For the last six years, scientists at the Centers for Disease Control and Prevention have been testing blood and urine samples from a cross-section of the U.S. population, the largest group ever studied, for exposure to environmental chemicals.

The good news in the bi-annual report released this week on the study is that children's exposure to lead and both children and adults' exposure to second-hand cigarette smoke has dropped over the last 10 years. Exposure to banned organic pesticides also appears to have declined, CDC officials said.

The not-so-good news is that significant levels of other potentially harmful chemicals continue to show up in our systems and scientists do not know how most of these chemicals might affect us.

Scientists generally do not dispute the idea that the chemicals people inadvertently ingest, drink, breathe and wash with show up in our bodies. Most agree that biomonitoring of bodily fluids for chemicals is a useful public health tool.

The debate about environmental chemicals instead revolves around whether or not what environmentalists refer to as the "body burden" of trace amounts of chemicals directly affect our health.

The tests show children are carrying higher levels of some chemicals -- like pyrethroids, which are found in household bug sprays and can harm the nervous system, and phthalates, which are found in some cosmetics and are thought to harm the reproductive organs -- in their bodies than adults are.

One in 20 women over 20 years old have high enough levels of the metal cadmium in their systems to be associated with subtle kidney injury and an increased risk for low bone density, said John Osterloh, the chief medical officer for the CDC's environmental health laboratory. The study's authors point to cigarette smoking as the likely cause of cadmium levels this high.

In samples taken from women of childbearing age, the levels of the type of mercury found in seafood were below the concentration that is associated with brain damage in newborn children. However, nearly 6 percent of these women had enough mercury in their systems to warrant careful monitoring.

The CDC study looked at 148 chemicals suspected of causing health problems. The aim was to inspire scientific researchers around the country to study the levels at which these chemicals might cause harm to people, Osterloh said.

Environmental advocacy groups say testing for the presence of harmful chemicals after they have been absorbed by the human body is backwards thinking.

"We don't know if these chemicals are toxic or not or at what levels. For a chemical to be marketed or used, the industries don't have to test its safety. The burden of proof is on the government, to prove that they're harmful," said George Sorvalis, of the Working Group on Community Right-to-Know in Washington, D.C.

Devra Lee Davis, a professor at the Heinz School of Public Policy and Management at Carnegie Mellon University and director of the Center for Environmental Oncology at the University of Pittsburgh, criticized the perils of testing chemicals after they have been released into the environment.

"We're conducting a global experiment on ourselves and nobody can really tell us what this will mean," she said.

The federal Environmental Protection Agency has approved tens of thousands of chemicals for everyday use. Each year the agency approves 700 new ones. The EPA requires chemical companies to provide data about new chemicals, but it does not require the companies to test their long-term health effects.

Chris VandenHeuvel, spokesman for American Chemistry Council, which represents most big chemical manufacturers in the country, said it is appropriate for the EPA to have the burden of proving a chemical is dangerous.

"I think the American people would rather have the federal government decide than rely on the judgment of a corporation. We have absolutely no interest in creating a product that is going to create health concerns," he said.

A report published in June by the Government Accounting Office found that the EPA only collected data about potential long-term effects of new chemicals on human health for 15 percent of the chemicals it approved in a 30-year period.

The GAO recommended that the EPA tighten its standards. U.S. Sens. Frank Lautenberg, D-N.J., and Independent Jim Jeffords, of Vermont, have introduced legislation that would give the agency more regulatory responsibilities in evaluating chemicals.

In some cases, consumers can reduce their exposure to harmful chemicals with the decisions they make for themselves. They may quit smoking or keep their distance from smokers. Or they may avoid eating fish containing traces of mercury.

In other instances, the study authors say, it will be up to scientists to pick up where their data leaves off, and research the health effects of the chemicals on the human body.

For example, years ago the extensive research spearheaded by Herbert Needleman, a physician at the University of Pittsburgh Medical Center, coupled with nationwide public health campaigns, about the effects of leaded gasoline and lead-based paint undoubtedly helped Americans reduce the lead levels in children.