Definitions covering ventilation and the flow of air into and out of a space include:

Purpose provided (intentional) ventilation: Ventilation is the process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. This may be accomplished by either natural or mechanical means.

Air infiltration and exfiltration: In addition to intentional ventilation, air inevitably enters a building by the process of ‘air infiltration’. This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks in the building envelope. The corresponding loss of air from an enclosed space is termed ‘exfiltration’. The rate of air infiltration is dependent on the porosity of the building shell and the magnitude of the natural driving forces of wind and temperature. Vents and other openings incorporated into a building as part of ventilation design can also become routes for unintentional air flow when the pressures acting across such openings are dominated by weather conditions rather than intentionally (e.g. mechanically) induced driving forces. Air infiltration not only adds to the quantity of air entering the building but may also distort the intended air flow pattern to the detriment of overall indoor air quality and comfort. Although the magnitude of air infiltration can be considerable, it is frequently ignored by the designer. The consequences are inferior performance, excessive energy consumption, an inability to provide adequate heating (or cooling) and drastically impaired performance from heat recovery devices. Some Countries have introduced air-tightness Standards to limit infiltration losses (Limb 1994).

Other air losses, e.g. duct leakage: Air leakage from the seams and joints of ventilation, heating and air conditioning circulation ducts can be substantial. When, as is common, such ducting passes through unconditioned spaces, significant energy loss may occur. Modera (1993), for example, estimates that as much as 20% of the heat from typical North American domestic warm air heating systems can be lost through duct leakage. Pollutants may also be drawn into the building through these openings. As a consequence, considerable research and development into the performance of duct sealing measures is being undertaken.

Air recirculation: Air recirculation is frequently used in commercial buildings to provide for thermal conditioning. Recirculated air is usually filtered for dust removal but, since oxygen is not replenished and metabolic pollutants are not removed, recirculation should not usually be considered as contributing towards ventilation need.

Ventilation is needed to provide oxygen for metabolism and to dilute metabolic pollutants (carbon dioxide and odour). It is also used to assist in maintaining good indoor air quality by diluting and removing other pollutants emitted within a space but should not be used as a substitute for proper source control of pollutants. Ventilation is additionally used for cooling and (particularly in dwellings) to provide oxygen to combustion appliances. Good ventilation is a major contributor to the health and comfort of building occupants.