Nobody is advocating for a face-shield as a replacement for an N-95 mask, but using them together, would reduce the odds of infection.

According to the MIT paper, a typical sneeze will travel over 20 feet.

Additionally, there is mention of the velocity of the particles may be able to penetrate the mask, so a face-shield could help with that, too.

Yes. Defense-in-depth. In China, most hospital red zones required 3 layers of PPE, whether that's two masks and a face shield or such. One layer alone is not very good, and a face shield alone is no good. A mask and a face shield are very good, especially for confined spaces like subways and busses where there are jerks coughing and sneezing all over without hygiene or manners.

What if I want to protect myself and my facemask from large droplets. I also want to protect myself from accidentally touching my face. Is a face shield okay then?

And if so, why not drop $300 million to get one for everyone in the US? They're cheap, wash easily, and that's less than 0.02% of the last stimulus.

If someone is facing me and a stream of airborne particles hits a plastic shield instead of streaming out to me, it seems pretty likely that I will be protected.

I'm certainly open to the idea that fluid mechanics are complicated and somehow air currents are going to do unhelpful things, but until I find someone with a paper on the topic I'd much rather be talking to someone wearing a face shield than not.

Plus obviously wearing a face shield will protect against self-inflicted infections from a contaminated finger.

> it seems pretty likely that I will be protected

And you'd be wrong. Sure, a face shield will protect you from liquid streaming out at you (partially). However, it isn't a liquid that you need to protect yourself from in this instance... it is aerosolized particles. And those travel in the air. So when you breathe in, that air can and will move around the face shield.

The sibling comment about smoke moving is exactly right. Smoke is a great example of how small particles can move and travel in the air. If you think that a face shield will protect you from something in the air, you'd be wrong.

Side note: this is how N95 masks are actually fit. (Well, not using smoke, but similar test). If you can smell the chemical used in the test, the fit is wrong.

If you really want to protect yourself from other people sneezing -- stay away from other people. That's why social distancing is still the primary, and best, recommendation.

Isn't the recommendations by many countries to wear even cloth masks (e.g. the recent reversal by the CDC) _because_ there's a good chance COVID-19 can spread via large (i.e. non-aerosolized) respiratory droplets? (Not that it doesn't also spread in other ways, but removing even one vector would be better in this case than not doing so.)

I'm not sure why there is a recommendation to wear cloth masks in other countries (not discounting it, I just haven't looked). However, I believe that the primary rationale for the CDC switching their recommendation that everyone should wear masks has to do with the great numbers of asymptomatic carriers. In this scenario, wearing a mask is not designed to protect the wearer from others. Wearing an ill-fitting mask is not protective to the individual. Instead, wearing a mask is designed to protect others from the wearer. Because people are carriers before they exhibit symptoms (if they ever have symptoms), you can't know if you are infected or not. If you are wearing a mask, you are limiting the potential spread of virus from you. This way, you are keeping any potential virus closer to you so that others are more protected. This goes hand in hand with social distancing. If you use a mask and don't distance yourself, you aren't helping at all. I'm looking for a CDC reference, but this was also the rationale presented in local media reports.

If you're really curious, Ars Technica has a really good writeup of all of the relevant studies.

https://arstechnica.com/science/2020/04/should-you-wear-a-fa...

Most of the research above assumed either surgical or N95-style masks. There was one study that looked at use of cloth (non-medical, non N95) masks, and this was the key finding (from Ars, not the paper):

Wearing cloth masks resulted in significantly higher rates of infection, the authors found. They also noted that in their test, the cloth masks were only 3 percent effective at blocking particles.

Regarding everyone wearing (surgical) masks to protect others:

In a study published April 3, 2020 in Nature Medicine, researchers found that surgical masks reduced the detection of respiratory viruses in aerosols generated by infected people breathing or coughing in a breath-collecting machine.

> Wearing cloth masks resulted in significantly higher rates of infection, the authors found. They also noted that in their test, the cloth masks were only 3 percent effective at blocking particles.

Hoo boy that's misleading by itself. The control group was "use what you normally use", and had the following numbers:

> In the control arm, 170/458 (37%) used medical masks, 38/458 (8%) used cloth masks, and 245/458 (53%) used a combination of both medical and cloth masks during the study period.

Given those numbers, it's basically impossible to use this data to tell us anything about the efficacy of cloth mask vs. no mask. The control group did wear their masks less often, but it's still very unhelpful data.

>https://arstechnica.com/science/2020/04/should-you-wear-a-fa...

> This may lead some members of the public to be lax about ... washing their hands frequently and thoroughly. Moreover, donning [a] mask may lead some people to touch their faces more, some argued.

> simply touching their mask could contaminate their hands if there are viral particles caught on the outside ... a mask wearer could then transfer virus particles from their mask to their face

"May". "May". "Some argued". "Could". "If". "Could".

> negating any benefit of having the mask.

So even with all the above steps that have to occur, it's still no worse than getting a face-full of Covid directly.

Almost every stupid, pointless, dangerous argument against masks boils down to:

People may not wear a mask properly. People may not wash their hands properly.

