The Toxicity Connection

During the Viet Nam War, Air Force planes sprayed the dioxin-laced defoliant Agent Orange on dense jungles to strip the ground of cover. American soldiers got sprayed too, and now thousands of veterans are sure that the exposure has caused them skin rashes, neurological disorders, cancer and birth defects in their offspring. Residents of the Love Canal area of Niagara Falls, N.Y., are no less distressed. They are convinced that the toxic wastes buried there have led to nerve damage, miscarriages and other ailments, including mental retardation among their children. Says Housewife Cynthia Bassett: "It's as if we're all mutants."

If contemporary toxicologists were to conjure up a cause-and-effect grammar for lethal chemicals, asbestos would stand for lung cancer, benzene for leukemia, Kepone for sterility, vinyl chloride for cancer of the liver. The links between these chemicals and certain ailments are now clearly limned, in medical circles as well as in popular mythology. But the connections with diseases for other substances are merely suspicions and likely to remain so for a long time.

The dismaying fact is that while toxic chemicals unquestionably affect human health, there is usually no way of knowing who will become ill in any given population or what ailments will be caused by the exposure. Scientists might be able to find out by giving selected people precise amounts of chemicals for specific lengths of time and comparing these human guinea pigs with a control group that has not been exposed. But such experiments would raise proper howls of indignation. So the disease detectives must rely on less direct methods.

One alternative approach is to expose laboratory animals or even individual cells to chemicals. Often such experiments will produce unwelcome changes--say, mutations in bacteria or bladder cancer in rats (as was the case with the animals fed huge amounts of saccharin). But what causes problems in one species may not be dangerous to another. In Michigan, researchers found that cows that licked barn wood treated with the preservative pentachlorophenol were starving to death. It turned out, explains Jerry Hook of Michigan State University's new Center for Environmental Toxicology, that "this substance is toxic to the bacteria in the cow rumen." Such toxicity did not show up in tests with rats.

Another strategy is to study groups of people who have been exposed to specific chemicals, either on the job or in accidents. By this means, scientists have established some cause-and-effect links--for example, between prolonged inhalation of asbestos particles and mesothelioma (cancer of the lining of the chest or abdomen). But often the results of such epidemiological studies are not entirely convincing. The problem: scientists must trace effect back to probable cause ratherthan identifying cause and looking for effects. Says Dr. Irving Selikoff, of Manhattan's Mount Sinai Medical Center, who established the asbestos-cancer connection: "You're always working backward in this field."

Researchers are also confronted with the task of isolating the factors that might have contributed to an illness. The chemical dioxin can produce the skin condition chloracne, but allergies and plants produce conditions with similar symptoms. Benzene can cause leukemia, but so does radiation. Many diseases take a long time to show up. Workers exposed to vinyl chloride at a B.F. Goodrich plant in Louisville did not develop angiosarcoma, a rare form of liver cancer, until 15 to 20 years later. Says Selikoff: "There is no classic set of symptoms for chemical diseases."

Still another difficulty is that chemicals may behave totally unpredictably. Some act synergistically--one enhancing the toxicity of another. By contrast, others interfere with one another. Michigan researchers have found that both polybrominated biphenyls (PCBs) and dioxin seem to prevent kidney damage from chloroform, but they are hardly beneficial. Another problem for the researcher: determining the level, or threshold, at which a chemical will begin to do its damage.

Faced with all these uncertainties, scientists are understandably cautious. Says Hook: "We know that ingesting these things is probably not good for you. At the same time we should not go shooting our mouths off and scare everyone." Still, few scientists would disagree with Selikoff when he urges an immediate cleanup of the chemical mess. Says Selikoff: "We're fouling our own nest, and we can't survive if we continue."

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