What Makes an Effective Face Mask? Investigating Fabrics and Filters
Founder member and self-confessed 'mask nerd' Laetitia Adkins takes us through the pros and cons of various commonly-used mask materials and filters.
Oh my days. All of the information on all the different materials, filters, types, etc. etc. etc. that you can get in a mask or put in a mask is OVERWHELMING (and some of those that get “recommended” can be quite bad for you). So we’re going to do a breakdown about the different materials you might find or put in a face mask.
Before we go into this, it’s worth saying that if you are making a mask, always test the breathability of your materials as your number 1 priority, because breathing happens to be your number 1 priority anyway. Even if you’re not thinking about it most of the time.
Your second priority should be avoiding materials that shed small fibres, or contain chemicals that aren’t designed to be next to your skin (let alone your face). Small fibres and strange chemicals are an inhalation risk. No point avoiding one batch of deadly particles if you’re just going to put another source of them right next to your nostrils.
This series has been written by one of our three masketeers, and accidental face mask designer, Letty. Since coronavirus hit the UK she has developed a habit of scrolling through nice instagram photos one moment, and then finding herself in a deep-dive of face mask material science 5 minutes later. She has a degree in physics and a wardrobe half-filled with homemade clothes, so it seemed like a natural process to co-develop an extremely decent non-medical face mask.
So, let's look at some of the post common mask-making materials.
Two cotton layers has become the industry go-to for face mask construction. The key reasons for this are that tightly woven or knitted cotton is very breathable, very easy to wash, has some filtration efficacy (more than say, a chiffon scarf), and pretty much everyone is able to source it (from a t-shirt, for example). Two layers are slightly more effective at filtration than one layer and are still easy to breathe through. Three layers don’t offer much improvement in filtration efficacy, but they are much harder to breathe through. When choosing a cotton fabric, tighter and thicker are better. That's what she said ;).
I am a big fan of thick unbleached 100% cotton calico or canvas - it’s super cheap, readily available, and breathable. When Smart Air tested a bunch of readily available materials, they had this to say:
“One material that’s easier to breathe through than a surgical mask, and still performed fairly well at filtering particles was the 0.4-0.5mm thick canvas material. This material even performed better than the 100% cotton T-shirt.”
“Non-woven” is typically the standard in recommending a filter material these days. If you look at the clothes you are wearing, you will likely see an orderly mesh of threads that indicates whether the fabric is knitted or woven. A non-woven fabric looks like all the fibres have just been smushed together. It’s this property of smushed fibres that makes the material a better filter.
Lightweight non-fusible interfacing is a great example of a non-woven breathable fabric, that is affordable, safe to use, washable, and easy to source from most fabric shops.
Empress Mills (a UK fabric shop) actually got in touch with Freudenberg, who are the parent company of Vlieseline (Vilene) - a widely-available interfacing brand. They had this to say: “We have been in contact with Freudenberg, the parent company of Vlieseline (Vilene).
They have informed us that their samples show that Vilene S13, Vilene M12, Vilene L11 and Vilene F220 are best suited for the construction of simple face masks – again these are not ‘medical grade’ options but will hopefully offer some protection.”* I have personally found 2 layers of Vilene L11 (the lightweight option) to be a good option for an inner filter - it’s sew-in (so no risk of inhaling any glues from the iron-on version), breathable, and passes the domestic aerosol test devised by Georgia Tech University (sounds fancy, but you just spray water through the material onto a mirror, and see if it mists).
*some of these are fusible versions, which are not recommended due to risk of inhaling the glues coating them.
Spunbond non-woven polypropylene (NWPP) is becoming a popular choice as a material that can be easily breathed through, and can filter small particles well. It is water-resistant (hydrophobic), electrostatic, and washable.
A medical grade lightweight (20 – 25 gsm) NWPP is used in commercial masks and respirators. This material shouldn’t be used to make non-medical face masks, because because of PPE shortages - keep it for our doctors and carers!
There are lots of other non-medical grade options available, though. A particularly good source of NWPP can be found in reusable conference or grocery bags. The material you are looking for should have a diamond-shape dimple pattern, and the construction label should read 100% polypropylene. NWPP grocery bags (if you can find them - I think they are more common in the US than the UK), are particularly good because they are made with the intention to carry food - this means that they are more durable and less likely to shed small fibres.
Melt-blown NWPP should be avoided because it is less durable, and more likely to shed small fibres that can be inhaled (which unsurprisingly, isn’t good for you).
HEPA FILTERS, FURNACE FILTERS, VACUUM CLEANER BAGS/FILTERS
Right, so there are various filtration materials that you might find in domestic products, such as air purifiers or vacuum cleaners, that you can repurpose into a face mask filter. Face masks that have a filter pocket may recommend using these kinds of materials in the pocket.
These materials are designed to be used under high air pressure to filter teeny tiny (0.3 micron) particles, so it’s a pretty good general filtration material (for reference, the coronavirus is about 0.12 microns, and N95 masks are 95% effective at filtering particles of this size).
HOWEVER, these materials (and also the blue paper towel rolls you find in many workplaces) are not designed to be worn next to the human body, let alone on the face, let alone breathed through. Some of these materials may contain fibreglass, or chemicals that should not be inhaled (even if there are other fabric layers around them). Manufacturers of these materials and scientists are now typically recommending that you DO NOT use them in face masks. It is worth noting that the discourse on the use of these materials has evolved over the pandemic; initially many people working to make ad-hoc PPE saw these as a pretty good option due to the filtration efficacy. You might see listings for face masks with a filter pocket "to be used with HEPA filter" or similar. These masks are still fine to use, just use them with something else, like paper kitchen towel or a safe non-woven fabric.
PAPER KITCHEN TOWEL
Kitchen towel is a really great option for a disposable filter. It’s affordable, readily available (well, now that the supermarkets are recovering), and better at filtering particles than a cotton t-shirt or bedsheet. If you are looking for something that you can put in your face mask pocket and throw away after a few hours, well you have found it.
Of course, the main drawback with kitchen towel is the disposability of the stuff. It makes it a less environmentally friendly option, and potentially more costly in the long run (yes, there might be a long run….sorry). If you are choosing paper kitchen towel, look for thicker versions (they’ll make more effective filters), and ideally ones that can be composted, if you are the composting type.
Other things to consider when choosing mask materials
Multiple layers are always offer more protection than one layer, and if you use the right materials won't compromise on breathability (test this if you're concerned).
Non-woven fabrics work well because of the fibre smushing. The irregular layered patten of the fibres mean that there aren't the straightforward gaps that you see if you pull apart a woven material.
When you layer materials, try to get patterns of the fabrics construction to overlap. What the hell do I mean by this? Well, when we make masks for UK Mask Makers, we cut one of the cotton layers on the bias (on a diagonal) - this means that the threads from each layer go across each other, hopefully closing some of the gaps that we would get if we cut them all at the same angle.
Fun fact: The testing team at smart air filters found that bra pads are actually both surprisingly effective at filtering air, and pretty breathable. Turns out we were onto something when we took this photo:
Not so good for social distancing though. Luckily, we were members of the same household. Are you a face mask materials nerd? Want to know more? Here are some great external resources: https://makermask.org/ https://smartairfilters.com/en/blog/ https://sites.gatech.edu/rapid-response/face-masks/