Police commander Dacid Vijakainen warned residents to avoid using generators and barbeques to heat their homes after a 52-year-old man and his 72-year-old mother died northeast of Toronto.
Police in Quebec said carbon monoxide poisoning is believed to be the cause of three deaths in a chalet on the province's North Shore.
In Toronto on Monday night, 11 people were taken to hospitals with signs of carbon monoxide poisoning. Mayor Rob Ford said the city received about six times as many calls about carbon monoxide Monday about 110 over 24 hours as it gets in a usual day.
Toronto issued an extreme cold weather alert, and temperatures were set to plunge to 6.8 degrees Fahrenheit (-14 degrees Celsius) Tuesday night. The freezing conditions will last throughout the week, according to Environment Canada.
Officials said 85,000 customers in Toronto were still without power, down from 300,000 people at the height of the outages over the weekend. Officials said some may not have power restored until after Christmas.
In Quebec, 31,700 customers remained without power as of early Tuesday. In New Brunswick, more than 40,000 customers were still in the dark.
Travelers trying to get home for the holidays continued to face delays and cancellations on airlines and trains.