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RCT on use of cloth vs surgical masks

Barry Dehlin writes:

See this blog post by “Scott Alexander” about an RCT on the use of cloth and surgical masks. As he points out, this issue is momumentally important RIGHT NOW and a real statistical expert should be evaluating this study and the conclusions the authors draw. Here, I think the value of your expertise would be huge and topical, and would encourage you to review and post on this issue.

I don’t know how relevant my statistical expertise is here, actually. I’ll get back to that at the end of this post.

Now to the details. First I took a look at the research paper in question, “A cluster randomised trial of cloth masks compared with medical masks in healthcare workers,” by C. Raina MacIntyre et al., and was published in the British Medical Journal in 2015. Here’s what they report:

The aim of this study was to compare the efficacy of cloth masks to medical masks in hospital healthcare workers (HCWs). The null hypothesis is that there is no difference between medical masks and cloth masks.

Setting: 14 secondary-level/tertiary-level hospitals in Hanoi, Vietnam. . . . 1607 hospital HCWs . . .

Hospital wards were randomised to: medical masks, cloth masks or a control group (usual practice, which included mask wearing). . . .

Results: The rates of all infection outcomes were highest in the cloth mask arm, with the rate of ILI statistically significantly higher . . . Penetration of cloth masks by particles was almost 97% and medical masks 44%. . . .

OK, the conclusions seem pretty clear. In this small study, medical masks worked better than cloth masks. That makes sense! To learn more, we’d want more data in other settings.

Next I went to Scott Alexander’s post, which begins:

The New York Times says that It’s Time To Make Your Own Face Mask. But MacIntyre et al (2015) says it isn’t.

Huh? Did MacIntyre et al. really say not to make your own face mask??? I missed that! Let me take a closer look . . . I guess Alexander is picking up on the following bits from the linked paper:

The results caution against the use of cloth masks . . . as a precautionary measure, cloth masks should not be recommended for HCWs . . . HCWs should not use cloth masks as protection against respiratory infection. . . it is important to consider the potential risk of using cloth masks. . . .

I guess it’s all how you read it. As a statistician, I read the MacIntyre et al. paper and immediately interpreted the result as a comparison: cloth masks vs. medical masks. Medical masks are better than cloth masks. As a practitioner, Alexander read the paper as making an absolute statement that cloth masks are no good for the general population. That’s not how I read the paper—after all, “compared with medical masks in healthcare workers” is right there in the title!—but I guess this is a warning to all of us who write research papers that, if we’re not super-careful, people can draw conclusions from our work that are not in our data. And, to be fair to Alexander, there are those quotes from the paper: “The results caution against the use of cloth masks,” etc.

Alexander then looks at the article more carefully and reports, “the authors themselves lean towards the hypothesis that that cloth masks are actively bad.” I guess I didn’t take that so seriously. At this point, I pretty much make it a point to focus on the details of the study (what’s actually being measured and compared) and to look carefully at the title and abstract (as this is the main message sent by the paper) and not worry so much about unsupported speculation. What I’m saying is, if the paper had some speculation that Alexander is skeptical of, that’s fine, it’s good to know. But I don’t think some speculation deep in the paper is the main message of the paper. The main message is first in the title, and second in the abstract, and in both it’s clear that the paper is comparing cloth to medical masks, and that it’s for health care workers, not the general population. Alexander is arguing against some claims made in the discussion section of the paper. I’m not disagreeing with Alexander’s arguments on that point; I just think it’s a mistake for him to read that paper as making any claims about the efficacy of cloth masks compared to no masks for the general population.

The background is that a lot of people are mad at public health authorities for discouraging mask-wearing. The evidence on homemade masks still seems unclear, but I’m sympathetic to the argument that there’s nothing to lose by wearing masks in crowded places. That research article from 2015 seems fine to me: it clearly separates its empirical conclusions from its speculations, and I’m sure that there have been some studies since then on the effectiveness of masks in other settings.

OK, so what about the business of my statistical expertise? It didn’t really come up here, except in the negative sense that I could see what they were doing in that paper so I didn’t have to look at the details too carefully. I had a good GRE score and I’m paid well, so I guess that gives me credibility in some quarters. Perhaps more relevant here was my expertise as a consumer of statistical results, as this led me to focus on the title, abstract, and main conclusions and not get distracted by the speculations in that paper. Alexander’s post is reasonable too and makes use of his expertise as a clinician.

