Haemoglobin consist of a protein globin and pigment heme. The four portion of iron in heme combine with molecule of oxygen. It is an easy reversible reaction to form oxyhaemoglobin

Whereas, the complex formed by the reaction of carbon monoxide and haemoglobin is incredibly strong

As a result of this strong reaction between the haemoglobin and carbon monoxide, the haemoglobin loses its affinity to oxygen thus may lead to choking or even death.

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