02/12/21 Update: The Centers for Disease Control now recommends double-masking for additional protection against the spread of coronavirus. You can read more here.

Public health policy for slowing the spread of coronavirus has made it clear that masks are a key intervention. By inhibiting both the distribution and inhalation of potentially infectious droplets, wearers are less likely to be involved in a contagious social encounter.

Recently, White House advisor Dr. Anthony Fauci suggested that if one mask is good, two may be better. “So if you have a physical covering with one layer, you put another layer on, it just makes common sense that it likely would be more effective,” Fauci told Today. “That’s the reason why you see people either double masking or doing a version of an N95.”

Fauci’s comments followed a number of high-profile individuals, including several people attending the January 20 inauguration of President Joe Biden, being photographed wearing two layers of masks, typically a surgical-style mask held in place by a cloth mask. In February, the Centers for Disease Control recommended double-masking to further reduce the spread of the coronavirus.

According to University of California, San Francisco professor and infectious disease expert Dr. Monica Gandhi, the advice is sound—so long as you’re doing it properly.

“The objective is to improve both fit and filtration by layering a surgical and cotton mask,” Gandhi tells Mental Floss. “The surgical mask made of polypropylene electrostatically repulses the virus, whereas the cotton mask fibers physically block it. The fit is improved by tamping down the masks with two layers.”

The term “double-masking” is a bit of a misnomer. While accurate, the objective is to add a filtering layer to a cloth mask. Surgical masks, which are the disposable coverings typically worn in health care settings, have a filtering effect similar to the superior N95 mask, which are in short supply. But the surgical mask doesn’t conform to the face very well. By layering a homemade cotton mask on top, the wearer receives the benefit of the polypropylene layer without the gaps commonly seen around the nose and cheeks.

Cloth masks combined with a surgical mask or other polypropylene filter offer improved protection.Courtesy of Monica Gandhi.

The effect is similar to cotton masks that already use a polypropylene layer. If you have one of these types of homemade masks, it’s not necessary to double-mask, Gandhi says.

In a new commentary [PDF] she co-authored with Virginia Tech engineering professor Linsey Marr for the journal Cell, Gandhi offered a reminder of how cloth masks work. Even though their woven fabric is permeable, the nature of cotton fibers means that pathogens have to navigate a kind of obstacle course, moving around bends and curves. Many aerosols can’t make the trip, slamming into fibers or sticking to them. This partial protection is bolstered by the addition of the surgical mask, which repels particles.

There are some caveats. While surgical masks are useful for double-masking, it’s important to make sure they’re made of polypropylene material. Gandhi says that masks labeled for “surgical” or “medical” use should be fine.

Double-masking is also no substitute for an N95 mask, which blocks 95 percent of airborne particles, is typically worn by health care professionals, and works best when it’s been fit-tested for a proper face seal. Double-masking is also no substitute for practicing the current social distancing guidelines.

If double-masking is superior, why are health experts advocating for it only now? It’s not exactly a new idea; filters for masks have been suggested previously. But Gandhi says the reinforcement is cropping up now for a good reason. “We always knew cloth masks alone didn’t block out all virus, but they were recommended to increase acceptability and because the surges were smaller before now,” she says. “We are now in our third and deadliest surge with a lot of circulating virus in this country, so increasing mask performance will help bring those transmission numbers down as we await mass vaccination to get out of the pandemic.”