An article I wrote for Prevention magazine in 2010 described the Environmental Working Group’s “Shopper’s Guide to Pesticides in Produce,” which listed a “Dirty Dozen” fruits and vegetables most highly contaminated with pesticides and a “Clean Fifteen” relatively low in pesticide residues. I interviewed horticulturalists and entomologists across the country about the use of pesticides on peaches, apples, bell peppers, and the like; learned that the fuzz on peaches traps a bit more pesticides than waxy-skinned nectarines; and read about watermelon vine decline, a devastating viral disease transmitted by whitefly.
I also discovered that farmers often spray pesticides not just to combat watermelon killers and similar pathogens but also to provide produce that is more pleasing to the consumer, free of blemishes or barely visible insect nibbles. In 2007, according to the U.S. Environmental Protection Agency, more than five billion pounds of pesticides were applied on farms and in homes and factories, and for water treatment. Among these applications were 33 million pounds of organophosphate insecticides, which are known nerve toxins.
Now newly published research has shown that pregnant women who live close to farms sprayed with pesticides have a two-thirds increased risk of having a child diagnosed with autism or other developmental delay, which refers to “significant delays” in cognitive development or in communication, social, and/or motor skills. The study, led by Janie Shelton and Irva Hertz-Picciotto of the MIND Institute at the University of California, Davis, examined 1,000 participants, including children with autism and developmental delay, as well as those who developed typically. One-third of the mothers in the study lived within slightly less than a mile of agricultural pesticide applications, and those in close proximity to organophosphate spraying during pregnancy had a 60 percent increased risk of giving birth to a child later diagnosed with autism spectrum disorder.
Most of those in the study live in the Sacramento Valley, Central Valley, and the greater San Francisco Bay Area—coincidentally, the regions where the majority of the 200 million pounds of pesticides used yearly in California are applied. The researchers identified 21 different organophosphate pesticides sprayed in these areas, including chlorpyrifos, a broad-spectrum insecticide effective against corn rootworms, lice, flies, and termites. “Because these pesticides are neurotoxic, in utero exposures during early development may distort the complex processes of structural development and neuronal signaling,” within the developing fetal brain, the researchers note in an accompanying press release, leading to alterations in the mechanisms that govern mood, learning, social interactions, and behavior.
The research confirms previous findings that link autism to organophosphate exposure, particularly chlorpyrifos, and adds evidence that pyrethroids—a class of synthetic chemicals often found in household insecticides—may also increase the risk, Shelton wrote to me via email. Although organophosphates and pyrethroids were, on average, more strongly associated with autism than developmental delay, Shelton wrote, another class of pesticides, called carbamates, were more strongly associated with developmental delay.
The use of organophosphates has declined over the past decade, but pyrethroids, which have replaced chlorpyrifos for indoor use, are not necessarily safer. One study has shown that cyfluthrin, a common pyrethroid, has either “an equivalent or higher toxic effect” as chlorpyrifos on the growth and survival of fetal human astrocytes, which physically support neurons within the central nervous system and sweep up debris within the brain.
Shelton’s research supports a growing body of evidence linking autism to environmental contaminants and provides a strong impetus for pregnant women to avoid pesticide-contaminated foods as well as indoor insect sprays. As with research into ultrafine particles of air pollution, the study could also partly explain the mystifying increase in autism diagnosis over the last decade. Her conclusion: “I do think, and evidence shows, environmental factors likely contribute to a significant number of additional cases beyond genetic risk alone.”
