Children with the highest exposure to traffic pollution are significantly more likely to develop autism, according to a study published Monday in a major psychiatric journal.

In 2010, lead researcher Heather Volk published findings that showed children born to mothers who lived within 309 metres of a freeway were twice as likely to develop autism. “In this study, we sought to improve on that work,” said Volk, an assistant professor of research at the University of Southern California’s department of preventative medicine and pediatrics.

Special section: The Autism Project

Volk’s team found that children who had the highest exposure to pollutants during the first year of life were more than three times as likely to develop autism as those with the least exposure.

Children whose mothers had the highest exposure to pollutants during pregnancy were nearly two times as likely to develop autism.

Many questions remain unanswered, says Volk.

“Right now, our research can’t tell us if there’s a particularly harmful pollutant — if there’s one pollutant that really we should be worried about,” she said.

The team also doesn’t know how that pollutant mix — including nitrogen dioxide and particulate matter under 10 micrometres in diameter — might act on children to trigger autism. The study found an association between pollution and autism, not that one causes the other.

Volk says she was prompted to look into links between traffic and autism because other studies have found associations between hazardous air pollutants and risks like pre-term birth, asthma, and even early-onset cardiovascular disease.

Expectant mothers should not run off to the woods: Monday’s study found that living in densely populated cities versus sparsely populated areas did not alter the risk. But Volk notes that good prenatal care is crucial, and that it can’t hurt to be careful during high-smog days.

The findings, published in the Archives of General Psychiatry, are the latest from the Childhood Autism Risks from Genetics and Environment (CHARGE) study, a huge population-based cohort that has recruited 1,500 children in California and is gathering reams of data on them to try and determine what mysterious combination of genetics and environmental triggers might cause autism.

As the CHARGE website explains, its researchers recognize that “no single factor accounts for all autism cases, nor is there one event or exposure that can be responsible for the rapid increase in diagnoses over the last few decades. Instead, each child’s path to altered brain development may be different.”

An estimated 1 in 88 children will be diagnosed with autism, figures that have prompted some to label the disorder an epidemic.

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Volk says finding environmental risk factors is an exciting avenue of study.

“Ultimately, for most of them, there’s some sort of intervention or broad-scale public health prevention measure that potentially could be undertaken.”