[AIVC, Air infiltration and ventilation glossary, 1992]:the average value of the Local Mean Ages of air flow for all points in a room. The Room Mean Age cannot therefore be measured easily. It is necessary to express the Room Mean Age in terms of a measurable quantity, such as a tracer gas concentration in the exhaust duct. This requires a mass balance equation to be constructed for the room.(see air change rate; Local Mean Age)