If I wear a regular dust mask, without edge sealing, and glasses, my glasses fog up when I exhale. Seems to me suggestive evidence that anything at all deflects normally invisible breath that would travel outward onto the surroundings.
> I have troubles understanding how droplets can distinguish the direction of passing through a face mask, i.e., how my face mask can help protect others from my droplets, but how it cannot protect me from other people's droplets.
Masks don't necessarily completely stop all the droplets, but the ones they let through are slowed down, which means they fall to earth at a much shorter distance than unimpeded ones.
Incoming droplets that doesn't matter all that much - they only need to go a few millimeters to reach your face. Outgoing droplets, though, need to go six feet before they hit someone if you're practicing proper distancing.
> A good mask will filter some aerosols when inhaling: but a cloth mask won't.
There's an issue with statements like that. "Cloth mask" is not well defined - are we talking literal layer of cloth, or the cloth-synthetic-cloth design recommended in some countries. Either way there's some gradient between 100% protection and nothing - cloth masks will still be better than nothing, but purpose-designed masks will outperform them.
This is not a trivial scenario to talk about, so the more specific we are the better.
> I can clearly see by the fabric's motion that air is substantially being pulled through the material rather than around it.
I suspect that the fabric’s motion doesn’t mean what you think. You’re just seeing motion from the pressure change. A non permeable plastic film will also move as the pressure changes. This doesn’t mean any air is going through it.
> If my mask is only filtering 80% of the air
If your mask is leaking around the edges, it is extremely unlikely that your mask is filtering anything close to 80% of the air.
Well, no: the fleece mask broke larger droplets into many smaller ones, resulting in an increase of 10% in the number of droplets compared to the no-mask baseline. Being smaller droplets, they would also hang in the air longer.
> which are covered by a cloth mask which vastly reduces how much makes it into the air
Where do you think the extra air goes?
It still makes it into the air, just in different directions. Some of the viral particles are probably caught in the mask, but I’m not sure how much of the total we can count on.
> A mask will not prevent you from getting the virus, since covering your own mouth from others coughing doesn't really help you. If you are sick though, it protects others.
What is the mechanism for this?
At first, I'd expect it to be the other way around--if I'm wearing a mask and cough the mask is going to bear the full force of my cough. All of the cough is hitting the mask, and the cough is moving at its fastest speed.
If I'm not wearing a mask and you are, my cough is going to be spread out some when it reaches you and slowed by air resistance, so your mask should only have to deal with a fraction of my cough and at lower velocity.
Thus, I'd expect your mask to have a much easier task than mine.
Is it that the drops of moisture (and phlegm and whatever else might be in a cough) are bigger at the point of emission and break apart as they disperse, and so they have a harder time going through a mask at my end than at your end?
If that is the case, I still have to wonder what happens on subsequent coughs. I cough, producing big drops that the mask stops. Still, the viruses in the drops have to go somewhere when the mask absorbs the droplets, right? Does my mask end up moistened with virus-contaminated water? Will my subsequent coughs as they blast air through the mask then be able to knock small virus laden droplets off my mask?
> Do you have a source for this? Because from what little I've been able to find, filtration efficiency for particulate in the aerosolized size range are practically negligible with cotton masks and even surgical masks. Which makes sense considering that aerosolized particles are around 2-3 orders of magnitude smaller than pore sizes in cotton and surgical masks, putting such masks in the realm of security theatre.
Pretend that you're blowing out a candle: make a small "stream" with your lips and blow as far as you can. When I do so, my stream can go at least as far out as my arm, which I think is ~1.5 feet or ~2feet or something (longer than that, because my breath is still "strong" at the full length of my arm in front of me).
Now put a cotton mask on and do the same thing. How far does the stream go? Inches, at best.
Note: oxygen, nitrogen, and CO2 are all much much smaller than mask fibers. But you'll find that these things behave as a __fluid__, and thus we have a fluid-dynamics question rather than one of physical objects hitting a screen door model.
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The distance your breath goes when talking is much reduced. Similarly, the "stream" where you draw in more air is also reduced to a smaller distance when you're masked up.
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Or heck, _do_ blow out a candle. This experiment is like, $2. One dollar for a mask, and a second dollar for a birthday candle. Just measure it yourself: the mask grossly affects the distance that oxygen / CO2 / Nitrogen leaves the body.
It might be false but it's not logically impossible, for example if you were breathing through a tiny but non-filtered hole the aerodynamics could make it very difficult to breathe while allowing aerosol droplets through in small quantities. In fact, there is a good lesson buried in the insanity, which is that having something over your face, even if it is making it harder to breathe, might not be perfect protection.
>Now put a cotton mask on and do the same thing. How far does the stream go? Inches, at best.
