Double-masking — specifically, wearing a surgical mask underneath a cloth mask — may provide an extremely high level of protection against the viral particles that cause COVID-19, the Centers for Disease Control and Prevention reported Wednesday.
A year into the pandemic, the report is the first from the CDC that breaks down how well certain mask-wearing techniques work to protect wearers and those around them.
"We know that masks work," said Dr. John Brooks, a medical epidemiologist at the CDC and co-author of the new report. "This is about how to help them work better."
Common surgical masks, often seen in blue, are one popular option. While these masks do a good job at protecting the wearer from large droplets — say, splashes of blood or other fluids — they're less effective against tiny viral particles, because they don't fit tightly on the wearer's face.
Surgical masks work in part because they're made of polypropylene, which creates a tiny electric charge to capture viral particles. But the gaps that are created by improper fit leave too much room for those particles to escape or sneak in through the sides of the mask.
Indeed, surgical masks "do not provide complete protection from germs and other contaminants because of the loose fit between the surface of the mask and your face," the Food and Drug Administration says on its website.
Adding a cloth mask over the surgical mask, or knotting and tucking the mask for a tighter fit, can help eliminate those gaps.
In the new CDC report, researchers found that in "simulated breathing" experiments, a surgical mask alone blocked 42% of particles small enough to be considered "most important for transmitting SARS-CoV-2." One cloth mask did not fare much better, blocking about 44% of particles from an unmasked individual.
Double-masking, with a surgical mask and a cloth mask, they found, upped the protection for the wearer significantly, blocking 83% of small particles from an unmasked individual.
And "knotting and tucking" a surgical mask reduced a person's exposure to viral particles by about 65%.
Brooks said his team was focused less on the material, and more on the fit of masks. While multiple layers may be important, the new research suggests it's really the fit of the mask that's key. "Both of those proved effective in the experimental simulations that we ran," Brooks said.
And protection increased even more when more people wore masks.
If two people are both wearing either the knotted and tucked surgical masks, or if both are double-masked, exposure to viral particles was cut by more than 95%.
"We've been waiting, frankly, for a kind of official CDC communication" on masking, said Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco.
"We always thought those surgical masks were going to be good," Gandhi said, "but if you do this knot and tuck, this study shows that it can block as much virus as an N95." N95 respirators are considered the gold standard, and are worn by health care workers.
"Wearing any type of mask performed significantly better than not wearing a mask," CDC director Dr. Rochelle Walensky said Wednesday during a White House COVID-19 briefing, adding that "well-fitting masks provided the greatest performance."
Still, some experts question whether messaging on double-masking is useful, given that many Americans either don't wear a single mask or if they do, don't wear it properly. That means covering both the nose and mouth.
"My concept of double masking is you wear a mask and I wear a mask," said Dr. Carlos del Rio, executive associate dean at Emory University School of Medicine in Atlanta. "If we're both wearing a mask, we will be double masking, and we will be both protected."
Walensky said the CDC continues to "recommend that masks should have two or more layers, and completely cover your nose and mouth and fit snugly against your nose and the sides of your face." The CDC's website was updated Wednesday with the latest information on masks.
Brooks also noted that proper masking is expected to be effective no matter what variant of SARS-CoV-2 is circulating, because masks are designed to prevent particles of a certain size from getting through a physical barrier.
Those variants "have not changed the physical properties of the virus," he said. "Regardless of what variants are out there, masks will work against them the same way."
According to the CDC, 14 states and Washington, D.C., have universal masking mandates. The Biden administration issued an order requiring that people wear masks while on federal property, as well as a directive that anyone using public transportation must also wear a mask.
This story was originally published on NBCNews.com.