A new study found exposure to air pollution in the womb or during the first year of life was linked to a dramatic increase in a child's chances of having the disorder.
Children from homes with the highest traffic pollution levels were three times more at risk than those from the least exposed homes, the 'Daily Mail' reported.
Experts described the finding as "important" but stressed it did not prove that traffic pollutants could affect brain development.
Autism, or autistic spectrum disorder (ASD), including Asperger's syndrome, is an umbrella term for a range of developmental disorders that have a lifelong effect on someone's ability to interact socially and communicate.
Scientists in California set out to investigate a possible link between traffic pollution and autism rates, saying they were on the increase.
They looked at data on 279 children affected by autism compared to 245 children without autism matched for age and background.
Air pollution records from the US Environmental Protection Agency were used to estimate exposure to nitrogen dioxide and small sooty particles, both produced from motor vehicle exhausts, at their mothers' addresses.
Children living in homes with the highest exposure to traffic pollution were three times as likely to have autism as those with least exposure.
The researchers found a twofold increase in risk of autism among children exposed in the womb or during the first year of life to higher levels of air pollution.
"This work has broad potential public health implications. We've known for a long time that air pollution is bad for lungs, and especially for children. We're now beginning to understand how air pollution may affect the brain," lead scientist Dr Heather Volk, from the Keck School of Medicine at the University of Southern California, said.
Volk was especially concerned about exposure to small and very fine pollution particles produced by diesel engines known as PM10s and PM2.5s.
"From studies conducted in the lab, we know that we can breathe in tiny particles and they can produce inflammation. Particles have varied composition, and there are many chemicals that can bind to them. The components of these particles could be hazardous to the brain," she said.
However, British experts were cautious about the findings, saying they did not prove pollution caused autism.
"Although traffic-related air pollution might be one of the contributing factors to the development of autism, other factors cannot be ruled out," Sophia Xiang Sun, from the Autism Research Centre at Cambridge University, said.
The findings were published in the journal Archives of General Psychiatry.
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