The best homemade facemask may be a combination of two fabrics

Combo of cotton and natural silk/chiffon can effectively filter out aerosol particles

facemask Painting, fabrics and sewing accessories for making an anti-virus mask | Shutterstock

Wearing facemasks in public settings has been made mandatory in many places across India. The US CDC advises the use of simple cloth face coverings to slow the spread of the novel coronavirus. People need to wear only cloth face coverings and not surgical masks or N-95 respirators. CDC guidance says that special masks like surgical masks or N-95 respirators are critical supplies that must be reserved for healthcare workers and other medical first responders. 

People everywhere are making their own facemasks rather than waiting for supplies and using it in public spaces. 

Now a new research study published in American Chemical Society’s ACS Nano says that a combination of cotton with natural silk or chiffon can effectively filter out aerosol particles—if the fit is good.

SARS-CoV-2, the new coronavirus that causes COVID-19, is thought to spread mainly through respiratory droplets when an infected person coughs, sneezes, speaks or breathes. These droplets form in a wide range of sizes, but the tiniest ones, called aerosols, can easily slip through the openings between certain cloth fibers, leading some people to question whether cloth masks can actually help prevent disease. 

Supratik Guha at the University of Chicago and colleagues studied the ability of common fabrics, alone or in combination, to filter out aerosols similar in size to respiratory droplets andpublished their report in American Chemical Society’s ACS Nano.

The team of researchers used an aerosol mixing chamber to produce particles ranging from 10 nm to 6 μm in diameter. A fan blew the aerosol across various cloth samples at an airflow rate corresponding to a person's respiration at rest, and the team measured the number and size of particles in air before and after passing through the fabric. One layer of a tightly woven cotton sheet combined with two layers of polyester-spandex chiffon—a sheer fabric often used in evening gowns—filtered out the most aerosol particles (80-99 percent, depending on particle size), with performance close to that of an N95 mask material.  

When the researchers substituted chiffon with natural silk or flannel, or simply using a cotton quilt with cotton-polyester batting, it produced similar results. 

The study points out that tightly woven fabric, such as cotton, can act as a mechanical barrier to particles, whereas fabrics that hold a static charge, like certain types of chiffon and natural silk, serve as an electrostatic barrier. 

However, if the gap is reduced by 1 per cent, the filtering efficiency of all masks will be reduced by half or more, emphasising the importance of a properly fitted mask.