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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C, £0460
'eceister 17, 1984
Honorable William D. Ruckelshaus
Administrator
U.S. Environmental Protection Agency OFFICE OF
.-,.._ ,__ S TM6 ftOM4NlSTRAT
401 M Street, SW
Washington, B.C. 2Q460
Dear Mr. Suckelshaus:
The Environmental Protection Agencyt in the implementation of its
authorizing statutes, utilizes a number of modeling techniques to develop
and enforce regulations and standards for various pollution sources. SPA
utilizes models for at least three .major predictive purposes. These
include 1) as a supplement to, and sometimes substitute for, direct measure
ments; 2) to clarify environmental mechanisms and processes; and 3) as a
tool for the development of policy guidance. Examples of the application
of modeling by the Agency include the projection of exposures from land-
based incinerators of liquid hazardous wastes and the determination of
industrial source compliance with the National Ambient Air Quality
Standards-
The widespread use of models by EPA, the states and the regulated
community has major economic implications. More attention should be
directed to the evaluation of the uncertainity and confidence limits to
be applied to model results* It is also important to identify the
relative fraction of the uncertainty that can be attributed to the various
subcomponents of large, complex models. Without explicit identification
of the assumptions and limitations of the submodels, it is impossible for
one to judge whether or not a large model is consistent overall.
From time to time the Science Advisory Board (SAB) is asked to
review models developed or used by the Agency. Many of these models have
not been adequately tested for their reliability. Nor has the proper mix
between monitoring and modeling been examined systematically. All models
adopt simplifying assumptions and are therefore incomplete. A model
validation effort should center around defining the extent of these
limitations.
Validation of these pivotal procedures is a priority issue for EPA's
research agenda. In view of this situation, the SAB makes the following
recommendations;
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o that EPA initiate a systematic effort at model validation. Given
the Agency's increasing reliance upon exposure assessment in
making risk assessment and risk management decisions, the SAB
recommends that the Agency focus its efforts initially on the
validation of exposure models*
o
that such an exposure model -validation effort address the appropriate
mix "between monitoring and modeling.
o that EPA seek to determine the relative utility of exposure ':
modeling approaches for several biological media and systems that
are of regulatory priority*
o that IPA prepare an analytical rationale for this effort in the
form of a case study that would discuss the approaches to be taken
and milestones by which to measure the degree of success.
o that the Agency seek SAB review of the model validation effort
during the life of the project.
A first step toward implementation of this initative would be to
submit for SAB review a specific model, now in use or about to be
introducedj so that its efficacy may be independently evaluated, including
whatever progress EPA itself has nade la validating it and exploring its
applicability to other situations.
The Board fully recognizes that such an undertaking will require
ample resources. By not taking a leadership position on the need to
validate models, however, the Agency takes an even larger risk that many
of its current procedures for assessing risk will soon be rendered obsolete,
The SAB is concerned that.this is already happening as current models are
utilized for purposes for which they were not designed and for which
there is insufficient scientific justification. The continuation and
extension of this practice will cause further strains upon the Agency's
scientific credibility.
I would like to discuss these SAB observations and recommendations
with you and your successor at a mutally convenient time*
Sincerely, ,
- Us-
/ \ V. ^ •'-
Norton Nelson, Chairman
Executive Committee
Science Advisory Board
cc: Alvin Aim
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