Exposure to air pollutants can dramatically increase a child's chances of developing autism, claims a new study by the University of Chicago.
The study is the largest to have examined the link between autism and environmental factors, with others studies having concentrated on possible genetic causes for the disorder.
As part of their research, scientists from the university examined the insurance claims of 100 million people from across the USA
The report examined birth defects associated with parental exposure to pollution, and found where there was a 1% increase in birth defects, there was a 283% increase in autism cases.
"Autism appears to be strongly correlated with rate of congenital malformations of the genitals in males across the country. This gives an indicator of environmental load and the effect is surprisingly strong,' said study author Andrey Rzhetsky of the university's department of human genetics.
"Essentially what happens is during pregnancy there are certain sensitive periods where the foetus is very vulnerable to a range of small molecules – from things like plasticisers, prescription drugs, environmental pesticides and other things," Rzhetsky said.
"Some of these small molecules essentially alter normal development. It's not really well known why, but it's an experimental observation."
Women living in the top fifth of areas with high levels of air pollution were more than twice as likely to give birth to a child with autism than those in less polluted areas.
Those with the greatest exposure to these pollutants were another 50 times more likely to have a child born with autism.
"The environment may play a very significant role in autism, and we should be paying more attention to it," Rzhetsky wrote in the study, which was published in the journal PLOS Computational Biology.
About one in 88 children has some form of autism, which is diagnosed on a sliding scale, according to the Centre for Disease Prevention.
It is far more likely to develop in boys than girls.
The disorder affects the ability to interact, and its cause has so far remained unknown.