When a man inhales air containing normal concentration of O₂ as well as CO he suffers from suffocation because

CO reacts with O₂ reducing its percentage in air

haemoglobin combines with CO instead of O₂ and forms carboxyhaemoglobin

CO affects diaphragm and intercostal muscle

CO affects the nerves of the lungs.

Solution

haemoglobin combines with CO instead of O₂ and forms carboxyhaemoglobin

Affinity of carbon monoxide for haemoglobin is 200 times more than oxygen. At 0.5 partial pressure, CO combines with 50% of haemoglobin. It produces a relatively stable compound called carboxy-haemoglobin. This causes low supply of oxygen to the body which is characterised by headache, dizziness, nausea, etc.