Carbon monoxide, chemical formula CO, is a colourless, odourles gas. It is a major product of the incomplete combustion of carbon and carbon-containing compounds.
It binds very strongly to the iron atom in hemoglobin, (the principal oxygen-carrying compund in blood); this renders the hemoglobin incapable of taking up and releasing oxygen. A sufficient exposure to carbon monoxide can reduce the oxygen carrying capacity of the blood to the point that the victim becomes unconscious, and can suffer brain damage or even death from anoxia. Hemoglobin becomes a bright red colour when bound to carbon monoxide, so a casualty of CO poisoning can actually look abnormally pink-cheeked and healthy, were it not for he or she being dead or comatose.
Town gas, used for illumination and heating from the 19th century, was made by passing steam through red-hot coke; the resultant reaction between the water and carbon generated a mixture of hydrogen and carbon monoxide. As other poisons such as opiates and arsenic had their availability placed under more and more stringent legal restrictions, the use of carbon monoxide in town gas became the principal method of suicide by poisoning. Nowadays, the town gas mix has been replaced by natural gas (methane), and this route of suicide has been replaced by other means.
The exhaust of the internal combustion engine, when burning a carbon based fuel (i.e. almost any fuel except pure hydrogen) contains carbon monoxide. During World War II, the Nazis devised a portable gas chamber for their murder of Jews and other minorities: a Diesel engine van, with the CO-laden exhaust piped into the back of the van. As town gas was replaced by natural gas, suicides by CO poisoning tended to be by this route, of using the exhaust fumes from a running engine. Air-quality regulations have begun to reduce suicide by this route, as catalytic converters designed to clean up the exhausts remove all but a trace of CO.
A major problem of accidental CO poisoning that still exists is the use of heaters, particularly gas water heaters and gas fires which are improperly vented. There are a number of deaths every year from this cause.
The metal nickel forms a volatile compound with carbon monoxide, known as nickel carbonyl. The carbonyl decomposes readily back to the metal and gas, and this was used as the basis for the industrial purification of nickel.