On a winter night, a group of workers slept in a closed room, having a burning furnace to keep themselves warm. Most of them died in their sleep. What is the reason for their fatality?

Due to deficiency of oxygen in the room, the burning furnace will emit carbon monoxide. This carbon monoxide when inhaled, combines with oxygen to form a stable compound, Carboxyhaemoglobin, which cuts off the supply of oxygen to tissues, leading to death due to asphyxiation.