Until more information becomes available on the relative sensitivity of different species and the effect of natural conditions on their response to novel stresses, some approach is needed to ensure that risk management decisions are made in a manner that is protective under most of the likely scenarios. The use of safety factors ensures that the risk estimates will be conservative; the challenge, of course, is to keep from being significantly overprotec-tive or defining regulatory values that are below natural background levels or levels of contamination that are known to have no discernible ecological effects. Selection of an appropriate safety factor most often is based on experience and best professional judgment and can be highly variable among regulatory agencies or between risk assessors. Documentation of reasoning for why particular safety factors were selected often is lacking in risk assessment reports but is critical if reviewers and the general public are to understand how risk management decisions are made and the degree of conservatism or bias incorporated into the supporting assessment. Fortunately, an effort is now being made to develop a scientific basis for the use of safety factors to at least narrow the selection range and make more accurate extrapolations. Additionally, other methods of estimating uncertainty are being incorporated more routinely into ecological risk assessments with the goal of moving completely away from the use of judgment-based safety factors. More sophisticated methods of toxicity estimation (e.g., quantitative structure-activity relationships based on genomic response patterns or physiologically based toxicokinetics models) may also provide a science-based estimate of comparative toxicity responses. Although all these methods show promise, it is likely that safety factors will continue to be used for at least the foreseeable future to ensure that risk management decisions are appropriately protective of the most sensitive valued ecological resources.

See also: Acute and Chronic Toxicity; Allometric Principles; Biogeochemical Approaches to Environmental Risk Assessment; Statistical Methods; Dose-Response; Ecotoxicology: The Focal Topics; Ecotoxicology Nomenclature: LC, LD, LOC, LOEC, MAC; Ecotoxicological Model of Populations, Ecosystems, and Landscapes; Ecological Risk Assessment.

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