By Denise Gellene
Los Angeles Times
One-third of attention deficit hyperactivity disorder cases are linked to prenatal exposures to cigarette smoke or childhood exposures to lead, researchers reported Monday.
The study, led by researchers at Cincinnati Children's Hospital Medical Center, was the first to estimate the number of ADHD cases attributable to environmental toxins.
The report "provides further evidence that we need to find ways to dramatically reduce prenatal tobacco smoke exposures and childhood lead exposures," said lead author Dr. Bruce Lanphear.
ADHD is a condition marked by impulsivity, poor concentration and hyperactivity, making it difficult for children to pay attention in school. About 2 million, or one of every 25 school-age children in the United States, have ADHD.
Researchers analyzed data gathered on 4,704 children ages 4 to 15 as part of the federal National Health and Nutrition Examination Survey. The survey was conducted from 1999 to 2002.
About 8 percent of children in the study had been diagnosed with ADHD and 4.2 percent were prescribed drugs to treat the condition.
The study contained information on prenatal exposure to cigarette smoke and the concentration of lead in blood samples taken from the children.
The researchers, who published their findings in the journal Environmental Health Perspectives, linked blood lead concentrations of 2 micrograms per deciliter or greater to an increased risk of ADHD. Children in that group had a four-fold risk of ADHD compared to children with the lowest blood lead levels - under 0.8 micrograms per deciliter.
Federal standards consider blood lead levels below 10 micrograms per deciliter to be safe.
The study confirmed the link found in previous studies between prenatal exposure to cigarette smoke and ADHD. The latest study found that children exposed to tobacco smoke prenatally had a 2.5-fold greater risk of ADHD than unexposed children.
There was no connection between childhood exposure to tobacco smoke and ADHD.
Lanphear said that taken together, prenatal tobacco smoke and childhood lead exposures accounted for 480,000 of and estimated 1.8 million cases of ADHD.