Carbon monoxide is a colorless, odorless, tasteless, toxic and flammable gas whose molecules each consist of one carbon atom and one oxygen atom. It is present in small amounts in the atmosphere from natural sources including photochemical reactions in the troposphere, forest fires and volcanoes, and it is also produced by human activity, particularly the burning of fossil fuels.
Although it is harmful and can be lethal when inhaled because it displaces oxygen in the blood and thus deprives the heart, brain and other organs of oxygen, carbon monoxide has important biological roles, and is produced in small quantities in many organisms, including humans. It is also a key ingredient in many industrial processes and is used in the production of many compounds including fuels, drugs, and fragrances.