Susanne Green tries to live her environmental beliefs - eating organic foods whenever she can and shunning meat altogether.
So the U.S. Senate staffer got a nasty surprise last year when she found out there were traces of chemical fire retardant in the breast milk she was feeding her newborn daughter, McKenzie.
"I'm quite perplexed," says Green, 35. "I wonder what I was exposed to."
Fire retardants, and much, much more, it seems. Armed with powerful new tools for detecting contaminants, researchers are finding dozens of chemicals in our bodies that probably don't belong there. Traces of pesticides - some banned years ago - show up routinely in samples of human blood, urine and breast-milk. So do well-known neurological toxins such as lead and mercury.
There are also many less familiar, polysyllabic compounds, such as polybrominated diphenyl ethers - the flame retardant in Green's breast milk - and di-2-ethylhexyl phthalate, used in countless plastic home and garden products, children's toys and food packaging.
How they wind up inside us and what, if any, health threat they pose isn't certain. Researchers suspect that they're picked up through the food and water we consume, the air we breathe or the clothing, cosmetics and myriad other products we wear or use daily.
Though the levels of most toxic substances are low, there has been relatively little research on long-term exposure to such doses.
One thing, however, is clear: The fledgling science of biomonitoring - finding out what pollutants make their way into our bodies - is ushering in a new era in environmental and public health research.
"I consider this kind of comparable to the mid-1970s," says Thomas Burke, professor of health policy and management at the Johns Hopkins Bloomberg School of Public Health. "Almost 30 years ago, we began to measure industrial chemicals in drinking-water supplies and in ground-water." Those studies led to federal anti-pollution laws, such as the Superfund act, which provides federal money to clean up hazardous waste sites.
The new biomonitoring studies are prompting officials to target chemicals that are showing up in people, not just in the air or water.
Health and environmental officers have been monitoring human exposure to a few toxic substances, such as lead, for a long time. But in the past decade researchers have broadened their inquiry to include more of the estimated 75,000 chemicals in industrial use.
Leading the way has been the federal Centers for Disease Control and Prevention in Atlanta, which pioneered tests for previously undetectable contaminants. In the late 1990s, CDC began analyzing blood and urine for contaminants as part of its regular nationwide health surveys.
The first round of testing, reported in 2001, looked for 27 chemicals, including lead, mercury, pesticides, nicotine and phthalates (used in plastics, soaps and other personal care products). CDC's second study, released last year, expanded to 116 chemicals. Another report is expected next year, and every two years thereafter.
The studies provide good news about a few old health nemeses. Nationwide, the average lead level in the blood of young children has fallen by half. A drop in the levels of cotinine, a breakdown product of nicotine, suggests efforts to crack down on public exposure to smoking have been effective.
But scientists also found that 30 years after the pesticide DDT was banned, traces of it continue to show up in people's blood.
Another potentially troubling finding: Levels of one type of phthalate - a class of chemicals widely used as a plastic softener - were nearly twice as high in children ages 6 to 11 as they were in older youths and adults. The substance, di-2-ethylhexyl phthalate, has been removed from most children's toys and food packaging here, but is still in home and garden products.
So, how dangerous are these contaminants?
"Just the mere fact we can measure these chemicals doesn't mean there's a health effect," says Dr. John Osterloh, deputy director of CDC's Division of Laboratory Sciences.
There are no established health standards for most of the chemicals found. Animal tests have found some to be toxic - capable of causing developmental or reproductive problems, or even cancer. Information is scarce, though, on the long-term effects of chronic, low-level exposure.
"Our knowledge is uncertain," says Hopkins' Burke. But he said finding even low levels of some substances in blood, urine and breast milk "raises a red flag."
Independent researchers and activists are filling in the gaps. The Washington-based Environmental Working Group, for example, had nine volunteers analyzed for 210 chemicals - and found an average of 91 in each.
In a separate study, EWG tested the breast milk of Susanne Green and 19 other pregnant women around the country. It found flame retardants in every sample, from 9.5 parts per billion to 1,078 parts per billion. Their levels, confirmed in other U.S. studies, are several times higher than European samples.
"The big concern here is that these chemicals are linked to neurological problems," says Jane Houlihan, the group's vice president for research.
CDC studies have prompted government action in at least one case. Based in part on the levels of mercury found in children and women of child-bearing age, the Food and Drug Administration is urging pregnant women to avoid swordfish, shark and other large predatory fish that store the toxic metal.
As with lead, even low levels of mercury can cause learning disabilities. Scientists recently found that mercury tends to concentrate even more in a pregnant mother's cord blood, heightening the risk of development problems.
With federal grants, states are getting into biomonitoring as well. In Maryland, the state health laboratory recently bought equipment to test urine samples for 22 organophosphate pesticides. By year's end, the lab also should be able to test for traces of 11 heavy metals, including arsenic and mercury.
The new capacity, available to any private physician through a local health department, "can help guide research into environmental causes of human health problems," says Diane Matuszak, deputy state health secretary.
For some activists, the discovery of widespread contaminants is further encouragement to avoid potential sources by eating organic foods and wearing only natural fibers. But others say that without more evidence, it's premature to ban many chemicals or radically alter our lifestyles.
"We only have half the risk picture," says David Ropeik, director of risk communication at Harvard University's Center for Risk Analysis. "Some people are jumping the gun before we know the presence of these things in our system is bad."
Indeed, some toxicologists argue that the body can benefit from low-level exposure to some harmful substances. Their theory, called hormesis, disputes the mainstream view that there are no safe doses of many toxic chemicals.
"These are things we've all used and we all benefit from," says Hopkins' Burke. But he doubts that tiny doses are beneficial. "We didn't evolve to have this battery of substances in our bodies," he says.