Does My Mask Protect Me Against Covid-19 If Others Don’t Wear One?

If I’m the only person wearing a mask on a plane or subway train, in a store or at another indoor location, am I really protected from infection?

It’s true that masks work best when everyone around you is wearing one. That’s because when an infected person wears a mask, a large percentage of the infectious particles they exhale are trapped, stopping viral spread at the source. And when fewer viral particles are floating around the room, the masks others are wearing are likely to block those particles that have escaped.

But there is also plenty of evidence showing that masks protect the wearer, even when others around them are mask-free.

The amount of protection depends on the quality of the mask and how well it fits. Health experts recommend using an N95, KN95 or KF94 to protect yourself against the Omicron subvariant BA.2, which is now the dominant version of the coronavirus and is far more infectious than previous strains.

Other variables, such as how much time you are exposed to an infected person and how well a space is ventilated also will affect your risk.

On most planes, for example, the cabin air is frequently pumped through high-efficiency particulate air (HEPA) filters that are pretty good at reducing virus transmission. But that doesn’t completely eliminate your risk. In a modeling study, published in December 2021, researchers found that passengers sitting in the same row or one row away from someone who had Covid-19 still had a high risk of getting infected through direct respiratory droplets. Wearing a mask reduced the risk of infection by 54 percent.

In closed settings like a plane, it can be hard to avoid a mixed group of vaccinated and unvaccinated people, symptomatic or asymptomatic people, all of whom may transmit the coronavirus at varying levels. And the flight may not even be the riskiest part of a trip.

Air circulation may be running at low strength or completely turned off during boarding and deplaning, so those periods, as well as the time you spend at the airport, may be when you are at most risk of exposure and may benefit from wearing a mask.

There is also real-world data supporting the protective effects of masking in other indoor locations. In a 2020 investigation of a hotel outbreak in Switzerland, for instance, several employees and a guest who tested positive for the coronavirus were wearing only face shields (with no masks); those who wore masks were not infected. And a Tennessee study found that communities with mask mandates had lower hospitalization rates than areas where masks weren’t required.

“Health care workers, scientists who work with nasty pathogens, and workers who may be exposed to hazardous airborne particles on the job rely on specialized masks like N95s for protection, so we know that properly fitted, high-efficiency masks work,” said Linsey Marr, a Virginia Tech engineering professor and a leading expert on viral transmission.

A number of laboratory studies, too, have documented that a mask protects the person who is wearing it, though the level of that protection can vary depending on the type of mask, the material it’s made from, the experimental setup and how particle exposure was measured.

But the bottom line of all the studies is that a mask reduces the potential exposure of the person wearing it. Here are some of the findings.

• One study from the Centers for Disease Control and Prevention found that a standard surgical mask only protected the wearer from about 7.5 percent of the particles generated by a simulated cough. But knotting the loops and tucking in the sides of the medical mask reduced exposure by nearly 65 percent. (Watch this video to see the “knot and tuck” method.) Covering the surgical mask with a cloth mask, a technique known as double masking, reduced exposure to the simulated cough particles by 83 percent.

• A Virginia Tech study looked at how well homemade masks, surgical masks and face shields protected the wearer, based on virus particle size. The research showed that most masks could block very large particles, like those from a sneeze. But when the researchers looked at smaller aerosolized particles that are hardest to block, protection ranged from near zero with a face shield to about 30 percent protection with a surgical mask. (The percentages in the study can’t be directly compared to the C.D.C. knot-and-tuck study because the testing methods were different.)

  Based on the findings, Dr. Marr and her colleagues concluded that a two-layer cloth mask made of flexible, tightly woven fabric, combined with a filter material (like a coffee filter or surgical mask), could offer good protection, reducing 70 percent of the most penetrating particles and trapping 90 percent or more of the larger particles. They also found that head straps or ties created a better fit than ear loops.

• A study from Tokyo tested how well different types of masks protected the wearer from actual coronavirus particles. The study showed that even a simple cotton mask offered some protection (17 percent to 27 percent) to the wearer. Medical masks performed better, including a surgical mask (47 percent to 50 percent protection), a loose fitting N95 (57 percent to 86 percent protection) and a tightly sealed N95 (79 percent to 90 percent protection).

• While many lab studies test masks using mannequin heads, a 2008 study used real people to measure how well masks could protect the wearer against a respiratory virus. The study subjects wore different kinds of masks fitted with special receptors that could measure particle concentration on both sides of the masks. In this study, cloth masks reduced exposure by 60 percent, surgical masks by 76 percent and N95 masks by 99 percent.

While the lab studies all show a mask can protect the wearer, how well the masks perform in the real world depends on a number of variables, including how consistently people use them, whether a person is in high-risk situations and the rate of infection in the community.

The laboratory studies showed that a high-quality medical mask, like an N95, KN95 or KF94, works best. While vaccination and a booster shot is the best protection against infection with the coronavirus, even vaccinated people should be cautious around crowds or large groups indoors when the vaccination status of others isn’t known.

Dr. Marr also recommended wearing the highest-quality mask possible when you can’t keep your distance or be outdoors — or when nobody around you is masking up.

“If I’m in a situation where I have to rely solely on my mask for protection — unvaccinated people may be present, it’s crowded, I don’t know anything about the ventilation — I would wear the best mask in my wardrobe, which is an N95,” said Dr. Marr. “We need to wear the best masks possible in high-risk situations.”