Carbon monoxide (CO) is a colorless, odorless, poisonous gas produced when fuels containing carbon are burned where there is too little oxygen. It also forms as a result of burning fuels at too high a temperature. It burns in air or oxygen with a blue flame and is slightly lighter than air.
In the presence of an adequate supply of O2 most carbon monoxide produced during combustion is immediately oxidised to carbon dioxide (CO2). However, this is not the case in spark ignition engines in motor cars, especially under idling and deceleration conditions. Thus, the major source of atmospheric carbon monoxide is road transport. Smaller contributions come from processes involving the combustion of organic matter, for example in power stations and waste incineration.
Natural background levels of carbon monoxide fall in the range of 10-200 parts per billion (ppb). Levels in urban areas are highly variable, depending upon weather conditions and traffic density. Concentrations are generally less than 10 parts per million (ppm) but can be as high as 500 ppm.
Carbon monoxide is poisonous when inhaled because it combines with haemoglobin, the oxygen-carrying substance in red blood cells. The hemoglobin then cannot take up oxygen from the air. Lack of oxygen causes cells and tissues to die.