This part of ISO 16000 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 and can be used for a) checking whether the building ventilation requirements are met, b) estimating the adequacy of ventilation in buildings with indoor air quality problems, and c) characterizing the strength and distribution of indoor emission sources. In principle, the methods can be applied to all indoor spaces, regardless of the type of ventilation used and the state of mixing of air between zones. The prevailing ventilation conditions need not be disturbed by the measurement. This part of ISO 16000 does not address the details of the analytical methods for tracer gases. The availability of such analysis services should be checked before planning actual field measurements.