Carbon monoxide is a colorless, odorless, tasteless gas. A molecule of carbon monoxide (CO) contains one carbon atom and one oxygen atom. This image shows four representations chemists use for carbon monoxide. In the colored models, carbon is light gray and oxygen is red.
Carbon monoxide is flammable and is quite toxic to humans and other oxygen-breathing organisms. Carbon monoxide is present in Earth's atmosphere at very low concentrations, typically around 100 parts per billion (ppb). Volcanoes and exhaust emissions from automobiles contribute carbon monoxide to Earth's atmosphere.