Children exposed to pesticides known as organophosphates could have a higher risk of attention-deficit/hyperactivity disorder (ADHD), according to a U.S. study.

The study urges parents to always wash produce thoroughly before serving.

Researchers tracked the pesticides' breakdown products in children' urine and found those with high levels were almost twice as likely to develop ADHD as those with undetectable levels.

The findings are based on data from the general U.S. population, meaning that exposure to the pesticides could be harmful even at levels commonly found in children's environment.

Researcher Marc Weisskopf of the Harvard School of Public Health, who worked on the study, said that even low concentrations could trigger the development of AHDH.

A compound known as Organophosphates was originally developed for chemical warfare, and they are known to be toxic to the nervous system.

There are about 40 organophosphate pesticides such as malathion registered in the United States, the researchers wrote in the journal Pediatrics.

Weisskopf said the compounds have been linked to behavioral symptoms common to ADHD -- for instance, impulsivity and attention problems -- but exactly how is not fully understood.

Although the researchers had no way to determine the source of the breakdown products they found, Weisskopf said the most likely culprits were pesticides and insecticides used on produce and indoors.

Garry Hamlin of Dow AgroSciences, which manufactures an organophosphate known as chlorpyrifos, said he had not had time to read the report closely.

Weisskopf and colleagues' sample included 1,139 children between 8 and 15 years. They interviewed the children's mothers, or another caretaker, and found that about one in 10 met the criteria for ADHD, which jibes with estimates for the general population.

After accounting for factors such as gender, age and race, they found the odds of having ADHD rose with the level of pesticide breakdown products.

For a 10-fold increase in one class of those compounds, the odds of ADHD increased by more than half. And for the most common breakdown product, called dimethyl triophosphate, the odds of ADHD almost doubled in kids with above-average levels compared to those without detectable levels.

That's a very strong association that, if true, is of very serious concern, said Weisskopf. These are widely used pesticides.

He emphasized that more studies are needed, especially following exposure levels over time, before contemplating a ban on the pesticides. Still, he urged parents to be aware of what insecticides they were using around the house and to wash produce.