It said the actual temperature is expected to fluctuate between high 20s and low 30s, with a humidex of high 30s to low 40s.

“Hot, humid air may also deteriorate air quality and cause health index to high-risk category,” Environment Canada said in a statement.

Temperatures will reach 31°C in many communities, while the index used by Canadian meteorologists to describe how hot the weather is for the average person, known as humidex, will range from 35 to 40°C.

The weather agency said 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.

Canada’s weather agency said it issues heat warnings when high temperatures pose an “elevated risk” of illnesses such as heat stroke or heat exhaustion.

Environment Canada said the heat warning is expected to last until Thursday for most of the province, including the Greater Toronto Area.

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