Investigators looking into a carbon monoxide leak that left dozens of people in Winnipeg in hospital are looking at the building's ventilation system for clues as to what caused the gas to escape.

Alex Forrest, the president of the United Fire Fighters of Winnipeg union, told CTV's Your Morning Wednesday that work was being done on the ventilation system at the time of the leak.

Forty-six people were taken to hospital, including 15 in critical condition, after emergency crews received an automatic alarm Tuesday about carbon monoxide detection from the boiler room of the Super 8 Hotel on Portage Avenue. Some of them reported symptoms including nausea, vomiting, dizziness and shortness of breath.

"This is probably the largest mass tragedy triage issue that I had ever seen – not only in Winnipeg, but in Canada – for carbon monoxide poisoning," Forrest said.

Firefighters said they detected carbon monoxide levels in the hotel at nearly 400 parts per million, or more than 20 times the level that would trigger an evacuation. CO levels fell to zero about 90 minutes after firefighters arrived and started using fans to ventilate the building.

Based on the symptoms of the victims, Forrest said people in the hotel had likely been exposed to the gas for an hour or two before the alarm was activated.

"If it would have been any longer than that … we would have had fatalities," he said.

Almost all of the 46 people taken to hospital had been discharged as of Tuesday night. The hotel reopened Wednesday morning.

An initial report into the incident from the Winnipeg Fire Department could be completed as soon as Wednesday, Forrest said.