It’s going to be a hot start to the week in much of southwestern Ontario. 

On Sunday afternoon, Environment Canada issued a heat warning for several regions, including Toronto, Hamilton, Burlington-Oakville, Halton Hills-Milton, Mississauga-Brampton, Pickering-Oshawa-Southern Durham, St. Catharines-Grimsby-Northern Niagara, and Vaughan-Richmond Hill, and Markham.

The national weather agency said that a “heat event” is expected to start on Monday and persist into Tuesday with daytime highs of near 31 degrees Celsius, which will feel more like 35 to 38 with the humidity.

Environment Canada said that there will be little relief in the overnight hours as lows are only expected to be 20 C or higher.

A cooler air mass is expected to arrive on Tuesday evening.

Older adults, infants and young children, pregnant people, people with physical and/or mental illnesses as well as those with disabilities or mobility issues may experience greater health risks due to the heat.

People are being urged to watch out for effects of heat illness, which include swelling, rash, cramps, fainting, heat exhaustion, heat stroke, and the worsening of some health conditions.

“Drink plenty of water regularly, even before you feel thirsty to decrease your risk of dehydration. Thirst is not a good indicator of dehydration,” Environment Canada urged.

“Never leave people, particularly children, or pets inside a parked vehicle.”