Growth of microorganisms on HVAC filters under controlled temperature and humidity conditions

A recirculating duct system was used to challenge a glass fiber media filter, a polymer fiber media filter, and a two-stage electrostatic precipitator (electronic air cleaner) with generated Cladosporium fungal spores and Flavobacterium bacteria. Over a period of a year, the continuous airflow in the ducts was maintained at 70°F (21°C) and 90% relative humidity (RH), but ambient loading was minimal. Filtration efficiency measurements were made, as well as microbial sampling on the surfaces of the filters, to determine filter removal and potential growth of the generated bioaerosols. The electrostatic precipitator demonstrated greater than 90% effectiveness on the fungal and bacterial bioaerosols. The media filters demonstrated initially lower efficiencies, which rose to more than 90% as the filters loaded. At the end of the first year, observable microbial growth took place only on the upstream side of the glass fiber filter. With subsequent nutrient loading and wetting, however, significant microbial growth was found on the downstream side as well as the upstream side of the fiber media filters.