Do improvised masks work?
Update as of Apr. 5, 20
Our first masks are set to be delivered tomorrow. There is still a lot of confusion around how effective cloth masks will be. My belief is that they will be of some benefit but it is difficult to quantify for now. The best strategy is still to keep up with all of the other recommended practices; social distancing, frequent hand washing, hand sanitizers, avoid touching your face, etc. The mask will just add a little extra protection to that.
Our first generation of masks use a synthetic fabric called dazzle on the inside. That is a comfortable fabric often used in athletic wear. That is not the primary filter. The idea is that it will wick water away and stay comfortable for hours. Some masks are very good filters but effective at preventing infection. They are uncomfortable so people constantly adjust them, allowing openings for exposure.
The outer layer is a brushed flannel. We are trying to mimic a HEPA filter but these are NOT HEPA filters. They will not be nearly as effective. But they are made from materials that we can source immediately. We will be studying materials in more depth to improve designs down the road but that won’t help with this current crisis. Viruses are tiny but they aren’t viable on their own. They travel from victim to victim in droplets of water or mucus. In the case of a virus, those can still be well under a micron (0.00004 inches) in diameter. The pores in a HEPA filter are much larger than that but they do not go straight through the fabric. They twist and turn. The idea is that the particles can’t make the turns fast enough and bang into a fiber where they are captured. That is achieved by building up the fabric with randomly oriented fibers, avoiding the straights pores generated in a perfectly woven fabric. When we were trying to think of readily available fabrics that mimic that structure, it always came back to natural fibres like cotton and wool.
A prototype mask was washed and dried to see if that had a negative impact on the mask. There was nothing obviously changed with the mask. Not scientific but enough to recommend the following as a strategy to start. These masks are meant to be re-useable. If they are doing their jobs, virus containing droplets will be trapped in the mask. We would recommend that you wash the mask immediately after wearing it. Soap and water has been shown to kill the virus. Please to not leave it uncleaned on a surface that you or others will be touching.
We will continue to learn how these things work and post new learnings here. Please check back periodically for updates. We have an agreement with a high-end commercial lab to test this design and alternative designs. The design may evolve as a result of this testing. We are also looking for a spray on material that will enhance the fabrics ability to capture droplets. Hopefully we find something that you can apply to your mask periodically to improve its effectiveness.
Finally, your feedback is crucial, both negative and positive. As mentioned earlier, the mask will only be used properly if it is comfortable. If its not used properly, it won’t be effective. What is comfortable for me may not be comfortable for you. It’s probably impossible to please everyone but the bigger the sample of feedback we get, the more people we will be able to serve well.
As of the time this was written (Mar. 29, 2020) I had been looking into this for 3 days, part time. I do have a technical background but this is WAY outside my area of expertise. I have been leaning on a number of others for help in this effort. This will not be the definitive work on this topic. It’s my best guess, at this point in time, at the best solution. By best solution, I am balancing efficacy, easy of production, cost and wearability. Others would weigh those things differently than I do. So take this for what it is, a somewhat educated guess and a work in progress. I believe we will learn and improve on it as we go. Unfortunately, Covid 19 is not allowing us the time to perfect it before taking action.
Do improvised masks work?
There has been work done on this in the past, notably a cluster of papers after the H1N1 pandemic in 2009. At that point it was recognized that the world might run short of proper certified masks. Several researchers looked into the possibility of using common household materials to fashion improvised masks. It is a bit difficult to draw hard conclusions with exact results. The materials used were described as cotton t-shirt, cotton/poly blend sweatshirt, tea towel, fleece scarf, etc. All of those come in different weights, weaves, etc. So the results are, at best, directional. Some trends do emerge though.
Tests have been done on the filtration efficiency of different fabrics. The pressure drops across the materials was measured. Fit tests have been done to check for leakage around the edges of masks. Field test we done to see how good they were at preventing infection in real world conditions. Again, I have only scratched the surface of this topic but this is my best interpretation of the data so far.
Wearing a mask, any mask, seems to help prevent infection. Even materials that don’t filter well have positive results. This is likely because it prevents you from touching your mouth. It seems that a lot of infections are the result of picking up the contagion on your hand and then touching your mouth or eyes. Having said that, don’t take addition risks because you think you are now protected. It’s better than not wearing a mask at the same risk exposure. If you then subject yourself to greater exposure, you could easily overwhelm any benefit you get from the mask.
That was the good news. The bad news is that nothing you make at home will compete with a certified N95 mask. The 95 stands for 95% removal of the fine particles of concern. Most do much better than that across the size spectrum. The advice we are getting today about social distancing is to stay at least 2 meters or 6 feet away from others. The best I can tell, that comes from a study that measured the travel of potentially infected mucus and saliva droplets when someone coughs. That showed that fewer than 20% make it that distance. I’ve seen an interview with a researcher who says that more recent works indicates that more droplets travel further but I haven’t seen that data. Lets say we can cut that by half or two thirds, so much the better. That is a WAG (engineering acronym; wild ass guess) but it still doesn’t get to N95 (wear that if you can!) but worth it. So what is the best way to make an improvised mask? As I mentioned earlier, it comes down to a balance of a number of factors.
I was given a great suggestion to look into 0.3 micron filters. You’ve probably heard of HEPA filters. They are good at capturing particles in the size range the virus resides in. They don’t do it by having 0.3 micron pores or holes. They have randomly oriented, matted fibers. That means there is no straight path through the fabric. Air has to twist and turn its way through. Droplets have momentum though so they can’t make the really sharp turns. They run into one of the fibers and get captured. There are a number
of common fabrics with this random orientation, though not necessarily as efficient as HEPA. Natural fabrics like cotton and wool for example, have more of those characteristics than most synthetic fabrics. So there is some reason to believe they would filter better than synthetics. Most of the testing seems to bear that out. This part is pure intuition on my part. I’m guessing that a brushed cotton with a “fuzzy” surface would be even better as it is even less structured. Fabrics like vacuum bags actually filter very well but they fall down on other measures like pressure drop (breathability) and ability to get a good seal. I actually went looking for rolls of HEPA quality fabric but delivery time is in months. Something to look at for the future.
The ability to get a snug fit to prevent leakage around the edges matters. The problem is that the types of fabrics that give a good seal tend to stretch. That stretching opens up the pores though and worsens filtration.
It’s also important for the mask to be comfortable. The pressure drop can’t be too high or it will be hard to breath. The fabric shouldn’t irritate the skin. If it isn’t comfortable, it will not be worn constantly or it will be worn incorrectly.
Cost and availability of materials is also important.
Balancing these things out, where did we land? To get started, we are going with a two ply mask. The outer layer will be a 100% cotton flannel with limited stretch. That is the filter. The inner layer will be a stretchy synthetic. That won’t contribute a lot to filtration but it will provide the good seal. It is also meant to be comfortable.
Now it’s your turn.
Please try the masks, when it is appropriate to do so. Again, these are not a replacement for N95 masks. People at greatest risk need the best equipment. If your risk exposure is lower, everything I’m seeing says these improvised masks are a significant improvement over no mask.
Once you’ve tried them, we really want your feedback. How do they fit? Are they comfortable to wear for a full shift? What do you think of the attachment mechanism (we’ll try a couple of different styles to start)? Do you have information that a different fabric might work better? Anything else? Feedback can be sent to firstname.lastname@example.org
Thanks for everything you are doing to fight this virus and stay safe.