How do three-layer surgical masks prevent SARS-CoV-2 aerosol transmission?

The three-layer surgical mask was recognized by the World Health Organization as an effective-protection tool for reducing SARS-CoV-2 transmission during the COVID-19 pandemic; however, the contribution of each layer of this mask to the particle size-dependent filtration performance resistance remains unclear. Here, both experimental work and numerical simulation were conducted to study the role of each mask layer in particle size-dependent filtration and respiratory resistance. By using scanning electron microscopy images of a commercial three-layer mask, composed of two spun-bond and one melt-blown nonwoven polypropylene fabric layers, four representative models were constructed, in which the computational fluid dynamics of multiphase flow were performed. The pressure drop of all models under different flow conditions was measured next. Numerical simulation was then verified by comparing the experimental results in the present study and other theoretical works. The filtration efficiency of the spun-bond polypropylene nonwoven fabric layer was much lower than that of the melt-blown nonwoven polypropylene fabric layer for the particle diameter in the range of 0.1-2.0 μm. Both the spun-bond and melt-blown nonwoven polypropylene fabric layers demonstrated extremely low filtration efficiency for particles was<0.3 μm in diameter, with the maximum filtration efficiency being only 30%. The present results may facilitate rational design of mask products in terms of layer number and structural design.