The age of the air in a room is normally determined either from a pulse response or from a step change response (up or down). There are a certain number of problems involved in applying these two theoretical models, especially those associated with the duration of the injection, which must either be infinitely short or infinitely long. A hybrid method that consists of injecting a known quantity of tracer for a given time offers the advantages of both methods. The equation for calculating age is exact, regardless of the type of flow considered, and is derived from the expressions already established for a pulse response to which a correction is included to account for the tracer generation function. If a rectangular pulse is used for the injection, the solution is particularly simple.