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Their basic chemical structure consists of two benzene rings connected by a pair of oxygen atoms; when substituents on the rings are chlorine atoms, the molecules are particularly toxic. The best-known, usually called simply dioxin, is 2,3,7,8-tetrachlorodibenzo- p -dioxin (2,3,7,8-TCDD). It is extremely stable chemically; it does not dissolve in water but dissolves in oils (and thus accumulates in body fat). The extent of its human toxicity is disputed and the subject of continuing research.