In summary

When it comes to the important question of whether we should all be wearing masks in crowded places, my statistical expertise isn’t so relevant! I was able to read the above-linked research article and focus on its data-based conclusions, and I’m here to remind you that the question isn’t so much, Should we wear masks? but How should we do it?, but that doesn’t take a lot of statistical expertise. Subject-matter expertise and relevant data are much more important here. If we had some data, then statistical analysis could become more relevant, but I’m guessing you can get most of the way there by following the usual principle of not trying to collapse uncertainty into certainty. Statistical expertise is also relevant for data collection, but again the basic idea seems pretty clear: get some people in the lab wearing no masks or different masks in real-world conditions and see what you find, then do something similar in the wild.


  1. paul says:

    I think that these studies are also different than the public health purposes of the masks.

    It is not about keeping the mask wearer from getting infected, but to keep asymptomatic people from spreading the virus.

    That is why it is “I wear a mask for you not me” slogan. I think that is why the whole notion of decontaminating the masks may be overblown as well. Not that it hurts. I deliver food, and wear a mask just for the possibility that if I am unknowingly infected/infectious I have less chance to pass it on.

    • Keith O’Rourke says:

      That’s my understanding too – along with if its just a placebo (really does not help much) there is not a big down side.

      But then if people feel safer or if they put them on without first washing their hands or touch their faces more often after putting it on or taking it off, there might be a big downside.

      As Andrew suggested above, right now unlikely if statistical advice can add much…

      • Jack says:

        I was a certified fit tester at Intel for N95 mask, APR’s and SCBA’s. If your mask is rated less than a “good” N95, and/or not sealed right, you are wearing a mask that blocks dust and dirt in a wind storm for a short period of time. You cannot wear it all day without cleaning it changing it. It is a false security and if people think it is safe because you have one on you are seriously mistaken. A virus is 1,000 times smaller than a bacteria and the mask isn’t even efficient enough for a bacteria. Many people don’t even know how to doff or don a mask properly (put it on or take it off), but in this case it really doesn’t matter since it probably more dangerous because people think they are safe they are taking more chance with them. Social distancing (21 ft.) or self quarantine is the safest measure one can take.

    • YES YES YES…. I am SO frustrated about this, it’s finally getting into some public health messaging though.

      The purpose of the cloth mask is NOT to protect the wearer from breathing in particles… It doesn’t do a good job of that. The purpose of the cloth mask is to act as a cheap filter of a HIGH CONCENTRATION SOURCE… your mouth.

      Even if it’s inefficient, taking maybe 50% of the viruses out of your exhalation is a huge win for reducing viral load in the environment. Imagine if we could take 50% of the plastic out of the rivers that dump plastic in the ocean? Same idea. Also taking the fluid energy out of the plume prevents you from exhaling and spreading the virus over a longer distance.

      Here’s a Schlieren imaging video of how fluid moves around your mouth as you exhale, with and without an N95 mask (they don’t show a cloth mask… but the principle is the same in terms of fluid energy)

      • Steve says:

        +1000 Agreed, the frustrating thing is when we hear someone say that “there is no evidence that wearing a mask reduces the risk.” Of course, we know that masks work. Yes, we don’t have a specific study that the general public will benefit from it. Maybe they will misuse the mask. Maybe they will become overconfident and stop social distancing. But, there are solutions to those things. Tell us the best way to use the mask. Tell us not to be overconfident. In Hong Kong, public health officials have made PSAs on how to wear a mask, how to take it off and dispose of it safely. I don’t know how the idea that you need a specific study for every claim became the dominant paradigm, but in a pandemic, it is killing us.

        • Alex says:

          Even if we find out tomorrow that masks don’t reduce the risk, I’m still going to sleep easy knowing I at least tried to protect others, and I did best I could do given the information I had at hand. There’s little things we can do to help stop the spread, and I wish more people did them. It’s not that difficult to reduce the chance of spread to others. Honestly, this comment section makes me feel better about the world.

      • Rahul says:

        Tangential question:

        Is the particle capture rate assymetric?

        i.e. for a given mask does it do better at capturing particles on the way out than in in terms of percent captured?

