Summary: Dioxins--chemical compounds that share structural and biological characteristics--have been linked to human illnesses, including cancer. Often the byproducts of combustion and industrial processes, complex mixtures of dioxins enter the food chain and human diet through emissions into the air. The Environmental Protection Agency (EPA) and the World Health Organization (WHO) noted the potential human health risks of dioxins in the 1970s when animal studies showed them to be among the most potent cancer-causing chemicals. EPA derived its estimates of human dietary exposure to dioxins in the United States from (1) chemically analyzed samples of 10 food types, (2) toxicity estimates of levels of individual dioxins in these foods, and (3) estimates of the quantities of these foods consumed by Americans. To develop more reliable national estimates of dietary exposure, EPA incorporated into its analysis some food studies that were nationally representative. Although both EPA and the WHO have assessed the human health risks of dioxins during the last decade, some of their objectives and processes have differed. Nonetheless, the analytical methods used and the conclusions reached have much in common. A major difference in the assessments is whether there are threshold levels below which exposure to dioxins would pose a negligible risk of cancer. EPA assumed there is no safe threshold level for cancer effects, but the WHO assumed there is. EPA's draft reassessment report reflects the recommendations and suggestions provided to the agency by the two most recent independent peer review panels. The panels, one consisting of 12 independent reviewers and the other convened by EPA's Science Advisory Board, concurred with many key assumptions and approaches that EPA used.
