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Rethinking Autism's Origins |
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Author: Bruce Lanphear, MD, MPH NOTE: This article was originally published on March 25, 2025 by Plagues, Pollution & Poverty on Substack. The spike in autism over just one generation is hard to ignore. In 2000, the rate among 8-year-old children was 1 in 150 children in the US. In 2022, the incidence among 8-year-old children is 1 in 32. That spike can’t be fully explained by changes in diagnosis, growing awareness or genetics; genes evolve slowly—on the scale of hundreds to thousands of years. Those factors matter—but they don’t tell the whole story. Something else is happening, and we need to look harder at the world around us. For 30 years, I’ve studied how toxic chemicals—lead, air pollution, pesticides—affect children’s brain development. My wife, a developmental pediatrician, works with children with behavioral conditions, including autism. Like many people drawn to this field, we have a personal stake: our granddaughter has autism. The Limits of Genetics Most autism research has focused on genes. Twin studies are often cited: identical twins are more likely to both have autism than fraternal twins. From that, people often say autism is “80 percent genetic.” It sounds like genes are the main cause—but that’s a misunderstanding. The number doesn’t mean genes cause autism in 80 percent of cases. It means that, in today’s world, genes help explain who develops autism. Change the environment, and that number could change too. If everyone is exposed to the same stew of chemicals, then genes appear to matter more than environment. But most children who are genetically vulnerable only show symptoms when exposed to certain environmental triggers. This is where most genetic studies fall short. They rarely ask how genes and environment interact. It’s like listening to only one half of a conversation and calling it a full story. And yet, the genome has remained an alluring place to search for answers. Rare genetic mutations account for only 5 to 10 percent of autism cases. More common variants may raise the risk, but if they’re so common, why doesn’t everyone develop autism? The answer: genes don’t act alone. A child may inherit a vulnerability, but whether it is expressed depends on what the developing brain is exposed to during development. That’s where genes and environment meet—and where we should be looking. A Toxic World We’ve learned some hard lessons from environmental disasters. Lead, mercury, PCBs—were once considered safe. Now we know they can damage the developing brain in ways that lasts a lifetime. Today, we’re all exposed to hundreds of synthetic chemicals. Some of them—pesticides, air pollution, flame retardants, PFAS (so-called “forever chemicals”)—are known to interfere with brain development. Over the past two decades, scientists have discovered troubling links between prenatal exposure to toxic chemicals and autism. Take air pollution. Numerous studies have found that children with the highest prenatal exposure—especially from traffic—are more likely to have autism. One study found that children with the highest prenatal exposure were twice as likely to be diagnosed with autism. Pollutants like nitrogen dioxide and polycyclic aromatic hydrocarbons can inflame the developing brain and alter its chemistry. Pesticides tells a similar story. Chlorpyrifos, once widely used in homes and still used in agriculture, harms the developing nervous system. Children born to mothers living near fields sprayed with chlorpyrifos and glyphosate had higher autism rates.
Phthalates, found in plastics and personal care products, disrupt hormones and are particularly risky during fetal development. Higher prenatal exposure, especially in boys, has been linked to autistic traits.
Different chemicals,
same pattern: exposures to toxic chemicals during critical stages of
brain development appear to raise the risk of autism.
Amid the risks, one protective factor stands out: folate, or vitamin B9. It’s essential for DNA synthesis and brain development. Women who take folic acid before conception or early in pregnancy lower their child’s risk of autism. Many forces can block folate or make it less available. Maternal diabetes or infection—which increase the risk for autism—stress the body and make it harder to use folate efficiently. Some genetic variants reduce the body’s ability to process folate. Some chemicals interfere with how folate works. Valproic acid (an anti-seizure drug which was linked with autism), tobacco smoke, and arsenic can block or deplete folate. When these factors combine, the risk of autism rises sharply. A Wider Lens |
