The carbon monoxide leak originated in a boiler room at a Super 8 hotel in Winnipeg, Manitoba's capital and largest city. An automatic alarm was triggered, bringing the Winnipeg Fire Paramedic Service to the scene, Chief John Lane said at a press conference Tuesday.
There were 52 people at the hotel, including staff, and 46 were transported to local hospitals, with at least 15 in critical condition and five in unstable condition, Lane said. One dog was also removed from the hotel and treated by Winnipeg Animal Services.
The readings inside the hotel were 385 parts per million, according to the City of Winnipeg. A reading of over 150 to 200 parts per million can be fatal, according to the U.S. government's Consumer Protection Safety Commission.
"The crew immediately put on their [masks] and began evacuating the hotel," Lane said.
Officials said they do not believe any of the patients in the hospital are at risk of dying.
"I'm very happy to say none of the patients were requiring resuscitation," Lane said. "None of them were intubated or anything. But their carbon monoxide levels were such that they were in the critical category."
Manitoba Hydro Electric Energy and Natural Gas is investigating the cause of the CO gas leak.
ABC News' Darren Reynolds contributed to this report.