> Cloth masks also outperformed both KN95 and surgical masks. Milton theorizes that cloth variants were the second-most effective masks tested because of greater coverage that wraps around the face and gives a better seal than either KN95 or surgical masks. Flow resistance is also lower, allowing breath to pass through rather than around the filter.
I'm always baffled by the people who went around arguing masks did nothing. They don't seem to have any problem accepting that a sneeze guard at a salad bar does something, or that a surgeon should put something on their face to prevent them from coughing into your open body cavity.
We can argue all day about specific masks under specific settings, or when it is worth it to bother, but "putting something in front of your face prevents you from breathing out particles" is in the category of "things that are pretty f&%*ing obvious".
The argument against mask efficacy by members of the general population was never done in good faith.
It was about less-educated people who were in arrested development and never moved past teenager rebellion. Their rebellion was egged on by malicious state actors through social media in order to sow large-scale chaos by unmitigated viral spread.
I think it is pretty widely accepted at this point that foreign actors regularly jump into online topics to fan flames and domestic distractions.
Russia pays for massive botfarms doing... something.
China funds massive social media investments into western markets for... something.
The topics don't even have to be relevant to anything geopolitical. As an example, the East Palestine train derailment was a weeks old news story that the Chinese media pushed into social media as a response to their balloon being shot down: https://www.bbc.com/news/world-asia-china-64633990
> China is trying to rewrite the narrative, he said, by suggesting that U.S. army officers brought the virus to Wuhan during military games in December — but Kremlin-controlled media outlets appear to be trying to prolong the pandemic outside of Russia.
> "We saw articles ... claiming that handwashing has no impact on stemming the spread of the virus, that smokers do not get coronavirus, that ginger can be used to prevent contracting the disease," said Nehring. "Tons of false information that's never been corrected, never been retracted and is still online today, being put out by the Russians."
Kneecapping a geopolitical enemy by spreading disinformation would hardly be a first.
I think the important thing is - the strongest benefit of masks are always for other people. But I think certain people cannot accept a selfless reason to do something.
> I think the important thing is - the strongest benefit of masks are always for other people
Not getting COVID in the first place seems like a huge benefit, and if you find yourself surrounded by people who no longer mask (most countries in 2024), then wearing an N95 respirator for your own protection is more personally beneficial compared to, say, being up to date on vaccination (since COVID vaccines don't prevent transmission, and repeated infections increase chances of acquiring long COVID).
I've consistently worn an N95 in all public spaces and have not contracted COVID (to my knowledge). Meanwhile, most of the people I know (some vaccinated, some not) are either often sick due to weakened immune systems and/or have gained one or more severe health issues since the pandemic began.
> It was never about science, it was about convenience.
There was an inverse problem coming from the other side though. There was also a large and vocal group who argued that the masks did nothing as a way to create unnecessary anxiety and force unnecessarily inconvenient lockdown restrictions - like banning people with "inferior" masks.
I think it was mostly motivated reasoning by people who didn’t want to wear masks. Also a “gotcha” because of the early advice/request that people stop buying masks (because they were needed for medical professionals and also because it wasn’t known the extent to which they would help).
Things seem "obvious" until the best available evidence doesn't support them.
As a physician, I had a hard time understanding the ferocity with which some of my colleagues defended the "science" of masking when the Cochrane review(s) -- largely held to be the pinnacle of evidence-based-medicine -- were so underwhelming. I never had a problem with people arguing for them based on face validity (hehe), personal preference, that they "might" help, were "better than nothing," or any number of softer reasons.
Don't get me wrong -- as a physician I wore an N95 every shift, because I believe that the odds that they might help were much greater than the odds that they might harm. (And because my hospital's N95s are much more comfortable for me than our surgical masks, and other trivial reasons.)
But I had a hard time finding other physicians who were both steadfastly convinced of the incredible benefits of masking and also had taken any time to try to find or read the available literature on the topic.
> Medical or surgical masks
>
> Seven studies took place in the community, and two studies in healthcare workers. Compared with wearing no mask, wearing a mask may make little to no difference in how many people caught a flu-like illness (9 studies; 3507 people); and probably makes no difference in how many people have flu confirmed by a laboratory test (6 studies; 3005 people). Unwanted effects were rarely reported, but included discomfort.
