Low temperatures overnight are expected to be in the high teens to low twenties providing little relief from the heat.
A cooldown isn't expected until Friday, when a cold front finally passes through.
Mylene Drouin, the Montreal Regional Director of Public Health, confirmed the deaths were due to heat.
The extreme heat event will continue through Thursday.
Firefighters are going door-to-door to check on at-risk residents in Montreal and health officials have set up drop-in cooling centres in the city to offer respite from sizzling temperatures. Mayor Valerie Plante urged people to check on vulnerable neighbors, including seniors.
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She says there has also been a rise in the number of calls to ambulance services in recent days and that the city wants to avoid what happened in 2010, when intense heat claimed 106 lives.
"After a couple of days of being hot and living in a place that has no air conditioning, the heat just overwhelms the body's capacity to adapt".
He said the Montreal heat exposure victims were between 50 and 80.
"Usually in a normal situation we have 10 deaths per day, so we're not in an excess of mortality", Drouin said.
Extreme heat affects everyone, Environment Canada said. A high of 26 and low of 14 is expected in Toronto on Friday, Ms. Hobelman said, with a chance of showers and a thunderstorm. "And also keep in mind to make sure you don't leave any people or pets in your auto, as temperatures will rise dramatically within minutes". However, the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.