According to the Calgary Fire Department, crews were dispatched around 5 a.m. to the 2700 block of 12 Avenue S.E. after a resident called 911 when their carbon monoxide alarm activated.
Firefighters entered the three-storey residential complex and immediately noticed a strong exhaust odour.
Carbon monoxide readings inside the building exceeded 150 parts per million (ppm), three times the 50 ppm level the department considers dangerous.
Crews activated the building's fire alarm and began evacuating residents.
Calgary Transit sent two buses to provide warm shelter, with 62 people boarding the buses while other residents found shelter elsewhere.
Firefighters traced the carbon monoxide to a rooftop ventilation unit. Gas to the equipment was shut off, and ATCO Gas attended to confirm the source.
Crews ventilated the building until carbon monoxide was no longer detected, allowing residents to safely return to their homes.
No injuries were reported.
The incident highlights the importance of working carbon monoxide alarms, particularly as temperatures begin to fall and heating systems see increased use.
Carbon monoxide is a colourless, odourless gas that can cause serious illness or death.
The Calgary Fire Department reminds residents that a repeating four-beep carbon monoxide alarm means everyone should leave the building immediately and call 911.
Officials recommend installing carbon monoxide alarms on every level of a home, particularly near sleeping areas, and having fuel-burning appliances regularly inspected.