When the fossil fuels are burnt in a limited amount of air (oxygen), the products of combustion are CO and H2O. Carbon monoxide is the most dangerous air pollutant. Carbon monoxide when inhaled combines with hemoglobin in the blood to form carboxyhaemoglobin. Carboxyhaemoglobin cannot carry oxygen. This causes a deficiency of oxygen in the tissues resulting in suffocation and even death.
It is due to the reason, which it is not advisable to sleep in a closed room with a coal-fired fireplace (Angithi) inside, on a cold wintry night. When the room is closed, no fresh air can enter the room. Initial burning of coal would consume whatever oxygen is the available in the room. Thus, coal/wood would burn in a limited amount of oxygen producing carbon monoxide.