The call came as temperatures dropped along the path of a nasty storm which had downed power lines, left trees and roads covered in ice and caused widespread travel delays since the weekend.
"It's really cold, so that's why the concern arises," Environment Canada meteorologist Marie-Eve Giguere said of the warning that was being sounded Tuesday in communities where some were in their third full day without power.
Much of southern Ontario and southern Quebec experienced temperatures between -15 and -20, with the mercury expected to drop further through the night. Parts of the Maritimes that had been hit by the ice storm on Monday weren't quite as cold, but still experienced temperatures below zero
"It usually happens behind a strong system, you get very cold air in behind," said Giguere.
Warmth-craving residents were being urged to take advantage of warming centres or the hospitality of friends and family rather than using gas-powered generators, charcoal stoves or even barbecues indoors — scenarios which led to a number of people being rushed to hospital.
At least two people died of carbon monoxide poisoning in the town of Newcastle, Ont., east of Toronto, after trying to keep warm with a gas generator running in a garage.
Carbon monoxide poisoning was also believed to be the cause of three deaths in a chalet on Quebec's north shore, although it was unclear if the incident involved a loss of power related to the ice storm.
In the hard-hit Toronto area — where some 90,000 customers remained without power — authorities reported a dramatic jump in calls for suspected carbon monoxide poisoning, responding to 110 calls in a 24 hour period. Officials said they typically see 20 such calls a day.
"We're looking at approximately six times as many calls," Toronto Mayor Rob Ford said as the city issued an extreme cold weather alert.
"I understand they want to keep warm but you cannot do this. This is deadly."
Fire officials warned residents not to use any appliance that burns inside a home, and even cautioned against using a lot of candles.
"People are using these in the house, I've never seen anything like it, I don't understand it," Toronto Deputy Fire Chief Mike McCoy said of residents using charcoal brickettes or other outdoor heat sources inside.
"If the home stays closed and there’s no way to bring in fresh air, oxygen, you’re going to run into a CO issue."
Adding to the risk was the fact that people often don't know they are being affected by the deadly colourless, odourless gas.
"It's known as the silent killer, that's the real danger," said McCoy. "People don't feel this coming on. They aren't aware of it, typically they may go to bed leaving something burning for the heat and worst-case scenario, you never wake up from it."
Ontario's premier added her voice to the cautionary calls, reminding residents that electric carbon monoxide detectors would not work in a power outage unless they had battery backups.
"It's vital that everyone heed the warnings of emergency officials," said Kathleen Wynne.
Meanwhile, crews continued to work around the clock to restore power to those who were still without hydro in parts of southern Ontario, Quebec, New Brunswick and Nova Scotia.
Officials warned, however, that while crews were doing their best, there would be some who would likely have no power through Christmas and even Boxing day.
Dianne Worrell expected to be among them and was making alternate arrangements for the holiday meal she planned to host.
"It's like a war zone," she said of her Markham, Ont., neighbourhood where a downed power line was still strewn across a neighbour's yard.
"There's lots of broken trees, the neighbours were trapped in their driveway for a couple of days because a tree had fallen across."
Power Stream, the utility which serves Markham, Richmond Hill and Vaughan among other communities, reported 12,100 customers were still down by early Wednesday. Veridian Connections, which serves the Pickering and Ajax areas east of Toronto, said about 4,000 customers were still affected.
In Toronto, where 300,000 lost power at the height of the storm, about 79,000 clients were waiting for power to restored as crews from Ottawa, Windsor, Manitoba and Michigan helped local teams with their efforts.
Hydro One, which serves 1.3 million customers in Ontario communities that include Guelph, Peterborough and Walkerton, had 28,000 customers still without power.
Hydro Quebec said some 28,00 customers were without electricity, primarily in the Eastern Townships.
In New Brunswick, just over 29,000 customers were still in the dark, almost half of them in the Rothesay area near. NB Power said it was aware of the frustration the outage was causing but warned that some would have to wait until Saturday before hydro is restored.
Crews made progress in Nova Scotia, where the number of outages had dropped to fewer than 300 by early Wednesday.
Airlines which had to make widespread cancellations during the ice storm were returning to regular service Tuesday with only a few weather-related delays expected.