Quebec health officials are investigating two deaths they say may be connected to the searing heat that has blanketed both Ontario and Quebec.

Richard Massé, the director of Montreal's public health service, has confirmed that one person died Tuesday, but the person had other health problems and was in a private institution. Officials are unsure whether the death is linked to the heat.

Another death was reported Wednesday in Montreal, and officials are investigating it in connection with the soaring temperatures. Massé is urging people to look out for each other.

A severe thunderstorm left more than 9,000 households and businesses without power in Montreal. At least half of those have since been restored to power. 

The temperature dropped 10 degrees in less than 45 minutes as the storm rolled into the city.

Residents prepared for possible flash flooding as Environment Canada issued a severe thunderstorm warning for Montreal on Wednesday afternoon.

In Ontario, Ottawa and Toronto endured a heat wave Wednesday, as a third straight day of high temperatures baked parts of southern Quebec and Ontario.

The heat will be relentless for the next few days.

"Humidex values in southern Ontario will be between 40 to 45," said CBC meteorologist Jay Scotland, with Ottawa likely having its hottest day of the year — expected to hit the mid-40s.

"North and northeastern Ontario and southern Quebec could experience some severe thunderstorms late in the afternoon and evening."

Scotland said the cool down will move in on the weekend.

"Until then, it's just going to be hot and sticky," said Scotland.

Public health officials are encouraging people to get indoors, get out of heat and into air-conditioned spaces.

The high temperatures pose a particular health risk to the elderly and those who work outside. Humidity makes matters worse, because the moist air impedes the body's ability to cool off by sweating.

People working outside are encouraged to take frequent breaks.

Symptoms of heat-related illness include dizziness, fainting, nausea, vomiting, rapid breathing and heartbeat, extreme thirst, and decreased urination.

Officials are urging anyone caught in the heat to drink lots of water and, if possible, seek relief in a local cooling centre, or even an air-conditioned movie theatre. At least one cooling centre was already open before sunrise in Toronto.

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Public health officials are encouraging people to get indoors, get out of heat and into air-conditioned spaces. In Toronto, emergency cooling stations have been set up to offer relief from the heat downtown. (Evan Mitsui/CBC)

A third straight day of temperatures above 32 degrees means locations like Ottawa and Toronto are officially in heat alerts.

Eighty Toronto city pools are open for extended hours as a result of the extreme heat.

Dr. David McKeown, Toronto's medical officer of health, says the alert will be in effect in Toronto until further notice.

The heat is also sparking high power usage.  In southwestern Ontario, many communities hit peak power usage. Guelph Hydro said it expects to break its all-time usage record for the second day in a row on Wednesday.

Usage on Tuesday clocked in at 297 megawatts, according to Guelph Hydro spokesperson Sandy Manners.

"Because it was so hot out, of course everybody was using their air conditioners at full blast and we hit our record demand of 297 megawatts. So, we've never used so much electricity, let's put it that way," she said.