March 3, 2003
Scientists call the accumulation of chemical contaminants – like PCBs, mercury, and pesticides – within a person’s body their body burden.
Body burden is just a number – a concentration in parts per billion or micrograms per liter. But, technical as it is, the term calls forth an image too; an image 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 Center for Disease Control we have a better sense than ever before of the typical American’s body burden. 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 says so clearly, 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.
This is not easy research. The questions are complex. Is the safe level different for children – who eat and respire more per pound of body weight – than for adults? What do you measure to determine safety? How relevant are animal studies to questions about human health?
These have always been the questions of toxicology, but there are new questions emerging too. The website associated with the book Our Stolen Future, which introduced the idea of endocrine receptors 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, like hormone systems, subtle impacts during early development can result in illness after a long latency. Traditional tests for the safety of chemicals look for immediate effects, not illness emerging years after exposure.
2. Has a given chemical been tested for low-dose effects? Traditionally, lower and lower doses of a chemical are tested until a concentration range that has no ill-effects is identified as the safe dose. All lower doses are usually assumed to be harmless. But for some chemicals the dose-response relationship is not so simple. At lower concentrations than the “safe dose” unexpected effects can appear as a biologically active chemical “hijacks” cellular processes. Traditional toxicology may miss this low-dose effect because it is focused on testing for outright damage by toxic chemicals.
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 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 they 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.
There was one definitive finding of the CDC study. For the chemicals lead, DDT, PCBs and hexachlorobenzene – all chemicals that have been banned or strictly regulated in the US – body burdens have decreased since they were last measured. That’s great news. It means that the pollution of our bodies, like the pollution of our rivers or our air, is reversible.
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. If a lower body burden seems like a sensible, desirable thing, then we are going to need to limit the chemicals to which we expose ourselves.
The number of manmade chemicals in our environment is astounding. The CDC tested for the presence of 116, but the 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 stop giving chemicals the benefit of the doubt until we are convinced they are safe?
Second National Report on Human Exposure to Environmental Chemicals, Centers for Disease Conrol and Prevention, January 31, 2003
Our Stolen Future website, with information about the book and regular updates about the cutting edge of science related to endocrine disruption, information about ongoing policy debates, and suggestions about what consumers and citizens can do to minimize risks related to hormonally-disruptive contaminants.
“Developmental Toxicity of a Commercial Herbicide Mixture in Mice: I. Effects on Embryo Implantation and Litter Size,” by María Fernanda Cavieres, James Jaeger, and Warren Porter, Environmental Health Perspectives, Volume 110, Number 11, November 2002.