Southern Ontario and Quebec continue to swelter through unusually hot, humid conditions as a nearly week-long heat wave rages on, with no relief in sight until Friday.

Temperatures for southern Ontario and Quebec are in the 30s but with the humidity, it feels like 40 Celsius or hotter.

A flow of hot air from the south -- called the Bermuda High -- has blanketed the area and is acting like a heat pump, moving warm, southerly air from the Gulf of Mexico northward.

The weather system is already believed to have caused the death of a 92-year-old woman in Philadelphia and a homeless woman in Detroit.

North of the border, health authorities were urging residents to take precautions against the heat -- especially the elderly and those who have heart or respiratory conditions.

"With the exposure to the high temperatures, your thermal regulation system will try to cool down your body," said Dr. Luc Lefebvre with the Montreal Public Health Authority.

"If it's not working, you're going to have symptoms," he added, such as headaches, vomiting, nausea, confusion, co-ordination problems or loss of consciousness.

The heat and humidity isn't expected to break until at least Friday, when a cold front is forecast to sweep in and drop temperatures closer to seasonal norms of about 25 or 26 Celsius.

Environment Canada's senior climatologist Dave Phillips says if it feels like the heat wave is hitting hard, that's because it's been so long since we've had a long, withering bout of the three H's: heat, humidity and haze.

"This is the first heat wave in, I think, three years for this area and that's why it's sort of noteworthy... And the humidity is so high, probably because of all that precipitation we had in June," he told CTV's Canada AM Tuesday morning.

Phillips noted that while we've already seen a few very hot days across Canada this summer, it's the humidity element that makes the heat so dangerous.

"In this kind of heat wave we have now, when you add in that humidity factor, the air is just saturated with moisture and the body just can't perspire. So we often see health problems associated with that," he said.

Once again today, Environment Canada issued smog and humidex advisories for parts of Ontario and Quebec. Humidex values are expected to be in the 40-degree range. The advisory covered southern Ontario and southern Quebec, from Windsor, to Montreal and the St. Lawrence River.

Heat warnings were in effect for a swath of southern Quebec, from Pontiac in the west to Montreal, the Eastern Townships and Quebec City.

The hot weather has also sparked a wave of calls to air conditioning repair companies.

"Obviously when it gets to this type of weather, because of the heat load things are running harder, running longer," said Artie Skinner of Laird and Son Air Conditioning. "Things break down."

Many affected cities have opened special cooling centres and are urging people to use them. Those most at risk of heat stroke, such as young children, seniors, and those on certain medications, should try to spend the day in air-conditioned facilities, such as shopping malls, libraries and community centres.

Rob Kuhn from Environment Canada recommended taking drinking "a lot of fluids, and take it easy if you're working outside or doing anything outside, especially between 12 noon and 6 p.m. because certainly, it's going to be quite hot," he told CTV News Channel.

"We also note that the UV index across southern Ontario will be ‘10' or ‘extreme'," he said.

The hot, humid conditions are expected to continue for a few more days, CTV Ottawa's weather anchor J.J. Clarke said, but a cold front may arrive in central Canada by Friday.

"If you don't like it hot, I guess misery loves company. But we're going to have to find ways to keep cool for another couple of days."

With so many residents expected to crank up the air conditioning today, power utilities across Ontario and the eastern U.S. are preparing for maximum demand, trying to avoid power outages.

The situation is different in Quebec, however. Flavie Cote, a spokesperson with Hydro-Quebec, said peak demand for electricity in the province is significantly higher during the winter months.

"(Quebecers) do use air conditioning as they do in Ontario, but in much lesser quantities," she told CTV Montreal.

On Monday, an explosion and fire at a transformer station in Toronto knocked out electricity for more than 300,000 local residents just before 5 p.m. ET, though it's unclear whether a surge in electricity demand sparked the blowout.

The blackout sent thousands of office workers out into the streets, where they found the downtown in a brief state of chaos from a lack of traffic signals. It was confined to a relatively small area, because the relay system at the transformer station was able to disconnect the disabled site from the rest of the grid.

That helped prevent the ripple effect that occurred in 2003, when a local Ohio outage led to a massive blackout throughout the northeastern U.S. and southern Ontario.

With a report from CTV Montreal's Caroline van Vlaardingen, CTV Toronto's Austin Delaney and files from The Canadian Press