Carbon monoxide gas is produced when any type of fuel is incompletely burned. Gasoline engines and Natural Gas and Propane burning appliances (furnace, fireplace, oven stove, water heater etc.) space heaters, gas and charcoal grills, etc. produce CO.Extended operation of unvented fuel burning appliances (range, oven, fireplace, etc.) can build up high CO levels.

CO build up can be caused by reverse/negative venting of fuel burning appliances including; 1) Clogged, loose or faulty stacks or chimneys of (clothes dryers, furnaces and water heaters, etc.), 2) wind direction and/or velocity, 3) simultaneous operation of multiple fuel burning appliances, and/or exhaust fans. An idling vehicle in an open or closed garage, or near a home can cause high levels of CO to build up in a residence.