New research suggests that exposure to high levels of the kind of pesticides commonly found on berries and other produce, could raise the odds that children will develop attention-deficit/hyperactivity disorder (ADHD).

The study doesn't prove that pesticide exposure can actually cause ADHD. But the study's lead author Maryse Bouchard, an adjunct researcher in the department of environmental and occupational health at the University of Montreal and at Sainte-Justine University Hospital Centre thinks it's safe to say that children should reduce their exposure to pesticides as much as possible.

"The results of this study support the hypothesis that exposure to organophosphate pesticides could play a role in the development of ADHD," said Bouchard.

"But further studies are needed," she said, because the study "is the first one to show this possible link."

Bouchard and her colleagues analyzed data on organophosphate pesticide exposure and ADHD in more than 1,100 American children aged eight to 15. All of the children were part of the U.S. government-funded National Health and Nutrition Examination Survey (NHANES) between 2000 and 2004.

They found detectable levels of the pesticides in the urine of almost all -- 94 per cent -- of the children. The study didn't determine how the children were exposed; it may have been from food, drinking water, or from the air.

As reported by their parents, 119 children in the study had attention-deficit hyperactivity disorder, or showed the severe inattention, hyperactivity and impulsivity symptoms of the disorder. Another 29 did not meet diagnostic criteria, but were taking ADHD medications regularly in the previous year.

The conclusions, published in the journal Pediatrics, state that children with higher pesticide levels in their urine were more likely to have ADHD.

In particular, those with higher levels of one pesticide called dimethyl alkylphosphate (DMAP) were more likely to be diagnosed with ADHD.

For the most frequent compound detected, 20 per cent of the children with above-average levels had ADHD. In children with no detectable amount in their urine, 10 per cent had ADHD.

According to the authors, high doses of organophosphates may inhibit acetylcholinesterase, a nervous system enzyme. Even lower doses of the pesticide might affect different growth factors and neurotransmitters.

The researchers say they couldn't prove that pesticides caused ADHD. In fact, they suggested it's possible it might even be the other way round: that that the behavior of ADHD children increased their exposure to pesticides.

"Future studies should use a prospective design, with multiple urine samples collected over time, for better assessment of chronic exposure and critical windows of exposure, and should establish appropriate temporality," the researchers wrote.

However, Dr. Philip Landrigan, director of the Children's Environmental Health Center at Mount Sinai Medical Center in New York City, noted that the study's main conclusion fits with previous research.

"It's consistent with other studies that have looked at organophosphate pesticides and have found that exposure early in life can cause brain injury," he said.

It's not clear how pesticides might affect children, but it's known that organophosphates interfere with the metabolism of acetylcholine, a key brain chemical in cognition.

Organophosphates were originally developed as nerve gases for warfare. Because they are neurotoxic, they are effective at killing pests, but worries have long remained about how they affect humans, especially the developing brains of small children.

The researchers noted that the study method was not ideal, since only one urine sample per child was used. Still, they called for further research into the link they found.

With a report by CTV's Genevieve Beauchemin in Montreal