Death while sleeping in closed room with burning coal furnace is due to

The risk of asphyxia (sleep death) is high when sleeping in a closed room with coal furnace. Carbon monoxide combines with hemoglobin to form carboxyhemoglobin at any or all of the oxygen-binding sites of hemoglobin, and also acts to increase the stability of the bond between hemoglobin and oxygen, reducing the ability of the hemoglobin molecule to release oxygen bound to other oxygen-binding sites. The excessive carbon monoxide in the room chokes the supply of blood to the brain, which can lead to haemorrhage and eventually death.