Carbon monoxide is highly poisonous gas. It is a colourless and odourless gas and hence its presence cannot be detected immediately. When inhaled, carbon monoxide readily combines with the haemoglobin of our blood. Haemoglobin, as you know, helps in carrying oxygen from longs to the tissues. Oxygen combines with haemoglobin to form oxy-haemoglobin. Similarly, CO combines with hemoglobin to form carboxyhaemoglobin.
The affinity of CO for haemoglobins much more (about 200 times) than that of oxygen. In the presence of CO, the oxygen carrying capacity of the hemoglobin decreases. The deficiency of oxygen in blood may cause headache, dizziness, cardiac and respiratory problems and even death. Because of this reason, it is dangerous to sleep in a closed room with a coke fire burning inside because burning of coke in an insufficient supply of oxygen produces a lot of carbon monoxide that can cause poisoning.