As part of the City of Montreal's response to the current heat wave, firefighters and police officers have been asked to check on potentially vulnerable residents. (CBC)

The first heat wave of the summer in Quebec has had fatal consequences, according to Montreal public health authorities.

Eighty deaths were reported Thursday, when the scorching heat reached its peak. The average number of deaths on a regular day in Montreal is 40.

The temperature at Montreal's Trudeau Airport reached 34 C, breaking the previous record of 33 C set in 2008.

Forty-three deaths were reported on Wednesday.

Officials said it was still too soon to confirm how many of the deaths were directly related to the heat.

"We should not panic," said Luc Lefebvre, head of the emergency measures unit with the Montreal public health department. "But, what is clear is that the population of Greater Montreal suffered during this heat wave."

Though the temperatures dropped off somewhat on Friday afternoon, when the region was hit by severe thunderstorms, Lefebvre said extreme heat measures would remain in effect.

"We must not let our guard down," said Lefebvre. "People who have been exposed to the heat and who have suffered could still see their health condition deteriorate over the next few hours or days."

Lefebvre encouraged Montrealers to check on people they know who are alone and may be vulnerable to the heat.

The city has opened 108 cooling centres, where people can find respite from the heat, and has extended public pool hours.

Police officers and firefighters have also been making door-to-door visits to check on potentially vulnerable people.

Montreal's ambulance service, Urgences-Santé, confirmed it had received a record-setting 1,391 calls Thursday. On an average day, the service usually receives about 800 calls.

"We were expecting this heat wave so we made sure that we had enough resources on the road and … enough operators in order to respond to the demand," said spokesman Benoît Garneau.