There has been a lot of speculation over what is causing the rise in autism rates, but a study out of the greater Vancouver area offers strong evidence that a pregnant woman's exposure to certain pollutants significantly increases the child's risk of autism.

It's considered one of the largest studies to date on the issue, published in JAMA Pediatrics, involving the health records of more than 132,000 children over a five-year period, with a five-year follow-up.

Researchers at the University of British Columbia and Frasier University found maternal exposure to nitric oxide during pregnancy was associated with a higher risk of autism among their offspring.

Nitric oxides are a key component of ozone pollution and smog, which has been notoriously hard to reduce in many cities around the country.

The study did not, however, find a link between autism and particulate matter pollution, which is tiny solid and liquid particles that come from car exhaust, for example.

Researchers say they believe reducing exposures to nitric oxide, could help slow or at least lower the rising rates of autism.