I see 2 reasons why [cloth] masks protect others more than they protect the wearer

1 – Masks reduce the exhale velocity, which helps stop spread throughout the room.

2 – [Cloth] Masks trap large particles more effectively than small. However the droplets that we exhale can quickly reduce in size by evaporative action (increasing the per-ml concentration of virus within the droplet during the process). It’s easier to stop them on the way out while they’re large than on the way in when they’re small.

Let’s talk more about each of these:

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1 – Masks reduce the exhale velocity, which helps stop spread throughout the room.

When you breathe, talk, scream, cough or sneeze, the air comes out with progressively higher velocity. Higher velocity helps the droplets travel farther faster and mix into a larger area of the room. The mask greatly slows that velocity which prevents it from mixing in the air as much and from traveling as far as fast. Now look at the opposite direction (inhalation), there is no counterpart to talking, screaming, coughing, sneezing so no counterpart that would be affected by wearing a mask during inhalation and no corresponding benefit. But even if we exclude those talking/screaming/coughing/sneezing activities and just look at simple breathing inhale and exhale, there are STILL velocity differences which make the mask more beneficial during exhale. Put your finger in front of your mouth or nostril and breathe in and out… which one can you feel with your finger? Only the exhale which is guided into a narrow stream inside our nostrils on the way out (narrow means higher velocity for a given flow rate). In contrast for the inhale the air is coming more from all directions into our nostrils, not a single direction (there is nothing to guide it in a single direction) so the velocity is lower.

So exhalation velocity is the high velocity that contributes to mixing and spreading and it can be knocked down by a mask. Inhalation velocity (for the air in front of your face during inhale) is much lower. Inhalation velocity might also be reduced by a mask but it was already low to begin with so it wasn’t contributing much to the transport and mixing of the virus within the room. ===================== [cloth masks]

2 – Evaporation causes droplets to get smaller after exhalation, so they are more easily filtered as large droplets on the way out than small droplets on the way in

I’ll bet this one is new and counterintuitive to a lot of people. You don’t see raindrops getting smaller as they fall, so why should exhaled respiratory droplets be any different?…The answer lies is surface area to volume ratio (4 pir 2)/ (4/3 * pi * r 3

) ~ (1/r). Respiratory droplets are much smaller than raindrops, so the r is smaller, and the ratio is larger. Since the evaporation depends on surface area and the amount of stuff to evaporate depends on volume, the respiratory droplets with higher surface area-to volume ratio (compared to raindrops) will evaporatively shrink much quicker and in fact the evaporation can be very significant before they reach the ground. This evaporation can turn larger respiratory droplets into small (non-filterable) aerosol droplets that linger in the air longer and travel further. And the droplet that starts out exhaled as a large droplet and then evaporatively reduces to a small droplet has the virus “concentrated” during the process (like evaporating a salt solution increases the concentration of salt in the water) which makes it potentially more potent / dangerous than other small droplets that were originally exhaled as small droplets.

The phenomenon of large droplets evaporating to small droplets and aerosols is well described in the wiki page for Wells’ curve where it is noted If the air is not saturated with water vapor, all droplets are also subject to evaporation as they fall, which gradually decreases their mass and thus slows the rate at which they are falling. Sufficiently large droplets still reach the ground or another surface, where they continue to dry, leaving potentially infectious residues called fomites. However,the high surface area to volume ratios of small droplets cause them to evaporate so rapidly that they dry out before they reach the ground. The dry residues of such droplets (called ‘droplet nuclei’ or ‘aerosol particles’) then cease falling and drift with the surrounding air. Thus, the continuous distribution of droplet sizes rapidly produces just two dichotomous outcomes, fomites on surfaces and droplet nuclei floating in the air

You might ask if there is any proof of relevance of this phenomenon for Covid-19. I would offer two things that support it is relevant:

First another quote from the wiki link above: Wells’ insight was widely adopted because of its relevance for the spread of respiratory infections[5] .[4] The efficiency of transmission of specific viruses and bacteria depends both on the types of droplets and droplet nuclei they cause and on their ability to survive in droplets, droplet nuclei and fomites. Diseases such as measles, whose causative viruses remain highly infectious in droplet nuclei, can be spread without personal contact, across a room or through ventilation systems and are said to have airborne transmission. [quantitative] Although later studies demonstrated that the droplet size at which evaporation outpaces falling is smaller than that described by Wells, and the settling time is longer, [qualitative]

his work remains important for understanding the physics of respiratory droplets

Second, consider the effect of relative humidity on virus transmission. The virus transmits much more in low relative humidity than high relative humidity as widely reported including here https://aaqr.org/articles/aaqr-20-06-covid-0302.pdf If you go back to the wiki page on Wells’ curve the first entry under “complicating factors” is relative humidity and you’ll see that indeed it is consistent with more transmission at lower relative humidity. Here is my attempt to explain it: The lower the relative humidity, the faster the droplets evaporatively reduce, the larger the particle that can evaporatively reduce to small droplet (aerosol size) before

reaching the ground, the more concentrated (more virus per particle) are the small droplets that can end up in the air.

========= Some other unrelated implications for relative humidity effects on transmission:

We attribute the very high rates of virus transmission in the winter in the northern states to driving people indoors and that is certainly true. BUT the relative humidity may be an additional important factor because relative humidity tends to be lower during winter (both outdoors and indoors where heating lowers it further). If you have an indoor space that you want to protect, then you might consider a humidifier either in addition to an air purifier (for better protection) or in lieu of an air purifier (to save money, although the tradeoff in effectiveness would need to be evaluated).

I haven’t heard anyone else say that this phenomenon #2 is part of explanation for why masks are more effective as source control, but it makes good sense to me as I tried to explain. So take it for what it’s worth.

I limited the post to cloth masks where both factors 1 and 2 apply. For better masks like KN95, factor 1 certainly applies but factor 2 is not as important (KN95 do a better job of stopping droplets across a range of sizes).