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.