Two men were taken by air ambulance to a Toronto hospital in life-threatening condition after a house explosion northwest of Uxbridge, Durham police say.

The explosion happened in the town of Zephyr in a house on Concession Road 3, according Duty Insp. Mitch Martin of Durham Regional Police. Officers were called to the home at about 6:30 p.m.

They found the two men, a resident and a furnace installer, with "fairly significant" burns to their bodies.

Martin said the two men were working on a newly installed furnace with the gas on when the blast happened.

"It appears to be an explosion related to the gas and the furnace," Martin said on Tuesday.

Durham paramedics took the men to a local hospital in Uxbridge. Then Orgne air ambulance airlifted them to a trauma centre in Toronto after it was determined that their injuries were more serious than initially thought.

Martin said neighbours on each side of the house were forced to leave for about two hours as police investigated the explosion and taped off the house. The neighbours have been allowed to return.

The explosion damaged the house structurally, with windows and siding blown out, but Martin did not have a dollar estimate.

Ontario's Office of the Fire Marshal and the Ontario Ministry of Labour have been asked to investigate.