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Friday, July 10, 2020

Explained: In homemade masks, two fabrics better than one

A study has examined the ability of various materials to keep out respiratory particles — and by extension the novel coronavirus — and identified a combination of two fabrics as possibly the best: either cotton plus natural silk, or cotton plus chiffon.

Written by Kabir Firaque | New Delhi | Published: April 28, 2020 4:20:29 am
Explained: In homemade masks, two fabrics better than one A worker makes a face mask during the global outbreak of the coronavirus disease (COVID-19), in California, US, April 9, 2020. (Reuters Photo: Mario Anzuoni)

With surgical and N95 masks in short supply, the Health Ministry recently published an advisory on how you can make your own face cover at home. For material, the Indian advisory recommends cotton, while the US Centers for Disease Control and Prevention recommend “cloth face coverings” without specifying the fabric.

Now, a study has examined the ability of various materials to keep out respiratory particles — and by extension the novel coronavirus — and identified a combination of two fabrics as possibly the best: either cotton plus natural silk, or cotton plus chiffon. If the fit is good, such homemade masks can effectively keep out aerosol particles, the researchers report in the American Chemical Society’s journal ACS Nano.

Why size matters

An aerosol is a suspension of particles or droplets in the air. The virus that causes COVID-19 is thought to spread mainly through respiratory droplets, particularly when an infected person coughs or sneezes.

The World Health Organization (WHO) defines two size ranges for droplets through which respiratory infections can be transmitted — 5-10 microns in diameter (respiratory droplets) and less than 5 microns in diameter (droplet nuclei). One micron is a millionth part of a metre.

Cotton acts as a mechanical barrier to particles; silk or chiffon act as an electrostatic barrier. (Source: ACS Nano)

It is respiratory droplets (the larger size range) that are thought to be the primary route of COVID-19 infection. Airborne transmission, which is through droplet nuclei (the smaller size range), was not reported in a large study in China, the WHO notes. It says airborne transmission may be possible “in specific circumstances and settings in which procedures… that generate aerosols are performed”. The bottom line is that while infection through the smaller droplet nuclei has not been ruled out, the evidence so far is that this virus spreads mainly by latching on to the larger respiratory particles.

Testing the fabrics

At the University of Chicago, molecular engineer Supratik Guha and colleagues tested common fabrics, alone and in combination, for their ability to filter out aerosols that are similar in size to respiratory droplets. They used an aerosol mixing chamber, which produced particles ranging from 10 nanometres to 6 microns in diameter. One nanometre is a billionth part of a metre. For context, 10 nanometres would fall within the WHO definition of droplet nuclei, while 6 microns would qualify a particle as a respiratory droplet.

A fan blew the aerosol across various cloth samples, at an airflow rate corresponding to a person’s respiration at rest, the American Chemical Society said in a statement. The researchers measured the number and size of particles in the air before and after passing through the fabric.

The best performance was by one layer of a tightly woven cotton sheet combined with two layers of chiffon (90 per cent polyester and 10 per cent Spandex). The cotton-chiffon combination filtered out 80-99% of the particles, depending on particle size. This is close to the performance of an N95 mask material, the researchers said.

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When the chiffon in the combination was replaced with natural silk or flannel, or when the experiments simply used a cotton quilt (rajai/razai) with cotton-polyester filling, the results were similar, they said.

Why it works

Tightly woven fabrics, such as cotton, can act as a mechanical barrier to particles, the researchers explained. Fabrics that hold a static charge, like certain types of chiffon and natural silk, serve as an electrostatic barrier.

The researchers, however, stressed the importance of a properly fitted mask. Even a 1% gap between the face contours and the mask reduced the filtering efficiency of all masks by half or more.

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