Levels of lead have dropped dramatically, exposure to second-hand smoke is down and most women are not burdened by unsafe levels of mercury, according to the latest U.S. government survey on chemical exposures, reported by Reuters.

The third National Report on Human Exposure to Environmental Chemicals (found on the Internet at http://www.cdc.gov/exposurereport/), released by the Centers for Disease Control and Prevention, details 148 different chemicals found in the blood and urine of 2,400 volunteers.

The reports details exposure levels, but more research is needed to determine if those levels pose any danger to people, CDC director Dr. Julie Gerberding said.

The latest report finds that 1.6 percent of U.S. children have elevated blood lead levels, compared to 4.4 percent in 1991-94 and 88.2 percent in 1976 to 1988.

"We don't know what is a safe level, so we continue to strive to ensure that all children are free of lead exposure," Gerberding said. Removing lead from gasoline was the main reason for the decline.

Exposure to second-hand tobacco smoke was analyzed using a measure of a chemical called cotinine, a breakdown product of nicotine. Cotinine levels in blood have fallen 68 percent in children aged 4 to 11 from a previous 1988-to-1991 test period, by 69 percent in 12- to 19-year-olds and by 75 percent in adults aged 20 to 74.

The report also contains details on pesticides, weed killers, pollutants known as polycyclic aromatic hydrocarbons, dioxins, furans, polychlorinated biphenyls or PCBs and phytoestrogens.

Gerberding said some people may have a genetic predisposition to be sensitive to some chemicals. "It is not just a matter of are you exposed to a chemical or not but how does your body or your unique composition respond to that chemical," she said.

"Mere detection of a chemical does not necessarily indicate a risk to health," the American Chemistry Council said in a statement.