The first one is used as an argument against masks, so why is the second not only not used as an argument against hand washing, but also used in arguments against masks?

People don't always practice social distancing. So why don't people argue against that? Oh, because that too is reserved for arguments against masks.

It's fucking ridiculous. Why do some people continue to spend so much time and effort trying to discourage use of potentially life-saving items?

> I'm not sure why there is a recommendation to wear cloth masks in other countries

You're spreading actively dangerous advice, that has already been proven wrong. DO look up why there is that recommendation.

A cloth mask can capture some sizes of droplet, but a flat wall that the all the air goes around is not really going to do anything.

You say that with great confidence and a lack of sources; and I'm guessing we are equally ignorant on the actual physics of how small droplets move.

The details aren't clear on how the virus is spreading. The virus might have multiple modes of transmission. Closing 2 of them reliably a great deal. There are important things to do with "catching COVID" that aren't binary; starting doses likely matter as well.

I'd rather not talk to you in person if you aren't going to cover your face, tyvm :)

> starting doses likely matter as well.

That is something that I've been wondering about too. I work in biotech but not as a scientist. These things are tricky to nail down as they seem to be very individual specific. Theoretically, in an exponential growth model, it wouldn't matter if you were inoculated with 20000 or 200000 virons, after the incubation period post-infection, you're going to be generating a huge amount, millions and millions, of virus particles in either case. Certainly, if you keep getting exposed, you're immune system is going to get overwhelmed quickly, but for a single event, I don't think it matters.

It could give your immune system an extra day or two to produce antibodies before it get overwhelmed.

Viral dose does indeed matter. https://www.nytimes.com/2020/04/01/opinion/coronavirus-viral...

It’s analogous to containing a new virus outbreak before it becomes an exponentially growing pandemic.

Thanks for taking the time to post the link, but it wasn't convincing as it lacked data and other relevant scientific information that I was looking for. Personally, I do not consider the NYT to a reliable source of information, but I know many do so maybe the link will help someone else.

> The virus might have multiple modes of transmission. Closing 2 of them reliably a great deal.

Sounds reasonable, but how is a face shield a reliable way to close a mode of transmission if people aren’t spitting liquid in your face?

It increases the effective time large droplets need to spend in the air to get to me, and decreases the airflow around me. It's about the same reason plexiglass screens help, and keeping a 6-foot distance helps.

Plus, it stops me from touching my face.

No, it doesn't stop everything, but it's a buck.

> Plus, it stops me from touching my face.

You would be touching your face if you had correctly fitting PPE, because it's uncomfortable. https://twitter.com/gabbyjackson95x/status/12479391789623255...

It’s okay to admit you’re wrong. It’s okay to not understand everything. It’s okay to defer to experts.

So many people on HN and in tech have the same attitude that if they just spend 5 minutes thinking about something then they can intuit an answer and that they must be right. This is a dangerous way of thinking. If anyone took what you’ve been saying as correct they’d be endangering themselves and depriving medical professionals of tools they need.

But I'm getting my opinions from the experts? See [0]. They recommend covering your face. I agree with them that cloth is more effective than a plastic shield but I'd expect a plastic shield to still be better than nothing. You've got to try and stop droplets from leaving your mouth and reaching other people.

A face shield would be better than failing to create a makeshift covering from a shirt for most people and it'd be resistant to decomposing so it could be stored long term in a home. It'd be a reasonable thing to stockpile.

[0] https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-si...

I don’t think it’s reasonable to stockpile given the huge demand for them by medical professionals.

I also can’t imagine a situation where you can order and wait for a shield but don’t have the time or resources to get a surgical mask or cloth.

And obviously the shield alone isn’t going to protect you from particles. And it might not help slow the spread if you’re infected yourself.

But again I’m speculating. If the health professionals say they need them and the public doesn’t, I’ll defer to them. I’m not an expert in their field and I don’t want to pretend to be or cause anyone to think I am.

This is just common sense. Aerosolized particles are not impeded by a shield. A shield is intended to obstruct ballistic projectiles. If those projectiles are aerosolized and are in the air that you breathe, then the shield will offer no protection at all. Even an N95 mask still can let in up to 5% of these particles. What efficacy is a piece of plastic in front of your face going to do against that?

> If those projectiles are aerosolized and are in the air that you breathe

It seems like you're imagining that the mucus and saliva particles would behave like a gas, but it's a suspension of liquid particles in the air, which behaves slightly differently. Think of breathing onto glass -- a lot of the aerosol I expel adheres to the glass. Some smaller droplets may move around it, but it's still preferable to receive a smaller dose of the virus.

>but it's still preferable to receive a smaller dose of the virus.

Could you explain why you believe that? I haven't found anything definitive on this topic.

[I'm no microbiology expert, so obviously take this with a grain of salt :-).]

I'm not sure whether the initial dose affects the severity of the disease, but it seems clear that it at least affects the probability of infection. The individual action hypothesis [1] posits that each individual pathogen has an independent chance of infecting a host, i.e.

    P(no infection | n pathogens) = P(no infection | single pathogen)^n

So if one is exposed to a single unit of the virus, infection is theoretically possible but unlikely. At higher doses infection becomes overwhelmingly likely.

