Carbon Monoxide forms a reversible bond with Haemoglobin much the same way Oxygen does. Unfortunately, it bonds 200 times more tightly then Oxygen forming Carboxyhaemoglobin.

Carboxyhaemoglobin has a half-life of several hours and impedes the transportation of Oxygen from the lungs to the rest of the body which is why ciggerette smoking prior to dives should be avoided.