Living near a busy road is associated with a dramatic increase in the risk of childhood autism, a study has shown.
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 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 genetic variants are known to be linked to the disorder, the role played by the environment has been less clear.
Air pollution records were used to estimate exposure to nitrogen dioxide and small sooty particles, both produced from motor 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 University of Southern California, said: "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."
Professor Emily Simonoff, from King's College London, said a number of factors had not been properly taken into account, including father's age and family history of autism.
"At present, pregnant women should continue to look after their health during pregnancy but should not be unduly concerned."