Carbon oxides and the carbonate system
Carbon forms two important gaseous oxides. Carbon monoxide, CO, is a linear diatomic molecule in which carbon has oxidation state +2. It is colourless, odourless and highly toxic because it binds strongly to haemoglobin and reduces the blood's ability to transport oxygen. It may form during incomplete combustion when oxygen is limited. A person cannot rely on smell to detect it; fuel-burning equipment must be maintained and certified CO detectors should be used.
Carbon dioxide, CO2, is a linear molecule in which carbon has oxidation state +4. It forms in complete combustion and respiration, is consumed in photosynthesis, and is an acidic oxide. CO2 is not poisonous in the same way as CO, but a high concentration can displace oxygen and cause asphyxiation. It is also a greenhouse gas whose concentration affects Earth's energy balance.