LOS ANGELES, Nov. 23 (Xinhua) -- Children may face a high risk of attention deficit hyper activity disorder, or ADHD, if they are exposed to tobacco smoke before being born or to lead during their childhood, a new study shows.

    In the study, researchers from Cincinnati Children's Hospital Medical Center evaluated 2001 - 2004 data from the National Center for Health Statistics at the Centers for Disease Control and Prevention among kids ranging from 8 to 15 years old.

    The research team measured prenatal tobacco exposure by maternal reports of cigarette use during pregnancy, while lead exposure was assessed by the participants' current blood lead levels.

    Based on the data, which is part of the National Health and Nutrition Examination Survey, children exposed to tobacco smoke prenatally were found 2.4 times more likely to suffer ADHD.

    Those with blood lead levels in the top third had a 2.3-times increase, despite the levels being well below the action level of 10 micrograms per deciliter prescribed by the Centers for Disease Control.

    "Tobacco and lead exposure each have their own adverse effects," said Tanya Froehlich, M.D., one of the study's leading researchers in the Division of Developmental and Behavioral Pediatrics at Cincinnati Children's. "But if children are exposed to both lead and prenatal tobacco inhalation, the combined effect is synergistic."

    According to the statistics, for children exposed to both tobacco and lead, the risk of ADHD will be enhanced by over eight times as much as the unexposed ones.

    The new study, which was for release online on Nov. 23 by Pediatrics magazine, also estimates that up to 35 percent of ADHD cases in children between the ages of 8 and 15 could be reduced by eliminating both of these environmental toxicants exposures. This could translate into up to 800,000 children in the U.S.

    "Our study suggests that reducing exposures to environmental toxicants might be an important way to lower rates of having ADHD," said Robert Kahn, the study's senior author.

    Attention deficit hyperactivity disorder is one of the most common mental disorders that develop in children. Children with ADHD have impaired functioning in multiple settings, including home, school, and in relationships with peers.