Environment Canada has issued a heat warning for much of the Greater Toronto Area with humidex values near 40 expected on Sunday and Monday.
The weather agency said the heat may also persist into Tuesday.
The warning is in effect for Toronto, much of Peel and Halton regions, as well as Vaughan, Richmond Hill and Markham.
“Daytime temperatures today and tomorrow will be in the low 30s with humidex values closer to 40,” Global News weather specialist Mike Arsenault said.
“There won’t be much relief through the overnight as overnight lows will stay above 20 C into Tuesday morning. Sleeping without air conditioning will be very uncomfortable.”
Arsenault said those who plan on spending a lot of time outside should drink plenty of water and spend time in shade.
Environment Canada said the hot and humid air may also cause air quality to deteriorate.
Young children, older adults and people with chronic illnesses are among those with the highest risk for heat-related illnesses.