An adequate air change is of fundamental importance for indoor air quality. Proper ventilation of all buildings is necessary for the health and comfort of the occupants, but also to protect against damage, for instance due to excessive atmospheric humidity. However, the present-day fitting of tightly-sealing windows, for example, to residential and office buildings can, in addition to the desired energy savings and noise reduction, lead to insufficient ventilation which leads to an increase in concentration of substances emitted indoors. In the present ISO 16000-8 methods are described for determining the local mean age of air in buildings based on measuring the concentration of tracer gases. As selected measurement methods the concentration-decay method as well as the active and passive homogeneous emission method are described. Important boundary conditions which are to be maintained during the measurement and interpretation of the results are considered in detailed examples.