Evaluation of Filters for Removal of Bacteriophages from Air

Abstract

Glass wool, nonabsorbent cotton, fiberglass filter medium, and a commercial absolute filter were tested for effectiveness in removing aerosolized bacterial viruses under low flow rate (1 ft 3 /min) and high flow rate (10 to 25 ft 3 /min) air-flow conditions. Special equipment was designed for measurement of filter efficiencies under the two air-flow conditions. Under low air-flow rate test conditions, glass wool was only 98.543 to 99.83% efficient, whereas cotton (five layers), fiberglass medium (three layers), and the commercial absolute filter were at least 99.900, 99.999, and 99.999 efficient, respectively. Glass wool and cotton were not used under higher air-flow conditions because they were difficult to assemble in leak-tight filters. The commercial absolute filter and fiberglass medium (three layers) were at least 99.990 and 99.999% efficient, respectively, under the higher air flow conditions. A stainless-steel filter of simple design and fitted with three layers of fiberglass medium was found to be greater than 99.999% efficient in removing high concentrations (20,000 to 70,000 plaque-forming units per cubic foot) of aerosolized bacteriophages from air moving at a low flow rate (1 ft 3 /min). Use of this filter on pressure-vacuum tanks in the fermentation industry is suggested. Several other uses of such a filter are proposed.

The impact of heating, ventilation, and air conditioning design features on the transmission of viruses, including the 2019 novel coronavirus: A systematic review of filtration

“…Five of seven aerosolized virus studies found that filtration was associated with virus removal using PRRSV [ 30 ], equine arteritis virus (EAV) [ 30 ], bovine enterovirus (BEV) [ 30 ], human adenovirus [ 29 ], and the bacteriophages T7 [ 27 , 28 ] and MS2 [ 31 ]. One study found that increasing MERV rating was associated with decreasing virus concentration for bacteriophage T4 at different locations within an apartment unit [ 40 ], and another found that increasing MERV rating was associated with increasing viral filtration efficiency using bacteriophage MS2 in a test duct set-up [ 26 ].…”

“…Zhang et al noted that the filter efficiency associated with E 1 (i.e., 0.30–1.0 μm; the smallest particle size range for MERV classification) was a good approximation of the viral filtration efficiency [ 26 ]. In addition, prior to the use of MERV ratings as developed in 1999, Washam et al identified cost differences between filters of similar efficiency [ 27 ].…”

“…Azimi and Stephens [ 42 ], using influenza, compared varying MERV filters and a HEPA filter and found that, for filters MERV 13 or greater, the risk reduction was not limited by the filter efficiency; instead by the amount of recirculating air through the HVAC system. Similar to Washam et al [ 27 ], Azimi and Stephens considered the cost of filtration [ 42 ]. Compared with the cost of an equivalent amount of outdoor air ventilation, they found filtration was the less expensive option for all MERV ratings except MERV 4 [ 42 ].…”

“…Diminishing returns are not limited to MERV ratings as Washam et al, prior to the use of MERV ratings, found that similar efficiency can be achieved using a lower cost option [ 27 ]. In addition, the cost increases with increasing MERV rating and HEPA filtration relative to the cost for MERV 13; however, the cost of filtration for these filters remains less than the cost of outdoor air ventilation with the equivalent transmission reduction [ 42 ].…”

“…The efficiency of virus removal is calculated as the difference between the virus concentration upstream of the filter and the virus concentration downstream of the filter divided by the virus concentration upstream of the filter expressed as a percentage. Only one aerosolized virus study used terminology that referred explicitly to virus, viral filtration efficiency [ 26 ], while the remaining studies on virus removal used a variety of terms typical for particle removal: removal efficiency [ 27 ], filtrating efficiency [ 28 ], filter efficiency [ 29 ], and filter reduction efficiency [ 30 , 31 ]. European ratings were presented with MERV rating conversions [ 32 ].…”