Compound formed when carbon monoxide combines with haemoglobin. The reaction is almost irreversable, and prevents the haemoglobin molecule from carrying oxygen, which can seriously affect the body's oxygen carrying capacity. This is one of the major problems caused by carbon monoxide poisoning, and will lead to death from lack of oxygen if enough is inhaled. It is also one of the health risks inherent in smoking, as carbon monoxide is one of the products of tobacco smoke, and smokers usually have about 5% of their haemoglobin combined with carbon monoxide. The only way carboxyhaemoglobin can be removed is when the red blood cell it is in dies and is broken down.