The Double Take on Dioxin

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In science, as in life, simple questions rarely have simple answers. That principle of uncertainty is especially frustrating when researchers try to determine the hazards of various chemicals to humans. Ten years after sounding an alarm over the dioxin-contaminated roadways of Times Beach, Mo., federal scientists wonder whether they acted too hastily in ordering the community's permanent evacuation. Perhaps, they say, dioxin was not such a serious threat after all. This kind of waffling only reinforces public skepticism about the credibility of scientists, who seem to change their mind with bewildering regularity whether the subject is the danger of dioxin or the benefits of oat bran.

Environmental groups still fear that even minute amounts of dioxin, which was an ingredient in the Vietnam-era defoliant Agent Orange, can cause epidemics of cancer. But Vernon Houk, the federal official who recommended the Times Beach evacuation, is no longer sure. Recent studies suggest that the chemical may not be so dangerous. In an interview with the St. Louis Post- Dispatch, Houk declared, "We should have been more up front with the Times Beach people and told them, 'We're doing our best with the estimates of the risk, but we may be wrong.' I think we never added 'but we may be wrong.' "

To get at the truth, the Environmental Protection Agency has ordered a reassessment of dioxin's risks and, depending on the findings, may relax rules on exposure to the chemical. That will be cold comfort to the displaced citizens of Times Beach. "Houk announced his decision with all the power and authority of science behind him," says Marcel LaFollette, a professor of science policy at George Washington University. "Now he's saying 'Never mind.' A reasonable person would ask the scientist, 'Why can't you make up your mind?' "

An unavoidable amount of uncertainty is built into every scientific investigation. To determine the risk of disease from trace amounts of dioxin, researchers had to assume that if it caused cancer in laboratory animals, then it could cause cancer in humans. In addition, because no one completely understands how toxins trigger cancer, scientists chose a mathematical model that assumes a linear relationship between the amount of toxin consumed and the incidence of malignancy. In other words, if a pound of dioxin caused cancer in 50 out of 100 subjects, then half a pound would trigger 25 cases out of 100, and so on.

Using such calculations for dioxin produced a conclusion that ingesting an infinitesimal amount of the compound each day over a lifetime -- about 0.006 trillionths of a gram per kilogram of body weight (or 0.014 trillionths of an ounce for a 150-lb. man) -- would cause 1 cancer among 1 million people. The contamination at Times Beach was 1,000 times as great as this safety limit.