"I don't want dioxin in my breast milk!" Hope exclaimed on an episode of "thirtysomething" a few seasons back, protesting the construction of a waste-incineration plant. Her objection expressed the popular terror of 2,3,7,8-tetrachlorodibenzo-p-dioxin, which has a reputation as "the most toxic synthetic chemical known to man."
Critics of government policy on dioxin have long argued that scientific evidence does not support that reputation, and recently they have begun to prevail. In August the Environmental Protection Agency announced that it is reevaluating the hazard posed by low concentrations of dioxin in the environment.
Two events helped prompt the review: a Long Island symposium on dioxin in October and a January report in The New England Journal of Medicine on the results of a major epidemiological study by Marilyn Fingerhut at the National Institute for Occupational Safety and Health.
"If dioxin is a human carcinogen…it is a relatively weak one and is a carcinogen only at extraordinary doses," Assistant Surgeon General Vernon N. Houk told Chemical & Engineering News. Houk, who urged the evacuation of Times Beach, Missouri, in 1982 because the town's soil was contaminated with dioxin, now says the move was unnecessary.
Studies of people exposed to concentrations of dioxin many times the level in Times Beach have repeatedly failed to find solid evidence of permanent, long-term health effects. Instead, the link between dioxin and disease, especially cancer, has been based largely on studies of laboratory animals.
Extrapolations from such studies assume that if large doses over a short period cause cancer in lab animals, small doses over a long period will cause a proportional amount of cancer in humans. But officials admit that this assumption is usually wrong. Many scientists believe that a chemical must be present at a level of at least 10,000 molecules per cell before it can cause cancer. Furthermore, large, toxic doses cause direct damage that accelerates cell division, which in itself increases the chances of mutation.
Finally, physiological differences between people and lab animals may render comparisons meaningless. For example, recent research indicates that the biochemical process by which gasoline causes kidney tumors in rats does not occur in humans.
Edward G. Remmers, vice president of the American Council on Science and Health, says the questions raised by the reevaluation of dioxin could lead the EPA to reassess the dangers of other substances, including PCBs, cyclamates, Alar, saccharin, formaldehyde, lead, and radon.