Two children and two adults in east-end Toronto have reportedly been taken to hospital to be treated for carbon monoxide poisoning.
Media reports said the apartment was without power and the occupants were burning coal to keep warm. The extent of injuries were not yet known.
This latest incident comes as thousands of people from Ontario to the Maritimes continue to await the return of electricity that was knocked out by an ice storm last weekend.
Authorities have strongly cautioned people against using gas-powered generators, charcoal stoves or even barbecues indoors to keep warm.
At least two people died of carbon monoxide poisoning in Newcastle, Ont., east of Toronto, on Monday after trying to keep warm with a gas generator 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 Toronto area – where some 70,000 customers remained without power Christmas morning – authorities reported a dramatic jump in calls for suspected carbon monoxide poisoning, responding to 110 calls in a 24-hour period. Officials said Tuesday they typically see 20 such calls a day.
The ice storm downed power lines, left trees and roads covered in ice and caused widespread travel delays.
Crews continue working non-stop to restore power in parts of southern Ontario, Quebec, New Brunswick and Nova Scotia.
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.
Hydro One, which serves 1.3 million customers in Ontario communities that include Guelph, Peterborough and Walkerton, had 31,000 customers still without power.
Hydro Quebec said some 28,00 customers were without electricity, primarily in the Eastern Townships.
In New Brunswick, just under 29,000 customers were still in the dark, with about half of them in the Rothesay area. In Nova Scotia, the number of outages had dropped to fewer than 300 by early Wednesday.