>
> N95/P2 respirators
>
> Four studies were in healthcare workers, and one small study was in the community. Compared with wearing medical or surgical masks, wearing N95/P2 respirators probably makes little to no difference in how many people have confirmed flu (5 studies; 8407 people); and may make little to no difference in how many people catch a flu-like illness (5 studies; 8407 people) or respiratory illness (3 studies; 7799 people). Unwanted effects were not well reported; discomfort was mentioned.
The Cochrane review is underwhelming because it is mediocre[0]. And its main author is far from dispassionate:
"The study's lead author, Tom Jefferson of the University of Oxford, promoted the misleading interpretation. When asked about different kinds of masks, including N95s, he declared, “Makes no difference—none of it.” In another interview, he called mask mandates scientifically baseless."
When the main author is misleading about the conclusions of his own report, you have to wonder how good his work was.
But if you want just arguments over data rather than doubting Jefferson on a personal basis, this paper[1] makes a compelling argument that the RCTs in the Cochrane review itself are good enough to make the argument masking works. GAVI has also a good answer to Cochrane[2].
> The Cochrane review is underwhelming because it is mediocre
I would consider the article you're referencing less than mediocre. "The Cochrane review didn't show what we expected it would show because their standards are too high." They are not conflating "absence of evidence" -- readers are.
I think it's odd how often people reference the Bangladesh study as evidence supporting masks, apparently without having read it.
Ultimately, I think masks probably do have a statistically significant benefit. I also think that if the rigorous studies (yes, I mean mostly RCTs) to date have been underpowered to show this benefit, that the effect size is likely to be small enough to make the costs of bickering potentially nullify those benefits.
Can we all just acknowledge how weird the debate has gotten though?
I mean clearly some types of masks are better than others, that's why there's standards for masks in the first place. This shouldn't be controversial. Worse by making it controversial the whole notion of doubting any kind of mask no matter how ill designed suddenly became a faux-pass. Severely obstructing any debate on the best way to make masks.
Heck as the article points out surgical masks (not designed to prevent infection from respiratory disease) only block around 70%. Chances are some people now want to shout how 70% is still a lot but 70% fewer particles is a whole lot different from a 70% lower chance of infection.
We could all have cooperated to figure out the best way to make masks, but somehow the debate was so heavily leaning towards whether masks were at all effective (which as you pointed out is really fucking obvious), that any notion of debating which masks were most effective and in which contexts was interpreted as a way to delegitimize masks as whole.
Edit: And really the lazy attempts to cast doubt on the effectiveness of masks should have been recognized as disingenuous. The idea that no masks work is easily dismissed because we've been testing and designing masks for all kinds of purposes for ages, so at best it is an argument for a higher class of mask, never no mask at all.
I would stake my fortune that the difference between 70% effectiveness and 99% effectiveness in a monitored study is going to be reduced into a rounding error in the real world. There are so many factors to control for when it comes to how they get used, how good they fit, how much they trap moisture over a long day, how much their performance degrades, etc.
For the purpose of preventing most incidental exposures, I would think that at the end of the day anything that prevents direct airflow is going to perform practically the same.
In my case, I was exclusively using N95s when I caught Covid back in the day, and it was probably for a myriad of ways that I was using the mask wrong or making other bad choices. The mask itself being 99% effective probably didn't significantly change my odds when I had to wear it 14 hours a day.
Well there's also the issue of range, those 30% of particles might well have far less velocity. Which might be all that you need in a well ventilated area, or might be a meaningless difference in a busy room.
Frankly I don't know, and I don't mind admitting I don't know. What worries me greatly though is that at some point saying you didn't know was misconstrued as an attack on ideology.
I mean if I had to guess I'd go for something blocking 99% of the viruses at least 70% of the time over something blocking 70% of the particles at least 99% of the time. Of course it depends, but one of those works to some extent at least 70% of the time, while for the other it is unclear.
But really it depends. If we're talking about umbrellas those two scenarios result in you getting roughly equally wet, yet if we're talking about bullet proof shields I'd take my chances with the shield stopping most bullets at least some of the time over the one stopping only 2 out of 3 bullets.
Cochrane's analysis is that they can't say if masks are effective or not. It's not evidence of harm or helpfulness. A similar way to think about this is if I show you a coin and ask you if it's weighted? You don't have enough information to make a determination (perhaps if I flip it a bunch of times you can claim one way but that's not part of the question/data).
