Environment Canada said much of southern Ontario, including Toronto, also remains under a heat warning, with a humidex reading of 43 expected on Tuesday.
They planned to visit thousands more as the heat was expected to continue until Friday. However, daytime highs in the low 30s will continue Tuesday through Thursday.
Toronto's Medical Officer of Health extended the city's heat warning until further notice over the weekend.
In the Eastern Townships region southeast of Montreal, four deaths are under investigation to determine if they are related to the extreme heat.
Sudbury has been under a heat warning since Friday.
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Overnight low temperatures are expected to be in the low 20s, with some rural areas dipping into the low teens - but no matter where you are in southwestern Ontario, there won't be much - if any - relief from the heat.
Humidex values are expected to make those temperatures feel like the low 40s.
Extreme heat affects everyone, Environment Canada said.
To prevent heat stress during a heat alert, Dr. Strang suggests to stay in a shaded, cool or air-conditioned area, drink plenty of water, wear light-coloured clothing and to not leave anyone or any animal unattended in vehicles. However, the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.
The weather agency says a "very warm and humid air mass" has settled over the Maritimes and above normal temperatures and humid conditions will stick around into Thursday. Watch for the effects of heat illness: swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.