        • The one paper I’ve read that addressed this topic found that with surgical masks they stopped roughly 75% of the small particles going in and 50% going out. I don’t remember exactly what aerosol size they used, sorry to say. The obvious reason is that surgical masks don’t have fitted seals so the negative pressure on an in-drawn breath will tend to reduce any gap but the positive pressure on an outgoing breath will tend puff the mask out and reduce the seal.

          But do remember that eyes are vulnerable too and those aren’t covered by surgical masks. Also, a patient will come into contact with far fewer nurses and doctors than nurses and doctors will come into contact with patients. A level of protection sufficient in the former case might be almost useless in the later.

        • paul says:

          This is important especially in respect to N95, P100 and other masks.

          These NIOSH style masks (the standard the follow) filter on the intake breathe. The mask is pulled against your face (you have to shave) and the air goes through the material.
          They don’t filter on the exhale (deflect flow and some stuff) but the either have a one way valve that opens to allow breathe out (the little plastic box you see on them), or the mask is pushed away from the face and the breathe escapes around the edges. This is a purposeful design feature. It is to ease breathing on the exhale so the user is more comfortable. The design assumption is the contamination is on the outside not the inside.

          There are obviously still some benefits to others, but I really think that cloth masks or surgical masks may be equivalent to protect others. Surgical masks are actually designed to protect the patient from the medical staff.

          This is why first responders, medical staff, cashiers, at risk people and their support staff, etc. should wear N95’s.

          I wear a cloth mask because if I am or become asymptomatic, I would pose a hazard that way.

        • paul says:

          Sorry if this comes up twice.

          N95, P100 and other NIOSH (design standard) filter on the inhale. The suction tightens the mask to a clean shaven face
          The exhale is released either through a one way valve (the little plastic box) or at the edges of the mask.

          The design assumption is the contamination is on the outside

          Surgical masks are designed to protect the patient not the medical staff

      • Keith O’Rourke says:

        Daniel – Thanks!

      • Kaiser says:

        We agree on this 100%! It doesn’t have to work perfectly and it has no downside.
        Also, we don’t have to choose to take just one action. It’s the aggregate of all the actions, each of which may have a small effect.
        Some stores started to put up signs saying you can’t enter unless you wear a mask. So the message is getting across.

        • Brent Hutto says:

          The main purpose of people putting cloth over their faces in public is virtue signaling.

          So the message is definitely getting across. As with everything in the response to this damned pandemic, the “message” is paramount and actual public health measures are secondary, at best.

          • Kaiser says:

            Brett: It’s possible that some of these actions are ineffective. And eventually, someone will publish a study to confirm or disprove it. Like Andrew, I don’t see the point of a RCT. This seems to require a lab experiment. Blast some particles at these masks and report the results.

            In terms of people doing “unscientific” things, it’s not limited to public health. I was annoyed about this until the day I realized that lots of unscientific things are perfectly legal and perfectly accepted in our society, e.g. astrology, bottled tap water, mindfulness, lots of high tech products, food fads…

          • Phil says:

            Brent, you say “The main purpose of people putting cloth over their faces in public is virtue signaling.”

            As Daniel points out, even a cloth mask has a pretty significant effect at reducing the spread of virus-containing droplets. It also makes it less likely the wearer will touch their lips with their hands, which may be contaminated. If you want to argue that virtue signaling is _also_ a purpose, at least for some people who wear masks, OK, I’m sure that’s true. But to say it’s the “main” purpose seems kind of ridiculous.

            • Kyle C says:

              “virtue signaling” is also the main reason we put cloth over most of our bodies in public even in warm weather, yet we do it without much complaint. Are libertarians simply incapable of recognizing norms of politeness?

            • Brent Hutto says:

              I could have more precisely stated it as…

              Covering your face with a piece of cloth in public is 100% effective as a form of virtue signaling, while being of limited and potentially even near-zero effectiveness for slowing down the pandemic of COVID-19.

              P.S. And hey, it sure makes for cool apocalyptic-looking stock photos and footage in the media. So that’s another side benefit I guess.