I didn't run any statistical analysis on their tables but given that their "Viral respiratory illness ‐ influenza/COVID‐like illness" for No Masks is 160/1000 while "Viral respiratory illness ‐ influenza‐like illness" for Masks is 50/1000 I'm guessing masks do have an effect but the amount of samples in their data is not enough to demonstrate it.
Where the oranges come from is the UMD paper is studying how many viruses appear in a plague doctor's mask [1] which is obviously different than how many individuals get sick (apples).
Sibling commenters also need to read the section on N95s in this same systematic review:
> The use of a N95/P2 respirators compared to medical/surgical masks probably makes little or no difference for the objective and more precise outcome of laboratory‐confirmed influenza infection (RR 1.10, 95% CI 0.90 to 1.34; 5 trials, 8407 participants; moderate‐certainty evidence).
More surprising is that cloth masks were better than KN95s! That's pretty cool. Sample size was quite small though (only 8 cloth mask participants) and the study in general had a pretty small sample size (surely it couldn't have been difficult to find covid-positive individuals?).
The title is misleading. The study found duckbill N95 masks "blocked 99% of large virus particles and 98% of small ones". This is important because the issue with masks was always to do with fit and longterm ware habits, not that N95 material was ineffective at blocking viral particles.
Read further down in the study. It discusses exactly what i'm referring to in that air was pushed out the sides with KN95 masks and one of the reasons cloth masks performed better then KN95s was a better fit.
And it showed that KN95s are NOT perfect at blocking COVID. This is why the general public now distrust scientists, because the media presents it to them inaccurately and tells them results the study never found. Science is about precision. The study did not find all N95s where nearly perfect at blocking covid, that's flat out inaccurate.
Also let's be really precise here. I'm not conflating anything and i think you don't realize you are.
1) KN95 and N95 use similar filter materials they both have to filter 95% of particles down to 3 microns, the difference is to do with certification N95 is a US certification and KN95 is a Chinese certification. So the material is essentially the same between them. The fits tend to be different.
2) What i'm getting at is the title said all N95s, that's not what the study found, it found that a specific fit style "duckbill" was "Nearly Perfect at Blocking COVID" the non duckbills did not.
So to be accurate what the study did NOT find which the title suggests is that all US certified masks that filter 95% of particles down to 3 microns were nearly perfect at blocking COVID.
Generally speaking, N95s fit better than KN95s, because earloops aren't allowed in the N95 standard, and headbands provide more tension so there's a better face seal. This study only evaluated one brand of N95, but there are many other people that have tested various respirators with generally the same conclusions.
No it wasn't. It was "I don't want to wear one"[1]. And the frame of the argument was fit to the desire. It was true then and remains true now that the single best protection for You Personally (absent isolation) is to put on and wear a mask. That remains true whether or not the "fit" is ideal or on the state of "longterm wear habits" of the rest of society.
[1] Though eventually the politically reframed corrolary of "You can't make me wear one" got more play.
And an important clarification: even if a virus particle is too small to make it through the mask - it is always riding some sort of host to transmit - water partical or something that help it transmit through the air.
Everything was moot though if you ripped off the mask and wiped your hands all over your face when you were done.
> Everything was moot though if you ripped off the mask and wiped your hands all over your face when you were done.
No, probably not. There's very little evidence of transmission via surface contact; you have to breathe a significant amount into the nose/lungs, and the surgical and N95 masks are additionally statically charged to keep viral particles stuck to them; this is how they block viruses much smaller than their own pores. (https://www.wired.com/story/the-physics-of-the-n95-face-mask...)
I think the thing with the N95 masks is they could stay moist all day from your breath's condensation. Covid was very dependent on moisture to spread, and much of the evidence about surface contact implied that the surface was dry or dried out between contacts.
Though to your point it was probably a lower risk than what might have been implied.
Honest question: Why do we need it to block _viruses_. Surely the virus isn't flying around with wings. Doesn't it travel on water droplets? Doesn't the mask do most of its good work by keeping water droplets in (or out)?
Generally the design and testing of the masks tries to take that into account.
I only looked into it informally during the early pandemic, so recollection is a bit hazy. But IIRC different amounts of virus travel on different sized droplets. Different sized droplets also can have different behaviors. Big enough and it doesn’t get through the mask. I think there’s also a “small enough” point where they don’t penetrate very well because they basically bounce around with brownian-like motion, don’t really create much flow. Also, IIRC larger droplets carry more viral load, but I’m pretty sure the smaller ones stay suspended in the air longer.
