Epistemic status: Pretty confident, seems obviously correct.

Summary: We should promote widespread use of reusable silicone masks to stop the spread of SARS-CoV-2.

Masks and transmission of SARS-CoV-2

SARS-CoV-2 spreads mainly through respiratory droplets. Most of the discussion of mask usage to prevent transmission has so far been centered on surgical masks and disposable filtering facepiece particle (FFP) masks (also called respirators sometimes). Surgical masks do not form an airtight seal to the face and thus can't reliably prevent transmission. Achieving a tight fit with FFP masks is tricky and at least requires some practice. A recent post on LessWrong suggested the use of reusable masks with replaceable filters and body made out of silicone, such as this:

(North 7700 Series Half Mask Air-Purifying Respirator / PD-USGOV-HHS-CDC)

These masks (also sometimes called respirators) are available in different sizes and can be equipped with filters of N99 / P3 standard. Safe usage should be much easier to achieve than with FFP masks due to the mask body having a flexible silicone edge that can form a tight seal with the face. Additionally, they are equipped with one-way valves that prevent the filters from being soaked by the wearers' breath. They are more expensive than the other mask types (~50$), but can be easily mass-produced using injection molding. Thus, widespread use of such masks seems highly desirable and might play a big part in reducing effective transmission rates without requiring severe social-distancing measures. Compared to the economic costs of these measures, the cost of providing these masks to everyone, at least in industrialized countries, seems to be negligible. (Much more detail can be found in the previously mentioned post.)

Call to action

Given the plausibility and potential benefit of the idea described above, it is urgent that we take action to promote it or figure out if it is wrong. I strongly encourage the readers of this post to:

  • Share the idea widely and try to convince influential people.
  • Buy a mask for yourself, friends, and family; wear them when you're in public (or generally close to people that you don't live with) to protect yourself and others and increase social acceptance.
  • Use 3D printers and share potential designs; distribute self-made masks to people in need of protective equipment.
  • Reach out to groups which are at high risk of infections and with an interest in protecting themselves (e.g. nurses in nursing homes).
  • Suggest further ideas and point out potential problems in the comments.
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The suggestion certainly fits broadly in with my current thinking. We simply cannot keep going as we are now. It is simply not sustainable.

I think wide/universal use of masks would allow more economic activities to resume. I think that needs to start being a focus (and can be without competing with a resources for getting treatments, vaccines and general sars-cov-2 research) I think this is an area where we do have slack that can be applied to the current binding constraint on "normal" economic life.

It doesn't need to be a complete solution right out of the box. For instance, the suggested masks could be used within the effort to increase the supply of the masks and improvements to them We might really want to have full face respirator masks that provide the eye protection as well as respiratory protection. If the first slice still includes some exhaust valves the second round can work on solving that -- mask with exhaust is still better than no mask.

With increased supply all those currently at risk but working in essential industries -- food production and distribution, power/utility/communications and some transportation (gas and oil) services.

As long as we have something that can be reasonable expected to produce the same level of spread as is occurring with the existing shutdown and social distancing we have a good zero start point. We can work on improving that and so allowing increased economic activity to follow. That starts getting us to the "good" spot. We can keep the economy going, keep people working, and getting others back to work. Debts get serviced. Perhaps at some point even larger social gathering can be supported (Think sports events without the beer and food maybe).

That happens while we are improving the the spread, keeping hospitals from being over loaded (and perhaps providing the health workers better tools for protecting themselves).

Now we can have a functioning economy and controlled infection spread so time to work on more difficult solutions (vaccines, treatments to cure/assist the body's efforts).

I think the more important point here is that we need to attack multiple problems here and they are not always competing for the same resources so are on the production possibility frontier. The approach we've been under doesn't quite appear to be doing that it's too virus-centric (and that was the critical starting point so not a bad thing, but we need to move forward).

Exactly! :)


Responding more to the other post but seems perhaps more sensible here as this seems more visible.

The use of these masks has have serious drawback in the case of verbal communications. So, out of the box off the shelf these would not be a long term solution -- assuming worst case and no cure/vaccine and the virus stays around for years and years. However, we can use written communications if needed. Moreover, it would not be that hard to put communications into the mask some way.

One, just a cheap internal mic and and external speaker. You could also integrated it via bluetooth and your smartphone or just dedicated bluetooth ear buds/head set getting paired with the internal mic. Obviously some new protocols for dealing with a a multi device setting would be needed but I cannot imagine that we don't already have solutions that are 80 to 90 percent ready to be apply to the specific setting.

The upside here is that such innovations to the mask may well have positive value in existing use cases as well.

