Exposure to traffic pollution while in the womb and the first year of life may be associated with a higher risk of autism, a US study has found.

Autism Spectrum Disorder has a variety of symptoms including problems with social interactions and a fondness for repetition.

The causes of autism have been notoriously hard to pin down but are thought to include a range of environmental and genetic factors.

A new study by the University of Southern California, titled Traffic-Related Air Pollution, Particulate Matter and Autism and published today in the Archives of General Psychiatry, analysed data on 279 children with autism and a control group of 245 typical children in California.

The researchers found that children with autism were more likely to live at residences that had the highest quartile of exposure to traffic-related air pollution during gestation and during the first year of life, compared with the control group – but stopped short of saying air pollution caused autism.

“Exposure to traffic-related air pollution, nitrogen dioxide, PM [particulate matter], and PM during pregnancy and during the first year of life was associated with autism,” the study said.

“Further epidemiological and toxicological examinations of likely biological pathways will help determine whether these associations are causal.”

Dr Andrew Whitehouse, an autism expert from the Telethon Institute for Child Health Research at the University of Western Australia, welcomed the study.

“This is a well conducted study that has great potential to advance our understanding of the biological pathways that lead to autism,” he said.

“There is an increasing recognition amongst researchers that certain aspects of our environment may interact with genetic make-up to cause autism. The hunt is on for these environmental risk factors, and this study suggests that traffic-related air pollution may be one of the many pieces of this puzzle.”

Dr Sophia Xiang Sun from the Autism Research Centre at the University of Cambridge said further research was needed and that other factors – including second-hand smoke or medical conditions during pregnancy, indoor air pollution or genetics – cannot be ruled out.

“Until further research is carried out, we will not know definitely if the association is there and, if it is there, how direct and to what degree,” she said in comments provided by the UK Science Media Centre.

It was “biologically plausible” to say traffic exhaust may play a role in pathways of autism but reducing air pollution would be a good idea anyway because it contributes to other health problems, she said.

“Regarding reports of an increase in prevalence of autism, there has been no clear evidence whether it is a real increase or not” because there have been many changes that could make it appear that the prevalence of autism is increasing, including changes in diagnosis, improved screening and changes in study methods, she said.

“We need more robust research before we will know if this is a real increase or whether it is simply due to these changes.”