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Issues in Risk Assessment safety. Work is needed to understand the mechanisms of the response that occur at the threshold in the stress-response function. Expression of Uncertainty The functional expression of the stress-response relationship is stochastic and distributional; the assessor must consider extremes and discontinuities, not just central tendencies. Assessments should recognize the natural variability in systems, and conclusions should be accompanied by a description of uncertainty and probability. Understanding the Stressor Qualitative and quantitative aspects of the stressors should be clearly articulated without bias with respect to desirability of outcome. The effect of other anthropogenic or natural stressors should be included in the analysis, because most ecological systems are affected by multiple stresses. For example, assessments of ecological risks of chemicals could increase reliance on field experiments in which test organisms are exposed to a suite of compounds and a range of natural conditions (this approach is already being widely used to set water quality criteria). One might also use a stressor classification to locate sensitive systems and sensitive components (e.g., species). Such a classification could include the components of the system potentially affected by the stressor, the pathway(s) of movement of the stressor, and the capacity of the affected component(s) to recover. Assessors should consider developing a matrix that considers the analytical method used to quantify stress versus class of stressor. A good understanding of mechanisms of action can substantially improve understanding of stress-response relationships. Knowledge of mechanisms is not, however, a prerequisite for a useful risk assessment. Before a theory of mechanisms is used in a risk assessment, it must be validated in a realistic and comprehensive fashion.