  highly
uncertain estimates of crop losses and are a result of a propagation of uncertainties.
They are rough estimates, and this should  be explicitly stated in this discussion.  The
Panel believes, however, that these Tables can be of some use in identifying rough
relative incremental benefits associated with a given NAAQS as long as it  is recognized
that small differences in benefits may have no significance because of these

      A related issue is the estimated yield losses and seedling biomass losses
displayed on the maps in Appendix E of the Staff Paper. Since these are also based
on the  results of open-top chamber experiments as well as the results of the CIS
technology approach, the uncertainties are large. The concern here is that the maps
will be  used out of context and the caveats ignored. The limitations and uncertainties
of the data need to be clearly stated in the  legend of each map.

      The SUM06 standard reflects a change in thinking over the current  1-hour
standard with respect to how plants respond to ambient ozone exposure.  This
proposed form of the standard implicitly recognizes that vegetation response to
ambient ozone is cumulative. However, there is disagreement over whether this is the
best form for a cumulative standard and what the level of the standard should be to
protect vegetation from damage by ozone.  One of the Panel's ecology experts thinks
the form and the range of between 25 to 38 ppm-hours proposed by the Agency is
appropriate.  A second expert thinks the form proposed by the Agency is appropriate

and biologically based, but feels that a level of 20 ppm-hours is necessary to
adequately protect natural resources. The other two experts are uncomfortable with a
SUM06 form because they feel it lacks a biological basis.  One member stated that he
feels very uncomfortable with SUM06 and would not want to defend it because he feels
there is too much uncertainty associated with its derivation.  The fourth expert is
concerned that a SUM06 form is unnecessarily complicated, and the level proposed by
the Agency would not eliminate ozone damage.  Instead, he proposes that the 1-hour
average ozone should not exceed 0.05 ppm for more than one hour between the hours
of 0700-1500. In his written comments, the Panel's economist noted that the welfare
benefits of a secondary standard depend on the decision regarding the primary
standard.  For example, he points out that if the primary standard remains at 0.12 ppm
for 1-hour, or is changed to an 8-hour standard of 0.09 ppm with one allowable
exceedence, Table Vll-5a suggests potentially significant incremental benefits
associated with a secondary standard based on SUM06. He further states that if the
primary standard is set at 0.07 or 0.08 ppm with one exceedence, there is little to be
gained by establishing a separate secondary standard.

      Although the three remaining CASAC members were neither biologists or
economists, they offered their opinion on the secondary standard proposals. Two think
the form proposed by the Agency  is appropriate. One thinks that the level proposed by
the Agency is appropriate, while the other feels that the Administrator's discretion
should be broader than  the range  presented in the Staff Paper. One of these members
pointed out, however, that the Staff Paper does not make it clear enough that the
SUM06 standard as  proposed is a practical choice being made as to the level of effects
that will be tolerated  and not a level that will prevent effects from occurring.  The third is
uncomfortable with SUM06 and based on the estimates in Tables VII-5a-VII-7,
recommends an 8-hour standard at the same level as the new primary standard. The
three members also  concurred that given the crudeness of the risk assessment
estimates, policy decisions cannot be based firmly on science.

      A number of the Panelists offered their insights as to why there are such
divergent opinions on the recommended form and level of the standard. The main
issues are the lack of sufficient rural ozone data, and the lack of relevant plant
exposure studies. There are serious deficiencies in terms of the distribution  of
monitoring sites, particularly in rural areas that prevent us from accurately assessing
exposure once ozone damage is observed.  The Panel is in agreement that plants are
being damaged by ozone and that the current secondary standard is not sufficiently
protective, but there  remain important limitations to our understanding of the extent of

the response of vegetation to ozone under field conditions. Five years from now, if we
do not have the results of research coupling ozone air quality and plant biology under
conditions more representative of ambient field conditions, to avoid the shortcomings of
the open-top chamber experiments, then we will continue to be hampered by our
inability to come to consensus on the levels of air quality that are protective of
vegetation and ecosystems at the most reasonable cost.  In addition, a number of
Panelists expressed the importance of  knowing the consequences of decisions
concerning National Ambient Air Quality Standards. Once a decision is made to
change the standard or to maintain the status quo, we must be able to determine, by
appropriate monitoring and research, what the consequences will be in terms of
ambient air quality and effects  on vegetation and ecosystems.

      In summary, a majority of the Panel has come to closure on the secondary part
of the ozone Staff Paper despite the desire of the Panel for additional significant
revisions.  These revisions have been communicated to your staff by this letter and in
written comments by individual Panel members.  The Panel trusts that your staff will
address these concerns.

      CASAC would appreciate being kept informed of progress on establishing a
revised or new ozone standard, and plans for research on ozone effects.  Please do
not hesitate to contact me if CASAC can be of further assistance in this matter. We
look forward to seeing the final version of the secondary standard portion of the Staff

                                    Dr. George T. Wolff, Chair
                                    Clean Air Scientific Advisory Committee


      This report has been written as part of the activities of the Science Advisory
Board, a public advisory group providing extramural scientific information and
advice to the Administrator and other officials of the Environmental Protection
Agency. The Board is structured to  provide balanced, expert assessment of
scientific matters related to problems facing the Agency. This report has not
been reviewed for approval by the Agency and, hence, the contents of this report
do not necessarily represent the views and policies of the Environmental
Protection Agency, nor of other agencies in the Executive Branch of the Federal
government, nor does mention of trade names or commercial products constitute
a recommendation for use.

                       SCIENCE ADVISORY BOARD
                      Secondary Standard Review

Dr. George T. Wolff, General Motors Environmental & Energy Staff, Detroit, Ml

Dr. Stephen M. Ayres, Office of International Health Programs, Virginia
      Commonwealth University, Medical College of Virginia, Richmond, VA

Dr. Philip K. Hopke, Department of Chemistry, Clark University, Potsdam, NY

Dr. Jay S. Jacobson, Boyce Thompson Institute, Cornell University, Ithaca, NY

Dr. Joe L. Mauderly, Inhalation Toxicology Research  Institute, Lovelace Biomedical &
      Environmental Research Institute, Albuquerque, NM

Dr. James H. Price, Jr., Research & Technology Section, Texas Natural Resource
      Conservation Commission, Austin, TX

Consultants to CASAC
Dr. A. Myrick Freeman, Professor, Department of Economics, Bowdoin College,
      Brunswick, ME

Dr. Allan Legge, Biosphere Solutions, Calgary, Alberta,  CANADA

Dr. William Manning, Department of Plant Pathology, University of Massachusetts,
      Amherst, MA

Dr. George Taylor, Biological Services Center, Desert Research Institute, University of
      Nevada, Reno, NV

Science Advisory Board Staff
Mr. A. Robert Flaak, Designated Federal Official, U. S. Environmental Protection
      Agency, Science Advisory Board (HOOF), Washington, DC 20460

Mrs.  Dorothy Clark, Staff Secretary, U. S. Environmental Protection Agency, Science
      Advisory Board (HOOF), Washington, DC  20460

                      DISTRIBUTION LIST
Deputy Administrator
Assistant Administrators
Director, Office of Air Quality Planning and Standards, OAR
Director, Office of Science Policy, ORD
EPA Regional Administrators
EPA Laboratory Directors
EPA Headquarters Library
EPA Regional Libraries
EPA Laboratory Libraries
Library of Congress
National Technical Information Service