The debate about whether people should wear facemasks in public to prevent the spread of Covid-19 has attracted plenty of experts, and many not-so-experts, on both sides. Irrespective of where you line up, we can probably agree on two things: facemasks shouldn’t be worn at the expense of frontline workers who need them desperately, and should be made of the most effective material available. To assist in both, molecular engineers have tested the best widely available fabrics for converting to homemade masks.
The surgical masks worn in hospitals are made of materials such as non-woven polypropylene, but that’s not easy to source at home. Instead, masks have been made out of old T-shirts or cotton quilts, but skeptics have pointed out the holes are large enough viral particles could slip through, even when carried on respiratory droplets emitted in coughing or sneezing.
Professor Supratik Guha of the University of Chicago used an aerosol mixing chamber and fan to blow particles and droplets ranging in size from 10 nanometres to 6 micrometers at samples of different types of fabric and measured how many made it through.
In the journal ACS nano, Guha and colleagues report the best results came by combining three layers of two different materials. Specifically, Guha stopped the most virus-carriers with a tightly woven sheet of cotton, such as from high thread-count sheets, and two layers of polyester-spandex chiffon. For some particle sizes, this was 99 percent effective, and even for smaller particles, up to 95 percent were captured. The combination, when well fitted, actually beat surgical masks at most particle sizes.
Low thread-count cotton, which many people have used for homemade masks, was only effective in certain size ranges. The paper also notes we don’t yet know whether larger or smaller respiratory droplets pose the greater threat when it comes to distributing SARS-CoV-2.
Polyester-spandex is popular in evening gowns. For those who don’t have one lying around they are willing to sacrifice, natural silk or even flannel proved almost as good, but only in combination with cotton.
The reason hybrid masks work so well, the authors believe, is because tightly woven cotton is an effective mechanical barrier, while chiffon and silk become electrically charged easily, forming an electrostatic barrier. The combination works better than trying to double up using multiple layers of the same form of protection.
Besides the danger of people taking masks away from hospitals, one reason some authorities have argued against the widespread use of masks is the fear they don’t work if badly fitted. Guha found there is a lot to this – even very small gaps between the mask and face reduced the particles filtered by 60 percent, so fitting the mask snugly is even more important than choosing the right material. A 2013 study on home-made masks for influenza prevention found most people are not good at making them fit well, so learning how to fit a mask to the face should be the priority.