Health Assessment Document for Polychlorinated Dibenzo-P-Dioxins. Parts 1 and 2. External Review Draft
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Dioxins are a class of compounds that contain the dibenzo-p-dioxin nucleus. In the chlorinated dioxins, the dibenzo-p-dioxin nucleus is substituted with chlorine at different positions of the fused benzene rings. Depending on the number and position of chlorine substitution, 75 congeners are possible for the chlorinated dioxins. This document deals with the most toxic chlorinated dioxins, namely, 2,3,7,8-tetrachloro-, 1,2,3,7,8-penta-chloro-, 1,2,3,6,7,8-hexachloro- and 1,2,3,7,8,9-hexachlorodibenzo-p-dioxon. Of these four congeners, the 2,3,7,8-tetrachlorodibenzo-p-dioxin has been studied extensively. This compound is often described in both popular and technical literature as 'TCDD' or simply 'dioxin'. A few documents exist at the present time that deal with selected aspects of polychlorinated dibenzo-p-dioxins in the environmental media. This document has been prepared to provide a comprehensive multi-media assessment of the analytical methodologies, environmental levels and ecological and health effects of the four chlorinated dioxins mentioned above.