Only CO2 is appropriate for doing experiment in the presence of occupants or the toxicity criteria [87]. The advantages of CO2 are that it has a density similar to that of atmospheric air, it is easy to detect (using portable and inexpensive infra-red detec- tors) and it is not absorbed by the room furniture. All tracer gas measurement techniques are based on a mass balance of the tracer gas within the building. This technique can provide an accurate measurement of ventilation rate in buildings because the air change rate is low and good mixing of tracer and air can be achieved by using a portable table fan. They are classified based on the method of injection and the form of the mass balance equation [3].