A link between autism and air pollution exposure during pregnancy has been suggested by scientists.
The Harvard School of Public Health team said high levels of pollution had been linked to a doubling of autism in their study of 1,767 children.
They said tiny particulate matter, which can pass from the lungs to the bloodstream, may be to blame.
Experts said pregnant women should minimise their exposure, although the link had still to be proven.
It analysed 245 children with autism and 1,522 without.
By looking at estimated pollution exposure during pregnancy, based on the mother's home address, the scientists concluded high levels of pollution were more common in children with autism.
The strongest link was with fine particulate matter - invisible specks of mineral dust, carbon and other chemicals - that enter the bloodstream and cause damage throughout the body.