University of California Berkeley researchers have linked pesticide exposure in pregnant mothers to attention deficit disorders in their children.

Published in the journal Environmental Health Perspectives, the study adds to a growing body of evidence showing how conventional pesticides adversely affect the human brain and condemns their use in conventional agriculture.

Researchers tested pregnant Mexican-American women from the Salinas Valley region (a heavily farmed area) for traces of organophosphate pesticides in their urine. Their children were followed for several years and given a regimen of standardized tests.

The results were telling: women with higher chemical traces of organophosphates gave birth to children more likely to exhibit attention deficit hyperactivity disorder, or ADHD.

"While results of this study are not conclusive, our findings suggest that prenatal exposure to organophosphate pesticides may affect young children's attention," wrote Amy Marks and her colleagues in the study.

Organophosphates work by attacking insects' nervous systems. They block message-carrying chemicals called neurotransmitters some of which are integral to human brain development.

By analyzing metabolites (or chemically broken down components) of pesticides in the children as they grew, researchers followed an interesting trend: while few ADHD symptoms in test group subjects were observed at age 3, it was found that symptoms dramatically heightened by age 5. A pregnant woman with ten times the amount of pesticide metabolites in her urine as another is 500 percent more likely to have a child with ADHD by age 5. If the children had metabolites in their urine, a smaller increase in risk was observed. The trend was stronger in boys.

These conclusions support research published in May when it was found that children with organophosphate metabolites in their urine were at twice the risk for developing ADHD than those with no detectable levels.

Organophosphates were originally developed for chemical warfare and are known neurotoxins. Currently, the Environmental Protection Agency allows about 40 organophosphate pesticides (such as malathion) to be used on US farm crops. Scientists have also linked pesticide exposure to other neurological diseases including Parkinson's.