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Face Mask Materials and Filters vs. VirusesDuring a time of surgical face mask and N95 respirator shortages, there have been many DIY homemade masks created from available fabrics, and some small businesses have been selling face masks which are actually not very effective. Indeed, an ineffective face mask can do more harm than good if the wearer believes they are well protected when they are actually not, and takes additional risks in public. Scientific tests have been done on alternative clothing materials which people may have at home and cut to make DIY homemade masks, during times of shortage of certified face mask materials. For example, in one study, five major categories of fabric materials were tested, including sweatshirts, T-shirts, towels, scarves, and cloth masks, and compared with N95 respirator materials. The results, in summary, are that of woven fabrics tested, they performed very poorly against viruses, and nowhere close to either an N95 or a sealed surgical face mask. However, just facial tissue combined with kitchen paper towels actually performed nearly as well as surgical masks. First, let's look at an N95 mask compared to some common clothing fabrics, according to one study:
Source: Simple Respiratory Protection—Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles The materials were tested against particles in various nanometer size range ranging from 20nm (0.002 microns) to 1000nm (1 micron). The COVID-19 coronavirus is in the approximately 100 nanometer range, so I've added a red line for this size range. The only material which gave good filtering was the N95 filter, which allowed only a tiny amount through, which is the only line at the bottom of the graph. All other lines for different materials show high penetration rates of virus size particles. They also tested it against a well marketed commercial face mask, and the results of that were not so good. I would prefer not to quote any particular brand name or model of face mask on the market. I just would urge people to look for certified N95 mask materials and/or use professional surgical face mask materials for a mask, if you can find them, and if not then at least try to analyze commercial masks using alternative materials in a more informed way. Above, towels performed best as alternative materials, followed by sweatshirts. However, even they perform poorly against N95 material, and also performed poorly against just thin surgical face masks (see below). Surgical mask material generally does not perform as well as N95 but is still usually much, MUCH better than alternative materials, letting through only about 3% to 7% of 100 nm particles for a sealed face mask, compared to roughly 40% to 90% for alternative materials. For example, tests were done with manikins with surgical masks:Manikin-tests-1
However, if the surgical mask is not sealed against the face, then there is a lot of air leakage around the nose and cheeks. Manikin tests came up with this:
What a big difference!! A sealed surgical mask on a manikin resulted in 3% to 7% of 100nm particles being breathed, whereas without a face seal it became around 25% to 35% inhalation of 100nm particles for normal breathing (and lower percentages for heavy breathing, probably because stronger breathing sucks the mask to make a better seal). If a surgical mask is giving you 95% protection, then wearing 2 surgical masks together, sealed well, might give you around 99% protection. The most common material for N95 and surgical masks is a "nonwoven" polypropylene fiber which is treated so that the smallest particles will tend to stick to the mask fibers instead of bounce off (by microscopic forces, called van der Waals forces, or electrostatic attraction), and nonwoven materials make the air coming in to take a very winding path to go through, thereby maximizing the probability that a particle will have a collision with the fibers and stick. Woven fabrics such as scarves, sweatshirts, and ordinary textiles simply haven't come close to these nonwoven polypropylene filters. Here are some images I took myself with one of my microscopes, whereby you can see the difference between woven vs. nonwoven, as the t-shirt is woven (2 photos, zoomed in and zoomed out) but the surgical mask is nonwoven, and the paper towel and facial tissue are also nonwoven (and discussed further down this page):
Woven t-shirt, zoomed out:
Woven t-shirt, same one, zoomed in:
Non-woven 3M N95 respirator face mask, zoomed out, where you can see the spot welds to hold it together:
Non-woven 3M N95 respirator face mask, zoomed in, where you can see both the filter and a spot weld to hold it together:
3M N95 respirator mask, zoom in, whereby you can see trapped particles which have stuck to fibers rather than captured by any smaller aperture. This is a mask I used for a long time against pollution such as while walking in downtown Bangkok and driving on the highway with diesel trucks. (It is a soft V-shaped mask which fits under my face mask sealer, not a more rigid type.)
