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By DICK THOMPSON WASHINGTON

Marilyn Leistner doesn't believe scientists anymore -- at least not the ones who once denounced dioxin but now downplay its dangers. Leistner was the last mayor of Times Beach, Mo., the town of 2,400 that the U.S. government evacuated and closed down in 1982 because it was contaminated with dioxin, considered by many to be one of the most fearsome of chemicals. The mayor saw dioxin's toxic effects all too clearly: the elderly forced out of their homes and into retirement centers, people so paranoid that every common illness was assumed to be dioxin poisoning, neighbors quarreling and even threatening to kill one another. "This chemical uprooted 801 families," she says. "The frustration, the divorces, the stress, the deaths can all be blamed on this chemical."

Well, no. It was not so much the chemical that caused the chaos as it was a questionable government judgment about the risks of dioxin. Now that some scientists are asserting -- 10 years too late -- that the concentrations of dioxin present at Times Beach were not harmful, the dispossessed residents, and the public in general, have every right to be confused.

There was a similar pattern of uncertainty in judgments about Alar, radon and even some forms of PCBs and asbestos. Citing government studies, environmentalists sounded the alarm about toxicity and cancer. The public fretted. Officials issued warnings and regulations. But then skeptical scientists re-evaluated the threat and began to argue that the risks had been exaggerated. After this series of debates, people are wondering if they have been unduly frightened by overzealous, if well-meaning, regulators.

At issue is risk assessment, the method of evaluating how dangerous a substance is to humans. In the U.S. officials have been quick to ban chemicals that according to lab tests are carcinogenic. But skeptics contend the system is so sensitive that if standard tests were applied to all chemicals, both natural and synthetic, fully half of them would appear to cause cancer. "It's a bit like the search for witches. You can always find them," says Colin Berry, chairman of the British government's pesticide advisory board and a critic of the way American scientists have evaluated risk. Now that U.S. system of risk assessment is itself being reassessed. Just last month, for example, the Environmental Protection Agency reversed an earlier decision and eased restrictions on the use of a class of pesticides known as EBDCs.

Proving that a chemical causes cancer is difficult, since the disease may not strike until years after exposure. Unable to wait that long, scientists have tried to speed up the process by feeding huge amounts of suspect chemicals to laboratory animals such as mice. Typically they are given what is known as the maximum tolerated dose, an amount just below the lethal level. In the case of the artificial sweetener saccharin, mice were given the equivalent of hundreds of cans of diet soda a day; similarly, a person would have had to eat thousands of apples a day to get the maximum tolerated dose of Alar, a fruit-ripening chemical used by growers until it was withdrawn from the market because of a cancer scare. If as few as five mice out of 200 given these megadoses develop tumors over two years, the substance is usually labeled a carcinogen.

Such a conclusion is based on a host of conservative assumptions, among them that the effect on mice is generally the same as on humans and that it makes no difference whether the chemical is swallowed, inhaled or rubbed on the skin. Another questionable premise is that there is no safe dose of a carcinogen. In fact, the body may have evolved methods of coping with small amounts of such chemicals. But when lab mice are given a megadose of a chemical, it could overwhelm their natural repair systems. Such a dose may also stimulate cells to divide rapidly, which magnifies normal genetic errors and produces cancer.

If traces of chemicals can cause cancer, then the peril is inescapable. Most fruits and vegetables contain natural pesticides -- chemicals that plants themselves have manufactured to ward off bugs and blights -- and about half these compounds have tested positive as carcinogens. "Just because something is natural doesn't make it good, and just because something is man-made doesn't make it bad," says Ronald Hart, director of the National Center for Toxicological Research.

A recent survey of the causes of cancer, published in Science, concluded that "the perception that environmental pollution is a major cancer hazard is incorrect." While estimates vary, many experts agree that pesticides and other environmental contaminants are responsible for no more than 1% of cancers and 5,000 deaths a year. The potential cost of erroneous risk assessments is enormous: America's bill for complying with environmental regulations could top $100 billion this year.

Congress is considering whether to loosen the Delaney clause, a 1958 law that bans any amount of any carcinogen from the food supply. In the 33 years since Delaney was written, science has developed an ability to identify substances at levels unimaginable to politicians who originally voted for the measure. Today one part per quintillion can be detected -- the same as a tablespoon of liquid in all the Great Lakes combined. As scientists become more sophisticated in detecting potential carcinogens and analyzing test results, lawmakers and consumers will have to become more sophisticated in deciding how to balance the risks against the benefits.