After more than a decade of urgent warnings about the effects of toxic chemicals on everything from the foods people eat to the towns they live in, scientists are taking a contentious second look at the way they decide the health risk of hundreds of chemicals.
There is by no means agreement among experts that it is time for an across-the-board lowering of official concern about the safety of chemicals. Depending on who you talk to, dioxin, which caused a whole Missouri town to be evacuated a decade ago, could either be as deadly as the plague or in low doses as non-threatening as the common cold.
But the question scientists and politicians are now asking is this: How far should the federal government go to protect the public's health when the best scientific opinion is still uncertain?
The stakes are enormous. Make the wrong decision and environmental regulators could risk thousands of lives or millions of dollars or both.
"I think we are overestimating the low-level risk for many compounds," said Dr. Curtis Travis, director of the Center for Risk Management at the Oak Ridge National Laboratory. "I don't think we need to spend $80 billion to clean up Superfund sites."
Among environmentalists and scientists, there is fierce disagreement because the risks of chemicals seem to be sometimes exaggerated and sometimes underestimated.
* The safe limit for the amount of lead in a child's bloodstream is being lowered as studies show children's behavior and intelligence can be effected by smaller doses.
* Alar, a pesticide used on apples, was taken off the market after the Natural Resources Defense Council sounded an alarm. But many experts now believe the risk of getting cancer from eating apples was tiny and the public overreacted.
* The ozone layer has depleted faster than was estimated.
* A growing body of evidence shows that people will suffer health effects from breathing air in supposedly smog-free cities because the standard is not conservative enough.
The debate has grown as scientists learn more about how chemicals cause cancer in the human body. It has been widely accepted that the size of the chemical dose is proportional to the damage on the body, and that even the tiniest dose will cause cancer. That thinking is beginning to change.
Dr. Vernon N. Houk, a director at the Centers for Disease Control who helped make the decision to evacuate Times Beach, Mo., in 1982 now says he made a mistake. "The overall cancer question is not settled, but if dioxin is a human carcinogen, it is, in my view, a weak one that is associated only with high-dose exposures," he said at a University of Missouri conference in May.
Dioxin is just one of many chemicals that the public has been told is a danger when it is not, said Dr. Bruce Ames, a biochemist at the University of California at Berkeley. "I think the whole thing is based on bad science."
But many other scientists and environmentalists strongly disagree with that assessment.
"I think we would be wrong to make a generalization about all chemicals," said David Doniger, a senior attorney at the Natural Resources Defense Council, a national environmental group. "Or will say, 'Oops we underestimated and a whole lot of people died who shouldn't have.' "
While the danger of some chemicals may be exaggerated, there are cases where a chemical has been found to be far worse than first thought.
On the same day this spring that the U.S. Environmental Protection Agency was announcing its decision to re-evaluate the risk of dioxin -- once believed to be the most potent cancer chemical known -- the Centers for Disease Control in Atlanta was announcing that it wanted to lower the amount of lead in a child's blood that it deems acceptable.
And early estimates of how quickly chlorofluorocarbons -- chemicals widely used in air conditioners, refrigerators and insulation -- would eat away at the Earth's protective ozone layer have proved wrong.
"If we had known in the 1970s what we know now we would have begun to phase these chemicals out immediately," Mr. Doniger said. The result may be thousands of additional skin cancer cases in the United States alone.
Better safe than sorry?
William Farland, director of EPA's Office of Health and Environmental Assessment, said it is not clear whether his agency's yearlong review will mean the estimates of the risk of exposure to dioxin will be reduced or increased.
But he said, "I think there is a general trend in the scientific community toward stepping away from the traditional approach of saying anything that produces tumors in animals is likely to produce tumors in humans and is likely to produce tumors at any level."
Whatever the EPA's pronouncement on dioxin will be next May, the review has raised the question of how sure the government must be before it proclaims a limit for a chemical. For years, environmental and health experts have assumed the worst-case scenario when evaluating risk. The idea was that it was better to be safe than sorry.