Yet more research has suggested that air pollution may be tied to a slightly increased risk of autism in children.

Researchers studied over 130,000 births in the Canadian city of Vancouver from 2004 to 2009. From this cohort, around 1 percent were diagnosed with an autism spectrum disorder (ASD) by age 5. However, if their mothers lived in a neighborhood with higher levels of airborne nitric oxide – a pollutant commonly associated with automobile traffic – while pregnant, the risk of ASD jumps to 1.7 percent.

Even when controlling for the child's sex, month and year of birth, mother's age and birthplace, and income, they found it was only prenatal exposure to higher levels of nitric oxide (and to a slightly lesser extent, PM2.5 and nitrogen dioxide) that appeared to increase odds for ASD.

The study – published this week in the journal JAMA Paediatrics – is robust with a huge sample size, but some independent experts are advising caution before declaring it a causal factor of autism, highlighting that the increase is relatively mild.

“Imagine 1,500 births in the Vancouver area, where the study took place. If they are like the births studied in this research, 15 of the children will be diagnosed with ASD,” Professor Kevin McConway, Emeritus Professor of Applied Statistics at The Open University, who was not involved in the study, explained.

“Imagine now that pollution round Vancouver got worse, so that the average level of nitric oxide in the atmosphere went up by 10 parts per billion. If nitric oxide is really affecting ASD risk to the extent measured in this study, that would lead to about 1 extra ASD diagnosis in the 1,500 children.”

Although other research has shown how air pollution can find its way into the placenta and harm the fetus, this research did not establish a causal link between air pollutants and autism. It’s also worth considering that the streets in Vancouver are not particularly associated with polluted air, compared to other cities.

“It’s easy to find coincidental changes in the environment at the same time that the number of people receiving autism diagnoses increases," Dr James Cusack, another independent expert and Director of Science at Autistica, commented on the findings. "This study does not provide evidence that air pollution causes autism.”

Nevertheless, the potential link between pollution and ASD continues to grow. Just a few weeks ago, another study was published that associated dirty air with an increase in ASD, adding to the growing list of similar studies that have also gently pointed into this direction. For the time being, however, more evidence is needed before anyone jumps to any bold conclusions.

“Perhaps air pollution does really affect ASD risk, or perhaps it doesn’t,” concluded Professor McConway. “This new study adds more evidence, but in my view, we’re a very long way from knowing.”