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The abrupt end of the federal mask mandate for public transportation and an uptick in coronavirus cases across the country have left some Americans wondering: Should I still wear a mask in certain situations or places?

The confusion comes after a federal judge struck down the transportation mandate, prompting airlines and transportation agencies to lift their mask rules just as cases are starting to tick up again. Most states and cities that still had indoor mask mandates lifted them weeks ago. President Joe Biden said Tuesday that people should decide for themselves if they want to wear masks or not.

Here's what we know about the science of masking to help you make decisions about if, when and where to cover your face.

How protective are masks when I'm wearing one and others are not?

Masks are most effective when they are blocking particles from entering your mouth and leaving others' mouths. But N95s and KN95s - considered the gold standard of masks - are still an effective shield against the coronavirus even when you are wearing them and others are not, experts say. The 95 in their name means the mask filters out 95% of particles that you would otherwise breathe in.

Linsey Marr, a Virginia Tech engineering professor who has conducted research on COVID and masking, emphasized that masks need to fit properly to effectively reduce exposure to coronavirus particles.

"The relationship between infection and exposure is not necessarily linear, so this does not necessarily translate to a 90-95% reduced risk of infection, but there will be a big drop in risk of infection," she said in an email.

Natascha Tuznik, an infectious-disease specialist at UC Davis Health, notes health-care workers wear N95s for hours on end and are well protected even when patients do not have their faces covered.

"Oftentimes the patients in the hospital aren't wearing masks ... and it confers very good protection," she said.

If I do wear a mask, what kind should I wear?

Lab researchers have found that various types of face masks, including cloth masks, surgical masks and N95 respirators, help prevent the spread of the coronavirus.

N95 masks provide the best protection because they generally fit tighter than cloth masks and are made with special material designed to block harmful particles. But the Centers for Disease Control and Prevention says any mask is better than no mask.

"I recommend a high-quality mask such as an N95, KN95, KF94, or high-filtration surgical mask," Laura Kwong, a researcher who has studied mask efficacy at the University of California at Berkeley's School of Public Health, said in an email.

One of the most important considerations is that the mask be snug across the nose and mouth and "conforms to your face without gaps," according to guidance from Johns Hopkins Medicine.

"It is important that most of the air you breathe in and out flows through the mask rather than around the mask through gaps at the sides, top or bottom," Johns Hopkins recommends. An ideal cloth mask consists of "several layers of tightly woven fabric and fits well over your nose and mouth to be an effective filter."

The CDC says a mask "should be a solid piece of material without slits, exhalation valves, or punctures." Using a mask fitter or tightening the mask's straps behind the ears to flatten any openings are both effective ways to increase the shielding effect, researchers have found. Face shields are not a substitute for a mask across the nose and mouth, the CDC says.

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How do I find reliable high-quality masks?

It can be hard to determine where to buy the best high-quality masks, particularly as the online market is ripe with counterfeits, particularly for KN95s, which are regulated differently.

The CDC has a list of respirator models and their manufacturers approved by the National Institute for Occupational Safety and Health at https://www.cdc.gov/coronavirus/2019-ncov/your-health/free-masks.html.

Many pharmacies provide free N95 respirators. To find one near you, use the CDC location finder at https://www.cdc.gov/coronavirus/2019-ncov/your-health/free-masks.html and call ahead to make sure they are still in stock.

What does research tell us about the effectiveness of masking?

Many studies support the use of masks to reduce transmission of the coronavirus, which is spread when an infected person expels tiny particles of virus into the air and someone else breathes those particles in. Masks work by erecting a barrier that can stop those airborne virus particles from being inhaled by an uninfected person, and KN95 and N95 masks provide the best protection.

A study that looked at mask use in California found that people who reported always wearing a cloth mask in indoor public spaces last year were 56% less likely to test positive compared with people who did not wear masks. The protection grew to 66% for those who consistently wore surgical masks and to 83% for those wearing N95 or KN95 masks.

Masks can offer another layer of protection as new variants evade vaccine-boosted immunity. A CDC study that looked at an anime convention in New York City - where 53,000 people gathered just as the omicron variant began to spread in the United States last November - found only a fraction of attendees contracted the virus at the mask-mandatory event. Those who did get sick were far more likely to report socializing in bars or nightclubs, participating in karaoke, and eating or drinking indoors near others for longer than 15 minutes, the study said.

A large-scale, randomized trial in Bangladesh led by researchers from Stanford Medicine and Yale University found that even modest mask use within a community can reduce transmission, particularly among older people.

"An approximately thirty percentage point increase in mask-wearing among all community members in public resulted in a 35% reduction in COVID-19 among individuals over 50 years old," Kwong, a co-author of the study, said in an email.

What if I am immunocompromised or have a child under age 5?

Doctors recommend that people with immune deficiencies keep wearing a mask in enclosed public spaces. Those who are immunocompromised tend to develop a lower level of antibodies to the coronavirus than others - even if vaccinated. A mask adds an extra layer of protection.

Children under age 5 are ineligible for a coronavirus vaccine. The American Academy of Pediatrics says that face masks are safe to wear for children age 2 or older and that "continued use of high-quality, well-fitting face masks in public settings may be warranted for these children and the individuals around them," especially children between 2 and 5 years old and those who are immunocompromised or at a higher risk of contracting the coronavirus.

"I definitely would wear a mask if you have any underlying immunocompromising condition or are under 5 years of age and are not eligible for a vaccine" while traveling, said Tina Q. Tan, a pediatrician and infectious-disease specialist at the Ann and Robert H. Lurie Children's Hospital of Chicago and professor at the Northwestern University Feinberg School of Medicine. "I would also encourage anyone who wants more protection to wear a mask, especially in crowded areas. The mask will provide protection to the person wearing it even if others are not."

What do we know about coronavirus risk while flying?

Experts say transmission risk is lower when a plane is flying because of the way the air is filtered. But they still recommend masking during air travel because of heightened risk inside airports and when filtration systems are turned off on the plane.

"So the air on the plane is extraordinarily safe. Of course, the people are the ones who bring COVID onto a plane," said Leonard Marcus of the Harvard T.H. Chan School of Public Health, who participated in an airline-funded study finding flying can be safe with proper precautionary measures. "It requires multiple layers. The air system itself won't do the job fully."

Masks are among those layers to reduce risk, experts say.

"The combination of good ventilation and filtration in any form of transportation or indoor setting, and an N95 - with those things, I think you are very well protected," said Marr, the Virginia Tech professor. "There are times, though, on the airplane, like during boarding and deplaning when they're not necessarily running the ventilation and filtration systems, and people are up in the aisle and everyone's moving around talking, that is a riskier time."

In a review of studies that looked at five eight-hour flights where some passengers tested positive for the coronavirus, the virus was not passed on to any other passengers when mask requirements were enforced. Meanwhile, on three flights where masks were not widely worn, between two and 15 additional passengers contracted the virus within two weeks of disembarking.