A Thousand Threats; Cancer-causing chemicals don't work alone, but in tandem. A scientist argues for increased vigilance.(Viewpoint essay)

Byline: Devra Davis (Davis is director of the Center for Environmental Oncology at the University of Pittsburgh Cancer Institute. Her book "The Secret History of the War on Cancer," from which this work is adapted, will be published in October 2007 by Basic Books.)

We know that children are not simply little adults. With their quick heartbeats, fast-growing organs and enviable metabolisms, the young absorb proportionally more pollutants than those who are older. Exposure to minute amounts of hormones, environmental tobacco smoke or pollutants early in the life of an animal or human embryo can deform reproductive tracts, lower birth weight and increase the chance of developing cancer. Yet results from an independent California chemical-testing laboratory, commissioned by activist David Steinman and released in February, found a probable human carcinogen, 1,4-dioxane (also known as paradioxane), in children's shampoos.

Scientists have long known that certain chemicals like paradioxane can cause cancer. Now we're beginning to realize that the total of a person's exposure to all the little amounts of cancerous agents in the environment may be just as harmful as big doses of a few well-known carcinogens. Over a lifetime, cigarettes deliver massive quantities of carcinogens that increase the risk of lung and other cancers. Our chances of getting cancer reflect the full gamut of carcinogens we're exposed to each day--in air, water and food pollution, and cancerous ingredients or contaminants in household cleaners, clothing, furniture and the dozens of personal-care products many of us use daily.

Of the many cancer risks we face, shampoos and bubble baths should not be among them. Europe has banned the use of paradioxane in all personal-care products and recently recalled all contaminated products. The United States and many other non-European countries have not done so. Whereas Europe tends to follow the precautionary principle--in which the burden is on manufacturers to prove that a product doesn't harm consumers--most other nations won't take regulatory action until enough people have already complained of harm. In light of what scientists are learning about exposure to trace chemicals, this makes little sense.

Scientists don't experiment on humans, for obvious reasons, but we have found some clues from lab and wildlife studies. Medical researchers have demonstrated that trace chemicals of some widely used synthetic organic materials can damage cultured human ...