N95 respirators, disposable procedure masks and reusable cloth face coverings: total inward leakage and filtration efficiency of materials against aerosol

Abstract

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.

Efficacy of face coverings in reducing transmission of COVID-19: Calculations based on models of droplet capture

“…73,82 Practical guidance on reducing leakage would therefore be required to pursue this strategy. For example, Duncan et al 82 found that surgical masks sealed via tie straps offered better face seal than ear loops.…”

Modeling the filtration efficiency of a woven fabric: The role of multiple lengthscales

“…Both Konda et al 12,13 and Duncan et al 14 have measured the filtration efficiency of woven fabrics, for particles up to five micrometers. Both groups find a large variability in filtration efficiency from one material to another, with filtration efficiencies in the range less than 10% to almost 100%, for particles with diameters of a few micrometers.…”

“…Konda et al , 12,13 Duncan et al , 14 and Sankhyan et al 15 have all measured filtration efficiencies for a number of fabrics. They studied the filtration of particles in the size range we consider, which is 1 μ m. Zangmeister et al 16 studied the mechanism of filtration for smaller particles.…”

A critical review on the role of leakages in the facemask protection against SARS‐CoV‐2 infection with consideration of vaccination and virus variants

“…Both the fit factor and the protection factor refer to the total inward leakage (penetration through the filter and the leaks); however, the fit factor is a reserved term to quantify the performances of a mask during a standardized fit test. [54][55][56][57] of relying on volunteers instead of fitting masks on manikins are the qualitative illustration of the inter-and intrapersonal variability of the fitting, and the generation of a realistic exhalation flow which would be difficult to reproduce with a manikin. However, this technique does not provide information about the trajectories of the respiratory droplets.…”

“…Both the fit factor and the protection factor refer to the total inward leakage (penetration through the filter and the leaks); however, the fit factor is a reserved term to quantify the performances of a mask during a standardized fit test. 54 , 55 , 56 , 57 …”