Carbon monoxide is odorless, colorless, and highly toxic. It kills by reducing the oxygen supply in the body and is a deadly poison. Properly installed and maintained heating appliances cause little threat from carbon monoxide. Poorly installed and maintained systems can be deadly.
All fossil fuels contain carbon.
During the combustion process the carbon in the fuel combines with oxygen in the air. With sufficient oxygen, sufficient turbulence, and at high ignition temperatures, nearly all of the carbon combines with two atoms of oxygen, producing the relatively innocuous carbon dioxide. These products of combustion from a properly maintained heating system will be vented outdoors through the chimney, and do not pose an undue risk to the building occupants. When insufficient oxygen is available for complete combustion, one atom of carbon combines with one atom of oxygen and carbon monoxide is produced. If the heating appliance or the venting system is defective, some or all of the carbon monoxide might be circulated into the building, posing an extremely hazardous health risk.