Outdoor pools provide respite from the heat and humidity. (CBC)

The stifling heat in Montreal has prompted a 25 per cent increase in the number of phone calls for ambulances.

The number of phone calls has jumped from an average of roughly 800 to more than 1,000 since Monday, officials with Urgences-Santé confirmed on Thursday.

For the fourth day in a row, Environment Canada issued a high heat and humidity alert Thursday for much of the province, including Montreal, Sherbrooke, Quebec City, Trois-Rivières and the Saguenay, where smog warnings are also in effect.

For the third straight day, record-breaking temperatures were recorded at Montreal's Trudeau airport. The temperature reached 34 C, topping the record of 33 C set in 2008.

Special measures have been put in place to help people cope with the extreme heat in many of the province's regions.

The City of Montreal has designated 108 different locations as cooling centres where people can seek refuge from the oppressive heat.

People can also call Montreal's 311 information line to find a cooling centre close to them.

On Wednesday, the director of Montreal's public health agency, David Levine, had said he was hopeful officials would be able to prevent heat-related deaths.

"Our public health studies have shown … there often is around 100 additional deaths during a period of heat wave than we would find during a non-heat-wave period," said Levine. "That is why these measures are being put into place."

Air-conditioned centres will be open in Sherbrooke, Magog, Asbestos, Trois-Rivières and the Saguenay.

Cities across the province have also extended their public pool hours to accommodate more people.

On Wednesday, one pool was forced to close because of excessive sunscreen mucking up its filters.

The popular outdoor pool in Montreal's Wilfrid-Laurier Park was forced to close early Wednesday night because of poor water quality.

Lifeguards at the pool told CBC News the filtration system in the 59-year-old pool hasn't been working properly.

Officials said the pool was able to reopen on Thursday, but the deep end was kept closed in order to reduce the number of people in the water.

For people without access to a pool or cooling centre, public health officials urged them to go to a mall or library where they can find air-conditioning.

Quebec's community health clinics (CLSCs) will also be reaching out to vulnerable populations such as seniors, to make sure they are getting by in the heat.