This varies from virus to virus; some generate droplets, some generate smaller particles ("aerosols" / airbone transmission). Measles is so infectious because it travels in smaller particles that linger in the air longer.
However, it doesn't matter how good the N95 material is if the virus travels on a water droplet _around_ the mask. This is what all the comments about fit means.
IIRC Covid is airborne so it doesn't need to travel on droplets.
But feel free to double-check that one. Everything surrounding Covid is so tied to politics and identity, it's easy to get things mixed up between actual studies that have actually demonstrated a thing and people just spouting off whatever just came out of their assholes.
In other words, I believe I read an article on how we know now that Covid doesn't need droplets to transmit, but it honestly could have just been someone saying that there was an article claiming that and I've just conflated being told there's an article with actually reading the thing.
>Rather they are designed to prevent you making others sick.
Except N95 masks weren't designed for that at all. N95s and better masks have been worn for decades to protect the wearer from all kinds of airborne problems, from dust to virus particles. They have proven quite effective at that task. But maybe you think all those workers in the Ebola ward are just wearing masks as a fashion statement?
Masks do prevent the wearer from getting sick, far more than wearing no mask at all. Is it 100% protection? No, but nobody should be complaining about that. I'll take 95% protection or even 50% protection over no protection at all.
Right. The classic "3M 8210 Plus N95 Performance Sanding And Fiberglass Disposable Respirator (20-Pack)" sells for about $1 in boxes of 20 at Home Depot, Lowes, etc. That's a good mask.
Wearing one properly isn't rocket science, but is non-obvious. There's an instruction sheet in the box.
You put it on, and then, using your palms, squeeze the part over your nose, which bends the mask and the metal stiffener. Check that you got it right by exhaling, then take a deep breath and feel the edges of the mask pushing against your face. If you don't feel the mask pushing, you did it wrong. Training video from OSHA: [1]
There's a serial number on the box. If you type it into 3M's web site, they will tell you if it's valid and if anyone ever queried that number before. This protects against counterfeit masks. Once you get an OK from the site, initial the box, so you don't ask the site again.
There's also a test kit you can buy, with a little bottle of a strong-smelling substance. If you can smell it with the mask on, you did it wrong.
The way these things work is subtle. There's an inner layer that's an electret, with a permanent electric field. Particles smaller than the mask mesh are caught and held by the electric charge. That's what makes these work against really small particles. They're actually more effective as particles get smaller. The "95%" is for the particle size for which they are least effective.
I've been wearing those since the beginning of COVID and never got COVID. I had a supply of those, because I used to use a water jet cutter at TechShop. This is all ordinary personal protective equipment, familiar to anyone who
works around moderately dangerous industrial processes.
I've been wearing the 3M 8210 N95 masks when indoors with other people and I have avoided getting sick at all for 4.5 years. Before covid lockdowns, I would get sick at least 5 times a year and the only thing that changed was wearing the mask when in higher risk situations.
Sure. Masks help the wearer. But the main purpose is to stop sick people spreading. And it’s more effective at preventing spread than preventing contraction.
> But the main purpose is to stop sick people spreading.
No, the main purpose of an N95 mask is definitely to protect the wearer first and foremost. It's why they were created and sold in the first place. These masks were intended to protect people from all manner of workplace environmental hazards, and they've been used for that for decades.
>And it’s more effective at preventing spread than preventing contraction.
Citation needed.
Also, many people purposely choose not to wear a mask during a pandemic, so a mask not worn is not protecting anyone. Protecting myself from people without masks is 100% the purpose of wearing an N95 mask. We aren't talking about cloth masks in this thread, this study was about N95 masks - the two are different in that a cloth mask protects others more than it protects the wearer, but an N95 mask will protect the wearer more than others (who may not be wearing any mask).
I haven’t even mentioned N95 masks. I’ve stated masks as a general thing. Surgical masks that are cheap and readily available are to prevent the spread of anything.
N95 masks to prevent you getting covid don’t help anymore than surgical when we know it was be contracted through skin and eyes etc.
If you’re wearing it in a workshop or some dusty place then obviously they are far superior to surgical masks as they are designed for the wearer.
And obviously wearing an N95 when you’re sick is more helpful than a surgical mask as they are much better fitting and don’t let as much through if you sneeze of cough.
