Environment Canada has issued a heat warning for the Ontario regions of Barrie, Collingwood, Hillsdale, Midland, Coldwater, Orr Lake, Orillia, Lagoon City and Washago.
The federal weather agency says a period of hot weather will begin Tuesday, with daytime high temperatures reaching in excess of 30 C.
Humidex levels in the high 30s and low 40s are expected this week, while overnight lows near 20 C are also expected.
A slightly cooler air mass may move into the area on the weekend.
Environment Canada says hot and humid air can bring deteriorating air quality and can result in the air quality health index approaching the “high-risk category.”
“The risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors,” Environment Canada says.
“Check on older family, friends and neighbours. Make sure they are cool and drinking water.”