21 June 2020

Snow Fences and Protective Masks in Times of COVID

I am not going to wade into the details of whether COVID exists or whether physical distancing and shutting down the economy in the hopes of saving lives is a good trade-off or even effective.

Rather, I want to talk about masks and what they do.

Two Views

I'm going to lead by saying the early guidance that masks were ineffective was a disastrous statement and policy. The subsequent reversal left millions of people confused.

Now we hear a lot of people saying that masks don't help the wearer, they help others. What is the truth?

There are two views that I have seen about masks. The main one is that they protect others--not he wearer. There's another group of people saying that the main purpose of masks is to protect the wearer and they're not effective at that.

Both of these views may be correct in the right context, but it also depends on the mask.

Masks as Snow Fences

If you grew up somewhere with snow, you have seen snow fences.

When I was a kid I thought it was preposterous that a fence made of slats of wood with big openings between the slats could possibly stop snow from drifting. Once I started studying physics and geology it made sense.

Snow fences don't stop snow like a solid wall would stop snow something, rather they rely on physics. When the wind blows, it carries particles. It could be dust, snow, leaves, sand, raindrops, etc. The stronger the wind, the larger the particles and the more of them it can carry. Think of a tornado full of dirt, cows, houses, and wicked witches on bicycles.

Once the moving air loses energy, the heaviest particles drop out and land, as it loses more energy, all but the smallest and least dense particles drop to the ground.

The snow fence then does not block the particles, rather, it robs energy from the wind, which then causes the particles to drop to the ground because there is no longer the energy to carry them.

When you cough, sneeze, or even breathe heavily, there is relatively high energy air coming out of your mouth. The energy in that carries air, moisture, and any other stuff like snot, mucus, or COVID 19.

Fortunately, snot drops out first, the other stuff though, depending on the density, temperature, can carry feet or yards. Air and moisture are not harmful, but of course, bacteria and virii can be.

Cheap cotton masks then work like a snow fence. When you breathe or cough or sneeze, through the cotton, the energy is reduced. There is no filtration at all for small virii, but the reduction of the energy means it may not go across the store and into other aisles.

What About Filtration

This graphic shows how effective the different masks are for filtration and protecting the wearer.

There are a few important things to note. The N95 mask in the graphic has a vent. This is an unfiltered vent that allows air to go out but not in. So while the N95 gives the wearer excellent protection, protection of others is much lower. It's not zero though. The vents are typically directed downwards, and in any case, there will be some reduction of energy as it passes through the mechanism of the vent.

Surgical masks are not quite as effective in terms of bacteria, but because there's no vent, it would be more effective at protecting others. There is strong filtration and a snow fence effect.

Once you get to the bottom from the main impact from a virus standpoint is as a snow fence. The wearer gets very little protection from viruses.

E4E Take:
All this can be summed up below.

Cotton masks at the bottom do very little to protect the wearer but have some positive (snow fence) impact in protecting others.

N95 masks with a vent do a good job of filtration but are less effective in protecting others. This is because the vent is unfiltered. N95 masks can feel stifling to some people.

Surgical masks are slightly less protective of the wearer than N95s but are also quite good in protecting others. There is both filtration and the snow fence effect going on.

There may be some downsides to wearing masks, but I have not seen empirical evidence that the net effect on public health would be negative.

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