Children whose mothers were exposed to certain types of pesticides while pregnant were more likely to have attention problems as they grew up, U.S. researchers reported on Thursday.

The study, published in Environmental Health Perspectives, adds to evidence that organophosphate pesticides can affect the human brain.

Researchers at the University of California Berkeley tested pregnant women for evidence that organophosphate pesticides had actually been absorbed by their bodies, and then followed their children as they grew.

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.

While results of this study are not conclusive, our findings suggest that prenatal exposure to organophosphate pesticides may affect young children's attention, Amy Marks and colleagues wrote in the study, available here:doi/10.1289/ehp.1002056

To test for ADHD, the researchers questioned the mothers and also gave the children standardized tests.

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.

The researchers tested Mexican-American women living in the Salinas Valley of California, an area of intensive agriculture.

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, Marks and colleagues found.

A tenfold increase in pesticide metabolites in the mother's urine correlated to a 500 percent 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.

In May 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. Studies have also linked exposure to Parkinson's, an incurable brain disease.

California Medicine