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A homemade coronavirus mask

British WWII poster: coughs and sneezes spread diseases. Source: National Library of Medicine, collections.nlm.nih.gov; also available at the Imperial War Museum, www.iwm.org.uk

Why are dust masks useful in the fight against coronavirus? Some people say that homemade masks are only useful to protect others, not you. This statement is really based on a lack of knowledge about masks and the way they work. A homemade mask does also protect the one wearing it. It might just protect to a lesser degree. The level of protection you get from a homemade mask depends only on its design (not the place where it is made ;-)

Almost everybody has now heard the term "N95 mask" or "FFP2 mask" (European standard) but few of us know what it really is. The US National Institute for Occupational Safety and Health (NIOSH) tests, certifies and classifies protective equipment. N means it is a mask for non oily environments (the mask is NOT resistant to petrolium solvents) and 95 means it filters about 95% particles with size of 0.3 μm (=300nm) or larger. This N-series of masks is tested with aerosols of sodium chloride (NaCl). That's salt dust. 5% of this dust is allowed to pass and it's certified to be a N95 mask.

What's the diameter of a coronavirus? About 0.12 μm (=120nm). This means a "3 lane coronavirus highway" could run through this mask with just minor narrowing in lanes. So why do we read in every newspaper that N95 masks are the protection that healthcare workers need when working with covid-19 patients? It seems like it's almost useless. These viruses will easily pass through the mask!?

The theory is that those masks work because the coronavirus is usually attached to water droplets coming out of the lungs of sick people. The virus together with the water droplets is bigger than 300nm. However a percentage might still pass. A mask reduces the virus load that the person carrying it receives. It does not offer 100% protection. Hopefully the person's immune system will eliminate the remaining percent and that person might not get sick from covid-19.

It means that even a basic cloth mask will protect you, just to a lesser degree. smartairfilters.com claims that the most primitive DIY mask can still catch 50% of coronavirus water droplets. They have as well an overview about the capabilities of different materials at https://smartairfilters.com/en/blog/best-materials-make-diy-face-mask-virus/?rel=1

Choosing the right fabric

Masks can be made out of any old piece of clothing but the filter efficiency depends on how dense the fabric is. If you hold it towards the light and you can see tiny holes between the threads then it is probably not very good.

I think of fabric catching small particles like humans running into a forest. The big tree trunks are the main threads the fabric is made of. It's not those trees that will slow you down. It's tree branches and and bushes. Thus a natural fiber that is a bit fuzzy is probably best. The hairs in wool and silk fibers as well as most synthetic fibers stay mostly as one thread. Other fibers such as cotton and linen will easily produce fine fuzzy ends and they are probably the best choice for such a mask. Pure cotton flannel is an interesting fabric for masks because it has many fine and fuzzy fibers.

Mask pattern for download

You can download my mask pattern here. The paper the pattern was drawn on is a standard US letter paper. This will help you to get the size right. It's probably not the easiest mask to build but I think it offers good protection. You stitch it together along dark line at the bottom. The triangles on the sides are folded and then stitched shut to give it a more round shape even on the sides. It should have a snug but comfortable fit.

Every face is different and I recommend to take a kitchen paper towel and build first a test mask. You can take needles or tape to hold it together along the lines where you would stitch the fabric together. Adapt it to your face and take scissors to cut it to the right size. Once the paper towel seems to fit you just remove the needles/tape and you have a flat pattern for cutting the fabric.

You could theoretically sew this by hand but a sewing machine will probably give better results.

The outer shell of the mask is made from a tightly woven cotton fabric.

Add a nose clip

N95 masks have a nose foam and a metal clip that can be adjusted to ensure a tight fit around the nose. A stripe cut from an aluminum metal sheet is best suited for that because it has almost no spring effect. A metal stripe cut out of can of beans will however work too. It has a bit of a spring effect and you will need to shape it with your hands but it will work. I took an aluminum metal stripe 1cm * 9cm and I drilled a few tiny holes into it to be able to sew it to the fabric. Use sandpaper to dull all the edges.
A metal nose clip ensures a tight fit around the nose
A N95 mask will have the nose clip on the outside but we can put it inside since we will add a liner

Maximum filter surface

A N95 mask has a dome shape and it keeps the filter surface away from your nose and mouth. A simple fabric mask will be blown away from your skin when your breath out and sucked towards your nose and mouth when you breath in. I am talking about the area just in-front of mouth and nose. This causes two problems:

We need something to keep the filter surface away from our face. This spacer is very important if you want to use this mask to protect you from the droplets of others.