Verbal communication through such masks is a minor drawback; voices are muffled and require only slightly more concentration to be understood while using one.

More expensive versions of respirators have already mostly solved even that inconvenience, with diaphragms that allow for sound to escape about as well as without a mask.


Exactly! Mic and so on is not necessary and too complicated.


Yep, Check off one of the "might be problematic" aspects of using masks as an alternative to stay at home/social distancing in limiting spread.

Seems to get back to production output constraints and general validation of efficacy.

Said one-way valve makes it mostly ineffective at preventing aerosols from exiting, and in addition coughs or sneezes while wearing a mask will force air out around the seal.

The filter pictured (yellow label) is appropriate for VOCs and acid gases- acceptable for working around untreated sewage, but water droplets will land on the filter media, evaporate, and the virus particles will be pulled through by inhalation.

The large variety in industrial respiration needs has resulted in a moderate variety of masks and a large variety of filters for those masks. The amount of education required to get the public up to speed on what they need to wear will be extensive, and there's still a supply problem.


Yes, this mask is more of a symbolic pic, perhaps Simon can briefly explain why he chose this one (copyright issues I think). As explained in my original post and mentioned above you should use P3/ N99 or N100 filters.

Current understanding is that CoV-2 becomes nonviable when dried out Results form this study, though not in the document: (https://www.land.nrw/sites/default/files/asset/document/zwischenergebnis_covid19_case_study_gangelt_0.pdf).

Same guy stated it here: https://www.zeit.de/wissen/gesundheit/2020-04/hendrik-streeck-covid-19-heinsberg-symptome-infektionsschutz-massnahmen-studie/seite-2

They looked at about 100 infected households and sampled surfaces. Found virus RNA everywhere but NO viable virus. Also if that were false we would see airborne spread with people being infected over much larger spacial and temporal distance.

The public needs not be educated on what to wear, correct masks need to be provided by public private partnership (state money is used to stimulate large scale production) to stand a chance of equipping most people in a few month. You may want to use the original post linked.

It's one thing to say that C19 can't survive long enough on dry surfaces or in the air to infect someone over a few feet away. The time between a water droplet drying on a carbon filter and the virus particle being in the airway of the mask-wearer is shorter than the time delay between sampling a surface and testing it for viable virus. There's a huge difference between tests that would tell if C19 remains viable for a few seconds after drying out and tests that would tell if it remains viable on the order of how long it takes to inhale.


You might find https://jamanetwork.com/journals/jama/fullarticle/2763852 of interest in the discussions as well. Both related to the question of just how fast the droplets might dry in air and how well the masks might work in terms of protecting others.

As I read the bit we might need to consider the protection as asymmetric -- keeps more out than, possible, in even without considering an exhaust valve.

It looks like an exhaust port that incorporated a heat sink and moisture separator is plausibly more effective at preventing pathogen escape, but it has to be high-volume enough to pass a sneeze without it blowing out along the face.

Yes, this mask is more of a symbolic pic, perhaps Simon can briefly explain why he chose this one (copyright issues I think).

Yep, it's simply the first one in open domain that I found. I hope it's not too misleading; it should get the general idea across.

Using a picture of a product to illustrate a discussion about it would be fair use even if there were copyrightable elements of the picture.

The use of an exhalation valve means that the filter fails to capture some of the outgoing virus particles from the wearer's breath. Some types of N95 also have valves that cause the same problem. Depending on the positioning of the valve, maybe it's not a big problem? Or it might be easy to mitigate by covering it with cloth or something.


Yes, this is a drawback of any mask with exhalation valve. It is, as you suggest, easy to mitigate by covering the exhalation valve with cloth or a surgical mask. This is however imho not really necessary under most conditions, since the idea is to not get infected in the first place, so you do not shed any virus particles anyway.

"The idea is to not get infected in the first place" is not good thinking.

Given the long asymptomatic infectious period, both the "protect myself" and "protect others" effectiveness matter in a proposal for universal public wearing, although of course compliance is more incentivized by "protect myself".

That said, I don't question that these are good overall.

Well, if your chances of getting infected are drastically reduced, then so is the use of the "protect others" effect of wearing the mask, so overall these masks are likely to be very useful.

That said, a slightly modified design that filters air both on the in- and the out- breath might be a good idea. This way, you keep your in-breath filters dry and have some "protect others" effect.

Everybody wearing a respirator could be one of the equilibrium point of the social evolution under the COVID-19, though may be not the only one. Unfortunately, I did not figure other equilibrium point yet. To my best knowledge, nobody gives other end point of the social evolution in a rigors way.


