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6 CONCLUSIONS AND RECOMMENDATIONS Far more chemicals are in the human environment than can be evaluated for potential toxicity with available methods and resources. Therefore, some chemicals have to be selected for testing for their potential impact on public health, and that requires a priority-setting process. The selection of chemicals for testing is made difficult by the existence of many possible health effects to test for and many possible tests and combinations of tests to choose from. Much of the information needed to set priorities for testing is fragmentary or lacking; little if any toxicity information is available on most chemicals, and the information on human exposure to or potential toxicity of only a few is more than minimal. Moreover, many of the data that do exist are not easily retrievable or are not in a form that makes . . . . them readily usable or veritable. An essential function or any priorityffetting system must be to serve as a guide to strategies for improving the available data through the most rapid and effective combinations and sequences of information-gathering procedures. On the basis of its review of existing priority-setting systems, the Committee on Priority Mechanisms has concluded that such systems must be logically and scientifically defensible, open to peer review, practical, and explicit about their underlying assumptions. Such analytic techniques as systems analysis and decision theory appear to permit design approaches with those characteristics. Application of these analytic techniques requires, however, that each information-gathering or testing procedure be described quantitatively with respect to its ability to identify potentially toxic chemicals; and the current art of toxicity testing and priority-setting does not readily permit such quantitative descriptions with great precision. Efforts toward further quantification of performance characteristics of toxicologic methods are essential to the development of an optimal priority-setting system. In addition to information on the accuracy of testing procedures, the design of a priority-setting system involves the number and types of chemicals to be considered by the system, the accuracy of selection procedures, the availability of testing resources, penalties for misclassifying chemicals, costs of selection and testing, and adequacy of information on exposure to and the potential health effects of the chemicals to be considered. Priority-setting systems must also be flexible, so that they can be applied by users with different missions. No system can anticipate and address all possible contingencies, and a given system should be adjustable to suit specific needs. Testing may be undertaken for a 295

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variety of reasons--e.g., to provide special scientific insights or to improve knowledge of testing procedures. At times, policy considerations may dictate testing of substances that would otherwise receive lower priorities. A priority-setting system should be able to respond to changes that result from advances in science and changes in the perceived relative importance of various toxicity and exposure problems. There is no evidence that any priority-setting system can remedy the basic problem of insufficient data. In the light of that impediment, the following may be considered as characteristics of a system that would advance the state of the art of priority-setting: Use of explicit, detailed, and formal decision-making procedures in selecting and ranking chemicals for testing, such as are illustrated in the approach described in this report, which is based on principles of systems analysis and decision theory. Validation and quantification of the performance characteristics of all tests and other information-gathering procedures used in the testing programe Development of relevant data bases, including those related to the production and use of chemicals, to the potential for human exposure, to structure-activity relationships, and to toxic activity. Characterization of the universe of chemicals to be tested. Estimation of the prevalence of substances that can cause different types of toxicity. Development of systematic procedures to ensure that the priority-setting process corrects and refines itself on the basis of experience. Inclusion of a procedure for systematic screening of the entire select universe of chemicals as defined in this report, as a basis for nominating substances to be tested. 296