              • Phil says:

                “Limited” effectiveness, sure. And _potentially_ near-zero effectiveness, yes, potentially. But I think your disdain is misplaced. It is extremely unlikely to hurt, it might help, and it’s very cheap.

                What level of effectiveness would be needed in order for you to endorse it? That’s a serious question, not a rhetorical one.

              • Carlos Ungil says:

                Another question would be what measures do you endorse, if any.

              • David J. Littleboy says:

                You might compare the rates of infections and fatalities in Japan (where masks are widely used) with the US (where masks aren’t) for a quick reality check on how useless masks are. (In case you don’t have the numbers at hand, they’re two orders of magnitude lower in Japan. So far.)

                As a Tokyo resident, I was sure Japan (with it’s massive dependence on public transportation) was going to be a world leader in this disaster. So far, it isn’t. So far. (The governor of Tokyo is pretty good, but she’s fighting gross incompetence and massive stupidity at the national level.)

                FWIW, the latest flaky theory from Japan is that taking shoes off and leaving them in the doorway reduces tracking the virus into the house, and explains Japan’s low rates of infection and deaths.

  2. Adrian K says:

    You wrote, “… but I guess this is a warning to all of us who write research papers that, if we’re not super-careful, people can draw conclusions from our work that are not in our data.” Not sure I agree. I think you can be super-careful, and other people can still draw completely unwarranted conclusions that are not in the data. Mainstream journalists do this all the time. In fact, I think it rather more the rule than the exception, which is why I don’t trust the MSM on much of anything.

  3. jim says:

    Here’s a relevant study

    From what I understand, a cloth mask provides 0% protection from aerosolized particles, but it should obviously be effective against spit going either in or out.

    • Phil says:

      “In 3 RCTs, wearing a facemask may very slightly reduce the odds of developing ILI/respiratory symptoms, by around 6% (OR 0.94, 95% CI 0.75 to 1.19, I2 29%, low certainty evidence).”

      I’m not sure I’d say a 20% reduction is ‘around 6%’, but it’s entirely consistent with their numbers, so they seem to not understand the implications of their own uncertainties. Their results are also consistent with 0% reduction, or even some increased risk, although that might be unlikely for known reasons. Anyway all I get from this, if we take their numbers seriously, is that the risk reduction is unlikely to be very large. I don’t see where they get “around 6%”, if that’s meant to imply, say, between 4-8% or something: It’s actually rather unlikely to be in that range, just because there’s so much probability in the rest of the range.

      But if we pretend the reduction is 6% then how is that “0% protection”, as you characterize it, jim? If I have a choice between 0% reduction and 6% reduction I am definitely choosing 6% reduction!

      • jim says:

        Phil the problem is understanding what risk is being reduced.

        The “0 %” I stated above is specifically with regard to “aerosolized” virus particles – that is, particles that are small enough to stay suspended in the air for an extended period. You walk down the grocery store aisle breathing out your particles, I come by 5-10 minutes later and I can still get infected.

        My understanding is that its common knowledge that cloth masks would not reduce this form of transmission. The particles are much smaller than the openings in cloth masks and would easily pass around the margins of a mask that doesn’t fit tight to one’s face. Supposedly, this is *not* the primary mode of transmission of corona virus.

        In the article in question, with regard to the 20% figure, they don’t specify the mode of transmission, they simply refer to wearing masks. If, as most epidemiologists have said, the primary mode of transmission of corona virus is through larger droplets from sneezing or spitting (possibly still not visible, but heavy enough to fall through the air without any suspension), then masks would reduce transmission.

        I advocate wearing masks. I just think we should understand specifically what’s being protected against and what’s not.

        • Phil says:

          Ah, I see, and I agree that’s an important distinction. To be fair, it was you who said that study was ‘relevant’!

          I’m sure we all wish we knew more about the risks of transmission via various routes, e.g. touching your face, touching your eyes, spittle touching your lips, free virus particles inhaled through the nose, free virus particles inhaled through the mouth, small droplets inhaled through nose or mouth, etc.

          As I mentioned elsewhere on this thread, I think it’s extremely unlikely that a good mask doesn’t help at all to reduce transmission risk. On the flip side, something extremely permeable, like a bandana, probably does very close to nothing (although even there, probably not exactly nothing). There’s a continuum in between. It may not even make sense to talk about the ‘effectiveness of masks’, given the very large amount of variability between them.

