Updated: Tue Nov. 24 2009 3:37:44 PM

CTV.ca News Staff

Children who are exposed to tobacco in the womb and to lead during childhood are almost eight times more likely to be diagnosed with attention deficit hyperactivity disorder (ADHD) than children with no such exposures, researchers have found.

The finding was made after researchers led by Dr. Tanya E. Froehlich, of the department of pediatrics at Cincinnati Children's Hospital Medical Center, analyzed U.S. government data on prenatal tobacco and childhood lead exposure.

All of the data came from the 2001-2004 National Health and Nutrition Examination Survey, which examined a representative sample of 2,600 U.S. children aged eight to 15.

The rate of ADHD in the whole group was 8.6 per cent. But the rate of ADHD was about 17 per cent in kids whose mothers smoked during pregnancy. And the rate was about 14 per cent in kids who had the highest levels of lead in their blood.

Most startling was that among those children who had a joint exposure to lead and prenatal smoking, 28.6 per cent had ADHD, found the study, published in the journal Pediatrics.

Exposure to tobacco alone was linked to a 2.4 times greater likelihood of ADHD, while a high blood level of lead was liked to a 2.3 greater risk. Combined, the tobacco and lead exposure increased the odds of ADHD by eight times.

Even children who were in the highest third of lead levels still had levels lower than what the U.S. government considers "elevated," the researchers noted.

The U.S. Centers for Disease Control and Prevention considers levels of 10 micrograms per deciliter "actionable," Froehlich said. But in the study, children in the upper third had blood lead levels that were only 1.3 micrograms per deciliter or greater; children in the middle group had levels between 0.9 and 1.3 micrograms per deciliter.

It's not clear why smoking and lead might increase the risk for ADHD, but it has been long believed that ADHD is at least in large part due to abnormalities of dopamine in the brain, and research has shown lead and smoking may alter the brain's metabolism of dopamine.

While the authors say their study was not designed to evaluate whether smoking and lead exposures caused ADHD, they conclude: "Our findings suggest that reduction of toxicant exposures may be an important avenue for ADHD prevention."

The researchers conclude that about 38 per cent of ADHD cases among children may be linked to prenatal exposure to tobacco smoke, while 25 per cent could be linked to lead exposure.