Explanation

If a person respires in air containing normal amount of oxygen (21%) and 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.