>The distance your breath goes when talking is much reduced. Similarly, the "stream" where you draw in more air is also reduced to a smaller distance when you're masked up.
So instead of breath being directed downward from the nose, the aerosolized particles are redirected upward, where they linger for longer? Moreover, the difference in velocity between speech and "blowing out a candle" makes this a bit of a contrived argument. The key here is that these aerosolized particles have very long settling times - so if they are not being filtered by mask media and instead being blown higher upwards with a mask as opposed to without, then as best masks are useless, at worst they are actually counterproductive.
Simplified explanation: when we breathe we exhale tiny "aerosols" and when we cough or sneeze we exhale far larger "droplets". The dust you see in a darkened room when a beam of sunlight enters is an aerosol. Aerosols are like marbles compared to droplets' boulders.
Covid-19 hitches a ride on both aerosols and droplets.
Cloth masks can stop droplets but cannot stop aerosols, which flow in/out a mask's gaps. To stop aerosols you'd have to wear something with an airtight seal, something akin to a swim mask. Cloth masks and even surgical masks do not do this.
Cloth masks don't work against aerosols; even surgical masks won't protect you (indeed they don't protect surgical patients or doctors either). Never trust a mask against Covid-19.
A final note from the interview with Michael Osterholm:
"When you wear your cloth mask in public, realize that it may only provide very limited protection. The most important thing that you can do to protect yourself and others is distance."
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And for God's sake put your goddam nose under the mask - if we can see your nostrils we're doomed, simply doomed!
1) I don't think its consensus that aerosols are a MAJOR source of spread.
2) Masks are much less effective for aerosols than for droplet transmission. I think the consensus is they at least somewhat reduce aerosol transmission. Only n95 w/o exhale vents that are properly worn are believed to be good at reducing aerosol transmission.
The fact that your glasses fog up shows that surgical masks certainly are very leaky.
> And regardless: there's not NEARLY enough air space behind your mask to store a full (~1L) exhalation. What you claim isn't even physically possible.
According to Wikipedia, normal level exhalation (tidal) volume is ~500 ml and has ~5 % CO_2. If only 10 % of exhalation (50 ml) is breathed back with 90 % of fresh air, then one is breathing equivalent of ~5000 ppm CO_2.
I am sure that improvized cloth facemasks have bigger air 'buffer' than this. Although measure it precisely would be hard.
> "All of the guidelines are about statistical safety..."
this bit is correct, but the rest is falling for false equivalency by throwing around the same "helps" with every case.
just being outside is overwhelmingly helpful, meaning it overwhelms every other factor by a large margin. relative to that, wearing a mask outside is of such negligible help to be effectively unhelpful. distance also overwhelms masks, both indoors and outdoors. distance outside helps only a little bit, but distance inside helps materially (because the positional and velocity vectors available to droplets and their virii and the dangers to them outside are exponentially greater). masks by themselves indoors are helpful only in limited situations (when in the direct exhaust of others for prolonged periods).
> "...do not drop your chances to zero"
for real-world situations, boolean evaluations like this are nearly always misleading, no matter in which direction. your chances of dying in a bathtub aren't zero either but we don't worry about it. relative magnitudes matter.
> cheap disposable surgical masks as a backup strategy. They won't prevent the air being polluted but they will still filter the tiny particulates in the air that cause so many problems.
This is such a common misconception. Surgical masks are meant to prevent the transmission of diseases by blocking blood droplets etc. from entering your mouth. They are NOT[1] intended nor capable of filtering particles from the air (except perhaps if they are visible to the eye). I see so many individuals in Japan, Thailand, Vietnam, and some in China wearing them, but the only role they can play is to (i) keep you warm, (ii) fit in with the locals, (iii) avoid spreading disease, (iv) keep bugs out of your mouth when riding with a moped down the street.
If you want to filter PM2.5 you need proper respirators[2] or activated charcoal masks that can do the job.
It is not logic, it is science. The mask prevents droplets from your lungs from dispersing throughout the air. It is not as good at sealing your respiratory system from droplets in the air.
How exactly does physics allow a disposable mask that completely lacks a barrier to entry on the sides filter the air?
> In hospitals, the word “airborne” is associated with a rigid set of protective methods, including the use of N95 respirators by workers and negative pressure rooms for patients. These are resource-intensive and legally required.
Medical professionals are not only required to use at least an n95 mask, they are required to keep it so tightly fitted to their face that unfiltered air cannot leak in through the sides. So tightly fitted that it digs into their skin.
> Exhausted doctors and nurses post images of their bruised faces after long shifts wearing protective gear
What is your basis for this statement?
If I wear a regular dust mask, without edge sealing, and glasses, my glasses fog up when I exhale. Seems to me suggestive evidence that anything at all deflects normally invisible breath that would travel outward onto the surroundings.
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