          • jim says:

            “I think it’s extremely unlikely that a good mask doesn’t help at all to reduce transmission risk”

            We’re 100% in total agreement on that. I guess my comment muddled things up!

            I bought KN95 masks for everyone in our family. We also have woodworking masks which I think are pretty good because they’re designed to protect against fine particles and shaped to fit against the face.

            • Phil says:

              I’m in California, where we’ve had severe smoke from wildfires the past few summers, so I have a reusable N95 that is supposedly a good brand. But I’m also going to get a bunch of cotton masks with pockets for putting in filter material (I plan to cut up some HEPA vacuum cleaner bags) so I can keep one in the car, one in each jacket pocket, etc. Why not? If it cuts my risk by even a few percentage points, it’s worth it to me.

              • Martha (Smith) says:

                Makes sense: the old Boy Scout motto, “Be Prepared” (so now the tune of Tom Lehrer’s song of that name is going through my head — such is life)

              • Phil says:

                Don’t make book if you cannot cover bets!

                I loved Tom Lehrer when I was a kid, and I still do now.

              • Martha (Smith) says:

                Don’t solicit for your sister, that’s not nice
                Unless you get a good percentage of her price

  4. Bill Spight says:

    I took a quick look at the NYT article. The caption under the picture talks about protective masks, which, if I hadn’t heard or read anything else, I would assume meant protective for the wearer. (I also used to live in Japan, and wore a mask when I caught a cold, so I understand that the purpose of the mask is to spread fewer germs, and also it keeps your nasal passages moist, which is less irritating.) The text is ambiguous about who is being protected, saying that wearing masks can help to mitigate the pandemic.

    The masks for health care workers are have, I suppose, a dual purpose, mainly to protect them when they come into close contact with infected patients, but also to protect other patients from them. The quote from the MacIntyre study that gives me pause is this: “Penetration of cloth masks by particles was almost 97% and medical masks 44%.” If infected wearer’s spewing out 97% of viral particles that come from their mouths, that is not much help.

    • I’m almost sure that 97% number is for *inhaled* particles. Also particles are not all the same size. Ever had someone accidentally sneeze on you and you felt actual droplets hit your face? Right, those droplets are WAY bigger than aerosol particles.

      figure an aerosol particle is ~ 1 um diameter, and a droplet you’d feel hit your face is 1mm diameter. that’s 1000 times bigger diameter, and therefore 1 billion times bigger volume. You’d also expect at least millions if not billions of times more virus in the droplet. If you prevent that droplet from exiting someone’s mouth and hitting a surface… you keep billions of viruses out of the environment where people could get them on their fingers and rub their eye… So even if only 1% of the particles are 1mm in diameter, still well over 99% of the virus is in those droplets and at 1mm they’ll be blocked essentially 100%

      • Rahul says:

        …which is why I think these percent capture studies are somewhat weak at evaluation of mask type.

        What would be illuminating is a study that gave cloth vs medical masks to cohorts and evaluated flu propensity (say) among wearers.

        Not sure what the ethics committee would say but would love to see if there’s any studies like this out there.

        • What would be illuminating is a study that gave cloth vs medical masks to cohorts and evaluated flu propensity (say) among wearers.

          You really want to give the mask to known flu patients and evaluate the flu propensity among the households they live with… it’s all about trapping the *expulsion* just pretend the mask does nothing or makes it slightly worse for the healthy wearer. There is some evidence that cloth masks might trap incoming droplets next to the face and expose the wearer to slightly more virus, particularly in a health-care setting.

          None of that is relevant to the public health purpose. The mask DOESN’T PROTECT THE WEARER, it protects the public FROM the wearer but reducing viral load in the environment, so people in grocery stores don’t touch surfaces and get heavy loads of virus on their fingers etc.

  5. Seth Green says:

    > but I’m sympathetic to the argument that there’s nothing to lose by wearing masks in crowded places.

    I wonder if people who wear masks are more comfortable standing closer to other people (with or without masks) than they otherwise would be. Anecdotally, I’ve observed this at a local deli — people without masks give each other a wider berth than do those wearing them.

    Perhaps in a *really* crowded place like a subway where you are going to be close to people no matter what, then there is nothing to lose. But in a semi-crowded space, like a grocery store, I’m not so sure.