Agree. My comments can certainly be seen as suggesting a starting approach and then refining that approach. However, I would actually expect to see a lot of zigs and zags, and possible multiple types of solutions that will work in different settings.

About the only thing I would say I have any high confidence about is that we need to start doing something. Masks, just like telecommuting for some, is one starting point. It might just end up being the crutch used to get over some hurdle. So I see things more as a stage where we will likely do some things that could be called trial and error approaches (but the error needs to be more on the "we cannot do this because I cannot stand doing X" side and not "we cannot do it this way because we just tripled R0!" side)

For sure people having a hard time breathing already will skimp on optional exit valve filtering, but you can sell it as helping-others to comply, and people will at least brag about how they're doing it.

The N95 mask does a poor job of keeping virus particles from being inhaled from an aerosol; the water droplets are largely caught by the filter media, and then evaporate, leaving the virus to be sucked through.

The virus isn't expected to spread by straight airborne transmission between people, but that's poor evidence that it won't be airborne from the distance from your mask to your lungs.


As EGI says that doesn't seem to be correct but he is talking about N99 not N95 so I'm wondering about this.

My understanding is that the N95 standard is about filtering 95% of the particles and the test level is for particle distributions with a mean of 75nm and standard deviation of less than 2nm. SARS-C0V-2 is thought to be 80 - 120 nm in size (and generally spherical). While the smallest would be close to the mean for N95 seems to suggest 95% should still be filtered (assuming the filter is not damaged).


Adsorption air filters are not sieves or membrane filters, particles are captured by adsorption to the filter medium, not by size exclusion. The pessimum of filtration efficiency is afaik around 1 µm with higher capture efficieny below that due to higher collision probability due to more brownian motion. Not completely sure of the numbers though.

I'd go with P 99 or 100 since they are not that much more expensive / unpleasant to wear and we want to have as little particle leakage as possible since we do not know how much dose reduction is needed to reduce infection probability by one to two orders of magnitude. A hundredfold seems plenty though.

Also note that virus particles do not fly alone since they are allway bound in liquid or whatever remains after the droplet dries. CoV-2 seems to be nonviable when dried though so you need not worry about dry stuff.


Interesting. While I knew some of the filtering was stuff just sticking together I had thought some level of particle size filtering was also present -- and not on a gross size scale.


See my reply to you above. That said, even if the virus was dry airborne as you assume, P3/N99 filters would still capture way above 99% of these particles as explained in my original post.

I don't think that it's safe to generalize "99% of particles" to "99% of virus particles". It's likely better than nothing and might even be adequate, especially since N95 seems to be the gold standard.


Yeah, it will be way more than 99% of virus particles since most virus particles are bound in larger droplets where filtration efficiency is much higher than 99 %.

And, as I've said above, I think that it's not sufficiently safe to assume that they inactivate within seconds of drying out.

I bought a half-mask and several filters almost two months ago, and it's definitely easier and safer for my once-per-week shopping trip than a single use mask. I don't think that it's a particularly effective general solution though, for the following reasons:

1. All of these are sold out. To make more, you need to manufacture both masks and filters.

2. All the replaceable filters are sold out. The filters need to be sequentially rotated or otherwise disinfected.

3. Wearing one for anything more than 30 minutes is still quite awful.

4. Most of them have exhaust valves which still spread the virus to others.


1. This was the point of my original post. States should begin stimulating large scale production of masks and filters to provide most people with such masks.

2. No they don't. Virus particles on the filter stay there as long as the filter does not get wet and decay quite quickly

3. Your mask either does not fit right or is low quality or has some kind of gas combination filter with very high flow resistance.

4. Yes, see discussion above.

Regarding point 2, how sure are you? Why are we even trying to disinfect N95 masks if that's true? I think your point is plausible but the filter technology in these masks isn't entirely trivial. Most filter materials actually depend on a static electric charge in the polypropylene to filter properly. Does the charge actually release the active virus particles after some time, and then you breathe them in? I have no idea. I was already surprised to find out that masks simply aren't just a dense material that filters particles, but a bit more complicated.


Pretty sure. You should not get your filter wet though since this may allow diffusion across the filter, which is why it is unsafe to wear (N 95 or other) fleece masks for extended periods. Also stuff that is bound in the filter is also attracted via Van-der-Waals forces which are really strong on this scale.

I bought 3M respirators and filters several months ago. I did not use it since I worked from home and did not go shopping. Those devices are more cheaper than N95 mask.


You have experience wearing a mask like above and are telling us it's awful to wear more than 30 minutes?

