Homemade cloth masks should have at least two layers to help prevent the dispersal of viral droplets from the nose and mouth that are associated with the transmission of COVID-19, according to a study published online Thursday in the journal Thorax.

“A single-layer cloth face covering reduced the droplet spread, but a double-layer covering performed better,” the authors of the study write.

Still, “even a single-layer face covering is better than nothing,” they add.

These findings are in line with those from a study published earlier this month, which reported that a stitched, double-layered cotton mask was more effective at keeping respiratory droplets from reaching others than a single-layer bandana-style covering, a non-stitched mask made from folding up a cotton handkerchief, or a non-sterile cone-style mask available in most pharmacies.

The findings also come as more and more states and businesses are instituting mask requirements. On Saturday, a statewide mask mandate issued by Gov. Tim Walz will take effect. Minnesotans will be required to wear some kind of facial covering while indoors in business and public settings, including on public transportation.

Health officials at the Centers for Disease Control and Prevention (CDC) recommend that everyone over the age of 2 wear some form of cloth face covering in public settings to help keep COVID-19 respiratory droplets from reaching others.

How the study was done

For the current study, a team of Australian researchers compared the effectiveness of three type of facial coverings: a single-layer, “no-sew” covering made from a folded piece of cotton; a sewn double-layer mask (as described by the CDC); and a three-layer surgical mask. The two cloth coverings were made from medium-weight cotton with a 170-thread count.

In a laboratory, a healthy volunteer with no respiratory infections spoke, coughed and sneezed while wearing no mask and then while wearing each of the three types of masks. The researchers used a LED lighting system and a high-speed video camera to capture the dispersal of airborne droplets in each scenario.

The video recordings “confirmed that even speaking generates substantial droplets,” the researchers write in an article for the Conversation. “Coughing and sneezing (in that order) generate even more.”

The recordings also showed that the surgical mask was significantly better than the two cloth masks at reducing the spread of droplets in all three (speaking, coughing, sneezing) scenarios. The cloth masks provided some protection too, although the single-layered mask let more droplets through than the two-layered one.

“We do not know how this translates to infection risk, which will depend on how many asymptomatic or mildly symptomatic infected people are around,” the researchers write. “However, it shows a single layer is not as good a barrier as a double layer.”

“In practice, we don’t yet know which has a greater effect — wearing masks to prevent infected people spreading to others or protecting well people from inhaling infected aerosols,” they add. “Probably both are equally important.”

The researchers point to a recent CDC study involving two hair stylists who continued working while infected with COVID-19. (They were awaiting the results of tests.) None of the stylists’ 139 clients became infected. The stylists and the clients had all worn either cloth or surgical masks.

“This is reassuring evidence that infection risk is reduced when everyone wears masks,” the researchers write.

Practical implications

Cloth masks with more than two layers were not tested in the current study, but the researchers believe that, generally, “more layers are better.” They point to a study that suggests a 12-layer cotton mask may be about as protective as a surgical mask.

Of course, stitching together 12 layers of cotton is not a practical thing to do. The study’s authors do have some other suggestions, however, for how you can make your cloth masks more effective:

• Increase the number of layers (at least three layers).
• Use a water-resistant fabric for the outer layer.
• Choose fabric with a high thread count (so a tighter weave, for instance from a good quality sheet is generally better than a fabric with a looser weave that you can clearly see light through).
• Hybrid fabrics such as cotton-silk, cotton-chiffon, or cotton-flannel may be good choices because they provide better filtration and are more comfortable to wear.
• Make sure your mask fits and seals well around your face.
• Wash your mask daily after using it.

FMI: You’ll find the study on the website for Thorax. You can find step-by-step instructions on how to make a cloth mask on the CDC’s website.