Risk assessment has been broadly defined as the characterization of the adverse health effects of human exposures to environmental hazards and can be divided into four major steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization (1). Hazard identification is defined as determining whether human exposure to an agent can cause an increased incidence of an adverse health effect (e.g., neurotoxicity). Dose-response assessment is the process of characterizing the relationship between the administered or effective dose of an agent and the incidence of an adverse health effect in exposed populations, estimating the incidence of the effect as a function of human exposure to the agent. A dose-response assessment should account for exposure intensity and duration, developmental age, and other factors that may modify the response (e.g., gender, diet). Exposure assessment is the process of measuring or estimating the intensity, frequency, and duration of human exposure to an agent found in the environment or an agent that may be released into the environment. Risk characterization integrates these preceding steps by estimating the incidence of a health effect under various conditions of human exposure.