A method of operating a dispensing device for at least one air treatment agent is described, the dispensing device comprising: a housing having one or more walls to define an interior adapted to receive at least one removable container of air treatment agent at least partially therein; and within said housing the device comprises: an airborne agent detector means operable to detect airborne agents in the air, wherein said means are provided with at least one aperture to the exterior of the device to permit, in use, air from outside of the device to enter said airborne agent detector means; receiving means for receiving said at least one container of air treatment agent; emanation means adapted, in use, to emanate the air treatment agent from the device through one or more exit orifices in the housing; control means in communication with the emanation means and in communication with said airborne agent detector means; the airborne agent detector means comprises one or more metal oxide semiconductor/metal oxide odour sensors; characterised in that the method comprises the application of power to the sensor(s) in pulses.