Across the world, the coronavirus ( COVID-19 ) pandemic has forced governments into mandating the use of face masks. The general public has been advised to use cloth masks and not hoard N95 and surgical masks which are reserved for the healthcare workers. But how do we make a good mask? Which material should we use? A new paper in ACS Nano shows that a combination of cotton and silk or chiffon is the answer.
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Supratik Guha and team from the University of Chicago carried out various studies on commonly available fabrics such as cotton, silk, chiffon, flannel and their combinations. They used an aerosol generation and mixing chamber to produce particles ranging from 10 nanometre to 10 micrometre in diameter. Using a fan, the aerosols were blown through the cloths at two different air flows corresponding to a person’s respiration at rest and moderate exertion.
Cotton and chiffon
The number and size of particles in air were measured before and after passing through the fabric. One layer of cotton along with two layers of chiffon showed the highest filtration efficiency of 97%.
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The researchers point out that the tightly woven cotton can act as a mechanical barrier to the aerosols and the chiffon serves as an electrostatic barrier. “As kids, we used to do this experiment where we would rub a comb on our dry hairs to lift small pieces of paper. Electrostatic filtration effectively works on the same principle. Some fabrics such as natural silk and chiffon can hold static charges. When small particles come in close proximity to these fabrics, they are attracted and trapped inside them, thus acting as filters,” explains Abhinav Prakash, a postdoctoral researcher and one of the authors of the paper in an email to The Hindu.
However, even a small 1% gap in the mask drastically reduced its efficiency. “The importance of a good fit needs to be emphasised. Even for a mask as good as an N95 mask, the filtration efficiency can drop significantly if they are nor properly worn,” he adds.
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“The idea behind using cloth masks using common fabrics is that they can be made readily available to the general public in the current COVID-19 situation where there is a shortage of N95 and surgical masks,”writes Dr. Prakash.
“Their reusability and best ways of treating them still need to be studied. Also, we haven't performed any tests on how long they can be used. We are currently working on studying the effect of different wash cycles on the filtration efficiencies.”
