Study author Dr. Robert Dales noted that "most urban areas" will have days when sulfur dioxide and nitrogen dioxide reach the levels that may increase SIDS risk.

However, he cautioned that SIDS is very rare, occurring in about 1 out of every 1,000 babies. And air pollution appears to exert only a small effect on an already uncommon condition, he told Reuters Health.

"The risk of SIDS is small, so the percentage increase in the small risk is even smaller," Dales explained.

For parents who are concerned about SIDS, Dales recommended protecting infants from clearly established risk factors, like secondhand smoke, sleeping on the stomach, and sleeping in couches or beds not made for infants.

The University of Ottawa researcher added that parents should bring their infants to a doctor if they notice they are having any trouble breathing.

To investigate the role air pollution plays in SIDS, Dales and his team reviewed daily air pollution data collected from 12 Canadian cities - with a total population of over 10 million people - between 1984 and 1999. The researchers compared that data to daily rates of SIDS.

In the report in the journal Pediatrics, the researchers defined SIDS as the sudden, unexplained death of a child less than 1 year old.

The investigators found a higher rate of SIDS on the day following an increase in levels of sulfur dioxide and nitrogen dioxide. This pattern persisted even when the researchers accounted for the influence of season, temperature, humidity and barometric pressure.

Ozone and carbon monoxide appeared to have no influence on SIDS rates, the authors report, but sulfur dioxide and nitrogen dioxide did.