The various meanings of ventilation efficiency are briefly summarised. The residual life time of a released tracer gas is chosen as the most meaningful and convenient basis for local efficiency measurements in large, occupied, mechanically ventilated buildings. Measurements were carried out in ten public swimming pool halls. Sulphur hexafluoride tracer gas was released from a 20 ml syringe at various locations around the pool hall and the integrated concentration with respect to time was measured at the exhaust air duct. This was extrapolated to infinite time using the measured decay rate. Hence the median residence time of the tracer after release was found and compared with the corresponding value for uniform and complete mixing using the measured exponential decay rate. The ratio of these two times was taken as a measure of the ventilation efficiency. None of the ventilation systems tested proved significantly more efficient than the others overall. Local variations were however detected at individual sites and the method shows promise for application in occupied buildings, where minimum intrusion is required.