30 Jan 2010, 1145 hrs IST

Scientists believe that exposure to lead might be linked to attention deficit hyperactivity disorder (ADHD) in kids.

They said while genes may account for as much as 70 percent of hyperactivity and inattention in children, rest 30pct could be down to levels of lead in the body.

Two new studies have provided strong evidence.

The first study compared children formally diagnosed with ADHD to controls, and found that the children with the disorder had slightly higher levels of lead in their blood. This study showed a link only between blood lead and hyperactivity/impulsivity symptoms, not inattention.

But a second study showed a robust link between blood lead and both parent and teacher ratings of ADHD symptoms, including both hyperactivity and attention problems.

Psychological scientist Joel Nigg of the Oregon Health and Science University offers a causal model for the disabling symptoms associated with ADHD.

Lead attaches to sites in the brain's striatum and frontal cortex, where it acts on the genes in these regions-causing them to turn on or remain inactive. Gene activity shapes the development and activity of these brain regions.

By disrupting brain activity, the toxin in turn alters psychological processes supported by these neurons, notably cognitive control. Finally, diminished cognitive control contributes to hyperactivity and lack of vigilance.

The findings appear in journal Current Directions in Psychological Science.

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