In an air conditioning system which includes a catalytic air purifier, a control system is provided for sensing the level of predetermined conditions in a space and for responsively modulating the air purification process in a manner which optimizes the process in terms of its effectiveness and its economical use. The conditions that are sensed include the levels of VOC's, CO2, and biological contaminants, as well as degree of occupancy, relative humidity and air flow. The air purification process is modulated by varying the power to a UV lamp, turning one or more UV lamps on or off, varying the effectiveness of the catalyst, varying the volume or speed of the air flowstream and/or varying the relative humidity in the space.