Non-woven surgical mask, zoom in:
Paper towel, zoom in:
Facial tissue, zoom in: However, polypropylene materials have been processed various ways to improve their suitability as particle filters, such as to make the fibers more attractive to nanoparticles, to make it attractive to water moisture (hydrophilic) for an inner layer to absorb the wearer's outgoing germs, and to make it repulsive to water moisture (hydrophobic) for an outer layer to protect the surgeon or other medical staff from blood and other things which might come from a patient. One way you can test whether a surgical mask is genuine, not a cheap copy, is by trying to put water on the outside surface, and comparing it to water on the inside surface. If you are sick, then face masks made of ordinary woven fabrics are better than nothing, if that's all you have, and might give some protection to others but not a lot. This is because when you cough or breathe, many of the largest particles coming out of your mouth and nose can be caught by your mask, instead of falling onto table surfaces, hand rails, and other things. However, viruses can easily leak out from an unsealed mask, and there can be a lot of spray coming out of the face mask fabric. Coughing into a handkerchief to protect others from yourself can be a lot more effective than breathing through a handkerchief to protect yourself from others. For these reasons, if no surgical face mask nor N95 mask is available, then it is better to wear another mask of available cloth then no mask at all, so I don't want to criticize other materials too much. It is at least considerate of others. However, it's much better to use a mask made of surgical face mask polypropylene material, or a certified N95 materials if available, than an ordinary cloth. Use an ordinary cloth only when there is no other option available, and understand the relative limitations of many masks being sold on the street. Don't get a false sense of security. A thick ordinary cloth with many layers is probably going to be very effective against coughs, but then you have the dirty hands handling the cloth which then touching things such as door knobs ... Disposable Tissue Paper MasksIn March 2020, another group of researchers tested tissue paper and kitchen paper towels as potential disposable mask material when there is a shortage of disposable N95 and surgical masks. Surprisingly, they found that these nonwoven paper tissues work much better than many woven fabrics, and that a mask made of 2 layers of paper towels plus 1 layer of tissue paper against the face actually filters 100 nm viruses almost as well as the surgical mask material they tested, as shown in this graph:
This is very exciting because kitchen paper towels and facial tissues are abundantly available, cheap, and they can just be used with our face mask sealer. No need to add straps, no need for any sewing machine. Our face mask sealer will hold them against your face, and seal around them. Actually, it makes sense, because paper towels and tissue paper and designed and processed to absorb things, so they can absorb viral material, too. Instantly, we have a fairly good solution to face mask shortages. We just needed a way to hold this facial tissue and paper towels against the face, and seal them. We have that in the reusable Cross Strap Spongy Face Mask Sealer. Just replace the facial tissues and paper towels from time to time with usage. Another AnalysisAnother research team took a somewhat different angle, not only (1) counting the particles trapped by various filters, but also (2) counting particles from participants in a "cough box" since coughing introduces unusual stresses, and also (3) measuring the pressure drop across each filter. The pressure drop is of particular interest because if a mask is not sealed well, and if there is more resistance to air flow thru the material, then more air will simply go around the mask, both in and out, unfiltered, viruses and all. Also, a good filter such as a vacuum cleaner bag (which I find overly promoted in some internet postings) might not be usable enough if people cannot breathe well enough through it. Some of the details are particularly interesting. They tested various household materials against two things:
For comparison, the COVID-19 coronovirus is around size 100 nanometers, which is of size in between the other two, so the range is relevant. It is interesting to compare how the materials tested against these two very different sized particles. Below are two different tables, easy vs. difficult for many laymen. First, I present their data, which might be a little overwhelming to some laymen, and below that present a much simplified table of my own where I removed many columns, sorted by filtration efficiency from high to low, and rounded numbers.
