Extracting Bacteria for Filter Forensics

Abstract Quantifying microbial concentration in HVAC or air cleaner filter dust is one useful method for characterizing indoor bioaerosols, however, the quantification depends on the effectiveness of the dust extraction method. This study investigates dry swabbing as an alternate dust extraction approach when compared with traditional methods such as vacuuming. Dust samples from 12 HVAC filters taken from two air handlers serving occupied and unoccupied office spaces in the same building were collected via dry swabbing and vacuuming, and total bacterial concentrations were quantified. The results suggest that multiple swab samples from a single filter may be required to capture the representativeness of total bacteria on the filter. Swabbing is more sensitive to local bacteria and inhibitors when compared to vacuuming and yields higher bacterial concentration in high dust loading conditions. Three to ten times higher bacterial concentrations were observed on filters from the occupied floor; however, swabbing displayed high uncertainties and variation in these results. The findings suggest that dry swabbing can be used for rapid screening of contaminants, especially in areas with low dust amounts or where vacuuming is not feasible. Further work should aim to resolve within-filter variation and associated uncertainties as well as extend this approach to characterize the microbial communities.