UC BERKELEY (US) — Children exposed prenatally to pesticides commonly used on food crops score as much as seven points lower on standardized intelligence tests when they reach the age of 7.
A new study finds that every tenfold increase in measures of organophosphates detected during a mother’s pregnancy corresponded to a 5.5 point drop in overall IQ scores.
“These associations are substantial, especially when viewing this at a population-wide level,” says Brenda Eskenazi, professor of epidemiology and of maternal and child health at the University of California, Berkeley.
“That difference could mean, on average, more kids being shifted into the lower end of the spectrum of learning, and more kids needing special services in school.”
While markers of prenatal OP pesticide exposure significantly correlated with childhood IQ, exposure to pesticides after birth didn’t, suggesting that exposure during fetal brain development is more critical than childhood exposure.
The study is one of a trio of papers showing an association between pesticide exposure and childhood IQ published online in the journal Environmental Health Perspectives.
The other two studies examined urban populations in New York City. The UC Berkeley study focused on children living in Salinas, an agricultural center in Monterey County, California.
“It is very unusual to see this much consistency across populations in studies, so that speaks to the significance of the findings,” says lead author Maryse Bouchard, who was a post-doctoral researcher with Eskenazi while the study was under way.
“The children are now at a stage where they are going to school, so it’s easier to get good, valid assessments of cognitive function.”
Indoor use of two common organophosphates (OP) pesticides has been phased out over the past decade, primarily because of health risks to children.
The 329 children in the study have been followed from before birth as part of the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS), an ongoing longitudinal study. The new findings come less than a year after another study found an association between prenatal pesticide exposure and attention problems in children at age 5.
Researchers began enrolling pregnant women in the study in 1999. During pregnancy and after the children were born, study participants regularly answered questionnaires and had the health and development of their children monitored.
During the visits, samples of urine were taken from the participants and tested for dialkyl phosphate (DAP) metabolites, the breakdown product of about 75 percent of the organophosphorus insecticides in use in the U.S.
Samples were taken twice during pregnancy, with the two results averaged, and after birth from the children at regular intervals between ages 6 months and 5 years.
The Wechsler Intelligence Scale for Children – Fourth Edition (WISC-IV) was used to assess cognitive abilities at age 7. The test includes subcategories for verbal comprehension, perceptual reasoning, working memory, and processing speed.
Each of the four cognitive development subcategories saw significant decreases in scores associated with higher levels of pesticide exposure when the mothers were pregnant. The findings held after researchers considered such factors as maternal education, family income, and exposure to other environmental contaminants, including DDT, lead, and flame retardants.
“There are limitations to every study; we used metabolites to assess exposure, so we cannot isolate the exposure to a specific pesticide chemical, for instance,” Eskenazi says.
“But the way this and the New York studies were designed—starting with pregnant women and then following their children—is one of the strongest methods available to study how environmental factors affect children’s health.”
Levels of maternal DAPs among the women were somewhat higher than average compared with the U.S. population, but they were not out of the range of measurements found among women in a national study.
“These findings are likely applicable to the general population,” Bouchard says. “In addition, the other two studies being published were done in New York City, so the connection between pesticide exposure and IQ is not limited to people living in an agricultural community.”
The prenatal exposures measured in this paper occurred in 1999-2000. Overall, OP pesticide use in the U.S. has been trending downward, declining more than 50 percent between 2001 and 2009, and, in California, about 45 percent since 2001.
According to the Centers for Disease Control, people are exposed to OP pesticides through eating foods from crops treated with them. Farm workers, gardeners, florists, pesticide applicators, and manufacturers of these insecticides may have greater exposure than the general population.
“Many people are also exposed when pesticides are used around homes, schools, or other buildings,” say co-author Asa Bradman.
To reduce exposure, consumers should reduce home use of pesticides, thoroughly wash fruits and vegetables, and switch to organic produce when possible.
“I’m concerned about people not eating right based on the results of this study,” says Eskenazi. “Most people already are not getting enough fruits and vegetables in their diet, which is linked to serious health problems in the United States. People, especially those who are pregnant, need to eat a diet rich in fruits and vegetables.”
Researchers at Emory University contributed to the study, that was funded in part by the National Institute of Environmental Health Sciences, the Environmental Protection Agency and the National Institute for Occupational Health and Safety.
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