In a recent government study, levels of lead have dropped dramatically, exposure to second-hand smoke is down, and most women are not burdened by unsafe levels of mercury.

The Third National Report on Human Exposure to Environmental Chemicals, released on Thursday by the Centers for Disease Control and Prevention, has details on 148 different chemicals found in the blood and urine of 2,400 volunteers.

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, noting that the removal of lead from gasoline was the main reason for the decline.