Since there are about 80,000 chemicals registered with the Environmental Protection Agency, getting a handle on how such substances affect us is a challenging feat. In the biggest report yet on toxic substances in our bodies, the federal Centers for Disease Control and Prevention examined levels of 148 of these substances in a sampling of 2,400 people from around the country.

Among the findings:

• The percentage of young children with toxic levels of lead, which are associated with low IQ and many medical problems, has "dropped precipitously," said Julie Gerberding, director of the CDC. Gerberding attributed the decline to deleading of gasoline and other public-health efforts. Levels of cotinine, a substance that indicates exposure to secondhand smoke, are also falling, though non-Hispanic blacks suffer an increased burden of this toxic substance in comparison with non-Hispanic whites and Mexican-Americans.
• There was concern over another metal, cadmium, with 1 in 20 adults over 19 having levels that may be associated with kidney dysfunction and low bone mineral density. While the CDC thinks such levels are likely due to smoking cigarettes, Tim Kropp, senior scientist at the Environmental Working Group, adds that burning fossil fuels may be a significant source of cadmium. "We need to figure out where that's coming from instead of chalking it up to many bad things about tobacco," said Kropp.
• All women of childbearing age had mercury levels below those associated with neurological impairment of the developing fetus. However, about 1 in 20 of them had levels that were within a zone of potential concern. This rate was slightly reduced from the 2003 report, although Jim Pirkle, deputy director for science at the CDC's Environmental Health Laboratory, said more data would be needed to assess whether this change was a trend.

Released yesterday, the Third National Report on Human Exposure to Environmental Chemicals documents the levels of many toxic substances, among them tobacco smoke, metals, dioxins, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), pesticides, and phthalates. The CDC examined blood and urine samples collected from 1999 to 2002 and looked at toxic levels in subgroups, such as age, gender, and race/ethnicity.

For the vast majority of substances evaluated, little research has been done regarding how much exposure, if any, is safe. Gerberding stressed that a chemical present in one's body does not necessarily cause disease. And the study was not designed to associate measured levels with distinct medical and developmental problems.

Kropp applauded the CDC for its work but also emphasized that the study looked at the toxics in a "chemical-by-chemical vacuum." He added, "Shouldn't how we regulate them mirror what happens in the world?" The Environmental Working Group recently published a study documenting an average of 200 chemicals present in samples of 10 babies' umbilical cord blood.

Future reports are scheduled for publication every two years, with certain chemicals already slated to be evaluated. These include polybrominated diphenyl ethers (PBDEs), flame retardants widely present in many consumer products such as furniture, mattresses, and computers. PBDEs have been shown to poison brain development and cause thyroid and reproductive dysfunction and cancerous tumors in laboratory animals.

"There is a reason that they test these [chemicals], and it's important that they test them," Kropp added.

For more information: The Agency for Toxic Substances and Disease Registry has a site with links in both English and Spanish to frequently asked questions about toxic substances.