The heat warning says the actual temperature is expected to fluctuate between high 20s and low 30s, with a humidex of high 30s to low 40s.
The weather agency warns while the extreme heat will affect everyone, it will pose greater risks to young children, pregnant women, older adults, people with chronic illnesses and those working outside.
It says the temperature in areas near Ontario’s lakes could be cooler compared to regions inland.
Environment Canada asks Ontarians to be cautious and drink plenty of water, stay in cool places and make sure not to leave people or pets inside parked vehicles.
Canada’s weather agency says it issues heat warnings when high temperatures pose an “elevated risk” of illnesses such as heat stroke or heat exhaustion.
