None of us can escape the myriad chemicals in the air, the water, our food and our homes. The Centers for Disease Control and Prevention periodically measures the so-called body burden of a sample of Americans; its most recent assessment occurred in December 2009. Though we still know little about whether the chemicals measured make us sick, the data are nonetheless vital so epidemiologists can spend years, even decades, studying the effects. The CDC tracks more than 200 substances. Here are some better-known ones.

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