Studies linking environmental substances to disease are coming fast and furious. Chemicals in plastics and common household goods have been associated with serious developmental problems, while a long inventory of other hazards are contributing to rising rates of modern ills: heart disease, obesity, diabetes, autism.

Add attention-deficit/hyperactivity disorder (ADHD) to the list. A new study in the journal Pediatrics associates exposure to pesticides with cases of ADHD in the U.S. and Canada. In the U.S. alone, an estimated 4.5 million children ages 5 to 17 have been diagnosed with ADHD, according to the Centers for Disease Control and Prevention, and rates of diagnosis have risen 3% a year between 1997 and 2006. Increasingly, research suggests that chemical influences, perhaps in combination with other environmental factors — like video games, hyperkinetically edited TV shows and flashing images in educational DVDs aimed at infants — may be contributing to the increase in attention problems.(See pictures of a school for autistic children.)

Led by Maryse Bouchard in Montreal, researchers based at the University of Montreal and Harvard University examined the potential relationship between ADHD and exposure to certain toxic pesticides called organophosphates. The team analyzed the levels of pesticide residue in the urine of more than 1,100 children ages 8 to 15 and found that those with the highest levels of dialkyl phosphates, which are the breakdown products of organophosphate pesticides, had the highest incidence of ADHD. Overall, they found a 35% increase in the odds of developing ADHD with every tenfold increase in urinary concentration of the pesticide residue. The effect was seen even at the low end of exposure: kids who had any detectable, above-average level of the most common pesticide metabolite in their urine were twice as likely as those with undetectable levels to record symptoms of the learning disorder.

“I was quite surprised to see an effect at lower levels of exposure,” says Bouchard, who used data on ADHD from the National Health and Nutrition Examination Survey, a long-term study of health parameters of a representative sample of U.S. citizens.(See how fidgeting can actually help kids with ADHD.)

Bouchard’s analysis is the first to home in on organophosphate pesticides as a potential contributor to ADHD in young children. But the author stresses that her study uncovers only an association, not a direct causal link between pesticide exposure and the developmental condition. There is evidence, however, that the mechanism of the link may be worth studying further: organophosphates are known to cause damage to the nerve connections in the brain — that’s how they kill agricultural pests, after all. The chemical works by disrupting a specific neurotransmitter, acetylcholinesterase, a defect that has been implicated in children diagnosed with ADHD. In animal models, exposure to the pesticides has resulted in hyperactivity and cognitive deficits as well. (Comment on this story.)

“I am very confident in the correlation in this study, because we controlled for quite a few things that we thought could play a role,” says Bouchard. “Adjusting for those things did not change the results very much. Which indicates that there is very little potential for confounding in this association between pesticides and ADHD.”

The results call for additional studies to determine exactly which foods and which residential uses of pesticides may be most likely to lead to harm in children. Although Bouchard’s study did not determine the exact method of exposure in the participants, youngsters are most likely to ingest the chemicals through their diet — by eating fruits and vegetables that have been sprayed while growing — according to the National Academy of Sciences. The study also raises the possibility of setting a national threshold for safe levels of exposure; the study authors note that according to the U.S. Pesticide Residue Program report, organophosphates were detected in 28% of frozen blueberries and in 19% of celery samples tested for pesticides. It is not clear whether those levels pose a threat to cognitive function in children, but the current study’s findings suggest it may be wise to figure that out.

In the meantime, Bouchard suggests that concerned parents try to avoid using bug sprays in the home and to feed their children organically grown fruits and vegetables, if possible. (Otherwise, parents should be careful to scrub all produce to reduce residue.) While pesticide-free fruits and greens may be more costly, Bouchard says they may be worth the price in terms of future health.