Ventilation is the changing air in any space to give back high indoor air quality. It is used to 'repair' excessive moisture/unpleasant smells. Additionally good working system introduces outside air to keep interior building air circulating, and prevents stagnation of the interior air. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. Ventilation has got priority in maintaining acceptable indoor air quality in buildings. It includes both the exchange of an air to the outside as well as circulation of air within the building.