GAO discussed the Environmental Protection Agency's (EPA) risk assessment process for Superfund hazardous waste sites, focusing on: (1) human health risks posed by Superfund sites; and (2) the assumptions and calculations used in risk assessments. GAO noted that: (1) EPA Superfund risk assessment is a 3-step process that evaluates exposure and toxicity risks, and estimates potential health risks; (2) scientific uncertainties and data gaps exist in the process despite the large amount of data EPA gathers about each site; (3) EPA generally assumes that residential use of contaminated land is possible in the future which greatly influences its exposure assessment and choice of remedy; (4) EPA believes that legislation requires it to use a conservative approach to protecting human health and the environment; (5) EPA prefers to use site-specific observations rather than standardized assumptions in making its assessments, but it is often difficult to gather site-specific information; (6) many interested parties believe that standardized assumptions overstate site risks; (7) EPA must rely on extrapolation of animal research data to assess toxicity risk because of the lack of data on human health responses to contaminants; (8) many industry groups believe that EPA risk characterization does not adequately reflect the uncertainties and variability of exposure to contaminants; and (9) the EPA risk formula sometimes overstates risk by combining exposure and toxicity estimates to account for risk uncertainties.