Americans have lower levels of lead and second-hand smoke byproducts in their bodies than they did a decade ago, according to a government study that is called the most extensive research ever of exposure to environmental chemicals.

The Centers for Disease Control and Prevention attributes the decline to stricter regulations on harmful chemicals, bans on smoking in the workplace, and programs to reduce children's exposure to lead paint.

The CDC took blood and urine samples from 2,500 people in 1999 and 2000 and tested for 116 chemicals, including metals, pesticides, insect repellents and disinfectants.

In the early 1990s, 4.4 percent of children ages 1 to 5 had elevated levels of lead, but that dropped to 2.2 percent in 1999-2000, the study found.

To gauge the effect of second-hand smoke, the CDC tested for cotinine, a product of nicotine after it enters the body, in nonsmokers. Levels dropped by 75 percent for nonsmoking adults and 58 percent for children in 1999-2000 compared with the early 1990s, the CDC said.

Cotinine levels for children were twice as high as levels for nonsmoking adults. CDC officials believe children's exposure to second-hand smoke may be high because public health efforts in the 1990s primarily focused on reducing second-hand smoke in adult areas, such as in the workplace.