A method to determine the release of low activities through natural air exchange from a decommissioned fuel fabrication plant is described. The method has been applied to the buildings of the NUKEM-A plant and was important in obtaining governmental authorization for the plant decommissioning. The air exchange rate in the NUKEM-A plant was measured by using a tracer gas method. For that purpose, N2O as inert gas was injected into representative rooms, and the decrease of concentration caused by exfiltration processes was measured by an infrared gas analyzer as a function of time. The knowledge of this decay curve allows the calculation of low activities, which may be released into the environment by the natural air exchange. The activity is determined according to the German radiation protection regulation. From this, an air exchange rate of ∼25 h−1 would be equivalent to 10% of the tolerable activity emission. The measured exchange rates are less than ∼0.5 h−1. This results—at least for the meteorological conditions during the measurement period—in a significantly lower activity release than that permitted. The measuring method was successfully performed and can be recommended for similar investigations.