Ecological risk assessments are management decision tools that organize and integrate information to estimate the likelihood and magnitude of an undesirable environmental response. Because ecological systems are highly complex, their responses to contaminants are inherently uncertain. This uncertainty results from a combination of factors, including ignorance of the true nature of ecological responses, intrinsic variability in biological and physical phenomena, and imprecision in measurement capabilities. As the consequences of underestimating risk are likely to be more severe than overestimates (e.g., irreversible harm to the environment vs. increased costs of less risky alternatives), risk managers prefer to err on the side of conservatism. This can be done in several ways, including using lower-end estimates of the range of potential effects and higher-end measurements of exposure. Alternatively, adding a safety factor to the analysis provides a simple way of ensuring that unknown differences (such as differing sensitivities between responses observed in the laboratory and those expected in the natural environment) are taken into account. This article summarizes the safety factors in use today in various countries (e.g., the United States, Canada, Britain, and the European Union), discusses the basis for their selection, and examines some of the biases and pitfalls in their use.

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