Introduction We have already seen that, along with fire, natural ventilation is the very oldest of environmental controls used by humans to make their shelters more habitable. Of course it was soon discovered that if you lit a fire indoors then smoke would travel upwards and it made sense to form an opening in the roof, whilst air would be drawn through openings at low level. This is what later became known as the ‘stack effect’ once humans learnt that combustion products could be carried away in stacks, both ejecting the pollutants clear of neighbouring buildings and increasing the amount of heat obtained from a combustion process.