Survival of Microorganisms on HEPA Filters

High Efficiency Particulate Air (HEPA) filters are required to minimize the release of microorganisms from laboratories and other settings. This study was carried out to assess whether a range of microorganisms captured on HEPA filters would survive under normal operating conditions. Bacillus atrophaeus (NCTC 10073), Staphylococcus epidermidis (NCIMB 12721), MS-2 coliphage, Escherichia coli (NCIMB 9481), Brevundimonas diminuta (NCIMB 11091), and Aspergillus brasiliensis (ATCC 16404) were individually aerosolized using a Collison nebulizer and captured on HEPA filter material. Clean air was drawn through the loaded filters for 6 days at a constant rate (face velocity of 0.4-0.5 m/s−1) for all organisms and for 210 days for B. atrophaeus to simulate the use of a HEPA filter. Pre-packed sterile filters were also contaminated with B. atrophaeus which survived on the HEPA filter material for 210 days with no significant loss of viability. MS-2 coliphage and A. brasiliensis survived over the 6 days with no significant loss of viability. There was a 5-log reduction in viability of S. epidermidis over 6 days, while both Gram-negative bacteria, E. coli and B. diminuta, were not recoverable after 48 hours of exposure. This study highlights the need for risk assessments and rigorous guidelines on the use and handling of air filtration membranes exposed to resistant pathogenic agents to minimize the risks of occupational exposure.