The mechanics of relative protection offered by masks:
Airborne diseases caused by viruses transmitted through droplets are
crudely divided into two large categories based on droplet size:
1. Aerosols: Droplets<10 µm diameter reaching the
alveoli where the gaseous exchange occurs. The N95 masks are designed to
filter out 95% of droplets smaller than 0.3 µm.
2. Spray Droplets : Droplets>10 µm diameter (up to
0.1 mm or more) are the larger spray droplets that get trapped in the
posterior nasal cavity and the nasopharynx.
A study published in Human Cell Atlas (HCA) by Sungnak et al. on
nasal expression of ACE-2 protein suggests that SARS-Cov-2 virus infects
cells in the posterior nasal cavity allowing viral attachment and host
cell entry. Additionally, nasal epithelial cells, specifically
goblet/secretory cells and the ciliated cells, display the highest ACE-2
expression out of all the epithelial cells.[2] A simple physical
barrier could effectively block this route of transmission.
As per
Xie
et al. on exhalation,droplets>0.1 mm in size,depending on
the size,humidity and temperature might either evaporate or fall upon a
surface within a 2 metres range.However, coughing or
sneezing could propel these droplets as projectiles with a “muzzle
velocity” of 50 meters/second (for sneezing) and 10 m/s (for
coughing),traveling distances as far as 6m away.In such cases,the
much-recommended advice of maintaining a ‘safe social distance’ of 6
feet in social gatherings may not suffice.[3]The Partial
filtering offered by masks in such situations would be better than no
protection at all.