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  Summary
   
Storm: No Danger
Dioxin has been called the most toxic man-made substance known, and the most potent man-made carcinogen. One millionth of a gram per kilogram body weight (1 μg/kg) can slowly kill a guinea pig or, administered chronically, give cancer to a rat. But, the levels to which human beings are exposed in the general environment (less than one trillionth of a gram per kilogram body weight or 1 pg/kg/day) are a million times smaller, still. Dioxin first came to public attention as an unwanted contaminant in Agent Orange, the defoliant used during the Vietnam War, and has generated an unusually high level of public concern, media attention, and government expenditure ever since. Yet, the only unequivocally adverse health effect that dioxin is known to have caused in humans is chloracne, a serious, but non-lethal, skin eruption which has been associated only with extremely high exposures in previous decades, exposures which are generally unattainable today. Although the data do not support a cause-and-effect relationship between dioxin exposure and cancer in humans, IARC, U.S. EPA, and NTP have all re-classified 2,3,7,8-TCDD as a “known” human carcinogen, based on "adequate" animal evidence and the assumption that humans share with laboratory animals a common mechanism of dioxin-induced carcinogenesis. However, the mechanism of action remains largely unknown for all of the adverse effects of dioxin, including cancer. Cancer risk assessment methodology produces conditional estimates of hypothetical risks by extrapolating beyond existing data. It is, therefore, unavoidable that every cancer risk assessment will be dependent on certain assumptions and inferences. Effective risk communication is possible only when all of those assumptions and inferences are fully and candidly disclosed.
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