Risk assessments must be conducted because it is generally not possible to obtain a direct measure of risk to people under actual conditions of exposure. Risks need to be understood in advance of exposure to insure proper safety precautions. In 1983, a special National Academy of Science committee reported that most human and environmental health hazards can be evaluated by dissecting the analysis into four parts. The four parts are hazard identification, dose-response evaluation, exposure assessment, and risk characterization. This methodology, described by the committee's report "Risk Assessment in the Federal Government", has been widely adopted. The first step of a human health risk assessment involves a review of published literature to obtain toxicity data on the chemicals of concern and identify the types of toxicity and their relevance to humans. Analytic epidemiologic studies are the primary means for determining causal relationships between specific environmental exposures and human health effects.