Carbon monoxide (chemical formula CO) is a colourless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxo carbon family. In coordination complexes the carbon monoxide ligand is called carbonyl.
Thermal combustion is the most common source of carbon monoxide, however there are numerous environmental and biological sources that generate and emit a significant amount of carbon monoxide. Humans utilize carbon monoxide for various industrial processes including synthetic chemical manufacturing and metallurgy, however it is also a problematic air pollutant arising from industrial activities. Upon emission into the atmosphere, carbon monoxide may have roles potentially affecting climate change.
Carbon monoxide has important biological roles across phylogenetic kingdoms. In mammalian physiology, carbon monoxide is a classic example of hormesis where low concentrations serve as an endogenous neurotransmitter (gas transmitter) and high concentrations are toxic resulting in carbon monoxide poisoning.