Environment Canada's heat alert will continue into Friday as daytime temperatures continue to surpass 30 C and overnight lows fall only to about 20.

"Daytime high temperatures between 31 to 34 Celsius are forecast each day, with warm overnight low temperatures. Afternoon humidex values are expected to peak in the high 30s to low 40s," the national weather agency said.

The weather service said that cooler temperatures are forecast for the weekend but that they will still be seasonal, in the mid-to-high 20s.

Who's at risk

Environment Canada said heat puts everyone at risk, but those who may be in the most distress from high temperatures are:

Older adults.

Infants and young children.

People with chronic illnesses, especially breathing difficulties, heart conditions or psychiatric illnesses.

People who work or exercise in the heat.

Homeless people.

Cities open cooling centres