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High levels of pesticide exposure in pregnant women have been linked to lower IQs in their children, according to three separate US studies.

Two studies were done in New York City and a third was in Salinas, a farming area of northern California. All spanned nearly a decade, tracking levels of pesticide in expectant mothers and testing nearly 1,000 children up to age nine.

Researchers looked at exposure to a family of pesticides known as organophosphates, which are commonly used on fruit and vegetable crops. The reports are published in the journal Environmental Health Perspectives.

In the California study involving 392 kids, "researchers found that every tenfold increase in measures of organophosphates detected during a mother's pregnancy corresponded to a 5.5 drop in overall IQ in the seven-year-olds."

The differences held even after researchers accounted for factors such as education, family income, and exposure to other environmental contaminants, the study, released on Thursday, said.

Researchers at Mount Sinai, New York measured 400 women and their children from 1998 onward.

They found that "exposure to organophosphates negatively impacted perceptual reasoning, a measure of non-verbal problem-solving skills" between the ages of six and nine.

They also found that about one-third of the mothers studied carried a gene variant that made them less able to metabolize the pesticides, and that the negative effects in children were limited to this subgroup.

The third study, done by researchers at New York's Columbia University, looked specifically at one pesticide, chlorpyrifos, which was widely used to kill cockroaches and termites until it was banned from residential use in 2001.

In the sample of 265 minority children born before the ban took effect, higher prenatal exposure was linked to lower intelligence scores and poorer memory.

Children in the top 25 percent of exposure levels scored 5.5 percent lower in working memory tests and 2.7 points lower in IQ.

"These observed deficits in cognitive functioning at seven years of age could have implications for school performance," said lead author Virginia Rauh of the Columbia Center for Children's Environmental Health.

"Working memory problems may interfere with reading comprehension, learning and academic achievement, even if general intelligence remains in the normal range."

Even though the studies were carried out independent of each other, the similarity in results raises concern, said lead author of the California study, Maryse Bouchard.

"It is very unusual to see this much consistency across populations in studies, so that speaks to the significance of the findings," she said.

Principal investigator Brenda Eskenazi described the associations as "substantial, especially when viewing this at a population-wide level."

Organophosphate pesticide use declined more than 50 percent between 2001 and 2009, the Berkeley researchers said.

However, both diazinon -- another common organophosphate that was banned from residential use in 2004 because it was a known neurotoxicant shown to have health risks for children -- and chlorpyrifos continue to be used in agricultural fields.

Most of the modern-day exposure to such chemicals would likely be through eating food treated with the pesticides. Experts recommend washing produce with running water and rubbing it to remove residue.

AFP