This International Standard describes the use of single tracer gas for determining the local mean age of air as an indicator of ventilation conditions in a building. The procedures include concentration decay and homogeneous constant emission.The described methods are intended for air quality studies, for example to determine whether poor air quality is caused by high emissions or inadequate ventilation. In principle, the methods can be applied to all indoor spaces, regardless of the ventilation principle and the state of mixing of air between zones in the measurement object. The prevailing ventilation conditions need not be disturbed by the measurement.NOT