Living near a busy road has been linked to a dramatic increase in the risk of childhood autism, a new study shows.
Early exposure to traffic pollution, either in the womb or during the first year of life, more than doubled a child's chances of having the disorder, scientists found.
Children from homes with the highest air pollution levels were three times more at risk than those from the least exposed homes.
Experts described the finding as "important" but stressed it did not prove a causal link between pollutant chemicals and impaired brain development.
Autism, a wide-ranging condition that affects communication and social skills, is believed to be caused by a combination of genetic and environmental factors.
While a number of genetic variants are known to be linked to the disorder, the role played by the environment has been less clear. Scientists in California set out to investigate a possible link between autism rates and traffic pollution.
The study looked at data on 245 children without the condition and 279 affected by autism. Air pollution records from the US Environmental Protection Agency were used to estimate exposure to nitrogen dioxide and small sooty particles, both produced from vehicle exhausts.
The researchers took into account how far mothers and babies lived from busy roads, and levels of pollutants in the air.
Lead scientist Dr Heather Volk, from the Keck School of Medicine at the University of Southern California, said: "This work has 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."
The findings are published in the latest online edition of the journal Archives of General Psychiatry.