    Tyler Cowen makes this point as well:

    > If you tell everyone to wear a mask, great! But people will feel safer as a result, and end up going out more. Some of the initial safety gains are given back through the subsequent behavioral adjustment.

  6. oncodoc says:

    A well done study answers a question, but it does not answer every question, nor does it answer questions that were not asked. Bad studies are often bad because it is unclear what the question was. Sometimes the answer is that with this experiment we cannot get to a definitive answer.
    My quick look at the study agrees with Andrew. A question was asked and answered, not every possible question, but one specific one. That’s all that can be required.
    Should you wear a homemade cloth mask? The downside is low. The upside is a bit hypothetical. Should police powers enforce the wearing of masks?

  7. Jonathan (another one) says:

    Scott has a great (IMO) piece yesterday which includes a followup on masks.

    ‘The real medical consensus on face masks came from pretty much the same process as t’he fake medical consensus on parachutes. Common sense said that they worked. But there weren’t many good RCTs. We couldn’t do more, because it would have been unethical to deliberately expose face-mask-less people to disease. In the end, all we had were some mediocre trials of slightly different things that we had to extrapolate out of range.

    Just like the legal term for “not proven guilty beyond a reasonable doubt” is “not guilty”, the medical term for “not proven to work in several gold-standard randomized controlled trials” is “it doesn’t work” (and don’t get me started on “no evidence”). So the CDC said masks didn’t work.’

    He goes on: ‘Goofus started with the position that masks, being a new idea, needed incontrovertible proof. When the few studies that appeared weren’t incontrovertible enough, he concluded that people shouldn’t wear masks.

    Gallant would have recognized the uncertainty – based on the studies we can’t be 100% sure masks definitely work for this particular condition – and done a cost-benefit analysis. Common sensically, it seems like masks probably should work. The existing evidence for masks is highly suggestive, even if it’s not utter proof. Maybe 80% chance they work, something like that? If you can buy an 80% chance of stopping a deadly pandemic for the cost of having to wear some silly cloth over your face, probably that’s a good deal. Even though regular medicine has good reasons for being as conservative as it is, during a crisis you have to be able to think on your feet.’

    • Brent Hutto says:

      The argument you guys are making seems like a variant on Pascal’s Wager.

      If there’s any chance at all that doing a certain thing might save millions of people from dying of COVID-19 then we ought to all do it, whether there’s evidence that thing (wearing a bandana over your face) actually saves anyone or not.

      P.S. Pascal’s Wager never convinced anyone to believe in God who didn’t already believe in God.

      • Jonathan (another one) says:

        A 1 percent chance of a large gain isn’t the same as as nonzero chance of an infinite gain.

      • Phil says:

        Brent, it may seem like a variant of Pascal’s Wager, but it isn’t: we understand the mechanism of disease transmission so we know there are plausible mechanisms by which wearing a mask could reduce it.

        It would be pretty shocking if wearing a really good mask didn’t reduce disease transmission. I assume you disagree, but if so I think you’re just wrong.

        On the other end of things, a bandana is ridiculous, I hope nobody is doing that. That really would be very close to zero effectiveness.

        In between 0 and ‘somewhat effective’ there’s (obviously) a continuum of options. There are people looking into what works best.

        • Phil, even a bandana worn appropriately would catch a lot of droplets, reducing the expulsion of the virus onto surfaces… Since that’s the MAIN purpose of public masks, in fact a bandana is fine, particularly if worn folded double and wrapped around the face (in the shape of a rectangle) with a rubber band to hold it in the back, rather than “cowboy” style (shape of a triangle, with flappy point at the bottom).

          • jim says:

            “…reducing the expulsion…”

            Exactly. People don’t seem to get: it’s not about what comes in. It’s about what goes out.

          • Phil says:

            I think a lot of people do get that!

            An ordinary cotton mask is not going to remove viruses that aren’t in droplets, so if a sick person coughs, (or, really, exhales) they are going to be putting viruses into the air. I know nobody disagrees with that.

