A new study from the University of Chicago reports that a multi-layered mask made from cotton fabric and chiffon or natural silk can be just as effective as N95 masks against the coronavirus.

There just aren’t enough masks to go around, and those that we do have should be earmarked for healthcare workers. How, then, are we to keep ourselves safe in the great (and pandemic) outdoors? Well, according to one new study, we should do like our forefathers before us — and sew!

The authors analyzed the filtration properties of fabrics against aerosols (the main method of transmission for the SARS-CoV-2 coronavirus) and reported on the types of materials to use in order to create an effective mask.

Cotton and chiffon

Although the U.S. Centers for Disease Control and Prevention recommends the use of face masks whenever going outside, the reality on the ground is that such equipment is often in short supply. Surgical masks are somewhat easier to come by, but they are much less effective than filtering masks such as the N95 model (although they’re still useful).

The real problem is that every mask we use is one that’s no longer available for the healthcare sector, and the medical personnel fighting to help the infected against the disease need such masks to be able to continue doing their jobs. So people have started making their own, which is awesome. Researchers are now pitching in, too, and are informing us of the best way, and the best materials, to use when making our masks.

Coronavirus is spread through saliva droplets that form aerosols when we breathe, talk, or cough. The heavier droplets fall to the floor, but the lighter ones remain in suspension around us and can travel (and infect) up to 4 meters away.

The team, led by Molecular Engineering Professor Supratik Guha, used an aerosol mixing chamber to produce particles ranging in diameter from 10 nm to 6 μm in diameter, roughly the same interval of the size seen in coronavirus-carrying aerosols. A fan was used to force them through various textile samples (the fan was set to generate airflow comparable to that of a person’s respiration at rest), and the team compared particle levels in the air before and after passing through the material. The study was carried out at the U.S. Department of Energy’s Center for Nanoscale Materials user facility at Argonne National Laboratory with funding from the U.S. Department of Defense’s Vannevar Bush Fellowship.

Their results show that one layer of “tightly-woven” cotton combined with two layers of polyester-spandex chiffon (a type of sheer fabric most commonly seen in evening gowns — can filter out between 80% to 99% of all aerosol particles in a sample (depending on their size). Such performance, they add, is close to that of an N95 respirator mask.

The chiffon can be swapped for natural silk or flannel without losing filtering ability, or the whole thing can be replaced with a cotton quilt with cotton-polyester batting. The combination of two materials is important, however. The team explains that the cotton creates a physical barrier to incoming aerosol particles, while materials such as chiffon and natural silk can become charged, and serve as an electrostatic barrier.

Another thing to keep in mind is that it’s essential for such masks to be perfectly fitted. Even the slightest gap between the mask’s edges and the user’s skin can reduce their filtering efficiency by 60%.

The paper “Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks” has been published in the journal ACS Nano.