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Synthetic chemicals added to consumer products to meet federal and state flammability standards are showing up in waterways, wildlife and even human breast milk.

Studies in laboratory animals and humans have linked the most scrutinized flame retardants, called polybrominated diphenyl ethers, or PBDEs, to thyroid disruption, memory and learning problems, delayed mental and physical development, lower IQ, advanced puberty and reduced fertility. Other flame retardants have been linked to cancer. At the same time, recent studies suggest the chemicals may not effectively reduce the flammability of treated products.

The potential risks of flame retardants have been known for some time. In 1977, brominated tris was banned from use in children's pajamas after researchers showed it could damage DNA in animals. Two PBDEs, penta and octa, were pulled from the U.S. market in 2004. But another chemical that was removed from pajamas decades ago based on evidence that it could mutate DNA is still being used in furniture and some other baby products.

Flame retardants rely on chemical reactions that counteract or inhibit the flammability of treated products. Since the 1970s, they have been applied to textiles, foam in couches and baby products, building insulation, carpets, drapes, personal computers, TV sets, car dashboards and many other products.

The brominated and chlorinated flame retardants commonly found in consumer goods belong to a class of chemicals called semi-volatile organic compounds. Because they are not chemically bound to material but incorporated during manufacturing or sprayed on afterward, they routinely escape as vapor or airborne particles that tend to stick to surfaces or settle in dust. Friction and heat generated through normal use of a product --sitting on a couch, for example -- can accelerate their release.

They can also escape during production or when treated products are recycled or disposed of in landfills or incinerators. Once released, they can build up in sewage sludge, soil and sediments. Scientists have detected flame retardants hundreds of miles from human sources, including in the tissue of sperm whales, which spend most of their time in deep ocean waters, and of Arctic marine mammals, suggesting long-distance transport by water and air currents.

Last year, Ronald Hites, a professor at the School of Public and Environmental Affairs at Indiana University, analyzed tree bark to track the spread of flame retardants in the environment worldwide. Tree bark, which soaks up chemicals in the atmosphere, has helped scientists document the presence of other persistent chemicals. He and Ph.D. student Amina Salamova collected bark samples from remote locations not associated with production or use of the chemicals on five continents, and flame retardants turned up in all of them, including a "very remote" region in Tasmania.

These compounds are building up in human fat, seminal fluid and breast milk. During the past 30 years, Hites reported in 2004, PBDE levels in human blood, milk and tissue increased by a factor of 100 -- essentially doubling every five years.

The Centers for Disease Control and Prevention measured PBDE levels in people by analyzing blood samples collected in 2003 and 2004 for its National Health and Nutrition Examination Survey. The study found 97 percent of Americans had flame retardants in their blood, and those ages 12 to 19 had the highest levels. It's unclear what a safe level of exposure might be, if it exists.