carbon monoxide, chemical compound, CO, a colorless, odorless, tasteless, extremely poisonous gas that is less dense than air under ordinary conditions. It is very slightly soluble in water and burns in air with a characteristic blue flame, producing carbon dioxide; it is a component of producer gas and water gas, which are widely used artificial fuels. It is a reducing agent, removing oxygen from many compounds and is used in the reduction of metals, e.g., iron (see blast furnace), from their ores. At high pressures and elevated temperatures it reacts with hydrogen in the presence of a catalyst to form methanol. Carbon monoxide is formed by combustion of carbon in oxygen at high temperatures when there is an excess of carbon. It is also formed (with oxygen) by decomposition of carbon dioxide at very high temperatures (above 2,000°C). It is present in the exhaust of internal-combustion engines (e.g., in automobiles) and is generated in coal stoves, furnaces, and gas appliances that do not get enough air (because of a faulty draft or for other reasons).
Carbon monoxide is an extremely poisonous gas. Breathing air that contains as little as 0.1% carbon monoxide by volume can be fatal; a concentration of about 1% can cause death within a few minutes. The gas is especially dangerous because it is not easily detected by human senses. Early symptoms of carbon monoxide poisoning include drowsiness and headache, followed by unconsciousness, respiratory failure, and death. First aid for a victim of carbon monoxide poisoning requires access to fresh air; administration of artificial respiration and, if available, oxygen; and, as soon as possible, expert medical attention. When carbon monoxide is inhaled, it reacts with hemoglobin, the red blood pigment that normally carries oxygen to all parts of the body. Because carbon monoxide is attracted to the hemoglobin about 210 times as strongly as is oxygen, it takes the place of oxygen in the blood, causing oxygen starvation throughout the body. Carbon monoxide detectors for homes are now readily available.
Carbon monoxide from automobile and industrial emissions is a dangerous pollutant that may contribute to the greenhouse effect and global warming. In urban areas carbon monoxide, along with aldehydes, react photochemically to produce peroxy radicals. Peroxy radicals react with nitrogen oxide to increase the ratio of NO2 to NO, which reduces the quantity of NO that is available to react with ozone (see smog). Carbon monoxide is also a constituent of tobacco smoke.