Air isa gasthat containsoxygen. It envelopstheplanet and fills all open spaces within buildings. The breath-ing function of human beings removes oxygen from air. Thus, a replacement supply of oxygen is needed to sustain life. Oxygen is also used up in burning fuels suchascoal, oil, orgastoprovide heatforhumancom-fort. Replacement of oxygenated air is needed for the heating process as well.

The process of providing a fresh supply of air for hu-man need in buildings and comfort is called ventila-tion. Sometimes the removal of odorous or contami-nated air is called ventilation as well. Building codes prescribe minimum amounts of ventilation.