Abstract
Based on the current knowledge of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) transmission, wearing a mask has been recommended during the COVID-19 pandemic. Bacterial filtration efficiency (BFE) measurements enable designing and regulating medical masks to prevent bioaerosol dissemination; however, despite the simplicity of these measurements, several scientific questions remain unanswered regarding BFE tests. Here, we investigated (1) the impact of substituting 100-mm Petri dishes with 90-mm disposable Petri dishes, (2) the impact of colony-counting methods on the bioaerosol aerodynamic size, and (3) the impact of colony-counting methods on the total viable particle counts. We demonstrated that disposable 90-mm Petri dishes can be used to replace the 100-mm dishes. We also showed that an automatic high-resolution colony counter can be used to directly count viable particles on collection substrates and to measure the bioaerosol size parameters. Our results enable possible modernization of the outdated testing methods recommended in the US and European standards for BFE measurements. Specifically, use of a modernized colony counter should be clearly regulated and permitted to avoid the counting of positive holes. The median aerodynamic diameter appears to be the most relevant parameter for characterizing bioaerosol size.
Highlights
Based on the current knowledge of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) transmission, wearing a mask has been recommended during the COVID-19 pandemic
To perform the Bacterial filtration efficiency (BFE) experiments in a class II biosafety cabinet, we reduced the length of the aerosol chamber in our experimental set-up (600 mm long and 80 mm in external diameter according to EN 14683:2019) without changing the features of the bioaerosol arriving at the top of the Andersen cascade impactor (ACI)
We found that (1) the bacterial challenge was similar between the positive runs using either the 100-mm or the 90-mm Petri dishes, (2) use of the 90-mm Petri dish yielded similar counts to those of the 100-mm Petri dishes for methods 1 and 3, and (3) the only impact from using the 90-mm plates occurred with method 2, which was irrelevant to counting the total bacteria/viable counts from the results in Fig. 5 (Fig. 6, Supplementary Table 2)
Summary
Based on the current knowledge of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) transmission, wearing a mask has been recommended during the COVID-19 pandemic. For some viral and bacterial pathogens, airborne transmission appears to be a major mode by which people are infected, as is the case with Mycobacterium tuberculosis (the bacterium that causes tuberculosis), rubeola (the virus that causes measles), and Varicella-Zoster (the virus that causes chicken pox) These diseases can be transmitted at close ranges, they are efficiently and frequently transmitted to people passing through a room in which an infectious person was present minutes to hours earlier. Among the different categories of face masks, medical masks are the only masks designed and regulated to prevent bioaerosol dissemination from the wearer into the environment (i.e., aerosol from the upper airways or saliva that may contain infectious agents transmissible by droplet or airborne routes). In addition to the BFE measurement, the performance requirements of medical face masks include a microbial cleanliness (i.e., the bioburden of the mask) < 30 colony-forming units (CFUs)/g, breathability (i.e., the air permeability of the mask determined from the differential pressure across the mask) < 40 Pa/cm[2] (for types I and II) or 60 Pa/ cm[2] (for type IIR), and a splash resistance for type IIR masks (i.e., resistance of the mask to penetration from splashing) > 16.0 kPa
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