In 10 years, diagnoses of attention deficit hyperactivity disorder (ADHD) increased 24% in southern California, according to a study published Monday in the journal JAMA Pediatrics.

Doctors reviewed anonymized medical records for children treated at the Kaiser Permanente Southern California physicians group between 2001 and 2010 - 842,830 children in all, according to the research.

Overall, in 2001, 2.5% of children aged 5 to 11 were diagnosed with ADHD, but that number crept up to 3.1% by 2010.

Researchers believe the study's method, reviewing actual medical records within a defined group, gives a more accurate picture of ADHD in Southern California than other estimates.

For example, the Centers for Disease Control and Prevention estimates a much higher rate of ADHD - about 9.5% of U.S. children aged 4 to 17 in 2007 – but that estimate relies on parents responding to telephone surveys, a relatively inaccurate method.

Also, the vast majority of ADHD diagnosis in the study were by specialists using a strict definition of ADHD, as defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). One study cited in the new research found that only 38% of primary care physicians actually used the DSM-IV for diagnosing ADHD, further complicating other estimates of the condition.

The lowest rate of ADHD - 1.1% - was seen in Asian and Pacific Islander children and did not increase in the 10-year period, though researchers noted that Asian-American children were particularly less likely to use mental health services and more likely to discontinue therapy.

Researchers found the largest percentage increase in ADHD diagnoses among African-American children. Rates increased 69.6% between 2001 and 2010 for that group.

Still, the overall rate of ADHD diagnosis was highest in white children, at 5.6% in 2010.

"Our study findings suggest that there may be a large number of factors that affect ADHD diagnosis rates, including cultural factors, that may influence the treatment-seeking behavior of some groups," said study lead author Dr. Darios Getahun from Kaiser Permanente Southern California's Department of Research & Evaluation. "These findings are particularly solid given that our study relied on clinical diagnoses of ADHD based on the criteria specified within the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders and that it represents a large and ethnically diverse population that can be generalized to other populations."

"Even if some of the reported increase is due to better awareness, these data seem to speak to a true increase. There's a fairly substantial list of environmental factors that have been linked to ADHD," says Dr. Philip Landrigan, Director of the Children's Environmental Health Center at Mount Sinai School of Medicine.

"That list includes prenatal exposure to maternal tobacco smoke, it includes lead; more recently, researchers at Mt. Sinai have linked prenatal exposure to phthalates, a plastics chemical, to ADHD. There's some evidence that prenatal exposure to organophosphate pesticides increases risk for ADHD. I think this is a list that we can expect to continue to grow in the years ahead because we now have very sophisticated scientific tools for testing linkage between prenatal exposure and conditions like ADHD in children."