I have experience with wearing 3M masks and I found them comfortable enough for 2 hours at a time, 8 hours per day. My experience is entirely with activated carbon filters, and the inherently higher resistance of the P100 or even P95 particulate with oil filters might impact comfort significantly.


I don't have experience with US P99/100 filters but modern European P3 filters which are between P99 and P100 have hardly noticeable resistance.

In amzon, the 2097 filtering is more expensive than before. But still available.


Not much more expensive; less than 2x the price that ordinary users would have paid before. Grainger still lists them for regular price, and doesn't claim that every p100 filter is sold out yet, just the less-expensive ones. If it's worth the extra cost to pay for a Olive/Black/Magenta filter when you just want a Magenta one... https://www.grainger.com/category/safety/respiratory-protection/cartridges-and-filters


In reading the comments here I see mention of both N95 and N99. It might not make any difference which is used other than the expected filtration rate (95% versus 99%)

But those filter materials may be something of a bottle neck too (and clearly do compete with providing masks to the medical workers.

We here that X distance between people is the acceptable social distance to be safe from spreading the infection. Current X is 6 feet but I think some have said 10 to 12 feet is still possible for the virus travel.

Anyone know of any studies or models that would allow some form of conversion between X feet and mask rating? The X distances has to be largely about gravity (but these are not parabolic paths but like leaves and feathers). Based on that we should be able to say something like at 6 feet we expect y% of the virus to reach you. At 4 feet y1 and at 10 feet y2%. My expectation would be y2 < y1 < y <<< Origin.

The interpretation I have for the mask is that we say y at any distance = mask rating (95 or 99).

So the mask filtration we could use would be the equivalent of 6 feet (or 8 or 10 or 12 -- whatever the number might be). Or is the potential problem that at X feet the expected exposure is so close to 0 we can call it that so even N99 would not be seen as a good trade off/alternative?

Alternatively, we might find that at X feet the expectation is that we're looking at 80% of the exposure to be reduces so an N80 mask filter would be equivalent. Then any filtration above that would be acceptable.

It might not make any difference which is used other than the expected filtration rate (95% versus 99%)

It's harder to breath in a N99 mask then in a N95 mask. It seems that this concern is enough to get hospitals to use N95 masks over N99 masks in health care settings where health care workers are constantly exposed to symptomatic ill patients.


I think it has more to do with N99 not being available in the filtering facepiece form which are the medical standard with very few exceptions.


I do think we need to get front line people taken care of first -- and that will include both medical and other industries with dangerous environments we need to keep open.

That said, unless we're claiming the 6 foot (or pick your number) limit is equivalent or better than the preferred mask ratings for the industry don't we want to consider just what the mask rating equivalent to the social distance rule might be. We may well find that we have a lot of other alternatives that are just as good as 6 feet. That would pretty much free up all the N95 and above materials for those that really need them.

I really have to wonder if we don't have some really low hanging fruit that for some reason no one wants to pick. The world has already conceded that any mask is better than none but we keep making the comparison of masks to the "gold standard" mask rather than the social distance standard. The latter seems more appropriate to me.

I am starting to look for the source analysis from Lydia Bourouiba's research to see if that might shed some light.

The interpretation I have for the mask is that we say y at any distance = mask rating (95 or 99).

This simple conversion does not work, since the drop of particle concentration over a certain distance is not fixed and drop size is not fixed. Under very favorable conditions you may get an infectious dose over 60 feet though that is quite unlikely.

But those filter materials may be something of a bottle neck too (and clearly do compete with providing masks to the medical workers.

That is my point. We should start manufacturing masks and filter material en masse NOW so we can provide everyone with a high quality mask in 2 to 3 months. Noone does this at the moment and what is produced is wasted on crappy one way products even most professionals do not use correctly.

Is this more (or less) comfortable than cloth masks? I support any/all masks but my inclination is to focus on whatever we can get people to adopt. I doubt I'm doing what I can, including tweeting #Masks4All once per day https://twitter.com/thatMikeBishop/status/1246501797512056834

I would consider them about the same. I've worn mine for ~5 hours at a time with minimal discomfort.

I have one, and it's far less comfortable.

Besides, buying these sorts of masks new should be left to healthcare workers or ("essential" workers) and those that most need them. The rest of us should wear cloth masks.

Healthcare workers workers use disposeable masks. Reuseable one's like the above are a good solution for people who can use masks that are not certified for healthcare use.


Exactly, though we should aim to change that since protection would be improved while logistic demands would fall.

Also we should try to get governments to stimulate large scale production of such masks. See my original post.


This highly depends on mask quality and fit. A well fiting high quality oneshould be about as comfortable as a cloth mask.