Determining the health risks to humans of exposure to toxic substances in the environment is made difficult by problems such as measuring the degree to which people have been exposed and determining causation--whether observed health effects are due to exposure to a suspected toxicant. Building on the well-received first volume, "Environmental Epidemiology: Hazardous Wastes and Public Health," this second volume continues the examination of ways to address these difficulties. It describes effective epidemiological methods for analyzing data and focuses on errors that may occur in the course of analyses. The book also investigates the utility of the gray literature in helping to identify the often elusive causative agent behind reported health effects. Although gray literature studies are often based on a study group that is quite small, use inadequate measures of exposure, and are not published, many of the reports from about 20 states that were examined by the committee were judged to be publishable with some additional work. The committee makes recommendations to improve the utility of the gray literature by enhancing quality and availability.