The risk of ADHD was higher among children exposed to tobacco and lead. (CBC)

More than a third of cases of ADHD in children age eight to 15 could be reduced by eliminating pregnant mothers' exposure to tobacco and children's exposure to lead, a new U.S. study estimates.

Children exposed to tobacco smoke prenatally were 2.4 times more likely to have attention deficit hyperactivity disorder. Those with the highest levels of lead (0.2 to 0.8 micrograms of lead per decilitre of blood) had a 2.3 increased likelihood of developing the disorder.

The results of the study, conducted by researchers at Cincinnati Children's Hospital Medical Center, were published in Monday's online issue of the journal Pediatrics.

"Although we tend to focus on ADHD treatment rather than prevention, our study suggests that reducing exposures to environmental toxicants might be an important way to lower rates of ADHD," said Dr. Robert Kahn, a physician and researcher at Cincinnati Children's and the study's senior author.

"But if children are exposed to both lead and prenatal tobacco, the combined effect is synergistic," he added in a news release.

The researchers found the risk of ADHD was 8.1 times higher for children exposed to both tobacco and lead compared to children who had exposure to only negligible levels of the substances. The findings were based on analysis of U.S. data on 2,588 children between the ages of eight and 15 gathered from 2001 to 2004.

A total of 8.7 per cent of children in the study met criteria for ADHD.

Researchers measured lead levels in a child's blood and evaluated tobacco exposure based on how much smoking a women reported during pregnancy.

In Canada,19 to 22 per cent of pregnant women smoke, according to a report published last year by Alberta Health. In the U.S., it's estimated that 15 per cent of women smoke during pregnancy, according to a 2004 study cited by the researchers.

Nearly two per cent of children in the U.S. have lead levels above what the U.S. Centers for Disease Control and Prevention calls "levels of concern."

Symptoms of ADHD can include a short attention span, a low level of organization, excessive talking, aggressive gestures and irritability. The incidence of ADHD in school-age children is between five per cent and 12 per cent, according to the Centre for ADD/ADHD Advocacy, Canada.

The study was funded by grants from the National Institutes of Health and the Academic Pediatrics Association, and a Robert Wood Johnson Generalist Physician Faculty Scholars Award.