When are two masks better than one for preventing COVID-19?

What’s better than one mask? The answer is rather obvious: two.

For the better part of the last year, public health officials have been advising people to wear a mask over their noses and mouths—sometimes to little avail. Now some medical experts are floating the option of adding a second layer to decrease the risk of getting or spreading COVID-19. It isn’t necessary in all scenarios, they say, but it can help protect the health of the general public in higher-risk situations.

When spending time indoors in public, for instance, adults may want to consider doubling up on masks, especially if they’re at high risk of severe COVID-19. Grocery stores, mass transit hubs, and doctor’s offices may warrant an extra level of care, though there are other considerations beyond time and place.

For starters, when doubling up, know that there are certain combinations of mask materials that offer more significant protection when than others. The best combo for the general public is one surgical mask and one cloth mask, says Monica Gandhi, an infectious disease specialist at the University of California, San Francisco. The cloth mask improves the fit and adds a layer of filtration, and the surgical mask helps block out aerosols and droplets that carry the coronavirus. “The surgical mask literally repels the virus electrostatically,” Gandhi says. “It’s not about thickness or the fibers.”

If you can’t access a surgical mask or are leaving them to health care workers, Gandhi and environmental engineer Linsey Marr provide another option in a recently published piece in Cell. They suggest wearing a cloth mask with two tightly woven layers and a filter made out of a type of plastic called polypropylene in between. Polypropylene works similarly to surgical masks, and is used in many everyday products, including vacuum bags.

You also don’t need to pile up your masks if you’re just getting some fresh air, Gandhi says: Wearing a two-layer cloth mask or a surgical mask is sufficient outdoors where transmission is less likely because it’s easier for virus particles to disperse on the wind. Double masking also isn’t a great fit for kids. “Children are not only less vulnerable [to virus symptoms], but there is something about choosing their own color and having a fun mask that matters to them,” Gandhi says.

In general, mask use has made a stark difference during the pandemic. Much of the evidence in support of it comes from reports of declining cases in areas that impose mask mandates. For example, Kansas counties that began mandating masks when COVID-19 cases surged over the summer saw a drop in cases compared to neighboring counties that didn’t. Meanwhile, in other parts of the country, higher rates of mask-wearing greatly curbed infection rates in some communities, a new study in The Lancet finds.

Lab studies, however, have proven that not all masks are equal. Gandhi and Marr’s masking recommendations stem from experiments that tested the effectiveness of different materials—both in terms of how many particles the wearer exhales through the mask, and how many get through to the wearer. One study found that surgical masks protect the wearer from influenza virus in droplets and aerosols by an average of 80 percent. Another found that homemade masks protect the wearer by 50 percent against air pollution particles.

Even if masks don’t totally block out the virus, they can still prevent a person from feeling the full force of COVID-19. The lower the viral load an individual comes in contact with, the less sick they’re likely to get, Gandhi says. Studies have shown this to be true for many viruses in lab animals and for the flu and rhinovirus in humans. Coronavirus research on hamsters also supports that the effect applies to the current pandemic. This is another important reason to wear a cloth mask with a filter or to double up on masks: If you do get infected, you may have a more mild or asymptomatic case of the illness.

The only major downside to double masking? It makes breathing harder. But then again, so does COVID-19. Pick and choose your battles.

Correction: The article previously had a photo of valved N95 masks, which may protect the wearer but also make others around them more susceptible to aerosolized viral particles. Read more about that here.