Environment Canada says a heat warning impacting most of southern Ontario, including Guelph and Waterloo Region, is expected to last all week.
Daytime high temperatures over 30 C, with humidity levels in the high thirties and low forties, are expected until at least Friday.
Overnight lows are expected near 20 C during the heatwave.
“A slightly cooler air mass may move into the area this weekend, however there is considerable uncertainty,” Environment Canada said in a post on its website.
It added that air quality may become an issue as the hot and humid weather could create elevated pollution levels.
The weather office said extreme heat affects everyone, but the risks are greater for young children, pregnant women, older adults and people with chronic illnesses.
Residents are being reminded to never leave pets inside a parked vehicle.
The effects of heat-related illnesses include swelling, rash, cramps, fainting, heat exhaustion and heatstroke.
Many usual cooling centres in Guelph and Waterloo Region are closed due to the novel coronavirus pandemic.
The region has posted a list of cooling centres that have opened in the tri-cities.
Most malls are also open and The Royal City Mission in downtown Guelph is open as a cooling centre from 11:30 a.m. to 3:30 p.m. on weekdays.
Splash pads and public pools have also begun to open in Guelph and Waterloo Region, but residents are asked to check their status ahead of time.