BAKERSFIELD, Calif. -- Studies suggest that nearly 1 in 10 school-age children has attention deficit hyperactivity disorder or ADHD -- putting them at greater risk for poor academic performance, dropping out, and substance abuse.

The causes of ADHD are still largely unknown, but new research has found that environmental toxins may play a role.

Doctors at Cincinnati Children's Hospital examined data from a national health survey and found 9 percent of kids ages 8 to 15 years old had ADHD.

Those who had the disorder were more likely to have been exposed to tobacco and lead. Kids whose mothers smoked during pregnancy and those who had higher amounts of lead in their blood had more than double the risk for ADHD.

But when combined, the two toxins increased ADHD risk by eight times.

Researchers estimated that 35 percent of ADHD diagnoses are caused by lead and tobacco, suggesting that reducing children's exposure could eliminate thousands of ADHD cases.