I don't know how accurate the IAH model is, but there seems to be a good amount of evidence that infection rates are at least positively correlated with dose, e.g. [2].

[1] https://royalsocietypublishing.org/doi/full/10.1098/rspb.200...

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2869824/pdf/120...

Well there's two things. There is the 'minimum viable dose' which you can estimate for various diseases, that will most likely result in an infection. And then there is the dose size (above that minimum).

With an exponential curve, whether you start off with 10,000 or 100,000, given that it takes about 2 weeks for our adaptive immunity to kick in, the virus is going to be in the hundreds of millions in either case.

[I work in vaccines, but not as a scientist, so I'm just trying to absorb as much as possible here..]

Yeah that's fair -- there might be some high-exposure situations (like being coughed on) where infection is very likely with or without a face shield.

I suspect that a lot of airbone infections are from inhaling small doses here and there though, considering how long mucus droplets can stay suspended [1] --

> Particles of diameters 1-3 μm remained suspended almost indefinitely, 10 μm took 17 min, 20 μm took 4 min, and 100 μm took 10 s to fall to the floor.

[1] https://www.mintie.com/assets/img/education/Aerosol%20transm...

I'm really not a medical expert at all. But, just thinking about it one could at least speculate or argue: A smaller payload means the virus takes a longer time to "become very active" in the body and delaying this "ramp up" period may give your body's immune response a longer lead time from when it first "sees" the virus to when it becomes dangerous to you.

Well with an exponential curve, even if the amount of the initial dose changes 10 fold, from 20,000 to 200,000 virons you will get to hundreds of millions in pretty much the same time. I believe adaptive immunity takes about a week or two to kick in, and in that time the virus is going to keep multiplying like crazy, regardless of the dose.

Also, suppose you were living with a partner and both of you got infected. Both of you will continually shed and "re-infect" the other, so would we expect that the infection will necessarily be severe?

There is a good New Yorker article on the matter this week:

https://www.newyorker.com/magazine/2020/04/06/how-does-the-c...

Stop spreading FUD.

> this is how N95 masks are actually fit. (Well, not using smoke, but similar test). If you can smell the chemical used in the test, the fit is wrong.

And if the smell is less strong than it would be if you weren’t wearing the mask the viral load you’ve been exposed to is smaller. You are less likely to be infected and if you are infected your immune system has more time to ramp up so you’re more likely to fight it off.

Go light a cigarette or incense and and see if a face shield prevents you from inhaling any smoke.

Viruses don't diffuse through the air like smoke. Viruses are contained within the aerosolised droplets of your bodily fluids that are ejected through sneezing, coughing, blowing the nose. The survival of the virus outside of the body exposed to the elements is limited. Viruses have limited lifespans and can only reproduce within the medium of the bodily fluid. Every virus has different characteristics, and may have a longer or shorter lifespan. These characteristics are more or less universal for viruses though. It's not impossible for them to travel longer distances through the air. For the most part they are limited to the initial propulsion from the body. A better analogy would be spraying an aerosol can and watching how the droplets settle.

Light a cigarette and see if you can smell it from six feet away.

I think a better comparison would be to try inhaling wet steam (i.e. a suspension of liquid H2O) from behind a face shield. You'll probably inhale some, but a lot of it will adhere to the shield.

Yeah, that is pretty much what I'm imagining. If someone tried to smoke behind a facemask it seems pretty likely that most of the smoke will be caught behind the mask then drift out behind them like a comet trail, or move up/down. Only small wisps would end up traveling forward to where I'd be standing for conversation. The doses of virus would be diluted very rapidly which is helpful.

Ideal scenario would be most of the smoke gets pushed down in a knee-wards direction, which is well within the realm of the imaginable.

>If someone tried to smoke behind a facemask it seems pretty likely that most of the smoke will be caught behind the mask then drift out behind them like a comet trail, or move up/down. Only small wisps would end up traveling forward to where I'd be standing for conversation.

You ARE the person in the faceshield (I assume you meant that instead of facemask), you don't control what other people do so assume they uncovered. The smoke is in the air, it can reach you. Will it get around that shield or not?

How could me wearing a face shield help protect me from catching the virus? It'd be mostly useless; although obviously as mentioned it'd stop me touching my face as much as normal.

This is all about containing the spread.

Although I suppose in hindsight that explains why people were downvoting me.

> it seems pretty likely that most of the smoke will be caught

If your plan is to only block most of the virus then perhaps you should rethink it.

It's like stating that a hole in the hull is not a problem because the rest of the ship keeps most of the water out.

Actually initial viral load seems to be a factor in how severe the disease gets. So keeping any percentage more out of you that you can helps..

This is potentially misleading. Viral load is a significant factor in the seriousness of most infectious diseases. This is why you'll likely be ok if you pass a coughing TB patient in the street and inhale a few bacteria, but are much more prone to contract TB if you spend an entire day cooped up in a small room with the same coughing, infected individual.

This is thought to be why an unexpectedly high proportion of young, healthy doctors and nurses are getting seriously ill from Covid-19. Statically, you'd expect almost all of them to get mild disease.

Strawman and wrong.