Americans have lower levels of lead, secondhand-smoke byproducts (search) and other potentially dangerous substances in their bodies than they did a decade ago, according to perhaps the most extensive government study ever of exposure to environmental chemicals (search).

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

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

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 ages 1 to 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 (search), Gerberding said.

Gauging the effect of secondhand smoke, the CDC tested for 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.

Gerberding said the decrease came from restrictions on smoking.

But more work needs to be done to reduce secondhand smoke, she said. Blacks still had more than twice the cotinine levels of whites or Mexican-Americans. Levels in children were more than twice those of nonsmoking adults.

The study looked at 38 chemicals, mainly pesticides, that were not measured during the last CDC analysis, in 2003.

Dr. Charles McKay, a medical toxicologist for Hartford Hospital in Connecticut, said researchers will be able to use the report as a reference to determine what levels of chemicals are typically found in Americans. It also will be helpful for doctors, he added.

"It allows us to reassure people if they are concerned ... that the actual amount that you take into your body for a large number of chemicals is trivial, is vanishingly small," said McKay, also associate medical director of the Connecticut Poison Control Center at the University of Connecticut Health Center.

Other findings:

--About 5 percent of smokers 20 or older had the heavy metal cadmium (search) 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 U.S., are either very low or undetectable in U.S. adults.

--No women in the survey had dangerous concentrations of methyl mercury (search), which can come from eating shellfish or fish. However, 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.

Kristin Schafer of the Pesticide Action Network, said the report is helpful but could be improved if the CDC provided more details on where those surveyed were from. Also, she said the substances that the CDC tested for represented only a "really small slice of all the chemicals we're exposed to in the environment."

For example, the CDC examined 43 pesticides in the report, but more than 1,200 are registered by the Environmental Protection Agency, she said.

"We're talking about the tip of the iceberg. It's very important indication — we're carrying multiple pesticides and other chemicals in our bodies, including our kids," Schafer said.