Cigarettes and lead linked to attention disorder.

Nearly half a million cases of attention-deficit hyperactivity disorder among U.S. children are reined to exposures to lead or their mothers' smoking while pregnant, a nationwide study suggests. The two environmental hazards might account for more than a quarter of drug-treated ADHD cases.

The finding bolsters earlier research that linked smoke exposure to ADHD and provides the best evidence yet that lead, a brain-damaging metal, might also contribute to the common behavioral disorder.

Published online on Sept. 19 for an upcoming Environ mental Health Perspectives, the study analyzed data on 4,704 children who were 4 to 15 years old when they participated in a recent nationwide survey of health and nutrition.

Surveyors recorded that 4.2 percent of the children had been both diagnosed with ADHD and prescribed stimulants to treat the condition. The researchers collected other data as well, including the concentration of lead in a blood sample from each child, whether the child's mother had smoked during pregnancy, and whether anyone subsequently smoked in the child's home.

Children whose mothers smoked before giving birth--but not those exposed later--were 2.5 times as likely as other children to have ADHD, the new analysis shows. And kids with the highest blood-lead concentrations were 4.1 times as likely to have the disorder as were children with the lowest concentrations, report researchers led by Bruce Lanphear of Cincinnati Children's Hospital Medical Center.

Given the percentage of surveyed children who are taking drugs for ADHD, at least 1.8 million children nationwide have the disorder, the researchers estimate. About 480,000 of those cases could be attributed to smoke exposure, lead exposure, or a combination of the two, the scientists say.