So Many Chemicals, So Few AnswersJohn Carey
In its rush to base all regulations on risk assessment, Congress is forgetting the method's dirty little secret: It doesn't give very good answers. When it comes to the crucial question of whether chemicals cause cancer in people, "I'm not sure we've made a lot of progress," says risk consultant Kenny S. Crump of ICF Kaiser in Ruston, La.
This doesn't mean science has been standing still. "There has been an explosion of information on the mechanisms by which chemicals cause cancer," says Dr. John Whysner of the American Health Foundation in Valhalla, N.Y. But the new research hasn't translated into greater certainty.
MASSIVE ZAPS. Witness dioxin. "Despite the fact that it has been studied to death, we still can't agree on how it acts or what it does," says Gail Charnley, executive director of the President's Commission on Risk Assessment & Risk Management. For other substances such as saccharin, assessments of carcinogenic potency differ by more than 1 billion--hardly a solid foundation for regulation.
The problem is the immense difficulty of linking chemicals with cancer or with other medical problems. With a few rare exceptions, Americans encounter such low levels of harmful substances that human studies cannot spot any connection. That's why toxicologists turn to animals. But thousands of rats and mice must be tested to spot risks posed by typical low-level exposures. So in the mid-1970s, scientists developed a method of zapping rodents with huge doses.
Such tests are of questionable relevance, however. Many rat carcinogens don't cause cancer in humans, and vice versa. What's more, what happens at massive doses often provides little guide to the effects of small exposures.
Indeed, as researchers probe the actual pathways of chemicals through the body, they are discovering how flawed the estimates derived from massive-dose tests can be. The body appears able to protect itself even against some substances, such as
formaldehyde, that directly damage DNA. And for hundreds of other chemicals, low doses fail to trigger the harmful mechanisms found at the higher doses. In fact, because dioxin prompts cells to make a protective enzyme, the substance may actually exert an anticancer effect in small doses, some scientists suggest.
Unfortunately, the new studies won't provide conclusive answers for years to come--if ever. Not only would ferreting out mechanisms of action for thousands of chemicals cost more than agencies can afford, but also there's no guarantee that each substance may not still be acting by an undiscovered, more harmful pathway. The inescapable conclusion: Science is decades away from being able to pinpoint the hazards of the thousands of chemicals that permeate our environment.
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