In the newest of several recent studies on air pollution and autism, researchers found that children with the greatest exposure to particulate matter had a doubled risk of autism.

"We're not saying that air pollution causes autism. We're saying it may be a risk factor for autism," lead author Heather Volk told Time. "Autism is a complex disorder and it's likely there are many factors contributing," she says.

The research team studied 500 California kids, about half of whom had been diagnosed with autism. Using the addresses from every home each child had lived in while in utero and during the first year of life, researchers estimated pollution exposure based on traffic volume, vehicle emissions, wind patterns and estimates of particulate matter, nitrogen oxide and ozone.

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The autistic kids were almost twice as likely to come from homes that topped the pollution charts, especially if their mothers lived in those homes while pregnant.

There are still many missing pieces in the puzzle, Volk told Time. For example, some children may be more susceptible to environmental factors because of genetic differences. And other environmental factors, such as indoor pollutin and second-hand smoke exposure, could also factor into the equation, she told Reuters.

"There are some potential pathways that we're examining in our current research that will be coming up next," Volk told Reuters.

In an accompanying editorial, Geraldine Dawson of the University of North Carolina at Chapel Hill highlights the rise in autism research. She points out that the current issue of the Archives of General Psychiatry, features three articles on autism; the journal used to run about three per year.

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"The alarming rise in prevalence has led to more scrutiny of environmental risk factors, such as the study on air pollution as a risk factor for autism published in the current issue," Dawson writes. "New research methods are providing a better understanding of the underlying neuropathology of autism."

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