To make a point about the saturating presence of toxic chemicals in the environment, field scientists will, on occasion, leave off looking for contaminants in big cities and abandoned industrial sites and travel to some of the world's most remote places. In recent years they have found petrochemicals--and breast cancer--in the bodies of beluga whales in Canada's St. Lawrence River. They have found PCBs--compounds used in electrical transformers that have been banned for thirty years--in the snow atop Aconcagua, the highest mountain in the Andes. They have even found flame retardants in the blubber of seals on Canada's Holman Island, far above the Arctic Circle. Synthetic chemicals, it turns out, circle the globe like the winds.

Despite such evidence--that toxic chemicals are, in essence, everywhere--human health advocates have struggled for decades to convince the public that there may be a link between so-called environmental toxins and individual and community health. After the stir caused by the publication of Silent Spring in the early 1960s, it took a full decade for the government to pass, and begin to enforce, pollution controls in factories and hazardous waste dumps. And thirty years after that, it remains more difficult than ever to convince people that the products they rely on every day--products that are made, after all, with these same toxic chemicals--might in any way be risky to use.

It's important to understand that your body is already full of toxic chemicals. This is true even if, as the saying goes, you were born yesterday. Long before you ever bought a flame retardant couch, or a sheet of plywood, or a can of ant spray, the chances are quite good that you absorbed toxins through your mother's placenta, her breast milk, or both. Given the ubiquity of chemicals in our lives, the accumulation grows from there.

In Maryland, where I live, a lot of attention is paid to the health of oysters, one of many endangered species suffering from toxic runoff in the Chesapeake Bay. Oysters spend their days on the bay floor, filtering water in one end and out the other. Whatever microscopic material is in the water passes through the oyster. Most of it exits; some of it stays inside. These toxins can be measured.

What is becoming clear is that we are all oysters. We are all exposed to all kinds of toxins. Some of these we filter out; others stay inside us.

In recent years, public health groups have come up with a new tactic to make this point: the body burden study. Such studies are not, at least primarily, invested in proving that toxic chemicals are "dangerous." This work is being done, with increasingly clear results, in scientific laboratories. What the body burden studies do is prove that these chemicals are everywhere--in the environment, in wild and domestic animals, and, with increasing frequency, in our bodies. Proving that toxic chemicals are dangerous hits people in their heads. Proving that people have chemicals in their bodies hits people in their guts. For decades, the chemical industry has been able to convince our heads that chemical harm is still in dispute, that "more research is needed." The authors of the new body burden studies are betting that the gut is less easily persuaded.

"Our experience with persistent chemicals of the past such as DDT and PCBs has shown what happens when we wait to gather conclusive evidence of a chemical's harm instead of acting on mounting evidence," the Public Interest Research Group reported in 2003. "By the time the chemicals were regulated, they had spread across the globe and left a path of damage from which we have yet to recover."

If lab science aspires to prove...