In recent semiconductor manufacturing clean rooms, air washers are used to remove airborne gaseous contaminants such as NH3, SOx and organic gases from outdoor air introduced into clean room. In order to improve the gas removal performance of the air washers, a hot water contact heat exchanger can be installed upstream of an air washer, heating and humidifying the incoming outdoor air before entering the air washer. In the present study, an experiment was carried out to examine closely the improvement of gas removal efficiency by the insertion of the hot water contact heat exchanger. The experiment showed that the gas removal efficiency was increased by the water vapor condensation effect. Keywords : Air washer, Hot water contact heat exchanger, Gas removal efficiency, Water vapor condensation