Recent events have drawn attention to the hypothesis that some xenobiotics in the environment may elicit toxicities in humans by modulating endocrine pathways. From the perspective of regulatory toxicology, pursuit of this hypothesis w ill prove difficult, because current risk assessment m ethods do not readily apply to substances with very high potencies, reversibility, transgener-ational effects, and subtle biological outcomes. Such xenobiotics typically persist in the body and bioaccumulate in the food chain. Yet the exposures are important only during critical periods of vulnerability, and no sustained bio-markers of these important exposures currently exist. This article describes some recent efforts by federal agencies to pursue the hypothesis, including research planning and screening of potential endocrine modulating xenobiotics and risk assessment of dioxin conflicts with its policy for substances that cause thyroid follicular carcinomas.
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