For healthy people, the benefits of exercising outside probably outweigh the risks of exposure to smog, public health officials say.

Previously, the public health officials urged people to avoid strenuous exercise during smog alert days.

"Smog days are the last things you want to hear as a triathlete," said Samantha McClone, a Canadian Olympian. "We do all of our training outdoors, we're on our bike for three or four hours and running for two or three hours."

Samantha McClone

Exercising outdoors in such conditions can pose health risks for some people. Those with asthma or emphysema may see their symptoms worsen.

Little is known about the long-term risks to people who are otherwise healthy, although some damage to the lungs is suspected.

"The poor quality of air is passing through the lungs at a higher rate than usual, and the cardiac and respiratory systems are under more strain," said Dr. Ron Olivenstein, a respirologist in Montreal.

Monica Campbell

Parts of southern Ontario have already had more than 30 days of smog alerts. The alerts prompted public health officials in Toronto to study air quality to see if smog levels were lower during certain times of the day.

"There is no ideal time of the day where all the pollution pollutants are low," concluded Monica Campbell of Toronto Public Health. "So we couldn't give some advice to the population, say be physically active in the morning versus the afternoon."

The study did find reduced levels before 6:30 a.m. or after 8:00 p.m., times that aren't practical for many people wishing to exercise.