Scientists call the accumulation of chemical contaminants (such as PCBs, mercury, and pesticides) within a person’s body the “body burden.” Body burden is just a number, a concentration in parts per billion or micrograms per liter. But the term calls forth an image, too, of a body bent over and struggling beneath a heavy load. When scientists start taking about body burden, I think about real bodies — my own and my children’s.

Thanks to a new report from the U.S. Centers for Disease Control and Prevention, we have a better sense than ever before of the body burden of the typical U.S. citizen. In the Second National Report on Human Exposure to Environmental Chemicals, CDC scientists measured the levels of 116 chemicals in the blood and urine of 2,500 volunteers. The study found detectable levels of 89 chemicals, including pesticides, phthalates, herbicides, pest repellents, and disinfectants.

Chemical by chemical, the report documents the average concentration of contaminants in the bodies of the people studied. But what does all this data mean? At what concentration do these chemicals become dangerous?

For all but a handful of chemicals, nobody knows the answer to this question. The report acknowledges as much, in one understated sentence: “Research studies, separate from the Report, are required to determine which blood or urine levels are safe and which cause disease.”

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This is not easy research. The questions involved are complex. What do you measure to determine safety? How relevant are animal studies to questions about human health? Do safe levels differ for children, who eat and respire more per pound of body weight than adults?

These have always been the questions of toxicology, but new questions are emerging, too. The website associated with the book Our Stolen Future, which introduced the idea of endocrine disruptors to the general public in 1996, collects scientific papers and news stories that track discoveries about the health effects of chemical exposures. The papers collected there make it clear that we need to add three new questions to the way we think about safe levels of chemicals.

1. Could a given chemical have health effects a long time after exposure? For chemicals that interfere with cell-signaling systems, such as hormone systems, subtle impacts during early development can cause trouble after a long latency. Traditional tests for the safety of chemicals look for immediate effects, not those that emerge years after exposure.

2. Has a given chemical been tested for low-dose effects? Traditionally, chemicals are tested for safety at lower and lower doses, until a concentration is discovered that has no ill effects. All doses below that threshold are usually assumed to be harmless. But for some chemicals, the dose-response relationship is not that simple. Unexpected effects can appear at lower concentrations than the “safe dose” as a biologically active chemical “hijacks” cellular processes. Because it focuses on testing for outright damage by toxic chemicals, traditional toxicology may miss this low-dose effect.

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3. Is a given chemical safe when mixed with other chemicals? Most studies of chemical safety examine the effects of one chemical at a time, but in real life, people are exposed to complex mixtures of contaminants. New studies, such as one on the impact of a commercially available weed-killer mixture on pregnant mice, suggest that mixtures of chemicals can have effects that none of the chemicals have on their own.

No wonder the CDC report can’t say much about the safe levels of the chemicals it measured. Looking for effects from very low doses over very long time periods is difficult enough. Try to do that for all possible real-world combinations of chemical exposure and the task grows exponentially.

Maybe some day our science will reach a level of sophistication that can give us solid assurances about chemical safety. But that’s a distant goal, not a current reality. Until then, we are all walking, breathing experiments in toxicology. Until then, we are all living with risk.

However, there was one definitive and telling finding in the CDC report: The body burdens of lead, DDT, PCBs, and hexachlorobenzene have all decreased since the last CDC study. Guess what? These are all chemicals that have been banned or strictly regulated in the U.S. That’s great news. It means that the pollution of our bodies, like the pollution of our rivers or our air, is reversible.

But a river doesn’t come back to life until the pollution is cut off at the source, and the same will be true for our bodies. So the CDC finding gives us a clear mandate of where to go from here: If a lower body burden seems like a sensible, desirable thing, then we need to limit the chemicals to which we are exposed. That’s no small task. The number of artificial chemicals in our environment is astounding. The CDC tested for the presence of 116, but the U.S. EPA estimates that at least 80,000 chemicals — 690 times the number tested by the CDC — are produced and used today.

If we keep assuming that all chemicals are harmless until we uncover the exact doses, combinations, and lag times that will make us sick, our bodies are going to remain polluted for a long, long time. Wouldn’t it make more sense to put the burden of proof of safety on the chemical manufacturers, rather than the burden of the chemicals on our bodies?

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