Sulphur hexafluoride, nitrous oxide and helium were compared as tracer gases for determining the mean residence time of air in a test room and the capture efficiency of a local exhaust hood. The mixtures of tracer gases were fed into the inlet air and two sites in the test room with and without mixing of room air. We determined the capture efficiency by releasing the tracer gases through the cylindrical injection system in front of the exhaust hood at different capture velocities and under various cross-draught conditions. The results of the measurements in the test room showed good agreement between the different tracer gases except in the stagnant zone. The first capture efficiency measurements indicated that SF6, the heaviest gas, gave smaller values obviously due to incomplete mixing of tracer gases. However, the difference disappeared when mixing of the tracer gas was speeded up by pumping extra air or by modifying the injection system.