            So then it comes down to particle sizes. Sure, even a bandana will block a loogie. But in my experience bandanas are usually extremely loosely woven, even looser than a typical tee-shirt weave. Little droplets with viruses in them will go right through a layer or two of bandana as if it weren’t even there. Doubling it up will help, I agree, and tripling will do even more, but when we talk about wearing a bandana over the mouth I think most of us picture a single bandana worn as a single layer. I wouldn’t claim that literally does nothing — it will remove the larger droplets — but…well, to find some common ground: it should be very very easy to do much much better, so nobody should be using just a bandana. Are ya with me?

            • Take a bandana and lay it out like a square. Fold the top to the center, and the bottom to the center. Now fold that in half… You have 4 layers of cloth. At what droplet size would it get 95% effectiveness for exhaling?


              Suggests that 3 layer folded bandanas had 40% filtration efficiency at 2.5 micron for volcanic ash type material. The vast majority of virus will be expelled at 10 to 100 micron droplets because the larger the droplet diameter the larger the volume like D^3

              2.5 micron is only like 25 times the virus diameter

              I conclude if you fold a bandana like I mention and wear that you’re doing just fine to filter exhaled virus, particularly cough or sneeze droplets.

            • For a while before I got my allergy shots, I was having pretty severe pollen allergies. So I have an elastomeric face piece with P95 pads. This really does reduce pollen size droplets by easily a factor of 100 to 200 under most circumstances…

              It would be a TERRIBLE mask to wear to the store for the other people in the store. Suppose I was asymptomatic… These facepieces have nice easy exhalation flapper valves, and they condense your breath. Large quantities of fluid drip out of those valves into the environment. It’s like a virus concentrator and distribution system.

              so what seems like it’d be a great thing for people to wear if you’re looking at inhalation efficiency, is WAY WORSE than a bandana for this particular public health purpose.

              Thinking clearly here is important, and the messaging should be much better than it is.

            • jim says:

              “So then it comes down to particle sizes”

              Well maybe not totally.

              Simple filter effects – particle size vs weave opening – is the first step. These effects are the same regardless of the flow direction – that is, whether you’re breathing in or breathing out. But other effects, such as the velocity of droplets, flow direction of air, and the effect of the bandana as a flow barrier, will change for breathing in vs breathing out.

              Presumably when you exhale the flow rate decreases if you’re wearing a bandana, which means the drops don’t go as far. Also the barrier may also force air down or up as well, where it will flow along the surface of the bandana and your face, drops will get caught on the surfaces and the velocity will decrease further.

              I can imagine a few other considerations but I’m really tired!

        • Luca Beltrame says:

          I’m not sure if the mechanism is really well understood. Currently droplets seem the most solid of the hypotheses, but aerosols are being debated (and the letter of the experts explicitly called for the precautionary principle, because there isn’t enough solid data).

          Back to masks. What is needed to know is not if they stop aerosols or droplets, but rather if they are useful in reducing transmission of this specific virus. Currently the evidence is scant, and I’m aware only of a Danish RCT with that objective.

  8. Nick Adams says:

    The literature on face masks for the prevention of transmission of infection is something I looked at a few years ago before any of this started. As with many things, the evidence is of varying quality (mostly low). The tentative conclusion I came to was that medical face masks do not appear to reduce disease transmission by an appreciable amount. If we take this as the baseline then evidence that cloth masks are worse than medical face masks is concerning. It has been theorised that wearing a mask induces people to touch their face more often unconsciously, and that this may increase disease transmission. So I don’t think we are justified in assuming that cloth face masks are harmless or of small benefit – the effect may go the other way. The current consensus (for what that is worth) is to put a mask on patients with respiratory infections as a visual signal. Medical staff wear masks to prevent being splashed in the face by various body fluids (yeah, gross, I know).
    Stop worrying about masks and start social distancing which demonstrably works (Japan/South Korea/Australia/NZ etc).

    • jim says:

      At least where I live in the US social distancing is pervasive. Almost all stores have 6-ft separation markers for waiting lines; most put barriers in front of cashiers and many have installed plexiglass; stores aren’t crowded and people give a wide berth when passing one another. For a while people were still going to parks, but that’s been stopped; but outdoor transmission rates would probably be very low anyway.

      The recommendation to wear masks was post-facto. Mask use has increased to the point where now almost everyone wears some kind of mask. The US Postal Service has managed to score masks for its tens of thousands of workers, so supply isn’t an issue.

