A colorless, odorless, very toxic gas CO that burns to carbon dioxide with a blue flame and is formed as a product of the incomplete combustion of carbon.

The toxicity of carbon monoxide (CO) is related to the high affinity of hemoglobin for CO. The affinity of hemoglobin to CO is about 200-300 times than its affinity of oxygen. The result is the formation of a stable carboxyhemoglobin complex instead of the formation of oxyhemoglobin. The carboxyhemoglobin will not accept any more oxygen, and thus oxygen transport will be severely hindered. Even low concentrations of CO can be extremely dangerous upon prolonged exposure.

Carbon monoxide is also an explosive gas. The lower explosive limit in air is 12.5%, and the upper explosive limit is 74%.

Carboxyhemoglobin (carboxyhaemoglobin), the complex formed when carbon monoxide reacts with hemoglobin (haemoglobin) is bright cherry-red in colour (as opposed to deoxygenated blood, which is bluish and oxygenated blood which is red).

Treatment is with supplemental oxygen. Severe carbon monoxide poisoning may require hyperbaric oxygen therapy, where the poor patient has to sit in a pressurised chamber (essentially the same kind of chamber used for curing divers of 'the bends') until the carbon monoxide in his system has dropped down to an acceptable level.