World Monitor Magazine, №1/2020 WM_March 2020_FOR WEB (12.03.) | Page 45

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FIGURE 3. Anatomy of airways and where droplets can end
up, depending to their size and what droplets are blocked by
what masks
(a) Droplets below a diameter of 10 um (micrometer), the up-
per size limit for the definition of ‘aerosol’ (particles so light
as to be able to float in the air). For brevity, let us call this
category “aerosols”. These small aerosols are carried by ven-
tilation or by winds and thus can travel across rooms. What
makes N95 facial masks different from the surgical masks
is that the former are designed (as per regulatory require-
ment) to stop aerosols: they have to filter out 95% of drop-
lets smaller than 0.3 um.
(b) Droplets larger than 10um (micrometer), reaching 100um
or more. Let us call these large particles “spray droplets”
here. (For a more detailed discussion, see Nicas and Jones,
2009). Of course, droplets can be even larger, up to a size vis-
ible to the naked eye in the spray generated by coughing or
sneezing (0.1 um diameter to above). Calculations by Xie et al
suggest that if exhaled, the >0.1 um droplets may evaporate
or fall to a surface within 2m, depending on size, air humidity
and temperature. But coughing or sneezing can shoot them
like projectiles out of the mouth with a “muzzle velocity” of
50 meters/second (for sneezing) or 10 m/s (for coughing),
and droplets can reach distances as far as 6m away. If so,
then the much mentioned “safe distance” of 6 feet in social
encounters may not suffice — except you wear a (simple)
mask –more on that later.
Here is the central biological implication of the distinction
between aerosols and spray droplets: For airborne particles
to be inspired and reach deep into the lung, through all the
air ducts down to the alveolar cells where gas-exchange
takes place, it has to be small (FIG. 3): only droplets below
10 micrometer diameter can reach the alveolae. By contrast,
the large spray droplets get stuck in the nose and throat (the
naso-pharyngeal space) and in the upper air ducts of the lung,
trachea and large bronchia. The droplets of a typical cough
expulsion have a size distribution such that approximately
half of the droplet are in the categories of aerosols, albeit
they collectively represent only less than 1/100,000 of the
expelled volume (Nicas et al 2005).
Itthus follows that the sophisticated N95 masks, designed
to filter out the smallest particles, help to prevent droplets
from carrying the virus down to the alveolae. But is this re-
ally relevant for flattening the curve? We shall see below. By
contrast, it is plausible that the large droplets that end up in
FIGURE 4.