Their more difficult and complicated table if you're a layman:
My extraction and summary: Some interesting notes:
A "tea towel" in British is basically a "dish towel" for most of the rest of us. These were British researchers. It's difficult to breathe through a quality dish towel, and doubling it does not seem realistic to me. Clearly, surgical masks are well designed for maximum filtration with minimum air resistance, as regards materials. Notably, they pushed air through these materials at a much higher rate than usual breathing rates, so the pressure numbers are not really this strong at normal breathing rates. However, when coughing, the pressure difference is so high that it could result in a lot of outward leakage of virus if the mask is not sealed well. "This instrument was used to deliver the challenge aerosol across each material at 30 L/min using the method of Wilkes et al2, which is about 3 to 6 times per minute the ventilation of a human at rest or doing light work, but is less than 0.1 the flow of an average cough."Davies-1 My take on their study is on some of the details, but I should also note their own summary, too, though I've added emphasis by shaded something in yellow and outlining it in red myself:
They did not test paper towels and facial tissue, unfortunately. The Hong Kong study referenced further above on this page found that paper towels combined with facial tissue was a significantly better filter than T-shirts and other household materials. AbsorbencyMaterials with high absorbency in general may perform better. If so, that would explain why kitchen paper towels and facial tissue would work so well, if made thick enough so that the viruses must bounce around more before they could penetrate. Also, in the experiment cited above, it was a tea towel aka dish towel which worked best, maybe because it is designed to absorb water to dry dishes or any tea spills, and is very thick. Cotton is a natural fiber suitable for clothing which is highly absorbent to water, but many artificial fibers are not. Fibers can be combined to provide various properties such as resistance to wrinkles, feel to the skin, desired water absorbency (hydrophilic) vs. water repellency (hydrophobic), antistatic, and other properties. Tissue papers such as kitchen paper towels and facial tissues are made of dried cellulose and extremely absorbent, as that's their main purpose. They are heated during the manufacturing process to become exceptionally dry. They absorb not only water but also oils and many other things. Viruses are basically a combination of proteins, lipids (fatty acids), and RNA. They tend to hold together due to ionic rather than covalent forces. Water tends to stick to many things because of its polar nature. I believe what we might be seeing in the data above is the effects of absorbency, which applies to both water and viruses though to different degrees. Misleading PromotionsOn the internet and on YouTube, I see too many promoters of face masks, with various credentials such as surgeons, saying that their own masks filter some high percentage (maybe 99%) of particulate matter, according to government standards, and things like that, when actually many such simple statements might not be relevant to 100 nm viruses. Some government standards can cave in to industry interests, and are not specific enough, so it's good to analyze exactly what they mean, and whether or not it applies to 100 nm viruses. Research scientists have raised these issues, too, not that it's made a difference in the politics and administration, much less public perception, but details do matter when it comes to the smallest particles. For example:
Unfortunately, I've seen certified doctors and others speak like they are an authority advising the public, overconfidently, when their wording is unclear and makes me question whether or not they really know the deeper scientific details. It is just too nice and easy to be a credentialed "expert" who just associates oneself with an establishment authority. What really counts is how well a filter captures 100nm particles like a virus, so be careful about believing what some businesses and spokespeople may say, and look for careful wording and attention to detail. Physically, there are different ways that filters capture particles, and it can vary significantly by size and the nature of the particles. For the tiniest particles such as viruses, it is by so-called Van der Waals or electrostatic forces whereby the viruses stick to fibers, such as the treated polypropylene used in professional surgical masks and most N95 masks. If you had a bag of particles, with 1 particle of size 10nm, 1 particle of size 20nm, 1 particle of size 30nm, 1 particle of size 40nm ... all the way up to 2500 nm (PM 2.5), i.e., ... 2480nm, 2490nm, 2500nm, so that you have a bag of 250 particles, with one particle of each size, and if you pass them through a filter which caught everything down to 300nm but let pass through everything under 300nm, then you would still be filtering over 99.99% of particulate matter by weight (mass), but only 88% of all particles by number. How do you think the manufacturer would promote their filter, as 88% or 99% efficient, to try to compete with the competition? (For simple calculation purposes, this is based on theoretical spheres of the same composition.) That said, many professional N95 masks made with nonwoven polypropylene fibers actually capture more than 95% of viruses and tiny nanoparticles which pass through them, by number of viruses, too, not just mass (weight) of particles, and they actually capture the tiniest particles quite well, because they are purposely made from materials which viruses tend to stick to. However, some other masks claiming 95% or higher filtration could make such a claim against particles in general by measuring the total particulate matter by mass (weight) but it might not actually apply to tiny viruses. The results of tests on facial tissue and kitchen paper towels against viruses, which are nonwoven paper fiber materials, is of keen interest because they were tested against particles of the size of the COVID-19 coronavirus and performed well, unlike many woven fabrics currently being used as face masks. Street and Online