Making your own Covid19 masks — a few pointers

Asit Mishra, Amit Mishra

When you study for an exam, you do not do that with a guarantee of securing 100% or 10 point GPA! You put your best effort. But your marks will depend on a number of things starting from how tough the questions are, to how did your classmates perform.

Similarly, wearing a mask does not guarantee you absolute protection from Covid-19. That does not mean you should skip the protection a mask can afford you. The efficiency of your mask will depend on a number of things including how the mask is made. In this article we shall discuss a little bit about what makes a good mask.

It does not have to be a fancy one — N95 — any kind of face covering helps out a little.

The most important criteria of a mask: It should be comfortable to wear and should fit around your nose and mouth snugly. An uncomfortable mask is one which you will rarely put on. If it is not on, it does not help.

Know thy powers: Before making a mask let us first understand what a mask does and how it does. In one of our previous articles (on if Covid19 viruses can fly or not!) we discussed the power of models. A model is a mental picture. Many people have a wrong mental picture of how a mask works.

A mask is not like a sieve.

Meaning, it does not just stop particles over a certain size. Filtration efficiency (fraction of particles filtered by the mask) behaves more like the figure below.

Masks work well over a certain size and under a certain size. To understand this we need to briefly look at how a mask stops particles. Filtration of small particles and large particles happens by different mechanisms. (So, masks have a most penetrating particle size — somewhere in the middle — for which they are tested and rated.)

Ways in which a mask filters particles.

Filtration occurs by one of the following mechanisms.

Image created based on https://blogs.cdc.gov/niosh-science-blog/2009/10/14/n95/

Out of all these modes only straining is similar to sieving. Inertial impact is when the particle directly collides with a strand of the mask. Interception is when the particle gets stuck to the fibre-strands even when its not a direct impact. As you may guess, mechanisms like straining, interception, and inertial impaction work for the larger particles. Diffusion works for smaller particles. Electrostatic attraction can work for all sizes of particles.

From this limited discussion let us try to think of the major desirable characteristics of an ideal mask.

1. An ideal mask should be able to take advantage of all the mechanisms.
2. It should be comfortable too.

For the second criterion the mask should have an outer layer of cotton which is the most comfortable to wear. To assure the first criterion, the masks should have

1. multiple layers (helps with diffusion interception),
2. compact, close woven fibres (for straining, impaction, and interception), and
3. blended fabric, even layering of different fabrics, to aid with a little bit of electrostatic attraction.

A good option is to use mixed fabric — cotton for comfort to make up the outermost layer and some synthetic fibres in between to create a mix of fibres for electrostatic effect. Outermost layer could be made from your old sweatshirts that had a cotton blend (https://doi.org/10.1093/annhyg/meq044).

We should try to use multiple layers (more the merrier!) for diffusive filtration and picking off the smaller particles. But, layers can only be added till the point it is comfortable for you to breathe. A study that used 16 layers of handkerchief fabric saw that the 16 layers led to a good filtration efficiency (> 60%) but so many layers made it difficult for the wearer to breathe (https://doi.org/10.17226/25776). The same problem comes up with using coffee filters — maybe good filtration but difficult to breathe.

Non-traditional fabrics — Some non-apparent choices can also be nice. A simple, easily available option is vacuum cleaner bags. They are pretty good at keeping the dust in the vacuum and particles out of your breathing. You could use it as a middle layer. Same goes for microfibre cloth — used for wiping down cars or screens. Surgical sterilization wraps can also work nicely for the intervening layers.

Let us summarize our discussions.

• Making your own mask is a worthwhile idea.
• Always try to use multiple layers when making your own mask.
• Keep a comfortable outer layer (like cotton or sweatshirt fabric).
• Some good options for the middle layers are vacuum cleaner bags, microfibre cloth, surgical sterilization wraps etc.)

Happy mask-making!