In the ADHD study, published today in Pediatrics, researchers at the University of Montreal and Harvard studied the levels of pesticide residue in the urine of more than 1,139 children ages 8 to 15, 119 of whom were diagnosed with ADHD. The children with highest levels of diakyl phosphates, the breakdown products of organophosphate pesticides, were 93 percent more likely to have the disorder than those with undetectable levels. Overall, the researchers, led by Maryse Bouchard, found a 35 percent increase in the odds of developing ADHD with every tenfold increase in concentration of the residue.
The exact causes of ADHD are still unknown. Bouchard stressed that while her analysis is the first to peg pesticides as a potential contributor to ADHD, the study proves only an association and not a direct causal link. Other studies have shown that exposure to organophosphates in early life can cause brain injuries.
According to the Centers for Disease Control and Prevention, up to 7 percent, or 4.5 million, of school-aged American children had been diagnosed with ADHD as of 2006. Boys are more likely to be diagnosed than girls, and prevalence rates are significantly higher among non-Hispanic, primarily English-speaking, and insured children.
While flashy TV programs and video games are often blamed for causing ADHD, research suggests that genetics and chemical influences are more likely to contribute to the increase in attention problems among kids. ADHD tends to run in families: according to the Mayo Clinic, about one in four kids with ADHD have at least one relative with the disorder. Children are more likely to have ADHD if their mothers smoked while pregnant, abused drugs or alcohol, or were exposed to environmental toxins such as polychlorinated biphenyls (PCBs). ADHD has also been linked to early childhood exposure to lead and PCBs.
A 2002 study found that 73 percent of conventionally grown sample crops had traces of pesticides. Five crops in particular -- apples, peaches, pears, strawberries, and celery -- had the most residues. Bouchard suggests that parents avoid using bug sprays at home and purchase organic produce when possible. It's also best to wash all fruits and vegetables under cold tap water and scrub firm-skinned produce with a brush.
CropLife America, a national association that represents pesticide manufacturers, issued a statement to ABC News saying it "fully supports continuous study to help better understand [ADHD's] cause." The organization noted that all pesticide products are "extensively reviewed" before approval for market use and that the particular pesticides studied here were approved and registered by the EPA.