The simplest ventilation system in a building uses external air as its source, the wind as its motive force, and openings in the external enclosure for fresh air intake on the windward side and stale air extraction on the leeward side . In a tightly constructed building, air infiltration is slow, but in a building with loose-fitting components, doors and windows, air movements will be excessive and cause draughts and high heat losses. The idea, therefore, is to create a naturally ventilated building using correctly fitting components of sizes and configurations which provide the optimum amount of air changes for the occupants, according to their activities, and also permit a minimum amount of heat loss. below Tables indicate the desirable minimum fresh-air requirements for persons taking part in various activities. These figures are also known as ventilation rates or air change rates.
In practice these rates may be very difficult to achieve by natural methods of ventilation because airflow will be governed by areas of openings, the degree to which their use can be controlled by obstruction within a building restricting air movements, and by the pressure differences causing the flow. Also, the recommended quantities of air indicated in Table will need some adjustment relative to the possible presence of offensive fumes or smells (including tobacco smoke), as well as for the moisture content of the ventilating air (relative humidity). It is usually considered that relative humidities of 30–70 per cent are acceptable as healthy, and increased ventilation rates will be required to reduce higher levels.