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Pursuant to a congressional request, GAO discussed the Environmental Protection Agency's (EPA) assessments of the health risks posed by Superfund hazardous waste sites. GAO noted that: (1) the EPA risk assessment process includes estimating the quantity of contaminants that could be reaching human populations and determining the contaminants' toxicity; (2) EPA makes assumptions about the quantity and toxicity of these contaminants because data on such factors are often incomplete; (3) although critics believe that EPA assumptions are often unrealistic, EPA believes its estimates must be conservative in order to minimize human exposure to toxins from Superfund sites; (4) although EPA risk assessors generally follow EPA policies, they have made inconsistent assumptions about the sites' future use and have not fully disclosed the basis for their conclusions as required; and (5) advisory groups generally support the EPA risk assessment process, but they believe that risk assumptions should be more explicit and more data should be collected on the range of possible exposure risks.

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