Environment Canada has issued a warning for Waterloo Region, Guelph and Wellington County as the area is expected to experience a mini heat wave on Wednesday and Thursday.
It says that temperatures in the area could reach between 30 C and 33 C with the humidex values being closer to 40 degrees.
There will be little relief overnight as the temperatures are still expected to be more than 20 C across much of southern Ontario.
The agency says that the sweltering weather will is forecast to continue into Thursday evening before the temperatures cool.
Environment Canada warns that the hot, humid air could result in poor air quality.
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It says that while everyone can be affected the risk is higher “for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.”
The agency says to drink plenty of water and to not leave people or pets inside a parked vehicle.