Carbon monoxide is usually generated by combustion- whether it’s a water heater, a car idling in or near the opening of a garage, or a fireplace, if something is burning, it’s likely generating carbon monoxide. In an enclosed space like a residential structure, it can quickly render the occupants unconscious before they even realize they are getting sick, and kill them. States are increasingly enforcing laws requiring the detection of this deadly gas. Carbon monoxide, often referred to in public health notices as a “silent killer,” can indeed silently and quickly permeate a home, apartment, or other structure.
The information for this answer was found on our Carbon Monoxide & Smoke Detector Laws answers.