City firefighters have responded to more than 125 carbon monoxide alarms going off so far this year, Sarnia Fire Rescue says.
Though in most cases the detector had malfunctioned or the CO was already dissipated, at 12 homes and apartments firefighters did find the deadly gas inside.
The grace period for residents to install CO detector ended last week.
The provincial law requires working alarms be installed outside the sleeping areas of any residence that has a fireplace, gas-burning appliance like a stove or furnace, or an attached garage.