Abstract
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.
Highlights
Values for the Fabric Protection Factor, Quality Factor, face velocity, inhalation pressure drop, and number penetration as a function of aerodynamic particle diameter, for each mask tested in all five mask groups in this study are provided in S1 Table and S2-S6 Figs in S1 Appendix
Our study has clearly shown that there is a variation in the level of particle penetration through materials used in the construction of masks
We found a difference of the means for the total inward leakage protection factor (TILPF) between the fabric 2-layer (1.42) and disposable procedure mask group (2.26), not the multi-layer group (1.77), but the improvement was small at best, and it is not possible to know whether such a difference was due to the filtration efficiency or the obvious difference in design between the mask groups
Summary
There seems little doubt, when taking into account macro air movements related to heating and ventilation, convection, people activity, and even more localised radiative warming due to equipment, the human body and sunshine through windows, a large fraction of respiratory generated particles will be entrained and suspended in air currents and may potentially travel significant distances over time [31,32,33,34]. This may certainly add to the airborne viral load in situations where influenza (e.g. COVID-19) symptomatic individuals are shedding virus
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