Health risks of "dioxins": a review of environmental and toxicological considerations.

Polychlorinated dibenzo-p-dioxins are the focus of great public concern because of the extreme toxicity of some congeners in animal studies, their apparent ubiquity in our food and the environment, and their involvement in some well-publicized environmental contamination incidents. This review summarizes and discusses some environmental and toxicological aspects of dioxins, with major focus on the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). In many animals, 2,3,7,8-TCDD is extremely toxic, but species differences are evident. Humans appear more resistant to the effects of 2,3,7,8-TCDD than most animal species tested. However, since human exposure has usually occurred to a mixture of chemicals, of which 2,3,7,8-TCDD is only 1 component, it is often difficult to clearly relate effects in humans to exposure to this compound. So far, it appears that chloracne is the only proven consequence of human exposure to 2,3,7,8-TCDD. Attempts to estimate the carcinogenic risk from 2,3,7,8-TCDD in humans have failed to provide reliable evidence. However, the International Agency for Research on Cancer has classified this compound as a possible human carcinogen based on animal studies. Whether 2,3,7,8-TCDD, in small quantities, poses any significant health risk is still debatable. It appears prudent to limit human exposure to this compound as much as possible.