Babies exposed to lots of traffic-related air pollution in the womb and during their first year of life are more likely to develop autism, according to a U.S. study.

The findings, which appeared in the Archives of General Psychiatry, support previous research linking how close children live to freeways to their risk of autism, the study's lead author says.

"We're not saying traffic pollution causes autism, but it may be a risk factor for it," said Heather Volk, an assistant professor at the Keck School of Medicine of the University of Southern California in Los Angeles.

Autism has become more commonly diagnosed over the past few years, and it's now estimated that the disorder - which runs a spectrum from a profound inability to communicate and mental retardation to milder symptoms seen in Asperger's Syndrome - affects one in every 88 children born in the United States.

The increase in autism diagnoses has also been accompanied by a growing body of research on the disorder. Volk's new study, however, is one of a series of looks into how environmental factors may be linked to a child's risk of being autistic, and done over the past few years.

"I think it's definitely an area that's been understudied until recently," Volk said.

While Volk and her colleagues used how close a child lived to a freeway as a substitute for pollution exposure in their last study, this time they looked at measures of air quality around the children's homes.

Compared to 245 California children without autism, the researchers found that 279 autistic children were almost twice as likely to have been exposed to the highest levels of pollution while in the womb, and about three times as likely to have been exposed to that level during their first year of life.

The found that children exposed to the highest amount of "particulate matter" - a mixture of acids, metals, soil and dust - had about a two-fold increase in autism risk.

Volk and her colleagues also saw a similar link between autism and nitrogen dioxide, which is in car, truck and other vehicle emissions.

"This is a risk factor that we can modify and potentially reduce the risk for autism," wrote Geraldine Dawson, of the University of North Carolina at Chapel Hill, in an email to Reuters Health. Dawson wrote an editorial that accompanied the study.

The researchers said certain pollutants could play a role in brain development, but that doesn't prove that being exposed to air pollution makes children autistic. They warned that there may be other factors that explain the association, including indoor pollution and second-hand smoke exposure.

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

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