N95 respirators, disposable procedure masks and reusable cloth face coverings: total inward leakage and filtration efficiency of materials against aerosol
Authors: Duncan Scott , Bodurtha Paul , Naqvi Syed 
Published: November 2020

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.