Explained: in homemade masks, two fabrics better than one

Tight fabrics, such as cotton, can act as a mechanical barrier against particles. (File photo)

With a shortage of surgical masks and N95, the Ministry of Health recently published a notice on how you can cover your face at home. For the material, the Indian notice recommends cotton, while the US Centers for Disease Control and Prevention. USA They recommend “fabric covers for the face” without specifying the fabric.

Now, a study has examined the ability of various materials to prevent respiratory particles, and by extension the new coronavirus, and identified a combination of two fabrics as arguably the best: cotton plus natural silk or cotton plus gauze. If the fit is good, such homemade masks can effectively prevent aerosol particles, the researchers report in the ACS Nano journal of the American Chemical Society.

Why size matters

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

The World Health Organization (WHO) defines two size ranges for the drops through which respiratory infections can be transmitted: 5-10 microns in diameter (respiratory drops) and less than 5 microns in diameter (droplet nuclei ). A micron is one millionth of a meter.

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

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

Test the fabrics

At the University of Chicago, molecular engineer Supratik Guha and colleagues tested common tissues, alone and in combination, for their ability to filter aerosols that are similar in size to respiratory drops. They used an aerosol mixing chamber, which produced particles ranging from 10 nanometers to 6 microns in diameter. A nanometer is a billionth of a meter. By context, 10 nanometers would be within the WHO definition of droplet nuclei, while 6 microns would qualify a particle as a respiratory droplet.

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

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

When the gauze in the blend was replaced with natural silk or flannel, or when the experiments simply used a cotton (rajai / racial) quilt with cotton and polyester padding, the results were similar, they said.

Why it works

Well-woven fabrics, such as cotton, can act as a mechanical barrier against particles, the researchers explained. Fabrics that maintain a static charge, such as certain types of gauze and natural silk, serve as an electrostatic barrier.

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

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