I will use one of those loosely woven microfiber cloth as a liner. This kind of microfiber cloth does not filter much. It's mainly a spacer.

The final mask with liner and rubber bands: We use a microfiber cloth as "spacer" too keep the filter fabric away from nose and mouth

The result

We attach two rubber bands to ensure a tight fit on the sides. This mask is bigger than a standard N95 mask because it closes below the chin and ends before the ears. I have used N95 masks before around the workshop and they make breathing harder. Despite the bigger filter area of this homemade mask it feels like an N95 mask with regards to the breathing effort that is required. I have no instruments to verify this mask or the claim that masks work in protecting against a virus. For now we have to trust the government experts that have equipment to test them. An advantage of this mask is that I can wash it and re-use it.

The DIY covid-19 mask

Re-using the mask

N95 masks can be disinfected by UV light (outside in the sun for several hours) but they are officially disposable masks. Since those N95 masks are non biodegradable they will accumulate in landfills.

The mask designed here can be washed in the washing machine. Cotton fabric will however thin out and become brittle after years of washing. I would try to reduce washing by washing it only every 3rd or 4th time and use sun light to disinfect it all the other times.

Testing and re-design

I have been using the above mask over the last two weeks and it worked well but similar to n95 masks it makes it harder to breath. In fact I think that my filter cloth is even denser than the polyester fabric used in n95 masks. It is certainly a good mask with regards to its filtering capabilities but it makes it uncomfortable to use this mask for more than a few hours. This is due to the very dense fabric that I used for the outside.

Let's design a simpler mask that is easier to build and more comfortable with regards to the required breathing effort.

The simple mask design:

I think that this new mask is still better than off the shelf commercial surgical masks since it has the nose clip and the spacer but it will be washable.

DIY covid-19 mask, simple design, cut the T-shirt fabric
Fold it up and sew over the foldes on the sides
DIY covid-19 mask, simple design, view from the outside, the nose clip is attached between the two fabric layers
DIY covid-19 mask, inside of the mask

Update: January 2022

You have probably heard by now that masks can't stop the virus because they are too small to be filtered out. That's true however it's not like throwing sand at a chain link fence and hoping that the fence will stop the sand. It's more like throwing wet sand balls because the virus is always inside water vapour droplets. The droplets are bigger than the virus.

With the original Wuhan variant it took about 15min until you got infected if you were around a sick person. As a result, both time and the number of viruses in the air play a role.

It's January with -22'C outside. Cold enough to see the breath of a person. A physical barrier must make some difference and I wanted to know what would happen. I took 3 different masks and tried to estimate the "clouds of breath" compared to no mask.

Conclusion: Mask do work by reducing the viral load. Thus they extend the time until you get infected but the difference they make is too small. E.g instead of 15min you can stay 20min in contaminated air. They will not work in a school or workplace. These mask make a too small difference to be useful.

The biggest problem with masks is however that they hide the facial expressions. We used to be able to judge if somebody had the flu or a cold just by looking them into the face. This is no longer the case with a mask.

Update: May 2022

There is now a good peer reviewed study on masks:
Correlation Between Mask Compliance and COVID-19 Outcomes in Europe:

Result: Countries with high levels of mask compliance did not perform better than those with low mask usage.

In other words: Masks don't work. Why do doctors use them? We don't know. They wear also lab-coats and those don't stop viruses either. It's just some kind of a uniform.

Here is a nice slide that shows the CDC makes mask recommendations over time:
CDC mask guidance over time, source: https://rumble.com/c/PettyPodcasts

Update: June 2022

A surprise article in the NY times. Even they are now trashing the masks! Their logic makes me laugh. Can you claim that something works when it does not make a difference?

It's time to be honest about how face-coverings made us an uglier society. The real purpose of masks was to keep everyone under the thumb of a ballooning health authority. It's time to face the truth and the NY times is not there yet.

Update: July 2022

There is now a US study showing that facemasks contribute to higher COVID-19 fatality rate:

Update: Aug 2022

Never forget what authorities did in the name of science:

Never forget what authorities did in the name of science!

Update: Nov 2022

If masks worked...


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