A study by the University of Southern California has found a link between autism and pesticides.
The study does not prove that pesticides cause autism, but does show that autism rates are higher where there are more pesticides used.
Pregnant women who lived close to fields and farms where chemical pesticides were applied experienced a two-thirds increased risk of having a child with autism spectrum disorder or other developmental delay, the researchers say.
The associations were stronger when the exposures occurred during the second and third trimesters of the women’s pregnancies.
The large, multisite California-based study examined associations between specific classes of pesticides, including organophosphates, pyrethroids and carbamates, applied during the study participants’ pregnancies and later diagnoses of autism and developmental delay in their offspring. It is published online today in Environmental Health Perspectives.
“This study validates the results of earlier research that has reported associations between having a child with autism and prenatal exposure to agricultural chemicals in California,” said lead study author Janie F. Shelton, a University of California at Davis graduate student who now consults with the United Nations.
“While we still must investigate whether certain sub-groups are more vulnerable to exposures to these compounds than others, the message is very clear: Women who are pregnant should take special care to avoid contact with agricultural chemicals whenever possible,” she said.
The study was conducted by examining commercial pesticide application using the California Pesticide Use Report and linking the data to the residential addresses of approximately 1,000 participants in the Northern California-based Childhood Risk of Autism from Genetics and the Environment (CHARGE) Study. The study includes families with children between two and five diagnosed with autism or developmental delay or with typical development.
It is led by principal investigator Irva Hertz-Picciotto, a MIND Institute researcher and professor and vice chair of the Department of Public Health Sciences at UC Davis.
The majority of study participants live in the Sacramento Valley, Central Valley and the greater San Francisco Bay Area.
Twenty-one chemical compounds were identified in the organophosphate class, including chlorpyrifos, acephate and diazinon.
The second most commonly applied class of pesticides was pyrethroids, one quarter of which was esfenvalerate, followed by lambda-cyhalothrin permethrin, cypermethrin and tau-fluvalinate.
Eighty percent of the carbamates were methomyl and carbaryl.
For the study, researchers used questionnaires to obtain study participants' residential addresses during the pre-conception and pregnancy periods. The addresses then were overlaid on maps with the locations of agricultural chemical application sites based on the pesticide-use reports to determine residential proximity. The study also examined which participants were exposed to which agricultural chemicals.
“We mapped where our study participants’ lived during pregnancy and around the time of birth.
“In California, pesticide applicators must report what they’re applying, where they’re applying it, dates when the applications were made and how much was applied,” Hertz-Picciotto said.
“What we saw were several classes of pesticides more commonly applied near residences of mothers whose children developed autism or had delayed cognitive or other skills.”