      The problem in the US is that we got behind the curve early. We’re catching up now.

  9. Kaiser says:

    There is a side benefit to wearing a mask (and glasses). I find that I don’t touch my face, which is how the virus gets into the body. Not sure if there is any study that supports this generally but it works for me that way.

  10. Rajesh says:

    The following preprint reviews existing evidence on the topic.

    • Kaiser says:

      Here’s an excerpt from their abstract: “The preponderance of evidence indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts. Public mask wearing is most effective at stopping spread of the virus when compliance is high. The decreased transmissibility could substantially reduce the death toll and economic impact while the cost of the intervention is low. Thus we recommend the adoption of public cloth mask wearing, as an effective form of source control, in conjunction with existing hygiene, distancing, and contact tracing strategies.”

      Note: Researchers are working very hard and fast. This is yet another paper labeled “not yet peer reviewed.” Is that a health warning or virtue signaling? I wonder what Andrew thinks.

    • Nick Adams says:

      It is not very good. The supportive clinical studies they quote aggregate face mask wearing with other infection control procedures such as hand washing.

  11. Martha (Smith) says:

    Many good comments relevant to the effectiveness of ordinary people wearing cloth face masks in everyday circumstances. But, as I understand it, there is still a shortage of the more effective
    “medical” masks for medical personnel, so they often have to resort to hand-made cloth face masks. (I have a cousin who has been busy the past few weeks making cloth masks for her physician son in Hawaii and her physician niece in England).

    • Carlos Ungil says:

      I guess we have all seen pictures from countries like Spain, UK or the US of hospital staff wearing plastic bags over their clothes and, if they were luck, a simple surgical mask. Unsurprisingly, over 25’000 healthcare workers in Spain have been infected (may be many more, because even when they develop symptoms they are not always tested).

  12. John says:

    Could there be a statistical analysis on whether the virus is less rampant in warmer temperatures?

  13. pb says:

    Here’s the interesting thing about “the control arm”:

    “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.”

  14. If aerosols are a primary path of deep lung infection at low infectious doses, and if woven cloth masks have pores small enough to allow a significant percentage of aerosol particles to pass through from the wearer to the surrounding air, and if aerosol particles seek the path of least resistance around mask edges (i.e., side leakage), and if many people predictably mishandle and misuse masks throughout their normal all-day work/life schedules (as I continually observe, where they have no clue of the concept of chain of contamination and no willingness to exercise the required discipline to deal with this), and if marketeers create an industry to supply fashionable face coverings, and if audience-hungry news media continue to pedal fearmongering forecasts about failure to cover faces to scare people into doing so, then really, … practically, … rationally, … reasonably, [and I ask this with impatient impoliteness] what the hell good is mass masking, other than to sustain a ritualistic practice of functionally sloppy behavior propagated to give a superficial appearance of exercising control over ones life in a state of self delusion?

    • Once more for the record… masks are primarily SOURCE CONTROL not personal protective equipment (though they may help as PPE somewhat).

      One of the main mechanisms of masks is that they are a barrier to fluid plumes. When you yell across a room or sing or whatever a plume of fluid carries forward multiple feet. With a mask on that fluid activity becomes a turbulent mess in a boundary layer near your face. The effect can be seen clearly with schlieren imaging:

      • Martha (Smith) says:

        Thanks for the (regrettably all too often necessary) repetition of why masks are of value — and for the link.

      • Anoneuoid says:

        What we really want is to stop SARS2 from spreading but not the other coronaviruses that offer partial protection to covid. Interrupting that transmission may end up making younger people tend to have more severe covid.

        A possible modification of COVID-19 severity by prior HCoV infection might account for the age distribution of COVID-19 susceptibility, where higher HCoV infection rates in children than in adults5,35,37, correlates with relative protection from COVID-1948 , and might also shape seasonal and geographical patterns of transmission.

        Public health measures intended to prevent the spread of SARS-CoV-2 will also prevent the spread of and, consequently, maintenance of herd immunity to HCoVs, particularly in children. It is, therefore, imperative that any effect, positive or negative, of pre-existing HCoV-elicited immunity on the natural course of SARS-CoV-2 infection is fully delineated.

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