TIME Autism

Pesticide Exposure During Pregnancy Strongly Linked to Autism

Prenatal exposure to commercial pest spraying can boost risk of autism by up to 60%

Autism cannot be attributed to any one risk factor—genes play a role, as does an expectant mom’s diet, some medications and exposure to environmental pollutants. While previous studies have connected autism to prenatal exposure to agricultural chemicals, it wasn’t clear whether other factors could account for the higher rates of autism among their children. A new study gets closer to the answer.

California laws require that commercial pesticide spraying be recorded. So Irva Hertz-Picciotto, professor of epidemiology at the Mind Institute at University of California, Davis, took advantage of the data showing where pesticides had been sprayed and matched it against pregnant women’s home addresses. About one-third of the mothers-to-be lived under a mile from at least one pesticide application during their pregnancy. If the pesticide was an organophosphate, a class of compounds that has largely been phased out of home bug and lawn sprays but remain in commercial applications, the women showed a 60% higher risk of having a child with an autism spectrum disorder (ASD). Living near a spraying of pyrethroids, which are commonly found in home insect sprays, just before conception or during the third trimester of pregnancy increased by up to two-fold the risk of both ASDs and developmental delays.

Insecticides are known to be toxic to nerves, and developing babies may be especially vulnerable, says Hertz-Picciotto, since their brains are just forming important brain structures and connections that can be disrupted by the chemicals. “Many pesticides operate through affecting the nervous system of lower organisms,” she says. “So they should be taken seriously, because they are by design neurotoxic. The question is at what dose.”

Still, while the study involved more than 1,000 participants, Hertz-Picciotto says it’s not definitive proof that pesticides cause autism. They adjusted for potential factors that could also contribute to higher risk of autism, such as parental age, mother’s health, and distance of the residences from freeways. But they did not have information on how many hours the pregnant women typically spent at home, or on whether they were actually at home during the sprayings. The scientists also did not have information on the mothers’ diets, which could introduce pesticide residue from foods, or their occupations, including whether their workplace exposures might have also played a role in their children’s autism risk.

The association does add to growing data that connects pesticide exposure to potential developmental problems in fetuses, however. The fact that the rates of autism were highest among women who lived closest to the pesticide applications, and lower among those who lived further away, suggests that the chemicals are worth studying further for what role, if any, they play in contributing to autism.

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