Americans have lower levels of lead, secondhand-smoke byproducts and other potentially dangerous substances in their bodies than they did a decade ago, according to the third government survey of exposure to environmental chemicals.

"These data help relieve worry and concern," Julie L. Gerberding, director of the Centers for Disease Control and Prevention, said Thursday.

The CDC released its first National Report on Human Exposure to Environmental Chemicals in 2001. For its latest findings, it took blood and urine samples from about 2,400 people in 2001 and 2002 and tested for 148 environmental chemicals, including metals, pesticides, insect repellants and disinfectants.

The CDC stressed that the presence of an environmental chemical in blood or urine "does not mean that the chemical causes disease."

In the early 1990s, 4.4 percent of U.S. children between the ages of 1 and 5 had elevated lead levels. That dropped to 1.6 percent between 1999 and 2002, according to the latest study.

"This is an astonishing public health achievement" that is related to the removal of lead from gasoline and other efforts to screen and treat children for lead exposure, Gerberding said.

To gauge the effect of secondhand smoke, the CDC tested nonsmokers' levels of cotinine, a product of nicotine after it enters the body. Levels dropped by 75 percent in adults and 68 percent in children between the early 1990s and 2002, the CDC said, as a result of restrictions on smoking.

But more work needs to be done to reduce secondhand smoke, Gerberding said. Blacks still had more than twice the cotinine levels of whites or Mexican Americans.

Other findings:

* No women in the survey had dangerous concentrations of methyl mercury, which can come from eating shellfish or fish. But the CDC said mercury levels in women of childbearing age should be monitored because 5.7 percent of women in this age group had levels close to what is believed to cause birth defects.

* About 5 percent of the U.S. population 20 or older had cadmium, a heavy metal, in their blood at a level that could cause a kidney injury. Cadmium can come from cigarette smoke.

* Traces of aldrin and dieldrin, pesticides for cotton and corn discontinued in 1970 in the United States, were very low or undetectable in U.S. adults.

CDC Director Julie L. Gerberding says work remains to reduce secondhand smoke.