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A federal judge in Florida struck down the U.S. Centers for Disease Control and Prevention (CDC)’s public transportation mask mandate on April 18, meaning that people no longer have to wear a mask on public transportation in most cases.

As mandates end, some public-health experts have advised people to keep masking on public transportation and in other crowded indoor areas, even if others around them aren’t. People at high risk for severe COVID-19—including those who are immunocompromised or have chronic conditions—may want to keep wearing a high-quality mask even when doing so isn’t recommended for everyone in their area, under CDC guidance.

But is “one-way masking”—or wearing a mask even if no one else around you does—really enough to protect you from getting infected by SARS-CoV-2, the virus that causes COVID-19?

“One-way masking is definitely better than nothing,” says Emily Sickbert-Bennett, director of infection prevention at UNC Medical Center—but she says it’s also not as good as universal masking.

A mask is meant to work in two different ways: by containing the wearer’s germs and filtering out other people’s germs. If two people are wearing masks that trap some of the particles they exhale, there will likely be fewer germs floating around their shared air, and both people will have a lower chance of getting sick. “One of the key things you want to do in infection control is block the source,” says Kimberly Prather, an aerosol expert and chair of atmospheric chemistry at the University of California, San Diego. “Never let it in the air in the first place.”

If one person is unmasked and freely exhaling germs, the burden of protection is all on the other person’s mask. While cloth and surgical masks provide some defense in this scenario, respirators such as N95s and KN95s are designed to filter out almost all particles, making them the best and most protective options for one-way masking.

A modeling study published in the journal PNAS in December 2021 estimated a masked person’s likelihood of getting sick after talking with an unmasked person who has COVID-19. Someone wearing a surgical mask had up to a 90% chance of getting infected after half an hour (even while seated about five feet apart from the sick person), while someone in a respirator had around a 20% chance after a full hour, the researchers estimated. If both the sick person and their companion wore respirators, the infection risk dropped to just 0.4% after an hour.

In other words, wearing a high-quality mask offers substantial protection even if others aren’t wearing one—but two-way masking is even better, the study suggests.

A relatively small CDC study published in February provides some real-world data. It found that people who said they always wore respirators in public indoor settings were 83% less likely to later test positive for COVID-19, compared to people who said they did not wear a mask indoors. People who wore surgical masks were 66% less likely and people who wore cloth masks were 56% less likely to test positive. Of course, someone who wears a respirator in public is likely more cautious about COVID-19 overall than someone who goes unmasked indoors—and the data are self-reported—but the CDC’s authors still concluded that masks help prevent people who wear them from getting sick.

Read More: What to Do If You Test Positive for COVID-19 While Traveling

Respirators like N95s are the most protective masks on the market, but they only work to their fullest extent when they fit properly. Your respirator should create a tight seal on your face, without gaps that let in air. (Experts say that a good way to test this is to cup your hands near your face and breathe in and out. If your mask fits well, you shouldn’t feel any air escaping.)

There are also lots of knockoff respirators for sale, so be careful when purchasing. Packaging for real N95s will be marked with an approval label from the National Institute for Occupational Safety and Health (NIOSH). The nonprofit Project N95 also has guidance on how to find authentic respirators, and a White House website directs people to where to find free N95 masks in their area.

If you don’t want to wear a respirator but still want to wear a mask in public, you can make a surgical mask more effective with a few simple adjustments, Sickbert-Bennett says. Knot the ear loops for a tighter fit, and use the metal nose bar to help the mask conform to the contours of your face. (Sickbert-Bennett recommends using both hands to smooth the bar across either side of your nose instead of pinching it, which can create a point at the bridge of your nose that lets in air.) If you want to wear a cloth mask, layer a surgical mask over it for added protection.

Read More: Which Airlines—and Other Transit Services—Are Still Requiring Masks

Aside from staying up-to-date on COVID-19 vaccinations, “having a well-fitted mask is the best thing you can do” to protect yourself in public indoor settings, Sickbert-Bennett says. But it’s not foolproof. If you’re around someone with COVID-19, lots of factors can influence whether or not you get infected: how contagious they are, if they’re masked, how much protection you have from vaccines and prior exposures, how strong your immune system is, and so on.

Even though one-way masking isn’t perfect, Dr. Monica Gandhi, associate chief of the University of California, San Francisco’s division of HIV, infectious diseases, and global medicine, argues it’s an appropriate strategy now that highly effective vaccines and boosters are widely accessible in the U.S. and therapies are available for people who don’t respond well to vaccination.

Mask mandates have proven to be imperfect, Gandhi says. Some people wore masks improperly or not at all, while others (initially at the urging of U.S. public-health officials) chose lower-quality cloth masks. She argues it’s time to recognize those shortcomings, shift away from mandates, and instead encourage people who want extra protection to wear high-quality respirators.

But the most vulnerable members of society may face an increased risk of infection now that masks are off in most shared indoor spaces. Kids under 5 can’t get vaccinated yet, and older people and those who are immunocompromised may still be cautious about getting infected. “It’s a much bigger burden now on the high-risk individuals,” Prather says.

Prather says she would feel more comfortable about mask mandates ending if the U.S. had better standards around measuring indoor air quality, properly ventilating spaces, and filtering public indoor air. Cleaner indoor air could act as “a mask for the whole room,” reducing or even eliminating individuals’ need for actual masks, she says.

Until and unless that happens, though, wearing a high-quality mask in indoor spaces is a good way to help protect yourself—even if others aren’t.

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