A widely used pesticide – banned in homes but still commonly used on farms – appears to harm boys’ developing brains more than girls’, according to a new study of children in New York City.

In boys, exposure to chlorpyrifos in the womb was associated with lower scores on short-term memory tests compared with girls exposed to similar amounts.

The study is the first to find gender differences in how the insecticide harms prenatal development. Scientists say the finding adds to evidence that boys’ brains may be more vulnerable to some chemical exposures.

“This suggests that the harmful effects of chlorpyrifos are stronger among boys, which indicates that perhaps boys are more vulnerable to this type of exposure,” said Virginia Rauh, a perinatal epidemiologist at Columbia University and co-author of the study published in July.

Chlorpyrifos is an organophosphate insecticide, a powerful class of pesticide that has toxic effects on nervous systems. It was widely used in homes and yards to kill cockroaches and other insects, but in 2001 the U.S. Environmental Protection Agency banned its residential use because of health risks to children. Since then, levels inside U.S. homes have dropped [PDF], but residue remains in many homes. In addition, many developing countries still use the pesticide indoors.

Known by the Dow trade name Lorsban, chlorpyrifos is still sprayed on some crops, including fruit trees and vegetables, and also is used on golf courses and for mosquito control. About 10 million pounds of chlorpyrifos are applied to agricultural fields annually, according to the EPA.

“There’s mounting evidence now from epidemiological studies that prenatal exposure to organophosphate pesticides, and chlorpyrifos in particular, may be associated with detriments with IQ in children,” said Kim Harley, an environmental epidemiologist with the University of California, Berkeley who has studied effects of pesticide exposure on children in California farm towns. She was not involved in the New York City study.

The environmental group Earthjustice has sued the EPA in an effort to ban all remaining uses of chlopyrifos.

“The exposures are to farmworkers and farmworker families, and people who live in those rural areas that are abutting the fields where chlorpyrifos is applied,” said Earthjustice attorney Kristen Boyles.

The 2007 lawsuit is still pending, although the EPA announced last month that it will require reductions in application rates and buffers to protect children and other bystanders. An EPA spokesperson said that the agency is re-evaluating chlorpyrifos and expects to make a decision in 2014.

Representatives of DowAgroSciences, a major manufacturer of pesticides containing chlorpyrifos, did not return requests seeking comment.

The 335 pairs of mothers and children in the new study were not farmworkers, but are part of a large group of Latino and African American children from low-income neighborhoods of Northern Manhattan and the South Bronx. Columbia University researchers have been tracking more than 700 of these kids since they were born, between 1998 and 2006. Children there have a history of health problems, including asthma rates that are among the nation’s highest, and low birth weight. Many were born before the residential ban on chlorpyrifos.

An earlier study of the children found that chlorpyrifos was linked to delayed mental and motor skill development even after controlling for poverty, dilapidated housing and other community factors. The scientists, in a more detailed follow-up, then found that IQs and memories were reduced in 7 year olds with higher prenatal exposure. Those with the highest exposures scored on average 5.3 points lower on a short-term memory test, and 2.7 points lower on an IQ test, than children with the lowest exposures.

In the new study, published last month in the journal Neurotoxicology and Teratology, umbilical cord blood was collected from the newborns, who were born before and slightly after the 2001 chlorpyrifos restrictions. When the kids were 3 years old, the researchers studied how well the mothers nurtured and educationally stimulated them. Then, at age 7, the children’s short-term memory ¬¬¬was tested, for example, by having them repeat a sequence of numbers. Memory is an important component of IQ tests.

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