Yet even though many health statistics have been improving over the past few decades, a few illnesses are rising mysteriously. From the early 1980s through the late 1990s, autism increased tenfold; from the early 1970s through the mid-1990s, one type of leukemia was up 62 percent, male birth defects doubled, and childhood brain cancer was up 40 percent. Some experts suspect a link to the man-made chemicals that pervade our food, water, and air. There's little firm evidence. But over the years, one chemical after another that was thought to be harmless turned out otherwise once the facts were in.

The classic example is lead. In 1971 the U.S. Surgeon General declared that lead levels of 40 micrograms per deciliter of blood were safe. It's now known that any detectable lead can cause neurological damage in children, shaving off IQ points. From DDT to PCBs, the chemical industry has released compounds first and discovered damaging health effects later. Regulators have often allowed a standard of innocent until proven guilty in what Leo Trasande, a pediatrician and environmental health specialist at Mount Sinai Hospital in New York City, calls "an uncontrolled experiment on America's children."

Each year the U.S. Environmental Protection Agency (EPA) reviews an average of 1,700 new compounds that industry is seeking to introduce. Yet the 1976 Toxic Substances Control Act requires that they be tested for any ill effects before approval only if evidence of potential harm exists—which is seldom the case for new chemicals. The agency approves about 90 percent of the new compounds without restrictions. Only a quarter of the 82,000 chemicals in use in the U.S. have ever been tested for toxicity.

Studies by the Environmental Working Group, an environmental advocacy organization that helped pioneer the concept of a "body burden" of toxic chemicals, had found hundreds of chemical traces in the bodies of volunteers. But until recently, no one had even measured average levels of exposure among large numbers of Americans. No regulations required it, the tests are expensive, and technology sensitive enough to measure the tiniest levels didn't exist.

Last year the Centers for Disease Control and Prevention (CDC) took a step toward closing that gap when it released data on 148 substances, from DDT and other pesticides to metals, PCBs, and plastic ingredients, measured in the blood and urine of several thousand people. The study said little about health impacts on the people tested or how they might have encountered the chemicals. "The good news is that we are getting real data about exposure levels," says James Pirkle, the study's lead author. "This gives us a place to start."