to assess the toxicity of these compounds to humans. Several episodes of high-level human exposure to TCDD have been found to cause a specific type of persistent, potentially disfiguring skin lesion called chloracne. In 2004, the media widely publicized the suspected intentional poisoning of Viktor Yushchenko with TCDD after he developed chloracne during the Ukraine presidential campaign. In contrast to the undisputed high-dose effects of chloracne, the potential adverse effects of TCDD, other dioxins, and DLCs in humans after long-term, low-level environmental exposures remain controversial. The major controversies include how to classify the potential of these compounds to cause cancer in humans (as either “carcinogenic to humans” or “likely to be carcinogenic to humans”), how to estimate the potential health risks at very low doses typical of actual population exposures, and how to assess the toxicity of each of the compounds and various mixtures of them in the environment.

TCDD, other dioxins, and DLCs have been regulated extensively worldwide. In the early 1980s, the U.S. Environmental Protection Agency (EPA) and other organizations, such as the World Health Organization (WHO), began collecting and evaluating scientific information about the sources, fate, and effects of the compounds. In 1985, EPA produced an initial assessment of the human health risks from environmental exposure to TCDD. Later, as new scientific information became available, EPA reassessed the human health risks in an open process involving participation of numerous scientists external to the agency, a series of public meetings, and peer review.

An Interagency Working Group (IWG) made up of representatives of seven federal agencies was established in 2000 to coordinate federal strategies for risk management of TCDD, other dioxins, and DLCs. Members of the IWG, EPA’s Science Advisory Board, and the public commented on earlier drafts of EPA’s dioxin risk assessment, and after further revisions, EPA released the 2003 draft document titled Exposure and Human Health Reassessment of Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds (referred to as the Reassessment). The IWG recommended further review of the new document, and in 2004, EPA asked the National Research Council (NRC) to convene an expert committee to review independently EPA’s 2003 draft Reassessment and to determine whether EPA’s risk estimates are scientifically robust and whether there is clear delineation of all substantial uncertainties and variabilities (Box S-1).

This report presents the committee’s conclusions and recommendations. In general, the committee recommends that EPA substantially augment its Reassessment to improve the transparency about assumptions used to estimate risk and how these assumptions affect estimates. The committee also recommends that EPA re-estimate the risks using several assumptions and communicate the uncertainty in these estimates to the public.

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