1/ Why universal mask wearing in buildings is important

First, the persons infected are unknown. So, if everyone wears a mask the infectors are also wearing masks. Second, everyone else (receptors) get an extra layer of protection for also wearing a mask.

2/ The % reduction in volume (or mass) of aerosol particles inhaled by a receptor is given by the following equation: Z = A + 0.01 x B x (100 – A)

3/ Where Z = combined % reduction in inhaled particle volume by receptors, A = % reduction in emissions by virtue of infector(s) wearing masks, B = % reduction in inhaled volume by virtue of receptors wearing masks.

4/ So (as an example), if the infector’s mask is 50% efficient at reducing emissions (A = 50%) and the receptor’s mask is 40% efficient at removing particles during the inhalation process (B = 40%), then Z = 50 + 0.01 x 40 (100 – 50) = 50 + 0.4 x 50 = 70%.

5/ That is a GR8 first layer in a layered risk reduction strategy. After that the additional layers (ventilation, filtration, etc.) are all focused on that remaining 30%. With right combo of increased ventilation and filtration it is possible to reduce dose by 90% or more!

6/ This analysis is for aerosol particles. If infectious viruses are uniformly distributed across particle sizes (it is not clear that this is the case), then the reduction in inhaled dose by particle volume described above also holds for virions.

7/ And there's extra benefit of infector(s) wearing masks as they will also emit less large (ballistic) droplets, thus reducing droplet impact w/ receptor’s face (including mouth, eyes, nostrils), hair/clothing, as well as other surfaces that can lead to transmission by fomites.

8/ And masks on receptors also protect the nose and mouth from large droplets.

9/ Moral of story = wear a mask in any indoor environment other than your own home & do not go in unless everyone else is wearing a mask. Layered DOSE reduction leads to layered RISK reduction. Step 1 (a very inexpensive step) is everyone wearing a mask. Just do it!