Researchers have found that children who live near freeways at birth have twice the risk of autism, suggesting that environmental factors may play a role in the disorder's growing incidence.

A study by researchers at the Saban Research Institute of Children's Hospital Los Angeles found that babies who lived within 1,000 feet of a freeway, but not a major road, were at risk.

Lead researcher Helen Volk says that may be due to the type and high level of pollutants on a freeway.

The study, published in the journal Environmental Health Perspectives, looked at 304 children with autism and 259 normally developing children.

A 2006 study also found autistic children were 50 percent more likely to have been born around contaminated air.