At least 15 people are in a critical condition, and five more unstable, following a carbon monoxide leak at a hotel in Canada.

A total of 46 people were taken to hospital after an automatic alarm alerted Super 8 hotel staff in Winnipeg to a leak in the boiler room on Tuesday.

Alex Forrest, president of the Winnipeg firefighters' union, told Global News it was "one of the worst incidents" the department had ever seen for carbon monoxide - and said firefighters were "absolutely in shock".

"They realised that there were extreme levels of carbon monoxide and their quick actions likely saved a lot of lives," he said.

Those in a critical condition were reported to have high carbon monoxide readings in their blood, with officials adding: "They were transported critical based on that reading and other symptoms, such as nausea, vomiting, shortness of breath, a decrease in level consciousness."

Fire crews detected carbon monoxide of various levels, up to 385 parts per million, throughout the building.

There were 52 people and one dog in the hotel at the time of the alarm.

Winnipeg Animal Services is providing care for the dog.

While the utility company Manitoba Hydro has shut off gas to the building, and is working to determine the cause.

The hotel's owner, Justin Schinkel, said the building had recently passed a fire inspection and had no history of carbon monoxide leaks.

"We're just super happy that the first responders are so helpful and they've been able to get here so quick and help us out here," he said.

Additional reporting: IRN