k order to determine the amount of micro-organism present before and after the filters ofHVAC systems, 6 systems in 5 buildings were monitored every 2 weeks for one year. Measurementswere taken in triplicate and simultaneously before and after the filters using a sixstage Andersen sampler. The reduction of total of micro-organisms through the air filtersranged from 20% up to 93% and was associated with filtration efficiency rating.In most of theHVAC system the total cfu levels were low or very low after the pre filters and the main filters.The micro-organisms were also analysed by particles mass (the six stages of the Andersensampler) to determine which micro-organisms were small enough to have the potential topenetrate deep into human lungs and cause adverse health effects (stages 4, 5 and 6 of thesampler). These smaller sized micro-organisms were significantly reduced by the filtration media.The results also revealed advantages and disadvantages between HVAC type and designincluding filter type, maintenance, and moisture damage. During the study, sources of microorganisms contamination werealso identified and mitigation could be performed.