Designing better masks

Optimizing design of masks to prevent spread of COVID-19:

(Originally a series of tweets that came out in the wrong order)

1.     COVID-19 is transmitted mainly by droplets and particles in the air we breathe, not by contact with contaminated surfaces.

2.     Surgical and cloth masks only poorly protect an uninfected wearer from becoming infected. 

3.     But these masks CAN reduce virus release by an infectious person, because exhalation produces large wet droplets that are relatively easy to trap on their way out but that rapidly evaporate to smaller dry particles that are hard to trap on their way in (see Wells Curve). 

4.     So the general public should wear masks not to protect themselves from infection but to protect other members of the community, in case the wearer is unknowingly infected. But design of surgical and cloth face masks has not been optimized for this function. 

5.     What properties should such a mask have?

a.     The fabric should block passage of most respiratory droplets.

b.     Most exhaled air should pass through the mask, not around it, even after a cough or sneeze.

c.      Any exhaled air that escapes should escape downward, not upward.

d.     Air and water molecules should pass easily through the mask fabric.

e.     For ease of breathing, exhaled and inhaled air should be filtered over a large area of mask. The mask should not be tightly pressed to the nostrils and mouth.

f.      To maximize air exchange, the mask should not normally enclose a large volume of air.

g.     The space inside the mask should expand in the event of a cough or sneeze, to trap the large volume of air and allow it to be gradually released through the mask (not around it). 

6.     These goals may best be met by long lightweight scarf-type masks that fit snugly around the nose, cheeks and ears, and settle loosely on the shoulders.