The study continues previous work, which had linked ADHD to environmental lead and smoke exposure. "It's a landmark paper that quantifies the number of cases of ADHD that can be attributed to very important environmental exposures," said Dr. Leo Trasande, assistant director of the Center for Children's Health and the Environment at Mount Sinai School of Medicine in New York.
The authors led by researcher Joe Braun of the University of Wisconsin-Milwaukee said that 25 percent of the cases are caused by environmental and genetic factors. "The findings of this study underscore the profound behavioral health impact of these prevalent exposures, and highlight the need to strengthen public health efforts to reduce prenatal tobacco smoke exposure and childhood lead exposure," they wrote.
The researchers examined data on 4,000 U.S. children ages 4 to 15 who were part of a 1999-2002 government health study. Among them 135 children had already been treated for the disease. They found that children whose mothers smoked when pregnant were 2 1/2 times more likely to have ADHD than children of mother who did not smoke.