Exposure to pesticides at levels previously considered harmless has been linked to behavioural problems in young children, reports The Washington Post.

When scientists at the University of Montreal and Harvard University looked at organophosphate pesticide metabolites, an indicator of pesticide exposure, in the urine of 1,139 children (aged eight to 15), they found that close to 95 percent had at least one of these chemical byproducts in their system.

The researchers showed that those children with the highest levels were 93 percent more likely to have received an ADHD diagnosis than children with none in their system. Children with above-average levels of the most common organophosphate byproduct —around a third of the whole group — were more than twice as likely as the rest to have ADHD.

Maryse Bouchard, a researcher at the University of Montreal, says the findings are significant. She told The Washington Post: “This is not a small effect and it is certainly cause for concern. We are talking about very low levels of exposure . . . levels that were believed previously to be safe and harmless but which are now associated with a serious health risk.