When appliances and vents work properly, and there is enough fresh air in your home to allow complete combustion, the trace amounts of CO produced are typically not dangerous. And normally, CO is safely vented outside your home. Problems arise when something goes wrong. An appliance can malfunction, a furnace heat exchanger can crack, vents can clog, or debris may block a chimney or flue. Fireplaces, wood burning stoves, charcoal grills, or gas logs can produce unsafe levels of CO if they are unvented or not properly vented.
Exhaust can seep into the home from vehicles left running in an attached garage.
All these sources can contribute to a CO problem in the home. The Consumer Product Safety Commission recommends that every home have at least one carbon monoxide alarm with an audible warning signal installed near the sleeping area. Look for the UL logo on the package. It's not like a smoke alarm. A smoke alarm triggers an alarm immediately when it detects smoke.