KN95 mask has FE above 90% for all particle sizes. Apart from this mask, all other masks were more or less permeable in the 250-500 nm range.

Northwestern University

8 face masks were tested using polydisperse indoor air. Their effectiveness was examined for aerosol of aerodynamic diameters of 0.006 μm to 10 μm. The commercially available masks used in this study were:
- Five were 3-layer mask (labeled M1, M2, M3, M4, and M5 in the study)
- One was a KN95 mask with one-way valve (labeled M6 in the study)
- Two were fabric masks (labeled F1- simple cloth mask, and F2- cloth mask with activated carbon)

Aerosol filtering efficiency of respiratory face masks used during the COVID-19 pandemic. 

Loupa G., Karali D., Rapsomanikis S. 21 July 2020. Aerosol filtering efficiency of respiratory face masks used during the COVID-19 pandemic. BioRxiv. https://doi.org/10.1101/2020.07.16.20155119

Loupa et al. (2020) Aerosol filtering efficiency of respiratory face masks used during the COVID-19 pandemic

F1- the simple cloth mask- had the lowest filtering efficiency (FE). F2- the cloth mask with activated carbon addition- had a much improved FE over F1. 

Results: Fabric masks (F1 and F2)

M1 and M2 (no brand name) masks had FEs in between that of F1 and F2 masks.However, they exhibited a larger FE than both F1 and F2 for particle sizes greater than 800 nm. 

Results: 3- layer masks (M1-M5)

Testing occurred in the University of Thrace in Greece. It was done in an indoor environment with natural air ventilation. Two instruments were utilized: PROMO 2000 (a light scattering particle counter) and SMPS- CPC (particle counter). Each mask was fitted over the inlet of the instrument and was tested for 30 mins with inlets open, and 30 mins with inlets closed (occasionally the mask was moved against the inlet to allow for homogeneous penetration).  This was repeated 6 times. Filtration efficiency was calculated as average particle size in background - average particle concentrations per size bin when the instrument sampled indoor air that passed through the mask) / (average particle size in background. Particle sizes tested by adjusting inlet size ranged from  size range of 0.006 μm to 10 μm.

Methods of Aerosol filtering efficiency of respiratory face masks used during the COVID-19 pandemic

The present findings are consistent with the reported results in other studies that have examined the FE in relation with particle size distribution, for example the study of Konda et al., (2020). 
The masks the public wears rarely fit properly, reducing their effectiveness. 

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