Calgary firefighters say they were called to a condominium building Friday morning.
Firefighters measured carbon monoxide levels of up to 310 parts per million — three times higher than levels that are dangerous to human health.
The evacuees sat in Calgary Transit buses to keep warm as crews searched for the problem and ventilated the building to get rid of the odourless gas.
The source of the carbon monoxide was a fresh air intake on the roof that was plugged with ice.
One person was treated by paramedics.