Masks

2020-04-26

DISCLAIMER: The information given in this post is a best effort attempt at making my family safe and spreading useful information, i am just an engineer.

Let's talk about masks.
Lets talk about you and me,
Let's talk about all the good and bad things that may be,
Let's talks about masks.

Ok, that was a bit tortured, but masks are important, even for the general population. You may have heard that makes have no significant effect in preventing, virus spread and that is not entirely wrong.

Many well founded scientific opinions can be found, one that I kept coming across was COMMENTARY: Masks-for-all for COVID-19 not based on sound data, which asserts, that

In sum, given the paucity of information about their performance as source control in real-world settings, along with the extremely low efficiency of cloth masks as filters and their poor fit, there is no evidence to support their use by the public or healthcare workers to control the emission of particles from the wearer.

Which is a reasonable assumptions about cloth masks in a health care setting. But this assertion is extended to the use of masks in the general populace with the added caveat

Leaving aside the fact that they are ineffective, telling the public to wear cloth or surgical masks could be interpreted by some to mean that people are safe to stop isolating at home.

At least in Germany, the mask carrying laws still emphasize social distancing and go only in effect in situations where the 1.5m distance rule can not easily be applied, such as in public transportation and during shopping.

But using the masks is not entirely useless, all comments and papers generally go back to Kellogg et.al. AN EXPERIMENTAL STUDY OF THE EFFICACY OF GAUZE FACE MASKS, which was a study of gauze masks after the 1918 Spanish Flu pandemic.

But there are 2 things to consider when looking at that study. For a start he used gauze masks, as it was common at the time and there is a notable reduction in ejecta with the help of the gauze mask.

You need to consider that around 1918, they did not have non woven polypropylene cloth, a commonly available material used for N95 masks. This should give us hope that we can manufacture or build at home masks that are significantly better than the ones they used in 1918.

On top of the social distancing laws, wearing masks should provide some benefit. The key point of the entire exercise is to reduce the spread of COVID-19 and keep the spread rate below 1.0, so that the health care is not overwhelmed before we get a vaccine or effective treatment.

The problem is, at the same time, you really do not want to take away the high end masks for the health care workers that actually need them. So it boils down to self made or commercially available masks that may or may not be any good. When it comes to fit and function I have so far not seen any masks that I consider sufficiently well made to not be just a alibi mask.

Also one thing that I really want to mention, the primary use of masks for the general population is source control, that is preventing other people catching your germs, so no vents!

But if we are going to wear masks, let's do it the right way. In this vain, when choosing, building and using a mask you should consider the following:

• you are wearing a mask to protect the people around you form your germs
• material is important, using a thin layer of woven cloth will not do
• fit is important, you need to breath through the material, not around the mask
• mind the gaps, stitching and seams should be avoided or mitigated

After extensive mostly fruitless research, I stumbled over Craig Turner's N95ish mask construction. Admittedly, I am quite lucky that You Tube suggested me this video and that I follow him for funny australian DIY videos.

This got my mask making jumpstarted, I had significantly more information and rather simple way to make the masks. By a quite of coincidence, I had just purchased a rinky dinky light box of a one of project and that had sheets of non woven polypropylene as backdrops of many colors.

I tried to follow Craig's instructions, but for a start I had only rather small sheets and I did not want to waste the material to I cut the template in half and traced each shell on the material. This allowed me to get about 2x more masks out of the sheets.

But this tuned out quite well, because I ended up doing some design changes that make the mask work better. First I was not able to make a head weld that actually held 3 layers securely together, especially not the 6 layers at the front of the mask. So I resorted to sowing the perimeter and then cutting the fabric down as close to the seam. To ensure that the seam is air tight I first melt the layers on the end together and then heat weld the seam, which should melt the material sufficiently to close any air gaps.

The outer perimeter gets the same treatment and I opted to use straps to tie the mask shut. That has been beneficial, as fitting elastic bands can be tedious, if the elastic bad is to big it won't hold and if it's to small it will be really unconformable to wear. I attach the strap with a cut I do in the extended flap, that obviously gets the heat treatment to bond the layers together. This is, I think acceptable since the air seal is between the mask and your cheek, not on the extended flap.

Fit is important. I opted to take the finished mask and fit it to the wearer, since the nose foam does change the shape and size a bit. Ensure that the nose "clap" presses the upper half to the cheek bones and no air "fogs the glasses". The second place is the chin, ensure no gap exists between chin / throat area and the lower lip of the mask. For both my wife and daughter needed adjustment. Simple trim the front part down by sowing a new smaller ark and going though the entire heat sealing process again. (The pictures are exactly from that stage.)

I have reversed the mask making the seem fold inwards. This has no real purpose, except to make the shape of the mask better and look nicer. We customized the masks to make them distinctive, since the tooth brush rule applies, don't use someone elses mask. The masks should be washable at 95°C, you may just need to reglue the nose foam and clamp back on, if the glue is water soluble.

And finally they just look cooler: