An exhaustive (and exhausting) list of mask filter options, with pros and cons
In part 1 and part 2 of this series, I talked about the new recommendations regarding mask construction and procedures, the disposable mask issue, and the pros and cons of 3-layered masks vs. 2-layered masks with filter pockets.
This time, let’s talk mask filters. Specifically, what your options are, their availability and safety, and why you might or might not want to use them. Of course, this isn’t a complete list, because if you can name a material, someone’s probably gone and tried it, however bonkers. These are just the things I’ve seen making the internet rounds.
DISCLAIMER: As always, nothing in this article is meant to be official medical or safety advice. Everything presented here is either publicly available information that’s considered accurate at the time of writing, or just my opinion. If you have health-related questions regarding the pandemic (or anything else), you should seek answers from your health care professional, not a sewing lady off the internet.
The boring science-y bit (skip at your own risk)
Sorry, I know this is kind of dull, but it still has to be said. Getting proper, reliable data from studies take time and funding and replication, and this pandemic is still pretty new in the grand scheme.
The good news is that lot of studies regarding mask materials and safety seem to be in the works, but I wasn’t able to find many that have been published and reviewed, never mind reproduced. This means there’s a lot of conflicting information out there, and what there is suffers from fuzzy definitions.
In the interest of not making this blog the length of a bible, I won’t be dwelling on the technical details of the studies I did find. OR going into detail on a laboratory’s set-up of the equipment. Or digressing into why sometimes it’s harder to capture larger particles than the smaller ones. OR complaining (much) about the non-standardized terminology some sources use when they publicize their findings.
However, both the scientist and seamstress parts of me are somewhat outraged when I see test items like “kitchen towel” or “cotton T-shirt.” How thick a towel, what weave density, and what fibre content? How thick a T-shirt, what kind of knit, and how tightly stretched was it, which hugely affects the space between thread loops in knit materials?
The point is that no one REALLY knows how well any of these materials work in real-life face masks right now, and even when there’s data being quoted, it might not mean what it sounds like. One T-shirt could block only 10% of tested particles, but a heavier one might block 80% but only if you stretch it no further than 12%, and so on. Test results for the same listed item (like “cotton T-shirt”) can vary wildly from study to study and source to source, partly due to that aforementioned lack of standardization. Basically, trust no one, and use your own best judgment.
Filter material safety
I’ll mention safety information provided by manufacturers or retailers when I have it, but it should all be taken with a giant pinch of salt. Different sources sometimes have conflicting opinions on an item — even on the exact same brand of an item. Read material safety info and/or data sheets yourself whenever you can find them, but remember they don’t tell the whole story either.
For your reference
A naked SARS-CoV-2 virus (not encased in a droplet of fluid) runs about 0.05 to 0.15 microns, depending on who you ask. Wearing the droplet, it’s 3-20 microns, give or take.
Whether it can get airborne without its protective droplet coat and survive for any length of time is currently a question mark. When we know this for sure, we’ll be much better equipped to decide what works and what doesn’t.
List of Potential Filter Materials.
With that out of the way, here comes the actual list of potential materials.
Non-woven polypropylene or “PP” fabric (the kind used in some reusable shopping bags)
PROS: This is what the official Canadian recommendations say is the preferred choice for a third layer. Available, if only by chopping up shopping bags (check to make sure it’s the right kind, though). Washable and reusable. Good breathability, good enough so you can use multiple layers to boost effectiveness if you’re so inclined. Higher-quality PP fabrics are pretty durable, though not like, say, high-thread-count cotton is.
CONS: PP fabric can develop thin spots or otherwise lose structural integrity in ways that aren’t obvious unless you look closely. Meaning you really have to keep checking. Some, though not all, shed little flecks and bits as they get more worn. One test found PP grocery bags only captured 11% of particles 0.3 microns and under, which is worse than bedsheets, kitchen towels, and coffee filters, among others — remember, that’s just in *this* study (others could say something totally different).
Cotton T-shirt material
PROS: Cheap, easily available, and easy to DIY into reusable, washable filters. Knit materials don’t fray, so you don’t need to finish the edges of your self-cut T-shirt filters. Very breathable. Mostly safe to have on your face. Using a double layer boosts effectiveness, and is doable for some, since it is so breathable. According to the Cambridge study (if you’ve been looking up mask material studies, you’ll have seen this one by now), one of the best for capturing droplets. However….
CONS: One of the *worst* materials for catching particles 0.3 micron and under (only about 3%) according to a more recent test, though this one apparently used a much thinner and more lightweight material than the Cambridge study (remember what I was saying about standardization? The specifics really do matter). Pieces cut from T-shirt knits tend to curl up when laundered, so can be a real hassle to insert into filter pockets.
Those blue shop towels
PROS: Fairly easy to find in home improvement stores. Easy to cut up into your own disposable filters. Okay breathability, but not great. Some tests say blue shop towels, especially those made from polyester hydro-knit or polypropylene, are quite effective at filtering droplets, though possibly less so than regular paper towels or cotton sheets.
CONS: Not reusable. Some safety data sheets say they contain toxic chemicals and/or carcinogenic particulates (if you inhale them, that is, not when you wipe your hands), while others don’t. This is a case where different sources don’t agree on the same product. One source says the tested brand filtered under 20% of 0.3 micron and smaller particles, which isn’t stellar.
Common-o’-garden kitchen paper towels
PROS: Surprisingly good at filtering larger virus-sized and droplet-sized particles, according to at least one source. Inexpensive, easily available, and easy to make DIY filters. Quite breathable.
CONS: Not reusable. Not as good at filtering Coronavirus-sized particles as droplet-sized ones. One source said paper towels filtered 33% of 0.3 micron particles (which is actually better than lots of other materials, but not great). So how well this works may depend on whether viruses can survive in the air without that droplet. Doubling paper towel layers doesn’t help much with effectiveness.
PROS: Cheap and easily available. Said to be very effective by at least one test, though results may vary between brands. Comes pre-cut into manageable sizes. The American CDC recommends it as a disposable layer for improvised face coverings, though they say nothing about why. Doesn’t cause moisture condensation.
CONS: Not reusable. It might just be me, but I found coffee filters REALLY hard to breathe through, which is a huge problem for those who agree with me.
Felt, synthetic or wool
PROS: Easily available and cheap if you stick to synthetic craft felt. Reusable and washable, though durability differs by type (wool is durable, synthetic less so). It doesn’t fray when cut, so you don’t need to finish the raw edges. Reasonably easy to breath through, though this will vary depending on thickness and density of fibres. Wool felt wicks moisture VERY well, and prevents condensation. Some sources say felt’s tangled structure makes an effective barrier for pathogens.
CONS: Can be expensive if you choose wool felt. Synthetic felt can cause moisture condensation, and some kinds may not hold up through multiple washings.
PROS: Easily available and made from non-woven material.
CONS: Full of who-knows-what chemical additives, and usually fragrances, none of which were meant to be snuffled close up. Breathability is hard to test through all that. I wouldn’t do this one.
Interfacing, non-woven (woven interfacing is just general fabric, really)
PROS: Meets the “non-woven fabric” recommendation for one of your three layers. Reusable and washable, at least somewhat. Comes in lots of different thicknesses and finishes, so you can pick one you like. Lightweight ones are fairly breathable, though possibly less of a barrier than the heavyweight ones. Doesn’t fray, so you don’t need to finish the edges. Easy to find in a fabric store or online.
CONS: Can get expensive, and heavier ones definitely don’t breathe well. Not durable through multiple launderings. Some types can develop irregular thin spots when washed. If anyone’s specifically conducted a test on interfacing’s filtering ability, I haven’t been able to find it.
PROS: Easy enough to get, if you know what it is. Washable and reusable. Cozy for outdoor winter masks, since it’s often used to add warmth to sewn items.
CONS: Nobody’s tested it that I know of, but since batting is actually designed to have lots of air space in it, I can’t imagine it would make a good barrier for anything. Also, washing and drying sometimes creates irregular thin and thick spots. Too warm to wear indoors, and not too breathable.
Dust and/or pollution filtration mask filters (usually rated PM2.5)
PROS: At least one study found these highly effective (97+%) at blocking tested particles, including air pollutants. Since they’re already meant to go into a mask, they’re reasonably safe to wear on your face. Supposedly quite breathable (I wasn’t able to get any, but it seems logical, for the same reason).
CONS: Supplies seem spotty right now. Can be expensive. Not reusable. The study didn’t test for particles as small as a solo coronavirus when it’s not encased in a droplet, so we have no idea how these performs against airborne viruses (many air pollution mask filters are rated to block 2.5 microns, which is a LOT bigger than the virus that causes COVID-19, but smaller than the droplet in which the virus usually travels).
HVAC system filters/home air filters
PROS: Fairly easy to find at home improvement stores. One layer is more breathable than you might suppose. Can be quite effective as a particle barrier, though how small a particle can get through depends on its rating (most will stop anything droplet-sized, though). One test found HEPA filters stopped more than 80% of 0.3 micron-sized particles.
CONS: Not reusable. Expensive. Not tested for safety as a face mask material. Even true HEPA-type filters aren’t usually rated to stop particles much smaller than 0.3 microns. Most manufacturers explicitly recommend against using home air filters for face masks. Some may contain components which are unsafe if inhaled.
Vacuum cleaner bags
PROS: Available, if somewhat spottily right now. Some health professionals have recommended HEPA vacuum bags as an option for face masks. Studies show they are very good at capturing even small droplets and large viruses (but not coronavirus-sized ones).
CONS: Not reusable. Not tested for safety in face masks; some do contain materials that may be quite dangerous if you inhale them. Most are very hard to breathe through, which creates problems all by itself. Personally, I’d stay away from this one.
Wet wipes/baby wipes/pre-moistened towelettes
PROS: Er…They come in a carrying case?
CONS: They’re *wet*, people. Wet = bad in masks, even if they’re wet with just water or something fairly harmless. Some are soaked in an alcohol-heavy solution, which may be virus-unfriendly, but it’s also unfriendly to your airways. Just don’t. I wouldn’t even have mentioned this one, except it really is floating around out there.
Pads/sanitary napkins/panty liners and disposable diapers
PROS: Still widely available.
CONS: Not at all breathable, even the thinnest types. Many contain chemicals or gelling agents which are NOT intended for you to inhale. Never even tested for effectiveness at filtering pathogens, as far as I can make out, but it surely doesn’t matter, as no one can breathe through these anyway. Another option I was surprised to even see existed.
Bandanas, socks, handkerchiefs, scarves, towels, bra pads, underwear, nylons, etc (assorted items of clothing or household fabric goods)
PROS: Everyone has them lying around. Usually washable. Some (and only some) of them probably work as well as anything else. In general, breathable material with a very close weave or tight knit pattern will do okay as barriers (you should NOT be able to see any spaces between threads or yarns when you hold it up to the light).
CONS: How effective any of these things may be depends ENTIRELY on its individual characteristics, so it’s all on you to decide. Loose-woven or loose-knit materials make terrible filters. The cut edges of some fabrics will need to be finished to prevent them from disintegrating when washed. Incidentally, cotton bandanas were cited as some of the worst materials at blocking particles by some studies, though they didn’t say what weight or weave (cheap bandanas are normally made from thin, low-thread-count cotton).
Gore-Tex (and other ePTFE fabrics)
PROS: Lets air and water vapour through, but not liquid droplets. In principle, this makes ePTFE fabric both waterproof and condensation-proof (exhaled moisture should pass through instead of condensing on it). I couldn’t find any studies regarding its effectiveness as a viral barrier, but it’s essentially a uniform film with micropores, so there are unlikely to be any thin spots or gaps. Very durable in structure.
CONS: The manufacturer specifically states that this fabric doesn’t allow for enough airflow when used in a face mask, and that it doesn’t protect against airborne pathogens. Expensive. Detergents can damage the waterproof qualities. Air-permeable or not, I find it hard to actually get a lungful of air through Gore-Tex, which in itself makes it a bad option.
