Children born to mothers who live near a freeway are twice as likely to develop autism as those who do not, a new study suggests.

The finding published online this week in the journal Environmental Health Perspectives bolsters the theory that environmental factors could play a role in the development of autism, researchers say. The study is the first to link contact with pollutants from vehicles to autism risk.

For the study, researchers looked at data from children living in Los Angeles, San Francisco and Sacramento, Calif. who were ages 24 and 60 months when the study began. Some of the kids had autism and some did not.

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Using prenatal records, researchers determined how close the child’s mother lived to a major road or freeway during the first, second and third trimesters of pregnancy and at the time the child was born.

“Children born to mothers living within 309 meters (about 1,000 feet) of a freeway appeared to be twice as likely to have autism,” said Heather Volk of Children’s Hospital Los Angeles who was the lead author on the study. Living close to a major road other than a freeway, however, did not appear to alter a child’s odds of developing autism.