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Issues in Risk Assessment assessment of risks associated with chemicals, ecological effects result from exposure to contaminants in ambient media, such as air, water, and sediment. These effects are functions of both the magnitude and duration of the exposure. Risk characterization requires that the assessor identify the dimensions of exposure and effects that are relevant to the risk estimate. The assessor must determine the relative positions of the expected exposure and effects in the (perhaps multidimensional) exposure-response space and then estimate the probability that the exposure exceeds some criterion of effects. The above description applies well to predictive risk assessments of chemicals; however, other integration approaches might be more appropriate to other types of assessments. For example, when epidemiological methods are used to assess risks associated with apparent environmental damage (decline of a population, decline of forest stands, etc.) risk characterization would include evaluation of the strength of association, the plausibility of causation (given information on mechanisms), and the extent and magnitude of the observed effects. Components of Risk Characterization The group determined that characterization of risks for each end point and action should include the following components (not all must be included in presentations to all audiences): An estimate of effects, including severity, frequency, spatial scope, temporal scope, and probability; A description of the sources and magnitudes of uncertainty in the risk estimate; An explanation of the assumptions used and a discussion of the plausible alternatives; A discussion of the nature and quality of the models used (types used and existence and credibility of validation studies); A discussion of the nature and quality of the data (quality assurance, quality control, relevance to the site, etc.); Supporting lines of evidence (alternative models, different types of laboratory test data, and field monitoring that support the risk estimate);