Carbon monoxide is the twelfth most abundant gas in the atmosphere. It makes up about 1.2 × 10-5% of a sample of dry air in the lower atmosphere. The major natural source of carbon monoxide is the combustion of wood, coal, and other naturally occurring substances on the earth's surface. Huge quantities of carbon monoxide are produced, for example, during a forest fire or a volcanic eruption. The amount of carbon monoxide produced in such reactions depends on the availability of oxygen and the combustion temperature. High levels of oxygen and high temperatures tend to produce complete oxidation of carbon, with carbon dioxide as the final product. Lower levels of oxygen and lower temperatures result in the formation of higher percentages of carbon monoxide in the combustion mixture.
Commercial methods for producing carbon monoxide often depend on the direct oxidation of carbon under controlled conditions. For example, producer gas is made by blowing air across very hot coke (nearly pure carbon). The final product consists of three gases, carbon monoxide, carbon dioxide, and nitrogen in the ratio of 6 to 1 to 18. Water gas is made by a similar process, by passing steam over hot coke. The products in this case are hydrogen (50%), carbon monoxide (40%), carbon dioxide (5%) and other gases (5%). Other methods of preparation are also available. One of the most commonly used involves the partial oxidation of hydrocarbons obtained from natural gas.