Microbial contamination of indoor air by air conditioning systems represents a major public health concern. Biofilm formation around condensation coils and drip pans takes place due to the combination of collection of particulate organic matter and condensation of moisture in an air conditioning unit, which not only reduces the efficiency of heat transfer in these systems but also leads to a high level of fungi and bacteria that become airborne and transported throughout the buildings via air circulation. The success of ozone as a disinfectant in water systems suggested its potential for aerial applications to prevent microbial growth in air conditioning systems. Preliminary work suggested that ozone prevented accumulation of microbial mass in an actual home air conditioning system, and thus a method and apparatus to control the microbial growth in air handling systems were developed