Citation needed (you still didn't provide the previous citation I requested)
Yes, it can be contracted through eyes, so eye protection is another mitigation that helps. When I had to travel in 2021, we wore eye protection on the plane, as well as P100 masks. We were well protected and did not get sick.
>If you’re wearing it in a workshop or some dusty place then obviously they are far superior to surgical masks as they are designed for the wearer.
Guess what they wear in Ebola wards. Guess why they wear them.
Not true of masks that comform to NIOSH's N95 specification such as the ones routinely worn in hospitals to provide care to COVID patients and which are readily available for $2 per mask without a prescription.
KF94s, (western tested)KN95s, and FFP2s were much better options for the general population rather than occupationally certified N95s. Yet, the US refused to acknowledge PPE for the commoner.
The public health initive was a collective one that tried to incentivize widespread usage as opposed as personal protective equipment (as that good masks are).
Unfortunately the initiave was a (many)day(s) late, and $1(00000000) short. Other nations [that used a higher standard of mask] did not have nearly as bad of numbers as the US had under the mask usage policies/regulations.
Read further down in the study. It discusses exactly what i'm referring to. The title says "N95 Masks Nearly Perfect at Blocking COVID" that's not what the study found. It found that duckbill N95s were nearly perfect at blocking COVID.
I dearly love my N95 duck bills and always wear them while flying. I'm not really worried about COVID now, but why would I want to risk getting from anything? A cold could ruin my trip. A few hours of discomfort, but I'm uncomfortable anyway.
I was surprised that the airlines were repeatedly trying to get people to remove their masks one time I flew on the tail end of the epidemic. Will they bring me chicken soup if I get sick?
Same. I wear masks on the outbound flight. If I'm dropping thousands on a vacation, I want to be healthy for it. I'll take my chances on the return flight.
Anecdotally, I’ve been wearing N95 indoors (outside the home) since the pandemic started and haven’t gotten sick yet, even when friends got sick from being in the same venue as me (e.g. office, airplane).
This is exactly what the guidance should have been about, right from the start.
Basically, if you are in a vulnerable group, wear a mask around others. Make it a good mask. Fit it well. Especially if the group is known to be 'risky' or large. And for those extremely vulnerable groups, wear a mask around them as a courtesy.
But it got warped into 'wear a mask around everyone at all times'. And that just didn't make sense to a lot of people.
But isn’t it also common sense that, if it’s already in high transmission then you wearing your mask isn’t going to budge the dial.
Thing is, at those times, responsibility needs to shift back to the individual to take care of themselves.
Most people judged they didn’t need substantial protection (else they’d have worn better masks, well fitted, always).
We sometimes look at human behaviour and find it superficially irrational. But this is an example where rationality simply doesn’t conform to what some would wish it to.
It’s still rational. And I would also argue it is common sense.
>But isn’t it also common sense that, if it’s already in high transmission then you wearing your mask isn’t going to budge the dial.
No, that isn't common sense at all. It's the opposite of common sense. Yes, you, yourself wearing a mask "budges the dial" ever so slightly. And the people in the house next to you wearing masks when appropriate also "budge the dial" just a tiny bit more. And thousands of others that wear masks "budge the dial" - well that dial is now spinning. That's how these things work. It's a group effort. But if the group is filled with morons that "think for themselves" (aka selfish assholes) then the dial will grind slower, because everyone's in it for themselves and don't believe that their actions have any effect on anyone else, which is pretty dumb.
>Thing is, at those times, responsibility needs to shift back to the individual to take care of themselves.
Yes, take care of your self. Wear the mask. Don't get sick so that you can't pass the sickness on to others. It's really that simple.
>Most people judged they didn’t need substantial protection
Think about the most average person you know. Half the people in the world are dumber than that. Most people are fucking stupid.
There's nothing rational about not wearing a mask in high risk situation during a global pandemic.
> […] if the group is filled with morons that "think for themselves" (aka selfish assholes) then the dial will grind slower […]
That’s basically my point. The group is already behaving as you describe. (Else transmission wouldn’t be high already.)
It’s OK to bemoan it. But it’s not reasonable to expect people to behave in a way that ignores reality on the ground.
> Most people are fucking stupid.
No, I don’t believe that. I can get on board with a criticism that many behaviours are automatic, and we are not always making conscious decisions. But in general, people are not stupid.
