When the objective is to minimize the average room exposure to contaminants the objective of a ventilating system is the removal of contaminants from the ventilated space as quickly as possible. The shorter the resident time of a contaminant in the ventilated space, the lower the exposure to the contaminant; therefore, the sooner the contaminant reaches the extract duct, the better is the ventilation. The important parameters in this context are, therefore, time and local flow rates, and not such traditional concepts in ventilation engineering, as for example air-change rates or mixing factors. The concept of age, or residence time, is applicable to characterize both how the supplied air or a contaminant is spread within an enclosed space and how quickly a contaminant is removed. A comprehensive discussion is presented in this paper on the application of the concept to any enclosed space with air intakes and extracts. The direct relationship between the age concept, exposure to contaminants, and equilibrium concentrations is established. Different tracer-gas techniques for measuring the age distributions and the derived quantities are also treated. Different definitions of ventilation efficiency based on maximum concentration or average room concentration, and a definition of "air diffusion efficiency" are also given. Results are presented from measurements carried out in a mechanically ventilated two-room test house, equipped in one case with one intake and one extract, and in another case with one intake and two extracts.

Finally, test results are presented from an office room equipped with terminals at different locations. The supplied air temperature was both lower and higher than the room air temperature. With a correct utilization of buoyancy, almost displacement flow systems may be achieved.