Governments worldwide, the World Health Organisation, the US Centers for Disease Control and Prevention have encouraged or made mask-wearing mandatory (for some countries).
As surgical and medical masks are reserved for first responders and frontlines in healthcare and have become scarce supply in many places across the world, the general public, or civilians, are wearing reusable fabric masks or homemade cloth face coverings outdoors for their safety and protection.
How does a fabric mask or cloth covering help?
The droplets released during sneezing and coughing are larger than those released while speaking and breathing, and any of these droplets may carry many virus particles. The larger droplets tend to fall nearby due to gravity, but the smaller ones can go far with the majority of them remaining within six feet of the infected individual. A covering across the face can prevent or reduce the spreading of infection by blocking these droplets, preventing the virus from entering our bodies. It is important that the face covering fits snug across the upper cheek and covers the nose and lower jaw completely to minimize the gaps through which droplets can enter into our bodies.
But exactly how effective are these fabric covering or masks?
In the current COVID-19 pandemic, some expressed uncertainty regarding the masks’ effectiveness in reducing exhaled droplets.
In response, many experts have performed studies on the effectiveness of common household fabrics for use in homemade masks.
Breathability and Permeability
The droplets released during coughing and sneezing come in various sizes and velocities. The fabric mask should be breathable and impermeable to high- and low-velocity droplets.
There is currently no universal consensus on the efficiency of different household materials against the Covid-19 coronavirus. However, from various sources, natural fabrics such as cotton, are mostly recommended because at the microscopic level, the natural fibers in cotton tend to have more three-dimensional structure than synthetic fibers, which are smoother. The roughness of the fibre and 3D structures creates more roadblocks to stop incoming particles, meaning cotton naturally has high filtering capabilities.
However, the effectiveness of filtration is not just dependent on the type of fabric fibre. Fabric Porosity, which is a measure of the tightness of weave of a material, is also an important factor.
What this basically means, is how much ‘gap’ there is in the weaving of the material. Fabric mills can label terms such as ‘thread count’ or by its’ weight in ‘oz’ for materials like cotton, which broadly gives an idea on how tightly woven the yarns are.
Which is why thicker materials, or materials with higher thread count were found to filter more particles.
When in doubt, a simple, quick ‘light test’ can also be done, by holding up the fabric to light and checking if one can see the individual fibres in it. If you can, as with this example, it’s likely too thin to provide adequate protection without a filter.
Doubling the layer can help to increase its filtration effectiveness, which is why a fabric mask should be 2-layered at the very least. Some fabrics however, continues to present large gaps and risks even multi-layered, such as linen, as the weaving is generally a lot looser, which one can see clearly with the naked eye.
Heavy or thick materials have the best filtration, but a highly impermeable fabric is likely to be less breathable. Low breathability will force airflow in and out through the sides which increases the risk of droplet inhalation.
In short, what’s tricky about most fabrics is that the diameter, permeability and thread count all determine its effectiveness as a mask. So the choice of fabric and the number of layers is a matter of compromise between breathability and droplet resistance.
So whilst most synthetic materials have smoother fibres compared to cotton, their drop resistance is notable because of their low fabric porosity.
How Experts test for Breathability and Droplet Resistance
“To quantify breathability, we simply measure the airflow velocity through the material. Measuring droplet blocking is a more complicated process that uses an inhaler to generate high-velocity droplets.
We fill the nozzle of an inhaler with distilled water mixed with 100-nanometer fluorescent particles, which mimic the coronavirus in size. When puffed, the inhaler forces the water through the nozzle and generates high-momentum droplets that we collect on a plastic dish placed vertically in front of the inhaler. We then repeat the process with the fabric we are testing over the collection dish. We measure how much water landed on the dish in both cases by counting the nanoparticles using a microscope. We can then use the ratio of the volume collected with and without the fabric to give us a measure of droplet-blocking efficiency.”
We’ve resources showing an interesting look at the fabric weave of various more commonly used materials for masks. Whilst it is not as microscopic as Lab-tested, it shows a 50x magnification of the fabric, placed over a red ipad cover.
With this, it provided enough insight to see how much of the ipad was showing through. Given that the Coronavirus particle spreads via droplets of water caused by coughing which are microscopic in size, you can be sure, any gaps seen here with the naked eye, definitely does not offer adequate protection from external droplets.
The images show that different weaving and fibre make affects the porosity of the fabric, particularly for cotton which comes in a wide range of production. Surprisingly, what we thought as ‘opaque’, and highly impermeable satin, turned out to be a poor filtration material. The material turned out surprisingly thin and levels of translucency with the red background color showing through.
With these findings, the right combinations of fabrics is important to make double-layered masks to increase filtration efficiency.
A look at 50x Magnified view of a surgical mask and N95 mask.