Carbon monoxide (CO) is one of the most common causes of accidental poisoning because it is a ubiquitous by-product of incomplete combustion. It is a non-irritating, colorless, tasteless, and odorless gas that is difficult to identify when it is present in the environment. It is produced during combustion of carbon compounds such as propane, gasoline, natural gas, wood, coal, and tobacco. The routine use of such fuels in automobiles, furnaces, space heaters, charcoal grills, and fireplaces creates numerous potential exposures in routine daily activities. Fortunately, with proper operation and adequate ventilation, these sources rarely result in toxic CO exposure. However, firefighters and fire victims are at risk of serious exposure. CO is a major contributor to morbidity and mortality in smoke inhalation and fire victims; hence these patients require evaluation for CO poisoning. A less common source of CO is the metabolism of methylene chloride in products such as paint thinner.

CO is produced naturally in the body as a result of enzymatic degradation of heme by heme oxygenase. Heme metabolism results in a very low basal level (1 per cent) of carboxyhemoglobin (COHb) unless there is hemolysis to increase it. COHb levels are increased during pregnancy and by some drugs such as diphenylhydantion. Smokers and urban dwellers exposed to automobile exhaust fumes commonly have COHb levels higher than normal, but these rarely exceed 10 per cent.

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