Carbon monoxide is produced as a result of incomplete burning of carbon containing fuels including coal, wood, charcoal, natural gas, and fuel oil.  It can be emitted by combustion sources such as unvented kerosene and gas space heaters, furnaces, wood stoves, gas stoves, fireplaces and water heaters, automobile exhaust from attached garages, and tobacco smoke.  Problems can arise as a result of improper installation, maintenance, or inadequate ventilation.