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.

Chlorpyrifos exposure had a larger association with working memory scores in the boys, who averaged three points lower than the girls with similar exposures, the study found.

“There are adverse effects overall, but you see that the effects are bigger in boys,” Rauh said. “The notion is that boys might be more vulnerable, for whatever reason.”

The researchers also looked to see if the parents could make up for the memory deficiencies. When the kids were 3 years old, the researchers measured the mom’s attentiveness, displays of affection, ability to control her negative reactions and other parenting skills. The study found “in terms of working memory, males benefit more from a nurturing environment than females,” the researchers wrote.

Rauh said the findings suggest that boys seemed to be “modestly” more influenced by environmental factors, whether chemical (chlorpyrifos) or social (the mother).

“It suggests that there may be some room for intervention,” Rauh said. “There’s a little wiggle room there, and that’s the good news.”

Why chlorpyrifos might affect boys more than girls is not fully understood, but a 2012 study of rats found that the pesticide reduced testosterone, which has a critical role in male brain development.

The new finding is consistent with what is known about how other chemicals affect boys more than girls, said David Carpenter, director of the Institute for Health and the Environment at the University at Albany.

“There’s such a variety of different chemicals that all do the same thing,” Carpenter said. “They reduce IQ, they appear to shorten the attention span and reduce the ability to deal with frustration.”

Lead, for example, seems to cause a greater IQ deficit in boys than girls, and some evidence suggests that polychlorinated biphenyls, PCBs, may have similar consequences, Carpenter said.

Previous research has shown that low to moderate exposure to chlorpyrifos during pregnancy can lead to irreversible changes in a child’s brain. According to a 2012 study of the New York City children, magnetic resonance imaging of 40 children, from about 6 to 11 years old, found that those with high exposures had more abnormalities in regions of the brain associated with memory. They also were significantly more likely than children exposed to low levels to experience attention problems and delays in cognitive and motor skills [PDF].

The new study is the first to measure chlorpyrifos in umbilical cord blood, which unequivocally shows if a mother and her fetus were exposed. Researchers do not know how those kids’ exposures, which occurred between 1998 and 2006, compare with levels in kids today because there are no data for comparison.
Dana Barr of Emory University, an expert in chemical exposure and a co-author of the new study, said chlorpyrifos exposure in the New York City children apparently has declined. The researchers did not detect it in the umbilical cord blood of babies enrolled in the study after the 2001 ban.

Data from California, which keeps detailed records of annual pesticide use, show that chlorpyrifos use on farm fields decreased 23 percent between 2001 and 2010. It is sprayed on corn, cotton, citrus and nut trees, alfalfa, grapes and other crops. Organophosphate exposure has been shown to have similar effects on children from farmworker families. In California’s Salinas Valley, Latino children whose mothers had the highest exposures to organophosphates, including chlorpyrifos, had a 7-point drop in IQ compared with children of moms with the lowest exposures.

“I think there’s no reason to believe that the exposure levels are very different” between the farm and the urban communities before the residential ban, Rauh said.

Virginia Ruiz, director of occupational and environmental health for Farmworker Justice, said chlorpyrifos is one of the most dangerous pesticides that farmworkers are exposed to. Her group is pushing for an end to all uses as soon as possible.

“Most farmworkers are foreign-born immigrants and have low levels of education and English proficiency,” Ruiz said. “Farmworkers don’t get a lot of education on the chemicals they’re working around.”

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.