Ventilation is the process of removing from an enclosed space foul air, laden with impurities, and replacing it with fresh air. An exact displacement, however, does not always take place. The incoming fresh air may merely dilute the foul air to a point suitable for healthful breathing. The standard of pure air is taken as that existing in the open country; it contains about four parts of carbon dioxide (C02) per ten thousand of air and is free from dust. An increase of two parts of carbon dioxide is accepted as the standard of pure air. Any excess above this is considered impure air. Badly ventilated rooms often contain as many as 80 parts of carbon dioxide per ten thousand of air.