Face coverings are a key component of preventive health measure strategies to mitigate the spread of respiratory illnesses. In this study five groups of masks were investigated that are of particular relevance to the SARS-CoV-2 pandemic: re-usable, fabric two-layer and multi-layer masks, disposable procedure/surgical masks, KN95 and N95 filtering facepiece respirators. Experimental work focussed on the particle penetration through mask materials as a function of particle diameter, and the total inward leakage protection performance of the mask system. Geometric mean fabric protection factors varied from 1.78 to 144.5 for the fabric two-layer and KN95 materials, corresponding to overall filtration efficiencies of 43.8% and 99.3% using a flow rate of 17 L/min, equivalent to a breathing expiration rate for a person in a sedentary or standing position conversing with another individual. Geometric mean total inward leakage protection factors for the 2-layer, multi-layer and procedure masks were <2.3, while 6.2 was achieved for the KN95 masks. The highest values were measured for the N95 group at 165.7. Mask performance is dominated by face seal leakage. Despite the additional filtering layers added to cloth masks, and the higher filtration efficiency of the materials used in disposable procedure and KN95 masks, the total inward leakage protection factor was only marginally improved. N95 FFRs were the only mask group investigated that provided not only high filtration efficiency but high total inward leakage protection, and remain the best option to protect individuals from exposure to aerosol in high risk settings. The Mask Quality Factor and total inward leakage performance are very useful to determine the best options for masking. However, it is highly recommended that testing is undertaken on prospective products, or guidance is sought from impartial authorities, to confirm they meet any implied standards.
Humans expel physiological particles continuously through normal respiratory activities such as breathing, talking, coughing and sneezing; a portion of these are aerosol in the size range <5.0 µm. Misconceptions exist on how to best implement face coverings as an effective preventive health measure against potentially infectious respiratory generated aerosol. The aim of this study was to characterise the performance of face coverings against aerosol when worn by individuals, and to quantify the maximum aerosol penetration through the material used in the construction of each mask. The former addresses their use as a means of possible protection against aerosol present in the environment and the latter having relevance to filtration and reducing human generated aerosol from reaching the environment. Face covering performance was assessed by measuring the total inward leakage of aerosol through the mask material and face seal. Aerosol penetration was measured on swatches of material taken from the face covering. An inert polydisperse charge-neutralized NaCl aerosol, with a distribution ranging from 0.023 µm to 5 μm in diameter, was used for the experiments.Total inward leakage tests were completed to assess the protection factor for nine variations of face coverings, including seven reusable cloth masks, of which six were homemade and one was commercially manufactured, and two styles of disposable procedure masks, one with ear loops and one with ties. Our results have shown that face coverings in general provide the wearer only limited protection against aerosol in the environment. All reusable cloth face coverings obtained a practical protection level of less than 2. The performance of the disposable procedure masks varied from 1.7 to 3.6. The mean practical protection level for the nine face coverings was 1.95 with a standard deviation of 0.89. Comparatively, a N95 respirator achieved a protection factor of 166. We have further shown that aerosol readily penetrates through most materials used in face coverings. Aerosol swatch penetration tests were completed on six different fabrics commonly available for reusable homemade face coverings, four different material systems comprised of multiple material types, eight different disposable procedure masks and the filtering material from three different N95 respirators. Maximum aerosol penetration through the six common fabrics varied from 39% to 91%; for systems comprised of multiple types of materials 4% to 23%; for materials used in disposable procedure masks 16% to 80%; and for filtering materials used in N95 respirators 1.0% to 1.9%. We also highlight that with the exception of some of the reusable cloth materials, penetration of particulates at 5 µm diameter, representing the minimum filtration efficiency that could be achieved against droplets, was insignificant; the six common fabrics showed penetration from 1% to 44%; the fabric systems comprised of multiple types of materials <0.9%; the materials used in disposable procedure masks <0.9% to 6%; and the filtering materials used in three different N95 respirators <0.9%. The observations from this study directly demonstrate that face coverings may be optimized by incorporating high filtration efficiency materials in their construction. Face coverings with an enhanced level of filtration would be of benefit in circumstances where SARS-CoV-2 may be present in the aerosol of infected individuals to reduce aerosol emission from respiratory activities penetrating through into the environment.
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