Carboxyhaemoglobin, a stable compound, is formed when haemoglobin readily combines with carbon monoxide. Carbon monoxide converts iron (II) to iron (III) in its reaction with haemoglobin. In this form haemoglobin does not carry oxygen resulting in its (oxygen) starvation and leads to asphyxiation and in extreme cases to death. The affinity of haemoglobin for CO is 250 times its affinity for O₂ and COHb liberates CO very slowly and also due to that compound the dissociation curve of the remaining HbO₂ shifts to the left, decreasing the amount of O₂ released.