MasksI don't want to criticize too much the home made masks or commercial masks I see, as long as they are made by a good faith effort, because something is better than nothing, and if that's all that people have access to, then wear them. However, it may be wise to also put good facial tissue and/or kitchen paper towels inside. If you are sick, then a homemade mask of any kind can significantly reduce your ability to give viruses to others, because particles are on average much larger when they first come out of your mouth and nose, so many are highly likely to be captured by any mask next to your face. However, it might not protect you much against others' viruses from particles small enough to stay suspended in air. A good thing about DIY home made masks is that they typically have no exhale valve, unlike many commercial masks. An exhale valve basically lets your breath and cough out without any filtering -- they bypass the filter, so if you are sick, then breathing out is almost like wearing no mask at all, even if your mask is an N95 mask but has an exhale valve. Masks with exhale valves are not good to wear in public during a pandemic. This is an exhale valve on a mask:
Masks with exhale valves are okay for protecting you against pollution and industrial chemicals, but they are not good to wear for protecting other people from yourself if you are sick. I have seen masks with exhale valves promoted for the COVID-19 epidemic, unfortunately. Actually, if you wear a second mask underneath, that may be okay, but wearing only a mask with an exhale valve can put other people around you at risk. If you see somebody wearing a mask with a valve, with no other mask underneath it, stay a longer distance away, and please point it out to them that their exhale valve could put others at risk. One of the ways I analyze home made masks is their resistance to air flow. If it's difficult to blow air through them, and if they do not seal the face, then you can be fairly sure that much of the air is just going around the mask, not through its filters, both going out and coming back in. I've also seen pockets for removable N95 and other filters in masks which are poorly designed, whereby people have been impressed with the removable filter feature without analyzing the rest of the mask. Any such pocket should be very easy to blow air through when there is no filter present, and the rest of the mask where there is no filter should be impossible or extremely difficult to blow air through. Unfortunately, I have seen many masks which are not like this, so that it's obvious that a lot of air is coming in without going through that filter, such as passing through other parts of the mask, and/or bypassing the mask because it's not sealed. Likewise, exhaled breath and cough material may bypass the filter of a poorly designed mask. A bad mask against a pandemic is no laughing matter. This is serious. Bad masks need to be improved. When I see bad masks, I can only wish that both consumers and manufacturers had analyzed things better. Whether it's just plain profiteering or innocent and well intentioned layman misunderstanding is something you can guess yourself on a case by case basis. It's good to try to inform well intentioned people so they can improve the design of their masks. The same thing applies to filters. Many have ratings stamped on them, but you should take a close look at the materials. I don't want to criticize vendors who sell masks or mask materials which may say PM2.5 or something like that, with no official certification, because on such short notice it simply may not be feasible to get government certification, which is a long and expensive process, and we might not have time for that during a pandemic with a shortage of masks. Something is better than nothing. However, it should look like a quality material such as a nonwoven polypropylene. If so, then maybe they might work well. It was originally my hope to find some abundant material which would be effective against viruses such as COVID-19, and to recommend the best materials, but reading the scientific literature, I have not found anything I would use besides N95 and surgical face mask polypropylene materials, based on the scientific research data I have seen to date, except a home made mask made from layers of plain facial tissue and kitchen paper towels which can work fairly well. If you see something else, please let us know. The problem is when N95 and professional surgical mask materials are in low supply. Perhaps factories which produce nonwoven polypropylene materials can produce large volumes of these materials for masks despite not having the time to get official government certification. A large factory should have the resources to test these masks, such as getting particle counters used for pollution monitoring or other things, and connecting the masks to the exhaust pipe of a diesel engine, and counting the number of particles in the gas stream with and without a filter. Diesel exhaust has a wide range of particle sizes. While diesel exhaust is not viruses, diesel engines are abundantly available for testing, unlike other particle generators, and it is relatively quick and easy to put together the test system if you have particle counters, too.
For example, Google this: Scientific literature on face mask testing details alternative methods for estimating the effectiveness in filtering viruses, and it would be good to acquire the necessary equipment for that, but until that is ordered, received, set up, and working properly, it's better to use a currently available diesel engine and whatever particle counter equipment is currently available than to not test at all and just wait. The Bigger PictureWhat about an analysis of airborne particles from sick people -- from coughing, or talking, or just breathing? Some of the results from just breathing are very surprising, and the result is alarming. What about viruses from people who are infected but not showing any symptoms yet -- no coughing, no fever? Evidence indicates they might be significant spreaders. This and more is on our page on Minimum Infective Dose.
You should see our page on air total inward leakage in face masks
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