> There's nothing rational about not wearing a mask in high risk situation […]
It can readily be rationalised, but on the basis that it’s not actually a high risk situation.
Same here, I haven't been sick once in 4.5 years, and before that I was sick many times a year. Wearing the mask indoors, and always seeking outdoor dining venues is the only thing that's different for me. I still travel and do lots of stuff but when in an enclosed space with other people I wear the mask.
So weird that the state of our discourse, even four years post-pandemic, is still stuck on "do masks reduce disease transmission", something that (1) has been paradigmatically settled science* since the dawn of the germ theory of disease, (2) has never been seriously challenged and (3) remains true every time (as here) it's been re-studied. Yet...
* Not quite the same thing as "true". Very rarely paradigms are wrong. But they aren't here, and... I mean duh.
I'm baffled that this is your take. I'm relieved to see some positive evidence, since the Cochrane reviews on the topic keep concluding "little to no benefit" for either surgical masks or N95s.
I believe that the conflict over masks stemmed from a fundamental difference in perspective.
Masks work great at blocking transmissions even if the fit is bad, much like covering your mouse when you sneeze.
Masks CAN work great at blocking absorption if fitted properly, but thats hard to do correctly
If you are a fundamentally selfish person you only care about whether masks prevent YOU from getting COVID, you don't care about spreading it.
If you are non-selfish, then you probably care about both yourself and the broader community well being and are not only concerned about catching covid yourself, but also spreading covid to other people.
I think its hard for these two groups to interact because there's not much common ground.
I would argue that if you only care about your own risk of catching COVID and don't have training on N95 masks, then the argument that masks aren't worth the hassle is probably right.
You don't really need "training" to effectively use an N95 mask. If you just read the package directions and do a pressure check as directed, and it passes, you're going to be getting very good protection.
> “Data from our study suggests that a mildly symptomatic person with COVID-19 who is not wearing a mask exhales a little over two infectious doses per hour says first author Jianyu Lai, a postdoctoral researcher at the PHAB Lab. “But when wearing an N95 mask, the risk goes down exponentially.”
I rarely saw people wearing masks over both nose and mouth. Favorite memory is the guy in the metro who took the mask off his mouth to sneeze (and no, it didn't begin by covering his nose)
During the early days, I think a good amount of those were "I'm required to wear one but they can't make me wear it properly".
I'm most baffled by the people I see today with a mask on but around their chin. They haven't been required for years, so I presume it's being particularly COVID-conscious (as someone on an immune suppressant, I still wear N95s in public)... but if you're that conscious you'd think you'd wear it right.
The right-wing spent a couple of years demonizing mask policies, and the major talking point was that masks didn’t work. You don’t need that many people to flag-kill a story and there’s far more than that number who will go to the grave not admitting they were wrong.
In that case HN has finally strayed from its "hacker ethos" and objectively wanting to find out about any system over time at least; truth. Eventually right (even) in online discussions? No more. OK.
Maybe I've been thinking about this forum too highly for too long already? Probably.
Could this be another potential signal for the trend of the attention economy's created culture taking over in this turbulent and in many ways regressive global state?
If so, this is starting to feel "on rails". Scary.
I think of it as a consequence of the field expanding and becoming a high-income profession: with more people in total, regression to the mean is going to inevitably show up and the hyper-focus on capitalism bleeds through in many places. Mask mandates were proclaimed to be bad for business so a certain level of opposition is largely baked in.
Moderation has some huge blind spots that's allowed a corrosive element to build a core in the community. But that's the fate of all social networks. We've yet to figure out how to scale social interaction. When a network gets large enough to grant relative anonymity to people, the worst comes out.
>I assume you wash your hands when you use the bathroom?
99% of the people I see in public restrooms don't wash their hands. They just sprinkle some water, when there's soap right there, and then go back out. And that's assuming they actually stop by the faucet.
So you know what? Forget masks, make people fucking wash their fucking hands after handling their literal fucking instruments.
I hate handshakes even though it's social ettiquete because I know those hands have, literally, seen shit.
I'm always baffled by the people who went around arguing masks did nothing. They don't seem to have any problem accepting that a sneeze guard at a salad bar does something, or that a surgeon should put something on their face to prevent them from coughing into your open body cavity.
We can argue all day about specific masks under specific settings, or when it is worth it to bother, but "putting something in front of your face prevents you from breathing out particles" is in the category of "things that are pretty f&%*ing obvious".