If a person respires in air containing a normal amount of oxygen (21%) and a small amount of carbon monoxide, he suffers from suffocation because haemoglobin combines with carbon monoxide to form a stable compound. The affinity of haemoglobin to carbon monoxide is about 250 times more than for oxygen. 0.1% of carbon monoxide blocks 50% Hb of the body due to which the oxygen-carrying capacity of blood is decreased. This is called hypoxia.