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Researchers in California have found a compelling association.

Tailpipe emissions from stalled cars and busy highways have been linked to a wide range of unpleasant health outcomes, to everything from heart disease to respiratory problems to damaged brain cells. If that isn’t worrisome enough, here is one more unnerving finding from the front edge of pollution research: All of this particulate matter may also be associated with an increased risk of autism.

Researchers in California, who’ve just published their findings in the Archives of General Psychiatry, studied 524 children in the state – 279 with autism, 245 without. The study reconstructed traffic and pollution levels around the home addresses of the pregnant mothers who were carrying these children, as well as around the homes where the children lived during their first year of life. The researchers found that children living in homes exposed to the highest modeled levels of traffic-related pollution were three times as likely to have autism, compared to children living in homes with the least exposure.

The magnitude of the association, the researchers conclude, appears strongest during late gestation and early life, although they couldn’t pinpoint a single period when exposure to pollution seemed most critical. Their findings now join the expanding and often controversial debate over the causes of autism.

"For a great deal of time, a lot of research has focused on genetic risk factors for autism," says the University of Southern California’s Heather Volk, the lead author of the study. "In the last few years, we really have begun to have additional efforts looking at environmental risk factors for autism. It’s increasingly likely that it’s a combination of [the two]."

Volk and her colleagues were following up on earlier research that found an association between autism and living near a freeway. In this latest study, instead of using freeway proximity as a proxy for pollution, they modeled individual exposure using historic meteorological and traffic data on roads of all kinds – freeways, state highways, arterials and collector roads – within 5 kilometers of the child’s home. And they concluded that this association couldn't be explained by demographic or socioeconomic factors (other variables, such as indoor pollution or a child’s nutrition, were not considered).

If nothing else, these findings are biologically plausible.

"We know that air pollution first of all can activate inflammatory and oxidative stress pathways in the body," Volk says. "And there are studies that have indicated that these same pathways are activated in children with autism."

Top image: Freer/Shutterstock.com

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