Organophosphates linked to ADHD

Researchers at the University of California Berkeley tested pregnant Mexican-American women living in the Salinas Valley of California, an area of intensive agriculture, for evidence that organophosphate pesticides had actually been absorbed by their bodies, and then followed their children as they grew.

To test for ADHD, the researchers questioned the mothers and also gave the children standardized tests. They looked for breakdown products or metabolites from pesticides in urine samples from the mothers during pregnancy and from their children as they grew.

Few symptoms showed up at age 3, but by age 5 the trend was significant. A tenfold increase in pesticide metabolites in the mother's urine correlated to a 500% increase in the chances of ADHD symptoms by age 5, with the trend stronger in boys. A smaller increase in risk was seen if the children had pesticide metabolites in the urine. Women with more chemical traces of the pesticides in their urine while pregnant had children more likely to have symptoms of attention deficit hyperactivity disorder, or ADHD, at age 5, the researchers found.

In May (see below) a different team found children with high levels of organophosphate traces in the urine were almost twice as likely to develop ADHD as those with undetectable levels.

There are about 40 organophosphate pesticides such as malathion registered in the United States. Organophosphates are designed to attack the nervous systems of bugs by affecting message-carrying chemicals called neurotransmitters including acetylcholine, which is important to human brain development. Studieshave also linked exposure to Parkinson's, an incurable brain disease.

Organophosphate Urinary Metabolite Levels during Pregnancy and after Delivery in Women Living in an Agricultural Community Environmental Health Perspectives 08/10

May 2010

A study by researchers at the Harvard School of Public Health interviewed the mother or other caretaker of 1,139 children between 8 and 15 years and found that about one in ten met the criteria for ADHD, which is around the estimate for the general population. They then tracked the pesticides' breakdown products in the childrens’ urine and found those with high levels were almost twice as likely to develop ADHD as those with undetectable levels.

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.

Although the researchers had no way to determine the source of the breakdown products they found, lead researcher Marc Weisskopf said the most likely culprits were pesticides and insecticides used on produce and indoors. These compounds have been linked to behavioral symptoms common to ADHD – such as impulsivity and attention problems - but exactly how is not fully understood.

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 children with above-average levels compared to those without detectable levels.

Professor Weisskopf stressed that although this was a very strong association and of very serious concern as they are widely used pesticides, 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.

Pediatrics – May 2010 and (updated) October 2010

Click here for more research reports on the causes of ADHD

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