Environment Canada and public health agencies issued the warning early this morning after winds pushed smoke from the fires burning near the Manicouagan Reservoir eastward.
The deteriorating air quality means that people with breathing problems or heart disease could be affected.
Karine Price, a toxicologist with Montreal's public health agency, said as long as the smoke persists, the smog will continue to be a problem for those who suffer from respiratory ailments.
"For those people, we recommend when you go outside, don't do very intense physical exercise: just go a bit slower," she said.
Quebec's public health director Horacio Arruda also issued a word of caution for those with pulmonary diseases.
"Diminish your exercise outside, because if you're going to do exercise outside, you're going to breathe in more often, and there can be more particles that can irritate your lungs," he said.The City of Montreal noted that the general public shouldn't feel any significant effects.
However, residents in affected areas are being asked to consider public transportation, reduce driving speed and avoid idling vehicles when possible.
The city of Laval, under the same air-quality warning, has reduced transit fares to $1 to encourage more commuters to leave their cars at home.
On the weekend, winds brought smoke down from the fire burning near the Cree community of Eastmain, Que., on James Bay.
That fire, estimated to be spread across 200,000 hectares, is now just 400 kilometres from the town.
"That contributed to the deteriorating air quality over most of southern Quebec," André Cantin, a meteorologist with Environment Canada, told CBC's Daybreak.
"The small particles coming from those forest fires gives a bad air quality this morning and that will probably continue like this for most of the day."
Several residents have been forced out of the community and flown to Val d'Or as a precaution.
Melanie Morin from Quebec's forest fire prevention agency said Monday that a change in the winds has somewhat halted the fire's movement toward the town, but it is still headed toward the access road to the community.
"With the weather that they're announcing, [it's] going to be an important day," she said. "There's going to be a lot of fire activity — keeping crews safe, minimizing the amount of smoke that's heading down our way."
On Friday, officials in the town of Wabush in western Labrador near the Quebec border issued a mandatory evacuation order as forest fires closed in on that community. The order was lifted late Sunday night, but crews are still working to extinguish the blaze.
The air in southern Quebec was expected to improve this afternoon, but officials now believe it will be tomorrow before the air quality index moves from the "poor" into the "acceptable" range.