Surgical sterilization wrap (Halyard and Medline are a couple of manufacturers)
PROS: Extremely effective at blocking particles (and just about everything else), which isn’t surprising, as this is what they use to keep instruments sterile for surgery in hospitals. Technically, it can block more particles than N95 masks. Definitely ticks the “non-woven polypropylene” box, in spades. Comes in lots of different weights. I don’t have access to any right now, and I can’t recall ever trying to snort air through it when I did, but I’m told it’s reasonably breathable.
CONS: Hard to source, unless you have a line on surgical supplies, and expensive if you do find it. Not reusable. Not actually tested for safety or effectiveness in face masks, though at least a few medical professionals are trying out the material in masks for themselves, so we’ll see if anyone does a study. The manufacturers say it’s not intended for masks, and they can’t support its use for the purpose.
The redundant option: Tightly-woven breathable fabric, like quilting cotton
Note: the reason I feel kind of silly mentioning this one is that if you’re going to use this as a filter, you might as well just make a three-layer mask and skip the filter pocket altogether. But if you want to use high-quality cotton as a removable filter for reasons of your own, here are the pros and cons.
PROS: Super-breathable. Most sources that tested this material found it to be at least moderate to good at capturing particles. Easy to find, very durable, washable, and available in lots of colours, finishes, and weights. Safe to wear against skin. Moisture won’t condense on it.
CONS: Cut edges will fray, so you’ll need to finish them in some way. Not a non-woven polypropylene-type thing, so not the first choice for official government recommendation for the third mask layer (they do mention cotton and linen as acceptable alternatives, though).
So what would Felix and Kitty use?
Your aged aunt Kitty doesn’t even carry a phone or a purse, because she’ll forget anything that isn’t sewn on. In the same vein, I just know that I’d forget to pop a filter into my mask pocket every time I step outside for some kibble.
For myself, I’m sticking with a mask made from high-thread-count, good-quality, breathable woven fabric. Only now, I’ll use three layers instead of the previously recommended two. Felix will most likely do the same, because he likes masks with pleats, which don’t play well with removable filters.
I’ll most likely switch entirely to a style with lots of 3D space over my nose and mouth, like my Anteater or Ziggurat masks, because that extra layer means I will want all the extra breathing room I can get. In fact, I’ve started to test out a few other styles with even more nose clearance, just in case we need them.
If I *had* to pick a filter for some reason, I’d try out the surgical wrap, if I could get it and assuming there was no danger of hospitals running short. Of the more easily available options, I might try out wool felt and thick cotton T-shirts (they seem safer to me), and in a pinch, plain old kitchen paper towels.
Why NOT polypropylene, like the official recommendation says?
This is just personal, but I don’t entirely trust a lot of non-woven materials. Over many years of sewing, I’ve observed that many shed some dubious bits and bobs (aka fibres and particulates) when they get a bit shabby. You see, dear Kittens, PP is basically a thermoplastic, and I feel like we swallow/breathe enough microscopic plastic particles every day without adding even more.
Also, some PP fabrics develop thin spots as they get worn, which are a hazard in masks. If you have a reusable PP shopping bag that’s been through the wash a dozen times or more, try holding it up against the light. Quite often, you’ll see semi-translucent patches. Compare that with a solid cotton bedsheet you’ve washed and dried a hundred times, and you may start to see my point about durability differences.
This has gone on far longer than any blog should, so I’ll wrap it up. Though I have much more to say on the subject of masks, filters, and updated safety data, which I’ll spout sooner or later.
I’ll put out that tutorial on retro-fitting filter pockets as soon as I can come up with a no-machine-sewing method that halfway works. It’s turning out harder than I anticipated.
And hopefully, I can come up with a 3-layer/pocket-enabled, 3D mask style that 1) provides as much breathing space as my Ziggurat masks but 2) can accommodate a filter for those who want to use one and 3) is a bit simpler in structure, so it’s more affordable to buy and/or easier to produce for donation.
Until next time, stay safe and warm, and try not to huff any thermoplastic crumbs….