The federal agency says high concentrations of ozone are expected and will result in poor air quality with high concentrations of pollutants expected to persist through Thursday evening.
Temperatures soared to 34 degrees Celsius (93 Fahrenheit) with a humidity that made it feel closer to 40 degrees, the meteorological service said.
Meanwhile, public-health officials in Quebec's Eastern Townships report four deaths suspected of being caused by the heat; one victim was a senior citizen and three were adults suffering from chronic illnesses.
Not included in those numbers was the death of a six-year-old boy who drowned in an apartment pool on Monday.
The spate of deaths may seem high, but Dr. Mylene Drouin, Montreal's regional director of Public Health, says it could have been much deadlier. Firefighters are knocking on doors in neighbourhoods with less greenery where temperatures can rise up to five degrees higher than the forecast high.
"Usually in a normal situation we have 10 deaths per day, so we're not in an excess of mortality", Drouin said.
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Montreal's emergency medical service says it has received an influx of 300 more calls than usual.
According to records, 106 people have died in Montreal due to heat since 2010.
"It's tragic, but whenever there are heat waves like this, and we will have more heat than before because of changes to the climate, the people who are weakened and vulnerable are the most affected", he said. Patients have also complained that apart from the emergency room, even hallways and other corridors of the hospital offer no refuge from the waves of heat that seems to be inescapable.
The Montreal city government unveiled an "extreme heat" plan Tuesday that includes distributing water to groups working with the homeless. Temperatures in Canada's largest city have topped 30 degrees every day since Friday.
Environment Canada has issued a heat warning for Barrie, Collingwood, Hillsdale, Midland, Coldwater, Orr Lake, Orillia, Washago, Lagoon City, Innisfil, New Tecumseth and Angus. The Atlantic region's heat wave is expected to break midweek, and Southern Ontario's should only last until Friday.