Environment Canada has issued a heat warning for Guelph, Waterloo Region and Wellington County that is expected to last right through Tuesday.
The agency says temperatures are expected to reach between 31 C and 34 C during the day but it will feel hotter as the humidex checks in closer to 40.
At night, there won’t be much relief as the temperatures will fall somewhere between 21 C and 25 C.
The heat wave will not come to an end until Tuesday evening, according to Environment Canada, which says a cold front is expected to arrive, with thunderstorms and showers in tow.
The agency warns that this type of hot, humid air can lead to deteriorating air quality and that the extreme heat affects everyone.
It says young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors are at the greatest risk.
Environment Canada says side effects could include swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.
It also warns to never leave people or pets inside a parked car.